A lithium-ion secondary battery

Abstract

This invention relates to the field of battery technology, specifically to a lithium-ion secondary battery. It includes a negative electrode, a positive electrode, a separator, and an electrolyte; the battery has a charging cut-off voltage ≥ 4.48V; the negative electrode comprises silicon-carbon material with a silicon content of 30%-80%; the positive electrode includes a positive active coating comprising lithium cobalt oxide, which contains aluminum, with an aluminum content of 6000ppm-15000ppm by mass; the separator includes an organic coating facing the positive electrode; the organic coating comprises polymer particles containing nitrogen, with a thickness of 0.5μm-4μm and a nitrogen content of 10.5%-55% by mass; the electrolyte comprises fluoroethylene carbonate, with a fluoroethylene carbonate content of 5%-30% by mass. The battery of this invention achieves high energy density, fast charging capability, excellent cycle stability, and high-temperature safety performance at a high charging cut-off voltage.

Description

Technical Field

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

[0002] To improve battery energy density, high-silicon-content silicon-based materials (e.g., silicon content of 30%-80%) are typically used as the negative electrode active material, and the battery's charging cut-off voltage is increased to enhance the capacity of the positive electrode, enabling it to work in conjunction with a high-capacity negative electrode, thereby achieving the goal of increasing battery energy density. However, the following problems arise during fast charging: First, high-silicon-content silicon-based materials undergo violent side reactions under fast charging conditions, generating a large amount of heat, seriously threatening battery safety. Second, under high voltage conditions, as the battery charges and discharges, the crystal structure of the positive electrode active material (e.g., lithium cobalt oxide) collapses, leading to poorer battery cycle stability. Especially during fast charging, the electrode potential of the negative electrode decreases, while the potential of the positive electrode increases, severely affecting the battery's cycle stability. Summary of the Invention

[0003] The purpose of this invention is to overcome the aforementioned problems in the prior art and provide a lithium-ion secondary battery. The lithium-ion secondary battery of this invention (hereinafter referred to as the battery) can achieve a balance of high energy density, fast charging capability, excellent cycle stability, and high-temperature safety performance at a high charging cutoff voltage (≥4.48V).

[0004] This invention provides a lithium-ion secondary battery, comprising a negative electrode, a positive electrode, a separator, and an electrolyte; the charging cut-off voltage of the lithium-ion secondary battery is ≥4.48V; the negative electrode comprises a silicon-carbon material, wherein the silicon content of the silicon-carbon material is 30%-80% by mass; the positive electrode comprises a positive active coating, wherein the positive active coating comprises lithium cobalt oxide, wherein the lithium cobalt oxide contains aluminum, and the aluminum content c in the positive active coating is 6000ppm-15000ppm; the separator comprises an organic coating, wherein the organic coating faces the positive electrode; the organic coating comprises polymer particles, wherein the polymer particles contain nitrogen, and the thickness h of the organic coating is 0.5μm-4μm; the electrolyte comprises fluoroethylene carbonate, wherein the fluoroethylene carbonate content in the electrolyte is 5%-30% by mass; the nitrogen content in the organic coating is 10.5%-55% by mass.

[0005] Due to the high silicon content in silicon-carbon materials, there are numerous reaction sites with the electrolyte, leading to increased side reactions. Therefore, it is necessary to add an appropriate amount of fluoroethylene carbonate to the electrolyte. Fluoroethylene carbonate helps form a stable SEI (Solid Electrolyte Interface) film on the surface of silicon-carbon materials, reducing the surface breakage and recombination of silicon particles during high-rate charge and discharge. This not only reduces side reactions between the silicon-carbon material and the electrolyte, thus improving the battery's high-temperature safety performance, but also reduces the consumption of active lithium due to SEI film breakage and recombination, preventing continuous delithiation of the positive electrode active material and contributing to its structural stability. However, the mass content of fluoroethylene carbonate in the electrolyte needs to be controlled. Too much will degrade the battery's high-temperature performance, affecting high-temperature safety; too little will fail to protect the silicon-carbon material, negatively impacting the battery's high-temperature safety and cycle stability.

[0006] As the battery charging cutoff voltage increases, the potential on the positive electrode side also increases, posing a greater challenge to the stability of the lithium cobalt oxide crystal structure. On the one hand, aluminum can form Al-O bonds with oxygen in lithium cobalt oxide, effectively suppressing the extraction of lattice oxygen; on the other hand, Al... 3+ The octahedral structure of lithium cobalt oxide provides greater stability, making it more difficult for lithium cobalt oxide to transform into a monoclinic crystal system. This reduces the kinetics of monoclinic crystal formation and suppresses the phase transition of lithium cobalt oxide. Therefore, controlling the mass content of aluminum in the positive electrode active coating can stabilize lithium cobalt oxide. However, it is not the case that the higher the aluminum content, the better. This is because an increase in aluminum content reduces the specific capacity of lithium cobalt oxide, thus affecting the overall energy density of the battery. Therefore, it is necessary to control the mass content of aluminum in the positive electrode active coating. When a specific range is met, it can ensure that the positive electrode operates stably at high voltages (e.g., 4.48V and above) and ensure that the positive electrode has sufficient capacity, without causing a loss in energy density due to excessive aluminum content.

[0007] Meanwhile, the inventors of this invention also conducted extensive targeted research and discovered that the direct cause of the poor stability of lithium cobalt oxide at high voltage lies in the silicon-carbon material in the negative electrode. This is because the silicon-carbon material undergoes volume expansion / contraction during battery charge-discharge cycles, causing the SEI film on the surface of the negative electrode to continuously break and recombine. This process continuously consumes the active lithium in the battery. With the continuous consumption of active lithium, lithium ions cannot return to the lattice structure of lithium cobalt oxide, resulting in less and less lithium in the lithium cobalt oxide lattice structure. Even by controlling the aluminum content, structural collapse cannot be prevented. However, when the organic coating includes polymer particles containing nitrogen, these polymer particles can complex the transition metal ions dissolved from the positive electrode. Furthermore, when the mass content of nitrogen in the organic coating is within a specific range, a stable CEI (Cathode Electrolyte Interphase) film can be formed on the surface of the positive electrode. Therefore, the inventors of this invention improved the separator by setting an organic coating in the separator and including polymer particles containing nitrogen in the organic coating, while positioning the organic coating directly opposite the positive electrode. Thus, the presence of the aforementioned polymer particles can stabilize the crystal structure of lithium cobalt oxide, reduce the release of active oxygen, and synergistically improve the overall cycle stability of the battery with the aluminum element in the positive electrode active coating.

[0008] Furthermore, this invention also controls the thickness of the organic coating, enabling the battery to better balance energy density and cycle stability. This is because: if the organic coating is too thick, the volumetric energy density of the battery will be low; while if it is too thin, it cannot effectively stabilize the lithium cobalt oxide crystal structure. Therefore, when the thickness of the organic coating is within a specific range, the battery can achieve a balance between energy density and cycle stability.

[0009] Through the above technical solution, the present invention has at least the following advantages compared with the prior art: the battery of the present invention can take into account high energy density, fast charging capability, excellent cycle stability and high temperature safety performance at a high charging cutoff voltage (≥4.48V).

[0010] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0011] Figure 1 The image shows the XRD diffraction pattern of the organic coating in an example of the present invention.

[0012] Figure 2The diagram shown is a top view of the groove on the surface of the negative electrode sheet in an embodiment of the present invention, wherein, Figure 2 (a) The grooves are continuously arranged in the middle; Figure 2 (b) The groove is segmented.

[0013] Figure 3 The diagram shown is a schematic representation of the width of the groove in an embodiment of the present invention, wherein... Figure 3 (a)- Figure 3 In (c), the two long sides of the groove are straight lines. Figure 3 The two long sides of the groove in (d) are curves.

[0014] Figure 4 The diagram shown is a schematic representation of the groove spacing in an embodiment of the present invention, wherein... Figure 4 (a) represents the case where two adjacent long sides are straight lines and parallel. Figure 4 (b) is the case where two adjacent long sides are straight lines and not parallel. Figure 4 (c) is the case where the two adjacent long sides are curves.

[0015] Figure 5 The diagram shown is a structural schematic of the core in an example of the present invention.

[0016] Figure 6 The diagram shown is a schematic representation of the positive electrode sheet in an embodiment of the present invention, wherein... Figure 6 (a) is a top view. Figure 6 (b) is a cross-sectional view along the thickness direction.

[0017] Figure 7 The diagram shown is a top view of the first surface of the positive electrode in an embodiment of the present invention. Detailed Implementation

[0018] 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.

[0019] This invention provides a lithium-ion secondary battery, comprising a negative electrode, a positive electrode, a separator, and an electrolyte. The charging cut-off voltage of the lithium-ion secondary battery is ≥4.48V, for example, 4.48V, 4.5V, or 4.53V. The term "charging cut-off voltage" has its conventional meaning in the art and typically refers to the maximum voltage value that the battery can safely reach during charging.

[0020] In this invention, the negative electrode comprises a silicon-carbon material. The silicon content of the silicon-carbon material can be 30%-80% by mass, for example, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or 80%.

[0021] In this invention, the mass content of silicon in the silicon-carbon material can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode is disassembled and removed. After soaking in dimethyl carbonate (DMC) solvent for 12 hours, it is rinsed with DMC solvent to remove the lithium salt attached to the negative electrode. The negative electrode is then cut with an argon ion milling machine (CP laser) and observed with a scanning electron microscope (SEM) (high voltage mode, back-scattered electrons, BSE). In this mode, the contrast of the silicon-carbon material is bright (which can be used to distinguish the graphite material and conductive agent in the negative electrode active coating). Combined with energy dispersive spectroscopy (EDS), at least 10 silicon-carbon materials are randomly selected and scanned to obtain the mass content of silicon in each particle, and the average value is taken.

