Lithium ion secondary battery, battery device, and power device

Abstract

The invention provides a lithium ion secondary battery, a battery device and a power utilization device. The lithium ion secondary battery comprises a positive pole piece, a negative pole piece and an electrolyte, the positive pole piece comprises a positive current collector and a positive film layer arranged on at least one side of the positive current collector, and the positive film layer comprises a positive active material; the positive electrode active material comprises lithium-containing transition metal phosphate particles, at least part of the surfaces of the lithium-containing transition metal phosphate particles are provided with carbon coating materials, and in the tangent plane of the positive electrode film layer in the thickness direction of the pole piece, the area proportion of particles with the particle size larger than or equal to 200 nm and smaller than 1500 nm is 73.0%-80.0%; in the tangent plane of the positive electrode film layer along the thickness direction of the pole piece, the area ratio of particles with the particle size greater than or equal to 1 mu m and less than 1.5 mu m is 15.0-25.0%; and the iron dissolution rate of the positive electrode film layer is 658 ppm to 1921 ppm.

Description

Lithium-ion secondary batteries, battery devices and electrical appliances

[0001] This application is a divisional application based on the invention with application number 202510514763.6, application date of April 23, 2025, applicant CATL, and invention title "Lithium-ion secondary battery, battery device and power consumption device". Technical Field

[0002] This application relates to the field of lithium-ion secondary battery technology, and in particular to a lithium-ion secondary battery, battery device, power supply device, a method for preparing positive electrode active material, and a method for preparing positive electrode sheet. Background Technology

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

[0004] Positive electrode active materials are a crucial component of secondary batteries. Lithium-containing transition metal phosphate materials possess characteristics such as structural stability, good safety, and long cycle life, demonstrating broad development prospects. However, with the increasing market demands for energy density and kinetics in lithium-containing transition metal phosphate system secondary batteries, it is difficult to simultaneously improve these properties using existing technologies. This has become a pressing technical problem that needs to be solved in this field. Summary of the Invention

[0005] In view of the above problems, this application provides a method for preparing a battery cell, a battery device, an electrical device, a positive electrode active material, and a method for preparing a positive electrode sheet, which are described below.

[0006] 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. In a cross-section of the positive electrode film along the thickness direction of the electrode, the area ratio of particles with a diameter greater than or equal to 1.5 μm is greater than or equal to 8.0% and less than or equal to 20.0%. The iron dissolution rate of the positive electrode film is 658 ppm to 1921 ppm.

[0007] In the cross-section of the positive electrode film along the thickness direction, the area ratio of particles with a diameter greater than or equal to 1.5 μm is less than 8%, making it difficult to achieve a high compaction density in the positive electrode. While a particle size ratio of more than 20% in the cross-section of the positive electrode film along the thickness direction is beneficial for achieving a high compaction density, it reduces the contact area between the electrolyte and the positive electrode active material, hinders the diffusion of lithium ions into the positive electrode film, increases the diffusion path of lithium ions within the particles, leads to severe localized polarization of the electrode, increases battery impedance, and significantly degrades the battery's kinetic performance.

[0008] Controlling the area ratio of particles with a diameter greater than or equal to 1.5 μm in the positive electrode film layer to be greater than or equal to 8.0% and less than or equal to 20.0% can reduce the significant bottleneck effect caused by large-sized particles, which is beneficial to maintaining the battery impedance at a low level and improving the battery's dynamic performance. However, this will limit the further improvement of the electrode compaction density. In the embodiments of this application, by controlling the iron dissolution rate of the positive electrode film layer to 658ppm-1921ppm, the density and integrity of the carbon coating on the surface of the positive electrode active material are improved, making it easier for particles to slide and reducing the lattice defects of lithium transition metal phosphates. This improves the pressure resistance of the particles, reduces the probability of particles collapsing and breaking under high rolling pressure, and improves the compaction density of the electrode. At the same time, the complete and dense carbon coating material is beneficial to improving the electrical contact between positive electrode active materials, improving the conductivity of positive electrode active materials, and reducing the polarization of positive electrode active materials. The low content of lattice defects is beneficial to unblocking lithium-ion transport channels and improving the lithium-ion transport rate of positive electrode active materials, thereby achieving a balance between battery dynamic performance and energy density.

[0009] In any embodiment, the iron dissolution rate of the positive electrode film is 658ppm-1485ppm. Positive electrode films with iron dissolution rates within this range have relatively fewer lattice defects and more complete and dense carbon coating material. Fewer crystal defects improve the compressibility and slippage of particles in the positive electrode film under high rolling pressure; high coating integrity facilitates particle slippage; and dense coating reduces the space occupancy rate of the carbon layer, further increasing the compaction density of the electrode and achieving a balance between battery kinetic performance and energy density.

[0010] In any embodiment, the median B of the coating value B obtained in the cumulative distribution curve of the positive electrode film layer in the laser microscopy confocal Raman spectroscopy instrument scanning mode is... 50 The value is 0.35-0.48, where the coating value B is I. P / I D , where I P This indicates that the Raman spectrum is at 948±100 cm⁻¹ -1 The intensity of the P peak at I DThis indicates that the Raman spectrum is at 1350±100 cm⁻¹ -1 The intensity of the D peak at that location. The coating value (B value) can indirectly reflect the thickness of the carbon coating material on the surface of lithium transition metal phosphate particles. The thinner the carbon coating material, the higher the intensity of the phosphate structure detected in the Raman spectrum, and the higher the coating value of the positive electrode film.

[0011] In any embodiment, the area percentage of particles with a diameter greater than or equal to 5 μm in the cross-section of the positive electrode film along the electrode thickness direction is 0. Studies have shown that particles with a diameter greater than or equal to 5 μm in the positive electrode film significantly deteriorate the wetting of the electrolyte in the positive electrode film and the diffusion of the electrolyte in the active material particles. Having an area percentage of particles with a diameter greater than or equal to 5 μm of 0 is beneficial for further reducing the battery's internal resistance and improving its kinetic performance.

[0012] 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 diameter greater than or equal to 1.5 μm and less than 5 μm is 9.0%-20.0%, optionally 10.0%-20.0%. The area ratio of particles with a diameter greater than or equal to 1.5 μm and less than 5 μm within the above range is beneficial for improving electrode compaction, while further reducing the hindering effect of large-sized particles on the electrode surface on the wetting and diffusion of electrolyte in the positive electrode film layer, improving the consistency of lithium ion diffusion rate in the positive electrode active material particles, reducing local polarization, and improving the kinetic 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 diameter greater than or equal to 1 μm and less than 1.5 μm is 15.0%-25.0%, optionally 16.0%-24.0%. The area ratio of particles with a diameter greater than or equal to 1 μm and less than 1.5 μm within the above range can fill larger gaps in the packing, forming a certain "support" structure, reducing the porosity between particles, and helping to enhance the contact between particles and the overall structural strength. While maintaining good dynamic performance of the battery, this further improves the compaction density of the electrode sheet and enhances the energy density of the battery.

[0014] In any embodiment, in the cross-section of the positive electrode film layer along the electrode thickness direction, the area ratio of particles with a diameter greater than or equal to 200 nm and less than 1500 nm is 73.0%-80.0%, optionally 73.0%-78.0%. The area ratio of particles with a diameter greater than or equal to 200 nm and less than 1500 nm can be obtained by testing using the method described above. The area ratio of particles with a diameter greater than or equal to 200 nm and less than 1500 nm in the cross-section of the positive electrode film layer along the electrode thickness direction is taken as the sum of the areas of particles with a diameter greater than or equal to 200 nm and less than 1500 nm divided by the total area of ​​the statistically analyzed particles.

[0015] In any embodiment, the median C50 of the graphitization degree C value obtained in the laser microscopy confocal Raman spectroscopy instrument scanning mode of the positive electrode film is 0.95-1.20, optionally 0.98-1.15, and further optionally 1.0-1.10; wherein the graphitization degree C value is IG / ID, where IG represents the intensity of the G peak in the Raman spectrum at 1580±100cm-1, and ID represents the intensity of the D peak in the Raman spectrum at 1350±100cm-1. The higher the degree of graphitization of carbon on the surface of the positive electrode active material, the higher the proportion of graphitic carbon in the positive electrode film, and the easier it is for particles to slip using the highly graphitized carbon structure in the coating material, thereby increasing the electrode compaction density.

[0016] In any embodiment, the median L of the spheroidal degree is obtained from the cumulative distribution curve of the area of ​​the particles obtained by slicing the positive electrode film along the thickness direction of the electrode sheet. A50 The median L of sphericity is 0.65-0.85, with a possible value of 0.70-0.80. 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.

[0017] In any embodiment, the median R of the particle roughness is obtained from the cumulative distribution curve of the particle roughness area obtained by slicing the positive electrode film along the electrode thickness direction. A50 The median roughness R is 0.92-0.96. A50 Within the aforementioned range, the particle surface is relatively smooth, and the friction between particles is relatively small. Under the action of external force, it is easy to slip, which can further improve the compaction density of the electrode and increase the energy density of the battery.

[0018] In any embodiment, based on the total mass of the positive electrode active material, the carbon content is 0.8%-1.8%, optionally 0.90%-1.50%. Compared with lithium phosphate-containing positive electrode active materials in the prior art, this positive electrode active material has a relatively low carbon coating content, which can further increase the lithium phosphate loading in the positive electrode sheet and improve the energy density of lithium-ion secondary batteries.

[0019] In any embodiment, the lithium iron antisite defect concentration of the positive electrode active material is 0.1%-1.5%, optionally 0.3%-1.0%. During preparation and cycling, certain lithium vacancies inevitably exist within the crystal structure of the positive electrode active material. These vacancies not only cause ferrous ions to oxidize to ferric ions but also induce partial migration of ferric ions to lithium sites, forming lithium iron antisite defects. This blocks the one-dimensional diffusion channels of lithium ions, adversely affecting the solid-phase transport of lithium ions. The positive electrode active material in this embodiment has low lithium iron antisite defects, which is beneficial for the uniform transport of lithium ions in the solid phase and further improves the kinetic performance of lithium-ion secondary batteries.

[0020] In any embodiment, the lithium-containing transition metal phosphate particles comprise 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 material during lithium insertion / extraction process, reduce oxygen activity on particle surface, improve the structural stability of material, thereby improving the specific capacity utilization level of material during cycling, and further improving the cycling stability of lithium-ion secondary batteries.

[0022] In any embodiment, the positive electrode active material includes one or more of lithium iron phosphate and its doped and modified materials, and coated and modified materials.

[0023] In any embodiment, the positive electrode active material includes titanium, and the mass content of titanium is 2000ppm-6000ppm based on the total mass of the positive electrode active material. The high addition of titanium did not form harmful impurity phases that negatively impact the battery's energy density and kinetic performance. While the reason for this is not yet clear, it is speculated that titanium, together with phosphate and other elements (e.g., lithium), forms a fast-ion conductor, which actually improves the battery's kinetic performance.

[0024] In any embodiment, the tap density of the positive electrode active material is 0.70 g / cm³. 3 -1.50g / cm 3 0.70 g / cm³ is an option. 3 -1.20g / cm 3 .

[0025] The effective gradation formed by the positive electrode active material in this embodiment is limited, resulting in a relatively low powder tap density. By leveraging the high integrity and density of the carbon coating material of the positive electrode active material in the positive electrode film layer, it is easy to compress the gaps between particles under external force to improve the powder tap density.

[0026] In any embodiment, the compacted density of the positive electrode active material under 3T pressure is 2.50 g / cm³. 3 -2.70g / cm 3 The option is 2.52g / cm³. 3 -2.68g / cm 3 .

[0027] Although the area ratio of particles with a diameter greater than or equal to 1.5 μm in this positive electrode film is low, the low iron dissolution rate of the positive electrode film indicates that the surface of the positive electrode active material has a high density and integrity of carbon coating material, which enables high compaction density to be achieved under external force. This provides a material basis for improving the compaction density of the electrode and preparing high energy density lithium-ion secondary batteries.

[0028] In any embodiment, the powder resistivity of the positive electrode active material at 8 MPa pressure is 0.5 Ω·cm-30.0 Ω·cm, optionally 2.0 Ω·cm-20.0 Ω·cm. The carbon coating material on the surface of this positive electrode active material has high integrity and density; therefore, the coating structure facilitates rapid electron conduction between particles, resulting in a low powder resistivity. This is beneficial for improving the solid-phase electron transport rate and further enhancing the battery's kinetic performance.

[0029] In any embodiment, the specific discharge capacity of the positive electrode active material at a 1C discharge rate is 135 mAh / g-150 mAh / g. This high specific discharge capacity at a 1C rate indicates that the positive electrode active material has good charge-discharge capability, which is beneficial for improving the battery's kinetic performance.

[0030] 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.

