Lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion secondary battery. The lithium ion secondary battery comprises a positive plate and a negative plate; the positive plate comprises a positive current collector; the negative plate comprises a negative current collector as well as a first negative coating and a second negative coating which are respectively positioned on two opposite sides of the negative current collector in the thickness direction; the first negative electrode coating comprises a first active material, the first negative electrode active material comprises a first carbon-based material and a first silicon-based material, the second negative electrode coating comprises a second active material, and the second negative electrode active material comprises a second carbon-based material; the mass content of the silicon element in the first negative electrode coating is greater than that of the silicon element in the second negative electrode coating; the positive electrode current collector comprises a positive electrode polymer layer and positive electrode metal layers located on the surfaces of the two sides of the positive electrode polymer layer, and the extension rate of the positive electrode current collector is 1%-20%. The lithium ion secondary battery provided by the invention is high in energy density, low in cycle expansion and low in risk of belt breakage of the positive electrode current collector.

Description

Technical Field

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

[0002] With the rapid development of the electric vehicle industry, the requirements for batteries are continuously increasing and the demand is constantly growing. Silicon anode materials have attracted much attention due to their high specific capacity. Existing silicon anode system batteries generally have the problem of cyclic swelling, which not only causes battery deformation, but also exerts pressure on the positive current collector, resulting in the breakage of the current collector tape. Therefore, effectively solving the existing problems of the above-mentioned silicon anode system is of great significance for the better practical application of the silicon anode system in actual high-energy density batteries. Summary of the Invention

[0003] The purpose of the present invention is to solve the problem of large cyclic swelling of silicon anode system batteries in the prior art and improve the breakage of the positive current collector tape, and provide a lithium-ion secondary battery (hereinafter referred to as the battery). By designing and regulating the structures of the positive electrode sheet and the negative electrode sheet, the current collector carried by the positive electrode is a composite current collector including a polymer layer and a metal layer, and the mass contents of silicon elements on both surfaces of the negative current collector are regulated to be different, effectively improving the cyclic swelling of the battery and also suppressing the breakage problem of the positive current collector caused by the extrusion of the current collector due to cyclic swelling.

[0004] In the prior art, silicon anode materials have a high specific capacity, but their volume changes greatly during the charge and discharge cycles of the battery. The swelling of the silicon anode reduces the structural stability of the negative electrode sheet. To alleviate the cyclic swelling of the negative electrode sheet, the inventors of the present invention found through experimental research that by regulating the different mass contents of silicon elements in the first negative electrode coating and the second negative electrode coating of the negative electrode sheet and combining them with the first carbon-based material and the second carbon-based material, the structural stability of the negative electrode sheet can be improved, and there is a significant inhibitory effect on the cyclic swelling of the silicon-based materials on both sides of the silicon anode, which can not only ensure high energy density but also alleviate swelling. The reason is that the negative electrode sheet has a structure with high silicon on one side and low silicon (or no silicon) on the other side, and the negative electrode coatings on both sides of the negative current collector respectively include the first carbon-based material and the second carbon-based material, which inhibit the volume change of the silicon-based material during the charge and discharge process of the battery; however, although the above structure can alleviate volume swelling, it still inevitably exerts pressure on the positive current collector. To further solve this problem, the inventors of the present invention use a composite current collector as the positive current collector. The composite current collector has the characteristics of light weight, high mass energy density and higher elongation rate compared with the conventional current collector, and can further alleviate or avoid the risk of breakage of the positive current collector caused by the swelling of the silicon anode in combination with the battery system under high energy density.

[0005] On each of the two surfaces of the negative electrode sheet in this system, coatings with different compositions are provided and combined with the positive electrode current collector. The two cooperate to not only improve the cycle life of high-energy-density batteries but also reduce the risks of cycle expansion and positive electrode tape breakage.

[0006] Based on this, the inventors of the present invention proposed the following technical solutions:

[0007] A lithium-ion secondary battery, the lithium-ion secondary battery comprising a positive electrode sheet and a negative electrode sheet; the positive electrode sheet includes a positive electrode current collector and a first positive electrode coating and a second positive electrode coating respectively located on opposite sides in the thickness direction of the positive electrode current collector; the negative electrode sheet includes a negative electrode current collector and a first negative electrode coating and a second negative electrode coating respectively located on opposite sides in the thickness direction of the negative electrode current collector; the first negative electrode coating includes a first negative electrode active material, the first negative electrode active material includes a first carbon-based material and a first silicon-based material, the second negative electrode coating includes a second negative electrode active material, the second negative electrode active material includes a second carbon-based material; the mass content of silicon element in the first negative electrode coating is greater than the mass content of silicon element in the second negative electrode coating; the positive electrode current collector includes a positive electrode polymer layer and positive electrode metal layers located on both surface sides of the positive electrode polymer layer, and the elongation rate of the positive electrode current collector is 1%-20%.

[0008] Compared with the prior art, the technical solutions of the present invention have at least the following beneficial effects:

[0009] (1) By regulating the mass content of silicon element in the first negative electrode coating and the second negative electrode coating respectively located on both sides of the current collector in the negative electrode sheet of the lithium-ion secondary battery of the present invention, the structural stability of the negative electrode sheet is improved, the problems of cycle expansion and battery deformation caused by the silicon-based material are effectively improved, and the flatness of the battery cell is improved.

[0010] (2) The positive electrode composite current collector of the lithium-ion secondary battery of the present invention not only improves the mass energy density but also can enhance the elongation rate and reduce the risk of tape breakage. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a schematic cross-sectional view of the positive electrode sheet along the thickness direction in an embodiment of the present invention.

[0012] Figure 2 It is a schematic cross-sectional view of the negative electrode sheet along the thickness direction in an embodiment of the present invention.

[0013] Figure 3 It is a schematic 3D contour diagram of the flatness of the battery cell in a comparative example of the present invention.

[0014] Figure 4 It is a schematic 3D contour diagram of the flatness of the battery cell in an embodiment of the present invention.

[0015] Figure 5 Schematic diagram of a stacked core structure in an embodiment of the present invention.

[0016] Figure 6 Schematic diagram of a wound core structure in an embodiment of the present invention.

[0017] Reference numerals: 1 is the positive electrode sheet, 2 is the negative electrode sheet, 11 is the first positive electrode coating, 12 is the second negative electrode coating, 13 is the positive electrode current collector, 131 is the positive electrode metal layer, 132 is the positive electrode polymer layer; 21 is the first negative electrode coating, 22 is the second negative electrode coating, 221 is the upper coating of the second negative electrode coating, 222 is the lower coating of the second negative electrode coating, 23 is the negative electrode current collector, 231 is the negative electrode metal layer, 232 is the negative electrode polymer layer; 41 is the first separator, 42 is the second separator. Detailed Description of the Invention

[0018] The following provides a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only for the purpose of illustrating and explaining the present invention, and are not intended to limit the present invention.

[0019] The present invention provides a lithium-ion secondary battery, and the lithium-ion secondary battery includes a positive electrode sheet and a negative electrode sheet.

[0020] In the present invention, the positive electrode sheet includes a positive electrode current collector and a first positive electrode coating and a second positive electrode coating respectively located on opposite sides in the thickness direction of the positive electrode current collector; the negative electrode sheet includes a negative electrode current collector and a first negative electrode coating and a second negative electrode coating respectively located on opposite sides in the thickness direction of the negative electrode current collector; the first negative electrode coating includes a first negative electrode active material, and the first negative electrode active material includes a first carbon-based material and a first silicon-based material, the second negative electrode coating includes a second negative electrode active material, and the second negative electrode active material includes a second carbon-based material; the mass content of silicon element in the first negative electrode coating is greater than the mass content of silicon element in the second negative electrode coating; the positive electrode current collector includes a positive electrode polymer layer and positive electrode metal layers located on both surface sides of the positive electrode polymer layer.

[0021] In the present invention, the elongation rate of the positive electrode current collector is 1% - 20%, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19% or 20%.

[0022] In one embodiment, the elongation rate of the positive electrode current collector is 5% - 20%.

[0023] In the present invention, the elongation rate of the positive electrode current collector can reflect the improvement of the problem of the positive electrode current collector breaking. The larger the elongation rate of the positive electrode current collector, when external stress is applied to the positive electrode current collector during rolling and winding processes or when the volume expansion during the charge and discharge cycles of the battery squeezes the positive electrode current collector, the positive electrode current collector is less likely to break, and the improvement of the problem of the positive electrode current collector breaking is more significant.

[0024] In the present invention, the thickness of the first positive electrode coating is l 1 , and the thickness of the second positive electrode coating is l 2 , l 1 and l 2 are set according to the areal capacity of the first positive electrode coating and the areal capacity of the second positive electrode coating. When the specific capacity of the positive electrode active material is the same, the larger the areal capacity, the larger the corresponding coating thickness.

[0025] As Figure 1 shown is a schematic cross-sectional view of the positive electrode sheet along the thickness direction in an embodiment of the present invention. It can be seen from the figure that the positive electrode sheet includes a first positive electrode coating 11, a second positive electrode coating 12, and a positive electrode current collector 13. The positive electrode current collector 13 further includes a positive electrode metal layer 131 and a positive electrode polymer layer 132.

