Lithium ion secondary battery, battery device, and power device

Abstract

The invention provides a lithium ion secondary battery, a battery device and an electric device. The lithium ion secondary battery comprises a positive pole piece, a negative pole piece and an electrolyte, the positive pole piece comprises a positive current collector and a positive film layer arranged on at least one side of the positive current collector, and the positive film layer comprises a positive active material; the positive electrode active material comprises lithium-containing transition metal phosphate particles, at least part of the surfaces of the lithium-containing transition metal phosphate particles are provided with carbon-coated materials, and in a particle sphericity-like area cumulative distribution curve with the particle size larger than or equal to 1 [mu] m obtained by the section of the positive electrode film layer in the thickness direction of the pole piece, the sphericity-like median LR1A50 is 0.71-0.85.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and particularly to a lithium-ion secondary battery, a battery device, an electrical device, a preparation method of a positive electrode active material, and a preparation method of 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 hydroelectric, thermal, wind, and solar power stations, as well as in multiple fields such as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace.

[0003] The positive electrode active material is an important component of a secondary battery. The lithium-containing transition metal phosphate material has the characteristics of stable structure, good safety, and long cycle life, and has broad development prospects. With the increasing requirements of the market for the energy density and kinetics of lithium-containing transition metal phosphate-based secondary batteries, it is difficult to simultaneously improve the above-mentioned performances in the prior art, which has become a technical problem urgently to be solved 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 preparation method of a positive electrode active material, and a preparation method of a positive electrode sheet, which will be described separately below.

[0005] A first aspect of the present application provides a lithium-ion secondary battery, including a positive electrode sheet, a negative electrode sheet, and an electrolyte. The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one side of the positive electrode current collector. The positive electrode film layer includes a positive electrode active material. The positive electrode active material includes lithium-containing transition metal phosphate particles with a carbon coating material provided on at least part of the surface. In the cumulative area distribution curve of the sphericity of particles with a particle size greater than or equal to 1 μm obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median L of the sphericity R1A50 is 0.71 - 0.85.

[0006] To achieve a relatively large compaction density, large particles need to be introduced as the framework for the stacking of the positive electrode film layer. The lithium-containing transition metal phosphate used in this application is a ceramic material. During the high-temperature sintering process of this material, the grain boundaries melt and the grains grow. The sintering of large particles often requires a higher sintering temperature or a longer sintering time and more grain boundary melting. Therefore, the sphericity of large particles in the prior art is relatively low. Research in this application shows that during the compaction of the electrode sheet, large particles are the main load-bearing objects. During the rolling process, bridging is likely to occur between large particles with poor sphericity, forming voids and stress concentration at the bridging points, which not only affects the further compaction of the electrode sheet and is not conducive to the further improvement of the electrode sheet compaction density, but also easily causes local fracture of the electrode sheet at the stress concentration points, damaging the conductive network inside the electrode sheet, and even generating isolated particles, resulting in an increase in the internal resistance of the battery and deteriorating the kinetic performance of the battery.

[0007] Research shows that the median L of the sphericity of particles with a particle size greater than or equal to 1 μm R1A50 Less than 0.71 means that the irregularity degree of the morphology of larger particles is still relatively high, and it is difficult to avoid stress concentration through slip during rolling. When the median L of the sphericity of particles with a particle size greater than or equal to 1 μm R1A50 Is greater than 0.85, the positive electrode active material needs to be formed by sintering after grinding the intermediate product into smaller particle sizes. Furthermore, the pursuit of improving the sphericity of large particles will reduce the proportion of large particles in the positive electrode film layer, which is not conducive to the improvement of the compaction density of the positive electrode film layer and the energy density of the battery.

[0008] In this application, through the processes of multiple sintering and multiple grinding, the median L of the sphericity of particles with a particle size greater than or equal to 1 μm R1A50 Is adjusted to the range of 0.71 - 0.85. By improving the problem of poor sphericity of large particles, reducing the stress concentration problem caused by particle bridging due to the irregular shape of large particles, and reducing the porosity in the film layer, the compaction density of the electrode sheet is improved, the energy density of the battery is effectively increased, the kinetic degradation is reduced, and with a more compact stacking, the electron conduction ability between particles is improved, thereby achieving the improvement of the kinetic performance.

[0009] In any implementation manner, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the particle size distribution index PDI of the particles is 0.80 - 1.10. The particle size distribution index PDI of the particles is the ratio of the sample standard deviation to the average value of the particle sizes of the particles in the cross-section of the positive electrode film layer along the thickness direction. In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the particle size distribution index PDI of the particles within the above range can achieve the grading of large and small particles, further improving the compaction density of the positive electrode film layer and the energy density of the battery monomer.

[0010] In any embodiment, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the particle size distribution index PDI of the particles is 0.85 - 1.00. The particle size distribution index of the particles within the above range is beneficial to further optimizing the grading of large and small particles, improving the uniformity of the lithium-ion diffusion path, reducing the probability of polarization generation, and taking into account both the energy density and kinetic performance of the battery.

[0011] In any embodiment, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the average value of the particle size of the particles is 150 nm - 400 nm, and can be optionally 150 nm - 300 nm. In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the number of medium and small particles accounts for the vast majority. Therefore, the average value of the particle size within the above range reflects the approximate particle size range center of the medium and small particles in the positive electrode film layer, and can play a role in filling the particle voids, thereby improving the compaction density of the electrode sheet.

[0012] In any embodiment, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the sample standard deviation of the particle size of the particles is 150 nm - 350 nm, and can be optionally 150 nm - 300 nm. In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the deviation between the particle size of the large particles and the average value of the particle size of the particles is larger compared to that of the medium and small particles. Therefore, it has a greater impact on the sample standard deviation S. The sample standard deviation S within the above interval reflects the presence of a certain number of large particles. This is beneficial for building a stacking skeleton through the large particles, playing a role in force transmission and support, enabling the positive electrode film layer to withstand higher rolling pressure, and improving the mechanical stability and compaction density of the electrode sheet.

[0013] In any embodiment, in the cumulative area distribution curve of the sphericity of the particles with a particle size greater than or equal to 1 μm obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median L of the sphericity R1A50 is 0.72 - 0.80. The median L of the sphericity of the particles with a particle size greater than or equal to 1 μm R1A50 within the above range, the particles with a particle size greater than or equal to 1 μm are closer to spherical, further reducing the possibility of voids and stress concentration caused by the lap of irregular particles during the rolling process. At the same time, maintaining a certain mechanical interlocking force between the large particles is beneficial for the construction of force chains, bearing higher rolling pressure and internal and external stresses, and further improving the compaction density and stability of the electrode sheet.

[0014] In any embodiment, in the cumulative area distribution curve of the sphericity of the particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median L of the sphericity A50is 0.70 - 0.85, and can be optionally 0.71 - 0.8. In the cumulative area distribution curve of the sphericity 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 within the above range means that the particles occupying most of the area and space within the positive electrode film layer are also relatively round, the morphology of the material is relatively uniform, and effective slip occurs under external force, participating in the connection and reconstruction of the force chain, thereby contributing to improving the compaction density, increasing the volumetric energy density, reducing the generation of particle breakage, and improving the kinetic performance.

[0015] In any embodiment, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, L R1A50 > L A50 , and can be optionally 0.01 ≤ L R1A50 -L A50 ≤ 0.1, and can be further optionally 0.01 ≤ L R1A50 -L A50 ≤ 0.05. In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, by improving the sphericity of the particles, the packing situation of the particles can be improved, but even if all the particles are perfect spheres, there will still be gaps. L R1A50 > L A50 makes the particles larger than 1 μm rounder, can be more evenly distributed, and helps to form a more regular skeleton structure; the sphericity of the smaller particles is smaller, and due to the diversity of their shapes (such as oval, wedge-shaped, etc.), they can form a more compact arrangement structure between the large particles, filling the gaps that are difficult to fill between the large particles. By the cooperation of high and low sphericity of different particle sizes, the space utilization rate of the aggregate is significantly improved, thereby improving the compaction density of the electrode sheet and the energy density of the battery.

[0016] In any embodiment, in the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of the laser confocal Raman spectrometer for the positive electrode film layer, the median of the graphitization degree C 50 is 0.98 - 1.20, and can be optionally 1.02 - 1.1; where the graphitization degree C value is I G / I D , where I G represents the intensity of the G peak of the Raman spectrum at 1580 ± 100 cm -1 , and I D represents the intensity of the D peak of the Raman spectrum at 1350 ± 100 cm -1 . The higher the graphitization degree of the carbon on the surface of the positive electrode active material, the higher the proportion of graphite-structured carbon in the positive electrode film layer, and the easier it is for the particles to slip during the rolling process by means of the carbon structure with high graphitization degree in the coating material, which can further reduce the possibility of stress concentration and improve the compaction density, and improve the kinetic performance of the battery by virtue of the good electronic conductivity of the coating material with high graphitization degree.

[0017] In any embodiment, in the cumulative area distribution curve of the roughness of the particles obtained from the cross-section of the positive electrode film layer in the thickness direction of the electrode sheet, the median roughness R A50 is 0.92 - 0.96. The median roughness R 50 of the particles within the above range is relatively smooth on the surface, and the frictional force between the particles is relatively small. Under the action of an external force, the particles are prone to slip, reducing the occurrence of the phenomenon of mutual meshing due to surface roughness during rolling. Combined with a reasonable particle size distribution index and particles with a high sphericity of greater than or equal to 1 μm, close packing can be formed under a relatively low rolling pressure, further improving the compaction density of the electrode sheet and increasing the energy density of the battery.

[0018] In any embodiment, the positive electrode active material includes iron element, and the iron dissolution rate of the positive electrode film layer is 400 ppm - 1800 ppm, and may be 500 ppm - 1500 ppm.

[0019] 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. When the iron dissolution rate is within the above range, it means that the surface of the positive electrode active material has relatively complete and dense carbon coating, 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 can play a role in optimizing the kinetic performance of the lithium-ion secondary battery. At the same time, the high-integrity carbon coating material structure makes the particles prone to stress slip during the rolling process, and can simultaneously improve the compaction density of the electrode sheet and the energy density of the battery.

[0020] 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 may be 0.90% - 1.5%. Compared with the lithium-containing transition metal phosphate positive electrode active material in the prior art, this positive electrode active material has a relatively low carbon coating amount, which can further increase the loading amount of lithium-containing transition metal phosphate in the positive electrode sheet and improve the energy density of the lithium-ion secondary battery.

[0021] In any embodiment, the lithium-iron antisite defect concentration of the positive electrode active material is 0.001% - 1.5%, and may be 0.01% - 1.0%. During the preparation and cycling process, there will inevitably be a certain amount of 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 partial migration of ferric ions to lithium sites, forming lithium-iron antisite defects, blocking the one-dimensional diffusion channels of lithium ions, and having an adverse effect on the solid-phase transport of lithium ions. The positive electrode active material in the embodiments of the present application has low lithium-iron antisite defects, which is beneficial to the uniform transport of lithium ions in the solid phase and further improves the kinetic performance of the lithium-ion secondary battery.

[0022] In any embodiment, the lithium-containing transition metal phosphate particles comprise a component having the following general formula: Li m Fe x P y O j Q q , wherein Q comprises one or more of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.8 ≤ m ≤ 1.15, 0.9 ≤ x ≤ 1, 0.95 ≤ y ≤ 1, 3.5 ≤ j ≤ 4, 0 < q ≤ 0.1.

[0023] Selecting an appropriate modification element Q can improve the lattice change rate of the cathode active material during the lithium deintercalation and insertion process, reduce the oxygen activity on the particle surface, improve the structural stability of the material, and further improve the specific capacity performance level of the material during the cycling process, and further improve the cycling stability of the lithium-ion secondary battery.

[0024] In any embodiment, the cathode active material comprises one or more of lithium iron phosphate and doping modification materials, coating modification materials.

[0025] In any embodiment, the cathode active material comprises titanium element, and based on the total mass of the cathode active material, the mass content of the titanium element is 4000 ppm - 8000 ppm. The high addition amount of the titanium element does not form a harmful impurity phase that has a negative impact on the battery energy density and kinetic performance. Although the reason is not yet clear, it is speculated that it may be that the titanium element and phosphate radicals and other elements (for example, lithium element) jointly form a fast ion conductor, which instead has a promoting effect on the kinetic performance of the battery.

[0026] In any embodiment, the tapped density of the powder of the cathode active material is 1.0 g / cm 3 -1.70 g / cm 3 ,optionally 1.20 g / cm 3 -1.50 g / cm 3 。The cathode active material particles form an effective grading, have a high tapped density, and due to the high sphericity of the particles larger than 1 μm in the cathode film layer, they can spontaneously roll to fill the pores, further improving the tapped density.

[0027] In any embodiment, the compacted density of the powder of the cathode active material under 3T pressure is 2.55 g / cm 3 -2.70 g / cm 3 ,optionally 2.58 g / cm 3 -2.68 g / cm3 The positive electrode active material particles form an effective grading, and the particles larger than 1 μm in the positive electrode film layer also have a high sphericity, enabling the positive electrode active material to achieve a higher tap density under external force, providing a material basis for improving the tap density of the electrode sheet and preparing a lithium-ion secondary battery with a high energy density.

[0028] In any embodiment, the powder resistivity of the positive electrode active material at a pressure of 8 MPa is 0.5 Ω·cm - 60 Ω·cm, and can be optionally 2.0 Ω·cm - 40.0 Ω·cm. The positive electrode active material is coated with a carbon material on the surface, and due to the sp 2 structure of the surface carbon, it is easy to achieve rapid conduction of electrons between particles, enabling the positive electrode active material to have a low powder resistivity, which is beneficial to improving the solid-phase transmission rate of electrons and improving the kinetic performance of the battery.

[0029] In any embodiment, the discharge specific capacity of the positive electrode active material at room temperature at a discharge rate of 1C is 135 mAh / g - 150 mAh / g. The positive electrode active material has a high discharge specific capacity at a rate of 1C, indicating its good charge-discharge ability, which is beneficial to improving the kinetic performance of the battery.

[0030] In any embodiment, the discharge capacity ratio η of the positive electrode active material discharged to 3.2V is η≥85%, and η is defined as: at room temperature, a button cell containing the positive electrode active material is charged and discharged at a constant current twice in the voltage range of 2.0V - 3.75V at a rate of 0.1C, and then charged and discharged at a constant current once at a rate of 1C. In the charge-discharge test at a rate of 1C, the capacity value at a discharge voltage of 3.2V is extracted and denoted as C 1 , and the capacity value when the discharge voltage reaches 2.0V is extracted as C 2 , η = C 1 / C 2 , where the charging process includes constant voltage charging, with a constant voltage of 3.75V and a constant voltage cut-off current of 50 μA.

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

[0032] In any embodiment, the positive electrode film layer further includes a conductive agent. Based on the total mass of the positive electrode film layer, the mass content of the conductive agent is 0.01% - 1.5%. The particles in the positive electrode film layer have good particle size distribution. The large particles have a high sphericity, so they can form a tight packing, enabling the particles to contact each other, having good electronic conductivity, forming an electron transport network, reducing the use of the conductive agent in the positive electrode film layer, facilitating further increasing the loading amount of the positive electrode active material, and improving the energy density of the lithium-ion secondary battery.

[0033] In any embodiment, 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 92% - 99.5%, optionally 96.5% - 99.5%; the mass content of the binder is 0.5% - 3%.