[0022] In this invention, the positive electrode sheet includes a positive electrode active coating. The positive electrode active coating includes lithium cobalt oxide. The lithium cobalt oxide contains aluminum, and the mass content c of the aluminum in the positive electrode active coating can be 6000ppm-15000ppm, for example, 6000ppm, 7000ppm, 8000ppm, 9000ppm, 10000ppm, 11000ppm, 12000ppm, 13000ppm, 14000ppm, or 15000ppm.

[0023] In one instance, c is 6500ppm-12000ppm.

[0024] In this invention, the mass content c of aluminum in the positive electrode active coating can be tested by conventional methods in the art, such as using an inductively coupled plasma-emission spectrometer (ICP-OES). The specific test method is as follows: discharge the battery to 0% SOC, disassemble and remove the positive electrode sheet, soak it in DMC solvent for 12 hours; then rinse it with DMC solvent to remove the lithium salt attached to the positive electrode sheet, calcine it in a muffle furnace at 400°C for 3 hours, gently scrape the positive electrode active coating off the surface of the positive electrode current collector, and measure the mass content of aluminum (in ppm, i.e., parts per million) by ICP-OES. The specific operation method is performed in accordance with GB / T 30902-2014.

[0025] In this invention, the separator includes an organic coating facing the positive electrode. The organic coating comprises polymer particles. The polymer particles contain nitrogen. The thickness h of the organic coating can be 0.5 μm-4 μm, for example, 0.5 μm, 1 μm, 2 μm, 3 μm, or 4 μm.

[0026] In one instance, h is 1μm-2μm.

[0027] In this invention, the thickness h of the organic coating can be obtained by conventional methods in the art, such as discharging the battery to 0% SOC, disassembling and removing the separator, soaking it in DMC solvent for 12 hours, rinsing it with DMC solvent to remove lithium salts adhering to the separator, cutting the separator along the thickness direction using an argon ion milling machine with a CP laser, and using SEM to randomly select at least 20 points on the organic coating, measuring the thickness of the organic coating at each point, and taking the average value.

[0028] In this invention, the mass content of nitrogen in the organic coating can be 10.5%-55%, for example, 10.5%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50% or 55%.

[0029] In one example, the organic coating contains 15%-35% nitrogen by mass.

[0030] In this invention, the average particle size of the polymer particles can be 0.2μm-5μm, for example, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 2μm, 3μm, 4μm or 5μm.

[0031] In one example, the average particle size of the polymer particles is 0.2 μm to 2 μm. The average particle size of the polymer particles can be obtained by laser particle size analyzer or SEM.

[0032] In this invention, the electrolyte comprises fluoroethylene carbonate. The mass content of fluoroethylene carbonate in the electrolyte can be 5%-30%, for example, 5%, 10%, 15%, 20%, 25% or 30%.

[0033] In this invention, the mass content of fluoroethylene carbonate in the electrolyte can be obtained by methods conventional in the art, such as gas chromatography (GC).

[0034] In this invention, the chemical formula of lithium cobalt oxide can be Li a Co b M 1 c O2, where 0.8 ≤ a ≤ 1.05, 0.85 ≤ b < 1, 0 <c≤0.15,M 1The lithium cobalt oxide comprises at least one of Al, Mg, Ti, Y, La, Ga, Ge, Sn, Si, Zr, Ca, Sb, In, Ni, and Mn. The lithium cobalt oxide may include a first particle and a second particle. The average particle size of the first particle may be 0.3 μm-7 μm (e.g., 0.3 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, or 7 μm), and the average particle size of the second particle may be 7.5 μm-40 μm (e.g., 7.5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, or 40 μm). Using first and second particles with different average particle sizes allows the difference in particle size distribution to directly affect the filling effect of the lithium cobalt oxide powder during the compression process, thereby affecting the compaction density and electronic conductivity of the lithium cobalt oxide, which is beneficial for improving the overall energy density and fast charging capability of the battery.

[0035] In this invention, the average particle size of the first particle and the average particle size of the second particle can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the positive electrode sheet is disassembled and removed. After soaking in DMC solvent for 12 hours, it is rinsed with DMC solvent to remove the lithium salt attached to the positive electrode sheet. After calcining in a muffle furnace at 400°C for 3 hours, the positive electrode active coating is gently scraped off from the surface of the positive electrode current collector. The particle size range of the first particle and the second particle can be obtained from the obtained curve.

[0036] In this invention, the silicon-carbon material comprises primary spherical particles. The average particle size of the primary spherical particles can be 1μm-6μm, for example, 1μm, 2μm, 3μm, 4μm, 5μm or 6μm.

[0037] In one example, the average particle size of the primary spherical particles is 3 μm-5 μm.

[0038] The silicon-carbon materials used in related technologies are in block form with an average particle size of approximately 6μm-12μm. This relatively large average particle size results in poor conductivity. Furthermore, the high hardness of silicon-carbon materials leads to low compaction density, making it difficult to meet the demands for high energy density, thus limiting the improvement in battery energy density. By further utilizing primary spherical silicon-carbon particles with an average particle size of 1μm-6μm and a high silicon content (e.g., 30%-80% by mass of silicon), the battery's energy density can be significantly improved. Moreover, the smaller particle size enhances the overall conductivity of the negative electrode, thereby improving the battery's charging speed.

[0039] In this invention, the average particle size of the primary spherical particles can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled, soaked in DMC solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt adhering to the negative electrode sheet. The negative electrode sheet is then cut using an argon-ion polisher with a CP laser and observed using SEM (high voltage mode (Back-scattered Electrons BSE)). In this mode, the contrast of silicon-carbon materials is brighter (which can be used to distinguish graphite materials and conductive agents in the negative electrode active coating). Measurements are taken at 5K magnification, at least 20 primary spherical particles are randomly selected, and the particle size of each primary spherical particle is measured, and the average value is taken. If the number of primary spherical particles at 5K magnification is less than 20, a micrograph is taken again until 20 primary spherical particles are measured.

[0040] In this invention, the mass content c of aluminum in the positive electrode active coating and the thickness h (in μm) of the organic coating satisfy the following condition: 0.003 ≤ c × h ≤ 0.06, for example, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.02, 0.03, 0.04, 0.05 or 0.06.

[0041] In one instance, 0.0065 ≤ c × h ≤ 0.024.

[0042] As mentioned earlier, the structural stability of lithium cobalt oxide improves with increasing organic coating thickness and with increasing aluminum content in the positive electrode active coating. Therefore, increasing both the organic coating thickness and the aluminum content in the positive electrode active coating can, to some extent, make the positive electrode more stable, thereby improving the overall cycle stability of the battery. However, increasing the organic coating thickness reduces the volumetric proportion of the active material in the battery, leading to a decrease in volumetric energy density. Furthermore, increasing the aluminum content in the positive electrode active coating reduces the capacity utilization of lithium cobalt oxide, resulting in a decrease in gravimetric energy density. Therefore, to further improve the battery's energy density while ensuring cycle stability, the relationship between the organic coating thickness *h* and the aluminum content *c* in the positive electrode active coating is controlled. When *c*×h is large (e.g., greater than 0.06), the battery experiences significant energy density loss, but the improvement in cycle stability is not significant. Conversely, when *c*h is small (e.g., less than 0.003), the battery's cycle stability deteriorates. When *c*h is within a specific range, the battery can better balance energy density and cycle stability.

[0043] In this invention, the polymer particles contain at least one of cyano, isocyanate, isocyanate, and triazine.

[0044] Cyanoyl, isocyano, isocyanate, and triazine groups can complex with transition metal ions dissolved from the positive electrode and form a stable CEI film on the surface of the positive electrode. In this way, organic cyanides containing the above groups can diffuse to the surface of the positive electrode, thereby stabilizing the crystal structure of lithium cobalt oxide, reducing the release of active oxygen, and synergistically improving the overall cycle stability of the battery with the aluminum element in the positive electrode active coating.

[0045] In this invention, the polymer particles further include carbon. The ratio of the mass content of carbon to the mass content of nitrogen in the organic coating can be 0.7-7.6, for example, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7 or 7.6.

[0046] In one example, the ratio of the mass content of carbon to the mass content of nitrogen in the organic coating is 1.5-4.

[0047] By adjusting the ratio of carbon to nitrogen content in the organic coating, the structural stabilizing effect of the organic coating on lithium cobalt oxide can be further controlled. When this ratio is within a specific range, the organic coating can further stabilize the structure of lithium cobalt oxide, thereby improving the cycle stability of the battery.

[0048] In this invention, the mass content of carbon and nitrogen in the organic coating can be obtained by conventional methods in the art. For example, the battery is discharged to 0% SOC, the battery is disassembled, the separator extending beyond the negative electrode area (i.e., the overhang region separator) is cut off, and the separator is soaked in DMC solvent for 12 hours. Then, it is rinsed with DMC solvent to remove the lithium salt adhering to the separator. The first adhesive layer on the outer surface of the organic coating is gently removed with tape (this step is omitted if no first adhesive layer is set on the outer surface of the organic coating). The organic coating is observed using SEM, and then combined with EDS, the particles in the organic coating are scanned at 30K magnification. At least 20 points are selected, and the mass content of carbon and nitrogen is tested respectively, and the average value is taken.

