Secondary battery and electronic device

Abstract

A secondary battery and an electronic device. The secondary battery comprises an electrode assembly. The electrode assembly comprises first electrode sheets, second electrode sheets, and a separator. The first electrode sheets comprise two outer first electrode sheets and an inner first electrode sheet, and the two outer first electrode sheets are respectively located on the outermost two sides of the electrode assembly. Each first electrode sheet comprises a first current collector and a first active material layer, and the first current collector comprises a first surface and a second surface. At least one outer first electrode sheet is a single-sided first electrode sheet, and the first active material layer provided on the second surface of the single-sided first electrode sheet is a first material layer. The first active material layer is provided on each of the first surface and the second surface of the inner first electrode sheet, and the first active material layer provided on each of the first surface and the second surface of the inner first electrode sheet is a second material layer. The electrical conductivity of the single-sided first electrode sheet is a S / cm, the electrical conductivity of the inner first electrode sheet is b S / cm, and a≤b. By means of the above arrangement, the risk of black spots and lithium plating on the outer electrode sheets of the electrode assembly is reduced.

Description

A secondary battery and electronic device

[0001] This application claims priority to Chinese Patent Application No. 202411216709.5, filed on August 31, 2024, entitled "A Secondary Battery and Electronic Device", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of electrochemical technology, and in particular to a secondary battery and electronic device. Background Technology

[0003] Secondary batteries, such as lithium-ion batteries, have the characteristics of high specific energy, high operating voltage, low self-discharge rate, small size, and light weight, and are widely used in the consumer electronics field.

[0004] Currently, electrode components in lithium-ion batteries often employ multi-tab and stacked structures to reduce battery impedance. To ensure the ultra-thinness and high energy density of lithium-ion batteries, the outermost electrode of the assembly typically uses a single-sided coated electrode. However, due to the manufacturing process and structural characteristics of single-sided coated electrodes, black spots or lithium plating are prone to occur during cycling, leading to a decrease in lithium-ion battery performance. Summary of the Invention

[0005] The purpose of this application is to provide a secondary battery and electronic device to reduce the risk of black spots and lithium plating on the outer electrode of the electrode assembly, and to improve the cycle performance of the secondary battery.

[0006] It should be noted that the invention description in this application uses lithium-ion batteries as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0007] In existing technologies, to reduce manufacturing costs and increase the energy density of secondary batteries, the outermost electrode of a multi-tab stacked electrode assembly in lithium-ion batteries is typically a single-sided coated electrode, meaning the material layer is only applied to the current collector side. The inner electrode is usually a double-sided coated electrode, and the proportions of various substances in the material layers of the inner and outer electrodes are generally the same. This results in a significantly higher current density on the outer side of the electrode assembly compared to the inner side. During secondary battery cycling, the electrolyte is consumed faster on the outer side of the electrode assembly, and black spots or lithium plating are prone to occur on the single-sided coated outer electrode, leading to a decrease in the cycle performance and safety of the secondary battery. Therefore, this application provides a secondary battery that can reduce the risk of black spots and lithium plating on the outer electrode of the electrode assembly, thereby improving the cycle performance of the secondary battery. The specific technical solution is as follows:

[0008] A first aspect of this application provides a secondary battery, including a casing and a stacked electrode assembly. Along the thickness direction of the electrode assembly, the electrode assembly includes a first electrode, a second electrode, and a separator disposed between the first and second electrodes. The electrode assembly also includes a first tab assembly and a second tab assembly. The first tab assembly includes multiple first tabs, and the second tab assembly includes multiple second tabs. Each first tab corresponds to a first electrode, and each second tab corresponds to a second electrode. The first electrode includes two outer first electrodes and at least one inner first electrode. Along the thickness direction of the electrode assembly, the two outer first electrodes are located on the outermost sides of the electrode assembly, and the inner first electrode is located between the two outer first electrodes. The first electrode includes a first current collector and a first active material layer. The first current collector includes a first surface and a second surface disposed opposite each other along the thickness direction of the electrode assembly, with the first surface closer to the casing than the second surface. At least one outer first electrode is a single-sided first electrode. The first surface of the single-sided first electrode does not have a first active material layer, while the second surface has a first active material layer, which is a first material layer. The inner first electrode has a first active material layer on both its first and second surfaces, which is a second material layer. The conductivity of the single-sided first electrode is a S / cm, and the conductivity of the inner first electrode is b S / cm, where a ≤ b. By setting different first active material layers on the outer and inner first electrodes of the electrode assembly, and making the conductivity of the outer first electrode less than or equal to that of the inner first electrode, the current density of the outer electrode of the electrode assembly is reduced. This reduces the electrolyte consumption rate during cycling, lowers the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, and improves the cycle performance of the secondary battery.

[0009] In one embodiment of this application, 0.3 ≤ a / b ≤ 1. In another embodiment of this application, 0.65 ≤ a / b ≤ 1. By adjusting the value of a / b within the above range, it is beneficial to reduce the risk of black spots and lithium plating on the outer electrode sheet of the electrode assembly during cycling, thereby improving the cycle performance of the secondary battery.

[0010] In one embodiment of this application, the first electrode is a positive electrode. This significantly reduces the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, thereby further improving the cycle performance of the secondary battery.

[0011] In one embodiment of this application, the first material layer includes 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 includes a second binder, and the mass percentage of the second binder is d based on the mass of the second material layer, where 0.6 ≤ c / d ≤ 4. By adjusting the value of c / d within the above range, compared with the outer electrode of a conventional electrode assembly, the impedance of the outer electrode of the electrode assembly is increased, thereby slowing down the consumption rate of the electrolyte on the outer side of the electrode assembly during cycling. This helps to reduce the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, thereby improving the cycle performance of the secondary battery.

[0012] In one embodiment of this 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, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, pure acrylic emulsion, or polyacrylic acid. By selecting the first binder and the second binder from the above-mentioned range and controlling the mass percentage of the first binder within the above-mentioned range, it is beneficial to reduce the risk of black spots and lithium plating on the outer electrode sheet of the electrode assembly during cycling, thereby improving the cycle performance of the secondary battery.

[0013] In one embodiment of this application, the first material layer includes a first conductive agent, and the second material layer includes a second conductive agent. The first and second conductive agents are each independently selected from at least one of conductive carbon black, carbon nanotubes, graphene, or conductive graphite. Based on the mass of the first material layer, the mass percentage content of the first conductive agent is 0.5% to 5%, and based on the mass of the second material layer, the mass percentage content of the second conductive agent is 1% to 10%. By selecting the first and second conductive agents within the aforementioned ranges and controlling the mass percentage content of the first and second conductive agents within these ranges, it is beneficial to reduce the risk of black spots and lithium plating on the outer electrode sheet of the electrode assembly during cycling, thereby improving the cycle performance of the secondary battery.

