A positive electrode sheet and a lithium-ion secondary battery

Abstract

This invention relates to the field of battery technology, specifically to a positive electrode sheet and a lithium-ion secondary battery including the positive electrode sheet. The positive electrode sheet includes a positive current collector and a positive electrode coating located on at least one side of the surface of the current collector. The positive electrode coating includes a first positive electrode coating and a second positive electrode coating along the thickness direction of the positive electrode sheet. The first positive electrode coating is disposed on at least one side of the current collector, and the second positive electrode coating is disposed on the surface of the first positive electrode coating facing away from the current collector. The first positive electrode coating includes first lithium iron phosphate with an average particle size of 500 nm-1600 nm, and the second positive electrode coating includes second lithium iron phosphate with an average particle size of 1.6 μm-15 μm. The second lithium iron phosphate comprises secondary particles, and the first lithium iron phosphate comprises single particles. The positive electrode sheet of this invention has high energy density and high Li... + The lithium-ion secondary battery, including the positive electrode of this invention, has a fast transmission speed and good energy density and cycle stability.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a positive electrode and a lithium-ion secondary battery. Background Technology

[0002] With technological advancements, lithium-ion rechargeable batteries have become widely used in various electronic devices and electric vehicles due to their advantages such as high energy density, long cycle life, and environmental friendliness. Double-layer coating technology is a commonly used technique in lithium-ion battery manufacturing; however, batteries with double-layer coated electrodes often suffer from low energy density and poor cycle performance. Summary of the Invention

[0003] The purpose of this invention is to overcome the aforementioned problems in the prior art and to propose a lithium-ion secondary battery (hereinafter referred to as the battery). The positive electrode of the lithium-ion secondary battery proposed in this invention has a first positive electrode coating and a second positive electrode coating stacked along the thickness direction; by controlling the difference between the materials of the first positive electrode coating and the second positive electrode coating and their average particle size, concentration polarization in lithium-ion diffusion can be improved, thereby ensuring that the battery has good energy density, cycle life and stability.

[0004] The inventors of this invention have discovered that when the positive electrode coating comprises a first positive electrode coating and a second positive electrode coating stacked along the thickness direction of the positive electrode sheet, with the first positive electrode coating disposed on at least one side surface of the positive electrode current collector and the second positive electrode coating disposed on the surface of the first positive electrode coating facing away from the positive electrode current collector, and the first and second positive electrode coatings respectively comprising first lithium iron phosphate and second lithium iron phosphate with different average particle sizes, when the first positive electrode coating is close to the positive electrode current collector and the second positive electrode coating is away from the positive electrode current collector, the migration path of lithium ions in the electrode is short and the transport speed is fast, which is beneficial to improving energy density, cycle life, and performance stability.

[0005] Based on this, the present invention proposes the following technical solution:

[0006] The first aspect of the present invention provides a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive electrode coating located on at least one side surface of the positive current collector, the positive electrode coating comprising a first positive electrode coating and a second positive electrode coating stacked along the thickness direction of the positive electrode sheet, the first positive electrode coating being disposed on at least one side surface of the positive current collector, and the second positive electrode coating being disposed on the surface of the first positive electrode coating facing away from the positive current collector; the first positive electrode coating comprising a first lithium iron phosphate, the second positive electrode coating comprising a second lithium iron phosphate, the first lithium iron phosphate having an average particle size of 500 nm-1600 nm, and the second lithium iron phosphate having an average particle size of 1.6 μm-15 μm; the second lithium iron phosphate comprising secondary particles, and the first lithium iron phosphate comprising single particles.

[0007] A second aspect of the present invention provides a lithium-ion secondary battery, the lithium-ion secondary battery comprising the positive electrode sheet described in the first aspect of the present invention.

[0008] The present invention has at least the following beneficial effects:

[0009] First, the positive electrode sheet of the present invention adopts a double-layer coating technology. By controlling the lithium iron phosphate grain structure and the average particle size and distribution of lithium iron phosphate in the first and second positive electrode coatings, the concentration polarization during the lithium ion diffusion process can be improved, the energy density can be increased, and the battery can be ensured to have good energy density, cycle stability and service life.

[0010] Secondly, the lithium-ion secondary battery of the present invention has good energy density, cycle stability and lifespan. Attached Figure Description

[0011] Figure 1 This is a cross-sectional scanning electron microscope (SEM) image of the positive electrode sheet in one embodiment of the present invention.

[0012] Figure 2 This is a cross-sectional SEM image of a pair of positive electrode plates in the present invention.

[0013] Figure 3 This is a cross-sectional SEM image of a pair of positive electrode plates in the present invention.

[0014] Figure 4 This is a cross-sectional SEM image of a pair of positive electrode plates in the present invention. Detailed Implementation

[0015] 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.

[0016] The first aspect of the present invention provides a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive electrode coating located on at least one side surface of the positive current collector, the positive electrode coating including a first positive electrode coating and a second positive electrode coating stacked along the thickness direction of the positive electrode sheet, the first positive electrode coating being disposed on at least one side surface of the positive current collector, and the second positive electrode coating being disposed on the surface of the first positive electrode coating opposite to the positive current collector.

[0017] In this invention, the first positive electrode coating comprises a first lithium iron phosphate, and the second positive electrode coating comprises a second lithium iron phosphate. The average particle size of the first lithium iron phosphate is 500nm-1600nm, for example, 500nm, 510nm, 520nm, 530nm, 540nm, 550nm, 560nm, 570nm, 580nm, 590nm, 600nm, 620nm, 640nm, 660nm, 680nm, 700nm, 740nm, 780nm, 820nm, 860nm. The average particle size of the second lithium iron phosphate is 1.6 μm-15 μm, for example, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm or 15 μm.

[0018] In one embodiment, the average particle size of the first lithium iron phosphate is 700nm-1500nm, and the average particle size of the second lithium iron phosphate is 2μm-10μm.

[0019] like Figure 1 The image shown is a cross-sectional SEM image of the positive electrode sheet in one embodiment of the present invention. As can be seen from the image, there are double-layered positive electrode coatings on both sides of the positive electrode current collector. On the positive electrode current collector side, the positive electrode coating is provided with a first positive electrode coating (including first lithium iron phosphate, with small average particle size) and a second positive electrode coating (including second lithium iron phosphate, with large average particle size) along the thickness direction.

[0020] In this invention, a second positive electrode coating comprising a second lithium iron phosphate and a first positive electrode coating comprising a first lithium iron phosphate are stacked along the thickness direction of the positive electrode current collector. The difference in average particle size between the second and first lithium iron phosphates is controlled. The first lithium iron phosphate, being closer to the positive electrode current collector, has a smaller average particle size and higher compaction density, which helps to improve energy density. Conversely, the second lithium iron phosphate, being further away from the positive electrode current collector, has a larger average particle size, reducing lithium-ion concentration polarization caused by internal diffusion, improving lithium-ion transport efficiency, and facilitating cold start of the lithium-ion battery. Using different lithium iron phosphates in the first and second positive electrode coatings allows for better control of the material differences between them, thereby ensuring battery cycle stability and lifespan. This helps balance the high and low temperature performance of the lithium-ion secondary battery and enhances high-temperature storage capacity. The differentiated arrangement of the first and second lithium iron phosphates effectively avoids the problem of poor battery stability caused by differences in average particle size, thereby reducing concentration polarization during lithium-ion diffusion and improving battery energy density, cycle stability, and lifespan.

[0021] If a single-layer coating is used instead of a double-layer coating, and the first lithium iron phosphate and the second lithium iron phosphate are mixed and coated uniformly, the first lithium iron phosphate will tightly coat the surface of the second lithium iron phosphate, hindering the contact between the positive electrode conductive agent and the second lithium iron phosphate, increasing the contact resistance. This not only reduces the transport speed of lithium ions in the positive electrode, but also reduces the energy density and cold start performance, which is detrimental to the cycle performance of the battery.

[0022] In this invention, the ratio of the vertical distance from the second lithium iron phosphate to the positive electrode current collector to the thickness of the positive electrode coating is 1:(1.2-12), for example, 1:1.2, 1:1.25, 1:3, 1:1.35, 1:4, 1:1.45, 1:5, 1:1.55, 1:6, 1:1.65, 1:7, 1:1.75, 1:8, 1:1.85, 1:9, 1:10, 1:11 or 1:12.