[0049] In this invention, the XRD diffraction pattern of the organic coating exhibits characteristic peaks in the 2θ range of 19.5°–23.5°. For example... Figure 1 The figure shows the XRD diffraction pattern of the organic coating in an example of the present invention. As can be seen from the figure, there is a characteristic peak at 2θ of 21.54°. The intensity of the characteristic peak is in the range of 6000-12000 (in au), specifically 11027 a.u.

[0050] When the XRD diffraction pattern of the organic coating has characteristic peaks within a specific angular range, and the intensity of the characteristic peaks is within a specific range, it indicates that the organic coating contains polymer particles containing at least one of cyano, isocyano, isocyanate, and triazine groups, and the content of the above groups can effectively improve the structural stability of lithium cobalt oxide without having a significant adverse effect on the energy density of the battery.

[0051] In this invention, the mass content of carbon in the organic coating can be 40%-80%, for example, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75% or 80%.

[0052] In one example, the organic coating contains 50%-70% carbon by mass.

[0053] In this invention, the diaphragm further includes at least one of a substrate layer, a first adhesive layer, and a second adhesive layer.

[0054] In one example, the organic coating includes the substrate layer, the organic coating located on one side surface of the substrate layer, a first adhesive layer located on the outer surface of the organic coating, and a second adhesive layer located on the other side surface of the substrate layer. The substrate layer may, for example, comprise polyethylene. The first adhesive layer may, for example, comprise polymethyl methacrylate (PMMA). The second adhesive layer may, for example, comprise polyvinylidene fluoride (PVDF) and / or PMMA.

[0055] Setting a first adhesive layer on the outer surface of the organic coating not only facilitates the adhesion between the organic coating and the positive electrode, but also allows for the storage of electrolyte, enabling the organic cyanide in the organic coating to diffuse better to the surface of the positive electrode, thereby playing a role in stabilizing the lattice stability of lithium cobalt oxide.

[0056] In this invention, the thickness of the substrate layer can be 3μm-6μm, for example, 3μm, 4μm, 5μm, or 6μm. The thickness of the first adhesive layer can be 0.2μm-3μm, for example, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 1.5μm, 2μm, 2.5μm, or 3μm. The thickness of the second adhesive layer can be 0.2μm-3μm, for example, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 1.5μm, 2μm, 2.5μm, or 3μm.

[0057] In one example, the thickness of the substrate layer is 4 μm-5 μm. The thickness of the first adhesive layer is 0.2 μm-1 μm. The thickness of the second adhesive layer is 1.5 μm-3 μm.

[0058] In one example, the thickness of the first adhesive layer is 0.3 μm-0.8 μm. The thickness of the second adhesive layer is 2 μm-2.5 μm.

[0059] By adjusting the thickness of the first adhesive layer, the battery can achieve a better balance between energy density and cycle stability. This is because: if the first adhesive layer is too thick, the volumetric energy density of the battery will be lower, and the diffusion of organic cyanides from the organic coating to the surface of the positive electrode will be hindered. Conversely, if the first adhesive layer is too thin, it will affect the adhesion between the separator and the positive electrode, potentially causing misalignment between the two, thus affecting the battery's cycle stability. Therefore, a first adhesive layer of appropriate thickness can further improve the battery's ability to balance energy density and cycle stability.

[0060] In this invention, the thickness of the substrate layer, the thickness of the first adhesive layer, and the thickness of the second adhesive layer can be obtained by methods conventional in the art, such as the test method for the thickness h of the organic coating, which will not be described in detail here.

[0061] In this invention, the polymer particles include, for example, at least one of polyacrylonitrile, nitrile rubber, 1,3,5-triazine-2,4,6-triamine, melamine cyanurate, and melamine thiocyanate.

[0062] In this invention, the adhesion force between the negative electrode and the separator is greater than the adhesion force between the positive electrode and the separator.

[0063] During high-rate charging, lithium ions are extracted from the positive electrode and inserted into the negative electrode. The extraction rate is much greater than the insertion rate. Therefore, increasing the lithium insertion rate of the negative electrode is key to improving charging speed. By increasing the adhesion between the negative electrode and the separator, the interfacial gap between them can be reduced, increasing the ion transport rate and thus improving the battery's charging speed.

[0064] In this invention, the adhesion force between the negative electrode and the separator can be 10 N / m to 40 N / m, for example, 10 N / m, 15 N / m, 20 N / m, 25 N / m, 30 N / m, 35 N / m, or 40 N / m. The adhesion force between the positive electrode and the separator is ≤10 N / m, for example, 10 N / m, 9 N / m, 8 N / m, 7 N / m, 6 N / m, 5 N / m, 4 N / m, 3 N / m, 2 N / m, or 1 N / m.

[0065] In this invention, the adhesion force between the negative electrode and the separator and the adhesion force between the positive electrode and the separator can be obtained by conventional methods in the art. For example, the battery is discharged to 0% SOC, the battery is disassembled, and the areas where the separator and the positive / negative electrode are completely bonded are taken. Using a knife, a strip with a size of 15mm is cut along the width direction (the width direction of the separator) (i.e., the length of the strip is the width of the separator, and the width of the strip is 15mm). Using a pull tester, the separator is fixed at one end, and the electrode (positive or negative electrode) is fixed at the other end. The separator and the electrode are peeled off until they are completely separated. The average peeling force of the entire peeling process is the adhesion force between the separator and the positive / negative electrode.

[0066] In this invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode active coating located on at least one side of the surface of the negative electrode current collector. The outer surface of the negative electrode active coating may have a first recess. The first recess on the surface of the negative electrode active coating can shorten the contact distance between the electrolyte and the negative electrode active material, reduce the polarization resistance in the thickness direction of the negative electrode sheet, improve the kinetic performance of the negative electrode sheet, and thus increase the charging speed of the battery.

[0067] In this invention, the first recess may include a recessed hole or a groove. The first recess can be obtained using laser drilling or wire drilling techniques. When the first recess is a groove, the groove can be continuously provided or segmented. Figure 2 The diagram shown is a top view of the groove on the surface of the negative electrode sheet in an embodiment of the present invention, wherein, Figure 2 (a) The grooves are continuously arranged in the middle; Figure 2 (b) The grooves are segmented. As can be seen from the figure, the surface of the negative electrode (i.e., the surface of the negative electrode active coating) has several grooves. Figure 2 In (a), the groove is continuously provided in the width direction of the negative electrode sheet. Figure 2 In (b), the groove is segmented along the width direction of the negative electrode sheet. It is understood that... Figure 2 The example only shows the case where the groove is set along the width direction of the negative electrode sheet; the groove can also be set along the length direction of the negative electrode sheet.

[0068] In this invention, the depth of the first recess can be 5μm-40μm, for example, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, or 40μm. The depth of the first recess has a conventional meaning in the art, referring to the vertical distance from the lowest point within the first recess to the surface of the negative electrode sheet. The depth of the first recess can be measured using conventional methods in the art, such as using a 3D profilometer to measure the depth of all or at least 20 first recesses on the surface of the negative electrode active coating and taking the average value.

[0069] In one example, the depth of the first recess is 15μm-30μm.

[0070] In this invention, the width of the first recess can be 40μm-200μm, for example, 40μm, 50μm, 60μm, 70μm, 80μm, 90μm, 100μm, 150μm or 200μm.

[0071] In one example, the width of the first recess is 60μm-100μm.

[0072] When the first recess is a concave hole, the width of the first recess refers to the diameter of the concave hole. The diameter of the concave hole has a conventional meaning in the art. When the shape of the orthographic projection of the concave hole on the surface of the negative electrode is a "regular circle", the diameter of the concave hole is the diameter of the regular circle; when the shape of the orthographic projection of the concave hole on the surface of the negative electrode is a non-"regular circle" (e.g., ellipse or irregular curved polygon), the diameter of the concave hole is the diameter of an equivalent circle with the same area as the non-"regular circle". The diameter of the concave hole can be tested by conventional means in the art, for example, by using a 3D profilometer to select all or at least 10 concave holes, measure the diameter, and take the average value.

[0073] When the first recess is a groove, the width of the first recess refers to the width of the groove. The width of the groove has a conventional meaning in the art. The orthographic projection of the groove onto the surface of the negative electrode includes two long sides, and the width of the groove refers to the average distance from one long side to the other along the length or width direction of the negative electrode. For example... Figure 3 The diagram shown is a schematic representation of the width of the groove in an embodiment of the present invention, wherein... Figure 3 (a)- Figure 3 In (c), the two long sides of the groove are straight lines. Figure 3 In (d), the two longer sides of the groove are curved. Figure 3 (a) and Figure 3 In (b), the two long sides are arranged parallel to each other. Therefore, in the width direction of the negative electrode sheet, the perpendicular distance from any point on one long side to the other long side is equal. At this time, the width of the groove is the perpendicular distance L1 from any point on one long side to the other long side in the length or width direction of the negative electrode sheet. Figure 3In (c), the two long sides of the groove are straight lines, but not parallel. Therefore, in the width direction, the distance from any point on one long side to the other long side is not equal. In this case, the width of the groove can be averaged. That is, on one long side, based on the length of that side, 50 points are selected at equal intervals (i.e., the distance between each point is equal, which makes the calculation result more accurate), and the width L1 corresponding to each point is measured. The average value is then taken to obtain the width of the groove. Figure 3 In (d), the two long sides are curves. Therefore, in the width direction, the distance from any point on one long side to the other long side is not equal. In this case, the width of the groove can also be taken as the average value, that is, 50 points are randomly selected on one long side (because...). Figure 3 In (d), the two longer sides are curves, and there is no... Figure 3 (c) Given the relationship between the two long sides, 50 points can be randomly selected for measurement. The width L1 corresponding to each point is measured, and the average value is taken to obtain the width of the groove. The width of the groove can be tested using conventional methods in the art, such as using a 3D profilometer to test the width of all grooves or at least 5 grooves on the surface of the negative electrode active coating, and taking the average value.

