Cathode material, preparation method thereof, cathode sheet and secondary battery

Abstract

The application provides a positive electrode material, a preparation method thereof, a positive electrode sheet and a secondary battery. The number ratio R of particles with a size of less than 1 micrometer in the positive electrode material satisfies 8%≤R≤20%; and the sheet conductivity T of the positive electrode material satisfies T>0.025 S / cm under a pressure of 25 MPa. The positive electrode material has high sheet conductivity, effectively reduces the resistance and power consumption of the prepared battery, and improves the energy transmission efficiency. In addition, by limiting the number ratio R of the microparticles in the positive electrode material to 8% to 20%, the positive electrode material has the advantages of high tap density and high material activity, and the problems of uncontrollable number of microparticles in the positive electrode material, increased surface side reactions and increased gas production are solved, and the safety and cycle performance are improved.

Description

Technical Field

[0001] The present application relates to the technical field of battery positive electrode materials, and in particular to a positive electrode material and a preparation method thereof, a positive electrode sheet and a secondary battery. Background Art

[0002] With the rapid development of new energy materials worldwide, the demand for lithium-ion batteries is increasing. Cathode materials, as key battery components, are undergoing rapid iteration and upgrading. Among various cathode materials, ternary cathode materials, due to their relatively high energy density, hold a significant market share. Currently commercialized ternary cathode materials are prone to structural collapse and intergranular cracking during charge and discharge, which impacts the cycle life of the cathode materials and urgently requires improvement. Summary of the Invention

[0003] In view of this, in order to solve at least one of the above defects, it is necessary to provide a positive electrode material.

[0004] In addition, it is also necessary to provide a method for preparing the aforementioned positive electrode material, as well as a positive electrode sheet and a secondary battery using the aforementioned positive electrode material.

[0005] In the first aspect, an embodiment of the present application provides a positive electrode material. In an image under a scanning electron microscope at a magnification of 3k, the number of particles less than 1 μm in the positive electrode material satisfies the following ratio R: 8%≤R≤20%; and under 25 MPa conditions, the electrode conductivity T of the positive electrode material satisfies the following ratio T>0.025S / cm.

[0006] In some possible embodiments, under a scanning electron microscope at a magnification of 3k, the positive electrode material has a first region and a second region, the number of particles below 1 μm in the first region accounts for R1, and the number of particles below 1 μm in the second region accounts for R2, wherein |R1-R2|≤9% and / or 0.5≤R1 / R2≤2.

[0007] In some possible embodiments, the two sides of the peak of the particle size distribution curve of the positive electrode material are asymmetric, and the area on the left side of the peak is larger than the area on the right side of the peak.

[0008] In some possible embodiments, the positive electrode material is a single crystal positive electrode material, and the average particle size of a single particle in the positive electrode material is 1 μm to 4 μm.

[0009] In some possible embodiments, the general formula of the positive electrode material is as follows: Li a Ni b Co c Q d M (1-a-b-c-d )N eO2, wherein the Q element is selected from at least one of Mn and Al, the M element is selected from at least one of Zr, Ti, Al, Mg and Y, the N element is selected from at least one of Zr, Ti, Al, Co, Mg, W, Ce, Y, K and Na, 0.95≤a≤1.2, 0<b≤1, 0≤c≤1, 0<d≤1, b+c+d+e≤1, 0≤e<1.

[0010] In some possible embodiments, the positive electrode material has a median particle size D50 of 2.5 μm to 5 μm, D10 of 1 μm to 2.5 μm, D90 of 6 μm to 10 μm, and 1.0≤(D90-D10) / D50≤1.5.

[0011] In some possible embodiments, the specific surface area of ​​the positive electrode material is 0.5 m 2 / g~1.4 m 2 / g.

[0012] In some possible embodiments, the compaction density of the positive electrode material is greater than 3.0 g / cm 3 .

[0013] In some possible embodiments, the tap density of the positive electrode material is greater than 1.5 g / cc.

[0014] In some possible embodiments, the bulk density of the positive electrode material is greater than 0.5 g / cm 3 .

[0015] In some possible embodiments, the discharge specific capacity of the positive electrode material at 2.8V-4.35V at 0.1C is greater than 175 mAh / g.

[0016] In the second aspect, an embodiment of the present application also provides a method for preparing a positive electrode material, comprising the following steps: mixing an oxide precursor with a lithium source and a dopant and sintering the mixture to obtain a positive electrode material matrix; and crushing the positive electrode material matrix and coating the mixture with a coating agent to obtain the positive electrode material, wherein, in an image under a scanning electron microscope at a magnification of 3k, the number of particles less than 1 μm in the positive electrode material accounts for R: 8%≤R≤20%; and under 25 MPa conditions, the electrode conductivity T of the positive electrode material satisfies: T>0.025S / cm.

[0017] In some possible embodiments, in the step of atomizing and pyrolyzing the metal mixed solution to obtain the oxide precursor, the total metal concentration in the solution is controlled to be in the range of 2 mol / L to 8 mol / L.

[0018] In some possible embodiments, in the step of atomizing and pyrolyzing the metal mixed solution to obtain the oxide precursor, the feed flow rate is controlled to be 0.5 m³ / h~5 m³ / h.

[0019] In some possible embodiments, in the step of atomizing and pyrolyzing the metal mixed solution to obtain the oxide precursor, the atomizing air flow rate is controlled to be 50m³ / h~300m³ / h, and the atomizing pressure is controlled to be 100kPa~700kPa.

[0020] In some possible embodiments, in the step of atomizing and pyrolyzing the metal mixed solution to obtain the oxide precursor, the pyrolysis temperature is controlled to be 400° C. to 1200° C.

[0021] In some possible embodiments, the sintering includes a first sintering stage, a second sintering stage, a third sintering stage and a fourth sintering stage performed in sequence, the sintering temperature of the first sintering stage is 300℃~600℃, and the time is 3h~5h; the sintering temperature of the second sintering stage is 700℃~1000℃, and the time is 4h~12h; the sintering temperature of the third sintering stage is 500℃~700℃, and the time is 2h~6h; the sintering temperature of the fourth sintering stage is 700℃~1000℃, and the time is 1h~3h.

[0022] In some possible embodiments, the doping element M in the dopant is selected from at least one of Zr, Ti, Al, Mg and Y.

[0023] In some possible embodiments, the coating element N in the coating agent is selected from at least one of Zr, Ti, Al, Co, Mg, W, Ce, Y, K and Na.

[0024] In a third aspect, an embodiment of the present application provides a positive electrode plate, comprising a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode material as described above or a positive electrode material prepared by the positive electrode material preparation method as described above.

[0025] In a fourth aspect, an embodiment of the present application provides a secondary battery, comprising the positive electrode material as described above or a positive electrode material prepared by the method for preparing the positive electrode material as described above.

[0026] The positive electrode material provided in the embodiment of the present application has a pole piece conductivity T at 25MPa that satisfies T>0.025S / cm. The higher pole piece conductivity enables the positive electrode material to have good electrical conductivity, effectively reduce the resistance and power consumption of the battery, and improve the efficiency of energy transmission. Moreover, by controlling the number of microparticles in the positive electrode material to account for 8%~20%, the presence of an appropriate amount of microparticles can, on the one hand, enable the positive electrode material to have the advantages of both high compaction density and high material activity. On the other hand, it can also solve the problem of a large number of material active sites caused by the uncontrollable number of microparticles in the positive electrode material, reduce side reactions on the material surface, reduce gas production, and improve the safety and cycle performance of the battery. In addition, by controlling the number of microparticles below 1μm in the positive electrode material to account for within the above range, the microparticles can also act as a buffer in the positive electrode material to disperse stress and suppress the volume change of the material caused by high conductivity during the charge and discharge process, thereby increasing the structural stability and cycle performance of the positive electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a schematic cross-sectional view of a lithium-ion battery using the positive electrode material of an embodiment of the present application during charging.

[0028] Figure 2 This is a schematic cross-sectional view of a lithium-ion battery using the positive electrode material of an embodiment of the present application during discharge.

[0029] Figure 3 This is a scanning electron microscope (SEM) photograph of the oxide precursor of Example 1 of the present application.

[0030] Figure 4 This is a scanning electron microscope (SEM) photograph of the positive electrode material of Example 1 of the present application.

[0031] Figure 5 This is a particle size distribution curve of Example 1 of the present application. DETAILED DESCRIPTION

[0032] The embodiments of the present application are described in detail below. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present application, and should not be understood as limiting the present application; it should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present application belongs; the embodiments of the present application and the features in the embodiments can be combined with each other unless there is a conflict; many specific details are set forth in the following description to facilitate a full understanding of the present application, and the embodiments described are only part of the embodiments of the present application, not all of the embodiments.

[0033] The positive electrode material has a particle number ratio R below 1 μm in a scanning electron microscope (SEM) 3k magnification image, wherein 8%≤R≤20%; and a positive electrode sheet conductivity T of the positive electrode material under 25 MPa satisfies T>0.025 S / cm.

