Lithium ion secondary battery, battery device, and power device

Abstract

The invention provides a lithium ion secondary battery, a battery device and an electric 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-coated materials, and in a particle sphericity-like area cumulative distribution curve with the particle size larger than or equal to 1 [mu] m obtained by the section of the positive electrode film layer in the thickness direction of the pole piece, the sphericity-like median LR1A50 is 0.71-0.85.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 202510515123.7, filed on April 23, 2025, entitled "Lithium-ion Secondary Battery, Battery Device and Power Consumption Device". Technical Field

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

[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 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 layer disposed on at least one side of the positive current collector. The positive electrode film layer includes a positive electrode active material, which comprises lithium transition metal phosphate particles with at least a portion of their surface coated with carbon material. In the cumulative distribution curve of the sphericity area of ​​particles with a particle size greater than or equal to 1 μm obtained from a cross-section of the positive electrode film layer along the thickness direction of the electrode, the median sphericity L... R1A50 It ranges from 0.71 to 0.85.

[0007] To achieve a higher compaction density, large particles are needed to serve as the framework for the positive electrode film stacking. The lithium-containing transition metal phosphate used in this application is a ceramic material. During high-temperature sintering, this material undergoes grain boundary melting and grain growth. The sintering of large particles often requires higher sintering temperatures or longer sintering times, and involves more grain boundary melting. Therefore, the sphericity of large particles in the prior art is relatively low. This application's research shows that large particles are the main load-bearing components during electrode compaction. During rolling, large particles with poor sphericity are prone to bridging, forming voids and stress concentrations at the bridging points. This not only affects further compaction of the electrode and hinders further improvement in electrode density, but also easily leads to localized fracture at stress concentration points, damaging the internal conductive network of the electrode and even creating isolated particles, resulting in increased internal resistance and deteriorated battery dynamic performance.

[0008] Studies have shown that the median L of sphericity for particles with a diameter greater than or equal to 1 μm is... R1A50 A value less than 0.71 indicates that the irregularity of the larger particle morphology remains high, making it difficult to avoid stress concentration through slippage during rolling. The median L of the sphericity for particles with a diameter greater than or equal to 1 μm... R1A50 If the particle size is greater than 0.85, the positive electrode active material needs to be formed by grinding the intermediate products into smaller particle sizes and then sintering them. The pursuit of improving the sphericity of large particles will reduce the proportion of large particles in the positive electrode film, which is not conducive to improving the compaction density of the positive electrode film and the energy density of the battery.

[0009] In this application, the median L of the sphericity of particles with a particle size greater than or equal to 1 μm is obtained through a process of multiple sintering and multiple grinding. R1A50 Adjusting the value to the range of 0.71-0.85 improves the sphericity difference of large particles, reduces stress concentration caused by particle bridging due to irregular shape of large particles, reduces porosity in the film layer, improves the compaction density of the electrode, effectively increases the energy density of the battery, reduces kinetic degradation, and improves the electron conduction between particles through closer packing, thereby improving kinetic performance.

[0010] In any embodiment, the particle size distribution index (PDI) of the particles in the cross-section along the thickness direction of the positive electrode film is 0.80-1.10. The PDI is the ratio of the sample standard deviation to the mean of the particle size in the cross-section along the thickness direction of the positive electrode film. A PDI within the above range in the cross-section along the thickness direction of the positive electrode film allows for particle size distribution, further improving the compaction density of the positive electrode film and the energy density of the battery cell.

[0011] In any embodiment, the particle size distribution index (PDI) of the particles in the cross-section along the thickness direction of the positive electrode film is 0.85-1.00. A PDI within this range is beneficial for further optimizing the gradation of large and small particles, improving the uniformity of the lithium-ion diffusion path, reducing the probability of polarization, and balancing the battery's energy density and kinetic performance.

[0012] In any embodiment, the average particle size in the cross-section of the positive electrode film layer along the electrode thickness direction is 150nm-400nm, optionally 150nm-300nm. In the cross-section of the positive electrode film layer along the electrode thickness direction, medium and small particles constitute the vast majority, therefore the average particle size... The above range represents the approximate particle size range of medium and small particles in the positive electrode film, which can fill the gaps between particles and increase the compaction density of the electrode sheet.

[0013] In any embodiment, the sample standard deviation of particle size in the cross-section of the positive electrode film layer along the electrode thickness direction is 150nm-350nm, optionally 150nm-300nm. In the cross-section of the positive electrode film layer along the electrode thickness direction, the deviation between the particle size of large particles and the average particle size is larger than that of medium and small particles, thus having a greater impact on the sample standard deviation S. The sample standard deviation S within the aforementioned range indicates a certain number of large particles. This is beneficial for building a stacking framework using large particles, which plays a role in force transmission and support, allowing the positive electrode film layer to withstand higher rolling pressure, thereby improving the mechanical stability and compaction density of the electrode.

[0014] In any embodiment, the median L of the sphericity in the cumulative distribution curve of the sphericity area of ​​particles with a particle size greater than or equal to 1 μm obtained from a cross-section of the positive electrode film along the electrode thickness direction is... R1A50 The median L of the sphericity for particles with a diameter greater than or equal to 1 μm is 0.72–0.80. R1A50 Within the aforementioned range, particles with a diameter greater than or equal to 1 μm are closer to spherical shapes. The possibility of irregular particle overlap during the rolling process leading to voids and stress concentration is further reduced. At the same time, maintaining a certain mechanical interlocking force between large particles is beneficial for the construction of force chains, enabling them to withstand higher rolling pressure and internal and external stresses, and further improving the compaction density and stability of the electrode sheet.

[0015] In any embodiment, the median L of the spheroidal density 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... A50The range is 0.70-0.85, with an optional range of 0.71-0.8. In the cumulative distribution curve of the spheroidal area of ​​particles obtained from a cross-section along the thickness direction of the positive electrode film, a median spheroidal value within the above range indicates that the particles occupying most of the area and space within the positive electrode film are relatively rounded, resulting in a more uniform material morphology. Under external force, this allows for effective slippage, participating in the connection and reconstruction of the force chain, thereby contributing to increased compaction density and volumetric energy density, while simultaneously reducing particle breakage and improving kinetic performance.

[0016] In any embodiment, in the cross-section of the positive electrode film layer along the electrode thickness direction, L R1A50 >L A50 The value can be selected as 0.01≤L R1A50 -L A50 ≤0.1, and can be further selected as 0.01≤L R1A50 -L A50 ≤0.05. In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, increasing the sphericity of the particles can improve the particle packing, but even if all particles are perfectly spherical, gaps will still exist. L R1A50 >L A50 This process makes particles larger than 1μm more spherical, allowing for a more uniform distribution and contributing to a more regular skeletal structure. Smaller particles have lower sphericity, and due to their diverse shapes (such as elliptical and wedge-shaped), they can form a tighter arrangement between larger particles, filling the gaps that are difficult to fill between them. By combining different particle sizes with high and low sphericity, the space utilization of the packing is significantly improved, thereby increasing the compaction density of the electrode and the energy density of the battery.

[0017] In any embodiment, 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 0.98-1.20, and can be selected as 1.02-1.1; 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 the D peak at the point. 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. The particles are more likely to slip during the rolling process by means of the highly graphitized carbon structure in the coating material, which can further reduce the possibility of stress concentration and increase the compaction density. In addition, the good electronic conductivity of the high graphitization coating material improves the dynamic performance of the battery.

[0018] In any embodiment, the median roughness R in the cumulative roughness area distribution curve of the particles obtained from a cross-section of the positive electrode film along the electrode thickness direction is... A50 The median roughness R is 0.92-0.96. 50 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, reducing the occurrence of mutual meshing due to surface roughness during rolling. With a reasonable particle size distribution index and particles with high sphericity of ≥1μm, a dense packing can be formed under lower rolling pressure, further improving the compaction density of the electrode and increasing the energy density of the battery.

[0019] In any embodiment, the positive electrode active material includes iron, and the iron dissolution rate of the positive electrode film is 400ppm-1800ppm, optionally 500ppm-1500ppm.

[0020] 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. An iron dissolution rate within the aforementioned range indicates a relatively complete and dense carbon coating on the surface of the positive electrode active material. This improves the electrical contact between positive electrode active materials, enhances their conductivity, and reduces polarization, thus optimizing the kinetic performance of lithium-ion secondary batteries. Simultaneously, the highly intact carbon coating structure facilitates stress slippage of particles during rolling, thereby simultaneously increasing the compaction density of the electrode and the energy density of the battery.

[0021] 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%-1.5%. Compared with existing lithium transition metal phosphate positive electrode active materials, this positive electrode active material has a relatively low carbon coating content, which can further increase the loading of lithium transition metal phosphate in the positive electrode sheet and improve the energy density of lithium-ion secondary batteries.

[0022] In any embodiment, the lithium iron antisite defect concentration of the positive electrode active material is 0.001%-1.5%, optionally 0.01%-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 the lithium-ion secondary battery.

[0023] 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.

[0024] 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.

[0025] In any embodiment, the positive electrode active material includes one or more of lithium iron phosphate and doped / modified materials and coated / modified materials.

[0026] In any embodiment, the positive electrode active material includes titanium, and the mass content of titanium is 4000ppm-8000ppm based on the total mass of the positive electrode active material. The high addition of titanium did not form a harmful impurity phase that negatively impacts 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.

[0027] In any embodiment, the tap density of the positive electrode active material is 1.0 g / cm³. 3 -1.70g / cm 3 1.20g / cm³ is an optional value. 3 -1.50g / cm 3 The positive electrode active material particles form an effective gradation and have a high tap density. Furthermore, the high sphericity of the particles larger than 1 μm in the positive electrode film allows them to spontaneously roll and fill the pores, further improving the tap density.

[0028] In any embodiment, the compacted density of the positive electrode active material under 3T pressure is 2.55 g / cm³. 3 -2.70g / cm 3 The option is 2.58g / cm³. 3 -2.68 g / cm3 The positive electrode active material particles form an effective gradation, and the particles larger than 1μm in the positive electrode film also have high sphericity, which enables the positive electrode active material to achieve a higher compaction density under external force, providing a material basis for improving the compaction density of the electrode sheet and preparing high-energy-density lithium-ion secondary batteries.

