Lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion secondary battery. The lithium ion secondary battery comprises an electrode assembly, wherein the electrode assembly is provided with an arc region and a straight region; the ratio of the thickness of the electrode assembly to the size L of the straight area in the length direction of the electrode assembly is (0.03-0.5): 1; the electrode assembly comprises a positive plate, a diaphragm and a negative plate which are stacked and wound; the positive plate comprises a positive active material, and the positive active material comprises a nickel-cobalt-manganese ternary material; the negative plate comprises a negative active material, the negative active material comprises a silicon-carbon material, the mass content of a silicon element in the silicon-carbon material is 35%-70%, the electrolyte comprises fluoroethylene carbonate, and the mass content of fluoroethylene carbonate in the electrolyte is 2%-20%. The pole piece of the lithium ion secondary battery has better toughness and is not easy to break, and the safety performance and the cycling stability of the battery are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly to a lithium-ion secondary battery. Background Art

[0002] As an important part of sustainable energy, battery technology has greatly promoted the sustainable development of society and the convenience and intelligence of human life. In order to obtain a higher energy density, ternary materials and silicon-carbon materials are often used in combination in batteries. However, due to the large volume change of silicon-carbon materials during the process of lithium deintercalation and intercalation, it is easy to cause the fracture of the electrode sheet. Especially in wound batteries, the first 3-fold electrode sheets inside the wound core are prone to fracture during cycling. After the electrode sheet fractures, the foil of the current collector is prone to produce burrs, which will pierce the separator and connect the positive and negative electrodes, causing a short circuit of the battery cell, and then leading to explosion or fire. Therefore, the safety performance of the battery is poor.

[0003] Therefore, how to ensure a large energy density while improving the cycling stability and safety performance of lithium-ion secondary batteries is a technical problem that we urgently need to solve. Summary of the Invention

[0004] The purpose of the present invention is to overcome the above problems existing in the prior art, and provide a lithium-ion secondary battery. By controlling the relationship between the thickness of the electrode assembly and the size of the flat area in the length direction of the electrode assembly, the pressure on the electrode sheets in the arc areas on both sides of the electrode assembly can be reduced. On the premise of not losing or losing less energy density, it is ensured that the electrode sheets of the lithium-ion secondary battery (hereinafter referred to as "battery") are not easily fractured during cycling, improving the safety performance of the battery. At the same time, by combining a positive electrode sheet including a nickel-cobalt-manganese ternary material and a negative electrode sheet including a silicon-carbon material, the cycling stability of the battery can be further improved, so that while the battery has a high energy density, both the safety performance and the cycling stability are improved.

[0005] The present invention provides a lithium-ion secondary battery, the lithium-ion secondary battery includes an electrode assembly formed by laminating and winding a positive electrode sheet, a separator, and a negative electrode sheet, and the electrode assembly has an arc area and a flat area; the ratio of the thickness of the electrode assembly to the size L of the flat area in the length direction of the electrode assembly is (0.03 - 0.5):1; the positive electrode sheet includes a positive electrode active material, the positive electrode active material includes a nickel-cobalt-manganese ternary material, and the nickel-cobalt-manganese ternary material includes a chemical formula of Li a Ni x Co y Mn z M bSubstances of O2, 0.9 ≤ a ≤ 1.1, 0.5 ≤ x ≤ 0.95, 0 < y ≤ 0.3, 0 < z ≤ 0.3, 0 ≤ b ≤ 0.05, M is selected from at least one of Al, Zr, B, Mg, B, Y, Sr, W, Ti and Nb; the negative electrode sheet includes a negative electrode active material, the negative electrode active material includes a silicon-carbon material, and the mass content of silicon element in the silicon-carbon material is 35%-70%; the lithium-ion secondary battery further includes an electrolyte, and the electrolyte includes fluoroethylene carbonate, and the mass content of fluoroethylene carbonate in the electrolyte is 2%-20%.

[0006] Through the above technical solutions, the present invention has at least the following advantages compared with the prior art:

[0007] (1) In the lithium-ion secondary battery of the present invention, the matching degree between the electrode assembly and the silicon-carbon material is relatively high, which can reduce the stress on the electrode sheet in the arc areas on both sides of the electrode assembly, keep the electrode sheet with strong toughness, and is not easy to break during the battery cycling process, with high safety performance.

[0008] (2) In the lithium-ion secondary battery of the present invention, the active material of the positive electrode sheet includes a nickel-cobalt-manganese ternary material with a high specific capacity, and the negative electrode sheet includes a silicon-carbon material with a high specific capacity, which can improve the energy density of the battery, and the nickel-cobalt-manganese ternary material has high high-temperature cycle stability, enabling the battery to have both high energy density and high cycle stability;

[0009] (3) The electrolyte in the lithium-ion secondary battery of the present invention includes fluoroethylene carbonate, which can form a CEI film with a relatively high strength on the surface of the silicon-carbon material, thereby reducing the expansion of the silicon-carbon material, reducing the adverse effects of the expansion of the silicon-carbon material on the electrode sheet in the arc areas on both sides, and also improving the high-temperature cycle performance of the battery.

[0010] In the ranges disclosed herein, the endpoints and any values are not limited to the exact 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 each range, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 The front view structural schematic diagram of the electrode assembly of the present invention is shown.

[0012] Figure 2 The top view structural schematic diagram of the electrode assembly of the present invention is shown.

[0013] Figure 3 The laminated schematic diagram (unwound) of the electrode assembly of the present invention is shown.

[0014] Figure 4 The SEM image of the cross-section of the positive electrode sheet according to an embodiment of the present invention is shown.

[0015] Figure 5 The SEM image of the cross-section of the positive electrode sheet according to an embodiment of the present invention is shown. Detailed Description of the Invention

[0016] The following provides a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only for the purpose of illustrating and explaining the present invention, and are not intended to limit the present invention.

[0017] Unless otherwise defined, all scientific and technical terms used in the present invention have the same meaning as commonly understood by those skilled in the technical field to which the present invention pertains.

[0018] The present invention provides a lithium-ion secondary battery. The lithium-ion secondary battery includes an electrode assembly formed by laminating and winding a positive electrode sheet, a separator, and a negative electrode sheet. The electrode assembly has an arc region and a flat region; the ratio of the thickness of the electrode assembly to the dimension L of the flat region in the length direction of the electrode assembly is (0.03 - 0.5):1, such as 0.03, 0.05, 0.1, 0.2, 0.3, 0.4, or 0.5; the positive electrode sheet includes a positive electrode active material, the positive electrode active material includes a nickel-cobalt-manganese ternary material, and the nickel-cobalt-manganese ternary material includes a chemical formula of Li a Ni x Co y Mn z M bA substance of O2, 0.9 ≤ a ≤ 1.1 (for example, 0.9, 0.95, 1, 1.05 or 1.1), 0.5 ≤ x ≤ 0.9 (for example, 0.5, 0.6, 0.7, 0.8 or 0.9), 0 < y ≤ 0.3 (for example, 0.05, 0.1, 0.15, 0.2, 0.25 or 0.3), 0 < z ≤ 0.3 (for example, 0.05, 0.1, 0.15, 0.2, 0.25 or 0.3), 0 ≤ b ≤ 0.05 (for example, 0, 0.01, 0.02, 0.03, 0.04 or 0.05), M is selected from at least one of Al, Zr, B, Y, Sr, W, Ti and Nb; the negative electrode sheet includes a negative electrode active material, and the negative electrode active material includes a silicon-carbon material; the mass content of silicon element in the silicon-carbon material is 35% - 70% (for example, 35%, 38%, 40%, 43%, 45%, 48%, 50%, 53%, 55%, 58%, 60%, 63%, 65%, 68% or 70%); the lithium-ion secondary battery further includes an electrolyte, and the electrolyte includes fluoroethylene carbonate, and the mass content of fluoroethylene carbonate in the electrolyte is 2% - 20% (for example, 2%, 5%, 7%, 10%, 12%, 15%, 17% or 20%).

[0019] As Figure 1 , Figure 2 and Figure 3 shown, the electrode assembly is formed by laminating and winding a positive electrode sheet 1, a separator 3, and a negative electrode sheet 2, and the electrode assembly has an arc region 5 and a straight region 4. Among them, based on the two folds on the innermost side of the electrode assembly, the arc region 5 is the region where the bending occurs, and the straight region 4 is the region where no bending occurs. From Figure 1 and Figure 2 it can be seen that the electrode assembly includes 2 arc regions and 1 straight region. The two arc regions are respectively located on both sides in the width direction of the electrode assembly, and the straight region is located in the middle region in the width direction of the electrode assembly. The thickness H of the electrode assembly refers to the diameter of the outermost arc of the arc region, that is, the distance between two tangents along the outermost arc of the arc region in the thickness direction. The dimension L of the straight region in the length direction of the electrode assembly is the length of the straight region in the length direction of the electrode assembly. From Figure 3 it can be seen that the positive electrode sheet 1 includes a positive electrode current collector 11 and a positive electrode active material layer located on at least one surface of the positive electrode current collector 11, and the negative electrode sheet 2 includes a negative electrode current collector 21 and a negative electrode active material layer 22 located on at least one surface of the negative electrode current collector 21.

