Lithium ion secondary battery, battery device, power utilization device, preparation method of positive active material and preparation method of positive pole piece

Abstract

The invention provides a lithium ion secondary battery, a battery device, a power utilization device, a preparation method of a positive electrode active material and a preparation method of a positive electrode plate. The lithium ion secondary battery comprises a positive pole piece, a negative pole piece and an electrolyte, the positive pole piece comprises a positive current collector and a positive film layer arranged on at least one side of the positive current collector, and the positive film layer comprises a positive active material; the positive electrode active material comprises lithium-containing transition metal phosphate particles, at least part of the surfaces of the lithium-containing transition metal phosphate particles are provided with carbon coating materials, and in the tangent plane of the positive electrode film layer in the thickness direction of the pole piece, DA90 of the particles is 1400 nm to 2100 nm; the particle size concentration ratio (DA90-DA10) / DA50 is 1.855-2.375, and DA90, DA50 and DA10 refer to the particle sizes of the corresponding particles when the cumulative area distribution of the particles reaches 90%, 50% and 10% in an area cumulative distribution curve of the particles.

Description

[0001] This application claims the priority of the PCT international application PCT / CN2025 / 085921, titled "Lithium-ion secondary battery, battery device, power-consuming device, preparation method of positive electrode active material and preparation method of positive electrode sheet", filed on March 28, 2025, and the entire content of this application is incorporated herein by reference. Technical Field

[0002] This application relates to the technical field of lithium-ion secondary batteries, and particularly to a lithium-ion secondary battery, a battery device, a power-consuming device, a preparation method of a positive electrode active material, and a preparation method of a positive electrode sheet. Background Art

[0003] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power stations, as well as in multiple fields such as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace.

[0004] The positive electrode active material is an important component of secondary batteries. Lithium-containing transition metal phosphate materials have the characteristics of stable structure, good safety, and long cycle life, and have broad development prospects. With the increasing market requirements for the energy density and kinetics of lithium-containing transition metal phosphate-based secondary batteries, it is difficult to simultaneously improve the above performances in the prior art, which has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] In view of the above problems, this application provides a lithium-ion secondary battery. By regulating the particle size concentration and large particle size in the positive electrode film layer, and forming a close packing through reasonable grading, while having a high energy density, the lithium-ion diffusion and conduction paths are optimized, and the kinetic performance of the secondary battery is taken into account.

[0006] The first aspect of this application provides a lithium-ion secondary battery, including a positive electrode sheet, a negative electrode sheet, and an electrolyte. The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one side of the positive electrode current collector. The positive electrode film layer includes a positive electrode active material. The positive electrode active material includes lithium-containing transition metal phosphate particles with at least part of their surfaces provided with a carbon coating material. In the cross-section of the positive electrode film layer along the thickness direction of the sheet, the D A90 of the particles is 1400 nm - 2100 nm, and the particle size concentration ((D A90 - D A10 ) / D A50 is 1.855 - 2.375, where D A90 , D A50、 D A10It refers to the particle sizes corresponding to when the cumulative area distribution of particles reaches 90%, 50%, and 10% in the area cumulative distribution curve of the particles.

[0007] In the prior art, the tap density of the electrode sheet is often improved and the energy density of the battery is increased by increasing the particle size concentration in the positive electrode film layer. However, research shows that, different from the understanding in the prior art, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, too large a particle size concentration has limited effect on improving the tap density of the electrode sheet, and it is difficult to further improve the tap density of the electrode sheet by adjusting the particle size concentration. When the particle size concentration of the particles is less than 1.855, it represents that the particle size distribution difference is small, the grading effect is not significant, and there is a lack of smaller particles to fill the gaps between larger particles. After the concentration exceeds 2.375, it means that the size difference of the active material particles is significant, making it impossible for the positive electrode film layer to achieve the optimal close packing, and it is difficult to further increase the tap density of the electrode sheet. The common method in the prior art to adjust the particle size concentration is to increase the size or proportion of large particles. However, the applicant further studies and finds that when the D A90 of the particles in the positive electrode film layer is greater than 2100 nm, the large particles in the positive electrode film layer will significantly deteriorate the kinetic performance of the battery.

[0008] In the embodiments of the present application, by controlling the particle size concentration of the particles in the positive electrode film layer (D A90 -D A10 ) / D A50 to be 1.855 - 2.375, and D A90 to be 1400 nm - 2100 nm, while improving the packing situation of the particles in the positive electrode film layer, increasing the tap density of the electrode sheet, and increasing the energy density of the battery, the battery maintains high kinetic performance, thereby achieving the balance between the kinetic performance and the energy density of the battery.

[0009] In any embodiment, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the D A90 of the particles is 1400 nm - 2000 nm.

[0010] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the D A90 of the particle size of the particles can reduce the internal resistance of the battery within the above range, and further suppress the negative impact of too large particles on the kinetic performance.

[0011] In any embodiment, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the D A50 of the particles is 600 nm - 900 nm, and can be 650 nm - 750 nm.

[0012] The D A50Within the above range, on the one hand, it is beneficial to maintain the supporting effect of the particles, enabling the rolling pressure to be evenly transmitted among the particles of the electrode sheet, and the electrode sheet to withstand a higher rolling pressure; on the other hand, it means that the overall particle size of the particles inside the positive electrode film layer is relatively small, keeping the kinetic performance at a high level and improving the energy density while taking into account the kinetics of the battery.

[0013] In any embodiment, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the D of the particles A10 is 100 nm - 300 nm, and can be optionally 120 nm - 250 nm.

[0014] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the D of the particles A10 Within the above range, on the one hand, it indicates that there are certain small particles in the film layer to form a graded filling of the voids between the particles, and at the same time, it will not cause agglomeration due to the overly small particle size; on the other hand, it indicates that the proportion of small-sized particles is limited, which can improve the compaction of the electrode sheet while reducing the side reactions between the small particles and the electrolyte, taking into account the cycle performance of the battery.

[0015] In any embodiment, in the cumulative distribution curve of the graphitization degree C value obtained by the positive electrode film layer in the surface scanning mode of a laser confocal Raman spectrometer, the median C of the graphitization degree 50 is 0.98 - 1.20, and can be optionally 1.02 - 1.10; where the graphitization degree C value is I G / I D where I G represents the intensity of the G peak of the Raman spectrum at 1580 ± 100 cm -1 and I D represents the intensity of the D peak of the Raman spectrum at 1350 ± 100 cm -1 .

[0016] The higher the graphitization degree of the carbon on the surface of the positive electrode active material, the higher the proportion of graphite-structured carbon in the positive electrode film layer, and the easier it is for the particles to slip during the rolling process by means of the carbon structure with a high graphitization degree in the coating material. On the basis of a reasonable grading of the particles in the electrode sheet, even at a low rolling pressure, it is possible to further increase the compaction density of the electrode sheet; the high graphitization degree of the carbon coating layer on the surface of the positive electrode active material is beneficial to electron transport, achieving a balance between energy density and kinetics.

[0017] In any embodiment, in the cumulative distribution curve of the sphericity area of the particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median L of the sphericity A50 is 0.70 - 0.85, and can be optionally 0.70 - 0.76.

[0018] The median L of the sphericity A50The particles within the above range are approximately spherical, which helps maintain good slidability during particle packing, facilitates filling the voids between particles, can further improve the compaction density of the electrode sheet, and enhance the energy density of the battery.

[0019] In any embodiment, in the cumulative area distribution curve of the roughness of the particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median roughness R A50 is 0.92 - 0.96.

[0020] The median roughness R 50 The surfaces of the particles within the above range are relatively smooth, and the frictional force between particles is relatively small. They are prone to slip under external forces, can further improve the compaction density of the electrode sheet, and enhance the energy density of the battery.

[0021] In any embodiment, in the cumulative area distribution curve of the roughness of the particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the concentration of roughness (R A90 -R A10 ) / R A50 is 0.05 - 0.10. The extremely small value of the roughness concentration indicates a high degree of roughness consistency of the overall particles, which is conducive to the relative sliding between particles, is more likely to form a high-density packing during rolling, and increases the compaction density of the electrode sheet and the energy density of the battery.

[0022] In any embodiment, the iron dissolution rate of the positive electrode film layer is 400 ppm - 1800 ppm.

[0023] The iron dissolution rate of the positive electrode film layer mainly comes from the lithium-containing transition metal phosphate positive electrode active material in the positive electrode film layer. The iron dissolution rate of the positive electrode film layer within the above range indicates that the surface of the positive electrode active material has a relatively complete and dense carbon coating material, which can improve the electrical contact between positive electrode active materials, improve the conductivity of the positive electrode active material, reduce the polarization of the positive electrode active material, and further optimize the kinetic performance of the lithium-ion secondary battery. At the same time, the high-integrity carbon coating material structure makes the particles prone to stress slip during the rolling process, and can simultaneously improve the compaction density of the electrode sheet and the energy density of the battery.

[0024] In any embodiment, based on the total mass of the positive electrode active material, the mass content of carbon element is 0.8% - 1.8%, and can be optionally 0.9% - 1.5%. Compared with the lithium-containing transition metal phosphate positive electrode active material in the prior art, this positive electrode active material has a relatively low content of carbon coating, which can further increase the loading of lithium-containing transition metal phosphate in the positive electrode sheet and improve the energy density of the lithium-ion secondary battery.

[0025] In any embodiment, the lithium-iron antisite defect concentration of the positive electrode active material is 0.001% - 1.5%, and may be optionally 0.01% - 1.0%. The positive electrode active material having low lithium-iron antisite defects is beneficial to the uniform transport of lithium ions in the solid phase, and further improves the kinetic performance of the lithium-ion secondary battery.

[0026] In any embodiment, the lithium-containing transition metal phosphate includes a component having the following general formula: Li m Fe x P y O j Q q , where Q includes one or more of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.8 ≤ m ≤ 1.15, 0.9 ≤ x ≤ 1, 0.95 ≤ y ≤ 1, 3.5 ≤ j ≤ 4, 0 < q ≤ 0.1.

[0027] Selecting an appropriate modification element Q can improve the lattice change rate of the positive electrode active material during the process of lithium deintercalation / insertion, reduce the oxygen activity on the particle surface, improve the structural stability of the material, and further improve the specific capacity utilization level of the material during the cycling process, and further improve the cycling stability of the lithium-ion secondary battery.

[0028] In any embodiment, the positive electrode active material includes one or more of lithium iron phosphate and its doped modification materials and coated modification materials.

[0029] In any embodiment, the positive electrode active material includes titanium element. Based on the total mass of the positive electrode active material, the mass content of titanium element is 4000 ppm - 8000 ppm. The high addition amount of titanium element does not form a harmful impurity phase that has a negative impact on the battery energy density and kinetic performance. Although the reason is not yet clear, it is speculated that titanium element may form a fast ion conductor together with phosphate groups and other elements (such as lithium element), which instead has a promoting effect on the kinetic performance of the battery.

[0030] In any embodiment, the tapped density of the powder of the positive electrode active material is 1.00 g / cm 3 -1.70 g / cm 3 , and may be optionally 1.20 g / cm 3 -1.50 g / cm 3 . The particles of this positive electrode active material not only have a wide particle size distribution, but also the particle size is in a reasonable range, forming an effective grading. The small particles can fill the voids between the particles, so it has a high tapped density.

[0031] In any embodiment, the powder tap density of the positive electrode active material under a pressure of 3T is 2.55 g / cm 3 - 2.70 g / cm 3 , and may be optionally 2.58 g / cm 3 - 2.68 g / cm 3 . The positive electrode active material particles form an effective gradation, enabling the positive electrode active material to construct a stacking structure with extremely small particle gaps under an external force, achieving a higher tap density, and providing a material basis for improving the tap density of the electrode sheet and preparing a lithium-ion secondary battery with a high energy density.

[0032] In any embodiment, the powder resistivity of the positive electrode active material under a pressure of 8 MPa is 0.5 Ω·cm - 60.0 Ω·cm, and may be optionally 2.0 Ω·cm - 40.0 Ω·cm.

[0033] The positive electrode active material is surface-coated with a carbon material. Due to the sp 2 structure of the surface carbon, it is easy to achieve rapid electron conduction between particles, endowing the positive electrode active material with a low powder resistivity, which is beneficial to improving the solid-phase transmission rate of electrons and the kinetic performance of the battery.

[0034] In any embodiment, the discharge specific capacity of the positive electrode active material at a discharge rate of 1C at room temperature is 135 mAh / g - 150 mAh / g. The high discharge specific capacity of the positive electrode active material at a rate of 1C indicates its good charge-discharge ability, which is beneficial to improving the kinetic performance of the battery.

[0035] In any embodiment, the discharge capacity ratio η of the positive electrode active material discharged to 3.2V ≥ 85%. η is defined as follows: at room temperature, a coin cell containing the positive electrode active material is charged and discharged at a constant current twice within a voltage range of 2.0V to 3.75V at a rate of 0.1C, and then charged and discharged at a constant current once at a rate of 1C. During the charge-discharge test at a rate of 1C, the capacity value at a discharge voltage of 3.2V is extracted and denoted as C 1 , the capacity value when the discharge voltage reaches 2.0V is C 2 , η = C 1 / C 2 , where the charging process includes constant voltage charging, with a constant voltage of 3.75V and a constant voltage cut-off current of 50 μA.

[0036] A high discharge capacity ratio of the positive electrode active material discharged to 3.2V in a lithium-ion secondary battery means that the positive electrode active material has good kinetic performance. At the same time, a high η value indicates that when the lithium-ion secondary battery containing the positive electrode active material is discharged to a low state of charge (SOC), it still has a high voltage, which is beneficial to maintaining good power performance.

[0037] In any embodiment, based on the total mass of the positive electrode film layer, the mass content of the conductive agent is 0.01 - 1.5%. Since the particles in the positive electrode film layer form a close packing, the positive electrode active material particles are coated with a highly graphitized carbon layer on the surface and are in sufficient contact with each other, having good electronic conductivity, which can reduce the use of the conductive agent in the positive electrode film layer, is beneficial to further increase the loading amount of the positive electrode active material, and improve the energy density of the lithium-ion secondary battery.

[0038] In any embodiment, the positive electrode film layer further includes a binder. Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 94.0% - 99.4%, optionally 96.5% - 99.4%; the mass content of the binder is 0.5% - 3.0%.

[0039] When the mass content of the positive electrode active material and the mass content of the binder are within the above ranges, it can effectively increase the loading amount of the active substance per unit volume of the positive electrode film layer, maintain good internal cohesion, reduce the occurrence probability of problems such as powder falling and swelling and cracking, and take into account the safety performance while improving the energy density of the secondary battery.

[0040] In any embodiment, the single-sided areal density of the positive electrode film layer is 300 mg / 1540 mm 2 - 450 mg / 1540 mm 2 . The positive electrode film layer with the areal density within the above range can help improve the energy density of the lithium-ion secondary battery.

[0041] In any embodiment, when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.52 g / cm 3 - 2.78 g / cm 3 .

[0042] In any embodiment, when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.55 g / cm 3 - 2.75 g / cm 3 .

[0043] When the tap density of the positive electrode film layer is within the above range, it is beneficial to improve the energy density of the lithium-ion secondary battery.

[0044] In any embodiment, the positive electrode film layer satisfies at least one of the following conditions: (1) When the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.52 g / cm 3 - 2.78 g / cm 3, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10% - 22%; (2) when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.55 g / cm 3 - 2.75 g / cm 3 , in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10% - 20%.

[0045] The lower the porosity in the cross-section of the positive electrode film layer, on the one hand, it means that the particle size distribution of large, medium and small particles in the positive electrode film layer is more optimal, and the tap density is high. On the other hand, after the same particle size distribution and rolling pressure, if the porosity is low, it means that the particles are easy to slip relative to each other, thereby reducing the risk of overpressure and stress concentration in the film layer, further reducing the probability of the positive electrode film peeling off during long-term cycling, which is beneficial to improving the long-term cycling performance of the battery.

[0046] In any implementation, the positive electrode sheet includes a bottom coating, and the bottom coating is disposed between the positive electrode film layer and the current collector; the bottom coating satisfies at least one of the following conditions: (1) the bottom coating includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating ≤ 10 pcs / 10 μm; (2) the tap density of the positive electrode sheet in the fully discharged state is greater than or equal to 2.4 g / cm 3 , the single-sided thickness of the bottom coating is 1 μm - 4 μm; (3) the tap density of the positive electrode sheet in the fully discharged state is greater than or equal to 2.5 g / cm 3 , the single-sided thickness of the bottom coating is 2 μm - 4 μm.

