Lithium ion secondary battery, battery device, and power device

Abstract

The invention provides a lithium ion secondary battery, a battery device and a power utilization device. The lithium ion secondary battery comprises a positive pole piece, a negative pole piece and an electrolyte, the positive pole piece comprises a positive current collector and a positive film layer arranged on at least one side of the positive current collector, and the positive film layer comprises a positive active material; the positive electrode active material comprises lithium-containing transition metal phosphate particles, at least part of the surfaces of the lithium-containing transition metal phosphate particles are provided with carbon coating materials, and in the tangent plane, in the thickness direction of the pole piece, of the positive electrode film layer, the area proportion of particles with the particle size larger than or equal to 1.5 micrometers is larger than or equal to 8.0% and smaller than or equal to 20.0%; and in a sphere-like degree area cumulative distribution curve of particles obtained by the positive electrode film layer along the tangent plane of the thickness direction of the pole piece, the median LA50 of the sphere-like degree is 0.70-0.74.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion secondary batteries, and in particular to a lithium-ion secondary battery, a battery device, an electrical device, a method for preparing a positive electrode active material, and a method for preparing a positive electrode sheet. Background Art

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

[0003] Positive electrode active materials are important components of secondary batteries. Lithium-containing transition metal phosphate materials have the characteristics of stable structure, good safety, and long cycle life, and have broad development prospects. As the market's requirements for energy density and kinetics of lithium-containing transition metal phosphate system secondary batteries increase, it is difficult to achieve simultaneous improvement of the above performance in the existing technology, which has become a technical problem that needs to be solved urgently in this field. Summary of the invention

[0004] In view of the above problems, the present application provides a battery cell, a battery device, an electrical device, a method for preparing a positive electrode active material, and a method for preparing a positive electrode sheet, which are described below respectively.

[0005] The first aspect of the present application provides a lithium-ion secondary battery, including a positive electrode plate, a negative electrode plate and an electrolyte, wherein the positive electrode plate includes a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes lithium-containing transition metal phosphate particles with a carbon coating material arranged on at least a portion of the surface, and in a cross section of the positive electrode film layer along the thickness direction of the electrode plate, 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%; and in a cumulative distribution curve of the sphericity area of ​​the particles obtained from the cross section of the positive electrode film layer along the thickness direction of the electrode plate, the median of the sphericity L A50 It is 0.70-0.74.

[0006] Research shows that although the presence of particles with a particle size of 1.5 μm or more in the positive electrode film layer is beneficial to improving the particle grading in the positive electrode film layer and increasing the compaction density of the electrode sheet, the presence of particles within the above range in the positive electrode film layer will produce a serious short-board effect. In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size of 1.5 μm or more accounts for less than 8%, which means that the proportion of large particles is seriously insufficient, and there is no reasonable grading, and it is difficult to achieve tight stacking. At the same time, the structural stability of the positive electrode film layer is also poor. In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size of 1.5 μm or more accounts for more than 20%, which is beneficial to the improvement of the compaction density of the positive electrode film layer, but it will sharply reduce the contact area between the electrolyte and the positive electrode active material, increase the diffusion path length of lithium ions in the particles, and cause serious polarization of the electrode sheet, increase the battery impedance, and significantly deteriorate the dynamic performance of the battery. The applicant further discovered that, under the condition of a low proportion of large particles, by adjusting the sphericity of the particles in the film layer, the particles can be easily slipped during rolling, thereby filling each other, and a positive electrode film layer with a high compaction density can also be obtained. A50 When it is less than 0.70, it means that the overall irregularity of the particles is still large and difficult to slide; when the median of the sphericity of the particles in the positive electrode film layer is L A50 When it is greater than 0.74, the mechanical interlocking force between the particles is too small. During the cycle, with the expansion and contraction of the volume, the particles inside the membrane layer are prone to displacement, resulting in a decrease in the stability of the electrode structure and affecting the cycle life of the battery.

[0007] In the present application, the area proportion of particles with a particle size greater than or equal to 1.5 μm in the positive electrode film layer is controlled to be greater than or equal to 8.0% and less than or equal to 20.0%, and the median of the sphericity L in the cumulative distribution curve of the sphericity area of ​​the particles obtained from the cross section of the positive electrode film layer along the thickness direction of the electrode sheet is A50 A value of 0.70-0.74 can reduce the short-board effect, help keep the battery impedance at a low level, and improve the battery's dynamic performance; and make the particles approximately spherical, making it easier for the particles to slide, thereby optimizing the compaction density of the pole piece, thereby achieving a balance between the battery's dynamic performance and energy density.

[0008] In any embodiment, in the cumulative distribution curve of the sphericity area of ​​particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the concentration (L A90 -L A10 ) / L A50 0.450-0.535, optional 0.450-0.500. (L A90 -L A10 ) / L A50It can reflect not only the sphericity of most particles, but also the asymmetry and width of the sphericity distribution of particles in the positive electrode film. The smaller the value, the more concentrated the distribution. Combined with the high median sphericity, it reflects that the particles are generally spherical, which is conducive to forming a tight stack, increasing the contact between particles, and further improving the dynamic performance of the battery.

[0009] In any embodiment, in the cumulative distribution curve of the sphericity area of ​​particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the sphericity L A10 0.40-0.60, spherical L A90 0.85-1.0. A10 , L A90 Within the above range, it indicates that most of the particles have a high degree of sphericity, avoiding extreme morphological differences, and will not cause excessive gaps and stress concentration due to bridging between particles. It is conducive to the formation of regular stacking and uniform slip, and improves the compaction density of the pole piece.

[0010] In any embodiment, in the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 5 μm is 0. Studies have shown 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. The area ratio of particles with a particle size greater than or equal to 5 μm is 0, which is conducive to further reducing the internal resistance of the battery and improving the battery kinetic performance.

[0011] In any embodiment, in the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 1.5 μm and less than 5 μm is 9.0%-20.0%, and can be optionally 10.0%-20.0%. In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 1.5 μm and less than 5 μm within the above range can further reduce the obstruction of large-sized particles in the electrode sheet to the infiltration and diffusion of the electrolyte in the positive electrode film layer, improve the consistency of the diffusion rate of lithium ions in the positive electrode active material particles, reduce local polarization, and improve the dynamic performance of the battery.

[0012] In any embodiment, in the cross section of the positive electrode film layer along the thickness direction of the pole piece, 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%, and can be optionally 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 within the above range can form a sufficient "support" structure in the stacking, which helps to enhance the overall structural strength between particles, and at the same time form a support structure between smaller particles, fill larger gaps, and reduce the void ratio between particles, which is conducive to further improving the compaction density of the pole piece and improving the energy density of the battery while maintaining good dynamic performance of the battery.

[0013] In any embodiment, in the cross section of the positive electrode film layer along the thickness direction of the pole piece, the area ratio of particles with a particle size greater than or equal to 200nm and less than 1500nm is 73.0%-80.0%, and can be optionally 73.0%-78.0%. The area ratio of particles with a particle size greater than or equal to 200nm and less than 1500nm in the positive electrode film layer within the above range can further improve the consistency of the lithium ion diffusion rate, thereby improving the dynamic performance of the lithium ion secondary battery, and can further improve the compaction density of the pole piece, thereby improving the energy density of the battery.

[0014] In any embodiment, in the cumulative distribution curve of the graphitization degree C value obtained by the laser microscopic confocal Raman spectrometer surface scanning mode, the median of the graphitization degree is 0.95-1.20, and can be 0.98-1.15; wherein the graphitization degree C value is I G / I D , I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at . The higher the degree of graphitization of the carbon on the surface of the positive electrode active material, the higher the proportion of graphite structure carbon in the positive electrode film layer, and the easier it is for the particles to slip with the help of the highly graphitized carbon structure in the coating material, thereby increasing the compaction density of the electrode sheet.

[0015] In any embodiment, in the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median roughness R A50 The median roughness R is 0.92-0.96. A50 The surface of particles within the above range is relatively smooth, the friction between particles is relatively small, and they are easy to slip under the action of external force, which can further improve the compaction density of the pole piece and increase the energy density of the battery.

[0016] In any embodiment, in the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the concentration of roughness (R A90 -R A10 ) / R A50 The concentration value of roughness is extremely small, indicating that the overall roughness consistency of the particles is high, which is conducive to the relative sliding between particles. On the basis of high sphericity, the particles are easier to form high-density accumulation during rolling, which increases the compaction density of the pole piece and the energy density of the battery.

[0017] In any embodiment, the positive electrode active material includes iron element, and the iron dissolution rate of the positive electrode film layer is 500ppm-2000ppm. The iron dissolution rate of the positive electrode film layer mainly comes from the lithium-containing transition metal phosphate positive electrode active material in the positive electrode film layer. The iron dissolution rate of the positive electrode film layer is within the above range, which means that the positive electrode active material has a relatively complete and dense carbon coating material, which can improve the electrical contact between the positive electrode active materials, improve the conductivity of the positive electrode active materials, reduce the polarization of the positive electrode active materials, and further optimize the kinetic performance of the lithium-ion secondary battery. At the same time, the densely coated carbon layer has a low space occupancy rate, and the gaps between the particles are easily compressed by stress during the rolling process, which can simultaneously improve the compaction density of the pole piece and the energy density of the battery.

[0018] In any embodiment, based on the total mass of the positive electrode active material, the mass content of carbon element is 0.8%-1.8%, and can be optionally 0.90%-1.5%. Compared with the lithium phosphate positive electrode active material in the prior art, the positive electrode active material has a relatively low content of carbon coating, which can further increase the loading amount of lithium phosphate in the positive electrode plate and improve the energy density of the lithium ion secondary battery.

[0019] In any embodiment, the lithium iron anti-site defect concentration of the positive electrode active material is 0.1%-1.5%, optionally 0.3%-1.0%. During the preparation and circulation process, there will inevitably be certain lithium vacancies in the crystal structure of the positive electrode active material. Lithium vacancies will not only cause ferrous ions to be oxidized to ferric ions, but also induce the partial migration of ferric ions to the lithium position, forming lithium iron anti-site defects, blocking the one-dimensional diffusion channel of lithium ions, and adversely affecting the solid phase transmission of lithium ions. The positive electrode active material in the embodiment of the present application has low lithium iron anti-site defects, which is conducive to the uniform transmission of lithium ions in the solid phase, and further improves the kinetic performance of lithium ion secondary batteries.

[0020] In any embodiment, 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 , wherein Q includes one or more of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.1.

[0021] Selecting an appropriate modifying element Q can improve the lattice change rate of the positive electrode active material during the lithium insertion and extraction process, reduce the oxygen activity on the particle surface, improve the structural stability of the material, and thereby increase the material's gram capacity during the cycle process, further improving the cycle stability of lithium-ion secondary batteries.

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

[0023] In any embodiment, the positive electrode active material includes titanium, and the mass content of titanium is 2000ppm-6000ppm based on the total mass of the positive electrode active material. The high addition of titanium does not form a harmful impurity phase that has a negative impact on the energy density and kinetic performance of the battery. Although the reason is not clear, it is speculated that titanium, phosphate and other elements (such as lithium) together form a fast ion conductor, which improves the kinetic performance of the battery.

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

[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 , optional 2.52g / cm 3 -2.68 g / cm 3 .

[0026] Although the area proportion of the particles with a particle size of 1.5 μm or more is low, the effective gradation formed independently is limited, and the powder tap density is relatively low, the positive electrode active material is easy to roll under the action of external force due to its high sphericity, thereby filling the gaps with each other to achieve a higher compaction density, providing a material basis for improving the compaction density of the pole piece and preparing a high energy density lithium-ion secondary battery.

[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 can be 2.0 Ω·cm-20.0 Ω·cm. The positive electrode active material is coated with carbon material on the surface, and the surface carbon sp 2The structure facilitates rapid conduction of electrons between particles, making the positive electrode active material have a low powder resistivity, which is beneficial to increasing the solid-phase transmission rate of electrons and improving the kinetic performance of the battery.

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

[0029] In any embodiment, the discharge capacity of the positive electrode active material discharged to 3.2V accounts for η≥85%, where η is defined as: at room temperature, a button cell containing the positive electrode active material is charged and discharged twice at a constant current rate of 0.1C in the voltage range of 2.0V to 3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at the rate of 1C, the capacity value at the discharge voltage of 3.2V is extracted and recorded as C 1 , extract the capacity value of the discharge voltage to 2.0V as C 2 , η = C 1 / C 2 The charging process includes constant voltage charging, constant voltage 3.75V, and constant voltage cut-off current 50μA.

[0030] The high discharge capacity of the positive electrode active material used in the lithium-ion secondary battery when discharged 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 conducive to maintaining good power performance.

[0031] In any embodiment, 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 to 2.9 V. This is conducive to increasing the discharge range of the battery and improving the energy density of the battery.

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

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

[0034] The particles in the positive electrode film layer have a high degree of sphericity, so the particles can be tightly packed during rolling, and the particles have good contact, so that the particles inside the positive electrode film layer have good electronic conductivity, which can reduce or even eliminate the use of conductive agents in the positive electrode film layer, which is conducive to further increasing the load of positive electrode active materials and improving the energy density of lithium-ion secondary batteries. In any embodiment, the positive electrode film layer also includes a binder, and 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, and can be optionally 96.5%-99.5%; the mass content of the binder is 0.5%-3%.

[0035] 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 per unit volume of the positive electrode film layer, maintain good internal bonding force, reduce the probability of powder loss, expansion and cracking problems, and improve the energy density of the secondary battery while taking into account safety performance.

[0036] In any embodiment, the single-side density of the positive electrode film layer is 300 mg / 1540 mm 2 -450mg / 1540mm 2 The positive electrode film layer having an area density within the above range can help to improve the energy density of the lithium-ion secondary battery.

[0037] In any embodiment, when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.51 g / cm 3 -2.73g / cm 3 .

[0038] In any embodiment, when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.55 g / cm 3 -2.70g / cm 3 .

[0039] The compaction density of the positive electrode film layer is within the above range, which is beneficial to improving the energy density of the lithium-ion secondary battery.

[0040] In any embodiment, the positive electrode film layer satisfies at least one of the following conditions: (1) when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.51 g / cm 3 -2.73g / cm 3 In the cross section of the positive electrode film along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-22%; (2) when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film is 2.55 g / cm 3 -2.70g / cm 3In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10%-20%.

[0041] The lower the porosity in the cross section of the positive electrode film, on the one hand, means that the gradation of large, medium and small particles in the positive electrode film is better and the compaction density is higher. On the other hand, after the same gradation and roller pressure, if the porosity is low, it means that the particles are easy to slip against each other, thereby reducing the risk of overpressure and stress concentration in the film, and further reducing the probability of demolding of the positive electrode film during long cycles, which is beneficial to improving the long cycle performance of the battery.

