Secondary battery and electronic device

Abstract

The present application relates to a secondary battery and an electronic device. The secondary battery comprises an electrode assembly, wherein the electrode assembly comprises a first electrode sheet, the first electrode sheet is a negative electrode sheet, one portion of the first electrode sheet comprises a second current collector and a first active layer, the other portion of the first electrode sheet comprises a first current collector and a second active layer, and a thickness of the first current collector in a first direction is greater than a thickness of the second current collector in the first direction; the first electrode sheet is a negative electrode sheet, the second current collector comprises a first metal layer, and the first metal layer comprises copper; and the first current collector comprises a second metal layer and a third metal layer stacked in the first direction, wherein the second metal layer comprises copper, and a resistivity of the third metal layer is greater than a resistivity of the second metal layer. The secondary battery and the electronic device of the present application can improve the problem of secondary battery capacity loss.

Description

Secondary batteries and electronic devices

[0001] Cross-reference to related applications

[0002] This application claims priority to Chinese Patent Application No. 202510037613.0, filed on January 9, 2025, entitled "Secondary Battery and Electronic Device", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of battery technology, and in particular to a secondary battery and electronic device. Background Technology

[0004] Secondary batteries, as the power source for electronic devices, are crucial for ensuring their normal operation. The outermost electrode of steel-cased secondary batteries is prone to black spots, leading to a loss of battery capacity. Summary of the Invention

[0005] The inventors of this application have discovered that secondary batteries typically include a casing and an electrode assembly housed within the casing. The electrode assembly includes an electrode sheet and a separator. The electrode sheet typically uses a metal layer as a current collector. To reduce the possibility of the outermost electrode sheet curling, the metal layer of the outermost electrode sheet is usually thicker than that of the middle electrode sheet. Since a thicker metal layer results in faster conductivity and faster electrolyte consumption, the outermost electrode sheet consumes electrolyte faster than the middle electrode sheet. This leads to black spots easily appearing on the interface of the outermost electrode sheet in the later stages of secondary battery cycling, thereby reducing the capacity of the secondary battery.

[0006] The purpose of this application is to provide a secondary battery and electronic device that aims to improve the problem of capacity loss in secondary batteries.

[0007] According to a first aspect of this application, a secondary battery is provided, including an electrode assembly. The electrode assembly includes a first electrode, a separator, and a second electrode stacked sequentially along a first direction. The first electrode and the second electrode have opposite polarities, and the first direction is the thickness direction of the electrode assembly. A portion of the first electrode includes a second current collector and a first active layer, while another portion of the first electrode includes a first current collector and a second active layer. The thickness of the first current collector along the first direction is greater than the thickness of the second current collector along the first direction. The first electrode is a negative electrode. The second current collector includes a first metal layer comprising copper. The first current collector includes a second metal layer and a third metal layer stacked along the first direction. The second metal layer comprises copper, and the resistivity of the third metal layer is greater than the resistivity of the second metal layer.

[0008] In the above technical solution, the thickness of the first current collector along the first direction is greater than that of the second current collector along the first direction, resulting in a higher conductivity rate for the first current collector compared to the second current collector. Consequently, the first electrode corresponding to the first current collector consumes electrolyte at a faster rate than the first electrode corresponding to the second current collector. This makes the interface of the first electrode corresponding to the first current collector more prone to black spots during the later stages of secondary battery cycling. This application configures the first current collector to include a stacked second metal layer and a third metal layer. The resistivity of the third metal layer is greater than that of the second metal layer. By including the third metal layer, the resistivity of the first current collector can be increased, thereby reducing its conductivity rate and the rate at which the first electrode corresponding to the first current collector consumes electrolyte. This, in turn, reduces the likelihood of black spots appearing at the interface of the first electrode corresponding to the first current collector. The second metal layer includes copper, which reduces the possibility of electrochemical corrosion between the second metal layer and the steel casing.

[0009] In some preferred embodiments, a portion of the first electrode is a first double-sided electrode, and a first active layer is disposed on both surfaces of the first metal layer along the first direction; another portion of the first electrode is a first single-sided electrode, and a second active layer is disposed on the surface of the first current collector facing the second electrode.