[0031] The high discharge capacity ratio of the positive electrode active material used in lithium-ion secondary batteries to 3.2V indicates that the positive electrode active material has good kinetic performance. At the same time, a high η value indicates that lithium-ion secondary batteries containing positive electrode active materials still have a high voltage when discharged to a low state of charge (SOC), which is beneficial for maintaining good power performance.

[0032] In any embodiment, the 0.1C discharge curve of the coin cell containing the positive electrode active material exhibits a discharge plateau within the voltage range of 2.5V to 2.9V. This is beneficial for increasing the discharge range of the battery and improving its energy density.

[0033] In any embodiment, the positive electrode film layer further includes a conductive agent, and the mass content of the conductive agent is 0.1%-1.5% based on the total mass of the positive electrode film layer.

[0034] In any embodiment, the positive electrode film layer does not include a conductive agent.

[0035] The particles in this positive electrode film have high sphericity, so the particles can be tightly packed during rolling and have good contact with each other. This results in good electronic conductivity between the particles inside the positive electrode film, which can reduce or even eliminate the use of conductive agents in the positive electrode film. This is beneficial to further increase the loading of positive electrode active materials and improve the energy density of lithium-ion secondary batteries.

[0036] In any embodiment, the positive electrode film layer further includes a binder, and based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 95.5%-99.5%, optionally 96.5%-99.5%; the mass content of the binder is 0.5%-3%.

[0037] When the mass content of the positive electrode active material and the mass content of the binder are within the above range, the loading of active material in the positive electrode film layer per unit volume can be effectively increased, while maintaining good internal adhesion, reducing the probability of powder shedding, expansion and cracking, and improving the energy density of the secondary battery while taking into account safety performance.

[0038] In any embodiment, the one-sided density of the positive electrode film is 300 mg / 1540 mm². 2 -450mg / 1540mm 2 A cathode film with an areal density within the above range can help improve the energy density of lithium-ion secondary batteries.

[0039] In any embodiment, the compaction density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery is 2.51 g / cm³. 3 -2.73g / cm 3 .

[0040] In any embodiment, the positive electrode film layer of the lithium-ion secondary battery in a fully discharged state has a compaction density of 2.55 g / cm³. 3 -2.70g / cm 3 .

[0041] 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.

[0042] In any embodiment, the positive electrode film layer satisfies at least one of the following conditions: (1) the compaction density of the positive electrode film layer is 2.51 g / cm³ when the lithium-ion secondary battery is in a fully discharged state. 3 -2.73g / cm 3 (1) In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-22%; (2) In the fully discharged state, the compaction density of the positive electrode film of the lithium-ion secondary battery is 2.55 g / cm³. 3 -2.70g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-20%.

[0043] The lower the porosity in the cross-section of the positive electrode film, the better the gradation of large, medium and small particles in the positive electrode film and the higher the compaction density. 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 overpressure and stress concentration in the film, further reducing the probability of the positive electrode film demolding during long cycles, which is beneficial to improving the long cycle performance of the battery.

[0044] In any embodiment, the positive electrode sheet includes a base coating layer disposed between the positive electrode film layer and the positive electrode current collector; the base coating layer satisfies at least one of the following conditions: (1) 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; (2) the compaction density of the positive electrode sheet in the 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 compaction density of the positive electrode sheet in the fully loaded state is greater than or equal to 2.5g / cm³. 3 The thickness of the base coating layer on one side is 2μm-4μm.

[0045] The undercoat layer helps improve the conductivity and adhesion between the positive electrode film and the current collector, reduces the detachment of the positive electrode film from the current collector during cycling, and improves the dynamic performance of the battery.

[0046] 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.

[0047] A second aspect of this application provides a battery device including the lithium-ion secondary battery of the first aspect of this application.

[0048] A third aspect of this application provides an electrical device, including at least one of the lithium-ion secondary battery of the first aspect of this application and the battery device of the second aspect of this application.

[0049] The fourth aspect of this application provides a method for preparing a positive electrode active material, comprising: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source, wherein the carbon source includes a polymer carbon source, the iron source contains less than or equal to 0.08% by mass of trivalent iron, and the molar ratio of lithium to iron in the mixed raw material is greater than or equal to 1 and less than or equal to 1.05; grinding to obtain a mixed slurry; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder and crushing it to obtain a positive electrode active material; the sintering includes a first temperature and a second temperature, wherein the second sintering temperature is 750℃-800℃.

[0050] Polymer carbon sources have relatively low decomposition and graphitization temperatures, allowing the carbon coating material on the surface of the cathode active material to decompose and form a carbon layer at lower sintering temperatures. This hinders the growth and sintering of lithium-containing transition metal phosphate grains, thus reducing the particle size of the cathode active material. Controlling the mass content of ferric iron helps improve the uniformity and consistency of the carbon coating material. The preparation method provided in this application optimizes the quality of the carbon coating and improves the particle size distribution and reduces crystal defects in the cathode active material by adjusting the carbon source, iron source, and sintering parameters, providing a material basis for the preparation of the cathode film.

[0051] The fifth aspect of this application provides a method for preparing an electrode sheet, the method comprising sequentially adding a binder, a conductive agent, and a positive electrode active material prepared by the method of the fourth aspect of this application, dry mixing them, adding a solvent, stirring, adjusting the viscosity, and obtaining a slurry; transferring the slurry to at least one side of a current collector, drying, and hot pressing to obtain a positive electrode film layer.

[0052] 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°C-80°C, and the electrode is heated to 40°C-50°C before the first pressing by the hot roller.

[0053] In any embodiment, the stirring includes pre-stirring and main stirring, wherein the main stirring has an orbital speed of 20 rpm-30 rpm and a rotational speed of 1450 rpm-1550 rpm.

[0054] The positive electrode active material prepared by the above-mentioned hot pressing process in combination with the preparation method in the fourth aspect is beneficial to further reduce the cross-sectional porosity of the positive electrode film, increase the ultimate compaction density of the electrode sheet, and improve the energy density of the battery.

[0055] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0056] Figure 1 is a scanning electron microscope image of a cross-section of the positive electrode film layer along the thickness direction of the electrode sheet according to an embodiment of this application; Figure 2 is a schematic diagram of a lithium-ion secondary battery according to an embodiment of this application; Figure 3 is an exploded view of a lithium-ion secondary battery according to an embodiment of this application; Figure 4 is a schematic diagram of a battery module according to an embodiment of this application; Figure 5 is a schematic diagram of a battery pack according to an embodiment of this application; Figure 6 is an exploded view of the battery pack shown in Figure 5; Figure 7 is a schematic diagram of a power supply device for the lithium-ion secondary battery according to an embodiment of this application; Figure 8 is a porosity test diagram of a cross-section of the positive electrode film layer along the thickness direction according to an embodiment of this application.

[0057] Explanation of reference numerals in the attached drawings: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Casing; 52 Electrode assembly; 53 Cover plate. Detailed Implementation

[0058] The following detailed description, with appropriate reference to the accompanying drawings, specifically discloses embodiments of the lithium-ion secondary battery, battery device, and power-consuming device 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] In this application, the terms "multiple" or "various" refer to two or more kinds.

[0064] Unless otherwise stated, the terms used in this application have the common meanings as commonly understood by those skilled in the art.

[0065] 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.

[0066] 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 modules, or battery packs.

[0067] 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.

[0068] Lithium-ion secondary batteries consist of electrode components and electrolyte.

[0069] 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).

[0070] 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.

[0071] 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.

[0072] 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.

[0073] In some implementations, the battery can be an energy storage device. Energy storage devices include energy storage containers, energy storage cabinets, etc.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] Lithium-containing transition metal phosphate materials have been widely used in lithium-ion batteries due to their stable structure, good safety, and long cycle life. However, they suffer from low electronic conductivity and low stacking efficiency, which makes it difficult to further increase the loading of lithium-containing phosphate in the electrode and thus fail to meet the needs of high-energy-density batteries.

[0079] To further improve battery energy density and increase electrode compaction density, a common industry practice is to increase the particle size distribution within the electrode. Improving particle size distribution requires increasing the proportion of large particles. However, research shows that exceeding a certain range of large particle proportion in the electrode can sacrifice battery kinetic performance. How to obtain a battery that simultaneously achieves high energy density and good kinetic performance is a pressing technical problem that needs to be solved in this field.

[0080] The first aspect of this application provides a lithium-ion secondary battery, which includes 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. In a cross-section of the positive electrode film along the thickness direction of the electrode, the area ratio of particles with a diameter greater than or equal to 1.5 μm is greater than or equal to 8.0% and less than or equal to 20.0%. The iron dissolution rate of the positive electrode film is 658 ppm to 1921 ppm.

[0081] In the cross-section of the positive electrode film along the thickness direction, the area ratio of particles with a diameter greater than or equal to 1.5 μm is less than 8%, making it difficult to achieve a high compaction density in the positive electrode. While a particle size ratio of more than 20% in the cross-section of the positive electrode film along the thickness direction is beneficial for achieving a high compaction density, it reduces the contact area between the electrolyte and the positive electrode active material, hinders the diffusion of lithium ions into the positive electrode film, increases the diffusion path of lithium ions within the particles, leads to severe localized polarization of the electrode, increases battery impedance, and significantly degrades the battery's kinetic performance.

[0082] Controlling the area ratio of particles with a diameter greater than or equal to 1.5 μm in the positive electrode film layer to be greater than or equal to 8.0% and less than or equal to 20.0% can reduce the significant bottleneck effect caused by large-sized particles, which is beneficial to maintaining the battery impedance at a low level and improving the battery's dynamic performance. However, this will limit the further improvement of the electrode compaction density. In the embodiments of this application, by controlling the iron dissolution rate of the positive electrode film layer to 658ppm-1921ppm, the density and integrity of the carbon coating on the surface of the positive electrode active material are improved, making it easier for particles to slide and reducing the lattice defects of lithium transition metal phosphates. This improves the pressure resistance of the particles, reduces the probability of particles collapsing and breaking under high rolling pressure, and improves the compaction density of the electrode. At the same time, the complete and dense carbon coating material is beneficial to improving the electrical contact between positive electrode active materials, improving the conductivity of positive electrode active materials, and reducing the polarization of positive electrode active materials. The low content of lattice defects is beneficial to unblocking lithium-ion transport channels and improving the lithium-ion transport rate of positive electrode active materials, thereby achieving a balance between battery dynamic performance and energy density.

[0083] 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).

[0084] Carbon-coated materials 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, carbon-coated materials 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 transmission electron microscopy coupled with energy dispersive spectroscopy.

[0085] In this application, the term "particle" refers to a particle in the positive electrode film layer that has a recognizable 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.

[0086] In some embodiments, in the cross-section of the positive electrode film layer along the electrode thickness direction, the area ratio of particles with a particle size greater than or equal to 1.5 μm is greater than or equal to 8.0% and less than or equal to 20.0%.

[0087] In some embodiments, the area percentage of particles with a diameter greater than or equal to 1.5 μm in the cross-section of the positive electrode film along the electrode thickness direction can be selected as 8.0%, 9.0%, 10.0%, 11.0%, 12.0%, 13.0%, 14.0%, 15.0%, 16.0%, 17.0%, 18.0%, 19.0%, 20.0%, or any value range between the two.

[0088] The specific method for particle identification in the cross-section of the positive electrode film along the electrode thickness direction is as follows: The positive electrode film is cut along the electrode thickness direction using an argon ion beam (for example, a Leica EM TIC 3X CP can be used; operating voltage: 6kV; operating time: 6h). After exposing the cross-section, a scanning electron microscope (SEM) is used (for example, a Hitachi SU8230 can be used; operating voltage: 3kV; beam current: high; probe model: U (LA100); working distance <5mm) to observe the cross-section of the positive electrode film along the electrode thickness direction. Images are acquired using a field emission scanning electron microscope at a non-edge location in the cross-section of the positive electrode film (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 in the image 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 during identification, 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 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 as follows: Particles located at the edges of the scanning electron microscope that are not fully displayed are deleted; It is determined whether other unidentified or misidentified particles have internal cracks or scratches. If no cracks or scratches are found, the particle is considered a single particle, and its boundary is manually marked based on observation; If cracks or scratches are found within the particle, it is determined whether the cracks or scratches penetrate the particle. If not, it is considered a single particle and manually marked; If the cracks or scratches penetrate the particle, it is determined whether the cracks or scratches are linear or irregular; If the cracks or scratches are irregular, they are considered the boundary between particles, and particles are divided along this boundary; If the cracks or scratches are linear, contrast is compared; If the contrast is not obvious and there is no crack-like appearance, it is considered a scratch and marked as a single particle; If the contrast is strong and there is a crack-like appearance, it is considered the boundary between particles and marked as two particles. After manual marking, irrelevant information from the automatic image processing is deleted, thus completing the particle identification and marking in the image.

