Positive electrode sheet, battery, and electric device

Abstract

The application provides a positive electrode sheet, a battery and an electric device, and relates to the technical field of batteries, and specifically relates to a positive electrode sheet, a battery and an electric device. The positive electrode sheet comprises: a positive electrode current collector; a first active material layer, which is located on at least one side surface of the positive electrode current collector, and comprises a first active material; and a second active material layer, which is located on a side surface of the first active material layer away from the positive electrode current collector, and comprises a second active material; wherein the circular degree of the first active material is greater than the circular degree of the second active material. Therefore, the damage of positive electrode active material particles to the positive electrode current collector during cold pressing can be reduced, and the structural stability of the positive electrode sheet can be improved.

Description

Technical Field

[0001] This invention relates to the field of new energy, specifically to a positive electrode sheet, a battery, and an electrical device. Background Technology

[0002] Lithium-ion batteries are widely used in 3C digital products, electric vehicles, and other fields due to their advantages such as high energy density, long cycle life, low self-discharge, and good safety. However, the cycle life and safety performance of current lithium-ion batteries still need to be improved.

[0003] Therefore, the current positive electrode plates, batteries, and electrical devices still need improvement. Summary of the Invention

[0004] This application is based on the inventor's discovery of the following problems:

[0005] Aluminum foil is easily oxidized in air, forming an oxide film on its surface. This oxide film acts as a barrier against external moisture and air corrosion, protecting the aluminum foil. In this application, the inventors, through extensive experimental research and theoretical analysis, discovered that in the lithium battery production process, cold pressing is required to increase the compaction density of the surface active material layer of the positive electrode current collector. When the positive electrode sheet is cold-pressed, due to the sharp edges on the surface of the positive electrode active material particles, under the pressure of the rollers, these edges come into close contact with the surface of the positive electrode current collector, thereby damaging the surface of the positive electrode current collector, such as the oxide film on the aluminum foil surface.

[0006] The present invention aims to alleviate or even solve the above-mentioned technical problems to a certain extent.

[0007] In one aspect of the present invention, a positive electrode sheet is provided, comprising: a positive current collector; a first active material layer located on at least one side surface of the positive current collector, the first active material layer comprising a first active material; and a second active material layer located on the side surface of the first active material layer away from the positive current collector, the second active material layer comprising a second active material, wherein the sphericity of the first active material is greater than the sphericity of the second active material. This reduces damage to the positive current collector by the positive active material particles during cold pressing, thereby improving the structural stability of the positive electrode sheet.

[0008] According to an embodiment of the present invention, the roundness 'a' of the first active material is 0.6-0.95; preferably, the roundness 'a' of the first active material is 0.8-0.95. This further improves the structural stability of the positive electrode sheet.

[0009] According to an embodiment of the present invention, the Dv90 particle size of the first active material is 2 μm - 5 μm. Thereby, the structural stability of the positive electrode sheet can be further improved.

[0010] According to an embodiment of the present invention, the Dv90 particle size of the second active material is 2 μm - 25 μm. Thereby, the energy density of the battery can be increased.

[0011] According to an embodiment of the present invention, the positive electrode current collector includes aluminum foil. Thereby, the structural stability of the positive electrode sheet can be improved.

[0012] According to an embodiment of the present invention, the thickness h0 of the first active material layer and the thickness h of the positive electrode current collector satisfy the relationship (h / 4) ≤ h0 ≤ (h / 2). Thereby, the positive electrode sheet can have both good structural stability and energy density.

[0013] According to an embodiment of the present invention, the thickness of the positive electrode current collector is 8 μm - 20 μm.

[0014] According to an embodiment of the present invention, the thickness of the first active material layer is 2 μm - 10 μm. Thereby, during the cold pressing process, the first active material layer can buffer the positive electrode current collector.

[0015] According to an embodiment of the present invention, the first active material layer is provided on both side surfaces of the positive electrode current collector. Thereby, the energy density of the positive electrode sheet can be further increased.

