Lithium-ion secondary battery, battery device, and electric device

Abstract

The present disclosure provides a lithium ion secondary battery, a battery device and a power utilization device. The lithium ion secondary battery comprises a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer comprises a positive electrode active material, the positive electrode active material comprises lithium-containing transition metal phosphate particles with carbon-coated materials arranged on at least part of the surfaces, and in a section of the positive electrode film layer along the thickness direction of the sheet, the area proportion of particles with a particle size greater than or equal to 1.5 μm is greater than or equal to 8.0% and less than or equal to 20.0%; the iron elution rate of the positive electrode film layer is 658 ppm-1921 ppm.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion secondary batteries, in particular to a lithium ion secondary battery, a battery device, a power utilization device, a preparation method of a positive electrode active material and a preparation method of a positive electrode sheet. BACKGROUND

[0002] In recent years, lithium ion secondary batteries are widely used in energy storage power systems such as hydraulic, thermal, wind and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, etc.

[0003] The positive electrode active material is an important component of the secondary battery, and the lithium-containing transition metal phosphate material has the characteristics of structural stability, good safety and long cycle life, and has broad development prospects. With the increasing requirements of the market for the energy density and kinetics of lithium-containing transition metal phosphate system secondary batteries, it is difficult to simultaneously improve the above-mentioned performances in the prior art, which becomes a technical problem that needs to be solved in the field. SUMMARY

[0004] In view of the above problems, the present application provides a battery monomer, a battery device, a power utilization device, a preparation method of a positive electrode active material and a preparation method of a positive electrode sheet, which are described below respectively.

[0005] The first aspect of the present application provides a lithium ion secondary battery, comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, the positive electrode sheet comprising a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer comprising a positive electrode active material, the positive electrode active material comprising lithium-containing transition metal phosphate particles having a carbon-coated material arranged on at least part of the surface, in a section of the positive electrode film layer along the thickness direction of the sheet, the area proportion of particles with a particle size greater than or equal to 1.5 μm is greater than or equal to 8.0% and less than or equal to 20.0%; the iron dissolution rate of the positive electrode film layer is 658 ppm-1921 ppm.

[0006] In a section of the positive electrode film layer along the thickness direction of the sheet, the area proportion of particles with a particle size greater than or equal to 1.5 μm is less than 8%, which is difficult to achieve high compaction density of the positive electrode sheet. In a section of the positive electrode film layer along the thickness direction of the sheet, the area proportion of particles with a particle size greater than or equal to 1.5 μm is more than 20%, which is beneficial to the realization of high compaction density of the positive electrode sheet, but reduces the contact area between the electrolyte and the positive electrode active material, hinders the diffusion of lithium ions to the positive electrode film layer, increases the diffusion path of lithium ions in the particles, causes serious polarization in the local sheet, increases the battery impedance, and significantly deteriorates the kinetic performance of the battery.

[0007] The area ratio of particles with a particle size greater than or equal to 1.5 μm in the positive electrode film layer is greater than or equal to 8.0% and less than or equal to 20.0%, which can reduce the significant short board effect caused by large-size particles, is beneficial to keeping the battery impedance at a low level, and improves the kinetic performance of the battery, but this limits the further improvement of the compaction density of the pole piece. In the embodiments of the present application, the iron dissolution rate of the positive electrode film layer is controlled to be 658 ppm-1921 ppm, that is, the density and integrity of the carbon coating on the surface of the positive electrode active material are improved, the particles are more easily to slip, and the lattice defects of the lithium-containing transition metal phosphate are reduced, the pressure resistance of the particles is improved, the probability of collapse and rupture of the particles under large roller pressure is reduced, and the compaction density of the pole piece is improved; at the same time, the complete and dense carbon coating material is beneficial to improve the electrical contact between the positive electrode active materials and improve the conductivity of the positive electrode active material, and reduce the polarization of the positive electrode active material; the low content of lattice defects is beneficial to the smooth transmission of lithium ions and the improvement of the lithium ion transmission rate of the positive electrode active material, thereby realizing the balance between the kinetic performance and the energy density of the battery.

[0008] In any embodiment, the iron dissolution rate of the positive electrode film layer is 658 ppm-1485 ppm. The positive electrode film layer with an iron dissolution rate in the above range has relatively fewer lattice defects and more complete and dense carbon coating materials. Fewer crystal defects are beneficial to improve the pressure resistance and easy sliding degree of the particles in the positive electrode film layer under large roller pressure, high coating integrity is beneficial to the sliding between the particles, dense coating is beneficial to reduce the space occupancy rate of the carbon layer, further improve the compaction density of the pole piece, and realize the balance between the kinetic performance and the energy density of the battery.

[0009] In any embodiment, in the cumulative distribution curve of the coating value B of the positive electrode film layer obtained by the laser microscopic confocal Raman spectrometer in the face scanning mode, the median of the coating value B 50 is 0.35-0.48, wherein the coating value B is I P / I D , I P represents the P peak intensity of the Raman spectrum at 948±100 cm -1 , and I D represents the D peak intensity of the Raman spectrum at 1350±100 cm -1 . The coating value B value can indirectly reflect the thickness of the carbon coating material on the surface of the lithium-containing transition metal phosphate particles. The thinner the thickness of the carbon coating material, the higher the phosphate structure intensity detected in the Raman spectrum, and the higher the coating value of the positive electrode film layer.

[0010] In any embodiment, the area ratio of particles with a particle size greater than or equal to 5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet is 0. Research shows that particles with a particle size greater than or equal to 5 μm in the positive electrode film layer will significantly deteriorate the infiltration of the electrolyte in the positive electrode film layer and the diffusion in the active material particles, and the area ratio of particles with a particle size greater than or equal to 5 μm being 0 is conducive to further reducing the internal resistance of the battery and improving the kinetic performance of the battery.

[0011] In any embodiment, the area ratio of particles with a particle size greater than or equal to 1.5 μm and less than 5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet is 9.0-20.0%, which can be 10.0-20.0%. The area ratio of particles with a particle size greater than or equal to 1.5 μm and less than 5 μm in the above range is conducive to improving the compaction of the electrode sheet, further reducing the hindering effect of large-size particles on the electrolyte infiltration and diffusion in the positive electrode film layer, improving the consistency of the lithium ion diffusion rate in the positive electrode active material particles, reducing local polarization, and improving the kinetic performance of the battery.

[0012] In any embodiment, the area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet is 15.0-25.0%, which can be 16.0-24.0%. The area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm in the above range can fill larger gaps in the accumulation, form a certain "support" structure, reduce the porosity between particles, and help to enhance the inter-particle contact and overall structural strength, further improve the compaction density of the electrode sheet on the basis of maintaining good kinetic performance of the battery, and improve the energy density of the battery.

[0013] In any embodiment, the area ratio of particles with a particle size greater than or equal to 200 nm and less than 1500 nm in the section of the positive electrode film layer along the thickness direction of the electrode sheet is 73.0-80.0%, which can be 73.0-78.0%. The area ratio of particles with a particle size greater than or equal to 200 nm and less than 1500 nm can be tested by the method described above. The area ratio of particles with a particle size greater than or equal to 200 nm and less than 1500 nm in the section of the positive electrode film layer along the thickness direction of the electrode sheet is the sum of the areas of the particles with a particle size greater than or equal to 200 nm and less than 1500 nm divided by the total area of the particles counted.

[0014] In any embodiment, the median value C50 of graphitization degree in the cumulative distribution curve of graphitization degree C value of the positive electrode film layer obtained under the laser micro confocal Raman spectroscopy face scanning mode is 0.95-1.20, which can be 0.98-1.15, and further can be 1.0-1.10; wherein the graphitization degree C value is IG / ID, wherein IG represents the G peak intensity of Raman spectrum at 1580±100 cm-1, and ID represents the D peak intensity of Raman spectrum at 1350±100 cm-1. The higher the graphitization degree of the carbon on the surface of the positive electrode active material, the higher the proportion of the carbon with graphite structure in the positive electrode film layer, and the more easily the particles slip by means of the carbon structure with high graphitization degree in the coating material, so as to improve the compaction density of the electrode sheet.

[0015] In any embodiment, the median value L50 of sphericity in the cumulative distribution curve of sphericity number area of the particles obtained from the section of the positive electrode film layer along the thickness direction of the electrode sheet is 0.65-0.85, which can be 0.70-0.80. The median value L A50 of sphericity is within the above range, the particles are approximately spherical, and the particles are easy to slip under the action of external force, so as to further improve the compaction density of the electrode sheet and improve the energy density of the battery. A50

[0016] In any embodiment, the median value R50 of roughness in the cumulative distribution curve of roughness area of the particles obtained from the section of the positive electrode film layer along the thickness direction of the electrode sheet is 0.92-0.96. The median value R A50 of roughness is within the above range, the surface of the particles is relatively smooth, and the friction between the particles is relatively small, so as to be easy to slip under the action of external force, further improve the compaction density of the electrode sheet, and improve the energy density of the battery. A50

[0017] In any embodiment, the mass content of carbon element is 0.8%-1.8% based on the total mass of the positive electrode active material, which can be 0.90%-1.50%. The positive electrode active material has a relatively low content of carbon coating compared with the lithium-containing phosphate positive electrode active material in the prior art, so as to further improve the load of the lithium-containing phosphate in the positive electrode sheet and improve the energy density of the lithium ion secondary battery.

[0018] ​​In any embodiment, the lithium-iron antisite defect concentration of the positive electrode active material is 0.1%-1.5%, optionally 0.3%-1.0%. In the preparation and cycling process, there are inevitably certain lithium vacancies in the crystal structure of the positive electrode active material. The lithium vacancies not only cause the oxidation of ferrous ions to ferric ions, but also induce the partial migration of ferric ions to lithium sites to form lithium-iron antisite defects, block the one-dimensional diffusion channel of lithium ions, and have an adverse effect on the solid-phase transmission of lithium ions. The positive electrode active material in the embodiment has a low lithium-iron antisite defect, which is beneficial to the uniform transmission of lithium ions in the solid phase, and further improves the kinetic performance of the lithium ion secondary battery.

[0019] In any embodiment, the lithium-containing transition metal phosphate particle comprises a component having the following general formula: Li m Fe x P y O j Q q wherein Q comprises 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, Br, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, 0

[0020] The selection of an appropriate modification element Q can improve the lattice change rate of the positive electrode active material in the process of lithium extraction, reduce the oxygen activity on the surface of the particle, improve the structural stability of the material, and further improve the specific capacity level of the material in the cycling process, thereby further improving the cycle stability of the lithium ion secondary battery.

[0021] In any embodiment, the positive electrode active material comprises one or more of lithium iron phosphate and doped and coated modified materials thereof.

[0022] In any embodiment, the positive electrode active material comprises titanium elements, and the mass content of the titanium elements is 2000-6000 ppm based on the total mass of the positive electrode active material. The high addition amount of titanium elements does not form harmful impurity phases that have a negative impact on the energy density and kinetic performance of the battery. Although the reason is not clear, it is speculated that the titanium elements, together with phosphate and other elements (e.g., lithium elements), form a fast ion conductor, which instead has a promoting effect on the kinetic performance of the battery.

[0023] In any embodiment, the tap density of the powder of the positive electrode active material is 0.70-1.50 g / cm 3 -1.50g / cm 3 , optionally 0.70-1.20 g / cm 3 -1.20g / cm 3 .

[0024] The positive electrode active material in the embodiments of the present application has a limited effective grading formed by itself, has a relatively low powder tap density, and is compressed under external force to fill the gap between particles to increase the powder compaction density, by virtue of the high integrity and density of the carbon coating material of the positive electrode active material in the positive electrode film layer.

[0025] In any embodiment, the powder compaction density of the positive electrode active material under a pressure of 3T is 2.50 g / cm 3 -2.70 g / cm 3 , and optionally 2.52 g / cm 3 -2.68 g / cm 3 .

[0026] Although the area ratio of particles with a particle size greater than or equal to 1.5 μm in the positive electrode film layer is low, the positive electrode film layer has a low iron elution rate, indicating that the carbon coating material on the surface of the positive electrode active material has high density and integrity, so that high compaction density can still be achieved 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.

[0027] In any embodiment, the powder resistivity of the positive electrode active material under a pressure of 8 MPa is 0.5 Ω·cm-30.0 Ω·cm, and optionally 2.0 Ω·cm-20.0 Ω·cm. The carbon coating material on the surface of the positive electrode active material has high integrity and density, so that the coated structure facilitates rapid conduction of electrons between particles, so that the positive electrode active material has a low powder resistivity, which is beneficial to improve the solid-phase transmission rate of electrons and further improve the kinetic performance of the battery.

[0028] In any embodiment, the discharge gram capacity of the positive electrode active material under a discharge rate of 1C is 135 mAh / g-150 mAh / g. The positive electrode active material has a high discharge gram capacity at a rate of 1C, indicating that it has good charge and discharge capacity, which is beneficial to improve the kinetic performance of the battery.

[0029] In any embodiment, the discharge capacity ratio η of the positive electrode active material discharged to 3.2 V is ≥85%, and the η is defined as follows: at room temperature, a button cell containing the positive electrode active material is charged and discharged twice in a voltage range of 2.0 V-3.75 V at a rate of 0.1C, and then charged and discharged once at a rate of 1C, in the charge and discharge test at a rate of 1C, the capacity value at a discharge voltage of 3.2 V is extracted as C1, and the capacity value at a discharge voltage of 2.0 V is extracted as C2, η=C1 / C2, wherein the charging process includes constant-voltage charging, constant-voltage 3.75 V, and constant-voltage cutoff current 50 μA.

[0030] A high proportion of the discharge capacity of the positive electrode active material used in the lithium ion secondary battery discharged to 3.2V means that the positive electrode active material has good kinetic performance. At the same time, a 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 to maintaining good power performance.

[0031] In any embodiment, there is a discharge platform in the 0.1C discharge curve of the button cell containing the positive electrode active material in the voltage range of 2.5V-2.9V. This is beneficial to increasing the discharge interval of the battery and improving the energy density of the battery.

[0032] In any embodiment, the positive electrode film layer further comprises a conductive agent, and the mass content of the conductive agent is 0.1%-1.5% based on the total mass of the positive electrode film layer.

[0033] In any embodiment, the positive electrode film layer does not comprise a conductive agent.

[0034] The particles in the positive electrode film layer have high sphericity, so the particles can be closely packed in rolling, and the contact between the particles is good, so that the electronic conductivity between the particles inside the positive electrode film layer is good, which can reduce or even eliminate the use of a conductive agent in the positive electrode film layer, which is beneficial to further increasing the loading capacity of the positive electrode active material and improving the energy density of the lithium ion secondary battery.

[0035] In any embodiment, the positive electrode film layer further comprises a binder, and the mass content of the positive electrode active material is 95.5%-99.5%, optionally 96.5%-99.5%, and the mass content of the binder is 0.5%-3% based on the total mass of the positive electrode film layer.

[0036] The mass content of the positive electrode active material and the mass content of the binder in the above range can effectively increase the active material loading capacity in the unit volume of the positive electrode film layer, maintain good internal adhesion, reduce the probability of problems such as powder falling, swelling and cracking, and improve the energy density of the secondary battery while taking into account the safety performance.

[0037] In any embodiment, the single-sided area density of the positive electrode film layer is 300mg / 1540mm 2 -450mg / 1540mm 2 The positive electrode film layer with the area density in the above range can help to improve the energy density of the lithium ion secondary battery.

[0038] In any embodiment, the compaction density of the positive electrode film layer of the lithium ion secondary battery in the full discharge state is 2.51g / cm 3 -2.73g / cm 3 .

[0039] In any embodiment, the compaction density of the positive electrode film layer is 2.55 g / cm 3 -2.70 g / cm 3 .

[0040] The compaction density of the positive electrode film layer within the above range is conducive to improving the energy density of the lithium ion secondary battery.

[0041] In any embodiment, the positive electrode film layer satisfies at least one of the following conditions: (1) the compaction density of the positive electrode film layer is 2.51 g / cm 3 -2.73 g / cm 3 , the porosity of the positive electrode film layer in the section along the thickness direction of the electrode sheet is 10%-22%; (2) the compaction density of the positive electrode film layer is 2.55 g / cm 3 -2.70 g / cm 3 , the porosity of the positive electrode film layer in the section along the thickness direction of the electrode sheet is 10%-20%.

[0042] The lower the porosity in the section of the positive electrode film layer, on the one hand, means that the size grading of the particles in the positive electrode film layer is more optimal, and the compaction density is high, and on the other hand, under the same grading and roll pressure, if the porosity is low, it means that the particles are easy to slide relative to each other, thereby reducing the risk of overpressure and stress concentration of the film layer, further reducing the probability of delamination of the positive electrode film sheet during long cycle, and being conducive to improving the long cycle performance of the battery.