[0074] In this invention, the spacing of the first recess can be 0.5mm-5mm, for example, 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, 4mm, 4.5mm or 5mm.

[0075] In one example, the spacing of the first recess is 0.8mm-1.5mm.

[0076] When the first recess is a hole, the spacing of the first recess refers to the spacing of the holes. The spacing of the holes has a conventional meaning in the art. It refers to the shortest distance between the edges of two adjacent holes on the surface of the negative electrode. The spacing of the holes can be tested by conventional means in the art, such as using a 3D profilometer to select all or at least 10 groups of adjacent holes, measure the spacing, and take the average value.

[0077] When the first recess is a groove, the spacing between the first recesses refers to the spacing between the grooves. It is understood that when there is only one groove on the surface of the negative electrode, there is no spacing between the grooves. The spacing between the grooves has a conventional meaning in the art, referring to the average distance between the two adjacent long sides of two adjacent grooves in the length or width direction of the negative electrode. Figure 4 The diagram shown is a schematic representation of the groove spacing in an embodiment of the present invention, wherein... Figure 4 (a) represents the case where two adjacent long sides are straight lines and parallel. Figure 4(b) is the case where two adjacent long sides are straight lines and not parallel. Figure 4 (c) represents the case where two adjacent long sides are curves. Figure 4 In (a), adjacent long sides are straight lines and parallel to each other. Therefore, in the width direction, the distance from any point on one long side to the other long side is equal. In this case, the spacing of the grooves is L2, the distance from any point on one long side to the other long side in the width direction. Figure 4 In (b), adjacent long sides are straight lines, but not parallel. Therefore, in the width direction, the distance from any point on one long side to the other long side is not equal. In this case, the spacing of the grooves can be averaged. That is, on one long side, based on the length of that side, 50 points are selected at equal intervals (i.e., the distance between each point is equal, which makes the calculation results more accurate), and the width L2 corresponding to each point is measured. The average value is then taken to obtain the spacing. Figure 4 In (c), two adjacent long sides are curves. Therefore, in the width direction, the distance from any point on one long side to the other long side is not equal. In this case, the spacing of the grooves can also be taken as an average value, that is, 50 points are randomly selected on one long side (because...). Figure 4 In (c), the two longer sides are curves, and there is no... Figure 4 (b) Given the relationship between the two long sides, 50 points can be randomly selected for measurement. The width L2 corresponding to each point is measured, and the average value is taken to obtain the spacing. The spacing of the grooves can be tested by conventional means in the art, such as using a 3D profilometer to test the spacing of all grooves or at least 5 grooves on the surface of the negative electrode active coating, and taking the average value.

[0078] In this invention, the positive electrode sheet includes a positive current collector and a positive active coating located on at least one surface of the positive current collector. The length of the positive active coating on the first surface of the positive current collector is greater than the length of the positive active coating on the second surface of the positive current collector. It is understood that during coating, the positive electrode sheet has a single-sided coating area and a double-sided coating area. The single-sided coating area is where the positive current collector has a positive active coating on only one surface; while the double-sided coating area is where the positive current collector has a positive active coating on both surfaces. This results in the positive electrode sheet having unequal lengths of the positive active coating on both surfaces. In this invention, the side of the positive active coating on the surface of the positive current collector with a relatively longer length is defined as the first surface, and the side with a relatively shorter length is defined as the second surface. The area where the projections of the positive active coating on the first surface and the positive active coating on the second surface overlap in the thickness direction of the positive electrode sheet is the double-sided coating area, and the area where the projections do not overlap is the single-sided coating area. Due to the special structure of wound batteries, the first surface usually faces the winding center of the core, while the second surface faces away from the winding center of the core.

[0079] In this invention, the surface of the positive electrode active coating located on the first surface has a second concave portion, and the surface of the positive electrode active coating located on the second surface has a convex portion. An embossing process can be performed on the surface of the positive electrode to obtain a structure with a concave portion on one side and a convex portion on the other. The storage locations of the electrolyte inside the battery are distributed in the gaps between the cell and the casing (e.g., aluminum-plastic film) and in the interlayer gaps between the positive and negative electrodes and the separator. The storage of electrolyte in the interlayer mainly relies on the pores of the electrodes (positive and negative electrodes) and capillary effects for slow wetting. Compared to the wetting in the gap between the cell and the casing, this process takes longer and is more difficult. Therefore, the amount of electrolyte stored in the interlayer gaps between the positive and negative electrodes and the separator is relatively small. Providing a second concave and convex portion on the surface of the positive electrode can provide more storage space, reduce the ion transport distance, thereby reducing the polarization of the positive and negative electrodes and improving the charging speed of the battery.

[0080] Furthermore, the inventors of this invention conducted a stress analysis on the positive electrode active coating facing the winding center and away from the winding center in the core, and found that when a concave part is provided on the first surface facing the winding center and a convex part is provided on the second surface away from the winding center, it is not only beneficial to the structural stability of the positive electrode sheet itself, but also provides better buffer space for the volume expansion of the negative electrode sheet, thereby improving the cycle life of the battery.

[0081] like Figure 5The following is a schematic structural diagram of a core in an example of the present invention. As can be seen from the figure, the surface of the positive electrode active coating on the first surface has a second recess, and the surface of the positive electrode active coating on the second surface has a protrusion. The first surface faces the winding center of the core, and the second surface faces away from the winding center of the core.

[0082] In the present invention, the shapes of the positive projections of the second recess and the protrusion on the surface of the positive electrode sheet are not limited, and may be shapes such as circular, oval, linear (including straight or wavy lines), polygonal, etc.

[0083] In the present invention, the positive electrode sheet includes a positive electrode tab welding area, a pasting area, and a bare foil area. The pasting area includes a double-sided coating area and a single-sided coating area. As Figure 6 The following is a schematic structural diagram of a positive electrode sheet in an example of the present invention, where Figure 6 (a) is a top view, Figure 6 (b) is a cross-sectional view along the thickness direction. As can be seen from the figure, the positive electrode sheet includes a positive electrode tab welding area 3, a pasting area 4, and a bare foil area 5, where the pasting area 4 includes a double-sided coating area 10 and a single-sided coating area 20.

[0084] In one example, the second recess and the protrusion are located in the pasting area.

[0085] In one example, the second recess and the protrusion are located in the double-sided coating area.

[0086] In one example, the second recess and the protrusion are located in the pasting area and in the double-sided coating area.

[0087] In the present invention, the distance from the second recess to the edge of the positive electrode tab welding area is w1, 0 mm < w1 ≤ 10 mm, for example, 0.1 mm, 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, or 10 mm.

[0088] In the present invention, the distance from the second recess to the edge of the first side of the pasting area is w2, 2 mm ≤ w2 ≤ 40 mm, for example, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm, 20 mm, 25 mm, 30 mm, 35 mm, or 40 mm. The first side is the side where the positive electrode tab welding area is provided.

[0089] In the present invention, the distance from the second recess to the edge of the second side of the pasting area is w3, where 2 mm ≤ w3 ≤ 25 mm, such as 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm, 20 mm or 25 mm. The second side is the side opposite to the side where the positive tab welding area is provided.

[0090] In the present invention, the distance from the second recess to the edge of the third side of the pasting area is w4, where 0 mm < w4 ≤ 20 mm, such as 0.1 mm, 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm or 20 mm. The third side is the side of the pasting area close to the starting end of winding.

[0091] In the present invention, the distance from the second recess to the boundary line between the double-sided coating area and the single-sided coating area is w5, where 0 mm < w5 ≤ 20 mm, such as 0.1 mm, 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm or 20 mm.

[0092] By controlling w1, w2, w3, w4 and w5, it is possible to ensure that the positive electrode sheet does not shed powder, which is beneficial to improving the cycle life of the battery.

[0093] As Figure 7 Shown is a top view schematic diagram of the first surface of the positive electrode sheet in an example of the present invention. As can be seen from the figure, the first surface has a plurality of second recesses 6. The distance from the second recess 6 to the edge of the positive tab welding area 3 is w1, the distance from the second recess 6 to the edge of the first side of the pasting area 4 is w2, the distance from the second recess 6 to the edge of the second side of the pasting area 4 is w3, the distance from the second recess 6 to the edge of the third side of the pasting area 4 is w4, and the distance from the second recess 6 to the boundary line between the double-sided coating area 10 and the single-sided coating area 20 is w5.

[0094] In the present invention, the depth of the second recess can be 3 μm - 40 μm, such as 3 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm or 40 μm. The height of the convex portion can be 3 μm - 40 μm, such as 3 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm or 40 μm.

[0095] In one example, the depth of the second recess is 10 μm-30 μm. The height of the convex portion is 10 μm-30 μm.

[0096] In this invention, the depth of the second recess and the height of the convex portion have conventional meanings in the art. The depth of the second recess refers to the vertical distance from the lowest point within the second recess to the surface of the positive electrode sheet. The height of the convex portion refers to the vertical distance from the highest point on the convex portion to the surface of the positive electrode sheet. The depth of the second recess and the height of the convex portion can be tested using conventional methods in the art, such as using a 3D profilometer. At least 10 second recesses and 10 convex portions are selected on the positive electrode sheet, and the depth of each second recess and the height of each convex portion are measured, and the average values ​​are taken.