[0016] According to an embodiment of the present invention, the first active material and the second active material each independently include LiMn2O4, LiFe a M (1-a) PO4, where 0 < a < 1, the M includes at least one of Co, Mn, Ni, Mg, Zn, and Al, and LiNi x Co y Q z R 1-x-y-z O2, where the Q includes at least one of Mn and Al, the R includes at least one of Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V, Ti, 0 ≤ x < 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1, and x + y + z ≤ 1.

[0017] In another aspect of the present invention, the present invention provides a battery, including a positive electrode sheet, a negative electrode sheet, and an electrolyte, and the positive electrode sheet is the aforementioned positive electrode sheet. Thereby, the battery has all the features and advantages of the aforementioned positive electrode sheet, which will not be elaborated herein.

[0018] According to an embodiment of the present invention, the electrolyte comprises a lithium salt, which includes a first lithium salt and a second lithium salt. The first lithium salt comprises lithium bis(fluorosulfonyl)imide, and the second lithium salt comprises lithium hexafluorophosphate. The concentration of the first lithium salt is C1, and the concentration of the second lithium salt is C2. The sphericity 'a' of the first active material satisfies the relationship a ≤ C1 / C2 ≤ 6a with respect to the concentrations of the first lithium salt C1 and the second lithium salt C2. This improves the safety performance and cycle life of the battery.

[0019] According to an embodiment of the present invention, the first lithium salt concentration C1 and the second lithium salt concentration C2 satisfy the relationship 1≤C1 / C2≤5 and / or 0.6mol / L≤C1+C2≤1.5mol / L, thereby further improving the safety performance and cycle life of the battery.

[0020] According to an embodiment of the present invention, the molar concentration of the first lithium salt in the electrolyte is 0.1 mol / L-1 mol / L, and the molar concentration of the second lithium salt in the electrolyte is 0.2 mol / L-1 mol / L.

[0021] In a third aspect, the present invention provides an electrical device comprising the aforementioned battery. Thus, the electrical device possesses all the features and advantages of the aforementioned battery, which will not be repeated here. Attached Figure Description

[0022] Figure 1 This is a partial structural schematic diagram of a positive electrode sheet according to an embodiment of the present invention;

[0023] Figure 2 This is a schematic diagram of the structure of a positive electrode sheet according to an embodiment of the present invention.

[0024] Figure label:

[0025] 10: First active material; 20: Second active material; 100: Positive current collector; 210: First active material layer; 220: Second active material layer. Detailed Implementation

[0026] The embodiments of this application are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0027] In one aspect of the invention, a positive electrode plate is provided, such as... Figure 2As shown, it includes: a positive current collector 100; and a first active material layer 210, the first active material layer 210 being located on at least one side surface of the positive current collector 100. Specifically, see [link to documentation]. Figure 1 The first active material layer 210 includes a first active material 10; the second active material layer 220 is located on the surface of the first active material layer 210 away from the positive current collector 100, and includes a second active material 20. The roundness of the first active material 10 is greater than that of the second active material 20. Because the roundness of the first active material is greater than that of the second active material, when the first active material directly contacts the positive current collector, its larger roundness (i.e., fewer surface edges) makes it less likely to damage the positive current collector, such as the oxide film on the surface of aluminum foil, when the edges of the positive active material come into contact with the surface of the positive current collector under the pressure of the cold press roller. This allows the oxide film on the surface of the positive current collector to better protect the positive current collector during use. Meanwhile, in this invention, the surface of the positive electrode current collector 100 has a first active material layer 210 and a second active material layer 220. The first active material layer can protect the oxide film on the surface of the aluminum foil and prevent the aluminum foil from being damaged by pressure; the second active material layer can provide capacity for the battery. In summary, the aforementioned positive electrode sheet improves the durability of the positive electrode current collector through the roundness design of the active material, and effectively improves the energy density through the setting of the second active material layer, reduces the occurrence of side reactions during battery operation, and improves the safety and cycle performance of the battery.