[0043] In any embodiment, the positive electrode sheet includes a primer layer, the primer layer being arranged between the positive electrode film layer and the positive electrode current collector; the primer layer satisfies at least one of the following conditions: (1) the primer layer includes carbon-based particles, the distribution density of carbon-based particles with a particle size greater than 100 nm in the primer layer is ≤10 pcs / 10 μm; (2) the compaction density of the positive electrode sheet in the full discharge state is greater than or equal to 2.4 g / cm 3 , the single-sided thickness of the primer layer is 1 μm-4 μm; (3) the compaction density of the positive electrode sheet in the full discharge state is greater than or equal to 2.5 g / cm 3 , the single-sided thickness of the primer layer is 2 μm-4 μm.

[0044] The primer layer is conducive to improving the conductivity and adhesion of the positive electrode film layer and the current collector, reducing the delamination of the positive electrode film layer and the current collector during the cycle process, and improving the dynamic performance of the battery.

[0045] With the increase of the compaction density of the pole piece, the extrusion effect of the lithium-containing phosphate material with large particles (for example, the particle size is greater than 1 μm) in the positive electrode film layer on the undercoat layer is more significant. Therefore, stress concentration is prone to occur at the large particle sites, and even damage the current collector by penetrating through the undercoat layer. Increasing the thickness of the undercoat layer is beneficial to improve the stress concentration phenomenon in the pole piece, and further improve the ultimate compaction density of the pole piece.

[0046] The second aspect of the present application provides a battery device comprising the lithium ion secondary battery of the first aspect of the present application.

[0047] The third aspect of the present application provides a power utilization device comprising at least one of the lithium ion secondary battery of the first aspect of the present application and the battery device of the second aspect of the present application.

[0048] The fourth aspect of the present application provides a preparation method of a positive electrode active material, comprising: obtaining a mixed raw material comprising a carbon source, a lithium source, an iron source, and a phosphorus source, the carbon source comprising a polymer carbon source, the mass content of trivalent iron elements in the iron source being less than or equal to 0.08%, and the molar ratio of lithium to iron in the mixed raw material being greater than or equal to 1 and less than or equal to 1.05; obtaining a mixed slurry after grinding; obtaining a precursor powder after drying the mixed slurry; sintering the precursor powder, and obtaining a positive electrode active material after crushing; the sintering comprises a first temperature and a second temperature, and the second temperature of the sintering is 750-800°C.

[0049] The polymer carbon source has a relatively low decomposition temperature and graphitization temperature, so that the carbon coating material on the surface of the positive electrode active material can be decomposed to form a carbon layer at a lower sintering temperature, hinder the growth and sintering of the lithium-containing transition metal phosphate crystal grains, and be beneficial to reducing the particle size of the positive electrode active material. Controlling the mass content of trivalent iron elements is helpful to improving the uniformity and consistency of the carbon coating material. The preparation method provided in the embodiments of the present application controls the carbon source, iron source, and sintering parameters, on the one hand optimizes the quality of the carbon coating, and on the other hand improves the particle size distribution of the positive electrode active material and reduces the crystal defects, thereby providing a material basis for the preparation of the positive electrode film layer.

[0050] The fifth aspect of the present application provides a preparation method of a pole piece, the preparation method comprising: sequentially adding a binder, a conductive agent, and the positive electrode active material prepared by the preparation method of the fourth aspect of the present application into a dry mixer, then adding a solvent, stirring, and adjusting the viscosity to obtain a delivery slurry; transferring and coating the delivery slurry to at least one side of a current collector, and then drying and hot pressing to obtain a positive electrode film layer.

[0051] In any embodiment, the hot pressing comprises at least three times of hot rolling, the hot rolling pressure increases successively, the hot rolling pressure is 20-50 tons, 50-70 tons, 70-90 tons successively; the hot rolling temperature is 40-80°C, the pole piece is heated before entering the hot rolling compaction for the first time, the heating temperature is 40-50°C.

[0052] In any embodiment, the stirring comprises pre-stirring and main stirring, the revolution speed of the main stirring is 20-30 rpm, the rotation speed is 1450-1550 rpm.

[0053] The positive electrode active material prepared by the preparation method of the fourth aspect in cooperation with the hot pressing process is advantageous to further reduce the pore porosity of the positive electrode film layer section, improve the ultimate compaction density of the pole piece, and improve the energy density of the battery.

[0054] The above description is only a summary of the technical solutions of the present application, in order to more clearly understand the technical means of the present application, the specific embodiments of the present application can be implemented according to the content of the description, and in order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the following specific embodiments of the present application are described. BRIEF DESCRIPTION OF DRAWINGS

[0055] Figure 1 is a scanning electron microscope image of the section of the positive electrode film layer along the thickness direction of the pole piece of an embodiment of the present application;

[0056] Figure 2 is a schematic diagram of a lithium ion secondary battery of an embodiment of the present application;

[0057] Figure 3 is a decomposition schematic diagram of a lithium ion secondary battery of an embodiment of the present application;

[0058] Figure 4 is a schematic diagram of a battery module of an embodiment of the present application;

[0059] Figure 5 is a schematic diagram of a battery pack of an embodiment of the present application;

[0060] Figure 6 is Figure 5 a decomposition schematic diagram of the battery pack shown in the figure;

[0061] Figure 7 is a schematic diagram of a power consumption device using a lithium ion secondary battery as a power supply of an embodiment of the present application;

[0062] Figure 8 is a pore porosity test diagram of the section of the positive electrode film layer along the thickness direction of an embodiment of the present application.

[0063] Explanation of reference signs:

[0064] 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 case; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0065] Hereinafter, embodiments of the lithium ion secondary battery, the battery device, and the electric device of the present application are specifically disclosed with appropriate reference to the accompanying drawings. However, there are cases where unnecessary detailed explanations are omitted. For example, there are cases where detailed explanations of matters well-known, repeated explanations of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following explanations are provided so that those skilled in the art can fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0066] The "ranges" disclosed in the present application are defined in the form of lower and upper limits, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The ranges defined in this way can be inclusive or exclusive of the end values, and can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to form a range. For example, if the ranges 60-120 and 80-110 are listed for a particular parameter, it is understood that the ranges 60-110 and 80-120 are also contemplated. Furthermore, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise stated, a numerical range "a-b" represents a shorthand manner of describing each and every numerical value that is contained within the range, wherein a and b are both real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed herein, and "0-5" is merely a shorthand manner of describing those numerical combinations. In addition, when it is stated that a parameter is an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and the like.

[0067] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of the present application.

[0068] If not specifically stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of the present application.

[0069] If not otherwise specified, all steps of the present application can be performed in sequence or randomly, preferably in sequence. For example, the method comprising steps (a) and (b) means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method mentioned above can further comprise step (c), which means that step (c) can be added to the method in any sequence, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0070] In the present application, the term "a plurality of" or "a plurality of" means two or more than two.

[0071] Unless otherwise specified, the terms used in the present application have the commonly known meanings as understood by a person skilled in the art.

[0072] Unless otherwise specified, the values of the parameters mentioned in the present application can be measured by various test methods commonly used in the art, for example, the test methods given in the examples of the present application. Unless otherwise specified, the test temperature of each parameter is 25°C.

[0073] The battery mentioned in the examples of the present 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 the present application can comprise a lithium ion secondary battery, a battery module or a battery pack, etc.

[0074] The lithium ion secondary battery is the smallest unit that makes up the battery, which can independently realize the function of charging and discharging. The lithium ion secondary battery can be in the shape of a cylinder, a cuboid or other shapes, which are not limited in the examples of the present application. For example, Figure 2 is a lithium ion secondary battery 5 in the shape of a cuboid as an example.

[0075] The lithium ion secondary battery comprises an electrode assembly and an electrolyte.

[0076] The lithium ion secondary battery can further comprise an outer package, which can be used to package the electrode assembly and the electrolyte. The outer package can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT) and polybutylene succinate (PBS).

[0077] In some embodiments, as Figure 3As shown, the outer package can include a housing 51 and a cover plate 53. The housing 51 can include a bottom plate and side plates connected to the bottom plate, which enclose a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. The electrode assembly 52 is packaged in the receiving cavity. The number of electrode assemblies 52 contained in the lithium ion secondary battery 5 can be one or more, which can be adjusted according to requirements.

[0078] The electrode assembly generally includes a positive electrode sheet and a negative electrode sheet. The negative electrode sheet is an electrode that undergoes a reaction of absorbing or lithiating lithium ions during charging and releasing or delithiating lithium during discharging. The positive electrode sheet is an electrode that undergoes a reaction of releasing or delithiating lithium ions during charging and absorbing or lithiating lithium during discharging.

[0079] When there are multiple lithium ion secondary batteries, the multiple lithium ion secondary batteries are connected in series, in parallel, or in a mixed connection through a busbar. In some embodiments, the battery can be a battery module; when there are multiple lithium ion secondary batteries, the multiple lithium ion secondary batteries are arranged and fixed to form a battery module. In some embodiments, the battery can be a battery pack, which includes a box body and lithium ion secondary batteries, and the lithium ion secondary batteries or the battery module are contained in the box body. In some embodiments, the box body can be part of the chassis structure of the vehicle. For example, part of the box body can be at least part of the floor of the vehicle, or part of the box body can be at least part of the cross beam and the longitudinal beam of the vehicle.

[0080] In some embodiments, the battery can be an energy storage device. The energy storage device includes an energy storage container, an energy storage cabinet, etc.

[0081] In some embodiments, the lithium ion secondary battery can be assembled into a battery module, and the number of lithium ion secondary batteries contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module. Figure 4 FIG. 4 is a schematic view of a battery module 4 as an example. As shown in FIG. 4, the battery module 4 includes a plurality of lithium ion secondary batteries 5. Figure 4 As shown in the battery module 4, the plurality of lithium ion secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, other arbitrary arrangements are also possible. Further, the plurality of lithium ion secondary batteries 5 can be fixed by fasteners.

[0082] Optionally, the battery module 4 can further include a housing having a receiving space, and the plurality of lithium ion secondary batteries 5 are contained in the receiving space.

[0083] In some embodiments, the above-mentioned battery module 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.

[0084] Figure 5 andFigure 6 is a schematic diagram of a battery pack 1 as an example. As shown in Figure 5 and Figure 6 shown, the battery pack 1 can include a box body and a plurality of battery modules 4 arranged in the box body. The box body includes an upper box body 2 and a lower box body 3, the upper box body 2 is used to cover the lower box body 3, and forms a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the box body in any manner.

[0085] Lithium-containing transition metal phosphate materials have been widely used in lithium ion batteries due to their structural stability, good safety, and long cycle life. However, they have low electronic conductivity and low packing efficiency, which makes it difficult to further effectively improve the loading capacity of lithium-containing phosphate in the electrode sheet, and cannot meet the needs of high energy density batteries.

[0086] In order to further improve the energy density of the battery and increase the compaction density of the electrode sheet, the common method in the industry is to increase the particle size distribution in the electrode sheet. In order to improve the particle size distribution, the proportion of large particles needs to be increased. However, research shows that if the proportion of large particles in the electrode sheet exceeds a certain range, the kinetic performance of the battery will be sacrificed. How to obtain a battery that takes into account both energy density and kinetic performance is a technical problem that needs to be solved in the field.

[0087] The first aspect of the present application provides a lithium ion secondary battery, which comprises a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer comprises a positive electrode active material, the positive electrode active material comprises lithium-containing transition metal phosphate particles with carbon-coated material arranged on at least part of the surface, and in the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area proportion of particles with a particle size greater than or equal to 1.5 μm is greater than or equal to 8.0% and less than or equal to 20.0%; the iron dissolution rate of the positive electrode film layer is 658 ppm-1921 ppm.

[0088] In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area proportion of particles with a particle size greater than or equal to 1.5 μm is less than 8%, which is difficult to achieve high compaction density of the positive electrode sheet. In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area proportion of particles with a particle size greater than or equal to 1.5 μm is more than 20%, which is beneficial to the realization of high compaction density of the positive electrode sheet, but reduces the contact area between the electrolyte and the positive electrode active material, hinders the diffusion of lithium ions to the positive electrode film layer, increases the diffusion path of lithium ions in the particles, and causes serious polarization in the local electrode sheet, increases the battery impedance, and significantly deteriorates the kinetic performance of the battery.

[0089] The area ratio of the particles with a particle size greater than or equal to 1.5 μm in the positive electrode film layer is greater than or equal to 8.0% and less than or equal to 20.0%, which can reduce the significant short board effect caused by large-size particles, is beneficial to keeping the battery impedance at a low level, and improves the kinetic performance of the battery, but this will limit the further improvement of the compaction density of the pole piece. In the embodiments of the present application, the iron dissolution rate of the positive electrode film layer is controlled to be 658 ppm-1921 ppm, that is, the density and integrity of the carbon coating on the surface of the positive electrode active material are improved, the particles are more easily to slip between each other, and the lattice defects of the lithium-containing transition metal phosphate are reduced, the pressure resistance of the particles is improved, the probability of collapse and rupture of the particles under large roller pressure is reduced, and the compaction density of the pole piece is improved; at the same time, the complete and dense carbon coating material is beneficial to improve the electrical contact between the positive electrode active materials and improve the conductivity of the positive electrode active material, and reduce the polarization of the positive electrode active material; the low content of lattice defects is beneficial to the smooth transmission of lithium ions and the improvement of the lithium ion transmission rate of the positive electrode active material, thereby realizing the balance between the kinetic performance and the energy density of the battery.

[0090] The lithium-containing transition metal phosphate refers to a phosphate material containing lithium and transition metal elements, which can be detected by any known method in the art. For example, it can be detected by X-ray diffractometer (XRD) and energy spectrum analyzer.

[0091] The carbon coating material arranged on at least part of the surface of the lithium-containing transition metal phosphate can be detected by any known method in the art. As an example, the carbon coating material arranged on at least part of the surface of the lithium-containing transition metal phosphate can be observed by transmission electron microscopy and energy spectrum analyzer.

[0092] In the present application, the term "particle" refers to a particle having a recognizable complete boundary in the field of view of the positive electrode film layer under a certain magnification, for example, 10 thousand times. Defects and scratches can exist inside the particle, but the complete boundary sufficient to divide the particle cannot be recognized inside the particle.

[0093] In some embodiments, the area ratio of the particles with a particle size greater than or equal to 1.5 μm in the section of the positive electrode film layer along the thickness direction of the pole piece is greater than or equal to 8.0% and less than or equal to 20.0%.

[0094] In some embodiments, the area ratio of the particles with a particle size greater than or equal to 1.5 μm in the section of the positive electrode film layer along the thickness direction of the pole piece is optionally 8.0%, 9.0%, 10.0%, 11.0%, 12.0%, 13.0%, 14.0%, 15.0%, 16.0%, 17.0%, 18.0%, 19.0%, 20.0%, or any numerical range between any two of them.

[0095] The specific method for particle identification in the cross-section of the positive electrode film along the electrode thickness direction is as follows: The positive electrode film is cut along the electrode thickness direction using an argon ion beam (for example, a Leica EM TIC 3X CP can be used; operating voltage: 6kV; operating time: 6h). After exposing the cross-section, a scanning electron microscope (SEM) is used (for example, a Hitachi SU8230 can be used; operating voltage: 3kV; beam current: high; probe model: U (LA100); working distance <5mm) to observe the cross-section of the positive electrode film along the electrode thickness direction. Images are acquired in secondary electron mode at a non-edge position in the cross-section of the positive electrode film 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 1The Cellpose plug-in software in the image is used to identify the particles, and manual correction is performed on this basis. Image J is used to read and count the data. The specific method of identifying particles by using the Cellpose plug-in software is as follows: set the segmentation diameter parameter (diameter in the Segmantation module) to 15 pixels, click "run cyto3" to identify the particles, and then manually identify the particles in the image that are not identified by the software or not completely identified by the software or have errors in identification. The particles in the image that are not identified by the software or not completely identified by the software or have errors in identification mainly include the following: 1. Due to the large size of the particles or the scratches on the surface of the particles, the particles cannot be identified or cannot be completely identified; 2. During the argon ion beam cutting process, scratches will be generated on the surface of the particles, and the software may misjudge the scratches as the boundaries of the particles during the identification process, thereby generating identification errors; 3. Due to the small size of the particles, the particles are not successfully identified; 4. The particles are located at the edge of the electron microscope field of view, the inside of the particles is penetrated by the edge, the morphology cannot be completely displayed, and the local instead of the whole is identified to cause identification errors. For the particles that are not identified or have identification errors, manual calibration is performed, and the specific process is as follows: delete the particles located at the edge of the scanning electron microscope that cannot be completely displayed; judge whether there are cracks in the inside of the particles that are not identified or have identification errors, if there are no cracks in the inside of the particles, judge that it is a particle, and manually identify it according to the particle boundary observed by the human; in response to the cracks in the inside of the particles, judge whether the cracks penetrate the particles, if the cracks do not penetrate the particles, judge that it is a particle, and manually identify it; in response to the cracks penetrating the particles, judge whether the cracks are linear or irregular; in response to the cracks being irregular, judge that it is the boundary between the particles, and divide the particles along the boundary; in response to the cracks being linear, perform contrast; in response to the contrast being not obvious and having no crack feeling, judge that it is a scratch, and identify it as a particle; in response to the contrast being strong and having a crack feeling, judge that it is the boundary between the particles, and identify it as two particles. After manual identification, delete the information unrelated to the particles in the image automatic processing process, that is, the determination and identification of the particles in the image are completed.

