Lithium-ion secondary battery, battery device, power-consuming device, method for producing positive electrode active material, and method for producing positive electrode sheet

Abstract

The application 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 sheet. The lithium ion secondary battery comprises a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer comprises a positive electrode active material, the positive electrode active material comprises lithium-containing transition metal phosphate particles provided with a carbon coating material on at least part of the surfaces, in a section of the positive electrode film layer along the thickness direction of the sheet, the area proportion of the particles with a particle size greater than or equal to 1 mu m is 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.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims priority to PCT Patent Application PCT / CN2025 / 085902 entitled "Lithium Ion Secondary Battery, Battery Device, Electric Device, Method for Preparing Positive Electrode Active Material, and Method for Preparing Positive Electrode Sheet" filed on March 28, 2025, the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

[0003] 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 electric device, a method for preparing a positive electrode active material, and a method for preparing a positive electrode sheet. BACKGROUND

[0004] In recent years, lithium ion secondary batteries are widely used in energy storage power systems such as hydroelectric, thermal, wind, and solar power stations, as well as in multiple fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, etc.

[0005] With the increasing demand for the endurance and safety of electric devices, higher requirements are placed on the energy density and storage performance of lithium ion secondary batteries. However, it is difficult to simultaneously improve the above-mentioned performances in the prior art, which is a technical problem that needs to be solved in the field. SUMMARY

[0006] The present application is made in view of the above-mentioned problems, and aims to provide a lithium ion secondary battery with high energy density and good storage performance.

[0007] A first aspect of the present application provides a lithium ion secondary battery, which includes a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein 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, the positive electrode active material includes lithium-containing transition metal phosphate particles with carbon-coated materials disposed on at least part of the surfaces, in a cross-section of the positive electrode film layer along the thickness direction of the sheet, the area proportion of particles with a particle size greater than or equal to 1 μm is 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.

[0008] The embodiment of the present application can effectively reduce the content of magnetic substances in the positive electrode film layer while improving the area ratio of large-size particles, so that the mass ratio 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, the compaction density of the pole piece in the lithium ion secondary battery is improved, and the self-discharge rate is kept low, which is beneficial to the improvement of the energy density of the battery and the long-term maintenance during storage.

[0009] In any embodiment, the area ratio of particles with a particle size of 1-5 μm in the cross section of the positive electrode film layer along the thickness direction of the pole piece is 30.0%-50.0%.

[0010] The area ratio of particles with a particle size of 1-5 μm in the cross section of the positive electrode film layer along the thickness direction of the pole piece is within the above range, which can control the content of magnetic substances in the battery while improving the compaction density of the pole piece through grading, and is beneficial to the improvement of the energy density of the lithium ion secondary battery and the long-term maintenance during storage.

[0011] In any embodiment, the area ratio of particles with a particle size of 1-5 μm in the cross section of the positive electrode film layer along the thickness direction of the pole piece is 30.0%-45.0%.

[0012] The area ratio of particles with a particle size of 1-5 μm in the cross section of the positive electrode film layer along the thickness direction of the pole piece 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 in the particle interior, thereby keeping the lithium ion secondary battery at a low impedance and improving the kinetic performance of the battery.

[0013] In any embodiment, 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 is 0.05%-0.20%.

[0014] 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 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 the kinetic performance of the lithium ion secondary battery is not seriously deteriorated due to the excessively large particle size of the large particles, and the energy density and kinetic performance of the battery are taken into account.

[0015] In any embodiment, the area ratio of particles with a particle size of 50-200 nm in the cross section of the positive electrode film layer along the thickness direction of the pole piece is 3.0%-15.0%, optionally 5.0%-12.0%, and further optionally 5.0%-10.0%.

[0016] The particle area ratio of the particles with a particle size of 50 nm-200 nm is within the above range, which means that there is a certain amount of particles with a particle size of 50 nm-200 nm, which is beneficial to improving the powder compaction density of the positive active material and the compaction density of the electrode sheet through grading, and further improving the energy density of the lithium ion secondary battery.

[0017] In any embodiment, the mass content of the magnetic substance in the positive electrode film layer is less than or equal to 300 ppm, which can be optionally 20-200 ppm.

[0018] 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.

[0019] In any embodiment, the magnetic substance includes one or more of Fe, Fe2P, FeP, γ-Fe2O3, and Fe2P2O7.

[0020] In any embodiment, the mass content of elemental iron in the positive electrode film layer is less than 20 ppm, which can be optionally less than or equal to 15 ppm.

[0021] Controlling the mass content of elemental iron within the above range is beneficial to improving the safety performance of the battery.

[0022] 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, Br, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.1.

[0023] Selecting an appropriate modification element Q can improve the lattice change rate of the positive active material during lithium extraction, reduce the oxygen activity on the particle surface, improve the structural stability of the material, and further improve the specific capacity level of the material, thereby further improving the energy density of the lithium ion secondary battery.

[0024] In any embodiment, the positive active material includes a titanium element, and the mass content of the titanium element is 1500 ppm-8000 ppm based on the total mass of the positive active material, which can be optionally 2500 ppm-8000 ppm, and further optionally 2500 ppm-6000 ppm.

[0025] The positive electrode active material includes titanium element and the mass content is controlled within the above range, which on the one hand plays a role of inhibiting and blocking the growth of particles, achieves the purpose of controlling the size of large particles, so that 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 is within a suitable range; on the other hand, the surface inertness thereof reduces the probability of uneven local chemical reaction of raw materials and the generation of magnetic substances. At the same time, the doping of titanium element in the positive electrode active material can improve the electronic conductivity and ion transmission rate of lithium-containing transition metal phosphate, and alleviate the negative impact of particles with relatively large particle size on the kinetic performance of the positive electrode active material. By affecting the particle size and lithium ion transmission path, the battery energy density and kinetic performance are balanced.

[0026] In any embodiment, the mass percentage of carbon element based on the total mass of the positive electrode active material is 0.9%-1.8%.

[0027] The mass percentage of carbon element based on the total mass of the positive electrode active material within the above range can improve the electrical conductivity of the positive electrode active material and improve the kinetic performance of the lithium ion secondary battery, and can also reduce the negative impact of excessive carbon content on the load of lithium-containing transition metal phosphate, balance the compaction density of the pole piece and the impedance of the lithium ion secondary battery, and improve the energy density and kinetic performance of the battery.

[0028] In any embodiment, in the cumulative distribution curve of the graphitization degree C value of the positive electrode film layer obtained under the face scanning mode of the laser microscope confocal Raman spectrometer, the median number C of the graphitization degree is greater than or equal to 0.9 and less than or equal to 1.3, which can be 0.99-1.2. 50 The graphitization degree C value is I G / I D , wherein I G represents the G peak intensity of the Raman spectrum at 1580±100 cm -1 , and I D represents the D peak intensity of the Raman spectrum at 1350±100 cm -1 .

[0029] The positive electrode active material with the graphitization degree within the above range can easily realize particle slipping in the process of rolling into a film by means 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 pole piece, and further improve the compaction density of the positive electrode film layer by means of particle slipping.

[0030] In any embodiment, in the cumulative distribution curve of the particle sphericity area of the cross section of the positive electrode film layer along the thickness direction of the pole piece, the median number L A50 of the sphericity is 0.60-0.85, which can be 0.65-0.80.

[0031] Median of sphericity L A50 The particles in the above range are approximately spherical, and the particles are prone to slip between particles under the action of external force, which can further improve the compaction density of the pole piece and increase the energy density of the battery.

[0032] In any embodiment, the powder compaction density of the positive electrode active material under 3T pressure is 2.48g / cm 3 -2.76g / cm 3 , and optionally 2.58g / cm 3 -2.76g / cm 3 .

[0033] The positive electrode active material has high powder compaction density, which provides a material basis for improving the compaction density of the pole piece and preparing high-energy-density lithium ion secondary batteries.

[0034] In any embodiment, the powder compaction density of the positive electrode active material under 3T pressure is 2.58g / cm 3 -2.76g / cm 3 .

[0035] The positive electrode active material with compaction density in the above range can further improve the compaction density of the pole piece and increase the energy density of the battery.

[0036] In any embodiment, the discharge gram capacity of the positive electrode active material at room temperature at a discharge rate of 1C is 135mAh / g-150mAh / g.

[0037] The positive electrode active material has high discharge gram capacity, indicating that the positive electrode active material has good kinetic performance and is conducive to improving the energy density of the lithium ion secondary battery.

[0038] In any embodiment, the discharge capacity ratio η of the positive electrode active material discharged to 3.2V is ≥85%, and the η is defined as follows: at room temperature, a button cell containing the positive electrode active material is charged and discharged twice in a voltage range of 2.0V-3.75V at a rate of 0.1C, and then charged and discharged once at a rate of 1C, in the charge and discharge test at the rate of 1C, the capacity value at the discharge voltage of 3.2V is extracted as C1, and the capacity value at the discharge voltage of 2.0V is extracted as C2, η=C1 / C2, wherein the charging process includes constant-voltage charging, constant-voltage 3.75V, and constant-voltage cutoff current 50μA.

[0039] The high proportion of the discharge capacity of the positive electrode active material in the battery to 3.2V in the embodiments of the present application means that the positive electrode active material has good kinetic performance although it has a certain proportion of large-size particles. Meanwhile, the high η value means that the lithium ion secondary battery containing the positive electrode active material still has a high voltage when discharged to a low state of charge (SOC), which is beneficial to maintaining good power performance.

[0040] In any embodiment, the positive electrode film layer further comprises a binder and a conductive agent, 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%, based on the total mass of the positive electrode film layer.

[0041] In any embodiment, the single-sided area density of the positive electrode film layer is 300mg / 1540mm 2 -450mg / 1540mm 2 .

[0042] The positive electrode film layer with the area density in the above range can help to improve the energy density of the lithium ion secondary battery.

[0043] In any embodiment, the compaction density of the positive electrode film layer of the lithium ion secondary battery in the full discharge state is 2.43g / cm 3 -2.78g / cm 3 .

[0044] In any embodiment, the compaction density of the positive electrode film layer of the lithium ion secondary battery in the full discharge state is 2.50g / cm 3 -2.75g / cm 3 .

[0045] In any embodiment, the compaction density of the positive electrode film layer of the lithium ion secondary battery in the full discharge state is 2.43g / cm 3 -2.78g / cm 3 , and the porosity of the positive electrode film layer in the section of the positive electrode film layer along the thickness direction of the electrode sheet is 10%-28%.

[0046] In any embodiment, the compaction density of the positive electrode film layer of the lithium ion secondary battery in the full discharge state is 2.5g / cm 3 -2.78g / cm 3 , and the porosity of the positive electrode film layer in the section of the positive electrode film layer along the thickness direction of the electrode sheet is 10%-22%.

[0047] The lower the porosity, on the one hand, means that the positive film layer has a more optimal large, medium and small particle gradation, and has a high compaction density. On the other hand, if the porosity is low under the same gradation and rolling pressure, it means that the particles are easy to slide relative to each other, thereby reducing the risk of overpressure and stress concentration of the positive film layer, further reducing the probability of positive film sheet demolding during long cycle process, and being beneficial to improve the long cycle performance of the battery. In any embodiment, the positive electrode sheet includes a bottom coating layer, the bottom coating layer is arranged between the positive film layer and the current collector; the bottom coating layer includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating layer is ≤10 pcs / 10 μm.

[0048] The bottom coating layer is beneficial to improve the conductivity and adhesion of the positive film layer and the current collector, reduce the demolding of the positive film layer from the current collector during the cycle process, and improve the dynamic performance of the battery. In the high compaction density electrode sheet of the embodiments of the present application, for example, the compaction density of the positive electrode sheet under full discharge state is greater than or equal to 2.4 g / cm 3 When the current collector is easy to be damaged during the compaction process of the electrode sheet with high pressure, and large-size particles are easy to produce pits on the current collector, controlling the distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating layer to be ≤10 pcs / 10 μm is beneficial to reduce the probability of damage of the current collector in the high pressure compaction electrode sheet, and further improve the limit compaction density of the positive electrode sheet.

[0049] In any embodiment, the positive electrode sheet includes a bottom coating layer, the bottom coating layer is arranged between the positive film layer and the current collector; the compaction density of the positive electrode sheet under full discharge state is greater than or equal to 2.4 g / cm 3 , and the single-sided thickness of the bottom coating layer is 1 μm-4 μm.

[0050] In any embodiment, the positive electrode sheet includes a bottom coating layer, the bottom coating layer is arranged between the positive film layer and the current collector; the compaction density of the positive electrode sheet under full discharge state is greater than or equal to 2.5 g / cm 3 , and the single-sided thickness of the bottom coating layer is 2 μm-4 μm.

[0051] With the increase of the compaction density of the electrode sheet, the extrusion effect of the lithium-containing phosphate material (for example, the particle size is greater than 1 μm) of the large particles in the positive film layer on the bottom coating layer is more significant. Therefore, stress concentration is easy to occur at the site of large particles, and even damage the current collector by penetrating through the bottom coating layer. Increasing the thickness of the bottom coating layer is beneficial to improve the stress concentration phenomenon in the electrode sheet, and further improve the limit compaction density of the electrode sheet.

[0052] The second aspect of the present application provides a battery device, including at least one of a battery module, a battery pack, and an energy storage battery provided by the first aspect of the present application.

[0053] The third aspect of the present application also provides a power utilization device, which comprises the lithium ion secondary battery provided by the first aspect of the present application or the battery device provided by the second aspect of the present application.

[0054] The fourth aspect of the present application also provides a preparation method of a positive electrode active material: obtaining a mixed raw material comprising 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 DV50 in the mixed slurry is 0.3 μm-0.4 μm; obtaining a precursor powder after drying the mixed slurry; and obtaining a positive electrode active material by sintering the precursor powder, wherein the sintering is performed in an inert gas environment, and the total gas flow rate during the sintering is 1100 m 3 / h-1400 m 3 / h; the sintering comprises a temperature rising interval and a constant temperature interval, the inert gas flow rate v1 in the temperature rising interval is higher than the inert gas flow rate v2 in the constant temperature interval; the constant temperature interval temperature of the sintering comprises 770℃-830℃; and the positive electrode active material comprises lithium-containing transition metal phosphate particles with at least part of the surfaces provided with a carbon-coated layer material.

[0055] The positive electrode active material prepared by the method has the advantages that the positive electrode film layer has a certain area ratio of particles with a size of more than 1 μm in the section along the thickness direction of the electrode sheet, and the positive electrode active material contains a small amount of magnetic substances, so that the energy density of the lithium ion secondary battery is improved, and the battery 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.

[0056] The fifth aspect of the present application provides a preparation method of a positive electrode sheet, which comprises sequentially adding a binder, a conductive agent and the positive electrode active material prepared by the method of the fourth aspect into a solvent for dry mixing, stirring to obtain a delivery slurry, transferring and coating the delivery slurry to at least one side of a current collector, and drying and hot pressing to obtain a positive electrode sheet.

