Positive electrode active material and preparation method and application thereof

Abstract

The invention provides a positive electrode active material and a preparation method and application thereof, the positive electrode active material comprises spherical or sphere-like secondary particles, the sphere center of each secondary particle is C, and the radius of each secondary particle is R; in the secondary particles, a region taking C as a starting point and 3 / 4 R as a radius is a first part, and a region except the first part is a second part; wherein the aperture r1 of the pores in the first part is greater than the aperture r2 of the pores in the second part. Due to the special structure of the positive electrode active material, the positive electrode active material has excellent lithium ion transmission capability on the premise of ensuring certain mechanical strength, so that the positive electrode active material is beneficial to improving the cycle performance and rate capability of the secondary battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of batteries, in particular to a positive electrode active material and a preparation method and application thereof. BACKGROUND

[0002] With the rapid development of energy storage technology, secondary batteries, as a kind of efficient and repeatable charge-discharge energy storage device, have penetrated into all aspects of social life. From portable electronic devices such as smart phones and notebook computers, to large-scale electric vehicle industry and grid-level energy storage systems, secondary batteries play an indispensable core role. Especially in the context of global green energy transformation, higher and more comprehensive requirements are put forward for the performance of secondary batteries, and the level of its technical development is directly related to the breadth and depth of clean energy utilization.

[0003] Among many positive electrode materials, ternary positive electrode active materials (lithium nickel cobalt manganese oxide NCM or lithium nickel cobalt aluminum oxide NCA) have become one of the preferred materials for current high-range electric vehicle power batteries due to their high energy density and high working voltage. By adjusting the ratio of elements such as nickel, cobalt and manganese, ternary positive electrode active materials can store more electric charges in unit mass or unit volume, thereby significantly improving the endurance of secondary batteries. This feature makes it dominant in application scenarios that pursue extreme energy density, driving the evolution of power battery technology.

[0004] However, as the market demands for fast charging capability and ultra-long cycle life of secondary batteries become increasingly demanding, the limitations of traditional ternary positive electrode active materials in fast charging and service life have gradually become apparent. Therefore, in-depth optimization and modification of ternary positive electrode active materials to overcome the bottleneck of fast charging and long cycle performance are of great significance for further expanding their application space and promoting the development of the next generation of high-performance secondary batteries. SUMMARY

[0005] The present application provides a positive electrode active material, which has a special structure that allows the positive electrode active material to have excellent lithium ion transmission capacity while ensuring a certain mechanical strength, thus helping to improve the cycle performance and rate performance of secondary batteries.

[0006] The present application also provides a preparation method of the above-mentioned positive electrode active material, which is used for preparing the above-mentioned positive electrode active material.

[0007] The present application also provides a positive electrode sheet comprising the above-mentioned positive electrode active material, which significantly improves the rate performance and cycle performance of secondary batteries.

[0008] The application also provides a secondary battery including the above positive electrode sheet, so that the secondary battery has a shorter fast charging time and a longer service life.

[0009] The application provides a positive electrode active material, which includes spherical or spherical-like secondary particles, a spherical center of the secondary particles is C, and a radius is R; in the secondary particles, a region with 3 / 4R as a radius and C as a starting point is a first part, and a region outside the first part is a second part; wherein an average pore size r1 of pores in the first part is greater than an average pore size r2 of pores in the second part.

[0010] The positive electrode active material as described above, wherein (r1-r2)≥50nm; and / or,

[0011] An average porosity P1 of the first part is less than an average porosity P2 of the second part; and / or,

[0012] The average porosity P1 of the first part is 2-10%, and the average porosity P2 of the second part is 5-20%; and / or,

[0013] r1 is 100-800nm, and r2 is 10-100nm.

[0014] The positive electrode active material as described above, wherein the positive electrode active material includes an inner core and a coating layer arranged on at least part of a surface of the inner core, the inner core includes the secondary particles, and the coating layer includes an electrophilic electrolyte organic functional group and / or an oxide of element Q, Q includes at least one of Zr, Y, Rb, Cs, W, Ce, Mo, Ba, Ti, Mg, Al, Ta and Nb.

[0015] The positive electrode active material as described above, wherein the electrophilic electrolyte organic functional group includes at least one of an ester group, an ether group, a hydroxyl group, a carbonyl group, an amino group, an imino group, a pyridyl group, an imidazolyl group, a quaternary ammonium salt group, a carboxyl group, a sulfonic acid group, a phosphate group, a thiol group, a sulfonamide group, a carboxylate group and a sulfonate group.

[0016] The positive electrode active material as described above, wherein a Span of the positive electrode active material is 0.9-1.5, and a specific surface area BET is 0.7-1.3m 2 / g; and / or,

[0017] The positive electrode active material has a chemical composition of Li a Ni x Co y A z T m Q n O2@OG swherein: 0.8≤a≤1.2, 0.5≤x≤0.96, 0≤y≤0.5, 0≤z≤0.3, 0≤m≤0.05, 0≤n≤0.1, s≥0, x+y+z+m+n=1, A comprises at least one of Mn and Al, T comprises at least one of Zr, Y, Rb, Cs, W, Ce, Mo, Ba, Ti, Mg, Ta and Nb, Q comprises at least one of Zr, Y, Rb, Cs, W, Ce, Mo, Ba, Ti, Mg, Al, Ta and Nb, and OG comprises an electrophilic electrolyte organic functional group.

[0018] The application also provides a preparation method of the positive electrode active material, comprising the following steps:

[0019] The first reaction is performed by using a first raw material solution comprising a metal source and a first pore-forming agent, and after an intermediate is obtained, the injection of the first pore-forming agent is stopped; a second reaction is performed by using a second raw material solution comprising the metal source and the intermediate, and a positive electrode active material precursor is obtained; the Dv50 of the intermediate is 3 / 4 of the target Dv50;

[0020] The sintering treatment is performed by using a mixed raw material comprising the positive electrode active material precursor, a second pore-forming agent and a lithium source, and the positive electrode active material is obtained;

[0021] The median particle size D1 of the first pore-forming agent is greater than the median particle size D2 of the second pore-forming agent, the melting point of the first pore-forming agent is less than the temperature of the sintering treatment, the melting point of the second pore-forming agent is greater than the temperature of the sintering treatment, and the second pore-forming agent is a water-soluble salt.

[0022] The preparation method described above, wherein the positive electrode active material is a ternary positive electrode active material, D1 is 100-800nm, the amount of substance of the first pore-forming agent is 2-5% of the total amount of substance of the metal source; D2 is 10-100nm, and the molar ratio of the second pore-forming agent to the positive electrode active material precursor is 0.001:1-0.02:1; and / or,

[0023] The first pore-forming agent comprises at least one of a metal hydroxide and a polymer particle, and the second pore-forming agent comprises at least one of a sulfate, a phosphate or a silicate.

[0024] The preparation method described above, wherein the preparation method further comprises a coating treatment on the sintered product after the sintering treatment;

[0025] The coating treatment comprises mixing and heating the sintered product with a solution containing an electrophilic electrolyte organic functional group until the solvent volatilizes.

[0026] The application provides a positive electrode sheet comprising the positive electrode active material according to any one of the preceding aspects, or the positive electrode active material prepared by any one of the preceding preparation methods.

[0027] The application provides a secondary battery comprising the positive electrode sheet according to the preceding aspect.