[0096] The specific method for statistically analyzing the particle area in the cross-section along the thickness direction of the positive electrode film is as follows. After particle identification and labeling, the image is imported into ImageJ software for analysis. The scale is set based on the scanning electron microscope image. The particle size in the image is analyzed using the "Feret Diameter," "Area," "Round," and "Solidity" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter represents the maximum spacing between all parallel lines in the two-dimensional projection of the particle, thus characterizing the particle size; the "Area" parameter represents the pixel area of ​​the particle. Since particles smaller than 50nm have a large error in the statistical process and are difficult to accurately identify, and the particle size of conductive agents is generally smaller than 50nm, which will introduce a large error into the statistical results, particles smaller than 50nm are not counted in the particle size statistics of this application, and the particle statistics corresponding to AR, Round, or Solidity values ​​displayed as "NaN" are deleted. Following the above method, to ensure a statistically significant sample size, at least 10 non-overlapping scanning electron microscope (SEM) images are acquired for each electrode. The areas of at least 5000 particles are counted. The sum of the "Area" parameters for particles with a diameter greater than or equal to 1.5 μm and the sum of the "Area" parameters for all particles are calculated, and these are respectively used as the area of ​​particles with a diameter greater than or equal to 1.5 μm and the total area of ​​the counted particles. The sum of the areas of particles with a diameter greater than or equal to 1.5 μm divided by the total area of ​​the counted particles is used as the percentage of the area of ​​particles with a diameter greater than or equal to 1.5 μm in the cross-section of the positive electrode film along the electrode thickness direction.

[0097] 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 area distribution within it.

[0098] During the compaction process, the positive electrode film undergoes compaction along its thickness. Therefore, compared to the surface of the positive electrode film, the cross-section along the thickness direction better reflects the actual compaction status of the particles within the film on a spatial scale. In the cross-section along the thickness direction, the area ratio of particles with a diameter greater than or equal to 1.5 μm directly reflects the ratio of the area of ​​some particles in that size range to the total area of ​​all particles, indicating the distribution of particles in that size range.

[0099] It can be understood that the particles in the section of the positive electrode film layer along the thickness direction of the electrode sheet, especially the particles of 50 nm or more, are mainly derived from the positive electrode active material. Therefore, the embodiments of the present application can accurately and objectively reflect the distribution of lithium-containing transition metal phosphate particles in the positive electrode film layer in the electrode sheet by observing and counting the particle area in the section of the positive electrode film layer.

[0100] In the prior art, Malvern laser diffraction method is usually used to count the particle size of the positive electrode active material. However, the applicant's research shows that because lithium-containing phosphate is prone to agglomeration, the test results obtained by Malvern laser diffraction method according to the principle of laser scattering are often the particle size of the particle agglomerates, and cannot truly reflect the particle size of the particles in the positive electrode active material, nor can they reflect the dispersion state of the positive electrode active material in the film layer, because the dispersion degree of the positive electrode active material in the film layer will be improved in the process of film forming roller pressing. The test results obtained by Malvern laser diffraction method are closely related to the particle size, specific surface area and agglomeration degree of the positive electrode active material. Therefore, the particle size obtained by Malvern laser diffraction method cannot be equivalent or analogized to the particle size obtained by the embodiments of the present application.

[0101] The skilled in the art can realize the regulation of the particle size of the particles by any known process. As an example, by regulating the temperature and time in the process of preparing the positive electrode material, the growth rate and time of the positive electrode material are controlled. The mechanical force of crushing and ore dressing process is used to process the raw materials to the target particle size distribution range, so as to realize the adjustment of the particle size; the particle size separation of the particle system is carried out by using screening and grading equipment, so as to obtain the required particle size proportion; by accurately controlling the feeding rate, the residence time and stress state of the particles in the equipment are adjusted, which is also helpful to realize the regulation of the particle size.

[0102] In some embodiments, the iron dissolution rate of the positive electrode film layer is 658 ppm-1921 ppm.

[0103] In some embodiments, the iron dissolution rate of the positive electrode film layer can be selected as 658 ppm, 700 ppm, 800 ppm, 890 pm, 900 ppm, 1000 ppm, 1058 pm, 1076 pm, 1100 ppm, 1143 pm, 1200 ppm, 1236 pm, 1300 ppm, 1311 pm, 1384 pm, 1349 pm, 1400 ppm, 1485 pm, 1500 ppm, 1531 pm, 1600 ppm, 1700 ppm, 1800 ppm, 1921 ppm or any numerical range between any two of them.

[0104] The iron dissolution rate of the positive electrode film layer can be tested by methods known in the art. As an example, the electrode tab is disassembled from the battery, washed, and then formed into a small disc with a diameter of 14 mm. Multiple small disc samples are taken so that the total mass of the samples is about 5 g. The samples are added to 100.3 g of an ascorbic acid solution with a mass concentration of 0.3% (the solvent is ultrapure water). After stirring at a speed of 500 revolutions per minute for 305 minutes, the solution is quickly drawn into a 5 mL syringe. The solution is filtered into a test tube using a filter head with a pore size of 0.45 μm. The supernatant is drawn into a 1 mL volume using a pipette, and then diluted 50 times in a glass volumetric flask. The iron element concentration in the solution is tested using an inductively coupled plasma optical emission spectrometer (ICP-OES). The iron dissolution rate of the positive electrode film layer is calculated using the formula: [(ICP test iron element concentration x solution volume / involved constant volume solution mass) x 100.3 g / (small disc electrode tab mass - small disc current collector mass)] with a solution volume of 50 mL and an involved constant volume solution mass of 1 g. Preferably, the current collector mass of the small disc is obtained by multiplying the thickness of the small disc by the area by the density. The thickness of the small disc can be measured by a thickness gauge to measure the thickness of the current collector in the uncoated area. It can be understood that although the coated area causes the current collector to spread during compaction, resulting in a slight decrease in thickness compared to the uncoated area, the decrease is negligible and does not significantly affect the test results. More preferably, when the current collector is an aluminum foil, the density is 2.7 g / cm 3 .

[0105] A person skilled in the art can control the iron dissolution rate of the positive electrode film layer by any known process. As an example, the iron dissolution rate of the positive electrode film layer can be controlled by adjusting the surface coating quality of the positive electrode material, the temperature, time, and pressure during preparation. In addition, during the use of the battery, the battery design, the content of oxidizing agents in the electrolyte, the battery operating temperature, and the battery charge and discharge intensity can also affect the iron dissolution rate of the positive electrode film layer. The iron dissolution rate of the positive electrode film layer is mainly derived from the lithium-containing transition metal phosphate positive electrode active material in the positive electrode film layer, which can indirectly reflect the completeness and density of the carbon coating on the surface of the positive electrode active material. The lower the iron dissolution rate, the less likely the iron ions are to be released from the carbon coating material after acid dissolution, i.e., the more complete and dense the carbon coating material on the surface of the positive electrode active material. An iron dissolution rate within the above range indicates that the positive electrode active material has a relatively complete and dense carbon coating material, which can improve the electrical contact between the positive electrode active materials, improve the conductivity of the positive electrode active material, reduce the polarization of the positive electrode active material, and further optimize the kinetic performance of the lithium ion secondary battery. At the same time, the high completeness of the carbon coating material makes the particles easy to slip under stress during rolling, which can simultaneously improve the compaction density of the electrode tab and the energy density of the battery.

[0106] In some embodiments, the iron dissolution rate of the positive electrode film layer is 658 ppm to 1485 ppm.

[0107] The positive electrode material with the iron dissolution rate of the positive electrode film layer in the above range has relatively fewer lattice defects and more complete and dense carbon coating material. Fewer crystal defects are conducive to improving the pressure resistance and easy sliding degree of the particles in the positive electrode film layer under large roller pressure, high coating completeness is conducive to the sliding between the particles, and dense coating is conducive to reducing the space occupancy of the carbon layer, further improving the compaction density of the electrode sheet, and achieving the balance of battery dynamics and energy density.

[0108] In some embodiments, the positive electrode film layer has a median value B of the coating value in the cumulative distribution curve of the coating value B obtained by the laser microscopic confocal Raman spectrometer in the face scanning mode, and the coating value B is 0.35-0.48. 50 In some embodiments, the positive electrode film layer has a median value B of the coating value in the cumulative distribution curve of the coating value B obtained by the laser microscopic confocal Raman spectrometer in the face scanning mode, and the coating value B is 0.35-0.48. P In some embodiments, the positive electrode film layer has a median value B of the coating value in the cumulative distribution curve of the coating value B obtained by the laser microscopic confocal Raman spectrometer in the face scanning mode, and the coating value B is 0.35-0.48. D In some embodiments, the positive electrode film layer has a median value B of the coating value in the cumulative distribution curve of the coating value B obtained by the laser microscopic confocal Raman spectrometer in the face scanning mode, and the coating value B is 0.35-0.48. P In some embodiments, the positive electrode film layer has a median value B of the coating value in the cumulative distribution curve of the coating value B obtained by the laser microscopic confocal Raman spectrometer in the face scanning mode, and the coating value B is 0.35-0.48. -1 In some embodiments, the positive electrode film layer has a median value B of the coating value in the cumulative distribution curve of the coating value B obtained by the laser microscopic confocal Raman spectrometer in the face scanning mode, and the coating value B is 0.35-0.48. D In some embodiments, the positive electrode film layer has a median value B of the coating value in the cumulative distribution curve of the coating value B obtained by the laser microscopic confocal Raman spectrometer in the face scanning mode, and the coating value B is 0.35-0.48. -1 In some embodiments, the positive electrode film layer has a median value B of the coating value in the cumulative distribution curve of the coating value B obtained by the laser microscopic confocal Raman spectrometer in the face scanning mode, and the coating value B is 0.35-0.48.

[0109] In the present application, the coating value B of the positive electrode film layer can be obtained by scanning with a laser microscopic confocal Raman spectrometer. As an example, specifically, a laser microscopic confocal Raman spectrometer (high-precision Renishaw laser microscopic confocal Raman spectrometer) is used, an excitation wavelength of 532 nm is selected, and an appropriate amount of positive electrode film layer is taken to scan the surface thereof or the section along the thickness direction of the electrode sheet. The scanning area is 45 μm x 45 μm, which is divided into 10 x 10 grids, the grid vertices are taken as the test points, the step size is 5 μm, and the total number of scanning points is 100 points, thereby obtaining the B values of different sites and the cumulative distribution curve of the B values of the face scanning area.

[0110] The positive electrode film layer in the present application can be a freshly prepared positive electrode film layer or a positive electrode film layer obtained by disassembling a battery. The surface of the positive electrode film layer obtained by disassembling the battery inevitably has residual electrolyte salt particles. In order to improve the testing accuracy, it is preferred to scan the section of the positive electrode film layer along the thickness direction of the electrode sheet to characterize the coating value of the positive electrode film layer.

[0111] The coating value B of the positive electrode film layer is obtained by the peak intensity ratio of the P peak (P-band) and the D peak (D-band) of the Raman spectrum, the P peak position is 948 ± 100 cm -1 , which represents the phosphate PO4 3- structure; the D peak position is 1350 ± 100 cm -1which represents the disordered structure of the carbon atoms in the structure. Raman spectroscopy is a surface analysis instrument with a detection depth of 10 nm, so the carbon structure peak of the positive electrode film layer under the face scanning mode of the laser microscopic confocal Raman spectrometer exhibits higher intensity than the phosphate structure peak with higher content in the bulk phase.

[0112] The skilled person in the art can control the coating value of the active material particles by any known process. As an example, adjusting the type of carbon source, the amount of carbon source added, the sintering temperature, the sintering time, the sintering pressure, and the sintering atmosphere can all achieve the adjustment of the coating value of the active material particles.

[0113] The coating value B value can indirectly reflect the thickness of the carbon coating material on the surface of the lithium-containing transition metal phosphate particles. The thinner the thickness of the carbon coating material, the higher the relative intensity of the phosphate structure detected in the Raman spectrum, and the higher the coating value of the positive electrode film layer.

[0114] The coating value B value cumulative distribution curve refers to a curve obtained by arranging at least 100 B values in order from small to large, taking the coating value as the horizontal axis, and taking the cumulative number ratio as the vertical axis. B 50 The B value corresponding to the cumulative number ratio of 50% on the vertical axis of the coating value B value cumulative distribution curve.

[0115] In some embodiments, the median of the coating value of the positive electrode film layer B 50 may be 0.35, 0.36, 0.37, 0.38, 0.39, 0.398, 0.4, 0.41, 0.42, 0.43, 0.432, 0.44, 0.443, 0.445, 0.448, 0.449, 0.45, 0.453, 0.459, 0.46, 0.47, 0.48, or any numerical range between any two of them.

[0116] In order to reduce the influence of the extreme value of the coating value caused by the non-particle area in the positive electrode film layer on the test results, the median of the coating value B 50 The thickness of the carbon coating material on the positive electrode active material is evaluated. The median of the coating value of the positive electrode film layer B 50 Within the above range, it indicates that the carbon coating material of the positive electrode active material has a lower thickness, which is beneficial to further reduce the volume occupied by the carbon coating material, further improve the compaction density of the pole piece, and achieve the balance of battery dynamics performance and energy density.

[0117] In some embodiments, the area ratio of particles with a particle size greater than or equal to 5 μm in the cross section of the positive electrode film layer along the thickness direction of the pole piece is 0.

[0118] The area ratio of the particles with a particle size greater than or equal to 5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet can be tested in the manner described above. The area ratio of the particles with a particle size greater than or equal to 5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet is taken as the area ratio of the particles with a particle size greater than or equal to 5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet.

[0119] Research shows that the particles with a particle size greater than or equal to 5 μm in the positive electrode film layer can significantly deteriorate the infiltration and diffusion of the electrolyte in the positive electrode film layer, and the area ratio of the particles with a particle size greater than or equal to 5 μm of 0 is conducive to further reducing the internal resistance of the battery and improving the dynamic performance of the battery.

[0120] In some embodiments, the area ratio of the particles with a particle size greater than or equal to 1.5 μm and less than 5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet is 8.0-20.0%, which can be 10.0-20.0%.

[0121] In some embodiments, the area ratio of the particles with a particle size greater than or equal to 1.5 μm and less than 5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet can be 8.0%, 9.0%, 9.02%, 10.0%, 11.0%, 12.0%, 12.81%, 13.0%, 14.0%, 14.58%, 14.62%, 14.92%, 15.0%, 15.31%, 15.39%, 15.46%, 16.0%, 16.02%, 16.89%, 17.0%, 18.0%, 19.0%, 20.0%, or any numerical range between any two of them.

[0122] The area ratio of the particles with a particle size greater than or equal to 1.5 μm and less than 5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet can be tested in the manner described above. The area ratio of the particles with a particle size greater than or equal to 1.5 μm and less than 5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet is taken as the area ratio of the particles with a particle size greater than or equal to 1.5 μm and less than 5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet.

[0123] In the process of improving the particle size distribution and increasing the area ratio of large particles, it is inevitable to introduce particles with a particle size greater than or equal to 1.5 μm and less than 5 μm. The area ratio of the particles with a particle size greater than or equal to 1.5 μm and less than 5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet within the above range is conducive to improving the compaction of the electrode sheet, further reducing the hindering effect of large-size particles on the infiltration and diffusion of the electrolyte in the positive electrode film layer, improving the consistency of the diffusion rate of lithium ions in the positive electrode active material particles, reducing local polarization, and improving the dynamic performance of the battery.

[0124] In some embodiments, the area ratio of the particles with a particle size greater than or equal to 1 μm and less than 1.5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet is 15.0%-25.0%, which can be 16.0%-24.0%.

[0125] In some embodiments, the area ratio of the particles with a particle size greater than or equal to 1 μm and less than 1.5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet can be 15.0%, 16.0%, 17.0%, 18.0%, 18.28%, 18.41%, 18.88%, 19.0%, 19.23%, 19.31%, 19.66%, 19.70%, 20.0%, 20.25%, 20.89%, 21.0%, 22.0%, 23.0%, 23.88%, 24.0%, 25.0%, or any numerical range between any two of them.

[0126] The area ratio of the particles with a particle size greater than or equal to 1 μm and less than 1.5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet can be tested in the manner described above. The area ratio of the particles with a particle size greater than or equal to 1 μm and less than 1.5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet is the area of the particles with a particle size greater than or equal to 1 μm and less than 1.5 μm divided by the total area of the particles counted in the section of the positive electrode film layer along the thickness direction of the electrode sheet.

