Secondary battery and electrical apparatus

Abstract

The present application belongs to the technical field of batteries, and discloses a secondary battery and an electrical apparatus. The secondary battery comprises a wound battery cell, the wound battery cell comprising a flat region and a corner region, and the wound battery cell comprising a positive electrode sheet, a negative electrode sheet, and a separator between the positive electrode sheet and the negative electrode sheet. The positive electrode sheet comprises a positive electrode current collector, and a flame retardant layer and a positive electrode active material layer which are sequentially disposed on at least one surface of the positive electrode current collector. In the present application, a thickness relationship between the flame retardant layer and the positive electrode active material layer in the flat region and the corner region is controlled, so that the phenomenon of lithium precipitation at a corner can be significantly ameliorated.

Description

Secondary batteries and electrical devices Technical Field

[0001] This application relates to the field of battery technology, specifically to a secondary battery and an electrical device. Background Technology

[0002] During the charging and discharging process of secondary batteries, especially lithium batteries, lithium plating may occur when overcharged or in an unstable operating environment. Lithium plating refers to the excessive reduction of lithium ions to lithium metal on the surface of the negative electrode material and its deposition, forming lithium dendrites. These dendrites not only reduce the battery capacity but may also puncture the battery separator, causing a short circuit, leading to thermal runaway, or even a fire or explosion.

[0003] Currently, most existing technologies reduce lithium plating by adjusting battery materials or optimizing the battery management system. For example, this involves improving the electrolyte, changing the negative electrode material, or optimizing the battery's charge and discharge algorithms. However, these methods often fail to effectively eliminate lithium plating, especially under conditions of high charging current, high current density, or prolonged use, where lithium plating still occurs.

[0004] Therefore, this application is submitted. Summary of the Invention

[0005] This application provides a secondary battery and power supply device that can significantly improve the lithium plating phenomenon at corners.

[0006] In a first aspect, this application provides a secondary battery, including a wound cell, the wound cell including a flat region and a corner region, the wound cell including a positive electrode, a negative electrode and a separator between the positive electrode and the negative electrode; the positive electrode includes a positive current collector and a flame retardant layer and a positive active material layer sequentially disposed on at least one surface of the positive current collector;

[0007] The flame-retardant layer includes a first flame-retardant layer located in the straight area and a second flame-retardant layer located in the corner area. The thickness of the first flame-retardant layer is A1, and the thickness of the second flame-retardant layer is A2, satisfying: A1 <A2;

[0008] The positive electrode active material layer includes a first positive electrode active material layer located in the flat region and a second positive electrode active material layer located in the corner region. The thickness of the first positive electrode active material layer is B1 and the thickness of the second positive electrode active material layer is B2, satisfying that B1>B2.

[0009] In some implementations, the following condition is satisfied: 1.5 ≤ A2 / A1 ≤ 10.

[0010] In some implementations, the following condition is satisfied: 1.08 ≤ B1 / B2 ≤ 1.3.

[0011] In some implementations, at least one of the following features is satisfied:

[0012] (1) 0.5μm≤A1≤10μm;

[0013] (2) 0.5μm≤A2≤10μm;

[0014] (3) 30μm≤B1≤100μm;

[0015] (4) 30μm≤B2≤100μm.

[0016] In some embodiments, the areal density of the first flame-retardant layer is C1, and the areal density of the second flame-retardant layer is C2, satisfying: C1 <C2。

[0017] In some embodiments, the areal density of the first positive electrode active material layer is D1, and the areal density of the second positive electrode active material layer is D2, satisfying: D2 <D1。

[0018] In some implementations, the following condition is satisfied: 1.5≤C2 / C1≤6.

[0019] In some implementations, the following condition is satisfied: 1.5≤D1 / D2≤5.

[0020] In some implementations, at least one of the following features is satisfied:

[0021] (5) 5mg / 1540.25mm 2 ≤C1≤30mg / 1540.25mm 2 ;

[0022] (6) 5mg / 1540.25mm 2 ≤C2≤30mg / 1540.25mm 2 ;

[0023] (7) 150mg / 1540.25mm 2 ≤D1≤500mg / 1540.25mm 2 ;

[0024] (8) 150mg / 1540.25mm 2 ≤D2≤500mg / 1540.25mm 2 .