[0047] The bottom coating is beneficial to improving the conductivity and adhesion between the positive electrode film layer and the current collector, reducing the peeling off of the positive electrode film layer from the current collector during cycling, and simultaneously improving the kinetic performance of the battery.

[0048] With the increase of the tap density of the electrode sheet, the extrusion effect of the lithium-containing phosphate material of large particles (such as particle size greater than 1 μm) in the positive electrode film layer on the bottom coating becomes more significant. Therefore, stress concentration is likely to occur at the large particle sites, and even damage the current collector by penetrating through the bottom coating. Increasing the thickness of the bottom coating is beneficial to improving the stress concentration phenomenon in the electrode sheet and further increasing the ultimate tap density of the electrode sheet.

[0049] The second aspect of the present application provides a battery device, including the lithium-ion secondary battery of the first aspect of the present application.

[0050] The third aspect of the present application provides an electrical device, including at least one of the lithium-ion secondary battery of the first aspect of the present application and the battery device of the third aspect of the present application.

[0051] The fourth aspect of the present application provides a method for preparing a positive electrode active material, and the preparation method includes: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source; adding a solvent and grinding to obtain a mixed slurry; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder to obtain the positive electrode active material; the sintering of the precursor powder to obtain the positive electrode active material includes at least two sinterings; after the first sintering, a first sintered product is obtained, the first sintered product and the carbon source are mixed to obtain an intermediate raw material, the intermediate raw material is divided into two groups, and grinding is respectively carried out to obtain a first group of ground products and a second group of ground products, the first group of ground products and the second group of ground products are mixed to obtain a mixed intermediate product; the mixed intermediate product is subjected to a second sintering to obtain the positive electrode active material; wherein, the D V50 of the first group of ground products is 0.8 μm - 1.2 μm; the D V50 of the second group of ground products is 0.3 μm - 0.5 μm.

[0052] By adjusting the mass ratio of the first group of ground products and the second group of ground products, positive electrode active substances with different grading ratios can be obtained, so that the area ratio of particles of each particle size can be finely adjusted, and the required packing structure can be obtained.

[0053] The two - sintering process can effectively shorten the sintering time in the high - temperature range, and further reduce the risk and probability of extremely large particles appearing during high - temperature sintering. By adjusting the particle sizes of the two groups of grinding during the second sintering, the activity of the particles can be controlled, so that the positive electrode active material has large particles with a certain area ratio, while improving the compaction density of the electrode sheet and the energy density of the battery cell, and making the battery cell have good kinetic performance at the same time.

[0054] The fifth aspect of the present application provides a method for preparing a positive electrode sheet, and the preparation method includes adding a binder, a conductive agent, and the positive electrode active material prepared by the preparation method of the fourth aspect of the present application in sequence, dry - mixing, adding a solvent, stirring, and adjusting the viscosity to obtain a shipping slurry; transferring and coating the shipping slurry on at least one side of a current collector, and drying and hot - pressing to obtain a positive electrode film layer.

[0055] In any embodiment, the drying temperature is 95°C - 105°C, and the speed is 2.0 m / min - 2.3 m / min.

[0056] In any embodiment, the hot - pressing includes at least three hot - roll pressings, the hot - roll pressure increases in sequence, and the hot - roll pressures are 20 tons - 50 tons, 50 tons - 70 tons, and 70 tons - 90 tons in sequence; the hot - roll temperature is 40°C - 80°C, and before the first entry into the hot - roll compaction, the electrode sheet is heated, and the heating temperature is 40°C - 50°C.

[0057] The positive electrode active material prepared by using the above hot pressing process in combination with the preparation method of the fourth aspect is beneficial to further reduce the porosity of the cut surface of the positive electrode film layer, improve the ultimate compaction density of the electrode sheet, and improve the energy density of the battery.

[0058] The above description is only an overview of the technical solution of the present application. In order to be able to understand the technical means of the present application more clearly, it can be implemented according to the content of the specification. And in order to make the above and other purposes, features and advantages of the present application more obvious and understandable, the following specifically illustrates the specific implementation manners of the present application. Brief Description of the Drawings

[0059] Figure 1 is a scanning electron microscope image of the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet in an embodiment of the present application; Figure 2 is a schematic diagram of a lithium-ion secondary battery in an embodiment of the present application; Figure 3 is an exploded schematic diagram of a lithium-ion secondary battery in an embodiment of the present application; Figure 4 is a schematic diagram of a battery module in an embodiment of the present application; Figure 5 is a schematic diagram of a battery pack in an embodiment of the present application; Figure 6 is Figure 5 an exploded schematic diagram of the battery pack shown; Figure 7 is a schematic diagram of an electrical device powered by a lithium-ion secondary battery in an embodiment of the present application.

[0060] Figure 8 is a porosity test chart of the cross-section of the positive electrode film layer along the thickness direction in an embodiment of the present application.

[0061] Description of the Reference Numerals: 1 Battery pack; 2 Upper box body; 3 Lower box body; 4 Battery module; 5 Secondary battery; 51 Housing; 52 Electrode assembly; 53 Cover plate. Detailed Description of the Embodiments

[0062] Hereinafter, embodiments of the lithium-ion secondary battery, battery device, and electrical device of the present application specifically disclosed will be described in detail with appropriate reference to the drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually identical structures are omitted. This is to avoid making the following descriptions unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following descriptions are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter recited in the claims.

[0063] The "ranges" disclosed in this application are defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The ranges defined in this way can include or exclude the end values, and can be combined arbitrarily, that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, ranges of 60-110 and 80-120 are understood to be contemplated. In addition, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise specified, the numerical range "a-b" represents an abbreviated representation of any real number combination between a and b, where both a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been fully listed herein, and "0-5" is only an abbreviated representation of these numerical combinations. Additionally, when stating that a certain parameter is an integer ≥2, it is equivalent to disclosing that the parameter is, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0064] If there is no special instruction, all embodiments and optional embodiments of this application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.

[0065] If there is no special instruction, all technical features and optional technical features of this application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.

[0066] If there is no special instruction, all steps of this application can be carried out sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method can include steps (a) and (b) carried out sequentially, or can also include steps (b) and (a) carried out sequentially. For example, it is mentioned that the method may further include step (c), which means that step (c) can be added to the method in any order. For example, the method can include steps (a), (b), and (c), or can also include steps (a), (c), and (b), or can also include steps (c), (a), and (b), etc.

[0067] In this application, the terms "a plurality of" and "a variety of" mean two or more than two.

[0068] Unless otherwise specified, the terms used in this application have the well-known meanings commonly understood by those skilled in the art.

[0069] Unless otherwise specified, the values of the various parameters mentioned in this application can be measured using various commonly used testing methods in the art. For example, they can be measured according to the testing methods given in the embodiments of this application. Unless otherwise specified, the test temperature for each parameter is 25°C.

[0070] The battery mentioned in the embodiments of this application can be a single physical module including one or more lithium-ion secondary batteries to provide higher voltage and capacity. For example, the battery mentioned in this application can include lithium-ion secondary batteries, battery cells, battery modules, or battery packs, etc.

[0071] The lithium-ion secondary battery is the smallest unit that makes up the battery and can independently perform the functions of charging and discharging. The lithium-ion secondary battery can be in the shape of a cylinder, a cuboid, or other shapes, and the embodiments of this application do not limit this. As Figure 2 is a lithium-ion secondary battery 5 in the shape of a cuboid as an example.

[0072] The lithium-ion secondary battery includes an electrode assembly and an electrolyte.

[0073] The lithium-ion secondary battery can also include an outer package, which can be used to encapsulate the electrode assembly and the electrolyte. The outer package can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0074] In some embodiments, as Figure 3 shown, the outer package can include a housing 51 and a cover plate 53. The housing 51 can include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. The electrode assembly 52 is encapsulated in the receiving cavity. The number of electrode assemblies 52 included in the lithium-ion secondary battery 5 can be one or more, which can be adjusted according to requirements.

[0075] The electrode assembly generally includes a positive electrode plate and a negative electrode plate. The negative electrode plate is the electrode where the reaction of absorbing or lithiating lithium ions occurs during charging and releasing or delithiating lithium during discharging. The positive electrode plate is the electrode where the reaction of releasing or delithiating lithium ions occurs during charging and absorbing or lithiating lithium during discharging.

[0076] When there are multiple lithium-ion secondary batteries, the multiple lithium-ion secondary batteries are connected in series, parallel or in a combined series-parallel manner through a busbar component. In some embodiments, the battery can be a battery module; when there are multiple lithium-ion secondary batteries, the multiple lithium-ion secondary batteries are arranged and fixed to form a battery module. In some embodiments, the battery can be a battery pack, which includes a box body and lithium-ion secondary batteries, and the lithium-ion secondary batteries or battery modules are accommodated in the box body. In some embodiments, the box body can be part of the chassis structure of a vehicle. For example, part of the box body can become at least part of the vehicle floor, or part of the box body can become at least part of the cross beams and longitudinal beams of the vehicle.

[0077] In some embodiments, the battery can be an energy storage device. The energy storage device includes an energy storage container, an energy storage electrical cabinet, etc.

[0078] In some embodiments, the lithium-ion secondary batteries can be assembled into a battery module, and the number of lithium-ion secondary batteries included in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module. Figure 4 It is a schematic diagram of a battery module 4 as an example. As Figure 4 shown, in the battery module 4, multiple lithium-ion secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other way. Further, the multiple lithium-ion secondary batteries 5 can be fixed by fasteners.

[0079] Optionally, the battery module 4 can further include a housing with an accommodation space, and the multiple lithium-ion secondary batteries 5 are accommodated in the accommodation space.

[0080] In some embodiments, the above battery module can be further assembled into a battery pack, and the number of battery modules included in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0081] Figure 5 and Figure 6 It is a schematic diagram of a battery pack 1 as an example. As Figure 5 and Figure 6 shown, the battery pack 1 can include a box body and multiple battery modules 4 arranged in the box body. The box body includes an upper box body 2 and a lower box body 3. The upper box body 2 is used to cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in the box body in any way.

[0082] Lithium-containing transition metal phosphate materials have been widely used in lithium-ion batteries due to their stable structure, good safety and long cycle life; however, they have problems such as low electronic conductivity and low stacking efficiency, which makes it difficult to further effectively increase the loading amount of lithium-containing transition metal phosphate per unit volume of the battery, and cannot meet the needs of high energy density batteries.

[0083] In order to further improve the battery energy density and increase the compaction density of the pole piece, the common method in the industry is to increase the particle grading. The commonly used method to improve the particle grading in the prior art is to increase the proportion of large particles to form a stacking skeleton to play a role in structural support. However, studies have shown that the increase in the proportion of large particles will increase the lithium ion diffusion path and deteriorate the kinetic performance of the battery. How to obtain a battery that takes into account both high energy density and kinetic performance is a technical problem that needs to be solved urgently in this field.

[0084] The first aspect of the present application provides a lithium-ion secondary battery, the lithium-ion secondary battery comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer comprises a positive electrode active material, the positive electrode active material comprises lithium-containing transition metal phosphate particles having a carbon coating material arranged on at least a portion of the surface, and in a cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the D A90 1400nm-2100nm, particle size concentration (D A90 -D A10 ) / D A50 is 1.855-2.375, where D A90 , D A50、 D A10 It refers to the particle size corresponding to when the cumulative area distribution of the particles reaches 90%, 50%, and 10% in the particle area cumulative distribution curve.

[0085] In the prior art, the electrode compaction density and battery energy density are often improved by increasing the particle size concentration of the particles in the positive electrode film layer. However, studies have shown that, contrary to the prior art, in the cross-section of the positive electrode film layer along the thickness direction of the electrode, excessive particle size concentration has limited effect on improving the compaction of the electrode, and it is difficult to further improve the compaction of the electrode by adjusting the particle size concentration. The particle size concentration of the particles is less than 1.855, which means that the difference in particle size distribution is small, the grading effect is not significant, and there is a lack of smaller particles to fill the gaps between larger particles. When the concentration exceeds 2.375, it means that the size of the active material particles is significantly different, so that the positive electrode film layer cannot achieve the optimal dense stacking, and it is difficult to further improve the compaction density of the electrode. In the prior art, the common way to adjust the particle size concentration is to increase the size or proportion of large particles. However, the applicant further found that when the D of the particles in the positive electrode film layer is too small, the particle size concentration of the particles in the positive electrode film layer is too small. A90When it is greater than 2100 nm, large particles in the positive electrode film layer will significantly deteriorate the kinetic performance of the battery.

[0086] In this application, by controlling the particle size concentration (D A90 -D A10 ) / D A50 is 1.855 - 2.375, D A90 is 1400 nm - 2100 nm. While improving the packing situation of particles in the positive electrode film layer, increasing the compaction density of the electrode sheet, and improving the energy density of the battery, the battery can maintain high kinetic performance, thereby achieving the balance between the kinetic performance and energy density of the battery.

[0087] Lithium-containing transition metal phosphate refers to a phosphate material containing lithium element and transition metal element, which can be detected by any well-known method in the art. For example, it can be detected by the combination of X-ray diffractometer (XRD) and energy spectrum analyzer.

[0088] The carbon coating material disposed on at least part of the surface of the lithium-containing transition metal phosphate can be detected by any well-known method in the art. As an example, by using the combination of transmission electron microscope and energy spectrum analyzer to characterize the lithium-containing transition metal phosphate, the carbon coating material disposed on at least part of the surface of the lithium-containing transition metal phosphate can be observed.

[0089] In this application, the term "particle" refers to a particle with a recognizable complete boundary in the field of view of the positive electrode film layer at a certain magnification, such as 10,000 times. There may be defects and scratches inside the particle, but no complete boundary sufficient to divide the particle can be recognized inside the particle.

[0090] The method for identifying particles is as follows: Cut the positive electrode film layer along the thickness direction of the electrode sheet by argon ion beam (as an example, the equipment model can be selected: Leica EM TIC 3X CP, working voltage: 6 kV, working duration: 6 h). After exposing the section, use a scanning electron microscope (as an example, the equipment model can be selected: Hitachi SU8230, working voltage: 3 kV, beam current: high, probe model: U(LA100), working distance <5 mm) to observe the section of the positive electrode film layer along the thickness direction of the electrode sheet. Collect images in the secondary electron mode at a non-edge position in the section of the positive electrode film layer (after observing the edge of the electrode sheet under the scanning electron microscope, adjust the field of view to the central part of the sample), take an electron microscope image at a magnification of 10k times, and analyze the particles in the electron microscope image with ImageJ software (1.46r, win64 version). The specific method of using ImageJ software is as follows: Load the scanning electron microscope image to be analyzed, such as Figure 1As shown; use the Cellpose plug-in software therein to identify particles, and perform manual correction on this basis; use Image J to read and count data. The specific method of using the Cellpose plug-in software therein to identify particles is as follows: set the segmentation diameter parameter (diameter in the Segmantation module) to 15 pixels, click "run cyto3" to perform particle identification; manually mark the particles in the image that are not recognized by the software, not fully recognized by the software, or have recognition errors. The particles in the image that are not recognized by the software, not fully recognized by the software, or have recognition errors mainly include the following types: 1. Due to the particle being too large or having scratches on the particle surface, the particle cannot be recognized or cannot be fully recognized; 2. During the argon ion beam cutting process, scratches will be generated on the particle surface, and the software may misjudge the scratches as the particle boundary during the recognition process, resulting in recognition errors; 3. Due to the particle being too small, it cannot be successfully recognized; 4. The particle is located at the edge of the electron microscope field of view, and the inside of the particle is penetrated by the edge, and the morphology cannot be completely displayed, and the local part is recognized instead of the whole, resulting in recognition errors. For the above unrecognized or misrecognized particles, manual calibration is performed, and the specific process is as follows: delete the particles located at the four edges of the scanning electron microscope that cannot be completely displayed; judge whether there are gap scratches inside other unrecognized or misrecognized particles. If there are no gap scratches inside the particle, judge it as a particle, and manually mark it according to the particle boundary observed manually; in response to the presence of gap scratches inside the particle, judge whether the gap scratches penetrate the particle. If the particle is not penetrated, judge it as a particle and perform manual marking; in response to the gap scratches penetrating the particle, judge whether the gap scratches are linear or irregular; in response to the gap scratches being irregular, judge it as the boundary between particles and divide the particles along this boundary; in response to the gap scratches being linear, perform contrast; in response to the contrast being not obvious and there being no sense of crack, judge it as a scratch and mark it as a particle; in response to the contrast being strong and there being a sense of crack, judge it as the boundary between particles and mark it as two particles. After manual marking, delete the information irrelevant to the particles during the automatic image processing process, that is, the determination and marking of the particles in the picture are completed.

