Secondary battery and electronic device

Abstract

The invention relates to a secondary battery and electronic equipment, the secondary battery comprises an electrode assembly, the electrode assembly comprises a first pole piece, the first pole piece is a negative pole piece, one part of the first pole piece comprises a second current collector and a first active layer, and the other part of the first pole piece comprises a first current collector and a second active layer; the thickness of the first current collector along the first direction is greater than that of the second current collector along the first direction; the first pole piece is a negative pole piece, the second current collector comprises a first metal layer, and the first metal layer comprises copper; the first current collector comprises a second metal layer and a third metal layer which are stacked in the first direction, the second metal layer comprises copper, and the resistivity of the third metal layer is larger than that of the second metal layer. According to the secondary battery and the electronic equipment, the problem of capacity loss of the secondary battery can be improved.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a secondary battery and an electronic device. Background Art

[0002] As the power source of electronic devices, secondary batteries are the key to ensure the normal use of electronic devices. The outermost electrode of steel-shell secondary batteries is prone to black spots, which will reduce the capacity of the secondary batteries. Summary of the invention

[0003] The inventors of the present application have discovered that a secondary battery generally includes a shell and an electrode assembly housed in the shell. The electrode assembly includes a pole piece and a diaphragm. The pole piece generally uses a metal layer as a current collector. In order to reduce the possibility of curling of the outermost pole piece of the electrode assembly, the metal layer of the outermost pole piece is generally thicker than the metal layer of the middle pole piece. Since the thicker the metal layer, the faster the conductivity and the faster the electrolyte is consumed, the outermost pole piece consumes the electrolyte faster than the middle pole piece, resulting in black spots on the interface of the outermost pole piece in the middle and late stages of the secondary battery cycle, thereby losing the capacity of the secondary battery.

[0004] The purpose of the present application is to provide a secondary battery and an electronic device, aiming to improve the problem of capacity loss of the secondary battery.

[0005] According to the first aspect of the present application, a secondary battery is provided, comprising an electrode assembly, the electrode assembly comprising a first pole piece, a diaphragm, and a second pole piece stacked in sequence along a first direction, the first pole piece and the second pole piece have opposite polarities, and the first direction is the thickness direction of the electrode assembly. A portion of the first pole piece comprises a second current collector and a first active layer, and another portion of the first pole piece comprises a first current collector and a second active layer, and 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 pole piece is a negative pole piece, and the second current collector comprises a first metal layer, and the first metal layer comprises copper. The first current collector comprises 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.

[0006] In the above technical solution, 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, resulting in the first current collector having a greater conductivity speed than the second current collector, and the first pole piece corresponding to the first current collector consumes electrolyte faster than the first pole piece corresponding to the second current collector, which makes the interface of the first pole piece corresponding to the first current collector more prone to black spots in the middle and late stages of the secondary battery cycle. The present application sets the first current collector to include a stacked second metal layer and a third metal layer, and the resistivity of the third metal layer is greater than the resistivity of the second metal layer. By setting the third metal layer, the resistivity of the first current collector can be increased, thereby reducing the conductivity speed of the first current collector, reducing the speed at which the first pole piece corresponding to the first current collector consumes electrolyte, and thereby reducing the possibility of black spots appearing at the interface of the first pole piece corresponding to the first current collector. The second metal layer includes copper, which can reduce the possibility of electrochemical corrosion between the second metal layer and the steel shell.

[0007] In some preferred embodiments, a portion of the first pole piece is a first double-sided pole piece, and a first active layer is provided on both surfaces of the first metal layer along a first direction; another portion of the first pole piece is a first single-sided pole piece, and a second active layer is provided on the surface of the first current collector facing the second pole piece.

[0008] In some preferred embodiments, a first double-sided pole piece is arranged between adjacent second pole pieces; the outermost pole piece of the electrode assembly along the first direction is a first single-sided pole piece. The outermost pole piece of the electrode assembly along the first direction is a first single-sided pole piece, and a second active layer is arranged on the surface of the first current collector facing the second pole piece. Compared with the second active layer being arranged on both opposite surfaces of the first current collector, the utilization rate of the active layer of the first single-sided pole piece can be improved, and the energy density of the secondary battery can be improved. In order to reduce the warping of the pole piece located at the outermost layer of the electrode assembly due to stress, the first current collector is usually thicker. The first pole piece located in the inner layer of the electrode assembly is located between the two layers of the second pole pieces, and the first pole piece of this part is provided with a first active layer on both surfaces of the first metal layer.

