Secondary battery and electronic device

Abstract

A secondary battery and an electronic device. The secondary battery comprises a housing and an electrode assembly disposed in the housing. The electrode assembly includes a negative electrode plate, a positive electrode plate, and a separator disposed between the negative electrode plate and the positive electrode plate. The negative electrode plate includes a negative electrode current collector, a negative electrode active material layer, and a plurality of negative electrode tabs. The negative electrode current collector is a copper foil. The negative electrode active material layer is disposed on the negative electrode current collector and includes a negative electrode active material. The negative electrode active material includes a carbon material, and a coating weight per unit area of the negative electrode active material layer is 70 mg / 1540.25 mm2 to 160 mg / 1540.25 mm2. The positive electrode plate includes a positive electrode current collector, a positive electrode active material layer, and a plurality of positive electrode tabs. The positive electrode current collector includes a first metal layer, a polymer layer, and a second metal layer that are stacked, and the positive electrode active material layer is disposed on the first metal layer and the second metal layer, respectively. The secondary battery has both a high energy density and good kinetic performance.

Description

Secondary batteries and electronic devices Technical Field

[0001] This application relates to the field of energy storage technology, and in particular to a secondary battery and an electronic device having the secondary battery. Background Technology

[0002] Secondary batteries (such as lithium-ion batteries) are widely used in electronic mobile devices, power tools, and electric vehicles. Secondary batteries typically have multiple tabs on their positive and negative electrodes to meet the demands of high-rate charging and discharging. However, these tabs occupy a significant amount of space within the casing, reducing the energy density of the secondary battery.

[0003] Related technologies employ a series of methods to address the issue of reduced energy density in multi-tab structures. However, improving energy density often comes at the cost of sacrificing kinetic performance, especially under high-rate discharge conditions, where reduced kinetic performance can easily lead to lithium plating on the negative electrode. Therefore, for rechargeable batteries with multi-tab structures, balancing high energy density with superior kinetic performance is a pressing issue that needs to be addressed. Summary of the Invention

[0004] Therefore, this application proposes a secondary battery that can achieve both high energy density and superior kinetic performance, and an electronic device having the secondary battery.

[0005] This application provides a secondary battery, including a casing and an electrode assembly disposed within the casing. The electrode assembly includes a negative electrode, a positive electrode, and a separator disposed between the negative and positive electrodes. The negative electrode includes a negative current collector, a negative active material layer, and a plurality of negative electrode tabs. The negative current collector is copper foil. The negative active material layer is disposed on the negative current collector. The negative active material layer contains a negative active material. The negative active material contains carbon material, and the coating weight w per unit area of ​​the negative active material layer is 70 mg / 1540.25 mm². 2 Up to 160mg / 1540.25mm 2 The positive electrode includes a positive current collector, a positive active material layer, and multiple positive electrode tabs. The positive current collector includes a first metal layer, a polymer layer, and a second metal layer stacked together, and the positive active material layer is disposed on the first metal layer and the second metal layer, respectively.

[0006] This application increases the coating weight per unit area of ​​the negative electrode active material layer, thereby mitigating the energy density reduction issue caused by the multi-tab structure in secondary batteries. Simultaneously, considering the decrease in the kinetic performance of the negative electrode sheet due to the increased coating weight per unit area of ​​the negative electrode active material layer, this application sets the positive electrode current collector as a composite current collector comprising a first metal layer, a polymer layer, and a second metal layer. This increases the impedance of the positive electrode current collector, thereby reducing the rate at which active ions are extracted from the positive electrode active material layer, better matching the kinetic performance of the positive and negative electrode sheets, and reducing the risk of lithium plating on the negative electrode sheet during high-rate charging. Therefore, based on theoretical analysis and extensive experimental verification, the inventors of this application have discovered that by increasing the coating weight per unit area of ​​the negative electrode active material layer in a multi-tab structure secondary battery and further setting the positive electrode current collector as a composite current collector structure, a balance can be achieved between the energy density and kinetic performance of the secondary battery, enabling the secondary battery to achieve both high energy density and superior kinetic performance.

[0007] Based on the first aspect, in some possible implementations, the compaction density ρ of the negative electrode active material layer is 1.63 g / cm³. 3 Up to 1.80 g / cm 3 By increasing the compaction density of the negative electrode active material layer, it is beneficial to further improve the problem of reduced energy density of secondary batteries caused by the multi-tab structure. Moreover, while further improving the energy density of secondary batteries, since the positive electrode current collector in this application is a composite current collector, it can better balance the kinetic performance of the positive electrode and the negative electrode, and reduce the risk of lithium plating of the negative electrode during high-rate charging. Therefore, the secondary battery can still achieve both high energy density and superior kinetic performance.

[0008] Based on the first aspect, in some possible implementations, the compaction density ρ of the negative electrode active material layer is 1.65 g / cm³. 3 Up to 1.78 g / cm 3 By further increasing the compaction density of the negative electrode active material layer, it is beneficial to further improve the problem of reduced energy density of secondary batteries caused by the multi-tab structure. Moreover, while further improving the energy density of secondary batteries, since the positive electrode current collector in this application is a composite current collector, it can better balance the kinetic performance of the positive electrode and the negative electrode, and reduce the risk of lithium plating of the negative electrode during high-rate charging. Therefore, the secondary battery can still achieve both high energy density and superior kinetic performance.

