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 disclosure 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 sheet. The lithium-ion secondary battery includes a positive electrode sheet, a negative electrode sheet, and an electrolyte. The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed 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 disposed on at least a portion of the surface. In a cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the D of the particles is A90 is 1400nm-2100nm, and the particle size concentration (D A90 ‑D A10 ) / D A50 is 1.855-2.375, where D A90 、D A50 、D A10 It refers to the particle size corresponding to the cumulative area distribution of the particles when the cumulative area distribution reaches 90%, 50%, and 10% in the particle area cumulative distribution curve.

Description

[0001] This application claims priority to PCT international application No. PCT / CN2025 / 085921 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, 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] Positive electrode active materials are a crucial component of 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 a pressing technical challenge in this field. Summary of the Invention

[0005] In view of the above problems, the present application provides a lithium-ion secondary battery, which regulates the particle size concentration and large particle size in the positive electrode film layer, forms a dense stacking through reasonable grading, has high energy density, optimizes the lithium ion diffusion and conduction path, and takes into account the kinetic performance of the secondary battery.

[0006] The first aspect of the present application provides a lithium-ion secondary battery, comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer disposed on at least one side of the positive electrode current collector, wherein the positive electrode film layer comprises a positive electrode active material, wherein the positive electrode active material comprises lithium-containing transition metal phosphate particles having a carbon coating material disposed on at least a portion of the surface thereof, wherein in a cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the D A90 1400nm-2100nm, particle size concentration (D A90 -D A10 ) / D A50 is 1.855-2.375, among which D A90 、D A50、 D A10It refers to the particle size corresponding to the cumulative area distribution of the particles when the cumulative area distribution reaches 90%, 50%, and 10% in the particle area cumulative distribution curve.

[0007] In the prior art, the electrode compaction density and battery energy density are often improved by increasing the particle size concentration of the particles in the positive electrode film layer. However, research shows that contrary to the prior art, in the cross-section of the positive electrode film layer along the thickness direction of the electrode, excessive particle size concentration has limited effect on improving the compaction of the electrode, and it is difficult to further improve the electrode compaction by adjusting the particle size concentration. The particle size concentration of the particles is less than 1.855, which means that the difference in particle size distribution is small, the grading effect is not significant, and there is a lack of smaller particles to fill the gaps between larger particles. When the concentration exceeds 2.375, it means that the size difference of the active material particles is significant, which makes it impossible for the positive electrode film layer to achieve optimal dense packing, and it is difficult to achieve further improvement in the electrode compaction density. The common way to adjust the particle size concentration in the prior art is to increase the size or proportion of large particles, but the applicant further found that when the D A90 When the particle size is larger than 2100 nm, the large particles in the positive electrode film layer will significantly deteriorate the kinetic performance of the battery.

[0008] In the embodiment of the present application, by controlling the particle size concentration (D A90 -D A10 ) / D A50 1.855-2.375, D A90 The particle size is 1400nm-2100nm, which improves the accumulation of particles in the positive electrode film layer, increases the compaction density of the pole piece, and improves the energy density of the battery while maintaining high dynamic performance of the battery, thereby achieving a balance between battery dynamic performance and energy density.

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

[0010] In the cross section of the positive electrode film along the thickness direction of the electrode, the particle size D A90 Within the above range, the internal resistance of the battery can be reduced, further suppressing the negative impact of excessively large particles on the dynamic performance.

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

[0012] Particle size D A50On the one hand, the above range is conducive to maintaining the supporting role of the particles, so that the rolling pressure is evenly transmitted between the electrode particles, and the electrode can withstand higher rolling pressure; on the other hand, it represents that the overall particle size of the particles inside the positive electrode film layer is smaller, so that the dynamic performance is maintained at a high level, and the energy density is improved while taking into account the dynamics of the battery.

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

[0014] In the cross section of the positive electrode film along the thickness direction of the electrode, the D A10 The above range indicates, on the one hand, that there are certain small particles in the film layer to form a graded gap between the filling particles, and at the same time, the particles will not agglomerate due to being too small. On the other hand, it indicates that the proportion of small-size particles is limited, which can improve the compaction of the electrode while reducing the side reaction between small particles and the electrolyte, taking into account the cycle performance of the battery.

[0015] In any embodiment, in the cumulative distribution curve of the graphitization degree C value 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 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 .

[0016] 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 during the rolling process with the help of the highly graphitized carbon structure in the coating material. On the basis of reasonable grading of the particles in the electrode sheet, the electrode sheet compaction density can be further improved even under low rolling pressure; the high degree of graphitization of the carbon layer coating the surface of the positive electrode active material is conducive to electron transmission, achieving a balance between energy density and dynamics.

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

[0018] The median of sphericity L A50The particles within the above range are approximately spherical, which helps the particles maintain good slidability when stacked, makes it easy to fill the gaps between the particles, and can further improve the compaction density of the pole piece and increase the energy density of the battery.

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

[0020] Median roughness R 50 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.

[0021] In any embodiment, in the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the concentration of roughness (R A90 -R A10 ) / R A50 The concentration value of roughness is extremely small, indicating that the overall roughness of the particles is highly consistent, which is conducive to the relative sliding between particles and makes it easier to form high-density accumulation during rolling, thereby increasing the compaction density of the electrode and the energy density of the battery.

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

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

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

[0025] In any embodiment, the positive electrode active material has a lithium iron antisite defect concentration of 0.001% to 1.5%, optionally 0.01% to 1.0%. The positive electrode active material has a low concentration of lithium iron antisite defects, which facilitates uniform transport of lithium ions in the solid phase, further improving the kinetic performance of the lithium-ion secondary battery.

[0026] In any embodiment, the lithium-containing transition metal phosphate includes a component having the following general formula: Li m Fe x P y O j Q q , 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.

[0027] Selecting an appropriate modifying element Q can improve the lattice change rate of the positive electrode active material during the lithium insertion and extraction process, reduce the oxygen activity on the particle surface, improve the structural stability of the material, and thereby increase the material's gram capacity during the cycle process, further improving the cycle stability of lithium-ion secondary batteries.

[0028] 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.

[0029] In any embodiment, the positive electrode active material includes titanium, and the mass content of titanium is 4000 ppm to 8000 ppm based on the total mass of the positive electrode active material. The high addition of titanium does not form a harmful impurity phase that negatively impacts the battery's energy density and kinetic performance. While the reason for this is unclear, it is speculated that titanium, phosphate, and other elements (e.g., lithium) may form a fast ion conductor, which in turn improves the battery's kinetic performance.

[0030] In any embodiment, the tap density of the positive electrode active material is 1.00 g / cm 3 -1.70g / cm 3 , optional 1.20g / cm 3 -1.50g / cm 3 The particles of the extremely active material not only have a wide particle size distribution, but also have a particle size within a reasonable range, forming an effective gradation. Small particles can fill the gaps between particles, thus having a higher tap density.

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

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

[0033] The positive electrode active material is coated with carbon material on the surface, and 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 improving the kinetic performance of the battery.

[0034] In any embodiment, the positive electrode active material has a discharge capacity of 135 mAh / g to 150 mAh / g at room temperature and a discharge rate of 1C. The positive electrode active material has a high discharge 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.

[0035] 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 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.

[0036] A high percentage of the cathode active material's discharge capacity at 3.2V indicates good kinetic performance. Furthermore, a high η value indicates that the cathode active material maintains a high voltage even when discharged to a low state of charge (SOC), contributing to good power performance.

[0037] In any embodiment, the mass content of the conductive agent is 0.01-1.5% based on the total mass of the positive electrode film layer. Because the particles in the positive electrode film layer are tightly packed, the positive electrode active material particles are coated with a highly graphitized carbon layer on their surface and are in full contact with each other, resulting in good electronic conductivity. This can reduce the use of the conductive agent in the positive electrode film layer, further increasing the loading of the positive electrode active material and improving the energy density of the lithium-ion secondary battery.

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

[0039] The mass content of the positive electrode active material and the mass content of the binder are within the above ranges, which can effectively increase the active material loading per unit volume of the positive electrode film layer, maintain good internal bonding strength, reduce the probability of powder loss, expansion and cracking problems, and improve the energy density of the secondary battery while taking into account safety performance.

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

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

[0042] 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 -2.75 g / cm 3 .

[0043] 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.

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

[0045] The lower the porosity in the cross section of the positive electrode film, on the one hand, means that the gradation of large, medium and small particles in the positive electrode film is better, and the compaction density is higher. 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.

[0046] 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 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 plate in a fully charged state is greater than or equal to 2.4 g / cm 3 The thickness of the bottom coating on one side is 1 μm-4 μm; (3) the compaction density of the positive electrode sheet in the full 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.

[0047] The primer layer is beneficial to improving the conductivity and adhesion between the positive electrode film layer and the current collector, reducing the demolding of the positive electrode film layer from the current collector during the cycle, and improving the dynamic performance of the battery.

[0048] 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 increasingly 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.

