Lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion secondary battery. The lithium ion secondary battery comprises a positive plate, a diaphragm and a negative plate which are stacked, the negative plate comprises a negative active material, the negative active material comprises a silicon-carbon material containing first silicon-carbon particles and second silicon-carbon particles, the sphericity degree of the first silicon-carbon particles is 0.8-0.99, the average particle size is 1-6 microns, and the sphericity degree of the first silicon-carbon particles is larger than that of the second silicon-carbon particles; the average particle size of the second silicon carbon particles is 5-15 [mu] m; the positive plate comprises a positive active material, the positive active material comprises lithium cobalt oxide containing yttrium element and lanthanum element, based on the total mass of the lithium cobalt oxide, the content of the yttrium element is C1, 150 < = C1 < = 700, the content of the lanthanum element is C2, 100 < = C2 < = 500, and the unit is ppm; the diaphragm comprises a base membrane and a nitrogen-containing coating located on the surface of at least one side of the base membrane, based on the total weight of the coating, the nitrogen content is C3, and C3 is larger than or equal to 10% and smaller than or equal to 50%; and the coating directly faces the positive plate. The lithium ion secondary battery provided by the invention has high energy density and good high-temperature safety and high-temperature cycle performance.

Description

Technical Field

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

[0002] With the development of high-energy-density lithium-ion batteries, silicon-carbon anodes with higher specific capacity are more frequently used. Commonly used silicon-carbon materials with high specific capacity such as silicon-carbon. However, with the increase in the content of silicon-carbon materials in the silicon-carbon anode, problems such as increased battery swelling and decreased tap density will occur. In the prior art, the size of silicon-carbon materials is often regulated, for example, silicon-carbon materials with larger average particle size and smaller average particle size are mixed and used together for the negative electrode sheet to improve the above problems. However, due to the large specific surface area of the silicon-carbon materials with smaller particle size, the side reaction with the electrolyte is aggravated. The side reaction products increase the internal resistance of the battery, and the temperature rise of the battery is aggravated during charge and discharge, affecting the structural stability of the positive electrode material, resulting in poor cycle performance and safety performance of the lithium-ion battery at high temperature, and affecting the use and popularization of high-energy-density lithium-ion batteries. Summary of the Invention

[0003] The purpose of the present invention is to solve the problem of how to improve the high-temperature safety performance and high-temperature cycle stability of the battery while using a silicon-carbon anode to increase the energy density of a lithium-ion secondary battery (hereinafter referred to as the battery).

[0004] In the related art, in order to increase the energy density of the battery, silicon-carbon particles with high specific capacity are often used for the negative electrode sheet. However, the volume change of the silicon-carbon particles during the charge and discharge cycle of the battery causes the battery volume to expand and the tap density to decrease. To alleviate the negative effects brought by the use of the silicon-carbon anode, the average particle size of the silicon-carbon particles is often regulated, and two kinds of silicon-carbon particles with different average particle sizes are mixed. However, inevitably, the side reaction is aggravated due to the increase in the specific surface area of the silicon-carbon particles with smaller average particle size, resulting in an increase in the internal resistance of the battery and having an adverse effect on the structural stability of the positive electrode material. Especially in the electrochemical system with a larger rate and a higher voltage, the cycle performance and safety performance of the battery at high temperature are further reduced.

[0005] To solve the above problems, the inventors of the present invention found that silicon-carbon particles with a large sphericity have a high specific surface area, which can increase the surface active sites and improve the battery performance. Silicon-carbon particles with a smaller sphericity have better adhesion with the carbon-based material and are not prone to peeling and demolding. Controlling the morphology of the silicon-carbon particles is beneficial to improving the stability and electrochemical performance of the negative electrode system. The inventors of the present invention combined the particle size and morphology of the silicon-carbon particles in the negative electrode system, and used first silicon-carbon particles with a small average particle size and a large sphericity (for example, larger than the second silicon-carbon particles) and second silicon-carbon particles with a relatively large average particle size and a smaller sphericity relative to the first silicon-carbon particles. This strategy can, on the one hand, enable the negative electrode active material to have both high energy density and cycle stability and maintain high adhesion, and can also, to a certain extent, alleviate the problem that the increase in the internal resistance of the battery is caused by the excessive side reaction between the silicon-carbon particles with a small particle size and the electrolyte, reduce the temperature rise during the charge and discharge process of the battery, and improve the stability and safety of the battery at high temperatures. On the other hand, controlling the sphericity of the second silicon-carbon particles to be smaller than that of the first silicon-carbon particles can improve the buffering effect of the negative electrode system on volume expansion, not only further alleviate the expansion problem, improve the overall high tap density, reduce the local enrichment of the electrolyte, and form a uniform and stable SEI film.

[0006] However, the inventors of the present invention found during the experiment that although this method has a certain alleviating effect on the increase in the internal resistance of the battery, it has little effect on the problem that the temperature rise during the charge and discharge process of the battery in a high-rate charging and high-voltage electrochemical system leads to a decrease in the structural stability of the positive electrode. There is still room for optimization in the safety performance and cycle performance of the battery at high temperatures. To further improve the above problems, the inventors of the present invention found through a large number of experimental studies that in a silicon negative electrode system formed by mixing first silicon-carbon particles with a large sphericity and a small average particle size and second silicon-carbon particles with a sphericity smaller than that of the first silicon-carbon particles and a relatively larger average particle size, the optimization effect of the structure and material of the positive electrode system and the separator on the high-temperature stability and high-temperature safety of the battery is significant. On the one hand, the content of yttrium and lanthanum elements in the positive electrode active material is controlled within a suitable range to improve the structural stability of the positive electrode active material at high temperatures. On the other hand, a coating is provided on the separator and the content of nitrogen elements in the coating is controlled to inhibit the structural phase change of the positive electrode active material and further improve the stability of the positive electrode sheet.

[0007] Based on this, the inventors of the present invention propose the following technical solutions:

[0008] A lithium-ion secondary battery, the lithium-ion secondary battery comprising an electrode assembly, the electrode assembly comprising a stacked positive electrode sheet, a separator, and a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active layer located on at least one surface of the negative electrode current collector, the negative electrode active layer comprising a negative electrode active material, the negative electrode active material comprising a silicon-carbon material, the silicon-carbon material comprising first silicon-carbon particles and second silicon-carbon particles, the sphericity of the first silicon-carbon particles being 0.8 - 0.99, the average particle size of the first silicon-carbon particles being 1 μm - 6 μm, and the sphericity of the first silicon-carbon particles > the sphericity of the second silicon-carbon particles; the average particle size of the second silicon-carbon particles being 5 μm - 15 μm;

[0009] The positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer located on at least one surface of the positive electrode current collector, the positive electrode active layer comprising a positive electrode active material, the positive electrode active material comprising lithium cobaltate, the lithium cobaltate comprising yttrium element and lanthanum element, based on the total mass of the lithium cobaltate, the content of the yttrium element being C1, 150 ≤ C1 ≤ 700, with the unit of ppm, and the content of the lanthanum element being C2, 100 ≤ C2 ≤ 500, with the unit of ppm;

[0010] The separator comprises a base film and a coating located on at least one surface of the base film, the coating comprising nitrogen element, based on the total weight of the coating, the content of the nitrogen element being C3, 10% ≤ C3 ≤ 50%; the coating faces the positive electrode sheet.

[0011] By the above technical solutions, the present invention has at least the following advantages compared with the prior art:

[0012] (1) In the present invention, the negative electrode active material uses first silicon-carbon particles and second silicon-carbon particles with different particle sizes, and the sphericity of the first silicon-carbon particles is regulated to be greater than that of the second silicon-carbon particles, so that the negative electrode active material has both high energy density and good structural stability, improves the tap density, and alleviates the cyclic expansion of the silicon-carbon negative electrode.

[0013] (2) In the present invention, the contents of the yttrium element and the lanthanum element in the positive electrode active material are regulated within a suitable range, a coating is provided on the separator, and the content of the nitrogen element in the coating is regulated to inhibit the structural phase change of the positive electrode active material, thereby improving the structural stability of the positive electrode active material at high temperatures.

[0014] (3) In the present invention, the high-temperature safety performance and high-temperature cycle performance of the battery are improved.

[0015] The endpoints and any values within the ranges disclosed herein are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the values between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. Description of the Drawings

[0016] Figure 1 Shown is a scanning electron microscope (SEM) image of the cross-section of the negative electrode sheet in an embodiment of the present invention.

[0017] Figure 2 Shown is a schematic structural diagram of the separator along the thickness direction in an embodiment of the present invention.

[0018] Figure 3 Shown is an SEM image of the separator in an embodiment of the present invention.

[0019] Figure 4 Shown is a schematic diagram of the difference in width between the separator and the negative electrode sheet in an embodiment of the present application.

[0020] Figure 5 Shown is a schematic structural diagram of the negative electrode sheet along the thickness direction in an embodiment of the present invention.

[0021] Figure 6 Shown is a schematic structural diagram of the positive electrode sheet along the thickness direction in an embodiment of the present invention.

[0022] Figure 7 Shown is a top view of the second positive electrode surface of the positive electrode sheet in an embodiment of the present invention.

[0023] Figure 8 Shown is a top view of the first pole ear groove in an embodiment of the present invention.

[0024] Figure 9 Shown is a sectional view of section 1 obtained by cutting the first pole ear groove along the S1 direction in an embodiment of the present invention.

[0025] Figure 10 Shown is a sectional view of section 2 obtained by cutting the first pole ear groove along the S2 direction in an embodiment of the present invention.

[0026] Figure 11 Shown is a top view schematic of the positive electrode pole ear groove and the third buried glue groove in an embodiment of the present invention.

[0027] Figure 12 Shown is a schematic structural diagram of the wound core in an embodiment of the present invention.

[0028] Reference numerals: 1-1 is the base film, 1-2 is the coating layer, and the particles in the coating layer represent nitrogen-containing particles, 1-3 is the adhesive layer, 2-1 is the groove, 2-2 is the negative electrode active layer, 2-3 is the negative electrode current collector, 3-1 is the positive electrode current collector, 3-2 is the bottom coating layer, 3-3 is the positive electrode active layer on the surface of the first positive electrode, 3-4 is the positive electrode active layer on the surface of the second positive electrode, 3-5 is the recess, 3-6 is the protrusion, 5-1 is the negative electrode tab, 5-2 is the positive electrode tab, 6-1 is the first tab groove, 6-2 is the second tab groove, 6-3 is the first adhesive embedding groove, 6-4 is the second adhesive embedding groove, 8-1 is the first tab adhesive tape, 8-2 is the second tab adhesive tape, 8-3 is the third adhesive embedding groove, 8-4 is the positive electrode tab groove, 9-1 is the positive electrode plate, 9-2 is the negative electrode plate, 9-3 is the separator. Detailed implementation manners

[0029] The following provides a detailed description of the specific implementation manners of the present invention. It should be understood that the specific implementation manners described herein are only for the purpose of illustrating and explaining the present invention, and are not used to limit the present invention.

[0030] A lithium-ion secondary battery, the lithium-ion secondary battery includes an electrode assembly, the electrode assembly includes a positive electrode plate, a separator, and a negative electrode plate stacked together, the negative electrode plate includes a negative electrode current collector and a negative electrode active layer located on at least one surface of the negative electrode current collector, the negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes a silicon-carbon material. The silicon-carbon material includes first silicon-carbon particles and second silicon-carbon particles. The sphericity of the first silicon-carbon particles is 0.8-0.99, for example, 0.8, 0.82, 0.84, 0.86, 0.88, 0.9, 0.92, 0.94, 0.96, 0.98 or 0.99; the average particle size of the first silicon-carbon particles is 1μm-6μm, for example, 1μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 5μm or 6μm. The sphericity of the first silicon-carbon particles > the sphericity of the second silicon-carbon particles. The average particle size of the second silicon-carbon particles is 5μm-15μm, for example, 5μm, 5.2μm, 5.4μm, 5.6μm, 5.8μm, 6μm, 6.2μm, 6.4μm, 6.6μm, 6.8μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm, 10μm, 11μm, 12μm, 13μm or 15μm.

[0031] In one embodiment, the sphericity of the first silicon-carbon particles is 0.9-0.99.

[0032] In one embodiment, the average particle size of the first silicon-carbon particles is 2μm-5μm.

[0033] In one embodiment, the average particle size of the second silicon carbide particles is 7 μm - 10 μm.

[0034] In the present invention, the sphericity of the second silicon carbide particles is 0.5 - 0.8, such as 0.5, 0.52, 0.54, 0.56, 0.58, 0.6, 0.62, 0.64, 0.66, 0.68, 0.7, 0.75 or 0.8.

[0035] In one embodiment, the sphericity of the second silicon carbide particles is 0.5 - 0.7.

[0036] In the present invention, the average particle sizes of the first silicon carbide particles and the second silicon carbide particles can be obtained by conventional testing methods in the art, for example, in the following manner: After polishing the cross-section of the negative electrode sheet with an argon ion milling instrument, observe it in a scanning electron microscope (SEM) using the backscattered imaging mode, and respectively find the first silicon carbide particles with a large sphericity and the second silicon carbide particles with a small sphericity. Taking any one of the silicon carbide particles as an example, take the two longest points on the cross-section contour of the silicon carbide particle to make a straight line segment, and record it as the particle size D of the silicon carbide particle. According to the appearance, take 10 particles within the sphericity ranges of the first silicon carbide particles and the second silicon carbide particles respectively, and calculate the average particle size respectively, which is recorded as the average particle size of the silicon carbide particles with this sphericity; it can also be obtained by focused ion beam (FIB) testing.

[0037] In the present invention, the sphericities of the first silicon carbide particles and the second silicon carbide particles can be obtained by conventional testing methods in the art, for example, in the following manner: After polishing the cross-section of the negative electrode sheet with an argon ion milling instrument, observe it in a scanning electron microscope (SEM) using the backscattered imaging mode, find the second silicon carbide particles with a continuous and smooth contour, connect the two longest points on the contour of the second silicon carbide particle to make a straight line segment, and the straight line segment is inside the second silicon carbide particle. Record the length of the line segment as Z1. Take the midpoint of this straight line and make a straight line segment. The two endpoints of this straight line segment are on the contour of the second silicon carbide particle and the length is the shortest. Record the length of this straight line segment as Z2. The sphericity of the second silicon carbide particle is Z2 / Z1. Select 20 second silicon carbide particles and take the average value after statistics; the method for testing the sphericity of the first silicon carbide particles is the same as that of the aforementioned second silicon carbide particles, except that when observing with SEM, 20 first silicon carbide particles are found for measurement.

