Negative plate and lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a negative plate and a lithium ion secondary battery comprising the negative plate. The negative plate comprises a negative current collector and a negative active coating located on at least one side surface of the negative current collector, the negative active coating comprises a negative active material, the negative active material comprises a silicon-carbon material, and the silicon-carbon material comprises first silicon-carbon particles and second silicon-carbon particles; the sphericity degree of the first silicon carbon particles is A1, the sphericity degree of the second silicon carbon particles is A2, and 0.3 < = A2lt; 0.8, A1 < gt >; a2; the OI value B of the negative plate is 9-25; a1, A2 and B satisfy 8.5 < = B / A1 < = 30, and 4 < = B * A2 < = 15; and the tensile strength sigma of the negative current collector in the width direction is greater than or equal to 600MPa. The lithium ion secondary battery disclosed by the invention has relatively high energy density and cycling stability, and the self-discharge phenomenon can be remarkably improved.

Description

Technical Field

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

[0002] High-energy density batteries usually add silicon-containing materials to the negative electrode active material to improve the specific capacity of the negative electrode. However, the blending of a large amount of silicon-containing materials in the negative electrode will bring some negative effects. For example, the expansion of the silicon-containing materials causes poor adhesion of the negative electrode active material, resulting in powder falling off, and the breakage of the positive electrode sheet, thereby causing the attenuation of the battery cycle capacity and the occurrence of self-discharge. In addition, for silicon-carbon materials with a low sphericity (for example, the sphericity is less than 0.8), the edges and corners will squeeze and even pierce the separator during the expansion process, causing micro-short circuits and exacerbating self-discharge.

[0003] Therefore, it is very important to improve the problems of attenuation of the cycle capacity and exacerbation of self-discharge of silicon-containing batteries. Summary of the Invention

[0004] The purpose of the present invention is to overcome the problems of attenuation of the cycle capacity and exacerbation of self-discharge of silicon-containing batteries existing in the prior art, and provide a negative electrode sheet and a lithium-ion secondary battery including the negative electrode sheet. The negative electrode sheet of the present invention uses silicon-carbon particles with different sphericities in combination, and regulates the relationship between the OI value of the negative electrode sheet and the sphericities of different silicon-carbon particles, and the tensile strength of the negative electrode current collector in the width direction, so as to improve the expansion caused by the blending of silicon-carbon materials in the battery negative electrode sheet, thereby improving the problems of powder falling off of the negative electrode active coating and breakage of the positive electrode sheet caused by the expansion; and improving the exacerbation of self-discharge caused by silicon-carbon materials with a low sphericity. The lithium-ion secondary battery (hereinafter simply referred to as the battery) of the present invention has both a high energy density and cycle stability, and can significantly improve the phenomenon of self-discharge.

[0005] In the related art, using silicon-carbon particles with low sphericity alone as silicon-carbon materials will exacerbate the occurrence of battery self-discharge. Through research, it is found that if silicon-carbon particles with high sphericity are used alone as silicon-carbon materials, although it can improve the occurrence of battery self-discharge, it is not conducive to the improvement of battery energy density and will also affect the cycle stability of the battery. Using silicon-carbon materials with high sphericity (for example, greater than or equal to 0.8) and low sphericity (for example, less than 0.8) in combination can reduce the volume expansion during the charging process of the battery. The reasons are as follows: First, the combination of the two can optimize the packing density. For the second silicon-carbon particles with lower sphericity, their edges and corners can be better embedded in the carbon-based material (such as graphite) to form the skeleton of the silicon-carbon material, preventing the silicon-carbon particles from being demolded during the expansion and contraction process; the first silicon-carbon particles with higher sphericity can fill the gaps between the second silicon-carbon particles and the carbon-based material, improving the packing tightness. Their higher sphericity makes the expansion direction more evenly distributed, reducing the expansion in a single direction. Second, the combination of the two can regulate the structure of the SEI (Solid Electrolyte Interphase) film and improve the diffusion kinetics of lithium ions on the negative electrode sheet. Generally, a thinner and denser SEI film is formed on the surface of the first silicon-carbon particles with higher sphericity; while a thicker and porous SEI film is generally formed on the surface of the second silicon-carbon particles with lower sphericity. Although the thickness of the porous SEI film is thicker, its porous structure can not only provide a certain buffer space for the volume expansion of the silicon-carbon material, but also be conducive to the transmission of lithium ions, reducing the risk of lithium dendrite formation. Therefore, combining the two is beneficial to suppressing the volume expansion of the silicon-carbon material and reducing the risk of self-discharge occurrence.

[0006] In addition, it is necessary to coordinately regulate the relationship between the OI value of the negative electrode sheet and the sphericity of the first silicon-carbon particles and the second silicon-carbon particles respectively. The reason is that: the larger the OI value of the negative electrode sheet, the more the expansion direction of the negative electrode sheet approaches the thickness direction of the battery. If the volume expansion of the silicon-carbon particles is larger at this time, it will exacerbate the volume expansion degree of the battery in the thickness direction. Within a certain range, when the sphericity of the first silicon-carbon particles is larger, its expansion direction is in all directions and the expansion is more uniform, which can reduce the expansion of the negative electrode sheet along the thickness direction. At this time, the OI value of the negative electrode sheet can be appropriately increased; when the sphericity of the second silicon-carbon particles is smaller, its expansion is more uneven and will expand in a specific direction. Therefore, only some of the second silicon-carbon particles expand in the thickness direction of the battery, reducing the number of particles expanding in the thickness direction. At this time, the OI value of the negative electrode sheet can be appropriately increased. Therefore, coordinately regulating the relationship between the OI value of the negative electrode sheet and the sphericity of the first silicon-carbon particles and the second silicon-carbon particles respectively can further reduce the volume expansion of the battery.

[0007] However, the inventors of the present invention have found that the improvement of the volume expansion of the negative electrode sheet by the above means alone is limited, and the problem of the positive electrode sheet breaking may still occur. After a large number of targeted studies, the inventors of the present invention further regulated the tensile strength of the negative electrode current collector in the width direction, and found that on the basis of the above regulation means, by regulating the tensile strength of the negative electrode current collector in the width direction, the problem of the positive electrode sheet breaking can be effectively improved. The reason is as follows: The negative electrode current collector is the support structure of the negative electrode sheet. The negative electrode current collector with high tensile strength can better resist the expansion force of the negative electrode active material, reduce the overall expansion amplitude of the negative electrode sheet, and reduce the internal stress; moreover, the negative electrode current collector with high tensile strength helps to more evenly distribute the stress generated by the expansion of the negative electrode active material, avoiding local damage of the positive electrode sheet caused by stress concentration; in addition, the interfacial bonding strength between the negative electrode current collector and the negative electrode active material also affects the limiting effect on the volume expansion of the negative electrode sheet. The negative electrode current collector with high tensile strength usually has better interfacial bonding performance and can more effectively transmit mechanical binding force. Therefore, by further regulating the tensile strength of the negative electrode current collector in the width direction, the mechanical support effect on the negative electrode sheet can be enhanced, the problem of uneven internal stress of the negative electrode sheet can be improved, and the overall structural stability of the negative electrode sheet can be improved, thereby limiting the volume expansion of the negative electrode sheet, and thus improving the problem of the positive electrode sheet breaking caused by the volume expansion of the negative electrode sheet. Based on this, the inventors of the present invention have proposed the following solutions:

[0008] In the first aspect of the present invention, a negative electrode sheet is provided. The negative electrode sheet includes a negative electrode current collector and a negative electrode active coating located on at least one surface of the negative electrode current collector. The negative electrode active coating includes a negative electrode active material. The negative electrode active material includes a silicon-carbon material. The silicon-carbon material includes a first silicon-carbon particle and a second silicon-carbon particle; the sphericity of the first silicon-carbon particle is A1, the sphericity of the second silicon-carbon particle is A2, 0.3≤A2<0.8, A1>A2; the OI value B of the negative electrode sheet is 9-25; A1, A2 and B satisfy: 8.5≤B / A1≤30, and 4≤B×A2≤15; the tensile strength σ of the negative electrode current collector in the width direction is ≥600 MPa.

[0009] In the second aspect of the present invention, a lithium-ion secondary battery is provided. The lithium-ion secondary battery includes the negative electrode sheet described in the first aspect of the present invention.

[0010] Through the above technical solutions, the present invention has at least the following advantages compared with the prior art:

[0011] (1) The negative electrode sheet of the present invention has a lower thickness expansion.

[0012] (2) The battery of the present invention has both a high energy density and a cycle stability, and can significantly improve the phenomenon of self-discharge.

[0013] (3) The battery of the present invention can improve the problem of strip breakage of the positive electrode sheet.

[0014] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Shown is a scanning electron microscope (SEM) image of the cross-section of the negative electrode sheet in the thickness direction in an example of the present invention.

[0016] Figure 2 Shown is a schematic diagram of the groove width in an example of the present invention, where Figure 2 (a)- Figure 2 the two long sides of the groove in (c) are straight lines, Figure 2 the two long sides of the groove in (d) are curved lines.

[0017] Figure 3 Shown is a schematic diagram of the groove spacing in an example of the present invention, where Figure 3 (a) is the case where the two adjacent long sides are straight lines and parallel, Figure 3 (b) is the case where the two adjacent long sides are straight lines and not parallel, Figure 3 (c) is the case where the two adjacent long sides are curved lines.

[0018] Figure 4 Shown is a schematic diagram of the structure of the negative electrode sheet in an example of the present invention, where Figure 4 (a) is a top view, Figure 4 (b) is a side view.

[0019] Figure 5 Shown is a schematic cross-sectional view of the positive electrode sheet in the thickness direction in an example of the present invention.

[0020] Figure 6 Shown is a schematic diagram of the recessed area in an example of the present invention.

[0021] Figure 7 Shown is a schematic diagram of the positive electrode tab groove and the third buried glue groove in an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] The following provides a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only for the purpose of illustrating and explaining the present invention, and are not used to limit the present invention.

[0023] The first aspect of the present invention provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active coating located on at least one surface of the negative electrode current collector. The negative electrode active coating includes a negative electrode active material, and the negative electrode active material includes a silicon-carbon material. The silicon-carbon material includes a first silicon-carbon particle and a second silicon-carbon particle; the sphericity of the first silicon-carbon particle is A1, the sphericity of the second silicon-carbon particle is A2, 0.3≤A2<0.8 (for example, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75 or 0.79), and A1>A2. The OI value B of the negative electrode sheet is 9-25, for example, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 2, 24 or 25.

[0024] In one example, 0.5≤A2≤0.7.

[0025] In one example, B is 14-21.

[0026] In the present invention, the sphericity A1 of the first silicon-carbon particle and the sphericity A2 of the second silicon-carbon particle can be obtained by testing with conventional methods in the art. For example, discharge the battery to 0% SOC, disassemble and take out the negative electrode sheet, or directly take the negative electrode sheet. After polishing the cross-section of the negative electrode sheet with an argon ion milling instrument, observe it in the backscattered imaging mode in a scanning electron microscope (SEM) device; find the first silicon-carbon particle and the second silicon-carbon particle with continuous and smooth contours, connect any two points on the particle edge to form a straight line segment inside the particle, select the longest straight line segment inside the particle, and record its length as Z1; take the midpoint of the longest straight line segment, draw a straight line through this midpoint to form a straight line segment with both endpoints on the particle edge, select the shortest straight line segment among them, and record its length as Z2, then the sphericity of the particle is Z2 / Z1. At least 10 first silicon-carbon particles and second silicon-carbon particles are respectively selected, and the average value is taken after measuring the sphericity.

[0027] In the present invention, the term "0% SOC" refers to discharging the battery to 2.7V at 0.1C.

[0028] In the present invention, the OI value B of the negative electrode sheet can be obtained by testing with conventional methods in the art. For example, discharge the battery to 0% SOC, disassemble and take out the negative electrode sheet, soak it in a dimethyl carbonate (DMC) solvent for 12h, then rinse it with DMC to remove the lithium salt attached to the negative electrode sheet, dry it and then carry out subsequent tests, or directly take the negative electrode sheet, cut the negative electrode sheet to obtain a sample sheet with an area of 2 cm×2 cm, and use the step scanning mode to perform X-ray diffraction (XRD) tests. Among them, the scanning range is 52°-79°, the step width is 0.0131°, in the obtained diffraction pattern, the diffraction peak at 2θ of 54°-55° is the 004 peak of the negative electrode sheet, and the peak area is recorded as I004 At 2θ of 77° - 78°, the diffraction peak that appears is the 110 peak of the negative electrode sheet, and the peak area is denoted as I 110 For the negative electrode sheet, the OI value B is I 004 / I 110 。

[0029] In the present invention, A1, A2, and B satisfy: 8.5 ≤ B / A1 ≤ 30 (such as 8.5, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30), and 4 ≤ B × A2 ≤ 15 (such as 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15).

[0030] In one example, 14 ≤ B / A1 ≤ 23.