[0028] It should be noted that in this invention, roundness refers to the degree to which the particle projection closely approximates a circle. The formula for calculating roundness is the particle's projected area multiplied by 4π and divided by the square of the particle's projected perimeter, i.e., e = (4π × particle projected area) / (particle projected perimeter × particle projected perimeter), where e is the roundness. The value of roundness is generally less than 1, and for regular spherical particles, its value is the largest, equal to 1. A higher roundness indicates that the surface of the active material has fewer sharp edges and is closer to a theoretical circle.

[0029] The roundness of this application can be tested using the following method: acquire optical images of the particles using a particle image analysis device "Morphologi G3" manufactured by Malvern Corporation, and perform filtering processing using a parameter of "solidity=0.93" based on the acquired optical images of more than 20,000 particles to determine the roundness.

[0030] According to some embodiments of the present invention, the roundness reflects the degree of sharpness of the surface of the first active material. The higher the roundness, the fewer sharp edges the material surface has. The roundness of the first active material is not particularly limited. For example, the roundness of the first active material can satisfy a of 0.6-0.95. Preferably, the roundness of the first active material can be 0.8-0.95. The higher the roundness, the fewer sharp edges the surface of the active material has, and the closer it is to the theoretical circle. Since the first active material is in direct contact with the surface of the current collector, when the roundness of the first active material is within the above range, the surface of the first active material has fewer sharp edges, and it is not easy to scratch the oxide film on the surface of the positive electrode current collector during cold pressing.

[0031] According to some embodiments of the present invention, the first active material is in direct contact with the current collector. Specifically, the particle size of the first active material is not particularly limited; for example, the Dv90 particle size of the first active material can be 2μm-5μm. When the Dv90 particle size of the first active material is less than 2μm, the particle size of the first active material is too small, the specific surface area of ​​the first active material is too large, and there are more side reactions between the active material and the electrolyte, which is not conducive to improving the electrical performance of the battery cell. When the Dv90 particle size of the first active material is greater than 5 μm, the contact area between the first active material and the positive electrode current collector is too small, resulting in weak adhesion between the first active material layer and the positive electrode current collector. When the Dv90 particle size of the first active material is between 2 μm and 5 μm, the first active material layer acts as a buffer layer during the cold pressing process of the positive electrode sheet, ensuring uniform stress on the surface of the positive electrode current collector. Therefore, the cold pressing process in battery production can alleviate significant pressure damage to the current collector caused by the active material and improve the longitudinal tensile strength of the positive electrode current collector. Furthermore, the larger contact area between the first active material and the positive electrode current collector is beneficial for improving the adhesion between the first active material layer and the positive electrode current collector.

[0032] According to some embodiments of the present invention, a second active material layer is located on the surface of the first active material layer away from the positive electrode current collector. The second active material layer includes a second active material, which can improve the compaction density of the positive electrode and the battery capacity. The particle size of the second active material is not limited; for example, the Dv90 particle size of the second active material can be 2 μm-25 μm. Further, the Dv90 particle size of the second active material can be 2-10 μm and / or 10 μm-25 μm, that is, the Dv90 of the second active material can be an active material with a particle size range of 2 μm-10 μm, an active material with a particle size range of 10 μm-25 μm, or a mixture of an active material with a particle size range of 2 μm-10 μm and an active material with a particle size range of 10 μm-25 μm. When the Dv90 particle size of the second active material is in the range of 2μm-10μm, the gaps between the second active materials are small, which is beneficial for the insertion and extraction of lithium ions during battery charging and discharging, thereby improving the rate performance of the battery. When the Dv90 particle size of the second active material is in the range of 10μm-25μm, the slurry made with the second active material has low viscosity and good fluidity, which can improve the thickness uniformity of the active material layer on the surface of the positive electrode sheet, as well as improve the stability and safety of the battery. When the second active material is a mixture of active materials with a Dv90 particle size range of 2μm-10μm and active materials with a Dv90 particle size range of 10μm-25μm, as mentioned above, both the rate performance and the safety performance of the battery can be improved.

[0033] According to some embodiments of the present invention, the second active material layer can be used to improve the compaction density of the positive electrode and the battery capacity. The thickness of the second active material layer is not limited. For example, the thickness of the second active material layer can be 30-60 μm, and preferably, the thickness of the second active material layer can be 45-55 μm.