[0025] In some embodiments, the flame-retardant layer comprises a flame-retardant material, which includes at least one of alumina, barium sulfate, titanium dioxide, silicon dioxide, aluminum hydroxide, magnesium oxide, boehmite, polyethylene microspheres, barium titanate, magnesium hydroxide, melamine, lithium iron phosphate, aluminum sol, and nanocellulose.

[0026] In some embodiments, the thickness of the positive current collector is 6–12 μm, and the areal density is 18–34 g / m³. 2 .

[0027] In a second aspect, this application provides an electrical device including the aforementioned secondary battery.

[0028] The beneficial effects of this application are as follows: This application sets a flame-retardant layer and a positive electrode active material layer sequentially on the surface of the positive electrode current collector, and controls the thickness relationship between the flame-retardant layer and the positive electrode active material layer in the flat region and the corner region. The thickness of the positive electrode active material layer is reduced in the corner region, the amount of positive electrode active material is reduced, and there are fewer lithium ions, which effectively reduces the lithium ions at the corner and improves the accumulation of lithium ions in the corner region. At the same time, due to the thickness change in the flat region and the corner region, the formation of stress concentration points caused by the structural changes in the flat region and the corner region is improved, thereby reducing the stress in the corner region. During the charging and discharging process, the lithium intercalation resistance is reduced, and the amount of lithium deposited on the negative electrode surface is reduced, thereby significantly improving the lithium deposition phenomenon at the corner. Attached Figure Description

[0029] Figure 1 is a schematic diagram of the structure of the secondary battery described in this application.

[0030] Figure 2 is a schematic diagram of the gravure roller of this application. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0032] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0033] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0034] In this application, there are no particular restrictions on the specific dispersion and mixing methods.

[0035] Unless otherwise specified, all components, raw materials, or instruments used in the embodiments and comparative examples of this application are commercially available, and the components and raw materials used in each parallel experiment are the same.

[0036] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0037] The term "rechargeable battery," also known as a rechargeable battery or accumulator, refers to a battery that can be recharged after being discharged to activate its active materials and continue to be used.

[0038] The term "surface density" (or "planar density") is the mass per unit area of ​​a material with a specified thickness. This term is typically used for materials with uniform or negligible thickness (such as paper, metal foil, and battery electrodes).

[0039] The inventors of this application discovered that in the winding structure of a battery cell, stress concentration points easily form at corners due to abrupt shape changes. During charging and discharging, the volume change of the negative electrode active material (especially silicon-based negative electrode active material) is more constrained at corners, leading to further stress increase and promoting lithium plating. As charging progresses, the intercalation space of lithium ions in the negative electrode active material gradually decreases, resulting in increased resistance to lithium intercalation. When the lithium intercalation process is hindered, some lithium ions may not be able to find suitable intercalation sites inside the negative electrode, thus depositing on the negative electrode surface, especially at corners where lithium plating is severe. Lithium plating leads to a decrease in cell performance, a shortened cycle life, and an increased risk of short circuits.

[0040] Therefore, based on the above problems, as shown in Figure 1, in some embodiments, this application provides a secondary battery including a wound cell, the wound cell including a flat area and a corner area, the wound cell including a positive electrode 1, a negative electrode 2 and a separator 3 between the positive electrode and the negative electrode; the positive electrode includes a positive current collector 11 and a flame retardant layer 12 and a positive active material layer 13 sequentially disposed on at least one surface of the positive current collector;

[0041] The flame-retardant layer includes a first flame-retardant layer 121 located in the straight area and a second flame-retardant layer 122 located in the corner area. The thickness of the first flame-retardant layer is A1, and the thickness of the second flame-retardant layer is A2, satisfying: A1 <A2;

[0042] The positive electrode active material layer includes a first positive electrode active material layer 131 located in the straight region and a second positive electrode active material layer 132 located in the corner region. The thickness of the first positive electrode active material layer is B1, and the thickness of the second flame retardant layer is B2, satisfying that B1>B2.

[0043] This application involves sequentially depositing a flame-retardant layer and a positive electrode active material layer on the surface of the positive electrode current collector, and controlling the thickness relationship between the flame-retardant layer and the positive electrode active material layer in the flat region and the corner region. Specifically, the thickness of the positive electrode active material layer is reduced in the corner region, resulting in less positive electrode active material and fewer lithium ions. This effectively reduces lithium ions at the corner and improves lithium ion accumulation in the corner region. Simultaneously, the thickness variation in the flat region and the corner region improves the formation of stress concentration points caused by structural changes in the flat region and the corner region, thereby reducing stress in the corner region. During charging and discharging, this reduces lithium intercalation resistance and decreases the amount of lithium deposited on the negative electrode surface, thus significantly improving the lithium deposition phenomenon at the corner.