[0028] The application provides a positive electrode active material, by arranging an ordered hierarchical pore structure in the positive electrode active material, on the one hand, the permeation efficiency of electrolyte in the positive electrode active material and the transmission capacity of lithium ions are significantly improved, on the other hand, the positive electrode active material also provides a buffer space for the volume expansion in the cycle, therefore, the positive electrode active material of the application can realize the rapid permeation of electrolyte, the efficient transmission of lithium ions and the improvement of structural stability, ultimately endowing the secondary battery with more excellent rate performance and cycle performance, shortening the fast charging time of the secondary battery, prolonging the service life of the secondary battery, and promoting the development and application of high-performance secondary batteries. BRIEF DESCRIPTION OF DRAWINGS

[0029] The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate embodiments consistent with the present application and, together with the description, further serve to explain the principles of the application.

[0030] Figure 1 SEM image of the positive electrode active material of Example 1 of the application;

[0031] Figure 2 SEM cross-sectional image of the positive electrode active material of Example 1 of the application.

[0032] The specific embodiments of the application have been shown and described in the foregoing drawings and specification, it will be understood by those skilled in the art that various other modifications can be made of the application concept without departing from the scope thereof. Accordingly, other embodiments are within the scope of the following claims. DETAILED DESCRIPTION

[0033] The exemplary embodiments are hereinafter described in detail with reference to the accompanying drawings. In the following description, the same drawings reference numbers are used to denote like or similar elements. The embodiments described in the following exemplary embodiments are not representative of all embodiments consistent with the present application. Rather, they are merely examples of apparatuses and methods consistent with some aspects of the present application, as detailed in the appended claims.

[0034] At present, the rate performance and cycle performance of secondary batteries still cannot meet the application expectations. The inventors have researched this and believe that the reason why the secondary batteries have this defect at present is that the gap between primary particles in the traditional polycrystalline particles is narrow and long, and the electrolyte is difficult to penetrate into the deep part of the particles, resulting in large lithium ion transmission resistance. Under the background of pursuing high energy density, the pores of the traditional material are compressed under high compaction, and the lithium ion diffusion path is blocked, so the capacity of the battery is limited when high-rate charging and discharging. At the same time, the traditional polycrystalline particles are prone to volume expansion and crack propagation during the cycle process, which is also the main reason for reducing the cycle life of the secondary battery. Therefore, how to ensure the smooth transmission of the electrolyte while maintaining the structural stability of the positive active material is the key to breaking this technical bottleneck.

[0035] Based on this, the first aspect of the present application provides a positive active material, the positive active material comprising a spherical or spheroid-like secondary particle, the spherical center of the secondary particle being C and the radius being R; in the secondary particle, the region with C as the starting point and with a radius of 3 / 4R is the first part, and the region outside the first part is the second part; wherein the average pore size r1 of the pores in the first part is greater than the average pore size r2 of the pores in the second part.

[0036] The spherical or spheroid-like secondary particle in the present application refers to that the basic unit of the positive active material is a secondary particle formed by agglomeration of a plurality of nanoscale or microscale primary particles. Its form is full spherical (the distance deviation of any point on the particle surface to the center is ≤5%) or spheroid-like (such as ellipsoidal, oblate spherical, the distance deviation of any point on the surface to the center is ≤15%), which can be confirmed by scanning electron microscope observation. Among them, the spherical center C refers to the geometric center of the spherical or spheroid-like secondary particle, for example, the contour of the secondary particle SEM image is fitted by image processing software, and the circumscribed circle (or similar circumscribed circle) of the particle contour is calculated, and the center of the circumscribed circle is the spherical center C; or for irregular spheroid-like particles in actual production, the intersection of the line connecting the midpoints of the longest diameter and the shortest diameter can be taken as the spherical center C. The radius R refers to the maximum distance from the spherical center C to the particle surface, that is, the radius of the above circumscribed circle (or 1 / 2 of the longest diameter), which can be obtained by particle size analyzer test.

[0037] In the present application, a spherical surface is formed by extending from the spherical center C to 3 / 4R in any direction, and the region surrounded by the spherical surface is the first part, and the annular region between the spherical surface of the first part and the outer surface of the secondary particle is the second part.

[0038] As aforementioned, the secondary particles in the positive electrode active material of the present application are formed by agglomeration of multiple primary particles, thus the secondary particles have multiple pores inside, wherein the average pore diameter r1 of the pores in the first part is larger than the average pore diameter r2 of the pores in the second part. On one hand, the first part has a larger volume with a radius of 3 / 4R, and the first part has pores with a larger average pore diameter r1, thus the first part has excellent liquid storage function, and can ensure that there is always sufficient supply of electrolyte deep in the secondary particles; while the pores with a relatively small average pore diameter r2 in the second part can form a capillary network with high specific surface area, quickly introducing electrolyte from the surface to the inside through short-range diffusion and capillary force, shortening the diffusion path of lithium ions. At the same time, the pores with a relatively small average pore diameter r2 divide the originally dense secondary particles into smaller regions closer to the pores, and lithium ions only need to diffuse a very short distance in a small dense region to reach the electrolyte interface in the pores, greatly shortening the distance of lithium ions from the bulk phase to the electrolyte / electrode interface. On the other hand, the pores with a larger average pore diameter r1 in the first part and the larger volume of the first part can provide a buffer space for the volume expansion of the positive electrode active material during cycling, directly avoiding the cracking of the positive electrode active material due to the expansion and extrusion, reducing the sudden rise of interface impedance caused by the crushing of the positive electrode active material, reducing the probability of side reactions between the electrolyte and the positive electrode active material, and significantly prolonging the cycle life of the battery. At the same time, the overall structural integrity of the electrode sheet can be maintained to ensure the continuous smoothness of the lithium ion transport channel.

[0039] Finally, the positive electrode active material of the present application can still achieve efficient electrolyte permeation and rapid lithium ion transport even at higher compaction density or higher charging voltage, significantly reduce the concentration polarization effect, maintain good structural stability, and improve the high-rate charge and discharge performance and cycle life.

[0040] The present application does not limit the specific values of the average pore diameter r1 of the pores in the first part and the average pore diameter r2 of the pores in the second part. In a specific embodiment, when (r1-r2)≥50nm, the significant pore diameter difference between the large pores and the small pores can form a more obvious capillary force gradient, further driving the transmission and permeation of electrolyte inside the secondary particles, and further improving the transmission rate of lithium ions.

[0041] Based on the further research of the inventors, it is found that when the average porosity P1 of the first part is less than the average porosity P2 of the second part, the large porosity of the second part provides a smooth channel for electrolyte permeation, shortens the diffusion path of lithium ions, reduces the interface impedance, especially improves the ion transmission rate during high-rate charge and discharge, and avoids excessive polarization. The small porosity of the first part not only can reduce the dead space inside the positive electrode active material, improve the packing density of the active material, and increase the electrode volume energy density, but also can enhance the bonding strength between the primary particles and maintain the structural integrity of the positive electrode sheet.

[0042] Further, when the average porosity P1 of the first portion is 2-10%, and the average porosity P2 of the second portion is 5-20%, the rate capability and cycle stability of the battery can be further improved.

