Positive plate and lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a positive plate and a lithium ion secondary battery. The positive plate comprises a positive current collector, a positive bottom coating and a positive active layer; the positive bottom coating is positioned between the positive current collector and the positive active layer; the positive current collector comprises a polymer layer and a metal layer positioned on at least one side surface of the polymer layer; the polymer layer comprises at least one of polyethylene glycol terephthalate, polypropylene, polyethylene and polyimide, the metal layer comprises an element Al, and the positive electrode primer layer comprises a first conductive agent containing graphene and a first binder; the thickness of the polymer layer is 2-8 [mu] m, and the thickness of the metal layer is 0.5-2 [mu] m; the positive electrode bottom coating comprises a plurality of protrusions, the height of the protrusions is X, and X is larger than or equal to 3 micrometers and smaller than or equal to 7 micrometers. The lithium ion secondary battery comprises the positive plate provided by the invention. The binding force between the positive current collector and the active layer is strong, and the conductivity is good; the lithium ion secondary battery is good in cycle performance and high in energy density.

Description

Technical Field

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

[0002] As the main new power battery and energy storage battery, lithium-ion batteries still have problems such as safety, energy density, and cost. Therefore, finding a lithium battery with high safety, low cost, and high energy density is the key to the current development breakthrough. The composite current collector is a new type of current collector, which is composed of a metal layer and an intermediate layer. The intermediate layer is a polymer layer. The composite current collector has a "metal layer - intermediate layer - metal layer" sandwich structure. The composite current collector is light in weight, consumes less raw materials and energy per unit area, can improve the thermal runaway of lithium-ion batteries, enhance the safety of lithium-ion batteries, and can solve the current development pain points of lithium batteries. Although the composite current collector has obvious cost advantages compared with traditional metal current collector materials, as the carrier of the positive electrode material, there are still problems such as poor adhesion and high contact resistance between the metal layer and the positive electrode active layer. Therefore, improving the above problems is very important for further realizing the application of the composite current collector in batteries. Summary of the Invention

[0003] The purpose of the present invention is to improve the problems of poor adhesion and high contact resistance between the metal layer of the composite current collector and the positive electrode active layer when the composite current collector is used in the positive electrode sheet, improve the energy density of the lithium-ion secondary battery (hereinafter referred to as the battery), improve the battery safety, reduce the thermal runaway risk, and improve the battery cycle stability.

[0004] In the prior art, the flexibility and ductility of traditional metal current collectors are poor, and they are prone to breakage under external force extrusion (such as negative electrode swelling) or collision, resulting in internal short circuit of the battery and reduced battery safety; while using a composite current collector instead of a metal current collector can not only reduce the battery weight and improve the energy density, but also utilize the high ductility of the polymer layer. When the metal layer breaks, the polymer layer can cut off the electronic path in time, reduce the thermal runaway risk, and improve the battery safety. However, the inventors of the present invention found during the research process that in the positive electrode sheet, only using a composite current collector to replace the metal layer can alleviate the fracture problem of the current collector, but there are still problems such as cracks or even fractures in the metal layer of the composite current collector caused by the volume change during the charge and discharge process of the battery, resulting in a decrease in conductivity and insufficient safety; therefore, the inventors of the present invention further provided a positive electrode bottom coating on the surface of the metal layer of the composite current collector, which can enhance the adhesion strength between the positive electrode active layer and the composite current collector on the one hand, enhance the battery cycle stability, and on the other hand, can provide and enhance the conductive network, reduce the contact resistance, and improve the safety performance of the battery.

[0005] In order to solve the above problems, the present invention proposes the following technical solutions:

[0006] In the first aspect of the present invention, a positive electrode current collector is provided. The positive electrode sheet includes a positive electrode current collector, a positive electrode bottom coating, and a positive electrode active layer. The positive electrode bottom coating is located between the positive electrode current collector and the positive electrode active layer; the positive electrode current collector includes a polymer layer and a metal layer located on at least one surface of the polymer layer; the polymer layer includes at least one of polyethylene terephthalate, polypropylene, polyethylene, and polyimide, the metal layer includes element Al, and the positive electrode bottom coating includes a first conductive agent and a first binder; the first conductive agent includes graphene; the thickness of the polymer layer is 2 μm - 8 μm, and the thickness of the metal layer is 0.5 μm - 2 μm; the positive electrode bottom coating includes a number of protrusions, and the height of the protrusions is X, 3 μm ≤ X ≤ 7 μm.

[0007] In the second aspect of the present invention, a lithium-ion secondary battery is provided, and the lithium-ion secondary battery includes the positive electrode sheet provided in the first aspect of the present invention.

[0008] Through the above technical solutions, the present invention has at least the following advantages compared with the prior art:

[0009] (1) The positive electrode sheet of the present invention can enhance the interfacial bonding force between the positive electrode current collector and the positive electrode active layer, and reduce the shedding of the positive electrode active material during the battery cycling process.

[0010] (2) The positive electrode sheet of the present invention can alleviate the decrease in conductivity caused by the rupture or fracture of the metal layer in the composite current collector, provide a conductive network for the positive electrode sheet, reduce the contact resistance between the positive electrode active layer and the positive electrode current collector, reduce the internal resistance, and enhance the current-carrying capacity of the composite current collector.

[0011] (3) The positive electrode sheet of the present invention has good conductivity and mechanical properties.

[0012] (4) The lithium-ion secondary battery of the present invention has good energy density and cycle stability, alleviates the risk of battery thermal runaway, and has good safety performance.

[0013] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 The structure diagram of the positive electrode current collector in an embodiment of the present invention is shown.

[0015] Figure 2 The structural schematic diagram of the first part in an embodiment of the present invention is shown.

[0016] Figure 3 The scanning electron microscope (SEM) image of the positive electrode bottom coating in an embodiment of the present invention is shown.

[0017] Figure 4 The SEM image of the positive electrode bottom coating in an embodiment of the present invention is shown.

[0018] Figure 5 The cross-sectional SEM image of the positive electrode sheet in an embodiment of the present invention is shown.

[0019] Figure 6 The EDS image of the surface of the positive electrode bottom coating in an embodiment of the present invention is shown.

[0020] Figure 7 The structural schematic diagram of the positive electrode current collector in an embodiment of the present invention is shown.

[0021] Reference numerals: 1 is an empty foil area, 2 is a pasted area, 31 is a positive electrode current collector, 32 is a positive electrode bottom coating, 312 is a metal layer, and 311 is a polymer layer. 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 explaining and illustrating the present invention, and are not used to limit the present invention.

[0023] In a first aspect of the present invention, a positive electrode sheet is provided. The positive electrode sheet includes a positive electrode current collector, a positive electrode bottom coating, and a positive electrode active layer. The positive electrode bottom coating is located between the positive electrode current collector and the positive electrode active layer; the positive electrode current collector includes a polymer layer and a metal layer located on at least one surface of the polymer layer; the polymer layer includes at least one of polyethylene terephthalate, polypropylene, polyethylene, and polyimide, the metal layer includes element Al, and the positive electrode bottom coating includes a first conductive agent and a first binder; the first conductive agent includes graphene.

[0024] In the present invention, the thickness of the polymer layer is 2 μm - 8 μm, for example, 2 μm, 2.2 μm, 2.4 μm, 2.6 μm, 2.8 μm, 3 μm, 3.5 μm, 4 μm, 5 μm, 6 μm, 7 μm, or 8 μm.

[0025] In one embodiment, the thickness of the polymer layer is 5 μm - 7 μm.

[0026] In the present invention, the thickness of the metal layer is 0.5 μm - 2 μm, for example, 0.5 μm, 0.6 μm, 0.8 μm, 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm or 2 μm.

[0027] In the present invention, the positive electrode bottom coating includes a plurality of protrusions, the height of the protrusions is X, 3 μm ≤ X ≤ 7 μm, for example, 3 μm, 3.2 μm, 3.4 μm, 3.6 μm, 3.8 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm or 7 μm.

[0028] In one embodiment, 4 μm ≤ X ≤ 5 μm.

[0029] In the present invention, the "a plurality of protrusions" means that the number of the protrusions ≥ 2.

[0030] In the present invention, the height of the protrusion refers to the maximum vertical distance from the highest point of the protrusion to the surface of the positive electrode bottom coating.