[0097] In this invention, the width of the second recess can be 0.2mm-8mm, for example, 0.2mm, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm or 8mm. The width of the protrusion can be 0.2mm-8mm, for example, 0.2mm, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm or 8mm.

[0098] In one example, the width of the second recess is 1mm-3mm. The width of the convex portion is 1mm-3mm.

[0099] In this invention, when the projection of the second recess onto the thickness direction of the positive electrode sheet is a regular circle, the width of the second recess is the diameter of the regular circle; when the projection of the second recess onto the thickness direction of the positive electrode sheet is not a regular circle, the width of the second recess is the equivalent diameter of a circle with the same area as the irregular circle. Similarly, when the projection of the convex portion onto the thickness direction of the positive electrode sheet is a regular circle, the width of the convex portion is the diameter of the regular circle; when the projection of the convex portion onto the thickness direction of the positive electrode sheet is not a regular circle, the width of the convex portion is the equivalent diameter of a circle with the same area as the irregular circle. The widths of the second recess and the convex portion can be obtained by conventional methods in the art, for example, by using a 3D profilometer to select at least 10 second recesses and 10 convex portions on the surface of the positive electrode sheet, measuring the width of each second recess and the width of each convex portion, and taking the average value.

[0100] In this invention, the spacing of the second recess can be 0.5mm-8mm, for example, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm or 8mm. The spacing of the protrusions can be 0.5mm-8mm, for example, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm or 8mm.

[0101] In one example, the spacing between the second recesses is 1mm-3mm. The spacing between the protrusions is 1mm-3mm.

[0102] In this invention, the spacing between the second recesses refers to the shortest distance between the edges of the orthographic projections of two adjacent second recesses onto the surface of the positive electrode sheet. Similarly, the spacing between the convexes refers to the shortest distance between the edges of the orthographic projections of two adjacent convexes onto the surface of the positive electrode sheet. The spacing between the second recesses and the spacing between the convexes can be tested using conventional methods in the art, such as using a 3D profilometer to select at least 10 sets of adjacent second recesses and 10 sets of adjacent convexes on the surface of the positive electrode sheet, measuring the spacing between each set of second recesses and the spacing between each set of convexes, and taking the average value.

[0103] In this invention, the silicon-carbon material further includes secondary spherical particles formed by a plurality of the primary spherical particles. The term "a plurality of" refers to a number of primary spherical particles forming the secondary spherical particles that is greater than or equal to two.

[0104] Conventional silicon-carbon materials are blocky with an average particle size of approximately 6μm-12μm. Therefore, they have poor conductivity and are difficult to compact, limiting the potential for increasing energy density. This invention uses silicon-carbon materials comprising primary spherical particles as the negative electrode active material. Smaller primary particles are beneficial for increasing battery energy density; however, their smaller particle size results in a larger specific surface area, increasing the risk of side reactions with the electrolyte and leading to poor stability. To further reduce side reactions between the silicon-carbon material and the electrolyte, secondary spherical particles formed from several primary spherical particles are added. These larger secondary spherical particles have a smaller specific surface area, thus reducing the risk of side reactions with the electrolyte and improving stability.

[0105] In this invention, the average particle size of the secondary spherical particles can be 3μm-20μm, for example, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm or 20μm.

[0106] In this invention, the average particle size of the secondary spherical particles can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled, soaked in DMC solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt adhering to the negative electrode sheet. The negative electrode sheet is then cut using an argon-ion milling machine with a CP laser, and observed using SEM (high voltage mode) at 5K magnification. At least 20 secondary spherical particles are randomly selected, and the particle size of each secondary spherical particle is measured, and the average value is taken. If the number of secondary spherical particles at 5K magnification is less than 20, a microscopic image is taken again until 20 secondary spherical particles are measured.

[0107] In this invention, in the negative electrode active coating, the number of primary spherical particles accounts for 0.1-0.9% of the total number of primary and secondary spherical particles, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 or 0.9%.

[0108] In one example, in the negative electrode active coating, the number of primary spherical particles accounts for 0.3-0.8% of the total number of primary and secondary spherical particles.

[0109] In this invention, the number of primary spherical particles and the number of secondary spherical particles in the negative electrode active coating can be tested using conventional methods in the art. For example, an argon ion milling machine with a CP laser can be used to cut along the thickness direction of the negative electrode sheet, and then a mirror image of the cross-section of the negative electrode sheet along the thickness direction can be obtained using SEM (high voltage mode). At least 20 mirror images with different cross-sections are selected, and the number of primary spherical particles and the number of secondary spherical particles in each mirror image are counted and the average value is taken.

[0110] In this invention, the negative electrode active coating includes a negative electrode active material, which includes the silicon-carbon material. The negative electrode active material may also include a graphite material. The graphite material includes, for example, artificial graphite and / or natural graphite. The graphite material includes secondary particles. The secondary particles are formed from a plurality of primary particles. "A plurality" means that the number of primary particles forming the secondary particles is greater than or equal to 2.

[0111] In this invention, the average particle size of the secondary particles can be 6μm-20μm, for example, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm or 20μm.

[0112] By combining specific graphite materials and controlling their structure and particle size, resulting in secondary particles formed from primary particles with an average particle size of 6μm-20μm, the energy density of the battery can be further improved. Compared to primary graphite materials, secondary particles of this specific particle size can increase the battery's energy density. Therefore, this graphite material has a high degree of compatibility with specific silicon-carbon materials, which is beneficial for improving battery energy density.

[0113] In this invention, the average particle size of the secondary particles can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled, soaked in DMC solvent for 12 hours, then rinsed with DMC to remove lithium salts adhering to the negative electrode sheet, and then the negative electrode active coating is rinsed off the negative electrode current collector with deionized water. After ultrasonication, the sample is centrifuged to remove the filtrate, dried, and dispersed in deionized water containing nonylphenol polyoxyethylene ether (wherein the mass content of nonylphenol polyoxyethylene ether is 0.02%-0.03%) to form a mixture. The mixture is ultrasonicated for 2 minutes, and then tested using a Malvern particle size analyzer. The median particle size Dv50 obtained is the average particle size of the secondary particles. Due to the specific composition and particle size of the graphite and silicon carbide materials in this invention, the silicon carbide material has a relatively small impact on the average particle size of the graphite secondary particles. Therefore, the data obtained by the above testing method is the average particle size of the secondary particles.

[0114] 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.

[0115] 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.

[0116] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0117] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0118] Example 1

[0119] The battery is prepared according to the following method:

[0120] (1) Preparation of positive electrode sheet

[0121] Lithium cobalt oxide (M) 1A positive electrode slurry is prepared by mixing Al, a positive electrode conductive agent (conductive carbon black), and a positive electrode binder (polyvinylidene fluoride) in a mass ratio of 97:1:2, adding N-methylpyrrolidone (NMP), and stirring until homogeneous. The positive electrode slurry is then coated onto the first and second surfaces of an aluminum foil (the coating length on the first surface is greater than the coating length on the second surface), baked, and rolled to obtain a positive electrode sheet with a thickness of 100 μm. A fixed-size positive electrode tab welding area (20 mm in width) is set on the coated area of ​​the positive electrode sheet, and nickel tabs are laser-welded into this area. After passing through a roller, an embossing process is performed on the double-sided coated area (avoiding the positive electrode tab welding area) from the first surface to the second surface to obtain a second concave portion (on the first surface) and a convex portion (on the second surface). The orthographic projections of the second concave portion and the convex portion onto the surface of the positive electrode sheet are circular.

[0122] The aluminum content (c) in the positive electrode active coating is 8166 ppm. The width of the second recess is 2 mm, the depth is 20 μm, and the spacing is 2 mm. The values ​​of w1, w2, w3, w4, and w5 are 7 mm.

[0123] (2) Preparation of negative electrode sheet

[0124] Artificial graphite (secondary particles), silicon-carbon material (the number of primary spherical particles accounts for 0.55 of the total number of primary and secondary spherical particles, the average particle size of the primary spherical particles is 4.2 μm, and the mass content of element Si in the silicon-carbon material is 40%), negative electrode conductive agent (carbon nanotubes), negative electrode dispersant (lithium carboxymethyl cellulose), and negative electrode binder (polyacrylic acid) are mixed in a mass ratio of 77:20:0.4:0.1:2.5, and deionized water is added to prepare a negative electrode slurry. The above negative electrode slurry is coated on both sides of a copper foil, baked, and rolled to obtain a negative electrode sheet with a thickness of 110 μm. A first concave part (groove) is created on the surface of the negative electrode sheet using a laser.

[0125] The grooves have a width of 80.3 μm, a depth of 23.2 μm, and a spacing of 1.2 mm.

[0126] (3) Preparation of electrolyte

[0127] In a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), ethylene carbonate, propylene carbonate, and diethyl carbonate were mixed in a weight ratio of 1:3:6 to obtain an organic solvent; fluoroethylene carbonate (FEC) and lithium hexafluorophosphate (LiPF6) were dissolved in the above organic solvent to obtain an electrolyte; wherein the mass content of FEC in the electrolyte was 15%, and the mass content of LiPF6 in the electrolyte was 12.5%.