[0009] Based on the first aspect, in some possible implementations, the thickness H3 of the polymer layer is 2 μm to 13 μm, the thickness H1 of the first metal layer is 0.2 μm to 1.3 μm, and the thickness H2 of the second metal layer is 0.2 μm to 1.3 μm. By setting the thicknesses of the polymer layer, the first metal layer, and the second metal layer, the positive electrode current collector has a higher impedance. Therefore, when the unit area coating weight and / or compaction density of the negative electrode active material layer is high, the kinetic performance of the positive and negative electrode sheets can be better matched, reducing the risk of lithium plating of the negative electrode sheet during high-rate charging.

[0010] Based on the first aspect, in some possible implementations, 6μm≤H3≤13μm, thereby further improving the impedance of the positive current collector, so that when the unit area coating weight and / or compaction density of the negative electrode active material layer is high, the dynamic performance of the positive electrode and the negative electrode can be better matched.

[0011] Based on the first aspect, in some possible implementations, 0.2μm≤H1≤0.8μm and 0.2μm≤H2≤0.8μm are used to further improve the impedance of the positive current collector. Thus, when the unit area coating weight and / or compaction density of the negative electrode active material layer is high, the dynamic performance of the positive electrode and the negative electrode can be better matched.

[0012] Based on the first aspect, in some possible implementations, the carbon material includes at least one of graphite, hard carbon, or soft carbon.

[0013] Based on the first aspect, in some possible implementations, the negative electrode active material also includes silicon. By adding silicon to the negative electrode active material, the energy density of the secondary battery can be further improved. Furthermore, while further improving the energy density, since the positive electrode current collector in this application is a composite current collector, it can better balance the kinetic performance of the positive and negative electrode sheets, reducing the risk of lithium plating on the negative electrode sheet during high-rate charging. Therefore, the secondary battery can still maintain both high energy density and superior kinetic performance. In addition, if the silicon material undergoes volume expansion due to the insertion of active ions, causing the negative electrode sheet to expand and deform, the positive electrode current collector has a higher elongation rate compared to a pure metal current collector of the same thickness. This improves the elongation of the positive electrode sheet, reducing the risk of breakage of the positive electrode sheet due to expansion and deformation of the negative electrode sheet.

[0014] Based on the first aspect, in some possible implementations, the silicon material includes at least one of silicon oxide, silicon carbide, and elemental silicon.

[0015] Based on the first aspect, in some possible implementations, the housing is an aluminum-plastic film. The negative electrode, separator, and positive electrode are stacked and wound into a flat structure. The electrode assembly includes a first bending section, with the positive electrode located at the outermost edge of the first bending section. By setting the outermost edge of the first bending section to be a highly ductile positive electrode, the risk of breakage of the outermost electrode due to the expansion and deformation of the negative electrode can be reduced. Moreover, the outermost positive electrode can also effectively isolate the negative electrode from the packaging bag. Even if the negative electrode at the first bending section has a sharp corner, the risk of electrochemical corrosion caused by the negative electrode piercing the packaging bag and the electrolyte in the electrolyte solution coming into contact with the metal layer (i.e., the aluminum layer) of the packaging bag can be reduced.

[0016] Based on the first aspect, in some possible implementations, the negative electrode, separator, and positive electrode are stacked and wound into a flat structure. Along the thickness direction of the electrode assembly, the electrode assembly includes N layers of positive electrode sheets. The number of positive electrode tabs is n, where n ≤ 2 / N. By appropriately reducing the number of positive electrode tabs, the impedance of the positive electrode sheet can be further increased while maintaining high rate performance in the secondary battery. This reduces the rate at which active ions are extracted from the positive electrode active material layer, better matches the kinetic performance of the positive and negative electrode sheets, and reduces the risk of lithium plating on the negative electrode sheet during high-rate charging.

[0017] Based on the first aspect, in some possible implementations, the polymer layer is made of at least one of polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and polyimide (PI), which have low density and good toughness and elongation.

[0018] Based on the first aspect, in some possible implementations, both the first metal layer and the second metal layer contain aluminum, thereby improving the conductivity, stability and mechanical strength of the positive current collector.

[0019] Based on the first aspect, in some possible implementations, the secondary battery further includes a negative electrode conductive plate and a positive electrode conductive plate. The projections of multiple negative electrode tabs along the thickness direction of the electrode assembly overlap, and these tabs are electrically connected to the negative electrode conductive plate. Similarly, the projections of multiple positive electrode tabs along the thickness direction of the electrode assembly overlap, and these tabs are electrically connected to the positive electrode conductive plate. The casing is a packaging bag, and the positive and negative electrode conductive plates extend from the same side of the packaging bag and are respectively electrically connected to an external circuit.

[0020] A second aspect of this application also provides an electronic device comprising a battery compartment and the aforementioned secondary battery disposed within the battery compartment. The electronic device is powered by the aforementioned secondary battery, which achieves both high energy density and superior kinetic performance. Attached Figure Description

[0021] Figure 1 is a schematic diagram of the structure of a secondary battery provided in one embodiment of this application.

[0022] Figure 2 is a cross-sectional view along II-II of the secondary battery shown in Figure 1 in some embodiments.

[0023] Figure 3 is a cross-sectional view along III-III of the secondary battery shown in Figure 1 in some embodiments.

[0024] Figure 4 is an enlarged view of the secondary battery in Figure 3 at point V.

[0025] Figure 5 is a cross-sectional view along II-II of the secondary battery shown in Figure 1 in some other embodiments.

[0026] Figure 6 is a schematic diagram of the structure of an electronic device provided in one embodiment of this application.

[0027] The following detailed description, in conjunction with the accompanying drawings, will further illustrate this application. Detailed Implementation

[0028] The technical solutions in the embodiments of this application are described clearly and in detail below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the specification of this application is for the purpose of describing particular embodiments only and is not intended to limit this application.