[0034] By controlling the positive electrode material to have a positive electrode sheet conductivity T under 25 MPa satisfying T>0.025 S / cm, the positive electrode material has good conductivity, effectively reducing the resistance and power consumption of the prepared battery, and improving the energy transmission efficiency. However, the higher the positive electrode sheet conductivity of the positive electrode material, the more active sites in the material, and too many active sites will cause a large number of side reactions. Therefore, by limiting the particle number ratio R of the positive electrode material to 8%-20%, the positive electrode material has the advantages of high tap density and high material activity, and solves the problem of uncontrollable particle number of the existing positive electrode material, which causes an increase in surface side reactions and gas production, thereby improving safety and cycle performance. In addition, the inventors have found through a large number of experimental studies that by controlling the particle number ratio of the positive electrode material below 1 μm within the above range, the particles can also act as a buffer in the positive electrode material, play a role in dispersing stress, and inhibit the volume change of the material caused by high conductivity during charging and discharging, thereby increasing the structural stability and cycle performance of the positive electrode material.

[0035] Exemplarily, the particle number ratio of the positive electrode material below 1 μm can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, or any value within the range formed by any two of the above values. Further, the particle number ratio of the positive electrode material below 1 μm is 12%-20%. Appropriately increasing the content of the particles in the positive electrode material can promote the dispersion of the particles and reduce the local stress concentration of the large particles during volume change.

[0036] In some embodiments, under a scanning electron microscope (SEM) 3k magnification, the positive electrode material has a first region and a second region, the first region has a particle number ratio R1, and the second region has a particle number ratio R2, wherein |R1-R2|≤9% and / or 0.5≤R1 / R2≤2. The particle number ratios of the first region and the second region are close, indicating that the particles are uniformly dispersed in the positive electrode material, which is conducive to further promoting the dispersion of the particles and reducing the local stress concentration of the large particles during volume change, thereby increasing the structural stability of the positive electrode material and improving the cycle performance of the positive electrode material.

[0037] In some embodiments, the peak of the particle size distribution curve of the positive electrode material is asymmetric, with the area to the left of the peak being larger than the area to the right of the peak. This indicates that the number of particles to the left of the peak is greater than the number of particles to the right. Specifically, particles with a particle size less than 2 μm are defined as small particles, where small particles include the aforementioned microparticles, and particles with a particle size greater than 2 μm are defined as large particles. That is, the proportion of small particles is greater than the proportion of large particles, and the distribution of large and small particles is wider, which further promotes the dispersion of stress in microparticles and reduces the local stress concentration on large particles during volume changes, thereby increasing the structural stability of the positive electrode material and improving the cycle performance of the positive electrode material.

[0038] In some embodiments, the average particle size of a single particle in the positive electrode material is 1µm~4µm, and the particles are primary particles. The presence of a certain proportion of microparticles in the positive electrode material can reduce the average particle size of the particles so that the average particle size of the single particles is within the above range, which is beneficial to the transmission of lithium ions during charging and discharging, improves ionic conductivity and thus reduces impedance; at the same time, particles in the above particle size range can control the specific surface area within an appropriate range, thereby improving the activity of the material while reducing side reactions between the material and the electrolyte.

[0039] In some embodiments, the median particle size D50 of the positive electrode material is 2.5μm~5μm, and the median particle size D50 represents the particle size of the material particles corresponding to the cumulative particle size distribution percentage reaching 50% volume ratio. The present application controls the median particle size of the positive electrode material within the above appropriate range, and the particle size is moderate, which is beneficial to control the specific surface area of ​​the particles within a suitable range, thereby improving the activity of the material while reducing the side reactions between the particles and the electrolyte, thereby improving the cycle life of the battery. In addition, particles of the above size range are also beneficial to reduce the internal stress of the particles and reduce the risk of electrochemical polarization of lithium ions inside and outside the particles, thereby increasing the capacity of the positive electrode material. Exemplarily, the median particle size D50 of the positive electrode material can be 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm or any value within the range composed of any two of the above values.

[0040] In some embodiments, the particle size D10 of the positive electrode material is 1 μm to 2.5 μm, the particle size D90 is 6 μm to 10 μm, and 1.0 ≤ (D90-D10) / D50 ≤ 1.5; D10 in the particle size distribution represents the particle size corresponding to the 10th percentile of the cumulative particle size distribution, that is, in the particle population, 10% of the particles are smaller than this particle size. Typically, D10 is used to describe the finer particles in the particle population. D90 represents the particle size corresponding to the 90th percentile of the cumulative particle size distribution, that is, in the particle population, 90% of the particles are smaller than this particle size. Typically, D90 is used to describe the coarser particles in the particle population. (D90-D10) / D50 represents the width of the particle size distribution in the positive electrode material. In this application, by controlling 1.0≤(D90-D10) / D50≤1.5, the particle size distribution of the positive electrode material is wide, which can improve the compaction density and material activity of the material while reducing the side reaction between the material and the electrolyte, thereby ensuring that the positive electrode material has high compaction density, high material activity, low impedance and excellent cycle performance. Exemplarily, (D90-D10) / D50 can be 1.0, 1.1, 1.2, 1.3, 1.4, 1.5 or any value within the range of any two of the above values. D10 can be 1.0, 1.3, 1.5, 1.6, 1.8, 1.9, 2.0, 2.2, 2.3, 2.5 or any value within the range of any two of the above values. D90 can be 6.0, 6.3, 6.5, 6.8, 6.9, 7.0, 7.2, 7.5, 8.1, 8.5, 9.2, 10 μm, or any value within the range formed by any two of the above values.

[0041] In some embodiments, the positive electrode material is a single crystal positive electrode material, and the average particle size of individual particles in the positive electrode material is 1 μm to 4 μm. For example, the average particle size of individual particles can be 1 μm, 2 μm, 3 μm, 4 μm, or any value within a range formed by any two of the above values.

[0042] In some embodiments, the specific surface area of ​​the positive electrode material is 0.5 m 2 / g~1.4m 2 / g, the gaps between the large particles in the positive electrode material contain a certain number of microparticles, which can control the specific surface area within the aforementioned appropriate range, which is beneficial to improving the activity of the positive electrode material and reducing the side reactions between the positive electrode material and the electrolyte, which is beneficial to improving the long-term cycle performance of the positive electrode material. For example, the specific surface area of ​​the positive electrode material can be 0.5m 2 / g, 0.6m 2 / g, 0.7m 2 / g, 0.8m 2 / g, 0.9 m 2 / g, 1.0m 2 / g, 1.1m 2 / g, 1.2m 2 / g, 1.3 m 2 / g, 1.4m 2 / g or any value within the range formed by any two of the above values.

[0043] In some embodiments, the compacted density of the positive electrode material is greater than 3.0 g / cm 3 By controlling the proportion of microparticles in the positive electrode material to 8%~20%, it can be ensured that during the rolling process of the positive electrode material, when the positive electrode material is subjected to external force, the microparticles fill the gaps between the large particles, thereby improving the uniform distribution of the microparticles in the positive electrode material, promoting the microparticles to disperse stress, and reducing the local stress concentration on the large particles during the volume change process, thereby increasing the structural stability of the positive electrode material and improving the cycle performance of the positive electrode material.

[0044] In some embodiments, the tap density of the positive electrode material is greater than 1.5 g / cm 3 By controlling the number of microparticles in the positive electrode material to 8%~20%, the tap density of the positive electrode material can be effectively improved, which is beneficial to improving the capacity and cycle performance of the battery.

[0045] In some embodiments, the bulk density of the positive electrode material is greater than 0.5 g / cm 3 By controlling the number of microparticles in the positive electrode material to 8%~20%, the bulk density of the material can be controlled to be greater than 0.5g / cm 3 The loose density is appropriate, which can effectively improve the capacity density of the battery while reducing the possibility of peeling of the active material during charging and discharging, which is beneficial to improving the battery capacity and cycle life.

[0046] In some embodiments, the general formula of the positive electrode material is as follows: Li a Ni b Co c Q d M (1-b-c-d )N e O2, wherein the Q element is selected from at least one of Mn and Al, the M element is selected from at least one of Zr, Ti, Al, Mg and Y, the N element is selected from at least one of Zr, Ti, Al, Co, Mg, W, Ce, Y, K and Na, 0.95≤a≤1.2, 0<b≤1, 0≤c≤1, 0<d≤1, b+c+d+e≤1, 0≤e<1.

[0047] The positive electrode material provided in the embodiment of the present application has a pole piece conductivity T at 25MPa that satisfies T>0.025S / cm. The higher pole piece conductivity makes the positive electrode material have good electrical conductivity, effectively reduces the resistance and power consumption of the prepared battery, and improves the efficiency of energy transmission. However, the higher the pole piece conductivity of the positive electrode material, the more active sites there are in the material. Too many active sites will lead to the generation of a large number of side reactions. To this end, the embodiment of the present application limits the proportion of the number of microparticles in the positive electrode material to R of 8%-20%, so that the positive electrode material has the advantages of high compaction density and high material activity at the same time, and solves the problems faced by the positive electrode material such as the uncontrollable number of microparticles leading to increased surface side reactions and increased gas production, thereby improving safety and cycle performance. In addition, by controlling the proportion of microparticles below 1μm in the positive electrode material to be within the above range, the microparticles can also act as buffers in the positive electrode material to disperse stress and suppress the volume change of the material caused by high conductivity during the charge and discharge process, thereby increasing the structural stability and cycle performance of the positive electrode material.