[0034] When the mass content of the positive electrode active material and the mass content of the binder are within the above ranges, it can effectively increase the loading amount of the active substance per unit volume of the positive electrode film layer, maintain good internal cohesion, reduce the probability of powder falling and swelling and cracking problems, and take into account the safety performance while improving the energy density of the secondary battery.

[0035] In any embodiment, the single-sided areal density of the positive electrode film layer is 300 mg / 1540 mm 2 - 450 mg / 1540 mm 2 . The positive electrode film layer with an areal density within the above range can help improve the energy density of the lithium-ion secondary battery.

[0036] In any embodiment, when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.52 g / cm 3 - 2.78 g / cm 3 .

[0037] In any embodiment, when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.55 g / cm 3 - 2.75 g / cm 3 .

[0038] When the tap density of the positive electrode film layer is within the above range, it is beneficial to improve the energy density of the lithium-ion secondary battery.

[0039] In any embodiment, the positive electrode film layer satisfies at least one of the following conditions: (1) When the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.52 g / cm 3 - 2.78 g / 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%; (2) when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.55 g / cm 3 - 2.75 g / 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%.

[0040] The lower the porosity in the cross-section of the positive electrode film layer, on the one hand, it means that the particle size distribution of large, medium and small particles in the positive electrode film layer is better, and the tap density is high. On the other hand, after the same particle size distribution and roll pressure, if the porosity is low, it means that the particles are easy to slip relative to each other, thus reducing the risk of overpressure and stress concentration in the film layer, further reducing the probability of the positive electrode film peeling off during long-term cycling, which is beneficial to improving the long-term cycling performance of the battery.

[0041] In any implementation, the positive electrode sheet includes a bottom coating, and the bottom coating is disposed between the positive electrode film layer and the positive electrode current collector; the bottom coating satisfies at least one of the following conditions: (1) the bottom coating includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating is ≤ 10 pcs / 10 μm; (2) when the positive electrode sheet is in a fully discharged state, the tap density is greater than or equal to 2.4 g / cm 3 , the single-sided thickness of the bottom coating is 1 μm - 4 μm; (3) when the positive electrode sheet is in a fully discharged state, the tap density is greater than or equal to 2.5 g / cm 3 , the single-sided thickness of the bottom coating is 2 μm - 4 μm.

[0042] The bottom coating is beneficial to improving the conductivity and adhesion between the positive electrode film layer and the current collector, reducing the peeling off of the positive electrode film layer from the current collector during cycling, and simultaneously improving the kinetic performance of the battery.

[0043] With the increase in the tap density of the electrode sheet, the extrusion effect of the lithium-containing phosphate material with large particles (such as particle size greater than 1 μm) in the positive electrode film layer on the bottom coating becomes more significant. Therefore, stress concentration is likely to occur at the large particle sites, and even damage the current collector by penetrating the bottom coating. Increasing the thickness of the bottom coating is beneficial to improving the stress concentration phenomenon in the electrode sheet and further increasing the ultimate tap density of the electrode sheet.

[0044] The second aspect of the present application provides a battery device, including the lithium-ion secondary battery of the first aspect of the present application.

[0045] The third aspect of the present application provides an electrical device, including 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.

[0046] The fourth aspect of the present application provides a method for preparing a cathode active material, including: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source; adding a solvent and grinding to obtain a mixed slurry; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder to obtain the cathode active material; the sintering of the precursor powder to obtain the cathode active material includes at least two sinterings; after the first sintering, a first sintered product is obtained, the first sintered product and the carbon source are mixed to obtain an intermediate raw material, the intermediate raw material is divided into two groups, and grinding is respectively carried out to obtain a first group of ground products and a second group of ground products, the first group of ground products and the second group of ground products are mixed to obtain a mixed intermediate product; grinding the mixed intermediate product to obtain a target mixture; carrying out a second sintering on the target mixture to obtain the cathode active material; wherein, the D V50 of the first group of ground products is 0.8 μm - 1.2 μm; the D V50 of the second group of ground products is 0.30 μm - 0.50 μm; the heat preservation time of the second sintering is 5 h - 12 h; the grinding time of the grinding of the mixed intermediate product is 0.5 h - 1.5 h.

[0047] Grinding the mixed intermediate product again helps to improve the particle size uniformity in the target mixture, reduce the presence of abnormally large particles, make the sintering uniform, and improve the sphericity of large particles.

[0048] Through the two - sintering process, the sintering time in the high - temperature range can be effectively shortened, thereby reducing the degree of grain boundary melting of large particles during high - temperature sintering, improving the sphericity of large - sized particles in the cathode active material, thus reducing the stress concentration problem caused by particle bridging due to the irregular shape of large particles, reducing the porosity in the film layer, improving the compaction density of the electrode sheet, effectively increasing the energy density of the battery, reducing kinetic degradation, and improving the electron conduction ability between particles by means of closer packing, thereby achieving the improvement of kinetic performance.

[0049] The fifth aspect of the present application provides a method for preparing an electrode sheet. The preparation method includes adding a binder, a conductive agent, and the cathode active material prepared by the preparation method of the fourth aspect of the present application in sequence, dry - mixing, then adding a solvent, stirring, and adjusting the viscosity to obtain a shipping slurry; transferring and coating the shipping slurry on at least one side of a current collector, drying and hot - pressing to obtain a positive electrode film layer.

[0050] In any implementation manner, the revolution speed of the dry - mixing is 20 rpm - 30 rpm, and the rotation speed is 750 rpm - 850 rpm.

[0051] In any embodiment, the hot pressing includes at least three hot rolling presses, and the hot rolling pressure increases successively. The hot rolling pressures are 20 tons - 50 tons, 50 tons - 70 tons, and 70 tons - 90 tons in sequence; the hot rolling temperature is 40°C - 80°C. Before the first entry into the hot rolling press, the electrode sheet is heated, and the heating temperature is 40°C - 50°C.

[0052] The positive electrode active material prepared by using the above hot pressing process in combination with the preparation method of the fourth aspect is beneficial to further reduce the porosity of the cut surface of the positive electrode film layer, improve the ultimate compaction density of the electrode sheet, and improve the energy density of the battery.

[0053] The above description is only an overview of the technical solution of the present application. In order to be able to understand the technical means of the present application more clearly, it can be implemented in accordance with the content of the specification. And in order to make the above and other purposes, features and advantages of the present application more obvious and understandable, the following specifically illustrates the specific embodiments of the present application. Description of the Drawings

[0054] Figure 1 is a scanning electron microscope image of the cut surface of the positive electrode film layer along the thickness direction of the electrode sheet in an embodiment of the present application; Figure 2 is a schematic diagram of a lithium-ion secondary battery in an embodiment of the present application; Figure 3 is an exploded schematic diagram of a lithium-ion secondary battery in an embodiment of the present application; Figure 4 is a schematic diagram of a battery module in an embodiment of the present application; Figure 5 is a schematic diagram of a battery pack in an embodiment of the present application; Figure 6 is Figure 5 an exploded schematic diagram of the battery pack shown; Figure 7 is a schematic diagram of an electrical device using a lithium-ion secondary battery as a power source in an embodiment of the present application; Figure 8 is a porosity test diagram of the cut surface of the positive electrode film layer along the thickness direction in an embodiment of the present application.

[0055] Description of the Reference Numerals: 1 Battery pack; 2 Upper box body; 3 Lower box body; 4 Battery module; 5 Secondary battery; 51 Housing; 52 Electrode assembly; 53 Cover plate. Detailed Embodiments

[0056] Hereinafter, embodiments of the lithium-ion secondary battery, battery device, and electrical device of the present application will be specifically described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary details are omitted. For example, there may be cases where details of well-known matters are omitted and repeated descriptions of actually identical structures are omitted. This is to prevent the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the accompanying 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 recited in the claims.

[0057] The "range" disclosed in the present application is defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a particular range. The range defined in this way can include the end values or not include the end values, and can be combined arbitrarily, that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, ranges of 60-110 and 80-120 are also contemplated. 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 ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise specified, the numerical range "a-b" represents an abbreviated representation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been fully listed herein, and "0-5" is only an abbreviated representation of these numerical combinations. In addition, when it is stated that a certain parameter is an integer ≥2, it is equivalent to disclosing that the parameter is, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0058] If there is no special description, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution, and such a technical solution should be considered to be included in the disclosure of the present application.

[0059] If there is no special description, all technical features and optional technical features of the present application can be combined with each other to form a new technical solution, and such a technical solution should be considered to be included in the disclosure of the present application.

[0060] Unless otherwise specified, all steps of this application can be carried out sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method can include steps (a) and (b) carried out sequentially, or can also include steps (b) and (a) carried out sequentially. For example, when it is mentioned that the method may further include step (c), it means that step (c) can be added to the method in any order. For example, the method can include steps (a), (b) and (c), or can also include steps (a), (c) and (b), or can also include steps (c), (a) and (b), etc.

[0061] In this application, the terms "a plurality of" and "a variety of" mean two or more than two.

[0062] Unless otherwise specified, the terms used in this application have the well-known meanings commonly understood by those skilled in the art.

[0063] Unless otherwise specified, the numerical values of the various parameters mentioned in this application can be measured by various commonly used testing methods in the art. For example, they can be measured according to the testing methods given in the embodiments of this application. Unless otherwise specified, the test temperature for each parameter is 25°C.

[0064] The battery mentioned in the embodiments of this application can 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 this application can include lithium-ion secondary batteries, battery cells, battery modules or battery packs, etc.

[0065] The lithium-ion secondary battery is the smallest unit that makes up the battery and can independently perform the functions of charging and discharging. The lithium-ion secondary battery can be in the shape of a cylinder, a cuboid or other shapes, etc., and the embodiments of this application do not limit this. For example, Figure 2 is a lithium-ion secondary battery 5 with a cuboid structure as an example.

[0066] The lithium-ion secondary battery includes an electrode assembly and an electrolyte.

[0067] The lithium-ion secondary battery may further include an outer package, and the outer package can be used to encapsulate the electrode assembly and the electrolyte. The outer package can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT) and polybutylene succinate (PBS).

[0068] In some embodiments, such as Figure 3As shown, the outer package may include a housing 51 and a cover plate 53. The housing 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose to form a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. The electrode assembly 52 is encapsulated in the receiving cavity. The number of electrode assemblies 52 included in the lithium-ion secondary battery 5 may be one or more, which can be adjusted according to requirements.

[0069] The electrode assembly generally includes a positive electrode plate and a negative electrode plate. The negative electrode plate is the electrode where the reaction of absorbing or lithiating lithium ions occurs during charging and releasing or delithiating lithium during discharging. The positive electrode plate is the electrode where the reaction of releasing or delithiating lithium ions occurs during charging and occluding or lithiating lithium during discharging.

[0070] When there are multiple lithium-ion secondary batteries, the multiple lithium-ion secondary batteries are connected in series, parallel or in a hybrid connection through a bus bar component. 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, and the battery pack includes a box body and lithium-ion secondary batteries, and the lithium-ion secondary batteries or battery modules are accommodated in the box body. In some embodiments, the box body may be a part of the chassis structure of a vehicle. For example, a part of the box body may become at least a part of the vehicle floor, or a part of the box body may become at least a part of the cross beams and longitudinal beams of the vehicle.

[0071] In some embodiments, the battery may be an energy storage device. The energy storage device includes an energy storage container, an energy storage cabinet, etc.

[0072] In some embodiments, the lithium-ion secondary batteries can be assembled into a battery module, and the number of lithium-ion secondary batteries included in the battery module may be multiple, and the specific number can be adjusted according to the application and capacity of the battery module. Figure 4 is a schematic diagram of a battery module 4 as an example. As Figure 4 shown, in the battery module 4, multiple lithium-ion secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other way. Further, the multiple lithium-ion secondary batteries 5 can be fixed by fasteners.

[0073] Optionally, the battery module 4 may further include a housing having a receiving space, and the multiple lithium-ion secondary batteries 5 are accommodated in the receiving space.

[0074] In some embodiments, the above battery module can be further assembled into a battery pack, and the number of battery modules included in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0075] Figure 5 and Figure 6FIG. 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.

[0076] 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-containing transition metal phosphate per unit volume of the battery, and cannot meet the needs of high energy density batteries.

[0077] In order to further improve the energy density of the battery and increase the compaction density of the pole piece, the common method in the industry is to improve the particle grading. The commonly used method to improve the particle grading in the prior art is to adjust the particle size or proportion of large and small particles to form a dense stacking. However, since the particles in the pole piece are generally sintered at high temperatures, large particles are not easy to form a uniform sphere. There will still be many gaps between the irregular large particles during the stacking process, making it difficult to further increase the compaction density; further increasing the rolling pressure will cause the irregular particles to break, and the conductive network inside the pole piece will be damaged, resulting in an increase in the internal resistance of the battery. How to prepare a high energy density battery while taking into account its dynamic performance is a technical problem that urgently needs to be solved in this field.

[0078] The first aspect of the present application provides a lithium-ion secondary battery, the lithium-ion secondary battery comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer comprises a positive electrode active material, the positive electrode active material comprises lithium-containing transition metal phosphate particles with a carbon coating material arranged on at least a portion of the surface, and in the cumulative distribution curve of the sphericity area of ​​particles with a particle size greater than or equal to 1 μm obtained from a section of the positive electrode film layer along the thickness direction of the electrode sheet, the median of the sphericity L R1A50 It is 0.71-0.85.

[0079] To achieve a relatively large compaction density, large particles need to be introduced as the framework for the stacking of the positive electrode film layer. The lithium-containing transition metal phosphate used in this application is a ceramic material. During the high-temperature sintering process of this material, the grain boundaries melt and the grains grow. The sintering of large particles often requires a higher sintering temperature or a longer sintering time and more grain boundary melting. Therefore, the sphericity of large particles in the prior art is relatively low. Research in this application shows that during the compaction of the electrode sheet, large particles are the main load-bearing objects. During the rolling process, bridging is likely to occur between large particles with poor sphericity, forming voids and stress concentration at the bridging points, which not only affects the further compaction of the electrode sheet and is not conducive to the further improvement of the compaction density of the electrode sheet, but also easily causes local fracture of the electrode sheet at the stress concentration points, damaging the conductive network inside the electrode sheet, and even generating isolated particles, resulting in an increase in the internal resistance of the battery and deteriorating the kinetic performance of the battery.

[0080] Research shows that the median L of the sphericity of particles with a particle size greater than or equal to 1 μm R1A50 Less than 0.71 means that the irregularity degree of the morphology of larger particles is still relatively high, and it is difficult to avoid stress concentration through slip during rolling. When the median L of the sphericity of particles with a particle size greater than or equal to 1 μm R1A50 Is greater than 0.85, the positive electrode active material needs to be formed by sintering the intermediate product after grinding it into a smaller particle size. Furthermore, the pursuit of improving the sphericity of large particles will reduce the proportion of large particles in the positive electrode film layer, which is not conducive to the improvement of the compaction density of the positive electrode film layer and the energy density of the battery.

[0081] In the embodiments of this application, through the processes of multiple sintering and multiple grinding, the median L of the sphericity of particles with a particle size greater than or equal to 1 μm R1A50 Is adjusted to the range of 0.71 - 0.85. By improving the problem of poor sphericity of large particles, reducing the stress concentration problem caused by particle bridging due to the irregular shape of large particles, and reducing the porosity in the film layer, the compaction density of the electrode sheet is improved, the energy density of the battery is effectively increased, the kinetic degradation is reduced, and with a more compact stacking, the electron conduction ability between particles is improved, thereby achieving the improvement of the kinetic performance.