[0014] In one embodiment of this application, the first material layer includes a first active material, and the second material layer includes a second active material. The first and second active materials are each independently selected from at least one of lithium cobalt oxide, lithium iron phosphate, lithium nickel oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, or lithium-rich manganese-based materials. Based on the mass of the first material layer, the mass percentage content of the first active material is 90% to 97.5%, and based on the mass of the second material layer, the mass percentage content of the second active material is 87% to 98.5%. By selecting the first and second active materials within the aforementioned ranges and controlling the mass percentage content of the first and second active materials within these ranges, it is beneficial to reduce the risk of black spots and lithium plating on the outer electrode sheet of the electrode assembly during cycling, thereby improving the cycle performance of the secondary battery.

[0015] In one embodiment of this application, 0.01 ≤ b ≤ 0.8. By adjusting the conductivity of the inner first electrode to within the above range, the current density of the inner electrode of the electrode assembly is relatively large during cycling, which is beneficial to improving the cycle performance of the secondary battery.

[0016] In one embodiment of this application, the thickness of the first current collector on the single-sided first electrode is T1 μm, and the thickness of the first current collector on the inner first electrode is T2 μm, where T1 ≥ T2. In another embodiment of this application, the first electrode is a positive electrode, where 12 ≤ T1 ≤ 25 μm and 6 ≤ T2 ≤ 16 μm. In yet another embodiment of this application, the first electrode is a negative electrode, where 14 ≤ T1 ≤ 25 μm and 4 ≤ T2 ≤ 16 μm. This configuration helps to mitigate the problem of warping caused by uneven stress on the first current collector of the single-sided first electrode due to only having a material layer on one surface. By providing a double-layer material layer on the outer single-sided first electrode of the electrode assembly, and ensuring that the conductivity of the single-sided first electrode is less than or equal to that of the inner first electrode, the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling is significantly reduced, resulting in better cycle performance of the secondary battery.

[0017] In one embodiment of this application, the casing is an aluminum-plastic film. Using an aluminum-plastic film as the casing helps reduce the risk of short circuits in the secondary battery and improves its cycle performance and safety.

[0018] In one embodiment of this application, the secondary battery further includes an electrolyte comprising a lithium salt, which is at least one selected from lithium tetrafluorophosphate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium dioxaborate, lithium bis(fluorosulfonyl)imide, or lithium difluorophosphate. The concentration of the lithium salt is from 1 mol / L to 5 mol / L. By selecting lithium salts within the above-mentioned range and controlling the concentration of the lithium salt within this range, the risk of black spots and lithium plating on the outer electrode plate of the electrode assembly during cycling is reduced. The secondary battery exhibits both high energy density and good cycle performance.

[0019] In one embodiment of this application, the first electrode is a negative electrode, with 0.5 ≤ a / b ≤ 1 and 0.1 ≤ b ≤ 10. When the first electrode is a negative electrode, different first active material layers are formed on the outer and inner first electrodes of the electrode assembly, and the values ​​of a / b and b are controlled within the above ranges. This helps to reduce the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, thereby improving the cycle performance of the secondary battery.

[0020] In one embodiment of this application, the casing is a steel casing. Using a steel casing helps reduce the risk of short circuits in the secondary battery, enhances the battery's rigidity, and improves its cycle performance and safety.

[0021] A second aspect of this application provides an electronic device comprising a secondary battery as described in any of the foregoing embodiments. The secondary battery of this application exhibits excellent cycle performance; therefore, the electronic device of this application has a long service life.

[0022] The beneficial effects of the embodiments of this application are as follows:

[0023] This application provides a secondary battery and electronic device. By adjusting the conductivity of the outermost first electrode plate of a single-sided electrode to be less than or equal to the conductivity of the innermost first electrode plate, the current density of the outermost electrode assembly decreases, and the electrolyte consumption rate slows down during cycling. This effectively reduces the risk of black spots and lithium plating on the outermost electrode plate of the electrode assembly during cycling, thereby improving the cycle performance of the secondary battery.

[0024] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these drawings.

[0026] Figure 1 is a schematic cross-sectional view of the secondary battery along its own length in one embodiment of this application.

[0027] Figure 2 is another cross-sectional view of the secondary battery in Figure 1 along its own length.

[0028] Reference numerals: Secondary battery 001; Electrode assembly 01; Housing 02; First electrode 10; Outer first electrode 101; Inner first electrode 102; First current collector 11; First active material layer 12; First material layer 121; Second material layer 122; Second electrode 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 Implementation

[0029] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0030] It should be noted that, in the specific embodiments of this application, a lithium-ion battery is used as an example of a secondary battery to explain this application; however, the secondary battery in this application is not limited to lithium-ion batteries. The specific technical solution is as follows:

[0031] A first aspect of this application provides a secondary battery, including a casing and a stacked electrode assembly. Along the thickness direction of the electrode assembly, the electrode assembly includes a first electrode, a second electrode, and a separator disposed between the first and second electrodes. The electrode assembly also includes a first tab assembly and a second tab assembly. The first tab assembly includes multiple first tabs, and the second tab assembly includes multiple second tabs. Each first tab corresponds to a first electrode, and each second tab corresponds to a second electrode. The first electrode includes two outer first electrodes and at least one inner first electrode. Along the thickness direction of the electrode assembly, the two outer first electrodes are located on the outermost sides of the electrode assembly, and the inner first electrode is located between the two outer first electrodes. The first electrode includes a first current collector and a first active material layer. The first current collector includes a first surface and a second surface disposed opposite each other along the thickness direction of the electrode assembly, with the first surface closer to the casing than the second surface. At least one outer first electrode is a single-sided first electrode. The first surface of the single-sided first electrode does not have a first active material layer, while the second surface of the single-sided first electrode has a first active material layer. The first active material layer on the second surface of the single-sided first electrode is a first material layer. The first surface and second surface of the inner first electrode both have first active material layers. The first active material layers on the first surface and second surface of the inner first electrode are second material layers. The conductivity of the single-sided first electrode is a S / cm, and the conductivity of the inner first electrode is b S / cm, where a ≤ b.

[0032] In this application, a stacked electrode assembly is defined with its width direction as the X direction, its length direction as the Y direction, and its thickness direction as the Z direction. It is understood that the first electrode, the second electrode, and the separator have the same length, width, and thickness directions as the electrode assembly. For example, as shown in Figures 1 and 2, the secondary battery 001 includes a stacked electrode assembly 01 and a housing 02. Along the thickness direction Z of the electrode assembly 01, the electrode assembly 01 includes a first electrode 10, a second electrode 20 stacked together, and a separator 30 disposed between the first electrode 10 and the second electrode 20. The electrode assembly 01 also includes a first tab assembly 13 and a second tab assembly 23. The first tab assembly 13 includes multiple first tabs 131, and the second tab assembly 23 includes multiple second tabs 231. Each first tab 131 corresponds to a first electrode 10, and each second tab 231 corresponds to a second electrode 20. The first electrode 10 includes two outer first electrode plates 101 and one inner first electrode plate 102. Along the thickness direction Z of the electrode assembly 01, the two outer first electrode plates 101 are located on the outermost sides of the electrode assembly 01, and the inner first electrode plate 102 is located between the two outer first electrode plates 101. The first electrode 10 includes a first current collector 11 and a first active material layer 12. The first current collector 11 includes a first surface (not shown in the figure) and a second surface (not shown in the figure) disposed opposite to each other 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 plates 101 are single-sided first electrode plates. The first surface of the single-sided first electrode plate does not have the first active material layer 12, while the second surface of the single-sided first electrode plate has the first active material layer 12. The first active material layer 12 disposed on the second surface of the single-sided first electrode plate is a first material layer 121. A first active material layer 12 is provided on both the first and second surfaces of the inner first electrode 102. The first active material layer 12 provided on the first and second surfaces of the inner first electrode 102 is a second material layer 122.

