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, the positive film layer comprises a positive active material, and the positive active material comprises lithium-containing transition metal phosphate particles of which at least partial surfaces are provided with carbon coating materials; in the tangent plane of the positive electrode film layer along the thickness direction of the pole piece, the area ratio of particles with the particle size of more than or equal to 1.5 microns is more than or equal to 8.0% and less than or equal to 20.0%; in a cumulative distribution curve of a graphitization degree C value obtained by the positive electrode film layer in a surface scanning mode of a laser microscopic confocal Raman spectrometer, the median C50 of the graphitization degree is greater than or equal to 0.95 and less than or equal to 1.20.

Description

[0001] This application claims the priority of International Application PCT / CN2025 / 085927 titled "Lithium-ion Secondary Battery, Battery Device, Electrical Device, Preparation Method of Positive Electrode Active Material and Preparation Method of Positive Electrode Plate" 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, an electrical device, a preparation method of a positive electrode active material, and a preparation method of a positive electrode plate. Background Art

[0003] In recent years, lithium-ion secondary batteries have been widely used in energy storage power systems such as hydroelectric, thermal, wind, 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] Positive electrode active material is an important component of lithium-ion 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-mentioned 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] This application is made in view of the above problems, and its purpose is to provide a lithium-ion secondary battery with both high energy density and good kinetic performance.

[0006] The first aspect of this application provides a lithium-ion secondary battery, which includes a positive electrode plate, a negative electrode plate, and an electrolyte. The positive electrode plate 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 electrode plate, the area ratio of particles with a particle size greater than or equal to 1.5 μm is greater than or equal to 8.0% and less than or equal to 20.0%; in the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of a laser confocal Raman spectrometer for the positive electrode film layer, the median C of the graphitization degree 50 is greater than or equal to 0.95 and less than or equal to 1.20, where the graphitization degree C value is I G / I D ,I G represents the intensity of the G peak in the Raman spectrum at 1580 ± 100 cm -1 , and I DIndicates the intensity of the D peak in the Raman spectrum at 1350 ± 100 cm -1 -1.

[0007] Controlling the area ratio of particles with a particle size greater than or equal to 1.5 μm in the positive electrode film layer to be greater than or equal to 8.0% and less than or equal to 20.0% can reduce the significant short-board effect caused by large-sized particles, which is beneficial to maintaining the battery impedance at a low level and improving the kinetic performance of the battery. However, this will limit the further increase of the electrode compaction density. In the embodiments of the present application, further by controlling the median C of the graphitization degree 50 to be greater than or equal to 0.95 and less than or equal to 1.20, the graphitization degree of the particles in the positive electrode film layer is increased, and the crystallinity of the carbon layer on the particle surface is increased, so that the particles are easy to slip during the roll compaction film formation process of the positive electrode active material. By virtue of the characteristic that the particles are easy to slip, the compaction density of the positive electrode film layer is further increased, and thus the balance between the battery kinetic performance and the energy density is achieved.

[0008] In any implementation manner, in the cumulative distribution curve of the C value of the graphitization degree obtained by the positive electrode film layer in the surface scanning mode of the laser confocal Raman spectrometer, the median C of the graphitization degree 50 is 0.97 - 1.13, and can be selected as 1.0 - 1.10.

[0009] The median C of the graphitization degree of the positive electrode film layer 50 within the above range is beneficial to further improving the easy slip degree between particles, so as to offset the insufficient grading caused by the small number of large particles in the positive electrode film layer, and further improve the compaction density of the electrode while maintaining the high kinetic performance of the battery, and achieve the balance between the battery kinetic performance and the energy density.

[0010] In any implementation manner, in the cumulative distribution curve of the C value of the graphitization degree obtained by the positive electrode film layer in the surface scanning mode of the laser confocal Raman spectrometer, the concentration degree of the C value (C 90 - C 10 ) / C 50 is 0.01 - 0.04.

[0011] The concentration degree of the C value of the positive electrode film layer being 0.01 - 0.04 indicates that the graphitization degree of the carbon coating on the surface of the positive electrode active material is relatively consistent, which means that the positive electrode active material has good coating uniformity and consistency, and can reduce the slip resistance caused by the inconsistent graphitization degree of the particles in the positive electrode active material and the resulting local stress concentration. Therefore, through the uniform and consistent slip between the positive electrode active material particles, the electrode can achieve a relatively high compaction density as a whole under a relatively low roll compaction pressure, and further improve the compaction density of the electrode and the energy density of the battery while maintaining the good kinetic performance of the battery.

[0012] In any embodiment, in the cumulative distribution curve of the graphitization degree C value obtained in the area scanning mode of a laser confocal Raman spectrometer for the positive electrode film layer, the concentration degree of the C value (C 90 -C 10 ) / C 50 is 0.02 - 0.04.

[0013] The concentration degree of the C value (C 90 -C 10 ) / C 50 Within the above range, it is beneficial to further improve the consistency of the surface carbon graphitization degree of the positive electrode active material, improve the slip degree between particles, and further improve the compaction density of the electrode sheet and the energy density of the battery on the basis of maintaining good kinetic performance of the battery.

[0014] In any embodiment, in the cumulative distribution curve of the graphitization degree C value obtained in the area scanning mode of a laser confocal Raman spectrometer for the positive electrode film layer, the C of the graphitization degree 90 is 1.00 - 1.30, and optionally 1.02 - 1.15.

[0015] The C of the graphitization degree 90 Within the above range, it is relatively close to the median C of the graphitization degree 50 , indicating that the distribution range of the graphitization degree of the positive electrode film layer is narrow, which is beneficial to the uniform slip between particles to improve the compaction density of the positive electrode sheet.

[0016] In any embodiment, in the cumulative distribution curve of the graphitization degree C value obtained in the area scanning mode of a laser confocal Raman spectrometer for the positive electrode film layer, the C of the graphitization degree 10 is 0.92 - 1.10, and optionally 0.98 - 1.08.

[0017] The C of the graphitization degree 10 Within the above range indicates that the positive electrode film layer has a high graphitization degree at different sites, which is beneficial to the uniform slip of particles, reduces the occurrence probability of local stress concentration phenomenon, and further improves the compaction density of the electrode sheet.

[0018] In any embodiment, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size of 1.5 μm - 5 μm is 9.0% - 20.0%.

[0019] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size of 1.5 μm - 5 μm within the above range can further reduce the hindrance of large-size particles on the electrode sheet surface to the infiltration and diffusion of the electrolyte in the positive electrode film layer, improve the consistency of the diffusion rate of lithium ions in the positive electrode active material particles, reduce local polarization, and improve the kinetic performance of the battery.

[0020] In any embodiment, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size of 1.5 μm - 5 μm is 10.0% - 20.0%.

[0021] In the process of improving the particle size distribution, increasing the size or proportion of large particles, it is inevitable to introduce particles with a particle size of 1.5 μm - 5 μm. The area ratio of particles with a particle size of 1.5 μm - 5 μm within the above range is beneficial to improving the compaction of the electrode sheet while taking into account the kinetic performance of the battery.

[0022] In any embodiment, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 5 μm is 0.

[0023] Research shows that particles with a particle size greater than or equal to 5 μm in the positive electrode film layer will significantly deteriorate the infiltration of the electrolyte in the positive electrode film layer and the diffusion in the active material particles. The area ratio of particles with a particle size greater than or equal to 5 μm being 0 is beneficial to further reducing the internal resistance of the battery and improving the kinetic performance of the battery.

[0024] In any embodiment, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm is 15.0% - 25.0%, optionally 16.0% - 24%, and further optionally 16% - 20%.

[0025] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm within the above range is beneficial to further improving the compaction density of the electrode sheet and the energy density of the battery on the basis of maintaining good kinetic performance of the battery.

[0026] In any embodiment, in the particle sphericity area cumulative distribution curve obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median sphericity L A50 is 0.60 - 0.85, optionally 0.65 - 0.80.

[0027] The median sphericity L A50 Particles within the above range are approximately spherical, and it is easy for particles to slip between each other under external force, which can further improve the compaction density of the electrode sheet and increase the energy density of the battery.

[0028] In any embodiment, in the particle roughness area cumulative distribution curve 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.

[0029] The median roughness R A50The surfaces of the particles within the above ranges are relatively smooth, and the frictional force between the particles is relatively small, making them prone to slippage under an external force, which can further improve the compaction density of the electrode sheet and increase the energy density of the battery.

[0030] In any embodiment, the iron dissolution rate of the positive electrode film layer is 500 ppm - 2000 ppm, and can be optionally 500 ppm - 1500 ppm.

[0031] The positive electrode active material with an iron dissolution rate within the above ranges has a relatively complete and dense carbon coating layer, which can improve the electrical contact between the positive electrode active materials, improve the conductivity of the positive electrode active materials, reduce the polarization of the positive electrode active materials, and further optimize the kinetic performance of the lithium-ion secondary battery. At the same time, the densely coated carbon layer has a low space occupancy rate, and the gaps between the particles are easily compressed by stress during the rolling process, which can simultaneously improve the compaction density of the electrode sheet and the energy density of the battery.

[0032] 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 0.90% - 1.5%.

[0033] 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 lithium-containing transition metal phosphate in the positive electrode sheet and improve the energy density of the lithium-ion secondary battery.

[0034] In any embodiment, the lithium-iron antisite defect concentration of the positive electrode active material is 0.1% - 1.5%, and can be optionally 0.3% - 1.0%.

[0035] The positive electrode active material in the embodiments of the present application has low lithium-iron antisite defects, which is beneficial to the uniform transmission of lithium ions in the solid phase and further improves the kinetic performance of the lithium-ion secondary battery.

[0036] In any embodiment, the lithium-containing transition metal phosphate includes components 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.

[0037] Selecting an appropriate modification element Q can improve the ion diffusion pathway of the cathode active material, improve the lithium ion diffusion rate of the cathode active material, and improve the kinetic performance of the battery.

[0038] In any embodiment, the cathode active material includes one or more of lithium iron phosphate and its doped modification materials and coated modification materials.

[0039] In any embodiment, the cathode active material includes titanium element, and based on the total mass of the cathode active material, the mass content of titanium element is 2000 ppm - 6000 ppm.

[0040] The cathode active material in the embodiments of the present application has a high content of titanium element. Moreover, surprisingly, the high addition amount of titanium element does not form harmful impurity phases that have 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 (for example, lithium element), which instead has a promoting effect on the kinetic performance of the battery.

[0041] In any embodiment, the tapped density of the powder of the cathode active material is 0.70 g / cm 3 -1.50 g / cm 3 , and can be optionally 0.70 g / cm 3 -1.20 g / cm 3 .

[0042] The effective grading independently formed by the cathode active material in the embodiments of the present application is limited, and it has a relatively low tapped density. However, due to the high graphitization degree of the cathode film layer, it is easy to slip under external force to achieve an increase in the compaction density.

[0043] In any embodiment, the compaction density of the powder of the cathode active material under 3T pressure is 2.50 g / cm 3 -2.70 g / cm 3 , and can be optionally 2.52 g / cm 3 -2.68 g / cm 3 .

[0044] Although the area ratio of particles with a particle size greater than or equal to 1.5 μm of this cathode active material is low, due to the high graphitization degree, the cathode active material can still achieve a high compaction density under external force, providing a material basis for improving the compaction density of the electrode sheet and preparing a lithium ion secondary battery with a high energy density.

[0045] In any embodiment, the powder resistivity of the cathode active material under 8 MPa pressure is 0.5 Ω·cm - 30.0 Ω·cm, and can be optionally 2 Ω·cm - 20.0 Ω·cm.

[0046] The positive electrode active material has a high degree of graphitization. Therefore, 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 further enhancing the kinetic performance of the battery.

[0047] In any embodiment, the discharge specific capacity of the positive electrode active material at room temperature at a discharge rate of 1C is 135 mAh / g - 150 mAh / g.

[0048] The positive electrode active material has a high discharge specific capacity at a rate of 1C, indicating its good charge-discharge ability, which is beneficial to improving the kinetic performance of the battery.

[0049] In any embodiment, the proportion η of the discharge capacity of the positive electrode active material discharged to 3.2V is η≥85%, where η is defined as: 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 - 3.75V at a rate of 0.1C, and then charged and discharged at a constant current once at a rate of 1C. In the charge-discharge test at a rate of 1C, the capacity value at a discharge voltage of 3.2V is denoted as C 1 and the capacity value at a discharge voltage of 2.0V is denoted as C 2 and 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.

[0050] The high proportion of the discharge capacity of the positive electrode active material used in the lithium-ion secondary battery according to the embodiments of the present application 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.

