A battery

Abstract

The application relates to the technical field of batteries, in particular to a battery. The battery comprises an electrode assembly formed by a positive electrode sheet, a diaphragm and a negative electrode sheet which are arranged in a stack, the positive electrode sheet comprises a first positive electrode sheet and a second positive electrode sheet located at the outermost side of the electrode assembly, the first positive electrode sheet comprises a first positive electrode current collector, the first positive electrode current collector comprises a first surface close to the center of the electrode assembly and a second surface away from the center of the electrode assembly, the first surface comprises a first positive electrode active layer; the difference between the yield strengths of the first positive electrode current collector and the negative electrode current collector arranged opposite to the first positive electrode current collector is A, 0 ‑1 ≤ A / h1 ≤ 0.14 micrometers ‑1 ; the difference between the thickness B1 of the first positive electrode current collector and the thickness B2 of the second positive electrode current collector is 2 micrometers-20 micrometers. The battery improves the purple stain problem and improves the cycle stability of the battery.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically to a secondary battery. Background Technology

[0002] With the rapid development of consumer electronics, communication devices, and new energy vehicles, consumer demand for lithium-ion batteries (LIBs) with high capacity, long lifespan, high stability, and high power characteristics is increasing. Currently, stacked and wound batteries are the mainstream in the lithium-ion battery market. Compared to wound structures, stacked structures have higher space utilization. When the outermost layer of the stack is a single-sided positive electrode, the high local current density of the single-sided positive electrode accelerates side reactions and continuous electrolyte consumption, leading to electrolyte depletion in this area during the later stages of cycling, resulting in a decrease in lithium-ion transport rate and purple spots on the corresponding negative electrode. In addition, the single-sided positive electrode is more prone to curling and warping during cycling. When using silicon-based negative electrodes, expansion and contraction stress can cause delamination between the single-sided positive and negative electrodes. After cycling, as the electrolyte is gradually consumed, the delamination space between the single-sided positive and negative electrodes is left unfilled by electrolyte, forming a broken bridge. Insufficient lithium intercalation on the negative electrode opposite the single-sided positive electrode also exacerbates the purple spot phenomenon on the negative electrode, affecting the cycle stability and capacity retention of the battery. Summary of the Invention

[0003] The purpose of this invention is to solve the above-mentioned problems existing in the prior art and to provide a battery that improves the purple spot problem of the negative electrode opposite the single-sided positive electrode in a stacked battery, thereby improving the cycle stability and capacity retention of the battery.

[0004] In related technologies, single-sided positive electrode sheets have inherently higher local current density. At the same time, since the positive active layer of a single-sided positive electrode sheet is only set on one side of the aluminum foil, it is more prone to curling and warping during battery cycling. Furthermore, when subjected to external compression and internal expansion stress, the mechanical response of the single-sided positive electrode sheet and the negative electrode sheet is inconsistent, leading to delamination between the electrodes. This accelerates electrolyte consumption and causes localized liquid shortage, cutting off the lithium-ion transport pathway and resulting in localized purple spots on the negative electrode sheet opposite to the single-sided positive electrode sheet, affecting the cycle stability and capacity retention of the battery.

[0005] To address this problem, the inventors of this invention discovered that by adjusting the difference A between the yield strength ratio Rc1 of the first positive current collector (i.e., the positive current collector of the single-sided positive electrode sheet) and the yield strength ratio Rc2 of the negative current collector of the negative electrode sheet opposite to the single-sided positive electrode sheet, and by providing a first protrusion on the single-sided positive electrode sheet (i.e., the first positive electrode sheet) that protrudes from the first positive active layer toward the empty foil surface, the thickness difference between the first positive current collector and the second positive current collector can be adjusted, thereby synergistically improving the delamination problem of the single-sided positive electrode sheet and the purple spot problem of the corresponding negative electrode sheet. Specifically, the yield strength ratio refers to the ratio of the yield strength to the tensile strength of a metal, which reflects the ductility and strength of the metal. This invention has found that if the yield strength ratio of the first positive current collector is low, while the yield strength ratio of the corresponding negative current collector is too high, the first positive current collector will undergo large plastic deformation when the battery is subjected to external force or internal expansion stress, while the negative current collector will deform less. This deformation mismatch will generate huge shear stress at the interface between the positive and negative electrodes. This shear stress will destroy the originally tightly attached electrode-separator interface, thereby generating a large number of gaps and affecting the transport of lithium ions. Therefore, by adjusting the difference between Rc1 and Rc2 within an appropriate range, it can be ensured that when the battery is subjected to external compression or internal expansion stress, the first positive current collector and the corresponding negative current collector enter and undergo plastic deformation in a coordinated manner. This avoids premature yielding, hardening, or brittle behavior of one side, which would generate destructive shear stress at the electrode-separator interface, destroying the originally tightly bonded electrode-separator interface and forming a large number of gaps. This ensures that the lithium-ion transport channel is unobstructed, thereby improving the delamination problem between the single-sided positive electrode and the negative electrode, as well as the purple spot problem of the single-sided positive electrode corresponding to the negative electrode. However, due to the uneven stress on the single-sided positive electrode sheet of the stacked battery, curling is prone to occur, which will aggravate poor interfacial adhesion. A first protrusion is provided on the surface of the single-sided positive electrode sheet and the first protrusion is made to protrude from the first positive electrode active layer to the empty foil surface. The stress generated by the first protrusion resists the curling stress of the single-sided positive electrode sheet, prevents the single-sided positive electrode sheet from curling and lifting, maintains the flatness of the electrode surface, and thus improves the electrode sheet delamination problem. In addition, the first protrusion can also improve the liquid storage capacity of the single-sided positive electrode sheet, further improving the problem of purple spots on the negative electrode sheet caused by local liquid shortage in the single-sided positive electrode sheet area in the later stage of cycling. However, for foils with high yield strength ratios, microcracks are easily generated when the first protrusion is formed during rolling. If the height of the first protrusion is too large, it will affect the safety performance of the battery. In addition, if the height of the first protrusion is too large, it will also weaken the interfacial adhesion between the first positive electrode and the separator. Furthermore, controlling A / h1 within a suitable range can take into account the effects of the first protrusion and the yield strength ratio of the single-sided positive electrode and the corresponding negative electrode, improve the mechanical properties of the electrode and the matching of the first protrusion, and avoid the decrease in the interfacial adhesion between the electrode and the separator due to excessive h1, as well as the easy generation of cracks when setting the first protrusion due to excessively high yield strength ratio, which is not conducive to improving the battery safety performance.The present invention also limits the thickness difference between the first positive current collector and the second positive current collector. This is because appropriately increasing the thickness of the first positive current collector can improve the anti-curling ability of the single-sided positive electrode sheet. At this time, in order to take into account the adhesion performance between the first positive electrode sheet and the separator as well as the stability of the electrode sheet, the height of the first protrusion can be appropriately reduced. However, if the thickness of the first positive current collector is too large, it will lead to an excessive current density of the single-sided positive electrode sheet, which will accelerate the consumption of electrolyte in this area and will not be conducive to improving the purple spot of the negative electrode.

[0006] Based on this, the present invention proposes the following technical solution: This invention proposes a battery comprising an electrode assembly, the electrode assembly including a positive electrode, a separator, and a negative electrode stacked together. The positive electrode includes a first positive electrode and a second positive electrode. The first positive electrode includes a first positive current collector, the first positive current collector including a first surface and a second surface disposed opposite to each other along its thickness direction. The first surface is close to the center of the electrode assembly, and the second surface is away from the center of the electrode assembly. The first surface includes a first positive active layer. The second positive electrode includes a second positive current collector and a second positive active layer located on both sides of the second positive current collector. The negative electrode includes a negative current collector and a second positive active layer located on both sides of the negative current collector. A negative electrode active layer; along the thickness direction of the electrode assembly, the first positive electrode sheet is located on the outermost side of the electrode assembly; the yield strength ratio of the first positive current collector is Rc1, and the yield strength ratio of the negative current collector opposite to the first positive electrode sheet is Rc2, A=|Rc1-Rc2|, 0<A≤0.15; the first positive electrode sheet includes a plurality of first protrusions and a plurality of first recesses opposite to the plurality of first protrusions, the first protrusions protrude from the second surface in a direction away from the center of the electrode assembly, the first recesses are recessed from the first surface to the second surface, the height of the first protrusion is h1 in μm, where A and h1 satisfy: 0.002μm -1 ≤A / h1≤0.14μm -1 The thickness of the first positive current collector is B1, in μm, and the thickness of the second positive current collector is B2, in μm, where 2μm≤B1-B2≤20μm.