[0029] The embodiments of this application will be described in detail below. However, this application may be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided to provide a thorough and detailed understanding of this application to those skilled in the art.

[0030] Additionally, for brevity and clarity, the dimensions or thicknesses of various components and layers may be enlarged in the accompanying drawings. Throughout the text, the same numerical values ​​refer to the same elements. As used herein, the terms "and / or" and "and / or" include any and all combinations of one or more of the associated enumerated items. Furthermore, it should be understood that when element A is referred to as "connecting" element B, element A may be directly connected to element B, or there may be an intermediate element C and element A and element B may be indirectly connected to each other.

[0031] Furthermore, when describing the implementation of this application, the word "may" refers to "one or more implementations of this application".

[0032] The technical terms used herein are for the purpose of describing particular embodiments and are not intended to limit this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. It should be further understood that the term "comprising," as used in this specification, means the presence of the described features, values, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, values, steps, operations, elements, components, and / or combinations thereof.

[0033] Spatial terms, such as "above," may be used herein for convenience in describing the relationship between one element or feature and another element (or feature) or feature (or feature) illustrated in the figures. It should be understood that, in addition to the directions depicted in the figures, spatial terms are intended to include different orientations of the device or apparatus during use or operation. For example, if the device in the figure is flipped, an element described as "above" or "on" other elements or features would be oriented "below" or "under" other elements or features. Therefore, the exemplary term "above" can include both above and below orientations. It should be understood that although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers, and / or portions, these elements, components, regions, layers, and / or portions should not be limited by these terms. These terms are used to distinguish one element, component, region, layer, or portion from another element, component, region, layer, or portion. Therefore, a first element, component, region, layer, or portion discussed below may be referred to as a second element, component, region, layer, or portion without departing from the teachings of the exemplary embodiments.

[0034] In this application, the design relationships of greater than, less than, or not equal to parameter values ​​need to exclude reasonable errors of the measuring equipment.

[0035] In related technologies, the problem of reduced energy density in multi-tab structures is improved by increasing the coating weight per unit area of ​​the active material layer on the electrode. However, when the coating weight per unit area of ​​the active material layer increases, the electrolyte wetting becomes worse, the migration impedance of active ions increases, and the kinetic performance is reduced. In particular, the polarization increases under high-rate discharge conditions, which can easily cause lithium plating on the negative electrode.

[0036] Referring to Figures 1 to 3, one embodiment of this application provides a secondary battery 100, including a housing 10, an electrode assembly 20, and an electrolyte (not shown). Both the electrode assembly 20 and the electrolyte are disposed within the housing 10. In some embodiments, the housing 10 is a packaging bag obtained by encapsulating with an aluminum-plastic film, which may include a receiving portion 11 and a sealing edge 12 connecting the receiving portion 11. The electrode assembly 20 and the electrolyte are disposed in the receiving portion 11. The sealing edge 12 is used to seal the receiving portion 11. In other embodiments, the housing 10 may also be a metal housing 10, such as a steel or aluminum housing.

[0037] As shown in Figure 2, in some embodiments, the electrode assembly 20 has a wound structure. The electrode assembly 20 includes a positive electrode 21, a negative electrode 22, and a separator 23, with the separator 23 disposed between the positive electrode 21 and the negative electrode 22. The positive electrode 21, the separator 23, and the negative electrode 22 are stacked and wound to form a flat structure. A three-dimensional coordinate system is established based on mutually perpendicular first directions X, second directions Y, and third directions Z. The first direction X is the thickness direction of the electrode assembly 20, and the second direction Y is the width direction of either the positive electrode 21 or the negative electrode 22. Along the winding direction D, the electrode assembly 20 includes a first straight section 201, a first bent section 202, a second straight section 203, and a second bent section 204 connected in sequence. The first direction X is the direction from the first straight section 201 to the second straight section 203, and the third direction Z is the direction from the first bent section 202 to the second bent section 204. In this application, a flat structure refers to an electrode assembly 20 whose dimension along the third direction Z (i.e., the width of the electrode assembly 20) is greater than its dimension along the first direction X (i.e., the thickness of the electrode assembly 20). In some embodiments, the positive electrode 21 is located on the outermost side of at least a portion of the electrode assembly 20. For example, the positive electrode 21 may be located on the outermost side of the first bent section 202 of the electrode assembly 20, or on the outermost side of the second straight section 203, or on the outermost side of the second bent section 204, or on the outermost side of the first straight section 201.

[0038] In other embodiments, the electrode assembly 20 may also be a stacked structure, comprising a plurality of positive electrode plates 21, a plurality of negative electrode plates 22, and a plurality of separators 23. In the stacked structure, the positive electrode plates 21 and negative electrode plates 22 are stacked alternately, with one negative electrode plate 22 in every two adjacent positive electrode plates 21 and one positive electrode plate 21 in every two adjacent negative electrode plates 22. The separators 23 are disposed between adjacent positive electrode plates 21 and negative electrode plates 22.