[0029] In any embodiment, the powder resistivity of the positive electrode active material at 8 MPa pressure is 0.5 Ω·cm-60 Ω·cm, optionally 2.0 Ω·cm-40.0 Ω·cm. This positive electrode active material is produced by coating a carbon material on its surface, utilizing the sp... 2 The structure facilitates rapid electron conduction between particles, resulting in low powder resistivity in the positive electrode active material. This is beneficial for improving the solid-phase electron transport rate and enhancing the battery's kinetic performance.

[0030] In any embodiment, the specific discharge capacity of the positive electrode active material at a 1C discharge rate at room temperature 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.

[0031] 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.

[0032] 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.

[0033] In any embodiment, the positive electrode film layer further includes a conductive agent, and the mass content of the conductive agent is 0.01%-1.5% based on the total mass of the positive electrode film layer. The particles in this positive electrode film layer have good gradation, and the large particles have high sphericity, thus they can form a close packing, allowing the particles to contact each other and have good electronic conductivity, forming an electron transport network. This can reduce the amount of conductive agent used in the positive electrode film layer, which is beneficial to further increase the loading of the positive electrode active material and improve the energy density of the lithium-ion secondary battery.

[0034] 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 92%-99.5%, optionally 96.5%-99.5%; the mass content of the binder is 0.5%-3%.

[0035] 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.

[0036] 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.

[0037] In any embodiment, the positive electrode film layer of the lithium-ion secondary battery in its fully discharged state has a compaction density of 2.52 g / cm³. 3 -2.78g / cm 3 .

[0038] 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 -2.75g / cm 3 .

[0039] 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.

[0040] 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 2.52 g / cm³ when the lithium-ion secondary battery is in a fully discharged state. 3 -2.78g / 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 -2.75g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-20%.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] A second aspect of this application provides a battery device including the lithium-ion secondary battery of the first aspect of this application.

[0046] 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.

[0047] 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; adding a solvent and grinding to obtain a mixed slurry; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder to obtain a positive electrode active material; the sintering of the precursor powder to obtain the positive electrode active material includes at least two sintering processes; obtaining a first sintered product after the first sintering; mixing the first sintered product and a carbon source to obtain an intermediate raw material; dividing the intermediate raw material into two groups and grinding them separately to obtain a first group of ground products and a second group of ground products; mixing the first group of ground products and the second group of ground products to obtain a mixed intermediate product; grinding the mixed intermediate product to obtain a target mixture; and performing a second sintering on the target mixture to obtain the positive electrode active material; wherein, the D of the first group of ground products is... V50 The particle size is 0.8μm-1.2μm; the D of the second group of milled products V50 The thickness is 0.30μm-0.50μm; the holding time for the second sintering is 5h-12h; the grinding time for grinding the mixed intermediate product is 0.5h-1.5h.

[0048] Grinding the intermediate mixture again helps to improve the uniformity of particle size in the target mixture, reduce the presence of abnormally large particles, make sintering uniform, and improve the sphericity of large particles.

[0049] The two-stage sintering process can effectively shorten the sintering time in the high-temperature range, thereby reducing the degree of grain boundary melting of large particles during high-temperature sintering. This improves the sphericity of large particles in the positive electrode active material, thus reducing stress concentration caused by particle bridging due to irregular shape of large particles, reducing porosity in the film layer, improving the compaction density of the electrode, effectively increasing the energy density of the battery, reducing kinetic degradation, and improving the electron conduction capacity between particles through closer packing, thereby achieving improved kinetic performance.

[0050] 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.

[0051] In any embodiment, the dry mix has a revolution speed of 20 rpm-30 rpm and a rotation speed of 750 rpm-850 rpm.

[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] 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.

[0054] 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

[0055] Figure 1 This is a scanning electron microscope image of a cross-section of the positive electrode film layer along the electrode thickness direction according to an embodiment of this application; Figure 2 This is a schematic diagram of a lithium-ion secondary battery according to one embodiment of this application; Figure 3 This is an exploded view of a lithium-ion secondary battery according to one embodiment of this application; Figure 4 This is a schematic diagram of a battery module according to one embodiment of this application; Figure 5 This is a schematic diagram of a battery pack according to one embodiment of this application; Figure 6 yes Figure 5 An exploded view of the battery pack shown; Figure 7 This is a schematic diagram of a power supply device for a lithium-ion secondary battery according to an embodiment of this application. Figure 8 This is a porosity test diagram of a cross-section along the thickness direction of the positive electrode film layer according to an embodiment of this application.

[0056] Explanation of reference numerals in the attached figures: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Housing; 52 Electrode assembly; 53 Cover plate. Detailed Implementation

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] In this application, the terms "multiple" or "various" refer to two or more kinds.

[0063] Unless otherwise stated, the terms used in this application have the common meanings as commonly understood by those skilled in the art.

[0064] 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.

[0065] The battery mentioned in the embodiments of this application can be a single physical module comprising one or more lithium-ion secondary batteries to provide higher voltage and capacity. For example, the battery mentioned in this application may include lithium-ion secondary batteries, battery cells, battery modules, or battery packs.

[0066] 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 in this respect. Figure 2 This is an example of a cuboid-structured lithium-ion secondary battery.

[0067] Lithium-ion secondary batteries consist of electrode components and electrolyte.

[0068] 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).

[0069] In some implementations, such as Figure 3As shown, 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 according to requirements.

[0070] 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.

[0071] 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.

[0072] In some implementations, the battery can be an energy storage device. Energy storage devices include energy storage containers, energy storage cabinets, etc.

[0073] In some implementations, lithium-ion secondary batteries can be assembled into battery modules, and the number of lithium-ion secondary batteries contained in a battery module can be multiple, the specific number of which can be adjusted according to the application and capacity of the battery module. Figure 4 This is a schematic diagram of battery module 4 as an example. Figure 4 As shown, in battery module 4, multiple lithium-ion secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple lithium-ion secondary batteries 5 can be fixed in place using fasteners.

[0074] 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.

[0075] 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.

[0076] Figure 5 and Figure 6This is a schematic diagram of battery pack 1 as an example. Figure 5 and Figure 6 As shown, 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.

[0077] Lithium-containing transition metal phosphate materials have been widely used in lithium-ion batteries due to their stable structure, good safety and long cycle life; however, they have problems such as low electronic conductivity and low stacking efficiency, which makes it difficult to further increase the loading of lithium-containing transition metal phosphates per unit volume of battery, thus failing to meet the needs of high energy density batteries.

[0078] To further improve battery energy density and increase electrode compaction density, the industry commonly uses particle size distribution improvement as a method. Existing technologies often employ methods to improve particle size distribution by adjusting the particle size or proportion of large and small particles to achieve close packing. However, since electrode particles are generally sintered at high temperatures, large particles are not easily formed into uniform spherical shapes. Irregularly shaped large particles still have many gaps between them during the packing process, making it difficult to further increase compaction density. Furthermore, increasing the rolling pressure can cause irregular particles to break, damaging the conductive network inside the electrode and increasing the battery's internal resistance. How to fabricate high-energy-density batteries while maintaining good kinetic performance is a pressing technical problem that needs to be solved in this field.

[0079] The first aspect of this application provides a lithium-ion secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive current collector and a positive electrode film disposed on at least one side of the current collector. The positive electrode film includes a positive active material, comprising lithium transition metal phosphate particles with at least a portion of their surface coated with carbon material. In the cumulative distribution curve of the sphericity area of ​​particles with a particle size greater than or equal to 1 μm obtained from a cross-section of the positive electrode film along the electrode thickness direction, the median sphericity L... R1A50 It ranges from 0.71 to 0.85.

[0080] To achieve a higher compaction density, large particles are needed to serve as the framework for the positive electrode film stacking. The lithium-containing transition metal phosphate used in this application is a ceramic material. During high-temperature sintering, this material undergoes grain boundary melting and grain growth. The sintering of large particles often requires higher sintering temperatures or longer sintering times, and involves more grain boundary melting. Therefore, the sphericity of large particles in the prior art is relatively low. This application's research shows that large particles are the main load-bearing components during electrode compaction. During rolling, large particles with poor sphericity are prone to bridging, forming voids and stress concentrations at the bridging points. This not only affects further compaction of the electrode and hinders further improvement in electrode density, but also easily leads to localized fracture at stress concentration points, damaging the internal conductive network of the electrode and even creating isolated particles, resulting in increased internal resistance and deteriorated battery dynamic performance.

[0081] Studies have shown that the median L of sphericity for particles with a diameter greater than or equal to 1 μm is... R1A50 A value less than 0.71 indicates that the irregularity of the larger particle morphology remains high, making it difficult to avoid stress concentration through slippage during rolling. The median L of the sphericity for particles with a diameter greater than or equal to 1 μm... R1A50 If the particle size is greater than 0.85, the positive electrode active material needs to be formed by grinding the intermediate products into smaller particle sizes and then sintering them. The pursuit of improving the sphericity of large particles will reduce the proportion of large particles in the positive electrode film, which is not conducive to improving the compaction density of the positive electrode film and the energy density of the battery.

[0082] In this embodiment, the median L of the sphericity of particles with a particle size greater than or equal to 1 μm is obtained through a process of multiple sintering and multiple grinding. R1A50 Adjusting the value to the range of 0.71-0.85 improves the sphericity difference of large particles, reduces stress concentration caused by particle bridging due to irregular shape of large particles, reduces porosity in the film layer, improves the compaction density of the electrode, effectively increases the energy density of the battery, reduces kinetic degradation, and improves the electron conduction between particles through closer packing, thereby improving kinetic performance.

[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) or inductively coupled plasma mass spectrometry (ICP-MS). As examples, lithium-containing transition metal phosphates include, but are not limited to, lithium iron phosphate, lithium manganese iron phosphate, and their doped materials.