[0051] In any embodiment, in the 0.1C discharge curve of the coin cell containing the positive electrode active material, there is a discharge plateau within the voltage range of 2.5V - 2.9V.

[0052] The coin cell containing the positive electrode active material in the embodiments of the present application shows a new charge-discharge plateau within the voltage range of 2.5V - 2.9V, which is beneficial to increasing the discharge range of the battery and improving the energy density of the battery.

[0053] In any embodiment, based on the total mass of the positive electrode film layer, the mass content of the conductive agent is 0 - 1.5%, and preferably 0.

[0054] The carbon layer of the positive electrode active material has a high degree of graphitization, enabling the positive electrode active material to have good electronic conductivity, reducing or even eliminating the use of conductive agents in the positive electrode film layer, facilitating further increase in the loading amount of the positive electrode active material, and improving the energy density of the lithium-ion secondary battery.

[0055] 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 95.5% - 99.5%, optionally 96.5% - 99.5%; the mass content of the binder is 0.5% - 3%.

[0056] In any embodiment, the areal density of the single side of the positive electrode film layer is 300 mg / 1540 mm 2 - 450 mg / 1540 mm 2 。

[0057] The positive electrode film layer with an areal density within the above range can contribute to improving the energy density of the lithium-ion secondary battery.

[0058] 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 2.51 g / cm 3 - 2.73 g / cm 3 。

[0059] 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 - 2.70 g / cm 3 。

[0060] The tap density of the positive electrode film layer within the above range is conducive to improving the energy density of the lithium-ion secondary battery.

[0061] 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 2.51 g / cm 3 - 2.73 g / cm 3 , and in the cross-section of the positive electrode film layer along the thickness direction of the electrode, the porosity of the positive electrode film layer is 10% - 22%.

[0062] 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 - 2.70 g / cm 3 , and in the cross-section of the positive electrode film layer along the thickness direction of the electrode, the porosity of the positive electrode film layer is 10% - 20%.

[0063] In the cross-section of the positive electrode film layer of the embodiments of the present application, the lower the porosity, 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 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.

[0064] In any embodiment, 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.

[0065] 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 at the same time improving the kinetic performance of the battery. In the high compaction density positive electrode tabs of the embodiments of the present application, 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 positive electrode tabs, and further improving the ultimate compaction density of the positive electrode tab.

[0066] In any embodiment, the positive electrode tab includes a bottom coating disposed between the positive electrode film layer and the current collector; the compaction density of the positive electrode tab 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.

[0067] In any embodiment, the positive electrode tab includes a bottom coating disposed between the positive electrode film layer and the current collector; the compaction density of the positive electrode tab 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.

[0068] With the increase in the compaction density of the positive electrode tab, the extrusion effect of the lithium-containing phosphate material with large particles (such as those with a 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 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 positive electrode tab and further increasing the ultimate compaction density of the positive electrode tab.

[0069] The second aspect of the present application provides a battery device, including the lithium-ion secondary battery provided in 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.

[0070] The third aspect of the present application further provides an electrical device, which includes the lithium-ion secondary battery provided by the first aspect of the present application or the battery device provided by the second aspect of the present application.

[0071] The fourth aspect of the present application further provides a method for preparing a positive electrode active material: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source; the carbon source includes polyethylene glycol; the iron source includes divalent iron; obtaining a mixed slurry after grinding in a solvent; the volume distribution particle size Dv of the particles in the mixed slurry 50 is 1 μm - 4 μm; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder to obtain a positive electrode active material; the sintering includes at least two-stage isothermal sintering, wherein the sintering temperature in the high-temperature stage is 750°C - 800°C.

[0072] The positive electrode active material prepared by this preparation method has a small proportion of the area of large-sized particles on the surface of the positive electrode film layer prepared, and this positive electrode active material has a high degree of graphitization, is easy to improve the compaction density of the electrode sheet through the slip between particles, and is beneficial to improving the energy density of the battery while improving the battery kinetic performance.

[0073] The fifth aspect of the present application provides a method for preparing a positive electrode sheet, which includes 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, and stirring 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 sheet.

[0074] In any embodiment, the stirring includes pre-stirring and main stirring, the stirring speed of the pre-stirring is lower than that of the main stirring, the revolution speed of the pre-stirring is 20 rpm - 30 rpm, the rotation speed of the pre-stirring is 450 rpm - 550 rpm, and the pre-stirring time is 10 min - 20 min.

[0075] 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.

[0076] Using the above hot-pressing process in combination with the preparation method of the fourth aspect to prepare the positive electrode active material is beneficial to further reducing the porosity of the cut surface of the positive electrode film layer, improving the ultimate compaction density of the electrode sheet, and improving the energy density of the battery. Description of the Drawings

[0077] 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 It is a schematic diagram of a lithium-ion secondary battery according to an embodiment of the present application; Figure 3 It is an exploded schematic diagram of a lithium-ion secondary battery according to an embodiment of the present application; Figure 4 It is a schematic diagram of a battery module according to an embodiment of the present application; Figure 5 It is a schematic diagram of a battery pack according to an embodiment of the present application; Figure 6 is Figure 5 an exploded schematic diagram of the battery pack shown; Figure 7 It is a schematic diagram of an electrical device using a lithium-ion secondary battery according to an embodiment of the present application as a power source; Figure 8 It is a porosity test diagram of a cross-section of a positive electrode film layer along the thickness direction of the electrode sheet according to an embodiment of the present application.

[0078] Explanation of reference numerals: 1 Battery pack; 2 Upper box body; 3 Lower box body; 4 Battery module; 5 Lithium-ion secondary battery; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed implementation manners

[0079] Hereinafter, embodiments of a lithium-ion secondary battery, a battery device, an electrical device, a method for preparing a positive electrode active material, and a method for preparing a positive electrode sheet of the present application are specifically disclosed 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 the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description 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.

[0080] 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 anticipated. 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 anticipated: 1 - 3, 1 - 4, 1 - 5, 2 - 3, 2 - 4, and 2 - 5. In this application, unless otherwise stated, the numerical range "a - b" represents an abbreviated representation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0 - 5" means that all real numbers between "0 - 5" are fully listed herein, and "0 - 5" is just 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] In this application, the terms "a plurality of", "a variety of" mean two or more than two.

[0085] Unless otherwise specified, the terms used in this application have the well-known meanings commonly understood by those skilled in the art.

[0086] Unless otherwise specified, the values of the various parameters mentioned in this application can be measured by 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 testing temperature of each parameter is 25°C.

[0087] 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 modules, battery packs, etc.

[0088] 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.

[0089] The lithium-ion secondary battery includes an electrode assembly and an electrolyte.

[0090] 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).

[0091] 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.

[0092] 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.

[0093] 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, and the battery pack 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.

[0094] In some embodiments, the battery can be an energy storage device. The energy storage device includes an energy storage container, an energy storage cabinet, etc.

[0095] 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 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.

[0096] Optionally, the battery module 4 can further include a housing having an accommodation space, and the multiple lithium-ion secondary batteries 5 are accommodated in the accommodation space.

[0097] In some embodiments, the above battery module can also be 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.

[0098] Figure 5 and Figure 6 are schematic diagrams 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 a plurality of battery modules 4 disposed 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.

[0099] 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.

[0100] In order to further improve the energy density of the battery and increase the compaction density of the pole piece, the common method in the industry is to increase the gradation of particles in the pole piece. In order to improve the particle gradation, it is necessary to increase the size or proportion of large particles. However, studies have shown that when the size and proportion of large particles in the pole piece exceed a certain range, the dynamic performance of the battery will be sacrificed. How to obtain a battery that takes into account both energy density and dynamic performance is a technical problem that needs to be solved urgently in this field.

[0101] The first aspect of the present application provides a lithium-ion secondary battery, the lithium-ion secondary battery comprising a positive electrode plate, a negative electrode plate and an electrolyte, wherein the positive electrode plate 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 with a carbon coating material arranged on at least a portion of the surface, in a cross section of the positive electrode film layer along the thickness direction of the electrode plate, the area proportion of particles with a particle size greater than or equal to 1.5 μm is greater than or equal to 8.0% and less than or equal to 20.0%; in a cumulative distribution curve of the graphitization degree C value of the positive electrode film layer obtained in a surface scanning mode of a laser microscopic confocal Raman spectrometer, the median value C of the graphitization degree is 50 Greater than or equal to 0.95 and less than or equal to 1.20, 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 .

[0102] In the cross section of the positive electrode film along the thickness direction of the pole piece, the area of ​​particles with a particle size greater than or equal to 1.5μm accounts for less than 8%, making it difficult to achieve a high compaction density of the positive electrode. In the cross section of the positive electrode film along the thickness direction of the pole piece, the area of ​​particles with a particle size greater than or equal to 1.5μm accounts for more than 20%, which is conducive to the realization of a high compaction density of the positive electrode, but it will reduce the contact area between the electrolyte and the positive electrode active material, hinder the diffusion of lithium ions into the positive electrode film layer, increase the diffusion path of lithium ions in the particles, and cause severe polarization in the local part of the pole piece, increase the battery impedance, and significantly deteriorate the dynamic performance of the battery.

[0103] Controlling the area ratio of particles with a particle size greater than or equal to 1.5 μm in the positive electrode film layer to be greater than or equal to 8.0% and less than or equal to 20.0% can reduce the significant short-board effect caused by large-sized particles, which is beneficial to maintaining the battery impedance at a low level and improving the kinetic performance of the battery. However, this will limit the further increase of the electrode compaction density. In the embodiments of the present application, the median C of the graphitization degree is further controlled 50 to be greater than or equal to 0.95 and less than or equal to 1.20, which can improve the graphitization degree of the particles in the positive electrode film layer and increase the crystallinity of the carbon layer on the particle surface, making it easier for the positive electrode active material to achieve particle slip during the roll-to-film process. By virtue of the characteristic that particles are easy to slip, the compaction density of the positive electrode film layer is further increased, thereby achieving the balance between the battery kinetic performance and the energy density.

[0104] In the present 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.

[0105] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode, the area ratio of particles with a particle size greater than or equal to 1.5 μm can be selected as 8.0%, 9.0%, 10.0%, 11.0%, 12.0%, 13.0%, 14.0%, 15.0%, 16.0%, 17.0%, 18.0%, 19.0%, 20.0% or any numerical range between any two of them.

[0106] In the cross-section of the positive electrode film layer along the thickness direction of the electrode, the area statistical method of the particles is as follows. The positive electrode film layer is cut along the thickness direction of the electrode by an argon ion beam (as an example, the equipment model: Leica EM TIC 3X CP, working voltage: 6 kV, working duration: 6 h). After exposing the cross-section, a scanning electron microscope (as an example, the equipment model: Hitachi SU8230, working voltage: 3 kV, beam current: high, probe model: U(LA100), working distance <5 mm) is used to observe the cross-section of the positive electrode film layer along the thickness direction of the electrode. An image is collected in the secondary electron mode at a non-edge position in the cross-section of the positive electrode film layer (after observing the edge of the electrode under the scanning electron microscope, the field of view is adjusted to the central part of the sample), and an electron microscope image is taken at a magnification of 10,000 times. The use method of the 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 identify particles, and then 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 interior of the particle is penetrated by the edge, and the morphology cannot be fully 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 carried out, and the specific process is as follows: delete the large particles that are located at the four edges of the scanning electron microscope and cannot be fully 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 they do not penetrate the particle, 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 unrelated to the particles during the automatic image processing process, and the determination and marking of the particles in the picture are completed.

[0107] The images after particle determination and labeling are imported into ImageJ software for analysis. The scale is set according to the scanning electron microscope image. The particle size and area of the particles in the image are analyzed through the "Feret diameter" and "Area" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter obtained from the analysis represents the maximum distance between all parallel lines in the two-dimensional projection of the particle, which characterizes the particle size; the "Area" parameter represents the pixel area of the particle, which characterizes the area of the particle. Since there are large errors in the statistics of particles with a particle size less than 50 nm and it is difficult to accurately identify them, 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 in Area is deleted. According to the above method, to meet the sample number with statistical significance, at least 10 non-overlapping scanning electron microscope images are collected for each electrode sheet, and the areas of at least 5000 particles are counted. Calculate the sum of the "Area" parameters of the particles with a particle size greater than or equal to 1.5 μm and the sum of the "Area" parameters of all particles, which are used as the area of the particles with a particle size greater than or equal to 1.5 μm and the total area of the counted particles, respectively. Divide the sum of the areas of the particles with a particle size greater than or equal to 1.5 μm by the total area of the counted particles to obtain the proportion of the area of the particles with a particle size greater than or equal to 1.5 μm in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet.