[0026] As Figure 2 shown is a schematic cross-sectional view of the negative electrode sheet along the thickness direction in an embodiment of the present invention. It can be seen from the figure that the negative electrode sheet includes a first negative electrode coating 21, a second negative electrode coating 22, and a negative electrode current collector 23. The second negative electrode coating 22 includes an upper coating 221 and a lower coating 222. The negative electrode current collector 23 further includes a negative electrode metal layer 231 and a negative electrode polymer layer 232.

[0027] In one embodiment, the thickness of the first negative electrode coating is h 1 , and the thickness of the second negative electrode coating is h 2 , h 2 > h 1 .

[0028] In the present invention, the first carbon-based material and the second carbon-based material each independently include at least one of artificial graphite, natural graphite, soft carbon, and hard carbon.

[0029] In the present invention, although the silicon-based material has a high specific capacity, its cyclic expansion problem is significant. Moreover, if a large amount of silicon-based material is provided on both sides of the negative electrode sheet, it is difficult to solve the problem of battery deformation caused by cyclic expansion. Based on this, after extensive research, the inventors found that a first negative electrode coating and a second negative electrode coating with different silicon element mass contents are provided on both sides of the negative electrode current collector of the negative electrode sheet, and the mass content of silicon element in the first negative electrode coating is controlled to be greater than that in the second negative electrode coating. The first negative electrode coating and the second negative electrode coating also respectively include a first carbon-based material and a second carbon-based material, forming a structure with high silicon on one side and low silicon (or no silicon) on the other side. The silicon-based material provides a high energy density, while the first carbon-based material and the second carbon-based material play an inhibitory role on cyclic expansion, reducing cyclic expansion and improving the flatness of the battery cell.

[0030] Although the structural design with high silicon on one side and low silicon (or no silicon) on the other side can alleviate and inhibit the cyclic expansion of the silicon-based material, this structural design cannot completely solve the problems of extrusion and stress on the positive electrode current collector caused by the volume expansion of the negative electrode. Therefore, the inventors of the present invention further use a composite current collector including a positive electrode polymer layer and a positive electrode metal layer as the positive electrode current collector to replace the metal current collector. The composite current collector has the characteristics of light weight, high energy density, and good ductility. When the positive electrode is equipped with the composite current collector, on the one hand, the composite current collector has good ductility, can effectively absorb deformation stress, alleviate and avoid the problem of the positive electrode current collector breaking due to the extrusion of the positive electrode current collector caused by the cyclic expansion of the silicon negative electrode system, and is beneficial to improving the battery safety; on the other hand, the composite current collector also has the effects of reducing the battery weight and increasing the energy density.

[0031] As Figure 3 shown is a schematic diagram of the 3D profile of the flatness of the battery cell in a comparative example of the present invention. As Figure 4 shown is a schematic diagram of the 3D profile of the flatness of the battery cell in an embodiment of the present invention. By comparison, it can be seen that in the present invention, the flatness of the battery cell in the comparative example is poor, while the flatness of the battery cell of the present invention is significantly improved.

[0032] In the present invention, the first positive electrode coating and the second positive electrode coating include a positive electrode active material, and the first positive electrode coating further includes a lithium supplement agent, and the first positive electrode coating faces the first negative electrode coating.

[0033] In the present invention, the lithium supplement agent includes at least one of lithium-rich lithium ferrite, lithium-rich lithium nickelate, lithium-rich manganese-based material, and lithium oxalate.

[0034] In the present invention, "lithium-rich" means that the content of lithium element in the material is 20%-50% (for example, 20%, 22%, 24%, 26%, 28%, 30%, 35%, 40%, 45% or 50%); for example, it means that the mass content of lithium element in lithium-rich lithium ferrite is 20%-50%.

[0035] In the present invention, the mass content of the lithium supplement agent in the cathode active material is 0.1% - 15%, for example, 0.1%, 0.2%, 0.4%, 0.6%, 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%.

[0036] In one embodiment, the mass content of the lithium supplement agent in the cathode active material is 1% - 10%.

[0037] In another embodiment, the mass content of the lithium supplement agent in the cathode active material is 2% - 8%.

[0038] Since the increase in the content of the silicon-based material in the silicon anode system reduces the first Coulombic efficiency of the battery, it is possible to add a lithium supplement agent with a higher active lithium content to the cathode to make up for the consumption of lithium ions during the first charge and discharge process. However, excessive lithium supplementation will also cause the lithium supplement agent to remain in the battery system, which is likely to react with the electrolyte system to generate lithium supplement residues and affect the battery life. The inventors of the present invention have found through a large number of studies that adding the lithium supplement agent only on one side surface of the cathode sheet can not only achieve a good lithium supplementation effect, thereby achieving the purpose of improving the first Coulombic efficiency of the battery, but also control a reasonable amount of lithium supplementation, neither causing side reactions due to excessive lithium supplementation nor resulting in the difficulty of the active material to exert its maximum capacity due to insufficient lithium supplementation and reducing the battery performance. In order to further enhance the lithium supplementation effect, a first cathode coating including a lithium supplement agent is arranged facing a first anode coating with a high silicon element mass content. The reason is that: the content of the silicon-based material in the first anode coating is large and the first efficiency is low. Facing the first anode coating with the first cathode coating can reduce the migration distance of lithium ions inside the battery, reduce the internal resistance of the battery, improve the charge and discharge efficiency of the battery, and thus improve the first efficiency of the battery; moreover, this arrangement can also improve the utilization rate of the lithium supplement agent, enabling the battery to significantly improve the first efficiency on the premise of having a small impact on the rate performance.

[0039] In the present invention, the cathode active material includes at least one of nickel-cobalt-manganese ternary material, lithium iron phosphate, and lithium cobalt oxide.

[0040] In one embodiment, the cathode active material includes a nickel-cobalt-manganese ternary material.

[0041] In the present invention, the nickel-cobalt-manganese ternary material includes a chemical formula of Li a Ni x Co y Mn z M b O 2of a substance, 0.9≤a≤1.1 (e.g., 0.9, 0.92, 0.94, 0.96, 0.98, 1 or 1.1), 0.6≤x≤0.98 (e.g., 0.6, 0.7, 0.8, 0.9, 0.92, 0.92, 0.94, 0.96 or 0.98), 0 <y≤0.2(例如为0.02、0.04、0.06、0.08、0.1、0.12、0.14、0.16、0.18或0.2),0<z≤0.2(例如为0.02、0.04、0.06、0.08、0.1、0.12、0.14、0.16、0.18或0.2),0≤b≤0.05(例如为0、0.01、0.02、0.03、0.04或0.05),M选自Al、Zr、B、Y、Sr、W、Ti和Nb中的至少一种;例如包括LiNi 0.93 Co 0.05 Mn 0.01 Al 0.01 O 2 、LiNi 0.95 Co 0.03 Mn 0.01 Zr 0.01 O 2 and LiNi 0.91 Co 0.04 Mn 0.02 B 0.03 O 2 At least one of .

[0042] In one embodiment, 0.6≤x≤0.95.

[0043] In the present invention, the first silicon-based material includes at least one of elemental silicon, silicon-oxygen material, silicon-carbon material and silicon alloy.

[0044] In one embodiment, the first silicon-based material includes the silicon-carbon material.

[0045] In the present invention, the ternary material as the positive electrode active material has high mass energy density, and the silicon-carbon material as the negative electrode material has high specific capacity, so the combination of the ternary material and the silicon-carbon material can further improve the energy density of the battery.

[0046] In the present invention, when 0.6 ≤ x ≤ 0.95, the nickel-cobalt-manganese ternary material is a medium- and high-nickel ternary material with the characteristic of high capacity. When the battery is charged and discharged for the first time, on the one hand, the formation of the SEI film consumes a part of lithium ions, resulting in capacity loss. On the other hand, due to the high content of nickel, its redox reaction causes lattice distortion or oxide precipitation, which also leads to irreversible loss of lithium. To make up for this loss, improve the first efficiency of the battery, and improve capacity attenuation, a lithium supplement agent is added to the positive electrode sheet, which can significantly improve the battery performance. Secondly, the positive electrode sheet of the present invention uses a medium- and high-nickel ternary material with high capacity, which is matched with a negative electrode sheet including a first negative electrode coating with a high silicon content and a second negative electrode coating with a low silicon content, giving play to the capacities of the positive and negative electrodes, improving the energy density of the battery, and alleviating the cyclic swelling of the high-silicon negative electrode system.

[0047] In the present invention, the contents of various elements in the positive electrode active material can be measured by conventional testing methods in the art, for example, by inductively coupled plasma mass spectrometry (ICP-MS).

[0048] In the present invention, the mass content of silicon element in the first negative electrode coating is 1% - 95%, for example, 1%, 2%, 3%, 4%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90% or 95%; the mass content of silicon element in the second negative electrode coating is 0% - 55%, for example, 0%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50% or 55%.

[0049] In one embodiment, the mass content of silicon element in the first negative electrode coating is 2% - 45%, and the mass content of silicon element in the second negative electrode coating is 2% - 30%.

[0050] In one embodiment, the second silicon-based material includes at least one of elemental silicon, silicon oxide material, silicon carbide material and silicon alloy.

[0051] In another embodiment, the second silicon-based material includes the silicon carbide material.