[0128] (4) Preparation of the diaphragm

[0129] Polyacrylonitrile was ground and then mixed with styrene-butadiene rubber and lithium polyacrylate in a mass ratio of 95:4:1 (polyacrylonitrile: styrene-butadiene rubber: lithium polyacrylate). NMP was added to obtain an organic coating slurry. The organic coating slurry was coated onto one side of a polyethylene film (4.5 μm thick) and dried (forming an organic coating with a thickness h of 1.6 μm and an average polymer particle size of 0.8 μm). A second adhesive layer, PMMA, was coated onto the other side of the polyethylene film (forming a second adhesive layer with a thickness of 2.3 μm). A first adhesive layer (including PVDF + PMMA, wherein the mass ratio of PVDF to PMMA is 7:3, forming a first adhesive layer with a thickness of 0.5 μm) was coated onto the outer surface of the organic coating to obtain a separator. The organic coating contains 23% nitrogen and 62% carbon, and the ratio of carbon to nitrogen in the organic coating is 2.7.

[0130] (5) Battery fabrication

[0131] The positive electrode sheet prepared in step (1), the separator prepared in step (4), and the negative electrode sheet prepared in step (2) are wound together to obtain a core (wherein the organic coating faces the positive electrode sheet); the battery is obtained through encapsulation, baking, liquid injection, formation, secondary sealing, sorting, and OCV.

[0132] Where c×h is 0.0131.

[0133] Example 2

[0134] The battery is prepared according to the following method:

[0135] (1) Preparation of positive electrode sheet

[0136] Lithium cobalt oxide (M) 1A positive electrode slurry is prepared by mixing Al, a positive electrode conductive agent (conductive carbon black), and a positive electrode binder (polyvinylidene fluoride) in a mass ratio of 97:1:2, adding N-methylpyrrolidone (NMP), and stirring until homogeneous. The positive electrode slurry is then coated onto the first and second surfaces of an aluminum foil (the coating length on the first surface is greater than the coating length on the second surface), baked, and rolled to obtain a positive electrode sheet with a thickness of 100 μm. A fixed-size positive electrode tab welding area (15 mm in width) is set on the coated area of ​​the positive electrode sheet, and nickel tabs are laser-welded into this area. After passing through a roller, an embossing process is performed on the double-sided coated area (avoiding the positive electrode tab welding area) from the first surface to the second surface to obtain a second concave portion (on the first surface) and a convex portion (on the second surface). The orthographic projections of the second concave portion and the convex portion onto the surface of the positive electrode sheet are circular.

[0137] The aluminum content (c) in the positive electrode active coating is 6523 ppm. The width of the second recess is 1 mm, the depth is 10 μm, and the spacing is 1 mm. w1 is 5 mm, w2 is 20 mm, w3 is 10 mm, w4 is 5 mm, and w5 is 5 mm.

[0138] (2) Preparation of negative electrode sheet

[0139] Artificial graphite (secondary particles), silicon-carbon material (the number of primary spherical particles accounts for 0.31 of the total number of primary and secondary spherical particles, the average particle size of the primary spherical particles is 3μm, and the mass content of element Si in the silicon-carbon material is 40%), negative electrode conductive agent (carbon nanotubes), negative electrode dispersant (lithium carboxymethyl cellulose), and negative electrode binder (polyacrylic acid) are mixed in a mass ratio of 77:20:0.4:0.1:2.5, and deionized water is added to prepare a negative electrode slurry. The above negative electrode slurry is coated on both sides of a copper foil, baked, and rolled to obtain a negative electrode sheet with a thickness of 110μm. A first concave part (groove) is created on the surface of the negative electrode sheet using a laser.

[0140] The grooves are 60.1 μm wide, 15.2 μm deep, and 0.8 mm apart.

[0141] (3) Preparation of electrolyte

[0142] In a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), ethylene carbonate, ethylene carbonate, and diethyl carbonate were mixed in a weight ratio of 1:3:6 to obtain an organic solvent; fluoroethylene carbonate (FEC) and lithium hexafluorophosphate (LiPF6) were dissolved in the above organic solvent to obtain an electrolyte; wherein the mass content of FEC in the electrolyte was 10%, and the mass content of LiPF6 in the electrolyte was 12.5%.

[0143] (4) Preparation of the diaphragm

[0144] 1,3,5-triazine-2,4,6-triamine and polyacrylonitrile were ground, then mixed with styrene-butadiene rubber and lithium polyacrylate in a mass ratio of 25:70:4:1 (1,3,5-triazine-2,4,6-triamine:polyacrylonitrile:styrene-butadiene rubber:lithium polyacrylate), and NMP was added to obtain an organic coating slurry. This organic coating slurry was coated onto one side of a polyethylene film (4 μm thick) and dried (forming an organic coating with a thickness h of 1 μm and an average polymer particle size of 0.6 μm). A second adhesive layer, PMMA, is coated on the other side of the ethylene membrane (forming a second adhesive layer with a thickness of 2.5 μm). A first adhesive layer (comprising PVDF + PMMA, wherein the mass ratio of PVDF to PMMA is 7:3, forming a first adhesive layer with a thickness of 0.3 μm) is coated on the outer surface of the organic coating to obtain a separator. The organic coating contains 33% nitrogen by mass and 50% carbon by mass, with a carbon-to-nitrogen mass ratio of 1.52.

[0145] (5) Battery fabrication

[0146] The positive electrode sheet prepared in step (1), the separator prepared in step (4), and the negative electrode sheet prepared in step (2) are wound together to obtain a core (wherein the organic coating faces the positive electrode sheet); the battery is obtained through encapsulation, baking, liquid injection, formation, secondary sealing, sorting, and OCV.

[0147] Where c×h is 0.0065.

[0148] Example 3

[0149] The battery is prepared according to the following method:

[0150] (1) Preparation of positive electrode sheet

[0151] Lithium cobalt oxide (M) 1A positive electrode slurry is prepared by mixing Al, a positive electrode conductive agent (conductive carbon black), and a positive electrode binder (polyvinylidene fluoride) in a mass ratio of 97:1:2, adding N-methylpyrrolidone (NMP), and stirring until homogeneous. The positive electrode slurry is then coated onto the first and second surfaces of an aluminum foil (the coating length on the first surface is greater than the coating length on the second surface), baked, and rolled to obtain a positive electrode sheet with a thickness of 100 μm. A fixed-size positive electrode tab welding area (25 mm in width) is set on the coated area of ​​the positive electrode sheet, and nickel tabs are laser-welded into this area. After passing through a roller, an embossing process is performed on the double-sided coated area (avoiding the positive electrode tab welding area) from the first surface to the second surface to obtain a second concave portion (on the first surface) and a convex portion (on the second surface). The orthographic projections of the second concave portion and the convex portion onto the surface of the positive electrode sheet are circular.

[0152] The aluminum content (c) in the positive electrode active coating is 11870 ppm. The width of the second recess is 3 mm, the depth is 30 μm, and the spacing is 3 mm. The dimensions of w1 are 9 mm, w2 is 34 mm, w3 is 20 mm, w4 is 10 mm, and w5 is 10 mm.

[0153] (2) Preparation of negative electrode sheet

[0154] Artificial graphite (secondary particles), silicon-carbon material (the number of primary spherical particles accounts for 0.75 of the total number of primary and secondary spherical particles, the average particle size of the primary spherical particles is 4.9 μm, and the mass content of element Si in the silicon-carbon material is 40%), negative electrode conductive agent (carbon nanotubes), negative electrode dispersant (lithium carboxymethyl cellulose), and negative electrode binder (polyacrylic acid) are mixed in a mass ratio of 77:20:0.4:0.1:2.5, and deionized water is added to prepare a negative electrode slurry. The above negative electrode slurry is coated on both sides of a copper foil, baked, and rolled to obtain a negative electrode sheet with a thickness of 110 μm. A first concave part (groove) is created on the surface of the negative electrode sheet using a laser.

[0155] The grooves are 99.7 μm wide, 29.9 μm deep, and 1.5 mm apart.

[0156] (3) Preparation of electrolyte

[0157] In a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), ethylene carbonate, ethylene carbonate, and diethyl carbonate were mixed in a weight ratio of 1:3:6 to obtain an organic solvent; fluoroethylene carbonate (FEC) and lithium hexafluorophosphate (LiPF6) were dissolved in the above organic solvent to obtain an electrolyte; wherein the mass content of FEC in the electrolyte was 20%, and the mass content of LiPF6 in the electrolyte was 12.5%.

[0158] (4) Preparation of the diaphragm

[0159] 1,3,5-triazine-2,4,6-triamine and polyacrylonitrile were ground, and then mixed with styrene-butadiene rubber and lithium polyacrylate in a mass ratio of 1:95:2:2 (1,3,5-triazine-2,4,6-triamine: polyacrylonitrile: styrene-butadiene rubber: lithium polyacrylate). NMP was added to obtain an organic coating slurry. The above organic coating slurry was coated onto one side of a polyethylene film (5 μm thick) and dried (forming an organic coating with a thickness h of 2 μm and an average polymer particle size of 0.9 μm). A second adhesive layer, PMMA, is coated on the other side of the polyethylene film (forming a second adhesive layer with a thickness of 2 μm). A first adhesive layer (comprising PVDF + PMMA, wherein the mass ratio of PVDF to PMMA is 7:3, forming a first adhesive layer with a thickness of 0.8 μm) is coated on the outer surface of the organic coating to obtain a separator. The organic coating contains 17% nitrogen by mass and 68% carbon by mass, with a carbon-to-nitrogen mass ratio of 4.

[0160] (5) Battery fabrication

[0161] The positive electrode sheet prepared in step (1), the separator prepared in step (4), and the negative electrode sheet prepared in step (2) are wound together to obtain a core (wherein the organic coating faces the positive electrode sheet); the battery is obtained through encapsulation, baking, liquid injection, formation, secondary sealing, sorting, and OCV.

[0162] Where c×h is 0.0237.