[0057] In any embodiment, the hot pressing comprises at least three times of hot rolling, and the hot rolling pressure increases sequentially, and the hot rolling pressure is 20 tons-50 tons, 50 tons-70 tons and 70 tons-90 tons, respectively; the hot rolling temperature is 40℃-80℃, and the electrode sheet is heated before being pressed for the first time, and the heating temperature is 40℃-50℃.

[0058] The positive electrode active material prepared by the hot pressing process and the preparation method of the fourth aspect has the advantages that the content of magnetic substances is low, the compaction density of the positive electrode sheet is effectively improved, and the battery has a low self-discharge and improved energy density.

[0059] In any embodiment, the transfer coating has a coating speed of 1 m / min to 25 m / min.

[0060] The transfer coating has a coating speed within the above range, which is beneficial to improve the uniformity of particle distribution in the coating process, reduce the risk of particle agglomeration in the positive electrode film layer, reduce the cross-section porosity of the positive electrode film layer, further improve the ultimate compaction density of the electrode sheet, and improve the energy density of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0061] 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 sheet according to an embodiment of the present application;

[0062] Figure 2 is a schematic diagram of a lithium ion secondary battery according to an embodiment of the present application;

[0063] Figure 3 is a schematic diagram of a lithium ion secondary battery according to an embodiment of the present application;

[0064] Figure 4 is a schematic diagram of a battery module according to an embodiment of the present application;

[0065] Figure 5 is a schematic diagram of a battery pack according to an embodiment of the present application;

[0066] Figure 6 is a schematic diagram of a battery pack 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;

[0067] Figure 7 is a schematic diagram of a battery pack according to an embodiment of the present application;

[0068] Figure 8 is a schematic diagram of a battery pack according to an embodiment of the present application;

[0069] BRIEF DESCRIPTION OF DRAWINGS

[0070] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 lithium ion secondary battery; 51 housing; 52 electrode assembly; 53 top cover assembly. DETAILED DESCRIPTION

[0071] Hereinafter, specific embodiments of the lithium-ion secondary battery, the battery device, the power-consuming device, the method for producing a positive electrode active material, and the method for producing a positive electrode sheet of the present application will be described in detail with appropriate reference to the accompanying drawings. However, there will be cases where unnecessary detailed description is omitted. For example, there will be cases where detailed description of matters that are already well known, repeated description of substantially identical structures are omitted. This is to avoid the following description becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided so that those skilled in the art can fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0072] The ranges disclosed herein are defined by their lower and upper limits. Ranges that include both endpoints are inclusive of the endpoints. Ranges that exclude both endpoints are not inclusive of the endpoints. Ranges that include one or both endpoints are inclusive of the endpoint(s) and ranges that exclude one or both endpoints are not inclusive of the endpoint(s). Ranges are defined by their lower and upper limits. Unless specifically stated otherwise, the use of a range of values for a parameter includes each and every value and sub-range within the range. Exemplary values for physical parameters are shown in the examples section herein. Unless otherwise stated, the use of a range of values for a parameter, for example, a chemical moiety, includes every value and sub-range within the range. Exemplary values for physical parameters are shown in the examples section herein. Exemplary ranges include, but are not limited to, 0-5, 0-4, 0-3, 0-2, 0-1, 0.1-5, 0.1-4, 0.1-3, 0.1-2, 0.1-1, 1-5, 1-4, 1-3, 1-2, 2-5, 2-4, 2-3, 3-5, 3-4, 4-5, 5-10, 5-9, 5-8, 5-7, 7-10, 7-9, 7-8, 8-10, 8-9, 9-10, and 10-20. It is specifically intended that the scope of each element includes at least the specific value and sub-range within the range. It is also specifically intended that the scope of each element includes the scope of at least the other embodiments specifically discussed herein.

[0073] If not specifically explained, all the embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of the present application.

[0074] If not specifically explained, all the technical features and optional technical features of the present application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of the present application.

[0075] If not otherwise specified, all steps of the present application can be performed in sequence or randomly, preferably in sequence. For example, the method comprising steps (a) and (b) means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method mentioned above can further comprise step (c), which means that step (c) can be added to the method in any sequence, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0076] In the present application, the term "a plurality of" or "a plurality" means two or more.

[0077] Unless otherwise specified, the terms used in the present application have the commonly understood meanings by those skilled in the art.

[0078] Unless otherwise specified, the values of the parameters mentioned in the present application can be measured by various test methods commonly used in the art, for example, the test methods given in the examples of the present application. Unless otherwise specified, the test temperature of each parameter is 25°C.

[0079] The battery mentioned in the examples of the present application can be a single physical module comprising one or more lithium ion secondary batteries to provide higher voltage and capacity. For example, the battery mentioned in the present application can comprise a lithium ion secondary battery, a battery cell, a battery module or a battery pack, etc.

[0080] The lithium ion secondary battery is the smallest unit that constitutes a battery, which can independently realize the function of charging and discharging. The lithium ion secondary battery can be in the shape of a cylinder, a cuboid or other shapes, which are not limited in the examples of the present application. For example, Figure 2 is a lithium ion secondary battery 5 in the shape of a cuboid as an example.

[0081] The lithium ion secondary battery comprises an electrode assembly and an electrolyte.

[0082] The lithium ion secondary battery can further comprise an outer package, which can be used to package the electrode assembly and the electrolyte. The outer package can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT) and polybutylene succinate (PBS).

[0083] In some embodiments, as Figure 3As shown, the outer package can include a housing 51 and a cover plate 53. The housing 51 can include a bottom plate and side plates connected to the bottom plate, which enclose a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. The electrode assembly 52 is packaged in the receiving cavity. The number of electrode assemblies 52 contained in the lithium ion secondary battery 5 can be one or more, which can be adjusted according to requirements.

[0084] The electrode assembly generally includes a positive electrode sheet and a negative electrode sheet. The negative electrode sheet is an electrode that undergoes a reaction of absorbing or lithiating lithium ions during charging and releasing or delithiating lithium during discharging. The positive electrode sheet is an electrode that undergoes a reaction of releasing or delithiating lithium ions during charging and absorbing or lithiating lithium during discharging.

[0085] When there are multiple lithium ion secondary batteries, the multiple lithium ion secondary batteries are connected in series, in parallel, or in a mixed connection through a busbar. In some embodiments, the battery can be a battery module; when there are multiple lithium ion secondary batteries, the multiple lithium ion secondary batteries are arranged and fixed to form a battery module. In some embodiments, the battery can be a battery pack, which includes a box body and lithium ion secondary batteries, and the lithium ion secondary batteries or battery modules are contained in the box body. In some embodiments, the box body can be part of the chassis structure of the vehicle. For example, part of the box body can be at least part of the floor of the vehicle, or part of the box body can be at least part of the cross beam and the longitudinal beam of the vehicle.

[0086] In some embodiments, the battery can be an energy storage device. The energy storage device includes an energy storage container, an energy storage cabinet, etc.

[0087] In some embodiments, the lithium ion secondary battery can be assembled into a battery module, and the number of lithium ion secondary batteries contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module. Figure 4 FIG. 4 is a schematic view of a battery module 4 as an example. As shown in FIG. 4, the battery module 4 includes a plurality of lithium ion secondary batteries 5. Figure 4 As shown in the battery module 4, the plurality of lithium ion secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, other arbitrary arrangements can also be used. Further, the plurality of lithium ion secondary batteries 5 can be fixed by fasteners.

[0088] Optionally, the battery module 4 can further include a housing having a receiving space, and the plurality of lithium ion secondary batteries 5 are contained in the receiving space.

[0089] In some embodiments, the above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0090] Figure 5 andFigure 6 is a schematic diagram of a battery pack 1 as an example. As shown in Figure 5 and Figure 6 shown, the battery pack 1 can include a box body and a plurality of battery modules 4 arranged in the box body. The box body includes an upper box body 2 and a lower box body 3, the upper box body 2 is used to cover the lower box body 3, and forms a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the box body in any manner.

[0091] Lithium-containing transition metal phosphate materials have been widely used in lithium ion batteries due to their structural stability, good safety, and long cycle life. However, their low electronic conductivity and low packing efficiency result in low active material loading in the battery, which cannot meet the needs of high energy density batteries.

[0092] Studies have shown that increasing the number and proportion of large-size particles in lithium-containing transition metal phosphate materials is an effective way to improve the powder compaction density and increase the loading of positive active materials in the battery. The formation 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 raw materials, and the higher the energy consumption and sintering temperature required. However, experimental results show that batteries with large-size lithium-containing transition metal phosphate particles often have high self-discharge. Researchers found that this is because as the sintering temperature of lithium-containing transition metal phosphate materials increases, carbon thermal reduction reactions are more likely to occur at lattice defects, and other reducing substances (such as hydrogen, carbon monoxide, etc.) generated during the preparation process are reduced to magnetic substances such as Fe and Fe2P. Therefore, the increase in large-size particles in the positive active material often leads to an increase in the content of magnetic substances. Magnetic substances are prone to cause the agglomeration and growth of organic substances in the electrolyte during the charging and discharging process of lithium ion secondary batteries, forming edges or spikes, which in turn easily pierce the separator, forming a micro-short circuit inside the lithium ion secondary battery, creating a leakage current path, resulting in a gradual decrease in battery capacity without an external load, i.e. increased self-discharge, which deteriorates the long-term performance of lithium ion secondary batteries.

[0093] 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 active material, the positive active material comprises lithium-containing transition metal phosphate particles with at least part of the surface coated with a carbon coating material, and as shown in Figure 1 , the area proportion 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 sheet is 30%-50%; 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.

[0094] 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 high tap density of the positive electrode tab; when the area ratio of particles with a particle size of 1 μm or more in the positive electrode film layer is more than 50%, the sintering temperature needs to be increased or the sintering time needs to be increased, and the content of magnetic substances will increase, resulting in an increase in the self-discharge K value of the battery cell. Controlling the area ratio of particles with a particle size of 1 μm or more to be 30.0%-50.0% and reducing the content of magnetic substances by adjusting the gas flow rate and the like can make the mass ratio of magnetic substances in the positive electrode film layer be 20 ppm or more and 1980 ppm or less, thereby improving the tap density of the tab while reducing the self-discharge phenomenon of the battery.

[0095] When the area ratio of particles with a particle size of 1 μm or more in the positive electrode film layer is 30.0%-50.0% along the thickness direction of the tab, the grading effect can be fully played in the tab production and cycling process, and the tap density of the tab can be effectively improved. However, the preparation of the positive electrode active material containing particles with an area ratio within the above range often needs to undergo high-temperature sintering for a long time, resulting in an increase in the content of magnetic substances. The embodiments of the present application effectively reduce the content of magnetic substances in the positive electrode film layer while increasing the area ratio of large-size particles, so that the mass ratio of magnetic substances in the positive electrode film layer is 20 ppm or more and 1980 ppm or less, the tap density of the tab in the lithium ion secondary battery is improved, and a low self-discharge rate can be maintained, which is conducive to the improvement of the energy density of the battery and the 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 under a certain magnification, for example, 10 thousand times. Defects and scratches can exist inside the particle, but the complete boundary inside the particle cannot be recognized enough to divide the particle.

[0096] The particle recognition method is as follows: the positive electrode film layer is cut along the thickness direction of the tab by an argon ion beam (as an example, the device model: Leica EM TIC 3X CP, working voltage: 6 kV, working time: 6 h), and the cut surface is observed by a scanning electron microscope (as an example, the device model: Hitachi SU8230, working voltage: 3 kV, beam current: high, probe model: U (LA100), working distance < 5 mm) after exposure. The cut surface of the positive electrode film layer along the thickness direction of the tab is observed by a field emission scanning electron microscope. The image is collected by a secondary electron mode under 10 k magnification, and the particles in the electron microscope image are analyzed by ImageJ software (1.46r, win64 version). The use method of the ImageJ software is as follows: the scanning electron microscope image to be analyzed is loaded, and the image is converted to 8-bit grayscale. The image is converted to binary image by adjusting the threshold, and the threshold is adjusted to 0. The number of particles in the image is counted, and the area of each particle is calculated.Figure 1 The specific method for identifying particles using the Cellpose plug-in software therein is as follows: set the segmentation diameter parameter (diameter in the Segmantation module) to 15 pixels, click "run cyto3" to identify particles, and then manually identify the particles in the image that are not identified by the software or are not completely identified by the software or have errors in identification. The particles in the image that are not identified by the software or are not completely identified by the software or have errors in identification mainly include the following: 1. The particles are too large or have scratches on the surface, which leads to the failure to identify or completely identify the particles; 2. During the argon ion beam cutting process, scratches are generated on the surface of the particles, and the software may misjudge the scratches as the boundaries of the particles during the identification process, thereby generating identification errors; 3. The particles are too small and are not successfully identified; 4. The particles are located at the edge of the electron microscope field of view, the inside of the particles is penetrated by the edge, the morphology is not completely displayed, and the local instead of the whole is identified, which leads to identification errors. The particles that are not identified or have identification errors are manually calibrated, and the specific process is as follows: delete the large particles that are located at the edge of the scanning electron microscope and are not completely displayed; determine whether there are cracks in the inside of the particles that are not identified or have identification errors, if there are no cracks in the inside of the particles, it is determined that the particles are one particle, and the particles are manually identified according to the particle boundaries observed by the human; in response to the existence of cracks in the inside of the particles, it is determined whether the cracks penetrate the particles, if the cracks do not penetrate the particles, it is determined that the particles are one particle, and the particles are manually identified; in response to the cracks being irregular, it is determined that the cracks are the boundaries between the particles, and the particles are divided along the boundaries; in response to the cracks being linear, the contrast is compared; in response to the contrast being not obvious and having no crack feeling, it is determined that the cracks are one particle; in response to the contrast being strong and having a crack feeling, it is determined that the cracks are the boundaries between the particles, and the cracks are identified as two particles. After the manual identification, the information unrelated to the particles in the image is deleted, that is, the determination and identification of the particles in the image are completed.

[0097] During the compaction process of the positive electrode film layer, compaction occurs in the thickness direction, so the section of the positive electrode film layer along the thickness direction of the pole piece can better reflect the real compaction condition of the particles in the film layer in the 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 pole piece, the area ratio of the particles with a particle size greater than or equal to 1 μm can directly reflect the proportional relationship between the area of the particles in this particle size range and the total particles, and reflect the area of the particles in this particle size range.

[0098] It can be understood that the particles in the section of the positive electrode film layer along the thickness direction of the electrode sheet, especially the particles with a size of 50 nm or more, mainly come from the positive electrode active material. Therefore, the application embodiments can accurately and objectively reflect the distribution of lithium-containing transition metal phosphate particles in the positive electrode film layer in the electrode sheet by observing and counting the particle area in the section of the positive electrode film layer.