[0052] In one embodiment, the silicon carbide material is a composite material formed by silicon deposited in the pores of carbon.

[0053] In the present invention, the mass content of the first silicon-based material in the first negative electrode active material is 5%-100%, such as 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100%; the mass content of the second silicon-based material in the second negative electrode active material is 0%-60%, such as 0%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55% or 60%.

[0054] In one embodiment, the mass content of the first silicon-based material in the first negative electrode active material > the mass content of the second silicon-based material in the second negative electrode active material.

[0055] In the present invention, the average particle size of the first silicon-based material is 0.5μm-20μm, such as 0.5μm, 1μm, 1.5μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm or 20μm, and the average particle size of the second silicon-based material is 0.5μm-20μm, such as 0.5μm, 1μm, 1.5μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm or 20μm.

[0056] In the present invention, the mass content of the silicon element can be measured by conventional testing methods in the art, for example, determined by inductively coupled plasma mass spectrometry (ICP-MS).

[0057] In the present invention, the average particle sizes of the first silicon-based material and the second silicon-based material can be obtained by conventional testing methods in the art. For example, first, high-resolution images of the first negative electrode coating and the second negative electrode coating are respectively obtained by a scanning electron microscope, and then particle size measurement and distribution statistics are respectively performed by electron microscope image analysis software such as ImageJ (for example, the particle sizes in the selected areas are measured in 10 different regions), and the average particle size is calculated. The model of the scanning electron microscope is Sigma300 / 500.

[0058] In the present invention, the positive electrode current collector includes a positive electrode polymer layer and positive electrode metal layers located on both surface sides of the positive electrode polymer layer.

[0059] In the present invention, the thickness of the positive electrode polymer layer is 0.1 μm - 10 μm, for example, 0.1 μm, 0.2 μm, 0.8 μm, 1 μm, 2 μm, 4 μm, 5 μm, 7 μm, 8.5 μm, 9 μm, 9.5 μm or 10 μm; the thickness of the positive electrode metal layer is 0.2 μm - 10 μm, for example, 0.2 μm, 0.8 μm, 1 μm, 2 μm, 4 μm, 5 μm, 7 μm, 8.5 μm, 9 μm, 9.5 μm or 10 μm.

[0060] In one embodiment, the thickness of the positive electrode polymer layer is 0.5 μm - 5 μm, and the thickness of the positive electrode metal layer is 0.5 μm - 5 μm.

[0061] In the present invention, the negative electrode current collector includes a negative electrode polymer layer and negative electrode metal layers on both surface sides of the negative electrode polymer layer.

[0062] In the present invention, the thickness of the negative electrode polymer layer is 0.1 μm - 10 μm, for example, 0.1 μm, 0.2 μm, 0.8 μm, 1 μm, 2 μm, 4 μm, 5 μm, 7 μm, 8.5 μm, 9 μm, 9.5 μm or 10 μm; the thickness of the negative electrode metal layer is 0.2 μm - 5 μm, for example, 0.2 μm, 0.4 μm, 0.6 μm, 0.8 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm or 5 μm.

[0063] In one embodiment, the thickness of the negative electrode polymer layer is 0.5 μm - 5 μm, and the thickness of the negative electrode metal layer is 0.5 μm - 5 μm.

[0064] In the present invention, the positive electrode polymer layer and the negative electrode polymer layer each independently include at least one of polypropylene, polyethylene, and polyethylene terephthalate.

[0065] In the present invention, the positive electrode metal layer includes Al.

[0066] In the present invention, the negative electrode metal layer includes Cu.

[0067] In the present invention, the thicknesses of the positive electrode polymer layer, the positive electrode metal layer, the negative electrode polymer layer, and the negative electrode metal layer can be measured by conventional testing methods in the art. For example, after cutting the negative electrode sheet along the thickness direction, the obtained cross-section is imaged using a scanning electron microscope, and the measurement is performed on the obtained image.

[0068] In addition, in the present invention, the negative electrode current collector includes a negative electrode polymer layer and a negative electrode metal layer. Both the positive electrode current collector and the negative electrode current collector are composite current collectors, and the composite current collector has the characteristics of light weight, high energy density, and good ductility. In the present invention, the negative electrode is equipped with a composite current collector. On the one hand, the composite current collector has good ductility, can effectively absorb deformation stress, relieve and avoid the problem of current collector tape breakage caused by the cyclic expansion of the silicon negative electrode system squeezing the negative electrode current collector and the positive electrode current collector, which is beneficial to improving battery safety; on the other hand, the composite current collector also has the effect of reducing the battery weight and increasing the energy density.

[0069] In the present invention, the areal capacity of the first negative electrode coating is 0.5 mAh / cm 2 -8 mAh / cm 2 , for example, 0.5 mAh / cm 2 , 1 mAh / cm 2 , 1.2 mAh / cm 2 , 1.6 mAh / cm 2 , 2 mAh / cm 2 , 2.5 mAh / cm 2 , 3 mAh / cm 2 , 3.2 mAh / cm 2 , 3.4 mAh / cm 2 , 3.6 mAh / cm 2 , 3.8 mAh / cm 2 , 4 mAh / cm 2 , 4.2 mAh / cm 2 , 4.4 mAh / cm 2 , 4.6 mAh / cm 2 , 4.8 mAh / cm 2 , 5 mAh / cm 2 , 5.5 mAh / cm 2 , 6 mAh / cm 2 , 6.5 mAh / cm 2 , 7 mAh / cm 2 , 7.5 mAh / cm 2 or 8 mAh / cm 2 ; the areal capacity of the second negative electrode coating is 0.5 mAh / cm 2 -8 mAh / cm 2 , for example, 0.5 mAh / cm 2 , 1 mAh / cm 2 , 1.2 mAh / cm 2 , 1.6 mAh / cm 2 , 2 mAh / cm 2 , 2.5 mAh / cm 2, 3 mAh / cm 2 , 3.2 mAh / cm 2 , 3.4 mAh / cm 2 , 3.6 mAh / cm 2 , 3.8 mAh / cm 2 , 4 mAh / cm 2 , 4.2 mAh / cm 2 , 4.4 mAh / cm 2 , 4.6 mAh / cm 2 , 4.8 mAh / cm 2 , 5 mAh / cm 2 , 5.5 mAh / cm 2 , 6 mAh / cm 2 , 6.5 mAh / cm 2 , 7 mAh / cm 2 , 7.5 mAh / cm 2 or 8 mAh / cm 2 .

[0070] In one embodiment, the areal capacity of the first negative electrode coating is 1 mAh / cm 2 -6 mAh / cm 2 , and the areal capacity of the second negative electrode coating is 1 mAh / cm 2 -4 mAh / cm 2 .

[0071] In the present invention, the areal capacity of the first positive electrode coating is 0.6 mAh / cm 2 -7 mAh / cm 2 , for example, 0.6 mAh / cm 2 , 0.8 mAh / cm 2 , 1 mAh / cm 2 , 1.2 mAh / cm 2 , 1.4 mAh / cm 2 , 1.6 mAh / cm 2 , 2 mAh / cm 2 , 2.5 mAh / cm 2 , 3 mAh / cm 2 , 3.5 mAh / cm 2 , 4.0 mAh / cm 2 , 5 mAh / cm 2 , 5.5 mAh / cm 2 , 6 mAh / cm 2 , 6.5 mAh / cm 2 or 7 mAh / cm 2 , and the areal capacity of the second positive electrode coating is 0.6 mAh / cm2 -7 mAh / cm 2 For example, it is 0.6 mAh / cm 2 、0.8 mAh / cm 2 、1 mAh / cm 2 、1.2 mAh / cm 2 、1.4 mAh / cm 2 、1.6 mAh / cm 2 、2 mAh / cm 2 、2.5 mAh / cm 2 、3 mAh / cm 2 、3.5 mAh / cm 2 、4.0 mAh / cm 2 、5 mAh / cm 2 、5.5 mAh / cm 2 、6 mAh / cm 2 、6.5 mAh / cm 2 or 7 mAh / cm 2 。

[0072] In one embodiment, the areal capacity of the first positive electrode coating is 1 mAh / cm 2 -5 mAh / cm 2 and the areal capacity of the second positive electrode coating is 1 mAh / cm 2 -5 mAh / cm 2 。

[0073] In the present invention, the specific capacity of the first negative electrode active material > the specific capacity of the second negative electrode active material.

[0074] In the present invention, the specific capacity of the first negative electrode coating is higher than that of the second negative electrode coating. This is because the mass content of silicon element in the first negative electrode coating is high, and the silicon-based material has a high specific capacity; the higher the specific capacity, the more lithium ions can be stored, thereby improving the energy density of the battery. Therefore, regulating the areal density on both sides of the positive electrode sheet and the negative electrode sheet within a certain range can better match the areal capacities of the positive and negative electrodes, make the energy density of the battery system higher, and improve the battery performance.

[0075] In the present invention, the areal capacity can be obtained by conventional calculation methods in the art, and the calculation method is as follows:

[0076] Areal capacity w = specific capacity of active material × content of active material × areal density;

[0077] Among them, the unit of areal capacity is mAh / cm 2 , the unit of specific capacity is mAh / g, the unit of content of active material is %, and the unit of areal density is g / cm 2 。

[0078] In the present invention, the second negative electrode coating includes an upper coating and a lower coating which are stacked in the thickness direction of the negative electrode sheet, the upper coating faces away from the negative electrode current collector, and the lower coating is close to the negative electrode current collector.