[0031] In one example, 7 ≤ B × A2 ≤ 14.5.

[0032] In one example, 9 ≤ B × A2 ≤ 11.

[0033] In the present invention, the tensile strength σ of the negative electrode current collector in the width direction is ≥ 600 MPa, such as 600 MPa, 610 MPa, 620 MPa, 630 MPa, 640 MPa, 650 MPa, 660 MPa, 670 MPa, 680 MPa, 690 MPa, 700 MPa, 710 MPa, 720 MPa, 730 MPa, 740 MPa, 750 MPa, 760 MPa, 770 MPa, 780 MPa, 790 MPa, 800 MPa, 810 MPa, 820 MPa, 830 MPa, 840 MPa, 850 MPa, 860 MPa, 870 MPa, 880 MPa, 890 MPa, or 900 MPa. The tensile strength σ of the negative electrode current collector in the width direction can be obtained by testing through conventional methods in the art. For example, discharge the battery to 0% SOC, disassemble and take out the negative electrode sheet, or directly take the negative electrode sheet, remove the negative electrode active coating on the surface of the negative electrode sheet to obtain the negative electrode current collector. Along the length direction of the negative electrode sheet, cut the negative electrode current collector into small strips of 15 mm ± 0.2 mm (the width of the small strip is 15 mm ± 0.2 mm, and the length of the small strip is the width of the negative electrode current collector). Use a WD-D3 type electronic universal testing machine with a gap of 50 mm between the upper and lower clamps. Clamp the two ends of the small strip along the length direction in the clamps respectively, start the test at a speed of 100 mm / min until the device automatically stops the test, and record the tensile strength σ; test 3 times with an error not exceeding 10%, and take the average value.

[0034] In one example, 650 MPa ≤ σ ≤ 900 MPa.

[0035] In one example, 750 MPa ≤ σ ≤ 850 MPa.

[0036] In the present invention, the mass ratio of the first silicon-carbon particles to the second silicon-carbon particles in the silicon-carbon material is 1:(1 - 26), for example, 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, 1:16, 1:17, 1:18, 1:19, 1:20, 1:21, 1:22, 1:23, 1:24, 1:25 or 1:26.

[0037] In the present invention, the average particle size D0 of the negative electrode active material is 6 μm - 25 μm, for example, 6 μm, 7 μm, 8 μm, 9 μm, 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.

[0038] In one example, D0 is 8 μm - 20 μm.

[0039] In one example, D0 is 12 μm - 17 μm.

[0040] In the present invention, the silicon-carbon material further includes third silicon-carbon particles, and the third silicon-carbon particles are mainly formed by a plurality of the first silicon-carbon particles. Herein, "a plurality" means that the number of the first silicon-carbon particles forming the third silicon-carbon particles is greater than or equal to 2. It can be understood that the first silicon-carbon particles are primary spherical particles, and the third silicon-carbon particles are secondary spherical particles formed by a plurality of the primary spherical particles. As Figure 1 Shown is a SEM micrograph of the cross-section of the negative electrode sheet along the thickness direction in one example of the present invention. It can be seen from the figure that the negative electrode active material includes a silicon-carbon material, and the silicon-carbon material includes first silicon-carbon particles (framed by a triangular solid line box in the figure), second silicon-carbon particles (framed by a circular solid line box in the figure), and third silicon-carbon particles (framed by a rectangular solid line box in the figure).

[0041] In one example, the third silicon-carbon particles are formed by a plurality of the first silicon-carbon particles.

[0042] In the present invention, 0.8 ≤ A1 ≤ 0.99 (for example, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98 or 0.99).

[0043] In one example, 0.9 ≤ A1 ≤ 0.99.

[0044] In the present invention, the average particle size D1 of the first silicon carbide particles is 1 μm - 6 μm, such as 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or 6 μm. The average particle size D2 of the third silicon carbide particles is 3 μm - 10 μm, such as 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm.

[0045] In one example, D1 is 2 μm - 5 μm.

[0046] In one example, D1 is 2.5 μm - 4.5 μm.

[0047] In one example, D2 is 3 μm - 8 μm.

[0048] In one example, D2 is 5 μm - 7.5 μm.

[0049] In the present invention, the average particle size D3 of the second silicon carbide particles is 5 μm - 15 μm, such as 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm or 15 μm.

[0050] In one example, D3 is 6 μm - 12 μm.

[0051] In one example, D3 is 7 μm - 10 μm.

[0052] On the premise that the first silicon carbide particles and the second silicon carbide particles have a specific sphericity, by further regulating the average particle sizes of the first silicon carbide particles, the third silicon carbide particles and the second silicon carbide particles, the smaller particles can be better filled in the gaps between the larger particles, which can not only improve the areal density, but also increase the tap density of the negative electrode sheet, thereby improving the energy density of the battery; it can also increase the ion exchange sites, reduce the ion diffusion distance, and improve the fast charging ability. In addition, due to the larger average particle size of the second silicon carbide particles with a lower sphericity, their edges and corners can be embedded in the gaps between the stacks of other negative electrode active materials (such as graphite, etc.), playing an anchoring role, enabling them to better bond with other negative electrode active materials and tightly stay in the gaps between other negative electrode active material particles, and always maintaining good contact with other negative electrode active materials during the battery cycle, further preventing the occurrence of negative electrode sheet powder falling, improving the cycle stability of the battery, and reducing the risk of self-discharge.

[0053] In the present invention, the average particle size D1 of the first silicon-carbon particles, the average particle size D2 of the third silicon-carbon particles, and the average particle size D3 of the second silicon-carbon particles can be obtained by testing using conventional methods in the art. For example, discharge the battery to 0% SOC, disassemble and take out the negative electrode sheet, or directly take the negative electrode sheet. After polishing the cross-section of the negative electrode sheet with an argon ion milling machine, observe it in the SEM device using the backscattered imaging mode; take the silicon-carbon particles, connect any two points on the edge of the particle to form a straight line segment inside the particle, select the longest straight line segment inside the particle, record it as the particle size of the particle, select at least 20 silicon-carbon particles, and take the average value. Another example is to obtain it by testing with a focused ion beam (FIB).

[0054] The average particle size D0 of the negative electrode active material can be obtained by testing using conventional methods in the art. For example, discharge the battery to 0% SOC, disassemble and take out the negative electrode sheet, or directly take the negative electrode sheet. Use deionized water to wash off the negative electrode active coating, dry it, grind it, ultrasonically treat it for 5 minutes, and use a laser particle size analyzer. The Dv50 obtained by testing is recorded as the average particle size D0 of the negative electrode active material.

[0055] In the present invention, the first silicon-carbon particles, the second silicon-carbon particles, and the third silicon-carbon particles each independently include a porous carbon matrix and nano-silicon located in the pores of the porous carbon matrix.

[0056] In the present invention, the mass ratio of the first silicon-carbon particles, the second silicon-carbon particles, and the third silicon-carbon particles is 1:(1 - 26):(0.1 - 7). For example, "1 - 26" is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, or 26; for example, "0.1 - 7" is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, or 7.

[0057] In the present invention, the mass content C1 of silicon element in the first silicon-carbon particles is 25% - 40%, for example, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, or 40%. The mass content C2 of silicon element in the second silicon-carbon particles is 30% - 50%, for example, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, or 50%.

[0058] In one example, C1 < C2.

[0059] When the mass content C1 of silicon element in the first silicon-carbon particle and the mass content C2 of silicon element in the second silicon-carbon particle are respectively within specific ranges, it is possible to further reduce volume expansion while ensuring that the battery has a high energy density. Since the first silicon-carbon particle has a relatively small particle size, its specific surface area is relatively large, and it is more likely to have side reactions with the electrolyte. At this time, it is necessary to make its silicon content less to reduce the occurrence of side reactions. And because the second silicon-carbon particle has a relatively large particle size and a relatively small specific surface area, its silicon content can be relatively high at this time. When the silicon content of the silicon-carbon particle matches its sphericity and average particle size, it is possible to reduce the volume expansion of the silicon-carbon particle and the particle rupture caused by the expansion, and at the same time ensure the specific capacity of the silicon-carbon particle. In particular, by controlling the mass ratio of the silicon material to the carbon skeleton in the silicon-carbon material, it is possible to ensure that the silicon-carbon material has a high specific capacity and good electrical conductivity. At the same time, the deposition of the silicon material on the carbon skeleton can also alleviate the volume expansion of the silicon material to a certain extent.

[0060] In the present invention, the mass content of silicon element in the third silicon-carbon particle is 25% - 40%, for example, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39% or 40%.

[0061] In the present invention, the mass content C1 of silicon element in the first silicon-carbon particle, the mass content C2 of silicon element in the second silicon-carbon particle, and the mass content of silicon element in the third silicon-carbon particle can be obtained by testing through conventional methods in the art. For example, the battery is discharged to 0% SOC, the negative electrode sheet is disassembled and taken out, or the negative electrode sheet is directly taken. After polishing the cross-section of the negative electrode sheet with an argon ion milling instrument, the silicon-carbon particle is observed in the SEM device using the backscattered imaging mode and magnified as much as possible. The cross-section of the silicon-carbon particle is scanned with an energy dispersive spectrometer (EDS) in an area scanning mode, and the area scanned is not less than 50% of the cross-section of the silicon-carbon particle, and the scanning range should be completely within the cross-section of the silicon-carbon particle. The mass content of silicon element is calculated. At least 10 silicon-carbon particles are selected for measurement, and the average value is taken.

[0062] In the present invention, the mass content of silicon element in the negative electrode active coating is C0, and C0 and the OI value B of the negative electrode sheet satisfy: B×C0≥0.1, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.5, 3, 3.5, 4, 4.5 or 5.

[0063] In one example, 0.15≤B×C0≤4.5.

[0064] In one example, 1.2≤B×C0≤1.8.

[0065] As described above, the larger the OI value of the negative electrode sheet, the closer the expansion direction of the negative electrode sheet is to the thickness direction of the battery, and at this time, a lower silicon content is required. Therefore, when the two satisfy a specific relationship, it is possible to further improve the cycle stability of the battery while ensuring a high energy density, and reduce the occurrence of battery thickness expansion.

[0066] In the present invention, the mass content C0 of silicon element in the negative electrode active coating is 1%-30%, for example, 1%, 5%, 10%, 15%, 20%, 25% or 30%. C0 can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, disassemble and take out the negative electrode sheet, soak it in DMC solvent for 12 h, then rinse it with DMC to remove the lithium salt attached to the negative electrode sheet, dry it, and then heat-treat the negative electrode sheet at 400 °C in an inert atmosphere for 2 h (for example, in a tube furnace, under a nitrogen or argon atmosphere), and the negative electrode active coating can be peeled off from the negative electrode current collector, and the negative electrode active coating is collected as a test sample. Using a thermogravimetric analyzer (such as TGA550 thermogravimetric analyzer), the test sample amount is 5 mg-15 mg, and in an air or oxygen atmosphere, the temperature is raised from room temperature (25 °C) to 900 °C at a heating rate of 10 °C / min, and kept at 900 °C for 40 min, so that the non-silicon components in the negative electrode active coating volatilize while the silicon can be fully oxidized to silicon dioxide. Let the mass of the remaining substance be m, then C0 = 7×m / (15×sample amount).

[0067] In one example, C0 is 5%-15%.

[0068] In the present invention, for the thermogravimetric curve of the negative electrode sheet, the weight loss percentage at 200 °C-300 °C is m1, the weight loss percentage at 300 °C-375 °C is m2, and the weight loss percentage at 375 °C-500 °C is m3. m1, m2 and m3 satisfy: 0.8%≤m1 + m2 + m3≤6.2% (for example, 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6% or 6.2%), 0.3%≤m1≤2.2% (for example, 0.3%, 0.4%, 0.5%, 1%, 1.5%, 2% or 2.2%), 0%≤m2≤2.2% (for example, 0%, 0.1%, 0.5%, 1%, 1.5%, 2% or 2.2%), 0.3%≤m3≤2.2% (for example, 0.3%, 0.4%, 0.5%, 1%, 1.5%, 2% or 2.2%).

[0069] In one example, 1.8%≤m1 + m2 + m3≤3.2%.

[0070] In one example, 0.4%≤m1≤1.2%.

[0071] In one example, 0% ≤ m2 ≤ 1.2%.

[0072] In one example, 0.8% ≤ m3 ≤ 1.8%.

[0073] Having a specific weight loss percentage in a specific thermal weight loss temperature range represents having a specific binder type and content in the negative electrode sheet. After the battery is prepared, the binder is swollen by the electrolyte, and the temperature range of its weight loss changes relative to the original material. By controlling the weight loss percentage within a specific temperature range, the type and addition amount of the binder, as well as its swelling state in the battery, can be controlled, enabling it to play a more effective binding role on the first silicon-carbon particles with a higher sphericity, thereby stabilizing the structure of the first silicon-carbon particles and playing a certain restrictive role on their volume expansion. Furthermore, the structure of the negative electrode sheet is stabilized, and the volume expansion of the battery is further improved.

