A positive electrode sheet and a battery

Abstract

The application relates to the technical field of batteries, in particular to a positive plate and a battery comprising the same. The positive plate comprises a first active layer, a second active layer and a third coating layer; the first active layer comprises first lithium iron phosphate particles, the second active layer comprises second lithium iron phosphate particles, and the third coating layer comprises a first lithium titanium aluminum phosphate material; the weight percentage of element Al in the third coating layer is greater than that in the first coating layer, and the weight percentage of element Al in the first coating layer is greater than that in the second coating layer; the weight percentage of element Ti in the third coating layer is greater than that in the first coating layer, and the weight percentage of element Ti in the first coating layer is greater than that in the second coating layer. The positive plate of the application can improve the high-temperature storage performance, the charge-discharge performance and the cycle performance of the battery while ensuring a high energy density of the battery.

Description

Technical Field

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

[0002] In recent years, with the rapid development of new energy vehicles and the demand for large-scale energy storage, the market's requirements for battery energy density have further increased.

[0003] To further address the range anxiety issue of new energy vehicles, it is often necessary to increase the energy density of lithium iron phosphate (LFP) batteries. Calculations show that the current maximum initial charge capacity of LFP batteries has reached 162-163 mAh / g, approaching the theoretical capacity. Currently, the compacted density is 3.6 g / cm³ away from the theoretical capacity of LFP. 3 There is still room for improvement in the true density; further increases in compaction density can be achieved to enhance energy density. Moreover, as the size of existing automotive and energy storage batteries increases, the requirements for capacity and energy efficiency are gradually rising. Therefore, increasing the compaction density of materials, and consequently the compaction density of electrode sheets, is a major development direction for improving energy density. However, with the increase in compaction density, the porosity of the electrode sheets will further decrease, especially when the electrode sheets are compacted to 2.8 g / cm³. 3 Subsequently, the porosity of the electrode drops below 20%. This decrease in porosity further reduces the kinetic performance of the electrode during the lithium insertion / extraction process. Moreover, with the increase in compaction, the battery's rate charge / discharge performance decreases, thereby reducing the state of charge (SOC) corresponding to the lithium plating potential of the negative electrode at high rates, increasing the battery charging time, further increasing the battery's DCR, increasing the resistance to lithium-ion charge transfer and ion diffusion, and also affecting the battery's high-temperature storage performance. Summary of the Invention

[0004] Research has revealed that the decrease in rate discharge performance and increase in discharge coefficient (DCR) of the battery when the compaction density of the lithium iron phosphate cathode increases is due to the following reasons: As the compaction density increases, the thickness of the cathode active layer decreases, resulting in a significant reduction in porosity. In the thickness direction of the cathode, the active layer consists of a surface layer near the separator and a bottom layer near the cathode current collector (regardless of whether the active layer is layered). Compared to the bottom layer, ion transport is faster on the surface layer. After ions are released, they dissociate, conduct to the electrolyte, and then pass through the separator to reach the negative electrode, resulting in a shorter path. However, electrons on the surface layer need to be conducted through the bottom layer to reach the cathode current collector, leading to a longer electron transport path and greater resistance. Conversely, compared to the surface layer, the bottom layer is closer to the cathode current collector, resulting in a shorter electron transport distance and a faster electron transport rate. However, ions need to overcome the resistance of particles and the longer path to reach the surface layer, then pass through the surface layer pores and the separator to reach the negative electrode, thus resulting in greater resistance to ion transport.

[0005] To address the issue of increased discharge rate (DCR) and decreased rate discharge performance in lithium iron phosphate (LFP) cathodes with high compaction density, where both surface electrons and bottom ions have longer transport paths, this invention provides a cathode and a battery incorporating the cathode. The cathode of this invention enables rapid transport of surface electrons to the bottom layer and rapid transport of bottom ions to the surface, reducing battery impedance. This improves the battery's high-temperature storage and charge / discharge performance while maintaining high energy density. Furthermore, it adsorbs HF generated during battery cycling, reducing HF damage to the electrode and its interface, thereby further enhancing the battery's cycle performance.

[0006] To achieve the above objectives, a first aspect of the present invention provides a positive electrode sheet, wherein 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 including a first active layer located on the surface of the positive current collector, a second active layer located on the side of the first active layer away from the positive current collector, and a third coating located between the first active layer and the second active layer; The first active layer includes first lithium iron phosphate particles, the second active layer includes second lithium iron phosphate particles, and the third coating includes first lithium titanium aluminum phosphate material. The weight percentage of element Al in the third coating is greater than that of element Al in the first coating, and the weight percentage of element Al in the first coating is greater than that of element Al in the second coating. The weight percentage of element Ti in the third coating is greater than that of element Ti in the first coating, and the weight percentage of element Ti in the first coating is greater than that of element Ti in the second coating.

[0007] A second aspect of the present invention provides a battery comprising an electrolyte and a positive electrode as described in the first aspect of the present invention.

[0008] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art: In the positive electrode sheet of the present invention, the positive electrode coating includes a first active layer close to the positive electrode current collector, a second active layer away from the positive electrode current collector, and a third coating located between the first active layer and the second active layer. The first active layer includes first lithium iron phosphate particles with a specific particle size distribution, the second active layer includes second lithium iron phosphate particles with a specific particle size distribution, and the third coating includes a first lithium titanium aluminum phosphate material with both high lithium ion transport and electron transport performance.

[0009] The third coating, located between the first and second active layers, includes a first lithium aluminum titanium phosphate material with high lithium-ion and electron transport performance. This material not only provides transport kinetics for ions and electrons during transport, thus improving their transport rate and high-temperature storage and charge / discharge performance, but also buffers the volume change of the positive electrode active material during battery cycling. Simultaneously, it adsorbs HF generated during cycling, enhancing the battery's cycle performance. Furthermore, the positive electrode sheet is controlled to ensure that the weight percentage of element Al in the third coating is greater than that in the first coating. The weight percentage of elemental Al in the third coating is greater than that in the first coating. The weight percentage of elemental Ti in the third coating is greater than that in the first coating. This, in turn, ensures the ion transport rate in the first active layer while simultaneously providing additional transport power to the ions during their journey. This reduces the gap between the ion diffusion rates of the second and first active layers, balances ion transport performance in the positive electrode active layer, reduces the risk of concentration polarization in the positive electrode active layer, and improves the battery's charge / discharge performance and high-temperature storage performance.

[0010] Other features and advantages of the present invention will be described in detail in the following detailed description section.

[0011] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0012] Figure 1 The diagram shown is a schematic diagram of the positive electrode sheet of the present invention.

[0013] Figure 2 The diagram shown is a schematic diagram of the positive electrode coating in the positive electrode sheet of the present invention.

[0014] Figure 3 The image shown is an SEM image of the first active layer in the positive electrode of the present invention.

[0015] Figure 4 The image shown is a SEM image of the second active layer in the positive electrode of the present invention. Detailed Implementation

[0016] 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 invention. Unless otherwise specified herein, data ranges include endpoints.

[0017] 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.

[0018] A first aspect of the present invention provides a positive electrode sheet, wherein 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 including a first active layer located on the surface of the positive current collector. Figure 3 The second active layer is located on the side of the first active layer away from the positive current collector. Figure 4 and a third coating layer located between the first active layer and the second active layer; The first active layer includes first lithium iron phosphate particles, the second active layer includes second lithium iron phosphate particles, and the third coating includes first lithium titanium aluminum phosphate material. The weight percentage of element Al in the third coating is greater than that of element Al in the first coating, and the weight percentage of element Al in the first coating is greater than that of element Al in the second coating. The weight percentage of element Ti in the third coating is greater than that of element Ti in the first coating, and the weight percentage of element Ti in the first coating is greater than that of element Ti in the second coating.

[0019] like Figure 1 and Figure 2 As shown, the positive electrode 1 includes a positive current collector 11 and a positive electrode coating 12 located on at least one side of the surface of the positive current collector. The positive electrode coating 12 includes a first active layer 121 located on the surface of the positive current collector, a second active layer 122 located on the side of the first active layer away from the current collector, and a third coating 123 located between the first active layer 121 and the second active layer 122.

[0020] In the positive electrode sheet of the present invention, the positive electrode coating includes a first active layer close to the positive electrode current collector, a second active layer away from the positive electrode current collector, and a third coating located between the first active layer and the second active layer. The third coating, comprising a first lithium titanium aluminum phosphate material, is disposed between the first and second active layers. This first lithium titanium aluminum phosphate material possesses both high lithium-ion and electron transport performance. On the one hand, the third coating enhances the transport rate of ions from the first active layer and electrons from the second active layer during the transport process, particularly by providing power for ion and electron transport, and simultaneously improving the transport rate of ions from the first active layer to the surface of the positive electrode sheet and the transport rate of electrons from the second active layer to the positive electrode current collector. On the other hand, during battery charging and discharging, the third coating also buffers the stress caused by volume changes in the positive electrode active material. Furthermore, the first lithium titanium aluminum phosphate material has a large number of microporous structures, which can adsorb HF generated during battery cycling through the displacement of lithium ions and hydrogen ions, thereby reducing the damage of HF to the electrode sheet and its interface and improving the battery's cycle performance.

[0021] Furthermore, both Ti and Al can enhance the lithium-ion transport rate. Simultaneously, by controlling the weight percentage of Al in the third coating to be greater than that in the first coating, and the weight percentage of Al in the first coating to be greater than that in the second coating, and the weight percentage of Ti in the third coating to be greater than that in the first coating, and the weight percentage of Ti in the first coating to be greater than that in the second coating, this approach ensures the ion transport rate in the first active layer while simultaneously providing additional transport power to the ions during transport. This reduces the gap between the ion diffusion rates of the second and first active layers, balances the ion transport performance in the positive electrode active layer, reduces the risk of concentration polarization in the positive electrode active layer, and improves the battery's charge / discharge performance and high-temperature storage performance. Specifically, controlling the weight percentage of element Al in the third coating to be greater than that in the first coating, and the weight percentage of element Al in the third coating to be greater than that in the second coating, as well as the weight percentage of element Ti in the third coating to be greater than that in the first coating, ensures a higher ion transport rate in the third coating. This allows the third coating to replenish the transport power of ions during transport, thereby improving the ion transport rate in subsequent stages. Controlling the weight percentages of element Al and Ti in the first and second coatings to be greater than those in the second coatings allows for a higher ion transport rate in the first lithium iron phosphate particles with a larger average particle size. While ensuring the ion transport rate in the first active layer, this reduces the gap between the ion diffusion rate in the second and first active layers, thereby balancing the ion transport performance in the positive electrode active layer and further improving the battery's charge / discharge performance and high-temperature storage performance.