[0038] In the present invention, increasing the proportion of silicon-carbon material in the negative electrode active material can improve the specific capacity of the negative electrode active material. In this process, in order to improve the tap density of the negative electrode sheet while alleviating the volume expansion of the silicon-carbon negative electrode, by filling the interstitial space formed by the close packing of silicon-carbon particles with larger particle sizes and carbon-based materials with silicon-carbon particles with smaller average particle sizes, it can not only ensure the bonding strength of the negative electrode active material and prevent peeling and demolding during the cyclic expansion process, but also improve the tap density of the negative electrode sheet and increase the energy density.

[0039] The inventor of the present invention mixes the first silicon-carbon particles with the second silicon-carbon particles, which can not only further alleviate the expansion while having a high energy density, but also improve the cycle stability of the battery. The reasons are as follows: First, the second silicon-carbon particles have a large average particle size but a sphericity smaller than that of the first silicon-carbon particles, and the contour smoothness of the second silicon-carbon particles is low, and its edges and corners can be well embedded in the carbon-based material to form a skeleton of the silicon-carbon material in the negative electrode active material, preventing the silicon-carbon particles from demolding caused by the expansion and contraction process; the first silicon-carbon particles have a small average particle size and high sphericity, and can be filled between the gaps formed by the second silicon-carbon particles and the carbon-based material to improve the packing tightness, so as to keep the negative electrode sheet having a high tap density. In addition, the high sphericity of the first silicon-carbon particles makes the expansion direction more evenly dispersed, reducing the expansion in a single direction; Second, the first silicon-carbon particles have a large specific surface area, providing more active sites for the transmission of lithium ions in the electrolyte. During high-rate charge and discharge processes, more lithium ions can be simultaneously embedded in the negative electrode active material, and at the same time, the smaller average particle size is beneficial to shortening the lithium ion diffusion distance and improving the charging rate of the battery.

[0040] In the present invention, the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer located on at least one surface of the positive electrode current collector. The positive electrode active layer includes a positive electrode active material, and the positive electrode active material includes lithium cobaltate. The lithium cobaltate includes yttrium element and lanthanum element. Based on the total mass of the lithium cobaltate, the content of the yttrium element is C1, 150 ≤ C1 ≤ 700, with the unit of ppm. For example, C1 is 150, 180, 200, 220, 240, 260, 300, 350, 400, 450, 500, 550, 600, 620, 640, 660, 680 or 700; the content of the lanthanum element is C2, 100 ≤ C2 ≤ 500, with the unit of ppm. For example, C2 is 100, 120, 140, 160, 180, 200, 240, 280, 320, 360, 400, 450 or 500.

[0041] In one embodiment, 300 ≤ C1 ≤ 580, with the unit of ppm.

[0042] In another embodiment, 350 ≤ C1 ≤ 580, with the unit of ppm.

[0043] In one embodiment, 200 ≤ C2 ≤ 330, in ppm.

[0044] In yet another embodiment, 220 ≤ C2 ≤ 330, in ppm.

[0045] In the present invention, the lithium cobaltate may be doped and / or coated and modified lithium cobaltate.

[0046] In the present invention, the contents of the yttrium element and the lanthanum element can be obtained by conventional testing methods in the art. For example, testing is carried out using an inductively coupled plasma emission spectrometer (ICP) as follows: After discharging the lithium-ion secondary battery to 0% SOC, disassemble and take out the positive electrode plate. Immerse it in dimethyl carbonate (DMC) solvent for 12 h, then rinse with DMC to remove the lithium salts attached to the positive electrode plate. Then keep the positive electrode plate at 450 °C for 180 min. Brush off the positive electrode active material from the cooled positive electrode plate with a brush to obtain positive electrode active material powder; Take the collected positive electrode active material powder as a sample. Add 0.1 g of the sample to 10 mL of hydrochloric acid and carry out microwave digestion for 10 min. After cooling, make up the volume to 100 mL, and then dilute it 10 times. Take a part of the solution and analyze it with an ICP spectroanalyzer.

[0047] In the present invention, the separator includes a base film and a coating located on at least one surface of the base film. The coating contains nitrogen element. Based on the total weight of the coating, the content of the nitrogen element is C3, 10% ≤ C3 ≤ 50%. For example, C3 is 10%, 12%, 14%, 16%, 18%, 20%, 25%, 30%, 35%, 40%, 45% or 50%.

[0048] In one embodiment, 30% ≤ C3 ≤ 48%.

[0049] In the present invention, the coating faces the positive electrode plate.

[0050] In the present invention, the content of the nitrogen element can be obtained by conventional testing methods in the art. For example, by the following method, a separator with a coating is tested by SEM-EDS (scanning electron microscope - energy dispersive X-ray spectrometer) surface scanning.

[0051] In the present invention, C1, C2 and C3 satisfy 75 ≤ (C1 + C2) × C3 ≤ 400. For example, they are 75, 76, 77, 78, 79, 80, 85, 90, 95, 100, 150, 200, 300 or 400.

[0052] In one embodiment, 240 ≤ (C1 + C2) × C3 ≤ 330.

[0053] In the present invention, although regulating the average particle size, sphericity of the first silicon-carbon particles, and the average particle size and sphericity of the second silicon-carbon particles can, to a certain extent, alleviate the increase in battery internal resistance and temperature rise caused by the increase in side reactions. However, when cycling under a battery chemical system with a charging rate above 3C or a cut-off voltage greater than or equal to 4.5V, when the negative electrode active material contains silicon-carbon particles with a smaller particle size, because the particle size of the silicon-carbon particles is too small, the specific surface area is significantly increased, and the active sites reacting with the electrolyte increase, resulting in intensified side reactions, increased contact internal resistance, and having an adverse effect on the positive electrode active material, especially the influence on the structural stability of lithium deintercalation / insertion of the positive electrode active material at high temperatures, leading to a reduction in the safety and life of the battery at high temperatures; In order to achieve stable cycling at the above high energy density and maintain safety under high temperature rise, the inventors further optimized the combination of the positive electrode active material and the separator under the above negative electrode system. On the one hand, yttrium (Y) and lanthanum (La) elements are added to lithium cobaltate. The ionic radius of Y 3+ and La 3+ is larger than that of Co 3+ . When replacing a part of Co in lithium cobaltate 3+ , it can expand the lattice spacing, alleviate the stress accumulation during lithium ion deintercalation / insertion, and also inhibit the phase transition trend of lithium cobaltate, stabilize the crystal structure and phase state of lithium cobaltate, and improve the structural stability of lithium cobaltate at high temperatures; On the other hand, a coating including nitrogen element is provided on the surface of the separator, and the coating is disposed facing the positive electrode sheet. When the nitrogen-containing particles in the coating diffuse to the surface of the positive electrode active material and contact the positive electrode active material, they can form chemical bonds with the metal atoms in the positive electrode active material, thereby stabilizing the metal atoms and crystal structure in the positive electrode active material, reducing the release of active oxygen, inhibiting the structural phase change caused by the attenuation of the electrochemical performance of the positive electrode active material during high voltage charge and discharge of lithium cobaltate, improving the stability of the positive electrode active material, and thus improving its stability at high temperatures. In addition, it is also necessary to regulate the contents of Y and La in lithium cobaltate within a reasonable range. If the contents of Y and La are too small, the contribution to the improvement of the structural stability of lithium cobaltate is insufficient, while if the contents of Y and La are too large, it will lead to a reduction in the specific capacity of lithium cobaltate, affecting the energy density. Therefore, the inventors of the present invention further explored the relationship between the contents of Y and La and the nitrogen element content in the coating of the separator within a certain range, so that the regulation of the positive electrode active material and the separator matches to achieve a better synergistic effect, improve the material stability of the positive electrode active material at high rates (≥3C) and / or high voltages (≥4.48V), improve the energy density of the battery, as well as the high temperature cycling performance and high temperature safety performance of the battery; In addition, facing the positive electrode sheet to the coating can improve the diffusion rate of the nitrogen-containing particles, making the effect more significant.

[0054] In the present invention, the silicon-carbon material further includes third silicon-carbon particles, and the average particle size of the third silicon-carbon particles is 3 μm - 10 μm, such as 3 μm, 3.2 μm, 3.4 μm, 3.6 μm, 3.8 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm or 10 μm.

[0055] In one embodiment, the average particle size of the third silicon-carbon particles is 3 μm - 8 μm.

[0056] In another embodiment, the average particle size of the third silicon-carbon particles is 5 μm - 7.5 μm.

[0057] In the present invention, the third silicon-carbon particles are mainly formed by a number of the first silicon-carbon particles.

[0058] In the present invention, the third silicon-carbon particles are formed by a number of the first silicon-carbon particles.

[0059] In the present invention, the first silicon-carbon particles are primary particles, and the third silicon-carbon particles are secondary particles formed by a number of the primary particles.

[0060] In the present invention, the "a number of" means that the number of the first silicon-carbon particles forming the third silicon-carbon particles ≥ 2; the "a number of" may also mean that the number of the primary particles forming the secondary particles ≥ 2.

[0061] As Figure 1 Shown is a SEM image of the cross-section of a negative electrode sheet in an embodiment of the present invention. Inside the white dotted line in the figure are the first silicon-carbon particles, inside the black solid line are the third silicon-carbon particles, and inside the white solid line are the second silicon-carbon particles.

[0062] In the present invention, the average particle size of the third silicon-carbon particles can be obtained by conventional testing methods in the art, for example, in the following manner: after polishing the cross-section of the negative electrode sheet with an argon ion milling instrument, observe it in a scanning electron microscope (SEM) using the backscattered imaging mode, find 10 different third silicon-carbon particles, take the two longest points on the cross-section contour of the particle to make a straight line segment, record it as the particle size D of the particle, and calculate the average particle size as the average particle size of the third silicon-carbon particles; the average particle size of the third silicon-carbon particles can also be obtained by a focused ion beam (FIB) test.

[0063] In the present invention, the first silicon-carbon particles have a high sphericity, a small average particle size, and many surface active sites. The average particle size of the third silicon-carbon particles formed by a number of the first silicon-carbon particles is still smaller than that of the second silicon-carbon particles, but the adhesiveness is increased. Therefore, the third silicon-carbon particles have both the good electrochemical activity of the first silicon-carbon particles and the good adhesiveness similar to that of the second silicon-carbon particles. While filling in the framework formed by the second silicon-carbon particles to improve the areal density, tap density, and energy density, they can also better adhere to the carbon-based material and tightly pack in the gaps formed by the second silicon-carbon particles and the carbon-based material to prevent delamination during the cycling process.

[0064] In the present invention, based on the total mass of the negative electrode active material, the content ratio of the first silicon-carbon particles to the second silicon-carbon particles is 1:(1 - 10), for example, 1:1, 1:2, 1:3, 1:5, 1:6, 1:8, or 1:10.

[0065] In one embodiment, based on the total mass of the negative electrode active material, the content ratio of the first silicon-carbon particles to the second silicon-carbon particles is 1:(1 - 5).

[0066] In the present invention, based on the total mass of the negative electrode active material, the content ratio of the first silicon-carbon particles, the second silicon-carbon particles, and the third silicon-carbon particles is 1:(1 - 10):(0.3 - 4), where "1 - 10" is, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10; and "0.3 - 4" is, for example, 0.3, 0.4, 0.5, 0.6, 0.8, 1, 1.5, 2, 2.5, 3, or 4.

[0067] In one embodiment, based on the total mass of the negative electrode active material, the content ratio of the first silicon-carbon particles, the second silicon-carbon particles, and the third silicon-carbon particles is 1:(1 - 5):(0.5 - 2).

[0068] In the present invention, the mass content of silicon element in the negative electrode active material is 4% - 25%, for example, 4%, 5%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, or 25%.

[0069] In the present invention, the mass content of the silicon-carbon material in the negative electrode active material is 8% - 50%, for example, 8%, 9%, 10%, 12%, 14%, 16%, 20%, 25%, 30%, 35%, 40%, 42%, 44%, 46%, 48%, or 50%.

[0070] In the present invention, the silicon-carbon material refers to a material containing elemental silicon and elemental carbon. For example, the silicon-carbon material includes a porous carbon matrix and nano-silicon located in the pores of the porous carbon matrix.

[0071] In the present invention, the mass content of silicon element in the negative electrode active material can be obtained by conventional testing methods in the art. For example, it can be tested by the following method: After discharging the lithium-ion secondary battery to 0% SOC, disassemble and take out the negative electrode sheet. Immerse it in dimethyl carbonate (DMC) solvent for 12 h, then rinse it with DMC to remove the lithium salts attached to the electrode sheet. After air drying, wash it with deionized water. The negative electrode active material can be peeled off from the current collector and collected, and then dried at 80 °C. Take the negative electrode active material with a mass of M1 as a sample. Under an air or oxygen atmosphere, heat it from room temperature (25 °C) to 900 °C at a heating rate of 10 °C / min, and keep it at 900 °C for 40 min. Weigh the mass of the calcined sample and record it as M2. The mass content of silicon element in the negative electrode active material is 7×M2 / (15×M1).

[0072] In the present invention, regulating the mass content of silicon element in the negative electrode active material and / or the mass content of the silicon-carbon material can, on the premise of good electrode structure stability, greatly exert the advantages of the high specific capacity of the silicon-carbon material, improve the overall energy density of the battery, optimize the electron conduction and ion diffusion paths of the electrode, and avoid excessive silicon causing too large volume expansion of the battery during cycling, resulting in damage to the electrode structure and causing cycling failure.

[0073] In the present invention, the negative electrode active material further includes a carbon-based material, and the carbon-based material includes at least one of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, and hard carbon.

[0074] In the present invention, the separator further includes an adhesive layer. The coating is located on at least one surface of the base film opposite in the thickness direction, and the adhesive layer is located on at least one outer surface of the separator.

[0075] In one embodiment, the adhesive layer is located on one surface of the coating facing away from the base film opposite to the positive electrode sheet and one surface of the base film opposite to the negative electrode sheet.

[0076] In one embodiment, the adhesive layer is located on one surface of the coating facing away from the base film opposite to the positive electrode sheet and one surface of the coating facing away from the base film opposite to the negative electrode sheet.

[0077] As Figure 2 shown is a schematic structural diagram of the separator in the thickness direction in one embodiment of the present invention, where 1-1 is the base film, 1-2 is the coating, and the particles in the coating represent nitrogen-containing particles, and 1-3 is the adhesive layer; Figure 3 is the SEM image of the separator in one embodiment of the present invention.

[0078] In the present invention, the base film includes polyethylene (PE) and / or polypropylene (PP).