[0049] A second aspect of the present application provides a battery device comprising the lithium-ion secondary battery of the first aspect of the present application.

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

[0051] The fourth aspect of the present application provides a method for preparing a positive electrode active material, the preparation method comprising: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source; adding a solvent and grinding to obtain a mixed slurry; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder to obtain a positive electrode active material; the sintering of the precursor powder to obtain the positive electrode active material comprises at least two sinterings; after the first sintering, a first sintered product is obtained, the first sintered product and a carbon source are mixed to obtain an intermediate raw material, the intermediate raw material is divided into two groups, and the first group of ground products and the second group of ground products are respectively ground to obtain a first group of ground products and a second group of ground products, the first group of ground products and the second group of ground products are mixed to obtain a mixed intermediate product; the mixed intermediate product is subjected to a second sintering to obtain a positive electrode active material; wherein, the D of the first group of ground products is less than 0.05. V50 The D of the second group of grinding products is 0.8μmm-1.2μm; V50 0.3μm-0.5μm.

[0052] By regulating the mass ratio of the first group of ground products and the second group of ground products, positive electrode active materials with different gradation ratios can be obtained, so that the area ratio of particles of different particle sizes can be finely controlled to obtain the required stacking structure.

[0053] The second sintering process can effectively shorten the sintering time in the high-temperature range, thereby reducing the risk and probability of extremely large particles appearing during high-temperature sintering. By adjusting the particle size of the two groups of grinds during the second sintering process, the activity of the particles can be controlled, so that the positive electrode active material has large particles with a certain area ratio at the same time, which increases the compaction density of the electrode sheet and improves the energy density of the battery cell while also allowing the battery cell to have better dynamic performance.

[0054] 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 of the present application 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 film layer.

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

[0056] In any embodiment, the hot pressing includes at least three hot roller pressings, and the hot roller pressure increases successively, and the hot roller pressure is 20 tons-50 tons, 50 tons-70 tons, and 70 tons-90 tons respectively; the hot roller temperature is 40°C-80°C, and the pole piece is heated before entering the hot roller compaction for the first time, and the heating temperature is 40°C-50°C.

[0057] 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.

[0058] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] 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;

[0060] Figure 2 is a schematic diagram of a lithium-ion secondary battery according to one embodiment of the present application;

[0061] Figure 3 is an exploded schematic diagram of a lithium-ion secondary battery according to one embodiment of the present application;

[0062] Figure 4 is a schematic diagram of a battery module according to one embodiment of the present application;

[0063] Figure 5 is a schematic diagram of a battery pack according to one embodiment of the present application;

[0064] Figure 6 yes Figure 5 An exploded schematic diagram of the battery pack shown;

[0065] Figure 7 It is a schematic diagram of an electrical device using a lithium-ion secondary battery as a power source according to one embodiment of the present application.

[0066] Figure 8 This is a porosity test diagram of a cross-section of the positive electrode film layer along the thickness direction according to one embodiment of the present application.

[0067] Description of reference numerals:

[0068] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0069] Below, the embodiments of the lithium-ion secondary battery, battery device and power device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be 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.

[0070] " 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] In this application, the terms "plurality" and "multiple" refer to two or more.

[0075] Unless otherwise specified, the terms used in this application have the common meanings that are commonly understood by those skilled in the art.

[0076] 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.

[0077] 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 cell, a battery module or a battery pack.

[0078] 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 present application embodiment does not limit this. Figure 2 As an example, a lithium-ion secondary battery 5 having a rectangular parallelepiped structure is shown.

[0079] A lithium-ion secondary battery includes an electrode assembly and an electrolyte.

[0080] 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).

[0081] In some embodiments, as Figure 3As 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.

[0082] 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.

[0083] 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.

[0084] In some embodiments, the battery may be an energy storage device, including an energy storage container, an energy storage cabinet, and the like.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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. A common method for improving particle size distribution in existing technologies is to increase the proportion of large particles to form a stacking skeleton that provides structural support. However, studies have shown that increasing the proportion of large particles increases the lithium ion diffusion path and degrades the battery's kinetic performance. Producing batteries that balance high energy density and kinetic performance is a technical problem that urgently needs to be addressed in this field.

[0091] 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 the D of the particles in a cross section along the thickness direction of the positive electrode film layer is A90 1400nm-2100nm, particle size concentration (D A90 -D A10 ) / D A50 is 1.855-2.375, among which D A90 、D A50、 D A10 It refers to the particle size corresponding to the cumulative area distribution of the particles when the cumulative area distribution reaches 90%, 50%, and 10% in the particle area cumulative distribution curve.

[0092] In the prior art, the electrode compaction density and battery energy density are often improved by increasing the particle size concentration of the particles in the positive electrode film layer. However, research shows that contrary to the prior art, in the cross-section of the positive electrode film layer along the thickness direction of the electrode, excessive particle size concentration has limited effect on improving the compaction of the electrode, and it is difficult to further improve the electrode compaction by adjusting the particle size concentration. The particle size concentration of the particles is less than 1.855, which means that the difference in particle size distribution is small, the grading effect is not significant, and there is a lack of smaller particles to fill the gaps between larger particles. When the concentration exceeds 2.375, it means that the size difference of the active material particles is significant, which makes it impossible for the positive electrode film layer to achieve optimal dense packing, and it is difficult to achieve further improvement in the electrode compaction density. The common way to adjust the particle size concentration in the prior art is to increase the size or proportion of large particles, but the applicant further found that when the D A90 When the particle size is larger than 2100 nm, the large particles in the positive electrode film layer will significantly deteriorate the kinetic performance of the battery.

[0093] The present invention controls the particle size concentration (D A90 -D A10 ) / D A50 1.855-2.375, D A90 The particle size is 1400nm-2100nm, which improves the accumulation of particles in the positive electrode film layer, increases the compaction density of the pole piece, and improves the energy density of the battery while maintaining high dynamic performance of the battery, thereby achieving a balance between battery dynamic performance and energy density.

[0094] Lithium-containing transition metal phosphate refers to a phosphate material containing lithium and a transition metal element, and can be detected by any method known in the art, for example, by combining an X-ray diffractometer (XRD) with an energy dispersive spectrometer.

[0095] 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 dispersive spectrometer.

[0096] 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.

[0097] The specific method for identifying particles is as follows: the positive electrode film layer is cut along the thickness direction of the electrode by an argon ion beam (as an example, the following 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 following 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 layer 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 layer (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, were not fully recognized, or were incorrectly identified. Particles that were not recognized, were not fully recognized, or were 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: particles located around the edges of the scanning electron microscope and 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 manually 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 determination and labeling of the particles in the image.

[0098] In some embodiments, in the cross section of the positive electrode film along the thickness direction of the electrode, the D A90 It is 1400nm-2100nm.

[0099] In the cross section of the positive electrode film along the thickness direction of the electrode, the D A90The calculation method is as follows. Import the image after particle determination and identification into ImageJ software for analysis, complete the scale setting according to the scanning electron microscope image, and analyze the particle size and area of ​​the particles in the cross-section of the positive electrode film along the thickness direction of the electrode through the "Feret diameter", "Area", "Round" and "Solidity" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter obtained by analysis represents the maximum spacing between all parallel lines in the two-dimensional projection of the particle, thereby characterizing the particle size of the particle; and the obtained "Area" parameter represents the pixel area of ​​the particle. Since particles with a diameter of less than 50nm 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 50nm, it will cause large errors in the statistical results. Therefore, in the particle size statistical process of this application, particles with a diameter of less than 50nm are not counted, and the particle statistical data corresponding to AR, Round or Solidity displayed as "NaN" are deleted. According to the above method, in order to meet the statistically significant sample number, each electrode piece is collected with no less than 10 non-overlapping scanning electron microscope images, and the particle sizes of no less than 5,000 particles are counted. The particle sizes of at least 5,000 particles obtained are arranged in order from small to large, with the particle size as the horizontal axis and the cumulative area percentage calculated by the particle "Area" as the vertical axis to obtain the area cumulative distribution curve of the particles in the positive electrode film layer. A90 、D A50 、D A10 They are the particle size values ​​corresponding to when the cumulative area of ​​the vertical axis in the area cumulative distribution curve accounts for 90%, 50%, and 10%, respectively.

[0100] In some embodiments, in the cross section of the positive electrode film along the thickness direction of the electrode, the D A90 It can be selected as 1400nm, 1486nm, 1500nm, 1556nm, 1600nm, 1674nm, 1700nm, 1800nm, 1900nm, 1984nm, 2000nm, 2100nm or any numerical range therebetween.

[0101] The cross-sectional morphology of the positive electrode film along the thickness direction of the electrode is shown in the figure below: Figure 1 As shown, the state of the positive electrode active material is different from that observed by Malvern laser scattering and scanning electron microscopy. The particles in the positive electrode film are well dispersed under roller pressure. Observing the positive electrode film is conducive to effectively characterizing the objective conditions of the particle size and distribution in the positive electrode film.

