Lithium ion secondary battery, battery device, power device, method for preparing positive electrode active material, and method for preparing positive electrode sheet

Abstract

The present application provides 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 plate. The lithium-ion secondary battery includes a positive electrode plate, a negative electrode plate, and an electrolyte, wherein 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 a carbon coating material provided on at least a portion of the surface, in a cross-section of the positive electrode film layer along the thickness direction of the plate, the area of ​​particles with a particle size greater than or equal to 1.5 μm accounts for 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 the surface scanning mode of a laser microscopic confocal Raman spectrometer, the median value of the graphitization degree C is greater than or equal to 0.0%. 50 Greater than or equal to 0.95 and less than or equal to 1.20.

Description

[0001] This application claims priority to international application PCT / CN2025 / 085927, filed on March 28, 2025, entitled “Lithium-ion secondary battery, battery device, electrical device, method for preparing positive electrode active material, and method for preparing positive electrode sheet,” the entire contents of which are incorporated herein by reference. Technical Field

[0002] The present application relates to the technical field of lithium-ion secondary batteries, and in particular to 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. Background Art

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

[0004] Cathode active materials are a crucial component of lithium-ion secondary batteries. Lithium-containing transition metal phosphate materials offer broad development prospects due to their structural stability, excellent safety, and long cycle life. As market demands for the energy density and kinetics of lithium-containing transition metal phosphate secondary batteries increase, achieving these performance improvements simultaneously is difficult with existing technologies, creating an urgent technical challenge in this field. Summary of the Invention

[0005] The present application has been made in view of the above-mentioned problems, and an object of the present application is to provide a lithium-ion secondary battery having both high energy density and good dynamic performance.

[0006] The first aspect of the present 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 a carbon coating material provided on at least a portion of the surface, in a cross-section of the positive electrode film layer along the thickness direction of the 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 the surface scanning mode of a laser microscopic confocal Raman spectrometer, the median value of the graphitization degree C is greater than or equal to 0.0%. 50 Greater than or equal to 0.95 and less than or equal to 1.20, where the graphitization degree C value is 1 G / I D , I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at I DIndicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at .

[0007] Controlling the area ratio of particles with a diameter of 1.5 μm or more 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 keeping the battery impedance at a low level and improving the battery's dynamic performance. However, this will limit the further improvement of the electrode compaction density. The embodiment of the present application further controls the median C of the graphitization degree. 50 Greater than or equal to 0.95 and less than or equal to 1.20, improve the graphitization degree of the particles in the positive electrode film layer, increase the crystallinity of the carbon layer on the surface of the particles, make it easy for the positive electrode active material to achieve particle slip during the roller pressing film forming process, and make use of the easy slip property of the particles to further improve the compaction density of the positive electrode film layer, thereby achieving a balance between battery kinetic performance and energy density.

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

[0009] The median graphitization degree of the positive electrode film C 50 Being within the above range is conducive to further improving the 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 while maintaining the high dynamic performance of the battery, and achieving a balance between battery dynamic performance and energy density.

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

[0011] The concentration of the C value of the positive electrode film layer is 0.01-0.04, indicating that the graphitization degree of the carbon coated 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, which can reduce the slip resistance caused by the inconsistent graphitization degree of the particles in the positive electrode active material and the local stress concentration caused thereby. Therefore, the electrode sheet can be compacted as a whole at a relatively low rolling pressure through the uniform and consistent slip between the particles of the positive electrode active material, thereby further improving the compaction density of the electrode sheet and the energy density of the battery while maintaining good dynamic performance of the battery.

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

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

[0014] In any embodiment, 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 microconfocal Raman spectrometer, the graphitization degree C 90 It is 1.00-1.30, and can be selected as 1.02-1.15.

[0015] Graphitization degree C 90 In the above range, the median value of graphitization degree C 50 The graphitization degree of the positive electrode film is relatively close, indicating that the distribution range of the graphitization degree of the positive electrode film is narrow, which is conducive to the uniform slip between the particles and improves the compaction density of the positive electrode sheet.

[0016] In any embodiment, 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 microconfocal Raman spectrometer, the graphitization degree C 10 It is 0.92-1.10, and can be selected as 0.98-1.08.

[0017] Graphitization degree C 10 The above range indicates that different sites in the positive electrode film layer have a high degree of graphitization, which is conducive to the uniform sliding of particles, reduces the probability of local stress concentration, and further improves the compaction density of the electrode.

[0018] In any embodiment, 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 of 1.5 μm-5 μm accounts for 9.0%-20.0%.

[0019] In the cross-section of the positive electrode film layer along the thickness direction of the electrode, the area proportion of particles with a particle size of 1.5μm-5μm is within the above range, which can further reduce the obstruction of large-sized particles on the electrode 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 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 of 1.5 μm-5 μm accounts for 10.0%-20.0%.

[0021] In the process of improving particle size distribution and increasing the size or proportion of large particles, it is inevitable that particles with a diameter of 1.5μm-5μm will be introduced. The area proportion of particles with a diameter of 1.5μm-5μm is within the above range, which is beneficial for improving the compaction of the electrode while taking into account the dynamic performance of the battery.

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

[0023] Studies have shown that particles with a diameter 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 diameter greater than or equal to 5 μm is 0, which is beneficial to further reduce the internal resistance of the battery and improve the battery kinetic performance.

[0024] In any embodiment, in a 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 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, the area ratio of particles with a particle size greater than or equal to 1μm and less than 1.5μm is within the above range, which is beneficial to further improve the compaction density of the electrode and improve the energy density of the battery while maintaining good dynamic performance of the battery.

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

[0027] The median of sphericity L A50 The particles within the above range are approximately spherical and are prone to slippage between particles under the action of external forces, which can further improve the compaction density of the electrode and increase the energy density of the battery.

[0028] In any embodiment, 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, the median of the roughness R A50 It is 0.92-0.96.

[0029] Median roughness R A50The surface of particles within the above range is relatively smooth, the friction between particles is relatively small, and they are easy to slip under the action of external force, which can further improve the compaction density of the electrode 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] Positive electrode active materials with iron dissolution rates within the above range possess a relatively complete and dense carbon coating, which can enhance electrical contact between the positive electrode active materials, improve their conductivity, reduce polarization, and further optimize the kinetic performance of lithium-ion secondary batteries. Furthermore, the densely coated carbon layer has a low spatial occupancy rate, making it easier for the interparticle spaces to be compressed by stress during rolling, thereby simultaneously increasing the electrode sheet's compaction density and the battery's energy density.

[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 optionally 0.90%-1.5%.

[0033] Compared with the lithium-containing transition metal phosphate positive electrode active materials in the prior art, the positive electrode active material has a relatively low carbon coating content, which can further increase the loading amount of the 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%, optionally 0.3%-1.0%.

[0035] The positive electrode active material in the embodiment of the present application has low lithium-iron antisite defects, which is conducive 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 a component having the following general formula: Li m Fe x P y O j Q q , wherein 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, and Br, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.1.

[0037] Selecting an appropriate modifying element Q can improve the ion diffusion path of the positive electrode active material, improve the lithium ion diffusion rate of the positive electrode active material, and improve the kinetic performance of the battery.

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

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

[0040] The positive electrode active material in the embodiments of the present application has a high titanium content, and surprisingly, the high addition of titanium does not form a harmful impurity phase that has a negative impact on the energy density and kinetic performance of the battery. Although the reason is not yet clear, it is speculated that titanium may form a fast ion conductor together with phosphate and other elements (for example, lithium), which in turn improves the kinetic performance of the battery.

[0041] In any embodiment, the tap density of the positive electrode active material is 0.70 g / cm 3 -1.50g / cm 3 , optional 0.70g / cm 3 -1.20g / cm 3 .

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

[0043] In any embodiment, the powder compaction density of the positive electrode active material at a pressure of 3T is 2.50 g / cm 3 -2.70g / cm 3 , optional 2.52g / cm 3 -2.68g / cm 3 .

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

[0045] In any embodiment, the powder resistivity of the positive electrode active material at a pressure of 8 MPa 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, so the sp 2 The structure facilitates rapid conduction of electrons between particles, making the positive electrode active material have low powder resistivity, which is beneficial to increasing the solid-phase transmission rate of electrons and further improving the kinetic performance of the battery.

[0047] In any embodiment, 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.

[0048] The positive electrode active material has a high discharge gram capacity at a 1C rate, indicating that it has good charge and discharge capabilities, which is beneficial to improving the kinetic performance of the battery.

[0049] In any embodiment, 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, a 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 at the discharge voltage to 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.

[0050] The high percentage of discharge capacity of the positive electrode active material used in the lithium-ion secondary battery of the present embodiment at 3.2V indicates that the positive electrode active material has good kinetic performance. Furthermore, a high η value indicates that the lithium-ion secondary battery containing the positive electrode active material maintains a high voltage when discharged to a low state of charge (SOC), which is conducive to maintaining good power performance.

[0051] In any embodiment, in a 0.1C discharge curve of a button cell comprising the positive electrode active material, a discharge platform exists in the voltage range of 2.5V to 2.9V.

[0052] The button cell containing the positive electrode active material in the embodiment of the present application shows a new charge and discharge platform in the voltage range of 2.5V~2.9V, which is conducive 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 can be 0.

[0054] The carbon layer of the positive electrode active material has a high degree of graphitization, which makes the positive electrode active material have good electronic conductivity, can reduce or even eliminate the use of conductive agents in the positive electrode film layer, and is conducive to further increasing 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 single-side density of the positive electrode film layer is 300 mg / 1540 mm 2 -450mg / 1540mm 2 .

[0057] The positive electrode film layer having an area 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 fully discharged, the compaction density of the positive electrode film layer is 2.51 g / cm 3 -2.73g / cm 3 .

[0059] In any embodiment, 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 .

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

[0061] In any embodiment, 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 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%.

[0062] In any embodiment, 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%.

[0063] The lower the porosity in the cross section of the positive electrode film layer in the embodiment of the present application, on the one hand, means that the gradation 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 gradation and roller pressure, if the porosity is low, it means that the particles are easy to slide against each other, thereby reducing the risk of overpressure and stress concentration in the film layer, and further reducing the probability of demolding of the positive electrode film during long cycles, which is beneficial to improving the long cycle performance of the battery.

[0064] In any embodiment, the positive electrode plate includes a primer layer, which is arranged between the positive electrode film layer and the current collector; 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.

[0065] The undercoat layer helps improve the conductivity and adhesion between the positive electrode film and the current collector, reduces the possibility of the positive electrode film separating from the current collector during cycling, and improves the battery's dynamic performance. In the high-density electrode of the present embodiment, the distribution density of carbon-based particles with a particle size greater than 100 nm in the undercoat layer is controlled to ≤10 pcs / 10 μm, which helps reduce the probability of current collector damage in the high-density electrode and further improves the ultimate compaction density of the positive electrode.

[0066] In any embodiment, the positive electrode plate includes a primer layer, which is disposed between the positive electrode film layer and the current collector; the compaction density of the positive electrode plate in a fully charged state is greater than or equal to 2.4 g / cm 3 The thickness of the primer layer on one side is 1 μm-4 μm.

[0067] In any embodiment, the positive electrode plate includes a primer layer, which is disposed between the positive electrode film layer and the current collector; the compaction density of the positive electrode plate in a fully charged state is greater than or equal to 2.5 g / cm 3 The thickness of the primer layer on one side is 2 μm-4 μm.

[0068] As the electrode packing density increases, the compressive effect of large particles of lithium-containing phosphate material (e.g., particles larger than 1μm) in the positive electrode film on the undercoat layer becomes more pronounced. Consequently, stress concentrations are more likely to occur at the sites of these large particles, and these particles can even penetrate the undercoat layer and damage the current collector. Increasing the thickness of the undercoat layer can help alleviate stress concentration in the electrode and further increase the electrode's ultimate packing density.

[0069] A second aspect of the present application provides a battery device, comprising the lithium-ion secondary battery provided in the first aspect of the present application, wherein the battery device comprises 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 in the first aspect of the present application or the battery device provided in the second aspect of the present application.

[0071] The fourth aspect of the present application also 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; grinding in a solvent to obtain a mixed slurry; the volume distribution particle size Dv of the particles in the mixed slurry is 50 The particle size is 1 μm-4 μm; the precursor powder is obtained after drying the mixed slurry; the precursor powder is sintered to obtain the positive electrode active material; the sintering includes at least two constant temperature sintering stages, wherein the sintering temperature of 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 large-sized particles on the surface of the positive electrode film layer, and the positive electrode active material has a high degree of graphitization, which is easy to increase the compaction density of the electrode sheet through slippage between particles, which is beneficial to improving the energy density of the battery while improving the battery's kinetic performance.

[0073] The fifth aspect of the present application provides a method for preparing a positive electrode sheet, which comprises adding a binder, a conductive agent, and the positive electrode active material prepared by the method of the fourth aspect in sequence, dry-mixing them, adding a solvent, and stirring to obtain a shipping slurry; transferring and coating the shipping slurry to 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 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.

[0075] In any embodiment, 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℃.