[0034] According to some embodiments of the present invention, the technical solution of the present invention is applicable to metal current collectors. When the surface of the metal current collector near the active material layer has a passivation layer, the above solution can better protect the passivation layer. Specifically, the positive electrode current collector may include aluminum foil. Aluminum foil has superior conductivity, which is beneficial for electron conduction, and is a superior positive electrode current collector material.

[0035] According to some embodiments of the present invention, the thickness of the first active material layer and the thickness of the positive current collector are not particularly limited. For example, the thickness h0 of the first active material layer and the thickness h of the positive current collector can satisfy the relationship h / 4≤h0≤h / 2. When h0<(h / 4), the first active material layer cannot effectively alleviate the pressure transmitted from the second active material layer to the positive current collector, and thus cannot play the role of a buffer layer. When h0>(h / 2), during the cold pressing process, the rolling pressure will increase with the increase of the thickness of the active material layer. When the active material layer is too thick, the pressure that needs to be applied to the surface of the positive electrode sheet is too large, which is prone to cold pressing strip breakage. When (h / 4)≤h0≤(h / 2), the first active material layer can effectively buffer the pressure transmitted from the second active material layer to the positive current collector when the rolling force is applied, play a buffering role for the positive current collector, and reduce the damage of the positive active material to the passivation layer on the surface of the positive current collector. According to other embodiments of the present invention, the thicknesses of the first active material layer and the positive current collector are not particularly limited. For example, the thickness h0 of the first active material layer can be 2 μm-10 μm, and the thickness h of the positive current collector can be 8 μm-20 μm. When the thickness of the first active material layer is less than 2 μm, the first active material layer is too thin and cannot effectively relieve the pressure of the second active material layer. When the thickness of the first active material layer is greater than 10 μm, the roller pressure will increase with the increase of the active material layer thickness during cold pressing. When the active material layer is too thick, the pressure required to be applied to the surface of the positive electrode sheet is too large, which easily leads to cold pressing strip breakage. When the thickness h of the positive current collector is less than 8 μm, the positive current collector, as an important carrier bearing the first and second active material layers, is prone to cold pressing strip breakage under the pressure of the rollers during cold pressing. When the thickness h of the positive electrode current collector is greater than 20μm, the thickness of the positive electrode current collector is too large. After the active material layer is set on the surface of the current collector to form the positive electrode sheet, the overall thickness of the positive electrode sheet is too large, which reduces the energy density of the battery. It is not suitable for use scenarios such as consumer batteries and power batteries that have high requirements for energy density and battery volume.

[0036] According to some embodiments of this application, the location of the first active material layer is not particularly limited. For example, the first active material layer may be disposed only on one side surface of the positive current collector, and the second active material layer may be located on the side surface of the first active material layer away from the positive current collector. Alternatively, the first active material layer may be disposed on both sides of the positive current collector, and in this case, the second active material layer may be located on the side surface of the two first active material layers away from the positive current collector.

[0037] According to some embodiments of this application, reference is made to Figure 1, in the first active material layer of the positive electrode sheet, a part of the first active material 10 will be embedded in the surface of the second active material layer close to the first active material layer. Specifically, the first active material 10 embedded in the second active material layer will fill the gaps between the second active materials 20 in the second active material layer. That is, there will be a phenomenon of cross-distribution of the first active material and the second active material at the layer interface between the first active material layer and the second active material layer.

[0038] According to some embodiments of the present application, the types of the first active material and the second active material are not particularly limited. For example, the first active material and the second active material can independently include LiMn2O4, LiFe a M (1-a) PO4, where 0 < a < 1, M includes at least one of Co, Mn, Ni, Mg, Zn, and Al, and LiNi x Co y Q z R 1-x-y-z O2, where Q includes at least one of Mn and Al, R includes at least one of Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V, and Ti, 0 ≤ x < 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1, and x + y + z ≤ 1. When the types of the first active material and the second active material are within the above range, the crystal structure stability of the positive electrode active material is good, and the battery made of the above positive electrode active material has a high energy density.