[0010] In some preferred embodiments, a first double-sided electrode is disposed between adjacent second electrodes; the outermost electrode of the electrode assembly along the first direction is a first single-sided electrode. The outermost electrode of the electrode assembly along the first direction is a first single-sided electrode, and a second active layer is disposed on the surface of the first current collector facing the second electrode. Compared to disposing second active layers on both opposite surfaces of the first current collector, this improves the utilization rate of the active layer of the first single-sided electrode and increases the energy density of the secondary battery. To reduce electrode warping caused by stress on the outermost electrode of the electrode assembly, the first current collector is typically thicker. The first electrode located in the inner layer of the electrode assembly is situated between two layers of second electrodes, and this portion of the first electrode has a first active layer disposed on both surfaces of the first metal layer.

[0011] In some preferred embodiments, the first metal layer is copper foil.

[0012] In some preferred embodiments, the resistivity of the first current collector is ≥1.75×10⁻⁶. -8 Ω·m can further reduce the conductivity velocity of the first current collector.

[0013] In some preferred embodiments, the third metal layer comprises nickel, and the third metal layer is disposed on at least one surface of the second metal layer along the first direction. The thickness of the third metal layer is 0.2 μm to 3 μm. The third metal layer comprises nickel, whose resistivity is greater than that of copper. The resistivity of the third metal layer can be greater than that of the second metal layer, allowing the resistivity of the first current collector to be greater than that of the second metal layer. When the thickness of the third metal layer is less than 0.2 μm, the increase in resistivity of the first current collector is not significant, and the third metal layer cannot provide sufficient protection, making it prone to cavitation on its surface. When the thickness of the third metal layer is greater than 3 μm, it increases the manufacturing cost of the secondary battery and also occupies internal space, affecting the battery's energy density. Here, the thickness of the third metal layer refers to the thickness of any one of the third metal layers.

[0014] In some preferred embodiments, the thickness of the third metal layer is 0.7 μm to 2.1 μm, which can further improve the resistivity of the first current collector, while reducing the manufacturing cost of the secondary battery and reducing the space occupied by the first current collector.

[0015] In some preferred embodiments, the thickness of the third metal layer is 1.3 μm to 2.1 μm, which can further improve the resistivity of the first current collector, while reducing the manufacturing cost of the secondary battery and reducing the space occupied by the first current collector.

[0016] In some preferred embodiments, a third metal layer is disposed on one surface of the second metal layer along the first direction, and the thickness of the second metal layer is 15 μm to 20 μm. When the third metal layer is a single layer, if the thickness of the second metal layer is less than 15 μm, the second metal layer is unlikely to provide support for the third metal layer; if the thickness of the second metal layer is greater than 20 μm, the second metal layer will lose a significant amount of energy density from the secondary battery.

[0017] In some preferred embodiments, the third metal layer is located on the surface of the second metal layer facing the second active layer. Compared with the third metal layer being located on the surface of the second metal layer away from the second active layer, the third metal layer can block the second metal layer from the second electrode, which can further reduce the conductivity of the first current collector.

[0018] In some preferred embodiments, a third metal layer is disposed on both opposite surfaces of the second metal layer along the first direction, and the thickness of the second metal layer is 13 μm to 18 μm. The presence of a third metal layer on both opposite surfaces of the second metal layer along the first direction can further reduce the conductivity velocity of the first current collector. When there are two third metal layers, the third metal layer can also provide support, and the thickness of the second metal layer can be further reduced. If the thickness of the second metal layer is less than 13 μm, it is difficult for the second metal layer to provide support for the third metal layer; if the thickness of the second metal layer is greater than 18 μm, the second metal layer will lose a significant amount of energy density from the secondary battery.

[0019] In some preferred embodiments, the second metal layer is a copper foil, the third metal layer is a nickel layer, and the third metal layer is attached to the surface of the second metal layer. The second metal layer can serve as a base layer, and the third metal layer can be attached to the surface of the second metal layer.

[0020] In some preferred embodiments, the third metal layer is disposed on the surface of the second metal layer by any one of electroplating, vapor deposition or vapor deposition.

[0021] In some preferred embodiments, the nickel content in the third metal layer is 99% to 99.8% by mass. A nickel content below 99% by mass in the third metal layer reduces its resistivity. Due to limitations in processing precision, the nickel content in the third metal layer is unlikely to exceed 99.8% by mass.

[0022] In some preferred embodiments, the electrode assembly further includes a first tab connected to a first current collector, the first tab including a second metal layer. To enable connection between the internal and external circuits of the battery, the electrode assembly further includes a first tab connected to the first current collector, the first tab being made of the same material as the second metal layer.