[0043] In a specific embodiment, r1 is 100-800 nm, and r2 is 10-100 nm. Within this average pore size range, the positive electrode active material not only has the advantage of the aforementioned size pores cooperating to improve the rate capability and cycle performance of the secondary battery, but also can significantly improve the energy density of the secondary battery. In detail, the above suitable average pore size can maximize the filling rate of the positive electrode active material in the secondary battery while retaining the necessary pores (ensuring electrolyte infiltration and ion transport), avoiding excessive reduction of the mass percentage content of the positive electrode active material in the positive electrode sheet due to an excessively large average pore size. In addition, this average pore size distribution can also avoid the shedding or structural collapse of the positive electrode active material during cycling, reduce capacity decay, and ensure the stability of the energy density of the battery during long-term use.

[0044] Further, the positive electrode active material of the present application includes a core and a coating layer arranged on at least part of the surface of the core, and the core includes secondary particles.

[0045] In a specific embodiment, the coating layer includes an electrolyte-philic organic functional group. The electrolyte-philic organic functional group is a type of organic molecular fragment containing a specific polar structure, heteroatom or ionization site, which can form a stable interaction with the electrolyte.

[0046] Specifically, the electrolyte-philic organic functional group in the coating layer establishes the starting point of the channel for the electrolyte to penetrate into the interior of the particle, can reduce the surface energy of the secondary particle, improve the spreading and wetting of the electrolyte on the outermost surface of the secondary particle, greatly reduce the resistance of the electrolyte into the pore network in the second portion, and accelerate the electrolyte absorption process driven by capillary force. At the same time, the coating layer physically separates the secondary particle from direct contact with the electrolyte, effectively inhibits the interface side reaction and transition metal dissolution, and protects the advantages brought by the pore classification structure in the secondary particle. In addition, the organic coating layer can also provide certain constraints and buffers outside the secondary particle, which combines with the first portion with larger pores to improve the structural stability of the positive electrode active material, helps to maintain the integrity of the porous structure, and prolongs the cycle life.

[0047] Illustratively, the electrolyte-philic organic functional group of the present application includes at least one of an ester group, an ether group, a hydroxyl group, a carbonyl group, an amino group, an imino group, a pyridyl group, an imidazolyl group, a quaternary ammonium salt group, a carboxyl group, a sulfonic acid group, a phosphate group, a thiol group, a sulfonamide group, a carboxylate group, and a sulfonate group.

[0048] In another specific embodiment, the coating layer comprises an oxide of element Q, Q comprising at least one of Zr, Y, Rb, Cs, W, Ce, Mo, Ba, Ti, Mg, Al, Ta and Nb. At this time, the coating layer of the oxide physically separates the secondary particles from direct contact with the electrolyte, effectively inhibiting the interface side reaction and transition metal dissolution, and protecting the advantages brought by the hierarchical pore structure in the secondary particles.

[0049] To further improve the rate performance and cycle performance of the secondary battery, the Span value of the positive electrode active material is also controlled in the application. Specifically, Span = (Dv90-Dv10) / Dv50, when the Span value of the positive electrode active material of the application is 0.9-1.5, both the large particle gap of the positive electrode active material can be avoided to be excessively filled by the small particles of the positive electrode active material, so as to form a continuous and uniform ion transmission channel, and the particle agglomeration can be reduced, the stress concentration in the electrode sheet is reduced, the particle rupture and interface impedance rise in the cycle process are inhibited, and the battery life is prolonged. Among them, Dv10 represents the particle size of the positive electrode active material particles when reaching 10% of the volume accumulation from the small particle size in the volume-based particle size distribution; Dv50 represents the particle size of the positive electrode active material particles when reaching 50% of the volume accumulation from the small particle size in the volume-based particle size distribution; Dv90 represents the particle size of the positive electrode active material when reaching 90% of the volume accumulation from the small particle size in the volume-based particle size distribution.

[0050] In addition, the specific surface area BET of the positive electrode active material also has a certain influence on the performance of its electrochemical reaction. The surface area of the positive electrode active material is further controlled to be 0.7-1.3 m 2 / g in the application, which ensures the appropriate contact area of the positive electrode active material and the electrolyte, accelerates the lithium ion deintercalation reaction, and further improves the battery rate performance and charge-discharge efficiency.

[0051] The chemical composition of the positive electrode active material is not specifically limited in the application. In one specific embodiment, the chemical composition of the positive electrode active material is Li a Ni x Co y A z T m Q n O2@OG swherein: 0.8≤a≤1.2, 0.5≤x≤96, 0≤y≤0.5, 0≤z≤0.3, 0≤m≤0.05, 0≤n≤0.1, s≥0, x+y+z+m+n=1, A comprises at least one of Mn and Al, T comprises at least one of Zr, Y, Rb, Cs, W, Ce, Mo, Ba, Ti, Mg, Al, Mn, Ta and Nb, Q comprises at least one of Zr, Y, Rb, Cs, W, Ce, Mo, Ba, Ti, Mg, Al, Ta and Nb, and OG comprises an electrophilic electrolyte organic functional group.

[0052] wherein Q is a coating element.

[0053] The second aspect of the present application provides a preparation method of the positive electrode active material of the first aspect, comprising the following steps:

[0054] After the intermediate is obtained by performing the first reaction on the first raw material solution comprising the metal source and the first pore-forming agent, the injection of the first pore-forming agent is stopped; the second raw material solution comprising the metal source and the intermediate is subjected to the second reaction to obtain the positive electrode active material precursor; the Dv50 of the intermediate is 3 / 4 of the target Dv50;

[0055] The sintering treatment is performed on the mixed raw material comprising the positive electrode active material precursor, the second pore-forming agent and the lithium source to obtain the positive electrode active material;

[0056] wherein the median particle size D1 of the first pore-forming agent is greater than the median particle size D2 of the second pore-forming agent, the melting point of the first pore-forming agent is less than the temperature of the sintering treatment, the melting point of the second pore-forming agent is greater than the temperature of the sintering treatment, and the second pore-forming agent is a water-soluble salt.

[0057] In detail, the metal source refers to a chemical composition for composing the secondary particles of the positive electrode active material. Taking a ternary positive electrode active material as an example, the metal source is a nickel source, a cobalt source and a manganese source (an aluminum source). The injection of the first pore-forming agent is stopped when the intermediate with the Dv50 of 3 / 4 of the target Dv50 is obtained by performing the first reaction on the first raw material solution comprising the metal source and the first pore-forming agent. The target Dv50 specifically refers to the Dv50 value of the positive electrode active material, which is determined as a fixed value before preparation.

[0058] Subsequently, the second reaction is stopped when the particle size of the intermediate gradually grows to the target Dv50 by performing the second reaction on the second raw material solution comprising the metal source and the intermediate to obtain the positive electrode active material precursor. In the first reaction, the first pore-forming agent is embedded into the particle interior to obtain the positive electrode active material precursor comprising the first pore-forming agent.

[0059] In addition to the metal source and the first pore-forming agent, the first raw material solution also includes a solvent, which can be deionized water, for example. The present application does not limit the specific conditions of the first reaction, which can be performed under heating (40-80°C) and stirring, for example.

[0060] Subsequently, the mixed raw materials including the positive electrode active material precursor, the second pore-forming agent, and the lithium source are subjected to a sintering process to obtain the positive electrode active material. Since the melting point of the first pore-forming agent is lower than the temperature of the sintering process, the first pore-forming agent is pyrolyzed during the sintering process, thereby forming larger pores in the first part of the positive electrode active material. In addition, since the surface of the positive electrode active material precursor is rough and porous, the second pore-forming agent can enter the positive electrode active material precursor along with the molten lithium source during the sintering process, and then the second pore-forming agent can occupy the second part of the positive electrode active material formed, thereby avoiding the porous structure of the positive electrode active material precursor from being filled during the sintering process. Since the second pore-forming agent is a water-soluble salt, it can be removed by a simple process such as water washing after the sintering process is completed, thereby leaving smaller pores with an average pore size in the second part.

