Positive electrode sheet and secondary battery

Abstract

The application relates to the secondary battery technical field and discloses a positive plate and a secondary battery, wherein the positive plate comprises first lithium iron phosphate particles, second lithium iron phosphate particles and third lithium iron phosphate particles in a quantity ratio of (1-10):(15-40):(50-80); the average particle size of the first lithium iron phosphate particles is 1.5-5 mu m, and the first lithium iron phosphate particles comprise first pores with an average pore size of 2-30 nm; the average particle size of the second lithium iron phosphate is 0.6-1.4 mu m, and the second lithium iron phosphate comprises second pores with an average pore size of 2-20 nm; and the average particle size of the third lithium iron phosphate particles is 0.35 mu m or below. The positive plate provided by the application adopts three kinds of lithium iron phosphate particles with different particle sizes and combines the pore structure of the lithium iron phosphate particles with a large size, so that the compaction density and the kinetic performance of the positive plate are balanced, and therefore the energy density, the fast-charging performance, the high-temperature storage performance and the cycle stability of the secondary battery are improved.

Description

Technical Field

[0001] This application relates to the field of secondary batteries, specifically to a positive electrode and a secondary battery. 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 the energy density of secondary batteries have further increased. To further address the range anxiety of new energy vehicles, it is often necessary to improve the energy density of lithium iron phosphate (LFP) secondary batteries. Calculations show that the current maximum initial charge capacity of LFP secondary batteries has reached 162 mAh / g to 163 mAh / g, which is close to the theoretical capacity. Further improving the energy density of lithium secondary batteries requires further increasing the compaction density of the electrode sheets. Currently, the compaction density of the positive electrode sheets containing LFP in existing secondary batteries is far from the theoretical true density of LFP (3.6 g / cm³). 3 It also has a considerable distance.

[0003] Furthermore, as the size of secondary batteries increases, the requirements for capacity and energy efficiency in existing energy storage batteries are gradually rising. Therefore, increasing the powder compaction density of materials, and consequently the compaction density of electrode sheets, to improve energy density is currently the main development direction. However, as the compaction density of the electrode sheets increases, the porosity of the sheets decreases further. This decrease in porosity further reduces the kinetic performance of the electrode during the lithium insertion / extraction process. Studies have shown that as the compaction density of the electrode sheets increases, the rate charge / discharge performance of the secondary battery decreases, thereby reducing the SOC (state of charge) corresponding to the lithium plating potential of the negative electrode at high rates, increasing the charging time and affecting the charging stability of the battery. It also further increases the charge transfer and ion diffusion resistance of lithium-ion secondary batteries, affecting the battery's fast-charging capability. In addition, the selection of lithium iron phosphate particles in the positive electrode also has a certain impact on the high-temperature storage performance of the secondary battery. Summary of the Invention

[0004] In view of this, in order to solve the problem that existing positive electrode sheets cannot simultaneously meet the requirements of high actual density, high kinetic performance and cycle stability, so that the positive electrode sheets make it impossible for secondary batteries to simultaneously achieve high energy density, high fast charging performance, excellent cycle performance and high temperature storage performance in practical applications, this application provides a positive electrode sheet and a secondary battery containing the positive electrode sheet.

[0005] According to an embodiment of this application, in a first aspect, this application provides a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector in the thickness direction, the positive active layer including a positive active material; The positive electrode active material includes lithium iron phosphate particles, which include first lithium iron phosphate particles, second lithium iron phosphate particles, and third lithium iron phosphate particles. In the cross-sectional SEM image along the thickness direction of the positive electrode active layer, within a 20 μm × 20 μm field of view, the ratio of the number of the first lithium iron phosphate particles to the number of the second lithium iron phosphate particles and the number of the third lithium iron phosphate particles is (1~10):(15~40):(50~80). The average particle size of the first lithium iron phosphate particle is denoted as D1 μm, which satisfies: 1.5≤D1≤5. The first lithium iron phosphate particle has a first channel, and the average pore size of the first channel is denoted as a nm, which satisfies: 2≤a≤30. The average particle size of the second lithium iron phosphate is denoted as D2 μm, which satisfies: 0.6≤D2≤1.4. The second lithium iron phosphate particles have a second channel, and the average pore size of the second channel is denoted as b nm, which satisfies: 2≤b≤20. The average particle size of the third lithium iron phosphate is denoted as D3 μm, which satisfies the condition: D3≤0.35.

[0006] Furthermore, in an optional embodiment, the average pore size a nm of the first channel satisfies: 4 ≤ a ≤ 20.

[0007] Furthermore, in an optional embodiment, the average pore diameter b nm of the second channel satisfies: 2≤b≤10.

[0008] In one alternative implementation, the tortuosity of the first channel is 2 to 4.

[0009] In an optional implementation, the depth of the first channel is denoted as h1 μm, satisfying: h1 / D1≤1 / 3.

[0010] In an optional embodiment, the number of pores x in the first channel of the first lithium iron phosphate particle satisfies: (D1 / 2-0.3)×8≤x≤(D1 / 2-0.3)×12, and x is rounded to the nearest integer.

[0011] In one alternative implementation, the tortuosity of the second channel is 1 to 2.

[0012] In an optional implementation, the depth of the second channel is denoted as h2 μm, satisfying: h2 / D2≤1 / 3.

[0013] In an optional embodiment, the number of pores y in the second channel of the second lithium iron phosphate particle satisfies: (D2 / 2-0.1)×9≤y≤(D2 / 2-0.1)×11, and y is rounded to the nearest integer.

[0014] In an optional embodiment, the number of pores x in the first channel of the first lithium iron phosphate particle is greater than the number of pores y in the second channel of the second lithium iron phosphate particle.

[0015] In an optional embodiment, the lithium iron phosphate particles in the positive electrode active layer have a particle size Dv50 of 0.6 μm to 2 μm.

[0016] In an optional embodiment, the lithium iron phosphate particles in the positive electrode active layer have a particle size span of 2 to 5.

[0017] In an optional embodiment, the resistivity of the lithium iron phosphate particles is 1 Ω·cm to 5 Ω·cm at 25±3°C.

[0018] In one optional embodiment, the compacted density of the lithium iron phosphate particles is 2.65 g·cm³ under a pressure of 3T. -3 above.

[0019] In an optional embodiment, the first lithium iron phosphate particle has a first carbon layer that coats at least a portion of the surface of the first lithium iron phosphate particle.

[0020] Furthermore, in an optional embodiment, the first lithium iron phosphate particle further includes a first ion-conducting material, which fills the first channel.

[0021] In one alternative embodiment, the second lithium iron phosphate particle has a second carbon layer that coats at least a portion of the surface of the second lithium iron phosphate particle.

[0022] Furthermore, in an optional embodiment, the second lithium iron phosphate particle further includes a second ion-conducting material, which fills the second channel.

[0023] In an optional embodiment, the third lithium iron phosphate particle has a third carbon layer that covers at least a portion of the surface of the third lithium iron phosphate particle.

[0024] In one optional embodiment, the first ion-conducting material includes at least one of lithium aluminum titanium phosphate (LATP), lithium lanthanum zirconium oxide (LLZO), and lithium lanthanum titanium oxide (LLTO).

[0025] In one optional embodiment, the mass content of the first ion-conducting material is 0.5% to 1% based on the total mass of the first lithium iron phosphate particles.

[0026] In one optional embodiment, the second ion-conducting material includes at least one of lithium aluminum titanium phosphate (LATP), lithium lanthanum zirconium oxide (LLZO), and lithium lanthanum titanium oxide (LLTO).

[0027] In one optional embodiment, the mass content of the second ion-conducting material is 0.5% to 1% based on the total mass of the second lithium iron phosphate particles.

[0028] In one optional embodiment, the mass content of the first carbon layer is 1% to 1.4% based on the total mass of the first lithium iron phosphate particles.

[0029] In one optional embodiment, the mass content of the second carbon layer is 1% to 1.4% based on the total mass of the second lithium iron phosphate particles.

[0030] In an optional embodiment, the mass content of the third carbon layer is 1% to 1.4% based on the total mass of the third lithium iron phosphate particles.

[0031] In one optional embodiment, the compaction density of the positive electrode is 2.65 g·cm³. -3 ~2.95 g·cm -3 .

[0032] In an optional embodiment, the positive electrode active layer includes a first active layer and a second active layer stacked together, the first active layer being located between the positive electrode current collector and the second active layer, the first active layer including the first lithium iron phosphate particles and the second lithium iron phosphate particles, and the second active layer including the third lithium iron phosphate particles.

[0033] Furthermore, in an optional embodiment, the thickness ratio of the first active layer to the second active layer is (7~9):(1~3).

[0034] Secondly, this application provides a secondary battery, which includes the positive electrode sheet described in the first aspect.

[0035] In an optional embodiment, the secondary battery further includes an electrolyte comprising a solvent, the solvent comprising a first solvent and a second solvent, the first solvent comprising ethylene carbonate and / or propylene carbonate, and the second solvent comprising ethyl acetate and / or methyl acetate.

[0036] Furthermore, in an optional embodiment, the mass content of the first solvent is 15% to 40% based on the total mass of the electrolyte.

[0037] Furthermore, in an optional embodiment, the mass content of the second solvent is 1% to 40% based on the total mass of the electrolyte.

[0038] Furthermore, in an optional embodiment, the mass content of the second solvent is 5% to 30% based on the total mass of the electrolyte.

[0039] In an optional embodiment, the electrolyte includes additives, which include a first additive and / or a second additive. The first additive includes at least one of lithium nitrate, lithium difluorophosphate, and lithium tetrafluoroborate, and the second additive is a nitrile additive, including at least one of acetonitrile, propionitrile, and fluoroacetonitrile.

[0040] Furthermore, in an optional embodiment, the mass content of the first additive is 0.1% to 3% based on the total mass of the electrolyte.

[0041] Furthermore, in an optional embodiment, the mass content of the second additive is 0.1% to 10% based on the total mass of the electrolyte.

