Lithium-ion secondary battery, battery apparatus, electric apparatus, preparation method for positive electrode active material, and preparation method for positive electrode sheet

Abstract

The present application provides a lithium-ion secondary battery, a battery apparatus, an electric apparatus, a preparation method for a positive electrode active material, and a preparation method for a positive electrode sheet. The lithium-ion secondary battery comprises a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer comprises a positive electrode active material, the positive electrode active material comprises lithium-containing transition metal phosphate particles having a carbon coating material disposed on at least a portion of surfaces thereof, and in a cross-section of the positive electrode film layer in a thickness direction of the electrode sheet, an area percentage of particles having a particle size greater than or equal to 1 μm is 30.0% to 50.0%; and a mass proportion of a magnetic substance in the positive electrode film layer is greater than or equal to 20 ppm and less than or equal to 1980 ppm.

Description

Methods for preparing lithium-ion secondary batteries, battery devices, electrical devices, positive electrode active materials, and positive electrode sheets. Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and in particular to a lithium-ion secondary battery, battery device, power device, a method for preparing positive electrode active material, and a method for preparing positive electrode sheet. Background Technology

[0002] In recent years, lithium-ion secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles, electric cars, military equipment, and aerospace.

[0003] As market demands for longer battery life and greater safety in electrical devices increase, higher requirements are being placed on the energy density and storage performance of lithium-ion rechargeable batteries. However, existing technologies struggle to simultaneously improve these performance characteristics, making this a critical technical problem that needs to be solved in this field. Summary of the Invention

[0004] This application is made in view of the above-mentioned issues, and its purpose is to provide a lithium-ion secondary battery that has both high energy density and good storage performance.

[0005] The first aspect of this application provides a lithium-ion secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive current collector and a positive electrode film disposed on at least one side of the positive current collector. The positive electrode film includes a positive active material, comprising lithium transition metal phosphate particles with at least a portion of their surface coated with carbon material. In a cross-section of the positive electrode film along the thickness direction of the electrode, the area percentage of particles with a diameter greater than or equal to 1 μm is 30.0%-50.0%. Furthermore, the mass percentage of magnetic material in the positive electrode film is greater than or equal to 20 ppm and less than or equal to 1980 ppm.

[0006] The embodiments of this application not only increase the area ratio of large-size particles, but also effectively reduce the content of magnetic materials in the positive electrode film, so that the mass ratio of magnetic materials in the positive electrode film is greater than or equal to 20ppm and less than or equal to 1980ppm. The electrode compaction density in the lithium-ion secondary battery can be increased while maintaining a low self-discharge rate, which is conducive to improving the battery energy density and maintaining it for a long time during storage.

[0007] In any embodiment, the area ratio of particles with a diameter of 1μm-5μm in the cross-section of the positive electrode film along the thickness direction of the electrode sheet is 30.0%-50.0%.

[0008] In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the area ratio of particles with a particle size of 1μm-5μm is within the above range. This can improve the compaction density of the electrode sheet through gradation while controlling the content of magnetic materials in the battery, which is beneficial to improving the energy density of lithium-ion secondary batteries and maintaining it for a long time during storage.

[0009] In any embodiment, the area ratio of particles with a diameter of 1μm-5μm in the cross-section of the positive electrode film along the thickness direction of the electrode sheet is 30.0%-45.0%.

[0010] In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the area ratio of particles with a diameter of 1μm-5μm is within the above range. This can improve the compaction density and capacity storage stability while further taking into account the migration distance of lithium ions inside the particles, thereby enabling the lithium-ion secondary battery to maintain a low impedance and improve the battery's dynamic performance.

[0011] In any embodiment, in the cross-section of the positive electrode film layer along the electrode thickness direction, the average equivalent area ratio of particles with a particle size of 1 μm or larger is 0.05%-0.20%.

[0012] If the average equivalent area ratio of particles with a diameter of 1 μm or larger in the cross-section of the positive electrode film along the thickness direction of the electrode sheet is within the above range, it means that the cross-section of the positive electrode film along the thickness direction of the electrode sheet has a certain number of large particles to improve the compaction density of the electrode sheet, but the large particle size will not seriously degrade the dynamic performance of the lithium-ion secondary battery, thus balancing the energy density and dynamic performance of the battery.

[0013] In any embodiment, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size of 50nm-200nm is 3.0%-15.0%, optionally 5.0%-12.0%, and further optionally 5.0%-10.0%.

[0014] The fact that the area ratio of particles with a diameter of 50nm-200nm is within the above range means that there are a certain number of particles with a diameter of 50nm-200nm. This is beneficial to improve the compaction density of the positive electrode active material powder and the compaction density of the electrode sheet through gradation, thereby further improving the energy density of the lithium-ion secondary battery.

[0015] In any embodiment, the mass percentage of the magnetic material in the positive electrode film layer is less than or equal to 300 ppm, and can be selected as 20-200 ppm.

[0016] Within the above-mentioned range, the mass content of magnetic materials can further mitigate self-discharge and improve the storage stability of battery capacity.

[0017] In any embodiment, the magnetic material includes one or more of Fe, Fe2P, FeP, γ-Fe2O3, and Fe2P2O7.

[0018] In any embodiment, the mass content of elemental iron in the positive electrode film layer is less than 20 ppm, and may be less than or equal to 15 ppm.

[0019] Controlling the mass content of elemental iron within the above range is beneficial to improving the safety performance of the battery.

[0020] 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.

[0021] Selecting appropriate modifying element Q can improve the lattice change rate of positive electrode active materials during lithium insertion / extraction, reduce oxygen activity on particle surface, improve the structural stability of materials, thereby improving the specific capacity utilization level of materials and further improving the energy density of lithium-ion secondary batteries.

[0022] In any embodiment, the positive electrode active material includes titanium, and the mass content of titanium is 1500ppm-8000ppm based on the total mass of the positive electrode active material, optionally 2500ppm-8000ppm, and further optionally 2500ppm-6000ppm.

[0023] The positive electrode active material includes titanium, and its mass content is controlled within the aforementioned range. On one hand, this inhibits particle growth, controlling the size of large particles and ensuring that the average equivalent area of ​​particles larger than 1 μm in the cross-section along the electrode thickness is within a suitable range. On the other hand, its surface inertness reduces the probability of uneven local chemical reactions and the formation of magnetic materials. Simultaneously, the doping of titanium in the positive electrode active material improves the electronic conductivity and ion transport rate of lithium-containing transition metal phosphates, mitigating the negative impact of relatively large particles on the kinetic performance of the positive electrode active material. By influencing particle size and lithium-ion transport paths, a balance between battery energy density and kinetic performance is achieved.

[0024] In any embodiment, the mass percentage of carbon element is 0.9%-1.8% based on the total mass of the positive electrode active material.

[0025] Based on the total mass of the positive electrode active material, a carbon content within the above-mentioned range can improve the conductivity of the positive electrode active material and enhance the kinetic performance of the lithium-ion secondary battery. It can also reduce the negative impact of excessive carbon content on the loading of lithium-containing transition metal phosphates, taking into account both the compaction density of the electrode and the impedance of the lithium-ion secondary battery, while simultaneously improving the energy density and kinetic performance of the battery.

[0026] In any embodiment, the median C of the graphitization degree C value in the cumulative distribution curve of the positive electrode film obtained in the laser microscopy confocal Raman spectroscopy instrument scanning mode is... 50 The value is greater than or equal to 0.9 and less than or equal to 1.3, and can be selected from 0.99 to 1.2, where the graphitization degree C value is I. G / I D , where I G This indicates that the Raman spectrum is at 1580±100 cm⁻¹ -1 The intensity of peak G at I D This indicates that the Raman spectrum is at 1350±100 cm⁻¹ -1 The intensity of peak D at that location.

[0027] Positive electrode active materials with graphitization within the above range can easily achieve particle slippage during the roll forming process by means of the graphitized carbon layer on the surface of the positive electrode active material, thereby offsetting the negative impact of particle size on the compaction density of the electrode sheet, and further improving the compaction density of the positive electrode film layer by means of particle slippage.

[0028] In any embodiment, the median L of the sphericity in the cumulative distribution curve of the particle sphericity area obtained from the cross-section of the positive electrode film along the electrode thickness direction is... A50 The range is 0.60-0.85, and the option is 0.65-0.80.

[0029] Median L of sphericity A50 Within the aforementioned range, the particles are approximately spherical, and under external force, they are prone to slippage between each other, which can further improve the compaction density of the electrode and increase the energy density of the battery.

[0030] In any embodiment, the compacted density of the positive electrode active material powder under 3T pressure is 2.48 g / cm³. 3 -2.76g / cm 3 The option is 2.58g / cm³. 3 -2.76g / cm 3 .

[0031] This positive electrode active material has a high powder compaction density, which provides a material basis for improving the compaction density of the electrode and preparing high-energy-density lithium-ion secondary batteries.

[0032] In any embodiment, the compacted density of the positive electrode active material powder under 3T pressure is 2.58 g / cm³. 3 -2.76g / cm 3 .

[0033] Positive electrode active materials with a compaction density within the above range can further improve the compaction density of the electrode and increase the energy density of the battery.

[0034] In any embodiment, the discharge specific capacity of the positive electrode active material at a 1C discharge rate at room temperature is 135mAh / g-150mAh / g.

[0035] The positive electrode active material has a high discharge specific capacity, indicating that the positive electrode active material has good kinetic performance and is beneficial to improving the energy density of lithium-ion secondary batteries.

[0036] In any embodiment, the discharge capacity η of the positive electrode active material discharged to 3.2V accounts for ≥85%, where η is defined as follows: at room temperature, a coin cell containing the positive electrode active material is charged and discharged twice at a constant current of 0.1C within a voltage range of 2.0V to 3.75V, followed by a constant current charge and discharge once at a constant current of 1C. In the 1C charge and discharge test, the capacity value extracted at a discharge voltage of 3.2V is recorded as C1, and the capacity value extracted at a discharge voltage of 2.0V is recorded as C2, where η = C1 / C2. The charging process includes constant voltage charging, with a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

[0037] The high discharge capacity ratio of the positive electrode active material used in the battery embodiment of this application to 3.2V indicates that although the positive electrode active material has a certain proportion of large-sized particles, it still maintains good kinetic performance. At the same time, the high η value indicates that the lithium-ion secondary battery containing the positive electrode active material still has a high voltage when discharged to a low state of charge (SOC), which is beneficial for maintaining good power performance.

[0038] In any embodiment, the positive electrode film layer further includes a binder and a conductive agent. Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 94%-99.4%, the mass content of the binder is 0.5%-3%, and the mass content of the conductive agent is 0.1%-3%.

[0039] In any embodiment, the one-sided density of the positive electrode film is 300 mg / 1540 mm². 2 -450mg / 1540mm 2 .

[0040] Positive electrode films with areal densities within the above range can help improve the energy density of lithium-ion secondary batteries.

[0041] In any embodiment, the compaction density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery is 2.43 g / cm³. 3 -2.78g / cm 3 .

[0042] In any embodiment, the compaction density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery is 2.50 g / cm³. 3 -2.75g / cm 3 .

[0043] In any embodiment, the positive electrode film layer of the lithium-ion secondary battery in its fully discharged state has a compaction density of 2.43 g / cm³. 3 -2.78g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-28%.

[0044] In any embodiment, the positive electrode film layer of the lithium-ion secondary battery in a fully discharged state has a compaction density of 2.5 g / cm³. 3 -2.78g / cm 3 In the cross-section along the thickness direction of the positive electrode film, the porosity of the positive electrode film is 10%-22%.

[0045] Lower porosity means, on the one hand, a better gradation of large, medium, and small particles in the positive electrode film, resulting in higher compaction density. On the other hand, under the same gradation and roller pressure, lower porosity means that particles are more likely to slide against each other, thereby reducing the risk of overpressure and stress concentration in the film, further reducing the probability of the positive electrode film demolding during long cycles, and improving the long-cycle performance of the battery. In any embodiment, the positive electrode sheet includes a base coating layer disposed between the positive electrode film and the current collector; the base coating layer includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the base coating layer is ≤10 pcs / 10 μm.

[0046] The undercoat layer helps improve the conductivity and adhesion between the positive electrode film and the current collector, reduces the likelihood of the positive electrode film detaching from the current collector during cycling, and improves the battery's kinetic performance. In the high compaction density electrode sheets of this application embodiment, for example, the compaction density of the positive electrode sheet in its fully stacked state is greater than or equal to 2.4 g / cm³. 3At this time, the current collector is easily damaged during the high-pressure compaction process of the electrode sheet, and large-sized particles are prone to creating pits on the current collector. Controlling the distribution density of carbon-based particles with a particle size greater than 100nm in the bottom coating layer to ≤10pcs / 10μm is beneficial to reducing the probability of damage to the current collector in the high-pressure compaction electrode sheet and further improving the ultimate compaction density of the positive electrode sheet.

[0047] In any embodiment, the positive electrode sheet includes a base coating layer disposed between the positive electrode film layer and the current collector; the compacted density of the positive electrode sheet in its fully loaded state is greater than or equal to 2.4 g / cm³. 3 The thickness of the base coating layer on one side is 1μm-4μm.

[0048] In any embodiment, the positive electrode sheet includes a base coating layer disposed between the positive electrode film layer and the current collector; the compacted density of the positive electrode sheet in its fully loaded state is greater than or equal to 2.5 g / cm³. 3 The thickness of the base coating layer on one side is 2μm-4μm.

[0049] As the compaction density of the electrode increases, the compressive effect of large lithium phosphate particles (e.g., particles larger than 1 μm) on the undercoat becomes more significant. Therefore, stress concentration easily occurs at large particle sites, and these particles can even penetrate the undercoat and damage the current collector. Increasing the thickness of the undercoat helps to mitigate stress concentration in the electrode, further improving the electrode's ultimate compaction density.

[0050] The second aspect of this application provides a battery device including the lithium-ion secondary battery provided in the first aspect of this application, wherein the battery device includes at least one of a battery module, a battery pack, and an energy storage battery.

[0051] A third aspect of this application also provides an electrical device, which includes the lithium-ion secondary battery provided in the first aspect of this application or the battery device provided in the second aspect of this application.

[0052] The fourth aspect of this application also provides a method for preparing a positive electrode active material: obtaining a mixed raw material comprising a carbon source, a lithium source, an iron source, and a phosphorus source, wherein the molar ratio of lithium to iron in the mixed raw material is greater than 1 and less than 1.05; grinding to obtain a mixed slurry, wherein the solid volume distribution particle size DV50 in the mixed slurry is 0.3 μm-0.4 μm; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder to obtain a positive electrode active material, wherein the sintering is carried out in an inert gas environment and the total gas flow rate during the sintering process is 1100 m³ / h. 3 / h-1400m 3 / h; the sintering includes a heating zone and a constant temperature zone, wherein the inert gas flow rate v1 in the heating zone is higher than the inert gas flow rate v2 in the constant temperature zone; the constant temperature zone temperature of the sintering includes 770℃-830℃; the positive electrode active material includes lithium transition metal phosphate particles with at least a carbon coating material on their surface.