[0127] The area ratio of the particles with a particle size greater than or equal to 1 μm and less than 1.5 μm in the section of the positive electrode film layer along the thickness direction of the electrode sheet within the above range can fill larger gaps in the packing, form a certain "support" structure, reduce the porosity between the particles, and help to enhance the contact between the particles and the overall structural strength, further improve the compaction density of the electrode sheet on the basis of maintaining good kinetic performance of the battery, and improve the energy density of the battery.

[0128] In some embodiments, the area ratio of the particles with a particle size greater than or equal to 200 nm and less than 1500 nm in the section of the positive electrode film layer along the thickness direction of the electrode sheet is 73.0%-80.0%, which can be 55%-65%.

[0129] In some embodiments, the area ratio of the particles with a particle size greater than or equal to 200 nm and less than 1500 nm in the section of the positive electrode film layer along the thickness direction of the electrode sheet can be 73.0%, 74.0%, 75.0%, 75.07%, 75.10%, 75.24%, 75.42%, 75.45%, 75.67%, 75.83%, 76%, 76.21%, 76.66%, 77.0%, 78.0%, 78.57%, 79.0%, 80.0%, or any numerical range between any two of them.

[0130] The area percentage of particles with a diameter greater than or equal to 200 nm and less than 1500 nm in the cross-section of the positive electrode film along the electrode thickness direction can be obtained by testing using the method described above. The area percentage of particles with a diameter greater than or equal to 200 nm and less than 1500 nm in the cross-section of the positive electrode film along the electrode thickness direction is calculated by dividing the sum of the areas of particles with a diameter greater than or equal to 200 nm and less than 1500 nm by the total area of ​​the statistically analyzed particles.

[0131] The area ratio of particles with a diameter greater than or equal to 200 nm and less than 1500 nm in the positive electrode film layer within the above range can further improve the consistency of lithium-ion diffusion rate, thereby improving the dynamic performance of lithium-ion secondary batteries, increasing the utilization rate of active materials, and thus improving the energy density of the battery.

[0132] In some embodiments, the median C of the graphitization degree C in the cumulative distribution curve of the positive electrode film obtained in laser microscopy confocal Raman spectroscopy instrument scanning mode is... 50 The value is 0.95-1.20, optionally 0.98-1.15, and further optionally 1.0-1.10; where the graphitization degree C value is I. G / I D , where I G This indicates that the Raman spectrum is at 1580±100 cm⁻¹ -1 The intensity of peak G at I D This indicates that the Raman spectrum is at 1350±100 cm⁻¹ -1 The intensity of peak D at that location.

[0133] 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.

[0134] The positive electrode film in this application can be either a freshly prepared positive electrode film or a positive electrode film obtained from disassembly of a battery. The surface of a positive electrode film obtained from disassembly of a battery inevitably contains residual electrolyte salt particles. To improve testing accuracy, it is preferable to perform a surface scan on a cross-section of the positive electrode film along the electrode thickness direction to characterize the degree of graphitization of the positive electrode film.

[0135] The graphitization degree C value of the positive electrode film layer is obtained by the peak intensity ratio of the G peak (G-band) and the D peak (D-band) of the Raman spectrum, the G peak position is 1580±100 cm -1 , which represents the sp 2 hybrid structure; the D peak position is 1350±100 cm -1 , which represents the disordered structure, wherein the disorder represents that there is no regular arrangement mode between the carbon atoms in the structure. In the graphite crystal, the carbon atoms in the same layer form covalent bonds with sp 2 hybrid, and the interlayer is van der Waals force, so that the carbon of the graphite structure is easy to slip. Therefore, the C value can represent the graphitization degree of the positive electrode film layer. It can be understood that the graphitization degree in the positive electrode film layer mainly comes from the carbon material in the positive electrode film layer which is subjected to graphitization treatment, i.e. the carbon coated material of the positive electrode active material. Although the carbon nanotube conductive agent with relatively high sp 2 hybrid structure also has relatively high I G / I D , but due to its small addition content and small tube diameter, its addition in the positive electrode film layer shows an extreme value in the Raman face scanning test of the positive electrode film layer, and does not affect the graphitization degree C 50 of the positive electrode film layer. Therefore, the graphitization degree of the positive electrode film layer can also be used to represent the graphitization degree of the positive electrode active material.

[0136] The cumulative distribution curve of the graphitization degree C value is that at least 100 C values obtained are arranged in order from small to large, and a curve is obtained with the graphitization degree as the horizontal axis and the cumulative number ratio as the vertical axis. C 50 is the C value corresponding to the cumulative number ratio of the vertical axis of the cumulative distribution curve of the graphitization degree C value being 50%. The median C 50 of the graphitization degree can reflect the graphitization degree of the particles as a whole in the positive electrode film layer, i.e. the easy slipping degree; compared with the mean value, it can reduce the influence of extreme values in the test process and improve the confidence of the test result.

[0137] The higher the graphitization degree of the surface carbon of the positive electrode active material, the higher the proportion of the graphite structure carbon in the positive electrode film layer, and the easier the particles slip with the help of the carbon structure with high graphitization degree in the coated material, so as to realize the improvement of the electrode sheet compaction density.

[0138] The person skilled in the art can realize the regulation of the graphitization degree of the active material particles by any known process. As an example, the regulation of the carbon source (the carbon source can be selected as a polymer carbon source, such as PEG), the sintering temperature, the sintering time, the sintering pressure, the sintering atmosphere, the nucleation process can all realize the adjustment of the graphitization degree of the active material particles. The higher the graphitization degree of the surface carbon of the positive electrode active material, the higher the proportion of the graphite structure carbon in the positive electrode film layer, and the easier the particles slip with the help of the carbon structure with high graphitization degree in the coated material, so as to realize the improvement of the electrode sheet compaction density.

[0139] In some embodiments, the median C of the graphitization degree C in the cumulative distribution curve of the positive electrode film obtained in laser microscopy confocal Raman spectroscopy instrument scanning mode is... 50 The value can be selected from 0.95, 0.96, 0.97, 0.98, 0.99, 0.993, 1, 1.005, 1.008, 1.01, 1.012, 1.015, 1.02, 1.021, 1.03, 1.032, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.20, or any value range between the two.

[0140] In some embodiments, the median L of the spheroidal density of the particles in the cumulative distribution curve of the particle spheroidal density obtained from a cross-section of the positive electrode film along the electrode thickness direction is... A50 The value is 0.65-0.85, and can be selected as 0.70-0.80, where the sphericity L value is the ratio of the particle area to the fitted circle area.

[0141] The specific method for testing the sphericity of particles in the cross-section of the positive electrode film along the electrode thickness direction is as follows: Particles in the cross-section of the positive electrode film are identified using the method described above in this application. The morphology and area of ​​the particles in the cross-section along the electrode thickness direction of the positive electrode film are analyzed using the "Shape Description" and "Area" analysis functions in ImageJ. According to the software manual (ImageJ User Guide IJ 1.46r), the "Area" parameter obtained from the analysis represents the pixel area of ​​the particle, and the "Round" parameter represents the ratio of the pixel area of ​​the particle to the area of ​​a circle with the fitted major axis as its diameter. The closer the particle is to a sphere, the closer the ratio of the pixel area to the area of ​​the circle with the fitted major axis as its diameter is to 1. Therefore, the "Round" parameter of the particle obtained from the analysis characterizes the sphericity of the particle. The sphericity of at least 5000 particles obtained are arranged in ascending order, and the cumulative distribution curve of the sphericity area of ​​the particles in the positive electrode film is obtained with sphericity as the horizontal axis and the cumulative area ratio as the vertical axis. L A50 This is the L-value of sphericity when the cumulative area under the vertical axis of the cumulative distribution curve of L-values ​​accounts for 50%.

[0142] 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... A50Optionally, 0.65, 0.66, 0.67, 0.68, 0.69, 0.70, 0.705, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.80, 0.81, 0.82, 0.83, 0.84, 0.85, or any numerical range between any two of them.

[0143] The person skilled in the art can realize the regulation of the sphericity of the particles by any process known. As an example, the regulation of the sphericity of the particles can be realized by processes such as grinding, polishing, chemical etching, mechanical stirring, extrusion, coating, granulation, adding surfactants, etc., and adjusting the parameters of each process.

[0144] The median value L of the sphericity A50 The particles in the above range are approximately spherical, and the particles are prone to slip between particles under the action of external force, which can further improve the compaction density of the electrode sheet and increase the energy density of the battery.

[0145] In some embodiments, the median value R of the roughness of the particles in the roughness area cumulative distribution curve of the particles obtained from the section of the positive electrode film layer along the thickness direction of the electrode sheet is A50 The median value R of the roughness of the particles in the roughness area cumulative distribution curve of the particles obtained from the section of the positive electrode film layer along the thickness direction of the electrode sheet is 0.92-0.96.

[0146] The median value R of the roughness of the particles in the roughness area cumulative distribution curve of the particles obtained from the section of the positive electrode film layer along the thickness direction of the electrode sheet is A50 The test method is as follows: the particles in the section of the positive electrode film layer are identified according to the method described above, and the shape of the particles in the section of the positive electrode film layer along the thickness direction of the electrode sheet is analyzed by using the "shape description" analysis function in ImageJ. According to the software manual (ImageJ User Guide IJ 1.46r), the "Solidity" parameter obtained by analysis represents the ratio of the pixel area to the convex area of the particle. Therefore, the "Solidity" parameter of the particles obtained by analysis is used to represent the roughness of the particles. According to the definition, the closer the roughness is to 1, the smoother the particle is. The sphericity of at least 5000 particles is arranged in order from small to large, and the roughness is taken as the horizontal axis and the cumulative area ratio is taken as the vertical axis to obtain the roughness area cumulative distribution curve of the particles in the positive electrode film layer. A50 The median value R of the roughness of the particles in the roughness area cumulative distribution curve of the particles obtained from the section of the positive electrode film layer along the thickness direction of the electrode sheet is

[0147] In some embodiments, the median value R of the roughness of the particles in the roughness area cumulative distribution curve of the particles obtained from the section of the positive electrode film layer along the thickness direction of the electrode sheet is A50 Optionally, 0.92, 0.93, 0.94, 0.95, 0.96, or any numerical range between any two of them.

[0148] The skilled person can control the roughness of the particles by any process known in the art. As an example, the roughness of the particles can be controlled by processes such as grinding, polishing, milling, blasting, plating, etching, and adjusting parameters of each process.

[0149] The median roughness Rz of the particles A50 The particle surface is relatively smooth within the above range, the friction between particles is relatively small, and the particles are prone to slip under external force, which can further improve the compaction density of the pole piece and increase the energy density of the battery.

[0150] In some embodiments, the mass content of carbon element is 0.8%-1.8%, or optionally 0.9%-1.5%, based on the total mass of the positive electrode active material.

[0151] The mass content of carbon element based on the total mass of the positive electrode active material can be measured by methods and equipment known in the art. For example, refer to GB / T 21023-2006 "Determination of total carbon and sulfur content of steel Infrared absorption method after high-frequency induction furnace combustion", and use a Deke HCS infrared carbon and sulfur analyzer for measurement.

[0152] In some embodiments, the mass content of carbon element is 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, or any numerical range between any two of them, based on the total mass of the positive electrode active material.

[0153] The positive electrode active material has a relatively low content of carbon coating compared to the lithium-containing phosphate positive electrode active material in the prior art, which can further improve the loading of lithium-containing phosphate in the positive electrode pole piece and improve the energy density of the lithium ion secondary battery.

[0154] In some embodiments, the lithium iron antisite defect concentration of the positive electrode active material is 0.1%-1.5%, or optionally 0.3%-1.0%.

[0155] XRD data of the sample is collected using an X-ray diffractometer, and phase analysis is performed on the sample. The CIF file of the phase obtained from an open source website is used as an initial model of the crystal structure, including defining the cell parameters, atomic positions, and occupancy probability, etc. In the initial model of the crystal structure, considering the possibility of Fe-Li antisite, the possible Li content in the Fe position and the possible Fe content in the Li position are set, and the initial value is set to 0.1%. The collected XRD data is fitted and refined using the FullProf Suite software. The parameters are refined in the order of background parameters, peak intensity, cell parameters, and peak shape. When the fitting peak shape and the experimental peak shape are best matched, and Rwp is less than 10, the occupancy probability of refined Li and Fe is obtained, which is used as the lithium-iron antisite defect concentration.

[0156] In some embodiments, the lithium-iron antisite defect concentration of the positive electrode active material can be selected as 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, or any numerical range between any two of them.

[0157] A person skilled in the art can realize the regulation of lithium-iron antisite defects of the positive electrode active material by any known process. As an example, the regulation of lithium-iron antisite defects of the positive electrode active material can be realized by regulating the sintering temperature, sintering time, preparation method, raw material metering ratio, etc.

[0158] During preparation and cycling, there will inevitably be a certain amount of lithium vacancies in the crystal structure of the positive electrode active material. Lithium vacancies not only cause the oxidation of ferrous ions to ferric ions, but also induce the partial migration of ferric ions to lithium sites, forming lithium-iron antisite defects, blocking the one-dimensional diffusion channel of lithium ions, and adversely affecting the solid-phase transmission of lithium ions.

[0159] The positive electrode active material in the embodiments of the present application has low lithium-iron antisite defects, which indirectly proves that the positive electrode active material in the embodiments of the present application has low lattice defect content, which is beneficial to reduce the risk of collapse and cracking of the particles under high roller pressure, improve the compaction density of the pole piece, and at the same time realize the uniform transmission of lithium ions in the solid phase, further improve the energy density and kinetic performance of the lithium ion secondary battery.

[0160] In some embodiments, the lithium-containing transition metal phosphate particles include a component having the following general formula:

[0161] Li m Fe x P y O j Q q ,

[0162] 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.

[0163] In some implementations, m can be selected as 0.8, 0.85, 0.9, 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, or a value within a range consisting of any two of the above values.

[0164] In some implementations, x can be selected as 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, or a value within a range of any two of the above values.

[0165] In some implementations, y can be selected as 0.95, 0.96, 0.97, 0.98, 0.99, 1.00, or a value within a range of any two of the above values.

[0166] In some implementations, j can be selected as a value from 3.5, 3.6, 3.7, 3.8, 3.9, 4, or any two of the above values.

[0167] In some implementations, q can be selected as 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, or a value within a range of any two of the above values.

[0168] 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.

[0169] 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.

[0170] In some embodiments, the positive electrode active material includes titanium, and the mass content of titanium is 2000ppm-6000ppm based on the total mass of the positive electrode active material.

[0171] The type and content of elements in the positive electrode active material can be tested by any known method in the art. As an example, the titanium element and content are tested by inductively coupled plasma emission spectrometry according to Appendix C of GB / T 33822-2017.

[0172] The doping of titanium element in the positive electrode active material is beneficial to cause lattice distortion, reduce Li-O bond energy, improve lithium ion transmission rate, and improve the kinetic performance of lithium ion secondary battery. However, in the prior art, the doping content of titanium element in lithium-containing phosphate is often not more than 3000 ppm, because too much titanium element is difficult to completely enter the lithium-containing phosphate phase, and is easy to become a harmful impurity phase remaining on the surface, which has a negative impact on the battery performance.

[0173] The positive electrode active material in the embodiments of the present application has a high content of titanium element, and surprisingly, the high addition amount of titanium element does not form a harmful impurity phase that has a negative impact on the energy density and kinetic performance of the battery. The reason is not yet clear, but it is speculated that the titanium element may form a fast ion conductor together with phosphate and other elements (e.g., lithium element), which in turn improves the kinetic performance of the battery.

[0174] In some embodiments, the tap density of the powder of the positive electrode active material is 0.70 g / cm 3 -1.50 g / cm 3 , and optionally 0.7-1.20 g / cm 3 .

[0175] The tap density of the powder can be tested by a method known in the art. As an example, open the electronic balance, first use the conical flask as the base on the electronic balance, then zero the electronic balance; take the tap cylinder and place it on the conical flask to weigh and record the weight of the cylinder; open the sample bag, first stir the sample in the sample bag 3-5 times with a clean sample spoon, then transfer the sample into the cylinder smoothly; wipe the powder adhered to the surface with a dust-free paper, then place it into the conical flask which has been zeroed; seal the cylinder with a sealing film, place the tap cylinder into the matching instrument rubber ring to ensure that the tap density cylinder is tightly attached to the rubber ring and is perpendicular to the surface of the instrument; set the vibration frequency to 250 times / min on the instrument, and the vibration times to 5000 times, press the button, and vibrate for 20 min; then remove the tap density cylinder, use a flashlight to irradiate the surface of the cylinder, and use visual method to read the highest scale V1 and the lowest scale V2, take the average value V of the two; subtract the mass of the cylinder m0 from the mass of the cylinder and sample m1 to obtain the mass of the powder m, and obtain the tap density of the sample by the density formula p=m / v.

[0176] In some embodiments, the tap density of the powder of the positive electrode active material is optionally 0.70 g / cm 30.80 g / cm 3 0.90 g / cm 3 1.00 g / cm 3 1.10 g / cm 3 1.20 g / cm 3 1.30 g / cm 3 1.40 g / cm 3 1.50 g / cm 3 or any numerical range between any two of the above values.