[0089] The specific method for statistically analyzing the particle area in the cross-section along the thickness direction of the positive electrode film is as follows. After particle identification and labeling, the image is imported into ImageJ software for analysis. The scale is set based on the scanning electron microscope image. The particle size in the image is analyzed using the "Feret Diameter," "Area," "Round," and "Solidity" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter represents the maximum spacing between all parallel lines in the two-dimensional projection of the particle, thus characterizing the particle size; the "Area" parameter represents the pixel area of ​​the particle. Since particles smaller than 50nm have a large error in the statistical process and are difficult to accurately identify, and the particle size of conductive agents is generally smaller than 50nm, which will introduce a large error into the statistical results, particles smaller than 50nm are not counted in the particle size statistics of this application, and the particle statistics corresponding to AR, Round, or Solidity values ​​displayed as "NaN" are deleted. Following the above method, to ensure a statistically significant sample size, at least 10 non-overlapping scanning electron microscope (SEM) images are acquired for each electrode. The areas of at least 5000 particles are counted. The sum of the "Area" parameters for particles with a diameter greater than or equal to 1.5 μm and the sum of the "Area" parameters for all particles are calculated, and these are respectively used as the area of ​​particles with a diameter greater than or equal to 1.5 μm and the total area of ​​the counted particles. The sum of the areas of particles with a diameter greater than or equal to 1.5 μm divided by the total area of ​​the counted particles is used as the percentage of the area of ​​particles with a diameter greater than or equal to 1.5 μm in the cross-section of the positive electrode film along the electrode thickness direction.

[0090] Figure 1 shows the cross-sectional morphology of the positive electrode film along the thickness direction of the electrode sheet. This differs from the state of the positive electrode active material observed by Malvern laser scattering and also from the state of the positive electrode active material when directly observed by scanning electron microscopy. Under the action of roller pressure, the particles in the positive electrode film exhibit a well-dispersed state. Observing the positive electrode film is beneficial for effectively characterizing the particle size and area distribution within it.

[0091] During the compaction process, the positive electrode film undergoes compaction along its thickness. Therefore, compared to the surface of the positive electrode film, the cross-section along the thickness direction better reflects the actual compaction status 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 1.5 μm directly reflects the ratio of the area of ​​some particles in that size range to the total area of ​​all particles, indicating the distribution of particles in that size range.

[0092] 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.

[0093] In existing technologies, Malvern laser diffraction is commonly used to statistically analyze the particle size of cathode active materials. However, the applicant's research indicates that because lithium phosphate readily agglomerates, the test results obtained using Malvern laser diffraction, based on the principle of laser scattering, often only reflect the particle size of the agglomerates. This does not accurately reflect the particle size within the cathode active material, nor does it reflect the dispersion state of the cathode active material in the film layer, as the dispersion of the cathode active material in the film layer increases during the film-forming rolling process. The test results obtained by Malvern laser diffraction are closely related to the particle size, specific surface area, and degree of agglomeration of the cathode active material. 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.

[0094] Those skilled in the art can control the particle size using any known process. For example, the growth rate and time of the cathode material can be controlled by adjusting the temperature and time during the preparation process. The particle size can be adjusted by utilizing the mechanical force of crushing and grinding processes to process the raw materials to the target particle size distribution range; particle size separation can be achieved by using sieving and grading equipment to obtain the required particle size ratio; and precise control of the feed rate, adjusting the residence time and stress state of the particles within the equipment, also helps to control the particle size.

[0095] In some embodiments, the iron dissolution rate of the positive electrode film is 658ppm-1921ppm.

[0096] In some embodiments, the iron dissolution rate of the positive electrode film can be selected from 658ppm, 700ppm, 800ppm, 890pm, 900ppm, 1000ppm, 1058pm, 1076pm, 1100ppm, 1143pm, 1200ppm, 1236pm, 1300ppm, 1311pm, 1384pm, 1349pm, 1400ppm, 1485pm, 1500ppm, 1531pm, 1600ppm, 1700ppm, 1800ppm, 1921ppm or any value range between the two.

[0097] The iron dissolution rate of the positive electrode film can be tested using methods known in the art. As an example, after disassembling and cleaning the electrode from the battery, it is formed into small discs with a diameter of 14 mm. Multiple disc samples are taken to make a total sample mass of approximately 5 g. These are added to 100.3 g of a 0.3% ascorbic acid solution (using ultrapure water as the solvent). The solution is stirred at 500 rpm for 305 minutes, and then quickly aspirated using a 5 mL syringe. The solution is then filtered through a 0.45 μm pore size filter. The solution is poured into a test tube, and 1 mL of the supernatant is pipetted into a glass volumetric flask and diluted 50 times. The iron concentration in the solution is measured using inductively coupled plasma mass spectrometry (ICP-OES). The iron dissolution rate of the positive electrode film is calculated using the formula: [(ICP test iron concentration × solution volume / mass of the solution used for volume adjustment) × 100.3 g / (mass of the small disc electrode - mass of the current collector in the small disc)]. The solution volume is 50 mL, and the mass of the solution used for volume adjustment is 1 g. Preferably, the mass of the current collector in the small disc is obtained by multiplying the thickness of the small disc by its area and density. The thickness of the small disc can be equivalently measured by measuring the current collector thickness in the uncoated area using a thickness gauge. It is understood that although the current collector in the coated area will extend during compaction, resulting in a slight reduction in thickness compared to the uncoated area, this reduction is negligible and will not significantly affect the test results. More preferably, when the current collector is aluminum foil, the density is 2.7 g / cm³. 3 .

[0098] Those skilled in the art can control the iron dissolution rate of the positive electrode film using any known process. For example, this can be achieved by controlling the surface coating quality of the positive electrode material, and the temperature, time, and pressure during the preparation process. Furthermore, during battery use, battery design, the oxidant content in the electrolyte, battery operating temperature, and battery charge / discharge intensity also affect the iron dissolution rate of the positive electrode film. The iron dissolution rate of the positive electrode film mainly originates from the lithium-containing transition metal phosphate positive electrode active material within the film, and can indirectly reflect the integrity and density of the carbon coating on the surface of the positive electrode active material. A lower iron dissolution rate means that iron ions dissolved by acid are less likely to precipitate from the carbon coating material, indicating a more complete and dense carbon coating on the surface of the positive electrode active material. An iron dissolution rate within the aforementioned range indicates that the positive electrode active material has a relatively complete and dense carbon coating, which can improve the electrical contact between positive electrode active materials, improve the conductivity of the positive electrode active material, reduce the polarization of the positive electrode active material, and further optimize the kinetic performance of the lithium-ion secondary battery. Meanwhile, the high integrity of the carbon coating material makes the particles more susceptible to stress slippage during the rolling process, which can simultaneously improve the compaction density of the electrode and the energy density of the battery.

[0099] In some embodiments, the iron dissolution rate of the positive electrode film is 658ppm-1485ppm.

[0100] Cathode materials with iron dissolution rates within the aforementioned range exhibit relatively fewer lattice defects and more complete and dense carbon coating. Fewer crystal defects improve the compressibility and slippage of particles in the cathode film under high rolling pressure, while high coating integrity facilitates particle slippage. Dense coating reduces the space occupancy of the carbon layer, further increasing the compaction density of the electrode and achieving a balance between battery kinetic performance and energy density.

[0101] In some embodiments, the median B of the coating value B obtained in the cumulative distribution curve of the coating value B obtained in the laser microscopy confocal Raman spectroscopy instrument scanning mode is... 50 The value is 0.35-0.48, where the coating value B is I. P / I D , where I P This indicates that the Raman spectrum is at 948±100 cm⁻¹ -1 The intensity of the P peak at I D This indicates that the Raman spectrum is at 1350±100 cm⁻¹ -1 The intensity of peak D at that location.

[0102] In this application, the coating value (B value) of the positive electrode film can be obtained by scanning with a laser confocal microscopy (LCM) spectrometer. Specifically, a high-precision Renishaw laser confocal microscopy (LCM) spectrometer is used, with an excitation wavelength of 532 nm. An appropriate amount of the positive electrode film is scanned on its surface or along a section of the electrode thickness. 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. This yields the B values ​​at different sites and the cumulative distribution curve of the B values ​​in the scanned area.

[0103] 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 salt particles. To improve testing accuracy, it is preferable to perform a surface scan on the cross-section of the positive electrode film along the electrode thickness direction to characterize the coating value of the positive electrode film.

[0104] The coating value B of the positive electrode film was obtained by the ratio of the peak intensities of the P-band and D-band in the Raman spectrum, with the P-band located at 948±100 cm⁻¹. -1 Its characterization of phosphate PO4 3- Structure; D peak position is 1350±100cm -1 Raman spectroscopy characterizes disordered structures, where disorder refers to the irregular arrangement of carbon atoms within the structure. As a surface analysis instrument with a detection depth of 10 nm, the carbon structure peaks in the positive electrode film, under laser microscopy confocal Raman spectroscopy surface scanning mode, exhibit higher intensity compared to the phosphate structure peaks, which are more abundant in the bulk phase.

[0105] Those skilled in the art can control the coating value of active material particles using any known process. For example, adjusting the carbon source type, carbon source addition amount, sintering temperature, sintering time, sintering pressure, and sintering atmosphere can all achieve adjustments to the coating value of active material particles.

[0106] The coating value (B value) can indirectly reflect the thickness of the carbon coating material on the surface of lithium transition metal phosphate particles. The thinner the carbon coating material, the higher the intensity of the phosphate structure detected in the Raman spectrum, and the higher the coating value of the cathode film.

[0107] The cumulative distribution curve of the coverage value (B value) is a curve obtained by arranging at least 100 B values ​​in ascending order, with the coverage value on the horizontal axis and the cumulative percentage on the vertical axis. 50 The B value is the value corresponding to the cumulative number of values ​​on the vertical axis of the cumulative distribution curve of the coverage value B value when the cumulative number accounts for 50%.

[0108] In some embodiments, the median B of the coating value of the positive electrode film layer 50 The value can be selected from 0.35, 0.36, 0.37, 0.38, 0.39, 0.398, 0.4, 0.41, 0.42, 0.43, 0.432, 0.44, 0.443, 0.445, 0.448, 0.449, 0.45, 0.453, 0.459, 0.46, 0.47, 0.48, or any value range between the two.

[0109] To reduce the impact of extreme coating values ​​caused by non-particle regions in the positive electrode film on the test results, the median coating value B was used. 50 Evaluate the thickness of the carbon coating material on the positive electrode active material. The median coating value (B) of the positive electrode film. 50 Within the above range, it is indicated that the carbon coating material of the positive electrode active material has a relatively low thickness, which is beneficial to further reduce the volume occupied by the carbon coating material, further improve the compaction density of the electrode, and achieve a balance between battery dynamic performance and energy density.

[0110] In some embodiments, the area ratio of particles with a diameter greater than or equal to 5 μm in the cross-section of the positive electrode film along the electrode thickness direction is 0.

[0111] The area percentage of particles with a diameter greater than or equal to 5 μm in the cross-section of the positive electrode film along the electrode thickness direction can be tested in the manner described above. The area of ​​particles with a diameter greater than or equal to 5 μm in the cross-section of the positive electrode film along the electrode thickness direction divided by the total area of ​​the statistically analyzed particles is taken as the area percentage of particles with a diameter greater than or equal to 5 μm in the cross-section of the positive electrode film along the electrode thickness direction.

[0112] Studies have shown that particles with a diameter greater than or equal to 5 μm in the positive electrode film layer will significantly deteriorate the wetting of the electrolyte in the positive electrode film layer and the diffusion in the active material particles. The area ratio of particles with a diameter greater than or equal to 5 μm is 0, which is beneficial to further reduce the internal resistance of the battery and improve the battery dynamic performance.

[0113] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 1.5 μm and less than 5 μm is 8.0%-20.0%, and can be selected as 10.0%-20.0%.

[0114] In some embodiments, the area percentage of particles with a diameter greater than or equal to 1.5 μm and less than 5 μm in the cross-section of the positive electrode film along the electrode thickness direction can be selected as 8.0%, 9.0%, 9.02%, 10.0%, 11.0%, 12.0%, 12.81%, 13.0%, 14.0%, 14.58%, 14.62%, 14.92%, 15.0%, 15.31%, 15.39%, 15.46%, 16.0%, 16.02%, 16.89%, 17.0%, 18.0%, 19.0%, 20.0%, or any value range between the two.

[0115] The area percentage of particles with a diameter greater than or equal to 1.5 μm and less than 5 μm in the cross-section of the positive electrode film along the electrode thickness direction can be tested in accordance with the method described above. The area of ​​particles with a diameter greater than or equal to 1.5 μm and less than 5 μm in the cross-section of the positive electrode film along the electrode thickness direction divided by the total area of ​​the statistically analyzed particles is taken as the area percentage of particles with a diameter greater than or equal to 1.5 μm and less than 5 μm in the cross-section of the positive electrode film along the electrode thickness direction.