[0053] The positive electrode active material prepared by this method can have particles with a certain area ratio of more than 1μm in the cross-section of the positive electrode film along the thickness direction of the electrode sheet, and at the same time, the positive electrode active material contains a small amount of magnetic material. While improving the electrode sheet compaction density and improving the energy density of lithium-ion secondary batteries, the battery also has a low self-discharge level, so that the energy density of lithium-ion secondary batteries can be maintained for a long time during battery storage and cycling.

[0054] The fifth aspect of this application provides a method for preparing a positive electrode sheet, the method comprising sequentially adding a binder, a conductive agent, and a positive active material prepared by the method of the fourth aspect, dry mixing them, adding a solvent, stirring to obtain a slurry; transferring the slurry to at least one side of a current collector, drying and hot pressing to obtain a positive electrode sheet.

[0055] In any embodiment, the hot pressing includes at least three hot roller pressings, with the hot roller pressure increasing sequentially to 20-50 tons, 50-70 tons, and 70-90 tons; the hot roller temperature is 40℃-80℃, and the electrode is heated to 40℃-50℃ before the first hot roller compaction.

[0056] The positive electrode active material prepared by the above-mentioned hot pressing process in combination with the preparation method in the fourth aspect effectively improves the compaction density of the positive electrode sheet while maintaining a low content of magnetic materials, thereby improving the energy density of the battery while maintaining low self-discharge.

[0057] In any embodiment, the coating speed of the transfer coating is 1 m / min to 25 m / min.

[0058] A coating speed within the above range is beneficial for improving the uniformity of particle distribution during the coating process, reducing the risk of particle agglomeration in the positive electrode film, reducing the cross-sectional porosity of the positive electrode film, further improving the ultimate compaction density of the electrode sheet, and improving the energy density of the battery. Attached Figure Description

[0059] Figure 1 is a scanning electron microscope image of a cross-section of the positive electrode film layer along the electrode thickness direction according to an embodiment of this application.

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

[0061] Figure 3 is an exploded view of a lithium-ion secondary battery according to an embodiment of this application;

[0062] Figure 4 is a schematic diagram of a battery module according to an embodiment of this application;

[0063] Figure 5 is a schematic diagram of a battery pack according to an embodiment of this application;

[0064] Figure 6 is an exploded view of the battery pack shown in Figure 5;

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

[0066] Figure 8 is a porosity test diagram of the positive electrode film layer in a cross section along the electrode thickness direction according to an embodiment of this application.

[0067] Explanation of reference numerals in the attached diagram: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Lithium-ion secondary battery; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0068] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the lithium-ion secondary battery, battery device, power-consuming device, method for preparing positive electrode active material, and method for preparing positive electrode sheet of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0069] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0070] Unless otherwise specified, all embodiments and optional embodiments of this application may be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.

[0071] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions, and such technical solutions shall be deemed to be included in the disclosure of this application.

[0072] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), indicating that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0073] In this application, the terms "multiple" or "various" refer to two or more kinds of things.

[0074] Unless otherwise stated, the terms used in this application have the common meanings as commonly understood by those skilled in the art.

[0075] Unless otherwise stated, the values ​​of the parameters mentioned in this application can be determined using various testing methods commonly used in the art, for example, according to the testing methods given in the embodiments of this application. Unless otherwise stated, the test temperature for each parameter is 25°C.

[0076] The battery mentioned in the embodiments of this application can be a single physical module comprising one or more lithium-ion secondary batteries to provide higher voltage and capacity. For example, the battery mentioned in this application may include lithium-ion secondary batteries, battery cells, battery modules, or battery packs.

[0077] A lithium-ion secondary battery is the smallest unit that makes up a battery, and it can independently perform the functions of charging and discharging. Lithium-ion secondary batteries can be cylindrical, cuboid, or other shapes, and the embodiments of this application are not limited to this. Figure 2 shows a cuboid lithium-ion secondary battery 5 as an example.

[0078] Lithium-ion secondary batteries consist of electrode components and electrolyte.

[0079] Lithium-ion secondary batteries may also include an outer packaging that encapsulates the electrode components and electrolyte. The outer packaging can be a rigid shell, such as a hard plastic shell, aluminum shell, or steel shell. It can also be a flexible package, such as a pouch. The material of the flexible package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0080] In some embodiments, as shown in FIG3, the outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates enclosing a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 is used to cover the opening to close the receiving cavity. Electrode assemblies 52 are encapsulated in the receiving cavity. The number of electrode assemblies 52 contained in the lithium-ion secondary battery 5 may be one or more, and can be adjusted as needed.

[0081] Electrode assemblies typically include positive and negative electrodes. The negative electrode is the electrode that absorbs or lithiates lithium ions during charging and releases or delithiates lithium during discharging. The positive electrode is the electrode that absorbs or delithiates lithium ions during charging and absorbs or lithiates lithium during discharging.

[0082] When multiple lithium-ion secondary batteries are present, they are connected in series, parallel, or mixed via a busbar. In some embodiments, the battery can be a battery module; when multiple lithium-ion secondary batteries are present, they are arranged and fixed to form a battery module. In some embodiments, the battery can be a battery pack, which includes a housing and lithium-ion secondary batteries, with the lithium-ion secondary batteries or battery modules housed within the housing. In some embodiments, the housing can be part of the vehicle's chassis structure. For example, a portion of the housing can be at least part of the vehicle's floor, or a portion of the housing can be at least part of the vehicle's crossbeams and longitudinal beams.

[0083] In some implementations, the battery can be an energy storage device. Energy storage devices include energy storage containers, energy storage cabinets, etc.

[0084] In some embodiments, lithium-ion secondary batteries can be assembled into battery modules. The number of lithium-ion secondary batteries in a battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module. Figure 4 is a schematic diagram of a battery module 4 as an example. As shown in Figure 4, in the battery module 4, multiple lithium-ion secondary batteries 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other arbitrary manner. Furthermore, the multiple lithium-ion secondary batteries 5 can be fixed in place using fasteners.

[0085] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of lithium-ion secondary batteries 5 are received.

[0086] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0087] Figures 5 and 6 are schematic diagrams of a battery pack 1 as an example. As shown in Figures 5 and 6, the battery pack 1 may include a housing and multiple battery modules 4 disposed within the housing. The housing includes an upper housing 2 and a lower housing 3. The upper housing 2 covers the lower housing 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the housing.

[0088] Lithium-containing transition metal phosphate materials have been widely used in lithium-ion batteries due to their stable structure, good safety and long cycle life; however, they have problems such as low electronic conductivity and low stacking efficiency, which result in low loading of active materials in the battery and cannot meet the needs of high energy density batteries.

[0089] Studies have shown that increasing the number and proportion of large-sized particles in lithium transition metal phosphate materials is an effective way to improve powder compaction density and increase the loading of positive electrode active materials in batteries. The molding of lithium transition metal phosphate materials often requires a high-temperature sintering process. Larger particle sizes generally mean more solid-phase diffusion and grain boundary melting between raw materials, requiring higher energy consumption and sintering temperatures. However, experimental results show that batteries with large-sized lithium transition metal phosphate particles are often accompanied by high self-discharge. Researchers have found that this is because with the increase of the sintering temperature of lithium transition metal phosphate materials, carbothermic reduction reactions easily occur at their lattice defects, reducing them to magnetic materials such as Fe and Fe2P by carbon on their surface and other reducing substances generated during the preparation process (such as hydrogen and carbon monoxide). Therefore, the increase of large-sized particles in the positive electrode active material is often accompanied by an increase in the content of magnetic materials. Magnetic materials can easily cause organic matter in the electrolyte to agglomerate and grow during the charging and discharging process of lithium-ion secondary batteries, forming sharp edges or spikes. These edges or spikes can easily pierce the separator, creating micro-short circuits inside the lithium-ion secondary battery and generating leakage current paths. As a result, the battery's capacity will gradually decrease even when no external load is connected, i.e., the self-discharge phenomenon will increase, which will deteriorate the long-term performance of lithium-ion secondary batteries.

[0090] The first aspect of this application provides a lithium-ion secondary battery, including a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive current collector and a positive electrode film disposed on at least one side of the positive current collector. The positive electrode film includes a positive active material, which includes lithium transition metal phosphate particles with at least a portion of their surface coated with carbon material. As shown in FIG1, in a cross-section of the positive electrode film along the thickness direction of the electrode, the area ratio of particles with a particle size greater than or equal to 1 μm is 30%-50%. Furthermore, in the positive electrode film, the mass ratio of magnetic material is greater than or equal to 20 ppm and less than or equal to 1980 ppm.

[0091] When the area ratio of particles with a diameter greater than or equal to 1 μm in the positive electrode film is less than 30%, it is difficult to achieve a high compaction density for the positive electrode sheet. When the area ratio of particles with a diameter greater than or equal to 1 μm in the positive electrode film exceeds 50%, it is necessary to increase the sintering temperature or sintering time, which will increase the content of magnetic materials and cause an increase in the self-discharge K value of the battery cell. While controlling the area ratio of particles with a diameter greater than or equal to 1 μm to 30.0%-50.0%, the content of magnetic materials can be reduced by adjusting the gas flow rate, so that the mass ratio of magnetic materials in the positive electrode film is greater than or equal to 20 ppm and less than or equal to 1980 ppm, thereby improving the compaction density of the electrode sheet while reducing the self-discharge phenomenon of the battery.

[0092] In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the area ratio of particles with a diameter greater than or equal to 1 μm is 30.0%-50.0%. This allows the particles to fully play a gradation role during electrode fabrication and cycling, effectively improving the compaction density of the electrode sheet. However, the preparation of positive electrode active materials containing particles with an area ratio within the above range often requires long-term high-temperature sintering, leading to an increase in the content of magnetic materials. The embodiments of this application, while increasing the area ratio of large-size particles, also effectively reduce the content of magnetic materials in the positive electrode film, making the mass ratio of magnetic materials in the positive electrode film greater than or equal to 20 ppm and less than or equal to 1980 ppm. This improves the compaction density of the electrode sheet in the lithium-ion secondary battery while maintaining a low self-discharge rate, which is beneficial for improving the battery energy density and maintaining it for a long time during storage. In this application, the term "particle" refers to a particle in the positive electrode film layer with a identifiable complete boundary in the field of view at a certain magnification, such as 10,000x. Defects and scratches may exist inside the particle, but a complete boundary sufficient to divide the particle cannot be identified inside the particle.

[0093] The particle identification method is as follows: The positive electrode film layer is cut along the thickness direction of the electrode sheet using an argon ion beam (for example, a Leica EM TIC 3X CP device can be used, operating voltage: 6kV, operating time: 6h). After exposing the cut surface, a scanning electron microscope (SEM) is used (for example, a Hitachi SU8230 device can be used, operating voltage: 3kV, beam current: high, probe model: U (LA100), working distance <5mm) to observe the cut surface along the thickness direction of the electrode sheet. Images are acquired using a field emission scanning electron microscope at a non-edge location in the cut surface of the positive electrode film layer (after observing the electrode edge under the SEM, the field of view is adjusted to the center of the sample) in secondary electron mode. Electron micrographs are taken at 10kx magnification, and the particles in the electron micrographs are analyzed using ImageJ software (1.46r, win64 version). The specific steps for using ImageJ software are as follows: Load the scanning electron microscope image to be analyzed, as shown in Figure 1; use the Cellpose plugin software to identify particles, and then perform manual corrections; use ImageJ to read and analyze data. The specific method for identifying particles using the Cellpose plugin software is as follows: Set the segmentation diameter parameter (diameter in the Segmentation module) to 15 pixels, click "run cyto3" to identify particles; manually mark particles in the image that were not identified by the software, were not fully identified by the software, or had identification errors. Particles that were not identified by the software, were not fully identified by the software, or had identification errors mainly include the following: 1. Particles that are too large or have scratches on their surface, making them unidentifiable or incompletely recognizable; 2. During argon ion beam cutting, scratches may be generated on the particle surface, and the software may misjudge these scratches as particle boundaries, leading to identification errors; 3. Particles that are too small and were not successfully identified; 4. Particles located at the edge of the electron microscope's field of view, with the particle's interior penetrated by the edge, preventing a complete display of the morphology, and resulting in identification errors due to partial identification replacing the whole.For the unidentified or misidentified particles mentioned above, manual calibration is performed. The specific process is as follows: 1. Delete large particles located around the edges of the scanning electron microscope that are not fully displayed. 2. Determine if any unidentified or misidentified particles have internal cracks or scratches. If no cracks or scratches are found, classify it as a single particle and manually mark it based on the observed particle boundary. 3. If cracks or scratches are found inside the particle, determine if they penetrate the particle. If not, classify it as a single particle and manually mark it. 4. If cracks or scratches penetrate the particle, determine if they are linear or irregular. 5. If the cracks or scratches are irregular, classify them as the boundary between particles and divide the particles along this boundary. 6. If the cracks or scratches are linear, perform contrast comparison. 7. If the contrast is not obvious and there is no crack-like appearance, classify it as a scratch and mark it as a single particle. 8. If the contrast is strong and there is a crack-like appearance, classify it as the boundary between particles and mark it as two particles. After manual marking, delete information irrelevant to the particles from the automatic image processing, thus completing the particle identification and marking in the image.

[0094] During the compaction process, the positive electrode film undergoes compaction along its thickness direction. Therefore, compared to the surface of the positive electrode film, the cross-section along the thickness direction provides a more accurate reflection of the actual compaction of the particles within the film on a spatial scale. In the cross-section along the thickness direction, the area ratio of particles with a diameter greater than or equal to 1 μm directly reflects the ratio of the area of ​​some particles in that size range to the total area of ​​the particles, thus indicating the size of the particles in that range.

[0095] It is understandable that the particles in the cross-section of the positive electrode film along the thickness direction of the electrode sheet, especially those larger than 50 nm, mainly originate from the positive electrode active material. Therefore, the embodiments of this application can accurately and objectively reflect the distribution of lithium-containing transition metal phosphate particles in the electrode sheet by observing and statistically analyzing the particle area in the cross-section of the positive electrode film.

[0096] In existing technologies, laser particle size analyzers are typically used to statistically analyze the particle size of positive electrode active materials using Malvern laser diffraction. However, the applicant's research indicates that because lithium-containing transition metal phosphate particles are prone to agglomeration, the test results obtained by Malvern laser diffraction based on the principle of laser scattering often only reflect the particle size of the agglomerates, and cannot accurately reflect the particle size of the positive electrode active material, let alone its dispersion state in the film layer. This is because the dispersion of the positive electrode active material in the film layer increases during slurry preparation and film forming rolling. The test results obtained by Malvern laser diffraction are affected by the particle size, specific surface area, and degree of agglomeration of the positive electrode active material. Compared to the actual dispersion in the electrode sheet, the number of large particles obtained by this test is lower than the actual value, and the number of small particles is higher than the actual value. Therefore, the particle size obtained by Malvern laser diffraction cannot be equated with or analogized to the particle size statistically obtained in the embodiments of this application.