[0020] The positive electrode active material of the positive electrode sheet of the present invention includes a nickel-cobalt-manganese ternary material (such as Figure 4 and Figure 5As shown, the negative active material of the negative electrode plate includes silicon-carbon material, which can enable both the positive and negative electrode plates to have a high specific capacity, improve the energy density of the battery, and the nickel-cobalt-manganese ternary material has good high-temperature cycle stability, which can further improve the cycle stability of the battery, so that while maintaining a high energy density, the safety performance and cycle stability of the battery are improved. When the ratio of the thickness of the electrode assembly to the dimension L of the flat area in the length direction of the electrode assembly meets the above range, the adaptability of the thickness of the electrode assembly to the dimension of the flat area electrode assembly in the length direction is relatively high. On the premise of not losing or losing little energy density, the pressure on the electrode plates in the two arc areas can be reduced, so that the electrode plates maintain high toughness and are not easily broken during the cycle. Therefore, the positive current collector (for example, foil) is not likely to generate burrs and is not likely to pierce the separator, and the battery cell is not likely to cause a short circuit. Moreover, because the volume of the silicon-carbon material changes greatly during the lithium insertion and extraction process, the pressure on the electrode plates in the two arc areas is relatively large. By controlling the mass content of silicon element in the silicon-carbon material, the present invention improves the adaptability between the electrode assembly and the silicon-carbon material, enabling the battery assembly of the present invention to reduce the pressure on the electrode plates in the two arc areas due to the volume expansion of the silicon-carbon material, reduce the risk of electrode plate fracture, and improve the safety performance of the battery. Especially for a multi-tab battery, the overall thickness of the multi-tab battery is relatively thick. Therefore, as the thickness of the electrode assembly increases, the flat area of the electrode assembly needs to meet a certain length so that the ratio of the thickness of the electrode assembly to the dimension of the flat area in the length direction of the electrode assembly meets the above range; when the electrode assembly of the multi-tab battery does not meet the above range, at this time, the pressure on the electrode plates in the two arc areas of the electrode assembly of the multi-tab battery is relatively large, and the electrode plates are prone to breakage, affecting the safety performance and cycle stability of the battery.

[0021] When the ratio of the thickness of the electrode assembly to the dimension L of the flat area in the length direction of the electrode assembly is lower than 0.03:1, the dimension of the flat area in the length direction of the electrode assembly is too long, the battery cell is thin and wide, the tension at the four corners of the battery cell is relatively large, and when the thickness expands, the four corners of the battery tilt in the same direction like a pot lid, causing the battery to deform, thus affecting the expansion performance of the battery and increasing the expansion rate of the battery; when the ratio of the thickness of the electrode assembly to the dimension L of the flat area in the length direction of the electrode assembly is higher than 0.5:1, the dimension of the flat area in the length direction of the electrode assembly is too short, the pressure on the electrode plates in the two arc areas of the electrode assembly is relatively large, and the electrode plates are prone to breakage, affecting the safety performance and cycle stability of the battery.

[0022] In the present invention, the ratio of the thickness of the electrode assembly to the dimension L of the flat area in the length direction of the electrode assembly can be achieved by adjusting the thickness of the electrode assembly and / or the dimension L of the flat area in the length direction of the electrode assembly.

[0023] The electrode assembly with the above characteristics, when paired with an electrolyte including fluoroethylene carbonate, and the mass content of fluoroethylene carbonate in the electrolyte is 2%-20%, helps to protect the negative active material. Fluoroethylene carbonate can form a SEI film with relatively high mechanical stability on the surface of the negative electrode sheet, thereby being able to inhibit the expansion of the silicon-carbon material, reduce the expansion performance of the electrode sheet, and the content of LiF in this SEI film is relatively high, which can effectively inhibit the rupture and recombination of the SEI film at high temperatures, thus being beneficial to improving the high-temperature cycle performance of the positive electrode. When the mass content of fluoroethylene carbonate in the electrolyte is greater than 20%, it will deteriorate the high-temperature cycle performance and gas generation performance during storage of the battery cell, be not conducive to reducing the expansion performance of the electrode sheet, and increase the expansion rate of the battery.

[0024] In some embodiments, the ratio of the thickness of the electrode assembly to the dimension L of the flat region in the length direction of the electrode assembly is (0.05 - 0.2):1.

[0025] In some embodiments, the thickness of the negative electrode sheet located in the arc region is greater than the thickness of the negative electrode sheet located in the flat region. When the thickness of the negative electrode sheet located in the arc region is greater than the thickness of the negative electrode sheet located in the flat region, it can ensure that the silicon-carbon material in the negative electrode sheets inside the arc regions on both sides of the electrode assembly will not cause the failure of the material structure due to overvoltage, resulting in lithium deposition at the interface and affecting the cycle of the battery cell.

[0026] In the present invention, the thickness of the negative electrode sheet includes the sum of the thickness of the negative electrode current collector and the thickness of the negative active material layer. Among them, when there is a negative active material layer on one surface of the negative electrode current collector, the thickness of the negative active material layer is the thickness of the negative active material layer on this side; when there are negative active material layers on both surfaces of the negative electrode current collector, the thickness of the negative active material layer is the sum of the thicknesses of the negative active material layers on both sides.

[0027] In some embodiments, the positive electrode active material further includes lithium cobalt oxide, and the mass proportion of the nickel-cobalt-manganese ternary material in the positive electrode active material is m, 0% <m≤50%,如为0.1%、5%、10%、15%、20%、25%、30%、35%、40%、45%或50%;所述镍钴锰三元材料包括单晶颗粒和 / 或多晶颗粒,所述镍钴锰三元材料的平均粒径为A,A为1-10,单位为μm,如为1μm、2μm、3μm、4μm、5μm或6μm;所述正极片还包括正极集流体,所述正极集流体的拉伸强度为B,B为120-300,单位为MPa,如为120、130、140、150、160、170、180、190、200、210、220、230、240、250、260、270、280、290或300;A和B满足:A / B为0.004-0.05,如为0.004、0.008、0.01、0.02、0.03、0.04或0.05。

[0028] In the present invention, by blending lithium cobalt oxide with nickel cobalt manganese ternary materials, the structural stability of the high-temperature cycle of the positive electrode active material can be improved, thereby extending the high-temperature cycle life of the battery and improving the safety performance under extreme conditions such as thermal shock and acupuncture. The specific reasons are as follows: In terms of safety performance, since the exothermic rate of the redox reaction of the nickel cobalt manganese ternary material in the delithiation state is relatively slow under extreme conditions (such as short circuit and combustion) (the main reason is that the step-by-step valence change reaction rate of the nickel element is not as fast as the single element and valence change rate of the cobalt element in pure lithium cobalt oxide), the overall safety performance of the battery cell after blending is greatly improved; in terms of cycle performance, due to the addition of manganese elements in the nickel cobalt manganese ternary material, the stability of the layered structure in the high delithiation state is stabilized, so the high-temperature cycle life of the blended material is better than that of pure lithium cobalt oxide. Therefore, the positive electrode active material includes nickel cobalt manganese ternary material and lithium cobalt oxide, and the mass proportion of nickel cobalt manganese ternary material in the positive electrode active material satisfies 0% <m≤50%时,能有效提升正极活性物质的克容量发挥及电池的倍率性能,同时提升了正极活性物质的高温循环结构稳定性,从而延长了电池高温循环寿命,提升了热冲击,针刺等极端条件下电池的安全性能。此外由于钴酸锂中Co元素的稀缺性,其原料(四氧化三钴)的成本较高,而镍钴锰三元材料中Ni元素及Mn元素含量较多,原料成本较低廉,且烧结加工温度较低,混掺使用后在相同克容量设计的电芯中,掺混的正极活性物质的成本更低。

[0029] By controlling the relationship between the average particle size of the nickel-cobalt-manganese ternary material and the tensile strength of the positive electrode current collector, the toughness of the positive electrode sheet can be improved, enabling the positive electrode sheet to withstand a higher roll pressure (for example, the roll pressure can be increased to 1000T). In this case, even if the thickness of the positive electrode current collector is low, the positive electrode sheet is still not easily broken under a high roll pressure. Thus, the compaction density of the positive electrode sheet can be increased. A higher compaction density, combined with a smaller average particle size of the nickel-cobalt-manganese ternary material, results in a higher stacking density of the positive electrode active material layer, which can shorten the transmission distance of lithium ions in the crystal, improve the low-temperature performance and rate performance of the battery. At the same time, the higher stacking density reduces the voids between the particles of the nickel-cobalt-manganese ternary material, reducing the direct contact sites between the electrolyte and the particles of the nickel-cobalt-manganese ternary material, thereby reducing the side reactions between the nickel-cobalt-manganese ternary material and the electrolyte under high-temperature cycling and improving the cycling performance of the battery.

