Lithium ion secondary battery, battery device, power utilization device, preparation method of positive active material and preparation method of positive pole piece

Abstract

The invention provides a lithium ion secondary battery, a battery device, a power utilization device, a preparation method of a positive electrode active material and a preparation method of a positive electrode plate. 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, the positive film layer comprises a positive active material, and the positive active material comprises lithium-containing transition metal phosphate particles of which at least partial surfaces are provided with carbon coating materials; in the tangent plane of the positive electrode film layer along the thickness direction of the pole piece, the area proportion of particles with the particle size greater than or equal to 1 mu m is 30.0-50.0%; and the mass ratio of the magnetic substance in the positive electrode film layer is greater than or equal to 20 ppm and less than or equal to 1980 ppm.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to PCT patent application PCT / CN2025 / 085902, filed on March 28, 2025, entitled “Lithium-ion secondary battery, battery device, electrical device, method for preparing positive electrode active material, and method for preparing positive electrode sheet,” the entire contents of which are incorporated herein by reference. Technical Field

[0002] The present application relates to the technical field of lithium-ion batteries, and in particular to a lithium-ion secondary battery, a battery device, an electrical device, a method for preparing a positive electrode active material, and a method for preparing a positive electrode sheet. Background Art

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

[0004] As the market demands for the endurance and safety of electrical devices increase, higher requirements are also placed on the energy density and storage performance of lithium-ion secondary batteries. However, it is difficult to simultaneously improve the above performances in the existing technology, which has become a technical problem that needs to be solved urgently in this field. Summary of the invention

[0005] The present application has been made in view of the above-mentioned problems, and an object of the present application is to provide a lithium ion secondary battery having both high energy density and good storage performance.

[0006] The first aspect of the present application provides a lithium-ion secondary battery, which includes a positive electrode plate, a negative electrode plate and an electrolyte, wherein the positive electrode plate includes a positive electrode collector and a positive electrode film layer arranged on at least one side of the positive electrode collector, the positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes lithium-containing transition metal phosphate particles with a carbon coating material arranged on at least a portion of the surface, in a cross-section of the positive electrode film layer along the thickness direction of the plate, the area of ​​particles with a particle size greater than or equal to 1 μm accounts for 30.0%-50.0%; and the mass proportion of magnetic substances in the positive electrode film layer is greater than or equal to 20 ppm and less than or equal to 1980 ppm.

[0007] The embodiment of the present application increases the area ratio of large-size particles while effectively reducing the content of magnetic substances in the positive electrode film layer, so that the mass ratio of magnetic substances in the positive electrode film layer is greater than or equal to 20ppm and less than or equal to 1980ppm. The compaction density of the pole pieces in the lithium-ion secondary battery can be increased while maintaining a low self-discharge rate, which is beneficial to the improvement of the battery energy density and its long-term maintenance during storage.

[0008] In any embodiment, in a cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size of 1 μm-5 μm accounts for 30.0%-50.0%.

[0009] In the cross-section of the positive electrode film layer along the thickness direction of the pole piece, the area proportion of particles with a particle size of 1μm-5μm is within the above range, which can control the content of magnetic materials in the battery while increasing the compaction density of the pole piece through grading, which is beneficial to the improvement of the energy density of lithium-ion secondary batteries and its long-term maintenance during storage.

[0010] In any embodiment, in a cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size of 1 μm-5 μm accounts for 30.0%-45.0%.

[0011] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size of 1μm-5μm is within the above range, which can improve the compaction density and capacity storage stability while further taking into account the migration distance of lithium ions inside the particles, thereby allowing the lithium-ion secondary battery to maintain a low impedance and improve the battery's kinetic performance.

[0012] In any embodiment, in a cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the average equivalent area of ​​particles with a particle size of 1 μm or more accounts for 0.05%-0.20%.

[0013] The average equivalent area ratio of particles with a particle size of 1 μm or more in the cross-section of the positive electrode film layer along the thickness direction of the electrode piece is within the above range, which means that the cross-section of the positive electrode film layer along the thickness direction of the electrode piece has a certain number of large particles to improve the compaction density of the electrode piece, but does not cause serious deterioration in the kinetic performance of the lithium-ion secondary battery due to the excessive particle size of the large particles, thereby taking into account both the energy density and kinetic performance of the battery.

[0014] In any embodiment, in a cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area proportion of particles with a particle size of 50nm-200nm is 3.0%-15.0%, optionally 5.0%-12.0%, and further optionally 5.0%-10.0%.

[0015] The area ratio of particles with a particle size of 50nm-200nm is within the above range, which means that a certain number of particles with a size of 50nm-200nm is beneficial to improving the powder compaction density of the positive electrode active material and the compaction density of the electrode through grading, and further improving the energy density of the lithium-ion secondary battery.

[0016] In any embodiment, the mass proportion of the magnetic substance in the positive electrode film layer is less than or equal to 300 ppm, and can be optionally 20-200 ppm.

[0017] The mass content of the magnetic substance within the above range can further alleviate the self-discharge phenomenon and improve the storage stability of the battery capacity.

[0018] In any embodiment, the magnetic material includes Fe, Fe 2 P, FeP, γ-Fe 2 O 3 , Fe 2 P 2 O 7 One or more of .

[0019] In any embodiment, the mass content of elemental iron in the positive electrode film layer is less than 20 ppm, and can be optionally less than or equal to 15 ppm.

[0020] Controlling the mass content of elemental iron within the above range is beneficial to improving the safety performance of the battery.

[0021] In any embodiment, the lithium-containing transition metal phosphate includes a component having the following general formula: Li m Fe x P y O j Q q , wherein Q includes one or more of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.1.

[0022] Selecting an appropriate modifying element Q can improve the lattice change rate of the positive electrode active material during the lithium insertion and extraction process, reduce the oxygen activity on the particle surface, improve the structural stability of the material, and thereby increase the material's gram capacity utilization level, further improving the energy density of lithium-ion secondary batteries.

[0023] In any embodiment, the positive electrode active material includes titanium element, and based on the total mass of the positive electrode active material, the mass content of titanium element is 1500ppm-8000ppm, optionally 2500ppm-8000ppm, and further optionally 2500ppm-6000ppm.

[0024] The positive electrode active material includes titanium and the mass content is controlled within the above range. On the one hand, it has an inhibitory and barrier effect on particle growth, achieving the purpose of controlling the size of large particles, so that the average equivalent area of ​​particles with a particle size of 1 μm or more in the cross-section of the positive electrode film layer along the thickness direction of the pole piece is within an appropriate range; on the other hand, its surface inertness reduces the probability of uneven local chemical reactions of raw materials and the generation of magnetic substances. At the same time, the doping of titanium in the positive electrode active material can improve the electronic conductivity and ion transfer rate of lithium-containing transition metal phosphates, and alleviate the negative impact of particles with relatively large particle sizes on the kinetic properties of the positive electrode active material. By affecting the particle size and lithium ion transmission path, the battery energy density and kinetic performance are taken into account.

[0025] In any embodiment, based on the total mass of the positive electrode active material, the mass proportion of carbon element is 0.9%-1.8%.

[0026] Based on the total mass of the positive electrode active material, the mass proportion of carbon element within the above range can not only improve the conductivity of the positive electrode active material and improve the kinetic performance of the lithium-ion secondary battery; it can also reduce the negative impact of excessive carbon content on the loading amount of lithium-containing transition metal phosphate, taking into account the compaction density of the electrode and the impedance of the lithium-ion secondary battery, while improving the energy density and kinetic performance of the battery.

[0027] In any embodiment, in the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the median graphitization degree C 50 Greater than or equal to 0.9 and less than or equal to 1.3, and can be selected as 0.99-1.2, wherein the graphitization degree C value is I G / I D , where I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at .

[0028] The positive electrode active material with a degree of graphitization within the above range can easily achieve particle sliding during the rolling film forming process with the help of the graphitized carbon layer on the surface of the positive electrode active material, offset the negative impact of particle size on the compaction density of the electrode sheet, and further improve the compaction density of the positive electrode film layer with the help of particle sliding.

[0029] In any embodiment, in the cumulative distribution curve of the particle sphericity area obtained from the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the median of the sphericity L A50 It is 0.60-0.85, and can be selected as 0.65-0.80.

[0030] The median of sphericity L A50 The particles within the above range are approximately spherical, and are prone to slippage between particles under the action of external forces, which can further improve the compaction density of the pole piece and increase the energy density of the battery.

[0031] In any embodiment, the powder compaction density of the positive electrode active material at 3T pressure is 2.48 g / cm 3 -2.76g / cm 3 , optional 2.58g / cm 3 -2.76g / cm 3 .

[0032] The positive electrode active material has a high powder compaction density, and provides a material basis for improving the compaction density of the pole piece and preparing a high energy density lithium ion secondary battery.

[0033] In any embodiment, the powder compaction density of the positive electrode active material at 3T pressure is 2.58 g / cm 3 -2.76g / cm 3 .

[0034] The positive electrode active material with a compaction density within the above range can further improve the compaction density of the electrode sheet and increase the energy density of the battery.

[0035] In any embodiment, the positive electrode active material has a discharge capacity of 135 mAh / g to 150 mAh / g at a 1 C discharge rate at room temperature.

[0036] The positive electrode active material has a high discharge gram capacity, indicating that the positive electrode active material has good kinetic performance and is beneficial to improving the energy density of the lithium ion secondary battery.

[0037] In any embodiment, 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 at the discharge voltage to 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.

[0038] The high proportion of the discharge capacity of the positive electrode active material used in the battery of the embodiment of the present application when discharged to 3.2V means that although the positive electrode active material has a certain proportion of large-sized particles, it still maintains good kinetic performance. At the same time, the high η value indicates that the lithium-ion secondary battery containing the positive electrode active material still has a high voltage when discharged to a low state of charge (SOC), which is conducive to maintaining good power performance.

[0039] In any embodiment, the positive electrode film layer further includes a binder and a conductive agent. Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 94%-99.4%, the mass content of the binder is 0.5%-3%, and the mass content of the conductive agent is 0.1%-3%.

[0040] In any embodiment, the single-side density of the positive electrode film layer is 300 mg / 1540 mm 2 -450mg / 1540mm 2 .

[0041] The positive electrode film layer having an area density within the above range can contribute to improving the energy density of the lithium ion secondary battery.

[0042] In any embodiment, when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.43 g / cm 3 -2.78g / cm 3 .

[0043] In any embodiment, when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.50 g / cm 3 -2.75g / cm 3 .

[0044] In any embodiment, when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.43 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%-28%.

[0045] In any embodiment, when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.5 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%.

[0046] The lower the porosity, on the one hand, means that the gradation 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 gradation and roller pressure, if the porosity is low, it means that the particles are easy to slip against each other, thereby reducing the risk of overpressure and stress concentration in the film layer, and further reducing the probability of demolding of the positive electrode film during long cycles, which is beneficial to improving the long cycle performance of the battery. In any embodiment, the positive electrode sheet includes a primer layer, which is arranged between the positive electrode film layer and the 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.

[0047] The bottom coating helps to improve the conductivity and adhesion between the positive electrode film and the current collector, reduce the release of the positive electrode film from the current collector during the cycle, and improve the dynamic performance of the battery. In the high compaction density electrode of the embodiment of the present application, for example, the compaction density of the positive electrode in the full discharge state is greater than or equal to 2.4g / cm 3 The current collector is easily damaged during the high-pressure compaction process of the electrode sheet, and large-sized particles are easy to form pits on the current collector. Controlling the distribution density of carbon-based particles with a particle size greater than 100nm in the primer layer to be ≤10pcs / 10μm is beneficial to reducing the probability of damage to the current collector in the high-pressure dense electrode sheet and further improving the ultimate compaction density of the positive electrode sheet.

[0048] In any embodiment, the positive electrode sheet includes a primer layer, and the primer layer is disposed between the positive electrode film layer and the 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.

[0049] In any embodiment, the positive electrode sheet includes a primer layer, and the primer layer is arranged between the positive electrode film layer and the 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.

[0050] As the compaction density of the pole piece increases, the large particles of lithium-containing phosphate materials (for example, particles larger than 1 μm) in the positive electrode film layer have a more significant squeezing effect on the bottom coating. Therefore, stress concentration is easily generated at the site of large particles, and even passes through the bottom coating to damage the current collector. Increasing the thickness of the bottom coating is conducive to improving the stress concentration phenomenon in the pole piece and further improving the ultimate compaction density of the pole piece.

[0051] A second aspect of the present application provides a battery device, including the lithium-ion secondary battery provided in the first aspect of the present application, and the battery device includes at least one of a battery module, a battery pack, and an energy storage battery.

[0052] The third aspect of the present application further provides an electrical device, which includes the lithium-ion secondary battery provided in the first aspect of the present application or the battery device provided in the second aspect of the present application.

[0053] The fourth aspect of the present application also provides a method for preparing a positive electrode active material: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source, wherein the molar ratio of lithium to iron in the mixed raw material is greater than 1 and less than 1.05; obtaining a mixed slurry after grinding, wherein the solid volume distribution particle size DV50 in the mixed slurry is 0.3 μm-0.4 μm; obtaining a precursor powder after drying the mixed slurry; sintering the precursor powder to obtain a positive electrode active material, wherein the sintering is performed in an inert gas environment, and the total gas flow rate during the sintering process is 1100 m / s. 3 / h-1400m 3 / h; the sintering includes a heating interval and a constant temperature interval, the inert gas introduction rate v1 in the heating interval is higher than the inert gas introduction rate v2 in the constant temperature interval; the temperature of the constant temperature interval of the sintering includes 770℃-830℃; the positive electrode active material includes lithium-containing transition metal phosphate particles with a carbon coating layer material provided on at least part of the surface.

[0054] The positive electrode active material prepared by this method can not only allow the positive electrode film layer to have particles larger than 1 μm with a certain area ratio in the cross-section along the thickness direction of the pole piece, but also allow the positive electrode active material to contain a small amount of magnetic substance. While increasing the compaction density of the pole piece and improving the energy density of the lithium-ion secondary battery, the battery also has a low self-discharge level, so that the energy density of the lithium-ion secondary battery can be maintained for a long time during battery storage and circulation.

[0055] The fifth aspect of the present application provides a method for preparing a positive electrode sheet, which comprises sequentially adding a binder, a conductive agent, and a positive electrode active material prepared by the method of the fourth aspect, dry-mixing them, adding a solvent, and stirring to obtain a shipping slurry; transferring and coating the shipping slurry to at least one side of a current collector, and drying and hot pressing to obtain a positive electrode sheet.