[0044] The flame-retardant layer, by controlling the thickness relationship between the flat and corner regions, contains more flame-retardant material (and less corresponding positive electrode active material) in the corner region. This effectively improves the safety performance of the secondary battery and increases the needle penetration pass rate. At the same time, the improved lithium plating interface in the corner region can suppress the formation of lithium dendrites, reduce the risk of short circuits, reduce the internal resistance of the secondary battery, improve the lithium ion transport efficiency, and effectively improve the cycle performance of the secondary battery.

[0045] It is understood that in this application, the phrase "the positive electrode sheet includes a positive current collector and a flame-retardant layer and a positive active material layer sequentially disposed on at least one surface of the positive current collector" refers to the substance coated / covered / attached on at least one surface of the positive current collector including a flame-retardant layer and a positive active material layer, wherein the flame-retardant layer is closer to the positive current collector than the positive active material layer (i.e., the positive active material layer is farther away from the positive current collector than the flame-retardant layer); it can also be understood that when the positive current collector is considered as the inner layer and the surface in contact with air is considered as the outer layer, the positive active material layer is located on the outermost layer compared to the flame-retardant layer.

[0046] In one embodiment, the following condition is satisfied: 1.5 ≤ A2 / A1 ≤ 10. For example, it can be a range consisting of 1.5, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 5, 5.2, 5.5, 5.8, 6, 7, 8, 9, 10, or any two of these values. In particular, controlling the ratio of A2 / A1 within this range can effectively achieve heat insulation and flame retardancy, effectively prevent heat from spreading inside the secondary battery, improve local overheating, reduce the risk of lithium plating, inhibit the formation of lithium dendrites, reduce the risk of short circuits, reduce the internal resistance of the secondary battery, and improve the safety performance of the secondary battery.

[0047] In one implementation, the following condition is satisfied: 1.8 ≤ A2 / A1 ≤ 6.

[0048] In one embodiment, the following condition is satisfied: 1.08 ≤ B1 / B2 ≤ 1.3. For example, it can be a range consisting of 1.08, 1.1, 1.15, 1.2, 1.25, 1.3, or any two of these values. In particular, controlling the ratio of B1 / B2 within this range can further shorten the diffusion path of lithium ions, reduce the transport resistance of lithium ions, reduce the stress in the corner region, reduce the resistance to lithium intercalation during charging and discharging, reduce the amount of lithium deposited on the negative electrode surface, reduce the risk of thermal runaway, further improve the lithium deposition phenomenon at the corner, and improve the safety performance of the secondary battery.

[0049] In one implementation, the following condition is satisfied: 1.1≤B1 / B2≤1.25.

[0050] In one embodiment, 0.5μm≤A1≤10μm, for example, can be a range of 0.5μm, 0.8μm, 1μm, 1.2μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm, 10μm or any two of these values.

[0051] In one embodiment, 2μm≤A1≤5μm.

[0052] In one embodiment, 0.5μm≤A2≤10μm, for example, can be a range of 0.5μm, 0.8μm, 1μm, 1.2μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm, 10μm or any two of these values.

[0053] In one embodiment, 1.8 μm ≤ A2 ≤ 6 μm.

[0054] Particularly when A1 and A2 are within this range, the flame retardant effect and heat insulation effect can be more effectively improved, effectively preventing the internal diffusion of heat in the secondary battery, further improving the safety performance of the secondary battery, while further improving the lithium deposition interface in the corner area, inhibiting the formation of lithium dendrites, reducing the risk of short circuit, reducing the internal resistance of the secondary battery, improving the lithium ion transmission efficiency, and effectively improving the cycle performance of the secondary battery.

[0055] In one embodiment, 30 μm ≤ B1 ≤ 100 μm. For example, it can be 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 95 μm, 100 μm or the range composed of any two of these values.

[0056] In one embodiment, 33 μm ≤ B1 ≤ 40 μm.

[0057] In some embodiments, 30 μm ≤ B2 ≤ 100 μm. For example, it can be 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 95 μm, 100 μm or the range composed of any two of these values.

[0058] In one embodiment, 30 μm ≤ B2 ≤ 32 μm.