[0082] The lithium-containing transition metal phosphate refers to a phosphate material containing lithium elements and transition metal elements, which can be detected by any well-known method in the art. For example, it can be detected by using an X-ray diffractometer (XRD) in combination with an energy spectrometer and an inductively coupled plasma mass spectrometer. As an example, the lithium-containing transition metal phosphate includes but is not limited to lithium iron phosphate, lithium manganese iron phosphate, and their doped materials.

[0083] The carbon-coated material disposed on at least a part of the surface of the lithium-containing transition metal phosphate can be detected by any well-known method in the art. As an example, the carbon-coated material disposed on at least a part of the surface of the lithium-containing transition metal phosphate can be observed by characterizing the lithium-containing transition metal phosphate using a transmission electron microscope and an energy spectrometer in combination.

[0084] In this application, the term "particle" refers to a particle with an identifiable complete boundary in the field of view of the positive electrode film layer at a certain magnification, such as 10,000 times. There may be defects and scratches inside the particle, but no complete boundary sufficient to divide the particle can be identified inside the particle.

[0085] The method for identifying particles is as follows: Cut the positive electrode film layer along the thickness direction of the electrode plate with an argon ion beam (as an example, the following can be selected: equipment model: Leica EM TIC 3X CP, working voltage: 6 kV, working duration: 6 h). After exposing the cut surface, use a scanning electron microscope (as an example, the following can be selected: equipment model: Hitachi SU8230, working voltage: 3 kV, beam current: high, probe model: U(LA100), working distance <5 mm) to observe the cut surface of the positive electrode film layer along the thickness direction of the electrode plate. Collect images in the secondary electron mode at a non-edge position on the cut surface of the positive electrode film layer (after observing the edge of the electrode plate under the scanning electron microscope, adjust the field of view to the central part of the sample), take an electron micrograph at a magnification of 10,000 times, and analyze the particles in the electron micrograph using ImageJ software (version 1.46r, win64). The usage method of ImageJ software is as follows: Load the scanning electron micrograph to be analyzed, such as Figure 1As shown; use the Cellpose plug-in software therein to identify particles, and on this basis, perform manual correction; use Image J to read and count data. The specific method of using the Cellpose plug-in software therein to identify particles is as follows: set the segmentation diameter parameter (diameter in the Segmantation module) to 15 pixels, click "run cyto3" to identify particles; manually mark the particles in the image that are not recognized by the software, not fully recognized by the software, or have recognition errors. The particles in the image that are not recognized by the software, not fully recognized by the software, or have recognition errors mainly include the following types: 1. Due to the particle being too large or having scratches on the particle surface, the particle cannot be recognized or cannot be fully recognized; 2. During the argon ion beam cutting process, scratches will be generated on the particle surface, and the software may misjudge the scratches as the particle boundary during the recognition process, resulting in recognition errors; 3. Due to the particle being too small, it is not successfully recognized; 4. The particle is located at the edge of the electron microscope field of view, and the inside of the particle is penetrated by the edge, and the morphology cannot be fully displayed, and the local part is recognized instead of the whole, resulting in recognition errors. For the above unrecognized or misrecognized particles, manual calibration is carried out, and the specific process is as follows: delete the particles located at the four edges of the scanning electron microscope that cannot be fully displayed; judge whether there are gap scratches inside other unrecognized or misrecognized particles. If there are no gap scratches inside the particle, judge it as a particle and manually mark it according to the particle boundary observed manually; in response to the presence of gap scratches inside the particle, judge whether the gap scratches penetrate the particle. If they do not penetrate the particle, judge it as a particle and perform manual marking; in response to the gap scratches penetrating the particle, judge whether the gap scratches are linear or irregular; in response to the gap scratches being irregular, judge it as the boundary between particles and divide the particles along this boundary; in response to the gap scratches being linear, perform contrast; in response to the contrast being not obvious and there being no sense of crack, judge it as a scratch and mark it as a particle; in response to the contrast being strong and there being a sense of crack, judge it as the boundary between particles and mark it as two particles. After manual marking, delete the information unrelated to the particles during the automatic image processing process, that is, the determination and marking of the particles in the picture are completed.

[0086] The statistical method for the particles in the positive electrode film layer is as follows: The picture after particle determination and identification is imported into ImageJ software for analysis. The scale is set according to the scanning electron microscope image, and the particle size and area of the particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode plate are analyzed through the analysis functions of "Feret diameter", "Area", "Round", and "Solidity". According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter obtained by analysis represents the maximum distance between all parallel lines in the two-dimensional projection of the particle, so as to characterize the particle size of the particle. Since the particles with a particle size less than 50 nm 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 50 nm, which will cause large errors to the statistical results. Therefore, in the particle size statistics process of this application, the particles with a particle size less than 50 nm are not counted, and the particle statistical data corresponding to "NaN" displayed by AR, Round, or Solidity are deleted. According to the above method, to meet the sample number with statistical significance, at least 10 non-overlapping scanning electron microscope images are collected for each electrode plate, and the particle sizes of at least 5000 particles are counted. The cross-sectional morphology diagram of the positive electrode film layer along the thickness direction of the electrode plate is as Figure 1 shown, which is different from the state of the positive electrode active material in the Malvern laser scattering method and also different from the state of the positive electrode active material when directly observing the positive electrode active material by scanning electron microscope. Under the action of the roll pressure, the particles in the positive electrode film layer show a good dispersion state. Observing the positive electrode film layer is beneficial to effectively characterizing the objective situation of the particle size and distribution of the particles in the positive electrode film layer.

[0087] During the compaction process of the positive electrode film layer, compaction occurs in the thickness direction. The cross-section of the positive electrode film layer along the thickness direction of the electrode plate can reflect the real compaction condition of the particles inside the film layer in the spatial scale more than the surface of the positive electrode film layer.

[0088] It can be understood that the particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode plate, especially the particles with a size above 50 nm, mainly come from the positive electrode active material. Therefore, through the observation and statistics of the particle size of the particles in the cross-section of the positive electrode film layer in the embodiments of this application, the distribution of the lithium-containing transition metal phosphate particles in the positive electrode film layer in the electrode plate can be accurately and objectively reflected.

[0089] In the prior art, a laser particle size analyzer is usually used to count the particle size of the cathode active material by the Malvern laser diffraction method. However, the applicant's research shows that due to the easy agglomeration of lithium-containing transition metal phosphate particles, the test results obtained by the Malvern laser diffraction method based on the laser scattering principle often measure the particle size of their particle aggregates, and cannot truly reflect the particle size of the particles in the cathode active material, let alone reflect the dispersion state of the cathode active material in the film layer, because the dispersion degree of the cathode active material in the film layer will increase during the processes of pulping and film forming and rolling. The test results obtained by the Malvern laser diffraction method are affected by the particle size, specific surface area, and agglomeration degree of the cathode active material. Compared with the actual dispersion situation in the electrode sheet, the number of large particles obtained by this test is lower than the actual value, and the number of small particles is higher than the actual value. Therefore, the particle size obtained by testing with the Malvern laser diffraction method cannot be equivalent to or analogized to the particle size statistically obtained in the embodiments of the present application.

[0090] In the cumulative area distribution curve of the sphericity of particles with a particle size greater than or equal to 1 μm obtained from the cross-section of the cathode film layer along the thickness direction of the electrode sheet, the sphericity test method is as follows: Refer to the method described above in this application to identify the particles in the cross-section of the cathode film layer. While using the "Feret" function in ImageJ to identify the particle size of the particles in the cathode film layer, use the "Shape Descriptor" and "Area" analysis functions to analyze the morphology of the particles and the area of the particles in the cross-section of the cathode film layer along the thickness direction of the electrode sheet. According to the software manual (ImageJ User Guide IJ 1.46r), the "Area" parameter obtained by analysis represents the pixel area of the particle, and the "Round" parameter represents the ratio of the pixel area of the particle to the area of a circle with the fitted major axis as the diameter. When the particle is closer to a sphere, the ratio of the pixel area to the area of a circle with the fitted major axis as the diameter is closer to 1. Therefore, the "Round" parameter of the obtained particle is used to characterize the sphericity of the particle.

[0091] Arrange the sphericities of the particles with a particle size greater than or equal to 1 μm among the obtained at least 5000 particles in ascending order. Use the sphericity as the horizontal axis and the cumulative area ratio calculated from the "Area" of the particles as the vertical axis to obtain the cumulative area distribution curve of the sphericity of the particles in the cathode film layer. The median of the sphericity refers to the sphericity corresponding to a cumulative area ratio of 50% on the vertical axis in the cumulative area distribution curve.

[0092] In some embodiments, in the cumulative area distribution curve of the sphericity of particles with a particle size greater than or equal to 1 μm obtained from the cross-section of the cathode film layer along the thickness direction of the electrode sheet, the median L of the sphericity R1A50It can be optionally 0.71, 0.716, 0.72, 0.73, 0.738, 0.74, 0.745, 0.75, 0.751, 0.755, 0.76, 0.764, 0.77, 0.78, 0.79, 0.793, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85 or the numerical range between any two of them.

[0093] Those skilled in the art can adjust the sphericity of the particles through any known process. As an example, through processes such as grinding, polishing, chemical etching, mechanical stirring, extrusion, coating, granulation, adding surfactants, etc., and adjusting the parameters of each process, the sphericity of the particles can be adjusted.

[0094] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the particle size distribution index PDI of the particles is 0.80 - 1.10. The particle size distribution index PDI is the ratio of the sample standard deviation to the average value of the particle sizes in the cross-section of the positive electrode film layer along the thickness direction.

[0095] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the calculation method of the particle size distribution index of the particles is as follows. The average particle size is calculated using the following formula and the sample standard deviation S. The particle size distribution index PDI of the particles is obtained by dividing the sample standard deviation by the average value. Generally speaking, the larger the PDI, the wider the particle size distribution, which is beneficial to the improvement of particle grading.

[0096]

[0097] Among them, is the particle size of the particles, is the number of samples, is the average particle size of the particles, and S is the sample standard deviation of the particle sizes.

[0098] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the particle size distribution index PDI of the particles can be optionally 0.8, 0.81, 0.82, 0.83, 0.84, 0.845, 0.85, 0.855, 0.86, 0.87, 0.874, 0.88, 0.89, 0.894, 0.897, 0.899, 0.9, 0.91, 0.919, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.973, 0.98, 0.99, 0.996, 1.00, 1.01, 1.02, 1.03, 1.04, 0.05, 1.06, 1.07, 1.08, 1.09, 1.10 or the numerical range between any two of them.

[0099] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the particle size distribution index PDI of the particles can intuitively reflect the size distribution of the overall particles.

[0100] Those skilled in the art can adjust the particle size distribution index, the average particle size, and the sample standard deviation of the particle size through any known process. As an example, by scientifically grading particles of different sizes, the particle size distribution index can be adjusted; by using the mechanical force of crushing and grinding processes, the raw materials are processed to the target particle size distribution range to adjust the average particle size and the sample standard deviation of the particle size; by using screening and classification equipment to separate the particle sizes of the particle system, a particle size distribution that meets the requirements can be obtained; by precisely controlling the feeding rate and adjusting the residence time and force state of the particles in the equipment, it also helps to adjust the particle size distribution index.

[0101] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, when the particle size distribution index PDI of the particles is within the above range, the grading of large and small particles can be achieved, further improving the compaction density of the positive electrode film layer and the energy density of the battery cell.

[0102] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the particle size distribution index PDI of the particles is 0.85 - 1.00.

[0103] When the particle size distribution index of the particles is within the above range, it is beneficial to further optimize the grading of large and small particles, improve the uniformity of the lithium-ion diffusion path, reduce the probability of polarization generation, and take into account both the energy density and kinetic performance of the battery.

[0104] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the average particle size of the particles is 150 nm - 400 nm, and can be optionally 150 nm - 300 nm.

[0105] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the average particle size of the particles can be optionally 150 nm, 160 nm, 170 nm, 180 nm, 189 nm, 190 nm, 191 nm, 200 nm, 210 nm, 220 nm, 230 nm, 236 nm, 240 nm, 250 nm, 257 nm, 258 nm, 260 nm, 262 nm, 270 nm, 280 nm, 281 nm, 284 nm, 290 nm, 300 nm, 310 nm, 320 nm, 330 nm, 340 nm, 350 nm, 360 nm, 370 nm, 380 nm, 390 nm, 400 nm or any numerical range between any two of them.

[0106] In this application, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the average particle size of the particles It can be obtained by testing according to the method described above.

[0107] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the number of medium and small particles accounts for the vast majority. Therefore, the average value of the particle size Within the above range, it reflects the approximate center of the particle size range of medium and small particles in the positive electrode film layer, which can play a role in filling the particle voids and improving the compaction density of the electrode sheet.

[0108] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the sample standard deviation of the particle size is 150 nm - 350 nm, and can be optionally 150 nm - 300 nm.

[0109] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the sample standard deviation of the particle size can be optionally 150 nm, 160 nm, 161 nm, 170 nm, 180 nm, 184 nm, 190 nm, 200 nm, 202 nm, 210 nm, 220 nm, 230 nm, 232 nm, 234 nm, 240 nm, 250 nm, 253 nm, 258 nm, 260 nm, 270 nm, 280 nm, 283 nm, 290 nm, 300 nm, 310 nm, 320 nm, 330 nm, 340 nm, 350 nm or the numerical range between any two of them.

[0110] In this application, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the sample standard deviation S of the particle size can be obtained by testing according to the method described above.

[0111] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the deviation between the particle size of the large particles and the average value of the particle size is larger than that of the medium and small particles. Therefore, it has a greater impact on the sample standard deviation S. The sample standard deviation S reflects the presence of a certain number of large particles within the above range. This is beneficial for building a stacking framework through the large particles, playing a role in force transmission and support, so that the positive electrode film layer can withstand higher rolling pressure to improve the mechanical stability and compaction density of the electrode sheet.

[0112] In some embodiments, in the cumulative area distribution curve of the sphericity of particles with a particle size greater than or equal to 1 μm obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median L of the sphericity R1A50 is 0.72 - 0.80.

[0113] The median L of the sphericity of particles with a particle size greater than or equal to 1 μm R1A50Within the above range, particles with a particle size greater than or equal to 1 μm are closer to spherical, and the possibility of irregular particle lap joints causing voids and stress concentration during the roll pressing process is further reduced. At the same time, a certain mechanical interlocking force between large particles is maintained, which is beneficial to the construction of force chains, can withstand higher roll pressing pressures and internal and external stresses, and further improves the compaction density and stability of the electrode sheet.

[0114] In some embodiments, in the cumulative area distribution curve of the sphericity of particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median L of the sphericity A50 is 0.70 - 0.85, and can be optionally 0.71 - 0.8.

[0115] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the specific method for testing the sphericity of particles can refer to the above text. Arrange the sphericities of the particles in the cross-section of the positive electrode film layer obtained along the thickness direction of the electrode sheet in ascending order, with the sphericity as the horizontal axis and the cumulative area ratio as the vertical axis to obtain the cumulative area distribution curve of the sphericity of particles in the positive electrode film layer. The median of the sphericity refers to the sphericity corresponding to the cumulative area ratio of 50% on the vertical axis in the cumulative area distribution curve.

[0116] In some embodiments, in the cumulative area distribution curve of the sphericity of particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median L of the sphericity A50 can be optionally 0.70, 0.71, 0.715, 0.72, 0.724, 0.725, 0.729, 0.73, 0.731, 0.733, 0.74, 0.749, 0.75, 0.76, 0.77, 0.78, 0.79, 0.80, 0.807, 0.81, 0.82, 0.83, 0.84, 0.85 or the numerical range between any two of them.