[0056] In any embodiment, the hot pressing includes at least three hot roller pressings, and the hot roller pressure increases successively, and the hot roller pressure is 20 tons-50 tons, 50 tons-70 tons, and 70 tons-90 tons respectively; the hot roller temperature is 40℃-80℃, and the pole piece is heated before entering the hot roller compaction for the first time, and the heating temperature is 40℃-50℃.

[0057] The positive electrode active material prepared by the hot pressing process described above in combination with the preparation method of the fourth aspect effectively improves the compaction density of the positive electrode sheet while maintaining a low magnetic substance content, so that the battery has improved energy density while having low self-discharge.

[0058] In any embodiment, the coating speed of the transfer coating is 1 m / min-25 m / min.

[0059] The coating speed of transfer coating within the above range is beneficial to improving the uniformity of particle distribution during the coating process, reducing the risk of particle agglomeration in the positive electrode film layer, reducing the porosity of the positive electrode film cross section, further increasing the ultimate compaction density of the electrode sheet, and improving the energy density of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 is a scanning electron microscope image of a cross section of the positive electrode film layer along the thickness direction of the electrode piece according to an embodiment of the present application; Figure 2 is a schematic diagram of a lithium-ion secondary battery according to one embodiment of the present application; Figure 3 is an exploded schematic diagram of a lithium-ion secondary battery according to one embodiment of the present application; Figure 4 is a schematic diagram of a battery module according to an embodiment of the present application; Figure 5 is a schematic diagram of a battery pack according to an embodiment of the present application; Figure 6 yes Figure 5 An exploded schematic diagram of a battery pack is shown; Figure 7 is a schematic diagram of an electrical device using a lithium-ion secondary battery according to an embodiment of the present application as a power source; Figure 8 This is a porosity test diagram of a cross-section of the positive electrode film layer along the thickness direction of the electrode sheet according to one embodiment of the present application.

[0061] Description of reference numerals: 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 lithium-ion secondary battery; 51 shell; 52 electrode assembly; 53 top cover assembly. DETAILED DESCRIPTION

[0062] Below, the embodiments of the lithium-ion secondary battery, battery device, power device, method for preparing positive electrode active material and method for preparing positive electrode sheet of the present application are specifically disclosed with appropriate reference to the accompanying drawings. However, there are cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following descriptions are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0063] "Scope" disclosed in the present application is limited in the form of lower limit and upper limit, and a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The scope limited in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "ab" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0064] If not otherwise specified, all embodiments and optional embodiments of the present application may be combined with each other to form new technical solutions, and such technical solutions should be deemed to be included in the disclosure of the present application.

[0065] Unless otherwise specified, all technical features and optional technical features of the present application may be combined with each other to form new technical solutions, and such technical solutions should be deemed to be included in the disclosure of the present application.

[0066] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0067] In the present application, the terms "plurality" and "multiple" refer to two or more.

[0068] Unless otherwise specified, the terms used in this application have the commonly understood meanings that are commonly understood by those skilled in the art.

[0069] Unless otherwise specified, the values ​​of the parameters mentioned in this application can be measured by various test methods commonly used in the art, for example, they can be measured according to the test methods given in the examples of this application. Unless otherwise specified, the test temperature of each parameter is 25°C.

[0070] The battery mentioned in the embodiments of the present application may be a single physical module including one or more lithium-ion secondary batteries to provide higher voltage and capacity. For example, the battery mentioned in the present application may include a lithium-ion secondary battery, a battery cell, a battery module or a battery pack.

[0071] A lithium-ion secondary battery is the smallest unit of a battery, which can independently realize the functions of charging and discharging. A lithium-ion secondary battery can be cylindrical, rectangular, or in other shapes, and the present application embodiment does not limit this. Figure 2 As an example, a lithium-ion secondary battery 5 having a rectangular parallelepiped structure is shown.

[0072] A lithium-ion secondary battery includes an electrode assembly and an electrolyte.

[0073] The lithium-ion secondary battery may further include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte. The outer package may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT) and polybutylene succinate (PBS).

[0074] In some embodiments, Figure 3 As shown, the outer package may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. The electrode assembly 52 is encapsulated in the receiving cavity. The number of electrode assemblies 52 contained in the lithium-ion secondary battery 5 can be one or more, which can be adjusted according to demand.

[0075] The electrode assembly usually includes a positive electrode plate and a negative electrode plate. The negative electrode plate is an electrode that absorbs or lithiates lithium ions during charging and releases or delithiates lithium during discharging. The positive electrode plate is an electrode that releases or delithiates lithium ions during charging and absorbs or lithiates lithium during discharging.

[0076] When there are multiple lithium-ion secondary batteries, the multiple lithium-ion secondary batteries are connected in series, in parallel, or in mixed connection through a busbar. In some embodiments, the battery may be a battery module; when there are multiple lithium-ion secondary batteries, the multiple lithium-ion secondary batteries are arranged and fixed to form a battery module. In some embodiments, the battery may be a battery pack, which includes a case and a lithium-ion secondary battery, and the lithium-ion secondary battery or the battery module is accommodated in the case. In some embodiments, the case may serve as part of the chassis structure of the vehicle. For example, a portion of the case may become at least a portion of the floor of the vehicle, or a portion of the case may become at least a portion of the crossbeam and longitudinal beam of the vehicle.

[0077] In some embodiments, the battery may be an energy storage device, which includes an energy storage container, an energy storage cabinet, and the like.

[0078] In some embodiments, lithium-ion secondary batteries may be assembled into a battery module. The battery module may contain a plurality of lithium-ion secondary batteries, and the specific number may be adjusted according to the application and capacity of the battery module. Figure 4 FIG. 4 is a schematic diagram of a battery module 4 as an example. Figure 4 As shown, in the battery module 4, the plurality of lithium-ion secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of lithium-ion secondary batteries 5 may be fixed by fasteners.

[0079] Optionally, the battery module 4 may further include a housing having a housing space, and the plurality of lithium-ion secondary batteries 5 are housed in the housing space.

[0080] In some embodiments, the battery modules described above may also be assembled into a battery pack, and the number of battery modules contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0081] Figure 5 and Figure 6 FIG. 1 is a schematic diagram of a battery pack 1 as an example. Figure 5 and Figure 6 As shown, the battery pack 1 may include a box body and a plurality of battery modules 4 disposed in the box body. The box body includes an upper box body 2 and a lower box body 3, and the upper box body 2 is used to cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 may be arranged in the box body in any manner.

[0082] 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, low stacking efficiency, and low active material loading in the battery, which cannot meet the needs of high energy density batteries.

[0083] Studies have shown that increasing the number and proportion of large-sized particles in lithium-containing transition metal phosphate materials is an effective way to increase the compaction density of the powder and increase the loading amount of positive electrode active materials in the battery. The molding of lithium-containing transition metal phosphate materials often requires a high-temperature sintering process of raw materials. The larger the particle size, the more solid-phase diffusion and grain boundary melting between the raw materials, and the higher the energy consumption and sintering temperature required. However, experimental results show that batteries with large-sized lithium-containing transition metal phosphate particles are often accompanied by high self-discharge. The researchers found that this is because as the sintering temperature of the lithium-containing transition metal phosphate material increases, its lattice defects are prone to carbon thermal reduction reactions and are reduced to Fe and Fe by the carbon on its surface and other reducing substances (such as hydrogen, carbon monoxide, etc.) generated during the preparation process. 2 P and other magnetic materials. Therefore, the increase of large-sized particles in the positive electrode active material is often accompanied by an increase in the content of magnetic materials. Magnetic materials can easily cause the organic matter in the electrolyte to grow in agglomeration to form edges or spikes during the charge and discharge process of lithium-ion secondary batteries, which can easily pierce the diaphragm, forming a micro-short circuit inside the lithium-ion secondary battery, generating a leakage current path, causing the battery power to gradually decrease when there is no external load connected, that is, the self-discharge phenomenon increases, deteriorating the long-term performance of lithium-ion secondary batteries.

[0084] The first aspect of the present application provides a lithium-ion secondary battery, comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, 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 the positive electrode film layer is cut along the thickness direction of the electrode sheet, as shown in FIG. Figure 1 As shown, the area proportion of particles with a particle size greater than or equal to 1 μm is 30%-50%; and in the positive electrode film layer, the mass proportion of magnetic substances is greater than or equal to 20 ppm and less than or equal to 1980 ppm.

[0085] When the area ratio of particles with a particle size of 1 μm or more in the positive electrode film layer is less than 30%, it is difficult to achieve a high compaction density of the positive electrode plate; when the area ratio of particles with a particle size of 1 μm or more in the positive electrode film layer exceeds 50%, the sintering temperature or sintering time needs to be increased, and the content of magnetic substances will increase accordingly, causing the self-discharge K value of the battery cell to increase. While controlling the area ratio of particles with a particle size of 1 μm or more to 30.0%-50.0%, the content of magnetic substances can be reduced by adjusting the gas flow rate, etc., so that the mass ratio of magnetic substances in the positive electrode film layer is greater than or equal to 20ppm and less than or equal to 1980ppm, thereby reducing the self-discharge of the battery while increasing the compaction density of the plate.

[0086] In the cross section of the positive electrode film layer along the thickness direction of the pole piece, the area ratio of particles with a particle size greater than or equal to 1 μm is 30.0%-50.0%, which can give full play to the grading effect during the manufacture and circulation of the pole piece, and effectively improve the compaction density of the pole piece. However, the preparation of positive electrode active materials containing particles with an area ratio within the above range often requires long-term high-temperature sintering, resulting in an increase in the content of magnetic substances. The embodiment of the present application not only increases the area ratio of large-size particles, but also effectively reduces the content of magnetic substances in the positive electrode film layer, so that the mass ratio of magnetic substances in the positive electrode film layer is greater than or equal to 20ppm and less than or equal to 1980ppm. The compaction density of the pole piece in the lithium-ion secondary battery can be improved while maintaining a low self-discharge rate, which is conducive to the improvement of the battery energy density and long-term maintenance during storage. In the present application, the term "particle" refers to a particle with a recognizable complete boundary in the field of view of the positive electrode film layer at a certain magnification, such as 10 thousand times. There may be defects and scratches inside the particles, but the particles cannot be identified inside the particles to separate the complete boundaries of the particles.

[0087] The specific method for identifying particles is as follows: Use an argon ion beam to cut the positive electrode film layer along the thickness direction of the electrode piece (as an example, you can choose: equipment model: Leica EM TIC 3X CP, working voltage: 6kV, working time: 6h), and after exposing the cross section, use a scanning electron microscope (as an example, you can choose: equipment model: Hitachi SU8230, working voltage: 3kV, beam current: high, probe model: U (LA100), working distance <5mm) to observe the cross section of the positive electrode film layer along the thickness direction of the electrode piece. Use a field emission scanning electron microscope to collect images in the secondary electron mode at the non-edge position of the cross section of the positive electrode film layer (after observing the edge of the electrode piece under the scanning electron microscope, adjust the field of view to the center of the sample), take electron microscope images at a magnification of 10k times, and use ImageJ software (1.46r, win64 version) to analyze the particles in the electron microscope image. The specific method of using ImageJ software is as follows: Load the scanning electron microscope image to be analyzed, such as Figure 1As shown; the Cellpose plug-in software is used to identify particles, and manual correction is performed on this basis; Image J is used to read and count the data. The specific method of using the Cellpose plug-in software to identify particles is as follows: set the segmentation diameter parameter (diameter in the Segmentation module) to 15 pixels, click "run cyto3" to identify particles; manually mark the particles in the image that are not recognized by the software or are not fully recognized by the software or are recognized with errors. The particles that are not recognized by the software or are not fully recognized by the software or are recognized with errors in the image mainly include the following: 1. The particles cannot be recognized or cannot be fully recognized because the particles are too large or there are scratches on the surface of the particles; 2. During the argon ion beam cutting process, scratches will be generated on the surface of the particles. The software may misjudge the scratches as particle boundaries during the recognition process, thereby generating recognition errors; 3. Because the particles are too small, they are not successfully recognized; 4. The particles are located at the edge of the electron microscope field of view, the inside of the particles is penetrated by the edge, the morphology cannot be fully displayed, and the local part is recognized instead of the whole, resulting in recognition errors. Manual calibration is performed for the above-mentioned particles that are not identified or have identification errors. The specific process is as follows: delete the large particles located at the edges of the scanning electron microscope that cannot be fully displayed; determine whether there are gap scratches inside other particles that are not identified or have identification errors. If there are no gap scratches inside the particles, it is determined to be one particle, and it is manually marked according to the particle boundary observed manually; in response to the presence of gap scratches inside the particles, determine whether the gap scratches penetrate the particles. If not, it is determined to be one particle and manually marked; in response to the gap scratches penetrating the particles, determine whether the gap scratches are linear or irregular; in response to the gap scratches being irregular, it is determined to be the boundary between particles, and the particles are divided along the boundary; in response to the gap scratches being linear, contrast comparison is performed; in response to the contrast contrast being not obvious and no crack feeling, it is determined to be a scratch and marked as one particle; in response to the contrast contrast being strong and having a crack feeling, it is determined to be the boundary between particles and marked as two particles. After manual marking, delete the information irrelevant to the particles in the automatic image processing process, and the determination and marking of the particles in the image are completed.

[0088] During the compaction process, the positive electrode film layer is compacted in the thickness direction. Therefore, the section of the positive electrode film layer along the thickness direction of the electrode sheet can better reflect the actual compaction status of the particles inside the film layer on a spatial scale than the surface of the positive electrode film layer. In the section of the positive electrode film layer along the thickness direction of the electrode sheet, the area proportion of particles with a particle size greater than or equal to 1μm can intuitively reflect the proportional relationship between the area of ​​some particles in this particle size segment and the area of ​​the entire particles, reflecting the area size of the particles in this particle size segment.

[0089] It is understandable that the particles in the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, especially the particles larger than 50 nm, mainly come from the positive electrode active material. Therefore, the embodiment of the present application can accurately and objectively reflect the distribution of lithium-containing transition metal phosphate particles in the positive electrode film layer in the electrode sheet by observing and counting the particle area in the cross section of the positive electrode film layer.