[0031] As Figure 1 shown is a schematic structural diagram of a positive electrode current collector in an embodiment of the present invention. Among them, 31 is the positive electrode current collector, 312 is the metal layer, and 311 is the polymer layer; as Figure 2 shown is a schematic structural diagram of a first part in an embodiment of the present invention. The first part includes the positive electrode current collector and the positive electrode bottom coating. Among them, 31 is the positive electrode current collector and 32 is the positive electrode bottom coating; as Figure 3 and Figure 4 shown is an SEM image of the positive electrode bottom coating in an embodiment of the present invention. Figure 3 and Figure 4 are taken at different magnifications respectively. It can be seen from the figure that the positive electrode bottom coating presents a network structure; as Figure 5 shown is a cross-sectional SEM image of a positive electrode sheet in an embodiment of the present invention. It can be seen from the figure that the positive electrode current collector includes a polymer layer and a metal layer, and the positive electrode bottom coating is located between the positive electrode current collector and the positive electrode active layer.

[0032] In the present invention, the positive electrode current collector includes a metal layer and a polymer layer. The polymer layer has high ductility, which can improve the tensile strength of the positive electrode current collector, thereby improving the physical integrity of the positive electrode sheet during the battery cycle and reducing or avoiding cracks or even fractures in the metal layer caused by battery swelling during the cycle. Moreover, the density of the polymer layer is lower than that of pure metal, which reduces the mass of the positive electrode current collector under the condition of the same thickness, while the proportion of the active material in the battery remains unchanged or increases, thereby increasing the energy density of the battery.

[0033] However, although the structure of the positive current collector can maintain the structural integrity of the positive electrode sheet to a certain extent, due to the different ductilities of the polymer layer and the metal layer, when the volume change is more significant during rolling or battery cycling, the positive current collector is subjected to stress, and the metal layer may break or fall off, resulting in damage to the metal layer, a decrease in conductivity, an increase in the resistance of the positive electrode sheet, and an impact on the electrochemical performance and safety of the battery. Therefore, the present invention further introduces a positive electrode bottom coating on the surface of the metal layer. The positive electrode bottom coating includes graphene and also has several protrusions in its structure. On the one hand, it can enhance the bonding force between the positive current collector and the positive electrode active layer, increase the contact area, and prevent the positive electrode active layer from peeling or falling off during battery cycling, resulting in cycle failure. On the other hand, even when the metal layer breaks and falls off after the positive electrode sheet is rolled or subjected to external pressure, the positive electrode bottom coating can still provide a conductive network, maintain a certain electronic path, and can relieve the battery temperature rise and improve the battery safety performance. In addition, the positive electrode bottom coating also has the effect of reducing the contact resistance between the positive electrode active layer and the positive current collector, blocking the direct contact between the electrolyte and the positive current collector, and inhibiting the corrosion of the positive current collector. The reasons are as follows: First, graphene has good conductivity, can improve the electron transport performance, provide a transport channel, and form a conductive network. Second, graphene also has good strength and toughness, can form a stable network structure between the positive electrode active layer and the positive current collector, and has both support and connection functions. Third, graphene is dispersed in the positive electrode bottom coating to form several protrusions. The protrusions can increase the roughness of the positive electrode bottom coating, and the protrusions can also extend to the bottom of the positive electrode active layer, forming an interlocking structure with the positive electrode active layer, improving the bonding strength between the positive electrode bottom coating and the positive electrode active layer, and improving the structural stability of the positive electrode.

[0034] In the present invention, the thickness of the metal layer, the thickness of the polymer layer, and the thickness of the positive electrode bottom coating can be obtained by conventional testing methods in the art, such as measured by a micrometer and a scanning electron microscope. Specifically, after discharging the lithium-ion secondary battery to 0% SOC, disassemble and take out the positive 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 positive electrode sheet, then keep the positive electrode sheet at 450 °C for 180 min, brush off the positive electrode active material from the cooled positive electrode sheet to obtain a positive current collector with a positive electrode bottom coating on its surface, cut the cross-section thereof by ion beam cutting, the thickness of the metal layer and the thickness of the polymer layer can be measured by a micrometer, and the thickness of the positive electrode bottom coating is measured by scanning electron microscope imaging of the obtained cross-section and using image analysis software to measure the thickness of the positive electrode bottom coating in the obtained image.

[0035] In the present invention, the height X of the protrusion can be obtained by conventional testing methods in the art, for example, measured by ion milling (CP) in combination with scanning electron microscopy (SEM), specifically as follows: After discharging the lithium-ion secondary battery to 0% SOC, disassemble and take out the positive electrode sheet, soak it in dimethyl carbonate (DMC) solvent for 12 h, then rinse it with DMC to remove the lithium salt attached to the positive electrode sheet, then keep the positive electrode sheet at 450 °C for 180 min, brush off the positive electrode active material from the cooled positive electrode sheet to obtain a positive electrode current collector with a positive electrode bottom coating on its surface, image its side cut cross-section, select 10 different protrusions to measure the protrusion height, and calculate the average value as the height X of the protrusion.

[0036] In the present invention, the thickness of the positive electrode bottom coating is 1 μm - 2 μm, for example, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm or 2 μm.

[0037] In the present invention, based on the weight of the metal current collector of the same thickness, the weight of the positive electrode current collector is reduced by 50% - 70%, for example, 50%, 52%, 54%, 56%, 58%, 60%, 62%, 64%, 66%, 68% or 70%.

[0038] In the present invention, the reduction in the weight of the positive electrode current collector helps to improve the mass energy density of the battery, enabling the battery to have a higher energy density under the same weight and improving energy storage.

[0039] In the present invention, the mass content ratio of element Al to element C on the surface of the positive electrode bottom coating is (0.43 - 1.1):1, for example, 0.43:1, 0.45:1, 0.48:1, 0.5:1, 0.55:1, 0.6:1, 0.65:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.05:1 or 1.1:1.

[0040] In one embodiment, the mass content ratio of element Al to element C on the surface of the positive electrode bottom coating is (0.55 - 0.9):1.

[0041] In the present invention, the mass content of element Al on the surface of the positive electrode bottom coating is 30% - 50%, for example, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 38%, 40%, 42%, 44%, 46%, 48% or 50%; the mass content of element C on the surface of the positive electrode bottom coating is 45% - 70%, for example, 45%, 46%, 47%, 48%, 50%, 52%, 54%, 56%, 58%, 60%, 62%, 64%, 66%, 68% or 70%.

[0042] In one embodiment, the mass content of element Al on the surface of the positive electrode bottom coating is 35%-45%, and the mass content of element C on the surface of the positive electrode bottom coating is 51%-62%.

[0043] In the present invention, by regulating the mass content ratio of element Al and element C on the surface of the positive electrode bottom coating, the coating amount of the positive electrode bottom coating on the surface of the metal layer can be regulated, thereby adjusting the overall conductivity and mechanical properties of the positive electrode sheet. Element Al is derived from the metal layer, and element C mainly comes from the first conductive agent in the positive electrode bottom coating. When the mass content ratio of element Al and element C is within an appropriate range, the first conductive agent can significantly improve the conductivity of the positive electrode sheet. Among them, both graphene and carbon nanotubes have good electrical properties and can quickly conduct electrons to form a continuous conductive path on the surface of the metal layer, improving the electron transport efficiency. On the other hand, the first conductive agent has a high strength and modulus, and being dispersed in the positive electrode bottom coating can improve the mechanical properties of the positive electrode current collector. When the mass content ratio of element Al and element C is less than 0.43:1, element C is excessive, and the first conductive agent is prone to agglomeration in the positive electrode bottom coating, resulting in uneven distribution and being unfavorable for performance improvement; when the mass content ratio of element Al and element C is greater than 1.1:1, element C is too little, and the positive electrode bottom coating cannot form a conductive network on the surface of the metal layer, so the conductivity of the positive electrode sheet cannot be effectively improved.

[0044] In the present invention, the mass content ratio of element Al and element C on the surface of the positive electrode bottom coating can be obtained by conventional testing methods in the art. For example, it can be obtained by performing element distribution analysis on the surface of the positive electrode bottom coating using energy-dispersive X-ray spectroscopy mapping (EDS). The specific method is as follows: After discharging the lithium-ion secondary battery to 0% SOC, disassemble and take out the positive electrode sheet, soak it in dimethyl carbonate (DMC) solvent for 12 h, then rinse it with DMC to remove the lithium salt attached to the positive electrode sheet. Then, keep the positive electrode sheet at 450 °C for 180 min. After cooling, brush off the positive electrode active material from the positive electrode sheet to obtain a positive electrode current collector with a positive electrode bottom coating on its surface, make a sample, perform SEM-EDS testing on it to obtain the element distribution and analyze it. The observation range of the SEM image can be selected to be magnified by 1.00K times.

[0045] As Figure 6 shown is the EDS diagram of the surface of the positive electrode bottom coating in an embodiment of the present invention. Among them, Figure 6 (a) is the SEM image of the surface of the positive electrode bottom coating, Figure 6 (b) is the distribution diagram of element O, Figure 6 (c) is the distribution diagram of element C, Figure 6 (d) is the distribution diagram of element Al, Figure 6(e) is the elemental spectral line diagram of the positive electrode current collector surface. It can be seen from the figure that the mass content of element Al on the surface of the positive electrode bottom coating is 38.6%, and the mass content of element C is 58.3%.