[0099] In the prior art, a laser particle size analyzer is usually used to count the particle size of the positive electrode active material by Malvern laser diffraction method. However, the applicant's research shows that the lithium-containing transition metal phosphate particles are prone to agglomeration, and the test results obtained by the Malvern laser diffraction method according to the laser scattering principle are often the particle size of the particle agglomerates, and cannot truly reflect the particle size of the particles in the positive electrode active material, and cannot reflect the dispersion state of the positive electrode active material in the film layer, because the dispersion degree of the positive electrode active material in the film layer will be improved in the process of slurry preparation and film rolling. The test results obtained by the Malvern laser diffraction method are affected by the particle size, specific surface area and agglomeration degree of the positive electrode active material, and compared with the true dispersion in the electrode sheet, 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, so the particle size obtained by the Malvern laser diffraction method cannot be equivalent or analogous to the particle size obtained by the application embodiments.

[0100] In some embodiments, in the section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of the particles with a particle size of 1 μm or more is 30%-50%.

[0101] The area ratio of the particles with a particle size greater than or equal to 1 pm in the section of the positive electrode film layer along the thickness direction of the electrode sheet is tested as follows: the particles in the positive electrode film layer are identified according to the method described above, and the picture after the particle determination and identification is imported into the ImageJ software for analysis. The scale is set according to the scanning electron microscope image, and the particle size, area, sphericity and roughness of the particles in the section of the positive electrode film layer along the thickness direction of the electrode sheet are statistically analyzed by using the analysis functions of "Feret diameter", "Area", "Round" and "Solidity". According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter obtained by analysis represents the maximum distance between all parallel lines in the two-dimensional projection of the particle, which represents the particle size of the particle; the "Area" parameter obtained represents the pixel area of the particle. Since the particles with a particle size less than 50 nm have a large error in the statistical process and are difficult to accurately identify, and the particle size of the conductive agent is generally less than 50 nm, which will cause a large error in the statistical result, therefore, in the particle size statistical process of the present application, the particles with a particle size less than 50 nm are not counted, and the particle statistical data corresponding to the particles with "NaN" displayed in AR, Round or Solidity are deleted. The sum of the "Area" parameters of the particles with a particle size greater than or equal to 1 pm and the sum of the "Area" parameters of all particles are calculated as the area of the particles with a particle size greater than or equal to 1 pm and the total area of the counted particles, respectively. The area ratio of the particles with a particle size greater than or equal to 1 pm in the section of the positive electrode film layer along the thickness direction of the electrode sheet is calculated by dividing the sum of the areas of the particles with a particle size greater than or equal to 1 pm by the total area of the counted particles.

[0102] In some embodiments, the area ratio of the particles with a particle size greater than or equal to 1 pm in the section of the positive electrode film layer along the thickness direction of the electrode sheet can 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 between any two of them.

[0103] The lithium-containing transition metal phosphate refers to a phosphate material containing lithium and transition metal elements, which can be detected by any known method in the art. For example, it can be detected by X-ray diffractometer (XRD) and energy spectrum analyzer, inductively coupled plasma mass spectrometer. As an example, the lithium-containing transition metal phosphate includes but is not limited to lithium iron phosphate, lithium manganese iron phosphate and its doped material.

[0104] The carbon coating layer disposed on at least part of the surface of the lithium-containing transition metal phosphate can be detected by any known method in the art. As an example, the lithium-containing transition metal phosphate is characterized by a combination of transmission electron microscopy and energy dispersive spectrometer, and the carbon coating layer disposed on at least part of the surface of the lithium-containing transition metal phosphate can be observed.

[0105] In order to fully exert the capacity of the lithium-containing transition metal phosphate material to improve the energy density thereof, 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.

[0106] In the present application, the "magnetic substance" refers to a substance capable of generating magnetism under the action of a magnetic field.

[0107] The "mass content of the magnetic substance in the positive electrode film layer" herein can be measured by the following steps. The battery is disassembled to obtain a positive electrode tab, which is soaked in dimethyl carbonate solvent for 8 hours, dried, and then sintered at 600°C for 2 hours under a nitrogen atmosphere. During the sintering process, the positive electrode current collector on the positive electrode tab falls off. The sintered tab is crushed with a mortar, sieved through a 200 mesh sieve, and a positive electrode material powder is obtained. Step 1, weigh the positive electrode material powder obtained by the above reverse method, for example 80g, and place it in a plastic bucket. Add 6L of deionized water, use a plastic tube to cover a φ24mm x 240mm magnetic bar (magnetic induction intensity is 6000GS), then use a heat sealing clamp to heat seal, place the magnetic bar in the plastic bucket and seal it together, then set the speed of the drum machine to 60rpm / min, set the stirring time to 15min, and place the sealed plastic bucket 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 substance on the plastic tube into the bucket until there is no magnetic substance with an area greater than or equal to 0.5cm 2Step 2, place the magnetic rod into a clean plastic bucket. Cover the clean bucket cover, continue to place the plastic bucket in the drum machine, and set the stirring speed of the drum machine to 60 rpm. Set the stirring time to 15 min, 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 amount of magnetic material extracted. Step 3, prepare a clean 500 mL beaker, remove the magnetic rod from the bucket, and place it in the beaker. Use a flushing bottle to flush all the magnetic material at the head of the plastic tube into the beaker. Use the degaussed scissors to cut the heat shrink tube head on both sides, and fold the upper edge of the heat shrink tube by 90°. Remove the magnetic rod and place it in the magnetic rod placement area. Use the flushing bottle to flush the heat shrink tube from top to bottom in a Z shape (at least three times on both sides), and flush the magnetic material into the beaker until there is no particle residue on the surface of the heat shrink tube (if there are difficult-to-flush clumps, use a clean ceramic knife to scrape them off, and flush the impurities adhered to the ceramic knife into the beaker). Lift the sleeve and flush the bottom of the sleeve at least three times to ensure that all the adsorbed magnetic material particles are collected. Step 4, ① Place the small magnetic block at the bottom of the beaker and rotate it clockwise at least three times from outside to inside, and then rotate it counterclockwise at least three times from outside to inside. ② Repeat ① three times, each time for no less than 10 s. ③ Fix the small magnetic block at the center of the beaker bottom with the palm, and slowly tilt it to pour out the solution after standing for more than 2 s. ④ Stand the beaker upright and flush the wall of the beaker with the flushing bottle to ensure that all the adhered magnetic material particles enter the solvent. Add 100-150 mL of solvent. ⑤ Repeat ③ for 2-4 times until the liquid in the beaker is clear (after the last rinse, no additional solvent is needed). Step 5, first use a syringe to add 70 mL of deionized water to the beaker, and then use another syringe to slowly add 70 mL of 36%-38% concentrated hydrochloric acid to the beaker. After completing the preparation of the diluted hydrochloric acid, transfer it to a fluorinated bottle with a sealed cover for storage. Step 6, ① Use a syringe to inject 15±2 mL of the hydrochloric acid solution prepared in step 5 into the beaker containing the extracted magnetic material, and then seal the beaker with a sealing film. Place it in an ultrasonic instrument for 2 min (power 200 w / frequency 53 KHz). After ultrasonic treatment, rinse the beaker with 100±10 mL of deionized water, and repeat the rinsing operation 2 times. Inject 100-150 mL of deionized water into the beaker for filtration. Use a filter membrane with a pore size of 0.45 μm to collect the magnetic material particles. Place the filter membrane with the magnetic material particles on a clean microscope slide and place it in an oven for drying at 45°C for (15±2) min. Weigh the dried filter membrane (containing magnetic material particles) using an electronic balance, and subtract the weight of the blank filter membrane to obtain the weight of the magnetic material. Calculate the mass content of the magnetic material relative to the mass of the positive electrode material powder sample as the mass ratio of the magnetic material in the positive electrode film layer, with the unit of ppm.

[0108] In some embodiments, the mass percentage of the magnetic substance in the positive electrode film layer can be 20 ppm, 100 ppm, 134.2 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm, 1000 ppm, 1061.7 ppm, 1450.2 ppm, 1500 ppm, 1980 ppm, or any numerical range between any two of the above values.

[0109] The skilled person can control the area percentage of the particles by any known process. For example, by scientific grading of particles of different sizes, the particle size concentration is adjusted; by mechanical force action of crushing and grinding processes, the raw materials are processed to the target particle size distribution range, so as to realize the adjustment of particle size and concentration; by using screening and grading equipment to separate the particle size of the particle system, the required particle size distribution can be obtained; by precisely controlling the feeding rate, the residence time and stress state of the particles in the equipment are adjusted, which also helps to realize the control of the particle concentration.

[0110] In some embodiments, the area percentage of particles with a particle size of 1-5 μm in the cross section of the positive electrode film layer along the thickness direction of the electrode sheet is 30-50%.

[0111] In some embodiments, the area percentage of particles with a particle size of 1-5 μm in the cross section of the positive electrode film layer along the thickness direction of the electrode sheet can 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 between any two of the above values.

[0112] The area percentage of particles with a particle size of 1-5 μm in the cross section of the positive electrode film layer along the thickness direction of the electrode sheet in the above range can improve the compaction density of the electrode sheet by grading while controlling the content of the magnetic substance in the battery, which is beneficial to the improvement of the energy density of the lithium ion secondary battery and the long-term maintenance during storage.

[0113] In some embodiments, the area percentage of particles with a particle size of 1-5 μm in the cross section of the positive electrode film layer along the thickness direction of the electrode sheet is 30-45%.

[0114] The area percentage of particles with a particle size of 1-5 μm in the cross section of the positive electrode film layer along the thickness direction of the electrode sheet in the above range can improve the compaction density and capacity storage stability while further taking into account the migration distance of lithium ions in the particle interior, thereby making the lithium ion secondary battery maintain a low impedance and improving the kinetic performance of the battery.

[0115] In some embodiments, the average equivalent area ratio of the particles with a particle size of 1 μm or more in the section of the positive electrode film layer along the thickness direction of the electrode sheet is 0.05%-0.20%.

[0116] In some embodiments, the average equivalent area ratio of the particles with a particle size of 1 μm or more in the section of the positive electrode film layer along the thickness direction of the electrode sheet can be selected from 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 between any two of them.

[0117] The average equivalent area ratio of the particles with a particle size of 1 μm or more in the section of the positive electrode film layer along the thickness direction of the electrode sheet is obtained by dividing the area ratio of the particles with a particle size of 1 μm or more in the section of the positive electrode film layer along the thickness direction of the electrode sheet by the total number of the particles with a particle size of 1 μm or more in the section of the positive electrode film layer along the thickness direction of the electrode sheet. The average equivalent area ratio of the particles with a particle size of 1 μm or more in the section of the positive electrode film layer along the thickness direction of the electrode sheet within the above range means that the section of the positive electrode film layer along the thickness direction of the electrode sheet has a certain number of large particles to improve the compaction density of the electrode sheet, and the large particles do not have a particle size that is too large to seriously deteriorate the kinetic performance of the lithium ion secondary battery, thus balancing the energy density and kinetic performance of the battery.

[0118] In some embodiments, the area ratio of the particles with a particle size of 50 nm-200 nm in the section of the positive electrode film layer along the thickness direction of the electrode sheet is 3%-15%, which can be selected from 5%-12%, and further selected from 5%-10%.

[0119] In some embodiments, the area ratio of the particles with a particle size of 50 nm-200 nm in the section of the positive electrode film layer along the thickness direction of the electrode sheet can be selected from 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or any numerical range between any two of them.

[0120] Theoretical studies show that in an ideal case, spherical particles with a diameter of 314 nm can be filled into the gaps formed by the packing of spherical particles with a diameter of 1 μm, thus improving the particle grading and the powder compaction density. The particles with a particle size of 50 nm-200 nm can be tightly packed in the gaps between the particles with a particle size of 1 μm or more, and cooperate with them to achieve dense packing. The area ratio of the particles with a particle size of 50 nm-200 nm within the above range means that there is a certain number of particles with a particle size of 50 nm-200 nm, which is beneficial to improving the powder compaction density of the positive electrode active material and the compaction density of the electrode sheet through grading, and further improving the energy density of the lithium ion secondary battery.

[0121] In some embodiments, the mass content of the magnetic substance in the positive electrode film layer is less than or equal to 300 ppm, and optionally is 20-200 ppm.

[0122] In some embodiments, the mass content of the magnetic substance in the positive electrode film layer is optionally 20 ppm, 50 ppm, 100 ppm, 150 ppm, 200 ppm, 250 ppm, 300 ppm, or a numerical range between any two of them.

[0123] The mass content of the magnetic substance in the above range can further alleviate the self-discharge phenomenon and improve the storage stability of the battery capacity.

[0124] In some embodiments, the magnetic substance includes one or more of Fe, Fe2P, FeP, γ-Fe2O3, and Fe2P2O7.

[0125] In some embodiments, the mass content of elemental iron in the positive electrode film layer is less than 20 ppm, and optionally is less than or equal to 15 ppm.

[0126] In some embodiments, the mass content of elemental iron in the positive electrode film layer is optionally 0, 5 ppm, 10 ppm, 15 ppm, 19 ppm, or a numerical range between any two of them.

[0127] It can be understood 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 below its detection limit.

[0128] Elemental iron is more likely to be oxidized in the positive electrode and then reduced in the negative electrode than other magnetic substances. When the elemental iron at the negative electrode accumulates to a certain extent, dendrites will be formed, causing the diaphragm to be perforated, causing internal short circuit of the battery, and even causing the battery to catch fire, explode, and have great safety hazards. Controlling the mass content of elemental iron in the above range is beneficial to improve the safety performance of the battery.

[0129] In some embodiments, the lithium-containing transition metal phosphate includes a component having the following general formula:

[0130] Li m Fe x P y O j Q q ,

[0131] wherein Q comprises one or more of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.8 < m < 1.15, 0.9 < x < 1, 0.95 < y < 1, 3.5 < j < 4, 0 < q < 0.1.

[0132] In some embodiments, m can be selected from 0.8, 0.85, 0.9, 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15 or a numerical range between any two of them; x can be selected from 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0 or a numerical range between any two of them; y can be selected from 0.95, 0.96, 0.97, 0.98, 0.99, 1.00 or a numerical range between any two of them; j can be selected from 3.5, 3.6, 3.7, 3.8, 3.9, 4 or a numerical range between any two of them; q can be selected from 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1 or a numerical range between any two of them.

[0133] Selecting appropriate modification element Q can improve the lattice change rate of the positive electrode active material during the process of deintercalating lithium, reduce the oxygen activity on the particle surface, improve the structural stability of the material, and thus improve the specific capacity level of the material, and further improve the energy density of the lithium ion secondary battery.

[0134] In some embodiments, the positive electrode active material comprises titanium element, and the mass content of the titanium element is 1500 ppm-8000 ppm, which can be selected from 2500 ppm-8000 ppm, and further selected from 2500 ppm-6000 ppm, based on the total mass of the positive electrode active material.