[0090] In the prior art, a laser particle size analyzer is usually used to count the particle size of the positive electrode active material through the Malvern laser diffraction method. However, the applicant's research shows that because lithium-containing transition metal phosphate particles are easy to agglomerate, the test results obtained by the Malvern laser diffraction method based on the laser scattering principle often measure the particle size of its particle agglomerates, which cannot truly reflect the particle size of the particles in the positive electrode active material, let alone the dispersion state of the positive electrode active material in the film layer, because the positive electrode active material in the film layer will be more dispersed during the pulping and film rolling process. The test results obtained by the Malvern laser diffraction method are affected by the particle size, specific surface area, and degree of agglomeration of the positive electrode active material. Compared with the actual dispersion in the electrode piece, the number of large particles obtained by the 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 the Malvern laser diffraction method test cannot be equivalent to or analogous to the particle size obtained by statistics in the embodiments of the present application.

[0091] In some embodiments, in a cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1 μm accounts for 30%-50%.

[0092] The specific test method for the area ratio of particles with a particle size greater than or equal to 1 μm in the cross section of the positive electrode film layer along the thickness direction of the electrode piece is as follows: refer to the method described above in this application to identify the particles in the positive electrode film layer, import the picture after the particle determination and identification is completed into the ImageJ software for analysis, complete the scale setting according to the scanning electron microscope image, and use the "Feret diameter", "Area", "Round" and "Solidity" analysis functions to statistically analyze the particle size, area, sphericity and roughness of the particles in the cross section of the positive electrode film layer along the thickness direction of the electrode piece. According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter obtained by analysis represents the maximum spacing between all parallel lines in the two-dimensional projection of the particle, thereby characterizing the particle size of the particle; the obtained "Area" parameter represents the pixel area of ​​the particle. Since particles with a particle size of less than 50 nm have large errors in the statistical process and are difficult to identify accurately, and the particle size of the conductive agent is generally less than 50 nm, it will cause large errors in the statistical results. Therefore, in the particle size statistical process of this application, particles with a particle size of less than 50 nm are not counted, and the particle statistical data corresponding to AR, Round or Solidity displayed as "NaN" are deleted. Calculate the sum of the "Area" parameters of particles with a particle size greater than or equal to 1 μm and the sum of the "Area" parameters of all particles as the area of ​​particles with a particle size greater than or equal to 1 μm and the total area of ​​the particles counted, respectively. The sum of the areas of particles with a particle size greater than or equal to 1 μm divided by the total area of ​​the particles counted is taken as the area ratio of particles with a particle size greater than or equal to 1 μm in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet.

[0093] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode piece, the area proportion of particles with a particle size greater than or equal to 1 μm can be selected as 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50% or any numerical range therebetween.

[0094] Lithium-containing transition metal phosphate refers to a phosphate material containing lithium and transition metal elements, which can be detected by any known method in the art. For example, it can be detected by combining an X-ray diffractometer (XRD) with an energy spectrum analyzer and an inductively coupled plasma mass spectrometer. As an example, lithium-containing transition metal phosphates include but are not limited to lithium iron phosphate, lithium manganese iron phosphate and doped materials thereof.

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

[0096] In order to fully utilize the capacity of the lithium-containing transition metal phosphate material to improve its energy density, a carbon layer is often coated on at least part of the surface of the lithium-containing transition metal phosphate material to improve the conduction efficiency of electrons between the positive electrode active materials.

[0097] In the present application, "magnetic material" refers to a material that can generate magnetism when subjected to a magnetic field.

[0098] The "mass content of magnetic material in the positive electrode film layer" herein can be measured by the following steps. Disassemble the battery, obtain the positive electrode plate, soak it in dimethyl carbonate solvent for 8 hours, dry it, and sinter the positive electrode plate at 600°C for 2 hours in a nitrogen atmosphere. During the sintering process, the positive current collector on the positive electrode plate falls off. Crush the sintered plate with a mortar, pass it through a 200-mesh sieve, and obtain the positive electrode material powder. Step 1, weigh the positive electrode material powder obtained by the above reverse method, for example, 80g, put it into a plastic bucket, add 6L of deionized water, use a plastic tube to cover a φ24mm×240mm magnetic bar (magnetic induction intensity is 6000GS), and then use a heat sealing clip to heat seal it, put the magnetic bar into the plastic barrel and seal it together, set the speed of the drum machine to 60 rpm, set the stirring time to 15min, and put the sealed plastic barrel on the drum machine for stirring. Step 2: Prepare another clean plastic bucket, add 5±0.2L of deionized water into the bucket, and flush the magnetic material on the plastic tube into the bucket until there is no area greater than or equal to 0.5cm on the surface of the magnetic rod. 2After removing the slurry block, put the magnetic bar into a clean plastic bucket. Cover the clean bucket cover, continue to place the plastic bucket behind the drum machine, set the speed of the drum machine mixing bucket to 60 rpm, set the stirring time to 15min, and place the sealed bucket on the equipment for stirring. Repeat the above step 2 at least 2 times to ensure the accuracy of the magnetic substance extraction amount. Step 3, prepare a clean 500mL beaker, take out the magnetic bar from the bucket, put it in the beaker, use the flushing bottle to rinse all the magnetic substances on the head of the plastic tube into the beaker, use the demagnetized scissors to cut the two sides of the head of the heat shrink tube, and fold the upper edge of the heat shrink tube 90°, pull out the magnetic bar, and place the magnetic bar in the magnetic bar placement area. Use the flushing bottle to rinse the heat shrink tube in a Z shape from top to bottom (rinse the front and back at least three times each), and rinse the magnetic substance into the beaker until there is no particle residue on the surface of the heat shrink tube (if there are lumps that are difficult to rinse, scrape them off with the back of a clean ceramic knife, and rinse the impurities bonded to the ceramic knife into the beaker). Lift the cannula and rinse the bottom of the cannula at least three times to ensure that all adsorbed magnetic particles are collected. Step 4, ① Place the small magnetic block on the bottom of the beaker and rotate clockwise from the outside to the inside for at least three circles, and then adsorb it counterclockwise from the outside to the inside along the bottom of the beaker for at least three circles. ② Repeat ① three times, with each round of adsorption not less than 10s. ③ Fix the small magnetic block with the palm of your hand at the center of the bottom of the beaker, let it stand for more than 2s, and then slowly tilt it to pour out the solution. ④ Stand the beaker upright and use a rinse bottle to rinse the wall of the cup to ensure that all the adhering magnetic particles enter the solvent. The added solution is 100-150mL. ⑤ Repeat ③ rinsing 2-4 times until the liquid in the beaker is clear (no solvent is needed after the last rinse). Step 5, first use a syringe to add 70mL of deionized water to the beaker, and then use another syringe to slowly add 70mL of 36%-38% hydrochloric acid to the beaker. After the hydrochloric acid dilution is completed, transfer it to a fluorinated bottle with a sealed lid for storage. Step 6, ① Use a syringe to inject 15±2mL of the hydrochloric acid solution prepared in step 5 into the beaker where the magnetic substance is extracted, and then seal the mouth of the beaker with a sealing film. Place it in an ultrasonic instrument for 2 minutes (power 200w / frequency 53KHz). After the ultrasonic is completed, inject 100±10mL of deionized water into the beaker for cleaning, and repeat the cleaning operation twice. Inject 100-150mL of deionized water into the beaker for filtration. Use a filter membrane with a pore size of 0.45μm to collect magnetic particles. Place the filter membrane with magnetic particles on the surface on the slide of the cleanliness microscope, put it in an oven, and dry it at 45°C for (15±2)min. Use an electronic balance to weigh the mass of the dried filter membrane (containing magnetic particles) and subtract the mass of the blank filter membrane to obtain the mass of the magnetic substance. Calculate the mass content of the magnetic substance relative to the mass content of the positive electrode material powder sample as the mass proportion of the magnetic substance in the positive electrode film layer, in ppm.

[0099] In some embodiments, in the positive electrode film layer, the mass proportion of the magnetic substance can be selected to be 20ppm, 100ppm, 134.2ppm, 200ppm, 300ppm, 400ppm, 500ppm, 1000ppm, 1061.7ppm, 1450.2ppm, 1500ppm, 1980ppm or any numerical range therebetween.

[0100] Those skilled in the art can achieve the regulation of the area ratio of particles through any known process. As an example, the particle size concentration can be adjusted through the scientific grading of particles of different sizes; the raw materials can be processed to the target particle size distribution range by the mechanical force of the crushing and grinding process to achieve the adjustment of particle size and concentration; the particle system can be separated by particle size using screening and grading equipment to obtain a particle size distribution that meets the requirements; and the precise control of the feed rate, the adjustment of the particle residence time and stress state in the equipment can also help to achieve the regulation of particle concentration.

[0101] In some embodiments, in a cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size of 1 μm-5 μm accounts for 30%-50%.

[0102] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode piece, the area proportion of particles with a particle size of 1 μm-5 μm can be selected to be 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50% or any numerical range therebetween.

[0103] In the cross-section of the positive electrode film layer along the thickness direction of the pole piece, the area proportion of particles with a particle size of 1μm-5μm is within the above range, which can control the content of magnetic materials in the battery while increasing the compaction density of the pole piece through grading, which is beneficial to the improvement of the energy density of lithium-ion secondary batteries and its long-term maintenance during storage.

[0104] In some embodiments, in a cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size of 1 μm-5 μm accounts for 30%-45%.

[0105] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a particle size of 1μm-5μm is within the above range, which can improve the compaction density and capacity storage stability while further taking into account the migration distance of lithium ions inside the particles, thereby allowing the lithium-ion secondary battery to maintain a low impedance and improve the battery's kinetic performance.

[0106] In some embodiments, in a cross section of the positive electrode film layer along the thickness direction of the electrode piece, the average equivalent area of ​​particles with a particle size of 1 μm or more accounts for 0.05%-0.20%.

[0107] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode piece, the average equivalent area proportion of particles with a particle size of 1 μm or more can be selected to be 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.2% or any numerical range therebetween.

[0108] In the cross section of the positive electrode film layer along the thickness direction of the pole piece, the average equivalent area ratio of particles with a particle size of 1 μm or more is obtained by dividing the area ratio of particles with a particle size of 1 μm or more in the cross section of the positive electrode film layer along the thickness direction of the pole piece by the total number of particles with a particle size of 1 μm or more in the cross section of the positive electrode film layer along the thickness direction of the pole piece. The average equivalent area ratio of particles with a particle size of 1 μm or more in the cross section of the positive electrode film layer along the thickness direction of the pole piece within the above range means that the cross section of the positive electrode film layer along the thickness direction of the pole piece has a certain number of large particles to improve the compaction density of the pole piece, and does not seriously deteriorate the dynamic performance of the lithium-ion secondary battery due to the excessive size of the large particles, taking into account both the energy density and dynamic performance of the battery.

[0109] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size of 50nm-200nm accounts for 3%-15%, optionally 5%-12%, and further optionally 5%-10%.

[0110] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode piece, the area proportion of particles with a particle size of 50nm-200nm can be selected as 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15% or any numerical range therebetween.

[0111] Theoretical studies have shown that under ideal conditions, spherical particles with a diameter of 314nm can be filled in the gaps formed by the accumulation of spherical particles with a diameter of 1μm, thereby improving the particle grading and powder compaction density. Particles with a particle size of 50nm-200nm can be tightly filled in the gaps between particles greater than or equal to 1μm, and cooperate with them to achieve dense stacking. The area ratio of particles with a particle size of 50nm-200nm is within the above range, which means that there are a certain number of particles of 50nm-200nm, which is conducive to improving the powder compaction density of the positive electrode active material and the compaction density of the pole piece through grading, and further improving the energy density of lithium-ion secondary batteries.

[0112] In some embodiments, in the positive electrode film layer, the mass content of the magnetic substance is less than or equal to 300 ppm, and can be optionally 20-200 ppm.

[0113] In some embodiments, in the positive electrode film layer, the mass content of the magnetic substance can be selected as 20 ppm, 50 ppm, 100 ppm, 150 ppm, 200 ppm, 250 ppm, 300 ppm or any numerical range therebetween.

[0114] The mass content of the magnetic substance within the above range can further alleviate the self-discharge phenomenon and improve the storage stability of the battery capacity.

[0115] In some embodiments, the magnetic material includes Fe, Fe 2 P, FeP, γ-Fe 2 O 3 , Fe 2 P 2 O 7 One or more of .

[0116] In some embodiments, the mass content of elemental iron in the positive electrode film layer is less than 20 ppm, and can be optionally less than or equal to 15 ppm.

[0117] In some embodiments, the mass content of elemental iron in the positive electrode film layer can be selected to be 0, 5 ppm, 10 ppm, 15 ppm, 19 ppm or any numerical range therebetween.

[0118] It is understandable that the mass content of elemental iron in the positive electrode film layer is 0, which does not necessarily mean that the positive electrode film layer does not contain elemental iron, but only means that the content of elemental iron therein is lower than its detection limit.

[0119] Compared with other magnetic materials, elemental iron is more likely to be oxidized at the positive electrode and then reduced at the negative electrode. When the iron at the negative electrode accumulates to a certain extent, it will form dendrites, causing perforation of the diaphragm, causing internal short circuit of the battery, and even causing the battery to catch fire or explode, which poses a great safety hazard. Controlling the mass content of elemental iron within the above range is conducive to improving the safety performance of the battery.

[0120] In some embodiments, the lithium-containing transition metal phosphate includes a component having the following general formula: Li m Fe x P y O j Q q , Among them, Q includes one or more of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, 0<q≤0.1.

[0121] In some embodiments, m can be selected as 0.8, 0.85, 0.9, 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, or any range therebetween; x can be selected as 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, or any range therebetween. range; y can be selected as 0.95, 0.96, 0.97, 0.98, 0.99, 1.00 or any range therebetween; j can be selected as 3.5, 3.6, 3.7, 3.8, 3.9, 4 or any range therebetween; q can be selected as 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1 or any range therebetween.

[0122] Selecting an appropriate modifying element Q can improve the lattice change rate of the positive electrode active material during the lithium insertion and extraction process, reduce the oxygen activity on the particle surface, improve the structural stability of the material, and thereby increase the material's gram capacity utilization level, further improving the energy density of lithium-ion secondary batteries.

[0123] 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 1500 ppm-8000 ppm, optionally 2500 ppm-8000 ppm, and further optionally 2500 ppm-6000 ppm.

[0124] In some embodiments, based on the total mass of the positive electrode active material, the mass content of the titanium element can be selected to be 1500ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm, 5500ppm, 6000ppm, 6500ppm, 7000ppm, 7500ppm, 8000ppm or any numerical range therebetween.