[0076] The positive electrode active material prepared by the hot pressing process described above in combination with the preparation method of the fourth aspect is beneficial for further reducing the cross-sectional porosity of the positive electrode film layer, increasing the ultimate compaction density of the electrode sheet, and improving the energy density of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0077] Figure 1 This is a scanning electron microscope image of a cross-section of the positive electrode film layer along the thickness direction of the electrode sheet according to one embodiment of the present application;

[0078] Figure 2 is a schematic diagram of a lithium-ion secondary battery according to one embodiment of the present application;

[0079] Figure 3 is an exploded schematic diagram of a lithium-ion secondary battery according to one embodiment of the present application;

[0080] Figure 4 is a schematic diagram of a battery module according to one embodiment of the present application;

[0081] Figure 5 is a schematic diagram of a battery pack according to one embodiment of the present application;

[0082] Figure 6 yes Figure 5 An exploded schematic diagram of the battery pack shown;

[0083] Figure 7 is a schematic diagram of an electrical device using a lithium-ion secondary battery according to one embodiment of the present application as a power source;

[0084] Figure 8 This is a porosity test diagram of a cross-section of the positive electrode film layer along the thickness direction of the electrode sheet in one embodiment of the present application.

[0085] Description of reference numerals:

[0086] 1. Battery pack; 2. Upper case; 3. Lower case; 4. Battery module; 5. Lithium-ion secondary battery; 5. Casing; 5. Electrode assembly; 5. Top cover assembly. DETAILED DESCRIPTION

[0087] Below, with appropriate reference to the accompanying drawings, the embodiments of the lithium-ion secondary battery, battery device, power device, method for preparing the positive electrode active material, and method for preparing the positive electrode sheet of the present application are described in detail. 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 the same structure 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 described in the claims.

[0088] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0089] Unless otherwise specified, all embodiments and optional embodiments of the present 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 the present application.

[0090] Unless otherwise specified, 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 deemed to be included in the disclosure of this application.

[0091] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b), and (c), or may include steps (a), (c), and (b), or may include steps (c), (a), and (b), etc.

[0092] In this application, the terms "plurality" and "multiple" refer to two or more.

[0093] Unless otherwise specified, the terms used in this application have the common meanings that are commonly understood by those skilled in the art.

[0094] Unless otherwise stated, the numerical values ​​of the various parameters mentioned in this application can be measured using various test methods commonly used in the art, for example, they can be measured according to the test methods given in the examples of this application. Unless otherwise stated, the test temperature of each parameter is 25°C.

[0095] The battery mentioned in the embodiments of the present 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 the present application can include a lithium-ion secondary battery, a battery module or a battery pack.

[0096] A lithium-ion secondary battery is the smallest unit of a battery, and it can independently realize the functions of charge and discharge. The lithium-ion secondary battery can be cylindrical, rectangular, or in other shapes, and the embodiments of the present application are not limited thereto. Figure 2 As an example, a lithium-ion secondary battery 5 having a rectangular parallelepiped structure is shown.

[0097] A lithium-ion secondary battery includes an electrode assembly and an electrolyte.

[0098] Lithium-ion secondary batteries may also include an outer packaging, which can be used to encapsulate the electrode assembly and electrolyte. The outer packaging can be a hard shell, such as a hard plastic shell, aluminum shell, or steel shell. Alternatively, the outer packaging can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0099] In some embodiments, as Figure 3 As shown, the outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a base plate and side plates connected to the base plate, with the base plate and side plates enclosing 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, thereby sealing 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 may be one or more, and can be adjusted according to needs.

[0100] The electrode assembly usually includes a positive electrode sheet and a negative electrode sheet. The negative electrode sheet is the electrode that absorbs or lithiates lithium ions during charging and releases or delithiates lithium during discharging. The positive electrode sheet is the electrode that releases or delithiates lithium ions during charging and absorbs or lithiates lithium during discharging.

[0101] When there are multiple lithium-ion secondary batteries, the multiple lithium-ion secondary batteries are connected in series, in parallel, or in a hybrid manner via a busbar. In some embodiments, the battery may 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 may be a battery pack, which includes a housing and lithium-ion secondary batteries, with the lithium-ion secondary batteries or battery modules housed in the housing. In some embodiments, the housing may serve as part of the vehicle's chassis structure. For example, a portion of the housing may form at least a portion of the vehicle's floor, or a portion of the housing may form at least a portion of the vehicle's crossbeam or longitudinal beam.

[0102] In some embodiments, the battery may be an energy storage device, including an energy storage container, an energy storage cabinet, and the like.

[0103] In some embodiments, lithium-ion secondary batteries can be assembled into a battery module. The battery module can contain multiple lithium-ion secondary batteries, and the specific number can be adjusted according to the application and capacity of the battery module. Figure 4 FIG. 4 is a schematic diagram of a battery module 4 as an example. Figure 4 As shown, in the battery module 4, the plurality of 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 manner. Further, the plurality of lithium-ion secondary batteries 5 can be fixed by fasteners.

[0104] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of lithium-ion secondary batteries 5 are received in the receiving space.

[0105] In some embodiments, the battery modules described above may also be assembled into a battery pack, and the number of battery modules contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0106] Figure 5 and Figure 6 FIG. 1 is a schematic diagram of a battery pack 1 as an example. Figure 5 and Figure 6 As shown, a battery pack 1 may include a housing and multiple battery modules 4 disposed therein. The housing comprises an upper housing 2 and a lower housing 3. The upper housing 2 covers the lower housing 3 and forms an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 may be arranged in any manner within the housing.

[0107] 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 suffer from 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.

[0108] To further improve battery energy density and increase the compaction density of the electrode, the industry's common approach is to increase the particle size distribution within the electrode. This requires increasing the size or proportion of large particles. However, research has shown that exceeding a certain range of large particle size and proportion in the electrode compromises battery kinetic performance. Achieving a battery that balances both energy density and kinetic performance is a pressing technical challenge in this field.

[0109] The first aspect of the present application provides a lithium-ion secondary battery, which includes a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one side of the positive electrode current collector, the positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes lithium-containing transition metal phosphate particles with a carbon coating material provided on at least a portion of the surface, and in a cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the 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 the surface scanning mode of a laser microconfocal Raman spectrometer, the median value of the graphitization degree C is greater than or equal to 0.0%. 50 Greater than or equal to 0.95 and less than or equal to 1.20, where the graphitization degree C value is 1 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 .

[0110] In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, 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 sheet. In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1.5μm accounts for more than 20%. Although this is conducive to achieving a high compaction density of the positive electrode sheet, 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 area of ​​the electrode sheet, increase the battery impedance, and significantly deteriorate the battery's dynamic performance.

[0111] Controlling the area ratio of particles with a diameter of 1.5 μm or more 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 keeping the battery impedance at a low level and improving the battery's dynamic performance. However, this will limit the further improvement of the electrode compaction density. The embodiment of the present application further controls the median C of the graphitization degree. 50 Greater than or equal to 0.95 and less than or equal to 1.20, improve the graphitization degree of the particles in the positive electrode film layer, increase the crystallinity of the carbon layer on the surface of the particles, make it easy for the positive electrode active material to achieve particle slip during the roller pressing film forming process, and make use of the easy slip property of the particles to further improve the compaction density of the positive electrode film layer, thereby achieving a balance between battery kinetic performance and energy density.

[0112] In this application, the term "particle" refers to particles with identifiable complete boundaries in the field of view of the positive electrode film layer under a certain magnification, such as 10,000 times. Defects and scratches may exist inside the particles, but complete boundaries sufficient to separate the particles cannot be identified inside the particles.

[0113] In some embodiments, in a 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 can be selected to be 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 therebetween.

[0114] The specific statistical method for the area of ​​particles in the cross section of the positive electrode film along the thickness direction of the electrode is as follows. The positive electrode film is cut along the thickness direction of the electrode by an argon ion beam (as an example, the equipment model can be selected: Leica EM TIC 3X CP, working voltage: 6kV, working time: 6h), and after the cross section is exposed, a scanning electron microscope is used (as an example, the equipment model can be selected: Hitachi SU8230, working voltage: 3kV, beam current: high, probe model: U (LA100), working distance <5mm) to observe the cross section of the positive electrode film along the thickness direction of the electrode. The field emission scanning electron microscope is used to collect images in the secondary electron mode at the non-edge position of the cross section of the positive electrode film (after observing the edge of the electrode under the scanning electron microscope, adjust the field of view to the center of the sample), and the electron microscope image is taken at a magnification of 10k times. The particles in the electron microscope image are analyzed using ImageJ software (1.46r, win64 version). The specific method of using ImageJ software is as follows: load the scanning electron microscope image to be analyzed, such as Figure 1As shown in the figure, the Cellpose plug-in software was used to identify particles, and manual correction was performed based on this. Image J was used to read and analyze the data. The specific method for particle identification using the Cellpose plug-in software is as follows: set the segmentation diameter parameter (diameter in the Segmentation module) to 15 pixels, click "run cyto3" to perform particle identification, and manually mark particles in the image that were not recognized, not fully recognized, or incorrectly identified by the software. Particles that were not recognized, not fully recognized, or incorrectly identified by the software in the image mainly include the following: 1. Particles that are too large or have scratches on the particle surface cannot be identified or cannot be completely identified; 2. Argon ion beam cutting creates scratches on the particle surface, which the software may mistakenly interpret as particle boundaries during identification, resulting in identification errors; 3. Particles that are too small cannot be successfully identified; 4. Particles located at the edge of the electron microscope field of view, with the particle interior penetrated by the edge, preventing the complete morphology from being displayed, and partial recognition instead of the entire particle, resulting in identification errors. Manual calibration is performed on the unidentified or misidentified particles. The specific process is as follows: large particles located around the edges of the scanning electron microscope that are not fully displayed are deleted; other unidentified or misidentified particles are determined to have internal cracks and scratches. If no cracks and scratches are present, the particle is determined to be a single particle and manually labeled based on the observed particle boundary; if a crack and scratch are present, it is determined whether the crack and scratch penetrate the particle. If not, the particle is determined to be a single particle and manually labeled; if a crack and scratch penetrate the particle, it is determined whether the crack and scratch are linear or irregular; if the crack and scratch are irregular, it is determined to be the boundary between particles and the particles are divided along this boundary; if the crack and scratch are linear, contrast comparison is performed; if the contrast and scratch are not obvious and there is no crack, it is determined to be a scratch and labeled as a single particle; if the contrast and scratch are strong and there is a crack, it is determined to be the boundary between particles and labeled as two particles. After manual labeling, information irrelevant to the particles during the automatic image processing is deleted, thus completing the identification and labeling of the particles in the image.

[0115] After particle identification and labeling, images were imported into ImageJ software for analysis. The scale was set based on the SEM image, and the particle size and area of ​​the particles in the image were analyzed using the "Feret Diameter" and "Area" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter represents the maximum distance between all parallel lines in the particle's two-dimensional projection, which characterizes the particle size; the "Area" parameter represents the pixel area of ​​the particle, which characterizes the particle area. Because particles with a diameter less than 50 nm have large errors during the statistical process and are difficult to accurately identify, and the particle size of conductive agents is generally less than 50 nm, which can cause significant errors in the statistical results, particles with a diameter less than 50 nm were not counted in the particle size statistics of this application, and the statistical data for particles with an Area displayed as "NaN" were deleted. According to the above method, to meet the statistically significant sample count, each electrode collected at least 10 SEM images with non-overlapping fields of view, and the areas of at least 5,000 particles were counted. Calculate the sum of the "Area" parameters for particles with a diameter greater than or equal to 1.5 μm and the sum of the "Area" parameters for all particles, and use these as the area of ​​particles with a diameter greater than or equal to 1.5 μm and the total area of ​​the particles counted, respectively. Divide the sum of the areas of particles with a diameter greater than or equal to 1.5 μm by the total area of ​​the particles counted as the area percentage of particles with a diameter greater than or equal to 1.5 μm in a cross-section of the positive electrode film along the thickness direction of the electrode sheet.

[0116] During compaction, the positive electrode film is compacted along its thickness. Therefore, a cross-section of the positive electrode film along the thickness of the electrode sheet better reflects the spatial compaction of the particles within the film than the surface. The area percentage of particles 1.5 μm or larger in a cross-section of the positive electrode film along the thickness of the electrode sheet directly reflects the ratio of the area of ​​particles in this size segment to the overall particle area, reflecting the distribution of particles in this size segment.

[0117] It is understood that the particles in the cross-section of the positive electrode film along the thickness of the electrode sheet, especially those larger than 50 nm, are primarily derived from the positive electrode active material. Therefore, the present embodiment, through the observation and statistics of the particle area in the cross-section of the positive electrode film, can accurately and objectively reflect the distribution of lithium-containing transition metal phosphate particles in the positive electrode film layer in the electrode sheet.

[0118] In the prior art, a laser particle size analyzer is usually used to count the particle size of the positive electrode active material through the Malvern laser diffraction method. However, the applicant's research shows that since lithium-containing transition metal phosphate particles are easy to agglomerate, the test results obtained by the Malvern laser diffraction method based on the laser scattering principle often measure the particle size of its particle agglomerates, which cannot truly reflect the particle size of the particles in the positive electrode active material, let alone the dispersion state of the positive electrode active material in the film layer, because the positive electrode active material in the film layer will be more dispersed during the process of slurrying and film rolling. The test results obtained by the Malvern laser diffraction method are affected by the particle size, specific surface area, and degree of agglomeration of the positive electrode active material. Compared with the actual dispersion in the electrode, the number of large particles obtained by the 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 test cannot be equivalent to or analogous to the particle size obtained by statistics in the embodiments of the present application.