[0007] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art: (1) In this invention, adjusting A can improve the mechanical matching degree between the first positive electrode current collector and the corresponding negative electrode current collector, so that they can enter the plastic deformation stage simultaneously without delamination when subjected to internal or external stress. At the same time, it avoids the generation of shear stress at the interface, which causes voids at the separator-electrode interface, affecting lithium ion transport and alleviating the purple spot problem of the negative electrode opposite to the first positive electrode. (2) In this invention, the first positive electrode is designed with a first protrusion protruding from the active layer to the empty foil surface to resist the curling stress of the first positive electrode itself and prevent the first positive electrode from curling, warping or even delaminating; it can also improve the electrolyte wettability and storage capacity in this area, avoid the lithium ion transport path being blocked due to lack of liquid in the later stage of the cycle, and further alleviate the purple spot problem of the negative electrode opposite to the first positive electrode. (3) In this invention, adjusting A / h1 allows the mechanical properties of the electrode and the convex structure to work synergistically, which can take into account both the interfacial adhesion and the structural stability of the first positive electrode, and jointly suppress the electrode delamination and the purple spot problem of the negative electrode opposite to the first positive electrode. (4) In this invention, appropriately increasing the thickness of the first positive current collector can improve the anti-curling ability of the single-sided positive electrode sheet. At this time, in order to take into account the bonding performance between the first positive electrode sheet and the separator as well as the stability of the electrode sheet, the height of the first protrusion can be appropriately reduced. However, if the thickness of the first positive current collector is too large, it will lead to an excessive current density of the single-sided positive electrode sheet, which will accelerate the consumption of electrolyte in this area and will not be conducive to improving the negative electrode purple spot. Limiting the thickness difference between the first positive current collector and the second positive current collector to the above range can take into account the above effects.

[0008] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0009] Figure 1 The diagram shown is a schematic diagram of the electrode assembly along the thickness direction in one embodiment of the present invention.

[0010] Figure 2 The diagram shown is a schematic diagram of the structure of the first positive electrode sheet in one embodiment of the present invention.

[0011] Figure 3 The diagram shown is a schematic diagram of the structure of the second positive electrode in one embodiment of the present invention.

[0012] Figure 4 The diagram shown is a schematic diagram of the first protrusion and the second protrusion in one embodiment of the present invention.

[0013] Figure 5 The image shown is a schematic diagram of the surface of the negative electrode sheet in one embodiment of the present invention.

[0014] Figure 6 The diagram shown is a schematic diagram of the negative electrode sheet in one embodiment of the present invention.

[0015] Figure 7The image shown is a cross-sectional scanning electron microscope image of silicon-carbon material in one embodiment of the present invention.

[0016] Reference numerals: 2, separator; 3, negative electrode sheet; 11, first positive electrode sheet; 111, first positive electrode current collector; 112, first positive electrode active layer; 113, first protrusion; 114, first concave portion; 115, first surface; 116, second surface; 12, second positive electrode sheet; 121, second positive electrode current collector; 122, second positive electrode active layer; 123, second protrusion; 21, negative electrode current collector; 22, negative electrode active layer. Detailed Implementation

[0017] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0018] The present invention provides a battery comprising an electrode assembly, the electrode assembly comprising a positive electrode, a separator, and a negative electrode stacked sequentially, the positive electrode comprising a first positive electrode and a second positive electrode, the first positive electrode comprising a first positive current collector, the first positive current collector comprising a first surface and a second surface disposed opposite to each other along its thickness direction, the first surface being close to the center of the electrode assembly, the second surface being away from the center of the electrode assembly, the first surface comprising a first positive active layer, the second positive electrode comprising a second positive current collector and a second positive active layer located on both sides of the second positive current collector, and the negative electrode comprising a negative current collector and a negative active layer located on both sides of the negative current collector.

[0019] In this invention, the first positive electrode is located on the outermost side of the electrode assembly along the thickness direction of the electrode assembly.

[0020] In this invention, the yield strength ratio of the first positive current collector is Rc1, and the yield strength ratio of the negative current collector disposed opposite to the first positive current collector is Rc2, A=|Rc1-Rc2|, 0<A≤0.15, for example 0.01, 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.15 or within any two of the above values.

[0021] In this invention, the yield strength ratio has the conventional meaning in the art, referring to the ratio of the metal's yield strength to its tensile strength. Yield strength is the stress value at which a metal begins to undergo plastic deformation during the stress process, and tensile strength is the maximum stress value that a metal can withstand during the stretching process.

[0022] In this invention, the first positive electrode includes a plurality of first protrusions and a plurality of first recesses disposed opposite to the plurality of first protrusions. The first protrusions protrude from the second surface in a direction away from the center of the electrode assembly, and the first recesses are recessed from the first surface toward the second surface. Here, "a plurality of" means that the number of the first recesses and the first protrusions is greater than or equal to 2.

[0023] In this invention, the height of the first protrusion is h1, in μm, wherein A and h1 satisfy: 0.002 μm -1 ≤A / h1≤0.14μm -1 For example, 0.002μm -1 0.005μm -1 0.006μm -1 0.007μm -1 0.008μm -1 0.01μm -1 0.02μm -1 0.04μm -1 0.06μm -1 0.08μm -1 0.1μm -1 0.12μm -1 0.14μm -1 Or it falls within the range formed by any two of the above values.

[0024] In this invention, the shape of the orthographic projection of the first protrusion onto the second surface is not limited; it can be circular or rectangular. The height of the first protrusion refers to the vertical distance from the highest point within the first protrusion to the surface of the first positive electrode sheet.

[0025] In this invention, the shape of the orthographic projection of the first recess onto the first surface is not limited; it can be circular or rectangular. The height of the first recess refers to the vertical distance from the lowest point within the first recess to the first surface.

[0026] like Figure 1The diagram shown is a schematic representation of an electrode assembly along its thickness direction in one embodiment of the present invention. The electrode assembly includes a positive electrode sheet, a separator 2, and a negative electrode sheet 3 stacked along the thickness direction. A first positive electrode sheet 11 is located on the outermost side and has a first surface 115 and a second surface 116. The first positive electrode sheet 11 includes a first positive current collector 111 and a first positive active layer 112 located on one side of the first positive current collector 111. The first positive active layer 112 is close to the center of the electrode assembly. The second positive electrode sheet 12 includes a second positive current collector 121 and a second positive active layer 122 located on both sides of the second positive current collector 121. The first positive electrode sheet 11 includes a plurality of first protrusions 113, which protrude from the second surface 116 in a direction away from the center of the electrode assembly. The second positive electrode sheet 12 includes a plurality of first protrusions 123. Figure 2 The diagram shown is a schematic diagram of the structure of the first positive electrode sheet in one embodiment of the present invention. The first positive electrode sheet 11 has a first protrusion 113 and a first recess 114 correspondingly provided.

[0027] In this invention, the thickness of the first positive current collector is B1, in μm, and the thickness of the second positive current collector is B2, in μm, where 2μm≤B1-B2≤20μm, for example, 2μm, 3μm, 4μm, 5μm, 6μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm, 20μm, or within any two of the above values.

[0028] Maintaining a low yield strength ratio results in better ductility and toughness for the current collector. However, an excessively low yield strength ratio can lead to a decrease in the current collector's strength, while an excessively high yield strength ratio results in poor plastic deformation capacity and poor buffering ability against external stress. Therefore, adjusting A ensures that the first positive current collector and the corresponding negative current collector undergo plastic deformation synchronously and in a coordinated manner when the electrode assembly is subjected to external compression or internal expansion stress. This avoids destructive shear stress at the electrode-diaphragm interface caused by premature yield hardening or brittle behavior of one side, leading to poor interfacial contact and significantly improving delamination and purple spot problems. When A is too large (e.g., >0.15), meaning the difference between Rc1 and Rc2 is too large, the mechanical properties of the first positive current collector and its corresponding negative current collector are mismatched. When the battery is subjected to external forces (such as winding tension, thermal pressure) or internal stress (such as the expansion stress of silicon-based negative electrodes), the excessive difference in their mechanical responses will generate huge shear stress at the interface between the first positive electrode and the negative electrode. This shear force will tear the originally tightly bonded electrode-separator interface, causing the electrode to wrinkle and arch, eventually forming tiny gaps in local areas, affecting lithium-ion transport. Especially when silicon-based negative electrodes are used, their repeated contraction and expansion during cycling will further compress the positive electrode. In addition, the mismatch in mechanical properties between the first positive current collector and its corresponding negative current collector can easily lead to physical delamination, exacerbating interface failure. When A is too small or even close to 0, Rc1 and Rc2 are close, which does not significantly improve the interface bonding between the first positive electrode and the negative electrode. Although the plastic deformation capacity between the first positive current collector and the opposite negative current collector is similar, the first positive current collector is a single-sided sheet, and the uneven stress on both sides is prone to curling. There will still be problems with poor interface bonding, which will lead to purple spots and capacity decay in the battery.