[0048] It should be noted that the difference between single-crystalline positive electrode materials and polycrystalline positive electrode materials (i.e., polycrystalline secondary particles) is that the smallest particles of polycrystalline secondary particles are secondary particles formed by the agglomeration of nanometer-sized primary particles. In contrast, the smallest particles of single-crystalline positive electrode materials are typically single micrometer-sized primary particles. Generally speaking, in addition to EBSD testing, characterization methods such as scanning electron microscopy (SEM) can also be used to determine whether the resulting positive electrode product is a single-crystalline material. For example, for single-crystalline positive electrode materials, the morphology of single-crystalline particles can be characterized by SEM, showing that the appearance of single-crystalline particles generally appears to be regular or irregular spherical, with no significant particle agglomeration. The orientation of single-crystalline positive electrode materials can also be characterized by EBSD. EBSD can observe that the color within at least one grain is the same, which can be used to determine that the orientation within at least one grain is the same. Grains with the same orientation are single crystals. It should be specifically noted that the "single-crystalline positive electrode material" known to those skilled in the art is not a "single crystal" in the strict crystallographic sense. In crystallography, an ideal single crystal refers to a crystal with completely identical arrangement and orientation. However, due to limitations in impurities, strain, and crystal defects, ideal single crystals are extremely rare and difficult to produce in the laboratory. Therefore, the single crystal cathode materials known in the art are actually more like "single crystal-like" cathode materials, which differ only in size from polycrystals, which are composed of numerous small primary particles, due to their large single crystal-like particles.

[0049] It is understandable that the single grain in the present application can be a single particle composed of a primary particle. The above-mentioned single crystal positive electrode material may also contain a small amount of "quasi-secondary particles" formed by the adhesion of several single particles. "Primary particle" refers to the smallest particle unit identified when observing the positive electrode active material through a scanning electron microscope, and "secondary particle" refers to a secondary structure formed by the agglomeration of multiple primary particles, showing a relatively rounded spherical morphology. "Quasi-secondary particles" refer to those formed by the adhesion of several single particles. Usually, the particle size of a single particle in the above-mentioned quasi-secondary particles is usually between 1μm and 5μm. In general, the roundness of the particles of "quasi-secondary particles" is lower than that of the above-mentioned conventional "secondary particles".

[0050] It should be further explained that the "single crystal" in the "single crystal positive electrode material" known to those skilled in the art is not a "single crystal" in the strict sense. In crystallography, an ideal single crystal refers to a crystal with completely the same arrangement and direction. However, due to impurities, strain and crystal defects, ideal single crystals are very rare and difficult to produce in the laboratory. Therefore, the single crystal positive electrode materials known in the art are actually more of a "single crystal morphology-like" positive electrode material, which only shows a large particle size similar to a single crystal in size, which is different from a polycrystal composed of many small primary particles.

[0051] The present application also provides a method for preparing the aforementioned positive electrode material, which specifically comprises the following steps:

[0052] Step S1, atomizing and pyrolyzing the metal mixed solution to obtain an oxide precursor.

[0053] Specifically, solutions of a nickel source, a cobalt source, and a manganese source or an aluminum source are mixed, and the mixture is atomized and then thermally decomposed to obtain an oxide precursor.

[0054] In some embodiments, the nickel source, cobalt source, manganese source, or aluminum source can be sulfate, hydrochloride, nitrate, acetate solution, or the like of the corresponding metal elements.

[0055] In some embodiments, the total metal concentration in the mixed solution is controlled to be 2 mol / L to 8 mol / L. By controlling the total metal concentration within the above range, the particle size of the oxide precursor primary particles can be controlled, making the particle size distribution of the precursor primary particles wider, which is conducive to the formation of microparticles in the positive electrode material. Exemplarily, the total metal concentration in the solution can be 2 mol / L, 3 mol / L, 4 mol / L, 5 mol / L, 6 mol / L, 7 mol / L, 8 mol / L, or any value within the range formed by any two of the above values.

[0056] In some embodiments, the feed rate is 0.5 m³ / h to 5 m³ / h. By controlling the feed rate within the above range, the growth rate of the precursor particles can be effectively controlled to form primary particles of appropriate particle size, thereby reducing the activity of the precursor particles and facilitating the formation of microparticles in the positive electrode material. For example, the feed rate can be 0.5 m³ / h, 1 m³ / h, 15 m³ / h, 2 m³ / h, 2.5 m³ / h, 3 m³ / h, 35 m³ / h, 4 m³ / h, 4.5 m³ / h, 5 m³ / h, or any value within the range formed by any two of the above values.

[0057] In some embodiments, during the spray pyrolysis process, the atomizing air flow rate is controlled to be 50m³ / h~300m³ / h, and the atomizing pressure can be 100kPa~700kPa. Since the size of the precursor particles is directly related to the subsequent reaction activity with the lithium source, the larger the particles, the weaker the activity. Therefore, by controlling the atomizing air flow rate and atomizing pressure, the atomizing intensity can be controlled, thereby allowing the various components in the mixture to fully decompose and react, thereby controlling the primary particle formation rate of the precursor and controlling the primary particle size of the precursor within an appropriate range, thereby reducing the subsequent reaction activity of the particles with the lithium source, which is conducive to the formation of microparticles in the positive electrode material. Furthermore, the atomizing air flow rate may be 100 m³ / h to 200 m³ / h. For example, the atomizing air flow rate may be 50 m³ / h, 80 m³ / h, 100 m³ / h, 120 m³ / h, 150 m³ / h, 180 m³ / h, 200 m³ / h, 230 m³ / h, 270 m³ / h, 300 m³ / h, or any value within the range formed by any two of the above values. Further, the atomizing pressure may be 100 kPa to 400 kPa. For example, the atomizing pressure may be 100 kPa, 200 kPa, 300 kPa, 400 kPa, 500 kPa, 600 kPa, 700 kPa, or any value within the range formed by any two of the above values.

[0058] In some embodiments, the nozzle diameter can be 0.1 mm to 20 mm. By controlling the nozzle diameter, the formation rate of the precursor and the size of the primary particles can also be regulated. For example, the nozzle diameter can be 0.1 mm, 1 mm, 3 mm, 5 mm, 8 mm, 10 mm, 12 mm, 14 mm, 16 mm, 18 mm, 20 mm, or any value within a range formed by any two of the foregoing values.

[0059] In some embodiments, the pyrolysis temperature is controlled to be 400°C to 1200°C. Since the uncontrollable microparticles in the positive electrode material can lead to an increase in side reactions on the material surface, the present application regulates the pyrolysis temperature within the above range to appropriately suppress the increase in the particle size of the precursor primary particles, thereby enhancing the activity of the primary particles. When reacting with the lithium salt, larger particles are formed, thereby suppressing the formation of microparticles to a certain extent, thereby achieving the purpose of regulating the proportion of microparticles in the positive electrode material, achieving controllable microparticle number, improving the activity of the positive electrode material, and reducing the occurrence of side reactions.

[0060] In some embodiments, during the pulverization process of the oxide precursor, by controlling the classifier frequency in the air flow pulverization process at 30HZ~50HZ and controlling the air pressure at 300kPa~500kPa, the size of the primary particles of the precursor can be further appropriately reduced to increase the activity of the primary particles, and the formation of microparticles can be further suppressed when reacting with lithium salts, and the proportion of the number of microparticles can be controlled within an appropriate range.

[0061] Step S2: mixing the oxide precursor with the lithium source and the dopant and sintering (defined as primary sintering here) to obtain a positive electrode material matrix.

[0062] The primary sintering includes a first sintering stage, a second sintering stage, a third sintering stage and a fourth sintering stage which are performed in sequence.

[0063] In some embodiments, the temperature T1 of the first sintering stage is 300°C to 600°C, and the sintering time is 3 hours to 5 hours. Heating the materials to a lower temperature first allows the lithium salt to fully melt and the dopant to diffuse evenly throughout the mixture. For example, the temperature of the first sintering stage can be 300°C, 400°C, 500°C, 600°C, or any value within a range consisting of any two of these values; the sintering time can be 3 hours, 4 hours, 5 hours, or any value within a range consisting of any two of these values.

[0064] In some embodiments, the temperature T2 of the second sintering stage is 700°C to 1000°C, and the time is 4h to 12h. This stage is the main stage of crystal formation and crystal growth. By controlling the sintering temperature and sintering time within the above range, lithium ion embedding can be achieved, crystal growth can be promoted, and the rate and size of particle size growth can be controlled. Exemplarily, the temperature of the second sintering stage can be 700°C, 800°C, 900°C, 1000°C or any value within the range composed of any two of the above values; the sintering time can be 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h or any value within the range composed of any two of the above values.

[0065] In some embodiments, the sintering temperature T3 of the third sintering stage is 500°C~700°C, and the time is 2h~6h. After the high-temperature sintering, by controlling the temperature and time of the low-temperature sintering within the above range, the lithium ions can be further embedded, while suppressing the growth of the particle size and controlling the particle size to form the required number of microparticles. In addition, the low-temperature sintering also has the effect of annealing, effectively eliminating the stress in the lattice, repairing the micrograin boundary, and reducing particle defects. Exemplarily, the temperature of the third sintering stage can be 500°C, 600°C, 700°C or any value in the range of any two values ​​above; the sintering time can be 2h, 3h, 4h, 5h, 6h or any value in the range of any two values ​​above.