[0030] The average particle size can be obtained by testing with a laser particle size analyzer.

[0031] In some examples, A / B is 0.008 - 0.042.

[0032] In some examples, A / B is 0.012 - 0.034.

[0033] In some embodiments, 20% ≤ m ≤ 40%.

[0034] In some embodiments, the nickel-cobalt-manganese ternary material includes single-crystal and / or polycrystalline particles, and the particle size number distribution of the nickel-cobalt-manganese ternary material is unimodal. When the particle size number distribution is unimodal, it indicates that the particle size of the nickel-cobalt-manganese ternary material is relatively uniform, which can ensure a suitable compaction density of the positive electrode sheet; when the particle size number distribution is bimodal, it indicates that the particle size uniformity of the nickel-cobalt-manganese ternary material is poor, and there are particles with smaller particle sizes, which has a certain impact on the compaction density of the positive electrode sheet.

[0035] In some embodiments, the nickel-cobalt-manganese ternary material includes single-crystal particles and polycrystalline particles, and the mass ratio of the single-crystal particles to the polycrystalline particles is (90% - 10%):(10% - 90%). When the mass ratio of the single-crystal particles to the polycrystalline particles is controlled within the above range, the combination of the single-crystal particles and the polycrystalline particles is more suitable, and it will not affect the kinetic performance of the battery due to excessive single-crystal particles, nor will it affect the high-temperature cycling performance due to excessive polycrystalline particles, thereby enabling the kinetic performance of the battery to be improved, and further improving the high and low-temperature cycling performance and rate performance of the battery.

[0036] It is understandable that in the nickel-cobalt-manganese ternary material, the mass ratio of single-crystal particles can take values within the range of 90%-10% (for example, 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20% or 10%), and the mass ratio of polycrystalline particles can take values within the range of 10%-90% (for example, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90%). However, it should be satisfied that the sum of the mass ratio of single-crystal particles and the mass ratio of polycrystalline particles is 100%.

[0037] In some embodiments, the mass ratio of the single-crystal particles to the polycrystalline particles is (50%-80%):(50%-20%).

[0038] In some embodiments, the average particle size A of the nickel-cobalt-manganese ternary material is 1 μm - 10 μm. When the average particle size of the nickel-cobalt-manganese ternary material is controlled within the above range, it is beneficial to improve the tap density of the positive active material layer after the nickel-cobalt-manganese ternary material is mixed with lithium cobaltate, thereby increasing the energy density of the lithium-ion secondary battery.

[0039] In some embodiments, the average particle size A of the nickel-cobalt-manganese ternary material is 1 μm - 10 μm.

[0040] In some embodiments, the average particle size of the polycrystalline particles is 1 μm - 10 μm.

[0041] In some embodiments, the average particle size of the single-crystal particles is 1 μm - 10 μm.

[0042] In the present invention, the average particle size of the nickel-cobalt-manganese ternary material can be measured by the following method: Disassemble the electrode assembly (cell) from the battery, take the positive electrode sheet, remove the pole piece adhesive tape and the welded tab, place the positive electrode sheet without adhesive tape and tab in a muffle furnace and calcine it at 450 °C for 4 h in an air atmosphere. After cooling to room temperature (25 °C ± 2), take out the positive electrode sheet and knead it (the kneading force here is small, just to make the nickel-cobalt-manganese ternary material fall off, and it will not cause the nickel-cobalt-manganese ternary material to be crushed) and collect the fallen positive electrode powder (the powder of the nickel-cobalt-manganese ternary material). After ultrasonic treatment in deionized water for 1 h, use a Malvern 3000 laser particle size analyzer to measure its particle size, and the average particle size of the nickel-cobalt-manganese ternary material can be obtained.

[0043] In some embodiments, the nickel cobalt manganese ternary material includes transition metal elements. The molar proportion of nickel in the transition metal elements of the nickel cobalt manganese ternary material is 50%-90%, such as 50%, 54%, 58%, 60%, 64%, 68%, 70%, 74%, 78%, 80%, 84%, 88% or 90%. Controlling the molar proportion of nickel in the transition metal elements of the nickel cobalt manganese ternary material within the above range can improve the stability of the nickel cobalt manganese ternary material and also make the average particle size of the nickel cobalt manganese ternary material more suitable.

[0044] In some embodiments, the tap density of the positive electrode sheet is greater than or equal to 3.45 g / cm 3 . When the tap density is controlled within the above range, the energy density of the lithium ion secondary battery can be improved. The test method of the tap density is calculated by the areal density of the electrode sheet and the micrometer measuring the thickness of the electrode sheet. The areal density of the electrode sheet can be measured by the following method: Cut out a 1 cm 2 positive electrode sheet and weigh it as W1. Scrape off the positive electrode active material layer and measure the weight of the positive electrode current collector as W2. Then the areal density is (W1 - W2) / 1.

[0045] In some embodiments, the nickel cobalt manganese ternary material includes LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.6 Co 0.1 Mn 0.3 O2, LiNi 0.7 Co 0.1 Mn 0.2 O2, LiNi 0.65 Co 0.15 Mn 0.2 O2 and LiNi 0.68 Co 0.09 Mn 0.23 One or more of O2.

[0046] In some examples, the nickel cobalt manganese ternary material includes LiNi 0.6 Co 0.1 Mn 0.3 O2.

[0047] In some embodiments, the specific surface area of the nickel cobalt manganese ternary material is 0.6 m 2 / g - 1 m 2 / g, such as 0.6 m 2 / g, 0.7 m 2 / g, 0.8 m2 / g, 0.9 m 2 / g or 1 m 2 / g. When the specific surface area of the nickel-cobalt-manganese ternary material is within the above range, it is beneficial to reduce the contact sites between the material and the electrolyte, reduce side reactions, and improve the cycling performance. The specific surface area can be directly measured by a specific surface area tester.

[0048] In some embodiments, the residual alkali content of the nickel-cobalt-manganese ternary material is less than or equal to 2000 ppm. The residual alkali content of the nickel-cobalt-manganese ternary material refers to the content of alkaline compounds remaining on the surface of the material, mainly including lithium hydroxide (LiOH) and lithium carbonate (Li2CO3). The sum of the contents of lithium hydroxide and lithium carbonate in the nickel-cobalt-manganese ternary material is less than or equal to 2000 ppm. The residual alkali content can be measured by potentiometric titration.

[0049] In some embodiments, the mass content of aluminum element (Al) in the nickel-cobalt-manganese ternary material is 1000 ppm - 2500 ppm (for example, 1000 ppm, 1200 ppm, 1500 ppm, 1700 ppm, 2000 ppm, 2200 ppm or 2500 ppm). In the nickel-cobalt-manganese ternary material, Al can exist in the form of a doping element.

[0050] In some embodiments, the mass content of titanium element (Ti) in the nickel-cobalt-manganese ternary material is 500 ppm - 1000 ppm (for example, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm or 1000 ppm). In the nickel-cobalt-manganese ternary material, Ti can exist in the form of a coating.

[0051] In some embodiments, the mass content of tungsten (W) element in the nickel-cobalt-manganese ternary material is 2000 ppm - 4000 ppm (for example, 2000 ppm, 2300 ppm, 2500 ppm, 2800 ppm, 3000 ppm, 3300 ppm, 3500 ppm, 3800 ppm or 4000 ppm). In the nickel-cobalt-manganese ternary material, W can exist in the form of a coating.

[0052] In some embodiments, the positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes a nickel-cobalt-manganese ternary material and lithium cobalt oxide; the negative electrode sheet includes a negative electrode active material, and the negative electrode active material includes a silicon-carbon material. Silicon-carbon materials have a higher gram capacity than graphite, and have a higher energy density when combined with lithium cobalt oxide and nickel-cobalt-manganese ternary materials, and can also improve the high-temperature cycle stability of the battery, and improve the safety performance of the battery under extreme conditions such as thermal shock and acupuncture. However, it will also increase the risk of pole piece fracture. When the nickel-cobalt-manganese ternary material and lithium cobalt oxide are combined with the electrode assembly of the present invention, by controlling the ratio of the thickness of the electrode assembly to the size of the straight area in the length direction of the electrode assembly, the stress on the pole piece in the arc area on both sides of the electrode assembly that is prone to fracture can be reduced, so that the pole piece can still maintain a high toughness, so that the pole piece can still withstand the risk of deterioration and fragmentation caused by the combination of lithium cobalt oxide and nickel-cobalt-manganese ternary materials, thereby making the battery not easy to break, and further improving the safety performance of the battery.

[0053] In some embodiments, the lithium cobalt oxide includes first particles and second particles, and the average particle size of the first particles is greater than the average particle size of the second particles. In the present invention, the test method for the average particle size of the first particles and the average particle size of the second particles can refer to the test method for the average particle size of the nickel-cobalt-manganese ternary material.

[0054] In some embodiments, the ratio of the average particle size of the first particles to the average particle size of the second particles is (2.1-8.5): 1. When the ratio of the average particle size of the first particles to the average particle size of the second particles is controlled within the above range, it is beneficial for the second particles to be evenly distributed in the pores formed between the first particles, thereby improving the compaction density of the positive electrode active material layer and improving the energy density of the battery.