[0046] In the present invention, the first conductive agent further includes at least one of carbon nanotubes, conductive carbon black, and acetylene black.

[0047] In the present invention, the first binder includes at least one of styrene-butadiene rubber, polyacrylic acid, styrene-acrylic rubber, lithium polyacrylate, sodium polyacrylate, polyacrylic acid, and polyvinylidene fluoride.

[0048] In one embodiment, the first conductive agent includes at least one of graphene, carbon nanotubes, and acetylene black.

[0049] In the present invention, acetylene black and conductive carbon black are zero-dimensional materials, the first carbon nanotubes are one-dimensional materials, and graphene is a two-dimensional material. When the first conductive agents in the positive electrode bottom coating are combined arbitrarily, the zero-dimensional materials and / or one-dimensional materials and / or two-dimensional materials can be combined by point-line, point-plane, or point-line-plane, and a uniform conductive network is formed after mixing, effectively enhancing the conductivity of the conductive positive electrode bottom coating, thereby reducing the resistance. And when the metal layer is stressed and fractured, an electron path can still be maintained, improving the battery safety.

[0050] In the present invention, the tensile strength of the positive electrode sheet in the MD direction ≥ 100 MPa, such as 100 MPa, 110 MPa, 120 MPa, 130 MPa, 140 MPa, 150 MPa, 160 MPa, 170 MPa, 180 MPa, 190 MPa, 200 MPa, 210 MPa, 220 MPa, 230 MPa, 240 MPa, or 250 MPa; the tensile strength in the TD direction ≥ 60 MPa, such as 60 MPa, 80 MPa, 100 MPa, 120 MPa, 140 MPa, 180 MPa, 190 MPa, 200 MPa, 210 MPa, or 220 MPa.

[0051] In one embodiment, the tensile strength of the positive electrode sheet in the MD direction ≥ 120 MPa, and the tensile strength in the TD direction ≥ 80 MPa.

[0052] In the present invention, the elongation at break of the positive electrode sheet in the MD direction ≥ 10%, such as 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, or 25%; the elongation at break in the TD direction ≥ 5%, such as 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or 15%.

[0053] In the present invention, the elongation at break in the MD direction of the positive current collector is ≥15%, such as 15%, 15.5%, 16%, 16.5%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24% or 25%; the elongation at break in the TD direction is ≥10%, such as 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5% or 15%.

[0054] In the present invention, the "MD direction" and the "TD direction" have their conventional meanings in the art. The "MD direction" represents..., and the "TD direction" represents perpendicular to the rolling direction of the positive electrode sheet. The "elongation at break MD" represents along the rolling direction of the positive electrode sheet.

[0055] In the present invention, the tensile strength in the MD direction and the tensile strength in the TD direction of the positive electrode sheet can be obtained by conventional testing methods in the art. For example, they are tested by the following method: 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. Then keep the positive electrode sheet at 450 °C for 180 min, and obtain the positive electrode sheet after cooling; Cut the positive electrode sheet into sheet samples of (2 cm - 3 cm) × (10 cm - 20 cm), and test them with a tensile testing machine to obtain the results.

[0056] In the present invention, the elongation at break in the MD direction and the elongation at break in the TD direction of the positive electrode sheet or the positive current collector can be obtained by conventional testing methods in the art. For example, they are tested by the following method: 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. Then keep the positive electrode sheet at 450 °C for 180 min, and obtain the positive electrode sheet after cooling; Cut it into sheet samples of (2 cm - 3 cm) × (10 cm - 20 cm), with an initial length of l0. Stretch it to break with a tensile testing machine, and measure its length as l1. Then the elongation at break is (l1 - l0) / l0.

[0057] In the present invention, the areal density of the positive electrode bottom coating is 0.3 g / m 2 -2 g / m 2 For example, it is 0.3 g / m 2 、0.32 g / m 2 、0.34 g / m 2 、0.36 g / m 2 、0.4 g / m 2 、0.5 g / m 2 、0.6 g / m 2 、0.7 g / m2 , 0.8g / m 2 , 0.9g / m 2 , 1g / m 2 , 1.2g / m 2 , 1.4g / m 2 , 1.6g / m 2 , 1.8g / m 2 or 2g / m 2 .

[0058] In one embodiment, the surface density of the positive electrode bottom coating is 0.5 g / m 2 -1.5g / m 2 .

[0059] Regulating the surface density of the positive electrode bottom coating within an appropriate range can optimize the conductive network, make more effective use of the active materials, and help improve the energy density. However, when the surface density of the positive electrode bottom coating is too large, based on the total weight of the positive electrode sheet, the proportion of positive electrode active materials will be relatively reduced, thereby reducing the energy density. When the surface density of the positive electrode bottom coating is too small, the conductivity of the positive electrode collector will not be significantly improved, and it will be less helpful to improve the activation degree of the positive electrode active materials, and the barrier effect on the contact between the electrolyte and the positive electrode collector will be reduced.

[0060] In the present invention, the area S1 of the positive electrode bottom coating and the area S2 of the positive electrode current collector satisfy: 50%≤S1 / S2<85%, for example, 50%, 52%, 54%, 56%, 58%, 60%, 64%, 68%, 72%, 76%, 80%, 81%, 82%, 83%, 84% or 84.5%.

[0061] In one embodiment, 70%≤S1 / S2≤80%.

[0062] In the present invention, the positive electrode sheet includes a hollow foil area and a paste-coated area, the positive electrode base coating is located in the paste-coated area, and the area ratio of the hollow foil area to the area ratio of the positive electrode sheet is S3, 15%≤S3≤25%, for example, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24% or 25%.

[0063] In one embodiment, the positive electrode bottom coating accounts for a smaller area than the positive electrode sheet than the paste coating area.

[0064] like Figure 7 FIG. 1 is a schematic diagram of the structure of the positive electrode current collector in one embodiment of the present invention, wherein: Figure 7 (a) is a top view, Figure 7(b) is a cross-sectional view along the thickness direction of the positive current collector. 1 is the empty foil area, 2 is the pasted area, 31 is the positive current collector, and 32 is the positive bottom coating.

[0065] In the present invention, by adjusting the relationship between the area S1 of the positive bottom coating and the area S2 of the positive current collector, the area ratio of the positive bottom coating on the surface of the positive current collector can be controlled within a suitable range. When S1 / S2 is greater than or equal to 85%, the area ratio of the positive bottom coating on the surface of the positive current collector is too high. At this time, when the content of the first conductive agent is the same, the too large coating area leads to a decrease in the density of the conductive network formed on the surface of the metal layer, reducing the risk of failure of the positive bottom coating caused by the edge effect and short circuit due to contact with other battery components, and improving the safety of the battery. When S1 / S2 is less than 50%, the area of the positive bottom coating is too small to form a conductive network to improve the conductivity of the positive current collector. Adjusting the area ratio S3 of the empty foil area on the positive current collector within a certain range can avoid contact between the active layer and other battery components, reduce the short circuit risk, improve safety, and can also increase extra space to adapt to the volume change of the battery during charge and discharge.

[0066] Furthermore, the pasted area includes a positive active layer. When the area ratio of the positive bottom coating on the positive electrode sheet is less than the area ratio of the pasted area on the positive electrode sheet, the positive bottom coating can provide a good conductive path for the positive active layer. When the area of the positive bottom coating is too large and exceeds the area of the pasted area, not only is it difficult for the positive bottom coating to effectively play a role in synergistic conduction with the positive active layer during the charge and discharge process of the battery, but also the safety performance of the battery will be reduced.

[0067] In the present invention, the surface density of the positive bottom coating is obtained through conventional tests in the art. For example, it is measured in the following manner: After discharging the lithium-ion secondary battery to 0% SOC, disassemble and take out the positive electrode sheet, soak it in dimethyl carbonate (DMC) solvent for 12 h, and then rinse it with DMC to remove the lithium salt attached to the electrode sheet. After drying, cut the positive electrode sheet along the extension direction of the positive electrode tab as a sample. Perpendicular to the junction of the empty foil area and the pasted area, take a pasted area with a width less than 2 mm in the direction of the pasted area, avoiding the positive bottom coating, and measure its surface density as ρ1. Then, take a positive bottom coating + pasted area with the same width in the area with both the pasted area and the positive bottom coating, and measure its surface density as ρ2. The surface density of the positive bottom coating is ρ2 - ρ1.