[0116] In the process of improving particle size distribution and increasing the area ratio of large particles, particles with a diameter greater than or equal to 1.5 μm and less than 5 μm are inevitably introduced. In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, a particle area ratio of particles with a diameter greater than or equal to 1.5 μm and less than 5 μm within the aforementioned range is beneficial for improving electrode compaction. It can further reduce the hindering effect of large particles on the electrode surface on the wetting and diffusion of the electrolyte in the positive electrode film layer, improve the consistency of lithium-ion diffusion rate in the positive electrode active material particles, reduce local polarization, and improve the battery's kinetic performance.

[0117] In some embodiments, the area ratio of particles with a diameter greater than or equal to 1 μm and less than 1.5 μm in the cross-section of the positive electrode film along the thickness direction of the electrode sheet is 15.0%-25.0%, and can be selected as 16.0%-24.0%.

[0118] In some embodiments, the area percentage of particles with a diameter greater than or equal to 1 μm and less than 1.5 μm in the cross-section of the positive electrode film along the electrode thickness direction can be selected as 15.0%, 16.0%, 17.0%, 18.0%, 18.28%, 18.41%, 18.88%, 19.0%, 19.23%, 19.31%, 19.66%, 19.70%, 20.0%, 20.25%, 20.89%, 21.0%, 22.0%, 23.0%, 23.88%, 24.0%, 25.0%, or any value range between the two.

[0119] The area percentage of particles with a diameter greater than or equal to 1 μm and less than 1.5 μm in the cross-section of the positive electrode film along the electrode thickness direction can be tested in accordance with the method described above. The area percentage of particles with a diameter greater than or equal to 1 μm and less than 1.5 μm in the cross-section of the positive electrode film along the electrode thickness direction is calculated by dividing the area of ​​particles with a diameter greater than or equal to 1 μm and less than 1.5 μm in the cross-section of the positive electrode film along the electrode thickness direction by the total area of ​​the particles counted.

[0120] 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 and less than 1.5 μm within the above range can fill larger gaps in the stacking, forming a certain "support" structure, reducing the porosity between particles, and helping to enhance the contact between particles and the overall structural strength. While maintaining good dynamic performance of the battery, it further improves the compaction density of the electrode sheet and improves the energy density of the battery.

[0121] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 200 nm and less than 1500 nm is 73.0%-80.0%, and can be selected as 55%-65%.

[0122] In some embodiments, the area percentage of particles with a diameter greater than or equal to 200 nm and less than 1500 nm in the cross-section of the positive electrode film along the electrode thickness direction can be selected as 73.0%, 74.0%, 75.0%, 75.07%, 75.10%, 75.24%, 75.42%, 75.45%, 75.67%, 75.83%, 76%, 76.21%, 76.66%, 77.0%, 78.0%, 78.57%, 79.0%, 80.0%, or any value range between the two.

[0123] The area percentage of particles with a diameter greater than or equal to 200 nm and less than 1500 nm in the cross-section of the positive electrode film along the electrode thickness direction can be obtained by testing using the method described above. The area percentage of particles with a diameter greater than or equal to 200 nm and less than 1500 nm in the cross-section of the positive electrode film along the electrode thickness direction is calculated by dividing the sum of the areas of particles with a diameter greater than or equal to 200 nm and less than 1500 nm by the total area of ​​the statistically analyzed particles.

[0124] The area ratio of particles with a diameter greater than or equal to 200 nm and less than 1500 nm in the positive electrode film layer within the above range can further improve the consistency of lithium-ion diffusion rate, thereby improving the dynamic performance of lithium-ion secondary batteries, increasing the utilization rate of active materials, and thus improving the energy density of the battery.

[0125] In some embodiments, the median C of the graphitization degree C 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 0.95-1.20, optionally 0.98-1.15, and further optionally 1.0-1.10; 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.

[0126] 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.

[0127] 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 salt particles. 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.

[0128] 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 peaks in the Raman spectrum. The position of the G-band peak was 1580±100 cm. -1 Its characterization of carbon sp 2 Hybrid structure; D peak position is 1350±100 cm⁻¹ -1 It characterizes a disordered structure, where disorder refers to the irregular arrangement of carbon atoms within the structure. In graphite crystals, carbon atoms in the same layer arrange themselves 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 film, i.e., the carbon coating material of the cathode active material. Although rich in sp... 2 Hybridized carbon nanotube conductive agents also have relatively high I0G / 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 This has an impact. Therefore, the degree of graphitization of the positive electrode film can also be used to characterize the degree of graphitization of the positive electrode active material.

[0129] 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 for graphitization degree C. The median C value for 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.

[0130] The higher the degree of graphitization of carbon on the surface of the positive electrode active material, the higher the proportion of graphite structure carbon in the positive electrode film layer, and the easier it is for particles to slip by means of the highly graphitized carbon structure in the coating material, thereby increasing the compaction density of the electrode sheet.

[0131] 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 (which can be a polymer carbon source, such as PEG), sintering temperature, sintering time, sintering pressure, sintering atmosphere, and nucleation process can all achieve adjustments to the degree of graphitization of active material particles. The higher the degree of graphitization of carbon on the surface of the positive electrode active material, the higher the proportion of graphitic carbon in the positive electrode film layer, and the easier it is for particles to slip using the highly graphitized carbon structures in the coating material, thereby increasing the electrode compaction density.

[0132] In some embodiments, the median C of the graphitization degree C 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 can be selected from 0.95, 0.96, 0.97, 0.98, 0.99, 0.993, 1, 1.005, 1.008, 1.01, 1.012, 1.015, 1.02, 1.021, 1.03, 1.032, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.20, or any value range between the two.

[0133] In some embodiments, the median L of the spheroidal density of the particles in the cumulative distribution curve of the particle spheroidal density obtained from a cross-section of the positive electrode film along the electrode thickness direction is... A50 The value is 0.65-0.85, and can be selected as 0.70-0.80, where the sphericity L value is the ratio of the particle area to the fitted circle area.

[0134] 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 "Area" parameter obtained from the analysis represents the pixel area of ​​the particle, and the "Round" parameter represents the ratio of the pixel area of ​​the particle to the area of ​​a circle with the fitted major axis as 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 particle obtained from the analysis characterizes the sphericity of the particle. The sphericity of at least 5000 particles obtained are 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%.

[0135] 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 0.65, 0.66, 0.67, 0.68, 0.69, 0.70, 0.705, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.80, 0.81, 0.82, 0.83, 0.84, 0.85, or any value range between the two.

[0136] 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.

[0137] 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.

[0138] In some embodiments, the median R of the particle roughness is obtained from the cumulative distribution curve of the particle roughness area obtained by slicing the positive electrode film along the electrode thickness direction. A50 It is 0.92-0.96.

[0139] In the cumulative roughness area distribution curve of the particles obtained from the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the median roughness R A50 The specific testing method is as follows: Particles in the cross-section of the positive electrode film layer were identified using the method described above in this application. The morphology of the particles in the cross-section along the thickness direction of the positive electrode film layer was analyzed using the "Shape Description" analysis function in ImageJ. According to the software manual (ImageJ User Guide IJ 1.46r), the "Solidity" parameter obtained from the analysis represents the ratio of the pixel area to the convex area of ​​the particle. Therefore, the "Solidity" parameter of the analyzed particles is used to characterize the roughness of the particles. By definition, the closer the roughness is to 1, the smoother the particle. The sphericity of at least 5000 particles was arranged in ascending order, and the cumulative area distribution curve of the particles in the positive electrode film layer was obtained with roughness as the horizontal axis and cumulative area ratio as the vertical axis. A50 This is the roughness R value when the cumulative area ratio of the vertical axis in the cumulative distribution curve of roughness R value is 50%.

[0140] In some embodiments, the median R of the particle roughness is obtained from the cumulative distribution curve of the particle roughness area obtained by slicing the positive electrode film along the electrode thickness direction. A50 The value can be selected as 0.92, 0.93, 0.94, 0.95, 0.96 or any range between two of them.

[0141] Those skilled in the art can control the roughness of particles using any known process. For example, particle roughness can be adjusted through processes such as grinding, polishing, abrasion, milligram energy processing, electroplating, and calendering, as well as by adjusting the parameters of each process.

[0142] Median roughness R A50 Within the aforementioned range, the particle surface is relatively smooth, and the friction between particles is relatively small. Under the action of external force, it is easy to slip, which can further improve the compaction density of the electrode and increase the energy density of the battery.

[0143] In some embodiments, the carbon content is 0.8%-1.8% by mass, and optionally 0.9%-1.5%, based on the total mass of the positive electrode active material.

[0144] 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".

[0145] In some embodiments, based on the total mass of the positive electrode active material, the mass content of carbon can be selected as 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, or any value range between the two.

[0146] Compared with existing lithium phosphate-containing positive electrode active materials, this positive electrode active material has a relatively low carbon coating content, which can further increase the lithium phosphate loading in the positive electrode sheet and improve the energy density of lithium-ion secondary batteries.

[0147] In some embodiments, the lithium iron ion reverse defect concentration of the positive electrode active material is 0.1%-1.5%, optionally 0.3%-1.0%.

[0148] XRD data of the samples were collected using an X-ray diffractometer, and phase analysis was performed. The CIF file of the phase obtained from an open-source website was used as the initial crystal structure model, including the definition of unit cell parameters, atomic positions, and occupancy probabilities. In the initial crystal structure model, considering the possibility of Fe-Li antisites, the possible Li content at Fe sites and the possible Fe content at Li sites were set to an initial value of 0.1%. The collected XRD data were fitted and refined using FullProf Suite software, refining the parameters in the order of background parameters, peak intensity, unit cell parameters, and peak shape. When the fitted peak shape and the experimental peak shape were optimally matched, and Rwp was less than 10, the refined Li and Fe occupancy probabilities were obtained, which were used as the concentration of lithium-iron antisite defects.

[0149] In some embodiments, the lithium iron ion site defect concentration of the positive electrode active material can be selected as 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, or any value range between the two.

[0150] Those skilled in the art can control the antisite defects of lithium iron phosphate in cathode active materials using any known process. For example, the antisite defects of lithium iron phosphate in cathode active materials can be controlled by adjusting the sintering temperature, sintering time, preparation method, and raw material stoichiometry.

[0151] During preparation and cycling, lithium vacancies inevitably exist within the crystal structure of the positive electrode active material. These vacancies not only cause ferrous ions to oxidize to ferric ions, but also induce some ferric ions to migrate to lithium sites, forming lithium-iron antisite defects. This blocks the one-dimensional diffusion channels of lithium ions and adversely affects the solid-phase transport of lithium ions.

[0152] The positive electrode active material in this embodiment has low lithium iron antisite defects, which indirectly confirms that the positive electrode active material in this embodiment has low lattice defect content. This is beneficial to reducing the risk of particle collapse and cracking under high rolling pressure, increasing the electrode compaction density, and achieving uniform transport of lithium ions in the solid phase, thereby further improving the energy density and kinetic performance of lithium-ion secondary batteries.

[0153] In some embodiments, the lithium-containing transition metal phosphate particles comprise 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.

[0154] 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 a value within a range consisting of any two of the above values.

[0155] In some implementations, 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 a value within a range of any two of the above values.

[0156] In some implementations, y can be selected as 0.95, 0.96, 0.97, 0.98, 0.99, 1.00, or a value within a range of any two of the above values.

[0157] In some implementations, j can be selected as a value from 3.5, 3.6, 3.7, 3.8, 3.9, 4, or any two of the above values.

[0158] In some implementations, q can be selected as 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, or a value within a range of any two of the above values.

[0159] Selecting appropriate modifying element Q can improve the lattice change rate of positive electrode active material during lithium insertion / extraction process, reduce oxygen activity on particle surface, improve the structural stability of material, thereby improving the specific capacity utilization level of material during cycling, and further improving the cycling stability of lithium-ion secondary batteries.

[0160] In some embodiments, the positive electrode active material includes one or more of lithium iron phosphate and its doped and modified materials, and coated and modified materials.

[0161] In some embodiments, the positive electrode active material includes titanium, and the mass content of titanium is 2000ppm-6000ppm based on the total mass of the positive electrode active material.

[0162] 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.

[0163] Doping titanium in cathode active materials can induce lattice distortion, reduce Li-O bond energy, increase lithium-ion transport rate, and improve the kinetic performance of lithium-ion secondary batteries. However, in existing technologies, the titanium doping content in lithium phosphates often cannot exceed 3000 ppm, because excessive titanium is difficult to completely enter the lithium phosphate bulk phase and easily becomes a harmful impurity phase remaining on the surface, negatively impacting battery performance.