[0119] Those skilled in the art can achieve the regulation of particle size through any known process. As an example, by regulating the temperature and time during the preparation of the positive electrode material, the growth rate and time of the positive electrode material can be controlled. The mechanical force of the crushing and grinding process is used to process the raw materials to the target particle size distribution range to achieve the adjustment of the particle size; the particle system is separated by particle size using screening and grading equipment to obtain the required particle size ratio; by precisely controlling the feed rate, adjusting the residence time and stress state of the particles in the equipment, it is also helpful to achieve the regulation of the particle size.

[0120] Lithium-containing transition metal phosphates refer to phosphate materials containing lithium and transition metal elements and can be detected by any method known in the art. For example, they can be detected using an X-ray diffractometer (XRD) coupled with an energy dispersive spectrometer or an inductively coupled plasma mass spectrometer. Examples of lithium-containing transition metal phosphates include, but are not limited to, lithium iron phosphate, lithium manganese iron phosphate, and their doped materials.

[0121] The carbon coating material disposed on at least a portion of the surface of the lithium-containing transition metal phosphate can be detected by any method known in the art. As an example, the carbon coating material disposed on at least a portion of the surface of the lithium-containing transition metal phosphate can be observed by characterizing the lithium-containing transition metal phosphate using a transmission electron microscope and an energy spectrum analyzer. It should be noted that the elements in the carbon coating material are not limited to carbon, but may also include other non-carbon elements. The carbon coating layer containing the carbon coating material is not limited to a film-like shape, but also includes an island-like, irregular, or discontinuous coating layer.

[0122] In some embodiments, 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 microconfocal Raman spectrometer, the median value of the graphitization degree C is 50Greater than or equal to 0.95 and less than or equal to 1.20, where the graphitization degree C value is 1 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 .

[0123] In this application, the graphitization degree C value of the positive electrode film layer can be obtained by the surface scanning mode of the laser microconfocal Raman spectrometer. As an example, specifically, a laser microconfocal Raman spectrometer (high-precision Renishaw laser microconfocal Raman spectrometer) is used, an excitation wavelength of 532nm is selected, and an appropriate amount of positive electrode film layer is taken to perform a surface scan on its surface or a cross-section along the thickness direction of the electrode piece. The scanning area is 45μm×45μm, divided into 10×10 grids, with the grid vertex as the test point, the step size is 5μm, and the total number of scanning points is 100 points, thereby obtaining the C values ​​of different sites and the cumulative distribution curve of the C value of the surface scan area.

[0124] The positive electrode film layer in this application can be either freshly prepared or obtained from a disassembled battery. The surface of a disassembled battery positive electrode film layer inevitably contains residual electrolyte salt. To improve test accuracy, it is preferred to perform a surface scan of the positive electrode film layer along the thickness direction of the electrode sheet to characterize the degree of graphitization of the positive electrode film layer.

[0125] The graphitization degree C value of the positive electrode film is obtained by the peak intensity ratio of the G peak (G-band) and the D peak (D-band) of the Raman spectrum. The G peak position is 1580±100cm -1 , which characterizes carbon sp 2 Hybrid structure; D peak position is 1350±100cm -1 , which characterizes the disordered structure of carbon, where disorder means that there is no regular arrangement between the carbon atoms in the structure. In graphite crystals, the carbon atoms in the same layer are arranged in sp 2 Hybridization forms covalent bonds, and the interlayer is van der Waals force, which makes the carbon in the graphite structure easy to slide. Therefore, the C value can characterize the degree of graphitization of the positive electrode film. It can be understood that the degree of graphitization in the positive electrode film mainly comes from the carbon material that has been graphitized in the positive electrode film, that is, the carbon coating layer of the positive electrode active material. Although it is rich in sp 2 The hybrid structure of carbon nanotube conductive agent also has a relatively high I G / I D However, due to its low content and small diameter, its addition to 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.

[0126] Therefore, the degree of graphitization of the positive electrode film can also be used to characterize the degree of graphitization of the positive electrode active material. The higher the degree of graphitization of the carbon on the surface of the positive electrode active material, the higher the proportion of graphite structure carbon in the positive electrode film, and the easier it is for the particles to slip during the rolling process with the help of the highly graphitized carbon structure in the coating layer, which can achieve an increase in the electrode sheet compaction density at a low rolling pressure.

[0127] The cumulative distribution curve of graphitization degree C value refers to a curve obtained by arranging at least 100 C values ​​obtained in ascending order, with graphitization degree as the horizontal axis and cumulative number percentage as the vertical axis. 50 The C value corresponding to the cumulative number of the vertical axis in the cumulative distribution curve of graphitization degree C is 50%. The median C value of 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 degree of slippage; compared with the mean value, it can reduce the influence of extreme values ​​during the test process and improve the confidence of the test results.

[0128] In some embodiments, 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 microconfocal Raman spectrometer, the median value of the graphitization degree C is 50 The amount can be selected from 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 any range therebetween.

[0129] Those skilled in the art can adjust the degree of graphitization of active material particles by any known process. As an example, adjusting the degree of graphitization of active material particles can be achieved by regulating the carbon source, optimizing the nucleation process, sintering temperature, sintering time, sintering pressure, and sintering atmosphere. The higher the degree of graphitization of the carbon on the surface of the positive electrode active material, the higher the proportion of graphite structure carbon in the positive electrode film layer, and the easier it is for the particles to slip with the help of the highly graphitized carbon structure in the coating material, thereby increasing the compaction density of the electrode sheet.

[0130] In some embodiments, the median graphitization degree of the positive electrode film is C 50 It is 0.97-1.13, and can be selected as 1.00-1.10.

[0131] The median graphitization degree of the positive electrode film C 50Being within the above range is conducive to further improving the 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 while maintaining the high dynamic performance of the battery, and achieving a balance between battery dynamic performance and energy density.

[0132] In some embodiments, 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 microconfocal Raman spectrometer, the concentration of the C value (C 90 -C 10 ) / C 50 It is 0.01-0.04.

[0133] Refer to the above, and so on, C 90 is the C value corresponding to the cumulative number of the vertical axis in the graphitization degree C value cumulative distribution curve when it accounts for 90%, C 10 The C value corresponding to the cumulative number of the vertical axis in the graphitization degree C value cumulative distribution curve accounts for 10%. The concentration of C value is expressed as (C 90 -C 10 ) / C 50 Indicates. (C 90 -C 10 ) / C 50 It can not only reflect the size of most C values, but also be unaffected by extreme values. It can also reflect the width of the distribution of the degree of graphitization of particles in the positive electrode film. The low concentration of the C value of the positive electrode film indicates that the distribution of the degree of graphitization of carbon on the surface of the positive electrode active material is narrow and well concentrated.

[0134] In some embodiments, 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 microconfocal Raman spectrometer, the concentration of the C value (C 90 -C 10 ) / C 50 The values ​​can be selected as 0.01, 0.02, 0.03, 0.04 or any range therebetween.

[0135] The concentration of the C value of the positive electrode film layer is 0.01-0.04, indicating that the graphitization degree of the carbon coated 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, which can reduce the slip resistance caused by the inconsistent graphitization degree of the particles in the positive electrode active material and the local stress concentration caused thereby. Therefore, the electrode sheet can be compacted as a whole at a relatively low rolling pressure through the uniform and consistent slip between the particles of the positive electrode active material, thereby further improving the compaction density of the electrode sheet and the energy density of the battery while maintaining good dynamic performance of the battery.

[0136] In some embodiments, 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 microconfocal Raman spectrometer, the concentration of the C value (C 90 -C 10 ) / C 50 It is 0.02-0.04.

[0137] The concentration of C value (C 90 -C 10 ) / C 50 The above range is conducive to further improving the consistency of carbon graphitization degree on the surface of the positive electrode active material, improving the slip degree between particles, and further improving the compaction density of the electrode and the energy density of the battery while maintaining good dynamic performance of the battery.

[0138] In some embodiments, 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 microconfocal Raman spectrometer, the graphitization degree C 90 It is 1.0-1.3, and can be selected as 1.02-1.15.

[0139] In some embodiments, 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 microconfocal Raman spectrometer, the graphitization degree C 90 1.24, 1.25, 1.26, 1.27, 1.28, 1.29, 1.3 or any range therebetween.

[0140] Graphitization degree C 90 In the above range, the median value of graphitization degree C 50 The graphitization degree of the positive electrode film is relatively close, indicating that the distribution range of the graphitization degree of the positive electrode film is narrow, which is conducive to the uniform slip between the particles and improves the compaction density of the positive electrode sheet.

[0141] In some embodiments, 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 microconfocal Raman spectrometer, the graphitization degree C 10 It is 0.92-1.1, and can be selected as 0.98-1.08.

[0142] In some embodiments, 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 microconfocal Raman spectrometer, the graphitization degree C 10The optional values ​​may 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 any range therebetween.

[0143] Graphitization degree C 10 The above range indicates that different sites in the positive electrode film layer have a high degree of graphitization, which is conducive to the uniform sliding of particles, reduces the probability of local stress concentration, and further improves the compaction density of the electrode.

[0144] In some embodiments, 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 of 1.5 μm-5 μm accounts for 9.0%-20.0%.

[0145] In some embodiments, 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 of 1.5 μm-5 μm can be selected as 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 therebetween.

[0146] The area ratio of particles with a particle size of 1.5 μm to 5 μm in a cross-section of the positive electrode film along the thickness direction of the electrode piece can be tested in the same manner as described above. The area ratio of particles with a particle size of 1.5 μm to 5 μm in a cross-section of the positive electrode film along the thickness direction of the electrode piece is calculated by dividing the sum of the areas of particles with a particle size of 1.5 μm to 5 μm in the cross-section of the positive electrode film along the thickness direction of the electrode piece by the total area of ​​the particles counted.

[0147] In the cross-section of the positive electrode film layer along the thickness direction of the electrode, the area proportion of particles with a particle size of 1.5μm-5μm is within the above range, which can further reduce the obstruction of large-sized particles on the electrode 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.

[0148] In some embodiments, 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 of 1.5 μm-5 μm accounts for 10.0%-20.0%.

[0149] In the process of improving particle size distribution and increasing the size or proportion of large particles, it is inevitable that particles with a diameter of 1.5μm-5μm will be introduced. The area proportion of particles with a diameter of 1.5μm-5μm is within the above range, which is beneficial for improving the compaction of the electrode while taking into account the dynamic performance of the battery.

[0150] In some embodiments, in a 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.

[0151] The area ratio of particles with a diameter of 5 μm or greater in a cross-section of the positive electrode film along the thickness of the electrode sheet can be measured using the method described above. The area ratio of particles with a diameter of 5 μm or greater in a cross-section of the positive electrode film along the thickness of the electrode sheet is calculated by dividing the sum of the areas of particles with a diameter of 5 μm or greater in the cross-section of the positive electrode film along the thickness of the electrode sheet by the total area of ​​the particles counted.

[0152] Studies have shown that particles with a diameter 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 diameter greater than or equal to 5 μm is 0, which is beneficial to further reduce the internal resistance of the battery and improve the battery kinetic performance.

[0153] In some embodiments, in a 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 μm and less than 1.5 μm is 15.0%-25.0%, optionally 16.0%-24%, and further optionally 16.0%-20.0%.

[0154] In some embodiments, 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 μm and less than 1.5 μm can be selected to be 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25% or any numerical range therebetween.

[0155] The area ratio of particles with a diameter greater than or equal to 1 μm and less than 1.5 μm in a cross-section of the positive electrode film along the thickness of the electrode sheet can be determined using the test method described above. The area ratio of particles with a diameter greater than or equal to 1 μm and less than 1.5 μm in a cross-section of the positive electrode film along the thickness of the electrode sheet can be calculated by dividing the sum of the areas of particles with a diameter greater than or equal to 1 μm and less than 1.5 μm by the total area of ​​the particles counted.

[0156] 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 1μm and less than 1.5μm is within the above range, which is beneficial to further improve the compaction density of the electrode and improve the energy density of the battery while maintaining good dynamic performance of the battery.

[0157] In some embodiments, in the cumulative distribution curve of the particle sphericity area obtained from the cross section of the positive electrode film along the thickness direction of the electrode, the median of the sphericity L A50 It is 0.60-0.85, and can be selected as 0.65-0.80.

[0158] The specific method for testing the sphericity of particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode 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 picture after the particle identification and marking is completed into the ImageJ software for analysis, complete the scale setting according to the scanning electron microscope image, and use the "Feret diameter", "Area" and "Round" analysis functions to analyze the particle size, area and sphericity of the particles in the picture. According to the software manual (ImageJUser 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 long diameter as the diameter, which can be used to characterize the sphericity of the particle. The closer the particle is to a sphere, the closer the ratio of the pixel area to the area of ​​the circle with the fitted long diameter as the diameter is to 1. Therefore, the "Round" parameter of the particle obtained by analysis is used to characterize the sphericity of the particle. Since particles with a diameter of less than 50 nm have large errors in the statistical process and are difficult to identify accurately, and the particle size of the conductive agent is generally less than 50 nm, which will cause large errors in the statistical results, particles with a diameter of less than 50 nm are not counted in the particle size statistics process of this application, and the statistical data of particles corresponding to Round displayed as "NaN" are deleted. According to the above method, in order to meet the number of samples with statistical significance, each electrode is collected with no less than 10 scanning electron microscope images with non-overlapping fields of view. The sphericity of at least 5,000 particles obtained is arranged in order from small to large, and the sphericity is used as the horizontal axis and the cumulative area ratio is used as the vertical axis to obtain the cumulative distribution curve of the sphericity of the particles in the positive electrode film layer. L A50 It is the sphericity L value corresponding to when the cumulative area of ​​the vertical axis in the cumulative distribution curve of sphericity L value accounts for 50%.