[0068] In the present invention, the area S1 of the positive electrode bottom coating and the area S2 of the positive electrode current collector can be obtained by the following method: After discharging the lithium-ion secondary battery to 0% SOC, disassemble and take out the positive electrode sheet, soak it in dimethyl carbonate (DMC) solvent for 12 h, then rinse it with DMC to remove the lithium salts attached to the electrode sheet, dry it, and cut the positive electrode sheet along the extension direction of the positive electrode tab as a sample. Measure the area of the sample, which is the area of the positive electrode current collector, denoted as S2. The size of the positive electrode sheet perpendicular to the extension direction of the tab is p1. Take SEM imaging of the obtained cross-section, and measure the interface length between the paste-coated area and the positive electrode bottom coating in the SEM image, denoted as d1. Then the area S1 of the positive electrode bottom coating = d1 × p1, and calculate S1 / S2 accordingly.

[0069] In the present invention, the aspect ratio of the first carbon nanotube is 100 - 20000, such as 40, 50, 100, 150, 200, 400, 600, 800, 1000, 1500, 2000, 3000, 5000, 10000, 15000 or 20000.

[0070] In one embodiment, the aspect ratio of the first carbon nanotube is 300 - 15000.

[0071] In the present invention, based on the total weight of the positive electrode bottom coating, the content W1 of the first carbon nanotube is 2% - 5%, such as 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, 3.4%, 3.8%, 4.2%, 4.6% or 5%.

[0072] In the present invention, the number of layers of the graphene is 3 - 10 layers, such as 3 layers, 4 layers, 5 layers, 6 layers, 7 layers, 8 layers, 9 layers or 10 layers.

[0073] In the present invention, the sheet diameter of the graphene is R, 0.5 μm ≤ R ≤ 30 μm, such as 0.5 μm, 0.6 μm, 0.8 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 25 μm or 30 μm.

[0074] In one embodiment, 2 μm ≤ R ≤ 20 μm.

[0075] In another embodiment, 5 μm ≤ R ≤ 15 μm.

[0076] In the present invention, the thickness of the graphene is D, 3 μm ≤ D ≤ 10 μm, such as 3 μm, 3.2 μm, 3.4 μm, 3.6 μm, 3.8 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm.

[0077] Adjusting the sheet diameter of graphene within a specific range can further optimize the conductivity of the positive electrode bottom coating. When the sheet diameter of graphene is too small (e.g., <0.5 μm), the specific surface area of graphene is large and the surface energy is high at this time, making it easy to agglomerate, and it is difficult to disperse uniformly during the preparation process, resulting in poor fluidity of the positive electrode bottom coating slurry; while when the sheet diameter of graphene is too large (e.g., >30 μm), the electron mobility of graphene is high at this time. When the sheet diameter of graphene further increases to be larger than the average particle size of the positive electrode active material, a steric hindrance effect will occur, thus hindering the migration of lithium ions.

[0078] In the present invention, graphene not only has good toughness, but also the property of "pre-shrinking" of graphene endows it with high flexibility. When graphene is used in the positive electrode bottom coating, when the positive electrode current collector is stressed, graphene is not easily completely broken, and at the same time, "horizontal sliding" can occur between graphene layers, thus playing a buffering role, reducing the damage to the conductive layer of the positive electrode current collector during the compaction process and the volume change during the charge and discharge process of the battery, avoiding cracks, and improving the stability of the battery.

[0079] It can be understood that the sheet diameter of the graphene has the conventional meaning in the art, which refers to: along the two-dimensional plane direction of the graphene, when the shape of the graphene is a regular shape, the sheet diameter of the graphene refers to the size along the long axis direction of the graphene, and when the shape of the graphene is an irregular shape, the sheet diameter of the graphene refers to the size of the equivalent diameter of a regular circle with an area equal to that of the irregular-shaped graphene.

[0080] In the present invention, the number of layers, the sheet diameter and the thickness of the graphene can be obtained by conventional testing methods in the art, for example, by the following methods: the number of layers of the graphene can obtain the adsorption isotherm through BET testing, calculate the actual specific surface area of the graphene using the BET equation in combination with the monolayer theory, and calculate the number of layers by comparing the specific surface area of monolayer graphene and the actual specific surface area of the graphene, and the transmission electron microscope (TEM) is used to assist in confirming the number of layers; the sheet diameter of the graphene can be obtained by scanning electron microscope (SEM) imaging, and then measuring the sheet diameters of 10-20 different graphene particles and taking the average value; the thickness of the graphene can be calculated according to the number of layers of the graphene.

[0081] In the present invention, based on the total weight of the positive electrode bottom coating, the sum of the contents of the first carbon nanotube and the graphene is Wt, 2% ≤ Wt ≤ 7%, for example, 2%, 2.2%, 2.4%, 2.6%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6% or 7%.

[0082] In the present invention, based on the total weight of the positive electrode bottom coating, wherein the content of the first carbon nanotube is W1 and the content of the graphene is W2, 0.4 ≤ W1 / W2 ≤ 2.5, for example, 0.4, 0.43, 0.45, 0.5, 0.6, 0.8, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4 or 2.5.

[0083] In one embodiment, 0.9 ≤ W1 / W2 ≤ 1.5.

[0084] In the present invention, 2% ≤ W1 ≤ 5%, for example, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, 3.4%, 3.8%, 4.2%, 4.6% or 5%.

[0085] In the present invention, 2% ≤ W1 ≤ 5%, for example, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, 3.4%, 3.8%, 4.2%, 4.6% or 5%.

[0086] In the present invention, adjusting the sum of the contents of the first carbon nanotube and the graphene helps to avoid the reduction of the dispersibility and the occurrence of agglomeration in the positive electrode bottom coating due to the excessive amount of the first carbon nanotube and the graphene in the positive electrode bottom coating, resulting in non-uniform conductivity; while regulating the ratio W1 / W2 of the contents of the first carbon nanotube and the graphene can optimize the dispersion stability of the two in the positive electrode bottom coating, which can not only avoid the stacking of the graphene but also avoid the entanglement between the first carbon nanotubes, improve the contact between the graphene and the first carbon nanotubes, enable the first carbon nanotubes to exert the electron conduction performance in the axial direction, and the planar structure of the graphene provides a transverse conduction channel, and the two are combined with each other to achieve the effect of optimizing the electron transport efficiency of the conductive network.

[0087] In the present invention, the roughness of the positive electrode bottom coating ≥ 2 μm, for example, 2 μm, 2.2 μm, 2.4 μm, 2.6 μm, 2.8 μm, 3 μm, 3.4 μm, 3.8 μm, 4.2 μm, 4.6 μm, 5 μm, 6 μm, 7 μm, 8 μm or 9 μm.

[0088] In one embodiment, the roughness of the positive electrode bottom coating is 3 μm - 6 μm.

[0089] In the present invention, the roughness can be obtained by conventional testing methods in the art, for example, measured by a surface roughness measuring instrument.

[0090] In the present invention, the peel strength between the positive electrode bottom coating and the metal layer ≥ 5 N / m, for example, 5 N / m, 6 N / m, 8 N / m, 10 N / m, 12 N / m, 14 N / m, 18 N / m, 22 N / mN, 26 N / m, 28 N / m or 30 N / m.

[0091] In the present invention, the positive electrode sheet includes a first part and a second part. The first part includes the positive electrode current collector and the positive electrode bottom coating, and the second part includes a positive electrode active layer. The sheet resistance of the first part is ≤ 40 mΩ, for example, 1 mΩ, 1.2 mΩ, 1.4 mΩ, 1.8 mΩ, 2.5 mΩ, 3 mΩ, 4 mΩ, 5 mΩ, 10 mΩ, 15 mΩ, 20 mΩ, 25 mΩ, 30 mΩ, 35 mΩ or 40 mΩ.

[0092] In the present invention, regulating the roughness of the positive electrode bottom coating can increase the contact area between the positive electrode bottom coating and the positive electrode active layer, cause the positive electrode active layer and the positive electrode bottom coating to interpenetrate each other, increase the adhesion between the positive electrode active layer and the current collector, increase the bonding strength, reduce the shedding of the active material during charge and discharge, and improve the electron conduction and ion diffusion in the battery.

[0093] In the present invention, the peel strength between the positive electrode bottom coating and the metal layer can be measured by the following method: Cut the positive electrode sheet into specimens with a size of 2 cm - 20 cm, paste them on a paper template, and test using a high-precision tensile testing machine; The sheet resistance of the first part can be measured by a surface resistance tester.

[0094] In the present invention, the positive electrode active layer includes a second carbon nanotube. Based on the total weight of the positive electrode active layer, the content of the second carbon nanotube is 0.8% - 1.5%, for example, 0.8%, 0.85%, 0.9%, 0.95%, 1%, 1.1%, 1.2%, 1.3%, 1.4% or 1.5%.

