A secondary battery and an electronic device

Abstract

The application provides a secondary battery and an electronic device, comprising an electrode assembly, the electrode assembly comprising a first electrode sheet, a second electrode sheet and a separator. The first electrode sheet comprises two outer first electrode sheets and an inner first electrode sheet, the two outer first electrode sheets being respectively located at the outermost two sides of the electrode assembly. 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. At least one outer first electrode sheet is a single-sided first electrode sheet, and the first active material layer arranged on the second surface of the single-sided first electrode sheet is a first material layer. The first surface and the second surface of the inner first electrode sheet are both provided with the first active material layer, and the first active material layers arranged on the first surface and the second surface of the inner first electrode sheet are second material layers. 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. 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, in particular to a secondary battery and an electronic device. BACKGROUND

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

[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 and an electronic device 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 used as an example of a secondary battery in the summary of the present application to explain the present application, but the secondary battery of the present application is not limited to a lithium ion battery.

[0006] In the prior art, 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 and laminated structure in the lithium ion battery is usually a single-sided coated tab, i.e., only a material layer is provided 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 generally have the same substance ratio, which will make the current density of the outer side of the electrode assembly significantly higher than that of the inner side. During the cycle process of the secondary battery, the electrolyte is consumed faster in 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 a shell and an electrode assembly of a laminated structure, the electrode assembly comprising, along the thickness direction of the electrode assembly, a first electrode tab, a second electrode tab, and a separator arranged between the first electrode tab and the second electrode tab; the electrode assembly further comprises a first tab group and a second tab group, the first tab group comprising a plurality of first tabs, the second tab group comprising a plurality of second tabs, the plurality of first tabs corresponding one-to-one to the first electrode tab, and the plurality of second tabs corresponding one-to-one to the second electrode tab. The first electrode tab comprises two outer first electrode tabs and at least one inner first electrode tab, the two outer first electrode tabs being respectively located at the outermost two sides of the electrode assembly along the thickness direction of the electrode assembly, and the inner first electrode tab being located between the two outer first electrode tabs. The first electrode tab 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 along the thickness direction of the electrode assembly, and the first surface being closer to the shell than the second surface. At least one outer first electrode tab is a single-sided first electrode tab, the first surface of the single-sided first electrode tab being free of the first active material layer, and the second surface of the single-sided first electrode tab being provided with the first active material layer, the first active material layer provided on the second surface of the single-sided first electrode tab being a first material layer. The first surface and the second surface of the inner first electrode tab are both provided with the first active material layer, the first active material layers provided on the first surface and the second surface of the inner first electrode tab being a second material layer. The electrical conductivity of the single-sided first electrode tab is a S / cm, the electrical conductivity of the inner first electrode tab is b S / cm, and a ≤ b. By providing different first active material layers on the single-sided outer first electrode tab and the inner first electrode tab of the electrode assembly, and making the electrical conductivity of the single-sided outer first electrode tab less than or equal to the electrical conductivity of the inner first electrode tab, the current density of the outer electrode tab of the electrode assembly is reduced, the consumption rate of the electrolyte during the cycle process is reduced, 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.

[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 tab of the electrode assembly during 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 tab is a positive electrode tab. The effect of reducing the risk of black spot and lithium precipitation of the outer electrode tab of the electrode assembly during 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 binder, the mass percentage of the first binder is c based on the mass of the first material layer; the second material layer comprises a second binder, the mass percentage of the second binder is d based on the mass of the second material layer, 0.6≤c / d≤4. By regulating the value of c / d within the above range, the impedance of the outer tab of the electrode assembly is increased compared with the outer tab of a conventional electrode assembly, 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, 2%≤c≤5%; 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. By selecting the first binder and the second binder within the above range and regulating the mass percentage of the first binder within the above range, the risk of black spot and lithium precipitation of the outer tab of the electrode assembly during the cycle process is reduced, thereby improving the cycle performance of the secondary battery.

[0012] 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 is 0.5% to 5% 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 range and regulating the mass percentage of the first conductive agent and the mass percentage of the second conductive agent within the above range, the risk of black spot and lithium precipitation of the outer tab of the electrode assembly during the cycle process is reduced, thereby improving the cycle performance of the secondary battery.

[0013] In an embodiment of the present application, the first material layer comprises a first active material, and the second material layer comprises a second active material, each of the first active material and the second active material being independently selected from at least one of lithium cobaltate, lithium iron phosphate, lithium nickelate, lithium manganate, a nickel-cobalt-manganese ternary material, or a lithium-rich manganese-based material. The mass percentage of the first active material is 90% to 97.5% based on the mass of the first material layer, and the mass percentage of the second active material is 87% to 98.5% based on the mass of the second material layer. By selecting the first active material and the second active material within the above-mentioned range and adjusting the mass percentages of the first active material and the second active material within the above-mentioned range, the risk of black spots and lithium precipitation on the outer electrode assembly tab during the cycle process is reduced, thereby improving the cycle performance of the secondary battery.

[0014] 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 assembly tab during the cycle process is larger, which is beneficial to improve the cycle performance of the secondary battery.

[0015] In an embodiment of the present application, the thickness of the first current collector of the single-sided first electrode tab is T1 μm, the thickness of the first current collector of the inner first electrode tab is T2 μm, and T1≥T2. In an embodiment of the present application, the first electrode tab is a positive electrode tab, 12≤T1≤25, and 6≤T2≤16. In an embodiment of the present application, the first electrode tab is a negative electrode tab, 14≤T1≤25, and 4≤T2≤16. By the above-mentioned 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 electrode tab is provided with the material layer is improved. At this time, the double-layer material layer is arranged on the outer single-sided first electrode tab of the electrode assembly, and the electrical conductivity of the single-sided first electrode tab is less than or equal to that of the inner first electrode tab, which reduces the risk of black spots and lithium precipitation on the outer electrode assembly tab during the cycle process, and the secondary battery has good cycle performance.

[0016] 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.

[0017] In an embodiment of the present application, the secondary battery further comprises an electrolyte, the electrolyte comprises a lithium salt, the lithium salt comprises at least one of lithium tetrafluorophosphate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium bisoxalate borate, lithium bisfluorosulfonylimide or lithium difluorophosphate, and the concentration of the lithium salt is 1-5 mol / L. When the lithium salt in the above range is selected and the concentration of the lithium salt is controlled in the above 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.