[0066] In some embodiments, the sintering temperature T4 of the fourth sintering stage is 700°C to 1000°C, and the sintering time is 1 hour to 3 hours. If the low-temperature sintering is too thick, a short high-temperature re-sintering can further increase the insertion of lithium ions and further improve the crystal structure. Exemplarily, the temperature of the fourth sintering stage can be 700°C, 800°C, 900°C, 1000°C, or any value within the range of any two of the above values; the sintering time can be 1 hour, 2 hours, 3 hours, or any value within the range of any two of the above values.

[0067] In some embodiments, the total sintering time can be 12 to 26 hours. By controlling the total sintering time, the particle size can be controlled, and the proportion of the formed microparticles can be controlled to be 8% to 20%. For example, the total sintering time can be 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 21 hours, 22 hours, 23 hours, 24 hours, 25 hours, 26 hours, or any value within the range consisting of any two of the above values.

[0068] In some embodiments, the temperature is increased from room temperature to the first sintering stage at a heating rate of 20°C / min, increased from the first sintering stage to the second sintering stage at a heating rate of 0.5~3°C / min, then decreased to the third sintering stage at a cooling rate of 0.5~3°C / min, then increased to the fourth sintering stage at a heating rate of 0.5~3°C / min, and finally decreased from the fourth sintering stage to room temperature at a cooling rate of 0.5~3°C / min to obtain a positive electrode material matrix.

[0069] In some embodiments, the sintering atmosphere may be an oxygen atmosphere or an air atmosphere.

[0070] In some embodiments, the doping element M in the dopant is selected from at least one of Zr, Ti, Al, Mg, and Y. The dopant can be an oxide of the aforementioned doping elements or a lithium ion conductor.

[0071] In some embodiments, the content of the dopant is 200 ppm to 10,000 ppm. For example, the content of the dopant is 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1,000 ppm, or any value within a range consisting of any two of the above values.

[0072] In some embodiments, the lithium source may be LiOH.H2O, Li2CO3, or a mixture of LiOH.H2O and Li2CO3.

[0073] In some embodiments, the molar ratio of the lithium content in the lithium salt to the nickel, cobalt and manganese content in the precursor is 0.98 to 1.10.

[0074] In some embodiments, the particle size D50 of the lithium salt may be 5 μm to 30 μm.

[0075] In this step, low-temperature sintering is first performed to achieve sufficient melting of the lithium salt and uniform diffusion of the dopant; the subsequent high-temperature sintering realizes the embedding of lithium ions and the initial growth of particles; the subsequent low-temperature sintering can further embed lithium ions, control the particle size, and facilitate the formation of microparticles, while also playing the role of annealing and repairing microcrystalline boundaries; finally, a short period of high-temperature sintering can further promote the embedding of lithium ions and further regulate the proportion of microparticles.

[0076] Step S3, crushing the positive electrode material matrix and coating it with a coating agent to obtain the positive electrode material, wherein in the image under a scanning electron microscope with a magnification of 3k, the number of microparticles less than 1 μm in the positive electrode material accounts for R: 8≤%R≤20%; and under 25 MPa conditions, the conductivity T of the electrode sheet prepared from the positive electrode material satisfies: T>0.025S / cm.

[0077] Specifically, the positive electrode material matrix obtained by the previous sintering step is crushed, and the particle size distribution of the matrix is ​​adjusted by controlling the classifier frequency in the air flow milling equipment between 40 Hz and 60 Hz and the air pressure between 400 kPa and 600 kPa. The crushed positive electrode material matrix undergoes a primary coating and a secondary sintering to obtain a primary coating material. The primary coating material undergoes a secondary coating and a tertiary sintering, and then is screened and ground to finally obtain the positive electrode material. The two coating and sintering steps allow the coating agent to be evenly coated on the surface of the positive electrode material matrix for surface modification, thereby further improving the structural stability and electrochemical performance of the positive electrode material.

[0078] The coating element N in each coating agent can be selected from at least one of Zr, Ti, Al, Co, Mg, W, Ce, Y, K, Na, etc., wherein the coating agent can be an oxide of the coating element N or a lithium ion conductor.

[0079] In some embodiments, the temperature of the second sintering can be 400-600℃, and the sintering time can be 6-12h. The temperature of the third sintering can be 400℃-600℃, and the sintering time can be 6h-10h. By controlling the temperature and time of the second sintering and the third sintering, the coating agent can be uniformly coated on the surface of the substrate in batches, and the surface structure of the positive electrode material is further optimized, so as to further improve the structural stability and electrical conductivity of the positive electrode material. For example, the temperature of the second sintering can be 400℃, 500℃, 600℃, or any value within the range formed by any two of the above values, and the sintering time can be 6h, 7h, 8h, 9h, 10h, 11h, 12h, or any value within the range formed by any two of the above values; the temperature of the third sintering can be 400℃, 500℃, 600℃, or any value within the range formed by any two of the above values, and the sintering time can be 6h, 7h, 8h, 9h, 10h, or any value within the range formed by any two of the above values.

[0080] In some embodiments, the content of the coating element can be 200ppm-5000ppm. For example, the content of the coating element can be 200ppm, 500ppm, 800ppm, 1000ppm, 1500ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm, or any value within the range formed by any two of the above values.

[0081] The hydroxide precursor prepared by the traditional co-precipitation method has low crystallinity and poor resistance to large radius element doping, and high valence elements are easily attached to the surface of the material, thereby inhibiting the growth of single crystal particles with good dispersion. Therefore, the preparation method of the positive electrode material provided in the embodiments of the present application uses a spray pyrolysis technology to prepare an oxide precursor, while appropriately reducing the reactivity of the oxide precursor, and then reacts the oxide precursor with lithium salt through multi-temperature stage sintering to prepare a positive electrode material containing appropriate microparticles.

[0082] First of all, unlike hydroxide precursors, oxide precursors have a certain crystal structure and are a two-phase mixture of spinel and rock salt phases. Oxide precursors have a larger diffusion width when sintering with lithium salts. A relatively wider particle size distribution is conducive to the formation of an appropriate number of microparticles. In other words, the particle size distribution of the prepared oxide precursor is wide, and the reaction activity with the lithium salt is low, so that the crystal particles are not fully diffused when growing into single crystals. Some particles are not sufficiently single-crystallized and will form microparticles with smaller particle sizes. By controlling the width of the particle size distribution of the aforementioned oxide precursor, the number of microparticles formed during the sintering process can be controlled to be within a suitable range of 8% to 20%, thereby improving the material compaction density and energy density. Moreover, the particle size distribution of the positive electrode material has good inheritance. The positive electrode material can well inherit the particle size distribution of the matrix, and the matrix can also inherit the particle size distribution of the oxide precursor to a certain extent.

[0083] Secondly, an appropriate proportion of microparticles is conducive to a more complete combination of the material and the conductive agent, reducing the proportion of micropores in the material. At the same time, coating the matrix material with N elements can also effectively improve the conductivity of the active material, thereby making the positive electrode material have a higher electrode conductivity.

[0084] In addition, the preparation method of the embodiment of the present application has the advantages of short reaction time, short-range high efficiency, low processing cost, low raw material cost, less pollution, no wastewater generation, and high recycling rate, and is suitable for industrial large-scale production.

[0085] An embodiment of the present application also provides a positive electrode sheet using the aforementioned positive electrode material, comprising a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises the aforementioned positive electrode material.

[0086] The positive electrode current collector may be aluminum foil or nickel foil, or any composite current collector disclosed in the prior art, such as but not limited to a current collector formed by combining the aforementioned conductive foil and a polymer substrate.

[0087] The present application also provides a secondary battery (such as a lithium-ion battery, sodium-ion battery, etc.) comprising a housing, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located within the housing. The electrode assembly includes a separator, a negative electrode sheet, and the aforementioned positive electrode sheet, with the separator being disposed between the positive and negative electrode sheets.

[0088] In some embodiments, the outer shell can be a packaging bag encapsulated with an encapsulation film (such as an aluminum-plastic film), such as when the secondary battery is a soft-pack battery. In other embodiments, the secondary battery can also be a steel-shell battery, an aluminum-shell battery, etc.

[0089] In some embodiments, the electrode assembly may be a laminated structure, formed by alternating layers of positive electrode sheets, separators, and negative electrode sheets. In other embodiments, the electrode assembly may be a wound structure, formed by stacking positive electrode sheets, separators, and negative electrode sheets in sequence and then winding them.

[0090] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode current collector can be at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or a carbon-based current collector, or any composite current collector disclosed in the prior art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foil and a polymer substrate. The negative electrode active material can include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material can be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material can be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, the present application is not limited to these materials; other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials can be used alone or in combination. The batteries provided in the embodiments of the present application have the advantages of high capacity, high initial efficiency, long cycle life, excellent rate performance, and low expansion. The battery can be a lithium-ion battery, a sodium-ion battery, a solid electrolyte battery, etc., which is not limited here.

[0091] like Figure 1 and Figure 2 As shown in FIG, they are schematic diagrams of lithium removal and lithium insertion during the charging and discharging processes of a lithium-ion battery prepared using the positive electrode material provided in the embodiment of the present application. Figure 1 As shown in Figure 1, when a lithium-ion battery is charged, lithium ions are released from the positive electrode and embedded in the negative electrode. Figure 2 As shown in the figure, when a lithium-ion battery is discharged, lithium ions are released from the negative electrode and embedded back into the positive electrode.