[0091] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode plate, the D of the particles A90 is 1400 nm - 2100 nm.

[0092] In the cross-section of the positive electrode film layer along the thickness direction of the electrode plate, the D of the particles A90The calculation method is as follows. The image after particle determination and labeling is imported into ImageJ software for analysis. The scale is set according to the scanning electron microscope image, and the particle size and area of the particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode are analyzed through the analysis functions of "Feret diameter", "Area", "Round", and "Solidity". According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter obtained by analysis represents the maximum distance between all parallel lines in the two-dimensional projection of the particle, which characterizes the particle size of the particle; and the obtained "Area" parameter represents the pixel area of the particle. Since particles with a particle size less than 50 nm have large errors in the statistical process and are difficult to accurately identify, and the particle size of the conductive agent is generally less than 50 nm, which will cause large errors in the statistical results. Therefore, in the particle size statistics process of this application, particles with a particle size less than 50 nm are not counted, and the particle statistical data corresponding to "NaN" displayed by AR or Round or Solidity are deleted. According to the above method, to meet the number of samples with statistical significance, at least 10 non-overlapping scanning electron microscope images are collected for each electrode, and the particle sizes of at least 5000 particles are counted. The particle sizes of at least 5000 obtained particles are arranged in ascending order. With the particle size as the horizontal axis and the cumulative area ratio calculated through the "Area" of the particles as the vertical axis, the area cumulative distribution curve of the particles in the positive electrode film layer is obtained. D A90 、D A50 、D A10 are the corresponding particle size values when the cumulative area ratio on the vertical axis in the area cumulative distribution curve is 90%, 50%, and 10% respectively.

[0093] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode, the D A90 of the particles can be selected from 1400 nm, 1486 nm, 1500 nm, 1556 nm, 1600 nm, 1674 nm, 1700 nm, 1800 nm, 1900 nm, 1984 nm, 2000 nm, 2100 nm or any numerical range between any two of them.

[0094] The cross-sectional morphology diagram of the positive electrode film layer along the thickness direction of the electrode is as Figure 1 shown, which is different from the state of the positive electrode active material in the Malvern laser scattering method and also different from the state of the positive electrode active material when directly observing the positive electrode active material by scanning electron microscope. The particles in the positive electrode film layer show a good dispersion state under the action of the rolling pressure, and observing the positive electrode film layer is beneficial to effectively characterizing the objective situation of the particle size and distribution of the particles in the positive electrode film layer.

[0095] During the compaction process of the positive electrode film layer, compaction occurs in the thickness direction. The cross-section of the positive electrode film layer along the thickness direction of the electrode plate can more accurately reflect the real compaction state of the particles inside the film layer at the spatial scale compared to the surface of the positive electrode film layer. In the cross-section of the positive electrode film layer along the thickness direction of the electrode plate, the D of the particles A50 can intuitively reflect the size of the overall particles.

[0096] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode plate, the particle size concentration (D A90 -D A10 ) / D A50 is 1.855 - 2.375.

[0097] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode plate, the particle size concentration (D A90 -D A10 ) / D A50 can be optionally 1.855, 1.86, 1.87, 1.88, 1.89, 1.9, 1.91, 1.92, 1.93, 1.94, 1.948, 1.95, 1.96, 1.97, 1.98, 1.99, 2.0, 2.01, 2.02, 2.03, 2.031, 2.04, 2.05, 2.06, 2.07, 2.08, 2.09, 2.10, 2.11, 2.12, 2.13, 2.14, 2.15, 2.16, 2.17, 2.18, 2.19, 2.2, 2.21, 2.22, 2.23, 2.24, 2.25, 2.26, 2.27, 2.28, 2.29, 2.3, 2.31, 2.32, 2.33, 2.34, 2.341, 2.35, 2.36, 2.37, 2.375 or the numerical range between any two of them.

[0098] In this application, in the cross-section of the positive electrode film layer along the thickness direction of the electrode plate, the particle size concentration can be tested by referring to the method described above. When the particle size concentration is within the above range, it helps to construct a reasonable particle gradation. In the positive electrode plate, a wide particle size distribution usually helps to increase the compaction density of the electrode plate. Specifically, a wider particle size distribution enables smaller particles to fill the gaps between larger particles, so that the particles form a more compact arrangement during the pressing process. This compact arrangement is beneficial to improving the compaction density of the electrode plate, and thus improving the energy density of the battery.

[0099] It can be understood that the particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode plate, especially particles above 50 nm, mainly come from the positive electrode active material. Therefore, through the observation and statistics of the particle sizes in the cross-section of the positive electrode film layer in the embodiments of this application, the distribution of lithium-containing transition metal phosphate particles in the positive electrode film layer in the electrode plate can be accurately and objectively reflected.

[0100] In the prior art, a laser particle size analyzer is usually used to statistically analyze the particle size of the positive electrode active material by the Malvern laser diffraction method. However, the applicant's research shows that due to the easy agglomeration of lithium-containing transition metal phosphate particles, the test results obtained by the Malvern laser diffraction method based on the laser scattering principle often measure the particle size of their particle aggregates, and cannot truly reflect the particle size of the particles in the positive electrode active material, let alone reflect the dispersion state of the positive electrode active material in the film layer, because the degree of dispersion of the positive electrode active material in the film layer will increase during the film forming and rolling process. The test results obtained by the Malvern laser diffraction method are affected by the particle size, specific surface area, and agglomeration degree of the positive electrode active material. Compared with the actual dispersion situation in the electrode sheet, the number of large particles obtained by this test is lower than the actual value, and the number of small particles is higher than the actual value. Therefore, the particle size obtained by the Malvern laser diffraction method cannot be equivalent to or analogized to the particle size statistically obtained in the embodiments of the present application.

[0101] Those skilled in the art can achieve the regulation of the particle size concentration, D A90 、D A10 、D A50 through any known process. As an example, by the scientific grading of particles with different particle sizes, the particle size concentration is adjusted; by using the mechanical force of the crushing and grinding processes, the raw materials are processed to the target particle size distribution range to achieve the adjustment of the particle size and concentration; by using screening and classification equipment to separate the particle sizes of the particle system, a particle size distribution that meets the requirements can be obtained; by precisely controlling the feeding rate, adjusting the residence time and force state of the particles in the equipment also helps to achieve the regulation of the particle concentration.

[0102] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the D A90 of the particles is 1400 nm - 2000 nm.

[0103] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, when the D A90 of the particle size of the particles is within the above range, the internal resistance of the battery can be reduced, and the negative impact of over-large particles on the kinetic performance can be further inhibited.

[0104] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the D A50 of the particles is 600 nm - 900 nm, and can be optionally 650 nm - 750 nm.

[0105] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the D A50It can be 600nm, 610nm, 620nm, 630nm, 640nm, 650nm, 656nm, 660nm, 670nm, 680nm, 688nm, 690nm, 700nm, 708nm, 710nm, 719nm, 720nm, 730nm, 740nm, 750nm, 757nm, 760nm, 770nm, 780nm, 785nm, 790nm, 800nm, 810nm, 820nm, 830nm, 840nm, 850nm, 860nm, 870nm, 880nm, 890nm, 900nm or a numerical range between any two of them.

[0106] D of the particle size of the particles A50 Within the above range, on the one hand, it is beneficial to maintain the supporting effect of the particles, enabling the roller pressing pressure to be evenly transmitted among the particles of the electrode sheet, and the electrode sheet can withstand a higher roller pressing force; on the other hand, it means that the overall particle size of the particles inside the positive electrode film layer is relatively small, keeping the kinetic performance at a relatively high level, and improving the energy density while taking into account the kinetics of the battery.

[0107] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, D of the particles A50 is 650nm - 750nm, which is beneficial to further balance the kinetic performance of the lithium-ion secondary battery.

[0108] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, D of the particles A10 is 100nm - 300nm, and can be 120nm - 250nm.

[0109] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, D of the particles A10 can be 100nm, 110nm, 120nm, 122nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 186nm, 190nm, 200nm, 210nm, 214nm, 220nm, 225nm, 230nm, 240nm, 250nm, 260nm, 262nm, 270nm, 280nm, 290nm, 300nm or a numerical range between any two of them.

[0110] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, D of the particles A10 Within the above range, on the one hand, it indicates that there are certain small particles in the film layer to form a graded filling of the voids between the particles, and at the same time, it will not cause agglomeration due to the too small particle size; on the other hand, it indicates that the proportion of small particle size particles is limited, which can improve the compaction of the electrode sheet while reducing the side reaction between the small particles and the electrolyte, and balance the cycle performance of the battery.

[0111] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the D A10 of the particles is 120 nm - 250 nm, which is beneficial to further improving the cycling performance of the lithium-ion secondary battery.

[0112] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of the laser confocal Raman spectrometer for the positive electrode film layer, the median C of the graphitization degree 50 is 0.98 - 1.20, and can be optionally 1.02 - 1.10; wherein the graphitization degree C value is I G / I D where I G represents the intensity of the G peak of the Raman spectrum at 1580 ± 100 cm -1 , and I D represents the intensity of the D peak of the Raman spectrum at 1350 ± 100 cm -1 .

[0113] In the present application, the graphitization degree C value of the positive electrode film layer can be obtained through the surface scanning mode of the laser confocal Raman spectrometer. As an example, specifically, a laser confocal Raman spectrometer (high-precision Renishaw laser confocal Raman spectrometer) is used, the excitation wavelength of 532 nm is selected, an appropriate amount of the positive electrode film layer is taken for surface scanning of its surface or the cross-section along the thickness direction of the electrode sheet, the scanning area is 45 μm × 45 μm, divided into 10 × 10 grids, the grid vertices are used as the test points, the step size is 5 μm, and the total number of scanning points is 100 points, thereby obtaining the C values at different sites and the cumulative distribution curve of the C value of the surface scanning area.

[0114] The positive electrode film layer in the present application can be either a freshly prepared positive electrode film layer or a positive electrode film layer disassembled from a battery. It is inevitable that there are residual electrolyte salt particles on the surface of the positive electrode film layer disassembled from the battery. To improve the test accuracy, it is preferable to perform surface scanning on the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet to characterize the graphitization degree of the positive electrode film layer.

[0115] The graphitization degree C value of the positive electrode film layer is obtained through the peak intensity ratio of the G peak (G-band) and D peak (D-band) of the Raman spectrum. The position of the G peak is 1580 ± 100 cm -1 , which characterizes the carbon sp 2 hybrid structure; the position of the D peak is 1350 ± 100 cm -1 , which characterizes the disordered structure, where disorder means that there is no regular arrangement between carbon atoms in the structure. In the graphite crystal, the carbon atoms in the same layer are in sp 2Hybridization forms covalent bonds, and the intermolecular force between layers is van der Waals force, making the carbon in the graphite structure easy to slip. Therefore, the C value can characterize the graphitization degree of the positive electrode film layer. It can be understood that the graphitization degree in the positive electrode film layer mainly comes from the carbon materials treated by graphitization in the positive electrode film layer, that is, the carbon coating material of the positive electrode active material. Although conductive agents such as carbon nanotubes with rich sp 2 hybrid structures also have relatively high I G / I D , but due to their low addition content and small tube diameter, their addition in the positive electrode film layer shows an extreme value in the Raman surface scan test of the positive electrode film layer and will not affect the graphitization degree C 50 in the positive electrode film layer. Therefore, the graphitization degree of the positive electrode film layer can also be used to characterize the graphitization degree of the positive electrode active material.

[0116] The cumulative distribution curve of the graphitization degree C value refers to the curve obtained by arranging at least 100 obtained C values in ascending order, with the graphitization degree as the horizontal axis and the cumulative quantity proportion as the vertical axis. C 50 is the C value corresponding to the cumulative quantity proportion of 50% on the vertical axis in the cumulative distribution curve of the graphitization degree C value. The median C 50 of the graphitization degree can reflect the overall graphitization degree of the positive electrode active material particles in the positive electrode film layer, that is, the ease of slippage, compared with the point value; compared with the mean value, it can reduce the influence of extreme values during the test and improve the confidence level of the test results.

[0117] Those skilled in the art can adjust the graphitization degree of the active material particles through any known process. As an example, adjusting the carbon source (the carbon source can be selected as cross-linked PEG), sintering temperature, sintering time, sintering pressure, sintering atmosphere, and nucleation process can all adjust the graphitization degree of the active material particles.

[0118] The higher the graphitization degree of the carbon on the surface of the positive electrode active material, the higher the proportion of graphite-structured carbon in the positive electrode film layer, and the easier the particles are to slip during the rolling process by means of the carbon structure with high graphitization degree in the coating material. On the basis of the reasonable grading of the particles in the electrode, even at a low rolling pressure, the compaction density of the electrode can be further improved; the high graphitization degree of the carbon coating layer on the surface of the positive electrode active material is beneficial to electron transport, achieving both energy density and kinetics.

[0119] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained by the positive electrode film layer in the surface scanning mode of the laser confocal Raman spectrometer, the median C 50It can be 0.98, 0.99, 1, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.10, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.20 or a numerical range between any two of them.

[0120] In some embodiments, in the cumulative area distribution curve of the sphericity of the particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median L of the sphericity A50 is 0.70 - 0.85, and can be optionally 0.70 - 0.76.

[0121] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median L of the sphericity 50 The test method is as follows: Refer to the method described above in this application to identify the particles in the cross-section of the positive electrode film layer, and use the "Shape Descriptor" and "Area" analysis functions in ImageJ to analyze the morphology and the area of the particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet. According to the software manual (ImageJ User Guide IJ 1.46r), the "Area" parameter obtained from the analysis represents the pixel area of the particle, and the "Round" parameter represents the ratio of the pixel area of the particle to the area of a circle with the fitted major axis as the diameter. When the particle is closer to a sphere, the ratio of the pixel area to the area of a circle with the fitted major axis as the diameter is closer to 1. Therefore, the "Round" parameter of the particles obtained from the analysis is used to characterize the sphericity of the particles. Arrange the sphericities of at least 5000 particles obtained in ascending order, and obtain the cumulative area distribution curve of the sphericity of the particles in the positive electrode film layer with the sphericity as the horizontal axis and the cumulative area ratio as the vertical axis. L A50 is the sphericity L value corresponding to the cumulative area ratio of 50% on the vertical axis in the cumulative distribution curve of the sphericity L value.

[0122] In some embodiments, in the cumulative area distribution curve of the sphericity of the particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median L of the sphericity A50 can be optionally 0.70, 0.709, 0.71, 0.719, 0.72, 0.725, 0.726, 0.73, 0.737, 0.74, 0.75, 0.751, 0.76, 0.77, 0.78, 0.79, 0.80, 0.81, 0.82, 0.83, 0.84, 0.85 or a numerical range between any two of them.

[0123] Those skilled in the art can adjust the sphericity of the particles through any known process. As an example, the sphericity of the particles can be adjusted by processes such as grinding, polishing, chemical etching, mechanical stirring, extrusion, coating, granulation, adding surfactants, etc., as well as by adjusting the parameters of each process.

[0124] Median L of sphericity A50 The particles within the above range are approximately spherical, which helps the particles maintain good slidability during packing, easily fill the voids between the particles, and can further improve the compaction density of the electrode sheet and increase the energy density of the battery.

[0125] In some embodiments, in the cumulative area distribution curve of the roughness of the particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median R of the roughness A50 is 0.92 - 0.96.

[0126] In the cumulative area distribution curve of the roughness of the particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median R of the roughness A50 The test method is as follows: Refer to the method described above in this application to identify the particles in the cross-section of the positive electrode film layer, and use the "Shape Descriptor" analysis function in ImageJ to analyze the morphology of the particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet. According to the software manual (ImageJ User Guide IJ 1.46r), the "Solidity" parameter obtained from the analysis represents the ratio of the pixel area of the particle to the convex area. Therefore, the "Solidity" parameter of the analyzed particles is used to characterize the roughness of the particles. According to the definition, the closer the roughness is to 1, the smoother the particle. Arrange the roughnesses of at least 5000 obtained particles in ascending order, and obtain the cumulative area distribution curve of the roughness of the particles in the positive electrode film layer with the roughness as the horizontal axis and the cumulative area ratio as the vertical axis. R A50 is the roughness R value corresponding to the cumulative area ratio of 50% on the vertical axis in the cumulative area distribution curve of the roughness R value.