[0108] During the compaction process of the positive electrode film layer, compaction occurs in the thickness direction. Therefore, compared with the surface of the positive electrode film layer, the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet can better reflect the real compaction condition of the particles inside the film layer at the spatial scale. In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the proportion of the area of the particles with a particle size greater than or equal to 1.5 μm can intuitively reflect the proportional relationship between the area of the particles in this particle size segment and the total area of the particles, and reflect the distribution of the particles in this particle size segment.

[0109] It can be understood that the particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, especially the particles above 50 nm, mainly come from the positive electrode active material. Therefore, through the observation and statistics of the particle areas in the cross-section of the positive electrode film layer in the embodiments of this application, the distribution of the lithium-containing transition metal phosphate particles in the positive electrode film layer in the electrode sheet can be accurately and objectively reflected.

[0110] In the prior art, a laser particle size analyzer is usually used to statistically analyze the particle size of the cathode active material by the Malvern laser diffraction method. However, the applicant's research shows that since the lithium-containing transition metal phosphate particles are prone to agglomeration, the test results obtained by the Malvern laser diffraction method based on the laser scattering principle often measure the particle size of the particle agglomerates, and cannot truly reflect the particle size of the particles in the cathode active material, let alone reflect the dispersion state of the cathode active material in the film layer, because the dispersion degree of the cathode active material in the film layer will increase during the processes of pulping and film forming and rolling. The test results obtained by the Malvern laser diffraction method are affected by the particle size, specific surface area, and agglomeration degree of the cathode active material. Compared with the actual dispersion situation in the electrode, 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.

[0111] Those skilled in the art can adjust the particle size of the particles through any known process. As an example, by adjusting the temperature and time during the preparation process of the cathode material, the growth rate and time of the cathode material are controlled. 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; screening and classification equipment are used to separate the particle size of the particle system to obtain the particle size ratio that meets the requirements; by precisely controlling the feeding rate, adjusting the residence time and force state of the particles in the equipment also helps to achieve the adjustment of the particle size of the particles.

[0112] Lithium-containing transition metal phosphate refers to a phosphate material containing lithium elements and transition metal elements, and can be detected by any well-known method in the art. For example, it can be detected by combining an X-ray diffractometer (XRD) with an energy spectrometer and an inductively coupled plasma mass spectrometer. As an example, lithium-containing transition metal phosphates include, but are not limited to, lithium iron phosphate, lithium manganese iron phosphate, and their doped materials.

[0113] The carbon coating material provided 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, the carbon coating material provided on at least part of the surface of the lithium-containing transition metal phosphate can be observed by characterizing the lithium-containing transition metal phosphate by combining a transmission electron microscope and an energy spectrometer. It should be noted that the elements in the carbon coating material are not limited to carbon elements, and there may also be other non-carbon elements. The carbon coating layer containing the carbon coating material is not limited to a film shape, but also includes island shapes, irregular shapes, or discontinuous coating layers.

[0114] 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 cathode film layer, the median C of the graphitization degree 50Greater than or equal to 0.95 and less than or equal to 1.20, where the graphitization degree C value is I G / I D , where I G represents the intensity of the G peak in the Raman spectrum at 1580 ± 100 cm -1 . I D represents the intensity of the D peak in the Raman spectrum at 1350 ± 100 cm -1 .

[0115] In the present application, the graphitization degree C value of the positive electrode film layer can be obtained by the surface scanning mode of a 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 and its surface or a cross-section along the thickness direction of the electrode is scanned. The scanning area is 45 μm × 45 μm, which is 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. Thus, the C values at different sites and the cumulative distribution curve of the C value of the surface scanning area are obtained.

[0116] 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 salts on the surface of the positive electrode film layer disassembled from the battery. To improve the test accuracy, it is preferred to perform a surface scan on the cross-section of the positive electrode film layer along the thickness direction of the electrode to characterize the graphitization degree of the positive electrode film layer.

[0117] The graphitization degree C value of the positive electrode film layer is obtained by the peak intensity ratio of the G peak (G-band) and the 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 of carbon, 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 form covalent bonds through sp 2 hybridization, and the intermolecular force between layers is the 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 material treated by graphitization in the positive electrode film layer, that is, the carbon coating layer of the positive electrode active material. Although the carbon nanotube conductive agent rich in sp 2 hybrid structure also has a relatively high I G / I D , but due to its small addition content and small tube diameter, its 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 in the positive electrode film layer 50Have an impact.

[0118] 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. The higher the graphitization degree of the carbon on the surface of the positive electrode active material, the higher the proportion of graphitic 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 layer, and the compaction density of the electrode sheet can be increased at a low rolling pressure.

[0119] The cumulative distribution curve of the C value of the graphitization degree 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 ratio as the vertical axis. C 50 Is the C value corresponding to the cumulative quantity ratio of 50% on the vertical axis in the cumulative distribution curve of the C value of the graphitization degree. The median C of the graphitization degree 50 Compared with the point value, it can reflect the overall graphitization degree of the particles in the positive electrode film layer, that is, the ease of slipping; 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.

[0120] In some embodiments, in the cumulative distribution curve of the C value of the graphitization degree 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 Can be optionally 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.20 or the numerical range between any two of them.

[0121] 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, optimizing the nucleation process, sintering temperature, sintering time, sintering pressure, and sintering atmosphere can all achieve the adjustment of the graphitization degree of the active material particles. The higher the graphitization degree of the carbon on the surface of the positive electrode active material, the higher the proportion of graphitic carbon in the positive electrode film layer, and the easier it is for the particles to slip by means of the carbon structure with a high graphitization degree in the coating material, and the compaction density of the electrode sheet is increased.

[0122] In some embodiments, the median C of the graphitization degree of the positive electrode film layer 50 Is 0.97 - 1.13, and can be optionally 1.00 - 1.10.

[0123] The median C of the graphitization degree of the positive electrode film layer 50Within the above range, it is beneficial to further improve the ease of slippage between particles, thereby offsetting the insufficient grading caused by the small number of large particles in the positive electrode film layer, further improving the compaction density of the electrode sheet while maintaining high battery kinetic performance, and achieving a balance between battery kinetic performance and energy density.

[0124] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of a laser microscopic confocal Raman spectrometer for the positive electrode film layer, the concentration (C 90 -C 10 ) / C 50 is 0.01 - 0.04.

[0125] Referring to the above, and so on, C 90 is the C value corresponding to when the cumulative quantity ratio on the vertical axis in the cumulative distribution curve of the graphitization degree C value is 90%, and C 10 is the C value corresponding to when the cumulative quantity ratio on the vertical axis in the cumulative distribution curve of the graphitization degree C value is 10%. The concentration of the C value is represented by (C 90 -C 10 ) / C 50 . (C 90 -C 10 ) / C 50 can not only reflect the magnitudes of most C values, but also be unaffected by extreme values, and can also reflect the width of the graphitization degree distribution of particles in the positive electrode film layer. A small concentration of the C value of the positive electrode film layer indicates a narrow width and good concentration of the graphitization degree distribution of carbon on the surface of the positive electrode active material.

[0126] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of a laser microscopic confocal Raman spectrometer for the positive electrode film layer, the concentration (C 90 -C 10 ) / C 50 can be optionally 0.01, 0.02, 0.03, 0.04 or the numerical range between any two of them.

[0127] When the concentration of the C value of the positive electrode film layer is 0.01 - 0.04, it indicates that the graphitization degree of the carbon coating on the surface of the positive electrode active material is relatively consistent, meaning that the positive electrode active material has good coating uniformity and consistency, which can reduce the slippage obstruction caused by the inconsistent graphitization degree of particles in the positive electrode active material and the resulting local stress concentration. Therefore, through the uniform and consistent slippage between the positive electrode active material particles, the electrode sheet can achieve a relatively high compaction density as a whole under a relatively low rolling pressure, further improving the compaction density of the electrode sheet and the energy density of the battery while maintaining good battery kinetic performance.

[0128] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained by the laser microscopic confocal Raman spectrometer in the surface scanning mode of the positive electrode film layer, the concentration degree of the C value (C 90 -C 10 ) / C 50 is 0.02 - 0.04.

[0129] The concentration degree of the C value (C 90 -C 10 ) / C 50 Within the above range, it is beneficial to further improve the consistency of the surface carbon graphitization degree of the positive electrode active material, improve the slip degree between particles, and further improve the compaction density of the electrode sheet and the energy density of the battery on the basis of maintaining good kinetic performance of the battery.

[0130] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained by the laser microscopic confocal Raman spectrometer in the surface scanning mode of the positive electrode film layer, the C of the graphitization degree 90 is 1.0 - 1.3, and can be optionally 1.02 - 1.15.

[0131] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained by the laser microscopic confocal Raman spectrometer in the surface scanning mode of the positive electrode film layer, the C of the graphitization degree 90 can be optionally 1.0, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.2, 1.21, 1.22, 1.23, 1.24, 1.25, 1.26, 1.27, 1.28, 1.29, 1.3 or the numerical range between any two of them.

[0132] The C of the graphitization degree 90 Within the above range, it is relatively close to the median C of the graphitization degree 50 indicating that the distribution range of the graphitization degree of the positive electrode film layer is narrow, which is beneficial to the uniform slip between particles to improve the compaction density of the positive electrode sheet.

[0133] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained by the laser microscopic confocal Raman spectrometer in the surface scanning mode of the positive electrode film layer, the C of the graphitization degree 10 is 0.92 - 1.1, and can be optionally 0.98 - 1.08.

[0134] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained by the laser microscopic confocal Raman spectrometer in the surface scanning mode of the positive electrode film layer, the C of the graphitization degree 10It can be 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1 or a numerical range between any two of them.

[0135] C of graphitization degree 10 Within the above range, it indicates that different sites in the positive electrode film layer have a high degree of graphitization, which is beneficial to the uniform slip of particles, reduces the occurrence probability of local stress concentration, and further improves the compaction density of the electrode sheet.

[0136] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size of 1.5 μm - 5 μm is 9.0% - 20.0%.

[0137] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size of 1.5 μm - 5 μm can be 9.0%, 10.0%, 11.0%, 12.0%, 13.0%, 14.0%, 15.0%, 16.0%, 17.0%, 18.0%, 19.0%, 20.0% or a numerical range between any two of them.

[0138] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size of 1.5 μm - 5 μm can be tested in the manner described above. The sum of the areas of particles with a particle size of 1.5 μm - 5 μm in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet is divided by the total area of the statistically counted particles as the area ratio of particles with a particle size of 1.5 μm - 5 μm in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet.

[0139] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, when the area ratio of particles with a particle size of 1.5 μm - 5 μm is within the above range, it can further reduce the hindrance of large-sized particles on the surface of the electrode sheet to the infiltration and diffusion of the electrolyte in the positive electrode film layer, improve the consistency of the diffusion rate of lithium ions in the positive electrode active material particles, reduce local polarization, and improve the kinetic performance of the battery.

[0140] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size of 1.5 μm - 5 μm is 10.0% - 20.0%.

[0141] In the process of improving the particle grading, increasing the large particle size or proportion, it is inevitable to introduce particles with a particle size of 1.5 μm - 5 μm. When the area ratio of particles with a particle size of 1.5 μm - 5 μm is within the above range, it is beneficial to improve the electrode sheet compaction while taking into account the kinetic performance of the battery.

[0142] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 5 μm is 0.

[0143] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 5 μm can be tested by referring to the method described above. The area ratio of particles with a particle size greater than or equal to 5 μm in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet is calculated by dividing the sum of the areas of particles with a particle size greater than or equal to 5 μm by the total area of the counted particles.

[0144] Research shows that particles with a particle size greater than or equal to 5 μm in the positive electrode film layer will significantly deteriorate the infiltration of the electrolyte in the positive electrode film layer and the diffusion in the active material particles. An area ratio of 0 for particles with a particle size greater than or equal to 5 μm is beneficial for further reducing the internal resistance of the battery and improving the battery kinetic performance.

[0145] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm is 15.0% - 25.0%, optionally 16.0% - 24%, and further optionally 16.0% - 20.0%.

[0146] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm can be optionally 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25% or any value range between any two of them.

[0147] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm can be obtained by testing according to the method described above. The area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet is calculated by dividing the sum of the areas of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm by the total area of the counted particles.

[0148] When the area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet is within the above range, it is beneficial to further increase the compaction density of the electrode sheet and improve the energy density of the battery on the basis of maintaining good battery kinetic performance.

[0149] In some embodiments, in the cumulative area distribution curve of particle sphericity obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median L of sphericity A50 is 0.60 - 0.85, optionally 0.65 - 0.80.