[0009] In some preferred embodiments, the first metal layer is copper foil.

[0010] In some preferred embodiments, the resistivity of the first current collector is ≥1.75×10 -8 Ω·m, which can further reduce the conduction speed of the first current collector.

[0011] In some preferred embodiments, the third metal layer includes nickel, and the third metal layer is provided on at least one surface of the second metal layer along the first direction, and the thickness of the third metal layer is 0.2 μm to 3 μm. The third metal layer includes nickel, and the resistivity of nickel is greater than the resistivity of copper. The resistivity of the third metal layer can be greater than the resistivity of the second metal layer, so that the resistivity of the first current collector can be greater than the resistivity of the second metal layer. When the thickness of the third metal layer is less than 0.2 μm, the resistivity of the first current collector is not greatly improved, and the third metal layer is difficult to provide sufficient protection, and holes are easily generated on the surface of the third metal layer. When the thickness of the third metal layer is greater than 3 μm, the manufacturing cost of the secondary battery will increase, and it will also occupy the space inside the secondary battery, affecting the energy density of the battery. Among them, the thickness of the third metal layer refers to the thickness of any third metal layer.

[0012] In some preferred embodiments, the thickness of the third metal layer is 0.7 μm to 2.1 μm, which can further increase the resistivity of the first current collector, while reducing the manufacturing cost of the secondary battery and the space occupied by the first current collector.

[0013] In some preferred embodiments, the thickness of the third metal layer is 1.3 μm to 2.1 μm, which can further increase the resistivity of the first current collector, while reducing the manufacturing cost of the secondary battery and the space occupied by the first current collector.

[0014] 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, it is difficult for the second metal layer to provide support for the third metal layer, and if the thickness of the second metal layer is greater than 20 μm, the second metal layer will lose more energy density of the secondary battery.

[0015] 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 facing away from the second active layer, the third metal layer can block the second metal layer and the second pole piece, and can further reduce the conduction speed of the first current collector.

[0016] In some preferred embodiments, the second metal layer is provided with a third metal layer on both surfaces relative to each other along the first direction, and the thickness of the second metal layer is 13 μm to 18 μm. The second metal layer is provided with a third metal layer on both surfaces relative to each other along the first direction, which can further reduce the conduction speed of the first current collector. When the third metal layer is two 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 more energy density of the secondary battery.

[0017] In some preferred embodiments, the second metal layer is copper foil, the third metal layer is nickel layer, 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.

[0018] In some preferred embodiments, the third metal layer is disposed on the surface of the second metal layer by any one of electroplating, evaporation or vapor deposition.

[0019] In some preferred embodiments, the mass percentage of nickel in the third metal layer is 99% to 99.8%. If the mass percentage of nickel in the third metal layer is lower than 99%, the resistivity of the third metal layer will be reduced. Due to the limitation of processing accuracy, the mass percentage of nickel in the third metal layer is difficult to be greater than 99.8%.

[0020] In some preferred embodiments, the electrode assembly further comprises a first pole ear, the first pole ear is connected to the first current collector, and the first pole ear comprises a second metal layer. To achieve the connection between the internal and external circuits of the battery, the electrode assembly further comprises a first pole ear, the first pole ear is connected to the first current collector, and the material of the first pole ear is the same as that of the second metal layer.

[0021] In some preferred embodiments, the first pole ear is integrally arranged with the first current collector, and the first pole ear includes a stacked second metal layer and a third metal layer. The first current collector includes an area coated with the second active layer and an area not coated with the second active layer. The first pole ear integrally arranged with the first current collector can be obtained by cutting the area of ​​the first current collector not coated with the second active layer by laser or cutting knife, wherein the material of the first pole ear is the same as that of the first current collector, and both include a stacked second metal layer and a third metal layer. The first pole ear is obtained by the above method, which can simplify the preparation process of the first pole ear.

[0022] In a second aspect, the present application further proposes an electronic device, comprising a secondary battery as in any embodiment of the first aspect above.

[0023] Additional aspects and advantages of the embodiments of the present application will be described, shown, or explained in part in the subsequent description through implementation of the embodiments of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] One or more embodiments are exemplarily described by corresponding drawings, which do not constitute limitations on the embodiments. Elements with the same reference numerals in the drawings represent similar elements, and unless otherwise stated, the dimensions in the drawings do not constitute proportional limitations.