[0023] In one embodiment, the ratio of the vertical distance from the second lithium iron phosphate to the positive electrode current collector to the thickness of the positive electrode coating is 1:(1.5-8).

[0024] In this invention, the dynamic performance of the battery is improved by adjusting the ratio of the vertical distance between the second lithium iron phosphate and the positive electrode current collector in the second positive electrode coating to the thickness of the positive electrode coating, thereby further improving the energy density and cycle performance of the battery. When the ratio of the vertical distance between the second lithium iron phosphate and the positive electrode current collector to the thickness of the positive electrode coating is >1:1.2, the compaction density during the coating or rolling process is too small, resulting in a low energy density. When the ratio of the vertical distance between the second lithium iron phosphate and the positive electrode current collector to the thickness of the positive electrode coating is <1:12, the second lithium iron phosphate is too close to the positive electrode current collector under compression, resulting in increased contact resistance and deterioration of cold start performance and cycle performance.

[0025] In this invention, the vertical distance from the second lithium iron phosphate to the positive electrode current collector is measured by the following method: the positive electrode sheet is laser-cut, the obtained cross-section is imaged using a scanning electron microscope (SEM), at least 10 different second lithium iron phosphates are selected on the obtained SEM image, and the average value of the shortest distance from the boundary of the second lithium iron phosphate to the positive electrode current collector is taken by image analysis software, which is the vertical distance from the second lithium iron phosphate to the positive electrode current collector.

[0026] In this invention, the thickness of the positive electrode coating can be obtained by conventional testing methods in the art, such as cutting the negative electrode sheet perpendicularly along the thickness direction to obtain a cross section, imaging the obtained cross section by scanning electron microscopy (SEM), and measuring the coating thickness by image analysis software; the average particle size of the first lithium iron phosphate and the second lithium iron phosphate can be obtained by conventional testing methods in the art, such as first obtaining a high-resolution image by scanning electron microscopy, and then measuring and statistically analyzing the particle size and distribution by electron microscopy image analysis software such as ImageJ to obtain the average particle size, or by laser diffraction testing, with the obtained Dv50 data being the average particle size.

[0027] In this invention, the second lithium iron phosphate comprises secondary particles, and the first lithium iron phosphate comprises single particles.

[0028] In this invention, the secondary particles comprise a plurality of primary particles.

[0029] In this invention, the "secondary particles" are formed by the agglomeration of several "primary particles" into lithium iron phosphate particles with a relatively large average particle size. The "primary particles" are lithium iron phosphate particles with a small average particle size that are easily agglomerated. "Several" refers to the number of primary particles forming the secondary particles being greater than or equal to two. The "single particle" is a lithium iron phosphate particle with a relatively large average particle size that is not prone to agglomeration.

[0030] In this invention, the ratio of the average particle size of the primary particles to the average particle size of the first lithium iron phosphate is 1:(1.25-40), for example, 1:1.25, 1:1.3, 1:1.35, 1:1.4, 1:1.45, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:11, 1:12, 1:13, 1:14, 1:16, 1:18, 1:20, 1:24, 1:28, 1:32, 1:36 or 1:40.

[0031] In one embodiment, the ratio of the average particle size of the primary particles to the average particle size of the first lithium iron phosphate is 1:(2.33-10).

[0032] In this invention, the positive electrode adopts a double-layer coating. The second lithium iron phosphate in the second positive electrode coating includes secondary particles formed by a number of primary particles. The primary particles are lithium iron phosphate particles with smaller average particle size and easy agglomeration. By adjusting the ratio of the average particle size of the primary particles to that of the first lithium iron phosphate in the first positive electrode coating, the average particle size of the primary particles in the second positive electrode coating can be within a suitable range and matched with that of the first lithium iron phosphate in the first positive electrode coating. This not only improves the compaction density of the double-layer coating and avoids excessive agglomeration of the primary particles, but also shortens the diffusion and migration path of lithium ions in each coating, thereby improving the transport rate and the energy density of the battery.

[0033] When the ratio of the average particle size of the primary particles to the average particle size of the first lithium iron phosphate is greater than 1:1.25, the average particle size of the first lithium iron phosphate is too small and they are densely packed. Due to insufficient conductive network, the contact internal resistance is increased, which reduces the cycle capability. When the ratio of the average particle size of the primary particles to the average particle size of the first lithium iron phosphate is less than 1:40, the average particle size of the primary particles that agglomerate into the second lithium iron phosphate is too small, resulting in excessive agglomeration and reduced compaction efficiency. Meanwhile, the average particle size of the first lithium iron phosphate is too large, which increases the diffusion and migration path of lithium ions inside the lithium iron phosphate, reduces the battery capacity, and affects the battery's energy density. When lithium ions diffuse on the surface and inside the first lithium iron phosphate with a larger average particle size, higher concentration polarization is generated, which reduces the battery voltage and is detrimental to cold start performance.

[0034] In this invention, the average particle size of the primary particles is 50nm-400nm, for example, 50nm, 52nm, 54nm, 56nm, 60nm, 64nm, 68nm, 72nm, 76nm, 80nm, 85nm, 90nm, 95nm, 100nm, 120nm, 140nm, 160nm, 180nm, 200nm, 240nm, 280nm, 320nm, 360nm or 400nm.

[0035] In one embodiment, the average particle size of the primary particles is 150nm-300nm.

[0036] In this invention, the positional relationship between the first positive electrode coating and the second positive electrode coating relative to the positive electrode current collector is also related to the average particle size of the first lithium iron phosphate and the second lithium iron phosphate. When the second positive electrode coating is positioned away from the positive electrode current collector and the first coating is positioned close to the positive electrode current collector, the average particle size of the first lithium iron phosphate is larger than that of the primary particles, resulting in a longer diffusion path for lithium ions inside the first lithium iron phosphate. Although the average particle size of the primary particles is smaller, their position close to the positive electrode current collector results in a longer migration path for the extracted lithium ions in the electrode, increasing diffusion resistance, creating significant concentration polarization, further increasing the battery internal resistance, reducing the terminal voltage of low-temperature high-rate discharge, and adversely affecting energy density and cold-start performance.

[0037] In this invention, the average particle size of the primary particles can be obtained by conventional testing methods in the art, such as the same testing methods used for the average particle size of the first lithium iron phosphate and the second lithium iron phosphate, or by other conventional testing methods.

[0038] In this invention, the first positive electrode coating further includes a first positive electrode conductive agent, and the second positive electrode coating further includes a second positive electrode conductive agent; the mass content of the second positive electrode conductive agent in the second positive electrode coating is less than or equal to the mass content of the first positive electrode conductive agent in the first positive electrode coating.

[0039] In this invention, the mass content of the second positive electrode conductive agent in the second positive electrode coating and the mass content of the first positive electrode conductive agent in the first positive electrode coating are 1:(1-6), for example, 1:1, 1:1.2, 1:1.4, 1:1.6, 1:1.8, 1:2, 1:2.4, 1:2.8, 1:3.2, 1:3.6, 1:4, 1:4.5, 1:5, 1:5.5, or 1:6.

[0040] In one embodiment, the mass content of the second positive electrode conductive agent in the second positive electrode coating and the mass content of the first positive electrode conductive agent in the first positive electrode coating are 1:(1.05-2.75).

[0041] In this invention, the mass content of the first positive electrode conductive agent in the first positive electrode coating is 3.9%-6%, for example, 3.9%, 4%, 4.1%, 4.2%, 4.3%, 4.4%, 4.5%, 4.6%, 4.7%, 4.8%, 4.9%, 5%, 5.1%, 5.2%, 5.3%, 5.4%, 5.5%, 5.6%, 5.7%, 5.8%, 5.9%, or 6%.

[0042] In one embodiment, the mass content of the first positive electrode conductive agent in the first positive electrode coating is 4%-5.5%.

[0043] In this invention, the mass content of the second positive electrode conductive agent in the second positive electrode coating is 1%-3.9%, for example, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, 3.2%, 3.4%, 3.6%, 3.8%, or 3.9%.

[0044] In one embodiment, the mass content of the second positive electrode conductive agent in the second positive electrode coating is 2%-3.8%.

