Positive electrode material, preparation method thereof, positive electrode sheet and secondary battery

Abstract

The present application provides a cathode material, a preparation method thereof, a cathode electrode sheet and a secondary battery. The ratio of the number of particles with cracks in the cathode material to the total number of particles is 1.5% to 11%, and the average aspect ratio M / N of the particles of the cathode material satisfies: 1 ≤ M / N ≤ 1.6, where M is the length of the longest straight side of a single particle, and N is the length of the straight side perpendicular to the midpoint position of the longest straight side. By controlling the proportion of the number of cracks and the aspect ratio of the particles in the cathode material, the present application can improve the cycling performance of the cathode material, improve the gas generation performance, and at the same time ensure that the material has good capacity.

Description

Technical Field

[0001] This application relates to the technical field of cathode materials for batteries, and specifically relates to a cathode material, a preparation method thereof, a cathode electrode sheet, and a secondary battery. Background Art

[0002] Currently, among numerous cathode materials, ternary cathode materials have become the most widely used cathode materials in the field of lithium-ion batteries due to their advantages of high specific capacity and high voltage platform. However, current cathode materials have obvious gas generation effects and poor cycle stability during the charge and discharge cycle process. 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 cathode material.

[0004] In addition, it is also necessary to provide a preparation method of the foregoing cathode material, as well as a cathode electrode sheet and a secondary battery using the foregoing cathode material.

[0005] In a first aspect, an embodiment of the present application provides a cathode material, where the ratio of the number of particles with cracks in the cathode material to the total number of particles is 1.5% - 11%, and the average aspect ratio M / N of the particles of the cathode material satisfies: 1 ≤ M / N ≤ 1.6, where M is the length of the longest straight line side of a single particle, and N is the length of the straight line side perpendicular to the midpoint position of the longest straight line side.

[0006] In some possible embodiments, the cathode material has an XRD diffraction peak in the range of 44° - 45°, and the full width at half maximum of the XRD diffraction peak is 0.150° - 0.200°.

[0007] In some possible embodiments, the proportion of the number of particles satisfying 1 ≤ M / N ≤ 1.6 in the cathode material is not less than 90%.

[0008] In some possible embodiments, the general formula composition of the cathode material is as follows: Li a Ni x Co y N 1-x-y-b Y b O2, where 0.95 ≤ a ≤ 1.05, 0.5 ≤ x < 1, 0 ≤ y ≤ 0.30, 0.001 ≤ b ≤ 0.01, the N element includes at least one of Mn and Al, and the Y element includes at least one element among Al, Ti, Zr, Sr, Mg, Y, Ba, Cu, W, Nb, La, Ce, Mo, Sn, Ta, and Ca.

[0009] In some possible embodiments, the Y element at least includes a Y1 element and a Y2 element, the Y1 element includes at least one element among Zr, Sr, Ba, and Ca, and the Y2 element includes at least one element among Al, Ti, Mg, Y, Cu, W, Nb, La, Ce, Mo, Sn, and Ta.

[0010] In some possible embodiments, the mass proportion of the Y element in the positive electrode material is 0.01 wt% to 0.5 wt%.

[0011] In some possible embodiments, the positive electrode material includes a plurality of primary particles, and the average particle size of the primary particles is 1 μm to 6 μm.

[0012] In some possible embodiments, the positive electrode material is a single crystal material, the positive electrode material contains a single crystal grain with the same orientation, and the average particle size of the single crystal grain is 1 μm to 6 μm.

[0013] In some possible embodiments, the particle size of the positive electrode material satisfies: D min <1 μm, 2.0 μm < D V,50 <4.5 μm, D max <15 μm, 0.8 < (D V,90 - D V,10 ) / D V,50 <1.6.

[0014] In some possible embodiments, the specific surface area of the positive electrode material is 0.1 m 2 / g ~ 1.6 m 2 / g.

[0015] In some possible embodiments, the oil absorption value per 100 grams of the positive electrode material is 10 mL to 40 mL.

[0016] In some possible embodiments, the surface residual alkali of the positive electrode material satisfies: LiOH ≤ 0.5 wt%, Li2CO3 ≤ 0.5 wt%.

[0017] In some possible embodiments, the tap density of the positive electrode material is 2.5 g / cm 3 ~ 3.6 g / cm 3 .

[0018] Second aspect: An embodiment of the present application provides a method for preparing a cathode material, including the following steps: mixing a precursor, a lithium salt, and an additive to form a mixture, and performing a first sintering on the mixture to obtain a first sintered product; crushing the first sintered product, and performing a second sintering on the crushed first sintered product, where the second sintering is a multi-stage gradient temperature sintering process to obtain a second sintered product; crushing the second sintered product to obtain a crushed second sintered product; and mixing the crushed second sintered product with a coating agent and performing sintering to obtain the cathode material, where the ratio of the number of particles with cracks in the cathode material to the total number of particles is 1.5% - 11%, and the average aspect ratio M / N of the particles of the cathode material satisfies: 1 ≤ M / N ≤ 1.6, where M is the length of the longest straight edge of a single particle, and N is the length of the straight edge perpendicular to the midpoint position of the longest straight edge.

[0019] Third aspect: An embodiment of the present application provides a positive electrode sheet, including a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, where the positive electrode active material layer includes the cathode material as described above or the cathode material prepared by the method for preparing the cathode material as described above.

[0020] Fourth aspect: An embodiment of the present application provides a secondary battery, including a positive electrode sheet, where the positive electrode sheet includes the cathode material as described above or the cathode material prepared by the method for preparing the cathode material as described above.

[0021] For the cathode material provided by the embodiment of the present application, by controlling the proportion of the number of cracks in the cathode material to be 1.5% - 11%, the gas generation of the cathode material during the battery cycle can be effectively reduced. In addition, by controlling the average aspect ratio M / N of single crystal particles in the cathode material to satisfy 1 ≤ M / N ≤ 1.6, the aspect ratio of crystal particles within this range is relatively low, and the crystal particles with a low aspect ratio have a higher roundness. The increased roundness is beneficial to reducing the specific surface area of the cathode material, and a smaller specific surface area is conducive to reducing the direct contact between the cathode material and the electrolyte, thereby reducing the generation of side reactions, which is beneficial to improving the long-term cycle performance, high-temperature storage performance, and gas generation performance of the cathode material. Therefore, the present application can effectively improve the cycle performance of the cathode material and improve the gas generation performance by controlling the proportion of the number of cracks and the aspect ratio of single crystal particles in the cathode material, while also ensuring that the cathode material has good capacity. Description of the Drawings

[0022] Figure 1 It is a schematic cross-sectional view when the lithium-ion battery using the cathode material of the embodiment of the present application is charged.

[0023] Figure 2Schematic cross-sectional view of a lithium-ion battery during discharge using the cathode material of the embodiments of the present application.

[0024] Figure 3 Scanning electron microscope (SEM) photograph of the cathode material of Embodiment 1 of the present application at a 3000-fold view.

[0025] Figure 4 Scanning electron microscope (SEM) photograph of the cathode material of Comparative Example 1 at a 3000-fold view.

[0026] Figure 5 Cross-sectional SEM photograph of the cathode material of Embodiment 1 of the present application at a 5000-fold view.

[0027] Figure 6 Cross-sectional SEM photograph of the cathode material of Comparative Example 1 at a 5000-fold view.

[0028] Figure 7 50-week cycle capacity retention rate curve of the batteries prepared from the cathode materials of Embodiment 1 and Comparative Example 1 of the present application.

[0029] Figure 8 Gas generation comparison chart of Embodiment 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 of the present application. Detailed Description of the Embodiments

[0030] The embodiments of the present application will be described in detail below. The embodiments described by referring to the accompanying drawings are exemplary and are only used to explain the present application, and should not be construed as a limitation to the present application. It should be noted that unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present application belongs. Without conflict, the embodiments of the present application and the features in the embodiments can be combined with each other. Many specific details are set forth in the following description to facilitate a full understanding of the present application. The described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments.

[0031] Since the cycle life of the existing cathode materials is not ideal and gas is easily generated, and the control means for particles in the existing solutions are relatively single, it is difficult to fully utilize the advantages of the particles.

