Positive electrode sheet and battery

Abstract

The present application provides a positive electrode sheet and a battery. The positive electrode sheet comprises a positive electrode current collector and a positive electrode coating located on the surface of one side of the positive electrode current collector, the positive electrode coating containing a positive electrode active material; the positive electrode coating comprises a first coating and a second coating located between the first coating and the positive electrode current collector; the positive electrode active material in the first coating comprises secondary particles, and the positive electrode active material in the second coating comprises primary particles; the particle diameters D90, D50, and D10 of the positive electrode active material in the first coating satisfy 1.6≤(D90-D10) / D50≤1.95; and the particle sizes D60 and D10 of the positive electrode active material in the second coating satisfy 3.7≤D60 / D10≤4.2. The present application can both reduce battery impedance and improve battery capacity and other performance.

Description

A positive electrode and a battery

[0001] This application claims priority to Chinese Patent Application No. 202411054550.1, filed on August 2, 2024, entitled “A Positive Electrode Sheet and Battery”, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of batteries, specifically to a positive electrode and a battery. Background Technology

[0003] The positive electrode is a crucial component of a battery, and its structure significantly impacts performance characteristics such as impedance and capacity. Specifically, to increase battery energy density, a high compaction density is typically required for the positive electrode. However, high compaction density hinders the insertion and extraction of active ions (such as lithium ions in lithium-ion batteries), resulting in high battery impedance. Therefore, reducing battery impedance and improving battery capacity and other performance characteristics are pressing technical challenges that require immediate attention from those skilled in the art. Summary of the Invention

[0004] This application provides a positive electrode and a battery that can reduce battery impedance and improve battery capacity and other performance, effectively overcoming the defects of the prior art.

[0005] In one aspect, this application provides a positive electrode sheet, including a positive current collector and a positive electrode coating located on one side surface of the positive current collector, the positive electrode coating containing a positive electrode active material; the positive electrode coating includes a first coating and a second coating located between the first coating and the positive current collector;

[0006] The positive electrode active material in the first coating includes secondary particles, and the positive electrode active material in the second coating includes primary particles;

[0007] The particle size D of the positive electrode active material in the first coating 90 D 50 and D 10 Satisfying 1.6≤(D) 90 -D 10 ) / D 50 ≤1.95; the particle size D of the positive electrode active material in the second coating 60 and D 10 Satisfying 3.7≤D 60 / D 10 ≤4.2.

[0008] According to one embodiment of this application, the particle size D of the positive electrode active material in the first coating is... 50 Satisfying 6μm≤D50 ≤9μm.

[0009] According to one embodiment of this application, the particle size D of the positive electrode active material in the first coating is... 90 Satisfying 8μm≤D 90 ≤18μm.

[0010] According to one embodiment of this application, the particle size D of the positive electrode active material in the first coating is... 10 Satisfying 1.2μm≤D 10 ≤3.5μm.

[0011] According to one embodiment of this application, the particle size D of the positive electrode active material in the second coating is... 10 Greater than or equal to 0.3 μm.

[0012] According to one embodiment of this application, the particle size D of the positive electrode active material in the second coating is... 10 Satisfying 0.3μm≤D 10 ≤1μm.

[0013] According to one embodiment of this application, the particle size D of the positive electrode active material in the second coating is... 60 Satisfying 0.5μm≤D 60 ≤3.9μm.

[0014] According to one embodiment of this application, the ratio of the thickness of the first coating to the thickness of the second coating is 0.6 to 1.5.

[0015] According to one embodiment of this application, the thickness of the first coating is 20 μm to 110 μm.

[0016] According to one embodiment of this application, the thickness of the second coating is 20 μm to 110 μm.

[0017] According to one embodiment of this application, the positive electrode active material includes lithium iron phosphate.

[0018] According to one embodiment of this application, the compaction density of the positive electrode coating is 2.2 g / cm³. 3 ~2.8g / cm 3 .

[0019] Another aspect of this application provides a battery including the aforementioned positive electrode.

[0020] The implementation of this application has at least the following beneficial effects: by providing a first coating on at least one side surface of the positive electrode current collector and a second coating located between the first coating and the positive electrode current collector, and controlling the particle size distribution of the positive electrode active material in the first coating to satisfy 1.6≤(D 90-D 10 ) / D 50 ≤1.95, the particle size distribution of the positive electrode active material in the second coating satisfies 3.7≤D 60 / D 10 ≤4.2, the two work together to maintain a high energy density of the positive electrode while reducing the impedance of the positive electrode and improving the ion transport capability of the positive electrode, thereby reducing the battery impedance, improving the battery dynamic performance, and also improving the battery capacity and other performance. Attached Figure Description

[0021] Figure 1 is a schematic diagram of the positive electrode structure according to an embodiment of this application.