[0061] The present application does not limit the temperature of the sintering process, as long as the lithium source can react with the positive electrode active material precursor.

[0062] The preparation method of the present application can precisely control the average pore size and distribution of the pores in the first part and the second part by adjusting the particle size difference and the addition sequence of the first pore-forming agent and the second pore-forming agent, thereby realizing the gradient design of the internal pores of the positive electrode active material. This step-by-step pore-forming preparation method is simple to operate and easy to scale up industrially, and can stably produce positive electrode active materials with specific pore structures, thereby providing a feasible industrialization path to solve the problem that the electrolyte transport and structural stability are difficult to balance in the prior art.

[0063] It should be noted that the water solubility of the second pore-forming agent of the present application refers to the solubility of the second pore-forming agent in deionized water at 25°C, which is not less than 5 g / 100 mL, so as to ensure that it can be fully removed by a conventional water washing process after sintering, thereby avoiding the influence of residual impurities on the electrochemical performance of the positive electrode active material.

[0064] Taking a ternary material as an example, the preparation method of the present application is explained in more detail as follows.

[0065] First, a bottom liquid is added to a reaction kettle, nitrogen is introduced to replace air, the stirring rate and the kettle temperature are stably controlled at a certain value, a metal salt solution containing Ni, Co, Mn (or Al) elements, a complexing agent, a precipitant, and a first pore-forming agent are continuously injected into the reaction kettle at a certain flow rate to perform a first reaction (co-precipitation reaction), and the pH value in the kettle needs to be maintained at 10.5-12 by controlling the flow rate of the precipitant during the reaction. The solid phase particle size is tested every 1h, and when the Dv50 reaches 3 / 4 of the target Dv50 of the product, the injection of the first pore-forming agent is stopped, and the remaining parameters remain unchanged to continue the second reaction, and finally a slurry including a positive electrode active material precursor is obtained. The temperature and stirring rate of the first reaction and the second reaction are not limited in the present application, for example, they can be 40-80℃ and 200-600rpm / min, respectively, and finally a precursor slurry is obtained.

[0066] The specific selection of various raw materials in the above first reaction is not additionally limited in the present application. For example, the metal salt solution can be a mixed aqueous solution of sulfate or nitrate of Ni, Co, Mn (or Al) elements, and further can be a mixed salt solution of 1-3mol / L, wherein the molar ratio of Ni, Co, Mn (or Al) elements can be determined according to the target composition. The precipitant can be, for example, a NaOH aqueous solution with a concentration of 2-15mol / L, and the complexing agent can be, for example, ammonia water with a concentration of 2-10mol / L. The bottom liquid can be a mixture of deionized water, complexing agent, and precipitant, and the pH thereof can be in the range of 12.0-12.5.

[0067] Subsequently, the precursor slurry is sequentially filtered, washed, centrifuged, and dried to obtain a positive electrode active material precursor containing the first pore-forming agent.

[0068] Finally, after sintering treatment, washing treatment, and drying treatment of the mixed raw materials including the positive electrode active material precursor, a lithium source, a dopant, and a second pore-forming agent, a ternary positive electrode active material is obtained. In the preparation process of the ternary positive electrode active material, the sintering temperature is not less than 500℃. At this time, the decomposition temperature of the first pore-forming agent is less than 500℃. It can be understood that the highest temperature of the sintering treatment is not limited in the present application, for example, when the melting point of the second pore-forming agent is greater than 1000℃, 1100℃, 1200℃, or 1500℃, the sintering temperature can be appropriately increased to be greater than the melting point of the second pore-forming agent. In addition, the sintering time can be 1-20h, and the sintering atmosphere can be an oxygen atmosphere, a nitrogen atmosphere, or an air atmosphere, and more further can be an oxygen atmosphere.

[0069] Further, the dopant refers to an oxide, a carbonate compound, a sulfate compound or a hydroxide of element T, the element T is selected from at least one of Zr, Y, Rb, Cs, W, Ce, Mo, Ba, Ti, Mg, Ta and Nb, and the molar ratio of the dopant to the positive electrode active material precursor is 0.001:1-0.01:1. The lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium acetate, lithium sulfate, lithium nitrate or lithium chloride and the like soluble lithium salt, and further is lithium hydroxide.

[0070] Further, the washing agent can be water or an ethanol solution, and further is water. When the washing agent is water, the mass ratio of water to the positive electrode active material precursor is 0.5:1-5:1, and further is 0.8:1-2:1. The washing time is, for example, 30s-30min, and further is 1min-15min. After the washing treatment, the washing system is further subjected to solid-liquid separation, and then the filter cake is subjected to drying treatment. Optionally, the drying treatment is performed at a temperature of 80-200℃, and further is 110-180℃. Optionally, the drying treatment is performed in an air atmosphere, a nitrogen atmosphere or a vacuum condition, and further is a vacuum condition.

[0071] In addition, the product after the drying treatment can be mixed with a coating agent, and then subjected to secondary calcination to obtain a positive electrode active material containing a coating layer of oxide of element Q. The molar ratio of the coating agent to the product after the drying treatment is 0.002:1-0.02:1. The coating agent includes an oxide of element Q, a carbonate compound of element Q, a sulfate compound of element Q or a hydroxide of element Q, and the element Q includes at least one of Zr, Y, Rb, Cs, W, Ce, Mo, Ba, Ti, Mg, Al, Ta and Nb. The secondary calcination is performed at a temperature of 150-750℃, and further is 250-700℃; the calcination is performed for a time of 2-20h, and further is 5-15h. The calcination is performed in an oxygen atmosphere, a nitrogen atmosphere or an air atmosphere, and further is an oxygen atmosphere.

[0072] Further, when the positive electrode active material is a ternary positive electrode active material, by controlling D1 to be 100-800nm, the mass of the first pore-forming agent to be 2-5% of the total molar mass of the metal source, D2 to be 10-100nm, and the molar ratio of the second pore-forming agent to the positive electrode active material precursor to be 0.001:1-0.02:1, the first part and the second part can have more suitable pore radius and average porosity, so that the rate performance and the cycle performance of the secondary battery are further improved.

[0073] In one specific embodiment, the first pore-forming agent comprises at least one of a metal hydroxide and polymer particles. The metal element in the metal hydroxide is selected consistent with the metal element in the positive electrode active material. For example, when the positive electrode active material is a ternary positive electrode active material, the metal hydroxide comprises at least one of cobalt hydroxide, nickel hydroxide, manganese hydroxide, and aluminum hydroxide; the polymer particles may be, for example, at least one of polystyrene microspheres and polymethyl methacrylate microspheres.

[0074] In one embodiment, the second pore-forming agent comprises at least one of a sulfate, a phosphate, or a silicate. Exemplarily, the silicate comprises at least one of sodium silicate, potassium silicate, sodium aluminum silicate, magnesium silicate, and lithium silicate. The sulfate comprises at least one of lithium sulfate and sodium sulfate. The phosphate comprises at least one selected from lithium phosphate and calcium phosphate.