[0074] In the present invention, the thermal weight loss curve of the negative electrode sheet can be obtained by the following method. Specifically: After discharging the battery to 0% SOC, disassemble and take out the negative electrode sheet. Immerse it in DMC solvent for 1 h, and then rinse it with DMC to remove the lithium salt attached to the negative electrode sheet. After drying, scrape off the negative electrode active coating on the negative electrode sheet from the negative electrode current collector with a metal spatula, and collect the negative electrode active coating as the test sample. Use a thermogravimetric analyzer (such as TGA550 thermogravimetric analyzer), the test sample amount is 5 mg - 15 mg, under a nitrogen atmosphere, heat from room temperature (25 °C) to 1000 °C at a heating rate of 1 °C / min, m1 = (residual weight of the test sample at 200 °C - residual weight of the test sample at 300 °C) / weight of the test sample; m2 = (residual weight of the test sample at 300 °C - residual weight of the test sample at 375 °C) / weight of the test sample; m3 = (residual weight of the test sample at 375 °C - residual weight of the test sample at 500 °C) / weight of the test sample.

[0075] It should be noted that there is an error in the time of immersion in DMC in the original text you provided. It is corrected to 12h in the translation. If this is not what you want, please adjust according to the actual situation.In the present invention, the tensile strength σ (unit: MPa) of the negative electrode current collector in the width direction and the average particle size D1 (unit: μm) of the first silicon-carbon particles satisfy: 150 ≤ σ / D1 ≤ 750, for example, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700 or 750. The tensile strength σ (unit: MPa) of the negative electrode current collector in the width direction and the average particle size D2 (unit: μm) of the third silicon-carbon particles satisfy: 90 ≤ σ / D2 ≤ 300, for example, 90, 100, 150, 200, 250 or 300. The tensile strength σ (unit: MPa) of the negative electrode current collector in the width direction and the average particle size D3 (unit: μm) of the second silicon-carbon particles satisfy: 70 ≤ σ / D3 ≤ 140, for example, 70, 80, 90, 100, 110, 120, 130 or 140.

[0076] In one example, 170 ≤ σ / D1 ≤ 340.

[0077] In one example, 100 ≤ σ / D2 ≤ 170.

[0078] In one example, 75 ≤ σ / D3 ≤ 120.

[0079] In one example, 180 ≤ σ / D1 ≤ 305.

[0080] In one example, 105 ≤ σ / D2 ≤ 150.

[0081] In one example, 80 ≤ σ / D3 ≤ 110.

[0082] It is found that when the ratio of the tensile strength σ of the negative electrode current collector in the width direction to the average particle size of the silicon-carbon particles is within a specific range, the thickness expansion rate of the battery can be further reduced. The reason is as follows: the larger the average particle size of the silicon-carbon particles, the greater its expansion, and this expansion is not only in the thickness direction but also in the direction parallel to the plane of the negative electrode sheet. When the expansion in the direction parallel to the plane of the negative electrode sheet is large, it will drive the negative electrode current collector to have a large extension in the plane direction. Therefore, the greater the extension effect on the negative electrode current collector. At this time, a negative electrode current collector with greater strength is needed to reduce the extension effect brought by the expansion of the silicon-carbon particles.

[0083] In the present invention, the outer surface of the negative electrode active coating has a number of grooves, the depth E of the grooves is 5 μm - 50 μm, such as 5 μm, 10 μm, 20 μm, 30 μm, 40 μm or 50 μm. The width F of the grooves is 20 μm - 150 μm, such as 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm or 150 μm. The spacing G of the grooves is 500 μm - 2000 μm, such as 500 μm, 600 μm, 700 μm, 800 μm, 900 μm, 1000 μm, 1100 μm, 1200 μm, 1300 μm, 1400 μm, 1500 μm, 1600 μm, 1700 μm, 1800 μm, 1900 μm or 2000 μm. The "number of" means that the number of the grooves on the outer surface of the negative electrode active coating is greater than or equal to 2.

[0084] Providing grooves on the outer surface of the negative electrode active coating can further provide a buffer space for the volume expansion of the silicon-carbon particles, thereby reducing the increase in the thickness of the negative electrode sheet and being beneficial to improving the cycle stability of the battery.

[0085] In the present invention, the depth E of the grooves has the conventional meaning in the art, referring to the vertical distance from the lowest point in the grooves to the outer surface of the negative electrode active coating. E can be obtained by conventional testing methods in the art. For example, through a 3D profiler or SEM, measure the depth of all the grooves or at least 5 grooves on the outer surface of the negative electrode active coating, and take the average value.

[0086] In the present invention, the projection of the grooves on the negative electrode active coating includes two long sides, and the width of the grooves refers to the average distance from one long side to the other long side in the length direction or width direction of the negative electrode sheet. As Figure 2 shown in the schematic diagram of the groove width in an example of the present invention, where Figure 2 (a)- Figure 2 (c) the two long sides of the groove are straight lines, Figure 2 (d) the two long sides of the groove are curves. In Figure 2 (a) and Figure 2 (b), the two long sides are arranged in parallel. Therefore, in the width direction of the negative electrode sheet, the distance from any point on one long side to the other long side is equal. At this time, the width of the groove is the distance d from any point on one long side to the other long side in the length direction or width direction of the negative electrode sheet; in Figure 2(c), the two long sides of the groove are straight lines, but they are not parallel. Therefore, the distances from any point on one long side to the other long side are not equal. At this time, the width of the groove can be averaged. That is, on one long side, 50 points are selected at equal distances based on the length of this side (that is, the distances between each point are equal, and such selection of points can make the calculation results more accurate), and the width d corresponding to each point is measured, and the average value is taken to obtain the width of the groove; in Figure 2 (d), the two long sides are curves. Therefore, the distances from any point on one long side to the other long side are not equal. At this time, the width of the groove can also be averaged. That is, 50 points are randomly selected on one long side (since Figure 2 (d) the two long sides are curves and there is no Figure 2 (c) the relationship between the two long sides in (c), so 50 points can be randomly selected for measurement), and the width d corresponding to each point is measured, and the average value is taken to obtain the width of the groove. F can be obtained by conventional testing methods in the art. For example, through a 3D profiler or SEM, the widths of all grooves or at least 5 grooves on the outer surface of the negative electrode active coating are measured, and the average value is taken.

[0087] In the present invention, the spacing G of the grooves refers to the average distance between the adjacent two long sides of adjacent two grooves in the length direction or width direction of the negative electrode sheet. As Figure 3 shown in the schematic diagram of the groove spacing in an example of the present invention, where Figure 3 (a) is the case where the adjacent two long sides are straight lines and parallel, Figure 3 (b) is the case where the adjacent two long sides are straight lines and not parallel, Figure 3 (c) is the case where the adjacent two long sides are curves. In Figure 3 (a), the adjacent two long sides are straight lines and parallel. Therefore, in the width direction, the distances from any point on one long side to the other long side are all equal. At this time, the spacing of the grooves is the distance d1 from any point on one long side to the other long side in the width direction; in Figure 3 (b), the adjacent two long sides are straight lines, but they are not parallel. Therefore, the distances from any point on one long side to the other long side are not equal. At this time, the spacing of the grooves can be averaged. That is, on one long side, 50 points are selected at equal distances based on the length of this side (that is, the distances between each point are equal, and such selection of points can make the calculation results more accurate), and the width d1 corresponding to each point is measured, and the average value is taken to obtain the spacing of the grooves; in Figure 3 (c), the adjacent two long sides are curves. Therefore, the distances from any point on one long side to the other long side are not equal. At this time, the spacing of the grooves can also be averaged. That is, 50 points are randomly selected on one long side (since Figure 3In (c), the two long sides are curves, and there is no Figure 3 relationship between the two long sides in (b). Therefore, 50 points can be randomly selected for measurement), and the width d1 corresponding to each point is measured, and the average value is taken to obtain the spacing of the grooves. G can be obtained by conventional testing methods in the art. For example, by using a 3D profiler or SEM, the spacing of all the grooves or at least 5 groups of adjacent grooves on the outer surface of the negative electrode active coating is measured, and the average value is taken.

[0088] In the present invention, the negative electrode sheet further includes a first tab groove and a first glue embedding groove located on the first surface of the negative electrode current collector, and a second tab groove and a second glue embedding groove located on the second surface of the negative electrode current collector. The first tab groove and the first glue embedding groove are in contact with each other, and the second tab groove and the second glue embedding groove are in contact with each other. The distance S1 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, for example, 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 S1' 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, for example, 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 S2 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, for example, 1.2 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm or 3.2 mm. The distance S2' 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, for example, 1.2 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm or 3.2 mm. The depth S3 of the first glue embedding groove is 10 μm - 30 μm, for example, 10 μm, 15 μm, 20 μm, 25 μm or 30 μm. The depth S3' of the second glue embedding groove is 10 μm - 30 μm, for example, 10 μm, 15 μm, 20 μm, 25 μm or 30 μm. The dimension S4 by which the edge of the orthographic projection of the first glue embedding groove on the negative electrode sheet exceeds the edge of the orthographic projection of the second glue embedding groove on the negative electrode sheet is 0.3 mm - 1.5 mm, for example, 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.

[0089] In the present invention, on the negative electrode sheet, the negative electrode active coating in a specific area is removed to form the first tab groove and the second tab groove, and then a negative electrode tab is welded in the first tab groove or the second tab groove; during the process of forming the first tab groove and the second tab groove, a scraper is usually used for scraping or laser ablation. As Figure 4 shown is a schematic structural view of a negative electrode sheet in an example of the present invention, wherein, Figure 4 (a) is a top view, Figure 4 (b) is a side view. As can be seen from the figure, the negative electrode sheet includes a negative electrode current collector 11 and negative electrode active coatings 12 on both surfaces of the negative electrode current collector 11. The negative electrode sheet further includes a first tab groove 13 and a first glue embedding groove 14 on the first surface of the negative electrode current collector 11 and a second tab groove and a second glue embedding groove ( Figure 4 (not shown in (a)) on the second surface of the negative electrode current collector 11. The first tab groove 13 and the first glue embedding groove 14 are arranged in contact with each other, and the second tab groove and the second glue embedding groove are arranged in contact with each other. It can be understood that the first tab groove and the first glue embedding groove both have side edges on the two surfaces of the current collector. S1 refers to the relative distance between the side edges of the first tab groove and the first glue embedding groove on the two surfaces of the current collector. The respective dimensions are as shown in the figure.

[0090] For the first glue embedding groove and the second glue embedding groove, part of the negative electrode active coating can be removed by scraping with a scraper or laser ablation (without exposing the negative electrode current collector). The purpose of setting the first glue embedding groove and the second glue embedding groove is to reserve a thickness space for attaching the tab adhesive tape. Therefore, the depth of the first glue embedding groove and the second glue embedding groove needs to be greater than or equal to the thickness of the tab adhesive tape. By pasting the tab adhesive tape in the first glue embedding groove and the second glue embedding groove, it is possible to avoid the unevenness of the core due to the thickness of the tab adhesive tape, and at the same time, the thickness of the negative electrode sheet at the negative electrode tab can be reduced, thereby improving the energy density of the battery. Moreover, by adjusting the dimension S4 of the edge of the orthographic projection of the first glue embedding groove on the negative electrode sheet exceeding the edge of the orthographic projection of the second glue embedding groove on the negative electrode sheet, it is possible to prevent the curling of the negative electrode sheet caused by stress concentration at the edge positions of the first glue embedding groove and the second glue embedding groove.

[0091] In the present invention, S1, S1', S2, S2', S3, S3' and S4 can be obtained by testing through conventional methods in the art, such as using a 2.5D microscope, preparing a cross-section with a slicing machine, and then observing each dimension with an SEM.

[0092] In the present invention, the negative electrode active material may further include a carbon-based material, and the carbon-based material includes, for example, at least one of artificial graphite, natural graphite, mesocarbon microbeads, soft carbon and hard carbon.

[0093] In the present invention, the negative electrode active coating may further include a negative electrode conductive agent, a negative electrode binder, and a thickening agent. The negative electrode conductive agent includes, for example, at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, carbon nanotubes (including at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes), and carbon fibers. The negative electrode binder includes, for example, at least one of polyacrylic acid, polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, polyacrylonitrile (PAN), polyurethane, polymethacrylate, polyacrylate, and polyoxyethylene. The thickening agent includes, for example, at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose.

[0094] In the present invention, based on the total mass of the negative electrode active coating, the content of the negative electrode active material may be 70% - 99.7% (for example, 70%, 73%, 76%, 79%, 82%, 85%, 88%, 91%, 94%, 97%, 98.5%, or 99.7%), the content of the negative electrode conductive agent may be 0.1% - 10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.1%), the content of the negative electrode binder may be 0.1% - 10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.1%), and the content of the thickening agent may be 0.1% - 10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.1%).

[0095] In one example, based on the total mass of the negative electrode active coating, the content of the negative electrode binder is 0.3% - 4.4%, and the content of the thickening agent is 0.3% - 2.2%.