[0095] In the present invention, the positive electrode active layer includes a positive electrode active material.

[0096] In the present invention, the second carbon nanotube and the first carbon nanotube may be the same or different. It is understood that when the second carbon nanotube and the first carbon nanotube are the same, the first carbon nanotube and the second carbon nanotube have the same aspect ratio and the same type.

[0097] In the present invention, the positive electrode active layer includes a ternary positive electrode material. The ternary positive electrode material includes a chemical formula of Li a Ni x Co y M Z A kSubstances of O2, 0.9 ≤ a ≤ 1.1 (such as 0.9, 0.92, 0.94, 0.96, 1, 1.05 or 1.1), 0.8 ≤ x ≤ 0.99 (such as 0.8, 0.82, 0.84, 0.86, 0.88, 0.9, 0.92, 0.94, 0.96, 0.98 or 0.99), 0 < y ≤ 0.1 (such as 0.01, 0.02, 0.04, 0.06, 0.08 or 0.1), 0 < z ≤ 0.1 (such as 0.01, 0.02, 0.04, 0.06, 0.08 or 0.1), 0 ≤ k ≤ 0.05 (such as 0, 0.01, 0.02, 0.03, 0.04 or 0.05), M is selected from Al and / or Mn, and A is selected from at least one of Mg, Ti, B, P, W, Zr, Mo, Y and La.

[0098] In one embodiment, the ternary cathode material includes transition metal elements, and the transition metal elements include nickel elements. Based on the sum of the molar contents of the transition metal elements, the content of the nickel elements ≥ 80%, such as 80%, 85%, 86%, 88%, 90%, 91%, 91.5%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100%.

[0099] In the present invention, based on the total weight of the cathode active layer, the content of the ternary cathode material is greater than 97%, such as 97%, 97.5%, 98%, 98.5%, 99% or 99.5%.

[0100] In the present invention, the cathode active layer includes a ternary cathode material. As the content of nickel elements in the ternary cathode material increases, the specific capacity of the ternary cathode material also increases, which is beneficial to further improving the energy density of the battery. However, the increase in the content of nickel elements is also accompanied by an increase in the risk of structural damage. Based on the sum of the molar contents of the transition metal elements, when the content of nickel elements ≥ 80%, its structural stability deteriorates. At this time, the high-nickel ternary cathode material (the proportion of nickel elements in the sum of the contents of transition metal elements ≥ 80%) is combined with the cathode current collector of the present invention. Due to its unique "sandwich" structure, it changes the fracture behavior of the metal layer under external force, forms multiple-point open circuits in the electrode sheet, slows down the temperature rise of the battery, effectively improves the battery safety, reduces the risk of thermal runaway, and effectively avoids the risk brought by the decline of the structural stability of the high-nickel ternary cathode material, which is beneficial to the use and popularization of the high-nickel ternary cathode material.

[0101] In the second aspect of the present invention, a lithium-ion secondary battery is provided, and the lithium-ion secondary battery includes the cathode sheet described in the first aspect of the present invention.

[0102] In the present invention, the lithium-ion secondary battery includes an anode sheet, and the anode sheet includes a carbon-based material and / or a silicon-based material.

[0103] In one embodiment, the silicon-based material includes at least one of a silicon-carbon material and a silicon-oxygen material.

[0104] In the present invention, the silicon-carbon material includes a porous carbon matrix and a silicon material located in the internal pores of the porous carbon matrix.

[0105] In one embodiment, the mass content of silicon element in the silicon-carbon material is 20%-70%, such as 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65% or 70%.

[0106] In the present invention, the negative electrode sheet includes a negative electrode active layer. Based on the total weight of the negative electrode active layer, the content of the carbon-based material and / or the silicon-based material is greater than 94%, such as 94%, 94.5%, 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, 99% or 99.5%.

[0107] In the present invention, the silicon-based negative electrode containing a silicon-based material has a higher capacity compared with a pure graphite negative electrode, and has a significant effect on improving the energy density of the battery. When combined with the positive electrode current collector of the present invention, it can alleviate the problem that the silicon-based negative electrode expands during the charge and discharge cycle and generates an external pressure on the positive electrode current collector, resulting in the rupture and breakage of the current collector, and can provide a complete conductive network to improve the battery efficiency.

[0108] In the present invention, the lithium-ion secondary battery includes a soft-pack battery.

[0109] In the present invention, the positive electrode active layer includes a positive electrode dispersant, a positive electrode conductive agent, and a positive electrode binder, and they are all conventional selections of those skilled in the art. For example, the positive electrode dispersant includes carboxymethyl cellulose (CMC), the positive electrode conductive agent includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene, and the positive electrode binder includes at least one of polyvinylidene fluoride (PVDF), polyacrylate, polyimide, and styrene-butadiene rubber.

[0110] In the present invention, the negative electrode sheet further includes a negative electrode current collector, a negative electrode conductive agent, and a negative electrode binder, and they are all conventional selections of those skilled in the art. For example, the negative electrode current collector includes copper foil, the negative electrode binder includes at least one of polyacrylic acid (PAA), sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose, polyimide, styrene-butadiene rubber (SBR), and polyvinylidene fluoride, and the negative electrode conductive agent includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

[0111] In the present invention, the lithium-ion secondary battery further includes an electrolyte, which is a conventional choice in the art.

[0112] It should be noted that in the present invention, the numerical representations such as "first" and "second" are only used to distinguish different substances or usage methods, and do not represent the difference in order.

[0113] 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 work belong to the scope of protection of the present invention.

[0114] In the following examples, unless otherwise specified, the materials used are commercially available analytical pure.

[0115] The following examples are used to illustrate the lithium-ion secondary battery of the present invention.

[0116] Example 1:

[0117] Preparation of the positive electrode sheet:

[0118] The first conductive agent and the first binder were uniformly dispersed in deionized water as a solvent in a mass ratio of 53:47 to obtain a positive electrode bottom coating slurry. The first conductive agent consists of graphene, the first carbon nanotube, and acetylene black, and the first binder is styrene-butadiene rubber. The positive electrode bottom coating slurry was uniformly coated on both surfaces of the positive electrode current collector using a bottom coating roller to form a positive electrode bottom coating, and a positive electrode current collector with a positive electrode bottom coating on its surface (i.e., the first part) was obtained.

[0119] The ternary positive electrode material (chemical formula: LiNi 0.9 Co 0.02 Mn 0.07 Zr 0.01 O2), the second carbon nanotube, conductive carbon black, and PVDF were dispersed in N-methylpyrrolidone (NMP) as a solvent in a mass ratio of 96.33:1.1:1.1:1.47 for uniform mixing to obtain a positive electrode active layer slurry; the positive electrode active layer slurry was uniformly coated on both surfaces of the above-mentioned first part, and after drying, a positive electrode sheet was obtained.

[0120] Among them, based on the total weight of the positive electrode bottom coating, the content W1 of the first carbon nanotube is 2.8%, the content W2 of graphene is 3%, the sum Wt of the content of the first carbon nanotube and graphene is 5.8%, W1 / W2 is 0.93, the sheet diameter R of graphene is 9.2 μm, and the aspect ratio of the first carbon nanotube is 11000; based on the total weight of the positive electrode active layer, the content of the second carbon nanotube is 1.1%.

[0121] The positive current collector is composed of a polymer layer and metal layers located on both surface sides of the polymer layer. The metal layer is aluminum foil with a thickness of 1.5 μm, and the polymer layer is polyethylene terephthalate (PET) with a thickness of 6 μm. The areal density of the positive bottom coating is 1 g / m 2 , the ratio of the area S1 of the positive bottom coating to the area S2 of the positive current collector is 80% (i.e., S1 / S2 is 80%), and the ratio S3 of the area of the empty foil region to the area of the positive electrode sheet is 18%;

[0122] The height X of the protrusion is 4.3 μm, the roughness of the positive bottom coating is 4.8 μm, and the sheet resistance of the first part is 15 mΩ.

[0123] At this time, the mass content ratio of element Al to element C on the surface of the positive bottom coating is 0.66:1, the mass content of element Al on the surface of the positive bottom coating is 38.6%, and the mass content of element C on the surface of the positive bottom coating is 58.3%.

[0124] Preparation of the negative electrode sheet:

[0125] Disperse the negative electrode active material (95% silicon-carbon material + 5% artificial graphite), conductive carbon black, and PVDF in deionized water according to a mass ratio of 95:2:3 and mix evenly to obtain the negative electrode active layer slurry; uniformly coat the negative electrode active layer slurry on both surface sides of the copper foil, and obtain the negative electrode sheet after drying.

