Lithium ion secondary battery

Abstract

This invention relates to the field of battery technology, specifically to a lithium-ion secondary battery. The lithium-ion secondary battery includes an electrode assembly, which includes a positive electrode sheet. The positive electrode sheet includes a positive current collector, a positive active layer, a protective layer, and a surface coating layer. The positive active layer is located on at least one side surface of the positive current collector, the protective layer is located between the positive current collector and the positive active layer, and the surface coating layer is located on the outer surface of the positive active layer away from the positive current collector. The thickness of the surface coating layer containing a solid electrolyte is 'a', where 'a' is 0.5 μm to 5 μm; the thickness of the protective layer is 'b', where 'b' is 0.2 μm to 5 μm; 0.1 ≤ a / b ≤ 10. The negative electrode sheet includes a negative current collector and a negative active layer comprising silicon-carbon material. One side outer surface of the positive electrode sheet has several protrusions. The lithium-ion secondary battery of this invention combines high energy density, low-temperature discharge performance, and needle penetration safety performance.

Description

Technical Field

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

[0002] Lithium-ion rechargeable batteries are characterized by high energy density, good cycle performance, and long service life, and are gradually gaining a larger market share in energy storage, power batteries, and 3C electronics, showing broad application prospects. However, with technological innovation and industrial development, higher demands are being placed on the energy density and safety performance of lithium-ion rechargeable batteries. Although silicon-based anodes can improve battery energy density, the poor conductivity and significant volume expansion of silicon-carbon materials not only increase lithium-ion transport resistance but may also cause local deformation of the electrode and separator during battery cycling, increasing the risk of short circuits between the positive and negative electrodes and posing challenges to needle penetration safety. Existing technologies often improve needle penetration by setting a protective layer between the positive electrode current collector and the positive electrode active layer. However, the inorganic particles used in the protective layer are mostly electronic insulators and / or ionic insulators, or have low ionic and / or electronic conductivity, which is not conducive to improving the dynamic performance of the positive electrode and makes it difficult to achieve a balance between needle penetration safety and low-temperature discharge performance. Therefore, it is of great significance to provide a high-energy-density lithium-ion rechargeable battery that combines needle penetration safety and low-temperature discharge performance. Summary of the Invention

[0003] The purpose of this invention is to provide a lithium-ion secondary battery (hereinafter referred to as the battery) that not only has high energy density characteristics, but also maintains good safety performance under mechanical abuse, and has good low-temperature discharge performance.

[0004] To address the aforementioned problems in the prior art, the inventors of this invention, through experimental research, discovered that by simultaneously providing a protective layer between the positive electrode active layer and the positive electrode current collector, and by providing a surface coating layer on at least one side of the positive electrode active layer to at least partially cover it, and by providing several protrusions on one side of the outer surface of the positive electrode sheet, and further adjusting the thickness of the surface coating layer and the protective layer, as well as the relationship between their thicknesses, the battery can achieve a balance between high energy density, low-temperature discharge performance, and needle penetration safety performance. Specifically: Firstly, the protective layer absorbs and disperses stresses on the positive electrode (such as expansion stress and rolling stress), preventing the positive electrode active layer from detaching or peeling off from the current collector surface and reducing the risk of battery short circuits. Secondly, it adheres firmly to the surface of the positive electrode current collector. When the stability of the electrode-separator interface decreases, it can cover the surface of the positive electrode current collector to form a protective layer, preventing battery short circuits caused by direct contact between the positive and negative electrodes, thereby improving the battery's needle penetration safety performance. Inorganic particles commonly used in protective layers have good interfacial compatibility with the positive electrode current collector and can adhere well to its surface. For example, alumina or boehmite can form Al-O-Al covalent bonds with the positive electrode current collector, improving adhesion. Secondly, the solid electrolyte in the surface coating has excellent ionic conductivity, which can reduce interfacial impedance, especially charge transfer impedance at low temperatures. The surface coating provides a continuous and rapid ion channel for lithium ions, reducing the diffusion barrier of lithium ions on the positive electrode surface and promoting lithium ion extraction. In addition, by utilizing the high ionic conductivity and electronic insulation characteristics of the solid electrolyte, the conduction rate of lithium ions in the surface coating is significantly improved compared to low-temperature electrolytes. At low temperatures, the battery no longer relies on the electrolyte to complete interfacial ion transport, resulting in less polarization during low-temperature charge and discharge, a more stable voltage plateau, and higher usable capacity. The surface coating can also achieve physical isolation between the positive electrode active material and the electrolyte, reducing the disordered growth of the CEI film at the positive electrode and maintaining low impedance and high stability at the interface. Finally, to balance the effects of the topcoat and protective layers and enhance their synergistic effect, the thicknesses 'a' of the topcoat and 'b' of the protective layer, as well as the thickness ratio 'a / b', were further adjusted within a suitable range. This not only ensured that the protective layer effectively protected the positive electrode current collector, preventing short circuits between the positive and negative electrodes and improving the battery's puncture safety, but also ensured that the topcoat could construct a uniform and stable ion transport channel on the surface of the positive electrode active layer and act as a physical barrier to isolate the electrolyte from the positive electrode active layer. Most importantly, adjusting 'a / b' allowed the topcoat and protective layers to complement each other. The topcoat could compensate for the deficiencies in ion conduction of the protective layer, ensuring smooth and efficient lithium-ion insertion / extraction and improving the battery's low-temperature discharge performance. Conversely, the protective layer could compensate for the compatibility issues between the topcoat and the positive electrode current collector when the topcoat was placed on the surface of the positive electrode current collector. When 'a / b' met the specified range, the battery's puncture safety and low-temperature discharge performance could be improved while ensuring energy density.

[0005] Only by combining a surface coating containing solid electrolyte and a protective layer with matching thicknesses can a vertical lithium-ion channel be constructed on the surface of the positive electrode, forming good interfacial adhesion. This systematically solves the defects of a single surface coating or protective layer. At the same time, the protrusion on one side of the outer surface of the positive electrode serves as a contact conduction area, making close contact with the surface coating and strengthening the contact between the positive electrode and the separator. This further promotes the transfer of lithium ions through the solid electrolyte in the surface coating, resulting in less polarization during low-temperature charge and discharge, a more stable voltage platform, higher usable capacity, and improved low-temperature discharge performance. The protrusion design also increases the wetting space of the electrolyte. Combined with the high ionic conductivity of the solid electrolyte, this improves lithium-ion migration efficiency, thereby reducing the impact of the protective layer on the positive electrode impedance. While ensuring good needle penetration safety performance, this achieves low impedance, high stability, and high kinetic interface construction, enabling the battery to have high energy density, low-temperature discharge performance, and needle penetration safety performance.

[0006] Based on this, the present invention proposes the following technical solution: This invention provides a lithium-ion secondary battery, comprising an electrode assembly including a positive electrode and a negative electrode. The positive electrode includes a positive current collector, a positive active layer, a protective layer, and a surface coating. The positive active layer is located on at least one side surface of the positive current collector, the protective layer is located between the positive current collector and the positive active layer, and the surface coating is located on the outer surface of the positive active layer away from the positive current collector. The surface coating includes a solid electrolyte, and the thickness of the surface coating is 'a', which is 0.5 μm to 5 μm. The thickness of the protective layer is 'b', which is 0.2 μm to 5 μm. 'a' and 'b' satisfy: 0.1 ≤ a / b ≤ 10. The negative electrode 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. One outer surface of the positive electrode has several protrusions.

[0007] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art: (1) In this invention, a protective layer is formed on the surface of the positive electrode current collector to reduce the risk of short circuit between the positive and negative electrodes and improve the needle penetration safety of the battery. (2) In this invention, the surface coating containing solid electrolyte isolates the electrolyte and the positive electrode active layer, reduces electrolyte side reactions, and constructs a uniform ion transport channel on the surface of the positive electrode, reducing interface impedance and improving the low-temperature discharge performance of the battery. (3) In this invention, the thickness of the surface coating and the protective layer and the relationship between their thicknesses are adjusted within a suitable range to achieve low-temperature discharge performance and needle penetration safety performance without affecting the battery energy density.

[0008] 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

[0009] Figure 1 The diagram shown is a schematic diagram of the positive electrode sheet along the thickness direction in one embodiment of the present invention.

[0010] Figure 2 The diagram shown is a structural schematic of an electrode assembly in one embodiment of the present invention.

[0011] Figure 3 The image shown is a cross-sectional scanning electron microscope image of the first positive electrode active layer in one embodiment of the present invention.

[0012] Figure 4 The image shown is a cross-sectional scanning electron microscope image of the positive electrode sheet in one embodiment of the present invention.

[0013] Reference numerals: 1. First region; 2. Second region; 3. Third region; 10. Positive current collector; 111. First positive active layer; 112. Second positive active layer; 12. Surface coating; 13. Protective layer; 14. Ceramic layer; 4. Positive electrode sheet; 5. Negative electrode sheet; 6. Separator. Detailed Implementation

[0014] 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.

[0015] The present invention provides a lithium-ion secondary battery, the lithium-ion secondary battery including an electrode assembly, the electrode assembly including a positive electrode sheet and a negative electrode sheet, the positive electrode sheet including a positive current collector, a positive active layer, a protective layer and a surface coating layer, the positive active layer being located on at least one side surface of the positive current collector, the protective layer being located between the positive current collector and the positive active layer, and the surface coating layer being located on the outer surface of the positive active layer away from the positive current collector.