[0023] In some preferred embodiments, the first electrode tab is integrally disposed with the first current collector, and the first electrode tab includes a stacked second metal layer and a third metal layer. The first current collector includes a region coated with the second active layer and a region not coated with the second active layer. The first electrode tab integrally disposed with the first current collector can be obtained by laser cutting or die-cutting the region of the first current collector not coated with the second active layer, wherein the material of the first electrode tab is the same as that of the first current collector, both including the stacked second metal layer and the third metal layer. Obtaining the first electrode tab in the above manner can simplify the manufacturing process of the first electrode tab.

[0024] Secondly, this application also proposes an electronic device including a secondary battery as described in any of the embodiments of the first aspect above.

[0025] Additional aspects and advantages of the embodiments of this application will be described, shown, or illustrated in part by way of implementation of the embodiments of this application in the following description. Attached Figure Description

[0026] One or more embodiments are illustrated by way of example with reference to the accompanying drawings. These illustrations do not constitute a limitation on the embodiments. Elements having the same reference numerals in the drawings are denoted as similar elements. Unless otherwise stated, the dimensions in the drawings do not constitute a limitation on scale.

[0027] Figure 1 is a schematic diagram of the structure of a secondary battery according to some embodiments of this application;

[0028] Figure 2 is a schematic diagram of the structure of an electrode assembly according to some embodiments of this application;

[0029] Figure 3 is a schematic diagram of the structure of the first current collector according to some embodiments of this application;

[0030] Figure 4 is a schematic diagram of the structure of the first single-sided electrode sheet in some embodiments of this application;

[0031] Figure 5 is a schematic diagram of the structure of the first current collector according to some embodiments of this application.

[0032] Explanation of reference numerals in the attached figures:

[0033] 100. Secondary battery; 10. Casing; 20. Electrode assembly; 21. First electrode; 211. First double-sided electrode; 2111. First metal layer; 2112. First active layer; 212. First single-sided electrode; 2121. First current collector; 2123. Second metal layer; 2124. Third metal layer; 2122. Second active layer; 22. Second electrode; 221. Third current collector; 222. Third active layer; 23. Separator; X. First direction. Embodiments of the present invention

[0034] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of this application, but not all embodiments.

[0035] In this application, the reference to "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment that is mutually exclusive with other embodiments.

[0036] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0037] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0038] The term "perpendicular" is used to describe an ideal state between two components. In actual production or use, two components can exist in a state that is approximately perpendicular. For example, in numerical terms, perpendicularity can refer to the angle between two straight lines within the range of 90 ± 10°, the dihedral angle between two planes within the range of 90 ± 10°, or the angle between a straight line and a plane within the range of 90 ± 10°. The two components described as "perpendicular" do not have to be absolutely straight lines or planes; they can be approximately straight lines or planes. From a macroscopic perspective, if the overall direction of extension is straight or plane, the component can be considered a "straight line" or "plane".

[0039] The first direction X of this application is a bidirectional direction, that is, the first direction X includes the direction shown by the arrow in the figure and its opposite direction.

[0040] The technical features involved in the different embodiments of this application described below can be combined with each other as long as they do not conflict with each other.

[0041] In a first aspect, embodiments of this application provide a secondary battery 100. Referring to FIG1, the secondary battery 100 includes a housing 10 and an electrode assembly 20. The housing 10 can accommodate the electrode assembly 20 and an electrolyte (not shown in the figure). The electrolyte wets the electrode assembly 20 within the housing 10.

[0042] Referring to Figure 2, which illustrates the stacked structure of the electrode assembly 20, the electrode assembly 20 includes a first electrode 21, a diaphragm 23, and a second electrode 22. The first electrode 21 and the second electrode 22 have opposite polarities. A diaphragm 23 is disposed between adjacent second electrode 22 and first electrode 21. The first electrode 21, diaphragm 23, and second electrode 22 are stacked. In the embodiments of this application, the electrode assembly 20 is described as a stacked structure. In other embodiments, the electrode assembly 20 may also be a wound structure. For example, the first electrode 21, diaphragm 23, and second electrode 22 are sequentially stacked and wound to form a wound electrode assembly 20.