[0079] In the present invention, the adhesive layer comprises at least one of polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose (CMC-Na), lithium carboxymethyl cellulose (CMC-Li), and polyvinylpyrrolidone (PVP).

[0080] In the present invention, the coating comprises nitrogen-containing particles, and the nitrogen-containing particles comprise at least one of polyacrylonitrile, nitrile rubber, and melamine and its derivatives.

[0081] In the present invention, the melamine and its derivatives comprise at least one of 1,3,5-triazine-2,4,6-triamine, melamine cyanurate, and melamine trithiocyanate and its derivatives.

[0082] In one embodiment, the nitrogen-containing particles comprise melamine and its derivatives.

[0083] In the present invention, the melamine trithiocyanate and its derivatives comprise at least one of melamine trithiocyanate, alkylated melamine trithiocyanate, acylated melamine trithiocyanate, and halogenated melamine trithiocyanate.

[0084] In the present invention, the thickness of the coating is 0.5 μm - 5 μm, for example, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, or 5 μm.

[0085] In one embodiment, the thickness of the coating is 0.5 μm - 2 μm.

[0086] In the present invention, the thickness of the base film is 3 μm - 10 μm, for example, 3 μm, 3.2 μm, 3.4 μm, 3.6 μm, 3.8 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 9 μm, or 10 μm.

[0087] In the present invention, the thicknesses of the separator, the base film, and the coating can be obtained by conventional testing methods in the art. For example, by the following method: After discharging the lithium-ion secondary battery to 0% SOC, disassemble and take out the separator. Immerse the separator in dimethyl carbonate (DMC) solvent for 12 h and then rinse it with DMC to remove the lithium salts attached to the electrode sheets to obtain the separator. Select the area of the separator that exceeds the negative electrode sheet in the battery, polish the cross-section of the separator with an argon ion milling device, observe and measure the thickness of the separator using SEM. The boundaries of the base film, coating, and adhesive layer can be seen in the SEM image. Measure the thicknesses of the base film and the coating according to the interfaces, and take the average value after measuring 10 different test points.

[0088] In the present invention, the average particle size of the nitrogen-containing particles is 100 nm - 1000 nm, such as 100 nm, 120 nm, 140 nm, 160 nm, 180 nm, 200 nm, 240 nm, 260 nm, 300 nm, 340 nm, 380 nm, 420 nm, 460 nm, 500 nm, 550 nm, 600 nm, 700 nm, 800 nm, 900 nm or 1000 nm.

[0089] In one embodiment, the average particle size of the nitrogen-containing particles is 120 nm - 800 nm.

[0090] In yet another embodiment, the average particle size of the nitrogen-containing particles is 120 nm - 600 nm.

[0091] In yet another embodiment, the average particle size of the nitrogen-containing particles is 120 nm - 500 nm.

[0092] In the present invention, the average particle size of the nitrogen-containing particles can be obtained by conventional testing methods in the art, such as by SEM measurement, specifically as follows: Take the side of the separator with the coating as the sample. After imaging with SEM, randomly select 20 nitrogen-containing particles, measure the distance between the two longest points on the edge contour of each nitrogen-containing particle, and take the average value, which is the average particle size of the nitrogen-containing particles; the average particle size of the nitrogen-containing particles can also be obtained by a laser particle size analyzer and a focused ion beam (FIB) test.

[0093] In the present invention, the coating can improve the structural stability of the positive electrode active material, the base film has the functions of ionic conduction and electron insulation, and the adhesive layer can provide good adhesion between the separator and the electrode sheet.

[0094] In the present invention, the tensile strength of the separator in the length direction is I, 1000 ≤ I ≤ 4000, with the unit of kgf / cm 2 , such as I being 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1800, 2000, 2200, 2400, 2600, 2800, 3000, 3200, 3400, 3600, 3800 or 4000.

[0095] In one embodiment, 1600 ≤ I ≤ 3200, with the unit of kgf / cm 2 .

[0096] In the present invention, the puncture resistance of the separator is J, 100 ≤ J ≤ 400, with the unit of gf, such as J being 100, 120, 140, 160, 180, 200, 220, 240, 260, 280, 300, 320, 340, 360, 380 or 400.

[0097] In one embodiment, 200 ≤ J ≤ 300, with the unit of gf.

[0098] In the present invention, the tensile strength of the separator in the length direction and the puncture resistance of the separator can be obtained by conventional testing methods in the art. For example, after discharging the lithium-ion secondary battery to 0% SOC, disassemble the battery to take out the separator, soak it in dimethyl carbonate (DMC) solvent for 12 h, and then rinse it with DMC to remove the lithium salts attached to the electrode sheet to obtain the separator. The tensile strength of the separator in the length direction can be tested using a WD-D3 universal testing machine after installing different test fixtures (there is no relative displacement between the separator and the fixture), and stretch it along the length direction between two fixtures until it breaks. The maximum tensile strength after breaking is the tensile strength of the separator in the length direction; the puncture resistance of the separator can be tested using a WD-D3 universal testing machine. Fix the separator on the base so that there is no relative displacement between them, and install different puncture force test heads for testing. Among them, the puncture resistance of the separator is tested in a state where the separator is flattened. The "flattened" means that the separator is neither bent nor stretched.

[0099] In the present invention, the thermal shrinkage rate of the separator in the width direction is K, K ≤ 3, with the unit of %, for example, K is 0.05, 0.1, 0.3, 0.5, 0.7, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.5 or 3.

[0100] In the present invention, the thermal shrinkage rate of the separator in the width direction can be obtained by conventional testing methods in the art. For example, through the following method: after discharging the lithium-ion secondary battery to 0% SOC, disassemble the battery to take out the separator, soak it in dimethyl carbonate (DMC) solvent for 12 h, and then rinse it with DMC to remove the lithium salts attached to the electrode sheet. Cut the separator to obtain a sample of 10 cm × 10 cm, clamp and fix it along the width direction with a fixture, place it in a forced-air drying oven at 105 °C for 1 h, take it out and measure the dimensional change of the separator after shrinking in the width direction, and calculate to obtain the thermal shrinkage rate of the separator in the width direction.

[0101] As Figure 4 shown, the direction indicated by the arrow is the "width direction" of the separator.

[0102] In the present invention, the difference between the width of the separator and the width of the negative electrode sheet is L, 1 ≤ L ≤ 4, with the unit of mm, for example, L is 1, 1.2, 1.4, 1.6, 1.8, 2, 2.5, 3, 3.5 or 4.

[0103] In the present invention, the difference between the width of the separator and the width of the negative electrode sheet can be obtained by conventional testing methods in the art. For example, it can be measured by the following method: Observe the width of the separator and the width of the negative electrode sheet using a 2.5D microscope, and subtract the width of the negative electrode sheet from the width of the separator, which is the difference between the width of the separator and the width of the negative electrode sheet.

[0104] As Figure 4 shown is a schematic diagram of the difference between the width of the separator and the width of the negative electrode sheet in an embodiment of the present application. It can be seen that there is a surplus in the width of the separator on both the upper and lower sides along the width direction of the negative electrode sheet, and L = L1 + L2.

[0105] In the present invention, 2800 ≤ I × L ≤ 11400, for example, 2800, 3000, 3500, 4000, 4500, 5000, 5500, 6000, 6500, 7000, 7500, 8000, 8500, 9000, 9500, 10000, 11000 or 11400.

[0106] In one embodiment, 3200 ≤ I × L ≤ 6300.

[0107] In the present invention, 290 ≤ J × L ≤ 1130, for example, 290, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1050, 1100 or 1130.

[0108] In one embodiment, 300 ≤ J × L ≤ 800.

[0109] In the present invention, 0.5 ≤ L / K ≤ 3.5, for example, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.5, 3 or 3.5.

[0110] In the present invention, by adjusting the difference (L) between the width of the separator and the width of the negative electrode sheet such that the width of the separator has a certain surplus relative to the width of the negative electrode sheet and is within a reasonable range, the risk of short circuit can be reduced. First, by adjusting the relationship between the difference between the width of the separator and the width of the negative electrode sheet and the tensile strength (I) of the separator in the length direction within a certain range, the risk of short circuit can be effectively reduced. This is because when the battery is wound, during the winding process, the separator will be subjected to a winding tension in the length direction. At this time, the edge position of the separator may be stretched and cause slight misalignment, resulting in contact between the positive and negative electrodes at the edge, triggering micro-short circuit or safety hazards. Therefore, L and I are inversely correlated, and adjusting the respective ranges of the two and the range of I×L can effectively avoid battery short circuit caused by winding tension. Second, by adjusting the relationship between the difference between the width of the separator and the width of the negative electrode sheet and the puncture force (J) that the separator can withstand within a certain range, the risk of the separator being punctured can be effectively reduced. This is because during the cutting process or the charging process, burrs or lithium dendrites may appear at the edge of the negative electrode sheet and pierce the separator. Increasing L can effectively reduce the risk of the separator being punctured at the edge of the electrode sheet and causing short circuit. Therefore, L and J are inversely correlated, and adjusting the respective ranges of the two and the range of J×L can effectively avoid the risk of the separator being punctured. Third, by adjusting the relationship between the difference between the width of the separator and the width of the negative electrode sheet and the thermal shrinkage rate (K) of the separator within a certain range, the short circuit caused by the thermal shrinkage of the separator can be avoided. When the battery is at an elevated temperature or under high-temperature conditions, the separator will undergo thermal shrinkage, resulting in the exposure of the negative electrode sheet and causing a short-circuit risk. Therefore, K and L are positively correlated, and adjusting the respective ranges of the two and the range of L / K can effectively avoid battery short circuit at high temperatures.

[0111] In the present invention, the lithium cobaltate includes first lithium cobaltate particles and second lithium cobaltate particles. The average particle size of the first lithium cobaltate particles is 2 μm - 10 μm, such as 2 μm, 2.2 μm, 2.4 μm, 2.6 μm, 2.8 μm, 3 μm, 3.4 μm, 3.8 μm, 4 μm, 4.5 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm.

[0112] In one embodiment, the average particle size of the first lithium cobaltate particles is 3 μm - 5.5 μm.

[0113] In the present invention, the average particle size of the second lithium cobaltate particles is 15 μm - 28 μm, such as 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm or 28 μm.

[0114] In one embodiment, the average particle size of the second lithium cobaltate particles is 16 μm - 23 μm.

[0115] In the present invention, the median particle size Dv50 of the positive electrode active material is 10 μm - 25 μm, such as 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm or 25 μm.

[0116] And / or, the particle size Dv99 of the positive electrode active material is 30 μm - 50 μm, such as 30 μm, 31 μm, 32 μm, 33 μm, 34 μm, 35 μm, 36 μm, 37 μm, 38 μm, 39 μm, 40 μm, 45 μm or 50 μm.

[0117] In the present invention, the average particle sizes of the first lithium cobaltate particles and the second lithium cobaltate particles, the median particle size Dv50 and the particle size Dv99 of the positive electrode active material can be obtained by conventional testing methods in the art. For example, the testing is carried out by the following method: after discharging the lithium ion secondary battery to 0% SOC, disassemble and take out the positive electrode plate, soak it in dimethyl carbonate (DMC) solvent for 12 h, then rinse it with DMC to remove the lithium salt attached to the positive electrode plate, then keep the positive electrode plate at 450 °C for 180 min, brush off the positive electrode active material from the cooled positive electrode plate with a brush to obtain the positive electrode active material powder; take the collected positive electrode active material powder as a sample, use a laser diffraction particle size analyzer to measure and obtain a volume distribution curve. Among them, the particle size value corresponding to the first peak of the volume distribution curve is the average particle size of the first lithium cobaltate particles, the particle size value corresponding to the second peak of the volume distribution curve is the average particle size of the second lithium cobaltate particles, and the median particle size Dv50 and the particle size Dv99 of the positive electrode active material are determined according to the equipment test values.

[0118] In the present invention, the positive electrode active material includes first lithium cobaltate particles with a small average particle size and second lithium cobaltate particles with a large average particle size, and the average particle sizes of the first lithium cobaltate particles and the second lithium cobaltate particles are regulated within reasonable ranges respectively. The average particle size of the second lithium cobaltate particles is larger, and it has a lower viscosity and good fluidity when preparing the positive electrode slurry, a high solid content, the tap density of the positive electrode sheet obtained by using the second lithium cobaltate particles is increased and the side reactions are less. However, when the average particle size of the second lithium cobaltate particles is too large and / or the proportion in the positive electrode active material is too high, the gaps inside the positive electrode active material will increase, resulting in energy density loss; while the average particle size of the first lithium cobaltate particles is smaller, which can provide a larger contact area, shorten the lithium ion transmission path, and has the effect of improving the electron conduction rate and the lithium deintercalation / insertion reaction rate. However, when the average particle size of the first lithium cobaltate particles is too small and / or the proportion in the positive electrode active material is too high, the specific surface area of the positive electrode active material is too large, and the side reactions with the electrolyte are intensified; therefore, it is also necessary to regulate the median particle size Dv50 of the positive electrode active material to realize the regulation of the contents of the first lithium cobaltate particles and the second lithium cobaltate particles in the positive electrode active material, so that the positive electrode active material has the advantages of both the first lithium cobaltate particles and the second lithium cobaltate particles, and improve the tap density, surface density and conductivity of the positive electrode sheet; in addition, it is also necessary to regulate the particle size Dv99 of the positive electrode active material to avoid the too large particle size of the second lithium cobaltate particles and the too high proportion of large particle sizes, so that a part of the positive electrode active material is broken during rolling.

[0119] In the present invention, the average particle size of the negative electrode active material is 6 μm - 25 μm, for example, 6 μm, 8 μm, 9 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 22 μm or 25 μm.

[0120] In one embodiment, the average particle size of the negative electrode active material is 8 μm - 20 μm.

[0121] In one embodiment, the average particle size of the negative electrode active material is 12 μm - 17 μm.

[0122] In the present invention, the average particle size of the primary particles can be obtained by conventional testing methods in the art, for example, in the following manner: after polishing the cross-section of the negative electrode sheet with an argon ion milling instrument, observing in a scanning electron microscope (SEM) using the backscattered imaging mode, arbitrarily taking 20 primary particles constituting the first silicon-carbon particles, respectively taking the two longest points of a single primary particle to make a straight line segment, and taking the average value after measurement, which is recorded as the average particle size of the primary particles, and it can also be obtained by a focused ion beam (FIB).