[0045] In this invention, by adjusting the mass content and ratio of the first and second positive electrode conductive agents according to the composition of the first and second positive electrode coatings and the average particle size of lithium iron phosphate, more conductive pathways can be formed between the first and second positive electrode coatings, reducing electron transport resistance, constructing a good conductive network, improving the cold start performance and cycle stability of the battery, ensuring the conductivity matching of the first and second positive electrode coatings, and avoiding large differences that would affect the conduction of ions in the positive electrode coating. When the mass ratio of the second positive electrode conductive agent in the second positive electrode coating to the mass ratio of the first positive electrode conductive agent in the first positive electrode coating is less than 1:6, then there is too little second positive electrode conductive agent or too much first positive electrode conductive agent. In this case, there are too few conductive channels formed in the second positive electrode coating. Due to the large particle size of the second lithium iron phosphate, the diffusion path of lithium ions is long. Too few conductive channels will hinder the transport of lithium ions and cause them to accumulate excessively inside the electrode, resulting in side reactions, reduced battery capacity, and affecting battery energy density and lifespan. On the other hand, when the mass ratio of the second positive electrode conductive agent in the second positive electrode coating to the mass ratio of the first positive electrode conductive agent in the first positive electrode coating is greater than 1:1, then there is too much second positive electrode conductive agent or too little first positive electrode conductive agent. The contact resistance between the positive electrode coating and the positive electrode current collector increases, and the electron transport resistance of the positive electrode sheet is too high, affecting the cold start and cycle performance of the battery.

[0046] In this invention, the thickness ratio of the first positive electrode coating to the second positive electrode coating is 1:(0.11-9.5), for example, 1:0.11, 1:0.2, 1:0.4, 1:0.6, 1:0.8, 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, 1:5.5, 1:6, 1:7, 1:8, 1:9 or 1:9.5.

[0047] In one embodiment, the thickness ratio of the first positive electrode coating to the second positive electrode coating is 1:(0.23-7).

[0048] In this invention, the thickness of the first positive electrode coating is 5μm-90μm, for example, 5μm, 6.5μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, 60μm, 70μm, 80μm or 90μm; the thickness of the second positive electrode coating is 5μm-90μm, for example, 5μm, 6.5μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, 60μm, 70μm, 80μm or 90μm.

[0049] In this invention, the thicknesses of the first positive electrode coating and the second positive electrode coating can be measured using conventional testing methods in the art. For example, the positive electrode sheet can be cut along the thickness direction, and the resulting cross-section can be imaged using a scanning electron microscope (SEM). On the obtained SEM image, the second lithium iron phosphate closest to the current collector can be selected, and the vertical distance from the lowest point of the second lithium iron phosphate (i.e., the position closest to the positive current collector) to the surface of the second positive electrode coating on that side can be measured using image analysis software to obtain the thickness of the second positive electrode coating. On the obtained SEM image, the first lithium iron phosphate farthest from the current collector can be selected, and the vertical distance from the highest point of the first lithium iron phosphate (i.e., the position farthest from the positive current collector) to the surface of the positive current collector on that side can be measured using image analysis software to obtain the thickness of the first positive electrode coating.

[0050] In this invention, the ratio of the average particle size of the second lithium iron phosphate to the thickness of the positive electrode coating on any side of the positive electrode current collector is 1:(1.6-85), for example, 1:1.6, 1:1.7, 1:1.8, 1:2, 1:2.25, 1:2.5, 1:2.75, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:12, 1:14, 1:16, 1:18, 1:20, 1:25, 1:30, 1:35, 1:40, 1:45, 1:50, 1:60, 1:70, 1:80, or 1:85.

[0051] In one embodiment, the ratio of the average particle size of the second lithium iron phosphate to the thickness of the positive electrode coating on either side of the positive electrode current collector is 1:(5-37).

[0052] The thickness of the positive electrode coating on any side of the positive electrode current collector is 25μm-130μm, for example, 25μm, 26μm, 27μm, 28μm, 29μm, 30μm, 32μm, 34μm, 36μm, 38μm, 40μm, 45μm, 50μm, 55μm, 60μm, 65μm, 70μm, 75μm, 80μm, 85μm, 90μm, 95μm, 100μm, 110μm, 120μm or 130μm.

[0053] In one embodiment, the thickness of the positive electrode coating on either side of the positive electrode current collector is 55μm-80μm.

[0054] In this invention, since the positive electrode sheet is coated with a double layer and the average particle size of lithium iron phosphate particles in the first and second positive electrode coatings is significantly different, if the thickness of the positive electrode coating and the average particle size are difficult to match, concentration polarization and coating scratches will occur. Therefore, the inventors thought that by controlling the relationship between the average particle size of the second lithium iron phosphate and the thickness of the positive electrode coating on either side of the positive electrode current collector, the coating scratch problem and the concentration polarization problem caused by lithium ion insertion and extraction can be improved, thereby improving the charging and discharging efficiency of the battery. When the ratio of the average particle size of the second lithium iron phosphate to the thickness of the positive electrode coating on either side of the positive electrode current collector is less than 1:85, the average particle size of the second lithium iron phosphate is too small and the thickness of the positive electrode coating on either side of the positive electrode current collector is too large. This leads to an extended lithium ion insertion and extraction path, resulting in a larger lithium ion concentration gradient and increased concentration polarization, which affects the cold start performance of the battery. Conversely, when the ratio of the average particle size of the second lithium iron phosphate to the thickness of the positive electrode coating on either side of the positive electrode current collector is greater than 1:1.6, the particle size of the second lithium iron phosphate is too large and the thickness of the positive electrode coating on either side of the positive electrode current collector is too thin, forming coating scratches and resulting in low coating amount. This further affects the battery capacity, reduces energy density, and worsens cycle performance.

[0055] In this invention, the porosity of the positive electrode sheet is 25%-60%, for example, 25%, 26%, 27%, 28%, 29%, 30%, 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54%, 56%, 58%, or 60%.

[0056] In one embodiment, the porosity of the positive electrode is 30%-50%.

[0057] In this invention, due to the double-layer coating structure, the porosity of the positive electrode sheet is affected by the first and second positive electrode coatings. Controlling the porosity of the positive electrode sheet within a certain range can promote the wetting ability of the electrolyte on the positive electrode active material, improve lithium-ion transport efficiency, and thus enhance battery performance. The porosity of the positive electrode sheet can be controlled by adjusting the thickness ratio of the first and second positive electrode coatings or by adjusting the distance between the second lithium iron phosphate layer and the positive electrode current collector. When the porosity of the positive electrode sheet is less than 25%, the electrolyte cannot fill the electrode pores, resulting in insufficient electrolyte wetting, slow lithium-ion transport speed, and weakened energy density and cold-start capability. Conversely, when the porosity of the positive electrode sheet is greater than 60%, the lithium-ion conduction speed through the electrolyte within the pores is too fast, forming lithium dendrites and increasing battery self-discharge.

[0058] In this invention, the porosity of the positive electrode sheet can be obtained by conventional testing methods in the art, such as disassembling the battery in a dry environment and separating the positive electrode sheet, soaking the positive electrode sheet in dimethyl carbonate at least 3 times, with each soaking time not less than 30 minutes, and then drying it in a forced-air oven; measuring the length, width and thickness of the positive electrode sheet; weighing the mass of the positive electrode sheet with an analytical balance, winding it and placing it in a sample cup for true density testing, and calculating the porosity.

[0059] In this invention, the sphericity of the second lithium iron phosphate is 0.2-1, for example, 0.22, 0.24, 0.26, 0.28, 0.3, 0.34, 0.38, 0.42, 0.46, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.9 or 1.

[0060] In one embodiment, the sphericity of the second lithium iron phosphate is 0.6-1.

[0061] In this invention, the second lithium iron phosphate is a spherical particle, and the sphericity is the ratio of the minor axis diameter to the major axis diameter of the sphere. When the sphericity of the second lithium iron phosphate is less than 0.2, the contact between the second lithium iron phosphate particles is too large, which leads to an increase in the contact internal resistance of the second lithium iron phosphate particles and affects the conductive path of the second positive electrode coating in which they are located, thus weakening the cycle performance and cold start performance of the battery.