[0125] The type and content of the elements in the positive electrode active material can be tested by any known method in the art. As an example, the titanium element and content are tested by inductively coupled plasma emission spectrometry according to Appendix C of GB / T 33822-2017.

[0126] Precursors of titanium, such as titanium dioxide, usually have surface inertness. Its addition during the preparation process can reduce the activity of the precursor raw material mixture. On the one hand, it has an inhibitory and barrier effect on particle growth, achieving the purpose of controlling the size of large particles, so that the average equivalent area of ​​particles with a particle size of more than 1 μm in the cross-section of the positive electrode film along the thickness direction of the pole piece is within an appropriate range; on the other hand, its surface inertness reduces the probability of uneven local chemical reactions of the raw materials and the generation of magnetic substances. At the same time, the doping of titanium in the positive electrode active material can improve the electronic conductivity and ion transfer rate of lithium-containing transition metal phosphates, and alleviate the negative impact of particles with relatively large particle size on the kinetic properties of the positive electrode active material. By affecting the particle size and lithium ion transmission path, the battery energy density and kinetic performance are taken into account.

[0127] In some embodiments, based on the total mass of the positive electrode active material, the mass proportion of carbon element is 0.9%-1.8%.

[0128] Based on the total mass of the positive electrode active material, the mass proportion of the 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 of steel - infrared absorption method after high-frequency induction furnace combustion", the carbon and sulfur content can be measured using a Dekai HCS infrared carbon and sulfur analyzer.

[0129] In some embodiments, based on the total mass of the positive electrode active material, the mass proportion of carbon element can be selected as 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8% or any numerical range therebetween.

[0130] Based on the total mass of the positive electrode active material, the mass proportion of carbon element within the above range can not only improve the conductivity of the positive electrode active material and improve the kinetic performance of the lithium-ion secondary battery; it can also reduce the negative impact of excessive carbon content on the loading amount of lithium-containing transition metal phosphate, taking into account the compaction density of the electrode and the impedance of the lithium-ion secondary battery, while improving the energy density and kinetic performance of the battery.

[0131] In some embodiments, in the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the surface scanning mode of the laser micro-confocal Raman spectrometer, the median graphitization degree C 50 Greater than or equal to 0.9 and less than or equal to 1.3, and can be selected as 0.99-1.2, wherein the graphitization degree C value is I G / I D , where I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at I DIndicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at .

[0132] In the present application, the graphitization degree C value of the positive electrode film layer can be obtained by the surface scanning mode of the laser micro-confocal Raman spectrometer. As an example, specifically, a laser micro-confocal Raman spectrometer (high-precision Renishaw laser micro-confocal Raman spectrometer) is used, an excitation wavelength of 532nm is selected, and an appropriate amount of positive electrode film layer is taken to perform a surface scan on its surface or a section along the thickness direction of the electrode piece. The scanning area is 45μm×45μm, divided into 10×10 grids, with the grid vertex as the test point, the step length is 5μm, and the total number of scanning points is 100 points, thereby obtaining the C values ​​of different sites and the cumulative distribution curve of the C value of the surface scanning area.

[0133] The positive electrode film layer in the present application can be either a freshly prepared positive electrode film layer or a positive electrode film layer disassembled from a battery. The surface of the positive electrode film layer disassembled from a battery inevitably has residual electrolyte salt. In order to improve the test accuracy, it is preferred to perform a surface scan on the cross section of the positive electrode film layer along the thickness direction of the electrode sheet to characterize the graphitization degree of the positive electrode film layer.

[0134] The graphitization degree C value of the positive electrode film is obtained by the peak intensity ratio of the G peak (G-band) and the D peak (D-band) of the Raman spectrum. The G peak position is 1580±100cm -1 , which characterizes carbon sp 2 Hybrid structure; D peak position is 1350±100cm -1 , which characterizes the disordered structure of carbon, where disorder means that there is no regular arrangement between the carbon atoms in the structure. In graphite crystals, carbon atoms in the same layer are arranged in sp 2 Hybridization forms covalent bonds, and the interlayer is Van der Waals force, which makes the carbon in the graphite structure easy to slide. Therefore, the C value can characterize the degree of graphitization of the positive electrode film. It can be understood that the degree of graphitization in the positive electrode film mainly comes from the carbon material that has been graphitized in the positive electrode film, that is, the carbon coating layer of the positive electrode active material. 2 The hybrid structure of carbon nanotube conductive agent also has a relatively high I G / I D However, due to its small content and small diameter, its addition to the positive electrode film layer shows an extreme value in the Raman surface scanning test of the positive electrode film layer, and will not affect the graphitization degree C in the positive electrode film layer. 50 have an impact.

[0135] Therefore, the graphitization degree of the positive electrode film can also be used to characterize the graphitization degree of the positive electrode active material. The higher the graphitization degree of the carbon on the surface of the positive electrode active material, the higher the proportion of graphite structure carbon in the positive electrode film, and the easier it is for the particles to slip during the rolling process with the help of the highly graphitized carbon structure in the coating layer, and the higher the compaction density of the pole piece can be achieved at a low rolling pressure.

[0136] The graphitization degree C value cumulative distribution curve refers to a curve obtained by arranging at least 100 C values ​​obtained in order from small to large, with the graphitization degree as the horizontal axis and the cumulative number percentage as the vertical axis. 50 The C value corresponding to the cumulative number of the vertical axis in the graphitization degree C value cumulative distribution curve is 50%. The median C of the graphitization degree 50 Compared with the point value, it can reflect the overall graphitization degree of the particles in the positive electrode film layer, that is, the degree of slip; compared with the mean value, it can reduce the influence of extreme values ​​during the test and improve the confidence of the test results.

[0137] Those skilled in the art can adjust the graphitization degree of the active material particles by any known process. As an example, adjusting the carbon source, sintering temperature, sintering time, sintering pressure, and sintering atmosphere can adjust the graphitization degree of the active material particles.

[0138] In some embodiments, in the cumulative distribution curve of the degree of graphitization C value obtained by the laser microconfocal Raman spectrometer surface scanning mode of the positive electrode active material, the median C50 of the degree of graphitization can be selected as 0.90, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1, 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, 1.25, 1.3 or any numerical range therebetween.

[0139] The positive electrode active material with a degree of graphitization within the above range can easily achieve particle sliding during the rolling film forming process with the help of the graphitized carbon layer on the surface of the positive electrode active material, offset the negative impact of particle size on the compaction density of the electrode sheet, and further improve the compaction density of the positive electrode film layer with the help of particle sliding.

[0140] In some embodiments, in the cumulative distribution curve of the particle sphericity area 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 0.6-0.85, and can be selected as 0.65-0.80.

[0141] The specific method for testing the sphericity of particles in the cross-section of the positive electrode film along the thickness direction of the electrode sheet is as follows: refer to the method described above in this application to identify the particles in the cross-section of the positive electrode film, and use the "Shape Description" and "Area" analysis functions in ImageJ to analyze the morphology of the particles and the area of ​​the particles in the cross-section of the positive electrode film 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 ​​the circle with the fitted major diameter as the diameter. The closer the particle is to a sphere, the closer the ratio of the pixel area to the area of ​​the circle with the fitted major diameter as the diameter is to 1. Therefore, the "Round" parameter of the particles obtained by analysis is used to characterize the sphericity of the particles. The sphericity of at least 5,000 particles obtained is arranged in order from small to large, and the sphericity is used as the horizontal axis and the cumulative area percentage is used as the vertical axis to obtain the cumulative distribution curve of the sphericity area of ​​the particles in the positive electrode film. L A50 It is the sphericity L value corresponding to when the cumulative area of ​​the vertical axis in the cumulative distribution curve of sphericity L value accounts for 50%.

[0142] Those skilled in the art can adjust the sphericity of the particles by any known process. For example, the sphericity of the particles can be adjusted by grinding, polishing, chemical etching, mechanical stirring, extrusion, coating, granulation, adding surfactants, and adjusting the parameters of each process.

[0143] In some embodiments, in the cumulative distribution curve of the sphericity area of ​​particles obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median of the sphericity L A50 The amount may be selected from 0.60, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.70, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.80, 0.81, 0.82, 0.83, 0.84, 0.85 or any range therebetween.

[0144] The median of sphericity L A50 The particles within the above range are approximately spherical, and are prone to slippage between particles under the action of external forces, which can further improve the compaction density of the pole piece and increase the energy density of the battery.

[0145] In some embodiments, the powder compaction density of the positive electrode active material under 3T pressure is 2.48 g / cm 3 -2.76g / cm 3 .

[0146] In this application, the term "powder compaction density" refers to the density of a compact with a certain density and strength formed during the external compression process. As the powder moves and deforms, larger gaps are filled, the contact area between particles increases, the attraction between atoms is generated, and the mechanical fit between particles is enhanced. The unit is g / cm 3 .

[0147] The powder compaction density of the positive electrode active material can be measured by methods and equipment known in the art. For example, it can be measured with a compaction density instrument with reference to GB / T 24533-2009. Specifically, a certain amount of positive electrode active material is placed on a special compaction mold (the mold diameter is known), and the mold is hollow in the middle with a metal disc on the top and bottom. The positive electrode active material is placed between the metal discs, a metal cylinder is placed on the top, and the mold is placed on the compaction density instrument. The pressure is set to 3T. The thickness of the positive electrode active material under a pressure of 3T can be read on the device. 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, and S is the bottom area of ​​the mold, 1.327cm 2 , H is the thickness of the positive electrode active material after compaction.

[0148] In some embodiments, the powder compaction density of the positive electrode active material under 3T pressure can be 2.48 g / cm 3 , 2.49g / cm 3 , 2.50g / cm 3 , 2.51g / cm 3 , 2.52g / cm 3 , 2.53g / cm 3 , 2.54g / cm 3 , 2.55g / cm 3 , 2.56g / cm 3 , 2.57g / cm 3 , 2.58g / cm 3 , 2.59g / cm 3 , 2.60g / cm 3 , 2.61g / cm 3 , 2.62g / cm 3 , 2.63g / cm 3 , 2.64g / cm 3 , 2.65g / cm 3 , 2.66g / cm 3 , 2.67g / cm 3 , 2.68g / cm 3 , 2.69g / cm 3 , 2.70g / cm 3 , 2.71g / cm3 , 2.72g / cm 3 , 2.73g / cm 3 , 2.74g / cm 3 , 2.75g / cm 3 , 2.76g / cm 3 Or any range of values ​​in between.

[0149] The positive electrode active material has a high powder compaction density, and provides a material basis for improving the compaction density of the pole piece and preparing a high energy density lithium ion secondary battery.

[0150] In some embodiments, the powder compaction density of the positive electrode active material under 3T pressure is 2.58 g / cm 3 -2.76 / cm 3 .

[0151] The positive electrode active material with a compaction density within the above range can further improve the compaction density of the electrode sheet and increase the energy density of the battery.

[0152] In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 8 MPa is 2.0 Ω·cm-40 Ω·cm.

[0153] The powder resistivity of the positive electrode active material can be measured by methods and equipment known in the art. For example, it can be measured with a powder resistivity meter (Suzhou Jingge, ST2722 model) with reference to GB / T 33822-2017. Specifically, a certain amount of positive electrode active material (e.g. 1 g) is weighed and added to the feeding chamber of the powder resistivity meter, and a pressure of 8 MPa is applied. The forward resistivity and reverse resistivity of the positive electrode active material are tested respectively, and the average value of the two is taken as the powder resistivity of the positive electrode active material.

[0154] In some embodiments, the powder resistivity of the positive electrode active material under a pressure of 8 MPa may be 2Ω·cm, 3Ω·cm, 4Ω·cm, 5Ω·cm, 6Ω·cm, 7Ω·cm, 8Ω·cm, 9Ω·cm, 10Ω·cm, 11Ω·cm, 12Ω·cm, 13Ω·cm, 14Ω·cm, 15Ω·cm, 16Ω·cm, 17Ω·cm, 18Ω·cm, 19Ω·cm, 20Ω·cm, ·cm, 21Ω·cm, 22Ω·cm, 23Ω·cm, 24Ω·cm, 25Ω·cm, 26Ω·cm, 27Ω·cm, 28Ω·cm, 29Ω·cm, 30Ω·cm, 31Ω·cm, 32Ω·cm, 33Ω·cm, 34Ω·cm, 35Ω·cm, 36Ω·cm, 37Ω·cm, 38Ω·cm, 39Ω·cm, 40Ω·cm or any range between two of them.

[0155] The positive electrode active material has a lower powder resistivity, which is beneficial to improving the capacity utilization level of the positive electrode active material and improving the energy density of the lithium ion secondary battery.

[0156] In some embodiments, the positive electrode active material has a discharge capacity of 135 mAh / g to 150 mAh / g at a 1 C discharge rate at room temperature.

[0157] In this application, the positive electrode active material is assembled into a button cell and the electrical performance is tested on a blue electric tester. At 25±5℃, within the voltage range of 2.0V~3.75V, it is charged to 3.75V at 1C constant current, then paused for 5 minutes, charged at constant voltage to a cut-off current of 50μA, and then discharged to 2.0V at 1C constant current. The discharge capacity of the button cell divided by the mass of the positive electrode active material is taken as the discharge gram capacity of the positive electrode active material at room temperature at a discharge rate of 1C.

[0158] The preparation and testing process of button cells is as follows: 2.0g of positive electrode active material, conductive carbon black, and PVDF are mixed at a mass ratio of 0.9:0.05:0.05, and then organic solvent NMP (N-methylpyrrolidone) is added. After fully mixing, a 150μm scraper is used for coating, and the mixture is dried at 100℃ for 2h. The compacted density is 2.0g / cm 3 -2.2g / cm 3 The positive electrode sheet was compacted and punched into a disc with a diameter of 14 mm using a hole puncher, then weighed and recorded, and the weighed positive electrode sheet was placed in a vacuum drying oven (105°C, 1-12 hrs, -90 kpa). After baking, the positive electrode sheet was placed in a glove box, and the battery was assembled in the order of negative electrode shell-nickel mesh-lithium sheet-diaphragm-positive electrode sheet-positive electrode shell, and 65-87 μL (pipette gun) of electrolyte (the electrolyte was a mixed solvent of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) in a volume ratio of 1:1, and the electrolyte LiPF 6 ), the negative electrode is on top and placed in the groove of the sealing machine, the sealing pressure is 650kg / cm 2 , use insulated tweezers to remove the button battery and put it into a dust-free bag, remove the glove box, and place it in a constant temperature room for 3 hours to obtain the button battery for testing.