[0033] The inventors discovered that coating the outer and inner first electrode plates of an electrode assembly with different first active material layers results in the conductivity of the outer first electrode plate being greater than or equal to that of the inner first electrode plate (A≤B). This reduces the current density on the outer electrode plate, slowing down the electrolyte consumption rate during cycling. This reduces the risk of black spots and lithium plating on the outer electrode plate, improving the cycle performance of the secondary battery. Conversely, when the outermost current density is greater than the inner current density (A>B), the electrolyte consumption rate on the outer electrode plate is too fast, making it prone to black spots or lithium plating, thus reducing the cycle performance and safety of the secondary battery. Therefore, by setting different first active material layers on the outer and inner first electrodes of the electrode assembly, and making the conductivity of the outer first electrode less than or equal to that of the inner first electrode, the current density of the outer electrode of the electrode assembly is reduced, which reduces the electrolyte consumption rate during cycling, lowers the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, and improves the cycle performance of the secondary battery.

[0034] It should be noted that the first active material layer of this application can be disposed in the entire area of ​​the surface of the first current collector, or it can be disposed in a part of the surface of the first current collector. This application has no particular restrictions, as long as the purpose of this application can be achieved.

[0035] In one embodiment of this 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 of any two values ​​therein. By adjusting the value of a / b within the above range, the current density of the outer electrode of the electrode assembly is greater than or equal to the current density of the inner electrode, which slows down the consumption rate of electrolyte during cycling. This helps to reduce the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, thereby improving the cycle performance of the secondary battery.

[0036] In one embodiment of this 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 of any two values ​​therein. By adjusting the value of a / b within the above range, the current density of the outer electrode of the electrode assembly is greater than or equal to the current density of the inner electrode, and the electrolyte consumption rate during cycling is slowed down. This helps to reduce the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, thereby improving the cycle performance of the secondary battery.

[0037] In one embodiment of this application, the first electrode is a positive electrode. When a positive electrode is selected as the first electrode, the outer first electrode of the electrode assembly is a positive electrode. This further slows down the consumption rate of the electrolyte on the outer side of the electrode assembly during cycling, and the effect of reducing the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling is more obvious, thereby further improving the cycle performance of the secondary battery.

[0038] In one embodiment of this application, the first material layer includes 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 includes a second binder, and the mass percentage of the second binder is d based on the mass of the second material layer, where 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 consisting of any two of these values. By adjusting the value of c / d within the above range, the impedance of the outer electrode of the electrode assembly is increased compared to the outer electrode of a conventional electrode assembly. The combined use of the contents of the first and second binders reduces the current density of the outer electrode of the electrode assembly, slows down the consumption rate of the electrolyte on the outer side of the electrode assembly during cycling, and reduces polarization during the conversion of active materials. This helps to reduce the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, thereby improving the cycle performance of the secondary battery.

[0039] In one embodiment of this 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 of any two of these values. The first adhesive and the second adhesive are each independently selected from at least one of polyvinylidene fluoride, polyvinylidene chloride, polyvinylidene chloride, aramid, polyamide, polyacrylonitrile, acrylate polymers, styrene-butadiene rubber, polyacrylate, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, pure acrylic emulsion, or polyacrylic acid. By selecting the first and second binders from the aforementioned range and controlling the mass percentage of the first binder within the aforementioned range, it is beneficial to increase the impedance of the outer electrode of the electrode assembly, reduce the conductivity of the outer electrode of the electrode assembly, thereby reducing the current density of the outer electrode of the electrode assembly, slowing down the consumption rate of the electrolyte on the outer side of the electrode assembly during cycling, and reducing the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, thereby improving the cycle performance of the secondary battery.

[0040] In one embodiment of this application, the first material layer includes a first conductive agent, and the second material layer includes a second conductive agent. The first and second conductive agents are each independently selected from at least one of conductive carbon black, carbon nanotubes, graphene, or conductive graphite. Based on the mass of the first material layer, the mass percentage W1 of the first conductive agent is 0.5% to 5%. For example, the mass percentage 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 consisting of any two of these values. Based on the mass of the second material layer, the mass percentage W2 of the second conductive agent is 1% to 10%. For example, the mass percentage 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 any combination of two of these values. By selecting the first and second conductive agents within the above-mentioned ranges and controlling the mass percentages of the first and second conductive agents within these ranges, the combined use of the first and second conductive agents helps to increase the impedance of the outer electrode plate of the electrode assembly, reduce the conductivity of the outer electrode plate of the electrode assembly, thereby reducing the current density of the outer electrode plate of the electrode assembly, slowing down the consumption rate of the electrolyte on the outer side of the electrode assembly during cycling, and reducing the risk of black spots and lithium plating on the outer electrode plate of the electrode assembly during cycling, thus improving the cycle performance of the secondary battery.

[0041] In one embodiment of this application, the first material layer includes a first active material, and the second material layer includes a second active material. The first active material and the second active material are each independently selected from at least one of lithium cobalt oxide, lithium iron phosphate, lithium nickel oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, or lithium-rich manganese-based materials. Based on the mass of the first material layer, the mass percentage W'1 of the first active material is 90% to 97.5%. For example, the mass percentage 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 consisting of any two of these values. Based on the mass of the second material layer, the mass percentage of the second active material is... The percentage content W'2 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 of any two of these values. By selecting a first active material and a second active material within the aforementioned range, and controlling the mass percentages of the first and second active materials within the aforementioned ranges, the combined use of the first and second active materials increases the impedance of the outer electrode plate of the electrode assembly, thereby reducing the current density of the outer electrode plate and slowing down the consumption rate of the electrolyte on the outer side of the electrode assembly during cycling. This helps to reduce the risk of black spots and lithium plating on the outer electrode plate during cycling, thus improving the cycle performance of the secondary battery. In this application, the nickel-cobalt-manganese ternary material includes, but is not limited to, 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 a binary or ternary layered material of Ni, Co, Mn or Ni, Co, Mn.