[0025] Figure 1 A schematic diagram of the structure of a secondary battery in some embodiments of the present application;

[0026] Figure 2 This is a schematic diagram of the structure of the electrode assembly of some embodiments of the present application;

[0027] Figure 3 This is a schematic structural diagram of a first current collector in some embodiments of the present application;

[0028] Figure 4 This is a schematic structural diagram of a first single-sided pole piece in some embodiments of the present application;

[0029] Figure 5 This is a schematic structural diagram of the first current collector of some embodiments of the present application.

[0030] Description of reference numerals:

[0031] 100, secondary battery; 10, shell; 20, electrode assembly; 21, first pole piece; 211, first double-sided pole piece; 2111, first metal layer; 2112, first active layer; 212, first single-sided pole piece; 2121, first current collector; 2123, second metal layer; 2124, third metal layer; 2122, second active layer; 22, second pole piece; 221, third current collector; 222, third active layer; 23, diaphragm; X, first direction. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments.

[0033] Reference to "embodiment" in this application means that a particular feature, structure, or characteristic described in conjunction with the embodiment may be included in at least one embodiment of the present application. The appearance of the phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments.

[0034] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0035] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0036] The term "perpendicular" is used to describe the ideal state between two components. In actual production or use, there may be a state that is approximately perpendicular between the two components. For example, in combination with numerical descriptions, perpendicularity can refer to the angle between two straight lines being between 90±10°, perpendicularity can also refer to the dihedral angle between two planes being between 90±10°, and perpendicularity can also refer to the angle between a straight line and a plane being between 90±10°. The two components described as "perpendicular" may not be absolute straight lines or planes, but may be roughly straight lines or planes. From a macroscopic perspective, a component can be considered a "straight line" or a "plane" if the overall extension direction is a straight line or a plane.

[0037] The first direction X of the present application is a bidirectional direction, that is, the first direction X includes the direction indicated by the arrow in the drawings and the opposite direction.

[0038] The technical features involved in the different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0039] In the first aspect, the embodiment of the present application provides a secondary battery 100, please refer to Figure 1 The secondary battery 100 includes a shell 10 and an electrode assembly 20 . The shell 10 can accommodate the electrode assembly 20 and an electrolyte (not shown in the figure). The electrolyte infiltrates the electrode assembly 20 in the shell 10 .

[0040] For the electrode assembly 20, please refer to Figure 2 , Figure 2The lamination structure of the electrode assembly 20 is shown. The electrode assembly 20 includes a first pole piece 21, a diaphragm 23 and a second pole piece 22. The polarities of the first pole piece 21 and the second pole piece 22 are opposite. A diaphragm 23 is provided between the adjacent second pole piece 22 and the first pole piece 21. The first pole piece 21, the diaphragm 23 and the second pole piece 22 are stacked. In the embodiment of the present application, the electrode assembly 20 is described as a lamination structure as an example. In some other embodiments, the electrode assembly 20 may also be a winding structure. For example, the first pole piece 21, the diaphragm 23 and the second pole piece 22 are stacked and wound in sequence to form a wound electrode assembly 20.

[0041] In some embodiments, the first pole piece 21 includes a first double-sided pole piece 211, and the first double-sided pole piece 211 is arranged between adjacent second pole pieces 22. The first double-sided pole piece 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 active layer 2112 is arranged on both surfaces of the first metal layer 2111 along the first direction X.

[0042] In some embodiments, the first metal layer 2111 may be copper foil.

[0043] In some embodiments, the first active layer 2112 is infiltrated by the electrolyte in the housing 10 to generate an electrochemical reaction. The first active layer 2112 includes a first active material, a conductive agent, an adhesive, etc. 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.

[0044] In some embodiments, the second pole piece 22 includes a third current collector 221 and a third active layer 222 , and the third active layer 222 is disposed on at least one surface of the third current collector 221 along the first direction X.

[0045] In some embodiments, the third current collector 221 may be an aluminum foil.

[0046] In some embodiments, the third active layer 222 is infiltrated by the electrolyte in the housing 10 to generate an electrochemical reaction. The third active layer 222 includes a third active material, a conductive agent, an adhesive, etc. The above materials are mixed and stirred evenly and coated on 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 iron manganese phosphate.