[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] The particle identification method is as follows: The positive electrode film layer is cut along the thickness direction of the electrode sheet using an argon ion beam (for example, a Leica EM TIC 3X CP device can be used, operating voltage: 6kV, operating time: 6h). After exposing the cut surface, a scanning electron microscope (SEM) is used (for example, a Hitachi SU8230 device can be used, operating voltage: 3kV, beam current: high, probe model: U (LA100), working distance <5mm) to observe the cut surface along the thickness direction of the positive electrode film layer. Images are acquired in secondary electron mode at a non-edge position in the cut surface of the positive electrode film layer using a field emission scanning electron microscope (after observing the electrode edge under the SEM, the field of view is adjusted to the center of the sample). 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 usage of ImageJ software is as follows: Load the SEM image to be analyzed, such as... Figure 1As shown, the Cellpose plugin software was used to identify particles, and manual corrections were performed based on this. ImageJ was used to read and analyze the 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, and then manually mark the particles in the image that were not identified by the software, were not fully identified by the software, or had identification errors. The particles that were not identified by the software, were not fully identified by the software, or had identification errors mainly include the following types: 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 the scratches as particle boundaries during the identification process, resulting in 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 interior of the particle penetrated by the edge, and the morphology not fully displayed, leading to 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.

[0087] The specific method for statistical analysis of particles in the positive electrode film is as follows: Images after particle identification and labeling are imported into ImageJ software for analysis. Scale settings are completed based on the scanning electron microscope (SEM) images. The particle size and area in the cross-section along the electrode thickness direction of the positive electrode film are 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 obtained from the analysis represents the maximum spacing between all parallel lines in the two-dimensional projection of the particle, thus characterizing the particle size. Since particles with a diameter less than 50 nm present significant errors during the statistical process and are difficult to accurately identify, and since the particle size of conductive agents is generally less than 50 nm, it will introduce significant errors into the statistical results. Therefore, in this application, particles with a diameter less than 50 nm are not counted in the particle size statistics process, and the statistical data for particles whose AR, Round, or Solidity values ​​are displayed as "NaN" are deleted. Following the above method, to ensure a statistically significant sample size, at least 10 non-overlapping scanning electron microscope images were acquired for each electrode, and the particle size of at least 5000 particles was statistically analyzed. The cross-sectional morphology of the positive electrode film along the thickness direction of the electrode sheet is shown in the figure. Figure 1 As shown, the state of the positive electrode active material differs from that observed by Malvern laser scattering and also from that observed directly by scanning electron microscopy. Under roller pressure, the particles in the positive electrode film exhibit good dispersion, making observation of the positive electrode film beneficial for effectively characterizing the particle size and distribution within it.

[0088] During the compaction process, the positive electrode film is compacted in the thickness direction. The cross-section of the positive electrode film along the thickness direction is more reflective of the actual compaction of the particles inside the film in a spatial scale than the surface of the positive electrode film.

[0089] 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 size in the cross-section of the positive electrode film.

[0090] In existing technologies, laser particle size analyzers are typically used to statistically analyze the particle size of positive electrode active materials using Malvern laser diffraction. However, the applicant's research indicates that because lithium-containing transition metal phosphate particles are prone to agglomeration, the test results obtained 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 positive electrode active material, nor does it reflect the dispersion state of the positive electrode active material in the film layer, as the dispersion of the positive electrode active material in the film layer increases during slurry preparation and film rolling. The test results obtained by Malvern laser diffraction are affected by the particle size, specific surface area, and degree of agglomeration of the positive electrode active material. Compared to the actual dispersion in the electrode sheet, the number of large particles obtained by this test is lower than the actual value, while the number of small particles is higher. 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.

[0091] The cumulative distribution curve of sphericity area of ​​particles with a diameter greater than or equal to 1 μm obtained from the cross-section of the positive electrode film along the electrode thickness direction is specifically tested using the following method: Particles in the cross-section of the positive electrode film are identified using the method described above in this application. While using the "Feret" function in ImageJ to identify the particle size in the positive electrode film, the "Shape Description" and "Area" analysis functions are used to analyze the morphology and area of ​​the particles in the cross-section along the electrode thickness direction. 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.

[0092] The sphericity of at least 5000 particles with a diameter greater than or equal to 1 μm was arranged in ascending order. A cumulative area distribution curve of sphericity in the positive electrode film was obtained, with sphericity as the horizontal axis and the cumulative area percentage calculated from the particle's "Area" as the vertical axis. The median sphericity refers to the sphericity corresponding to a cumulative area percentage of 50% on the vertical axis of the cumulative area distribution curve.

[0093] In some embodiments, the median L of the sphericity is obtained from the cumulative distribution curve of the sphericity area of ​​particles with a particle size greater than or equal to 1 μm obtained from a cross-section of the positive electrode film along the electrode thickness direction. R1A50The value can be selected from 0.71, 0.716, 0.72, 0.73, 0.738, 0.74, 0.745, 0.75, 0.751, 0.755, 0.76, 0.764, 0.77, 0.78, 0.79, 0.793, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, or any value range between the two.

[0094] 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.

[0095] In some embodiments, the particle size distribution index (PDI) of the particles in the cross-section of the positive electrode film along the thickness direction of the electrode sheet is 0.80-1.10. The PDI is the ratio of the sample standard deviation to the mean of the particle size in the cross-section of the positive electrode film along the thickness direction.

[0096] The particle size distribution index in a cross-section of the positive electrode film along the electrode thickness direction is calculated as follows: The average particle size is calculated using the following formula. The particle size distribution index (PDI) is calculated by dividing the sample standard deviation (S) by the mean. Generally, a larger PDI indicates a wider particle size distribution, which is beneficial for improving particle gradation.

[0097]

[0098] in, The particle size is [value missing]. For the number of samples, Let S be the average particle size, and S be the sample standard deviation of particle size.

[0099] In some embodiments, the particle size distribution index (PDI) of the particles in the cross-section along the thickness direction of the positive electrode film layer can be selected as 0.8, 0.81, 0.82, 0.83, 0.84, 0.845, 0.85, 0.855, 0.86, 0.87, 0.874, 0.88, 0.89, 0.894, 0.897, 0.899, 0.9, 0.91, 0.919, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.973, 0.98, 0.99, 0.996, 1.00, 1.01, 1.02, 1.03, 1.04, 0.05, 1.06, 1.07, 1.08, 1.09, 1.10, or any value range between the two.

[0100] In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the particle size distribution index (PDI) can intuitively reflect the overall particle size distribution.

[0101] Those skilled in the art can control the particle size distribution index, average particle size, and sample standard deviation of particle size using any known process. For example, the particle size distribution index can be adjusted through the scientific gradation of particles of different sizes; the average particle size and sample standard deviation can be adjusted by using the mechanical force of crushing and grinding processes to process raw materials to the target particle size distribution range; particle size separation can be achieved by using screening and grading equipment to obtain a particle size distribution that meets the requirements; and precise control of the feed rate to adjust the residence time and stress state of particles within the equipment also helps to control the particle size distribution index.

[0102] In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the particle size distribution index (PDI) of the particles within the above range can achieve particle size gradation, further improving the compaction density of the positive electrode film and the energy density of the battery cell.

[0103] In some embodiments, the particle size distribution index (PDI) of the particles in the cross-section of the positive electrode film along the thickness direction of the electrode sheet is 0.85-1.00.

[0104] A particle size distribution index within the above range is beneficial for further optimizing the gradation of large and small particles, improving the uniformity of lithium-ion diffusion paths, reducing the probability of polarization, and balancing the energy density and kinetic performance of the battery.

[0105] In some embodiments, the average particle size in the cross-section of the positive electrode film along the thickness direction of the electrode sheet is 150nm-400nm, and can be selected as 150nm-300nm.

[0106] In some embodiments, the average particle size in the cross-section of the positive electrode film along the thickness direction of the electrode sheet can be selected as 150nm, 160nm, 170nm, 180nm, 189nm, 190nm, 191nm, 200nm, 210nm, 220nm, 230nm, 236nm, 240nm, 250nm, 257nm, 258nm, 260nm, 262nm, 270nm, 280nm, 281nm, 284nm, 290nm, 300nm, 310nm, 320nm, 330nm, 340nm, 350nm, 360nm, 370nm, 380nm, 390nm, 400nm, or any value range between the two.

[0107] In this application, the average particle size in the cross-section of the positive electrode film layer along the electrode thickness direction is... This can be obtained by referring to the method described above.

[0108] In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, medium and small particles account for the vast majority, therefore the average particle size is... The above range represents the approximate particle size range of medium and small particles in the positive electrode film, which can fill the gaps between particles and increase the compaction density of the electrode sheet.

[0109] In some embodiments, the sample standard deviation of the particle size in the cross-section of the positive electrode film along the thickness direction of the electrode sheet is 150nm-350nm, and can be selected as 150nm-300nm.

[0110] In some embodiments, the sample standard deviation of the particle size in the cross-section of the positive electrode film along the thickness direction of the electrode sheet can be selected as 150nm, 160nm, 161nm, 170nm, 180nm, 184nm, 190nm, 200nm, 202nm, 210nm, 220nm, 230nm, 232nm, 234nm, 240nm, 250nm, 253nm, 258nm, 260nm, 270nm, 280nm, 283nm, 290nm, 300nm, 310nm, 320nm, 330nm, 340nm, 350nm, or any range between two of these values.

[0111] In this application, the sample standard deviation S of the particle size in the cross-section of the positive electrode film along the thickness direction of the electrode sheet can be obtained by referring to the method described above.

[0112] In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the deviation between the particle size of large particles and the average particle size is larger than that of medium and small particles, thus having a greater impact on the sample standard deviation S. The sample standard deviation S within the aforementioned range reflects a certain number of large particles. This is beneficial for building a packing framework with large particles, which plays a role in force transmission and support, allowing the positive electrode film layer to withstand higher rolling pressure, thereby improving the mechanical stability and compaction density of the electrode sheet.

[0113] In some embodiments, the median L of the sphericity is obtained from the cumulative distribution curve of the sphericity area of ​​particles with a particle size greater than or equal to 1 μm obtained from a cross-section of the positive electrode film along the electrode thickness direction. R1A50 It is 0.72-0.80.

[0114] The median L of sphericity for particles with a diameter greater than or equal to 1 μm R1A50Within the aforementioned range, particles with a diameter greater than or equal to 1 μm are closer to spherical shapes. The possibility of irregular particle overlap during the rolling process leading to voids and stress concentration is further reduced. At the same time, maintaining a certain mechanical interlocking force between large particles is beneficial for the construction of force chains, enabling them to withstand higher rolling pressure and internal and external stresses, and further improving the compaction density and stability of the electrode sheet.