[0032] The inventors of the present application studied the particle morphology of the cathode material and found that the performance of the cathode material is greatly affected by the number of cracks in the particles and the roundness of the particles. When there are too many cracks in the cathode material, the intergranular displacement during the cycle will expose new surfaces of the particles. In addition, particles with a large aspect ratio have poor roundness and will break during the pole piece rolling process, also exposing more new surfaces. These newly exposed surfaces will react with the electrolyte, resulting in the deterioration of the cycle performance and gas generation performance.

[0033] To this end, the embodiments of the present application provide a cathode material. The ratio of the number of particles with cracks in the cathode material to the total number of particles is 1.5% to 11%, and the average aspect ratio M / N of a single crystal particle of the cathode material satisfies: 1 ≤ M / N ≤ 1.6, where M is the length of the longest straight edge of a single particle, and N is the length of the straight edge perpendicular to the midpoint position of the longest straight edge.

[0034] First, the sources of cracks in the cathode material mainly include crystal cell anisotropy, process control during material pulverization, and intercrystalline displacement after cycling. The cracks in the cathode material of the present application mainly come from crystal cell anisotropy and cracks generated during the pulverization process. It should be noted that the cracks in the present application refer to cracks with a size between 0.5 nm and 30 nm. The present application controls the proportion of the number of cracks in the cathode material within the range of 1.5% to 11%, which can effectively reduce the gas generation of the cathode material during the battery cycling process. When the proportion of the number of cracks in the cathode material ≤ 1.5%, the crystals are too round at this time, resulting in a lower powder compaction density of this type of cathode material, and further resulting in a lower compaction density of the electrode sheet; when the proportion of the number of cracks in the cathode material > 11%, there are too many crystal cracks. After this type of cathode material is made into a battery, the gas generation during the cycling process will deteriorate significantly. Exemplarily, the proportion of the number of cracks in the cathode material can be 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11% or any value within the range composed of any two of the above values.

[0035] Although the present application controls the number of cracks in the cathode material, during the rolling process of the electrode sheet, the cathode material may still break and generate fine powder, resulting in gas generation. To this end, on the premise of controlling the proportion of the number of cracks in the material, the present application further controls the average aspect ratio M / N of a single particle in the cathode material to satisfy 1 ≤ M / N ≤ 1.6. The aspect ratio of the particles within this range is relatively low, and the roundness of the particles with a low aspect ratio is relatively high. The increase in roundness is beneficial to reducing the specific surface area of the cathode material, and a smaller specific surface area is beneficial to reducing the direct contact between the cathode material and the electrolyte, thereby reducing the generation of side reactions, being beneficial to improving the cycling performance of the cathode material. In addition, a lower aspect ratio can also reduce the breakage of the cathode material during the rolling process of the electrode sheet and improve the gas generation performance. Exemplarily, the aspect ratio of the particles can be 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6 or any value within the range composed of any two of the above values.

[0036] Therefore, by jointly controlling the proportion of the number of cracks in the cathode material and the average aspect ratio of the particles within the foregoing range, the present application can effectively improve the cycling stability of the cathode material, improve the cycling performance of the material, and can also improve the compaction density of the electrode sheet, enabling the cathode material to have good capacity, and at the same time can effectively reduce the gas generation after the cathode material is made into a battery.

[0037] In some embodiments, in the XRD pattern of the positive electrode material, the positive electrode material has a diffraction peak within 44° to 45°, and the full width at half maximum (FWHM) of the diffraction peak is 0.150° to 0.200°. Within the above FWHM value range, the crystal cell size of the positive electrode material is moderate, which is beneficial to the capacity performance of the positive electrode material. The FWHM range of the XRD diffraction peak is further 0.155° to 0.175°. Exemplarily, the FWHM of the XRD diffraction peak can be 0.150°, 0.155°, 0.160°, 0.165°, 0.170°, 0.175°, 0.180°, 0.185°, 0.190°, 0.195°, 0.200° or any value within the range composed of any two of the above values.

[0038] In some embodiments, the positive electrode material is a single crystal material, and the positive electrode material contains single particles with the same orientation. The single particles with the same orientation are primary particles. The positive electrode material includes a plurality of primary particles. The primary particles have good dispersibility, and the average particle size of the primary particles is 1 μm to 6 μm. The primary particles within the above range have a moderate particle size, which is beneficial to the full performance of the capacity of the positive electrode material. Moreover, the primary particles have good dispersibility, grow more fully, and less form aggregates, which is beneficial to improving the cycling performance of the positive electrode material. The average particle size of the primary particles is further 1.5 μm to 3 μm. Exemplarily, the average particle size of the primary particles can be 1 μm, 1.5 μm, 1.8 μm, 2.0 μm, 2.1 μm, 2.4 μm, 2.7 μm, 3 μm, 3.1 μm, 3.4 μm, 3.5 μm, 3.7 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm or any value within the range composed of any two of the above values.

[0039] In some embodiments, the proportion of the number of crystal particles satisfying 1 ≤ X / Y ≤ 1.6 in the positive electrode material is more than 90%. By controlling the aspect ratio of more than 90% of the crystal particles in the material within the range of 1 to 1.6, it is beneficial to improve the roundness of the overall particles in the positive electrode material, further reduce the specific surface area of the positive electrode material, reduce the generation of side reactions, and thus further improve the long-term cycling performance, high-temperature storage performance of the positive electrode material and improve the gas generation performance. Exemplarily, the proportion of the number of crystal particles satisfying 1 ≤ X / Y ≤ 1.6 in the positive electrode material can be 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, etc.

[0040] In some embodiments, the median diameter D of the positive electrode material V,50 is 2 μm to 4.5 μm. The median diameter D V,50It represents the particle size of the material particles corresponding to when the cumulative particle size distribution percentage reaches 50% by volume. In this application, by controlling the median particle size of the positive electrode material within the appropriate range above, the particle size is moderate, which is beneficial to reducing the specific surface area of the particles, reducing the side reactions between the particles and the electrolyte, thereby improving the safety and cycle life of the material; at the same time, particles within the above range are also beneficial to reducing the internal stress of the particles and reducing the risk of lithium-ion electrochemical polarization inside and outside the particles, thereby improving the capacity of the positive electrode material. The minimum particle size D of the positive electrode material min < 1 μm, and the maximum particle size D max < 15 μm, where the smallest particle size in the positive electrode material is below 1 μm, and the particle size of the largest particle is less than 15 μm. The presence of particles smaller than 1 μm is beneficial to improving the tap density and capacity of the positive electrode material. Controlling the maximum particle size below 15 μm is to reduce the number of large agglomerates and affect the cycle performance. Exemplarily, the median particle size D of the positive electrode material V,50 is 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm or any value within the range composed of any two of the above values.

[0041] In some embodiments, 0.8 ≤ (D V,90 - D V,10 ) / D V,50 ≤ 1.6 for the positive electrode material. D V,10 represents the particle size corresponding to when the cumulative particle size distribution reaches 10% in the particle size distribution, that is, in the particle population, 10% of the particles are smaller than this particle size. Usually, D V,10 is used to describe the finer particles in the particle population. D V,90 represents the particle size corresponding to when the cumulative particle size distribution reaches 90%, that is, in the particle population, 90% of the particles are smaller than this particle size. D V,90 is usually used to describe the coarser particles in the particle population. (D V,90 - D V,10 ) / D V,50 represents the width of the particle size distribution of the positive electrode material. In this application, by controlling 0.8 ≤ (D V,90 - D V,10 ) / D V,50 ≤ 1.6, the particle size distribution of the positive electrode material is moderate, which can reduce the specific surface area of the particles while maintaining the tap density of the material, thereby ensuring that the positive electrode material has excellent cycle performance, storage performance, and capacity. Exemplarily, (D V,90 - D V,10 ) / D V,50 of the positive electrode material can be 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6 or any value within the range composed of any two of the above values.

[0042] This application controls the particle size of the positive electrode material to meet the requirements: D min < 1 μm, 2.0 μm < D V,50 < 4.5 μm, D max < 15 μm, 0.8 < (D V,90 - D V,10 ) / D V,50 < 1.6, which can make the particle size of the positive electrode material moderate, so that while reducing the specific surface area of the particles, the tap density of the material can be maintained, which is beneficial to ensuring that the positive electrode material has excellent cycle performance, storage performance and capacity.