[0047] In some embodiments, the first pole piece 21 also includes a first single-sided pole piece 212. The outermost pole piece of the electrode assembly 20 along the first direction X is the first single-sided pole piece 212. The first single-sided pole piece 212 includes a first current collector 2121 and a second active layer 2122. The second active layer 2122 is arranged on the surface of the first current collector 2121 facing the second pole piece 22. Compared with the first current collector 2121 having the second active layer 2122 arranged on both surfaces, the utilization rate of the active layer of the first single-sided pole piece 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 curling of the first single-sided pole piece 212, 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 pole piece 212 consumes electrolyte faster than the first double-sided pole piece 211, resulting in black spots on the interface of the first single-sided pole piece 212 in the middle and late stages of the cycle of the secondary battery 100, thereby losing the capacity of the secondary battery 100.

[0048] To improve the above problems, please refer to Figure 2 and Figure 3 In the embodiment of the present 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 the resistivity of the second metal layer 2123. The second metal layer 2123 includes copper, which 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 likely to be greater than the first metal layer 2111, and the first single-sided pole piece 212 consumes electrolyte faster than the first double-sided pole piece 211, resulting in black spots on the interface of the first single-sided pole piece 212 in the middle and late stages of the secondary battery 100 cycle. In the present application, the first current collector 2121 is provided to include a stacked second metal layer 2123 and a third metal layer 2124, and the resistivity of the third metal layer 2124 is greater than the resistivity of the second metal layer 2123. This can make the resistivity of the first current collector 2121 greater than the resistivity of the second metal layer 2123, and the conduction speed of the first current collector 2121 can be lower than the conduction speed of the second metal layer 2123, which can reduce the speed at which the first single-sided pole piece 212 consumes electrolyte and reduce the possibility of black spots appearing on the interface of the first single-sided pole piece 212.

[0049] In some embodiments, the first electrode sheet 21 is a negative electrode sheet, and the first metal layer 2111 includes copper. For a secondary battery 100 whose housing 10 is a steel shell, if the outermost layer of the electrode assembly 20 is a positive electrode sheet, the aluminum foil of the positive electrode sheet is prone to electrochemical corrosion with the steel shell. By setting the first electrode sheet 21 as a negative electrode sheet, and the first single-sided electrode sheet 212 of the outermost layer of the electrode assembly 20 is also a negative electrode sheet, the possibility of electrochemical corrosion between the first single-sided electrode sheet 212 and the steel shell can be reduced. The first metal layer 2111 includes copper, which can make the resistivity of the first current collector 2121 greater than the resistivity of the first metal layer 2111, and the conductivity speed of the first current collector 2121 can be less than the conductivity speed of the first metal layer 2111, and the speed at which the first single-sided electrode sheet 212 consumes electrolyte can be less than the speed at which the first double-sided electrode sheet 211 consumes electrolyte, which can reduce the possibility of black spots appearing on the interface of the first single-sided electrode sheet 212.

[0050] In some embodiments, the resistivity of the first current collector 2121 is ≥ 1.75×10 -8 Ω·m, which can further reduce the conduction speed of the first current collector 2121.

[0051] In some embodiments, please refer to Figure 2 and Figure 4 , the third metal layer 2124 includes nickel, and the third metal layer 2124 is provided on the surface of the second metal layer 2123 facing the second active layer 2122 along the first direction X, and the thickness of the third metal layer 2124 is 0.2 μm to 3 μm. The third metal layer 2124 includes nickel, and the resistivity of nickel is greater than the resistivity of copper. The resistivity of the third metal layer 2124 can be greater than the resistivity of the second metal layer 2123, which can make the resistivity of the first current collector 2121 greater than the resistivity 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 with 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 and the second electrode 22, which can further reduce the conduction speed 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 is not greatly improved, and the third metal layer 2124 is difficult to provide sufficient protection, and holes are easily generated on the surface of the third metal layer 2124. When the thickness of the third metal layer 2124 is greater than 3 μm, the manufacturing cost of the secondary battery 100 will be increased, and the space inside the secondary battery 100 will also be occupied, affecting the energy density of the battery. Among them, the thickness of the third metal layer 2124 refers to the thickness of any third metal layer 2124.