[0042] In any embodiment, the positive electrode plate includes a primer layer, which is disposed 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, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the primer layer is ≤10 pcs / 10 μm; (2) the compaction density of the positive electrode plate in a fully charged state is greater than or equal to 2.4 g / cm 3 The thickness of the bottom coating on one side is 1 μm-4 μm; (3) the compaction density of the positive electrode sheet in the fully charged state is greater than or equal to 2.5 g / cm 3 The single-side thickness of the primer layer is 2 μm-4 μm.

[0043] The primer layer is beneficial to improving the conductivity and adhesion between the positive electrode film layer and the current collector, reducing the demolding of the positive electrode film layer from the current collector during the cycle, and improving the dynamic performance of the battery.

[0044] As the compaction density of the pole piece increases, the large particles of lithium-containing phosphate materials (for example, particles larger than 1 μm) in the positive electrode film layer have a more significant squeezing effect on the bottom coating. Therefore, stress concentration is easily generated at the site of large particles, and even passes through the bottom coating to damage the current collector. Increasing the thickness of the bottom coating is conducive to improving the stress concentration phenomenon in the pole piece and further improving the ultimate compaction density of the pole piece.

[0045] A second aspect of the present application provides a battery device, comprising the lithium-ion secondary battery of the first aspect of the present application.

[0046] A third aspect of the present application provides an electrical device, comprising at least one of the lithium-ion secondary battery of the first aspect of the present application and the battery device of the second aspect of the present application.

[0047] The fourth aspect of the present application provides a method for preparing a positive electrode active material, comprising: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source, wherein the molar ratio of lithium to iron in the mixed raw material is greater than or equal to 1 and less than or equal to 1.05; 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 includes a first temperature and a second temperature, and the second temperature of the sintering is 750°C-800°C; the crushing includes air flow crushing, the grading frequency of the air flow crushing is 18Hz-24Hz, and the crushing air pressure is 0.45MPa-0.65MPa.

[0048] At high temperatures, the diffusion of the particle surface increases, the defects in the particles are repaired, and the lattice is rearranged. Through recrystallization, the defects on the particle surface are eliminated, the grain structure of the particles is more orderly, the size of the particles gradually increases, and it helps to smooth the particle surface and promote the particle to develop into a spherical shape.

[0049] The classification frequency in airflow milling refers to the working frequency of the classification device in airflow milling, which is usually related to the classification efficiency and particle size distribution of the particles. A higher classification frequency will screen the particles in the airflow more times, so that larger particles are screened out and smaller particles are left. And a higher classification frequency may increase the number of particle collisions, causing irregular particles to be further impacted, making the particle surface smoother and the shape tending to be spherical.

[0050] The preparation method provided in the embodiment of the present application obtains a positive electrode active material with a small proportion of large particles by regulating the carbon source, iron source and crushing process. On the one hand, it improves the sphericity of the particles in the positive electrode active material and provides a material basis for the preparation of the positive electrode film layer.

[0051] The fifth aspect of the present application provides a method for preparing a pole piece, which comprises sequentially adding a binder, a conductive agent, and a positive electrode active material prepared by the preparation method of the fourth aspect of the present application, dry-mixing the mixture, adding a solvent, stirring, and adjusting the viscosity to obtain a shipping slurry; transfer-coating the shipping slurry to at least one side of a current collector, and obtaining a positive electrode film layer after drying and hot pressing.

[0052] In any embodiment, the stirring includes pre-stirring and main stirring, and the stirring time of the main stirring is 200min-250min and the temperature is 5°C-50°C.

[0053] In any embodiment, the hot pressing includes at least three hot roller pressings, and the hot roller pressure increases successively, and the hot roller pressure is 20 tons-50 tons, 50 tons-70 tons, and 70 tons-90 tons respectively; the hot roller temperature is 40°C-80°C, and before entering the hot roller compaction for the first time, the positive electrode plate is heated, and the heating temperature is 40°C-50°C.

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

[0055] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 is a scanning electron microscope image of a cross section of a positive electrode film layer along the thickness direction of the electrode sheet according to an embodiment of the present application; Figure 2 is a schematic diagram of a lithium-ion secondary battery according to one embodiment of the present application; Figure 3 is an exploded schematic diagram of a lithium-ion secondary battery according to one embodiment of the present application; Figure 4 is a schematic diagram of a battery module according to an embodiment of the present application; Figure 5 is a schematic diagram of a battery pack according to an embodiment of the present application; Figure 6 yes Figure 5 An exploded schematic diagram of a battery pack is shown; Figure 7 It is a schematic diagram of an electrical device using a lithium-ion secondary battery as a power source according to one embodiment of the present application; Figure 8 This is a porosity test diagram of a cross section of the positive electrode film layer along the thickness direction according to one embodiment of the present application.

[0057] Description of reference numerals: 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0058] Hereinafter, the embodiments of the lithium-ion secondary battery, battery device and power device of the present application are specifically disclosed in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0059] "Scope" disclosed in the present application is limited in the form of lower limit and upper limit, and a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The scope limited in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "ab" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0060] If not otherwise specified, all embodiments and optional embodiments of the present application may be combined with each other to form new technical solutions, and such technical solutions should be deemed to be included in the disclosure of the present application.

[0061] Unless otherwise specified, all technical features and optional technical features of the present application may be combined with each other to form new technical solutions, and such technical solutions should be deemed to be included in the disclosure of the present application.

[0062] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0063] In the present application, the terms "plurality" and "multiple" refer to two or more.

[0064] Unless otherwise specified, the terms used in this application have the commonly understood meanings that are generally understood by those skilled in the art.

[0065] Unless otherwise specified, the values ​​of the parameters mentioned in this application can be measured by various test methods commonly used in the art, for example, they can be measured according to the test methods given in the examples of this application. Unless otherwise specified, the test temperature of each parameter is 25°C.

[0066] The battery mentioned in the embodiments of the present application may be a single physical module including one or more lithium-ion secondary batteries to provide higher voltage and capacity. For example, the battery mentioned in the present application may include a lithium-ion secondary battery, a battery module or a battery pack.

[0067] A lithium-ion secondary battery is the smallest unit of a battery, which can independently realize the functions of charging and discharging. A lithium-ion secondary battery can be cylindrical, rectangular, or in other shapes, and the present application embodiment does not limit this. Figure 2 As an example, a lithium-ion secondary battery 5 having a rectangular parallelepiped structure is shown.

[0068] A lithium-ion secondary battery includes an electrode assembly and an electrolyte.

[0069] The lithium-ion secondary battery may further include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte. The outer package may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT) and polybutylene succinate (PBS).

[0070] In some embodiments, Figure 3 As shown, the outer package may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. The electrode assembly 52 is encapsulated 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 demand.

[0071] The electrode assembly usually includes a positive electrode plate and a negative electrode plate. The negative electrode plate is an electrode that absorbs or lithiates lithium ions during charging and releases or delithiates lithium during discharging. The positive electrode plate is an electrode that releases or delithiates lithium ions during charging and absorbs or lithiates lithium during discharging.

[0072] When there are multiple lithium-ion secondary batteries, the multiple lithium-ion secondary batteries are connected in series, in parallel, or in mixed connection through a busbar. In some embodiments, the battery may be a battery module; when there are multiple lithium-ion secondary batteries, the multiple lithium-ion secondary batteries are arranged and fixed to form a battery module. In some embodiments, the battery may be a battery pack, which includes a case and a lithium-ion secondary battery, and the lithium-ion secondary battery or the battery module is accommodated in the case. In some embodiments, the case may serve as part of the chassis structure of the vehicle. For example, a portion of the case may become at least a portion of the floor of the vehicle, or a portion of the case may become at least a portion of the crossbeam and longitudinal beam of the vehicle.

[0073] In some embodiments, the battery may be an energy storage device, which includes an energy storage container, an energy storage cabinet, and the like.

[0074] In some embodiments, lithium-ion secondary batteries may be assembled into a battery module. The battery module may contain a plurality of lithium-ion secondary batteries, and the specific number may be adjusted according to the application and capacity of the battery module. Figure 4 FIG. 4 is a schematic diagram of a battery module 4 as an example. Figure 4 As shown, in the battery module 4, the plurality of lithium-ion secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of lithium-ion secondary batteries 5 may be fixed by fasteners.

[0075] Optionally, the battery module 4 may further include a housing having a housing space, and the plurality of lithium-ion secondary batteries 5 are housed in the housing space.

[0076] In some embodiments, the battery modules described above may also be assembled into a battery pack, and the number of battery modules contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0077] Figure 5 and Figure 6 FIG. 1 is a schematic diagram of a battery pack 1 as an example. Figure 5 and Figure 6 As shown, the battery pack 1 may include a box body and a plurality of battery modules 4 disposed in the box body. The box body includes an upper box body 2 and a lower box body 3, and the upper box body 2 is used to cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 may be arranged in the box body in any manner.

[0078] Lithium-containing transition metal phosphate materials have been widely used in lithium-ion batteries due to their stable structure, good safety and long cycle life; however, they have problems such as low electronic conductivity and low stacking efficiency, which makes it difficult to further effectively increase the loading amount of lithium phosphate in the electrode and cannot meet the needs of high energy density batteries.

[0079] In order to further improve the battery energy density and increase the compaction density of the pole piece, the common method in the industry is to increase the particle grading in the pole piece. In order to improve the particle grading, it is necessary to increase the proportion of large particles. However, studies have shown that when the proportion of large particles in the pole piece exceeds a certain range, the dynamic performance of the battery will be sacrificed. How to obtain a battery that takes into account both energy density and dynamic performance is a technical problem that needs to be solved urgently in this field.

[0080] The first aspect of the present application provides a lithium-ion secondary battery, the lithium-ion secondary battery comprising a positive electrode plate, a negative electrode plate and an electrolyte, wherein the positive electrode plate 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 a carbon coating material arranged on at least a portion of the surface, in a cross section of the positive electrode film layer along the thickness direction of the electrode plate, 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%; and in a cumulative distribution curve of the sphericity area of ​​the particles obtained from the cross section of the positive electrode film layer along the thickness direction of the electrode plate, the median of the sphericity L A50 It is 0.70-0.74.

[0081] In the cross section of the positive electrode film along the thickness direction of the pole piece, the area of ​​particles with a particle size greater than or equal to 1.5um accounts for less than 8%, which means that the proportion of large particles is seriously insufficient, and there is no reasonable grading, so it is difficult to achieve tight stacking and high compaction density of the positive electrode pole piece. In the cross section of the positive electrode film along the thickness direction of the pole piece, the area of ​​particles with a particle size greater than or equal to 1.5um accounts for more than 20%, which is conducive to the improvement of the compaction density of the positive electrode film, but it will sharply reduce the contact area between the electrolyte and the positive electrode active material, increase the diffusion path length of lithium ions in the particles, and cause serious polarization of the pole piece locally, increase the battery impedance, and significantly deteriorate the dynamic performance of the battery.

[0082] Controlling the area ratio of particles with a diameter of 1.5 μm or more in the positive electrode film layer to be greater than or equal to 8.0% and less than or equal to 20.0% can reduce the significant short-board effect caused by large-sized particles, which is beneficial to keep the battery impedance at a low level and improve the battery's dynamic performance, but this will limit the further improvement of the pole piece compaction density. The applicant further discovered that by adjusting the sphericity of the particles in the film layer, the particles can easily slide during rolling and fill each other, and a positive electrode film layer with a high compaction density can also be obtained. When the median L of the sphericity of the particles in the positive electrode film layer is A50 When it is less than 0.70, it indicates that the overall irregularity of the particles is still large and difficult to slide; the compaction density of the pole piece should be further improved.

[0083] In the embodiment of the present application, the area proportion of particles with a particle size greater than or equal to 1.5 μm in the positive electrode film layer is controlled to be greater than or equal to 8.0% and less than or equal to 20.0%, and the median of the sphericity L in the cumulative distribution curve of the sphericity area of ​​the particles obtained from the cross section of the positive electrode film layer along the thickness direction of the electrode sheet is A50 A value of 0.70-0.74 can reduce the short-board effect, help keep the battery impedance at a low level, and improve the battery's dynamic performance; and make the particles approximately spherical, making it easier for the particles to slide, thereby optimizing the compaction density of the pole piece, thereby achieving a balance between the battery's dynamic performance and energy density.

[0084] In the present application, the term "particle" refers to particles with identifiable complete boundaries in the field of view of the positive electrode film layer under a certain magnification, such as 10,000 times. Defects and scratches may exist inside the particles, but complete boundaries sufficient to separate the particles cannot be identified inside the particles.

[0085] In some embodiments, in a cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1.5 μm accounts for greater than or equal to 8.0% and less than or equal to 20.0%.

[0086] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode piece, the area proportion of particles with a particle size greater than or equal to 1.5 μm can be selected to be 8.0%, 9.0%, 9.08%, 10.0%, 11.0%, 12.0%, 12.78%, 13.0%, 13.23%, 14.0%, 14.49%, 15.0%, 15.41%, 15.46%, 16.0%, 17.0%, 18.0%, 19.0%, 19.51%, 19.96%, 20.0% or any numerical range therebetween.

[0087] In the cross section of the positive electrode film layer along the thickness direction of the electrode piece, the method for identifying particles is as follows: the positive electrode film layer is cut along the thickness direction of the electrode piece by an argon ion beam (as an example, the equipment model can be selected: Leica EM TIC 3X CP, working voltage: 6kV, working time: 6h), and after the cross section is exposed, a scanning electron microscope is used (as an example, the equipment model can be selected: Hitachi SU8230, working voltage: 3kV, beam current: high, probe model: U (LA100), working distance <5mm) to observe the cross section of the positive electrode film layer along the thickness direction of the electrode piece. The field emission scanning electron microscope is used to collect images in the secondary electron mode at the non-edge position of the cross section of the positive electrode film layer (after observing the edge of the electrode piece under the scanning electron microscope, adjust the field of view to the center of the sample), and the electron microscope image is taken at a magnification of 10k times. The particles in the electron microscope image are analyzed using ImageJ software (1.46r, win64 version). The specific method of using the ImageJ software is as follows: Load the scanning electron microscope image to be analyzed, such as Figure 1 As shown; the Cellpose plug-in software is used to identify particles, and manual correction is performed on this basis; Image J is used to read and count the data. The specific method of using the Cellpose plug-in software to identify particles is as follows: set the segmentation diameter parameter (diameter in the Segmentation module) to 15 pixels, click "run cyto3" to identify particles; manually mark the particles in the image that are not recognized by the software or are not fully recognized by the software or are recognized with errors. The particles that are not recognized by the software or are not fully recognized by the software or are recognized with errors in the image mainly include the following: 1. The particles cannot be recognized or cannot be fully recognized because the particles are too large or there are scratches on the surface of the particles; 2. During the argon ion beam cutting process, scratches will be generated on the surface of the particles. The software may misjudge the scratches as particle boundaries during the recognition process, thereby generating recognition errors; 3. Because the particles are too small, they are not successfully recognized; 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 fully displayed, and the local part is recognized instead of the whole, resulting in recognition errors. Manual calibration is performed for the above-mentioned particles that are not identified or have identification errors. The specific process is as follows: delete the particles located on the edges of the scanning electron microscope and cannot be fully displayed; determine whether there are gap scratches inside other particles that are not identified or have identification errors. If there are no gap scratches inside the particles, it is determined to be one particle, and it is manually marked according to the particle boundary observed manually; in response to the presence of gap scratches inside the particles, determine whether the gap scratches penetrate the particles. If not, it is determined to be one particle and manually marked; in response to the gap scratches penetrating the particles, determine whether the gap scratches are linear or irregular; in response to the gap scratches being irregular, it is determined to be the boundary between particles, and the particles are divided along the boundary; in response to the gap scratches being linear, contrast comparison is performed; in response to the contrast contrast being not obvious and no crack feeling, it is determined to be a scratch and marked as one particle; in response to the contrast contrast being strong and having a crack feeling, it is determined to be the boundary between particles and marked as two particles. After manual marking, delete the information irrelevant to the particles in the automatic image processing process, and the determination and marking of the particles in the image are completed.