[0043] In some embodiments, the specific surface area of the positive electrode material is 0.1 m 2 / g ~ 1.6 m 2 / g. The crystal particles in the positive electrode material have a small aspect ratio, the particles are relatively round, and the specific surface area is small. A small specific surface area is beneficial to reducing the direct contact between the positive electrode material and the electrolyte, thereby reducing the generation of side reactions, which is beneficial to improving the long-term cycle performance, high-temperature storage performance and gas generation performance of the positive electrode material. Exemplarily, the specific surface area of the positive electrode material can be 0.1 m2 / g, 0.2 m2 / g, 0.3 m2 / g, 0.4 m2 / g, 0.5 m2 / g, 0.6 m2 / g, 0.7 m2 / g, 0.8 m2 / g, 0.9 m2 / g, 1.0 m2 / g, 1.1 m2 / g, 1.2 m2 / g, 1.3 m2 / g, 1.4 m2 / g, 1.5 m2 / g, 1.6 m2 / g or any value within the range composed of any two of the above values. Further, the specific surface area of the positive electrode material is further 0.6 m 2 / g ~ 1.2 m 2 / g.

[0044] In some embodiments, the oil absorption value per 100 g of the positive electrode material is 10 mL~40 mL. A lower oil absorption value is beneficial to improving the processing performance of the positive electrode material. Exemplarily, the oil absorption value per 100 g of the positive electrode material can be 10 mL, 15 mL, 20 mL, 25 mL, 30 mL, 35 mL, 40 mL or any value within the range composed of any two of the above values.

[0045] In some embodiments, the tap density of the positive electrode material under a pressure of 3 t is 2.5 g / cm 3 ~ 3.6 g / cm 3 , by controlling the number of cracks in the material and the aspect ratio of the crystal particles, the tap density of the positive electrode material powder can be effectively improved, so as to ensure that the positive electrode material has a high tap density after being made into a pole piece, so as to improve the capacity of the positive electrode material. Exemplarily, the tap density of the positive electrode material can be 2.5 g / cm3 , 2.6 g / cm 3 , 2.7 g / cm 3 , 2.8 g / cm 3 , 2.9 g / cm 3 , 3.0 g / cm 3 , 3.1 g / cm 3 , 3.2 g / cm 3 , 3.3 g / cm 3 , 3.4 g / cm 3 , 3.5 g / cm 3 , 3.6 g / cm 3 or any value within the range formed by any two of the above values.

[0046] In some embodiments, the residual alkali in the cathode material is LiOH < 0.5 wt%, Li2CO3 < 0.5 wt%. By controlling the number of cracks and the aspect ratio of crystal grains in the cathode material, it is beneficial to reduce the specific surface area of the grains while increasing the tap density of the material, so as to reduce the alkalinity of the material and improve the storage performance. At the same time, the residual alkali on the grain surface is reduced, effectively enhancing the electrochemical activity of the cathode material and ensuring the smooth diffusion of lithium ions in the material, thereby improving the battery performance. Exemplarily, the amount of residual alkali can be 0.01 wt%, 0.05 wt%, 0.1 wt%, 0.15 wt%, 0.2 wt%, 0.25 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.35 wt%, 0.5 wt% or any value within the range formed by any two of the above values.

[0047] In some embodiments, the general formula composition of the cathode material is as follows: Li a Ni x Co y N 1-x-y-b Y b O2, where 0.95 ≤ a ≤ 1.05, 0.5 ≤ x < 1, 0 ≤ y ≤ 0.30, 0.001 ≤ b ≤ 0.01, the N element includes at least one of Mn and Al, and the Y element includes at least one element among Al, Ti, Zr, Sr, Mg, Y, Ba, Cu, W, Nb, La, Ce, Mo, Sn, Ta, and Ca, etc. By doping the above elements into the cathode material, the crystal structure of the cathode material can be optimized, the surface morphology of single crystal grains can be further improved, cracks can be reduced, and at the same time, the grains can be controlled to have a lower aspect ratio to make the grains more rounded. It can also further improve the structural stability and conductivity of the cathode material.

[0048] In some embodiments, the mass percentage of the Y element in the cathode material is 0.01 wt% to 0.5 wt%. Exemplarily, the mass percentage of the Y element in the cathode material can be 0.01 wt%, 0.03 wt%, 0.05 wt%, 0.08 wt%, 0.1 wt%, 0.12 wt%, 0.15 wt%, 0.18 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt% or any value within the range formed by any two of the above values.

[0049] Among them, the Y element at least includes the Y1 element, and the Y1 element contains at least one element among Zr, Sr, Ba, Ca, etc. Doping such elements into the bulk phase can inhibit the mixing of lithium and nickel, reduce the change in unit cell volume during charge and discharge, and thus can effectively inhibit the generation of cracks in the crystal during charge and discharge, improving the first Coulomb efficiency and discharge capacity of the material.

[0050] The Y element at least further includes the Y2 element, and the Y2 element contains at least one of the elements such as Al, Ti, Mg, Y, Cu, W, Nb, La, Ce, Mo, Sn, and Ta. Doping such elements into the bulk phase can limit the irregular growth of crystal particles, limit the formation of particles with an excessive aspect ratio, and is beneficial to the aspect ratio of crystal particles being 1 to 1.6.

[0051] The cathode material provided by the embodiments of the present application can effectively reduce the gas generation of the cathode material during the battery cycle by controlling the proportion of the number of cracks in the cathode material to be 1.5% to 11%. In addition, by controlling the average aspect ratio M / N of a single particle in the cathode material to satisfy 1 ≤ M / N ≤ 1.6, the aspect ratio of the particles in this range is relatively low, and the crystal particles with a low aspect ratio have a higher roundness. The increased roundness is beneficial to reducing the specific surface area of the cathode material, and the smaller specific surface area is beneficial to reducing the direct contact between the cathode material and the electrolyte, thereby reducing the generation of side reactions, which is beneficial to improving the long-term cycle performance, high-temperature storage performance of the cathode material, and improving the gas generation performance. Therefore, by jointly controlling the proportion of the number of cracks in the cathode material and the average aspect ratio of the particles within the foregoing ranges, the present application can effectively improve the cycle stability of the cathode material, improve the cycle performance of the material, and can also increase the compaction density of the electrode sheet, enabling the cathode material to have good capacity, while effectively reducing the gas generation after the cathode material is made into a battery.

[0052] It should be noted that the difference between single crystal positive electrode materials and polycrystalline positive electrode materials (i.e., polycrystalline secondary particles) is that the smallest particle of polycrystalline secondary particles is a secondary particle formed by the agglomeration of nano-scale primary particles. For single crystal positive electrode materials, the smallest particle is usually a single primary particle of micrometer scale. Generally speaking, in addition to the EBSD test method, it is also possible to determine whether the obtained positive electrode product is a single crystal material by characterization methods such as scanning electron microscopy (SEM). For example, for single crystal positive electrode materials, the morphology of single crystal particles can be characterized by SEM, and it can be seen that the shape of single crystal particles is generally regular or irregular spherical, and there is no significant particle agglomeration. The orientation of single crystal positive electrode materials can also be characterized by EBSD. Through EBSD, it can be observed that the color in at least one grain is the same, so as to judge that the orientation in at least one grain is the same, and the grains with the same orientation are single crystals. It should be specifically noted that the "single crystal 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 the same arrangement and direction. However, due to impurities, strains and crystal defects, ideal single crystals are very rare and difficult to produce in the laboratory. Therefore, the single crystal cathode materials known in the art are actually more "single crystal morphology" cathode materials, which only show the size of large single crystal particles and are different from polycrystalline particles composed of many small primary particles.

[0053] It can be understood that a 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-6μm. In general, the roundness of the particles of "quasi-secondary particles" is lower than that of the above-mentioned conventional "secondary particles".

[0054] 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 exactly the same arrangement and orientation. 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 "single crystal-like morphology" positive electrode materials, which only show a large particle size similar to a single crystal in size, which is different from a polycrystal composed of many small primary particles.

[0055] The embodiments of the present application also provide a secondary battery (such as a lithium-ion battery, a sodium-ion battery, etc.), which includes a housing, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located inside the housing. The electrode assembly includes a separator, a negative electrode tab, and a positive electrode tab, and the separator is disposed between the positive electrode tab and the negative electrode tab. Among them, the positive electrode tab includes the aforementioned positive electrode material.

[0056] Specifically, the positive electrode tab includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, and the positive electrode active material layer includes the aforementioned positive electrode material. The positive electrode current collector can be made of aluminum foil, nickel foil, etc., or can also be a composite current collector disclosed in any prior art, such as but not limited to the current collector formed by combining the aforementioned conductive foil and a polymer substrate.