[0062] In this invention, the sphericity of the second lithium iron phosphate can be tested using conventional methods in the art. For example, after discharging the battery to 0% SOC, the positive electrode is disassembled and removed. It is then soaked in dimethyl carbonate (DMC) solvent for 12 hours, rinsed with DMC solvent to remove lithium salts adhering to the positive electrode, and the second positive electrode coating is washed off the positive electrode current collector with deionized water. After ultrasonication and centrifugation to remove the filtrate, the powder is dried and observed using SEM. Image analysis of the second lithium iron phosphate particles in the SEM image at a certain magnification (e.g., 2000x) is performed using image processing software (e.g., Image Pro Plus). At least 10 second lithium iron phosphate particles are selected from the image to obtain the major axis diameter and minor axis diameter of each particle. The sphericity of each particle is calculated, and the average value is taken.

[0063] In this invention, the first lithium iron phosphate and the second lithium iron phosphate contain element Ti. The mass content of element Ti in the first lithium iron phosphate is C1, and the mass content of element Ti in the second lithium iron phosphate is C2. The ratio of C2 to C1 is 1:(1.5-50), for example, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, 1:7, 1:10, 1:14, 1:18, 1:22, 1:24, 1:28, 1:32, 1:36, 1:40, 1:45, or 1:50.

[0064] In one embodiment, the ratio of C2 to C1 is 1:(2.78-16).

[0065] In this invention, C1 is 3000ppm-10000ppm, for example, 3000ppm, 3200ppm, 3400ppm, 3600ppm, 3800ppm, 4000ppm, 4400ppm, 4800ppm, 5200ppm, 5600ppm, 6000ppm, 6500ppm, 7000ppm, 7500ppm, 8000ppm, 8500ppm, 9000ppm, 9500ppm, or 10000ppm; C2 is... 200ppm-2000ppm, for example, 200ppm, 210ppm, 220ppm, 230ppm, 240ppm, 260ppm, 280ppm, 300ppm, 340ppm, 380ppm, 420ppm, 460ppm, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1200ppm, 1400ppm, 1600ppm, 1800ppm or 2000ppm.

[0066] In one embodiment, C1 is 5000ppm-8000ppm and C2 is 500ppm-1800ppm.

[0067] In this invention, to enhance the stability of the double-coated cathode layer, element Ti is introduced into the first lithium iron phosphate and the second lithium iron phosphate. During battery charging, after lithium ions are released, element Ti can promptly occupy lithium vacancies, thus maintaining the structural stability of the first and second lithium iron phosphate layers and enhancing the stability of the first and second cathode coatings. Since the average particle size of the first lithium iron phosphate is larger and the average particle size of the second lithium iron phosphate is smaller, the mass content of element Ti in the first and second lithium iron phosphate layers satisfies C1 > C2. This allows for maintaining a smaller C2 to avoid the risk of self-discharge while ensuring a larger C1 to fill the gaps in the second lithium iron phosphate layer and improve stability.

[0068] When the C2 to C1 ratio is less than 1:50, the Ti element in the second lithium iron phosphate is too small, resulting in too many lithium vacancies that cannot be filled, thus affecting the structure of the second lithium iron phosphate. There are not enough sites to accept the insertion of lithium ions during discharge, which weakens the cycle performance of the battery. When the C2 to C1 ratio is greater than 1:1.5, the Ti element in the first lithium iron phosphate is too small to fill the lithium vacancies, which reduces the structural stability of the first lithium iron phosphate. Excessive Ti element in the second lithium iron phosphate dissolves out, which easily forms dendrites and increases the self-discharge capacity of the battery.

[0069] In this invention, the mass content of element Ti in the first lithium iron phosphate and the second lithium iron phosphate can be determined by conventional testing methods in the art, such as by inductively coupled plasma atomic emission spectrometry (ICP-AES).

[0070] A second aspect of the present invention provides a lithium-ion secondary battery, the lithium-ion secondary battery comprising the positive electrode sheet described in the first aspect of the present invention.

[0071] In this invention, the lithium-ion secondary battery further includes a negative electrode and a separator.

[0072] In this invention, the negative electrode sheet includes a negative electrode active material, which includes hard carbon.

[0073] In this invention, hard carbon possesses a high specific surface area and good structural stability, making it a good negative electrode active material with excellent rate performance and long cycle life, especially showing application advantages under low-temperature conditions. Lithium iron phosphate, as a positive electrode active material, also possesses good structural stability, long cycle life, and high energy density. The combination of these two materials results in a battery that not only exhibits excellent rate and cycle performance but is also suitable for low-temperature conditions, demonstrating good cold-start performance. Furthermore, the double-layer coating design of the positive electrode and the particle size differentiation of lithium iron phosphate in the first and second positive electrode coatings reduce concentration polarization, increase the battery's energy density, and enhance the battery's cycle stability and cycle life.

[0074] In this invention, the lithium-ion secondary battery comprises a wound core and / or a stacked core.

[0075] In this invention, the ratio of the average particle size of the negative electrode active material to the thickness of the membrane is 1:(0.4-5), for example, 1:0.4, 1:0.6, 1:0.8, 1:1, 1:1.2, 1:1.4, 1:1.6, 1:1.8, 1:2, 1:2.4, 1:2.8, 1:3.2, 1:3.6, 1:4, 1:4.5 or 1:5.

[0076] In one embodiment, the ratio of the average particle size of the negative electrode active material to the thickness of the membrane is 1:(0.8-3.6).

[0077] In this invention, the average particle size of the negative electrode active material is 4μm-20μm, for example, 4μm, 4.2μm, 4.4μm, 4.6μm, 4.8μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 7.5μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm or 20μm.

[0078] In one embodiment, the average particle size of the negative electrode active material is 5 μm-15 μm.

[0079] In this invention, the thickness of the diaphragm is 8μm-20μm, for example, 8μm, 8.2μm, 8.4μm, 8.6μm, 8.8μm, 9μm, 9.5μm, 10μm, 10.5μm, 11μm, 11.5μm, 12μm, 12.5μm, 13μm, 13.5μm, 14μm, 14.5μm, 15μm, 15.5μm, 16μm, 17μm, 18μm, 19μm or 20μm.

[0080] In one embodiment, the thickness of the diaphragm is 12 μm-18 μm.

[0081] In this invention, the diaphragm can be a conventional choice in the art.

[0082] In this invention, controlling the ratio of the average particle size of the negative electrode active material to the thickness of the separator plays an important role in controlling the lithium ion transport speed and increasing capacity and energy density. When the ratio of the average particle size of the negative electrode active material to the thickness of the separator is less than 1:5, the average particle size of the negative electrode active material is too small or the separator thickness is too large. Excessive separator thickness increases the lithium-ion transport path, leading to increased internal resistance and thus worsening cold start and cycle performance. Conversely, an excessively small average particle size of the negative electrode active material results in more lithium ions being consumed during SEI film formation, increasing irreversible capacity loss and further reducing energy density and capacity retention during storage. When the ratio of the average particle size of the negative electrode active material to the thickness of the separator is greater than 1:0.4, the average particle size of the negative electrode active material is either too large or the separator thickness is too small. An excessively small separator thickness causes lithium ions to pass through quickly during charging and deposit excessively on the negative electrode side, forming lithium dendrites and increasing self-discharge. An excessively large average particle size of the negative electrode active material increases the contact area between the separator and the negative electrode, causing uneven distribution of lithium ions on the surface of the negative electrode active material, resulting in lithium plating, which is detrimental to improving battery energy density, cycle stability, and lifespan.

[0083] In this invention, the average particle size of the negative electrode active material can be obtained using conventional testing methods in the art, such as scanning electron microscopy imaging, and the testing method is the same as that used for testing the average particle size of the first lithium iron phosphate and the second lithium iron phosphate; the membrane thickness can be obtained using conventional testing methods in the art, such as imaging the obtained cross-section using a scanning electron microscope (SEM) and measuring the membrane thickness using image analysis software.

[0084] In this invention, the first positive electrode conductive agent and the second positive electrode conductive agent are each independently selected from at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes and graphene.