[0117] In the cumulative area distribution curve of the sphericity of 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 within the above range means that the particles occupying most of the area and space within the positive electrode film layer are also relatively round, the morphology of the material is relatively uniform, and effective slip occurs under external force, participating in the connection and reconstruction of force chains, thereby helping to improve the compaction density, increase the volumetric energy density, reduce the generation of particle breakage, and improve the kinetic performance.

[0118] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, L R1A50 > L A50 , and can be optionally 0.01 ≤ L R1A50 - L A50 ≤ 0.1, and further optionally 0.01 ≤ L R1A50 - L A50 ≤ 0.05.

[0119] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, L R1A50 -L A50 can be optionally 0.005, 0.01, 0.014, 0.02, 0.022, 0.024, 0.03, 0.031, 0.04, 0.043, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1 or the numerical range between any two of them.

[0120] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, by improving the sphericity of the particles, the packing situation of the particles can be improved. However, even if all the particles are perfect spheres, there will still be gaps. L R1A50 >L A50 makes the particles larger than 1 μm more round, can be more evenly distributed, and helps to form a more regular framework structure; the sphericity of the smaller particles is smaller, and due to the diversity of their shapes (such as oval, wedge-shaped, etc.), they can form a more compact arrangement structure between the large particles, filling the gaps that are difficult to fill between the large particles. By the cooperation of high and low sphericity of different particle sizes, the space utilization rate of the stacked body is significantly improved, thereby improving the compaction density of the electrode sheet and the energy density of the battery.

[0121] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of the laser confocal Raman spectrometer for the positive electrode film layer, the median C of the graphitization degree 50 is 0.98 - 1.20, and can be optionally 1.02 - 1.1; where the graphitization degree C value is I G / I D where I G represents the intensity of the G peak of the Raman spectrum at 1580 ± 100 cm -1 and I D represents the intensity of the D peak of the Raman spectrum at 1350 ± 100 cm -1 .

[0122] In the present application, the graphitization degree C value of the positive electrode film layer can be obtained through the surface scanning mode of the laser confocal Raman spectrometer. As an example, specifically, a laser confocal Raman spectrometer (high-precision Renishaw laser confocal Raman spectrometer) is used, the excitation wavelength of 532 nm is selected, an appropriate amount of the positive electrode film layer is taken for surface scanning on its surface or the cross-section along the thickness direction of the electrode sheet, the scanning area is 45 μm × 45 μm, divided into 10 × 10 grids, the grid vertices are used as the test points, the step size is 5 μm, and the total number of scanning points is 100 points, thereby obtaining the C values at different sites and the cumulative distribution curve of the C value of the surface scanning area.

[0123] The C value of the graphitization degree of the positive electrode film layer is obtained from the peak intensity ratio of the G peak (G-band) and the D peak (D-band) in Raman spectroscopy. The position of the G peak is 1580±100 cm -1 , which characterizes the carbon sp 2 hybrid structure; the position of the D peak is 1350±100 cm -1 , which characterizes the disordered structure, where disorder means that there is no regular arrangement between carbon atoms in the structure. In a graphite crystal, carbon atoms in the same layer form covalent bonds through sp 2 hybridization, and the intermolecular force between layers is the van der Waals force, making the carbon in the graphite structure prone to slip. Therefore, the C value can characterize the graphitization degree of the positive electrode film layer. The larger the value, the higher the graphitization degree of the carbon material. It can be understood that the graphitization degree in the positive electrode film layer mainly comes from the carbon material treated by graphitization in the positive electrode film layer, that is, the carbon coating material of the positive electrode active material. Although carbon nanotube conductive agents rich in sp 2 hybrid structures also have a relatively high I G / I D , due to their low addition content and small tube diameter, their addition in the positive electrode film layer shows an extreme value in the Raman surface scan test of the positive electrode film layer and will not affect the graphitization degree C 50 in the positive electrode film layer. Therefore, the graphitization degree of the positive electrode film layer can also be used to characterize the graphitization degree of the positive electrode active material.

[0124] The higher the graphitization degree of the carbon on the surface of the positive electrode active material, the higher the proportion of graphite-structured carbon in the positive electrode film layer, and the easier it is for the particles to slip during the rolling process by means of the carbon structure with a high graphitization degree in the coating material, which can further reduce the possibility of stress concentration and increase the compaction density, and improve the kinetic performance of the battery by virtue of the good electronic conductivity of the coating material with a high graphitization degree.

[0125] The cumulative distribution curve of the graphitization degree C value refers to a curve obtained by arranging at least 100 obtained C values in ascending order, with the graphitization degree on the horizontal axis and the cumulative quantity proportion on the vertical axis. C 50 is the C value corresponding to the cumulative quantity proportion of 50% on the vertical axis in the cumulative distribution curve of the graphitization degree C value. The median C 50 of the graphitization degree can reflect the overall graphitization degree of the particles in the positive electrode film layer, that is, the ease of slipping, compared with the point value; compared with the mean value, it can reduce the influence of extreme values during the test and improve the confidence level of the test results.

[0126] Those skilled in the art can adjust the graphitization degree of the active material particles through any known process. As an example, adjusting the carbon source (the carbon source can be selected as cross-linked PEG), sintering temperature, sintering time, sintering pressure, sintering atmosphere, and nucleation process can all achieve the adjustment of the graphitization degree of the active material particles.

[0127] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of a laser confocal Raman spectrometer for the positive electrode film layer, the median C of the graphitization degree 50 can be optionally 0.98, 0.99, 1.00, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.20 or a numerical range between any two of them.

[0128] In some embodiments, in the cumulative distribution curve of the roughness 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 R of the roughness A50 is 0.92 - 0.96.

[0129] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the specific method for testing the roughness of the particles is as follows: Identify the particles in the cross-section of the positive electrode film layer with reference to the method described above in this application. Use the "Shape Descriptor" and "Area" analysis functions 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 sheet. According to the software manual (ImageJ User Guide IJ 1.46r), the "Solidity" parameter obtained from the analysis represents the ratio of the pixel area of the particle to the convex area. Therefore, the "Solidity" parameter of the particles obtained from the 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 5000 obtained particles in ascending order, and obtain the cumulative area distribution curve of the roughness of the particles in the positive electrode film layer with the roughness as the horizontal axis and the cumulative area ratio as the vertical axis. R A50 is the roughness R value corresponding to the cumulative area ratio of 50% on the vertical axis in the cumulative area distribution curve of the roughness R value.

[0130] In some embodiments, in the cumulative distribution curve of the roughness 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 R of the roughness A50 can be optionally 0.92, 0.93, 0.94, 0.95, 0.96 or a numerical range between any two of them.

[0131] Those skilled in the art can adjust the roughness of the particles through any known process. As an example, the roughness of the particles can be adjusted by processes such as grinding, polishing, grinding, mechano-chemical energy, electroplating, calendaring, etc. and by adjusting the parameters of each process.

[0132] The median R of the roughness 50The surfaces of the particles within the above range are relatively smooth, and the frictional force between particles is relatively small, making it easy for them to slip under the action of an external force, reducing the occurrence of the intermeshing phenomenon due to rough surfaces during roll pressing. Coupled with a reasonable particle size distribution index and particles with a high sphericity of greater than or equal to 1 μm, close packing can be formed at a relatively low roll pressing pressure, further improving the compaction density of the electrode and increasing the energy density of the battery.

[0133] In some embodiments, the positive electrode active material includes iron element, and the iron dissolution rate of the positive electrode film layer is 400 ppm - 1800 ppm, and can be optionally 500 ppm - 1500 ppm.

[0134] The iron dissolution rate of the positive electrode film layer can be tested in the following manner. Specifically, after disassembling and washing the electrode from the battery, it is made 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 5 minutes, then quickly suck the solution with a 5 mL syringe, filter the solution through a 0.45 μm pore size filter head into a test tube, suck 1 mL of the supernatant with a pipette, 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 - measured iron element concentration × solution volume / mass of the solution involved in volume fixing) × 100.3 g / (mass of the electrode 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 fixing 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 。

[0135] In some embodiments, the iron dissolution rate of the positive electrode film layer can be optionally 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 900.2 ppm, 1000 ppm, 1100 ppm, 1200 ppm, 1300 ppm, 1400 ppm, 1500 ppm, 1600 ppm, 1700 ppm, 1800 ppm or any numerical range between any two of them.

[0136] 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 material is regulated by controlling the surface coating quality 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 the oxidant in the electrolyte, the battery operating temperature, and the battery charge-discharge intensity will also affect the iron dissolution rate of the positive electrode film layer. 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 coating material, that is, the more complete and dense the carbon coating 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 surface of the positive electrode active material has a relatively complete and dense carbon coating, 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 can play a role in optimizing the kinetic performance of the lithium-ion secondary battery. At the same time, the high-integrity carbon coating material makes the particles prone to stress slip during the rolling process, and can improve the compaction density of the electrode sheet and the energy density of the battery at the same time.

[0137] 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%.

[0138] Based on the total mass of the positive electrode active material, the mass content 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 the De Kai HCS infrared carbon-sulfur analyzer.

[0139] 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.

[0140] Compared with the lithium-containing transition metal phosphate positive electrode active material in the prior art, this positive electrode active material has a relatively low carbon coating amount, which can further increase the loading amount of lithium-containing transition metal phosphate in the positive electrode sheet and improve the energy density of the lithium-ion secondary battery.

[0141] In some embodiments, the lithium-iron anti-site defect concentration of the positive electrode active material is 0.001% - 1.5%, and can be optionally 0.01% - 1.0%.

[0142] XRD data of the sample was collected using an X-ray diffractometer, and phase analysis of the sample was performed. The CIF file of this phase obtained from an open-source website was used as the initial crystal structure model, including defining unit cell parameters, atomic positions, and occupancy probabilities, etc. In the initial crystal structure model, considering the possibility of Fe-Li antisite, 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 was 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 best matched and Rwp was less than 10, the refined occupancy probabilities of Li and Fe were obtained, which were used as the concentration of Fe-Li antisite defects.

[0143] In some embodiments, the concentration of Fe-Li antisite defects in the positive electrode active material can be selected from 0.001%, 0.01%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% or the numerical range between any two of them.

[0144] During the preparation and cycling process, there will inevitably be a certain amount of 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 Fe-Li antisite defects and blocking the one-dimensional diffusion channels of lithium ions, which has an adverse effect on the solid-phase transport of lithium ions. The positive electrode active material in the embodiments of the present application has low Fe-Li antisite defects, which is beneficial to the uniform transport of lithium ions in the solid phase and further improves the kinetic performance of lithium-ion secondary batteries.

[0145] In some embodiments, the lithium-containing transition metal phosphate particles include components having the following general formula: Li m Fe x P y O j Q q , wherein Q includes one or more of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.8 ≤ m ≤ 1.15, 0.9 ≤ x ≤ 1, 0.95 ≤ y ≤ 1, 3.5 ≤ j ≤ 4, 0 < q ≤ 0.1.

[0146] In some embodiments, m can be optionally 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 numerical range between any two of them; x can be optionally 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0 or a numerical range between any two of them; y can be optionally 0.95, 0.96, 0.97, 0.98, 0.99, 1.00 or a numerical range between any two of them; j can be optionally 3.5, 3.6, 3.7, 3.8, 3.9, 4 or a numerical range between any two of them; q can be optionally 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1 or a numerical range between any two of them.

[0147] Selecting an appropriate modification element Q can improve the lattice change rate of the positive electrode active material during the lithium deintercalation / insertion process, reduce the oxygen activity on the particle surface, improve the structural stability of the material, and further improve the specific capacity performance of the material during the cycling process, and further improve the cycling stability of the lithium ion secondary battery.

[0148] In some embodiments, the positive electrode active material includes one or more of lithium iron phosphate and its doped and modified materials, and coated and modified materials.

[0149] In some embodiments, the positive electrode active material includes titanium element, and based on the total mass of the positive electrode active material, the mass content of the titanium element is 4000 ppm - 8000 ppm.

[0150] The types and contents of elements in the positive electrode active material can be tested by any well-known method in the art. As an example, the inductively coupled plasma emission spectrometry is used to test the titanium element and its content with reference to Appendix C of GB / T 33822-2017.

[0151] The doping of titanium element in the positive electrode active material is beneficial to causing lattice distortion, reducing the Li-O bond energy, increasing the lithium ion transmission rate, and improving the kinetic performance of the lithium ion secondary battery. However, in the prior art, the doping content of titanium element in lithium-containing transition metal phosphates often cannot exceed 3000 ppm, because too much titanium element is difficult to completely enter the bulk phase of the lithium-containing transition metal phosphate and is easy to become a harmful impurity phase remaining on the surface, which has a negative impact on the battery performance.

[0152] The positive electrode active material in the embodiments of the present application has a high titanium element content. Moreover, surprisingly, the high addition amount of titanium element does not form harmful impurity phases that have a negative impact on the battery energy density and kinetic performance. Although the reason is not yet clear, it is speculated that it may be that titanium element forms a fast ion conductor together with phosphate groups and other elements (for example, lithium element), which instead has a promoting effect on the kinetic performance of the battery.

[0153] In some embodiments, the tapped density of the powder of the positive electrode active material is 1.00 g / cm 3 -1.70 g / cm 3 , and may be optionally 1.20 g / cm 3 -1.50 g / cm 3 .

[0154] The tapped density of the powder can be measured by any well-known method in the art.

[0155] The tapped density of the powder can be measured by any well-known method 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 zero the electronic balance; place the tapped density measuring cylinder on the conical flask and weigh it to 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 circles to mix it evenly, and then transfer the sample smoothly into the measuring cylinder; wipe the powder contaminated on the surface of the connection with dust-free paper, and then put it into the conical flask that has been zeroed and weigh it; seal the mouth of the measuring cylinder with a sealing film, place the tapped density measuring cylinder in the matching instrument rubber ring, ensure that the tapped density measuring cylinder fits tightly with the rubber ring and is perpendicular to the instrument surface; set the vibration frequency to 250 times / min on the instrument, the number of vibrations to 5000 times, press the button, and vibrate for 20 min; then remove the tapped density measuring cylinder, use a flashlight to irradiate the surface of the measuring cylinder, and by visual inspection, read the highest scale V 1 and the lowest scale V 2 , and take the average value V of the two; the mass of the measuring cylinder and the sample m 1 minus the mass of the measuring cylinder m 0 , to obtain the powder mass m, and the tapped density of the sample can be obtained from the density formula ρ = m / v.

[0156] In some embodiments, the tapped density of the powder of the positive electrode active material may be optionally 1.00 g / cm 3 , 1.05 g / cm 3 , 1.10 g / cm 3 , 1.15 g / cm 3 , 1.20 g / cm 3 , 1.25 g / cm 3 , 1.30 g / cm 3 , 1.35 g / cm 3 , 1.40 g / cm3 , 1.45 g / cm 3 , 1.50 g / cm 3 , 1.55 g / cm 3 , 1.60 g / cm 3 , 1.65 g / cm 3 , 1.70 g / cm 3 or the numerical range between any two of them.

[0157] The positive electrode active material particles in the embodiments of the present application form an effective grading, have a high tap density, and due to the high sphericity of the particles larger than 1 μm in the positive electrode film layer, they can spontaneously roll to fill the pores, further improving the tap density.