[0135] In some embodiments, the mass content of the titanium element can be selected from 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm, 5500 ppm, 6000 ppm, 6500 ppm, 7000 ppm, 7500 ppm, 8000 ppm or a numerical range between any two of them, based on the total mass of the positive electrode active material.

[0136] The type and content of elements in the positive electrode active material can be tested by any known method in the art. As an example, the titanium element and content can be tested by inductively coupled plasma emission spectrometry according to Appendix C of GB / T 33822-2017.

[0137] The precursor of titanium element, such as titanium dioxide, generally has surface inertness. The addition thereof in the preparation process can reduce the activity of the precursor raw material mixture, on the one hand, to inhibit the growth of particles, to achieve the purpose of controlling the size of large particles, so that 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 is within a suitable range; on the other hand, by reducing the local chemical reaction of the raw material, the probability of generating magnetic substances is reduced. At the same time, the doping of titanium element in the positive electrode active material can improve the electronic conductivity and ion transmission rate of lithium-containing transition metal phosphate, and alleviate the negative impact of particles with relatively large particle size on the kinetic performance of the positive electrode active material. By affecting the particle size and lithium ion transmission path, the battery energy density and kinetic performance are balanced.

[0138] In some embodiments, the mass percentage of carbon element is 0.9%-1.8% based on the total mass of the positive electrode active material.

[0139] The mass percentage of carbon element based on the total mass of the positive electrode active material can be measured by methods and devices known in the art. For example, refer to GB / T 21023-2006 "Determination of total carbon and sulfur content in steel - High frequency induction furnace combustion-infrared absorption method", and use a Deke HCS infrared carbon and sulfur analyzer for measurement.

[0140] In some embodiments, the mass percentage of carbon element is 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8% or any numerical range between any two of them based on the total mass of the positive electrode active material.

[0141] The mass percentage of carbon element within the above range based on the total mass of the positive electrode active material can not only improve the electrical conductivity of the positive electrode active material and improve the kinetic performance of the lithium ion secondary battery, but also reduce the negative impact of excessive carbon content on the loading amount of lithium-containing transition metal phosphate, balance the compaction density of the pole piece and the impedance of the lithium ion secondary battery, and improve the energy density and kinetic performance of the battery.

[0142] In some embodiments, the median value C of the graphitization degree in the cumulative distribution curve of the graphitization degree C value of the positive electrode film layer obtained under the face scanning mode of the laser microscopic confocal Raman spectrometer is greater than or equal to 0.9 and less than or equal to 1.3, which can be 0.99-1.2, wherein the graphitization degree C value is I 50 / I G / I D / I G , wherein I -1 represents the G peak intensity of the Raman spectrum at 1580±100 cm DD peak intensity at 1350±100 cm -1 D peak intensity at 1350±100 cm

[0143] In the present application, the graphitization degree C value of the positive electrode film layer can be obtained by laser microscopic confocal Raman spectrometer area scanning mode. As an example, specifically, a laser microscopic confocal Raman spectrometer (high-precision Renishaw laser microscopic confocal Raman spectrometer) is used, the excitation wavelength of 532 nm is selected, an appropriate amount of positive electrode film layer is taken to scan the surface thereof or the section along the thickness direction of the electrode sheet, the scanning area is 45 μm x 45 μm, which is divided into 10 x 10 grids, the grid vertices are taken as the test points, the step length is 5 μm, and the total number of scanning points is 100 points, thereby obtaining the C values of different sites and the C value cumulative distribution curve of the area scanning region.

[0144] The positive electrode film layer in the present application can be a freshly prepared positive electrode film layer or a positive electrode film layer obtained by disassembling a battery. The surface of the positive electrode film layer obtained by disassembling the battery inevitably has residual electrolyte salt. In order to improve the testing accuracy, it is preferred to scan the section along the thickness direction of the electrode sheet to characterize the graphitization degree of the positive electrode film layer.

[0145] The graphitization degree C value of the positive electrode film layer is obtained by the peak intensity ratio of the G peak (G-band) and the D peak (D-band) of the Raman spectrum, the G peak position is 1580±100 cm -1 , which represents the sp 2 hybrid structure of carbon; the D peak position is 1350±100 cm -1 , which represents the disordered structure of carbon, wherein disorder means that there is no regular arrangement mode between carbon atoms in the structure. In the graphite crystal, the carbon atoms in the same layer form covalent bonds in sp 2 hybrid, and the interlayer is van der Waals force, so that the carbon of the graphite structure is easy to slip. Therefore, the C value can represent the graphitization degree of the positive electrode film layer. It can be understood that the graphitization degree in the positive electrode film layer mainly comes from the carbon material in the positive electrode film layer which is subjected to graphitization treatment, i.e., the carbon coating layer of the positive electrode active material. Although the carbon nanotube conductive agent with relatively high sp 2 hybrid structure also has a relatively high I G / I D , but due to its small addition content and small tube diameter, its addition in the positive electrode film layer shows an extreme value in the Raman area scanning test of the positive electrode film layer, and does not affect the graphitization degree C 50 of the positive electrode film layer.

[0146] Therefore, the graphitization degree of the positive electrode film layer can also be used to characterize the graphitization degree of the positive electrode active material. The higher the degree of graphitization of the carbon on the surface of the positive electrode active material, the higher the proportion of carbon with a graphite structure in the positive electrode film layer, and the easier it is for the particles to slip during the rolling process by means of the carbon structure with a high degree of graphitization in the coating layer, so that the improvement in the electrode piece compaction density can be achieved at a low rolling pressure.

[0147] The graphitization degree C value cumulative distribution curve refers to a curve obtained by arranging at least 100 C values in ascending order, taking the graphitization degree as the horizontal axis, and taking the cumulative number ratio as the vertical axis. C 50 The median C50 of the graphitization degree is the C value corresponding to the cumulative number ratio of 50% on the vertical axis of the graphitization degree C value cumulative distribution curve. The median C 50 Compared with the point value, the graphitization degree of the particles in the positive electrode film layer as a whole, i.e., the degree of easy slipping, can be reflected; compared with the mean value, the influence of extreme values in the test process can be reduced, and the confidence of the test result can be improved.

[0148] A person skilled in the art can realize the regulation of the graphitization degree of the active material particles by any known process. As an example, the regulation of the carbon source, the sintering temperature, the sintering time, the sintering pressure, and the sintering atmosphere can all realize the adjustment of the graphitization degree of the active material particles.

[0149] In some embodiments, in the graphitization degree C value cumulative distribution curve obtained by the laser microconfocal Raman spectrometer in the face scanning mode, the median C50 of the graphitization degree 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 between any two of them.

[0150] The positive electrode active material with the graphitization degree in the above range can easily realize particle slipping in the process of rolling into a film by means of the graphitized carbon layer on the surface of the positive electrode active material, offset the negative impact of the particle size on the electrode piece compaction density, and further improve the positive electrode film layer compaction density by means of particle slipping.

[0151] In some embodiments, in the particle spheroidicity area cumulative distribution curve obtained by the section of the positive electrode film layer along the thickness direction of the electrode piece, the median L50 of the spheroidicity can be selected as 0.6-0.85, and can be selected as 0.65-0.80. A50 The median L50 of the spheroidicity is the spheroidicity value corresponding to the cumulative area ratio of 50% on the vertical axis of the particle spheroidicity area cumulative distribution curve.

[0152] The sphericity of the particles in the section of the positive electrode film layer along the thickness direction of the electrode tab is tested as follows: The particles in the section of the positive electrode film layer are identified according to the method described above, and the shape and area of the particles in the section of the positive electrode film layer along the thickness direction of the electrode tab are analyzed using the "shape description" and "area" analysis functions in ImageJ. According to the software manual (ImageJ User Guide IJ 1.46r), the "Area" parameter obtained by analysis represents the pixel area of the particle, and the "Round" parameter represents the ratio of the pixel area of the particle to the area of a circle with a diameter equal to the fitted long diameter. The closer the particle is to a sphere, the closer the ratio of the pixel area to the area of a circle with a diameter equal to the fitted long diameter is to 1. Therefore, the "Round" parameter of the particle obtained by analysis is used to represent the sphericity of the particle. The sphericity of at least 5000 particles obtained is arranged in order from small to large, and the sphericity area cumulative distribution curve of the particles in the positive electrode film layer is obtained with the sphericity as the horizontal axis and the cumulative area ratio as the vertical axis. A50 The sphericity L value corresponding to the cumulative area ratio of 50% on the vertical axis of the sphericity L value cumulative distribution curve.

[0153] A person skilled in the art can control the sphericity of the particles by any known process. As an example, the sphericity of the particles can be adjusted by processes such as grinding, polishing, chemical etching, mechanical stirring, extrusion, coating, granulation, adding a surfactant, and adjusting the parameters of each process.

[0154] In some embodiments, the sphericity area cumulative distribution curve of the particles obtained from the section of the positive electrode film layer along the thickness direction of the electrode tab has a median sphericity L A50 Optionally, 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 numerical range between any two of them.

[0155] The median sphericity L A50 Particles with a sphericity within the above range are approximately spherical, and under the action of an external force, the particles are prone to slip between particles, which can further improve the compaction density of the electrode tab and increase the energy density of the battery.

[0156] In some embodiments, the powder compaction density of the positive electrode active material under a pressure of 3T is 2.48 g / cm 3 -2.76 g / cm 3 .

[0157] In the present application, the term "powder compaction density" refers to the density of the compact formed with certain density and strength during the compression process of the powder under external force, with the larger voids being filled, the contact area between particles increasing, the interatomic attractive force and the mechanical bonding between particles being enhanced, and the unit being g / cm 3 .

[0158] The powder compaction density of the positive electrode active material can be measured by methods and devices known in the art. For example, GB / T 24533-2009 can be referred to for measurement by using a compaction density instrument. Specifically, a certain amount of positive electrode active material is placed on a compaction special mold (the mold diameter is known), and each of the upper and lower hollows in the middle of the mold has a metal disc. The positive electrode active material is placed between the metal discs, and a metal cylinder is placed on top. The mold is placed on the compaction density instrument, and the pressure is set to 3T. The thickness of the positive electrode active material under 3T pressure can be read on the device, and the powder compaction density of the positive electrode active material is p = m / v, where v = (S x H), m is the mass of the positive electrode active material, S is the bottom area of the mold 1.327 cm 2 , and H is the thickness of the positive electrode active material after compaction.

[0159] In some embodiments, the powder compaction density of the positive electrode active material under 3T pressure can be selected from 2.48 g / cm 3 , 2.49 g / cm 3 , 2.50 g / cm 3 , 2.51 g / cm 3 , 2.52 g / cm 3 , 2.53 g / cm 3 , 2.54 g / cm 3 , 2.55 g / cm 3 , 2.56 g / cm 3 , 2.57 g / cm 3 , 2.58 g / cm 3 , 2.59 g / cm 3 , 2.60 g / cm 3 , 2.61 g / cm 3 , 2.62 g / cm 3 , 2.63 g / cm 3 , 2.64 g / cm 3 , 2.65 g / cm 3 , 2.66 g / cm 3 , 2.67 g / cm 3 , 2.68 g / cm 3 , 2.69 g / cm 3 , 2.70 g / cm 3 , 2.71 g / cm3 2.72 g / cm3 3 2.73 g / cm3 3 2.74 g / cm3 3 2.75 g / cm3 3 2.76 g / cm3 3 or any numerical range between any two of the above values.

[0160] The positive electrode active material has a high powder compaction density, which can improve the compaction density of the electrode sheet and provide a material basis for preparing high-energy-density lithium ion secondary batteries.

[0161] In some embodiments, the powder compaction density of the positive electrode active material under a pressure of 3T is 2.58 g / cm3 3 2.76 / cm3 3 .

[0162] The positive electrode active material with a compaction density in the above range can further improve the compaction density of the electrode sheet and increase the energy density of the battery.

[0163] In some embodiments, the powder resistivity of the positive electrode active material under a pressure of 8 MPa is 2.0 Ω·cm-40 Ω·cm.

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

[0165] In some embodiments, the powder resistivity of the positive electrode active material under a pressure of 8 MPa can be selected as 2 Ω·cm, 3 Ω·cm, 4 Ω·cm, 5 Ω·cm, 6 Ω·cm, 7 Ω·cm, 8 Ω·cm, 9 Ω·cm, 10 Ω·cm, 11 Ω·cm, 12 Ω·cm, 13 Ω·cm, 14 Ω·cm, 15 Ω·cm, 16 Ω·cm, 17 Ω·cm, 18 Ω·cm, 19 Ω·cm, 20 Ω·cm, 21 Ω·cm, 22 Ω·cm, 23 Ω·cm, 24 Ω·cm, 25 Ω·cm, 26 Ω·cm, 27 Ω·cm, 28 Ω·cm, 29 Ω·cm, 30 Ω·cm, 31 Ω·cm, 32 Ω·cm, 33 Ω·cm, 34 Ω·cm, 35 Ω·cm, 36 Ω·cm, 37 Ω·cm, 38 Ω·cm, 39 Ω·cm, 40 Ω·cm, or any numerical range between any two of the above values.

[0166] The positive electrode active material has a low powder resistivity, which is beneficial to improve the capacity release level of the positive electrode active material and improve the energy density of the lithium ion secondary battery.

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

[0168] In the present application, the positive electrode active material is assembled into a button cell for testing the electrical performance on a blue cell tester. At 25±5℃, in a voltage range of 2.0V-3.75V, after constant current charging at 1C to 3.75V, pause for 5 minutes, constant voltage charging to the cutoff current of 50μA, then constant current discharging at 1C to 2.0V. The discharge capacity of the button cell divided by the mass of the positive electrode active material is taken as the discharge gram capacity of the positive electrode active material at room temperature at a 1C discharge rate.

[0169] The preparation and testing process of the button cell is as follows: 2.0g of the positive electrode active material, conductive carbon black, and PVDF are mixed in a mass ratio of 0.9:0.05:0.05, then NMP (N-methyl pyrrolidone) is added, and after being mixed uniformly, a 150μm doctor blade is used for coating, and the coated product is dried at 100℃ for 2h, and the compaction density is 2.0g / cm 3 -2.2g / cm 3 The positive electrode sheet is compacted, punched into a 14mm diameter disc using a puncher, then weighed and the weight is recorded, the weighed positive electrode sheet is placed in a vacuum drying oven (105℃, 1-12hrs, -90kpa), and after drying, the positive electrode sheet is placed in a glove box, and the battery is assembled in the order of negative electrode shell-nickel mesh-lithium sheet-separator-positive electrode sheet-positive electrode shell, 65-87μL (pipette) of electrolyte (the electrolyte is a mixed solvent of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) in a volume ratio of 1:1, and the electrolyte is LiPF6) is added dropwise, the negative electrode is on top, and placed in the groove of a sealing machine, and the sealing pressure is 650kg / cm 2 The button cell is taken out using an insulating tweezer and placed in a dust-free bag, the glove box is removed, and placed in a constant temperature room for 3h to obtain the button cell for testing.