[0055] In some embodiments, the average particle size of the first particles is 10 μm-30 μm, such as 10 μm, 15 μm, 20 μm, 25 μm or 30 μm, and the average particle size of the second particles is 2 μm-10 μm, such as 2 μm, 4 μm, 6 μm, 8 μm or 10 μm.

[0056] In some embodiments, the mass ratio of the first particles to the second particles is (50%-99%):(50%-1%). When the mass ratio of the first particles to the second particles is controlled within the above range, it is beneficial to increase the compaction density of the positive electrode active material layer and increase the energy density of the battery.

[0057] It is understandable that the mass ratio of the first particles in the lithium cobaltate can range from 50% to 99% (for example, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 99%), and the mass ratio of the second particles in the lithium cobaltate can range from 50% to 1% (for example, 50%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, 10%, 5% or 1%). However, it should be satisfied that the sum of the mass ratio of the first particles and the mass ratio of the second particles in the lithium cobaltate is 100%.

[0058] In some embodiments, the mass ratio of the first particles to the second particles is (70% - 80%) : (30% - 20%).

[0059] In some embodiments, the first particles and / or the second particles are doped with aluminum element.

[0060] In some embodiments, the content of aluminum element in the first particles is 6000 ppm - 8000 ppm.

[0061] In some embodiments, the content of aluminum element in the second particles is 5000 ppm - 7000 ppm.

[0062] In some embodiments, the positive electrode active material includes nickel cobalt manganese ternary material and lithium cobaltate. Among them, the content of Mg in the positive electrode active material is 500 - 1000 ppm, the content of Y is 1000 - 1500 ppm, and the content of Zr is 1000 - 1500 ppm.

[0063] In some embodiments, the positive electrode current collector includes a first substrate layer and first metal layers located on both surface sides of the first substrate layer (as Figure 5 shown). The first substrate layer includes a first polymer. The first polymer of the first substrate layer helps to reduce the extension of the metal, and the addition of the first substrate layer helps to avoid the metal fatigue effect after multiple foldings, making the positive electrode current collector not easily break, enhancing the toughness of the electrode sheet. At the same time, under high rolling pressure, the first substrate layer has a certain deformation elasticity, which can relieve the deformation of the positive electrode current collector and has good supporting performance; in addition, the first substrate layer has a high melting point and has higher safety performance in extreme cases. The positive electrode current collector with the above structure can increase the upper limit of the rolling pressure that the positive electrode sheet can withstand, not easily cause belt breakage. At the same time, after the positive electrode sheet is rolled and folded twice, the crease is opaque, meeting the toughness requirements of the lithium battery production electrode sheet, and can adapt to the extrusion under high external pressure without breaking to generate metal burrs, improving the safety performance of the battery cell.

[0064] In some embodiments, the first polymer includes at least one of polypropylene, polyethylene, poly(ethylene terephthalate), polyethylene naphthalate, polyimide, polycarbonate, polyvinyl chloride, polyvinylidene fluoride, polystyrene, polytetrafluoroethylene, polyvinyl alcohol, and modified polymers of the above substances.

[0065] In some embodiments, the first metal layer includes aluminum and / or aluminum alloy.

[0066] In some embodiments, the thickness of the first substrate layer is 2 μm - 28 μm, such as 2 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, or 28 μm.

[0067] In some embodiments, the thickness of the first substrate layer is 2.5 μm - 10 μm.

[0068] In some embodiments, the thickness of the first metal layer is 0.5 μm - 2.5 μm, such as 0.5 μm, 1 μm, 1.5 μm, 2 μm, or 2.5 μm.

[0069] In some embodiments, the thickness of the first metal layer is 1 μm - 2 μm.

[0070] In the present invention, the thickness of the first metal layer is the thickness of the first metal layer on one side. For example, when the first metal layer is located on one side surface of the first substrate layer, the thickness of the first metal layer is the thickness of the first metal layer on that side (i.e., the side with the first metal layer). When the first metal layer is located on both side surfaces of the first substrate layer, the thicknesses of the first metal layers on both sides are the same, and the thickness of the first metal layer is the thickness of the first metal layer on one of the sides.

[0071] In some embodiments, the thickness of the positive current collector is 3 μm - 30 μm, such as 3 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, or 30 μm. The thickness of the positive current collector is the overall thickness of the positive current collector. For example, when the first metal layer is located on one side surface of the first substrate layer, the thickness of the positive current collector is the sum of the thickness of the first substrate layer and the thickness of the first metal layer on one side (i.e., the side with the first metal layer). When the first metal layer is located on both side surfaces of the first substrate layer, the thickness of the positive current collector is the sum of the thickness of the first substrate layer and the thicknesses of the first metal layers on both sides.

[0072] In some embodiments, the thickness of the positive current collector is 4 μm - 14 μm.

[0073] In some embodiments, the elongation rate of the positive current collector is 0.5%-1.8% (for example, 0.5%, 0.6%, 0.7%, 0.8%, 1%, 1.3%, 1.5% or 1.8%). The test method for the tensile fracture elongation rate is to stretch the positive current collector under the tensile strength B. Before stretching, the length of the positive current collector is measured as L1, and the length of the positive current collector at the time of tensile fracture is L2. Then the elongation rate of the positive current collector is [(L2 - L1) / L1]×100%.

[0074] In some embodiments, the elongation rate of the positive current collector is 0.5%-0.8%.

[0075] In some embodiments, the positive active material further includes lithium cobaltate. The mass ratio of the nickel-cobalt-manganese ternary material in the positive active material is m, where 0% < m ≤ 50%; the nickel-cobalt-manganese ternary material includes single crystal particles, and the average particle size of the single crystal particles is A, where A is 1-10, and the unit is μm; the positive electrode sheet further includes a positive current collector, and the tensile strength of the positive current collector is B, where B is 120-300, and the unit is MPa; A and B satisfy: A / B is 0.004-0.05, and the positive current collector includes a first substrate layer and first metal layers on both surface sides of the first substrate layer. The first polymer of the first substrate layer includes at least one of polypropylene, polyethylene, polyethylene terephthalate, polyethylene naphthalate, polyimide, polycarbonate, polyvinyl chloride, polyvinylidene fluoride, polystyrene, polytetrafluoroethylene, polyvinyl alcohol and its derivatives. Due to the relatively small average particle size of the single crystal particles of the nickel-cobalt-manganese ternary material, when paired with the positive current collector added with the polymer in the present invention, the positive electrode sheet can have high toughness without breaking the belt, can withstand greater pressure, thereby reducing the risk of fracture when the positive electrode sheet is subjected to greater pressure in the two side arc regions, and improving the safety performance of the battery.

[0076] In some embodiments, the positive electrode sheet includes a positive current collector and a positive active material layer on one or both surface sides of the positive current collector. The positive active material layer includes a positive active material, a positive binder and a positive conductive agent.

[0077] In some embodiments, the positive binder includes one or more of polyvinylidene fluoride (PVDF), acrylic acid modified PVDF, polyacrylate polymers, polytetrafluoroethylene, perfluorosulfonic acid ionomer, polyacrylonitrile, polyimide, styrene-butadiene rubber and styrene-acrylonitrile rubber.

[0078] In some embodiments, the positive conductive agent includes one or more of carbon black and carbon nanotubes.

[0079] In some examples, based on the total weight of the positive electrode active material layer, the weight content of the positive electrode active material is 90%-99% (for example, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99%), the weight content of the positive electrode conductive agent is 0.1%-5% (for example, 0.1%, 0.5%, 1%, 2%, 3%, 4% or 5%), and the weight content range of the positive electrode binder is 0.1%-5% (for example, 0.1%, 0.5%, 1%, 2%, 3%, 4% or 5%).

[0080] In some embodiments, the positive electrode sheet further includes a positive electrode tab, the positive electrode tab extends from the positive electrode current collector along the width direction of the positive electrode sheet, the positive electrode tab is electrically connected to the positive electrode current collector, and the number of the positive electrode tabs is greater than or equal to 2.

[0081] In some embodiments, the lithium-ion secondary battery further includes a negative electrode sheet, the negative electrode sheet includes a negative electrode tab, and the number of the negative electrode tabs is greater than or equal to 2.

[0082] Since the silicon-carbon material of the negative electrode active material and the nickel-cobalt-manganese ternary material of the positive electrode active material have relatively poor conductivity (compared with graphite and lithium cobaltate), there are deficiencies in kinetics, and the multi-tab cell structure can greatly reduce the internal resistance of the cell and reduce the adverse effects brought by polarization through a parallel-like method.

[0083] By arranging soft tabs on the current collectors of the positive and negative electrode sheets in the width direction of the electrode sheets and welding them to the aluminum / nickel tabs uniformly after winding. During the charge and discharge process, the current in different regions is output simultaneously through multiple soft tabs on the corresponding current collectors, which is equivalent to multiple regions in parallel. Compared with the single-tab welding structure, it reduces the electron transmission path, effectively reduces the overall electron transfer impedance of the cell, and helps to improve the rate performance and low-temperature discharge performance of the cell.