[0016] In this invention, the surface coating comprises a solid electrolyte, and the thickness of the surface coating is a, which is 0.5μm to 5μm, for example, 0.5μm, 0.6μm, 0.8μm, 1μm, 1.5μm, 2μm, 3μm, 4μm or 5μm.

[0017] In one embodiment, a is 1 μm to 3 μm.

[0018] In this invention, the thickness of the protective layer is b, which is 0.2μm to 5μm, for example, 0.2μm, 0.4μm, 0.6μm, 0.8μm, 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm or 5μm.

[0019] In this invention, the protective layer comprises inorganic particles, which include at least one of alumina, boehmite, titanium dioxide, zirconium oxide, lithium iron phosphate, boron nitride, barium sulfate, barium titanate, and silicon dioxide.

[0020] In this invention, a and b satisfy: 0.1≤a / b≤10, for example, 0.1, 0.2, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.

[0021] In this invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on at least one side surface of the negative electrode current collector, wherein the negative electrode active layer comprises silicon-carbon material.

[0022] In this invention, one side of the outer surface of the positive electrode has a plurality of protrusions. The term "a plurality of" has a conventional meaning in the art, referring to a number of protrusions greater than or equal to two.

[0023] In this invention, the shape of the orthographic projection of the protrusion onto the outer surface of the positive electrode is not limited; it can be circular or rectangular. The protrusion can be obtained by conventional methods in the art, for example, by using an embossing roller with raised dots.

[0024] Adjusting the a / b ratio within a suitable range can improve the battery's puncture safety and low-temperature discharge performance while ensuring energy density. When a / b is too large (e.g., >10), a is too large and / or b is too small. While an excessively thick surface coating helps increase the lithium-ion transport rate at the positive electrode interface, it has a significant impact on the battery's energy density, which is not conducive to balancing energy density and low-temperature discharge performance. On the other hand, if the protective layer is too thin, it is insufficient to resist the impact of external forces such as punctures, which is not conducive to further improving the battery's puncture safety performance. When a / b is too small (e.g., <0.15), a is too small and / or b is too large. The protective layer is thick, while the surface coating is thin. Prioritizing the improvement of lithium-ion transport capacity on the positive electrode side, it cannot offset the increased impedance caused by the thick base coating, resulting in increased battery impedance. At the same time, the interface formed by the surface coating on the surface of the positive electrode active layer has limited effect on isolating the electrolyte and reducing interface impedance, which is not conducive to further improving the battery's low-temperature discharge performance.

[0025] Only by placing a surface coating containing a solid electrolyte on the surface of the positive electrode active layer and placing a protective layer between the positive electrode active layer and the positive electrode current collector can both needle penetration safety performance and low-temperature discharge performance be achieved. This is because, although the solid electrolyte in the surface coating has excellent ion conduction performance, its compatibility with the positive electrode current collector is poor. The contact between the solid electrolyte particles and the positive electrode current collector is a point-to-point physical contact with a small effective contact area. When a coating containing a solid electrolyte is used as a protective layer, on the one hand, a large number of microscopic voids will be formed between the positive electrode current collector and the protective layer, resulting in increased interfacial impedance and affecting the rate performance and charge-discharge polarization of the battery. On the other hand, the coating containing a solid electrolyte is brittle and has high internal stress, resulting in poor mechanical matching with the positive electrode current collector. When the electrode undergoes volume expansion / contraction, the coating will crack and warp rapidly, or even peel off from the positive electrode current collector, forming an open circuit. In contrast, the protective layer used in this invention has an Al-O-Al chemical bond with the positive electrode current collector, resulting in better interfacial compatibility. Therefore, the electrode structure and coating configuration in this invention can construct vertical and continuous ion channels on the surface of the positive electrode active layer, and directionally build an efficient lithium-ion transport interface. Without sacrificing the mechanical processing performance of the electrode, a uniform, dense and highly conductive interface film is formed, which blocks the erosion of electrolyte side reactions, reduces interface polarization, and achieves a balance between needle penetration safety performance and low-temperature discharge performance when combined with the protective layer.

[0026] In this invention, 'a' can be obtained using conventional testing methods in the art, such as by scanning electron microscopy (SEM). Specifically, after discharging the lithium-ion secondary battery to 0% SOC, the positive electrode sheet is disassembled and removed. It is then soaked in dimethyl carbonate (DMC) solvent for 12 hours, followed by rinsing with DMC to remove the lithium salt adhering to the positive electrode sheet. The positive electrode sheet located in the middle region is taken and cut using an argon-ion polishing machine to expose its cross-section. The obtained cross-section is imaged in an SEM device, and the area where the surface coating is located is located. The thickness of the surface coating located in the positive electrode active layer is measured using image analysis software. Ten measurements are taken, and the average value is calculated to obtain 'a'. It should be noted that when the surface coating thickness is thin, due to the high surface roughness of the positive electrode sheet, there may be severe thickness unevenness, or even missed areas. In this case, it is necessary to test the thickness of 10 areas without missed coatings and take the average value.

[0027] In this invention, b can be obtained by conventional testing methods in the art, such as by testing with SEM equipment, specifically as follows: After discharging the lithium-ion secondary battery to 0% SOC, the positive electrode sheet is disassembled and removed. It is then soaked in DMC solvent for 12 hours and rinsed with DMC to remove the lithium salt attached to the positive electrode sheet. The positive electrode sheet in this area is then cut using an argon ion milling machine to expose its cross-section. The cross-section of the obtained positive electrode sheet is imaged in an SEM device. The location of the protective layer is located by the interface. The thickness of at least 10 different points is measured using image analysis software, and the average value is taken to obtain b.

[0028] In this invention, the solid electrolyte includes at least one of oxide solid electrolytes, sulfide solid electrolytes, phosphate solid electrolytes, and borate solid electrolytes.

[0029] In one embodiment, the oxide-based solid electrolyte includes at least one of lithium oxynitride phosphate, lithium lanthanum zirconium gallium oxide, lithium lanthanum zirconium oxide (LLZO), lithium lanthanum titanate, lithium titanium aluminum phosphate (LATP), and lithium silicate-based oxides. In one embodiment, the sulfide-based solid electrolyte includes Li₂S-P₂S₅ glass, Li₂S-P₂S₅ glass-ceramic, and Li₂S-P₂S₅ glass. 10 GeP2S 12 At least one of them.

[0030] In one embodiment, the phosphate solid electrolyte includes Li3PO4 and / or LiPO3.

[0031] In one embodiment, the borate solid electrolyte includes Li3B7O. 12 .

[0032] In this invention, the positive electrode active layer includes a positive electrode active material, and the positive electrode active material includes lithium cobalt oxide.

[0033] In this invention, the lithium-ion secondary battery at 0% SOC has a total mass of the surface coating and the positive electrode active layer on the outer surface with the surface coating, and the mass ratio of Li element to Co element is k, where k is 0.95~1.5, for example 0.95, 0.96, 0.97, 0.98, 0.99, 1, 1.01, 1.02, 1.03, 1.05, 1.1, 1.2, 1.3, 1.4 or 1.5.

[0034] In one embodiment, k is 1.01 to 1.5.

[0035] In this invention, based on the total mass of the surface coating and the positive electrode active layer on the outer surface where the surface coating is provided, the mass content of Ti element is 500ppm to 4000ppm, for example, 500ppm, 520ppm, 540ppm, 560ppm, 58ppm, 600ppm, 650ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1500ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm or 4000ppm.

[0036] In this invention, based on the total mass of the surface coating and the positive electrode active layer on the outer surface where the surface coating is provided, the mass content of La element is 300ppm to 2000ppm, for example, 300ppm, 310ppm, 320ppm, 340ppm, 360ppm, 380ppm, 400ppm, 450ppm, 500ppm, 600ppm, 800ppm, 1000ppm, 1200ppm, 1600ppm or 2000ppm.

[0037] In this invention, based on the total mass of the surface coating and the positive electrode active layer on the outer surface where the surface coating is provided, the mass content of Al element is 7000ppm~15000ppm, for example, 7000ppm, 7200ppm, 7400ppm, 7600ppm, 7800ppm, 8000ppm, 8500ppm, 9000ppm, 9500ppm, 10000ppm or 15000ppm.

[0038] The solid electrolyte in the surface coating is mostly lithium-rich, containing excess extractable / contributable active lithium. This precisely fills the irreversible lithium gap caused by the formation of the SEI film on the silicon anode surface, improving the battery's initial coulombic efficiency. High initial coulombic efficiency can be achieved without additional lithium replenishment agents. At the same time, the solid electrolyte itself can block lithium dendrite growth, reducing the risk of separator puncture.

[0039] In this invention, based on the total mass of the surface coating and the positive electrode active layer on the outer surface with the surface coating, the content of elements Ti, La, Al, Li and Co can be obtained by conventional testing methods in the art, for example, by the following method: after discharging the lithium-ion secondary battery to 0% SOC, disassemble and remove the positive electrode sheet, soak it in DMC solvent for 12 hours and then rinse it with DMC to remove the lithium salt attached to the electrode sheet, calcine it in a high-temperature muffle furnace at 400°C for 3 hours, and then gently scrape the positive electrode active material and the surface coating from the aluminum foil surface, and measure the content of various elements in the standard solution according to the test method in GBT30902-2014.