[0102] During the compaction process, the positive electrode film layer is compacted in the thickness direction. Compared with the surface of the positive electrode film layer, the cross section of the positive electrode film layer along the thickness direction of the electrode layer can better reflect the actual compaction status of the particles inside the film layer on a spatial scale. A50 It can intuitively reflect the size of the overall particles.

[0103] In some embodiments, the particle size concentration (D A90 -D A10 ) / D A50 It is 1.855-2.375.

[0104] In some embodiments, the particle size concentration (D A90 -D A10 ) / D A50 Available options are 1.855, 1.86, 1.87, 1.88, 1.89, 1.9, 1.91, 1.92, 1.93, 1.94, 1.948, 1.95, 1.96, 1.97, 1.98, 1.99, 2.0, 2.01, 2.02, 2.03, 2.031, 2.04, 2.05, 2.06, 2.07, 2.08, 2.09, 2.10, 2.11, 2 2.15, 2.16, 2.17, 2.18, 2.19, 2.2, 2.21, 2.22, 2.23, 2.24, 2.25, 2.26, 2.27, 2.28, 2.29, 2.3, 2.31, 2.32, 2.33, 2.34, 2.341, 2.35, 2.36, 2.37, 2.375 or any range therebetween.

[0105] In the present application, the particle size concentration in the cross-section of the positive electrode film layer along the thickness direction of the electrode can be tested with reference to the method described above. The particle size concentration within the above range is helpful to construct a reasonable particle grading. In the positive electrode electrode, a wide particle size distribution generally helps to improve the electrode compaction density. Specifically, a wider particle size distribution enables smaller particles to fill the gaps between larger particles, thereby forming a tighter arrangement of particles during the pressing process. This tight arrangement is conducive to improving the compaction density of the electrode, thereby improving the energy density of the battery.

[0106] 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 size in the cross-section of the positive electrode film layer, can accurately and objectively reflect the distribution of lithium-containing transition metal phosphate particles in the positive electrode film layer in the electrode sheet.

[0107] 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 dispersion degree of the positive electrode active material in the film layer will be improved during the film forming rolling process. The test results obtained by the Malvern laser diffraction method are affected by the particle size, specific surface area, and agglomeration degree of the positive electrode active material. Compared with the actual dispersion 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.

[0108] Those skilled in the art can achieve the particle size concentration, D A90 、D A10 、D A50 For example, the particle size concentration can be adjusted by scientifically grading particles of different sizes; the raw materials can be processed to the target particle size distribution range by the mechanical force of the crushing and grinding process to achieve the adjustment of particle size and concentration; the particle system can be separated by particle size using screening and grading equipment to obtain the required particle size distribution; and the precise control of the feed rate and the adjustment of the particle residence time and stress state in the equipment can also help to achieve the control of particle concentration.

[0109] In some embodiments, in the cross section of the positive electrode film along the thickness direction of the electrode, the D A90 It is 1400nm-2000nm.

[0110] In the cross section of the positive electrode film along the thickness direction of the electrode, the particle size D A90 Within the above range, the internal resistance of the battery can be reduced, further suppressing the negative impact of excessively large particles on the dynamic performance.

[0111] In some embodiments, in the cross section of the positive electrode film along the thickness direction of the electrode, the D A50 It is 600nm-900nm, and can be optionally 650nm-750nm.

[0112] In some embodiments, the particle size D of the positive electrode film in the cross section along the thickness direction of the electrode is A50It can be selected as 600nm, 610nm, 620nm, 630nm, 640nm, 650nm, 656nm, 660nm, 670nm, 680nm, 688nm, 690nm, 700nm, 708nm, 710nm, 719nm, 720nm, 730nm, 740nm, 750nm, 757nm, 760nm, 770nm, 780nm, 785nm, 790nm, 800nm, 810nm, 820nm, 830nm, 840nm, 850nm, 860nm, 870nm, 880nm, 890nm, 900nm or any numerical range therebetween.

[0113] Particle size D A50 On the one hand, the above range is conducive to maintaining the supporting role of the particles, so that the rolling pressure is evenly transmitted between the electrode particles, and the electrode can withstand higher rolling pressure; on the other hand, it represents that the overall particle size of the particles inside the positive electrode film layer is smaller, so that the dynamic performance is maintained at a high level, and the energy density is improved while taking into account the dynamics of the battery.

[0114] In some embodiments, in the cross section of the positive electrode film along the thickness direction of the electrode, the D A50 The thickness is 650nm-750nm, which is beneficial to further take into account the dynamic performance of lithium-ion secondary batteries.

[0115] In some embodiments, in the cross section of the positive electrode film along the thickness direction of the electrode, the D A10 It is 100nm-300nm, and can be optionally 120nm-250nm.

[0116] In some embodiments, in the cross section of the positive electrode film along the thickness direction of the electrode, the D A10 It can be selected as 100nm, 110nm, 120nm, 122nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 186nm, 190nm, 200nm, 210nm, 214nm, 220nm, 225nm, 230nm, 240nm, 250nm, 260nm, 262nm, 270nm, 280nm, 290nm, 300nm or any numerical range therebetween.

[0117] In the cross section of the positive electrode film along the thickness direction of the electrode, the D A10 The above range indicates, on the one hand, that there are certain small particles in the film layer to form a graded gap between the filling particles, and at the same time, the particles will not agglomerate due to being too small. On the other hand, it indicates that the proportion of small-size particles is limited, which can improve the compaction of the electrode while reducing the side reaction between small particles and the electrolyte, taking into account the cycle performance of the battery.

[0118] In some embodiments, in the cross section of the positive electrode film along the thickness direction of the electrode, the D A10 The thickness is 120nm-250nm, which is beneficial to further improve the cycle performance of lithium-ion secondary batteries.

[0119] 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 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 .

[0120] 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.

[0121] 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 particles. 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.

[0122] 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 a disordered structure, 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 2Hybridization 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 graphitization degree of the positive electrode film. It can be understood that the graphitization degree 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 material of the positive electrode active material. Although it is rich in sp 2 Conductive agents such as hybrid carbon nanotubes also have 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. 50 Therefore, the graphitization degree of the positive electrode film can also be used to characterize the graphitization degree of the positive electrode active material.

[0123] 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 positive electrode active material 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.

[0124] Those skilled in the art can adjust the degree of graphitization of the active material particles by any known process. For example, the degree of graphitization of the active material particles can be adjusted by adjusting the carbon source (cross-linked PEG can be selected as the carbon source), sintering temperature, sintering time, sintering pressure, sintering atmosphere, and nucleation process.

[0125] 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 during the rolling process with the help of the highly graphitized carbon structure in the coating material. On the basis of reasonable grading of the particles in the electrode sheet, the electrode sheet compaction density can be further improved even under low rolling pressure; the high degree of graphitization of the carbon layer coating the surface of the positive electrode active material is conducive to electron transmission, achieving a balance between energy density and dynamics.

[0126] 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 50The amount can be selected from 0.98, 0.99, 1, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.10, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.20 or any range therebetween.

[0127] In some embodiments, in the cumulative distribution curve of the sphericity of particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median of the sphericity L A50 It is 0.70-0.85, and can be selected as 0.70-0.76.

[0128] The median of the sphericity L in the cross section of the positive electrode film along the thickness direction of the electrode is 50 The specific test method is as follows: Refer to the method described above in this application to identify the particles in the cross-section of the positive electrode film layer, and use the "Shape Description" and "Area" analysis functions in ImageJ to analyze the morphology of the particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet and the area of ​​the particles. According to the software manual (ImageJ User Guide IJ 1.46r), the "Area" parameter obtained by analysis represents the pixel area of ​​the particle, and the "Round" parameter represents the ratio of the pixel area of ​​the particle to the area of ​​the circle with the fitted long diameter as the diameter. When the particle is closer to a sphere, the ratio of the pixel area to the area of ​​the circle with the fitted long diameter as the diameter is closer to 1. Therefore, the "Round" parameter of the particles obtained by analysis is used to characterize the sphericity of the particles. 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 area 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%.

[0129] In some embodiments, in the cumulative distribution curve of the sphericity of particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median of the sphericity L A50 The optional values ​​are 0.70, 0.709, 0.71, 0.719, 0.72, 0.725, 0.726, 0.73, 0.737, 0.74, 0.75, 0.751, 0.76, 0.77, 0.78, 0.79, 0.80, 0.81, 0.82, 0.83, 0.84, 0.85 or any range therebetween.

[0130] 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.

[0131] The median of sphericity L A50 The particles within the above range are approximately spherical, which helps the particles maintain good slidability when stacked, makes it easy to fill the gaps between the particles, and can further improve the compaction density of the pole piece and increase the energy density of the battery.

[0132] In some embodiments, in the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode piece, the median roughness R A50 It is 0.92-0.96.