[0115] 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 is 0.70-0.85, and can be selected as 0.71-0.8.

[0116] The method for testing the sphericity of particles in the cross-section of the positive electrode film along the electrode thickness direction can be found above. The sphericity of the particles in the obtained cross-section along the electrode thickness direction is arranged in ascending order. A 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 percentage as the vertical axis. The median sphericity refers to the sphericity corresponding to a cumulative area percentage of 50% on the vertical axis of the cumulative area distribution curve.

[0117] 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 0.70, 0.71, 0.715, 0.72, 0.724, 0.725, 0.729, 0.73, 0.731, 0.733, 0.74, 0.749, 0.75, 0.76, 0.77, 0.78, 0.79, 0.80, 0.807, 0.81, 0.82, 0.83, 0.84, 0.85, or any value range between two of these.

[0118] In the cumulative distribution curve of the spheroidal area of ​​particles obtained by slicing the positive electrode film along the thickness direction of the electrode sheet, if the median of spheroidal area is within the above range, it means that the particles occupying most of the area and space in the positive electrode film are also relatively rounded, the material morphology is relatively uniform, and they can effectively slip under the action of external force, participate in the connection and reconstruction of the force chain, thereby helping to improve the compaction density and volumetric energy density, while reducing the occurrence of particle breakage and improving the dynamic performance.

[0119] In some embodiments, in the cross-section of the positive electrode film layer along the electrode thickness direction, L R1A50 >L A50 The value can be selected as 0.01≤L R1A50 -L A50 ≤0.1, and can be further selected as 0.01≤L R1A50 -L A50 ≤0.05.

[0120] In some embodiments, in the cross-section of the positive electrode film layer along the electrode thickness direction, L R1A50 -L A50 The value can be selected as 0.005, 0.01, 0.014, 0.02, 0.022, 0.024, 0.03, 0.031, 0.04, 0.043, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, or any value range between two of these.

[0121] In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, increasing the sphericity of the particles can improve the particle packing, but even if all particles are perfectly spherical, gaps will still exist. R1A50 >L A50 This process makes particles larger than 1μm more spherical, allowing for a more uniform distribution and contributing to a more regular skeletal structure. Smaller particles have lower sphericity, and due to their diverse shapes (such as elliptical and wedge-shaped), they can form a tighter arrangement between larger particles, filling the gaps that are difficult to fill between them. By combining different particle sizes with high and low sphericity, the space utilization of the packing is significantly improved, thereby increasing the compaction density of the electrode and the energy density of the battery.

[0122] 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 0.98-1.20, and can be selected as 1.02-1.1; 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.

[0123] 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.

[0124] 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 slippage. Therefore, the C value can characterize the degree of graphitization of the cathode film. A higher C value indicates a higher degree of graphitization of the carbon material. 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 I0 G / I D However, due to its low content and small tube diameter, its addition to the positive electrode film results in an extreme value in the Raman surface scan test of the positive electrode film, and does not affect the graphitization degree C in the positive electrode film. 50 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.

[0125] 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. The particles are more likely to slip during the rolling process by means of the highly graphitized carbon structure in the coating material, which can further reduce the possibility of stress concentration and increase the compaction density. Furthermore, the good electronic conductivity of the high-graphitized coating material improves the dynamic performance of the battery.

[0126] 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.

[0127] Those skilled in the art can control the degree of graphitization of active material particles using any known process. As an example, adjusting the carbon source (which can be cross-linked PEG), sintering temperature, sintering time, sintering pressure, sintering atmosphere, and nucleation process can all achieve the adjustment of the degree of graphitization of active material particles.

[0128] 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 0.98, 0.99, 1.00, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.20, or any value range between the two.

[0129] In some embodiments, the median roughness R in the cumulative roughness area distribution curve of the particles obtained from a cross-section of the positive electrode film along the electrode thickness direction is... A50 It is 0.92-0.96.

[0130] The specific method for testing the roughness 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 of the particles in the cross-section along the electrode thickness direction of the positive electrode film is analyzed using the "Shape Description" and "Area" analysis functions 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 is arranged in ascending order, and the cumulative area distribution curve of the particles in the positive electrode film is 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 area distribution curve of the roughness R value is 50%.

[0131] In some embodiments, the median roughness R in the cumulative roughness area distribution curve 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 as 0.92, 0.93, 0.94, 0.95, 0.96 or any range between two of them.

[0132] 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.

[0133] Median roughness R 50Within 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, reducing the occurrence of mutual meshing due to surface roughness during rolling. With a reasonable particle size distribution index and particles with high sphericity of ≥1μm, a dense packing can be formed under lower rolling pressure, further improving the compaction density of the electrode and increasing the energy density of the battery.

[0134] In some embodiments, the positive electrode active material includes iron, and the iron dissolution rate of the positive electrode film is 400ppm-1800ppm, optionally 500ppm-1500ppm.

[0135] The iron dissolution rate of the positive electrode film can be tested in the following ways. Specifically, after disassembling and cleaning the electrode from the battery, it is formed into small discs with a diameter of 14 mm. Multiple small disc samples are taken to make the total sample mass about 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 5 minutes, and then the solution is quickly aspirated using a 5 mL syringe. The solution is filtered into a test tube using a 0.45 μm pore size filter. 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 an inductively coupled plasma mass spectrometer (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 small disc electrode - mass of small disc current collector)]. 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 disc's thickness by its area and density. The disc's thickness can be equivalently measured by using a thickness gauge to measure the current collector thickness in the uncoated area. It is understood that although the current collector in the coated area will expand during compaction, resulting in a slight decrease in thickness compared to the uncoated area, this decrease 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 .

[0136] In some embodiments, the iron dissolution rate of the positive electrode film can be selected as 400ppm, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 900.2ppm, 1000ppm, 1100ppm, 1200ppm, 1300ppm, 1400ppm, 1500ppm, 1600ppm, 1700ppm, 1800ppm or any value range between the two.

[0137] Those skilled in the art can control the iron dissolution rate of the positive electrode film using any known process. For example, the iron dissolution rate of the positive electrode material can be controlled by adjusting the surface coating quality of the positive electrode material, the temperature, time, and pressure during the preparation process. Furthermore, during battery use, battery design, the oxidant content in the electrolyte, the battery operating temperature, and the 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 in 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, i.e., the carbon coating on the surface of the positive electrode active material is more complete and dense. An iron dissolution rate within the above range indicates that the surface of 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 thus 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.

[0138] In some embodiments, the carbon content is 0.8%-1.8% by mass, and optionally 0.90%-1.5%, based on the total mass of the positive electrode active material.

[0139] Based on the total mass of the positive electrode active material, the mass content 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 and 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".

[0140] 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.

[0141] Compared with existing lithium transition metal phosphate cathode active materials, this cathode active material has a relatively low carbon coating content, which can further increase the loading of lithium transition metal phosphate in the cathode sheet and improve the energy density of lithium-ion secondary batteries.

[0142] In some embodiments, the lithium iron ion reverse defect concentration of the positive electrode active material is 0.001%-1.5%, and may be 0.01%-1.0%.

[0143] 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.

[0144] In some embodiments, the lithium iron ion site defect concentration of the positive electrode active material can be selected as 0.001%, 0.01%, 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.

[0145] During preparation and cycling, lithium vacancies inevitably exist within the crystal structure of the positive electrode active material. These vacancies not only lead to the oxidation of ferrous ions 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 application 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 the lithium-ion secondary battery.

[0146] In some embodiments, the lithium-containing transition metal phosphate particles comprise components 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.

[0147] In some implementations, m can be selected as 0.8, 0.85, 0.9, 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, or any value range between two of these; x can be selected as 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, or any value range between two of these. y can be 0.95, 0.96, 0.97, 0.98, 0.99, 1.00 or any value between two of these; j can be 3.5, 3.6, 3.7, 3.8, 3.9, 4 or any value between two of these; q can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1 or any value between two of these.

[0148] 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.

[0149] 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.

[0150] In some embodiments, the positive electrode active material includes titanium, and the mass content of titanium is 4000ppm-8000ppm based on the total mass of the positive electrode active material.

[0151] 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.

[0152] 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 doping content of titanium in lithium-containing transition metal phosphates often cannot exceed 3000 ppm, because excessive titanium is difficult to completely enter the bulk phase of lithium-containing transition metal phosphates and easily becomes a harmful impurity phase remaining on the surface, negatively impacting battery performance.

[0153] 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.

[0154] In some embodiments, the tap density of the positive electrode active material is 1.00 g / cm³. 3 -1.70g / cm 3 1.20g / cm³ is an optional value. 3 -1.50g / cm 3 .

[0155] The tap density of powder can be obtained by any method known in the art.

[0156] The tap density of powder can be obtained by any method 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.

[0157] In some embodiments, the tap density of the positive electrode active material powder can be selected as 1.00 g / cm³. 3 1.05g / cm 3 1.10 g / cm 3 1.15g / cm 3 1.20g / cm 3 1.25g / cm 3 1.30g / cm 3 1.35g / cm 3 1.40g / cm 3 1.45g / cm 3 1.50g / cm 31.55g / cm 3 1.60g / cm 3 1.65g / cm 3 1.70g / cm 3 Or the range of values ​​between any two.

[0158] In the embodiments of this application, the positive electrode active material particles form an effective gradation and have a high tap density. Furthermore, the high sphericity of the particles larger than 1 μm in the positive electrode film layer allows them to spontaneously roll and fill the pores, further improving the tap density.

[0159] In some embodiments, the compacted density of the positive electrode active material at a pressure of 3T is 2.55 g / cm³. 3 -2.70g / cm 3 The option is 2.58g / cm³. 3 -2.68g / cm 3 .

[0160] 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 .