[0042] In one embodiment of this 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 of any two of these values. By adjusting the conductivity of the inner first electrode to within the above range, the current density of the outer electrode of the electrode assembly is relatively low during cycling, which is beneficial to improving the cycle performance of the secondary battery.

[0043] In one embodiment of this application, the thickness of the first current collector of the single-sided first electrode is T1 μm, and the thickness of the first current collector of the inner first electrode is T2 μm, where T1 ≥ T2. In another embodiment of this application, the first electrode is a positive electrode, where 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 of any two of these values, and the value of T2 can be 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 or a range of any two of these values. In one embodiment of this application, the first electrode is a negative electrode, where 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 of any two of these values; the value of T2 can be 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, or a range of any two of these values. This configuration helps to improve the problem of uneven stress and warping caused by a single-sided first electrode where only one surface of the first current collector has a material layer. In this case, a double-layer material layer is provided on the outer single-sided first electrode of the electrode assembly, and the conductivity of the single-sided first electrode is less than or equal to the conductivity of the inner first electrode. This significantly reduces the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, resulting in a rechargeable battery with good cycle performance.

[0044] In one embodiment of this application, the casing is an aluminum-plastic film. When an aluminum-plastic film is selected as the casing, and the first electrode on the outer side of the electrode assembly is used as the positive electrode, it helps to reduce the risk of short circuit in the secondary battery and improves the cycle performance and safety performance of the secondary battery.

[0045] In one embodiment of this application, the secondary battery further includes an electrolyte comprising a lithium salt, which is at least one selected from lithium tetrafluorophosphate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium dioxaborate, lithium bis(fluorosulfonyl)imide, or lithium difluorophosphate. The concentration ω of the lithium salt is from 1 mol / L to 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 consisting of any two of these values. By selecting lithium salts within the aforementioned range and adjusting their concentration within that range, ion transport in the electrolyte is facilitated. When used in conjunction with the binder in the first active material layer, it helps to form a stable interface with good interface consistency, reducing the conductivity of the outer electrode of the electrode assembly, thereby lowering the current density of the outer electrode of the electrode assembly, slowing down the consumption rate of the electrolyte on the outer side of the electrode assembly during cycling, and reducing the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, thus improving the cycle performance of the secondary battery.

[0046] In this application, when the first or second electrode is a positive electrode, there are no particular restrictions on the positive current collector, as long as it can achieve the purpose of this application. For example, the positive current collector may include aluminum foil, aluminum alloy foil, or composite current collector (e.g., aluminum-carbon composite current collector).

[0047] In one embodiment of this application, the first electrode is a negative electrode, with 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 any two of these 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 any two of these values. When the first electrode is a negative electrode, different first active material layers are coated on the outer and inner first electrodes of the electrode assembly, and the values ​​of a / b and b are controlled within the above ranges. Compared with the outer electrode of a conventional electrode assembly, this increases the impedance of the outer electrode of the electrode assembly, thereby reducing the current density of the outer electrode of the electrode assembly, slowing down the consumption rate of the electrolyte on the outer side of the electrode assembly during cycling, which helps to reduce the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, and thus improves the cycle performance of the secondary battery.

[0048] In one embodiment of this application, the casing is a steel casing. When a steel casing is selected as the casing, and the first electrode on the outer side of the electrode assembly is used as the negative electrode, it helps to reduce the risk of short circuit in the secondary battery and improves the cycle performance and safety performance of the secondary battery.

[0049] In one embodiment of this application, the first electrode is a negative electrode. 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, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, pure acrylic emulsion, or polyacrylic acid. Based on the mass of the first material layer, the mass percentage content of the first binder is 0.5% to 5%, and based on the mass of the second material layer, the mass percentage content of the second binder is 0.5% to 5%. When the first electrode is a negative electrode, selecting the first binder and the second binder within the above-mentioned ranges, and controlling the mass percentage content of the first binder and the second binder within the above-mentioned ranges, the combined use of the first binder and the second binder increases the impedance of the outer electrode of the electrode assembly, thereby reducing the current density of the outer electrode of the electrode assembly, slowing down the consumption rate of the electrolyte on the outer side of the electrode assembly during cycling, which helps to reduce the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, thereby improving the cycle performance of the secondary battery.

[0050] In one embodiment of this application, the first electrode is a negative electrode. 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 nanotubes (CNTs), or graphene. Based on the mass of the first material layer, the mass percentage content of the first conductive agent is 0.5% to 10%, and based on the mass of the second material layer, the mass percentage content of the second conductive agent is 0.5% to 10%. When the first electrode is a negative electrode, selecting the first and second conductive agents within the above-mentioned ranges and controlling the mass percentage content of the first and second conductive agents within the above-mentioned ranges, the combined use of the first and second conductive agents increases the impedance of the outer electrode of the electrode assembly, thereby reducing the current density of the outer electrode of the electrode assembly, slowing down the consumption rate of the electrolyte on the outer side of the electrode assembly during cycling, which helps to reduce the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, thereby improving the cycle performance of the secondary battery.

[0051] In one embodiment of this application, the first electrode is a negative electrode. 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 materials, titanate materials, or boron nitride materials. Based on the mass of the first material layer, the mass percentage of the first active material is 90% to 99%, and based on the mass of the second material layer, the mass percentage of the second active material is 90% to 99%. When the first electrode is a negative electrode, selecting the first and second active materials within the above-mentioned ranges and controlling the mass percentages of the first and second active materials within these ranges, the combination of the first and second active materials increases the impedance of the outer electrode of the electrode assembly, thereby reducing the current density of the outer electrode of the electrode assembly, slowing down the consumption rate of the electrolyte on the outer side of the electrode assembly during cycling, and helping to reduce the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, thereby improving the cycle performance of the secondary battery.

[0052] In this application, when the first or second electrode is a negative electrode, there are no particular restrictions on the negative current collector, as long as it can achieve the purpose of this application. For example, the negative current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or composite current collectors (such as lithium copper composite current collectors, carbon copper composite current collectors, nickel copper composite current collectors, titanium copper composite current collectors, etc.).

[0053] This application does not impose any particular restrictions on the preparation method of the outer first electrode sheet, as long as it can achieve the purpose of this application. For example, the preparation method of the outer first electrode sheet includes, but is not limited to, the following steps: (1) mixing the first active material, the first binder and the first conductive agent in a certain mass ratio and adding a solvent to prepare a first material layer slurry; (2) coating the first material layer slurry onto one surface of the first current collector, and drying it to obtain a single-sided outer first electrode sheet 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 sheet coated with the first material layer; (4) cold pressing, cutting, and welding the first electrode tab to obtain the outer first electrode sheet.