[0088] In the cross section of the positive electrode film layer along the thickness direction of the electrode, the area statistics of the particles are specifically as follows. Import the image after particle determination and identification into the ImageJ software for analysis, and complete the scale setting according to the scanning electron microscope image; analyze the particle size of the particles in the image through the "Feret diameter", "Area", "Round" and "Solidity" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter obtained by analysis represents the maximum spacing between all parallel lines in the two-dimensional projection of the particle, which characterizes the particle size of the particle; the "Area" parameter represents the pixel area of ​​the particle. Because particles with a particle size of less than 50nm have large errors in the statistical process and are difficult to accurately identify, and the particle size of the conductive agent is generally less than 50nm, it will cause large errors in the statistical results. Therefore, in the particle size statistical process of this application, particles with a particle size of less than 50nm are not counted, and the particle statistical data corresponding to AR, Round or Solidity displayed as "NaN" are deleted. According to the above method, in order to meet the statistically significant sample number, each pole piece collects no less than 10 scanning electron microscope images with non-overlapping fields of view, counts the area of ​​no less than 5,000 particles, and calculates the sum of the "Area" parameters of particles with a particle size greater than or equal to 1.5 μm and the sum of the "Area" parameters of all particles, which are respectively used as the area of ​​particles with a particle size greater than or equal to 1.5 μm and the total area of ​​the particles counted. The sum of the areas of particles with a particle size greater than or equal to 1.5 μm divided by the total area of ​​the particles counted is used as the area ratio of particles with a particle size greater than or equal to 1.5 μm in the cross-section of the positive electrode film layer along the thickness direction of the pole piece.

[0089] The cross-sectional morphology of the positive electrode film along the thickness direction of the electrode is shown in Figure 1 As shown, it is different from the state of the positive electrode active material in the Malvern laser scattering method and the state of the positive electrode active material when the positive electrode active material is directly observed by scanning electron microscopy. The particles in the positive electrode film layer are well dispersed under the roller pressure. Observing the positive electrode film layer is conducive to effectively characterizing the objective situation of the particle size and area ratio in the positive electrode film layer.

[0090] During the cold pressing process, the positive electrode film layer is compacted in the thickness direction. Therefore, the cross section of the positive electrode film layer along the thickness direction of the electrode sheet can better reflect the actual compaction status of the particles inside the film layer on a spatial scale than the surface of the positive electrode film layer. 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 1.5μm can intuitively reflect the proportional relationship between the area of ​​some particles in this particle size segment and the area of ​​the entire particles, reflecting the distribution of particles in this particle size segment.

[0091] It is understandable that the particles in the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, especially the particles larger than 50 nm, mainly come from the positive electrode active material. Therefore, the embodiment 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 cross section of the positive electrode film layer.

[0092] In the prior art, the 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 phosphates are easy to agglomerate, the test results obtained by the Malvern laser diffraction method based on the laser scattering principle often measure the particle size of its particle agglomerates, which cannot truly reflect the particle size of the particles in the positive electrode active material, let alone the dispersion state of the positive electrode active material in the film layer, because the positive electrode active material in the film layer will be more dispersed during the film forming rolling process. The test results obtained by the Malvern laser diffraction method are closely related to the particle size, specific surface area, and degree of agglomeration of the positive electrode active material. Therefore, the particle size obtained by the Malvern laser diffraction method test cannot be equivalent to or analogous to the particle size obtained by statistics in the embodiments of the present application.

[0093] Those skilled in the art can control the particle size of the particles by any known process. As an example, the growth rate and time of the positive electrode material can be controlled by regulating the temperature and time during the preparation of the positive electrode material. The raw materials are processed to the target particle size distribution range by the mechanical force of the crushing and grinding process to adjust the particle size; the particle system is separated by particle size by screening and grading equipment to obtain a particle size ratio that meets the requirements; by accurately controlling the feed rate, adjusting the residence time and stress state of the particles in the equipment, it is also helpful to achieve the control of the particle size.

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

[0095] The carbon coating material disposed on at least a portion of the surface of the lithium-containing transition metal phosphate can be detected by any known method in the art. As an example, the carbon coating material disposed on at least a portion of the surface of the lithium-containing transition metal phosphate can be observed by characterizing the lithium-containing transition metal phosphate by using a transmission electron microscope and an energy spectrum analyzer.

[0096] In some embodiments, in the cumulative distribution curve of the sphericity area of ​​particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median of the sphericity L A50 It is 0.70-0.74.

[0097] The specific method for testing the sphericity of particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode piece is as follows: refer to the method described above in this application to identify the particles in the cross-section of the positive electrode film layer, and use the "shape description" and "Area" analysis functions to analyze the morphology of the particles and the area of ​​the particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode piece. According to the software manual (ImageJ User Guide IJ 1.46r), the "Round" parameter obtained by analysis represents the ratio of the pixel area of ​​the particle to the area of ​​the circle with the fitted major diameter as the diameter, which can be used to characterize the sphericity of the particle. 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 diameter as the diameter is to 1. Therefore, the "Round" parameter of the particle obtained by analysis characterizes the sphericity of the particle.

[0098] The sphericity of at least 5000 particles obtained is arranged in order from small to large, and the sphericity area cumulative distribution curve of the particles in the positive electrode film layer is obtained with the sphericity as the horizontal axis and the cumulative area percentage as the vertical axis. A50 It is the sphericity L value corresponding to when the cumulative area of ​​the vertical axis in the cumulative distribution curve of sphericity L value accounts for 50%.

[0099] In some embodiments, in the cumulative distribution curve of the sphericity area of ​​particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median of the sphericity L A50 The options can be 0.70, 0.707, 0.71, 0.717, 0.72, 0.721, 0.723, 0.73, 0.736, 0.738, 0.74 or any numerical range therebetween.

[0100] Those skilled in the art can adjust the sphericity of the particles by any known process. For example, the sphericity of the particles can be adjusted by grinding, polishing, chemical etching, mechanical stirring, extrusion, coating, granulation, adding surfactants, and adjusting the parameters of each process.

[0101] The median of sphericity L A50 The particles within the above range are approximately spherical, and are prone to slippage between particles under the action of external forces, which can further improve the compaction density of the pole piece and increase the energy density of the battery.

[0102] In some embodiments, in the cumulative distribution curve of the sphericity area of ​​particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the concentration of sphericity (L A90 -L A10 ) / L A50 It is 0.450-0.535, and can be selected as 0.450-0.500.

[0103] In some embodiments, in the cumulative distribution curve of the sphericity area of ​​the particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode piece, the concentration of sphericity can be selected as 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, 0.51, 0.52, 0.523, 0.526, 0.528, 0.53, 0.534, 0.535 or any numerical range therebetween.

[0104] In the cumulative distribution curve of the sphericity area of ​​the particles obtained from the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the concentration test method of the sphericity is as follows: Referring to the sphericity test method described above in this application, by analogy, L A90 is the L value corresponding to the cumulative area of ​​the vertical axis in the sphericity L value cumulative distribution curve when the cumulative area accounts for 90%, L A10 is the L value corresponding to the cumulative area of ​​the vertical axis in the cumulative distribution curve of sphericity L value when the cumulative area accounts for 10%. The concentration of sphericity is expressed by (L A90 -L A10 ) / L A50 Indicates. (L A90 -L A10 ) / L A50 It can reflect not only the sphericity of most particles, but also the asymmetry and width of the sphericity distribution of particles in the positive electrode film. The smaller the value, the more concentrated the distribution. Combined with the high median sphericity, it reflects that the particles are generally spherical, which is conducive to forming a tight stack, increasing the contact between particles, and further improving the dynamic performance of the battery.

[0105] In some embodiments, in the cumulative distribution curve of the sphericity area of ​​particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the sphericity L A10 0.40-0.60, spherical L A90 It is 0.85-1.

[0106] In some embodiments, in the cumulative distribution curve of the sphericity area of ​​particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the sphericity L A10 The amount may be selected from 0.40, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.50, 0.507, 0.509, 0.51, 0.512, 0.514, 0.516, 0.52, 0.524, 0.526, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.60 or any range therebetween.

[0107] In some embodiments, in the cumulative distribution curve of the sphericity area of ​​particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the sphericity LA90 The options include 0.85, 0.86, 0.865, 0.87, 0.88, 0.886, 0.887, 0.89, 0.894, 0.898, 0.90, 0.91, 0.918, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1 or any range therebetween.

[0108] Sphericity L A10 , L A90 Within the above range, it indicates that most of the particles have a high degree of sphericity, avoiding extreme morphological differences, and will not cause excessive gaps and stress concentration due to bridging between particles. It is conducive to the formation of regular stacking and uniform slip, and improves the compaction density of the pole piece.

[0109] In some embodiments, in a cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles having a particle size greater than or equal to 5 μm is 0.

[0110] Studies have shown that particles with a diameter greater than or equal to 5 μm in the positive electrode film layer will significantly deteriorate the infiltration of the electrolyte in the positive electrode film layer and the diffusion in the active material particles. The area ratio of particles with a diameter greater than or equal to 5 μm is 0, which is beneficial to further reduce the internal resistance of the battery and improve the battery kinetic performance.

[0111] In some embodiments, in a cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1.5 μm and less than 5 μm accounts for 9.0%-20.0%, and can be optionally 10.0%-20.0%.

[0112] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode piece, the area proportion of particles with a particle size greater than or equal to 1.5 μm and less than 5 μm can be selected to be 9.0%, 9.08%, 10.0%, 11.0%, 12.0%, 12.78%, 13.0%, 13.23%, 14.0%, 14.49%, 15.0%, 15.41%, 15.46%, 16.0%, 17.0%, 18.0%, 19.0%, 19.51%, 19.96%, 20.0% or any numerical range therebetween.

[0113] 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 along the thickness direction of the electrode piece can be tested in the manner described above. 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 along the thickness direction of the electrode piece is divided by the total area of ​​the particles counted as 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 along the thickness direction of the electrode piece.

[0114] In the process of improving particle grading, increasing the size or area ratio of large particles, it is inevitable to introduce particles with a particle size greater than or equal to 1.5μm and less than 5μm. In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 1.5μm and less than 5μm within the above range can further reduce the obstruction of large-sized particles in the electrode sheet to the infiltration and diffusion of the electrolyte in the positive electrode film layer, improve the consistency of the diffusion rate of lithium ions in the positive electrode active material particles, reduce local polarization, and improve the dynamic performance of the battery.

[0115] In some embodiments, in a cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1 μm and less than 1.5 μm accounts for 15.0%-25.0%, and can be optionally 16.0%-24.0%.

[0116] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode piece, the area proportion of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm can be selected to be 15.0%, 15.65%, 15.91%, 16.0%, 17.0%, 18.0%, 18.33%, 18.38%, 19.0%, 19.65%, 20%, 20.45%, 20.87%, 21%, 22%, 23%, 23.88%, 24%, 25% or any numerical range therebetween.

[0117] The area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm in the cross section of the positive electrode film layer along the thickness direction of the electrode piece can be tested in the manner described above. The area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm in the cross section of the positive electrode film layer along the thickness direction of the electrode piece is divided by the total area of ​​the particles counted as the area ratio of particles with a particle size greater than or equal to 1 μm and less than 1.5 μm in the cross section of the positive electrode film layer along the thickness direction of the electrode piece.

[0118] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size greater than or equal to 1μm and less than 1.5μm is within the above range, which can form a sufficient "support" structure in the stacking, which helps to enhance the overall structural strength between the particles. At the same time, a support structure is formed between smaller particles, filling larger gaps and reducing the void ratio between particles. This is beneficial to further improve the compaction density of the electrode sheet and improve the energy density of the battery while maintaining good battery dynamic performance.

[0119] In some embodiments, in a cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 200 nm and less than 1500 nm accounts for 73.0%-80.0%, and can optionally be 73.0%-78.0%.

[0120] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode piece, the area proportion of particles with a particle size greater than or equal to 200 nm and less than 1500 nm can be selected to be 73.0%, 73.01%, 73.15%, 74.0%, 75.0%, 75.16%, 75.20%, 75.48%, 75.78%, 75.88%, 76.0%, 77.0%, 78.0%, 78.69%, 79.0%, 80.0% or any numerical range therebetween.

[0121] The area ratio of particles with a particle size greater than or equal to 200nm and less than 1500nm in the cross section of the positive electrode film layer along the thickness direction of the electrode piece can be obtained by referring to the test method described above. The area ratio of particles with a particle size greater than or equal to 200nm and less than 1500nm in the cross section of the positive electrode film layer along the thickness direction of the electrode piece is the sum of the areas of particles with a particle size greater than or equal to 200nm and less than 1500nm divided by the total area of ​​the particles counted.

[0122] The area ratio of particles with a diameter greater than or equal to 200nm and less than 1500nm in the positive electrode film layer is within the above range, which can further improve the consistency of the lithium ion diffusion rate, thereby improving the kinetic performance of the lithium ion secondary battery, and can further increase the compaction density of the electrode sheet and improve the energy density of the battery.

[0123] In some embodiments, in the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the surface scanning mode of the laser micro-confocal Raman spectrometer, the median graphitization degree C 50 is 0.95-1.20, and can be further selected as 0.98-1.15; wherein the graphitization degree C value is I G / I D , where I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at .