[0057] In some embodiments, the housing can be a packaging bag obtained by encapsulating with a packaging film (such as an aluminum-plastic film), such as when the secondary battery is a soft-pack battery. In some other embodiments, the secondary battery can also be a steel-shell battery, an aluminum-shell battery, etc.

[0058] In some embodiments, the electrode assembly can be a laminated structure, which is formed by alternately laminating the positive electrode tab, the separator, and the negative electrode tab in sequence. In some other embodiments, the electrode assembly can also be a wound structure, which is formed by laminating the positive electrode tab, the separator, and the negative electrode tab in sequence and then winding them.

[0059] In some embodiments, the negative electrode tab 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 made of at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or a carbon-based current collector, or can also be a composite current collector disclosed in any prior art, such as but not limited to the 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, etc. The silicon-based materials can be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based materials can be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other conventional materials that can be used as the negative electrode active material of the battery can also be used. These negative electrode active materials can be used alone or in combination of two or more. The battery provided by the embodiments of the present application has the advantages of high capacity, high initial efficiency, long cycle life, excellent rate performance, and low swelling. The battery can be a lithium-ion battery, a sodium-ion battery, a solid electrolyte battery, etc., which is not limited herein.

[0060] Such as Figure 1 And Figure 2As shown, they are respectively schematic diagrams of lithium deintercalation and intercalation during the charging and discharging processes of a lithium-ion battery prepared using the cathode material provided in the embodiments of the present application. As Figure 1 shown, when the lithium-ion battery is charged, lithium ions are deintercalated from the positive electrode plate and intercalated into the negative electrode plate; as Figure 2 shown, when the lithium-ion battery is discharged, lithium ions are deintercalated from the negative electrode plate and intercalated back into the positive electrode plate.

[0061] Among them, the aforementioned cathode material is used in a coin cell to evaluate the cycle performance of the battery. At room temperature (25°C), after 50 cycles at 1C / 1C, the cycle retention rate of the battery is ≥96%. It shows that using the aforementioned cathode material can effectively improve the cycle performance of the battery, and the gas generation amount during the cycling process of the battery is relatively low, not exceeding 1.4 mL, and even reduced to below 1 mL.

[0062] The present application also provides a preparation method of the aforementioned cathode material, which specifically includes the following steps:

[0063] Step S1, mixing a precursor, a lithium salt, and an additive containing element Y to form a mixture, and performing a first sintering on the mixture to obtain a first sintered product.

[0064] Among them, Y is selected from at least one element of Al, Ti, Zr, Sr, Mg, Y, Ba, Cu, W, Nb, La, Ce, Mo, Sn, Ta, Ca, including oxides, peroxides, hydroxides, chlorides, carbonates, sulfates, phosphates, or combinations thereof.

[0065] In some embodiments, based on the mass of the precursor, the mass ratio of the additive containing element Y is 0.01 wt% - 0.5 wt%. Exemplarily, the mass ratio of the additive containing element Y can be 0.01 wt%, 0.03 wt%, 0.05 wt%, 0.08 wt%, 0.1 wt%, 0.12 wt%, 0.15 wt%, 0.18 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, or any value within the range composed of any two of the above values.

[0066] The Y element contains at least the Y1 element, and the Y1 element includes at least one element among Zr, Sr, Ba, Ca, etc. Additives containing such elements can be used as fluxes. Doping such elements into the bulk phase can inhibit the mixing of lithium and nickel, reduce the change in unit cell volume during charge and discharge, and thus effectively inhibit the generation of cracks in the crystal during charge and discharge, improving the first Coulomb efficiency and discharge capacity of the material. By adjusting the addition amount of such flux components containing the Y1 element, it is beneficial to form primary particles with good dispersibility, a suitable grain size, and less lithium-nickel mixing in the lithium nickel cobalt oxide single crystal cathode material. Well-dispersed primary particles can form a uniform coating layer, preventing new surface interfaces from being exposed due to the fragmentation of agglomerated particles during the rolling process of the electrode sheet, avoiding direct contact between the newly exposed surface particles of the cathode material and the electrolyte, and inhibiting side reactions, which is beneficial to improving the first Coulomb efficiency, capacity, structural stability, thermal stability, and long-cycle stability of the material. Among them, based on the mass of the precursor, the mass ratio of the effective element in the flux containing the Y1 element is 0.01wt% - 0.5wt%. For example, it can be 0.01wt%, 0.05wt%, 0.1wt%, 0.15wt%, 0.2wt%, 0.25wt%, 0.3wt%, 0.35wt%, 0.4wt%, 0.45wt%, 0.5wt% or any value within the range composed of any two of the above values.

[0067] The Y element at least further includes the Y2 element, and the Y2 element can contain at least one element among Al, Ti, Mg, Y, Cu, La, Ce, Sn, W, Mo, Nb, Ta, etc. Additives containing such elements can be used as inhibitors. Doping such elements into the bulk phase can limit the irregular growth of particles, limit the formation of particles with an excessive aspect ratio, and is beneficial for the aspect ratio of the particles to be 1 - 1.6. By adjusting the addition amount of such inhibitor components of the Y2 element, it is beneficial to form primary particles with good dispersibility and a lithium nickel manganese cobalt oxide cathode material with a smaller aspect ratio. The cathode material with a smaller aspect ratio has a more regular shape, which can prevent particle breakage and exposure of new surface interfaces during the rolling process of the electrode sheet, avoid direct contact between the newly formed surface interface and the electrolyte, and inhibit side reactions, which is beneficial to improving the structural stability, thermal stability, long-term cycle performance, and gas generation performance of the material. Among them, based on the mass of the precursor, the mass ratio of the effective element in the inhibitor containing the Y2 element is 0.01wt% - 0.5wt%. For example, it can be 0.01wt%, 0.05wt%, 0.1wt%, 0.15wt%, 0.2wt%, 0.25wt%, 0.3wt%, 0.35wt%, 0.4wt%, 0.45wt%, 0.5wt% or any value within the range composed of any two of the above values.

[0068] In addition, substances such as CaO2, Na2O2, KMnO4, etc. can be added to the mixture as oxygen supplement agents, aiming to increase the oxygen partial pressure, which is beneficial to the growth of particles, inhibits the formation of the surface rock salt phase, and is conducive to improving the first Coulomb efficiency, capacity, structural stability, thermal stability, and long cycle performance of the cathode material. Among them, based on the mass of the precursor, the mass ratio of such oxygen supplement agents is 0.01wt% - 1wt%.

[0069] Specifically, in step S1, a pressure of 5MPa - 50MPa is applied to the mixture, the pressure holding time is 1min - 5min, and then it is placed in an oxygen atmosphere for a first sintering. Among them, the first sintering includes two-stage sintering processes. The temperature of the first stage sintering is not higher than 500°C (usually 300°C - 500°C), and the time is 6h - 10h, aiming to melt the molten salt and make the additives uniformly diffuse in the mixture. The temperature of the second stage sintering is not higher than 880°C (usually 550°C - 880°C), and the time is 4h - 12h, which can make the molten salt fully react with the precursor in an oxygen atmosphere, being beneficial to the formation of single crystal materials. Through two-stage gradient temperature sintering, it is beneficial to the formation of particles, and at the same time, the number of particle cracks can be controlled. Among them, within the above range, with the increase of temperature and the prolongation of the holding time, the further fusion of small particles and fine powders can be achieved, reducing the number of particle cracks.

[0070] In some embodiments, the temperature of the first stage sintering is further 400°C - 450°C. Exemplarily, this temperature can be 410°C, 420°C, 430°C, 440°C, 450°C or any value within the range composed of any two of the above values. The holding time of the first stage sintering is further 7h - 9h. Exemplarily, this holding time can be 7h, 8h, 9h or any value within the range composed of any two of the above values.

[0071] In some embodiments, the temperature of the second stage sintering can be further 700°C - 860°C. Exemplarily, the temperature of the second stage sintering can be 700°C, 720°C, 740°C, 760°C, 780°C, 800°C, 820°C, 840°C, 860°C, 880°C or any value within the range composed of any two of the above values. The holding time of the second stage sintering is further 6 - 10h. Exemplarily, this holding time can be 6h, 7h, 8h, 9h, 10h or any value within the range composed of any two of the above values.