[0054] In this application, the conductivity of the outer first electrode can be controlled by adjusting the mass percentage of the first binder, the first conductive agent, and the first active material. For example, when other conditions are constant, if the mass percentage of the first active material remains unchanged, and the mass percentage of the first binder increases, the mass percentage of the first conductive agent decreases accordingly, resulting in a decrease in the conductivity of the outer first electrode; conversely, if the mass percentage of the first binder decreases, the mass percentage of the first conductive agent increases accordingly, resulting in an increase in the conductivity of the outer first electrode. In this application, the mass percentages of the first active material, the first binder, and the first conductive agent can be controlled by adjusting their mass ratios. In this application, the conductivity of the outer first electrode is also related to the overall thickness of the outer first electrode and the coating weight of the corresponding material layer. For example, for outer first electrodes of the same specifications, when other conditions are the same, a smaller thickness and a smaller coating weight result in a larger conductivity for the outer first electrode. This application does not impose any particular restrictions on the solvent and solid content of the slurry used in the first material layer in step (1) above, as long as the purpose of this application can be achieved. This application does not impose any particular restrictions on the drying temperature and time in step (2) above; those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved. This application does not impose any particular restrictions on the process parameters for cold pressing, cutting, and welding the first electrode tab in step (4) above; those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved. This application does not impose any particular restrictions on the size of the first material layer above, as long as the purpose of this application can be achieved.

[0055] This application does not impose any particular restrictions on the preparation method of the inner first electrode sheet, as long as it can achieve the purpose of this application. 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 in a certain mass ratio and adding a solvent to prepare a second material layer slurry; (2) coating the second material layer slurry onto one surface of the first current collector, and drying it to obtain a single-sided inner first electrode sheet coated with the 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 the second material layer; (4) cold pressing, cutting, and welding the first electrode tab to obtain the inner first electrode sheet.

[0056] In this application, the conductivity of the inner first electrode can be controlled by adjusting the mass percentages of the second binder, the second conductive agent, and the second active material. For example, when other conditions are constant, if the mass percentage of the second active material remains unchanged, and the mass percentage of the second binder increases, the mass percentage of the second conductive agent decreases accordingly, and the conductivity of the inner first electrode decreases; conversely, if the mass percentage of the second binder decreases, the mass percentage of the second conductive agent increases accordingly, and the conductivity of the inner first electrode increases. In this application, the mass percentages of the second active material, the second binder, and the second conductive agent can be controlled by adjusting their mass ratio. In this application, the conductivity of the inner first electrode is also related to the overall thickness of the inner first electrode and the coating weight of the corresponding material layer. For example, for inner first electrodes of the same specifications, when other conditions are the same, a smaller thickness and a smaller coating weight result in a larger conductivity for the inner first electrode. This application does not impose any particular restrictions on the solvent and solid content of the slurry used in the second material layer in step (1) above, as long as the purpose of this application can be achieved. This application does not impose any particular restrictions on the drying temperature and time in step (2) above; those skilled in the art can select according to actual needs, as long as the purpose of this application can be achieved. This application does not impose any particular restrictions on the process parameters for cold pressing, cutting, and welding the first electrode tab in step (4) above; those skilled in the art can select according to actual needs, as long as the purpose of this application can be achieved. This application does not impose any particular restrictions on the size of the second material layer above, as long as the purpose of this application can be achieved.

[0057] In one embodiment of this application, the electrolyte further includes an organic solvent, which includes at least one selected from ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), propylene carbonate (PC), ethyl propylene (EA), or methyl propionate (MA). When an organic solvent within the above-mentioned range is selected and used in combination with a lithium salt, it is beneficial to reduce the conductivity of the outer electrode of the electrode assembly, thereby reducing the current density of the outer electrode and slowing down the consumption rate of the electrolyte on the outer electrode during cycling. This helps to reduce the risk of black spots and lithium plating on the outer electrode during cycling, thereby improving the cycle performance of the secondary battery.

[0058] The electrolyte of this application also includes other solvents. This application does not particularly limit the other solvents, as long as they achieve the purpose of this application. For example, other solvents may include, but are not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorinated carbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dipropyl carbonate, methyl propyl carbonate, or ethyl propyl carbonate. The aforementioned cyclic carbonates may include, but are not limited to, at least one of butylene carbonate or vinyl ethylene carbonate. Fluorocarbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate, 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, propyl propionate, γ-butyrolactone, decanoic acid lactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0059] This application does not impose any particular limitation on the diaphragm, as long as it achieves the purpose of this application. For example, the diaphragm material may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) mainly composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of diaphragm may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane. The diaphragm of this application may have a porous structure, and this application does not impose any particular limitation on the size of the pores in the porous structure of the diaphragm, as long as it achieves the purpose of this application. For example, the pore size may be from 0.01 μm to 1 μm. This application does not impose any particular limitation on the thickness of the diaphragm, as long as it achieves the purpose of this application. For example, the thickness of the diaphragm may be from 5 μm to 40 μm.

[0060] The secondary battery described in this application is not particularly limited and may include any device in which an electrochemical reaction occurs. In one or more embodiments, the secondary battery may 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.

[0061] The fabrication process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the fabrication process of the secondary battery may include, but is not limited to, the following steps: stacking the outer first electrode, separator, second electrode, separator, inner first electrode, separator, second electrode, separator, and outer first electrode in sequence; then fixing the four corners of the entire stacked structure with tape to obtain the electrode assembly of the stacked structure; placing the electrode assembly into the housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent the internal pressure of the secondary battery from rising and overcharging / discharging.

[0062] A second aspect of this application provides an electronic device comprising a secondary battery as described in any of the foregoing embodiments. The secondary battery of this application exhibits excellent cycle performance; therefore, the electronic device of this application has a long service life.

[0063] The electronic device described in this application is not particularly limited and can be any electronic device known in the prior art. For example, the electronic device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors.

[0064] Example

[0065] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0066] Test methods and equipment:

[0067] Electrode conductivity testing:

[0068] A lithium-ion battery discharged at 0.2C to 3.0V was disassembled. The outer first electrode of the electrode assembly was removed, and the inner first electrode of the electrode assembly was removed according to the arrangement of the electrodes in the assembly. The electrodes were soaked in dimethyl carbonate (DMC) for 20 minutes, and then rinsed with DMC and acetone respectively. The outer and inner first electrodes were then placed in an oven and baked at 80°C for 12 hours to obtain the processed outer and inner first electrode samples.

[0069] The outer and inner first electrodes were tested separately using a resistivity meter (Yuaneng Technology IEST, BER1200). Before use, the resistivity meter was reset for resistance and pressure. The electrode sample to be tested was placed between the electrodes of the meter. Three electrodes with a cross-sectional area of ​​30mm × 30mm were taken at three different locations on the sample for testing. The resistance values ​​of the three electrodes with a cross-sectional area of ​​30mm × 30mm were measured, and the average value was recorded as the resistance value of the electrode under test. The resistance was then calculated according to the following formula:

[0070] R=ρ×l / s; γ=1 / ρ=l / Rs

[0071] Where R is the measured resistance value; ρ is the resistivity; l is the thickness of the measured electrode; s is the cross-sectional area; and γ is the measured electrode conductivity.

[0072] The conductivity 'a' of the outer first electrode and the conductivity 'b' of the inner first electrode were obtained respectively.

