A secondary battery, a method of manufacturing a secondary battery, and an electronic device

Abstract

The application provides a secondary battery, a preparation method of the secondary battery and an electronic device. The secondary battery comprises an electrode assembly, the electrode assembly comprising a first electrode sheet, a second electrode sheet and a separator. The first electrode sheet comprises an outer first electrode sheet and an inner first electrode sheet, and the outer first electrode sheet is located at the outermost side of the electrode assembly. The first electrode sheet comprises a first current collector and a first active material layer, and the first current collector comprises a first surface and a second surface. The first surface is closer to the shell than the second surface. The first active material layer is arranged on the first surface and the second surface of the inner first electrode sheet. At least one outer first electrode sheet is a single-sided first electrode sheet. The first active material layer arranged on the second surface of the single-sided first electrode sheet is composed of a first material layer and a second material layer. The conductivity of the single-sided first electrode sheet is A S / cm, the conductivity of the inner first electrode sheet is B S / cm, and A≤B. Through the above arrangement, the risk of black spots and lithium precipitation of the outer electrode sheet of the electrode assembly is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemistry, and in particular to a secondary battery, a preparation method of the secondary battery and an electronic device. BACKGROUND

[0002] Secondary batteries, such as lithium ion batteries, have characteristics such as high specific energy, high working voltage, low self-discharge rate, small volume, light weight, etc., and have a wide range of applications in the consumer electronics field.

[0003] At present, the electrode assembly in the lithium ion battery often adopts a multi-tab and laminated structure to reduce the impedance of the battery. In order to ensure the performance of the lithium ion battery such as ultra-thin and high energy density, the outermost tab of the electrode assembly usually adopts a single-sided coated tab. However, due to the manufacturing process and the particularity of the structure of the single-sided coated tab, black spots or lithium precipitation problems are prone to occur during the cycle process, which reduces the performance of the lithium ion battery. SUMMARY

[0004] The purpose of the present application is to provide a secondary battery, a preparation method of the secondary battery and an electronic device, so as to reduce the risk of black spots and lithium precipitation on the outer tab of the electrode assembly and improve the cycle performance of the secondary battery.

[0005] It should be noted that the lithium ion battery is taken as an example to explain the present application in the summary of the present application, but the secondary battery of the present application is not limited to the lithium ion battery.

[0006] At present, in order to reduce the process cost and improve the energy density of the secondary battery, the outermost tab of the electrode assembly with a multi-tab laminated structure in the lithium ion battery is usually a single-sided coated tab, i.e., only a material layer is arranged on one side of the current collector of the tab, the inner tab is usually a double-sided coated tab, and the material layers of the inner and outer tabs are the same, which will make the current density on the outer side of the electrode assembly significantly higher than that on the inner side. During the cycle process of the secondary battery, the electrolyte is consumed faster on the outer side of the electrode assembly, and black spots or lithium precipitation are prone to occur on the single-sided coated tab on the outer side, which reduces the cycle performance and safety performance of the secondary battery. Based on this, the present application provides a secondary battery which can reduce the risk of black spots and lithium precipitation on the outer tab of the electrode assembly and improve the cycle performance of the secondary battery. The specific technical solutions are as follows:

[0007] The first aspect of the present application provides a secondary battery, comprising an electrolyte, a shell, and an electrode assembly of a laminated structure, the electrode assembly comprising a first electrode sheet, a second electrode sheet, and a separator arranged between the first electrode sheet and the second electrode sheet in a thickness direction of the electrode assembly; the electrode assembly further comprises a first tab assembly and a second tab assembly, the first tab assembly comprising a plurality of first tabs, and the second tab assembly comprising a plurality of second tabs, the plurality of first tabs corresponding to the first electrode sheet one by one, and the plurality of second tabs corresponding to the second electrode sheet one by one. The first electrode sheet comprises two outer first electrode sheets and at least one inner first electrode sheet, the two outer first electrode sheets being respectively located at the outermost two sides of the electrode assembly in the thickness direction of the electrode assembly, and the inner first electrode sheet being located between the two outer first electrode sheets; the first electrode sheet comprises a first current collector and a first active material layer, the first current collector comprising a first surface and a second surface arranged oppositely in the thickness direction of the electrode assembly, the first surface being closer to the shell than the second surface, and the first active material layer being arranged on the first surface and the second surface of the inner first electrode sheet; at least one outer first electrode sheet is a single-sided first electrode sheet, the first active material layer is not arranged on the first surface of the single-sided first electrode sheet, and the first active material layer is arranged on the second surface of the single-sided first electrode sheet, the first active material layer arranged on the second surface of the single-sided first electrode sheet being composed of a first material layer and a second material layer arranged in layers, and the first material layer being located between the first current collector and the second material layer. The electrical conductivity of the single-sided first electrode sheet is AS / cm, the electrical conductivity of the inner first electrode sheet is BS / cm, and A≤B. By arranging a double-layer material layer on the single-sided outer electrode sheet of the electrode assembly and making the electrical conductivity of the single-sided outer electrode sheet less than or equal to the electrical conductivity of the inner first electrode sheet, the current density of the outer electrode sheet of the electrode assembly is reduced, the consumption rate of the electrolyte in the cycle process is reduced, the risk of black spot and lithium precipitation of the outer electrode sheet of the electrode assembly in the cycle process is reduced, and the cycle performance of the secondary battery is improved.

[0008] In an embodiment of the present application, 0.3≤A / B≤1. In an embodiment of the present application, 0.65≤A / B≤1. By adjusting the value of A / B within the above range, the risk of black spot and lithium precipitation of the outer electrode sheet of the electrode assembly in the cycle process is reduced, thereby further improving the cycle performance of the secondary battery.

[0009] In an embodiment of the present application, the first electrode sheet is a positive electrode sheet. The effect of reducing the risk of black spot and lithium precipitation of the outer electrode sheet of the electrode assembly in the cycle process is more obvious, thereby further improving the cycle performance of the secondary battery.

[0010] In an embodiment of the present application, the first material layer comprises a first active material, the first active material comprises at least one of lithium iron phosphate, lithium manganese iron phosphate or lithium manganese oxide, and the second material layer comprises a second active material, the second active material comprises at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, nickel cobalt manganese ternary material or lithium-rich manganese-based material. Compared with the outer tab of the conventional electrode assembly, the impedance of the outer tab of the electrode assembly is increased, thereby slowing down the consumption rate of the electrolyte outside the electrode assembly during the cycle process, which is conducive to reducing the risk of black spot and lithium precipitation of the outer tab of the electrode assembly during the cycle process, thereby improving the cycle performance of the secondary battery.

[0011] In an embodiment of the present application, the Dv50 of the first active material is 3 μm to 25 μm, and the Dv99 of the first active material is 10 μm to 65 μm; the Dv50 of the second active material is 4.5 μm to 24.5 μm, and the Dv99 of the second active material is 45 μm to 65 μm. By adjusting the Dv50 and Dv99 of the first active material, and the Dv50 and Dv99 of the second active material within the above range, it is conducive to reducing the risk of black spot and lithium precipitation of the outer tab of the electrode assembly during the cycle process, thereby improving the cycle performance of the secondary battery.

[0012] In an embodiment of the present application, the first material layer comprises a first binder, and the second material layer comprises a second binder, the first binder and the second binder are each independently selected from at least one of polyvinylidene fluoride, polyvinylidene chloride, polyvinylidene chloride, aramid, polyamide, polyacrylonitrile, acrylate polymer, styrene butadiene rubber, polyacrylate, carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, pure acrylic emulsion or polyacrylic acid; the mass percentage of the first binder is 0.5% to 5% based on the mass of the first material layer, and the mass percentage of the second binder is 0.4% to 5% based on the mass of the second material layer. By selecting the first binder and the second binder within the above range, and adjusting the mass percentage of the first binder and the mass percentage of the second binder within the above range, it is conducive to reducing the risk of black spot and lithium precipitation of the outer tab of the electrode assembly during the cycle process, thereby improving the cycle performance of the secondary battery.

[0013] In an embodiment of the present application, the first material layer comprises a first conductive agent, and the second material layer comprises a second conductive agent, each of the first conductive agent and the second conductive agent is independently selected from at least one of conductive carbon black, carbon nanotube, graphene or conductive graphite; the mass percentage of the first conductive agent is 1% to 10% based on the mass of the first material layer, and the mass percentage of the second conductive agent is 1% to 10% based on the mass of the second material layer. By selecting the first conductive agent and the second conductive agent within the above-mentioned range and adjusting the mass percentage of the first conductive agent and the mass percentage of the second conductive agent within the above-mentioned range, the risk of black spot and lithium precipitation of the outer electrode tab of the electrode assembly during the cycle process is reduced, thereby improving the cycle performance of the secondary battery.

[0014] In an embodiment of the present application, the first active material layer arranged on the first surface and the second surface of the inner first electrode tab is a third material layer, the third material layer comprises a third active material, the third active material comprises at least one of lithium cobaltate, lithium nickelate, lithium manganate, nickel-cobalt-manganese ternary material or lithium-rich manganese-based material, and the mass percentage of the third active material is 85% to 98.6% based on the mass of the third material layer. By selecting the third active material within the above-mentioned range and adjusting the mass percentage of the third active material within the above-mentioned range, the risk of black spot and lithium precipitation of the outer electrode tab of the electrode assembly during the cycle process is reduced, and the secondary battery has good cycle performance.

[0015] In an embodiment of the present application, the thickness of the first material layer is 20 μm to 60 μm, and the thickness of the second material layer is 20 μm to 150 μm. By adjusting the thickness of the first material layer and the thickness of the second material layer within the above-mentioned range, the risk of black spot and lithium precipitation of the outer electrode tab of the electrode assembly during the cycle process is reduced, the cycle performance of the secondary battery is improved while the high energy density of the secondary battery is taken into account.

[0016] In an embodiment of the present application, the thickness of the third material layer is 80 μm to 250 μm. By adjusting the thickness of the third material layer within the above-mentioned range, the risk of black spot and lithium precipitation of the outer electrode tab of the electrode assembly during the cycle process is reduced, and the secondary battery has high energy density and good cycle performance.

[0017] In an embodiment of the present application, 0.01≤B≤0.8. By adjusting the electrical conductivity of the inner first electrode tab within the above-mentioned range, the current density of the inner electrode tab of the electrode assembly during the cycle process is larger, which is beneficial to improve the cycle performance of the secondary battery.