[0170] It can be understood that the discharge gram capacity of the positive electrode active material can also be obtained by disassembling the battery, obtaining the positive electrode sheet, and then testing the button cell assembled according to the method described above.

[0171] In some embodiments, the positive electrode active material has a discharge gram capacity at room temperature at 1C discharge rate of 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 numerical range between any two of the aforementioned values.

[0172] 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 improve the energy density of the lithium ion secondary battery.

[0173] In some embodiments, the positive electrode active material has a discharge capacity ratio η≥85% at 3.2V, wherein the η is defined as follows: at room temperature, a button cell containing the positive electrode active material is subjected to constant current charge and discharge twice in a voltage range of 2.0V-3.75V at a rate of 0.1C, and then subjected to constant current charge and discharge once at a rate of 1C, in the charge and discharge test at a rate of 1C, the capacity value at a discharge voltage of 3.2V is taken as C1, and the capacity value at a discharge voltage of 2.0V is taken as C2, and η=C1 / C2, wherein the charging process includes constant voltage charging, constant voltage 3.75V, and constant voltage cutoff current 50μA.

[0174] The η value of the positive electrode active material can be measured by methods and devices known in the art. As an example, first refer to the method described above to prepare a button cell, and test the electrical performance of the prepared button cell on a blue light tester, specifically, at room temperature, the button cell is subjected to constant current charge and discharge twice in a voltage range of 2.0V-3.75V at a rate of 0.1C, and then subjected to constant current charge and discharge once at a rate of 1C. In the charge and discharge test at a rate of 1C, the capacity value from 3.75V to a voltage of 3.2V is taken as C1, and the capacity value from 3.75V to 2.0V is taken as C2, and η=C1 / C2.

[0175] In some embodiments, η can be selected as 85%, 86%, 87%, 88%, 88.1%, 89%, 90%, 90.1%, 91%, 92%, 92.2%, 93%, 94%, 94.1%, 94.5%, 95%, 95.1%, or any numerical range between any two of the aforementioned values.

[0176] In some embodiments, the proportion of the discharge capacity of the positive electrode active material in the freshly prepared lithium ion secondary battery discharged to 3.2V is η≥88%. After the freshly prepared lithium ion secondary battery is subjected to constant current charging and discharging at a rate of 0.1C within a voltage range of 2.0V-3.75V for a period of time, the proportion of the discharge capacity of the positive electrode active material discharged to 3.2V is η≥85%.

[0177] The high proportion of the discharge capacity of the positive electrode active material in the battery of the embodiments of the present application discharged to 3.2V means that the positive electrode active material, although having a certain proportion of large-size particles, still maintains good kinetic performance. Meanwhile, the high η value indicates that the lithium ion secondary battery containing the positive electrode active material still has a high voltage when discharged to a low state of charge (SOC), which is beneficial to maintaining good power performance.

[0178] In some embodiments, the positive electrode film layer further comprises a binder and a conductive agent, 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%, based on the total mass of the positive electrode film layer.

[0179] In some embodiments, the binder comprises at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylic ester resin.

[0180] In some embodiments, the conductive agent comprises at least one of super-conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0181] In some embodiments, the mass content of the positive electrode active material can be selected as 94%, 95%, 96%, 97%, 98%, 99%, 99.4%, or a numerical range between any two of them, based on the total mass of the positive electrode film layer.

[0182] In some embodiments, the mass content of the binder can be selected as 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or a numerical range between any two of them, based on the total mass of the positive electrode film layer.

[0183] In some embodiments, 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 a numerical range between any two of them, based on the total mass of the positive electrode film layer.

[0184] In some embodiments, the single-sided areal density of the positive electrode film layer is 300 mg / 1540 mm 2 - 450 mg / 1540 mm 2 .

[0185] In the present application, the single-sided areal density of the positive electrode film layer is the meaning well known in the art, which can be tested by methods known in the art. For example, a single-sided coated and compacted positive electrode sheet (if it is a double-sided coated positive electrode sheet, the positive electrode film layer on one side can be wiped off first) is punched into a small round piece with an area of S1, weighed, and recorded as M1. Then the positive electrode film layer of the above weighed positive electrode sheet is wiped off, the weight of the current collector is weighed, and recorded as M0. The single-sided areal density of the positive electrode film layer is (M1-M0) / S1. In order to ensure the accuracy of the test results, multiple groups (for example, 10 groups) of samples to be tested can be tested, and the average value is calculated as the test result.

[0186] In some embodiments, the single-sided areal density of the positive electrode film layer can be selected from 300 mg / 1540 mm 2 , 310 mg / 1540 mm 2 , 320 mg / 1540 mm 2 , 330 mg / 1540 mm 2 , 340 mg / 1540 mm 2 , 350 mg / 1540 mm 2 , 360 mg / 1540 mm 2 , 370 mg / 1540 mm 2 , 380 mg / 1540 mm 2 , 390 mg / 1540 mm 2 , 400 mg / 1540 mm 2 , 410 mg / 1540 mm 2 , 420 mg / 1540 mm 2 , 430 mg / 1540 mm 2 , 440 mg / 1540 mm 2 , 450 mg / 1540 mm 2 or any numerical range between any two of them.

[0187] The positive electrode film layer with an areal density within the above range can help to improve the energy density of the lithium ion secondary battery.

[0188] In some embodiments, the lithium ion secondary battery has a compacted density of the positive electrode film layer of 2.43 g / cm 3 - 2.78 g / cm 3 .

[0189] In the present application, the full discharge state refers to placing the battery in a 25℃ oven environment, standing for 2h, waiting for the battery temperature to remain 25℃, discharging the battery at 1 / 3C constant current to 2.5V, and then discharging at 0.1C constant current to 2.0V.

[0190] The compaction density of the positive electrode film layer can be tested by methods known in the art. As an example, place the battery in a 25℃ oven environment, stand for 2h, wait for the battery temperature to remain 25℃, discharge the battery at 1 / 3C constant current to 2.5V, and then discharge at 0.1C constant current to 2.0V, disassemble the battery, obtain the positive electrode sheet of the lithium ion secondary battery in the full discharge state, treat the residual electrolyte with dimethyl carbonate solvent, dry the sheet, cut into small round pieces with an area of S, weigh W1, and measure the thickness T1 of the positive electrode sheet using a micrometer, then wipe off the positive electrode film layer of the above weighed sheet, weigh the weight of the current collector, denoted as W2, and measure the thickness T2 of the current collector, then the compaction density PD of the positive electrode film layer = (W1-W2) / [(T1-T2)×S].

[0191] In some embodiments, the compaction density of the positive electrode film layer of the lithium ion secondary battery in the full discharge state can be selected from 2.43g / 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 / cm 3 , 2.67g / cm 3 , 2.68g / cm 32.75 g / cm3 3 2.76 g / cm3 3 2.77 g / cm3 3 2.78 g / cm3 3 2.79 g / cm3 3 2.80 g / cm3 3 2.81 g / cm3 3 2.82 g / cm3 3 2.83 g / cm3 3 2.84 g / cm3 3 2.85 g / cm3

[0192] The compaction density of the positive electrode film layer within the above range is beneficial to improve the energy density of the lithium ion secondary battery.

[0193] In some embodiments, the compaction density of the positive electrode film layer is 2.50 g / cm3 3 - 2.75 g / cm3 3 .

[0194] In some embodiments, the compaction density of the positive electrode film layer after the compaction process is 2.55 g / cm3 3 - 2.90 g / cm3 3 .

[0195] In some embodiments, the compaction density of the positive electrode film layer after the compaction process can be selected from 2.55 g / cm3 3 2.63 g / cm3 3 2.64 g / cm3 3 2.65 g / cm3 3 2.66 g / cm3 3 2.67 g / cm3 3 2.68 g / cm3 3 2.69 g / cm3 3 2.70 g / cm3 3 2.71 g / cm3 3 2.72 g / cm3 3 2.73 g / cm3 3 2.74 g / cm3 3 2.75 g / cm3 3 2.76 g / cm3 3 2.77 g / cm3 3 2.78 g / cm3 3 2.79 g / cm3 3 2.80 g / cm3 3 2.81 g / cm3 3 2.82 g / cm33 2.82 g / cm3 3 2.83 g / cm3 3 2.84 g / cm3 3 2.85 g / cm3 3 2.90 g / cm3 3 2.91 g / cm3 or any numerical range between any two of the above values.

[0196] In the present application, “compaction” refers to the process of compacting the positive electrode film by mechanical pressure during the battery assembly process to improve its density and conductivity.

[0197] In some embodiments, the compacted density of the positive electrode film layer after the formation process is 2.43 g / cm3 3 2.48 g / cm3 3 2.49 g / cm3

[0198] In some embodiments, the compacted density of the positive electrode film layer after the formation process can be selected from 2.43 g / cm3 3 2.48 g / cm3 3 2.49 g / cm3 3 2.50 g / cm3 3 2.51 g / cm3 3 2.52 g / cm3 3 2.53 g / cm3 3 2.54 g / cm3 3 2.55 g / cm3 3 2.56 g / cm3 3 2.57 g / cm3 3 2.58 g / cm3 3 2.59 g / cm3 3 2.60 g / cm3 3 2.61 g / cm3 3 2.62 g / cm3 3 2.63 g / cm3 3 2.64 g / cm3 3 2.65 g / cm3 3 2.66 g / cm3 3 2.67 g / cm3 3 2.68 g / cm3 3 2.69 g / cm3 3 2.70 g / cm3 3 2.71 g / cm3

[0199] In the present application, formation refers to the process of forming a stable solid electrolyte interface (SEI film) and electrode structure through electrochemical reaction during the first charge and discharge process of the battery.

[0200] It can be understood that the compaction density of the positive electrode film layer of the lithium ion secondary battery in the full discharge state is slightly lower than that of the positive electrode film layer after compaction and formation, along with the rebound of the pole piece in the circulation process.

[0201] In some embodiments, the compaction density of the positive electrode film layer of the lithium ion secondary battery in the full discharge state is 2.43-2.78 g / cm 3 The porosity of the positive electrode film layer in the section along the thickness direction of the pole piece is 10%-28%.

[0202] In some embodiments, the compaction density of the positive electrode film layer of the lithium ion secondary battery in the full discharge state is 2.5-2.78 g / cm 3 The porosity of the positive electrode film layer in the section along the thickness direction of the pole piece is 10%-22%.

[0203] In some embodiments, the porosity of the positive electrode film layer in the section along the thickness direction of the pole piece can be selected as 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, or any numerical range between any two of them.

[0204] The porosity of the positive electrode film layer in a cross-section along the electrode thickness direction can be tested as follows: Import the scanning electron microscope (SEM) image of the positive electrode film layer in a cross-section along the electrode thickness direction obtained as described above into ImageJ software. Select the line tool and use a line to mark the length of the scale bar in the image. Click "Analyze Set Scale" and set the scale parameters in the software according to the scale bar length in the image. Select the rectangle tool and select the part of the image outside the scale bar area. Use "Image Duplicate" to copy the selected area and use "Image Type 8 bit" to adjust the image format. Select "Analyze Set Measurements" and choose the following 5 options: "Area", "Mean gray value", "Area Fraction", "Limit to threshold", and "Feret's diameter". Select 3 for "Decimal places". Then select "Image" - "Adjust" - "Threshold" in sequence and set 0 and 100 in the "Threshold" box. You can then use the Analyze-Measure function to export the porosity data in the SEM image of this cross-section. Export using "Image" - "Overlay" - "Flatten" to obtain a pore image; click "Apply" in "Threshold", then click "Analyze" - "Analyze Particles", check the four columns on the left to obtain pore statistics.

[0205] It can be understood that in the embodiments of this application, "pores" in the cross-section of the positive electrode film are identified through image color difference and threshold. For example... Figure 8 As shown, the "porosity" is not the porosity data obtained from the venting test. It is mainly used to characterize the cross-sectional area between particles in the cross-section of the positive electrode film. This method is superior to the venting method because the porosity obtained by the venting method is related to the porosity between particles and the pores in the carbon layer coating the surface of the lithium iron phosphate particles, thus failing to objectively reflect the porosity between particles. The lower the porosity in the cross-section of the positive electrode film tested by this method, the better the particle gradation of large, medium and small particles in the positive electrode film and the higher the compaction density. On the other hand, under the same gradation and roller pressure, if the porosity is low, it means that the particles are more likely to slide against each other, thereby reducing the risk of film overpressure and stress concentration, further reducing the probability of the positive electrode film demolding during long cycles, which is beneficial to improving the long cycle performance of the battery.

[0206] In some embodiments, the positive electrode tab includes a primer layer, the primer layer is arranged between the positive electrode film layer and the current collector; the primer layer includes carbon-based particles, the distribution density of carbon-based particles with a particle size greater than 100 nm in the primer layer is ≤10 pcs / 10 μm.

[0207] The carbon-based particles refer to particles with carbon elements as the main component, including but not limited to conductive carbon, carbon black, etc.

[0208] The primer layer is beneficial to improve the conductivity and adhesion of the positive electrode film layer and the current collector, reduce the delamination of the positive electrode film layer and the current collector during the cycle process, and improve the kinetic performance of the battery. In the high-density electrode tab of the embodiments of the present application, for example, the tap density of the positive electrode tab under full discharge state is greater than or equal to 2.4 g / cm 3 When the current collector is easily damaged during the compaction process of the electrode tab under high pressure, large-size particles are easy to produce pits on the current collector, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the primer layer is controlled to be ≤10 pcs / 10 μm, which is beneficial to reduce the probability of damage of the current collector in the high-pressure compaction electrode tab, and further improve the limit compaction density of the positive electrode tab.

[0209] In some embodiments, the positive electrode tab includes a primer layer, the primer layer is arranged between the positive electrode film layer and the current collector; the primer layer includes carbon-based particles, the distribution density of carbon-based particles with a particle size greater than 100 nm in the primer layer 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 / 10 μm, 4 pcs / 10 μm, 4.5 pcs / 10 μm, 5 pcs / 10 μm, 5.5 pcs / 10 μm, 6 pcs / 10 μm, 6.5 pcs / 10 μm, 7 pcs / 10 μm, 7.5 pcs / 10 μm, 8 pcs / 10 μm, 8.5 pcs / 10 μm, 9 pcs / 10 μm, 9.5 pcs / 10 μm, 10 pcs / 10 μm, or any numerical range between any two of them.

[0210] The distribution density of carbon-based particles with a particle size greater than 100 nm in the primer layer can be detected by the above-mentioned method, the positive electrode film layer is cut along the thickness direction of the electrode tab by argon ion beam, the scanning electron microscope image or the microscope image is taken, the size of the carbon particles in the primer layer is detected by statistical method, and the number of carbon-based particles with a particle size greater than 100 nm contained in every 10 μm of the primer layer is counted, the counting is not less than 5 times, and the average value is obtained.