[0075] As mentioned above, when the positive electrode active material of this application is a core-shell coated structure containing electrolyte-loving organic functional groups, it also includes coating treatment of the sintered product after sintering treatment; the coating treatment includes: mixing the sintered product with a solution containing electrolyte-loving organic functional groups and heating until the solvent evaporates.

[0076] The solution containing electrophilic organic functional groups includes a solvent and an organic coating agent containing these groups. The electrophilic organic functional groups include at least one selected from the following groups: ester, ether, hydroxyl, carbonyl, amino, imino, pyridyl, imidazolyl, quaternary ammonium salt, carboxyl, sulfonic acid, phosphate ester, thiol, sulfonamide, carboxylate, and sulfonate. For example, the organic coating agent includes at least one selected from polyethylene glycol, polyvinylpyrrolidone, polycarbonate, polyvinyl alcohol, and polymethyl methacrylate; the solvent includes at least one selected from ethanol, glycerol, ethylene glycol, methanol, isopropanol, N-methylpyrrolidone, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and tetrahydrofuran.

[0077] Further, the heating temperature is 50-250℃, the stirring speed is 60-1200 rpm, and the mass ratio of organic coating agent to sintered product is (0.2:1)-(1:1). After the solution has completely evaporated, the remaining solid product is dried to obtain a core-shell structured positive electrode active material, wherein the core of the positive electrode active material includes the aforementioned secondary particles, and the coating layer includes electrolyte-loving organic functional groups. Exemplarily, the drying atmosphere is air or vacuum, and more specifically, vacuum drying, the drying time is 1-10 hours, and the drying temperature is 80-150℃.

[0078] The third aspect of the present application provides a positive electrode sheet, which comprises the positive electrode active material of the first aspect or the positive electrode active material prepared in the second aspect. Therefore, the positive electrode sheet has the advantage of being able to effectively improve the rate performance and cycle performance of the secondary battery.

[0079] In a specific embodiment, the positive electrode sheet of the present application specifically comprises a positive electrode current collector and a positive electrode active layer comprising a positive electrode active material arranged on the surface of the positive electrode current collector.

[0080] In the specific preparation of the positive electrode sheet, for example, the positive electrode active material of the present application can be dispersed with a conductive agent and a binder in an appropriate amount of N-methylpyrrolidone (NMP) solvent, and the mixture is sufficiently stirred to form a uniform positive electrode slurry; the positive electrode slurry is uniformly coated on the positive electrode current collector, and then dried, rolled and cut to obtain the positive electrode sheet. In a specific embodiment, the positive electrode active layer comprises 70-99 wt% of the positive electrode active material, 0.5-15 wt% of the conductive agent, and 0.5-15 wt% of the binder in terms of mass percentage, and further comprises 80-98 wt% of the positive electrode active material, 1-10 wt% of the conductive agent, and 1-10 wt% of the binder.

[0081] The material of the positive electrode current collector can be at least one of an aluminum foil and a nickel foil; the conductive agent can be at least one selected from carbon black, acetylene black, graphene, ketjen black, and carbon fiber; and the binder can be at least one selected from polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, an oxirane-containing polymer, polyvinylpyrrolidone, and polyurethane.

[0082] The fourth aspect of the present application provides a secondary battery, which comprises the positive electrode sheet of the third aspect. Therefore, the secondary battery of the present application has excellent rate performance and cycle performance.

[0083] It is conceivable that the secondary battery of the present application comprises, in addition to the above-mentioned positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator.

[0084] The application is not strictly limited to the negative active material in the negative electrode sheet, and can be at least one of the currently commonly used negative active materials, such as graphite, hard carbon, soft carbon, mesocarbon microbeads, silicon-based negative materials (mainly including silicon monoxide, silicon-carbon negative electrode), tin-based negative materials (mainly including tin, tin alloy) and the like. The application is not strictly limited to the selection of electrolyte, which can include one or more of the commonly used solvents in the electrolyte, and the commonly used electrolyte lithium salt in the electrolyte at present, for example: the solvent can be ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, methyl ethyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, γ-butyrolactone and the like; the lithium salt can be one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0085] The application is not strictly limited to the material selection of the separator, which can be one of the commonly used separator materials in the battery at present, such as polypropylene separator (PP), polyethylene separator (PE), polypropylene / polyethylene double-layer composite film (PP / PE), polyimide electrospun separator (PI), polypropylene / polyethylene / polypropylene three-layer composite film (PP / PE / PP), cellulose non-woven fabric separator, ceramic-coated separator.

[0086] In the preparation of the secondary battery, the positive electrode sheet, the separator and the negative electrode sheet are wound or laminated to obtain a bare battery core, and the bare battery core is packaged into an aluminum-plastic film bag pre-punched and formed. After the packaged battery is dried at 85°C, the electrolyte is injected into the dried battery, and the battery is subjected to standing, formation and secondary sealing to complete the preparation of the secondary battery.

[0087] Hereinafter, the positive active material of the application is described in more detail through specific examples.

[0088] Example 1

[0089] The preparation method of the positive active material of the present embodiment comprises the following steps:

[0090] 1) According to the molar ratio of Ni:Co:Mn=8:1:1, the nickel sulfate, cobalt sulfate and manganese sulfate are prepared into a mixed salt solution with a metal ion concentration of 2 mol / L by using deionized water. The sodium hydroxide precipitator is prepared into an alkali solution with a concentration of 3 mol / L as a precipitator by using deionized water. 5 mol / L ammonia is used as a complexing agent. The complexing agent and the precipitator are added to deionized water to prepare a bottom solution, the ammonia concentration in the bottom solution is 8 mol / L, and the precipitator is used to adjust the pH to 12.3.

[0091] In the reaction kettle, 20% of the total volume of the bottom liquid was added, nitrogen was introduced for air replacement, stirring and heating were opened, the reaction temperature in the kettle was kept at 50℃, the stirring speed was 600 rpm / min, the mixed salt solution containing Ni, Co, Mn elements, the complexing agent, the precipitating agent and the cobalt hydroxide (the first pore-forming agent, the amount of substance was 2% of the total amount of substance of the mixed salt and the median particle size Dv50=200 nm) were continuously injected into the reaction kettle for co-precipitation reaction. During the reaction, the pH value in the kettle was kept at 11.7 by controlling the flow rate of the precipitating agent, and the particle size of the current solid phase particles was tested every 1h, when 11μm≤Dv50<13μm, the injection of the first pore-forming agent was stopped, and the intermediate was obtained, and the Dv50 of the intermediate was 12μm.

[0092] The second reaction was continued with the remaining parameters unchanged, and a slurry including a positive electrode active material precursor was obtained, and the Dv50 of the positive electrode active material precursor was 16μm. The precursor slurry was sequentially filtered, washed, centrifuged, and dried to obtain the positive electrode active material precursor Ni 0.8 Co 0.1 Mn 0.1 (OH)2.