[0177] The effective grading of the positive electrode active material autonomously formed in the embodiments of the present application is limited, and the tap density of the powder is relatively low. With the aid of the high integrity and compactness of the carbon coating material of the positive electrode active material in the positive electrode film layer, the inter-particle voids are easily compressed under external force to increase the powder compaction density.

[0178] In some embodiments, the powder compaction density of the positive electrode active material under 3T pressure is 2.50 g / cm 3 -2.70 g / cm 3 , and can be 2.52 g / cm 3 -2.68 g / cm 3 .

[0179] In the present application, the term "powder compaction density" refers to the density of the compaction body with certain density and strength formed during the external force compression process, with the movement and deformation of the powder, the filling of larger voids, the increase of inter-particle contact area, the generation of inter-atomic attractive force, and the enhancement of inter-particle mechanical bonding, with the unit of g / cm 3 .

[0180] The powder compaction density of the positive electrode active material can be measured by methods and devices known in the art. For example, GB / T 24533-2009 can be referred to for measurement by using a compaction density instrument. Specifically, a certain amount of positive electrode active material is placed on a compaction special mold (the mold diameter is known), and each of the upper and lower hollow centers in the middle of the mold has a metal disc. The positive electrode active material is placed between the metal discs, and a metal cylinder is placed on top. The mold is placed on the compaction density instrument, and the bottom area of the mold is 1.327 cm 2 . The pressure is set to 3T, and the thickness of the positive electrode active material under 3T pressure can be read on the device. The powder compaction density of the positive electrode active material is p = m / v, where v = (S x H), m is the mass of the positive electrode active material, S is the bottom area of the mold, and H is the thickness of the positive electrode active material after compaction.

[0181] In some embodiments, the powder compaction density of the positive electrode active material under 3T pressure can be 2.50 g / cm 32.51 g / cm 3 2.52 g / cm 3 2.53 g / cm 3 2.54 g / cm 3 2.55 g / cm 3 2.56 g / cm 3 2.57 g / cm 3 2.58 g / cm 3 2.59 g / cm 3 2.60 g / cm 3 2.61 g / cm 3 2.62 g / cm 3 2.63 g / cm 3 2.64 g / cm 3 2.65 g / cm 3 2.66 g / cm 3 2.67 g / cm 3 2.68 g / cm 3 2.69 g / cm 3 2.70 g / cm 3 or any numerical range between any two of the above values.

[0182] Although the area ratio of particles with a particle size greater than or equal to 1.5 μm in the positive electrode film layer is low, the positive electrode film layer has a low iron elution rate, which indicates that the carbon-coated material on the surface of the positive electrode active material has high density and integrity, so that high compaction density can still be achieved 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.

[0183] In some embodiments, the powder resistivity of the positive electrode active material is 0.5 Ω·cm-30.0 Ω·cm, and optionally 2.0 Ω·cm-20.0 Ω·cm, under a pressure of 8 MPa.

[0184] The powder resistivity of the positive electrode active material can be measured by methods and devices known in the art. For example, GB / T 33822-2017 can be referred to, and a powder resistivity meter (Suzhou Crystal, ST2722 type) can be used for measurement. Specifically, a certain amount of positive electrode active material (for example, 1 g) is weighed and added to the charging cavity of the powder resistivity meter, a pressure of 8 MPa is applied, and the positive resistivity and the reverse resistivity of the positive electrode active material are measured, and the average value of the two is taken as the powder resistivity of the positive electrode active material.

[0185] In some embodiments, the positive electrode active material has a powder resistivity of 0.5 Ω-cm, 1 Ω-cm, 2 Ω-cm, 3 Ω-cm, 4 Ω-cm, 5 Ω-cm, 6 Ω-cm, 7 Ω-cm, 8 Ω-cm, 9 Ω-cm, 10 Ω-cm, 15 Ω-cm, 20 Ω-cm, 25 Ω-cm, 30 Ω-cm, or any range between any two of the listed values, at a pressure of 8 MPa.

[0186] The carbon coating material on the surface of the positive electrode active material has high integrity and compactness, and therefore, the rapid conduction of electrons between particles is easily achieved by means of the coating structure, so that the positive electrode active material has a low powder resistivity, which is beneficial to improve the solid-phase transmission rate of electrons and further improve the kinetic performance of the battery.

[0187] In some embodiments, the positive electrode active material has a discharge gram capacity of 135 mAh / g-150 mAh / g at a 1C discharge rate.

[0188] In the present application, the positive electrode active material is assembled into a button cell for testing the electrical performance on a blue light tester. At 25±5°C, in a voltage range of 2.0V-3.75V, after 1C constant current charging to 3.75V, pause for 5 minutes, constant voltage charging to the cutoff current 50μA, then 1C constant current discharging to 2.0V. The discharge capacity of the button cell divided by the mass of the positive electrode active material is taken as the discharge gram capacity of the positive electrode active material at room temperature at a 1C discharge rate.

[0189] The preparation and testing process of the button cell is as follows: 2.0g of the positive electrode active material, conductive carbon black, and PVDF are mixed in a mass ratio of 0.9:0.05:0.05, then NMP (N-methyl pyrrolidone) is added, and after being mixed uniformly, a 150μm doctor blade is used for coating, and the coated product is dried at 100°C for 2h, and then pressed to a density of 2.0g / cm 3 -2.2g / cm 3 The positive electrode sheet is pressed, punched into a 14mm diameter disc using a puncher, then weighed and recorded, and the weighed positive electrode sheet is placed in a vacuum drying oven (105°C, 1-12hrs, -90kpa), and after drying, the positive electrode sheet is placed in a glove box, and assembled into a battery in the order of negative electrode shell-nickel mesh-lithium sheet-separator film-positive electrode sheet-positive electrode shell, and 65-87μL (pipette) of electrolyte (electrolyte is a mixed solvent of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) in a volume ratio of 1:1, electrolyte LiPF6) is added dropwise, the negative electrode is on top, and placed in the groove of a sealing machine, and the sealing pressure is 650kg / cm 2 The button cell is taken out with an insulating tweezer and placed in a dust-free bag, the glove box is removed, and placed in a constant temperature room for 3h to obtain the button cell for testing.

[0190] It can be understood that the discharge gram capacity of the positive active material can also be obtained by disassembling the battery, obtaining the positive electrode sheet, and then testing the assembled button cell according to the method described above.

[0191] In some embodiments, the discharge gram capacity of the positive active material at 1C discharge rate can be selected as 135 mAh / g, 140 mAh / g, 142.4 mAh / g, 145 mAh / g, 150 mAh / g, or a numerical range between any two of them.

[0192] The positive active material has a high discharge gram capacity at 1C rate, indicating that it has good charge and discharge capacity, which is beneficial to improve the kinetic performance of the battery.

[0193] In some embodiments, the discharge capacity ratio η of the positive active material discharged to 3.2V is ≥85%, and the η is defined as: at room temperature, the button cell containing the positive active material is charged and discharged at a rate of 0.1C in the voltage range of 2.0V~3.75V twice, and then charged and discharged at a rate of 1C once, in the charge and discharge test at the rate of 1C, the capacity value at the discharge voltage of 3.2V is extracted as C1, and the capacity value at the discharge voltage of 2.0V is extracted as C2, and η=C1 / C2, wherein the charging process includes constant voltage charging, constant voltage 3.75V, and constant voltage cutoff current 50μA.

[0194] In some embodiments, the discharge capacity ratio η of the positive active material discharged to 3.2V is ≥88%.

[0195] The η value of the positive active material can be measured by methods and devices known in the art. As an example, first refer to the method described above to prepare a button cell, and test the electrical performance of the prepared button cell at room temperature on a blue instrument, specifically, the button cell is charged and discharged at a rate of 0.1C in the voltage range of 2.0V~3.75V twice, and then charged and discharged at a rate of 1C once after constant voltage charging to a cutoff voltage. In the charge and discharge test at the rate of 1C, the capacity value discharged from 3.75V to a voltage of 3.2V is recorded as C1, and the capacity value discharged from 3.75V to 2.0V is recorded as C2, and η=C1 / C2.

[0196] In some embodiments, η can be selected as 85%, 86%, 87%, 88%, 88.1%, 89%, 90%, 90.1%, 91%, 91.1%, 92%, 92.2%, 93%, 94%, 94.1%, 94.5%, 95%, 95.1%, or a range or a value in the range composed of any two of the above numerical intervals.

[0197] In some embodiments, the discharge capacity ratio of the positive electrode active material in the freshly prepared lithium ion secondary battery to 3.2V is η≥88%. After the freshly prepared lithium ion secondary battery is subjected to constant current charging and discharging at a rate of 0.1C in a voltage range of 2.0V-3.75V for a period of time, the discharge capacity ratio of the positive electrode active material to 3.2V can be maintained at η≥85%.

[0198] The high discharge capacity ratio of the positive electrode active material in the lithium ion secondary battery of the embodiments of the present application to 3.2V means 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 to maintaining good power performance.

[0199] In some embodiments, there is a discharge platform in the 0.1C discharge curve of the button cell containing the positive electrode active material in a voltage range of 2.5V-2.9V.

[0200] The discharge platform generally refers to a region in which the voltage is relatively stable during the charging and discharging process of the battery. During the discharging process of the battery, the current flows out of the battery, and the voltage of the battery decreases at the beginning, but then enters a relatively stable region, in which the voltage changes little. This stable voltage region is referred to as a discharge platform.

[0201] The button cell can be prepared by disassembling the positive electrode sheet in the lithium ion secondary battery and combining it with lithium metal. It can also be prepared by assembling according to the method described above. In the present application, the positive electrode active material is assembled into a button cell for testing the electrical performance on a blue light tester. After being charged at 0.1C to 3.75V in a voltage range of 2.0V-3.75V, it is paused for 5 minutes, and then charged at constant voltage to a cutoff current of 50μA, and then discharged at 0.1C to 2.0V.

[0202] The discharge curve shows that the standard charging and discharging platform voltage of lithium-containing phosphate is usually between 3.2V and 3.65V. The button cell containing the positive electrode active material in the embodiments of the present application shows a new charging and discharging platform in a voltage range of 2.5V-2.9V, which is beneficial to increasing the discharge interval of the battery and improving the energy density of the battery. At the same time, this also verifies the guess that the positive electrode active material of the embodiments of the present application contains a fast ion conductor.

[0203] In some embodiments, the positive electrode film layer further comprises a conductive agent, and the mass content of the conductive agent based on the total mass of the positive electrode film layer is 0.1%-1.5%.

[0204] In some embodiments, the positive electrode film layer further comprises a conductive agent, and a mass content of the conductive agent can be selected as 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, or a numerical range between any two of them, based on a total mass of the positive electrode film layer.

[0205] In some embodiments, the conductive agent comprises at least one of super-conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0206] The positive electrode active material has a high-density and high-integrity coated carbon material, so that the positive electrode active material has good electronic conductivity, which can reduce the use of conductive agents in the positive electrode film layer, and is conducive to further improving the loading capacity of the positive electrode active material and improving the energy density of the lithium ion secondary battery.

[0207] In some embodiments, the positive electrode film layer does not comprise a conductive agent.

[0208] The positive electrode active material has extremely high electronic conductivity, so that the positive electrode film layer can even not add a conductive agent, which is conducive to further improving the loading capacity of the positive electrode active material and improving the energy density of the lithium ion secondary battery.

[0209] In some embodiments, the positive electrode film layer further comprises a binder, and a mass content of the positive electrode active material is 95.5%-99.5%, which can be selected as 96.5%-99.5%, and a mass content of the binder is 0.5%-3.0%, based on a total mass of the positive electrode film layer.

[0210] In some embodiments, the binder comprises at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylic ester resin.

[0211] In some embodiments, a mass content of the positive electrode active material can be selected as 95.5%, 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, 99%, 99.5%, or a numerical range between any two of them, based on a total mass of the positive electrode film layer.

[0212] In some embodiments, a mass content of the binder can be selected as 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or a numerical range between any two of them, based on a total mass of the positive electrode film layer.

[0213] The mass content of the positive electrode active material and the mass content of the binder are within the above range, which can effectively increase the active material loading in the unit volume of the positive electrode film layer, maintain good internal adhesion, reduce the probability of powder falling, swelling and cracking, and improve the energy density of the secondary battery while considering the safety performance.

[0214] In some embodiments, the single-sided area density of the positive electrode film layer is 300 mg / 1540 mm 2 - 450 mg / 1540 mm 2 .

[0215] In the present application, the single-sided area density of the positive electrode film layer has the meaning known in the art and can be tested by methods known in the art. For example, a single-sided coated and compacted positive electrode sheet (if it is a double-sided coated positive electrode sheet, the positive electrode film layer on one side can be wiped off first) is punched into a small disc with an area of S1, weighed, and recorded as M1. Then the positive electrode film layer of the above weighed positive electrode sheet is wiped off, the weight of the current collector is weighed and recorded as M0. The single-sided area density of the positive electrode film layer is (M1-M0) / S1. In order to ensure the accuracy of the test results, multiple groups (for example, 10 groups) of samples to be tested can be tested, and the average value is calculated as the test result.

[0216] In some embodiments, the single-sided area density of the positive electrode film layer can be selected from 300 mg / 1540 mm 2 , 310 mg / 1540 mm 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 mm 2 , 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 any numerical range between any two of them.

[0217] The positive electrode film layer with the area density within the above range can help to improve the energy density of the lithium ion secondary battery.

[0218] In some embodiments, the lithium ion secondary battery has a compaction density of the positive electrode film layer of 2.51 g / cm3in a full discharge state. 3 - 2.73 g / cm3 3 .

[0219] In some embodiments, the lithium ion secondary battery has a compaction density of the positive electrode film layer of 2.55 g / cm3in a full discharge state. 3 - 2.70 g / cm3 3 .

[0220] In the present application, the full discharge state refers to a state after the battery is placed in a 25°C oven environment, left for 2 h, the battery temperature is kept at 25°C, the battery is discharged at 1 / 3C constant current to 2.5V, and then discharged at 0.1C constant current to 2.0V.

[0221] The compaction density of the positive electrode film layer can be tested by methods known in the art. As an example, the battery is placed in a 25°C oven environment, left for 2 h, the battery temperature is kept at 25°C, the battery is discharged at 1 / 3C constant current to 2.5V, and then discharged at 0.1C constant current to 2.0V, the battery is disassembled, the positive electrode tab is obtained, the residual electrolyte is treated with dimethyl carbonate solvent, the tab is dried, cut into small round pieces with an area of S, the mass W1 is obtained, and the thickness T1 of the positive electrode tab is measured using a micrometer, then the positive electrode film layer of the above weighed tab is wiped off, the mass of the current collector is weighed and recorded as W2, and the thickness T2 of the current collector is measured using a micrometer, then the compaction density PD of the positive electrode film layer is (W1-W2) / [(T1-T2) x S].

[0222] In some embodiments, the lithium ion secondary battery has a compaction density of the positive electrode film layer of 2.51 g / cm3in a full discharge state. 3 , 2.52 g / cm3 3 , 2.53 g / cm3 3 , 2.54 g / cm3 3 , 2.55 g / cm3 3 , 2.56 g / cm3 3 , 2.57 g / cm3 3 , 2.58 g / cm3 3 , 2.59 g / cm3 3 , 2.60 g / cm3 3 , 2.61 g / cm3 3 , 2.62 g / cm3 3 , 2.63 g / cm3 3 , 2.64 g / cm3 3 , 2.65 g / cm3 3 , 2.66 g / cm3 3 , 2.67 g / cm33 2.68g / cm 3 2.69 g / cm 3 2.70 g / cm 3 2.71 g / cm 3 2.72 g / cm 3 2.73 g / cm 3 Or any value in between.

[0223] In some embodiments, the compaction density of the positive electrode film layer after the compaction process is 2.63 g / cm³. 3 -2.85g / cm 3 .

[0224] In some embodiments, the compaction density of the positive electrode film layer after the compaction process can be selected as 2.63 g / cm³. 3 2.64 g / cm 3 2.65g / cm 3 2.66 g / cm 3 2.67 g / cm 3 2.68g / cm 3 2.69 g / cm 3 2.70 g / cm 3 2.71 g / cm 3 2.72 g / cm 3 2.73 g / cm 3 2.74 g / cm 3 2.75g / cm 3 2.76 g / cm 3 2.77 g / cm 3 2.78g / cm 3 g / cm 3 2.79 g / cm 3 2.80g / cm 3 2.81 g / cm 3 2.82 g / cm 3 2.83 g / cm 3 2.84 g / cm 3 2.85g / cm 3 Or the range of values ​​between any two.

[0225] 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.

[0226] In some embodiments, the compaction density of the positive electrode film layer after formation processing is 2.51 g / cm³. 3 -2.73g / cm3 .