[0052] In some embodiments, the thickness of the third metal layer 2124 is 0.7 μm to 2.1 μm, which can further increase the resistivity of the first current collector 2121 , while reducing the manufacturing cost of the secondary battery 100 and the space occupied by the first current collector 2121 .

[0053] In some embodiments, the thickness of the third metal layer 2124 is 1.3 μm to 2.1 μm, which can further increase the resistivity of the first current collector 2121 , while reducing the manufacturing cost of the secondary battery 100 and reducing the occupied space of the first current collector 2121 .

[0054] 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, it is difficult for the second metal layer 2123 to provide support for the third metal layer 2124, and if the thickness of the second metal layer 2123 is greater than 20 μm, the second metal layer 2123 will consume more energy density of the secondary battery 100.

[0055] In some embodiments, please refer to Figure 2 and Figure 5 , the second metal layer 2123 is provided with a third metal layer 2124 on both opposite surfaces along the first direction X, and the thickness of the second metal layer 2123 is 13 μm to 18 μm. The second metal layer 2123 is provided with a third metal layer 2124 on both opposite surfaces along the first direction X, which can further reduce the conduction speed of the first current collector 2121. When the third metal layer 2124 is two layers, the third metal layer 2124 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.

[0056] 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.

[0057] In some embodiments, the third metal layer 2124 is disposed on the surface of the second metal layer 2123 by any one of electroplating, evaporation or vapor deposition.

[0058] In some embodiments, the mass percentage of nickel in the third metal layer 2124 is 99% to 99.8%. When the mass percentage of nickel in the third metal layer 2124 is lower than 99%, the resistivity of the third metal layer 2124 is reduced. Due to the limitation of processing accuracy, the mass percentage of nickel in the third metal layer 2124 is difficult to be greater than 99.8%.

[0059] In the second aspect of the present application, an electronic device is further proposed, including a secondary battery 100 as in any embodiment of the first aspect described above. The electronic device of the embodiment of the present application is not particularly limited, and it 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, electric tools, battery vehicles, electric cars, ships, spacecraft, and the like. Among them, electric toys can include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, and the like, and spacecraft can include airplanes, rockets, space shuttles, and spacecraft, and the like. Test section:

[0060] 1. Cycle test of secondary batteries:

[0061] The test temperature was adjusted to a constant temperature of 25°C, and the following steps were performed on the secondary battery sample:

[0062] (1) 2C constant current charging to 4.25V, constant voltage charging to 1.5C;

[0063] (2) 1.5C constant current charging to 4.35V, constant voltage charging to 1.0C;

[0064] (3) 1C constant current charging to 4.5V, constant voltage charging to 0.7C;

[0065] (4) 0.7C constant current charging to 4.545V, constant voltage charging to 0.17C;

[0066] (5) Let stand for 5 minutes;

[0067] (6) 0.7C constant current discharge to 3.0V

[0068] (7) Let stand for 5 minutes;

[0069] (8) Steps (1) to (7) are repeated 300 times;

[0070] Finish.

[0071] 2. Resistivity test of current collector:

[0072] Remove the active material layer on the surface of the current collector, such as by scraping it with a scraper, and then perform the four-probe method measurement steps: First, cut the current collector into small blocks of 10cm*10cm, select four locations on the surface of the sample to be tested, and apply four electrodes respectively. Then, apply current through two electrodes and measure voltage through the other two electrodes. When measuring voltage, it is necessary to pay attention to the contact quality between the electrode 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.

[0073] 3. Battery volume energy density test:

[0074] The volume energy density test steps are as follows: 1) Under the environmental conditions of 25°C, let the secondary battery stand for 10 minutes, charge to 4.5V at a constant current of 0.2C, charge to 0.02C at a constant voltage, and let it stand for 5 minutes; then discharge to 3V at a constant current of 0.2C, let it stand for 5 minutes, and record the discharge capacity C0; 2) Measure the length, width and thickness of the secondary battery with a PPG battery thickness gauge, and calculate the volume energy density using the following formula: Volume energy density = 3.92C0 / (length×width×thickness).

[0075] Example 1

[0076] <Preparation of the first pole piece>:

[0077] The first electrode sheet is the negative electrode sheet. The negative electrode active material graphite, the binder styrene butadiene rubber (SBR) and the thickener sodium carboxymethyl cellulose (CMC) are mixed in a weight ratio of 96:2:2, deionized water is added as a solvent, and stirred under the action of a vacuum mixer to a negative electrode slurry with a solid content of 50-70wt% and a uniform system.