[0018] In an embodiment of the present application, the first electrode tab is a negative electrode tab, 0.5≤a / b≤1, and 0.1≤b≤10. When the first electrode tab is a negative electrode tab, different first active material layers are arranged on the outer first electrode tab and the inner first electrode tab of the electrode assembly, and the values of a / b and b are controlled in the above range, which is beneficial to reduce the risk of black spot and lithium precipitation of the outer electrode tab of the electrode assembly during the cycle process, thereby improving the cycle performance of the secondary battery.

[0019] 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, the hardness of the secondary battery is enhanced, and the cycle performance and safety performance of the secondary battery are improved.

[0020] The second 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 long service life.

[0021] The beneficial effects of the embodiments of the present application are as follows:

[0022] The present application provides a secondary battery and an electronic device. By controlling the electrical conductivity of the outer first electrode tab to be less than or equal to that of the inner first electrode tab, the current density of the outermost electrode tab of the electrode assembly is reduced, the consumption rate of the electrolyte 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 effectively reduced, and the cycle performance of the secondary battery is improved.

[0023] Of course, implementing any product or method of the present application does not necessarily require all the advantages described above to be achieved at the same time. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other embodiments can also be obtained by those skilled in the art based on these drawings.

[0025] Figure 1 A cross-sectional structure schematic diagram of a secondary battery according to an embodiment of the present application, viewed in the length direction of the secondary battery itself;

[0026] Figure 2 A cross-sectional structure schematic diagram of a secondary battery according to an embodiment of the present application, viewed in the length direction of the secondary battery itself; Figure 1 A cross-sectional structure schematic diagram of a secondary battery according to an embodiment of the present application, viewed in the length direction of the secondary battery itself.

[0027] 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; 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

[0028] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. All other embodiments obtained by a person skilled in the art based on the present application belong to the scope of protection of the present application.

[0029] 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:

[0030] The first aspect of the present application provides a secondary battery, comprising 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 in layers 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 sheet, and the plurality of second tabs corresponding one-to-one to the second electrode sheet. 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 along 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 along the thickness direction of the electrode assembly, the first surface being closer to the shell than the second surface. At least one outer first electrode sheet is a single-sided first electrode sheet, the first surface of the single-sided first electrode sheet being free of the first active material layer, and the second surface of the single-sided first electrode sheet being provided with the first active material layer, the first active material layer provided on the second surface of the single-sided first electrode sheet being a first material layer. The first surface and the second surface of the inner first electrode sheet are both provided with the first active material layer, the first active material layer provided on the first surface and the second surface of the inner first electrode sheet being a second material layer. The electrical conductivity of the single-sided first electrode sheet is a S / cm, and the electrical conductivity of the inner first electrode sheet is b S / cm, a≤b.

[0031] In the present application, the electrode assembly of the laminated structure is defined with its own width direction as the X direction, its own length direction as the Y direction, and its own 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 length direction, width direction and thickness direction as the electrode assembly. Exemplarily, as shown in Figure 1 and Figure 2As shown, the secondary battery 001 comprises an electrode assembly 01 in a laminated structure and a housing 02. The electrode assembly 01 comprises a first electrode sheet 10, a second electrode sheet 20 and a separator 30 arranged in a stack along a thickness direction Z of the electrode assembly 01. The electrode assembly 01 further comprises a first tab assembly 13 comprising a plurality of first tabs 131 corresponding to the first electrode sheet 10 and a second tab assembly 23 comprising a plurality of second tabs 231 corresponding to the second electrode sheet 20. The first electrode sheet 10 comprises two outer first electrode sheets 101 and one inner first electrode sheet 102. The two outer first electrode sheets 101 are respectively located at the outermost sides of the electrode assembly 01 along the thickness direction Z of the electrode assembly 01. 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) and a second surface (not shown) arranged oppositely along the thickness direction Z of the electrode assembly 01. The first surface is closer to the housing 02 than the second surface. The two outer first electrode sheets 101 are single-sided first electrode sheets. The first active material layer 12 is not provided on the first surface of the single-sided first electrode sheet. The first active material layer 12 is provided on the second surface of the single-sided first electrode sheet. The first active material layer 12 provided on the second surface of the single-sided first electrode sheet is a first material layer 121. The first active material layer 12 is provided on the first surface and the second surface of the inner first electrode sheet 102. The first active material layer 12 provided on the first surface and the second surface of the inner first electrode sheet 102 is a second material layer 122.

[0032] The inventor found that, by coating the first active material layer on the outer first tab and the inner first tab of the electrode assembly, the conductivity of the outer first tab is greater than or equal to the conductivity of the inner first tab, 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 outermost layer of the electrode assembly is greater than the current density of the inner layer, i.e. A>B, the consumption rate of the electrolyte in the outer tab of the electrode assembly is too fast, and the outer tab of the electrode assembly is prone to black spots or lithium precipitation, thereby reducing the cycle performance and safety performance of the secondary battery. Therefore, by setting the first active material layer on the outer first tab and the inner first tab of the electrode assembly, and making the conductivity of the outer first tab less than or equal to the conductivity of the inner first tab, 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.

[0033] 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.

[0034] 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 the current density 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 improve the cycle performance of the secondary battery.

[0035] 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 the current density 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 improve the cycle performance of the secondary battery.

[0036] In an embodiment of the present application, the first tab is a positive electrode tab. When the positive electrode tab is used 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.

[0037] In an embodiment of the present application, the first material layer comprises a first binder, and the mass percentage of the first binder is c based on the mass of the first material layer; the second material layer comprises a second binder, and the mass percentage of the second binder is d based on the mass of the second material layer, and 0.6≤c / d≤4. For example, the value of c / d can be 0.6, 0.8, 1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, or a range formed by any two of the above values. By adjusting the value of c / d within the above range, compared with the outer tab of the conventional electrode assembly, the impedance of the outer tab of the electrode assembly is increased, and the content of the first binder and the second binder is used in combination, which reduces the current density of the outer tab of the electrode assembly, slows down the consumption rate of the electrolyte outside the electrode assembly during the cycle process, reduces the polarization during the conversion of the active material, reduces the risk of black spot and lithium precipitation of the outer tab of the electrode assembly during the cycle process, and further improves the cycle performance of the secondary battery.

[0038] In an embodiment of the present application, 2%≤c≤5%; for example, the value of c can be 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 formed by any two of the above values. 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 sodium, polyvinylpyrrolidone, polyvinyl ether, pure acrylic emulsion, or polyacrylic acid. By selecting the first binder and the second binder within the above range and adjusting the mass percentage of the first binder within the above range, the impedance of the outer tab of the electrode assembly is increased, the conductivity of the outer tab of the electrode assembly is reduced, the current density of the outer 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 tab of the electrode assembly during the cycle process is reduced, and the cycle performance of the secondary battery is further improved.