[0079] In the present invention, the upper coating includes a second carbon-based material A, and the average particle size of the second carbon-based material A is 1 μm - 30 μm, such as 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 17 μm, 19 μm, 21 μm, 23 μm, 25 μm, 27 μm or 30 μm; the lower coating includes a second carbon-based material B, and the average particle size of the second carbon-based material B is 1 μm - 30 μm, such as 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 17 μm, 19 μm, 21 μm, 23 μm, 25 μm, 27 μm or 30 μm.

[0080] In one embodiment, the average particle size of the second carbon-based material A is 6 μm - 25 μm, and the average particle size of the second carbon-based material B is 3 μm - 20 μm.

[0081] In one embodiment, the average particle size of the second carbon-based material A > the average particle size of the second carbon-based material B.

[0082] In the present invention, the second carbon-based material A and the second carbon-based material B each independently include at least one of artificial graphite, natural graphite, soft carbon and hard carbon.

[0083] In the present invention, the second negative electrode coating includes an upper coating and a lower coating in the thickness direction of the negative electrode sheet. The average particle size of the second carbon-based material A in the upper coating is large, which can increase the porosity of the negative electrode surface layer and is beneficial to the infiltration of the electrolyte. The average particle size of the second carbon-based material B in the lower coating is small and the reaction activity is strong. The closer it is to the negative electrode current collector, the higher the potential during the charging process, and it is more difficult to react. Placing the second carbon-based material B with strong reaction activity in the lower coating close to the negative electrode current collector can improve the reaction activity and utilization rate. This structure enables the negative electrode sheet to have uniform reaction activity in the thickness direction, improves the cycle life, and further solves the problems of poor kinetics, low charging window, and poor rate performance in the silicon negative electrode system battery in the prior art.

[0084] In the present invention, the average particle sizes of the second carbon-based material A in the upper coating and the second carbon-based material B in the lower coating can be obtained by conventional testing methods in the art. For example, first obtain a high-resolution image through a scanning electron microscope, and then perform particle size measurement and distribution statistics through electron microscope image analysis software such as ImageJ, etc. to obtain the average particle size. The model of the scanning electron microscope is Sigma300 / 500.

[0085] In the present invention, the graphitization degree of the silicon-carbon material is 0 - 0.85, for example, 0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.14, 0.18, 0.22, 0.26, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8 or 0.85.

[0086] The graphitization degree reflects the crystallization degree of the material. Controlling the graphitization degree of the silicon-carbon material can improve the battery performance. A good graphitization degree increases the specific capacity, and can also balance the conductivity and structural stability, which is beneficial to improving the charge and discharge efficiency and structural stability of the battery.

[0087] In the present invention, the graphitization degree of the silicon-carbon material can be measured by conventional testing methods in the art, for example, measured by an X-ray diffractometer (XRD).

[0088] In the present invention, the oil absorption value of the silicon-carbon material is 10 ml / 100 g - 100 ml / 100 g, for example, 10 ml / 100 g, 12 ml / 100 g, 14 ml / 100 g, 16 ml / 100 g, 18 ml / 100 g, 20 ml / 100 g, 25 ml / 100 g, 30 ml / 100 g, 35 ml / 100 g, 40 ml / 100 g, 45 ml / 100 g, 50 ml / 100 g, 60 ml / 100 g, 70 ml / 100 g, 80 ml / 100 g, 90 ml / 100 g or 100 ml / 100 g.

[0089] The oil absorption value is an indicator for measuring the surface roughness and porosity of a material. Controlling the oil absorption value can regulate the porosity and specific surface area of the silicon-carbon material to vary within a suitable range, which helps the infiltration of the electrolyte, regulates the contact area between the active material and the electrolyte, and thus improves the capacity and rate performance of the battery.

[0090] In the present invention, the oil absorption value of the silicon-carbon material can be measured by conventional testing methods in the art. For example: scrape off the negative active material layer of the negative electrode sheet, wash away the binder, and after drying (in a vacuum drying oven at 100 °C for 12 h), weigh the mass m of the clean beaker and glass rod. 1 , add 5 g of the silicon-carbon material and record the total mass m. 2 , gradually add dioctyl phthalate (DOP) drop by drop using a burette and stir well. Stop adding (DOP) when a lump is formed, and weigh the total weight m of the beaker at this time. 3 , then the oil absorption value = (m 3 - m 2 ) / (m 2 - m 1 ) × 100.

[0091] In the present invention, the OI value of the first negative electrode coating is 5 - 30, for example, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 or 30.

[0092] The OI value reflects the degree of orientation of the first negative electrode coating. Controlling the OI value can regulate the deintercalation and intercalation of lithium ions, thereby improving the charge-discharge performance and cycle life of the battery.

[0093] In the present invention, the OI value of the first negative electrode coating can be obtained by conventional testing methods in the art. For example, by using an X-ray powder diffractometer, the testing method is as follows: after discharging the lithium-ion secondary battery to 0% SOC, disassemble and take out the negative electrode sheet, soak it in a dimethyl carbonate (DMC) solvent for 12 h, then rinse it with DMC to remove the lithium salt attached to the electrode sheet, dry it, and test it with an X-ray powder diffraction instrument (such as a Shimadzu XRD-6100 type X-ray diffractometer). The diffraction peak appearing at 2θ = 54 - 55° in the obtained diffraction pattern is the (004) peak of graphite, and its intensity is denoted as I004. The diffraction peak appearing at 2θ = 77 - 78° is the (110) peak of graphite, and its intensity is denoted as I110. The OI value of the first negative electrode coating is I004 / I110.

[0094] In the present invention, the lithium-ion secondary battery includes a stacked core or a wound core, and the stacked core or the wound core includes the positive electrode sheet and the negative electrode sheet.

[0095] In the present invention, the stacked core further includes a separator. The stacked core includes a plurality of alternately stacked positive electrode sheets and negative electrode sheets, and the separator is disposed between the positive electrode sheet and the negative electrode sheet.

[0096] In the present invention, the wound core further includes a first separator and a second separator. The wound core includes a winding structure formed by sequentially laminating the first separator, the positive electrode sheet, the second separator, and the negative electrode sheet.

[0097] In the present invention, the arrangement of the first coating layer and the second coating layer of the negative electrode sheet in the stacked core or the wound core is not limited, and good effects can be achieved in both cases.

[0098] In one embodiment, the first negative electrode coating layer is located on a side of the negative electrode sheet facing away from the winding center of the wound core, and the second negative electrode coating layer is located on a side of the negative electrode sheet close to the winding center of the wound core.

[0099] In another embodiment, the first coating layer is located on a side of the negative electrode sheet close to the winding center of the wound core, and the second coating layer is located on a side of the negative electrode sheet facing away from the winding center of the wound core.

[0100] In one embodiment, the first negative electrode coating layer is located on a side of the negative electrode sheet close to the center of the stacked core, and the second negative electrode coating layer is located on a side of the negative electrode sheet facing away from the center of the stacked core.

[0101] In another embodiment, the first coating layer is located on a side of the negative electrode sheet facing away from the center of the stacked core, and the second coating layer is located on a side of the negative electrode sheet close to the center of the stacked core.

[0102] As Figure 5 shown is a schematic diagram of the structure of the stacked core in an embodiment of the present invention. It can be seen from the figure that the negative electrode sheet and the positive electrode sheet are sequentially stacked. The negative electrode sheet includes the first negative electrode coating layer 21 and the second negative electrode coating layer 22, and the positive electrode sheet includes the first positive electrode coating layer 11 and the second positive electrode coating layer 12. Among them, the first positive electrode coating layer 11 faces the first negative electrode coating layer 21; As Figure 6 shown is a schematic diagram of the structure of the wound core in an embodiment of the present invention, where 1 is the positive electrode sheet, 2 is the negative electrode sheet, 41 is the first separator, and 42 is the second separator.

[0103] In the present invention, the distribution of the negative electrode sheet in the battery cell structure is optimized. In the wound core, on the side of the negative electrode sheet facing away from the winding center, the tensile force is stronger, the gap between particles and the swelling space are larger. Since the mass content of silicon element in the first negative electrode coating is high, setting the first negative electrode coating on this side can relieve the deformation of the battery cell caused by cyclic swelling and improve the edge ridge formation; while on the side of the negative electrode sheet close to the winding center, the pressing force is large, and the second negative electrode coating including the second carbon-based material is set on this side. By utilizing the good kinetics of graphite, the comprehensive performance of the battery is improved; in the stacked core, setting the second negative electrode coating on the side of the negative electrode sheet facing away from the stacked core center can exert the inhibitory effect of the second carbon-based material on cyclic swelling and reduce the cyclic swelling rate.

[0104] In the present invention, the positive electrode sheet further includes at least one of a positive electrode conductive agent and a positive electrode binder. The positive electrode conductive agent and the positive electrode binder are both conventional selections in the art. For example, the positive electrode conductive agent includes at least one of carbon nanotubes and conductive carbon black, the positive electrode binder includes polyacrylic acid (PAA), and the positive electrode dispersant includes carboxymethyl cellulose (CMC).