[0018] In an embodiment of the present application, the thickness of the first current collector of the single-sided first tab is T1 μm, the thickness of the first current collector of the inner first tab is T2 μm, and T1≥T2. In an embodiment of the present application, the first tab is a positive tab, 12≤T1≤25, and 6≤T2≤16. In an embodiment of the present application, the first tab is a negative tab, 14≤T1≤25, and 4≤T2≤16. By the above arrangement, the problem of warping caused by uneven stress of the first current collector due to the fact that only one surface of the first current collector of the single-sided first tab is provided with a material layer is improved, the double-layer material layer is provided on the outer single-sided first tab of the electrode assembly, the electrical conductivity of the single-sided first tab is less than or equal to that of the inner first tab, the effect of reducing the risk of black spot and lithium precipitation of the outer tab of the electrode assembly during the cycle is more obvious, and the secondary battery has good cycle performance.

[0019] In an embodiment of the present application, the shell is an aluminum plastic film. When the aluminum plastic film is selected as the shell, the risk of short circuit of the secondary battery is reduced, and the cycle performance and safety performance of the secondary battery are improved.

[0020] In an embodiment of the present application, the first tab is a negative tab, the first material layer comprises a first active material, the electrical conductivity of the first active material is c S / cm, the second material layer comprises a second active material, the electrical conductivity of the second active material is d S / cm, and c

[0021] In an embodiment of the present application, the shell is a steel shell. When the steel shell is selected as the shell, the risk of short circuit of the secondary battery is reduced, and the cycle performance and safety performance of the secondary battery are improved.

[0022] The second aspect of the present application provides a preparation method of a secondary battery, comprising the following steps: preparing at least one single-sided first pole piece, at least one inner first pole piece, a plurality of second pole pieces, a plurality of separators, an electrolyte and a shell, and assembling to obtain a secondary battery; wherein the preparation method of the single-sided first pole piece comprises the following steps: sequentially arranging a first material layer and a second material layer on a second surface of a first current collector to obtain a single-sided first pole piece; and the preparation method of the inner first pole piece comprises the following steps: arranging a first active material layer on the first surface and the second surface of the first current collector to obtain an inner first pole piece. The secondary battery prepared by the above preparation method is beneficial to process operation and production, and the risk of black spot and lithium precipitation of the outer pole piece of the electrode assembly in the cycle process is low, and the secondary battery has good cycle performance.

[0023] In an embodiment of the present application, the first material layer comprises a first active material, the electrical conductivity of the first active material is a S / cm, the second material layer comprises a second active material, the electrical conductivity of the second active material is b S / cm, and a < b. Compared with the outer pole piece of the conventional electrode assembly, the impedance of the outer pole piece of the electrode assembly is increased, thereby slowing down the consumption speed of the electrolyte outside the electrode assembly in the cycle process, which is beneficial to reduce the risk of black spot and lithium precipitation of the outer pole piece of the electrode assembly in the cycle process, and further improve the cycle performance of the secondary battery.

[0024] The third aspect of the present application provides an electronic device comprising the secondary battery in any of the foregoing embodiments. The secondary battery of the present application has good cycle performance, and therefore the electronic device of the present application has a longer service life.

[0025] The beneficial effects of the embodiments of the present application are as follows:

[0026] The present application provides a secondary battery, a preparation method of a secondary battery and an electronic device. By arranging a double-layer material layer on the single-sided outer first pole piece of the electrode assembly, and making the electrical conductivity of the single-sided outer first pole piece less than or equal to the electrical conductivity of the inner first pole piece, the current density of the outermost electrode assembly is reduced, the consumption speed of the electrolyte in the cycle process is reduced, the risk of black spot and lithium precipitation of the outer pole piece of the electrode assembly in the cycle process is reduced, and the cycle performance of the secondary battery is improved.

[0027] Of course, implementing any product or method of the present application does not necessarily require achieving all the advantages described above at the same time. BRIEF DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description only constitute some embodiments of the present application, and other embodiments can also be obtained by those skilled in the art based on these drawings.

[0029] Figure 1 A cross-sectional structure schematic diagram of a secondary battery according to an embodiment of the present application viewed in a length direction of the secondary battery.

[0030] Figure 2 A cross-sectional structure schematic diagram of a secondary battery according to another embodiment of the present application viewed in a length direction of the secondary battery. Figure 1

[0031] Reference signs: secondary battery 001; electrode assembly 01; case 02; first electrode sheet 10; outer first electrode sheet 101; inner first electrode sheet 102; first current collector 11; first active material layer 12; first material layer 121; second material layer 122; third material layer 123; second electrode sheet 20; second current collector 21; second active material layer 22; first tab assembly 13; first tab 131; second tab assembly 23; second tab 231; separator 30. DETAILED DESCRIPTION

[0032] The technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments only constitute some of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on the present application are within the scope of protection of the present application.

[0033] It should be noted that in the specific embodiments of the present application, the lithium ion battery is taken as an example of the secondary battery to explain the present application, but the secondary battery of the present application is not limited to the lithium ion battery. The specific technical solutions are as follows:

[0034] ​The first aspect of the present application provides a secondary battery, comprising an electrolyte, a shell, and an electrode assembly of a laminated structure, the electrode assembly comprising a first electrode sheet, a second electrode sheet, and a separator arranged between the first electrode sheet and the second electrode sheet in a thickness direction of the electrode assembly; the electrode assembly further comprising a first tab assembly and a second tab assembly, the first tab assembly comprising a plurality of first tabs, and the second tab assembly comprising a plurality of second tabs, the plurality of first tabs corresponding to the first electrode sheet one by one, and the plurality of second tabs corresponding to the second electrode sheet one by one. The first electrode sheet comprises two outer first electrode sheets and at least one inner first electrode sheet, the two outer first electrode sheets being respectively located at the outermost two sides of the electrode assembly in the thickness direction of the electrode assembly, and the inner first electrode sheet being located between the two outer first electrode sheets; the first electrode sheet comprises a first current collector and a first active material layer, the first current collector comprising a first surface and a second surface arranged oppositely in the thickness direction of the electrode assembly, the first surface being closer to the shell than the second surface, and the first active material layer being arranged on the first surface and the second surface of the inner first electrode sheet; at least one outer first electrode sheet is a single-sided first electrode sheet, the first active material layer is not arranged on the first surface of the single-sided first electrode sheet, and the first active material layer is arranged on the second surface of the single-sided first electrode sheet, the first active material layer arranged on the second surface of the single-sided first electrode sheet being composed of a first material layer and a second material layer arranged in layers, the first material layer being located between the first current collector and the second material layer. The electrical conductivity of the single-sided first electrode sheet is A S / cm, the electrical conductivity of the inner first electrode sheet is B S / cm, and A≤B.

[0035] In the present application, the electrode assembly of the laminated structure is defined as having a self-width direction as the X direction, a self-length direction as the Y direction, and a self-thickness direction as the Z direction. It can be understood that the first electrode sheet, the second electrode sheet, and the separator have the same self-length direction, self-width direction, and self-thickness direction as the electrode assembly. Exemplarily, as shown in FIG. 1, the self-length direction of the electrode assembly is the Y direction, the self-width direction of the electrode assembly is the X direction, and the self-thickness direction of the electrode assembly is the Z direction. Figure 1 and Figure 2As shown, the secondary battery 001 comprises an electrode assembly 01 in a laminated structure and a housing 02, along a thickness direction Z direction of the electrode assembly 01, the electrode assembly 01 comprises a first electrode sheet 10, a second electrode sheet 20 and a separator 30 arranged in a stack between the first electrode sheet 10 and the second electrode sheet 20; the electrode assembly 01 further comprises a first tab assembly 13 and a second tab assembly 23, the first tab assembly 13 comprises a plurality of first tabs 131, the second tab assembly 23 comprises a plurality of second tabs 231, the plurality of first tabs 131 correspond to the first electrode sheet 10 one by one, and the plurality of second tabs 231 correspond to the second electrode sheet 20 one by one. The first electrode sheet 10 comprises two outer first electrode sheets 101 and one inner first electrode sheet 102, along the thickness direction Z direction of the electrode assembly 01, the two outer first electrode sheets 101 are respectively located at the two outermost sides of the electrode assembly 01, and the inner first electrode sheet 102 is located between the two outer first electrode sheets 101; the first electrode sheet 10 comprises a first current collector 11 and a first active material layer 12, the first current collector 11 comprises a first surface (not shown in the figure) and a second surface (not shown in the figure) arranged oppositely along the thickness direction Z direction of the electrode assembly 01, the first surface is closer to the housing 02 than the second surface, and the first active material layer 12 is arranged on the first surface and the second surface of the inner first electrode sheet 102; the two outer first electrode sheets 101 are single-sided first electrode sheets 10, the first active material layer 12 is not arranged on the first surface of the single-sided first electrode sheet 10, and the first active material layer 12 is arranged on the second surface of the single-sided first electrode sheet 10, the first active material layer 12 arranged on the second surface of the single-sided first electrode sheet 10 is composed of a first material layer 121 and a second material layer 122 arranged in a stack, and the first material layer 121 is located between the first current collector 11 and the second material layer 122.

[0036] The inventors have found that the addition of a material layer between the active material layer and the current collector of the single-face outer tab of the electrode assembly, i.e., the addition of a second material layer between the first current collector and the second material layer, increases the self-resistance of the single-face outer tab, and further makes the electrical conductivity of the single-face first tab on the outer side of the electrode assembly greater than or equal to the electrical conductivity of the first tab on the inner side, i.e., A≤B, the current density of the outer tab of the electrode assembly is reduced, the consumption rate of the electrolyte in the outer tab of the electrode assembly during the cycle process is slowed down, thereby reducing the risk of black spots and lithium precipitation in the outer tab of the electrode assembly, and improving the cycle performance of the secondary battery. When the current density of the outer tab of the electrode assembly is greater than that of the inner tab, i.e., A>B, the electrolyte in the outer tab of the electrode assembly is consumed too quickly, and black spots or lithium precipitation problems are prone to occur in the outer tab of the electrode assembly, thereby reducing the cycle performance and safety performance of the secondary battery. Therefore, by providing a double-layer material layer on the single-face outer tab of the electrode assembly and making the electrical conductivity of the single-face first tab on the outer side less than or equal to the electrical conductivity of the first tab on the inner side, the current density of the outer tab of the electrode assembly is reduced, the consumption rate of the electrolyte during the cycle process is reduced, the risk of black spots and lithium precipitation in the outer tab of the electrode assembly during the cycle process is reduced, and the cycle performance of the secondary battery is improved.