[0133] In the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode piece, the median of the roughness R A50 The specific test method is as follows: Refer to the method described above in this application to identify the particles in the cross-section of the positive electrode film layer, and use the "shape description" analysis function in ImageJ to analyze the morphology of the particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode. 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. Arrange the roughness of at least 5,000 particles obtained in order from small to large, and use the roughness as the horizontal axis and the cumulative area ratio as the vertical axis to obtain the roughness area cumulative distribution curve of the particles in the positive electrode film layer. R A50 It is the roughness R value corresponding to when the cumulative area of ​​the vertical axis in the roughness R value area cumulative distribution curve accounts for 50%.

[0134] In some embodiments, in the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode piece, 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.

[0135] 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.

[0136] Median roughness R 50 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.

[0137] In some embodiments, in the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode piece, the concentration of roughness (R A90 -R A10 ) / R A50 The optional range is 0.05-0.10.

[0138] In some embodiments, in the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode piece, the concentration of roughness (R A90 -R A10 ) / R A50 The optional values ​​are 0.05, 0.06, 0.07, 0.08, 0.09, 0.10 or any range therebetween.

[0139] In the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the roughness concentration test method is as follows: Referring to the roughness test method described above in this application, by analogy, R A90 The R value corresponding to the cumulative area of ​​the vertical axis in the sphericity R value area cumulative distribution curve accounts for 90%, R A10 The R value corresponding to the cumulative area of ​​the vertical axis in the roughness R value area cumulative distribution curve accounts for 10%. The concentration of roughness is expressed by (R A90 -R A10 ) / R A50 express.

[0140] 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.

[0141] The concentration value of roughness is extremely small, indicating that the overall roughness consistency of the particles is high, which is conducive to the relative sliding between the particles and is easier to form high-density accumulation during rolling, thereby increasing the compaction density of the electrode and the energy density of the battery.

[0142] In some embodiments, the iron dissolution rate of the positive electrode film layer is 400 ppm-1800 ppm.

[0143] 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 .

[0144] In some embodiments, the iron dissolution rate of the positive electrode film layer can be selected as 400ppm, 500ppm, 600ppm, 700ppm, 800ppm, 860.7ppm, 900ppm, 1000ppm, 1100ppm, 1200ppm, 1300ppm, 1400ppm, 1500ppm, 1600ppm, 1700ppm, 1800ppm or any numerical range therebetween.

[0145] Those skilled in the art can control the iron dissolution rate of the positive electrode material by any known process. As an example, the iron dissolution rate of the positive electrode material can be controlled by regulating the surface coating quality of the positive electrode material, the temperature, time, and pressure during the preparation process. In addition, during battery use, the battery design, the oxidant content in the electrolyte, the battery operating temperature, and the battery charge and discharge intensity will also affect the iron dissolution rate of the positive electrode film layer.

[0146] The iron dissolution rate of the positive electrode film layer mainly comes from the lithium-containing transition metal phosphate positive electrode active material in the positive electrode film layer, which can 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 the iron ions are to precipitate from the carbon coating material after acid dissolution, that is, the more complete and dense the carbon coating material on the surface of the positive electrode active material. The iron dissolution rate of the positive electrode film layer within the above range indicates that the positive electrode active material has a relatively complete and dense carbon coating material, which can improve the electrical contact between the positive electrode active materials, improve the conductivity of the positive electrode active materials, reduce the polarization of the positive electrode active materials, and further optimize the kinetic performance of the lithium-ion secondary battery. At the same time, the high integrity of the carbon coating material makes the particles easy to slip under stress during the rolling process, which can simultaneously improve the compaction density of the electrode sheet and the energy density of the battery.

[0147] In some embodiments, based on the total mass of the positive electrode active material, the mass content of carbon element is 0.8%-1.8%, and optionally 0.9%-1.5%.

[0148] 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."

[0149] 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.

[0150] Compared with the lithium 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 transition metal lithium phosphate in the positive electrode plate and improve the energy density of the lithium ion secondary battery.

[0151] In some embodiments, the lithium iron antisite defect concentration of the positive electrode active material is 0.001%-1.5%, and can be optionally 0.01%-1.0%.

[0152] XRD data were collected using an X-ray diffractometer, and phase analysis was performed on the sample. 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. The collected XRD data were fitted and refined using FullProf Suite software, with parameters refined in the order of background parameters, peak intensity, unit cell parameters, and peak shape. When the fitted peak shape matched the experimental peak shape optimally, with an Rwp of less than 10, the refined Li and Fe occupancy probabilities were obtained, and the probability of Fe occupying the lithium site was used as the lithium-iron antisite defect concentration.

[0153] In some embodiments, the lithium iron antisite defect concentration of the positive electrode active material may be selected as 0.001%, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07, 0.08%, 0.09%, 0.1%, 0.2%, 0.22%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% or any range therebetween.

[0154] 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.

[0155] 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 the oxidation of ferrous ions to ferric ions, but also induce the partial 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.

[0156] In some embodiments, the lithium-containing transition metal phosphate includes a component having the following general formula:

[0157] Li m Fe x P y O j Q q ,

[0158] 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.

[0159] 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.

[0160] Selecting an appropriate modifying element Q can improve the lattice change rate of the positive electrode active material during the lithium insertion and extraction process, reduce the oxygen activity on the particle surface, improve the structural stability of the material, and thereby increase the material's gram capacity during the cycle process, further improving the cycle stability of lithium-ion secondary batteries.

[0161] 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.

[0162] In some embodiments, the positive electrode active material includes titanium, and the mass content of the titanium is 4000 ppm-8000 ppm based on the total mass of the positive electrode active material.

[0163] 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.

[0164] Doping positive electrode active materials with titanium can induce lattice distortion, reduce Li-O bond energy, increase lithium ion diffusion 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.

[0165] 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.

[0166] In some embodiments, the tap density of the positive electrode active material is 1.00 g / cm 3 -1.70g / cm 3 , optional 1.20 g / cm 3 -1.50g / cm 3 .

[0167] The tap density of powder can be obtained by any known method in the art. 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 the tapped measuring cylinder and place it on the conical flask to weigh and record the weight of the measuring cylinder; open the sample bag, use a clean sample spoon to stir the sample in the sample bag for 3-5 times to mix it, and then transfer the sample steadily into the measuring cylinder; wipe the powder on the surface of the measuring cylinder with dust-free paper, and then put it into the cleared conical flask to weigh; seal the mouth of the measuring cylinder with sealing film, and place the tapped measuring cylinder into the matching instrument rubber ring to ensure the vibration The tap density measuring cylinder is tightly fitted to the rubber ring and kept 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 tap density measuring cylinder, use a flashlight to illuminate the surface of the measuring cylinder, and use visual inspection 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 tap density of the sample is obtained by the density formula ρ=m / v.

[0168] In some embodiments, the tap density of the positive electrode active material powder can be 1.00 g / 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 , 1.55g / cm 3 , 1.60g / cm 3 , 1.65g / cm 3 , 1.70g / cm 3 or any range of values ​​between them.

[0169] The positive electrode active material particles in the embodiments of the present application not only have a wide particle size distribution, but also have a particle size within a reasonable range, forming an effective gradation. Small particles can fill the gaps between particles, and thus have a higher tap density.

[0170] In some embodiments, the powder compaction density of the positive electrode active material at a pressure of 3T is 2.55 g / cm 3 -2.70g / cm 3 , optional 2.58g / cm 3 -2.68g / cm 3 .

[0171] 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 .

[0172] The powder compaction density of the positive electrode active material can be measured by methods and equipment known in the art. For example, it can be measured with a compaction density instrument with reference to GB / T 24533-2009. Specifically, a certain amount of positive electrode active material is placed on a special compaction mold (the mold diameter is known), and the mold is hollow in the middle with a metal disc on the top and bottom. The positive electrode active material is placed between the metal discs, a metal cylinder is placed on the top, and the mold is placed on the compaction density instrument. The pressure is set to 3T. The thickness of the positive electrode active material under a pressure of 3T can be read on the device. 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, and S is the bottom area of ​​the mold, 1.327cm. 2 , H is the thickness of the positive electrode active material after compaction.

[0173] In some embodiments, the powder compaction density of the positive electrode active material at a pressure of 3T can be 2.55 g / cm3 , 2.56g / cm 3 , 2.57g / cm 3 , 2.58g / cm 3 , 2.59 g / cm 3 , 2.60 g / cm 3 , 2.61 g / cm 3 , 2.62 g / cm 3 , 2.63 g / cm 3 , 2.64 g / cm 3 , 2.65 g / cm 3 , 2.66 g / cm 3 , 2.67 g / cm 3 , 2.68 g / cm 3 , 2.69 g / cm 3 , 2.70 g / cm 3 or any range of values ​​between them.

[0174] The positive electrode active material particles form an effective gradation, so that the positive electrode active material can construct a stacking structure with extremely small particle gaps under the action of external force, which can achieve a higher compaction density and provide a material basis for improving the compaction density of the electrode and preparing high-energy density lithium-ion secondary batteries.

[0175] In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 8 MPa is 0.5 Ω·cm-60.0 Ω·cm, and can be optionally 2.0 Ω·cm-40.0 Ω·cm.