[0042] The technical solution of this application has the following advantages: 1. This application provides a positive electrode sheet, comprising a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector in the thickness direction. The positive active layer comprises a positive active material, which comprises lithium iron phosphate particles. The lithium iron phosphate particles include first lithium iron phosphate particles, second lithium iron phosphate particles, and third lithium iron phosphate particles. In a cross-sectional SEM image of the positive active layer in the thickness direction, within a 20 μm × 20 μm field of view, the ratio of the number of first lithium iron phosphate particles to the number of second and third lithium iron phosphate particles is (1~10):(15~40):(50~80). The average particle size of the first lithium iron phosphate particles is denoted as D1 μm, satisfying: 1.5≤D1≤5. The first lithium iron phosphate particles have a first channel, the average pore size of which is denoted as a nm, satisfying: 2≤a≤30. The average particle size of the second lithium iron phosphate particles is denoted as D2. The second lithium iron phosphate particle has a second channel, and the average pore size of the second channel is denoted as b nm, which satisfies: 2≤b≤20; the average particle size of the third lithium iron phosphate particle is denoted as D3 μm, which satisfies: D3≤0.35. The positive electrode active material in this application uses three types of lithium iron phosphate particles with different average particle sizes. The first type of lithium iron phosphate particles has a larger average particle size, ranging from 1.5 μm to 5 μm. The larger particle size increases the volumetric efficiency of the lithium iron phosphate particles in the positive electrode, thereby improving the compaction density of the electrode. The introduction of second type lithium iron phosphate particles with an average particle size of 0.6 μm to 1.4 μm fills the pores between the larger first type of lithium iron phosphate particles, further increasing the mass within the limited electrode volume and constructing a lithium iron phosphate stacking structure with high volumetric efficiency. This increases the material's packing density and, consequently, the compaction density of the positive electrode, which is beneficial for improving the energy density of the secondary battery and enhancing its high-temperature storage performance. The first type of lithium iron phosphate particles has an average particle size of 0.35 μm. Smaller lithium iron phosphate particles with a size of less than μm can improve the rate and smoothness of lithium ion insertion and extraction in the electrode, which is conducive to improving the rapid transfer and diffusion of lithium ions in the active layer, improving the kinetic performance of the positive electrode, and thus improving the fast charging performance of the battery.Furthermore, in this application, the third lithium iron phosphate particles with a smaller average particle size accelerate the charging process, the second lithium iron phosphate particles with a medium average particle size bridge the gap, and the first lithium iron phosphate particles with a larger average particle size stabilize the battery. These three types of lithium iron phosphate particles with different particle sizes work together to construct a low-resistance, highly stable electrode system. This allows the positive electrode to be used in the secondary battery cycle process, effectively suppressing the rapid voltage drop caused by increased internal resistance and intensified polarization in electrodes formed by lithium iron phosphate particles with a single particle size. The three types of lithium iron phosphate particles with different particle sizes ensure that lithium iron phosphate particles of the corresponding particle size participate in the battery at different charging and discharging time periods, thereby improving and extending the cycle stability of the secondary battery.

[0043] Furthermore, this application research found that the first and second lithium iron phosphate particles with larger average particle sizes suffer from low ion diffusion rates. To address this, this application modifies the interior of the first and second lithium iron phosphate particles by designing specific first and second pore structures, respectively. The average pore size of the first pore is controlled within the range of 2 nm to 30 nm, and the average pore size of the second pore is controlled within the range of 2 nm to 20 nm. While effectively increasing the compaction density of the positive electrode, the designed pore structures also improve the electrolyte's wetting of the positive electrode and optimize its liquid retention capacity, as well as provide channels for ion and electron transport. This effectively solves the problem of decreased ion and electron transport performance in high-compact-density electrodes, thereby improving the material's kinetic performance and thus effectively improving the fast-charging performance of the secondary battery. On the other hand, the pore structures in the first and second lithium iron phosphate particles also provide buffer space to effectively alleviate the volume changes of the positive electrode active material during battery charging and discharging, releasing the stress generated by the volume changes during charging and discharging, thereby improving… The high material stability enables the positive electrode to effectively improve the energy density of the secondary battery while also enhancing its cycle stability in practical applications. Thirdly, the average pore size of the first channel provided in this application matches the average particle size of the first lithium iron phosphate particles, and the average pore size of the second channel matches the average particle size of the second lithium iron phosphate particles. This ensures a better balance between electrode compaction density and kinetic performance, while also making the ion transport rate in the first and second lithium iron phosphate particles more uniform. This results in uniform reaction and potential change within the electrode, thereby improving the cycle stability, fast charging performance, and high-temperature storage performance of the secondary battery.

[0044] Secondly, this application controls the number of first, second, and third lithium iron phosphate particles in a specific field of view in the cross-sectional SEM image of the positive electrode active layer in the thickness direction to be (1~10):(15~40):(50~80), so as to ensure excellent matching between the first, second, and third lithium iron phosphate particles. This ensures that there are enough lithium iron phosphate particles with larger particle sizes to improve the compaction density of the positive electrode sheet, while also having a sufficient number of lithium iron phosphate particles with smaller particle sizes to improve the kinetic performance of the electrode sheet. This allows the compaction density and kinetic performance of the positive electrode sheet to achieve a better balance. In the actual application of the positive electrode sheet, this ensures that the secondary battery simultaneously has high energy density, fast charging capability, and excellent high-temperature storage performance. The fact that the number of first, second, and third lithium iron phosphate particles is within the above range ensures that there are enough second lithium iron phosphate particles to connect the first and third lithium iron phosphate particles with large size differences. This prevents the voltage changes caused by the small number of second lithium iron phosphate particles during the charging and discharging of the secondary battery from being too rapid, which in turn helps to improve the cycle stability of the secondary battery. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0046] Figure 1 This is a schematic diagram of the structure of the first lithium iron phosphate particle in the positive electrode sheet provided in Embodiment 1 of this application; Figure 1 In the middle, 1, the first lithium iron phosphate particle; 1-1, the first lithium iron phosphate; 1-2, the first channel; 1-3, the first carbon layer.

[0047] Figure 2 This is a schematic diagram of the structure of the second lithium iron phosphate particle in the positive electrode sheet provided in Embodiment 1 of this application; Figure 2 In the middle, 2, the second lithium iron phosphate particle; 2-1, the second lithium iron phosphate; 2-2, the second channel; 2-3, the second carbon layer.

[0048] Figure 3 This is a schematic diagram of the structure of the third lithium iron phosphate particle in the positive electrode sheet provided in Embodiment 1 of this application; Figure 3 In the middle, 3, the third lithium iron phosphate particle; 3-1, the third lithium iron phosphate; 3-2, the third carbon layer. Detailed Implementation

[0049] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0050] It should be noted in the description of this application that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, the technical features involved in the different embodiments of this application described below may be combined with each other as long as they do not conflict with each other.

[0051] To address the problem that existing cathode materials cannot simultaneously achieve high actual density, high kinetic performance, and long cycle stability, thus preventing secondary batteries from simultaneously achieving high energy density, fast charging performance, excellent cycle performance, and high-temperature storage performance in practical applications, this application proposes the following solution.

[0052] In a first aspect, this application provides a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector in the thickness direction, the positive active layer comprising a positive active material; The positive electrode active material includes lithium iron phosphate particles, which include first lithium iron phosphate particles, second lithium iron phosphate particles, and third lithium iron phosphate particles. In the cross-sectional SEM image along the thickness direction of the positive electrode active layer, within a 20 μm × 20 μm field of view, the ratio of the number of the first lithium iron phosphate particles to the number of the second lithium iron phosphate particles and the number of the third lithium iron phosphate particles is (1~10):(15~40):(50~80). The average particle size of the first lithium iron phosphate particle is denoted as D1 μm, which satisfies: 1.5≤D1≤5. The first lithium iron phosphate particle has a first channel, and the average pore size of the first channel is denoted as a nm, which satisfies: 2≤a≤30. The average particle size of the second lithium iron phosphate is denoted as D2 μm, which satisfies: 0.6≤D2≤1.4. The second lithium iron phosphate particles have a second channel, and the average pore size of the second channel is denoted as b nm, which satisfies: 2≤b≤20. The average particle size of the third lithium iron phosphate is denoted as D3 μm, which satisfies the condition: D3≤0.35.

[0053] The positive electrode active material in this application uses three types of lithium iron phosphate particles with different average particle sizes. The first type of lithium iron phosphate particles has a larger average particle size, ranging from 1.5 μm to 5 μm. The larger particle size increases the volumetric efficiency of the lithium iron phosphate particles in the positive electrode, thereby improving the compaction density of the electrode. The introduction of second type lithium iron phosphate particles with an average particle size of 0.6 μm to 1.4 μm fills the pores between the larger first type of lithium iron phosphate particles, further increasing the mass within the limited electrode volume and constructing a lithium iron phosphate stacking structure with high volumetric efficiency. This increases the material's packing density and, consequently, the compaction density of the positive electrode, which is beneficial for improving the energy density of the secondary battery and enhancing its high-temperature storage performance. The first type of lithium iron phosphate particles has an average particle size of 0.35 μm. Smaller third lithium iron phosphate particles (below μm) improve the rate and smoothness of lithium-ion insertion / extraction in the electrode, facilitating rapid transfer and diffusion of lithium ions in the active layer, enhancing the kinetic performance of the positive electrode, and thus improving the fast-charging performance of the battery. Furthermore, this application utilizes three types of lithium iron phosphate particles with different average sizes: smaller third lithium iron phosphate particles for acceleration, medium-sized second lithium iron phosphate particles for bridging, and larger first lithium iron phosphate particles for stabilization. These three particles work synergistically to construct a low-resistance, highly stable electrode system. When the positive electrode is used in the secondary battery cycle, it effectively suppresses the phenomenon of increased internal resistance and intensified polarization leading to rapid voltage drop in electrodes formed by lithium iron phosphate particles of a single size. The presence of lithium iron phosphate particles of corresponding sizes at different charge / discharge stages ensures that the battery has lithium iron phosphate particles of the appropriate size participating in different charge / discharge periods, thereby improving the cycle stability of the secondary battery.

[0054] Furthermore, this application research found that the first and second lithium iron phosphate particles with larger average particle sizes suffer from low ion diffusion rates. To address this, this application modifies the interior of the first and second lithium iron phosphate particles by designing specific first and second channels, respectively. The average pore size of the first channel is controlled within the range of 2 nm to 30 nm, and the average pore size of the second channel is controlled within the range of 2 nm to 20 nm. This ensures that the large-size particles effectively increase the compaction density of the cathode sheet, while also improving the electrolyte's wetting of the cathode sheet and optimizing its liquid retention capacity through the designed channel structure. It also provides a channel structure for ion and electron transport, effectively solving the problem of decreased ion and electron transport performance in high-compact-density electrodes, thereby improving the material's kinetic performance and thus effectively improving the fast-charging performance of the secondary battery. On the other hand, the channel structure in the first and second lithium iron phosphate particles also provides buffer space to effectively mitigate the volume changes of the cathode active material during battery charging and discharging, which is beneficial for releasing the volume changes generated during battery charging and discharging. The stress is reduced, thereby improving the stability of the material. This allows the positive electrode to effectively improve the energy density of the secondary battery and enhance its cycle stability during practical applications. Thirdly, the average pore size of the first channel designed in this application matches the average particle size of the first lithium iron phosphate particle, and the average pore size of the second channel matches the average particle size of the second lithium iron phosphate particle. This ensures a better balance between the electrode compaction density and kinetic performance, while also making the ion transport rate in the first and second lithium iron phosphate particles more uniform. This results in uniform reaction and potential change within the electrode, thereby improving the cycle stability, fast charging performance, and high-temperature storage performance of the secondary battery.