[0043] In some embodiments, the first electrode 21 includes a first double-sided electrode 211, and the first double-sided electrode 211 is disposed between adjacent second electrodes 22. The first double-sided electrode 211 includes a second current collector and a first active layer 2112. The second current collector includes a first metal layer 2111, and the first metal layer 2111 has a first active layer 2112 disposed on both surfaces along the first direction X.

[0044] In some embodiments, the first metal layer 2111 may be a copper foil.

[0045] In some embodiments, the first active layer 2112 is immersed in the electrolyte within the housing 10 to undergo an electrochemical reaction. The first active layer 2112 includes a first active material, a conductive agent, an adhesive, etc., and the above materials are mixed and stirred evenly and coated on both surfaces of the first metal layer 2111 along the first direction X, thereby obtaining the first active layer 2112. The first active material may include at least one of graphite, silicon, hard carbon, and carbon fiber.

[0046] In some embodiments, the second electrode 22 includes a third current collector 221 and a third active layer 222, wherein the third current collector 221 has the third active layer 222 disposed on at least one surface along the first direction X.

[0047] In some embodiments, the third current collector 221 may be an aluminum foil.

[0048] In some embodiments, the third active layer 222 is immersed in the electrolyte within the housing 10 to undergo an electrochemical reaction. The third active layer 222 includes a third active material, a conductive agent, an adhesive, etc., and the above materials are mixed and stirred evenly and coated onto at least one surface of the third current collector 221 along the first direction X, thereby obtaining the third active layer 222. The third active material may include at least one of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt aluminum oxide, lithium manganese oxide, and lithium manganese iron phosphate.

[0049] In some embodiments, the first electrode 21 further includes a first single-sided electrode 212. The outermost electrode of the electrode assembly 20 along the first direction X is the first single-sided electrode 212. The first single-sided electrode 212 includes a first current collector 2121 and a second active layer 2122. The surface of the first current collector 2121 facing the second electrode 22 is provided with the second active layer 2122. Compared with the first current collector 2121 having the second active layer 2122 on both opposite surfaces, the utilization rate of the active layer of the first single-sided electrode 212 can be improved, and the energy density of the secondary battery 100 can be improved. The first current collector 2121 is usually a metal layer. In order to reduce the possibility of the first single-sided electrode 212 curling, the first current collector 2121 is usually thicker than the first metal layer 2111. Since the thicker the metal layer, the faster the conductivity and the faster the electrolyte is consumed, the first single-sided electrode 212 consumes electrolyte faster than the first double-sided electrode 211. This causes black spots to easily appear on the interface of the first single-sided electrode 212 in the later stages of the cycle of the secondary battery 100, thereby losing the capacity of the secondary battery 100.

[0050] To improve the above problems, please refer to Figures 2 and 3. In the embodiments of this application, the first current collector 2121 includes a stacked second metal layer 2123 and a third metal layer 2124. The second metal layer 2123 includes copper, and the resistivity of the third metal layer 2124 is greater than that of the second metal layer 2123. The inclusion of copper in the second metal layer 2123 can reduce the possibility of electrochemical corrosion between the second metal layer 2123 and the steel shell 10. The first current collector 2121 is usually a thicker second metal layer 2123, so the conductivity of the first current collector 2121 is more likely to be greater than that of the first metal layer 2111. The first single-sided electrode 212 consumes electrolyte at a faster rate than the first double-sided electrode 211, which leads to black spots easily appearing at the interface of the first single-sided electrode 212 in the later stages of the secondary battery 100 cycle. This application sets the first current collector 2121 to include a stacked second metal layer 2123 and a third metal layer 2124. The resistivity of the third metal layer 2124 is greater than that of the second metal layer 2123. This makes the resistivity of the first current collector 2121 greater than that of the second metal layer 2123. The conductivity of the first current collector 2121 can be less than that of the second metal layer 2123. This can reduce the rate at which the first single-sided electrode 212 consumes electrolyte and reduce the possibility of black spots appearing at the interface of the first single-sided electrode 212.