[0022] Explanation of reference numerals in the attached figures: 1. Positive current collector; 2. Positive electrode coating; 21. First coating; 22. Second coating; H1. Thickness of the first coating; H2. Thickness of the second coating. Detailed Implementation

[0023] To enable those skilled in the art to better understand the solutions of this application, a further detailed description of this application is provided below. The specific embodiments listed below are merely descriptions of the principles and features of this application; the examples are only for explaining this application and are not intended to limit its scope. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this application.

[0024] This application provides a positive electrode sheet, including a positive current collector 1 and a positive electrode coating 2 located on at least one side surface of the positive current collector 1. The positive electrode coating 2 contains a positive electrode active material. The positive electrode coating 2 includes a first coating 21 and a second coating 22 located between the first coating 21 and the positive current collector 1. The positive electrode active material in the first coating 21 includes secondary particles, and the positive electrode active material in the second coating 22 includes primary particles. The particle size D of the positive electrode active material in the first coating 21 is... 90 D 50 and D 10 Satisfying 1.6≤(D) 90 -D 10 ) / D 50 ≤1.95; Particle size D of the positive electrode active material in the second coating 22 60 and D 10 Satisfying 3.7≤D 60 / D 10 ≤4.2.

[0025] According to the inventors' research, it is difficult to achieve high compaction of positive electrode active materials with a single particle size distribution. The reasonable matching of particle sizes of positive electrode active materials can improve the ultimate compaction density of positive electrode active materials, thereby improving the energy density of the battery. However, excessively high compaction density will also affect ion channels to a certain extent, thereby affecting the transport and diffusion ability of active ions in the positive electrode sheet and increasing the impedance of the positive electrode sheet.

[0026] Therefore, according to the inventor's research, under the above-mentioned positive electrode structure system, the particle size distribution of the positive electrode active material in the first coating 21 is adjusted to satisfy 1.6≤(D 90 -D 10 ) / D 50 With a particle size distribution ≤1.95, the positive electrode active material in the first coating 21 (upper layer) has a narrow particle size distribution, which is conducive to forming a high-porosity, low-torsion upper electrode structure. This provides pathways for the transport and diffusion of active ions, improves the liquid-phase diffusion capability of active ions, reduces the impedance of the positive electrode sheet, and thus reduces the battery impedance and improves the battery's dynamic performance. Simultaneously, by controlling the particle size distribution of the positive electrode active material in the second coating 22 (lower layer), it is made to satisfy 3.7 ≤ D. 60 / D 10 ≤4.2 enables the second coating 22 to have a high compaction structure, ensuring the high energy density of the positive electrode sheet, thereby improving the battery's capacity and other performance.

[0027] Therefore, based on the influence of the particle size distribution of the positive electrode active material in the thickness direction of the positive electrode sheet on the impedance and other properties of the positive electrode sheet, the above-mentioned double-layer electrode structure is constructed. The high-energy layer (second coating 22) is used as the lower layer and the high-porosity layer (first coating 21) is used as the upper layer. This constructs a fast ion transport path on the surface of the positive electrode sheet, which can simultaneously take into account both high compaction density and a positive electrode structure that facilitates ion diffusion, thereby taking into account both reducing battery impedance and improving battery capacity and other electrochemical performance.

[0028] Specifically, the positive electrode active material in the first coating 21 may include secondary particles, and the positive electrode active material in the second coating 22 may include primary particles, which is beneficial to both improve the compaction density and ion diffusion capability of the positive electrode sheet, thereby reducing battery impedance and improving battery capacity and other electrochemical performance.

[0029] Specifically, the particle size distribution of the positive electrode active material in the second coating 22 satisfies 3.7 ≤ D 60 / D 10 ≤4.2, wherein the positive electrode active material includes primary particles. By using high-compacted, low-porosity primary particles with a reasonable particle size distribution as the positive electrode active material, the compaction density of the second coating 22 can be improved, thereby ensuring the energy density of the positive electrode sheet; the particle size distribution of the positive electrode active material in the first coating 21 satisfies 1.6≤(D 90-D 10 ) / D 50 ≤1.95, wherein the positive electrode active material includes secondary particles (i.e., the first coating 21 uses secondary particles with a narrow particle size distribution as the positive electrode active material). The secondary particles are composed of multiple primary particles and have a large number of pore structures inside, which can provide pathways for active ion transport. This allows the first coating 21 to have a high porosity, improve ion transport capability, and reduce the impedance of the positive electrode sheet.