[0085] In this invention, the positive electrode sheet further includes a positive electrode binder, and the negative electrode sheet further includes a negative electrode conductive agent and a negative electrode binder. The positive electrode conductive agent, positive electrode binder, negative electrode conductive agent, and negative electrode binder are all conventional choices in the art. For example, the positive electrode binder includes at least one of polyvinylidene fluoride (PVDF), polyacrylate, polyimide, and styrene-butadiene rubber; the negative electrode binder includes at least one of polyacrylic acid (PAA), sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose, polyimide, styrene-butadiene rubber (SBR), and polyvinylidene fluoride; and the negative electrode conductive agent includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

[0086] In this invention, the lithium-ion secondary battery further includes an electrolyte or electrolyte solution, which can be a conventional choice in the art.

[0087] The lithium-ion secondary battery of the present invention is suitable for automotive power batteries and has good performance, performance stability and lifespan.

[0088] 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.

[0089] 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.

[0090] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0091] In the following embodiments, the average particle size of the first lithium iron phosphate is denoted as d1, the average particle size of the second lithium iron phosphate is denoted as d2, the average particle size of the primary particles is denoted as d0, the thickness of the positive electrode coating on the current collector side (i.e., the single-sided positive electrode coating) is denoted as L, the thickness of the first positive electrode coating is denoted as L1, and the thickness of the second positive electrode coating is denoted as L2; ​​the mass content of the first positive electrode conductive agent in the first positive electrode coating is denoted as w1, the mass content of the second positive electrode conductive agent in the second positive electrode coating is denoted as w2, and the vertical distance from the second lithium iron phosphate to the positive electrode current collector is denoted as x1.

[0092] Example 1:

[0093] (1) Preparation of positive electrode

[0094] The first lithium iron phosphate (with an average particle size of d1, d1 being 1150 nm, and the mass content of element Ti in the first lithium iron phosphate being C1, C1 being 6840.4 ppm), PVDF, and the first positive electrode conductive agent (conductive carbon black SP1, with a mass content of w1 in the first positive electrode coating, w1 being 5.2%) were mixed by stirring to form a uniform and stable mixture, thus obtaining the first positive electrode slurry. The solid components included 92 wt% first lithium iron phosphate, 3.5 wt% PVDF, and 4.5 wt% conductive carbon black SP2, and the solvent was N-methylpyrrolidone (NMP).

[0095] The second lithium iron phosphate (with an average particle size of d2, 5.1 μm, sphericity of 1, and Ti content of C2 of the second lithium iron phosphate, 684.3 ppm), PVDF, and the second positive electrode conductive agent (conductive carbon black SP2, with a mass content of w2 of 2.6% in the first positive electrode coating) are mixed by stirring to form a uniform and stable mixture, thus obtaining the second positive electrode slurry. The solid components include 93.5 wt% second lithium iron phosphate, 3.5 wt% PVDF, and 3 wt% conductive carbon black SP1. The solvent is NMP. The second lithium iron phosphate contains several primary particles (with an average particle size of d0, 209 nm). The ratio of the average particle size of the primary particles to the average particle size of the first lithium iron phosphate (denoted as d0:d1) is 1:5.5.

[0096] The second positive electrode slurry is coated on the upper layer, and the first positive electrode slurry is coated on the lower layer. The areal density ratio of the second positive electrode slurry coating to the first positive electrode slurry coating is 1:4.5. The coating is evenly applied to both sides of the aluminum foil. After drying and compaction by a roller press, the positive electrode sheet is obtained.

[0097] At this point, the ratio of the vertical distance (x1) from the second lithium iron phosphate to the positive electrode current collector to the thickness (L) of the single-sided positive electrode coating is x1:L = 1:6.25, x1 = 9.6 μm, L = 60 μm; the ratio of the average particle size of the second lithium iron phosphate to the thickness of the single-sided positive electrode coating is d2:L = 1:11.76; the ratio of the thickness of the first positive electrode coating to the thickness of the second positive electrode coating (L1:L2) is 1:4.5; the ratio of the mass content of the second positive electrode conductive agent in the second positive electrode coating to the mass content of the first positive electrode conductive agent in the first positive electrode coating (w2:w1) is 1:2; the mass content of element Ti in the second lithium iron phosphate to the mass content of element Ti in the first lithium iron phosphate (C2:C1) is 1:10; and the porosity of the positive electrode sheet is 40.56%.

[0098] (2) Preparation of negative electrode

[0099] The negative electrode active material (90% graphite + 10% hard carbon with an average particle size of 10 μm), SBR, and conductive carbon black E1 are mixed and stirred to form a uniform and stable negative electrode slurry (solid content of 46 wt%). The solid components include 95 wt% negative electrode active material, 3 wt% SBR, and 2 wt% conductive agent E1, and the solvent is water. The negative electrode slurry is uniformly coated on both sides of copper foil, dried, and compacted by a roller press to obtain the negative electrode sheet.

[0100] (3) Assembly of lithium-ion secondary batteries

[0101] The obtained positive and negative electrode sheets are wound / stacked to form bare cells, and aluminum tabs and copper-plated nickel tabs are welded after hot pressing. After perforation in the aluminum-plastic film, the cells are encapsulated and vacuum-baked at 95℃ for 24 hours. The electrolyte used is a 1M lithium hexafluorophosphate electrolyte, with a solvent of ethylene carbonate: dimethyl carbonate: 1,2-propylene glycol carbonate in a 1:1:1 (volume ratio). After electrolyte injection, the batteries undergo formation, secondary sealing, sorting, and OCV testing to obtain soft-pack batteries.

[0102] At this point, the ratio of the average particle size of the negative electrode active material to the thickness of the membrane is 1:1.5, and the thickness of the membrane is 15 μm.

[0103] Example 2:

[0104] (1) Preparation of positive electrode

[0105] The first lithium iron phosphate (d1 is 703 nm, C1 is 5023.2 ppm), PVDF and the first positive electrode conductive agent (conductive carbon black SP1, w1 is 4%) are mixed by stirring to form a uniform and stable mixture to obtain the first positive electrode slurry. The solid components include 92 wt% first lithium iron phosphate, 3.5 wt% PVDF and 4.5 wt% conductive carbon black SP2, and the solvent is NMP.

[0106] Lithium iron phosphate (d2 = 10 μm, sphericity = 0.6, C2 = 1807.1 ppm), PVDF, and a second positive electrode conductive agent (conductive carbon black SP2, w2 = 3.8%) were mixed by stirring to form a uniform and stable mixture, thus obtaining a second positive electrode slurry. The solid components included 93.5 wt% lithium iron phosphate, 3.5 wt% PVDF, and 3 wt% conductive carbon black SP1. The solvent was NMP. The lithium iron phosphate contained several primary particles (d0 = 297 nm) with a d0:d1 ratio of 1:2.37.

[0107] The second positive electrode slurry is coated on the upper layer, and the first positive electrode slurry is coated on the lower layer. The ratio of the areal density of the second positive electrode slurry coating to that of the first positive electrode slurry coating is 1:7. The coating is evenly applied to both sides of the aluminum foil. After drying and compaction by a roller press, the positive electrode sheet is obtained.

[0108] At this point, x1:L is 1:8, x1 = 6.9 μm, L = 55 μm; d2:L is 1:5.5, L1:L2 is 1:7; w2:w1 is 1:1.05; C2:C1 is 1:2.78; and the porosity of the positive electrode is 49.17%.

[0109] (2) Preparation of negative electrode

[0110] The negative electrode active material (80% graphite + 20% hard carbon with an average particle size of 15 μm), SBR, and conductive carbon black E1 are mixed and stirred to form a uniform and stable negative electrode slurry (solid content of 46 wt%). The solid components include 95 wt% negative electrode active material, 3 wt% SBR, and 2 wt% conductive agent E1, and the solvent is water. The negative electrode slurry is uniformly coated on both sides of copper foil, dried, and compacted by a roller press to obtain the negative electrode sheet.

[0111] (3) Assembly of lithium-ion secondary batteries

[0112] The obtained positive and negative electrode sheets are wound / stacked to form bare cells, and aluminum tabs and copper-plated nickel tabs are welded after hot pressing. After perforation in the aluminum-plastic film, the cells are encapsulated and vacuum-baked at 95℃ for 24 hours. The electrolyte used is a 1M lithium hexafluorophosphate electrolyte, with a solvent of ethylene carbonate: dimethyl carbonate: 1,2-propylene glycol carbonate in a 1:1:1 (volume ratio). After electrolyte injection, the batteries undergo formation, secondary sealing, sorting, and OCV testing to obtain soft-pack batteries.