[0123] In the present invention, the average particle size of the negative electrode active material can be obtained by conventional testing methods in the art, for example, in the following manner: after discharging the lithium ion secondary battery to 0% SOC, disassemble and take out the negative electrode sheet, soak it in dimethyl carbonate (DMC) solvent for 12 h, and then rinse it with DMC to remove the lithium salts attached to the negative electrode sheet. Wash the negative electrode active layer on the negative electrode sheet with water, and then test it with a laser particle size analyzer after ultrasonic treatment.

[0124] Regulating the average particle size of the negative electrode active material within a certain range can not only meet the bonding strength between the silicon-carbon particles and / or the carbon-based material, improve the compaction density of the negative electrode sheet, but also increase the conductivity and alleviate the cycle expansion.

[0125] In the present invention, the lithium ion secondary battery further includes an electrolyte, and the electrolyte includes fluoroethylene carbonate. Based on the total weight of the electrolyte, the content of fluoroethylene carbonate is 10% - 30%, for example, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 25% or 30%.

[0126] In one embodiment, the content of fluoroethylene carbonate is 10% - 20%.

[0127] In the present invention, the content of the fluoroethylene carbonate (FEC) can be obtained by conventional testing methods in the art, for example, by using a gas chromatograph for testing.

[0128] In the present invention, adding FEC to the electrolyte can effectively form a SEI film rich in LiF on the surface of the silicon-carbon negative electrode, promote the formation of the SEI film, and protect the negative electrode active material. At this time, it is necessary to regulate the content of FEC in the electrolyte within a certain range. The reason is that when the content of FEC is too high, it may cause the battery to bulge, increase the thickness of the interface film, and lose active lithium. When the content of FEC is too low, the interface film will be incomplete, resulting in an aggravation of side reactions.

[0129] In the present invention, the lithium ion secondary battery further includes a tab and tab glue, and the melting point of the tab glue is 80°C - 180°C, for example, 80°C, 82°C, 84°C, 86°C, 88°C, 90°C, 95°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C or 180°C.

[0130] In the present invention, the tab glue includes an outer layer, a middle layer and an inner layer, the lithium ion secondary battery includes a housing, the outer layer is arranged in contact with the housing, and the inner layer is arranged in contact with the tab.

[0131] In the present invention, the tab includes a positive tab and / or a negative tab.

[0132] In the present invention, the intermediate layer is located between the outer layer and the inner layer.

[0133] In the present invention, the outer layer and the inner layer comprise an acid-modified resin, and the melting points of the outer layer and the inner layer are independently 80°C - 140°C, such as 80°C, 82°C, 84°C, 86°C, 90°C, 94°C, 98°C, 102°C, 106°C, 110°C, 114°C, 118°C, 120°C, 125°C, 130°C, 135°C or 140°C.

[0134] In the present invention, the intermediate layer comprises polypropylene or crosslinked polyethylene, and the melting point of the intermediate layer is 130°C - 180°C, such as 130°C, 132°C, 134°C, 136°C, 138°C, 140°C, 145°C, 150°C, 155°C, 160°C, 165°C, 170°C or 180°C.

[0135] In the present invention, the melting point of the tab glue can be obtained by conventional testing methods in the art, such as by testing with differential scanning calorimetry (DSC).

[0136] It can be understood that the melting point of the tab glue refers to the critical temperature at which the tab glue begins to transform from a solid state to a liquid state. For the tab glue with a layered structure, the melting point of the tab glue is the melting point of the part with the lowest melting point in the layered structure.

[0137] In the present invention, the tab glue can enhance the bonding performance with the tab or the aluminum-plastic film. Since the battery will generate a large amount of gas at high temperatures, and even catch fire or explode, by using a low-melting-point tab glue, when the melting temperature is reached at high temperatures, the top seal is released, and the gas inside the battery is discharged, which can prevent the battery from exploding at high temperatures.

[0138] In the present invention, the positive electrode sheet further comprises a bottom coating, the bottom coating is located between the positive electrode active layer and the positive electrode current collector, and the bottom coating comprises inorganic particles.

[0139] In the present invention, the average particle size of the inorganic particles is 20nm - 1000nm, such as 20nm, 40nm, 60nm, 80nm, 100nm, 150nm, 200nm, 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, 900nm or 1000nm.

[0140] In one embodiment, the average particle size of the inorganic particles is 20nm - 200nm.

[0141] In another embodiment, the average particle size of the inorganic particles is 100nm - 160nm.

[0142] In the present invention, the thickness of the bottom coating is 0.5 μm - 5 μm, for example, 0.5 μm, 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2 μm, 2.4 μm, 2.8 μm, 3 μm, 3.5 μm, 4 μm or 5 μm.

[0143] In one embodiment, the thickness of the bottom coating is 1 μm - 3 μm.

[0144] In the present invention, the bottom coating further includes a binder and a conductive agent. The inorganic particles include at least one of aluminum oxide, magnesium oxide, boehmite, titanium dioxide, lithium iron phosphate, magnesium hydroxide, magnesium hydroxide and silicon dioxide. The binder includes polyvinylidene fluoride and / or polyacrylic acid. The conductive agent includes conductive carbon black and / or carbon nanotubes.

[0145] In the present invention, the average particle size of the inorganic particles can be obtained by conventional testing methods in the art. For example, taking the bottom coating as a sample and observing it in the SEM using the backscattered imaging mode, finding 20 inorganic particles and measuring the distance between the two longest points on the edge contour of each inorganic particle, and taking the average value as the average particle size of the inorganic particles; or for example, testing by a laser particle size analyzer.

[0146] In the present invention, the thickness of the bottom coating can be obtained by conventional testing methods in the art. For example, by the following method: after polishing the cross-section of the positive electrode sheet with an argon ion milling machine, observing the obtained cross-section in the SEM device using the backscattered imaging mode, and measuring to obtain the thickness of the bottom coating.

[0147] In the present invention, the positive electrode sheet further includes a positive electrode current collector, and the bottom coating is disposed on at least one surface of the positive electrode current collector opposite in the thickness direction.

[0148] In one embodiment, the bottom coating is disposed on both surfaces of the positive electrode current collector opposite in the thickness direction.

[0149] In the present invention, the bottom coating can cover one surface and / or both surfaces of the positive electrode current collector opposite in the thickness direction, or can cover one surface and / or both surfaces of the positive electrode current collector opposite in the thickness direction except for the area where the positive electrode tab groove is located.

[0150] In one embodiment, the length of the bottom coating in the length direction is greater than the length of the positive electrode active layer; or the bottom coating extends from the positive electrode active layer.

[0151] In the present invention, the setting of the bottom coating can avoid that when the battery is damaged by external force, the aluminum foil fragments generated by the rupture of the positive electrode current collector directly contact the negative electrode sheet, causing the battery to short-circuit and catch fire, and reducing the safety hazard of the battery.

[0152] In the present invention, the negative electrode active layer includes a plurality of grooves, the width of the grooves being 20 μm - 150 μm, such as 20 μm, 25 μm, 30 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm or 150 μm; the depth of the grooves being 5 μm - 50 μm, such as 5 μm, 7 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm or 50 μm, and the spacing between the grooves being 500 μm - 2000 μm, such as 500 μm, 520 μm, 540 μm, 560 μm, 600 μm, 650 μm, 700 μm, 800 μm, 900 μm, 1000 μm, 1100 μm, 1200 μm, 1300 μm, 1400 μm, 1500 μm or 2000 μm.

[0153] In the present invention, the term "a plurality of" means that the number of the grooves ≥ 2.

[0154] In the present invention, the grooves can be obtained by conventional techniques in the art, for example, by a laser scribing process.

[0155] It can be understood that the width, the depth and the spacing of the grooves have their conventional meanings in the art. The width of the grooves refers to the shortest distance between the two opposite edges of the positive projection of a single groove on the negative electrode sheet; the depth of the grooves refers to the vertical distance from the lowest point in the grooves to the surface of the negative electrode active layer; and the spacing between the grooves refers to the average distance between the adjacent two long sides of two adjacent grooves in the length direction or the width direction of the negative electrode sheet.

[0156] In the present invention, the width, the depth and the spacing of the grooves can be obtained by conventional testing methods in the art. For example, the depth of the grooves is measured by a 3D profiler or SEM to measure the depth of all the grooves or at least 5 grooves in the negative electrode active layer, and the average value is taken; the width of the grooves is measured by a 3D profiler or SEM to measure the width of all the grooves or at least 5 grooves in the negative electrode active layer, and the average value is taken; the spacing between the grooves is measured by a 3D profiler or SEM to measure the spacing between all the grooves or at least 5 groups of adjacent grooves in the negative electrode active layer, and the average value is taken.

[0157] Such as Figure 5The following is a schematic cross-sectional view of the negative electrode sheet in the thickness direction according to an embodiment of the present invention. Among them, 2-1 is a groove, 2-2 is a negative electrode active layer, 2-3 is a negative electrode current collector, T1 represents the depth of the groove, T2 represents the width of the groove, and T3 represents the spacing between the grooves.

[0158] In the present invention, by providing grooves on the negative electrode active layer, it can provide expansion space, buffer the volume change caused by the expansion of the silicon-carbon negative electrode, and reduce the increase in the thickness of the negative electrode sheet. In addition, the grooves can also improve the wetting performance of the electrolyte and enhance the cycle life.

[0159] In the present invention, the positive electrode sheet includes a first positive electrode surface and a second positive electrode surface that are oppositely arranged along the thickness direction of the positive electrode current collector. The first positive electrode surface includes a convex portion area, and the convex portion area includes a plurality of convex portions; the second positive electrode surface includes a concave portion area, and the concave portion area has a plurality of concave portions.

[0160] In the present invention, the depth of the concave portion is 10 μm - 30 μm, for example, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 25 μm, or 30 μm; the spacing between the concave portions is 2 mm - 6 mm, for example, 2 mm, 2.2 mm, 2.4 mm, 2.6 mm, 2.8 mm, 3 mm, 3.5 mm, 4 mm, 4.5 mm, 5 mm, 5.5 mm, or 6 mm; the width of the concave portion is 1 mm - 5 mm, for example, 1 mm, 1.2 mm, 1.4 mm, 1.6 mm, 1.8 mm, 2 mm, 2.5 mm, 3 mm, 3.5 mm, 4 mm, 4.5 mm, 5 mm; the area of the projection of the concave portion area on the second positive electrode surface accounts for 40% - 90% of the area of the projection of the positive electrode active layer located on the second positive electrode surface on the second positive electrode surface, for example, 40%, 42%, 44%, 46%, 48%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, or 90%.

[0161] In one embodiment, the area of the projection of the concave portion area on the second positive electrode surface accounts for 60% - 70% of the area of the projection of the positive electrode active layer located on the second positive electrode surface on the second positive electrode surface.

[0162] In the present invention, the height of the convex portion is 10 μm - 30 μm, such as 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 25 μm or 30 μm; the spacing between the convex portions is 2 mm - 6 mm, such as 2 mm, 2.2 mm, 2.4 mm, 2.6 mm, 2.8 mm, 3 mm, 3.5 mm, 4 mm, 4.5 mm or 5 mm; the width of the convex portion is 1 mm - 5 mm, such as 1 mm, 1.2 mm, 1.4 mm, 1.6 mm, 1.8 mm, 2 mm, 2.5 mm, 3 mm, 3.5 mm, 4 mm, 4.5 mm, 5 mm; the area of the projection of the convex portion region on the first positive electrode surface accounts for 40% - 90% of the area of the positive electrode active layer located on the first positive electrode surface in the projection on the first positive electrode surface, such as 40%, 42%, 44%, 46%, 48%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85% or 90%.

[0163] In one embodiment, the area of the projection of the convex portion region on the first positive electrode surface accounts for 60% - 70% of the area of the positive electrode active layer located on the first positive electrode surface in the projection on the first positive electrode surface.

[0164] In the present invention, the positions of the convex portion region and the concave portion region correspond to each other, and the positions of the convex portion and the concave portion correspond to each other.

[0165] In the present invention, the concave portion and the convex portion can be obtained by conventional techniques in the art, for example, by using an embossing roll (with bumps) through an embossing process.

[0166] In one embodiment, the positions of the concave portions on the second positive electrode surface correspond one-to-one with the positions of the convex portions on the first positive electrode surface.

[0167] In one embodiment, the second positive electrode surface where the concave portions are located is away from the winding center of the core, and the first positive electrode surface where the convex portions are located is close to the winding center of the core.

[0168] In the present invention, the shape of the projection of the concave portion on the second positive electrode surface is not limited, and it can be circular or rectangular. When the shape of the projection of the concave portion on the second positive electrode surface is circular, the width of the concave portion is the diameter of the circle; when the shape of the projection of the concave portion on the second positive electrode surface is non-circular, the width of the concave portion is the equivalent diameter of a circle with an area equal to that of the non-circular shape; the spacing between the concave portions refers to the distance between the lowest points of two adjacent concave portions; the depth of the concave portion refers to the vertical distance from the lowest point inside the concave portion to the second positive electrode surface.

[0169] In the present invention, the shape of the projection of the convex portion on the surface of the first positive electrode is not limited and can be circular or rectangular. When the shape of the projection of the convex portion on the surface of the first positive electrode is circular, the width of the convex portion is the diameter of the circle; when the shape of the projection of the convex portion on the surface of the first positive electrode is non-circular, the width of the convex portion is the equivalent diameter of the circle with the same area as the non-circular shape; the spacing between the convex portions refers to the distance between the highest points of two adjacent convex portions; the height of the convex portion refers to the vertical distance from the lowest point inside the convex portion to the surface of the first positive electrode.

[0170] In the present invention, the depth of the concave portion, the spacing of the concave portions, the height of the convex portions, and the spacing of the convex portions can be obtained by conventional testing methods in the art. For example, by using a 3D profiler to test, 20 groups are measured respectively, and the average value is taken.

[0171] As Figure 6 As shown in the schematic cross-sectional view of the positive electrode sheet in the thickness direction in an embodiment of the present invention, wherein 3-1 is the positive electrode current collector, 3-2 is the bottom coating, 3-3 is the positive electrode active layer on the surface of the first positive electrode, 3-4 is the positive electrode active layer on the surface of the second positive electrode, 3-5 is the concave portion, and 3-6 is the convex portion.