[0176] 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.

[0177] 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, 10.95Ω·cm, 15Ω·cm, 20Ω·cm, 25Ω·cm, 30Ω·cm, 35Ω·cm, 40Ω·cm, 45Ω·cm, 50Ω·cm, 55Ω·cm, 60Ω·cm, or any range therebetween.

[0178] The positive electrode active material is coated with carbon material on the surface, and 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 improving the kinetic performance of the battery.

[0179] 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.

[0180] 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.

[0181] 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°C for 2h. The compacted density is 2.0g / cm 3 -2.2g / cm 3 Compact 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.

[0182] 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.

[0183] In some embodiments, the discharge capacity of the positive electrode active material at room temperature at a 1C discharge rate may be 135 mAh / g, 140 mAh / g, 140.8 mAh / g, 145 mAh / g, 150 mAh / g, or any range therebetween.

[0184] 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.

[0185] 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.

[0186] In some embodiments, the discharge capacity of the positive electrode active material discharged to 3.2V accounts for η≥88%.

[0187] The η value of the positive electrode active material can be measured by methods and equipment known in the art. As an example, a button cell is first prepared with reference to the method described above, and the electrical performance of the prepared button cell is tested on a blue light tester at room temperature. Specifically, the button cell is charged and discharged twice at a constant current of 0.1C in the voltage range of 2.0V to 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.

[0188] 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.

[0189] 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%.

[0190] 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.

[0191] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the conductive agent is 0.01-1.5%.

[0192] 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 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% or any numerical range therebetween.

[0193] 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.

[0194] Because the particles in the positive electrode film layer form a dense stack, the positive electrode active material particles are coated with a highly graphitized carbon layer on the surface and are in full contact with each other, having good electronic conductivity, which can reduce 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.

[0195] In some embodiments, the positive electrode film layer further includes a binder. Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 94.0%-99.4%, optionally 96.5%-99.4%; the mass content of the binder is 0.5%-3.0%.

[0196] 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.

[0197] 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 94.0%, 94.5%, 95.0%, 95.5%, 96.0%, 96.5%, 97%, 97.5%, 98%, 98.5%, 99%, 99.4% or any numerical range therebetween.

[0198] 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%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0% or any numerical range therebetween.

[0199] The mass content of the positive electrode active material and the mass content of the binder are within the above ranges, which can effectively increase the active material loading per unit volume of the positive electrode film layer, maintain good internal bonding strength, reduce the probability of powder loss, expansion and cracking problems, and improve the energy density of the secondary battery while taking into account safety performance.

[0200] In some embodiments, the single-side density of the positive electrode film layer is 300 mg / 1540 mm 2 -450mg / 1540mm 2 .

[0201] 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.

[0202] In some embodiments, the surface density of the positive electrode film layer on one side may be 300 mg / 1540 mm 2 、310mg / 1540mm 2 、320 mg / 1540mm 2 、330 mg / 1540mm 2 、340 mg / 1540mm 2、350 mg / 1540mm 2 、360 mg / 1540mm 2 、370 mg / 1540mm 2 、380 mg / 1540mm 2 、390 mg / 1540mm 2 、400 mg / 1540mm 2 、410 mg / 1540mm 2 、420 mg / 1540mm 2 、430 mg / 1540mm 2 、440 mg / 1540mm 2 、450 mg / 1540mm 2 or any range of values ​​between them.

[0203] 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.

[0204] In some embodiments, when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.52 g / cm 3 -2.78 g / cm 3 .

[0205] 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 -2.75 g / cm 3 .

[0206] In this application, the full discharge state refers to the state where the battery is placed at 25°C, left to stand for 2 hours, and when the battery temperature remains at 25°C, the battery is discharged at a constant current of 1 / 3C to 2.5V and then at a constant current of 0.1C to 2.0V.

[0207] 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. Once 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 to obtain T2. ​​The compaction density of the positive electrode film layer PD = (W1-W2) / [(T1-T2)×S].

[0208] In some embodiments, the lithium-ion secondary battery has a compaction density of the positive electrode film layer of 2.52 g / cm2 when fully discharged. 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 , 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 or any range of values ​​between them.

[0209] In some embodiments, after the compaction process, the compaction density of the positive electrode film layer is 2.63-2.90 g / cm 3 .

[0210] 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 / cm 3 , 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 , 2.86g / cm 3 , 2.87g / cm 3 , 2.88g / cm 3 , 2.89g / cm 3 , 2.90g / cm 3 or any range of values ​​between them.

[0211] In this application, “compaction” refers to compacting the positive electrode film layer by mechanical pressure during the battery assembly process to improve its density and conductivity.

[0212] In some embodiments, after the chemical formation process, the compaction density of the positive electrode film layer is 2.52-2.78 g / cm 3 .

[0213] In some embodiments, after the chemical formation process, the compaction density of the positive electrode film layer can be selected to be 2.52 g / 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 / cm3 , 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 or any range of values ​​between them.

[0214] 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.

[0215] 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.

[0216] 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.

[0217] In some embodiments, when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.52 g / cm 3 -2.78g / cm 3 In 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%.

[0218] 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 -2.75g / cm 3 In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10%-20%.

[0219] 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% or any numerical range therebetween.

[0220] 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.

[0221] It can be understood that in the embodiment of the present application, the “pores” in the positive electrode film section are identified by image color difference and threshold value. The “pores” are not the pore data obtained in the exhaust test, but are mainly used to characterize the cross-sectional area between the particles in the positive electrode film section. This method is better than the exhaust method because the porosity obtained by the exhaust method is related to the pores between the particles and the pores in the carbon layer coated on the surface of the lithium iron phosphate particles, which cannot objectively reflect the pores between the particles. The lower the porosity in the positive electrode film section tested by this method, on the one hand, 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 slip 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 diaphragm during long cycles, which is beneficial to improving the long cycle performance of the battery.

[0222] 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.

[0223] Among them, carbon-based particles refer to particles with carbon as the main component, including but not limited to conductive carbon, carbon black, etc.

[0224] 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.

[0225] 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.

[0226] 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 20-60nm, 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.

[0227] 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.

[0228] 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.

[0229] 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 increasingly 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.

[0230] 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.

[0231] 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.

[0232] 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.

[0233] 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.).

[0234] In some embodiments, 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 -215mg / 1540mm 2 ; and / or the compaction density of the negative electrode film layer is 1.40g / cm 3 -1.75g / cm 3 .

[0235] 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.

[0236] The surface density and compaction density of the negative electrode film layer being within the above ranges are beneficial to improving the energy density of the lithium-ion secondary battery.

[0237] 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.).

[0238] 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.

[0239] 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).

[0240] 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.

[0241] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0242] 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.

[0243] 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.

[0244] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0245] 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.

[0246] 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.

[0247] 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.

[0248] 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.

[0249] 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.

[0250] 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.

[0251] 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.

[0252] 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.

[0253] 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.

[0254] 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.

[0255] The fourth aspect of the 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; adding a solvent and grinding to obtain a mixed slurry; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder to obtain a positive electrode active material; sintering the precursor powder to obtain the positive electrode active material comprises at least two sinterings; after the first sintering, a first sintered product is obtained, the first sintered product and a carbon source are mixed to obtain an intermediate raw material, the intermediate raw material is divided into two groups, and the first group of ground products and the second group of ground products are respectively ground to obtain a first group of ground products and a second group of ground products, the first group of ground products and the second group of ground products are mixed to obtain a mixed intermediate product; the mixed intermediate product is subjected to a second sintering to obtain a positive electrode active material; D of the first group of ground products is obtained. V50 The D of the second group of grinding products is 0.8μm-1.2μm; V50 0.30μm-0.50μm.

[0256] The preparation method can effectively control the content and size of large particles in the positive electrode active material by a process method including two sintering and two grinding processes, so that the positive electrode active material has a certain content of large particles without causing the large particles to be too large. A90 1400nm-2100nm, particle size concentration (D A90 -D A10 ) / D A50 Provides material basis for the positive electrode film layer of 1.855-2.375.

[0257] In some embodiments, the iron source is an iron-containing compound. In some embodiments, the iron source includes at least one of ferric hydroxide, ferrous chloride, ferric oxide, ferric phosphate, ferric pyrophosphate, ferrous oxalate, iron powder, ferric nitrate, ferrosoferric oxide, and ferric oxyhydroxide.

[0258] In some embodiments, the phosphorus source is a phosphoric acid compound. In some embodiments, the phosphorus source includes at least one of phosphoric acid, ferric phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0259] In some embodiments, the iron source and the phosphorus source can be the same substance. In some embodiments, ferric phosphate is used as the iron source and the phosphorus source.

[0260] In some embodiments, the lithium source includes one or more of lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium oxide, lithium hydroxide, and lithium acetate.

[0261] In some embodiments, the lithium source comprises lithium carbonate.