[0105] In the present invention, the negative electrode sheet further includes at least one of a negative electrode conductive agent, a negative electrode binder, and a dispersant. The negative electrode conductive agent, the negative electrode binder, and the dispersant are all conventional selections in the art. For example, the negative electrode conductive agent includes at least one of carbon nanotubes and conductive carbon black, the negative electrode binder includes polyacrylic acid (PAA), and the negative electrode dispersant includes carboxymethyl cellulose (CMC).

[0106] In the present invention, the lithium-ion secondary battery further includes a separator and an electrolyte, and both the separator and the electrolyte are conventional selections in the art.

[0107] The lithium-ion secondary battery of the present invention is applicable to the direction of automotive power batteries and has the characteristics of high energy density and good rate performance.

[0108] It should be noted that in the present invention, the numerical representation methods such as "first" and "second" are only used to distinguish different substances or usage methods and do not represent the difference in order.

[0109] The present invention will be described in detail below through embodiments. The embodiments described in the present invention are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative work fall within the scope of protection of the present invention.

[0110] In the following examples, unless otherwise specified, the materials used are commercially available analytical pure.

[0111] The following examples are used to illustrate the lithium-ion secondary battery of the present invention.

[0112] Example 1:

[0113] Preparation of the positive electrode sheet:

[0114] Dissolve 94% of the positive electrode active material (LiNi 0.93 Co 0.05 Mn 0.01 M 0.01 O 2 , where M is Al element), 1.5% of conductive carbon black, 1% of polyvinylidene fluoride (PVDF), and 3.5% of lithium supplement agent in N-methylpyrrolidone (NMP). After mixing evenly, the first positive electrode coating slurry is obtained. The first positive electrode coating slurry is evenly coated on one side of the positive electrode current collector to form the first positive electrode coating; Dissolve 97.5% of the positive electrode active material (LiNi 0.93 Co 0.05 Mn 0.01 M 0.01 O 2 , where M is Al element), 1.5% of conductive carbon black and 1% of PVDF in NMP. After mixing evenly, the second positive electrode coating slurry is obtained. The second positive electrode coating slurry is evenly coated on the other side of the positive electrode current collector to form the second positive electrode coating slurry; After baking, drying and slitting, the positive electrode sheet is obtained;

[0115] Among them, the lithium supplement agent is a mixture of lithium-rich nickelate and lithium-rich ironate in a mass ratio of 1:1. The positive electrode current collector is a composite current collector, the positive electrode metal layer is aluminum foil with a thickness of 3.5 μm, and the positive electrode polymer layer is polypropylene with a thickness of 2.6 μm;

[0116] At this time, the areal capacity of the first positive electrode coating is 4 mAh / cm 2 , and the areal capacity of the second positive electrode coating is 3.5 mAh / cm 2 .

[0117] Preparation of the negative electrode sheet:

[0118] Dissolve 96.5% of the first negative electrode active material (70% artificial graphite + 30% silicon-carbon material, the specific capacity of the first negative electrode active material is 818.5 mAh / g), 0.8% of the conductive agent (carbon nanotubes and carbon black are mixed according to a mass ratio of 9:1), 1.5% of PAA, and 1.2% of CMC in deionized water to obtain the first negative electrode coating slurry. Coat the first negative electrode coating slurry evenly on one side of the negative electrode current collector to form the first negative electrode coating; dissolve 96.5% of the second negative electrode active material (85% artificial graphite + 15% silicon-carbon material, the specific capacity of the second negative electrode active material is 586.75 mAh / g), 0.8% of the negative electrode conductive agent (carbon nanotubes and carbon black are mixed according to a mass ratio of 9:1), 1.5% of PAA, and 1.2% of CMC in deionized water to obtain the second negative electrode coating slurry. Coat the second negative electrode coating slurry evenly on the other side of the negative electrode current collector to form the second negative electrode coating; then obtain the negative electrode sheet after drying, cold pressing, and slitting.

[0119] Among them, in the second negative electrode coating, the average particle size of the second carbon-based material A in the upper coating is 12 μm, the average particle size of the second carbon-based material B in the lower coating is 7 μm. The negative electrode current collector is a composite current collector, the negative electrode metal layer is copper foil with a thickness of 2.8 μm, and the negative electrode polymer layer is polypropylene with a thickness of 2.6 μm;

[0120] At this time, the mass content of silicon element in the first negative electrode coating is 12%, and the areal capacity of the first negative electrode coating is 4.35 mAh / cm 2 , the mass content of silicon element in the second negative electrode coating is 6%, and the areal capacity of the second negative electrode coating is 3.11 mAh / cm 2 .

[0121] Assembly of the lithium-ion battery:

[0122] For the positive electrode sheet and the negative electrode sheet prepared by the method described above, perform tab making (welding the tab), prepare the stacked core (positive electrode sheet + separator + negative electrode sheet, where the first positive electrode coating of the positive electrode sheet faces the first negative electrode coating of the negative electrode sheet), and the separator uses a coated separator of base film + ceramic + glue; then perform encapsulation, liquid injection, and formation, then perform secondary sealing, and finally perform sorting to complete the production of the square lithium-ion battery, and then report for inspection and testing.

[0123] Example 2:

[0124] Preparation of the positive electrode sheet:

[0125] Dissolve 89.7% of the positive electrode active material (LiNi 0.95 Co 0.03 Mn 0.01 M 0.01 O 2, M is Zr element), 1.5% of conductive carbon black, 1% of PVDF and 7.8% of lithium supplement agent are dissolved in NMP, and after mixing evenly, the first positive electrode coating slurry is obtained. The first positive electrode coating slurry is evenly coated on one side of the positive electrode current collector to form the first positive electrode coating; 97.5% of the positive electrode active material (LiNi 0.95 Co 0.03 Mn 0.01 M 0.01 O 2 , M is Zr element), 1.5% of conductive carbon black and 1% of PVDF are dissolved in NMP, and after mixing evenly, the second positive electrode coating slurry is obtained. The second positive electrode coating slurry is evenly coated on the other side of the positive electrode current collector to form the second positive electrode coating; after baking, drying and slitting, the positive electrode sheet is obtained;

[0126] Among them, the lithium supplement agent is a mixture of lithium-rich nickelate and lithium-rich ferrate with a mass ratio of 1:5. The positive electrode current collector is a composite current collector. The positive electrode metal layer is aluminum foil with a thickness of 2.9 μm, and the positive electrode polymer layer is polypropylene with a thickness of 3.1 μm;

[0127] At this time, the areal capacity of the first positive electrode coating is 4 mAh / cm 2 , and the areal capacity of the second positive electrode coating is 3.5 mAh / cm 2 .

[0128] Preparation of negative electrode sheet:

[0129] 96.5% of the first negative electrode active material (32% silicon-carbon material + 68% artificial graphite, the specific capacity of the first negative electrode active material is 849.4 mAh / g), 0.8% of the conductive agent (conductive carbon nanotubes and conductive carbon black are mixed according to a mass ratio of 9:1), 1.5% of PAA and 1.2% of CMC are dissolved in deionized water to obtain the first negative electrode coating slurry. The first negative electrode coating slurry is evenly coated on one side of the negative electrode current collector to form the first negative electrode coating; 96.5% of the second negative electrode active material (13% silicon-carbon material + 87% artificial graphite, the specific capacity of the second negative electrode active material is 555.85 mAh / g), 0.8% of the negative electrode conductive agent (conductive carbon nanotubes and conductive carbon black are mixed according to a mass ratio of 9:1), 1.5% of PAA and 1.2% of CMC are dissolved in deionized water to obtain the second negative electrode coating slurry. The second coating slurry is evenly coated on the other side of the negative electrode current collector to form the second negative electrode coating; after drying, cold pressing and slitting, the negative electrode sheet is obtained;

[0130] Among them, in the second negative electrode coating, the average particle size of the second carbon-based material A in the upper coating is 6 μm, and the average particle size of the second carbon-based material B in the lower coating is 3 μm. The negative electrode current collector is a composite current collector, the negative electrode metal layer is a copper foil with a thickness of 3.8 μm, and the negative electrode polymer layer is polyethylene terephthalate with a thickness of 2.4 μm;

[0131] At this time, the mass content of silicon element in the first negative electrode coating is 12.9%, and the areal capacity of the first negative electrode coating is 4.35 mAh / cm 2 , the mass content of silicon element in the second negative electrode coating is 5.2%, and the areal capacity of the second negative electrode coating is 3.11 mAh / cm 2 .

[0132] Assembly of the lithium-ion battery:

[0133] The positive electrode sheet and the negative electrode sheet prepared by the method described above are subjected to sheet making (welding the tab), preparing a stacked core (positive electrode sheet + separator + negative electrode sheet, the first positive electrode coating of the positive electrode sheet faces the first negative electrode coating of the negative electrode sheet), and the separator uses a coated separator of base film + ceramic + glue; then it is subjected to encapsulation, liquid injection and formation, then secondary encapsulation, and finally sorting to complete the production of a prismatic lithium-ion battery, and then reported for inspection and testing.