[0113] At this point, the ratio of the average particle size of the negative electrode active material to the thickness of the membrane is 1:0.8, and the thickness of the membrane is 12 μm.

[0114] Example 3:

[0115] (1) Preparation of positive electrode

[0116] The first positive electrode slurry is obtained by mixing lithium iron phosphate (d1 is 1496nm, C1 is 8000.9ppm), PVDF and the first positive electrode conductive agent (conductive carbon black SP1, w1 is 5.5%) by stirring to form a uniform and stable mixture. The solid components include 92wt% lithium iron phosphate, 3.5wt% PVDF and 4.5wt% conductive carbon black SP2, and the solvent is NMP.

[0117] Lithium iron phosphate (d2 = 2.2 μm, sphericity = 0.8, C2 = 500.8 ppm), PVDF, and a second positive electrode conductive agent (conductive carbon black SP2, w2 = 2%) are mixed by stirring to form a uniform and stable mixture, thus obtaining a second positive electrode slurry. The solid components include 93.5 wt% lithium iron phosphate, 3.5 wt% PVDF, and 3 wt% conductive carbon black SP1. The solvent is NMP. The lithium iron phosphate contains several primary particles (d0 = 151 nm), and the d0:d1 ratio is 1:9.91.

[0118] The second positive electrode slurry is coated on the upper layer, and the first positive electrode slurry is coated on the lower layer. The areal density ratio of the second positive electrode slurry coating to the first positive electrode slurry coating is 1:0.23. The coating is evenly applied to both sides of the aluminum foil. After drying and compaction by a roller press, the positive electrode sheet is obtained.

[0119] At this point, x1:L is 1:2, x1 = 40 μm, L = 80 μm; d2:L is 1:36.36, L1:L2 is 1:0.23; w2:w1 is 1:2.75; C2:C1 is 1:16; the porosity of the positive electrode is 30.28%.

[0120] (2) Preparation of negative electrode

[0121] The negative electrode active material (85% graphite + 15% hard carbon with an average particle size of 5μm), SBR, and conductive carbon black E1 are mixed and stirred to form a uniform and stable negative electrode slurry (solid content of 46wt%). The solid components include 95wt% negative electrode active material, 3wt% SBR, and 2wt% conductive agent E1, and the solvent is water. The negative electrode slurry is uniformly coated on both sides of copper foil, dried, and compacted by a roller press to obtain the negative electrode sheet.

[0122] (3) Assembly of lithium-ion secondary batteries

[0123] The obtained positive and negative electrode sheets are wound / stacked to form bare cells, and aluminum tabs and copper-plated nickel tabs are welded after hot pressing. After perforation in the aluminum-plastic film, the cells are encapsulated and vacuum-baked at 95℃ for 24 hours. The electrolyte used is a 1M lithium hexafluorophosphate electrolyte, with a solvent of ethylene carbonate: dimethyl carbonate: 1,2-propylene glycol carbonate in a 1:1:1 (volume ratio). After electrolyte injection, the batteries undergo formation, secondary sealing, sorting, and OCV testing to obtain soft-pack batteries.

[0124] At this point, the ratio of the average particle size of the negative electrode active material to the thickness of the membrane is 1:3.6, and the thickness of the membrane is 18 μm.

[0125] Example 4:

[0126] (1) Preparation of positive electrode

[0127] The first positive electrode slurry is obtained by mixing lithium iron phosphate (d1 is 1328nm, C1 is 7410.3ppm), PVDF and the first positive electrode conductive agent (conductive carbon black SP1, w1 is 5.5%) by stirring to form a uniform and stable mixture. The solid components include 92wt% lithium iron phosphate, 3.5wt% PVDF and 4.5wt% conductive carbon black SP2, and the solvent is NMP.

[0128] Lithium iron phosphate (d2 = 5.9 μm, sphericity = 1, C2 = 1235.2 ppm), PVDF, and a second positive electrode conductive agent (conductive carbon black SP2, w2 = 2%) were mixed by stirring to form a uniform and stable mixture, thus obtaining a second positive electrode slurry. The solid components consisted of 93.5 wt% lithium iron phosphate, 3.5 wt% PVDF, and 3 wt% conductive carbon black SP1. The solvent was NMP. The lithium iron phosphate contained several primary particles (d0 = 162 nm), with a d0:d1 ratio of 1:8.2.

[0129] The second positive electrode slurry is coated on the upper layer, and the first positive electrode slurry is coated on the lower layer. The areal density ratio of the second positive electrode slurry coating to the first positive electrode slurry coating is 1:0.4. The coating is evenly applied to both sides of the aluminum foil. After drying and compaction by a roller press, the positive electrode sheet is obtained.

[0130] At this point, x1:L is 1:1.5, x1 = 46.7 μm, L = 70 μm; d2:L is 1:11.86, L1:L2 is 1:0.4; w2:w1 is 1:2.75; C2:C1 is 1:6; the porosity of the positive electrode is 32.89%.

[0131] (2) Preparation of negative electrode

[0132] The negative electrode active material (70% graphite + 30% hard carbon with an average particle size of 5 μm), SBR, and conductive carbon black E1 are mixed and stirred to form a uniform and stable negative electrode slurry (solid content of 46 wt%). The solid components include 95 wt% negative electrode active material, 3 wt% SBR, and 2 wt% conductive agent E1, and the solvent is water. The negative electrode slurry is uniformly coated on both sides of copper foil, dried, and compacted by a roller press to obtain the negative electrode sheet.

[0133] (3) Assembly of lithium-ion secondary batteries

[0134] The obtained positive and negative electrode sheets are wound / stacked to form bare cells, and aluminum tabs and copper-plated nickel tabs are welded after hot pressing. After perforation in the aluminum-plastic film, the cells are encapsulated and vacuum-baked at 95℃ for 24 hours. The electrolyte used is a 1M lithium hexafluorophosphate electrolyte, with a solvent of ethylene carbonate: dimethyl carbonate: 1,2-propylene glycol carbonate in a 1:1:1 (volume ratio). After electrolyte injection, the batteries undergo formation, secondary sealing, sorting, and OCV testing to obtain soft-pack batteries.

[0135] At this point, the ratio of the average particle size of the negative electrode active material to the thickness of the membrane is 1:3.6, and the thickness of the membrane is 18 μm.

[0136] Example 5 group:

[0137] This set of examples is used to verify the effect of the ratio of "the vertical distance (x1) from the second lithium iron phosphate to the positive electrode current collector to the thickness (L) of the single-sided positive electrode coating", i.e., "x1:L". Keeping L constant, this is achieved by adjusting the areal density and compaction density of the first and second positive electrode coatings, as follows:

[0138] Example 5a is based on Example 3, except that the positive electrode is different. Specifically, the ratio of x1 to L is 1:1.24 and x1 is 64.5 μm. In this case, the porosity of the positive electrode is 25.13%.

[0139] Example 5b is based on Example 2, except that the positive electrode is different. Specifically, x1:L is 1:11.27 and x1 is 4.9 μm. In this case, the porosity of the positive electrode is 36.34%.

[0140] Example 5c is based on Example 3, except that the positive electrode is different. Specifically, x1: L is 0 and x1 is 0 μm. In this case, the porosity of the positive electrode is 27.34%.

[0141] Example 6 group:

[0142] This set of examples is used to verify the effect of changes in the ratio of the average particle size of primary particles to the average particle size of the first lithium iron phosphate, i.e., "d0:d1". This is achieved by adjusting the average particle size (d1) of the first lithium iron phosphate and the average particle size (d0) of the primary particles, as follows:

[0143] Example 6a is based on Example 3, except that d1 = 505 nm and d0 = 400 nm. In this case, d0:d1 = 1:1.25 and the porosity of the positive electrode is 28.71%.

[0144] Example 6b is based on Example 3, except that d1 = 1593 nm and d0 is 50 nm. In this case, d0:d1 is 1:32 and the porosity of the positive electrode is 31.77%.