[0159] It can be understood that the discharge capacity in grams of the positive electrode active material can also be obtained by disassembling the battery, obtaining the positive electrode sheets, assembling them into button cells according to the method described above, and then testing them.

[0160] In some embodiments, the discharge capacity in grams of the positive electrode active material at room temperature at a discharge rate of 1C may be selected as 135 mAh / g, 140 mAh / g, 141 mAh / g, 142 mAh / g, 143 mAh / g, 144 mAh / g, 145 mAh / g, 146 mAh / g, 147 mAh / g, 148 mAh / g, 149 mAh / g, 150 mAh / g or any range of values ​​therebetween.

[0161] The positive electrode active material has a high discharge gram capacity, indicating that the positive electrode active material has good kinetic performance and is beneficial to improving the energy density of the lithium ion secondary battery.

[0162] In some embodiments, 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, a 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 a rate of 1C, the capacity value extracted at a discharge voltage of 3.2V is recorded as C1, and the capacity value extracted to a 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.

[0163] The η value of the positive electrode active material can be measured by methods and equipment known in the art. As an example, first prepare a button cell with reference to the method described above, and test the electrical performance of the prepared button cell on a blue electric tester. Specifically, at room temperature, the button cell is charged and discharged twice at a constant current of 0.1C in the voltage range of 2.0V~3.75V, and then charged to the cut-off voltage with a constant current and charged to a current of 50μA with a constant voltage, and then charged and discharged once with a constant current of 1C. In the charge and discharge test at a rate of 1C, the capacity value from 3.75V to 3.2V is recorded as C1, the capacity value from 3.75V to 2.0V is C2, and η=C1 / C2.

[0164] In some embodiments, n can be selected as 85%, 86%, 87%, 88%, 88.1%, 89%, 90%, 90.1%, 91%, 92%, 92.2%, 93%, 94%, 94.1%, 94.5%, 95%, 95.1%, or any range therebetween.

[0165] In some embodiments, the positive electrode active material in the freshly prepared lithium ion secondary battery has a discharge capacity ratio η of ≥88% when discharged to 3.2V. After the freshly prepared lithium ion secondary battery is charged and discharged at a constant current rate of 0.1C in a voltage range of 2.0V to 3.75V for a period of time, the discharge capacity ratio η of the positive electrode active material when discharged to 3.2V can be maintained at ≥85%.

[0166] The high proportion of the discharge capacity of the positive electrode active material used in the battery of the embodiment of the present application when discharged to 3.2V means that although the positive electrode active material has a certain proportion of large-sized particles, it still maintains good kinetic performance. At the same time, the high η value indicates that the lithium-ion secondary battery containing the positive electrode active material still has a high voltage when discharged to a low state of charge (SOC), which is conducive to maintaining good power performance.

[0167] In some embodiments, the positive electrode film layer further includes a binder and a conductive agent. Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 94%-99.4%, the mass content of the binder is 0.5%-3%, and the mass content of the conductive agent is 0.1%-3%.

[0168] In some embodiments, the binder includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

[0169] 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.

[0170] 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 94%, 95%, 96%, 97%, 98%, 99%, 99.4% or any numerical range therebetween.

[0171] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the binder can be selected to be 0.5%, 1%, 1.5%, 2%, 2.5%, 3% or any numerical range therebetween.

[0172] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the conductive agent can be selected as 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3% or any numerical range therebetween.

[0173] In some embodiments, the single-side density of the positive electrode film layer is 300 mg / 1540 mm 2 -450mg / 1540mm 2 .

[0174] In this application, the single-side density of the positive electrode film layer is a well-known meaning in the art and can be tested by methods known in the art. For example, take a single-sided coated and 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), and punch it into an area of ​​S 1 Weigh the small disc and record it as M 1 Then wipe off the positive electrode film layer of the weighed positive electrode sheet, weigh the weight of the current collector, and record it as M 0 . Single-side density of positive electrode film = (M 1 -M 0 ) / S 1 In order to ensure the accuracy of the test results, multiple groups (eg, 10 groups) of samples to be tested may be tested, and the average value may be calculated as the test result.

[0175] In some embodiments, the surface density of the positive electrode film layer on one side may be 300 mg / 1540 mm 2 、310mg / 1540mm 2 、320mg / 1540mm 2 、330mg / 1540mm 2 、340mg / 1540mm 2 、350mg / 1540mm 2 、360mg / 1540mm 2 、370mg / 1540mm 2 、380mg / 1540mm 2 、390mg / 1540mm 2 , 400mg / 1540mm 2 、410mg / 1540mm 2 、420mg / 1540mm 2 、430mg / 1540mm 2 、440mg / 1540mm 2 、450mg / 1540mm 2 Or any range of values ​​in between.

[0176] The positive electrode film layer having an area density within the above range can contribute to improving the energy density of the lithium ion secondary battery.

[0177] In some embodiments, when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.43 g / cm3 -2.78g / cm 3 .

[0178] In this application, the full discharge state refers to the state in which the battery is placed in a 25°C oven environment, left to stand for 2 hours, and the battery temperature is maintained at 25°C, and the battery is discharged at a constant current of 1 / 3C to 2.5V and then discharged at a constant current of 0.1C to 2.0V.

[0179] The compaction density of the positive electrode film layer can be tested by methods known in the art. As an example, the battery is placed in a 25°C oven environment and left to stand for 2 hours. When the battery temperature is maintained 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 of the lithium-ion secondary battery in a fully discharged state, and the residual electrolyte is treated with a dimethyl carbonate solvent, the plate is dried, and cut into small discs with an area of ​​S, weighed as W1, and the thickness of the positive electrode plate T1 is measured using a micrometer, and then the positive electrode film layer of the weighed plate is wiped off, and the weight of the current collector is weighed, recorded as W2, and the thickness of the current collector T2 is measured using a micrometer, then the compaction density of the positive electrode film layer PD = (W1-W2) / [(T1-T2)×S].

[0180] In some embodiments, when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer can be 2.43 g / cm 3 , 2.44g / cm 3 , 2.45g / cm 3 , 2.46g / cm 3 , 2.47g / cm 3 , 2.48g / cm 3 , 2.49g / cm 3 , 2.50g / cm 3 , 2.51g / cm 3 , 2.52g / cm 3 , 2.53g / cm 3 , 2.54g / cm 3 , 2.55g / cm 3 , 2.56g / cm 3 , 2.57g / cm 3 , 2.58g / cm 3 , 2.59g / cm 3 , 2.60g / cm 3 , 2.61g / cm 3 , 2.62g / cm 3 , 2.63g / cm 3 , 2.64g / cm 3 , 2.65g / cm 3 , 2.66g / cm3 , 2.67g / cm 3 , 2.68g / cm 3 , 2.69g / cm 3 , 2.70g / cm 3 , 2.71g / cm 3 , 2.72g / cm 3 , 2.73g / cm 3 , 2.74g / cm 3 , 2.75g / cm 3 , 2.76g / cm 3 , 2.77g / cm 3 , 2.78g / cm 3 Or any range of values ​​in between.

[0181] The compaction density of the positive electrode film layer is within the above range, which is beneficial to improving the energy density of the lithium-ion secondary battery.

[0182] In some embodiments, when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.50 g / cm 3 -2.75g / cm 3 .

[0183] In some embodiments, after the compaction process, the compaction density of the positive electrode film layer is 2.55 g / cm 3 -2.90g / cm 3 .

[0184] In some embodiments, after the compaction process, the compaction density of the positive electrode film layer can be selected to be 2.55 g / cm 3 , 2.63g / cm 3 , 2.64g / cm 3 , 2.65g / cm 3 , 2.66g / cm 3 , 2.67g / cm 3 , 2.68g / cm 3 , 2.69g / cm 3 , 2.70g / cm 3 , 2.71g / cm 3 , 2.72g / cm 3 , 2.73g / cm 3 , 2.74g / cm 3 , 2.75g / cm 3 , 2.76g / cm 3 , 2.77g / cm 3 , 2.78g / cm 3 g / cm 3, 2.79g / cm 3 , 2.80g / cm 3 , 2.81g / cm 3 , 2.82g / cm 3 , 2.83g / cm 3 , 2.84g / cm 3 , 2.85g / cm 3 , 2.90g / cm 3 Or any range of values ​​in between.

[0185] In this application, "compaction" refers to compacting the positive electrode film layer by mechanical pressure during the battery assembly process to improve its density and conductivity.

[0186] In some embodiments, after the chemical formation process, the compaction density of the positive electrode film layer is 2.43 g / cm 3 -2.78g / cm 3 .

[0187] In some embodiments, after the chemical formation process, the compaction density of the positive electrode film layer can be selected to be 2.43 g / cm 3 , 2.52g / cm 3 , 2.53g / cm 3 , 2.54g / cm 3 , 2.55g / cm 3 , 2.56g / cm 3 , 2.57g / cm 3 , 2.58g / cm 3 , 2.59g / cm 3 , 2.60g / cm 3 , 2.61g / cm 3 , 2.62g / cm 3 , 2.63g / cm 3 , 2.64g / cm 3 , 2.65g / cm 3 , 2.66g / cm 3 , 2.67g / cm 3 , 2.68g / cm 3 , 2.69g / cm 3 , 2.70g / cm 3 , 2.71g / cm 3 , 2.72g / cm 3 , 2.73g / cm 3 , 2.78g / cm 3 Or any range of values ​​in between.

[0188] In this application, formation refers to the formation of a stable solid electrolyte interface (SEI film) and electrode structure through electrochemical reactions during the first charge and discharge process of the battery.

[0189] It can be understood that, with the rebound of the electrode during the cycle 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 the compaction density of the positive electrode film layer after compaction and formation.

[0190] In some embodiments, when the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.43-2.78 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%-28%.

[0191] In some embodiments, the lithium-ion secondary battery has a compaction density of 2.5-2.78 g / cm2 in a fully discharged state. 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%.

[0192] In some embodiments, in a cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer may be selected to be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28% or any numerical range therebetween.

[0193] In the cross section of the positive electrode film layer along the thickness direction of the electrode piece, the porosity of the positive electrode film layer can be tested in the following manner. Import the cross section scanning electron microscope image of the positive electrode film layer along the thickness direction of the electrode piece obtained in the manner described above into the ImageJ software, select the straight line tool, use the straight line to mark the ruler length in the image, click "Analyze SetScale", and set the ruler parameters in the software according to the ruler length in the image. Select the rectangle tool, select the part of the image outside the ruler area, use "Image Duplicate" to copy the selected area, and use "Image Type 8 bit" to adjust the image format; select "Analyze Set Measurements", select the following 5 options: "Area", "Mean gray value", "Area Fraction", "Limit to threshold", "Feret's diameter", where "Decimal places" selects 3, selects "Image"-"Adjust"-"Threshold" in turn, and sets 0 and 100 in the "Threshold" box position in turn, and the Analyze-Measure function can be used to export the pore data in the cross section electron microscope image. Use "Image"-"Overlay"-"Flatten" to export and obtain the pore image; click "Apply" in "Threshold", then click "Analyze"-"Analyze Particles", check the four columns on the left, and you can get the pore statistics.

[0194] It can be understood that in the embodiment of the present application, the "pores" in the cross section of the positive electrode film layer are identified by using the color difference of the image and the threshold. Figure 8 As shown in the figure, the "pores" are not the pore data obtained in the exhaust test, but are mainly used to characterize the cross-sectional area between particles in the section of the positive electrode film layer. This method is better than the exhaust method because the porosity obtained by the exhaust method is related to the pores between particles and the pores in the carbon layer coated on the surface of the lithium iron phosphate particles, which cannot objectively reflect the pores between particles. The lower the porosity in the section of the positive electrode film layer tested by this method, on the one hand, it means that the gradation 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 gradation and roller pressure, if the porosity is low, it means that the particles are easy to slip against each other, thereby reducing the risk of overpressure and stress concentration in the film layer, and further reducing the probability of demolding of the positive electrode film during long cycles, which is beneficial to improving the long cycle performance of the battery.

[0195] In some embodiments, the positive electrode plate includes a primer layer, which is disposed between the positive electrode film layer and the current collector; the primer layer includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the primer layer is ≤10 pcs / 10 μm.

[0196] Among them, carbon-based particles refer to particles with carbon as the main component, including but not limited to conductive carbon, carbon black, etc.

[0197] The bottom coating helps to improve the conductivity and adhesion between the positive electrode film and the current collector, reduce the release of the positive electrode film from the current collector during the cycle, and improve the dynamic performance of the battery. In the high compaction density electrode of the embodiment of the present application, for example, the compaction density of the positive electrode in the full discharge state is greater than or equal to 2.4g / cm 3 The current collector is easily damaged during the high-pressure compaction process of the electrode sheet, and large-sized particles are easy to form pits on the current collector. Controlling the distribution density of carbon-based particles with a particle size greater than 100nm in the primer layer to be ≤10pcs / 10μm is beneficial to reducing the probability of damage to the current collector in the high-pressure dense electrode sheet and further improving the ultimate compaction density of the positive electrode sheet.

[0198] In some embodiments, the positive electrode plate includes a primer layer, which is disposed between the positive electrode film layer and the current collector; the primer layer includes carbon-based particles, and the distribution density of carbon-based particles having a particle size greater than 100 nm in the primer layer can be 0.1 pcs / 10 μm, 1 pcs / 10 μm, 1.5 pcs / 10 μm, 2 pcs / 10 μm, 2.5 pcs / 10 μm, 3 pcs / 10 μm, 3.5 pcs / 10μm, 4pcs / 10μm, 4.5pcs / 10μm, 5pcs / 10μm, 5.5pcs / 10μm, 6pcs / 10μm, 6.5pcs / 10μm, 7pcs / 10μm, 7.5pcs / 10μm, 8pcs / 10μm, 8.5pcs / 10μm, 9pcs / 10μm, 9.5pcs / 10μm, 10pcs / 10μm or any numerical range between them.

[0199] The distribution density of carbon-based particles with a particle size greater than 100 nm in the base coating layer can be determined using the method described above, by cutting the positive electrode film layer along the thickness direction of the electrode piece with an argon ion beam, taking a scanning electron microscope image or a microscope image, and detecting the size of the carbon particles in the base coating layer by a statistical method, and counting the number of carbon-based particles with a particle size greater than 100 nm per 10 μm in the base coating layer, and counting for no less than 5 times to find the average value.