[0159] In some embodiments, in the cumulative distribution curve of particle sphericity obtained from the cross section of the positive electrode film along the thickness direction of the electrode, the sphericity L A50 The amount may be selected from 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 any range therebetween.

[0160] Those skilled in the art can adjust the sphericity of the particles by any known process. For example, the sphericity of the particles can be adjusted by grinding, polishing, chemical etching, mechanical stirring, extrusion, coating, granulation, adding surfactants, and adjusting the parameters of each process.

[0161] The median of sphericity LA50 The particles within the above range are approximately spherical and are prone to slippage between particles under the action of external forces, which can further improve the compaction density of the electrode and increase the energy density of the battery.

[0162] In some embodiments, 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, the median roughness R A50 It is 0.92-0.96.

[0163] The roughness test method for the particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet 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 image after the particle identification and marking is completed into ImageJ software for analysis, complete the scale setting according to the scanning electron microscope image, and use the "Feret diameter", "Area", and "Solidity" analysis functions to analyze the particle size, area and roughness of the particles in the image. According to the software manual (ImageJ User Guide IJ 1.46r), the "Solidity" parameter obtained by analysis represents the ratio of the pixel area to the convex area of ​​the particle. Therefore, the "Solidity" parameter of the particles obtained by analysis 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 particles with a diameter of less than 50 nm have large errors in the statistical process and are difficult to identify accurately, and the particle size of the conductive agent is generally less than 50 nm, which will cause large errors in the statistical results, particles with a diameter of less than 50 nm are not counted in the particle size statistics process of this application, and the particle statistics corresponding to the Solidity display of "NaN" are deleted. According to the above method, in order to meet the number of statistically significant samples, each electrode is collected with no less than 10 scanning electron microscope images with non-overlapping fields of view. The roughness of at least 5,000 particles obtained are arranged in order from small to large, and the roughness cumulative distribution curve of the particles in the positive electrode film layer is obtained with the roughness as the horizontal axis and the cumulative area ratio as the vertical axis. R A50 It is the roughness R value corresponding to when the cumulative area of ​​the vertical axis in the roughness R value cumulative distribution curve accounts for 50%.

[0164] In some embodiments, 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, the median roughness R A50 The values ​​can be selected as 0.92, 0.93, 0.94, 0.95, 0.96 or any range therebetween.

[0165] Those skilled in the art can adjust the roughness of the particles by any known process. For example, the roughness of the particles can be adjusted by grinding, polishing, grinding, milling, electroplating, calendering, and other processes, as well as by adjusting the parameters of each process.

[0166] Median roughness R A50 The surface of particles within the above range is relatively smooth, the friction between particles is relatively small, and they are easy to slip under the action of external force, which can further improve the compaction density of the electrode and increase the energy density of the battery.

[0167] 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.

[0168] The iron dissolution rate of the positive electrode film layer can be tested in the following way. Specifically, the electrode was disassembled and cleaned from the battery, and then filled into small discs with a diameter of 14 mm. Several small disc samples were taken so that the total mass of the sample was about 5 g. The samples were added to 100.3 g of ascorbic acid solution with a mass concentration of 0.3% (the solvent was ultrapure water). After stirring at 500 revolutions per minute for 5 minutes, the solution was quickly aspirated using a 5 mL syringe, and the solution was filtered into a test tube using a 0.45 μm pore filter. 1 mL of the supernatant was aspirated with a pipette, added to a glass volumetric flask and diluted 50 times. The solution was tested using an inductively coupled plasma mass spectrometer (ICP-OES) to obtain the iron concentration in the solution. The formula: [(ICP test iron concentration × solution volume / mass of the fixed volume solution) × 100.3 g / (mass of the electrode of the small disc - mass of the current collector of the small disc)], the solution volume was 50 mL, and the mass of the fixed volume solution was 1 g, and the iron dissolution rate of the positive electrode film was calculated. Preferably, the mass of the current collector of the small disc is obtained by multiplying the thickness of the small disc by the area and the density. The thickness of the small disc can be equivalent to the thickness of the current collector in the uncoated area by measuring it with a thickness gauge. It is understandable that although the current collector in the coated area will expand during the compaction process, resulting in a slight decrease in thickness compared to the uncoated area, the decrease is negligible and 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 .

[0169] In some embodiments, the iron dissolution rate of the positive electrode film layer can be selected as 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1100ppm, 1200ppm, 1300ppm, 1400ppm, 1500ppm, 1600ppm, 1700ppm, 1800ppm, 1900ppm, 2000ppm or any numerical range therebetween.

[0170] Those skilled in the art can control the iron dissolution rate of the positive electrode film layer by any known process. As an example, the iron dissolution rate of the positive electrode film layer can be controlled by controlling the surface coating quality of the positive electrode film layer, the temperature, time, and pressure during the preparation process.

[0171] The iron dissolution rate can indirectly reflect the integrity and density of the carbon coating on the surface of the positive electrode active material. The lower the iron dissolution rate, the less likely it is that iron ions will precipitate from the carbon coating after acid dissolution, that is, the more complete and dense the carbon coating on the surface of the positive electrode active material. Positive electrode active materials with an iron dissolution rate within the above range have a relatively complete and dense carbon coating, 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 lithium-ion secondary batteries. 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 pole piece and the energy density of the battery.

[0172] In some embodiments, based on the total mass of the positive electrode active material, the mass content of carbon element is 0.8%-1.8%, and optionally 0.90%-1.5%.

[0173] The mass content of carbon element can be measured based on the total mass of the positive electrode active material by methods and equipment known in the art. For example, the mass content can be measured using a Dekai HCS infrared carbon and sulfur analyzer, referring to GB / T 21023-2006 "Determination of total carbon and sulfur content of steel - Infrared absorption method after high-frequency induction furnace combustion."

[0174] In some embodiments, based on the total mass of the positive electrode active material, the mass content of carbon element can be selected as 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 therebetween.

[0175] Compared with the lithium-containing transition metal phosphate positive electrode active materials in the prior art, the positive electrode active material has a relatively low carbon coating content, which can further increase the loading amount of the lithium-containing transition metal phosphate in the positive electrode sheet and improve the energy density of the lithium-ion secondary battery.

[0176] In some embodiments, the positive electrode active material has a lithium iron antisite defect concentration of 0.1% to 1.5%.

[0177] XRD data for the sample were collected using an X-ray diffractometer, and phase analysis was performed. A CIF file of the phase, obtained from an open-source website, was used as an initial crystal structure model, defining unit cell parameters, atomic positions, and occupancy probabilities. In this initial crystal structure model, the possible Li content at the Fe site and the possible Fe content at the Li site were set to 0.1%, taking into account the possibility of Fe-Li antisites. FullProf Suite software was used to fit and refine the collected XRD data, refining parameters in the order of background parameters, peak intensity, unit cell parameters, and peak shape. When the fitted peak shape and experimental peak shape were optimally matched, with Rwp less than 10, the refined Li and Fe occupancy probabilities were obtained and used as the lithium-iron antisite defect concentration.

[0178] In some embodiments, the lithium iron antisite defect concentration of the positive electrode active material can be selected as 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 any range therebetween.

[0179] Those skilled in the art can control the antisite defects of the lithium-iron cathode active material by any known process. For example, the antisite defects of the lithium-iron cathode active material can be controlled by controlling the sintering temperature, sintering time, preparation method, raw material stoichiometry, etc.

[0180] During the preparation and circulation process, it is inevitable that there will be certain lithium vacancies in the crystal structure of the positive electrode active material. Lithium vacancies not only cause ferrous ions to be oxidized to ferric ions, but also induce the migration of ferric ions to the lithium position, forming lithium-iron anti-site defects, blocking the one-dimensional diffusion channel of lithium ions, and adversely affecting the solid-phase transmission of lithium ions. The positive electrode active material in the embodiment of the present application has low lithium-iron anti-site defects, which is conducive to the uniform transmission of lithium ions in the solid phase, further improving the kinetic performance of lithium-ion secondary batteries.

[0181] In some embodiments, the positive electrode active material has a lithium iron antisite defect concentration of 0.3% to 1.0%.

[0182] In some embodiments, the lithium-containing transition metal phosphate includes a component having the following general formula:

[0183] Li m Fe x P y O j Q q ,

[0184] 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, and Br, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.1.

[0185] In some embodiments, m can be selected from 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 any range therebetween; x can be selected from 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, or any range therebetween. ; y can be selected as 0.95, 0.96, 0.97, 0.98, 0.99, 1.00 or any numerical range therebetween; j can be selected as 3.5, 3.6, 3.7, 3.8, 3.9, 4 or any numerical range therebetween; 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 any numerical range therebetween.

[0186] Selecting an appropriate modifying element Q can improve the ion diffusion path of the positive electrode active material, improve the lithium ion diffusion rate of the positive electrode active material, and improve the kinetic performance of the battery.

[0187] In some embodiments, the positive electrode active material includes one or more of lithium iron phosphate and its doped modified materials and coated modified materials.

[0188] In some embodiments, the positive electrode active material includes titanium, and the mass content of the titanium is 2000 ppm-6000 ppm based on the total mass of the positive electrode active material.

[0189] The type and content of elements in the positive electrode active material can be tested by any method known in the art. As an example, titanium element and content are tested using inductively coupled plasma optical emission spectrometry according to Appendix C of GB / T 33822-2017.

[0190] In some embodiments, based on the total mass of the positive electrode active material, the mass content of titanium element can be selected as 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm, 5500ppm, 6000ppm or any numerical range therebetween.

[0191] Doping positive electrode active materials with titanium can induce lattice distortion, reduce Li-O bond energy, increase lithium ion transport rate, and improve the kinetic performance of lithium-ion secondary batteries. However, in existing technologies, the titanium doping level in lithium-containing transition metal phosphates often cannot exceed 3000 ppm. This is because excessive titanium cannot fully enter the bulk of the lithium-containing transition metal phosphate and easily remains as a harmful impurity on the surface, negatively impacting battery performance.

[0192] The positive electrode active material in the embodiments of the present application has a high titanium content, and surprisingly, the high addition of titanium does not form a harmful impurity phase that has a negative impact on the energy density and kinetic performance of the battery. Although the reason is not yet clear, it is speculated that titanium may form a fast ion conductor together with phosphate and other elements (for example, lithium), which in turn improves the kinetic performance of the battery.

[0193] In some embodiments, the powder compaction density of the positive electrode active material under a pressure of 3T is 2.50 g / cm 3 -2.70g / cm 3 .

[0194] In this application, the term "powder compaction density" refers to the density of a compact with a certain density and strength formed during the external compression process. As the powder moves and deforms, larger gaps are filled, the contact area between particles increases, the attraction between atoms is generated, and the mechanical fit between particles is enhanced. The unit is g / cm 3 .

[0195] The compaction density of the positive electrode active material powder can be measured using methods and equipment known in the art. For example, it can be measured using a compaction density instrument, as described in GB / T 24533-2009. Specifically, a certain amount of positive electrode active material is placed in a dedicated compaction mold (of known diameter). The mold has a hollow center with two metal discs above and below. The positive electrode active material is placed between the metal discs, with a metal cylinder placed on top. The mold is then placed on the compaction density instrument. The mold bottom area should be 1.327 cm. 2 , set the pressure to 3T, and the thickness of the positive electrode active material under a pressure of 3T can be read on the equipment. The powder compaction density of the positive electrode active material is ρ=m / v, where v=(S×H), m is the mass of the positive electrode active material, S is the bottom area of ​​the mold, and H is the thickness of the positive electrode active material after compaction.

[0196] In some embodiments, the powder compaction density of the positive electrode active material under 3T pressure can be 2.50 g / cm 3 , 2.51g / cm 3 , 2.52g / cm 3, 2.53g / cm 3 , 2.54g / cm 3 , 2.55g / cm 3 , 2.56g / cm 3 , 2.57g / cm 3 , 2.58g / cm 3 , 2.59g / cm 3 , 2.60g / cm 3 , 2.61g / cm 3 , 2.62g / cm 3 , 2.63g / cm 3 , 2.64g / cm 3 , 2.65g / cm 3 , 2.66g / cm 3 , 2.67g / cm 3 , 2.68g / cm 3 , 2.69g / cm 3 , 2.70g / cm 3 or any range of values ​​between them.

[0197] Although the area proportion of particles with a particle size greater than or equal to 1.5 μm in this positive electrode active material is low, due to its high degree of graphitization, the positive electrode active material can still achieve a high compaction density under the action of external force, providing a material basis for improving the compaction density of the electrode sheet and preparing high-energy-density lithium-ion secondary batteries.

[0198] In some embodiments, the powder compaction density of the positive electrode active material at a pressure of 3T is 2.52 g / cm 3 -2.68g / cm 3 .

[0199] In some embodiments, the tap density of the positive electrode active material is 0.70 g / cm 3 -1.50g / cm 3 , optional 0.70g / cm 3 -1.20g / cm 3 .

[0200] The tap density of the powder can be measured by any method known in the art.