[0039] In the second aspect of the present invention, the present invention also provides a battery, which includes a positive electrode sheet, a negative electrode sheet, and an electrolyte. It should be noted that the process of assembling the positive electrode sheet and the negative electrode sheet of the battery can be the stacking method or the winding method. The positive electrode sheet of this battery is the positive electrode sheet designed by the technical solution of the present invention, so this battery includes all the advantages of this positive electrode sheet.

[0040] According to some embodiments of the present invention, the composition of the electrolyte is not particularly limited. For example, the electrolyte of the battery may include lithium salts, which include various lithium-containing compounds. After dissolving in the solvent of the electrolyte, the lithium salts release a large number of active lithium ions. Specifically, the lithium salts may include a first lithium salt and a second lithium salt. For example, the first lithium salt may include lithium bis(fluorosulfonyl)imide (LiFSI), and the second lithium salt may include lithium hexafluorophosphate (LiPF6). In this application, the inventors have discovered that during lithium battery cycling, LiFSI has a high thermal decomposition temperature and low sensitivity to water, and a high degree of dissociation in the electrolyte. Therefore, LiFSI can effectively improve the kinetic performance of the electrolyte. The decomposition product LiF of LiPF6 in the electrolyte can, to some extent, alleviate the corrosion of aluminum foil by LiFSI. Thus, by adding a first lithium salt and a second lithium salt to the electrolyte, the corrosion of aluminum foil by the first lithium salt in the electrolyte can be effectively alleviated, thereby improving the safety performance and cycle life of the battery.

[0041] According to some embodiments of the present invention, the concentrations of the first lithium salt and the second lithium salt are not particularly limited. For example, the concentration of the first lithium salt is C1, and the concentration of the second lithium salt is C2. When the sphericity a of the first active material is 0.6-0.95, the sphericity a of the first active material and the concentrations of the first lithium salt C1 and the second lithium salt C2 can satisfy the relationship a≤C1 / C2≤6a. When the sphericity a of the first positive electrode active material and the concentrations of the first lithium salt C1 and the second lithium salt C2 satisfy the aforementioned relationship, the corrosion of the aluminum foil by the electrolyte can be effectively mitigated, thereby improving the safety performance and cycle life of the battery.

[0042] According to some embodiments of the present invention, the concentrations of the first lithium salt and the second lithium salt are not particularly limited. For example, the concentration of the first lithium salt is C1 and the concentration of the second lithium salt is C2. The concentrations of the first lithium salt C1 and the second lithium salt C2 can satisfy the relationship 1≤C1 / C2≤5 and / or 0.6mol / L≤C1+C2≤1.5mol / L. When the concentrations of the first lithium salt and the second lithium salt satisfy the aforementioned relationship, the lithium ions released by the lithium salt can effectively alleviate the corrosion of the aluminum foil, thereby improving the safety performance and cycle life of the battery.

[0043] In the description of this application, "A and / or B" can include any of the cases of A alone, B alone, or A and B, where A and B are merely examples and can be any technical feature connected by "and / or" in this application.

[0044] According to some embodiments of the present invention, the concentrations of the first lithium salt and the second lithium salt are not particularly limited. For example, the molar concentration of the first lithium salt in the electrolyte can be 0.1 mol / L to 1 mol / L, and the molar concentration of the second lithium salt in the electrolyte can be 0.2 mol / L to 1 mol / L. According to other embodiments of the present invention, the mass fractions of the first lithium salt and the second lithium salt are not particularly limited based on the total mass of the electrolyte. For example, the mass percentage of the first lithium salt in the electrolyte can be 7.7 wt% to 15.4 wt%, and the mass percentage of the second lithium salt in the electrolyte can be 2.5 wt% to 6.25 wt%.