[0055] Secondly, this application controls the number of first, second, and third lithium iron phosphate particles in a specific field of view in the cross-sectional SEM image of the positive electrode active layer in the thickness direction to be (1~10):(15~40):(50~80), so as to ensure excellent matching between the first, second, and third lithium iron phosphate particles. This ensures that there are enough lithium iron phosphate particles with larger particle sizes to improve the compaction density of the positive electrode sheet, while also having a sufficient number of lithium iron phosphate particles with smaller particle sizes to improve the kinetic performance of the electrode sheet. This allows the compaction density and kinetic performance of the positive electrode sheet to achieve a better balance. In the actual application of the positive electrode sheet, this ensures that the secondary battery simultaneously has high energy density, fast charging capability, and excellent high-temperature storage performance. The fact that the number of first, second, and third lithium iron phosphate particles is within the above range ensures that there are enough second lithium iron phosphate particles to connect the first and third lithium iron phosphate particles with large size differences. This avoids the polarization caused by the small number of second lithium iron phosphate particles during the charging and discharging of the secondary battery, thus helping to improve the cycle stability of the secondary battery.

[0056] This study found that within a 20 μm × 20 μm field of view of a cross-sectional SEM image along the thickness direction of the positive electrode active layer, if the number of first lithium iron phosphate particles is too low, the powder compaction density of the material cannot be effectively improved, which is detrimental to improving the compaction density of the positive electrode sheet. Consequently, the energy density and high-temperature storage performance of the secondary battery cannot be effectively improved. If the number of first lithium iron phosphate particles is too high, the number of second and third lithium iron phosphate particles in the positive electrode sheet will be low, the kinetic performance of the positive electrode sheet cannot be effectively improved, and the compaction density and high-temperature storage performance of the electrode sheet cannot reach the optimal effect, which is detrimental to improving the energy density, high-temperature storage performance, and fast-charging kinetic performance of the secondary battery. If the number of second lithium iron phosphate particles is too low or too high, the compaction density and kinetic performance of the electrode sheet will not reach the optimal level. An unbalanced approach leads to a decrease in the energy density, high-temperature storage performance, and fast-charging kinetics of the secondary battery. Furthermore, if the number of second lithium iron phosphate (LFP) particles is too low or too high, the bonding effect between the second LFP and third LFP particles with significantly different particle sizes during battery cycling will be weakened, affecting the cycle stability of the secondary battery. If the number of third LFP particles is too low, the kinetic performance of the electrode will decrease, affecting the fast-charging kinetics of the secondary battery. If the number of third LFP particles is too high, the number of large-diameter LFP particles will be low, reducing the compaction density of the electrode, thus failing to effectively improve the energy density and high-temperature storage performance of the secondary battery. In addition, if the ratio of the number of first, second, and third LFP particles within a specific field of view defined in this application is not within the specific range of (1~10):(15~40):(50~80), it will also affect the bonding effect of LFP particles of different sizes during voltage changes during battery charging and discharging, thereby affecting the cycle stability of the secondary battery.

[0057] This study found that if the average particle size of the first lithium iron phosphate particles is less than 1.5 μm, the average particle size of the lithium iron phosphate particles in the positive electrode is low, resulting in a decrease in the volume utilization rate and tap density of the positive electrode, which is not conducive to improving the energy density and high-temperature storage performance of the secondary battery. If the average particle size of the first lithium iron phosphate particles is greater than 5 μm, the kinetic performance of lithium iron phosphate with excessively large particle size will decrease exponentially, failing to achieve a good balance between the tap density and kinetic performance of the positive electrode, thus leading to a deterioration in the fast-charging kinetic performance of the secondary battery.

[0058] This study found that if the average particle size of the second lithium iron phosphate (LFP) particles is less than 0.6 μm, it affects the mass filling rate of LFP particles per unit volume of the positive electrode, leading to a decrease in the compaction density of the electrode and thus hindering the improvement of the energy density and high-temperature storage performance of the secondary battery. If the average particle size of the second LFP particles is greater than 1.4 μm, it leads to a poorer pore filling effect between the second LFP particles and the first LFP particles, affecting the material's packing density and consequently the compaction density of the positive electrode. Furthermore, an excessively high average particle size of the second LFP particles also results in poor material kinetic properties, which is detrimental to improving the energy density, fast-charging kinetic performance, and high-temperature storage performance of the secondary battery. Moreover, an average particle size of the second LFP particles less than 0.6 μm or greater than 1.4 μm both lead to a poorer connection effect between the second LFP particles and the first and third LFP particles, which is detrimental to improving the stability of voltage changes during charge-discharge cycles and thus affecting the cycle stability of the secondary battery.

[0059] This study found that if the average particle size D3 of the third lithium iron phosphate particles is higher than 0.35 μm, it will hinder the rapid and smooth transport and transfer of lithium ions, leading to a decrease in the kinetic performance of the cathode and hindering the improvement of the fast charging performance of the secondary battery.

[0060] This study found that if the average pore size of the first channel in the first lithium iron phosphate particle is less than 2 nm, or the average pore size of the second channel in the second lithium iron phosphate particle is less than 2 nm, it will affect the wetting effect of the electrolyte on the positive electrode active material and the transport of ions and electrons in the positive electrode, thus significantly reducing the fast-charging kinetics of the secondary battery in practical applications. If the average pore size of the first channel in the first lithium iron phosphate particle is greater than 30 nm, or the average pore size of the second channel in the second lithium iron phosphate particle is greater than 20 nm, the compaction density of the lithium iron phosphate particles cannot be effectively improved, and the structural stability of the lithium iron phosphate particles will be affected, thus affecting the energy density and cycle stability of the secondary battery. The third lithium iron phosphate particle itself has a small particle size and excellent kinetic performance, and therefore does not require channels.

[0061] It should be noted that the ratio of the number of the first lithium iron phosphate particles to the second and third lithium iron phosphate particles can be obtained through the following process: Specifically, the cross-section of the positive electrode active layer is tested using a scanning electron microscope. Within any 20 μm × 20 μm field of view, the cross-sectional image of the positive electrode active layer is filled. The particle size in the image is measured using a scale, and the particles are counted according to the corresponding particle size values. The ratio is then calculated. Particles with an exposed particle area of ​​less than 50% of the total particle area are not counted, while particles with an exposed particle area of ​​≥ 50% of the total particle area are counted, and the longest length is taken as the particle size. For example, the quantity of the first lithium iron phosphate particles can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or a value within the range of any two of the above values; the quantity of the second lithium iron phosphate particles can be 15, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, or a value within the range of any two of the above values; the quantity of the second lithium iron phosphate particles can be 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, or a value within the range of any two of the above values.

[0062] It should be noted that the average particle size D1 μm of the first lithium iron phosphate particles can be obtained by averaging the particle sizes of 50 first lithium iron phosphate particles at any location in the positive electrode active layer using a scanning electron microscope (SEM). For example, the average particle size D1 μm of the first lithium iron phosphate particles can be 1.5 μm, 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, 5.0 μm, or a value within any two of the above ranges.

[0063] It should be noted that the average pore size 'a nm' of the first channel can be obtained by averaging the values ​​using a scanning electron microscope (SEM). For example, the average pore size 'a nm' of the first channel can be 2 nm, 4 nm, 6 nm, 8 nm, 10 nm, 12 nm, 14 nm, 16 nm, 18 nm, 20 nm, 22 nm, 24 nm, 26 nm, 28 nm, 30 nm, or a value within any two of the above ranges.

[0064] It should be noted that the average particle size D2 μm of the second lithium iron phosphate particles can be obtained by averaging the particle sizes of 50 second lithium iron phosphate particles at any location in the positive electrode active layer using a scanning electron microscope (SEM). For example, the average particle size D2 μm of the second lithium iron phosphate particles can be 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, or a value within any two of the above ranges.

[0065] It should be noted that the average pore size b nm of the second channel can be obtained by averaging the values ​​using a scanning electron microscope (SEM). For example, the average pore size b nm of the second channel can be 2 nm, 4 nm, 6 nm, 8 nm, 10 nm, 12 nm, 14 nm, 16 nm, 18 nm, 20 nm, or a value within any two of the above ranges.

[0066] It should be noted that the average particle size D3 μm of the third lithium iron phosphate can be obtained by averaging the particle sizes of 50 third lithium iron phosphate particles at any location in the positive electrode active layer using a scanning electron microscope (SEM). For example, the average particle size D3 μm of the third lithium iron phosphate can be 0.35 μm, 0.34 μm, 0.32 μm, 0.30 μm, 0.28 μm, 0.26 μm, 0.24 μm, 0.22 μm, 0.20 μm, 0.18 μm, or a value within any two of the above ranges.

[0067] In this application, the first lithium iron phosphate particle refers to lithium iron phosphate particles with a particle size in the range of 1.45 μm to 5.50 μm.

[0068] In this application, the second lithium iron phosphate particle refers to lithium iron phosphate particles with a particle size greater than 0.4 μm and less than 1.45 μm.

[0069] In this application, the third lithium iron phosphate particle refers to lithium iron phosphate particles with a particle size in the range of 0.05 μm to 0.4 μm.

[0070] Optionally, in some embodiments, the positive electrode active layer on one side comprises a single active layer or a double active layer.

[0071] Furthermore, when the positive electrode active layer comprises a single active layer, the first lithium iron phosphate particle, the second lithium iron phosphate particle, and the third lithium iron phosphate particle are all distributed in the same single active layer.

[0072] Furthermore, when the positive electrode active layer includes a double active layer, the first lithium iron phosphate particle, the second lithium iron phosphate particle, and the third lithium iron phosphate particle can be located in different active layers of the double active layer.

[0073] Furthermore, in some embodiments, the average pore size α nm of the first channel satisfies: 4 ≤ α ≤ 20. This further improves the electrode compaction density while also enhancing the electron and ion transport within the first lithium iron phosphate particles and further improving the electrolyte's wetting effect on the positive electrode active material, thereby further balancing the compaction density and kinetic performance of the first lithium iron phosphate particles.

[0074] Furthermore, in some embodiments, the average pore size b nm of the second channel satisfies: 2 ≤ b ≤ 10. This further increases the electrode compaction density while also improving the electron and ion transport within the second lithium iron phosphate particles and enhancing the electrolyte's wetting effect on the positive electrode active material, thereby further balancing the compaction density and kinetic performance of the second lithium iron phosphate particles.

[0075] In some embodiments, the tortuosity of the first channel is 2 to 4. This expands the diffusion channels for lithium ions inside the first lithium iron phosphate particle, which is more conducive to improving the transport effect of lithium ions in the first lithium iron phosphate particle and improving the kinetic performance of the particle. At the same time, it can also ensure the structural stability of the first lithium iron phosphate particle, thereby further improving the fast charging kinetic performance and cycle stability of the secondary battery.

[0076] This study found that if the tortuosity of the first channel is less than 2, the contact area between lithium ions and the pore structure inside the first lithium iron phosphate will be smaller, resulting in poorer lithium ion transport and deteriorating kinetic performance of the first lithium iron phosphate particles, thus leading to a decrease in the fast-charging kinetic performance of the secondary battery. Conversely, if the tortuosity of the first channel is greater than 4, the structural stability of the first lithium iron phosphate particles will be poor, thus affecting the cycle stability of the secondary battery.

[0077] In this application, the tortuosity of the first channel refers to the ratio between the distance R from the outermost side of the first channel along the channel wall to the innermost side of the first channel and the straight-line distance (i.e., the channel depth) h1 from the outermost side of the first channel to the innermost side of the first channel.