[0040] In this invention, the outer surface of the other side of the positive electrode sheet also has a plurality of recesses, and the recesses and the protrusions are respectively arranged in the thickness direction of the positive electrode sheet.

[0041] In this invention, the depth of the recess is h, which is 2μm to 50μm, for example, 2μm, 3μm, 4μm, 5μm, 8μm, 10μm, 15μm, 20μm, 30μm, 40μm, or 50μm. The spacing between the recesses is 0.5mm to 10mm, for example, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, or 10mm. The width of the recess is 0.5mm to 10mm, for example, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, or 10mm.

[0042] In this invention, the height of the protrusion is h', which is 2μm to 50μm, for example, 2μm, 3μm, 4μm, 5μm, 8μm, 10μm, 15μm, 20μm, 30μm, 40μm, or 50μm. The spacing between the protrusions is 0.5mm to 10mm, for example, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, or 10mm. The width of the protrusion is 0.5mm to 10mm, for example, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, or 10mm.

[0043] In this invention, the positions of the recess and the convex portion correspond one-to-one.

[0044] In this invention, the term "a number" has the conventional meaning in the art, referring to a number of recesses greater than or equal to 2 and a number of protrusions greater than or equal to 2.

[0045] In this invention, the shape of the orthographic projection of the protrusion onto the outer surface of one side of the positive electrode sheet where the first positive electrode active layer is located is not limited; it can be circular or rectangular. Similarly, the shape of the orthographic projection of the concave portion onto the outer surface of one side of the positive electrode sheet where the second positive electrode active layer is located is not limited; it can be circular or rectangular.

[0046] In this invention, the depth h of the recess refers to the vertical distance from the lowest point of the recess to one side of the outer surface of the positive electrode sheet where the second positive electrode active layer is located. The width of the recess is the maximum distance between any two points on the contour of the recess. The spacing between the recesses refers to the distance between the lowest points of two adjacent recesses.

[0047] In this invention, the height h' of the protrusion refers to the vertical distance from the highest point of the protrusion to one side of the outer surface of the positive electrode sheet where the first positive electrode active layer is located. The width of the protrusion is the maximum distance between any two points on the contour of the concave portion. The spacing between the protrusions refers to the distance between the highest points of two adjacent protrusions.

[0048] In this invention, the depth, width, and spacing of the recesses can be measured using conventional testing methods in the art, such as a 3D profilometer. Specifically, after discharging the lithium-ion secondary battery to 0% SOC, the negative electrode is disassembled and removed. It is then soaked in DMC solvent for 12 hours and rinsed with DMC to remove lithium salts adhering to the electrode. A 3D profilometer is used to test the flatness of the outer surface of one side of the positive electrode where the second positive electrode active layer of the negative electrode is located. The depth of the recesses on the surface of the negative electrode is measured through image analysis. The depth of at least 10 recesses is measured and the average value is recorded as h. Two adjacent recesses are grouped together, and the spacing between at least 5 groups of adjacent recesses is measured using the 3D profilometer scale, and the average value is taken as the spacing of the recesses. The width of at least 10 recesses is measured using the 3D profilometer scale, and the average value is taken as the width of the recesses. During testing, when the number of recesses is less than 10, technicians can select a representative sample size based on the actual number. The testing methods for the height, width, and spacing of the protrusions can refer to the testing methods for the recesses.

[0049] In this invention, the electrode assembly includes a positive electrode sheet, a separator, and a negative electrode sheet that are stacked and wound together. Along the winding direction of the electrode assembly from the inside to the outside, the positive electrode sheet includes at least a first region and a second region. The positive electrode active layer includes a first positive electrode active layer and a second positive electrode active layer that are disposed opposite to each other along the thickness direction. The length of the first positive electrode active layer is greater than the length of the second positive electrode active layer.

[0050] In one embodiment, the surface coating is located on the surface of at least a portion of the first positive electrode active layer and at least a portion of the second positive electrode active layer opposite to the positive electrode current collector.

[0051] In one embodiment, the surface coating is located on at least a portion of the surface of the first positive electrode active layer opposite to the positive electrode current collector.

[0052] The phrase "at least part" can be understood as meaning that the area of ​​the orthographic projection of the surface coating layer on the first positive electrode active layer can be less than 100% or equal to 100%. Similarly, the area of ​​the orthographic projection of the surface coating layer on the second positive electrode active layer can be less than 100% or equal to 100%.

[0053] In this invention, the first region includes the positive current collector and the positive active layer located on both sides of the positive current collector, the protective layer is located between the positive current collector and the positive active layer, and the surface coating layer is located on at least a portion of the outer surface of the first positive active layer. The term "at least a portion" can be understood as meaning that the projected area of ​​the surface coating layer on the first positive active layer can be less than 100% or equal to 100%.

[0054] In this invention, the second region includes the positive current collector and the first positive active layer, and the protective layer is located between the positive current collector and the first positive active layer.

[0055] In one embodiment, the second region further includes the surface coating layer, which is located on the outer surface of the first positive electrode active layer.

[0056] In another embodiment, the surface coating is located on the outer surface of the first positive electrode active layer.

[0057] In this invention, the outer surface of one side of the positive electrode sheet where the first positive electrode active layer is located has the convex portion, and the outer surface of one side of the positive electrode sheet where the second positive electrode active layer is located has a plurality of concave portions.

[0058] The positive electrode sheet has concave and convex structures. The concave areas are stress concentration zones that can disperse and absorb the expansion stress generated during battery cycling, maintaining structural stability. The convex areas, as contact conduction regions, enhance the contact between the positive electrode and the separator, further promoting lithium-ion transfer. When a surface coating including a solid electrolyte is applied to the side of the convex area, the high ionic conductivity of the solid electrolyte, combined with the contact conduction effect of the convex area, constructs a vertical lithium-ion transport channel on the surface of the first positive electrode active layer. This promotes the lithium-ion transport efficiency at the positive electrode-separator interface, thereby further improving the battery's low-temperature discharge performance. When h and h' are too large (e.g., >50μm), the contact area between the electrolyte and the surface coating increases. Moreover, the overly sharp protrusion structure may cause cracking or damage to the surface coating, exacerbating electrolyte side reactions and hindering the stability of the positive electrode and further improvement of the battery's cycle stability. When h and h' are too small (e.g., <2μm), the concave / convex portion cannot provide sufficient buffer space for the volume expansion of the silicon-based negative electrode, and the contact area with the electrolyte is even lower. Wetting between the positive electrode and the electrolyte is more difficult at low temperatures, which is detrimental to the improvement of the battery's rate performance and low-temperature discharge performance.

[0059] In this invention, along the inside-out winding direction of the electrode assembly, the positive electrode sheet further includes a third region, which is adjacent to the second region, and the second region is located between the first region and the third region. The third region includes the positive current collector.

[0060] It is understood that the positive electrode active layer is not provided on the positive electrode current collector located in the third region.

[0061] In this invention, the positive current collector includes a first surface and a second surface, the first surface of the positive current collector is provided with a first positive active layer, and the second surface of the positive current collector is provided with a second positive active layer.

[0062] In this invention, the second surface of the positive current collector located in the second region and the third region includes a ceramic layer. For example... Figure 1 The diagram shows a schematic representation of the positive electrode sheet along its thickness in one embodiment of the present invention. Each of the two sides of the positive electrode sheet includes a plurality of protrusions 15 and a plurality of recesses 16. Region 1 is a first region, comprising a positive current collector 10, a first positive active layer 111, and a second positive active layer 112 disposed opposite to the first positive active layer 111. A surface coating 12 is provided on the surface of the first positive active layer 111, and a protective layer 13 is provided between the first positive active layer 111, the second positive active layer 112, and the positive current collector 10. Region 2 is a second region, comprising the positive current collector 10, the first positive active layer 111, and the protective layer 13 located between them. The surface coating 12 is provided on the surface of the first positive active layer 111, and a ceramic layer 14 is provided on the second surface of the positive current collector 10. Region 3 is a third region, comprising the positive current collector 10 and the ceramic layer 14 located on its second surface.

[0063] like Figure 2 The diagram shown is a schematic diagram of the electrode assembly in one embodiment of the present invention, wherein 4 is a positive electrode sheet, 5 is a negative electrode sheet, 6 is a separator, the surface of the first positive electrode active layer 111 has several protrusions and a surface coating, the surface of the second positive electrode active layer 112 has several concave portions, a protective layer is provided between the positive electrode active layer and the positive electrode current collector, and a ceramic layer 14 is provided at the winding tail.

[0064] In this invention, the ceramic layer comprises ceramic particles, which include at least one of lithium iron phosphate, alumina, boehmite, titanium dioxide, zirconium oxide, boron nitride, barium sulfate, barium titanate, and silicon dioxide.

[0065] In this invention, the thickness of the ceramic layer is c, which is 0.5μm to 10μm, for example, 0.5μm, 1μm, 2μm, 3μm, 4μm, 6μm, 8μm or 10μm.

[0066] In this invention, a and c satisfy: 0.07≤a / c≤12, for example, 0.07, 0.08, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or 12.

[0067] In one embodiment, 0.5 ≤ a / c ≤ 2.