[0227] In some embodiments, the compacted density of the positive electrode film layer after the chemical conversion process can be selected from 2.51 g / cm 3 , 2.52 g / cm 3 , 2.53 g / cm 3 , 2.54 g / cm 3 , 2.55 g / cm 3 , 2.56 g / cm 3 , 2.57 g / cm 3 , 2.58 g / cm 3 , 2.59 g / cm 3 , 2.60 g / cm 3 , 2.61 g / cm 3 , 2.62 g / cm 3 , 2.63 g / cm 3 , 2.64 g / cm 3 , 2.65 g / cm 3 , 2.66 g / cm 3 , 2.67 g / cm 3 , 2.68 g / cm 3 , 2.69 g / cm 3 , 2.70 g / cm 3 , 2.71 g / cm 3 , 2.72 g / cm 3 , 2.73 g / cm 3 , or any range of values between any two of them.

[0228] In this application, chemical conversion refers to the process of forming a stable solid electrolyte interface (SEI film) and electrode structure through electrochemical reaction during the first charge-discharge of the battery.

[0229] It can be understood that the compacted density of the positive electrode film layer of the lithium ion secondary battery in the full discharge state is slightly lower than that of the positive electrode film layer after compaction and chemical conversion, accompanied by the rebound of the electrode sheet during the cycle process.

[0230] The compacted density of the positive electrode film layer in the above range is beneficial to improve the energy density of the lithium ion secondary battery.

[0231] In some embodiments, the compacted density of the positive electrode film layer is 2.51 g / cm 3 - 2.73 g / cm 3 , and the porosity of the positive electrode film layer in the section along the thickness direction of the electrode sheet is 10%-22%.

[0232] In some embodiments, the compacted density of the positive electrode film layer is 2.55 g / cm3 -2.70 g / cm 3 In the section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10-20%.

[0233] In some embodiments, the porosity of the positive electrode film layer in the section of the positive electrode film layer along the thickness direction of the electrode sheet can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, or any numerical range between any two of them.

[0234] The porosity of the positive electrode film layer in the section of the positive electrode film layer along the thickness direction of the electrode sheet can be tested in the following way. The section of the positive electrode film layer along the thickness direction of the electrode sheet obtained in the manner described above is imported into ImageJ software, a straight line tool is selected, a ruler length in the picture is marked using a straight line, “Analyze SetScale” is clicked, and a ruler parameter is set in the software according to the ruler length in the picture. A rectangular tool is selected, the picture part outside the ruler area is selected, the selected area is copied using “Image Duplicate”, the picture format is adjusted using “Image Type 8 bit”; “Analyze Set Measurements” is selected, the following five options are selected: “Area”, “Mean gray value”, “Area Fraction”, “Limit to threshold”, and “Feret’s diameter”, “Decimal places” is selected as 3, “Image”-“Adjust”-“Threshold” is selected in turn, 0 and 100 are set in the “Threshold” box selected position, and the porosity data in the section of the electron microscope picture can be exported using the Analyze-Measure function. The porosity picture is obtained using “Image”-“Overlay”-“Flatten”; “Apply” in “Threshold” is clicked, and “Analyze”-“Analyze Particles” is clicked, the four columns on the left are checked, and the porosity statistical data can be obtained.

[0235] As Figure 8As shown, it can be understood that in the embodiments of the present application, the "pores" in the positive electrode film layer section are identified by the picture color difference and the threshold value. The "pores" are not the pore data obtained in the exhaust test, and are mainly used to represent the cross-sectional area between the particles in the positive electrode film layer section. This method is better than the exhaust method because the porosity obtained by the exhaust method is related to the pores between the particles and the pores in the carbon layer coated on the surface of the lithium iron phosphate particles, so it cannot objectively reflect the pores between the particles. The lower the porosity of the positive electrode film layer section tested by this method, on the one hand, means that the particle size distribution of large, medium and small particles in the positive electrode film layer is more optimal, and the compaction density is high, and on the other hand, under the same particle size distribution and roll pressure, if the porosity is low, it means that the particles are easy to slide relative to each other, thereby reducing the risk of overpressure and stress concentration of the film layer, further reducing the probability of positive electrode film delamination in the long cycle process, and helping to improve the long cycle performance of the battery.

[0236] In some embodiments, the positive electrode tab includes a primer layer disposed between the positive electrode film layer and the positive electrode current collector; the primer layer includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the primer layer is ≤10 pcs / 10 μm.

[0237] The carbon-based particles refer to particles with carbon elements as the main component, including but not limited to conductive carbon, carbon black, etc.

[0238] The primer layer is beneficial to improve the conductivity and adhesion of the positive electrode film layer and the current collector, reduce the delamination of the positive electrode film layer and the current collector during the cycle process, and improve the dynamic performance of the battery. In the high compaction density tab of the embodiments of the present application, for example, the compaction density of the positive electrode tab under the full discharge state is greater than or equal to 2.4 g / cm 3 When the current collector is easily damaged in the high pressure compaction process of the tab, and large-size particles are easy to produce pits on the current collector, controlling the distribution density of carbon-based particles with a particle size greater than 100 nm in the primer layer to be ≤10 pcs / 10 μm is beneficial to reduce the probability of damage of the current collector in the high pressure compaction tab, and further improve the limit compaction density of the positive electrode tab.

[0239] The distribution density of carbon-based particles with a particle size greater than 100 nm in the primer layer can be detected by the method described above, by cutting the positive electrode film layer along the thickness direction of the tab with an argon ion beam, taking a scanning electron microscope image or a microscope image, detecting the size of the carbon particles in the primer layer by a statistical method, and counting the number of carbon-based particles with a particle size greater than 100 nm contained in every 10 μm of the primer layer. The counting is not less than 5 times, and the average value is obtained.

[0240] The undercoat layer in the embodiments of the present application can be prepared by any known preparation process, such as pre-separation or centrifugation in the preparation process of carbon-based particles to remove large-particle carbon-based materials, so that the D50 of the carbon-based particles added in the undercoat preparation process is less than 70 nm. V50 at 20-60 nm, D50 V90 at 20-60 nm, D50

[0241] In some embodiments, the positive electrode tab has a compaction density greater than or equal to 2.4 g / cm3in the full discharge state. 3 The single-side thickness of the undercoat layer is 1-4 μm.

[0242] In some embodiments, the positive electrode tab has a compaction density greater than or equal to 2.5 g / cm3in the full discharge state. 3 The single-side thickness of the undercoat layer is 2-4 μm.

[0243] With the increase of the compaction density of the tab, the extrusion of the large-particle lithium-containing phosphate material (e.g., particle size greater than 1 μm) in the positive electrode film layer on the undercoat layer is more significant. Therefore, stress concentration is prone to occur at the large-particle sites, and even damage the current collector by penetrating the undercoat layer. Increasing the thickness of the undercoat layer is beneficial to improve the stress concentration phenomenon in the tab and further increase the ultimate compaction density of the tab.

[0244] The single-side thickness of the undercoat layer can be tested as follows. The positive electrode film layer is cut along the thickness direction of the tab by argon ion beam, and a scanning electron microscope image is taken. In the length direction of the tab, the thickness of the single-side undercoat layer is measured at every 1 m, and the average value is obtained after measuring the thickness of the undercoat layer at 10 points. It should be noted that abnormal points, i.e., the thickness of the undercoat layer is less than 50 nm and greater than 4 m, should be avoided during the measurement. These abnormal points are mainly caused by the extreme fluctuation of the thickness of individual regions due to abnormal stress concentration and extrusion during the compaction process of the tab, and have no statistical significance.

[0245] In some embodiments, the thickness of the positive electrode current collector is less than or equal to 17 μm, which can be 13-15 μm.

[0246] In some embodiments, the thickness of the positive electrode current collector is 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, or any numerical range between any two of them.

[0247] In some embodiments, the positive current collector can employ a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material base material (e.g., a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0248] In some embodiments, the negative electrode tab includes a negative current collector and a negative film layer disposed on at least one side of the negative current collector, the negative film layer having a single-sided areal density of 140 mg / 1540 mm 2 -221 mg / 1540 mm 2 ; and / or a compaction density of the negative film layer is 1.40 g / cm 3 -1.75 g / cm 3 .

[0249] The single-sided areal density and the compaction density of the negative film layer can be tested by a method similar to that of the positive film layer described above.

[0250] The areal density and the compaction density of the negative film layer in the above range are advantageous in improving the energy density of the lithium ion secondary battery.

[0251] In some embodiments, the negative current collector can employ a metal foil or a composite current collector. For example, as the metal foil, a copper foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base material. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material base material (e.g., a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0252] In some embodiments, the negative electrode film layer includes a negative electrode active material. The negative electrode active material can employ a negative electrode active material known in the art for use in a battery. As an example, the negative electrode active material can include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy. The tin-based material can be selected from at least one of elemental tin, tin oxide compound, and tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as a negative electrode active material for a battery can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0253] In some embodiments, the negative electrode film layer can also optionally include a binder. The binder can 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).

[0254] In some embodiments, the negative electrode film layer can also optionally include a conductive agent. The conductive agent can be selected from at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dot, carbon nanotube, graphene, and carbon nanofiber.

[0255] In some embodiments, the negative electrode film layer can also optionally include other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0256] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on a negative electrode current collector, and then drying, compacting, and the like to obtain the negative electrode sheet.

[0257] In some embodiments, the lithium ion secondary battery includes an electrolyte. The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The present application does not have a specific limitation on the type of electrolyte, which can be selected as needed. For example, the electrolyte can be liquid, gel, or all-solid.

[0258] In some embodiments, the electrolyte employs an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0259] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoro oxalato borate, lithium difluoro dioxalato borate, lithium difluoro dioxalato phosphate, and lithium tetrafluoro oxalato phosphate.

[0260] In some embodiments, the solvent can 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, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclobutane sulfone, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0261] In some embodiments, the electrolyte solution can further optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain performance of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, etc.

[0262] In some embodiments, the lithium ion secondary battery further includes a separator. The type of the separator is not particularly limited in the present application, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0263] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the respective layers can be the same or different, and are not particularly limited.

[0264] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be made into an electrode assembly through a winding process or a stacking process.

[0265] In some embodiments, the lithium ion secondary battery can include an outer package. The outer package can be used to package the above-described electrode assembly and the electrolyte.

[0266] In some embodiments, the outer package of the lithium ion secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and as the plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, etc. can be listed.

[0267] The second aspect of the present application provides a battery device, including at least one of the lithium ion secondary battery provided by the first aspect of the present application, a battery module, a battery pack, and an energy storage battery.

[0268] The third aspect of the present application provides a power utilization device, including the lithium ion secondary battery provided by the first aspect of the present application.

[0269] The fourth aspect of the present application provides a preparation method of a positive electrode active material, including: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source, the carbon source including a polymer carbon source, the mass content of trivalent iron elements in the iron source being less than or equal to 0.08%, and the molar ratio of lithium to iron in the mixed raw material being greater than or equal to 1 and less than or equal to 1.05; obtaining a mixed slurry after grinding; obtaining a precursor powder after drying the mixed slurry; sintering the precursor powder, and obtaining a positive electrode active material after crushing; the sintering including a first temperature and a second temperature, and the second temperature of the sintering being 750-800°C.

[0270] The preparation method provided by the embodiments of the present application optimizes the quality of carbon coating on one hand and improves the particle size distribution in the positive electrode active material and reduces crystal defects on the other hand, so that the area ratio of particles with a particle size greater than or equal to 1.5 μm in the section along the thickness direction of the electrode sheet is greater than or equal to 8.0% and less than or equal to 20.0%, and the preparation of a positive electrode film layer with an iron leaching rate of 658-1921 ppm is provided with a material basis.

[0271] In some embodiments, the iron source includes divalent iron, which can be one or more of ferrous oxalate, ferrous carbonate, and ferrous nitrate.

[0272] In the sintering process, the divalent iron source is preferentially decomposed to generate a large amount of ferrous oxide as a nucleation site to generate a nanocrystalline core of lithium-containing transition metal phosphate. At the same time, the polymer carbon source has a relatively low decomposition temperature, and the iron elements located on the surface of the nanocrystalline core will further catalyze the decomposition of the carbon source, so that the carbon coating material on the surface of the positive electrode active material can have a relatively high graphitization degree at a relatively low sintering temperature, thereby reducing the resistivity of the positive electrode active material and improving the compactness and uniformity of the carbon coating material on the surface of the lithium-containing transition metal phosphate. In addition, the uniform deposition of carbon on the surface of the lithium-containing transition metal phosphate will further hinder the growth of lithium-containing transition metal phosphate grains, thereby reducing the probability of the positive electrode active material particles growing into large particles with a particle size greater than 1.5 μm.

[0273] In some embodiments, the particle size D 10 is greater than or equal to 3 μm, the particle size D 50 is 50-80 μm, and the particle size D 90 is less than or equal to 150 μm.

[0274] In the present application, the terms "D 10 ", "D 50 ", and "D 90 " correspond to the particle sizes at 10%, 50%, and 90% of the cumulative particle size distribution of the sample, respectively, as determined by Malvern laser scattering method.

[0275] Controlling the particle size D 10 of ferrous oxalate to be greater than or equal to 3 pm can reduce the proportion of small-particle ferrous oxalate particles and control their reactivity during the grinding process. Controlling the particle sizes D 50 , D 90 of ferrous oxalate helps to uniformly mix the raw materials during the grinding process, to obtain a mixed slurry with consistent components and uniform particle sizes, and to improve the particle size consistency of the prepared lithium-containing transition metal phosphate.

[0276] In some embodiments, the mass content of the trivalent iron element is less than or equal to 0.08%.

[0277] In some embodiments, the mass content of the trivalent iron element can be selected as 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, or a numerical range between any two of them.

[0278] Controlling the mass content of the trivalent iron element helps to improve the uniformity and consistency of the carbon-coated material. Too high a content of the trivalent iron element will preferentially consume the carbon source, resulting in poor consistency in the mass and thickness of the carbon layer between particles, which on the one hand will affect the compaction between particles, and on the other hand will affect the lapping of the conductive network between particles, which is not conducive to effective improvement of the compaction density of the electrode plate and improvement of the kinetics.

[0279] In some embodiments, the lithium source includes one or more of lithium dihydrogen phosphate, lithium phosphate, lithium carbonate, and lithium acetate.

[0280] In some embodiments, the carbon source includes a polymer carbon source, which can be selected as one or more of polyethylene glycol and polyvinyl alcohol.

[0281] In some embodiments, the mass content of the carbon source is 1-4% based on the total mass of the mixed raw materials.

[0282] The polymer carbon source has a relatively low decomposition temperature and graphitization temperature, so that the carbon-coated material on the surface of the positive active material can be decomposed to form a carbon layer at a lower sintering temperature, which hinders the growth and sintering of lithium-containing transition metal phosphate grains, and is conducive to reducing the particle size of the positive active material particles.

[0283] Meanwhile, the polymer carbon source usually has a high molecular weight or a long molecular chain, which is easy to form a stable skeleton structure through cross-linking or orientation in the heat treatment process. The orderliness is preserved in the high-temperature carbonization process, which is beneficial to the directional growth of graphite crystals. Meanwhile, the entanglement and cross-linking between long chains are beneficial to reducing structural defects and reducing lattice disorder caused by chain rupture during carbonization, thereby improving the graphitization degree.

[0284] The organic molecules in the carbon source decompose at high temperatures to release carbon atoms, which can cover and fill the small gaps or defects on the surface of the active material, reducing the surface roughness. The coated material formed by the polymer carbon source has a high graphitization degree and a tight carbon structure, which is beneficial to the optimization of the surface roughness of the positive active material.

[0285] In some embodiments, the weight average molecular weight of the polyethylene glycol is 10000 or less.

[0286] The polyethylene glycol with a weight average molecular weight of less than 10000 has a short carbon chain, which is easy to control the decomposition rate during sintering to form a carbon-coated material with a suitable and uniform thickness.

[0287] In some embodiments, the water content of the polyethylene glycol is less than or equal to 0.5%.

[0288] If the water content in the polyethylene glycol is high, the water may affect the decomposition process, causing incomplete decomposition or uneven decomposition rate during sintering. Excessive water may also cause uneven distribution of molten polyethylene glycol during sintering, affecting the uniformity of the carbon layer, leading to instability or peeling of the carbon-coated material.

[0289] In some embodiments, the water content of the polyethylene glycol can be selected as 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5% or any numerical range between any two of them.

[0290] In some embodiments, the pH of the polyethylene glycol is 5-7.

[0291] The polyethylene glycol with a pH of 5-7 has high stability and will not degrade during mixing, especially under high temperature conditions, which may cause rapid decomposition and affect the coating quality. If the polyethylene glycol is alkaline, it may affect the stability of other ingredients, causing metal ions to dissolve or oxidize, affecting the performance of the final positive active material.

[0292] In some embodiments, the phosphorus source includes one or more of lithium dihydrogen phosphate, phosphoric acid, and ammonium dihydrogen phosphate.

[0293] In some embodiments, the lithium source and the phosphorus source can be the same substance.

[0294] In some embodiments, the iron source comprises ferrous oxalate, the lithium source and the phosphorus source comprise lithium dihydrogen phosphate, and the carbon source comprises polyethylene glycol.