[0262] In some embodiments, the carbon source comprises one or more of glucose, polyethylene glycol, citric acid, sucrose, starch, fructose, lactose, polyaniline, polyacrylonitrile, and polyvinyl pyrrolidone.

[0263] In some embodiments, the carbon source comprises glucose or polyethylene glycol.

[0264] In some embodiments, the mass content of polyethylene glycol is 20%-75% based on the total mass of the carbon source.

[0265] The polymer carbon source has a relatively low graphitization temperature, so that the carbon coating material on the surface of the positive electrode active material can decompose 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.

[0266] 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.

[0267] 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 material formed by the sugar carbon source has a low degree of graphitization and a loose flocculent carbon structure, which may itself serve as a new rough point. Therefore, the higher the proportion of polymer carbon source in the carbon source, the more conducive it is to the optimization of the surface roughness of the positive electrode active material.

[0268] 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.

[0269] 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.

[0270] 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.

[0271] 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.

[0272] In some embodiments, the atomic molar ratio of iron element to phosphorus element in the iron source and the phosphorus source is 0.95-1.

[0273] In some embodiments, the atomic molar ratio of iron to phosphorus in the lithium source and the iron source can be selected from 0.95, 0.955, 0.96, 0.965, 0.97, 0.975, 0.98, 0.985, 0.99, 0.995, 1 or any range therebetween.

[0274] When synthesizing lithium-containing transition metal oxides, the iron-to-phosphorus ratio influences the reaction rate and crystal growth process. A low iron-to-phosphorus ratio increases the reaction rate of the iron source, accelerating its conversion to lithium-containing transition metal phosphates. A high phosphorus content inhibits the interaction between iron and carbon, reducing the formation of iron-based compounds such as Fe3C, and promoting the graphitization of the carbon layer. This rate-determining step prolongs crystal growth time, promoting the formation of larger particles. This allows for the control of particle size and area distribution to achieve the desired stacking structure.

[0275] 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%.

[0276] 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.

[0277] 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.

[0278] In some embodiments, the solvent includes water and mixtures thereof.

[0279] 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.

[0280] In some embodiments, the grinding to obtain the mixed slurry includes a first grinding, and the first grinding satisfies one or more of the following conditions:

[0281] (1) The grinding balls used in the first grinding are one or more of zirconia balls, silicon nitride zirconium balls, and ceramic zirconium balls;

[0282] (2) The diameter of the grinding balls used in the first grinding is 0.5 mm to 0.7 mm;

[0283] (3) The rotation speed of the first grinding is 450 rpm-550 rpm;

[0284] (4) The first grinding time is 0.5h-1.5h;

[0285] (5) The grinding chamber pressure during the first grinding is 0.01 MPa-0.3 MPa.

[0286] In some embodiments, the grinding to obtain the mixed slurry comprises performing a second grinding after the first grinding, wherein the second grinding satisfies one or more of the following conditions:

[0287] (1) The grinding balls used in the second grinding are one or more of zirconia balls, silicon nitride zirconium balls, and ceramic zirconium balls;

[0288] (2) The diameter of the grinding balls for the second grinding is 0.25 mm to 0.35 mm;

[0289] (3) The second grinding speed is 470 rpm-530 rpm;

[0290] (4) The second grinding time is 3.0h-5.0h;

[0291] (5) The grinding chamber pressure during the second grinding is 0.01 MPa-0.3 MPa.

[0292] In some embodiments, the particle size D of the mixed slurry is V50 0.3μm-0.5μm.

[0293] Grinding at least twice is beneficial to controlling the slurry temperature and viscosity, reducing the excessive viscosity of the slurry caused by excessive temperature and the resulting agglomeration of raw materials, improving the uniformity of particle size in the product, reducing the generation of oversized particles, and being beneficial to the particle size of the positive electrode film layer. A90 control.

[0294] The first grinding process can process large particles, while the second grinding process further refines the material and adjusts the particle size distribution. This can effectively reduce the uneven particle size caused by the grinding process, reduce the agglomeration of particles, improve the conductivity and cycle stability of the battery, and also improve overall production efficiency while meeting the final product performance.

[0295] In some embodiments, drying the mixed slurry to obtain the precursor powder includes spray drying the mixed slurry to obtain the precursor powder.

[0296] In some embodiments, the first sintering of the at least two sinterings satisfies one or more of the following conditions:

[0297] (1) Heating rate is 2°C / min-10°C / min;

[0298] (2) The holding temperature is 720°C-790°C;

[0299] (3) The insulation time is 5h-12h.

[0300] In some embodiments, the carbon source added to the intermediate raw material obtained by mixing the first sintered product and the carbon source includes glucose and polyethylene glycol.

[0301] In some embodiments, based on the mass of the first sintered product, the mass content of glucose in the intermediate raw material is 1%-3%, and the mass content of polyethylene glycol in the intermediate raw material is 0.5%-6%.

[0302] In some embodiments, the D of the first group of ground products is V50 0.8μm-1.2μm.

[0303] In some embodiments, the D of the second group of ground products is V50 0.30μm-0.50μm.

[0304] Compared with large particles, small particles have a relatively higher proportion of surface atoms, larger surface energy, higher activity of surface atoms, and a larger surface area per unit volume. Therefore, the rate of surface diffusion is faster, which makes small particles prone to more intense surface rearrangement during the sintering process, pushing the particles to evolve into a spherical form. Therefore, controlling the grinding particle size to a smaller value, including the grinding of the mixed slurry and the grinding of the intermediate raw materials, is conducive to the formation of products with higher sphericity and smoother and flatter surfaces during the subsequent sintering process. At the same time, particles with smaller grinding particle sizes are easier to be coated, which improves the coating integrity.

[0305] In some embodiments, the grinding conditions of the first set of ground products meet one or more of the following conditions:

[0306] (1) The speed is 550 rpm ± 50 rpm;

[0307] (2) The grinding time is 0.5h-1.5h.

[0308] In some embodiments, the grinding conditions of the second set of ground products meet one or more of the following conditions:

[0309] (1) The speed is 500 rpm ± 50 rpm;

[0310] (2) The grinding time is 3h-5h.

[0311] In some embodiments, the mass ratio of the first group of ground products to the second group of ground products is (60:40)-(80:20).

[0312] By regulating the mass ratio of the first group of ground products and the second group of ground products, positive electrode active materials with different gradation ratios can be obtained, so that the area ratio of particles of different particle sizes can be finely controlled to obtain the required stacking structure.

[0313] In some embodiments, the second sintering satisfies one or more of the following conditions:

[0314] (1) The holding temperature is 770°C-830°C;

[0315] (2) The heat preservation time is 5h-12h;

[0316] (3) The heating rate is 2°C / min-10°C / min.

[0317] The second sintering process can effectively shorten the sintering time in the high-temperature range, thereby reducing the risk and probability of extremely large particles appearing during high-temperature sintering. By adjusting the particle size of the two groups of grinds during the second sintering process, the activity of the particles can be controlled, so that the positive electrode active material has large particles with a certain area ratio at the same time, which increases the compaction density of the electrode sheet and improves the energy density of the battery cell while also allowing the battery cell to have better dynamic performance.

[0318] By controlling the sintering temperature during both the primary and secondary sintering processes, the diffusion rate can be controlled. At high temperatures, diffusion on the particle surface increases, defects within the particles are repaired, and the crystal lattice rearranges. Recrystallization eliminates surface defects, resulting in a more ordered grain structure and a gradual increase in particle size. This helps smooth the particle surface and promotes spherical particle development. The sintering temperature also affects the graphitization rate of the carbon source. Kinetically, carbon atoms gain more energy, enabling them to overcome the original energy barrier and undergo a more vigorous rearrangement within the crystal lattice. The sintering time, on the other hand, influences the extent of the reaction. Too short a sintering time prevents complete diffusion and rearrangement of the lithium-containing transition metal phosphate and the carbon source. Too long a sintering time leads to abnormal particle growth, coarsening of the grains within the particles, unstable material structure, increased adhesion between particles, and agglomeration.

[0319] In some embodiments, after sintering the precursor, the product is subjected to air flow milling to obtain the positive electrode active material.

[0320] In some embodiments, the classification frequency of the air flow milling is 20 Hz-30 Hz, and the milling pressure is 0.45 MPa-0.55 MPa.

[0321] 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.

[0322] 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.

[0323] 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 making the carbon-coated material structure incomplete. This is manifested 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, which is detrimental to the battery's cycle performance and lifespan. Therefore, it is necessary to control the classification frequency and pulverization pressure of airflow milling within an appropriate range.

[0324] 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 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 film layer.

[0325] In some embodiments, the drying temperature is 95°C-105°C, and the drying speed is 2.0m / min-2.3m / min.

[0326] In some embodiments, the hot pressing includes at least three hot roller pressings, and the hot roller pressure increases successively, and the hot roller pressure is 20 tons-50 tons, 50 tons-70 tons, and 70 tons-90 tons respectively; the hot roller temperature is 40°C-80°C, and the pole piece is heated before entering the hot roller compaction for the first time, and the heating temperature is 40°C-50°C.