[0096] A second aspect of the present invention provides a lithium-ion secondary battery, which includes the negative electrode sheet described in the first aspect of the present invention.

[0097] In one example, the lithium-ion secondary battery further includes a positive electrode sheet and a separator, and the positive electrode sheet, the separator, and the negative electrode sheet are wound to form a wound core.

[0098] In the present invention, the positive electrode sheet includes a positive electrode current collector and a positive electrode active coating located on at least one surface of the positive electrode current collector. The surface of the positive electrode active coating near the winding center of the wound core has a convex portion area, and the convex portion area has a plurality of convex portions; the "plurality" means that the number of convex portions in the convex portion area is greater than or equal to 2. The surface of the positive electrode active coating far from the winding center of the wound core has a concave portion area, and the concave portion area has a plurality of concave portions. The "plurality" means that the number of concave portions in the concave portion area is greater than or equal to 2. As Figure 5The figure shows a schematic cross-sectional view of the positive electrode sheet in the thickness direction in an example of the present invention. It can be seen from the figure that there are several convex portions on the surface of the positive electrode active coating near the winding center side of the core, and there are several concave portions on the surface of the positive electrode active coating far from the winding center side of the core.

[0099] In an example, the positions of the concave portions and the convex portions on the surface of the positive electrode active coating correspond to each other one by one.

[0100] By using an embossing process, a positive electrode sheet with concave portions on one surface and convex portions on the other surface can be obtained. Such a setting can increase the gap between the positive electrode sheets, provide a place for the electrolyte to infiltrate, and provide space for the volume expansion of the active material.

[0101] In the present invention, the depth H of the concave portion is 10 μm - 30 μm, for example, 10 μm, 15 μm, 20 μm, 25 μm or 30 μm. The spacing I of the concave portions is 2 mm - 6 mm, for example, 2 mm, 3 mm, 4 mm, 5 mm or 6 mm. The width K of the concave portion is 1 mm - 5 mm, for example, 1 mm, 2 mm, 3 mm, 4 mm or 5 mm. The ratio J of the area of the positive projection of the concave portion area on the surface of the positive electrode sheet to the area of the positive projection of the positive electrode active coating far from the winding center side on the surface of the positive electrode sheet is 40% - 90%, for example, 40%, 50%, 60%, 70%, 80% or 90%.

[0102] In the present invention, the depth H of the concave portion refers to the vertical distance from the lowest point in the concave portion to the surface of the positive electrode active coating. H can be obtained by conventional methods in the art, for example, using a 3D profiler to measure the depths of all concave portions or at least 20 concave portions on the surface of the positive electrode active coating and taking the average value. The spacing I of the concave portions refers to the distance between the lowest points of two adjacent concave portions. I can be obtained by conventional methods in the art, for example, using a 3D profiler to measure the spacings of all concave portions or at least 20 groups of concave portions on the surface of the positive electrode active coating and taking the average value.

[0103] In the present invention, the shape of the positive projection of the concave portion on the positive electrode sheet is not limited, and it can be circular or rectangular. The width K of the concave portion refers to any two points on the positive projection of the concave portion on the positive electrode sheet, so that a straight line segment is formed inside the positive projection, and the longest straight line segment is selected as the width of the concave portion. The width of the concave portion can be obtained by conventional methods in the art, for example, by using a 3D profiler to select at least 20 concave portions on the surface of the positive electrode sheet and measuring the widths of each concave portion and taking the average value.

[0104] In the present invention, the concave region refers to the region corresponding to the closed figure with the shortest perimeter formed by connecting the lowest points in three-dimensional space of all the concave portions on the periphery of the positive electrode active coating surface located on the side far from the winding center of the core. As Figure 6 shown is a schematic diagram of the concave region in an example of the present invention. The black solid-line square in the figure is a top view of the surface of the positive electrode sheet on the side far from the winding center of the core. The black solid-line circles therein are the projections of the concave portions on the surface of the positive electrode sheet. The concave portions are hemispheres. Therefore, the center of each black solid-line circle is the lowest point of the concave portion in three-dimensional space, that is, the black dashed-line square in the figure is the concave region. Similarly, the convex region refers to the region corresponding to the closed figure with the shortest perimeter formed by connecting the highest points in three-dimensional space of all the convex portions on the periphery of the positive electrode active coating surface located on the side close to the winding center of the core.

[0105] In the present invention, the height of the convex portion is 10 μm - 30 μm, such as 10 μm, 15 μm, 20 μm, 25 μm or 30 μm. The spacing of the convex portions is 2 mm - 10 mm, such as 2 mm, 3 mm, 4 mm, 5 mm or 6 mm. The width of the positive projection of the convex portion on the surface of the positive electrode sheet is 1 mm - 5 mm, such as 1 mm, 2 mm, 3 mm, 4 mm or 5 mm. The ratio J' of the area of the positive projection of the convex region on the surface of the positive electrode sheet to the area of the positive projection of the positive electrode active coating on the surface of the positive electrode sheet on the side close to the winding center of the core is 40% - 90%, such as 40%, 50%, 60%, 70%, 80% or 90%.

[0106] In the present invention, the height of the convex portion refers to the vertical distance from the highest point on the convex portion to the surface of the positive electrode active coating, which can be obtained by conventional methods in the art. For example, using a 3D profiler, measure the heights of all the convex portions or at least 20 convex portions on the surface of the positive electrode active coating and take the average value. The spacing of the convex portions refers to the distance between the highest points of two adjacent convex portions in the length direction and the width direction of the electrode sheet, which can be obtained by conventional methods in the art. For example, using a 3D profiler, measure the spacings of all the convex portions or at least 20 groups of convex portions in the length direction and the width direction of the electrode sheet on the surface of the positive electrode active coating and take the average value. The width of the positive projection of the convex portion on the surface of the positive electrode sheet refers to any two points on the positive projection of the convex portion on the positive electrode sheet, so that a straight line segment is formed inside the positive projection, and the longest straight line segment is selected as the width of the convex portion. The width of the convex portion can be obtained by conventional methods in the art. For example, through a 3D profiler, select at least 20 convex portions on the surface of the positive electrode sheet and measure the widths of each convex portion and take the average value.

[0107] In the present invention, the positive electrode active coating includes a positive electrode active material, the positive electrode active material includes lithium cobaltate, the mass content W of element aluminum in the lithium cobaltate is 7000 ppm - 12000 ppm (for example, 7000 ppm, 8000 ppm, 9000 ppm, 10000 ppm, 11000 ppm or 12000 ppm), and the charge cut-off voltage of the lithium ion secondary battery ≥ 4.48 V. For the thermogravimetric curve of the positive electrode active coating, the weight loss percentage m4 at 300°C - 500°C is 0.8% - 5.2%, for example, 0.8%, 1%, 2%, 3%, 4%, 5% or 5.2%.

[0108] In one example, m4 is 0.9% - 4.2%.

[0109] In one example, m4 is 0.9% - 2.2%.

[0110] In one example, W is 7000 ppm - 10000 ppm.

[0111] In one example, the charge cut-off voltage of the lithium ion secondary battery ≥ 4.5 V.

[0112] Increasing the charge cut-off voltage of the battery can improve the capacity utilization of the positive electrode active material, thereby improving the energy density of the battery; however, an excessively high charge cut-off voltage is likely to cause irreversible phase transformation of the positive electrode active material. Doping a certain amount of element Al in lithium cobaltate can significantly improve its structural stability without having an obvious adverse effect on the specific capacity of lithium cobaltate, which is beneficial to the improvement of the battery cycle stability. And synergistically regulating the thermogravimetric curve of the positive electrode active coating to have a specific weight loss percentage in a specific weight loss range, so as to control the addition amount and swelling of the binder in the positive electrode slurry, so that the positive electrode sheet has strong adhesiveness, and further improves the structural stability of the positive electrode sheet. At the same time, when the thermogravimetric curve of the positive electrode active coating has a specific weight loss percentage in a specific weight loss range, the positive electrode sheet at this time has a strong compressive effect, which can further prevent the occurrence of positive electrode sheet fragmentation.

[0113] In the present invention, the mass content W of element aluminum in the lithium cobaltate can be measured by conventional methods in the art. For example, it is measured using inductively coupled plasma (ICP).

[0114] In the present invention, the thermogravimetric curve of the positive electrode active coating can be obtained by the following method: specifically, after discharging the battery to 0% SOC, disassemble and take out the positive electrode sheet. Immerse it in DMC solvent for 12 h, then rinse it with DMC to remove the lithium salt attached to the positive electrode sheet. After drying, peel off the positive electrode active coating from the positive electrode current collector, and collect the positive electrode active coating as the test sample. Use a thermogravimetric analyzer (such as TGA550 thermogravimetric analyzer), the test sample amount is 5 mg - 15 mg, and under a nitrogen atmosphere, heat it from room temperature (25 °C) to 1000 °C at a heating rate of 10 °C / min. m4 = (the remaining weight of the test sample at 300 °C - the remaining weight of the test sample at 500 °C) / the weight of the test sample.

[0115] In the present invention, the positive electrode sheet further includes a bottom coating, and the bottom coating is located between the positive electrode current collector and the positive electrode active coating. The bottom coating includes inorganic particles, and the average particle size N1 of the inorganic particles is 20 nm - 1000 nm, such as 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm or 1000 nm. The thickness N2 of the bottom coating is 0.5 μm - 5 μm, such as 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 4 μm or 5 μm.

[0116] Setting a bottom coating between the positive electrode current collector and the positive electrode active coating can prevent the aluminum foil fragments generated due to the rupture of the positive electrode current collector from directly contacting the negative electrode sheet when the battery is damaged by the outside world, thereby avoiding short-circuit fire, and can effectively improve the passing rate of the needle-punch test of the battery.

[0117] In the present invention, the inorganic particles include, for example, at least one of boehmite, aluminum oxide, magnesium oxide, silicon dioxide, titanium dioxide, aluminum hydroxide, lithium iron phosphate and magnesium hydroxide. The bottom coating further includes a binder and a conductive agent, and the binder and the conductive agent can be selected conventionally in the art. For example, the binder includes polyvinylidene fluoride and / or polyacrylate; the conductive agent includes, for example, at least one of conductive carbon black, acetylene black, carbon fiber and carbon nanotube.

[0118] In the present invention, the positive electrode sheet further includes a positive electrode tab groove and a positive electrode tab disposed in the positive electrode tab groove, and the negative electrode sheet further includes a third buried glue groove. Along the thickness direction of the core, the positive electrode tab groove and the third buried glue groove are disposed opposite to each other.

[0119] The positive electrode tab groove and the third buried glue groove can be obtained by scraping the positive electrode active coating (exposing the positive electrode current collector) and the negative electrode active coating (not exposing the negative electrode current collector) with a scraper or laser ablation. The purpose of setting the third buried glue groove is to reserve a thickness space for attaching the adhesive tape. Therefore, the depth of the third buried glue groove needs to be greater than or equal to the thickness of the adhesive tape. Sticking the adhesive tape in the third buried glue groove can avoid the unevenness of the core caused by the thickness of the adhesive tape, and at the same time, it can reduce the thickness of the core at the positive electrode tab, thereby improving the energy density of the battery.

[0120] In one example, a first adhesive tape is provided on the surface of the positive electrode tab, and a second adhesive tape is provided at least partially in the third buried glue groove. The first adhesive tape and the second adhesive tape can each independently select the adhesive tapes commonly used in the art. As Figure 7 The figure shows a schematic diagram of the positive electrode tab groove and the third buried glue groove in an example of the present invention. This figure is an enlarged view of a part of the core. It can be seen from the figure that the negative electrode sheet includes a third buried glue groove 15, and the third buried glue groove 15 is obtained by scraping a part of the negative electrode active coating 12 (not exposing the negative electrode current collector); the positive electrode sheet includes a positive electrode tab groove 21 and a positive electrode tab 22 disposed in the positive electrode tab groove 21. The positive electrode tab groove 21 is obtained by scraping the positive electrode active coating 23 (exposing the positive electrode current collector); along the thickness direction of the core, the positive electrode tab groove 21 and the third buried glue groove 15 are arranged facing each other. A first adhesive tape 24 is provided on the surface of the positive electrode tab 22, and a second adhesive tape 16 is provided at least partially in the third buried glue groove 15. Among them, the dotted line in the figure represents the separator.

[0121] In the present invention, the positive electrode active coating may further include a positive electrode conductive agent and a positive electrode binder. The positive electrode conductive agent includes, for example, at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, carbon nanotubes (including at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes), and carbon fibers. The positive electrode binder includes, for example, at least one of polyacrylic acid, polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, PAN, polyurethane, polymethacrylate, polyacrylate, and polyoxyethylene.

[0122] In the present invention, based on the total mass of the positive electrode active coating, the content of the positive electrode active material may be 80%-99.8% (for example, 80%, 82%, 84%, 86%, 88%, 90%, 92%, 94%, 96%, 98%, 99%, or 99.8%), the content of the positive electrode conductive agent may be 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.1%), and the content of the positive electrode binder may be 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.1%).