[0172] In the present invention, the concave portion area refers to the closed figure with the shortest perimeter formed by connecting the lowest points in the three-dimensional space of the concave portions located on the periphery of the surface of the second positive electrode. The area of the projection of the closed figure on the surface of the second positive electrode is the area of the concave portion area. Taking the attached drawing as an example, as Figure 7 As shown in the top view of the surface of the second positive electrode of the positive electrode sheet in an embodiment of the present invention, wherein the area formed by the white solid line is the concave portion area. It can be understood that the white solid line is a closed figure formed by sequentially connecting the lowest points in the three-dimensional space of the concave portions located on the periphery with a continuous and non-branching line. When the line between any two of the lowest points is a straight line, the perimeter of the formed closed figure is the shortest, and this closed figure is the concave portion area. The area of its projection on the surface of the second positive electrode is the area of the concave portion area. The area of the concave portion area can be measured and calculated by the following method: Along the width direction of the positive electrode sheet, sequentially connect the lowest points in the three-dimensional space of the outermost concave portions, and the dimension is denoted as Y1. Along the length direction of the positive electrode sheet, sequentially connect the lowest points in the three-dimensional space of the outermost concave portions, and the dimension is denoted as Y2. The width of the positive electrode sheet is Y3, and the length of the positive electrode sheet is Y4. Then, the area range delimited by the line segment Y1 and the line segment Y2 is the concave portion area, and the area of the concave portion area is Y1×Y2. The proportion of the area of the concave portion area in the area of the positive electrode sheet is (Y1×Y2) / (Y3×Y4).

[0173] In the present invention, the area of the concave portion region on the positive electrode sheet can be obtained by conventional testing methods in the art, for example, by measuring Y1, Y2, Y3, and Y4 with a tape measure and calculating.

[0174] In the present invention, a plurality of convex portions and a plurality of concave portions are respectively provided on the first positive electrode surface and the second positive electrode surface of the positive electrode sheet, which can increase the gap between the positive and negative electrode sheets, increase the contact area between the electrolyte and the positive electrode active material, and provide a buffer space for the volume expansion of the negative electrode active material. At the same time, for a wound battery, there may be a problem of uneven thermal pressing softening and melting of the adhesive layer of the separator in the arc region of the battery, forming some spaces. Especially for the adhesive layer obtained by spraying, its flatness is poor, and the same reserves some gaps between the positive and negative electrode sheets, which is beneficial to releasing the stress generated by expansion, reducing the expansion stress on the positive and negative electrode sheets, and improving the cycle life and charge-discharge performance of the battery.

[0175] In the present invention, the negative electrode sheet has a first negative electrode surface and a second negative electrode surface oppositely arranged along the thickness direction of the negative electrode current collector. The negative electrode sheet further includes a first tab groove and a first adhesive embedding groove located on the first negative electrode surface, and a second tab groove and a second adhesive embedding groove located on the second negative electrode surface. The first tab groove and the first adhesive embedding groove are connected, and the second tab groove and the second adhesive embedding groove are connected.

[0176] In the present invention, the "connected" means that the first tab groove and the first adhesive embedding groove have a common boundary, and the second tab groove and the second adhesive embedding groove have a common boundary.

[0177] In the present invention, the negative electrode sheet further includes a negative electrode tab. The first tab groove and the second tab groove are formed by the absence of the negative electrode active layer. The negative electrode tab is located in the first tab groove and is electrically connected to the negative electrode current collector.

[0178] In the present invention, the positive electrode sheet further includes a positive electrode tab groove formed by the absence of the positive electrode active layer, and a positive electrode tab disposed in the positive electrode tab groove. The negative electrode sheet includes a third adhesive embedding groove formed by the absence of the negative electrode active layer. Along the thickness direction of the electrode assembly, the positive electrode tab groove and the third adhesive embedding groove are arranged facing each other.

[0179] In the present invention, a first tab adhesive tape is provided on the surface of the positive electrode tab; at least part of the third adhesive embedding groove is provided with a second tab adhesive tape.

[0180] In the present invention, the first adhesive embedding groove and / or the second adhesive embedding groove further includes a third tab adhesive tape.

[0181] In the present invention, the depth of the first adhesive embedding groove is greater than the thickness of the third tab adhesive tape.

[0182] In the present invention, the depth of the second glue-embedding groove is greater than the thickness of the third tab glue paper.

[0183] In the present invention, the depth of the third glue-embedding groove is greater than the thickness of the second tab glue paper.

[0184] In the present invention, the distance X1 between the side edge of the first tab groove and the side edge of the first glue-embedding groove is 0.4 mm - 1.5 mm, such as 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1 mm, 1.1 mm, 1.2 mm, 1.3 mm, 1.4 mm or 1.5 mm; the distance X1' between the side edge of the second tab groove and the side edge of the second glue-embedding groove is 0.4 mm - 1.5 mm, such as 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1 mm, 1.1 mm, 1.2 mm, 1.3 mm, 1.4 mm or 1.5 mm.

[0185] In the present invention, the distance X2 between the bottom edge of the first tab groove and the bottom edge of the first glue-embedding groove is 1.2 mm - 3.2 mm, such as 1.2 mm, 1.3 mm, 1.4 mm, 1.6 mm, 1.8 mm, 2 mm, 2.2 mm, 2.4 mm, 2.6 mm, 2.8 mm, 3 mm or 3.2 mm; the distance X2' between the bottom edge of the second tab groove and the bottom edge of the second glue-embedding groove is 1.2 mm - 3.2 mm, such as 1.2 mm, 1.3 mm, 1.4 mm, 1.6 mm, 1.8 mm, 2 mm, 2.2 mm, 2.4 mm, 2.6 mm, 2.8 mm, 3 mm or 3.2 mm.

[0186] In the present invention, the depth X3 of the first glue-embedding groove is 10 μm - 30 μm, such as 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 25 μm or 30 μm; the depth X3' of the second glue-embedding groove is 10 μm - 30 μm, such as 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 25 μm or 30 μm.

[0187] In the present invention, the negative electrode sheet includes the negative electrode current collector, the glue-embedding groove includes a first glue-embedding groove and a second glue-embedding groove oppositely arranged along the thickness direction of the negative electrode current collector, and the distance X4 between the side edge of the first glue-embedding groove and the side edge of the second glue-embedding groove is 0.3 mm - 1.5 mm, such as 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1 mm, 1.1 mm, 1.2 mm, 1.3 mm, 1.4 mm or 1.5 mm.

[0188] In the present invention, the lithium-ion secondary battery includes a wound core, which is formed by laminating and winding the positive electrode sheet, the separator, and the negative electrode sheet.

[0189] In the present invention, X1, X1', X2, and X2' can be obtained by conventional testing methods in the art, such as by testing with a 2.5D microscope; X3, X3', and X4 can be obtained by conventional testing methods in the art, such as by preparing a cross-section of the embedded glue groove area using a slicing machine and then observing and measuring with an SEM.

[0190] As Figure 8 shown is a top view of the first tab groove in an embodiment of the present invention. Among them, 5-1 is the negative tab. Cutting along the S1 direction can obtain cross-section 1, and cutting along the S2 direction can obtain cross-section 2; as Figure 9 shown is a sectional view of cross-section 1 obtained by cutting the first tab groove along the S1 direction in an embodiment of the present invention. As Figure 10 shown is a sectional view of cross-section 2 obtained by cutting the first tab groove along the S2 direction in an embodiment of the present invention. Among them, 6-1 is the first tab groove, 6-2 is the second tab groove, 6-3 is the first embedded glue groove, and 6-4 is the second embedded glue groove.

[0191] As Figure 11 shown is a schematic top view of the positive tab groove and the third embedded glue groove in an embodiment of the present invention. Among them, 5-2 is the positive tab, 9-1 is the positive electrode sheet, 8-1 is the first tab adhesive tape, 8-2 is the second tab adhesive tape, 8-3 is the third embedded glue groove, and 8-4 is the positive tab groove.

[0192] As Figure 12 shown is a schematic structural view of the wound core in an embodiment of the present invention. Among them, 9-1 is the positive electrode sheet, 9-2 is the negative electrode sheet, and 9-3 is the separator.

[0193] In the present invention, the first glue-embedding groove and the second glue-embedding groove can scrape off or laser ablate part of the negative active material layer (without exposing the negative current collector) by using a scraper. The purpose of setting the first glue-embedding groove and the second glue-embedding groove is to reserve a thickness space for attaching the third tab sticker. Therefore, the depth of the glue-embedding groove is greater than the thickness of the third tab sticker. At this time, when the third tab sticker is pasted in the glue-embedding groove, the unevenness of the core caused by the thickness of the third tab sticker can be avoided, and at the same time, the thickness of the core at the tab can be reduced, improving the energy density. In addition, the distance between the side edges of the first glue-embedding groove and the second glue-embedding groove is also adjusted so that the sizes of the glue-embedding grooves on both sides of the negative current collector are different, which can prevent the curling of the electrode sheet at the tab position caused by stress concentration at the edge position of the glue-embedding groove. Combined with the negative electrode material of the present application, mixing the first silicon-carbon particles with a large sphericity and a small average particle size and the second silicon-carbon particles with a small sphericity (for example, smaller than the first silicon-carbon particles) and a large average particle size can further relieve swelling, reduce stress and avoid the bending of the electrode sheet.

[0194] In the present invention, the positive current collector includes aluminum foil, and the negative current collector includes coated copper foil and / or copper foil.

[0195] In the present invention, the positive electrode sheet further includes at least one of a positive electrode conductive agent, a positive electrode dispersant, and a positive electrode binder, and they are all conventional selections for those skilled in the art. For example, the positive electrode dispersant includes at least one of carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose (CMC-Na), and lithium carboxymethyl cellulose (CMC-Li), the positive electrode binder includes at least one of polyvinylidene fluoride (PVDF), polyacrylate, polyimide, and styrene-butadiene rubber, and the positive electrode conductive agent includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

[0196] In the present invention, the negative electrode sheet further includes a negative electrode conductive agent, a negative electrode binder, and a negative electrode thickener, and they are all conventional selections for those skilled in the art. For example, the negative electrode binder includes at least one of polyacrylic acid (PAA), sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose (CMC-Na), polyimide, styrene-butadiene rubber (SBR), and polyvinylidene fluoride, the negative electrode conductive agent includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene, and the negative electrode thickener includes lithium carboxymethyl cellulose (CMC-Li).

[0197] In the present invention, the electrolyte further includes at least one of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), propyl propionate (PP), ethyl propionate (EP), ethyl butyrate (EB), ethyl acetate (EA), difluoroethyl acetate (DFEA), ethylene methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0198] In the present invention, the electrolyte further includes a lithium salt, and the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium difluorophosphate.

[0199] In the present invention, the electrolyte further includes an additive, and the additive includes at least one of adiponitrile (AND), succinonitrile

[0200] (SN), glutaronitrile, adiponitrile (AND), pimelonitrile, suberonitrile, sebaconitrile, 1,3,6-hexanetricarbonitrile (HTCN), glycerol trinitrile, and propylene sulfate (PS).

[0201] In the present invention, the charging cut-off voltage of the lithium ion secondary battery is ≥ 4.48V.

[0202] In the present invention, the charging cut-off voltage of the lithium ion secondary battery is ≥ 4.5V.

[0203] In the present invention, the term "0% SOC" refers to the battery being discharged to 2.7V at 0.1C.

[0204] It should be noted that in the present invention, the numerical representations such as "first", "second", etc. are only used to distinguish different substances or usage methods, and do not represent the difference in order.

[0205] The present invention will be described in detail below through examples. The examples described in the present invention are only a part of the examples of the present invention, rather than all the examples. All other examples obtained by those of ordinary skill in the art based on the examples in the present invention without creative efforts belong to the scope of protection of the present invention.

[0206] In the following examples, unless otherwise specified, the materials used are commercially available analytical pure.

[0207] The following examples are used to illustrate the lithium ion secondary battery of the present invention.

[0208] Example 1:

[0209] (1) Preparation of the positive electrode sheet:

[0210] Dissolve and disperse 4 parts of CMC in deionized water, then add 80 parts of boehmite (average particle size of 135 nm), 5 parts of conductive carbon black and 5 parts of carbon nanotubes, stir evenly, and then add 6 parts of styrene-butadiene rubber and stir evenly to prepare a bottom coating slurry; coat the bottom coating slurry on both surfaces of the aluminum foil to form a bottom coating (thickness of 1.8 μm) to obtain a bottom-coated aluminum foil.

[0211] Mix the positive electrode active material (20% first lithium cobalt oxide particles + 80% second lithium cobalt oxide particles, C1 = 473 ppm, C2 = 295 ppm), conductive carbon black, multi-walled carbon nanotubes and PVDF in a mass ratio of 95:2:1:2, and add NMP, stir evenly to obtain a positive electrode active slurry; coat the positive electrode active slurry on both surfaces of the bottom-coated aluminum foil, and after baking, rolling and slitting, remove all the positive electrode active layers in a specific area to form a positive electrode tab groove, and weld the positive electrode tab in the positive electrode tab groove, and attach the first tab adhesive tape (SBS rubber adhesive tape); use a special roller with protrusions for processing to obtain a positive electrode sheet with a concave area on one surface (i.e., the second positive electrode surface) and a convex area on the other surface (i.e., the first positive electrode surface).

[0212] Among them, the average particle size of the first lithium cobalt oxide particles is 4.5 μm, the average particle size of the second lithium cobalt oxide particles is 18.3 μm, the median particle size Dv50 of the positive electrode active material is 15.5 μm, and the particle size Dv99 of the positive electrode active material is 36.8 μm.

[0213] On the second positive electrode surface, the depth of the concave part is 19 μm, the spacing of the concave part is 4 mm, and the width of the concave part is 2.5 mm; correspondingly, on the first positive electrode surface, the height of the convex part is 19 μm, the spacing of the convex part is 4 mm, and the width of the convex part is 2.5 mm; the area of the concave area accounts for 65% of the area of the positive electrode sheet.

[0214] (2) Preparation of the negative electrode sheet:

[0215] Mix the negative electrode active material (15% silicon-carbon material + 85% artificial graphite), conductive carbon black, carbon nanotubes, and PVDF in a mass ratio of 95:2:1:2, and add deionized water, stir evenly to obtain the negative electrode active paste; coat the two surfaces of the carbon-coated copper foil with the negative electrode active paste, after baking, rolling, die-cutting, and cold pressing, through laser processing technology, etch grooves on the outer surface of the negative electrode active layer (the width of the groove is 48 μm, the depth is 23 μm, and the spacing is 1100 μm), and then remove all / part of the negative electrode active layer in a specific area to form a first tab groove, a second tab groove, a first glue embedding groove, a second glue embedding groove, and a third glue embedding groove. Ultrasonically weld the negative electrode tab (copper-nickel plated tab) in the first tab groove. The first glue embedding groove and the second glue embedding groove are attached with a third tab glue paper (SBS rubber glue paper), and the second tab glue paper (SBS rubber glue paper) is attached in the third glue embedding groove. The thickness of the third tab glue paper < the depth of the first glue embedding groove and the second glue embedding groove, and the thickness of the second tab glue paper < the depth of the third glue embedding groove.