[0073] Lithium salt concentration test in electrolyte:

[0074] The lithium-ion battery was discharged to 3V at 0.2C and then disassembled. The electrolyte was collected, and the disassembled first electrode, second electrode, and separator were centrifuged. The liquid obtained after centrifugation was mixed evenly with the electrolyte to obtain a liquid sample. The liquid sample was subjected to ion chromatography (IC) to determine the concentration of lithium salt in the electrolyte.

[0075] Lithium plating performance test:

[0076] The lithium-ion batteries from the examples and comparative examples were placed in a constant temperature chamber at 10°C for 60 minutes. They were then charged at a constant current of 2C to 4.5V, followed by constant voltage charging at 4.5V until the current reached 0.05C. After resting for 5 minutes, they were discharged at a constant current of 0.5C to 3.0V. This constituted one cycle. After 100 cycles of the above charge-discharge process, the batteries were charged again at a constant current of 2C to 4.5V, followed by constant voltage charging at 4.5V until the current reached 0.05C. After resting for 5 minutes, the lithium-ion batteries were disassembled, and the outer electrode of the electrode assembly was removed. The lithium plating state on the surface of the outer electrode was observed. The non-lithium-plated areas on the outer electrode surface were golden yellow, while the lithium-plated areas were grayish-white.

[0077] The criteria for judging the degree of lithium plating in lithium-ion batteries are as follows: 0% lithium plating area is considered no lithium plating, i.e., the degree of lithium plating is zero; lithium plating area greater than 0 and less than or equal to 2% is considered mild lithium plating; lithium plating area greater than 2% and less than or equal to 20% is considered moderate lithium plating; and lithium plating area greater than 20% and less than or equal to 100% is considered severe lithium plating. The percentage of lithium plating area is calculated based on the total area of ​​the first active material layer of the outer first electrode.

[0078] Dark spot test:

[0079] The lithium-ion battery was placed in a 0℃ constant temperature chamber and left to stand for 60 minutes to allow it to reach a constant temperature. The battery was then charged at a constant current of 1C at 0℃ to its full charge voltage of 4.5V, followed by constant voltage charging at 4.5V to 0.025C. After standing for 5 minutes, it was discharged at a constant current of 1C to 3.0V. This constituted one charge-discharge cycle. After 500 cycles, the battery was charged again at a constant current of 1C to its full charge voltage of 4.5V, and then charged at a constant voltage of 4.5V to 0.025C, resulting in a fully charged battery with 500 cycles. The lithium-ion battery was disassembled in a dry room with humidity less than 5%, and photographs were taken to record whether any black spots appeared on the surface of the outer electrode.

[0080] The criteria for judging the degree of black spots in lithium-ion batteries are as follows: 0% black spot area means no black spot appears, black spot area less than or equal to 2% means mild black spot, black spot area between 2% and 20% means moderate black spot, and black spot area greater than 20% means severe black spot. The percentage of black spot area is calculated based on the total area of ​​the first active material layer of the outer first electrode.

[0081] Cyclic performance test:

[0082] The lithium-ion battery was placed in a 25°C constant-temperature test chamber and allowed to stand for 30 minutes to reach a constant temperature of 25°C. It was then charged at a constant current of 1C to 4.5V, followed by constant voltage charging at 4.5V to a current of 0.025C. After standing for 5 minutes, it was discharged at a constant current of 0.2C to 3.0V. This was the first cycle, and the initial discharge capacity was recorded as C0. The lithium-ion battery was cycled according to the above process. The test was stopped after 400 cycles (cls), and the discharge capacity after 400 cycles (cls) was recorded as C1. The capacity retention rate at 400cls was calculated as an indicator for evaluating the cycle performance of the lithium-ion battery.

[0083] 400cls capacity retention rate (%) = C1 / C0 × 100%.

[0084] A higher capacity retention rate (400cls) indicates better cycle performance of the lithium-ion battery.

[0085] Example 1

[0086] <Preparation of the first electrode>

[0087] A positive electrode sheet was selected as the first electrode sheet. The first active material, lithium cobalt oxide, 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-methylpyrrolidone (NMP) was added. The mixture was stirred evenly under vacuum to obtain a first material layer slurry with a solid content of 70 wt%. This first material layer slurry was coated onto one surface of a 14 μm thick positive electrode current collector aluminum foil. After baking at 120°C for 1 hour, a positive electrode sheet with the first material layer coated on one side (i.e., a single-sided outer positive electrode sheet) was obtained. This outer positive electrode sheet, with dimensions of 50 mm × 90 mm, was then cold-pressed, cut, and welded with positive electrode tabs. The coating weight of the first material layer was 16 mg / mm². 2 The thickness of the first material layer is 50 μm. Based on the mass of the first material layer, the mass percentage of the first adhesive c is 3%, the mass percentage of the first conductive agent W1 is 0.5%, and the mass percentage of the first active material W'1 is 96.5%.

[0088] Lithium cobalt oxide (the second active material), polyvinylidene fluoride (the second binder), and conductive carbon black (the second conductive agent) were mixed in a mass ratio of 97.5:1.5:1, and N-methylpyrrolidone (NMP) was added. The mixture was stirred evenly under vacuum to obtain a second material layer slurry with a solid content of 70 wt%. This second material layer slurry was coated onto one surface of a 14 μm thick aluminum foil used as a positive electrode current collector. After baking at 120°C for 1 hour, a single-sided coated positive electrode sheet was obtained. The above steps were repeated on the other surface of the aluminum foil to obtain a double-sided coated positive electrode sheet. After drying under vacuum at 120°C for 1 hour, the sheet was cold-pressed, cut, and welded with positive electrode tabs to obtain an inner positive electrode sheet with dimensions of 50 mm × 90 mm. The coating weight of the second material layer was 16 mg / mm². 2 The thickness of the double-sided second material layer is 100 μm. Based on the mass of the second material layer, the mass percentage of the second adhesive d is 1.5%, the mass percentage of the second conductive agent W2 is 1%, and the mass percentage of the second active material W'2 is 97.5%.

[0089] <Preparation of the Second Electrode>

[0090] A negative electrode sheet was selected as the second electrode sheet. The negative electrode active material, artificial graphite, conductive carbon black, and the 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 evenly under vacuum to obtain a negative electrode slurry with a solid content of 65 wt%. The negative electrode slurry was uniformly coated onto one surface of a 12 μm thick copper foil current collector and baked at 120°C for 1 hour to obtain a negative electrode sheet with a single-sided negative electrode material layer. The above steps were repeated on the other surface of the copper foil current collector to obtain a negative electrode sheet with a double-sided negative electrode material layer. After drying under vacuum at 120°C for 1 hour, the sheet was cold-pressed, cut, and welded with negative electrode tabs to obtain a negative electrode sheet with a specification of 50 mm × 90 mm. The coating weight of the negative electrode material layer was 11 mg / mm². 2 The thickness of the double-sided negative electrode material layer is 110 μm.