[0039] As shown in Figures 2 and 3, the negative electrode sheet 22 includes a negative electrode current collector 220, a negative electrode active material layer 221, and multiple negative electrode tabs 222. The negative electrode current collector 220 is a copper foil. The negative electrode active material layer 221 is disposed on the negative electrode current collector 220 and contains negative electrode active material. The negative electrode active material includes a carbon material capable of reversible insertion / extraction of active ions. The multiple negative electrode tabs 222 can be integrally formed with the negative electrode current collector 220; for example, the negative electrode tabs 222 can be cut from the negative electrode current collector 220. The coating weight w per unit area of ​​the negative electrode active material layer 221 is 70 mg / 1540.25 mm². 2 Up to 160mg / 1540.25mm 2By setting the range of coating weight per unit area of ​​the negative electrode active material layer 221, it is beneficial to improve the energy density of the secondary battery 100. In some embodiments, the carbon material includes at least one of graphite (such as artificial graphite, natural graphite, etc.), hard carbon, or soft carbon. In this application, the coating weight per unit area w can be measured in the following way: the secondary battery 100 is discharged to 0 SOC%, the negative electrode sheet 22 is disassembled, cleaned with dimethyl carbonate (DMC), and dried; then, a negative electrode sheet 22 sample of a certain area S is punched out, and it is weighed using a balance, and the weight is recorded as W1; the negative electrode active material layer 221 of the sample is washed away with the solvent N-methylpyrrolidone (NMP), dried, and the weight of the negative electrode current collector 220 is weighed and recorded as W2; the coating weight per unit area is calculated by the following formula: w = (W1 - W2) / S.

[0040] The positive electrode 21 includes a positive current collector 210, a positive active material layer 211, and a plurality of positive electrode tabs 212. The positive current collector 210 is a composite current collector, comprising a first metal layer 2101, a polymer layer 2103, and a second metal layer 2102 stacked together. The positive active material layer 211 is disposed on the first metal layer 2101 and the second metal layer 2102. The positive active material layer 211, disposed on the first metal layer 2101 and the second metal layer 2102, contains a positive active material, including compounds that reversibly insert and deintercalate lithium ions (i.e., lithium-ionized intercalation compounds). The plurality of positive electrode tabs 212 can be integrally formed with the positive current collector 210; for example, the positive electrode tabs 212 can be formed by cutting the positive current collector 210. Referring to Figure 4, the positive electrode tab 212 may include a first sub-layer 2121, a third sub-layer 2123, and a second sub-layer 2122 stacked together. The first sub-layer 2121 is integrally formed with the first metal layer 2101, the third sub-layer 2123 is integrally formed with the polymer layer 2103, and the second sub-layer 2122 is integrally formed with the second metal layer 2102. When the secondary battery 100 is charged and discharged through the positive electrode tab 212 and the negative electrode tab 222 connected to an external circuit, the current can flow through multiple positive electrode tabs 212 and multiple negative electrode tabs 222 respectively. Therefore, providing multiple positive electrode tabs 212 and multiple negative electrode tabs 222 helps to make the current flowing through the positive electrode plate 21 and the negative electrode plate 22 more uniform, reducing the electrode impedance and thus supporting higher charge and discharge rates. In some embodiments, the positive electrode active material may include a lithium transition metal composite oxide. This lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, and nickel. In some embodiments, the positive electrode active material is selected from lithium cobalt oxide (LiCoO2), lithium nickel manganese cobalt ternary materials (NCM), lithium manganese oxide (LiMn2O4), and lithium nickel manganese oxide (LiNi). 0.5 Mn 1.5At least one of lithium iron phosphate (LiFePO4) or lithium iron phosphate (LiFePO4).

[0041] In some embodiments, the polymer layer 2103 is made of at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide, which have low density and good toughness and elongation. The first metal layer 2101 and the second metal layer 2102 may respectively contain aluminum or nickel, thereby improving the conductivity, stability, and mechanical strength of the positive electrode current collector 210. In some embodiments, both the first metal layer 2101 and the second metal layer 2102 contain aluminum.

[0042] As shown in Figure 3, the negative current collector 220 includes a first end edge 220A and a second end edge 220B disposed opposite to each other in the second direction Y. In the second direction Y, the first end edge 220A is closer to the sealing edge 12 than the second end edge 220B. The negative electrode tab 222 extends from the first end edge 220A to form a negative electrode sheet 22. The positive current collector 210 includes a third end edge 210A and a fourth end edge 210B disposed opposite to each other in the second direction Y. In the second direction Y, the third end edge 210A is closer to the sealing edge 12 than the fourth end edge 210B. The positive electrode tab 212 extends from the third end edge 210A to form a positive electrode sheet 21. In the embodiments of this application, the overlapping of multiple negative electrode tabs 222 or multiple positive electrode tabs 212 means that the multiple positive electrode tabs 212 or multiple negative electrode tabs 222 at least partially cover each other in the first direction X.

[0043] The secondary battery 100 may further include a negative electrode conductive plate 40 and a positive electrode conductive plate 30. The projections of multiple negative electrode tabs 222 in the first direction X overlap, and the multiple negative electrode tabs 222 are connected to the negative electrode conductive plate 40. The projections of multiple positive electrode tabs 212 in the first direction X of the electrode assembly 20 overlap, and the multiple positive electrode tabs 212 are connected to the positive electrode conductive plate 30. When the housing 10 is a packaging bag, the positive electrode conductive plate 30 and the negative electrode conductive plate 40 extend from the same side of the packaging bag (e.g., from the sealing edge 12) and are configured to be electrically connected to an external circuit (not shown).