[0039] 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 content W1 of the first conductive agent is 0.5% to 5% based on the mass of the first material layer. For example, the mass percentage content W1 of the first conductive agent 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 the above values. The mass percentage content W2 of the second conductive agent is 1% to 10% based on the mass of the second material layer. For example, the mass percentage content W2 of the second conductive agent can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, or a range between any two of the above values. By selecting the first conductive agent and the second conductive agent within the above ranges and adjusting the mass percentage content of the first conductive agent and the mass percentage content of the second conductive agent within the above ranges, the combination of the first conductive agent and the second conductive agent can increase the impedance of the outer electrode tab of the electrode assembly, reduce the conductivity of the outer electrode tab of the electrode assembly, thereby reducing the current density of the outer electrode 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 electrode tab of the electrode assembly during the cycle process, thereby improving the cycle performance of the secondary battery.

[0040] In an embodiment of the present application, the first material layer comprises a first active material, and the second material layer comprises a second active material, each of the first active material and the second active material is independently selected from at least one of lithium cobaltate, lithium iron phosphate, lithium nickelate, lithium manganate, a nickel-cobalt-manganese ternary material, or a lithium-rich manganese-based material. The mass percentage content W'1 of the first active material is 90% to 97.5%, for example, the mass percentage content W'1 of the first active material can be 90%, 90.5%, 91%, 91.5%, 92%, 92.5%, 93%, 93.5%, 94%, 94.5%, 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, or a range between any two of them; the mass percentage content W'2 of the second active material is 87% to 98.5%, for example, the mass percentage content W'2 of the second active material can be 87%, 87.5%, 88%, 88.5%, 89%, 89.5%, 90%, 90.5%, 91%, 91.5%, 92%, 92.5%, 93%, 93.5%, 94%, 94.5%, 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, or a range between any two of them. The first active material and the second active material are selected within the above-mentioned range, and the mass percentage content of the first active material and the mass percentage content of the second active material are regulated within the above-mentioned range. The first active material and the second active material are used in combination, which increases the impedance of the outer electrode tab of the electrode assembly, thereby reducing the current density of the outer electrode 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 spots and lithium precipitation on the outer electrode 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 cathode material Li2MnO3·LiMO2 based on Li2MnO3, where M is Ni, Co, Mn, or a binary or ternary layered material of Ni, Co, and Mn.

[0041] 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, 0.05, 0.08, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or a range between any two of them. By adjusting the electrical conductivity of the inner first tab within the above range, the current density of the outer tab of the electrode assembly during the cycle is small, which is conducive to improving the cycle performance of the secondary battery.

[0042] In an embodiment of the 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 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 between 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 between any two of them. In an embodiment of the 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 between 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 between 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 outer tab of the electrode assembly during the cycle more obvious, and the secondary battery has good cycle performance.

[0043] In an embodiment of the 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, the risk of short circuit of the secondary battery is reduced, and the cycle performance and safety performance of the secondary battery are improved.

[0044] In an embodiment of the present application, the secondary battery further comprises an electrolyte, the electrolyte comprises a lithium salt, the lithium salt comprises at least one of lithium tetrafluorophosphate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium bis(fluorosulfonyl)imide or lithium difluorophosphate, and the concentration ω of the lithium salt is 1-5 mol / L. For example, the concentration ω of the lithium salt can be 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L, 2 mol / L, 2.2 mol / L, 2.5 mol / L, 2.8 mol / L, 3 mol / L, 3.2 mol / L, 3.5 mol / L, 3.8 mol / L, 4 mol / L, 4.2 mol / L, 4.5 mol / L, 4.8 mol / L, 5 mol / L or a range between any two of the above values. By selecting the lithium salt within the above range and regulating the concentration of the lithium salt within the above range, the ion transmission in the electrolyte is facilitated, and when used in combination with the binder in the first active material layer, a stable interface is formed, the interface consistency is good, the electrical conductivity of the outer tab of the electrode assembly is reduced, the current density of the outer 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 tab of the electrode assembly during the cycle process is reduced, and the cycle performance of the secondary battery is improved.

[0045] 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 the purpose of the present application can be achieved. For example, the positive electrode current collector can include an aluminum foil, an aluminum alloy foil, or a composite current collector (e.g., an aluminum-carbon composite current collector), etc.

[0046] In an embodiment of the present application, the first tab is a negative electrode tab, 0.5≤a / b≤1, and 0.1≤b≤10. For example, the value of a / b can be 0.5, 0.6, 0.7, 0.8, 0.9, 1 or a range between any two of the above values; the value of b can be 0.1, 0.3, 0.5, 0.8, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or a range between any two of the above values. When the first tab is a negative electrode tab, different first active material layers are coated on the outer and inner first tabs of the electrode assembly, and the values of a / b and b are regulated within the above ranges, compared with the outer tab of the conventional electrode assembly, the impedance of the outer tab of the electrode assembly is increased, the current density of the outer 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 tab of the electrode assembly during the cycle process is reduced, and the cycle performance of the secondary battery is improved.

[0047] In an embodiment of the present application, the shell is a steel shell. When the steel shell is used as the shell, and the first tab on the outer side of the electrode assembly is a negative tab, the risk of short circuit of the secondary battery is reduced, and the cycle performance and safety performance of the secondary battery are improved.

[0048] In an embodiment of the present application, the first tab is a negative tab, and 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 0.5% to 5% based on the mass of the second material layer. When the first tab is a negative tab, the first binder and the second binder are selected within the above-mentioned range, and the mass percentage of the first binder and the mass percentage of the second binder are controlled within the above-mentioned range. The first binder and the second binder are used in combination, the impedance of the tab on the outer side of the electrode assembly is increased, the current density of the tab on the outer side of the electrode assembly is reduced, the consumption rate of the electrolyte on the outer side of the electrode assembly during the cycle process is slowed down, the risk of black spots and lithium precipitation on the tab on the outer side of the electrode assembly during the cycle process is reduced, and the cycle performance of the secondary battery is improved.