[0084] In some embodiments, the positive electrode active material includes a nickel-cobalt-manganese ternary material and lithium cobaltate, the positive electrode sheet further includes a positive electrode tab, the positive electrode tab extends from the positive electrode current collector along the width direction of the positive electrode sheet, the positive electrode tab is electrically connected to the positive electrode current collector, the number of the positive electrode tabs is greater than or equal to 2, the lithium-ion secondary battery further includes a negative electrode sheet, the negative electrode sheet includes a negative electrode tab, the negative electrode tab extends from the negative electrode current collector along the width direction of the negative electrode sheet, the negative electrode tab is electrically connected to the negative electrode current collector, and the number of the negative electrode tabs is greater than or equal to 2. Due to the relatively large change in the valence of transition metals during the charge and discharge process of the nickel-cobalt-manganese ternary material, its rate performance and low-temperature discharge capacity retention rate are weaker than those of lithium cobaltate. Mixing lithium cobaltate with the nickel-cobalt-manganese ternary material and using the multi-tab structure can further improve the overall rate performance and low-temperature discharge performance of the cell.

[0085] In some embodiments, the negative electrode sheet further includes a negative electrode current collector and a negative electrode active material layer located on at least one surface of the negative electrode current collector; the negative electrode active material layer includes the negative electrode active material, and the negative electrode active material further includes a silicon-carbon material. The silicon-carbon material includes silicon element.

[0086] In some embodiments, the silicon-carbon material includes a porous carbon matrix and a silicon material located in the internal pores of the porous carbon matrix.

[0087] In some embodiments, the porous carbon matrix includes carbon nanotubes (such as multi-walled carbon nanotubes and / or single-walled carbon nanotubes).

[0088] In one example, the negative electrode active material further includes a graphite material, and the graphite material includes artificial graphite and / or natural graphite.

[0089] In some embodiments, the mass content c of silicon element in the negative electrode active material layer is 1.5% - 22% (for example, 1.5%, 5%, 8%, 10%, 13%, 18%, 20% or 22%).

[0090] In some embodiments, the mass content of the silicon-carbon material in the negative electrode active material is 5% - 30%, such as 5%, 10%, 15%, 20%, 25% or 30%. When the mass content of the silicon-carbon material in the negative electrode active material is controlled within the above range, it is beneficial to improve the specific capacity of the negative electrode sheet, thereby improving the overall energy density of the battery cell.

[0091] In some embodiments, the volume distribution particle size Dv10 of the silicon-carbon material is 1μm - 6μm, Dv50 is 6μm - 15μm, and Dv90 is 12μm - 30μm. When the volume distribution particle size of the silicon-carbon material meets the above range, it is beneficial to improve the specific capacity of the negative electrode sheet and improve the overall energy density of the battery cell.

[0092] In some embodiments, the negative electrode current collector includes a second substrate layer and second metal layers located on both surfaces of the second substrate layer; the second substrate layer includes a second polymer; the mass content c of silicon element in the negative electrode active material layer and the thickness T1 of the second metal layer satisfy: c / T1 is 2 - 13 (for example, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 or 13).

[0093] In some examples, c / T1 is 2.5 - 9.

[0094] In the present invention, the thickness of the second metal layer is the thickness of the second metal layer on one side. For example, when the second metal layer is located on one surface of the second substrate layer, the thickness of the second metal layer is the thickness of the second metal layer on this side (i.e., the side with the first metal layer). When the second metal layer is located on both surfaces of the second substrate layer, the thicknesses of the second metal layers on both sides are the same, and the thickness of the second metal layer is the thickness of the second metal layer on one of the sides.

[0095] In some embodiments, the value V of the negative electrode active material layer OI and the thickness T2 of the negative electrode current collector satisfy: V OI / T2 is 0.5 - 8 (for example, 0.5, 1, 2, 3, 4, 5, 6, 7, or 8). V OI The value refers to the oxidation induction period value of the graphite negative electrode. V OI The value can be obtained from the XRD spectrum of the negative electrode sheet. V OI The value is the ratio OI = I(004) / (110) of the intensities of the (004) diffraction peak and the (110) diffraction peak in the XRD spectrum. The diffraction angle of the 004 peak is approximately 54° - 55°, and the diffraction peak angle of the 110 crystal plane is approximately 77° - 78°. V OI The magnitude of the value is directly related to the perfection degree of the crystal structure of the negative electrode active material. V OI The larger the value, the more complete the crystal structure, the higher the electron cloud density, the more stable the chemical properties, and the more conducive to improving the lithium ion insertion and extraction rate. However, V OI The larger the value, the higher the compaction density of the negative electrode sheet, and the greater the thickness of the negative electrode current collector required, that is, the thicknesses of the second substrate layer and the second metal layer also need to be increased. Otherwise, it is easy to cause the negative electrode sheet to break during rolling. However, if the thickness of the negative electrode current collector is too large, it is easy to cause a decrease in the energy density of the battery cell. Therefore, it is necessary to control the relationship between V OI of the negative electrode active material layer and the thickness T2 of the negative electrode current collector to make their matching degree relatively high. When the value V OI of the negative electrode active material layer and the thickness of the negative electrode current collector satisfy the above relationship, while maintaining a relatively high energy density, the negative electrode active layer has a high specific capacity, and it can also alleviate the problems of high lithium insertion and the resulting large negative electrode swelling force caused by the high specific capacity, which helps to relieve the stress of the electrode sheet caused by the swelling of the battery cell, prevent the deformation of the electrode sheet, helps to relieve the volume deformation of the battery cell, and improves the comprehensive performance of the battery cell.

[0096] In some examples, V OI / T2 is 0.7 - 6.

[0097] In the present invention, the thickness of the negative electrode current collector is the overall thickness of the negative electrode current collector. For example, when the second metal layer is located on one surface of the second substrate layer, the thickness of the negative electrode current collector is the sum of the thickness of the second substrate layer and the thickness of the second metal layer on one side (i.e., the side with the second metal layer); when the second metal layer is located on both surfaces of the second substrate layer, the thickness of the negative electrode current collector is the sum of the thickness of the second substrate layer and the thicknesses of the second metal layers on both sides.

[0098] In some embodiments, the value V of the negative electrode active material layer OI is 5 - 30.

[0099] In some embodiments, the second metal layer includes Cu.

[0100] In some embodiments, the second substrate layer includes a second polymer, and the second polymer includes at least one of polypropylene, polyethylene, polyethylene terephthalate, polyethylene naphthalate, polyimide, polycarbonate, polyvinyl chloride, polyvinylidene fluoride, polystyrene, polytetrafluoroethylene, polyvinyl alcohol, and modified polymers of the above substances.

[0101] In some embodiments, the negative electrode sheet includes a positive electrode current collector and a negative electrode active material layer located on one or both surfaces of the negative electrode current collector, and the negative electrode active material layer includes a negative electrode active substance, a negative electrode binder, and a negative electrode conductive agent.

[0102] In some embodiments, the negative electrode conductive agent includes conductive carbon black and / or acetylene black.

[0103] In some embodiments, the negative electrode binder includes one or more of styrene-butadiene rubber (SBR), lithium carboxymethyl cellulose (CMC-Li), and sodium carboxymethyl cellulose (CMC-Na).

[0104] In some examples, based on the total weight of the negative electrode active material layer, the weight content of the negative electrode active substance is 90% - 99% (for example, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99%), the weight content range of the negative electrode binder is 1% - 10% (for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10%), and the weight content of the negative electrode conductive agent is 0 - 5% (for example, 0, 0.1%, 0.5%, 1%, 2%, 3%, 4%, or 5%).

[0105] In some embodiments, the lithium ion secondary battery further includes an electrolyte, and the electrolyte includes fluoroethylene carbonate.

[0106] In some examples, the electrolyte further includes a lithium salt, an organic solvent, and an additive.

[0107] In some examples, in one example, the lithium salt includes one or more of lithium hexafluorophosphate (LiFP6), lithium tetrafluoroborate (LiBF4), lithium bis(oxalato)borate (LiBOB), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium difluoro(oxalato)borate (LiODFB), and lithium bis(fluorosulfonyl)imide (LiFSI).

[0108] In some examples, the organic solvent includes one or more of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0109] In some examples, the additive includes vinylene carbonate (VC).

[0110] In some examples, based on the total weight of the electrolyte, the weight content of the lithium salt is 10 wt% - 25 wt% (e.g., 10%, 13%, 15%, 18%, 20%, 23%, or 25%), the weight content of the organic solvent is 50 wt% - 80 wt% (e.g., 50%, 55%, 60%, 65%, 70%, 75%, or 80%), and the weight content of the additive is 10 wt% - 30 wt% (e.g., 10%, 15%, 20%, 25%, or 30%).