[0200] The primer layer in the embodiment of the present application can be prepared by any known preparation process, such as pre-screening or centrifugation in the process of preparing carbon-based particles to remove large particles of carbon-based materials, so that the carbon-based particles added in the process of preparing the primer layer have a D V 50 in 20-60nm, D V 90 is less than or equal to 70nm, and the carbon-based material and the binder are mixed, stirred, and coated on the current collector to obtain a primer layer.

[0201] In some embodiments, the positive electrode sheet includes a primer layer, and the primer layer is disposed between the positive electrode film layer and the 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 1-4 μm.

[0202] In some embodiments, the positive electrode sheet includes a primer layer, and the primer layer is disposed between the positive electrode film layer and the 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 2-4 μm.

[0203] As the compaction density of the pole piece increases, the large particles of lithium-containing phosphate materials (for example, particles larger than 1 μm) in the positive electrode film layer have a more significant squeezing effect on the bottom coating. Therefore, stress concentration is easily generated at the site of large particles, and even passes through the bottom coating to damage the current collector. Increasing the thickness of the bottom coating is conducive to improving the stress concentration phenomenon in the pole piece and further improving the ultimate compaction density of the pole piece.

[0204] In some embodiments, the thickness of the primer layer on one side may be 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, or any range therebetween.

[0205] The single-sided thickness of the primer layer can be tested in the following way. As described above, the positive electrode film layer is cut along the thickness direction of the pole piece by an argon ion beam, and a scanning electron microscope image is taken. In the length direction of the pole piece, points are taken at intervals of 1 μm to measure the thickness of the single-sided primer layer. After measuring the thickness of the primer layer at 10 points, the average value is calculated. It should be noted that in the process of measuring points, it is necessary to avoid abnormal points, that is, the primer layer areas with a thickness of less than 50 nm and a thickness of more than 4 μm; these abnormal points are mainly due to the extreme fluctuations in thickness of individual areas caused by abnormal stress concentration and extrusion during the compaction of the pole piece, and are not statistically significant. In some embodiments, the thickness of the positive electrode current collector is less than or equal to 17 μm, and can be optionally 13 μm-15 μm.

[0206] In some embodiments, the thickness of the positive electrode current collector is 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, or any range therebetween.

[0207] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0208] A positive electrode current collector having a thickness within the above range helps to increase the load per unit mass of the battery and improve the energy density of the lithium-ion secondary battery.

[0209] In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector, and the surface density of the negative electrode film layer on one side is 130 mg / 1540 mm 2 -220mg / 1540mm 2 ; and / or the compaction density of the negative electrode film layer is 1.40g / cm 3 -1.75g / cm 3 .

[0210] In some embodiments, the surface density of the negative electrode film layer on one side may be 130 mg / 1540 mm 2 、140mg / 1540mm 2 、150mg / 1540mm 2 、160mg / 1540mm 2 、170mg / 1540mm 2 、180mg / 1540mm 2 、190mg / 1540mm 2 、200mg / 1540mm 2 、210mg / 1540mm 2 , 220mg / 1540mm 2 Or any range of values ​​in between.

[0211] In some embodiments, the compaction density of the negative electrode film layer is 1.40 g / cm 3 -1.75g / cm 3 .

[0212] In some embodiments, the compaction density of the negative electrode film layer can be selected to be 1.40 g / cm 3 , 1.45g / cm 3 , 1.50g / cm3 , 1.55g / cm 3 , 1.60g / cm 3 , 1.70g / cm 3 , 1.75g / cm 3 Or any range of values ​​in between.

[0213] The single-side density and compaction density of the negative electrode film layer can be tested by methods similar to those of the positive electrode film layer described above.

[0214] The surface density and compaction density of the negative electrode film layer are within the above ranges, which is conducive to matching with the positive electrode film layer and improving the energy density of the lithium-ion secondary battery.

[0215] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, a copper foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0216] In some embodiments, the negative electrode film layer includes a negative electrode active material. The negative electrode active material may be a negative electrode active material for a battery known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0217] In some embodiments, the negative electrode film layer may further include a binder, which may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS).

[0218] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

[0219] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0220] In some embodiments, the negative electrode sheet can be prepared in the following manner: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode collector, and after drying, compacting and other processes, the negative electrode sheet can be obtained.

[0221] The electrolyte plays the role of conducting ions between the positive electrode and the negative electrode. The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs. For example, the electrolyte can be liquid, gel or all-solid.

[0222] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0223] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0224] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane sulfone, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0225] In some embodiments, the electrolyte may further include additives, such as negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high or low temperature performance, etc.

[0226] In some embodiments, the lithium-ion secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical stability and mechanical stability can be selected.

[0227] In some embodiments, the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0228] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

[0229] In some embodiments, the lithium-ion secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0230] In some embodiments, the outer packaging of the lithium ion secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, and as the plastic, polypropylene, polybutylene terephthalate, and polybutylene succinate can be listed.

[0231] A second aspect of the present application provides a battery device, including the lithium-ion secondary battery provided by the first aspect of the present application, and the battery device includes at least one of a battery module, a battery pack, and an energy storage battery.

[0232] The third aspect of the present application provides an electrical device, including the lithium-ion secondary battery provided in the first aspect of the present application. The fourth aspect of the present application provides a method for preparing a positive electrode active material: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source, wherein the molar ratio of lithium to iron in the mixed raw material is greater than 1 and less than 1.05; obtaining a mixed slurry after grinding, wherein the solid phase volume distribution particle size D V50 The mixed slurry is dried to obtain a precursor powder; the precursor powder is sintered to obtain a positive electrode active material, the sintering is performed in an inert gas environment, and the total gas flow rate during the sintering process is 1100m 3 / h-1400m 3 / h; the sintering includes a heating interval and a constant temperature interval, the inert gas introduction rate v1 in the heating interval is higher than the inert gas introduction rate v2 in the constant temperature interval; the temperature of the constant temperature interval of the sintering includes 770℃-830℃; the positive electrode active material includes lithium-containing transition metal phosphate particles with carbon coating material provided on at least part of the surface.

[0233] In some embodiments, the constant temperature interval of the sintering may be selected as 770° C., 780° C., 790° C., 800° C., 810° C., 820° C., 830° C. or any range therebetween.

[0234] High temperature sintering is beneficial to increasing the area ratio of particles with a particle size greater than or equal to 1 μm in the positive electrode film layer.

[0235] In some embodiments, the molar ratio of lithium to iron in the mixed raw material can be selected as 1.01, 1.02, 1.03, 1.04 or any range therebetween.

[0236] In the mixed raw materials, the molar ratio of lithium to iron is greater than 1, which helps to replenish lithium during the sintering process of the positive electrode active material, improve the crystallinity, and improve the capacity of the positive electrode active material; however, studies have shown that too high a molar ratio of lithium to iron in the raw materials will lead to incomplete chemical reactions in local areas during the sintering process of the positive electrode active material, thereby increasing the magnetic substances such as Fe 2 The molar ratio of lithium to iron in the mixed raw material within the above range helps to increase the compaction density of the pole piece and improve the energy density of the lithium-ion secondary battery while making the battery have a low self-discharge level, so that the energy density of the lithium-ion secondary battery can be maintained for a long time during battery storage and cycling.

[0237] In this application, the term "D V50 " refers to the particle size corresponding to 50% of the sample volume cumulative particle size distribution percentage obtained by Malvern laser scattering method.

[0238] In some embodiments, the solid volume distribution particle size D in the mixed slurry is V50 The optional value may be 0.3 μm, 0.31 μm, 0.32 μm, 0.33 μm, 0.34 μm, 0.35 μm, 0.36 μm, 0.37 μm, 0.38 μm, 0.39 μm, 0.4 μm or any numerical range therebetween.

[0239] The solid phase particle size in the mixed slurry after grinding is within the above range, indicating that the raw material particle size is small, so that the raw material has a relatively high activity, and solid phase diffusion is easy to occur during the high-temperature sintering process, growing into particles with a particle size of more than 1μm with a certain area share. At the same time, the solid phase particle size in the mixed slurry after grinding within the above range helps to reduce the probability of generating magnetic substances due to uneven chemical reactions in local areas caused by excessive raw material activity.

[0240] In some embodiments, the inert gas includes one or more of nitrogen, neon, and helium.

[0241] In some embodiments, the sintering is performed in an inert gas environment, and the total gas flow rate during the sintering process can be 1100m 3 / h、1200m 3 / h、1300m 3 / h、1350m 3 / h、1400m3 / h or any range of values ​​therebetween.

[0242] During the sintering process, the total gas flow rate within the above range is beneficial to reducing the partial pressure of the reducing atmosphere and reducing the possibility of local reduction and increase of magnetic substances.

[0243] During the heating process, violent chemical reactions occur between the precursor raw materials. Increasing the ventilation rate is beneficial to reducing the phenomenon of excessive local reducing atmosphere, uneven reaction, and high content of magnetic substances. During the constant temperature process, slow solid-phase diffusion occurs between the precursor raw materials to achieve particle growth. Maintaining a relatively low ventilation rate is beneficial to maintaining the stability of the temperature field during sintering and achieving uniform growth of particles.

[0244] In some embodiments, v1:v2 can be selected as 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1 or any range therebetween.

[0245] The positive electrode active material prepared by this method can not only allow the positive electrode film layer to have particles larger than 1 μm with a certain area ratio in the cross-section along the thickness direction of the pole piece, but also allow the positive electrode active material to contain a small amount of magnetic substance. While increasing the compaction density of the pole piece and improving the energy density of the lithium-ion secondary battery, the battery also has a low self-discharge level, so that the energy density of the lithium-ion secondary battery can be maintained for a long time during battery storage and circulation.

[0246] In some embodiments, the iron source is an iron-containing compound.

[0247] In some embodiments, the iron source includes at least one of ferric hydroxide, ferrous chloride, ferric oxide, ferric phosphate, ferric pyrophosphate, ferrous oxalate, iron powder, ferric nitrate, ferrosoferric oxide, and ferric oxyhydroxide.

[0248] In some embodiments, the phosphorus source is a phosphoric acid compound.

[0249] In some embodiments, the phosphorus source includes at least one of phosphoric acid, ferric phosphate, monoammonium phosphate, and diammonium phosphate.

[0250] In some embodiments, the iron source and the phosphorus source can be the same substance. In some embodiments, iron phosphate is used as the iron source and the phosphorus source.

[0251] 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.

[0252] In some embodiments, the lithium source comprises lithium carbonate.

[0253] In some embodiments, the carbon source includes one or more of glucose, polyethylene glycol, citric acid, sucrose, starch, fructose, lactose, polyaniline, polyacrylonitrile, and polyvinyl pyrrolidone.

[0254] In some embodiments, the carbon source comprises glucose or polyethylene glycol.

[0255] 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.

[0256] In some embodiments, the solid volume distribution coefficient (D V90 -D V10 ) / D V50 It is 1.8-3.0.

[0257] In some embodiments, the solid volume distribution coefficient (D V90 -D V10 ) / D V50 The options can be 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0 or any numerical range therebetween.

[0258] The solid phase volume distribution coefficient in the mixed slurry being within the above range indicates that the raw material particle size distribution is uniform, which is beneficial to improving the uniformity of the solid phase reaction during subsequent sintering and reducing the risk of local concentration of reducing substances, enhanced reducing property, and aggregation of magnetic substances due to uneven mixing of raw materials.

[0259] In some embodiments, the volume distribution particle size D of the precursor powder is V50 5μm-60μm.

[0260] In some embodiments, the precursor powder is obtained by spray-drying the mixed slurry.

[0261] In some embodiments, the sintering is a single sintering, including at least two constant temperature intervals, the constant temperature of the first constant temperature interval is 400℃-500℃, and the constant temperature time of the first constant temperature interval is 3h-8h; the highest constant temperature of the single sintering is 770℃-820℃, and the constant temperature treatment is performed at the highest temperature for 8h-15h.

[0262] In some embodiments, the maximum constant temperature of the primary sintering may be 770° C., 780° C., 790° C., 800° C., 810° C., 820° C. or any range therebetween.

[0263] In some embodiments, the maximum temperature is maintained at 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, or any range therebetween.

[0264] High temperature sintering within the above temperature range helps to increase the particle size of the positive electrode active material, so that large particles larger than 1 μm can occupy a certain area in the cross section of the positive electrode film along the thickness direction of the electrode, increase the compaction density of the electrode, and at the same time, reduce the increase of reducing atmosphere caused by excessively high sintering temperature, and control the reduction to generate Fe 2 The probability of magnetic materials such as P, thereby taking into account the energy density and storage stability of the battery.

[0265] In some embodiments, the sintering is performed at least twice, a primary sintered product is obtained after the first sintering, and the primary sintered product is ground and then sintered for the second time.

[0266] In some embodiments, the sintering temperature of the first sintering is 720° C.-780° C., and the sintering time is 6 h-12 h.

[0267] In some embodiments, the sintering temperature of the first sintering may be 720° C., 730° C., 740° C., 750° C., 760° C., 770° C., 780° C. or any range therebetween.

[0268] In some embodiments, the sintering time of the first sintering may be selected as 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h or any range therebetween.

[0269] In some embodiments, the sintering temperature of the second sintering is 770° C.-830° C., and the sintering time is 6 h-12 h.

[0270] In some embodiments, the sintering temperature of the second sintering may be 770° C., 780° C., 790° C., 800° C., 810° C., 820° C., 830° C. or any range therebetween.

[0271] In some embodiments, the sintering time of the second sintering may be selected as 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h or any range therebetween.

[0272] In some embodiments, the grinding of the primary calcined product and then performing a second sintering comprises: adding a carbon source to the primary calcined product and then grinding the product separately, wherein the D of the first group of ground particles is V50 The D of the second group of ground particles is 1.40 μm-2.0 μm. V50The first group of ground particles and the second group of ground particles are mixed in a mass ratio of 30:70-70:30 to obtain mixed intermediate particles, and the mixed intermediate particles are sintered for a second time.

[0273] In some embodiments, the grinding of the primary calcined product and then performing a second sintering comprises: adding a carbon source to the primary calcined product and then grinding the product separately, wherein the D of the first group of ground particles is V50 The optional value may be 1.4 μm, 1.45 μm, 1.5 μm, 1.55 μm, 1.6 μm, 1.65 μm, 1.7 μm, 1.75 μm, 1.8 μm, 1.9 μm, 2 μm or any numerical range therebetween.