[0127] In some embodiments, in the cumulative area distribution curve of the roughness of the particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median R of the roughness A50 can be optionally 0.92, 0.93, 0.94, 0.95, 0.96 or the numerical range between any two of them.

[0128] Those skilled in the art can adjust the sphericity of the particles through any known process. As an example, the sphericity of the particles can be adjusted by processes such as grinding, polishing, chemical etching, mechanical stirring, extrusion, coating, granulation, adding surfactants, etc., as well as by adjusting the parameters of each process.

[0129] Median R of roughness 50 The particle surfaces within the above range are relatively smooth, the frictional force between particles is relatively small, and they are prone to slip under an external force, which can further improve the compaction density of the electrode sheet and increase the energy density of the battery.

[0130] In some embodiments, in the cumulative area distribution curve of the roughness of the particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the concentration of roughness (R A90 -R A10 ) / R A50 is optionally 0.05 - 0.10.

[0131] In some embodiments, in the cumulative area distribution curve of the roughness of the particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the concentration of roughness (R A90 -R A10 ) / R A50 is optionally 0.05, 0.06, 0.07, 0.08, 0.09, 0.10 or the numerical range between any two of them.

[0132] In the cumulative area distribution curve of the roughness of the particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the specific method for testing the concentration of roughness is as follows: Referring to the roughness testing method described above in this application, and so on, R A90 is the R value corresponding to when the cumulative area ratio of the vertical axis in the cumulative area distribution curve of the sphericity-like R value accounts for 90%, and R A10 is the R value corresponding to when the cumulative area ratio of the vertical axis in the cumulative area distribution curve of the roughness R value accounts for 10%. The concentration of roughness is represented by (R A90 -R A10 ) / R A50 .

[0133] Those skilled in the art can adjust the roughness of the particles through any known process. As an example, the roughness of the particles can be adjusted by processes such as grinding, polishing, grinding, mechanical energy, electroplating, calendaring, etc. and by adjusting the parameters of each process.

[0134] The numerical value of the concentration of roughness is extremely small, indicating that the overall roughness of the particles is highly consistent, which is beneficial to the relative sliding between particles, is more likely to form a high-density accumulation during rolling, and increases the compaction density of the electrode sheet and the energy density of the battery.

[0135] In some embodiments, the iron dissolution rate of the positive electrode film layer is 400 ppm - 1800 ppm.

[0136] The iron dissolution rate of the positive electrode film layer can be tested in the following manner. Specifically, after disassembling and washing the electrode sheet from the battery, it is made into small round pieces with a diameter of 14 mm. Take multiple small round piece samples so that the total mass of the samples is about 5 g. Add them to 100.3 g of ascorbic acid solution with a mass concentration of 0.3% (the solvent is ultrapure water). Stir at a speed of 500 revolutions per minute for 5 minutes, then quickly use a 5 mL syringe to rapidly suck the solution. Filter the solution through a 0.45 μm pore size filter head into a test tube. Use a pipette to suck 1 mL of the supernatant and add it to a glass volumetric flask for dilution 50 times. Test it with an inductively coupled plasma optical emission spectrometer (ICP-OES) to obtain the iron element concentration in the solution. Through the formula: [(ICP test iron element concentration × solution volume / mass of the solution involved in volume fixation) × 100.3 g / (mass of the electrode sheet of the small round piece - mass of the current collector of the small round piece)], the solution volume is 50 mL, and the mass of the solution involved in volume fixation is 1 g, and calculate the iron dissolution rate of the positive electrode film layer. Preferably, the mass of the current collector of the small round piece is obtained by multiplying the thickness of the small round piece by the area and the density. The thickness of the small round piece can be equivalently measured by measuring the thickness of the current collector in the uncoated area with a thickness gauge. It can be understood that although the current collector will be extended during the compaction process in the coated area, resulting in a slightly lower thickness compared to the uncoated area, since the reduction amplitude is negligible, it will not have a significant impact on the test results. More preferably, when the current collector is aluminum foil, the density is 2.7 g / cm 3 .

[0137] In some embodiments, the iron dissolution rate of the positive electrode film layer can be selected as 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 860.7 ppm, 900 ppm, 1000 ppm, 1100 ppm, 1200 ppm, 1300 ppm, 1400 ppm, 1500 ppm, 1600 ppm, 1700 ppm, 1800 ppm or the numerical range between any two of them.

[0138] Those skilled in the art can regulate the iron dissolution rate of the positive electrode material through any known process. As an example, regulate the iron dissolution rate of the positive electrode material by regulating the surface coating mass of the positive electrode material, the temperature, time, and pressure during the preparation process. In addition, during the use of the battery, the battery design, the content of oxidants in the electrolyte, the battery operating temperature, and the battery charge and discharge intensity will also affect the iron dissolution rate of the positive electrode film layer.

[0139] The iron dissolution rate of the positive electrode film layer mainly comes from the lithium-containing transition metal phosphate positive electrode active material in the positive electrode film layer, which can reflect the integrity and density of the carbon coating on the surface of the positive electrode active material from the side. The lower the iron dissolution rate, the less likely the iron ions after acid dissolution are to precipitate from the carbon-coated material, that is, the more complete and dense the carbon-coated material on the surface of the positive electrode active material. The iron dissolution rate of the positive electrode film layer within the above range indicates that the positive electrode active material has a relatively complete and dense carbon-coated material, which can improve the electrical contact between the positive electrode active materials, improve the conductivity of the positive electrode active material, reduce the polarization of the positive electrode active material, and further optimize the kinetic performance of the lithium-ion secondary battery. At the same time, the highly complete carbon-coated material makes the particles prone to stress slip during the rolling process, which can improve the compaction density of the electrode sheet and the energy density of the battery.

[0140] In some embodiments, based on the total mass of the positive electrode active material, the mass content of carbon element is 0.8% - 1.8%, and optionally 0.9% - 1.5%.

[0141] Based on the total mass of the positive electrode active material, the mass content of carbon element can be measured by methods and equipment known in the art. For example, referring to GB / T 21023 - 2006 "Determination of total carbon and sulfur content in steel - Infrared absorption method after combustion in high-frequency induction furnace", it is measured by De Kai HCS infrared carbon-sulfur analyzer.

[0142] In some embodiments, based on the total mass of the positive electrode active material, the mass content of carbon element can be optionally 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8% or the numerical range between any two of them.

[0143] Compared with the lithium-containing transition metal phosphate positive electrode active material in the prior art, this positive electrode active material has a relatively low content of carbon coating, which can further increase the loading amount of transition metal lithium phosphate in the positive electrode sheet and improve the energy density of the lithium-ion secondary battery.

[0144] In some embodiments, the lithium-iron antisite defect concentration of the positive electrode active material is 0.001% - 1.5%, and optionally 0.01% - 1.0%.

[0145] XRD data of the sample was collected using an X-ray diffractometer, and phase analysis of the sample was carried out. The CIF file of this phase obtained from an open-source website was used as the initial crystal structure model, including defining unit cell parameters, atomic positions, and occupancy probabilities, etc. In the initial crystal structure model, considering the possibility of Fe-Li anti-site, the possible Li content at the Fe position and the possible Fe content at the Li position were set, and the initial value was set to 0.1%. The collected XRD data was fitted and refined using the FullProf Suite software, and the parameters were refined in the order of background parameters, peak intensity, unit cell parameters, and peak shape. When the fitted peak shape and the experimental peak shape were best matched and Rwp was less than 10, the occupancy probabilities of Li and Fe after refinement were obtained, and the probability of Fe occupying the Li position was used as the concentration of Li-Fe anti-site defects.

[0146] In some embodiments, the concentration of Li-Fe anti-site defects in the positive electrode active material can be selected from 0.001%, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07, 0.08%, 0.09%, 0.1%, 0.2%, 0.22%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% or the numerical range between any two of them.

[0147] Those skilled in the art can regulate the Li-Fe anti-site defects of the positive electrode active material through any known process. As an example, the regulation of Li-Fe anti-site defects of the positive electrode active material can be achieved by regulating the sintering temperature, sintering time, preparation method, raw material stoichiometric ratio, etc.

[0148] During the preparation and cycling process, there will inevitably be certain lithium vacancies in the crystal structure of the positive electrode active material. Lithium vacancies will not only cause the oxidation of ferrous ions to ferric ions, but also induce the partial migration of ferric ions to the lithium position, forming Li-Fe anti-site defects and blocking the one-dimensional diffusion channels of lithium ions, which has an adverse effect on the solid-phase transport of lithium ions. The positive electrode active material in the embodiments of the present application has low Li-Fe anti-site defects, which is beneficial to the uniform transport of lithium ions in the solid phase and further improves the kinetic performance of lithium-ion secondary batteries.

[0149] In some embodiments, the lithium-containing transition metal phosphate includes components having the following general formula: Li m Fe x P y O j Q q , Among them, Q includes one or more of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.8 ≤ m ≤ 1.15, 0.9 ≤ x ≤ 1, 0.95 ≤ y ≤ 1, 3.5 ≤ j ≤ 4, 0 < q ≤ 0.1.

[0150] In some embodiments, m can be selected as 0.8, 0.85, 0.9, 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15 or the numerical range between any two of them; x can be selected as 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0 or the numerical range between any two of them; y can be selected as 0.95, 0.96, 0.97, 0.98, 0.99, 1.00 or the numerical range between any two of them; j can be selected as 3.5, 3.6, 3.7, 3.8, 3.9, 4 or the numerical range between any two of them; q can be selected as 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1 or the numerical range between any two of them.

[0151] Selecting an appropriate modification element Q can improve the lattice change rate of the cathode active material during the lithium deintercalation and intercalation process, reduce the oxygen activity on the particle surface, improve the structural stability of the material, and further improve the specific capacity utilization level of the material during the cycling process, and further improve the cycling stability of the lithium-ion secondary battery.

[0152] In some embodiments, the cathode active material includes one or more of lithium iron phosphate and its doped modification materials and coated modification materials.

[0153] In some embodiments, the cathode active material includes titanium element, and based on the total mass of the cathode active material, the mass content of the titanium element is 4000 ppm - 8000 ppm.

[0154] The types and contents of elements in the cathode active material can be tested by any well-known method in the art. As an example, the inductively coupled plasma emission spectrometry is used to test the titanium element and its content with reference to Appendix C of GB / T 33822-2017.

[0155] The doping of titanium element in the cathode active material is beneficial to causing lattice distortion, reducing the Li-O bond energy, increasing the lithium ion diffusion rate, and improving the kinetic performance of the lithium ion secondary battery. However, in the prior art, the doping content of titanium element in the lithium-containing transition metal phosphate often cannot exceed 3000 ppm, because too much titanium element is difficult to completely enter the bulk phase of the lithium-containing transition metal phosphate and is likely to become a harmful impurity phase remaining on the surface, which has a negative impact on the battery performance.

[0156] The cathode active material in the embodiment of the present application has a high titanium element content. Moreover, surprisingly, the high addition amount of titanium element does not form a harmful impurity phase that has a negative impact on the battery energy density and kinetic performance. Although the reason is not yet clear, it is speculated that it may be that titanium element and phosphate radical and other elements (for example, lithium element) jointly form a fast ion conductor, which instead has a promoting effect on the kinetic performance of the battery.

[0157] In some embodiments, the tapped density of the powder of the cathode active material is 1.00 g / cm 3 -1.70 g / cm 3 , and can be optionally 1.20 g / cm 3 -1.50 g / cm 3 .

[0158] The tapped density of the powder can be measured by any well-known method in the art. As an example, turn on the electronic balance, first use a conical flask as a base and place it on the electronic balance, and then zero the electronic balance; place the tapped density measuring cylinder on the conical flask and weigh it to record the weight of the measuring cylinder; open the sample bag, use a clean sample spoon to stir the sample in the sample bag for 3 - 5 circles to mix it evenly, and then transfer the sample smoothly into the measuring cylinder; wipe the powder stained on the surface of the connection with dust-free paper, and then put it into the conical flask that has been zeroed and weigh it; seal the mouth of the measuring cylinder with a sealing film, place the tapped density measuring cylinder in the matching instrument rubber ring, ensure that the tapped density measuring cylinder fits tightly with the rubber ring and is perpendicular to the surface of the instrument; set the vibration frequency to 250 times / min on the instrument, the vibration times to 5000 times, press the button, and vibrate for 20 min; then remove the tapped density measuring cylinder, use a flashlight to irradiate the surface of the measuring cylinder, and by visual inspection, read the highest scale V 1 and the lowest scale V 2 , and take the average value V of the two; the mass m 1 of the measuring cylinder and the sample minus the mass m 0 of the measuring cylinder to obtain the powder mass m, and the tapped density of the sample can be obtained from the density formula ρ = m / v.

[0159] In some embodiments, the tapped density of the powder of the cathode active material can be optionally 1.00 g / cm 3 , 1.05 g / cm 3 , 1.10 g / cm3 , 1.15 g / cm 3 , 1.20 g / cm 3 , 1.25 g / cm 3 , 1.30 g / cm 3 , 1.35 g / cm 3 , 1.40 g / cm 3 , 1.45 g / cm 3 , 1.50 g / cm 3 , 1.55 g / cm 3 , 1.60 g / cm 3 , 1.65 g / cm 3 , 1.70 g / cm 3 or the numerical range between any two of them.

[0160] The positive electrode active material particles in the embodiments of the present application not only have a wide particle size distribution, but also have a reasonable particle size range, forming an effective grading. The small particles can fill the voids between the particles, so they have a high tap density.

[0161] In some embodiments, the powder compaction density of the positive electrode active material under 3T pressure is 2.55 g / cm 3 -2.70 g / cm 3 , and can be optionally 2.58 g / cm 3 -2.68 g / cm 3 .

[0162] In the present application, the term "powder compaction density" refers to the density of a green compact with a certain density and strength formed during the external force compression process. As the powder moves and deforms, larger voids are filled, the contact area between particles increases, the atomic attraction is generated, and the mechanical fitting effect between particles is enhanced. The unit is g / cm 3 .

[0163] The powder compaction density of the positive electrode active material can be measured by methods and equipment known in the art. For example, reference can be made to GB / T 24533-2009 and measured using a compaction density instrument. Specifically, a certain amount of positive electrode active material is placed on a special compaction mold (the diameter of the mold is known), and there is a hollow in the middle of the mold with a metal disc at each end. The positive electrode active material is placed between the metal discs, and a metal cylinder is placed on the top. The mold is placed on the compaction density instrument, the pressure is set to 3T, and the thickness of the positive electrode active material under 3T pressure can be read on the equipment. The powder compaction density of the positive electrode active material is ρ = m / v, where v = (S×H), m is the mass of the positive electrode active material, S is the bottom area of the mold 1.327 cm 2 , and H is the thickness of the positive electrode active material after compaction.

[0164] In some embodiments, the powder tap density of the positive electrode active material under a pressure of 3T can be selected as 2.55 g / cm 3 , 2.56 g / cm 3 , 2.57 g / cm 3 , 2.58 g / cm 3 , 2.59 g / cm 3 , 2.60 g / cm 3 , 2.61 g / cm 3 , 2.62 g / cm 3 , 2.63 g / cm 3 , 2.64 g / cm 3 , 2.65 g / cm 3 , 2.66 g / cm 3 , 2.67 g / cm 3 , 2.68 g / cm 3 , 2.69 g / cm 3 , 2.70 g / cm 3 or the numerical range between any two of them.

[0165] The positive electrode active material particles form an effective grading, enabling the positive electrode active material to construct a stacking structure with extremely small particle gaps under an external force, achieving a higher tap density and providing a material basis for improving the tap density of the electrode sheet and preparing a lithium-ion secondary battery with a high energy density.

[0166] In some embodiments, the powder resistivity of the positive electrode active material under a pressure of 8 MPa is 0.5 Ω·cm - 60.0 Ω·cm, and can be selected as 2.0 Ω·cm - 40.0 Ω·cm.