[0158] In some embodiments, the powder compaction density of the positive electrode active material under 3T pressure is 2.55 g / cm 3 -2.70 g / cm 3 , and can be optionally 2.58 g / cm 3 -2.68 g / cm 3 .

[0159] In the present application, the term "powder compaction density" refers to the density of the green compact formed during the external force compression process. As the powder moves and deforms, larger voids are filled, the contact area between particles increases, the atomic attraction is generated between atoms, and the mechanical fitting effect between particles is enhanced, so as to form a green compact with a certain density and strength, and the unit is g / cm 3 .

[0160] The powder compaction density of the positive electrode active material can be measured by methods and equipment known in the art. For example, reference can be made to GB / T 24533-2009 and measured using a compaction density instrument. Specifically, a certain amount of positive electrode active material is placed on a special compaction mold (the diameter of the mold is known), and there is a hollow in the middle of the mold with a metal disc at each end. The positive electrode active material is placed between the metal discs, and a metal cylinder is placed on the top. The mold is placed on the compaction density instrument, the pressure is set to 3T, and the thickness of the positive electrode active material under 3T pressure 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 1.327 cm 2 , and H is the thickness of the positive electrode active material after compaction.

[0161] In some embodiments, the powder compaction density of the positive electrode active material under 3T pressure can be optionally 2.55 g / cm 3 , 2.56 g / cm 3 , 2.57 g / cm 3 , 2.58 g / cm3 , 2.59 g / cm 3 , 2.60 g / cm 3 , 2.61 g / cm 3 , 2.62 g / cm 3 , 2.63 g / cm 3 , 2.64 g / cm 3 , 2.65 g / cm 3 , 2.66 g / cm 3 , 2.67 g / cm 3 , 2.68 g / cm 3 , 2.69 g / cm 3 , 2.70 g / cm 3 or the numerical range between any two of them.

[0162] The positive electrode active material particles form an effective gradation, and the particles larger than 1 μm in the positive electrode film layer also have a high sphericity, enabling the positive electrode active material to achieve a higher tap density under an external force, providing a material basis for improving the tap density of the electrode sheet and preparing a lithium-ion secondary battery with a high energy density.

[0163] In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 8 MPa is 0.5 Ω·cm - 60.0 Ω·cm, and can be optionally 2.0 Ω·cm - 40.0 Ω·cm.

[0164] The powder resistivity of the positive electrode active material can be measured by methods and equipment known in the art. For example, referring to GB / T 33822-2017, a powder resistivity meter (Suzhou Crystal, model ST2722) can be used for measurement. Specifically, a certain amount of the positive electrode active material (such as 1 g) is weighed and added to the feeding chamber of the powder resistivity meter, a pressure of 8 MPa is applied, and the forward resistivity and reverse resistivity of the positive electrode active material are respectively measured, and the average value of the two is taken as the powder resistivity of the positive electrode active material.

[0165] In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 8 MPa can be optionally 0.5 Ω·cm, 1 Ω·cm, 2 Ω·cm, 3 Ω·cm, 4 Ω·cm, 5 Ω·cm, 6 Ω·cm, 7 Ω·cm, 8 Ω·cm, 9 Ω·cm, 10 Ω·cm, 11.96 Ω·cm, 15 Ω·cm, 20 Ω·cm, 25 Ω·cm, 30 Ω·cm, 35 Ω·cm, 40 Ω·cm, 45 Ω·cm, 50 Ω·cm, 55 Ω·cm, 60 Ω·cm or the numerical range between any two of them.

[0166] The positive electrode active material is surface-coated with a carbon material, relying on the sp of the surface carbon 2The structure is easy to achieve rapid conduction of electrons between particles, enabling the positive electrode active material to have a low powder resistivity, which is beneficial to improving the solid-phase transmission rate of electrons and enhancing the kinetic performance of the battery.

[0167] In some embodiments, the discharge specific capacity of the positive electrode active material at room temperature at a discharge rate of 1C is 135 mAh / g - 150 mAh / g.

[0168] In this application, the positive electrode active material is assembled into a coin cell to test its electrical performance on a blue battery tester. At 25 ± 5°C within the voltage range of 2.0V - 3.75V, after constant current charging at 1C to 3.75V, pause for 5 minutes, then constant voltage charge until the cut-off current is 50 μA, and then discharge at 1C to 2.0V. The discharge capacity of the coin cell is divided by the mass of the positive electrode active material to obtain the discharge specific capacity of the positive electrode active material at room temperature at a discharge rate of 1C.

[0169] The preparation and testing process of the coin cell is as follows: Mix 2.0 g of the positive electrode active material, conductive carbon black, and PVDF in a mass ratio of 0.9∶0.05∶0.05, then add the organic solvent NMP (N-methylpyrrolidone), mix well, coat with a 150 μm doctor blade, dry at 100°C for 2 h, and compact the positive electrode plate according to a compaction density of 2.0 g / cm 3 - 2.2 g / cm 3 Compact the positive electrode plate, punch it into a circular piece with a diameter of 14 mm, then weigh and record the weight. Put the weighed positive electrode plate into a vacuum drying oven (105°C, 1 - 12 hrs, -90 kPa). After drying, put the positive electrode plate into a glove box and assemble it into a battery in the order of negative electrode shell - nickel mesh - lithium sheet - separator - positive electrode sheet - positive electrode shell. Drop 65 - 87 μL (pipette) of electrolyte (the electrolyte is a mixed solvent of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) with a volume ratio of 1:1, and the electrolyte is LiPF 6 ) with the negative electrode on top, place it in the groove of the sealing machine, with a sealing pressure of 650 kg / cm 2 , use insulating tweezers to remove the coin cell and put it into a dust-free bag, remove the glove box, and place it in a constant temperature room for 3 h to obtain the coin cell for testing.

[0170] It can be understood that the discharge specific capacity of the positive electrode active material can also be obtained by disassembling the battery, obtaining the positive electrode plate, assembling it into a coin cell according to the method described above, and then testing.

[0171] In some embodiments, the discharge specific capacity of the positive electrode active material at room temperature at a discharge rate of 1C can be selected as 135 mAh / g, 140 mAh / g, 141.1 mAh / g, 145 mAh / g, 150 mAh / g, or the numerical range between any two of them.

[0172] The positive electrode active material has a high discharge specific capacity at a rate of 1C, indicating its good charge-discharge ability, which is beneficial to improving the kinetic performance of the battery.

[0173] In some embodiments, the proportion η of the discharge capacity of the positive electrode active material discharged to 3.2V satisfies η≥85%. The η is defined as follows: at room temperature, a coin cell containing the positive electrode active material is charged and discharged at a constant current twice at a rate of 0.1C within a voltage range of 2.0V to 3.75V, and then charged and discharged at a constant current once at a rate of 1C. In the charge-discharge test at a rate of 1C, the capacity value when the discharge voltage is 3.2V is denoted as C 1 , and the capacity value when the discharge voltage reaches 2.0V is C 2 , η = C 1 / C 2 , where the charging process includes constant voltage charging, with a constant voltage of 3.75V and a constant voltage cut-off current of 50μA.

[0174] In some embodiments, the proportion η of the discharge capacity of the positive electrode active material discharged to 3.2V satisfies η≥88%.

[0175] 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 coin cell according to the method described above. At room temperature, test the electrical performance of the prepared coin cell on a Blue-Electro tester. Specifically, the coin cell is charged and discharged at a constant current twice at a rate of 0.1C within a voltage range of 2.0V to 3.75V. After constant current charging to the cut-off voltage, it is charged at a constant voltage until the current is 50μA, and then charged and discharged at a constant current once at a rate of 1C. In the charge-discharge test at a rate of 1C, the capacity value discharged from 3.75V to a voltage of 3.2V is denoted as C 1 , the capacity value discharged from 3.75V to 2.0V is C 2 , and η = C 1 / C 2 .

[0176] In some embodiments, η can be selected from 85%, 86%, 87%, 88%, 88.1%, 89%, 90%, 90.1%, 91%, 92%, 92.2%, 93%, 94%, 94.1%, 94.5%, 95%, 95.1% or any numerical range between any two of them.

[0177] In some embodiments, the proportion η of the discharge capacity of the positive electrode active material in a freshly prepared lithium-ion secondary battery discharged to 3.2V satisfies η≥88%. After the freshly prepared lithium-ion secondary battery is charged and discharged at a constant current twice at a rate of 0.1C within a voltage range of 2.0V to 3.75V for a period of time, the proportion η of the discharge capacity of the positive electrode active material discharged to 3.2V can remain ≥85%.

[0178] In the lithium-ion secondary battery according to the embodiment of the present application, the high proportion of the discharge capacity of the positive electrode active material 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 when the lithium-ion secondary battery containing the positive electrode active material is discharged to a low state of charge (SOC), it still has a high voltage, which is beneficial to maintaining good power performance.

[0179] 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 0.01%-1.5%.

[0180] 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 can be 0.01%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% or any value range between any two of them.

[0181] 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.

[0182] The particles in the positive electrode film layer have good grading, and the large particles have a high sphericity, so they can form a tight packing, enabling the particles to contact each other, having good electron conductivity, forming an electron transport network, which can reduce the use of the conductive agent in the positive electrode film layer, is beneficial to further increasing the loading amount of the positive electrode active material, and improving the energy density of the lithium-ion secondary battery.

[0183] The positive electrode active material has extremely high electron conductivity, so that the conductive agent can even not be added to the positive electrode film layer, which is beneficial to further increasing the loading amount of the positive electrode active material and improving the energy density of the lithium-ion secondary battery.

[0184] In some embodiments, the positive electrode film layer further includes a binder, and based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 92%-99.5%, and can be 96.5%-99.5%; the mass content of the binder is 0.5%-3%.

[0185] 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 fluorinated acrylate resin.

[0186] 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 92%, 93%, 94%, 95%, 96%, 96.5%, 97%, 98%, 99%, 99.5% or any value range between any two of them.

[0187] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the binder may be 0.5%, 1%, 1.5%, 2%, 2.5%, 3% or any value range between any two of them.

[0188] When the mass content of the positive electrode active material and the mass content of the binder are within the above ranges, the loading amount of the active substance in the positive electrode film layer per unit volume can be effectively increased, and good internal adhesion can be maintained, reducing the occurrence probability of powder falling, swelling and cracking problems, and taking into account the safety performance while improving the energy density of the secondary battery.

[0189] In some embodiments, the single-sided areal density of the positive electrode film layer is 300 mg / 1540mm 2 -450 mg / 1540mm 2 .

[0190] In the present application, the single-sided areal density of the positive electrode film layer has the meaning well-known in the art and can be tested by methods known in the art. For example, take a single-sided coated and compacted positive electrode sheet (if it is a double-sided coated positive electrode sheet, the positive electrode film layer on one side can be wiped off first), punch it into small round pieces with an area of S 1 , weigh it, and record it as M 1 . Then wipe off the positive electrode film layer of the above-mentioned weighed positive electrode sheet and weigh the weight of the current collector, and record it as M 0 . The single-sided areal density of the positive electrode film layer = (M 1 - M 0 ) / S 1 . In order to ensure the accuracy of the test results, multiple groups (for example, 10 groups) of samples to be tested can be tested, and the average value can be calculated as the test result.

[0191] In some embodiments, the areal density of the single-sided positive electrode film layer may be 300 mg / 1540mm 2 , 310 mg / 1540mm 2 , 320 mg / 1540mm 2 , 330 mg / 1540mm 2 , 340 mg / 1540mm 2 , 350 mg / 1540mm 2 , 360 mg / 1540mm 2 , 370 mg / 1540mm 2, 380 mg / 1540mm 2 , 390 mg / 1540mm 2 , 400 mg / 1540mm 2 , 410 mg / 1540mm 2 , 420 mg / 1540mm 2 , 430 mg / 1540mm 2 , 440 mg / 1540mm 2 , 450 mg / 1540mm 2 or the numerical range between any two of them.

[0192] The positive electrode film layer with areal density within the above range can help improve the energy density of the lithium-ion secondary battery.

[0193] In some embodiments, when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.52 g / cm 3 -2.78 g / cm 3 .

[0194] In some embodiments, when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.55 g / cm 3 -2.75 g / cm 3 .

[0195] In this application, the fully discharged state means that the battery is placed at 25°C and left standing for 2 h. After the battery temperature remains at 25°C, the battery is discharged at a constant current of 1 / 3C to 2.5V and then at a constant current of 0.1C to 2.0V.

[0196] The tap density of the positive electrode film layer can be tested by methods known in the art. As an example, the battery is placed in an oven environment at 25°C and left standing for 2 h. After the battery temperature remains at 25°C, the battery is discharged at a constant current of 1 / 3C to 2.5V and then at a constant current of 0.1C to 2.0V. The battery is disassembled to obtain the positive electrode plate. The residual electrolyte is treated with dimethyl carbonate solvent, the electrode plate is dried, cut into small round pieces with an area of S, and its mass is obtained as W 1 , and the thickness T of the positive electrode plate is measured using a micrometer 1 . Then, the positive electrode film layer of the above weighed electrode plate is wiped off, and the mass of the current collector is weighed and recorded as W 2 , and the thickness T of the current collector is measured using a micrometer 2 . Then, the tap density PD of the positive electrode film layer = (W 1 -W 2 ) / [(T 1 -T 2 )×S].

[0197] In some embodiments, in the fully discharged state of the lithium-ion secondary battery, the tap density of the positive electrode film layer can be selected from 2.52 g / cm 3 , 2.53 g / cm 3 , 2.54 g / cm 3 , 2.55 g / cm 3 , 2.56 g / cm 3 , 2.57 g / cm 3 , 2.58 g / cm 3 , 2.59 g / cm 3 , 2.60 g / cm 3 , 2.61 g / cm 3 , 2.62 g / cm 3 , 2.63 g / cm 3 , 2.64 g / cm 3 , 2.65 g / cm 3 , 2.66 g / cm 3 , 2.67 g / cm 3 , 2.68 g / cm 3 , 2.69 g / cm 3 , 2.70 g / cm 3 , 2.71 g / cm 3 , 2.72 g / cm 3 , 2.73 g / cm 3 , 2.74 g / cm 3 , 2.75 g / cm 3 , 2.76 g / cm 3 , 2.77 g / cm 3 , 2.78 g / cm 3 or any value within the range between any two of them.

[0198] In some embodiments, after being processed by the compaction process, the tap density of the positive electrode film layer is 2.63 g / cm 3 -2.90 g / cm 3 .

[0199] In some embodiments, after being processed by the compaction process, the tap density of the positive electrode film layer can be selected from 2.63 g / cm 3 , 2.64 g / cm 3 , 2.65 g / cm 3 , 2.66 g / cm 3 , 2.67 g / cm 3 , 2.68 g / cm 3 , 2.69 g / cm 3 , 2.70 g / cm 3 , 2.71 g / cm3 , 2.72 g / cm 3 , 2.73 g / cm 3 , 2.74 g / cm 3 , 2.75 g / cm 3 , 2.76 g / cm 3 , 2.77 g / cm 3 , 2.78 g / cm 3 g / cm 3 , 2.79 g / cm 3 , 2.80 g / cm 3 , 2.81 g / cm 3 , 2.82 g / cm 3 , 2.83 g / cm 3 , 2.84 g / cm 3 , 2.85 g / cm 3 , 2.86 g / cm 3 , 2.87 g / cm 3 , 2.88 g / cm 3 , 2.89 g / cm 3 , 2.90 g / cm 3 or any value range between any two of them.

[0200] In this application, "compaction" means that during the battery assembly process, the positive electrode film layer is compacted by mechanical pressure to improve its density and conductivity.