[0124] In the present application, the graphitization degree C value of the positive electrode film layer can be obtained by the surface scanning mode of the laser micro-confocal Raman spectrometer. As an example, specifically, a laser micro-confocal Raman spectrometer (high-precision Renishaw laser micro-confocal Raman spectrometer) is used, an excitation wavelength of 532nm is selected, and an appropriate amount of positive electrode film layer is taken to perform a surface scan on its surface or a section along the thickness direction of the electrode piece. The scanning area is 45μm×45μm, divided into 10×10 grids, with the grid vertex as the test point, the step length is 5μm, and the total number of scanning points is 100 points, thereby obtaining the C values ​​of different sites and the cumulative distribution curve of the C value of the surface scanning area.

[0125] The graphitization degree C value of the positive electrode film is obtained by the peak intensity ratio of the G peak (G-band) and the D peak (D-band) of the Raman spectrum. The G peak position is 1585±100cm -1 , which characterizes carbon sp 2 Hybrid structure; D peak position is 1350±50cm -1 , which characterizes a disordered structure, where disorder means that there is no regular arrangement between the carbon atoms in the structure. In graphite crystals, the carbon atoms in the same layer are arranged in sp 2 Hybridization forms covalent bonds, and the interlayer is van der Waals force, which makes the carbon in the graphite structure easy to slide. Therefore, the C value can characterize the graphitization degree of the positive electrode active material. It can be understood that the graphitization degree of the positive electrode active material containing lithium phosphate in this application mainly comes from the carbon coating material on its surface. 2 The hybrid structure of carbon nanotube conductive agent also has a relatively high I G / I D However, due to its small content and small diameter, its addition to the positive electrode film layer shows an extreme value in the Raman surface scanning test of the positive electrode film layer, and will not affect the graphitization degree C in the positive electrode film layer. 50 Therefore, the graphitization degree of the positive electrode film can also be used to characterize the graphitization degree of the positive electrode active material.

[0126] The graphitization degree C value cumulative distribution curve refers to a curve obtained by arranging at least 100 C values ​​obtained in order from small to large, with the graphitization degree as the horizontal axis and the cumulative number percentage as the vertical axis. 50 The C value corresponding to the cumulative number of the vertical axis in the graphitization degree C value cumulative distribution curve is 50%. The median C of the graphitization degree 50 Compared with the point value, it can reflect the overall graphitization degree of the particles in the positive electrode film layer, that is, the degree of slip; compared with the mean value, it can reduce the influence of extreme values ​​during the test and improve the confidence of the test results.

[0127] The higher the degree of graphitization of the carbon on the surface of the positive electrode active material, the higher the proportion of graphite structure carbon in the positive electrode film layer, and the easier it is for the particles to slip with the help of the highly graphitized carbon structure in the coating material, thereby increasing the compaction density of the electrode.

[0128] Those skilled in the art can adjust the graphitization degree of the active material particles by any known process. As an example, adjusting the carbon source (the carbon source can be a polymer carbon source, such as PEG), sintering temperature, sintering time, sintering pressure, sintering atmosphere, and nucleation process can all adjust the graphitization degree of the active material particles.

[0129] In some embodiments, in the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the surface scanning mode of the laser micro-confocal Raman spectrometer, the median graphitization degree C 50 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.20, or any range therebetween.

[0130] In some embodiments, in the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median roughness R A50 It is 0.92-0.96.

[0131] In the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median of the roughness R A50 The specific test method is as follows: Refer to the method described above in this application to identify the particles in the cross-section of the positive electrode film layer, and use the "shape description" analysis function in ImageJ to analyze the morphology of the particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode. According to the software manual (ImageJ User Guide IJ 1.46r), the "Solidity" parameter obtained by analysis represents the ratio of the pixel area to the convex area of ​​the particle. Therefore, the "Solidity" parameter of the particles obtained by analysis is used to characterize the roughness of the particles. According to the definition, the closer the roughness is to 1, the smoother the particle. Arrange the sphericity of at least 5,000 particles obtained in order from small to large, and use the roughness as the horizontal axis and the cumulative area percentage as the vertical axis to obtain the roughness cumulative distribution curve of the particles in the positive electrode film layer. R A50 It is the roughness R value corresponding to when the cumulative area of ​​the vertical axis in the roughness R value cumulative distribution curve accounts for 50%.

[0132] In some embodiments, in the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the median roughness of the particles R A50 The values ​​may be 0.92, 0.93, 0.94, 0.95, 0.96 or any range therebetween.

[0133] Those skilled in the art can adjust the roughness of the particles by any known process. For example, the roughness of the particles can be adjusted by grinding, polishing, grinding, milling, electroplating, calendering and other processes and by adjusting the parameters of each process.

[0134] Median roughness R A50 The surface of particles within the above range is relatively smooth, the friction between particles is relatively small, and they are easy to slip under the action of external force, which can further improve the compaction density of the pole piece and increase the energy density of the battery.

[0135] In some embodiments, in the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the concentration (R A90 -R A10 ) / R A50 It is 0.06-0.15.

[0136] In some embodiments, in the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the concentration (R A90 -R A10 ) / R A50 It can be selected as 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15 or any numerical range therebetween.

[0137] In the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the roughness concentration test method is as follows: Referring to the roughness test method described above in this application, by analogy, R A90 is the R value corresponding to the cumulative area of ​​the vertical axis in the sphericity R value cumulative distribution curve when it accounts for 90%, R A10 The R value corresponding to the cumulative area of ​​the vertical axis in the roughness R value cumulative distribution curve is 10%. The concentration of roughness is expressed by (R A90 -R A10 ) / R A50 The concentration value of roughness is extremely small, indicating that the overall roughness consistency of the particles is high, which is conducive to the relative sliding between particles. On the basis of high sphericity, the particles are more likely to form high-density accumulation during rolling, increasing the compaction density of the pole piece and the energy density of the battery.

[0138] In some embodiments, the positive electrode active material includes iron element, and the iron dissolution rate of the positive electrode film layer is 500 ppm-2000 ppm.

[0139] The iron dissolution rate of the positive electrode film layer can be tested by methods well-known in the art. After disassembling and washing the electrode sheet from the battery, it is formed into small round pieces with a diameter of 14 mm. Take multiple small round piece samples so that the total mass of the samples is about 5 g, and add them to 100.3 g of ascorbic acid solution with a mass concentration of 0.3% (the solvent is ultrapure water). Stir at a speed of 500 revolutions per minute for 305 minutes, then quickly use a 5 mL syringe to rapidly suck the solution, filter the solution through a 0.45 μm pore size filter head into a test tube, pipette 1 mL of the supernatant, add it to a glass volumetric flask and dilute it 50 times, and test it with an inductively coupled plasma mass spectrometer (ICP-OES) to obtain the iron element concentration in the solution. Through the formula: [(ICP test iron element concentration × solution volume / mass of the solution involved in volume fixation) × 100.3 g / (mass of the electrode sheet of the small round piece - mass of the current collector of the small round piece)], the solution volume is 50 mL, and the mass of the solution involved in volume fixation is 1 g, and the iron dissolution rate of the positive electrode film layer is calculated. Preferably, the mass of the current collector of the small round piece is obtained by multiplying the thickness of the small round piece by the area and the density. The thickness of the small round piece can be equivalently measured by measuring the thickness of the current collector in the uncoated area with a thickness gauge. It can be understood that although the current collector will be extended during the compaction process in the coated area, resulting in a slightly lower thickness compared to the uncoated area, since the reduction amplitude is negligible, it will not have a significant impact on the test results. More preferably, when the current collector is aluminum foil, the density is 2.7 g / cm 3 .

[0140] In some embodiments, the positive electrode active material includes iron element, and the iron dissolution rate of the positive electrode film layer can be selected from 500 ppm, 575 ppm, 600 ppm, 700 ppm, 800 ppm, 816 ppm, 900 ppm, 1000 ppm, 1051 ppm, 1062 ppm, 1100 ppm, 1187 ppm, 1200 ppm, 1300 ppm, 1334 ppm, 1400 ppm, 1500 ppm, 1600 ppm, 1700 ppm, 1761 ppm, 1800 ppm, 1894 ppm, 1900 ppm, 2000 ppm or the numerical range between any two of them.

[0141] Those skilled in the art can regulate the iron dissolution rate of the positive electrode film layer through any known process. As an example, the iron dissolution rate of the positive electrode film layer is regulated by regulating the surface coating mass of the positive electrode material, the temperature, time, and pressure during the preparation process. In addition, during the use of the battery, the battery design, the content of oxidants in the electrolyte, the battery operating temperature, and the battery charge and discharge intensity will also affect the iron dissolution rate of the positive electrode film layer.

[0142] The iron dissolution rate of the positive electrode film layer mainly comes from the lithium-containing transition metal phosphate positive electrode active material in the positive electrode film layer, which can reflect the integrity and density of the carbon coating on the surface of the positive electrode active material from the side. The lower the iron dissolution rate, the less likely the iron ions after acid dissolution are to precipitate from the carbon-coated material, that is, the more complete and dense the carbon-coated material on the surface of the positive electrode active material. When the iron dissolution rate of the positive electrode film layer is within the above range, it means that the positive electrode active material has a relatively complete and dense carbon-coated 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 densely coated carbon layer has a low space occupancy rate, and the particle gaps are easily compressed by stress during the rolling process, which can improve the compaction density of the electrode sheet and the energy density of the battery at the same time.

[0143] In some embodiments, based on the total mass of the positive electrode active material, the mass content of carbon element is 0.8% - 1.8%, and can be optionally 0.90% - 1.5%.

[0144] Based on the total mass of the positive electrode active material, the mass ratio of carbon element can be measured by methods and equipment known in the art. For example, referring to GB / T 21023-2006 "Determination of total carbon and sulfur content in steel - Infrared absorption method after combustion in high-frequency induction furnace", it is measured by a Dekai HCS infrared carbon-sulfur analyzer.

[0145] In some embodiments, based on the total mass of the positive electrode active material, the mass content of carbon element can be optionally 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.

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

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

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

[0149] In some embodiments, the lithium iron antisite defect concentration of the positive electrode active material may 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 range therebetween.

[0150] Those skilled in the art can achieve the regulation of the anti-site defects of lithium iron in the positive electrode active material by any known process. As an example, the regulation of the anti-site defects of lithium iron in the positive electrode active material can be achieved by regulating the sintering temperature, sintering time, preparation method, raw material stoichiometry, etc.

[0151] During the preparation and circulation process, there will inevitably be certain lithium vacancies in the crystal structure of the positive electrode active material. Lithium vacancies will not only cause the oxidation of ferrous ions to ferric ions, but also induce the partial migration of ferric ions to the lithium position, forming lithium-iron anti-site defects, blocking the one-dimensional diffusion channel of lithium ions, and adversely affecting the solid phase transmission of lithium ions. The positive electrode active material in the embodiment of the present application has low lithium-iron anti-site defects, which is conducive to the uniform transmission of lithium ions in the solid phase, and further improves the kinetic performance of lithium-ion secondary batteries.

[0152] In some embodiments, 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 , Among them, Q includes one or more of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, 0<q≤0.1.

[0153] In some embodiments, 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 in the range consisting of any two of the above values.

[0154] In some embodiments, 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 in a range consisting of any two of the above values.

[0155] In some embodiments, y may be 0.95, 0.96, 0.97, 0.98, 0.99, 1.00, or a value in a range consisting of any two of the above values.

[0156] In some embodiments, j can be selected as 3.5, 3.6, 3.7, 3.8, 3.9, 4 or a value in the range consisting of any two of the above values.

[0157] In some embodiments, 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 in a range consisting of any two of the above values.

[0158] Selecting an appropriate modifying element Q can improve the lattice change rate of the positive electrode active material during the lithium insertion and extraction process, reduce the oxygen activity on the particle surface, improve the structural stability of the material, and thereby increase the material's gram capacity during the cycle process, further improving the cycle stability of lithium-ion secondary batteries.

[0159] In some embodiments, the positive electrode active material includes one or more of lithium iron phosphate and its doped modified materials and coated modified materials.

[0160] In some embodiments, the positive electrode active material includes titanium element, and the mass content of the titanium element is 2000 ppm-6000 ppm based on the total mass of the positive electrode active material.

[0161] The type and content of the 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.

[0162] The doping of titanium in the positive electrode active material is conducive to causing lattice distortion, reducing Li-O bond energy, increasing lithium ion transmission rate, and improving the dynamic performance of lithium ion secondary batteries. However, in the prior art, the doping content of titanium in lithium-containing phosphates often cannot exceed 3000ppm, because too much titanium 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 battery performance.

[0163] The positive electrode active material in the embodiment of the present application has a high titanium content, and surprisingly, the high addition of titanium does not form a harmful impurity phase that has a negative impact on the energy density and kinetic performance of the battery. Although the reason is not yet clear, it is speculated that titanium, phosphate and other elements (for example, lithium) together form a fast ion conductor, which in turn improves the kinetic performance of the battery.

[0164] In some embodiments, the tap density of the positive electrode active material is 0.70 g / cm 3 -1.50g / cm 3 , optional 0.7-1.20g / cm 3 .

[0165] The tap density of powders can be tested by methods known in the art. As an example, turn on the electronic balance, first use a conical flask as a base and place it on the electronic balance, then reset the electronic balance; take a tapped measuring cylinder and place it on the conical flask to weigh and record the weight of the measuring cylinder; open the sample bag, use a clean sample spoon to stir the sample in the sample bag for 3-5 times to mix, and then transfer the sample smoothly into the measuring cylinder; wipe the powder contaminated on the surface of the measuring cylinder with dust-free paper, and then put it into the zeroed conical flask to weigh; seal the mouth of the measuring cylinder with a sealing film, and place the tapped measuring cylinder into the matching instrument rubber ring to ensure that the tapped density measuring cylinder fits tightly to the rubber ring and remains perpendicular to the instrument surface; set the vibration frequency on the instrument to 250 times / min and the number of vibrations to 5000 times, press the button, and vibrate for 20 minutes; then remove the tapped density measuring cylinder, use a flashlight to illuminate the surface of the measuring cylinder, and use visual inspection to read the highest scale V 1 and the minimum scale V 2 , take the average value V of the two; the mass of the measuring cylinder and the sample m 1 Subtract the mass of the measuring cylinder m 0 , the powder mass m is obtained, and the sample tap density is obtained by the density formula ρ=m / v.

[0166] In some embodiments, the tap density of the positive electrode active material powder can be 0.70 g / cm 3 、0.80g / cm 3 , 0.90g / cm 3 , 1.00g / cm 3, 1.10g / cm 3 , 1.20g / cm 3 , 1.30g / cm 3 , 1.40g / cm 3 , 1.50g / cm 3 Or any range of values ​​in between.

[0167] In some embodiments, the powder compaction density of the positive electrode active material under 3T pressure is 2.50 g / cm 3 -2.70g / cm 3 , optional 2.52g / cm 3 -2.68 g / cm 3 .