[0216] Among them, the silicon-carbon material is composed of first silicon-carbon particles, second silicon-carbon particles, and third silicon-carbon particles in a mass ratio of 4:4:2. The sphericity of the first silicon-carbon particles is 0.99, and the average particle size of the first silicon-carbon particles is 3.5 μm; the sphericity of the second silicon-carbon particles is 0.62, the average particle size of the second silicon-carbon particles is 8.5 μm, the average particle size of the third silicon-carbon particles is 6.7 μm, and the average particle size of the negative electrode active material is 14.8 μm; the mass content of silicon element in the negative electrode active material is 7%.

[0217] Among them, the distance X1 between the side edge of the first tab groove and the side edge of the first glue embedding groove is 1 mm, and the distance X1' between the side edge of the second tab groove and the side edge of the second glue embedding groove is 1 mm; the distance X2 between the bottom edge of the first tab groove and the bottom edge of the first glue embedding groove is 2.5 mm, and the distance X2' between the bottom edge of the second tab groove and the bottom edge of the second glue embedding groove is 2.5 mm; the depth X3 of the first glue embedding groove is 23 μm, and the depth X3' of the second glue embedding groove is 23 μm; the size X4 of the edge of the positive projection of the first glue embedding groove on the negative electrode sheet exceeding the edge of the positive projection of the second glue embedding groove on the negative electrode sheet is 0.8 mm.

[0218] (3) Preparation of the separator:

[0219] Mix the nitrogen-containing particles (40% polyacrylonitrile + 60% melamine cyanurate) and sodium polyacrylate in a mass ratio of 96:6 (C3 is 38.5%), among which the average particle size of the nitrogen-containing particles is 235 nm to obtain the coating paste; mix PVDF, PMMA, CMC, and PVP in a mass ratio of 40:44:15:1 to obtain the glue layer paste.

[0220] The base film (PE, with a thickness of 4.5 μm) is prepared by a wet biaxial stretching process and wound up. The wound base film is unwound and fed into the coating area. The coating slurry is coated on one surface of the base film using an intaglio roll to obtain a coating (with a thickness of 1.6 μm). After drying the moisture in an oven, the adhesive layer slurry is sprayed on the surface of the coating and the other surface of the base film by spraying to obtain an adhesive layer. Finally, the moisture is dried in an oven and wound up to obtain a separator.

[0221] Among them, the tensile strength I of the separator in the length direction is 1983 kgf / cm 2 , the difference L between the width of the separator and the width of the negative electrode sheet is 2.85 mm, the puncture resistance J of the separator is 255 gf, the thermal shrinkage rate K of the separator in the width direction is 0.82%, at this time, I×L is 5651.55, J×L is 726.75, and L / K is 3.48.

[0222] (4) Preparation of the electrolyte:

[0223] EC, PC, EP, PP, and DEC are mixed in a weight ratio of 1:2:1:3:3 to obtain a basic electrolyte, and then FEC and LiPF6 are added to the basic electrolyte to obtain an electrolyte. Based on the total weight of the electrolyte, 3% of PS, 2% of SN, 1.5% of AND, 1.5% of HTCN, 15% of FEC, and 12.5% of LiPF6 are added.

[0224] (5) Preparation of the lithium-ion secondary battery:

[0225] After the above positive electrode sheet and negative electrode sheet are slit and made into sheets, the positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, and a wound core is obtained after winding. Then, through encapsulation, baking, liquid injection, formation, secondary sealing, sorting, and OCV, a lithium-ion secondary battery is obtained. The outer layer of the tab glue is in contact with the outer shell, and the inner layer is in contact with the tab. The inner and outer layers of the tab glue are acid-modified resins, the middle layer is polypropylene, the melting point of the tab glue is 103.9 °C, the melting point of the outer layer is 103.9 °C, and the melting point of the middle layer is 159 °C.

[0226] The side of the separator with the coating is arranged opposite to the positive electrode sheet, and the first positive electrode surface of the positive electrode sheet is close to the winding center of the wound core, and the second positive electrode surface of the positive electrode sheet faces away from the winding center of the wound core.

[0227] At this time, (C1 + C2)×C3 is 295.68.

[0228] Example 2:

[0229] (1) Preparation of the positive electrode sheet:

[0230] Dissolve and disperse 4 parts of CMC in deionized water, then add 80 parts of boehmite (average particle size is 103 nm), 5 parts of conductive carbon black and 5 parts of carbon nanotubes, stir evenly, and then add 6 parts of styrene-butadiene rubber and stir evenly to prepare a bottom coating slurry; coat the bottom coating slurry on both surfaces of the aluminum foil to form a bottom coating (thickness is 1.1 μm) to obtain a bottom-coated aluminum foil.

[0231] Mix the positive electrode active material (20% first lithium cobaltate particles + 80% second lithium cobaltate particles, C1 = 352 ppm, C2 = 328 ppm), conductive carbon black, multi-walled carbon nanotubes and PVDF in a mass ratio of 95:2:1:2, and add NMP, stir evenly to obtain a positive electrode active slurry; coat the positive electrode active slurry on both surfaces of the bottom-coated aluminum foil, and after baking, rolling and then slitting, remove all the positive electrode active layers in a specific area to form a positive electrode tab groove, and weld the positive electrode tab in the positive electrode tab groove, and attach the first tab adhesive tape (SBS rubber adhesive tape); use a special roller with protrusions for processing to obtain a positive electrode sheet with a concave area on one surface (i.e., the second positive electrode surface) and a convex area on the other surface (i.e., the first positive electrode surface).

[0232] Among them, the average particle size of the first lithium cobaltate particles is 3 μm, the average particle size of the second lithium cobaltate particles is 16 μm, the median particle size Dv50 of the positive electrode active material is 13.7 μm, and the particle size Dv99 of the positive electrode active material is 31.7 μm.

[0233] Use a special roller with protrusions for processing to obtain a positive electrode sheet with a concave area on one surface (i.e., the second positive electrode surface) and a convex area on the other surface (i.e., the first positive electrode surface). On the second positive electrode surface, the depth of the concave part is 11 μm, the spacing of the concave parts is 2 mm, and the width of the concave parts is 1 mm; correspondingly, on the first positive electrode surface, the height of the convex part is 11 μm, the spacing of the convex parts is 2 mm, and the width of the convex parts is 1 mm; the area of the concave area accounts for 60% of the area of the positive electrode sheet.

[0234] (2) Preparation of the negative electrode sheet:

[0235] Mix the negative electrode active material (15% silicon-carbon material + 85% artificial graphite), conductive carbon black, carbon nanotubes, and PVDF in a mass ratio of 95:2:1:2, and add deionized water, stir evenly to obtain the negative electrode active paste; coat the negative electrode active paste on both surfaces of the carbon-coated copper foil, after baking, rolling, die-cutting, and cold pressing, through laser processing technology, etch grooves on the outer surface of the negative electrode active layer (the width of the grooves is 21 μm, the depth is 5 μm, and the spacing is 500 μm), then remove all / part of the negative electrode active layer in a specific area, form a first tab groove and a first glue embedding groove on the first negative electrode surface of the negative electrode sheet, form a second tab groove and a second glue embedding groove at the corresponding position on the second negative electrode surface of the negative electrode sheet, weld the negative electrode tab (copper-nickel plated tab) in the first tab groove by ultrasonic welding, the bottom of the first glue embedding groove and the second glue embedding groove is flat against the third tab glue paper, and the thickness of the third tab glue paper < the depth of the first glue embedding groove and the second glue embedding groove.

[0236] Among them, the silicon-carbon material is composed of first silicon-carbon particles, second silicon-carbon particles, and third silicon-carbon particles in a mass ratio of 1:2:1. The sphericity of the first silicon-carbon particles is 0.95, and the average particle size of the first silicon-carbon particles is 4.4 μm; the sphericity of the second silicon-carbon particles is 0.68, and the average particle size of the second silicon-carbon particles is 10 μm; the average particle size of the third silicon-carbon particles is 7.5 μm, and the average particle size of the negative electrode active material is 16.9 μm; the mass content of silicon element in the negative electrode active material is 7%.

[0237] At a certain position on the first negative electrode surface of the negative electrode sheet, there are a first tab groove, a second tab groove, a first glue embedding groove, a second glue embedding groove, and a third glue embedding groove. Weld the negative electrode tab (copper-nickel plated tab) in the first tab groove by ultrasonic welding. The first glue embedding groove and the second glue embedding groove are attached to the third tab glue paper (SBS rubber glue paper), and the second tab glue paper (SBS rubber glue paper) is attached in the third glue embedding groove. The thickness of the third tab glue paper < the depth of the first glue embedding groove and the second glue embedding groove, and the thickness of the second tab glue paper < the depth of the third glue embedding groove.

[0238] Among them, X1 is 0.4 mm, X1' is 0.4 mm; X2 is 3.2 mm, X2' is 3.2 mm; X3 is 12 μm, X3' is 12 μm; X4 is 1.5 mm.

[0239] (3) Preparation of the separator:

[0240] Mix the nitrogen-containing particles (42% polyacrylonitrile + 58% 1,3,5-triazine-2,4,6-triamine) and sodium polyacrylate in a mass ratio of 96:6 (C3 is 48%). Among them, the average particle size of the nitrogen-containing particles is 122 nm to obtain the coating paste; mix PVDF, PMMA, CMC, and PVP in a mass ratio of 40:44:15:1 to obtain the glue layer paste.

[0241] The base film (PE, with a thickness of 5 μm) is prepared by a wet biaxial stretching process and wound up. The wound base film is unwound and fed into the coating area. The coating slurry is coated on one surface of the base film using an intaglio roll to obtain a coating layer (with a thickness of 2 μm). After drying the moisture in an oven, the adhesive layer slurry is sprayed on the surface of the coating layer and the other surface of the base film using a spraying method to obtain an adhesive layer. Finally, it is dried in an oven to remove moisture and wound up to obtain a separator.

[0242] Among them, the tensile strength I of the separator in the length direction is 3198 kgf / cm 2 , the difference L between the width of the separator and the width of the negative electrode sheet is 1.03 mm, the puncture resistance J of the separator is 297 gf, the thermal shrinkage rate K of the separator in the width direction is 0.5%. At this time, I×L is 3293.94, J×L is 305.9, and L / K is 2.06.

[0243] (4) Preparation of the electrolyte:

[0244] The electrolyte is obtained by mixing EC, PC, EP, PP, and DEC in a weight ratio of 1:2:1:3:3 to obtain a basic electrolyte, and then adding FEC and LiPF6 to the basic electrolyte. Based on the total weight of the electrolyte, 3% of PS, 2% of SN, 1.5% of AND, 1.5% of HTCN, 10% of FEC, and 12.5% of LiPF6 are added.

[0245] (4) Preparation of the lithium-ion secondary battery:

[0246] After the above positive electrode sheet and negative electrode sheet are slit and made into pieces, the positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, and a wound core is obtained after winding. Then, through encapsulation, baking, liquid injection, formation, secondary encapsulation, sorting, and OCV, a lithium-ion secondary battery is obtained. The outer layer of the tab glue is in contact with the outer shell, and the inner layer is in contact with the tab. The inner layer and outer layer of the tab glue are acid-modified resins, the middle layer is cross-linked polyethylene, the melting point of the tab glue is 123 °C, the melting point of the outer layer is 123 °C, and the melting point of the middle layer is 164 °C.

[0247] The side of the separator with the coating layer is arranged opposite to the positive electrode sheet, and the first positive surface of the positive electrode sheet is close to the winding center of the wound core, and the second positive surface of the positive electrode sheet faces away from the winding center of the wound core.

[0248] At this time, (C1 + C2)×C3 is 326.4.

[0249] Example 3:

[0250] (1) Preparation of the positive electrode sheet:

[0251] Dissolve and disperse 4 parts of CMC in deionized water, then add 80 parts of boehmite (average particle size is 158 nm), 5 parts of conductive carbon black and 5 parts of carbon nanotubes, stir evenly, and then add 6 parts of PAA and stir evenly to prepare a bottom coating slurry; coat the bottom coating slurry on both surfaces of the aluminum foil to form a bottom coating (thickness is 3 μm) to obtain a bottom-coated aluminum foil.

[0252] Mix the positive electrode active material (20% first lithium cobaltate particles + 80% second lithium cobaltate particles, C1 = 578 ppm, C2 = 223 ppm), conductive carbon black, multi-walled carbon nanotubes and PVDF in a mass ratio of 95:2:1:2, and add NMP, stir evenly to obtain a positive electrode active slurry; coat the positive electrode active slurry on both surfaces of the bottom-coated aluminum foil, perform baking, rolling and then slitting treatments, remove all the positive electrode active layers in a specific area to form a positive electrode tab groove, and weld the positive electrode tab in the positive electrode tab groove, attach the first tab adhesive tape (SBS rubber adhesive tape); use a special roller with protrusions for processing to obtain a positive electrode sheet with a concave area on one surface (i.e., the second positive electrode surface) and a convex area on the other surface (i.e., the first positive electrode surface).

[0253] Among them, the average particle size of the first lithium cobaltate particles is 5.5 μm, the average particle size of the second lithium cobaltate particles is 23 μm, the median particle size Dv50 of the positive electrode active material is 18.3 μm, and the particle size Dv99 of the positive electrode active material is 37.4 μm.

[0254] After the positive electrode sheet is embossed, on the second positive electrode surface, the depth of the concave part is 30 μm, the spacing of the concave parts is 6 mm, and the width of the concave parts is 5 mm; correspondingly, on the first positive electrode surface, the height of the convex part is 30 μm, the spacing of the convex parts is 6 mm, and the width of the convex parts is 5 mm; the area of the concave area accounts for 70% of the area of the positive electrode sheet.

[0255] (2) Preparation of the negative electrode sheet:

[0256] Mix the negative electrode active material (15% silicon-carbon material + 85% artificial graphite), conductive carbon black, carbon nanotubes, and PVDF in a mass ratio of 95:2:1:2, and add deionized water, stir evenly to obtain the negative electrode active paste; coat the negative electrode active paste on both surfaces of the carbon-coated copper foil, after baking, rolling, die-cutting, and cold pressing, through laser processing technology, etch grooves (the width of the grooves is 140 μm, the depth is 47 μm, and the spacing is 2000 μm) on the outer surface of the negative electrode active layer, and then remove all / part of the negative electrode active layer in a specific area to form a first tab groove, a second tab groove, a first glue embedding groove, a second glue embedding groove, and a third glue embedding groove on the first negative electrode surface of the negative electrode sheet. Weld the negative electrode tab (copper-nickel plated tab) in the first tab groove by ultrasonic welding. Stick the third tab adhesive paper (SBS rubber adhesive paper) to the first glue embedding groove and the second glue embedding groove, and stick the second tab adhesive paper (SBS rubber adhesive paper) in the third glue embedding groove. The thickness of the third tab adhesive paper < the depth of the first glue embedding groove and the second glue embedding groove, and the thickness of the second tab adhesive paper < the depth of the third glue embedding groove.