[0029] A first protrusion is formed on the first positive electrode sheet, extending from the first positive electrode active layer towards the empty foil surface. The stress generated by this protrusion effectively counteracts the inherent curling stress of the first positive electrode sheet, preventing it from warping and maintaining its flatness. Simultaneously, the first protrusion optimizes Rc1 and Rc2, keeping them at a low level and reducing the difference A between them, thus improving the matching degree of mechanical properties and interface adhesion. Furthermore, the first protrusion enhances the liquid storage capacity of a single-sided positive electrode sheet, further mitigating the purple spot problem on the negative electrode sheet caused by localized liquid shortage in the later stages of cycling. However, for foils with high yield strength ratios, microcracks are easily generated when the first protrusion is formed during rolling. Excessive height of the first protrusion can negatively impact battery safety. Furthermore, an excessively high first protrusion weakens the interfacial adhesion between the first positive electrode and the separator. Adjusting A / h1 within a suitable range balances the effects of the first protrusion and the yield strength ratio between the single-sided positive electrode and the corresponding negative electrode, improving the matching of electrode mechanical properties and the first protrusion. This avoids excessively high h1 leading to decreased electrode-separator interfacial adhesion and excessively high yield strength ratio making the first protrusion prone to cracking, which is detrimental to battery safety. When A / h1 is too large (e.g., >0.14μm),... -1 When A is too large and / or h1 is too small, physical delamination occurs between the first positive electrode and its corresponding negative electrode. If h1 is too small, it is difficult to provide sufficient electrolyte storage space and to resist the curling stress of the first positive electrode, leading to interlayer electrolyte deficiency and purple spots on the battery. Simultaneously, if Rc1 is too large, the first protrusion will cause microcracks to form on the surface of the first current collector, which is detrimental to improving battery safety performance. When A / h1 is too small (e.g., <0.002μm),... -1 If A is too small and / or h1 is too large, the height of the first protrusion will be too large, which will reduce the interfacial adhesion between the first positive electrode and the separator, which is not conducive to suppressing the formation of purple spots in the battery.

[0030] Further limiting the thickness difference between the first positive current collector and the second positive current collector to a suitable range, appropriately increasing the thickness of the first positive current collector can improve the anti-curling ability of the single-sided positive electrode sheet. At this time, in order to take into account both the adhesion performance between the first positive electrode sheet and the separator and the stability of the electrode sheet, the height of the first protrusion can be appropriately reduced without increasing the local current density, further suppressing the formation of purple spots.

[0031] In this invention, Rc1 and Rc2 can be obtained by conventional testing methods in the art, for example, by the following method (taking Rc1 as an example): Discharge the battery to 0% SOC, disassemble and remove the first positive electrode sheet or directly remove the first positive electrode sheet, remove the negative electrode active coating on its surface to obtain the first positive current collector, and after preparing the obtained first positive current collector samples (more than 5), vertically install them on the fixture of a universal testing machine, ensuring that the sample axis coincides with the tensile direction, and that the clamping length of the fixture on the sample is not less than 20 mm, with moderate clamping force to prevent the sample from sliding or being broken during the tensile process, the tensile speed is 20 mm / min, and the force and displacement data during the tensile process are recorded in real time, and the sample is stretched at a uniform speed. The tensile process is continuously observed, and when the sample breaks, the equipment automatically stops the tensile process, and the stress-strain curve is obtained. (1) Tensile strength test: Record the maximum tensile force F1 at the moment of fracture, and calculate the tensile strength φ1 (unit: MPa) of each specimen by φ1 = F1 / S, where F1 is the maximum tensile force (N) at the moment of fracture and S is the original cross-sectional area of ​​the specimen (mm). 2 (2) Test of yield strength: Record the stress value F2 (in N) corresponding to the entry of the specimen into plastic deformation. The yield strength φ2 = F2 / S, where S is the original cross-sectional area of ​​the specimen (mm). 2 Rc1 is the ratio of yield strength to tensile strength. The test of Rc2 can be carried out with reference to Rc1, except that the sample is replaced with the negative current collector of the negative electrode corresponding to the first positive electrode.

[0032] In this invention, the thickness B1 of the first positive current collector is 10μm-30μm, for example, 10μm, 12μm, 14μm, 16μm, 18μm, 20μm, 22μm, 24μm, 26μm, 28μm, 30μm, or within any two of the above values.

[0033] In one embodiment, the thickness B1 of the first positive current collector is 15 μm-20 μm.

[0034] In this invention, the elongation γ1 of the first positive current collector is 1%-10%, for example, 1%, 1.5%, 2%, 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or within any two of the above values.

[0035] In one embodiment, the elongation γ1 of the first positive current collector is 2%-6%.

[0036] Limiting B1 and γ1 improves the anti-curling ability of the first positive electrode without significantly enhancing its current-carrying capacity. It also prevents the bonding between the positive electrode and the separator from weakening due to excessive difference in foil elongation between the first positive electrode and the corresponding negative electrode, thus avoiding delamination between the first positive electrode and the overall electrode assembly and suppressing purple spot formation. When B1 is too large (e.g., >30μm), the current-carrying capacity of the first positive electrode current collector is enhanced, resulting in excessively high current density in the first positive electrode. This accelerates electrolyte consumption in this area, increases side reactions, and easily leads to electrolyte depletion in this area during the later stages of cycling, which is detrimental to improving negative electrode purple spots. When B1 is too small (e.g., <10μm), the anti-curling ability of the first positive electrode current collector weakens, making it prone to curling during cycling, leading to interfacial delamination and failing to improve negative electrode purple spots. When γ1 is too low (e.g., <1%), the elongation of the first positive electrode current collector is too low, and the deformation under compressive stress is too low. When the first protrusion is set, local stress is easily generated, which can cause cracks in the current collector and is not conducive to improving the battery safety performance. When γ1 is too high (e.g., >10%), the elongation of the first positive electrode current collector is too high, the strength of the first positive electrode current collector is too low, the interfacial adhesion decreases, and it is not conducive to improving purple spots.

[0037] In this invention, B1 can be obtained by conventional testing methods in the art, such as by scanning electron microscopy (SEM), specifically as follows: discharge the battery to 0% SOC, disassemble and remove the first positive electrode sheet, soak it in DMC solvent for 12 hours, then rinse it with dimethyl carbonate (DMC) solvent to remove the lithium salt attached to the first positive electrode sheet, dry the first positive electrode sheet and polish its cross-section with an argon ion polisher, image the obtained cross-section in an SEM device, and measure the thickness of the first positive current collector at least 10 points randomly selected and take the average value as B1.

[0038] In this invention, 1μm≤h1≤30μm, for example, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm, 20μm, 22μm, 24μm, 26μm, 28μm, 30μm, or within any two of the above values.

[0039] In this invention, h1 can be obtained by conventional testing methods in the art, such as by measuring with a 3D microscope, as follows: After discharging the battery to 0% SOC, the first positive electrode is disassembled and removed. It is then soaked in dimethyl carbonate (DMC) solvent for 12 hours and rinsed with DMC to remove the lithium salt attached to the electrode. The flatness of the second surface of the positive electrode is tested using a 3D profilometer. The height of the first protrusion on the second surface is measured by image analysis. The height of at least 10 first protrusions on the first positive electrode is measured and the average value is taken as h1.

[0040] In this invention, the thickness B2 of the second positive current collector is 4μm-15μm, for example, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, or within any two of the above values.

[0041] In one embodiment, the thickness B2 of the second positive current collector is 8 μm-10 μm.