[0051] In some embodiments, the first electrode 21 is a negative electrode, and the first metal layer 2111 includes copper. For a secondary battery 100 with a steel casing 10, if the outermost layer of the electrode assembly 20 is a positive electrode, the aluminum foil of the positive electrode is prone to electrochemical corrosion with the steel casing. By setting the first electrode 21 as a negative electrode, and the outermost first single-sided electrode 212 of the electrode assembly 20 also being a negative electrode, the possibility of electrochemical corrosion between the first single-sided electrode 212 and the steel casing can be reduced. The inclusion of copper in the first metal layer 2111 allows the resistivity of the first current collector 2121 to be greater than that of the first metal layer 2111, and the conductivity of the first current collector 2121 to be less than that of the first metal layer 2111. This reduces the rate at which the first single-sided electrode 212 consumes electrolyte compared to the rate at which the first double-sided electrode 211 consumes electrolyte, thus reducing the possibility of black spots appearing at the interface of the first single-sided electrode 212.

[0052] In some embodiments, the resistivity of the first current collector 2121 is ≥1.75×10⁻⁶. -8 Ω·m can further reduce the conductivity velocity of the first current collector 2121.

[0053] In some embodiments, referring to Figures 2 and 4, the third metal layer 2124 comprises nickel. Along the first direction X, the third metal layer 2124 is disposed on the surface of the second metal layer 2123 facing the second active layer 2122. The thickness of the third metal layer 2124 is 0.2 μm to 3 μm. The resistivity of nickel is greater than that of copper, and the resistivity of the third metal layer 2124 can be greater than that of the second metal layer 2123, thus making the resistivity of the first current collector 2121 greater than that of the second metal layer 2123. The third metal layer 2124 is located on the surface of the second metal layer 2123 facing the second active layer 2122. Compared to the third metal layer 2124 being located on the surface of the second metal layer 2123 away from the second active layer 2122, the third metal layer 2124 can block the second metal layer 2123 from the second electrode 22, further reducing the conductivity of the first current collector 2121. When the thickness of the third metal layer 2124 is less than 0.2 μm, the resistivity of the first current collector 2121 does not increase significantly, and the third metal layer 2124 cannot provide sufficient protection, making it prone to cavitation on its surface. When the thickness of the third metal layer 2124 is greater than 3 μm, it increases the manufacturing cost of the secondary battery 100 and also occupies internal space, affecting the battery's energy density. Here, the thickness of the third metal layer 2124 refers to the thickness of any one of the third metal layers 2124.

[0054] In some embodiments, the thickness of the third metal layer 2124 is 0.7 μm to 2.1 μm, which can further improve the resistivity of the first current collector 2121, while reducing the manufacturing cost of the secondary battery 100 and reducing the space occupied by the first current collector 2121.

[0055] In some embodiments, the thickness of the third metal layer 2124 is 1.3 μm to 2.1 μm, which can further improve the resistivity of the first current collector 2121, while reducing the manufacturing cost of the secondary battery 100 and reducing the space occupied by the first current collector 2121.

[0056] In some embodiments, a third metal layer 2124 is disposed on one surface of the second metal layer 2123 along the first direction X, and the thickness of the second metal layer 2123 is 15 μm to 20 μm. When the third metal layer 2124 is a single layer, if the thickness of the second metal layer 2123 is less than 15 μm, the second metal layer 2123 will have difficulty providing support for the third metal layer 2124; if the thickness of the second metal layer 2123 is greater than 20 μm, the second metal layer 2123 will lose a significant amount of energy density from the secondary battery 100.

[0057] In some embodiments, referring to Figures 2 and 5, a third metal layer 2124 is disposed on both opposite surfaces of the second metal layer 2123 along the first direction X, and the thickness of the second metal layer 2123 is 13 μm to 18 μm. The presence of the third metal layer 2124 on both opposite surfaces of the second metal layer 2123 along the first direction X can further reduce the conductivity velocity of the first current collector 2121. When the third metal layer 2124 is two layers, it can also provide support, and the thickness of the second metal layer 2123 can be further reduced. If the thickness of the second metal layer 2123 is less than 13 μm, it is difficult for the second metal layer 2123 to provide support for the third metal layer 2124. If the thickness of the second metal layer 2123 is greater than 18 μm, the second metal layer 2123 will lose more energy density of the secondary battery 100.

[0058] In some embodiments, the second metal layer 2123 is a copper foil, the third metal layer 2124 is a nickel layer, the third metal layer 2124 is attached to the surface of the second metal layer 2123, the second metal layer 2123 can serve as a base layer, and the third metal layer 2124 can be attached to the surface of the second metal layer 2123.

[0059] In some embodiments, the third metal layer 2124 is disposed on the surface of the second metal layer 2123 by any one of electroplating, vapor deposition or vapor deposition.