[0097] In some embodiments, the area of ​​particles with a diameter greater than or equal to 1 μm in the cross-section of the positive electrode film along the thickness direction of the electrode sheet accounts for 30%-50%.

[0098] The specific method for testing the area ratio of particles with a diameter greater than or equal to 1 μm in the cross-section of the positive electrode film along the electrode thickness direction is as follows: Particles in the positive electrode film are identified using the method described above in this application. The images after particle identification and labeling are imported into ImageJ software for analysis. A scale is set based on the scanning electron microscope image. The particle diameter, area, sphericity, and roughness of the particles in the cross-section along the electrode thickness direction of the positive electrode film are statistically analyzed using the "Feret diameter," "Area," "Round," and "Solidity" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the obtained "Feret" parameter represents the maximum spacing between all parallel lines in the two-dimensional projection of the particle, thus characterizing the particle diameter; the obtained "Area" parameter represents the pixel area of ​​the particle. Because particles smaller than 50 nm are prone to significant errors during statistical analysis and are difficult to accurately identify, and because the particle size of conductive agents is generally smaller than 50 nm, which can also introduce large errors into the statistical results, particles smaller than 50 nm are not counted in the particle size statistics process of this application, and the statistical data of particles whose AR, Round, or Solidity values ​​are displayed as "NaN" are deleted. The sum of the "Area" parameters of particles with a diameter greater than or equal to 1 μm and the sum of the "Area" parameters of all particles are calculated and used as the area of ​​particles with a diameter greater than or equal to 1 μm and the total area of ​​the counted particles, respectively. The sum of the areas of particles with a diameter greater than or equal to 1 μm divided by the total area of ​​the counted particles is used as the proportion of the area of ​​particles with a diameter greater than or equal to 1 μm in the cross-section of the positive electrode film along the thickness direction of the electrode sheet.

[0099] In some embodiments, the area percentage of particles with a diameter greater than or equal to 1 μm in the cross-section of the positive electrode film along the electrode thickness direction can be selected as 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, or any value range between the two.

[0100] Lithium-containing transition metal phosphates refer to phosphate materials containing lithium and transition metal elements, and can be detected by any method known in the art. For example, they can be detected by combining X-ray diffraction (XRD) with energy dispersive spectroscopy (EDS) or inductively coupled plasma mass spectrometry (ICP-MS). As examples, lithium-containing transition metal phosphates include, but are not limited to, lithium iron phosphate, lithium manganese iron phosphate, and their doped materials.

[0101] The carbon coating layer disposed on at least a portion of the surface of lithium-containing transition metal phosphates can be detected by any method known in the art. As an example, the carbon coating layer disposed on at least a portion of the surface of lithium-containing transition metal phosphates can be observed by characterizing lithium-containing transition metal phosphates using a combination of transmission electron microscopy and energy dispersive spectroscopy.

[0102] In order to fully utilize the capacity of lithium-containing transition metal phosphate materials to improve their energy density, at least part of the surface of lithium-containing transition metal phosphate materials is often coated with a carbon layer to improve the electron conduction efficiency between positive electrode active materials.

[0103] In this application, "magnetic material" refers to a substance that can generate magnetism when subjected to a magnetic field.

[0104] The "mass content of magnetic material in the positive electrode film" mentioned in this article can be measured through the following steps. Disassemble the battery to obtain the positive electrode sheet, soak it in dimethyl carbonate solvent for 8 hours, dry it, and then sinter the positive electrode sheet at 600°C for 2 hours under a nitrogen atmosphere. During the sintering process, the positive current collector on the positive electrode sheet will detach. Crush the sintered electrode sheet with a mortar and pestle, and pass it through a 200-mesh sieve to obtain positive electrode material powder. Step 1: Weigh 80g of the positive electrode material powder obtained by the reverse method above, place it in a plastic bucket, add 6L of deionized water, and cover it with a plastic tube. A magnetic rod (magnetic induction intensity of 6000GS) is used. It is then heat-sealed using heat-sealing clamps and placed inside a plastic bucket. The bucket is then sealed, and the drum mixer is set to a speed of 60 rpm and a stirring time of 15 minutes. The sealed bucket is placed on the drum mixer for stirring. Step 2: Prepare another clean plastic bucket and add 5±0.2L of deionized water. Pour the magnetic material from the plastic tube into the bucket until the surface of the magnetic rod has no area greater than or equal to 0.5 cm². 2After removing the slurry clumps, place the magnetic rod into a clean plastic bucket. Cover the bucket with a clean lid and place the bucket back into the drum mixer. Set the drum mixer speed to 60 rpm and the mixing time to 15 minutes. Place the sealed bucket on the equipment for mixing. Repeat step 2 at least twice to ensure the accuracy of the magnetic material extraction. Step 3: Prepare a clean 500mL beaker. Remove the magnetic rod from the bucket and place it in the beaker. Use a rinsing bottle to rinse all the magnetic material from the plastic tube head into the beaker. Use demagnetized scissors to cut open both sides of the heat shrink tubing head and fold the upper edge of the heat shrink tubing 90°. Remove the magnetic rod and place it in the magnetic rod placement area. Use a rinsing bottle to rinse the heat shrink tubing from top to bottom in a Z-shape (rinse at least three times on each side) to rinse the magnetic material into the beaker until there are no particles remaining on the surface of the heat shrink tubing (if there are clumps that are difficult to rinse, scrape them off with the back of a clean ceramic knife and rinse any impurities adhering to the ceramic knife into the beaker). Lift the sleeve and rinse the bottom of the sleeve at least three times to ensure that all adsorbed magnetic particles are collected. Step 4: ① Place the small magnetic block at the bottom of the beaker and rotate it clockwise at least three times from the outside to the inside, then rotate it counterclockwise at least three times from the outside to the inside of the beaker to adsorb. ② Repeat step ① three times, each time adsorbing for at least 10 seconds. ③ Fix the small magnetic block in the center of the bottom of the beaker with your palm, let it stand for more than 2 seconds, then slowly tilt and pour out the solution. ④ Stand the beaker upright and rinse the walls of the beaker with a rinsing bottle to ensure that all the adsorbed magnetic particles are put into the solvent. The amount of solution added is 100-150 mL. ⑤ Repeat step ③ rinsing 2-4 times until the liquid in the beaker is clear (no need to add solvent after the last rinse). Step 5: First, add 70 mL of deionized water to the beaker using one syringe, then slowly add 70 mL of 36%-38% hydrochloric acid to the beaker using another syringe. After the hydrochloric acid dilution is completed, transfer it to a fluorinated bottle with a sealed cap for storage. Step 6: ① Inject 15±2 mL of the hydrochloric acid solution prepared in Step 5 into the beaker containing the extracted magnetic material using a syringe, and then seal the beaker opening with sealing film. Place it in an ultrasonic bath and sonicate for 2 minutes (power 200 W / frequency 53 kHz). After sonication, inject 100±10 mL of deionized water into the beaker for rinsing, repeating the rinsing operation twice. Inject 100-150 mL of deionized water into the beaker for filtration. Use a filter membrane with a pore size of 0.45 μm to collect the magnetic material particles. Place the filter membrane with the magnetic material particles on its surface on a slide of a cleanliness microscope and place it in an oven to dry at 45℃ for (15±2) minutes. Weigh the dried filter membrane (containing magnetic material particles) using an electronic balance and subtract the mass of the blank filter membrane to obtain the mass of the magnetic material. Calculate the mass content of the magnetic material relative to the mass of the cathode material powder sample, as the mass percentage of the magnetic material in the cathode film layer, in ppm.

[0105] In some embodiments, the mass percentage of magnetic material in the positive electrode film layer can be selected as 20ppm, 100ppm, 134.2ppm, 200ppm, 300ppm, 400ppm, 500ppm, 1000ppm, 1061.7ppm, 1450.2ppm, 1500ppm, 1980ppm or any value range between the two.

[0106] Those skilled in the art can control the area ratio of particles using any known process. For example, particle size distribution can be adjusted through the scientific gradation of particles of different sizes; the mechanical force of crushing and grinding processes can be used to process raw materials to the target particle size distribution range, thus adjusting particle size and concentration; particle size separation can be achieved by using screening and grading equipment to obtain a particle size distribution that meets the requirements; and precise control of the feed rate can also help control particle concentration by adjusting the residence time and stress state of particles within the equipment.

[0107] In some embodiments, the area of ​​particles with a diameter of 1μm-5μm in the cross-section of the positive electrode film along the thickness direction of the electrode sheet accounts for 30%-50%.

[0108] In some embodiments, the area percentage of particles with a diameter of 1μm-5μm in the cross-section of the positive electrode film along the thickness direction of the electrode sheet can be selected as 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, or any value range between the two.

[0109] In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the area ratio of particles with a particle size of 1μm-5μm is within the above range. This can improve the compaction density of the electrode sheet through gradation while controlling the content of magnetic materials in the battery, which is beneficial to improving the energy density of lithium-ion secondary batteries and maintaining it for a long time during storage.

[0110] In some embodiments, the area of ​​particles with a diameter of 1μm-5μm in the cross-section of the positive electrode film along the thickness direction of the electrode sheet accounts for 30%-45%.

[0111] In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the area ratio of particles with a diameter of 1μm-5μm is within the above range. This can improve the compaction density and capacity storage stability while further taking into account the migration distance of lithium ions inside the particles, thereby enabling the lithium-ion secondary battery to maintain a low impedance and improve the battery's dynamic performance.

[0112] In some embodiments, the average equivalent area of ​​particles with a diameter of 1 μm or larger in the cross-section of the positive electrode film along the electrode thickness direction accounts for 0.05%-0.20%.

[0113] In some embodiments, the average equivalent area percentage of particles with a diameter of 1 μm or larger in the cross-section of the positive electrode film along the electrode thickness direction can be selected as 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.2%, or any value range between the two.

[0114] The average equivalent area ratio of particles with a diameter of 1 μm or larger in the cross-section of the positive electrode film along the electrode thickness direction is obtained by dividing the area ratio of particles with a diameter of 1 μm or larger in the cross-section of the positive electrode film along the electrode thickness direction by the total number of particles with a diameter of 1 μm or larger in the cross-section of the positive electrode film along the electrode thickness direction. An average equivalent area ratio of particles with a diameter of 1 μm or larger in the cross-section of the positive electrode film along the electrode thickness direction within the above range means that the cross-section of the positive electrode film along the electrode thickness direction has a certain number of large particles to improve the compaction density of the electrode, but without the large particle size causing serious deterioration of the kinetic performance of the lithium-ion secondary battery, thus balancing the battery's energy density and kinetic performance.

[0115] In some embodiments, the area ratio of particles with a diameter of 50nm-200nm in the cross-section of the positive electrode film along the thickness direction is 3%-15%, optionally 5%-12%, and further optionally 5%-10%.

[0116] In some embodiments, the area percentage of particles with a diameter of 50nm-200nm in the cross-section of the positive electrode film along the thickness direction of the electrode sheet can be selected as 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or any value range between the two.

[0117] Theoretical studies show that, under ideal conditions, spherical particles with a diameter of 314 nm can fill the gaps formed by the accumulation of spherical particles with a diameter of 1 μm, thereby improving particle size distribution and powder compaction density. Particles with a diameter of 50 nm-200 nm can tightly fill the gaps between particles of 1 μm or larger, achieving dense packing. The fact that the area ratio of particles with a diameter of 50 nm-200 nm falls within this range indicates a certain number of such particles, which is beneficial for improving the powder compaction density of the positive electrode active material and the compaction density of the electrode sheet through gradation, further enhancing the energy density of lithium-ion secondary batteries.

[0118] In some embodiments, the mass content of the magnetic material in the positive electrode film layer is less than or equal to 300 ppm, and can be selected as 20-200 ppm.

[0119] In some embodiments, the mass content of the magnetic material in the positive electrode film layer can be selected as 20ppm, 50ppm, 100ppm, 150ppm, 200ppm, 250ppm, 300ppm or any value range between the two.

[0120] Within the above-mentioned range, the mass content of magnetic materials can further mitigate self-discharge and improve the storage stability of battery capacity.

[0121] In some embodiments, the magnetic material includes one or more of Fe, Fe2P, FeP, γ-Fe2O3, and Fe2P2O7.

[0122] In some embodiments, the mass content of elemental iron in the positive electrode film is less than 20 ppm, and may be less than or equal to 15 ppm.

[0123] In some embodiments, the mass content of elemental iron in the positive electrode film can be selected as 0, 5 ppm, 10 ppm, 15 ppm, 19 ppm or any value range between the two.

[0124] It is understandable that the mass content of elemental iron in the positive electrode film is 0, which does not necessarily mean that the positive electrode film does not contain elemental iron. It simply means that the content of elemental iron in it is lower than its detection limit.

[0125] Compared to other magnetic materials, elemental iron is more prone to oxidation at the positive electrode and reduction at the negative electrode. When elemental iron accumulates to a certain extent at the negative electrode, it forms dendrites, causing perforation of the separator, resulting in internal short circuits, and even fires and explosions, posing a significant safety hazard. Controlling the mass content of elemental iron within the aforementioned range is beneficial to improving battery safety performance.

[0126] In some embodiments, the lithium-containing transition metal phosphate includes a component having the following general formula: Li m Fe x P y O j Q q ,

[0127] 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.

[0128] In some implementations, m can be selected as 0.8, 0.85, 0.9, 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, or any value between two of these; x can be selected as 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, or any value between two of these. The range of values ​​is defined as follows: y can be 0.95, 0.96, 0.97, 0.98, 0.99, 1.00, or any value between two of these ranges; j can be 3.5, 3.6, 3.7, 3.8, 3.9, 4, or any value between two of these ranges; q can be 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, or any value between two of these ranges.

[0129] Selecting appropriate modifying element Q can improve the lattice change rate of positive electrode active materials during lithium insertion / extraction, reduce oxygen activity on particle surface, improve the structural stability of materials, thereby improving the specific capacity utilization level of materials and further improving the energy density of lithium-ion secondary batteries.

[0130] In some embodiments, the positive electrode active material includes titanium, and the mass content of titanium is 1500ppm-8000ppm based on the total mass of the positive electrode active material, optionally 2500ppm-8000ppm, and further optionally 2500ppm-6000ppm.

[0131] In some embodiments, based on the total mass of the positive electrode active material, the mass content of titanium can be selected as 1500ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm, 5500ppm, 6000ppm, 6500ppm, 7000ppm, 7500ppm, 8000ppm or any range between the two.

[0132] The types and contents of elements in positive electrode active materials can be tested using any method known in the art. As an example, inductively coupled plasma atomic emission spectrometry (ICP-AES) is used to test the titanium content, referring to Appendix C of GB / T 33822-2017.