[0167] The powder resistivity of the positive electrode active material can be measured by methods and equipment known in the art. For example, referring to GB / T 33822-2017, a powder resistivity meter (Suzhou Crystal, model ST2722) can be used for measurement. Specifically, a certain amount of positive electrode active material (such as 1 g) is weighed and added to the feeding chamber of the powder resistivity meter, and a pressure of 8 MPa is applied. The forward resistivity and reverse resistivity of the positive electrode active material are respectively measured, and the average value of the two is taken as the powder resistivity of the positive electrode active material.

[0168] In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 8 MPa can be selected from 0.5 Ω·cm, 1 Ω·cm, 2 Ω·cm, 3 Ω·cm, 4 Ω·cm, 5 Ω·cm, 6 Ω·cm, 7 Ω·cm, 8 Ω·cm, 9 Ω·cm, 10 Ω·cm, 10.95 Ω·cm, 15 Ω·cm, 20 Ω·cm, 25 Ω·cm, 30 Ω·cm, 35 Ω·cm, 40 Ω·cm, 45 Ω·cm, 50 Ω·cm, 55 Ω·cm, 60 Ω·cm, or the numerical range between any two of them.

[0169] The positive electrode active material is coated with a carbon material on the surface, and due to the sp 2 structure of the surface carbon, it is easy to achieve rapid conduction of electrons between particles, enabling the positive electrode active material to have a low powder resistivity, which is beneficial to improving the solid-phase transmission rate of electrons and enhancing the kinetic performance of the battery.

[0170] In some embodiments, the discharge specific capacity of the positive electrode active material at a discharge rate of 1C at room temperature is 135 mAh / g - 150 mAh / g.

[0171] In this application, the positive electrode active material is assembled into a coin cell to test its electrical performance on a Blue Electric Tester. At 25 ± 5 °C within the voltage range of 2.0 V to 3.75 V, after constant current charging at 1C to 3.75 V, pause for 5 minutes, then constant voltage charge until the cut-off current is 50 μA, and then discharge at 1C to 2.0 V. The discharge capacity of the coin cell is divided by the mass of the positive electrode active material to obtain the discharge specific capacity of the positive electrode active material at a discharge rate of 1C at room temperature.

[0172] The preparation and testing process of the coin cell is as follows: Mix 2.0 g of the positive electrode active material, conductive carbon black, and PVDF in a mass ratio of 0.9∶0.05∶0.05, then add the organic solvent NMP (N-methylpyrrolidone), mix well, coat with a 150 μm doctor blade, dry at 100 °C for 2 h, and press the positive electrode plate with a compaction density of 2.0 g / cm 3 - 2.2 g / cm 3 Press the positive electrode plate, punch it into a circular piece with a diameter of 14 mm, then weigh and record the weight. Put the weighed positive electrode plate into a vacuum drying oven (105 °C, 1 - 12 hrs, -90 kPa). After drying, put the positive electrode plate into a glove box and assemble it into a battery in the order of negative electrode shell - nickel mesh - lithium sheet - separator - positive electrode sheet - positive electrode shell. Drop 65 - 87 μL (pipette) of electrolyte (the electrolyte is a mixed solvent of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) with a volume ratio of 1:1, and the electrolyte is LiPF 6 ) with the negative electrode on top, place it in the groove of the sealer, and the sealing pressure is 650 kg / cm 2, use insulated tweezers to remove the button cell and place it in a dust-free bag. Remove the glove box and place it in a constant temperature room for 3 hours to obtain the button cell for testing.

[0173] It can be understood that the discharge specific capacity of the positive electrode active material can also be obtained by disassembling the battery, obtaining the positive electrode plate, assembling it into a button cell according to the method described above, and then testing.

[0174] In some embodiments, the discharge specific capacity of the positive electrode active material at room temperature at a discharge rate of 1C can be selected from 135 mAh / g, 140 mAh / g, 140.8 mAh / g, 145 mAh / g, 150 mAh / g, or the numerical range between any two of them.

[0175] The positive electrode active material has a high discharge specific capacity at a rate of 1C, indicating that it has good charge and discharge capabilities, which is beneficial to improving the kinetic performance of the battery.

[0176] In some embodiments, the discharge capacity ratio η of the positive electrode active material discharged to 3.2V ≥ 85%, and η is defined as: at room temperature, the button cell containing the positive electrode active material is charged and discharged at a constant current of 0.1C twice within the voltage range of 2.0V to 3.75V, and then charged and discharged at a constant current of 1C once. In the charge and discharge test at a rate of 1C, the capacity value when the discharge voltage is 3.2V is recorded as C 1 , and the capacity value when the discharge voltage reaches 2.0V is C 2 , η = C 1 / C 2 , where the charging process includes constant voltage charging, with a constant voltage of 3.75V and a constant voltage cut-off current of 50 μA.

[0177] In some embodiments, the discharge capacity ratio η of the positive electrode active material discharged to 3.2V ≥ 88%.

[0178] The η value of the positive electrode active material can be measured by methods and equipment known in the art. As an example, first prepare a button cell according to the method described above, and test the electrical performance of the prepared button cell on a blue electricity tester at room temperature. Specifically, the button cell is charged and discharged at a constant current of 0.1C twice within the voltage range of 2.0V to 3.75V, and after constant current charging to the cut-off voltage, it is charged at a constant voltage until the current is 50 μA, and then charged and discharged at a constant current of 1C once. In the charge and discharge test at a rate of 1C, the capacity value discharged from 3.75V to a voltage of 3.2V is recorded as C 1 , and the capacity value discharged from 3.75V to 2.0V is C 2 , and η = C 1 / C 2 .

[0179] In some embodiments, η can be optionally 85%, 86%, 87%, 88%, 88.1%, 89%, 90%, 90.1%, 91%, 92%, 92.2%, 93%, 94%, 94.1%, 94.5%, 95%, 95.1% or a numerical range between any two of them.

[0180] In some embodiments, the proportion η of the discharge capacity of the positive electrode active material in the freshly prepared lithium-ion secondary battery discharged to 3.2V is ≥88%. After the freshly prepared lithium-ion secondary battery is charged and discharged at a constant current rate of 0.1C for a period of time within the voltage range of 2.0V to 3.75V, the proportion η of the discharge capacity of the positive electrode active material discharged to 3.2V can remain ≥85%.

[0181] In the lithium-ion secondary battery of the embodiment of the present application, the high proportion of the discharge capacity of the positive electrode active material discharged to 3.2V means that the positive electrode active material has good kinetic performance. At the same time, a high η value indicates that when the lithium-ion secondary battery containing the positive electrode active material is discharged to a low state of charge (SOC), it still has a high voltage, which is beneficial to maintaining good power performance.

[0182] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the conductive agent is 0.01-1.5%.

[0183] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the conductive agent can be optionally 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% or a numerical range between any two of them.

[0184] In some embodiments, the conductive agent includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

[0185] Since the particles in the positive electrode film layer form a close packing, the positive electrode active material particles are coated with a highly graphitized carbon layer on the surface and are in full contact with each other, having good electron conductivity, which can reduce the use of the conductive agent in the positive electrode film layer, is beneficial to further increasing the loading amount of the positive electrode active material, and improving the energy density of the lithium-ion secondary battery.

[0186] In some embodiments, the positive electrode film layer further includes a binder. Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 94.0% - 99.4%, optionally 96.5% - 99.4%; the mass content of the binder is 0.5% - 3.0%.

[0187] In some embodiments, the binder includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride - tetrafluoroethylene - propylene terpolymer, vinylidene fluoride - hexafluoropropylene - tetrafluoroethylene terpolymer, tetrafluoroethylene - hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0188] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material can be optionally 94.0%, 94.5%, 95.0%, 95.5%, 96.0%, 96.5%, 97%, 97.5%, 98%, 98.5%, 99%, 99.4% or the numerical range between any two of them.

[0189] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the binder can be optionally 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0% or the numerical range between any two of them.

[0190] When the mass content of the positive electrode active material and the mass content of the binder are within the above ranges, the active material loading amount per unit volume of the positive electrode film layer can be effectively increased, and good internal adhesion can be maintained, reducing the occurrence probability of powder falling, swelling and cracking problems, and taking into account the safety performance while improving the energy density of the secondary battery.

[0191] In some embodiments, the single - side areal density of the positive electrode film layer is 300mg / 1540mm 2 - 450mg / 1540mm 2 .

[0192] In the present application, the single - side areal density of the positive electrode film layer has the meaning well - known in the art and can be tested by methods known in the art. For example, take a single - side coated and compacted positive electrode plate (if it is a double - side coated positive electrode plate, the positive electrode film layer on one side can be wiped off first), punch it into small circular pieces with an area of S 1 , weigh it, and record it as M 1 . Then wipe off the positive electrode film layer of the above - weighed positive electrode plate and weigh the weight of the current collector, and record it as M 0 . The single - side areal density of the positive electrode film layer = (M 1 - M 0 ) / S 1To ensure the accuracy of test results, multiple groups (e.g., 10 groups) of samples to be tested can be tested, and the average value can be calculated as the test result.

[0193] In some embodiments, the areal density of the positive electrode film layer on one side can be selected from 300 mg / 1540mm 2 , 310mg / 1540mm 2 , 320 mg / 1540mm 2 , 330 mg / 1540mm 2 , 340 mg / 1540mm 2 , 350 mg / 1540mm 2 , 360 mg / 1540mm 2 , 370 mg / 1540mm 2 , 380 mg / 1540mm 2 , 390 mg / 1540mm 2 , 400 mg / 1540mm 2 , 410 mg / 1540mm 2 , 420 mg / 1540mm 2 , 430 mg / 1540mm 2 , 440 mg / 1540mm 2 , 450 mg / 1540mm 2 or any value range between any two of them.

[0194] The positive electrode film layer with an areal density within the above range can help improve the energy density of the lithium-ion secondary battery.

[0195] In some embodiments, when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.52g / cm 3 -2.78 g / cm 3 .

[0196] In some embodiments, when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.55 g / cm 3 -2.75 g / cm 3 .

[0197] In this application, the fully discharged state means that the battery is placed at 25°C, left standing for 2h, and after the battery temperature remains at 25°C, the battery is discharged at a constant current of 1 / 3C to 2.5V and then at a constant current of 0.1C to 2.0V.

[0198] The tap density of the positive electrode film layer can be tested by methods known in the art. As an example, place the battery in an oven environment at 25°C and let it stand for 2 h. After the battery temperature reaches 25°C, discharge the battery at a constant current of 1 / 3C until 2.5V and then at a constant current of 0.1C until 2.0V. Disassemble the battery to obtain the positive electrode sheet. Treat the residual electrolyte with dimethyl carbonate solvent, dry the electrode sheet, cut it into small round pieces with an area of S, and obtain its mass W 1 , and measure the thickness T of the positive electrode sheet using a micrometer 1 , then wipe off the positive electrode film layer of the above-weighted electrode sheet, weigh the mass of the current collector, denoted as W 2 , and measure the thickness T of the current collector using a micrometer 2 , then the tap density PD of the positive electrode film layer = (W 1 - W 2 ) / [(T 1 - T 2 ) × S].

[0199] In some embodiments, when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.52 g / cm 3 , 2.53 g / cm 3 , 2.54 g / cm 3 , 2.55 g / cm 3 , 2.56 g / cm 3 , 2.57 g / cm 3 , 2.58 g / cm 3 , 2.59 g / cm 3 , 2.60 g / cm 3 , 2.61 g / cm 3 , 2.62 g / cm 3 , 2.63 g / cm 3 , 2.64 g / cm 3 , 2.65 g / cm 3 , 2.66 g / cm 3 , 2.67 g / cm 3 , 2.68 g / cm 3 , 2.69 g / cm 3 , 2.70 g / cm 3 , 2.71 g / cm 3 , 2.72 g / cm 3 , 2.73 g / cm 3 , 2.74 g / cm 3 , 2.75 g / cm 3 , 2.76 g / cm 3 , 2.77 g / cm 3 , 2.78 g / cm 3or a numerical range between any two of them.

[0200] In some embodiments, after being processed by the compaction process, the compaction density of the positive electrode film layer is 2.63 - 2.90 g / cm 3 .

[0201] In some embodiments, after being processed by the compaction process, the compaction density of the positive electrode film layer can be selected as 2.63 g / cm 3 , 2.64 g / cm 3 , 2.65 g / cm 3 , 2.66 g / cm 3 , 2.67 g / cm 3 , 2.68 g / cm 3 , 2.69 g / cm 3 , 2.70 g / cm 3 , 2.71 g / cm 3 , 2.72 g / cm 3 , 2.73 g / cm 3 , 2.74 g / cm 3 , 2.75 g / cm 3 , 2.76 g / cm 3 , 2.77 g / cm 3 , 2.78 g / cm 3 g / cm 3 , 2.79 g / cm 3 , 2.80 g / cm 3 , 2.81 g / cm 3 , 2.82 g / cm 3 , 2.83 g / cm 3 , 2.84 g / cm 3 , 2.85 g / cm 3 , 2.86 g / cm 3 , 2.87 g / cm 3 , 2.88 g / cm 3 , 2.89 g / cm 3 , 2.90 g / cm 3 or a numerical range between any two of them.

[0202] In this application, "compaction" means that during the battery assembly process, the positive electrode film layer is compacted by mechanical pressure to improve its density and conductivity.

[0203] In some embodiments, after being processed by the formation process, the compaction density of the positive electrode film layer is 2.52 - 2.78 g / cm 3 .

[0204] In some embodiments, after the formation process, the tap density of the positive electrode film layer can be selected as 2.52 g / cm 3 , 2.53 g / cm 3 , 2.54 g / cm 3 , 2.55 g / cm 3 , 2.56 g / cm 3 , 2.57 g / cm 3 , 2.58 g / cm 3 , 2.59 g / cm 3 , 2.60 g / cm 3 , 2.61 g / cm 3 , 2.62 g / cm 3 , 2.63 g / cm 3 , 2.64 g / cm 3 , 2.65 g / cm 3 , 2.66 g / cm 3 , 2.67 g / cm 3 , 2.68 g / cm 3 , 2.69 g / cm 3 , 2.70 g / cm 3 , 2.71 g / cm 3 , 2.72 g / cm 3 , 2.73 g / cm 3 , 2.74 g / cm 3 , 2.75 g / cm 3 , 2.76 g / cm 3 , 2.77 g / cm 3 , 2.78 g / cm 3 or any value range between any two of them.

[0205] In the present application, formation refers to the process of forming a stable solid electrolyte interface (SEI film) and electrode structure through electrochemical reactions during the first charge and discharge of the battery.

[0206] It can be understood that along with the rebound of the electrode sheet during the cycling process, the tap density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery is slightly lower than the tap density of the positive electrode film layer after compaction and formation.

[0207] When the tap density of the positive electrode film layer is within the above range, it is beneficial to improve the energy density of the lithium-ion secondary battery.

[0208] In some embodiments, when the lithium-ion secondary battery is in the fully discharged state, the tap density of the positive electrode film layer is 2.52 g / cm 3 - 2.78 g / cm 3, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10% - 22%.

[0209] In some embodiments, when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.55 g / cm 3 - 2.75 g / cm 3 , in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10% - 20%.

[0210] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer can be selected as 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20% or the numerical range between any two of them.

[0211] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer can be tested in the following manner. Import the scanning electron microscope image of the cross-section of the positive electrode film layer obtained in the manner described above along the thickness direction of the electrode sheet into ImageJ software. Select the straight line tool, use the straight line to mark the scale length in the picture, click "Analyze SetScale", and set the scale parameters in the software according to the scale length in the picture. Select the rectangular tool, select the part of the picture outside the scale area, use "Image Duplicate" to copy the selected area, and use "Image Type 8 bit" to adjust the picture format; select "Analyze Set Measurements", and select the following 5 options: "Area", "Mean gray value", "Area Fraction", "Limit to threshold", "Feret’s diameter", where "Decimal places" is selected as 3, and then select "Image" - "Adjust" - "Threshold" in sequence, and set 0 and 100 in the "Threshold" box selection position in sequence, then the pore data in the scanning electron microscope image of this cross-section can be exported using the Analyze-Measure function. Use "Image" - "Overlay" - "Flatten" to export to obtain the pore picture; click "Apply" in "Threshold", and then click "Analyze" - "Analyze Particles", and check the left four columns to obtain the pore statistical data.