[0049] In an embodiment of the present application, the first tab is a negative tab, and the first conductive agent and the second conductive agent are each independently selected from at least one of conductive carbon black, conductive graphite, carbon fiber (VGCF), carbon nanotube (CNT), or graphene. The mass percentage of the first conductive agent is 0.5% to 10% based on the mass of the first material layer, and the mass percentage of the second conductive agent is 0.5% to 10% based on the mass of the second material layer. When the first tab is a negative tab, the first conductive agent and the second conductive agent are selected within the above-mentioned range, and the mass percentage of the first conductive agent and the mass percentage of the second conductive agent are controlled within the above-mentioned range. The first conductive agent and the second conductive agent are used in combination, the impedance of the tab on the outer side of the electrode assembly is increased, the current density of the tab on the outer side of the electrode assembly is reduced, the consumption rate of the electrolyte on the outer side of the electrode assembly during the cycle process is slowed down, the risk of black spots and lithium precipitation on the tab on the outer side of the electrode assembly during the cycle process is reduced, and the cycle performance of the secondary battery is improved.

[0050] In an embodiment of the present application, the first electrode tab is a negative electrode tab, the first active material and the second active material are each independently selected from at least one of natural graphite, artificial graphite, soft carbon, hard carbon, activated carbon, lithium-alloy, lithium-titanium alloy, lithium-silicon dioxide, lithium-phosphorus oxide, lithium-sulfide, silicon-based material, titanate material or boron nitride material, the mass percentage of the first active material is 90% to 99% based on the mass of the first material layer, and the mass percentage of the second active material is 90% to 99% based on the mass of the second material layer. When the first electrode tab is a negative electrode tab, the first active material and the second active material are selected within the above-mentioned range, and the mass percentage of the first active material and the mass percentage of the second active material are regulated within the above-mentioned range, the first active material and the second active material are used in combination, the impedance of the electrode assembly outer electrode tab is increased, thereby reducing the current density of the electrode assembly outer electrode tab, 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 spots and lithium precipitation of the electrode assembly outer electrode tab during the cycle process, thereby improving the cycle performance of the secondary battery.

[0051] In the present application, when the first electrode tab or the second electrode tab is a negative electrode tab, the negative electrode current collector is not particularly limited as long as the purpose of the present application can be achieved. For example, the negative electrode 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 (such as 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.

[0052] The preparation method of the outer first electrode tab is not particularly limited in the present application as long as the purpose of the present application can be achieved. For example, the preparation method of the outer first electrode tab includes but is not limited to the following steps: (1) mixing the first active material, the first binder and the first conductive agent according to a certain mass ratio, and then adding a solvent to prepare a first material layer slurry; (2) coating the first material layer slurry on one surface of the first current collector, and drying to obtain a single-sided outer first electrode tab coated with the first material layer; (3) repeating the above steps on the other surface of the first current collector to obtain a double-sided outer first electrode tab coated with the first material layer; (4) cold pressing, cutting and welding the first tab, to obtain the outer first electrode tab.

[0053] In the present application, the electrical conductivity of the outer first electrode sheet can be regulated by regulating the mass percentage content of the first binder, the mass percentage content of the first conductive agent and the mass percentage content of the first active material. For example, when other conditions are constant, the mass percentage content of the first active material is unchanged, the mass percentage content of the first binder is increased, and the mass percentage content of the corresponding first conductive agent is reduced, the electrical conductivity of the outer first electrode sheet is reduced; the mass percentage content of the first binder is reduced, and the mass percentage content of the corresponding first conductive agent is increased, the electrical conductivity of the outer first electrode sheet is increased. In the present application, the mass percentage contents 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 present application, the electrical conductivity of the outer first electrode sheet is also related to the overall thickness of the outer first electrode sheet, the coating weight of the corresponding material layer, etc. For example, for the outer first electrode sheet of the same specification, when other conditions are constant, the electrical conductivity of the outer first electrode sheet is greater when the thickness of the outer first electrode sheet is smaller and the coating weight is less. The present application does not have special restrictions on the solvent and the solid content of the first material layer slurry in the above step (1), as long as the purpose of the present application can be achieved. The present application does not have special restrictions on the temperature and time of drying in the above step (2), which can be selected by a person skilled in the art according to actual needs, as long as the purpose of the present application can be achieved. The present application does not have special restrictions on the process parameters of cold pressing, sheet cutting and welding of the first tab in the above step (4), which can be selected by a person skilled in the art according to actual needs, as long as the purpose of the present application can be achieved. The present application does not have special restrictions on the size of the above first material layer, as long as the purpose of the present application can be achieved.

[0054] The present application does not have special restrictions on the preparation method of the inner first electrode sheet, as long as the purpose of the present application can be achieved. For example, the preparation method of the inner first electrode sheet includes but is not limited to the following steps: (1) mixing the second active material, the second binder and the second conductive agent according to a certain mass ratio, and then adding a solvent to prepare a second material layer slurry; (2) coating the second material layer slurry on one surface of the first current collector, and drying to obtain a single-sided inner first electrode sheet coated with a second material layer; (3) repeating the above steps on the other surface of the first current collector to obtain a double-sided inner first electrode sheet coated with a second material layer; (4) cold pressing, sheet cutting and welding of the first tab, to obtain the inner first electrode sheet.

[0055] In the present application, the electrical conductivity of the inner first electrode sheet can be regulated by regulating the mass percentage content of the second binder, the mass percentage content of the second conductive agent, and the mass percentage content of the second active material. For example, when other conditions are constant, the mass percentage content of the second active material remains unchanged, the mass percentage content of the second binder increases, and the mass percentage content of the corresponding second conductive agent decreases, the electrical conductivity of the inner first electrode sheet decreases; the mass percentage content of the second binder decreases, and the mass percentage content of the corresponding second conductive agent increases, the electrical conductivity of the inner first electrode sheet increases. In the present application, 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 present application, the electrical conductivity of the inner first electrode sheet is also related to the overall thickness of the inner first electrode sheet, the coating weight of the corresponding material layer, etc. For example, for the inner first electrode sheet of the same specification, when other conditions are constant, the electrical conductivity of the inner first electrode sheet is greater when the thickness of the inner first electrode sheet is smaller and the coating weight is less. The present application does not have special restrictions on the solvent and solid content of the second material layer slurry used in the above step (1), as long as the purpose of the present application can be achieved. The present application does not have special restrictions on the temperature and time of drying in the above step (2), which can be selected by a person skilled in the art according to actual needs, as long as the purpose of the present application can be achieved. The present application does not have special restrictions on the process parameters of cold pressing, sheet cutting, and welding the first tab in the above step (4), which can be selected by a person skilled in the art according to actual needs, as long as the purpose of the present application can be achieved. The present application does not have special restrictions on the size of the above second material layer, as long as the purpose of the present application can be achieved.