[0042] In this invention, the elongation γ2 of the second positive current collector is 1%-10%, for example, 1%, 1.5%, 2%, 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or within any two of the above values.

[0043] In one embodiment, the elongation γ2 of the second positive current collector is 2%-5%.

[0044] In this invention, the difference in elongation between the first positive current collector and the second positive current collector is Δγ, where Δγ = γ1 - γ2, and Δγ < 2%, for example, 1.9%, 1.8%, 1.7%, 1.6%, 1.5%, 1.4%, 1.2%, 1%, 0.8%, 0.6%, 0.4%, 0.2%, 0.1%, 0.05%, or within any two of the above values.

[0045] The second positive electrode is a double-sided positive electrode. The stress on both sides of the second positive current collector is balanced, making it less prone to curling and wrinkling compared to the first positive electrode. By limiting B2 and γ2, the adhesion between the second and first positive electrodes and the separator can be prevented from weakening due to excessive difference in foil elongation, thus avoiding delamination between the first positive electrode and the entire electrode assembly and suppressing purple spot formation. When Δγ is too high (e.g., >2%), the difference in elongation between the first and second positive current collectors becomes too large, leading to decreased adhesion between the positive electrode and the separator, poor interfacial contact, and delamination between the first positive electrode and the entire electrode assembly, which is detrimental to improving the purple spot problem.

[0046] In this invention, the thickness of the negative current collector of the negative electrode sheet disposed opposite to the first positive electrode sheet is 3μm-10μm, for example, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm or within any two of the above values.

[0047] In one embodiment, the thickness of the negative current collector of the negative electrode plate disposed opposite to the first positive electrode plate is 5μm-7μm.

[0048] In this invention, the elongation γ3 of the negative current collector of the negative electrode plate disposed opposite to the first positive electrode plate is 2%-11%, for example, 2%, 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, or within any two of the above values.

[0049] In one embodiment, the elongation γ3 of the negative current collector of the negative electrode plate disposed opposite to the first positive electrode plate is 3%-9%.

[0050] Limiting γ3 can prevent the negative current collector of the negative electrode sheet opposite to the first positive electrode sheet from having a small deformation capacity, which would cause cracks or breakage when subjected to external force. When γ3 is too low (e.g., <2%), the elongation of the negative current collector of the negative electrode sheet opposite to the first positive electrode sheet is too low, and the deformation is too low when subjected to compressive stress, which is easy to cause cracks and is not conducive to improving battery safety performance. When γ3 is too high (e.g., >11%), the elongation of the negative current collector of the negative electrode sheet opposite to the first positive electrode sheet is too high, the strength is too low, the interfacial adhesion decreases, which is not conducive to improving purple spots.

[0051] In this invention, the thickness of B2 and the negative current collector of the negative electrode sheet disposed opposite to the first positive electrode sheet can be measured with reference to the test method of B1. The difference is that the measurement of B2 replaces the sample with the second positive electrode sheet, and the measurement of the thickness of the negative current collector replaces the sample with the negative electrode sheet disposed opposite to the first positive electrode sheet.

[0052] In this invention, γ1 can be obtained by conventional testing methods in the art. For example, after discharging the battery to 0% SOC, the first positive electrode sheet is disassembled and removed. After soaking in DMC solvent for 12 hours, it is rinsed with DMC solvent to remove the lithium salt attached to the empty foil area and the negative electrode active coating on its surface, thus obtaining the first positive electrode current collector. After drying, a knife is used to cut the first positive electrode current collector into samples with a size of 15 mm along the width direction and a size of more than 50 mm along the length direction. Using a WD-D3 type electronic universal testing machine (accuracy grade 0.5, accuracy of ±1% of the indicated value), the gauge length (L0) is set to 50 mm, and the speed is 50 mm / min. The above samples are subjected to tensile testing along the length direction. The gauge length at the moment of sample breakage is L1. The elongation γ1 of the first positive electrode current collector is measured, γ1 = (L1-L0) / L0 × 100%. γ2 and γ3 can be tested using the same method as γ1.

[0053] In this invention, the tensile strength of the negative current collector of the negative electrode plate disposed opposite to the first positive electrode plate can be obtained by conventional testing methods in the art, such as the tensile strength testing method in the test of Rc2.

[0054] In this invention, the thickness of the first positive electrode active layer is d1, and the thickness of the negative electrode active layer disposed opposite to the first positive electrode active layer is d2. The ratio of d1 / d2 is 1.1-1.8, for example, 1.1, 1.15, 1.2, 1.25, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8 or within any two of the above values.

[0055] The positive electrode active material (such as nickel-cobalt-manganese ternary materials or lithium iron phosphate) and the negative electrode active material (such as graphite or silicon-carbon materials) have high modulus. The addition of a positive or negative electrode active layer will reduce the elongation of the positive or negative electrode sheet. Adjusting d1 / d2 can optimize the matching degree of electrode elongation, ensuring that when the battery is subjected to external compression or internal expansion stress, the two electrodes enter and undergo plastic deformation in a coordinated manner, thereby maintaining good interface contact and improving purple spots. When d1 / d2 is too large (e.g., >1.8), the excessively large d1 leads to an excessively low elongation of the first positive electrode sheet. During the expansion / contraction process of battery cycling, it is difficult to maintain the mechanical behavior matching with the negative electrode sheet, resulting in coating cracking or peeling, which is not conducive to improving battery purple spots and safety performance. When d1 / d2 is too small (e.g., <1.1), the excessively small d1 leads to a relatively large compaction density of the first positive electrode sheet, which exacerbates the reduction of electrolyte storage in the single-sided positive electrode area, easily leading to negative electrode purple spots in the later stages of cycling.

[0056] In this invention, d1 and d2 can be measured using conventional testing methods in the art, such as SEM, specifically as follows: The battery is discharged to 0% SOC, the first positive electrode is disassembled and removed, and then immersed in DMC solvent for 12 hours. Afterward, it is rinsed with DMC solvent to remove the lithium salt adhering to the first positive electrode. The first positive electrode is dried and its cross-section is polished using an argon-ion polishing machine. The cross-section is then imaged using backscatter imaging mode in an SEM device. At least 10 points are randomly selected to measure the thickness of the first positive electrode active layer, and the average value is taken as d1. The test for d2 can be performed similarly to d1, except that the negative electrode opposite the first positive electrode is disassembled and the thickness of the negative electrode active layer opposite the first surface of the first positive electrode is measured; this thickness is d2.

[0057] In this invention, the second positive electrode includes a plurality of second protrusions, the height of which is h2, 5μm≤h2≤40μm, for example, 5μm, 6μm, 7μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm, 20μm, 24μm, 28μm, 32μm, 36μm, 40μm, or within any two of the above values. The term "a plurality of" refers to a number greater than or equal to two for the first concave portion and the first protrusion portion.

[0058] In one embodiment, h1 < h2.

[0059] In this invention, the shape of the orthographic projection of the second protrusion onto the surface of the second positive electrode active layer is not limited; it can be circular or rectangular. The height of the second protrusion refers to the vertical distance from the highest point within the second protrusion to the surface of the second positive electrode.

[0060] In this invention, the shape of the orthographic projection of the second recess onto the surface of the second positive electrode active layer is not limited; it can be circular or rectangular. The height of the second recess refers to the vertical distance from the lowest point within the second recess to the surface of the second positive electrode.

[0061] In this invention, the protrusion direction of the second protrusion on the second positive electrode can be consistent with the protrusion direction of the first protrusion of any of the first positive electrodes located on the outermost side of the electrode assembly.

[0062] In this invention, the diameter t1 of the circumcircle of the orthographic projection of the first protrusion onto the first positive electrode sheet is 1mm-8mm, for example, 1mm, 1.2mm, 1.4mm, 1.6mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, or within any two of the above values.

[0063] In this invention, the second positive electrode includes a plurality of second protrusions, and the diameter t2 of the circumcircle of the orthographic projection of the second protrusion on the second positive electrode is 1mm-8mm, for example, 1mm, 1.2mm, 1.4mm, 1.6mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm or within any two of the above values.

[0064] In this invention, the first protrusion includes a first outer surface and a first inner surface, where t1 refers to the diameter of the first outer surface. The second protrusion includes a second outer surface and a second inner surface, where t2 refers to the diameter of the second outer surface.

[0065] like Figure 3 The diagram shown is a schematic representation of the structure of the second positive electrode in one embodiment of the present invention. Figure 4 The diagram shown is a schematic diagram of the first protrusion and the second protrusion in one embodiment of the present invention.