[0211] The undercoat layer in the embodiments of the present application can be prepared by any known preparation process, such as pre-separation or centrifugation in the preparation process of carbon-based particles to remove large-particle carbon-based materials, so that the D50 of the carbon-based particles added in the undercoat preparation process is less than 70 nm. V 50 in 20-60 nm, D V 90 less than or equal to 70 nm, mixing, stirring, and coating the carbon-based material and the binder on the current collector to obtain the undercoat layer.

[0212] In some embodiments, the positive electrode tab includes an undercoat layer disposed between the positive electrode film layer and the current collector; the positive electrode tab has a compaction density greater than or equal to 2.4 g / cm3 in the full discharge state. 3 The single-sided thickness of the undercoat layer is 1-4 μm.

[0213] In some embodiments, the positive electrode tab includes an undercoat layer disposed between the positive electrode film layer and the current collector; the positive electrode tab has a compaction density greater than or equal to 2.5 g / cm3 in the full discharge state. 3 The single-sided thickness of the undercoat layer is 2-4 μm.

[0214] With the increase of the compaction density of the tab, the extrusion of the large-particle lithium-containing phosphate material (e.g., particle size greater than 1 μm) in the positive electrode film layer on the undercoat layer is more significant. Therefore, stress concentration is prone to occur at the large-particle sites, and even damage the current collector through the undercoat layer. Increasing the thickness of the undercoat layer is beneficial to improve the stress concentration phenomenon in the tab and further increase the ultimate compaction density of the tab.

[0215] In some embodiments, the single-sided thickness of the undercoat layer can be 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, or any numerical range between any two of them.

[0216] The single-sided thickness of the undercoat layer can be tested as follows. As described above, the positive electrode film layer is cut along the thickness direction of the tab by an argon ion beam, a scanning electron microscope image is taken, and the single-sided undercoat layer thickness is measured at every 1 μm interval in the length direction of the tab. After measuring the undercoat layer thickness of 10 points, the average value is obtained. It should be noted that abnormal points, i.e., undercoat layer regions with a thickness less than 50 nm and a thickness greater than 4 μm, should be avoided during the measurement point taking process; these abnormal points are mainly due to the extreme fluctuation of the thickness of individual regions caused by abnormal stress concentration extrusion during the compaction process of the tab, and do not have statistical significance. In some embodiments, the thickness of the positive electrode current collector is less than or equal to 17 μm, which can be optionally 13 μm-15 μm.

[0217] In some embodiments, the thickness of the positive electrode current collector is 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, or any numerical range between any two of them.

[0218] In some embodiments, the positive current collector can employ a metal foil or a composite current collector. For example, as a metal foil, an aluminum foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base material (e.g., a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0219] The positive current collector having a thickness within the above range is helpful to increase the load per unit mass of the battery and to increase the energy density of the lithium ion secondary battery.

[0220] In some embodiments, the negative electrode sheet includes a negative current collector and a negative film layer disposed on at least one side of the negative current collector, the areal density of the negative film layer on one side is 130 mg / 1540 mm 2 -220 mg / 1540 mm 2 ; and / or the compaction density of the negative film layer is 1.40 g / cm 3 -1.75 g / cm 3 .

[0221] In some embodiments, the areal density of the negative film layer on one side can be selected as 130 mg / 1540 mm 2 , 140 mg / 1540 mm 2 , 150 mg / 1540 mm 2 , 160 mg / 1540 mm 2 , 170 mg / 1540 mm 2 , 180 mg / 1540 mm 2 , 190 mg / 1540 mm 2 , 200 mg / 1540 mm 2 , 210 mg / 1540 mm 2 , 220 mg / 1540 mm 2 , or any numerical range between any two of them.

[0222] In some embodiments, the compaction density of the negative film layer is 1.40 g / cm 3 -1.75 g / cm 3 .

[0223] In some embodiments, the compaction density of the negative film layer can be selected as 1.40 g / cm 3 , 1.45 g / cm 3 , 1.50 g / cm3 1.55 g / cm 3 1.60 g / cm 3 1.70 g / cm 3 1.75 g / cm 3 or any numerical range falling within the above ranges.

[0224] The single-sided density and the compacted density of the negative electrode film layer can be tested by a similar method as described above for the positive electrode film layer.

[0225] The single-sided density and the compacted density of the negative electrode film layer within the above ranges are advantageous for matching with the positive electrode film layer and improving the energy density of the lithium ion secondary battery.

[0226] In some embodiments, the negative current collector can be a metal foil or a composite current collector. For example, as the metal foil, a copper foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material base layer such as a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0227] In some embodiments, the negative electrode film layer includes a negative electrode active material. The negative electrode active material can be a negative electrode active material known in the art for use in a battery. As an example, the negative electrode active material can include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy. The tin-based material can be selected from at least one of elemental tin, tin oxide compound, and tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as a negative electrode active material for a battery can also be used. These negative electrode active materials can be used alone or in combination of two or more.

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

[0229] In some embodiments, the negative electrode film layer can further optionally include a conductive agent. The conductive agent can be selected from at least one of super P, acetylene black, carbon black, ketjen black, carbon dot, carbon nanotube, graphene, and carbon nanofiber.

[0230] In some embodiments, the negative electrode film layer can also optionally include other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0231] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on a negative electrode current collector, and after drying, compacting, and the like, the negative electrode sheet can be obtained.

[0232] The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The type of electrolyte is not particularly limited in the present application and can be selected as needed. For example, the electrolyte can be liquid, gel, or all-solid.

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

[0234] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorobisoxalate borate, lithium bisoxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorodioxalate phosphate.

[0235] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclobutane sulfone, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0236] In some embodiments, the electrolyte solution can also optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can also include an additive capable of improving certain properties of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, and the like.

[0237] In some embodiments, the lithium ion secondary battery also includes a separator. The type of separator is not particularly limited in the present application and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0238] In some embodiments, the material of the separator film can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator film can be a single-layer film or a multi-layer composite film, and is not particularly limited. When the separator film is a multi-layer composite film, the materials of the respective layers can be the same or different, and are not particularly limited.

[0239] In some embodiments, the positive electrode tab, the negative electrode tab, and the separator film can be used to make an electrode assembly through a winding process or a stacking process.

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

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

[0242] The second aspect of the present application provides a battery device, which includes the lithium ion secondary battery provided by the first aspect of the present application. The battery device includes at least one of a battery module, a battery pack, and an energy storage battery.

[0243] The third aspect of the present application provides a power consumption device, which includes the lithium ion secondary battery provided by the first aspect of the present application. The fourth aspect of the present application provides a preparation method of 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 0.3 μm-0.4 μm; obtaining a precursor powder after drying the mixed slurry; and 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 is 1100 m 3 / h-1400 m 3 / h; the sintering includes a temperature rising interval and a constant temperature interval, wherein the inert gas flow rate v1 in the temperature rising interval is higher than the inert gas flow rate v2 in the constant temperature interval; the constant temperature interval temperature of the sintering is 770°C-830°C; and the positive electrode active material includes lithium-containing transition metal phosphate particles with carbon-coated material on at least part of the surfaces.

[0244] In some embodiments, the constant temperature interval temperature of the sintering can be selected as 770°C, 780°C, 790°C, 800°C, 810°C, 820°C, 830°C, or any numerical range between any two of them.

[0245] The high-temperature sintering is beneficial to increasing the area ratio of the particles with a particle size of 1 μm or more in the positive electrode film.

[0246] In some embodiments, the molar ratio of lithium to iron in the mixed raw materials can be 1.01, 1.02, 1.03, 1.04, or any numerical range between any two of them.

[0247] In the mixed raw materials, the molar ratio of lithium to iron is greater than 1, which is helpful for lithium supplementation during the sintering of the positive electrode active material, improving the crystallinity of the crystal, and improving the capacity of the positive electrode active material. However, research shows that too high a molar ratio of lithium to iron in the raw materials can lead to incomplete chemical reaction in local areas during the sintering of the positive electrode active material, thereby increasing the probability of the generation of magnetic substances such as Fe2P. The molar ratio of lithium to iron in the mixed raw materials within the above range is helpful for improving the compaction density of the electrode sheet, improving 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 the battery storage and cycling process.

[0248] In the present application, the term "D V50 " refers to the particle size corresponding to the cumulative particle size distribution percentage of 50% of the sample obtained by Malvern laser scattering method.

[0249] In some embodiments, the solid phase volume distribution particle size D V50 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 between any two of them.

[0250] The solid phase particle size in the mixed slurry after grinding within the above range indicates that the raw material particle size is small, so that the raw material has a relatively high activity, and is easy to undergo solid phase diffusion during high-temperature sintering, and grow into particles with a particle size of 1 μm or more with a certain area ratio. At the same time, the solid phase particle size in the mixed slurry after grinding within the above range is helpful for reducing the probability of the generation of magnetic substances due to the uneven chemical reaction in local areas caused by the excessively high activity of the raw material.

[0251] In some embodiments, the inert gas includes one or more of nitrogen, neon, and helium.

[0252] In some embodiments, the sintering is carried out in an inert gas environment, and the total gas flow during the sintering process can be selected to be 1100 m 3 / h, 1200 m 3 / h, 1300 m 3 / h, 1350 m 3 / h, 1400 m 3or any numerical range between any two of the foregoing.

[0253] The total gas flow rate in the sintering process within the above range is conducive to reducing the partial pressure of the reducing atmosphere and reducing the possibility of local reduction and increased magnetic material.

[0254] During the temperature rising process, a violent chemical reaction occurs between the precursor raw materials, and increasing the ventilation rate is conducive to reducing the phenomenon of excessively high local reducing atmosphere, uneven reaction, and high magnetic material content. During the constant temperature process, slow solid-phase diffusion occurs between the precursor raw materials, realizing particle growth, and maintaining a relatively low ventilation rate is conducive to maintaining the stability of the temperature field in the sintering process and realizing uniform growth of the particles.

[0255] 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 numerical range between any two of the foregoing.

[0256] The positive electrode active material prepared by the method can not only make the positive electrode film layer have a certain area ratio of particles of 1 μm or more in the section along the thickness direction of the pole piece, but also contain a small amount of magnetic material. In this way, the energy density of the lithium ion secondary battery can be improved, and the battery 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 cycling.

[0257] In some embodiments, the iron source is an iron-containing compound.

[0258] In some embodiments, the iron source includes at least one of iron hydroxide, ferrous chloride, diiron trioxide, iron phosphate, iron pyrophosphate, ferrous oxalate, iron powder, ferric nitrate, magnetite, iron hydroxide.

[0259] In some embodiments, the phosphorus source is a phosphoric acid compound.

[0260] In some embodiments, the phosphorus source includes at least one of phosphoric acid, iron phosphate, ammonium dihydrogen phosphate, and dihydrogen phosphate.

[0261] 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.

[0262] In some embodiments, the lithium source includes one or more of lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium oxide, lithium hydroxide, lithium acetate.

[0263] In some embodiments, the lithium source includes lithium carbonate.

[0264] In some embodiments, the carbon source comprises one or more of glucose, polyethylene glycol, citric acid, sucrose, starch, fructose, lactose, polyaniline, polyacrylonitrile, polyvinylpyrrolidone.

[0265] In some embodiments, the carbon source comprises glucose, polyethylene glycol.

[0266] In some embodiments, the slurry further comprises a titanium source, optionally, the titanium source comprises one or more of titanium dioxide, tetrabutyl titanate, titanium nitrate, titanate.

[0267] In some embodiments, the solid phase volume distribution coefficient (D V90 -D V10 ) / D V50 is 1.8-3.0.

[0268] In some embodiments, the solid phase volume distribution coefficient (D V90 -D V10 ) / D V50 may 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 between any two of them.

[0269] The solid phase volume distribution coefficient in the mixed slurry within the above range indicates that the raw material particle size distribution is uniform, which is conducive to improving the uniformity of solid phase reaction during subsequent sintering, reducing the risk of local reduction material concentration and enhanced reducibility, and magnetic material aggregation due to uneven mixing of raw materials.

[0270] In some embodiments, the volume distribution particle size D V50 of the precursor powder is 5 μm-60 μm.

[0271] In some embodiments, the precursor powder is obtained by spray drying the mixed slurry.

[0272] In some embodiments, the sintering is one-time sintering, which comprises at least two constant temperature intervals, the constant temperature temperature of the first constant temperature interval is 400-500°C, and the constant temperature time of the first constant temperature interval is 3-8h; the highest constant temperature temperature of the one-time sintering is 770-820°C, and the highest temperature is treated for 8-15h.

[0273] In some embodiments, the highest constant temperature temperature of the one-time sintering may be 770°C, 780°C, 790°C, 800°C, 810°C, 820°C, or any numerical range between any two of them.

[0274] In some embodiments, the highest temperature is kept constant for 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, or any numerical range between any of these values.

[0275] High-temperature sintering in the above temperature range helps to increase the particle size of the positive electrode active material, so that large particles of 1 μm or more have a certain area ratio in the section of the positive electrode film layer along the thickness direction of the electrode sheet, increasing the electrode compaction density, while also reducing the increase in reducing atmosphere caused by too high a sintering temperature, controlling the probability of the reduction of Fe2P and other magnetic substances, and thus balancing the energy density and storage stability of the battery.

[0276] In some embodiments, the sintering is at least twice, and the first sintering obtains a primary sintering product, and the second sintering is performed after grinding the primary sintering product.

[0277] In some embodiments, the sintering temperature of the first sintering is 720-780°C, and the sintering time is 6-12h.

[0278] In some embodiments, the sintering temperature of the first sintering can be selected as 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, or any numerical range between any of these values.

[0279] In some embodiments, the sintering time of the first sintering can be selected as 6h, 7h, 8h, 9h, 10h, 11h, 12h, or any numerical range between any of these values.

[0280] In some embodiments, the sintering temperature of the second sintering is 770-830°C, and the sintering time is 6-12h.

[0281] In some embodiments, the sintering temperature of the second sintering can be selected as 770°C, 780°C, 790°C, 800°C, 810°C, 820°C, 830°C, or any numerical range between any of these values.

[0282] In some embodiments, the sintering time of the second sintering can be selected as 6h, 7h, 8h, 9h, 10h, 11h, 12h, or any numerical range between any of these values.

[0283] In some embodiments, the second sintering after grinding the primary sintering product comprises: adding a carbon source to the primary sintering product and grinding separately, the D V50 of the first group of ground particles is 1.40-2.0 μm; and the D V500.35 μm-0.45 μm; the 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 subjected to second sintering.

[0284] In some embodiments, the second sintering after grinding the primary sintering product comprises: adding a carbon source to the primary sintering product and grinding separately, wherein the first group of ground particles has a D V50 Optionally, 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 between any two of them.