[0078] Copper foil was selected as the first metal layer, the thickness of the first metal layer was 6 μm, the above negative electrode slurry was evenly coated on one surface of the first metal layer, and the negative electrode slurry was dried to obtain a first double-sided pole piece coated with a negative electrode active material layer on one side. Then, the above steps were repeated on the other surface of the first metal layer to obtain a first double-sided pole piece coated with a negative electrode active material layer on both sides.

[0079] A 20μm thick copper foil is selected as the second metal layer, and a third metal layer is plated on two opposite surfaces of the second metal layer to form a first current collector. The third metal layer is a nickel layer. The thickness of the third metal layer is 0.2μm, and the mass percentage of nickel in the third metal layer is 99.8%. The above-mentioned negative electrode slurry is evenly coated on one surface of the first current collector, and the negative electrode slurry is dried to obtain a first single-sided electrode sheet coated with a negative electrode active material layer on one side.

[0080] <Preparation of the second pole piece>:

[0081] The second electrode is a positive electrode. The positive electrode active material is lithium cobalt oxide, the positive electrode conductive agent is acetylene black, and the positive electrode binder is polyvinylidene fluoride (PVDF, with a weight average molecular weight of 5×10 5 ) were mixed in a mass ratio of 94:3:3, N-methylpyrrolidone (NMP) was added as a solvent, and the mixture was stirred under a vacuum stirrer until a positive electrode slurry with a solid content of 75 wt% and a uniform system was obtained.

[0082] A 10 μm aluminum foil was selected as the current collector of the second electrode sheet, and the positive electrode slurry was coated on one surface of the aluminum foil, and the positive electrode slurry was dried to obtain a second electrode sheet coated with a positive electrode active material layer on one side. Then, the above steps were repeated on the other surface of the aluminum foil to obtain a second electrode sheet coated with a positive electrode active material layer on both sides.

[0083] <Preparation of diaphragm>:

[0084] A polyethylene (PE) porous film with a thickness of 8 μm was used as the separator.

[0085] <Electrolyte Preparation>:

[0086] In a dry argon atmosphere, ethylene carbonate, ethyl methyl carbonate and diethyl carbonate were mixed in a mass ratio of 30:50:20 to obtain an organic solution, and then lithium hexafluorophosphate was added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0087] <Preparation of Secondary Battery>:

[0088] The first pole piece, the diaphragm, and the second pole piece prepared above are stacked in sequence to form an electrode assembly, and the electrode assembly is placed in a steel shell. After drying, 0.82 g of electrolyte is injected, and the secondary battery is packaged.

[0089] The relevant parameters in Comparative Example 1 and Examples 1 to 18 are shown in Table 1 below.

[0090] The first current collector in Comparative Example 1 is a 20 μm thick copper foil. The first current collector in Examples 1 to 18 is a nickel-plated copper foil, the second metal layer is a copper foil, and the third metal layer is a nickel layer. The nickel-plated layer of the first current collector in Examples 1 to 9 is two layers. The nickel-plated layer of the first current collector in Examples 10 to 18 is one layer, and the nickel-plated layer (third metal layer) is located on the surface of the copper foil (second metal layer) facing the negative electrode active material layer (second active layer).

[0091] Table 1

[0092]

[0093]

[0094] Note: “\” in Table 1 means that the parameter is not included.

[0095] According to the above Table 1, combined 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 speed of the first current collector is likely to be greater than the first metal layer (copper foil), and the first single-sided pole piece consumes electrolyte faster than the first double-sided pole piece, resulting in black spots on the interface of the first single-sided pole piece in the middle and late 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), the resistivity of the third metal layer is greater than the resistivity of the second metal layer, the resistivity of the first current collector can be greater than the resistivity of the second metal layer, the conductivity speed of the first current collector can be less than the conductivity speed of the second metal layer, the speed at which the first single-sided pole piece consumes electrolyte can be reduced, and the possibility of black spots on the interface of the first single-sided pole piece can be reduced. Furthermore, the resistivity of the first current collector is ≥1.75×10 -8 Ω·m, which can further reduce the conduction speed of the first current collector.