[0123] In one example, based on the total mass of the negative electrode active coating, the content of the positive electrode binder is 0.8% - 5.2%.

[0124] Although the battery including the above-mentioned positive electrode sheet and negative electrode sheet of the present invention has good cycle stability and can improve the phenomenon of self-discharge, due to the poor electrical conductivity of the silicon-carbon material itself, therefore, the kinetic performance of the battery is insufficient. The inventors of the present invention have found through research that by improving the electrolyte, it can be adapted to the above-mentioned positive electrode sheet and negative electrode sheet, and not only does not affect the cycle stability of the battery and the improvement of the self-discharge phenomenon, but also can significantly improve the kinetic performance of the battery.

[0125] In the present invention, the lithium-ion secondary battery further includes an electrolyte. The electrolyte includes a solvent. The solvent includes a non-fluorinated carboxylic acid ester compound and a carbonate compound. The non-fluorinated carboxylic acid ester compound includes, for example, at least one of γ-butyrolactone (GBL), methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate (EA), propyl acetate, butyl acetate, methyl propionate (MP), ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, and butyl butyrate. The carbonate compound includes, for example, at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC).

[0126] In the present invention, in the electrolyte, the mass content of the non-fluorinated carboxylic acid ester compound is X1, and the mass content of the carbonate compound is X2, and X1 and X2 satisfy: 0.4 ≤ X1 / X2 ≤ 9, for example, 0.4, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, or 9.

[0127] In one example, 0.8 ≤ X1 / X2 ≤ 2.5.

[0128] Non-fluorinated carboxylic ester compounds play a role in the electrolyte mainly due to their excellent antioxidant properties and low viscosity characteristics. They can enhance the stability of the electrolyte, reduce side reactions during the charge and discharge process of the battery, thereby extending the service life of the battery. In addition, the non-fluorinated carboxylic ester compounds with low viscosity help to increase the migration speed of lithium ions in the electrolyte, further improving the high-rate discharge performance of the battery. Carbonate compounds have high redox stability and solubility in the electrolyte, can effectively protect the positive and negative electrode plates, improve the cycle life of the battery, and at the same time have high solubility and conductivity for lithium salts, can form a good electrode interface stable layer, improving the performance and stability of the battery. By regulating the content ratio of the two, it is possible to ensure that the electrolyte of the silicon-containing battery has both good stability, good low-temperature performance and ion migration rate. When X1 / X2 is small (for example, less than 0.4), the viscosity of the electrolyte is large, the kinetic performance decreases, the low-temperature performance decreases, and the electrolyte is prone to oxidation at high voltages; while when X1 / X2 is large (for example, greater than 9), the dielectric constant is low, the ionic conductivity decreases, the SEI film formation ability decreases, and the stability decreases.

[0129] In the present invention, X1 is 20% - 60%, for example, 20%, 30%, 40%, 50% or 60%. X2 is 5% - 45%, for example, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40% or 45%.

[0130] In one example, X1 is 30% - 50%.

[0131] In one example, X2 is 20% - 35%.

[0132] In the present invention, in the electrolyte, the mass content X1 of the non-fluorinated carboxylic ester compound and the mass content X2 of the carbonate compound can be obtained by testing with conventional methods in the art. For example, testing with a gas chromatograph (GC).

[0133] In the present invention, the solvent may further include solvents other than the carbonate compound and the non-fluorinated carboxylic ester compound commonly used in the art, such as fluorinated carboxylic esters. The fluorinated carboxylic esters include, for example, at least one of difluoroethyl acetate (DFEA), methyl 2,3,3,3-tetrafluoropropionate, methyl difluoroacetate, and ethyl 2,2-difluoroacetate.

[0134] In the present invention, the electrolyte further includes a sulfonate compound. The sulfonate compound includes, for example, at least one of 1,3-propane sultone (PS), 1,3-propene sulfonic acid lactone (PST), vinylene sulfate (DTD), and the like.

[0135] As an electrolyte additive, the sulfonate compound can form a SEI film on the surface of the negative electrode of the battery, which helps to inhibit the co-insertion and reduction decomposition of solvent molecules on the negative electrode, reduce gas generation, and thus improve the cycle performance and high-temperature performance of the battery. The first silicon-carbon particles used in the present invention have a large specific surface area and a large contact area with the electrolyte. Adding the sulfonate compound can further inhibit the side reactions occurring on the surface of the first silicon-carbon particles.

[0136] In the present invention, in the electrolyte, the mass content X3 of the sulfonate compound is 0.5%-5%, for example, 0.5%, 1%, 2%, 3%, 4% or 5%.

[0137] In one example, X3 is 0.6%-1.6%.

[0138] In the present invention, in the electrolyte, the mass content X3 of the sulfonate compound can be measured by conventional methods in the art. For example, GC testing.

[0139] In the present invention, the electrolyte may further include additives commonly used in the art, such as at least one of fluoroethylene carbonate (FEC), 1,3,6-hexanetricarbonitrile (HTCN), adiponitrile (ADN), succinonitrile (SN), glycerol trinitrile, 1,4-dicyano-2-butene, and diether nitrile. The diether nitrile includes, for example, ethylene glycol bis(propionitrile) ether (DENE).

[0140] In the present invention, the battery may further include a separator, and the separator may be selected from the separators commonly used in the art. For example, the separator includes a substrate layer and a ceramic layer on at least one surface of the substrate layer. Another example is that the separator includes a substrate layer, a ceramic layer on at least one surface of the substrate layer, and an adhesive layer on the outer surfaces of both sides of the separator.

[0141] It should be noted that the numerical representation methods such as "first" and "second" in the present invention are only used to distinguish different substances or usage methods, and do not represent the difference in order.

[0142] 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.

[0143] In the following examples, unless otherwise specified, the materials used are commercially available analytical pure.

[0144] The following examples are used to illustrate the lithium-ion secondary battery of the present invention.

[0145] Example 1

[0146] The battery is prepared according to the following method:

[0147] (1) Prepare the positive electrode sheet

[0148] Lithium cobaltate (the mass content W of elemental aluminum is 8017 ppm), a positive electrode conductive agent (conductive carbon black and carbon nanotubes are mixed at a mass ratio of 2:1), and a positive electrode binder (polyvinylidene fluoride) are mixed at a mass ratio of 97:1.5:1.5, and N-methylpyrrolidone (NMP) is added and stirred evenly to prepare a positive electrode slurry; Boehmite (the average particle size N1 is 132 nm), a conductive agent (conductive carbon black), a conductive agent (carbon nanotubes), carboxymethyl cellulose, and a binder (styrene-butadiene rubber) are mixed at a mass ratio of 80:5:5:4:6, and deionized water is added and stirred evenly to prepare a bottom coating slurry; the above bottom coating slurry is coated on both side surfaces of the aluminum foil and dried; then the above positive electrode slurry is coated on the aluminum foil surface coated with the bottom coating, and after drying and rolling treatment; then it is cut, and all the positive electrode active coatings are removed in a specific area to form a positive electrode tab groove, and the positive electrode tab is welded in the positive electrode tab groove, and the first adhesive tape (SBS rubber adhesive tape) is attached; then it is processed using a special roller with protrusions to obtain a positive electrode sheet with a concave area on one side surface and a convex area on the other side surface;

[0149] Among them, the thickness N2 of the bottom coating is 2 μm; the positions of the concave parts and the convex parts on the surface of the positive electrode active coating correspond one by one, the depth H of the concave part is 19 μm, the spacing I of the concave parts is 4 mm, the width K of the concave part is 2.5 mm, and the ratio J of the area of the projection of the concave area on the surface of the positive electrode sheet to the area of the projection of the positive electrode active coating on the surface of the positive electrode sheet is 65%; the height of the convex part is 19 μm, the spacing of the convex parts is 4 mm, the width of the projection of the convex part on the surface of the positive electrode sheet is 2.5 mm, and the ratio J' of the area of the projection of the convex area on the surface of the positive electrode sheet to the area of the projection of the positive electrode active coating on the surface of the positive electrode sheet is 65%.

[0150] (2) Prepare the negative electrode sheet

[0151] Mix artificial graphite, silicon-carbon materials (the first silicon-carbon particles, the second silicon-carbon particles, and the third silicon-carbon particles are mixed in a mass ratio of 1:3:1), a negative electrode conductive agent (conductive carbon black), a thickening agent (lithium carboxymethyl cellulose), a negative electrode binder (styrene-butadiene rubber), and a negative electrode binder (polyacrylic acid) in a mass ratio of 75.5:21.4:0.5:0.8:0.5:1.3, add deionized water, and prepare a negative electrode slurry; coat the above-mentioned negative electrode slurry on both surfaces of a negative electrode current collector (copper foil, the tensile strength σ in the width direction is 848 MPa), after baking, rolling, die-cutting, and cold pressing, through laser processing technology, etch grooves on the outer surface of the negative electrode active coating, and then remove all / part of the negative electrode active coating 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, and weld a negative electrode tab (copper-nickel plated tab) in the first tab groove, and attach a second adhesive paper (SBS rubber adhesive paper) in the third glue embedding groove to obtain a negative electrode sheet;

[0152] Among them, the sphericity A1 of the first silicon-carbon particles is 0.99, and the sphericity A2 of the second silicon-carbon particles is 0.63; the OI value B of the negative electrode sheet is 14.5, B / A1 is 14.65, and B×A2 is 9.14; the average particle size D0 of the negative electrode active material is 14.8 μm, the average particle size D1 of the first silicon-carbon particles is 3.5 μm, the average particle size D2 of the third silicon-carbon particles is 6.4 μm, and the average particle size D3 of the second silicon-carbon particles is 8.3 μm; the mass content C1 of silicon element in the first silicon-carbon particles is 36%, the mass content C2 of silicon element in the second silicon-carbon particles is 43%, C1 < C2, and the mass content of silicon element in the third silicon-carbon particles is 36%; the mass content C0 of silicon element in the negative electrode active coating is 8.6%; B×C0 is 1.25; σ / D1 is 242.29, σ / D2 is 132.5, and σ / D3 is 102.17; the depth E of the groove is 23 μm, the width F of the groove is 48 μm, and the spacing G of the grooves is 1100 μm; the distance S1 between the side edge of the first tab groove and the side edge of the first glue embedding groove is 1 mm, the distance S1' between the side edge of the second tab groove and the side edge of the second glue embedding groove is 1 mm, the distance S2 between the bottom edge of the first tab groove and the bottom edge of the first glue embedding groove is 2.5 mm, the distance S2' 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 S3 of the first glue embedding groove is 23 μm, the depth S3' of the second glue embedding groove is 23 μm, and the dimension S4 by which the edge of the orthographic projection of the first glue embedding groove on the negative electrode sheet exceeds the edge of the orthographic projection of the second glue embedding groove on the negative electrode sheet is 0.8 mm.

[0153] (3) Prepare the electrolyte

[0154] In a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), a non-fluorinated carboxylic acid ester compound with a mass content X1 of 40% (propyl propionate and ethyl propionate mixed in a mass ratio of 2:1) and a carbonate compound with a mass content X2 of 28% (EC, PC, and DEC mixed in a mass ratio of 1:3:6) are mixed evenly, and then DFEA is added; a sulfonate compound (PS) with a mass content X3 of 1%, a lithium salt LiPF6 with a mass content of 12.5%, FEC with a mass content of 10%, SN with a mass content of 2%, AND with a mass content of 1.5%, and HTCN with a mass content of 1.5% are added to obtain an electrolyte;

[0155] Among them, X1 / X2 is 1.43.

[0156] (4) Prepare the battery

[0157] The positive electrode sheet prepared in step (1), the separator (a polyethylene film with a thickness of 5 μm, a boehmite ceramic layer with a thickness of 2 μm on one surface of the polyethylene film, a mixed adhesive layer of polyvinylidene fluoride and polymethyl methacrylate with a thickness of 1 μm on the outer surface of the boehmite ceramic layer, and a polymethyl methacrylate adhesive layer with a thickness of 1 μm on the other surface of the polyethylene film) and the negative electrode sheet prepared in step (2) are wound to obtain a core; the battery is obtained through encapsulation, baking, liquid injection, formation, secondary sealing, sorting, and OCV. Among them, the positive electrode active coating with a convex part area is located closer to the winding center side of the core, and the positive electrode active coating with a concave part area is located farther from the winding center side of the core; and the positive electrode tab groove and the third buried glue groove are arranged facing each other.