[0030] In this embodiment of the application, the particle size distribution width ε of the positive electrode active material in the first coating 21 is (D 90 -D 10 ) / D 50 That is, 1.6≤ε≤1.95, where ε is, for example, a range consisting of 1.6, 1.65, 1.7, 1.75, 1.8, 1.85, 1.9, 1.95 or any two of them.

[0031] In this embodiment, the particle size non-uniformity coefficient C of the positive electrode active material in the second coating 22 is... u =D 60 / D 10 That is, 3.7≤C u ≤4.2, C u For example, a range consisting of 3.7, 3.8, 3.9, 4, 4.1, 4.2, or any two of them.

[0032] In this embodiment of the application, the positive electrode active material may include lithium iron phosphate, that is, the positive electrode active material in the first coating 21 includes lithium iron phosphate, and the positive electrode active material in the second coating 22 includes lithium iron phosphate.

[0033] According to the inventors' research, in the above-mentioned positive electrode structure system, the second coating 22 mainly uses high-compact, low-porosity primary lithium iron phosphate particles (primary particles) as the positive electrode active material, which enables the second coating 22 to have high compaction and maintain a high energy density of the positive electrode. Simultaneously, the first coating 21 mainly uses narrow-size secondary lithium iron phosphate particles (secondary particles) as the positive electrode active material. These secondary lithium iron phosphate particles are composed of multiple primary lithium iron phosphate particles and have a large number of pores, which helps the first coating 21 to have high porosity, providing pathways for the transport of active ions, thereby improving the liquid-phase transport capability of active ions and reducing the impedance of the positive electrode. Therefore, the embodiments of this application, targeting the design of a dual-layer positive electrode structure of primary / secondary lithium iron phosphate particles, can simultaneously reduce the impedance of lithium iron phosphate batteries and improve their capacity and other performance characteristics.

[0034] According to further research by the inventors, the particle size D of the positive electrode active material in the first coating 21 50 It can satisfy 6μm≤D 50≤9μm, the particle size D of the positive electrode active material in the first coating 21 50 For example, a range of 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm or any two of these ranges can help to further reduce battery impedance and improve battery capacity and other performance characteristics.

[0035] In some embodiments, the particle size D of the positive electrode active material in the first coating 21 90 Satisfying 8μm≤D 90 ≤18μm, the particle size D of the positive electrode active material in the first coating 21 90 For example, a range of 8μm, 10μm, 13μm, 15μm, 16μm, 17μm, 18μm or any two of these.

[0036] In some embodiments, the particle size D of the positive electrode active material in the first coating 21 10 Satisfying 1.2μm≤D 10 ≤3.5μm, the particle size D of the positive electrode active material in the first coating 21 10 For example, a range of 1.2μm, 1.5μm, 1.8μm, 2μm, 2.2μm, 2.5μm, 2.8μm, 3μm, 3.3μm, 3.5μm or any two of these.

[0037] Further research revealed that the particle size D of the positive electrode active material in the second coating 22... 10 Greater than or equal to 0.3 μm, specifically satisfying 0.3 μm ≤ D 10 ≤1μm, the particle size D of the positive electrode active material in the second coating 22 10 For example, a range of 0.3μm, 0.32μm, 0.35μm, 0.4μm, 0.45μm, 0.5μm, 0.55μm, 0.6μm, 0.65μm, 0.7μm, 0.75μm, 0.8μm, 0.85μm, 0.9μm, 0.95μm, 1μm, or any combination thereof, is beneficial for further reducing battery impedance and improving battery capacity and other performance characteristics.

[0038] In some embodiments, the particle size D of the positive electrode active material in the second coating 22 60 Satisfying 0.5μm≤D 60 ≤3.9μm.

[0039] Furthermore, the ratio of the thickness H1 of the first coating 21 to the thickness H2 of the second coating 22 can be 0.6 to 1.5, for example, a range of 0.6, 0.8, 1, 1.1, 1.3, 1.5 or any two of them, which is beneficial for reducing battery impedance and improving battery capacity and other performance characteristics.