[0201] In some embodiments, after being treated by the formation process, the compaction density of the positive electrode film layer is 2.52 g / cm 3 - 2.78 g / cm 3 .

[0202] In some embodiments, after being treated by the formation process, the compaction density of the positive electrode film layer can be selected as 2.52 g / cm 3 , 2.53 g / cm 3 , 2.54 g / cm 3 , 2.55 g / cm 3 , 2.56 g / cm 3 , 2.57 g / cm 3 , 2.58 g / cm 3 , 2.59 g / cm 3 , 2.60 g / cm 3 , 2.61 g / cm 3 , 2.62 g / cm 3 , 2.63 g / cm 3 , 2.64 g / cm 3 , 2.65 g / cm 3 , 2.66 g / cm3 , 2.67 g / cm 3 , 2.68 g / cm 3 , 2.69 g / cm 3 , 2.70 g / cm 3 , 2.71 g / cm 3 , 2.72 g / cm 3 , 2.73 g / cm 3 , 2.74 g / cm 3 , 2.75 g / cm 3 , 2.76 g / cm 3 , 2.77 g / cm 3 , 2.78 g / cm 3 or the numerical range between any two of them.

[0203] In the present application, formation refers to the process of forming a stable solid electrolyte interface (SEI film) and electrode structure through electrochemical reactions during the first charge and discharge of the battery.

[0204] It can be understood that along with the rebound of the electrode sheet during the cycling process, the compaction density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery is slightly lower than that of the positive electrode film layer after compaction and formation.

[0205] When the compaction density of the positive electrode film layer is within the above range, it is beneficial to improve the energy density of the lithium-ion secondary battery.

[0206] In some embodiments, when the lithium-ion secondary battery is in the fully discharged state, the compaction density of the positive electrode film layer is 2.52 g / cm 3 - 2.78 g / cm 3 , and 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%.

[0207] In some embodiments, when the lithium-ion secondary battery is in the fully discharged state, the compaction density of the positive electrode film layer is 2.55 g / cm 3 - 2.75 g / cm 3 , and 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%.

[0208] In some embodiments, when the lithium-ion secondary battery is in the fully discharged state, 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 can be selected from 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22% or the numerical range between any two of them.

[0209] 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 can be tested in the following manner. Import the scanning electron microscope image of the cross-section of the positive electrode film layer obtained in the manner described above along the thickness direction of the electrode sheet into the ImageJ software. Select the straight line tool, use the straight line to mark the scale length in the picture, click "Analyze SetScale", and set the scale parameters in the software according to the scale length in the picture. Select the rectangular tool, select the part of the picture outside the scale area, use "Image Duplicate" to copy the selected area, and use "Image Type 8 bit" to adjust the picture format; select "Analyze Set Measurements", and select the following 5 options: "Area", "Mean gray value", "Area Fraction", "Limit to threshold", "Feret’s diameter", where "Decimal places" is selected as 3. Then, in sequence, select "Image"-"Adjust"-"Threshold", and set 0 and 100 in turn at the box selection position of "Threshold". Then the pore data in the cross-section electron microscope image can be exported using the Analyze-Measure function. Use "Image"-"Overlay"-"Flatten" to export and obtain the pore picture; click "Apply" in "Threshold", then click "Analyze"-"Analyze Particles", and check the left four columns to obtain the pore statistical data.

[0210] As Figure 8 shown, it can be understood that in the embodiments of the present application, the "pores" in the cross-section of the positive electrode film layer are identified through the picture color difference and threshold. This "pore" is not the pore data obtained from the exhaust test, and is mainly used to characterize the cross-sectional area between the particles in the cross-section of the positive electrode film layer. This method is superior to the exhaust method because the porosity obtained by the exhaust method is related to the pores between the particles and also to the mesopores in the carbon layer coated on the surface of the lithium iron phosphate particles, thus unable to objectively reflect the pores between the particles. The lower the porosity of the cross-section of the positive electrode film layer measured by this method means that, on the one hand, the grading of large, medium, and small particles in the positive electrode film layer is better and the compaction density is high; on the other hand, after the same grading and rolling pressure, if the porosity is low, it means that the particles are easy to slip relative to each other, thereby reducing the risk of overpressure and stress concentration in the film layer, further reducing the probability of the positive electrode film peeling off during the long cycle process, which is beneficial to improving the long cycle performance of the battery.

[0211] In some embodiments, the positive electrode tab includes a bottom coating disposed between the positive electrode film layer and the positive electrode current collector; the bottom coating includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating is ≤ 10 pcs / 10 μm.

[0212] Among them, carbon-based particles refer to particles mainly composed of carbon elements, including but not limited to conductive carbon, carbon black, etc.

[0213] The bottom coating is beneficial to improving the conductivity and adhesion between the positive electrode film layer and the current collector, reducing the release of the positive electrode film layer from the current collector during the cycling process, and simultaneously improving the kinetic performance of the battery. In the high-compaction-density electrode tab of the embodiment of the present application, for example, the compaction density of the positive electrode tab in the fully discharged state is greater than or equal to 2.4 g / cm 3 When the current collector is prone to damage during the compaction process of the high pressure of the electrode tab, large-size particles are prone to generate pits on the current collector. Controlling the distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating to be ≤ 10 pcs / 10 μm is beneficial to reducing the probability of damage to the current collector in the high-compaction-density electrode tab and further improving the ultimate compaction density of the positive electrode tab.

[0214] The distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating can adopt the method described above. By cutting the positive electrode film layer along the thickness direction of the electrode tab with an argon ion beam, taking a scanning electron microscope image or a microscope image, detecting the size of carbon particles in the bottom coating by a statistical method, and counting the number of carbon-based particles with a particle size greater than 100 nm contained in every 10 μm in the bottom coating, and counting not less than 5 times and taking the average value.

[0215] The bottom coating in the embodiment of the present application can be realized by any known preparation process. For example, in the preparation process of carbon-based particles, operations such as pre-screening or centrifugation are carried out in advance to remove large-particle carbon-based materials, so that the D of the carbon-based particles added during the bottom coating preparation process V50 is between 20 - 60 nm, and D V90 is less than or equal to 70 nm. The carbon-based material and the binder are mixed, stirred, and coated on the current collector to obtain the bottom coating.

[0216] In some embodiments, the compaction density of the positive electrode tab in the fully discharged state is greater than or equal to 2.4 g / cm 3 , and the single-sided thickness of the bottom coating is 1 μm - 4 μm.

[0217] In some embodiments, the compaction density of the positive electrode tab in the fully discharged state is greater than or equal to 2.5 g / cm 3 , and the single-sided thickness of the bottom coating is 2 μm - 4 μm.

[0218] With the increase in the compaction density of the electrode, the extrusion effect of large particle-containing lithium phosphate materials (e.g., particle size greater than 1 μm) in the positive electrode film layer on the bottom coating becomes more significant. Therefore, stress concentration is prone to occur at large particle sites, and even penetrate through the bottom coating to damage the current collector. Increasing the thickness of the bottom coating is beneficial to improving the stress concentration phenomenon in the electrode and further increasing the ultimate compaction density of the electrode.

[0219] The single-sided thickness of the bottom coating can be tested in the following way. As described above, the positive electrode film layer is cut along the thickness direction of the electrode by an argon ion beam, and a scanning electron microscope image is taken. In the length direction of the electrode, points are taken every 1 m to measure the single-sided bottom coating thickness. After measuring the bottom coating thickness at 10 points, the average value is calculated. It should be noted that during the process of taking measurement points, abnormal points need to be avoided, that is, the bottom coating areas with a thickness less than 50 nm and greater than 4 m; these abnormal points are mainly caused by extreme fluctuations in the thickness of individual areas due to abnormal stress concentration during the electrode compaction process and do not have statistical significance.

[0220] In some embodiments, the thickness of the positive current collector is less than or equal to 17 μm, and can be selected as 13 μm - 15 μm.

[0221] In some embodiments, the thickness of the positive current collector is 13 μm, 14 μm, 15 μm, 16 μm, 17 μm or the numerical range between any two of them.

[0222] In some embodiments, the positive current collector can be a metal foil or a composite current collector. For example, as the metal foil, aluminum foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as substrates of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0223] In some embodiments, the negative electrode includes a negative current collector and a negative electrode film layer provided on at least one side of the negative current collector. The areal density of the single-sided negative electrode film layer is 140 mg / 1540 mm 2 -221 mg / 1540 mm 2 ; and / or the compaction density of the negative electrode film layer is 1.40 g / cm 3 -1.75 g / cm 3 .

[0224] The areal density and compaction density of the single-sided negative electrode film layer can be tested by a method similar to that of the positive electrode film layer described above.

[0225] When the areal density and the compaction density of the negative electrode film layer are within the above ranges, it is beneficial to improve the energy density of the lithium-ion secondary battery.

[0226] In some embodiments, the negative electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, copper foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (such as 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.).

[0227] In some embodiments, the negative electrode film layer includes a negative electrode active material. The negative electrode active material can be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material can include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material can 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 can 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 conventional materials that can be used as the negative electrode active material of the battery can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0228] In some embodiments, the negative electrode film layer may also optionally include a binder. The binder can be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0229] In some embodiments, the negative electrode film layer may also optionally include a conductive agent. The conductive agent can be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0230] In some embodiments, the negative electrode film layer may also optionally include other additives, such as thickeners (such as sodium carboxymethyl cellulose (CMC-Na)), etc.

[0231] In some embodiments, the negative electrode sheet can be prepared in the following manner: the components for preparing the negative electrode sheet described above, 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 current collector, and after processes such as drying and compaction, the negative electrode sheet can be obtained.

[0232] In some embodiments, the lithium-ion secondary battery includes an electrolyte. The electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. There is no specific limitation on the type of the electrolyte in this application, and it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or all-solid-state.

[0233] In some embodiments, the electrolyte uses an electrolytic solution. The electrolytic solution includes an electrolyte salt and a solvent.

[0234] 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 difluoro(oxalato)borate, lithium bis(oxalato)borate, lithium difluoro bis(oxalato)phosphate, and lithium tetrafluoro(oxalato)phosphate.

[0235] 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, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0236] In some embodiments, the electrolytic solution may further optionally include additives. For example, the additives can include negative electrode film-forming additives, positive electrode film-forming additives, and can also include additives that can improve certain battery performances, such as additives for improving the overcharge performance of the battery, additives for improving the high-temperature or low-temperature performance of the battery, etc.

[0237] In some embodiments, the lithium-ion secondary battery further includes a separator. There is no particular limitation on the type of the separator in this application, and any well-known porous structure separator with good chemical stability and mechanical stability can be selected.

[0238] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0239] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be made into an electrode assembly through a winding process or a stacking process.

[0240] In some embodiments, the lithium-ion secondary battery may include an outer package. The outer package may be used to encapsulate the above-mentioned electrode assembly and electrolyte.

[0241] In some embodiments, the outer package of the lithium-ion secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery may also be a soft package, such as a pouch soft package. The material of the soft package may be plastic. As plastics, polypropylene, polybutylene terephthalate, and polybutylene succinate, etc. may be listed.

[0242] The 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. The battery device includes at least one of a battery module, a battery pack, and an energy storage battery.

[0243] The third aspect of the present application provides an electrical device, including the lithium-ion secondary battery provided by the first aspect of the present application.

[0244] The fourth aspect of the present application provides a method for preparing a positive electrode active material, including: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source; adding a solvent and grinding to obtain a mixed slurry; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder to obtain a positive electrode active material; the sintering the precursor powder to obtain a positive electrode active material includes at least two sinterings; after the first sintering, a first sintered product is obtained, the first sintered product and a carbon source are mixed to obtain an intermediate raw material, the intermediate raw material is divided into two groups, and grinding is respectively carried out to obtain a first group of ground products and a second group of ground products, the first group of ground products and the second group of ground products are mixed to obtain a mixed intermediate product; the mixed intermediate product is ground again to obtain a target mixture; the target mixture is subjected to a second sintering to obtain a positive electrode active material; wherein, the D V50 of the first group of ground products is 0.8 μm - 1.2 μm; the D V50 of the second group of ground products is 0.30 μm - 0.50 μm; the heat preservation time of the second sintering is 5 h - 12 h; the grinding time of the mixed intermediate product when it is ground again is 0.5 h - 1.5 h.

[0245] The preparation method provided by the embodiments of the present application obtains a positive electrode active material with a certain particle size distribution through three grindings and two sinterings, and on the other hand, improves the sphericity of large-size particles in the positive electrode active material. For the cumulative area distribution curve of the sphericity of particles with a particle size greater than or equal to 1 μm obtained from the cross-section along the thickness direction of the electrode sheet, the median L R1A50Provide a material basis for the preparation of the positive electrode film layer with a value of 0.71 - 0.85.

[0246] In some embodiments, the iron source is an iron-containing compound. In some embodiments, the iron source includes at least one of iron hydroxide, ferrous chloride, ferric oxide, iron phosphate, ferric pyrophosphate, ferrous oxalate, iron powder, ferric nitrate, iron tetroxide, and iron oxyhydroxide.

[0247] In some embodiments, the phosphorus source is a phosphoric acid compound. In some embodiments, the phosphorus source includes at least one of phosphoric acid, iron phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0248] In some embodiments, the iron source and the phosphorus source can be the same substance. In some embodiments, iron phosphate is used as both the iron source and the phosphorus source.

[0249] In some embodiments, the lithium source includes one or more of lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium oxide, lithium hydroxide, and lithium acetate.

[0250] In some embodiments, the lithium source includes lithium carbonate.

[0251] In some embodiments, the carbon source includes one or more of glucose, polyethylene glycol, citric acid, sucrose, starch, fructose, lactose, polyaniline, polyacrylonitrile, and polyvinylpyrrolidone.

[0252] In some embodiments, the carbon source includes glucose and polyethylene glycol.

[0253] In some embodiments, based on the total mass of the carbon source, the mass content of polyethylene glycol is 20% - 75%.

[0254] The polymer carbon source has a relatively low graphitization temperature, enabling the carbon coating material on the surface of the positive electrode active material to decompose and form a coating structure at a relatively low sintering temperature, hindering the growth and sintering of lithium-containing transition metal phosphate grains, and facilitating the reduction of the particle size of the positive electrode active material particles.

[0255] At the same time, the polymer carbon source usually has a relatively high molecular weight or a long molecular chain, which is prone to form a stable skeleton structure through cross-linking or orientation during the heat treatment process. This orderliness is retained during the high-temperature carbonization process, facilitating the directional growth of graphite crystals; meanwhile, the entanglement and cross-linking between long chains are conducive to reducing structural defects and decreasing the lattice disorder caused by chain breakage during the carbonization process, thereby enhancing the graphitization degree.

[0256] At high temperatures, organic molecules in the carbon source decompose, releasing carbon atoms. These carbon atoms can cover and fill the tiny voids or defects on the surface of the active material, reducing the surface roughness. The coating material formed from the sugar carbon source has a relatively low degree of graphitization, and the flocculent carbon structure is relatively loose, which may itself act as new rough points. Therefore, the higher the proportion of the polymer carbon source in the carbon source, the more beneficial it is to optimize the surface roughness of the cathode active material.

[0257] 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.

[0258] The titanium source often has a relatively low surface activity. Including a titanium source in the slurry can reduce the activity of the lithium-containing transition metal phosphate precursor and inhibit the growth of particles of the lithium-containing transition metal phosphate during high-temperature sintering, resulting in the formation of smaller particles of the lithium-containing transition metal phosphate during sintering.