[0133] Titanium precursors, such as titanium dioxide, typically possess surface inertness. Their addition during preparation reduces the activity of the precursor mixture, inhibiting particle growth and controlling particle size to ensure the average equivalent area of ​​particles larger than 1 μm in the cross-section along the electrode thickness is within a suitable range. Furthermore, their surface inertness reduces the probability of uneven local chemical reactions and the formation of magnetic materials. Simultaneously, titanium doping in the cathode active material improves the electronic conductivity and ion transport rate of lithium-containing transition metal phosphates, mitigating the negative impact of relatively large particles on the kinetic performance of the cathode active material. By influencing particle size and lithium-ion transport pathways, a balance between battery energy density and kinetic performance can be achieved.

[0134] In some embodiments, the mass percentage of carbon element is 0.9%-1.8% based on the total mass of the positive electrode active material.

[0135] Based on the total mass of the positive electrode active material, the mass percentage of carbon can be measured using methods and equipment known in the art. For example, it can be determined using a Dekai HCS infrared carbon-sulfur analyzer, referring to GB / T 21023-2006 "Determination of Total Carbon and Sulfur Content in Iron and Steel - Infrared Absorption Method After Combustion in a High-Frequency Induction Furnace".

[0136] In some embodiments, the mass percentage of carbon element, based on the total mass of the positive electrode active material, can be selected as 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, or any value range between the two.

[0137] Based on the total mass of the positive electrode active material, a carbon content within the above-mentioned range can improve the conductivity of the positive electrode active material and enhance the kinetic performance of the lithium-ion secondary battery. It can also reduce the negative impact of excessive carbon content on the loading of lithium-containing transition metal phosphates, taking into account both the compaction density of the electrode and the impedance of the lithium-ion secondary battery, while simultaneously improving the energy density and kinetic performance of the battery.

[0138] In some embodiments, the median C of the graphitization degree C in the cumulative distribution curve of the positive electrode film obtained in laser microscopy confocal Raman spectroscopy instrument scanning mode is... 50 The value is greater than or equal to 0.9 and less than or equal to 1.3, and can be selected from 0.99 to 1.2, where the graphitization degree C value is I. G / I D , where I G This indicates that the Raman spectrum is at 1580±100 cm⁻¹ -1 The intensity of peak G at I DThis indicates that the Raman spectrum is at 1350±100 cm⁻¹ -1 The intensity of peak D at that location.

[0139] In this application, the graphitization degree C value of the positive electrode film can be obtained by surface scanning mode of a laser confocal Raman spectrometer. As an example, specifically, a laser confocal Raman spectrometer (high-precision Renishaw laser confocal Raman spectrometer) is used, with an excitation wavelength of 532 nm. An appropriate amount of the positive electrode film is taken and surface scanned on its surface or along the thickness direction of the electrode. The scanning area is 45 μm × 45 μm, divided into 10 × 10 grids, with grid vertices as test points, a step size of 5 μm, and a total of 100 scan points. Thus, the C values ​​at different sites and the cumulative distribution curve of the C values ​​in the surface scan area are obtained.

[0140] The positive electrode film in this application can be either a freshly prepared positive electrode film or a positive electrode film obtained from disassembly of a battery. The surface of a positive electrode film obtained from disassembly of a battery inevitably contains residual electrolyte salts. To improve testing accuracy, it is preferable to perform a surface scan on a cross-section of the positive electrode film along the electrode thickness direction to characterize the degree of graphitization of the positive electrode film.

[0141] The graphitization degree C of the positive electrode film was obtained by the ratio of the peak intensities of the G-band and D-band in the Raman spectrum. The position of the G-band peak was 1580±100 cm⁻¹. -1 Its characterization of carbon sp 2 Hybrid structure; D peak position is 1350±100 cm⁻¹ -1 It characterizes the disordered structure of carbon, where disorder refers to the irregular arrangement of carbon atoms within the structure. In graphite crystals, carbon atoms in the same layer are arranged in an sp... 2 Hybridization forms covalent bonds, while interlayer bonding is facilitated by van der Waals forces, making the carbon in the graphite structure prone to slip. Therefore, the C value can characterize the degree of graphitization in the cathode film. It is understandable that the degree of graphitization in the cathode film mainly originates from the graphitized carbon material within the cathode film, i.e., the carbon coating layer of the cathode active material. Although rich in sp... 2 Hybridized carbon nanotube conductive agents also have relatively high I0 G / I D However, due to its low content and small tube diameter, its addition to the positive electrode film results in an extreme value in the Raman surface scan test of the positive electrode film, and does not affect the graphitization degree C in the positive electrode film. 50 It has an impact.

[0142] Therefore, the degree of graphitization of the cathode film can also be used to characterize the degree of graphitization of the cathode active material. The higher the degree of graphitization of carbon on the surface of the cathode active material, the higher the proportion of graphitic carbon in the cathode film, and the easier it is for particles to slip during the rolling process by means of the highly graphitized carbon structure in the coating layer, thus achieving an increase in electrode compaction density under low rolling pressure.

[0143] The cumulative distribution curve of graphitization degree C value refers to the curve obtained by arranging at least 100 C values ​​in ascending order, with graphitization degree as the horizontal axis and the cumulative percentage as the vertical axis. 50 This represents the C value corresponding to a cumulative percentage of 50% on the vertical axis of the cumulative distribution curve of graphitization degree C. The median C value of graphitization degree. 50 Compared to point values, it can reflect the overall graphitization degree of particles in the positive electrode film, i.e., the degree of slippage; compared to the mean value, it can reduce the influence of extreme values ​​during the test and improve the confidence of the test results.

[0144] Those skilled in the art can control the degree of graphitization of active material particles using any known process. For example, adjusting the carbon source, sintering temperature, sintering time, sintering pressure, and sintering atmosphere can all achieve adjustments to the degree of graphitization of active material particles.

[0145] In some embodiments, the median C50 of the graphitization degree C value in the cumulative distribution curve of the positive electrode active material obtained by laser microscopy confocal Raman spectroscopy instrument scanning mode can be selected from 0.90, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.20, 1.25, 1.3 or any value range between the two.

[0146] Positive electrode active materials with graphitization within the above range can easily achieve particle slippage during the roll forming process by means of the graphitized carbon layer on the surface of the positive electrode active material, thereby offsetting the negative impact of particle size on the compaction density of the electrode sheet, and further improving the compaction density of the positive electrode film layer by means of particle slippage.

[0147] In some embodiments, the median L of the spheroidal area distribution curve of the particles obtained from the cross-section of the positive electrode film along the electrode thickness direction is... A50 The range is 0.6-0.85, and can be set to 0.65-0.80.

[0148] The specific method for testing the sphericity of particles in the cross-section of the positive electrode film along the electrode thickness direction is as follows: Particles in the cross-section of the positive electrode film are identified using the method described above in this application. The morphology and area of ​​the particles in the cross-section along the electrode thickness direction of the positive electrode film are analyzed using the "Shape Description" and "Area" analysis functions in ImageJ. According to the software manual (ImageJ User Guide IJ 1.46r), the analyzed "Area" parameter represents the pixel area of ​​the particle, and the "Round" parameter represents the ratio of the pixel area of ​​the particle to the area of ​​a circle with the fitted major axis as its diameter. The closer the particle is to a sphere, the closer the ratio of the pixel area to the area of ​​the circle with the fitted major axis as its diameter is to 1. Therefore, the "Round" parameter of the analyzed particles characterizes the sphericity of the particles. The sphericity of at least 5000 particles is arranged in ascending order, and the cumulative distribution curve of the sphericity area of ​​the particles in the positive electrode film is obtained with sphericity as the horizontal axis and the cumulative area ratio as the vertical axis. L A50 This is the L-value of sphericity when the cumulative area under the vertical axis of the cumulative distribution curve of L-values ​​accounts for 50%.

[0149] Those skilled in the art can control the sphericity of particles using any known process. For example, the sphericity of particles can be adjusted through processes such as grinding, polishing, chemical etching, mechanical stirring, extrusion, coating, granulation, and adding surfactants, as well as by adjusting the parameters of each process.

[0150] In some embodiments, the median L of the spheroidal density in the cumulative distribution curve of the particle spheroidal density of the particles obtained from a cross-section of the positive electrode film along the electrode thickness direction is... A50 The value can be selected from 0.60, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.70, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.80, 0.81, 0.82, 0.83, 0.84, 0.85, or any value range between the two.

[0151] Median L of sphericity A50 Within the aforementioned range, the particles are approximately spherical, and under external force, they are prone to slippage between each other, which can further improve the compaction density of the electrode and increase the energy density of the battery.

[0152] In some embodiments, the compacted density of the positive electrode active material at a pressure of 3T is 2.48 g / cm³. 3 -2.76g / cm 3 .

[0153] In this application, the term "powder compaction density" refers to the density of a compacted compact with a certain density and strength, formed during the external force compression process. This density is measured in g / cm³, as the powder moves and deforms, larger voids are filled, the contact area between particles increases, resulting in attractive forces between atoms and enhanced mechanical cohesion between particles. 3 .

[0154] The compacted density of the positive electrode active material powder can be measured using methods and equipment known in the art. For example, it can be measured using a compaction density instrument, referring to GB / T 24533-2009. Specifically, a certain amount of positive electrode active material is placed on a compaction mold (the mold diameter is known). The mold is hollow in the middle and has a metal disc at the top and bottom. The positive electrode active material is placed between the metal discs, and a metal cylinder is placed on top. The mold is placed on a compaction density instrument, and the pressure is set to 3T. The thickness of the positive electrode active material under 3T pressure can be read on the instrument. The compacted density of the positive electrode active material powder 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 (1.327 cm²). 2 H represents the thickness of the positive electrode active material after compaction.

[0155] In some embodiments, the compaction density of the positive electrode active material under 3T pressure can be selected as 2.48 g / cm³. 3 2.49 g / cm 3 2.50g / cm 3 2.51g / cm 3 2.52g / cm 3 2.53g / cm 3 2.54 g / cm 3 2.55g / cm 3 2.56 g / cm 3 2.57g / cm 3 2.58g / cm 3 2.59g / cm 3 2.60g / cm 3 2.61 g / cm 3 2.62 g / cm 3 2.63 g / cm 3 2.64 g / cm 3 2.65g / cm 3 2.66 g / cm 3 2.67 g / cm 3 2.68g / cm 3 2.69 g / cm 3 2.70 g / cm 3 2.71 g / cm3 2.72 g / cm 3 2.73 g / cm 3 2.74 g / cm 3 2.75g / cm 3 2.76 g / cm 3 Or the range of values ​​between any two.

[0156] This positive electrode active material has a high powder compaction density, which provides a material basis for improving the compaction density of the electrode and preparing high-energy-density lithium-ion secondary batteries.

[0157] In some embodiments, the compacted density of the positive electrode active material at a pressure of 3T is 2.58 g / cm³. 3 -2.76 / cm 3 .

[0158] Positive electrode active materials with a compaction density within the above range can further improve the compaction density of the electrode and increase the energy density of the battery.

[0159] In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 8 MPa is 2.0 Ω·cm to 40 Ω·cm.

[0160] 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, ST2722 type) according to GB / T 33822-2017. Specifically, a certain amount of positive electrode active material (e.g., 1g) is weighed and added to the feeding chamber of the powder resistivity meter. A pressure of 8MPa is applied, and the forward and reverse resistivity of the positive electrode active material are measured separately. The average value of the two is taken as the powder resistivity of the positive electrode active material.

[0161] In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 8 MPa can be selected as 2 Ω·cm, 3 Ω·cm, 4 Ω·cm, 5 Ω·cm, 6 Ω·cm, 7 Ω·cm, 8 Ω·cm, 9 Ω·cm, 10 Ω·cm, 11 Ω·cm, 12 Ω·cm, 13 Ω·cm, 14 Ω·cm, 15 Ω·cm, 16 Ω·cm, 17 Ω·cm, 18 Ω·cm, 19 Ω·cm, or 20 Ω·cm. ·cm, 21Ω·cm, 22Ω·cm, 23Ω·cm, 24Ω·cm, 25Ω·cm, 26Ω·cm, 27Ω·cm, 28Ω·cm, 29Ω·cm, 30Ω·cm, 31Ω·cm, 32Ω·cm, 33Ω·cm, 34Ω·cm, 35Ω·cm, 36Ω·cm, 37Ω·cm, 38Ω·cm, 39Ω·cm, 40Ω·cm, or any range of two.

[0162] This positive electrode active material has a low powder resistivity, which is beneficial to improving the capacity utilization level of the positive electrode active material and increasing the energy density of lithium-ion secondary batteries.

[0163] In some embodiments, the positive electrode active material has a discharge capacity of 135 mAh / g to 150 mAh / g at a 1C discharge rate at room temperature.

[0164] In this application, the positive electrode active material is assembled into a coin cell, and its electrical performance is tested using a blue electric current tester. At 25±5℃ and within a voltage range of 2.0V to 3.75V, it is charged at a constant current of 1C to 3.75V, paused for 5 minutes, charged at a constant voltage until the cutoff current reaches 50μA, and then discharged at a constant current of 1C to 2.0V. The discharge capacity of the coin cell is divided by the mass of the positive electrode active material to obtain the specific discharge capacity of the positive electrode active material at a 1C discharge rate at room temperature.

[0165] The preparation and testing process of the coin cell 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. Then, the organic solvent NMP (N-methylpyrrolidone) is added and thoroughly mixed. The mixture is then coated using a 150μm doctor blade, dried at 100℃ for 2 hours, and compacted to a density of 2.0g / cm³. 3 -2.2g / cm 3 Compact the positive electrode sheet, punch it into a 14mm diameter circle, weigh it and record the weight. Place the weighed positive electrode sheet in a vacuum drying oven (105℃, 1-12hrs, -90kPa). After drying, place the positive electrode sheet in a glove box. Assemble the battery in the following order: negative electrode shell - nickel mesh - lithium sheet - separator - positive electrode sheet - positive electrode shell. Add 65-87μL (pipette) of electrolyte (a 1:1 volume ratio of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) mixed solvent, with LiPF6 as the electrolyte). Place the negative electrode on top and put it in the groove of the sealing machine. The sealing pressure is 650kg / cm. 2 The button was removed with insulated tweezers, placed in a cleanroom bag, removed from the glove box, and left to stand in a constant temperature room for 3 hours to obtain the button for testing.

[0166] It is understandable that the discharge capacity of the positive electrode active material can also be obtained by disassembling the battery, obtaining the positive electrode sheet, assembling it into a coin cell according to the method described above, and then testing it.

[0167] In some embodiments, the discharge specific capacity of the positive electrode active material at a 1C discharge rate at room temperature can be selected as 135mAh / g, 140mAh / g, 141mAh / g, 142mAh / g, 143mAh / g, 144mAh / g, 145mAh / g, 146mAh / g, 147mAh / g, 148mAh / g, 149mAh / g, 150mAh / g, or any value range between the two.