[0212] It can be understood that in the embodiments of the present application, the "pores" in the cross-section of the positive electrode film layer are identified through the picture color difference and threshold. These "pores" are not the pore data obtained from the exhaust test, and are mainly used to characterize the cross-sectional area between the particles in the cross-section of the positive electrode film layer. This method is superior to the exhaust method because the porosity obtained by the exhaust method is related to the pores between the particles and also to the mesopores in the carbon layer coated on the surface of the lithium iron phosphate particles, thus unable to objectively reflect the pores between the particles. The lower the porosity of the cross-section of the positive electrode film layer measured by this method, on the one hand, means that the particle size distribution of large, medium and small particles in the positive electrode film layer is better and the compaction density is high. On the other hand, after the same particle size distribution and roll pressure, if the porosity is low, it means that the particles are easy to slide relative to each other, thereby reducing the risk of overpressure and stress concentration in the film layer, further reducing the probability of the positive electrode film peeling off during long-term cycling, which is beneficial to improving the long-term cycling performance of the battery.

[0213] In some embodiments, the positive electrode tab includes a bottom coating disposed between the positive electrode film layer and the current collector; the bottom coating includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating is ≤ 10 pcs / 10 μm.

[0214] Among them, carbon-based particles refer to particles mainly composed of carbon elements, including but not limited to conductive carbon, carbon black, etc.

[0215] The bottom coating is beneficial to improving the conductivity and adhesion between the positive electrode film layer and the current collector, reducing the peeling off of the positive electrode film layer from the current collector during cycling, and simultaneously improving the kinetic performance of the battery. In the high compaction density electrode tab of the embodiments of the present application, for example, when the compaction density of the positive electrode tab in the fully discharged state is greater than or equal to 2.4 g / cm 3 during the compaction process of the current collector under high pressure of the electrode tab, the current collector is prone to damage, and large-sized particles are easy to produce pits on the current collector. Controlling the distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating to be ≤ 10 pcs / 10 μm is beneficial to reducing the probability of damage to the current collector in the high compaction density electrode tab and further improving the ultimate compaction density of the positive electrode tab.

[0216] The distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating can adopt the method described above. By cutting the positive electrode film layer along the thickness direction of the electrode tab with an argon ion beam, taking a scanning electron microscope image or a microscope image, detecting the size of the carbon particles in the bottom coating by a statistical method, and counting the number of carbon-based particles with a particle size greater than 100 nm contained in every 10 μm in the bottom coating, and counting not less than 5 times and taking the average value.

[0217] The bottom coating in the embodiments of the present application can be realized through any well-known preparation process. For example, in the preparation process of carbon-based particles, operations such as pre-screening or centrifugation are carried out in advance to remove large particle carbon-based materials, so that the D of the carbon-based particles added during the preparation of the bottom coatingV50 At 20 - 60 nm, D V90 Less than or equal to 70 nm, the carbon-based material is mixed with a binder, stirred, and coated on a current collector to obtain a bottom coating.

[0218] In some embodiments, the compaction density of the positive electrode sheet in the fully discharged state is greater than or equal to 2.4 g / cm 3 , and the single-sided thickness of the bottom coating is 1 μm - 4 μm.

[0219] In some embodiments, the compaction density of the positive electrode sheet in the fully discharged state is greater than or equal to 2.5 g / cm 3 , and the single-sided thickness of the bottom coating is 2 μm - 4 μm.

[0220] With the increase in the compaction density of the electrode sheet, the extrusion effect of the lithium-containing phosphate material with large particles (such as particle size greater than 1 μm) in the positive electrode film layer on the bottom coating becomes more significant. Therefore, stress concentration is prone to occur at the large particle sites, and even penetrate through the bottom coating to damage the current collector. Increasing the thickness of the bottom coating is beneficial to improving the stress concentration phenomenon in the electrode sheet and further increasing the ultimate compaction density of the electrode sheet.

[0221] The single-sided thickness of the bottom coating can be tested in the following way. As described above, the positive electrode film layer is cut along the thickness direction of the electrode sheet by an argon ion beam, and a scanning electron microscope image is taken. In the length direction of the electrode sheet, points are taken every 1 m to measure the single-sided bottom coating thickness. After measuring the bottom coating thickness at 10 points, the average value is calculated. It should be noted that during the process of taking measurement points, abnormal points need to be avoided, that is, the bottom coating areas with a thickness less than 50 nm and a thickness greater than 4 m; these abnormal points are mainly caused by extreme fluctuations in the thickness of individual areas due to abnormal stress concentration during the compaction process of the electrode sheet and do not have statistical significance.

[0222] In some embodiments, the thickness of the positive electrode current collector is less than or equal to 17 μm, and can be optionally 13 μm - 15 μm.

[0223] In some embodiments, the thickness of the positive electrode current collector is 13 μm, 14 μm, 15 μm, 16 μm, 17 μm or the numerical range between any two of them.

[0224] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, as the metal foil, aluminum foil may be used. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0225] In some embodiments, the negative electrode sheet includes a negative current collector and a negative electrode film layer provided on at least one side of the negative current collector. The areal density of the negative electrode film layer on one side is 140 mg / 1540 mm 2 -215 mg / 1540 mm 2 ; and / or the tap density of the negative electrode film layer is 1.40 g / cm 3 -1.75 g / cm 3 .

[0226] The areal density and tap density of the negative electrode film layer can be tested by a method similar to that of the positive electrode film layer described above.

[0227] When the areal density and tap density of the negative electrode film layer are within the above ranges, it is beneficial to improve the energy density of the lithium-ion secondary battery.

[0228] In some embodiments, the negative current collector may be a metal foil or a composite current collector. For example, as the metal foil, copper foil may be used. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (such as copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0229] In some embodiments, the negative electrode film layer includes a negative electrode active material. The negative electrode active material can be a negative electrode active material for batteries well-known in the art. As an example, the negative electrode active material can include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material can be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other conventional materials that can be used as the negative electrode active material of the battery can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0230] In some embodiments, the negative electrode film layer may further optionally include a binder. The binder can be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0231] In some embodiments, the negative electrode film layer may further optionally include a conductive agent. The conductive agent can be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0232] In some embodiments, the negative electrode film layer may further optionally include other additives, such as thickeners (such as sodium carboxymethyl cellulose (CMC-Na)), etc.

[0233] In some embodiments, the negative electrode plate can be prepared in the following manner: the components for preparing the negative electrode plate, such as the negative electrode active material, the conductive agent, the binder, and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after processes such as drying and compaction, the negative electrode plate can be obtained.

[0234] In some embodiments, the lithium-ion secondary battery includes an electrolyte. The electrolyte plays a role in conducting ions between the positive electrode plate and the negative electrode plate. The present application does not specifically limit the type of the electrolyte, and it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or all-solid-state.

[0235] In some embodiments, the electrolyte uses an electrolytic solution. The electrolytic solution includes an electrolyte salt and a solvent.

[0236] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, lithium difluoro bis(oxalato)phosphate, and lithium tetrafluoro(oxalato)phosphate.

[0237] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0238] In some embodiments, the electrolyte may optionally further include additives. For example, the additives may include anode film-forming additives, cathode film-forming additives, and may also include additives that can improve certain battery performances, such as additives for improving the overcharge performance of the battery, additives for improving the high-temperature or low-temperature performance of the battery, etc.

[0239] In some embodiments, the lithium-ion secondary battery further includes a separator. The present application has no particular limitation on the type of the separator, and any well-known porous structure separator with good chemical stability and mechanical stability can be selected.

[0240] In some embodiments, the material of the separator may be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0241] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be made into an electrode assembly by a winding process or a stacking process.

[0242] In some embodiments, the lithium-ion secondary battery may include an outer package. The outer package can be used to encapsulate the above-mentioned electrode assembly and electrolyte.

[0243] In some embodiments, the outer package of the lithium-ion secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate, etc.

[0244] The second aspect of the present application provides a battery device, including the lithium-ion secondary battery provided by the first aspect of the present application. The battery device includes at least one of a battery module, a battery pack, and an energy storage battery.

[0245] The third aspect of the present application provides an electrical device, including the lithium-ion secondary battery provided by the first aspect of the present application.

[0246] The fourth aspect of the application provides a method for preparing a positive electrode active material, including: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source; adding a solvent and grinding to obtain a mixed slurry; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder to obtain the positive electrode active material; the sintering of the precursor powder to obtain the positive electrode active material includes at least two sinterings; after the first sintering, a first sintered product is obtained, the first sintered product and the carbon source are mixed to obtain an intermediate raw material, the intermediate raw material is divided into two groups, and grinding is respectively performed to obtain a first group of ground products and a second group of ground products, and the first group of ground products and the second group of ground products are mixed to obtain a mixed intermediate product; performing a second sintering on the mixed intermediate product to obtain the positive electrode active material; the D V50 of the first group of ground products is 0.8 μm - 1.2 μm; the D V50 of the second group of ground products is 0.30 μm - 0.50 μm.

[0247] Through the process method including two sinterings and two grindings, the content and size of large particles in the positive electrode active material can be effectively controlled, so that the positive electrode active material has a certain content of large particles and does not cause the large particle size to be too large; for preparing a positive electrode film layer with a D A90 of 1400 nm - 2100 nm and a particle size concentration ratio (D A90 - D A10 ) / D A50 of 1.855 - 2.375, it provides a material basis.

[0248] In some embodiments, the iron source is an iron-containing compound. In some embodiments, the iron source includes at least one of iron hydroxide, ferrous chloride, iron(III) oxide, iron phosphate, ferric pyrophosphate, ferrous oxalate, iron powder, iron nitrate, magnetite, and iron oxyhydroxide.

[0249] In some embodiments, the phosphorus source is a phosphoric acid compound. In some embodiments, the phosphorus source includes at least one of phosphoric acid, iron phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0250] In some embodiments, the iron source and the phosphorus source can be the same substance. In some embodiments, iron phosphate is used as both the iron source and the phosphorus source.

[0251] In some embodiments, the lithium source includes one or more of lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium oxide, lithium hydroxide, and lithium acetate.

[0252] In some embodiments, the lithium source includes lithium carbonate.

[0253] In some embodiments, the carbon source includes one or more of glucose, polyethylene glycol, citric acid, sucrose, starch, fructose, lactose, polyaniline, polyacrylonitrile, and polyvinylpyrrolidone.

[0254] In some embodiments, the carbon source includes glucose and polyethylene glycol.

[0255] In some embodiments, based on the total mass of the carbon source, the mass content of polyethylene glycol is 20% - 75%.

[0256] The polymer carbon source has a relatively low graphitization temperature, enabling the carbon coating material on the surface of the cathode active material to decompose and form a carbon layer at a relatively low sintering temperature, hindering the growth and sintering of lithium-containing transition metal phosphate grains and facilitating the reduction of the particle size of the cathode active material particles.

[0257] At the same time, the polymer carbon source usually has a relatively high molecular weight or a long molecular chain, and it is prone to form a stable skeleton structure through cross-linking or orientation during the heat treatment process. This orderliness is retained during the high-temperature carbonization process, facilitating the directional growth of graphite crystals; meanwhile, the entanglement and cross-linking between long chains are conducive to reducing structural defects and decreasing the lattice disorder caused by chain breakage during the carbonization process, thereby enhancing the graphitization degree.

[0258] The organic molecules in the carbon source will decompose at high temperatures, releasing carbon atoms. These carbon atoms can cover and fill the tiny voids or defects on the surface of the active material, reducing the surface roughness. The coating material formed by the sugar carbon source has a relatively low graphitization degree and a loose flocculent carbon structure, and itself may act as a new rough point. Therefore, the higher the proportion of the polymer carbon source in the carbon source, the more beneficial it is to optimize the surface roughness of the cathode active material.

[0259] In some embodiments, the slurry further includes a titanium source. Optionally, the titanium source includes one or more of titanium dioxide, tetrabutyl titanate, titanium nitrate, and titanic acid.

[0260] The titanium source often has a relatively low surface activity. Including the titanium source in the slurry can reduce the activity of the lithium-containing transition metal phosphate precursor and inhibit the particle growth of the lithium-containing transition metal phosphate during the high-temperature sintering process, enabling the lithium-containing transition metal phosphate to form smaller particles during the sintering process.

[0261] Titanium, as a lattice stabilizer, the titanium element is usually in the form of Ti 4+In the form of entering the lattice of lithium-containing transition metal phosphate, some titanium ions can replace the position of iron ions, making the crystal structure more stable and reducing the possibility of inversion of lithium and iron ions, especially during high-temperature or high-current charge and discharge.

[0262] At the same time, the doping of titanium helps to improve the sphericity of the particles and reduce the roughness of the particles, thereby enhancing the overall structural stability of the material.

[0263] In some embodiments, the atomic molar ratio of iron element to phosphorus element in the iron source and the phosphorus source is 0.95 - 1.

[0264] In some embodiments, the atomic molar ratio of iron element to phosphorus element in the lithium source and the iron source can be selected from 0.95, 0.955, 0.96, 0.965, 0.97, 0.975, 0.98, 0.985, 0.99, 0.995, 1 or the numerical range between any two of them.

[0265] When synthesizing lithium-containing transition metal oxides, the iron-phosphorus ratio affects the reaction rate and the crystal growth process. A low iron-phosphorus ratio can increase the reaction rate of the iron source and accelerate the conversion of the iron source into lithium-containing transition metal phosphate; a high phosphorus content can inhibit the interaction between iron and carbon and reduce iron-based compounds such as Fe 3 The formation of C etc. helps the graphitization of the carbon layer. After this step becomes the rate-determining step, the crystal growth time is prolonged, thereby promoting the formation of larger particles. Thus, the particle size distribution and area distribution are regulated to obtain the desired packing structure.

[0266] In some embodiments, based on the total mass of the mixed raw materials, the mass proportion of the carbon source in the mixed raw materials is 5% - 7%.

[0267] In some embodiments, based on the total mass of the mixed raw materials, the mass proportion of the carbon source in the mixed raw materials can be selected from 5%, 5.1%, 5.2%, 5.3%, 5.4%, 5.5%, 5.6%, 5.7%, 5.8%, 5.9%, 6.0%, 6.1%, 6.2%, 6.3%, 6.4%, 6.5%, 6.6%, 6.7%, 6.8%, 6.9%, 7% or the numerical range between any two of them.

[0268] By controlling the mass of the carbon source containing lithium within the above range, the conductivity of the material can be enhanced, and the negative impact on the specific capacity of the positive electrode sheet and the battery energy density can be reduced. An overly thick carbon layer not only occupies the effective active material space but may also cause the instability of the material structure.

[0269] In some embodiments, the solvent includes water and its mixtures.

[0270] In some embodiments, obtaining the mixed raw materials including a carbon source, a lithium source, an iron source, and a phosphorus source includes: adding the carbon source, the lithium source, the phosphorus source, the iron source, and the carbon source into a solvent and mixing and stirring, and the rotation speed of the stirring is 1400 rpm - 2200 rpm.

[0271] In some embodiments, obtaining the mixed slurry after grinding includes primary grinding, and the primary grinding satisfies one or more of the following conditions: (1) The grinding balls for the primary grinding are one or more of zirconia balls, silicon nitride zirconia balls, and ceramic zirconia balls; (2) The diameter of the grinding balls for the primary grinding is 0.5 mm - 0.7 mm; (3) The rotation speed of the primary grinding is 450 rpm - 550 rpm; (4) The time of the primary grinding is 0.5 h - 1.5 h; (5) The pressure in the grinding chamber for the primary grinding is 0.01 MPa - 0.3 MPa.

[0272] In some embodiments, obtaining the mixed slurry after grinding includes secondary grinding after the primary grinding, and the secondary grinding satisfies one or more of the following conditions: (1) The grinding balls for the secondary grinding are one or more of zirconia balls, silicon nitride zirconia balls, and ceramic zirconia balls; (2) The diameter of the grinding balls for the secondary grinding is 0.25 mm - 0.35 mm; (3) The rotation speed of the secondary grinding is 470 rpm - 530 rpm; (4) The time of the secondary grinding is 3.0 h - 5.0 h; (5) The pressure in the grinding chamber for the secondary grinding is 0.01 MPa - 0.3 MPa.

[0273] In some embodiments, the particle size D of the mixed slurry V50 is 0.3 μm - 0.5 μm.