[0150] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the sphericity test method of the particles 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. Import the pictures after the particles are determined and marked into the ImageJ software for analysis. Complete the scale setting according to the scanning electron microscope image. Analyze the particle size, the area of the particle, and the sphericity of the particles in the picture through the "Feret diameter", "Area", and "Round" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Round" parameter obtained by analysis represents the ratio of the pixel area of the particle to the area of the circle with the fitted major axis as the diameter, and can be used to characterize the sphericity of the particle. When the particle is closer to a sphere, the ratio of the pixel area to the area of the circle with the fitted major axis as the diameter is closer to 1. Therefore, the "Round" parameter of the obtained particle is used to characterize the sphericity 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 to the statistical results. Therefore, in the particle size statistics process of this application, particles with a particle size less than 50 nm are not statistically counted, and the particle statistical data corresponding to "NaN" displayed in Round are deleted. According to the above method, to meet the sample number with statistical significance, each electrode sheet collects at least 10 non-overlapping scanning electron microscope images in the field of view. Arrange the sphericities of at least 5000 obtained particles in ascending order. Use the sphericity as the horizontal axis and the cumulative area ratio as the vertical axis to obtain the sphericity cumulative distribution curve of the particles in the positive electrode film layer. L A50 It is the sphericity L value corresponding to the cumulative area ratio of 50% on the vertical axis in the sphericity L value cumulative distribution curve.

[0151] In some embodiments, in the particle sphericity area cumulative distribution curve obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the L of the sphericity A50 Can be selected as 0.60, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.70, 0.705, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.80, 0.81, 0.82, 0.83, 0.84, 0.85 or the numerical range between any two of them.

[0152] Those skilled in the art can adjust the sphericity of the particles through any known process. As an example, through processes such as grinding, polishing, chemical etching, mechanical stirring, extrusion, coating, granulation, adding surfactants, etc., and adjusting the parameters of each process, the adjustment of the particle sphericity can be achieved.

[0153] The median L of the sphericityA50 The particles within the above range are approximately spherical, and it is easy for the particles to slip between each other under the action of an external force, which can further improve the compaction density of the electrode sheet and increase the energy density of the battery.

[0154] In some embodiments, in the cumulative area distribution curve of the particle roughness 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.

[0155] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the specific method for testing the roughness of the particles is as follows: Identify the particles in the cross-section of the positive electrode film layer according to the method described above in this application. Import the picture after the particles are determined and marked into the ImageJ software for analysis. Complete the scale setting according to the scanning electron microscope image, and analyze the particle size, the area of the particle, and the roughness of the particles in the picture through the "Feret diameter", "Area", and "Solidity" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Solidity" parameter obtained by 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. Since the 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, the particles with a particle size less than 50 nm are not counted, and the particle statistical data corresponding to "NaN" displayed by Solidity is deleted. According to the above method, to meet the sample number with statistical significance, each electrode sheet collects at least 10 non-overlapping scanning electron microscope images of the field of view. Arrange the roughness of at least 5000 obtained particles in ascending order, and obtain the cumulative distribution curve of the particle roughness 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 distribution curve of the roughness R value.

[0156] In some embodiments, in the cumulative area distribution curve of the particle roughness obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median roughness R A50 can be optionally 0.92, 0.93, 0.94, 0.95, 0.96 or the numerical range between any two of them.

[0157] 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, mechano-chemical treatment, electroplating, calendering, etc. and by adjusting the parameters of each process.

[0158] Median roughness R A50 The particle surfaces within the above range are relatively smooth, and the frictional force between particles is relatively small. Under the action of an external force, they are prone to slip, which can further improve the compaction density of the electrode sheet and increase the energy density of the battery.

[0159] In some embodiments, the iron dissolution rate of the positive electrode film layer is 500 ppm - 2000 ppm, and can be optionally 500 ppm - 1500 ppm.

[0160] 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 punched 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, and 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, pipette 1 mL of the supernatant with a pipette gun, add it to a glass volumetric flask and dilute it 50 times, and test it with an inductively coupled plasma mass spectrometer (ICP - OES) to obtain the iron element concentration in the solution. Through the formula: [(ICP - measured iron element concentration × solution volume / mass of the solution participating 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 participating in volume fixation is 1 g, and the iron dissolution rate of the positive electrode film layer is calculated. 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 by 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 。

[0161] In some embodiments, the iron dissolution rate of the positive electrode film layer can be optionally 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1100 ppm, 1200 ppm, 1300 ppm, 1400 ppm, 1500 ppm, 1600 ppm, 1700 ppm, 1800 ppm, 1900 ppm, 2000 ppm or any numerical range between any two of them.

[0162] Those skilled in the art can regulate the iron dissolution rate of the positive electrode film layer through any known process. As an example, the iron dissolution rate of the positive electrode film layer is regulated by controlling the coating quality on the surface of the positive electrode film layer, the temperature, time, and pressure during the preparation process.

[0163] The iron dissolution rate 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 coating layer, that is, the more complete and dense the carbon coating layer on the surface of the positive electrode active material. The positive electrode active material with an iron dissolution rate within the above range has a relatively complete and dense carbon coating layer, which can improve the electrical contact between the positive electrode active materials, improve the conductivity of the positive electrode active materials, reduce the polarization of the positive electrode active materials, and further optimize the kinetic performance of the lithium-ion secondary battery. At the same time, the densely coated carbon layer has a low space occupancy rate, and the particle gaps are easily compressed by stress during the rolling process, which can simultaneously increase the compaction density of the electrode sheet and the energy density of the battery.

[0164] 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 can be optionally 0.90% - 1.5%.

[0165] 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 a Dekai HCS infrared carbon-sulfur analyzer.

[0166] 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 any numerical range between any two of them.

[0167] 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 lithium-containing transition metal phosphate in the positive electrode sheet and improve the energy density of the lithium-ion secondary battery.

[0168] In some embodiments, the lithium-iron anti-site defect concentration of the positive electrode active material is 0.1% - 1.5%.

[0169] 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 refined occupancy probabilities of Li and Fe were obtained, which were used as the concentration of Li-Fe anti-site defects.

[0170] In some embodiments, the concentration of Li-Fe anti-site defects in the positive electrode active material can be selected from 0.1%, 0.2%, 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.

[0171] 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 metering ratio, etc.

[0172] During the preparation and cycling process, there will inevitably be a certain amount of 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, blocking the one-dimensional diffusion channel of lithium ions, and having 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 the lithium-ion secondary battery.

[0173] In some embodiments, the concentration of Li-Fe anti-site defects in the positive electrode active material is 0.3% - 1.0%.

[0174] 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.

[0175] 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.

[0176] Selecting an appropriate modification element Q can improve the ion diffusion pathway of the cathode active material, improve the lithium ion diffusion rate of the cathode active material, and improve the kinetic performance of the battery.

[0177] In some embodiments, the cathode active material includes one or more of lithium iron phosphate and its doped modification materials and coated modification materials.

[0178] 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 titanium element is 2000 ppm - 6000 ppm.

[0179] 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.

[0180] In some embodiments, based on the total mass of the cathode active material, the mass content of titanium element can be selected as 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm, 5500 ppm, 6000 ppm or the numerical range between any two of them.

[0181] 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 transmission 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.

[0182] 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 forms a fast ion conductor together with phosphate and other elements (for example, lithium element), which instead has a promoting effect on the kinetic performance of the battery.

[0183] In some embodiments, the powder compaction density of the cathode active material under 3T pressure is 2.50 g / cm 3 -2.70 g / cm 3 .

[0184] In the present application, the term "powder compaction density" refers to the density of a green compact with 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, resulting in an increase in the atomic attraction and the mechanical fit between particles. The unit is g / cm 3 .

[0185] The powder compaction density of the cathode 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 cathode 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 cathode 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 bottom area of the mold is 1.327 cm 2 , the pressure is set to 3T, and the thickness of the cathode active material under 3T pressure can be read on the equipment. The powder compaction density of the cathode active material is ρ = m / v, where v = (S×H), m is the mass of the cathode active material, S is the bottom area of the mold, and H is the thickness of the compacted cathode active material.

[0186] In some embodiments, the powder compaction density of the cathode active material under 3T pressure can be selected as 2.50 g / cm 3 , 2.51 g / cm 3 , 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 or the numerical range between any two of them.

[0187] Although the area ratio of the particles with a particle size greater than or equal to 1.5 μm in the positive electrode active material is low, due to the high degree of graphitization, the positive electrode active material can still achieve a high tap density under an external force, 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.

[0188] In some embodiments, the tap density of the powder of the positive electrode active material under a pressure of 3 T is 2.52 g / cm 3 -2.68 g / cm 3 .

[0189] In some embodiments, the bulk density of the powder of the positive electrode active material is 0.70 g / cm 3 -1.50 g / cm 3 , and may be optionally 0.70 g / cm 3 -1.20 g / cm 3 .

[0190] The bulk density of the powder can be measured by any well-known method in the art.

[0191] As an example, turn on the electronic balance. First, use a conical flask as a base and place it on the electronic balance, then zero the electronic balance; place the tapped density measuring cylinder on the conical flask, weigh it and 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 smoothly transfer the sample into the measuring cylinder; wipe the powder contaminated on the surface of the connection with dust-free paper, and then put it into the zeroed conical flask 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 instrument surface; set the vibration frequency on the instrument to 250 times / min and the vibration times to 5000 times, press the button, and vibrate for 20 min; then remove the TD tube, 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 ; take the average value V of the two; the mass m of the measuring cylinder and the sample 1 minus the mass m of the measuring cylinder 0 , to obtain the powder mass m, and obtain the tapped density of the sample from the density formula ρ = m / v.

[0192] In some embodiments, the tapped density of the powder of the positive electrode active material can be selected as 0.70 g / cm 3 , 0.75 g / cm 3 , 0.80 g / cm 3 , 0.85 g / cm 3 , 0.90 g / cm 3 , 0.95 g / cm 3 , 1.00 g / cm 3 , 1.05 g / cm 3 , 1.10 g / cm 3 , 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 or any numerical range between any two of them.

[0193] The effective grading independently formed by the positive electrode active material in the embodiments of the present application is limited, and has a relatively low tapped density. However, with the high graphitization degree of the positive electrode film layer, it is easy to slip under external force to improve the compaction density.

[0194] In some embodiments, the powder resistivity of the positive electrode active material under a pressure of 8 MPa is 0.5 Ω·cm - 30.0 Ω·cm.

[0195] The powder resistivity of the positive electrode active material can be measured by methods and equipment known in the art. For example, it can be measured with a powder resistivity meter (Suzhou Jingge, ST2722 model) with reference to GB / T 33822-2017. Specifically, a certain amount of positive electrode active material (e.g. 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 tested respectively, and the average value of the two is taken as the powder resistivity of the positive electrode active material.

[0196] In some embodiments, the powder resistivity of the positive electrode active material under a pressure of 8 MPa may be selected as 0.5Ω·cm, 1Ω·cm, 2Ω·cm, 3Ω·cm, 4Ω·cm, 5Ω·cm, 6Ω·cm, 7Ω·cm, 8Ω·cm, 9Ω·cm, 10Ω·cm, 15Ω·cm, 20Ω·cm, 25Ω·cm, 30Ω·cm or any range therebetween.

[0197] The positive electrode active material has a high degree of graphitization, so the sp 2 The structure facilitates rapid conduction of electrons between particles, making the positive electrode active material have a low powder resistivity, which is beneficial to increasing the solid-phase transmission rate of electrons and further improving the kinetic performance of the battery.

[0198] In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 8 MPa is 2.0 Ω·cm-20.0 Ω·cm.

[0199] In some embodiments, the positive electrode active material has a discharge capacity of 135 mAh / g to 150 mAh / g at a 1 C discharge rate at room temperature.

[0200] In this application, the positive electrode active material is assembled into a button cell and the electrical performance is tested on a blue electric tester. At 25±5℃, within the voltage range of 2.0V~3.75V, it is charged to 3.75V at 1C constant current, then paused for 5 minutes, charged at constant voltage to a cut-off current of 50μA, and then discharged to 2.0V at 1C constant current. The discharge capacity of the button cell divided by the mass of the positive electrode active material is taken as the discharge gram capacity of the positive electrode active material at room temperature at a discharge rate of 1C.