[0066] The first positive electrode sheet has a first positive active layer only on the first surface of the first positive current collector, resulting in an imbalance of stress on both sides of the first positive current collector. The first surface generates shrinkage stress during the rolling process, while the second surface is relatively free. This imbalance causes the first positive electrode sheet to tend to curl towards the second surface during battery manufacturing (such as winding and hot pressing) and cycling. In contrast, the second positive electrode sheet has a second positive active layer on both sides of the second positive current collector, resulting in stress balance on both sides and offsetting the internal stress of the electrode curling. The structure of the second positive electrode sheet is more stable. Therefore, limiting h1 to h2 can maintain the structural stability of the first and second positive electrodes while ensuring that the first and second protrusions perform their respective functions. This not only allows for the storage of more electrolyte and delays electrolyte consumption during cycling, but the liquid phase transport channels formed also facilitate the rapid wetting of the electrolyte to the center of the electrode assembly, ensuring uniform wetting of the electrode sheet, improving battery purple spots, and enhancing cycle stability.

[0067] When h1 > h2, if h1 is too high and / or h2 is too low, the stress on the first positive electrode sheet will be more likely to concentrate at the edge of the first protrusion when subjected to external pressure, increasing the risk of wrinkles, microcracks, or even breakage of the first positive electrode sheet. If h2 is too low, the protrusions on the surface of the second positive electrode sheet will be short and flat, resulting in decreased interlayer adhesion, which is not conducive to improving the purple spot of the battery and further enhancing the cycle stability of the battery.

[0068] In this invention, t1 can be obtained by conventional testing methods in the art, such as 3D microscopy testing, specifically as follows: After discharging the battery to 0% SOC, the first positive electrode is disassembled and removed. It is then immersed in dimethyl carbonate (DMC) solvent for 12 hours, followed by rinsing with DMC to remove lithium salts adhering to the electrode. The maximum distance between the edges of the first protrusion contour on the first outer surface of the positive electrode is measured using a 3D profilometer. At least 10 measurements are taken, and the average value is obtained as t1. The testing of t2 can be performed with reference to t1.

[0069] In this invention, h2 can be obtained by conventional testing methods in the art, for example, by referring to the testing method of h1.

[0070] In this invention, the surface of the negative electrode active layer opposite to the first positive electrode sheet and away from the negative electrode current collector includes a plurality of grooves. Here, "a plurality of" means that the number of grooves is greater than or equal to 2.

[0071] In this invention, the width of the groove is 20μm-150μm, for example, 20μm, 22μm, 24μm, 26μm, 28μm, 30μm, 35μm, 40μm, 50μm, 60μm, 70μm, 80μm, 100μm, 120μm, 140μm, 150μm, or within any two of the above values. The distance between two adjacent grooves is 0.5mm-5mm, for example, 0.5mm, 0.6mm, 0.8mm, 1mm, 2mm, 3mm, 4mm, 5mm, or within any two of the above values. The depth of the groove is 5μm-50μm, for example, 5μm, 6μm, 7μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm, 20μm, 25μm, 30μm, 35μm, 40μm, 50μm, or within any two of the above values.

[0072] Since the current density of the first positive electrode is inherently high, it will accelerate the side reactions and electrolyte consumption in this area, leading to electrolyte shortage in this area in the later stages of the cycle. The above-mentioned groove is provided on the surface of the negative electrode active layer of the negative electrode that is set opposite to the first positive electrode, and the parameters of the groove are controlled within the above range to increase the electrolyte storage capacity of the negative electrode and further improve the purple spots.

[0073] In one embodiment, the negative electrode sheet may include the groove, wherein the depth of the groove on the surface of the negative electrode sheet opposite to the first positive electrode sheet may be greater than the depth of the groove on the other surfaces of the negative electrode sheet, and / or the spacing between two adjacent grooves on the surface of the negative electrode sheet opposite to the first positive electrode sheet may be smaller than the spacing between two adjacent grooves on the other surfaces of the negative electrode sheet.

[0074] The width of the groove, the depth of the groove, and the spacing between two adjacent grooves have conventional meanings in the art. The width of the groove refers to the shortest distance between two opposite edges of the orthographic projection of a single groove onto the negative electrode active layer. The depth of the groove refers to the vertical distance from the lowest point in the groove to the surface of the negative electrode active layer. The spacing between two adjacent grooves refers to the average distance between the same edge of two adjacent grooves in the length or width direction of the negative electrode sheet.

[0075] In this invention, the electrode assembly further includes a tab. Along the extending direction of the tab, the negative electrode includes a first edge and a second edge disposed opposite to each other. The angle θ formed by the groove and the first edge is 30°-80°, for example, 30°, 32°, 34°, 36°, 38°, 40°, 45°, 50°, 55°, 60°, 65°, 70°, 75°, 80°, or within any two of the above values.

[0076] In one embodiment, θ is 50°-70°.

[0077] like Figure 5 The diagram shown is a schematic representation of the surface of a negative electrode sheet in one embodiment of the present invention. The negative electrode sheet has a first edge 201 and a second edge 202, and the angle between the first edge and the groove is θ. Figure 6 The diagram shown is a schematic diagram of the negative electrode sheet in one embodiment of the present invention, including a negative electrode current collector 22 and a negative electrode active layer 21. The depth of the groove is e, the width is f, and the distance between two adjacent grooves is g.

[0078] By creating obliquely arranged grooves on the surface of the negative electrode, additional three-dimensional channels and micro-spaces can be constructed between the negative electrode and the separator, improving the electrolyte storage capacity and increasing the electrolyte retention of the negative electrode, which is positioned opposite the first positive electrode, which is prone to electrolyte shortage. This ensures that in the later stages of battery cycling, when the overall electrolyte is consumed, these stored electrolytes can continuously provide an effective and uniformly distributed transport channel for lithium ion migration, avoiding the formation of purple spots caused by localized electrolyte shortage and excessively high concentrations of some lithium ions. Adjusting the angle θ formed by the groove and the first edge within the above range not only ensures that the groove can play a sufficient role in liquid storage and promote lithium-ion transport, but also avoids the edge powder shedding problem that exists when the groove is set along the tab direction or perpendicular to the tab direction. On the one hand, it will lead to a reduction in negative electrode active material, damage to the conductive network, and hinder ion transport, which is not conducive to further improvement of single-sided purple spots and battery rate performance. On the other hand, the transport path of lithium ions inside the negative electrode sheet is more tortuous, which increases the difficulty of lithium ion migration. Even during cycling, the negative electrode active material will continue to fall off, resulting in a decrease in the structural stability of the negative electrode sheet, which is not conducive to further improvement of battery fast charging performance and cycle stability.

[0079] In this invention, the diaphragm includes a substrate layer, the substrate layer includes a third surface and a fourth surface, the third surface includes a heat-resistant layer and a first adhesive layer stacked along the thickness direction, and the heat-resistant layer is located between the substrate layer and the first adhesive layer.

[0080] In this invention, the first adhesive layer includes a coated area and a blank area, the coated area comprising first polymer particles. It is understood that the first adhesive layer is discontinuous, and the blank area does not contain a coated layer.

[0081] In this invention, the fourth surface includes a second adhesive layer, which comprises a continuous coating layer formed by filler particles and a second polymer. The first adhesive layer is disposed opposite to the negative electrode sheet, and the second adhesive layer is disposed opposite to the positive electrode sheet.

[0082] In this invention, the first polymer particles comprise polymers formed by polymerizing at least one of the following monomers: styrene, ethylene, propylene, acrylonitrile, methyl methacrylate, butyl acrylate, isooctyl acrylate, octadecyl acrylate, ethyl acrylate, amide, or imide.

[0083] In this invention, the filler particles comprise at least one of boehmite, alumina, barium sulfate, magnesium oxide, magnesium hydroxide, silicon dioxide, tin dioxide, titanium dioxide, barium titanate, 1,3,5-triazine-2,4,6-triamine, melamine cyanurate, and melamine trithiocyanate. The filler particles are capable of filling a porous structure in which the second polymer is a continuous phase.

[0084] In this invention, the second polymer includes a fluorinated polymer, which includes polymers formed by polymerizing at least one of the following monomers: vinylidene fluoride, tetrafluoroethylene, hexafluoroethylene, or hexafluoropropylene.