[0044] In this application, by setting multiple positive electrode tabs 212 and multiple negative electrode tabs 222, the current flowing through the positive electrode 21 and negative electrode 22 is more uniform, reducing electrode impedance. Therefore, it can support higher charge and discharge rates, improving the rate performance of the secondary battery 100. Furthermore, this application increases the coating weight per unit area of ​​the negative electrode active material layer 221, thereby mitigating the energy density reduction problem caused by the multi-tab structure in the secondary battery 100. Simultaneously, this application sets the positive electrode current collector 210 as a composite current collector including a first metal layer 2101, a polymer layer 2103, and a second metal layer 2102. This increases the impedance of the positive electrode current collector 210, thereby reducing the rate at which active ions are extracted from the positive electrode active material layer 211. This better matches the kinetic performance of the positive electrode 21 and negative electrode 22, reducing the risk of lithium plating on the negative electrode 22 during high-rate charging. Therefore, this application achieves a balance between the energy density and kinetic performance of the secondary battery 100 by increasing the coating weight per unit area of ​​the negative electrode active material layer 221 in the multi-tab structure and further configuring the positive electrode current collector 210 as a composite current collector structure, thus enabling the secondary battery 100 to achieve both high energy density and superior kinetic performance. Furthermore, when the positive electrode 21 is located on the outermost side of at least part of the electrode assembly 20, the composite current collector structure can also improve the pin penetration rate of the secondary battery 100.

[0045] In some embodiments, the compaction density ρ of the negative electrode active material layer 221 is 1.63 g / cm³. 3 Up to 1.80 g / cm 3 By increasing the compaction density of the negative electrode active material layer 221, it is beneficial to further improve the problem of reduced energy density of the secondary battery 100 caused by the multi-tab structure. Moreover, while further improving the energy density of the secondary battery 100, since the positive electrode current collector 210 in this application is a composite current collector including a first metal layer 2101, a polymer layer 2103 and a second metal layer 2102, it can better balance the dynamic performance of the positive electrode 21 and the negative electrode 22, and reduce the risk of lithium plating of the negative electrode 22 during high-rate charging. Therefore, the secondary battery 100 can still achieve both high energy density and better dynamic performance.

[0046] In some embodiments, the compaction density ρ of the negative electrode active material layer 221 is 1.65 g / cm³. 3 Up to 1.78 g / cm 3By further increasing the compaction density of the negative electrode active material layer 221, it is beneficial to further improve the problem of reduced energy density of the secondary battery 100 caused by the multi-tab structure. Moreover, while further improving the energy density of the secondary battery 100, since the positive electrode current collector 210 in this application is a composite current collector including the first metal layer 2101, the polymer layer 2103 and the second metal layer 2102, it can better balance the dynamic performance of the positive electrode 21 and the negative electrode 22, and reduce the risk of lithium plating of the negative electrode 22 during high-rate charging. Therefore, the secondary battery 100 can still achieve both high energy density and better dynamic performance. In this application, the compaction density ρ can be measured as follows: The secondary battery 100 is discharged to 0% SOC, disassembled to obtain the negative electrode sheet 22, cleaned with dimethyl carbonate (DMC) and dried; then, a negative electrode sheet 22 sample of a certain area S is punched out, weighed using a balance, and the weight is recorded as W1; the thickness of the sample is measured using a micrometer and recorded as d; the negative electrode active material layer 221 of the sample is washed away using the solvent N-methylpyrrolidone (NMP), dried, the weight of the negative electrode current collector 220 is weighed and recorded as W2, and the thickness of the negative electrode current collector 220 is measured using a micrometer and recorded as d2. Then, the compaction density ρ of the negative electrode active material layer 221 is calculated using the following formula: ρ=(W1-W2) / [(d-d2)×S].

[0047] In some embodiments, the thickness H3 of the polymer layer 2103 can be set to 2 μm to 13 μm, the thickness H1 of the first metal layer 2101 to 0.2 μm to 1.3 μm, and the thickness H2 of the second metal layer 2102 to 0.2 μm to 1.3 μm. By setting the thicknesses of the polymer layer 2103, the first metal layer 2101, and the second metal layer 2102, the positive electrode current collector 210 can have a higher impedance. Therefore, when the unit area coating weight and / or compaction density of the negative electrode active material layer 221 is high, the kinetic performance of the positive electrode 21 and the negative electrode 22 can be better matched, reducing the risk of lithium plating on the negative electrode 22 during high-rate charging. The thicknesses of the first metal layer 2101 and the second metal layer 2102 can be the same or different. In some embodiments, the thickness can be set to 6μm≤H3≤13μm to further improve the impedance of the positive current collector 210. This allows for better matching of the kinetic performance of the positive electrode 21 and the negative electrode 22 when the coating weight per unit area and / or compaction density of the negative electrode active material layer 221 is high. Alternatively, the thickness can be set to 0.2μm≤H1≤0.8μm and 0.2μm≤H2≤0.8μm to further improve the impedance of the positive current collector 210. This also allows for better matching of the kinetic performance of the positive electrode 21 and the negative electrode 22 when the coating weight per unit area and / or compaction density of the negative electrode active material layer 221 is high.

[0048] In some embodiments, the negative electrode active material may further include silicon. By adding silicon to the negative electrode active material, the negative electrode active material has a higher specific capacity, thereby improving the energy density of the secondary battery 100. Moreover, while further improving the energy density of the secondary battery 100, since the positive electrode current collector 210 in this application is a composite current collector including a first metal layer 2101, a polymer layer 2103, and a second metal layer 2102, it can better balance the kinetic performance of the positive electrode 21 and the negative electrode 22, and reduce the risk of lithium plating of the negative electrode 22 during high-rate charging. Therefore, the secondary battery 100 can still achieve both high energy density and superior kinetic performance. Furthermore, if the negative electrode active material undergoes volume expansion due to the insertion of active ions, causing the negative electrode sheet 22 to expand and deform, the positive electrode current collector 210 of this application is a composite current collector including a first metal layer 2101, a polymer layer 2103, and a second metal layer 2102. Compared to conventional pure metal current collectors of the same thickness, it has a higher elongation, thus improving the elongation of the positive electrode sheet 21 containing the positive electrode current collector 210 and reducing the risk of breakage of the positive electrode sheet 21 due to the expansion and deformation of the negative electrode sheet 22. The silicon material can be at least one of silicon oxide, silicon carbide, or elemental silicon. The silicon material can also be dispersed within the pores of the carbon material, which is used to suppress the volume expansion of the silicon material during cycling.