[0078] It should be noted that the tortuosity of the first channel can be obtained by averaging the tortuosity of each channel structure of the first lithium iron phosphate particle measured in the SEM image obtained by scanning electron microscopy (SEM). For example, the tortuosity of the first channel can be 2.0, 2.2, 2.4, 2.6, 2.8, 3.0, 3.2, 3.4, 3.6, 3.8, 4.0, or a value within any two of the above values.

[0079] In some embodiments, the pore depth of the first channel is denoted as h1 μm, and the ratio of the pore depth of the first channel to the average particle size of the first lithium iron phosphate particle satisfies: h1 / D1 ≤ 1 / 3. This ensures that the pore depth of the first channel in the first lithium iron phosphate particle matches the average particle size, guaranteeing that the first lithium iron phosphate particle has a certain pore depth in the first channel. This further ensures superior lithium-ion transport performance and provides the first lithium iron phosphate particle with a larger compaction density and a more stable structure. This further improves the balance between the kinetic performance, compaction density, and structural stability of the first lithium iron phosphate particle, thus better ensuring that the secondary battery simultaneously possesses high energy density, fast charging capability, excellent high-temperature storage performance, and cycle stability.

[0080] It should be noted that the pore depth h1 of the first channel can be obtained by averaging the pore depth h2 of the first channel in each first lithium iron phosphate particle measured by scanning electron microscopy (SEM). For example, the ratio h1 / D1 between the pore depth of the first channel and the average particle size of the first lithium iron phosphate particle can be, for example, 1 / 3, 1 / 4, 1 / 5, 1 / 6, 1 / 7, 1 / 8, 1 / 9, 1 / 10, 1 / 11, 1 / 12, 1 / 13, or a value within any range of two of the above values.

[0081] In some embodiments, the number of pores x in the first channel of the first lithium iron phosphate particle satisfies: (D1 / 2-0.3)×8≤x≤(D1 / 2-0.3)×12, where x is rounded to the nearest integer. Thus, by controlling the relationship between the number of pores in the first channel of the first lithium iron phosphate particle and its average particle size, this application can further improve the transport effect of lithium ions in the first lithium iron phosphate particle, control the diffusion rate of lithium ions, and thereby further improve the fast-charging performance of the secondary battery. Simultaneously, it can avoid the structural stability of the particle and the capacity performance of the material from being reduced due to an excessive number of pores in the first channel. Using first lithium iron phosphate particles with an appropriate number of pores in this application can also further improve the stability of the particles and the capacity performance of the material, thereby improving the cycle stability, high-temperature storage performance of the secondary battery, and ensuring the acquisition of a high-energy-density secondary battery.

[0082] It is understood that the condition that the number of pores x in the first channel satisfies "(D1 / 2-0.3)×8≤x≤(D1 / 2-0.3)×12" means that the number of pores is determined by the average particle size of the first lithium iron phosphate particles. The larger the size of the first lithium iron phosphate particles, the more pores are needed inside the particles for lithium insertion and extraction. 0.3 is a coefficient, 8 is the minimum coefficient required to maintain lithium diffusion, and 12 is the maximum coefficient required to maintain structural stability. For example, the number of pores x in the first channel can be (D1 / 2-0.3)×8, (D1 / 2-0.3)×9, (D1 / 2-0.3)×10, (D1 / 2-0.3)×11, (D1 / 2-0.3)×12, or a value within the range of any two of the above values.

[0083] In some embodiments, the tortuosity of the second channel is 1 to 2. This expands the internal pore structure of the second lithium iron phosphate particle, further improving the diffusion channels of lithium ions in the particle, enhancing the kinetic performance of the second lithium iron phosphate particle, and ensuring the structural stability of the second lithium iron phosphate particle, thereby further improving the cycle stability and fast charging performance of the secondary battery.

[0084] It should be noted that the tortuosity of the second channel can be obtained by averaging the tortuosity of each channel structure of the second lithium iron phosphate particle under a scanning electron microscope (SEM). For example, the tortuosity of the second channel can be 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, or a value within any two of the above ranges.

[0085] In this application, the tortuosity of the second channel refers to the ratio between the distance R from the outermost side of the second channel along the hole wall to the innermost side of the second channel and the straight-line distance (i.e., hole depth) h2 from the outermost side of the second channel to the innermost side of the second channel.

[0086] In some embodiments, the pore depth of the second channel is denoted as h² μm, and the ratio of the pore depth of the second channel to the average particle size of the second lithium iron phosphate particle satisfies: h² / D² ≤ 1 / 3. This ensures a good match between the pore depth and the average pore size of the second lithium iron phosphate particle, guaranteeing superior lithium-ion transport within the particle and further improving the kinetic performance of the cathode. Simultaneously, it ensures higher compaction density and structural stability of the second lithium iron phosphate particle, thereby improving the balance between kinetics, compaction density, and structural stability. This ultimately contributes to ensuring that the secondary battery simultaneously possesses high energy density, fast charging capability, excellent high-temperature storage performance, and cycle stability.

[0087] It should be noted that the pore depth h2 of the second channel can be obtained by averaging the pore depth h2 of the first channel in each first lithium iron phosphate particle measured by scanning electron microscopy (SEM). For example, the ratio between the pore depth of the second channel and the average particle size of the second lithium iron phosphate particle can be, for example, 1 / 3, 1 / 4, 1 / 5, 1 / 6, 1 / 7, 1 / 8, 1 / 9, 1 / 10, 1 / 11, 1 / 12, 1 / 13, or a value within any range of two of the above values.

[0088] In some embodiments, the number of pores y in the second channel of the second lithium iron phosphate particle satisfies: (D² / 2 - 0.1) × 9 ≤ y ≤ (D² / 2 - 0.1) × 11, where y is rounded to the nearest integer. Thus, this application controls the number of pores in the second channel by controlling the average particle size of the second lithium iron phosphate particle, further regulating the diffusion rate of lithium ions in the second lithium iron phosphate particle to match the number of pores in the first channel of the first lithium iron phosphate particle. This ensures a more uniform diffusion rate of lithium ions in both the first and second lithium iron phosphate particles, improving the kinetic performance of the electrode while maintaining its stability. Furthermore, controlling the number of pores in the lithium iron phosphate particle further ensures a higher compaction density, which is more conducive to ensuring that the secondary battery simultaneously possesses high energy density, fast charging capability, excellent high-temperature storage performance, and cycle stability.

[0089] It is understandable that the condition "(D² / 2-0.1)×9≤y≤(D² / 2-0.1)×11" for the number of pores x in the second channel means that the number of pores is determined by the average particle size of the second lithium iron phosphate particles. The larger the size of the second lithium iron phosphate particles, the more pores are needed inside the particles for lithium insertion and extraction. 0.1 is a coefficient, 9 is the minimum coefficient required to maintain lithium diffusion, and 11 is the maximum coefficient required to maintain structural stability. For example, the number of pores y in the second channel can be (D² / 2-0.1)×9, (D² / 2-0.1)×10, (D² / 2-0.1)×11, or a value within the range of any two of the above values.

[0090] In some embodiments, the number of pores (x) in the first channel of the first lithium iron phosphate particle is greater than the number of pores (y) in the second channel of the second lithium iron phosphate particle. Thus, by providing a larger number of pores in the first lithium iron phosphate particle, the difference in lithium ion transport speed between the first and second lithium iron phosphate particles can be balanced, thereby ensuring the uniformity of lithium ion transport rates in both particles and further improving the cycle stability of the secondary battery.

[0091] This study found that if the number of pores x in the first channel is less than or equal to the number of pores y in the second channel, the diffusion rate of lithium ions in the first lithium iron phosphate particle will be significantly lower than that in the second lithium iron phosphate particle, resulting in poorer uniformity of lithium ion diffusion and thus affecting the cycle performance of the secondary battery.

[0092] In some embodiments, the particle size Dv50 of the lithium iron phosphate particles in the positive electrode active layer is 0.6 μm to 2 μm. This ensures a suitable particle size distribution of lithium iron phosphate particles, resulting in a sufficient number of small and large lithium iron phosphate particles. This further balances the compaction density and kinetic performance of the positive electrode, making it easier to obtain a positive electrode that simultaneously meets the requirements of high compaction density and high kinetic performance. Consequently, it further contributes to obtaining a secondary battery with high compaction density and high fast-charging performance.

[0093] It should be noted that the particle size Dv50 of the lithium iron phosphate particles can be obtained by measuring the particle size using a Malvern laser particle size analyzer, arranging the particles from smallest to largest, and assigning the particle size corresponding to 50% of the total volume. For example, the particle size Dv50 of the lithium iron phosphate particles can be 0.6 μm, 0.8 μm, 1.0 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2.0 μm, or a value within any two of the above ranges.

[0094] In this application, the particle size Dv50 of the lithium iron phosphate particles refers to the particle size when the first lithium iron phosphate particle, the second lithium iron phosphate particle, and the third lithium iron phosphate particle are arranged in ascending order of particle size in the positive electrode sheet, and the cumulative particle size distribution volume fraction reaches 50%.

[0095] In some embodiments, the lithium iron phosphate particles in the positive electrode active layer have a particle size span of 2 to 5. This results in a wide particle size distribution, with large, medium, and small lithium iron phosphate particles coexisting. This ensures synergy among particles of varying sizes, further increasing the packing density of the positive electrode and improving its compaction density. This, in turn, contributes to increasing the energy density of the secondary battery and enhances the stability of the positive electrode, thereby further improving the cycle stability of the secondary battery.

[0096] It should be noted that the particle size span value of the lithium iron phosphate particles can be calculated using (Dv90-Dv10) / Dv50, where Dv10 is the particle size corresponding to a cumulative volume percentage of 10% when the lithium iron phosphate particles are arranged from smallest to largest, and Dv90 is the particle size corresponding to a cumulative volume percentage of 90% when the lithium iron phosphate particles are arranged from smallest to largest. For example, the particle size span value of the lithium iron phosphate particles can be, for example, 2.0, 2.2, 2.4, 2.6, 2.8, 3.0, 3.2, 3.4, 3.6, 3.8, 4.0, 4.2, 4.4, 4.6, 4.8, 5.0, or a value within the range of any two of the above values.

[0097] In some embodiments, the resistivity of the lithium iron phosphate particles is 1 Ω·cm to 5 Ω·cm at 25±3°C.

[0098] It should be noted that the resistivity of the lithium iron phosphate particles can be obtained by testing with a powder resistivity instrument. For example, the resistivity of the lithium iron phosphate particles can be 1.0 Ω·cm, 1.5 Ω·cm, 2.0 Ω·cm, 2.5 Ω·cm, 3.0 Ω·cm, 3.5 Ω·cm, 4.0 Ω·cm, 4.5 Ω·cm, 5.0 Ω·cm, or a value within any two of the above ranges.

[0099] In this application, the resistivity of the lithium iron phosphate particles refers to the average resistivity of the first lithium iron phosphate particle, the second lithium iron phosphate particle, and the third lithium iron phosphate particle in the positive electrode at 25±3℃.

[0100] In some embodiments, the compacted density of the lithium iron phosphate particles is 2.65 g·cm³ under a pressure of 3T. -3 above.