[0168] The positive electrode active material has a high discharge specific capacity, indicating that the positive electrode active material has good kinetic performance and is beneficial to improving the energy density of lithium-ion secondary batteries.

[0169] In some embodiments, the discharge capacity η of the positive electrode active material discharged to 3.2V accounts for ≥85%, where η is defined as follows: at room temperature, a coin cell containing the positive electrode active material is charged and discharged twice at a constant current of 0.1C within a voltage range of 2.0V to 3.75V, followed by a constant current charge and discharge once at a constant current of 1C. In the 1C charge and discharge test, the capacity value extracted at a discharge voltage of 3.2V is recorded as C1, and the capacity value extracted at a discharge voltage of 2.0V is recorded as C2, where η = C1 / C2. The charging process includes constant voltage charging, with a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

[0170] The η value of the positive electrode active material can be measured using methods and equipment known in the art. As an example, a coin cell is first prepared according to the method described above. The electrical performance of the prepared coin cell is then tested using a blue-light tester. Specifically, at room temperature, the coin cell is charged and discharged twice at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V. After constant current charging to the cutoff voltage, it is charged at a constant voltage to a current of 50μA, followed by a single constant current charge and discharge at a rate of 1C. In the 1C charge and discharge test, the capacity value discharged from 3.75V to 3.2V is recorded as C1, and the capacity value discharged from 3.75V to 2.0V is recorded as C2, where η = C1 / C2.

[0171] In some implementations, η can be selected as 85%, 86%, 87%, 88%, 88.1%, 89%, 90%, 90.1%, 91%, 92%, 92.2%, 93%, 94%, 94.1%, 94.5%, 95%, 95.1%, or any range between the two.

[0172] In some embodiments, the discharge capacity percentage η of the positive electrode active material in the freshly prepared lithium-ion secondary battery when discharged to 3.2V is ≥88%. After the freshly prepared lithium-ion secondary battery is charged and discharged at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V for a period of time, the discharge capacity percentage η of the positive electrode active material when discharged to 3.2V can be maintained at ≥85%.

[0173] The high discharge capacity ratio of the positive electrode active material used in the battery embodiment of this application to 3.2V indicates that although the positive electrode active material has a certain proportion of large-sized particles, it still maintains good kinetic performance. At the same time, the high η value indicates that the lithium-ion secondary battery containing the positive electrode active material still has a high voltage when discharged to a low state of charge (SOC), which is beneficial for maintaining good power performance.

[0174] In some embodiments, the positive electrode film layer further includes a binder and a conductive agent. Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 94%-99.4%, the mass content of the binder is 0.5%-3%, and the mass content of the conductive agent is 0.1%-3%.

[0175] In some embodiments, the adhesive includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0176] 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.

[0177] In some embodiments, based on the total mass of the positive electrode film, the mass content of the positive electrode active material can be selected as 94%, 95%, 96%, 97%, 98%, 99%, 99.4%, or any value range between the two.

[0178] In some embodiments, based on the total mass of the positive electrode film, the mass content of the binder can be selected as 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or any value range between the two.

[0179] In some embodiments, based on the total mass of the positive electrode film, the mass content of the conductive agent can be selected as 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or any value range between the two.

[0180] In some embodiments, the one-sided density of the positive electrode film is 300 mg / 1540 mm². 2 -450mg / 1540mm 2 .

[0181] In this application, the unilateral density of the positive electrode film layer has a meaning known in the art and can be tested using methods known in the art. For example, take a positive electrode sheet that has been coated on one side and compacted (if it is a double-sided coated positive electrode sheet, the positive electrode film layer on one side can be wiped off first), cut it into small circular pieces with an area of ​​S1, weigh them, and record their weight as M1. Then wipe off the positive electrode film layer of the above-weighed positive electrode sheet, weigh the current collector, and record it as M0. The unilateral density of the positive electrode film layer = (M1-M0) / S1. To ensure the accuracy of the test results, multiple sets (e.g., 10 sets) of samples can be tested, and the average value can be calculated as the test result.

[0182] In some embodiments, the areal density of the positive electrode film layer on one side may be selected as 300 mg / 1540 mm². 2 310mg / 1540mm 2 320mg / 1540mm 2 330mg / 1540mm 2 340mg / 1540mm 2 350mg / 1540mm 2 360mg / 1540mm 2 370mg / 1540mm 2 380mg / 1540mm 2 390mg / 1540mm 2 400mg / 1540mm 2 410mg / 1540mm 2 420mg / 1540mm 2 430mg / 1540mm 2 440mg / 1540mm 2 450mg / 1540mm 2 Or the range of values ​​between any two.

[0183] Positive electrode films with areal densities within the above range can help improve the energy density of lithium-ion secondary batteries.

[0184] In some embodiments, the positive electrode film layer of the lithium-ion secondary battery, in its fully discharged state, has a compaction density of 2.43 g / cm³. 3 -2.78g / cm 3 .

[0185] In this application, the fully discharged state refers to the state after the battery is placed in a 25°C oven environment, left to stand for 2 hours, and the battery temperature is maintained at 25°C, and then discharged at a constant current of 1 / 3C to 2.5V and then discharged at a constant current of 0.1C to 2.0V.

[0186] The compaction density of the positive electrode film can be tested using methods known in the art. As an example, the battery is placed in a 25°C oven environment and left to stand for 2 hours. After the battery temperature is maintained at 25°C, the battery is discharged at a constant current of 1 / 3C to 2.5V and then discharged at a constant current of 0.1C to 2.0V. The battery is disassembled to obtain the positive electrode sheet in the fully discharged state of the lithium-ion secondary battery. The residual electrolyte is treated with dimethyl carbonate solvent, the electrode sheet is dried, and it is cut into small circular pieces with an area of ​​S. The weight is W1, and the thickness T1 of the positive electrode sheet is measured using a micrometer. Then, the positive electrode film layer of the above-weighed electrode sheet is wiped off, the weight of the current collector is weighed and recorded as W2, and the thickness T2 of the current collector is measured using a micrometer. Then, the compaction density of the positive electrode film PD = (W1-W2) / [(T1-T2)×S].

[0187] In some embodiments, the compaction density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery can be selected as 2.43 g / cm³. 3 2.44 g / cm 3 2.45g / cm 3 2.46 g / cm 3 2.47 g / cm 3 2.48 g / cm 3 2.49 g / cm 3 2.50g / cm 3 2.51g / cm 3 2.52g / cm 3 2.53g / cm 3 2.54 g / cm 3 2.55g / cm 3 2.56 g / cm 3 2.57g / cm 3 2.58g / cm 3 2.59g / cm 3 2.60g / cm 3 2.61 g / cm 3 2.62 g / cm 3 2.63 g / cm 3 2.64 g / cm 3 2.65g / cm 3 2.66 g / cm 3 2.67 g / cm 3 2.68g / cm 32.69 g / cm 3 2.70 g / cm 3 2.71 g / cm 3 2.72 g / cm 3 2.73 g / cm 3 2.74 g / cm 3 2.75g / cm 3 2.76 g / cm 3 2.77 g / cm 3 2.78g / cm 3 Or the range of values ​​between any two.

[0188] A compaction density of the positive electrode film within the above-mentioned range is beneficial to improving the energy density of lithium-ion secondary batteries.

[0189] In some embodiments, the positive electrode film layer of the lithium-ion secondary battery, in its fully discharged state, has a compaction density of 2.50 g / cm³. 3 -2.75g / cm 3 .

[0190] In some embodiments, after compaction processing, the compaction density of the positive electrode film layer is 2.55 g / cm³. 3 -2.90g / cm 3 .

[0191] In some embodiments, the compaction density of the positive electrode film layer after the compaction process can be selected as 2.55 g / cm³. 3 2.63 g / cm 3 2.64 g / cm 3 2.65g / cm 3 2.66 g / cm 3 2.67 g / cm 3 2.68g / cm 3 2.69 g / cm 3 2.70 g / cm 3 2.71 g / cm 3 2.72 g / cm 3 2.73 g / cm 3 2.74 g / cm 3 2.75g / cm 3 2.76 g / cm 3 2.77 g / cm 3 2.78g / cm 3 g / cm 3 2.79 g / cm 3 2.80g / cm 3 2.81 g / cm3 2.82 g / cm 3 2.83 g / cm 3 2.84 g / cm 3 2.85g / cm 3 2.90g / cm 3 Or the range of values ​​between any two.

[0192] In this application, "compaction" refers to the process of compacting the positive electrode film layer with mechanical pressure during battery assembly to improve its density and conductivity.

[0193] In some embodiments, the compaction density of the positive electrode film layer after formation processing is 2.43 g / cm³. 3 -2.78g / cm 3 .

[0194] In some embodiments, the compaction density of the positive electrode film layer after formation processing can be selected as 2.43 g / cm³. 3 2.52g / cm 3 2.53g / cm 3 2.54 g / cm 3 2.55g / cm 3 2.56 g / cm 3 2.57g / cm 3 2.58g / cm 3 2.59g / cm 3 2.60g / cm 3 2.61 g / cm 3 2.62 g / cm 3 2.63 g / cm 3 2.64 g / cm 3 2.65g / cm 3 2.66 g / cm 3 2.67 g / cm 3 2.68g / cm 3 2.69 g / cm 3 2.70 g / cm 3 2.71 g / cm 3 2.72 g / cm 3 2.73 g / cm 3 2.78g / cm 3 Or the range of values ​​between any two.

[0195] 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 of the battery.

[0196] It is understandable that, with the rebound of the electrode during the cycle, the compaction density of the positive electrode film in the fully discharged state of the lithium-ion secondary battery is slightly lower than that of the positive electrode film after compaction and formation.

[0197] In some embodiments, the positive electrode film of the lithium-ion secondary battery, in its fully discharged state, has a compaction density of 2.43-2.78 g / cm³. 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-28%.

[0198] In some embodiments, the positive electrode film of the lithium-ion secondary battery, in its fully discharged state, has a compaction density of 2.5-2.78 g / cm³. 3 In the cross-section along the thickness direction of the positive electrode film, the porosity of the positive electrode film is 10%-22%.

[0199] In some embodiments, the porosity of the positive electrode film layer in a cross-section along the thickness direction of the electrode sheet can be selected as 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, or any value range between the two.

[0200] The porosity of the positive electrode film layer in a cross-section along the electrode thickness direction can be tested as follows: Import the scanning electron microscope (SEM) image of the positive electrode film layer in a cross-section along the electrode thickness direction obtained as described above into ImageJ software. Select the line tool and use a line to mark the length of the scale bar in the image. Click "Analyze Set Scale" and set the scale parameters in the software according to the scale bar length in the image. Select the rectangle tool and select the part of the image outside the scale bar area. Use "Image Duplicate" to copy the selected area and use "Image Type 8bit" to adjust the image format. Select "Analyze Set Measurements" and choose the following 5 options: "Area", "Mean gray value", "Area Fraction", "Limit to threshold", and "Feret's diameter". Select 3 for "Decimal places". Then select "Image" - "Adjust" - "Threshold" in sequence and set 0 and 100 in the "Threshold" box. You can then use the Analyze-Measure function to export the porosity data in the SEM image of this cross-section. Export using "Image" - "Overlay" - "Flatten" to obtain a pore image; click "Apply" in "Threshold", then click "Analyze" - "Analyze Particles", check the four columns on the left to obtain pore statistics.

[0201] It is understood that the "pores" in the cross-section of the positive electrode film layer are identified by image color difference and threshold in the embodiments of this application. As shown in Figure 8, the "pores" are not the pore data obtained from the degassing test, but are mainly used to characterize the cross-sectional area between particles in the cross-section of the positive electrode film layer. This method is superior to the degassing method because the porosity obtained by the degassing method is related to the pores between particles and the pores in the carbon layer covering the surface of the lithium iron phosphate particles, thus failing to objectively reflect the pores between particles. The lower the porosity in the cross-section of the positive electrode film layer tested by this method, on the one hand, it means that the gradation of large, medium and small particles in the positive electrode film layer is better and the compaction density is higher. On the other hand, under the same gradation and roller pressure, if the porosity is low, it means that the particles are more likely to slide against each other, thereby reducing the risk of film overpressure and stress concentration, further reducing the probability of the positive electrode film demolding during long cycle, which is beneficial to improving the long cycle performance of the battery.

[0202] In some embodiments, the positive electrode includes a base coating layer disposed between the positive electrode film layer and the current collector; the base coating layer includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the base coating layer is ≤10 pcs / 10 μm.

[0203] Carbon-based particles refer to particles whose main component is carbon, including but not limited to conductive carbon and carbon black.

[0204] The undercoat layer helps improve the conductivity and adhesion between the positive electrode film and the current collector, reduces the likelihood of the positive electrode film detaching from the current collector during cycling, and improves the battery's kinetic performance. In the high compaction density electrode sheets of this application embodiment, for example, the compaction density of the positive electrode sheet in its fully stacked state is greater than or equal to 2.4 g / cm³. 3 At this time, the current collector is easily damaged during the high-pressure compaction process of the electrode sheet, and large-sized particles are prone to creating pits on the current collector. Controlling the distribution density of carbon-based particles with a particle size greater than 100nm in the bottom coating layer to ≤10pcs / 10μm is beneficial to reducing the probability of damage to the current collector in the high-pressure compaction electrode sheet and further improving the ultimate compaction density of the positive electrode sheet.

[0205] In some embodiments, the positive electrode includes a base coating layer disposed between the positive electrode film and the current collector; the base coating layer includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the base coating layer can be 0.1 pcs / 10 μm, 1 pcs / 10 μm, 1.5 pcs / 10 μm, 2 pcs / 10 μm, 2.5 pcs / 10 μm, 3 pcs / 10 μm, or 3.5 pcs / 10 μm. 10μm, 4pcs / 10μm, 4.5pcs / 10μm, 5pcs / 10μm, 5.5pcs / 10μm, 6pcs / 10μm, 6.5pcs / 10μm, 7pcs / 10μm, 7.5pcs / 10μm, 8pcs / 10μm, 8.5pcs / 10μm, 9pcs / 10μm, 9.5pcs / 10μm, 10pcs / 10μm or any range of two.

[0206] The distribution density of carbon-based particles with a diameter greater than 100 nm in the undercoat can be determined by the method described above. The positive electrode film is cut along the thickness direction of the electrode by an argon ion beam, and scanning electron microscope or microscopic image is taken. The size of carbon particles in the undercoat is detected by statistical methods, and the number of carbon-based particles with a diameter greater than 100 nm per 10 μm in the undercoat is counted. The count is repeated at least 5 times and the average value is calculated.

[0207] The base coating in this embodiment can be achieved through any known preparation process, such as pre-sieving or centrifuging during the preparation of carbon-based particles to remove large carbon-based material particles, thereby reducing the D of the carbon-based particles added during the preparation of the base coating. V 50 in 20-60nm, D V For a wavelength of 90 nm or less than or equal to 70 nm, a base coating is obtained by mixing, stirring, and applying the carbon-based material with a binder onto the current collector.