[0168] In this application, the term "powder compaction density" refers to the density of a compact with a certain density and strength formed during the external compression process. As the powder moves and deforms, larger gaps are filled, the contact area between particles increases, the attraction between atoms is generated, and the mechanical fit between particles is enhanced. The unit is g / cm 3 .

[0169] The compaction density of the positive electrode active material powder can be measured by methods and equipment known in the art. For example, it can be measured by using a compaction density instrument with reference to GB / T 24533-2009. Specifically, a certain amount of positive electrode active material is placed on a special compaction mold (the mold diameter is known), and the mold has a metal disc above and below the hollow center. The positive electrode active material is placed between the metal discs, a metal cylinder is placed on the top, and the mold is placed on the compaction density instrument. The bottom area of ​​the mold is 1.327cm 2 , set the pressure to 3T, the thickness of the positive electrode active material under the pressure of 3T can be read on the equipment, the powder compaction density of the positive electrode active material is ρ=m / v, where v=(S×H), m is the mass of the positive electrode active material, S is the bottom area of ​​the mold, and H is the thickness of the positive electrode active material after compaction.

[0170] In some embodiments, the powder compaction density of the positive electrode active material under 3T pressure can be 2.50 g / cm 3 , 2.51g / cm 3 , 2.52g / cm 3 , 2.53g / cm 3 , 2.54g / cm 3 , 2.55g / cm 3 , 2.56g / cm 3 , 2.57g / cm 3 , 2.58g / cm 3 , 2.59g / cm3 , 2.60g / cm 3 , 2.61g / cm 3 , 2.62g / cm 3 , 2.63g / cm 3 , 2.64g / cm 3 , 2.65g / cm 3 , 2.67g / cm 3 , 2.68g / cm 3 , 2.69g / cm 3 , 2.70g / cm 3 Or any range of values ​​in between.

[0171] Although the area proportion of the particles with a particle size of 1.5 μm or more is low, the effective gradation formed independently is limited, and the powder tap density is relatively low, the positive electrode active material is easy to roll under the action of external force due to its high sphericity, thereby filling the gaps with each other to achieve a higher compaction density, providing a material basis for improving the compaction density of the pole piece and preparing a high energy density lithium-ion secondary battery.

[0172] In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 8 MPa is 0.5 Ω·cm-30.0 Ω·cm, and can be optionally 2.0-20.0 Ω·cm.

[0173] The powder resistivity of the positive electrode active material can be measured by methods and equipment known in the art. For example, it can be measured with a powder resistivity meter (Suzhou Jingge, ST2722 model) with reference to GB / T 33822-2017. Specifically, a certain amount of positive electrode active material (e.g. 1 g) is weighed and added to the feeding chamber of the powder resistivity meter, and a pressure of 8 MPa is applied. The forward resistivity and reverse resistivity of the positive electrode active material are tested respectively, and the average value of the two is taken as the powder resistivity of the positive electrode active material.

[0174] In some embodiments, the powder resistivity of the positive electrode active material under a pressure of 8 MPa may be selected as 0.5Ω·cm, 1Ω·cm, 2Ω·cm, 3Ω·cm, 4Ω·cm, 5Ω·cm, 6Ω·cm, 6.3Ω·cm, 7Ω·cm, 8Ω·cm, 9Ω·cm, 10Ω·cm, 15Ω·cm, 20Ω·cm, 25Ω·cm, 30Ω·cm or any range therebetween.

[0175] The positive electrode active material is coated with carbon material on the surface, and the sp 2 The structure facilitates rapid conduction of electrons between particles, making the positive electrode active material have a low powder resistivity, which is beneficial to increasing the solid-phase transmission rate of electrons and improving the kinetic performance of the battery.

[0176] In some embodiments, the positive electrode active material has a discharge capacity of 135 mAh / g to 150 mAh / g at a 1C discharge rate.

[0177] In this application, the positive electrode active material is assembled into a button cell and the electrical performance is tested on a blue electric tester. At 25±5℃, within the voltage range of 2.0V~3.75V, it is charged to 3.75V at 1C constant current, then paused for 5 minutes, charged at constant voltage to a cut-off current of 50μA, and then discharged to 2.0V at 1C constant current. 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 discharge rate of 1C.

[0178] The preparation and testing process of button cells is as follows: 2.0g of positive electrode active material, conductive carbon black, and PVDF are mixed at a mass ratio of 0.9:0.05:0.05, and then organic solvent NMP (N-methylpyrrolidone) is added. After fully mixing, a 150μm scraper is used for coating, and the mixture is dried at 100℃ for 2h. The compacted density is 2.0g / cm 3 -2.2g / cm 3 The positive electrode sheet was cold pressed and punched into a 14 mm diameter disc using a hole puncher, then weighed and recorded, and the weighed positive electrode sheet was placed in a vacuum drying oven (105°C, 1-12 hrs, -90 kpa). After baking, the positive electrode sheet was placed in a glove box, and the battery was assembled in the order of negative electrode shell-nickel mesh-lithium sheet-isolation membrane-positive electrode sheet-positive electrode shell, and 65-87 μL (pipette gun) of electrolyte (the electrolyte was a mixed solvent of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) in a volume ratio of 1:1, and the electrolyte LiPF 6 ), the negative electrode is on top and placed in the groove of the sealing machine, the sealing pressure is 650kg / cm 2 , use insulated tweezers to remove the button battery and put it into a dust-free bag, remove the glove box, and place it in a constant temperature room for 3 hours to obtain the button battery for testing.

[0179] It can be understood that the discharge capacity in grams of the positive electrode active material can also be obtained by disassembling the entire battery to obtain the positive electrode sheets, assembling them into button cells according to the method described above, and then testing them.

[0180] In some embodiments, the discharge capacity in grams of the positive electrode active material at a 1C discharge rate may be 135 mAh / g, 140 mAh / g, 142.3 mAh / g, 145 mAh / g, 150 mAh / g, or any range therebetween.

[0181] The positive electrode active material has a high discharge gram capacity at a 1C rate, indicating that it has good charge and discharge capabilities and is beneficial to improving the kinetic performance of the battery.

[0182] In some embodiments, the discharge capacity of the positive electrode active material discharged to 3.2V accounts for η≥85%, where η is defined as: At room temperature, a button cell containing the positive electrode active material is charged and discharged twice at a constant current rate of 0.1C in the voltage range of 2.0V to 3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at a rate of 1C, the capacity value at the discharge voltage of 3.2V is extracted and recorded as C 1 , extract the capacity value of the discharge voltage to 2.0V as C 2 , η = C 1 / C 2 The charging process includes constant voltage charging, constant voltage 3.75V, and constant voltage cut-off current 50μA.

[0183] In some embodiments, the discharge capacity of the positive electrode active material discharged to 3.2V accounts for n≥88%.

[0184] The η value of the positive electrode active material can be measured by methods and equipment known in the art. As an example, first prepare a button cell with reference to the method described above, and test the electrical performance of the prepared button cell on a blue electric tester at room temperature. Specifically, the button cell is charged and discharged twice at a constant current of 0.1C in the voltage range of 2.0V~3.75V, and then charged to the cut-off voltage with a constant current and then charged and discharged once with a constant current of 1C. In the charge and discharge test at a rate of 1C, the capacity value from 3.75V to 3.2V is recorded as C 1 , the capacity value of discharge from 3.75V to 2.0V is C 2 , and η=C 1 / C 2 .

[0185] In some embodiments, n can be selected as 88%, 88.1%, 89%, 90%, 90.1%, 91%, 92%, 92.1%, 92.2%, 93%, 94%, 94.1%, 94.5%, 95%, 95.1%, or a range consisting of any two of the above numerical intervals, or a value within the range.

[0186] In some embodiments, the positive electrode active material in the freshly prepared lithium ion secondary battery has a discharge capacity ratio η of ≥88% when discharged to 3.2V. After the freshly prepared lithium ion secondary battery is charged and discharged at a constant current rate of 0.1C in a voltage range of 2.0V to 3.75V for a period of time, the discharge capacity ratio η of the positive electrode active material when discharged to 3.2V can be maintained at ≥85%.

[0187] The high discharge capacity ratio of the positive electrode active material used in the lithium ion secondary battery of the present application embodiment 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 conducive to maintaining good power performance.

[0188] In some embodiments, in a 0.1C discharge curve of a button cell comprising the positive electrode active material, there is a discharge platform in the voltage range of 2.5V to 2.9V.

[0189] The discharge platform usually refers to a region where the voltage is relatively stable during the battery charging and discharging process. During the battery discharge process, current flows out of the battery, and the battery voltage will drop at the beginning, but then it will enter a relatively stable region where the voltage changes very little. This stable voltage region is called the discharge platform.

[0190] The button cell can be assembled by disassembling the positive electrode sheet in the lithium-ion secondary battery and combining it with lithium metal. It can also be assembled and prepared according to the method described above. In this application, the positive electrode active material is assembled into a button cell and the electrical performance is tested on a blue electric tester. In the voltage range of 2.0V~3.75V, after charging to 3.75V at a constant current of 0.1C, pause for 5 minutes, charge at a constant voltage to a cut-off current of 50μA, and then discharge to 2.0V at a constant current of 0.1C.

[0191] The discharge curve shows that the standard charge and discharge platform voltage of lithium phosphate is usually between 3.2V and 3.65V. The button battery containing the positive electrode active material in the embodiment of the present application shows a new charge and discharge platform in the voltage range of 2.5V~2.9V, which is conducive to increasing the discharge range of the battery and improving the energy density of the battery. At the same time, this also verifies the conjecture that the positive electrode active material in the embodiment of the present application contains a fast ion conductor.

[0192] In some embodiments, the positive electrode film layer further includes a conductive agent, and based on the total mass of the positive electrode film layer, the mass content of the conductive agent is 0.1%-1.5%.

[0193] In some embodiments, the positive electrode film layer also includes a conductive agent. Based on the total mass of the positive electrode film layer, the 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 any numerical range therebetween.

[0194] In some embodiments, the conductive agent includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0195] The particles in the positive electrode film layer have a high sphericity, so the particles can be densely stacked during rolling, and the particles are in good contact with each other, so that the particles inside the positive electrode film layer have good electronic conductivity, which can reduce or even eliminate the use of conductive agents in the positive electrode film layer, which is beneficial to further increase the loading amount of positive electrode active materials and improve the energy density of lithium-ion secondary batteries.

[0196] In some embodiments, the positive electrode film layer does not include a conductive agent.

[0197] The particles inside the positive electrode film layer have extremely high electronic conductivity, so that no conductive agent is even added to the positive electrode film layer, which is beneficial to further increase the loading amount of the positive electrode active material and improve the energy density of the lithium-ion secondary battery.

[0198] In some embodiments, the positive electrode film layer further includes a binder. Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 95.5%-99.5%, optionally 96.5%-99.5%; the mass content of the binder is 0.5%-3%.

[0199] In some embodiments, the binder includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin.

[0200] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material may be 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, 99%, 99.5% or any numerical range therebetween.

[0201] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the binder can be selected to be 0.5%, 1%, 1.5%, 2%, 2.5%, 3% or any numerical range therebetween.

[0202] 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 per unit volume of the positive electrode film layer, maintain good internal bonding force, reduce the probability of powder loss, expansion and cracking problems, and improve the energy density of the secondary battery while taking into account safety performance.

[0203] In some embodiments, the single-side density of the positive electrode film layer is 300 mg / 1540 mm 2 -450mg / 1540mm 2 .

[0204] In this application, the single-side density of the positive electrode film layer is a well-known meaning in the art and can be tested by methods known in the art. For example, take a single-sided coated and cold-pressed 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), and punch it into an area of ​​S 1 Weigh the small disc and record it as M 1 Then wipe off the positive electrode film layer of the weighed positive electrode sheet, weigh the weight of the current collector, and record it as M 0 . Single-side density of positive electrode film = (M 1 -M 0 ) / S 1 In order to ensure the accuracy of the test results, multiple groups (eg, 10 groups) of samples to be tested may be tested, and the average value may be calculated as the test result.

[0205] In some embodiments, the surface density of the positive electrode film layer on one side may be 300 mg / 1540 mm 2 、310mg / 1540mm 2 、320mg / 1540mm 2 、330mg / 1540mm 2 、340mg / 1540mm 2 、350mg / 1540mm 2 、360mg / 1540mm 2 、370mg / 1540mm 2 、380mg / 1540mm 2 、390 mg / 1540mm 2 、400mg / 1540mm 2 、410mg / 1540mm 2 、420mg / 1540mm 2 、430mg / 1540mm 2 、440mg / 1540mm 2 450 mg / 1540 mm 2 Or any range of values ​​in between.

[0206] The positive electrode film layer having an area density within the above range can contribute to improving the energy density of the lithium ion secondary battery.

[0207] In some embodiments, when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.51 g / cm 3 -2.73 g / cm 3 .

[0208] In some embodiments, when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.55 g / cm 3 -2.70g / cm 3 .

[0209] In this application, the full discharge state refers to placing the battery at 25°C, leaving it to stand for 2 hours, and when the battery temperature remains at 25°C, discharging the battery at a constant current of 1 / 3C to 2.5V and then at a constant current of 0.1C to 2.0V.

[0210] The compaction density of the positive electrode film layer can be tested by methods known in the art. As an example, place the battery in a 25°C oven environment and let it stand for 2 hours. When the battery temperature is maintained at 25°C, discharge it at a constant current of 1 / 3C to 2.5V and then at a constant current of 0.1C to 2.0V. Disassemble the battery to obtain the positive electrode sheet, treat the residual electrolyte with dimethyl carbonate solvent, dry the sheet, cut it into small discs with an area of ​​S, and obtain its mass W. 1 The thickness T of the positive electrode sheet is measured using a micrometer 1 Then wipe off the positive electrode film layer of the weighed electrode and weigh the mass of the current collector, which is recorded as W. 2 Measure the thickness of the current collector T using a micrometer 2 , then the compaction density of the positive electrode film layer PD = (W 1 -W 2 ) / [(T 1 -T 2 )×S].

[0211] In some embodiments, the compaction density of the positive electrode film layer is 2.51 g / cm 3 , 2.52g / cm 3 , 2.53g / cm 3 , 2.54g / cm 3 , 2.55g / cm 3 , 2.56g / cm 3 , 2.57g / cm 3 , 2.58g / cm 3 , 2.59g / cm 3 , 2.60g / cm 3 , 2.61g / cm 3 , 2.62g / cm 3 , 2.63g / cm 3 , 2.64g / cm3 , 2.65g / cm 3 , 2.66g / cm 3 , 2.67g / cm 3 , 2.68g / cm 3 , 2.69g / cm 3 , 2.70g / cm 3 , 2.71g / cm 3 , 2.72g / cm 3 , 2.73g / cm 3 or any value in between.