[0201] The preparation and testing process of button cells is as follows: 2.0g of positive electrode active material, conductive carbon black, and PVDF are mixed at a mass ratio of 0.9:0.05:0.05, and then organic solvent NMP (N-methylpyrrolidone) is added. After fully mixing, a 150μm scraper is used for coating, and the mixture is dried at 100℃ for 2h. The compacted density is 2.0g / cm 3 -2.2g / cm 3Compress the positive electrode sheet, punch it into a circular sheet with a diameter of 14 mm using a hole puncher, then weigh and record the weight. Place the weighed positive electrode sheet in a vacuum drying oven (105 °C, 1 - 12 hrs, -90 kPa). After drying, put the positive electrode sheet into a glove box and assemble it into a battery in the order of negative electrode case - nickel mesh - lithium sheet - separator - positive electrode sheet - positive electrode case. 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, and place it in the groove of the sealer. The sealing pressure is 650 kg / cm 2 . Use insulated tweezers to remove the button cell and put it into a dust-free bag, remove the glove box, and place it in a constant temperature room for 3 h to obtain the button cell for testing.

[0202] 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 sheet, assembling it into a coin-type battery according to the method described above, and then testing.

[0203] In some embodiments, the discharge specific capacity of the positive electrode active material at a discharge rate of 1C at room temperature can be selected from 135 mAh / g, 136 mAh / g, 137 mAh / g, 138 mAh / g, 139 mAh / g, 140 mAh / g, 141 mAh / g, 142 mAh / g, 143 mAh / g, 144 mAh / g, 145 mAh / g, 146 mAh / g, 147 mAh / g, 148 mAh / g, 149 mAh / g, 150 mAh / g or the numerical range between any two of them.

[0204] 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.

[0205] In some embodiments, the discharge capacity ratio η of the positive electrode active material discharged to 3.2V ≥ 85%. The η is defined as follows: at room temperature, the coin-type battery containing the positive electrode active material is charged and discharged at a constant current twice at a rate of 0.1C in the voltage range of 2.0V - 3.75V, and then charged and discharged at a constant current once at a rate of 1C. In the charge and discharge test at a rate of 1C, the capacity value with a discharge voltage of 3.2V is extracted and recorded as C1, and the capacity value with a discharge voltage to 2.0V is C2, η = C1 / C2. Among them, the charging process includes constant voltage charging, with a constant voltage of 3.75V and a constant voltage cut-off current of 50 μA.

[0206] 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 by referring to the method described above, and test the electrical performance of the prepared button cell on a Blue Electric Tester. Specifically, the button cell is charged and discharged at a constant current of 0.1C twice within a voltage range of 2.0V to 3.75V. 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 denoted as C1, and the capacity value discharged from 3.75V to 2.0V is C2, and η = C1 / C2.

[0207] In some embodiments, η can be selected as 85%, 86%, 87%, 88%, 88.1%, 89%, 90%, 90.1%, 91%, 92%, 92.2%, 93%, 94%, 94.1%, 94.5%, 95%, 95.1% or any numerical range between any two of them.

[0208] In some embodiments, the proportion of the discharge capacity of the positive electrode active material in the freshly prepared lithium-ion secondary battery discharged to 3.2V, η ≥ 88%. After the freshly prepared lithium-ion secondary battery is charged and discharged at a constant current of 0.1C for a period of time within a voltage range of 2.0V to 3.75V, the proportion of the discharge capacity of the positive electrode active material discharged to 3.2V, η can remain ≥ 85%.

[0209] The high proportion of the discharge capacity of the positive electrode active material used in the lithium-ion secondary battery of the embodiment of the present application 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.

[0210] In some embodiments, in the 0.1C discharge curve of the button cell containing the positive electrode active material, there is a discharge plateau within a voltage range of 2.5V to 2.9V.

[0211] The discharge plateau generally refers to a region where the voltage remains relatively stable during the charge and discharge process of the battery. During the discharge process of the battery, the current flows out of the battery, and the voltage of the battery will decrease at the beginning, but then it will enter a relatively stable region where the voltage change is very small, and this stable voltage region is called the discharge plateau.

[0212] The button cell can be formed by disassembling the positive electrode sheet in a lithium-ion secondary battery and combining it with lithium metal. It can also be assembled and prepared by referring to the method described above. In this application, the positive electrode active material is assembled into a button cell and the electrical performance is tested on a BlueTester. In the voltage range of 2.0V to 3.75V, it is charged at a constant current of 0.1C to 3.75V, paused for 5 minutes, then charged at a constant voltage until the cut-off current is 50μA, and then discharged at a constant current of 0.1C to 2.0V.

[0213] The discharge curve shows that the standard charge-discharge platform voltage of the lithium-containing transition metal phosphate is usually between 3.2V and 3.65V. The button cell containing the positive electrode active material in the embodiment of this application shows a new charge-discharge platform in the voltage range of 2.5V to 2.9V, which is beneficial to increasing the discharge range of the battery and improving the energy density of the battery. At the same time, this also verifies the conjecture that the positive electrode active material in the embodiment of this application contains a fast ion conductor.

[0214] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the conductive agent is 0-1.5%.

[0215] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the conductive agent can be selected from 0, 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 the numerical range between any two of them.

[0216] 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.

[0217] The carbon layer of the positive electrode active material has a high degree of graphitization, enabling the positive electrode active material to have good electronic conductivity, which can reduce or even eliminate the use of the conductive agent in the positive electrode film layer, facilitating further increasing the loading amount of the positive electrode active material and improving the energy density of the lithium-ion secondary battery.

[0218] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the conductive agent is 0.

[0219] The positive electrode active material has extremely high electronic conductivity, such that the conductive agent can even not be added to the positive electrode film layer, which 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.

[0220] 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 95.5% - 99.5%, optionally 96.5% - 99.5%; the mass content of the binder is 0.5% - 3%.

[0221] 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.

[0222] 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 95.5%, 96%, 96.5%, 97%, 98%, 99%, 99.5% or the numerical range between any two of them.

[0223] 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%, 1.5%, 2%, 2.5%, 3% or the numerical range between any two of them.

[0224] In some embodiments, the single - sided areal density of the positive electrode film layer is 300 mg / 1540 mm 2 - 450 mg / 1540 mm 2 .

[0225] In the present application, the single - sided 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 - sided coated and compacted positive electrode sheet (if it is a double - sided coated positive electrode sheet, the positive electrode film layer on one side can be wiped off first), punch it into small round 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 sheet and weigh the weight of the current collector, record it as M 0 . The single - sided areal density of the positive electrode film layer = (M 1 - M 0 ) / S 1 . To ensure the accuracy of the test results, multiple groups (for example, 10 groups) of samples to be tested can be tested and the average value can be calculated as the test result.

[0226] In some embodiments, the single - sided areal density of the positive electrode film layer can be optionally 300 mg / 1540 mm 2 , 310 mg / 1540 mm 2 , 320 mg / 1540 mm 2 , 330 mg / 1540 mm 2, 340 mg / 1540 mm 2 , 350 mg / 1540 mm 2 , 360 mg / 1540 mm 2 , 370 mg / 1540 mm 2 , 380 mg / 1540 mm 2 , 390 mg / 1540 mm 2 , 400 mg / 1540 mm 2 , 410 mg / 1540 mm 2 , 420 mg / 1540 mm 2 , 430 mg / 1540 mm 2 , 440 mg / 1540 mm 2 , 450 mg / 1540 mm 2 or a numerical range between any two of them.

[0227] The positive electrode film layer with areal density within the above range can help improve the energy density of the lithium-ion secondary battery.

[0228] 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 2.51 g / cm 3 -2.73 g / cm 3 .

[0229] 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 2.55 g / cm 3 -2.70 g / cm 3 .

[0230] In this application, the fully discharged state means that the battery is placed in an oven environment at 25 °C, left standing for 2 h, and after the battery temperature remains at 25 °C, the battery is discharged at a constant current of 1 / 3C to 2.5 V and then at a constant current of 0.1C to 2.0 V.

[0231] The tap density of the positive electrode film layer can be tested by methods known in the art. As an example, the battery is placed in an oven environment at 25 °C, left standing for 2 h, and after the battery temperature remains at 25 °C, the battery is discharged at a constant current of 1 / 3C to 2.5 V and then at a constant current of 0.1C to 2.0 V. The battery is disassembled to obtain the positive electrode plate. The residual electrolyte is treated with dimethyl carbonate solvent, the electrode plate is dried, cut into small round pieces with an area of S, and its mass is obtained as W 1 , and the thickness T of the positive electrode plate is measured using a micrometer 1 , and then the positive electrode film layer of the weighed electrode plate is wiped off, and the mass of the current collector is weighed and recorded as W 2 , and the thickness T of the current collector is measured using a micrometer 2, then the compaction density PD of the positive electrode film layer = (W 1 -W 2 ) / [(T 1 -T 2 ) × S].

[0232] In some embodiments, when the lithium-ion secondary battery is in a fully discharged state, the compaction density of the positive electrode film layer can be selected as 2.51 g / cm 3 , 2.52 g / cm 3 , 2.53 g / cm 3 , 2.54 g / cm 3 , 2.55 / 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 or any value range between any two of them.

[0233] In some embodiments, after being treated by the compaction process, the compaction density of the positive electrode film layer is 2.63 g / cm 3 -2.85 g / cm 3 .

[0234] In some embodiments, after being treated 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 / cm3 , 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 or any value range between any two of them.

[0235] In the present application, the term "compaction" refers to compacting the positive electrode film layer by mechanical pressure during battery assembly to improve its density and conductivity.

[0236] In some embodiments, after being treated by the formation process, the compaction density of the positive electrode film layer is 2.53 g / cm 3 -2.73 g / cm 3 .

[0237] In some embodiments, after being treated by the formation process, the compaction density of the positive electrode film layer can be selected as 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 or a numerical range between any two of them.

[0238] In this 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.

[0239] It can be understood that along with the rebound of the electrode sheet during the cycling process, the compaction density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery is slightly lower than that of the positive electrode film layer after compaction and formation.

[0240] When the compaction 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.

[0241] In some embodiments, the compaction density of the positive electrode film layer is 2.51 g / cm 3 - 2.73 g / cm 3 , and 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%.

[0242] In some embodiments, the compaction density of the positive electrode film layer is 2.55 g / cm 3 - 2.70 g / cm 3 , and 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%.

[0243] 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 from 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22% or a numerical range between any two of them.

[0244] 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 the 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. 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 cross-section electron microscope image of this section can be exported using the Analyze-Measure function. Use "Image"-"Overlay"-"Flatten" to export and obtain the pore picture; click "Apply" in "Threshold", then click "Analyze"-"Analyze Particles", and check the left four columns to obtain the pore statistical data.

[0245] 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. This "pore" is not the pore data obtained from the exhaust test, and is mainly used to characterize the cross-sectional area between 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 particles and also related to the mesopores in the carbon layer coated on the surface of the lithium iron phosphate particles, so it cannot objectively reflect the pores between particles.

[0246] As Figure 8 shown, the lower the porosity in the cross-section of the positive electrode film layer tested by this method, on the one hand, it 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 it is easy for particles 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 cycle processes, which is beneficial to improving the long cycle performance of the battery.

[0247] 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.

[0248] Among them, carbon-based particles refer to particles mainly composed of carbon elements, including but not limited to conductive carbon, carbon black, etc.

[0249] The bottom coating is beneficial to improving the conductivity and adhesion between the positive electrode film layer and the current collector, reducing the delamination 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 embodiment of the present application, for example, the compaction density of the positive electrode tab in the fully discharged state is greater than or equal to 2.4 g / cm 3 When the pressure is applied, the current collector is likely to be damaged during the compaction process of the high-pressure electrode tab, and large-sized particles are likely 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.

[0250] 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. The positive electrode film layer is cut along the thickness direction of the electrode tab by an argon ion beam, and a scanning electron microscope image or a microscope image is taken. The size of the carbon particles in the bottom coating is detected by a statistical method, and the number of carbon-based particles with a particle size greater than 100 nm contained in every 10 μm of the bottom coating is counted. The statistics are performed no less than 5 times, and the average value is calculated.

[0251] The bottom coating in the embodiment of the present application can be realized through any known preparation process. For example, in the preparation process of carbon-based particles, operations such as pre-screening or centrifugation are performed in advance to remove large-particle carbon-based materials, so that the D of the carbon-based particles added during the bottom coating preparation process V50 is between 20 nm and 60 nm, and D V90 is less than or equal to 70 nm. The carbon-based material and the binder are mixed, stirred, and coated on the current collector to obtain the bottom coating.

[0252] In some embodiments, the compaction density of the positive electrode tab 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.

[0253] In some embodiments, the compaction density of the positive electrode tab 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.

[0254] With the increase in the compaction density of the electrode, the extrusion effect of large particle-containing lithium phosphate materials (such as particles with a 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 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 and further increasing the ultimate compaction density of the electrode.