[0101] It should be noted that the powder compaction density of the lithium iron phosphate particles can be obtained by testing with a powder compaction density meter. For example, the powder compaction density of the lithium iron phosphate particles can be, for instance, 2.65 g·cm³. -3 2.67 g·cm -3 2.69 g·cm -3 2.71 g·cm -3 2.73 g·cm -3 2.75 g·cm -3 2.77 g·cm -3 Values ​​equal to or within the range of any two of the above values.

[0102] In this application, the powder compaction density of the lithium iron phosphate particles refers to the overall powder compaction density of the first lithium iron phosphate particles, the second lithium iron phosphate particles, and the third lithium iron phosphate particles in the positive electrode sheet.

[0103] In some embodiments, the first lithium iron phosphate particle has a first carbon layer that coats at least a portion of the surface of the first lithium iron phosphate particle. Thus, the carbon layer on the surface of the first lithium iron phosphate particle can further reduce the erosion of the first lithium iron phosphate particle by the electrolyte, reduce the risk of iron dissolution, and further improve the wetting of the particle by the electrolyte and the particle's liquid retention capacity. Furthermore, the carbon layer coating structure can stabilize the structure of the first lithium iron phosphate particle, alleviate the pressure on the first lithium iron phosphate particle during battery charging and discharging, and thus further contribute to improving the cycle stability of the secondary battery. Moreover, the carbon layer coating can also effectively improve the high-temperature storage performance of the electrode.

[0104] It is understandable that when carbon is applied to the first iron phosphate particles with specific first channels, the carbon source will not only coat the surface of the particles but also enter the channel structure.

[0105] Optionally, in some embodiments, the carbon source of the first carbon layer comprises an organic polymer and a CN-based carbon source in a mass ratio of (7-9):(1-3).

[0106] Optionally, in some embodiments, the CN-based carbon source includes at least one of polyacrylonitrile, melamine, urea, and polypyrrole.

[0107] Furthermore, in some embodiments, the first lithium iron phosphate particle further includes a first ion-conducting material, which fills the first channel. Thus, filling the first channel of the first lithium iron phosphate particle with the first ion-conducting material increases the ion-conducting capacity inside the first lithium iron phosphate particle and further stabilizes the channel framework structure of the first lithium iron phosphate particle.

[0108] In some embodiments, the second lithium iron phosphate particle has a second carbon layer that coats at least a portion of the surface of the second lithium iron phosphate particle. Thus, coating the surface of the second lithium iron phosphate particle with a carbon layer can further reduce the erosion of the first lithium iron phosphate particle by the electrolyte, reduce the risk of iron dissolution, and further improve the wetting of the particle by the electrolyte and the particle's liquid retention capacity, as well as stabilize the structure of the second lithium iron phosphate particle, alleviating the pressure on the second lithium iron phosphate particle during battery charging and discharging. This further contributes to improving the cycle stability of the secondary battery. Furthermore, the carbon layer coating can effectively improve the high-temperature storage performance of the electrode.

[0109] Optionally, in some embodiments, the carbon source of the second carbon layer comprises an organic polymer and a CN-based carbon source in a mass ratio of (7-9):(1-3).

[0110] Optionally, in some embodiments, the CN-based carbon source includes at least one of polyacrylonitrile, melamine, urea, and polypyrrole.

[0111] Furthermore, in some embodiments, the second lithium iron phosphate particle further includes a second ion-conducting material, which fills the second channel. Thus, filling the second channel of the second lithium iron phosphate particle with the second ion-conducting material increases the ion-conducting energy inside the second lithium iron phosphate particle and further stabilizes the channel framework structure of the second lithium iron phosphate particle.

[0112] In some embodiments, the third lithium iron phosphate particle has a third carbon layer that covers at least a portion of the surface of the third lithium iron phosphate particle.

[0113] Optionally, in some embodiments, the carbon source of the third carbon layer comprises an organic polymer and a CN-based carbon source in a mass ratio of (7-9):(1-3).

[0114] Optionally, in some embodiments, the CN-based carbon source includes at least one of polyacrylonitrile, melamine, urea, and polypyrrole.

[0115] In some embodiments, the first ion-conducting material includes at least one of lithium aluminum titanium phosphate (LATP), lithium lanthanum zirconium oxide (LLZO), and lithium lanthanum titanium oxide (LLTO).

[0116] In some embodiments, the mass content of the first ion-conducting material is 0.5% to 1% based on the total mass of the first lithium iron phosphate particles. This can further improve the ion-conducting ability of the first lithium iron phosphate particles and more effectively stabilize the pore structure of the first lithium iron phosphate particles.

[0117] It should be noted that the mass content of the first ion-conducting material can be obtained by inductively coupled plasma (ICP) testing. For example, the mass content of the first ion-conducting material can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, or a value within any two of the above ranges.

[0118] In some embodiments, the second ion-conducting material includes at least one of lithium aluminum titanium phosphate (LATP), lithium lanthanum zirconium oxide (LLZO), and lithium lanthanum titanium oxide (LLTO).

[0119] In some embodiments, the mass content of the second ion-conducting material is 0.5% to 1% based on the total mass of the second lithium iron phosphate particles. This further improves the ion-conducting ability of the second lithium iron phosphate particles and more effectively stabilizes the pore structure of the second lithium iron phosphate particles.

[0120] It should be noted that the mass content of the second ion-conducting material can be obtained by inductively coupled plasma (ICP) testing. For example, the mass content of the second ion-conducting material can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, or a value within any two of the above ranges.

[0121] In some embodiments, the mass content of the first carbon layer is 1% to 1.4% based on the total mass of the first lithium iron phosphate particles. This further enhances the protective effect of the carbon layer on the first lithium iron phosphate particles, better reduces electrolyte erosion of the particles and iron dissolution, further improves the wetting performance of the electrolyte and the electrolyte retention capacity of the particles, and also further enhances the structural stability of the first lithium iron phosphate particles, thereby contributing to improved cycle performance of the secondary battery.

[0122] It should be noted that the mass content of the first carbon layer can be obtained by testing with a carbon-sulfur analyzer. For example, the mass content of the first carbon layer can be 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, or a value within any two of the above values.

[0123] In some embodiments, the mass content of the second carbon layer is 1% to 1.4% based on the total mass of the second lithium iron phosphate particles. This further enhances the protective effect of the second carbon layer on the second lithium iron phosphate particles, further reduces the risk of electrolyte erosion and iron leaching of the particles, further improves the wetting effect of the electrolyte on the particles and the particles' own liquid retention capacity, and further stabilizes the stability of the second lithium iron phosphate particles, thus further improving the cycle performance of the secondary battery.

[0124] It should be noted that the mass content of the second carbon layer can be obtained by testing with a carbon-sulfur analyzer. For example, the mass content of the first carbon layer can be 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, or a value within any two of the above ranges.

[0125] In some embodiments, the mass content of the third carbon layer is 1% to 1.4% based on the total mass of the third lithium iron phosphate particles.

[0126] It should be noted that the mass content of the third carbon layer can be obtained by testing with a carbon-sulfur analyzer. For example, the mass content of the third carbon layer can be 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, or a value within any two of the above values.

[0127] In some embodiments, the compaction density of the positive electrode is 2.65 g·cm³. -3 ~2.95 g·cm -3 .

[0128] It should be noted that the compaction density of the positive electrode sheet can be obtained by testing with a compaction density meter. For example, the compaction density of the positive electrode sheet can be, for instance, 2.65 g·cm³. -3 2.70 g·cm -3 2.75 g·cm -3 2.80 g·cm -3 2.85 g·cm -3 2.90 g·cm -3 2.95 g·cm -3 Values ​​equal to or within the range of any two of the above values.

[0129] Optionally, in some embodiments, the thickness of the positive electrode active layer is 80 μm to 100 μm.

[0130] In some embodiments, the positive electrode active layer includes a first active layer and a second active layer stacked together, the first active layer being located between the positive electrode current collector and the second active layer, the first active layer including the first lithium iron phosphate particles and the second lithium iron phosphate particles, and the second positive electrode active layer including the third lithium iron phosphate particles. Thus, introducing first and second lithium iron phosphate particles into the active layer near the current collector further improves the compaction density of the cathode. Furthermore, the first and second lithium iron phosphate particles, with their different average particle sizes, mutually fill each other, constructing a lithium iron phosphate stacking structure with excellent volumetric utilization in the first active layer. This increases the material's packing density and, consequently, the compaction density of the cathode, which is beneficial for improving the energy density and high-temperature storage performance of the secondary battery. Simultaneously, the porous structure in the first and second lithium iron phosphate particles further enhances the diffusion effect of lithium ions in the first active layer while maintaining high energy density. Using third lithium iron phosphate particles in the active layer away from the current collector allows for faster and smoother insertion and extraction of lithium ions in the second active layer composed of these third lithium iron phosphate particles, further improving the kinetic performance of the electrode and consequently enhancing the fast-charging kinetic performance of the secondary battery. Furthermore, by using smaller average particle size third lithium iron phosphate particles in the second active layer away from the current collector, and larger particle size first lithium iron phosphate particles and medium particle size second lithium iron phosphate particles in the first active layer near the current collector, it is possible to further ensure that large, medium and small lithium iron phosphate particles are involved in different charging and discharging time periods, thereby reducing the polarization of the secondary battery during the cycle and further extending the cycle stability of the secondary battery.

[0131] In some embodiments, the thickness ratio of the first active layer to the second active layer is (7~9):(1~3). This allows for a better balance between the compaction density and kinetic performance of the positive electrode, ensuring higher compaction density and kinetic performance while avoiding an excessively thick second active layer, leading to an excessive number of small-sized third lithium iron phosphate particles and poor bonding between lithium iron phosphate particles of different sizes. It also avoids an excessive number of third lithium iron phosphate particles resulting in an excessively high specific surface area of ​​the second active layer on the side furthest from the current collector, thereby reducing side reactions between the electrolyte and the positive electrode, and ultimately improving the high-temperature storage performance of the secondary battery.

[0132] It should be noted that the thickness ratio of the first active layer to the second active layer can be obtained by cross-sectional scanning electron microscopy of the positive electrode active layer. For example, the thickness ratio of the first active layer to the second active layer can be 7.0:3.0, 7.5:3.5, 8.0:2.0, 8.5:2.5, 9.0:1.0, or a value within any two of the above ranges.

[0133] It is understandable that when the positive electrode active layer includes a first active layer and a second active layer stacked together, the first lithium iron phosphate particle and the second lithium iron phosphate particle are located in the first active layer, and the third lithium iron phosphate particle is located in the second active layer, in the cross-sectional SEM image along the thickness direction of the positive electrode active layer, taking the direction perpendicular to the thickness direction of the positive electrode active layer as the first direction and the direction parallel to the thickness direction of the positive electrode active layer as the second direction, within a field of view of (A×20) μm×20 μm formed by a dimension of 0 μm~(A×20) μm away from the positive current collector along the second direction and 20 μm in the first direction, the number of the first lithium iron phosphate particle and the number of the second lithium iron phosphate particle are calculated; within the second active layer, within a field of view of (B×20) μm×20 μm formed by a dimension of 0 μm~(B×20) μm away from the surface of the second active layer away from the first active layer along the second direction and 20 μm in the first direction, the number of the first lithium iron phosphate particle and the number of the second lithium iron phosphate particle are calculated. Within a field of view of μm, the number of the third lithium iron phosphate particles is calculated, and the percentage of the number of the first, second and third lithium iron phosphate particles can be obtained. Among them, A is the percentage of the thickness of the first active layer on one side to the total thickness of the positive electrode active layer on one side, and B is the percentage of the thickness of the second active layer on one side to the total thickness of the positive electrode active layer on one side.