[0059] Particularly when B1 and B2 are controlled within this range, the diffusion path of lithium ions can be further reduced, the diffusion resistance can be reduced, and at the same time, the content of lithium ions in the corner area can be reduced, improving the accumulation of lithium ions in the corner area. During the charge and discharge process, the lithium intercalation resistance can be reduced, and the precipitation amount on the negative electrode surface can be reduced, thereby significantly improving the lithium deposition phenomenon at the corner.

[0060] In one embodiment, the surface density of the first flame retardant layer is C1, and the surface density of the second flame retardant layer is C2, satisfying: C1 < C2. By controlling C1 < C2, the safety of the secondary battery in case of faults such as overcharge, short circuit, and thermal runaway can be improved, preventing the spread of fire or thermal runaway, while contributing to the accumulation of heat, reducing the risks of lithium deposition and thermal runaway.

[0061] In one embodiment, the areal density of the first positive electrode active material layer is D1, and the areal density of the second positive electrode active material layer is D2, satisfying: D2 < D1. By controlling D2 < D1, the content of lithium ions in the corner region can be further reduced, the accumulation of lithium ions in the corner region can be improved, and at the same time, the local current density can be avoided from being too large, the risk of thermal runaway can be reduced, and the phenomenon of lithium deposition at the corner can be further improved.

[0062] In one embodiment, it satisfies: 1.5 ≤ C2 / C1 ≤ 6. For example, it can be 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 5, 5.2, 5.5 or the range composed of any two of these values.

[0063] In one embodiment, it satisfies: 2 ≤ C2 / C1 ≤ 5.

[0064] In one embodiment, it satisfies: 1.5 ≤ D1 / D2 ≤ 5. For example, it can be 1.5, 2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 5 or the range composed of any two of these values.

[0065] In one embodiment, it satisfies: 2 ≤ D1 / D2 ≤ 3.

[0066] Especially when C2 / C1 and D1 / D2 are controlled within the above ranges, the safety performance can be further improved and the phenomenon of lithium deposition can be improved.

[0067] In one embodiment, 5 mg / 1540.25 mm 2 ≤ C1 ≤ 30 mg / 1540.25 mm 2 , for example, it can be 5 mg / 1540.25 mm 2 , 6 mg / 1540.25 mm 2 , 8 mg / 1540.25 mm 2 , 10 mg / 1540.25 mm 2 , 15 mg / 1540.25 mm 2 , 20 mg / 1540.25 mm 2 , 25 mg / 1540.25 mm 2 , 30 mg / 1540.25 mm 2 or the range composed of any two of these values.

[0068] In one embodiment, 5 mg / 1540.25 mm 2 ≤ C1 ≤ 12 mg / 1540.25 mm 2 .

[0069] In one embodiment, 5mg / 1540.25mm 2 ≤C2≤30mg / 1540.25mm 2 For example, it could be 5mg / 1540.25mm 2 6mg / 1540.25mm 2 8mg / 1540.25mm 2 10mg / 1540.25mm 2 15mg / 1540.25mm 2 20mg / 1540.25mm 2 25mg / 1540.25mm 2 30mg / 1540.25mm 2 Or a range consisting of any two of these values.

[0070] In one embodiment, 12mg / 1540.25mm 2 ≤C2≤30mg / 1540.25mm 2 .

[0071] In one embodiment, 150mg / 1540.25mm 2 ≤D1≤500mg / 1540.25mm 2 For example, it could be 150mg / 1540.25mm 2 200mg / 1540.25mm 2 300mg / 1540.25mm 2 400mg / 1540.25mm 2 500mg / 1540.25mm 2 Or a range consisting of any two of these values.

[0072] In one embodiment, 300mg / 1540.25mm 2 ≤D1≤500mg / 1540.25mm 2 .

[0073] In one embodiment, 150mg / 1540.25mm 2 ≤D2≤500mg / 1540.25mm 2 For example, it could be 150mg / 1540.25mm 2 200mg / 1540.25mm 2 300mg / 1540.25mm 2 400mg / 1540.25mm 2 500mg / 1540.25mm2 Or a range consisting of any two of these values.

[0074] In one embodiment, 150mg / 1540.25mm 2 ≤D2≤250mg / 1540.25mm 2 .

[0075] In one embodiment, the thickness of the positive current collector is 6 to 12 μm, for example, it can be 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm or any two of these values.