[0285] In some embodiments, the second sintering after grinding the primary sintering product comprises: adding a carbon source to the primary sintering product and grinding separately, wherein the second group of ground particles has a D V50 Optionally, 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 between any two of them.

[0286] In some embodiments, the first group of ground particles and the second group of ground particles can be mixed in a mass ratio of 30:70, 40:60, 50:50, 60:40, 70:30 to obtain mixed intermediate particles

[0287] 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 ground particles in the second sintering process, the activity of the particles can be controlled, so that the positive active material has large particles with a certain area ratio. This can improve the compaction density of the electrode sheet, improve the energy density of the lithium ion secondary battery, and at the same time make 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.

[0288] 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 active material prepared by the method of the fourth aspect to a solvent, stirring to obtain a delivery slurry; transferring and coating the delivery slurry to at least one side of a current collector, and drying and hot pressing to obtain a positive electrode sheet.

[0289] In some embodiments, the hot pressing comprises at least three times of hot rolling, the hot rolling pressure increases successively, and the hot rolling pressure is 20-50 tons, 50-70 tons, and 70-90 tons successively; the hot rolling temperature is 40-80℃, and the pole piece is heated before entering the hot rolling compaction for the first time, and the heating temperature is 40-50℃.

[0290] The positive electrode active material prepared by the hot pressing process and the preparation method of the fourth aspect in the embodiments of the present application can effectively improve the compaction density of the positive electrode pole piece while maintaining a low content of magnetic substances, so that the battery improves the energy density while having low self-discharge.

[0291] In some embodiments, the transfer coating has a coating speed of 1 m / min-25 m / min. In some embodiments, the transfer coating has a coating speed of 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 numerical range between any two of them.

[0292] The transfer coating has a coating speed within the above range, which is beneficial to improve the uniformity of particle distribution in the coating process, reduce the risk of particle agglomeration in the positive electrode film layer, reduce the cross-section porosity of the positive electrode film layer, further improve the ultimate compaction density of the pole piece, and improve the energy density of the battery.

[0293] In addition, the present application also provides a power utilization device, which comprises at least one of the lithium ion secondary battery, the battery module, the battery pack, and the energy storage battery provided by the present application. The lithium ion secondary battery, the battery module, and the battery pack can be used as a power supply of the power utilization device, or can be used as an energy storage unit of the power utilization device. The power utilization device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0294] As the power utilization device, the lithium ion secondary battery, the battery module, or the battery pack can be selected according to the use requirement thereof.

[0295] Figure 7is an example of a power consuming device. The power consuming device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the demand of the power consuming device for high power and high energy density of the secondary battery, a battery pack or a battery module can be used.

[0296] The device as another example can be a mobile phone, a tablet, a notebook computer, etc. The device generally requires thinning, and a secondary battery can be used as a power source.

[0297] Embodiment

[0298] Hereinafter, an embodiment of the present application will be described. The embodiment described below is exemplary and is for the purpose of explanation of the present application only and is not to be understood as a limitation of the present application. In the embodiment, a specific technique or condition not mentioned is performed according to a technique or condition described in a document in the art or according to a product manual. The reagent or instrument not mentioned by the manufacturer is a conventional product that can be obtained on the market.

[0299] Embodiment 1

[0300] (1) Positive electrode active material preparation

[0301] Lithium carbonate, iron phosphate, titanium dioxide, glucose, and polyethylene glycol were added to water, mixed in a premixing tank at a rotation speed of 1400 rpm, and de-magnetized by a de-magnetizing bar with a magnetic field strength of 10000 Gs, wherein the lithium carbonate and the iron phosphate were mixed in a molar ratio of lithium to iron of 1.03:1.0, the mass content of the glucose was 6% compared to the total amount of raw materials, and the mass content of the polyethylene glycol was 5% compared to the total amount of raw materials, and the mixture was uniformly mixed to obtain a mixed raw material with a solid content of 40%;

[0302] The number of magnetic substance particles in the lithium carbonate was less than or equal to 500 pcs / kg, the particle size D V10 was greater than or equal to 1 μm, the particle size D V50 was 6 μm, and the particle size D V90 was less than or equal to 40 μm. The number of magnetic substance particles in the iron phosphate was less than or equal to 95 pcs / kg, and the morphology was spherical. The number of magnetic substance particles in the glucose was less than or equal to 500 pcs / kg. The molecular weight of the polyethylene glycol was 1500, and the number of magnetic substance particles was less than or equal to 150 pcs / kg.

[0303] The mixed raw material was ground twice in a sand mill for a de-magnetization cycle. After coarse grinding for 1 h, the de-magnetization of the coarsely ground raw material was performed using a permanent magnet de-ironing device with a de-magnetization strength of greater than or equal to 8000 Gs. The de-magnetized raw material was further finely ground, 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 V50 is 0.35 μm, spray drying to obtain a dry precursor powder, and the D50 of the dry precursor powder is 55.0 μm; and the magnetic substance is less than or equal to 70 pcs / kg.

[0304] The precursor powder is subjected to two-stage temperature rising sintering in a nitrogen atmosphere to obtain a lithium iron phosphate positive electrode material: the temperature is raised from 25 ℃ to 450 ℃ at a temperature rising rate of 2 ℃ / min (first temperature rising stage), and the temperature is kept constant for 3 h; the temperature is raised from 450 ℃ to 780 ℃ at a temperature rising rate of 5 ℃ / min (second temperature rising stage), and the temperature is kept constant for 12 h; wherein the air flow rate in the temperature rising stage is greater than that in the constant temperature stage, and the ratio is 1.5:1, and the total air flow rate is 1350 cm 3 / h, and the temperature is lowered after the end; airflow crushing is performed to obtain a D V 50 of 1.0 μm -2.0 μm carbon-coated lithium iron phosphate material.

[0305] The above D50, D V 50, D V 90 refers to the data obtained by the Malvern laser scattering method.

[0306] The mass percentage of carbon element is 1.25% and the mass percentage of titanium element is 4000 ppm based on the total mass of the positive electrode active material. The powder compaction density of the positive electrode active material under a pressure of 3T is 2.52 g / cm 3 , and the discharge capacity at room temperature at a discharge rate of 1C is 141.4 mAh / g. The discharge capacity percentage η of the positive electrode active material discharged to 3.2V is 92.5%.

[0307] (2) Preparation of the positive electrode sheet:

[0308] 2.2wt% of PVDF, 0.8wt% of conductive carbon black, and 97.0wt% of the positive electrode active material are sequentially added and dry-mixed, and then added into N-methyl pyrrolidone to perform stirring and viscosity adjustment to obtain a delivery slurry; the delivery slurry is transferred and coated onto a primer layer of a current collector aluminum foil, the primer layer comprising carbon black and PVDF, and the mass ratio of the two is 1:1, the distribution density of carbon-based particles with a particle size greater than 100 nm in the primer layer is ≤10 pcs / 10 μm, and the thickness of the primer layer is 2 μm. After drying and hot pressing, a positive electrode film layer with a single-sided area density of 350 mg / 1540 cm 2 is obtained. The speed of the transfer coating is 20 m / min.

[0309] The hot pressing process includes three times of hot roller pressing process, and the hot roller pressing pressure increases in turn, and the hot roller pressing pressure is 40 tons, 60 tons, and 80 tons in turn; the hot roller temperature is 60 ℃, and the sheet is heated before entering the hot roller compaction for the first time, and the heating temperature is 40 ℃.

[0310] The compaction density of the pole piece is the pole piece limit compaction density, and the limit compaction density test method of the pole piece is described below; the limit compaction density of the pole piece in this embodiment is 2.63 g / cm 3 .

[0311] In the section of the positive electrode film layer along the thickness direction of the pole piece, the area ratio of particles with a particle size of 1-5 μm is 30%, the average equivalent area ratio of particles with a particle size of 1 μm or more in the section of the positive electrode film layer along the thickness direction of the pole piece is 0.127%, the area ratio of particles with a particle size of 50-200 nm in the section of the positive electrode film layer along the thickness direction of the pole piece is 6.34%, the mass content of the magnetic substance in the positive electrode film layer is 20 ppm, and the mass content of elemental iron is 0 (lower than the detection lower limit, recorded as 0). In the cumulative distribution curve of the graphitization degree C value obtained by the positive electrode film layer in the laser microscopic confocal Raman spectrometer face scanning mode, the median number C 50 of the graphitization degree is 1.00, the median number L A50 of the sphericity of the particles in the positive electrode film layer is 0.715, and the porosity of the section of the positive electrode film layer is 15.991%.

[0312] (3) Preparation of the negative pole piece:

[0313] 95.5 wt% of the negative active material (artificial graphite), 1.0 wt% of the conductive agent (conductive carbon black), 2.0 wt% of the binder (styrene-butadiene rubber (SBR)), and 1.5 wt% of the thickening agent (sodium carboxymethyl cellulose (CMC)) are mixed, deionized water is added for stirring and dispersion to prepare a negative electrode slurry. Then the negative electrode slurry is coated on the double-sided surface of the Cu foil, both sides are completed, and then dried, compacted, cut, and sheeted to prepare the negative pole piece. The single-sided density of the coating is 164 mg / 1540.25 cm 2 , and the compaction density is 1.60 g / cm 3 .

[0314] (4) Preparation of the isolation film

[0315] The polypropylene film is used as the isolation film.

[0316] (5) Preparation of the electrolyte

[0317] In an argon atmosphere glove box (H2O<0.1 ppm, O2<0.1 ppm), organic solvents ethylene carbonate (EC) / dimethyl carbonate (DMC) are mixed uniformly in a volume ratio of 1 / 1, lithium salt LiPF6 is dissolved in the organic solvent, the content of LiPF6 in the solution is 1 mol / L, and stirring is uniform to obtain the electrolyte.

[0318] (6) Preparation of the battery:

[0319] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, the separator can play a role of separating the anode and the cathode, a bare battery cell is obtained by winding, the bare battery cell is placed in an outer package, electrolyte is injected, and finally a lithium ion secondary battery is obtained through processes such as packaging, formation, and degassing.

[0320] Example 2

[0321] The preparation method of Example 2 is basically the same as that of Example 1, except that the sintering process of the positive active material is different, specifically:

[0322] The precursor powder is sintered in two stages in a nitrogen atmosphere to obtain a lithium iron phosphate positive material: the temperature is raised from 25 ℃ to 450 ℃ at a rate of 2 ℃ / min (first temperature raising stage), and the temperature is kept constant for 3 h; the temperature is raised from 450 ℃ to 800 ℃ at a rate of 5 ℃ / min (second temperature raising stage), and the temperature is kept constant for 12 h; the gas flow in the temperature raising stage is greater than that in the constant temperature stage, the ratio is 1.5:1, and the total gas flow is 1350 cm 3 / h, and the temperature is lowered after the end; the D V 50 is a 1.0 μm -2.0 μm carbon-coated lithium iron phosphate material.

[0323] Example 3

[0324] The preparation method of Example 3 is basically the same as that of Example 1, except that the sintering process of the positive active material is different, specifically:

[0325] The precursor powder is sintered in two stages in a nitrogen atmosphere to obtain a lithium iron phosphate positive material: the temperature is raised from 25 ℃ to 450 ℃ at a rate of 2 ℃ / min (first temperature raising stage), and the temperature is kept constant for 3 h; the temperature is raised from 450 ℃ to 820 ℃ at a rate of 5 ℃ / min (second temperature raising stage), and the temperature is kept constant for 12 h; the gas flow in the temperature raising stage is greater than that in the constant temperature stage, the ratio is 1.5:1, and the total gas flow is 1350 cm 3 / h, and the temperature is lowered after the end; the D V 50 is a 1.0 μm -2.0 μm carbon-coated lithium iron phosphate material.

[0326] Example 4

[0327] The preparation method of Example 4 is basically the same as that of Example 1, except that the addition content of the titanium source in the preparation process of the positive active material is adjusted, so that the mass content of titanium element is 2500 ppm based on the mass of the positive active material.

[0328] Example 5

[0329] Example 5 is substantially the same as the preparation method of Example 1, except that the content of the titanium source added in the preparation process of the positive electrode active material is adjusted, so that the mass content of titanium element is 1500 ppm based on the mass of the positive electrode active material.

[0330] Example 6

[0331] The preparation method of Example 6 is substantially the same as that of Example 1, except that the preparation process of the positive electrode active material is slightly different, and the difference points specifically include the following two points:

[0332] (1) The carbon source in the mixed raw materials is only glucose, and the mass of glucose is 5.7 wt% compared to the mass of iron phosphate;

[0333] (2) The temperature sintering process is different. The precursor powder is sintered at least twice in a nitrogen atmosphere, the first sintering temperature is 750°C, and the holding time is 8 hours to obtain a primary sintered product.

[0334] 1.5 wt% (based on the mass of the primary sintered product) of glucose, 3.0 wt% (based on the mass of the primary sintered product) of polyethylene glycol and a titanium source are added to the primary sintered product, and after grinding, it is divided into two groups for secondary grinding. The grinding parameters of the two groups are different, and the D V 50 of the particles after grinding of the first group is 2.0 μm, and the D V 50 of the particles after grinding of the second group is 0.35 μm. The particles after grinding of the first group and the second group are mixed in a mass ratio of 30:70, and spray dried and sintered for the second time. The second sintering temperature is 800°C, and the holding time is 10 hours.

[0335] The ratio of titanium element in the titanium source in the mixed raw materials to the titanium element in the titanium source added in the primary sintered product is 5:2, and the mass content of titanium element is 6000 ppm based on the total mass of the positive electrode active material.

[0336] Example 7

[0337] The preparation method of Example 7 is substantially the same as that of Example 6, except that

[0338] The D V 50 of the particles after grinding of the first group is 1.50 μm, and the D V 50 of the particles after grinding of the second group is 0.40 μm; the particles after grinding of the first group and the second group are mixed in a mass ratio of 70:30, and spray dried and sintered for the second time.

[0339] Comparative Example 1

[0340] Comparative Example 1 is substantially the same as the preparation method of Example 1, except that in the preparation process of the positive electrode active material, the following methods are different:

[0341] The mixed raw materials were ground twice in a sand mill, and the raw materials after coarse grinding were de-magnetized using a permanent magnet de-ironing device after coarse grinding for 1 h, and the de-magnetization intensity was greater than or equal to 8000 Gs; the de-magnetized raw materials were further finely ground, and the slurry temperature was controlled to be less than 40 ℃ during the grinding process to obtain a mixed slurry; the solid phase particle size D V 50 was 0.5 pm, and spray drying was performed to obtain dry precursor powder.