[0134] Example 3:

[0135] Preparation of the positive electrode sheet:

[0136] Dissolve 92.1% of the positive electrode active material (LiNi 0.91 Co 0.04 Mn 0.02 M 0.03 O 2 , M is element B), 1.5% of conductive carbon black, 1% of PVDF and 5.4% of lithium supplement agent in de-NMP, and after mixing evenly, the first positive electrode coating slurry is obtained. The first positive electrode coating slurry is evenly coated on one side of the positive electrode current collector to form the first positive electrode coating; dissolve 97.5% of the positive electrode active material (LiNi 0.91 Co 0.04 Mn 0.02 Al 0.03 O 2 , M is element B), 1.5% of conductive carbon black and 1% of PVDF in de-NMP, and after mixing evenly, the second positive electrode coating slurry is obtained. The second positive electrode coating slurry is evenly coated on the other side of the positive electrode current collector to form the second coating slurry; after baking, drying and slitting, the positive electrode sheet is obtained;

[0137] Among them, the lithium supplement agent is a mixture of lithium-rich nickelate and lithium-rich ironate with a mass ratio of 5:1. The positive electrode current collector is a composite current collector. The positive electrode metal layer is aluminum foil with a thickness of 3.7 μm, and the positive electrode polymer layer is polyethylene terephthalate with a thickness of 2.3 μm;

[0138] At this time, the areal capacity of the first positive electrode coating is 4 mAh / cm 2 , and the areal capacity of the second positive electrode coating is 3.5 mAh / cm 2 .

[0139] Negative electrode sheet preparation:

[0140] Dissolve 96.5% of the first negative electrode active material (27% silicon-carbon material + 73% artificial graphite, the specific capacity of the first negative electrode active material is 772.15 mAh / g), 0.8% of the conductive agent (carbon nanotubes and carbon black are mixed according to a mass ratio of 9:1), 1.5% of PAA and 1.2% of CMC in deionized water to obtain the first negative electrode coating slurry. Coating the first negative electrode coating slurry uniformly on one side of the negative electrode current collector to form the first negative electrode coating; Dissolve 96.5% of the second negative electrode active material (14% silicon-carbon material + 86% artificial graphite, the specific capacity of the second negative electrode active material is 571.3 mAh / g), 0.8% of the negative electrode conductive agent (carbon nanotubes and carbon black are mixed according to a mass ratio of 9:1), 1.5% of PAA and 1.2% of CMC in deionized water to obtain the second negative electrode coating slurry. Coating the second negative electrode coating slurry uniformly on the other side of the negative electrode current collector to form the second negative electrode coating; Then, after drying, cold pressing, and slitting, a negative electrode sheet is obtained;

[0141] Among them, in the second negative electrode coating, the average particle size of the second carbon-based material A in the upper coating is 25 μm, and the average particle size of the second carbon-based material B in the lower coating is 20 μm. The negative electrode current collector is a composite current collector. The negative electrode metal layer is copper foil with a thickness of 2.6 μm, and the negative electrode polymer layer is polyethylene with a thickness of 3.2 μm;

[0142] At this time, the mass content of silicon element in the first negative electrode coating is 10.8%, and the areal capacity of the first negative electrode coating is 4.35 mAh / cm 2 , and the mass content of silicon element in the second negative electrode coating is 5.6%, and the areal capacity of the second negative electrode coating is 3.11 mAh / cm 2 .

[0143] Assembly of the lithium-ion battery:

[0144] The positive electrode sheet and the negative electrode sheet prepared by the method described above are subjected to sheet production (welding tabs), and a core is prepared (positive electrode sheet + separator + negative electrode sheet, wherein the first positive electrode coating of the positive electrode sheet faces the first negative electrode coating of the negative electrode sheet). The separator is a coated separator with a base film + ceramic + glue; then it is encapsulated, filled with electrolyte, and formed, followed by secondary encapsulation, and finally sorted to complete the production of a prismatic lithium-ion battery, and then reported for inspection and testing.

[0145] 4 groups of examples:

[0146] This group of examples is used to verify the influence brought by the "lithium supplement agent", which is achieved by changing the mass content of the lithium supplement agent in the positive electrode active material, specifically as follows:

[0147] Example 4a, based on Example 1, is different in that the mass content of the lithium supplement agent in the positive electrode active material is 0.1%. At this time, the first positive electrode coating slurry is composed of 97.4% of the positive electrode active material (LiNi 0.93 Co 0.05 Mn 0.01 M 0.01 O 2 , where M is Al element), 1.5% of conductive carbon black, 1% of PVDF, and 0.1% of the lithium supplement agent.

[0148] Example 4b, based on Example 1, is different in that the mass content of the lithium supplement agent in the positive electrode active material is 15%. At this time, the first positive electrode coating slurry is composed of 82.5% of the positive electrode active material (LiNi 0.93 Co 0.05 Mn 0.01 M 0.01 O 2 , where M is Al element), 1.5% of conductive carbon black, 1% of PVDF, and 15% of the lithium supplement agent.

[0149] 5 groups of examples:

[0150] This group of examples is used to verify the influence brought by the change of "the areal capacity of the positive electrode coating and the areal capacity of the negative electrode coating", which is achieved by adjusting the coating amounts of the first positive electrode coating slurry, the second positive electrode coating slurry on the surface of the positive electrode current collector and the coating amounts of the first negative electrode coating slurry, the second negative electrode coating slurry on the surface of the negative electrode current collector, specifically as follows:

[0151] Example 5a, based on Example 1, is different in that the areal capacity of the first positive electrode coating is 1.5 mAh / cm 2 , the areal capacity of the second positive electrode coating is 1.5 mAh / cm 2 , the areal capacity of the first negative electrode coating is 1.3 mAh / cm 2 , and the areal capacity of the second negative electrode coating is 1.2 mAh / cm2 .

[0152] Example 5b, based on Example 1, differs in that the areal capacity of the first positive electrode coating is 4.7 mAh / cm 2 , and the areal capacity of the second positive electrode coating is 4.6 mAh / cm 2 , the areal capacity of the first negative electrode coating is 5.5 mAh / cm 2 , and the areal capacity of the second negative electrode coating is 3.8 mAh / cm 2 .

[0153] Example 5c, based on Example 1, differs in that the areal capacity of the first positive electrode coating is 0.62 mAh / cm 2 , and the areal capacity of the second positive electrode coating is 0.64 mAh / cm 2 , the areal capacity of the first negative electrode coating is 0.55 mAh / cm 2 , and the areal capacity of the second negative electrode coating is 0.5 mAh / cm 2 .

[0154] Example 5bd, based on Example 1, differs in that the areal capacity of the first positive electrode coating is 6.8 mAh / cm 2 , and the areal capacity of the second positive electrode coating is 7 mAh / cm 2 , the areal capacity of the first negative electrode coating is 7.7 mAh / cm 2 , and the areal capacity of the second negative electrode coating is 7.9 mAh / cm 2 .

[0155] Group of Example 6:

[0156] This group of examples is used to verify the influence brought by the change of "the mass content of silicon element on both sides of the negative electrode current collector surface", which is achieved by changing the mass content of silicon element in the first negative electrode coating and the mass content of silicon element in the second negative electrode coating, specifically as follows:

[0157] Example 6a, based on Example 1, differs in that the mass content of silicon element in the first negative electrode coating is 10%, and the mass content of silicon element in the second negative electrode coating is 4%; at this time, the first negative electrode active material consists of 25% silicon-carbon material + 75% artificial graphite, the specific capacity of the first negative electrode active material is 741.25 mAh / g, the second negative electrode active material consists of 10% silicon-carbon material + 90% artificial graphite, the specific capacity of the second negative electrode active material is 509.5 mAh / g, the areal capacity of the first negative electrode coating is 5.57 mAh / cm 2 , and the areal capacity of the second negative electrode coating is 3.77 mAh / cm 2 .

[0158] Example 6b, based on Example 1, is different in that the mass content of silicon element in the first negative electrode coating is 24%, and the mass content of silicon element in the second negative electrode coating is 13%. At this time, the first negative electrode active material is composed of 60% silicon-carbon material + 40% artificial graphite, the specific capacity of the first negative electrode active material is 1282 mAh / g, the second negative electrode active material is composed of 32% silicon-carbon material + 68% artificial graphite, the specific capacity of the second negative electrode active material is 849.4 mAh / g, and the areal capacity of the first negative electrode coating is 6.8 mAh / cm 2 , and the areal capacity of the second negative electrode coating is 4.51 mAh / cm 2 .

[0159] Example 6c, based on Example 1, is different in that the mass content of silicon element in the first negative electrode coating is 2%, and the mass content of silicon element in the second negative electrode coating is 0%. At this time, the first negative electrode active material is composed of 5% silicon-carbon material + 95% artificial graphite, the specific capacity of the first negative electrode active material is 432.25 mAh / g, the second negative electrode active material is composed of 0% silicon-carbon material + 100% artificial graphite, the specific capacity of the second negative electrode active material is 355 mAh / g, and the areal capacity of the first negative electrode coating is 2.29 mAh / cm 2 , and the areal capacity of the second negative electrode coating is 1.88 mAh / cm 2 .