[0126] Preparation of the lithium-ion secondary battery:

[0127] After laminating and winding the obtained positive electrode sheet and negative electrode sheet with a separator, inject the electrolyte and encapsulate and form it to obtain the secondary battery. The electrolyte is composed of propylene carbonate, ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate mixed according to a mass ratio of 15:15:50:20, and then add 1 M lithium salt LiPF6. The separator is polypropylene (PP).

[0128] Example 2:

[0129] Preparation of the positive electrode sheet:

[0130] Uniformly disperse the first conductive agent and the first binder according to a mass ratio of 53:47 and then disperse them in the solvent deionized water to obtain the positive bottom coating slurry. The first conductive agent is composed of graphene, the first carbon nanotubes, and acetylene black, and the first binder is styrene-butadiene rubber. Use a bottom coating roller to uniformly coat the positive bottom coating slurry on both surface sides of the positive current collector to form the positive bottom coating, and obtain the positive current collector with the positive bottom coating on the surface (i.e., the first part).

[0131] Disperse the ternary positive electrode material (chemical formula LiNi 0.92 Co 0.04 Mn 0.03 Al 0.01O2), the second carbon nanotube, conductive carbon black, and PVDF are dispersed in the solvent NMP in a mass ratio of 93:1.2:1.2:1.6 for uniform mixing to obtain a positive electrode active layer slurry; the positive electrode active layer slurry is uniformly coated on both side surfaces of the above-mentioned first part and dried to obtain a positive electrode sheet.

[0132] Among them, based on the total weight of the positive electrode bottom coating, the content W1 of the first carbon nanotube is 2.5%, the content W2 of graphene is 2.2%, the sum Wt of the content of the first carbon nanotube and graphene is 4.7%, W1 / W2 is 1.14, the sheet diameter R of graphene is 14.5 μm, and the aspect ratio of the first carbon nanotube is 300; based on the total weight of the positive electrode active layer, the content of the second carbon nanotube is 1.2%.

[0133] The positive electrode current collector is composed of a polymer layer and metal layers located on both side surfaces of the polymer layer. The metal layer is aluminum foil with a thickness of 2 μm, the polymer layer is polypropylene (PP) with a thickness of 5 μm, and the areal density of the positive electrode bottom coating is 0.5 g / m 2 , the ratio of the area S1 of the positive electrode bottom coating to the area S2 of the positive electrode current collector is 70% (i.e., S1 / S2 is 70%), and the area ratio S3 of the empty foil area to the area of the positive electrode sheet is 15%.

[0134] The height X of the protrusion is 4.9 μm, the roughness of the positive electrode bottom coating is 5.3 μm, and the sheet resistance of the first part is 28 mΩ.

[0135] At this time, the mass content ratio of element Al and element C on the surface of the positive electrode bottom coating is 0.57:1, the mass content of element Al on the surface of the positive electrode bottom coating is 35.2%, and the mass content of element C on the surface of the positive electrode bottom coating is 61.5%.

[0136] Preparation of the negative electrode sheet:

[0137] The negative electrode active material (95% silicon-carbon material + 5% artificial graphite), conductive carbon black, and PVDF are dispersed in deionized water in a mass ratio of 95:2:3 for uniform mixing to obtain a negative electrode active layer slurry; the negative electrode active layer slurry is uniformly coated on both side surfaces of the copper foil and dried to obtain a negative electrode sheet.

[0138] Preparation of the lithium-ion secondary battery:

[0139] The obtained positive electrode sheet and negative electrode sheet are laminated and wound with a separator, and then injected with electrolyte and encapsulated and formed to obtain a secondary battery. The electrolyte is composed of propylene carbonate, ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate mixed in a mass ratio of 15:15:50:20, and then added with 1 M lithium salt LiPF6. The separator is polypropylene (PP).

[0140] Example 3:

[0141] Preparation of the positive electrode sheet:

[0142] The first conductive agent and the first binder are uniformly dispersed in deionized water as the solvent according to a mass ratio of 53:47 to obtain the positive electrode bottom coating slurry. The first conductive agent is composed of graphene, the first carbon nanotubes, and acetylene black, and the first binder is styrene-butadiene rubber. A bottom coating roller is used to uniformly coat the positive electrode bottom coating slurry on both surfaces of the positive electrode current collector to form a positive electrode bottom coating, and a positive electrode current collector with a positive electrode bottom coating on its surface (i.e., the first part) is obtained.

[0143] The ternary positive electrode material (chemical formula: LiNi 0.9 Co 0.05 Al 0.05 O2), the second carbon nanotubes, and PVDF are dispersed in NMP as the solvent according to a mass ratio of 96.7:1:1:1.3 for uniform mixing to obtain the positive electrode active layer slurry; the positive electrode active layer slurry is uniformly coated on both surfaces of the above-mentioned first part and dried to obtain the positive electrode sheet.

[0144] Among them, based on the total weight of the positive electrode bottom coating, the content W1 of the first carbon nanotubes is 3.4%, the content W2 of graphene is 2.3%, the sum Wt of the contents of the first carbon nanotubes and graphene is 5.7%, W1 / W2 is 1.48, the sheet diameter R of graphene is 5.3 μm, and the aspect ratio of the first carbon nanotubes is 15,000; based on the total weight of the positive electrode active layer, the content of the second carbon nanotubes is 1%.

[0145] The positive electrode current collector is composed of a polymer layer and metal layers on both surfaces of the polymer layer. The metal layer is aluminum foil with a thickness of 1 μm, and the polymer layer is polyimide (PI) with a thickness of 7 μm. The surface density of the positive electrode bottom coating is 1.45 g / m 2 , and the ratio of the area S1 of the positive electrode bottom coating to the area S2 of the positive electrode current collector is 75% (i.e., S1 / S2 is 75%), and the area ratio S3 of the empty foil area to the area of the positive electrode sheet is 23%.

[0146] The height X of the protrusion is 4 μm, the roughness of the positive electrode bottom coating is 4.6 μm, and the sheet resistance of the first part is 34 mΩ.

[0147] At this time, the mass content ratio of element Al to element C on the surface of the positive electrode bottom coating is 0.87:1, the mass content of element Al on the surface of the positive electrode bottom coating is 44.9%, and the mass content of element C on the surface of the positive electrode bottom coating is 51.7%.

[0148] Preparation of the negative electrode sheet:

[0149] Disperse the negative electrode active material (95% silicon-carbon material + 5% artificial graphite), conductive carbon black, and PVDF in deionized water according to a mass ratio of 95:2:3 and mix evenly to obtain the negative electrode active layer slurry; coat the negative electrode active layer slurry evenly on both surfaces of the copper foil and dry to obtain the negative electrode sheet.

[0150] Preparation of lithium-ion secondary battery:

[0151] After laminating and winding the obtained positive electrode sheet and negative electrode sheet with a separator, inject the electrolyte, encapsulate and form it to obtain the secondary battery. The electrolyte is composed of propylene carbonate, ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate mixed according to a mass ratio of 15:15:50:20, and then add 1M lithium salt LiPF6. The separator is polypropylene (PP).

[0152] 4 groups of examples:

[0153] This group of examples is used to verify the influence brought by the change of "protrusion height X", which is achieved by changing the mesh number of the bottom coating roller, specifically as follows:

[0154] Example 4a, based on Example 1, the difference is that the protrusion height X is 3.2 μm, the roughness of the positive electrode bottom coating is 2 μm, and at this time, the sheet resistance of the first part is 35 mΩ.

[0155] Example 4b, based on Example 1, the difference is that the protrusion height X is 6.7 μm, the roughness of the positive electrode bottom coating is 8.5 μm, and at this time, the sheet resistance of the first part is 20 mΩ.

[0156] 5 groups of examples:

[0157] This group of examples is used to verify the influence brought by the change of "polymer layer thickness", specifically as follows:

[0158] Example 5a, based on Example 1, the difference is that the thickness of the polymer layer is 2 μm, and at this time, the sheet resistance of the first part is 28 mΩ.

[0159] Example 5b, based on Example 1, the difference is that the thickness of the polymer layer is 8 μm, and at this time, the sheet resistance of the first part is 37 mΩ.

[0160] 6 groups of examples:

[0161] This group of examples is used to verify the influence brought by the change of "mass content ratio of element Al and element C on the surface of the positive electrode bottom coating", which is achieved by regulating the coating amount of the positive electrode bottom coating on the surface of the positive electrode current collector, specifically as follows:

[0162] Example 6a, based on Example 1, is different in that the mass ratio of element Al to element C on the surface of the positive electrode bottom coating is 0.433:1. At this time, the mass content of element Al on the surface of the positive electrode bottom coating is 30.1%, the mass content of element C on the surface of the positive electrode bottom coating is 69.5%, and the sheet resistance of the first part is 38 mΩ.