[0158] Example 2

[0159] Prepare the battery according to the following method:

[0160] (1) Prepare the positive electrode sheet

[0161] Lithium cobaltate (the mass content W of elemental aluminum is 7213 ppm), a positive electrode conductive agent (conductive carbon black and carbon nanotubes are mixed in a mass ratio of 2:1), and a positive electrode binder (polyvinylidene fluoride) are mixed in a mass ratio of 97:1.9:1.1, and N-methylpyrrolidone (NMP) is added and stirred evenly to prepare a positive electrode paste; Boehmite (the average particle size N1 is 103 nm), a conductive agent (conductive carbon black), a conductive agent (carbon nanotubes), carboxymethyl cellulose, and a binder (styrene-butadiene rubber) are mixed in a mass ratio of 80:5:5:4:6, deionized water is added, and stirred evenly to prepare a bottom coating paste; the above bottom coating paste is coated on both side surfaces of the aluminum foil and dried; then the above positive electrode paste is coated on the aluminum foil surface coated with the bottom coating, and after drying and rolling treatment; then it is slit, and all the positive electrode active coatings are removed in a specific area to form a positive electrode tab groove, and the positive electrode tab is welded in the positive electrode tab groove, and the first adhesive tape (SBS rubber adhesive tape) is attached; then it is processed using a special roller with protrusions to obtain a positive electrode sheet with a concave portion area on one side surface and a convex portion area on the other side surface;

[0162] Among them, the thickness N2 of the bottom coating is 1 μm; the positions of the concave portions and the convex portions on the surface of the positive electrode active coating correspond one by one. The depth H of the concave portion is 11 μm, the spacing I between the concave portions is 2 mm, the width K of the concave portion is 1 mm, and the ratio J of the area of the positive projection of the concave portion area on the surface of the positive electrode sheet to the area of the positive projection of the positive electrode active coating on the surface of the positive electrode sheet is 60%; the height of the convex portion is 11 μm, the spacing of the convex portions is 2 mm, the width of the positive projection of the convex portion on the surface of the positive electrode sheet is 1 mm, and the ratio J' of the area of the positive projection of the convex portion area on the surface of the positive electrode sheet to the area of the positive projection of the positive electrode active coating on the surface of the positive electrode sheet is 60%.

[0163] (2) Preparation of the negative electrode sheet

[0164] Artificial graphite, silicon-carbon materials (the first silicon-carbon particles, the second silicon-carbon particles, and the third silicon-carbon particles are mixed in a mass ratio of 3:77:20), a negative electrode conductive agent (conductive carbon black), a thickening agent (lithium carboxymethyl cellulose), and a negative electrode binder (polyacrylic acid) are mixed in a mass ratio of 74.5:22.8:0.5:0.6:1.6, and deionized water is added to prepare a negative electrode paste; the above negative electrode paste is coated on both side surfaces of the negative electrode current collector (copper foil, the tensile strength σ in the width direction is 806 MPa), and after baking, rolling, die-cutting, and cold pressing, grooves are etched on the outer surface of the negative electrode active coating through laser processing technology, and then all / part of the negative electrode active coating is removed in a specific area to form a first tab groove, a second tab groove, a first buried adhesive groove, a second buried adhesive groove, and a third buried adhesive groove, and the negative electrode tab (nickel-plated copper tab) is welded in the first tab groove, and the second adhesive tape (SBS rubber adhesive tape) is attached in the third buried adhesive groove to obtain a negative electrode sheet;

[0165] Among them, the sphericity A1 of the first silicon-carbon particle is 0.95, and the sphericity A2 of the second silicon-carbon particle is 0.7; the OI value B of the negative electrode sheet is 15.7, B / A1 is 16.53, and B×A2 is 10.99; the average particle size D0 of the negative electrode active material is 16.9 μm, the average particle size D1 of the first silicon-carbon particle is 4.4 μm, the average particle size D2 of the third silicon-carbon particle is 7.5 μm, and the average particle size D3 of the second silicon-carbon particle is 10 μm; the mass content C1 of silicon element in the first silicon-carbon particle is 30%, the mass content C2 of silicon element in the second silicon-carbon particle is 40%, C1 < C2, and the mass content of silicon element in the third silicon-carbon particle is 30%; the mass content C0 of silicon element in the negative electrode active coating is 8.6%; B×C0 is 1.35; σ / D1 is 183.18, σ / D2 is 107.47, and σ / D3 is 80.6; the depth E of the groove is 7 μm, the width F of the groove is 28 μm, and the spacing G of the grooves is 500 μm; the distance S1 between the side edge of the first tab groove and the side edge of the first glue-embedding groove is 0.4 mm, the distance S1' between the side edge of the second tab groove and the side edge of the second glue-embedding groove is 0.4 mm, the distance S2 between the bottom edge of the first tab groove and the bottom edge of the first glue-embedding groove is 3.2 mm, the distance S2' between the bottom edge of the second tab groove and the bottom edge of the second glue-embedding groove is 3.2 mm, the depth S3 of the first glue-embedding groove is 12 μm, the depth S3' of the second glue-embedding groove is 12 μm, and the dimension S4 by which the edge of the orthographic projection of the first glue-embedding groove on the negative electrode sheet exceeds the edge of the orthographic projection of the second glue-embedding groove on the negative electrode sheet is 1.5 mm.

[0166] (3) Preparation of electrolyte

[0167] In a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), a non-fluorinated carboxylic ester compound (propyl propionate and ethyl propionate are mixed in a mass ratio of 2:1) with a mass content X1 of 30% and a carbonate compound (EC, PC, and DEC are mixed in a mass ratio of 1:3:6) with a mass content X2 of 35% are mixed evenly, and then DFEA is added; a sulfonate compound (PS) with a mass content X3 of 0.6%, a lithium salt LiPF6 with a mass content of 12.5%, FEC with a mass content of 10%, SN with a mass content of 2%, AND with a mass content of 1.5%, and HTCN with a mass content of 1.5% are added to obtain the electrolyte;

[0168] Among them, X1 / X2 is 0.86.

[0169] (4) Preparation of battery

[0170] The positive electrode sheet prepared in step (1), the separator (a polyethylene film with a thickness of 5 μm, a boehmite ceramic layer with a thickness of 2 μm on one surface of the polyethylene film, a mixed glue layer of polyvinylidene fluoride and polymethyl methacrylate with a thickness of 1 μm on the outer surface of the boehmite ceramic layer, and a polymethyl methacrylate glue layer with a thickness of 1 μm on the other surface of the polyethylene film) and the negative electrode sheet prepared in step (2) are wound to obtain a core; through encapsulation, baking, liquid injection, formation, secondary sealing, sorting and OCV, a battery is obtained. Among them, the positive electrode active coating with a convex portion area is located on the side close to the winding center of the core, and the positive electrode active coating with a concave portion area is located on the side far from the winding center of the core; and the positive electrode tab groove and the third buried glue groove are arranged facing each other.

[0171] Example 3

[0172] The battery is prepared according to the following method:

[0173] (1) Prepare the positive electrode sheet

[0174] Lithium cobaltate (the mass content W of elemental aluminum is 8467 ppm), a positive electrode conductive agent (conductive carbon black and carbon nanotubes are mixed according to a mass ratio of 2:1), and a positive electrode binder (polyvinylidene fluoride) are mixed according to a mass ratio of 97:1:2, and N-methylpyrrolidone (NMP) is added and stirred evenly to prepare a positive electrode slurry; Boehmite (the average particle size N1 is 149 nm), a conductive agent (conductive carbon black), a conductive agent (carbon nanotubes), carboxymethyl cellulose and a binder (styrene-butadiene rubber) are mixed according to a mass ratio of 80:5:5:4:6, deionized water is added and stirred evenly to prepare a bottom coating slurry; the above bottom coating slurry is coated on both surfaces of the aluminum foil and dried; then the above positive electrode slurry is coated on the aluminum foil coated with the bottom coating, and after drying and rolling treatment; then cutting treatment is carried out, all the positive electrode active coating is removed in a specific area to form a positive electrode tab groove, and the positive electrode tab is welded in the positive electrode tab groove, and the first adhesive paper (SBS rubber adhesive paper) is attached; then processing is carried out using a special roller with protrusions to obtain a positive electrode sheet with a concave portion area on one surface and a convex portion area on the other surface;

[0175] Among them, the thickness N2 of the bottom coating is 3 μm; the positions of the concave portions and the convex portions on the surface of the positive electrode active coating correspond one by one. The depth H of the concave portion is 30 μm, the spacing I between the concave portions is 6 mm, the width K of the concave portion is 5 mm, and the ratio J of the area of the positive projection of the concave portion area on the surface of the positive electrode sheet to the area of the positive projection of the positive electrode active coating on the surface of the positive electrode sheet is 70%; the height of the convex portion is 30 μm, the spacing of the convex portions is 6 mm, the width of the positive projection of the convex portion on the surface of the positive electrode sheet is 5 mm, and the ratio J' of the area of the positive projection of the convex portion area on the surface of the positive electrode sheet to the area of the positive projection of the positive electrode active coating on the surface of the positive electrode sheet is 70%.

[0176] (2) Prepare the negative electrode sheet

[0177] Mix artificial graphite, silicon-carbon materials (the first silicon-carbon particles, the second silicon-carbon particles, and the third silicon-carbon particles are mixed in a mass ratio of 20:78:2), a negative electrode conductive agent (conductive carbon black), a thickening agent (lithium carboxymethyl cellulose), a negative electrode binder (styrene-butadiene rubber), and a negative electrode binder (polyacrylic acid) in a mass ratio of 78.5:18:0.5:1:1:1, add deionized water, and prepare a negative electrode slurry; coat the above negative electrode slurry on both sides of a negative electrode current collector (copper foil, with a tensile strength σ in the width direction of 753 MPa), bake, roll, die-cut, and cold-press, and then, through laser processing technology, etch grooves on the outer surface of the negative electrode active coating, and then remove all / part of the negative electrode active coating 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, and weld a negative electrode tab (nickel-plated copper tab) in the first tab groove, and attach a second adhesive paper (SBS rubber adhesive paper) in the third glue embedding groove to obtain a negative electrode sheet;

[0178] Among them, the sphericity A1 of the first silicon-carbon particles is 0.9, and the sphericity A2 of the second silicon-carbon particles is 0.51; the OI value B of the negative electrode sheet is 20.3, B / A1 is 22.56, and B×A2 is 10.35; the average particle size D0 of the negative electrode active material is 12.3 μm, the average particle size D1 of the first silicon-carbon particles is 2.5 μm, the average particle size D2 of the third silicon-carbon particles is 5.1 μm, and the average particle size D3 of the second silicon-carbon particles is 7.1 μm; the mass content C1 of silicon element in the first silicon-carbon particles is 40%, the mass content C2 of silicon element in the second silicon-carbon particles is 50%, C1 < C2, and the mass content of silicon element in the third silicon-carbon particles is 40%; the mass content C0 of silicon element in the negative electrode active coating is 8.6%; B×C0 is 1.75; σ / D1 is 301.2, σ / D2 is 147.65, and σ / D3 is 106.06; the depth E of the groove is 42 μm, the width F of the groove is 146 μm, and the spacing G between the grooves is 2000 μm; the distance S1 between the side edge of the first tab groove and the side edge of the first glue embedding groove is 1.5 mm, the distance S1' between the side edge of the second tab groove and the side edge of the second glue embedding groove is 1.5 mm, the distance S2 between the bottom edge of the first tab groove and the bottom edge of the first glue embedding groove is 1.2 mm, the distance S2' between the bottom edge of the second tab groove and the bottom edge of the second glue embedding groove is 1.2 mm, the depth S3 of the first glue embedding groove is 29 μm, the depth S3' of the second glue embedding groove is 29 μm, and the dimension S4 by which the edge of the orthographic projection of the first glue embedding groove on the negative electrode sheet exceeds the edge of the orthographic projection of the second glue embedding groove on the negative electrode sheet is 0.3 mm.

[0179] (3) Prepare the electrolyte

[0180] In a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), a non-fluorinated carboxylic acid ester compound (propyl propionate) with a mass content X1 of 50% and a carbonate compound (a mixture of EC, PC, and DEC in a mass ratio of 1:3:6) with a mass content X2 of 20% were mixed evenly, and then DFEA was added; a sulfonate compound (PS) with a mass content X3 of 1.6%, a lithium salt LiPF6 with a mass content of 12.5%, FEC with a mass content of 10%, SN with a mass content of 2%, AND with a mass content of 1.5%, and HTCN with a mass content of 1.5% were added to obtain an electrolyte;

[0181] Among them, X1 / X2 is 2.5.

[0182] (4) Preparation of the battery

[0183] The positive electrode sheet prepared in step (1), the separator (a polyethylene film with a thickness of 5 μm, a boehmite ceramic layer with a thickness of 2 μm on one surface of the polyethylene film, a mixed adhesive layer of polyvinylidene fluoride and polymethyl methacrylate with a thickness of 1 μm on the outer surface of the boehmite ceramic layer, and a polymethyl methacrylate adhesive layer with a thickness of 1 μm on the other surface of the polyethylene film), and the negative electrode sheet prepared in step (2) were wound to obtain a core; the battery was obtained through encapsulation, baking, liquid injection, formation, secondary sealing, sorting, and OCV. Among them, the positive electrode active coating with a convex part area is located closer to the winding center side of the core, and the positive electrode active coating with a concave part area is located farther from the winding center side of the core; and the positive electrode tab groove and the third buried adhesive groove are arranged facing each other.