[0201] As an example, turn on the electronic balance, first use the conical flask as a base and place it on the electronic balance, then reset the electronic balance to zero; take a vibrating graduated cylinder and place it on the conical flask to weigh and record the weight of the graduated cylinder; open the sample bag, use a clean sample spoon to stir the sample in the sample bag for 3-5 times to mix it, and then transfer the sample steadily into the graduated cylinder; wipe the powder on the surface of the tube with dust-free paper, and then put it into the zeroed conical flask to weigh; seal the mouth of the graduated cylinder with sealing film, and place the vibrating graduated cylinder into the matching instrument rubber ring to keep it in the vacuum chamber. Verify that the vibrated measuring cylinder fits tightly to the rubber ring and remains perpendicular to the instrument surface; set the vibration frequency on the instrument to 250 times / min and the number of vibrations to 5000 times, press the button and vibrate for 20 minutes; then remove the TD tube, use a flashlight to illuminate the surface of the measuring cylinder, and use the visual method to read the highest scale V1 and the lowest scale V2, and take the average value V of the two; subtract the mass of the measuring cylinder m0 from the mass of the measuring cylinder and sample m1 to obtain the powder mass m, and the sample tap density is obtained by the density formula ρ=m / v.

[0202] In some embodiments, the tap density of the positive electrode active material powder can be 0.70 g / cm 3 , 0.75g / cm 3 、0.80g / cm 3 , 0.85g / cm 3 , 0.90g / cm 3 , 0.95g / cm 3 , 1.00g / cm 3 , 1.05g / cm 3 , 1.10g / cm 3 , 1.15g / cm 3 , 1.20g / cm 3 , 1.25g / cm 3 , 1.30g / cm 3 , 1.35g / cm 3 , 1.40g / cm 3 , 1.45g / cm 3 , 1.50g / cm 3 or any range of values ​​between them.

[0203] The positive electrode active material in the embodiment of the present application has limited effective grading formed independently and has a relatively low tap density. However, due to the high degree of graphitization of the positive electrode film, it is easy to slip under the action of external force to achieve an increase in the compaction density.

[0204] In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 8 MPa is 0.5 Ω·cm-30.0 Ω·cm.

[0205] The powder resistivity of the positive electrode active material can be measured using methods and equipment known in the art. For example, it can be measured using a powder resistivity meter (Suzhou Jingge, model ST2722) in accordance with 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. A pressure of 8 MPa is applied, and the forward and reverse resistivities of the positive electrode active material are measured. The average of the two values ​​is taken as the powder resistivity of the positive electrode active material.

[0206] In some embodiments, the powder resistivity of the positive electrode active material under a pressure of 8 MPa may be 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.

[0207] 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 low powder resistivity, which is beneficial to increasing the solid-phase transmission rate of electrons and further improving the kinetic performance of the battery.

[0208] 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.

[0209] 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.

[0210] In this application, the positive electrode active material was assembled into a button cell and its electrical performance was tested on a blue battery tester. At 25±5°C, within the voltage range of 2.0V to 3.75V, the battery was charged at a constant current of 1C to 3.75V. After a 5-minute pause, the battery was charged at a constant voltage to a cutoff current of 50μA, and then discharged at a constant current of 1C to 2.0V. The discharge capacity of the button cell divided by the mass of the positive electrode active material was calculated as the discharge capacity in grams of the positive electrode active material at room temperature and a 1C discharge rate.

[0211] 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 in a mass ratio of 0.9:0.05:0.05, and then organic solvent NMP (N-methylpyrrolidone) is added. After thorough mixing, the mixture is coated with a 150μm scraper and dried at 100℃ for 2h. The compacted density is 2.0g / cm 3 -2.2g / cm 3Compact the positive electrode sheet and use a hole punch to punch it into a disc with a diameter of 14 mm. Then weigh and record the weight. Place the weighed positive electrode sheet in a vacuum drying oven (105°C, 1-12hrs, -90kPa). After drying, place the positive electrode sheet in a glove box and assemble the battery in the order of negative electrode shell-nickel mesh-lithium sheet-diaphragm-positive electrode sheet-positive electrode shell. Add 65-87μL (pipette gun) 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 LiPF6). The negative electrode is on top and placed in the groove of the sealing machine with a sealing pressure of 650kg / cm 2 , use insulated tweezers to remove the button battery and put it into a dust-free bag, remove the glove box, and place it in a constant temperature room for 3 hours to obtain the button battery for testing.

[0212] It is understood that the discharge capacity in grams of the positive electrode active material can also be obtained by disassembling the battery, obtaining the positive electrode sheets, assembling them into button cells according to the method described above, and then testing them.

[0213] In some embodiments, the discharge capacity of the positive electrode active material at room temperature at a discharge rate of 1C may be 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 any range therebetween.

[0214] The positive electrode active material has a high discharge gram capacity at a 1C rate, indicating that it has good charge and discharge capabilities, which is beneficial to improving the kinetic performance of the battery.

[0215] In some embodiments, 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, a 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 a rate of 1C, the capacity value extracted at the discharge voltage of 3.2V is recorded as C1, and the capacity value extracted at the discharge voltage to 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.

[0216] 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 with reference 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 twice at a constant current of 0.1C in the voltage range of 2.0V~3.75V. After constant current charging to the cut-off voltage, it is charged at a constant voltage to a current of 50μA, and then charged and discharged once at a constant current of 1C. In the charge and discharge test at a rate of 1C, the capacity value from 3.75V to 3.2V is recorded as C1, the capacity value from 3.75V to 2.0V is C2, and η=C1 / C2.

[0217] In some embodiments, n 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 range therebetween.

[0218] In some embodiments, the discharge capacity ratio n of the positive electrode active material in a freshly prepared lithium ion secondary battery when discharged to 3.2 V is ≥ 88%. After the freshly prepared lithium ion secondary battery is charged and discharged at a constant current rate of 0.1C within a voltage range of 2.0 V to 3.75 V for a period of time, the discharge capacity ratio n of the positive electrode active material when discharged to 3.2 V can be maintained at ≥ 85%.

[0219] The high percentage of discharge capacity of the positive electrode active material used in the lithium-ion secondary battery of the present embodiment at 3.2V indicates that the positive electrode active material has good kinetic performance. Furthermore, a high η value indicates that the lithium-ion secondary battery containing the positive electrode active material maintains a high voltage when discharged to a low state of charge (SOC), which is conducive to maintaining good power performance.

[0220] In some embodiments, a 0.1C discharge curve of a button cell comprising the positive electrode active material has a discharge platform within a voltage range of 2.5V to 2.9V.

[0221] A discharge plateau typically refers to a region of relatively stable voltage during the battery's charge and discharge process. During discharge, current flows out of the battery, and the battery voltage initially drops, but then enters a relatively stable region with minimal voltage fluctuations. This stable voltage region is called a discharge plateau.

[0222] Button cells can be made by disassembling the positive electrode sheet from a lithium-ion secondary battery and combining it with lithium metal. They can also be assembled and prepared according 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 blue light tester. Within the voltage range of 2.0V to 3.75V, it is charged at a constant current of 0.1C to 3.75V, then paused for 5 minutes, and then charged at a constant voltage to a cutoff current of 50μA, and then discharged at a constant current of 0.1C to 2.0V.

[0223] The discharge curves show that the standard charge-discharge platform voltage for lithium-containing transition metal phosphates is typically between 3.2V and 3.65V. Coin-type batteries containing the cathode active material of the present embodiment exhibit a new charge-discharge platform within the 2.5V-2.9V voltage range, which helps increase the battery's discharge range and improve its energy density. This also confirms the hypothesis that the cathode active material of the present embodiment contains a fast ion conductor.

[0224] 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%.

[0225] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the conductive agent can be selected as 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 any numerical range therebetween.

[0226] 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.

[0227] The carbon layer of the positive electrode active material has a high degree of graphitization, which makes the positive electrode active material have good electronic conductivity, can reduce or even eliminate the use of conductive agents in the positive electrode film layer, and is conducive to further increasing the loading amount of the positive electrode active material and improving the energy density of the lithium ion secondary battery.

[0228] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the conductive agent is 0.

[0229] The positive electrode active material has extremely high electronic conductivity, so that no conductive agent may even be added to the positive electrode film layer, which is beneficial to further increase the loading amount of the positive electrode active material and improve the energy density of the lithium ion secondary battery.

[0230] 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%.

[0231] 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 fluorine-containing acrylate resin.

[0232] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material may be 95.5%, 96%, 96.5%, 97%, 98%, 99%, 99.5% or any range therebetween.

[0233] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the binder can be selected as 0.5%, 1%, 1.5%, 2%, 2.5%, 3% or any numerical range therebetween.

[0234] In some embodiments, the single-side density of the positive electrode film layer is 300 mg / 1540 mm 2 -450mg / 1540mm 2 .

[0235] In the present application, the single-side density of the positive electrode film layer has a 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 discs with an area of ​​S1, weigh it, and record it as M1. Then wipe off the positive electrode film layer of the above-mentioned weighed positive electrode sheet, weigh the weight of the current collector, and record it as M0. The single-side density of the positive electrode film layer = (M1-M0) / S1. In order to ensure the accuracy of the test results, multiple groups (for example, 10 groups) of test samples can be tested, and the average value can be calculated as the test result.

[0236] In some embodiments, the single-side density of the positive electrode film layer can be selected to be 300 mg / 1540 mm 2 、310mg / 1540mm 2 、320mg / 1540mm 2 、330mg / 1540mm 2 、340mg / 1540mm 2 、350mg / 1540mm 2 、360mg / 1540mm2 、370mg / 1540mm 2 、380mg / 1540mm 2 、390mg / 1540mm 2 , 400mg / 1540mm 2 、410mg / 1540mm 2 、420mg / 1540mm 2 、430mg / 1540mm 2 , 440mg / 1540mm 2 、450mg / 1540mm 2 or any range of values ​​between them.

[0237] The positive electrode film layer having an area density within the above range can contribute to improving the energy density of the lithium-ion secondary battery.

[0238] In some embodiments, 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 .

[0239] In some embodiments, 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 .

[0240] In this application, the fully discharged state refers to the state after the battery is placed in a 25°C oven environment, left to stand for 2 hours, and the battery temperature is maintained at 25°C, the battery is discharged at a constant current of 1 / 3C to 2.5V, and then discharged at a constant current of 0.1C to 2.0V.

[0241] The compaction density of the positive electrode film layer can be tested using methods known in the art. As an example, place the battery in a 25°C oven environment and let it stand for 2 hours. When the battery temperature remains at 25°C, discharge the battery at a constant current of 1 / 3C to 2.5V and then at a constant current of 0.1C to 2.0V. Disassemble the battery to obtain the positive electrode plate. Treat the residual electrolyte with dimethyl carbonate solvent, dry the plate, and cut it into small discs with an area of ​​S. The mass is W1, and the thickness of the positive electrode plate is measured using a micrometer to obtain T1. Then, wipe off the positive electrode film layer of the weighed plate, weigh the mass of the current collector, record it as W2, and use a micrometer to measure the thickness of the current collector T2. The compaction density of the positive electrode film layer PD = (W1-W2) / [(T1-T2)×S].

[0242] In some embodiments, when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer can be selected to be 2.51 g / cm 3 , 2.52g / cm3 , 2.53g / cm 3 , 2.54g / cm 3 , 2.55 / cm 3 , 2.56g / cm 3 , 2.57g / cm 3 , 2.58g / cm 3 , 2.59g / cm 3 , 2.60g / cm 3 , 2.61g / cm 3 , 2.62g / cm 3 , 2.63g / cm 3 , 2.64g / cm 3 , 2.65g / cm 3 , 2.66g / cm 3 , 2.67g / cm 3 , 2.68g / cm 3 , 2.69g / cm 3 , 2.70g / cm 3 , 2.71g / cm 3 , 2.72g / cm 3 , 2.73g / cm 3 or any range of values ​​between them.

[0243] In some embodiments, after the compaction process, the compaction density of the positive electrode film layer is 2.63 g / cm 3 -2.85g / cm 3 .

[0244] In some embodiments, after the compaction process, the compaction density of the positive electrode film layer can be selected to be 2.63 g / cm 3 , 2.64g / cm 3 , 2.65g / cm 3 , 2.66g / cm 3 , 2.67g / cm 3 , 2.68g / cm 3 , 2.69g / cm 3 , 2.70g / cm 3 , 2.71g / cm 3 , 2.72g / cm 3 , 2.73g / cm 3 , 2.74g / cm 3 , 2.75g / cm 3 , 2.76g / cm 3 , 2.77g / cm 3 , 2.78g / cm 3 g / cm3 , 2.79g / cm 3 , 2.80g / cm 3 , 2.81g / cm 3 , 2.82g / cm 3 , 2.83g / cm 3 , 2.84g / cm 3 , 2.85g / cm 3 or any range of values ​​between them.

[0245] In this application, the term "compaction" refers to compacting the positive electrode film layer by mechanical pressure during the battery assembly process to improve its density and conductivity.

[0246] In some embodiments, after the chemical formation process, the compaction density of the positive electrode film layer is 2.53 g / cm 3 -2.73g / cm 3 .

[0247] In some embodiments, after the chemical formation process, the compaction density of the positive electrode film layer can be selected to be 2.53 g / cm 3 , 2.54g / cm 3 , 2.55g / cm 3 , 2.56g / cm 3 , 2.57g / cm 3 , 2.58g / cm 3 , 2.59g / cm 3 , 2.60g / cm 3 , 2.61g / cm 3 , 2.62g / cm 3 , 2.63g / cm 3 , 2.64g / cm 3 , 2.65g / cm 3 , 2.66g / cm 3 , 2.67g / cm 3 , 2.68g / cm 3 , 2.69g / cm 3 , 2.70g / cm 3 , 2.71g / cm 3 , 2.72g / cm 3 , 2.73g / cm 3 or any range of values ​​between them.