[0068] At high temperatures, the lattice oxygen released from the positive electrode active material can undergo slight oxidation or lattice doping reactions with ceramic particles (such as alumina), causing the ceramic layer structure to collapse and lose its isolation and protection functions. In fact, the debris generated by the cracking of the ceramic layer may cause internal short circuits in the battery, exacerbating the risk of thermal runaway. In the positive electrode sheet located in the third region, the ceramic layer is exposed on the surface, making it more susceptible to reaction with the lattice oxygen released from the positive electrode, leading to ceramic layer failure. Therefore, the thickness of c needs to be increased to prevent failure.

[0069] The ceramic layer also reacts with HF, a byproduct of the electrolyte, to capture some of the HF. The resulting AlF3 is a non-conductive substance that accumulates at the interface between the positive electrode active layer and the ceramic layer, blocking lithium-ion transport channels. This leads to lithium deposition on the corresponding negative electrode side at the interface, causing the battery to develop ridges and deform, which is detrimental to improving battery cycle stability. Therefore, adjusting the a / c ratio within a suitable range can ensure the ceramic layer provides good protection while avoiding lithium deposition. This is because as the ceramic layer thickness increases, high-resistivity byproducts are more easily generated. In this case, it is necessary to increase the thickness of the surface coating to promote lithium-ion transport and prevent lithium deposition. When a / c is too large (e.g., >10), the ceramic layer is too thin, which limits its protective function. At the same time, there is no need for a thicker surface coating to improve lithium-ion transport. An excessively large a will occupy space in the positive electrode, which is not conducive to improving energy density. When a / c is too small (e.g., <0.05), the ceramic layer is too thick, which generates a large number of high-resistivity byproducts. The surface coating is too thin, which cannot provide more low-resistivity transport channels for lithium ions, which is not conducive to further improvement of the battery's low-temperature discharge performance.

[0070] In this invention, the thickness c of the ceramic layer can be obtained by conventional testing methods in the art, such as by SEM equipment, as follows: After discharging the lithium-ion secondary battery to 0% SOC, the positive electrode sheet is disassembled and removed. It is then soaked in DMC solvent for 12 hours and rinsed with DMC to remove the lithium salt attached to the positive electrode sheet. The positive electrode sheet (wound tail) located in the third region is selected, and the positive electrode sheet in this region is cut by an argon ion mill to expose its cross-section. The cross-section of the obtained positive electrode sheet located in the third region is imaged in an SEM device, and the thickness of the ceramic layer is measured. The average value of at least 5 different points is taken to obtain c.

[0071] In this invention, the average particle size of the ceramic particles is d1, which is 0.02μm to 2μm, for example, 0.02μm, 0.04μm, 0.06μm, 0.1μm, 0.2μm, 0.3μm, 0.4μm, 0.6μm, 0.8μm, 1μm, 1.2μm, 1.4μm, 1.6μm or 2μm.

[0072] The average particle size of the ceramic particles is limited to the range of 0.02μm to 2μm. At this range, the ceramic particles can stack on the surface of the positive electrode current collector to form a ceramic layer with a rigid skeleton and a rough surface. The gaps between the ceramic particles are large, and when subjected to external forces such as needle piercing, a heat insulation layer can be formed to prevent direct contact between the positive and negative electrodes. At the same time, when heated, it can block heat conduction and delay the spread of heat.

[0073] In this invention, the protective layer comprises inorganic particles with an average particle size of d2, which is 0.01 μm to 1.5 μm, for example, 0.01 μm, 0.02 μm, 0.04 μm, 0.06 μm, 0.08 μm, 0.1 μm, 0.2 μm, 0.4 μm, 0.6 μm, 0.8 μm, 1 μm, 1.2 μm or 1.5 μm.

[0074] Using inorganic particles with a smaller average particle size in the protective layer can reduce the coating thickness of the protective layer and reduce its adverse effect on energy density. At the same time, it can allow more inorganic particles to be deposited in the protective layer at the same thickness, so that the prepared protective layer has better structural density and stability, and improves the needle penetration safety performance of the battery.

[0075] In this invention, d1 and d2 satisfy: 1.3≤d1 / d2≤45, for example, 1.3, 1.5, 2, 3, 4, 6, 8, 10, 12, 14, 16, 20, 25, 30, 35, 40 or 45.

[0076] When a battery is subjected to external forces such as needle penetration, the nail first contacts the ceramic layer located on the outer fold of the electrode assembly. The ceramic particles in this layer are large and hard; under the force of the needle penetration, these particles break, dissipating kinetic energy and hindering further penetration. As the nail penetrates deeper, the dense protective layer composed of small-diameter inorganic particles blocks electron channels, isolates heat diffusion, and improves the battery's needle penetration safety performance. When d1 / d2 is too large (e.g., >45), the ceramic particle size is too large compared to the inorganic particles, resulting in decreased adhesion and uneven thickness of the ceramic layer. The gaps between particles are not only large but also interconnected, reducing the insulation breakdown voltage and decreasing insulation reliability, which is detrimental to improving battery safety performance. When d1 / d2 is too small (e.g., <1.3), the ceramic particle size is too small compared to the inorganic particles, requiring more binder to encapsulate them. At high temperatures, these particles are prone to cracking and powdering, losing their protective function and failing to form an effective needle penetration resistant, heat-insulating, and energy-absorbing structure, further hindering the improvement of needle penetration safety performance.

[0077] In this invention, the average particle size d1 of the ceramic particles can be obtained by conventional testing methods in the art, such as by SEM testing, specifically as follows: After discharging the lithium-ion secondary battery to 0% SOC, the positive electrode sheet is disassembled and removed. It is then soaked in DMC solvent for 12 hours and rinsed with DMC to remove the lithium salt attached to the positive electrode sheet. The positive electrode sheet located in the third region (the winding tail) is selected and cut using an argon ion milling machine to expose its cross-section. The cross-section of the positive electrode sheet located in the third region is imaged in an SEM device and magnified to a certain magnification (e.g., 20,000 times). The particle size of at least 15 different ceramic particles is determined using image analysis software, and the average value is taken as d1.

[0078] In this invention, the average particle size of the inorganic particles can be obtained by conventional testing methods in the art, such as by SEM testing, specifically as follows: After discharging the lithium-ion secondary battery to 0% SOC, the positive electrode sheet is disassembled and removed. It is then soaked in DMC solvent for 12 hours and rinsed with DMC to remove the lithium salt attached to the positive electrode sheet. The positive electrode sheet located in the first or second region (avoiding the coiled tail) is selected and cut using an argon ion mill to expose its cross-section. The cross-section of the obtained positive electrode sheet is imaged in an SEM device and magnified to a certain magnification (e.g., 20,000 times). The location of the protective layer is determined by the interface. The particle size of at least 15 different inorganic particles is measured using image analysis software, and the average value is taken as the average particle size of the inorganic particles.

[0079] In this invention, at least a portion of the protective layer is embedded in the positive electrode active layer, and at least a portion of the surface coating layer is embedded in the positive electrode active layer.

[0080] In this invention, at least a portion of the surface coating layer is embedded in the positive electrode active layer, and at least a portion of the protective layer is embedded in the positive electrode active layer. The terms "at least a portion" and "embedded" have conventional meanings in the art. "Embedded" can be understood as: along the thickness direction of the positive electrode sheet, the orthographic projection of the surface coating layer (or the protective layer) overlaps with the orthographic projection of the positive electrode active layer, and the area of ​​the overlapping portion on the surface coating layer (or the protective layer) can be less than 100% or equal to 100%. "At least a portion" can also be understood as: along the length direction of the positive electrode sheet, the surface coating layer (or the protective layer) can be partially embedded in the positive electrode active layer, or it can be completely embedded in the positive electrode active layer. "Partially embedded" means that within the surface coating layer (or the protective layer), both "embedded in the positive electrode active layer" and "not embedded in the positive electrode active layer" can coexist.

[0081] In this invention, the average embedding depth of the protective layer in the positive electrode active layer is L1, where L1 is 0.1 μm to 3 μm, for example, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.8 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, or 3 μm.

[0082] In this invention, the average embedding depth of the surface coating in the positive electrode active layer is L2, where L2 is 0.1 μm to 3 μm, for example, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.8 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, or 3 μm.

[0083] In some embodiments, the surface coating is not embedded in the positive electrode active layer, i.e., L1 = 0 μm.

[0084] In some embodiments, the protective layer is not embedded in the positive electrode active layer, i.e., L2 = 0 μm.

[0085] By adjusting L1 and L2 within a suitable range, the positive electrode active layer forms a gradual structure of bottom infiltration layer, body layer and surface infiltration layer in the direction from the positive electrode current collector to the opposite direction of the positive electrode current collector. This reduces interfacial stress, prevents cracking of the positive electrode active layer under high compaction density, and enables continuous conduction and transition of ions / electrons. It also reduces abrupt changes in interfacial impedance, resulting in a gradient infiltration structure without obvious interfaces within the active material layer.