[0184] Example 4

[0185] It is used to verify the influence brought by the change of "the sphericity A1 of the first silicon-carbon particles".

[0186] It was carried out with reference to Example 1, the difference being that A1 was changed. Specifically: A1 is 0.81; among them, B / A1 is 17.9.

[0187] Example 5 group

[0188] This group of examples is used to verify the influence brought by the change of "the sphericity A2 of the second silicon-carbon particles".

[0189] This group of examples was carried out with reference to Example 1, the difference being that A2 was changed, specifically as follows:

[0190] Example 5a, A2 is 0.32; among them, B×A2 is 4.64;

[0191] Example 5b, A2 is 0.78; among them, B×A2 is 11.31.

[0192] Example 6 group

[0193] This group of embodiments is used to verify the influence brought about by the change of "OI value B of the negative electrode sheet".

[0194] This group of embodiments is carried out with reference to Embodiment 1. The difference is that B is regulated by changing the baking temperature and time of the negative electrode current collector after coating the negative electrode paste, specifically as follows:

[0195] In Example 6a, B is 9.3; among them, B / A1 is 9.39, and B×A2 is 5.86;

[0196] In Example 6b, B is 23.8; among them, B / A1 is 24.04, and B×A2 is 14.99.

[0197] Example 7 group

[0198] This group of embodiments is used to verify the influence brought about by the change of "B×A2". <0_{

[0199] This group of embodiments is carried out with reference to Embodiment 1 and Embodiment 3 respectively. The difference is that B×A2 is regulated by changing the sphericity A2 of the second silicon-carbon particles, specifically as follows:

[0200] In Example 7a, carried out with reference to Embodiment 1. The difference is that A2 is 0.51; among them, B×A2 is 7.4;

[0201] In Example 7b, carried out with reference to Embodiment 3. The difference is that A2 is 0.7; among them, B×A2 is 14.21.

[0202] Example 8

[0203] Used to verify the influence brought about by the change of "tensile strength σ of the negative electrode current collector in the width direction".

[0204] Carried out with reference to Embodiment 1. The difference is that σ is regulated by changing the negative electrode current collector. Specifically: σ is 632 MPa; among them, σ / D1 is 180.57, σ / D2 is 98.75, and σ / D3 is 76.14.

[0205] Example 9 group

[0206] This group of embodiments is used to verify the influence brought about by the change of "average particle size D3 of the second silicon-carbon particles".

[0207] This group of embodiments is carried out with reference to Embodiment 1. The difference is that D3 is changed, specifically as follows:

[0208] In Example 9a, D3 is 6.2 μm; among them, σ / D3 is 136.77;

[0209] In Example 9b, D3 is 11.8 μm; among them, σ / D3 is 71.86.

[0210] 10 groups of Examples

[0211] This group of examples is used to verify the influence brought about by the change of "the average particle size D0 of the negative electrode active material".

[0212] This group of examples is carried out with reference to Example 1. The difference is that D0 is regulated by changing the average particle size of artificial graphite, specifically as follows:

[0213] Example 10a, D0 is 8.1 μm;

[0214] Example 10b, D0 is 19.7 μm.

[0215] 11 groups of Examples

[0216] This group of examples is used to verify the influence brought about by the change of "the average particle size D1 of the first silicon-carbon particle".

[0217] This group of examples is carried out with reference to Example 1. The difference is that D1 is changed, specifically as follows:

[0218] Example 11a, D1 is 1.2 μm; among which, σ / D1 is 706.67;

[0219] Example 11b, D1 is 4.9 μm; among which, σ / D1 is 173.06.

[0220] 12 groups of Examples

[0221] This group of examples is used to verify the influence brought about by the change of "the average particle size D2 of the third silicon-carbon particle".

[0222] This group of examples is carried out with reference to Example 1. The difference is that D2 is changed, specifically as follows:

[0223] Example 12a, D2 is 3.2 μm; among which, σ / D2 is 265;

[0224] Example 12b, D2 is 8 μm; among which, σ / D2 is 106.

[0225] 13 groups of Examples

[0226] This group of examples is used to verify the influence brought about by the change of "the mass content C0 of silicon element in the negative electrode active coating".

[0227] This group of examples is carried out with reference to Example 1. The difference is that C0 is regulated by changing the mass ratio of the carbon-based material and the silicon-carbon material in the negative electrode slurry, specifically as follows:

[0228] Example 13a, C0 is 5.2%; wherein, the mass ratio of the carbon-based material, silicon-carbon material, negative electrode conductive agent, thickening agent and negative electrode binder (styrene-butadiene rubber and polyacrylic acid) is 84:12.9:0.5:0.8:0.5:1.3; B×C0 is 0.75;

[0229] Example 13b, C0 is 14.7%; wherein, the mass ratio of the carbon-based material, silicon-carbon material, negative electrode conductive agent, thickening agent and negative electrode binder (styrene-butadiene rubber and polyacrylic acid) is 61:35.9:0.5:0.8:0.5:1.3; B×C0 is 2.09;

[0230] Example 13c, C0 is 1.3%; wherein, the mass ratio of the carbon-based material, silicon-carbon material, negative electrode conductive agent, thickening agent and negative electrode binder (styrene-butadiene rubber and polyacrylic acid) is 94:2.9:0.5:0.8:0.5:1.3; B×C0 is 0.17;

[0231] Example 13d, C0 is 28.5%; wherein, the mass ratio of the carbon-based material, silicon-carbon material, negative electrode conductive agent, thickening agent and negative electrode binder (styrene-butadiene rubber and polyacrylic acid) is 26.9:70.9:0.8:0.5:1.3; B×C0 is 4.13.

[0232] Example 14 group

[0233] This group of examples is used to verify the influence brought by the change of "m1 + m2 + m3". [[ID=ID=16]]

[0234] This group of examples is carried out with reference to Example 1. The difference is that by changing the addition amounts of the thickening agent and the negative electrode binder (styrene-butadiene rubber and polyacrylic acid) in the negative electrode slurry, m1 + m2 + m3 is regulated as follows:

[0235] Example 14a, m1 + m2 + m3 is 3.18%; wherein, the mass ratio of the carbon-based material, silicon-carbon material, negative electrode conductive agent, thickening agent and negative electrode binder (styrene-butadiene rubber and polyacrylic acid) is 75:21.5:0.5:0.5:0.5:2;

[0236] Example 14b, m1 + m2 + m3 is 4.6%; wherein, the mass ratio of the carbon-based material, silicon-carbon material, negative electrode conductive agent, thickening agent and negative electrode binder (styrene-butadiene rubber and polyacrylic acid) is 73.5:21.5:0.5:2:2:0.5.

[0237] Example 15 group

[0238] This group of examples is used to verify the influence brought by the change of "σ / D1".

[0239] This group of embodiments is carried out by referring to Embodiment 1 and Embodiment 3 respectively. The difference is that σ / D1 is regulated by changing D1, specifically as follows:

[0240] Embodiment 15a is carried out by referring to Embodiment 1. The difference is that D1 is 2.5 μm; among them, σ / D1 is 339.2;

[0241] Embodiment 15b is carried out by referring to Embodiment 3. The difference is that D1 is 4.4 μm; among them, σ / D1 is 171.14.

[0242] Embodiment 16 group

[0243] This group of embodiments is used to verify the influence brought by the change of "σ / D2".

[0244] This group of embodiments is carried out by referring to Embodiment 1 and Embodiment 3 respectively. The difference is that σ / D2 is regulated by changing D2, specifically as follows:

[0245] Embodiment 16a is carried out by referring to Embodiment 1. The difference is that D2 is 5.1 μm; among them, σ / D2 is 166.27;

[0246] Embodiment 16b is carried out by referring to Embodiment 3. The difference is that D2 is 7.5 μm; among them, σ / D2 is 100.4.

[0247] Embodiment 17 group

[0248] This group of embodiments is used to verify the influence brought by the change of "σ / D3".

[0249] This group of embodiments is carried out by referring to Embodiment 1 and Embodiment 3 respectively. The difference is that σ / D3 is regulated by changing D3, specifically as follows:

[0250] Embodiment 17a is carried out by referring to Embodiment 1. The difference is that D3 is 7.1 μm; among them, σ / D3 is 119.44;

[0251] Embodiment 17b is carried out by referring to Embodiment 3. The difference is that D3 is 10 μm; among them, σ / D3 is 75.3.

[0252] Embodiment 18

[0253] It is used to verify the influence brought by "the outer surface of the negative electrode active coating does not have grooves".

[0254] It is carried out by referring to Embodiment 1. The difference is that grooves are not etched on the outer surface of the negative electrode active coating.

[0255] Embodiment 19

[0256] To verify the impact brought about by "the negative electrode sheet does not have the first glue-embedding groove and the second glue-embedding groove".

[0257] It is carried out with reference to Example 1, except that the first glue-embedding groove and the second glue-embedding groove are not provided.

[0258] Example 20

[0259] To verify the impact brought about by "the surface of the positive electrode active coating does not have a concave region and a convex region".

[0260] It is carried out with reference to Example 1, except that after the slitting process, a special roller with protrusions is not used for processing.

[0261] Example 21 group

[0262] This group of examples is used to verify the impact brought about by the change of "the ratio J of the area of the positive projection of the concave region on the surface of the positive electrode sheet to the area of the positive projection of the positive electrode active coating on the surface of the positive electrode sheet".

[0263] This group of examples is carried out with reference to Example 1, except that J is regulated by changing the rolling area of the special roller with protrusions, specifically as follows:

[0264] Example 21a, J is 40%, J' is 40%;

[0265] Example 21b, J is 90%, J' is 90%.

[0266] Example 22 group

[0267] This group of examples is used to verify the impact brought about by the change of "the mass content W of element aluminum in lithium cobaltate".

[0268] This group of examples is carried out with reference to Example 1, except that W is changed, specifically as follows:

[0269] Example 22a, W is 7041 ppm;

[0270] Example 22b, W is 9947 ppm.

[0271] Example 23 group

[0272] This group of examples is used to verify the impact brought about by the change of "m4".

[0273] This group of examples is carried out with reference to Example 1, except that m4 is regulated by changing the addition amount of the positive electrode binder in the positive electrode slurry, specifically as follows:

[0274] Example 23a, the mass ratio of lithium cobaltate, positive electrode conductive agent and positive electrode binder is 94.5:1.5:4;

[0275] Example 23b, the mass ratio of lithium cobaltate, positive electrode conductive agent and positive electrode binder is 93.5:1.5:5.

[0276] Example 24

[0277] Used to verify the influence brought by "the positive electrode sheet does not have a bottom coating".

[0278] Carried out with reference to Example 1, the difference is that no bottom coating is provided.

[0279] Example 25 group

[0280] This group of examples is used to verify the influence brought by the change of "the average particle size N1 of inorganic particles".

[0281] This group of examples is carried out with reference to Example 1, the difference is that N1 is changed, specifically as follows:

[0282] Example 25a, N1 is 25 nm;

[0283] Example 25b, N1 is 684 nm.

[0284] Example 26

[0285] Used to verify the influence brought by the change of "the mass content X1 of non-fluorinated carboxylic acid ester compound".

[0286] Carried out with reference to Example 1, the difference is that X1 is changed, specifically: X1 is 20%; among them, X1 / X2 is 0.71.

[0287] Example 27

[0288] Used to verify the influence brought by the change of "the mass content X2 of carbonate compound".

[0289] Carried out with reference to Example 1, the difference is that X2 is changed, specifically: X2 is 5%; among them, X1 / X2 is 8.

[0290] Example 28 group

[0291] This group of examples is used to verify the influence brought by the change of "the mass content X3 of sulfonate compound".

[0292] This group of examples is carried out with reference to Example 1, the difference is that X3 is changed, specifically as follows:

[0293] Example 28a, X3 is 0.5%;

[0294] Example 28b, X3 is 40%.

[0295] Comparative Example 1

[0296] Performed with reference to Example 1, except that all the silicon-carbon materials are the first silicon-carbon particles, where C0 is 7.7%.

[0297] Comparative Example 2

[0298] Performed with reference to Example 1, except that all the silicon-carbon materials are the second silicon-carbon particles, where C0 is 7.7%.

[0299] Comparative Example 3a

[0300] Performed with reference to Example 1, except that A1 is 0.75.

[0301] Comparative Example 3b

[0302] Performed with reference to Example 1, except that A2 is 0.25.

[0303] Comparative Example 4a

[0304] Performed with reference to Example 1, except that B is 5.3.

[0305] Comparative Example 4b

[0306] Performed with reference to Example 1, except that B is 39.6.

[0307] Comparative Example 5

[0308] Performed with reference to Example 1, except that σ is 585 MPa.

[0309] The silicon-carbon materials used in the above examples and preparation examples are porous carbon matrices and nano-silicon materials located in the pores of the porous carbon matrices. And the copper foils are all commercially available.