[0092] The embodiment of the present application uses the aforementioned positive electrode material to prepare a secondary battery. By controlling the number of microparticles in the positive electrode material to account for R of 8% to 20%, and the conductivity of the positive electrode sheet at 25MPa of T>0.025S / cm, the positive electrode material has both high compaction density and high material activity, thereby improving the capacity and activity of the secondary battery. Moreover, by limiting the number of microparticles in the positive electrode material to account for R of 8% to 20%, the positive electrode material has the advantages of both high compaction density and high material activity, and solves the problems faced by existing positive electrode materials, such as the uncontrollable number of microparticles leading to increased surface side reactions and increased gas production, thereby improving safety and cycle performance. In addition, by controlling the number of microparticles below 1μm in the positive electrode material to account for within the above range, the microparticles can also act as buffers in the positive electrode material, dispersing stress and suppressing the volume change of the material caused by high conductivity during the charge and discharge process, thereby increasing the structural stability and cycle performance of the positive electrode material. The secondary battery using the above-mentioned positive electrode material has a discharge specific capacity of 2.8V~4.35V at 0.1C greater than 175mAh / g, and a cycle retention rate of 0.5C charge and 1C discharge for 50 cycles greater than 95%.

[0093] The present invention will be explained below in conjunction with the embodiments. It will be understood by those skilled in the art that the following examples are only used to explain the present invention and are not to be construed as limiting the present invention. Unless otherwise indicated, the reagents, software, and instruments not specifically described in the following examples are all conventional commercially available products or open source.

[0094] Example 1:

[0095] Step 1: Prepare a metal salt mixed solution with a molar ratio of Ni:Co:Mn of 50:20:30, control the total metal concentration in the solution to 2 mol / L, control the feed flow rate to 1.2 m³ / h, control the atomizing air flow rate to 150 m³ / h, control the atomizing air pressure to 300 kPa, select a nozzle with a diameter of 10 mm, control the pyrolysis temperature to 550 ° C, and calcine in an air atmosphere to obtain a precursor. The precursor is pulverized by air flow, and during the pulverization process, the classifier frequency is controlled to 40 Hz and the air pressure is controlled to 400 kPa to obtain an oxide precursor with a particle size of D50 = 4.1 μm. Its main element chemical composition can be expressed as Ni 0.50 Co 0.20 Mn 0.30 O 1.2 (The deviation of each element content is within ±1%).

[0096] Step 2: The oxide precursor prepared in step 1 is mixed evenly with LiOH·H2O at a molar ratio of 1:1.03, wherein the average particle size of LiOH·H2O is 13 μm. Based on the mass of the oxide precursor, 1200 ppm of nano-ZrO2 is added.

[0097] Step 3: Place the mixture of step 2 in an atmosphere furnace. Under an oxygen atmosphere, ① heat up from room temperature to the first sintering stage temperature: 300℃ at a heating rate of 20℃ / min, and keep warm for 3h; ② heat up from the first sintering stage to the second sintering stage temperature: 800℃ at a heating rate of 3℃ / min, and keep warm for 5h; ③ cool down from the second sintering stage to the third sintering stage temperature: 600℃ at a cooling rate of 2℃ / min, and keep warm for 4h; ④ heat up again from the third sintering stage to the fourth sintering stage temperature: 960℃ at a heating rate of 3℃ / min, and keep warm for 3h; ⑤ cool down from the fourth sintering stage to room temperature at a cooling rate of 3℃ / min. During the sintering process, the oxygen flow rate was controlled at 0.14m³ / min and the furnace pressure was 7Pa to obtain the positive electrode material matrix (LiNi 0.50 Co 0.20 Mn 0.30 O2).

[0098] In step 4, the positive electrode material matrix of step 3 is coarsely crushed by a roller mill to obtain a coarse particle sample with an average particle size of 10 μm to 20 μm, and the coarse particle sample is crushed by a jet mill to obtain a sample with a D50 of 4.3 μm, wherein the grinding air pressure during the jet milling process is 600 kPa and the classifier frequency is 52 Hz.

[0099] Step 5: Mix the sample in step 4 with 1000 ppm of nano-sized Al2O3, and heat to 450°C at a rate of 4°C / min in an oxygen atmosphere and keep the temperature for 6 hours to obtain a primary coated material.

[0100] Step 6: After uniformly mixing the primary coating material in step 5 with 800 ppm of NaOH and KOH, the mixture is heated to 400° C. at a rate of 4° C. / min in an oxygen atmosphere and kept at that temperature for 6 h to obtain a secondary coating material.

[0101] Step 7: The secondary coating material of step 6 is screened and ground for 30 minutes to obtain a positive electrode material.

[0102] Example 2

[0103] The difference from Example 1 is that in step 1, the atomizing air flow rate is adjusted to 120 m³ / h and the atomizing air pressure is adjusted to 250 kPa. The other steps are basically the same as those in Example 1. Please refer to Example 1.

[0104] Example 3

[0105] The difference from Example 1 is that in step 1, the pyrolysis temperature is 500° C. The other steps are basically the same as those in Example 1, please refer to Example 1.

[0106] Example 4

[0107] The difference from Example 1 is that in step 1, the frequency of the classifier during the crushing of the large-particle oxide precursor is adjusted to 45 Hz to obtain a precursor with a particle size D50 = 3.5 μm. The other steps are basically the same as in Example 1, please refer to Example 1.

[0108] Example 5

[0109] The difference from Example 1 is that: in step 3, ① the temperature is increased from room temperature to the first sintering stage temperature: 300°C at a heating rate of 20°C / min, and kept warm for 3 hours; ② the temperature is increased from the first sintering stage to the second sintering stage temperature: 800°C at a heating rate of 3°C / min, and kept warm for 5 hours; ③ the temperature is decreased from the second sintering stage to the third sintering stage temperature: 600°C at a cooling rate of 2°C / min, and kept warm for 4 hours; ④ the temperature is increased again from the third sintering stage to the fourth sintering stage temperature: 990°C at a heating rate of 3°C / min, and kept warm for 3 hours; ⑤ the temperature is decreased from the fourth sintering stage to room temperature at a cooling rate of 3°C / min.

[0110] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0111] Example 6

[0112] The difference from Example 1 is that: in step 3, ① the temperature is increased from room temperature to the first sintering stage temperature: 300°C at a heating rate of 20°C / min, and kept warm for 3 hours; ② the temperature is increased from the first sintering stage to the second sintering stage temperature: 800°C at a heating rate of 3°C / min, and kept warm for 5 hours; ③ the temperature is decreased from the second sintering stage to the third sintering stage temperature: 600°C at a cooling rate of 2°C / min, and kept warm for 4 hours; ④ the temperature is increased again from the third sintering stage to the fourth sintering stage temperature: 930°C at a heating rate of 3°C / min, and kept warm for 3 hours; ⑤ the temperature is decreased from the fourth sintering stage to room temperature at a cooling rate of 3°C / min.

[0113] In step 7, the grinding time is 10 min.

[0114] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0115] Example 7

[0116] The difference from Example 1 is that in step 5, the coating agent is 1000 ppm of WO3.

[0117] In step 6, the total coating amount of NaOH and KOH was increased to 1000 ppm.

[0118] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0119] Example 8

[0120] The difference from Example 1 is that in step 3, the lithium source is Li2CO3 and the sintering atmosphere is air atmosphere.

[0121] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0122] Example 9

[0123] The difference from Example 1 is that in step S3, the lithium source is a mixture of Li2CO3:LiOH·H2O=1:1, and the sintering atmosphere is air atmosphere.

[0124] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0125] Example 10

[0126] The difference from Example 1 is that in step 3, the gas flow rate is increased to 0.18 m³ / min and the furnace pressure is 10 Pa.

[0127] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0128] Example 11

[0129] The difference from Example 8 is that in step 1, the molar ratio of Ni:Co:Mn is controlled to be 60:10:30, and the oxide precursor Ni is obtained by pyrolysis. 0.60 Co 0.10 Mn 0.30 O 1.2 In step 3, ④ the temperature was raised again in the third sintering stage to the fourth sintering stage temperature of 920℃ at a heating rate of 3℃ / min and kept at this temperature for 3h. The elemental composition of the obtained positive electrode material is LiNi 0.60 Co 0.10 Mn 0.30 O2.

[0130] The other steps are basically the same as those in Example 8, please refer to Example 8.

[0131] Example 12

[0132] The difference from Example 8 is that the molar ratio of Ni:Co:Mn is controlled to be 80:10:10, and the oxide precursor Ni is obtained by pyrolysis. 0.80 Co 0.10 Mn 0.10 O 1.2 In step 3, ④ the temperature was raised again in the third sintering stage to the fourth sintering stage temperature of 880℃ at a heating rate of 3℃ / min and kept at this temperature for 3h. The elemental composition of the obtained positive electrode material is LiNi 0.80 Co 0.10 Mn 0.10 O2.

[0133] Other steps are basically the same as those in Example 8, please refer to Example 8.