[0040] In some embodiments, the thickness H1 of the first coating 21 can be 20μm to 110μm, for example, a range of 20μm, 40μm, 50μm, 70μm, 90μm, 100μm, 110μm or any two of these, which is beneficial for reducing battery impedance and improving battery capacity and other performance characteristics.

[0041] In some embodiments, the thickness H2 of the second coating 22 can be 20μm to 110μm, for example, a range of 20μm, 40μm, 50μm, 70μm, 90μm, 100μm, 110μm or any combination thereof, which is beneficial for reducing battery impedance and improving battery capacity and other performance characteristics.

[0042] In this embodiment, the compaction density of the positive electrode coating 2 can be 2.2 g / cm³. 3 ~2.8g / cm 3 For example, 2.2g / cm 3 2.3g / cm 3 2.4g / cm 3 2.5g / cm 3 2.6g / cm 3 2.65g / cm 3 2.7g / cm 3 2.8g / cm 3 The range of either or both of these is beneficial for increasing the energy density of the positive electrode while reducing its impedance, thereby achieving a balance between reducing battery impedance and increasing battery capacity.

[0043] The embodiments of this application may employ a conventional positive current collector 1 in the art, for example, the positive current collector 1 includes aluminum foil.

[0044] In this embodiment, a positive electrode coating 2 (positive electrode active material layer) can be provided on one side surface of the positive electrode current collector 1, or a positive electrode coating 2 can be provided on both sides of the positive electrode current collector 1 in the thickness direction (as shown in Figure 1). When a positive electrode coating 2 is provided on both sides of the positive electrode current collector 1, the positive electrode coating 2 on one side surface can be the positive electrode coating 2 with the first coating 21 and the second coating 22, or the positive electrode coating 2 on both sides of the positive electrode current collector 1 can be the positive electrode coating 2 with the first coating 21 and the second coating 22.

[0045] Generally, the first coating 21 also includes a conductive agent and a binder. Based on the total mass of the first coating 21, the mass fraction of the positive electrode active material (i.e., the ratio of the mass of the positive electrode active material to the total mass of the first coating 21) can be 70% to 99%, for example, 70%, 75%, 80%, 85%, 90%, 93%, 95%, 97%, 99%, or any combination thereof. The mass fraction of the conductive agent can be 0.5% to 15%, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 8%, 10%, 13%, 15%, or any combination thereof. The mass fraction of the binder can be 0.5% to 15%, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 8%, 10%, 13%, 15%, or any combination thereof.

[0046] In addition, the second coating 22 also includes a conductive agent and a binder. Based on the total mass of the second coating 22, the mass fraction of the positive electrode active material (i.e., the ratio of the mass of the positive electrode active material to the total mass of the second coating 22) can be 70% to 99%, for example, a range of 70%, 75%, 80%, 85%, 90%, 93%, 95%, 97%, 99%, or any two of these. The mass fraction of the conductive agent can be 0.5% to 15%, for example, a range of 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 8%, 10%, 13%, 15%, or any two of these. The mass fraction of the binder can be 0.5% to 15%, for example, a range of 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 8%, 10%, 13%, 15%, or any two of these.

[0047] In this embodiment, the conductive agent and binder in the positive electrode coating 2 (first coating 21, second coating 22) can both be conventional materials in the art. For example, the conductive agent may include one or more of conductive carbon black, carbon nanotubes (CNT), acetylene black, graphene, Ketjen black, and carbon fiber; the binder may include one or more of polyvinylidene fluoride (PVDF), polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, etc.

[0048] In this embodiment, the positive electrode sheet can be prepared by a conventional coating method in the art. Specifically, the components used to form the first coating 21, such as the positive electrode active material, conductive agent, and binder, can be dispersed in a first solvent, such as N-methylpyrrolidone (NMP). Following this process, a first slurry for forming the first coating 21 and a second slurry for forming the second coating 22 are prepared. Then, a double-layer coating process is used to coat the second slurry and the first slurry onto the surface of the positive electrode current collector 1. After drying, rolling, and other processes, the positive electrode sheet is obtained. The coating, drying, and rolling processes involved are conventional operations for preparing positive electrode sheets using the coating method and are not particularly limited thereto.