[0160] Example 6d, based on Example 1, is different in that the mass content of silicon element in the first negative electrode coating is 38.2%, and the mass content of silicon element in the second negative electrode coating is 24.1%. At this time, the first negative electrode active material is composed of 95% silicon-carbon material + 5% artificial graphite, the specific capacity of the first negative electrode active material is 1822.75 mAh / g, the second negative electrode active material is composed of 60% silicon-carbon material + 40% artificial graphite, the specific capacity of the second negative electrode active material is 1282 mAh / g, and the areal capacity of the first negative electrode coating is 10.08 mAh / cm 2 , and the areal capacity of the second negative electrode coating is 6.8 mAh / cm 2 .

[0161] Group of Example 7:

[0162] This group of examples is used to verify the influence brought by the change of "the average particle size of the second carbon-based material A in the upper coating and the second carbon-based material B in the lower coating in the second negative electrode coating", and the details are as follows:

[0163] Example 7a, based on Example 1, is different in that in the second negative electrode coating, the average particle size of the second carbon-based material A in the upper coating is 25.6 μm, and the average particle size of the second carbon-based material B in the lower coating is 2.8 μm.

[0164] Example 7b, based on Example 1, which is different in that in the second negative electrode coating, the average particle size of the second carbon-based material A in the upper coating is 16.5 μm, and the average particle size of the second carbon-based material B in the lower coating is 5.2 μm.

[0165] Group of Example 8:

[0166] This group of examples is used to verify the influence brought by the change of "positive electrode current collector and negative electrode current collector", which is achieved by changing the thickness of the positive electrode polymer layer and the positive electrode metal layer of the positive electrode current collector, and the thickness of the negative electrode polymer layer and the negative electrode metal layer of the negative electrode current collector, specifically as follows:

[0167] Example 8a, based on Example 1, which is different in that the thickness of the negative electrode metal layer is 0.3 μm, and the thickness of the negative electrode polymer layer is 0.2 μm; the thickness of the positive electrode metal layer is 0.3 μm, and the thickness of the positive electrode polymer layer is 0.3 μm.

[0168] Example 8b, based on Example 1, which is different in that the thickness of the negative electrode metal layer is 5 μm, and the thickness of the negative electrode polymer layer is 9.5 μm; the thickness of the positive electrode metal layer is 8 μm, and the thickness of the positive electrode polymer layer is 9.5 μm.

[0169] Group of Example 9:

[0170] This group of examples is used to verify the influence brought by "lithium supplementation for the positive electrode", which is achieved by changing the content and setting position of the lithium supplementing agent, specifically as follows:

[0171] Example 9a, based on Example 1, which is different in that the first positive electrode coating slurry does not contain a lithium supplementing agent. At this time, the first positive electrode coating slurry is composed of the following components: 97.5% of positive electrode active material (LiNi 0.95 Co 0.03 Mn 0.01 M 0.01 O 2 , M is Al element), 1.5% of conductive carbon black, and 1% of PVDF.

[0172] Example 9b, based on Example 1, which is different in that the content of the lithium supplementing agent in the first positive electrode coating slurry is 18%. At this time, the first positive electrode coating slurry is composed of the following components: 79.5% of positive electrode active material (LiNi 0.95 Co 0.03 Mn 0.01 M 0.01 O 2 , M is Al element), 1.5% of conductive carbon black, 1% of PVDF, and 15% of lithium supplementing agent.

[0173] Example 9c, based on Example 1, is different in that the second positive electrode coating slurry also contains a lithium supplement agent and has the same lithium supplement agent content as that in the first positive electrode coating slurry. At this time, the compositions of the first positive electrode coating slurry and the second positive electrode coating slurry are as follows: 94% of positive electrode active material (LiNi 0.93 Co 0.05 Mn 0.01 Al 0.01 O 2 , where M is Al element), 1.5% of conductive carbon black, 1% of PVDF, and 3.5% of lithium supplement agent.

[0174] Example 10:

[0175] Based on Example 1, it is different in that the first positive electrode coating faces the second negative electrode coating.

[0176] Example 11:

[0177] Based on Example 1, the first negative electrode active material consists of 70% artificial graphite + 30% silicon-oxygen material, and the specific capacity of the first negative electrode active material is 563.5 mAh / g. The second negative electrode active material consists of 85% artificial graphite + 15% silicon-oxygen material, and the specific capacity of the second negative electrode active material is 459.25 mAh / g. The mass content of silicon element in the first negative electrode coating is 19.12%, and the mass content of silicon element in the second negative electrode coating is 9.56%.

[0178] In the above examples, the graphitization degree of the silicon-carbon material is within the range of 0 - 0.85, the oil absorption value of the silicon-carbon material is within the range of 10 mL / 100g - 100 mL / 100g, and the OI value of the first negative electrode coating is within the range of 5 - 30.

[0179] Comparative Example 1:

[0180] Based on Example 1, it is different in that the positive electrode current collector and the negative electrode current collector do not use a composite current collector. The positive electrode current collector is an aluminum foil with a thickness of 10 μm, and the negative electrode current collector is a copper foil with a thickness of 6 μm.

[0181] Comparative Example 2:

[0182] Based on Example 1, it is different in that the mass content of silicon element in the first negative electrode coating is 12%, and the mass content of silicon element in the second negative electrode coating is 12%. At this time, both the first negative electrode active material and the second negative electrode active material consist of 30% silicon-carbon material + 70% artificial graphite. The specific capacity of the first negative electrode active material is 818.5 mAh / g, and the specific capacity of the second negative electrode active material is 818.5 mAh / g. The areal capacity of the first negative electrode coating is 4.35 mAh / cm 2, the areal capacity of the second negative electrode coating is 4.35 mAh / cm 2 .

[0183] Test Example:

[0184] (1) Positive electrode current collector elongation rate test:

[0185] Specimen preparation: After discharging the lithium-ion secondary battery to 0% SOC, disassemble and take out the positive electrode sheet. Immerse it in dimethyl carbonate (DMC) solvent for 12 h, then rinse it with DMC to remove the lithium salts attached to the positive electrode sheet. After drying, scrape off the surface active material layer to obtain the positive electrode current collector, and cut the positive electrode current collector into standard dumbbell-shaped or rectangular specimens.

[0186] Clamping method: Fix both ends of the specimen using a fixture suitable for the type and size of the positive electrode sheet material to avoid slippage.

[0187] Tensile test: Stretch the specimen on a tensile testing machine at a constant speed until it breaks.

[0188] Data recording: Measure the length after fracture and the original length, and calculate the elongation rate.

[0189] Calculation formula: Positive electrode current collector elongation rate = (fracture length - original length) / original length × 100%.

[0190] Among them, the fracture length is the length when the positive electrode current collector breaks.

[0191] The test results of the positive electrode sheet elongation rates of Examples 1-3, Example 6 groups, Example 8 groups and Comparative Example 1 of the present invention are recorded in Table 1.

[0192] (2) Mass energy density:

[0193] At 25°C, the lithium-ion secondary batteries obtained in the examples and comparative examples of the present invention are measured using a charge-discharge regime of 0.2C charging, 0.5C discharging, and 0.025C cut-off; the plateau voltage of the lithium-ion secondary battery is the plateau voltage at 0.2C discharge rate; among them, the charge-discharge cut-off voltage is 2.5V - 4.25V.

[0194] The mass energy density (WED, unit: Wh / kg) is calculated using the following formula:

[0195] WED = capacity × plateau voltage / (battery mass).

[0196] (3) Capacity retention rate:

[0197] At 25°C, the lithium-ion secondary batteries obtained in the examples and comparative examples of the present invention were charged at 2C, discharged at 0.5C, and cycled 800T under a cycling regime with a cut-off of 0.025C; wherein, the charge and discharge cut-off voltages were 2.5V - 4.25V.

[0198] The capacity retention rate at 25°C can be calculated by the following formula:

[0199] Capacity retention rate = discharge capacity (per cycle) / initial capacity.

[0200] (4) Cycling expansion rate:

[0201] At 25°C, the lithium-ion secondary batteries obtained in the examples and comparative examples of the present invention were charged at 2C, discharged at 0.5C, and cycled 800T under a cycling regime with a cut-off of 0.025C; wherein, the charge and discharge cut-off voltages were 2.5V - 4.25V.

[0202] The cycling expansion rate at 25°C can be calculated by the following formula:

[0203] Cycling expansion rate = (thickness after cycling - initial thickness) / initial thickness.

[0204] (5) First Coulombic efficiency:

[0205] Under the condition of 25°C, if the first discharge capacity of the battery is denoted as q and the first charge capacity is denoted as Q, then the first efficiency of the battery is: η = q / Q.

[0206] The performance test records of the batteries prepared in the examples and comparative examples of the present invention are shown in Table 2.

[0207] (6) Whether the positive current collector is broken after cycling:

[0208] After cycling 800T on the lithium-ion secondary batteries obtained in the examples and comparative examples of the present invention according to the cycling regime described in (4), the battery was disassembled to observe whether the positive current collector was broken.

[0209] The results of whether the positive current collectors of Examples 1 - 3, Example Group 6, Example Group 8, and Comparative Example 1 of the present invention were broken are recorded in Table 1.