[0134] Example 13

[0135] The difference from Example 8 is that the molar ratio of Ni:Co:Mn is controlled to be 82.5:6.5:11, and the oxide precursor Ni 0.825 Co 0.065 Mn 0.11 O 1.2 In step 3, ④ the third sintering stage is heated to the fourth sintering stage temperature again at a heating rate of 3℃ / min: 870℃, and the temperature is kept for 3h. The chemical formula of the obtained positive electrode material is LiNi 0.825 Co 0.065 Mn 0.11 O2.

[0136] Other steps are basically the same as those in Example 8, please refer to Example 8.

[0137] Example 14

[0138] The difference from Example 1 is that in step 2, the precursor used is the oxide precursor Ni 0.50 Co 0.20 Mn 0.30 O 1.2 Co 0.50 Co 0.20 Mn 0.30 (OH)2, and the molar ratio of the oxide precursor to the hydroxide precursor is 1:1. This hydroxide precursor is prepared by a conventional co-precipitation method, and the preparation method is as follows: a salt solution (such as NiSO4, CoSO4, MnSO4, etc.) and an alkali solution (such as NaOH, NH3, etc.) with a certain concentration are continuously added into a reactor at a certain flow rate. The reaction is carried out at a suitable reaction temperature (40~60℃), stirring rate, and pH value (10~13) to generate a hydroxide precipitate. The generated precipitate is washed, filtered, and dried to obtain the hydroxide precursor material finally.

[0139] In step 2, the following steps are further included: the precursor mixture is mixed with LiOH·H2O at a molar ratio of 1:1.05, and 1200ppm of ZrO2 relative to the mass of the precursor is added.

[0140] Step 3 also includes the following steps: in an oxygen atmosphere, ① at a heating rate of 20°C / min, from room temperature to the first sintering stage temperature: 300°C, and keep warm for 3 hours; ② at a heating rate of 3°C / min, from the first sintering stage to the second sintering stage temperature: 800°C, and keep warm for 7 hours; ③ at a cooling rate of 2°C / min, from the second sintering stage to the third sintering stage temperature: 600°C, and keep warm for 4 hours; ④ at a heating rate of 3°C / min, the third sintering stage is heated again to the fourth sintering stage temperature: 960°C, and keep warm for 3 hours; ⑤ at a cooling rate of 3°C / min, from the fourth sintering stage to room temperature. During the sintering process, the oxygen flow rate is controlled at 0.20m³ / min and the furnace pressure is 10Pa to obtain the positive electrode material matrix (LiNi 0.50 Co 0.20 Mn 0.30 O2).

[0141] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0142] Example 15

[0143] The difference between this embodiment and embodiment 12 is that the metal salt mixed solution is prepared according to the molar ratio of Ni:Co:Al of 80:10:10, and the element composition of the obtained positive electrode material is LiNi 0.80 Co 0.10 Al 0.10 O2.

[0144] The other steps are basically the same as those in Example 12, please refer to Example 12.

[0145] Comparative Example 1

[0146] The difference from Example 4 is that in step 4, during the crushing process of the positive electrode material matrix, the grinding air pressure is 600 kPa and the classifying wheel frequency is 60 Hz.

[0147] The other steps are basically the same as those in Example 4, please refer to Example 4.

[0148] Comparative Example 2

[0149] The difference from Example 1 is that in step 3, ① the temperature is raised from room temperature to the first sintering stage temperature: 300℃ at a heating rate of 20℃ / min, and kept warm for 3h; ② the temperature is raised from the first sintering stage to the second sintering stage temperature: 700℃ at a heating rate of 3℃ / min, and kept warm for 5h; ③ the temperature is lowered from the second sintering stage to the third sintering stage temperature: 500℃ at a cooling rate of 2℃ / min, and kept warm for 4h; ④ the temperature is raised again from the third sintering stage to the fourth sintering stage temperature: 820℃ at a heating rate of 3℃ / min, and kept warm for 3h; ⑤ the temperature is lowered from the fourth sintering stage to room temperature at a cooling rate of 3℃ / min. The positive electrode material LiNi formed at this time 0.5 Co 0.2 Mn 0.3 O2 presents a polycrystalline morphology.

[0150] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0151] Comparative Example 3

[0152] The difference from Example 1 is that in step 2, the coprecipitated hydroxide precursor in Example 14 is selected as the raw material, and its chemical formula is Ni 0.50 Co 0.2 Mn 0.3 (OH)2. The formed cathode material exhibits a single crystal morphology.

[0153] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0154] The performance of the positive electrode materials obtained in Examples 1-15 and Comparative Examples 1-3 was tested using the following method.

[0155] 1. Characterization of the proportion of micro powder:

[0156] Sample Preparation: Use tweezers or other tools to pick up the powder sample, spread it flat on the conductive adhesive, and flatten it. Test the sample under 5kV / 10mA electron beam conditions. For cross-section testing, the material is coated on aluminum foil using conventional electrode preparation techniques, then cut using a focused ion beam (FIB). The sample is then tested under the same SEM conditions as the powder sample.

[0157] Characterization: Single crystal particle size can be determined using software to identify the particle size in scanning electron microscope (SEM) images. Take a single image at 3K magnification. Measure the longest diameter of all particles in the image and count the individual particle sizes. The total number of particles is recorded as N, and the number of particles with a diameter less than 1 μm is recorded as n. R represents the percentage of particles, where R = n / N * 100%.

[0158] 2. Specific surface area characterization:

[0159] Equipment: Specific surface area and pore structure analyzer.

[0160] Method: Weigh the empty sample tube (m1). Add 3 g of sample to the tube via a long-necked funnel. Degas under vacuum at 300°C for 1 hour. After cooling, weigh the tube (m2). The sample mass is calculated as m2 - m1. The tube is placed in liquid nitrogen, and the nitrogen adsorption capacity (V) of the sample is measured at a range of relative pressures (P / P0) of 0.05, 0.1, 0.15, 0.20, 0.25, and 0.30. The isotherm is fitted, and the monolayer saturation adsorption capacity (Vm) is calculated based on the slope and intercept. The specific surface area is then calculated based on Vm.

[0161] 3. Compaction density characterization:

[0162] Using the American Carver 4350 tester, 1g of sample was weighed and placed in a mold with a pressure of 3T for 30s. After compaction, the height was measured to calculate the compaction density.

[0163] 4. Characterization of bulk density:

[0164] Equipment: Bulk density meter

[0165] Method: Take an appropriate amount of sample and let it flow naturally into the fabric box, alternately pass through the four glass plates with an inclined angle of 25° and the square funnel in the fabric box, and flow into a cylindrical cup with a known volume (25 ml). Finally, weigh the mass of the powder in the cylindrical cup.

[0166] 5. Tap density test:

[0167] The American Quantacon tap density tester DAT-4-220 was used.

[0168] The test steps include: cleaning the graduated cylinder and weighing its mass as m1; adding the sample to the cylinder, keeping the sample surface as horizontal as possible and wiping the surrounding area with a paper towel; weighing the total mass of the sample and cylinder as m2; placing the cylinder on a vibrating table and securing it with three symmetrical legs; turning on the instrument and switching on the vibration switch, which will automatically stop after the specified number of vibrations; removing the cylinder and reading the sample volume. If the sample surface is horizontal after vibration, read the volume directly; if it is diagonal, take the average of the highest and lowest readings (V).

[0169] Calculation formula: tap density = (m2-m1) / V.

[0170] 6. SEM and EDS characterization:

[0171] Equipment: Scanning electron microscope, energy dispersive spectrometer.

[0172] SEM test method: When testing powder samples, use tweezers or other tools to pick up the powder sample, spread it flat on the conductive glue and flatten it to make a test sample, and test it under 5kV / 10mA electron beam conditions. In the test of the percentage of particle number, randomly select an area of ​​the test sample as the first area, and collect an image of the first area under a 3K magnification of the electron microscope, and calculate the percentage of particles below 1μm in the SEM image of the first area as R1; then randomly select another area of ​​the test sample as the second area, and collect an image of the second area under a 3K magnification of the electron microscope, and calculate the percentage of particles below 1μm in the SEM image of the second area as R2. It can be understood that the aforementioned first area and second area can be multiple. The percentage of particles below 1μm R is the average percentage of particles below 1μm in n areas randomly selected from the sample, and n≥10.

[0173] EDS testing method: Adjust the electron beam voltage of the SEM to above 15kV, then use EDS to perform a selected area test, with the selected area being the center of each single crystal particle. To calculate the average Ni / Mn element ratio, select 10 particles with the longest diameter less than 1.5μm and 10 particles with the longest diameter greater than 2.5μm, and perform EDS tests on each of the 10 obtained Ni / Mn ratio values. Remove the maximum and minimum values, and then average the remaining 8 values.

[0174] 7. Particle size characterization:

[0175] Equipment: Wet particle size analyzer.

[0176] Method: Before testing, observe the laser intensity. If it is lower than 70%, notify the testing department to clean the lens and water inlet and outlet hoses. After starting background measurement, perform external ultrasound to dissolve the sample (using the instrument's automatic light alignment and background measurement time to perform external ultrasound to dissolve the sample can reduce the probability of the sample being contaminated by long-term storage). Perform external ultrasound for 30 seconds while stirring back and forth. Before adding the sample, add 3 drops of sodium hexametaphosphate to the sample cell, stir, and pour all the sample into the sample cell at once. When all the sample is poured in, wait 10 seconds and click to start the sample test. The laser light shielding degree should be maintained between 7% and 12%.