[0056] In an embodiment of the present application, the electrolyte further comprises an organic solvent, and the organic solvent comprises at least one of ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), propylene carbonate (PC), ethyl acrylate (EA), or methyl propionate (MA). When an organic solvent within the above-mentioned category is selected and used in combination with a lithium salt, it is beneficial to reduce the electrical conductivity of the outer electrode sheet of the electrode assembly, thereby reducing the current density of the outer electrode sheet 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 electrode sheet of the electrode assembly during the cycle process, thereby improving the cycle performance of the secondary battery.

[0057] The electrolyte of the present application can further include other solvents. The present application does not particularly limit the other solvents as long as the purpose of the present application can be achieved. For example, the other solvents can include, but are not limited to, at least one of a carbonate compound, a carboxylate compound, an ether compound, or other organic solvents. The above-mentioned carbonate compound can include, but is not limited to, at least one of a chain carbonate compound, a cyclic carbonate compound, or a fluoro-carbonate compound. The above-mentioned chain carbonate compound can include, but is not limited to, at least one of dipropyl carbonate, methyl propyl carbonate, or ethyl propyl carbonate. The above-mentioned cyclic carbonate can include, but is not limited to, at least one of butylene carbonate or vinyl ethylene carbonate. The fluoro-carbonate compound can include, but is not limited to, at least one of fluoro-vinylene carbonate, carbonic acid-1,2-difluoro ethylene ester, carbonic acid-1,1-difluoro ethylene ester, carbonic acid-1,1,2-trifluoro ethylene ester, carbonic acid-1,1,2,2-tetrafluoro ethylene 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 above-mentioned 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, propyl propionate, γ-butyrolactone, decanolactone, pentolactone, or hexolactone. The above-mentioned ether compound can include, but is not limited to, at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxy ethane, 1,2-diethoxy ethane, 1-ethoxy-1-methoxy ethane, 2-methyl tetrahydrofuran, or tetrahydrofuran. The above-mentioned other organic solvents can include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0058] The present application does not particularly limit the separator as long as the purpose of the present application can be achieved. For example, the material of the separator 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 separator can include at least one of a woven film, a non-woven film, a microporous film, a composite film, a calendered film, or a spunlaid film. The separator of the present application can have a porous structure, and the present application does not particularly limit the size of the pore diameter of the porous structure of the separator as long as the purpose of the present application can be achieved. For example, the size of the pore diameter can be 0.01 μm to 1 μm. The present application does not particularly limit the thickness of the separator as long as the purpose of the present application can be achieved. For example, the thickness of the separator can be 5 μm to 40 μm.

[0059] The secondary battery of the present application is not particularly limited, and can include any device that undergoes an electrochemical reaction. 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.

[0060] The preparation process of the secondary battery of the present application is well known to those skilled in the art, and the present application is not particularly limited, for example, the preparation process of the secondary battery can include, but is not limited to, the following steps: stacking 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 in order, then fixing the four corners of the entire stack structure with adhesive tape to obtain an electrode assembly of the stack structure, placing the electrode assembly into a housing, injecting electrolyte into the housing and sealing, to obtain a secondary battery. In addition, a current protection element, a guide plate, etc. can also be placed in the housing as needed, thereby preventing the pressure inside the secondary battery from rising and overcharging and discharging.

[0061] The second aspect of the present application provides an electronic device including the secondary battery of any of the preceding embodiments. The secondary battery of the present application has good cycle performance, and thus the electronic device of the present application has a longer service life.

[0062] The electronic device of 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 computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, 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 recorder, 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 clock, a power tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor.

[0063] Embodiments

[0064] 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 on a mass basis.

[0065] Test methods and apparatuses:

[0066] Conductivity test of the pole piece:

[0067] 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 minutes, the outer first pole piece and the inner first pole piece were rinsed with DMC and acetone respectively. 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.

[0068] The treated outer first pole piece and the treated inner first pole piece were tested using a resistivity tester (IEST, BER1200). Before testing, the resistance tester was reset for resistance and pressure. The tested pole piece sample was placed between the electrodes of the tester, and the resistance values of three pole pieces with a cross-sectional area of 30mm×30mm were measured at three different positions on the sample. The average value was recorded as the resistance value of the pole piece with a cross-sectional area of 30mm×30mm, which was recorded as the resistance value of the tested pole piece. The following formula was used for calculation:

[0069] R = p x l / s; y = 1 / p = l / Rs

[0070] Wherein, R is the measured resistance value; p is the resistivity; l is the thickness of the measured pole piece, s is the cross-sectional area, and y is the measured pole piece conductivity.

[0071] The conductivity a of the outer first pole piece and the conductivity b of the inner first pole piece were obtained respectively.

[0072] Lithium salt concentration test in electrolyte:

[0073] The lithium ion battery was disassembled after being discharged at 0.2C to 3V, the electrolyte was collected, and the first pole piece, the second pole piece and the separator were centrifuged. The liquid obtained after centrifugation was mixed with the above electrolyte to obtain a liquid sample. The liquid sample was tested by ion chromatography (IC) to measure the concentration of lithium salt in the electrolyte.

[0074] Lithium precipitation performance test:

[0075] The lithium ion battery in the examples and comparative examples was placed in a thermostat at 10°C for 60 minutes, then charged at 2C to 4.5V, charged at 4.5V to a current of 0.05C, and rested for 5 minutes. Then discharged at 0.5C to 3.0V, which was one cycle. After 100 cycles according to the above charging and discharging process, the lithium ion battery was disassembled, the outer pole piece of the electrode assembly was taken out, and the lithium precipitation state on the surface of the outer pole piece was observed. The area without lithium precipitation on the surface of the outer pole piece was golden yellow, and the area with lithium precipitation was grayish white.

[0076] The criteria for judging the degree of lithium precipitation of the lithium ion battery are as follows: no lithium precipitation, i.e. the degree of lithium precipitation is none, when the lithium precipitation area is 0%; mild lithium precipitation, i.e. the degree of lithium precipitation is mild, when the lithium precipitation area is greater than 0% and less than or equal to 2%; moderate lithium precipitation, i.e. the degree of lithium precipitation is moderate, when the lithium precipitation area is greater than 2% and less than or equal to 20%; and severe lithium precipitation, i.e. the degree of lithium precipitation is severe, when the lithium precipitation area is greater than 20% and less than or equal to 100%, 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.