[0164] The positive electrode active material in this application embodiment has a high titanium content. Surprisingly, the high amount of titanium added did not form a harmful impurity phase that negatively affects the battery's energy density and kinetic performance. Although the reason is not yet clear, it is speculated that titanium, together with phosphate and other elements (e.g., lithium), forms a fast ion conductor, which instead improves the battery's kinetic performance.

[0165] In some embodiments, the tap density of the positive electrode active material is 0.70 g / cm³. 3 -1.50g / cm 3 The selectable value is 0.7-1.20 g / cm³. 3 .

[0166] The tap density of powder can be tested using methods known in the art. As an example, turn on the electronic balance, place a conical flask as a base on the balance, and then zero the balance. Place the tapped graduated cylinder on the conical flask, weigh it, and record the weight. Open the sample bag, use a clean sample spoon to stir the sample in the bag 3-5 times to mix it thoroughly, and then smoothly transfer the sample into the graduated cylinder. Wipe the powder adhering to the surface of the cylinder with lint-free paper, and then place it into the zeroed conical flask and weigh it. Seal the mouth of the graduated cylinder with sealing film, and place the tapped graduated cylinder into the matching instrument rubber ring to ensure proper vibration. The tapped density cylinder is tightly fitted to the rubber ring and kept perpendicular to the instrument surface. The vibration frequency is set to 250 times / min and the number of vibrations is 5000. The button is pressed and the cylinder vibrates for 20 minutes. Then, the tapped density cylinder is removed, and the surface of the cylinder is illuminated with a flashlight. The highest scale value V1 and the lowest scale value V2 are read visually, and the average value V is taken. The mass of the cylinder is obtained by subtracting the mass of the cylinder m0 from the mass of the sample m1. The mass of the powder m is obtained by using the density formula ρ=m / v.

[0167] In some embodiments, the tap density of the positive electrode active material powder can be selected as 0.70 g / cm³. 3 0.80g / cm 3 0.90g / cm 3 1.00g / cm 3 1.10 g / cm 3 1.20g / cm 3 1.30g / cm 3 1.40g / cm 3 1.50g / cm 3 Or the range of values ​​between any two.

[0168] The effective gradation formed by the positive electrode active material in this embodiment is limited, resulting in a relatively low powder tap density. By leveraging the high integrity and density of the carbon coating material of the positive electrode active material in the positive electrode film layer, it is easy to compress the gaps between particles under external force to improve the powder tap density.

[0169] In some embodiments, the compacted density of the positive electrode active material at a pressure of 3T is 2.50 g / cm³. 3 -2.70 g / cm 3 The option is 2.52g / cm³. 3 -2.68 g / cm 3 .

[0170] 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 .

[0171] 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 then placed on a compaction density instrument. The bottom area of ​​the mold is 1.327 cm². 2 The pressure is set to 3T. The thickness of the positive electrode active material under 3T pressure can be read on the equipment. The compaction 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, S is the bottom area of ​​the mold, and H is the thickness of the positive electrode active material after compaction.

[0172] In some embodiments, the compaction density of the positive electrode active material under 3T pressure can be selected as 2.50 g / cm³. 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 2.59 g / cm 3 2.60 g / cm 3 2.61 g / cm 3 2.62 g / cm 3 2.63 g / cm 3 2.64 g / cm 3 2.65 g / cm 3 2.66 g / cm 3 2.67 g / cm 3 2.68 g / cm 3 2.69 g / cm 3 2.70 g / cm 3 Or the range of values ​​between any two.

[0173] Although the area ratio of particles with a diameter greater than or equal to 1.5 μm in this positive electrode film is low, the low iron dissolution rate of the positive electrode film indicates that the surface of the positive electrode active material has a high density and integrity of carbon coating material, which enables high compaction density to be achieved under external force. This provides a material basis for improving the compaction density of the electrode and preparing high energy density lithium-ion secondary batteries.

[0174] In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 8 MPa is 0.5 Ω·cm-30.0 Ω·cm, and can be selected as 2.0 Ω·cm-20.0 Ω·cm.

[0175] 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 8 MPa 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.

[0176] In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 8 MPa can be selected as 0.5 Ω·cm, 1 Ω·cm, 2 Ω·cm, 3 Ω·cm, 4 Ω·cm, 5 Ω·cm, 6 Ω·cm, 7 Ω·cm, 8 Ω·cm, 9 Ω·cm, 10 Ω·cm, 15 Ω·cm, 20 Ω·cm, 25 Ω·cm, 30 Ω·cm, or any value range between the two.

[0177] The carbon coating material on the surface of the positive electrode active material has high integrity and density. Therefore, the coating structure facilitates the rapid conduction of electrons between particles, resulting in a low powder resistivity of the positive electrode active material. This is beneficial for improving the solid-phase electron transport rate and further improving the kinetic performance of the battery.

[0178] In some embodiments, the discharge specific capacity of the positive electrode active material at a 1C discharge rate is 135mAh / g-150mAh / g.

[0179] 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.

[0180] 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 using a punch, 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.

[0181] 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.

[0182] In some embodiments, the discharge specific capacity of the positive electrode active material at a 1C discharge rate can be selected as 135mAh / g, 140mAh / g, 142.4mAh / g, 145mAh / g, 150mAh / g, or any value range between the two.

[0183] The positive electrode active material exhibits a high discharge specific capacity at a 1C rate, indicating that it has good charge and discharge capabilities, which is beneficial for improving the battery's dynamic performance.

[0184] 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.

[0185] In some embodiments, the discharge capacity η of the positive electrode active material when discharged to 3.2V accounts for ≥88%.

[0186] 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 prepared coin cell is then tested for electrical performance at room temperature using a blue-light tester. Specifically, 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 until the current reaches 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.

[0187] In some implementations, η can be selected as 85%, 86%, 87%, 88%, 88.1%, 89%, 90%, 90.1%, 91%, 91.1%, 92%, 92.2%, 93%, 94%, 94.1%, 94.5%, 95%, 95.1%, or a range consisting of any two of the above numerical intervals, or a value within that range.

[0188] 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%.

[0189] The high discharge capacity ratio of the positive electrode active material used in the lithium-ion secondary battery of this application to 3.2V indicates that the positive electrode active material has 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.

[0190] In some embodiments, the 0.1C discharge curve of the coin cell containing the positive electrode active material exhibits a discharge plateau in the voltage range of 2.5V to 2.9V.

[0191] A discharge plateau typically refers to a region where the voltage remains relatively stable during the charging and discharging process of a battery. During battery discharge, current flows out of the battery, and the battery voltage initially drops, but then enters a relatively stable region where voltage fluctuations are minimal; this stable voltage region is called the discharge plateau.

[0192] Coin cells can be assembled by disassembling the positive electrode of a lithium-ion secondary battery and combining it with lithium metal. Alternatively, they can be assembled using the method described above. In this application, the positive electrode active material is assembled into a coin cell, and its electrical performance is tested using a blue electrode tester. Within a voltage range of 2.0V to 3.75V, it is charged at a constant current of 0.1C to 3.75V, paused for 5 minutes, charged at a constant voltage to a cutoff current of 50μA, and then discharged at a constant current of 0.1C to 2.0V.

[0193] The discharge curves show that the standard charge-discharge plateau voltage for lithium phosphate batteries is typically between 3.2V and 3.65V. The coin cell containing the positive electrode active material described in this application exhibits a new charge-discharge plateau within the 2.5V to 2.9V voltage range, which is beneficial for increasing the battery's discharge range and improving its energy density. This also verifies the hypothesis that the positive electrode active material in this application contains a fast-ion conductor.

[0194] In some embodiments, the positive electrode film layer further includes a conductive agent, and the mass content of the conductive agent is 0.1%-1.5% based on the total mass of the positive electrode film layer.

[0195] In some embodiments, the positive electrode film layer further includes a conductive agent, and the mass content of the conductive agent, based on the total mass of the positive electrode film layer, can be selected as 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, or any value range between the two.

[0196] 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.

[0197] This positive electrode active material has a highly dense and intact carbon coating, which gives it good electronic conductivity. This reduces the amount of conductive agent used in the positive electrode film, which is beneficial to further increase the loading of the positive electrode active material and improve the energy density of lithium-ion secondary batteries.

[0198] In some embodiments, the positive electrode film does not include a conductive agent.

[0199] This positive electrode active material has extremely high electronic conductivity, which means that conductive agents can be omitted from the positive electrode film, thus improving the loading of the positive electrode active material and enhancing the energy density of lithium-ion secondary batteries.

[0200] In some embodiments, the positive electrode film layer further includes a binder, and based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 95.5%-99.5%, optionally 96.5%-99.5%; the mass content of the binder is 0.5%-3.0%.

[0201] 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.

[0202] 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 95.5%, 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, 99%, 99.5%, or any range between the two.

[0203] 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.

[0204] When the mass content of the positive electrode active material and the mass content of the binder are within the above range, the loading of active material in the positive electrode film layer per unit volume can be effectively increased, while maintaining good internal adhesion, reducing the probability of powder shedding, expansion and cracking, and improving the energy density of the secondary battery while taking into account safety performance.

[0205] In some embodiments, the one-sided density of the positive electrode film is 300 mg / 1540 mm². 2 -450mg / 1540mm 2 .

[0206] 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 a small circular piece with an area of ​​S1, weigh it, and record its 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.

[0207] In some embodiments, the areal density of the positive electrode film layer on one side may be selected as 300 mg / 1540 mm². 2310mg / 1540mm 2 320mg / 1540mm 2 330mg / 1540mm 2 340mg / 1540mm 2 350mg / 1540mm 2 360mg / 1540mm 2 370 mg / 1540 mm 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.

[0208] Positive electrode films with areal densities within the above range can help improve the energy density of lithium-ion secondary batteries.

[0209] In some embodiments, the positive electrode film layer of the lithium-ion secondary battery, in its fully discharged state, has a compaction density of 2.51 g / cm³. 3 -2.73g / cm 3 .

[0210] In some embodiments, the positive electrode film layer of the lithium-ion secondary battery, in its fully discharged state, has a compaction density of 2.55 g / cm³. 3 -2.70g / cm 3 .

[0211] 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.

[0212] 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 and left to stand for 2 hours. Once the battery temperature is maintained at 25°C, it is discharged at a constant current of 1 / 3C to 2.5V, then discharged at a constant current of 0.1C to 2.0V. The battery is then disassembled to obtain the positive electrode sheet. 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, yielding a mass of W1. The thickness T1 of the positive electrode sheet is measured using a micrometer. The positive electrode film layer of the weighed electrode sheet is then wiped off, and the mass of the current collector is weighed and recorded as W2. The thickness T2 of the current collector is measured using a micrometer. The compaction density PD of the positive electrode film layer is then calculated as follows: PD = (W1 / (W2)) / (W2 / (W2)) W2) / [(T1-T2)×S。

[0213] In some embodiments, the positive electrode film layer of the lithium-ion secondary battery, in its fully discharged state, has a compaction density of 2.51 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 Or any value in between.

[0214] In some embodiments, after compaction processing, the compaction density of the positive electrode film layer is 2.63 g / cm³. 3 -2.85g / cm 3 .

[0215] In some embodiments, the compaction density of the positive electrode film layer after the compaction process can be selected as 2.63 g / cm³.3 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 / cm 3 2.82 g / cm 3 2.83 g / cm 3 2.84 g / cm 3 2.85g / cm 3 Or the range of values ​​between any two.

[0216] 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.

[0217] In some embodiments, the compaction density of the positive electrode film layer after formation processing is 2.51 g / cm³. 3 -2.73g / cm 3 .

[0218] In some embodiments, the compaction density of the positive electrode film layer after formation processing can be selected as 2.51 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 / cm3 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 Or the range of values ​​between any two.

[0219] 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.

[0220] 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.

[0221] 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.

[0222] In some embodiments, the compaction density of the positive electrode film is 2.51 g / cm³. 3 -2.73g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-22%.

[0223] In some embodiments, the compaction density of the positive electrode film is 2.55 g / cm³. 3 -2.70g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-20%.

[0224] 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%, or any value range between the two.

[0225] 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 8 bit" 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.

[0226] As shown in Figure 8, it can be understood that the "pores" in the cross-section of the positive electrode film layer are identified by image color difference and threshold in this embodiment. These "pores" are not the pore data obtained from the venting 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 venting method because the porosity obtained by the venting method is related to the pores between particles and also to the pores in the carbon layer coating 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, the better the particle gradation of the positive electrode film layer and the higher the compaction density. Furthermore, under the same gradation and roller pressure, a 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 layer, further reducing the probability of the positive electrode film demolding during long cycles, which is beneficial to improving the long-cycle performance of the battery.