[0086] In this invention, L1 and L2 can be obtained by conventional testing methods in the art, such as by scanning electron microscopy, specifically as follows: The lithium-ion secondary battery is discharged to 0% SOC, the positive electrode is disassembled and removed, or the positive electrode is directly removed, soaked and cleaned with dimethyl carbonate (DMC), and then dried. After processing with argon ion polishing technology, a cross-sectional sample of the positive electrode can be obtained. The obtained cross-section is imaged under a scanning electron microscope (SEM), and the positive electrode located in the second region (containing both a surface coating and a protective layer) is selected. In the selected region, the straight line containing the point where the distance from the outline of the positive electrode active material particle to the positive electrode current collector is the largest (parallel to the positive electrode current collector) is used as the first reference line (see [reference]). Figure 3 (White dashed line) Select at least 5 different points on the outline of the surface coating embedded in the positive electrode active layer, measure their vertical distances to the first reference line, and take the average value as L1; In the selected area, the straight line containing the point with the shortest distance from the outline of the positive electrode active material particle to the positive electrode current collector (parallel to the positive electrode current collector) is taken as the second reference line (see Figure 4 (White dashed line) Select at least 5 different points on the outline of the protective layer embedded in the positive electrode active layer, measure their vertical distance to the second baseline and take the average value as L2.

[0087] like Figure 3 The image shown is a cross-sectional scanning electron microscope image of the first positive electrode active layer in one embodiment of the present invention. Figure 4 The image shown is a cross-sectional scanning electron microscope image of the positive electrode sheet in one embodiment of the present invention.

[0088] In this invention, based on the total mass of the negative electrode active layer, the mass content of silicon element is w, where w is 1% to 70%, for example, 1%, 3%, 5%, 8%, 10%, 15%, 20%, 30%, 40%, 50%, 60%, or 70%.

[0089] In this invention, based on the total mass of the silicon-carbon material, the mass content of silicon element is 30% to 80%, for example, 30%, 32%, 34%, 36%, 38%, 40%, 45%, 50%, 55%, 60%, 65%, 70% or 80%.

[0090] In this invention, the silicon-carbon material comprises a porous carbon matrix and silicon particles located in the pores within the porous carbon matrix.

[0091] In this invention, the average particle size of the silicon-carbon material is 1μm to 20μm, for example, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm or 20μm.

[0092] In this invention, the negative electrode active layer further includes graphite material.

[0093] When the mass content of silicon in silicon-carbon materials and the mass content of silicon in the negative electrode active layer meet the above ranges, the battery can have good cycle life, excellent safety performance, and charge / discharge performance while ensuring high energy density.

[0094] Adjusting the average particle size of silicon-carbon materials within a suitable range is beneficial to improving the overall electrochemical performance of the negative electrode active material. When the average particle size of silicon-carbon materials is too small (e.g., <1μm), the negative electrode active material has a high specific surface area and a large contact area with the electrolyte, resulting in the consumption of more active lithium during the first charging cycle, thus leading to a lower initial coulombic efficiency. At the same time, the side reactions with the electrolyte are intense, which is not conducive to improving the battery's cycle stability. When the average particle size of silicon-carbon materials is too large (e.g., >20μm), the diffusion path of lithium ions within the negative electrode active material is long, resulting in poor kinetic performance of the negative electrode active material, which is not conducive to further improving the battery's low-temperature discharge performance and rate performance.

[0095] In this invention, the average particle size of the silicon-carbon material can be obtained by conventional methods in the art. For example, after discharging a lithium-ion secondary 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. Alternatively, the negative electrode sheet before soaking in electrolyte can be directly taken, and the negative electrode sheet can be cut using an argon-ion milling machine with a CP laser. Then, it can be observed using SEM in backscattered imaging mode. In this mode, the contrast of the silicon-carbon material is brighter (which can be used to distinguish the carbon-based material and conductive agent in the negative electrode active layer). Measured at 5000x magnification, at least 10 silicon-carbon particles with significantly different particle sizes are randomly selected, and the particle size of each silicon-carbon particle is measured, and the average value is taken. If the number of particles is less than 10 at 5000x magnification, another microscopic image is taken until 10 particles are measured. When the particles in the mirror image are regular circles, the particle diameter is the diameter of the regular circle; when the particles in the mirror image are not "regular circles", connect any two points on the edge of the particle to form a straight line segment inside the particle, and select the longest straight line segment inside the particle as the particle diameter.

[0096] In this invention, the mass content of silicon in the silicon-carbon material can be obtained using conventional testing methods in the art. For example, a lithium-ion secondary battery is discharged to 0% SOC, the negative electrode is disassembled and removed, or the negative electrode is taken directly before immersion in electrolyte. The cross-section of the negative electrode is polished using an argon-ion polishing machine, and the silicon-carbon material is observed in SEM equipment using backscatter imaging mode, magnifying it as much as possible. The cross-section of the silicon-carbon material particles is scanned using an energy dispersive spectroscopy (EDS) instrument, with the scanned area not less than 50% of the particle cross-section, and the scanning range should be completely within the particle cross-section. The mass content of elemental Si is then calculated. At least 10 particles are selected for measurement, and the average value is taken as the mass content of silicon in the silicon-carbon material.

[0097] In this invention, w can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled and removed. It is then soaked in dimethyl carbonate (DMC) solvent for 12 hours, followed by rinsing with DMC solvent to remove lithium salts 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. 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 the ash. The calculation formula is as follows: w = 7 × mass of ash / (15 × mass of test sample).

[0098] In this invention, w and k satisfy: 2≤k / w≤120, for example, 2, 3, 4, 5, 6, 8, 10, 15, 20, 30, 40, 50, 60, 80, 100 or 120.

[0099] In one embodiment, 9 ≤ k / w ≤ 25.

[0100] Adjusting k / w within a suitable range allows the surface coating and silicon-based anode to work synergistically, achieving a balance between energy density, safety performance, and initial coulombic efficiency. When k / w is too large (e.g., >120), the silicon doping of the anode is low, which is detrimental to improving energy density. However, the demand for lithium replenishment is also reduced, as there is no need for excessive additional lithium sources to compensate for the loss of active lithium. When k / w is too small (e.g., <2), the silicon doping of the anode is high, but the Li / Co ratio is low. The amount of active lithium in the system is insufficient to compensate for the loss of active lithium caused by the formation of the SEI film, which is detrimental to further improvement of the battery's initial coulombic efficiency and cycle stability.

[0101] In this invention, the thickness of the protective layer located in the first region is b1, and the thickness of the protective layer located in the second region is b2, where b2 ≥ b1.

[0102] In one embodiment, c ≥ b2 ≥ b1.

[0103] In this invention, a and b1 satisfy: 0.1≤a / b1≤10, for example, 0.1, 0.2, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.

[0104] In one embodiment, 0.2 ≤ a / b1 ≤ 6.

[0105] Adjusting a / b1 within a suitable range can improve the battery's puncture safety and low-temperature discharge performance while ensuring energy density. When a / b1 is too large (e.g., >10), a is too large and / or b1 is too small. Although the excessively thick surface coating improves the lithium-ion transport capacity on the positive electrode side, the increased thickness of the positive electrode sheet is detrimental to further improvement of battery energy density. On the other hand, the protective layer is too thin and insufficient to resist the impact of external forces such as puncture, which is detrimental to further improvement of battery puncture safety performance. When a / b1 is too small (e.g., <0.1), a is too small and / or b1 is too large. The protective layer is thick, while the surface coating is thin. Prioritizing the improvement of lithium-ion transport capacity on the positive electrode side, it cannot offset the increased impedance caused by the thick bottom coating, resulting in increased battery impedance. At the same time, the interface formed by the surface coating on the surface of the positive electrode active layer has limited effect on isolating the electrolyte and reducing interfacial impedance, which is detrimental to further improving the battery's low-temperature discharge performance.

[0106] It is understandable that when b=b1, a / b and a / b1 have the same meaning; when b=b2, a / b and a / b2 have the same meaning.

[0107] In this invention, b1 and b2 can be obtained by conventional testing methods in the art, such as the test method for b, except that when b1 is measured, a positive electrode located in the first region is selected, and when b2 is measured, a positive electrode located in the second region is selected.

[0108] In this invention, the diaphragm includes a base membrane, a nitrogen-containing coating, and an adhesive layer. The nitrogen-containing coating is located on at least one side surface of the base membrane, and the adhesive layer is located on at least one side outer surface of the diaphragm.

[0109] In one embodiment, the nitrogen-containing coating is located on one side surface of the base film, and the adhesive layer is located on both outer surfaces of the diaphragm.

[0110] In one embodiment, the nitrogen-containing coating is located on at least one side surface of the base film, and the adhesive layer is located on the outer surface of the nitrogen-containing coating.

[0111] In this invention, the base film comprises polyethylene (PE) and / or polypropylene (PP).

[0112] In this invention, the nitrogen-containing coating comprises nitrogen-containing particles, which include at least one of polyacrylonitrile, nitrile rubber, 1,3,5-triazine-2,4,6-triamine, melamine cyanurate, and melamine thiocyanate.

[0113] In this invention, the adhesive layer comprises polyvinylidene fluoride (PVDF) and / or polymethyl methacrylate (PMMA).

[0114] In one embodiment, the nitrogen-containing coating and the positive electrode are disposed opposite to each other.

[0115] The nitrogen-containing particles in the nitrogen-containing coating contain cyano groups, which can diffuse and contact the surface of the positive electrode active material, stabilize the transition metal atoms and the crystal structure of the positive electrode active material, reduce the release of active oxygen, alleviate the structural collapse of the positive electrode active material, reduce the generation of electrolyte side reactions, thereby cutting off the gas source and further improving the cycle stability of the battery.

[0116] In this invention, the term "0% SOC" refers to the battery being discharged at 0.1C to the lower limit voltage (e.g., 3.0V).

[0117] 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.

[0118] 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.