[0049] As shown in Figure 2, in some embodiments, when the housing 10 is a packaging bag obtained by aluminum-plastic film encapsulation, the positive electrode 21 is located on the outermost side of the first bending section 202 of the wound electrode assembly 20. Therefore, if the negative electrode active material expands in volume due to the embedding of active ions (especially when silicon material is added to the negative electrode active material), the negative electrode 22 will expand and deform, resulting in a certain risk of breakage of the outermost electrode of the electrode assembly 20. Therefore, by setting the outermost side of the first bending section 202 to be a positive electrode 21 with high ductility, the risk of breakage of the outermost electrode due to the expansion and deformation of the negative electrode 22 can be reduced. Moreover, the outermost positive electrode 21 can also effectively isolate the negative electrode 22 from the packaging bag. Even if the negative electrode 22 at the first bending section 202 produces a sharp corner, the risk of electrochemical corrosion caused by the negative electrode 22 piercing the packaging bag and the electrolyte in the electrolyte contacting the metal layer (i.e., the aluminum layer) of the packaging bag can be reduced. Furthermore, the positive electrode 21 can also be located on the outermost side of the second bending section 204 of the electrode assembly 20, thereby further reducing the risk of breakage of the outermost electrode when the negative electrode 22 expands and deforms, and further reducing the risk of electrochemical corrosion caused by the negative electrode 22 puncturing the packaging bag.

[0050] As shown in Figures 2 and 3, in some embodiments, when the electrode assembly 20 is a wound structure, a layer of positive current collector 210 and a layer of positive active material 211 on the surface of the positive current collector 210 are defined as a layer of positive electrode sheet 21. Along the first direction X, the electrode assembly 20 includes N layers of positive electrode sheets 21, and each layer of positive electrode sheet 21 is provided with a positive electrode tab 212 (i.e., a full tab structure). Correspondingly, a layer of negative current collector 220 and a layer of negative active material 221 on the surface of the negative current collector 220 are defined as a layer of negative electrode sheet 22. Along the first direction X, the electrode assembly 20 includes M layers of negative electrode sheets 22, and each layer of negative electrode sheet 22 is provided with a negative electrode tab 222. In this way, the secondary battery 100 can have a higher rate performance.

[0051] As shown in Figure 5, in some other embodiments, the number of positive electrode tabs 212 can be less than or equal to 2 / N. Specifically, the number of positive electrode tabs 212 is defined as n, where 2 ≤ n ≤ 2 / N. By appropriately reducing the number of positive electrode tabs 212, the impedance of the positive electrode 21 can be further increased while maintaining a high rate performance of the secondary battery 100. This reduces the rate at which active ions are extracted from the positive electrode active material layer 211, better matches the kinetic performance of the positive electrode 21 and the negative electrode 22, and reduces the risk of lithium plating on the negative electrode 22 during high-rate charging.

[0052] The secondary battery 100 of this application includes all devices capable of undergoing electrochemical reactions. Specifically, the secondary battery 100 includes all types of primary batteries, secondary batteries, fuel cells, solar cells, and capacitors (e.g., supercapacitors). Optionally, the secondary battery 100 can be a lithium secondary battery, including lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, and lithium-ion polymer secondary batteries.

[0053] Referring to Figure 6, one embodiment of this application also provides an electronic device 1, which includes a battery compartment 101 and a secondary battery 100 housed within the battery compartment 101. The electronic device 1 is powered by the aforementioned secondary battery 100, and the secondary battery 100 can achieve both high energy density and superior fast-charging performance. The secondary battery 100 of this application is applicable to electronic devices 1 in various fields. In one embodiment, the electronic device 1 of this application may be, but is not limited to, laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors, etc.

[0054] The following uses a secondary battery 100 as an example of a lithium-ion pouch battery to illustrate the preparation method and performance of the secondary battery 100 provided in this application through specific embodiments and comparative examples. Those skilled in the art should understand that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application.

[0055] Example 1

[0056] (1) Preparation of positive electrode 21: Lithium cobalt oxide (LiCoO2), conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 96.5:1.5:2. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75 wt%, and the mixture was stirred evenly. A composite current collector (1 μm Al foil + 5 μm PET layer + 1 μm Al foil) was used as the positive electrode current collector. The slurry was uniformly coated on one surface of the composite current collector, leaving a blank foil area at the edge of the composite current collector. The mixture was dried at 110 °C. The above steps were repeated on the other surface of the composite current collector to obtain a double-sided coated positive electrode 21. The initial positive electrode 21 was cold-pressed to obtain a single coating layer 211 with a thickness of 77 μm. Then, the positive electrode 21 was pre-cut, and the reserved blank foil area was die-cut to obtain multiple positive electrode tabs 212, thus obtaining the positive electrode 21.