[0091] <Preparation of Electrolyte>

[0092] 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 hexafluorophosphate (LiPF6) was dissolved in the organic solvent and mixed thoroughly to obtain an electrolyte. The concentration ω of the lithium salt LiPF6 was 3 mol / L.

[0093] <Septum>

[0094] A polyethylene (PE) membrane with a thickness of 7 μm was used as the separator.

[0095] <Preparation of Lithium-ion Batteries>

[0096] The outer positive electrode, separator, negative electrode, separator, inner positive electrode, separator, negative electrode, separator, and outer positive electrode are stacked sequentially. The four corners of the stacked structure are then secured with tape to obtain the electrode assembly. The electrode assembly is placed inside an aluminum-plastic film and dried in an 80°C vacuum oven for 12 hours to remove moisture. The prepared electrolyte is then injected, and the lithium-ion battery is obtained through vacuum sealing, settling, formation, degassing, and edge trimming. The designed potential range for the lithium-ion battery is 3.0V to 4.5V.

[0097] Examples 2 to 18

[0098] Except for adjusting the relevant preparation parameters according to Table 1, everything else is the same as in Example 1. When the lithium salt concentration in the electrolyte changes, the concentration of the organic solvent changes accordingly, while the mass ratio of ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) remains constant.

[0099] Example 19

[0100] Except for preparing the first electrode, the second electrode, and the lithium-ion battery according to the following steps, and adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1.

[0101] <Preparation of the first electrode>

[0102] A 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 nanotubes (CNTs), were mixed in a mass ratio of 96.5:3:0.5 and added to deionized water. The mixture was stirred evenly under vacuum to obtain a first material layer slurry with a solid content of 65 wt%. This first material layer slurry was coated onto one surface of a 12 μm thick copper foil used as a negative electrode current collector. After baking at 120°C for 1 hour, a negative electrode sheet with the first material layer coated on one side (i.e., a single-sided outer negative electrode sheet) was obtained. Through cold pressing, cutting, and welding of negative electrode tabs, an outer negative electrode sheet with a specification of 50 mm × 90 mm was obtained. The coating weight of the first material layer was 12 mg / mm². 2 The thickness of the first material layer is 60 μm. Based on the mass of the first material layer, the mass percentage of the first adhesive c is 3%, the mass percentage of the first conductive agent W1 is 0.5%, and the mass percentage of the first active material W'1 is 96.5%.

[0103] The second active material, artificial graphite, the second binder, styrene-butadiene rubber (SBR), and the second conductive agent, carbon nanotubes (CNTs), were mixed in a mass ratio of 96.5:1.5:2 and then added to deionized water. The mixture was stirred evenly under vacuum to obtain a second material layer slurry with a solid content of 65 wt%. This second material layer slurry was coated onto one surface of a 12 μm thick copper foil used as a negative electrode current collector. After baking at 120°C for 1 hour, a negative electrode sheet with a single-sided second material layer was obtained. The above steps were repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided second material layer. After drying under vacuum at 120°C for 1 hour, the sheet was cold-pressed, cut, and welded with negative electrode tabs to obtain an inner negative electrode sheet with dimensions of 50 mm × 90 mm. The coating weight of the second material layer was 12 mg / mm². 2 The thickness of the double-sided second material layer is 120 μm. Based on the mass of the second material layer, the mass percentage of the second adhesive d is 1.5%, the mass percentage of the second conductive agent W2 is 2%, and the mass percentage of the second active material W'2 is 96.5%.

[0104] <Preparation of the Second Electrode>

[0105] A positive electrode sheet was selected as the second electrode sheet. Lithium cobalt oxide (positive electrode active material), polyvinylidene fluoride (PVDF) binder, and conductive carbon black (conductive agent) were mixed in a mass ratio of 92.5:1.5:6. N-methylpyrrolidone (NMP) was added, and the mixture was stirred evenly under vacuum to obtain a positive electrode slurry with a solid content of 70 wt%. The positive electrode slurry was uniformly coated onto one surface of a 14 μm thick aluminum foil current collector and baked at 120°C for 1 hour to obtain a positive electrode sheet with a single-sided coating of positive electrode material. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After drying under vacuum at 120°C for 1 hour, the sheet was cold-pressed, cut, and welded with positive electrode tabs to obtain a positive electrode sheet with a specification of 60 mm × 90 mm. The coating weight of the positive electrode material layer was 18 mg / mm². 2 The thickness of the double-sided cathode material layer is 108 μm.

[0106] <Preparation of Lithium-ion Batteries>

[0107] The outer negative electrode, separator, positive electrode, separator, inner negative electrode, separator, positive electrode, separator, and outer negative electrode are stacked sequentially. The four corners of the stacked structure are then secured with tape to obtain the stacked electrode assembly. The electrode assembly is placed inside a steel shell and dried in an 80°C vacuum oven for 12 hours to remove moisture. The prepared electrolyte is then injected, and the lithium-ion battery is obtained through vacuum sealing, settling, formation, degassing, and edge trimming. The designed potential range for the lithium-ion battery is 3.0V to 4.5V.

[0108] Examples 20 to 21

[0109] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 19.

[0110] Comparative Examples 1 to 3

[0111] Except for adjusting the relevant preparation parameters according to Table 1, everything else is the same as in Example 1.

[0112] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Table 1 and Table 2.

[0113] Table 2

[0114] As can be seen from Examples 1 to 21 and Comparative Examples 1 to 3, by adjusting the conductivity of the single-sided first electrode to be less than or equal to the conductivity of the inner first electrode, the black spot area and lithium plating area of ​​the outer electrode of the electrode assembly are smaller, and the 400cls capacity retention rate of the lithium-ion battery is improved. This indicates that the lithium-ion battery of this application can reduce the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling, and the lithium-ion battery has good cycle performance. The parameters of the first and second material layers in Comparative Example 1 are exactly the opposite of those in Example 1; the conductivity of the single-sided first electrode in Comparative Examples 1 to 3 is greater than that of the inner first electrode; the black spot area of ​​the lithium-ion battery in Comparative Examples 1 to 3 is larger; the lithium plating area is larger, and the 400cls capacity retention rate is lower. In the lithium-ion batteries of Examples 1 to 21, the area of ​​black spots and lithium plating on the outer first electrode is small, and the capacity retention rate of 400cls is high, indicating that the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling is low, and the lithium-ion battery has good cycling performance.

[0115] The c / d value typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 3, by adjusting the c / d value within the scope of this application, the area of ​​black spots and lithium plating on the outer first electrode of the lithium-ion battery is smaller, and the 400cls capacity retention rate is higher. This indicates that the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling is low, and the lithium-ion battery exhibits good cycle performance.