[0248] In this application, formation refers to the formation of a stable solid electrolyte interface (SEI film) and electrode structure through electrochemical reactions during the first charge and discharge process of the battery.

[0249] It can be understood that, with the rebound of the electrode during the cycle, the compaction density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery is slightly lower than the compaction density of the positive electrode film layer after compaction and formation.

[0250] The compaction density of the positive electrode film layer is within the above range, which is beneficial to improving the energy density of the lithium-ion secondary battery.

[0251] In some embodiments, the compaction density of the positive electrode film layer is 2.51 g / cm 3 -2.73g / cm 3 In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10%-22%.

[0252] In some embodiments, 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%.

[0253] In some embodiments, 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 may be selected to be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22% or any numerical range therebetween.

[0254] The porosity of the positive electrode film layer in the cross-section along the thickness direction of the electrode can be tested as follows. Import the cross-section scanning electron microscope image of the positive electrode film layer along the thickness direction of the electrode obtained in the manner described above into ImageJ software, select the straight line tool, use the straight line to mark the ruler length in the image, click "Analyze SetScale", and set the ruler parameters in the software according to the ruler length in the image. Select the rectangle tool, select the part of the image outside the ruler area, use "Image Duplicate" to copy the selected area, and use "Image Type 8 bit" to adjust the image format; select "Analyze Set Measurements", select the following 5 options: "Area", "Mean gray value", "Area Fraction", "Limit to threshold", "Feret's diameter", among which "Decimal places" selects 3, select "Image"-"Adjust"-"Threshold" in sequence, set 0 and 100 in the "Threshold" box position, and then use the Analyze-Measure function to export the pore data in the cross-section scanning electron microscope image. Use "Image" - "Overlay" - "Flatten" to export and obtain the pore image; click "Apply" in "Threshold", then click "Analyze" - "Analyze Particles", check the four columns on the left, and you can get the pore statistics.

[0255] It is understood that the "pores" in the positive electrode film cross-section are identified by image color difference and threshold in the embodiments of this application. This "pore" is not the pore data obtained in the exhaust test, but is mainly used to characterize the cross-sectional area between particles in the positive electrode film cross-section. This method is superior to the exhaust method because the porosity obtained by the exhaust method is related to the pores between particles and the pores in the carbon layer covering the surface of the lithium iron phosphate particles, and thus cannot objectively reflect the pores between particles.

[0256] like Figure 8 As shown in the figure, the lower the porosity in the cross section of the positive electrode film tested by this method, on the one hand, it means that the gradation of large, medium and small particles in the positive electrode film is better, and the compaction density is high. On the other hand, after the same gradation and roller pressure, if the porosity is low, it means that the particles are easy to slide against each other, thereby reducing the risk of overpressure and stress concentration in the film layer, and further reducing the probability of demolding of the positive electrode film during long cycles, which is beneficial to improving the long cycle performance of the battery.

[0257] In some embodiments, the positive electrode plate includes a primer layer, which is arranged between the positive electrode film layer and the current collector; 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.

[0258] Among them, carbon-based particles refer to particles with carbon as the main component, including but not limited to conductive carbon, carbon black, etc.

[0259] The undercoat helps to improve the conductivity and adhesion between the positive electrode film and the current collector, reduce the release of the positive electrode film from the current collector during the cycle, and improve the dynamic performance of the battery. In the high compaction density electrode of the embodiment of the present application, for example, the compaction density of the positive electrode in the fully charged state is greater than or equal to 2.4g / cm 3 The current collector is easily damaged during the high-pressure compaction process of the electrode, and large-sized particles are prone to form pits on the current collector. Controlling the distribution density of carbon-based particles with a particle size greater than 100nm in the primer layer to be ≤10pcs / 10μm is beneficial to reducing the probability of damage to the current collector in the high-pressure dense electrode and further improving the ultimate compaction density of the positive electrode.

[0260] The distribution density of carbon-based particles with a particle size greater than 100 nm in the base coating layer can be determined by the method described above, by cutting the positive electrode film layer along the thickness direction of the electrode piece with an argon ion beam, taking a scanning electron microscope image or a microscope image, and detecting the size of the carbon particles in the base coating layer by a statistical method. The number of carbon-based particles with a particle size greater than 100 nm contained in every 10 μm of the base coating layer is counted, and the number is counted for no less than 5 times to calculate the average value.

[0261] The primer layer in the embodiment of the present application can be prepared by any known preparation process, such as pre-screening or centrifugation in the process of preparing carbon-based particles to remove large particles of carbon-based materials, so that the carbon-based particles added in the primer layer preparation process have a low D V50 At 20nm-60nm, D V90 The thickness is less than or equal to 70 nm, and the carbon-based material and the binder are mixed, stirred, and coated on the current collector to obtain a primer layer.

[0262] In some embodiments, the compaction density of the positive electrode sheet in the full state is greater than or equal to 2.4 g / cm 3 The thickness of the primer layer on one side is 1 μm-4 μm.

[0263] In some embodiments, the compaction density of the positive electrode sheet in the full state is greater than or equal to 2.5g / cm 3 The thickness of the primer layer on one side is 2 μm-4 μm.

[0264] As the electrode packing density increases, the compressive effect of large particles of lithium-containing phosphate material (e.g., particles larger than 1μm) in the positive electrode film on the undercoat layer becomes more pronounced. Consequently, stress concentrations are more likely to occur at the sites of these large particles, and these particles can even penetrate the undercoat layer and damage the current collector. Increasing the thickness of the undercoat layer can help alleviate stress concentration in the electrode and further increase the electrode's ultimate packing density.

[0265] The thickness of the primer layer on one side 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, the thickness of the primer layer on one side is measured at intervals of 1m. After measuring the thickness of the primer layer at 10 points, the average value is calculated. It should be noted that in the process of measuring points, it is necessary to avoid abnormal points, that is, the primer layer areas with a thickness of less than 50nm and a thickness of more than 4m; these abnormal points are mainly due to the extreme fluctuations in the thickness of individual areas caused by abnormal stress concentration and extrusion during the compaction of the electrode, and are not statistically significant.

[0266] In some embodiments, the thickness of the positive electrode current collector is less than or equal to 17 μm, and can be optionally 13 μm-15 μm.

[0267] In some embodiments, the thickness of the positive electrode current collector is 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, or any range therebetween.

[0268] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy) on a polymer substrate (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0269] In some embodiments of the lithium-ion secondary battery, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector, and the surface density of the negative electrode film layer on one side is 140 mg / 1540 mm 2 -221mg / 1540mm 2 ; and / or the compaction density of the negative electrode film layer is 1.40 g / cm 3 -1.75g / cm 3 .

[0270] The single-side density and compaction density of the negative electrode film layer can be tested by methods similar to those described above for the positive electrode film layer.

[0271] The surface density and compaction density of the negative electrode film layer are within the above ranges, which is beneficial to improving the energy density of the lithium-ion secondary battery.

[0272] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy) on a polymer substrate (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0273] In some embodiments, the negative electrode film layer includes a negative electrode active material. The negative electrode active material can be a negative electrode active material for a battery that is well known in the art. 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, lithium titanate, etc. The silicon-based material can 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 can 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 traditional materials that can be used as negative electrode active materials for batteries can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0274] In some embodiments, the negative electrode film layer may further 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).

[0275] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0276] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0277] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode collector, and after drying, compaction and other processes, the negative electrode sheet can be obtained.

[0278] In some embodiments, a lithium-ion secondary battery includes an electrolyte. The electrolyte conducts ions between the positive and negative electrodes. This application does not specifically limit the type of electrolyte, and the electrolyte may be selected based on needs. For example, the electrolyte may be liquid, gel, or solid.

[0279] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0280] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0281] 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, methylpropyl carbonate, ethylpropyl 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, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0282] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0283] In some embodiments, the lithium-ion secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0284] 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.

[0285] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0286] In some embodiments, the lithium-ion secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0287] In some embodiments, the outer packaging of the lithium-ion secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0288] A 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, and the battery device includes at least one of a battery module, a battery pack, and an energy storage battery.

[0289] A third aspect of the present application provides an electrical device comprising the lithium-ion secondary battery provided in the first aspect of the present application.

[0290] The fourth aspect of the present application provides a method for preparing a positive electrode active material, comprising: 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; grinding in a solvent to obtain a mixed slurry; and the volume distribution particle size Dv of the particles in the mixed slurry is 50 The particle size is 1 μm-4 μm; the precursor powder is obtained after drying the mixed slurry; the precursor powder is sintered to obtain the positive electrode active material; the sintering includes at least two constant temperature sintering stages, wherein the sintering temperature of the high temperature stage is 750°C-800°C.

[0291] The preparation method provided in the embodiment of the present application adjusts the content of large particles in the positive electrode active material by controlling the sintering temperature and the precursor particle size. At the same time, by using polyethylene glycol as a carbon source, the graphitization degree of the positive electrode active material is further improved by combining sintering temperature control and divalent iron catalytic reduction. In order to prepare a cross section 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%, and the median graphitization degree C 50 The positive electrode film layer with a viscosity greater than or equal to 0.95 and less than or equal to 1.20 provides the material basis.

[0292] In some embodiments, the volume distribution particle size Dv of the particles in the mixed slurry is 50 1μm-4μm.

[0293] In this application, the term "Dv 50" refers to the particle size corresponding to 50% of the sample volume cumulative particle size distribution percentage obtained by Malvern laser scattering method;

[0294] In some embodiments, the volume distribution particle size Dv of the particles in the mixed slurry is 50 The thickness can be selected as 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4.0 μm or any range therebetween.

[0295] Volume distribution particle size Dv of the mixed slurry 50 Within the above range, on the one hand, the activity of the particles can be increased to a certain extent, and some positive electrode active material particles with a particle size of 1μm-2μm can be generated at the same temperature, thereby improving the compaction density of the pole piece and the energy density of the battery; on the other hand, the catalytic decomposition efficiency of the iron element on the surface of the crystal core to the carbon source can be improved, the coating quality of the carbon source can be improved, and the uniformity and graphitization degree of the carbon coating layer can be improved, thereby further improving the compaction density of the pole piece and the energy density of the battery.

[0296] The positive electrode active material prepared by this preparation method has a small proportion of large-sized particles on the surface of the positive electrode film layer, and the positive electrode active material has a high degree of graphitization, which is easy to increase the compaction density of the electrode sheet through slippage between particles, which is beneficial to improving the energy density of the battery while improving the battery's kinetic performance.

[0297] In some embodiments, the iron source includes ferrous iron, which can be one or more of ferrous oxalate, ferrous carbonate, and ferrous nitrate.

[0298] In some embodiments, the lithium source includes one or more of lithium dihydrogen phosphate, lithium phosphate, lithium carbonate, and lithium acetate.

[0299] In some embodiments, the carbon source comprises a polymer carbon source, which may be one or more of polyethylene glycol and polyvinyl alcohol.

[0300] In some embodiments, the phosphorus source includes one or more of lithium dihydrogen phosphate, phosphoric acid, and ammonium dihydrogen phosphate.

[0301] In some embodiments, the lithium source and the phosphorus source may be the same substance.

[0302] In some embodiments, the iron source includes ferrous oxalate, the lithium and phosphorus sources include lithium dihydrogen phosphate, and the carbon source includes polyethylene glycol.

[0303] During the sintering process, the divalent iron source will preferentially decompose to generate a large amount of ferrous oxide, which will serve as a nucleation site to generate nanocrystalline cores containing lithium transition metal phosphate. At the same time, the polymer carbon source has a relatively low decomposition temperature. The iron element located on the surface of the nanocrystalline core will further catalyze the decomposition of the carbon source, so that the carbon coating layer on the surface of the positive electrode active material can have a relatively high degree of graphitization at a lower sintering temperature, reducing the resistivity of the positive electrode active material and improving the density and uniformity of the carbon coating layer on the surface of the lithium transition metal phosphate. In addition, the uniform deposition of carbon on the surface of the lithium transition metal phosphate will further hinder the growth of the lithium transition metal phosphate grains, reducing the probability of the positive electrode active material particles growing into large particles with a particle size greater than 1.5μm.

[0304] In some embodiments, the particle size D of ferrous oxalate is 10 Greater than or equal to 3μm, particle size D 50 50μm-80μm, particle size D 90 Less than or equal to 150μm.

[0305] In this application, the term "D 10 ”, “D 50 ” and “D 90 " respectively correspond to the particle sizes when the cumulative particle size distribution percentages of the sample measured by Malvern laser scattering method reach 10%, 50%, and 90%.

[0306] Control the particle size D of ferrous oxalate 10 Greater than or equal to 3μm can reduce the proportion of small-sized ferrous oxalate particles and control their reactivity during the grinding process. 50 、D 90 It helps to evenly mix the raw materials during the grinding process, obtain a mixed slurry with consistent components and uniform particle size, and improve the particle size consistency of the prepared lithium-containing transition metal phosphate.

[0307] In some embodiments, the mass content of trivalent iron is less than or equal to 0.08%.

[0308] In some embodiments, the mass content of the trivalent iron element may be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08% or any range therebetween.