[0134] Secondly, this application provides a secondary battery, which includes the positive electrode sheet described in the first aspect. Thus, the secondary battery provided by this application, employing a specific positive electrode sheet, can collectively improve the energy density, fast-charging capability, high-temperature storage performance, and cycle stability of the secondary battery.

[0135] In some embodiments, the secondary battery further includes an electrolyte comprising a solvent, which includes a first solvent and a second solvent. The first solvent comprises ethylene carbonate and / or propylene carbonate, and the second solvent comprises ethyl acetate and / or methyl acetate. Thus, the introduction of high-viscosity solvents such as ethylene carbonate and / or propylene carbonate into the electrolyte ensures that the electrolyte has a sufficient dielectric constant to promote lithium salt dissociation and form a stable solid-state electrolyte interface (SEI) film on the electrode surface, thereby further improving the cycle performance of the secondary battery. Simultaneously, the introduction of low-viscosity, small-molecule solvents such as ethyl acetate and / or methyl acetate into the electrolyte reduces the overall viscosity of the electrolyte and the solvation barrier of lithium ions, constructing a smaller solvation structure to carry lithium ions within the pores of the positive electrode active material at a faster rate, thereby further enhancing the lithium ion transport capability.

[0136] Furthermore, in some embodiments, the mass content of the first solvent is 15% to 40% based on the total mass of the electrolyte.

[0137] It should be noted that the mass content of the first solvent can be obtained by gas chromatography (GC). For example, the mass content of the first solvent may be 15%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, or a value within any two of the above values.

[0138] Furthermore, in some embodiments, the mass content of the second solvent is 1% to 40% based on the total mass of the electrolyte.

[0139] It should be noted that the mass content of the second solvent can be obtained by gas chromatography (GC). For example, the mass content of the second solvent can be 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or a value within any two of the above ranges.

[0140] Furthermore, in some embodiments, the mass content of the second solvent is 5% to 30% based on the total mass of the electrolyte.

[0141] For example, the mass content of the second solvent may be 5%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30%, or a value within the range of any two of the above values.

[0142] In some embodiments, the electrolyte includes additives, which include a first additive and / or a second additive. The first additive includes at least one of lithium nitrate, lithium difluorophosphate, and lithium tetrafluoroborate, and the second additive is a nitrile additive, including at least one of acetonitrile, propionitrile, and fluoroacetonitrile. Thus, by adding small-molecule lithium salt additives such as lithium nitrate, lithium difluorophosphate, and lithium tetrafluoroborate to the electrolyte, this application can achieve precise interface repair and optimized ion transport on the inner walls of the pores of the first and second lithium iron phosphate particles. Furthermore, the small-molecule additives have smaller molecular structure characteristics, allowing them to enter the pores along with the solvation structure of lithium ions, forming a dense inorganic protective layer on the active inner wall surface not completely covered by the carbon layer. This further effectively isolates the electrolyte from corrosion and more effectively inhibits side reactions and iron dissolution. Meanwhile, the electrolyte of this application also contains nitrile additives such as acetonitrile, propionitrile, and fluoroacetonitrile, which can effectively inhibit the dissolution of iron ions in the lithium iron phosphate active material used in the positive electrode, thereby preventing the precipitation of lithium dendrites in the negative electrode. Acetonitrile, propionitrile, and fluoroacetonitrile are all short-chain nitrile additives. On the one hand, their cyano groups (-CN) can effectively complex the dissolved Fe through strong coordination. 2+ / Fe 3+ On the one hand, ions are inhibited from migrating to the negative electrode and damaging the SEI film and catalyzing the growth of lithium dendrites. On the other hand, nitrile molecules can preferentially adsorb onto the surface of the positive electrode material to form a dynamic protective film, reducing the dissolution of Fe ions from the source. At the same time, these additives can also significantly reduce the viscosity of the electrolyte and improve its wetting and penetration efficiency on the electrodes and separator, especially the high-pressure positive electrode, thereby improving ion transport dynamics and enhancing the cycle stability and safety of the battery.

[0143] Furthermore, in some embodiments, the mass content of the first additive is 0.1% to 3% based on the total mass of the electrolyte.

[0144] It should be noted that the mass content of the first additive can be obtained by gas chromatography (GC). For example, the mass content of the first additive may be 0.1%, 0.3%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, or a value within any two of the above values.

[0145] Furthermore, in some embodiments, the mass content of the second additive is 0.1% to 10% based on the total mass of the electrolyte.

[0146] It should be noted that the mass content of the second additive can be obtained by gas chromatography (GC). For example, the mass content of the second additive may be 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a value within any two of the above ranges.

[0147] Example 1 This embodiment provides a method for preparing a secondary battery, including the following steps: (1) Preparation of lithium iron phosphate particles: Lithium carbonate, iron oxide (Fe₂O₃), and phosphorus dihydrogen phosphate were weighted and dispersed in water according to a Li:Fe:P molar ratio of 1.03:1:1. Glucose and polyethylene glycol were then added and mixed further. The mass of glucose accounted for 4% of the total mass of lithium carbonate, iron oxide, and ammonium dihydrogen phosphate, and the mass of polyethylene glycol accounted for 1% of the total mass of lithium carbonate, iron oxide, and ammonium dihydrogen phosphate. The mixture was then rapidly dispersed and transferred to a grinding jar for grinding with 0.5 mm zirconium balls. After grinding for 6 hours, a first grinding slurry was obtained. This slurry was then transferred into a mesoporous carbon template (CMK-3 template agent) with controlled pore size, pore depth, tortuosity, and pore number. A first high-temperature calcination was then performed at 750 °C for 20 hours under a nitrogen atmosphere to react the precursor and generate crystalline lithium iron phosphate. A second high-temperature calcination was then performed at 400 °C to remove the carbon template, leaving lithium iron phosphate with specific internal first channels. The second high-temperature calcination was carried out at ℃, in a dry air atmosphere for 8 hours. Finally, the first lithium iron phosphate particles with a specific first channel structure were carbon-coated using a chemical vapor deposition method with polyethylene glycol and melamine in a mass ratio of 8:2 as carbon sources to obtain first lithium iron phosphate particles.

[0148] (2) Preparation of lithium iron phosphate particles: Lithium carbonate, iron oxide (Fe₂O₃), and phosphorus dihydrogen phosphate were weighted and dispersed in water according to a Li:Fe:P molar ratio of 1.03:1:1. Glucose and polyethylene glycol were then added and mixed further. The mass of glucose was 4% of the total mass of lithium carbonate, iron oxide, and ammonium dihydrogen phosphate, and the mass of polyethylene glycol was 1%. The mixture was then rapidly dispersed and transferred to a grinding jar for grinding with 0.3 mm zirconium balls. After grinding for 8 hours, a second grinding slurry was obtained. This slurry was then transferred into a mesoporous carbon template (CMK-3 template agent) with controlled pore size, depth, tortuosity, and number of pores. A third high-temperature calcination was then performed at 750 °C for 20 hours under a nitrogen atmosphere to react the precursor and generate crystalline lithium iron phosphate. A fourth high-temperature calcination was then performed at 400 °C to remove the carbon template, leaving lithium iron phosphate with an internal second-channel structure. The fourth high-temperature calcination was carried out at ℃, in a dry air atmosphere for 8 hours. Finally, the lithium iron phosphate with a second channel structure was carbon-coated using a chemical vapor deposition method with polyethylene glycol and melamine in a mass ratio of 8:2 as carbon sources to obtain lithium iron phosphate particles.

[0149] (3) Preparation of lithium iron phosphate particles: Lithium iron phosphate particles were obtained by carbon coating of lithium iron phosphate using a chemical vapor deposition method with polyethylene glycol and melamine in a mass ratio of 8:2 as carbon sources.

[0150] (4) Preparation of the positive electrode: The first lithium iron phosphate particles prepared in step (1), the second lithium iron phosphate particles prepared in step (2), and the third lithium iron phosphate particles prepared in step (3) are mixed with a conductive agent, a binder, and N-methylpyrrolidone according to the formula to obtain a positive electrode mixed slurry. In the positive electrode mixed slurry, based on the total mass of solid matter in the positive electrode mixed slurry, the total mass content of the first lithium iron phosphate particles, the second lithium iron phosphate particles, and the third lithium iron phosphate particles is 96.5%; the conductive agent adopts a composite conductive agent system, which is composed of conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene in a mass ratio of 8:1:0.5:0.5, and the mass content of the conductive agent is 1.4%; the binder is polyvinylidene fluoride, and its mass content is 2.1%.

[0151] A positive electrode slurry was coated onto both sides of a positive electrode current collector aluminum foil. After drying and rolling, a positive electrode active layer with a thickness of 80 μm was formed on both sides of the aluminum foil, resulting in a positive electrode sheet with a compaction density of 2.75 g·cm³. -3 .

[0152] In the positive electrode sheet, within a 20μm × 20μm field of view taken in the cross-sectional SEM image along the thickness direction of the positive electrode active layer, the ratio of the first, second, and third lithium iron phosphate particles is 1:40:59. The particle size Dv50 of the lithium iron phosphate particles in the positive electrode sheet is 1.21 μm, and the span value is 2.31. At 25℃, the powder resistivity of the lithium iron phosphate particles in the positive electrode sheet is 2.3 Ω·cm. Under a pressure of 3T, the powder compaction density of the lithium iron phosphate particles in the positive electrode sheet is 2.67 g·cm³. A schematic diagram of the structure of the first lithium iron phosphate particle is shown below. Figure 1 As shown, according to Figure 1 It can be seen that the first lithium iron phosphate particle 1 is composed of first lithium iron phosphate 1-1, first channel 1-2, and first carbon layer 1-3. The first channel 1-2 is disposed in the first lithium iron phosphate 1-1, and the first carbon layer 1-3 covers the surface of the first lithium iron phosphate 1-1. Specifically, the average particle size D1 of the first lithium iron phosphate particle 1 is 1.512 μm, the average pore size a of the first channel 1-2 is 4 nm, the tortuosity (the distance R from the outermost edge of the first channel 1-2 along the pore wall to the innermost edge of the first channel 1-2 / the straight-line distance h1 from the outermost edge to the innermost edge of the first channel 1-2) is 2.11, and the ratio of the pore depth h1 to the average particle size D1 of the first lithium iron phosphate particle 1 satisfies: h1 / D1=0.30, h1 / D1<1 / 3, and h1 is 0.45. μm; the number of pores x in the first channel 1-2 satisfies (1.512 / 2-0.3)×10≈5. Based on the total mass of the first lithium iron phosphate particle 1, the mass content of the first carbon layer 1-3 is 1.1%. A schematic diagram of the structure of the second lithium iron phosphate particle is shown below. Figure 2 As shown, according to Figure 2 It can be seen that the second lithium iron phosphate particle 2 consists of a second lithium iron phosphate 2-1, a second channel 2-2, and a second carbon layer 2-3. The second channel 2-2 is disposed in the second lithium iron phosphate 2-1, and the second carbon layer 2-3 covers the surface of the second lithium iron phosphate 2-1. Specifically, the average particle size D2 of the second lithium iron phosphate particle 2 is 0.68 μm, the average pore size b of the second channel 2-2 is 4 nm, the tortuosity (the distance R from the outermost edge of the second channel 2-2 along the pore wall to the innermost edge of the second channel 2-2 / the straight-line distance h2 from the outermost edge to the innermost edge of the second channel 2-2) is 1.41, and the ratio of the pore depth h2 to the average particle size D2 of the second lithium iron phosphate particle 2 satisfies: h2 / D2=0.31, h2 / D2<1 / 3, and h2 is 0.21. μm; the number of pores y in the second channel 2-2 satisfies (0.68 / 2-0.1)×10≈2. Based on the total mass of the second lithium iron phosphate particle 2, the mass content of the second carbon layer 2-3 is 1.1%. A schematic diagram of the structure of the third lithium iron phosphate particle is shown below. Figure 3 As shown, according to Figure 3It can be seen that the third lithium iron phosphate particle 3 is composed of third lithium iron phosphate 3-1 and a third carbon layer 3-2 coated on the surface of third lithium iron phosphate 3-1. The average particle size D3 of the third lithium iron phosphate particle 3 is 0.32 μm. Based on the total mass of the third lithium iron phosphate particle 3, the mass content of the third carbon layer 3-2 is 1.1%.