[0227] In some embodiments, the positive electrode sheet includes a base coating layer disposed between the positive electrode film layer and the positive electrode 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.

[0228] Carbon-based particles refer to particles whose main component is carbon, including but not limited to conductive carbon and carbon black.

[0229] 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 dynamic 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 loaded 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.

[0230] 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.

[0231] 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. V50 In the 20-60nm range, D V90 For materials less than or equal to 70 nm, a base coating is obtained by mixing, stirring, and coating carbon-based materials with a binder onto a current collector.

[0232] In some embodiments, 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.

[0233] In some embodiments, 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.

[0234] 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.

[0235] 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 1m 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, namely, areas with a thickness less than 50nm and areas with a thickness greater than 4m. These outliers are mainly due to extreme thickness fluctuations in individual areas caused by abnormal stress concentration and compression during electrode compaction, and are not statistically significant.

[0236] In some embodiments, the thickness of the positive current collector is less than or equal to 17 μm, and can be selected as 13 μm-15 μm.

[0237] 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.

[0238] 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.).

[0239] 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 140 mg / 1540 mm². 2 -221mg / 1540mm 2 ; and / or the compaction density of the negative electrode film is 1.40 g / cm³. 3 -1.75g / cm 3 .

[0240] 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.

[0241] Having the areal density and compaction density of the negative electrode film within the above-mentioned range is beneficial for improving the energy density of lithium-ion secondary batteries.

[0242] 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.).

[0243] 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.

[0244] In some embodiments, the negative electrode film layer may optionally include an adhesive. The adhesive 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).

[0245] 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.

[0246] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0247] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as 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 after drying, compaction and other processes, the negative electrode sheet can be obtained.

[0248] In some embodiments, the lithium-ion secondary battery includes an electrolyte. The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific limitations on the type of electrolyte, which can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or entirely solid-state.

[0249] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0250] 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.

[0251] 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.

[0252] 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.

[0253] 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.

[0254] 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.

[0255] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0256] 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.

[0257] 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.

[0258] 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.

[0259] A third aspect of this application provides an electrical device, including the lithium-ion secondary battery provided in the first aspect of this application.

[0260] The fourth aspect of this application provides a method for preparing a positive electrode active material, comprising: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source, wherein the carbon source includes a polymer carbon source, the iron source contains less than or equal to 0.08% by mass of trivalent iron, and the molar ratio of lithium to iron in the mixed raw material is greater than or equal to 1 and less than or equal to 1.05; grinding to obtain a mixed slurry; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder and crushing it to obtain a positive electrode active material; the sintering includes a first temperature and a second temperature, wherein the second sintering temperature is 750℃-800℃.

[0261] The preparation method provided in this application optimizes the quality of carbon coating and improves the particle size distribution and reduces crystal defects in the positive electrode active material by controlling the carbon source, iron source and sintering parameters. This provides a material basis for the preparation of a positive electrode film with a particle size greater than or equal to 1.5 μm accounting for more than 8.0% and less than or equal to 20.0% of the area of ​​particles with a particle size greater than or equal to 1.5 μm in the cross-section along the thickness direction of the electrode, and an iron dissolution rate of 658 ppm-1921 ppm.

[0262] In some embodiments, the iron source includes ferrous iron, which may be one or more of ferrous oxalate, ferrous carbonate, and ferrous nitrate.

[0263] During sintering, the ferrous iron source preferentially decomposes to generate a large amount of ferrous oxide, which serves as nucleation sites for the formation of lithium transition metal phosphate nanocrystals. Simultaneously, the polymer carbon source has a relatively low decomposition temperature, and the iron elements on the surface of the nanocrystals further catalyze the decomposition of the carbon source. This allows the carbon-coated material on the surface of the cathode active material to achieve a relatively high degree of graphitization at a lower sintering temperature, reducing the resistivity of the cathode active material and improving the density and uniformity of the carbon coating on the lithium transition metal phosphate surface. Furthermore, the uniform deposition of carbon on the lithium transition metal phosphate surface further hinders the growth of lithium transition metal phosphate grains, reducing the probability of the cathode active material particles growing into large particles with a diameter greater than 1.5 μm.

[0264] In some embodiments, the particle size D of ferrous oxalate 10 ≥3μm, particle size D 50 With a particle size of 50-80 μm and a diameter D 90 Less than or equal to 150 μm.

[0265] In this application, the term "D" 10 “D” 50 "and "D 90 "These correspond to the particle sizes at which the cumulative particle size distribution percentage of the sample obtained by the Malvern laser scattering method reaches 10%, 50%, and 90%, respectively."

[0266] Controlling the particle size D of ferrous oxalate 10 A particle size of 3 μm or larger can reduce the proportion of small-sized ferrous oxalate particles and control their reactivity during the grinding process. Controlling the particle size D of ferrous oxalate... 50 D 90 It helps to uniformly mix the raw materials during the grinding process, obtain a mixed slurry with consistent composition and uniform particle size, and improve the particle size consistency of the prepared lithium transition metal phosphate.

[0267] In some implementations, the mass content of ferric iron is less than or equal to 0.08%.

[0268] In some implementations, the mass content of ferric iron can be selected as 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, or any value range between the two.

[0269] Controlling the mass content of ferric iron helps improve the uniformity and consistency of carbon-coated materials. Excessive ferric iron content will preferentially consume the carbon source, resulting in poor consistency in the mass and thickness of the carbon layer coating between particles. On the one hand, the uneven thickness of the carbon-coated material will affect the compaction between particles; on the other hand, local carbon depletion will affect the overlap of the conductive network between particles, which is not conducive to effectively improving the compaction density of the electrode and the improvement of kinetics.

[0270] In some embodiments, the lithium source includes one or more of lithium dihydrogen phosphate, lithium phosphate, lithium carbonate, and lithium acetate.

[0271] In some embodiments, the carbon source includes a polymeric carbon source, which may be one or more of polyethylene glycol and polyvinyl alcohol.

[0272] In some implementations, the carbon source content is 1-4% based on the total mass of the mixed raw materials.

[0273] The polymer carbon source has a relatively low decomposition temperature and graphitization temperature, which allows the carbon coating material on the surface of the positive electrode active material to decompose and form a carbon layer at a lower sintering temperature. This hinders the growth and sintering of lithium transition metal phosphate grains, which is beneficial for reducing the particle size of the positive electrode active material.

[0274] Meanwhile, polymer carbon sources typically have high molecular weights or long molecular chains, which are easy to form stable framework structures through cross-linking or orientation during heat treatment. This orderliness is preserved during high-temperature carbonization, which is beneficial for the directional growth of graphite crystals. At the same time, the entanglement and cross-linking between long chains help reduce structural defects and reduce lattice disorder caused by chain breakage during carbonization, thereby improving the degree of graphitization.

[0275] Organic molecules in the carbon source decompose at high temperatures, releasing carbon atoms. These carbon atoms can cover and fill tiny gaps or defects on the surface of the active material, reducing surface roughness. The coating material formed by the polymer carbon source has a high degree of graphitization and a denser carbon structure, which is beneficial for optimizing the surface roughness of the positive electrode active material.

[0276] In some embodiments, the weight-average molecular weight of polyethylene glycol is 10,000 or less.

[0277] Polyethylene glycol with a weight average molecular weight of less than 10,000 has a shorter carbon chain, making it easier to control the decomposition rate during sintering to form a carbon coating material with appropriate and uniform thickness.

[0278] In some implementations, the water content of polyethylene glycol is less than or equal to 0.5%.

[0279] If the moisture content in polyethylene glycol is high, it may affect the decomposition process, resulting in incomplete decomposition or uneven decomposition rate during sintering. Excessive moisture may also cause uneven distribution of molten polyethylene glycol during sintering, affecting the uniformity of the carbon layer and leading to instability or detachment of the carbon coating material.

[0280] In some embodiments, the water content of polyethylene glycol can be selected as 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, or any value between two of these.

[0281] In some implementations, the pH of polyethylene glycol is 5-7.

[0282] Polyethylene glycol (PEG) with a pH of 5-7 exhibits high stability and will not degrade during mixing due to excessive acidity, especially under high-temperature conditions, which would lead to excessively rapid decomposition and affect the coating quality. If PEG is alkaline, it may affect the stability of other components, causing metal ions to dissolve or oxidize, thus impacting the final performance of the positive electrode active material.

[0283] In some embodiments, the phosphorus source includes one or more of lithium dihydrogen phosphate, phosphoric acid, and ammonium dihydrogen phosphate.

[0284] In some implementations, the lithium source and the phosphorus source can be the same substance.

[0285] In some embodiments, the iron source includes ferrous oxalate, the lithium and phosphorus sources include lithium dihydrogen phosphate, and the carbon source includes polyethylene glycol.

[0286] In some embodiments, the atomic molar ratio of lithium to iron in the lithium source and the iron source is 1.00-1.05.

[0287] In some embodiments, the atomic molar ratio of lithium to iron in the lithium source and the iron source can be selected as 1.00, 1.01, 1.02, 1.03, 1.04, 1.05 or any value range between the two.

[0288] A lithium-iron molar ratio of 1 is considered an ideal stoichiometric ratio, maintaining optimal electrochemical performance, optimizing the reversible insertion / extraction of lithium ions during charge / discharge, and exhibiting good crystal structure stability to improve cycle life, while also reducing the probability of impurity phases. However, in actual production, to compensate for lithium loss during sintering, the lithium-iron molar ratio needs to be adjusted to slightly higher than 1.

[0289] 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.

[0290] Titanium sources often have low surface activity. Including titanium sources in the slurry can reduce the activity of lithium transition metal phosphate precursors and inhibit the particle growth of lithium transition metal phosphates during high-temperature sintering, resulting in smaller particles of lithium transition metal phosphates during sintering.

[0291] Titanium is used as a lattice stabilizer; the element titanium is usually represented by Ti. 4+ In the form of lithium transition metal phosphate, some titanium ions can replace iron ions in the crystal lattice, making the crystal structure more stable and reducing the possibility of lithium and iron ions being reversed, especially at high temperatures or during high-current charging and discharging.

[0292] Meanwhile, titanium doping helps improve the sphericity of the particles and reduce their roughness, thereby enhancing the overall structural stability of the material.

[0293] In some embodiments, lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide are mixed evenly in an organic solvent and then ground to obtain a mixed raw material.

[0294] Organic solvents can effectively reduce the occurrence of side reactions and improve the purity and consistency of materials. Furthermore, organic solvents have good volatility, making them easier to remove during subsequent drying processes and preventing residues inside the material that could create pores and affect its density and structural stability.

[0295] In some embodiments, the obtained mixed slurry after grinding includes at least two ball milling-demagnetization cycles, each of which independently satisfies one or more of the following conditions: (1) the grinding balls used in the ball milling are one or more of zirconium oxide balls, silicon nitride zirconium balls, and ceramic zirconium balls; (2) the diameter of the grinding balls used in the first ball milling is 5 mm-6 mm, and the diameter of the grinding balls used in the second ball milling is 0.5 mm-0.7 mm; (3) the rotation speed of the first ball milling is 1400 rpm-1600 rpm, and the rotation speed of the second ball milling is 400 rpm-600 rpm; (4) the grinding time of the first ball milling is 150 min-200 min, and the grinding time of the second ball milling is 140 min-180 min; (5) the demagnetization method is permanent magnet demagnetization; and (6) the demagnetization intensity is greater than or equal to 8000 GS.

[0296] By combining ball milling and demagnetization at least twice, large particles can be processed quickly and further refined in a short time. This effectively avoids particle size inhomogeneity caused by ball milling, reduces particle agglomeration, improves battery conductivity and cycle stability, and enhances overall production efficiency while ensuring the performance of the final product.

[0297] In some embodiments, the particle size D of the mixed slurry is...v50 The range is 1.0μm-4.0μm.

[0298] In this application, the term "D" V50 "" refers to the particle size at which the cumulative particle size distribution percentage of the sample volume obtained by Malvern laser scattering method reaches 50%; in some embodiments, the particle size D of the volume distribution of particles in the mixed slurry V50 The value can be selected as 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4.0μm or any value range between the two.

[0299] Volume distribution of particles in the mixed slurry, particle size D V50 Within the aforementioned range, on the one hand, it can increase the activity of particles to a certain extent, and at the same temperature, some positive electrode active material particles with a particle size of 1μm-1.5μm can be generated, improving the compaction density of the electrode and the energy density of the battery; on the other hand, it can improve the catalytic decomposition efficiency of the iron element on the crystal nucleus surface for the carbon source, improve the coating quality of the carbon source, improve the coating uniformity and graphitization degree of the carbon material, and further improve the compaction density of the electrode and the energy density of the battery.

[0300] In some embodiments, obtaining precursor powder after drying the mixed slurry includes obtaining precursor powder after spray drying the mixed slurry.