[0177] 8. Battery electrode production and conductivity characterization:

[0178] a. Battery electrode production

[0179] The obtained positive electrode material was mixed with conductive carbon black and binder PVDF (polyvinylidene fluoride) in a mass ratio of 80:10:10, and then NMP (N-methylpyrrolidone) was added to form a uniform slurry, which was coated on copper foil, dried in an oven, and rolled at a pressure of 5 MPa before being cut into circular electrodes with a diameter of 14 mm.

[0180] b. Conductivity characterization:

[0181] Equipment: Electrode resistivity meter.

[0182] Method: Tested using the Kelvin four-wire method:

[0183] ①Set the test parameters to experimental area = 153.94mm 2 , the pressure before the pole piece rolling = 5MPa, the pressure after the pole piece rolling = 25MPa, and the holding time = 15s.

[0184] ②Data recording: the resistivity data COV% collected in parallel is less than 5%. Otherwise, the electrode needs to be re-prepared and re-tested until the requirements are met.

[0185] ③The measured resistivity is recorded as ρ, and the conductivity σ is the reciprocal of the resistivity, σ=1 / ρ.

[0186] 9. Battery production and performance testing

[0187] a. Preparation of electrochemical performance test samples:

[0188] Button battery production:

[0189] ① Ingredients: NCM: SP: 5% PVDF glue = 93: 5: 2 = 9.3 g: 0.5 g: 4.0 g, NMP=9g, stir at high speed to disperse evenly.

[0190] ② Coating and drying: evenly coat the slurry on 20μm thick aluminum foil with a 210um coater, set the coating length to the maximum length of the coater, cut the back half of the electrode and put it into a 100℃ forced air drying oven to dry for more than 1.5 hours.

[0191] ③ Rolling, punching and drying:

[0192] When rolling, adjust the roller machine to 1 roller gap and roll for 3 times; use a 14mm punching machine to punch holes, weigh, and place in a vacuum drying oven for vacuum drying at 85°C for more than 8 hours.

[0193] ④ Assembly of the LIR2016 power button:

[0194] Positive electrode shell - 2 drops of electrolyte - positive electrode sheet (14mm) - 3 drops of electrolyte - 20μm separator - 2 drops of electrolyte - φ16*1.0mm lithium sheet - 150μm nickel foam - negative electrode shell (dried at 50℃). After the battery is assembled, seal it and take it out of the glove box for testing.

[0195] b. Power-off test:

[0196] After standing for 12 hours, perform battery testing according to the following system:

[0197] 0.1C charge and 0.1C discharge for 2 weeks (first effect), constant voltage cut-off current 0.005C;

[0198] 0.5C charge and 0.5C discharge for 1 week (rate), constant voltage cut-off current 0.05C;

[0199] 0.5C charge and 1C discharge for 1 week (rate), constant voltage cut-off current 0.05C;

[0200] 0.5C charge and 2C discharge for 1 week (rate), constant voltage cut-off current 0.05C;

[0201] 0.5C charge and 1C discharge for 50 cycles (cycles), constant voltage cut-off current 0.05C.

[0202] The corresponding test results of the positive electrode materials of Examples 1-15 and Comparative Examples 1-3 are shown in Table 1 and Table 2 below.

[0203] Table 1

[0204]

[0205] Table 2

[0206]

[0207] like Figure 3 The following is a SEM image of the oxide precursor prepared in Example 1. Figure 3 It can be seen that the particles of the oxide precursor have a certain crystal structure, are large in size, and are evenly dispersed.

[0208] like Figure 4 The SEM image of the cathode material prepared in Example 1 is shown in FIG. Figure 4 It can be seen that Example 1 can form single crystal particles with good dispersion, and the gaps between large particles are evenly dispersed with microparticles with smaller particle sizes. Moreover, different regions in the SEM image contain microparticles, and the number of microparticles in different regions is close. In addition, the number of small particles with a particle size of less than 2 microns accounts for a large proportion.

[0209] Further integration Figure 5 As shown, it is the particle size distribution curve of the positive electrode material in Example 1. Figure 5 It can be seen that the peak (about 2 μm) of the particle size distribution curve of the positive electrode material of Example 1 is asymmetric on both sides, and the area on the left side of the peak is larger than the area on the right side of the peak, indicating that the number of particles on the left side of the peak is greater than the number of particles on the right side. Specifically, the proportion of small particles with a particle size less than 2 μm (including microparticles with a particle size of less than 1 μm) is greater than that of large particles with a particle size greater than 2 μm. The distribution of large and small particles is wider, which is conducive to further improving the compaction density and electrical conductivity of the positive electrode material while reducing impedance.

[0210] It can be seen from Table 1 and Table 2 that the positive electrode materials prepared in Examples 1-15 have a content of microparticles with a particle size of less than 1 μm in the range of 8% to 20%. An appropriate content of microparticles is beneficial to improving the compaction density of the positive electrode materials, and the compaction density is greater than 3 g / cm 3 , so that the prepared battery can obtain a higher energy density, and the discharge specific capacity of 2.8V~4.35V at 0.1C is greater than 175mAh / g. The discharge specific capacity of the battery prepared by the positive electrode material of Example 13 is as high as 206.6mAh / g. Secondly, the presence of microparticles can reduce the average particle size of the particles, which is beneficial to shorten the transmission path of lithium ions during charging and discharging, improve the charge transfer effect, and improve the electrode conductivity. The electrode conductivity T at 25MPa is greater than 0.025S / cm, and the positive electrode material of Example 5 has an electrode conductivity of up to 0.0796S / cm. The higher the electrode conductivity, the better the conductivity of the positive electrode material, which effectively reduces the resistance and power consumption of the prepared battery and improves the efficiency of energy transmission. At the same time, the higher the conductivity of the positive electrode material, the more active sites there are in the material. Too many active sites will lead to a large number of side reactions. For this reason, in Examples 1-15, by limiting the proportion R of microparticles to 8%-20%, the positive electrode material has the advantages of high compaction density and high material activity, and solves the problems faced by existing positive electrode materials such as uncontrollable number of microparticles leading to increased surface side reactions and increased gas production, thereby improving safety and cycle performance. In addition, a proper amount of microparticles dispersed in the gaps between large particles can, on the one hand, provide good support and reduce the risk of particle breakage of large particles during the rolling process of the pole piece, thereby improving the cycle performance of the battery. On the other hand, the microparticles can also act as a buffer in the positive electrode material to disperse stress and suppress the volume change of the material caused by high conductivity during the charge and discharge process, thereby increasing the structural stability and cycle performance of the positive electrode material. The batteries prepared from the positive electrode materials of Examples 1-15 all have good cycle performance, among which the 50-week cycle retention rate is greater than 95%. In Examples 1-15, by changing different process parameters, the proportion of microparticles in the positive electrode material can be adjusted, thereby optimizing the pole piece conductivity of the positive electrode material and the capacity and cycle performance of the prepared battery.

[0211] Among them, compared with Example 1, the atomization intensity in Example 2 is reduced, the particle size of the primary particles of the oxide precursor is increased, the reaction activity is weakened, and the distribution of the single crystal particles is widened, thereby increasing the proportion of the number of microparticles in the positive electrode material and the compaction density of the powder. The discharge capacity of the battery is increased to 177.4mAh / g, the first coulombic efficiency is increased to 87.43%, and the electrode conductivity is increased to 0.0621S / cm.

[0212] Compared with example 1, the pyrolysis temperature is reduced in example 3, the primary particle size of the generated oxide precursor is reduced, the reaction activity is increased, the growth of single crystal particles is facilitated, the number of microparticles is reduced, the compaction density of the powder is reduced, and the conductivity of the pole piece is reduced.

[0213] Compared with example 1, in example 4, the gas crushing strength in the precursor crushing process is increased, the D50 of the oxide precursor is reduced, the reaction activity is enhanced, the growth of the single crystal particles of the positive electrode material is facilitated, the number of microparticles is reduced, and therefore, the compaction density of the powder and the conductivity of the pole piece are reduced.

[0214] Compared with example 1, in example 5, the sintering temperature of the fourth sintering stage (high-temperature sintering stage) is increased, the primary particle size of the positive electrode material is increased, the content of microparticles after the matrix is broken is increased, and the conductivity of the pole piece, the capacity and the efficiency of the battery are obviously improved.

[0215] Compared with example 1, in example 6, the sintering temperature of the fourth sintering stage (high-temperature sintering stage) is reduced, the primary particle size of the positive electrode material is reduced, the content of microparticles after the matrix is broken is reduced, and in addition, the grinding time is reduced, the number of large particles is increased, the number of microparticles is reduced, and the conductivity is reduced.

[0216] Compared with example 1, in example 7, the coating of Al2O3 is changed to the coating of 1000 ppm of WO3, and the coating amount of NaOH and KOH is increased to 1000 ppm, the activity of the coating substance is stronger, the amount of the coating active substance is more, and it is beneficial to improve the conductivity of the pole piece.

[0217] Compared with example 1, in examples 8 and 9, the lithium source and the sintering atmosphere in the primary sintering process are changed, the air atmosphere and lithium carbonate, which is beneficial to the growth of single crystal particles of the positive electrode material, the number of microparticles is reduced, and the more the amount of lithium carbonate is, the lower the content of microparticles is.