[0255] 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 by an argon ion beam, and a scanning electron microscope image is taken. In the length direction of the electrode, 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 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 electrode compaction process and do not have statistical significance.

[0256] In some embodiments, the thickness of the positive electrode current collector is less than or equal to 17 μm, and can be selected from 13 μm - 15 μm.

[0257] 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.

[0258] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, aluminum foil can be used. The composite current collector can include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can 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 substrates of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0259] In some embodiments of the lithium ion secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode film layer provided on at least one side of the negative electrode current collector. The areal density of the single-sided negative electrode film layer is 140 mg / 1540 mm 2 -221 mg / 1540 mm 2 ; and / or the compaction density of the negative electrode film layer is 1.40 g / cm 3 -1.75 g / cm 3 .

[0260] The areal density and compaction density of the single-sided negative electrode film layer can be tested by a method similar to that of the positive electrode film layer described above.

[0261] When the areal density and the compaction 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.

[0262] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, a copper foil may be used. The composite current collector may include a polymer material base layer 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.).

[0263] In some embodiments, the negative electrode film layer includes a negative electrode active material. The negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may 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 may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, 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 may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0264] In some embodiments, the negative electrode film layer may also optionally include a binder. The binder may 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).

[0265] In some embodiments, the negative electrode film layer may also optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0266] In some embodiments, the negative electrode film layer may also optionally include other additives, such as a thickening agent (such as sodium carboxymethyl cellulose (CMC-Na)), etc.

[0267] In some embodiments, the negative electrode sheet can be prepared in the following manner: the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material, conductive agent, 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 sheet can be obtained.

[0268] In some embodiments, the lithium-ion secondary battery includes an electrolyte. The electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. There is no specific limitation on the type of the electrolyte in this application, and it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or all-solid-state.

[0269] In some embodiments, the electrolyte uses an electrolytic solution. The electrolytic solution includes an electrolyte salt and a solvent.

[0270] In some embodiments, the electrolyte salt can 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.

[0271] In some embodiments, the solvent can 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.

[0272] In some embodiments, the electrolytic solution may further optionally include additives. For example, the additives can include negative electrode film-forming additives, positive electrode film-forming additives, and can 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.

[0273] In some embodiments, the lithium-ion secondary battery further includes a separator. There is no particular limitation on the type of the separator in this application, and any well-known porous structure separator with good chemical stability and mechanical stability can be selected.

[0274] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can 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 can be the same or different, without particular limitation.

[0275] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be made into an electrode assembly through a winding process or a stacking process.

[0276] In some embodiments, the lithium-ion secondary battery may include an outer package. The outer package may be used to encapsulate the above-mentioned electrode assembly and electrolyte.

[0277] In some embodiments, the outer package of the lithium-ion secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery may also be a soft package, such as a pouch soft package. The material of the soft package may be plastic. As plastics, polypropylene, polybutylene terephthalate, and polybutylene succinate, etc. may be cited.

[0278] 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.

[0279] 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.

[0280] The fourth aspect of the present 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; the carbon source includes polyethylene glycol; the iron source includes divalent iron; obtaining a mixed slurry after grinding in a solvent; the volume distribution particle size Dv of the particles in the mixed slurry 50 is 1 μm - 4 μm; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder to obtain a positive electrode active material; the sintering includes at least two-stage isothermal sintering, wherein the sintering temperature in the high-temperature stage is 750 °C - 800 °C.

[0281] The preparation method provided by the embodiments of the present application adjusts the content of large particles in the positive electrode active material by controlling the sintering temperature and the particle size of the precursor. At the same time, by using polyethylene glycol as the carbon source and combining sintering temperature control and divalent iron catalytic reduction, the graphitization degree of the positive electrode active material is further improved. For the positive electrode film layer with the area ratio of particles with a particle size greater than or equal to 1.5 μm being greater than or equal to 8.0% and less than or equal to 20.0% and the median C of the graphitization degree 50 being greater than or equal to 0.95 and less than or equal to 1.20, a material basis is provided.

[0282] In some embodiments, the volume distribution particle size Dv of the particles in the mixed slurry 50 is 1 μm - 4 μm.

[0283] In the present application, the term "Dv" 50refers to the particle size corresponding to when the volume cumulative particle size distribution percentage of the sample measured by the Malvern laser scattering method reaches 50%; In some embodiments, the volume distribution particle size Dv of the particles in the mixed slurry 50 can be optionally 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4.0μm or a numerical range between any two of them.

[0284] The volume distribution particle size Dv of the particles in the mixed slurry 50 Within the above range, on the one hand, it can increase the activity of the particles to a certain extent, and at the same temperature, some cathode active material particles with a particle size of 1μm - 2μm can be generated, improving the compaction density of the electrode sheet and the energy density of the battery; on the other hand, it can improve the catalytic decomposition efficiency of iron elements on the surface of the crystal nuclei for the carbon source, improve the coating quality of the carbon source, and improve the uniformity and graphitization degree of the carbon coating layer, thereby further improving the compaction density of the electrode sheet and the energy density of the battery.

[0285] The large - sized particle area ratio on the surface of the cathode film layer prepared from the cathode active material prepared by this preparation method is small, and this cathode active material has a high graphitization degree, and it is easy to improve the compaction density of the electrode sheet through the slip between particles, which is beneficial to improving the energy density of the battery while improving the battery kinetic performance.

[0286] In some embodiments, the iron source includes divalent iron, and can be optionally one or more of ferrous oxalate, ferrous carbonate, and ferrous nitrate.

[0287] In some embodiments, the lithium source includes one or more of lithium dihydrogen phosphate, lithium phosphate, lithium carbonate, and lithium acetate.

[0288] In some embodiments, the carbon source includes a polymer carbon source, and can be optionally one or more of polyethylene glycol and polyvinyl alcohol.

[0289] In some embodiments, the phosphorus source includes one or more of lithium dihydrogen phosphate, phosphoric acid, and ammonium dihydrogen phosphate.

[0290] In some embodiments, the lithium source and the phosphorus source can be the same substance.

[0291] In some embodiments, the iron source includes ferrous oxalate, the lithium source and the phosphorus source include lithium dihydrogen phosphate, and the carbon source includes polyethylene glycol.

[0292] During the sintering process, the divalent iron source will preferentially decompose to generate a large amount of ferrous oxide, which serves as the nucleation site to form nanocrystallization nuclei of lithium-containing transition metal phosphate. Meanwhile, the polymer carbon source has a relatively low decomposition temperature. The iron element on the surface of the nanocrystallization nuclei will further catalyze the decomposition of the carbon source, enabling the carbon coating layer on the surface of the cathode active material to have a relatively high degree of graphitization at a relatively low sintering temperature, reducing the resistivity of the cathode active material, and improving the compactness and uniformity of the carbon coating layer on the surface of the lithium-containing transition metal phosphate. In addition, the uniform deposition of carbon on the surface of the lithium-containing transition metal phosphate will further hinder the growth of the grains of the lithium-containing transition metal phosphate, reducing the probability of the cathode active material particles growing into large particles with a particle size greater than 1.5 μm.

[0293] In some embodiments, the particle size D of iron oxalate 10 is greater than or equal to 3 μm, the particle size D 50 is 50 μm - 80 μm, and the particle size D 90 is less than or equal to 150 μm.

[0294] In the present application, the terms "D 10 ", "D 50 ", and "D 90 " respectively correspond to the particle sizes when the cumulative particle size distribution percentage of the sample measured by the Malvern laser scattering method reaches 10%, 50%, and 90%.

[0295] Controlling the particle size D of iron oxalate 10 to be greater than or equal to 3 μm can reduce the proportion of small-particle-size iron oxalate particles and control its reactivity during the grinding process. Controlling the particle sizes D 50 , D 90 helps to uniformly mix the raw materials during the grinding process, obtain a mixed slurry with consistent components and uniform particle sizes, and improve the particle size consistency of the prepared lithium-containing transition metal phosphate.

[0296] In some embodiments, the mass content of trivalent iron element is less than or equal to 0.08%.

[0297] In some embodiments, the mass content of trivalent iron element can be selected as 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08% or the numerical range between any two of them.

[0298] Controlling the mass content of ferric iron helps to improve the uniformity and consistency of the carbon coating layer. An excessively high content of ferric iron will preferentially consume the carbon source, resulting in poor consistency in the mass and thickness of the carbon layers coated between the particles. On the one hand, the uneven carbon coating layer in thickness will affect the compaction between the particles, and on the other hand, local carbon deficiency will affect the connection of the conductive network between the particles, which is not conducive to effectively improving the compaction density of the electrode sheet and the improvement of kinetics.

[0299] In some embodiments, the atomic molar ratio of lithium element to iron element in the lithium source and the iron source is 1.0:1.0 - 1.05:1.0.

[0300] In some embodiments, the atomic molar ratio of lithium element to iron element in the lithium source and the iron source can be 1.0:1.0, 1.01:1.0, 1.02:1.0, 1.03:1.0, 1.04:1.0, 1.05:1.0 or the numerical range between any two of them. In some embodiments, the carbon source includes a polymer carbon source, and can be one or more of polyethylene glycol and polyvinyl alcohol.

[0301] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the carbon source is 1 - 4%.

[0302] The polymer carbon source has a relatively low decomposition temperature and graphitization temperature, so that the carbon coating layer on the surface of the positive electrode active material can be decomposed to form a carbon layer at a relatively low sintering temperature, hindering the growth and sintering of the lithium-containing transition metal phosphate grains, and is conducive to reducing the particle size of the positive electrode active material particles.

[0303] At the same time, the polymer carbon source usually has a relatively high molecular weight or a long molecular chain, and it is easy 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, which is conducive to the directional growth of graphite crystals; at the same time, the entanglement and cross-linking between long chains are conducive to reducing structural defects and reducing the lattice disorder caused by chain breakage during the carbonization process, thereby improving the graphitization degree.

[0304] 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 layer formed by the polymer carbon source has a relatively high graphitization degree and a relatively tight carbon structure, which is conducive to optimizing the surface roughness of the positive electrode active material.

[0305] In some embodiments, the weight-average molecular weight of polyethylene glycol is less than 10000.

[0306] In some embodiments, the weight-average molecular weight of polyethylene glycol can be 1500, 2000, 3000, 4000, 6000, 8000 or the numerical range between any two of them.

[0307] Using polyethylene glycol with a weight-average molecular weight below 10,000 can control the decomposition rate during sintering to form a carbon coating layer with appropriate and uniform thickness.

[0308] In some embodiments, the water content of the polyethylene glycol is less than or equal to 0.5%.

[0309] If the water content in the polyethylene glycol is high, the water may affect the decomposition process, resulting in incomplete decomposition or uneven decomposition rate during sintering. Excessive water may also cause uneven distribution of the molten polyethylene glycol during sintering, affecting the uniformity of the carbon layer and leading to instability or peeling of the carbon coating layer.

[0310] In some embodiments, the water content of the polyethylene glycol can be selected as 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5% or the numerical range between any two of them.

[0311] In some embodiments, the pH of the polyethylene glycol is 5 - 7.

[0312] Polyethylene glycol with a pH of 5 - 7 has high stability and will not degrade during the mixing process due to excessive acidity. Especially under high-temperature conditions, it will cause too fast decomposition and affect the quality of the coating layer. If the polyethylene glycol is alkaline, it may affect the stability of other components, leading to dissolution or oxidation reactions of metal ions and affecting the performance of the final positive electrode active material.

[0313] 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.

[0314] The titanium source often has low surface activity. Including the titanium source in the slurry can reduce the activity of the lithium-containing transition metal phosphate precursor, inhibit the grain growth of the lithium-containing transition metal phosphate during high-temperature sintering, and enable the lithium-containing transition metal phosphate to form smaller particles during sintering.

[0315] As a lattice stabilizer, titanium usually enters the lattice of the lithium-containing transition metal phosphate in the form of Ti 4+ Some titanium ions can replace the position of iron ions, making the crystal structure more stable and reducing the possibility of lithium and iron ions being in the wrong position, especially during high-temperature or high-current charge and discharge.

[0316] 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.

[0317] In some embodiments, the sintering includes at least two stages of isothermal sintering, wherein the sintering temperature in the low-temperature stage is 300°C - 400°C, and the heat preservation time is 2 hours - 6 hours; the sintering temperature in the high-temperature stage is 750°C - 800°C, and the heat preservation time is 8 hours - 15 hours.

[0318] In some embodiments, the heating rate from the low-temperature stage to the high-temperature stage is greater than or equal to 5°C / min.

[0319] Rapid heating to the target temperature at a higher heating rate is beneficial to the uniform growth of particles and reduces the presence of particles with a particle size greater than or equal to 1.5 μm.