[0119] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0120] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0121] Example 1: (1) Preparation of the positive electrode: Alumina (d2 = 1.4 μm), conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 89:2:9 and stirred evenly with deionized water as a solvent to prepare a protective layer slurry. The resulting protective layer slurry was coated on the first and second surfaces of an aluminum foil to obtain a protective layer. The length of the protective layer on the first surface was the same as the length of the first positive electrode active layer, and the length of the protective layer on the second surface was the same as the length of the second positive electrode active layer. After baking, an aluminum foil with a protective layer was obtained. Lithium cobalt oxide, conductive carbon black, and PVDF are mixed in a mass ratio of 97:1:2, and N-methylpyrrolidone (NMP) is added and stirred evenly to prepare a positive electrode active slurry. The positive electrode active slurry is coated on the outer surface of the protective layer on the first surface to obtain the first positive electrode active layer, and coated on the outer surface of the protective layer on the second surface to obtain the second positive electrode active layer. The length of the first positive electrode active layer is greater than that of the second positive electrode active layer. After baking, a positive electrode sheet is obtained. LATP and PVDF are mixed at a mass ratio of 97:3, and NMP is added and stirred evenly to obtain a surface coating slurry. The surface coating slurry is applied to the outer surface of the first positive electrode active layer, and after baking and rolling, a positive electrode sheet with a surface coating is obtained. After roll forming, the positive electrode sheet is embossed by a embossing roller to form a protrusion on one side of the first positive electrode active layer and a concave part on one side of the second positive electrode active layer. The depth h of the concave part is 28μm, the width of the concave part is 4.5mm, and the spacing between the concave parts is 3.8mm. The height h' of the protrusion part is 28μm, the width of the protrusion part is 4.5mm, and the spacing between the protrusion parts is 3.8mm. The positive electrode sheet has a groove of a fixed size at a specific position, and the nickel electrode tab is laser welded into this groove. Finally, alumina (d1 is 1.9 μm) and PVDF are mixed at a mass ratio of 7:3, and then stirred evenly with deionized water as a solvent to prepare a ceramic slurry. The ceramic slurry is then coated on the second surface of the positive current collector located at the tail (the surface has no second positive active layer) to obtain a ceramic layer and the final positive electrode sheet. Where a is 1.8 μm, b1 is 1.9 μm, b2 is 1.9 μm, L1 is 0.8 μm, L2 is 1 μm, d1 / d2 is 1.36, c is 2.4 μm, k is 1.17, and the contents of Ti, La and Al are 1354 ppm, 270 ppm and 7560 ppm respectively, based on the total mass of the surface coating and the positive electrode active layer with the surface coating on the outer surface.

[0122] (2) Preparation of the negative electrode: Artificial graphite, silicon carbide material (average particle size 14.3 μm), conductive carbon black, lithium carboxymethyl cellulose (CMC-Li), and polyacrylic acid (PAA) were mixed in a mass ratio of 77.2:20:0.05:0.35:2.4, and deionized water was added to obtain a negative electrode active slurry. The negative electrode active slurry was coated on both sides of a carbon-coated copper foil, and after baking and rolling, a negative electrode sheet was obtained. The negative electrode sheet has grooves of fixed size at specific positions, and copper-plated nickel tabs are laser-welded into these grooves, with a w of 9%.

[0123] (3) Preparation of electrolyte: Ethylene carbonate (EC), propylene carbonate (PC), and propyl propionate (PP) were mixed in a weight ratio of 1:3:6 as a base solvent. Then, fluoroethylene carbonate and LiPF6 were dissolved in the base solvent to obtain the electrolyte. Based on the total mass of the electrolyte, the mass percentage of LiPF6 was 13%, and the mass percentage of fluoroethylene carbonate was 15%.

[0124] (4) Diaphragm: The diaphragm is a composite diaphragm with a base membrane of polyethylene (PE) with a thickness of 5μm. One side of the base membrane is provided with a melamine organic coating with a thickness of 2μm and an adhesive layer on the outer surface of the diaphragm. The surface of the organic coating is provided with a polymethyl methacrylate (PMMA) + PVDF mixed adhesive layer with a thickness of 2μm. The other side of the base membrane is provided with a pure PMMA adhesive layer with a thickness of 0.5μm.

[0125] (5) Preparation of lithium-ion secondary batteries: After the obtained positive and negative electrode sheets are slit and formed, they are wound with a separator to obtain a core. After encapsulation, baking, liquid injection, formation, secondary sealing, sorting and OCV, and edge folding and glue application, a lithium-ion secondary battery is obtained.

[0126] Among them, a / b1 is 0.95, a / c is 0.75, and k / w is 13.

[0127] Example 2: (1) Preparation of the positive electrode: Boehmite (d2 is 0.04μm), conductive carbon black and PVDF are mixed in a mass ratio of 89:2:9 and stirred evenly with deionized water as solvent to prepare a protective layer slurry. The resulting protective layer slurry is coated on the first and second surfaces of an aluminum foil to obtain a protective layer. The length of the protective layer on the first surface is the same as the length of the first positive electrode active layer, and the length of the protective layer on the second surface is the same as the length of the second positive electrode active layer. After baking, an aluminum foil with a protective layer is obtained. Lithium cobalt oxide, conductive carbon black, and PVDF are mixed in a mass ratio of 97:1:2, and NMP is added. The mixture is stirred evenly to prepare a positive electrode active slurry. The positive electrode active slurry is coated on the outer surface of the protective layer on the first surface to obtain the first positive electrode active layer, and coated on the outer surface of the protective layer on the second surface to obtain the second positive electrode active layer. The length of the first positive electrode active layer is greater than that of the second positive electrode active layer. The positive electrode sheet is obtained by baking. LLZO and PVDF are mixed at a mass ratio of 97:3, and NMP is added and stirred evenly to obtain a surface coating slurry. The surface coating slurry is applied to the outer surface of the first positive electrode active layer, and after baking and rolling, a positive electrode sheet with a surface coating is obtained. After roll forming, the positive electrode sheet is embossed by a embossing roller to form a protrusion on one side of the first positive electrode active layer and a concave part on one side of the second positive electrode active layer. The depth h of the concave part is 3μm, the width of the concave part is 9.8mm, and the spacing between the concave parts is 0.7mm. The height h' of the protrusion part is 3μm, the width of the protrusion part is 9.8mm, and the spacing between the protrusion parts is 0.7mm. The positive electrode sheet has a groove of a fixed size at a specific position, and the nickel electrode tab is laser welded into this groove. Finally, boehmite (d1 is 1.8 μm) and PVDF are mixed at a mass ratio of 7:3, and then stirred evenly with deionized water as a solvent to prepare a ceramic slurry. The ceramic slurry is then coated on the second surface of the positive current collector located at the tail (the surface has no second positive active layer) to obtain a ceramic layer and the final positive electrode sheet. Where a is 1 μm, b1 is 4.5 μm, b2 is 4.5 μm, L1 is 2.7 μm, L2 is 0.3 μm, d1 / d2 is 45, c is 0.6 μm; k is 1.1, based on the total mass of the surface coating and the positive electrode active layer with the surface coating on the outer surface, the contents of Ti, La and Al are 557 ppm, 988 ppm and 7021 ppm respectively.

[0128] (2) Preparation of the negative electrode: Artificial graphite, silicon carbide (average particle size 5.4 μm), conductive carbon black, CMC-Li, and PAA were mixed in a mass ratio of 77.2:20:0.05:0.35:2.4, and deionized water was added to obtain a negative electrode active slurry. The negative electrode active slurry was coated on both sides of a carbon-coated copper foil, and after baking and rolling, a negative electrode sheet was obtained. The negative electrode sheet has grooves of fixed size at specific positions, and copper-plated nickel tabs are laser-welded into these grooves, with a w of 9%.

[0129] (3) Preparation of electrolyte: EC, PC, and PP were mixed in a weight ratio of 1:3:6 as a base solvent. Then, fluoroethylene carbonate and LiPF6 were dissolved in the base solvent to obtain the electrolyte. Based on the total mass of the electrolyte, the mass percentage of LiPF6 was 13%, and the mass percentage of fluoroethylene carbonate was 15%.

[0130] (4) Diaphragm: The diaphragm is a composite diaphragm with a PE base membrane of 5 μm thickness. One side of the base membrane is provided with a melamine organic coating of 2 μm thickness and an adhesive layer on the outer surface of the diaphragm. The organic coating surface is provided with a PMMA+PVDF mixed adhesive layer of 2 μm thickness. The other side of the base membrane is provided with a pure PMMA adhesive layer of 0.5 μm thickness.

[0131] (5) Preparation of lithium-ion secondary batteries: After the obtained positive and negative electrode sheets are slit and formed, they are wound with a separator to obtain a core. After encapsulation, baking, liquid injection, formation, secondary sealing, sorting and OCV, and edge folding and glue application, a lithium-ion secondary battery is obtained.

[0132] Among them, a / b1 is 0.22, a / c is 1.67, and k / w is 12.2.