[0093] 2) The positive electrode active material precursor Ni 0.8 Co 0.1 Mn 0.1 (OH)2, lithium hydroxide monohydrate (lithium source), zirconium dioxide (doping agent) and lithium silicate (second pore-forming agent, and the median particle size Dv50=20 nm) were mixed in a molar ratio of 1:1.04:0.002:0.002 and uniformly mixed in a high-speed mixer. In an oxygen atmosphere furnace, it was raised to 500℃ at a rate of 2℃ / min and kept constant for 2h, and then raised to 750℃ at a rate of 2℃ / min and kept constant for 12h, to obtain an intermediate;

[0094] The intermediate and deionized water were washed with water at a weight ratio of 1:1 for 20min, and the liquid was filtered and dehydrated by a Buchner filter flask for 30min, then the filter cake obtained by solid-liquid separation was crushed and placed in a vacuum drying machine at a temperature of 150℃ for drying, to obtain a dried material;

[0095] The dried material and aluminum oxide were uniformly mixed in a high-speed mixer at a molar ratio of 1:0.005, and then the mixture was calcined at 600℃ for 10h in an oxygen atmosphere to obtain a calcined product;

[0096] 3) Dissolve the polymer coating agent polyethylene glycol PEG in N-methyl pyrrolidone solvent, wherein the mass ratio of polyethylene glycol PEG (average molecular weight 400 g / mol) to calcined product is 0.5:1, to obtain a solution with a concentration of 30 g / L containing electrophilic electrolyte organic functional groups; immerse the calcined product in the solution containing electrophilic electrolyte organic functional groups, the stirring temperature is 200°C, the stirring speed is 1000 rpm, heat and stir until the solution is completely volatilized, take out the solid phase, dry, and obtain the ternary positive electrode active material of the present embodiment.

[0097] The chemical composition of the positive electrode active material of the present embodiment is Li(Ni 0.8 Co 0.1 Mn 0.1 ) 0.988 Zr 0.002 Al 0.01 O2@HO(CH2CH2O) n H.

[0098] Figure 1 The TEM image of the positive electrode active material of the present embodiment is shown in FIG. 1. Figure 1 It can be seen that there is a coating layer on the outside of the positive electrode active material.

[0099] Example 2

[0100] The preparation method of the present embodiment is basically the same as that of Example 1, except that the median particle size Dv50 of the first pore-forming agent cobalt hydroxide is 800 nm, and the Dv50 of the second pore-forming agent lithium silicate is 10 nm. The chemical composition of the positive electrode active material of the present embodiment is the same as that of Example 1.

[0101] Example 3

[0102] The preparation method of the present embodiment is basically the same as that of Example 1, except that the median particle size Dv50 of the first pore-forming agent cobalt hydroxide is 800 nm, and the Dv50 of the second pore-forming agent lithium silicate is 100 nm. The chemical composition of the positive electrode active material of the present embodiment is the same as that of Example 1.

[0103] Example 4

[0104] The preparation method of the present embodiment is basically the same as that of Example 1, except that the median particle size Dv50 of the first pore-forming agent cobalt hydroxide is 100 nm, and the Dv50 of the second pore-forming agent lithium silicate is 10 nm. The chemical composition of the positive electrode active material of the present embodiment is the same as that of Example 1.

[0105] Example 5

[0106] The preparation method of this example is basically the same as that of Example 1, except that the mass ratio of polyethylene glycol PEG to calcined product is 0.2:1. The chemical composition of the positive electrode active material of this example is the same as that of Example 1.

[0107] Example 6

[0108] The preparation method of this example is basically the same as that of Example 1, except that the mass ratio of polyethylene glycol PEG to calcined product is 1:1. The chemical composition of the positive electrode active material of this example is the same as that of Example 1.

[0109] Example 7

[0110] The preparation method of this example is basically the same as that of Example 1, except that the coating treatment of step 3) is not performed, and the calcined product of step 2) is the positive electrode active material of this example. The chemical composition of the positive electrode active material of this example is Li(Ni 0.8 Co 0.1 Mn 0.1 ) 0.988 Zr 0.002 Al 0.01 O2.

[0111] Example 8

[0112] The preparation method of this example is basically the same as that of Example 1, except that polyethylene glycol PEG is replaced by polymethyl methacrylate in step 3). The chemical composition of the positive electrode active material of this example is Li(Ni 0.8 Co 0.1 Mn 0.1 ) 0.988 Zr 0.002 Al 0.01 O2@ (C5O2H8) n .

[0113] Example 9

[0114] The preparation method of this example is basically the same as that of Example 1, except that step 2) does not include the mixing and calcination process of aluminum trioxide. The chemical composition of the positive electrode active material of this example is Li(Ni 0.8 Co 0.1 Mn 0.1 ) 0.998 Zr 0.002 O2@ (C5O2H8) n .

[0115] Example 10

[0116] 1) According to the molar ratio of Ni:Co:Mn=9:0.5:0.5, nickel sulfate, cobalt sulfate and manganese sulfate were prepared into a mixed salt solution with a metal ion concentration of 3 mol / L using deionized water. Sodium hydroxide precipitant was prepared into an alkali solution with a concentration of 2 mol / L using deionized water as a precipitant. 4 mol / L ammonia was used as a complexing agent. The complexing agent and the precipitant were added in deionized water to prepare a stock solution, and the ammonia concentration in the stock solution was 8 mol / L. The pH was adjusted to 12.3 using the precipitant.

[0117] The total volume of the stock solution was 20% of the total volume of the reactor. Nitrogen was introduced to replace the air, and the stirring and heating were turned on. The reaction temperature in the reactor was maintained at 60°C, and the stirring speed was 600 rpm / min. The mixed salt solution containing Ni, Co and Mn elements, the complexing agent, the precipitant and cobalt hydroxide (the first pore-forming agent, the amount of substance was 3% of the total amount of substance of the mixed salt, and the median particle size Dv50=200 nm) were continuously injected into the reactor for co-precipitation reaction. During the reaction, the pH in the reactor was maintained at 10.7 by controlling the flow rate of the precipitant, and the particle size of the current solid phase was tested every 1 h. The intermediate was obtained, and the Dv50 of the intermediate was 9 μm.

[0118] The second reaction was continued with the same parameters to obtain a slurry containing a positive electrode active material precursor, and the Dv50 of the positive electrode active material precursor was 12 μm. The precursor slurry was sequentially filtered, washed, centrifuged, and dried to obtain a positive electrode active material precursor Ni 0.9 Co 0.05 Mn 0.05 (OH)2.

[0119] 2) The positive electrode active material precursor Ni 0.9 Co 0.05 Mn 0.05 (OH)2, lithium hydroxide monohydrate (lithium source), zirconium dioxide (doping agent) and lithium silicate (second pore-forming agent, and the median particle size Dv50=20 nm) were mixed in a molar ratio of 1:1.04:0.002:0.003 and uniformly mixed in a high-speed mixer. In an oxygen atmosphere furnace, the temperature was raised to 600°C at a rate of 2°C / min and kept constant for 2 h, and then raised to 800°C at a rate of 2°C / min and kept constant for 12 h to obtain an intermediate;

[0120] The intermediate and deionized water were washed with water at a weight ratio of 1:1 for 20 min, and the liquid was filtered and dehydrated for 30 min through a Buchner filter bottle. The filter cake obtained by solid-liquid separation was crushed and placed in a vacuum drying machine at a temperature of 150°C for drying to obtain a dried material.

[0121] The calcined product is obtained by mixing the dried material and aluminum oxide in a high-speed mixer at a molar ratio of 1:0.005, and then calcining the mixture at 600°C for 10h in an oxygen atmosphere.

[0122] 3) Dissolve the high-molecular coating agent polyethylene glycol PEG in N-methyl pyrrolidone solvent, wherein the mass ratio of polyethylene glycol PEG to the calcined product is 0.5:1, to obtain a solution with a concentration of 30g / L containing electrophilic electrolyte organic functional groups; immerse the calcined product in the solution containing electrophilic electrolyte organic functional groups, and stir at a temperature of 200°C and a speed of 1000r / min; heat and stir until the solution is completely volatilized, take out the solid phase, and dry to obtain the ternary positive electrode active material of the present embodiment.