[0060] In some embodiments, the nickel mass percentage in the third metal layer 2124 is 99% to 99.8%. A nickel mass percentage below 99% in the third metal layer 2124 would reduce the resistivity of the third metal layer 2124. Due to limitations in processing precision, the nickel mass percentage in the third metal layer 2124 is unlikely to exceed 99.8%.

[0061] A second aspect of this application also provides an electronic device including a secondary battery 100 as described in any embodiment of the first aspect above. The electronic device in this application is not particularly limited and can be any electronic device known in the prior art. For example, electronic devices include, but are not limited to, Bluetooth headsets, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc., while spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0062] Test section:

[0063] 1. Cycle test of secondary batteries:

[0064] The test temperature was adjusted to a constant 25℃, and the following steps were performed on the secondary battery sample:

[0065] (1) Charge at a constant current of 2C to 4.25V, then charge at a constant voltage of 1.5C;

[0066] (2) Charge at a constant current of 1.5C to 4.35V, then charge at a constant voltage of 1.0C;

[0067] (3) Charge at a constant current of 1C to 4.5V, then charge at a constant voltage of 0.7C;

[0068] (4) Charge at a constant current of 0.7C to 4.545V, then charge at a constant voltage of 0.17C;

[0069] (5) Let stand for 5 minutes;

[0070] (6) Discharge at a constant current of 0.7C to 3.0V

[0071] (7) Let stand for 5 minutes;

[0072] (8) Repeat steps (1) to (7) 300 times;

[0073] Finish.

[0074] 2. Resistivity test of current collector:

[0075] Remove the active material layer from the surface of the current collector, for example, by scraping it off with a scraper, and then perform the four-probe method measurement: First, cut the current collector into small 10cm × 10cm samples. Select four locations on the sample surface and apply four electrodes to each. Then, apply current through two electrodes and measure the voltage through the other two electrodes. When measuring the voltage, pay attention to the contact quality between the electrodes and the sample surface to ensure the accuracy of the measurement results. Based on the measured current and voltage data, the resistivity of the sample can be calculated.

[0076] 3. Battery volumetric energy density test:

[0077] The volumetric energy density test steps are as follows: 1) Under the environmental conditions of 25℃, let the secondary battery stand for 10 min, charge it to 4.5V with a constant current of 0.2C, charge it to 0.02C with a constant voltage, and let it stand for 5 min; then discharge it to 3V with a constant current of 0.2C, and let it stand for 5 min, and record the discharge capacity C0; 2) Measure the length, width and thickness of the secondary battery with a PPG battery thickness measuring instrument, and calculate the volumetric energy density using the following formula: Volumetric energy density = 3.92 C0 / (length × width × thickness).

[0078] Example 1

[0079] <Preparation of the first electrode>:

[0080] The first electrode is the negative electrode. The negative electrode active material graphite, the binder styrene-butadiene rubber (SBR) and the thickener sodium carboxymethyl cellulose (CMC) are mixed in a mass ratio of 96:2:2. Deionized water is added as a solvent and the mixture is stirred under vacuum until a negative electrode slurry with a solid content of 50-70 wt% and a uniform system is obtained.

[0081] Copper foil was selected as the first metal layer, with a thickness of 6 μm. The aforementioned negative electrode slurry was uniformly coated on one surface of the first metal layer, and the negative electrode slurry was dried to obtain a first double-sided electrode sheet with a single-sided coating of negative electrode active material. Then, the above steps were repeated on the other surface of the first metal layer to obtain a first double-sided electrode sheet with a double-sided coating of negative electrode active material.

[0082] A 20μm thick copper foil is selected as the second metal layer. A third metal layer is deposited on both opposite surfaces of the second metal layer to form the first current collector. The third metal layer is a nickel layer with a thickness of 0.2μm and a nickel mass percentage of 99.8%. The above negative electrode slurry is uniformly coated on one surface of the first current collector. The negative electrode slurry is dried to obtain the first single-sided electrode with a single-sided coating of negative electrode active material layer.

[0083] <Preparation of the Second Electrode>:

[0084] The second electrode is the positive electrode, consisting of lithium cobalt oxide as the positive active material, acetylene black as the positive conductive agent, and polyvinylidene fluoride (PVDF) as the positive binder (weight-average molecular weight 5 × 10⁻⁶). 5 Mix according to a mass ratio of 94:3:3, and add N. Methylpyrrolidone (NMP) was used as a solvent and stirred in a vacuum mixer until a positive electrode slurry with a solid content of 75 wt% and a homogeneous system was obtained.