[0022] Therefore, the positive electrode of the present invention, on the one hand, by setting a third coating comprising a first lithium aluminum titanium phosphate material with high lithium-ion transport and electron transport performance between the first active layer and the second active layer, provides transport power for ions and electrons during transport, thereby improving the transport dynamics performance of ions and electrons and increasing the transport rate of ions and electrons. On the other hand, by simultaneously controlling the weight proportion of element Al in the third coating to be greater than the weight proportion of element Al in the first coating, the weight proportion of element Al in the first coating to be greater than the weight proportion of element Al in the second coating, and the weight proportion of element Ti in the third coating to be greater than the weight proportion of element Ti in the first coating to be greater than the weight proportion of element Ti in the second coating, it is possible to ensure the transport rate of ions in the first active layer while enabling the third coating to provide transport power for ions during transport, thereby shortening the difference between the ion diffusion rate of the second active layer and the ion diffusion rate of the first active layer, balancing the transport performance of ions in the positive electrode active layer, reducing the risk of concentration polarization of ions in the positive electrode active layer, and improving the charge-discharge performance and high-temperature storage performance of the battery.

[0023] In this invention, by providing a third coating comprising a first lithium aluminum titanium phosphate material between the first and second active layers, and simultaneously controlling the weight ratios of element Ti and element Al in the first, second, and third active layers respectively, it is possible to improve the battery's charge-discharge performance, high-temperature storage performance, and cycle performance while maintaining a high energy density, compared to existing technologies. To further enhance the effect, one or more of the technical features can be further optimized.

[0024] In some examples, lithium titanium aluminum phosphate materials include elements Si and C. The combination of Si and C can further enhance the ion transport and electron transport performance of lithium titanium aluminum phosphate materials. Therefore, lithium titanium aluminum phosphate materials including elements Si and C have both higher lithium ion transport and electron transport performance.

[0025] In this invention, element Si is located in the first lithium titanium aluminum phosphate material in the form of a dopant, and element C is located on the surface of at least a portion of the lithium titanium aluminum phosphate in the form of a carbon coating layer. It is understood that the lithium titanium aluminum phosphate material includes a core and a carbon coating layer located on the surface of the core. The core is lithium titanium aluminum phosphate containing doped elements, including element Si.

[0026] In some instances, the carbon coating layer covers the entire surface of the core.

[0027] In some instances, the first lithium titanium aluminum phosphate material comprises elements Al, Ti, C, Si, and phosphate.

[0028] In some instances, based on the total weight of the first lithium titanium aluminum phosphate material, the weight percentage of element Si is 0.1%-1.5% (e.g., 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.3%, or 1.5%), and the weight percentage of element C is 1%-2% (e.g., 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, or 2%).

[0029] In some instances, based on the total amount of the third coating, the weight percentage of element Si is 0.1%-1.42% (e.g., 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.3%, or 1.42%), and the weight percentage of element C is 2.5%-7% (e.g., 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, or 7%). In this invention, a cross-sectional view of the positive electrode coating is obtained by cutting the positive electrode sheet using an argon ion milling machine, and the result can be measured using EDS spot scanning.

[0030] In some instances, the first active layer includes element Ti, and the weight percentage of element Ti in the first active layer is 3000ppm-6000ppm (e.g., 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm, 5500ppm, 6000ppm, or within any two of the above values). Controlling the weight percentage of element Ti in the first active layer within the above range can improve the ion transport rate in the first active layer.

[0031] It is understood that the element Ti in the first active layer may originate from the doped first lithium iron phosphate particles, or from the second lithium titanium aluminum phosphate material added to the first active layer, or from the doped first lithium iron phosphate particles and the second lithium titanium aluminum phosphate material added to the first active layer.

[0032] In some instances, the second lithium titanium aluminum phosphate material comprises a lithium titanium aluminum phosphate-carbon composite material.

[0033] In some instances, the second active layer includes element Ti, with the weight percentage of Ti in the second active layer being 2000ppm-5000ppm (e.g., 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm, 5500ppm, or within any two of these values). The second active layer is located on the surface of the positive electrode and possesses high porosity and high ion diffusion capability. Simultaneously, the average particle size of the second lithium iron phosphate particles in the second active layer is small, resulting in high ion insertion / extraction and diffusion energy. By controlling the weight percentage of Ti in the second active layer within the aforementioned range, while ensuring the ion diffusion rate in the second active layer, the difference between the ion diffusion rate of the second active layer and that of the first active layer can be reduced, achieving a balance in the overall performance of the positive electrode and reducing the risk of localized concentration polarization in the positive electrode.

[0034] It is understood that the element Ti in the second active layer may originate from the doped second lithium iron phosphate particles, or from the third lithium titanium aluminum phosphate material added to the second active layer, or from the doped second lithium iron phosphate particles and the second lithium titanium aluminum phosphate material added to the second active layer.

[0035] In some instances, the third titanium aluminum lithium phosphate material comprises titanium aluminum lithium phosphate.

[0036] In some instances, the third coating includes element Ti, and the weight percentage of Ti in the third coating is between 200,000 ppm and 250,000 ppm (e.g., 200,000 ppm, 205,000 ppm, 210,000 ppm, 215,000 ppm, 220,000 ppm, 225,000 ppm, 230,000 ppm, 235,000 ppm, 240,000 ppm, 245,000 ppm, 250,000 ppm, or within any two of the above values). Controlling the weight percentage of Ti in the third coating within the above range enables the third coating to provide sufficient power for ion transport.

[0037] According to some specific implementation methods, the positive electrode sheet satisfies the following relationship: T 3 >T 1 >T 2 The weight percentage T of element Ti in the first active layer 1 The weight percentage T of element Ti in the second active layer is 3000ppm-6000ppm. 2 The weight percentage T of element Ti in the third coating is 2000ppm-5000ppm. 3The value is 200,000 ppm to 250,000 ppm.

[0038] In this invention, an argon ion mill is used to cut the positive electrode sheet to obtain cross-sections of the first active layer, the third coating layer, and the second active layer. Then, energy dispersive spectroscopy (EDS) is performed on the first active layer, the third coating layer, and the second active layer to obtain the weight percentage A of element Al in the first active layer. 1 The weight percentage A of element Al in the second active layer 2 The weight percentage A of element Al in the third coating. 3 The weight percentage T of element Ti in the first active layer 1 The weight percentage T of element Ti in the second active layer 2 and the weight percentage T of element Ti in the third coating 3 .

[0039] In some instances, the first active layer includes element Al, and the weight percentage of element Al in the first active layer is 500ppm-1500ppm (e.g., 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1100ppm, 1200ppm, 1300ppm, 1400ppm, 1500ppm, or within any two of the above values). Controlling the weight percentage of element Al in the first active layer within the above range can improve the ion transport rate in the first active layer, enabling ions to be rapidly transported to the surface of the positive electrode.

[0040] It is understood that the element Al in the first active layer may originate from the doped first lithium iron phosphate particles, or from the second lithium titanium aluminum phosphate material added to the first active layer, or from the doped first lithium iron phosphate particles and the second lithium titanium aluminum phosphate material added to the first active layer.

[0041] In some instances, the second active layer includes element Al, and the weight percentage of Al in the second active layer is 0-1000 ppm (e.g., 0, 1 ppm, 50 ppm, 100 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, or any two of the above values). It is understood that when the weight percentage of Al in the second active layer is 0, it indicates that Al is absent from the second active layer. The second active layer is located on the surface of the positive electrode and has high porosity and high ion diffusion capacity. Simultaneously, the average particle size of the second lithium iron phosphate particles in the second active layer is small, resulting in high ion insertion / extraction and diffusion energy. Controlling the weight percentage of Al in the second active layer within the aforementioned range can reduce the difference between the ion diffusion rate of the second active layer and the ion diffusion rate of the first active layer, achieving a balance in the overall performance of the positive electrode and reducing the risk of localized concentration polarization in the positive electrode.

[0042] It is understood that the element Al in the second active layer may originate from the doped second lithium iron phosphate particles, or from the third lithium titanium aluminum phosphate material added to the second active layer, or from the doped second lithium iron phosphate particles and the third lithium titanium aluminum phosphate material added to the second active layer.

[0043] In some instances, the third coating includes element Al, and the weight percentage of Al in the third coating is between 5000 ppm and 8000 ppm (e.g., 5000 ppm, 5300 ppm, 5500 ppm, 5800 ppm, 6000 ppm, 6300 ppm, 6500 ppm, 6800 ppm, 7000 ppm, 7300 ppm, 7500 ppm, 7800 ppm, 8000 ppm, or within any two of the above values). Controlling the weight percentage of Al in the third coating within the above range enables the third coating to provide sufficient power for ion transport.

[0044] According to some specific implementation methods, the positive electrode sheet satisfies the following relationship: A 3 >A 1 >A 2 The weight percentage A of element Al in the first active layer 1 The weight percentage A of element Al in the second active layer is 500ppm-1500ppm. 2 The weight percentage A of element Al in the third coating is 0-1000 ppm. 3 The concentration is 5000ppm-8000ppm.

[0045] In some instances, the particle size distribution of the first lithium iron phosphate particles satisfies the following relationship: 2 < (Dv90 - Dv10) / Dv50 < 10 (e.g., 2.1, 2.3, 2.5, 2.8, 3, 3.5, 3.8, 4, 4.3, 4.5, 4.8, 5, 5.3, 5.5, 5.8, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10), 400 nm ≤ Dv50 ≤ 1500 nm (e.g., 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, 1100 nm, 1200 nm, 1300 nm, 1400 nm, 1500 nm or within any two of the above values).

[0046] It is understood that the Dv10 of the first lithium iron phosphate particle is the particle size corresponding to 10% of the cumulative particle size distribution in the particle size distribution curve of the first lithium iron phosphate particle, in nm; the Dv50 of the first lithium iron phosphate particle is the particle size corresponding to 50% of the cumulative particle size distribution in the particle size distribution curve of the first lithium iron phosphate particle, in nm; and the Dv90 of the first lithium iron phosphate particle is the particle size corresponding to 90% of the cumulative particle size distribution in the particle size distribution curve of the first lithium iron phosphate particle, in nm.

[0047] In some instances, the second active layer comprises second lithium iron phosphate particles, the particle size distribution of which satisfies the following relationship: 0.1 < (Dv90 - Dv10) / Dv50 < 2 (e.g., 1.9, 1.8, 1.5, 1.3, 1, 0.8, 0.5, 0.3, 0.11), 200 nm ≤ Dv50 < 400 nm (e.g., 200 nm, 230 nm, 250 nm, 280 nm, 300 nm, 330 nm, 350 nm, 380 nm, 399 nm or within any two of the above values).