[0037] It should be noted that the first active material layer of the present application can be provided on the entire surface of the first current collector, or on a part of the surface of the first current collector, and the present application does not have a particular limitation as long as the purpose of the present application can be achieved.

[0038] In an embodiment of the present application, 0.3≤A / B≤1. For example, the value of A / B can be 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or a range composed of any two of the above values. By adjusting the value of A / B within the above range, the current density of the outer tab of the electrode assembly is greater than or equal to that of the inner tab, the consumption rate of the electrolyte during the cycle process is slowed down, which is beneficial to reduce the risk of black spots and lithium precipitation in the outer tab of the electrode assembly during the cycle process, and further improves the cycle performance of the secondary battery.

[0039] In an embodiment of the present application, 0.65≤A / B≤1. For example, the value of A / B can be 0.65, 0.68, 0.7, 0.72, 0.75, 0.78, 0.8, 0.82, 0.85, 0.88, 0.9, 0.92, 0.95, 0.98, 1, or a range composed of any two of the above values. By adjusting the value of A / B within the above range, the current density of the outer tab of the electrode assembly is greater than or equal to that of the inner tab, the consumption rate of the electrolyte during the cycle process is slowed down, which is beneficial to reduce the risk of black spots and lithium precipitation in the outer tab of the electrode assembly during the cycle process, and further improves the cycle performance of the secondary battery.

[0040] In an embodiment of the present application, the first tab is a positive electrode tab. When the positive electrode tab is selected as the first tab, the outer first tab of the electrode assembly is a positive electrode tab, which further slows down the consumption rate of the electrolyte outside the electrode assembly during the cycle process, and the effect of reducing the risk of black spot and lithium precipitation of the outer tab of the electrode assembly during the cycle process is more obvious, thereby further improving the cycle performance of the secondary battery.

[0041] In an embodiment of the present application, the first material layer includes a first active material, the first active material includes at least one of lithium iron phosphate (LFP), lithium manganese iron phosphate (LMFP) or lithium manganate (LiMn2O4), and the second material layer includes a second active material, the second active material includes at least one of lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMn2O4), nickel-cobalt-manganese ternary material or lithium-rich manganese-based material. The appropriate first active material and second active material are selected within the above-mentioned category range, so that the electrical conductivity of the first active material and the second active material is within the appropriate range, the impedance of the outer tab of the electrode assembly is increased compared with the outer tab of the conventional electrode assembly, thereby reducing the current density of the outer tab of the electrode assembly, slowing down the consumption rate of the electrolyte outside the electrode assembly during the cycle process, and reducing the risk of black spot and lithium precipitation of the outer tab of the electrode assembly during the cycle process, thereby improving the cycle performance of the secondary battery. In the present application, the nickel-cobalt-manganese ternary material includes but is not limited to at least one of LiNi 0.90 Co 0.05 Mn 0.05 O2(NCM955), NCM811, NCM622, NCM523 or NCM111. The lithium-rich manganese-based material is a composite positive electrode material Li2MnO3·LiMO2 based on Li2MnO3, wherein M is Ni, Co, Mn or a binary or ternary layered material of Ni, Co and Mn.

[0042] In an embodiment of the present application, the Dv50 of the first active material is 3 μm to 25 μm, for example, the Dv50 of the first active material can be 3 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 22 μm, 25 μm, or a range defined by any two of them; the Dv99 of the first active material is 10 μm to 65 μm, for example, the Dv99 of the first active material can be 10 μm, 13 μm, 15 μm, 18 μm, 20 μm, 22 μm, 25 μm, 28 μm, 30 μm, 32 μm, 35 μm, 38 μm, 40 μm, 42 μm, 45 μm, 48 μm, 50 μm, 52 μm, 55 μm, 58 μm, 60 μm, 62 μm, 65 μm, or a range defined by any two of them; the Dv50 of the second active material is 4.5 μm to 24.5 μm, for example, the Dv50 of the second active material can be 4.5 μm, 4.8 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 22 μm, 23 μm, 24 μm, 24.2 μm, 24.5 μm, or a range defined by any two of them; the Dv99 of the second active material is 45 μm to 65 μm, for example, the Dv99 of the second active material can be 45 μm, 48 μm, 50 μm, 52 μm, 55 μm, 58 μm, 60 μm, 62 μm, 65 μm, or a range defined by any two of them. By adjusting the Dv50 and Dv99 of the first active material, the Dv50 and Dv99 of the second active material are within the above range, so that the electrical conductivity of the first active material and the second active material is within a suitable range, which is conducive to further increasing the impedance of the first material layer and the second material layer, and in turn increasing the impedance of the outer tab of the electrode assembly, reducing the electrical conductivity of the outer tab of the electrode assembly, thereby reducing the current density of the outer tab of the electrode assembly, slowing down the consumption speed of the electrolyte outside the electrode assembly during the cycle process, and reducing the risk of black spots and lithium precipitation on the outer tab of the electrode assembly during the cycle process, thereby improving the cycle performance of the secondary battery.

[0043] In the present application, Dv50 refers to the particle size at which the volume accumulation is 50% in the particle size distribution of the active material on a volume basis, measured from small particle size. Dv99 refers to the particle size at which the volume accumulation is 99% in the particle size distribution of the active material on a volume basis, measured from small particle size. The present application does not have a particular limitation on the adjustment method of the Dv50 and Dv99 of the active material, as long as the purpose of the present application can be achieved. For example, the Dv50 and Dv99 of the active material can be adjusted by grading and grinding the particles of the active material, etc. Illustratively, when other conditions are unchanged, the Dv50 and Dv99 of the active material decrease with the extension of the grinding time, and the Dv50 and Dv99 of the active material increase with the shortening of the grinding time.

[0044] In an embodiment of the present application, the first material layer comprises a first binder, and the second material layer comprises a second binder, each of the first binder and the second binder is independently selected from at least one of polyvinylidene fluoride, polyvinylidene chloride, polyvinylidene chloride, aramid, polyamide, polyacrylonitrile, acrylate polymer, styrene butadiene rubber, polyacrylate, carboxymethylcellulose, polyvinylpyrrolidone, polyvinyl ether, pure acrylic emulsion or polyacrylic acid. The mass percentage of the first binder W1 is 0.5% to 5% based on the mass of the first material layer, for example, the mass percentage of the first binder W1 can be 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5% or a range between any two of them; the mass percentage of the second binder W2 is 0.4% to 5% based on the mass of the second material layer, for example, the mass percentage of the second binder W2 can be 0.4%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5% or a range between any two of them. By selecting the first binder and the second binder within the above-mentioned range and adjusting the mass percentage of the first binder and the mass percentage of the second binder within the above-mentioned range, the impedance of the outer electrode of the electrode assembly is increased, the electrical conductivity of the outer electrode of the electrode assembly is reduced, the current density of the outer electrode of the electrode assembly is reduced, the consumption rate of the electrolyte outside the electrode assembly during the cycle is slowed down, the risk of black spot and lithium precipitation of the outer electrode of the electrode assembly during the cycle is reduced, and the cycle performance of the secondary battery is improved.

[0045] In an embodiment of the present application, the first material layer comprises a first conductive agent, the second material layer comprises a second conductive agent, the first conductive agent and the second conductive agent are each independently selected from at least one of conductive carbon black, carbon nanotube, graphene or conductive graphite; the mass percentage of the first conductive agent W'1 is 1% to 10% based on the mass of the first material layer, for example, the mass percentage of the first conductive agent W'1 can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or a range formed by any two of the above values; the mass percentage of the second conductive agent W'2 is 1% to 10% based on the mass of the second material layer, for example, the mass percentage of the second conductive agent W'2 can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or a range formed by any two of the above values. By selecting the first conductive agent and the second conductive agent within the above range and adjusting the mass percentage of the first conductive agent and the mass percentage of the second conductive agent within the above range, the impedance of the outer electrode tab of the electrode assembly is increased, the conductivity of the outer electrode tab of the electrode assembly is reduced, the current density of the outer electrode tab of the electrode assembly is reduced, the consumption rate of the electrolyte outside the electrode assembly during the cycle process is slowed down, the risk of black spot and lithium precipitation of the outer electrode tab of the electrode assembly during the cycle process is reduced, and the cycle performance of the secondary battery is improved.

[0046] In an embodiment of the present application, the mass percentage of the first active material W''1 is 85% to 98.5% based on the mass of the first material layer; and / or, the mass percentage of the second active material W''2 is 85% to 98.6% based on the mass of the second material layer. For example, the mass percentage of the first active material W''1 can be 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 98.3%, 98.5% or a range formed by any two of the above values; the mass percentage of the second active material W''2 can be 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 98.2%, 98.4%, 98.5%, 98.6% or a range formed by any two of the above values. By adjusting the mass percentage of the first active material and / or the mass percentage of the second active material within the above range, the impedance of the outer electrode tab of the electrode assembly is increased, the consumption rate of the electrolyte outside the electrode assembly during the cycle process is slowed down, the risk of black spot and lithium precipitation of the outer electrode tab of the electrode assembly during the cycle process is reduced, and the secondary battery has good cycle performance.

[0047] In an embodiment of the present application, as Figure 1 and Figure 2As shown, the first active material layer 12 arranged on the first surface and the second surface of the inner first tab 102 is a third material layer 123. The third material layer comprises a third active material, the third active material comprises at least one of lithium cobaltate, lithium nickelate, lithium manganate, nickel cobalt manganese ternary material or lithium-rich manganese-based material, and the mass percentage content W”3 of the third active material in the third material layer is 85% to 98.6%. For example, the mass percentage content W”3 of the third active material can be 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 98.2%, 98.5%, 98.6%, or a range formed by any two of them. By selecting the third active material of the above type and adjusting the mass percentage content of the third active material within the above range, the inner tab of the electrode assembly has a higher electrical conductivity, which is beneficial to shorten the electrical conductivity difference between the inner and outer tabs of the electrode assembly. While the inner tab of the electrode assembly has a higher electrical conductivity, the electrical conductivity of the outer tab of the electrode assembly is relatively low, thereby slowing down the consumption speed of the electrolyte on the outer side of the electrode assembly during the cycle process, which is beneficial to reduce the risk of black spot and lithium precipitation on the outer tab of the electrode assembly during the cycle process, and at the same time the secondary battery has good cycle performance.