[0077] Black spot test:

[0078] The lithium ion battery was placed in a 0°C constant temperature oven and allowed to stand for 60 min to reach a constant temperature. The lithium ion battery reaching a constant temperature was charged at 1C constant current to a full charge voltage of 4.5V at 0°C, and then charged at 4.5V constant voltage to 0.025C, allowed to stand for 5 min, and then discharged at 1C constant current to 3.0V. This was one cycle of charging and discharging. After 500 cycles of charging and discharging, the lithium ion battery was charged at 1C constant current to a full charge voltage of 4.5V, and then charged at 4.5V constant voltage to 0.025C to obtain a full charged battery after 500 cycles. The lithium ion battery was disassembled in a dry room with a humidity of less than 5%, and photographed to record whether black spots appeared on the surface of the outer electrode tab.

[0079] The criteria for judging the degree of black spot of the lithium ion battery are as follows: no black spot, i.e. the degree of black spot is none, when the black spot area is 0%; mild black spot, i.e. the degree of black spot is mild, when the black spot area is less than or equal to 2%; moderate black spot, i.e. the degree of black spot is moderate, when the black spot area is 2% to 20%; and severe black spot, i.e. the degree of black spot is severe, when the black spot area is greater than 20%, 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.

[0080] Cycle performance test:

[0081] The lithium ion battery was placed in a 25°C constant temperature test box and allowed to stand for 30 min to reach a constant temperature of 25°C. The lithium ion battery was charged at 1C constant current to 4.5V, and then charged at 4.5V constant voltage to a current of 0.025C, allowed to stand for 5 min, and then discharged at 0.2C constant current to 3.0V. This was the first cycle of charging and discharging, and the initial discharge capacity was recorded as C0. The lithium ion battery was subjected to charging and discharging according to the above process, and the test was stopped when the cycle number reached 400 (cls). 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.

[0082] 400 cls capacity retention rate (%) = C1 / C0 x 100%.

[0083] The higher the 400 cls capacity retention rate, the better the cycle performance of the lithium ion battery.

[0084] Example 1

[0085] <Preparation of the first electrode sheet>

[0086] The positive electrode sheet was selected as the first electrode sheet. The first active material lithium cobaltate, the first binder polyvinylidene fluoride, and the first conductive agent conductive carbon black were mixed in a mass ratio of 96.5:3:0.5, and N-methyl pyrrolidone (NMP) was added. The mixture was stirred uniformly under the action of a vacuum stirrer to obtain a first material layer slurry with a solid content of 70 wt%. The first material layer slurry was coated on one surface of a positive electrode current collector aluminum foil with a thickness of 14 μm. After baking treatment at 120 °C for 1 hour, a positive electrode sheet coated with a single first material layer was obtained. The above steps were repeated on the other surface of the positive electrode current collector aluminum foil to obtain a positive electrode sheet coated with double second material layers. After drying at 120 °C under vacuum for 1 hour, the positive electrode sheet was cold-pressed, cut, and the positive electrode tabs were welded to obtain an inner positive electrode sheet with a size of 50 mm x 90 mm. The coating weight of the first material layer was 16 mg / mm 2 , the thickness of the first material layer was 50 μm, the mass percentage content of the first binder c was 3% based on the mass of the first material layer, the mass percentage content of the first conductive agent W1 was 0.5%, and the mass percentage content of the first active material W'1 was 96.5%.

[0087] The second active material lithium cobaltate, the second binder polyvinylidene fluoride, and the second conductive agent conductive carbon black were mixed in a mass ratio of 97.5:1.5:1, and N-methyl pyrrolidone (NMP) was added. The mixture was stirred uniformly under the action of a vacuum stirrer to obtain a second material layer slurry with a solid content of 70 wt%. The second material layer slurry was coated on one surface of a positive electrode current collector aluminum foil with a thickness of 14 μm. After baking treatment at 120 °C for 1 hour, a positive electrode sheet coated with a single second material layer was obtained. The above steps were repeated on the other surface of the positive electrode current collector aluminum foil to obtain a positive electrode sheet coated with double second material layers. After drying at 120 °C under vacuum for 1 hour, the positive electrode sheet was cold-pressed, cut, and the positive electrode tabs were welded to obtain an inner positive electrode sheet with a size of 50 mm x 90 mm. The coating weight of the second material layer was 16 mg / mm 2 , the thickness of the double second material layers was 100 μm, the mass percentage content of the second binder d was 1.5% based on the mass of the second material layer, the mass percentage content of the second conductive agent W2 was 1%, and the mass percentage content of the second active material W'2 was 97.5%.

[0088] <Preparation of the second electrode sheet>

[0089] The negative electrode sheet was selected as the second electrode sheet. A negative electrode active material artificial graphite, conductive carbon black, and a negative electrode binder styrene-butadiene rubber were mixed in a mass ratio of 94:2:4, deionized water was added, and the mixture was stirred uniformly under the action of a vacuum stirrer to obtain a negative electrode slurry with a solid content of 65wt%. The negative electrode slurry was uniformly coated on one surface of a negative electrode current collector copper foil with a thickness of 12μm, and the coated copper foil was baked at 120℃ for 1 hour to obtain a negative electrode sheet with a single-side coated negative electrode material layer. The above steps were repeated on the other surface of the negative electrode current collector copper foil to obtain a negative electrode sheet with a double-side coated negative electrode material layer. Then, the negative electrode sheet was dried in a vacuum at 120℃ for 1 hour, and then cold-pressed, cut, and welded to obtain a negative electrode sheet with a size of 50mm×90mm. The coated weight of the negative electrode material layer was 11mg / mm 2 , and the thickness of the double-side negative electrode material layer was 110μm.

[0090] <Preparation of electrolyte>

[0091] In a dry argon atmosphere glove box, 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 lithium hexafluorophosphate (LiPF6) was dissolved in the above organic solvent, and the mixture was stirred uniformly to obtain an electrolyte. The concentration ω of the lithium salt LiPF6 was 3mol / L.

[0092] <Separator>

[0093] A polyethylene (PE) film with a thickness of 7μm was used as the separator.

[0094] <Preparation of lithium ion battery>

[0095] The outer positive electrode sheet, the separator, the negative electrode sheet, the separator, the inner positive electrode sheet, the separator, the negative electrode sheet, the separator, and the outer positive 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 an aluminum plastic film and dried in a vacuum oven at 80℃ for 12 hours to remove water. The electrolyte prepared above was injected, and the lithium ion battery was obtained after vacuum packaging, standing, formation, degassing, and edge cutting processes. The design potential interval of the lithium ion battery was 3.0V to 4.5V.