[0048] It is understood that the Dv10 of the second lithium iron phosphate particle is the particle size corresponding to 10% of the cumulative particle size distribution in the particle size distribution curve of the second lithium iron phosphate particle, in nm; the Dv50 of the second lithium iron phosphate particle is the particle size corresponding to 50% of the cumulative particle size distribution in the particle size distribution curve of the second lithium iron phosphate particle, in nm; and the Dv90 of the second lithium iron phosphate particle is the particle size corresponding to 10% of the cumulative particle size distribution in the particle size distribution curve of the second lithium iron phosphate particle, in nm.

[0049] In this invention, the particle size distribution curve of the first lithium iron phosphate particle and the particle size distribution region of the second lithium iron phosphate particle can be obtained by the following method: Taking the "particle size distribution curve of the first lithium iron phosphate particle" as an example, the positive electrode sheet is cut using an argon ion mill to obtain the cross-section of the positive electrode sheet. Taking the position of the positive electrode current collector as a reference, a first line parallel to the positive electrode current collector is drawn in a direction away from the positive electrode current collector. The distance from the first line to the positive electrode current collector in the thickness direction of the positive electrode sheet is less than 60% of the total thickness of the positive electrode coating. The region formed by the first line and the positive electrode current collector is the first region. 500 phosphorus particles are continuously measured in the first region. The particle size of lithium iron phosphate (LFP) particles is determined by arranging the measured particle sizes in ascending order to obtain the particle size distribution curve of the first LFP particle. Based on the obtained particle size distribution of the first LFP particle, Dv10, Dv50, and Dv90 can be directly obtained, and (Dv90-Dv10) / Dv50 can be calculated. This operation is repeated twice (meaning that the particle size of 500 LFP particles is measured again during the repetition). The average of the three measured Dv50 values ​​of the first LFP particle is taken as the test result, and the average of the three measured (Dv90-Dv10) / Dv50 values ​​is taken as the test result. In the cross-sectional SEM image of the positive electrode sheet, the smallest square or rectangle that completely surrounds a LFP particle is drawn, that is, the square or long direction where the edge of the LFP particle meets the four sides of the square or rectangle is drawn. The length of one side of the square or the length of the long side of the rectangle is the particle size of the LFP particle. The measurement of the particle size distribution (Dv90-Dv10) / Dv50 of the second lithium iron phosphate particles can be carried out with reference to the particle size distribution (Dv90-Dv10) / Dv50 of the first lithium iron phosphate particles. The difference is that 500 second lithium iron phosphate particles are selected consecutively in the second region. The surface of the second active layer away from the positive electrode current collector is the second surface. Based on the position of the second surface, a second line parallel to the second surface is drawn in the direction closer to the positive electrode current collector. The distance from the second line to the second surface in the thickness direction of the positive electrode sheet accounts for less than 20% of the total thickness of the positive electrode coating. The area formed by the second line and the second surface is the second region.

[0050] The particle size distribution of the first lithium iron phosphate particles in the first active layer satisfies the following relationship: 2 < (Dv90 - Dv10) / Dv50 < 10, 400 nm ≤ Dv50 ≤ 1500 nm. The particle size distribution of the second lithium iron phosphate particles in the second active layer satisfies the following relationship: 0.1 < (Dv90 - Dv10) / Dv50 < 2, 200 nm ≤ Dv50 < 400 nm. nm, meaning the Dv50 of the first lithium iron phosphate particle in the first active layer is greater than the Dv50 of the second lithium iron phosphate particle, and the (Dv90-Dv10) / Dv50 of the first lithium iron phosphate particle in the first active layer is greater than the (Dv90-Dv10) / Dv50 of the second lithium iron phosphate particle in the second active layer, allows the porosity of the second active layer to be greater than that of the first active layer. This is beneficial for increasing the diffusion rate of lithium ions in the second active layer and reducing the diffusion resistance of ions in the first active layer to the second active layer. Furthermore, the particle size distribution of the first lithium iron phosphate particle and the particle size distribution of the second lithium iron phosphate particle are also considered. Under the synergistic effect of the diameter distribution, while ensuring the pore distribution of the first active layer, the gap between the pore distribution of the first and second active layers can be further widened, thereby improving the pore distribution of the second active layer, increasing the diffusion rate of lithium ions in the second active layer, and improving the high-temperature storage performance and charge-discharge performance of the battery. At the same time, it can also increase the compaction density of the first active layer, thereby improving the continuity and integrity of the conductive network in the first active layer while ensuring that the battery has a high energy density, thus further improving the electron transport rate in the first active layer, and further improving the charge-discharge performance and high-temperature storage performance of the battery.

[0051] By controlling the particle size distribution of the first lithium iron phosphate particles in the first active layer and the second lithium iron phosphate particles in the second active layer, and through the synergistic cooperation of the first lithium iron phosphate particles with specific particle size distributions in the first active layer and the second lithium iron phosphate particles with specific particle size distributions in the second active layer, the compaction density of the first active layer is increased while ensuring a suitable pore distribution, thus ensuring a high energy density of the battery. This further improves the electron transport rate in the first active layer and increases the pore distribution in the second active layer, making the pore distribution in the second active layer larger than that in the first active layer. This further improves the ion transport rate in the second active layer, thereby enhancing the battery's charge-discharge performance and high-temperature storage performance. Moreover, the third coating located between the first and second active layers can also buffer the volume change of the positive electrode active material during battery cycling, while adsorbing HF generated during cycling, thus improving the battery's cycle performance.

[0052] In some instances, the first lithium titanium aluminum phosphate material includes a core and a carbon coating layer on the surface of the core. By providing a coating layer on the surface of the first lithium titanium aluminum phosphate material, the electronic conductivity of the first lithium titanium aluminum phosphate material can be further improved, thereby further enhancing its electron transport dynamics.

[0053] In some instances, the thickness of the carbon coating layer is 2nm-10nm (e.g., 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm). Controlling the average thickness of the carbon coating layer within the above range can improve the electron transport rate through the first lithium titanium aluminum phosphate material. Furthermore, the carbon coating layer with the specific average thickness can not only absorb a certain amount of electrolyte and improve the ion transport rate, but also does not affect the speed at which ions pass through the carbon coating layer to the core of the first lithium titanium aluminum phosphate material. At the same time, the coating layer further hinders the erosion of the first lithium titanium aluminum phosphate material by the electrolyte, ensuring the stability of the interface of the first lithium titanium aluminum phosphate material and maintaining the overall structure of the first lithium titanium aluminum phosphate material.

[0054] In this invention, a projection scanning electron microscope (TEM) is used to observe the first lithium titanium aluminum phosphate material on the surface of the third coating. Ten points are randomly selected on the surface of a single lithium titanium aluminum phosphate particle, and the thickness of the carbon coating layer at each of the ten points is measured. The average value is taken as the thickness of the single lithium titanium aluminum phosphate material. Twenty lithium titanium aluminum phosphate particles are randomly selected and their thicknesses are tested. The average thickness of the carbon coating layer of the 20 lithium titanium aluminum phosphate particles is taken as the thickness of the carbon coating layer of the first lithium titanium aluminum phosphate material.

[0055] In some examples, the average particle size of the first lithium titanium aluminum phosphate material is 150nm-400nm (e.g., 150nm, 180nm, 200nm, 230nm, 250nm, 280nm, 300nm, 330nm, 350nm, 380nm, 400nm, or any two of the above values). An excessively large average particle size of the first lithium titanium aluminum phosphate material will reduce the electron and ion transport rates; an excessively small average particle size will reduce electrode stability and increase processing difficulty. Controlling the average particle size of the first lithium titanium aluminum phosphate material within the above range allows for a balance between ion and electron transport rates and electrode stability.

[0056] In this invention, the average particle size of the first lithium titanium aluminum phosphate material can be obtained by the following method: The positive electrode sheet is cut using an argon ion mill to obtain its cross-section. Using the position of the positive electrode current collector as a reference, a third line parallel to the positive electrode current collector is drawn in a direction away from it. The distance from the third line to the positive electrode current collector in the thickness direction of the positive electrode sheet accounts for 70% of the total thickness of the positive electrode coating. Using the position of the positive electrode current collector as a reference, a fourth line parallel to the positive electrode current collector is drawn in a direction away from it. The distance from the fourth line to the positive electrode current collector in the thickness direction of the positive electrode sheet accounts for 71% of the total thickness of the positive electrode coating. The distance from the third line to the fourth line is then calculated. The plane region where the line is located is the third region. Within the third region, the particle size of 500 lithium titanium aluminum phosphate materials is continuously measured, and the average value is taken as the average particle size. The above operation is repeated twice (understandably, the particle size of 500 lithium titanium aluminum phosphate materials is measured again during the repetition of the above operation), and the average value of the three measurements is taken as the final measurement result. In the cross-sectional SEM image of the positive electrode, the smallest square or rectangle that completely surrounds a lithium titanium aluminum phosphate material particle is drawn, that is, the square or long direction where the edge of the lithium titanium aluminum phosphate material particle is connected to the four sides of the square or rectangle is drawn. The length of one side of the square or the length of the long side of the rectangle is the particle size of the lithium titanium aluminum phosphate material.

[0057] In some instances, the ionic conductivity of the first lithium titanium aluminum phosphate material is 0.8 × 10⁻⁶. -3 S·cm-1.5×10 -3 S·cm. Controlling the ionic conductivity of the first lithium titanium aluminum phosphate material within the above range can satisfy the ion transport rate of the first lithium iron phosphate particles under high-rate charge and discharge, especially under high density conditions and poor electrolyte wetting, it can still maintain rapid lithium ion transport.

[0058] In this invention, the ionic conductivity of the first lithium titanium aluminum phosphate material can be obtained by the following method: after the first lithium titanium aluminum phosphate material is sputtered with gold, a certain weight (the weight is determined by the size of the mold) is weighed and placed in the mold. External pressure is applied to it. When the test pressure is 290-310 MPa (for example, 300 MPa), an electrochemical workstation is used to scan EIS at a scanning frequency of 1 MHz-1 Hz. The ionic conductivity can be directly calculated by combining the EIS data.

[0059] In some instances, the weight percentage of the first lithium aluminum titanium phosphate material in the third coating is 90%-95% (e.g., 90%, 90.5%, 91%, 91.5%, 92%, 92.5%, 93%, 93.5%, 94%, 94.5%, 95%, or within any two of the above values).