[0049] In specific implementation, when preparing the first slurry, multiple (e.g., three or more) positive electrode active material particles of different particle sizes can be used for compounding to ensure that the positive electrode active material in the formed first coating 21 meets the preset particle size (e.g., D). 90 D 10 D 50 Features such as particle size distribution width ε; when preparing the second slurry, multiple (e.g., three or more) positive electrode active material particles with different particle sizes can be compounded to ensure that the positive electrode active material in the formed second coating 22 meets the preset particle size (e.g., D). 60 D 10 ) and particle size inhomogeneity coefficient C u Features such as...

[0050] This application also provides a battery including the above-described positive electrode sheet, which has advantages corresponding to the above-described positive electrode sheet, and will not be described in detail here.

[0051] The battery in this application embodiment can be a lithium-ion battery, specifically a lithium iron phosphate battery (i.e., the positive electrode active material in its positive electrode sheet is lithium iron phosphate).

[0052] Generally, a battery includes an electrolyte, a battery cell, and a casing that encapsulates the battery cell. The electrolyte is injected into the battery cell inside the casing. The battery cell includes a positive electrode, a negative electrode, and a separator located between the positive and negative electrode. The battery cell can be a stacked battery cell, meaning it is composed of a positive electrode, a separator, and a negative electrode stacked together.

[0053] Specifically, the negative electrode sheet includes a negative current collector and a negative electrode coating located on at least one side surface of the negative current collector. Specifically, the negative electrode coating can be provided on one side surface of the negative current collector, or negative electrode coatings can be provided on both opposite sides of the negative current collector in the thickness direction.

[0054] Specifically, the negative electrode coating (negative electrode active material layer) may include a negative electrode active material, a conductive agent, and a binder, all of which can be conventional materials in the art. For example, the negative electrode active material may include graphite; the conductive agent may include one or more of conductive carbon black, carbon nanotubes (CNT), acetylene black, graphene, Ketjen black, and carbon fiber; and the binder may include one or more of sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate.

[0055] The embodiments of this application may employ conventional negative electrode current collectors in the art, for example, negative electrode current collectors may include copper foil.

[0056] In this embodiment, the negative electrode sheet can be prepared by conventional methods in the art, such as by coating. Specifically, the components used to form the negative electrode coating, such as the negative electrode active material, conductive agent, and binder, can be dispersed in a second solvent, such as water, to prepare a negative electrode slurry. The slurry is then coated on the surface of the negative electrode current collector and, after drying, rolling and other processes, the negative electrode sheet is obtained.

[0057] The electrolyte in this application embodiment can be a conventional electrolyte in the art. For example, the electrolyte is a non-aqueous electrolyte, which may specifically include organic solvents, additives and electrolyte salts. Organic solvents include one or more of ethylene carbonate (EC), diethyl carbonate (DEC) and propylene carbonate (PC). Additives include, for example, fluoroethylene carbonate (FEC). Electrolyte salts may include lithium salts, such as lithium hexafluorophosphate (LiPF6), but are not limited thereto.

[0058] In this embodiment, the separator is used to separate the positive electrode and the negative electrode to prevent the positive electrode and the negative electrode from short-circuiting due to contact. Conventional separators in the art can be used in this embodiment, and there are no special restrictions.

[0059] In this embodiment, the battery cell can be packaged using conventional housing materials in the art, such as flexible packaging materials like aluminum-plastic film, but is not limited to these.

[0060] The embodiments of this application can assemble components such as positive electrode, separator and negative electrode into a battery using conventional methods in the art. For example, positive electrode, separator and negative electrode can be stacked to obtain a stacked cell; then the cell is placed in a casing (outer packaging) and after conventional processes such as electrolyte injection and encapsulation, a battery is obtained.

[0061] In this embodiment of the application, after obtaining the positive electrode sheet, the cross-section of the positive electrode coating (which is basically parallel to the thickness direction of the positive electrode sheet) can be observed by scanning electron microscopy (SEM). Based on the characteristics such as particle size and morphology in different regions (for example, the first coating 21 contains secondary lithium iron phosphate particles, and the second coating 22 contains primary lithium iron phosphate particles, which have significant differences in size and morphology; for example, the sphericity of secondary lithium iron phosphate particles is significantly better than that of primary lithium iron phosphate particles), the first coating and the second coating are determined, and the thickness H1 of the first coating and the thickness H2 of the second coating are measured.

[0062] In the embodiments of this application, the particle size D of the positive electrode active material is... 60 (or D60) refers to the particle size on the particle size distribution curve of the positive electrode active material when the volume accounts for 60% of the total volume, starting from the small particle size side. In other words, the particle size D60 of the positive electrode active material refers to the particle size when the volume accumulates to 60% in the particle size distribution of the positive electrode active material, starting from the small particle size side.