[0257] Among them, the silicon-carbon material is composed of the first silicon-carbon particle, the second silicon-carbon particle, and the third silicon-carbon particle in a mass ratio of 1.3:6:2.7. The sphericity of the first silicon-carbon particle is 0.9, and the average particle size of the first silicon-carbon particle is 2.5 μm; the sphericity of the second silicon-carbon particle is 0.5, and the average particle size of the second silicon-carbon particle is 7.2 μm; the average particle size of the third silicon-carbon particle is 5.1 μm, and the average particle size of the negative electrode active material is 12 μm; the mass content of silicon element in the negative electrode active material is 7%.

[0258] At a certain position on the first negative electrode surface of the negative electrode sheet, a first tab groove and a first glue embedding groove are provided. At the corresponding position on the second negative electrode surface of the negative electrode sheet, a second tab groove and a second glue embedding groove are provided. Weld the negative electrode tab (copper-nickel plated tab) in the first tab groove by ultrasonic welding. The bottom of the first glue embedding groove and the second glue embedding groove is flat against the tab adhesive paper, and the thickness of the tab adhesive paper < the depth of the first glue embedding groove and the second glue embedding groove.

[0259] Among them, X1 is 1.5 mm, X1' is 1.5 mm; X2 is 1.2 mm, X2' is 1.2 mm; X3 is 29 μm, X3' is 29 μm; X4 is 0.3 mm.

[0260] The inner layer and the outer layer of the tab glue are acid-modified resins, the middle layer is polypropylene, the melting point of the tab glue is 92 °C, the melting point of the outer layer is 92 °C, and the melting point of the middle layer is 140 °C.

[0261] (3) Preparation of the separator:

[0262] Mix nitrogen-containing particles (79% polyacrylonitrile + 21% melamine cyanurate) and sodium polyacrylate in a mass ratio of 96:6 (C3 is 30%), where the average particle size of the nitrogen-containing particles is 330 nm, to obtain a coating slurry; mix PVDF, PMMA, CMC, and PVP in a mass ratio of 40:44:15:1 to obtain an adhesive layer slurry.

[0263] Prepare and wind up a base film (PE, with a thickness of 4 μm) through a wet biaxial stretching process. Unwind the wound base film and feed it into the coating area. Use an intaglio roll to coat the coating slurry on one surface of the base film to obtain a coating (with a thickness of 0.5 μm). Dry the moisture in an oven, and then spray the adhesive layer slurry on the surface of the coating and the other surface of the base film by spraying to obtain an adhesive layer. Finally, dry the moisture in an oven and wind up to obtain a separator.

[0264] Among them, the tensile strength I of the separator in the length direction is 1602 kgf / cm 2 , the difference L between the width of the separator and the width of the negative electrode sheet is 3.92 mm, the puncture resistance J of the separator is 200 gf, the thermal shrinkage rate K of the separator in the width direction is 2.97%, and at this time, I×L is 6279.84, J×L is 784, and L / K is 1.32.

[0265] (4) Preparation of electrolyte:

[0266] Mix EC, PC, EP, PP, and DEC in a weight ratio of 1:2:1:3:3 to obtain a basic electrolyte, and then add FEC and LiPF6 to the basic electrolyte to obtain an electrolyte. Based on the total weight of the electrolyte, add 3% of PS, 2% of SN, 1.5% of AND, 1.5% of HTCN, 18% of FEC, and 12.5% of LiPF6.

[0267] (5) Preparation of lithium-ion secondary battery:

[0268] After slitting and making the above positive electrode sheet and negative electrode sheet, stack the positive electrode sheet, separator, and negative electrode sheet in sequence, wind them up to obtain a wound core, and then obtain a lithium-ion secondary battery through encapsulation, baking, liquid injection, formation, secondary encapsulation, sorting, and OCV. The outer layer of the tab glue is in contact with the outer shell, and the inner layer is in contact with the tab. The inner and outer layers of the tab glue are acid-modified resins, and the middle layer is cross-linked polyethylene. The melting point of the tab glue is 92 °C, the melting point of the outer layer is 92 °C, and the melting point of the middle layer is 140 °C.

[0269] The side of the separator with the coating is arranged opposite to the positive electrode sheet, and the first positive surface of the positive electrode sheet is close to the winding center of the wound core, and the second positive surface of the positive electrode sheet faces away from the winding center of the wound core.

[0270] At this time, (C1 + C2)×C3 is 240.3.

[0271] Example 4:

[0272] Based on Example 1, the difference is that the sphericity of the first silicon-carbon particle is 0.81.

[0273] Example 5:

[0274] Based on Example 1, the difference is that the sphericity of the second silicon-carbon particle is 0.78.

[0275] Example 6 group:

[0276] This group of examples is used to verify the influence brought by the change of "the average particle size of the third silicon-carbon particle", as follows:

[0277] Example 6a, based on Example 3, the difference is that the average particle size of the third silicon-carbon particle is 3.2 μm. Since the average particle size of the third silicon-carbon particle is larger than that of the first silicon-carbon particle, this example refers to Example 3.

[0278] Example 6b, based on Example 1, the difference is that the average particle size of the third silicon-carbon particle is 7.9 μm.

[0279] In this group of examples, the average particle size of the artificial graphite in the negative electrode active material is changed to keep the average particle size of the negative electrode active material unchanged.

[0280] Example 7 group:

[0281] This group of examples is used to verify the influence brought by the change of "the average particle size of the second silicon-carbon particle", as follows:

[0282] Example 7a, based on Example 3, the difference is that the average particle size of the second silicon-carbon particle is 6.1 μm. Since the average particle size of the third silicon-carbon particle is smaller than that of the second silicon-carbon particle, this example refers to Example 3.

[0283] Example 7b, based on Example 1, the difference is that the average particle size of the second silicon-carbon particle is 14.7 μm.

[0284] In this group of examples, the average particle size of the artificial graphite in the negative electrode active material is changed to keep the average particle size of the negative electrode active material unchanged.

[0285] Example 8 group:

[0286] This group of examples is used to verify the influence brought by the change of "C1", which is achieved by changing the content of yttrium element in lithium cobaltate, as follows:

[0287] Example 8a, based on Example 1, is different in that C1 = 152 ppm, and at this time, (C1 + C2) × C3 is 172.

[0288] Example 8b, based on Example 1, is different in that C1 = 697 ppm, and at this time, (C1 + C2) × C3 is 381.92.

[0289] Group of Example 9:

[0290] This group of examples is used to verify the influence brought by the change of "C2", which is achieved by changing the content of lanthanum element in lithium cobalt oxide, specifically as follows:

[0291] Example 9a, based on Example 1, is different in that C2 = 105 ppm, and at this time, (C1 + C2) × C3 is 222.5.

[0292] Example 9b, based on Example 1, is different in that C2 = 499 ppm, and at this time, (C1 + C2) × C3 is 374.22.

[0293] Group of Example 10:

[0294] This group of examples is used to verify the influence brought by the change of "C3", which is achieved by changing the content of nitrogen-containing particles in the separator coating, specifically as follows:

[0295] Example 10a, based on Example 1, is different in that the nitrogen-containing particles (polyacrylonitrile), SBR and sodium polyacrylate are mixed to prepare a coating slurry according to a mass ratio of 39:10:51, C3 = 10.3%, and at this time, (C1 + C2) × C3 is 79.1.

[0296] Example 10b, based on Example 1, is different in that the nitrogen-containing particles are composed of 39% polyacrylonitrile + 61% 1,3,5-triazine-2,4,6-triamine, C3 = 49.8%, and at this time, (C1 + C2) × C3 is 382.46.

[0297] Group of Example 11:

[0298] This group of examples is used to verify the influence brought by the change of "C1 × C2 × C3", which is achieved by regulating the content C1 of yttrium element, the content C2 of lanthanum element and the content C3 of nitrogen element in the coating, specifically as follows:

[0299] Example 11a, based on Example 1, is different in that C1 is 352 ppm, C2 is 223 ppm, C3 is 30%, and the nitrogen-containing particles are composed of 79% polyacrylonitrile + 21% melamine cyanurate, and at this time, (C1 + C2) × C3 is 172.5.

[0300] Example 12a, based on Example 1, is different in that C1 is 578 ppm, C2 is 328 ppm, C3 is 38.5%, and the nitrogen-containing particles are composed of 40% polyacrylonitrile + 60% melamine cyanurate. At this time, (C1 + C2) × C3 is 348.8.

[0301] Group of Example 12:

[0302] This group of examples is used to verify the influence brought by the change of "average particle size of the first silicon-carbon particles", specifically as follows:

[0303] Example 12a, based on Example 1, is different in that the average particle size of the first silicon-carbon particles is 1.2 μm.

[0304] Example 12b, based on Example 1, is different in that the average particle size of the first silicon-carbon particles is 6 μm.

[0305] Group of Example 13:

[0306] This group of examples is used to verify the influence brought by the change of "content of silicon element in the negative electrode active material", which is achieved by changing the proportion of silicon-carbon material in the negative electrode active material, specifically as follows:

[0307] Example 13a, based on Example 1, is different in that the mass content of silicon element in the negative electrode active material is 4.1%. At this time, the negative electrode active material is composed of 8.6% silicon-carbon material and 91.4% artificial graphite.

[0308] Example 13b, based on Example 1, is different in that the mass content of silicon element in the negative electrode active material is 24.7%. At this time, the negative electrode active material is composed of 50% silicon-carbon material and 50% artificial graphite.

[0309] In this group of examples, the average particle size of the artificial graphite in the negative electrode active material is changed to keep the average particle size of the negative electrode active material unchanged.

[0310] Example 14:

[0311] Based on Example 1, it is different in that the thickness of the coating on the separator is 4.8 μm.

[0312] Group of Example 15:

[0313] This group of examples is used to verify the influence brought by the change of "average particle size of the nitrogen-containing particles", specifically as follows:

[0314] Example 15a, based on Example 1, is different in that the average particle size of the nitrogen-containing particles is 104 nm.

[0315] Example 15b, based on Example 1, is different in that the average particle size of the nitrogen-containing particles is 780 nm.

[0316] Group of Example 16:

[0317] This group of examples is used to verify the influence brought by the change of "I", which is achieved by changing the tensile strength of the separator in the length direction, specifically as follows:

[0318] Example 16a, based on Example 1, is different in that the tensile strength I of the separator in the length direction is 1012 kgf / cm 2 , and at this time, I×L is 2884.2.

[0319] Example 16b, based on Example 1, is different in that the tensile strength I of the separator in the length direction is 3996 kgf / cm 2 , and at this time, I×L is 11388.6.

[0320] Group of Example 17:

[0321] This group of examples is used to verify the influence brought by the change of "I×L", which is achieved by changing the tensile strength of the separator in the length direction and the difference between the width of the separator and the width of the negative electrode sheet, specifically as follows:

[0322] Example 17a, based on Example 1, is different in that the tensile strength I of the separator in the length direction is 3198 kgf / cm 2 , the difference L between the width of the separator and the width of the negative electrode sheet is 2.85 mm, and at this time, I×L is 9114.3.

[0323] Example 17b, based on Example 1, is different in that the tensile strength I of the separator in the length direction is 1602 kgf / cm 2 , the difference L between the width of the separator and the width of the negative electrode sheet is 1.03 mm, and at this time, I×L is 1650.

[0324] Group of Example 18:

[0325] This group of examples is used to verify the influence brought by the change of "J", which is achieved by changing the puncture resistance of the separator, specifically as follows:

[0326] Example 18a, based on Example 1, is different in that the puncture resistance J of the separator is 102 gf, and at this time, J×L is 290.7.

[0327] Example 18b, based on Example 1, is different in that the puncture resistance J of the separator is 396 gf, and at this time, J×L is 1128.6.

[0328] Group of Example 19:

[0329] This group of embodiments is used to verify the influence brought about by the change of "J×L", which is achieved by changing the puncture resistance of the separator and the difference between the width of the separator and the width of the negative electrode sheet, specifically as follows:

[0330] Example 19a, based on Example 1, is different in that the puncture resistance J of the separator is 297 gf, and the difference L between the width of the separator and the width of the negative electrode sheet is 3.92 mm. At this time, J×L is 1164.2.

[0331] Example 19b, based on Example 1, is different in that the puncture resistance J of the separator is 200 gf, and the difference L between the width of the separator and the width of the negative electrode sheet is 1.03 mm. At this time, J×L is 206.

[0332] Example 20:

[0333] Based on Example 1, it is different in that the thermal shrinkage rate K of the separator in the width direction is 5%, and the difference L between the width of the separator and the width of the negative electrode sheet is 1.03 mm. At this time, L / K is 0.206.

[0334] Example 21:

[0335] Based on Example 1, it is different in that the side of the separator with the coating is arranged away from the positive electrode sheet.

[0336] Example 22 group:

[0337] This group of embodiments is used to verify the influence brought about by the change of "the median particle size Dv50 of the positive electrode active material", which is achieved by changing the average particle sizes of the first lithium cobaltate particles and the second lithium cobaltate particles, specifically as follows:

[0338] Example 22a, based on Example 1, is different in that the median particle size Dv50 of the positive electrode active material is 10.3 μm. At this time, the average particle size of the first lithium cobaltate particles is 2.3 μm, the average particle size of the second lithium cobaltate particles is 15.1 μm, and the particle size Dv99 of the positive electrode active material is 30.2 μm.

[0339] Example 22b, based on Example 1, is different in that the median particle size Dv50 of the positive electrode active material is 19.7 μm. At this time, the average particle size of the first lithium cobaltate particles is 9.5 μm, the average particle size of the second lithium cobaltate particles is 24.3 μm, and the particle size Dv99 of the positive electrode active material is 49.8 μm.

[0340] Example 23 group:

[0341] This group of embodiments is used to verify the influence brought about by the change of "the average particle size of the inorganic particles in the bottom coating and the thickness of the bottom coating", specifically as follows:

[0342] Example 23a, based on Example 1, is different in that the average particle size of boehmite is 23 nm and the thickness of the bottom coating is 0.5 μm.