[0153] (5) Preparation of negative electrode: The negative electrode uses artificial graphite as the negative electrode active material, conductive carbon black as the conductive agent, sodium carboxymethyl cellulose (CMC) and polyacrylic acid as binders, and deionized water as the solvent. After being mixed evenly, the mixture is coated onto both sides of the copper foil of the negative electrode current collector. After drying and rolling, the negative electrode sheet is obtained. The mass ratio of artificial graphite, conductive carbon black, sodium carboxymethyl cellulose, and polyacrylic acid is 96:1:1.8:1.2.

[0154] (6) Preparation of electrolyte: In a glove box filled with argon gas and containing <0.1 ppm of water and <0.1 ppm of oxygen, ethylene carbonate (first solvent), ethyl acetate (second solvent), and methyl ethyl carbonate (third solvent) were mixed. A fully dried lithium hexafluorophosphate (LiPF6) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) mixed lithium salt were added, followed by lithium nitrate (first additive), acetonitrile (second additive), and ethylene carbonate (third additive) to form the electrolyte. Based on the total mass of the electrolyte, the mass content of LiPF6 was 10%, LiTFSI was 4%, lithium nitrate was 1%, acetonitrile was 5%, ethylene carbonate was 1.3%, ethylene carbonate was 20%, ethyl acetate was 22%, and methyl ethyl carbonate was 36.7%.

[0155] (7) Preparation of the diaphragm: A ceramic coating with a thickness of 2 μm is coated on one side of a polyethylene film with a thickness of 7 μm. Then, an adhesive layer with a thickness of 1 μm is coated on the side of the polyethylene film without the ceramic coating and on the side of the polyethylene film with the ceramic coating away from the ceramic film to obtain a diaphragm. The ceramic particles in the ceramic coating are alumina, and the adhesive layer is composed of polyvinylidene fluoride (PVDF).

[0156] (8) Assembly of secondary batteries: The positive electrode sheet obtained in step (4) and the negative electrode sheet obtained in step (5) are die-cut. The die-cut positive electrode sheet, the separator obtained in step (7) and the die-cut negative electrode sheet are stacked alternately in a Z-shaped stacking method. After ultrasonic welding, the positive electrode is welded to the aluminum electrode and the negative electrode is welded to the copper electrode. Then, it is heat-sealed in a shell made of aluminum-plastic film. The electrolyte obtained in step (6) is injected under vacuum and vacuum secondary sealing is performed. The separator is a polyethylene separator. The secondary battery is obtained through standing, formation and sorting processes.

[0157] Example 37 The difference between this embodiment and Embodiment 1 is only that: the first lithium iron phosphate particles prepared in step (1) and the second lithium iron phosphate particles prepared in step (2) are mixed with conductive agent, binder, and N-methylpyrrolidone according to the formula to obtain the first positive electrode slurry. In the first positive electrode slurry, based on the total mass of solid matter in the first positive electrode slurry, the total mass content of the first lithium iron phosphate particles and the second lithium iron phosphate particles is 96.5%. The conductive agent adopts a three-dimensional composite conductive agent system, which is composed of conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes and graphene in a mass ratio of 8:1:0.5:0.5. The mass content of the conductive agent is 1.4%. The binder is polyvinylidene fluoride, and its mass content is 2.1%.

[0158] The lithium iron phosphate particles obtained in step (3) are mixed with a conductive agent, a binder, and N-methylpyrrolidone to obtain a second positive electrode slurry. In the second positive electrode slurry, based on the total mass of solid matter in the slurry, the mass content of the lithium iron phosphate particles is 96.5%. The conductive agent is a composite conductive agent system composed of conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene in a mass ratio of 8:1:0.5:0.5, and the mass content of the conductive agent is 1.4%. The binder is polyvinylidene fluoride, and its mass content is 2.1%.

[0159] A first positive electrode slurry is coated onto both sides of the positive electrode current collector aluminum foil. After drying, a first active layer is formed on both sides of the positive electrode current collector aluminum foil. A second positive electrode slurry is coated onto the side of the first active layer away from the positive electrode current collector. After drying and rolling, a second active layer is formed. The thickness ratio of the first active layer to the second active layer is 8:2, resulting in a positive electrode sheet with a compaction density of 2.75 g·cm³. -3 .

[0160] The preparation methods and parameter settings of the remaining embodiments and comparative examples are basically the same as those of Example 1. The differences are shown in Tables 1 and 2. In Tables 1 and 2, D1 represents the average particle size of the first lithium iron phosphate particle, in μm; D2 represents the average particle size of the second lithium iron phosphate particle, in μm; D3 represents the average particle size of the third lithium iron phosphate particle, in μm; a represents the average pore size of the first channel in the first lithium iron phosphate particle, in nm; b represents the average pore size of the second channel in the second lithium iron phosphate particle, in nm; h1 represents the pore depth of the first channel, in μm; x represents the number of pores in the first channel; h2 represents the pore depth of the second channel, in μm; y represents the number of pores in the second channel.

[0161] Table 1

[0162] Table 2

[0163] In addition to the differences shown in Tables 1 and 2 above, some embodiments also differ from Embodiment 1 in the following ways: The difference between Example 29 and Example 1 is that, based on the total mass of the electrolyte, the mass content of LiPF6 is 8%, the mass content of LiTFSI is 4%, the mass content of vinylene carbonate as the third additive is 1.3%, and the mass content of methyl ethyl carbonate as the third solvent is 48.5%. All other contents are the same as in Example 1.

[0164] The difference between Example 30 and Example 1 is that, based on the total mass of the electrolyte, the mass content of LiPF6 is 10%, the mass content of LiTFSI is 4%, the mass content of vinylene carbonate as the third additive is 1%, and the mass content of methyl ethyl carbonate as the third solvent is 33%. All other contents are the same as in Example 1.

[0165] The difference between Example 31 and Example 1 is that, based on the total mass of the electrolyte, the mass content of LiPF6 is 10%, the mass content of LiTFSI is 4%, the mass content of vinylene carbonate as the third additive is 7.1%, and the mass content of methyl ethyl carbonate as the third solvent is 58.2%. All other contents are the same as in Example 1.

[0166] The difference between Example 32 and Example 1 is that, based on the total mass of the electrolyte, the mass content of LiPF6 is 10%, the mass content of LiTFSI is 4%, the mass content of vinylene carbonate as the third additive is 1%, and the mass content of methyl ethyl carbonate as the third solvent is 13%. All other contents are the same as in Example 1.

[0167] The difference between Example 33 and Example 1 is that, based on the total mass of the electrolyte, the mass content of LiPF6 is 10%, the mass content of LiTFSI is 4%, the mass content of vinylene carbonate as the third additive is 7.15%, and the mass content of methyl ethyl carbonate as the third solvent is 57.7%. All other contents are the same as in Example 1.

[0168] The difference between Example 34 and Example 1 is that, based on the total mass of the electrolyte, the mass content of LiPF6 is 10%, the mass content of LiTFSI is 4%, the mass content of vinylene carbonate as the third additive is 1%, and the mass content of methyl ethyl carbonate as the third solvent is 14%. All other contents are the same as in Example 1.

[0169] The difference between Example 35 and Example 1 is also as follows: The first lithium iron phosphate particle also has a first ion-conducting material Li filled in the first channel. 1.3 Al 0.3 Ti 1.7 (PO4)3, based on the total mass of the first lithium iron phosphate particles, Li 1.3 Al 0.3 Ti 1.7 The mass content of the (PO4)3 first ion-conducting material is 0.5%. The second lithium iron phosphate particle also contains a second ion-conducting material, Li, filled in the second channel. 1.3 Al 0.3 Ti 1.7 (PO4)3, based on the total mass of the second lithium iron phosphate particles, Li 1.3 Al 0.3 Ti 1.7 The mass content of the (PO4)3 second ion-conducting material is 0.5%.

[0170] Accordingly, the specific process differs from that of Example 1 as follows: In step (1), after the second high-temperature calcination, the first lithium iron phosphate with the first channel and the first ion-conducting material Li in the specified amount are... 1.3 Al 0.3 Ti 1.7 A mixed solution of (PO4)3 and ethanol solvent is mixed, then dried, and then carbon-coated with first lithium iron phosphate particles filled with first ion-conducting material in the first channel using chemical vapor deposition with polyethylene glycol and melamine in a mass ratio of 8:2 as carbon sources. In step (2), after the fourth high-temperature calcination, the second lithium iron phosphate with the second channel is mixed with the formulated amount of second ion-conducting material Li 1.3 Al 0.3 Ti 1.7A mixed solution of (PO4)3 and ethanol solvent is mixed, then dried, and then carbon-coated with second lithium iron phosphate particles filled with second ion-conducting material in the second channel is performed by chemical vapor deposition using polyethylene glycol and melamine in a mass ratio of 8:2 as carbon sources.

[0171] Furthermore, Example 35 differs from Example 1 in that, based on the total mass of the electrolyte, the mass content of LiPF6 is 10%, the mass content of LiTFSI is 4%, the mass content of vinylene carbonate as the third additive is 1.3%, and the mass content of methyl ethyl carbonate as the third solvent is 58.7%.

[0172] The rest of the content is the same as in Example 1.

[0173] The difference between Example 36 and Example 1 is also as follows: The first lithium iron phosphate particle also contains a first ion-conducting material, Li7La3Zr2O, filled in the first channel. 12 Based on the total mass of the first lithium iron phosphate particles, Li7La3Zr2O 12 The mass content of the first ion-conducting material is 1%. The second lithium iron phosphate particle also contains a second ion-conducting material, Li7La3Zr2O, filled in the second channel. 12 Based on the total mass of the second lithium iron phosphate particles, Li7La3Zr2O 12 The mass content of the second ion-conducting material is 1%.