[0076] In one embodiment, the areal density of the positive current collector is 18–34 g / m³. 2 For example, it could be 18g / m 2 20g / m 2 25g / m 2 30g / m 2 32g / m 2 34g / m 2 Or a range consisting of any two of these values.

[0077] In one embodiment, the positive current collector has a tensile strength ≥17 kgf / mm² and a surface dyne value ≥38 Dyne / cm.

[0078] In one embodiment, the flame-retardant layer comprises a flame-retardant material, which includes at least one of alumina, barium sulfate, titanium dioxide, silicon dioxide, aluminum hydroxide, magnesium oxide, boehmite, polyethylene (PE) microspheres, barium titanate, magnesium hydroxide, melamine, lithium iron phosphate, aluminum sol, and nanocellulose. In some embodiments, the flame-retardant material includes at least one of alumina, barium sulfate, titanium dioxide, silicon dioxide, aluminum hydroxide, magnesium oxide, boehmite, and PE microspheres.

[0079] In one embodiment, the flame-retardant material comprises 50% to 99% by mass in the flame-retardant layer, for example, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 90%, 94%, 99%, or any two of these values. In another embodiment, the flame-retardant material comprises 50% to 80% by mass in the flame-retardant layer.

[0080] In one embodiment, the flame-retardant layer further includes a conductive agent and a binder.

[0081] In one embodiment, the conductive agent has a mass percentage content of 1 to 10% in the flame retardant layer, for example, it can be 1%, 2%, 4%, 6%, 8%, 10%, or any two of these values.

[0082] In one embodiment, the adhesive has a mass percentage content of 5% to 20% in the flame-retardant layer, for example, it can be 5%, 6%, 8%, 10%, 12%, 14%, 15%, 16%, 18%, 20% or any two of these values.

[0083] In one embodiment, the positive electrode active material layer includes a positive electrode active material, which includes at least one of lithium cobalt oxide (LCO), nickel cobalt manganese ternary material (NCM), nickel cobalt aluminum ternary material (NCA), nickel cobalt manganese aluminum quaternary material (NCMA), lithium iron phosphate (LFP), lithium manganese phosphate (LMP), lithium vanadium phosphate (LVP), and lithium manganese oxide (LMO).

[0084] In one embodiment, the mass percentage of the positive electrode active material in the positive electrode active material layer is 93% to 99%, for example, it can be 93%, 94%, 95%, 96%, 97%, 98%, 99% or any two of these values.

[0085] In one embodiment, the positive electrode active material layer further includes a conductive agent and a binder.

[0086] In one embodiment, the conductive agent has a mass percentage content of 0.5% to 5% in the positive electrode active material layer, for example, it can be 0.5%, 1%, 2%, 3%, 4%, 5% or any two of these values.

[0087] In one embodiment, the binder has a mass percentage content of 0.5% to 3% in the positive electrode active material layer, for example, it can be 0.5%, 1%, 2%, 3% or any two of these values.

[0088] In this application, there is no particular limitation on the type of positive electrode current collector; it can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector includes metallic materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum, as well as carbon materials such as carbon cloth and carbon paper. In one embodiment, the positive electrode current collector is a metallic material. In one embodiment, the positive electrode current collector is aluminum.

[0089] There are no particular limitations on the form of the positive electrode current collector. In some embodiments, when the positive electrode current collector is a metallic material, its form can be a metal foil, a metal cylinder, a metal strip, a metal plate, a metal foil mesh, stamped metal, foamed metal, etc. In some embodiments, when the positive electrode current collector is a carbon material, its form can include, but is not limited to, a carbon plate, a carbon film, a carbon cylinder, etc.

[0090] In one embodiment, the flame-retardant layer is formed on the surface of the positive electrode current collector using a gravure printing coating machine, and the first and second flame-retardant layers are formed using a multi-depth gravure roller with different lateral depths. For example, the flame-retardant layer can be formed on the surface of the positive electrode current collector using the gravure roller shown in FIG2. In some embodiments, the thickness of the first and second flame-retardant layers is varied by controlling the lateral depth.

[0091] In one embodiment, the positive electrode active material layer is formed on the surface of the flame retardant layer by extrusion coating using an extrusion coating machine.

[0092] In one embodiment, the negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, the negative active material layer comprising a negative active material.

[0093] In this application, there are no particular restrictions on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, or composite current collector, etc.

[0094] In one embodiment, the negative electrode active material can be natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, or spinel-structured lithium titanate Li4Ti5O. 12 At least one of Li-Al alloys and metallic lithium.