[0163] Example 6b, based on Example 1, is different in that the mass ratio of element Al to element C on the surface of the positive electrode bottom coating is 1.1:1. At this time, the mass content of element Al on the surface of the positive electrode bottom coating is 49.7%, the mass content of element C on the surface of the positive electrode bottom coating is 45.2%, and the sheet resistance of the first part is 32 mΩ.

[0164] Example 7:

[0165] Based on Example 1, it is different in that acetylene black in the first conductive agent is replaced by conductive carbon black, that is, the first conductive agent is composed of graphene, the first carbon nanotube and conductive carbon black, and the first binder is polyacrylic acid. At this time, the sheet resistance of the first part is 33 mΩ.

[0166] Example 8 group:

[0167] This group of examples is used to verify the influence caused by the change of "the areal density of the positive electrode bottom coating", which is achieved by changing the mesh number of the bottom coating roller. Specifically as follows:

[0168] Example 8a, based on Example 1, is different in that the areal density of the positive electrode bottom coating is 1.8 g / m 2 , and at this time, the sheet resistance of the first part is 32 mΩ.

[0169] Example 8b, based on Example 1, is different in that the areal density of the positive electrode bottom coating is 0.5 g / m 2 , and at this time, the sheet resistance of the first part is 34 mΩ.

[0170] Example 9 group:

[0171] This group of examples is used to verify the influence brought by the change of "S1 / S2", which is achieved by changing the area of the positive electrode bottom coating. Specifically as follows:

[0172] Example 9a, based on Example 1, is different in that S1 / S2 is 51.4%. At this time, the sheet resistance of the first part is 31 mΩ.

[0173] Example 9b, based on Example 2, is different in that S1 / S2 is 84.2%. At this time, the sheet resistance of the first part is 35 mΩ.

[0174] Example 10 group:

[0175] This group of embodiments is used to verify the influence brought about by the change of "the sheet diameter R of graphene", specifically as follows:

[0176] Example 10a, based on Example 1, the difference is that the sheet diameter R of graphene is 2.6 μm. At this time, the sheet resistance of the first part is 36 mΩ.

[0177] Example 10b, based on Example 1, the difference is that the sheet diameter R of graphene is 22.7 μm. At this time, the sheet resistance of the first part is 33 mΩ.

[0178] Group of Example 11:

[0179] This group of embodiments is used to verify the influence brought about by the change of "the aspect ratio of the first carbon nanotube", specifically as follows:

[0180] Example 11a, based on Example 1, the difference is that the aspect ratio of the first carbon nanotube is 100. At this time, the sheet resistance of the first part is 34 mΩ.

[0181] Example 11b, based on Example 1, the difference is that the aspect ratio of the first carbon nanotube is 20000. At this time, the sheet resistance of the first part is 31 mΩ.

[0182] Group of Example 12:

[0183] This group of embodiments is used to verify the influence brought about by the change of "W1 / W2", which is achieved by changing the content W1 of the first carbon nanotube and the content W2 of graphene, specifically as follows:

[0184] Example 12a, based on Example 1, the difference is that W1 is 2.1%, W2 is 4.9%. At this time, W1 / W2 is 0.43, Wt is 7%, the first conductive agent and the first binder are in a mass ratio of 53:47, and the sheet resistance of the first part is 31 mΩ.

[0185] Example 12b, based on Example 1, the difference is that W1 is 4.8%, W2 is 2%. At this time, W1 / W2 is 2.4, Wt is 6.8%, the first conductive agent and the first binder are in a mass ratio of 53:47, and the sheet resistance of the first part is 33 mΩ.

[0186] Example 12c, based on Example 1, the difference is that W1 is 2.5%, W2 is 3%. At this time, W1 / W2 is 0.833, Wt is 5.5%, the first conductive agent and the first binder are in a mass ratio of 53:47, and the sheet resistance of the first part is 34 mΩ.

[0187] Example 12d, based on Example 1, is different in that W1 is 3.4%, W2 is 2.2%, at this time, W1 / W2 is 1.545, Wt is 5.6%, the first conductive agent and the first binder are in a mass ratio of 53:47, and the sheet resistance of the first part is 33 mΩ.

[0188] Group of Example 13:

[0189] This group of examples is used to verify the influence brought by the change of "the content of the second carbon nanotube", specifically as follows:

[0190] Example 13a, based on Example 1, is different in that based on the total weight of the positive electrode active layer, the content of the second carbon nanotube is 0.83%, at this time, the mass ratio of the ternary positive electrode material, the second carbon nanotube, the conductive carbon black and PVDF in the positive electrode active layer slurry is 97.24:0.83:0.83:1.1, and the sheet resistance of the first part is 34 mΩ.

[0191] Example 13b, based on Example 1, is different in that based on the total weight of the positive electrode active layer, the content of the second carbon nanotube is 1.45%, at this time, the mass ratio of the ternary positive electrode material, the second carbon nanotube, the conductive carbon black and PVDF in the positive electrode active layer slurry is 95.2:1.45:1.45:1.9, and the sheet resistance of the first part is 32 mΩ.

[0192] Example 14:

[0193] Based on Example 1, it is different in that W1 is 0, W2 is 5.8%, at this time, W1 / W2 is 0, Wt is 5.8%, the first conductive agent and the first binder are in a mass ratio of 53:47, and the sheet resistance of the first part is 17 mΩ.

[0194] Group of Example 15:

[0195] This group of examples is used to verify the influence brought by the change of "the mass content ratio of element Al and element C on the surface of the positive electrode bottom coating", which is achieved by regulating the coating amount of the positive electrode bottom coating on the surface of the positive electrode current collector, specifically as follows:

[0196] Example 15a, based on Example 1, is different in that the mass content ratio of element Al and element C on the surface of the positive electrode bottom coating is 0.343:1, at this time, the mass content of element Al on the surface of the positive electrode bottom coating is 24.8%, the mass content of element C on the surface of the positive electrode bottom coating is 72.4%, and the sheet resistance of the first part is 37 mΩ.

[0197] Example 15b, based on Example 1, is different in that the mass ratio of element Al to element C on the surface of the positive electrode bottom coating is 1.345:1. At this time, the mass content of element Al on the surface of the positive electrode bottom coating is 55.4%, and the mass content of element C on the surface of the positive electrode bottom coating is 41.2%. The sheet resistance of the first part is 39 mΩ.

[0198] In the above examples, the number of layers of graphene is within the range of 3 - 10 layers, and the thickness D of graphene is within the range of 3 μm - 10 μm. The elongation at break in the MD direction of the positive electrode sheet is ≥10%, and the elongation at break in the TD direction is ≥5%. The elongation at break in the MD direction of the positive electrode current collector is ≥15%, and the elongation at break in the TD direction is ≥10%. The peel strength between the positive electrode bottom coating and the metal layer is ≥5 N / m. Based on the weight of the metal current collector of the same thickness, the weight of the positive electrode current collector is reduced by 50% - 70%.

[0199] In Examples 1 - 3, the tensile strength in the MD direction of the positive electrode sheet is ≥120 MPa, and the tensile strength in the TD direction is ≥80 MPa; in Examples 4 - 15 groups, the tensile strength in the MD direction of the positive electrode sheet is ≥100 MPa, and the tensile strength in the TD direction is ≥60 MPa.

[0200] Comparative Example 1 group:

[0201] This group of comparative examples is used to verify the influence brought by the change of "protrusion height X", which is achieved by changing the mesh number of the bottom coating roller, as follows:

[0202] Comparative Example 1a, based on Example 1, is different in that the protrusion height X is 1.5 μm, and the roughness of the positive electrode bottom coating is 1.9 μm. At this time, the sheet resistance of the first part is 40 mΩ.

[0203] Example 1b, based on Example 1, is different in that the protrusion height X is 8.2 μm, and the roughness of the positive electrode bottom coating is 9.4 μm. At this time, the sheet resistance of the first part is 38 mΩ.

[0204] Comparative Example 2:

[0205] Based on Example 1, it is different in that the positive electrode current collector does not include the positive electrode bottom coating. At this time, the mass ratio of element Al to element C on the surface of the positive electrode bottom coating does not exist, the mass content of element Al on the surface of the positive electrode bottom coating is 98.8%, the mass content of element C on the surface of the positive electrode bottom coating is 0%, and the sheet resistance of the first part is 42 mΩ.