[0259] As a lattice stabilizer, titanium usually enters the lattice of the lithium-containing transition metal phosphate in the form of Ti 4+ Some titanium ions can replace the position of iron ions, making the crystal structure more stable and reducing the possibility of lithium and iron ions being in the wrong position, especially during high-temperature or high-current charge and discharge.

[0260] At the same time, the doping of titanium helps to improve the sphericity of the particles and reduce the roughness of the particles, thereby enhancing the overall structural stability of the material.

[0261] In some embodiments, the atomic molar ratio of iron element to phosphorus element in the iron source and the phosphorus source is 0.95 - 1.

[0262] In some embodiments, the atomic molar ratio of iron element to phosphorus element in the lithium source and the iron source can be selected from 0.95, 0.955, 0.96, 0.965, 0.97, 0.975, 0.98, 0.985, 0.99, 0.995, 1 or the numerical range between any two of them.

[0263] When synthesizing the lithium-containing transition metal oxide, the iron-to-phosphorus ratio affects the reaction rate and the crystal growth process. A high iron-to-phosphorus ratio may inhibit the reaction rate of the iron source, resulting in a slowdown in the reaction rate of the iron source to be converted into the lithium-containing transition metal phosphate. After this step becomes the rate-determining step, the crystal growth time is extended, thus promoting the formation of larger particles.

[0264] In some embodiments, based on the total mass of the mixed raw materials, the mass percentage of the carbon source in the mixed raw materials is 5% - 7%.

[0265] In some embodiments, based on the total mass of the mixed raw materials, the mass ratio of the carbon source in the mixed raw materials can be optionally 5%, 5.1%, 5.2%, 5.3%, 5.4%, 5.5%, 5.6%, 5.7%, 5.8%, 5.9%, 6.0%, 6.1%, 6.2%, 6.3%, 6.4%, 6.5%, 6.6%, 6.7%, 6.8%, 6.9%, 7% or the numerical range between any two of them.

[0266] By controlling the lithium content of the carbon source within the above range, the conductivity of the material can be enhanced, and the negative impact on the specific capacity of the positive electrode sheet and the energy density of the battery can be reduced. An overly thick carbon coating material not only occupies the effective active material space but also may cause the instability of the material structure.

[0267] In some embodiments, the solvent includes water and its mixtures.

[0268] In some embodiments, obtaining the mixed raw materials including a carbon source, a lithium source, an iron source, and a phosphorus source includes: adding the carbon source, the lithium source, the phosphorus source, the iron source, and the carbon source into a solvent and mixing and stirring, and the rotation speed of the stirring is 1400 rpm - 2200 rpm.

[0269] In some embodiments, the obtained mixed slurry after grinding includes a first grinding, and the first grinding satisfies one or more of the following conditions: (1) The grinding balls for the first grinding are one or more of zirconia balls, silicon nitride zirconia balls, and ceramic zirconia balls; (2) The diameter of the grinding balls for the first grinding is 0.5 mm - 0.7 mm; (3) The rotation speed of the first grinding is 450 rpm - 550 rpm; (4) The grinding time of the first grinding is 0.5 h - 1.5 h; (5) The pressure in the grinding chamber for the first grinding is 0.01 MPa - 0.3 MPa.

[0270] In some embodiments, the obtained mixed slurry after grinding includes a second grinding after the first grinding, and the second grinding satisfies one or more of the following conditions: (1) The grinding balls for the second grinding are one or more of zirconia balls, silicon nitride zirconia balls, and ceramic zirconia balls; (2) The diameter of the grinding balls for the second grinding is 0.25 mm - 0.35 mm; (3) The rotation speed of the second grinding is 470 rpm - 530 rpm; (4) The grinding time of the second grinding is 3.0 h - 5.0 h; (5)The pressure in the grinding chamber for the secondary grinding is 0.01 MPa - 0.3 MPa.

[0271] In some embodiments, the particle size D of the mixed slurry V50 is 0.3 μm - 0.5 μm.

[0272] Performing at least two grindings is beneficial for controlling the temperature and viscosity of the slurry, reducing the excessive viscosity of the slurry caused by too high temperature and the resulting raw material agglomeration.

[0273] The first grinding can process large particle materials, and the second grinding further refines the materials and adjusts the particle size distribution. In this way, the non-uniformity of particle size caused during grinding can be effectively reduced, the agglomeration phenomenon between particles can be reduced, the conductivity and cycle stability of the battery can be improved, and at the same time, the overall production efficiency can be increased while meeting the performance of the final product.

[0274] In some embodiments, obtaining the precursor powder after drying the mixed slurry includes obtaining the precursor powder after spray-drying the mixed slurry.

[0275] In some embodiments, sintering the precursor powder to obtain the positive electrode active material includes at least two sinterings.

[0276] In some embodiments, the first sintering in the at least two sinterings satisfies one or more of the following conditions: (1) The heating rate is 2 °C / min - 10 °C / min; (2) The holding temperature is 720 °C - 790 °C; (3) The holding time is 5 h - 12 h.

[0277] In some embodiments, after obtaining the first sintering product from the first sintering, mixing the first sintering product and a carbon source to obtain an intermediate raw material, dividing the intermediate raw material into two groups, respectively performing grinding (third grinding) to obtain a first group of grinding products and a second group of grinding products, and mixing the first group of grinding products and the second group of grinding products to obtain a mixed intermediate product.

[0278] In some embodiments, the carbon sources added in mixing the first sintering product and the carbon source to obtain the intermediate raw material include glucose and polyethylene glycol.

[0279] In some embodiments, based on the mass of the first sintering product, the mass content of glucose in the intermediate raw material is 1% - 3%, and the mass content of polyethylene glycol in the intermediate raw material is 0.5% - 6%.

[0280] In some embodiments, the D of the first group of grinding products V50 is 0.8 μm - 1.2 μm.

[0281] In some embodiments, the D of the second group of grinding products V50 is 0.30 μm - 0.50 μm.

[0282] For small particles relative to large particles, the proportion of surface atoms is relatively high, the surface energy is relatively large, the surface atoms have relatively high activity, and the surface area per unit volume is larger. Therefore, the rate of surface diffusion is relatively fast, which makes small particles prone to relatively intense surface rearrangement during the sintering process, promoting the evolution of particles into a spherical shape. Therefore, controlling the grinding particle size to a relatively small value, including the grinding of the mixed slurry and the grinding of the intermediate raw material, is beneficial for the formation of products with higher sphericity, smoother and flatter surfaces during the subsequent sintering process. At the same time, particles with a smaller grinding particle size are more easily coated, improving the coating integrity.

[0283] In some embodiments, the grinding conditions of the first group of grinding products satisfy one or more of the following conditions: (1) The rotation speed is 550 rpm ± 50 rpm; (2) The grinding time is 0.5 h - 1.5 h.

[0284] In some embodiments, the grinding conditions of the second group of grinding products satisfy one or more of the following conditions: (1) The rotation speed is 500 rpm ± 50 rpm; (2) The grinding time is 3 h - 5 h. In some embodiments, the mass ratio of the first group of grinding products to the second group of grinding products is (60:40) - (80:20).

[0285] By adjusting the mass ratio of the first group of grinding products to the second group of grinding products, cathode active materials with different grading ratios can be obtained, so that the quantity ratio and area ratio of particles of each particle size can be finely adjusted, and the required packing structure can be obtained.

[0286] In some embodiments, the mixed intermediate product is ground under the grinding conditions of 500 rpm ± 50 rpm and a grinding time of 0.5 h - 1.5 h to obtain a target mixture.

[0287] Grinding the mixed intermediate product again helps to improve the uniformity of the particle size in the target mixture, reduce the presence of abnormally large particles, make the sintering uniform, and improve the sphericity of large particles.

[0288] In some embodiments, the target mixture is subjected to a second sintering.

[0289] In some embodiments, the second sintering satisfies one or more of the following conditions: (1) The heat preservation temperature is 770°C - 830°C; (2) The heat preservation time is 5h - 12h; (3) The heating rate is 2°C / min - 10°C / min.

[0290] Through the secondary sintering process, the sintering time in the high-temperature range can be effectively shortened. Furthermore, the degree of grain boundary melting of large particles during high-temperature sintering can be reduced, the sphericity of large-sized particles in the cathode active material can be improved, thereby reducing the stress concentration problem caused by particle bridging due to the irregular shape of large particles, and reducing the porosity in the film layer, improving the compaction density of the electrode sheet, effectively increasing the energy density of the battery, reducing kinetic degradation, and improving the electron conduction ability between particles through closer packing, thus achieving the improvement of kinetic performance.

[0291] By controlling the sintering temperature in the primary and secondary sintering processes, the speed of the sintering diffusion rate can be controlled. At high temperatures, the diffusion on the particle surface increases, the defects in the particles are repaired, and the crystal lattice rearranges. Through recrystallization, the defects on the particle surface are eliminated, the crystal grain structure of the particles becomes more ordered, the size of the particles gradually increases, and it helps to smooth the particle surface and promote the particles to develop towards a spherical shape. The sintering temperature also affects the graphitization rate of the carbon source. Kinetically, carbon atoms obtain more energy and can overcome the original energy barrier, causing them to rearrange more violently in the crystal lattice. The sintering time affects the degree of reaction progress. If the sintering time is too short, the diffusion and rearrangement of lithium-containing transition metal phosphate and carbon source are not completely completed; if the sintering time is too long, the particles will grow abnormally, the crystal grains inside the particles will coarsen, the material structure tends to be unstable, the adhesion force between particles increases, and agglomeration occurs.

[0292] In some embodiments, after sintering the precursor, the product is subjected to air jet milling to obtain the cathode active material.

[0293] In some embodiments, the classification frequency of air jet milling is 20Hz - 30Hz, and the crushing air pressure is 0.45MPa - 0.55MPa.

[0294] The classification frequency in air jet milling refers to the working frequency of the classification device in air jet milling, which is usually related to the classification efficiency and particle size distribution of particles. A higher classification frequency will screen the particles in the air flow 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 tend to be spherical.

[0295] Higher air pressure will cause the particles to be subjected to greater impact force, and the collisions between particles will be more intense, resulting in stronger impact and wear on the particle surface. It can crush large particles into small particles, and the collisions between particles will be more intense, and the surface is more easily trimmed, improving the sphericity and surface flatness of the particles.

[0296] However, too high classification frequency and crushing air pressure will cause the agglomerated particles to disperse into primary particles and then further crack and break, affecting the predetermined particle size distribution, and making the carbon coating structure incomplete, manifested as an increase in iron dissolution, which has a negative impact on the slip of particles during roll pressing, and increases the contact and reaction between lithium-containing transition metal phosphates and external factors such as electrolytes, which is not conducive to maintaining the cycle performance and life of the battery. Therefore, it is necessary to control the classification frequency and crushing air pressure of air jet milling within a suitable range.

[0297] The fifth aspect of the present application provides a method for preparing a positive electrode sheet, the preparation method includes adding a binder, a conductive agent, and a positive electrode active material prepared by the preparation method of the fourth aspect in sequence, dry mixing, and then adding a solvent, stirring and adjusting the viscosity to obtain a shipping slurry; transferring and coating the shipping slurry on at least one side of a current collector, drying and hot pressing to obtain a positive electrode film layer.

[0298] In some embodiments, the revolution speed of the dry mixing is 20 rpm - 30 rpm, and the rotation speed is 750 rpm - 850 rpm.

[0299] In some embodiments, the hot pressing includes at least three hot roll pressings, the hot roll pressure increases in sequence, and the hot roll pressures are 20 tons - 50 tons, 50 tons - 70 tons, and 70 tons - 90 tons in sequence; the hot roll temperature is 40°C - 80°C, and before the first entry into the hot roll compaction, the electrode sheet is heated, and the heating temperature is 40°C - 50°C.

[0300] The positive electrode active material prepared by the present application example using the above hot pressing process in combination with the preparation method of the fourth aspect is beneficial to further reduce the porosity of the cut surface of the positive electrode film layer, improve the ultimate compaction density of the electrode sheet, and improve the energy density of the battery.

[0301] In addition, the present application also provides an electrical device, the electrical device includes at least one of the secondary battery, battery module or battery pack provided by the present application. The secondary battery, battery module or battery pack can be used as the power source of the electrical device, or can be used as the energy storage unit of the electrical device. The electrical device can 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 not limited thereto.

[0302] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0303] Figure 7 It is an electrical device as an example. The electrical device is a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the secondary battery for this electrical device, a battery pack or a battery module can be adopted.

[0304] As another example of the device, it can be a mobile phone, a tablet computer, a laptop computer, etc. This device usually requires being thin and light, and a secondary battery can be adopted as the power source.

[0305] Embodiment Hereinafter, 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 a limitation to the present application. For those without specific technical or conditions noted in the embodiments, the technologies or conditions described in the literature in this field or according to the product specifications are followed. For the reagents or instruments without the producer noted, they are all conventional products that can be obtained through commercial purchase.

[0306] Embodiment 1 (1) Preparation of the positive electrode active material S1. Lithium carbonate, iron phosphate and glucose are added to water and mixed in a premixing tank at a rotation speed of 1800 rpm, and demagnetization is carried out through a demagnetizing rod with a magnetic field strength of 8000 - 12000 Gs to obtain a mixed raw material. Among them, the ratio of lithium carbonate to iron phosphate is such that the molar ratio of iron to phosphorus is 0.975, the mass content of glucose relative to the total amount of the raw materials is 5.7%, and the addition amount of titanium dioxide is such that the doping amount of titanium element in the positive electrode active material is 5000 ppm; S2. The mixed raw material is subjected to two grinding - demagnetization cycles in a sand mill. The first grinding is carried out under the conditions of using zirconia balls with a diameter of 0.6 mm and a rotation speed of 260 rpm for 1 hour, and the pressure in the grinding chamber is less than 0.2 MPa. The raw material after the first grinding is demagnetized using a permanent magnet demagnetizer, and the demagnetization intensity is greater than or equal to 8000 Gs; the demagnetized raw material is ground for the second time to obtain a mixed slurry, and the particle size D V50 is 0.40 μm ± 0.10 μm; S3. The mixed slurry is spray - dried to obtain a precursor powder, S4. The precursor powder is sintered to obtain a lithium iron phosphate positive electrode material. The sintering process includes: First sintering: Sinter the precursor powder in a nitrogen atmosphere, heat it from 25°C to 760°C at a heating rate of 5°C / min, hold for 10 h, and obtain the first sintered product after cooling; Grinding and mixing: Add 1.5% glucose and 3.0% polyethylene glycol based on the total mass of the first sintered product to the first sintered product; Grind it in two groups (the third grinding), where the grinding stops when the particle D V50 reaches 1.0 μm ± 0.20 μm (grinding conditions: 550 rpm ± 50 rpm, grinding time 1 h), and the grinding stops when the particle D V50 in the second group reaches 0.38 μm ± 0.05 μm (grinding conditions: 500 rpm ± 50 rpm, grinding time 4 h); Mix the first group and the second group according to a mass ratio of 70:30 to obtain a mixed intermediate product; To ensure the mixing uniformity, grind the mixed product again for the fourth time before spraying, grinding conditions: 500 rpm ± 50 rpm, grinding time 1 h, to obtain the target mixture; Spray-dry the target mixture; Second sintering: Sinter the dried target mixture in a nitrogen atmosphere, heat it from 25°C to 800°C at a heating rate of 5°C / min, hold for 8 h, and obtain the second sintered product after cooling.