[0060] In some instances, the third coating includes a third adhesive comprising one or more of polybenzimidazole (PBI), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), modified polyvinylidene fluoride (modified PVDF), polyimide (PI), polyacrylic acid (PAA), polyamide-imide (PAI), and polyethyleneimine (PEI).

[0061] In some instances, the modified PVDF includes at least one of carboxyl and hydroxyl groups.

[0062] In some instances, the third adhesive includes one or more of polyacrylic acid (PAA), polyamide-imide (PAI), polyethyleneimine (PEI), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), modified polyvinylidene fluoride including at least one of carboxyl and hydroxyl groups, and mixed adhesives of polyvinylidene fluoride and polyimide.

[0063] In some instances, the third adhesive in the third coating comprises 2%-5% by weight (e.g., 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, 3.5%, 4%, 4.5%, 5%, or within any two of the above values).

[0064] In some instances, the third coating includes a third conductive agent, which includes one or more of carbon black (SP), carbon nanotubes, graphene, and graphite.

[0065] In some instances, the third conductive agent includes carbon black and carbon nanotubes.

[0066] In this invention, the carbon nanotubes include one or more of single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0067] In some instances, the weight percentage of the third conductive agent in the third coating is 2%-5% (e.g., 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or within any two of the above values).

[0068] In some instances, the first lithium iron phosphate particle comprises lithium iron phosphate, LiFe... (1-α) M 1 α PO4, M 1Including one or more of Ti, V, Mg, and Mn, 0.001≤α<0.01 (e.g., 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01).

[0069] In some instances, the second lithium iron phosphate particle comprises lithium iron phosphate, LiFe... (1-β) M 2 α PO4, M 2 Including one or more of Ti, V, Mg, and Mn, 0.001≤β<0.01 (e.g., 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01).

[0070] In some instances, the surface of the first lithium iron phosphate particle includes a first coating layer comprising elemental carbon. It is understood that at least a portion of the surface of the first lithium iron phosphate particle includes the first coating layer.

[0071] In some instances, the entire surface of the first lithium iron phosphate particle includes a first coating layer.

[0072] In some instances, the first coating layer is a carbon coating layer.

[0073] In some instances, the surface of the second lithium iron phosphate particle includes a second coating layer comprising elemental carbon and elemental nitrogen. This allows for a higher degree of graphitization in the second lithium iron phosphate particle, resulting in a denser second coating layer, thereby improving the electronic conductivity of the second lithium iron phosphate particle and enhancing electron transport in the second active layer. It is understood that at least a portion of the surface of the second lithium iron phosphate particle includes this second coating layer.

[0074] In some instances, the entire surface of the second lithium iron phosphate particle includes a second coating layer.

[0075] In some instances, the ratio of the D peak intensity to the G peak intensity in the Raman spectrum of the first lithium iron phosphate particle is I. 1 It is between 0.95 and 1.05 (e.g., 0.95, 0.96, 0.97, 0.98, 0.99, 1, 1.01, 1.02, 1.03, 1.04, 1.05, or within any two of the above values).

[0076] In some instances, the ratio of the D peak intensity to the G peak intensity in the Raman spectrum of the second lithium iron phosphate particle is I. 2It is 0.85-0.95 (for example, 0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95 or within any two of the above values).

[0077] In some instances, the positive electrode satisfies the following relationship: I 1 >I 2 , among which, I 1 I is the ratio of the D peak intensity to the G peak intensity in the Raman spectrum of the first lithium iron phosphate particle. 2 This represents the ratio of the D peak intensity to the G peak intensity in the Raman spectrum of the second lithium iron phosphate particle. The first active layer is closer to the positive electrode current collector than the second active layer. Electron transport resistance is relatively low in the first active layer, while electrons in the second active layer need to overcome a longer transport distance to reach the positive electrode. The positive electrode is controlled to satisfy the following relationship: I 1 >I 2 This can improve the electron transport rate in the second active layer, enhance the electron conductivity in the positive electrode, reduce the risk of local concentration polarization inside the positive electrode, and further improve the battery's charge-discharge performance and high-temperature storage performance.

[0078] According to some specific implementation methods, the positive electrode sheet satisfies the following relationship: I 1 >I 2 The ratio of the D peak intensity to the G peak intensity in the Raman spectrum of the first lithium iron phosphate particle is I. 1 The ratio of the intensity of the D peak to the intensity of the G peak in the Raman spectrum of the second lithium iron phosphate particle is 0.95-1.05. 1 It is 0.85-0.95.

[0079] In this invention, by controlling the first coating layer to include elemental carbon and the second coating layer to include elemental carbon and elemental nitrogen, the positive electrode can satisfy the following relationship: I 1 >I 2 That is, the first lithium iron phosphate particle has a lower degree of graphitization, the second lithium iron phosphate particle has a higher degree of graphitization, the second coating layer is more dense, and the electronic conductivity is better.

[0080] In some instances, 0.05 ≤ I 1 -I 2≤0.25 (e.g., 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.18, 0.2, 0.23, 0.25, or within any two of the above values). This can further improve the electron conductivity in the positive electrode, further reduce the risk of local concentration polarization within the positive electrode, and further improve the battery's charge / discharge performance and high-temperature storage performance.

[0081] According to some specific implementation methods, the positive electrode sheet satisfies the following relationship: 0.05≤I 1 -I 2 ≤0.15, the ratio of the D peak intensity to the G peak intensity in the Raman spectrum of the first lithium iron phosphate particle is I 1 The ratio of the intensity of the D peak to the intensity of the G peak in the Raman spectrum of the second lithium iron phosphate particle is 0.95-1.05. 1 It is 0.85-0.95.

[0082] In this invention, the Raman spectra of the first lithium iron phosphate particle and the Raman spectra of the second lithium iron phosphate particle can be obtained by spot scanning the first lithium iron phosphate particle and the second lithium iron phosphate particle respectively using a Raman spectrometer, wherein the D peak is located at approximately 1250 cm⁻¹. -1 -1450cm -1 (For example, 1250cm) -1 1280cm -1 1300cm -1 1330cm -1 1350cm -1 1380cm -1 1400cm -1 1430cm -1 Or 1450cm -1 It reflects the defects and disordered structure of the material. The higher the defect density, the greater the intensity of the D peak, and the G peak is located at approximately 1480 cm⁻¹. -1 -1680cm -1 (For example, 1480cm) -1 1500cm -1 1530cm -1 1550cm -1 1580cm -1 1600cm -1 1630cm -1 1650cm -1 Or 1680cm -1 Near ) it reflects SP 2The ordered vibrations of hybrid carbon indicate that a higher intensity of the G peak indicates better crystallinity of the material. The ratio of the D peak intensity to the G peak intensity is I. d / I g Reflects the degree of graphitization of the material, I d / I g The smaller the value, the fewer defects there are in the material, the higher the degree of graphitization, and the better the electrical conductivity.

[0083] In some instances, the resistivity of the first lithium iron phosphate particle is 5 Ω·cm to 25 Ω·cm (e.g., 5 Ω·cm, 8 Ω·cm, 10 Ω·cm, 13 Ω·cm, 15 Ω·cm, 18 Ω·cm, 20 Ω·cm, 23 Ω·cm, 25 Ω·cm or within any two of the above values).

[0084] In some instances, the resistivity of the second lithium iron phosphate particle is 1 Ω·cm to 5 Ω·cm (e.g., 1 Ω·cm, 1.5 Ω·cm, 2 Ω·cm, 2.5 Ω·cm, 3 Ω·cm, 3.5 Ω·cm, 4 Ω·cm, 4.5 Ω·cm, 5 Ω·cm or within any two of the above values).

[0085] In some instances, the resistivity of the first lithium iron phosphate particle is greater than that of the second lithium iron phosphate particle. Controlling the resistivity of the first lithium iron phosphate particle to be greater than that of the second lithium iron phosphate particle, i.e., having a lower resistivity in the second lithium iron phosphate particle, facilitates the diffusion of electrons from the second active layer towards the positive electrode current collector. Furthermore, since the first active layer is closer to the positive electrode current collector, the electron diffusion of the first lithium iron phosphate particle has a certain advantage, and even if the resistivity of the first lithium iron phosphate particle is higher, its impact on electron diffusion is relatively small.

[0086] According to some specific implementation methods, the resistivity of the first lithium iron phosphate particle is greater than that of the second lithium iron phosphate particle, the resistivity of the first lithium iron phosphate particle is 5Ω·cm-25Ω·cm, and the resistivity of the second lithium iron phosphate particle is 1Ω·cm-5Ω·cm.

[0087] In this invention, the resistivity of both the first and second lithium iron phosphate particles can be measured using a four-probe powder resistivity meter. During the test, the pressure is gradually increased, and the value at 50.93 MPa is taken as the test result. The first and second lithium iron phosphate particles can be obtained by, for example, discharging the battery to 0% SOC (e.g., discharging the battery to 2V), disassembling and removing the positive electrode sheet, soaking it in dimethyl carbonate (DMC) solvent for 12 hours, then rinsing it with DMC solvent to remove the lithium salt adhering to the positive electrode sheet, calcining the positive electrode sheet in air at 450°C for 2-4 hours, and then scraping the lithium iron phosphate particles off the positive electrode sheet with a ceramic knife. The lithium iron phosphate particles peeled off from the first active layer are the first lithium iron phosphate particles, and the lithium iron phosphate particles peeled off from the second active layer are the second lithium iron phosphate particles.

[0088] In some instances, the resistivity of the first active layer at 26 MPa is 10 Ω·cm to 25 Ω·cm (e.g., 10 Ω·cm, 13 Ω·cm, 15 Ω·cm, 18 Ω·cm, 20 Ω·cm, 23 Ω·cm, 25 Ω·cm or within any two of the above values).

[0089] In some instances, the resistivity of the second active layer at 26 MPa is 4 Ω·cm to 8 Ω·cm (e.g., 4 Ω·cm, 4.5 Ω·cm, 5 Ω·cm, 5.5 Ω·cm, 6 Ω·cm, 6.5 Ω·cm, 7 Ω·cm, 7.5 Ω·cm, 8 Ω·cm or within any two of the above values).

[0090] In this invention, the resistivity of both the first active layer and the second active layer can be measured using a resistivity meter. For example, the first and second active layers can be peeled off from the positive electrode coating, cleaned with DMC, and then degummed and pulverized at high temperature to form electrode sheets containing only the first active layer and electrode sheets containing only the second active layer, respectively. Then, the resistivity can be measured using a resistivity meter.