[0057] (2) Preparation of negative electrode sheet 22: Artificial graphite, conductive carbon black (Super P), polyacrylic acid binder (PAA), and lithium difluorophosphate (LDPF) were mixed in a weight ratio of 74:6:19:1. Deionized water was added as a solvent to prepare a slurry with a weight percentage of 55 wt%, and the mixture was stirred evenly. The slurry was uniformly coated onto one surface of a 5 μm thick negative electrode current collector 220, i.e., a copper foil, leaving an empty foil area at the edge of the copper foil. The slurry was dried at 110°C. The above steps were repeated on the other surface of the copper foil to obtain a double-sided coated negative electrode sheet 22. The coating weight w per unit area of ​​the negative electrode active material layer 221 was 70 mg / 1540.25 mm. 2 The compacted density ρ is 1.65 g / cm³. 3 The initial negative electrode sheet 22 is rolled to obtain a negative electrode active material layer 221 with a coating thickness of 70 μm. Then, the negative electrode sheet 22 is pre-cut, and the reserved empty foil area is die-cut to obtain multiple negative electrode tabs 222, thus obtaining the negative electrode sheet 22.

[0058] (3) Preparation of electrolyte: In a dry argon atmosphere, the organic solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are first mixed in a mass ratio of EC:EMC:DEC = 30:50:20. Then, lithium salt lithium hexafluorophosphate (LiPF6) 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.

[0059] (4) Preparation of the isolation membrane 23: A polyethylene (PE) membrane with a thickness of 9 μm was selected.

[0060] (5) Assembly of the secondary battery 100: The positive electrode 21, the separator 23, and the negative electrode 22 are stacked and wound to obtain a flat electrode assembly 20, wherein each layer of positive electrode 21 has a positive electrode tab 212 and each layer of negative electrode 22 has a negative electrode tab 222. The negative electrode tabs 222 are stacked and welded to the negative electrode conductive plate 40, which is made of copper. The positive electrode tabs 212 are stacked and welded to the positive electrode conductive plate 30, which is also made of copper. Then, the dented aluminum-plastic film (150 μm thick) is placed in the assembly fixture with the dent facing upward, and the electrode assembly 20 is placed in the dent. Electrolyte is injected into the dent of the aluminum-plastic film, and the positive electrode conductive plate 30 and the negative electrode conductive plate 40 are led out of the aluminum-plastic film. Then, formation and encapsulation are performed to obtain the secondary battery 100 shown in Figures 1 to 3.

[0061] Examples 2-7

[0062] The difference from Example 1 is that the values ​​of w, H1, H2, H3, etc. are specifically recorded in Table 1.

[0063] Comparative Example 1

[0064] The difference from Example 1 is the value of w, and the positive current collector uses aluminum foil of the same thickness as the composite current collector described above.

[0065] Comparative Example 2

[0066] The difference from Example 2 is that the positive current collector uses aluminum foil of the same thickness as the composite current collector described above.

[0067] Comparative Example 3

[0068] The difference from Example 1 lies in the value of w.

[0069] The energy density and kinetic performance of the secondary batteries 100 in each comparative example and embodiment were tested, and the test results are recorded in Table 1.

[0070] The energy density test steps are as follows: 1) At a test temperature of 25℃, the secondary battery 100 is left to stand for 5 minutes, discharged to the termination voltage with a constant current of 0.2C (CC), left to stand for another 5 minutes, charged to the limit voltage with a constant current of 0.2C, and then charged under the limit voltage constant voltage condition (CV) until the current decreases to 0.02C. After standing for 5 minutes, it is discharged to the termination voltage with a constant current of 0.2C. The capacity of the secondary battery 100 is recorded, and the energy D is obtained by multiplying the capacity by the plateau voltage; 2) The length, width, and height of the secondary battery 100 are measured with an optical detector to calculate the volume T. The energy density (ED) = D / T, with the unit being Wh / L.

[0071] The kinetic performance test steps for the secondary battery 100 are as follows: 1) At a test temperature of 25℃, the secondary battery 100 is left to stand for 5 minutes, then charged at a constant current of 3C to 4.53V, and then charged at a constant voltage of 4.53V to a current of 0.05C; 2) The electrode assembly 20 is disassembled, and the surface of the negative electrode 22 is inspected. If a gray area is present, it indicates lithium plating; if no gray area is present, there is no lithium plating. The degree of lithium plating is divided into slight lithium plating, moderate lithium plating, and severe lithium plating. Slight lithium plating is defined as lithium plating area less than 0.5% of the entire negative electrode 22 area; moderate lithium plating is defined as lithium plating area between 0.5% and 5% of the entire negative electrode 22 area; and severe lithium plating is defined as lithium plating area greater than 5% of the entire negative electrode 22 area.

[0072] Table 1

[0073] As shown in Table 1, compared to Example 1, Comparative Example 1 did not increase the coating weight per unit area of ​​the negative electrode active material layer, and the positive electrode current collector used pure aluminum foil. Therefore, the energy density of the secondary battery was low, making it difficult to meet the increasingly higher energy density requirements of portable devices, and the pin penetration rate was also low. Compared to Example 2, Comparative Example 2 increased the coating weight per unit area of ​​the negative electrode active material layer, but the positive electrode current collector used pure aluminum foil. Therefore, the energy density of the secondary battery increased accordingly, but the kinetic performance of the negative electrode decreased. Under high-rate discharge, polarization increased, and severe lithium plating occurred on the negative electrode, resulting in a low pin penetration rate. Although Comparative Example 3 used a composite current collector for the positive electrode, significantly improving the pin penetration rate, the coating weight per unit area of ​​the negative electrode active material layer was too large. Therefore, it was difficult to match the kinetic performance of the positive and negative electrodes, resulting in increased polarization under high-rate discharge and severe lithium plating on the negative electrode.