[0163] Example 4 group

[0164] This set of examples is used to verify the impact of changes in the "mass content of silicon element in silicon-carbon materials".

[0165] This set of embodiments is based on Embodiment 1, except that the mass content of silicon in the silicon-carbon material is changed, as follows:

[0166] Example 4a: The silicon content in the silicon-carbon material is 51% by mass.

[0167] Example 4b: The silicon content in the silicon-carbon material is 62% by mass.

[0168] Example 4c: The silicon content in the silicon-carbon material is 30% by mass.

[0169] In Example 4d, the silicon content in the silicon-carbon material was 71% by mass.

[0170] Example 5 group

[0171] This set of examples is used to verify the impact of changes in the "average particle size of primary spherical particles".

[0172] This set of embodiments is based on Embodiment 1, except that the average particle size of the spherical particles is changed, as follows:

[0173] In Example 5a, the average particle size of the primary spherical particles was 1.2 μm;

[0174] In Example 5b, the average particle size of the primary spherical particles was 5.9 μm.

[0175] Example 6 group

[0176] This set of examples is used to verify the effect of changing the "mass content c of aluminum element in the positive electrode active coating".

[0177] This set of embodiments is based on Embodiment 1, except that c is changed, as follows:

[0178] Example 6a, c is 6021 ppm; where c×h is 0.0096;

[0179] Example 6b, c is 14955 ppm; where c×h is 0.0239.

[0180] Example 7 group

[0181] This set of examples is used to verify the effect of changing the thickness h of the organic coating.

[0182] This set of embodiments is based on Embodiment 1, except that h is changed, as follows:

[0183] Example 7a, h is 0.5 μm; where c × h is 0.0041;

[0184] Example 7b, h is 4 μm; where c × h is 0.0327.

[0185] Example 8 group

[0186] This set of examples is used to verify the effect of changing "the product of the mass content of aluminum in the positive electrode active coating c and the thickness of the organic coating h c×h".

[0187] This set of embodiments follows the same procedure as Embodiment 1, except that c×h is adjusted by changing c and h, as detailed below:

[0188] Example 8a, c is 6122 ppm, h is 0.5 μm; where c × h is 0.0031;

[0189] Example 8b, c1 is 14876 ppm, h is 4 μm; where c×h is 0.0595.

[0190] Example 9 group

[0191] This set of examples is used to verify the effect of changing the "ratio of the mass content of carbon to the mass content of nitrogen in the organic coating".

[0192] This set of embodiments follows the same principles as Embodiment 1, except that the ratio of carbon to nitrogen content in the organic coating is changed by altering the formulation of the organic coating slurry, as detailed below:

[0193] In Example 9a, the mass ratio of 1,3,5-triazine-2,4,6-triamine, polyacrylonitrile, styrene-butadiene rubber, and lithium polyacrylate was 70:25:3:2. The organic coating contained 53% nitrogen and 43% carbon, and the ratio of carbon to nitrogen in the organic coating was 0.81.

[0194] In Example 9b, the mass ratio of 1,3,5-triazine-2,4,6-triamine, polyacrylonitrile, styrene-butadiene rubber, and lithium polyacrylate was 5:90:3:2. The organic coating contained 12% nitrogen and 76% carbon, with a carbon-to-nitrogen mass ratio of 6.33.

[0195] Example 10 group

[0196] This set of examples is used to verify the impact of changes in the thickness of the first adhesive layer.

[0197] This set of embodiments is based on Embodiment 1, except that the thickness of the first adhesive layer is changed, as follows:

[0198] Example 10a: The thickness of the first adhesive layer is 1 μm;

[0199] Example 10b: The thickness of the first adhesive layer is 0.2 μm;

[0200] In Example 10c, the thickness of the first adhesive layer was 3 μm.

[0201] Example 11 group

[0202] This set of examples is used to verify the impact of changes in the thickness of the second adhesive layer.

[0203] This set of embodiments is based on Embodiment 1, except that the thickness of the second adhesive layer is changed, as follows:

[0204] In Example 11a, the thickness of the second adhesive layer is 1.5 μm;

[0205] In Example 11b, the thickness of the second adhesive layer is 0.2 μm;

[0206] In Example 11c, the thickness of the second adhesive layer is 3 μm.

[0207] Example 12

[0208] This embodiment is used to verify the impact of changing the "type of the first recess".

[0209] The procedure was carried out in accordance with Example 1, except that a laser was used to create recesses on the surface of the negative electrode. The recesses had a width of 93.4 μm, a depth of 25.5 μm, and a spacing of 0.5 mm.

[0210] Example 13 group

[0211] This set of examples is used to verify the impact of changing the "value of the number of primary spherical particles as a percentage of the total number of primary and secondary spherical particles".

[0212] This set of embodiments is based on Embodiment 1, except that the proportion of spherical particles is changed, as follows:

[0213] In Example 13a, the number of primary spherical particles accounts for 0.1% of the total number of primary and secondary spherical particles;

[0214] In Example 13b, the number of primary spherical particles accounted for 0.89% of the total number of primary and secondary spherical particles;

[0215] In Example 13c, all silicon-carbon materials are primary spherical particles, meaning that the number of primary spherical particles accounts for 1% of the total number of primary and secondary spherical particles.

[0216] Example 14

[0217] The procedure was carried out in accordance with Example 1, except that the artificial graphite was primary particles with an average particle size of 6.1 μm.

[0218] Example 15

[0219] This embodiment is used to verify the effect of whether the second concave portion is located on the surface of the positive active coating of the first surface and whether the convex portion is located on the surface of the positive active coating of the second surface.

[0220] The process is carried out with reference to Example 1, except that the embossing process is performed from the second surface to the first surface, that is, the surface of the positive electrode active coating on the second surface has a second concave portion, and the surface of the positive electrode active coating on the first surface has a convex portion.

[0221] Example 16

[0222] This embodiment is used to verify the effect of "not setting a second concave and convex part on the surface of the positive electrode active coating".

[0223] The procedure was carried out in accordance with Example 1, except that no embossing was performed.

[0224] Example 17

[0225] This embodiment is used to verify the impact of whether the single-sided coating area has a second recess.

[0226] The process is carried out in accordance with Example 1, except that embossing is performed on the single-sided coating area and the double-sided coating area (and avoiding the positive electrode tab welding area).

[0227] Example 18 group

[0228] This set of examples is used to verify the impact of changes to "w1, w2, w3, w4 and w5".

[0229] This set of embodiments is based on Embodiment 1, except that w1, w2, w3, w4, and w5 are changed, as follows:

[0230] Example 18a, w1 is 0.5mm, w2 is 2mm, w3 is 2mm, w4 is 0.5mm, and w5 is 0.5mm;

[0231] Example 18b: w1 is 10mm, w2 is 40mm, w3 is 25mm, w4 is 20mm, and w5 is 20mm.

[0232] All the above embodiments satisfy the following: the XRD diffraction pattern of the organic coating has a characteristic peak in the 2θ range of 19.5°-23.5°. The average particle size of the first lithium cobalt oxide particle is 0.3μm-7μm, and the average particle size of the second lithium cobalt oxide particle is 7.5μm-40μm.

[0233] Except for Example 16, all the above embodiments satisfy the following conditions: the height of the protrusions is 3μm-40μm, the width of the protrusions is 0.2mm-8mm, and the spacing between the protrusions is 0.2mm-8mm. Except for Example 13c, all satisfy the following condition: the average particle size of the secondary spherical particles is 3μm-20μm. Except for Example 14, all satisfy the following condition: the average particle size of the secondary particles is 6μm-20μm.

[0234] Test Case I

[0235] Adhesion test

[0236] The batteries prepared in the examples were tested for the adhesion between the negative electrode and the separator, and the adhesion between the positive electrode and the separator. The results showed that all examples met the following requirements: the adhesion between the negative electrode and the separator was 10 N / m-40 N / m; the adhesion between the positive electrode and the separator was ≤10 N / m.

[0237] Comparative Example 1

[0238] The procedure was carried out in accordance with Example 1, except that the average particle size of the spherical particles was changed, as follows:

[0239] Comparative Example 1a: The average particle size of the primary spherical particles was 0.7 μm;

[0240] Comparative Example 1b shows that the average particle size of the primary spherical particles is 6.5 μm.

[0241] Comparative Example 2

[0242] The procedure was carried out in accordance with Example 1, except that the mass content c of aluminum in the positive electrode active coating was changed, as follows:

[0243] Comparative Examples 2a and 2c were 5643 ppm;

[0244] Comparative Examples 2b and 2c were 17965 ppm.

[0245] Comparative Example 3

[0246] The procedure was carried out in accordance with Example 1, except that the organic coating was replaced with a boehmite ceramic coating of the same thickness.

[0247] Comparative Example 4

[0248] The procedure was carried out in accordance with Example 1, except that the organic coating was placed directly opposite the negative electrode.

[0249] Comparative Example 5 Groups

[0250] The procedure was carried out in accordance with Example 1, except that the thickness h of the organic coating was changed, as follows:

[0251] Comparative Example 5a, h is 0.3 μm;

[0252] Comparative Example 5b, h is 4.5 μm.

[0253] Comparative Example 6

[0254] The procedure was carried out in accordance with Example 1, except that FEC was not added to the electrolyte.

[0255] Test Case II

[0256] (1) Volumetric energy density test

[0257] The volumetric energy density of the batteries prepared in the examples and comparative examples was tested, and the specific testing methods are as follows:

[0258] The battery was charged to 4.48V at a current of 0.2C, then charged at a constant voltage until the current dropped to 0.02C. It was then discharged at a current of 0.2C to 3.0V, and the energy discharged was denoted as E. The thickness, width, and length of the battery were measured, and their product was calculated to obtain the battery volume, denoted as V. The formula for calculating the volumetric energy density is VED = E / V, and the results are recorded in Table 1.