[0091] In some instances, the resistivity of the positive electrode at 26 MPa is 5 Ω·cm to 15 Ω·cm (e.g., 5 Ω·cm, 6 Ω·cm, 7 Ω·cm, 8 Ω·cm, 9 Ω·cm, 10 Ω·cm, 11 Ω·cm, 12 Ω·cm, 13 Ω·cm, 14 Ω·cm, 15 Ω·cm).

[0092] In this invention, the resistivity of the positive electrode can be obtained by measuring a resistivity meter.

[0093] In some instances, the first lithium iron phosphate particle is a primary particle, and the second lithium iron phosphate particle includes primary particles and / or secondary particles.

[0094] In some instances, the mechanical strength of the secondary particles of the second lithium iron phosphate particle is greater than 40 MPa (e.g., 41 MPa, 45 MPa, 50 MPa, 55 MPa, 60 MPa, 70 MPa, 75 MPa, 80 MPa, 85 MPa, 90 MPa, 95 MPa, 100 MPa, 105 MPa, 110 MPa, 115 MPa, 120 MPa, 125 MPa, 130 MPa, 135 MPa, 140 MPa, 145 MPa, 150 MPa).

[0095] In this invention, the mechanical strength of the secondary particles can be obtained by testing with a mechanical strength tester. Specifically, the secondary particles are gradually pressurized using a mechanical strength tester, and the pressure strength at which the secondary particles break is taken as the test result.

[0096] In some instances, the interior of the secondary particles is filled with pores, and the carbon layer between the primary particles inside the secondary particles enables conductive connections within the secondary particles, thereby facilitating the diffusion of ions on the surface of the secondary particles.

[0097] In some instances, the weight percentage of the first lithium iron phosphate particles in the first active layer is 96%-98.5% (e.g., 96%, 96.3%, 96.5%, 96.8%, 97%, 97.3%, 97.5%, 97.8%, 98%, 98.3%, 98.5%, or within any two of the above values).

[0098] In some instances, the weight percentage of the second lithium iron phosphate in the second active layer is 93%-97% (e.g., 93%, 93.5%, 94%, 94.5%, 95%, 95.3%, 95.5%, 95.8%, 96%, 96.3%, 96.5%, 96.8%, 97% or within any two of the above values).

[0099] In some instances, the weight percentage of the first lithium iron phosphate particles in the first active layer is greater than the weight percentage of the second lithium iron phosphate particles in the second active layer. Maintaining this weight percentage ensures the overall capacity of the cathode, thereby ensuring a higher energy density for the battery.

[0100] In some instances, the first active layer includes a first adhesive, which is a polyimide.

[0101] In some instances, the polyimide in the first active layer comprises 1%-1.4% by weight (e.g., 1%, 1.1%, 1.2%, 1.3%, 1.4%, or any combination of the above values).

[0102] According to some specific embodiments, the first binder is polyimide, and the weight percentage of the polyimide in the first active layer is 1%-1.4%. By setting the first binder to polyimide and controlling the weight percentage of the polyimide in the first active layer, the adhesion of the first active layer can be ensured while increasing the weight percentage of the second lithium iron phosphate particles, thereby improving the energy density of the battery.

[0103] In some instances, the peak value of the first weight loss peak in the thermogravimetric analysis (TGA) spectrum of the first active layer is located between 550℃ and 600℃ (e.g., 550℃, 560℃, 570℃, 580℃, 590℃, 600℃, or any two of these values). In the TGA analysis of the first active layer, the initial decomposition temperature is 500℃-550℃, and the decomposition stage involves a primary weight loss process, with the peak value of the first weight loss peak representing the maximum weight loss rate. The presence of the first weight loss peak between 550℃ and 600℃ indicates good adhesion and strong thermal stability of the first active layer.

[0104] In some instances, the second active layer includes a second adhesive, which is polyvinylidene fluoride.

[0105] In some instances, the weight percentage of polyvinylidene fluoride (PVDF) in the second active layer is 1.7%-2.1% (e.g., 1.7%, 1.8%, 1.9%, 2%, 2.1%, or any two of these values). Because the average particle size of the second lithium iron phosphate particles in the second active layer is relatively small, controlling the weight percentage of PVDF in the second active layer within the aforementioned range ensures that the second active layer will not crack during electrode processing.

[0106] In some instances, the peak value of the second weight loss peak in the thermogravimetric analysis (TGA) spectrum of the second active layer is located between 400°C and 450°C (e.g., 400°C, 410°C, 420°C, 430°C, 440°C, 450°C, or any two of these values). In the TGA analysis of the second active layer, the initial decomposition temperature is 460°C–500°C, and a major weight loss process occurs during the decomposition stage, with the peak value of the second weight loss peak representing the maximum weight loss rate. The peak value of the second weight loss peak being located between 400°C and 450°C indicates that the second active layer includes polyvinylidene fluoride (PVDF).

[0107] According to some specific embodiments, the first binder is polyimide, the second binder is polyvinylidene fluoride (PVDF), and the third binder includes one or more of polyacrylic acid (PAA), polyamide-imide (PAI), polyethyleneimine (PEI), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), modified PVDF including at least one of carboxyl and hydroxyl groups, and mixed binders of PVDF and polyimide. In this case, the third binder can form dipoles with the second binder PVDF and F atoms in the second active layer, and can also form hydrogen bonds with the first binder polyimide in the first active layer, thereby achieving a bridging effect, further improving the adhesion between the first active layer, the third coating layer, and the second active layer, improving the overall adhesion of the positive electrode coating, and further improving the cycle performance of the battery.

[0108] In some instances, the first active layer includes a first conductive agent, which includes carbon black.

[0109] In some instances, the carbon black in the first active layer comprises 0.5%-1% by weight (e.g., 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1% or within any two of the above values).

[0110] In some instances, the second active layer includes a second conductive agent, which may include carbon black, carbon nanotubes, and graphene. The three-dimensional conductive system constructed from carbon black, carbon nanotubes, and graphene can enhance the rapid conduction of electrons transported from the second lithium iron phosphate particles in the second active layer to the first active layer, and then facilitate their export through the positive electrode current collector, thereby reducing the internal electron diffusion resistance and further improving electron transport dynamics.

[0111] In some instances, based on the total weight of the second active layer, the carbon black accounts for 0.6%-0.8% by weight (e.g., 0.6%, 0.63%, 0.65%, 0.68%, 0.7%, 0.73%, 0.75%, 0.78%, 0.8%, or any two of the above values), and the carbon nanotubes account for 0.3%-0.6% by weight (e.g., 0.3%, 0.33%, 0.35%, 0.38%, 0.4%, 0.43%, 0.45%, 0.4%). The graphene content is 0.2%-0.6% by weight (e.g., 0.2%, 0.23%, 0.25%, 0.28%, 0.3%, 0.35%, 0.38%, 0.4%, 0.43%, 0.45%, 0.48%, 0.5%, 0.53%, 0.55%, 0.58%, 0.6%, or within the range of any two of the above values).

[0112] In some instances, the first active layer further includes a second lithium titanium aluminum phosphate material, which comprises at least one of a lithium titanium aluminum phosphate-carbon composite material and a lithium titanium aluminum phosphate-carbon silicon composite material. It is understood that the lithium titanium aluminum phosphate-carbon composite material can be prepared by mixing and grinding a carbon source and lithium titanium aluminum phosphate material, followed by spray drying, high-temperature sintering coating, and pulverization / depolymerization. Alternatively, it can be prepared by directly atomizing the carbon source and coating the lithium titanium aluminum phosphate material particles with CVD vapor deposition. The chemical formula of the lithium titanium aluminum phosphate-carbon composite material is Li. (1+x) Al x Ti (2-x) (PO4)3-C, where 0 < x < 1 (e.g., 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9).

[0113] In this invention, the lithium titanium aluminum phosphate-carbon composite material includes one or more of the following: a mixture of carbon materials and lithium titanium aluminum phosphate, and lithium titanium aluminum phosphate with at least a partial carbon coating on its surface.

[0114] In this invention, the lithium titanium aluminum phosphate-silicon carbon composite material includes a core and a carbon coating layer on the surface of the core. The core is lithium titanium aluminum phosphate containing doped elements, including element Si.

[0115] In some instances, the weight percentage of the second lithium titanium aluminum phosphate material in the first active layer is 0-1% (e.g., 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or within any two of the above values). When the weight percentage of the lithium titanium aluminum phosphate-carbon composite material in the first active layer is 0, it indicates that the lithium titanium aluminum phosphate-carbon composite material is not present in the first active layer.

[0116] In some instances, the second active layer also includes a third lithium titanium aluminum phosphate material.

[0117] In some instances, the third lithium titanium aluminum phosphate material includes at least one of lithium titanium aluminum phosphate-carbon composite material, lithium titanium aluminum phosphate-carbon silicon composite material, and lithium titanium aluminum phosphate. The chemical formula of the lithium titanium aluminum phosphate is Li. (1+y) Al y Ti (2-y) (PO4)3, where 0 < y < 1 (e.g., 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 or 0.9).

[0118] According to some specific implementations, the first lithium titanium aluminum phosphate material includes a core and a carbon coating layer on the surface of the core. The core is lithium titanium aluminum phosphate containing doped elements, including element Si. The second lithium titanium aluminum phosphate material includes a lithium titanium aluminum phosphate-carbon composite material. The third lithium titanium aluminum phosphate material includes lithium titanium aluminum phosphate.

[0119] In some instances, the weight percentage of lithium titanium aluminum phosphate in the second active layer is 0-0.5% (e.g., 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, or any two of the above values). When the weight percentage of lithium titanium aluminum phosphate in the second active layer is 0, it indicates that lithium titanium aluminum phosphate is not present in the second active layer.

[0120] In some instances, the compaction density of the first active layer is 2.65 g / cm³. 3 -3.1g / cm 3 (For example, 2.65g / cm) 3 2.68g / cm 3 2.7g / cm 3 2.73 g / cm 3 2.75g / cm 3 2.78g / cm 3 2.8g / cm 3 2.85g / cm 3 2.9g / cm 32.95g / cm 3 3g / cm 3 3.05g / cm 3 3.1g / cm 3 Or it falls within the range formed by any two of the above values.

[0121] In some instances, the compaction density of the second active layer is 2.2 g / cm³. 3 -2.5g / cm 3 (For example, 2.2g / cm) 3 2.3g / cm 3 2.4g / cm 3 2.5g / cm 3 Or it falls within the range formed by any two of the above values.

[0122] In some instances, the compaction density of the first active layer is greater than that of the second active layer. Controlling the compaction density of the first active layer to be greater than that of the second active layer ensures a higher energy density for the battery, while a lower compaction density allows for a larger porosity distribution in the second active layer.