[0301] In some embodiments, sintering the precursor powder to obtain the positive electrode active material includes at least two sintering processes.

[0302] In some embodiments, the first sintering of the at least two sinterings satisfies one or more of the following conditions: (1) the heating rate is greater than or equal to 2ºC / min; (2) the holding temperature is 300℃-400℃; and (3) the holding time is 2h-6h.

[0303] In some embodiments, the second sintering in the at least two sinterings satisfies one or more of the following conditions: (1) the heating rate is greater than or equal to 3ºC / min; (2) the holding temperature is 750℃-800℃; and (3) the holding time is 8h-15h.

[0304] Using a high heating rate to quickly heat to the target temperature is beneficial for uniform particle growth and reduces the presence of particles with a diameter greater than or equal to 1.5 μm.

[0305] By controlling the sintering temperature in the first and second sintering processes, the sintering diffusion rate can be controlled. At high temperatures, diffusion on the particle surface increases, defects within the particles are repaired, and the crystal lattice rearranges. Through recrystallization, surface defects are eliminated, the grain structure of the particles becomes more ordered, the particle size gradually increases, and it helps to smooth the particle surface and promotes the development of spherical particles. Sintering temperature also affects the graphitization rate of the carbon source. Kinetically, carbon atoms gain more energy, enabling them to overcome the original energy barrier, resulting in more vigorous rearrangement within the crystal lattice. Sintering time affects the extent of the reaction. Too short a sintering time results in incomplete diffusion and rearrangement of the lithium transition metal phosphate and the carbon source; too long a sintering time leads to abnormal particle growth, coarsening of the internal grains, instability of the material structure, increased adhesion between particles, and agglomeration.

[0306] In some embodiments, the product is subjected to air jet milling after sintering the precursor to obtain the positive electrode active material.

[0307] In some embodiments, the grading frequency of the air jet mill is 18Hz-24Hz, and the milling pressure is 0.45MPa-0.65MPa.

[0308] In air jet milling, the classification frequency refers to the operating frequency of the classification device, which is typically related to the classification efficiency and particle size distribution. A higher classification frequency results in more frequent sieving of particles in the airflow, removing larger particles and leaving smaller ones. Furthermore, a higher classification frequency may increase the number of particle collisions, subjecting irregular particles to further impact, resulting in smoother particle surfaces and a more spherical shape.

[0309] High air pressure causes particles to be subjected to greater impact force, and the collisions between particles are more intense. This results in stronger impact and wear on the particle surface, which can crush large particles into smaller particles. The more intense collisions between particles make the surface easier to be trimmed, improving the sphericity and surface smoothness of the particles.

[0310] However, excessively high classification frequency and pulverizing pressure can cause agglomerated particles to disperse into primary particles, leading to further cracking and breakage. This affects the intended particle size distribution and results in incomplete carbon coating material, manifested as increased iron dissolution. This negatively impacts particle slippage during rolling and increases the contact and reaction between lithium-containing transition metal phosphates and external factors such as the electrolyte, which is detrimental to battery cycle performance and lifespan. Therefore, it is necessary to control the classification frequency and pulverizing pressure of the air jet pulverizer within a suitable range.

[0311] 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, adjusting the viscosity, and obtaining a slurry; transferring the slurry to at least one side of a current collector, drying, and hot pressing to obtain a positive electrode film layer.

[0312] In some embodiments, the stirring includes pre-stirring and main stirring, wherein the main stirring has an orbital speed of 20 rpm-30 rpm and a rotational speed of 1450 rpm-1550 rpm.

[0313] 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°C-80°C, and the electrode is heated to 40°C-50°C before the first hot roller compaction.

[0314] 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 is beneficial to further reduce the cross-sectional porosity of the positive electrode film, increase the ultimate compaction density of the electrode sheet, and improve the energy density of the battery.

[0315] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0316] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0317] 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.

[0318] 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.

[0319] The following are 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.

[0320] Example 1 (1) Preparation of positive electrode active material: Lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide were mixed evenly in methanol and ground to obtain a mixed raw material. The ratio of lithium dihydrogen phosphate to ferrous oxalate resulted in an atomic molar ratio of lithium to iron of 1.03; the particle size D of ferrous oxalate was... 90 It is 100nm in size and contains 0.03% ferric iron by mass.

[0321] The mixed raw materials are ball-milled multiple times in a ball mill and then demagnetized to obtain a mixed slurry.

[0322] The spray-dried slurry yields a dried precursor powder, which is light yellow in appearance and uniform in color.

[0323] The precursor powder was placed in a sintering furnace and heated from 25°C to a first temperature of 350°C at a rate of 2°C / min under a nitrogen atmosphere and held at that temperature for 3 hours. Then, the temperature was increased to a second temperature of 770°C at a rate of 5°C / min and held at that temperature for 10 hours. After the process, the temperature was lowered and cooled.

[0324] The obtained material was crushed using an airflow pulverization method with a staged frequency of 22Hz and a pulverizing airflow of 0.55MPa to obtain carbon-coated lithium iron phosphate cathode active material.

[0325] The prepared positive electrode active material has a carbon content of 1.144% by mass, a lithium iron antisite defect concentration of 0.58%, and a powder tap density of 1.05 g / cm³. 3 The compacted density of the powder under 3T pressure is 2.57 g / cm³. 3 The powder resistivity at 8MPa is 6.0Ω·cm; the discharge capacity at 1C discharge rate is 142.4mAh / g; there is a discharge plateau in the voltage range of 2.5V to 2.9V, and the discharge capacity of the 3.2V discharge plateau accounts for 91.1%.

[0326] (2) Preparation of the positive electrode sheet: 2.2 wt% PVDF, 0.8 wt% conductive carbon black, and 97.0 wt% positive electrode active material were added sequentially and dry-mixed. N-methylpyrrolidone was then added, and the mixture was stirred and the viscosity adjusted to obtain a slurry. The slurry was transferred and coated onto the base coating of the current collector aluminum foil. The base coating consisted of carbon black and PVDF in a mass ratio of 1:1. The distribution density of carbon-based particles with a particle size greater than 100 nm in the base coating was ≤10 pcs / 10 μm, and the thickness of the base coating 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 stirring includes pre-stirring and main stirring, wherein the main stirring has an orbital speed of 25 rpm and a rotational speed of 1500 rpm.

[0327] The hot pressing process includes three hot roller pressing processes, with the hot roller pressing pressure increasing sequentially to 35 tons, 55 tons, and 75 tons. The hot roller temperature is 65°C. Before the first entry into the hot roller for compaction, the electrode sheet is heated to 50°C.

[0328] The compacted density of the electrode sheet is its ultimate compacted density. The method for testing the ultimate compacted density of the electrode sheet is described below. In this example, the ultimate compacted density of the electrode sheet is 2.67 g / cm³. 3 .

[0329] Statistical results from cross-sections along the thickness direction of the prepared positive electrode film show that: particles with a diameter greater than or equal to 1.5 μm and less than 5 μm account for 14.58% of the area; particles with a diameter greater than 5 μm account for 0% of the area; particles with a diameter greater than or equal to 1 μm and less than 1.5 μm account for 19.70% of the area; and particles with a diameter greater than or equal to 200 nm and less than 1500 nm account for 75.07% of the area.

[0330] The median B of the coating value obtained by the positive electrode film in the surface scanning mode of laser microscopy confocal Raman spectroscopy. 50 The median C of graphitization degree obtained in laser microscopy confocal Raman spectroscopy instrument scanning mode is 0.443. 50 The median L of the sphericity is 1.021. This is the cumulative distribution curve of particle sphericity obtained from a cross-section along the electrode thickness direction of the positive electrode film. 50 The median roughness R is 0.722. In the cumulative distribution curve of particle roughness number obtained from a cross-section along the electrode thickness direction of the positive electrode film, the median roughness R is... 50 The value is 0.943. The iron dissolution rate of the positive electrode film is 1058 ppm.

[0331] (3) Preparation of negative electrode sheet: 95.5 wt% of negative electrode active material (artificial graphite), 1.0 wt% of conductive agent (conductive carbon black), 2.0 wt% of binder (styrene-butadiene rubber (SBR)) and 1.5 wt% of thickener (sodium carboxymethyl cellulose (CMC)) were mixed, and deionized water was added and stirred to disperse and prepare a negative electrode slurry. The negative electrode slurry was then coated onto both sides of a Cu foil. After both sides were coated, the foil was dried, compacted, slit, and sheeted to obtain the negative electrode sheet. The density of the coated single side was 165 mg / 1540 mm². 2 The compacted density is 1.60 g / cm³. 3 .

[0332] (4) The separation membrane is prepared using polypropylene membrane as the separation membrane.

[0333] (5) Preparation of electrolyte: In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), the organic solvents ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed evenly at a volume ratio of 1 / 1. Lithium salt LiPF6 is added and dissolved in the organic solvent. The content of LiPF6 in the solution is 1mol / L. The mixture is stirred evenly to obtain the electrolyte.

[0334] (6) Battery preparation: The positive electrode, separator, and negative electrode are stacked in sequence. The separator should be able to isolate the anode and cathode. The bare cell is obtained by winding. The bare cell is placed in the outer packaging, the electrolyte is injected, and the battery is then encapsulated, formed, and degassed to finally obtain a lithium-ion battery.

[0335] The preparation method of Example 2 is basically the same as that of Example 1. The difference is that in the preparation steps of the positive electrode active material, the ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.02.

[0336] The preparation method of Example 3 is basically the same as that of Example 1. The difference is that in the preparation steps of the positive electrode active material, the ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.01.

[0337] The preparation method of Example 4 is basically the same as that of Example 1. The difference is that in the preparation steps of the positive electrode active material, the ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.05.

[0338] The preparation method of Example 5 is basically the same as that of Example 1, except that the mass content of ferric iron in ferrous oxalate is 0.80% in the preparation step of the positive electrode active material.

[0339] The preparation method of Example 6 is basically the same as that of Example 1, except that in the preparation step of the positive electrode active material, the carbon source is replaced with polyethylene glycol + glucose, wherein the mass ratio of polyethylene glycol to glucose is 3:1.

[0340] The preparation method of Example 7 is basically the same as that of Example 1, except that in the preparation step of the positive electrode active material, the carbon source is replaced with polyethylene glycol + glucose, wherein the mass ratio of polyethylene glycol to glucose is 2:1.

[0341] The preparation method of Example 8 is basically the same as that of Example 1, except that the carbon source is replaced with polyethylene glycol + glucose in the preparation step of the positive electrode active material, wherein the mass ratio of polyethylene glycol to glucose is 1:2.

[0342] The preparation method of Example 9 is basically the same as that of Example 1, except that the carbon source is replaced with polyethylene glycol + glucose in the preparation step of the positive electrode active material, wherein the mass ratio of polyethylene glycol to glucose is 1:3.

[0343] The preparation method of Example 10 is basically the same as that of Example 1, except that conductive carbon black is not added in the preparation steps of the positive electrode sheet.

[0344] The preparation method of Example 11 is basically the same as that of Example 1, except that the second temperature in the preparation step of the positive electrode active material is 755°C.

[0345] The preparation method of Example 12 is basically the same as that of Example 1, except that in the preparation steps of the positive electrode active material, the carbon source is replaced with polyethylene glycol + glucose, wherein the mass ratio of polyethylene glycol to glucose is 1:3; the ratio of lithium dihydrogen phosphate and ferrous oxalate makes the atomic molar ratio of lithium to iron 1.05.

[0346] The preparation method of Comparative Example 1 is basically the same as that of Example 1, except that in the preparation steps of the positive electrode active material, the carbon source polyethylene glycol is replaced with glucose; the ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.05; and the second temperature is 810°C.

[0347] The preparation method of Comparative Example 2 is basically the same as that of Example 1, except that in the preparation steps of the positive electrode active material, the ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.01; and the second temperature is 745°C.

[0348] The preparation method of Comparative Example 3 is basically the same as that of Example 1, except that in the preparation steps of the positive electrode active material, the carbon source polyethylene glycol is replaced with glucose; the ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the molar ratio of lithium to iron is 1.06; and the second temperature is 740°C.

[0349] Performance Testing 1. Ultimate Compacted Density of Electrode Sheets: Double-sided coated electrodes are compacted using a roller press. The elongation of the compacted electrode is tested, and its flexibility is evaluated. By increasing the pressure of the roller press, electrodes with different compacted densities are obtained. As the pressure increases, the compacted density increases, the elongation increases, and the flexibility decreases. Excessive elongation can easily lead to electrode warping, while insufficient flexibility can easily cause brittle fracture. Therefore, the smaller of the compacted density corresponding to an 8% elongation or a 3-fold flexible fold is defined as the ultimate compacted density of the electrode.

[0350] The compaction density is calculated by dividing the mass of the positive electrode film by the volume of the positive electrode film.