[0048] In an embodiment of the present application, as shown in Figure 1 and Figure 2 As shown, the thickness H1 of the first material layer 121 is 20 μm to 60 μm, and the thickness H2 of the second material layer 122 is 20 μm to 150 μm. For example, the thickness H1 of the first material layer can be 20 μm, 23 μm, 25 μm, 28 μm, 30 μm, 32 μm, 35 μm, 38 μm, 40 μm, 42 μm, 45 μm, 48 μm, 50 μm, 52 μm, 55 μm, 58 μm, 60 μm, or a range formed by any two of them; the thickness H2 of the second material layer can be 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 95 μm, 100 μm, 105 μm, 110 μm, 115 μm, 120 μm, 125 μm, 130 μm, 135 μm, 140 μm, 145 μm, 150 μm, or a range formed by any two of them. By adjusting the thickness of the first material layer and the thickness of the second material layer within the above range, it is beneficial to increase the impedance of the outer tab of the electrode assembly, thereby slowing down the consumption speed of the electrolyte on the outer side of the electrode assembly during the cycle process, which is beneficial to reduce the risk of black spot and lithium precipitation on the outer tab of the electrode assembly during the cycle process, while taking into account the high energy density of the secondary battery, the cycle performance of the secondary battery is improved.

[0049] In an embodiment of the application, as shown in Figure 1 and Figure 2 The thickness H3 of the third material layer 123 is 80 μm to 250 μm. For example, the thickness H3 of the third material layer can be 80 μm, 85 μm, 90 μm, 95 μm, 100 μm, 105 μm, 110 μm, 115 μm, 120 μm, 125 μm, 130 μm, 135 μm, 140 μm, 145 μm, 150 μm, 155 μm, 160 μm, 165 μm, 170 μm, 175 μm, 180 μm, 185 μm, 190 μm, 195 μm, 200 μm, 205 μm, 210 μm, 215 μm, 220 μm, 225 μm, 230 μm, 235 μm, 240 μm, 245 μm, 250 μm, or a range defined by any two of the above values. By adjusting the thickness of the third material layer to be within the above range, the secondary battery has a higher energy density, while the inner electrode assembly has a higher electrical conductivity, the difference in electrical conductivity between the inner and outer electrode assembly is smaller, which is conducive to slowing down the consumption rate of the electrolyte on the outer electrode assembly during the cycle process, reducing the risk of black spots and lithium precipitation on the outer electrode assembly during the cycle process, and the secondary battery has a higher energy density while having good cycle performance.

[0050] In the present application, the thickness of the material layer can be adjusted by means known to those skilled in the art, for example, when the slurry is coated on the surface of the current collector, the thickness of the material layer can be increased by increasing the coating weight based on a certain solid content of the slurry, or the thickness of the material layer can be decreased by decreasing the coating weight; the thickness of the material layer can also be decreased by increasing the cold pressing pressure when the electrode is cold pressed, or the thickness of the material layer can be increased by decreasing the cold pressing pressure.

[0051] In an embodiment of the application, 0.01≤B≤0.8. For example, the value of B can be 0.01, 0.012, 0.015, 0.018, 0.02, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or a range defined by any two of the above values. By adjusting the electrical conductivity of the inner first electrode to be within the above range, the current density of the outer electrode assembly during the cycle process is smaller, which is conducive to improving the cycle performance of the secondary battery.

[0052] In an embodiment of the present application, the thickness of the first current collector of the single-sided first tab is T1 μm, the thickness of the first current collector of the inner first tab is T2 μm, and T1≥T2. In an embodiment of the present application, the first tab is a positive electrode tab, 12≤T1≤25, and 6≤T2≤16; for example, the value of T1 can be 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, or a range defined by any two of them, and the value of T2 can be 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, or a range defined by any two of them. In an embodiment of the present application, the first tab is a negative electrode tab, 14≤T1≤25, and 4≤T2≤16; for example, the value of T1 can be 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, or a range defined by any two of them, and the value of T2 can be 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, or a range defined by any two of them. By the above arrangement, the problem of warping caused by uneven stress of the first current collector due to the fact that only one surface of the first current collector of the single-sided first tab is provided with a material layer is improved, at the same time, a double-layer material layer is provided on the outer single-sided first tab of the electrode assembly, and the electrical conductivity of the single-sided first tab is less than or equal to that of the inner first tab, which makes the effect of reducing the risk of black spot and lithium precipitation of the electrode assembly on the outside during the cycle process more obvious, and the secondary battery has good cycle performance.

[0053] In an embodiment of the present application, the shell is an aluminum plastic film. When the aluminum plastic film is used as the shell, and the outer first tab of the electrode assembly is a positive electrode tab, it is beneficial to reduce the risk of short circuit of the secondary battery, and improve the cycle performance and safety performance of the secondary battery.

[0054] In the present application, when the first tab or the second tab is a positive electrode tab, the positive electrode current collector is not particularly limited as long as it can achieve the purpose of the present application. For example, the positive electrode current collector can include an aluminum foil, an aluminum alloy foil, or a composite current collector (such as an aluminum-carbon composite current collector), etc.

[0055] In an embodiment of the present application, the first electrode tab is a negative electrode tab, the first material layer comprises a first active material, the first active material has an electrical conductivity of c S / cm, the second material layer comprises a second active material, the second active material has an electrical conductivity of d S / cm, c < d; the first active material and the second active material are each independently selected from at least one of natural graphite, artificial graphite, mesophase carbon microbeads, hard carbon, soft carbon, silicon or silicon-carbon composite. When the first electrode tab is a negative electrode tab, by arranging the double-layer material layer on the outer tab of the electrode assembly, and selecting the first active material and the second active material from the above-mentioned categories, the electrical conductivity of the first active material and the second active material is within a suitable range, compared with the outer tab of the conventional electrode assembly, the impedance of the outer tab of the electrode assembly is increased, thereby reducing the current density of the outer tab of the electrode assembly, slowing down the consumption rate of the electrolyte on the outer side of the electrode assembly during the cycle process, which is conducive to reducing the risk of black spots and lithium precipitation on the outer tab of the electrode assembly during the cycle process, thereby improving the cycle performance of the secondary battery.

[0056] In an embodiment of the present application, the shell is a steel shell. When the steel shell is selected as the shell, and the first electrode tab on the outer side of the electrode assembly is a negative electrode tab, it is conducive to reducing the risk of short circuit of the secondary battery, and improves the cycle performance and safety performance of the secondary battery.

[0057] In an embodiment of the present application, the first active material layer arranged on the first surface and the second surface of the inner first electrode tab is a third material layer, and the third material layer comprises a third active material, and the third active material comprises at least one of natural graphite, artificial graphite, mesophase carbon microbeads, hard carbon, soft carbon, silicon or silicon-carbon composite. Figure 1 and Figure 2 As shown in FIGS. 1 and 2, the first active material layer 12 arranged on the first surface and the second surface of the inner first electrode tab 102 is a third material layer 123. By selecting the third active material from the above-mentioned categories, the inner tab of the electrode assembly has a relatively high electrical conductivity, which is conducive to shortening the difference in electrical conductivity between the inner and outer tabs of the electrode assembly. While the inner tab of the electrode assembly has a relatively high electrical conductivity, the electrical conductivity of the outer tab of the electrode assembly is relatively low, thereby slowing down the consumption rate of the electrolyte on the outer side of the electrode assembly during the cycle process, which is conducive to reducing the risk of black spots and lithium precipitation on the outer tab of the electrode assembly during the cycle process, and the secondary battery has good cycle performance.

[0058] In the present application, when the first or second pole piece is a negative pole piece, the negative current collector is not particularly limited in the present application as long as the purpose of the present application can be achieved. For example, the negative current collector can include a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, or a composite current collector (e.g., a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.), and the like. At this time, the first material layer, the second material layer, and the third material layer are all negative material layers. The first material layer further includes a first conductive agent and a first binder, the second material layer further includes a second conductive agent and a second binder, the third material layer further includes a third conductive agent and a third binder, the first conductive agent, the second conductive agent, and the third conductive agent can each independently be selected from at least one of conductive carbon black, carbon nanotubes, graphene, or conductive graphite, and the first binder, the second binder, and the third binder can each independently be selected from at least one of polyvinylidene fluoride, polyvinylidene chloride, polyvinylidene dichloride, aramid, polyamide, polyacrylonitrile, acrylate polymer, styrene butadiene rubber, polyacrylate, carboxymethyl cellulose sodium, polyvinylpyrrolidone, polyvinyl ether, pure acrylic emulsion, or polyacrylic acid. In the present application, when the first pole piece is a negative pole piece, the mass ratio of the first active material, the first conductive agent, and the first binder; the mass ratio of the second active material, the second conductive agent, and the second binder; and the mass ratio of the third active material, the third conductive agent, and the third binder can be adjusted by a person skilled in the art according to actual needs as long as the purpose of the present application can be achieved.

[0059] The second aspect of the present application provides a preparation method of a secondary battery, which comprises the following steps: preparing at least one single-sided first pole piece, at least one inner first pole piece, a plurality of second pole pieces, a plurality of separators, an electrolyte, and a shell, and assembling to obtain a secondary battery; wherein the preparation method of the single-sided first pole piece comprises the following steps: sequentially arranging a first material layer and a second material layer on the second surface of the first current collector to obtain a single-sided first pole piece; and the preparation method of the inner first pole piece comprises the following steps: arranging a first active material layer on the first surface and the second surface of the first current collector to obtain an inner first pole piece. The secondary battery prepared by the above preparation method is beneficial to process operation and production, and the risk of black spots and lithium precipitation on the outer pole piece of the electrode assembly during the cycle process is low, and the secondary battery has good cycle performance.