[0212] In some embodiments, after the cold pressing process, the compaction density of the positive electrode film layer is 2.63 g / cm 3 -2.85g / cm 3 .

[0213] In some embodiments, after the cold pressing process, the compaction density of the positive electrode film layer can be selected to be 2.63 g / cm 3 , 2.64g / cm 3 , 2.65g / cm 3 , 2.66g / cm 3 , 2.67g / cm 3 , 2.68g / cm 3 , 2.69g / cm 3 , 2.70g / cm 3 , 2.71g / cm 3 , 2.72g / cm 3 , 2.73g / cm 3 , 2.74g / cm 3 , 2.75g / cm 3 , 2.76g / cm 3 , 2.77g / cm 3 , 2.78g / cm 3 g / cm 3 , 2.79g / cm 3 , 2.80g / cm 3 , 2.81g / cm 3 , 2.82g / cm 3 , 2.83g / cm 3 , 2.84g / cm 3 , 2.85g / cm 3 Or any range of values ​​in between.

[0214] In this application, cold pressing refers to compacting the positive electrode film layer by mechanical pressure during the battery assembly process to improve its density and conductivity.

[0215] In some embodiments, after the chemical formation process, the compaction density of the positive electrode film layer is 2.51 g / cm 3 -2.73g / cm 3 .

[0216] In some embodiments, after the chemical formation process, the compaction density of the positive electrode film layer can be selected to be 2.51 g / cm 3 , 2.52g / cm 3 , 2.53g / cm 3 , 2.54g / cm 3 , 2.55g / cm 3 , 2.56g / cm 3 , 2.57g / cm 3 , 2.58g / cm 3 , 2.59g / cm 3 , 2.60g / cm 3 , 2.61g / cm 3 , 2.62g / cm 3 , 2.63g / cm 3 , 2.64g / cm 3 , 2.65g / cm 3 , 2.66g / cm 3 , 2.67g / cm 3 , 2.68g / cm 3 , 2.69g / cm 3 , 2.70g / cm 3 , 2.71g / cm 3 , 2.72g / cm 3 , 2.73g / cm 3 Or any range of values ​​in between.

[0217] In this application, formation refers to the formation of a stable solid electrolyte interface (SEI film) and electrode structure through electrochemical reactions during the first charge and discharge process of the battery.

[0218] It can be understood that, with the rebound of the electrode during the cycle, the compaction density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery is slightly lower than the compaction density of the positive electrode film layer after cold pressing and formation.

[0219] The compaction density of the positive electrode film layer is within the above range, which is beneficial to improving the energy density of the lithium-ion secondary battery.

[0220] In some embodiments, the compaction density of the positive electrode film layer is 2.51 g / cm 3 -2.73g / cm 3 In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10%-22%.

[0221] In some embodiments, the compaction density of the positive electrode film layer is 2.55 g / cm 3 -2.70g / cm 3 In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10%-20%.

[0222] In some embodiments, in a cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer may be selected to be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22% or any numerical range therebetween.

[0223] In the cross section of the positive electrode film layer along the thickness direction of the electrode piece, the porosity of the positive electrode film layer can be tested in the following manner. Import the cross section scanning electron microscope image of the positive electrode film layer along the thickness direction of the electrode piece obtained in the manner described above into the ImageJ software, select the straight line tool, use the straight line to mark the ruler length in the image, click "Analyze SetScale", and set the ruler parameters in the software according to the ruler length in the image. Select the rectangle tool, select the part of the image outside the ruler area, use "Image Duplicate" to copy the selected area, and use "Image Type 8 bit" to adjust the image format; select "Analyze Set Measurements", select the following 5 options: "Area", "Mean gray value", "Area Fraction", "Limit to threshold", "Feret's diameter", where "Decimal places" selects 3, selects "Image"-"Adjust"-"Threshold" in turn, and sets 0 and 100 in the "Threshold" box position in turn, and the Analyze-Measure function can be used to export the pore data in the cross section electron microscope image. Use "Image"-"Overlay"-"Flatten" to export and obtain the pore image; click "Apply" in "Threshold", then click "Analyze"-"Analyze Particles", check the four columns on the left, and you can get the pore statistics.

[0224] like Figure 8As shown, it can be understood that in the embodiment of the present application, the "pores" in the positive electrode film section are identified by the color difference of the picture and the threshold. The "pores" are not the pore data obtained in the exhaust test, but are mainly used to characterize the cross-sectional area between the particles in the positive electrode film section. This method is better than the exhaust method, because the porosity obtained by the exhaust method is related to the pores between the particles, and is also related to the pores in the carbon layer coated on the surface of the lithium iron phosphate particles, so it is impossible to objectively reflect the pores between the particles. The lower the porosity in the positive electrode film section tested by this method, on the one hand, it means that the gradation of large, medium and small particles in the positive electrode film is better, and the compaction density is high. On the other hand, after the same gradation and roller pressure, if the porosity is low, it means that the particles are easy to slip with each other, thereby reducing the risk of overpressure and stress concentration in the film layer, and further reducing the probability of demolding of the positive electrode film during the long cycle, which is beneficial to improve the long cycle performance of the battery.

[0225] In some embodiments, the positive electrode plate includes a primer layer, which is 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.

[0226] Among them, carbon-based particles refer to particles with carbon as the main component, including but not limited to conductive carbon, carbon black, etc.

[0227] The bottom coating helps to improve the conductivity and adhesion between the positive electrode film and the current collector, reduce the release of the positive electrode film from the current collector during the cycle, and improve the dynamic performance of the battery. In the high compaction density electrode of the embodiment of the present application, for example, the compaction density of the positive electrode in the full discharge state is greater than or equal to 2.4g / cm 3 The current collector is easily damaged during the high-pressure compaction process of the electrode sheet, and large-sized particles are easy to form pits on the current collector. Controlling the distribution density of carbon-based particles with a particle size greater than 100nm in the primer layer to be ≤10pcs / 10μm is beneficial to reducing the probability of damage to the current collector in the high-pressure dense electrode sheet and further improving the ultimate compaction density of the positive electrode sheet.

[0228] The distribution density of carbon-based particles with a particle size greater than 100 nm in the base coating layer can be determined using the method described above, by cutting the positive electrode film layer along the thickness direction of the electrode piece with an argon ion beam, taking a scanning electron microscope image or a microscope image, and detecting the size of the carbon particles in the base coating layer by a statistical method, and counting the number of carbon-based particles with a particle size greater than 100 nm per 10 μm in the base coating layer, and counting for no less than 5 times to find the average value.

[0229] The primer layer in the embodiment of the present application can be prepared by any known preparation process, such as pre-screening or centrifugation in the process of preparing carbon-based particles to remove large particles of carbon-based materials, so that the carbon-based particles added in the process of preparing the primer layer have a D V50 At 20-60nm, D V90 The thickness is less than or equal to 70 nm, and the carbon-based material is mixed with a binder, stirred, and coated on a current collector to obtain a primer layer.

[0230] In some embodiments, the compaction density of the positive electrode sheet in a fully charged state is greater than or equal to 2.4 g / cm 3 The single-side thickness of the primer layer is 1 μm-4 μm.

[0231] In some embodiments, the compaction density of the positive electrode sheet in a fully charged state is greater than or equal to 2.5 g / cm 3 The single-side thickness of the primer layer is 2 μm-4 μm.

[0232] As the compaction density of the pole piece increases, the large particles of lithium-containing phosphate materials (for example, particles larger than 1 μm) in the positive electrode film layer have a more significant squeezing effect on the bottom coating. Therefore, stress concentration is easily generated at the site of large particles, and even passes through the bottom coating to damage the current collector. Increasing the thickness of the bottom coating is conducive to improving the stress concentration phenomenon in the pole piece and further improving the ultimate compaction density of the pole piece.

[0233] The single-sided thickness of the primer layer can be tested in the following way. As described above, the positive electrode film layer is cut along the thickness direction of the pole piece by an argon ion beam, and a scanning electron microscope image is taken. In the length direction of the pole piece, points are taken every 1m to measure the thickness of the single-sided primer layer. After measuring the thickness of the primer layer at 10 points, the average value is calculated. It should be noted that in the process of measuring points, it is necessary to avoid abnormal points, that is, the primer layer areas with a thickness of less than 50nm and a thickness of more than 4m; these abnormal points are mainly due to the extreme fluctuations in the thickness of individual areas caused by abnormal stress concentration extrusion during the compaction of the pole piece, and are not statistically significant.

[0234] In some embodiments, the thickness of the positive electrode current collector is less than or equal to 17 μm, and can be 13 μm-15 μm.

[0235] In some embodiments, the thickness of the positive electrode current collector is 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, or any range therebetween.

[0236] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0237] In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector, and the surface density of the negative electrode film layer on one side is 140 mg / 1540 mm 2 -221mg / 1540mm 2 ; and / or the compaction density of the negative electrode film layer is 1.40g / cm 3 -1.75g / cm 3 .

[0238] The single-side density and compaction density of the negative electrode film layer can be tested by methods similar to those of the positive electrode film layer described above.

[0239] The surface density and compaction density of the negative electrode film layer are within the above ranges, which is beneficial to improving the energy density of the lithium-ion secondary battery.

[0240] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, a copper foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0241] In some embodiments, the negative electrode film layer includes a negative electrode active material. The negative electrode active material may be a negative electrode active material for a battery known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0242] In some embodiments, the negative electrode film layer may further include a binder, which may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS).

[0243] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

[0244] In some embodiments, the negative electrode film layer may further optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0245] In some embodiments, the negative electrode sheet can be prepared in the following manner: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0246] In some embodiments, the lithium-ion secondary battery includes an electrolyte. The electrolyte plays a role in conducting ions between the positive electrode and the negative electrode. The present application does not specifically limit the type of electrolyte, and it can be selected according to needs. For example, the electrolyte can be liquid, gel or all-solid.

[0247] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0248] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0249] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, 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, cyclopentane sulfone, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0250] In some embodiments, the electrolyte may further include additives, such as negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high or low temperature performance, etc.

[0251] In some embodiments, the lithium-ion secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical stability and mechanical stability can be selected.

[0252] In some embodiments, the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0253] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

[0254] In some embodiments, the lithium-ion secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0255] In some embodiments, the outer packaging of the lithium ion secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, and as the plastic, polypropylene, polybutylene terephthalate, and polybutylene succinate can be listed.

[0256] A second aspect of the present application provides a battery device, including the lithium-ion secondary battery provided by the first aspect of the present application, and the battery device includes at least one of a battery module, a battery pack, and an energy storage battery.

[0257] A third aspect of the present application provides an electrical device, comprising the lithium-ion secondary battery provided in the first aspect of the present application.

[0258] The fourth aspect of the present application provides a method for preparing a positive electrode active material: comprising: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source, wherein the molar ratio of lithium to iron in the mixed raw material is greater than or equal to 1 and less than or equal to 1.05; 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 includes a first temperature and a second temperature, and the second sintering temperature is 750°C-800°C; the crushing includes air flow crushing, the grading frequency of the air flow crushing is 18Hz-24Hz, and the crushing air pressure is 0.45MPa-0.65MPa.

[0259] The preparation method provided in the embodiment of the present application obtains a positive electrode active material with a small proportion of large particles by regulating the lithium-iron ratio, the sintering temperature and the crushing process. On the one hand, the sphericity of the particles in the positive electrode active material is improved, that is, in the section 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%; and in the cumulative distribution curve of the sphericity area of ​​the particles obtained from the section along the thickness direction of the electrode sheet, the median L of the sphericity is A50 Provides material basis for the preparation of 0.70-0.74 positive electrode film layer.

[0260] In some embodiments, the iron source includes ferrous iron, which can be one or more of ferrous oxalate, ferrous carbonate, and ferrous nitrate.

[0261] During the sintering process, the divalent iron source will preferentially decompose to generate a large amount of ferrous oxide, which will serve as a nucleation site to generate nanocrystalline cores containing lithium transition metal phosphates. At the same time, the polymer carbon source has a relatively low decomposition temperature, and the iron element 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 degree of graphitization at a lower sintering temperature, reduce the resistivity of the positive electrode active material, and improve the density and uniformity of the carbon coating material on the surface of the lithium transition metal phosphate. In addition, the uniform deposition of carbon on the surface of the lithium transition metal phosphate will further hinder the growth of the lithium transition metal phosphate grains, reducing the probability of the positive electrode active material particles growing into large particles with a particle size greater than 1.5μm.

[0262] In some embodiments, the particle size D of ferrous oxalate is 10Greater than or equal to 3μm, particle size D 50 50-80μm, particle size D 90 Less than or equal to 150μm.

[0263] In this application, the term "D 10 ”, “D 50 ” and “D 90 " respectively correspond to the particle sizes when the cumulative particle size distribution percentage of the sample particle size measured by Malvern laser scattering method reaches 10%, 50%, and 90%.

[0264] Controlling the particle size D of ferrous oxalate 10 Greater than or equal to 3μm can reduce the proportion of small-sized ferrous oxalate particles and control their reactivity during the grinding process. 50 , D 90 It helps to evenly mix the raw materials during the grinding process, obtain a mixed slurry with consistent components and uniform particle size, and improve the particle size consistency of the prepared lithium-containing transition metal phosphate.

[0265] In some embodiments, the mass content of trivalent iron is less than or equal to 0.08%.

[0266] In some embodiments, the mass content of the trivalent iron element may be selected as 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08% or any numerical range therebetween.

[0267] Controlling the mass content of trivalent iron helps improve the uniformity and consistency of carbon-coated materials. Excessive trivalent iron content will preferentially consume the carbon source, resulting in poor consistency in the quality and thickness of the carbon layer coated between particles. On the one hand, uneven thickness of carbon-coated materials will affect the compaction between particles, and on the other hand, local carbon deficiency will affect the overlap of the conductive network between particles, which is not conducive to the effective improvement of the compaction density of the pole piece and the improvement of dynamics.

[0268] In some embodiments, the lithium source includes one or more of lithium dihydrogen phosphate, lithium phosphate, lithium carbonate, and lithium acetate.

[0269] In some embodiments, the carbon source includes a polymer carbon source, which can be one or more of polyethylene glycol and polyvinyl alcohol.

[0270] In some embodiments, the mass content of polyethylene glycol is 1.0%-4.0% based on the total mass of the carbon source.

[0271] 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, which hinders the growth and sintering growth of lithium-containing transition metal phosphate grains, and is beneficial to reducing the particle size of the positive electrode active material particles.

[0272] At the same time, polymer carbon sources usually have a higher molecular weight or a longer molecular chain, which can easily form a stable skeleton structure through cross-linking or orientation during heat treatment. This orderliness is retained during high-temperature carbonization, which is beneficial to the directional growth of graphite crystals. At the same time, the entanglement and cross-linking between long chains are beneficial to reducing structural defects and reducing the lattice disorder caused by chain breakage during carbonization, thereby improving the degree of graphitization.