[0072] In some embodiments, in step S1, the lithium salt can include any one or a combination of at least two of lithium carbonate, lithium hydroxide, lithium acetate, lithium nitrate, or lithium oxalate, and is preferably lithium hydroxide.

[0073] In some embodiments, the mixing method in step S1 is mechanical mixing, which can improve the mixing effect. For example, a high-speed mixer or a VC mixer can be used for material mixing.

[0074] In some embodiments, the mixing time in step S1 can be 0.3h to 2.0h. An appropriate mixing time can improve the material dispersion effect. Exemplarily, the mixing time can be 0.3h, 0.4h, 0.5h, 0.7h, 0.8h, 1.0h, 1.2h, 1.5h, 2.0h, or any value within the range composed of any two of the above values.

[0075] In some embodiments, the mixing temperature in step S1 can be 10°C to 50°C. An appropriate mixing temperature can improve the material dispersion effect. Within this range, the materials can be fully and evenly mixed, and side reactions of the mixed raw materials caused by too high a temperature can be prevented. The mixing temperature is further 20°C to 40°C. Exemplarily, the mixing temperature can be 10°C, 15°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, or any value within the range composed of any two of the above values.

[0076] In some embodiments, the materials are sintered once in step S1. In terms of molar ratio, the content of lithium in the lithium salt (i.e., lithium compound) relative to the total amount of Ni, Co, and N elements in the above-mentioned precursor is 0.95 to 1.05. Specifically, the general formula of the precursor can be Ni x Co y N 1-x-y O2, 0.5 ≤ x < 1, 0 ≤ y ≤ 0.30, and the N element includes at least one of Mn and Al. When the lithium content ratio is within the above range, it can not only reduce the Li / Ni cation mixing degree but also prevent excessive residual lithium on the surface of the once-sintered product, which affects the processing performance and safety performance. The content ratio of lithium in the lithium salt is further 0.99 to 1.02. Exemplarily, the content ratio of lithium in the lithium salt can be 0.95, 0.96, 0.97, 0.98, 0.99, 1.00, 1.05, or any value within the range composed of any two of the above values.

[0077] In some embodiments, the sintering in step S1 is carried out in an atmosphere with an oxygen content ≥ 95%. Sufficient oxygen can promote the conversion of Ni 2+ to Ni 3+ and reduce the Li / Ni cation mixing, thereby enhancing the capacity of the cathode material.

[0078] Step S2: Crush the once-sintered product and perform secondary sintering on the crushed once-sintered product. The secondary sintering is a multi-stage gradient temperature sintering process to obtain a secondary-sintered product.

[0079] Specifically, the primary sintered product is subjected to air jet milling. The particles of the milled material have an aspect ratio of 1 to 1.4 and a particle size of 1 μm to 3 μm. Then, the milled material is subjected to a second sintering. The reduced particle size of the milled material is beneficial to improving the dispersibility of single crystal particles, forming a uniform coating layer, and inhibiting the direct contact between the electrolyte and the active material, which is conducive to improving the first Coulomb efficiency, capacity, structural stability, thermal stability, and long cycle stability of the material.

[0080] The second sintering is set at a two-stage temperature, first high temperature and then low temperature. The high temperature sintering temperature is 750°C to 980°C, and the time is 8 to 16 h, which can effectively promote the growth of particles. After high temperature sintering, low temperature sintering is carried out. The low temperature sintering temperature is 500 to 900°C, and the time is 3 to 10 h. The low temperature sintering process can effectively eliminate the internal stress in the lattice. In addition, the temperature difference between high temperature and low temperature is less than 300°C.

[0081] In some embodiments, the temperature in the high temperature sintering stage is further 800 - 950°C. Exemplarily, the temperature can be 750°C, 800°C, 830°C, 860°C, 890°C, 920°C, 950°C, 980°C or any value within the range composed of any two of the above values.

[0082] In some embodiments, the temperature in the low temperature sintering stage is further 600 - 800°C. Exemplarily, the temperature can be 500°C, 600°C, 650°C, 700°C, 750°C, 800°C, 850°C, 900°C, 950°C or any value within the range composed of any two of the above values.

[0083] In this step, by first controlling the temperature and time in the high temperature sintering stage, the intercalation of lithium ions can be achieved, the growth of crystals can be promoted, the aspect ratio of crystal particles can be controlled to be relatively low, the particles are more rounded, and the specific surface area is reduced. Then, by controlling the temperature and time of low temperature sintering, the further intercalation of lithium ions can be achieved, and at the same time, the annealing effect can be achieved, effectively eliminating the internal stress in the lattice, repairing the microcrystalline boundaries, and reducing the number of cracks in the positive electrode material.

[0084] Step S3, pulverize the secondary sintered product to obtain the pulverized secondary sintered product.

[0085] Among them, the average aspect ratio M / N of the particles of the positive electrode material satisfies: 1 ≤ M / N ≤ 1.6, where M is the length of the longest straight side of a single particle, and N is the length of the straight side perpendicular to the midpoint position of the longest straight side.

[0086] Specifically, the secondary sintered product is subjected to air flow pulverization, the process parameters of the pulverization are controlled, and the mass of the grinding body is controlled to be 10% - 60% of the upper limit of the sample amount processed by the equipment, the pulverization air pressure is controlled to be 20% - 90% of the upper limit of the equipment pressure, the aspect ratio of the pulverized crystal particles is 1 - 1.6, and the crystal particle size is 1 μm - 6 μm.

[0087] In this step, the mass of the grinding body is 10% - 60% of the upper limit of the equipment. For example, the upper limit of the sample amount processed by a small air flow pulverizer is 3 kg, and the mass of the grinding body is 0.3 kg - 1.8 kg. Within this range, the particles can be evenly dispersed, so that cracks will not be generated due to excessive particle collisions, and the efficiency of air flow crushing can be ensured. Further, the mass of the grinding body is 20% - 50% of the upper limit of the equipment. Exemplarily, the mass of the grinding body can be 10%, 20%, 30%, 40%, 50%, 60% of the upper limit of the equipment or any value within the range composed of any two of the above values.

[0088] In this step, the pulverization air pressure is controlled to be 20% - 90% of the upper limit of the equipment pressure. For example, the upper limit of the pulverization gas pressure of a small air flow pulverizer is 1 MPa, and the pulverization gas pressure is 0.2 MPa - 0.9 MPa. Within this range, the pulverization air pressure is moderate, so that excessive cracks will not be generated due to strong collisions caused by too high pulverization air pressure, nor will the particles lack collisions due to too low pulverization pressure, resulting in the failure to disperse the primary particles. The pulverization air pressure is further controlled to be 30% - 80% of the upper limit of the equipment pressure, and the pulverization air pressure is further controlled to be 40% - 70% of the upper limit of the equipment pressure. Exemplarily, the pulverization air pressure can be controlled to be 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% of the upper limit of the equipment or any value within the range composed of any two of the above values.

[0089] Step S4: Mix the pulverized secondary sintered product with a coating agent and sinter the mixture to obtain the positive electrode material.

[0090] In some embodiments, the temperature of the coating sintering can be 250°C - 850°C for 4 h - 10 h. Exemplarily, the sintering temperature can be 250°C, 300°C, 400°C, 500°C, 600°C, 700°C, 800°C, 850°C or any value within the range composed of any two of the above values, and the sintering time can be 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h or any value within the range composed of any two of the above values.

[0091] In some embodiments, the coating agent can include at least one of substances such as Al2O3, AlOOH, ZrO2, Co3O4, Co(OH)2, CoOOH, TiO2, WO3, Nb2O5, MoO3, etc.

[0092] In some embodiments, the mass ratio of the coating agent to the pulverized secondary sintered product may be 0.05 wt% to 3 wt%. Exemplarily, the proportion of the coating agent may be 0.05 wt%, 0.1 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt% or any value within the range formed by any two of the above values.

[0093] In the preparation process of the positive electrode material of the present application, firstly, by controlling the flux, oxygen replenishing agent, sintering temperature curve and air-crushing process parameters, the crack proportion of the powder material can be effectively optimized, and the gas generation performance of the positive electrode material can be improved. Additionally, on the basis of optimizing the crack proportion of the powder material, further optimizing the inhibitor and sintering temperature curve can effectively control the aspect ratio of the crystal particles, reduce the breakage of the positive electrode material during the pole piece rolling process, and improve the gas generation performance of the battery.