[0060] The application does not have special restrictions on the manner of disposing the first material layer on the second surface of the first current collector and disposing the second material layer, as long as the purpose of the application can be achieved. For example, the first active material, the first binder, and the first conductive agent are mixed in a certain mass ratio to prepare a first material layer slurry by adding a solvent, the first material layer slurry is coated on the second surface of the first current collector, and after drying, a first pole piece coated with the first material layer is obtained; the second active material, the second binder, and the second conductive agent are mixed in a certain mass ratio to prepare a second material layer slurry by adding a solvent, the second material layer slurry is coated on the surface of the first material layer away from the first current collector, and after drying, a single-sided first pole piece coated with the first material layer and the second material layer is obtained. The application does not have special restrictions on the manner of obtaining the single-sided first pole piece, as long as the purpose of the application can be achieved. For example, the single-sided first pole piece coated with the first material layer and the second material layer is cold-pressed, cut, and welded with the first tab to obtain the single-sided first pole piece. The application does not have special restrictions on the solvent and solid content of the first material layer slurry and the second material layer slurry, as long as the purpose of the application can be achieved. The application does not have special restrictions on the temperature and time of drying, which can be selected by a person skilled in the art according to actual needs, as long as the purpose of the application can be achieved. The application does not have special restrictions on the process parameters of cold-pressing, cutting, and welding the first tab, which can be selected by a person skilled in the art according to actual needs, as long as the purpose of the application can be achieved. The application does not have special restrictions on the size of the first material layer and the second material layer, as long as the purpose of the application can be achieved.

[0061] In the application, the mass percentage content of the first active material, the first binder, and the first conductive agent can be regulated by regulating the mass ratio of the first active material, the first binder, and the first conductive agent in the first material layer slurry; the mass percentage content of the second active material, the second binder, and the second conductive agent can be regulated by regulating the mass ratio of the second active material, the second binder, and the second conductive agent in the second material layer slurry.

[0062] The application has no particular restrictions on the manner of arranging the first active material layer on the first surface and the second surface of the first current collector, i.e., arranging the third material layer on both surfaces of the first current collector, as long as the purpose of the application can be achieved. For example, the third active material, the third binder and the third conductive agent are mixed in a certain mass ratio, then a solvent is added to prepare a third material layer slurry; the third material layer slurry is coated on the first surface of the first current collector, and after drying, a single-sided inner first pole piece coated with the third material layer is obtained; the above steps are repeated on the other surface of the first current collector to obtain a double-sided inner first pole piece coated with the third material layer. The application has no particular restrictions on the manner of obtaining the inner first pole piece, as long as the purpose of the application can be achieved. For example, the double-sided inner first pole piece coated with the third material layer is cold-pressed, cut and welded with the first tab to obtain the inner first pole piece. The application has no particular restrictions on the solvent and solid content of the third material layer slurry, as long as the purpose of the application can be achieved. The application has no particular restrictions on the temperature and time of drying, which can be selected by those skilled in the art according to actual needs, as long as the purpose of the application can be achieved. The application has no particular restrictions on the process parameters of cold-pressing, cutting and welding the first tab, which can be selected by those skilled in the art according to actual needs, as long as the purpose of the application can be achieved. The application has no particular restrictions on the size of the third material layer, as long as the purpose of the application can be achieved.

[0063] In the application, the mass percentage content of the third active material, the third binder and the third conductive agent can be regulated by regulating the mass ratio of the third active material, the third binder and the third conductive agent.

[0064] The application has no particular restrictions on the manner of assembling the secondary battery, as long as the purpose of the application can be achieved. For example, the outer first pole piece, the separator, the second pole piece, the separator, the inner first pole piece, the separator, the second pole piece, the separator and the outer first pole piece are stacked in order, then the four corners of the entire stack structure are fixed with adhesive tape to obtain an electrode assembly of the stack structure, the electrode assembly is placed in the shell, the electrolyte is injected into the shell and sealed to obtain the secondary battery.

[0065] In an embodiment of the present application, the first material layer comprises a first active material, the first active material has an electrical conductivity of a S / cm, the second material layer comprises a second active material, the second active material has an electrical conductivity of b S / cm, and a < b. By arranging the double-layer material layer on the outer tab of the electrode assembly and regulating the electrical conductivity of the first active material in the first material layer between the second material layer and the first current collector to be less than the electrical conductivity of the second active material in the second material layer, the impedance of the outer tab of the electrode assembly is increased compared with the outer tab of a conventional electrode assembly, thereby reducing the current density of the outer tab of the electrode assembly, slowing down the consumption speed of the electrolyte outside the electrode assembly during the cycle process, and being conducive to reducing the risk of black spot and lithium precipitation of the outer tab of the electrode assembly during the cycle process, thereby improving the cycle performance of the secondary battery.

[0066] In the present application, the electrical conductivity of the outer first tab can be regulated by regulating the electrical conductivity of the first active material and the second active material, and further, the electrical conductivity of the outer first tab can be regulated by regulating the type and particle size of the first active material and the type and particle size of the second active material. For example, for the same active material, when the particle size of the active material particles is smaller, i.e., Dv50 and Dv99 are smaller, the electrical conductivity of the active material is larger. In the present application, the electrical conductivity of the inner first tab can be regulated by regulating the type and mass percentage of the third active material. In the present application, the electrical conductivity of the outer first tab and / or the inner first tab is also related to the overall thickness of the outer first tab and / or the inner first tab, the coating weight of the corresponding material layer, etc. For example, for the same specification of the inner first tab, when other conditions are the same, when the thickness of the inner first tab is smaller and the coating weight is less, the electrical conductivity of the inner first tab is larger.

[0067] The electrolyte of the present application includes a lithium salt and a nonaqueous solvent. The lithium salt can include at least one of LiPF6, LiNO3, LiBF4, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), or lithium difluorophosphate. The content of the lithium salt in the electrolyte is not limited in the present application as long as the object of the present application is achieved. The nonaqueous solvent is not particularly limited in the present application as long as the object of the present application is achieved. For example, the nonaqueous solvent can include, but is not limited to, at least one of a carbonate compound, a carboxylate compound, an ether compound, or other organic solvents. The carbonate compound can include, but is not limited to, at least one of a chain carbonate compound, a cyclic carbonate compound, or a fluorinated carbonate compound. The chain carbonate compound can include, but is not limited to, at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, or methyl ethyl carbonate. The cyclic carbonate can include, but is not limited to, at least one of vinylene carbonate, propylene carbonate (PC), butylene carbonate, or vinyl ethylene carbonate. The fluorinated carbonate compound can include, but is not limited to, at least one of fluorinated vinylene carbonate, carbonic acid-1,2-difluoroethylene ester, carbonic acid-1,1-difluoroethylene ester, carbonic acid-1,1,2-trifluoroethylene ester, carbonic acid-1,1,2,2-tetrafluoroethylene ester, carbonic acid-1-fluoro-2-methyl ethylene ester, carbonic acid-1-fluoro-1-methyl ethylene ester, carbonic acid-1,2-difluoro-1-methyl ethylene ester, carbonic acid-1,1,2-trifluoro-2-methyl ethylene ester, or carbonic acid trifluoromethyl ethylene ester. The carboxylate compound can include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone, or caprolactone. The ether compound can include, but is not limited to, at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The other organic solvents can include, but is not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0068] The diaphragm according to the present application is not particularly limited as long as the object of the present application can be achieved. For example, the material of the diaphragm can include, but is not limited to, at least one of polyethylene (PE), polyolefin (PO) based on polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of the diaphragm can include at least one of a woven film, a nonwoven film, a microporous film, a composite film, a calendered film, or a spunlaced film. The diaphragm according to the present application can have a porous structure, and the size of the pore diameter of the porous structure of the diaphragm according to the present application is not particularly limited as long as the object of the present application can be achieved. For example, the size of the pore diameter can be 0.01 μm to 1 μm. The thickness of the diaphragm according to the present application is not particularly limited as long as the object of the present application can be achieved. For example, the thickness of the diaphragm can be 5 μm to 40 μm.

[0069] The secondary battery according to the present application is not particularly limited and can include any device in which an electrochemical reaction occurs. In one or more embodiments, the secondary battery can include, but is not limited to, a lithium ion secondary battery (lithium ion battery), a lithium polymer secondary battery, or a lithium ion polymer secondary battery, etc.

[0070] The third aspect of the present application provides an electronic device including the secondary battery according to any one of the foregoing embodiments. The secondary battery according to the present application has good cycle performance, and thus the electronic device according to the present application has a long service life.

[0071] The electronic device according to the present application is not particularly limited and can be any electronic device known in the art. For example, the electronic device can include, but is not limited to, a notebook computer, a pen input type computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile, a portable copier, a portable printer, a headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable audio player, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a timepiece, a power tool, a flashlight, a camera, a home-use large storage battery, and a lithium ion capacitor.

[0072] Embodiments

[0073] Hereinafter, embodiments and comparative examples are presented to more specifically describe the embodiments of the present application. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0074] Test methods and apparatuses:

[0075] Test of the electrical conductivity of the electrode tab:

[0076] The lithium ion battery discharged at 0.2C to 3.0V was disassembled, the outer first pole piece of the electrode assembly was taken out, and the inner first pole piece of the electrode assembly was taken out according to the arrangement mode of the pole piece in the electrode assembly. After being soaked in dimethyl carbonate (DMC) for 20 min, the outer first pole piece and the inner first pole piece were rinsed with DMC and acetone in turn. Then the outer first pole piece and the inner first pole piece were placed in an oven and baked at 80°C for 12 hours to obtain the treated outer first pole piece and the treated inner first pole piece.

[0077] The treated outer first pole piece and the treated inner first pole piece were tested using a resistivity tester (IEST, BER1200). Before use, the resistance tester was reset for resistance and pressure. The measured pole piece sample was placed between the electrodes of the tester, and the pole piece with a cross-sectional area of 30mm×30mm was taken at three different positions on the sample for testing. The resistance values of the three pole pieces with a cross-sectional area of 30mm×30mm were measured and averaged to obtain the resistance value of the pole piece with a cross-sectional area of 30mm×30mm. The following formula was used for calculation:

[0078] R=ρ×l / s; γ=1 / ρ=l / Rs

[0079] Wherein, R is the measured resistance value; ρ is the resistivity; l is the thickness of the measured pole piece, s is the cross-sectional area, and γ is the measured conductivity of the pole piece.

[0080] The conductivity A of the outer first pole piece and the conductivity B of the inner first pole piece were obtained respectively.