[0096] Examples 2 to 18

[0097] Except for adjusting the relevant preparation parameters according to Table 1, the rest was the same as Example 1. When the lithium salt concentration in the electrolyte changed, the concentration of the organic solvent changed accordingly, and the mass ratio of ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) remained unchanged.

[0098] Example 19

[0099] The first electrode sheet, the second electrode sheet and the lithium ion battery were prepared according to the following steps, and the related preparation parameters were adjusted according to Table 1. The rest was the same as Example 1.

[0100] <Preparation of the first electrode sheet>

[0101] The negative electrode sheet was selected as the first electrode sheet. The first active material artificial graphite, the first binder styrene-butadiene rubber (SBR) and the first conductive agent carbon nanotube (CNT) were mixed in a mass ratio of 96.5:3:0.5 and added to deionized water. The mixture was stirred uniformly under the action of a vacuum stirrer to obtain a first material layer slurry with a solid content of 65wt%. The first material layer slurry was coated on one surface of a negative current collector copper foil with a thickness of 12μm. After baking treatment at 120℃ for 1 hour, a negative electrode sheet coated with a single first material layer was obtained. The above steps were repeated on the other surface of the negative current collector copper foil to obtain a negative electrode sheet coated with double second material layers. Then, the negative electrode sheet was dried at 120℃ under vacuum for 1 hour. After cold pressing, cutting and welding of the negative electrode tabs, an inner negative electrode sheet with a size of 50mm×90mm was obtained. The coating weight of the first material layer was 12mg / mm 2 , the thickness of the first material layer was 60μm, the mass percentage content c of the first binder was 3% based on the mass of the first material layer, the mass percentage content W1 of the first conductive agent was 0.5% based on the mass of the first material layer, and the mass percentage content W'1 of the first active material was 96.5% based on the mass of the first material layer.

[0102] The second active material artificial graphite, the second binder styrene-butadiene rubber (SBR) and the second conductive agent carbon nanotube (CNT) were mixed in a mass ratio of 96.5:1.5:2 and added to deionized water. The mixture was stirred uniformly under the action of a vacuum stirrer to obtain a second material layer slurry with a solid content of 65wt%. The second material layer slurry was coated on one surface of a negative current collector copper foil with a thickness of 12μm. After baking treatment at 120℃ for 1 hour, a negative electrode sheet coated with a single second material layer was obtained. The above steps were repeated on the other surface of the negative current collector copper foil to obtain a negative electrode sheet coated with double second material layers. Then, the negative electrode sheet was dried at 120℃ under vacuum for 1 hour. After cold pressing, cutting and welding of the negative electrode tabs, an inner negative electrode sheet with a size of 50mm×90mm was obtained. The coating weight of the second material layer was 12mg / mm 2 , the thickness of the double second material layers was 120μm, the mass percentage content d of the second binder was 1.5% based on the mass of the second material layer, the mass percentage content W2 of the second conductive agent was 2% based on the mass of the second material layer, and the mass percentage content W'2 of the second active material was 96.5% based on the mass of the second material layer.

[0103] <Preparation of the second electrode sheet>

[0104] The positive electrode sheet was selected as the second electrode sheet. The positive electrode active material lithium cobaltate, the binder polyvinylidene fluoride, and the conductive agent conductive carbon black were mixed in a mass ratio of 92.5:1.5:6, and N-methyl pyrrolidone (NMP) was added. 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 coated positive electrode sheet was baked at 120 °C for 1 hour to obtain a positive electrode sheet coated with a single-layer positive electrode material layer. The above steps were repeated on the other surface of the positive electrode current collector aluminum foil to obtain a positive electrode sheet coated with a double-layer positive electrode material layer. Then, the positive electrode sheet was dried at 120 °C under vacuum for 1 hour, and then cold-pressed, cut, and welded to obtain a positive electrode sheet with a size of 60 mm x 90 mm. The coating weight of the positive electrode material layer was 18 mg / mm 2 , and the thickness of the double-layer positive electrode material layer was 108 μm.

[0105] <Preparation of a lithium ion battery>

[0106] 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. The electrolyte prepared above was injected, and the lithium ion battery was obtained after vacuum packaging, standing, formation, degassing, and edge cutting. The design potential interval of the lithium ion battery was 3.0 V to 4.5 V.

[0107] Examples 20 to 21

[0108] Except for adjusting the relevant preparation parameters according to Table 1, the rest was the same as in Example 19.

[0109] Comparative Examples 1 to 3

[0110] Except for adjusting the relevant preparation parameters according to Table 1, the rest was the same as in Example 1.

[0111] The preparation parameters and performance parameters of each example and comparative example are shown in Tables 1 and 2.

[0112]

[0113]

[0114] Table 2

[0115]

[0116] As can be seen from Examples 1 to 21, 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 black spot area of the outer tab of the electrode assembly is smaller, the lithium precipitation area is smaller, and the 400 cls capacity retention of the lithium ion battery is improved, indicating that the lithium ion battery of the present application can reduce the risk of black spot and lithium precipitation 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, the parameters of the first material layer and the second material layer are exactly opposite to those of the first material layer and the second material layer in Example 1; the electrical conductivity of the single-sided first tab in Comparative Examples 1 to 3 is greater than that of the inner first tab; the black spot area of the lithium ion battery in Comparative Examples 1 to 3 is larger; the lithium precipitation area is larger, and the 400 cls capacity retention is lower. However, the black spot area and the lithium precipitation area of the outer first tab in the lithium ion battery in Examples 1 to 21 are smaller, and the 400 cls capacity retention is higher, indicating that the risk of black spot and lithium precipitation of the outer tab of the electrode assembly during the cycle process is lower, and the lithium ion battery has good cycle performance.

[0117] The value of c / d usually affects the cycle performance of the lithium ion battery. As can be seen from Examples 1 to 3, by regulating the value of c / d within the range of the present application, the black spot area and the lithium precipitation area of the outer first tab in the lithium ion battery are smaller, and the 400 cls capacity retention is higher, indicating that the risk of black spot and lithium precipitation of the outer tab of the electrode assembly during the cycle process is lower, and the lithium ion battery has good cycle performance.

[0118] The type and content of the first binder and the type and content of the second binder usually affect the cycle performance of the lithium ion battery. As can be seen from Examples 1 to 14, when the first binder and the second binder within the range of the present application are selected, and the content of the first binder and the content of the second binder are regulated within the range of the present application, the black spot area and the lithium precipitation area of the outer first tab in the lithium ion battery are smaller, and the 400 cls capacity retention is higher, indicating that the risk of black spot and lithium precipitation of the outer tab of the electrode assembly during the cycle process is lower, and the lithium ion battery has good cycle performance.