[0342] The precursor powder was sintered in two stages in a nitrogen atmosphere to obtain a lithium iron phosphate positive electrode material: the temperature was raised from 25 ℃ to 450 ℃ at a rate of 2 ℃ / min (first temperature raising stage), and the temperature was kept constant for 3 h; the temperature was raised from 450 ℃ to 765 ℃ at a rate of 5 ℃ / min (second temperature raising stage), and the temperature was kept constant for 12 h; wherein the gas flow rate in the temperature raising stage was greater than that in the constant temperature stage, and the ratio was 1:1, and the total gas flow rate was 1350 cm 3 / h, and the temperature was lowered after the end; airflow crushing was performed to obtain a D V 50 was 1.0 pm -2.0 pm carbon-coated lithium iron phosphate material.

[0343] Comparative Example 2

[0344] Comparative Example 2 and Example 1 have basically the same preparation method, the difference lies in the following methods during the preparation of the positive active material:

[0345] Lithium carbonate, iron phosphate, titanium dioxide, glucose and polyethylene glycol were added to water, mixed in a premixing tank at a speed of 1400 rpm, and de-magnetized by a de-magnetizing rod with a magnetic field strength of 8000-12000 Gs, wherein the molar ratio of lithium to iron was 1.05:1.0, the mass content of glucose was 6% compared to the total amount of raw materials, and the mass content of polyethylene glycol was 5% compared to the total amount of raw materials, and the mixture was uniformly mixed to obtain a mixed raw material with a solid content of 40%;

[0346] The mixed raw materials were ground twice in a sand mill, and the raw materials after coarse grinding were de-magnetized using a permanent magnet de-ironing device after coarse grinding for 1 h, and the de-magnetization intensity was greater than or equal to 8000 Gs; the de-magnetized raw materials were further finely ground, and the slurry temperature was controlled to be less than 40 ℃ during the grinding process to obtain a mixed slurry; the solid phase particle size D V 50 was 0.35 pm, and spray drying was performed to obtain dry precursor powder, and the D50 after drying was 50-60 pm; the magnetic substance was less than or equal to 70 pcs / kg.

[0347] The precursor powder is sintered in two stages in a nitrogen atmosphere to obtain the lithium iron phosphate positive electrode material: heating from 25 °C to 450 °C at a heating rate of 2 °C / min (first heating stage), holding for 3 h; heating from 450 °C to 820 °C at a heating rate of 5 °C / min (second heating stage), holding for 12 h; wherein the gas volume ratio of the heating stage to the constant temperature stage is 1:1, and the total gas flow is 900 cm 3 / h, and then cooling; and airflow crushing to obtain D V 50 is 1.0 μm -2.0 μm carbon-coated lithium iron phosphate material.

[0348] Performance test

[0349] Self-discharge K value test: after 0.05C constant current charging to 3.0V at 25℃, constant voltage charging to current of 0.05C, after standing at 25℃ for 24h, test open circuit voltage V1, unit: V, continue to stand for 24h, then test open circuit voltage V2, unit: V, self-discharge K value is 1000x(V1-V2) / 48, unit: mV / h.

[0350] DCR test: after 0.33C constant current charging to 3.65V at 25℃, constant voltage charging to current of 0.05C, then 0.33C discharging to 20% SOC, after standing for 5min, 3C pulse discharging for 30s, after standing for 40s, 3C charging for 40s, after standing for 5min, 0.33C constant current charging to 3.65V, constant voltage charging to 0.05C, then 0.33C discharging to 10% SOC, after standing for 5min, 3C pulse discharging for 30s, after standing for 40s, 3C charging for 40s, after standing for 5min, then 0.33C full charging, then 0.33C discharging to 50% SOC, then 1C pulse discharging for 30s after standing for 2h at-25℃, standing for 10min, then standing for 2h at 25℃, 0.33C constant current charging to 3.65V, constant voltage charging to 0.05C, then 0.33C discharging to 20% SOC, then 1C pulse discharging for 30s after standing for 2h at-25℃, standing for 10min.

[0351] Record the voltage at each pulse discharge before and after, calculate the DCR under different conditions, the calculation formula is DCR=(voltage before pulse discharge after standing- voltage after pulse discharge) / pulse current.

[0352] 3. Limiting compaction density test of electrode

[0353] The double-coated electrode sheet is compacted by a roller compactor, and the elongation of the compacted electrode sheet is tested, and the flexibility of the compacted electrode sheet is evaluated. By increasing the pressure of the roller compactor, electrode sheets with different compacted densities are obtained. As the pressure increases, the compacted density of the electrode sheet increases, the elongation of the electrode sheet increases, and the flexibility of the electrode sheet decreases. If the elongation of the electrode sheet is too high, the electrode sheet is prone to warping. If the flexibility of the electrode sheet is too low, the electrode sheet is prone to brittle fracture. Therefore, the smaller of the compacted density corresponding to the elongation of 8% of the electrode sheet or the flexibility of the electrode sheet folded 3 times is defined as the limit compacted density of the electrode sheet.

[0354] The compacted density is calculated by the mass of the positive electrode film layer / the volume of the positive electrode film layer.

[0355] 4. Test of elongation

[0356] The electrode sheet is laid flat on a horizontal table, and the electrode sheet is cut into sections, each about 100 cm long. Remove the copper foil edge substrate of the electrode sheet, and note that the cut edge of the electrode sheet is parallel to the MD direction of the electrode sheet (perpendicular to the direction of the compression roller), and ensure that the electrode sheet is completely covered with the coating. Use a steel ruler to measure the length between the marked points at the same position in the length direction, and estimate to 0.1 mm. Record the length before compaction. Record the length after compaction, and take the elongation of the electrode sheet as (length after compaction-length before compaction) / length before compaction.

[0357] 5. Test of number of flexible folds

[0358] The positive electrode sheet is cut into 20x100mm 2 The test sample is folded in the positive direction, flattened with a 2kg roller, and unfolded to check for light transmission. If there is no light transmission, fold it in the reverse direction, flatten it with a 2kg roller, and check it again against the light. Repeat until light transmission occurs, and record the number of folds. Repeat the test three times and take the average as the reference data for the flexibility of the electrode sheet.

[0359] Test results

[0360] Table 1

[0361]

[0362] *0 in the examples means that the accurate content is difficult to identify because it is below the lower limit of detection.

[0363] As can be seen from the comparison of the examples and the comparative examples, in the section of the positive electrode film layer along the thickness direction of the pole piece, the area ratio of the particles with a particle size of 1 μm or more is 30%-50%; and in the positive electrode film layer, the mass ratio of the magnetic substance is greater than or equal to 20 ppm and less than or equal to 1980 ppm, which is helpful for the lithium ion secondary battery to achieve high pole piece compaction density while making the battery have low self-discharge K value, so that the battery has both high energy density and good storage performance.

[0364] When the area ratio of the particles with a particle size of 1 μm-5 μm in the section of the positive electrode film layer along the thickness direction of the pole piece is 30%-45%, the lithium ion secondary battery has good kinetic performance and high capacity on the basis of achieving high pole piece compaction density while making the battery have low self-discharge K value.

[0365] When the mass content of titanium element is 2500 ppm-8000 ppm based on the total mass of the positive electrode active material, the activity of the raw material can be reduced by the surface inertness of the titanium source, the probability of uneven local chemical reaction and the generation of high content of magnetic substance is reduced, and the average equivalent area ratio of particles above 1 μm is controlled, so that the storage stability and kinetic performance of the lithium ion secondary battery are balanced.

[0366] As can be seen from the comparison of examples 1, 2, 6, 7 and other examples, when the mass content of the magnetic substance in the positive electrode film layer is 20-200 ppm, it is helpful for the lithium ion secondary battery to have low self-discharge K value while maintaining high pole piece compaction density, and the lithium ion secondary battery further improves the kinetic performance while having high energy density and good storage performance.

[0367] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and embodiments having the same technical idea and playing the same role and effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, within the scope of the main idea of the present application, various modifications of the embodiments that can be thought of by those skilled in the art, and other ways constructed by combining part 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, Includes positive electrode, negative electrode, and electrolyte. The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector. The positive electrode film layer includes a positive electrode active material, which comprises lithium transition metal phosphate particles with at least a portion of their surface coated with carbon material. In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with a diameter greater than or equal to 1 μm is 30.0%-50.0%, and the average equivalent area ratio of particles with a diameter greater than 1 μm is greater than or equal to 0.15% and less than 0.2%; and The mass percentage of magnetic material in the positive electrode film 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 ratio of particles with a particle size of 1μm-5μm is 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 ratio of particles with a diameter of 1μm-5μm is 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 along the thickness direction of the electrode sheet, the area ratio of particles with a diameter of 50nm-200nm is 3.0%-15.0%.

5. The lithium-ion secondary battery according to claim 1, characterized in that, In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the area ratio of particles with a diameter of 50nm-200nm is 5.0%-12.0%.

6. The lithium-ion secondary battery according to claim 1, characterized in that, In the cross-section of the positive electrode film along the thickness direction of the electrode sheet, the area ratio of particles with a diameter of 50nm-200nm is 5.0%-10.0%.

7. The lithium-ion secondary battery according to claim 1, characterized in that, In the positive electrode film layer, the mass percentage of the magnetic material is less than or equal to 300 ppm.

8. The lithium-ion secondary battery according to claim 7, characterized in that, In the positive electrode film, the mass percentage of the magnetic material is 20ppm-200ppm.

9. 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.

10. The lithium-ion secondary battery according to claim 9, characterized in that, The mass content of elemental iron in the positive electrode film is less than 20 ppm.

11. The lithium-ion secondary battery according to claim 10, characterized in that, The mass content of elemental iron in the positive electrode film is less than or equal to 15 ppm.

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

1.

13. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode active material includes titanium, and the mass content of titanium is 1500ppm-8000ppm based on the total mass of the positive electrode active material.

14. The lithium-ion secondary battery according to claim 13, characterized in that, The positive electrode active material includes titanium, and the mass content of titanium is 2500ppm-8000ppm based on the total mass of the positive electrode active material.

15. The lithium-ion secondary battery according to claim 13, characterized in that, The positive electrode active material includes titanium, and the mass content of titanium is 2500ppm-6000ppm based on the total mass of the positive electrode active material.

16. 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 percentage of carbon is 0.9%-1.8%.

17. The lithium-ion secondary battery according to claim 1, characterized in that, In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the laser microscopy confocal Raman spectroscopy instrument scanning mode, the median C of the graphitization degree is... 50 ≥0.9 and ≤1.3, wherein the degree of graphitization C is I. G / I D , where I G This indicates that the Raman spectrum is at 1580±100 cm⁻¹ -1 The intensity of peak G at I D This indicates that the Raman spectrum is at 1350±100 cm⁻¹ -1 The intensity of peak D at that location.

18. The lithium-ion secondary battery according to claim 17, characterized in that, In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the laser microscopy confocal Raman spectroscopy instrument scanning mode, the median C of the graphitization degree is... 50 The value is 0.99-1.

2.

19. 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 electrode thickness direction, the median L of sphericity is... A50 It ranges from 0.60 to 0.

85.

20. The lithium-ion secondary battery according to claim 19, characterized in that, In the cumulative distribution curve of particle sphericity area obtained from the cross-section of the positive electrode film along the electrode thickness direction, the median L of sphericity is... A50 It is 0.65-0.

80.

21. The lithium-ion secondary battery according to claim 1, characterized in that, The compacted density of the positive electrode active material under 3T pressure is 2.48 g / cm³. 3 -2.76g / cm 3 .

22. The lithium-ion secondary battery according to claim 21, characterized in that, The compacted density of the positive electrode active material under 3T pressure is 2.58 g / cm³. 3 -2.76g / cm 3 .

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

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

25. 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%.

26. The lithium-ion secondary battery according to claim 1, characterized in that, The single-sided density of the positive electrode film is 300 mg / 1540 mm². 2 -450mg / 1540mm 2 .

27. The lithium-ion secondary battery according to claim 1, characterized in that, In the fully discharged state, the compaction density of the positive electrode film in the lithium-ion secondary battery is 2.43 g / cm³. 3 -2.78g / cm 3 .

28. The lithium-ion secondary battery according to claim 1, characterized in that, In the fully discharged state, the compaction density of the positive electrode film in the lithium-ion secondary battery is 2.50 g / cm³. 3 -2.75g / cm 3 .

29. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode film layer satisfies the following conditions: In the fully discharged state, the compaction density of the positive electrode film in the lithium-ion secondary battery is 2.43 g / cm³. 3 -2.78g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-28%.

30. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode film layer satisfies the following conditions: In the fully discharged state, the positive electrode film of the lithium-ion secondary battery has a compaction density of 2.50 g / cm³. 3 -2.78g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-22%.

31. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode sheet includes a base coating layer disposed between the positive electrode film layer and the positive electrode current collector; the base coating layer satisfies at least one of the following conditions: (1) The base coating includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the base coating is ≤10 pcs / 10 μm; (2) The compacted density of the positive electrode sheet in the fully loaded state is greater than or equal to 2.4 g / cm³. 3 The thickness of the base coating layer on one side is 1μm-4μm.

32. The lithium-ion secondary battery according to claim 31, characterized in that, The compacted density of the positive electrode sheet in its fully loaded state is greater than or equal to 2.5 g / cm³. 3 The thickness of the base coating layer on one side is 2μm-4μm.

33. A battery device, characterized in that, The battery device includes any one of claims 1 to 32, wherein the battery device comprises at least one of a battery module, a battery pack, and an energy storage battery.

34. An electrical appliance, characterized in that, The lithium-ion secondary battery includes any one of claims 1 to 32.

35. 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; after grinding, a mixed slurry is obtained, wherein the solid phase volume distribution particle size D in the mixed slurry is... V50 The particle size is 0.3μm-0.4μm; after drying the mixed slurry, a precursor powder is obtained; 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 32, wherein the sintering is carried out in an inert gas environment and the total gas flow rate during the sintering process is 1100 m³ / h. 3 / h-1400m 3 / h; the sintering includes a heating zone and a constant temperature zone, wherein the inert gas flow rate v1 in the heating zone is higher than the inert gas flow rate v2 in the constant temperature zone; the constant temperature zone temperature of the sintering includes 770℃-830℃; the positive electrode active material includes lithium transition metal phosphate particles with carbon-coated material on at least part of their surface.

36. A method for preparing a positive electrode sheet, characterized in that, The preparation method includes: The binder, conductive agent, and positive electrode active material prepared by the preparation method of claim 35 are added sequentially, dry mixed, and then a solvent is added and stirred to obtain a slurry. The slurry is then transferred and coated onto at least one side of the positive electrode current collector, dried, and hot-pressed to obtain the positive electrode sheet used in the lithium-ion secondary battery according to any one of claims 1 to 32.

37. The preparation method according to claim 36, characterized in that, The hot pressing includes at least three hot roller pressings, with the hot roller pressure increasing sequentially to 20-50 tons, 50-70 tons, and 70-90 tons; the hot roller temperature is 40℃-80℃. Before the first hot roller compaction, the electrode is heated to a temperature of 40℃-50℃.

38. The preparation method according to claim 36, characterized in that, The transfer coating speed is 1 m / min - 25 m / min.

Images