[0133] Example 3: (1) Preparation of the positive electrode: Titanium dioxide (d2 is 0.02μm), conductive carbon black and PVDF are mixed in a mass ratio of 89:2:9 and stirred evenly with deionized water as solvent to prepare a protective layer slurry. The obtained protective layer slurry is coated on the first and second surfaces of aluminum foil to obtain a protective layer. The length of the protective layer on the first surface is the same as the length of the first positive electrode active layer, and the length of the protective layer on the second surface is the same as the length of the second positive electrode active layer. After baking, an aluminum foil with a protective layer is obtained. Lithium cobalt oxide, conductive carbon black, and PVDF are mixed in a mass ratio of 97:1:2, and NMP is added. The mixture is stirred evenly to prepare a positive electrode active slurry. The positive electrode active slurry is coated on the outer surface of the protective layer on the first surface to obtain the first positive electrode active layer, and coated on the outer surface of the protective layer on the second surface to obtain the second positive electrode active layer. The length of the first positive electrode active layer is greater than that of the second positive electrode active layer. The positive electrode sheet is obtained by baking. Solid electrolyte and PVDF are mixed at a mass ratio of 97:3, and NMP is added and stirred evenly to obtain a surface coating slurry. The surface coating slurry is applied to the outer surface of the first positive electrode active layer. After baking and rolling, a positive electrode sheet with a surface coating is obtained. The solid electrolyte is obtained by mixing LATP and LLZO at a mass ratio of 1:1. After roll forming, the positive electrode sheet is embossed by a embossing roller to form a protrusion on one side of the first positive electrode active layer and a concave part on one side of the second positive electrode active layer. The depth h of the concave part is 47μm, the width of the concave part is 0.6mm, and the spacing between the concave parts is 9.7mm. The height h' of the protrusion part is 47μm, the width of the protrusion part is 0.6mm, and the spacing between the protrusion parts is 9.7mm. The positive electrode sheet has a groove of a fixed size at a specific position, and the nickel electrode tab is laser welded into this groove. Finally, alumina (d1 is 0.1 μm) and PVDF are mixed at a mass ratio of 7:3, and then stirred evenly with deionized water as a solvent to prepare a ceramic slurry. The ceramic slurry is then coated on the second surface of the positive current collector located at the tail (the surface has no second positive active layer) to obtain a ceramic layer and the final positive electrode sheet. Where a is 2.7 μm, b1 is 0.5 μm, b2 is 0.5 μm, L1 is 0.4 μm, L2 is 2.2 μm, d1 / d2 is 5, c is 4.8 μm; k is 1.25, and the contents of Ti, La and Al are 1380 ppm, 990 ppm and 7590 ppm respectively, based on the total mass of the surface coating and the positive electrode active layer with the surface coating on the outer surface.

[0134] (2) Preparation of the negative electrode: Artificial graphite, silicon carbide material (average particle size 19.7 μm), conductive carbon black, CMC-Li, and PAA were mixed in a mass ratio of 77.2:20:0.05:0.35:2.4, and deionized water was added to obtain a negative electrode active slurry. The negative electrode active slurry was coated on both sides of a carbon-coated copper foil, and after baking and rolling, a negative electrode sheet was obtained. The negative electrode sheet has grooves of fixed size at specific positions, and copper-plated nickel tabs are laser-welded into these grooves, with a w of 9%.

[0135] (3) Preparation of electrolyte: EC, PC, and PP were mixed in a weight ratio of 1:3:6 as a base solvent. Then, fluoroethylene carbonate and LiPF6 were dissolved in the base solvent to obtain the electrolyte. Based on the total mass of the electrolyte, the mass percentage of LiPF6 was 13%, and the mass percentage of fluoroethylene carbonate was 15%.

[0136] (4) Diaphragm: The diaphragm is a composite diaphragm with a PE base membrane of 5 μm thickness. One side of the base membrane is provided with a melamine organic coating of 2 μm thickness and an adhesive layer on the outer surface of the diaphragm. The organic coating surface is provided with a PMMA+PVDF mixed adhesive layer of 2 μm thickness. The other side of the base membrane is provided with a pure PMMA adhesive layer of 0.5 μm thickness.

[0137] (5) Preparation of lithium-ion secondary batteries: After the obtained positive and negative electrode sheets are slit and formed, they are wound with a separator to obtain a core. After encapsulation, baking, liquid injection, formation, secondary sealing, sorting and OCV, and edge folding and glue application, a lithium-ion secondary battery is obtained.

[0138] Among them, a / b1 is 5.4, a / c is 0.56, and k / w is 13.9.

[0139] Example 4 group: This set of examples is used to verify the impact of changes in "a / b", as detailed below: Example 4a is based on Example 3, except that the solid electrolyte in the surface coating is replaced with the same mass of LATP, a is 4.8 μm, L2 is 3 μm, and k is 1.49. Based on the total mass of the surface coating and the positive electrode active layer with the surface coating on the outer surface, the contents of Ti, La and Al are 2133 ppm, 270 ppm and 8125 ppm respectively. Example 4b is based on Example 2, except that the solid electrolyte in the surface coating is replaced with the same mass of LATP, where a is 0.7 μm, L2 is 0.1 μm, and k is 1.03. Based on the total mass of the surface coating and the positive electrode active layer with the surface coating on the outer surface, the contents of Ti, La and Al are 952 ppm, 273 ppm and 7321 ppm, respectively.

[0140] Example 5 group: This set of examples is used to verify the impact of the change in "d1 / d2", as follows: Example 5a is based on Example 1, except that d1 is 1.9 μm and d2 is 1.4 μm; Example 5b is based on Example 1, except that the inorganic particles in the protective layer are replaced with boehmite of equal mass, with d1 being 0.04 μm and d2 being 1.9 μm.

[0141] Example 6 group: This set of examples is used to verify the impact of changes in "a / c", as detailed below: Example 6a is based on Example 4b, except that c is 9.6 μm; Example 6b is based on Example 4a, except that c is 0.4 μm; Example 6c is based on Example 1, except that c is 11 μm; Example 6d is based on Example 1, except that a is 0.5 μm, L2 is 0.1 μm, c is 9.6 μm, k is 1.01, and the contents of Ti, La and Al are 937 ppm, 265 ppm and 7053 ppm respectively, based on the total mass of the surface coating and the positive electrode active layer with the surface coating on the outer surface. Example 6e is based on Example 1, except that a is 4.8 μm, L2 is 2.9 μm, c is 0.3 μm, k is 1.49, and the contents of Ti, La and Al are 2126 ppm, 277 ppm and 8092 ppm respectively, based on the total mass of the surface coating and the positive electrode active layer with the surface coating on the outer surface.

[0142] Example 7 group: This set of examples is used to verify the impact of changes in "k / w", as detailed below: Example 7a, based on Example 3, except that the negative electrode active material is obtained by mixing artificial graphite, silicon carbide, conductive carbon black, CMC-Li and PAA in a mass ratio of 67.2:30:0.05:0.35:2.4, with w being 13.5%; Example 7b is based on Example 2, except that the negative electrode active material is obtained by mixing artificial graphite, silicon carbide, conductive carbon black, CMC-Li and PAA in a mass ratio of 87.2:10:0.05:0.35:2.4, with w being 4.5%. Example 7c is based on Example 4a, except that the negative electrode active material is obtained by mixing artificial graphite, silicon carbide, conductive carbon black, CMC-Li and PAA in a mass ratio of 7.2:90:0.05:0.35:2.4, with w being 67.5%. Example 7d is based on Example 3, except that the negative electrode active material is obtained by mixing artificial graphite, silicon carbide, conductive carbon black, CMC-Li and PAA in a mass ratio of 94.2:3:0.05:0.35:2.4, with w being 1.1%.

[0143] Comparative Example 1: Based on Example 1, the difference is that in the preparation of the positive electrode sheet, the protective layer slurry is obtained by mixing LLZO and PVDF in a mass ratio of 97:3 and adding NMP and stirring evenly.

[0144] Comparative Example 2: This set of proportions is used to verify the impact of changes in "a / b", as detailed below: Comparative Example 2a, based on Example 1, except that no surface coating is provided, a=0μm, and the contents of Ti, La and Al are 508ppm, 254ppm and 7010ppm respectively, based on the total mass of the surface coating and the positive active layer with the surface coating on the outer surface. Comparative Example 2b is based on Example 1, except that a is 5.7 μm, L2 is 3.2 μm, k is 1.53, and the contents of Ti, La and Al are 2784 ppm, 271 ppm and 8368 ppm respectively, based on the total mass of the surface coating and the positive electrode active layer with the surface coating on the outer surface. Comparative Example 2c is based on Example 2, except that b1 is 5.2 μm, b2 is 5.5 μm, and L1 is 3.5 μm; Comparative Example 2d, based on Example 1, except that a is 0.5 μm, b1 is 4.8 μm, b2 is 4.8 μm, L1 is 3 μm, L2 is 0.1 μm, k is 1.01, and the contents of Ti, La and Al are 924 ppm, 273 ppm and 7057 ppm respectively, based on the total mass of the surface coating and the positive electrode active layer with the surface coating on the outer surface.

[0145] Comparative Example 3: Based on Example 7c, the difference is that no surface coating and protective layer are provided in the positive electrode sheet.

[0146] Test example: 1. Low-temperature discharge test: The lithium-ion secondary batteries prepared in the embodiments and comparative examples of this invention were fully charged at 25℃±3℃ using a constant current of 0.5C, with a cutoff current of 0.02C. They were then discharged at 0.5C to the lower limit voltage of 3.0V, allowed to stand for 10 minutes, and the initial discharge capacity W0 was recorded. The batteries were then fully charged again using a constant current of 0.5C, with a cutoff current of 0.02C. The batteries were then placed in a -20℃ constant temperature chamber and stored for 6 hours. Afterward, the batteries were discharged at a constant current of 0.5C, and the discharge capacity W1 was recorded. The low-temperature discharge capacity retention rate is calculated as W1 / W0 × 100%.