[0081] Conductivity test of active material:

[0082] 1.5g of the first active material powder sample was weighed, and a cylindrical sheet with a diameter of 10mm and a thickness of 2mm was prepared under a pressure of 80MPa using a circular grinding tool. The treated cylindrical sheet was measured using a powder resistivity & compaction density instrument (IEST, PRCD3100). The conductivity was calculated by the following formula:

[0083] ρ=2πlU / I; γ=1 / ρ

[0084] Wherein, ρ is the resistivity; l is the probe coefficient of the instrument used, U is the direct current voltage value measured by the two inner electrodes of the instrument used, I is the direct current value applied by the two outer electrodes of the instrument used, and γ is the measured conductivity of the active material.

[0085] The conductivity a of the first active material and the conductivity b of the second active material were obtained respectively.

[0086] Particle size test:

[0087] The lithium ion battery discharged at 0.2C to 3.0V was disassembled, and the outer first pole piece of the electrode assembly was taken out. After being soaked in dimethyl carbonate (DMC) for 20 min, it was rinsed with DMC and acetone in turn. Then the outer first pole piece was placed in an oven and baked at 80°C for 12 hours to obtain the treated outer first pole piece sample.

[0088] The cross section of the outer first pole piece was observed by scanning electron microscope. The double-layer coating boundary line was observed clearly. The second material layer was scraped off along the double-layer coating boundary line with a scraper and cleaned with dimethyl carbonate (DMC) to obtain a second active material powder sample. In a 50 mL clean beaker, the particles of the second active material were added, 20 mL of deionized water was added, and the 120W ultrasonic cleaner was used for ultrasonic cleaning for 5 min, so that the particles of the second active material were completely dispersed in the deionized water to obtain a sample dispersion liquid. The particle size analyzer (model MasterSizer 2000) was used to test the sample dispersion liquid to obtain the particle size Dv50 and Dv99 of the second active material.

[0089] The first material layer was scraped off with a scraper and cleaned with dimethyl carbonate (DMC) to obtain a first active material powder sample. In a 50 mL clean beaker, the particles of the first active material were added, 20 mL of deionized water was added, and the 120W ultrasonic cleaner was used for ultrasonic cleaning for 5 min, so that the particles of the first active material were completely dispersed in the deionized water to obtain a sample dispersion liquid. The particle size analyzer (model MasterSizer 2000) was used to test the sample dispersion liquid to obtain the particle size Dv50 and Dv99 of the first active material.

[0090] Material layer thickness test:

[0091] The lithium ion battery discharged at 0.2C to 3.0V was disassembled, and the outer first pole piece and the inner first pole piece of the electrode assembly were taken out. After being soaked in dimethyl carbonate (DMC) for 20 min, it was rinsed with DMC and acetone in turn. Then the outer first pole piece and the inner first pole piece were placed in an oven and baked at 80°C for 12 hours to obtain the treated outer first pole piece sample and the inner first pole piece sample.

[0092] The outer first pole piece was polished by argon ion to obtain the cross section of the outer first pole piece. The morphology of the cross section of the outer first pole piece along the thickness direction was observed by field emission scanning electron microscope (Philips, XL-30 type), and scanning electron microscope photos were taken. The thickness H1 of the first material layer and the thickness H2 of the second material layer were measured by scanning electron microscope.

[0093] The first inner electrode tab was argon ion polished to obtain a first outer electrode tab cross section. The morphology of the cross section of the first inner electrode tab along the thickness direction was observed by field emission scanning electron microscopy (Philips, model XL-30), and a scanning electron microscope photograph was taken. The thickness H3 of the third material layer was measured by scanning electron microscopy.

[0094] Lithium precipitation performance test:

[0095] The lithium ion battery in the examples and comparative examples was placed in a thermostat at 10°C, and after 60 minutes, was charged at 2C constant current to 4.5V, and charged at 4.5V constant voltage to a current of 0.05C, and after 5 minutes of standing, was discharged at 0.5C constant current to 3.0V, which was one cycle. After 100 cycles according to the above charging and discharging process, the lithium ion battery was charged at 2C constant current to 4.5V, and charged at 4.5V constant voltage to a current of 0.05C, and after 5 minutes of standing, the lithium ion battery was disassembled, and the first outer electrode tab of the electrode assembly was removed, and the lithium precipitation state on the surface of the first outer electrode tab was observed. The area of the first outer electrode tab surface where lithium was not precipitated was golden yellow, and the area where lithium was precipitated was off-white.

[0096] The judgment standard for the degree of lithium precipitation of the lithium ion battery is as follows: 0% of the lithium precipitation area is no lithium precipitation, i.e., the degree of lithium precipitation is none, greater than 0 and less than or equal to 2% of the lithium precipitation area is mild lithium precipitation, i.e., the degree of lithium precipitation is mild, greater than 2% and less than or equal to 20% of the lithium precipitation area is moderate lithium precipitation, i.e., the degree of lithium precipitation is moderate, and greater than 20% and less than or equal to 100% of the lithium precipitation area is severe lithium precipitation, i.e., the degree of lithium precipitation is severe, wherein the percentage of the lithium precipitation area is calculated based on the total area of the first active material layer of the first outer electrode tab.

[0097] Black spot test:

[0098] The lithium ion battery was placed in a thermostat at 0°C, and after 60 minutes of standing, the lithium ion battery reached a constant temperature. The lithium ion battery that reached a constant temperature was charged at 1C constant current to the full charge voltage 4.5V at 0°C, and then charged at 4.5V constant voltage to 0.025C, and after 5 minutes of standing, was discharged at 1C constant current to 3.0V; this was one charging and discharging cycle. After 500 cycles of charging and discharging, the lithium ion battery was charged at 1C constant current to the full charge voltage 4.5V, and charged at 4.5V constant voltage to 0.025C, and a full battery after 500 cycles was obtained. The lithium ion battery was disassembled in a dry room with a humidity of less than 5%, and a photograph was taken to record whether black spots appeared on the surface of the first outer electrode tab.

[0099] The judgment standard for the degree of black spot of the lithium ion battery is as follows: 0% of the black spot area is no black spot, less than or equal to 2% of the black spot area is mild black spot, 2% to 20% of the black spot area is moderate black spot, and greater than 20% of the black spot area is severe black spot, wherein the percentage of the black spot area is calculated based on the total area of the first active material layer of the first outer electrode tab.

[0100] Cycle performance test:

[0101] The lithium ion battery was placed in a 25℃ constant temperature test box and rested for 30 min to make the lithium ion battery reach a 25℃ constant temperature state. It was charged at 1C constant current to 4.5V, charged at 4.5V constant voltage to a current of 0.025C, rested for 5 min, and discharged at 0.2C constant current to 3.0V. This was the first cycle, and the initial discharge capacity was recorded as C0. The lithium ion battery was subjected to the above charging and discharging cycles, and when the cycle reached 400 cycles (cls), the test was stopped, and the discharge capacity after 400 cycles (cls) was recorded as C1. The 400 cls capacity retention rate was calculated as an index for evaluating the cycle performance of the lithium ion battery.

[0102] 400 cls capacity retention rate (%) = C1 / C0 x 100%.

[0103] The higher the 400 cls capacity retention rate, the better the cycle performance of the lithium ion battery.

[0104] Example 1-1

[0105] Preparation of the first electrode sheet

[0106] The positive electrode sheet was selected as the first electrode sheet. The first active material lithium iron phosphate (LFP, Dv50 = 5 μm, Dv99 = 30 μm), the first binder polyvinylidene fluoride, and the first conductive agent conductive carbon black were mixed in a mass ratio of 92:3:5, added to N-methyl pyrrolidone (NMP), and stirred uniformly under the action of a vacuum stirrer to obtain a first material layer slurry with a solid content of 70 wt%. The second active material lithium manganate (LiMn2O4, Dv50 = 8 μm, Dv99 = 50 μm), the second binder polyvinylidene fluoride, and the second conductive agent conductive carbon black were mixed in a mass ratio of 92:3:5, added to N-methyl pyrrolidone (NMP), and stirred uniformly under the action of a vacuum stirrer to obtain a second material layer slurry with a solid content of 70 wt%. The first material layer slurry was coated on one surface of the positive electrode current collector aluminum foil with a thickness of 14 μm, and after baking treatment at 120℃ for 1 hour, a positive electrode sheet coated with the first material layer was obtained. The second material layer slurry was coated on the surface of the first material layer away from the positive electrode current collector aluminum foil, and after baking treatment at 120℃ for 1 hour, a single-sided outer positive electrode sheet coated with the first material layer and the second material layer was obtained. After cold pressing, cutting, and welding of the positive electrode tabs, an outer positive electrode sheet with a specification of 50 mm x 90 mm was obtained. The coating weight of the first material layer was 6.5 mg / mm 2 , the thickness of the first material layer was 40 μm, the mass percentage content W1 of the first binder based on the mass of the first material layer was 3%, and the mass percentage content W'1 of the first conductive agent was 5%; the coating weight of the second material layer was 13 mg / mm 2, the thickness of the second material layer is 80 pm, the mass percentage content of the second binder W2 is 3% and the mass percentage content of the second conductive agent W'2 is 5% based on the mass of the second material layer.

[0107] The third active material lithium manganate (LiMn2O4, Dv50 = 8 pm, Dv99 = 50 pm), the third binder polyvinylidene fluoride and the third conductive agent conductive carbon black are mixed in a mass ratio of 92:3:5, added into N-methyl pyrrolidone (NMP) and stirred uniformly under the action of a vacuum stirrer to obtain a third material layer slurry with a solid content of 70 wt%. The third material layer slurry is coated on one surface of the positive current collector aluminum foil with a thickness of 14 pm, and after baking treatment at 120°C for 1 hour, a positive electrode tab coated with the third material layer is obtained. The above steps are repeated on the other surface of the positive current collector aluminum foil to obtain a positive electrode tab coated with the third material layer on both surfaces. Then after drying at 120°C under vacuum for 1 hour, cold pressing, cutting and welding of the positive electrode tabs are performed to obtain an inner positive electrode tab with a size of 50 mm x 90 mm. The coating weight of the third material layer is 18 mg / mm 2 , the thickness of the third material layer is 160 pm and the mass percentage content of the third active material W"3 is 92% based on the mass of the third material layer.