[0095] In one embodiment, the negative electrode active material layer further includes a conductive agent and a binder.

[0096] In one embodiment, there is no limitation on the type of conductive agent mentioned in this application (including the conductive agent in the flame retardant layer, the positive electrode active material layer, and the negative electrode active material layer), and any known conductive agent can be used.

[0097] In one embodiment, the conductive agent (including the conductive agent in the flame retardant layer, the positive electrode active material layer, and the negative electrode active material layer) includes at least one of carbon materials such as acetylene black, needle coke, carbon nanotubes, graphene, and conductive carbon black.

[0098] In one embodiment, there is no limitation on the type of binder mentioned in this application (including binders in the flame retardant layer, the positive electrode active material layer, and the negative electrode active material layer), and any known positive electrode binder can be used.

[0099] In one embodiment, the binder (including the conductive agent in the flame-retardant layer, the positive electrode active material layer, and the negative electrode active material layer) includes at least one of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, styrene-acrylic latex, pure styrene latex, aromatic polyamide, cellulose (e.g., sodium carboxymethyl cellulose), nitrocellulose, styrene-butadiene rubber, nitrile rubber, fluororubber, isoprene rubber, polybutadiene rubber, ethylene-propylene rubber, styrene-butadiene-styrene block copolymer or its hydrogenation, ethylene-propylene-diene terpolymer, styrene-ethylene-butadiene-ethylene copolymer, styrene-isoprene-styrene block copolymer, syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, propylene-α-olefin copolymer, polyvinylidene fluoride, polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, polytetrafluoroethylene-ethylene copolymer, and acrylics.

[0100] In one embodiment, the diaphragm comprises a porous sheet-like or nonwoven material with excellent liquid retention properties. Materials for resin or glass fiber diaphragms include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, and polyethersulfone.

[0101] In one embodiment, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The materials of the diaphragm described above can be used alone or in any combination.

[0102] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0103] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. In some embodiments, the outer packaging of the secondary battery can also be a soft pack, such as a pouch-type soft pack. The material of the soft pack can be plastic, and non-limiting examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0104] In some embodiments, the type of electrolyte is not specifically limited. The electrolyte includes an electrolyte salt and an organic solvent, and the specific types of the electrolyte salt and organic solvent are not specifically limited and can be selected according to actual needs. The electrolyte may also include additives, and the type of additives is not particularly limited. These additives can be film-forming additives for the positive and / or negative electrodes, or additives that can improve certain battery performance, such as additives that improve the battery's high or low temperature performance.

[0105] This application does not impose any particular restrictions on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape.

[0106] One embodiment of this application provides an electrical device including the secondary battery described above, wherein the secondary battery serves as the power supply for the electrical device.

[0107] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0108] The following embodiments are provided to facilitate understanding of this application. These embodiments are provided not to limit the scope of the claims.

[0109] Example 1

[0110] The preparation of a secondary battery includes the following steps:

[0111] (1) Preparation of flame-retardant slurry: Alumina is preferred as the main material, and acrylic binder is used. 94wt% alumina, 5wt% binder (selected from at least one of the binders mentioned above), and 1wt% conductive carbon black are mixed evenly to obtain a slurry. Water is added, and the mixture is stirred to prepare the flame-retardant slurry. The particle size is D50≤1.0um, D90≤2.0um, and the viscosity is 30-500 (mPa.s).

[0112] (2) Prepare the positive electrode slurry by mixing 95.2 wt% lithium cobalt oxide, 2 wt% conductive carbon black, 0.8 wt% carbon nanotubes and 2 wt% polyvinylidene fluoride (PVDF), adding N-methylpyrrolidone (NMP), and stirring to prepare the positive electrode slurry.

[0113] (3) Prepare negative electrode slurry by mixing 15wt% silicon, 81wt% artificial graphite, 1wt% conductive carbon black, 1.5wt% styrene-butadiene rubber (SBR) and 1.5wt% sodium carboxymethyl cellulose (CMC), adding deionized water, and stirring to prepare negative electrode slurry.

[0114] (4) Flame retardant coating: Using a gravure roller and a gravure coating machine, the flame retardant slurry is coated onto the positive electrode current collector (thickness 10μm, areal density 25g / m²). 2 The aluminum foil surface is baked and rolled to form a first flame-retardant layer in the flat area and a second flame-retardant layer in the corner area.