[0307] S5, After sintering, cool it to below 100 °C, and crush the second sintered product by air flow crushing to obtain the carbon-coated lithium iron phosphate cathode active material, where the classification frequency of air flow crushing is 25 Hz and the crushing air pressure is 0.55 MPa.

[0308] The mass content of carbon element in the prepared cathode active material is 1.269%; the concentration of lithium-iron inversion defects is 0.20%, and the tapped density of the powder is 1.40 g / cm 3 ³, and the compacted density of the powder under 3T pressure is 2.62 g / cm 3 ³, and the powder resistivity under 8 MPa pressure is 11.96 Ω·cm; the discharge specific capacity at 1C discharge rate is 141.1 mAh / g; the discharge capacity ratio of the 3.2V discharge platform is 90.0%.

[0309] (2) Preparation of the cathode electrode sheet: Add 2.2 wt% of PVDF, 0.8 wt% of conductive carbon black, and 97.0 wt% of the cathode active material in sequence, dry-mix them, then add N-methylpyrrolidone, stir and adjust the viscosity to obtain the shipping slurry; transfer the shipping slurry and coat it on the bottom coating of the current collector aluminum foil. The bottom coating includes carbon black and PVDF, and the mass ratio of the two is 1:1. The distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating is ≤10 pcs / 10 μm, and the thickness of the bottom coating is 2 μm. After drying and hot pressing, a cathode film layer with a single-sided areal density of 350 mg / 1540 cm 2 is obtained. Among them, the revolution speed of the dry mixing is 25 rpm, and the rotation speed is 800 rpm.

[0310] The hot pressing process includes three hot roll pressing processes, and the hot roll pressing pressure increases in sequence. The hot roll pressures are 35 tons, 60 tons, and 80 tons in sequence; the hot roll temperature is 60 °C. Before the first entry into the hot roll compaction, the electrode sheet is heated, and the heating temperature is 50 °C.

[0311] The compaction density of the electrode sheet is the ultimate compaction density of the electrode sheet. The test method for the ultimate compaction density of the electrode sheet is as follows; in this example, the ultimate compaction density of the electrode sheet is 2.73 g / cm 3 .

[0312] In the cross-section of the prepared cathode film layer along the thickness direction of the electrode sheet, the particle size distribution index PDI of the particles is 0.899, the average value of the particle size is 258 nm, and the sample standard deviation of the particle size is 232 nm. In the sphericity area distribution curve of particles with a particle size greater than or equal to 1 μm obtained from the cross-section of the cathode film layer along the thickness direction of the electrode sheet, the median L R1A50 of the sphericity is 0.751. In the sphericity area cumulative distribution curve of particles obtained from the cross-section of the cathode film layer along the thickness direction of the electrode sheet, the median L A50 of the sphericity is 0.731. The median C 50 of the graphitization degree obtained in the surface scanning mode of the laser confocal Raman spectrometer is 1.05. In the roughness area cumulative distribution curve of particles obtained from the cross-section of the cathode film layer along the thickness direction of the electrode sheet, the median R A50 of the roughness is 0.950. The iron dissolution rate of the cathode film layer is 900.2 ppm.

[0313] (3) Preparation of the negative electrode sheet: Mix 95.5 wt% of the negative electrode active material (artificial graphite), 1.0 wt% of the conductive agent (conductive carbon black), 2.0 wt% of the binder (styrene-butadiene rubber (SBR)), and 1.5 wt% of the thickener (sodium carboxymethyl cellulose (CMC)), add deionized water and stir to disperse and make the negative electrode slurry. Then coat the negative electrode slurry on both sides of the Cu foil. After both sides are completed, dry, compact, slit, and slice to prepare the negative electrode plate. The single-sided coating density is 165 mg / 1540 mm 2 , and the compaction density is 1.60 g / cm 3 .

[0314] (4) Preparation of the separator Use a polypropylene film as the separator.

[0315] (5) Preparation of the electrolyte In a glove box under an argon atmosphere (H 2 O < 0.1 ppm, O 2 < 0.1 ppm), mix the organic solvents ethylene carbonate (EC) / dimethyl carbonate (DMC) evenly according to a volume ratio of 1 / 1, add the lithium salt LiPF 6 and dissolve it in the organic solvent. The content of LiPF 6 in the solution is 1 mol / L, stir evenly to obtain the electrolyte.

[0316] (6) Preparation of the battery: Stack the positive electrode plate, the separator, and the negative electrode plate in sequence. The separator should be able to isolate the anode and cathode. Wind to obtain a bare battery cell. Place the bare battery cell in the outer package, inject the electrolyte well, and go through processes such as encapsulation, formation, and degassing to finally obtain a lithium-ion battery.

[0317] The preparation method of Example 2 is basically the same as that of Example 1, except that in step S4, when the D V50 of the particles in the first group reaches 0.80 μm ± 0.20 μm, stop grinding; when the D V50 of the particles in the second group reaches 0.30 μm ± 0.05 μm, stop grinding.

[0318] The preparation method of Example 3 is basically the same as that of Example 1, except that in step S4, when the D V50 of the particles in the first group reaches 0.80 μm ± 0.20 μm, stop grinding; when the D V50 of the particles in the second group reaches 0.45 μm ± 0.05 μm, stop grinding.

[0319] The preparation method of Example 4 is basically the same as that of Example 1, except that in step S4, when the D V50Stop grinding when it reaches 1.20μm ± 0.20μm; D of the particles in the second group V50 Stop grinding when it reaches 0.30μm ± 0.05μm.

[0320] The preparation method of Example 5 is basically the same as that of Example 1, except that in step S4, D of the particles in the first group V50 Stop grinding when it reaches 1.20μm ± 0.20μm; D of the particles in the second group V50 Stop grinding when it reaches 0.45μm ± 0.05μm.

[0321] The preparation method of Example 6 is basically the same as that of Example 1, except that in step S4, there is no fourth grinding, and after obtaining the mixed intermediate product, directly perform the second sintering.

[0322] The preparation method of Example 7 is basically the same as that of Example 1, except that in step S4, the grinding duration of the fourth grinding is 1.5h.

[0323] The preparation method of Example 8 is basically the same as that of Example 1, except that in step S4, the heat preservation time of the second sintering is 5h.

[0324] The preparation method of Example 9 is basically the same as that of Example 1, except that in step S4, the heat preservation time of the second sintering is 12h.

[0325] The preparation method of Comparative Example 1 is basically the same as that of Example 1, except that in step S4, D of the particles in the first group V50 Stop grinding when it reaches 0.75μm ± 0.20μm; D of the particles in the second group V50 Stop grinding when it reaches 0.50μm ± 0.05μm.

[0326] The preparation method of Comparative Example 2 is basically the same as that of Example 1, except that in step S4, D of the particles in the first group V50 Stop grinding when it reaches 1.50μm ± 0.20μm; Dv50 of the particles in the second group stops grinding when it reaches 0.50μm ± 0.05μm.

[0327] The preparation method of Comparative Example 3 is basically the same as that of Example 1, except that in step S2, the particle size D of the mixed slurry V50 is 0.35μm ± 0.10μm; in step S4, directly perform the second sintering after obtaining the first sintering product, and the heat preservation time of the second sintering is 12h.

[0328] Performance Test 1. Ultimate Compaction Density of the Electrode The double-sided coated electrode sheet is compacted by a roll press, and the elongation of the compacted electrode sheet is tested, while the flexibility of the electrode sheet after compaction is evaluated. By increasing the pressure of the roll press, electrode sheets with different compaction densities can be obtained. As the pressure increases, the compaction density of the electrode sheet increases, the elongation of the electrode sheet increases, and the flexibility of the electrode sheet decreases. If the elongation of the electrode sheet is too high, warping of the electrode sheet is likely to occur. If the flexibility of the electrode sheet is too low, brittle fracture of the electrode sheet is likely to occur. Therefore, the smaller of the compaction densities corresponding to an elongation of 8% of the electrode sheet or a folding number of 3 times for the flexibility of the electrode sheet is defined as the ultimate compaction density of the electrode sheet.

[0329] The compaction density is calculated by the mass of the positive electrode film layer / the volume of the positive electrode film layer.

[0330] The test method for elongation is as follows: Lay the electrode sheet flat on a horizontal tabletop, cut the electrode sheet into segments. Each electrode sheet is about 100 cm long; remove the copper foil of the substrate at the edge of the electrode sheet, and note to keep the cut edge of the electrode sheet parallel to the MD direction of the electrode sheet (perpendicular to the direction of the pressure roller), ensure that the electrode sheet part is completely covered by the coating, use a steel ruler to measure the length between the marked points at the same position in the width direction of the head and tail of the electrode sheet, estimate to 0.1 mm, and record the length before compaction; record the length after compaction between the corresponding marked points after compaction, and use (length after compaction - length before compaction) / length before compaction as the elongation of the electrode sheet.

[0331] The test method for the number of flexible folds is as follows.

[0332] Cut the positive electrode sheet into test specimens with a size of 20×100 mm 2 ; after folding it forward, flatten it with a 2 kg pressure roller, and unfold it to check whether light passes through the gap against the light. If no light passes through, fold it backward, flatten it with a 2 kg pressure roller, and check again against the light. Repeat this process until light passes through the gap, and record the number of folds; repeat the test three times and take the average value as the reference data for the flexibility of the electrode sheet.

[0333] 2. Energy density test Let the lithium-ion secondary battery stand for 2 h at 25°C to ensure that the temperature of the lithium-ion secondary battery is 25°C. At 25°C, charge the lithium-ion secondary battery at 0.33C until the charge cut-off voltage of 3.65V, and then continue to perform constant voltage charging at this charge cut-off voltage until the current is 0.05C, and the charging is cut off (where C represents the rated capacity of the lithium-ion secondary battery). After the lithium-ion secondary battery stands for 1 h at 25°C, discharge the lithium-ion secondary battery at 0.33C at 25°C until the discharge cut-off voltage of 2.5V, and record the total discharge energy of the lithium-ion secondary battery as E 0 .

[0334] 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

[0335] The volume energy density of the lithium-ion secondary battery = the discharge energy E of the lithium-ion secondary battery 0 / the volume V of the lithium-ion secondary battery 0 。

[0336] 3. DCR test method At 25°C, after constant current charging to 3.65V at 0.33C, constant voltage charging until the current is 0.05C, then discharging at 1 / 3C to 20% SOC, standing for 5 min, then discharging with a 3C pulse for 30 s, standing for 40 s, then charging at 3C for 40 s, standing for 5 min, then constant current charging at 1 / 3C to 3.65V, constant voltage charging to 0.05C, then discharging at 1 / 3C to 10% SOC, standing for 5 min, then discharging with a 3C pulse for 30 s, standing for 40 s, charging at 3C for 40 s, standing for 5 min, then fully charging at 1 / 3C and then discharging at 1 / 3C to 50% SOC, then standing at -25°C for 2 h and then discharging with a 1C pulse for 30 s, standing for 10 min, then standing at 25°C for 2 h, constant current charging at 1 / 3C to 3.65V and then constant voltage charging to 0.05C, then discharging at 1 / 3C to 20% SOC, then standing at -25°C for 2 h and then discharging with a 1C pulse for 30 s, standing for 10 min

[0337] Record the voltage at this time before and after each pulse discharge, calculate the DCR under different conditions, and the calculation formula is DCR = (voltage before pulse discharge after standing - voltage before standing after pulse discharge) / pulse current

[0338] Experimental parameters and test results Prepare the batteries of each example and comparative example according to the above method, and measure various performance parameters. The results are shown in Table 1 below

[0339] Table 1

[0340] Table 2

[0341] From the data comparison of the examples and comparative examples, it can be seen that in the cumulative area distribution curve of the sphericity of particles with a particle size greater than or equal to 1μm obtained by controlling the cross-section of the positive electrode film layer along the thickness direction of the electrode plate in the examples of the present application, the median of the sphericity is 0.71 - 0.85, which reduces the battery impedance while increasing the compaction density of the positive electrode plate, and improves the energy density of the battery

[0342] As can be seen from the comparison between Example 3 and other examples, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the particle size distribution index PDI of the particles is 0.85 - 1.00, which is beneficial to further improve the compaction density of the electrode sheet and the energy density of the battery.

[0343] As can be seen from the comparison between Example 5 and other examples, in the cumulative area distribution curve of the sphericity of particles with a particle size greater than or equal to 1 μm obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median of the sphericity is 0.72 - 0.80, which is beneficial to further reduce the battery impedance while improving the compaction density of the electrode sheet and improving the battery dynamic performance.

[0344] As can be seen from the comparison between Example 5 and other examples, in the cumulative area distribution curve of the sphericity of the particles obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median L of the sphericity A50 is 0.71 - 0.80, which is beneficial to further reduce the battery impedance while improving the compaction density of the electrode sheet and improving the battery dynamic performance.

[0345] It should be noted that this application is not limited to the above embodiments. The above embodiments are only examples, and embodiments with the same structure and the same effect as the technical idea within the technical solution scope of this application are all included in the technical scope of this application. In addition, within the scope of not departing from the main idea of this application, various deformations that can be thought of by those skilled in the art are applied to the embodiments, and other ways constructed by combining some constituent elements of the embodiments are also included in the scope of this 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 cumulative distribution curve of the sphericity area of ​​particles with a particle size greater than or equal to 1 μm obtained from the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the median of the sphericity L R1A50 It is 0.71-0.

85.

2. The lithium ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the particle size distribution index PDI of the particles is 0.80-1.10, and the particle size distribution index PDI of the particles is the ratio of the sample standard deviation of the particle size in the cross section of the positive electrode film layer along the thickness direction to the average value.

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

00.

4. The lithium-ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the average particle size of the particles is 150nm-400nm.

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

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 sample standard deviation of the particle size of the particles is 150nm-350nm.

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 sample standard deviation of the particle size of the particles is 150nm-300nm.

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

80.

9. 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 median of the sphericity L A50 It is 0.70-0.

85.

10. 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 median of the sphericity L A50 It is 0.71-0.

8.

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

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

1.

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

05.

14. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C of the positive electrode film layer obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the median value of the graphitization degree C 50 is 0.98-1.20; among which 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 .

15. The lithium ion secondary battery according to claim 14, characterized in that: In the cumulative distribution curve of the graphitization degree C of the positive electrode film layer obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the median value of the graphitization degree C 50 It is 1.02-1.

1.

16. 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 of the roughness R A50 It is 0.92-0.

96.

17. 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 400ppm-1800ppm.

18. 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-1500ppm.

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

20. 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%.

21. 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 0.001%-1.5%.

22. 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 0.01%-1.0%.

23. The lithium ion secondary battery according to claim 1, characterized in that: The lithium-containing transition metal phosphate particles include components 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.

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

25. 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 4000ppm-8000ppm based on the total mass of the positive electrode active material.

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

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

28. 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 2.55g / cm 3 -2.70g / cm 3 .

29. 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 2.58g / cm 3 -2.68g / cm 3 .

30. 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-60 Ω·cm.

31. 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-40.0 Ω·cm.

32. 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 at room temperature.

33. 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.

34. 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 0.01%-1.5%.

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 92%-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 2.52 g / cm 3 -2.78g / 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 -2.75g / 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 2.52 g / cm 3 -2.78g / 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 -2.75g / 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 cell, a battery module, a battery pack, and an energy storage battery.

47. An electrical device, characterized in that: A lithium ion secondary battery comprising the lithium ion secondary battery according to any one of claims 1 to 45.

48. An electrical device, characterized in that: A battery device comprising the battery device of claim 46.

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