[0111] The present invention will be described in detail below by way of examples. The examples described in the present invention are only a part of the examples of the present invention, rather than all of the examples. All other examples obtained by those of ordinary skill in the art based on the examples in the present invention without creative efforts belong to the scope of protection of the present invention.

[0112] In the following examples, unless otherwise specified, the materials used are commercially available analytical pure.

[0113] The following examples are used to illustrate the lithium ion secondary battery of the present invention.

[0114] Example 1

[0115] (1) Positive electrode sheet

[0116] Component preparation: Positive electrode current collector: The positive electrode current collector includes a first base material layer and first metal layers on both surface sides of the first base material layer, wherein the first base material layer: polypropylene (PP), with a thickness of 6 μm, and the first metal layer: aluminum foil, with a thickness of 1.5 μm; wherein the tensile strength B of the positive electrode current collector is 250 MPa, and the elongation of the positive electrode current collector is 0.7%;

[0117] Positive electrode active material: Nickel-cobalt-manganese ternary material (chemical formula LiNi0.6 Co 0.096 Mn 0.299 Al 0.005 O2, the average particle size A of the nickel-cobalt-manganese ternary material is 3.62 μm) and lithium cobaltate material (wherein, the mass ratio m of the nickel-cobalt-manganese ternary material is 40%, and the mass ratio of the lithium cobaltate material is 60%) total 97.3 parts by weight, the positive electrode binder: PVDF, 1.1 parts by weight, the positive electrode conductive agent: carbon nanotubes and carbon black (wherein, 0.7 parts by weight of carbon nanotubes, 0.9 parts by weight of carbon black) total 1.6 parts by weight;

[0118] Where A / B = 3.62 / 250 = 0.01448, the nickel-cobalt-manganese ternary material includes single crystal particles and polycrystalline particles, the average particle size of the single crystal particles is 3.8 μm, the average particle size of the polycrystalline particles is 3.2 μm, the mass ratio of the single crystal particles to the polycrystalline particles is 70%:30%, the molar ratio of nickel in the nickel-cobalt-manganese ternary material among the transition metal elements is 60%, the residual alkali content of the nickel-cobalt-manganese ternary material is 900 ppm, and the specific surface area of the nickel-cobalt-manganese ternary material is 0.85 m 2 / g; the average particle size of the first particles is 15 μm, the aluminum element content in the first particles is 7000 ppm, the average particle size of the second particles is 4 μm, the aluminum element content in the second particles is 6000 ppm, and the mass ratio of the first particles to the second particles is 80%:20%.

[0119] Mix the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent, add N-methylpyrrolidone (NMP), and stir under the action of a vacuum mixer until the mixed system becomes a positive electrode slurry with uniform fluidity; uniformly coat the positive electrode slurry on both surfaces of the positive electrode current collector; bake the above-coated aluminum foil in an oven with 5 different temperature gradients (95°C ± 5°C, 100°C ± 5°C, 103°C ± 5°C, 100°C ± 5°C, 95°C ± 5°C), and then dry it in an oven at 120°C for 8 h, and then obtain the required positive electrode sheet through rolling and slitting.

[0120] (2) Negative electrode sheet

[0121] Composition preparation: Negative electrode current collector: The negative electrode current collector includes a second substrate layer and second metal layers on both surfaces of the second substrate layer, wherein the second substrate layer: polypropylene (PP), with a thickness of 3 μm, the second metal layer: copper foil, the thickness T1 of the single-sided second metal layer is 1.5 μm; the thickness T2 of the negative electrode current collector is 6 μm.

[0122] Negative electrode active material: silicon-carbon material (9.86 parts by weight), graphite material (artificial graphite 88.74 parts by weight) total 98.6 parts by weight, negative electrode binder: CMC and SBR (the weight ratio of CMC to SBR is 65%:35%), total 1.4 parts by weight;

[0123] Mix the negative electrode active material and the negative electrode binder, and make a slurry by a wet process (add water to the mixture formed above, stir under the action of a vacuum mixer until the mixed system becomes a homogeneous and flowing negative electrode slurry), and coat it on the surfaces on both sides of the negative electrode current collector. After drying (temperature: 85 °C, time: 5 h), rolling and die-cutting, a negative electrode sheet is obtained. Among them, the mass content of silicon element in the silicon-carbon material is 50%, the mass content c of silicon element in the negative electrode active material layer is 4.93%, c / T1 = 4.93 / 1.5 = 3.29, and the V OI value of the negative electrode active material is 18, and V OI / T2 = 18 / 6 = 3.

[0124] (3) Electrolyte

[0125] Component preparation: Lithium salt: LiFP6, 18 parts by weight;

[0126] Organic solvent: 70 parts by weight, among which the weight ratio of PC, EC and DMC is 40:30:30;

[0127] Additive: FEC, 12 parts by weight;

[0128] In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), mix the organic solvents evenly to obtain a mixed solution 1. Add the additive to the mixed solution 1 to obtain a mixed solution 2. Slowly add the lithium salt to the mixed solution 2 and stir evenly to obtain a non-aqueous electrolyte.

[0129] (4) Separator

[0130] Base film (thickness: 5 μm), one side surface of the base film is coated with a ceramic coating (thickness: 1 μm, this side is adjacent to the positive electrode sheet), and the other side surface of the base film is coated with a PMMA coating (thickness: 1.5 μm, this side is adjacent to the negative electrode sheet).

[0131] (5) Preparation of lithium-ion secondary battery

[0132] Wind the positive electrode sheet in step (1), the separator in step (4), and the negative electrode sheet in step (2) to obtain an un-injected bare battery, and apply 5 MPa for 10 min to the wound electrode assembly; place the bare battery in an outer packaging foil, inject the electrolyte in step (3) into the dried bare battery, and through processes such as vacuum packaging, standing, formation, shaping, sorting, etc., the required lithium-ion secondary battery is obtained, where the number of positive electrode tabs is 15 and the number of negative electrode tabs is 15. Among them, the ratio of the thickness of the electrode assembly to the size L of the flat area in the length direction of the electrode assembly is 0.035, the thickness of the negative electrode sheet in the arc area is 114 μm, and the thickness of the flat area is 114 μm.

[0133] Example 2

[0134] (1) Positive electrode sheet

[0135] Performed with reference to Example 1, except that the thickness of the first substrate layer is 8 μm, the thickness of the first metal layer is 2 μm, the tensile strength B of the positive electrode current collector is 220 MPa, and the elongation of the positive electrode current collector is 0.5%; the chemical formula of the nickel-cobalt-manganese ternary material is LiNi 0.5 Co 0.2 Mn 0.3 O2, the mass ratio m of the nickel-cobalt-manganese ternary material in the positive electrode active material is 30%, the mass ratio of lithium cobalt oxide in the positive electrode active material is 70%, the average particle size A of the nickel-cobalt-manganese ternary material is 2.85 μm, A / B = 2.85 / 220 = 0.013, the nickel-cobalt-manganese ternary material includes single crystal particles and polycrystalline particles, the average particle size of the single crystal particles is 3 μm, the average particle size of the polycrystalline particles is 2.5 μm, the molar proportion of nickel in the transition metal elements in the nickel-cobalt-manganese ternary material is 50%, the residual alkali content of the nickel-cobalt-manganese ternary material is 700 ppm, and the specific surface area of the nickel-cobalt-manganese ternary material is 0.8 m 2 / g; the average particle size of the first particles is 1.0 μm, the aluminum element content in the first particles is 6200 ppm, the average particle size of the second particles is 2 μm, and the aluminum element content in the second particles is 0 ppm.

[0136] (2) Negative electrode sheet

[0137] Performed with reference to Example 1, except that the thickness T1 of the single-sided second metal layer is 0.8 μm, the thickness T2 of the negative electrode current collector is 4.6 μm, c / T1 = 4.93 / 0.8 = 6.16, and the V OI value of the negative electrode active material is 10, V OI / T2 = 10 / 4.6 = 2.2.

[0138] (3) Electrolyte

[0139] Performed with reference to Example 1, except that the mass content of fluoroethylene carbonate in the electrolyte is 15%.

[0140] (4) Separator: Performed with reference to Example 1.

[0141] (5) Preparation of the lithium-ion secondary battery: Performed with reference to Example 1. Among them, the ratio of the thickness of the electrode assembly to the size L of the flat area in the length direction of the electrode assembly is 0.15.

[0142] Example 3 group

[0143] This group of examples is used to illustrate the influence generated when the ratio of the thickness of the electrode assembly to the size L of the flat area in the length direction of the electrode assembly changes.

[0144] Example 3a

[0145] It is carried out with reference to Example 1, except that the ratio of the thickness of the electrode assembly to the dimension L of the flat region in the length direction of the electrode assembly is 0.05.

[0146] Example 3b

[0147] It is carried out with reference to Example 1, except that the ratio of the thickness of the electrode assembly to the dimension L of the flat region in the length direction of the electrode assembly is 0.2.

[0148] Example 3c

[0149] It is carried out with reference to Example 1, except that the ratio of the thickness of the electrode assembly to the dimension L of the flat region in the length direction of the electrode assembly is 0.03.