[0085] A 10μm aluminum foil was selected as the current collector for the second electrode. The aforementioned positive electrode slurry was coated on one surface of the aluminum foil, and the positive electrode slurry was dried to obtain a second electrode with a positive electrode active material layer coated on one side. Then, the above steps were repeated on the other surface of the aluminum foil to obtain a second electrode with a positive electrode active material layer coated on both sides.

[0086] <Preparation of the diaphragm>:

[0087] A porous polyethylene (PE) membrane with a thickness of 8 μm was used as the diaphragm.

[0088] <Electrolyte Preparation>:

[0089] In a dry argon atmosphere, ethylene carbonate, methyl ethyl carbonate and diethyl carbonate are mixed in a mass ratio of 30:50:20 to obtain an organic solution. Then, lithium hexafluorophosphate is added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0090] <Preparation of Secondary Batteries>:

[0091] The first electrode, separator, and second electrode prepared above are stacked in sequence to form an electrode assembly. The electrode assembly is placed in a steel shell, dried, and then injected with 0.82g of electrolyte. The assembly is then encapsulated to obtain a secondary battery.

[0092] The relevant parameters in Comparative Example 1 and Examples 1 to 18 are shown in Table 1 below.

[0093] In Comparative Example 1, the first current collector was a 20 μm thick copper foil. In Examples 1 to 18, the first current collector was a nickel-plated copper foil, the second metal layer was a copper foil, and the third metal layer was a nickel layer. In Examples 1 to 9, the nickel plating layer of the first current collector consisted of two layers. In Examples 10 to 18, the nickel plating layer of the first current collector was a single layer, and the nickel plating layer (third metal layer) was located on the surface of the copper foil (second metal layer) facing the negative electrode active material layer (second active layer).

[0094] Table 1

[0095]

[0096] Note: In Table 1, "\" indicates that the parameter is not included.

[0097] According to Table 1 above, and in conjunction with Comparative Example 1 and Examples 1 to 18, it can be seen that if the first current collector is a thicker second metal layer (copper foil), the conductivity of the first current collector is likely to be greater than that of the first metal layer (copper foil). This means the first single-sided electrode consumes electrolyte at a faster rate than the first double-sided electrode, leading to black spots easily appearing at the interface of the first single-sided electrode during the later stages of the secondary battery cycle. By setting the first current collector to include a stacked second metal layer (copper foil) and a third metal layer (nickel layer), where the resistivity of the third metal layer is greater than that of the second metal layer, the resistivity of the first current collector can be greater than that of the second metal layer. This allows the conductivity of the first current collector to be less than that of the second metal layer, reducing the rate at which the first single-sided electrode consumes electrolyte and decreasing the likelihood of black spots appearing at the interface of the first single-sided electrode. Furthermore, the resistivity of the first current collector is ≥1.75 × 10⁻⁶. -8 Ω·m can further reduce the conductivity velocity of the first current collector.

[0098] As can be seen from Examples 1 to 18, a thickness of 0.2 μm to 3 μm for the third metal layer allows the resistivity of the first current collector to be greater than that of the second metal layer. This results in a lower conductivity rate for the first current collector compared to the second metal layer, reducing the rate at which the first single-sided electrode consumes electrolyte and decreasing the likelihood of black spots appearing at the interface of the first single-sided electrode. When the thickness of the third metal layer is less than 0.2 μm, the increase in resistivity of the first current collector is minimal, and the third metal layer fails to provide sufficient protection, making it prone to cavitation on its surface. When the thickness of the third metal layer is greater than 3 μm, it increases the manufacturing cost of the secondary battery, occupies internal space, and affects the battery's energy density. Furthermore, the excessively high resistance also increases the risk of lithium plating on the outermost electrode. Further, a thickness of 0.7 μm to 2.1 μm for the third metal layer can further improve the resistivity of the first current collector, while simultaneously reducing the manufacturing cost of the secondary battery, reducing the space occupied by the first current collector, increasing the volumetric energy density of the secondary battery, and reducing the increased risk of lithium plating due to excessive resistance. Furthermore, in order to balance reducing the risk of lithium plating and improving the volumetric energy density of the secondary battery, the thickness of the third metal layer is preferably 1.3 μm to 2.1 μm.