[0145] Example 7 group:

[0146] This set of examples is used to verify the impact of changes in the "average particle size of lithium iron phosphate," i.e., "d2," which is achieved by adjusting the particle size of lithium iron phosphate, as follows:

[0147] Example 7a is based on Example 3, except that d2 = 1.7 μm, and the ratio of d2:L is 1:47.06, with a porosity of 29.08% for the positive electrode.

[0148] Example 7b is based on Example 3, except that d2 = 14.5 μm, and the ratio of d2:L is 1:5.52, with a porosity of 34.01% for the positive electrode.

[0149] Example 8 group:

[0150] This set of embodiments is used to verify the impact of changes in the ratio of the average particle size of the second lithium iron phosphate to the thickness of the single-sided cathode coating, i.e., "d2:L". This is achieved by adjusting the average particle size of the second lithium iron phosphate and the areal density of the first and second cathode coatings, as detailed below:

[0151] Example 8a is based on Example 3, except that d2 is 14.9 μm, L is 25.3 μm, and L1:L2 is 1:7; at this time, d2:L is 1:1.7, x1:L is 1:8.16, x1 is 3.1 μm, and the porosity of the positive electrode is 40.9%.

[0152] Example 8b is based on Example 3, except that d2 is 2.2 μm, L is 128 μm, and L1:L2 is 1:0.8; at this time, d2:L is 1:80, x1:L is 1:2.84, x1 is 45 μm, and the porosity of the positive electrode is 30.66%.

[0153] Example 9 group:

[0154] This set of embodiments is used to verify the effect of changes in the "thickness ratio of the first positive electrode coating to the second positive electrode coating", i.e., "L1:L2". This is achieved by adjusting the areal density of the first positive electrode coating and the areal density of the second positive electrode coating, as detailed below:

[0155] Example 9a is based on Example 5a, except that L1:L2 is 1:0.15, and the porosity of the positive electrode is 25.2%.

[0156] Example 9b is based on Example 5b, except that L1:L2 is 1:9.5, and the porosity of the positive electrode is 55.65%.

[0157] Example 10 group:

[0158] This set of examples is used to verify the effect of changes in the ratio of the mass content of the second positive electrode conductive agent in the second positive electrode coating to the mass content of the first positive electrode conductive agent in the first positive electrode coating, i.e., "w2:w1". This is achieved by adjusting the amounts of the first and second positive electrode conductive agents, as follows:

[0159] Example 10a is based on Example 3, except that w1 is 3.9% and w2 is 3.9%, and w2:w1 is 1:1.

[0160] Example 10b is based on Example 3, except that w1 is 5.95% and w2 is 1.02%, and the ratio of w2 to w1 is 1:5.85.

[0161] Example 11:

[0162] Based on Example 3, the difference is that the sphericity of the second lithium iron phosphate is 0.22.

[0163] Example 12 group:

[0164] This set of examples is used to verify the impact of changes in the "C2 to C1 ratio". This is achieved by adjusting the mass content of element Ti in the first lithium iron phosphate and the mass content of element Ti in the second lithium iron phosphate, as detailed below:

[0165] Example 12a is based on Example 3, except that C1 is 3080.7 ppm and C2 is 1990.1 ppm, and the ratio of C2 to C1 is 1:1.55.

[0166] Example 12b is based on Example 3, except that C1 is 9991.6 ppm and C2 is 206.4 ppm, and the ratio of C2 to C1 is 1:48.5.

[0167] Example 13 group:

[0168] This set of examples is used to verify the effect of changes in the ratio of the average particle size of the negative electrode active material to the membrane thickness. This is achieved by adjusting the average particle size of the negative electrode active material and the membrane thickness, as detailed below:

[0169] Example 13a is based on Example 3, except that the average particle size of the negative electrode active material is 19.5 μm and the membrane thickness is 8.2 μm. At this time, the ratio of the average particle size of the negative electrode active material to the membrane thickness is 1:0.42.

[0170] Example 13b is based on Example 3, except that the average particle size of the negative electrode active material is 4.1 μm and the membrane thickness is 19.8 μm. In this case, the ratio of the average particle size of the negative electrode active material to the membrane thickness is 1:4.85.

[0171] Example 14

[0172] Based on Example 3, the difference is that the negative electrode active material is 100% graphite.

[0173] Comparative Example 1

[0174] Based on Example 3, the difference is that only a single layer of the second positive electrode slurry is coated on the positive electrode sheet to obtain a positive electrode coating with a thickness of 80 μm. At this time, the porosity of the positive electrode sheet is 43.18%.

[0175] like Figure 2 The image shown is a cross-sectional SEM image of the positive electrode sheet in Comparative Example 1 of this invention. As can be seen from the image, when only a single layer of the second positive electrode slurry is coated, the resulting positive electrode coating contains the second lithium iron phosphate.

[0176] Comparative Example 2

[0177] Based on Example 3, the difference is that only a single layer of the first positive electrode slurry is coated on the positive electrode sheet to obtain a positive electrode coating with a thickness of 80 μm. At this time, the porosity of the positive electrode sheet is 28.41%.

[0178] like Figure 3 The image shown is a cross-sectional SEM image of the positive electrode sheet in Comparative Example 2 of this invention. As can be seen from the image, when only a single layer of the first positive electrode slurry is coated, the resulting positive electrode coating contains the first lithium iron phosphate.

[0179] Comparative Example 3

[0180] Based on Example 3, the difference is that the first positive electrode slurry and the second positive electrode slurry are mixed in a ratio of 8:2 and then coated onto the surface of the positive electrode sheet in a single layer to obtain a positive electrode coating with a thickness of 80 μm. At this time, the porosity of the positive electrode sheet is 26.13%.

[0181] like Figure 4 The image shown is a cross-sectional SEM image of the positive electrode sheet in Comparative Example 3 of this invention. As can be seen from the image, when the first positive electrode slurry and the second positive electrode slurry are mixed and then combined into a single layer, the first lithium iron phosphate and the second lithium iron phosphate in the resulting positive electrode coating are difficult to distinguish.

[0182] Comparative Example 4

[0183] Based on Example 3, the difference is that the first positive electrode slurry is coated on the upper layer and the second positive electrode slurry is coated on the lower layer, at which time the porosity of the positive electrode sheet is 29.83%.

[0184] In the above embodiments, the thickness L1 of the first positive electrode coating is in the range of 5μm-90μm, and the thickness L2 of the second positive electrode coating is in the range of 5μm-90μm.

[0185] In the above embodiments, the solid content of the first positive electrode slurry is in the range of 50wt%-58wt%, and the solid content of the second positive electrode slurry is in the range of 58wt%-64wt%.

[0186] Test example:

[0187] (1) Battery cold start test:

[0188] Under (25±2)℃ conditions, discharge at 1C standard constant current until the discharge termination voltage is 2.2V, and let stand for 30 minutes; then charge at 1C standard constant current and constant voltage until the charging limit voltage is 3.65V, with a cutoff current of 0.05C, and let stand for 30 minutes; discharge at 1C standard constant current until the discharge termination voltage is 2.2V to obtain the actual battery capacity C0, and let stand for 30 minutes; charge at 1C standard constant current and constant voltage until the charging limit voltage is 3.65V, with a cutoff current of 0.05C, and discharge at 1C0 for 30 minutes, which is 50% SOC; after standing at (25±2)℃ for 2 hours, place the battery in a -28℃ constant temperature chamber and maintain the constant temperature for 4 hours; test the terminal voltage value of 3C constant current discharge for 2 seconds, which is the battery cold start terminal voltage, in V.

[0189] (2) Battery cycle test:

[0190] Discharge the battery at 25°C using a 3C standard constant current until the discharge termination voltage is 2.2V, and let it rest for 30 minutes. Then charge it using a 3C standard constant current and constant voltage until the charging limit voltage is 3.65V and the cutoff current is 0.05C, and let it rest for 30 minutes. Discharge it using a 3C standard constant current until the discharge termination voltage is 2.2V, and let it rest for 30 minutes. Repeat the above full charge and discharge steps until the capacity decays to 80% and then stops. The number of repeats is the cycle number, which is used to evaluate the cycle performance of the battery after aging.