[0273] The organic molecules in the carbon source will decompose at high temperatures to release carbon atoms, which can cover and fill the tiny gaps or defects on the surface of the active material, reducing the surface roughness. The coating material formed by the polymer carbon source has a high degree of graphitization and a compact carbon structure, which is beneficial to the optimization of the surface roughness of the positive electrode active material.

[0274] In some embodiments, the weight average molecular weight of polyethylene glycol is 10,000 or less.

[0275] By using polyethylene glycol with a weight average molecular weight lower than 10,000, the carbon chain is shorter and the decomposition rate during sintering is easily controlled to form a carbon coating material with a suitable and uniform thickness.

[0276] In some embodiments, the water content of the polyethylene glycol is less than or equal to 0.5%.

[0277] If the water content in polyethylene glycol is high, the water content may affect the decomposition process, causing incomplete decomposition or uneven decomposition rate during sintering. Excessive water content may also cause uneven distribution of molten polyethylene glycol during sintering, affecting the uniformity of the carbon layer, causing the carbon coating material to be unstable or fall off.

[0278] In some embodiments, the water content of polyethylene glycol can be selected as 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5% or any range therebetween.

[0279] In some embodiments, the pH of the polyethylene glycol is 5-7.

[0280] Polyethylene glycol with a pH of 5-7 is highly stable and will not degrade during the mixing process due to excessive acidity, especially under high temperature conditions, which will cause too fast decomposition and affect the quality of the coating material. If polyethylene glycol is alkaline, it may affect the stability of other components, causing metal ions to dissolve or oxidize, affecting the performance of the final positive electrode active material.

[0281] In some embodiments, the phosphorus source includes one or more of lithium dihydrogen phosphate, phosphoric acid, and ammonium dihydrogen phosphate.

[0282] In some embodiments, the lithium source and the phosphorus source may be the same substance.

[0283] In some embodiments, the iron source includes ferrous oxalate, the lithium source and phosphorus source include lithium dihydrogen phosphate, and the carbon source includes polyethylene glycol.

[0284] 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.

[0285] In some embodiments, the atomic molar ratio of lithium element to iron element in the lithium source and the iron source can be selected as 1.00, 1.01, 1.02, 1.03, 1.04, 1.05 or any numerical range therebetween.

[0286] When the atomic molar ratio of lithium and iron is 1, it is an ideal stoichiometric ratio, which can maintain the best electrochemical performance, optimize the reversible insertion and extraction ability of lithium ions during charge and discharge, and have good crystal structure stability to improve cycle life and reduce the probability of the appearance of impurities. However, in actual production, in order to compensate for the loss of lithium during sintering, the molar ratio of lithium and iron needs to be adjusted to slightly higher than 1.

[0287] In some embodiments, the slurry further includes a titanium source. Optionally, the titanium source includes one or more of titanium dioxide, tetrabutyl titanate, titanium nitrate, and titanic acid.

[0288] Titanium sources often have lower surface activity. Including titanium sources in the slurry can reduce the activity of the lithium-containing transition metal phosphate precursor, inhibit the particle growth of the lithium-containing transition metal phosphate during high-temperature sintering, and form smaller particles of the lithium-containing transition metal phosphate during the sintering process.

[0289] Titanium is used as a lattice stabilizer. Titanium is usually 4+ In the form of titanium, some titanium ions can replace the iron ions, making the crystal structure more stable and reducing the possibility of anti-position between lithium and iron ions, especially during high temperature or high current charging and discharging.

[0290] At the same time, titanium doping helps to improve the sphericity of the particles and reduce the roughness of the particles, thereby enhancing the overall structural stability of the material.

[0291] In some embodiments, lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide are uniformly mixed and ground in an organic solvent to obtain a mixed raw material.

[0292] Organic solvents can effectively reduce the occurrence of side reactions and improve the purity and consistency of materials. In addition, organic solvents have good volatility and are easier to remove in the subsequent drying process. They will not remain inside the material, causing pores inside the material and affecting the density and structural stability of the material.

[0293] In some embodiments, the grinding to obtain the mixed slurry comprises at least two ball milling-demagnetization cycles, and the ball milling-demagnetization cycles each independently satisfy one or more of the following conditions: (1) The grinding balls of the ball mill are one or more of zirconium oxide balls, silicon nitride zirconium balls, and ceramic zirconium balls. (2) The diameter of the first ball milling ball is 5mm-6mm, and the diameter of the second ball milling ball is 0.5mm-0.7mm; (3) The first ball milling speed is 1400rpm-1600rpm, and the second ball milling speed is 400rpm-600rpm; (4) The first ball milling time is 150-200 min, and the second ball milling time is 140-180 min; (5) The demagnetization method is permanent magnet iron removal; (6) The demagnetization intensity of the demagnetization is greater than or equal to 8000GS.

[0294] Through at least two ball milling-demagnetization combinations, large particles can be processed quickly and further refined in a short time. In this way, uneven particle size caused by ball milling can be effectively avoided, the agglomeration of particles can be reduced, the conductivity and cycle stability of the battery can be improved, and the overall production efficiency can be improved while ensuring the performance of the final product.

[0295] In some embodiments, the volume distribution particle size D of the particles in the mixed slurry is V50 1.0μm-4.0μm.

[0296] In this application, the term "D V50 ” refers to the particle size corresponding to 50% of the sample volume cumulative particle size distribution percentage obtained by Malvern laser scattering method; In some embodiments, the volume distribution particle size D of the particles in the mixed slurry is V50 The thickness can be selected as 1.0 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4.0 μm or any range therebetween.

[0297] Volume distribution particle size D of the particles in the mixed slurry V50Within the above range, on the one hand, the activity of the particles can be increased to a certain extent, and some positive electrode active material particles with a particle size of 1μm-1.5μm can be generated at the same temperature, thereby improving the compaction density of the pole piece and the energy density of the battery; on the other hand, the catalytic decomposition efficiency of the iron element on the surface of the crystal core to the carbon source can be improved, the coating quality of the carbon source can be improved, and the uniformity and graphitization degree of the carbon coating material coating can be improved, thereby further improving the compaction density of the pole piece and the energy density of the battery.

[0298] In some embodiments, drying the mixed slurry to obtain the precursor powder includes spray drying the mixed slurry to obtain the precursor powder.

[0299] In some embodiments, sintering the precursor powder to obtain the positive electrode active material includes at least two sintering steps.

[0300] In some embodiments, the first sintering of the at least two sinterings satisfies one or more of the following conditions: (1) The heating rate is greater than or equal to 2°C / min; (2) The holding temperature is 300℃-400℃; (3) The insulation time is 2h-6h.

[0301] In some embodiments, the second sintering in the at least two sinterings satisfies one or more of the following conditions: (1) The heating rate is greater than or equal to 3°C / min; (2) The holding temperature is 750℃-800℃; (3) The insulation time is 8h-15h.

[0302] Rapidly heating to the target temperature at a higher heating rate is beneficial to uniform particle growth and reduces the presence of particles with a particle size greater than or equal to 1.5 μm.

[0303] By controlling the sintering temperature in the first and second sintering processes, the speed of the sintering diffusion rate can be controlled. At high temperatures, the diffusion of the particle surface increases, the defects in the particles will be repaired, and the lattice will be rearranged. Through recrystallization, the defects on the particle surface are eliminated, the grain structure of the particles is more orderly, the size of the particles gradually increases, and it helps to smooth the particle surface and promote the particle to develop into a spherical shape. The sintering temperature also affects the graphitization rate of the carbon source. Kinetics, carbon atoms gain more energy and can overcome the original energy barrier, causing them to rearrange more violently in the 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 completely completed; if the sintering time is too long, the particles will grow abnormally, the grains inside the particles will coarsen, the material structure will tend to be unstable, the bonding force between the particles will be enhanced, and agglomeration will occur.

[0304] In some embodiments, after sintering the precursor, the product is subjected to air flow milling to obtain the positive electrode active material.

[0305] In some embodiments, the classification frequency of the air flow milling is 18 Hz-24 Hz, and the milling air pressure is 0.45 MPa-0.65 MPa.

[0306] The classification frequency in airflow milling refers to the working frequency of the classification device in airflow milling, which is usually related to the classification efficiency and particle size distribution of the particles. A higher classification frequency will screen the particles in the airflow more times, so that larger particles are screened out and smaller particles are left. And a higher classification frequency may increase the number of particle collisions, causing irregular particles to be further impacted, making the particle surface smoother and the shape tending to be spherical.

[0307] High air pressure will cause the particles to be subjected to greater impact force, and the collisions between particles will be more intense, which will cause the surface of the particles to be subjected to stronger impact and wear, and can crush large particles into small particles. The collisions between particles will be more intense, and the surface will be easier to be trimmed, thereby improving the sphericity and surface flatness of the particles.

[0308] However, too high classification frequency and crushing pressure will cause the agglomerated particles to further crack and break after being dispersed into primary particles, affecting the predetermined particle gradation distribution and making the carbon coating material incomplete, which is manifested as increased iron dissolution, negatively affecting the slip of particles during roller pressing, and increasing the contact and reaction between lithium-containing transition metal phosphates and external factors such as electrolytes, which is not conducive to the cycle performance and life of the battery. Therefore, it is necessary to control the classification frequency and crushing pressure of airflow crushing within a suitable range.

[0309] The fifth aspect of the present application provides a method for preparing a positive electrode plate, which comprises sequentially adding a binder, a conductive agent, and a positive electrode active material prepared by the preparation method of the fourth aspect, dry-mixing the mixture, adding a solvent, stirring, and adjusting the viscosity to obtain a shipping slurry; transfer-coating the shipping slurry to at least one side of a current collector, and obtaining a positive electrode film layer after drying and hot pressing.

[0310] In some embodiments, the stirring includes pre-stirring and main stirring, and the stirring time of the main stirring is 200 min-250 min and the temperature is 5°C-50°C.

[0311] In some embodiments, the hot pressing includes at least three hot roller pressings, and the hot roller pressure increases successively, and the hot roller pressure is 20-50 tons, 50-70 tons, and 70-90 tons respectively; the hot roller temperature is 40-80°C, and the positive electrode sheet is heated before entering the hot roller compaction for the first time, and the heating temperature is 40-50°C.

[0312] The positive electrode active material prepared by the above-mentioned hot pressing process in combination with the preparation method of the fourth aspect in the embodiment of the present application is beneficial to further reduce the cross-sectional porosity of the positive electrode film layer, increase the ultimate compaction density of the electrode piece, and improve the energy density of the battery.

[0313] In addition, the present application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited to these.

[0314] As the electrical device, a secondary battery, a battery module or a battery pack may be selected according to its usage requirements.

[0315] Figure 7 The power consumption device is taken as an example. The power consumption device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the power consumption device's requirements for high power and high energy density of secondary batteries, a battery pack or a battery module can be used.

[0316] Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a secondary battery may be used as a power source.

[0317] Example Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If no specific techniques or conditions are indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. If the manufacturer is not indicated in the reagents or instruments used, they are all conventional products that can be obtained commercially.

[0318] Example 1 (1) Preparation of positive electrode active materials The lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol and titanium dioxide are mixed evenly in methanol and ground to obtain a mixed raw material. The ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.03.

[0319] The mixed slurry is spray dried to obtain a dry precursor powder material, and the dry precursor powder material has a light yellow appearance and a uniform color.

[0320] The precursor powder was placed in a sintering furnace, and the temperature was increased from 25°C to 350°C at a rate of 2°C / min under a nitrogen atmosphere and kept at this temperature for 3 hours. The temperature was then increased to a second temperature of 770°C at a rate of 5°C / min and kept at this temperature for 10 hours. Afterwards, the temperature was lowered and cooled.

[0321] The obtained material was crushed by air flow pulverization method with a classification frequency of 22 Hz and a pulverization pressure of 0.55 MPa to obtain a carbon-coated lithium iron phosphate positive electrode active material.

[0322] The mass content of carbon in the prepared positive electrode active material is 1.177%, the concentration of lithium iron antisite defects is 0.53%, and the tap density of the powder is 1.19 g / cm 3 The compacted density of the powder under 3T pressure is 2.57g / cm 3 The powder resistivity is 6.3Ω·cm at 8MPa; the discharge capacity at 1C discharge rate is 142.3mAh / g; there is a discharge platform in the voltage range of 2.5V~2.9V, and the discharge capacity of the 3.2V discharge platform accounts for 92.1%.

[0323] (2) Preparation of positive electrode sheet: 2.2wt% PVDF, 0.8wt% conductive carbon black, and 97.0wt% positive electrode active material were added in sequence and dry-mixed, and then N-methylpyrrolidone was added, stirred, and the viscosity was adjusted to obtain a shipping slurry; the shipping slurry was transferred and coated on the bottom coating of the current collector aluminum foil, the bottom coating included carbon black and PVDF, the mass ratio of the two was 1:1, the distribution density of carbon-based particles with a particle size greater than 100nm in the bottom coating was ≤10pcs / 10μm, and the thickness of the bottom coating was 2μm. After drying and hot pressing, the single-side density was 350mg / 1540mm 2 wherein the stirring comprises pre-stirring and main stirring, the stirring time of the main stirring is 220 min and the temperature is 40°C.

[0324] The hot pressing process includes three hot roller pressing processes, and the hot roller pressing pressure increases successively, and the hot roller pressure is 40 tons, 55 tons, and 80 tons respectively; the hot roller temperature is 65°C, and the pole piece is heated before entering the hot roller compaction for the first time, and the heating temperature is 45°C.

[0325] The compaction density of the pole piece is the ultimate compaction density of the pole piece. The ultimate compaction density test method of the pole piece is as follows; the ultimate compaction density of the pole piece in this embodiment is 2.68 g / cm 3 .

[0326] The statistical results of the cross section of the prepared positive electrode film along the thickness direction of the electrode sheet show that the area of ​​particles with a particle size of greater than or equal to 1.5μm and less than 5μm in the cross section of the positive electrode film accounts for 14.49%; the area of ​​particles with a particle size greater than 5μm accounts for 0; the area of ​​particles with a particle size greater than 1μm and less than 1.5μm accounts for 19.65%; the area of ​​particles with a particle size greater than or equal to 200nm and less than 1500nm accounts for 75.16%. The iron dissolution rate of the positive electrode film is 1062ppm.

[0327] The median C of the graphitization degree obtained in the surface scanning mode of the laser microscopy confocal Raman spectrometer 50 The median of the sphericity L is 1.025. A50 The median value of the roughness R is 0.72. A50 It is 0.942.