[0208] In some embodiments, the positive electrode sheet includes a base coating layer disposed between the positive electrode film layer and the current collector; the compacted density of the positive electrode sheet in its fully loaded state is greater than or equal to 2.4 g / cm³. 3 The thickness of the base coating on one side is 1-4 μm.

[0209] In some embodiments, the positive electrode sheet includes a base coating layer disposed between the positive electrode film layer and the current collector; the compacted density of the positive electrode sheet in its fully loaded state is greater than or equal to 2.5 g / cm³. 3 The thickness of the base coating on one side is 2-4 μm.

[0210] As the compaction density of the electrode increases, the compressive effect of large lithium phosphate particles (e.g., particles larger than 1 μm) on the undercoat becomes more significant. Therefore, stress concentration easily occurs at large particle sites, and these particles can even penetrate the undercoat and damage the current collector. Increasing the thickness of the undercoat helps to mitigate stress concentration in the electrode, further improving the electrode's ultimate compaction density.

[0211] In some embodiments, the thickness of the base coating on one side can be 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm or any value between the two.

[0212] The thickness of the base coating on one side can be tested as follows: As described above, the positive electrode film is cut along the thickness direction of the electrode using an argon ion beam, and a scanning electron microscope image is taken. The thickness of the base coating on one side is measured at 1 μm intervals along the length of the electrode. After measuring the thickness of the base coating at 10 points, the average value is calculated. It is important to note that outliers should be avoided during the measurement process, i.e., areas with a thickness less than 50 nm and areas with a thickness greater than 4 μm. These outliers are mainly due to extreme thickness fluctuations in individual areas caused by abnormal stress concentration during electrode compaction and are not statistically significant. In some embodiments, the thickness of the positive electrode current collector is less than or equal to 17 μm, and can be selected as 13 μm-15 μm.

[0213] In some embodiments, the thickness of the positive current collector is 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, or any value between the two.

[0214] In some embodiments, the positive 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, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0215] A positive electrode current collector with a thickness within the above range helps to increase the load per unit mass of the battery and improve the energy density of lithium-ion secondary batteries.

[0216] In some embodiments, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one side of the negative current collector, wherein the areal density of the negative electrode film layer on one side is 130 mg / 1540 mm². 2 -220mg / 1540mm 2 ; and / or the compaction density of the negative electrode film is 1.40 g / cm³. 3 -1.75g / cm 3 .

[0217] In some embodiments, the areal density of the negative electrode film layer on one side may be selected as 130 mg / 1540 mm². 2 140mg / 1540mm 2 150mg / 1540mm 2 160mg / 1540mm 2 170mg / 1540mm 2 180mg / 1540mm 2 190mg / 1540mm 2 200mg / 1540mm 2 210mg / 1540mm 2 220mg / 1540mm 2 Or the range of values ​​between any two.

[0218] In some embodiments, the compaction density of the negative electrode film is 1.40 g / cm³. 3 -1.75g / cm 3 .

[0219] In some embodiments, the compaction density of the negative electrode film can be selected as 1.40 g / cm³. 3 1.45g / cm 3 1.50g / cm3 1.55g / cm 3 1.60g / cm 3 1.70g / cm 3 1.75g / cm 3 Or the range of values ​​between any two.

[0220] The unilateral density and compaction density of the negative electrode film can be tested using a method similar to that used for the positive electrode film described above.

[0221] Having the areal density and compaction density of the negative electrode film within the above-mentioned range is beneficial for matching with the positive electrode film and improving the energy density of the lithium-ion secondary battery.

[0222] 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, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0223] In some embodiments, the negative electrode film layer includes a negative electrode active material. The negative electrode active material may be any negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0224] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0225] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0226] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0227] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, compaction and other processes.

[0228] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0229] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0230] 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 difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0231] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0232] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0233] In some embodiments, the lithium-ion secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0234] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven 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.

[0235] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0236] In some embodiments, the lithium-ion secondary battery may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0237] In some implementations, the outer packaging of the lithium-ion secondary battery can be a hard shell, such as a hard plastic shell, aluminum shell, or steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0238] The second aspect of this application provides a battery device including the lithium-ion secondary battery provided in the first aspect of this application. The battery device includes at least one of a battery module, a battery pack, and an energy storage battery.

[0239] A third aspect of this application provides an electrical device, including the lithium-ion secondary battery provided in the first aspect of this application. A fourth aspect of this application provides a method for preparing a positive electrode active material: obtaining a mixed raw material comprising a carbon source, a lithium source, an iron source, and a phosphorus source, wherein the molar ratio of lithium to iron in the mixed raw material is greater than 1 and less than 1.05; and grinding to obtain a mixed slurry, wherein the solid phase volume distribution particle size D in the mixed slurry is... V50 The particle size is 0.3μm-0.4μm; after drying the mixed slurry, a precursor powder is obtained; the precursor powder is sintered to obtain a positive electrode active material, wherein the sintering is carried out in an inert gas environment and the total gas flow rate during the sintering process is 1100m³. 3 / h-1400m 3 / h; the sintering includes a heating zone and a constant temperature zone, wherein the inert gas flow rate v1 in the heating zone is higher than the inert gas flow rate v2 in the constant temperature zone; the constant temperature zone temperature of the sintering includes 770℃-830℃; the positive electrode active material includes lithium transition metal phosphate particles with carbon-coated material on at least part of their surface.

[0240] In some embodiments, the isothermal temperature range for sintering can be selected as 770°C, 780°C, 790°C, 800°C, 810°C, 820°C, 830°C, or any value range between the two.

[0241] High-temperature sintering helps to increase the area ratio of particles with a diameter greater than or equal to 1 μm in the positive electrode film.

[0242] In some embodiments, the molar ratio of lithium to iron in the mixed raw materials can be selected as 1.01, 1.02, 1.03, 1.04 or any value range between the two.

[0243] In mixed raw materials, a lithium to iron molar ratio greater than 1 helps to replenish lithium during the sintering process of the positive electrode active material, improve crystallinity, and enhance the capacity of the positive electrode active material. However, studies have shown that an excessively high lithium to iron molar ratio in the raw materials can lead to incomplete chemical reactions in localized areas during the sintering process of the positive electrode active material, thereby increasing the probability of the formation of magnetic materials such as Fe2P. Maintaining a lithium to iron molar ratio within the aforementioned range in the mixed raw materials helps to improve electrode compaction density and the energy density of lithium-ion secondary batteries while simultaneously ensuring a low self-discharge level, allowing the energy density of lithium-ion secondary batteries to be maintained over a long period during battery storage and cycling.

[0244] In this application, the term "D" V50 "" refers to the particle size at which the cumulative particle size distribution percentage of the sample volume reaches 50% as measured by the Malvern laser scattering method.

[0245] In some embodiments, the solid volume distribution particle size D in the mixed slurry V50 The value can be selected as 0.3μm, 0.31μm, 0.32μm, 0.33μm, 0.34μm, 0.35μm, 0.36μm, 0.37μm, 0.38μm, 0.39μm, 0.4μm or any value range between the two.

[0246] The solid particle size in the ground slurry is within the above-mentioned range, indicating that the raw material has a small particle size, resulting in relatively high activity. During high-temperature sintering, solid-phase diffusion easily occurs, leading to the growth of particles with a diameter of 1 μm or larger and a certain area proportion. Simultaneously, the solid particle size in the ground slurry being within the above-mentioned range helps reduce the probability of generating magnetic materials due to uneven local chemical reactions caused by excessively high raw material activity.

[0247] In some embodiments, the inert gas includes one or more of nitrogen, neon, and helium.

[0248] In some embodiments, the sintering is carried out in an inert gas environment, and the total gas flow rate during the sintering process can be selected as 1100 m³ / s. 3 / h, 1200m 3 / h, 1300m 3 / h, 1350m 3 / h, 1400m 3 / h or any numerical range between the two.

[0249] During sintering, a total gas flow rate within the above range is beneficial for reducing the partial pressure of the reducing atmosphere and decreasing the possibility of localized reduction and an increase in magnetic materials.

[0250] During the heating process, vigorous chemical reactions occur between the precursor raw materials. Increasing the ventilation rate helps reduce the phenomena of excessively high local reducing atmosphere, uneven reaction, and high magnetic material content. During the isothermal process, slow solid-phase diffusion occurs between the precursor raw materials, enabling particle growth. Maintaining a relatively low ventilation rate helps maintain the stability of the temperature field during sintering and achieve uniform particle growth.

[0251] In some implementations, v1:v2 can be selected as 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1 or any range between the two.

[0252] The positive electrode active material prepared by this method can have particles with a certain area ratio of more than 1μm in the cross-section of the positive electrode film along the thickness direction of the electrode sheet, and at the same time, the positive electrode active material contains a small amount of magnetic material. While improving the electrode sheet compaction density and improving the energy density of lithium-ion secondary batteries, the battery also has a low self-discharge level, so that the energy density of lithium-ion secondary batteries can be maintained for a long time during battery storage and cycling.

[0253] In some implementations, the iron source is an iron-containing compound.

[0254] 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, iron(II) oxide, and ferric hydroxide.

[0255] In some implementations, the phosphorus source is a phosphoric acid compound.

[0256] In some embodiments, the phosphorus source includes at least one of phosphoric acid, ferric phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0257] In some embodiments, the iron source and the phosphorus source can be the same substance. In some embodiments, ferric phosphate is used as both the iron source and the phosphorus source.

[0258] 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.

[0259] In some embodiments, the lithium source includes lithium carbonate.

[0260] In some embodiments, the carbon source includes one or more of glucose, polyethylene glycol, citric acid, sucrose, starch, fructose, lactose, polyaniline, polyacrylonitrile, and polyvinylpyrrolidone.

[0261] In some embodiments, the carbon source includes glucose and polyethylene glycol.

[0262] In some embodiments, the slurry further includes a titanium source, optionally including one or more of titanium dioxide, tetrabutyl titanate, titanium nitrate, and titanic acid.

[0263] In some embodiments, the solid volume distribution coefficient (D) in the mixed slurry V90 -D V10 ) / D V50 It ranges from 1.8 to 3.0.

[0264] In some embodiments, the solid volume distribution coefficient (D) in the mixed slurry V90 -D V10 ) / D V50 The value can be selected as 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0 or any range between the two.

[0265] The solid volume distribution coefficient in the mixed slurry is within the above range, which indicates that the raw material particle size distribution is uniform. This is beneficial to improve the uniformity of the solid phase reaction during subsequent sintering and reduces the risk of local concentration of reducing substances, enhanced reducing power, and aggregation of magnetic materials due to uneven mixing of raw materials.

[0266] In some embodiments, the volume distribution particle size D of the precursor powder V50 The range is 5μm-60μm.

[0267] In some embodiments, the precursor powder is obtained by spray drying the mixed slurry.

[0268] In some embodiments, the sintering is a single sintering, including at least two isothermal intervals. The isothermal temperature of the first isothermal interval is 400℃-500℃, and the isothermal time of the first isothermal interval is 3h-8h. The highest isothermal temperature of the single sintering is 770℃-820℃, and the temperature is maintained at the highest temperature for 8h-15h.

[0269] In some embodiments, the maximum isothermal temperature for the first sintering can be selected as 770°C, 780°C, 790°C, 800°C, 810°C, 820°C or any range between the two.

[0270] In some embodiments, the isothermal treatment at the highest temperature is within a range of 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, or any two of these values.

[0271] High-temperature sintering within the aforementioned temperature range helps to increase the particle size of the positive electrode active material, allowing large particles larger than 1μm to have a certain area ratio in the cross-section of the positive electrode film along the thickness direction of the electrode sheet, increasing the electrode sheet compaction density. At the same time, it can also reduce the increase of reducing atmosphere caused by excessively high sintering temperature, control the probability of reducing and generating magnetic materials such as Fe2P, and thus balance the energy density and storage stability of the battery.

[0272] In some embodiments, the sintering is at least two sintering processes, wherein a preliminary sintering product is obtained after the first sintering, and the preliminary sintering product is ground and then subjected to a second sintering.

[0273] In some embodiments, the sintering temperature of the first sintering is 720℃-780℃, and the sintering time is 6h-12h.

[0274] In some embodiments, the sintering temperature for the first sintering can be selected as 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, or any value range between the two.

[0275] In some embodiments, the sintering time for the first sintering can be selected as 6h, 7h, 8h, 9h, 10h, 11h, 12h or any value range between the two.

[0276] In some embodiments, the sintering temperature of the second sintering is 770℃-830℃, and the sintering time is 6h-12h.

[0277] In some embodiments, the sintering temperature of the second sintering can be selected as 770°C, 780°C, 790°C, 800°C, 810°C, 820°C, 830°C or any value range between the two.

[0278] In some embodiments, the sintering time for the second sintering can be selected as 6h, 7h, 8h, 9h, 10h, 11h, 12h or any value range between the two.

[0279] In some embodiments, the second sintering after grinding the initial calcined product includes: adding a carbon source to the initial calcined product and grinding it separately, wherein the D of the first group of ground particles is... V50 The particle size ranges from 1.40 μm to 2.0 μm; the D of the particles after grinding in the second group is... V50The particle size is 0.35μm-0.45μm; the first group of ground particles and the second group of ground particles are mixed at a mass ratio of 30:70-70:30 to obtain mixed intermediate particles, and the mixed intermediate particles are sintered for a second time.

[0280] In some embodiments, the second sintering after grinding the initial calcined product includes: adding a carbon source to the initial calcined product and grinding it separately, wherein the D of the first group of ground particles is... V50 The value can be selected as 1.4μm, 1.45μm, 1.5μm, 1.55μm, 1.6μm, 1.65μm, 1.7μm, 1.75μm, 1.8μm, 1.9μm, 2μm or any value range between the two.

[0281] In some embodiments, the second sintering after grinding the initial calcined product includes: adding a carbon source to the initial calcined product and grinding it separately, and the D of the second set of ground particles... V50 The value can be selected as 0.35μm, 0.36μm, 0.37μm, 0.38μm, 0.39μm, 0.4μm, 0.41μm, 0.42μm, 0.43μm, 0.44μm, 0.45μm or any range between the two.

[0282] In some embodiments, the first group of ground particles and the second group of ground particles can be mixed in a mass ratio of 30:70, 40:60, 50:50, 60:40, or 70:30 to obtain mixed intermediate particles.

[0283] The two-stage sintering process can effectively shorten the sintering time in the high-temperature range, thereby reducing the risk and probability of magnetic materials appearing during high-temperature sintering. By adjusting the particle size of the two sets of grinding processes during the second sintering, the activity of the particles can be controlled, so that the positive electrode active material has large particles with a certain area ratio. This improves the electrode compaction density and the energy density of the lithium-ion secondary battery, while also ensuring that the battery has a low self-discharge level, allowing the energy density of the lithium-ion secondary battery to be maintained for a long time during battery storage and cycling.