[0123] In some instances, the ratio of the thickness of the first active layer, the thickness of the third coating, and the thickness of the second active layer is (6-7):(0.1-1.5):(1.5-2.9) (e.g., 6:1.1:2.9, 7:1:2, 6:1.5:2.5, 6.5:1.5:2, 6.5:1:2.5, 6:1.2:2.8). Controlling the ratio of the thickness of the first active layer, the third coating, and the second active layer within the above range is beneficial for the transport and diffusion of electrons and ions in the positive electrode coating, while also balancing the energy density of the battery.

[0124] In this invention, the ratio of the thickness of the first active layer, the thickness of the third coating, and the thickness of the second active layer can be obtained by scanning a cross-sectional view of the positive electrode using a scanning electron microscope, for example, SEM.

[0125] In some instances, the compaction density of the positive electrode coating is 2.7 g / cm³. 3 -3g / cm 3 (For example, 2.7g / cm) 3 2.75g / cm 3 2.76 g / cm 3 2.77 g / cm 3 2.78g / cm 3 2.79 g / cm 32.8g / cm 3 2.85g / cm 3 2.9g / cm 3 2.95g / cm 3 3g / cm 3 Or it falls within the range formed by any two of the above values.

[0126] In this invention, the compaction density can be obtained by testing with a compaction density tester, and the control pressure is 3T.

[0127] A second aspect of the present invention provides a battery comprising an electrolyte and a positive electrode as described in the first aspect of the present invention.

[0128] Except for the positive electrode, all materials used in the battery can be manufactured in accordance with the methods described in this art, and can achieve high charge-discharge performance, high high-temperature storage performance, and high cycle performance.

[0129] In some instances, the electrolyte comprises a lithium salt, including one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide. The silicon element in the modified lithium titanium aluminum phosphate material preferentially adsorbs trace amounts of HF generated from the decomposition of lithium salts (such as LiPF6) in the electrolyte. This adsorption process not only reduces but can even eliminate the risk of HF erosion of the electrode interfaces (including the positive and negative electrode interfaces) and the modified lithium titanium aluminum phosphate material.

[0130] In some instances, the lithium salt constitutes 16%-20% by weight in the electrolyte (e.g., 16%, 16.5%, 17%, 17.5%, 18%, 18.5%, 19%, 19.5%, 20%, or any combination of the above values). Using a higher concentration of lithium salt, i.e., lithium salt with the aforementioned weight percentage, ensures efficient conductivity during high-rate cycling, allowing the silicon element in the modified lithium titanium aluminum phosphate material to interact with the lithium salt, thereby achieving the adsorption of HF.

[0131] In some instances, the electrolyte includes a phosphorus-containing additive, which includes one or more of lithium difluorophosphate, lithium tetrafluorophosphate, lithium difluorodioxarate phosphate, lithium tripolyphosphate, tris(trimethylsilane) phosphate, and tris(trimethylsilane) phosphite. The modified lithium titanium aluminum phosphate material includes elements Al and phosphate. The phosphorus-containing additive preferentially adsorbs onto the surface of the modified lithium titanium aluminum phosphate material in the electrolyte and chemically reacts with the aluminum and free phosphate on its surface, thereby promoting the in-situ formation of a uniform lithium phosphate interface layer on the surface of the modified lithium titanium aluminum phosphate material. This interface layer exhibits excellent lithium-ion conduction electrochemical properties, effectively promoting lithium-ion interfacial conduction and reducing the desolvation energy barrier at the interface. Simultaneously, this interface layer can suppress side reactions between the modified lithium titanium aluminum phosphate material and the electrolyte, enhancing interface stability. Therefore, the modified lithium titanium aluminum phosphate material, together with the phosphorus-containing additive in the electrolyte, can improve the ion transport efficiency and rate performance of the battery by optimizing the lithium-ion transport kinetics at the solid-liquid interface.

[0132] In some instances, the phosphorus-containing additive in the electrolyte accounts for 0.1% to 3% by weight (e.g., 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or within any two of the above values).

[0133] In some instances, the electrolyte includes sulfur-containing additives that can participate in the in-situ formation of an interface film rich in Li2S and Li3P on the surface of lithium iron phosphate particles. This interface film can not only suppress Ti in the lithium iron phosphate particles, but also... 4+ Restore to Ti 3+ This reduces the occurrence of side reactions, and the interface film also has high lithium-ion transport performance, thereby reducing the increase in interface impedance and improving the charge-discharge performance and cycle performance of the battery. The sulfur-containing additive includes one or more of vinylene sulfate and vinyl sulfate.

[0134] In some instances, the sulfur-containing additive in the electrolyte comprises 1%-5% by weight (e.g., 1%, 2%, 3%, 4%, 5%, or any combination of two of these values). Controlling the weight percentage of the sulfur-containing additive in the electrolyte ensures that the electrolyte has a sufficient dielectric constant to promote the dissociation of lithium salts and to form a stable interfacial film on the electrode surface.

[0135] In some instances, the electrolyte includes a first solvent, which includes one or more of ethylene carbonate and propylene carbonate.

[0136] In some instances, the weight percentage of the first solvent in the electrolyte is 15%-60% (e.g., 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, or within any two of the above values).

[0137] In some instances, the electrolyte includes a second solvent, which includes one or more of propyl propionate, ethyl propionate, ethyl acetate, methyl acetate, acetonitrile, acetone, methyl ethyl carbonate, and dimethyl carbonate.

[0138] In some instances, the weight percentage of the second solvent in the electrolyte is 20%-70% (e.g., 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or within any two of the above values). Controlling the weight percentage of the second solvent in the electrolyte within the above range can reduce the overall viscosity of the electrolyte and the solvation energy barrier of lithium ions, thereby accelerating the migration rate of lithium ions and significantly reducing the desolvation energy of lithium ions at the interface, thus improving the lithium ion transport rate.

[0139] In some instances, the battery further includes a negative electrode and a separator, the separator being located between the positive electrode and the negative electrode.

[0140] The negative electrode sheet can be a conventional negative electrode sheet in the art. For example, the negative electrode sheet includes a current collector and a negative electrode coating located on one or both sides of the current collector. The current collector is a copper foil or a composite copper foil. The negative electrode coating includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder. The negative electrode conductive agent includes one or more of graphite, carbon black, graphene, carbon nanofibers, and carbon nanotubes, preferably carbon black. The negative electrode binder is one or more of styrene-butadiene rubber, styrene-acrylic emulsion, polyacrylic acid, sodium carboxymethyl cellulose, or lithium carboxymethyl cellulose, preferably lithium carboxymethyl cellulose and lithium polyacrylate. The negative electrode active material is at least one of graphite, silicon, silicon suboxide, hard carbon, soft carbon, and lithium titanate, preferably graphite.

[0141] The separator can be any material suitable for secondary battery separators in the art, such as, but not limited to, at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers.

[0142] 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.

[0143] The following examples illustrate the positive electrode sheet and battery of the present invention.

[0144] Example 1 (1) Positive electrode plate Ingredient preparation: First active layer: 98 parts by weight of first lithium iron phosphate particles (in primary particle form), lithium titanium aluminum phosphate-carbon composite material (chemical formula Li). 1.3 Al 0.3 Ti 1.7 -C) 0.5 parts by weight, carbon black 0.5 parts by weight, polyimide 1 part by weight; Second active layer: 95.5 parts by weight of second lithium iron phosphate particles (in primary particle form), lithium titanium aluminum phosphate (chemical formula Li) 1.3 Al 0.3 Ti 1.7 0.4 parts by weight, carbon black 0.8 parts by weight, carbon nanotubes 0.6 parts by weight, graphene 0.6 parts by weight, polyvinylidene fluoride 2.1 parts by weight; The third coating consists of 95.5 parts by weight of lithium titanium aluminum phosphate material (with an average particle size of 220 nm, and the weight percentage of element Si in the lithium titanium aluminum phosphate material is 0.5% and the weight percentage of element C is 1.5%), 1.5 parts by weight of carbon black, 1 part by weight of carbon nanotubes, and 2 parts by weight of PVDF. The components of the first active layer are mixed in N-methylpyrrolidone (NMP) solvent and continuously stirred under the action of a stirrer to form a homogeneous and flowing first slurry. The components of the second active layer are mixed in N-methylpyrrolidone (NMP) solvent and continuously stirred under the action of a stirrer to form a homogeneous and flowing second slurry. The components of the third active layer are mixed in N-methylpyrrolidone (NMP) solvent and continuously stirred under the action of a stirrer to form a homogeneous and flowing third slurry. The first, second, and third slurries are filtered and impurity removed, and then fed into the inner mold cavity of a three-layer coating die for the first, third, and second active layers. According to the thickness ratio of the first, third, and second active layers of 7:1:2, the first active layer is first coated on both sides of the positive electrode current collector (aluminum foil) of the bottom test piece. After passing the coating, the third and second slurry test pieces are added on the bottom coating. After the test pieces pass the coating, continuous coating is performed to obtain the positive electrode sheet of the present invention. The compaction density of the first active layer is 2.75 g / cm³. 3 The compaction of the second active layer is 2.25 g / cm³. 3 The compaction density of the positive electrode is 2.75 g / cm³. 3 .

[0145] (2) Negative electrode plate Graphite, conductive carbon black, sodium carboxymethyl cellulose (CMC), and polyacrylic acid were mixed in an aqueous solvent at a weight ratio of 97:1:1.5:0.5 and continuously stirred under the action of a stirrer to form a homogeneous and fluid negative electrode slurry. Subsequently, the slurry was coated on both sides of the negative electrode current collector (copper foil), and then dried, rolled, and slit to obtain the negative electrode sheet.

[0146] (3) Diaphragm A base membrane (polyethylene) is used, and a ceramic layer (the ceramic particles in the ceramic layer are alumina) is coated on one side of the base membrane. A PVDF adhesive layer is coated on the surface of the ceramic layer and the other side of the base membrane. The thickness of the membrane is 11 μm.

[0147] (4) Electrolyte In an argon-filled glove box (moisture <1 ppm, oxygen <1 ppm), ethylene carbonate (EC), propylene carbonate (PC), and ethyl propionate were mixed to form a homogeneous solvent. LiPF6, lithium bis(fluorosulfonyl)imide, lithium difluorophosphate, and ethylene sulfate were then slowly added and stirred until homogeneous to obtain the electrolyte. The electrolyte contained 10 wt% LiPF6, 4 wt% lithium bis(fluorosulfonyl)imide, 1.4 wt% lithium difluorophosphate, 2.5 wt% ethylene sulfate, 25.1 wt% ethylene carbonate, 25 wt% propylene carbonate, and 32% ethyl propionate.