[0085] In this invention, the heat-resistant layer comprises heat-resistant particles, which include at least one of boehmite, alumina, barium sulfate, magnesium oxide, magnesium hydroxide, silicon dioxide, tin dioxide, titanium dioxide, barium titanate, 1,3,5-triazine-2,4,6-triamine, melamine cyanurate, and melamine thiocyanate.

[0086] In this invention, the coating area further includes third polymer particles, wherein the average particle size of the primary particles of the third polymer particles is 0.5μm-0.85μm, for example, 0.5μm, 0.52μm, 0.54μm, 0.56μm, 0.58μm, 0.6μm, 0.65μm, 0.7μm, 0.75μm, 0.8μm, 0.85μm, or within any two of the above values.

[0087] In this invention, the average particle size of the primary particles of the first polymer particle is 150nm-500nm, for example, 150nm, 160nm, 170nm, 180nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm, or within any two of the above values. The average particle size of the secondary particles of the first polymer particle is 2μm-12μm, for example, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, or within any two of the above values.

[0088] In this invention, the third polymer particles comprise polymers formed by polymerizing at least one of the following monomers: vinylidene fluoride, tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, styrene, acrylate, or methyl methacrylate.

[0089] In one embodiment, based on the total mass of the second adhesive layer, the mass content of the second polymer is 20%-70%, for example, 20%, 22%, 24%, 26%, 28%, 30%, 35%, 40%, 50%, 60%, 70%, or within any two of the above values.

[0090] In this invention, the term "polymerization" has the conventional meaning in the art, for example, it can refer to the polymerization of one monomer to form a homopolymer, or it can refer to the polymerization of multiple monomers to form a copolymer.

[0091] Because the first positive electrode has a first protrusion, the adhesion between the first positive electrode and the separator is weakened. Using a separator with a specific structure, with the second adhesive layer positioned opposite the first positive electrode, can improve the interfacial adhesion between the two. The formation of the second polymer in the second adhesive layer can increase the contact area with the positive electrode, effectively improving the adhesion between the separator and the positive electrode. Meanwhile, the first adhesive layer positioned opposite the negative electrode can increase the electrolyte storage space. Utilizing the gaps between the first polymer particles in the first adhesive layer, a liquid storage space can be constructed at the negative electrode-separator interface. During long-term battery cycling and as the electrolyte is gradually consumed, this continuously provides the interface with the liquid phase transport channels required for lithium-ion migration, effectively alleviating the problem of electrolyte shortage that is more likely to occur with the first positive electrode due to its higher current density, and further mitigating the formation of purple spots.

[0092] In this invention, the average particle size of the primary particles of the third polymer particle, the average particle size of the primary particles of the first polymer particle, and the average particle size of the secondary particles of the first polymer particle can be obtained by conventional testing methods in the art, such as using a laser particle size analyzer.

[0093] In this invention, the negative electrode active layer includes a negative electrode active material, which includes a silicon-carbon material. The silicon-carbon material includes a porous carbon matrix and silicon particles located in the internal channels of the porous carbon matrix. The outer surface of the porous carbon matrix includes a coating layer, which includes amorphous carbon and / or a solid electrolyte.

[0094] In one embodiment, the coating layer comprises amorphous carbon.

[0095] In another embodiment, the coating layer includes a solid electrolyte, which includes at least one of lithium aluminum titanium phosphate (LATP), lithium lanthanum zirconium oxide (LLZO), and lithium lanthanum zirconium titanium oxide (LLZTO).

[0096] In this invention, in a 10,000x scanning electron microscope image of the cross-section of the silicon-carbon material, the cross-sectional profile of the silicon-carbon material includes a first interior angle, the degree of which is greater than 180°, for example 182°, 184°, 186°, 190°, 190°, 200°, 220°, 240°, 260°, 280°, 300°, 320°, 350° or within any two of the above values.

[0097] It can be understood that "first interior angle" refers to the angle formed by drawing two straight lines tangent to the cross-sectional profile of a silicon carbide particle in a cross-sectional SEM image of the particle, starting from a point on the cross-sectional profile closer to the center of the particle. The angle between these two tangent lines is greater than 180 degrees. Figure 7 As shown in the figure (A represents the first interior angle, and A1, A2, and A3 represent different first interior angles), the cross-sectional profile of the silicon-carbon material of the present invention includes such interior angles. The surface of the silicon-carbon material corresponding to the area where such interior angles (first interior angles) are located forms a groove-like structure. This groove-like structure can store electrolyte. During battery cycling, the first positive electrode plate will accelerate electrolyte consumption due to its high current density. When the electrolyte inside the first positive electrode plate is consumed, the electrolyte stored at the corner of the silicon-carbon surface can continuously replenish the area through the pore network, thereby effectively alleviating the interruption of the local lithium-ion transport channel and further suppressing the purple spots on the negative electrode.

[0098] It should be noted that the silicon-carbon material in this invention is prepared by vapor deposition, where silicon particles (by introducing silane gas into a fluidized bed reactor) are deposited onto a porous carbon matrix (using methods well-known to those skilled in the art). The preparation of silicon-carbon differs from conventional methods in that it uses phenol and formaldehyde aqueous solutions as raw materials, reacting them under sodium hydroxide catalysis to obtain phenolic resin microspheres. Using silica-supported sulfonic acid as a catalyst, the obtained phenolic resin is reacted with polyethylene glycol to obtain modified resin microspheres. This modified resin is dispersed in a mixed solvent of ethanol and water, and ammonia is added for reaction, followed by a hydrothermal reaction to obtain spherical phenolic resin microspheres. The obtained microspheres are mixed with potassium hydroxide, carbonized at a certain temperature under an inert atmosphere, and then heated to a high temperature and held for treatment to finally obtain a spherical porous carbon matrix material (reaction conditions can be designed according to actual needs).

[0099] In this invention, the tensile strength of the first positive electrode in the TD direction is greater than or equal to the tensile strength of the second positive electrode in the TD direction.

[0100] In this invention, the tensile strength of the first positive electrode in the MD direction is greater than or equal to the tensile strength of the second positive electrode in the MD direction.

[0101] In this invention, the tensile strength of the first positive current collector is P1, which is 250-400 MPa, for example, 250 MPa, 260 MPa, 270 MPa, 280 MPa, 290 MPa, 300 MPa, 320 MPa, 340 MPa, 360 MPa, 380 MPa, 400 MPa, or within any two of the above values. The height of the first protrusion is h1, which is μm, and h1 and P1 satisfy: 0.003 ≤ h1 / P1 ≤ 0.08, for example, 0.003, 0.004, 0.005, 0.007, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, or within any two of the above values.

[0102] The larger h1 is, the greater the compressive stress it exerts on the first positive electrode current collector, requiring an increase in the tensile strength of the first positive electrode current collector to prevent its breakage. Adjusting h1 / P1 within the aforementioned range can synergize the mechanical properties of the first protrusion and the first positive electrode current collector, enabling the first positive electrode current collector to have both additional electrolyte storage space and resistance to the winding stress of the first positive electrode sheet. This avoids interlayer liquid shortage caused by poor interface contact and interlayer separation caused by plastic deformation of the first positive electrode sheet, thereby suppressing the formation of purple spots in the battery and improving the battery's cycle capacity retention rate.

[0103] In this invention, the tensile strength P1 of the first positive current collector can be obtained by conventional methods in the art, such as the test method for testing tensile strength with reference to Rc1.

[0104] It should be noted that the numerical designations such as "first" and "second" in this invention are only used to distinguish different substances or methods of use, and do not represent a difference in order.

[0105] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0106] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0107] The following examples illustrate the battery of the present invention.

[0108] Example 1: (1) Preparation of the first positive electrode: Lithium cobalt oxide, a conductive agent (a mixture of single-walled and multi-walled carbon nanotubes), and polyvinylidene fluoride (PVDF) were placed in N-methylpyrrolidone (NMP) at a mass ratio of 97.6:1.35:1.05 and stirred until homogeneous to obtain a first positive electrode slurry. This first positive electrode slurry was then uniformly coated onto one side of an aluminum foil. After drying, rolling, and die-cutting, a first positive electrode sheet was obtained. A special roller with protrusions was used to form a first protrusion on the surface of the first positive electrode sheet. The first protrusion protrudes from the second surface of the first positive electrode sheet in a direction away from the center of the electrode assembly. P1 is 260 MPa, B1 is 14 μm, γ1 is 9.7%, and h1 is 10 μm.