[0210] Table 1:

[0211] Positive electrode sheet elongation rate / % Positive electrode current collector breaks after 800T cycles Example 1 7.5％ No break Example 2 8.3％ No break Example 3 6.6％ No break Example 6a 7.5％ No break Example 6b 7.3％ No break Example 6c 7.4％ No break Example 6d 7.4％ Break Example 8a 3.3％ No break Example 8b 10.4％ No break Comparative Example 1 2.2％ Break

[0212] Table 2:

[0213]

[0214] Compared with Example 1, when the content of the lithium supplement agent is too small (such as Example 4a), the lithium supplement effect is not significant, and the initial Coulombic efficiency is not effectively improved. However, compared with Example 9a, a small amount of the lithium supplement agent can improve both the mass energy density and the initial Coulombic efficiency of the battery compared to no lithium supplementation; as the content of the lithium supplement agent increases, the initial Coulombic efficiency and the energy density increase. However, when the content of the lithium supplement agent increases to a certain extent, side reactions will increase, resulting in gas generation in the battery and causing an increase in the cyclic swelling of the battery (such as Example 4b). Therefore, the content of the lithium supplement agent also needs to be limited within a reasonable range. When it exceeds the range (such as Example 9b), not only will the battery experience a significant attenuation in capacity and an increase in the swelling rate due to the intensification of side reactions, but also the improvement of the energy density and the initial Coulombic efficiency of the battery will show a slow trend. On the other hand, the mass content of silicon elements in the first negative electrode coating and the second negative electrode coating should not be too large (such as Example 6d). When the silicon content on both sides is at a relatively high level, the structural design with high silicon on one side and low silicon (or no silicon) on the other side is also difficult to effectively suppress cyclic swelling. Although the energy density is significantly increased, the capacity retention rate is significantly attenuated compared to Example 1. Moreover, due to the volume change squeezing the current collector, it will cause the positive current collector to break after cycling.

[0215] As can be seen from Table 1, the lithium-ion secondary battery prepared by the present invention improves the energy density, cyclic performance, and initial Coulombic efficiency of the battery compared to the comparative example.

[0216] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A lithium ion secondary battery, characterized in that: The lithium-ion secondary battery comprises a positive electrode sheet and a negative electrode sheet; The positive electrode sheet comprises a positive electrode current collector and a first positive electrode coating and a second positive electrode coating respectively located on two opposite sides of the positive electrode current collector in a thickness direction; The negative electrode sheet comprises a negative electrode current collector and a first negative electrode coating and a second negative electrode coating respectively located on opposite sides of the negative electrode current collector in a thickness direction; the first negative electrode coating comprises a first negative electrode active material, the first negative electrode active material comprises a first carbon-based material and a first silicon-based material, the second negative electrode coating comprises a second negative electrode active material, the second negative electrode active material comprises a second carbon-based material; The mass content of silicon in the first negative electrode coating is greater than the mass content of silicon in the second negative electrode coating; The positive electrode current collector comprises a positive electrode polymer layer and a positive electrode metal layer located on both side surfaces of the positive electrode polymer layer; The elongation rate of the positive electrode current collector is 1%-20%.

2. The lithium ion secondary battery according to claim 1, wherein The first positive electrode coating and the second positive electrode coating include positive electrode active materials, the first positive electrode coating also includes a lithium supplement, and the first positive electrode coating faces the first negative electrode coating; And / or, the mass content of the lithium supplement agent in the positive electrode active material is 0.1%-15%; preferably 2%-8%; And / or, the lithium supplement includes at least one of lithium-rich lithium iron oxide, lithium-rich lithium nickel oxide, lithium-rich manganese-based material and lithium oxalate; And / or, the first silicon-based material includes at least one of elemental silicon, silicon-oxygen material, silicon-carbon material and silicon alloy; preferably includes the silicon-carbon material.

3. The lithium ion secondary battery according to claim 1, wherein The first negative electrode coating The mass content of silicon in the negative electrode coating is 1%-95%, and the mass content of silicon in the second negative electrode coating is 0%-55%; Preferably, the mass content of silicon in the first negative electrode coating is 2%-45%, and the mass content of silicon in the second negative electrode coating is 2%-30%.

4. The lithium ion secondary battery according to claim 1, wherein The second negative electrode coating further includes a second silicon-based material; And / or, the mass content of the first silicon-based material in the first negative electrode active material is 5%-100%, and the mass content of the second silicon-based material in the second negative electrode active material is 0%-60%; Preferably, the mass content of the first silicon-based material in the first negative electrode active material is greater than the mass content of the second silicon-based material in the second negative electrode active material; Preferably, the second silicon-based material includes at least one of elemental silicon, silicon-oxygen material, silicon-carbon material and silicon alloy; more preferably includes the silicon-carbon material; Preferably, the average particle size of the first silicon-based material is 0.5 μm-20 μm, and the average particle size of the second silicon-based material is 0.5 μm-20 μm.

5. The lithium ion secondary battery according to claim 2, wherein: The positive electrode active material includes at least one of a nickel-cobalt-manganese ternary material, lithium iron phosphate and lithium cobalt oxide; Preferably, the positive electrode active material includes the nickel-cobalt-manganese ternary material; the nickel-cobalt-manganese ternary material includes a substance with the chemical formula Li a Ni x Co y Mn z M b O2, where 0.9 ≤ a ≤ 1.1, 0.6 ≤ x ≤ 0.98, 0 < y ≤ 0.2, 0 < z ≤ 0.2, 0 ≤ b ≤ 0.05, and M is selected from at least one of Al, Zr, B, Y, Sr, W, Ti, and Nb; Preferably, 0.6≤x≤0.

95.

6. The lithium ion secondary battery according to claim 1, wherein The negative electrode current collector comprises a negative electrode polymer layer and a negative electrode metal layer located on both sides of the negative electrode polymer layer; And / or, the thickness of the positive electrode polymer layer is 0.1 μm-10 μm, and the thickness of the positive electrode metal layer is 0.2 μm-10 μm; preferably, the thickness of the positive electrode polymer layer is 0.5 μm-5 μm, and the thickness of the positive electrode metal layer is 0.5 μm-5 μm; And / or, the thickness of the negative electrode polymer layer is 0.1 μm-10 μm, and the thickness of the negative electrode metal layer is 0.2 μm-5 μm; preferably, the thickness of the negative electrode polymer layer is 0.5 μm-5 μm, and the thickness of the negative electrode metal layer is 0.5 μm-5 μm; and / or, the positive electrode polymer layer and the negative electrode polymer layer each independently include at least one of polypropylene, polyethylene and polyethylene terephthalate; And / or, the positive electrode metal layer includes Al; And / or, the negative electrode metal layer includes Cu.

7. The lithium ion secondary battery according to claim 1, wherein The surface capacity of the first negative electrode coating is 0.5 mAh / cm 2 -8mAh / cm 2 The surface capacity of the second negative electrode coating is 0.5 mAh / cm 2 -8mAh / cm 2 Preferably, the surface capacity of the first negative electrode coating is 1 mAh / cm 2 -6mAh / cm 2 The surface capacity of the second negative electrode coating is 1 mAh / cm 2 -4mAh / cm 2 ; And / or, the surface capacity of the first positive electrode coating is 0.6 mAh / cm 2 -7mAh / cm 2 The surface capacity of the second positive electrode coating is 0.6 mAh / cm 2 -7mAh / cm 2 Preferably, the surface capacity of the first cathode coating is 1 mAh / cm 2 -5mAh / cm 2 The surface capacity of the second positive electrode coating is 1 mAh / cm 2 -5mAh / cm 2 ; Preferably, the gram capacity of the first negative electrode active material is greater than the gram capacity of the second negative electrode active material.

8. The lithium ion secondary battery according to claim 1, wherein The second negative electrode coating comprises an upper coating and a lower coating stacked in the thickness direction of the negative electrode sheet, the upper coating is away from the negative electrode current collector, and the lower coating is close to the negative electrode current collector; the upper coating comprises a second carbon-based material A, and the lower coating comprises a second carbon-based material B, the average particle size of the second carbon-based material A is 1 μm-30 μm, and the average particle size of the second carbon-based material B is 1 μm-30 μm; And / or, the second carbon-based material A and the second carbon-based material B each independently include at least one of artificial graphite, natural graphite, soft carbon and hard carbon; Preferably, the average particle size of the second carbon-based material A is greater than the average particle size of the second carbon-based material B.

9. The lithium ion secondary battery according to claim 2, wherein: The oil absorption value of the silicon-carbon material is 10ml / 100g-100ml / 100g; And / or, the OI value of the first negative electrode coating is 5-30.

10. The lithium ion secondary battery according to any one of claims 1 to 9, wherein: The lithium-ion secondary battery comprises a stacked core or a rolled core; The stacked core further includes a diaphragm, the stacked core includes a plurality of the positive electrode sheets and the negative electrode sheets alternately stacked, and the diaphragm is arranged between the positive electrode sheets and the negative electrode sheets; The winding core further includes a first separator and a second separator, and the winding core includes a winding structure formed by sequentially stacking the first separator, the positive electrode sheet, the second separator, and the negative electrode sheet; The first negative electrode coating is located on a side of the negative electrode sheet away from the winding center of the winding core, and the second negative electrode coating is located on a side of the negative electrode sheet close to the winding center of the winding core.

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