[0161] The compacted density of the positive electrode active material powder can be measured using methods and equipment known in the art. For example, it can be measured using a compaction density instrument, referring to GB / T 24533-2009. Specifically, a certain amount of positive electrode active material is placed on a compaction mold (the mold diameter is known). The mold is hollow in the middle and has a metal disc at the top and bottom. The positive electrode active material is placed between the metal discs, and a metal cylinder is placed on top. The mold is placed on a compaction density instrument, and the pressure is set to 3T. The thickness of the positive electrode active material under 3T pressure can be read on the instrument. The compacted density of the positive electrode active material powder is ρ=m / v, where v=(S×H), m is the mass of the positive electrode active material, and S is the bottom area of ​​the mold (1.327 cm²). 2 H represents the thickness of the positive electrode active material after compaction.

[0162] In some embodiments, the compaction density of the positive electrode active material under 3T pressure can be selected as 2.55 g / cm³. 3 2.56 g / cm 3 2.57g / cm 3 2.58g / cm 3 2.59 g / cm 3 2.60 g / cm 32.61 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.

[0163] The positive electrode active material particles form an effective gradation, and the particles larger than 1μm in the positive electrode film also have high sphericity, which enables the positive electrode active material to achieve higher compaction density under external force, providing a material basis for improving the compaction density of the electrode sheet and preparing high-energy-density lithium-ion secondary batteries.

[0164] In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 8 MPa is 0.5 Ω·cm-60.0 Ω·cm, and can be selected as 2.0 Ω·cm-40.0 Ω·cm.

[0165] 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.

[0166] 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, 11.96 Ω·cm, 15 Ω·cm, 20 Ω·cm, 25 Ω·cm, 30 Ω·cm, 35 Ω·cm, 40 Ω·cm, 45 Ω·cm, 50 Ω·cm, 55 Ω·cm, 60 Ω·cm, or any value range between the two.

[0167] This positive electrode active material is produced by coating a carbon material on its surface, utilizing the spt properties of the surface carbon. 2The structure facilitates rapid electron conduction between particles, resulting in low powder resistivity in the positive electrode active material. This is beneficial for improving the solid-phase electron transport rate and enhancing the battery's kinetic performance.

[0168] In some embodiments, the positive electrode active material has a discharge capacity of 135 mAh / g to 150 mAh / g at a 1C discharge rate at room temperature.

[0169] 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°C 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.

[0170] 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°C 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 hole puncher, weigh it and record the weight. Place the weighed positive electrode sheet in a vacuum drying oven (105°C, 1-12hrs, -90kPa). After drying, place the positive electrode sheet in a glove box. 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.

[0171] 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.

[0172] In some embodiments, the discharge specific capacity of the positive electrode active material at a 1C discharge rate at room temperature can be selected as 135mAh / g, 140mAh / g, 141.1mAh / g, 145mAh / g, 150mAh / g, or any value range between the two.

[0173] 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.

[0174] 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.

[0175] In some embodiments, the discharge capacity η of the positive electrode active material when discharged to 3.2V accounts for ≥88%.

[0176] 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.

[0177] In some implementations, η can be selected as 85%, 86%, 87%, 88%, 88.1%, 89%, 90%, 90.1%, 91%, 92%, 92.2%, 93%, 94%, 94.1%, 94.5%, 95%, 95.1%, or any value range between the two.

[0178] 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%.

[0179] 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.

[0180] In some embodiments, the positive electrode film layer further includes a conductive agent, the mass content of which is 0.01%-1.5% based on the total mass of the positive electrode film layer.

[0181] 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 0.01%, 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.

[0182] 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.

[0183] The particles in this positive electrode film have good gradation, and the large particles have high sphericity, so they can form a dense packing, which allows the particles to contact each other and have good electronic conductivity, forming an electron transport network. This can reduce the use of conductive agents in the positive electrode film, which is conducive to further increasing the loading of positive electrode active materials and improving the energy density of lithium-ion secondary batteries.

[0184] 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.

[0185] 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 92%-99.5%, optionally 96.5%-99.5%; the mass content of the binder is 0.5%-3%.

[0186] 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.

[0187] 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 92%, 93%, 94%, 95%, 96%, 96.5%, 97%, 98%, 99%, 99.5%, or any value range between the two.

[0188] 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.

[0189] 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.

[0190] In some embodiments, the one-sided density of the positive electrode film is 300 mg / 1540 mm². 2 -450mg / 1540mm 2 .

[0191] 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.

[0192] In some embodiments, the areal density of the positive electrode film layer on one side may be selected as 300 mg / 1540 mm². 2 310mg / 1540mm 2 320 mg / 1540 mm 2 330 mg / 1540 mm 2 340 mg / 1540 mm 2 350 mg / 1540 mm 2 360 mg / 1540 mm 2 370 mg / 1540 mm 2 380 mg / 1540 mm 2 390 mg / 1540 mm 2 400 mg / 1540 mm2 410 mg / 1540 mm 2 420 mg / 1540 mm 2 430 mg / 1540 mm 2 440 mg / 1540 mm 2 450 mg / 1540 mm 2 Or the range of values ​​between any two.

[0193] Positive electrode films with areal densities within the above range can help improve the energy density of lithium-ion secondary batteries.

[0194] In some embodiments, the positive electrode film layer of the lithium-ion secondary battery, in its fully discharged state, has a compaction density of 2.52 g / cm³. 3 -2.78 g / cm 3 .

[0195] 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 -2.75 g / cm 3 .

[0196] In this application, the fully discharged state refers to the state in which the battery is placed at 25°C, left to stand for 2 hours, 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.

[0197] 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: (W1...) W2) / [(T1-T2)×S。

[0198] In some embodiments, the compaction density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery can be selected as 2.52 g / cm³. 3 2.53g / cm 3 2.54 g / cm 3 2.55g / cm 3 2.56 g / cm 32.57g / cm 3 2.58g / cm 3 2.59g / cm 3 2.60g / cm 3 2.61 g / cm 3 2.62 g / cm 3 2.63 g / cm 3 2.64 g / cm 3 2.65g / cm 3 2.66 g / cm 3 2.67 g / cm 3 2.68g / cm 3 2.69 g / cm 3 2.70 g / cm 3 2.71 g / cm 3 2.72 g / cm 3 2.73 g / cm 3 2.74 g / cm 3 2.75g / cm 3 2.76 g / cm 3 2.77 g / cm 3 2.78g / cm 3 Or the range of values ​​between any two.

[0199] In some embodiments, after compaction processing, the compaction density of the positive electrode film layer is 2.63 g / cm³. 3 -2.90g / cm 3 .

[0200] 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 3g / 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 2.86 g / cm 3 2.87 g / cm 3 2.88g / cm 3 2.89 g / cm 3 2.90g / cm 3 Or the range of values ​​between any two.

[0201] 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.

[0202] In some embodiments, the compaction density of the positive electrode film layer after formation processing is 2.52 g / cm³. 3 -2.78g / cm 3 .

[0203] In some embodiments, the compaction density of the positive electrode film layer after formation processing can be selected as 2.52 g / cm³. 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 32.74 g / cm 3 2.75g / cm 3 2.76 g / cm 3 2.77 g / cm 3 2.78g / cm 3 Or the range of values ​​between any two.

[0204] 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.

[0205] 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.

[0206] 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.

[0207] In some embodiments, the positive electrode film layer of the lithium-ion secondary battery, in its fully discharged state, has a compaction density of 2.52 g / cm³. 3 -2.78g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-22%.

[0208] 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 -2.75g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-20%.

[0209] In some embodiments, when the lithium-ion secondary battery is in a fully discharged state, the porosity of the positive electrode film layer in the 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.

[0210] 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.

[0211] like Figure 8 As shown, in this embodiment of the application, "pores" in the cross-section of the positive electrode film are identified through image color difference and threshold. 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. 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 tested by this method, the better the particle gradation of the positive electrode film, resulting in higher compaction density. Furthermore, under the same gradation and roller pressure, lower porosity means that particles are more likely to slide against each other, thereby reducing the risk of overpressure and stress concentration in the film, further reducing the probability of the positive electrode film demolding during long cycles, and improving the long-cycle performance of the battery.

[0212] 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.

[0213] Carbon-based particles refer to particles whose main component is carbon, including but not limited to conductive carbon and carbon black.

[0214] 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.

[0215] 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.

[0216] 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.

[0217] 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.

[0218] 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.

[0219] 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.

[0220] 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.

[0221] 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.

[0222] 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.

[0223] 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.).

[0224] 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 .

[0225] 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.

[0226] 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.

[0227] 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.).

[0228] 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.

[0229] 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).

[0230] 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.

[0231] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0232] 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.

[0233] 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.

[0234] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0235] 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.

[0236] 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.

[0237] 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.

[0238] 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.

[0239] 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.

[0240] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0241] 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.

[0242] 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.

[0243] 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.

[0244] A third aspect of this application provides an electrical device, including the lithium-ion secondary battery provided in the first aspect of this application.

[0245] 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; adding a solvent and grinding to obtain a mixed slurry; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder to obtain a positive electrode active material; the sintering of the precursor powder to obtain the positive electrode active material includes at least two sintering processes; obtaining a first sintered product after the first sintering; mixing the first sintered product and a carbon source to obtain an intermediate raw material; dividing the intermediate raw material into two groups and grinding them separately to obtain a first group of ground products and a second group of ground products; mixing the first group of ground products and the second group of ground products to obtain a mixed intermediate product; grinding the mixed intermediate product again to obtain a target mixture; and performing a second sintering on the target mixture to obtain the positive electrode active material; wherein, the D of the first group of ground products is... V50 The particle size is 0.8 μm - 1.2 μm; the D of the second group of milled products V50 The thickness is 0.30μm-0.50μm; the holding time for the second sintering is 5h-12h; the grinding time for the second grinding of the mixed intermediate product is 0.5h-1.5h.

[0246] The preparation method provided in this application, through three grinding processes and two sintering processes, not only obtains a positive electrode active material with a certain particle size distribution, but also improves the sphericity of large particles in the positive electrode active material. Specifically, in the cumulative distribution curve of sphericity area for particles with a diameter greater than or equal to 1 μm obtained along the thickness direction of the electrode sheet, the median L of sphericity is... R1A50This provides a material basis for the preparation of positive electrode films with a thickness of 0.71-0.85.