[0063] In the embodiments of this application, the particle size D of the positive electrode active material is... 10 (or D10) refers to the particle size on the particle size distribution curve of the positive electrode active material, starting from the small particle size side and accounting for 10% of the total volume. In other words, the particle size D10 of the positive electrode active material refers to the particle size in the particle size distribution of the positive electrode active material, starting from the small particle size side and accumulating to 10% of the total volume.

[0064] In the embodiments of this application, the particle size D of the positive electrode active material is... 50 (or D50) is the average particle size of the positive electrode active material. It refers to the particle size on the particle size distribution curve of the positive electrode active material when the volume accounts for 50% of the total volume, starting from the small particle size side. In other words, the particle size D50 of the positive electrode active material refers to the particle size in the particle size distribution of the positive electrode active material when the volume accumulates to 50% from the small particle size side.

[0065] In the embodiments of this application, the particle size D of the positive electrode active material is... 90 (or D90) refers to the particle size on the particle size distribution curve of the positive electrode active material when the volume accounts for 90% of the total volume, starting from the small particle size side. In other words, the particle size D90 of the positive electrode active material refers to the particle size when the volume accumulates to 90% in the particle size distribution of the positive electrode active material, starting from the small particle size side.

[0066] In this embodiment of the application, after obtaining the positive electrode sheet, when testing the particle size of the positive electrode active material in the above-mentioned coating (first coating or second coating), the coating can be scraped off from the positive electrode sheet, and then washed with NMP to obtain solid particulate products. Then, conventional particle size testing instruments in the art, such as laser particle size analyzers, are used to measure the particle size characteristics of the positive electrode active material, such as D10, D50, D60, D90.

[0067] In this embodiment, the test process for the compaction density of the electrode coating (such as the positive electrode coating) may include: taking a positive electrode sample (specifically, a cutter can be used to cut the positive electrode sheet to obtain a positive electrode sample of suitable size), testing the total mass m1 of the positive electrode sample, and the total thickness T1 of the positive electrode sample (T1 = total thickness of the positive electrode coating + thickness of the positive electrode current collector; when both the positive and negative surfaces of the positive electrode current collector are respectively provided with a positive electrode coating, the total thickness of the positive electrode coating = thickness of the positive electrode coating on one side of the positive electrode current collector + ... The thickness of the positive electrode coating on the other side of the positive electrode current collector and the surface area S on one side of the positive electrode sample in the thickness direction are measured. Then, the positive electrode coating on the positive electrode sample is scraped off, and the mass m2 and thickness T2 of the positive electrode current collector are measured. Then, the total thickness of the positive electrode coating = T1-T2, the surface density of the positive electrode coating = (m1-m2) / S, and the compaction density of the positive electrode coating = surface density of the positive electrode coating / total thickness of the positive electrode coating = (m1-m2) / (S×(T1-T2)).

[0068] In this embodiment, the battery can be disassembled to obtain the positive electrode sheet, which is then cleaned with an organic solvent (such as diethyl carbonate (DEC)) to remove impurities such as electrolyte salts. The positive electrode sheet is then dried to remove the organic solvent. Specifically, after disassembling the battery and removing the positive electrode sheet, it can be immersed in DEC for 10 seconds, then dried. This immersion-drying process is repeated three times. Then, the positive electrode sheet undergoes the aforementioned SEM, particle size, and compaction density tests to measure characteristics such as the thickness H1 of the first coating, the thickness H2 of the second coating, the particle size of the positive active material in the first coating 21, the particle size of the positive active material in the second coating 22, and the compaction density of the positive coating.

[0069] The present application will be further described below through specific embodiments.

[0070] Example 1

[0071] 1. Preparation of positive electrode sheet

[0072] Lithium iron phosphate secondary particles, conductive carbon black, and PVDF are mixed in a mass ratio of 100:2:3, and NMP is added to prepare a first slurry for forming the first coating (upper layer).

[0073] Lithium iron phosphate primary particles, conductive carbon black, and PVDF are mixed in a mass ratio of 100:2:3, and NMP is added to prepare a second slurry for forming the second coating (lower layer).

[0074] A double-layer coating process is used to coat the first and second slurries onto the front and back surfaces of the aluminum foil. After drying and rolling (rolling pressure is about 1.5 MPa), the positive electrode sheet is obtained.