[0218] Compared with example 1, in example 10, the gas flow and the furnace pressure in the primary sintering process are increased, the contact between the material and oxygen in the thermal reaction process is increased, which is helpful for the growth of large-grained single crystal material, and the number of microparticles is reduced.

[0219] Compared with example 8, in examples 11, 12, 13 and 15, the proportion of each metal element in the oxide precursor is adjusted, and the positive electrode material with the content of microparticles in the range of 8% to 20% can also be obtained, the conductivity of the pole piece under 25 MPa is greater than 0.033 S / cm, and with the increase of nickel ions, the discharge specific capacity of the battery is increased and is obviously higher than that of the ternary positive electrode material of 5 series.

[0220] Compared with Example 1, Example 14 uses a mixture of oxide precursors and hydroxide precursors, which allows for a larger diffusion width during sintering reaction with lithium salts, and significantly increases the proportion of microparticles. At the same time, by extending the holding time in a single sintering process and increasing the oxygen flow rate and furnace pressure, the proportion of microparticles can be controlled within the range of 8% to 20%, thereby optimizing the electrode conductivity and primary effect of the positive electrode material.

[0221] Compared with Example 1, the value of R1 / R2 in Example 12 is larger, indicating that the distribution uniformity of particles below 1 μm in Example 12 is weaker than that in Example 1, resulting in a decrease in the first efficiency and cycle performance of the positive electrode material.

[0222] Compared with Example 1, the value of |R1-R2| in Example 14 is larger, indicating that the distribution uniformity of particles below 1 μm in Example 14 is weaker than that in Example 1, resulting in a decrease in the cycle performance of the positive electrode material.

[0223] The proportion of microparticles in the positive electrode materials prepared in Comparative Examples 1-3 was not within the range of 8% to 20%. The number of microparticles was small, and the transmission path of lithium ions during charging and discharging was longer, which reduced the charge transfer efficiency and the electrode conductivity. Among them, in Comparative Example 1, the grinding air pressure and the frequency of the grading wheel were increased during the crushing process of the positive electrode material matrix, which aggravated the particle crushing degree and the proportion of microparticles exceeded 20%. The increase in the number of microparticles resulted in an electrode conductivity of only 0.0212S / cm. Moreover, the large number of microparticles exacerbated the occurrence of side reactions, and the cycle performance of the prepared battery was reduced. In Comparative Example 2, the temperature was low during the primary sintering, forming polycrystalline particles, not single-crystal positive electrode materials. Polycrystalline materials have a small number of microparticles, low compaction, and insufficient growth of positive electrode particles, which affects the electrochemical performance. Comparative Example 3 uses a hydroxide precursor prepared by coprecipitation. Due to its low crystallinity, it has a narrow diffusion width when reacting with lithium salts. The proportion of microparticles is reduced during the formation of positive electrode single crystal materials, and the compaction density and tap density are low, while the conductivity performance is also correspondingly reduced.

[0224] Therefore, the cathode materials prepared by the methods of Examples 1-15 of the present application have a plate conductivity T at 25 MPa that satisfies T>0.025S / cm. The high plate conductivity gives the cathode materials good electrical conductivity, effectively reducing the resistance and power consumption of the prepared batteries and improving the efficiency of energy transmission. In addition, the proportion of microparticles in the cathode materials of Examples 1-15 is in the range of 8%-20%, giving the cathode materials the advantages of both high compaction density and high material activity. This also solves the problems faced by the cathode materials, such as increased surface side reactions and increased gas production due to the uncontrollable number of microparticles, thereby improving safety and cycle performance.

[0225] The above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit the present application. Although the present application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that modifications or equivalent replacements of the technical solutions of the present application should not depart from the spirit and scope of the technical solutions of the present application.

Claims

1. A positive electrode material, characterized in that In an image obtained under a scanning electron microscope at a magnification of 3k, the proportion R of particles smaller than 1 μm in the positive electrode material satisfies the following conditions: 8% ≤ R ≤ 20%. Under a scanning electron microscope at a magnification of 3k, the positive electrode material has a first region and a second region, the proportion of particles smaller than 1 μm in the first region is R1, and the proportion of particles smaller than 1 μm in the second region is R2, wherein |R1-R2| ≤ 9% and / or 0.5 ≤ R1 / R2 ≤ 2. Under conditions of 25 MPa, the electrical conductivity T of the electrode sheet of the positive electrode material satisfies the following conditions: T > 0.025 S / cm. The two sides of the peak of the particle size distribution curve of the positive electrode material are asymmetrical, and the area on the left side of the peak is larger than the area on the right side of the peak.

2. The positive electrode material according to claim 1, wherein The positive electrode material is a single crystal positive electrode material, and the average particle size of a single particle in the positive electrode material is 1 μm to 4 μm.

3. The positive electrode material according to claim 1, wherein The general formula of the positive electrode material is as follows: Li a Ni b Co c Q d M (1-b-c-d) N e O2, wherein the Q element is selected from at least one of Mn and Al, the M element is selected from at least one of Zr, Ti, Al, Mg and Y, the N element is selected from at least one of Zr, Ti, Al, Co, Mg, W, Ce, Y, K and Na, 0.95≤a≤1.2, 0<b≤1, 0≤c≤1, 0<d≤1, b+c+d+e≤1, 0≤e<1.

4. The positive electrode material according to claim 1, wherein The positive electrode material satisfies at least one of the following characteristics: (1) The median particle size D50 of the positive electrode material is 2.5 μm to 5 μm, D10 is 1 μm to 2.5 μm, D90 is 6 μm to 10 μm, and 1.0≤(D90-D10) / D50≤1.5; (2) The specific surface area of ​​the positive electrode material is 0.5 m2 / g to 1.4 m2 / g; (3) The compaction density of the positive electrode material is greater than 3.0 g / cm3; (4) The tap density of the positive electrode material is greater than 1.5 g / cc; (5) The bulk density of the positive electrode material is greater than 0.5 g / cm3; (6) The discharge specific capacity of the positive electrode material at 2.8V to 4.35V at 0.1C is greater than 175mAh / g.

5. A method for preparing a positive electrode material, characterized in that: The following steps are involved: mixing an oxide precursor with a lithium source and a dopant and sintering the mixture to obtain a cathode material matrix; and The positive electrode material matrix is ​​crushed and coated with a coating agent to obtain the positive electrode material, wherein, in an image under a scanning electron microscope at a magnification of 3k, the number of particles less than 1 μm in the positive electrode material accounts for R: 8%≤R≤20%, and under a scanning electron microscope at a magnification of 3k, the positive electrode material has a first region and a second region, the number of particles less than 1 μm in the first region accounts for R1, and the number of particles less than 1 μm in the second region accounts for R2, wherein |R1-R2|≤9% and / or 0.5≤R1 / R2≤2; and under the condition of 25 MPa, the electrode sheet conductivity T of the positive electrode material satisfies: T>0.025S / cm; the two sides of the peak of the particle size distribution curve of the positive electrode material are asymmetric, and the area on the left side of the peak is larger than the area on the right side of the peak.

6. The method for preparing the positive electrode material according to claim 5, wherein: Before mixing the oxide precursor with the lithium source and the dopant and sintering, the method further includes atomizing and pyrolyzing the metal mixed solution to obtain the oxide precursor. The preparation method further satisfies at least one of the following characteristics: (1) In the step of atomizing and pyrolyzing the metal mixed solution to obtain the oxide precursor, the total metal concentration in the solution is controlled to be in the range of 2 mol / L to 8 mol / L; (2) In the step of atomizing and pyrolyzing the metal mixed solution to obtain the oxide precursor, the feed flow rate is controlled to be 0.5m 3 / h~5m 3 / h; (3) In the step of atomizing and pyrolyzing the metal mixed solution to obtain the oxide precursor, the atomizing air flow rate is controlled to be 50m 3 / h~300m 3 / h, atomization pressure is 100kPa~700kPa; (4) in the step of atomizing and pyrolyzing the metal mixed solution to obtain the oxide precursor, controlling the pyrolysis temperature to be 400° C. to 1200° C.; (5) The sintering includes a first sintering stage, a second sintering stage, a third sintering stage and a fourth sintering stage, which are carried out in sequence. The sintering temperature of the first sintering stage is 300°C to 600°C, and the time is 3h to 5h; the sintering temperature of the second sintering stage is 700°C to 1000°C, and the time is 4h to 12h; the sintering temperature of the third sintering stage is 500°C to 700°C, and the time is 2h to 6h; the sintering temperature of the fourth sintering stage is 700°C to 1000°C, and the time is 1h to 3h; (6) The doping element M in the dopant is selected from at least one of Zr, Ti, Al, Mg and Y; (7) The coating element N in the coating agent is selected from at least one of Zr, Ti, Al, Co, Mg, W, Ce, Y, K and Na.

7. A positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, characterized in that: The positive electrode active material layer includes the positive electrode material according to any one of claims 1 to 4 or a positive electrode material prepared by the method for preparing a positive electrode material according to claim 5 or 6.

8. A secondary battery, characterized in that: The positive electrode material comprises the positive electrode material according to any one of claims 1 to 4 or the positive electrode material prepared by the method for preparing the positive electrode material according to claim 5 or 6.

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