[0148] (5) Lithium-ion batteries The positive electrode sheet prepared in step (1) and the negative electrode sheet prepared in step (2) are cut into the designed shape by die cutting. The positive electrode sheet, the separator prepared in step (3) and the negative electrode sheet are stacked alternately in a Z-shaped stacking method. After the electrode core and electrode tab are ultrasonically welded, the positive electrode is welded to the aluminum electrode tab and the negative electrode is welded to the copper electrode tab. Then, they are placed in a shell made of aluminum-plastic film by heat sealing. The stacked core is sealed in the shell by heating. The electrolyte prepared in step (4) is injected and sealed. Formation, degassing, and resealing are performed to obtain a stacked lithium-ion battery.

[0149] Example 2 group This set of examples illustrates the effects of changes in the particle size distribution of the second lithium iron phosphate particles.

[0150] This embodiment is based on Embodiment 1, except that the particle size distribution of the second lithium iron phosphate particles is changed, as detailed in Table 1-1. It is understood that parameters that are slightly altered due to the particle size distribution of the second lithium iron phosphate particles are not listed separately, but are all within the scope of this invention.

[0151] Table 1-1 Example 2 group This set of examples illustrates the effects of changes in the particle size distribution of the first lithium iron phosphate particles.

[0152] This embodiment is based on Embodiment 1, except that the particle size distribution of the first lithium iron phosphate particles is changed, as detailed in Tables 1-2. It is understood that parameters that are slightly altered due to the particle size distribution of the first lithium iron phosphate particles are not listed separately, but are all within the scope of this invention.

[0153] Table 1-2 Example 4 group This set of examples is used to illustrate when I 1 -I 2 The impact of changes.

[0154] This implementation group follows Example 1, except that the process for preparing lithium iron phosphate particles is changed to modify I. 1 -I 2 For details, please refer to Table 1-3.

[0155] Table 1-3 Example 5 group This set of embodiments is used to illustrate the effects of changes in the weight percentage of element Ti in the first active layer, the second active layer, and the third coating layer.

[0156] This embodiment group is based on Embodiment 1, except that by adjusting one or more of the following: the weight ratio of lithium titanium aluminum phosphate-carbon composite material in the first active layer, the preparation process of lithium titanium aluminum phosphate-carbon composite material, the weight ratio of lithium titanium aluminum phosphate in the second active layer, the preparation process of lithium titanium aluminum phosphate in the second active layer, the weight of lithium titanium aluminum phosphate material in the third coating, and the preparation process of lithium titanium aluminum phosphate material, the weight ratio of element Ti in the first active layer, the weight ratio of element Ti in the second active layer, and the weight ratio of element Ti in the third coating are changed. See Tables 1-4 for details.

[0157] Table 1-4 Example 6 group This set of embodiments is used to illustrate the effects of changes in the weight percentage of element Al in the first active layer, the second active layer, and the third coating layer.

[0158] This embodiment group is based on Embodiment 1, except that one or more of the following are adjusted: the weight ratio of lithium titanium aluminum phosphate-carbon composite material in the first active layer, the preparation process of lithium titanium aluminum phosphate-carbon composite material, the weight ratio of lithium titanium aluminum phosphate in the second active layer, the preparation process of lithium titanium aluminum phosphate in the second active layer, the weight of lithium titanium aluminum phosphate material in the third coating, and the preparation process of lithium titanium aluminum phosphate material. This changes the weight ratio of element Al in the first active layer, the weight ratio of element Al in the second active layer, and the weight ratio of element Al in the third coating. See Tables 1-5 for details.

[0159] Table 1-5 Example 7 The procedure was carried out in accordance with Example 1, except that the morphology of the second lithium iron phosphate particles was adjusted to that of secondary particles. Example 8 group This set of embodiments is used to illustrate the effects of changes in the weight ratio of the first lithium iron phosphate particles in the first active layer and the weight ratio of the second lithium iron phosphate particles in the second active layer.

[0160] The same procedure was followed as in Example 1, except that the weight ratio of the first lithium iron phosphate particles in the first active layer and the weight ratio of the second lithium iron phosphate particles in the second active layer were changed. See Tables 1-6 and 1-7 for details.

[0161] Table 1-6 Table 1-7 Example 9 group This set of examples illustrates the effects of changes in the weight percentage of lithium salt in the electrolyte.

[0162] Example 9a This embodiment is based on Example 1, except that the weight percentage of lithium salt in the electrolyte is adjusted to 16%, while keeping the weight ratio of each component in the lithium salt unchanged. Correspondingly, the weight percentage of the first solvent in the electrolyte is adjusted to ensure that the sum of the weight percentages of each component in the electrolyte is 100%.

[0163] Example 9b This embodiment is based on Example 1, except that the weight ratio of lithium salt in the electrolyte is adjusted to 20%, while keeping the weight ratio of each component in the lithium salt unchanged. Correspondingly, the weight ratio of the first solvent in the electrolyte is adjusted to ensure that the sum of the weight ratios of each component in the electrolyte is 100%.

[0164] Example 10 group This set of examples illustrates the effects of changes in the weight percentage of phosphorus-containing additives in the electrolyte.

[0165] Example 10a This embodiment is based on Example 1, except that the weight percentage of phosphorus-containing additives in the electrolyte is adjusted to 1%, and the weight percentage of the first solvent in the electrolyte is adjusted accordingly to ensure that the sum of the weight percentages of all components in the electrolyte is 100%.

[0166] Example 10b This embodiment is based on Example 1, except that the weight percentage of phosphorus-containing additives in the electrolyte is adjusted to 3%, and the weight percentage of the first solvent in the electrolyte is adjusted accordingly to ensure that the sum of the weight percentages of all components in the electrolyte is 100%.

[0167] Example 11 group This set of examples illustrates the effects of changes in the weight percentage of sulfur-containing additives in the electrolyte.

[0168] Example 11a This embodiment is based on Example 1, except that the weight percentage of sulfur-containing additives in the electrolyte is adjusted to 1%, and the weight percentage of the first solvent in the electrolyte is adjusted accordingly to ensure that the sum of the weight percentages of all components in the electrolyte is 100%.

[0169] Example 11b This embodiment is based on Example 1, except that the weight percentage of sulfur-containing additives in the electrolyte is adjusted to 5%, and the weight percentage of the first solvent in the electrolyte is adjusted accordingly to ensure that the sum of the weight percentages of all components in the electrolyte is 100%.

[0170] Example 12 The procedure was carried out in accordance with Example 1, except that the compaction density of the second active layer was 2.48 g / cm³. 3 The second active layer contains 1.7% polyvinylidene fluoride by weight; the third coating contains lithium titanium aluminum phosphate with an average particle size of 380 nm; the third conductive agent in the third coating contains 2% by weight; the first lithium iron phosphate particles have a powder resistivity of 21.13 Ω·cm; and the first active layer has a compaction density of 2.75 g / cm³. 3 The carbon black content in the first active layer is 1% by weight, the polyimide content in the first active layer is 1.4% by weight, and the thickness ratio of the first active layer, the third coating, and the second active layer is 8:1:1.

[0171] Example 13 The procedure was carried out in accordance with Example 1, except that the compaction density of the second active layer was 2.2 g / cm³. 3The second active layer contains 2.1% polyvinylidene fluoride by weight, the third coating contains lithium titanium aluminum phosphate material with an average particle size of 164 nm, the third conductive agent in the third coating contains 5% by weight, the first lithium iron phosphate particles have a powder resistivity of 5.4 Ω·cm, the first active layer contains 0.5% carbon black by weight, the first active layer contains 1% polyimide by weight, and the thickness ratio of the first active layer, the third coating, and the second active layer is 7.5:0.5:2.

[0172] Comparative Example 1 The procedure was carried out as in Example 1, except that the positive electrode was adjusted, specifically: Positive electrode sheet: Composition preparation: Refer to Example 1.

[0173] The components of the first active layer, the third coating layer, and the second active layer are mixed in N-methylpyrrolidone (NMP) solvent and continuously stirred under the action of a stirrer to form a homogeneous, flowing positive electrode slurry. The positive electrode slurry is then coated onto both sides of the positive electrode current collector (aluminum foil) to form a positive electrode sheet.

[0174] Comparative Example 2 The same procedure is followed as in Example 1, except that no third coating is provided between the first active layer and the second active layer, and the second active layer is located on the surface of the first active layer.

[0175] Comparative Example 3 The experiment was carried out in accordance with Example 1, except that the weight percentage of element Ti in the second active layer, T2, was 4679 ppm, the weight percentage of element Ti in the first active layer, T1, was 3059 ppm, and the weight percentage of element Ti in the third coating was 227600 ppm.

[0176] Comparative Example 4 The experiment was carried out in accordance with Example 1, except that the weight percentage of element Al in the second active layer, A2, was 981 ppm, the weight percentage of element Al in the first active layer, A1, was 517 ppm, and the weight percentage of element Al in the third coating was 6819 ppm.

[0177] Test case The batteries prepared in the examples and comparative examples were subjected to the following tests.

[0178] 1. Volumetric energy density test The battery was left to stand for 1 hour at (25±2)℃, then charged at a constant current of 0.5C to 3.8V, and then charged at a constant voltage of 3.8V to a current of 0.05C. It was then left to stand for 10 minutes; next, it was discharged at a constant current of 0.5C to 2.0V and left to stand for 10 minutes. The discharge capacity was recorded as C, the average discharge plateau voltage as V, the lithium-ion battery thickness as X, the lithium-ion battery length as Y, the lithium-ion battery width as Z, and the volumetric energy density as (C×V) / (X×Y×Z).

[0179] 2. High-temperature storage test 1) Initial capacity Q3 test: Place the battery in an environment of 25℃±2℃ and let it stand for 10 minutes; then, discharge it at 1C to the lower limit voltage of 2V, and let it stand for another 10 minutes; then, charge it at 1C constant current to the upper limit voltage of 3.8V (100% SOC), switch to constant voltage charging until the current cutoff is 0.05C, and let it stand for another 10 minutes; discharge it again at 1C constant current to the lower limit voltage of 2V, and record the discharge capacity at this time as the initial capacity Q3; (2) Test storage state: After completing the initial capacity test, let it stand for 10 minutes, charge it to full charge with 1C constant current, and then switch to constant voltage charging until the current is cut off at 0.05C; let the fully charged battery stand for 2 hours in an environment of 25℃±2℃. (3) High temperature storage capacity Q4 test: The battery was placed in a constant temperature chamber at 60℃±2℃ for 30 days. After storage, the sample was taken out and left to stand at 25℃±2℃ for 2 hours to allow the battery to return to room temperature. After the sample returned to room temperature, it was discharged at 1C constant current to the lower limit voltage of 2V. The discharge capacity at this time was recorded as the capacity Q4 after high temperature storage. The high temperature storage capacity retention rate is (Q4 / Q3)×100%.