[0045] According to some embodiments of the present invention, the composition of the electrolyte is not particularly limited. For example, the battery electrolyte may include additives, which may include at least one selected from vinylene carbonate, vinyl sulfate, lithium difluorooxalate borate, lithium difluorophosphate, lithium tetrafluoroborate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, lithium trifluoromethanesulfonate, and lithium diacetate borate. Using the above-mentioned additives can improve the conductivity and stability of the electrolyte, thereby improving the rate performance, cycle performance, and capacity of the battery.

[0046] According to some embodiments of the present invention, the composition of the electrolyte is not particularly limited. For example, the electrolyte of the battery may include a non-aqueous organic solvent, which may include at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, ethyl propyl carbonate, γ-butyrolactone, methyl formate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, methyl propionate, and tetrahydrofuran. The above-mentioned non-aqueous solvents have good conductivity in the electrolyte. During battery cycling, the use of electrolytes containing these non-aqueous solvents can promote the migration of lithium ions between the positive and negative electrodes, thereby improving the battery's electrical performance.

[0047] According to some embodiments of the present invention, the content of non-aqueous solvents in the electrolyte is not particularly limited. For example, the mass percentage of non-aqueous organic solvents in the electrolyte can be 65wt%-85wt%. When the mass percentage of non-aqueous solvents in the electrolyte is 65wt%-85wt%, the conductivity of the electrolyte can be improved. Using an electrolyte with the aforementioned range of non-aqueous solvent content during battery cycling can promote the smooth movement of lithium ions between the positive and negative electrodes, thereby improving the electrical performance of the battery.

[0048] According to some embodiments of the present invention, the type of active material in the negative electrode is not particularly limited. For example, the negative electrode active material may include natural graphite, artificial graphite, mesophase micro carbon spheres, hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, TiO2-Li4Ti5O 12 And at least one of Li-Al alloys. The aforementioned negative electrode active materials have advantages such as good conductivity, low resistance, and good compatibility with electrolyte, thereby improving the battery capacity and cycle performance.

[0049] According to some embodiments of the present invention, during the battery assembly process, the positive electrode sheet needs to be cold-pressed to meet the preset thickness requirements of the battery according to the battery capacity design, thus reducing the thickness of the positive electrode sheet in the battery. The thickness of the positive electrode sheet in the battery is not particularly limited. For example, when both sides of the positive electrode sheet are provided with the aforementioned first active material layer and second active material layer, the total thickness of the positive electrode sheet can be 50μm-116μm, wherein the thickness of the positive current collector can be 10μm-15μm, the thickness of the first active material layer can be 2.5μm-7.5μm, and the thickness of the second active material layer can be 15μm-50μm.

[0050] In a third aspect, the present invention provides an electrical device comprising the aforementioned battery. Thus, the electrical device possesses all the features and advantages of the aforementioned battery, which will not be repeated here.

[0051] The following specific embodiments illustrate the solution of this application. It should be noted that these embodiments are for illustrative purposes only and should not be considered as limiting the scope of this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0052] Example 1:

[0053] Electrolyte preparation: In an argon-filled glove box (water content <10ppm, oxygen content <1ppm), a first organic solvent with a mass ratio of ethylene carbonate to dimethyl carbonate of 3:7 was provided. 2% vinylene carbonate was added to the first organic solvent and mixed thoroughly to obtain a second organic solvent. 82.1g of the second organic solvent was taken, and 15.4g of lithium bis(fluorosulfonyl)imide (LiFSI) and 2.5g of LiPF6 were added. After the lithium salts were completely dissolved, the target electrolyte was obtained. The concentration of LiFSI (C1) in the target electrolyte was 1 mol / L, and the concentration of LiPF6 (C2) was 0.2 mol / L.

[0054] Preparation of the positive electrode: A primary active material, LiNi, with a sphericity of 0.9 and a Dv90 particle size range of 2μm-5μm, was prepared. 0.5 Co 0.2 Mn 0.3 The first positive electrode slurry is prepared by reacting O2, a conductive agent, and polyvinylidene fluoride in N-methylpyrrolidone. The first positive electrode slurry has a solid content of 50 wt%, of which LiNi is the most abundant solid component. 0.5 Co 0.2 Mn 0.3 The mass ratio of O2, conductive agent, and polyvinylidene fluoride is 8:1:1.