[0191] (3) Energy density test:

[0192] The weight of the lithium-ion battery after the second sealing process is recorded as the battery weight. The energy of the battery discharged to 2.2V after being fully charged to 3.65V and left to stand for 5 minutes during the sorting process is recorded as the discharge energy. After the lithium-ion battery is removed from the OCV, the width, height and thickness of the lithium-ion battery are tested. The volume of the lithium-ion battery = width × height × thickness. The ratio of discharge energy to battery volume is the energy density of the battery, with the unit being Wh / L.

[0193] (4) Battery storage capacity retention rate:

[0194] Under an environment of (25±2)℃, the battery was discharged at a standard constant current of 1C until the discharge termination voltage of 2.2V, and then left to stand for 30 minutes; then charged at a standard constant current and constant voltage of 1C until the charging limit voltage of 3.65V was reached, with a cutoff current of 0.05C, and left to stand for 30 minutes; finally, it was discharged at a standard constant current of 1C until the discharge termination voltage of 2.2V, thus obtaining the initial battery capacity C. 始 The sample was left to stand for 30 minutes; then charged with a 1C standard constant current and constant voltage to the charging limit voltage of 3.65V, with a cutoff current of 0.05C. It was then left to stand open-circuit at 60℃ for 30 days; finally, it was left to stand at (25±2)℃ for 1 hour, discharged with a 1C standard constant current to the discharge termination voltage of 2.2V, then charged again with a 1C standard constant current and constant voltage to the charging limit voltage of 3.65V, with a cutoff current of 0.05C. Finally, it was discharged with a 1C standard constant current to the discharge termination voltage of 2.2V, yielding the recovered capacity C. 末 The battery's recovery capacity retention rate is C. 末 / C 始 ×100%.

[0195] (5) Self-discharge rate test:

[0196] At (25±2)℃, the battery was discharged at a standard constant current of 1C until the discharge termination voltage of 2.2V, and then left to stand for 30 minutes. It was then charged at a standard constant current and constant voltage of 1C until the charging limit voltage of 3.65V was reached, with a cutoff current of 0.05C, and left to stand for 30 minutes. The battery was then discharged at a standard constant current of 1C until the discharge termination voltage of 2.2V was reached, and the actual battery capacity C0 was recorded, and left to stand for 30 minutes. The battery was then charged at a constant current of 1C0 to 36% SOC, and then discharged at 1C0 to remove 3% SOC, resulting in 33% SOC, and left to stand for 30 minutes. The initial voltage OCV1 was recorded. After storage at room temperature for 30 days, the final voltage OCV2 was recorded. The battery self-discharge rate = (OCV1 - OCV2) / storage time, in mV / h.

[0197] The performance test records of the batteries obtained in the embodiments and comparative examples of this invention are shown in Table 1.

[0198] Table 1:

[0199]

[0200] As can be seen from Table 1, the lithium-ion secondary battery prepared in this invention has better cold start performance, energy density and cycle stability compared with the comparative example.

[0201] 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 lithium-ion secondary battery, characterized in that, The lithium-ion secondary battery includes a positive electrode sheet, a negative electrode sheet, and a separator. The positive electrode sheet includes a positive current collector and a positive electrode coating located on at least one side surface of the positive current collector. The positive electrode coating includes a first positive electrode coating and a second positive electrode coating stacked along the thickness direction of the positive electrode sheet. The first positive electrode coating is disposed on at least one side surface of the positive current collector, and the second positive electrode coating is disposed on the surface of the first positive electrode coating facing away from the positive current collector. The first positive electrode coating includes a first lithium iron phosphate, and the second positive electrode coating includes a second lithium iron phosphate; the average particle size of the first lithium iron phosphate is 500nm-1600nm, and the average particle size of the second lithium iron phosphate is 1.6μm-15μm. The second lithium iron phosphate comprises secondary particles, while the first lithium iron phosphate comprises single particles; The thickness of the positive electrode coating on either side of the positive electrode current collector is 25μm-80μm; The ratio of the average particle size of the second lithium iron phosphate to the thickness of the positive electrode coating on any side of the positive electrode current collector is 1:(1.6-85); The negative electrode sheet includes a negative electrode active material; the negative electrode active material includes hard carbon; the ratio of the average particle size of the negative electrode active material to the thickness of the separator is 1:(0.4-5).

2. The lithium-ion secondary battery according to claim 1, wherein, The first positive electrode coating further includes a first positive electrode conductive agent, and the second positive electrode coating further includes a second positive electrode conductive agent; the mass content of the second positive electrode conductive agent in the second positive electrode coating and the mass content of the first positive electrode conductive agent in the first positive electrode coating are 1:(1.05-6).

3. The lithium-ion secondary battery according to claim 1, wherein, The ratio of the vertical distance from the second lithium iron phosphate to the positive electrode current collector to the thickness of the positive electrode coating is 1:(1.2-12). The vertical distance from the second lithium iron phosphate to the positive electrode current collector is measured by the following method: the positive electrode sheet is laser-cut, the obtained cross-section is imaged using a scanning electron microscope (SEM), at least 10 different second lithium iron phosphates are selected on the obtained SEM image, and the average value of the shortest distance from the boundary of the second lithium iron phosphate to the positive electrode current collector is taken as the vertical distance from the second lithium iron phosphate to the positive electrode current collector by image analysis software. And / or, the average particle size of the first lithium iron phosphate is 700nm-1500nm, and the average particle size of the second lithium iron phosphate is 2μm-10μm.

4. The lithium-ion secondary battery according to claim 3, wherein, The ratio of the vertical distance from the second lithium iron phosphate to the positive electrode current collector to the thickness of the positive electrode coating is 1:(1.5-8).

5. The lithium-ion secondary battery according to claim 1, wherein, The secondary particles include a plurality of primary particles, and the ratio of the average particle size of the primary particles to the average particle size of the first lithium iron phosphate is 1:(1.25-40). And / or, the average particle size of the primary particles is 50nm-400nm.

6. The lithium-ion secondary battery according to claim 5, wherein, The secondary particles comprise a plurality of primary particles, and the ratio of the average particle size of the primary particles to the average particle size of the first lithium iron phosphate is 1:(2.33-10).

7. The lithium-ion secondary battery according to claim 5, wherein, The average particle size of the primary particles is 150nm-300nm.

8. The lithium-ion secondary battery according to claim 1, wherein, The thickness of the first positive electrode coating is 5μm-90μm, and the thickness of the second positive electrode coating is 5μm-90μm.

9. The lithium-ion secondary battery according to claim 1, wherein, The ratio of the average particle size of the second lithium iron phosphate to the thickness of the positive electrode coating on either side of the positive electrode current collector is 1:(5-37).

10. The lithium-ion secondary battery according to claim 1, wherein, The porosity of the positive electrode is 25%-60%; And / or, the sphericity of the second lithium iron phosphate is 0.2-1.

11. The lithium-ion secondary battery according to claim 10, wherein, The porosity of the positive electrode is 30%-50%.

12. The lithium-ion secondary battery according to claim 10, wherein, The sphericity of the second lithium iron phosphate is 0.6-1.

13. The lithium-ion secondary battery according to claim 1, wherein, The first lithium iron phosphate and the second lithium iron phosphate contain element Ti. The mass content of element Ti in the first lithium iron phosphate is C1, and the mass content of element Ti in the second lithium iron phosphate is C2. The ratio of C2 to C1 is 1:(1.5-50). And / or, C1 is 3000ppm-10000ppm, and C2 is 200ppm-2000ppm.

14. The lithium-ion secondary battery according to claim 13, wherein, The first lithium iron phosphate and the second lithium iron phosphate contain element Ti. The mass content of element Ti in the first lithium iron phosphate is C1, and the mass content of element Ti in the second lithium iron phosphate is C2. The ratio of C2 to C1 is 1:(2.78-16).

15. The lithium-ion secondary battery according to claim 13, wherein, C1 is 5000ppm-8000ppm, and C2 is 500ppm-1800ppm.

16. The lithium-ion secondary battery according to claim 1, wherein, The average particle size of the negative electrode active material is 4μm-20μm.

17. The lithium-ion secondary battery according to claim 1, wherein, The ratio of the average particle size of the negative electrode active material to the thickness of the membrane is 1:(0.8-3.6).

18. The lithium-ion secondary battery according to claim 1, wherein, The average particle size of the negative electrode active material is 5μm-15μm.

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