[0096] In combination with Example 1 to Example 18, it can be known that the thickness of the third metal layer is 0.2μm to 3μm, which can make the resistivity of the first current collector greater than the resistivity of the second metal layer, and the conductivity of the first current collector can be less than the conductivity of the second metal layer, which can reduce the speed at which the first single-sided electrode consumes electrolyte, and can reduce the possibility of black spots appearing on the interface of the first single-sided electrode. When the thickness of the third metal layer is less than 0.2μm, the resistivity of the first current collector is not greatly improved, and the third metal layer is difficult to provide sufficient protection, and holes are easily generated on the surface of the third metal layer. When the thickness of the third metal layer is greater than 3μm, the manufacturing cost of the secondary battery will be increased, and the space inside the secondary battery will be occupied, affecting the energy density of the battery. In addition, due to the excessive resistance value, the risk of lithium plating of the outermost electrode will also increase. Further, the thickness of the third metal layer is 0.7μm to 2.1μm, which can further increase the resistivity of the first current collector, reduce the manufacturing cost of the secondary battery, and reduce the occupied space of the first current collector, improve the volume energy density of the secondary battery, and reduce the increase in the risk of lithium plating due to excessive resistance value. Furthermore, in order to reduce the risk of lithium plating and improve the volume energy density of the secondary battery, the thickness of the third metal layer is further preferably 1.3 μm to 2.1 μm.

[0097] The relevant parameters in Examples 19 to 21 are shown in Table 2 below.

[0098] The mass percentage of nickel in the third metal layer in Example 6 and Examples 19 to 21 is different.

[0099] Table 2

[0100]

[0101] According to the above Table 2, combined with Example 6 and Examples 19 to 21, it can be seen that the mass percentage of nickel in the third metal layer is 99% to 99.8%, the first current collector has a higher resistivity, and the area of ​​black spots appearing on the interface of the first single-sided pole piece is smaller. If the mass percentage of nickel in the third metal layer is lower than 99%, the resistivity of the first current collector will be reduced, and the area of ​​black spots appearing on the interface of the first single-sided pole piece will increase. In addition, other impurity metals other than nickel in the third metal layer are easily melted into the electrolyte, and other impurity metals become dendrites after obtaining electrons, which are easy to pierce the diaphragm. Limited by processing accuracy, the mass percentage of nickel in the third metal layer is difficult to be greater than 99.8%.

[0102] The above descriptions are merely embodiments of the present application and are not intended to limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the contents of the present application specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A secondary battery, comprising an electrode assembly, wherein the electrode assembly comprises a first electrode sheet, a separator, and a second electrode sheet stacked in sequence along a first direction, wherein the first electrode sheet and the second electrode sheet have opposite polarities, and the first direction is a thickness direction of the electrode assembly; characterized in that: A portion of the first pole piece includes a second current collector and a first active layer, and another portion of the first pole piece includes a first current collector and a second active layer, and a thickness of the first current collector along the first direction is greater than a thickness of the second current collector along the first direction; The first electrode sheet is a negative electrode sheet, the second current collector includes a first metal layer, and the first metal layer 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 includes copper, and the resistivity of the third metal layer is greater than that of the second metal layer.

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

3. The secondary battery according to claim 2, characterized in that: The first double-sided pole piece is arranged between adjacent second pole pieces; the outermost pole piece of the electrode assembly along the first direction is the first single-sided pole piece.

4. The secondary battery according to claim 1, characterized in that: The first metal layer is copper foil.

5. The secondary battery according to claim 1, characterized in that: The resistivity of the first current collector is ≥1.75×10 -8 Ω·m.

6. The secondary battery according to claim 1, characterized in that: The third metal layer includes nickel. 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.

7. The secondary battery according to claim 6, characterized in that: The thickness of the third metal layer is 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 claim 6, characterized in that: A third metal layer is disposed on one surface of the second metal layer along the first direction, and a thickness of the second metal layer is 15 μm to 20 μm.

10. The secondary battery according to claim 6, characterized in that: The third metal layer is disposed on two 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.

11. The secondary battery according to claim 6, characterized in that: 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.

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

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

14. The secondary battery according to claim 1, characterized in that: The electrode assembly further includes a first electrode tab connected to the first current collector, and the first electrode tab includes the second metal layer.

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

16. An electronic device, characterized in that: The invention comprises the secondary battery according to any one of claims 1 to 15.

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