[0310] Test Example I

[0311] (1) Thermogravimetric test of the negative electrode sheet

[0312] The negative electrode sheets of the batteries prepared in the examples were subjected to thermogravimetric tests, and the specific test results are shown in Table 1.

[0313] (2) Thermogravimetric test of the positive electrode active coating

[0314] The positive electrode active coatings of the batteries prepared in the examples were subjected to thermogravimetric tests, and the specific test results are shown in Table 1.

[0315] Table 1

[0316]

[0317]

[0318] Note: The "*" in Table 1 indicates that the value is the same as that of the example it refers to in this example.

[0319] Test Example II

[0320] (1) Energy density test

[0321] The batteries prepared in the examples and comparative examples were subjected to energy density tests. The specific test methods are as follows:

[0322] Using a Neware battery tester, charge at 0.2C to the upper limit voltage of 4.53V (cutoff at 0.02C), discharge at 0.2C to the lower limit voltage of 3.0V, repeat 3 times, and take the discharge energy of the 3rd time as Q. Use a 2.5D microscope tester to measure the width and height of the battery, and use a PPG thickness tester to measure the full charge thickness of the battery; then the energy density is Q / (width × height × full charge thickness), with the unit of Wh / L. Record the results in Table 2.

[0323] (2) Cycle test

[0324] The batteries prepared in the examples and comparative examples were subjected to cycle tests. The specific test methods are as follows:

[0325] At 25°C, charge at 1C to the cutoff voltage of 4.53V, charge at constant voltage to the cutoff current of 0.05C, discharge at 1C to 3.0V, and repeat the above charge and discharge steps 200 times; the capacity retention rate is the discharge capacity of the 200th time divided by the maximum value of the first 3 discharge capacities; measure the full charge thickness of the battery before the cycle test and the full charge thickness of the battery after 200 cycles. The thickness expansion rate is the difference between the two divided by the full charge thickness of the battery before the cycle test; after the cycle is completed, disassemble the battery and observe whether there are cracks or fractures in the positive electrode plate. If there are no cracks and fractures in the positive electrode plate, record it as "no"; if there are only cracks but no fractures in the positive electrode plate, record it as "crack"; if the positive electrode plate is fractured, record it as "fracture"; the degree ranges from light to heavy as "no", "crack", and "fracture". Record the above results in Table 2.

[0326] (3) Self-discharge test

[0327] The batteries prepared in the examples and comparative examples were subjected to self-discharge tests. The specific test methods are as follows: After the battery is fully charged, discharge it at 0.1C to 3.6V, let it stand for 10 hours, and test the battery voltage V after standing. The self-discharge K = (3.6 - V) / 10, with the unit of mV / h. Record the results in Table 2.

[0328] (4) 25°C constant current charging ratio test

[0329] The batteries prepared in the examples and comparative examples were subjected to 25°C constant current charging ratio tests. The specific test methods are as follows:

[0330] At 25 °C, discharge at 0.2C to 3V, then charge at a constant current of 2C to 4.53V, record the constant current charge capacity R1, continue the constant voltage charge until the cut-off current of 0.05C, record the constant voltage charge capacity R2, and the constant current charge ratio is R1 / (R1 + R2). Record the results in Table 2.

[0331] Table 2

[0332]

[0333]

[0334]

[0335] As can be seen from Table 2, compared with the comparative example, the battery prepared from the negative electrode sheet of the present invention has both a high energy density and a cyclic stability, and can significantly improve the phenomenon of self-discharge.

[0336] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative electrode current collector and a negative electrode active coating located on at least one surface of the negative electrode current collector. The negative electrode active coating includes a negative electrode active material, the negative electrode active material includes a silicon-carbon material, and the silicon-carbon material includes first silicon-carbon particles and second silicon-carbon particles. The sphericity of the first silicon-carbon particles is A1, the sphericity of the second silicon-carbon particles is A2, 0.3 ≤ A2 < 0.8, and A1 > A2. The OI value B of the negative electrode sheet is 9 - 25. A1, A2, and B satisfy: 8.5 ≤ B / A1 ≤ 30, and 4 ≤ B×A2 ≤ 15. The tensile strength σ of the negative electrode current collector in the width direction is ≥ 600 MPa.

2. The negative electrode sheet according to claim 1, wherein, 0.8 ≤ A1 ≤ 0.99; preferably, 0.9 ≤ A1 ≤ 0.

99. And / or, 0.5 ≤ A2 ≤ 0.

7. And / or, B is 14 - 21. And / or, 14 ≤ B / A1 ≤ 23. And / or, 7 ≤ B×A2 ≤ 14.

5. And / or, 650 MPa ≤ σ ≤ 900 MPa; preferably, 750 MPa ≤ σ ≤ 850 MPa.

3. The negative electrode sheet according to claim 1 or 2, wherein The average particle size D0 of the negative electrode active material is 6 μm - 25 μm. And / or, the silicon-carbon material further includes third silicon-carbon particles, and the third silicon-carbon particles are mainly formed by a plurality of the first silicon-carbon particles. And / or, the average particle size D3 of the second silicon-carbon particles is 5 μm - 15 μm. 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, mesophase carbon microspheres, soft carbon, and hard carbon. Preferably, the average particle size D1 of the first silicon-carbon particles is 1 μm - 6 μm, and the average particle size D2 of the third silicon-carbon particles is 3 μm - 10 μm; more preferably, D1 is 2 μm - 5 μm, and D2 is 3 μm - 8 μm. Preferably, the first silicon-carbon particles, the second silicon-carbon particles, and the third silicon-carbon particles each independently include a porous carbon matrix and nano-silicon located in the pores of the porous carbon matrix.

4. The negative electrode sheet according to claim 1 or 2, wherein, The mass content C1 of silicon element in the first silicon-carbon particles is 25% - 40%, and the mass content C2 of silicon element in the second silicon-carbon particles is 30% - 50%; preferably, C1 < C2. And / or, the mass content of silicon element in the negative electrode active coating is C0, and C0 and B satisfy: B×C0 ≥ 0.1; preferably, 0.15 ≤ B×C0 ≤ 4.5; more preferably, 1.2 ≤ B×C0 ≤ 1.

8. Preferably, C0 is 1% - 30%; more preferably, 2% - 15%.

5. The negative electrode sheet according to claim 1 or 2, wherein For the thermogravimetric curve of the negative electrode sheet, the weight loss percentage at 200°C - 300°C is m1, the weight loss percentage at 300°C - 375°C is m2, and the weight loss percentage at 375°C - 500°C is m3. m1, m2, and m3 satisfy: 0.8% ≤ m1 + m2 + m3 ≤ 6.2%, 0.3% ≤ m1 ≤ 2.2%, 0% ≤ m2 ≤ 2.2%, 0.3% ≤ m3 ≤ 2.2%. Preferably, 1.8% ≤ m1 + m2 + m3 ≤ 3.2%, 0.4% ≤ m1 ≤ 1.2%, 0% ≤ m2 ≤ 1.2%, 0.8% ≤ m3 ≤ 1.8%; Preferably, the negative electrode active coating further includes a negative electrode binder and a thickener. The negative electrode binder includes at least one of polyacrylic acid, polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, polyacrylonitrile, polyurethane, polymethacrylate, polyacrylate, and polyethylene oxide. The thickener includes at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose. Based on the total mass of the negative electrode active coating, the content of the negative electrode binder is 0.1% - 10%, and the content of the thickener is 0.1% - 10%; More preferably, based on the total mass of the negative electrode active coating, the content of the negative electrode binder is 0.3% - 4.4%, and the content of the thickener is 0.3% - 2.2%.

6. The negative electrode sheet according to claim 3, wherein The tensile strength σ of the negative electrode current collector in the width direction, in MPa, and the average particle size D1 of the first silicon-carbon particles, in μm, satisfy: 150 ≤ σ / D1 ≤ 750; preferably, 170 ≤ σ / D1 ≤ 340; more preferably, 180 ≤ σ / D1 ≤ 305; And / or, the tensile strength σ of the negative electrode current collector in the width direction, in MPa, and the average particle size D2 of the third silicon-carbon particles, in μm, satisfy: 90 ≤ σ / D2 ≤ 300; preferably, 100 ≤ σ / D2 ≤ 170; more preferably, 105 ≤ σ / D2 ≤ 150; And / or, the tensile strength σ of the negative electrode current collector in the width direction, in MPa, and the average particle size D3 of the second silicon-carbon particles, in μm, satisfy: 70 ≤ σ / D3 ≤ 140; preferably, 75 ≤ σ / D3 ≤ 120; more preferably, 80 ≤ σ / D3 ≤ 110.

7. The negative electrode sheet according to claim 1 or 2, wherein, The outer surface of the negative electrode active coating has a plurality of grooves. The depth E of the grooves is 5 μm - 50 μm, the width F of the grooves is 20 μm - 150 μm, and the spacing G of the grooves is 500 μm - 2000 μm; And / or, the negative electrode sheet further includes a first tab groove and a first glue-embedding groove on the first surface of the negative electrode current collector and a second tab groove and a second glue-embedding groove on the second surface of the negative electrode current collector. The first tab groove and the first glue-embedding groove are in contact with each other, and the second tab groove and the second glue-embedding groove are in contact with each other.

8. A lithium ion secondary battery, characterized in that, The lithium ion secondary battery includes the negative electrode sheet according to any one of claims 1 - 7; Preferably, the lithium ion secondary battery further includes a positive electrode sheet and a separator, and the positive electrode sheet, the separator, and the negative electrode sheet are wound to form a wound core.

9. The lithium ion secondary battery according to claim 8, wherein, The lithium ion secondary battery further includes a positive electrode sheet. The positive electrode sheet includes a positive electrode current collector and a positive electrode active coating on at least one surface of the positive electrode current collector; The surface of the positive electrode active coating located on the winding center side close to the core has a convex region with a number of convex portions; the surface of the positive electrode active coating located on the side away from the winding center of the core has a concave region with a number of concave portions; Preferably, the depth H of the concave portion is 10 μm - 30 μm, the spacing I between the concave portions is 2 mm - 6 mm, and the width K of the concave portion is 1 mm - 5 mm; Preferably, the ratio J of the area of the positive projection of the concave region on the surface of the positive electrode sheet to the area of the positive projection of the positive electrode active coating on the surface of the positive electrode sheet on the side away from the winding center of the core is 40% - 90%; 10. The lithium ion secondary battery according to claim 9, wherein, The positive electrode active coating includes a positive electrode active material, the positive electrode active material includes lithium cobaltate, the mass content W of element aluminum in the lithium cobaltate is 7000 ppm - 12000 ppm, and the cut-off voltage of the lithium ion secondary battery is ≥ 4.48 V; Preferably, for the thermogravimetric curve of the positive electrode active coating, the weight loss percentage m4 at 300 °C - 500 °C is 0.8% - 5.2%; more preferably 0.9% - 2.2%; Preferably, the positive electrode sheet further includes a bottom coating located between the positive electrode current collector and the positive electrode active coating. The bottom coating includes inorganic particles, the average particle size N1 of the inorganic particles is 20 nm - 1000 nm, and the thickness N2 of the bottom coating is 0.5 μm - 5 μm; more preferably, the inorganic particles include at least one of boehmite, aluminum oxide, magnesium oxide, silicon dioxide, titanium dioxide, aluminum hydroxide, lithium iron phosphate, and magnesium hydroxide; Preferably, the positive electrode sheet further includes a positive electrode tab groove and a positive electrode tab disposed in the positive electrode tab groove. The negative electrode sheet further includes a third buried glue groove. Along the thickness direction of the core, the positive electrode tab groove and the third buried glue groove are arranged facing each other; more preferably, a first adhesive tape is provided on the surface of the positive electrode tab, and a second adhesive tape is provided at least partially in the third buried glue groove.

11. The lithium ion secondary battery according to claim 8 or 9, wherein, The lithium ion secondary battery further includes an electrolyte. The electrolyte includes a solvent, and the solvent includes a non-fluorinated carboxylic acid ester compound and a carbonate compound; in the electrolyte, the mass content of the non-fluorinated carboxylic acid ester compound is X1, and the mass content of the carbonate compound is X2, and X1 and X2 satisfy 0.4 ≤ X1 / X2 ≤ 9; preferably, 0.8 ≤ X1 / X2 ≤ 2.5; And / or, X1 is 20% - 60%; preferably 30% - 50%; And / or, X2 is 5% - 45%; Preferably, the non-fluorinated carboxylic acid ester compound includes at least one of γ-butyrolactone, methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, and butyl butyrate; Preferably, the carbonate compound includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

12. The lithium ion secondary battery according to claim 11, wherein, The electrolyte further includes a sulfonate compound; Preferably, in the electrolyte, the mass content X3 of the sulfonate compound is 0.5%-5%; Preferably, the sulfonate compound includes at least one of 1,3-propane sultone, 1,3-propene sultone, and ethylene sulfate; and the like.

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