[0119] The type and content of the first conductive agent and the type and content of the second conductive agent usually affect the cycle performance of the lithium ion battery. As can be seen from Examples 1 to 14, when the first conductive agent and the second conductive agent within the range of the present application are selected, and the content of the first conductive agent and the content of the second conductive agent are regulated within the range of the present application, the black spot area and the lithium precipitation area of the outer first tab in the lithium ion battery are smaller, and the 400 cls capacity retention is higher, indicating that the risk of black spot and lithium precipitation of the outer tab of the electrode assembly during the cycle process is lower, and the lithium ion battery has good cycle performance.

[0120] The type and content of the first active material, the type and content of the second active material generally affect the cycle performance of the lithium ion battery. As can be seen from Example 1 to Example 14, when the first active material, the second active material within the scope of the application are selected, and the content of the first active material and the content of the second active material are controlled within the scope of the application, the black spot area and lithium precipitation area of the outer first pole piece in the lithium ion battery are smaller, and the 400 cls capacity retention rate is higher, 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.

[0121] The conductivity b of the inner first pole piece generally affects the cycle performance of the lithium ion battery. As can be seen from Example 1 to Example 18, when the value of b is controlled within the scope of the application, the black spot area and lithium precipitation area of the outer first pole piece in the lithium ion battery are smaller, and the 400 cls capacity retention rate is higher, 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.

[0122] The type and concentration of lithium salt generally affect the cycle performance of the lithium ion battery. As can be seen from Example 6, Example 15 to Example 18, when the lithium salt within the scope of the application is selected, and the concentration of the lithium salt is controlled within the scope of the application, the black spot area and lithium precipitation area of the outer first pole piece in the lithium ion battery are smaller, and the 400 cls capacity retention rate is higher, 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.

[0123] The type of the first pole piece generally affects the cycle performance of the lithium ion battery. As can be seen from Example 1, Example 19, when the first pole piece is a positive pole piece or a negative pole piece, the black spot area and lithium precipitation area of the outer first pole piece in the lithium ion battery are smaller, and the 400 cls capacity retention rate is higher, 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.

[0124] When the first pole piece is a negative pole piece, the value of a / b and the value of b generally affect the cycle performance of the lithium ion battery. As can be seen from Example 19 to Example 21, when the value of a / b and the value of b are controlled within the scope of the application, the black spot area and lithium precipitation area of the outer first pole piece in the lithium ion battery are smaller, and the 400 cls capacity retention rate is higher, 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.

[0125] It is to be noted that, in the present 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 variation 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.

[0126] Each of the embodiments in the present specification is described in a related manner, and the same or similar parts among the embodiments can be referred to each other. Each of the embodiments focuses on the difference from other embodiments.

[0127] The above only describes the preferred embodiments 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 protection scope of the present application.

Claims

1. A secondary battery, comprising a casing and an electrode assembly of a laminated 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 to the first electrode plate one by one, and the plurality of second tabs corresponding to the second electrode plate one by one; 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 the outermost two 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; at least one of the outer first electrode plates being a single-sided first electrode plate, the first surface of the single-sided first electrode plate being free of the first active material layer, and the second surface of the single-sided first electrode plate being provided with the first active material layer, the first active material layer provided on the second surface of the single-sided first electrode plate being a first material layer; the first surface and the second surface of the inner first electrode plate being both provided with the first active material layer, the first active material layer provided on the first surface and the second surface of the inner first electrode plate being a second material layer; the electrical conductivity of the single-sided first electrode plate being a S / cm, and the electrical conductivity of the inner first electrode plate being b S / cm, a ≤ b; 0.3 ≤ a / b ≤ 1; 0.65 ≤ a / b ≤ 1; the first electrode plate being a positive electrode plate; the first material layer comprising a first binder, the mass percentage of the first binder based on the mass of the first material layer being c; the second material layer comprising a second binder, the mass percentage of the second binder based on the mass of the second material layer being d, 0.6 ≤ c / d ≤ 4; 2% ≤ c ≤ 5%; the first binder and the second binder each being 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 first material layer comprising a first conductive agent, and the second material layer comprising a second conductive agent, the first conductive agent and the second conductive agent each being independently selected from at least one of conductive carbon black, carbon nanotube, graphene or conductive graphite; the mass percentage of the first conductive agent based on the mass of the first material layer being 0.5% to 5%, and the mass percentage of the second conductive agent based on the mass of the second material layer being 1% to 10%. ​ ​ ​ ​ ​ 2. The secondary battery according to claim 1, wherein ​ 3. The secondary battery according to claim 2, wherein ​ 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 ​ 8. The secondary battery according to claim 4, wherein The first material layer comprises a first active material, and the second material layer comprises a second active material, each of the first active material and the second active material is independently selected from at least one of lithium cobaltate, lithium iron phosphate, lithium nickelate, lithium manganate, a nickel-cobalt-manganese ternary material or a lithium-rich manganese-based material; the mass percentage of the first active material is 90% to 97.5% based on the mass of the first material layer, and the mass percentage of the second active material is 87% to 98.5% based on the mass of the second material layer.

9. The secondary battery according to claim 4, wherein 0.01≤b≤0.8。 10. The secondary battery according to claim 1 or 2, wherein The thickness of the first current collector of the single-sided first pole piece is T1 μm, and the thickness of the first current collector of the inner first pole piece is T2 μm, T1 ≥ T2.

11. The secondary battery according to claim 10, wherein The first pole piece is a positive pole piece, 12 ≤ T1 ≤ 25, and 6 ≤ T2 ≤ 16.

12. The secondary battery according to claim 10, wherein The first pole piece is a negative pole piece, 14 ≤ T1 ≤ 25, and 4 ≤ T2 ≤ 16.

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

14. The secondary battery according to claim 5, wherein The secondary battery further comprises an electrolyte, the electrolyte comprises a lithium salt, the lithium salt comprises at least one of lithium tetrafluorophosphate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium bis (oxalato) borate, lithium bis (fluorosulfonyl) imide or lithium difluorophosphate, and the concentration of the lithium salt is 1 mol / L to 5 mol / L.

15. The secondary battery according to claim 1 or 2, wherein The first pole piece is a negative pole piece, 0.5 ≤ a / b ≤ 1, and 0.1 ≤ b ≤ 10.

16. The secondary battery according to claim 12, wherein The shell is a steel shell.

17. An electronic device comprising the secondary battery of any one of claims 1 to 16.

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