[0284] The fifth aspect of this application provides a method for preparing a positive electrode sheet, the method comprising sequentially adding a binder, a conductive agent, and a positive active material prepared by the method of the fourth aspect, dry mixing them, adding a solvent, stirring to obtain a slurry; transferring the slurry to at least one side of a current collector, drying and hot pressing to obtain a positive electrode sheet.

[0285] In some embodiments, the hot pressing includes at least three hot roller pressings, with the hot roller pressure increasing sequentially to 20-50 tons, 50-70 tons, and 70-90 tons; the hot roller temperature is 40-80°C, and the electrode is heated to 40-50°C before the first hot roller compaction.

[0286] The positive electrode active material prepared by the above-mentioned hot pressing process in combination with the preparation method of the fourth aspect in this application embodiment effectively improves the compaction density of the positive electrode sheet while maintaining a low content of magnetic materials, thereby improving the energy density of the battery while maintaining low self-discharge.

[0287] In some embodiments, the transfer coating speed is 1 m / min to 25 m / min. In some embodiments, the transfer coating speed can be selected from 1 m / min, 2 m / min, 3 m / min, 4 m / min, 5 m / min, 6 m / min, 7 m / min, 8 m / min, 9 m / min, 10 m / min, 11 m / min, 12 m / min, 13 m / min, 14 m / min, 15 m / min, 16 m / min, 17 m / min, 18 m / min, 19 m / min, 20 m / min, 21 m / min, 22 m / min, 23 m / min, 24 m / min, 25 m / min, or any value range between the two.

[0288] A coating speed within the above range is beneficial for improving the uniformity of particle distribution during the coating process, reducing the risk of particle agglomeration in the positive electrode film, reducing the cross-sectional porosity of the positive electrode film, further improving the ultimate compaction density of the electrode sheet, and improving the energy density of the battery.

[0289] In addition, this application also provides an electrical device, which includes at least one of the following: a lithium-ion secondary battery, a battery module, a battery pack, and an energy storage battery. The lithium-ion secondary battery, battery module, and 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 (e.g., mobile phones, laptops), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks), electric trains, ships and satellites, energy storage systems, etc.

[0290] As the electrical device, a lithium-ion secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0291] Figure 7 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0292] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0293] Example

[0294] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0295] Example 1

[0296] (1) Preparation of positive electrode active material

[0297] Lithium carbonate, iron phosphate, titanium dioxide, glucose, and polyethylene glycol were added to water and mixed in a premixing tank at 1400 rpm. The mixture was demagnetized by a demagnetizing rod with a magnetic field strength of 10000 Gs. The ratio of lithium carbonate to iron phosphate was such that the molar ratio of lithium to iron was 1.03:1.0. The mass content of glucose was 6% of the total raw materials, and the mass content of polyethylene glycol was 5% of the total raw materials. After uniform mixing, a mixed raw material with a solid content of 40% was obtained.

[0298] Among them, the number of magnetic material particles in lithium carbonate is less than or equal to 500 pcs / kg, and the particle size D of the material is... V10 ≥1μm, particle size D V50 The particle size is 6 μm, and the particle size D is... V90 The magnetic particle size is less than or equal to 40 μm; the number of magnetic particles in ferric phosphate is less than or equal to 95 pcs / kg, and the morphology is quasi-spherical; the number of magnetic particles in glucose is less than or equal to 500 pcs / kg; the molecular weight of polyethylene glycol is 1500, and the number of magnetic particles is less than or equal to 150 pcs / kg.

[0299] The mixed raw materials were subjected to two grinding-demagnetization cycles in a sand mill. After coarse grinding for 1 hour, the coarsely ground raw materials were demagnetized using a permanent magnet separator with a demagnetization intensity greater than or equal to 8000 Gs. The demagnetized raw materials were then finely ground, with the slurry temperature controlled below 40℃ during the grinding process to obtain a mixed slurry. The solid particle size D in the mixed slurry was... VThe particle size was 0.35 μm. After spray drying, the precursor powder was obtained, and the D50 after drying was 55.0 μm; the magnetic material content was less than 70 pcs / kg.

[0300] The precursor powder was subjected to a two-stage heating sintering process in a nitrogen atmosphere to obtain lithium iron phosphate cathode material: the temperature was increased from 25℃ to 450℃ at a heating rate of 2℃ / min (first heating stage), and held for 3 hours; the temperature was then increased from 450℃ to 780℃ at a heating rate of 5℃ / min (second heating stage), and held for 12 hours; the air flow rate in the heating stage was greater than that in the isothermal stage, with a ratio of 1.5:1, and the total air flow rate was 1350 cm³. 3 / h, after which cooling is completed; D is obtained by airflow pulverization. V 50 is a lithium iron phosphate material with 1.0μm-2.0μm carbon coating.

[0301] The above D50, D V 50. D V 90 refers to the data obtained through the Malvern laser scattering method.

[0302] Based on the total mass of the positive electrode active material, carbon accounts for 1.25% by mass, and titanium accounts for 4000 ppm by mass. The compacted density of the positive electrode active material powder under 3T pressure is 2.52 g / cm³. 3 The discharge specific capacity at 1C discharge rate at room temperature is 141.4 mAh / g. The discharge capacity η of the positive electrode active material discharged to 3.2V accounts for 92.5%.

[0303] (2) Preparation of the positive electrode sheet:

[0304] 2.2 wt% PVDF, 0.8 wt% conductive carbon black, and 97.0 wt% positive electrode active material were sequentially added and dry-mixed, followed by the addition of N-methylpyrrolidone. The mixture was stirred and the viscosity adjusted to obtain a slurry. This slurry was then transferred and coated onto a base coating of current collector aluminum foil. The base coating consisted of carbon black and PVDF in a 1:1 mass ratio. The density of carbon-based particles larger than 100 nm in the base coating was ≤10 pcs / 10 μm, and the base coating thickness was 2 μm. After drying and hot pressing, a single-sided surface density of 350 mg / 1540 cm³ was obtained. 2 The positive electrode film layer. The transfer coating speed is 20 m / min.

[0305] The hot pressing process includes three hot roller pressing processes, with the hot roller pressing pressure increasing sequentially to 40 tons, 60 tons, and 80 tons. The hot roller temperature is 60℃. Before the first hot roller compaction, the electrode sheet is heated to 40℃.

[0306] The compacted density of the electrode sheet is the ultimate compacted density of the electrode sheet. The test method for the ultimate compacted density of the electrode sheet is described below; in this example, the ultimate compacted density of the electrode sheet is 2.63 g / cm³. 3 .

[0307] In the cross-section along the thickness direction of the positive electrode film, particles with a diameter of 1μm-5μm account for 30% of the area; particles with a diameter greater than 1μm account for 0.127% of the average equivalent area; and particles with a diameter of 50nm-200nm account for 6.34%. The mass content of magnetic material in the positive electrode film is 20ppm, and the mass content of elemental iron is 0 (below the detection limit, recorded as 0). In the cumulative distribution curve of graphitization degree C obtained by laser microscopy confocal Raman spectroscopy in instrument scanning mode, the median C of graphitization degree is... 50 The median L of the sphericity of particles in the positive electrode film is 1.00. A50 The value is 0.715, and the porosity of the positive electrode film cross-section is 15.991%.

[0308] (3) Preparation of negative electrode sheet:

[0309] A mixture of 95.5 wt% negative electrode active material (artificial graphite), 1.0 wt% conductive agent (conductive carbon black), 2.0 wt% binder (styrene-butadiene rubber (SBR)), and 1.5 wt% thickener (sodium carboxymethyl cellulose (CMC)) was prepared by mixing with deionized water and stirring to disperse the mixture into a negative electrode slurry. The negative electrode slurry was then coated onto both sides of a Cu foil. After coating both sides, the coating was dried, compacted, slit, and sheeted to obtain the negative electrode sheet. The density of the coating on one side was 164 mg / 1540.25 cm⁻¹. 2 The compacted density is 1.60 g / cm³. 3 .

[0310] (4) Preparation of the separating membrane

[0311] Polypropylene film is used as the separator.

[0312] (5) Preparation of electrolyte

[0313] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate (EC) and dimethyl carbonate (DMC) organic solvents were mixed evenly at a volume ratio of 1 / 1. Lithium salt LiPF6 was added and dissolved in the organic solvent. The concentration of LiPF6 in the solution was 1 mol / L. The mixture was stirred evenly to obtain the electrolyte.

[0314] (6) Battery fabrication:

[0315] The positive electrode, separator, and negative electrode are stacked in sequence. The separator must be able to isolate the anode and cathode. The bare cell is obtained by winding. The bare cell is placed in the outer packaging, electrolyte is injected, and after processes such as encapsulation, formation, and venting, a lithium-ion secondary battery is finally obtained.

[0316] Example 2

[0317] The preparation method of Example 2 is basically the same as that of Example 1, except that the sintering process of the positive electrode active material is different, specifically:

[0318] The precursor powder was subjected to a two-stage heating sintering process in a nitrogen atmosphere to obtain lithium iron phosphate cathode material: the temperature was increased from 25℃ to 450℃ at a heating rate of 2℃ / min (first heating stage), and held for 3 hours; the temperature was then increased from 450℃ to 800℃ at a heating rate of 5℃ / min (second heating stage), and held for 12 hours; the air volume in the heating stage was greater than that in the isothermal stage, with a ratio of 1.5:1, and the total air volume was 1350 cm³. 3 / h, after which cooling is completed; D is obtained by airflow pulverization. V 50 is a lithium iron phosphate material with 1.0μm-2.0μm carbon coating.

[0319] Example 3

[0320] The preparation method of Example 3 is basically the same as that of Example 1, except that the sintering process of the positive electrode active material is different, specifically:

[0321] The precursor powder was subjected to a two-stage heating sintering process in a nitrogen atmosphere to obtain lithium iron phosphate cathode material: the temperature was increased from 25℃ to 450℃ at a heating rate of 2℃ / min (first heating stage), and held for 3 hours; the temperature was then increased from 450℃ to 820℃ at a heating rate of 5℃ / min (second heating stage), and held for 12 hours; the air flow rate in the heating stage was greater than that in the isothermal stage, with a ratio of 1.5:1, and the total air flow rate was 1350 cm³. 3 / h, after which cooling is completed; D is obtained by airflow pulverization. V 50 is a lithium iron phosphate material with 1.0μm-2.0μm carbon coating.

[0322] Example 4

[0323] The preparation method of Example 4 is basically the same as that of Example 1. The difference is that the content of titanium source added during the preparation of positive electrode active material is adjusted so that the mass content of titanium element is 2500ppm based on the mass of positive electrode active material.

[0324] Example 5

[0325] The preparation method of Example 5 is basically the same as that of Example 1, except that the content of titanium source added during the preparation of positive electrode active material is adjusted so that the mass content of titanium element is 1500ppm based on the mass of positive electrode active material.

[0326] Example 6

[0327] The preparation method of Example 6 is basically the same as that of Example 1, except that the preparation process of the positive electrode active material is slightly different. The specific differences are as follows:

[0328] (1) The carbon source in the mixed raw materials is only glucose, and the mass of glucose is 5.7 wt% compared to the mass of iron phosphate;

[0329] (2) The heating and sintering processes are different. The precursor powder is sintered at least twice in a nitrogen atmosphere. The first sintering temperature is 750℃ and the holding time is 8 hours to obtain the initial sintered product.

[0330] 1.5 wt% (based on the mass of the initial calcined product) of glucose, 3.0 wt% (based on the mass of the initial calcined product) of polyethylene glycol and titanium source were added to the initial calcined product. After homogenization, the mixture was divided into two groups for secondary grinding. The grinding parameters for the two groups were different, and the particle size distribution (D) of the particles after the first group of grinding was controlled. V 50 is 2.0μm, and the D of the particles after the second group of grinding is... V 50 represents 0.35 μm. The ground particles from the first and second groups were mixed at a mass ratio of 30:70, spray-dried, and then subjected to a second sintering. The second sintering temperature was 800℃, and the temperature was maintained for 10 hours.

[0331] The ratio of titanium element in the titanium source in the mixed raw materials to that in the titanium source added to the initial calcination product is 5:2. Based on the total mass of the positive electrode active material, the mass percentage of titanium element is 6000ppm.

[0332] Example 7

[0333] The preparation method of Example 7 is basically the same as that of Example 6, except that...

[0334] D of the first group of ground particles V 50 represents 1.50 μm, and the D of the particles after the second group of grinding is... V 50 represents 0.40 μm; the ground particles from the first and second groups are mixed at a mass ratio of 70:30, spray-dried, and then sintered a second time.

[0335] Comparative Example 1

[0336] The preparation methods of Comparative Example 1 and Example 1 are basically the same, except that the following methods differ in the preparation of the positive electrode active material:

[0337] The mixed raw materials were subjected to two grinding-demagnetization cycles in a sand mill. After coarse grinding for 1 hour, the coarsely ground raw materials were demagnetized using a permanent magnet separator with a demagnetization intensity greater than or equal to 8000 Gs. The demagnetized raw materials were then finely ground, with the slurry temperature controlled below 40℃ during the grinding process to obtain a mixed slurry. The solid particle size D in the mixed slurry was... V The sample was 0.5 μm thick and spray-dried to obtain a dried precursor powder.

[0338] The precursor powder was subjected to a two-stage heating sintering process in a nitrogen atmosphere to obtain lithium iron phosphate cathode material: the temperature was increased from 25℃ to 450℃ at a heating rate of 2℃ / min (first heating stage), and held for 3 hours; the temperature was then increased from 450℃ to 765℃ at a heating rate of 5℃ / min (second heating stage), and held for 12 hours; the air volume in the heating stage was greater than that in the isothermal stage, with a ratio of 1:1, and the total air volume was 1350 cm³. 3 / h, after which cooling is completed; D is obtained by airflow pulverization. V 50 is a lithium iron phosphate material with 1.0μm-2.0μm carbon coating.

[0339] Comparative Example 2

[0340] The preparation methods of Comparative Example 2 and Example 1 are basically the same, except that the following methods differ in the preparation of the positive electrode active material:

[0341] Lithium carbonate, iron phosphate, titanium dioxide, glucose, and polyethylene glycol were added to water and mixed in a premixing tank at 1400 rpm. The mixture was demagnetized by a demagnetizing rod with a magnetic field strength of 8000 Gs-12000 Gs. The ratio of lithium carbonate to iron phosphate was such that the molar ratio of lithium to iron was 1.05:1.0. The mass content of glucose was 6% of the total raw materials, and the mass content of polyethylene glycol was 5% of the total raw materials. After uniform mixing, a mixed raw material with a solid content of 40% was obtained.