[0055] The second active material, LiNi, has a Dv90 particle size range of 2μm-25μm. 0.5 Co 0.2 Mn 0.3 O2, a conductive agent, and polyvinylidene fluoride were used to prepare a second positive electrode slurry in N-methylpyrrolidone. The solid content of the second positive electrode slurry was 50 wt%, of which LiNi was the main solid component. 0.5 Co 0.2 Mn 0.3 The mass ratio of O2, conductive agent, and polyvinylidene fluoride is 8:1:1.

[0056] First, the first slurry is coated onto the current collector aluminum foil and dried at 85°C to form the first active material layer; then the second slurry is coated onto the surface of the first active material layer and dried at 85°C, followed by cold pressing, and then edge trimming, cutting, and slitting to form the positive electrode sheet.

[0057] Preparation of the negative electrode sheet: Graphite was selected as the negative electrode active material. The negative electrode active material, conductive agent, thickener, and styrene-butadiene rubber were mixed evenly in deionized water to prepare a negative electrode slurry. The solid content of the negative electrode slurry was 30 wt%, and the mass ratio of graphite, conductive agent, thickener, and styrene-butadiene rubber in the solid components was 80:15:3:2. The negative electrode slurry was coated on the current collector copper foil and dried at 85℃. Then, it was cold-pressed, trimmed, cut into sheets, and slit. Finally, it was dried under vacuum at 120℃ for 12 hours to prepare the negative electrode sheet.

[0058] Lithium-ion battery fabrication: A 16μm polyethylene film (PE) is used as the separator. The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to isolate them. A bare cell is obtained by winding, tabs are welded on, and the bare cell is placed in outer packaging. The prepared electrolyte is injected into the dried cell, followed by encapsulation, settling, formation, shaping, and capacity testing to complete the lithium-ion battery fabrication.

[0059] Examples 2-11 and Comparative Examples 1-8 follow the same process as in Example 1, except for the sphericity of the first active material and the concentrations of LiFSI and LiPF6 in the electrolyte, as detailed in Table 1.

[0060] Table 1

[0061]

[0062] For Examples 1-11 and Comparative Examples 1-8, the transverse tensile strength test was performed on the positive electrode sheet. The test method is as follows: Take a sample with a length of 50 mm, a width of 20 mm, and an aluminum foil thickness of 15 mm. Fix both ends of the sample in the length or width direction on the clamp of the Instrom 3365 high-speed rail tensile testing machine. Pull at a speed of 10 mm / min until the sample breaks to complete the test. The width direction tensile force value F (N) is obtained according to the software of the instrument. The transverse tensile strength can be measured by R = F / (width of aluminum foil * thickness of aluminum foil).

[0063] Thermal runaway performance and cycle life tests were conducted on the batteries in Examples 1-11 and Comparative Examples 1-8. The test methods are as follows:

[0064] (1) Thermal runaway performance: The ambient temperature was adjusted to 25℃, the battery cell was charged at 1C to 4.25V, and then charged at a constant voltage to 0.05C. The battery cell was placed in a heating furnace, and the furnace was heated at 10℃ / min and held at that temperature for 10min until the battery cell experienced thermal runaway, at which point the process was terminated. The temperature monitored by the furnace at the time of battery cell thermal runaway was recorded.

[0065] (2) Cycle life test: The cycle life performance test of lithium-ion batteries was conducted at an ambient temperature of 25°C. The cells were charged to 4.25V at 1C, then charged to 0.05C at constant voltage, and left to stand for 10 minutes.

[0066] The test results are shown in Table 2:

[0067] Table 2

[0068]

[0069]

[0070] The test results in Table 2 show that the overall performance of Examples 1-11 is significantly improved compared with that of Comparative Examples 1-8 in terms of thermal runaway performance and cycle performance. This indicates that when the sphericity 'a' of the first active material and the concentrations of the first lithium salt C1 and the second lithium salt C2 satisfy the relationship a≤C1 / C2≤6a, the corrosion of the aluminum foil by the electrolyte can be effectively suppressed, thereby improving the overall performance of the battery cell.