[0109] (2) Preparation of the second positive electrode: Lithium cobalt oxide, a conductive agent (a mixture of single-walled and multi-walled carbon nanotubes), and polyvinylidene fluoride (PVDF) were placed in N-methylpyrrolidone (NMP) at a mass ratio of 97.6:1.35:1.05 and stirred until homogeneous to obtain a second positive electrode slurry. This second positive electrode slurry was then uniformly coated onto both sides of an aluminum foil. After drying, rolling, and die-cutting, a second positive electrode sheet was obtained. A special roller with protrusions was used to form a second protrusion on the surface of the first positive electrode sheet. B2 was 4 μm, γ2 was 8.7%, and h2 was 23 μm.

[0110] (3) Preparation of the negative electrode: Artificial graphite, silicon carbide, polyacrylic acid (PAA), sodium carboxymethyl cellulose, and acetylene black were added to a vacuum mixer in a mass ratio of 76.8:19.2:2.7:0.65:0.65, along with an appropriate amount of deionized water. The mixture was thoroughly stirred under vacuum until a uniform, free-flowing negative electrode slurry was formed. This negative electrode slurry was then uniformly coated onto both sides of a copper foil with a thickness of 8 μm and a γ3 content of 10.5%. After drying, rolling, and slitting, a negative electrode sheet was obtained. The slit negative electrode sheet was then laser-etched with grooves on its entire surface. The grooves were 100 μm wide, 2.5 mm apart, 30 μm deep, and had an θ of 70°. The laser-treated negative electrode sheet was then cleaned and prepared into a sheet. The cross-sectional profile of the silicon carbide material does not have a first interior angle.

[0111] (4) Preparation of the diaphragm: Preparation of the heat-resistant layer: Heat-resistant alumina particles, polyacrylate, carboxymethyl cellulose (CMC), sodium dodecylbenzenesulfonate, and deionized water were blended to obtain a ceramic slurry with a solid content of 35% by mass. The mass ratio of alumina:polyacrylate:CMC:sodium dodecylbenzenesulfonate was 94.5:5:0.4:0.1 based on 100% solid mass. After thorough stirring and dispersion, the slurry was coated onto the third surface of a porous polyethylene (PE) base film using a gravure roller and dried in a multi-section oven at 60°C to form a heat-resistant layer with a thickness of 1 μm.

[0112] Preparation of the first adhesive layer: First polymer particles, third polymer particles, and deionized water were blended to obtain a first mixed slurry with a solid content of 5% by mass. The mass ratio of the first polymer particles to polyacrylate was 95:5 (based on 100% solids). After thorough stirring and dispersion, the first mixed slurry was coated onto the surface of the heat-resistant layer using a gravure roller and dried in a multi-section oven at 60°C to form a first adhesive layer with a thickness of 3 μm. The first polymer particles (purchased from Shenzhen Bairou New Material Technology Co., Ltd., model DWP4201A) were secondary particle aggregates with an average primary particle size of 354 nm and an average secondary particle size of 7.2 μm. The monomers included styrene, isooctyl acrylate, and methyl methacrylate in a copolymerization ratio of 64:8:28. The third polymer was polyacrylate with an average primary particle size of 0.63 μm.

[0113] Preparation of the second adhesive layer: The second polymer and the organic solvent dimethylacetamide (DMAC) were blended and thoroughly stirred until dissolved. Alumina filler particles were then added and stirred until uniformly dispersed to obtain a second mixed slurry with a solid content of 8% by mass. The mass ratio of the second polymer to alumina was 4:6. The second mixed slurry was coated onto the fourth surface of the porous PE base membrane using a gravure roller. The organic solvent was extracted in a water bath to create pores. The membrane was then dried in a multi-section oven at 60°C to form a second adhesive layer with a thickness of 1.5 μm, thus obtaining the separator. The second polymer was PVDF, specifically Arkema's LBG.

[0114] (5) Electrolyte preparation: Under an argon atmosphere and in an environment with a water content of less than 10 ppm, lithium hexafluorophosphate was mixed with a non-aqueous organic solvent (ethylene carbonate (EC): propylene carbonate (PC): propyl propionate (PP): diethyl carbonate (DEC) = 1:1:1:1, mass percentage) to prepare an electrolyte with a lithium salt concentration of 1.05 mol / L. 15% fluoroethylene carbonate and 2.5% succinate based on the total mass of the electrolyte were added. After stirring evenly, the electrolyte was obtained after passing the tests for moisture and free acid.

[0115] (6) Battery fabrication: The prepared first positive electrode, second positive electrode, negative electrode and separator are stacked in the following order to form an electrode assembly: the outermost electrode is the first positive electrode, and the middle one is arranged in the following cycle: separator, negative electrode, second positive electrode, wherein the first adhesive layer of the separator is set opposite to the positive electrode, and the second adhesive layer of the separator is set opposite to the negative electrode to obtain a bare cell; the bare cell is welded with tabs; a stacked cell is obtained, the obtained cell is placed in an aluminum-plastic film of matching size and sealed, the electrolyte of step (5) is injected under vacuum conditions and vacuum sealed, and the battery is obtained through standing, formation and sorting processes.

[0116] At this time, Δγ is 1%, d1 / d2 is 1.5, h1 / P1 is 0.0385, B1-B2 is 10μm, A is 0.104, and A / h1 is 0.01.

[0117] Example 2: (1) Preparation of the first positive electrode: Based on Example 1, the difference is that P1 is 290 MPa, B1 is 20 μm, γ1 is 7.6%, and h1 is 1 μm.

[0118] (2) Preparation of the second positive electrode: Based on Example 1, the difference is that B2 is 15 μm, γ2 is 5.7%, and h2 is 6 μm.

[0119] (3) Preparation of the negative electrode: Based on Example 1, the difference is that the copper foil thickness is 3μm, γ3 is 6.4%, the groove width is 22μm, the spacing between adjacent grooves is 0.5mm, the groove depth is 5μm, and θ is 32°.

[0120] (4) Preparation of the diaphragm: The average particle size of the primary particles of the third polymer is 0.51 μm, the average particle size of the primary particles of the first polymer is 152 nm, and the average particle size of the secondary particles of the first polymer is 2 μm.

[0121] At this time, Δγ is 1.9%, d1 / d2 is 1.1, h1 / P is 0.0034, B1-B2 is 5μm, A is 0.133, and A / h1 is 0.133.

[0122] Example 3: (1) Preparation of the first positive electrode: Based on Example 1, the difference is that P1 is 390 MPa, B1 is 30 μm, γ1 is 1.3%, and h1 is 29 μm.

[0123] (2) Preparation of the second positive electrode: Based on Example 1, the difference is that B2 is 13 μm, γ2 is 1%, and h2 is 40 μm.

[0124] (3) Preparation of the negative electrode: Based on Example 1, the difference is that the copper foil thickness is 10 μm, γ3 is 2.3%, the groove width is 147 μm, the spacing between adjacent grooves is 4.8 mm, the groove depth is 50 μm, and θ is 78°.

[0125] (4) Preparation of the diaphragm: The average particle size of the primary particles of the third polymer is 0.85 μm, the average particle size of the primary particles of the first polymer is 497 nm, and the average particle size of the secondary particles of the first polymer is 11.7 μm.

[0126] At this time, Δγ is 0.3%, d1 / d2 is 1.8, h1 / P is 0.0744, B1-B2 is 17μm, A is 0.057, and A / h1 is 0.002.

[0127] The specific settings of the embodiments and comparative examples are shown in Tables 1 and 2. In Embodiment 9, unlike Embodiment 1, the first adhesive layer of the diaphragm is arranged opposite to the negative electrode, and the second adhesive layer is arranged opposite to the positive electrode. In Comparative Example 4, unlike Embodiment 1, the first protrusion protrudes from the first surface of the first positive electrode sheet toward the center of the electrode assembly.

[0128] Table 1: Table 2: Note:" "" indicates that the data is the same as or similar to the referenced embodiment, and " / " indicates that the data does not exist.

[0129] In the above embodiments, t1 and t2 are both in the range of 1mm-8mm, and the proportion of the second polymer in the second adhesive layer is 20%-70%.