[0094] The solution of the present application will be explained below in conjunction with embodiments. Those skilled in the art will understand that the following examples are only for explaining the present application and should not be construed as a limitation to the present application. Unless otherwise specified, the reagents, software and instruments involved in the following examples that are not specifically described are all commercially available products or open source.

[0095] Example 1:

[0096] Step S1, mix the precursor Ni 0.6 Co 0.1 Mn 0.3 O, LiOH·H2O and the flux SrO, the inhibitor H2WO4, and the oxygen replenishing agent CaO2 evenly in a high-speed mixer. Among them, the molar ratio of LiOH·H2O to Ni 0.6 Co 0.1 Mn 0.3 O is 1.03, and the mass ratios of SrO, H2WO4, and CaO2 to Ni 0.6 Co 0.1 Mn 0.3 O are 0.15 wt%, 0.15 wt%, and 0.15 wt% respectively.

[0097] Apply a pressure of 50 MPa to the above mixture, and keep the pressure for 5 minutes. Keep the temperature at 450 °C for 7 hours in an oxygen atmosphere with a furnace pressure of 12 ± 4 Pa, and then raise the temperature to 800 °C and keep it for 8 hours to obtain a primary sintered product.

[0098] Step S2, the primary sintered product is pulverized by a small air flow mill, the grinding mass is 0.6 kg, and the pulverizing pressure is 0.4 MPa. The gas-crushed material is subjected to secondary sintering, the first stage temperature is set at 950°C, and the temperature is kept for 10 hours, and the second stage temperature is set at 800°C, and the temperature is kept for 6 hours, to obtain a secondary sintered product.

[0099] Step S3, the above-mentioned secondary sintered product is subjected to air flow pulverization by a small air flow pulverizer, the pulverization process parameters are controlled, the grinding body amount is controlled to be 0.8kg, the pulverization air pressure is controlled to be 0.5MPa, the aspect ratio of the crystal particles after pulverization is 1.48, and the average particle size is 2.2μm, to obtain the secondary sintered product after air pulverization.

[0100] Step S4, mixing the gas-crushed secondary sintered product with the coating agent Al2O3, the mass ratio of the coating agent to the gas-crushed secondary sintered product is 0.2wt%, the shearing action during the mixing process can further reduce the aspect ratio of the single crystal particles, and sintering the mixture at 500°C for 8h in an oxygen atmosphere to obtain an Al-coated ternary single crystal material.

[0101] Example 2

[0102] The difference from Example 1 is that in step S1, based on the mass of the precursor, the mass proportion of SrO is 0.30wt%. The other steps are basically the same as Example 1, please refer to Example 1.

[0103] Example 3

[0104] The difference from Example 1 is that in step S3, the amount of the grinding body is 1.6 kg and the crushing gas pressure is 0.7 MPa. The other steps are basically the same as those in Example 1, please refer to Example 1.

[0105] Example 4

[0106] The difference from Example 1 is that in step S1, based on the mass of the precursor, the mass of H2WO4 accounts for 0.4wt%. The other steps are basically the same as Example 1, please refer to Example 1.

[0107] Example 5

[0108] The difference from Example 1 is that in step S1, based on the mass of the precursor, the mass proportion of H2WO4 is 0.05wt%. The other steps are basically the same as Example 1, please refer to Example 1.

[0109] Example 6

[0110] The difference from Example 1 is that in step S1, the primary sintering process is adjusted from keeping warm at 450° C. for 7 hours to keeping warm at 300° C. for 5 hours. The other steps are basically the same as Example 1, please refer to Example 1.

[0111] Example 7

[0112] The difference from Example 1 is that in step S1, during the primary sintering process, the holding time at 800°C for 8 h is adjusted to the holding time at 700°C for 6 h. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0113] Example 8

[0114] The difference from Example 1 is that in step S2, during the secondary sintering process, the holding time at 950°C for 10 h in the first stage is adjusted to the holding time at 920°C for 6 h. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0115] Example 9

[0116] The difference from Example 1 is that in step S2, during the secondary sintering process, the holding time at 800°C for 6 h in the second stage is adjusted to the holding time at 600°C for 4 h. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0117] Example 10

[0118] The difference from Example 1 is that in step S1, based on the mass of the precursor, the mass ratio of the flux SrO is 0.2 wt%, and the mass ratio of the inhibitor H2WO4 is 0.1 wt%; the sintering process in step S2 is: after holding at 945°C for 9 h, the temperature is lowered to 820°C and held for 5 h.

[0119] Other steps are basically the same as those in Example 1, please refer to Example 1.

[0120] Example 11

[0121] The difference from Example 1 is that in step S1, the precursor is Ni 0.7 Co 0.1 Mn 0.2 O, LiOH·H2O is changed to Li2CO3, and the flux SrO is changed to ZrO2. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0122] Example 12

[0123] The difference from Example 1 is that in step S1, the precursor is Ni 0.8 Co 0.1 Mn 0.1 O, the flux SrO is changed to ZrO2, and the inhibitor is changed to MoO3. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0124] Comparative Example 1

[0125] The difference from Example 1 is that SrO is not added in step S1. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0126] Comparative Example 2

[0127] The difference from Example 1 is that the airflow pulverization process is not carried out in step S3. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0128] Comparative Example 3

[0129] The difference from Example 1 is that the pulverization air pressure in step S3 is controlled at 95% of the upper limit. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0130] Comparative Example 4

[0131] The difference from Example 1 is that the grinding mass in step S3 is 70% of the upper limit of the equipment. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0132] Comparative Example 5

[0133] The difference from Example 1 is that the mass ratio of the flux SrO is 0.05 wt%, and the mass ratio of the inhibitor H2WO4 is 0.05 wt%. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0134] Comparative Example 6

[0135] The difference from Example 1 is that the mass ratio of the flux SrO is 0.3 wt%, and the mass ratio of the inhibitor H2WO4 is 0.5 wt%. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0136] Comparative Example 7

[0137] The difference from Example 1 is that the mass ratio of the flux SrO is 0.4 wt%, and the mass ratio of the inhibitor H2WO4 is 0.1 wt%; the heat preservation at 950 °C for 10 h in step S2 is reduced to heat preservation at 930 °C for 10 h. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0138] The following method is used to test the performance of the cathode materials obtained in the above Examples 1-12 and Comparative Examples 1-7.

[0139] 1. Aspect ratio test:

[0140] Sample preparation: Take the pulverized cathode material matrix, use a conductive adhesive with a length of 0.5 mm to pick up the material. After the material is picked up, use an ear bulb to blow the conductive adhesive surface with the material 10 times.

[0141] Electron microscopy imaging: Using a scanning electron microscope (SEM), the test was carried out under the electron beam conditions of 5 kV / 10 mA. Select the image with more particles and take 10 electron microscope photos at a magnification of 3K of the electron microscope.

[0142] Aspect ratio test: Use Nano Measure software to measure the length M of the longest straight edge of the single crystal particles in each electron microscope photo, and the length N of the straight edge perpendicular to the midpoint position of the longest straight edge. The average aspect ratio of each single crystal particle is M / N.

[0143] 2. Crack test:

[0144] Test for the proportion of the number of particles with cracks in the positive electrode material: First, take the powder positive electrode material. After mixing the powder positive electrode material, conductive carbon black, and binder PVDF (polyvinylidene fluoride) in a mass ratio of 80:10:10, add NMP (N-methylpyrrolidone) to make a uniform slurry and coat it on the aluminum foil, and dry it in an oven to make a pole piece. Take this pole piece as a sample. According to the standardized process of sample preparation for cross-section, use an electron transmission microscope to focus the ion beam into a small-sized ion beam to bombard the surface of the sample to achieve sample peeling. Conduct a scanning electron microscope (SEM) test on the peeled sample. Under a field of view of 3 k, count the fields of view in different regions, and count the proportion of the number of positive electrode material particles with cracks in the total number of positive electrode material particles counted. Characterize the amount of cracks by the high or low proportion. The test results of this application are the statistical results obtained by taking the cross-sections of about 300 positive electrode material particles randomly selected in the SEM image as samples. The SEM test requires random sampling of the positive electrode material and random selection of regions. The SEM images obtained by the test can represent the average level of the positive electrode material to be measured. Since in this test method, the pole piece is not rolled during the process of making the pole piece, therefore, the proportion of the number of cracks obtained by the test can be regarded as the proportion of the number of cracked particles in the positive electrode material in the powder state.