[0327] 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.

[0328] In some embodiments, the stirring and viscosity adjustment to obtain the shipping slurry specifically includes pre-stirring to obtain a first slurry; re-stirring the first slurry to obtain a second slurry; and slowly stirring the second slurry and adjusting the viscosity to obtain the shipping slurry.

[0329] 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, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, 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, satellites, energy storage systems, etc.

[0330] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0331] 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.

[0332] 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.

[0333] Example

[0334] 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.

[0335] Example 1

[0336] (1) Preparation of positive electrode active materials

[0337] S1, lithium carbonate, iron phosphate, glucose and titanium dioxide are added to water, mixed in a premixing tank at a speed of 1800 rpm, and demagnetized by passing through a demagnetizing rod with a magnetic field strength of 8000-12000 Gs, wherein the ratio of lithium carbonate and iron phosphate is such that the molar ratio of iron to phosphorus is 0.975, the mass content of glucose relative to the total amount of iron phosphate is 5.7%, and the amount of titanium dioxide doped is such that the mass content of titanium doped in the carbon-coated lithium iron phosphate positive active material product is 5000 ppm;

[0338] S2, the mixed raw material is subjected to two grinding-demagnetization cycles in a sand mill, the first grinding is carried out under the conditions of using zirconia balls with a diameter of 0.6 mm and a rotation speed of 500 rpm, the grinding time is 1 hour, the grinding chamber pressure is less than 0.3 MPa, the raw material after the first grinding is demagnetized using a permanent magnetic iron remover, and the demagnetization intensity is greater than or equal to 8000 Gs; the raw material after demagnetization is then ground for a second time to obtain a mixed slurry, the particle size D of the mixed slurry V50 0.40μm±0.10μm;

[0339] S3, spray drying the mixed slurry to obtain a precursor powder,

[0340] S4, sintering the precursor powder to obtain a lithium iron phosphate positive electrode material, the sintering process comprising:

[0341] First sintering: sintering the precursor powder in a nitrogen atmosphere at a heating rate of 5°C / min from 25°C to 760°C, and keeping the temperature for 10 hours, and then cooling to obtain a first sintered product;

[0342] Grinding and mixing: add 1.5% of the total mass of the first sintered product to the first sintered product, and 3.0% of the total mass of the first sintered product to the first sintered product; divide into two groups for grinding (third grinding), wherein the D V50 When the particle size reaches 1.0 μm ± 0.20 μm, the grinding is stopped (grinding conditions: 550 rpm ± 50 rpm, grinding time 1 h), and the first group of grinding products is obtained; the D V50 When the particle size reaches 0.38 μm ± 0.05 μm, grinding is stopped (grinding conditions: 500 rpm ± 50 rpm, grinding time 4 h) to obtain a second group of ground products; the first group of ground products and the second group of ground products are mixed in a mass ratio of 70:30 to obtain a mixed intermediate product; and the mixed intermediate product is spray-dried;

[0343] Second sintering: The dried mixed intermediate product was sintered in a nitrogen atmosphere at a heating rate of 5°C / min from 25°C to 800°C, and kept at this temperature for 10 hours. After cooling, a second sintered product was obtained.

[0344] S5, after sintering is completed, cooling to below 100 ° C, crushing the second sintered product by air flow milling to obtain a carbon-coated lithium iron phosphate positive electrode active material, wherein the classification frequency of the air flow milling is 25 Hz and the crushing pressure is 0.55 MPa.

[0345] The mass content of carbon element in the prepared positive electrode active material is 1.248%, the concentration of lithium iron antisite defects is 0.22%, and the tap density of the powder is 1.30g / cm 3 The compacted density of the powder under 3T pressure is 2.62g / cm 3 The powder resistivity is 10.95Ω·cm under a pressure of 8MPa; the discharge capacity at a discharge rate of 1C is 140.8mAh / g; and the discharge capacity of the 3.2V discharge platform accounts for 90.0%.

[0346] (2) Preparation of positive electrode sheet:

[0347] 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 The drying temperature is 95°C and the drying speed is 2.0 m / min.

[0348] 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.

[0349] 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 2.71g / cm 3 .

[0350] 14827 particles were counted in the cross section of the positive electrode film along the thickness direction of the electrode sheet. The results showed that the D A50 719nm, D A10 214nm, D A90 is 1674nm, particle size concentration (D A90 -D A10 ) / D A50 It is 2.031.

[0351] The median C of the graphitization degree of the prepared cathode film obtained in the surface scanning mode of the laser microconfocal Raman spectrometer 50 The median of the sphericity L is 1.04. A50 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 roughness concentration is 0.08 and the iron dissolution rate of the positive electrode film is 860.7 ppm.

[0352] (3) Preparation of negative electrode sheet:

[0353] 95.5 wt% of the negative electrode active material (artificial graphite), 1.0 wt% of the conductive agent (conductive carbon black), 2.0 wt% of the binder (styrene-butadiene rubber (SBR)) and 1.5 wt% of the thickener (sodium carboxymethyl cellulose (CMC)) were mixed, added with deionized water and stirred, and dispersed to form a negative electrode slurry. The negative electrode slurry was then coated on both sides of the Cu foil. After both sides were completed, the negative electrode sheet was prepared by drying, compacting, slitting and sheeting. The density of the coated single side was 166 mg / 1540.25 mm 2 , compacted density is 1.60g / cm 3 .

[0354] (4) Preparation of isolation membrane

[0355] Polypropylene film is used as the isolation film.

[0356] (5) Preparation of electrolyte

[0357] 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.

[0358] (6) Preparation of batteries:

[0359] 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.

[0360] The preparation method of Example 2 is basically the same as that of Example 1, except that in step S4, the D of the particles in the first group is V50 Stop grinding when the particle size reaches 0.80μm±0.20μm; D V50 Stop grinding when the thickness reaches 0.30 μm ± 0.05 μm.

[0361] The preparation method of Example 3 is basically the same as that of Example 1, except that in step S4, the D V50 Stop grinding when the particle size reaches 0.80μm±0.20μm; D V50 Stop grinding when the thickness reaches 0.45 μm ± 0.05 μm.

[0362] The preparation method of Example 4 is basically the same as that of Example 1, except that in step S4, the D V50Stop grinding when the particle size reaches 0.80μm±0.20μm; D V50 The grinding was stopped when the particle size reached 0.45 μm±0.05 μm; the first group of ground products and the second group of ground products were mixed in a mass ratio of 60:40.

[0363] The preparation method of Example 5 is basically the same as that of Example 1, except that in step S4, the D V50 Stop grinding when the particle size reaches 1.20 μm ± 0.20 μm; D V50 The grinding was stopped when the particle size reached 0.45 μm±0.05 μm; the first group of ground products and the second group of ground products were mixed in a mass ratio of 80:20.

[0364] The preparation method of Example 6 is basically the same as that of Example 1, except that in step S4, the D V50 Stop grinding when the particle size reaches 1.20 μm ± 0.20 μm; D V50 Stop grinding when the thickness reaches 0.30 μm ± 0.05 μm.

[0365] The preparation method of Comparative Example 1 is basically the same as that of Example 1, except that in step S4, the D V50 Stop grinding when the particle size reaches 0.75μm±0.20μm; D V50 Stop grinding when the thickness reaches 0.50 μm ± 0.05 μm.

[0366] The preparation method of Comparative Example 2 is basically the same as that of Example 1, except that in step S4, the D V50 Stop grinding when the particle size reaches 1.50μm±0.20μm; D V50 Stop grinding when the thickness reaches 0.38 μm ± 0.05 μm.

[0367] The preparation method of Comparative Example 3 is basically the same as that of Example 1, except that in step S2, the particle size D of the mixed slurry is V50 is 0.35 μm ± 0.10 μm; in step S4, there is only the first sintering, and the sintering process includes: sintering the precursor powder in a nitrogen atmosphere, heating from 25 ° C to 800 ° C at a heating rate of 5 ° C / min, and keeping warm for 10 hours, and obtaining a sintered product after cooling.

[0368] Performance Testing

[0369] 1. Pole piece ultimate compaction density

[0370] The double-sided coated electrode is compacted by a roller press, and the elongation of the electrode after compaction is tested, and the flexibility of the electrode after compaction is evaluated. By increasing the pressure of the roller press, electrodes with different compaction densities will be obtained. As the pressure increases, the compaction density of the electrode increases, the elongation of the electrode increases, and the flexibility of the electrode decreases. Too high an elongation of the electrode can easily cause the electrode to warp, and too low a flexibility of the electrode can easily lead to brittle fracture of the electrode. Therefore, the smaller of the compaction density corresponding to the elongation of the electrode of 8% or the number of times the electrode flexibility is folded is defined as the ultimate compaction density of the electrode.

[0371] The compacted density is calculated by dividing the mass of the positive electrode film layer by the volume of the positive electrode film layer.

[0372] The test method for elongation is as follows:

[0373] 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.