[0206] Comparative Example 3:

[0207] Based on Example 1, the difference is that the positive current collector is a metal current collector, the metal current collector is aluminum foil with a thickness of 10 μm. At this time, the sheet resistance of the first part is 5 mΩ, the elongation at break in the MD direction of the positive current collector is 3.5%, the elongation at break in the TD direction is 3%, the elongation at break in the MD direction of the positive electrode sheet is 2.3%, the elongation at break in the TD direction is 2.3%, the tensile strength in the MD direction of the positive electrode sheet is 200 MPa, and the tensile strength in the TD direction is 180 MPa.

[0208] Comparative Example 4:

[0209] Based on Example 1, the difference is that the positive electrode bottom coating does not contain graphene, the content W2 of graphene is 0%, the content W1 of the first carbon nanotube is 5.8%, and W1 / W2 does not exist. At this time, the sheet resistance of the first part is 37 mΩ.

[0210] Test Example:

[0211] (1) Cycle capacity retention rate:

[0212] At 25 ± 5 °C, the batteries prepared in the examples and comparative examples are left standing for 5 minutes, charged at a constant current of 1C to 4.25V, then charged at a constant voltage until the current is less than or equal to 0.05C to full charge, then left standing for 5 minutes, and then discharged at a constant current of 0.33C to 2.75V. The discharge capacity at this time is recorded as the discharge capacity of the first cycle of the battery. The battery is left standing for 5 minutes, charged at a constant current of 1C to 4.25V, then charged at a constant voltage until the current is less than or equal to 0.05C to full charge, then left standing for 5 minutes, and then discharged at a constant current of 1C to 2.75V; the lithium-ion battery is subjected to 500 cycles of charge and discharge tests according to the above method, and the discharge capacity of the 500th cycle is recorded. Then the 500-cycle battery cycle capacity retention rate (%) = discharge capacity of the 500th cycle / discharge capacity of the first cycle × 100%;

[0213] (2) Energy density:

[0214] At 25 ± 5 °C, the batteries prepared in the examples and comparative examples are left standing for 5 minutes, charged at a constant current of 1C to 4.25V, then charged at a constant voltage until the current is less than or equal to 0.05C to full charge, then left standing for 5 minutes, and then discharged at a constant current of 0.33C to 2.75V. The discharge energy at this time is recorded as the initial energy of the battery, and the weight of the packaged battery is recorded as the mass of the battery. Energy density = energy of the battery / mass of the battery.

[0215] (3) Resistivity of the positive electrode sheet:

[0216] The positive electrode sheets prepared in the examples and comparative examples are cut into small pieces of 4 cm × 6 cm and placed on a bulk resistivity tester for testing.

[0217] (4) Penetration test:

[0218] Under the environment of 25°C ± 5°C, the batteries prepared in the examples and comparative examples are charged at a rate of 0.7C to the upper limit voltage (4.2V), with a cut-off current of 0.02C, and then left standing for 10 min; use an iron nail (with a diameter of 5 mm and a tip length of 15 mm); invert the iron nail, and use the flat head surface of the iron nail to pass through the center position of the battery respectively at a needle speed of 30 mm / s, and the nail remains in the battery; keep for 5 min. Ten batteries are tested for each example and comparative example. If there is no smoking, no fire, and no explosion, it is considered to pass. Record the number of passes / the number of tests as n / 10 in Table 1. The larger this value is, the higher the penetration safety of the battery is proved.

[0219] The performance test results of the lithium-ion secondary batteries prepared in the examples and comparative examples of the present invention are recorded in Table 1:

[0220] Table 1:

[0221]

[0222] It can be seen from Table 1 that compared with the comparative example, the positive electrode sheet prepared in the present invention has improved conductivity. Compared with the comparative example, the lithium-ion battery prepared in the present invention has good energy density and cycle stability, alleviates the risk of battery thermal runaway, and has good safety performance.

[0223] 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 positive electrode sheet, characterized in that, The positive electrode sheet includes a positive electrode current collector, a positive electrode bottom coating layer, and a positive electrode active layer. The positive electrode bottom coating layer is located between the positive electrode current collector and the positive electrode active layer; the positive electrode current collector includes a polymer layer and a metal layer located on at least one surface of the polymer layer; the polymer layer includes at least one of polyethylene terephthalate, polypropylene, polyethylene, and polyimide, the metal layer includes element Al, and the positive electrode bottom coating layer includes a first conductive agent and a first binder; the first conductive agent includes graphene; The thickness of the polymer layer is 2 μm - 8 μm, and the thickness of the metal layer is 0.5 μm - 2 μm; The positive electrode bottom coating layer includes a number of protrusions, and the height of the protrusions is X, 3 μm ≤ X ≤ 7 μm.

2. The positive electrode sheet according to claim 1, wherein, 4 μm ≤ X ≤ 5 μm; And / or, the thickness of the positive electrode bottom coating layer is 1 μm - 2 μm; And / or, the mass content ratio of element Al to element C on the surface of the positive electrode bottom coating layer is (0.43 - 1.1):1; Preferably, the mass content ratio of element Al to element C on the surface of the positive electrode bottom coating layer is (0.55 - 0.9):

1.

3. The positive electrode sheet according to claim 1 or 2, wherein, The first conductive agent further includes at least one of first carbon nanotubes, conductive carbon black, and acetylene black; And / or, the first binder includes at least one of styrene-butadiene rubber, styrene-acrylic rubber, lithium polyacrylate, sodium polyacrylate, polyacrylic acid, and polyvinylidene fluoride.

4. The positive electrode sheet according to claim 1, wherein, The tensile strength of the positive electrode sheet in the MD direction ≥ 100 MPa, the tensile strength in the TD direction ≥ 60 MPa, the elongation at break in the MD direction ≥ 10%, and the elongation at break in the TD direction ≥ 5%; Preferably, the tensile strength of the positive electrode sheet in the MD direction ≥ 120 MPa, and the tensile strength in the TD direction ≥ 80 MPa.

5. The positive electrode sheet according to claim 1, wherein, The areal density of the positive electrode bottom coating is 0.3 g / m 2 -2 g / m 2 ; preferably 0.5 g / m 2 -1.5 g / m 2 ; And / or, the area S1 of the positive electrode bottom coating layer and the area S2 of the positive electrode current collector satisfy: 50% ≤ S1 / S2 < 85%; preferably 70% ≤ S1 / S2 ≤ 80%; And / or, the positive electrode sheet includes an empty foil area and a pasted area. The positive electrode bottom coating layer is located in the pasted area, and the area ratio of the empty foil area to the area of the positive electrode sheet is S3, 15% ≤ S3 ≤ 25%.

6. The positive electrode sheet according to claim 3, wherein, The aspect ratio of the first carbon nanotubes is 100 - 20000; preferably 300 - 15000; And / or, based on the total weight of the positive electrode bottom coating layer, the content W1 of the first carbon nanotubes is 2% - 5%; And / or, the number of layers of the graphene is 3 - 10 layers; And / or, the sheet diameter of the graphene is R, 0.5 μm ≤ R ≤ 30 μm; preferably 5 μm ≤ R ≤ 15 μm; And / or, the thickness of the graphene is D, 3 μm ≤ D ≤ 10 μm.

7. The positive electrode sheet according to claim 3, wherein, Based on the total weight of the positive electrode bottom coating layer, the sum of the content of the first carbon nanotubes and the content of the graphene is Wt, 2% ≤ Wt ≤ 7%; And / or, based on the total weight of the positive electrode bottom coating layer, where the content of the first carbon nanotubes is W1 and the content of the graphene is W2, 0.4 ≤ W1 / W2 ≤ 2.5; preferably 0.9 ≤ W1 / W2 ≤ 1.

5.

8. The positive electrode sheet according to claim 1, wherein, The roughness of the positive electrode bottom coating layer ≥ 2 μm; And / or, the peel strength between the positive electrode bottom coating and the metal layer is ≥5 N / m; And / or, the positive electrode sheet includes a first part and a second part, the first part includes the positive electrode current collector and the positive electrode bottom coating, the second part includes a positive electrode active layer, and the sheet resistance of the first part is ≤40 mΩ.

9. The positive electrode sheet according to claim 1, wherein, The positive electrode active layer includes a second carbon nanotube, and based on the total weight of the positive electrode active layer, the content of the second carbon nanotube is 0.8%-1.5%; And / or, the positive electrode active layer includes a positive electrode active material, and the positive electrode active material includes a substance with the chemical formula Li a Ni x Co y M z A k O2, where 0.9 ≤ a ≤ 1.1, 0.8 ≤ x ≤ 0.99, 0 < y ≤ 0.1, 0 < z ≤ 0.1, 0 ≤ k ≤ 0.05, M is selected from Al and / or Mn, and A is selected from at least one of Mg, Ti, B, P, W, Zr, Mo, Y, and La.

10. A lithium-ion secondary battery, characterized in that, The lithium ion secondary battery includes the positive electrode sheet according to any one of claims 1-9.

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