[0150] Example 3d

[0151] It is carried out with reference to Example 1, except that the ratio of the thickness of the electrode assembly to the dimension L of the flat region in the length direction of the electrode assembly is 0.5.

[0152] Example 4

[0153] It is carried out with reference to Example 1, except that the thickness of the negative electrode sheet in the arc region is 116 μm and the thickness of the flat region is 114 μm.

[0154] Example 5 group

[0155] This group of examples is used to illustrate the effects when the mass fraction m of the nickel-cobalt-manganese ternary material in the positive active material changes.

[0156] Example 5a

[0157] It is carried out with reference to Example 1, except that the mass fraction m of the nickel-cobalt-manganese ternary material in the positive active material is 20%.

[0158] Example 5b

[0159] It is carried out with reference to Example 1, except that the mass fraction m of the nickel-cobalt-manganese ternary material in the positive active material is 50%.

[0160] Example 6 group

[0161] This group of examples is used to illustrate the effects when A / B changes.

[0162] This group of examples is carried out with reference to Example 1, except that A / B is changed. For details, see Table 1-1.

[0163] Table 1-1

[0164]

[0165]

[0166] Example Group 7

[0167] This group of examples is used to illustrate the effects produced when the mass ratio of single-crystal particles to polycrystalline particles changes.

[0168] Example 7a

[0169] Carried out with reference to Example 1, the difference is that the mass ratio of single-crystal particles to polycrystalline particles is 50%:50%, the average particle size of the nickel-cobalt-manganese ternary material is 3.5 μm, and A / B = 3.5 / 250 = 0.014.

[0170] Example 7b

[0171] Carried out with reference to Example 1, the difference is that the mass ratio of single-crystal particles to polycrystalline particles is 80%:20%, the average particle size of the nickel-cobalt-manganese ternary material is 3.68 μm, and A / B = 3.68 / 250 = 0.01472.

[0172] Example 7c

[0173] Carried out with reference to Example 1, the difference is that the mass ratio of single-crystal particles to polycrystalline particles is 90%:10%, the average particle size of the nickel-cobalt-manganese ternary material is 3.74 μm, and A / B = 3.5 / 250 = 0.01496.

[0174] Example Group 8

[0175] This group of examples is used to illustrate the effects produced when the aluminum element in the first particles or the aluminum element in the second particles changes.

[0176] Example 8a

[0177] Carried out with reference to Example 1, the difference is that the aluminum element content in the first particles is 6000 ppm.

[0178] Example 8b

[0179] Carried out with reference to Example 1, the difference is that the aluminum element content in the first particles is 8000 ppm.

[0180] Example 8c

[0181] Carried out with reference to Example 1, the difference is that the aluminum element content in the second particles is 5000 ppm.

[0182] Example 8d

[0183] It was carried out with reference to Example 1, except that the content of aluminum element in the second particles was 7000 ppm.

[0184] Example 9

[0185] It was carried out with reference to Example 1, except that the average particle size of the first particles was the same as that of the second particles, both being 10 μm.

[0186] Example 10 group

[0187] This group of examples is used to illustrate the influence produced when c / T1 changes.

[0188] This group of examples was carried out with reference to Example 1, except that c / T1 was changed. For details, see Table 1-2.

[0189] Example 11 group

[0190] This group of examples is used to illustrate when V OI / T2 changes, the resulting influence.

[0191] This group of examples was carried out with reference to Example 1, except that V OI / T2 was changed. For details, see Table 1-2.

[0192] Table 1-2

[0193]

[0194] Example 12 group

[0195] This group of examples is used to illustrate the influence produced when the mass content of fluoroethylene carbonate in the electrolyte changes.

[0196] Example 12a

[0197] It was carried out with reference to Example 1, except that the mass content of fluoroethylene carbonate in the electrolyte was 20%.

[0198] Example 12b

[0199] It was carried out with reference to Example 1, except that the mass content of fluoroethylene carbonate in the electrolyte was 2%.

[0200] Comparative Example 1

[0201] It was carried out with reference to Example 1, except that the ratio of the thickness of the electrode assembly to the dimension L of the flat region in the length direction of the electrode assembly was 0.01.

[0202] Comparative Example 2

[0203] Performed with reference to Example 1, except that the ratio of the thickness of the electrode assembly to the dimension L of the flat region in the length direction of the electrode assembly is 0.6.

[0204] Comparative Example 3

[0205] Performed with reference to Example 1, except that there is no fluoroethylene carbonate in the electrolyte.

[0206] Test Example

[0207] The lithium-ion secondary batteries prepared in the examples and comparative examples were respectively tested as follows:

[0208] (1) V OI Value test

[0209] Take out the negative electrode sheet in the battery, rinse it with the organic solvent DMC, dry it, cut it into a suitable size, attach it to the sample plate, and use an X-ray diffractometer to test the X-ray diffraction pattern of the negative electrode. The excitation source of the X-ray diffractometer is CuKα, the scanning angle range is 10° - 90°, and the scanning speed is 2° / min. The V OI value is the ratio of the peak area of (004) to the peak area of (110).

[0210] (2) High-temperature cycle performance test

[0211] a) Place the lithium-ion battery at 45°C, charge it at a constant current of 1C to 4.42V, then switch to a constant current of 0.7C to charge to the upper limit voltage (4.45V), then charge at a constant voltage of 4.45V to 0.05C, and let it stand for 5 minutes; then discharge at a constant current of 1C to 3V, and the discharge capacity at this time is the initial discharge capacity, denoted as C0, and let it stand for 5 minutes;

[0212] b) Cycle 700T according to step a), and record the discharge capacity of the 700th cycle as C1.

[0213] Capacity retention rate: C = (C1 / C0) × 100%, thickness expansion rate = [(H1 - H0) / H0] × 100%.

[0214] (3) Test for the situation of the electrode strip breaking

[0215] Cycle the lithium-ion battery according to the method of the high-temperature cycle performance test. After 700T of cycling, disassemble the battery and observe the first fold and the second fold inside the battery cell. If both the first fold and the second fold are opaque, it means passing; if there is dot-like light transmission or cracked edges in the first fold or the second fold, it means failing; a total of 10 batteries are tested, and the result is expressed as "number of passes PASS / 10". For example, if only 8 out of 10 batteries pass the test, the result is expressed as "8PASS / 10".

[0216] (4) Safety performance test

[0217] Place the battery in an environment of 25°C ± 3°C and discharge it at 0.2C to the cut-off voltage of 3.0V; let it stand for 5 minutes, then charge it at a constant current and constant voltage of 0.5C to the upper limit voltage (4.45V), with a cut-off current of 0.05C (at this time, the battery is fully charged). Put the fully charged battery cell into the test chamber, and the test chamber heats up at a temperature rise rate of (5 ± 2)°C / min. When the temperature in the chamber reaches 130°C ± 2°C and 145°C ± 2°C respectively, keep it constant for 60 minutes. If the battery catches fire or explodes, it fails; if the battery does not catch fire or explode, it passes. Test 10 batteries at 130°C and 145°C respectively, and the results are expressed as "10 / number of passes pass". For example, if only 8 out of 10 batteries pass the test, the result is expressed as "10 / 8pass".

[0218] (5) Volume energy density test

[0219] Place the lithium-ion battery at 25°C and charge it at a constant current of 1C to 4.42V, then switch to 0.7C to charge to the upper limit voltage (4.45V), and then charge at a constant voltage of 4.45V to 0.05C. Record the discharge capacity of the battery at this time, denoted as C. Measure the length L, width S, and thickness H of the battery with a micrometer. Then the volume energy density of the battery = C / (L × S × H).

[0220] (6) Lithium plating situation

[0221] Place the lithium-ion battery at 25°C and charge it at a constant current of 1C to 4.42V, then switch to 0.7C to charge to the upper limit voltage (4.45V), and then charge at a constant voltage of 4.45V to 0.05C. After that, disassemble the battery and observe whether there is lithium plating on the negative electrode. If there is no lithium plating, the result is expressed as "no lithium plating"; if there is lithium plating, it is expressed as the proportion of the lithium plating area to the surface area of the negative electrode.

[0222] (7) Rate performance

[0223] (1) Place the lithium-ion battery at 25 ± 5 °C and let it stand for 10 min; discharge it at 0.2C to the lower limit voltage; let it stand for 10 min. (2) Charge it to full capacity at 0.7C in a constant-temperature chamber (the temperature of the constant-temperature chamber is 25 ± 5 °C), with a cut-off current of 0.025C, and let it stand for 10 min. In the constant-temperature chamber or constant-temperature box environment (the temperature of the constant-temperature chamber is 25 ± 5 °C), discharge it at 0.2C to the cut-off voltage (3V), and record the discharge capacity at this time as Q1, then let it stand for 10 min. (3) Charge it to full capacity at 0.7C in a constant-temperature chamber (the temperature of the constant-temperature chamber is 25 ± 5 °C), with a cut-off current of 0.025C, and let it stand for 10 min. In the constant-temperature chamber or constant-temperature box environment (the temperature of the constant-temperature chamber is 25 ± 5 °C), discharge it at 2C to the cut-off voltage (3V), and record the discharge capacity at this time as Q2, then let it stand for 10 min. The rate performance % = (Q2 / Q1) × 100%.