[0180] 3. Charge and discharge performance test The lithium-ion battery was placed at 25℃±3℃, discharged at 1C to 2V, left to stand for 5 minutes, then charged at 1C constant current to the upper limit voltage (3.8V), then charged at 3.8V constant voltage to 0.05C, left to stand for 5 minutes; then discharged at 1C constant current to 2V, then charged at 5C constant current to the upper limit voltage (3.8V), and the constant current charging capacity was recorded as Q5. Then charged at 3.8V constant voltage to 0.05C, and the constant voltage charging capacity was recorded as Q6. The 5C constant current charging ratio = [Q5 / (Q5+Q6)]×100%.

[0181] 4. Room temperature cycling performance test The lithium-ion battery was placed at 25℃±3℃, then charged at a constant current of 1C to the upper limit voltage (3.8V), then charged at a constant voltage of 3.8V to 0.05C, and left to stand for 5 minutes; next, it was discharged at a constant current of 1C to 2V, and the discharge capacity at this point was recorded as Q1. After standing for 5 minutes, this constituted one charge-discharge cycle. After 500 such charge / discharge cycles, the discharge capacity Q2 of the lithium-ion battery after 500T cycles was recorded. The capacity retention rate (%) is then calculated as (Q2 / Q1)×100%.

[0182] The test results are recorded in Table 2.

[0183] Table 2 As can be seen from Table 2, by comparing the comparative examples and the embodiments, the battery prepared in the embodiments has a higher energy density, improved high-temperature storage capacity, improved 5C constant current charge ratio, and improved cycle capacity retention. This indicates that by setting a third coating including lithium titanium aluminum phosphate material between the first and second active layers, and controlling the weight ratio of element Ti and element Al in the first, second, and third active layers respectively, the battery's charge-discharge performance, high-temperature storage performance, and cycle performance are improved while maintaining a high energy density.

[0184] 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 positive electrode plate, characterized in that, 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 positive current collector. The positive electrode coating includes a first active layer located on the surface of the positive current collector, a second active layer located on the side of the first active layer away from the positive current collector, and a third coating located between the first active layer and the second active layer. The first active layer includes first lithium iron phosphate particles, the second active layer includes second lithium iron phosphate particles, and the third coating includes first lithium titanium aluminum phosphate material. The weight percentage of element Al in the third coating is greater than that of element Al in the first coating, and the weight percentage of element Al in the first coating is greater than that of element Al in the second coating. The weight percentage of element Ti in the third coating is greater than that of element Ti in the first coating, and the weight percentage of element Ti in the first coating is greater than that of element Ti in the second coating.

2. The positive electrode according to claim 1, wherein, The weight percentage of element Ti in the first active layer is 3000ppm-6000ppm; And / or, the weight percentage of element Ti in the second active layer is 2000ppm-5000ppm; And / or, the weight percentage of element Ti in the third coating is 200,000 ppm to 250,000 ppm; And / or, the weight percentage of element Al in the first active layer is 500ppm-1500ppm; And / or, the weight percentage of element Al in the second active layer is 0-1000 ppm; And / or, the weight percentage of element Al in the third coating is 5000ppm-8000ppm; And / or, the first lithium titanium aluminum phosphate material includes the elements Si and C; And / or, the first lithium titanium aluminum phosphate material includes a core and a carbon coating layer on the surface of the core, wherein the core is lithium titanium aluminum phosphate including doped elements, and the doped elements include element Si.

3. The positive electrode according to claim 1, wherein, The particle size distribution of the first lithium iron phosphate particles satisfies the following relationship: 2<(Dv90-Dv10) / Dv50<10, 400 nm≤Dv50≤1500 nm; And / or, the particle size distribution of the second lithium iron phosphate particles satisfies the following relationship: 0.1<(Dv90-Dv10) / Dv50<2, 200 nm≤Dv50<400 nm; And / or, the first active layer further includes a second lithium titanium aluminum phosphate material, preferably, the weight percentage of the second lithium titanium aluminum phosphate material in the first active layer is 0-1%; And / or, the second active layer further includes a third lithium titanium aluminum phosphate material, preferably, the weight percentage of the third lithium titanium aluminum phosphate material in the second active layer is 0-0.5%; And / or, the resistivity of the first lithium iron phosphate particle is greater than the resistivity of the second lithium iron phosphate particle.

4. The positive electrode sheet according to claim 1, wherein, The resistivity of the first lithium iron phosphate particle is 5 Ω·cm - 25 Ω·cm; And / or, the resistivity of the second lithium iron phosphate particle is 1 Ω·cm-5 Ω·cm; And / or, the first lithium titanium aluminum phosphate material includes a carbon coating layer, the thickness of which is 2nm-10nm; And / or, the first lithium titanium aluminum phosphate material has a porous structure; And / or, the average particle size of the first lithium titanium aluminum phosphate material is 150nm-400nm; And / or, the ionic conductivity of the first lithium titanium aluminum phosphate material is 0.8 × 10⁻⁶. -3 S·cm-1.5×10 -3 S·cm.

5. The positive electrode according to claim 1, wherein, The positive electrode plate satisfies the following relationship: I 1 >I 2 , among which, I 1 I is the ratio of the D peak intensity to the G peak intensity in the Raman spectrum of the first lithium iron phosphate particle. 2 The ratio of the D peak intensity to the G peak intensity in the Raman spectrum of the second lithium iron phosphate particle is preferred to be 0.05 ≤ I. 1 -I 2 ≤0.25; And / or, the ratio of the D peak intensity to the G peak intensity in the Raman spectrum of the first lithium iron phosphate particle is I. 1 The value is 0.95-1.2; And / or, the ratio of the D peak intensity to the G peak intensity in the Raman spectrum of the second lithium iron phosphate particle is I. 2 The value is 0.7-0.95; And / or, the resistivity of the positive electrode at 26 MPa is 5 Ω·cm-15 Ω·cm; And / or, at 26 MPa, the resistivity of the first active layer is 10 Ω·cm-25 Ω·cm; And / or, at 26 MPa, the resistivity of the second active layer is 4 Ω·cm-8 Ω·cm.

6. The positive electrode according to claim 1, wherein, The second lithium iron phosphate particle includes primary particles and secondary particles, wherein the mechanical strength of the secondary particles is greater than 40 MPa; And / or, the first lithium iron phosphate particles include primary particles; And / or, the ratio of the thickness of the first active layer, the thickness of the third coating, and the thickness of the second active layer is (6-7):(0.1-1.5):(1.5-2.9); And / or, the weight percentage of the first lithium iron phosphate particles in the first active layer is 96%-98.5%; And / or, the weight percentage of the second lithium iron phosphate in the second active layer is 93%-97%; And / or, the weight percentage of the first lithium iron phosphate particles in the first active layer is greater than the weight percentage of the second lithium iron phosphate in the second active layer; And / or, the weight percentage of the first lithium titanium aluminum phosphate material in the third coating is 90%-95%.

7. The positive electrode according to claim 1, wherein, The first active layer includes a first adhesive, which includes polyimide; And / or, the second active layer includes a second adhesive, the second adhesive including polyvinylidene fluoride; And / or, in the thermogravimetric analysis spectrum of the first active layer, the peak value of the first weight loss peak is located at 550℃-600℃, and the weight loss rate of the first weight loss peak is 1%-1.4%; And / or, in the thermogravimetric analysis spectrum of the second active layer, the peak value of the second weight loss peak is located at 400℃-450℃, and the weight loss rate of the second weight loss peak is 1.7%-2.1%; And / or, the third coating includes a third binder, which includes one or more of polybenzimidazole, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, modified polyvinylidene fluoride, polyimide, polyacrylic acid, polyamide-imide, and polyethyleneimine. Preferably, the weight percentage of the third binder in the third coating is 2%-3%.

8. The positive electrode sheet according to any one of claims 1-6, wherein, The first active layer includes a first conductive agent, which includes carbon black. Preferably, the carbon black accounts for 0.5%-1% of the weight of the first active layer. And / or, the second active layer includes a second conductive agent, which includes carbon black, carbon nanotubes, and graphene. Preferably, based on the total weight of the second active layer, the weight percentage of carbon black is 0.6%-0.8%, the weight percentage of carbon nanotubes is 0.3%-0.6%, and the weight percentage of graphene is 0.2%-0.6%. And / or, the third coating includes a third conductive agent, which includes one or more of carbon black, carbon nanotubes, graphene, and graphite. Preferably, the weight percentage of the third conductive agent in the third coating is 2%-5%. And / or, the compaction density of the first active layer is greater than the compaction density of the second active layer; And / or, the compaction density of the first active layer is 2.65 g / cm³. 3 -3.1g / cm 3 ; And / or, the compaction density of the second active layer is 2.2 g / cm³. 3 -2.5g / cm 3 ; And / or, the compaction density of the positive electrode coating is 2.7 g / cm³. 3 -3g / cm 3 .

9. A battery, characterized in that, The battery includes an electrolyte and a positive electrode sheet according to any one of claims 1-8.

10. The battery according to claim 9, wherein, The electrolyte includes a lithium salt, which includes one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide, and the weight percentage of the lithium salt in the electrolyte is 10%-20%. And / or, the electrolyte includes a phosphorus-containing additive, which includes one or more of lithium difluorophosphate, lithium tetrafluorophosphate, lithium difluorodioxarate phosphate, lithium tripolyphosphate, tris(trimethylsilane) phosphate, and tris(trimethylsilane) phosphite, wherein the weight percentage of the phosphorus-containing additive in the electrolyte is 0.1%-3%; And / or, the electrolyte includes a sulfur-containing additive, which includes one or more of vinylene sulfate and vinyl sulfate, and the sulfur-containing additive accounts for 1%-5% of the weight of the electrolyte; And / or, the electrolyte includes a first solvent, which includes one or more of ethylene carbonate and propylene carbonate, and the weight percentage of the first solvent in the electrolyte is 15%-60%; And / or, the electrolyte includes a second solvent, which includes one or more of ethyl propionate, ethyl acetate, methyl acetate, acetonitrile, acetone, propyl propionate, methyl ethyl carbonate, and dimethyl carbonate, wherein the weight percentage of the second solvent in the electrolyte is 20%-70%.

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