[0320] In some embodiments, lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide are mixed evenly and ground in an organic solution to obtain a mixed raw material.

[0321] The organic solvent can effectively reduce the occurrence of side reactions, improve the purity and consistency of the material. Moreover, the organic solvent has good volatility and is easier to remove during the subsequent drying process, without remaining inside the material, causing pores inside the material and affecting the denseness and structural stability of the material.

[0322] In some embodiments, based on the total mass of the mixed raw material, the mass proportion of the carbon source in the mixed raw material is 5% - 7%.

[0323] In some embodiments, based on the total mass of the mixed raw material, the mass proportion of the carbon source in the mixed raw material 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 any numerical range between any two of them.

[0324] 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 plate and the energy density of the battery 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.

[0325] In some embodiments, the solvent includes water and its mixtures.

[0326] In some embodiments, the obtaining of the mixed raw material 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.

[0327] In some embodiments, obtaining the precursor powder after drying the mixed slurry includes obtaining the precursor powder after spray-drying the mixed slurry.

[0328] In some embodiments, after sintering the precursor, the product is subjected to air jet milling to obtain the positive electrode active material.

[0329] In some embodiments, the classification frequency of air jet milling is 18 Hz - 24 Hz, and the milling air pressure is 0.45 MPa - 0.65 MPa.

[0330] In air jet milling, the classification frequency 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 stream more times, causing larger particles to be screened out and leaving smaller particles. And a higher classification frequency may increase the number of particle collisions, causing irregular particles to be further impacted, making the particle surface smoother and the shape tending to be spherical.

[0331] A high air pressure will cause the particles to receive a greater impact force, and the collisions between particles will be more intense, resulting in a strong impact and wear on the particle surface. It can crush large particles into small particles, and the collisions between particles are more intense, and the surface is more easily trimmed, improving the sphericity and surface flatness of the particles.

[0332] However, excessive 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 carbon coating layer incomplete, manifested as an increase in iron dissolution, having a negative impact on the slippage of particles during rolling, and increasing the contact and reaction between lithium-containing transition metal phosphates and external factors such as electrolytes, 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 air jet milling within a suitable range.

[0333] The fifth aspect of the present application provides a method for preparing a positive electrode plate, 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 in sequence, dry-mixing, then 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, drying and hot-pressing to obtain a positive electrode plate.

[0334] In some embodiments, the stirring includes pre-stirring and main stirring, the stirring speed of the pre-stirring is lower than that of the main stirring, the revolution speed of the pre-stirring is 20 rpm - 30 rpm, the rotation speed of the pre-stirring is 450 rpm - 550 rpm, and the pre-stirring time is 10 min - 20 min.

[0335] In some embodiments, the hot pressing includes at least three hot roll pressings, and the hot roll pressure increases successively. The hot roll pressures are successively 20 tons - 50 tons, 50 tons - 70 tons, and 70 tons - 90 tons; the hot roll temperature is 40°C - 80°C. Before the first entry into the hot roll compaction, the electrode sheet is heated, and the heating temperature is 40°C - 50°C.

[0336] The positive electrode 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 reducing the porosity of the cut surface of the positive electrode film layer, increasing the ultimate compaction density of the electrode sheet, and improving the energy density of the battery.

[0337] 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 can 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.

[0338] As the electrical device, the secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0339] Figure 7 is an electrical device as an example. 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 electrical device for the secondary battery, a battery pack or battery module can be used.

[0340] Another example of the device can be a mobile phone, tablet computer, laptop computer, etc. This device usually requires thinness and lightness, and a secondary battery can be used as the power source.

[0341] 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 not specified in the embodiments in terms of specific technology or conditions, the technology or conditions described in the literature in the art or the product specification are followed. For reagents or instruments not specified in terms of the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0342] Embodiment 1 (1) Preparation of positive electrode active material Mix lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide evenly in methanol and grind to obtain a mixed raw material. Among them, the ratio of lithium dihydrogen phosphate to ferrous oxalate is such that the molar ratio of lithium to iron is 1.03:1.0. The particle size D of ferrous oxalate 10 is 6.1 μm, the particle size D 50 is 60.5 μm, the particle size D 90 is 105.5 μm, and the mass content of Fe element in ferrous oxalate is 30.9%, and the mass content of trivalent iron element is 0.03%.

[0343] Perform ball milling on the mixed raw material in a ball mill multiple times and demagnetize to obtain a mixed slurry. Control the number of grinding times and time. The particle size Dv of the mixed slurry after grinding 50 is 3.0 μm.

[0344] Spray-dry the mixed slurry to obtain a dried precursor powder. The appearance of the dried precursor powder material is light yellow and the color is uniform.

[0345] Place the precursor powder in a sintering furnace. Under a nitrogen atmosphere, heat from 25°C to 350°C at a rate of 2°C / min and hold at this temperature for 3 h, then heat to the second temperature of 770°C at a rate of 5°C / min and hold at this temperature for 10 h, and then cool down after completion.

[0346] Use the method of air flow pulverization to crush the obtained material with a classification frequency of 22 Hz and a pulverization air volume with a pressure of 0.55 MPa to obtain a carbon-coated lithium iron phosphate cathode active material.

[0347] The mass content of carbon element in the cathode active material is 1.2%, the median L of the sphericity A50 is 0.719, the median R of the roughness A50 is 0.939, the lithium iron anti-site defect concentration is 0.62%, the tapped density of the powder is 1.04 g / cm 3 , and the compacted density of the powder under a pressure of 3 T is 2.57 g / cm 3 , and the powder resistivity under a pressure of 8 Mpa is 5.58 Ω·cm; the discharge specific capacity at a 1C discharge rate is 141.4 mAh / g; there is a discharge plateau in the voltage range of 2.5V - 2.9V, and the discharge capacity ratio of the 3.2V discharge plateau is 90.52%.

[0348] (2) Preparation of the cathode electrode sheet: Add 2.2 wt% of PVDF, 0.8 wt% of conductive carbon black, and 97.0 wt% of cathode active material in sequence, dry-mix them, then add N-methylpyrrolidone, stir and adjust the viscosity to obtain the shipping slurry; transfer the shipping slurry and coat it on 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, a cathode film layer with a single-sided density of 350 mg / 1540 cm 2 is obtained.

[0349] Among them, the stirring includes pre-stirring and main stirring. The stirring speed of pre-stirring is lower than that of main stirring. The revolution speed of pre-stirring is 25 rpm, the rotation speed is 500 rpm, and the pre-stirring time is 15 min.

[0350] The hot pressing process includes three hot roll pressing processes, and the hot roll pressing pressure increases in sequence. 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.

[0351] The compaction density of the electrode sheet is the ultimate compaction density of the electrode sheet. The test method for the ultimate compaction density of the electrode sheet is as follows; in this example, the ultimate compaction density of the electrode sheet is 2.68 g / cm 3 .

[0352] Statistical analysis of 17,707 particles in the cross-section of the cathode film layer along the thickness direction of the electrode sheet shows that there are 79 particles with a particle size between 1.5 μm - 5 μm in the cross-section of the cathode film layer, and the area ratio is 14.58%. There are no particles with a particle size greater than 5 μm, and the area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm is 19.70%. The median C of the graphitization degree obtained by the cathode film layer in the surface scanning mode of the laser confocal Raman spectrometer 50 is 1.02, C 90 is 1.04, C 10 is 1.0, and the concentration degree of the C value (C 90 -C 10 ) / C 50 is 0.034.

[0353] The iron dissolution rate of the cathode film layer is 1076 ppm.

[0354] (3) Preparation of the negative electrode sheet: Mix 95.5 wt% of the anode active material (artificial graphite), 1.0 wt% of the conductive agent (conductive carbon black), 2.0 wt% of the binder (styrene-butadiene rubber (SBR)), and 1.5 wt% of the thickener (sodium carboxymethyl cellulose (CMC)), add deionized water and stir to disperse and make the anode slurry. Then coat the anode slurry on both sides of the Cu foil. After both sides are completed, dry, cold press, slit, and cut into pieces to prepare the anode electrode sheet. The single-sided coating density is 165 mg / 1540 mm 2 , and the compaction density is 1.60 g / cm 3 .

[0355] (4) Preparation of the separator Use a polypropylene film as the separator.

[0356] (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.

[0357] (6) Preparation of the battery: Stack the positive electrode sheet, separator, and negative electrode sheet 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, and go through processes such as encapsulation, formation, and degassing to finally obtain a lithium-ion battery.

[0358] The preparation methods of Examples 2 and 3 are basically the same as that of Example 1, except that the sintering temperature of the precursor powder is adjusted.

[0359] Example 2 Place the precursor powder in a sintering furnace. Under a nitrogen atmosphere, heat it from 25°C to 350°C at a rate of 2°C / min and hold at this temperature for 3 h, then heat it to the second temperature of 755°C at a rate of 5°C / min and hold at this temperature for 10 h. After that, cool down.

[0360] Example 3 Place the precursor powder in a sintering furnace. Under a nitrogen atmosphere, heat it from 25°C to 350°C at a rate of 2°C / min and hold at this temperature for 3 h, then heat it to the second temperature of 790°C at a rate of 5°C / min and hold at this temperature for 10 h. After that, cool down.

[0361] The preparation methods of Examples 4 and 5 are basically the same as that of Example 1, except that the particle size Dv of the mixed slurry after grinding is adjusted. 50 .

[0362] Example 4 The mixed raw materials are ball-milled multiple times in a ball mill and demagnetized to obtain a mixed slurry. The number of grinding times and time are controlled, and the particle size Dv of the mixed slurry after grinding 50 is 4.0 μm.

[0363] Example 5 The mixed raw materials are ball-milled multiple times in a ball mill and demagnetized to obtain a mixed slurry. The number of grinding times and time are controlled, and the particle size Dv of the mixed slurry after grinding 50 is 1.5 μm.

[0364] Example 6 The preparation method of Example 6 is basically the same as that of Example 1, except that conductive carbon black is not added when preparing the positive electrode sheet: 97.8 wt% of the positive electrode active material and 2.2 wt% of PVDF are mixed, and then N-methylpyrrolidone is added and stirred and dispersed to form a positive electrode slurry.

[0365] The preparation methods of Examples 7 and 8 are basically the same as that of Example 1, except that the carbon source in the preparation method of the positive electrode active material is adjusted.

[0366] Example 7 Lithium dihydrogen phosphate, ferrous oxalate, a mixture of polyethylene glycol and glucose, and titanium dioxide are mixed evenly and ground in methanol. Among them, the ratio of lithium dihydrogen phosphate to ferrous oxalate is such that the molar ratio of lithium to iron is 1.03:1.0; the mass ratio of polyethylene glycol to glucose is 3:1.

[0367] Example 8 Lithium dihydrogen phosphate, ferrous oxalate, a mixture of polyethylene glycol and glucose, and titanium dioxide are mixed evenly and ground in methanol. Among them, the ratio of lithium dihydrogen phosphate to ferrous oxalate is such that the molar ratio of lithium to iron is 1.03:1.0; the mass ratio of polyethylene glycol to glucose is 1:3.

[0368] The preparation method of Comparative Example 1 is basically the same as that of Example 1, except that the carbon source is replaced with glucose and the sintering temperature of the precursor powder is adjusted.

[0369] Comparative Example 1 Mix lithium dihydrogen phosphate, iron oxalate, glucose, and titanium dioxide evenly in methanol and grind them to obtain a mixed raw material. Place the precursor powder in a sintering furnace. Under a nitrogen atmosphere, heat it from 25°C to 350°C at a rate of 2°C / min and hold at this temperature for 3 h. Then, heat it to the second temperature of 803°C at a rate of 5°C / min and hold at this temperature for 10 h. After that, cool it down.

[0370] The preparation method of Comparative Example 2 is basically the same as that of Example 1, except that the sintering temperature of the precursor powder and the particle size Dv of the mixed slurry after grinding are adjusted. 50 And the carbon source is replaced with glucose.

[0371] Comparative Example 2 Mix lithium dihydrogen phosphate, iron oxalate, glucose, and titanium dioxide evenly in methanol and grind them. Perform multiple ball milling operations on the mixed raw material in a ball mill and demagnetize it to obtain a mixed slurry. Control the number of grinding times and time, and the particle size Dv of the mixed slurry after grinding 50 is 4.0 μm. Spray-dry the mixed slurry to obtain a dried precursor powder. The appearance of the dried precursor powder material is light yellow and the color is uniform. Place the precursor powder in a sintering furnace. Under a nitrogen atmosphere, heat it from 25°C to 350°C at a rate of 2°C / min and hold at this temperature for 3 h. Then, heat it to the second temperature of 750°C at a rate of 5°C / min and hold at this temperature for 10 h. After that, cool it down.