[0147] 2. Needle puncture safety performance test: At room temperature, the lithium-ion secondary batteries prepared in the embodiments and comparative examples of the present invention were charged at a constant current of 1.5C to 4.53V, with a cut-off current of 0.02C. Within 48h after charging, a steel needle with a diameter of 4mm was used to vertically penetrate the left, middle, and right positions of the battery at a speed of 30mm / s. If there was no fire or explosion within 5 minutes, it was considered to pass. 15 batteries were measured for each group of embodiments / comparative examples, and the passing rate was recorded as n / 15, which refers to n batteries passing the test out of 15 batteries. The test results were recorded in Table 1.

[0148] 3. Volume energy density: The lithium-ion secondary batteries prepared in the embodiments and comparative examples of the present invention were charged to the upper limit voltage of 4.53V (cut-off at 0.02C) at 0.2C and discharged to the lower limit voltage of 3.0V at 0.2C using a Neware battery tester, and repeated 3 times. The discharge energy of the third time was taken as the battery energy Q. The width and height of the battery were measured using a 2.5D microscope tester, and the full charge thickness of the battery was measured using a PPG thickness tester. Then the volume energy density was Q / (width × height × thickness), and the test results were recorded in Table 1.

[0149] 4. Cycle capacity retention rate: The lithium-ion secondary batteries prepared in the embodiments and comparative examples of the present invention were tested in a LAND test system: at 25°C ± 3°C, charged at a constant current of 2C to 4.53V, charged at a constant voltage to 0.02C, left to stand for 10 minutes, and then discharged at 1C to 3.0V and left to stand for 10 minutes; cycled 1000 times, and the battery cycle capacity was calculated based on the initial discharge capacity and the weekly discharge capacity of the battery.

[0150] Table 1: For Example 7c, due to the high silicon content in the negative electrode active layer, its energy density was significantly improved. By comparing Example 7c and Comparative Example 3, it can be found that the setting of the surface coating and the bottom coating for high-silicon system batteries significantly improved the low-temperature discharge performance and the puncture safety performance of the battery. As can be seen from Table 1, the batteries prepared by the present invention have high energy density, low-temperature discharge performance, and puncture safety performance compared with the comparative examples.

[0151] 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 technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A lithium-ion secondary battery, characterized in that, The lithium-ion secondary battery includes an electrode assembly, which includes a positive electrode sheet and a negative electrode sheet. The positive electrode sheet includes a positive current collector, a positive active layer, a protective layer, and a surface coating layer. The positive active layer is located on at least one side surface of the positive current collector. The protective layer is located between the positive current collector and the positive active layer. The surface coating layer is located on the outer surface of the positive active layer away from the positive current collector. The surface coating includes a solid electrolyte, and the thickness of the surface coating is a, where a is 0.5 μm to 5 μm. The thickness of the protective layer is b, where b is 0.2 μm to 5 μm; a and b satisfy: 0.1 ≤ a / b ≤ 10; The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on at least one side surface of the negative electrode current collector, wherein the negative electrode active layer includes a silicon-carbon material; The outer surface of one side of the positive electrode has several protrusions.

2. The lithium-ion secondary battery according to claim 1, wherein, The solid electrolyte includes at least one of oxide solid electrolytes, sulfide solid electrolytes, phosphate solid electrolytes, and borate solid electrolytes; Preferably, the oxide-based solid electrolyte includes at least one of lithium oxynitride phosphate, lithium lanthanum zirconium gallium oxide, lithium lanthanum zirconium oxide, lithium lanthanum titanate, lithium titanium aluminum phosphate, and lithium silicate-based oxides; Preferably, the sulfide-based solid electrolyte includes Li₂S-P₂S₅ glass, Li₂S-P₂S₅ glass-ceramic, and Li₂S-P₂S₅ glass. 10 GeP2S 12 At least one of them; Preferably, the phosphate solid electrolyte includes Li3PO4 and / or LiPO3; Preferably, the borate solid electrolyte includes Li3B7O. 12 .

3. The lithium-ion secondary battery according to claim 1 or 2, wherein, The positive electrode active layer includes a positive electrode active material, and the positive electrode active material includes lithium cobalt oxide; And / or, at 0% SOC, the mass ratio of Li to Co in the lithium-ion secondary battery is k, based on the total mass of the surface coating and the positive electrode active layer on the outer surface where the surface coating is provided, where k is 0.95~1.5; preferably 1.01~1.5; And / or, based on the total mass of the surface coating and the positive electrode active layer on the outer surface where the surface coating is provided, the mass content of Ti element is 500ppm~4000ppm; And / or, based on the total mass of the surface coating and the positive electrode active layer on the outer surface where the surface coating is provided, the mass content of La element is 300ppm~2000ppm; And / or, based on the total mass of the surface coating and the positive electrode active layer on the outer surface where the surface coating is provided, the mass content of Al element is 7000ppm~15000ppm.

4. The lithium-ion secondary battery according to claim 1 or 2, wherein, The outer surface of the other side of the positive electrode sheet also has several recesses, and the recesses and the protrusions are respectively arranged in the thickness direction of the positive electrode sheet; And / or, the depth of the recess is h, where h is 2μm~50μm, the spacing between the recesses is 0.5mm-10mm, and the width of the recess is 0.5mm-10mm; And / or, the height of the protrusion is h', h' is 2μm~50μm, the spacing between the protrusions is 0.5mm-10mm, and the width of the protrusion is 0.5mm-10mm.

5. The lithium-ion secondary battery according to claim 1 or 2, wherein, The electrode assembly includes a positive electrode sheet, a separator, and a negative electrode sheet that are stacked and wound together. Along the winding direction of the electrode assembly from the inside to the outside, the positive electrode sheet includes at least a first region and a second region. The positive electrode active layer includes a first positive electrode active layer and a second positive electrode active layer that are disposed opposite to each other along the thickness direction. The length of the first positive electrode active layer is greater than the length of the second positive electrode active layer. And / or, the first region includes the positive current collector and the positive active layer located on both sides of the positive current collector, the protective layer is located between the positive current collector and the positive active layer, and the surface coating is located on at least a portion of the outer surface of the first positive active layer; And / or, the second region includes the positive current collector and the first positive active layer, and the protective layer is located between the positive current collector and the first positive active layer; Preferably, the second region further includes the surface coating layer, which is located on the outer surface of the first positive electrode active layer; Preferably, the outer surface of one side of the positive electrode sheet where the first positive electrode active layer is located has the convex portion, and the outer surface of one side of the positive electrode sheet where the second positive electrode active layer is located has a plurality of concave portions.

6. The lithium-ion secondary battery according to claim 5, wherein, Along the inside-out winding direction of the electrode assembly, the positive electrode sheet further includes a third region, which is adjacent to the second region, and the second region is located between the first region and the third region. The third region includes the positive current collector. And / or, the positive current collector includes a first surface and a second surface, the first surface of the positive current collector is provided with a first positive active layer, the second surface of the positive current collector is provided with a second positive active layer, and the second surface of the positive current collector located in the second region and the third region includes a ceramic layer; Preferably, the ceramic layer comprises ceramic particles, which include at least one of lithium iron phosphate, alumina, boehmite, titanium dioxide, zirconium oxide, boron nitride, barium sulfate, barium titanate, and silicon dioxide. Preferably, the protective layer comprises inorganic particles, which include at least one of alumina, boehmite, titanium dioxide, zirconium oxide, lithium iron phosphate, boron nitride, barium sulfate, barium titanate, and silicon dioxide.

7. The lithium-ion secondary battery according to claim 6, wherein, The thickness of the ceramic layer is c, where c is 0.5μm to 10μm; And / or, the average particle size of the ceramic particles is d1, where d1 is 0.02 μm to 2 μm; And / or, the average particle size of the inorganic particles is d2, where d2 is 0.01 μm to 1.5 μm; Preferably, d1 and d2 satisfy: 1.3 ≤ d1 / d2 ≤ 45; Preferably, a and c satisfy: 0.07≤a / c≤12.

8. The lithium-ion secondary battery according to claim 1 or 2, wherein, At least a portion of the protective layer is embedded in the positive electrode active layer, and at least a portion of the surface coating layer is embedded in the positive electrode active layer; And / or, the average embedding depth of the protective layer in the positive electrode active layer is L1, where L1 is 0.1 μm to 3 μm; And / or, the average embedding depth of the surface coating in the positive electrode active layer is L2, where L2 is 0.1 μm to 3 μm.

9. The lithium-ion secondary battery according to claim 1 or 2, wherein, Based on the total mass of the negative electrode active layer, the mass content of silicon element is w, where w is 1%~70%; And / or, the silicon-carbon material comprises a porous carbon matrix and silicon particles located in the internal channels of the porous carbon matrix; And / or, the average particle size of the silicon-carbon material is 1 μm to 20 μm.

10. The lithium-ion secondary battery according to claim 5, wherein, The thickness of the protective layer located in the first region is b1, and the thickness of the protective layer located in the second region is b2, where b2 ≥ b1; And / or, a and b1 satisfy: 0.1 ≤ a / b1 ≤ 10; Preferably, c ≥ b2 ≥ b1; More preferably, 0.2≤a / b1≤6.

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