[0328] (3) Preparation of negative electrode sheet: 95.5 wt% of negative electrode active material (artificial graphite), 1.0 wt% of conductive agent (conductive carbon black), 2.0 wt% of binder (styrene-butadiene rubber (SBR)) and 1.5 wt% of thickener (sodium carboxymethyl cellulose (CMC)) were mixed, deionized water was added and stirred, and the negative electrode slurry was dispersed. Then the negative electrode slurry was coated on both sides of the Cu foil. After both sides were completed, it was dried, cold pressed, cut and sliced ​​to prepare the negative electrode sheet. The density of the coated single side is 165 mg / 1540 mm 2 , compacted density is 1.60g / cm 3 .

[0329] (4) Preparation of isolation membrane Polypropylene film is used as the isolation film.

[0330] (5) Preparation of electrolyte In an argon atmosphere glove box (H 2 O<0.1ppm, O 2 <0.1ppm), mix the organic solvent ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1 / 1, add lithium salt LiPF 6 Dissolved in organic solvents, LiPF 6 The content in the solution is 1 mol / L, stir evenly to obtain an electrolyte.

[0331] (6) Preparation of batteries: Stack the positive electrode sheet, isolation film, and negative electrode sheet in order. The isolation film must be able to isolate the anode and cathode. The bare battery cell is obtained by winding, and the bare battery cell is placed in an outer package. The electrolyte is injected, and after packaging, formation, exhaust and other processes, a lithium-ion battery is finally obtained.

[0332] Example 2 This preparation example is basically the same as preparation example 1, except that, in the preparation step of the positive electrode active material, the maximum temperature of the second sintering process is 750°C.

[0333] Example 3 This preparation example is basically the same as preparation example 1, except that, in the preparation step of the positive electrode active material, the maximum temperature of the second sintering process is 790°C.

[0334] Example 4 This preparation example is substantially the same as preparation example 1, except that, in the preparation step of the positive electrode active material, the ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.01.

[0335] Example 5 This preparation example is substantially the same as preparation example 1, except that, in the preparation step of the positive electrode active material, the ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.05.

[0336] Example 6 This preparation example is basically the same as preparation example 1, except that, in the preparation step of the positive electrode active material, the ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the molar ratio of lithium to iron is 1.05; and the maximum sintering temperature of the second sintering process is 790°C.

[0337] Example 7 This preparation example is substantially the same as preparation example 1, except that the pulverization frequency of the air flow pulverization is 18 Hz.

[0338] Example 8 This preparation example is substantially the same as preparation example 1, except that, in the preparation step of the positive electrode active material, the pulverization frequency of the air flow pulverization is 24 Hz.

[0339] Comparative Example 1 This preparation example is basically the same as preparation example 1, except that, in the positive electrode active material preparation step, the ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.05; and the maximum sintering temperature of the second sintering process is 810°C.

[0340] Comparative Example 2 This preparation example is basically the same as preparation example 1, except that, in the positive electrode active material preparation step, the ratio of lithium dihydrogen phosphate and ferrous oxalate is such that the atomic molar ratio of lithium to iron is 1.01; and the maximum sintering temperature of the second sintering process is 740°C.

[0341] Comparative Example 3 This preparation example is basically the same as the preparation example 1, except that, in the positive electrode active material preparation step, the preparation is carried out by the following method: The lithium carbonate, iron phosphate, titanium dioxide, glucose and polyethylene glycol are mixed evenly in water and ground to obtain a mixed raw material, wherein the ratio of lithium carbonate to iron phosphate is such that the atomic molar ratio of lithium to iron is 1.03:1.0.

[0342] The mixed slurry is spray dried to obtain a dry precursor powder material, and the dry precursor powder material has a light yellow appearance and a uniform color.

[0343] The precursor powder was placed in a sintering furnace, and in a nitrogen atmosphere, the temperature was increased from 25°C to 450°C at a rate of 2°C / min and kept at this temperature for 3 hours, and then the temperature was increased to a second temperature of 780°C at a rate of 5°C / min and kept at this temperature for 10 hours. After completion, the temperature was lowered and cooled.

[0344] The obtained material was crushed by air flow pulverization method with a classification frequency of 22 Hz and a pulverization pressure of 0.55 MPa to obtain a carbon-coated lithium iron phosphate positive electrode active material.

[0345] Performance Testing 1. Pole piece ultimate compaction density The double-sided coated pole piece is compacted by a roller press, and the elongation of the pole piece after compaction is tested, and the flexibility of the pole piece after compaction is evaluated. By increasing the pressure of the roller press, pole pieces with different compaction densities will be obtained. As the pressure increases, the compaction density of the pole piece increases, the elongation of the pole piece increases, and the flexibility of the pole piece decreases. Too high an elongation of the pole piece can easily cause the pole piece to warp, and too low a flexibility of the pole piece can easily cause the pole piece to break brittlely. Therefore, the smaller of the compaction density corresponding to the elongation of the pole piece of 8% or the number of times the pole piece's flexibility is folded is defined as the limit compaction density of the pole piece.

[0346] The compacted density is calculated by the mass of the positive electrode film layer / the volume of the positive electrode film layer.

[0347] The test method for elongation is as follows: Lay the electrode flat on a horizontal table and cut it into sections, with each electrode being about 100 cm long; remove the copper foil of the substrate at the edge of the electrode, and pay attention to keeping the cut edge of the electrode parallel to the MD direction of the electrode (perpendicular to the direction of the pressing roller), ensuring that the electrode part is completely covered by the coating, and use a steel ruler to measure the length between the marking points at the same position of the length and width at the head and tail of the electrode, and estimate it to 0.1 mm, and record the length before compaction; after compaction, record the length between the corresponding marking points after compaction, and use (length after compaction - length before compaction) / length before compaction as the elongation rate of the electrode.

[0348] The test method for the number of flexible folds is shown below.

[0349] Cut the positive electrode sheet into 20×100mm 2 Fold the specimen in the forward direction, flatten it with a 2kg roller, unfold it and check against the light to see if there is light transmittance between the gaps. If not, fold it in the reverse direction, flatten it with a 2kg roller, and check against the light again. Repeat this process until light transmittance between the gaps. Record the number of folds. Repeat the test three times and take the average value as the reference data for the flexibility of the electrode.

[0350] 2. Energy density test The lithium-ion secondary battery was left at 25°C for 2 hours to ensure that the temperature of the lithium-ion secondary battery was 25°C. After charging the lithium-ion secondary battery at 0.33C at 25°C to a charge cut-off voltage of 3.65V, constant voltage charging was continued at the charge cut-off voltage until the current reached 0.05C and the charge was cut off (where C represents the rated capacity of the lithium-ion secondary battery). After the lithium-ion secondary battery was left at 25°C for 1 hour, it was discharged at 0.33C at 25°C to a discharge cut-off voltage of 2.5V. The total discharge energy of the lithium-ion secondary battery was recorded as E. 0 .

[0351] Measure the length, width and height of the lithium-ion secondary battery and calculate the volume value V of the lithium-ion secondary battery 0 =length*width*height.

[0352] The mass energy density of lithium-ion secondary battery = the discharge energy E of lithium-ion secondary battery 0 / Lithium-ion secondary battery volume V 0 .

[0353] 3. DC resistance (DCR) test method At 25°C, charge to 3.65V at 0.33C constant current, charge to 0.05C at constant voltage, then discharge to 20% SOC at 0.33C, let stand for 5min, discharge at 3C pulse for 30s, let stand for 40s, charge at 3C for 40s, let stand for 5min, charge to 3.65V at 0.33C constant current, charge to 0.05C at constant voltage, then discharge to 10% SOC at 0.33C, let stand for 5min, discharge at 3C pulse for 30s, let stand for 40s Then charge at 3C for 40s, let it stand for 5min, then fully charge at 0.33C, then discharge to 50% SOC at 0.33C, then let it stand at -25°C for 2h, then pulse discharge at 1C for 30s, let it stand for 10min, then let it stand at 25°C for 2h, charge to 3.65V at 0.33C, then charge to 0.05C at constant voltage, then discharge to 20% SOC at 0.33C, then let it stand at -25°C for 2h, then pulse discharge at 1C for 30s, let it stand for 10min.

[0354] The voltage was recorded before and after each pulse discharge, and the DCR under different conditions was calculated using the formula DCR = (voltage before pulse discharge after rest - voltage before rest after pulse discharge) / pulse current. Experimental parameters and test results The batteries of the embodiments and comparative examples were prepared according to the above method, and various performance parameters were measured. The results are shown in Table 1 below.

[0355] Table 1

[0356] Table 1

[0357] From the comparison of the data of the embodiment and the comparative example, it can be seen that 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%; and in the cumulative distribution curve of the sphericity area of ​​the particles obtained from the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the median of the sphericity L A50 When it is 0.70-0.74, the battery has a higher positive electrode sheet compaction density and energy density, and the battery has good dynamic performance.

[0358] From the comparison between Examples 3 and 5 and other Examples, it can be seen that in the cumulative distribution curve of the sphericity area of ​​the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the concentration (L A90 -L A10 ) / L A50 When it is 0.450-0.535, it is conducive to forming a dense stack, increasing the contact between particles, and further improving the dynamic performance of the battery.

[0359] From the comparison between Example 2 and other examples, it can be seen that in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1.5 μm and less than 5 μm accounts for 9.0%-20.0%. When the area further accounts for 10.0%-20.0%, it is beneficial to achieve a balance between battery kinetic performance and energy density.

[0360] From the comparison between Examples 3 and 6 and other examples, it can be seen that in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1 μm and less than 1.5 μm accounts for 15.0%-25.0%, and further accounts for 16.0%-24.0%, which is beneficial to reducing the DC internal resistance of the battery and improving the battery's dynamic performance.

[0361] From the comparison between Example 2 and other examples, it can be seen that in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 200nm and less than 1500nm accounts for 73.0%-80.0%, and further accounts for 73.0%-78.0%, which can achieve a high compaction density of the positive electrode sheet and further improve the energy density of the battery.

[0362] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A lithium ion secondary battery, characterized in that: The invention 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 includes a positive electrode active material, and the positive electrode active material includes lithium-containing transition metal phosphate particles with a carbon coating material disposed on at least a portion of the surface. In 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%; In the cumulative distribution curve of the sphericity area of ​​the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median of the sphericity L A50 It is 0.70-0.

74.

2. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the sphericity area of ​​the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the concentration (L A90 -L A10 ) / L A50 It is 0.450-0.

535.

3. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the sphericity area of ​​the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the concentration (L A90 -L A10 ) / L A50 It is 0.450-0.

500.

4. The lithium-ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the sphericity area of ​​particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the sphericity L A10 0.40-0.60, spherical L A90 It is 0.85-1.

0.

5. The lithium ion secondary battery according to claim 1, characterized in that: In a cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 5 μm accounts for 0.

6. The lithium ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1.5 μm and less than 5 μm accounts for 9.0%-20.0%.

7. The lithium ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1.5 μm and less than 5 μm accounts for 10.0%-20.0%.

8. The lithium ion secondary battery according to claim 1, characterized in that: In a cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1 μm and less than 1.5 μm accounts for 15.0%-25.0%.

9. The lithium ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1 μm and less than 1.5 μm accounts for 16.0%-24.0%.

10. The lithium ion secondary battery according to claim 1, characterized in that: In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 200nm and less than 1500nm accounts for 73.0%-80.0%.

11. The lithium ion secondary battery according to claim 1, characterized in that: In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 200nm and less than 1500nm accounts for 73.0%-78.0%.

12. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C value obtained by the laser microscopic confocal Raman spectrometer surface scanning mode, the median of the graphitization degree is 0.95-1.20; wherein the graphitization degree C value is I G / I D , I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at .

13. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C value obtained by the laser microscopic confocal Raman spectrometer surface scanning mode, the median of the graphitization degree is 0.98-1.15; wherein the graphitization degree C value is I G / I D , I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at .

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

96.

15. The lithium ion secondary battery according to claim 1, characterized in that: In the roughness area cumulative distribution curve of the particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the concentration of roughness (R A90 -R A10 ) / R A50 It is 0.06-0.

15.

16. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode active material includes iron element, and the iron dissolution rate of the positive electrode film layer is 500ppm-2000ppm.

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

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

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

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

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

1.

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

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

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

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

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

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

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

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

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

31. The lithium ion secondary battery according to claim 1, characterized in that: The discharge capacity of the positive electrode active material discharged to 3.2V accounts for η≥85%, and η is defined as: at room temperature, the button battery containing the positive electrode active material is charged and discharged twice at a constant current rate of 0.1C in the voltage range of 2.0V~3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at the rate of 1C, the capacity value extracted at the discharge voltage of 3.2V is recorded as C1, and the capacity value extracted to the discharge voltage of 2.0V is C2, η=C1 / C2, wherein the charging process includes constant voltage charging, a constant voltage of 3.75V, and a constant voltage cut-off current of 50μA.

32. The lithium ion secondary battery according to claim 1, characterized in that: 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 2.5V to 2.9V.

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

34. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode film layer does not include a conductive agent.

35. The lithium ion secondary battery according to claim 1, characterized in that: Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 95.5%-99.5%.

36. The lithium ion secondary battery according to claim 1, characterized in that: Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 96.5%-99.5%.

37. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode film layer further includes a binder, and based on the total mass of the positive electrode film layer, the mass content of the binder is 0.5%-3%.

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

39. The lithium ion secondary battery according to claim 1, characterized in that: When the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.51 g / cm 3 -2.73g / cm 3 .

40. The lithium ion secondary battery according to claim 1, characterized in that: When the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.55 g / cm 3 -2.70g / cm 3 .

41. The lithium ion secondary battery according to claim 1, characterized in that: When the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.51 g / cm 3 -2.73g / cm 3 In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10%-22%.

42. The lithium ion secondary battery according to claim 1, characterized in that: The lithium-ion secondary battery is in a fully discharged state, and the compaction density of the positive electrode film layer is 2.55 g / cm 3 -2.70g / cm 3 In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10%-20%.

43. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode plate includes a primer layer, which is arranged 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 100nm in the primer layer is ≤10pcs / 10μm.

44. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode sheet includes a primer layer, which is disposed between the positive electrode film layer and the positive electrode current collector; the compaction density of the positive electrode sheet in a fully charged state is greater than or equal to 2.4 g / cm 3 The single-side thickness of the primer layer is 1 μm-4 μm.

45. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode sheet includes a primer layer, which is disposed between the positive electrode film layer and the positive electrode current collector; the compaction density of the positive electrode sheet in a fully charged state is greater than or equal to 2.5 g / cm 3 The single-side thickness of the primer layer is 2 μm-4 μm.

46. ​​A battery device, characterized in that: A lithium-ion secondary battery comprising any one of claims 1 to 45, wherein the battery device comprises at least one of a battery module, a battery pack, and an energy storage battery.

47. An electrical device, characterized in that: A lithium ion secondary battery according to any one of claims 1 to 45 or a battery device according to claim 46.

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