[0372] Performance Test For the energy density test, let the lithium-ion secondary battery stand at 25°C for 2 h to ensure that the temperature of the lithium-ion secondary battery is 25°C. At 25°C, charge the lithium-ion secondary battery at 0.33C until the charging cut-off voltage of 3.65V, and then continue to perform constant-voltage charging at this charging cut-off voltage until the current is 0.05C, at which point the charging stops (where C represents the rated capacity of the lithium-ion secondary battery). After the lithium-ion secondary battery stands at 25°C for 1 h, discharge the lithium-ion secondary battery at 0.33C at 25°C until the discharge cut-off voltage of 3.65V, and record the total discharge energy of the lithium-ion secondary battery as E 0 .

[0373] Measure the length, width, and height of the battery cell and calculate the volume value V of the battery cell 0 = length * width * height. 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 .

[0374] The DC resistance (DCR) test method is as follows: at 25°C, it is charged at a constant current of 0.33C until 3.65V, then charged at a constant voltage until the current reaches 0.05C, then discharged at 0.33C until 20% SOC, left standing for 5 minutes, then discharged with a 3C pulse for 30 seconds, left standing for 40 seconds, then charged with 3C for 40 seconds, left standing for 5 minutes, then charged at a constant current of 0.33C until 3.65V, then charged at a constant voltage until 0.05C, then discharged at 0.33C until 10% SOC, left standing for 5 minutes, then discharged with a 3C pulse for 30 seconds, left standing for 40 seconds, charged with 3C for 40 seconds, left standing for 5 minutes, then fully charged at 0.33C and then discharged at 0.33C until 50% SOC, then left standing at -25°C for 2 hours and then discharged with a 1C pulse for 30 seconds, left standing for 10 minutes, then left standing at 25°C for 2 hours, charged at a constant current of 0.33C until 3.65V and then charged at a constant voltage until 0.05C, then discharged at 0.33C until 20% SOC, then left standing at -25°C for 2 hours and then discharged with a 1C pulse for 30 seconds, left standing for 10 minutes.

[0375] Record the voltage at this time before and after each pulse discharge, calculate the DCR under different conditions, and the calculation formula is DCR = (voltage before pulse discharge at the end of standing - voltage before standing after pulse discharge) / pulse current.

[0376] The ultimate compaction density of the electrode sheet: After double-sided coating, the electrode sheet is compacted by a roller press, the elongation rate of the compacted electrode sheet is tested, and the flexibility of the electrode sheet after compaction is evaluated at the same time. By increasing the pressure of the roller press, electrode sheets with different compaction densities can be obtained. As the pressure increases, the compaction density of the electrode sheet increases, the elongation rate of the electrode sheet increases, and the flexibility of the electrode sheet decreases. If the elongation rate of the electrode sheet is too high, it is easy to cause warping of the electrode sheet; if the flexibility of the electrode sheet is too low, it is easy to cause brittle fracture of the electrode sheet. Therefore, the smaller of the compaction density corresponding to an elongation rate of 8% of the electrode sheet or a folding number of 3 times for the flexibility of the electrode sheet is defined as the ultimate compaction density of the electrode sheet.

[0377] The compaction density is calculated by the mass of the positive electrode film layer / the volume of the positive electrode film layer.

[0378] The test method for the elongation rate is as follows: Lay the electrode sheet flat on a horizontal table, cut the electrode sheet into segments, and the length of each electrode sheet is about 100 cm; remove the copper foil of the base material at the edge of the electrode sheet, and pay attention to keeping the cut edge of the electrode sheet parallel to the MD direction of the electrode sheet (perpendicular to the direction of the pressure roller), ensure that the electrode sheet 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 sheet, estimate to 0.1 mm, and record the length before compaction; record the length after compaction between the corresponding marked points after compaction, and use (length after compaction - length before compaction) / length before compaction as the elongation rate of the electrode sheet.

[0379] The test method for the flexible folding number is as follows.

[0380] Cut the positive electrode plate into test specimens with a size of 20×100 mm 2 ; after folding it forward, flatten it with a 2 kg roller, and unfold it to check whether light passes through the gap against the light. If no light passes through, fold it backward, flatten it with a 2 kg roller, and check again against the light. Repeat this process until light passes through the gap, and record the number of folding times; repeat the test three times and take the average as the reference data for the flexibility of the electrode plate.

[0381] Test results Table 1

[0382] Table 2

[0383] Table 3

[0384] It can be seen from the comparison between the examples and the comparative examples that in the cross-section of the positive electrode film layer along the thickness direction of the electrode plate, the area ratio of particles with a particle size greater than or equal to 1.5 μm is greater than or equal to 8.0% and less than or equal to 20.0%; 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 of the graphitization degree 50 When it is greater than or equal to 0.95 and less than or equal to 1.20, while the battery maintains a low internal resistance (especially has a low impedance at low SOC), the compaction density of the positive electrode plate is improved, enabling the battery to take into account both good energy density and kinetic performance.

[0385] It can be seen from the comparison between Examples 1-8 and Comparative Example 2 that 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 of the graphitization degree 50 is 0.97 - 1.13, which is beneficial to improving the compaction density of the electrode plate on the basis of maintaining the low impedance of the battery, and improving the energy density of the battery on the basis of maintaining the good kinetic performance of the battery.

[0386] It can be seen from the comparison between Examples 2 and 8 and Examples 1 and 3-7 that 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 of the graphitization degree 50 is 1.0 - 1.10, which is beneficial to having a relatively high compaction density of the electrode plate while maintaining the low impedance of the battery, and achieving the balance between the kinetic performance and energy density of the battery.

[0387] As can be seen from the comparison between Example 2 and Examples 1, 3-8, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size of 1.5 μm - 5 μm is 10.0% - 20.0%, which is beneficial to improving the compaction density of the electrode sheet on the basis of maintaining the low impedance of the battery and improving the energy density of the battery on the basis of maintaining good kinetic performance of the battery.

[0388] As can be seen from the comparison between Example 6 and Example 1, the lithium-ion secondary battery of the embodiment of the present application still has good kinetic performance without adding a conductive agent, so that the energy density of the lithium-ion secondary battery can be further improved.

[0389] 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 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 conceived by those skilled in the art to the embodiments, and other ways constructed by combining some constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A lithium ion secondary battery, characterized in that: Including positive electrode sheet, negative electrode sheet and electrolyte, The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed 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 layer along the thickness direction of the electrode sheet, the area proportion of particles with a particle size greater than or equal to 1.5 μm is greater than or equal to 8.0% and less than or equal to 20.0%; 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 Greater than or equal to 0.95 and less than or equal to 1.20, where the graphitization degree C value is I G / I D , 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 .

2. 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 It is 0.97-1.

13.

3. 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 It is 1.0-1.

10.

4. The lithium-ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C value obtained by the laser micro-confocal Raman spectrometer in the surface scanning mode, the concentration of C value (C 90 -C 10 ) / C 50 It is 0.01-0.

04.

5. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C value obtained by the laser micro-confocal Raman spectrometer in the surface scanning mode, the concentration of C value (C 90 -C 10 ) / C 50 It is 0.02-0.

04.

6. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the graphitization degree C 90 It is 1.00-1.

30.

7. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the graphitization degree C 90 It is 1.02-1.

15.

8. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the graphitization degree C 10 It is 0.92-1.

10.

9. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the graphitization degree C 10 It is 0.98-1.

08.

10. The lithium ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size of 1.5 μm-5 μm accounts for 9.0%-20.0%.

11. The lithium ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size of 1.5 μm-5 μm accounts for 10.0%-20.0%.

12. The lithium ion secondary battery according to claim 1, characterized in that: In a cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 5 μm accounts for 0.

13. The lithium ion secondary battery according to claim 1, characterized in that: In a cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1 μm and less than 1.5 μm accounts for 15.0%-25.0%.

14. The lithium ion secondary battery according to claim 13, characterized in that: In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1 μm and less than 1.5 μm accounts for 16.0%-24%.

15. The lithium ion secondary battery according to claim 13, characterized in that: In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1 μm and less than 1.5 μm accounts for 16%-20%.

16. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of particle sphericity area 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 0.60-0.

85.

17. The lithium ion secondary battery according to claim 16, characterized in that: In the cumulative distribution curve of particle sphericity area 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 0.65-0.

80.

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

96.

19. The lithium ion secondary battery according to claim 1, characterized in that: The iron dissolution rate of the positive electrode film layer is 500ppm-2000ppm.

20. The lithium ion secondary battery according to claim 1, characterized in that: The iron dissolution rate of the positive electrode film layer is 500ppm-1500ppm.

21. 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%.

22. The lithium ion secondary battery according to claim 21, characterized in that: Based on the total mass of the positive electrode active material, the mass content of carbon element is 0.90%-1.5%.

23. 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 0.1%-1.5%.

24. The lithium ion secondary battery according to claim 23, characterized in that: The lithium iron antisite defect concentration of the positive electrode active material is 0.3%-1.0%.

25. 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: The 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.

26. 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.

27. 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 2000ppm-6000ppm based on the total mass of the positive electrode active material.

28. The lithium ion secondary battery according to claim 1, characterized in that: The tap density of the positive electrode active material powder is 0.70 g / cm 3 -1.50g / cm 3 ; And / or the cathode active material has a powder compaction density of 2.50 g / cm under a pressure of 3T 3 -2.70g / cm 3 .

29. The lithium ion secondary battery according to claim 28, characterized in that: The tap density of the positive electrode active material powder is 0.70 g / cm 3 -1.20g / cm 3 ; And / or the cathode active material powder compaction density under 3T pressure is 2.52g / cm 3 -2.68g / cm 3 .

30. 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-30.0 Ω·cm.

31. The lithium ion secondary battery according to claim 30, characterized in that: The powder resistivity of the positive electrode active material under a pressure of 8 MPa is 2Ω·cm-20.0Ω·cm.

32. 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 at room temperature.

33. 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.

34. The lithium ion secondary battery according to claim 1, characterized in that: In the 0.1C discharge curve of the button cell containing the positive electrode active material, there is a discharge platform in the voltage range of 2.5V to 2.9V.

35. 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 conductive agent is 0-1.5%.

36. The lithium ion secondary battery according to claim 35, characterized in that: Based on the total mass of the positive electrode film layer, the mass content of the conductive agent is 0.

37. 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 95.5%-99.5%; the mass content of the binder is 0.5%-3%.

38. The lithium ion secondary battery according to claim 37, 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%-99.5%.

39. 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 .

40. 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 2.51 g / cm 3 -2.73g / cm 3 .

41. The lithium ion secondary battery according to claim 40, 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 -2.70g / cm 3 .

42. 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 2.51 g / cm 3 -2.73g / 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 -2.70g / 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%.

43. 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 single-side thickness of the primer layer is 1 μm-4 μm.

44. The lithium ion secondary battery according to claim 43, characterized in that: 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.

45. A battery device, characterized in that: A lithium-ion secondary battery comprising any one of claims 1 to 44, wherein the battery device comprises at least one of a battery module, a battery pack, and an energy storage battery.

46. ​​An electrical device, characterized in that: A lithium ion secondary battery according to any one of claims 1 to 44 or a battery device according to claim 45.

47. A method for preparing a positive electrode active material, characterized in that: A mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source is obtained; the carbon source includes polyethylene glycol; the iron source includes divalent iron; a mixed slurry is obtained after grinding in a solvent; and the volume distribution particle size Dv of the particles in the mixed slurry is 50 The mixed slurry is dried to obtain a precursor powder; the precursor powder is sintered to obtain the positive electrode active material used in the lithium-ion secondary battery according to any one of claims 1 to 44; the sintering includes at least two stages of constant temperature sintering, wherein the sintering temperature of the high temperature stage is 750°C-800°C.

48. 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 47 in sequence, dry-mixing them, adding a solvent, and stirring to obtain a delivery slurry; transfer-coating the delivery slurry to at least one side of a current collector, and obtaining a positive electrode sheet after drying and hot pressing.

49. The preparation method according to claim 48, characterized in that The stirring includes pre-stirring and main stirring. The stirring speed of the pre-stirring is lower than that of the main stirring. The revolution speed of the pre-stirring is 20rpm-30rpm, the rotation speed of the pre-stirring is 450rpm-550rpm, and the pre-stirring time is 10min-20min.

50. The preparation method according to claim 48, 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℃-80℃, and the pole piece is heated before entering the hot roller compaction for the first time, and the heating temperature is 40℃-50℃.

Images