[0115] (5) Positive electrode slurry coating: Using an extrusion coating process, the positive electrode slurry is coated on the surface of the flame retardant layer, baked and rolled to form the first positive electrode active material layer and the second positive electrode active material layer, thus obtaining the positive electrode sheet.

[0116] (6) The negative electrode paste is coated on the surface of copper foil (10 μm thick), baked and rolled to form a negative electrode sheet;

[0117] (7) The positive and negative electrode sheets are rolled, slit, sheeted, wound, packaged, injected with liquid, formed, capacity tested, and OCV are used to prepare secondary batteries.

[0118] The parameters of Example 1 are shown in Table 1.

[0119] Examples 2-14, Comparative Examples 1-4

[0120] Examples 2-14 and Comparative Examples 1-4 differ from Example 1 in that the parameter settings are different, as shown in Tables 1 and 2.

[0121] Table 1

[0122] Table 2

[0123] Performance testing

[0124] 1. Lithium plating test

[0125] The lithium-ion battery was placed in a 0℃ constant temperature chamber and left to stand for 60 minutes to allow it to reach a constant temperature. The battery was then charged at 0℃ with a constant current of 1C to 4.45V, followed by constant voltage charging at 4.45V to 0.025C. After standing for 5 minutes, it was discharged at a constant current of 1C to 3.0V; this constitutes one charge-discharge cycle. After 10 charge-discharge cycles, the battery was charged again with a constant current of 1C to 4.45V, followed by constant voltage charging at 4.45V to 0.025C, resulting in a fully charged battery with 10 cycles. The battery was disassembled in a dry room with humidity less than 5%, and the state of the negative electrode was photographed and recorded. If lithium plating was present, the lithium-ion battery was marked as "yes"; if lithium plating was not present, the lithium-ion battery was marked as "no".

[0126] The degree of lithium plating in lithium-ion batteries can be judged according to the following criteria:

[0127] No lithium deposition: No lithium is deposited on the surface of the negative electrode.

[0128] Slight lithium plating: The lithium deposition area on the surface of the negative electrode is less than 10%.

[0129] Moderate lithium deposition: The lithium deposition area on the surface of the negative electrode is 10% to 30%.

[0130] Severe lithium plating: The lithium deposition area on the surface of the negative electrode is greater than 30%.

[0131] 2. Security Testing

[0132] At 25°C, the lithium-ion battery was charged at 0.7C to 4.5V with a cutoff rate of 0.02C; then, it was charged at a constant voltage to 0.05C at 4.5V. Afterward, it was discharged at a current of 10C to 2.8V, completing one charge-discharge cycle. After 500 cycles, a side-crush safety test was performed.

[0133] Process description:

[0134] 1) Use right-angled edge extrusion;

[0135] 2) Both the right-angled edge of the extrusion head and the edge of the backing plate are directly machined right angles, without beveling.

[0136] 3) Descent range: approximately 130mm;

[0137] 4) Descent speed: 9000 mm / min;

[0138] 5) The lateral gap width between the extrusion head and the pad is set to 2.0mm;

[0139] 6) Target height for extrusion head descent: The distance between the lower plane of the extrusion head and the base plate should be controlled at 7mm;

[0140] 7) The shallow pit side of the battery cell faces upwards;

[0141] 8) Press half of the battery cell at the head and rotate the other half 180° to press the tail; (half aluminum tab, half nickel tab);

[0142] 9) Dwell time after the extrusion head reaches the target stroke: 10 seconds;

[0143] 10) Speed ​​control: 100%, no deceleration;

[0144] 11) Let it sit for 1 hour and save the photos after the test.

[0145] If a fire or explosion occurs, the battery is considered a failure; if neither fire nor explosion occurs, it is considered a pass. Each group is tested with 10 lithium-ion batteries. Safety performance is characterized by the extrusion pass rate, calculated as: Extrusion pass rate (%) = (Number of lithium-ion batteries passing / 10) × 100%.

[0146] Table 3

[0147] As can be seen from Table 3, in this application, a flame retardant layer and a positive electrode active material layer are sequentially provided on the surface of the positive electrode current collector, and the thickness relationship between the flame retardant layer and the positive electrode active material layer in the straight region and the corner region is controlled. Among them, the thickness of the positive electrode active material layer decreases in the corner region, the amount of the positive electrode active material decreases, there are fewer lithium ions, effectively reducing the lithium ions at the corner, improving the accumulation of lithium ions in the corner region. At the same time, due to the thickness change between the straight region and the corner region, it improves the formation of stress concentration points caused by the structural change between the straight region and the corner region, thereby reducing the stress in the corner region. During the charge and discharge process, the lithium intercalation resistance is reduced, and the precipitation amount on the negative electrode surface is reduced, so that the lithium precipitation phenomenon at the corner can be significantly improved.