[0259] (2) Fast charging performance test

[0260] The batteries prepared in the examples and comparative examples were subjected to fast charging performance tests. The specific test methods are as follows:

[0261] Place the device in a 25℃ constant temperature chamber for 2 hours, discharge it to 3.0V using a 0.2C constant current, and let it stand for 5 minutes; then charge it to 4.2V using a 3C constant current, switch to a 2.5C constant current to 4.25V, and then charge it to 4.48V using a 2C constant current (1.2C cutoff). Record the time it takes to charge to 4.2V using a 3C constant current (the longer the time, the better the fast charging performance). Set the Blue Electric to collect data every 1 second and record the results in Table 1.

[0262] (3) Loop test

[0263] The batteries prepared in the examples and comparative examples were subjected to cycle tests, and the specific test methods are as follows:

[0264] The capacitor was left to stand in a constant temperature chamber at 45℃ for 2 hours, then charged to 4.2V with a constant current of 3C, then charged to 4.25V with a constant current and constant voltage of 2.5C, then charged to 4.48V with a constant current and constant voltage of 2C, and finally stopped at 0.05C and left to stand for 10 minutes. Then it was discharged to 3.0V with a constant current of 0.7C. This cycle was repeated 500 times. The discharge capacity was C1, and the discharge capacity at the first full charge was C0. C1 / C0 is the capacity retention rate after 500 cycles. The results are recorded in Table 1.

[0265] (4) High temperature safety test

[0266] The batteries prepared in the examples and comparative examples were subjected to high-temperature safety testing. The specific testing methods are as follows:

[0267] At room temperature (25℃), the battery was charged to 4.48V under constant current and voltage at 0.5C, and then stopped at 0.05C to ensure full charge. The fully charged battery was then placed in a high-temperature chamber, and the temperature was increased to 130℃ at a rate of 5℃ / min. The temperature was maintained for 1 hour. 100 batteries were tested, and the pass rate was recorded in Table 1. The pass criterion was that the battery did not catch fire or explode.

[0268] Table 1

[0269]

[0270]

[0271] As can be seen from Table 1, compared with the comparative example, the battery of the present invention can balance high energy density, fast charging capability, excellent cycle stability and high temperature safety performance under high charging cutoff voltage.

[0272] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A lithium-ion secondary battery, characterized in that, It includes a negative electrode, a positive electrode, a separator, and an electrolyte; the charging cut-off voltage of the lithium-ion secondary battery is ≥4.48V; The negative electrode sheet comprises silicon-carbon material, wherein the silicon content of silicon element in the silicon-carbon material is 30%-80% by mass. The positive electrode sheet includes a positive electrode active coating, which includes lithium cobalt oxide, and the lithium cobalt oxide contains aluminum. The mass content c of the aluminum in the positive electrode active coating is 6000ppm-15000ppm. The separator includes an organic coating facing the positive electrode; the organic coating includes polymer particles containing nitrogen, and the thickness h of the organic coating is 0.5 μm-4 μm. The electrolyte includes fluoroethylene carbonate, and the mass content of fluoroethylene carbonate in the electrolyte is 5%-30%. The organic coating contains 10.5%-55% nitrogen by mass.

2. The lithium-ion secondary battery according to claim 1, wherein, The silicon-carbon material comprises primary spherical particles, the average particle size of which is 1μm-6μm; And / or, the mass content c of aluminum in the positive electrode active coating is 6500ppm-12000ppm; And / or, the negative electrode sheet may further include graphite material.

3. The lithium-ion secondary battery according to claim 2, wherein, The average particle size of the primary spherical particles is 3μm-5μm.

4. The lithium-ion secondary battery according to claim 1 or 2, wherein, The mass content c of aluminum in the positive electrode active coating and the thickness h of the organic coating satisfy the following condition: 0.003 ≤ c × h ≤ 0.06, where the unit of h is μm.

5. The lithium-ion secondary battery according to claim 4, wherein, 0.0065≤c×h≤0.

024.

6. The lithium-ion secondary battery according to claim 1 or 2, wherein, The polymer particles include at least one of cyano, isocyano, isocyanate, and triazine groups; And / or, the polymer particles further contain carbon, and the ratio of the mass content of carbon to the mass content of nitrogen in the organic coating is 0.7-7.6; And / or, the XRD diffraction pattern of the organic coating has a characteristic peak in the 2θ range of 19.5°–23.5°.

7. The lithium-ion secondary battery according to claim 6, wherein, The polymer particles include at least one of polyacrylonitrile, nitrile rubber, 1,3,5-triazine-2,4,6-triamine, melamine cyanurate, and melamine trithiocyanate. And / or, the ratio of the mass content of carbon to the mass content of nitrogen in the organic coating is 1.5-4.

8. The lithium-ion secondary battery according to claim 6, wherein, The organic coating contains 40%-80% carbon by mass. And / or, the nitrogen content in the organic coating is 15%-35% by mass.

9. The lithium-ion secondary battery according to claim 8, wherein, The organic coating contains 50%-70% carbon by mass.

10. The lithium-ion secondary battery according to claim 1 or 2, wherein, The diaphragm further includes at least one of a substrate layer, a first adhesive layer, and a second adhesive layer.

11. The lithium-ion secondary battery according to claim 10, wherein, The thickness of the substrate layer is 3μm-6μm; And / or, the thickness of the first adhesive layer is 0.2μm-3μm; And / or, the thickness of the second adhesive layer is 0.2μm-3μm.

12. The lithium-ion secondary battery according to claim 10, wherein, The diaphragm includes the substrate layer, the organic coating on one side of the substrate layer, the first adhesive layer on the outer surface of the organic coating, and the second adhesive layer on the other side of the substrate layer.

13. The lithium-ion secondary battery according to claim 12, wherein, The thickness of the first adhesive layer is 0.2 μm-1 μm; And / or, the thickness of the second adhesive layer is 1.5μm-3μm.

14. The lithium-ion secondary battery according to claim 13, wherein, The thickness of the first adhesive layer is 0.3 μm - 0.8 μm; and / or, the thickness of the second adhesive layer is 2 μm - 2.5 μm.

15. The lithium-ion secondary battery according to claim 1 or 2, wherein, The adhesion force between the negative electrode sheet and the separator is greater than the adhesion force between the positive electrode sheet and the separator.

16. The lithium-ion secondary battery according to claim 15, wherein, The adhesion force between the negative electrode sheet and the separator is 10 N / m - 40 N / m; and / or, the adhesion force between the positive electrode sheet and the separator ≤ 10 N / m.

17. The lithium-ion secondary battery according to claim 1 or 2, wherein, The negative electrode sheet includes a negative current collector and a negative active coating located on at least one surface of the negative current collector; the outer surface of the negative active coating has a first recess.

18. The lithium-ion secondary battery according to claim 17, wherein, The depth of the first recess is 5 μm - 40 μm; and / or, the width of the first recess is 40 μm - 200 μm; and / or, the spacing of the first recesses is 0.5 mm - 5 mm.

19. The lithium-ion secondary battery according to claim 1 or 2, wherein, The positive electrode sheet includes a positive current collector and the positive active coating located on at least one surface of the positive current collector; the length of the positive active coating located on the first surface of the positive current collector is greater than the length of the positive active coating located on the second surface of the positive current collector; The region where the projection of the positive active coating located on the first surface overlaps with the projection of the positive active coating located on the second surface in the thickness direction of the positive electrode sheet is the double-sided coating region, and the region where the projections do not overlap is the single-sided coating region; The surface of the positive active coating located on the first surface has a second recess, and the surface of the positive active coating located on the second surface has a protrusion.

20. The lithium-ion secondary battery according to claim 19, wherein, The positive electrode sheet includes a pasting region, the pasting region includes the double-sided coating region and the single-sided coating region, and the second recess and the protrusion are located in the double-sided coating region; and / or, the positive electrode sheet includes a positive electrode tab welding region, a pasting region and a bare foil region, and the second recess and the protrusion are located in the pasting region; and / or, the depth of the second recess is 3 μm - 40 μm, the width of the second recess is 0.2 mm - 8 mm, and the spacing of the second recesses is 0.5 mm - 8 mm; and / or, the height of the protrusion is 3 μm - 40 μm, the width of the protrusion is 0.2 mm - 8 mm, and the spacing of the protrusions is 0.5 mm - 8 mm.

21. The lithium-ion secondary battery according to claim 19, wherein, The positive electrode sheet includes a positive electrode tab welding region, a pasting region and a bare foil region, and the pasting region includes the double-sided coating region and the single-sided coating region; The distance from the second recess to the edge of the positive electrode tab welding region is w1, 0 mm < w1 ≤ 10 mm; and / or, the distance from the second recess to the edge of the first side of the pasting region is w2, 2 mm ≤ w2 ≤ 40 mm, and the first side is the side where the positive electrode tab welding region is provided; and / or, the distance from the second recess to the edge of the second side of the pasting region is w3, 2 mm ≤ w3 ≤ 25 mm, and the second side is the side opposite to the side where the positive electrode tab welding region is provided; and / or, the distance from the second recess to the edge of the third side of the pasting region is w4, 0 mm < w4 ≤ And / or, the distance from the second recess to the boundary line between the double-sided coating area and the single-sided coating area is w5.0mm. <w5≤20mm。

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