[0123] The chemical composition of the positive electrode active material of the present embodiment is Li(Ni 0.9 Co 0.05 Mn 0.05 ) 0.988 Zr 0.002 Al 0.01 O2@HO(CH2CH2O) n H.

[0124] Example 11

[0125] The preparation method of the present embodiment is basically the same as that of Example 1, except that in step 1), the amount of the first pore-forming agent is 4% of the total amount of the mixed salt, and the median particle size Dv50 of the first pore-forming agent is 100nm; and in step 2), the positive electrode active material precursor Ni 0.8 Co 0.1 Mn 0.1 (OH)2, lithium hydroxide monohydrate (lithium source), zirconium dioxide (dopant), and lithium silicate (second pore-forming agent, median particle size Dv50=80nm) are mixed at a molar ratio of 1:1.04:0.002:0.001. The chemical composition of the positive electrode active material of the present embodiment is the same as that of Example 1.

[0126] Example 12

[0127] The preparation method of the present embodiment is basically the same as that of Example 1, except that in step 1), the amount of the first pore-forming agent is 8% of the total amount of the mixed salt; and in step 2), the positive electrode active material precursor Ni 0.8 Co 0.1 Mn 0.1 (OH)2, lithium hydroxide monohydrate (lithium source), zirconium dioxide (dopant), and lithium silicate (second pore-forming agent) are mixed at a molar ratio of 1:1.04:0.002:0.005. The chemical composition of the positive electrode active material of the present embodiment is the same as that of Example 1.

[0128] Comparative Example 1

[0129] The preparation method of the present comparative example is basically the same as that of Example 1, except that the median particle size Dv50 of the first pore-forming agent cobalt hydroxide is 20 nm, and the Dv50 of the second pore-forming agent lithium silicate is 120 nm. The chemical composition of the positive electrode active material of the present comparative example is the same as that of Example 1.

[0130] Comparative Example 2

[0131] The preparation method of the present comparative example is basically the same as that of Example 1, except that no first pore-forming agent and second pore-forming agent are added. The chemical composition of the positive electrode active material of the present comparative example is the same as that of Example 1.

[0132] Comparative Example 3

[0133] The preparation method of the present comparative example is basically the same as that of Example 1, except that in step 1), the particle size of the current solid particles is tested every 1 h, and when 7.5 μm≤Dv50≤8 μm, the injection of the first pore-forming agent is stopped to obtain an intermediate, and the Dv50 of the intermediate is 8 μm. The remaining steps are the same as those of Example 1. The chemical composition of the positive electrode active material of the present comparative example is the same as that of Example 1.

[0134] Test Example 1

[0135] The following tests were performed on the internal structure of the positive electrode active materials of all examples and comparative examples, and the results are shown in Table 1A and Table 1B.

[0136] 1) Average pore size

[0137] The secondary particles of the positive electrode active material were cut open using an argon ion polishing cutter (CP), and the section passed through the geometric center of the secondary particle. The measurement was performed using SEM measurement, and the pore size of at least 10 secondary particles was counted. Among them, with 3 / 4 of the section diameter as the boundary, the size of the pores inside the boundary (i.e. the first part) and outside the boundary (i.e. the second part) were counted respectively, and then the average pore size of each part was calculated. Among them, Figure 2 Figure 1 is a SEM section of the positive electrode active material of Example 1.

[0138] Among them, the boundary of Comparative Example 3 is 1 / 2 of the section diameter.

[0139] The data of Examples and Comparative Examples 1-2 are shown in Table 1A, and the data of Comparative Example 3 are shown in Table 1B. In Table 1B, with 1 / 2 of the diameter of the particles of Comparative Example 3 as the boundary, the average pore size r3 of the pores inside the boundary and the average pore size r4 of the pores outside the boundary were calculated according to the above method.

[0140] 2) Average porosity

[0141] The secondary particles of the positive electrode active material are cut open with an argon ion polishing cutter (CP), and the cut surface passes through the geometric center of the secondary particles. A scanning electron microscope (SEM) is used to take a SEM image of the cut surface. Then, Image J software is used to quantitatively analyze the SEM grayscale of the cut surface, wherein the boundary is 3 / 4 of the diameter of the cut surface, and the SEM grayscale of the first part (i.e., the first part) and the second part (i.e., the second part) inside and outside the boundary is quantitatively analyzed, respectively.

[0142] Specifically, first, the RGB threshold is set to 90, and the area ratio of the region with an RGB range of 0-90 in the first part and the second part of the single secondary particle cut surface region picture is obtained, denoted as X1; similarly, the area ratio of the region with an RGB range of 0-252 in the first part and the second part of the same selected region picture of the same single secondary particle is denoted as X2, and the material cross-sectional porosity K = X1 / X2 x 100% is obtained. Take the average of 10 groups of data to obtain the average porosity of the first part and the second part of the positive electrode material.

[0143] In Comparative Example 3, the boundary is 1 / 2 of the diameter of the cut surface.

[0144] The data of Examples and Comparative Examples 1-2 are shown in Table 1A, and the data of Comparative Example 3 are shown in Table 1B. In Table 1B, the average porosity P3 of the pores inside the boundary and the average porosity P4 of the pores outside the boundary of the particles of Comparative Example 3 are calculated according to the above method, with the boundary being 1 / 2 of the diameter of the particles of Comparative Example 3.

[0145] 3) Span value

[0146] The Dv90, Dv10, and Dv50 of the positive electrode active material are detected by a particle size tester, and the Span value is calculated according to (Dv90-Dv10) / Dv50.

[0147] 4) Specific surface area BET

[0148] The BET of the positive electrode active material is detected by nitrogen adsorption-desorption method.

[0149] 5) Detection of electrophilic electrolyte organic functional groups

[0150] The surface functional groups of the positive electrode active material are detected by a Fourier infrared spectrometer, and the attenuated total reflection infrared spectrum (ATR-FTIR) test method is used. A small amount of dried positive electrode active material powder (the amount used is determined according to the instrument test standard) is placed on the ATR crystal, and a fixed pressure arm is used to tightly press the sample on the crystal to ensure good optical contact. Scanning is performed to obtain the spectrum of the sample, and the functional group information is obtained from the spectral data.

[0151] Table 1A

[0152]

[0153] Table 1B

[0154]

[0155] Test Example 2

[0156] After the positive electrode active materials of the examples and comparative examples were prepared into positive electrode sheets and assembled into batteries with negative electrode sheets, electrolyte, etc., the following performances were detected, and the results are shown in Table 1.

[0157] 1) Contact angle of electrode sheet

[0158] The positive electrode active materials were prepared into positive electrode sheets (including 95% of the positive electrode active material, 3% of conductive carbon black SP, and 2% of PVDF by mass percentage), and rolled to the same compactness of 3.4 g / cm 3 The used electrolyte (including LiPF6, EC, EMC, and DEC, the content of LiPF6 in the electrolyte was 1 mol / L, and the volume ratio of EC, EMC, and DEC was 1:1:1) was added dropwise to the rolled positive electrode material sheet, and the contact angle was measured by a contact angle measuring instrument after standing for 3 s. The smaller the angle, the better the wettability.