[0274] Grinding at least twice is beneficial to controlling the temperature and viscosity of the slurry, reducing the excessive viscosity of the slurry caused by too high temperature and the resulting raw material agglomeration, improving the uniformity of the particle size in the product, reducing the generation of oversize particles, and being beneficial to the control of particle D in the positive electrode film layer. A90 control.

[0275] The first grinding can process large particle materials, and the second grinding further refines the materials and adjusts the particle size distribution. In this way, the particle size non-uniformity caused during the grinding process can be effectively reduced, the agglomeration phenomenon between particles can be reduced, the conductivity and cycle stability of the battery can be improved, and at the same time, the overall production efficiency can be improved while meeting the performance of the final product.

[0276] In some embodiments, obtaining the precursor powder after drying the mixed slurry includes obtaining the precursor powder after spray-drying the mixed slurry.

[0277] In some embodiments, the first sintering in the at least two sinterings satisfies one or more of the following conditions: (1) The heating rate is 2 °C / min - 10 °C / min; (2) The holding temperature is 720 °C - 790 °C; (3) The holding time is 5 h - 12 h.

[0278] In some embodiments, the carbon sources added in obtaining the intermediate raw material by mixing the first sintering product and the carbon source include glucose and polyethylene glycol.

[0279] In some embodiments, based on the mass of the first sintering product, the mass content of glucose in the intermediate raw material is 1% - 3%, and the mass content of polyethylene glycol in the intermediate raw material is 0.5% - 6%.

[0280] In some embodiments, the D of the first group of grinding products V50 is 0.8 μm - 1.2 μm.

[0281] In some embodiments, the D of the second group of grinding products V50 is 0.30 μm - 0.50 μm.

[0282] For small particles relative to large particles, the proportion of surface atoms is relatively high, the surface energy is large, the surface atoms have high activity, and the surface area per unit volume is larger. Therefore, the surface diffusion rate is faster, which makes small particles prone to relatively intense surface rearrangement during the sintering process, promoting the evolution of particles into a spherical shape. Therefore, controlling the grinding particle size to a smaller value, including the grinding of the mixed slurry and the grinding of the intermediate raw material, is beneficial for the formation of products with higher sphericity, smoother and flatter surfaces during the subsequent sintering process. At the same time, particles with a smaller grinding particle size are more easily coated, improving the coating integrity.

[0283] In some embodiments, the grinding conditions of the first group of grinding products satisfy one or more of the following conditions: (1) The rotation speed is 550 rpm ± 50 rpm; (2) The grinding time is 0.5 h - 1.5 h.

[0284] In some embodiments, the grinding conditions of the second group of ground products satisfy one or more of the following conditions: (1) The rotation speed is 500 rpm ± 50 rpm; (2) The grinding time is 3 h - 5 h. In some embodiments, the mass ratio of the first group of ground products to the second group of ground products is (60:40) - (80:20).

[0285] By adjusting the mass ratio of the first group of ground products to the second group of ground products, cathode active materials with different grading ratios can be obtained, so that the area ratio of particles of each particle size can be finely adjusted, and the required packing structure can be obtained.

[0286] In some embodiments, the second sintering satisfies one or more of the following conditions: (1) The holding temperature is 770°C - 830°C; (2) The holding time is 5 h - 12 h; (3) The heating rate is 2°C / min - 10°C / min.

[0287] Through the two - firing process, the sintering time in the high - temperature range can be effectively shortened, thereby reducing the risk and probability of extremely large particles appearing during high - temperature sintering. By adjusting the particle sizes of the two groups of grinding during the second sintering, the activity of the particles can be controlled, so that the cathode active material has large particles with a certain area ratio. While improving the compaction density of the electrode sheet and the energy density of the battery monomer, the battery monomer also has good kinetic performance.

[0288] By controlling the sintering temperature in the first - firing and second - firing processes, the speed of the sintering diffusion rate can be controlled. At high temperatures, the diffusion on the particle surface increases, the defects in the particles are repaired, and the crystal lattice is rearranged. Through recrystallization, the defects on the particle surface are eliminated, the crystal grain structure of the particles becomes more ordered, the particle size gradually increases, and it helps to smooth the particle surface and promote the particles to develop into a spherical shape. The sintering temperature also affects the graphitization rate of the carbon source. Kinetically, carbon atoms obtain more energy and can overcome the original energy barrier, making them rearrange more violently in the crystal lattice. The sintering time affects the degree of the reaction. If the sintering time is too short, the diffusion and rearrangement of the lithium - containing transition metal phosphate and the carbon source are not completely completed; if the sintering time is too long, the particles will grow abnormally, the crystal grains inside the particles will coarsen, the material structure tends to be unstable, and the adhesion force between the particles will increase, resulting in agglomeration.

[0289] In some embodiments, after sintering the precursor, the product is subjected to air jet milling to obtain the positive electrode active material.

[0290] In some embodiments, the classification frequency of the air jet milling is 20 Hz - 30 Hz, and the milling air pressure is 0.45 MPa - 0.55 MPa.

[0291] The classification frequency in air jet milling refers to the working frequency of the classification device in air jet milling, which is usually related to the classification efficiency and particle size distribution of particles. A higher classification frequency will screen the particles in the air flow more times, so that larger particles are screened out, leaving smaller particles. And a higher classification frequency may increase the number of particle collisions, making irregular particles receive further impacts, making the particle surface smoother and the shape tend to be spherical.

[0292] A high air pressure will cause the particles to receive a greater impact force, and the collisions between the particles will be more intense, which will cause the particle surface to be strongly impacted and worn. It can crush large particles into small particles, and the collisions between the particles are more intense, and the surface is more easily trimmed, improving the sphericity and surface flatness of the particles.

[0293] However, too high a classification frequency and milling air pressure will cause the agglomerated particles to disperse into primary particles and then further crack and break, affecting the predetermined particle size distribution, and making the structure of the carbon-coated material incomplete, manifested as an increase in iron dissolution, having a negative impact on the slip of particles during rolling, and increasing the contact and reaction between the lithium-containing transition metal phosphate and external factors such as the electrolyte, which is not conducive to maintaining the cycle performance and life of the battery. Therefore, it is necessary to control the classification frequency and milling air pressure of the air jet milling within a suitable range.

[0294] The fifth aspect of the present application provides a method for preparing a positive electrode sheet, the preparation method including sequentially adding a binder, a conductive agent, and the positive electrode active material prepared by the method of the fourth aspect, dry mixing, adding a solvent, stirring, and adjusting the viscosity to obtain a shipping slurry; transferring and coating the shipping slurry on at least one side of a current collector, and drying and hot pressing to obtain a positive electrode film layer.

[0295] In some embodiments, the drying temperature is 95°C - 105°C, and the speed is 2.0 m / min - 2.3 m / min.

[0296] In some embodiments, the hot pressing includes at least three hot rolling presses, and the hot rolling pressures increase in sequence, and the hot rolling pressures are 20 tons - 50 tons, 50 tons - 70 tons, and 70 tons - 90 tons in sequence; the hot rolling temperature is 40°C - 80°C, and before first entering the hot rolling press, the sheet is heated, and the heating temperature is 40°C - 50°C.

[0297] The cathode active material prepared by the above hot pressing process in combination with the preparation method of the fourth aspect in the embodiments of the present application is beneficial to further reduce the porosity of the cut surface of the cathode film layer, improve the ultimate compaction density of the electrode sheet, and improve the energy density of the battery.

[0298] In some embodiments, the steps of stirring and adjusting the viscosity to obtain the shipped slurry specifically include performing pre-stirring to obtain a first slurry; re-stirring the first slurry to obtain a second slurry; and slowly stirring and adjusting the viscosity of the second slurry to obtain the shipped slurry.

[0299] In addition, the present application further provides an electrical device, which includes at least one of the secondary battery, battery module or battery pack provided by the present application. The secondary battery, battery module or battery pack can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device may include mobile devices (such as mobile phones, laptop computers, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships, satellites, energy storage systems, etc., but is not limited thereto.

[0300] As the electrical device, the secondary battery, battery module or battery pack can be selected according to its usage requirements.

[0301] Figure 7 Here is an example of an electrical device. The electrical device is a pure electric vehicle, hybrid electric vehicle or plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the secondary battery for this electrical device, a battery pack or battery module can be used.

[0302] Another example of the device can be a mobile phone, tablet computer, laptop computer, etc. This device usually requires being thin and light, and a secondary battery can be used as the power source.

[0303] Embodiment Hereinafter, embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application and should not be construed as a limitation to the present application. For those technologies or conditions not specified in the embodiments, they shall be carried out according to the technologies or conditions described in the literature in the art or according to the product specifications. For the reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0304] Embodiment 1 (1) Preparation of the cathode active material S1. Add lithium carbonate, iron phosphate, glucose, and titanium dioxide to water, mix them in a premixing tank at a rotation speed of 1800 rpm, and demagnetize them with a demagnetizing rod with a magnetic field strength of 8000 - 12000 Gs. Among them, the ratio of lithium carbonate to iron phosphate is such that the molar ratio of iron to phosphorus is 0.975, the mass content of glucose compared to the total amount of iron phosphate is 5.7%, and the doping amount of titanium dioxide is such that the mass content of doped titanium in the carbon-coated lithium iron phosphate cathode active material product is 5000 ppm; S2. Conduct two grinding - demagnetization cycles on the mixed raw materials in a sand mill. For the first grinding, use zirconia balls with a diameter of 0.6 mm and a rotation speed of 500 rpm, with a grinding duration of 1 h and the pressure in the grinding chamber less than 0.3 MPa. Demagnetize the raw materials after the first grinding with a permanent magnet demagnetizer, and the demagnetization intensity is greater than or equal to 8000 Gs. Then conduct a second grinding on the demagnetized raw materials to obtain a mixed slurry, and the particle size D V50 of the mixed slurry is 0.40 μm ± 0.10 μm; S3. Spray - dry the mixed slurry to obtain a precursor powder, S4. Sinter the precursor powder to obtain a lithium iron phosphate cathode material. The sintering process includes: First sintering: Sinter the precursor powder in a nitrogen atmosphere, heat it from 25°C to 760°C at a heating rate of 5°C / min, and keep it warm for 10 h. After cooling, obtain the first sintering product; Grinding and mixing: Add 1.5% of glucose and 3.0% of polyethylene glycol based on the total mass of the first sintering product to the first sintering product. Divide it into two groups for grinding (the third grinding). Among them, stop grinding when the particle size D V50 of the first group reaches 1.0 μm ± 0.20 μm (grinding conditions: 550 rpm ± 50 rpm, grinding time 1 h) to obtain the first - group grinding product; stop grinding when the particle size D V50 of the second group reaches 0.38 μm ± 0.05 μm (grinding conditions: 500 rpm ± 50 rpm, grinding time 4 h) to obtain the second - group grinding product; Mix the first - group grinding product and the second - group grinding product according to a mass ratio of 70:30 to obtain a mixed intermediate product; Spray - dry the mixed intermediate product; Second sintering: Sinter the dried mixed intermediate product in a nitrogen atmosphere, heat it from 25°C to 800°C at a heating rate of 5°C / min, and keep it warm for 10 h. After cooling, obtain the second sintering product.

[0305] S5. After sintering is completed, it is cooled to below 100 °C, and the second sintered product is crushed by air jet milling to obtain the carbon-coated lithium iron phosphate cathode active material. Among them, the classification frequency of air jet milling is 25 Hz, and the crushing air pressure is 0.55 MPa.

[0306] The mass content of carbon element in the prepared cathode active material is 1.248%, the concentration of lithium-iron inversion defects is 0.22%, and the tapped density of the powder is 1.30 g / cm 3 , and the compacted density of the powder under 3T pressure is 2.62 g / cm 3 , and the powder resistivity under 8 MPa pressure is 10.95 Ω·cm; the discharge specific capacity at 1C discharge rate is 140.8 mAh / g; the discharge capacity ratio of the 3.2V discharge platform is 90.0%.

[0307] (2) Preparation of the cathode electrode sheet: 2.2 wt% of PVDF, 0.8 wt% of conductive carbon black, and 97.0 wt% of the cathode active material are sequentially added and dry-mixed, and then N-methylpyrrolidone is added, and stirred and the viscosity is adjusted to obtain the shipping slurry; the shipping slurry is transferred and coated onto the bottom coating of the current collector aluminum foil. The bottom coating includes carbon black and PVDF, and the mass ratio of the two is 1:1. The distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating is ≤10 pcs / 10 μm, and the thickness of the bottom coating is 2 μm. After drying and hot pressing, the single-sided areal density is 350 mg / 1540 cm 2 of the cathode film layer. Among them, the drying temperature is 95 °C and the speed is 2.0 m / min.

[0308] The hot pressing process includes three hot roll pressing processes, and the hot roll pressing pressure increases sequentially. The hot roll pressures are 35 tons, 55 tons, and 75 tons in sequence; the hot roll temperature is 65 °C. Before the first entry into the hot roll compaction, the electrode sheet is heated, and the heating temperature is 50 °C.

[0309] The compacted density of the electrode sheet is the ultimate compacted density of the electrode sheet. The test method for the ultimate compacted density of the electrode sheet is as follows; the ultimate compacted density of the electrode sheet in this example is 2.71 g / cm 3 .

[0310] 14,827 particles are counted in the cross-section of the cathode film layer along the thickness direction of the electrode sheet. The results show that the D A50 of the particles in the cross-section of the cathode film layer is 719 nm, the D A10 is 214 nm, the D A90 is 1674 nm, and the particle size concentration ratio (D A90 -D A10 ) / D A50 is 2.031.

[0311] The median value C of the graphitization degree obtained by the positive electrode film layer prepared in the surface scanning mode of a laser confocal Raman spectrometer 50 is 1.04. In the cumulative area distribution curve of the sphericity of particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode plate, the median value L of the sphericity A50 is 0.725. In the cumulative area distribution curve of the roughness of particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode plate, the median value R of the roughness A50 is 0.952, and the concentration degree of the roughness is 0.08. The iron dissolution rate of the positive electrode film layer is 860.7 ppm.

[0312] (3) Preparation of the negative electrode plate: Mix 95.5 wt% of negative electrode active material (artificial graphite), 1.0 wt% of conductive agent (conductive carbon black), 2.0 wt% of binder (styrene-butadiene rubber (SBR)), and 1.5 wt% of thickener (sodium carboxymethyl cellulose (CMC)), add deionized water and stir to disperse to make a negative electrode slurry. Then coat the negative electrode slurry on both surfaces of the Cu foil. After both sides are completed, dry, press, slit, and slice to prepare the negative electrode plate. The single-sided coating density is 166 mg / 1540.25 mm 2 , and the pressed density is 1.60 g / cm 3 .

[0313] (4) Preparation of the separator Use a polypropylene film as the separator.

[0314] (5) Preparation of the electrolyte In a glove box under an argon atmosphere (H 2 O < 0.1 ppm, O 2 < 0.1 ppm), mix the organic solvents ethylene carbonate (EC) / dimethyl carbonate (DMC) evenly according to a volume ratio of 1 / 1, add the lithium salt LiPF 6 and dissolve it in the organic solvent. The content of LiPF 6 in the solution is 1 mol / L, stir evenly to obtain the electrolyte.

[0315] (6) Preparation of the battery: Stack the positive electrode plate, separator, and negative electrode plate in sequence. The separator should be able to isolate the anode and cathode. Wind to obtain a bare battery cell, place the bare battery cell in the outer package, inject the electrolyte well, and go through processes such as encapsulation, formation, and degassing to finally obtain a lithium-ion battery.

[0316] The preparation method of Example 2 is basically the same as that of Example 1, except that in step S4, the D of the particles in the first group V50Stop grinding when it reaches 0.80μm ± 0.20μm; for the particles in the second group, D V50 Stop grinding when it reaches 0.30μm ± 0.05μm.

[0317] The preparation method of Example 3 is basically the same as that of Example 1, except that in step S4, for the particles in the first group, D V50 Stop grinding when it reaches 0.80μm ± 0.20μm; for the particles in the second group, D V50 Stop grinding when it reaches 0.45μm ± 0.05μm.

[0318] The preparation method of Example 4 is basically the same as that of Example 1, except that in step S4, for the particles in the first group, D V50 Stop grinding when it reaches 0.80μm ± 0.20μm; for the particles in the second group, D V50 Stop grinding when it reaches 0.45μm ± 0.05μm; mix the grinding products of the first group and the second group according to a mass ratio of 60:40.