[0343] Example 23b, based on Example 1, is different in that the average particle size of boehmite is 658 nm and the thickness of the bottom coating is 4.7 μm.

[0344] Example 24:

[0345] Based on Example 1, it is different in that the positive electrode sheet is not embossed. At this time, the depth of the concave part, the spacing of the concave part, the width of the concave part, the height of the convex part, the spacing of the convex part, the width of the convex part, and the area of the concave part area accounting for the area of the positive electrode sheet do not exist.

[0346] Example 25 group:

[0347] This group of examples is used to verify the influence brought by the change of "the area of the concave part area accounting for the area of the positive electrode sheet", which is achieved by changing the embossing area, specifically as follows:

[0348] Example 25a, based on Example 1, is different in that the area of the concave part area accounts for 40% of the area of the positive electrode sheet.

[0349] Example 25b, based on Example 1, is different in that the area of the concave part area accounts for 90% of the area of the positive electrode sheet.

[0350] Example 26:

[0351] Based on Example 1, it is different in that no grooves are provided on both sides of the negative electrode sheet.

[0352] Example 27:

[0353] Based on Example 1, it is different in that the first buried glue groove and the second buried glue groove are not provided.

[0354] Example 28 group:

[0355] This group of examples is used to verify the influence brought by the change of "(C1 + C2) × C3", which is achieved by regulating the content C1 of yttrium element, the content C2 of lanthanum element and the content C3 of nitrogen element in the coating, specifically as follows:

[0356] Example 28a, based on Example 1, is different in that C1 is 473 ppm, C2 is 399 ppm, C3 is 49.8%, and the nitrogen-containing particles are composed of 39% polyacrylonitrile + 61% 1,3,5-triazine-2,4,6-triamine. At this time, (C1 + C2) × C3 = 434.26.

[0357] Example 28b, based on Example 1, which is different in that C1 is 352 ppm, C2 is 295 ppm, C3 is 10.3%, and the nitrogen-containing particles (polyacrylonitrile), SBR, and sodium polyacrylate are mixed in a mass ratio of 39:10:51 to prepare a coating slurry. At this time, (C1 + C2) × C3 = 66.64.

[0358] Comparative Example 1:

[0359] Based on Example 1, which is different in that there is only the first silicon-carbon particle in the silicon-carbon material. At this time, the average particle size of the artificial graphite in the negative electrode active material is adjusted to make the average particle size of the negative electrode active material the same as that in Example 1.

[0360] Comparative Example 2:

[0361] Based on Example 1, which is different in that there is only the second silicon-carbon particle in the silicon-carbon material. At this time, the average particle size of the artificial graphite in the negative electrode active material is adjusted to make the average particle size of the negative electrode active material the same as that in Example 1.

[0362] Comparative Example 3:

[0363] Based on Example 1, which is different in that C1 is 860 ppm.

[0364] Comparative Example 4:

[0365] Based on Example 1, which is different in that C2 is 735 ppm.

[0366] Comparative Example 5:

[0367] Based on Example 1, which is different in that C3 is 58.2%, and the nitrogen-containing particles are composed of 15% polyacrylonitrile + 85% 1,3,5-triazine-2,4,6-triamine.

[0368] Comparative Example 6:

[0369] Based on Example 1, which is different in that the content of yttrium element C1 = 0 ppm and the content of lanthanum element C2 = 0 ppm.

[0370] Comparative Example 7:

[0371] Based on Example 1, which is different in that the separator is not provided with a coating. At this time, the content of nitrogen element C3 in the coating = 0.

[0372] Note: In the above examples and comparative examples, "%" represents mass content. For example, "20% first lithium cobaltate particles + 80% second lithium cobaltate particles" means that based on the total weight of lithium cobaltate, the content of the first lithium cobaltate particles is 20% and the content of the second lithium cobaltate particles is 80%.

[0373] Test Example:

[0374] 1. Energy density test:

[0375] Battery Energy Testing: Batteries prepared in the Examples and Comparative Examples were tested using a Xinwei battery tester. The battery cycle was repeated three times: 0.2C charging to an upper voltage limit of 4.53V (cutoff at 0.02C) and 0.2C discharge to a lower voltage limit. The third discharge energy was taken as the battery energy Q. The width W and height H of the battery were measured using a 2.5D microscope, and the thickness L of the battery at full charge was measured using a PPG thickness tester. The energy density is then calculated as Q / (W × H × L).

[0376] 2. 45°C Cycling Expansion: Batteries prepared in the Examples and Comparative Examples were placed at 45°C for 1 hour. The pre-cycling batteries were fully charged and the pre-cycling battery thickness (Z1) was measured. After 500 cycles, the batteries were fully charged and the battery thickness (Z2) was measured. Expansion = (Z2 - Z1) / Z1. Cycling conditions: 0°C, 1C constant current charge with a cutoff voltage of 4.53V, constant voltage charge with a cutoff current of 0.2C, and a constant current discharge of 0.7C with a cutoff voltage of 3V.

[0377] 3. 45℃ cycle capacity retention rate:

[0378] The batteries prepared in the examples of the present invention and the comparative examples were placed at 45°C for 1 hour, charged at 1C to a cut-off voltage of 4.53V, charged at a constant voltage to a cut-off current of 0.05C, and discharged at 1C to 3V. The above charge and discharge steps were repeated 500 times. The capacity retention rate was calculated by dividing the 500th discharge capacity by the maximum value of the first three discharge capacities.

[0379] 4. 132℃ full electric furnace temperature:

[0380] Batteries prepared in the Examples and Comparative Examples of the present invention were fully charged and placed in a thermostat. The temperature was raised from room temperature to 132°C at a rate of 5°C / min. After reaching 132°C, the temperature was maintained for 1 hour. The batteries were observed for ignition and explosion. Three batteries were tested for each Example and Comparative Example. Those that did not ignite or explode were considered to have passed. The number of passing batteries was recorded as n / 3.

[0381] 5. Acupuncture experiment:

[0382] Batteries prepared in the examples and comparative examples of the present invention were fully charged. A 15 mm long, 4 mm diameter steel needle was used to pierce the center of the battery at a speed of 30 mm / s to observe whether fire occurred. Five batteries were tested for each example and comparative example. Batteries that did not emit smoke, fire, or explosion were considered to have passed. The number of passing batteries was recorded as n / 5.

[0383] The above test results are recorded in Table 1.

[0384] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including the combination of each technical feature in any other suitable manner. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

[0385] Table 1:

[0386]

[0387]

[0388] As can be seen from Table 1, the lithium-ion secondary battery prepared by the present invention has good energy density, improved cycle expansion, and both good high-temperature safety performance and high-temperature cycle performance compared with the comparative example.

[0389] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including the combination of each technical feature in any other suitable manner. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A lithium-ion secondary battery, characterized in that, The lithium-ion secondary battery includes an electrode assembly. The electrode assembly includes a stacked positive electrode sheet, a separator, and a negative electrode sheet. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on at least one surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material. The negative electrode active material includes a silicon-carbon material. The silicon-carbon material includes first silicon-carbon particles and second silicon-carbon particles. The sphericity of the first silicon-carbon particles is 0.8 - 0.

99. The average particle size of the first silicon-carbon particles is 1 μm - 6 μm. The sphericity of the first silicon-carbon particles > the sphericity of the second silicon-carbon particles. The average particle size of the second silicon-carbon particles is 5 μm - 15 μm; The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer located on at least one surface of the positive electrode current collector. The positive electrode active layer includes a positive electrode active material. The positive electrode active material includes lithium cobaltate. The lithium cobaltate includes yttrium element and lanthanum element. Based on the total mass of the lithium cobaltate, the content of the yttrium element is C1, 150 ≤ C1 ≤ 700, in ppm. The content of the lanthanum element is C2, 100 ≤ C2 ≤ 500, in ppm; The separator includes a base film and a coating located on at least one surface of the base film. The coating includes nitrogen element. Based on the total weight of the coating, the content of the nitrogen element is C3, 10% ≤ C3 ≤ 50%. The coating faces the positive electrode sheet.

2. The lithium ion secondary battery according to claim 1, wherein, The silicon-carbon material further includes third silicon-carbon particles. The third silicon-carbon particles are mainly formed by a plurality of the first silicon-carbon particles. The average particle size of the third silicon-carbon particles is 3 μm - 10 μm, preferably 3 μm - 8 μm; and / or, the sphericity of the first silicon-carbon particles is 0.9 - 0.99; and / or, the sphericity of the second silicon-carbon particles is 0.5 - 0.8; and / or, 300 ≤ C1 ≤ 580, in ppm; and / or, 200 ≤ C2 ≤ 330, in ppm; and / or, 30% ≤ C3 ≤ 48%; Preferably, C1, C2, and C3 satisfy 75 ≤ (C1 + C2) × C3 ≤ 400; More preferably, 240 ≤ (C1 + C2) × C3 ≤ 330.

3. The lithium ion secondary battery according to claim 1, wherein, The separator further includes an adhesive layer. The coating is located on at least one surface of the base film opposite in the thickness direction. The adhesive layer is located on at least one outer surface of the separator; and / or, the coating includes nitrogen-containing particles. The nitrogen-containing particles include at least one of polyacrylonitrile, nitrile rubber, and melamine and its derivatives; Preferably, the melamine and its derivatives include at least one of 1,3,5-triazine-2,4,6-triamine, melamine cyanurate, and melamine trithiocyanate and its derivatives; and / or, the thickness of the coating is 0.5 μm - 5 μm; preferably 0.5 μm - 2 μm; and / or, the thickness of the base film is 3 μm - 10 μm; Preferably, the average particle size of the nitrogen-containing particles is 100 nm - 1000 nm; more preferably 120 nm - 600 nm.

4. The lithium ion secondary battery according to claim 1, wherein, The tensile strength of the diaphragm in the length direction is I, where 1000 ≤ I ≤ 4000, and the unit is kgf / cm 2 ; preferably 1600 ≤ I ≤ 3200, and the unit is kgf / cm 2 ; And / or, the puncture resistance of the separator is J, 100 ≤ J ≤ 400, in gf; preferably 200 ≤ J ≤ 300, in gf; And / or, the thermal shrinkage rate of the separator in the width direction is K, K ≤ 3, in %; And / or, the difference between the width of the separator and the width of the negative electrode sheet is L, 1 ≤ L ≤ 4, in mm; Preferably, 2800 ≤ I × L ≤ 11400; more preferably 3200 ≤ I × L ≤ 6300; Preferably, 290 ≤ J × L ≤ 1130; more preferably 300 ≤ J × L ≤ 800; Preferably, 0.5 ≤ L / K ≤ 3.

5.

5. The lithium ion secondary battery according to claim 1, wherein, The lithium cobalt oxide includes first lithium cobalt oxide particles and second lithium cobalt oxide particles, and the average particle size of the first lithium cobalt oxide particles is 2 μm - 10 μm; And / or, the average particle size of the second lithium cobalt oxide particles is 15 μm - 28 μm; And / or, the median particle size Dv50 of the positive electrode active material is 10 μm - 25 μm; And / or, the particle size Dv99 of the positive electrode active material is 30 μm - 50 μm.

6. The lithium ion secondary battery according to claim 1, wherein, The average particle size of the negative electrode active material is 6 μm - 25 μm; And / or, the mass content of silicon element in the negative electrode active material is 4% - 25%; And / or, the silicon-carbon material includes a porous carbon matrix and nano-silicon located in the pores of the porous carbon matrix; And / or, the negative electrode active material further includes a carbon-based material, and the carbon-based material includes at least one of artificial graphite, natural graphite, mesocarbon microbeads, soft carbon, and hard carbon.

7. The lithium ion secondary battery according to claim 1, wherein, The lithium ion secondary battery further includes a tab and tab glue, and the melting point of the tab glue is 80°C - 180°C; Preferably, the tab glue includes an outer layer, an intermediate layer, and an inner layer, the lithium ion secondary battery further includes a housing, the outer layer is in contact with the housing, and the inner layer is in contact with the tab; the intermediate layer is located between the outer layer and the inner layer; More preferably, the melting points of the outer layer and the inner layer are each independently 80°C - 140°C; More preferably, the melting point of the intermediate layer is 130°C - 180°C.

8. The lithium ion secondary battery according to claim 1, wherein, The positive electrode sheet further includes a bottom coating, the bottom coating is located between the positive electrode active layer and the positive electrode current collector, and the bottom coating includes inorganic particles; Preferably, the average particle size of the inorganic particles is 20 nm - 1000 nm; Preferably, the thickness of the bottom coating is 0.5 μm - 5 μm, more preferably 1 μm - 3 μm.

9. The lithium ion secondary battery according to claim 1, wherein, The negative electrode active layer includes a plurality of grooves; the width of the grooves is 20 μm - 150 μm, the depth of the grooves is 5 μm - 50 μm, and the spacing between the grooves is 500 μm - 2000 μm; And / or, the positive electrode sheet has a first positive electrode surface and a second positive electrode surface along the thickness direction of the positive electrode current collector, the first positive electrode surface includes a convex portion area, and the convex portion area includes a plurality of convex portions; the second positive electrode surface includes a concave portion area, and the concave portion area has a plurality of concave portions; Preferably, the depth of the concave portion is 10 μm - 30 μm, the spacing of the concave portions is 2 mm - 6 mm, the width of the concave portion is 1 mm - 5 mm, and the area of the projection of the concave portion region on the second positive electrode surface accounts for 40% - 90% of the area of the positive electrode active layer located on the second positive electrode surface in the projection on the second positive electrode surface.

10. The lithium ion secondary battery according to claim 1, wherein, The negative electrode sheet has a first negative electrode surface and a second negative electrode surface oppositely arranged along the thickness direction of the negative electrode current collector. The negative electrode sheet further includes a first tab slot and a first glue embedding slot located on the first negative electrode surface, and a second tab slot and a second glue embedding slot located on the second negative electrode surface. The first tab slot and the first glue embedding slot are connected, and the second tab slot and the second glue embedding slot are connected; And / or, the positive electrode sheet further includes a positive electrode tab slot formed by the absence of the positive electrode active layer, and a positive electrode tab disposed in the positive electrode tab slot. The negative electrode sheet further includes a third glue embedding slot formed by the absence of the negative electrode active layer. Along the thickness direction of the electrode assembly, the positive electrode tab slot and the third glue embedding slot are arranged facing each other; Preferably, a first tab adhesive tape is provided on the surface of the positive electrode tab; at least a part of the third glue embedding slot is provided with a second tab adhesive tape.

11. The lithium ion secondary battery according to claim 1, wherein, The cut-off voltage of the lithium ion secondary battery is ≥ 4.48V.

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