[0159] 2) Rate performance and cycle performance

[0160] 1. The positive electrode active materials were prepared into positive electrode sheets, and assembled into button cells with negative electrode sheets, electrolyte, and separators according to the following method. Specifically, each positive electrode active material was mixed with conductive carbon black (SP) and PVDF at a weight ratio of 95:3:2, and a positive electrode slurry was obtained by dispersion. The slurry was coated on an aluminum foil current collector, dried in a 120°C air oven for 30 min, and rolled to prepare a positive electrode sheet with a compactness of 3.4 g / cm 3 Then, the positive electrode sheet was punched into a small round sheet with a diameter of 15 mm using a mold, dried and weighed, and assembled into a button cell in an Ar protective atmosphere glove box using a 2430 button cell shell, a Li metal round sheet as the negative electrode, and an electrolyte composed of LiPF6, EC, EMC, and DEC, wherein the content of LiPF6 was 1 mol / L, and the volume ratio of EC, EMC, and DEC was 1:1:1.

[0161] 2. Rate test: the battery was subjected to charge-discharge rate test at 25°C using a battery charge-discharge tester, and the charge-discharge rate system was as follows: 0.5C constant current charging to 4.25V, converted to 4.25V constant voltage charging until the current decreased to 0.05C, standing for 5 min, 0.1C constant current discharging to 2.8V, and recording the discharge capacity Q 0.1c; after standing for 5 min, 0.5 C constant current charging to 4.25 V, converted to 4.25 V constant voltage charging to current reduced to 0.05 C, after standing for 5 min, 3 C constant current discharging to 2.5 V, record the discharge capacity Q 1c ; after standing for 5 min, 0.5 C constant current charging to 4.25 V, converted to 4.25 V constant voltage charging to current reduced to 0.05 C, after standing for 5 min, 3 C constant current discharging to 2.5 V, record the discharge capacity Q 3c :

[0162] 1C discharge rate capacity retention rate Q1 = Q 1c / Q 0.1c x 100%

[0163] 3C discharge rate capacity retention rate Q3 = Q 3c / Q 0.1c x 100%

[0164] 3, 1 C constant current charging to 4.25 V, converted to 4.25 V constant voltage charging to current reduced to 0.05 C, then discharged to 2.8 V at a discharge rate of 1 C, repeat 50 times such charge-discharge cycle, to determine the discharge capacity Q1 at the first cycle and the discharge capacity Q at the 50th cycle 50 ;

[0165] Capacity retention rate Q = Q 50 / Q1 x 100%

[0166] Table 2

[0167]

[0168] According to Table 2, the positive active material of the application helps to improve the cycle performance and rate performance of the secondary battery.

[0169] Finally, it should be noted that: other embodiments of the application will be readily apparent to those skilled in the art after considering the specification and practice of the application disclosed herein. The application is intended to cover any variations, uses or adaptive changes to the application following the general principles of the application and including common knowledge or conventional technical means in the art not disclosed by the application, and is not limited to the precise structure described above and shown in the drawings, and various modifications and changes can be made without departing from the scope thereof. The scope of the application is only limited by the appended claims.

Claims

1. A positive electrode active material, characterized in that, The positive electrode active material includes spherical or near-spherical secondary particles, the center of the secondary particles being C and the radius being R; in the secondary particles, the region with C as the starting point and 3 / 4R as the radius is the first part, and the region outside the first part is the second part; wherein, the average pore diameter r1 of the pores in the first part is greater than the average pore diameter r2 of the pores in the second part.

2. The positive electrode active material according to claim 1, characterized in that, (r1-r2) ≥ 50 nm; and / or, The average porosity P1 of the first portion is less than the average porosity P2 of the second portion; and / or, The average porosity P1 of the first part is 2-10%, and the average porosity P2 of the second part is 5-20%; and / or, r1 is 100-800nm ​​and r2 is 10-100nm.

3. The positive electrode active material according to claim 1 or 2, characterized in that, The device includes a core and a coating layer disposed on at least a portion of the surface of the core. The core includes the secondary particles, and the coating layer includes an oxide of an electrophilic organic functional group and / or an element Q, wherein Q includes at least one of Zr, Y, Rb, Cs, W, Ce, Mo, Ba, Ti, Mg, Al, Ta, and Nb.

4. The positive electrode active material according to claim 3, characterized in that, The electrolyte-loving organic functional groups include at least one of the following: ester group, ether group, hydroxyl group, carbonyl group, amino group, imino group, pyridyl group, imidazolyl group, quaternary ammonium salt group, carboxyl group, sulfonic acid group, phosphate ester group, thiol group, sulfonamide group, carboxyl salt group, and sulfonate group.

5. The positive electrode active material according to any one of claims 1-4, characterized in that, The positive electrode active material has a Span of 0.9-1.5 and a specific surface area (BET) of 0.7-1.3 m². 2 / g; and / or, The chemical composition of the positive electrode active material is Li a Ni x Co y A z T m Q n O2@OG s Where: 0.8≤a≤1.2, 0.5≤x≤0.96, 0≤y≤0.5, 0≤z≤0.3, 0≤m≤0.05, 0≤n≤0.1, s≥0, x+y+z+m+n=1, A includes at least one of Mn and Al, T includes at least one of Zr, Y, Rb, Cs, W, Ce, Mo, Ba, Ti, Mg, Ta and Nb, Q includes at least one of Zr, Y, Rb, Cs, W, Ce, Mo, Ba, Ti, Mg, Al, Ta and Nb, and OG includes an electrophilic organic functional group.

6. A method for preparing the positive electrode active material according to any one of claims 1-5, characterized in that, Includes the following steps: A first reaction is carried out using a first raw material solution including a metal source and a first pore-forming agent to obtain an intermediate. After obtaining the intermediate, the injection of the first pore-forming agent is stopped. A second reaction is carried out using a second raw material solution including the metal source and the intermediate to obtain a positive electrode active material precursor. The Dv50 of the intermediate is 3 / 4 of the target Dv50. The positive electrode active material is obtained by sintering a mixture of raw materials including the positive electrode active material precursor, a second pore-forming agent and a lithium source. Wherein, the median particle size D1 of the first pore-forming agent is greater than the median particle size D2 of the second pore-forming agent, the melting point of the first pore-forming agent is lower than the sintering temperature, the melting point of the second pore-forming agent is higher than the sintering temperature, and the second pore-forming agent is a water-soluble salt.

7. The preparation method according to claim 6, characterized in that, The positive electrode active material is a ternary positive electrode active material, with D1 of 100-800 nm, and the amount of the first pore-forming agent is 2-5% of the total amount of the metal source; D2 is 10-100 nm, and the molar ratio of the second pore-forming agent to the positive electrode active material precursor is 0.001:1-0.02:1; and / or, The first pore-forming agent includes at least one of metal hydroxide and polymer particles, and the second pore-forming agent includes at least one of sulfate, phosphate or silicate.

8. The preparation method according to claim 6 or 7, characterized in that, It also includes coating treatment of the sintered products after sintering; The coating process includes: mixing the sintered product with a solution containing electrolyte-loving organic functional groups and heating until the solvent evaporates.

9. A positive electrode plate, characterized in that, It includes the positive electrode active material according to any one of claims 1-5, or the positive electrode active material prepared by any one of claims 6-8.

10. A secondary battery, characterized in that, Includes the positive electrode sheet as described in claim 9.

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