[0075] The structural diagram of the positive electrode sheet is shown in Figure 1. It includes a positive current collector (aluminum foil), positive electrode coatings on both the front and back surfaces of the positive current collector, and each side of the positive electrode coating includes a first coating and a second coating located between the first coating and the positive current collector.

[0076] 2. Preparation of negative electrode sheet

[0077] Graphite, conductive carbon black, SBR, and CMC are mixed in a mass ratio of 100:1:2:2, and deionized water is added. The mixture is stirred evenly to prepare a negative electrode slurry.

[0078] The negative electrode slurry is coated onto both sides of a copper foil. After drying and rolling, a negative electrode coating is formed on both sides of the copper foil, thus producing a negative electrode sheet (the compacted density of the negative electrode coating is 1.60 g / cm³). 3 ).

[0079] (2) Assembly of pouch lithium-ion batteries

[0080] Seven positive electrode sheets, eight separator sheets, and sixteen negative electrode sheets are alternately stacked to assemble a stacked cell. The stacked cell is placed in an aluminum-plastic film and assembled into a soft-pack lithium-ion battery through processes such as electrolyte injection and encapsulation. The excess ratio of the negative electrode sheet is 15% ((excess ratio of negative electrode sheet = theoretical capacity of negative electrode sheet - theoretical capacity of positive electrode sheet) / theoretical capacity of positive electrode sheet). The preparation process of the electrolyte is as follows: ethylene carbonate (EC), diethyl carbonate (DEC), and propylene carbonate (PC) are mixed in a mass ratio of 2:5:3. Then, 5% of fluoroethylene carbonate (FEC) and 13% of lithium hexafluorophosphate (LiPF6) are added to the mixture, and the mixture is stirred evenly to obtain the electrolyte.

[0081] Examples 1 to 20, and Comparative Examples 1 to 4: The difference from Example 1 is that the lithium iron phosphate particle size and particle size distribution width ε in the first coating, the lithium iron phosphate particle size and particle size non-uniformity coefficient Cu in the second coating, and the ratio of the thickness H1 of the first coating to the thickness H2 of the second coating are different, as shown in Table 1. Except for the differences shown in Table 1, the other conditions are the same as in Example 1.

[0082] The positive electrode or battery of each embodiment and comparative example were subjected to the following performance tests, and the results are shown in Table 2.

[0083] (1) Positive electrode tortuosity test:

[0084] The test was conducted using a FIB (Helios 5UC) instrument. The test procedure was as follows: The positive electrode was placed on the FIB (Focused Ion Beam Analysis) sample plate, a Pt protective layer was deposited, and the cross-section was marked for automatic cutting and image acquisition. Data acquisition was performed by sequentially slicing the material (slice size 30×30×80μm). 3 Left and right cubes), sliced ​​(slice thickness 0.1μm), reconstruct a specific micro-region (i.e. 30×30×80μm). 3 The three-dimensional structure of the cube (left and right) is used to obtain the tortuosity (the tortuosity is the ratio of the actual transport path of lithium ions in the electrode coating to the thickness of the electrode coating).

[0085] (2) Liquid phase diffusion impedance test:

[0086] The two positive electrode plates and the separator are assembled sequentially to form the electrode core (the separator is located between the two positive electrode plates); the electrode core is placed in an aluminum-plastic film, baked, and injected with electrolyte. After encapsulation and impregnation processes, a liquid-phase diffusion impedance battery is obtained; the liquid-phase diffusion impedance battery is tested using an electrochemical workstation in the frequency range of 300,000 Hz-0.05 Hz. The preparation process of the electrolyte is as follows: ethylene carbonate (EC), diethyl carbonate (DEC), and propylene carbonate (PC) are mixed in a mass ratio of 2:5:3, and then 5% fluoroethylene carbonate (FEC) and 13% lithium hexafluorophosphate (LiPF6) are added to the mixture. The mixture is stirred evenly to obtain the electrolyte.

[0087] (3) Energy density test:

[0088] At room temperature (25±5℃), the soft-pack lithium-ion battery is discharged to 2.0V at a constant current of 1 / 3C and charged to 3.8V at a constant current and constant voltage of 1 / 3C, with a cutoff current of 0.05C, for 3 cycles; the discharge energy of the third cycle is the battery energy, and the energy density = battery energy / battery volume.