[0174] Accordingly, the specific process differs from that of Example 1 as follows: In step (1), after the second high-temperature calcination, the first lithium iron phosphate with the first channel and the first ion-conducting material Li7La3Zr2O in the specified amount are mixed. 12 The mixture is prepared by mixing with ethanol solvent, then dried, and then carbon-coated with first lithium iron phosphate particles filled with first ion-conducting material in the first channel using chemical vapor deposition with polyethylene glycol and melamine in a mass ratio of 8:2 as carbon sources. In step (2), after the fourth high-temperature calcination, the second lithium iron phosphate with second channels is mixed with the formulated amount of second ion-conducting material Li7La3Zr2O. 12 The mixture is mixed with an ethanol solvent, then dried, and then carbon-coated with second lithium iron phosphate particles filled with second ion-conducting material in the second channel by chemical vapor deposition using polyethylene glycol and melamine in a mass ratio of 8:2 as carbon sources.

[0175] Furthermore, Example 36 differs from Example 1 in that, based on the total mass of the electrolyte, the mass content of LiPF6 is 10%, the mass content of LiTFSI is 4%, the mass content of vinylene carbonate as the third additive is 1.3%, and the mass content of methyl ethyl carbonate as the third solvent is 8.7%.

[0176] The rest of the content is the same as in Example 1.

[0177] Test example: The secondary batteries provided in the above embodiments and comparative examples were tested as follows: (1) Energy density testing process: The volume of the secondary batteries provided in the above embodiments and comparative examples after the second sealing process is recorded as the cell volume. In the sorting process, the cells are fully charged at 0.33 C with constant current and constant voltage until the charging limit voltage of 3.8 V is reached, the cutoff current is 0.05 C, and the cells are left to stand for 5 min. Then, they are discharged at 0.33 C with constant current until the discharge termination voltage of 2.0 V is recorded as the discharge capacity. The average voltage of the discharge voltage plateau is also recorded. The ratio of (discharge capacity × average voltage) / cell volume is the energy density of the cell, with the unit being Wh / L.

[0178] (2) Testing process for cycle capacity retention: At 25°C, the secondary batteries provided in the above embodiments and comparative examples were activated at 0.33 C, then charged at a constant current of 1 C to 3.8 V, followed by constant voltage charging to a cutoff current of 0.05 C, allowed to stand for 5 min, and then discharged at a constant current of 1 C to 2.0 V. This initial discharge specific capacity was recorded as C1. Cyclic testing was performed according to the following procedure: charging at a constant current of 1 C to 3.8 V, then charging at a constant voltage to a cutoff current of 0.05 C, allowed to stand for 5 min, and then discharging at a constant current of 1 C to 2.0 V. This cycle was repeated for 1000 cycles, and the discharge specific capacity C2 after 1000 cycles was recorded. The capacity retention rate after 1000 cycles was calculated using the formula: Capacity retention rate (%) = C2 / C1 × 100%.

[0179] (3) Testing process for fast charging performance: At room temperature of 25°C, the secondary batteries provided in the above embodiments and comparative examples were charged to 3.8V at a constant current of 5C, and then charged to the cutoff current of 0.05C at a constant voltage. The capacity charged during the constant current stage (C3) and the capacity charged during the constant voltage stage (C4) were recorded. The ratio of the capacity charged during the constant current stage to the total capacity charged was calculated as the constant current charging ratio, i.e., constant current charging ratio = C3 / (C3+C4)×100%, to evaluate the fast charging performance of the battery.

[0180] (4) Testing process for high-temperature storage performance: At room temperature (25°C), the secondary batteries provided in the above embodiments and comparative examples were charged to 3.8V at 0.5C, then charged at a constant voltage to a cutoff current of 0.05C, left to stand for 5 minutes, and then discharged at a constant current of 0.5C to a discharge termination voltage of 2.0V. The discharge capacity was recorded as C5. Next, the battery was charged at a constant current of 0.5C to 3.8V, then charged at a constant voltage to a cutoff current of 0.05C. The battery was then placed in an environment of 60°C±2 for 30 days. The secondary batteries were then discharged at 0.5C to 2.0V, and the discharge capacity was recorded as C6. Wherein C6 / C5×100% is the residual capacity retention rate of the battery after high-temperature storage.

[0181] The test results are shown in Table 3.

[0182] Table 3

[0183] As can be seen from Tables 1-3, the positive electrode sheet provided in this application uses three types of lithium iron phosphate particles with specific particle sizes. By modifying the channels of the first and second lithium iron phosphate particles and controlling the pore size of the channels in the first and second lithium iron phosphate particles respectively, the optimal balance between the compaction density and kinetic performance of the positive electrode sheet is achieved, thereby improving the energy density, fast charging performance, high-temperature storage performance and cycle stability of the secondary battery.

[0184] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active layer disposed on at least one surface of the positive current collector in the thickness direction, the positive active layer comprising a positive active material, characterized in that, The positive electrode active material includes lithium iron phosphate particles, which include first lithium iron phosphate particles, second lithium iron phosphate particles, and third lithium iron phosphate particles. In the cross-sectional SEM image along the thickness direction of the positive electrode active layer, within a 20 μm × 20 μm field of view, the ratio of the number of the first lithium iron phosphate particles to the number of the second lithium iron phosphate particles and the number of the third lithium iron phosphate particles is (1~10):(15~40):(50~80). The average particle size of the first lithium iron phosphate particle is denoted as D1 μm, which satisfies: 1.5≤D1≤5. The first lithium iron phosphate particle has a first channel, and the average pore size of the first channel is denoted as a nm, which satisfies: 2≤a≤30. The average particle size of the second lithium iron phosphate particle is denoted as D2 μm, which satisfies: 0.6≤D2≤1.

4. The second lithium iron phosphate particle has a second channel, and the average pore size of the second channel is denoted as b nm, which satisfies: 2≤b≤20. The average particle size of the third lithium iron phosphate particles is denoted as D3 μm, which satisfies the condition: D3≤0.

35.

2. The positive electrode sheet according to claim 1, characterized in that, At least one of the following conditions must be met: (a) 4 ≤ a ≤ 20; (b) 2 ≤ b ≤ 10; (c) The tortuosity of the first channel is 2 to 4; (d) The depth of the first channel is denoted as h1 μm, which satisfies: h1 / D1≤1 / 3; (e) In the first lithium iron phosphate particle, the number of pores x in the first channel satisfies: (D1 / 2-0.3)×8≤x≤(D1 / 2-0.3)×12, and x is rounded to the nearest integer. (f) The tortuosity of the second channel is 1~2; (g) The depth of the second channel is denoted as h2 μm, which satisfies: h2 / D2≤1 / 3; (h) In the second lithium iron phosphate particle, the number of pores y of the second channel satisfies: (D2 / 2-0.1)×9≤y≤(D2 / 2-0.1)×11, and y is rounded to the nearest integer. (i) The number of pores x in the first channel of the first lithium iron phosphate particle is greater than the number of pores y in the second channel of the second lithium iron phosphate particle.

3. The positive electrode sheet according to claim 1, characterized in that, In the positive electrode active layer, the particle size Dv50 of the lithium iron phosphate particles is 0.6 μm to 2 μm; And / or, in the positive electrode active layer, the particle size span value of the lithium iron phosphate particles is 2~5; And / or, at 25±3℃, the resistivity of the lithium iron phosphate particles is 1 Ω·cm~5 Ω·cm; And / or, under a pressure of 3T, the compacted density of the lithium iron phosphate particles is 2.65 g·cm³. -3 above.

4. The positive electrode sheet according to claim 1, characterized in that, The first lithium iron phosphate particle has a first carbon layer, which coats at least a portion of the surface of the first lithium iron phosphate particle; preferably, the first lithium iron phosphate particle further includes a first ion-conducting material, which fills the first channel; And / or, the second lithium iron phosphate particle has a second carbon layer, the second carbon layer covering at least a portion of the surface of the second lithium iron phosphate particle; preferably, the second lithium iron phosphate particle further includes a second ion-conducting material, the second ion-conducting material filling the second channel; And / or, the third lithium iron phosphate particle has a third carbon layer that covers at least a portion of the surface of the third lithium iron phosphate particle.

5. The positive electrode sheet according to claim 4, characterized in that, The first ion-conducting material includes at least one of lithium aluminum titanium phosphate, lithium lanthanum zirconium oxide, and lithium lanthanum titanium oxide; And / or, based on the total mass of the first lithium iron phosphate particles, the mass content of the first ion-conducting material is 0.5% to 1%; And / or, the second ion-conducting material includes at least one of lithium aluminum titanium phosphate, lithium lanthanum zirconium oxide, and lithium lanthanum titanium oxide; And / or, based on the total mass of the second lithium iron phosphate particles, the mass content of the second ion-conducting material is 0.5% to 1%; And / or, based on the total mass of the first lithium iron phosphate particles, the mass content of the first carbon layer is 1% to 1.4%; And / or, based on the total mass of the second lithium iron phosphate particles, the mass content of the second carbon layer is 1% to 1.4%; And / or, based on the total mass of the third lithium iron phosphate particles, the mass content of the third carbon layer is 1% to 1.4%.

6. The positive electrode sheet according to any one of claims 1 to 5, characterized in that, The compaction density of the positive electrode is 2.65 g·cm³. -3 ~2.95 g·cm -3 .

7. The positive electrode sheet according to any one of claims 1 to 5, characterized in that, The positive electrode active layer includes a first active layer and a second active layer stacked together. The first active layer is located between the positive electrode current collector and the second active layer. The first active layer includes the first lithium iron phosphate particles and the second lithium iron phosphate particles. The second active layer includes the third lithium iron phosphate particles. Preferably, the thickness ratio of the first active layer to the second active layer is (7~9):(1~3).

8. A secondary battery, characterized in that, The secondary battery includes the positive electrode sheet as described in any one of claims 1 to 7.

9. The secondary battery according to claim 8, characterized in that, The secondary battery further includes an electrolyte, which includes a solvent, comprising a first solvent and a second solvent. The first solvent includes ethylene carbonate and / or propylene carbonate, and the second solvent includes ethyl acetate and / or methyl acetate. Preferably, based on the total mass of the electrolyte, the mass content of the first solvent is 15% to 40%; Preferably, based on the total mass of the electrolyte, the mass content of the second solvent is 1% to 40%, more preferably 5% to 30%.

10. The secondary battery according to claim 9, characterized in that, The electrolyte includes additives, which include a first additive and / or a second additive. The first additive includes at least one of lithium nitrate, lithium difluorophosphate, and lithium tetrafluoroborate. The second additive is a nitrile additive, including at least one of acetonitrile, propionitrile, and fluoroacetonitrile. Preferably, based on the total mass of the electrolyte, the mass content of the first additive is 0.1% to 3%; Preferably, the mass content of the second additive is 0.1% to 10% based on the total mass of the electrolyte.

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