[0247] In some embodiments, the iron source is an iron-containing compound. In some embodiments, the iron source includes at least one of ferric hydroxide, ferrous chloride, ferric oxide, ferric phosphate, ferric pyrophosphate, ferrous oxalate, iron powder, ferric nitrate, magnetite, and ferric hydroxide.

[0248] In some embodiments, the phosphorus source is a phosphoric acid compound. In some embodiments, the phosphorus source includes at least one selected from phosphoric acid, ferric phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0249] In some embodiments, the iron source and the phosphorus source can be the same substance. In some embodiments, ferric phosphate is used as both the iron source and the phosphorus source.

[0250] In some embodiments, the lithium source includes one or more of lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium oxide, lithium hydroxide, and lithium acetate.

[0251] In some embodiments, the lithium source includes lithium carbonate.

[0252] In some embodiments, the carbon source includes one or more of glucose, polyethylene glycol, citric acid, sucrose, starch, fructose, lactose, polyaniline, polyacrylonitrile, and polyvinylpyrrolidone.

[0253] In some embodiments, the carbon source includes glucose and polyethylene glycol.

[0254] In some implementations, the mass content of polyethylene glycol is 20%-75% based on the total mass of the carbon source.

[0255] The polymer carbon source has a relatively low graphitization temperature, which allows the carbon coating material on the surface of the positive electrode active material to decompose and form a coating structure 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.

[0256] 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.

[0257] Organic molecules in carbon sources decompose at high temperatures, releasing carbon atoms. These carbon atoms can cover and fill tiny gaps or defects on the surface of active materials, reducing surface roughness. The coating material formed by sugar carbon sources has a low degree of graphitization and a loose flocculent carbon structure, which may itself act as a new roughness point. Therefore, the higher the proportion of polymer carbon sources in the carbon source, the more beneficial it is to optimizing the surface roughness of the positive electrode active material.

[0258] 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.

[0259] 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.

[0260] 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.

[0261] Meanwhile, titanium doping helps improve the sphericity of the particles and reduce their roughness, thereby enhancing the overall structural stability of the material.

[0262] In some embodiments, the atomic molar ratio of iron to phosphorus in the iron source and the phosphorus source is 0.95-1.

[0263] In some embodiments, the atomic molar ratio of iron to phosphorus in the lithium source and the iron source can be selected as 0.95, 0.955, 0.96, 0.965, 0.97, 0.975, 0.98, 0.985, 0.99, 0.995, 1, or any value range between the two.

[0264] In the synthesis of lithium-containing transition metal oxides, the iron-to-phosphorus ratio affects the reaction rate and crystal growth process. A high iron-to-phosphorus ratio may inhibit the reaction rate of the iron source, leading to a slower rate of iron source conversion into lithium-containing transition metal phosphates. This step becomes the rate-determining step, prolonging crystal growth time and thus promoting the formation of larger particles.

[0265] In some embodiments, the carbon source in the mixed raw materials accounts for 5%-7% of the total mass of the mixed raw materials.

[0266] In some embodiments, based on the total mass of the mixed raw materials, the mass percentage of the carbon source in the mixed raw materials can be selected as 5%, 5.1%, 5.2%, 5.3%, 5.4%, 5.5%, 5.6%, 5.7%, 5.8%, 5.9%, 6.0%, 6.1%, 6.2%, 6.3%, 6.4%, 6.5%, 6.6%, 6.7%, 6.8%, 6.9%, 7%, or any value range between the two.

[0267] By controlling the quality of the carbon source and ensuring the lithium content remains within the aforementioned range, the conductivity of the material can be enhanced, and the negative impact on the specific capacity of the positive electrode and the energy density of the battery can be reduced. Excessively thick carbon coating not only occupies valuable space for the active material but may also lead to structural instability.

[0268] In some embodiments, the solvent includes water and mixtures thereof.

[0269] In some embodiments, obtaining the mixed raw material including carbon source, lithium source, iron source and phosphorus source includes: adding carbon source, lithium source, phosphorus source, iron source and carbon source to a solvent and mixing and stirring, wherein the stirring speed is 1400rpm-2200rpm.

[0270] In some embodiments, obtaining the mixed slurry after grinding includes an initial grinding, which satisfies one or more of the following conditions: (1) The grinding balls used in the first grinding are one or more of the following: zirconium oxide balls, silicon nitride zirconium balls, and ceramic zirconium balls. (2) The diameter of the grinding balls used in the first grinding is 0.5mm-0.7mm; (3) The rotation speed of the first grinding is 450rpm-550rpm; (4) The grinding time for the first grinding is 0.5h-1.5h; (5) The grinding chamber pressure during the first grinding is 0.01MPa-0.3MPa.

[0271] In some embodiments, obtaining a mixed slurry after grinding includes performing a second grinding after the first grinding, the second grinding satisfying one or more of the following conditions: (1) The grinding balls used in the secondary grinding are one or more of the following: zirconium oxide balls, silicon nitride zirconium balls, and ceramic zirconium balls. (2) The diameter of the grinding balls used in the secondary grinding is 0.25mm-0.35mm; (3) The rotation speed of the secondary grinding is 470rpm-530rpm; (4) The grinding time for the secondary grinding is 3.0h-5.0h; (5) The grinding chamber pressure of the secondary grinding is 0.01MPa-0.3MPa.

[0272] In some embodiments, the particle size D of the mixed slurry V50 The thickness is 0.3μm-0.5μm.

[0273] Grinding at least twice helps control the slurry temperature and viscosity, reducing excessive slurry viscosity caused by high temperature and the resulting agglomeration of raw materials.

[0274] The first grinding process handles large particles, while the second grinding further refines the material and adjusts the particle size distribution. This effectively reduces particle size inhomogeneity caused during grinding, minimizes particle agglomeration, improves battery conductivity and cycle stability, and enhances overall production efficiency while meeting final product performance requirements.

[0275] In some embodiments, obtaining precursor powder after drying the mixed slurry includes obtaining precursor powder after spray drying the mixed slurry.

[0276] In some embodiments, sintering the precursor powder to obtain the positive electrode active material includes at least two sintering processes.

[0277] In some embodiments, the first sintering of the at least two sintering processes satisfies one or more of the following conditions: (1) The heating rate is 2°C / min-10°C / min; (2) The insulation temperature is 720°C-790°C; (3) The heat preservation time is 5h-12h.

[0278] In some embodiments, after the first sintering, a first sintered product is obtained. The first sintered product and a carbon source are mixed to obtain intermediate raw materials. The intermediate raw materials are divided into two groups and ground (a third grinding) to obtain a first group of ground products and a second group of ground products. The first group of ground products and the second group of ground products are mixed to obtain a mixed intermediate product.

[0279] In some embodiments, the carbon source added to the intermediate raw material obtained by mixing the first sintered product and the carbon source includes glucose and polyethylene glycol.

[0280] In some embodiments, based on the mass of the first sintered product, the glucose content in the intermediate raw material is 1%-3% by mass, and the polyethylene glycol content in the intermediate raw material is 0.5%-6% by mass.

[0281] In some embodiments, the D of the first set of milled products V50 The thickness ranges from 0.8μm to 1.2μm.

[0282] In some embodiments, the D of the second set of milled products V50 The range is 0.30μm-0.50μm.

[0283] Compared to larger particles, smaller particles have a higher proportion of surface atoms, resulting in greater surface energy and higher surface activity. This leads to a larger surface area per unit volume and a faster surface diffusion rate. Consequently, smaller particles are more prone to undergoing more intense surface rearrangement during sintering, driving them towards a spherical shape. Therefore, controlling the grinding particle size to a smaller value, including grinding the mixed slurry and the intermediate raw materials, is beneficial for forming products with higher sphericity and smoother, flatter surfaces during subsequent sintering. Simultaneously, smaller particle sizes are easier to coat, improving coating integrity.

[0284] In some embodiments, the grinding conditions of the first group of grinding products satisfy one or more of the following conditions: (1) The rotational speed is 550 rpm ± 50 rpm; (2) The grinding time is 0.5h-1.5h.

[0285] In some embodiments, the grinding conditions of the second set of grinding products satisfy one or more of the following conditions: (1) The rotational speed is 500 rpm ± 50 rpm; (2) The grinding time is 3h-5h. In some embodiments, the mass ratio of the first group of grinding products to the second group of grinding products is (60:40) to (80:20).

[0286] By adjusting the mass ratio of the first group of grinding products and the second group of grinding products, positive electrode active materials with different gradation ratios can be obtained. This allows for precise control of the number and area ratios of particles of various sizes, thus obtaining the desired packing structure.

[0287] In some embodiments, the mixed intermediate product is ground at 500 rpm ± 50 rpm for 0.5 h to 1.5 h to obtain the target mixture.

[0288] Grinding the intermediate mixture again helps to improve the uniformity of particle size in the target mixture, reduce the presence of abnormally large particles, make sintering uniform, and improve the sphericity of large particles.

[0289] In some embodiments, the target mixture is subjected to a second sintering.

[0290] In some embodiments, the second sintering satisfies one or more of the following conditions: (1) The heat preservation temperature is 770°C-830°C; (2) The heat preservation time is 5h-12h; (3) The heating rate is 2°C / min-10°C / min.

[0291] The two-stage sintering process can effectively shorten the sintering time in the high-temperature range, thereby reducing the degree of grain boundary melting of large particles during high-temperature sintering. This improves the sphericity of large particles in the positive electrode active material, thus reducing stress concentration caused by particle bridging due to irregular shape of large particles, reducing porosity in the film layer, improving the compaction density of the electrode, effectively increasing the energy density of the battery, reducing kinetic degradation, and improving the electron conduction capacity between particles through closer packing, thereby achieving improved kinetic performance.

[0292] 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.

[0293] In some embodiments, the product is subjected to air jet milling after sintering the precursor to obtain the positive electrode active material.

[0294] In some embodiments, the staged frequency of the air jet mill is 20Hz-30Hz, and the milling pressure is 0.45MPa-0.55MPa.

[0295] 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.

[0296] 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.