[0089] (4) 50% SOC DC internal resistance test:

[0090] At room temperature (25±5℃), the soft-pack lithium-ion battery was discharged to 2.0V at a constant current of 1 / 3C, charged to 50% SOC at a constant current of 1 / 3C, and left to stand for 30 minutes; then discharged at a constant current of 1.5C for 30 seconds, and the DC internal resistance at 50% SOC was measured.

[0091] Table 1

[0092] Table 2

[0093] As can be seen from Table 2, Examples 1 to 20 can reduce the tortuosity of the positive electrode and the liquid phase diffusion resistance, thus reducing the internal resistance of the battery and increasing the energy density of the battery.

[0094] Specifically, relative to Comparative Example 1 (C u <3.7) and Comparative Example 2 (C u >4.2), the second coating in Examples 1 to 3 satisfies 3.7≤D 60 / D 10 With a value ≤4.2, it can significantly reduce the tortuosity of the positive electrode and the liquid phase diffusion resistance, reduce the internal resistance of the battery, and at the same time improve the energy density of the battery.

[0095] Specifically, compared to Comparative Example 3 (ε < 1.6) and Comparative Example 4 (ε > 1.95), the first coating in Examples 1 and 4 to 6 satisfies 1.6 ≤ ε ≤ 1.95, which can significantly reduce the tortuosity of the positive electrode and the liquid phase diffusion resistance, reduce the internal resistance of the battery, and improve the energy density of the battery.

[0096] Furthermore, compared to Examples 10 and 11, in Examples 1 and 7 to 9, the lithium iron phosphate particle size D50 in the first coating is ≤6μm. 50 Within the range of ≤9μm, it is beneficial to further balance reducing the battery's internal resistance and increasing its energy density.

[0097] Furthermore, compared to Examples 16 and 20, in Examples 1, 17 to 19, the ratio of the thickness H1 of the first coating to the thickness of the second coating (H1 / H2) is in the range of 0.6 to 1.5, which is beneficial to further reduce the internal resistance of the battery and increase the energy density of the battery.

[0098] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A positive electrode plate, wherein, It includes a positive current collector and a positive electrode coating located on one side surface of the positive current collector, the positive electrode coating containing a positive electrode active material; The positive electrode coating includes a first coating and a second coating located between the first coating and the positive electrode current collector; the positive electrode active material in the first coating includes secondary particles, and the positive electrode active material in the second coating includes primary particles; The particle size D of the positive electrode active material in the first coating 90 D 50 and D 10 Satisfying 1.6≤(D) 90 -D 10 ) / D 50 ≤1.95; The particle size D of the positive electrode active material in the second coating 60 and D 10 Satisfying 3.7≤D 60 / D 10 ≤4.

2.

2. The positive electrode according to claim 1, wherein, The particle size D of the positive electrode active material in the first coating 50 Satisfying 6μm≤D 50 ≤9μm.

3. The positive electrode according to claim 1 or 2, wherein, The particle size D of the positive electrode active material in the first coating 90 Satisfying 8μm≤D 90 ≤18μm.

4. The positive electrode according to any one of claims 1-3, wherein, The particle size D of the positive electrode active material in the first coating 10 Satisfying 1.2μm≤D 10 ≤3.5μm.

5. The positive electrode according to any one of claims 1-4, wherein, The particle size D of the positive electrode active material in the second coating 10 Greater than or equal to 0.3 μm.

6. The positive electrode according to any one of claims 1-5, wherein, The particle size D of the positive electrode active material in the second coating 10 Satisfying 0.3μm≤D 10 ≤1μm.

7. The positive electrode according to any one of claims 1-6, wherein, The particle size D of the positive electrode active material in the second coating 60 Satisfying 0.5μm≤D 60 ≤3.9μm.

8. The positive electrode according to any one of claims 1-7, wherein, The ratio of the thickness of the first coating to the thickness of the second coating is 0.6 to 1.

5.

9. The positive electrode according to any one of claims 1-8, wherein, The thickness of the first coating is 20 μm to 110 μm.

10. The positive electrode according to any one of claims 1-9, wherein, The thickness of the second coating is 20μm to 110μm.

11. The positive electrode according to any one of claims 1-10, wherein, The positive electrode active material includes lithium iron phosphate.

12. The positive electrode according to any one of claims 1-11, wherein, The compaction density of the positive electrode coating is 2.2 g / cm³. 3 ~2.8g / cm 3 .

13. A battery, wherein, Includes the positive electrode sheet as described in any one of claims 1-12.

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