[0108] <Preparation of the second tab>

[0109] The negative electrode tab is selected as the second tab, the negative electrode active material artificial graphite, the conductive carbon black and the negative electrode binder styrene-butadiene rubber are mixed in a mass ratio of 94:2:4, deionized water is added and stirred uniformly under the action of a vacuum stirrer to obtain a negative electrode slurry with a solid content of 65 wt%. The negative electrode slurry is uniformly coated on one surface of the negative current collector copper foil with a thickness of 12 pm, baked at 120°C for 1 hour to obtain a negative electrode tab coated with a negative electrode material layer on one surface. The above steps are repeated on the other surface of the negative current collector copper foil to obtain a negative electrode tab coated with a negative electrode material layer on both surfaces. Then after drying at 120°C under vacuum for 1 hour, cold pressing, cutting and welding of the negative electrode tabs are performed to obtain a negative electrode tab with a size of 50 mm x 90 mm. The coating weight of the negative electrode material layer is 12 mg / mm 2 , the thickness of the negative electrode material layer is 120 pm.

[0110] <Preparation of the electrolyte>

[0111] In a glove box under dry argon atmosphere, ethylene carbonate (EC), propylene carbonate (PC) and diethyl carbonate (DEC) were mixed in a mass ratio of 1:1:1 to obtain an organic solvent; lithium salt LiPF6 was dissolved in the above organic solvent, and fluoroethylene carbonate (FEC) was added and mixed uniformly to obtain an electrolyte. The mass percentage of LiPF6 was 12.5% and the mass percentage of FEC was 5% based on the total mass of the electrolyte, and the rest was the organic solvent.

[0112] <Separator>

[0113] A polyethylene (PE) film with a thickness of 7 μm was used as the separator.

[0114] <Preparation of lithium ion battery>

[0115] The above-prepared outer positive electrode sheet, separator, negative electrode sheet, separator, inner positive electrode sheet, separator, negative electrode sheet, separator, outer positive electrode sheet were sequentially stacked in order, and then the four corners of the entire stack structure were fixed with adhesive tape to obtain an electrode assembly of the stack structure. The electrode assembly was placed in an aluminum plastic film and dried in a vacuum oven at 80°C for 12 hours to remove water, and the above-prepared electrolyte was injected. After vacuum packaging, standing, formation, degassing, and edge cutting, a lithium ion battery was obtained. The design potential interval of the lithium ion battery was 3.0 V to 4.5 V.

[0116] Examples 1-2 to 1-29

[0117] Except for adjusting the relevant preparation parameters according to Table 1, the rest was the same as Example 1-1. When the Dv50 and Dv99 of the same active material changed, the grinding time was adjusted so that the values of Dv50 and Dv99 were as shown in Table 1; when the thickness of the material layer changed, the coating weight was adjusted so that the thickness of the material layer was as shown in Table 1. When the type and / or Dv50 and Dv99 of the second active material changed, the type and / or Dv50 and Dv99 of the third active material changed accordingly.

[0118] Example 2-1

[0119] Except for preparing the positive electrode sheet, negative electrode sheet and lithium ion battery according to the following steps, and adjusting the relevant preparation parameters according to Table 3, the rest was the same as Example 1-1.

[0120] <Preparation of first electrode sheet>

[0121] The negative electrode sheet is selected as the first electrode sheet, the first active material artificial graphite, the first binder styrene-butadiene rubber, and the first conductive agent conductive carbon black are mixed in a mass ratio of 92:3:5, and deionized water is added, and stirred uniformly under the action of a vacuum stirrer to obtain a first material layer slurry with a solid content of 65wt%. The second active material silicon-carbon composite SiC, the second binder styrene-butadiene rubber, and the second conductive agent conductive carbon black are mixed in a mass ratio of 92:3:5, and deionized water is added, and stirred uniformly under the action of a vacuum stirrer to obtain a second material layer slurry with a solid content of 65wt%. The first material layer slurry is coated on one surface of a negative electrode current collector copper foil with a thickness of 12μm, and after baking treatment at 120℃ for 1 hour, a negative electrode sheet coated with the first material layer is obtained. The second material layer slurry is coated on the surface of the first material layer away from the negative electrode current collector copper foil, and after baking treatment at 120℃ for 1 hour, a single-sided outside negative electrode sheet coated with the first material layer and the second material layer is obtained. After cold pressing, cutting, and welding of the negative electrode tabs, a specification of 50mm×90mm outside negative electrode sheet is obtained. The coating weight of the first material layer is 4mg / mm 2 , the thickness of the first material layer is 40μm, the mass percentage content W1 of the first binder based on the mass of the first material layer is 3%, and the mass percentage content W'1 of the first conductive agent based on the mass of the first material layer is 5%; the coating weight of the second material layer is 12mg / mm 2 , the thickness of the second material layer is 80μm, the mass percentage content W2 of the second binder based on the mass of the second material layer is 3%, and the mass percentage content W'2 of the second conductive agent based on the mass of the second material layer is 5%.

[0122] The third active material silicon-carbon composite SiC, the third binder styrene-butadiene rubber, and the third conductive agent conductive carbon black are mixed in a mass ratio of 92:3:5, and deionized water is added, and stirred uniformly under the action of a vacuum stirrer to obtain a third material layer slurry with a solid content of 65wt%. The third material layer slurry is coated on one surface of a negative electrode current collector copper foil with a thickness of 12μm, and after baking treatment at 120℃ for 1 hour, a negative electrode sheet coated with the third material layer is obtained. The above steps are repeated on the other surface of the negative electrode current collector copper foil to obtain a negative electrode sheet coated with the third material layer on both surfaces. Then after drying at 120℃ under vacuum conditions for 1 hour, cold pressing, cutting, and welding of the negative electrode tabs are performed to obtain an inside negative electrode sheet with a specification of 50mm×90mm. The coating weight of the third material layer is 14mg / mm 2 , the thickness of the third material layer is 160μm, and the mass percentage content W"3 of the third active material based on the mass of the third material layer is 92%.

[0123] <Preparation of the second electrode sheet>

[0124] The positive electrode sheet was selected as the second electrode sheet. The positive electrode active material lithium manganate (LiMn2O4), the binder polyvinylidene fluoride, and the conductive agent conductive carbon black were mixed in a mass ratio of 92:3:5, N-methyl pyrrolidone (NMP) was added, and the mixture was stirred uniformly under the action of a vacuum stirrer to obtain a positive electrode slurry with a solid content of 70 wt%. The positive electrode slurry was uniformly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 14 μm, and the positive electrode sheet with a single-sided coated positive electrode material layer was obtained after baking treatment at 120°C for 1 hour. The above steps were repeated on the other surface of the positive electrode current collector aluminum foil, and the positive electrode sheet with a double-sided coated positive electrode material layer was obtained. Then, after drying at 120°C under vacuum for 1 hour, cold pressing, sheet cutting, and positive electrode tab welding were performed, and the positive electrode sheet with a size of 50 mm x 90 mm was obtained. The coating weight of the positive electrode material layer was 19 mg / mm 2 , and the thickness of the positive electrode material layer was 180 μm.

[0125] <Preparation of a lithium ion battery>

[0126] The outer negative electrode sheet, the separator, the positive electrode sheet, the separator, the inner negative electrode sheet, the separator, the positive electrode sheet, the separator, and the outer negative electrode sheet prepared above were sequentially stacked in order, and the four corners of the entire stack structure were fixed with adhesive tape to obtain an electrode assembly with a stack structure. The electrode assembly was placed in a steel shell and dried in a vacuum oven at 80°C for 12 hours to remove water, and the electrolyte prepared above was injected. After vacuum packaging, standing, formation, degassing, and edge cutting, a lithium ion battery was obtained. The design potential interval of the lithium ion battery was 3.0 V to 4.5 V.

[0127] Examples 2-2 to 2-4

[0128] Except for adjusting the relevant preparation parameters according to Table 3, the rest was the same as Example 2-1.

[0129] Comparative Examples 1 and 2

[0130] Except for not setting the first material layer on the outer positive electrode sheet and only setting the second material layer, and adjusting the relevant preparation parameters according to Table 1, the rest was the same as Example 1-1.

[0131] Comparative Example 3

[0132] Except for adjusting the relevant preparation parameters according to Table 1, the rest was the same as Example 1-1.

[0133] The preparation parameters and performance parameters of each example and comparative example are shown in Tables 1 to 3.

[0134]

[0135]

[0136]

[0137]

[0138] Table 2

[0139]

[0140] As can be seen from Examples 1-1 to 1-29, Comparative Examples 1 to 3, by regulating the electrical conductivity of the single-sided first tab to be less than or equal to the electrical conductivity of the inner first tab, the lithium ion battery has a smaller lithium precipitation and black spot area of the outer tab of the electrode assembly, and a higher 400cls capacity retention rate, indicating that the lithium ion battery of the present application can reduce the risk of lithium precipitation and black spot of the outer tab of the electrode assembly during the cycle process, and the lithium ion battery has good cycle performance. In Comparative Example 1 and Comparative Example 2, only the second material layer is provided on the outer single-sided first tab; in Comparative Example 3, the parameters of the first material layer and the second material layer are exactly opposite to the parameters of the first material layer and the second material layer of Example 1-1, and the parameters of the third material layer in Comparative Example 3 are the same as the parameters of the second material layer; the lithium ion batteries in Comparative Examples 1 to 3 have larger lithium precipitation and black spot areas; and the 400cls capacity retention rate is lower. However, the lithium ion batteries in Examples 1-1 to 1-29 have smaller lithium precipitation and black spot areas of the outer first tab, and higher 400cls capacity retention rates, indicating that the risk of lithium precipitation and black spot of the outer tab of the electrode assembly during the cycle process is lower, and the lithium ion battery has good cycle performance.

[0141] The types of the first active material and the second active material usually affect the cycle performance of the lithium ion battery. As can be seen from Examples 1-1 to 1-13, when the first active material and the second active material within the scope of the present application are selected, the lithium ion battery has a smaller lithium precipitation and black spot area of the outer first tab, and a higher 400cls capacity retention rate, indicating that the risk of lithium precipitation and black spot of the outer tab of the electrode assembly during the cycle process is lower, and the lithium ion battery has good cycle performance.