[0074] Compared to Comparative Examples 1-3, Examples 1-3, while increasing the coating weight per unit area of ​​the negative electrode active material layer to a specific range, further set the positive electrode current collector to a composite current collector structure. Therefore, not only is the energy density of the secondary battery improved, but the kinetic performance of the positive and negative electrode plates can also be better matched. No lithium is deposited at the interface during high-rate charging. Therefore, the secondary battery can take into account both high energy density and better kinetic performance.

[0075] Furthermore, compared to Examples 6-7, Examples 1 and 4-5 increase the compaction density of the negative electrode active material layer to a specific range, which further improves the energy density of the secondary battery while still maintaining better kinetic performance.

[0076] Examples 8-15

[0077] The difference from Example 3 is that the values ​​of H1, H2, H3, etc. are specifically recorded in Table 2.

[0078] Example 16

[0079] The difference from Example 3 is that the number of positive electrode tabs 212 is n = 2 / N.

[0080] Table 2

[0081] As can be seen from the data in Table 2, based on Examples 3 and 11, Examples 8-10 increase the thickness of the polymer layer of the positive current collector, thereby increasing the impedance of the positive current collector. Therefore, it can better match the dynamic performance of the positive and negative electrodes, and no lithium is deposited at the interface during high-rate charging.

[0082] Based on Example 12, Examples 13-15 reduce the thickness of the first and second metal layers of the positive current collector, thereby increasing the impedance of the positive current collector. This better matches the dynamic performance of the positive and negative electrodes, and no lithium is deposited at the interface during high-rate charging.

[0083] Compared to Example 3, Example 16 sets the number of positive electrode tabs n = 2 / N. Since the number of positive electrode tabs is reduced, the impedance of the positive electrode sheet is further increased. Therefore, the dynamic performance of the positive electrode sheet and the negative electrode sheet can be better matched, and no lithium is deposited at the interface during high-rate charging.

[0084] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the spirit and scope of the technical solutions of this application.

Claims

1. A secondary battery, comprising a casing and an electrode assembly disposed within the casing, the electrode assembly comprising a negative electrode, a positive electrode, and a separator disposed between the negative electrode and the positive electrode, wherein, The negative electrode sheet includes a negative current collector, a negative active material layer, and multiple negative electrode tabs; the negative current collector is a copper foil, and the negative active material layer is disposed on the negative current collector; the negative active material layer contains a negative active material, which includes a carbon material, and the coating weight w per unit area of ​​the negative active material layer is 70 mg / 1540.25 mm². 2 Up to 160mg / 1540.25mm 2 ; The positive electrode sheet includes a positive current collector, a positive active material layer, and multiple positive electrode tabs; the positive current collector includes a first metal layer, a polymer layer, and a second metal layer stacked together, and the positive active material layer is respectively disposed on the first metal layer and the second metal layer.

2. The secondary battery as described in claim 1, wherein, The compaction density ρ of the negative electrode active material layer is 1.63 g / cm³. 3 Up to 1.80 g / cm 3 .

3. The secondary battery as described in claim 1, wherein, The compaction density ρ of the negative electrode active material layer is 1.65 g / cm³. 3 Up to 1.78 g / cm 3 .

4. The secondary battery as described in claim 1, wherein, The thickness H3 of the polymer layer is 2 μm to 13 μm, the thickness H1 of the first metal layer is 0.2 μm to 1.3 μm, and the thickness H2 of the second metal layer is 0.2 μm to 1.3 μm.

5. The secondary battery as described in claim 4, wherein, 6μm≤H3≤13μm; and / or, 0.2μm≤H1≤0.8μm, 0.2μm≤H2≤0.8μm.

6. The secondary battery as described in claim 1, wherein, The carbon material includes at least one of graphite, hard carbon, or soft carbon.

7. The secondary battery according to any one of claims 1 to 6, wherein, The negative electrode active material also includes silicon material.

8. The secondary battery as described in claim 7, wherein, The silicon material includes at least one of silicon oxide, silicon carbide, or elemental silicon.

9. The secondary battery as described in claim 1, wherein, The housing is an aluminum-plastic film. The negative electrode, the separator, and the positive electrode are stacked and wound to form a flat structure. The electrode assembly includes a first bending section, and the positive electrode is located on the outermost side of the first bending section.

10. The secondary battery as claimed in claim 1, wherein, The negative electrode sheet, the separator, and the positive electrode sheet are stacked and wound to form a flat structure. Along the thickness direction of the electrode assembly, the electrode assembly includes N layers of the positive electrode sheet; the number of positive electrode tabs is n, where n≤2 / N.

11. The secondary battery as claimed in claim 1, wherein, The polymer layer is made of at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide.

12. The secondary battery as claimed in claim 1, wherein, Both the first metal layer and the second metal layer contain aluminum.

13. The secondary battery according to any one of claims 1 to 12, wherein, The secondary battery also includes a negative electrode conductive plate and a positive electrode conductive plate. The projections of the plurality of negative electrode tabs in the thickness direction of the electrode assembly overlap, and the plurality of negative electrode tabs are electrically connected to the negative electrode conductive plate. The projections of the plurality of positive electrode tabs in the thickness direction overlap, and the plurality of positive electrode tabs are electrically connected to the positive electrode conductive plate. The shell is a packaging bag, and the positive conductive plate and the negative conductive plate extend out of the packaging bag from the same side and are respectively electrically connected to the external circuit.

14. An electronic device, wherein, It includes a battery compartment and a secondary battery as described in any one of claims 1 to 13 disposed within the battery compartment.

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