[0145] Test on the proportion of the number of cathode material particles with cracks in the cathode electrode sheet: First, take the powder cathode material. After mixing the powder material, the cathode material, conductive carbon black, and binder PVDF (polyvinylidene fluoride) in a mass ratio of 80:10:10, add NMP (N-methylpyrrolidone) to make a uniform slurry, coat it on the aluminum foil, dry it in an oven, and roll it under a pressure of 10 Mpa to make an electrode sheet. Take this electrode sheet as a sample. According to the standardized sample preparation process for the cross-section, use an electron transmission microscope to focus the ion beam into a small-sized ion beam to bombard the surface of the sample to achieve sample peeling. Perform scanning electron microscope (SEM) testing on the peeled sample. Under a field of view of 3 k, count the fields of view in different regions, count the number of cathode material particles with cracks accounting for the total number of cathode material particles counted, and characterize the amount of cracks by the level of the proportion. The test results of this application are the statistical results obtained by taking the cross-sections of about 300 cathode material particles randomly selected in the SEM image as samples. The SEM test requires random sampling of the cathode material, randomly selecting regions, and the obtained SEM images can represent the average level of the measured cathode material.

[0146] 3. XRD Test of Cathode Material:

[0147] X-ray diffraction phase analysis (XRD): Use a Rigaku X-ray diffractometer (XRD) in Japan to analyze the phase structure of the cathode material. The specific parameters are as follows: scanning range 10° - 80°, scanning speed 2° / min, step size 0.005°.

[0148] 4. Particle Size Test:

[0149] Use Nano Measurer software to test the average particle size of single-crystal materials. The specific method is as follows: Use a scanning electron microscope to perform morphological analysis on single-crystal materials, use Nano Measurer software to measure the particles in the SEM at a 3000-fold view, take the long side value as the particle size, count all the particles with clear morphology in the SEM, and take the average value.

[0150] 5. Compaction Density Test:

[0151] Use an American carver 4350 tester. Weigh 1 g of the sample, put it into the mold, press it with a pressure of 3 T for 30 s, and then test the height after pressing to calculate the compaction density.

[0152] 6. Specific Surface Area Test: Use the Micromeritics 3020 nitrogen adsorption method to test the specific surface area of the material.

[0153] Pretreatment of the finished product: Weigh the mass m1 of the empty sample tube; take 3 g of the sample, evacuate and degas it at 300 °C for 1 h, and after cooling, weigh the mass of the sample tube as m2; the sample mass m = m2 - m1.

[0154] Sample test: The sample tube is placed in liquid nitrogen, and the nitrogen adsorption volume V of the sample is measured at a series of relative pressures P / P0 to obtain the adsorption isotherm.

[0155] Fit the isothermal adsorption curve, calculate the monolayer saturation adsorption volume Vm according to the slope and intercept; then calculate the specific surface area according to Vm.

[0156] 7. Surface residual lithium test:

[0157] Use a potentiometric titrator to test the total amount of lithium compounds on the surface of the cathode material. Specifically, titrate with hydrochloric acid and then convert the amount of lithium compounds according to the amount of hydrochloric acid consumed.

[0158] 8. Battery fabrication and performance test

[0159] Use a coin-type half-cell to evaluate the electrochemical performance of the prepared cathode material. The specific method is as follows: Weigh the cathode material, SP, and polyvinylidene fluoride (PVDF) according to a mass ratio of 93:5:2, add NMP according to a solid content of 50%, and use a high-speed disperser to make a viscous slurry. Then, evenly coat it on the aluminum foil with a scraper, bake it dry in an 80°C oven, roll it, and cut it into a cathode electrode sheet with a diameter of 14 mm. Use a lithium sheet with a diameter of 16 mm as the anode electrode sheet, a Celgard polyethylene PP membrane as the separator, and a 1 mol / L LiPF6 carbonate (DEC / EC volume ratio 1:1) solution as the electrolyte, and assemble it in a glove box filled with argon.

[0160] Use a LAND battery test system to conduct capacity, initial efficiency, and rate performance tests at 25°C and 3.0V - 4.4V. The reference capacity is set to 200 mA / g, and 1C corresponds to a current density of 200 mA / g.

[0161] 9. Gas generation performance test:

[0162] Use a primitive soft-pack battery to evaluate the gas generation performance of the cathode material during storage. The specific method is as follows: Weigh the cathode material, PVDF, SP, and CNT according to a mass ratio of 97.0:1.0:1.5:0.5, add NMP according to a solid content of 50%, and use a high-speed dispersant to make a viscous slurry. Then, evenly coat it on the aluminum foil with a scraper, bake it dry in an 80°C oven, roll it, and cut it into a cathode electrode sheet with a size of 4 cm × 8 cm; Weigh the graphite anode, SP, CMC, and SBR according to a mass ratio of 96:1.0:1.2:1.8, disperse them into a slurry, coat them on the copper foil, and cut them into an anode electrode sheet with a size of 4 cm × 8 cm. Assemble the anode electrode sheet and the cathode electrode sheet into a primitive soft-pack battery, charge it to the target voltage value after formation, and then store it in a 60°C oven for 21 days. Use the water displacement method to test the volume difference before and after storage, which is the gas generation amount.

[0163] Among them, the corresponding test results of the cathode materials of Examples 1-12 and Comparative Examples 1-7 are shown in Table 1 and Table 2 below.

[0164] Table 1

[0165]

[0166] Table 2

[0167]

[0168] Figure 3 and Figure 4 are SEM photos of the cathode materials of Example 1 and Comparative Example 1 at a magnification of 3000 times, respectively. It can be seen from Figure 3 that the primary particles of the cathode material obtained in Example 1 are well-dispersed and uniform; it can be seen from Figure 4 that the primary particles of the cathode material obtained in Comparative Example 1 have obvious agglomeration, and there are some particles that have not fully grown. Figure 5 and Figure 6 are cross-sectional SEMs of the cathode materials of Example 1 and Comparative Example 1 at a magnification of 5000 times, respectively. It can be seen from Figure 5 that the cathode material obtained in Example 1 has fewer cracks; it can be seen from Figure 6 that the cathode material obtained in Comparative Example 1 has more cracks.

[0169] It can be seen from Figure 7 that the single-crystal cathode material obtained in Example 1 has significantly better cycling performance than Comparative Example 1. In addition, it can be seen from Table 1 that the cycling performance of the batteries prepared with the cathode materials of Examples 1-12 is significantly better than that of Comparative Examples 1-7. It can be seen from Table 1 that after the single-crystal cathode material obtained in Example 1 is made into a battery, the stored gas production is significantly lower than that of Comparative Examples 1-3.

[0170] In Examples 1, 11, and 12, the content of Ni in the precursor was adjusted. For the ternary cathode materials with different Ni contents, the preparation method of the embodiments of the present application can ensure that the aspect ratio of the single-crystal particles in the cathode material is in the range of 1 to 1.6 (specifically 1.3), the particles are relatively round, and the proportion of crack numbers is below 11%. The single-crystal cathode material that meets the above conditions has good cycling performance, and can effectively reduce the gas production of the battery and improve the storage performance after being made into a battery.

[0171] Compared with Example 1, Example 2 increased the addition amount of the flux, which can significantly reduce the crack proportion in the cathode material, reduce the crack proportion from 8.3% in Example 1 to 1.9%, improve the cycling performance of the battery, reduce the gas production, and the gas production can be reduced from 0.91 mL in Example 1 to 0.76 mL.

[0172] Compared with Example 1, Example 3 increases the amount of grinding material and the crushing air pressure in Step S3, which can effectively disperse the single crystal particles evenly, and prevent cracks from being generated due to excessive particle collisions. At the same time, the efficiency of air flow crushing can be ensured.

[0173] Compared with Example 1, Example 4 increases the addition amount of the inhibitor, which can reduce the aspect ratio of the particles of the positive electrode material, reducing the aspect ratio from 1.42 in Example 1 to 1.13. It can also reduce the crack proportion in the positive electrode material, reducing the crack proportion from 8.3% in Example 1 to 7.8%, improving the cycle performance of the battery, and reducing the gas generation amount at the same time.