[0342] The mixed raw materials were subjected to two grinding-demagnetization cycles in a sand mill. After coarse grinding for 1 hour, the coarsely ground raw materials were demagnetized using a permanent magnet separator with a demagnetization intensity greater than or equal to 8000 Gs. The demagnetized raw materials were then finely ground, with the slurry temperature controlled below 40℃ during the grinding process to obtain a mixed slurry. The solid particle size D in the mixed slurry was... V The D50 is 0.35μm. Spray drying is performed to obtain a dried precursor powder. After drying, the D50 is 50-60μm; the magnetic material is less than 70pcs / kg.

[0343] The precursor powder was sintered in a nitrogen atmosphere in two stages to obtain lithium iron phosphate cathode material: the temperature was increased from 25℃ to 450℃ at a heating rate of 2℃ / min (first heating stage), and held for 3 hours; the temperature was increased from 450℃ to 820℃ at a heating rate of 5℃ / min (second heating stage), and held for 12 hours; the gas volume ratio between the heating stage and the isothermal stage was 1:1, and the total gas flow rate was 900 cm³ / min. 3 / h, after which cooling is completed; D is obtained by airflow pulverization. V 50 is a lithium iron phosphate material with 1.0μm-2.0μm carbon coating.

[0344] Performance testing

[0345] 1. Self-discharge K-value test

[0346] At 25℃, it is charged at a constant current of 0.05C to 3.0V, and then charged at a constant voltage to a current of 0.05C. After standing at 25℃ for 24 hours, the open circuit voltage V1 is measured in V. After standing for another 24 hours, the open circuit voltage V2 is measured again in V. The self-discharge K value is 1000×(V1-V2) / 48, in mV / h.

[0347] 2. DCR Test

[0348] At 25℃, the voltage was charged at a constant current of 0.33C to 3.65V, then charged at a constant voltage to a current of 0.05C, then discharged at 0.33C to 20% SOC. After resting for 5 minutes, it was pulsed discharged at 3C for 30 seconds, rested for 40 seconds, charged at 3C for 40 seconds, rested for 5 minutes, charged at a constant current of 0.33C to 3.65V, then charged at a constant voltage to 0.05C, then discharged at 0.33C to 10% SOC. After resting for 5 minutes, it was pulsed discharged at 3C for 30 seconds, and then rested for 40 seconds. After charging at 3C for 40 seconds, let it stand for 5 minutes, then fully charge it at 0.33C, then discharge it at 0.33C to 50% SOC, then let it stand at -25℃ for 2 hours, then pulse discharge it at 1C for 30 seconds, let it stand for 10 minutes, then let it stand at 25℃ for 2 hours, then charge it at 0.33C constant current to 3.65V, then charge it at constant voltage to 0.05C, then discharge it at 0.33C to 20% SOC, then let it stand at -25℃ for 2 hours, then pulse discharge it at 1C for 30 seconds, and let it stand for 10 minutes.

[0349] Record the voltage before and after each pulse discharge, and calculate the DCR under different conditions. The calculation formula is DCR = (voltage before pulse discharge after resting - voltage after pulse discharge) / pulse current.

[0350] 3. Electrode ultimate compaction density test

[0351] The double-coated electrode sheets were compacted using a roller press, and the elongation and flexibility of the compacted electrode sheets were tested. By increasing the pressure of the roller press, electrode sheets with different compaction densities were obtained. As the pressure increased, the compaction density of the electrode sheet increased, the elongation of the electrode sheet increased, and the flexibility of the electrode sheet decreased. Excessive elongation of the electrode sheet can easily lead to warping, while insufficient flexibility can easily lead to brittle fracture. Therefore, the lower of the compaction density corresponding to an elongation of 8% or the compaction density corresponding to a flexible folding number of times is defined as the ultimate compaction density of the electrode sheet.

[0352] The compaction density is calculated by dividing the mass of the positive electrode film by the volume of the positive electrode film.

[0353] 4. Elongation test

[0354] Lay the electrode flat on a horizontal table and cut it into sections, each about 100cm long. Remove the copper foil from the edges of the electrode, ensuring the cut edges are parallel to the MD direction (perpendicular to the pressure roller) to guarantee the electrode is completely covered by the coating. Use a steel ruler to measure the length between marked points at the beginning and end of the electrode, estimating to 0.1mm, and record the length before compaction. After compaction, record the length between the corresponding marked points. Use (compacted length - uncompacted length) / uncompacted length as the electrode's elongation.

[0355] 5. Test of the number of flexible folds

[0356] Cut the positive electrode sheet to 20×100mm. 2 Test the size of the sample; fold it in half with the front side facing up, flatten it with a 2kg roller, unfold it and check the gap against the light to see if light passes through. If no light passes through, fold it in half again with the back side facing up, flatten it with a 2kg roller, and check it against the light again. Repeat this process until light passes through the gap. 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 sheet.

[0357] Test Results

[0358] Table 1

[0359] *In the examples, 0 means below the detection limit, making it difficult to identify its accurate content.

[0360] As can be seen from the comparison between the examples and the comparative examples, in the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the area ratio of particles with a diameter greater than or equal to 1 μm is 30%-50%; and in the positive electrode film layer, the mass ratio of magnetic material is greater than or equal to 20 ppm and less than or equal to 1980 ppm, which helps the lithium-ion secondary battery to achieve high electrode sheet compaction density while having a low self-discharge K value, so that the battery has both high energy density and good storage performance.

[0361] When the area ratio of particles with a diameter of 1μm-5μm in the cross-section of the positive electrode film along the thickness direction is 30%-45%, the lithium-ion secondary battery can achieve high electrode compaction density, low self-discharge K value, good dynamic performance and high capacity at the same time.

[0362] Based on the total mass of the positive electrode active material, when the mass content of titanium element is 2500ppm-8000ppm, the surface inertness of the titanium source can reduce the activity of the raw material, reduce the probability of uneven local chemical reaction and the generation of high-content magnetic materials, and at the same time help control the average equivalent area ratio of particles larger than 1μm, so as to achieve a balance between the storage stability and dynamic performance of lithium-ion secondary batteries.

[0363] As can be seen from the comparison of Examples 1, 2, 6, and 7 with other examples, when the mass content of magnetic material in the positive electrode film layer is 20-200 ppm, it helps the lithium-ion secondary battery to maintain a high electrode compaction density while having a low self-discharge K value. The lithium-ion secondary battery can further improve its dynamic performance while having both high energy density and good storage performance.

[0364] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A lithium-ion secondary battery, characterized in that, Includes positive electrode, negative electrode, and electrolyte. The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector. The positive electrode film layer includes a positive electrode active material, which comprises lithium transition metal phosphate particles with at least a portion of their surface coated with carbon material. In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a diameter greater than or equal to 1 μm is 30.0%-50.0%; and The mass percentage of magnetic material in the positive electrode film is greater than or equal to 20 ppm and less than or equal to 1980 ppm.

2. The lithium-ion secondary battery according to claim 1, characterized in that, In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size of 1μm-5μm is 30.0%-50.0%.

3. The lithium-ion secondary battery according to claim 1, characterized in that, In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a diameter of 1μm-5μm is 30.0%-45.0%.

4. The lithium-ion secondary battery according to claim 1, characterized in that, In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the average equivalent area ratio of particles with a diameter of 1 μm or larger is 0.05%-0.20%.

5. The lithium-ion secondary battery according to claim 1, characterized in that, In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the area ratio of particles with a diameter of 50nm-200nm is 3.0%-15.0%.

6. The lithium-ion secondary battery according to claim 1, characterized in that, In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the area ratio of particles with a diameter of 50nm-200nm is 5.0%-12.0%.

7. The lithium-ion secondary battery according to claim 1, characterized in that, In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the area ratio of particles with a diameter of 50nm-200nm is 5.0%-10.0%.

8. The lithium-ion secondary battery according to claim 1, characterized in that, In the positive electrode film layer, the mass percentage of the magnetic material is less than or equal to 300 ppm.

9. The lithium-ion secondary battery according to claim 8, characterized in that, In the positive electrode film, the mass percentage of the magnetic material is 20ppm-200ppm.

10. The lithium-ion secondary battery according to claim 1, characterized in that, The magnetic material includes one or more of Fe, Fe2P, FeP, γ-Fe2O3, and Fe2P2O7.

11. The lithium-ion secondary battery according to claim 1, characterized in that, The mass content of elemental iron in the positive electrode film is less than 20 ppm.

12. The lithium-ion secondary battery according to claim 11, characterized in that, The mass content of elemental iron in the positive electrode film is less than or equal to 15 ppm.

13. The lithium-ion secondary battery according to claim 1, characterized in that, The lithium-containing transition metal phosphate comprises 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.

14. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode active material includes titanium, and the mass content of titanium is 1500ppm-8000ppm based on the total mass of the positive electrode active material.

15. The lithium-ion secondary battery according to claim 14, characterized in that, The positive electrode active material includes titanium, and the mass content of titanium is 2500ppm-8000ppm based on the total mass of the positive electrode active material.

16. The lithium-ion secondary battery according to claim 14, characterized in that, The positive electrode active material includes titanium, and the mass content of titanium is 2500ppm-6000ppm based on the total mass of the positive electrode active material.

17. The lithium-ion secondary battery according to claim 1, characterized in that, Based on the total mass of the positive electrode active material, the mass percentage of carbon is 0.9%-1.8%.

18. The lithium-ion secondary battery according to claim 1, characterized in that, In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the laser microscopy confocal Raman spectroscopy instrument scanning mode, the median C of the graphitization degree is... 50 ≥0.9 and ≤1.3, wherein the degree of graphitization C is I. G / I D , where I G This indicates that the Raman spectrum is at 1580±100 cm⁻¹ -1 The intensity of peak G at I D This indicates that the Raman spectrum is at 1350±100 cm⁻¹ -1 The intensity of peak D at that location.

19. The lithium-ion secondary battery according to claim 18, characterized in that, In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the laser microscopy confocal Raman spectroscopy instrument scanning mode, the median C of the graphitization degree is... 50 The value is 0.99-1.

2.

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

85.

21. The lithium-ion secondary battery according to claim 20, characterized in that, In the cumulative distribution curve of particle sphericity area obtained from the cross-section of the positive electrode film along the electrode thickness direction, the median L of sphericity is... A50 It is 0.65-0.

80.

22. The lithium-ion secondary battery according to claim 1, characterized in that, The compacted density of the positive electrode active material under 3T pressure is 2.48 g / cm³. 3 -2.76g / cm 3 .

23. The lithium-ion secondary battery according to claim 22, characterized in that, The compacted density of the positive electrode active material under 3T pressure is 2.58 g / cm³. 3 -2.76g / cm 3 .

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

25. The lithium-ion secondary battery according to claim 1, characterized in that, The discharge capacity percentage η of the positive electrode active material discharged to 3.2V is ≥85%. η is defined as follows: at room temperature, a coin cell containing the positive electrode active material is charged and discharged twice at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V, followed by a constant current charge and discharge once at a constant current rate of 1C. In the 1C charge and discharge test, the capacity value extracted at a discharge voltage of 3.2V is recorded as C1, and the capacity value extracted at a discharge voltage of 2.0V is recorded as C2. η = C1 / C2. The charging process includes constant voltage charging at a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

26. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode film layer also includes a binder and a conductive agent. Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 94%-99.4%, the mass content of the binder is 0.5%-3%, and the mass content of the conductive agent is 0.1%-3%.

27. The lithium-ion secondary battery according to claim 1, characterized in that, The single-sided density of the positive electrode film is 300 mg / 1540 mm². 2 -450mg / 1540mm 2 .

28. The lithium-ion secondary battery according to claim 1, characterized in that, In the fully discharged state, the compaction density of the positive electrode film in the lithium-ion secondary battery is 2.43 g / cm³. 3 -2.78g / cm 3 .

29. The lithium-ion secondary battery according to claim 1, characterized in that, In the fully discharged state, the compaction density of the positive electrode film in the lithium-ion secondary battery is 2.50 g / cm³. 3 -2.75g / cm 3 .

30. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode film layer satisfies at least one of the following conditions: (1) In the fully discharged state, the compaction density of the positive electrode film layer of the lithium-ion secondary battery is 2.43 g / cm³. 3 -2.78g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-28%. (2) In the fully discharged state, the positive electrode film of the lithium-ion secondary battery has a compaction density of 2.50 g / cm³. 3 -2.78g / cm 3 In the cross-section along the thickness direction of the positive electrode film, the porosity of the positive electrode film is 10%-22%.

31. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode sheet includes a base coating layer disposed between the positive electrode film layer and the current collector; the base coating layer satisfies at least one of the following conditions: (1) The base coating includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the base coating is ≤10 pcs / 10 μm; (2) The compacted density of the positive electrode sheet in its fully loaded state is greater than or equal to 2.4 g / cm³. 3 The thickness of the base coating on one side is 1μm-4μm; (3) The compacted density of the positive electrode sheet in its fully loaded state is greater than or equal to 2.5 g / cm³. 3 The thickness of the base coating layer on one side is 2μm-4μm.

32. A battery device, characterized in that, The battery device includes any one of claims 1 to 31, wherein the battery device comprises at least one of a battery module, a battery pack, and an energy storage battery.

33. An electrical appliance, characterized in that, The lithium-ion secondary battery includes any one of claims 1 to 31.

34. A method for preparing a positive electrode active material, characterized in that, A mixed raw material comprising a carbon source, a lithium source, an iron source, and a phosphorus source is obtained, wherein the molar ratio of lithium to iron in the mixed raw material is greater than 1 and less than 1.05; after grinding, a mixed slurry is obtained, wherein the solid phase volume distribution particle size D in the mixed slurry is... V50 The particle size is 0.3μm-0.4μm; after drying the mixed slurry, a precursor powder is obtained; the precursor powder is sintered to obtain a positive electrode active material, wherein the sintering is carried out in an inert gas environment and the total gas flow rate during the sintering process is 1100m³. 3 / h-1400m 3 / h; the sintering includes a heating zone and a constant temperature zone, wherein the inert gas flow rate v1 in the heating zone is higher than the inert gas flow rate v2 in the constant temperature zone; the constant temperature zone temperature of the sintering includes 770℃-830℃; the positive electrode active material includes lithium transition metal phosphate particles with carbon-coated material on at least part of their surface.

35. A method for preparing a positive electrode sheet, characterized in that, The preparation method includes sequentially adding a binder, a conductive agent, and the positive electrode active material prepared by the preparation method of claim 34, dry mixing them, adding a solvent, stirring, and obtaining a slurry; transferring the slurry to at least one side of the current collector, drying, and hot pressing to obtain a positive electrode sheet.

36. The method of claim 35, wherein the method is performed in a single step. The hot pressing includes at least three hot roller pressings, with the hot roller pressure increasing sequentially to 20-50 tons, 50-70 tons, and 70-90 tons; the hot roller temperature is 40℃-80℃. Before the first hot roller compaction, the electrode is heated to a temperature of 40℃-50℃.

37. The preparation method according to claim 35, characterized in that, The transfer coating speed is 1 m / min - 25 m / min.

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