[0295] In some embodiments, the atomic molar ratio of lithium element to iron element in the lithium source and the iron source is 1.00-1.05.

[0296] In some embodiments, the atomic molar ratio of lithium element to iron element in the lithium source and the iron source can be 1.00, 1.01, 1.02, 1.03, 1.04, 1.05, or any numerical range between any two of them.

[0297] When the atomic molar ratio of lithium element to iron element is 1, it belongs to the ideal stoichiometric ratio, which can maintain the best electrochemical performance, optimize the reversible de-intercalation ability of lithium ions in the charging and discharging process, and has good crystal structure stability to improve the cycle life, and can also reduce the probability of the occurrence of impurities. However, in actual production, in order to compensate for the loss of lithium in the sintering process, the molar ratio of lithium to iron element needs to be adjusted to be slightly higher than 1.

[0298] In some embodiments, the slurry further comprises a titanium source, and optionally, the titanium source comprises one or more of titanium dioxide, tetrabutyl titanate, titanium nitrate, and titanate.

[0299] The titanium source often has low surface activity, and the inclusion of the titanium source in the slurry can reduce the activity of the lithium-containing transition metal phosphate precursor, inhibit the particle growth of the lithium-containing transition metal phosphate during high-temperature sintering, and enable the lithium-containing transition metal phosphate to form smaller particles during sintering.

[0300] As a lattice stabilizer, titanium element usually enters the crystal lattice of lithium-containing transition metal phosphate in the form of Ti 4+ , and part of the titanium ions can replace the position of iron ions, making the crystal structure more stable and reducing the possibility of lithium and iron ions translocation, especially at high temperature or large current charging and discharging.

[0301] Meanwhile, the doping of titanium helps to improve the sphericity of the particles and reduce the roughness of the particles, thereby enhancing the overall structural stability of the material.

[0302] In some embodiments, lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide are mixed uniformly in an organic solvent and ground to obtain a mixed raw material.

[0303] The organic solvent can effectively reduce the occurrence of side reactions and improve the purity and consistency of the material. Moreover, the organic solvent has good volatility and is easier to remove in the subsequent drying process, and will not be left inside the material, causing pores in the material and affecting the density and structural stability of the material.

[0304] In some embodiments, the mixed slurry obtained after the grinding comprises at least two ball-milling-magnetic-removing cycles, each of the ball-milling-magnetic-removing independently satisfies one or more of the following conditions:

[0305] (1) the grinding balls of the ball-milling are one or more of zirconia balls, silicon nitride zirconium balls, and ceramic zirconium balls,

[0306] (2) the first ball-milling grinding balls have a diameter of 5 mm-6 mm, and the second ball-milling grinding balls have a diameter of 0.5 mm-0.7 mm;

[0307] (3) the first ball-milling has a rotation speed of 1400 rpm-1600 rpm, and the second ball-milling has a rotation speed of 400 rpm-600 rpm;

[0308] (4) the first ball-milling has a grinding time of 150 min-200 min, and the second ball-milling has a grinding time of 140 min-180 min;

[0309] (5) the magnetic-removing is permanent magnet removing;

[0310] (6) the magnetic-removing has a magnetic-removing strength of greater than or equal to 8000 GS.

[0311] Through the combination of at least two ball-milling-magnetic-removing, large particle materials can be quickly processed, and further refinement can be achieved in a shorter time. In this way, the non-uniform particle size caused by ball-milling can be effectively avoided, the agglomeration phenomenon between particles can be reduced, the electrical conductivity and cycle stability of the battery can be improved, and at the same time, the overall production efficiency can be improved while ensuring the performance of the final product.

[0312] In some embodiments, the volume distribution particle size D v50 of the particles in the mixed slurry is 1.0 μm-4.0 μm.

[0313] In the present application, the term "D V50 " refers to the particle size corresponding to the percentage of 50% of the volume cumulative particle size distribution of the sample obtained by Malvern laser scattering method;

[0314] In some embodiments, the volume distribution particle size D V50 of the particles in the mixed slurry can be 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4.0 μm, or any numerical range between any two of them.

[0315] The volume distribution particle size D V50On the one hand, the activity of the particles can be increased to some extent in the above range, and the positive active material particles with a particle size of 1 μm-1.5 μm can be generated at the same temperature, the compaction density of the electrode sheet and the energy density of the battery are improved; on the other hand, the catalytic decomposition efficiency of the iron element on the surface of the crystal nucleus to the carbon source can be improved, the coating quality of the carbon source is improved, the uniformity of the carbon material coating and the graphitization degree are improved, and the compaction density of the electrode sheet and the energy density of the battery are further improved.

[0316] In some embodiments, drying the mixed slurry to obtain a precursor powder comprises spray drying the mixed slurry to obtain the precursor powder.

[0317] In some embodiments, sintering the precursor powder to obtain the positive active material comprises at least two sintering processes.

[0318] In some embodiments, the first sintering in the at least two sintering processes satisfies one or more of the following conditions:

[0319] (1) the heating rate is greater than or equal to 2ºC / min;

[0320] (2) the holding temperature is 300ºC-400ºC;

[0321] (3) the holding time is 2h-6h.

[0322] In some embodiments, the second sintering in the at least two sintering processes satisfies one or more of the following conditions:

[0323] (1) the heating rate is greater than or equal to 3ºC / min;

[0324] (2) the holding temperature is 750ºC-800ºC;

[0325] (3) the holding time is 8h-15h.

[0326] Using a higher heating rate to quickly heat to the target temperature is beneficial for uniform particle growth and reduces the presence of particles with a particle size greater than or equal to 1.5 μm.

[0327] By controlling the sintering temperature in the first and second sintering process, the sintering diffusion rate can be controlled. At high temperature, the diffusion on the surface of the particle increases, the defects in the particle are repaired, and the crystal lattice is rearranged. Through recrystallization, the defects on the surface of the particle are eliminated, the crystal grain structure of the particle is more ordered, the size of the particle is gradually increased, and the particle surface is smoothed and the particle is developed to a spherical shape. The sintering temperature also affects the graphitization rate of the carbon source. In dynamics, carbon atoms obtain more energy and can overcome the original energy barrier, so that they can rearrange more violently in the crystal lattice. The sintering time affects the degree of reaction. If the sintering time is too short, the diffusion and rearrangement of the lithium-containing transition metal phosphate and the carbon source are not completed. If the sintering time is too long, the particles will abnormally grow, the crystal grains in the particles will coarsen, the material structure will tend to be unstable, the adhesion between the particles will increase, and agglomeration will occur.

[0328] In some embodiments, the product is subjected to jet milling after sintering the precursor to obtain the positive electrode active material.

[0329] In some embodiments, the classification frequency of the jet milling is 18-24 Hz, and the milling gas pressure is 0.45-0.65 MPa.

[0330] The classification frequency in jet milling refers to the working frequency of the classification device in jet milling, which is usually related to the classification efficiency and particle size distribution of the particles. Higher classification frequency will screen the particles in the gas more times, so that larger particles are screened out and smaller particles are left. Higher classification frequency may also increase the number of particle collisions, so that irregular particles are further impacted, making the particle surface smoother and the shape more spherical.

[0331] High gas pressure will cause the particles to be subjected to greater impact force, and the collisions between the particles will be more intense, which will cause the particle surface to be subjected to strong impact and wear, enabling large particles to be crushed into small particles. The collisions between the particles are more intense, and the surface is more likely to be modified, improving the sphericity and surface smoothness of the particles.

[0332] However, too high classification frequency and milling gas pressure will cause the agglomerated particles to be dispersed into primary particles and then further crack and break, affecting the predetermined particle size distribution and making the carbon-coated material incomplete, which will increase iron dissolution, have a negative impact on the slip of the particles in rolling, and increase the contact and reaction between the lithium-containing transition metal phosphate and the electrolyte and other external factors, which is not conducive to the cycle performance and service life of the battery. Therefore, the classification frequency and milling gas pressure of jet milling need to be controlled within a suitable range.

[0333] The fifth aspect of the present application provides a preparation method of a positive electrode tab, which comprises sequentially adding a binder, a conductive agent, and a positive electrode active material prepared by the preparation method of the fourth aspect into a dry mixer, adding a solvent, stirring, and adjusting the viscosity to obtain a delivery slurry; transferring and coating the delivery slurry to at least one side of a current collector, and drying and hot pressing to obtain a positive electrode film layer.

[0334] In some embodiments, the stirring comprises pre-stirring and main stirring, and the main stirring has an orbital speed of 20 rpm-30 rpm and a rotational speed of 1450 rpm-1550 rpm.

[0335] In some embodiments, the hot pressing comprises at least three times of hot rolling, and the hot rolling pressure increases sequentially, and the hot rolling pressure is 20 tons-50 tons, 50 tons-70 tons, and 70 tons-90 tons, respectively; the hot roller temperature is 40°C-80°C, and the tab is heated before entering the hot roller for the first time, and the heating temperature is 40°C-50°C.

[0336] The positive electrode active material prepared by the preparation method of the fourth aspect in combination with the hot pressing process of the present application is beneficial to further reduce the pore porosity of the positive electrode film layer, improve the ultimate compaction density of the tab, and improve the energy density of the battery.

[0337] In addition, the present application also provides an electric device, which comprises at least one of the secondary battery, the battery module, or the battery pack provided by the present application. The secondary battery, the battery module, or the battery pack can be used as a power supply of the electric device, and can also be used as an energy storage unit of the electric device. The electric device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0338] As the electric device, the secondary battery, the battery module, or the battery pack can be selected according to the use requirement thereof.

[0339] Figure 7 The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. as an example. In order to meet the requirement of high power and high energy density of the secondary battery for the electric device, the battery pack or the battery module can be used.

[0340] The device as another example can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and the secondary battery can be used as a power supply.

[0341] Embodiments

[0342] Hereinafter, the examples of the present application will be described. The examples described below are exemplary and are for the purpose of explanation of the present application and cannot be understood as a limitation of the present application. In the examples, the specific techniques or conditions not mentioned are performed according to the techniques or conditions described in the literature in the art or according to the product manual. The reagents or instruments not mentioned by the manufacturer are all conventional products that can be obtained commercially.

[0343] Example 1

[0344] (1) Preparation of positive electrode active material

[0345] Lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol and titanium dioxide were mixed uniformly in methanol and ground to obtain a mixed raw material. The ratio of lithium dihydrogen phosphate and ferrous oxalate was such that the atomic molar ratio of lithium to iron was 1.03; the particle size D 90 of the ferrous oxalate was 100 nm, and the mass content of trivalent iron element was 0.03%.

[0346] The mixed raw material was ball milled in a ball mill for multiple times and de-magnetized to obtain a mixed slurry.

[0347] The mixed slurry was spray dried to obtain a dry precursor powder material, and the appearance of the dry precursor powder material was light yellow and uniform in color.

[0348] The precursor powder material was placed in a sintering furnace, and the temperature was raised from 25°C to a first temperature of 350°C at a rate of 2°C / min under a nitrogen atmosphere and maintained at the temperature for 3h, then raised to a second temperature of 770°C at a rate of 5°C / min, and maintained at the temperature for 10h, and after the end of the process, the temperature was lowered for cooling.

[0349] The obtained material was broken by the method of airflow crushing with a classification frequency of 22Hz and a wind volume of 0.55MPa, to obtain a carbon-coated lithium iron phosphate positive electrode active material.

[0350] The mass content of carbon element in the prepared positive electrode active material was 1.144%, the lithium iron antisite defect concentration was 0.58%, the powder tap density was 1.05g / cm 3 , the powder compaction density under 3T pressure was 2.57g / cm 3 , the powder resistivity under 8MPa was 6.0Ω·cm; the discharge capacity under 1C discharge rate was 142.4mAh / g; there was a discharge platform in the voltage range of 2.5V~2.9V, and the discharge capacity ratio of the 3.2V discharge platform was 91.1%.

[0351] (2) Preparation of positive electrode sheet:

[0352] A positive electrode film layer with a single side density of 350 mg / 1540 cm 2 is prepared by sequentially adding 2.2 wt% of PVDF, 0.8 wt% of conductive carbon black, and 97.0 wt% of a positive electrode active material into a dry mixer, adding N-methyl pyrrolidone, stirring, and adjusting the viscosity. The slurry is transferred and coated onto a bottom coating layer of a current collector aluminum foil, the bottom coating layer including carbon black and PVDF in a mass ratio of 1:1, the distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating layer being ≤10 pcs / 10 μm, and the thickness of the bottom coating layer being 2 μm. After drying and hot pressing, the positive electrode film layer is obtained. The stirring includes pre-stirring and main stirring, and the main stirring has an orbital speed of 25 rpm and a rotational speed of 1500 rpm.

[0353] The hot pressing process includes three hot roller pressing processes, and the hot roller pressing pressure increases sequentially, being 35 tons, 55 tons, and 75 tons, respectively. The hot roller temperature is 65°C. Before the first time entering the hot roller pressing process, the electrode piece is heated, and the heating temperature is 50°C.

[0354] The compaction density of the electrode piece is the limit compaction density of the electrode piece, and the limit compaction density of the electrode piece is tested according to the method described below. In this embodiment, the limit compaction density of the electrode piece is 2.67 g / cm 3 .

[0355] The positive electrode film layer obtained in the preparation is cut along the thickness direction of the electrode piece, and the statistical results show that the area ratio of particles with a particle size greater than or equal to 1.5 μm and less than 5 μm in the cross section of the positive electrode film layer is 14.58%; the area ratio of particles with a particle size greater than 5 μm is 0; the area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm is 19.70%; and the area ratio of particles with a particle size greater than or equal to 200 nm and less than 1500 nm is 75.07%.

[0356] The median number B 50 of the coating value of the positive electrode film layer obtained in the face scanning mode of the laser microscopic confocal Raman spectrometer is 0.443. The median number C 50 of the graphitization degree obtained in the face scanning mode of the laser microscopic confocal Raman spectrometer is 1.021. In the particle sphericity number cumulative distribution curve obtained in the cross section of the positive electrode film layer along the thickness direction of the electrode piece, the median number L 50 of the sphericity is 0.722. In the particle roughness number cumulative distribution curve obtained in the cross section of the positive electrode film layer along the thickness direction of the electrode piece, the median number R 50 of the roughness is 0.943. The iron dissolution rate of the positive electrode film layer is 1058 ppm.

[0357] (3) Preparation of a negative electrode piece:

[0358] 95.5 wt% of the negative active material (artificial graphite), 1.0 wt% of the conductive agent (conductive carbon black), 2.0 wt% of the binder (styrene-butadiene rubber (SBR)), and 1.5 wt% of the thickening agent (sodium carboxymethyl cellulose (CMC)) are mixed, deionized water is added for stirring and dispersion to prepare a negative electrode slurry. Then the negative electrode slurry is coated on the double-sided surface of a Cu foil, both sides are completed, and then dried, compacted, cut, and sheeted to prepare a negative electrode sheet. The single-sided density of the coating is 165 mg / 1540 mm 2 , and the compacted density is 1.60 g / cm 3 .

[0359] (4) Preparation of the separator film

[0360] The polypropylene film is used as the separator film.

[0361] (5) Preparation of the electrolyte

[0362] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), organic solvent ethylene carbonate (EC) / dimethyl carbonate (DMC) is mixed uniformly at a volume ratio of 1 / 1, lithium salt LiPF6 is dissolved in the organic solvent, the content of LiPF6 in the solution is 1 mol / L, and stirring is uniform to obtain the electrolyte.

[0363] (6) Preparation of the battery:

[0364] The positive electrode sheet, the separator film, and the negative electrode sheet are stacked in order, the separator film can play a role of separating the cathode and the anode, a bare cell is obtained by winding, the bare cell is placed in an outer package, the electrolyte is injected, and the process of packaging, formation, and exhaust is performed to finally obtain the lithium ion battery.

[0365] The preparation method of Example 2 is basically the same as that of Example 1, except that in the preparation step of the positive active material, the ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.02.

[0366] The preparation method of Example 3 is basically the same as that of Example 1, except that in the preparation step of the positive active material, the ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.01.

[0367] The preparation method of Example 4 is basically the same as that of Example 1, except that in the preparation step of the positive active material, the ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.05.

[0368] The preparation method of Example 5 is basically the same as that of Example 1, except that in the preparation step of the positive active material, the mass content of trivalent iron element in the ferrous oxalate is 0.80%.

[0369] The preparation method of Example 6 is basically the same as that of Example 1, except that in the preparation step of the positive active material, the carbon source is replaced by polyethylene glycol + glucose, and the mass ratio of polyethylene glycol to glucose is 3:1.

[0370] The preparation method of Example 7 is basically the same as that of Example 1, except that in the preparation step of the positive active material, the carbon source is replaced by polyethylene glycol + glucose, and the mass ratio of polyethylene glycol to glucose is 2:1.