[0224] (8) Thickness expansion rate

[0225] a) Place the lithium-ion battery at 55 °C and charge it at a constant current of 1C to 4.42V, then switch to 0.7C to charge to the upper limit voltage (4.45V), then charge at a constant voltage of 4.45V to 0.05C, and let it stand for 5 minutes. Measure the thickness of the battery at this time as the initial thickness H0; then discharge it at a constant current of 1C to 3V and let it stand for 5 minutes.

[0226] b) Repeat step a) 300 times, and record the thickness of the battery at the 300th cycle as H1.

[0227] Thickness expansion rate = [(H1 - H0) / H0] × 100%.

[0228] After 300 cycles, observe the bulging situation of the battery. When there is no bulging, it means no bulging. When there is bulging, if the thickness expansion rate is not higher than 10%, it means slight bulging; if it is higher than 10% but not higher than 20%, it means general bulging.

[0229] Record the obtained results in Table 2.

[0230] Table 2

[0231]

[0232]

[0233]

[0234] When the lithium-ion secondary battery of Comparative Example 1 was subjected to high-temperature cycle testing, the capacity retention rate dropped suddenly to below 50% after about 300 cycles. Therefore, its rate performance and volume energy data do not exist.

[0235] The thickness expansion rate of the lithium-ion secondary battery of Comparative Example 1 was 12%, but there was no bulging, indicating that the thickness of the lithium-ion secondary battery of Comparative Example 1 changed during the cycle, but no gas was generated in the battery, so there was no bulging.

[0236] As can be seen from Table 2, through the comparison between the comparative examples and the examples, it can be seen that the pole breakage of the lithium-ion secondary battery of the examples was significantly reduced, the passing rate of the hot box test was significantly improved, and the cycle capacity retention rate was significantly improved. It shows that by controlling the thickness of the battery components of the nickel-cobalt-manganese ternary material and the silicon-carbon material system and the size of the flat area in the length direction of the electrode assembly, the mass content of silicon element in the silicon-carbon material, and the mass content of fluoroethylene carbonate in the electrolyte, while maintaining a high energy density, the pole breakage situation in the battery can be improved, the safety performance and cycle performance of the battery can be improved, and the thickness expansion rate of the battery can be reduced.

[0237] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the technical concept scope 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 formed by laminating and winding a positive electrode sheet, a separator, and a negative electrode sheet. The electrode assembly has an arc region and a flat region. The ratio of the thickness of the electrode assembly to the dimension L of the flat region in the length direction of the electrode assembly is (0.03 - 0.5):

1. The positive electrode sheet includes a positive electrode active material, the positive electrode active material includes a nickel-cobalt-manganese ternary material, and the nickel-cobalt-manganese ternary material includes a substance with the chemical formula Li a Ni x Co y Mn z M b O2, where 0.9 ≤ a ≤ 1.1, 0.5 ≤ x ≤ 0.95, 0 < y ≤ 0.3, 0 < z ≤ 0.3, 0 ≤ b ≤ 0.05, and M is selected from at least one of Al, Zr, B, Mg, B, Y, Sr, W, Ti, and Nb; The negative electrode sheet includes a negative electrode active material, and the negative electrode active material includes a silicon-carbon material. The mass content of silicon element in the silicon-carbon material is 35% - 70%. The lithium-ion secondary battery further includes an electrolyte, and the electrolyte includes fluoroethylene carbonate. The mass content of fluoroethylene carbonate in the electrolyte is 2% - 20%.

2. The lithium ion secondary battery according to claim 1, wherein The ratio of the thickness of the electrode assembly to the dimension L of the flat region in the length direction of the electrode assembly is (0.05 - 0.2):

1. And / or, the thickness of the negative electrode sheet located in the arc region is greater than the thickness of the negative electrode sheet located in the flat region. And / or, the mass content of aluminum element in the nickel-cobalt-manganese ternary material is 1000 - 2500 ppm, the mass content of titanium element is 500 - 1000 ppm, and the mass content of tungsten element is 2000 - 4000 ppm.

3. The lithium ion secondary battery according to claim 1 or 2, wherein: The positive electrode active material further includes lithium cobaltate. The mass proportion of the nickel-cobalt-manganese ternary material in the positive electrode active material is m, where 0% < m ≤ 50%. The nickel-cobalt-manganese ternary material includes single crystal and / or polycrystalline particles, and the average particle size of the nickel-cobalt-manganese ternary material is A, and A is 1 - 10, with the unit of μm. The positive electrode sheet further includes a positive electrode current collector, and the tensile strength of the positive electrode current collector is B, and B is 120 - 300, with the unit of MPa. A and B satisfy: A / B is 0.004 - 0.

05. And / or, the mass proportion m of the nickel-cobalt-manganese ternary material in the positive electrode active material is 20% ≤ m ≤ 40%. And / or; the silicon-carbon material includes a porous carbon matrix and silicon materials located in the internal pores of the porous carbon matrix.

4. The lithium ion secondary battery according to claim 3, wherein: The positive electrode current collector includes a first substrate layer and first metal layers located on both surface sides of the first substrate layer. The first substrate layer includes a first polymer. Preferably, the first polymer includes at least one of polypropylene, polyethylene, polyethylene terephthalate, polyethylene naphthalate, polyimide, polycarbonate, polyvinyl chloride, polyvinylidene fluoride, polystyrene, polytetrafluoroethylene, polyvinyl alcohol, and modified polymers of the above substances. Preferably, the first metal layer includes aluminum and / or aluminum alloy.

5. The lithium ion secondary battery according to claim 4, wherein: The thickness of the first substrate layer is 2 μm - 28 μm. And / or, the thickness of the first metal layer is 0.5 μm - 2.5 μm. And / or, the thickness of the positive electrode current collector is 3 μm - 30 μm. And / or, the elongation rate of the positive electrode current collector is 0.5% - 1.8%.

6. The lithium ion secondary battery according to claim 1 or 2, wherein: The specific surface area of ​​the nickel-cobalt-manganese ternary material is 0.6 m 2 / g-1m 2 / g; And / or, the residual alkali content of the nickel-cobalt-manganese ternary material is less than or equal to 2000 ppm. And / or, the nickel-cobalt-manganese ternary material further includes single crystal particles and / or polycrystalline particles, and the mass ratio of the single crystal particles to the polycrystalline particles is (90% - 10%):(10% - 90%). And / or, the nickel-cobalt-manganese ternary material further includes polycrystalline particles, and the average particle size of the polycrystalline particles is 1 μm-10 μm; And / or, the compaction density of the positive electrode sheet is greater than or equal to 3.45 g / cm 3 .

7. The lithium ion secondary battery according to claim 3, wherein: The lithium cobalt oxide includes first particles and second particles, and the average particle size of the first particles is greater than the average particle size of the second particles; Preferably, the average particle size of the first particles is 10 μm-30 μm, and the average particle size of the second particles is 2 μm-10 μm; Preferably, the mass ratio of the first particles to the second particles is (50%-99%):(50%-1%).

8. The lithium ion secondary battery according to claim 1 or 2, wherein: The positive electrode sheet comprises a positive electrode current collector and a positive electrode tab, wherein the positive electrode tab extends from the positive electrode current collector along a width direction of the positive electrode sheet, the positive electrode tab is electrically connected to the positive electrode current collector, and the number of the positive electrode tabs is greater than or equal to 2; And / or, the negative electrode sheet includes a negative electrode collector and a negative electrode tab, the negative electrode tab extends from the negative electrode collector along the width direction of the negative electrode sheet, the negative electrode tab is electrically connected to the negative electrode collector, and the number of the negative electrode tabs is greater than or equal to 2.

9. The lithium ion secondary battery according to claim 1 or 2, wherein: The negative electrode sheet further comprises a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector; the negative electrode active material layer comprises the negative electrode active material, and the negative electrode active material further comprises a graphite material; Preferably, the mass content c of silicon in the negative electrode active material layer is 1.5%-22%; Preferably, the mass content of the silicon-carbon material in the negative electrode active material is 5%-30%.

10. The lithium ion secondary battery according to claim 9, wherein The negative electrode current collector comprises a second substrate layer and a second metal layer located on both sides of the second substrate layer; the second substrate layer comprises a second polymer; the mass content c of silicon in the negative electrode active material layer and the thickness T1 of the second metal layer on one side satisfy: c / T1 is 2-13; And / or, the value V of the negative electrode active material layer OI The thickness T2 of the negative electrode current collector satisfies: V OI / T2 is 0.5-8, where V OI It is the ratio of the intensity of the 004 diffraction peak to the intensity of the 110 diffraction peak in the XRD spectrum of the negative electrode; And / or, the value V of the negative electrode active material layer OI 5-30.

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