[0174] Compared with Example 1, Example 5 reduces the addition amount of the inhibitor, weakening the inhibitory effect on the crystal growth of the positive electrode material. Therefore, the aspect ratio of the particles of the positive electrode material increases compared with Example 1, and the proportion of particles satisfying 1≤X / Y≤1.6 is less than 90%. The gas generation amount increases, and the cycle performance decreases.

[0175] Compared with Example 1, both Example 6 and Example 7 reduce the temperature of the first sintering in Step S1 and shorten the heat preservation time. The fusion of small particles and fine powder is not as sufficient as that in Example 1. Therefore, the number of cracks in the single crystal particles of Example 6 and Example 7 increases, and the gas generation amount also increases.

[0176] Compared with Example 1, Example 8 reduces the temperature of the second sintering in Step S2 and shortens the heat preservation time. The growth time of the single crystal particles decreases, the crystal particles are not as well repaired as those in Example 1, and the unit cells of the crystal grains are smaller. Therefore, the number of cracks in the positive electrode material of Example 8 increases, the full width at half maximum is larger, and the gas generation amount increases.

[0177] Compared with Example 1, Example 9 reduces the heat preservation temperature and time of the second sintering in Step S2. The internal stress in the lattice of the single crystal particles cannot be effectively eliminated, and the crystal particles are not as well repaired as those in Example 1. The number of cracks in the positive electrode material obtained in Example 9 increases, and the gas generation amount also increases.

[0178] Compared with Example 1, Example 10 increases the flux content, reduces the inhibitor content, and adjusts the sintering temperature and sintering time at the same time, which can reduce the number of cracks and the aspect ratio of the particles of the positive electrode material, and can obtain the optimal cycle performance and the lowest gas generation amount.

[0179] Compared with Example 1, Example 11 increases the nickel content in the precursor, which can improve the capacity of the positive electrode material on the premise of ensuring a low aspect ratio and a low crack proportion, enabling the positive electrode material to have good cycle performance, a high capacity, and a low gas generation amount.

[0180] Compared with Examples 1 and 11, Example 12 further increases the nickel content. By cooperating with corresponding fluxes and inhibitors, it can further improve the capacity of the cathode material while ensuring a low aspect ratio and a low crack proportion, enabling the cathode material to have good cycling performance, a high capacity, and a low gas generation amount.

[0181] In Comparative Example 1, no pole flux was added, and the proportion of the number of cracks was relatively high, reaching 12.6%, exceeding 11%. The cycling performance of the cathode material was significantly reduced, and the gas generation amount was significantly increased. Compared with Example 1, in Comparative Example 2, after secondary sintering, airflow pulverization was not carried out, and the proportion of the number of cracks in the cathode material was relatively high, reaching 13.5%, exceeding 11%. The cycling performance of the cathode material was significantly reduced, and the gas generation amount was significantly increased.

[0182] In Comparative Example 3, the pulverization air pressure in Step S3 was increased. The pulverization air pressure was too high, causing excessive collisions of crystal particles and generating too many cracks. The proportion of the number of cracks was 11.8%, exceeding 11%, resulting in more gas generation and poorer cycling performance of the prepared battery.

[0183] In Comparative Example 4, the grinding amount in Step S3 was increased. The grinding amount was too high, causing excessive collisions of crystal particles and generating too many cracks. The proportion of the number of cracks was 12.0%, exceeding 11%, resulting in more gas generation and poorer cycling performance of the prepared battery.

[0184] Therefore, compared with Examples 1-12, the proportion of the number of cracks in the cathode materials in Comparative Examples 1-7 is not within the range of 1.5% - 11%, and the aspect ratio is not within the range of 1 - 1.6. The cycling performance and gas generation performance of the cathode materials are both poor.

[0185] The above embodiments are only used to illustrate the technical solutions of the present application and are not restrictive. Although the present application has been described in detail with reference to the above preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present application can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present application.

Claims

1. A positive electrode material, characterized in that: The positive electrode material in powder form is made into a pole piece, and no rolling is performed during the production process of the pole piece. The pole piece is used as a sample, and the sample is peeled off using an electron transmission microscope, and the peeled sample is tested by a scanning electron microscope. Under a field of view of 3k, the ratio of the number of particles with cracks in the positive electrode material to the total number of particles is 1.5%~8.3%, and the average aspect ratio M / N of the particles of the positive electrode material satisfies: 1≤M / N≤1.54, wherein M is the length of the longest straight side of a single particle, and N is the length of the straight side perpendicular to the midpoint of the longest straight side. The number of particles in the positive electrode material that satisfy 1≤M / N≤1.54 accounts for no less than 88%, The general formula of the positive electrode material is as follows: Li a Ni x Co y N 1-x-y-b Y b O2, wherein 0.95≤a≤1.05, 0.5≤x<1, 0≤y≤0.30, 0.001≤b≤0.01, the N element includes at least one of Mn and Al, the Y element includes at least a Y1 element and a Y2 element, the Y1 element includes at least one of Zr, Sr, Ba and Ca, and the Y2 element includes at least one of Al, Ti, Mg, Y, Cu, W, Nb, La, Ce, Mo, Sn and Ta.

2. The positive electrode material according to claim 1, characterized in that In the XRD spectrum of the positive electrode material, the positive electrode material has a diffraction peak within 44°~45°, and the half-peak width of the diffraction peak is 0.150°~0.200°.

3. The positive electrode material according to claim 1, characterized in that The mass proportion of the Y element in the positive electrode material is 0.01wt%~0.5wt%.

4. The positive electrode material according to claim 1, characterized in that The positive electrode material satisfies at least one of the following characteristics: (1) The positive electrode material comprises a plurality of primary particles, and the average particle size of the primary particles is 1 μm to 6 μm; (2) The positive electrode material is a single crystal material, and the positive electrode material contains single particles with the same orientation; (3) The particle size of the positive electrode material satisfies: D min <1μm,2.0μm<D V,50 <4.5μm, D max <15μm,0.8<(D V,90 -D V,10 ) / D V,50 <1.6; (4) The specific surface area of ​​the positive electrode material is 0.1 m 2 / g ~1.6 m 2 / g; (5) The oil absorption value of the positive electrode material is 10 mL to 40 mL per 100 g; (6) The surface residual alkali of the positive electrode material satisfies: LiOH≤0.5wt%, Li2CO3≤0.5wt%; (7) The compaction density of the positive electrode material is 2.5 g / cm 3 ~3.6g / cm 3 .

5. A method for preparing a positive electrode material, characterized in that: The following steps are involved: Mixing a precursor, a lithium salt and an additive to form a mixture, and sintering the mixture once to obtain a primary sintered product; The primary sintered product is crushed, and the crushed primary sintered product is subjected to secondary sintering, wherein the secondary sintering is a multi-stage gradient temperature sintering process to obtain a secondary sintered product; crushing the secondary sintered product to obtain a crushed secondary sintered product; as well as The crushed secondary sintered product is mixed with a coating agent and sintered to obtain the positive electrode material, and the powdered positive electrode material is made into a pole piece. No rolling is performed during the production of the pole piece. The pole piece is used as a sample, and the sample is peeled off using an electron transmission microscope, and the peeled sample is tested by a scanning electron microscope. Under a field of view of 3k, the ratio of the number of particles with cracks in the positive electrode material to the total number of particles is 1.5%~8.3%, and the average aspect ratio M / N of the particles of the positive electrode material satisfies: 1≤M / N≤1.54, wherein M is the length of the longest straight side of a single particle, and N is the length of the straight side perpendicular to the midpoint of the longest straight side. The number of particles in the positive electrode material that satisfy 1≤M / N≤1.54 accounts for no less than 88%, The general formula of the positive electrode material is as follows: Li a Ni x Co y N 1-x-y-b Y b O2, wherein 0.95≤a≤1.05, 0.5≤x<1, 0≤y≤0.30, 0.001≤b≤0.01, the N element includes at least one of Mn and Al, the Y element includes at least a Y1 element and a Y2 element, the Y1 element includes at least one of Zr, Sr, Ba and Ca, and the Y2 element includes at least one of Al, Ti, Mg, Y, Cu, W, Nb, La, Ce, Mo, Sn and Ta.

6. A positive electrode sheet, characterized in that: It comprises a positive electrode current collector and a positive electrode active material layer arranged 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 in any one of claims 1 to 4 or the positive electrode material prepared by the method for preparing the positive electrode material as described in claim 5.

7. A secondary battery, characterized in that: It comprises a positive electrode plate, wherein the positive electrode plate 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.

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