[0148] By comparing the examples with Comparative Examples 1-2, it can be seen that in this application, by controlling A1 < A2, the safety performance and lithium precipitation phenomenon of the secondary battery are significantly improved.

[0149] By comparing the examples with Comparative Examples 3-4, it can be seen that in this application, by controlling B1 > B2, the lithium precipitation phenomenon is significantly improved.

[0150] By comparing Examples 1-5 with Comparative Examples 6-7, it can be seen that in this application, by adjusting: 1.8 ≤ A2 / A1 ≤ 6 and 1.08 ≤ B1 / B2 ≤ 1.3, the safety performance and lithium precipitation phenomenon of the secondary battery are further improved.

[0151] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of this application and not to limit the protection scope of this application. Although this application has been described in detail with reference to the preferred examples, those of ordinary skill in the art should understand that the technical solutions of this application can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of this application.

Claims

1. A secondary battery, characterized in that, It includes a wound cell, the wound cell includes a flat region and a corner region, and the wound cell includes a positive electrode tab, a negative electrode tab, and a separator between the positive electrode tab and the negative electrode tab; the positive electrode tab includes a positive current collector and a flame retardant layer and a positive active material layer sequentially provided on at least one surface of the positive current collector; The flame retardant layer includes a first flame retardant layer located in the flat region and a second flame retardant layer located in the corner region. The thickness of the first flame retardant layer is A1, and the thickness of the second flame retardant layer is A2, satisfying: A1 < A2; The positive active material layer includes a first positive active material layer located in the flat region and a second positive active material layer located in the corner region. The thickness of the first positive active material layer is B1, and the thickness of the second positive active material layer is B2, satisfying: B1 > B2.

2. The secondary battery according to claim 1, characterized in that, Satisfying: 1.5 ≤ A2 / A1 ≤ 10.

3. The secondary battery according to claim 1, characterized in that, Satisfying: 1.08 ≤ B1 / B2 ≤ 1.

3.

4. The secondary battery according to claim 1, characterized in that, Satisfying at least one of the following characteristics: (1) 0.5 μm ≤ A1 ≤ 10 μm; (2) 0.5 μm ≤ A2 ≤ 10 μm; (3) 30 μm ≤ B1 ≤ 100 μm; (4) 30 μm ≤ B2 ≤ 100 μm.

5. The secondary battery according to claim 1, characterized in that, The surface density of the first flame retardant layer is C1, and the surface density of the second flame retardant layer is C2, satisfying: C1 < C2; and / or The surface density of the first positive active material layer is D1, and the surface density of the second positive active material layer is D2, satisfying: D2 < D1.

6. The secondary battery according to claim 5, characterized in that, Satisfying: 1.5 ≤ C2 / C1 ≤ 6; and / or: 1.5 ≤ D1 / D2 ≤ 5.

7. The secondary battery according to any one of claims 5 to 6, characterized in that, Satisfying at least one of the following characteristics: (5)5mg / 1540.25mm 2 ≤C1≤30mg / 1540.25mm 2 ; (6)5mg / 1540.25mm 2 ≤C2≤30mg / 1540.25mm 2 ; (7)150mg / 1540.25mm 2 ≤D1≤500mg / 1540.25mm 2 ; (8)150mg / 1540.25mm 2 ≤D2≤500mg / 1540.25mm 2 。 8. The secondary battery according to any one of claims 1-6, characterized in that, The flame retardant layer includes a flame retardant material, and the flame retardant material includes at least one of aluminum oxide, barium sulfate, titanium dioxide, silicon dioxide, aluminum hydroxide, magnesium oxide, boehmite, polyethylene microspheres, barium titanate, magnesium hydroxide, melamine, lithium iron phosphate, aluminum sol, and nanocellulose.

9. The secondary battery according to any one of claims 1-6, characterized in that, The thickness of the positive electrode current collector is 6–12 μm, and the areal density is 18–34 g / m³. 2 .

10. An electrical device, characterized in that, It includes a secondary battery according to any one of claims 1 to 9.

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