[0374] The test method for the number of flexible folds is shown below.

[0375] 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.

[0376] 2. Energy density test

[0377] Allow the lithium-ion secondary battery to rest at 25°C for 2 hours to ensure the battery temperature is 25°C. Charge the battery at 0.33°C to a charge cutoff voltage of 3.65V at 25°C. Continue constant-voltage charging 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). Allow the battery to rest at 25°C for 1 hour. Then, discharge the battery at 0.33°C at 25°C to a discharge cutoff voltage of 2.5V. Record the total discharge energy of the battery as E0.

[0378] Measure the length, width and height of the lithium-ion secondary battery and calculate the volume value V0 of the lithium-ion secondary battery = length * width * height.

[0379] The volume energy density of a lithium-ion secondary battery = the discharge energy E0 of the lithium-ion secondary battery / the volume V0 of the lithium-ion secondary battery.

[0380] 3. DCR test method

[0381] At 25°C, charge to 3.65V at 0.33C constant current, charge to 0.05C at constant voltage, then discharge to 20% SOC at 1 / 3C, let it stand for 5 minutes, then discharge at 3C pulse for 30s, let it stand for 40s, then charge at 3C for 40s, let it stand for 5 minutes, then charge to 3.65V at 1 / 3C constant current, charge to 0.05C at constant voltage, then discharge to 10% SOC at 1 / 3C, let it stand for 5 minutes, then discharge at 3C pulse for 30s, let it stand for 40s. s, then charge at 3C for 40s, let it stand for 5min, then fully charge at 1 / 3C, then discharge at 1 / 3C to 50% SOC, then let it stand at -25°C for 2h, then pulse discharge at 1C for 30s, let it stand for 10min, then let it stand at 25°C for 2h, charge at 1 / 3C constant current to 3.65V, then charge at constant voltage to 0.05C, then discharge at 1 / 3C to 20% SOC, then let it stand at -25°C for 2h, then pulse discharge at 1C for 30s, let it stand for 10min.

[0382] 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.

[0383] Experimental parameters and test results

[0384] Batteries of various examples and comparative examples were prepared according to the above methods, and various performance parameters were measured. The results are shown in Table 1 below.

[0385] Table 1

[0386]

[0387] Table 1

[0388]

[0389] From the data comparison of the embodiment and the comparative example, it can be seen that, different from the prior art, in the section of the positive electrode film along the thickness direction of the electrode, the particle size concentration is too high, which has limited effect on improving the compaction of the electrode, and it is difficult to further improve the compaction of the electrode by adjusting the particle size concentration. A90 1400nm-2100nm, particle size concentration (DA90 -D A10 ) / D A50 When it is 1.855-2.375, the battery has a higher positive electrode sheet compaction density and energy density and enables the battery to have good dynamic performance.

[0390] From the comparison of Example 5 and other examples, it can be seen that the D A90 The battery's DC internal resistance can be further reduced at 1400nm-2000nm.

[0391] From the comparison between Examples 5 and 6 and other examples, it can be seen that in the cross section of the positive electrode film along the thickness direction of the electrode, the D A50 A diameter of 650nm-750nm is beneficial for further reducing the impedance of the battery while increasing the compaction density of the electrode, taking into account the dynamic performance of the battery.

[0392] By comparing Example 2, Example 4 and Comparative Example 1, it can be seen that in the cross section of the positive electrode film along the thickness direction of the electrode sheet, the particle size concentration (D A90 -D A10 ) / D A50 The particle size distribution in the positive electrode film layer is 1.855-2.375, which improves the particle stacking and increases the compaction density of the electrode and the energy density of the battery.

[0393] 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: It 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 arranged on at least one side of the positive electrode current collector. The positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes lithium-containing transition metal phosphate particles with a carbon coating material provided on at least a portion of the surface. In the cross section of the positive electrode film along the thickness direction of the electrode, the D A90 1400nm-2100nm, particle size concentration (D A90 -D A10 ) / D A50 is 1.855-2.375, among which D A90 、D A50、 D A10 It refers to the particle size corresponding to the cumulative area distribution of the particles when the cumulative area distribution of the particles reaches 90%, 50%, and 10% in the area cumulative distribution curve of the particles; the median of the sphericity L in the area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode A50 It is 0.70-0.

85.

2. The lithium-ion secondary battery according to claim 1, wherein In the cross section of the positive electrode film along the thickness direction of the electrode, the particle size concentration (D A90 -D A10 ) / D A50 It is 2.031-2.

341.

3. The lithium-ion secondary battery according to claim 1, wherein In the cross section of the positive electrode film along the thickness direction of the electrode, the D A90 It is 1400nm-2000nm.

4. The lithium-ion secondary battery according to claim 1, wherein In the cross section of the positive electrode film along the thickness direction of the electrode, the D A90 It is 1400nm-1674nm.

5. The lithium-ion secondary battery according to claim 1, wherein In the cross section of the positive electrode film along the thickness direction of the electrode, the D A50 It is 600nm-900nm.

6. The lithium-ion secondary battery according to claim 1, wherein In the cross section of the positive electrode film along the thickness direction of the electrode, the D A50 It is 650nm-750nm.

7. The lithium-ion secondary battery according to claim 1, wherein In the cross section of the positive electrode film along the thickness direction of the electrode, the D A10 100nm-300nm.

8. The lithium-ion secondary battery according to claim 1, wherein In the cross section of the positive electrode film along the thickness direction of the electrode, the D A10 120nm-250nm.

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 median value of the graphitization degree C 50 is 0.98-1.20; wherein the graphitization degree C value is I G / I D , where I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at .

10. 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 is 1.02-1.10, 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 .

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

76.

12. The lithium-ion secondary battery according to claim 1, wherein In the cumulative distribution curve of the sphericity of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median of the sphericity L A50 It is 0.709-0.

751.

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

96.

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

10.

15. The lithium-ion secondary battery according to claim 1, wherein The iron dissolution rate of the positive electrode film layer is 400ppm-1800ppm.

16. 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%.

17. 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 0.9%-1.5%.

18. The lithium-ion secondary battery according to claim 1, wherein The lithium iron antisite defect concentration of the positive electrode active material is 0.001%-1.5%.

19. The lithium-ion secondary battery according to claim 1, wherein The lithium iron antisite defect concentration of the positive electrode active material is 0.01%-1.0%.

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

1.

21. The lithium-ion secondary battery according to claim 1, wherein The positive electrode active material includes one or more of lithium iron phosphate and its doping modified materials and coating modified materials.

22. The lithium-ion secondary battery according to claim 1, wherein 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 4000 ppm-8000 ppm.

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

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

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

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

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

28. 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.

29. The lithium-ion secondary battery according to claim 1, wherein The positive electrode film layer further includes a conductive agent. Based on the total mass of the positive electrode film layer, the mass content of the conductive agent is 0.01%-1.5%.

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

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

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

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

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

35. 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 2.52 g / cm 3 -2.78g / 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 -2.75g / cm 3 In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10%-20%.

36. 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; (3) The compaction density of the positive electrode sheet in the fully charged 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.

37. A battery device, characterized in that: The lithium-ion secondary battery according to any one of claims 1 to 36 is included, wherein the battery device comprises at least one of a battery module, a battery pack, and an energy storage battery.

38. An electrical device, characterized in that: A lithium ion secondary battery according to any one of claims 1 to 36 or a battery device according to claim 37.

39. A method for preparing a positive electrode active material, characterized in that: The preparation method is used to prepare the positive electrode active material in the lithium ion secondary battery according to any one of claims 1 to 36, and the preparation method comprises: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source; adding a solvent and grinding to obtain a mixed slurry; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder to obtain the positive electrode active material; the sintering of the precursor powder to obtain the positive electrode active material comprises at least two sinterings; after the first sintering, a first sintered product is obtained, the first sintered product and a carbon source are mixed to obtain an intermediate raw material, the intermediate raw material is divided into two groups, and the first group of ground products and the second group of ground products are respectively ground to obtain a first group of ground products and a second group of ground products, the first group of ground products and the second group of ground products are mixed to obtain a mixed intermediate product; the mixed intermediate product is subjected to a second sintering to obtain the positive electrode active material; wherein, the D of the first group of ground products is V50 The D of the second group of grinding products is 0.8μm-1.2μm; V50 0.3μm-0.5μm.

40. 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 39 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 the current collector, and drying and hot pressing to obtain a positive electrode film layer.

41. The preparation method according to claim 40, characterized in that The drying temperature is 95°C-105°C, and the drying speed is 2.0m / min-2.3m / min.

42. The preparation method according to claim 40 or 41, characterized in that The hot pressing includes at least three hot roller pressings, and the hot roller pressure increases successively, and the hot roller pressure is 20 tons-50 tons, 50 tons-70 tons, and 70 tons-90 tons respectively; the hot roller temperature is 40°C-80°C, and the pole piece is heated before entering the hot roller compaction for the first time, and the heating temperature is 40°C-50°C.

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