[0274] In some embodiments, the grinding of the primary calcined product and then performing a second sintering comprises: adding a carbon source to the primary calcined product and then grinding the product separately, wherein the D of the second group of ground particles is V50 The optional value may be 0.35 μm, 0.36 μm, 0.37 μm, 0.38 μm, 0.39 μm, 0.4 μm, 0.41 μm, 0.42 μm, 0.43 μm, 0.44 μm, 0.45 μm or any numerical range therebetween.

[0275] In some embodiments, the first group of ground particles and the second group of ground particles can be mixed at a mass ratio of 30:70, 40:60, 50:50, 60:40, 70:30 to obtain mixed intermediate particles. The second sintering process can effectively shorten the sintering time in the high-temperature range, thereby reducing the risk and probability of magnetic substances appearing during high-temperature sintering. By adjusting the particle size of the two groups of grinds during the second sintering process, the activity of the particles can be controlled, so that the positive electrode active material has large particles with a certain area ratio. While increasing the compaction density of the pole piece and improving the energy density of the lithium-ion secondary battery, the battery also has a low self-discharge level, so that the energy density of the lithium-ion secondary battery can be maintained for a long time during battery storage and circulation.

[0276] The fifth aspect of the present application provides a method for preparing a positive electrode sheet, which comprises sequentially adding a binder, a conductive agent, and a positive electrode active material prepared by the method of the fourth aspect, dry-mixing them, adding a solvent, and stirring to obtain a shipping slurry; transferring and coating the shipping slurry to at least one side of a current collector, and drying and hot pressing to obtain a positive electrode sheet.

[0277] In some embodiments, the hot pressing includes at least three hot roller pressings, and the hot roller pressure increases successively, and the hot roller pressure is 20-50 tons, 50-70 tons, and 70-90 tons respectively; the hot roller temperature is 40-80°C, and the pole piece is heated before entering the hot roller compaction for the first time, and the heating temperature is 40-50°C.

[0278] The embodiment of the present application adopts the above-mentioned hot pressing process in combination with the preparation method of the fourth aspect to prepare the positive electrode active material, which effectively improves the compaction density of the positive electrode plate while maintaining a low magnetic substance content, so that the battery has improved energy density while reducing self-discharge.

[0279] In some embodiments, the coating speed of the transfer coating is 1 m / min-25 m / min. In some embodiments, the coating speed of the transfer coating can be selected from 1 m / min, 2 m / min, 3 m / min, 4 m / min, 5 m / min, 6 m / min, 7 m / min, 8 m / min, 9 m / min, 10 m / min, 11 m / min, 12 m / min, 13 m / min, 14 m / min, 15 m / min, 16 m / min, 17 m / min, 18 m / min, 19 m / min, 20 m / min, 21 m / min, 22 m / min, 23 m / min, 24 m / min, 25 m / min or any range between the two.

[0280] The coating speed of transfer coating within the above range is beneficial to improving the uniformity of particle distribution during the coating process, reducing the risk of particle agglomeration in the positive electrode film layer, reducing the porosity of the positive electrode film cross section, further increasing the ultimate compaction density of the electrode sheet, and improving the energy density of the battery.

[0281] In addition, the present application also provides an electrical device, which includes at least one of the lithium-ion secondary battery, battery module, battery pack, and energy storage battery provided in the present application. The lithium-ion secondary battery, battery module, and battery pack can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0282] As the electrical device, a lithium-ion secondary battery, a battery module or a battery pack may be selected according to its usage requirements.

[0283] Figure 7The power consumption device is taken as an example. The power consumption device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. In order to meet the high power and high energy density requirements of the power consumption device for the secondary battery, a battery pack or a battery module can be used.

[0284] Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a secondary battery may be used as a power source.

[0285] Example Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If no specific techniques or conditions are indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. If the manufacturer is not indicated in the reagents or instruments used, they are all conventional products that can be obtained commercially.

[0286] Example 1 (1) Preparation of positive electrode active materials Add lithium carbonate, iron phosphate, titanium dioxide, glucose and polyethylene glycol into water, mix in a premixing tank, rotate at 1400 rpm, and demagnetize by a demagnetizing rod with a magnetic field strength of 10000 Gs, wherein the ratio of lithium carbonate and iron phosphate is such that the molar ratio of lithium to iron is 1.03:1.0, the mass content of glucose relative to the total amount of raw materials is 6%, and the mass content of polyethylene glycol relative to the total amount of raw materials is 5%, and after uniform mixing, a mixed raw material with a solid content of 40% is obtained; Among them, the number of magnetic particles in lithium carbonate is less than or equal to 500pcs / kg, and the particle size D V10 Greater than or equal to 1μm, particle size D V50 6μm, particle size D V90 Less than or equal to 40μm; the number of magnetic particles in iron phosphate is less than or equal to 95pcs / kg, and the morphology is spherical; the number of magnetic particles in glucose is less than or equal to 500pcs / kg; the molecular weight of polyethylene glycol is 1500, and the number of magnetic particles is less than or equal to 150pcs / kg.

[0287] The mixed raw materials were subjected to two grinding-demagnetization cycles in a sand mill. After coarse grinding for 1 hour, the coarsely ground raw materials were demagnetized using a permanent magnetic demagnetizer, and the demagnetization intensity was greater than or equal to 8000 Gs. The demagnetized raw materials were finely ground again, and the slurry temperature was controlled to be less than 40°C during the grinding process to obtain a mixed slurry. The solid phase particle size D in the mixed slurry V 50 is 0.35μm, and spray drying is performed to obtain a dry precursor powder. After drying, D50 is 55.0μm; the magnetic material is less than 70pcs / kg.

[0288] The precursor powder was sintered in a nitrogen atmosphere in two stages to obtain lithium iron phosphate positive electrode material: the temperature was increased from 25 °C to 450 °C at a heating rate of 2 °C / min (the first heating stage) and kept warm for 3 h; the temperature was increased from 450 °C to 780 °C at a heating rate of 5 °C / min (the second heating stage) and kept warm for 12 h; the ventilation volume in the heating stage was greater than that in the constant temperature stage, with a ratio of 1.5:1, and the total ventilation volume was 1350 cm 3 / h, and then cool down after finishing; D V 50 is 1.0μm-2.0μm carbon-coated lithium iron phosphate material.

[0289] The above D50, D V 50. D V 90 refers to the data obtained by Malvern laser scattering method test.

[0290] Based on the total mass of the positive electrode active material, the mass proportion of carbon element is 1.25%, and the mass proportion of titanium element is 4000ppm. The powder compaction density of the positive electrode active material under 3T pressure is 2.52 g / cm 3 The discharge capacity at room temperature and 1C discharge rate is 141.4 mAh / g. The discharge capacity of the positive electrode active material discharged to 3.2 V accounts for 92.5%.

[0291] (2) Preparation of positive electrode sheet: 2.2wt% PVDF, 0.8wt% conductive carbon black, and 97.0wt% positive electrode active material were added in sequence and dry-mixed, and then N-methylpyrrolidone was added, stirred, and the viscosity was adjusted to obtain a shipping slurry; the shipping slurry was transferred and coated on the bottom coating of the current collector aluminum foil, the bottom coating included carbon black and PVDF, the mass ratio of the two was 1:1, the distribution density of carbon-based particles with a particle size greater than 100nm in the bottom coating was ≤10pcs / 10μm, and the thickness of the bottom coating was 2μm. After drying and hot pressing, the single-side density was 350mg / 1540cm 2 The transfer coating speed is 20 m / min.

[0292] The hot pressing process includes three hot roller pressing processes. The hot roller pressing pressure increases successively, and the hot roller pressure is 40 tons, 60 tons, and 80 tons respectively; the hot roller temperature is 60°C, and the pole piece is heated before entering the hot roller compaction for the first time, and the heating temperature is 40°C.

[0293] The compaction density of the pole piece is the ultimate compaction density of the pole piece. The ultimate compaction density test method of the pole piece is as follows; the ultimate compaction density of the pole piece in this embodiment is 2.63 g / cm 3 .

[0294] In the cross section of the positive electrode film along the thickness direction of the pole piece, the area of ​​particles with a particle size of 1μm-5μm accounts for 30%, the average equivalent area of ​​particles with a particle size of 1μm or more in the cross section of the positive electrode film along the thickness direction of the pole piece accounts for 0.127%, and the area of ​​particles with a particle size of 50nm-200nm in the cross section of the positive electrode film along the thickness direction of the pole piece accounts for 6.34%. The mass content of magnetic substances in the positive electrode film is 20ppm, and the mass content of elemental iron is 0 (below the detection limit, recorded as 0). In the cumulative distribution curve of the graphitization degree C value obtained by the laser microscopic confocal Raman spectrometer in the surface scanning mode, the median graphitization degree C 50 is 1.00, and the median of the sphericity of the particles in the positive electrode film layer is L A50 It is 0.715, and the porosity of the cross section of the positive electrode film is 15.991%.

[0295] (3) Preparation of negative electrode sheet: 95.5 wt% of negative electrode active material (artificial graphite), 1.0 wt% of conductive agent (conductive carbon black), 2.0 wt% of binder (styrene-butadiene rubber (SBR)) and 1.5 wt% of thickener (sodium carboxymethyl cellulose (CMC)) were mixed, deionized water was added, stirred, and dispersed to form negative electrode slurry. Then the negative electrode slurry was coated on both sides of the Cu foil. After both sides were completed, it was dried, compacted, cut, and sliced ​​to prepare the negative electrode sheet. The density of the coated single side is 164 mg / 1540.25 cm 2 , compacted density is 1.60g / cm 3 .

[0296] (4) Preparation of isolation membrane Polypropylene film is used as the isolation film.

[0297] (5) Preparation of electrolyte In an argon atmosphere glove box (H 2 O<0.1ppm, O 2 <0.1ppm), mix the organic solvent ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1 / 1, add lithium salt LiPF 6 Dissolved in organic solvents, LiPF 6 The content in the solution is 1 mol / L, stir evenly to obtain an electrolyte.

[0298] (6) Preparation of batteries: The positive electrode sheet, the isolation film, and the negative electrode sheet are stacked in order. The isolation film should be able to isolate the positive and negative electrodes. The bare battery cell is obtained by winding, and the bare battery cell is placed in an outer package. The electrolyte is injected, and after packaging, formation, exhaust and other processes, a lithium-ion secondary battery is finally obtained.

[0299] Example 2 The preparation method of Example 2 is basically the same as that of Example 1, except that the sintering process of the positive electrode active material is different. Specifically: The precursor powder was sintered in a nitrogen atmosphere in two stages to obtain lithium iron phosphate positive electrode material: the temperature was increased from 25 °C to 450 °C at a heating rate of 2 °C / min (the first heating stage) and kept at this temperature for 3 h; the temperature was increased from 450 °C to 800 °C at a heating rate of 5 °C / min (the second heating stage) and kept at this temperature for 12 h; the ventilation volume in the heating stage was greater than that in the constant temperature stage, with a ratio of 1.5:1, and the total ventilation volume was 1350 cm 3 / h, and then cool down after finishing; D V 50 is 1.0μm-2.0μm carbon-coated lithium iron phosphate material.

[0300] Example 3 The preparation method of Example 3 is basically the same as that of Example 1, except that the sintering process of the positive electrode active material is different. Specifically: The precursor powder was sintered in a nitrogen atmosphere in two stages to obtain lithium iron phosphate positive electrode material: the temperature was increased from 25 °C to 450 °C at a heating rate of 2 °C / min (the first heating stage) and kept warm for 3 h; the temperature was increased from 450 °C to 820 °C at a heating rate of 5 °C / min (the second heating stage) and kept warm for 12 h; the ventilation volume in the heating stage was greater than that in the constant temperature stage, with a ratio of 1.5:1, and the total ventilation volume was 1350 cm 3 / h, and then cool down after finishing; D V 50 is 1.0μm-2.0μm carbon-coated lithium iron phosphate material.

[0301] Example 4 The preparation method of Example 4 is basically the same as that of Example 1, except that the added content of the titanium source during the preparation of the positive electrode active material is adjusted so that the mass content of the titanium element is 2500 ppm based on the mass of the positive electrode active material.

[0302] Example 5 The preparation method of Example 5 is basically the same as that of Example 1, except that the added content of the titanium source during the preparation of the positive electrode active material is adjusted so that the mass content of the titanium element is 1500 ppm based on the mass of the positive electrode active material.

[0303] Example 6 The preparation method of Example 6 is basically the same as that of Example 1, except that the preparation process of the positive electrode active material is slightly different. The differences specifically include the following two points: (1) The carbon source in the mixed raw material is only glucose, and the mass of glucose is 5.7wt% compared to the mass of iron phosphate; (2) The temperature rise sintering process is different. The precursor powder is sintered at least twice in a nitrogen atmosphere, with the first sintering temperature at 750°C and the holding time at 8 hours to obtain a primary sintered product.

[0304] 1.5 wt% (based on the mass of the primary calcined product) of glucose, 3.0 wt% (based on the mass of the primary calcined product) of polyethylene glycol and a titanium source were added to the primary calcined product, and after being evenly ground, the product was divided into two groups for secondary grinding. The grinding parameters of the two groups were different, and the D of the particles after the first group of grinding was controlled. V 50 is 2.0 μm, and the D of the second group of ground particles is V 50 is 0.35 μm. The ground particles of the first group and the second group are mixed in a mass ratio of 30:70, spray-dried, and sintered for the second time. The temperature of the second sintering is 800°C, and the temperature is kept for 10 hours.

[0305] The ratio of the titanium element in the titanium source in the mixed raw material to the titanium element in the titanium source added to the primary calcined product is 5:2. Based on the total mass of the positive electrode active material, the mass proportion of the titanium element is 6000ppm.

[0306] Example 7 The preparation method of Example 7 is basically the same as that of Example 6, except that D of the first group of ground particles V 50 is 1.50 μm, and the D of the second group of ground particles is V 50 is 0.40 μm; the ground particles of the first group and the second group are mixed in a mass ratio of 70:30, spray-dried, and sintered for the second time.

[0307] Comparative Example 1 The preparation methods of Comparative Example 1 and Example 1 are basically the same, except that in the preparation process of the positive electrode active material, the following methods are different: The mixed raw materials were subjected to two grinding-demagnetization cycles in a sand mill. After coarse grinding for 1 hour, the coarsely ground raw materials were demagnetized using a permanent magnetic demagnetizer, and the demagnetization intensity was greater than or equal to 8000 Gs. The demagnetized raw materials were finely ground again, and the slurry temperature was controlled to be less than 40°C during the grinding process to obtain a mixed slurry. The solid phase particle size D in the mixed slurry V 50 is 0.5 μm, and spray drying is performed to obtain a dry precursor powder.