[0297] However, excessively high classification frequency and pulverizing pressure can cause agglomerated particles to further crack and break down after being dispersed into primary particles, affecting the intended particle size distribution and resulting in an incomplete carbon coating structure. This manifests as increased iron dissolution, negatively impacting particle slippage during rolling and increasing the contact and reaction between lithium-containing transition metal phosphates and external factors such as the electrolyte, which is detrimental to the battery's cycle performance and lifespan. Therefore, it is necessary to control the classification frequency and pulverizing pressure of the air jet pulverizer within a suitable range.

[0298] 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.

[0299] In some embodiments, the dry mix has a revolution speed of 20 rpm-30 rpm and a rotation speed of 750 rpm-850 rpm.

[0300] 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.

[0301] 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.

[0302] 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.

[0303] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0304] Figure 7 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0305] 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.

[0306] Example The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0307] Example 1 (1) Preparation of positive electrode active material S1, lithium carbonate, iron phosphate, and glucose are added to water and mixed in a premixing tank at 1800 rpm. The mixture is then demagnetized by a demagnetizing rod with a magnetic field strength of 8000-12000 Gs to obtain a mixed raw material. The ratio of lithium carbonate to iron phosphate results in a molar ratio of iron to phosphorus of 0.975. The glucose content is 5.7% of the total raw material mass. The addition of titanium dioxide results in a titanium doping amount of 5000 ppm in the positive electrode active material. S2, the mixed raw materials are subjected to two grinding-demagnetization cycles in a sand mill. The first grinding is performed using 0.6 mm diameter zirconia balls at a speed of 260 rpm for 1 hour, with a grinding chamber pressure of less than 0.2 MPa. After the first grinding, the raw materials are demagnetized using a permanent magnet separator with a demagnetization intensity greater than or equal to 8000 Gs. The demagnetized raw materials are then ground a second time to obtain a mixed slurry. The particle size D of the mixed slurry is... V50 It is 0.40μm ± 0.10μm; S3, the mixed slurry is spray-dried to obtain precursor powder. S4, the precursor powder is sintered to obtain lithium iron phosphate cathode material. The sintering process includes: First sintering: The precursor powder was sintered in a nitrogen atmosphere, heated from 25°C to 760°C at a heating rate of 5°C / min, and held at that temperature for 10 hours. After cooling, the first sintered product was obtained. Grinding and mixing: Add 1.5% glucose and 3.0% polyethylene glycol (by weight of the first sintered product) to the first sintered product; divide into two groups and grind (third grinding), wherein the particle size D of the first group is... V50 Grinding was stopped when the particle size reached 1.0μm ± 0.20μm (grinding conditions: 550rpm ± 50rpm, grinding time 1h). The D-value of the particles in the second group... V50 Grinding was stopped when the particle size reached 0.38μm±0.05μm (grinding conditions: 500rpm±50rpm, grinding time 4h); the first group and the second group were mixed at a mass ratio of 70:30 to obtain a mixed intermediate product; to ensure the uniformity of the mixture, the mixed product was ground for a fourth time before spraying, with grinding conditions: 500rpm±50rpm, grinding time 1h, to obtain the target mixture; the target mixture was then spray-dried. Second sintering: The dried target mixture is sintered in a nitrogen atmosphere, heated from 25°C to 800°C at a heating rate of 5°C / min, and held at that temperature for 8 hours. After cooling, the second sintered product is obtained.

[0308] S5. After sintering, the product is cooled to below 100 °C and then crushed using an air jet milling method to obtain carbon-coated lithium iron phosphate cathode active material. The air jet milling stage frequency is 25 Hz and the milling pressure is 0.55 MPa.

[0309] The prepared positive electrode active material has a carbon content of 1.269% by mass, a lithium iron antisite defect concentration of 0.20%, and a powder tap density of 1.40 g / cm³. 3 The compacted density of the powder under 3T pressure is 2.62 g / cm³. 3 The powder resistivity at 8 MPa pressure is 11.96 Ω·cm; the discharge capacity at 1C discharge rate is 141.1 mAh / g; and the discharge capacity at 3.2V discharge platform accounts for 90.0%.

[0310] (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 sequentially added and dry-mixed, followed by the addition of N-methylpyrrolidone. The mixture was stirred and the viscosity adjusted to obtain a slurry. This slurry was then transferred and coated onto a base coating of current collector aluminum foil. The base coating consisted of carbon black and PVDF in a 1:1 mass ratio. The density of carbon-based particles larger than 100 nm in the base coating was ≤10 pcs / 10 μm, and the base coating thickness was 2 μm. After drying and hot pressing, a single-sided surface density of 350 mg / 1540 cm³ was obtained. 2 The positive electrode film layer. The dry-mixed electrode has a revolution speed of 25 rpm and a rotation speed of 800 rpm.

[0311] The hot pressing process includes three hot roller pressing processes, with the hot roller pressing pressure increasing sequentially to 35 tons, 60 tons, and 80 tons. The hot roller temperature is 60°C. Before the first hot roller compaction, the electrode sheet is heated to 50°C.

[0312] The compacted density of the electrode sheet is the ultimate compacted density of the electrode sheet. The test method for the ultimate compacted density of the electrode sheet is described below; in this example, the ultimate compacted density of the electrode sheet is 2.73 g / cm³. 3 .

[0313] In the cross-section along the thickness direction of the prepared positive electrode film, the particle size distribution index (PDI) is 0.899, the average particle size is 258 nm, and the sample standard deviation of the particle size is 232 nm. In the spheroidity area distribution curve of particles with a diameter greater than or equal to 1 μm obtained from the cross-section along the thickness direction of the positive electrode film, the median L of spheroidity is... R1A50 The median L of the spheroidality is 0.751. This is from the cumulative distribution curve of the particle spheroidality area obtained along the thickness direction of the positive electrode film. A50 The median C of graphitization degree obtained in laser microscopy confocal Raman spectroscopy instrument scanning mode is 0.731. 50 The median roughness R is 1.05. In the cumulative area distribution curve of particle roughness obtained from a cross-section along the electrode thickness direction, the median roughness R is... A50 The value is 0.950. The iron dissolution rate of the positive electrode film is 900.2 ppm.

[0314] (3) Preparation of negative electrode sheet: A mixture of 95.5 wt% negative electrode active material (artificial graphite), 1.0 wt% conductive agent (conductive carbon black), 2.0 wt% binder (styrene-butadiene rubber (SBR)), and 1.5 wt% thickener (sodium carboxymethyl cellulose (CMC)) was prepared by mixing with deionized water and stirring to disperse the mixture into a negative electrode slurry. The negative electrode slurry was then coated onto both sides of a Cu foil. After coating both sides, the foil was dried, compacted, slit, and sheeted to obtain the negative electrode sheet. The coating density on one side was 165 mg / 1540 mm². 2 The compacted density is 1.60 g / cm³. 3 .

[0315] (4) Preparation of the separating membrane Polypropylene film is used as the separator.

[0316] (5) Preparation of electrolyte In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate (EC) and dimethyl carbonate (DMC) organic solvents were mixed evenly at a volume ratio of 1 / 1. Lithium salt LiPF6 was added and dissolved in the organic solvent. The concentration of LiPF6 in the solution was 1 mol / L. The mixture was stirred evenly to obtain the electrolyte.

[0317] (6) Battery fabrication: The positive electrode, separator, and negative electrode are stacked in sequence. The separator must be able to isolate the anode and cathode. The bare cell is obtained by winding. The bare cell is placed in the outer packaging, electrolyte is injected, and after processes such as encapsulation, formation, and degassing, a lithium-ion battery is finally obtained.

[0318] The preparation method of Example 2 is basically the same as that of Example 1, except that in step S4, the D of the particles in the first group... V50 Grinding was stopped when the particle size reached 0.80μm ± 0.20μm; the D of the particles in the second group... V50 Stop grinding when the thickness reaches 0.30μm±0.05μm.

[0319] The preparation method of Example 3 is basically the same as that of Example 1, except that in step S4, the D of the particles in the first group... V50 Grinding was stopped when the particle size reached 0.80μm ± 0.20μm; the D of the particles in the second group... V50 Stop grinding when the thickness reaches 0.45μm±0.05μm.

[0320] The preparation method of Example 4 is basically the same as that of Example 1, except that in step S4, the D of the particles in the first group... V50 Grinding was stopped when the particle size reached 1.20μm ± 0.20μm; the D of the particles in the second group... V50 Stop grinding when the thickness reaches 0.30μm±0.05μm.

[0321] The preparation method of Example 5 is basically the same as that of Example 1, except that in step S4, the D of the particles in the first group... V50 Grinding was stopped when the particle size reached 1.20μm ± 0.20μm; the D of the particles in the second group... V50 Stop grinding when the thickness reaches 0.45μm±0.05μm.

[0322] The preparation method of Example 6 is basically the same as that of Example 1, except that in step S4, there is no fourth grinding, and the mixed intermediate product is directly subjected to the second sintering.

[0323] The preparation method of Example 7 is basically the same as that of Example 1, except that in step S4, the grinding time for the fourth grinding is 1.5 hours.

[0324] The preparation method of Example 8 is basically the same as that of Example 1, except that the holding time for the second sintering in step S4 is 5 hours.

[0325] The preparation method of Example 9 is basically the same as that of Example 1, except that the holding time for the second sintering in step S4 is 12h.

[0326] The preparation method of Comparative Example 1 is basically the same as that of Example 1, except that in step S4, the D of the particles in the first group... V50 Grinding was stopped when the particle size reached 0.75μm ± 0.20μm; the D of the particles in the second group... V50 Stop grinding when the thickness reaches 0.50μm±0.05μm.

[0327] The preparation method of Comparative Example 2 is basically the same as that of Example 1, except that in step S4, the D of the particles in the first group... V50 Grinding was stopped when the particle size reached 1.50μm±0.20μm; grinding was stopped when the particle size Dv50 in the second group reached 0.50μm±0.05μm.

[0328] The preparation method of Comparative Example 3 is basically the same as that of Example 1, except that in step S2, the particle size D of the mixed slurry is... V50 The thickness is 0.35μm ± 0.10μm; in step S4, after obtaining the first sintering product, a second sintering is performed directly, and the holding time for the second sintering is 12h.