[0371] The preparation method of Example 8 is basically the same as that of Example 1, except that in the preparation step of the positive active material, the carbon source is replaced by polyethylene glycol + glucose, and the mass ratio of polyethylene glycol to glucose is 1:2.

[0372] The preparation method of Example 9 is basically the same as that of Example 1, except that in the preparation step of the positive active material, the carbon source is replaced by polyethylene glycol + glucose, and the mass ratio of polyethylene glycol to glucose is 1:3.

[0373] The preparation method of Example 10 is basically the same as that of Example 1, except that in the preparation step of the positive electrode sheet, no conductive carbon black is added.

[0374] The preparation method of Example 11 is basically the same as that of Example 1, except that in the preparation step of the positive active material, the second temperature is 755°C.

[0375] The preparation method of Example 12 is basically the same as that of Example 1, except that in the preparation step of the positive active material, the carbon source is replaced by polyethylene glycol + glucose, and the mass ratio of polyethylene glycol to glucose is 1:3; and the ratio of lithium dihydrogen phosphate to ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.05.

[0376] The preparation method of Comparative Example 1 is basically the same as that of Example 1, except that in the preparation step of the positive active material, the carbon source polyethylene glycol is replaced by glucose; the ratio of lithium dihydrogen phosphate to ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.05; and the second temperature is 810°C.

[0377] The preparation method of Comparative Example 2 is basically the same as that of Example 1, except that in the preparation step of the positive active material, the ratio of lithium dihydrogen phosphate to ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.01; and the second temperature is 745°C.

[0378] The preparation method of Comparative Example 3 is basically the same as that of Example 1, except that in the preparation step of the positive active material, the carbon source polyethylene glycol is replaced by glucose; the ratio of lithium dihydrogen phosphate to ferrous oxalate is such that the molar ratio of lithium to iron is 1.06; and the second temperature is 740°C.

[0379] Performance test

[0380] 1. Limiting compaction density of electrode sheet

[0381] The double-coated electrode sheet was compacted by a roller compactor, and the elongation of the compacted electrode sheet was tested, and the flexibility of the compacted electrode sheet was evaluated. By increasing the pressure of the roller compactor, electrode sheets with different compaction densities were obtained. As the pressure increases, the compaction density of the electrode sheet increases, the elongation of the electrode sheet increases, and the flexibility of the electrode sheet decreases. Too high elongation of the electrode sheet can easily cause the electrode sheet to warp, and too low flexibility of the electrode sheet can easily cause the electrode sheet to break. Therefore, the limiting compaction density of the electrode sheet is defined as the smaller one of the elongation of the electrode sheet of 8% or the flexibility of the electrode sheet of 3 times.

[0382] The compaction density was calculated by the mass of the positive electrode film / the volume of the positive electrode film.

[0383] The test method of elongation is as follows:

[0384] The electrode sheet was laid flat on a horizontal table, and the electrode sheet was cut into sections, each about 100 cm long. The edge substrate copper foil of the electrode sheet was removed, and the cutting edge of the electrode sheet was kept parallel to the MD direction of the electrode sheet (perpendicular to the direction of the compression roller), and it was ensured that the electrode sheet was completely covered by the coating. The length between the marked points at the same position in the length direction was measured using a steel ruler, and the reading was estimated to 0.1 mm. The length before compaction was recorded. The length after compaction was recorded, and the elongation of the electrode sheet was calculated as (length after compaction-length before compaction) / length before compaction.

[0385] The test method of flexible folding times is as follows.

[0386] The positive electrode sheet was cut into 20x100mm 2 The test sample was cut into 20x100mm, and then it was folded forward, pressed flat with a 2kg roller, and unfolded to check if the gap was transparent. If there was no light transmission, it was folded backward, pressed flat with a 2kg roller, and checked again. This was repeated until the gap was transparent, and the folding times were recorded. The test was repeated three times, and the average value was taken as the reference data of the flexibility of the electrode sheet.

[0387] 2. Energy density test

[0388] The lithium ion secondary battery was allowed to stand at 25°C for 2 h, and the temperature of the lithium ion secondary battery was ensured to be 25°C. After the lithium ion secondary battery was charged at 25°C at 0.33 C to a charge cut-off voltage of 3.65 V, constant voltage charging was continued at the charge cut-off voltage until the current was 0.05 C, and the charge cut-off (wherein C represents the rated capacity of the lithium ion secondary battery). After the lithium ion secondary battery was allowed to stand at 25°C for 1 h, the lithium ion secondary battery was discharged at 25°C at 0.33 C to a discharge cut-off voltage of 2.5 V, and the total discharge energy of the lithium ion secondary battery was recorded as E0.

[0389] The length, width and height of the lithium ion secondary battery were measured, and the volume value V0 of the lithium ion secondary battery was calculated as length* width* height.

[0390] The volumetric energy density of the lithium ion secondary battery was calculated as the discharge energy E0 of the lithium ion secondary battery / the volume V0 of the lithium ion secondary battery.

[0391] 3. DCR test method

[0392] After 0.33 C constant current charging to 3.65 V at 25°C, constant voltage charging to 0.05 C, then 1 / 3 C discharging to 20% SOC, standing for 5 min, then 3 C pulse discharging for 30 s, standing for 40 s, then 3 C charging for 40 s, standing for 5 min, then 1 / 3 C constant current charging to 3.65 V, constant voltage charging to 0.05 C, then 1 / 3 C discharging to 10% SOC, standing for 5 min, then 3 C pulse discharging for 30 s, standing for 40 s, then 3 C charging for 40 s, standing for 5 min, then 1 / 3 C full charging, then 1 / 3 C discharging to 50% SOC, then 1 C pulse discharging for 30 s after standing for 2 h at -25°C, standing for 10 min, then 1 / 3 C constant current charging to 3.65 V at 25°C, constant voltage charging to 0.05 C, then 1 / 3 C discharging to 20% SOC, then 1 C pulse discharging for 30 s after standing for 2 h at -25°C, standing for 10 min.

[0393] The voltage at each pulse discharge was recorded, and the DCR under different conditions was calculated, with the calculation formula being DCR = (voltage before pulse discharge after standing - voltage before pulse discharge after standing) / pulse current.

[0394] Experimental parameters and test results

[0395] The batteries of each example and comparative example were prepared according to the above method, and each performance parameter was measured, and the results are shown in Table 1 below.

[0396] Table 1

[0397]

[0398] Table 1 (continued)

[0399]

[0400] As can be seen from the data comparison of the examples and comparative examples, when the area ratio of particles with a particle size of 1.5 μm or more is 8.0% or more and 20.0% or less, and the iron dissolution rate of the positive electrode film layer is 658 ppm-1921 ppm, the battery has a high positive electrode tab compactness, energy density, and good kinetic performance.

[0401] As can be seen from the comparison of Example 9, Example 12 and other examples, when the iron dissolution rate of the positive electrode film layer is 658 ppm-1485 ppm, the battery DC internal resistance can be further reduced.

[0402] As can be seen from the data comparison of the examples and comparative examples, when the area ratio of particles with a particle size of 1.5 μm or more and less than 5 μm is 9.0%-20.0%, the positive electrode tab compactness and energy density can be further improved.

[0403] As can be seen from the comparison of Example 11 and other examples, when the area ratio of particles with a particle size of 200 nm or more and less than 1500 nm in the cross section of the positive electrode film layer along the thickness direction of the tab is 73.0%-78.0%, the battery energy density can be further improved.

[0404] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and embodiments having the same technical idea and playing the same role and effect within the scope of the technical solutions of the present application are all included in the technical scope of the present application. In addition, within the scope of the main idea of the present application, various modifications that can be thought of by those skilled in the art, other modes constructed by combining part of the components of the embodiments are also included in the scope of the present application.

Claims

1. A lithium-ion secondary battery, characterized by comprising: The positive electrode sheet, the negative electrode sheet and the electrolyte, The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, The positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes lithium-containing transition metal phosphate particles having a carbon coating material arranged on at least part of the surface, In a section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 1.5 μm and less than 5 μm is greater than or equal to 8.0% and less than or equal to 20.0%. The iron dissolution rate of the positive electrode film layer is 658 ppm-1921 ppm. The median value B of the coating value in the cumulative distribution curve of the coating value B obtained by the positive electrode film layer under the laser microscopic confocal Raman spectrometer face scanning mode 50 is 0.35-0.48, wherein the coating value B is I P / I D , wherein I P represents the P peak intensity of the Raman spectrum at 948±100cm -1 , I D represents the D peak intensity of the Raman spectrum at 1350±100cm -1 .

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

3. The lithium-ion secondary battery according to claim 1, characterized by In a section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 5 μm is 0.

4. The lithium-ion secondary battery according to claim 1, characterized by In a section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 1.5 μm and less than 5 μm is 9.0%-20.0%.

5. The lithium-ion secondary battery according to claim 1, characterized by In a section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 1.5 μm and less than 5 μm is 10.0%-20.0%.

6. The lithium-ion secondary battery according to claim 1, characterized by In a section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm is 15.0%-25.0%.

7. The lithium-ion secondary battery according to claim 1, characterized by In a section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm is 16.0%-24.0%.

8. The lithium-ion secondary battery according to claim 1, characterized by In a section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 200 nm and less than 1500 nm is 73.0%-80.0%.

9. The lithium-ion secondary battery according to claim 1, characterized by In a section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 200 nm and less than 1500 nm is 73.0%-78.0%.

10. The lithium-ion secondary battery according to claim 1, characterized by The median value C of the graphitization degree in the cumulative distribution curve of the graphitization degree C value obtained by the positive electrode film layer in the laser microscopic confocal Raman spectrometer face scanning mode is 0.95-1.20; wherein the graphitization degree C value is I 50 / I G / I D , wherein I G represents the G peak intensity of the Raman spectrum at 1580±100cm -1 , I D represents the D peak intensity of the Raman spectrum at 1350±100cm -1 .

11. The lithium-ion secondary battery according to claim 1, characterized by The median value C of graphitization degree in the cumulative distribution curve of graphitization degree C value obtained by the positive electrode film layer in the laser microscopic confocal Raman spectrometer face scanning mode 50 is 0.98-1.15; wherein the graphitization degree C value is I G / I D , wherein I G represents the G peak intensity of Raman spectrum at 1580±100 cm -1 , and I D represents the D peak intensity of Raman spectrum at 1350±100 cm -1 .

12. The lithium-ion secondary battery according to claim 1, characterized by The median value C of graphitization degree in the cumulative distribution curve of graphitization degree C value obtained by the positive electrode film layer in the laser microscopic confocal Raman spectrometer face scanning mode 50 is 1.0-1.10; wherein the graphitization degree C value is I G / I D , wherein I G represents the G peak intensity of Raman spectrum at 1580±100 cm -1 , I D represents the D peak intensity of Raman spectrum at 1350±100 cm -1 .

13. The lithium-ion secondary battery according to claim 1, characterized by The sphericity median number L of the particles obtained from the section of the positive electrode film layer along the thickness direction of the electrode tab in the sphericity number area cumulative distribution curve of the particles A50 is 0.65-0.

85.

14. The lithium-ion secondary battery according to claim 1, characterized by The sphericity median number L of the particles obtained from the section of the positive electrode film layer along the thickness direction of the electrode tab in the sphericity number area cumulative distribution curve of the particles A50 is 0.70-0.

80.

15. The lithium-ion secondary battery according to claim 1, characterized by The median value Rz of the roughness of the particles in the roughness area cumulative distribution curve of the particles obtained from the section of the positive electrode film layer in the thickness direction of the electrode tab A50 is 0.92-0.

96.

16. The lithium-ion secondary battery according to claim 1, characterized by The mass content of carbon element is 0.8%-1.8% based on the total mass of the positive electrode active material.

17. The lithium-ion secondary battery according to claim 1, characterized by The mass content of carbon element is 0.90%-1.50% based on the total mass of the positive electrode active material.

18. The lithium-ion secondary battery according to claim 1, characterized by, The lithium-iron antisite defect concentration of the positive electrode active material is 0.1%-1.5%.

19. The lithium-ion secondary battery according to claim 1, characterized by, The lithium-iron antisite defect concentration of the positive electrode active material is 0.3%-1.0%.

20. The lithium-ion secondary battery according to claim 1, characterized by The lithium-containing transition metal phosphate particles include a component having the following general formula: Li m Fe x P y O j Q q , 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, Br, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, 0<q≤0.

1.

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

22. The lithium-ion secondary battery according to claim 1, characterized by The positive electrode active material includes titanium element, and the mass content of titanium element is 2000 ppm-6000 ppm based on the total mass of the positive electrode active material.

23. The lithium-ion secondary battery according to claim 1, characterized by, The tap density of the powder of the positive electrode active material is 0.70 g / cm 3 -1.50 g / cm 3 .

24. The lithium-ion secondary battery according to claim 1, characterized by, The tap density of the powder of the positive electrode active material is 0.70 g / cm 3 - 1.20 g / cm 3 .

25. The lithium-ion secondary battery according to claim 1, characterized by, The powder compaction density of the positive electrode active material under 3T pressure is 2.50 g / cm 3 - 2.70 g / cm 3 .

26. The lithium-ion secondary battery according to claim 1, characterized by The powder compaction density of the positive electrode active material under 3T pressure is 2.52 g / cm 3 - 2.68 g / cm 3 .

27. The lithium-ion secondary battery according to claim 1, characterized by, The powder resistivity of the positive electrode active material under a pressure of 8 MPa is 0.5 Ω·cm-30.0 Ω·cm.

28. The lithium-ion secondary battery according to claim 1, characterized by, The powder resistivity of the positive electrode active material under a pressure of 8 MPa is 2.0 Ω·cm-20.0 Ω·cm.

29. The lithium-ion secondary battery according to claim 1, characterized by, The discharge gram capacity of the positive electrode active material under a discharge rate of 1C is 135 mAh / g-150 mAh / g.

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

31. The lithium-ion secondary battery according to claim 1, characterized by, In the 0.1C discharge curve of the button cell containing the positive electrode active material, there is a discharge platform in the voltage range of 2.5 V-2.9 V.

32. The lithium-ion secondary battery according to claim 1, characterized by, The positive electrode film layer further comprises a conductive agent, and the mass content of the conductive agent is 0.1%-1.5% based on the total mass of the positive electrode film layer.

33. The lithium-ion secondary battery according to claim 1, characterized by, The positive electrode film layer does not comprise a conductive agent.

34. The lithium-ion secondary battery according to claim 1, characterized by, The mass content of the positive electrode active material is 95.5%-99.5% based on the total mass of the positive electrode film layer.

35. The lithium-ion secondary battery according to claim 1, characterized by, The mass content of the positive electrode active material is 96.5%-99.5% based on the total mass of the positive electrode film layer.

36. The lithium-ion secondary battery according to claim 1, characterized by, The positive electrode film layer further comprises 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 by, The single side surface density of the positive electrode film layer is 300 mg / 1540 mm 2 - 450 mg / 1540 mm 2 .

38. The lithium-ion secondary battery according to claim 1, characterized by, The lithium ion secondary battery has a compaction density of the positive electrode film layer of 2.51 g / cm 3 -2.73 g / cm 3 .

39. The lithium-ion secondary battery according to claim 1, characterized by, The lithium ion secondary battery has a compaction density of the positive electrode film layer of 2.55 g / cm 3 -2.70 g / cm 3 .

40. The lithium-ion secondary battery according to claim 1, characterized by, The compaction density of the positive electrode film layer of the lithium ion secondary battery in a full discharge state is 2.51 g / cm3-2.73 g / cm3, and the porosity of the positive electrode film layer in the section along the thickness direction of the electrode sheet is 10%-22%.

41. The lithium-ion secondary battery according to claim 1, characterized by, The lithium ion secondary battery has a positive electrode film layer with a compaction density of 2.55 g / cm 3 -2.70 g / cm 3 In a section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10%-20%.

42. The lithium-ion secondary battery according to claim 1, characterized by, The positive electrode sheet comprises a primer layer, which is arranged between the positive electrode film layer and the positive electrode current collector; the primer layer comprises carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the primer layer is ≤10 particle numbers / 10 μm.

43. The lithium-ion secondary battery according to claim 1, characterized by, The positive electrode plate comprises a bottom coating layer, which is arranged between the positive electrode film layer and the positive electrode current collector; the compaction density of the positive electrode plate in a full discharge state is greater than or equal to 2.4 g / cm 3 The single-side thickness of the bottom coating layer is 1-4 μm.

44. The lithium-ion secondary battery according to claim 1, characterized by, The positive electrode tab comprises a base coating layer, which is arranged between the positive electrode film layer and the positive electrode current collector; the compaction density of the positive electrode tab in a full discharge state is greater than or equal to 2.5 g / cm 3 The single-side thickness of the base coating layer is 2 μm-4 μm.

45. A battery device, comprising: The battery device comprises at least one of a battery cell, a battery module, and a battery pack.

46. An energy storage cell, comprising: The battery device comprises at least one of a battery cell, a battery module, and a battery pack.

47. An electrical device, comprising: The battery device comprises at least one of a battery cell, a battery module, and a battery pack.

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