[0116] The type and content of the first binder, and the type and content of the second binder, typically affect the cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 14, when the first and second binders within the scope of this application are used, and the contents of the first and second binders are controlled within the scope of this application, the black spot area and lithium plating area of ​​the outer first electrode in the lithium-ion battery are small, and the 400cls capacity retention rate is high. This indicates that the risk of black spots and lithium plating forming on the outer electrode of the electrode assembly during cycling is low, and the lithium-ion battery exhibits good cycle performance.

[0117] The type and content of the first conductive agent, and the type and content of the second conductive agent, typically affect the cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 14, when the first and second conductive agents within the scope of this application are used, and the contents of the first and second conductive agents are controlled within the scope of this application, the black spot area and lithium plating area of ​​the outer first electrode in the lithium-ion battery are small, and the 400cls capacity retention rate is high. This indicates that the risk of black spots and lithium plating forming on the outer electrode of the electrode assembly during cycling is low, and the lithium-ion battery exhibits good cycle performance.

[0118] The type and content of the first active material, and the type and content of the second active material, typically affect the cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 14, when the first and second active materials within the scope of this application are used, and the content of the first and second active materials is controlled within the scope of this application, the black spot area and lithium plating area of ​​the outer first electrode in the lithium-ion battery are small, and the 400cls capacity retention rate is high. This indicates that the risk of black spots and lithium plating forming on the outer electrode of the electrode assembly during cycling is low, and the lithium-ion battery exhibits good cycle performance.

[0119] The conductivity 'b' of the inner first electrode typically affects the cycle performance of a lithium-ion battery. As can be seen from Examples 1 to 18, by adjusting the value of 'b' within the range of this application, the black spot area and lithium plating area of ​​the outer first electrode in the lithium-ion battery are smaller, and the 400cls capacity retention rate is higher. This indicates that the risk of black spots and lithium plating forming on the outer electrode of the electrode assembly during cycling is low, and the lithium-ion battery exhibits good cycle performance.

[0120] The type and concentration of lithium salts typically affect the cycle performance of lithium-ion batteries. As can be seen from Examples 6, 15 to 18, when lithium salts within the scope of this application are used and their concentrations are controlled within this scope, the black spot area and lithium plating area of ​​the outer first electrode in the lithium-ion battery are small, and the 400cls capacity retention rate is high. This indicates that the risk of black spots and lithium plating forming on the outer electrode of the electrode assembly during cycling is low, and the lithium-ion battery exhibits good cycle performance.

[0121] The type of the first electrode typically affects the cycle performance of a lithium-ion battery. As can be seen from Examples 1 and 19, when the first electrode is either a positive or negative electrode, the area of ​​black spots and lithium plating on the outer first electrode of the lithium-ion battery is smaller, and the 400cls capacity retention rate is higher. This indicates that the risk of black spots and lithium plating on the outer electrode of the electrode assembly during cycling is low, and the lithium-ion battery has good cycle performance.

[0122] When the first electrode is a negative electrode, the values ​​of a / b and b typically affect the cycle performance of the lithium-ion battery. As can be seen from Examples 19 to 21, by adjusting the values ​​of a / b and b within the range specified in this application, the black spot area and lithium plating area of ​​the outer first electrode in the lithium-ion battery are smaller, and the 400cls capacity retention rate is higher. This indicates that the risk of black spots and lithium plating forming on the outer electrode of the electrode assembly during cycling is low, and the lithium-ion battery exhibits good cycle performance.

[0123] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0124] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0125] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A secondary battery, comprising a casing and an electrode assembly with a stacked structure, wherein along the thickness direction of the electrode assembly, the electrode assembly includes a first electrode, a second electrode, and a separator disposed between the first electrode and the second electrode; the electrode assembly further includes a first tab assembly and a second tab assembly, wherein the first tab assembly includes a plurality of first tabs, the second tab assembly includes a plurality of second tabs, the plurality of first tabs corresponding one-to-one with the first electrode, and the plurality of second tabs corresponding one-to-one with the second electrode; The first electrode includes two outer first electrodes and at least one inner first electrode. Along the thickness direction of the electrode assembly, the two outer first electrodes are located on the outermost two sides of the electrode assembly, and the inner first electrode is located between the two outer first electrodes. The first electrode includes a first current collector and a first active material layer. The first current collector includes a first surface and a second surface disposed opposite to each other along the thickness direction of the electrode assembly. The first surface is closer to the housing than the second surface. At least one of the outer first electrodes is a single-sided first electrode, the first surface of the single-sided first electrode is not provided with the first active material layer, the second surface of the single-sided first electrode is provided with the first active material layer, and the first active material layer provided on the second surface of the single-sided first electrode is a first material layer. The first active material layer is provided on both the first surface and the second surface of the inner first electrode, and the first active material layer provided on the first surface and the second surface of the inner first electrode is a second material layer. The conductivity of the single-sided first electrode is a S / cm, and the conductivity of the inner first electrode is b S / cm, where a≤b.

2. The secondary battery according to claim 1, wherein, 0.3≤a / b≤1.

3. The secondary battery according to claim 2, wherein, 0.65≤a / b≤1.

4. The secondary battery according to any one of claims 1 to 3, wherein, The first electrode is a positive electrode.

5. The secondary battery according to claim 4, wherein, The first material layer includes a first adhesive, and the mass percentage of the first adhesive is c based on the mass of the first material layer; the second material layer includes a second adhesive, and the mass percentage of the second adhesive is d based on the mass of the second material layer, where 0.6 ≤ c / d ≤ 4.

6. The secondary battery according to claim 5, wherein, 2% ≤ c ≤ 5%; the first adhesive and the second adhesive are each independently selected from at least one of polyvinylidene fluoride, polyvinylidene chloride, polyvinylidene chloride, aramid, polyamide, polyacrylonitrile, acrylate polymer, styrene-butadiene rubber, polyacrylate, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, pure acrylic emulsion or polyacrylic acid.

7. The secondary battery according to claim 4, wherein, The first material layer includes a first conductive agent, and the second material layer includes 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 nanotubes, graphene, or conductive graphite. Based on the mass of the first material layer, the mass percentage of the first conductive agent is 0.5% to 5%, and based on the mass of the second material layer, the mass percentage of the second conductive agent is 1% to 10%.

8. The secondary battery according to claim 4, wherein, The first material layer includes a first active material, and the second material layer includes a second active material. The first active material and the second active material are each independently selected from at least one of lithium cobalt oxide, lithium iron phosphate, lithium nickel oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, or lithium-rich manganese-based materials. Based on the mass of the first material layer, the mass percentage of the first active material is 90% to 97.5%, and based on the mass of the second material layer, the mass percentage of the second active material is 87% to 98.5%.

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 electrode is T1 μm, and the thickness of the first current collector of the inner first electrode is T2 μm, where T1 ≥ T2.

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

12. The secondary battery according to claim 10, wherein, The first electrode is the negative electrode, 14≤T1≤25, 4≤T2≤16.

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

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

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

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

17. An electronic device comprising a secondary battery as claimed in any one of claims 1 to 16.

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