[0308] The precursor powder was sintered in a nitrogen atmosphere in two stages to obtain lithium iron phosphate positive electrode material: the temperature was increased from 25 °C to 450 °C at a heating rate of 2 °C / min (the first heating stage) and kept warm for 3 h; the temperature was increased from 450 °C to 765 °C at a heating rate of 5 °C / min (the second heating stage) and kept warm for 12 h; the ventilation volume in the heating stage was greater than that in the constant temperature stage, with a ratio of 1:1, and the total ventilation volume was 1350 cm 3 / h, and then cool down after finishing; D V 50 is 1.0μm-2.0μm carbon-coated lithium iron phosphate material.

[0309] Comparative Example 2 The preparation method of Comparative Example 2 is basically the same as that of Example 1, except that in the preparation process of the positive electrode active material, the following methods are different: Add lithium carbonate, iron phosphate, titanium dioxide, glucose and polyethylene glycol into water, mix in a premixing tank, rotate at 1400 rpm, and demagnetize with a demagnetizing rod with a magnetic field strength of 8000 Gs-12000 Gs, wherein the ratio of lithium carbonate and iron phosphate is such that the molar ratio of lithium to iron is 1.05:1.0, the mass content of glucose relative to the total amount of raw materials is 6%, and the mass content of polyethylene glycol relative to the total amount of raw materials is 5%, and after uniform mixing, a mixed raw material with a solid content of 40% is obtained; The mixed raw materials were subjected to two grinding-demagnetization cycles in a sand mill. After coarse grinding for 1 hour, the coarsely ground raw materials were demagnetized using a permanent magnetic demagnetizer, and the demagnetization intensity was greater than or equal to 8000 Gs. The demagnetized raw materials were finely ground again, and the slurry temperature was controlled to be less than 40°C during the grinding process to obtain a mixed slurry. The solid phase particle size D in the mixed slurry V 50 is 0.35μm, and spray drying is performed to obtain a dry precursor powder. After drying, D50 is 50-60μm; the magnetic material is less than 70pcs / kg.

[0310] The precursor powder was sintered in a nitrogen atmosphere in two stages to obtain lithium iron phosphate positive electrode material: the temperature was increased from 25 °C to 450 °C at a heating rate of 2 °C / min (the first heating stage) and kept at this temperature for 3 h; the temperature was increased from 450 °C to 820 °C at a heating rate of 5 °C / min (the second heating stage) and kept at this temperature for 12 h; the ventilation volume ratio of the heating stage to the constant temperature stage was 1:1, and the total ventilation volume was 900 cm 3 / h, and then cool down after finishing; D V 50 is 1.0μm-2.0μm carbon-coated lithium iron phosphate material.

[0311] Performance Testing The self-discharge K value test is at 25℃. Charge to 3.0V with a constant current of 0.05C, then charge to 0.05C with a constant voltage. After standing at 25℃ for 24h, test the open circuit voltage V1 in V. After standing for another 24h, test the open circuit voltage V2 again in V. The self-discharge K value is 1000×(V1-V2) / 48, in mV / h.

[0312] DCR test at 25℃, after charging to 3.65V at 0.33C constant current, charging to 0.05C at constant voltage, then discharging to 20% SOC at 0.33C, standing for 5min, discharging with 3C pulse for 30s, standing for 40s, charging at 3C for 40s, standing for 5min, charging to 3.65V at 0.33C constant current, charging to 0.05C at constant voltage, then discharging to 10% SOC at 0.33C, standing for 5min, discharging with 3C pulse for 30s, standing for After standing for 40s, charge at 3C for 40s, stand for 5min, then fully charge at 0.33C, then discharge to 50% SOC at 0.33C, then stand at -25℃ for 2h, then pulse discharge at 1C for 30s, stand for 10min, then stand at 25℃ for 2h, charge to 3.65V at 0.33C, then charge to 0.05C at constant voltage, then discharge to 20% SOC at 0.33C, then stand at -25℃ for 2h, then pulse discharge at 1C for 30s, stand for 10min.

[0313] The voltage was recorded before and after each pulse discharge, and the DCR under different conditions was calculated. The calculation formula was DCR = (voltage before pulse discharge after standing still - voltage after pulse discharge) / pulse current.

[0314] 3. Pole piece ultimate compaction density test The double-sided coated pole piece is compacted by a roller press, and the elongation of the pole piece after compaction is tested, and the flexibility of the pole piece after compaction is evaluated. By increasing the pressure of the roller press, pole pieces with different compaction densities will be obtained. As the pressure increases, the compaction density of the pole piece increases, the elongation of the pole piece increases, and the flexibility of the pole piece decreases. Too high an elongation of the pole piece can easily cause the pole piece to warp, and too low a flexibility of the pole piece can easily cause the pole piece to break brittlely. Therefore, the smaller of the compaction density corresponding to the elongation of the pole piece of 8% or the number of times the pole piece's flexibility is folded is defined as the limit compaction density of the pole piece.

[0315] The compacted density is calculated by the mass of the positive electrode film layer / the volume of the positive electrode film layer.

[0316] 4. Elongation test Lay the electrode flat on a horizontal table and cut it into sections, with each electrode being about 100 cm long; remove the copper foil of the substrate at the edge of the electrode, and pay attention to keeping the cut edge of the electrode parallel to the MD direction of the electrode (perpendicular to the direction of the pressing roller), ensuring that the electrode part is completely covered by the coating, and use a steel ruler to measure the length between the marking points at the same position of the length and width at the head and tail of the electrode, and estimate it to 0.1 mm, and record the length before compaction; after compaction, record the length between the corresponding marking points after compaction, and use (length after compaction - length before compaction) / length before compaction as the elongation rate of the electrode.

[0317] 5. Test of flexible folding times Cut the positive electrode sheet into 20×100mm 2 Fold the specimen in the forward direction, flatten it with a 2kg roller, unfold it and check against the light to see if there is light transmittance between the gaps. If not, fold it in the reverse direction, flatten it with a 2kg roller, and check against the light again. Repeat this process until light transmittance between the gaps. Record the number of folds. Repeat the test three times and take the average value as the reference data for the flexibility of the electrode.

[0318] Test Results Table 1

[0319] * In the examples, 0 means that the content is below the detection limit and it is difficult to identify its exact content.

[0320] From the comparison between the embodiment and the comparative example, it can be seen that in the cross-section of the positive electrode film layer along the thickness direction of the pole piece, the area of ​​particles with a particle size greater than or equal to 1 μm accounts for 30%-50%; and in the positive electrode film layer, the mass proportion of magnetic substances is greater than or equal to 20 ppm and less than or equal to 1980 ppm, which helps the lithium-ion secondary battery to achieve a high pole piece compaction density while allowing the battery to have a low self-discharge K value, so that the battery has both high energy density and good storage performance.

[0321] When the area of ​​particles with a diameter of 1μm-5μm in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet accounts for 30%-45%, the lithium-ion secondary battery can achieve a high electrode sheet compaction density while having a low self-discharge K value, while also having good dynamic performance and high capacity.

[0322] Based on the total mass of the positive electrode active material, when the mass content of titanium element is 2500ppm-8000ppm, the surface inertness of the titanium source can reduce the activity of the raw material, reduce the probability of local chemical reaction unevenness and the generation of high-content magnetic substances, and at the same time help to control the average equivalent area ratio of particles larger than 1μm, so as to achieve a balance between storage stability and kinetic performance of lithium-ion secondary batteries.

[0323] From the comparison between Examples 1, 2, 6, and 7 and other examples, it can be seen that when the mass content of the magnetic substance in the positive electrode film layer is 20-200ppm, it helps the lithium-ion secondary battery to have a low self-discharge K value while maintaining a high pole piece compaction density. The lithium-ion secondary battery further improves the kinetic performance while having both high energy density and good storage performance.

[0324] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A lithium ion secondary battery, characterized in that: Including positive electrode sheet, negative electrode sheet and electrolyte, The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one side of the positive electrode current collector. The positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes lithium-containing transition metal phosphate particles with a carbon coating material disposed on at least a portion of the surface. In the cross section of the positive electrode film along the thickness direction of the electrode sheet, the area of ​​particles with a particle size greater than or equal to 1 μm accounts for 30.0%-50.0%; and The mass proportion of the magnetic substance in the positive electrode film layer is greater than or equal to 20 ppm and less than or equal to 1980 ppm.

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 area of ​​particles with a particle size of 1 μm-5 μm accounts for 30.0%-50.0%.

3. The lithium ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size of 1 μm-5 μm accounts for 30.0%-45.0%.

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 equivalent area of ​​particles with a particle size of 1 μm or more accounts for 0.05%-0.20%.

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 area of ​​particles with a particle size of 50nm-200nm accounts for 3.0%-15.0%.

6. The lithium ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size of 50nm-200nm accounts for 5.0%-12.0%.

7. The lithium ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the area of ​​particles with a particle size of 50nm-200nm accounts for 5.0%-10.0%.

8. The lithium ion secondary battery according to claim 1, characterized in that: In the positive electrode film layer, the mass proportion of the magnetic substance is less than or equal to 300 ppm.

9. The lithium ion secondary battery according to claim 8, characterized in that: In the positive electrode film layer, the mass proportion of the magnetic substance is 20ppm-200ppm.

10. The lithium ion secondary battery according to claim 1, characterized in that: The magnetic material includes one or more of elemental iron, Fe2P, FeP, γ-Fe2O3, and Fe2P2O7.

11. The lithium ion secondary battery according to claim 10, characterized in that: In the positive electrode film layer, the mass content of elemental iron is less than 20 ppm.

12. The lithium ion secondary battery according to claim 11, characterized in that: In the positive electrode film layer, the mass content of elemental iron is less than or equal to 15 ppm.

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

1.

14. 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 1500ppm-8000ppm based on the total mass of the positive electrode active material.

15. The lithium ion secondary battery according to claim 14, characterized in that: The positive electrode active material includes titanium element, and the mass content of the titanium element is 2500ppm-8000ppm based on the total mass of the positive electrode active material.

16. The lithium ion secondary battery according to claim 14, characterized in that: The positive electrode active material includes titanium element, and the mass content of the titanium element is 2500ppm-6000ppm based on the total mass of the positive electrode active material.

17. The lithium ion secondary battery according to claim 1, characterized in that: Based on the total mass of the positive electrode active material, the mass proportion of carbon element is 0.9%-1.8%.

18. 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 Greater than or equal to 0.9 and less than or equal to 1.3, wherein the graphitization degree C value is I G / I D , where I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at .

19. The lithium ion secondary battery according to claim 18, 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 0.99-1.

2.

20. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of particle sphericity area 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 0.60-0.

85.

21. The lithium ion secondary battery according to claim 20, characterized in that: In the cumulative distribution curve of particle sphericity area 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 0.65-0.

80.

22. 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 2.48 g / cm 3 -2.76g / cm 3 .

23. The lithium ion secondary battery according to claim 22, characterized in that: The powder compaction density of the positive electrode active material under 3T pressure is 2.58g / cm 3 -2.76g / cm 3 .

24. 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.

25. 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.

26. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode film layer also includes a binder and a conductive agent. Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 94%-99.4%, the mass content of the binder is 0.5%-3%, and the mass content of the conductive agent is 0.1%-3%.

27. 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 .

28. 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 2.43 g / cm 3 -2.78g / cm 3 .

29. 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 2.50 g / cm 3 -2.75g / cm 3 .

30. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode film layer satisfies at least one of the following conditions: (1) When the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.43 g / cm 3 -2.78g / cm 3 , in a cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10%-28%; (2) When the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.50 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%.

31. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode plate includes a primer layer, which is disposed between the positive electrode film layer and the positive electrode current collector; the primer layer satisfies at least one of the following conditions: (1) The primer layer includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the primer layer is ≤10 pcs / 10 μm; (2) The compaction density of the positive electrode sheet in the fully charged state is greater than or equal to 2.4 g / cm 3 , the thickness of the primer layer on one side is 1 μm-4 μm; (3) The compaction density of the positive electrode sheet in the fully charged state is greater than or equal to 2.5 g / cm 3 The single-side thickness of the primer layer is 2 μm-4 μm.

32. A battery device, characterized in that: A lithium-ion secondary battery comprising any one of claims 1 to 31, wherein the battery device comprises at least one of a battery module, a battery pack, and an energy storage battery.

33. 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 31.

34. A method for preparing a positive electrode active material, characterized in that: A mixed raw material comprising a carbon source, a lithium source, an iron source, and a phosphorus source is obtained, wherein the molar ratio of lithium to iron in the mixed raw material is greater than 1 and less than 1.05; a mixed slurry is obtained after grinding, wherein the solid phase volume distribution particle size D V50 The mixed slurry is dried to obtain a precursor powder; the precursor powder is sintered to obtain the positive electrode active material used in the lithium ion secondary battery according to any one of claims 1 to 31, wherein the sintering is carried out in an inert gas environment and the total gas flow rate during the sintering process is 1100m 3 / h-1400m 3 / h; the sintering includes a heating interval and a constant temperature interval, the inert gas introduction rate v1 in the heating interval is higher than the inert gas introduction rate v2 in the constant temperature interval; the temperature of the constant temperature interval of the sintering includes 770℃-830℃; the positive electrode active material includes lithium-containing transition metal phosphate particles with carbon coating material provided on at least part of the surface.

35. A method for preparing a positive electrode sheet, characterized in that: The preparation method comprises: A binder, a conductive agent, and a positive electrode active material prepared by the preparation method according to claim 34 are added in sequence, dry-mixed, and then a solvent is added, and the mixture is stirred to obtain a shipping slurry; the shipping slurry is transfer-coated to at least one side of the positive electrode current collector, and after drying and hot pressing, a positive electrode sheet for a lithium-ion secondary battery according to any one of claims 1 to 31 is obtained.

36. The preparation method according to claim 35, characterized in that: The hot pressing includes at least three hot roller pressings, and the hot roller pressure increases successively, and the hot roller pressure is 20 tons-50 tons, 50 tons-70 tons, and 70 tons-90 tons respectively; the hot roller temperature is 40℃-80℃, and the pole piece is heated before entering the hot roller compaction for the first time, and the heating temperature is 40℃-50℃.

37. The preparation method according to claim 35, characterized in that: The transfer coating speed is 1 m / min-25 m / min.

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