[0329] Performance testing 1. Ultimate compaction density of electrode sheets The double-coated electrode sheets were compacted using a roller press, and the elongation and flexibility of the compacted electrode sheets were tested. By increasing the pressure of the roller press, electrode sheets with different compaction densities were obtained. As the pressure increased, the compaction density of the electrode sheet increased, the elongation of the electrode sheet increased, and the flexibility of the electrode sheet decreased. Excessive elongation of the electrode sheet can easily lead to warping, while insufficient flexibility can easily lead to brittle fracture. Therefore, the lower of the compaction density corresponding to an elongation of 8% or the compaction density corresponding to a flexible folding number of times is defined as the ultimate compaction density of the electrode sheet.

[0330] The compaction density is calculated by dividing the mass of the positive electrode film by the volume of the positive electrode film.

[0331] The method for testing elongation 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 edges of the electrode, ensuring the cut edges are parallel to the MD direction (perpendicular to the pressure roller) to guarantee the electrode is completely covered by the coating. Use a steel ruler to measure the length between marked points at the beginning and end of the electrode, estimating to 0.1mm, and record the length before compaction. After compaction, record the length between the corresponding marked points. Use (compacted length - uncompacted length) / uncompacted length as the electrode's elongation.

[0332] The test method for the number of flexible folds is as follows.

[0333] 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.

[0334] 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 25°C at 0.33C to the discharge cutoff voltage of 2.5V. The total discharge energy of the lithium-ion secondary battery was recorded as E0.

[0335] 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.

[0336] 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.

[0337] 3. DCR Test Method At 25°C, the system is charged at a constant current of 0.33C to 3.65V, then charged at a constant voltage to a current of 0.05C, followed by a discharge at 1 / 3C to 20% SOC. After resting for 5 minutes, it is pulsed discharged at 3C for 30 seconds, rested for 40 seconds, charged at 3C for 40 seconds, rested for 5 minutes, charged at a constant current of 1 / 3C to 3.65V, then charged at a constant voltage to 0.05C, then discharged at 1 / 3C to 10% SOC. After resting for 5 minutes, it is pulsed discharged at 3C for 30 seconds, and then rested for 40 seconds. After charging at 3C for 40 seconds, let it stand for 5 minutes, then fully charge it at 1 / 3C, then discharge it at 1 / 3C to 50% SOC, then let it stand at -25°C for 2 hours, then pulse discharge it at 1C for 30 seconds, let it stand for 10 minutes, then let it stand at 25°C for 2 hours, then charge it at 1 / 3C constant current to 3.65V, then charge it at constant voltage to 0.05C, then discharge it at 1 / 3C to 20% SOC, then let it stand at -25°C for 2 hours, then pulse discharge it at 1C for 30 seconds, and let it stand for 10 minutes.

[0338] 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.

[0339] 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.

[0340] Table 1

[0341] Table 2

[0342] As can be seen from the data comparison of the embodiments and comparative examples, in the cumulative distribution curve of the sphericity area of ​​particles with a particle size greater than or equal to 1 μm obtained by controlling the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median of sphericity is 0.71-0.85, which reduces the battery impedance while increasing the compaction density of the positive electrode sheet, thereby improving the energy density of the battery.

[0343] As can be seen from the comparison between Example 3 and other examples, the particle size distribution index (PDI) of the particles in the cross-section along the thickness direction of the positive electrode film is 0.85-1.00, which is beneficial to further improve the compaction density of the electrode and improve the energy density of the battery.

[0344] As can be seen from the comparison between Example 5 and other examples, in the cumulative distribution curve of the sphericity area of ​​particles with a particle size greater than or equal to 1 μm obtained by the cross-section of the positive electrode film along the electrode thickness direction, the median of sphericity is 0.72-0.80, which is beneficial to further reduce battery impedance and improve battery dynamic performance while increasing the electrode compaction density.

[0345] As can be seen from the comparison between Example 5 and other examples, in the cumulative distribution curve of the spheroidal area of ​​particles obtained by slicing the positive electrode film along the thickness direction of the electrode sheet, the median L of spheroidality is... A50 A value of 0.71-0.80 is beneficial for increasing the electrode compaction density while further reducing battery impedance and improving battery dynamic performance.

[0346] 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 electrode comprises 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 layer includes a positive electrode active material, which comprises lithium transition metal phosphate particles with at least a portion of their surface coated with carbon material. In the cumulative distribution curve of spheroidal area of ​​particles with a diameter greater than or equal to 1 μm obtained from the cross-section of the positive electrode film along the electrode thickness direction, the median L of spheroidality is... R1A50 The average particle size is 0.71-0.85; in the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the average particle size is 150nm-400nm.

2. The lithium-ion secondary battery according to claim 1, characterized in that, In the cross-section of the positive electrode film along the thickness direction of the electrode, the particle size distribution index (PDI) of the particles is 0.80-1.

10. The PDI is the ratio of the sample standard deviation to the mean of the particle size in the cross-section of the positive electrode film along the thickness direction.

3. The lithium-ion secondary battery according to claim 2, characterized in that, In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the particle size distribution index (PDI) of the particles is 0.85-1.

00.

4. The lithium-ion secondary battery according to claim 1, characterized in that, The average particle size of the particles in the cross-section of the positive electrode film along the thickness direction of the electrode sheet is 150nm-300nm.

5. The lithium-ion secondary battery according to claim 1, characterized in that, The standard deviation of the particle size in the cross-section of the positive electrode film along the thickness direction of the electrode sheet is 150nm-350nm.

6. The lithium-ion secondary battery according to claim 1, characterized in that, The standard deviation of the particle size in the cross-section of the positive electrode film along the thickness direction of the electrode sheet is 150nm-300nm.

7. The lithium-ion secondary battery according to claim 1, characterized in that, In the cumulative distribution curve of spheroidal area of ​​particles with a diameter greater than or equal to 1 μm obtained from the cross-section of the positive electrode film along the electrode thickness direction, the median L of spheroidality is... R1A50 It is 0.72-0.

80.

8. The lithium-ion secondary battery according to claim 1, characterized in that, In the cumulative distribution curve of the spheroidal 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 spheroidality is... A50 It is 0.70-0.

85.

9. The lithium-ion secondary battery according to claim 1, characterized in that, In the cumulative distribution curve of the spheroidal 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 spheroidality is... A50 It is 0.71-0.

8.

10. The lithium-ion secondary battery according to claim 8 or 9, characterized in that, In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, L R1A50 >L A50 .

11. The lithium-ion secondary battery according to claim 10, characterized in that, In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, 0.01 ≤ L R1A50 -L A50 ≤0.

1.

12. The lithium-ion secondary battery according to claim 10, characterized in that, In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, 0.01 ≤ L R1A50 -L A50 ≤0.

05.

13. The lithium-ion secondary battery according to claim 1, characterized in that, In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the laser microscopy confocal Raman spectroscopy instrument scanning mode, the median C of the graphitization degree is... 50 The value ranges from 0.98 to 1.20; 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.

14. The lithium-ion secondary battery according to claim 13, 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 It is 1.02-1.

1.

15. The lithium-ion secondary battery according to claim 1, 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 roughness R A50 It is 0.92-0.

96.

16. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode active material includes iron, and the iron dissolution rate of the positive electrode film is 400ppm-1800ppm.

17. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode active material includes iron, and the iron dissolution rate of the positive electrode film is 500ppm-1500ppm.

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

19. The lithium-ion secondary battery according to claim 1, characterized in that, Based on the total mass of the positive electrode active material, the carbon content is 0.90%-1.5%.

20. The lithium-ion secondary battery according to claim 1, characterized in that, The concentration of lithium iron ion reverse defects in the positive electrode active material is 0.001%-1.5%.

21. The lithium-ion secondary battery according to claim 1, characterized in that, The concentration of lithium iron ion reverse defects in the positive electrode active material is 0.01%-1.0%.

22. The lithium-ion secondary battery according to claim 1, characterized in that, The lithium-containing transition metal phosphate particles comprise components having the following general formula: The 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.

23. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode active material includes one or more of lithium iron phosphate and doped / modified materials and coated / modified materials.

24. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode active material includes titanium, and the mass content of titanium is 4000ppm-8000ppm based on the total mass of the positive electrode active material.

25. The lithium-ion secondary battery according to claim 1, characterized in that, The tap density of the positive electrode active material is 1.0 g / cm³. 3 -1.70g / cm 3 .

26. The lithium-ion secondary battery according to claim 1, characterized in that, The tap density of the positive electrode active material is 1.20 g / cm³. 3 -1.50g / cm 3 .

27. The lithium-ion secondary battery according to claim 1, characterized in that, The compacted density of the positive electrode active material under 3T pressure is 2.55 g / cm³. 3 -2.70g / cm 3 .

28. The lithium-ion secondary battery according to claim 1, characterized in that, The compacted density of the positive electrode active material under 3T pressure is 2.58 g / cm³. 3 -2.68g / cm 3 .

29. The lithium-ion secondary battery according to claim 1, characterized in that, The resistivity of the positive electrode active material at 8 MPa pressure is 0.5 Ω·cm-60 Ω·cm.

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

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

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

33. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode film also includes a conductive agent, and the mass content of the conductive agent is 0.01%-1.5% based on the total mass of the positive electrode film.

34. The lithium-ion secondary battery according to claim 1, characterized in that, Based on the total mass of the positive electrode film, the mass content of the positive electrode active material is 92%-99.5%.

35. The lithium-ion secondary battery according to claim 1, 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 claim 1, 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 claim 1, 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 claim 1, characterized in that, In the fully discharged state, the positive electrode film of the lithium-ion secondary battery has a compaction density of 2.52 g / cm³. 3 -2.78g / cm 3 .

39. The lithium-ion secondary battery according to claim 1, 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 -2.75g / cm 3 .

40. The lithium-ion secondary battery according to claim 1, characterized in that, In the fully discharged state, the positive electrode film of the lithium-ion secondary battery has a compaction density of 2.52 g / cm³. 3 -2.78g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-22%.

41. The lithium-ion secondary battery according to claim 1, 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 -2.75g / 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 claim 1, 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 claim 1, 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 claim 1, 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, The lithium-ion secondary battery includes any one of claims 1 to 44.

47. An electrical appliance, characterized in that, Includes the battery device as described in claim 45.

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