[0309] Controlling the mass content of ferric iron helps improve the uniformity and consistency of the carbon coating. Excessive ferric iron content will preferentially consume the carbon source, resulting in poor consistency in the quality and thickness of the carbon coating between particles. On the one hand, the uneven thickness of the carbon coating will affect the compaction between particles. On the other hand, local carbon deficiency will affect the overlap of the conductive network between particles, which is not conducive to effectively improving the compaction density of the electrode and improving the dynamics.

[0310] In some embodiments, the atomic molar ratio of lithium to iron in the lithium source and the iron source is 1.0:1.0-1.05:1.0.

[0311] In some embodiments, the atomic molar ratio of lithium to iron 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 any range therebetween. In some embodiments, the carbon source comprises a polymeric carbon source, which can be one or more of polyethylene glycol and polyvinyl alcohol.

[0312] In some embodiments, the mass content of the carbon source is 1-4% based on the total mass of the positive electrode film layer.

[0313] 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 lower sintering temperature, hindering the growth and sintering growth of lithium-containing transition metal phosphate grains, which is beneficial to reducing the particle size of the positive electrode active material particles.

[0314] At the same time, polymer carbon sources usually have a higher molecular weight or a longer molecular chain, which can easily form a stable skeleton structure through cross-linking or orientation during heat treatment. 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 degree of graphitization.

[0315] The organic molecules in the carbon source will decompose at high temperatures, releasing carbon atoms. These carbon atoms can cover and fill the tiny gaps or defects on the surface of the active material, reducing the surface roughness. The coating layer formed by the polymer carbon source has a high degree of graphitization and a tighter carbon structure, which is beneficial to the optimization of the surface roughness of the positive electrode active material.

[0316] In some embodiments, the polyethylene glycol has a weight average molecular weight of less than 10,000.

[0317] In some embodiments, the weight average molecular weight of polyethylene glycol can be selected to be 1500, 2000, 3000, 4000, 6000, 8000 or any range therebetween.

[0318] Using polyethylene glycol with a weight average molecular weight of less than 10,000 can control the decomposition rate during sintering to form a carbon coating layer with a suitable and uniform thickness.

[0319] In some embodiments, the water content of the polyethylene glycol is less than or equal to 0.5%.

[0320] If the polyethylene glycol contains too much water, the water may affect the decomposition process, causing incomplete decomposition or uneven decomposition during sintering. Excessive water may also cause the molten polyethylene glycol to be unevenly distributed during sintering, affecting the uniformity of the carbon layer and causing the carbon coating to be unstable or fall off.

[0321] In some embodiments, the water content of polyethylene glycol can be selected to be 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5% or any range therebetween.

[0322] In some embodiments, the pH of the polyethylene glycol is 5-7.

[0323] Polyethylene glycol with a pH of 5-7 is highly stable and will not degrade during mixing due to excessive acidity, especially at high temperatures, which can lead to rapid decomposition and affect the quality of the coating. If the polyethylene glycol is alkaline, it may affect the stability of other components, causing metal ions to dissolve or oxidize, affecting the performance of the final positive electrode active material.

[0324] 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.

[0325] Titanium sources often have low surface activity. Including titanium sources in the slurry can reduce the activity of the lithium-containing transition metal phosphate precursor, inhibit the particle growth of the lithium-containing transition metal phosphate during high-temperature sintering, and form smaller particles of the lithium-containing transition metal phosphate during the sintering process.

[0326] Titanium is used as a lattice stabilizer. Titanium is usually 4+ In the form of titanium ions, some titanium ions can replace the iron ions, making the crystal structure more stable and reducing the possibility of anti-position between lithium and iron ions, especially during high temperature or high current charging and discharging.

[0327] At the same time, titanium doping helps to improve the sphericity of the particles and reduce the roughness of the particles, thereby enhancing the overall structural stability of the material.

[0328] In some embodiments, the sintering includes at least two constant temperature sintering stages, wherein the sintering temperature of the low temperature stage is 300°C-400°C, the holding time is 2 hours-6 hours, and the sintering temperature of the high temperature stage is 750°C-800°C, the holding time is 8 hours-15 hours.

[0329] In some embodiments, the heating rate from the low temperature section to the high temperature section is greater than or equal to 5°C / min.

[0330] Using a higher heating rate to quickly heat up to the target temperature is conducive to uniform particle growth and reduces the presence of particles with a particle size greater than or equal to 1.5 μm.

[0331] In some embodiments, lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide are uniformly mixed and ground in an organic solution to obtain a mixed raw material.

[0332] Organic solvents can effectively reduce the occurrence of side reactions and improve the purity and consistency of the material. Furthermore, organic solvents are highly volatile, making them easier to remove during the subsequent drying process. They will not remain within the material, causing pores and affecting its density and structural stability.

[0333] In some embodiments, based on the total mass of the mixed feedstock, the mass proportion of the carbon source in the mixed feedstock is 5%-7%.

[0334] 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 as 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 therebetween.

[0335] By controlling the mass of the carbon source and the lithium content within the above range, the conductivity of the material can be enhanced and the negative impact on the specific capacity of the positive electrode and the energy density of the battery can be reduced. Too thick a carbon layer not only occupies the effective active material space, but may also lead to structural instability of the material.

[0336] In some embodiments, the solvent includes water and mixtures thereof.

[0337] In some embodiments, obtaining the mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source comprises: adding the carbon source, the lithium source, the phosphorus source, the iron source, and the carbon source into a solvent and mixing and stirring at a stirring speed of 1400 rpm-2200 rpm.

[0338] In some embodiments, drying the mixed slurry to obtain the precursor powder includes spray drying the mixed slurry to obtain the precursor powder.

[0339] In some embodiments, after sintering the precursor, the product is subjected to air flow milling to obtain the positive electrode active material.

[0340] In some embodiments, the classification frequency of the air flow milling is 18 Hz-24 Hz, and the milling pressure is 0.45 MPa-0.65 MPa.

[0341] The classification frequency in airflow milling refers to the operating frequency of the classifier, and is generally correlated with the particle classification efficiency and particle size distribution. A higher classification frequency screens the particles in the airflow more frequently, removing larger particles and retaining smaller ones. Furthermore, a higher classification frequency may increase the number of particle collisions, subjecting irregular particles to further impact, resulting in a smoother surface and a more spherical shape.

[0342] High air pressure will cause the particles to be subjected to greater impact force, the collisions between particles will be more intense, and the surface of the particles will be subjected to stronger impact and wear, which can crush large particles into small particles. The collisions between particles will be more intense, and the surface will be easier to be trimmed, thereby improving the sphericity and surface flatness of the particles.

[0343] However, excessively high classification frequency and pulverization pressure can cause the agglomerated particles to further crack and break after being dispersed into primary particles, affecting the predetermined particle size distribution and resulting in an incomplete carbon coating. This can manifest as increased iron dissolution, negatively impacting particle slip during roller compaction, and increasing the contact and reaction between lithium-containing transition metal phosphates and external factors such as the electrolyte, negatively impacting battery cycle performance and lifespan. Therefore, it is necessary to control the classification frequency and pulverization pressure of airflow milling within an appropriate range.

[0344] The fifth aspect of the present application provides a method for preparing a positive electrode sheet, which comprises adding a binder, a conductive agent, and a positive electrode active material prepared by the preparation method of the fourth aspect in sequence, dry-mixing them, adding a solvent, stirring, and adjusting the viscosity to obtain a shipping slurry; transferring and coating the shipping slurry to at least one side of a current collector, and drying and hot pressing to obtain a positive electrode sheet.

[0345] In some embodiments, 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.

[0346] In some embodiments, 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℃.

[0347] The positive electrode active material prepared by the embodiment of the present application using the above-mentioned hot pressing process in combination with the preparation method of the fourth aspect is beneficial to further reduce the cross-sectional porosity of the positive electrode film layer, increase the ultimate compaction density of the electrode sheet, and improve the energy density of the battery.

[0348] In addition, the present application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, 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 and satellites, energy storage systems, etc., but is not limited thereto.

[0349] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0350] Figure 7 This is an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0351] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0352] Example

[0353] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0354] Example 1

[0355] (1) Preparation of positive electrode active materials

[0356] Lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide are mixed uniformly in methanol and ground to obtain a mixed raw material. The ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the molar ratio of lithium to iron is 1.03:1.0. The particle size of ferrous oxalate is D 10 The particle size is 6.1 μm and the particle size D 50 The particle size is 60.5 μm and the particle size D 90 The particle size is 105.5 μm, the mass content of Fe element in ferrous oxalate is 30.9%, and the mass content of trivalent iron element is 0.03%.

[0357] The mixed raw materials are ball milled several times in a ball mill and demagnetized to obtain a mixed slurry. The grinding times and time are controlled, and the particle size of the mixed slurry after grinding is Dv 50 3.0μm.

[0358] The mixed slurry is spray-dried to obtain a dry precursor powder material, and the dry precursor powder material has a light yellow appearance and a uniform color.

[0359] The precursor powder was placed in a sintering furnace, and the temperature was raised from 25°C to 350°C at a rate of 2°C / min under a nitrogen atmosphere and kept at this temperature for 3 hours. The temperature was then raised to a second temperature of 770°C at a rate of 5°C / min and kept at this temperature for 10 hours. After that, the temperature was lowered and cooled.

[0360] The obtained material was crushed by air flow pulverization with a classification frequency of 22 Hz and a pulverization pressure of 0.55 MPa to obtain a carbon-coated lithium iron phosphate positive electrode active material.

[0361] The mass content of carbon element in the positive electrode active material is 1.2%, and the median sphericity is L A50 The median roughness R is 0.719. A50 The lithium iron antisite defect concentration is 0.62%, and the powder tap density is 1.04 g / cm 3 The compacted density of the powder under 3T pressure is 2.57g / cm 3 The powder resistivity is 5.58Ω·cm under a pressure of 8Mpa; the discharge capacity at a discharge rate of 1C is 141.4mAh / g; there is a discharge platform in the voltage range of 2.5V~2.9V, and the discharge capacity of the 3.2V discharge platform accounts for 90.52%.

[0362] (2) Preparation of positive electrode sheet:

[0363] 2.2wt% PVDF, 0.8wt% conductive carbon black, and 97.0wt% positive electrode active material were added and dry-mixed in sequence, and then N-methylpyrrolidone was added, stirred, and the viscosity was adjusted to obtain a shipping slurry; the shipping slurry was transferred and coated onto the bottom coating of the current collector aluminum foil. The bottom coating included carbon black and PVDF in a mass ratio of 1:1. The distribution density of carbon-based particles with a particle size greater than 100nm in the bottom coating was ≤10pcs / 10μm, and the thickness of the bottom coating was 2μm. After drying and hot pressing, the single-side density was 350mg / 1540cm 2 positive electrode film layer.

[0364] 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 25 rpm, the rotation speed is 500 rpm, and the pre-stirring time is 15 minutes.

[0365] The hot pressing process includes three hot roller pressing processes, and the hot roller pressing pressure increases successively, and the hot roller pressure is 35 tons, 55 tons, and 75 tons respectively; the hot roller temperature is 65°C, and before entering the hot roller compaction for the first time, the electrode is heated to 50°C.

[0366] The compaction density of the pole piece is the ultimate compaction density of the pole piece. The ultimate compaction density test method of the pole piece is as follows; the ultimate compaction density of the pole piece in this embodiment is 2.68g / cm 3 .

[0367] 17,707 particles were counted in the cross-section of the positive electrode film along the thickness direction of the electrode. The results showed that there were 79 particles with a diameter between 1.5μm and 5μm in the cross-section of the positive electrode film, accounting for 14.58% of the area. There were no particles larger than 5μm. The area of ​​particles with a diameter greater than or equal to 1μm and less than 1.5μm accounted for 19.70%. The median graphitization degree C of the positive electrode film obtained in the surface scanning mode of the laser microconfocal Raman spectrometer was 50 is 1.02, C 90 is 1.04, C 10 is 1.0, the concentration of C value (C 90 -C 10 ) / C 50 It is 0.034.

[0368] The iron dissolution rate of the positive electrode film layer is 1076ppm.

[0369] (3) Preparation of negative electrode sheet:

[0370] 95.5wt% of negative electrode active material (artificial graphite), 1.0wt% of conductive agent (conductive carbon black), 2.0wt% of binder (styrene-butadiene rubber (SBR)) and 1.5wt% of thickener (sodium carboxymethyl cellulose (CMC)) are mixed, added with deionized water and stirred, and dispersed to form negative electrode slurry. Then the negative electrode slurry is coated on both sides of the Cu foil. After both sides are completed, it is dried, cold pressed, cut and sliced ​​to prepare the negative electrode sheet. The density of the coated single side is 165mg / 1540mm 2 , compacted density is 1.60g / cm 3 .

[0371] (4) Preparation of isolation membrane

[0372] Polypropylene film is used as the isolation film.

[0373] (5) Preparation of electrolyte

[0374] In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), organic solvents ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of 1 / 1, and lithium salt LiPF6 was added and dissolved in the organic solvent. The content of LiPF6 in the solution was 1 mol / L. The mixture was stirred evenly to obtain an electrolyte.

[0375] (6) Preparation of batteries:

[0376] The positive electrode sheet, isolation film, and negative electrode sheet are stacked in order. The isolation film must be able to isolate the anode and cathode. The bare battery cell is obtained by winding, and the bare battery cell is placed in the outer packaging. The electrolyte is injected and the lithium-ion battery is finally obtained after the packaging, formation, exhaust and other processes.