[0142] The Dv50 and Dv99 of the first active material, and the Dv50 and Dv99 of the second active material usually affect the cycle performance of the lithium ion battery. As can be seen from Examples 1-1 to 1-13, 1-28 and 1-29, when the Dv50 and Dv99 of the first active material, and the Dv50 and Dv99 of the second active material are within the scope of the present application, the lithium ion battery has a smaller lithium precipitation and black spot area of the outer first tab, and a higher 400cls capacity retention rate, indicating that the risk of lithium precipitation and black spot of the outer tab of the electrode assembly during the cycle process is lower, and the lithium ion battery has good cycle performance.

[0143] The kind and content of the first binder and the kind and content of the second binder generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-14 to Example 1-17, when the first binder and the second binder within the scope of the application are selected, and the content of the first binder and the content of the second binder are controlled within the scope of the application, the lithium ion battery has smaller lithium precipitation and black spot area of the outer first pole piece, and higher 400 cls capacity retention rate, indicating that the risk of black spot and lithium precipitation of the outer pole piece of the electrode assembly during the cycle process is lower, and the lithium ion battery has good cycle performance.

[0144] The kind and content of the first conductive agent and the kind and content of the second conductive agent generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-18 to Example 1-21, when the first conductive agent and the second conductive agent within the scope of the application are selected, and the content of the first conductive agent and the content of the second conductive agent are controlled within the scope of the application, the lithium ion battery has smaller lithium precipitation and black spot area of the outer first pole piece, and higher 400 cls capacity retention rate, indicating that the risk of black spot and lithium precipitation of the outer pole piece of the electrode assembly during the cycle process is lower, and the lithium ion battery has good cycle performance.

[0145] The kind and content of the third active material generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1-1 to Example 1-13, when the third active material within the scope of the application is selected, and the content of the third active material is controlled within the scope of the application, the lithium ion battery has smaller lithium precipitation and black spot area of the outer first pole piece, and higher 400 cls capacity retention rate, indicating that the risk of black spot and lithium precipitation of the outer pole piece of the electrode assembly during the cycle process is lower, and the lithium ion battery has good cycle performance.

[0146] The thickness of the first material layer and the thickness of the second material layer generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-22 to Example 1-25, when the thickness of the first material layer and the thickness of the second material layer are controlled within the scope of the application, the lithium ion battery has smaller lithium precipitation and black spot area of the outer first pole piece, and higher 400 cls capacity retention rate, indicating that the risk of black spot and lithium precipitation of the outer pole piece of the electrode assembly during the cycle process is lower, and the lithium ion battery has good cycle performance.

[0147] The thickness of the third material layer generally affects the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-26 to Example 1-27, when the thickness of the third material layer is controlled within the scope of the application, the lithium ion battery has smaller lithium precipitation and black spot area of the outer first pole piece, and higher 400 cls capacity retention rate, indicating that the risk of black spot and lithium precipitation of the outer pole piece of the electrode assembly during the cycle process is lower, and the lithium ion battery has good cycle performance.

[0148] Table 3

[0149]

[0150] The type of the first electrode sheet generally affects the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-1, when the first electrode sheet is a positive electrode sheet or a negative electrode sheet, the lithium precipitation and the black spot area of the outer first electrode sheet in the lithium ion battery are small, and the 400cls capacity retention rate is high, indicating that the risk of black spot and lithium precipitation of the outer electrode sheet of the electrode assembly during the cycle process is low, and the lithium ion battery has good cycle performance.

[0151] When the first electrode sheet is a negative electrode sheet, the type of the first active material and the type of the second active material generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-1 to Example 2-4, when the first active material and the second active material within the scope of the present application are selected, the lithium precipitation and the black spot area of the outer first electrode sheet in the lithium ion battery are small, and the 400cls capacity retention rate is high, indicating that the risk of black spot and lithium precipitation of the outer electrode sheet of the electrode assembly during the cycle process is low, and the lithium ion battery has good cycle performance.

[0152] It should be noted that in this document, relational terms such as first and second and the like can be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0153] Each of the embodiments in the specification is described in a relevant manner, and the same and similar parts between the embodiments can be referred to each other, and each embodiment mainly explains the difference from other embodiments.

[0154] The above description is only the preferred embodiment of the present application, and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the scope of protection of the present application.

Claims

1. A secondary battery, comprising an electrolyte, a casing, and an electrode assembly of a jelly-roll structure, the electrode assembly comprising a first electrode plate, a second electrode plate, and a separator disposed between the first electrode plate and the second electrode plate, stacked along a thickness direction of the electrode assembly; the electrode assembly further comprising a first tab assembly and a second tab assembly, the first tab assembly comprising a plurality of first tabs, the second tab assembly comprising a plurality of second tabs, the plurality of first tabs corresponding one-to-one to the first electrode plate, and the plurality of second tabs corresponding one-to-one to the second electrode plate; the first electrode plate comprising two outer first electrode plates and at least one inner first electrode plate, the two outer first electrode plates being respectively located at two outermost sides of the electrode assembly along the thickness direction of the electrode assembly, and the inner first electrode plate being located between the two outer first electrode plates; the first electrode plate comprising a first current collector and a first active material layer, the first current collector comprising a first surface and a second surface oppositely disposed along the thickness direction of the electrode assembly, the first surface being closer to the casing than the second surface, and the first active material layer being disposed on both the first surface and the second surface of the inner first electrode plate; at least one of the outer first electrode plates being a single-sided first electrode plate, the first active material layer being not disposed on the first surface of the single-sided first electrode plate, and the first active material layer being disposed on the second surface of the single-sided first electrode plate, the first active material layer disposed on the second surface of the single-sided first electrode plate being composed of a first material layer and a second material layer stacked together, the first material layer being located between the first current collector and the second material layer; the single-sided first electrode plate having an electrical conductivity of A S / cm, and the inner first electrode plate having an electrical conductivity of B S / cm, A≤B. 0.3≤A / B≤1. 0.65≤A / B≤1. The first electrode plate is a positive electrode plate.

2. The secondary battery according to claim 1, wherein The first material layer comprises a first active material, the first active material comprising at least one of lithium iron phosphate, lithium manganese iron phosphate, or lithium manganate; and the second material layer comprises a second active material, the second active material comprising at least one of lithium cobaltate, lithium nickelate, lithium manganate, a nickel-cobalt-manganese ternary material, or a lithium-rich manganese-based material.

3. The secondary battery according to claim 2, wherein The first active material has a Dv50 of 3 μm to 25 μm and a Dv99 of 10 μm to 65 μm; and the second active material has a Dv50 of 4.5 μm to 24.5 μm and a Dv99 of 45 μm to 65 μm.

4. The secondary battery according to any one of claims 1 to 3, wherein ​ 5. The secondary battery according to claim 4, wherein ​ 6. The secondary battery according to claim 5, wherein ​ 7. The secondary battery according to claim 4, wherein The first material layer comprises a first binder, and the second material layer comprises a second binder, each of the first binder and the second binder is independently selected from at least one of polyvinylidene fluoride, polyvinylidene chloride, polyvinylidene chloride, aramid, polyamide, polyacrylonitrile, acrylate polymer, styrene butadiene rubber, polyacrylate, carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, pure acrylic emulsion or polyacrylic acid; the mass percentage of the first binder is 0.5% to 5% based on the mass of the first material layer, and the mass percentage of the second binder is 0.4% to 5% based on the mass of the second material layer.

8. The secondary battery according to claim 4, wherein The first material layer comprises a first conductive agent, and the second material layer comprises a second conductive agent, each of the first conductive agent and the second conductive agent is independently selected from at least one of conductive carbon black, carbon nanotube, graphene or conductive graphite; the mass percentage of the first conductive agent is 1% to 10% based on the mass of the first material layer, and the mass percentage of the second conductive agent is 1% to 10% based on the mass of the second material layer.

9. The secondary battery according to claim 4, wherein The first active material layer arranged on the first surface and the second surface of the inner first tab is a third material layer, and the third material layer comprises a third active material, the third active material comprises at least one of lithium cobaltate, lithium nickelate, lithium manganate, nickel cobalt manganese ternary material or lithium-rich manganese-based material, and the mass percentage of the third active material is 85% to 98.6% based on the mass of the third material layer.

10. The secondary battery according to claim 4, wherein The thickness of the first material layer is 20μm to 60μm, and the thickness of the second material layer is 20μm to 150μm.

11. The secondary battery according to claim 9, wherein The thickness of the third material layer is 80μm to 250μm.

12. The secondary battery according to claim 4, wherein 0.01≤B≤0.8。 13. The secondary battery according to claim 1 or 2, wherein The thickness of the first current collector of the single-sided first tab is T1μm, the thickness of the first current collector of the inner first tab is T2μm, and T1≥T2.

14. The secondary battery according to claim 13, wherein The first tab is a positive electrode tab, 12≤T1≤25, and 6≤T2≤16.

15. The secondary battery according to claim 13, wherein The first tab is a negative electrode tab, 14≤T1≤25, and 4≤T2≤16.

16. The secondary battery according to claim 4, wherein The shell is an aluminum plastic film.

17. The secondary battery according to claim 1 or 2, wherein The first tab is a negative electrode tab, the first material layer comprises a first active material, the electrical conductivity of the first active material is c S / cm, the second material layer comprises a second active material, the electrical conductivity of the second active material is d S / cm, c 18. The secondary battery according to claim 17, wherein The shell is a steel shell.

19. A method of producing the secondary battery according to any one of claims 1 to 18, comprising the steps of: At least one of the single-sided first tab, at least one of the inner first tab, a plurality of the second tabs, a plurality of the separators, the electrolyte and the shell are prepared to obtain the secondary battery; The preparation method of the single-sided first tab comprises the following steps: sequentially arranging the first material layer and the second material layer on the second surface of the first current collector to obtain the single-sided first tab. The preparation method of the inner first pole piece comprises the following steps: arranging the first active material layer on the first surface and the second surface of the first current collector, to obtain the inner first pole piece.

20. The method of making according to claim 19, wherein, The first material layer comprises a first active material, and the conductivity of the first active material is a S / cm; the second material layer comprises a second active material, and the conductivity of the second active material is b S / cm, a < b. 21.An electronic device comprising the secondary battery of any one of claims 1 to 18, or the secondary battery prepared by the preparation method of claim 19 or 20.

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