[0071] In Comparative Examples 1, 3, 4, and 8, the sphericity of the active material particles was less than 0.6, resulting in numerous sharp edges on the surface. During cold pressing, these edges easily damaged the oxide film on the aluminum foil surface, leading to easy corrosion of the aluminum foil when the electrolyte contained lithium fluorosulfonylimide. In Comparative Examples 2, 5, 6, and 7, although the sphericity of the active materials was within an appropriate range, the concentration of LiFSI was too high at this sphericity level, failing to effectively inhibit the corrosion of the aluminum foil by the electrolyte.

[0072] 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 indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified. In this invention, unless otherwise explicitly stated and limited, "above" or "below" a second feature may mean that the first and second features are in direct contact, or that the first and second features are in indirect contact through an intermediate medium. Moreover, "above," "over," and "on top" of a second feature may mean that the first feature is directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" a second feature may mean that the first feature is directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0073] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0074] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A positive electrode sheet, characterized by, The positive electrode current collector comprises: a first active material layer on at least one side surface of the positive electrode current collector, the first active material layer comprising a first active material; a second active material layer on a side surface of the first active material layer away from the positive electrode current collector, the second active material layer comprising a second active material, wherein the circularity of the first active material is greater than the circularity of the second active material. The circularity a of the first active material is 0.6-0.

95.

2. The cathode electrode of claim 1, wherein, The circularity of the first active material is 0.8-0.

95.

3. The cathode electrode of claim 2, wherein, The Dv90 particle size of the first active material is 2-5 μm.

4. The cathode electrode of claim 1, wherein The Dv90 particle size of the second active material is 2-25 μm.

5. The cathode electrode of claim 4, wherein, The positive electrode current collector comprises an aluminum foil.

6. The cathode sheet of claim 1, wherein, The thickness h0 of the first active material layer and the thickness h of the positive electrode current collector satisfy the relationship (h / 4)≤h0≤(h / 2).

7. The cathode sheet of claim 1, wherein, The thickness of the positive electrode current collector is 8-20 μm.

8. The cathode sheet of claim 1, wherein, The thickness of the first active material layer is 2-10 μm.

9. The cathode electrode of claim 1, wherein, The first active material layer is arranged on both side surfaces of the positive electrode current collector.

10. The cathode sheet of claim 1, wherein, The positive electrode sheet comprises the positive electrode sheet of any one of claims 1-11.

11. The cathode electrode of claim 1, wherein, LiMn2O4, LiFe a M (1-a) PO4, where 0 < a < 1, the M comprises at least one of Co, Mn, Ni, Mg, Zn, and Al, and LiNi x Co y Q z R 1-x-y-z O2, where the Q comprises at least one of Mn and Al, the R comprises at least one of Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V, Ti, 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z < 1.

12. A battery, characterized by The electrolyte comprises a lithium salt, the lithium salt comprising a first lithium salt and a second lithium salt, the first lithium salt comprising lithium bisfluorosulfonylimide, the second lithium salt comprising lithium hexafluorophosphate, the concentration of the first lithium salt being C1, the concentration of the second lithium salt being C2, the circularity a of the first active material and the concentration C1 of the first lithium salt and the concentration C2 of the second lithium salt satisfying the relationship a≤C1 / C2≤6a.

13. The battery of claim 12, wherein, The concentration C1 of the first lithium salt and the concentration C2 of the second lithium salt satisfy the relationship 1≤C1 / C2≤5 and / or 0.6 mol / L≤C1+C2≤1.5 mol / L, the units of the concentration of the first lithium salt and the concentration of the second lithium salt being mol / L.

14. The battery of claim 13, wherein, The molar concentration of the first lithium salt in the electrolyte is 0.1-1 mol / L, and the molar concentration of the second lithium salt in the electrolyte is 0.2-1 mol / L.

15. The battery of claim 13, wherein, 16. An electrical device comprising the battery of any one of claims 12-15. ​

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