[0130] Test example: 1. Test for the degree of purpura: The batteries obtained in the embodiments and comparative examples of this invention were subjected to room temperature cycling tests at 25°C. The room temperature cycling test regime was as follows: charging at 2.0C to 4.18V, charging at 1.8C to 4.3V, charging at 1C to 4.4V, charging at 0.5C to the upper limit voltage of 4.53V, resting for 5 minutes, and discharging at 0.7C to 3.0V. The fully charged batteries with 400T cycles were disassembled to observe the degree of purple spots. The degree of purple spots was judged by the area of ​​the purple spots, as follows: 0% for no purple spots, ≤5% for very slight purple spots, 5%-10% for slight purple spots, 10%-20% for purple spots, 20%-50% for severe purple spots, and >50% for very severe purple spots. The test results are recorded in Table 3.

[0131] 2. Cycle retention rate test: The batteries obtained in the embodiments and comparative examples of the invention were subjected to cyclic testing on a Blue Electric Test Cabinet. The cyclic mechanism was as follows: (1) After standing at 25℃±2℃ for 5 minutes, the batteries were discharged at 0.2C to the lower limit voltage (3.0V); (2) After standing at 25℃±2℃ for 5 minutes, the batteries were discharged at 0.7C to 3.0V, and the discharge capacity at this time was measured as Q1. After standing for 5 minutes, the batteries were charged at 2.0C to 4.1V, 1.8C to 4.3V, 1C to 4.4V, and 0.5C to 4.53V. The batteries were then charged to 0.05C, stood for 5 minutes, and discharged at 0.7C to 3V. This process was repeated 500 times. The discharge capacity at 0.7C to 3V on the 500th cycle was recorded as Q2. The cycle capacity retention rate of the battery was (Q2 / Q1)×100%. The test results are recorded in Table 3.

[0132] Table 3: As can be seen from Table 3, the battery prepared in this invention, compared with the comparative example, suppresses purple spots and improves the cycle capacity retention rate.

[0133] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A battery, characterized in that, The battery includes an electrode assembly, which includes a positive electrode, a separator, and a negative electrode stacked together. The positive electrode includes a first positive electrode and a second positive electrode. The first positive electrode includes a first positive current collector. The first positive current collector includes a first surface and a second surface disposed opposite to each other along its thickness direction. The first surface is close to the center of the electrode assembly, and the second surface is away from the center of the electrode assembly. The first surface includes a first positive active layer. The second positive electrode includes a second positive current collector and a second positive active layer located on both sides of the second positive current collector. The negative electrode includes a negative current collector and a negative active layer located on both sides of the negative current collector. Along the thickness direction of the electrode assembly, the first positive electrode is located on the outermost side of the electrode assembly; The yield strength ratio of the first positive current collector is Rc1, and the yield strength ratio of the negative current collector disposed opposite to the first positive current collector is Rc2, A=|Rc1-Rc2|, 0<A≤0.15; The first positive electrode includes a plurality of first protrusions and a plurality of first recesses disposed opposite to the plurality of first protrusions. The first protrusions protrude from the second surface in a direction away from the center of the electrode assembly, and the first recesses are recessed from the first surface toward the second surface. The height of the first protrusion is h1, in μm, where h1 and h1 satisfy: 0.002 μm. -1 ≤A / h1≤0.14μm -1 ; The thickness of the first positive current collector is B1, in μm, and the thickness of the second positive current collector is B2, in μm, where 2μm≤B1-B2≤20μm.

2. The battery according to claim 1, wherein, The thickness B1 of the first positive electrode current collector is 10μm-30μm; preferably 15μm-20μm. And / or, 1μm≤h1≤30μm; And / or, the elongation γ1 of the first positive current collector is 1%-10%; preferably 2%-6%; And / or, the thickness B2 of the second positive current collector is 4μm-15μm; preferably 8μm-10μm; And / or, the elongation γ2 of the second positive current collector is 1%-10%; preferably 2%-5%; And / or, the elongation γ3 of the negative current collector of the negative electrode sheet disposed opposite to the first positive electrode sheet is 2%-11%; preferably 3%-9%.

3. The battery according to claim 2, wherein, The difference in elongation between the first positive electrode current collector and the second positive electrode current collector is Δγ, where Δγ = γ1 - γ2, and Δγ < 2%. Preferably, the thickness of the first positive electrode active layer is d1, and the thickness of the negative electrode active layer disposed opposite to the first positive electrode active layer is d2, and the ratio of d1 / d2 is 1.1-1.

8.

4. The battery according to claim 1 or 2, wherein, The second positive electrode includes a plurality of second protrusions, the height of which is h2, 5μm≤h2≤40μm; And / or, the diameter t1 of the circumcircle of the orthographic projection of the first protrusion onto the first positive electrode sheet is 1mm-8mm; And / or, the second positive electrode includes a plurality of second protrusions, wherein the diameter t2 of the circumcircle of the orthographic projection of the second protrusion onto the second positive electrode is 1mm-8mm; Preferably, h1 < h2.

5. The battery according to claim 1 or 2, wherein, The surface of the negative electrode active layer opposite to the first positive electrode sheet and away from the negative electrode current collector includes a plurality of grooves. The width of the grooves is 20μm-150μm, the spacing between two adjacent grooves is 0.5mm-5mm, and the depth of the grooves is 5μm-50μm. Preferably, the electrode assembly further includes a tab, and along the extending direction of the tab, the negative electrode includes a first edge and a second edge disposed opposite to each other, and the angle θ formed by the groove and the first edge is 30°-80°; more preferably, θ is 50°-70°.

6. The battery according to claim 1 or 2, wherein, The separator includes a substrate layer, which includes a third surface and a fourth surface. The third surface includes a heat-resistant layer and a first adhesive layer stacked along the thickness direction. The heat-resistant layer is located between the substrate layer and the first adhesive layer. The first adhesive layer includes a coated area and a blank area. The coated area includes first polymer particles. The fourth surface includes a second adhesive layer, which includes a continuous coating layer formed by filler particles and a second polymer. The first adhesive layer is disposed opposite to the negative electrode, and the second adhesive layer is disposed opposite to the positive electrode.

7. The battery according to claim 6, wherein, The coating area also includes third polymer particles, wherein the average particle size of the primary particles of the third polymer particles is 0.5 μm-0.85 μm; And / or, the average particle size of the primary particles of the first polymer particle is 150nm-500nm, and the average particle size of the secondary particles of the first polymer particle is 2μm-12μm.

8. The battery according to claim 7, wherein, The first polymer particles comprise polymers formed by polymerizing at least one of the following monomers: styrene, ethylene, propylene, acrylonitrile, methyl methacrylate, butyl acrylate, isooctyl acrylate, octadecyl acrylate, ethyl acrylate, amides, or imides; And / or, the filler particles include at least one of boehmite, alumina, barium sulfate, magnesium oxide, magnesium hydroxide, silicon dioxide, tin dioxide, titanium dioxide, barium titanate, 1,3,5-triazine-2,4,6-triamine, melamine cyanurate, and melamine trithiocyanate. And / or, the second polymer includes a fluorinated polymer, which includes a polymer formed by polymerizing at least one of the following monomers: vinylidene fluoride, tetrafluoroethylene, hexafluoroethylene, or hexafluoropropylene; And / or, the third polymer particles comprise polymers formed by polymerizing at least one monomer selected from vinylidene fluoride, tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, styrene, acrylate, and methyl methacrylate. Preferably, the mass content of the second polymer is 20%-70% based on the total mass of the second adhesive layer.

9. The battery according to claim 1 or 2, wherein, The negative electrode active layer includes a negative electrode active material, which includes a silicon-carbon material. The silicon-carbon material includes a porous carbon matrix and silicon particles located in the internal channels of the porous carbon matrix. The outer surface of the porous carbon matrix includes a coating layer, which includes amorphous carbon and / or a solid electrolyte. In a 10,000x scanning electron microscope image of the cross-section of the silicon-carbon material, the cross-sectional profile of the silicon-carbon material includes a first interior angle, the degree of which is greater than 180°.

10. The battery according to claim 1 or 2, wherein, The tensile strength of the first positive electrode in the TD direction is greater than or equal to the tensile strength of the second positive electrode in the TD direction; And / or, the tensile strength of the first positive electrode in the MD direction is greater than or equal to the tensile strength of the second positive electrode in the MD direction; And / or, the tensile strength of the first positive current collector is P1, P1 is 250-400, the unit is MPa, the height of the first protrusion is h1, the unit is μm, and h1 and P1 satisfy: 0.003≤h1 / P1≤0.08.

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