[0377] The preparation methods of Examples 2 and 3 are basically the same as those of Example 1, except that the sintering temperature of the precursor powder is adjusted.

[0378] Example 2

[0379] The precursor powder was placed in a sintering furnace, and heated from 25°C to 350°C at a rate of 2°C / min under a nitrogen atmosphere and kept at this temperature for 3 hours. The temperature was then raised to a second temperature of 755°C at a rate of 5°C / min and kept at this temperature for 10 hours. Afterwards, the temperature was lowered and cooled.

[0380] Example 3

[0381] The precursor powder was placed in a sintering furnace, and the temperature was raised from 25°C to 350°C at a rate of 2°C / min under a nitrogen atmosphere and kept at this temperature for 3 hours. The temperature was then raised to a second temperature of 790°C at a rate of 5°C / min and kept at this temperature for 10 hours. After that, the temperature was lowered and cooled.

[0382] The preparation methods of Examples 4 and 5 are basically the same as those of Example 1, except that the particle size Dv of the mixed slurry after grinding is adjusted. 50 .

[0383] Example 4

[0384] The mixed raw materials are ball milled several times in a ball mill and demagnetized to obtain a mixed slurry. The grinding times and time are controlled, and the particle size of the mixed slurry after grinding is Dv 50 is 4.0μm.

[0385] Example 5

[0386] The mixed raw materials are ball milled several times in a ball mill and demagnetized to obtain a mixed slurry. The grinding times and time are controlled, and the particle size of the mixed slurry after grinding is Dv 50 1.5μm.

[0387] Example 6

[0388] 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:

[0389] 97.8 wt% of the positive electrode active material and 2.2 wt% of PVDF were mixed, and then N-methylpyrrolidone was added, stirred, and dispersed to prepare a positive electrode slurry.

[0390] The preparation methods of Examples 7 and 8 are basically the same as those of Example 1, except that the carbon source in the preparation method of the positive electrode active material is adjusted.

[0391] Example 7

[0392] Lithium dihydrogen phosphate, ferrous oxalate, a mixture of polyethylene glycol and glucose, and titanium dioxide are mixed uniformly in methanol and ground. The lithium dihydrogen phosphate and ferrous oxalate are mixed in a ratio such that the molar ratio of lithium to iron is 1.03:1.0; and the mass ratio of polyethylene glycol to glucose is 3:1.

[0393] Example 8

[0394] Lithium dihydrogen phosphate, ferrous oxalate, a mixture of polyethylene glycol and glucose, and titanium dioxide are mixed uniformly in methanol and ground. The lithium dihydrogen phosphate and ferrous oxalate are mixed in a ratio such that the molar ratio of lithium to iron is 1.03:1.0; and the mass ratio of polyethylene glycol to glucose is 1:3.

[0395] 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.

[0396] Comparative Example 1

[0397] Lithium dihydrogen phosphate, ferrous oxalate, glucose, and titanium dioxide were mixed and ground in methanol to obtain a mixed raw material. The precursor powder was placed in a sintering furnace and heated from 25°C to 350°C at a rate of 2°C / min under a nitrogen atmosphere. The temperature was then increased to a second temperature of 803°C at a rate of 5°C / min and held at this temperature for 10 hours. The temperature was then cooled.

[0398] 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 changing the carbon source to glucose.

[0399] Comparative Example 2

[0400] Mix lithium dihydrogen phosphate, ferrous oxalate, glucose and titanium dioxide in methanol and grind them evenly. Mill the mixed raw materials in a ball mill several times and demagnetize them to obtain a mixed slurry. Control the grinding times and time, and the particle size of the mixed slurry after grinding is Dv 50 The mixed slurry was spray-dried to obtain a dried precursor powder. The dried precursor powder had a light yellow, uniform appearance. The precursor powder was placed in a sintering furnace and heated from 25°C to 350°C at a rate of 2°C / min under a nitrogen atmosphere. The temperature was then raised to a second temperature of 750°C at a rate of 5°C / min and held at this temperature for 10 hours. The temperature was then cooled.

[0401] Performance Testing

[0402] Energy density testing: The lithium-ion secondary battery is left at rest at 25°C for 2 hours, ensuring the battery temperature is 25°C. After charging the battery at 0.33C at 25°C to a charge cutoff voltage of 3.65V, constant voltage charging is continued at this charge cutoff voltage until the current reaches 0.05C, at which point charging is terminated (where C represents the rated capacity of the lithium-ion secondary battery). After leaving the battery at 25°C for 1 hour, the battery is discharged at 0.33C at 25°C to a discharge cutoff voltage of 3.65V. The total discharge energy of the battery is recorded as E0.

[0403] Measure the length, width, and height of a battery cell and calculate its volume (V0) = length * width * height. The volumetric energy density of a lithium-ion secondary battery is calculated as: discharge energy (E0) / volume (V0).

[0404] The DC resistance (DCR) test method is at 25°C. After charging to 3.65V at a constant current of 0.33C, charging to a current of 0.05C at a constant voltage, then discharging to 20% SOC at 0.33C. After standing for 5 minutes, discharge with a 3C pulse for 30s. After standing for 40s, charge with 3C for 40s. After standing for 5 minutes, charge to 3.65V at a constant current of 0.33C. After charging to 0.05C at a constant voltage, discharge to 10% SOC at 0.33C. After standing for 5 minutes, discharge with a 3C pulse for 30s. 0s, stand for 40s, charge at 3C for 40s, stand for 5min, then fully charge at 0.33C, then discharge to 50% SOC at 0.33C, then stand at -25℃ for 2h, then pulse discharge at 1C for 30s, stand for 10min, then stand at 25℃ for 2h, charge at 0.33C constant current to 3.65V, then charge at constant voltage to 0.05C, then discharge to 20% SOC at 0.33C, then stand at -25℃ for 2h, then pulse discharge at 1C for 30s, stand for 10min.

[0405] The voltage before and after each pulse discharge was recorded, and the DCR under different conditions was calculated. The calculation formula was DCR = (voltage before pulse discharge after the end of static state - voltage before static state after pulse discharge) / pulse current.

[0406] The pole piece's ultimate compaction density is achieved by compacting the double-sided coated pole piece through a roller press, testing the pole piece's elongation after compaction, and evaluating the pole piece's flexibility after compaction. By increasing the pressure of the roller press, pole pieces with different compaction densities will be obtained. As the pressure increases, the pole piece's compaction density increases, the pole piece's elongation increases, and the pole piece's flexibility decreases. A pole piece with an elongation that is too high can easily cause the pole piece to warp, and a pole piece with too low flexibility can easily lead to brittle fracture. Therefore, the smaller of the compaction densities corresponding to when the pole piece's elongation is 8% or when the pole piece's flexibility is folded 3 times is defined as the pole piece's ultimate compaction density.

[0407] The compacted density is calculated by dividing the mass of the positive electrode film layer by the volume of the positive electrode film layer.

[0408] The test method for elongation is as follows:

[0409] Lay the electrode flat on a horizontal table and cut it into sections, with each electrode about 100 cm long; remove the copper foil at the edge of the electrode substrate, and be careful to keep the cut edge of the electrode parallel to the MD direction of the electrode (perpendicular to the direction of the pressing roller), ensuring that the electrode part is completely covered by the coating, and use a steel ruler to measure the length between the marking points at the same position of the length and width at the head and tail of the electrode, estimate it to 0.1mm, and record the length before compaction; after compaction, record the length between the corresponding marking points after compaction, and use (length after compaction - length before compaction) / length before compaction as the elongation of the electrode.

[0410] The test method for the number of flexible folds is shown below.

[0411] Cut the positive electrode sheet into 20×100mm 2 Fold the test specimen in the forward direction, flatten it with a 2kg roller, unfold it and check against the light to see if there is any light transmittance. If there is no light transmittance, fold it in the reverse direction, flatten it with a 2kg roller, and check against the light again. Repeat this process until there is light transmittance. Record the number of folds. Repeat the test three times and take the average value as the reference data for the flexibility of the electrode.

[0412] Test results

[0413] Table 1

[0414]

[0415] Table 2

[0416]

[0417] Table 3

[0418]

[0419] From the comparison between the embodiment and the comparative example, it can be seen that in the cross section of the positive electrode film 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 value of the positive electrode film obtained in the surface scanning mode of the laser microconfocal Raman spectrometer, the median value of the graphitization degree C is 0.01. 50 When it is greater than or equal to 0.95 and less than or equal to 1.20, the compaction density of the positive electrode sheet is improved while maintaining low internal resistance (especially low impedance at low SOC), allowing the battery to have both good energy density and dynamic performance.

[0420] From the comparison between Examples 1-8 and Comparative Example 2, it can be seen that in the cumulative distribution curve of the graphitization degree C value of the positive electrode film layer obtained in the surface scanning mode of the laser microconfocal Raman spectrometer, the median value of the graphitization degree C is 50 It is 0.97-1.13, which is beneficial to increase the electrode compaction density while maintaining low battery impedance, and improve the battery energy density while maintaining good battery dynamic performance.

[0421] From the comparison between Examples 2 and 8 and Examples 1 and Examples 3-7, it can be seen that in the cumulative distribution curve of the graphitization degree C value of the positive electrode film layer obtained in the surface scanning mode of the laser microconfocal Raman spectrometer, the median value of the graphitization degree C is 50 It is 1.0-1.10, which is conducive to maintaining a high electrode compaction density while maintaining low battery impedance, thereby achieving a balance between the battery's dynamic performance and energy density.

[0422] From the comparison between Example 2 and Examples 1 and 3-8, it can be seen that in the cross-section of the positive electrode film layer along the thickness direction of the electrode, the area of ​​particles with a particle size of 1.5μm-5μm accounts for 10.0%-20.0%, which is beneficial to improving the electrode compaction density while maintaining low impedance of the battery, and improving the energy density of the battery while maintaining good dynamic performance of the battery.

[0423] From the comparison between Example 6 and Example 1, it can be seen that the lithium ion secondary battery of the embodiment of the present application still has good dynamic performance when no conductive agent is added, so that the energy density of the lithium ion secondary battery is further improved.

[0424] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the 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 provided 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 provided on at least a portion of the surface. In a cross-section of the positive electrode film along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1.5 μm accounts for 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 obtained in the surface scanning mode of the laser microconfocal 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 1 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, wherein In the cumulative distribution curve of the graphitization degree C of the positive electrode film obtained in the surface scanning mode of the laser microconfocal 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, wherein In the cumulative distribution curve of the graphitization degree C of the positive electrode film obtained in the surface scanning mode of the laser microconfocal 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, wherein 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 microconfocal Raman spectrometer, the concentration of the C value (C 90 -C 10 ) / C 50 It is 0.01-0.

04.

5. The lithium-ion secondary battery according to claim 1, wherein 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 microconfocal Raman spectrometer, the concentration of the C value (C 90 -C 10 ) / C 50 It is 0.02-0.

04.

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

30.

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

15.

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

10.

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

08.

10. The lithium-ion secondary battery according to claim 1, wherein 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 of 1.5 μm-5 μm accounts for 9.0%-20.0%.

11. The lithium-ion secondary battery according to claim 1, wherein 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 of 1.5 μm-5 μm accounts for 10.0%-20.0%.

12. The lithium-ion secondary battery according to claim 1, wherein In a 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 5 μm is 0.

13. The lithium-ion secondary battery according to claim 1, wherein 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, wherein 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 16.0%-24%.

15. The lithium-ion secondary battery according to claim 13, wherein 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 16%-20%.

16. The lithium-ion secondary battery according to claim 1, wherein In the cumulative distribution curve of particle sphericity obtained from the cross section of the positive electrode film along the thickness direction of the electrode, the median of sphericity L A50 It is 0.60-0.

85.

17. The lithium-ion secondary battery according to claim 16, wherein: In the cumulative distribution curve of particle sphericity obtained from the cross section of the positive electrode film along the thickness direction of the electrode, the median of sphericity L A50 It is 0.65-0.

80.

18. The lithium-ion secondary battery according to claim 1, wherein 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, the median of the roughness R A50 It is 0.92-0.

96.

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

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

21. The lithium-ion secondary battery according to claim 1, wherein 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, wherein 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, and Br, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 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 doping modified materials and coating modified materials.

27. The lithium-ion secondary battery according to claim 1, characterized in that The positive electrode active material includes titanium element. Based on the total mass of the positive electrode active material, the mass content of the titanium element is 2000 ppm-6000 ppm.

28. The lithium-ion secondary battery according to claim 1, wherein 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 has a powder compaction density of 2.52 g / cm under a pressure of 3T 3 -2.68g / cm 3 .

30. The lithium-ion secondary battery according to claim 1, wherein 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 discharge rate of 1 C 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 at the discharge voltage to 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, a discharge platform exists 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, wherein 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, wherein The lithium-ion secondary battery is in a fully discharged state, and 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, wherein 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 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 2.4g / cm 3 The thickness of the primer layer on one side 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 full state is greater than or equal to 2.5g / cm 3 The thickness of the primer layer on one side 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 comprising a carbon source, a lithium source, an iron source, and a phosphorus source is obtained; the carbon source comprises polyethylene glycol; the iron source comprises 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 constant temperature sintering stages, 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 according to claim 47 in sequence, dry-mixing them, adding a solvent, and stirring to obtain a delivery slurry; transferring and coating the delivery slurry to at least one side of a current collector, and drying and hot pressing to obtain a positive electrode sheet.

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℃.

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