[0099] The relevant parameters in Examples 19 to 21 are shown in Table 2 below.

[0100] The mass percentage of nickel in the third metal layer differs in Examples 6 and Examples 19 to 21.

[0101] Table 2

[0102]

[0103] According to Table 2 above, and in conjunction with Examples 6 and 19 to 21, when the nickel mass percentage in the third metal layer is 99% to 99.8%, the first current collector has a high resistivity, and the area of ​​black spots appearing at the interface of the first single-sided electrode is small. If the nickel mass percentage in the third metal layer is less than 99%, the resistivity of the first current collector will decrease, the area of ​​black spots appearing at the interface of the first single-sided electrode will increase, and other impurity metals in the third metal layer, besides nickel, will easily dissolve into the electrolyte. After gaining electrons, these impurity metals will form dendrites that easily pierce the separator. Due to limitations in processing precision, the nickel mass percentage in the third metal layer is difficult to exceed 99.8%.

[0104] The above description is merely an embodiment of this application and does not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A secondary battery, comprising an electrode assembly, the electrode assembly comprising a first electrode, a separator, and a second electrode sequentially stacked along a first direction, the first electrode and the second electrode having opposite polarities, the first direction being the thickness direction of the electrode assembly; characterized in that, A portion of the first electrode includes a second current collector and a first active layer, and another portion of the first electrode includes a first current collector and a second active layer, wherein the thickness of the first current collector along the first direction is greater than the thickness of the second current collector along the first direction. The first electrode is a negative electrode, and the second current collector includes a first metal layer, which includes copper. The first current collector includes a second metal layer and a third metal layer stacked along the first direction, the second metal layer including copper, and the resistivity of the third metal layer being greater than that of the second metal layer.

2. The secondary battery according to claim 1, characterized in that, The first electrode sheet in part is a first double-sided electrode sheet, and the first active layer is disposed on both surfaces of the first metal layer along the first direction; The other part of the first electrode is a first single-sided electrode, and the surface of the first current collector facing the second electrode is provided with the second active layer.

3. The secondary battery according to claim 2, characterized in that, The first double-sided electrode is disposed between adjacent second electrode sheets; the outermost electrode sheet of the electrode assembly along the first direction is the first single-sided electrode.

4. The secondary battery according to any one of claims 1 to 3, characterized in that, The first metal layer is copper foil.

5. The secondary battery according to any one of claims 1 to 4, characterized in that, The resistivity of the first current collector is ≥1.75×10⁻⁶. -8 Ω·m.

6. The secondary battery according to any one of claims 1 to 5, characterized in that, The third metal layer comprises nickel, and the third metal layer is disposed on at least one surface of the second metal layer along the first direction, the thickness of the third metal layer being 0.2 μm to 3 μm.

7. The secondary battery according to claim 6, characterized in that, The thickness of the third metal layer is from 0.7 μm to 2.1 μm.

8. The secondary battery according to claim 7, characterized in that, The thickness of the third metal layer is 1.3 μm to 2.1 μm.

9. The secondary battery according to any one of claims 6 to 8, characterized in that, A third metal layer is disposed on one surface of the second metal layer along the first direction, and the thickness of the second metal layer is 15 μm to 20 μm.

10. The secondary battery according to any one of claims 6 to 8, characterized in that, The second metal layer has the third metal layer disposed on both opposite surfaces along the first direction, and the thickness of the second metal layer is 13 μm to 18 μm.

11. The secondary battery according to any one of claims 6 to 10, characterized in that, The second metal layer is a copper foil, and the third metal layer is a nickel layer, which is attached to the surface of the second metal layer.

12. The secondary battery according to claim 11, characterized in that, The third metal layer is formed on the surface of the second metal layer by any one of electroplating, vapor deposition or vapor deposition.

13. The secondary battery according to any one of claims 6 to 12, characterized in that, The third metal layer contains 99% to 99.8% nickel by mass.

14. The secondary battery according to any one of claims 1 to 13, characterized in that, The electrode assembly further includes a first tab connected to the first current collector, and the first tab includes the second metal layer.

15. The secondary battery according to claim 14, characterized in that, The first electrode tab is integrally disposed with the first current collector, and the first electrode tab includes a stacked second metal layer and the third metal layer.

16. An electronic device, characterized in that, Includes the secondary battery as described in any one of claims 1 to 15.

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