[0319] The preparation method of Example 5 is basically the same as that of Example 1, except that in step S4, for the particles in the first group, D V50 Stop grinding when it reaches 1.20μm ± 0.20μm; for the particles in the second group, D V50 Stop grinding when it reaches 0.45μm ± 0.05μm; mix the grinding products of the first group and the second group according to a mass ratio of 80:20.

[0320] The preparation method of Example 6 is basically the same as that of Example 1, except that in step S4, for the particles in the first group, D V50 Stop grinding when it reaches 1.20μm ± 0.20μm; for the particles in the second group, D V50 Stop grinding when it reaches 0.30μm ± 0.05μm.

[0321] The preparation method of Comparative Example 1 is basically the same as that of Example 1, except that in step S4, for the particles in the first group, D V50 Stop grinding when it reaches 0.75μm ± 0.20μm; for the particles in the second group, D V50 Stop grinding when it reaches 0.50μm ± 0.05μm.

[0322] The preparation method of Comparative Example 2 is basically the same as that of Example 1, except that in step S4, for the particles in the first group, D V50 Stop grinding when it reaches 1.50μm ± 0.20μm; for the particles in the second group, D V50 Stop grinding when it reaches 0.38μm ± 0.05μm.

[0323] The preparation method of Comparative Example 3 is basically the same as that of Example 1, except that in step S2, the particle size D of the mixed slurry V50 is 0.35 μm ± 0.10 μm; in step S4, there is only the first sintering, and the sintering process includes: sintering the precursor powder in a nitrogen atmosphere, heating from 25°C to 800°C at a heating rate of 5°C / min, and holding for 10 h, and obtaining a sintered product after cooling.

[0324] Performance Test 1. Ultimate Compaction Density of the Electrode The double-sided coated electrode is compacted by a roll press, and the elongation of the compacted electrode is tested, and the flexibility of the electrode after compaction is evaluated at the same time. By increasing the pressure of the roll press, electrodes with different compaction densities can be obtained. As the pressure increases, the compaction density of the electrode increases, the elongation of the electrode increases, and the flexibility of the electrode decreases. If the elongation of the electrode is too high, it is easy to cause warping of the electrode, and if the flexibility of the electrode is too low, it is easy to cause brittle fracture of the electrode. Therefore, the smaller of the compaction densities corresponding to an elongation of 8% of the electrode or a folding number of 3 times for the flexibility of the electrode is defined as the ultimate compaction density of the electrode.

[0325] The compaction density is calculated by the mass of the positive electrode film layer / the volume of the positive electrode film layer.

[0326] The test method for elongation is as follows: Lay the electrode flat on a horizontal table, cut the electrode into segments, and the length of each electrode is about 100 cm; remove the copper foil of the substrate at the edge of the electrode, and pay attention to keeping the cut edge of the electrode parallel to the MD direction of the electrode (perpendicular to the direction of the pressure roller), ensure that the electrode part is completely covered by the coating, use a steel ruler to measure the length between the marked points at the same position of the head, tail and width in the length direction of the electrode, estimate to 0.1 mm, and record the length before compaction; record the length after compaction between the corresponding marked points, and use (length after compaction - length before compaction) / length before compaction as the elongation of the electrode.

[0327] The test method for the number of flexible folds is as follows.

[0328] Cut the positive electrode into test specimens with a size of 20×100 mm 2 ; after folding it forward, flatten it with a 2 kg pressure roller, and unfold it to check whether there is light transmission through the gap against the light. If there is no light transmission, fold it backward, flatten it with a 2 kg pressure roller, and check against the light again. Repeat this process until there is light transmission through the gap, and record the number of folds; repeat the test three times and take the average value as the reference data for the flexibility of the electrode.

[0329] 2. Energy Density Test Leave the lithium-ion secondary battery at rest at 25°C for 2 h to ensure that the temperature of the lithium-ion secondary battery is 25°C. After charging the lithium-ion secondary battery at 0.33C to the charging cut-off voltage of 3.65V at 25°C, continue the constant voltage charging at this charging cut-off voltage until the current is 0.05C, and the charging is cut off (where C represents the rated capacity of the lithium-ion secondary battery). Leave the lithium-ion secondary battery at rest at 25°C for 1 h, then discharge the lithium-ion secondary battery at 0.33C to the discharge cut-off voltage of 2.5V at 25°C, and record the total discharge energy of the lithium-ion secondary battery as E 0 。

[0330] Measure the length, width and height of the lithium-ion secondary battery, and calculate the volume value V of the lithium-ion secondary battery 0 = length * width * height

[0331] The volume energy density of the lithium-ion secondary battery = the discharge energy E of the lithium-ion secondary battery 0 / the volume V of the lithium-ion secondary battery 0 。

[0332] 3. DCR Test Method At 25°C, after constant current charging at 0.33C to 3.65V, constant voltage charging until the current is 0.05C, then discharging at 1 / 3C to 20% SOC, leaving at rest for 5 min, then discharging with a 3C pulse for 30 s, leaving at rest for 40 s, then charging with 3C for 40 s, leaving at rest for 5 min, then constant current charging at 1 / 3C to 3.65V, constant voltage charging to 0.05C, then discharging at 1 / 3C to 10% SOC, leaving at rest for 5 min, then discharging with a 3C pulse for 30 s, leaving at rest for 40 s, charging with 3C for 40 s, leaving at rest for 5 min, then fully charging at 1 / 3C and then discharging at 1 / 3C to 50% SOC, then leaving at rest at -25°C for 2 h and then discharging with a 1C pulse for 30 s, leaving at rest for 10 min, then leaving at rest at 25°C for 2 h, constant current charging at 1 / 3C to 3.65V and then constant voltage charging to 0.05C, then discharging at 1 / 3C to 20% SOC, then leaving at rest at -25°C for 2 h and then discharging with a 1C pulse for 30 s, leaving at rest for 10 min

[0333] Record the voltage at this time before and after each pulse discharge, calculate the DCR under different conditions, and the calculation formula is DCR = (the voltage before the pulse discharge after the end of rest - the voltage before rest after the pulse discharge) / the pulse current

[0334] Experimental Parameters and Test Results Prepare the batteries of each example and comparative example according to the above method, and measure each performance parameter. The results are shown in Table 1 below

[0335] Table 1

[0336] Continued Table 1

[0337] From the data comparison between the examples and the comparative examples, different from the existing understanding, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the excessively high particle size concentration degree has limited effect on improving the compaction of the electrode sheet, and it is difficult to further improve the electrode sheet compaction by adjusting the particle size concentration degree. Control the D of the particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet A90 to be 1400nm - 2100nm, and when the particle size concentration degree (D A90 -D A10 ) / D A50 is 1.855 - 2.375, the battery has a relatively high compaction density of the positive electrode sheet and energy density and enables the battery to have good kinetic performance.

[0338] From the comparison between Example 5 and other examples, it can be seen that when the D of the particles A90 is in the range of 1400nm - 2000nm, the DC internal resistance of the battery can be further reduced.

[0339] From the comparison between Examples 5 and 6 and other examples, it can be seen that in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, when the D of the particles A50 is 650nm - 750nm, it is beneficial to further reduce the impedance of the battery while improving the compaction density of the electrode sheet, taking into account the kinetic performance of the battery.

[0340] From the comparison between Example 2, Example 4 and Comparative Example 1, it can be seen that in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, when the particle size concentration degree (D A90 -D A10 ) / D A50 is 1.855 - 2.375, the particles in the positive electrode film layer have a better particle size distribution, improving the particle packing situation and increasing the compaction density of the electrode sheet and the energy density of the battery.

[0341] It should be noted that the present application is not limited to the above embodiments. The above embodiments are only examples, and embodiments with the same structure and the same effect as the technical idea within the technical solution scope of the present application are all included in the technical scope of the present application. In addition, within the scope not departing from the gist of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other ways constructed by combining some constituent elements of the embodiments are also included in the scope of the present application.

Claims

1. A lithium ion secondary battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector. The positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes lithium-containing transition metal phosphate particles with a carbon coating material disposed on at least a portion of the surface. In the cross section of the positive electrode film along the thickness direction of the electrode sheet, the D A90 1400nm-2100nm, particle size concentration (D A90 -D A10 ) / D A50 is 1.855-2.375, where D A90 , D A50、 D A10 It refers to the particle size corresponding to when the cumulative area distribution of the particles reaches 90%, 50%, and 10% in the particle area cumulative distribution curve.

2. The lithium ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film along the thickness direction of the electrode sheet, the D A90 It is 1400nm-2000nm.

3. The lithium ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film along the thickness direction of the electrode sheet, the D A50 It is 600nm-900nm.

4. The lithium-ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film along the thickness direction of the electrode sheet, the D A50 It is 650nm-750nm.

5. The lithium ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film along the thickness direction of the electrode sheet, the D A10 It is 100nm-300nm.

6. The lithium ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film along the thickness direction of the electrode sheet, the D A10 It is 120nm-250nm.

7. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C of the positive electrode film layer obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the median value of the graphitization degree C 50 is 0.98-1.20; among which the graphitization degree C value is I G / I D , where I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at .

8. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C of the positive electrode film layer obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the median value of the graphitization degree C 50 is 1.02-1.10, where the graphitization degree C value is I G / I D , where I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at .

9. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the sphericity area of ​​the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median of the sphericity L A50 It is 0.70-0.

85.

10. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the sphericity area of ​​the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median of the sphericity L A50 It is 0.70-0.

76.

11. The lithium ion secondary battery according to claim 1, characterized in that: In the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median of the roughness R A50 It is 0.92-0.

96.

12. The lithium ion secondary battery according to claim 1, characterized in that: In the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode, the concentration of roughness (R A90 -R A10 ) / R A50 It is 0.05-0.

10.

13. The lithium ion secondary battery according to claim 1, characterized in that: The iron dissolution rate of the positive electrode film layer is 400ppm-1800ppm.

14. The lithium ion secondary battery according to claim 1, characterized in that: Based on the total mass of the positive electrode active material, the mass content of carbon element is 0.8%-1.8%.

15. The lithium ion secondary battery according to claim 1, characterized in that: Based on the total mass of the positive electrode active material, the mass content of carbon element is 0.9%-1.5%.

16. The lithium ion secondary battery according to claim 1, characterized in that: The lithium iron antisite defect concentration of the positive electrode active material is 0.001%-1.5%.

17. The lithium ion secondary battery according to claim 1, characterized in that: The lithium iron antisite defect concentration of the positive electrode active material is 0.01%-1.0%.

18. The lithium ion secondary battery according to claim 1, characterized in that: The lithium-containing transition metal phosphate includes a component having the following general formula: Li m Fe x P y O j Q q , Among them, Q includes one or more of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, 0<q≤0.

1.

19. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode active material includes one or more of lithium iron phosphate and its doped modified materials and coated modified materials.

20. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode active material includes titanium element, and the mass content of the titanium element is 4000ppm-8000ppm based on the total mass of the positive electrode active material.

21. The lithium ion secondary battery according to claim 1, characterized in that: The tap density of the positive electrode active material powder is 1.00 g / cm 3 -1.70g / cm 3 ; And / or the cathode active material has a powder compaction density of 2.55 g / cm under a pressure of 3T 3 -2.70g / cm 3 .

22. The lithium ion secondary battery according to claim 1, characterized in that: The tap density of the positive electrode active material powder is 1.20 g / cm 3 -1.50g / cm 3 ; And / or the cathode active material has a powder compaction density of 2.58 g / cm under a pressure of 3T 3 -2.68g / cm 3 .

23. The lithium ion secondary battery according to claim 1, characterized in that: The powder resistivity of the positive electrode active material under a pressure of 8 MPa is 0.5 Ω·cm-60.0 Ω·cm.

24. The lithium ion secondary battery according to claim 1, characterized in that: The powder resistivity of the positive electrode active material under a pressure of 8 MPa is 2.0 Ω·cm-40.0 Ω·cm.

25. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode active material has a discharge capacity of 135 mAh / g to 150 mAh / g at a 1C discharge rate.

26. The lithium ion secondary battery according to claim 1, characterized in that: The discharge capacity of the positive electrode active material discharged to 3.2V accounts for η≥85%, and η is defined as: at room temperature, the button battery containing the positive electrode active material is charged and discharged twice at a constant current rate of 0.1C in the voltage range of 2.0V~3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at the rate of 1C, the capacity value extracted at the discharge voltage of 3.2V is recorded as C1, and the capacity value extracted to the discharge voltage of 2.0V is C2, η=C1 / C2, wherein the charging process includes constant voltage charging, a constant voltage of 3.75V, and a constant voltage cut-off current of 50μA.

27. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode film layer further includes a conductive agent, and based on the total mass of the positive electrode film layer, the mass content of the conductive agent is 0.01%-1.5%.

28. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode film layer further includes a binder. Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 94.0%-99.4%; the mass content of the binder is 0.5%-3.0%.

29. The lithium ion secondary battery according to claim 1, characterized in that: Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 96.5%-99.4%.

30. The lithium ion secondary battery according to claim 1, characterized in that: The single-side density of the positive electrode film layer is 300 mg / 1540 mm 2 -450mg / 1540mm 2 .

31. The lithium ion secondary battery according to claim 1, characterized in that: When the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.52 g / cm 3 -2.78g / cm 3 .

32. The lithium ion secondary battery according to claim 1, characterized in that: When the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.55 g / cm 3 -2.75g / cm 3 .

33. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode film layer satisfies at least one of the following conditions: (1) When the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.52 g / cm 3 -2.78g / cm 3 , in a cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10%-22%; (2) When the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.55 g / cm 3 -2.75g / cm 3 In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10%-20%.

34. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode sheet includes a primer layer, which is disposed between the positive electrode film layer and the current collector; the primer layer satisfies at least one of the following conditions: (1) The primer layer includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the primer layer is ≤10 pcs / 10 μm; (2) The compaction density of the positive electrode sheet in the fully charged state is greater than or equal to 2.4 g / cm 3 , the thickness of the primer layer on one side is 1 μm-4 μm; (3) The compaction density of the positive electrode sheet in the fully charged state is greater than or equal to 2.5 g / cm 3 The single-side thickness of the primer layer is 2 μm-4 μm.

35. A battery device, characterized in that: A lithium-ion secondary battery comprising any one of claims 1 to 34, wherein the battery device comprises at least one of a battery module, a battery pack, and an energy storage battery.

36. An electrical device, characterized in that: A lithium ion secondary battery according to any one of claims 1 to 34 or a battery device according to claim 35.

37. A method for preparing a positive electrode active material, characterized in that: The preparation method is used to prepare the positive electrode active material in the lithium ion secondary battery according to any one of claims 1 to 34, and the preparation method comprises: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source; adding a solvent and grinding to obtain a mixed slurry; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder to obtain the positive electrode active material; the sintering of the precursor powder to obtain the positive electrode active material comprises at least two sinterings; after the first sintering, a first sintering product is obtained, the first sintering product and a carbon source are mixed to obtain an intermediate raw material, the intermediate raw material is divided into two groups, and the first group of grinding products and the second group of grinding products are respectively ground to obtain a first group of grinding products and a second group of grinding products, the first group of grinding products and the second group of grinding products are mixed to obtain a mixed intermediate product; the mixed intermediate product is subjected to a second sintering to obtain the positive electrode active material; wherein the D V50 The D of the second group of grinding products is 0.8 μm-1.2 μm; V50 It is 0.3μm-0.5μm.

38. A method for preparing a positive electrode sheet, characterized in that: The preparation method comprises adding a binder, a conductive agent, and a positive electrode active material prepared by the preparation method of claim 37 in sequence, dry-mixing them, adding a solvent, stirring, and adjusting the viscosity to obtain a shipping slurry; transfer-coating the shipping slurry to at least one side of the current collector, and obtaining a positive electrode film layer after drying and hot pressing.

39. The preparation method according to claim 38, characterized in that: The drying temperature is 95°C-105°C, and the drying speed is 2.0m / min-2.3m / min.

40. The preparation method according to claim 38 or 39, characterized in that: The hot pressing includes at least three hot roller pressings, and the hot roller pressure increases successively, and the hot roller pressure is 20 tons-50 tons, 50 tons-70 tons, and 70 tons-90 tons respectively; the hot roller temperature is 40°C-80°C, and the pole piece is heated before entering the hot roller compaction for the first time, and the heating temperature is 40°C-50°C.

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