[0351] The elongation test method is as follows: Lay the electrode flat on a horizontal table and cut it into sections, each about 100cm long; remove the copper foil from the electrode edge, ensuring that the cut edge is parallel to the MD direction of the electrode (perpendicular to the direction of the pressure roller), and ensure that the electrode is completely covered by the coating. Use a steel ruler to measure the length between the marked points at the same position along the length direction of the electrode at the beginning and end, estimating to 0.1mm, and record the length before compaction; after compaction, record the length between the corresponding marked points. The elongation of the electrode is calculated as (length after compaction - length before compaction) / length before compaction.

[0352] The test method for the number of flexible folds is as follows.

[0353] 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.

[0354] 2. Energy Density Test: The lithium-ion secondary battery was left to stand at 25°C for 2 hours to ensure the temperature remained at 25°C. At 25°C, the battery was charged at 0.33C to the charging cutoff voltage of 3.65V, and then continued to be charged at this cutoff voltage under constant voltage until the current reached 0.05C, at which point charging was stopped (where C represents the rated capacity of the lithium-ion secondary battery). After leaving the battery to stand at 25°C for 1 hour, it was discharged at 0.33C at 25°C to the discharge cutoff voltage of 2.5V. The total discharge energy of the lithium-ion secondary battery was recorded as E0.

[0355] Measure the length, width, and height of the lithium-ion secondary battery, and calculate the volume value V0 of the lithium-ion secondary battery = length * width * height.

[0356] The volumetric energy density of a lithium-ion secondary battery = discharge energy of the lithium-ion secondary battery E0 / volume of the lithium-ion secondary battery V0.

[0357] 3. DCR Test Method: At 25℃, charge at a constant current of 0.33C to 3.65V, then charge at a constant voltage to a current of 0.05C, then discharge at 1 / 3C to 20% SOC. After resting for 5 minutes, pulse discharge at 3C for 30 seconds, rest for 40 seconds, charge at 3C for 40 seconds, rest for 5 minutes, charge at a constant current of 1 / 3C to 3.65V, then charge at a constant voltage to 0.05C, then discharge at 1 / 3C to 10% SOC. After resting for 5 minutes, pulse discharge at 3C for 30 seconds. After standing for 40 seconds, charge at 3C for 40 seconds, stand for 5 minutes, then fully charge at 1 / 3C, then discharge at 1 / 3C to 50% SOC, then stand at -25℃ for 2 hours, then pulse discharge at 1C for 30 seconds, stand for 10 minutes, then stand at 25℃ for 2 hours, charge at 1 / 3C constant current to 3.65V, then charge at constant voltage to 0.05C, then discharge at 1 / 3C to 20% SOC, then stand at -25℃ for 2 hours, then pulse discharge at 1C for 30 seconds, then stand for 10 minutes.

[0358] 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 before resting after pulse discharge) / pulse current.

[0359] Experimental parameters and test results: Batteries for each embodiment and comparative example were prepared according to the above method, and various performance parameters were measured. The results are shown in Table 1 below.

[0360] Table 1

[0361] Continued from Table 1

[0362] As can be seen from the data comparison of the examples and comparative examples, when the area ratio of particles with a particle size of 1.5 μm or greater is greater than or equal to 8.0% and less than or equal to 20.0%, and the iron dissolution rate of the positive electrode film is 658ppm-1921ppm, the battery has high positive electrode compaction density and energy density, and the battery also has good dynamic performance.

[0363] As can be seen from the comparison of Examples 9 and 12 with other examples, when the iron dissolution rate of the positive electrode film is 658ppm-1485ppm, the DC internal resistance of the battery can be further reduced.

[0364] As can be seen from the data comparison of the examples and comparative examples, when the area ratio of particles with a particle size greater than or equal to 1.5 μm and less than 5 μm is 9.0%-20.0%, it is beneficial to further improve the compaction density and energy density of the positive electrode sheet.

[0365] As can be seen from the comparison between Example 11 and other examples, when the area ratio of particles with a diameter greater than or equal to 200 nm and less than 1500 nm in the cross-section of the positive electrode film along the thickness direction of the electrode sheet is 73.0%-78.0%, it is beneficial to further improve the energy density of the battery.

[0366] 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, The device includes 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. In the 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 200 nm and less than 1500 nm is 73.0%-80.0%. In the 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 and less than 1.5 μm is 15.0%-25.0%. The iron dissolution rate of the positive electrode film is 658 ppm-1921 ppm.

2. The lithium-ion secondary battery according to claim 1, characterized in that, The iron dissolution rate of the positive electrode film is 658ppm-1485ppm.

3. The lithium-ion secondary battery according to claim 1 or 2, characterized in that, In the cumulative distribution curve of the coating value B obtained by laser microscopy confocal Raman spectroscopy in instrument scanning mode, the median B of the coating value is... 50 The value is 0.35-0.48, where the coating value B is I. P / I D , where I P This indicates that the Raman spectrum is at 948±100 cm⁻¹ -1 The intensity of the P peak at I D This indicates that the Raman spectrum is at 1350±100 cm⁻¹ -1 The intensity of peak D at that location.

4. The lithium-ion secondary battery according to any one of claims 1-3, 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 greater than or equal to 5 μm is 0.

5. The lithium-ion secondary battery according to any one of claims 1-4, 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 greater than or equal to 1.5 μm is greater than or equal to 8.0% and less than or equal to 20.0%.

6. The lithium-ion secondary battery according to any one of claims 1-5, 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 greater than or equal to 1.5 μm and less than 5 μm is 9.0%-20.0%.

7. The lithium-ion secondary battery according to any one of claims 1-6, 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 greater than or equal to 1.5 μm and less than 5 μm is 10.0%-20.0%.

8. The lithium-ion secondary battery according to any one of claims 1-7, 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 greater than or equal to 1 μm and less than 1.5 μm is 16.0%-24.0%.

9. The lithium-ion secondary battery according to any one of claims 1-8, 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 greater than or equal to 200 nm and less than 1500 nm is 73.0%-78.0%.

10. The lithium-ion secondary battery according to any one of claims 1-9, 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 ranges from 0.95 to 1.20; where 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.

11. The lithium-ion secondary battery according to any one of claims 1-10, 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 ranges from 0.98 to 1.15; where 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.

12. The lithium-ion secondary battery according to any one of claims 1-11, 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 1.0-1.10; where 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.

13. The lithium-ion secondary battery according to any one of claims 1-12, characterized in that, In the cumulative distribution curve of the spheroidal degree area of ​​the particles obtained from the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the median L of the spheroidal degree is... A50 It ranges from 0.65 to 0.

85.

14. The lithium-ion secondary battery according to any one of claims 1-13, characterized in that, In the cumulative distribution curve of the spheroidal degree area of ​​the particles obtained from the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the median L of the spheroidal degree is... A50 It is 0.70-0.

80.

15. The lithium-ion secondary battery according to any one of claims 1-14, characterized in that, In the cumulative roughness area distribution curve of the particles obtained from the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the median R of the particle roughness is... A50 It is 0.92-0.

96.

16. The lithium-ion secondary battery according to any one of claims 1-15, characterized in that, Based on the total mass of the positive electrode active material, the carbon content is 0.8%-1.8%.

17. The lithium-ion secondary battery according to any one of claims 1-16, characterized in that, Based on the total mass of the positive electrode active material, the carbon content is 0.90%-1.50%.

18. The lithium-ion secondary battery according to any one of claims 1-17, characterized in that, The concentration of lithium iron ion reverse defects in the positive electrode active material is 0.1%-1.5%.

19. The lithium-ion secondary battery according to any one of claims 1-18, characterized in that, The concentration of lithium iron ion reverse defects in the positive electrode active material is 0.3%-1.0%.

20. The lithium-ion secondary battery according to any one of claims 1-19, characterized in that, The lithium-containing transition metal phosphate particles comprise 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.

21. The lithium-ion secondary battery according to any one of claims 1-20, characterized in that, The positive electrode active material includes one or more of lithium iron phosphate and its doped and modified materials, and coated and modified materials.

22. The lithium-ion secondary battery according to any one of claims 1-21, characterized in that, The positive electrode active material includes titanium, and the mass content of titanium is 2000ppm-6000ppm based on the total mass of the positive electrode active material.

23. The lithium-ion secondary battery according to any one of claims 1-22, characterized in that, The tap density of the positive electrode active material is 0.70 g / cm³. 3 -1.50g / cm 3 .

24. The lithium-ion secondary battery according to any one of claims 1-23, characterized in that, The tap density of the positive electrode active material is 0.70 g / cm³. 3 -1.20g / cm 3 .

25. The lithium-ion secondary battery according to any one of claims 1-24, characterized in that, The compacted density of the positive electrode active material under 3T pressure is 2.50 g / cm³. 3 -2.70g / cm 3 .

26. The lithium-ion secondary battery according to any one of claims 1-25, characterized in that, The compacted density of the positive electrode active material under 3T pressure is 2.52 g / cm³. 3 -2.68g / cm 3 .

27. The lithium-ion secondary battery according to any one of claims 1-26, characterized in that, The resistivity of the positive electrode active material at 8 MPa pressure is 0.5 Ω·cm-30.0 Ω·cm.

28. The lithium-ion secondary battery according to any one of claims 1-27, characterized in that, The resistivity of the positive electrode active material at 8 MPa pressure is 2.0 Ω·cm-20.0 Ω·cm.

29. The lithium-ion secondary battery according to any one of claims 1-28, characterized in that, The discharge capacity of the positive electrode active material at a 1C discharge rate is 135mAh / g-150mAh / g.

30. The lithium-ion secondary battery according to any one of claims 1-29, 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.

31. The lithium-ion secondary battery according to any one of claims 1-30, characterized in that, In the 0.1C discharge curve of the coin cell containing the positive electrode active material, there is a discharge plateau in the voltage range of 2.5V to 2.9V.

32. The lithium-ion secondary battery according to any one of claims 1-31, characterized in that, The positive electrode film layer also includes a conductive agent, and the mass content of the conductive agent is 0.1%-1.5% based on the total mass of the positive electrode film layer.

33. The lithium-ion secondary battery according to any one of claims 1-31, characterized in that, The positive electrode film does not include a conductive agent.

34. The lithium-ion secondary battery according to any one of claims 1-33, characterized in that, Based on the total mass of the positive electrode film, the mass content of the positive electrode active material is 95.5%-99.5%.

35. The lithium-ion secondary battery according to any one of claims 1-34, characterized in that, Based on the total mass of the positive electrode film, the mass content of the positive electrode active material is 96.5%-99.5%.

36. The lithium-ion secondary battery according to any one of claims 1-35, characterized in that, The positive electrode film layer also includes a binder, and the mass content of the binder is 0.5%-3% based on the total mass of the positive electrode film layer.

37. The lithium-ion secondary battery according to any one of claims 1-36, characterized in that, The single-sided density of the positive electrode film is 300 mg / 1540 mm². 2 -450mg / 1540mm 2 .

38. The lithium-ion secondary battery according to any one of claims 1-37, characterized in that, In the fully discharged state, the compaction density of the positive electrode film in the lithium-ion secondary battery is 2.51 g / cm³. 3 -2.73g / cm 3 .

39. The lithium-ion secondary battery according to any one of claims 1-38, characterized in that, In the fully discharged state, the positive electrode film of the lithium-ion secondary battery has a compaction density of 2.55 g / cm³. 3 -2.70g / cm 3 .

40. The lithium-ion secondary battery according to any one of claims 1-39, characterized in that, In the fully discharged state, the compaction density of the positive electrode film in the lithium-ion secondary battery is 2.51 g / cm³. 3 -2.73g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-22%.

41. The lithium-ion secondary battery according to any one of claims 1-39, characterized in that, In the fully discharged state, the positive electrode film of the lithium-ion secondary battery has a compaction density of 2.55 g / cm³. 3 -2.70g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-20%.

42. The lithium-ion secondary battery according to any one of claims 1-41, characterized in that, The positive electrode sheet includes a base coating layer disposed between the positive electrode film layer and the positive electrode 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.

43. The lithium-ion secondary battery according to any one of claims 1-42, characterized in that, The positive electrode sheet includes a base coating layer disposed between the positive electrode film layer and the positive electrode 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.

44. The lithium-ion secondary battery according to any one of claims 1-42, characterized in that, The positive electrode sheet includes a base coating layer disposed between the positive electrode film layer and the positive electrode 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.

45. A battery device, characterized in that, The battery device includes any one of claims 1 to 44, wherein the battery device comprises at least one of a battery cell, a battery module, a battery pack, and an energy storage battery.

46. ​​An electrical appliance, characterized in that, Includes the lithium-ion secondary battery according to any one of claims 1 to 44 or the battery device according to claim 45.

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