A ternary polycrystalline positive electrode material and a preparation method thereof

Abstract

This invention discloses a ternary polycrystalline cathode material and its preparation method. The cathode material is a secondary particle formed by the agglomeration of primary particles. The primary particles grow and are arranged radially along the secondary particles, and the primary particles on the cross-section of the secondary particles are strip-shaped with a long axis of 220nm < L < 350nm and a short axis of 100nm < N < 200nm, and an aspect ratio of 1.55 < K < 1.90. The method includes: S1, mixing and sintering a cathode material precursor, a lithium source, and a dopant to obtain a sintered matrix; S2, washing and drying the sintered matrix and mixing it uniformly with a coating agent, and then performing a secondary sintering. The special structure of the ternary polycrystalline cathode material of this invention can alleviate the internal stress of the cathode material, improve the cycle stability, cell safety, and discharge capacity of the cathode material. This invention promotes stable and uniform directional growth of the cathode material through process control and controls the size of the primary particles to obtain the desired structural morphology of the cathode material; moreover, the preparation process is simple and suitable for large-scale industrial production.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a cathode material and its preparation method. Background Technology

[0002] Currently, lithium-ion batteries have become the primary power source for electric vehicles due to their high energy density and long lifespan. However, electric vehicle technology faces significant performance and economic challenges, such as limited driving range and battery durability, long charging times, and high battery costs. These issues are directly related to the limitations of lithium-ion batteries. Therefore, improving the energy density and cycle stability of lithium-ion batteries, increasing fast charging capabilities, and reducing costs are prerequisites for the widespread adoption of electric vehicles. The overall performance of current lithium-ion batteries primarily depends on their cathode materials. Among them, high-nickel ternary polycrystalline cathode materials are considered one of the most promising high-energy lithium-ion battery cathode materials due to their high specific capacity and relatively low cost, and can meet the needs of rapidly upgrading applications.

[0003] However, the higher the nickel content in ternary polycrystalline cathode materials, the less stable their structure becomes during cycling, making them prone to bulk structure damage and intergranular microcracks. This hinders electron / ion transport, reduces rate capability and cycle stability, and causes severe side reactions due to electrolyte erosion of the cathode material's fresh surface, posing significant safety hazards. Therefore, controlling the crystal structure of primary particles and the construction of secondary particles is crucial for improving energy density and cycle stability. Existing methods for improving cycle stability generally involve doping and coating, but this often results in capacity loss. Capacity enhancement for cathode materials mainly focuses on precursor design, such as increasing the specific surface area of ​​the precursor or constructing hollow structures within the precursor to increase the contact area between the electrolyte and the cathode material, thus opening up lithium-ion transport pathways. However, cathode materials obtained by sintering high-specific-surface-area precursors may have a high risk of gas generation, and porous structures have lower pressure resistance, making them prone to structural collapse, side reactions with the electrolyte, and heat release, leading to cell safety issues. It is evident that existing cathode material preparation methods struggle to simultaneously achieve both capacity and cycle stability. Summary of the Invention

[0004] This invention provides a ternary polycrystalline cathode material and its preparation method, which solves the technical problem mentioned in the background art that the preparation methods of existing cathode materials, especially high-nickel cathode materials, are difficult to simultaneously take into account the cycle stability and capacity of the material.

[0005] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows: A ternary polycrystalline cathode material, wherein the ternary polycrystalline cathode material is a secondary particle formed by the agglomeration of primary particles; the primary particles grow and are arranged in a radial pattern along the secondary particles, and the primary particles on the cross-section of the secondary particles are strip-shaped, having a major axis L and a minor axis N, wherein 220nm < L < 350nm, 100nm < N < 200nm, and the aspect ratio of the primary particles is K, where 1.55 < K < 1.90.

[0006] In the cathode material of this invention, the primary particles grow and are arranged radially along the secondary particles in a divergent pattern, which can alleviate the microcrack problem caused by strain accumulation in the cathode material. Furthermore, the specific design of the long axis, short axis, and aspect ratio of the primary particles improves the crystallinity of the grains, while limiting the increase of grain boundaries, avoiding the increase of crystal surface defect density, reducing the risk of thermal runaway in the cell, and effectively improving the cycle stability and cell safety of the cathode material. At the same time, the orderly radial arrangement of the primary particles on the cross-section of the secondary particles is also conducive to the full wetting of the cathode material by the electrolyte. The increased contact area between the electrolyte and the cathode material is beneficial to lithium-ion transport during the charging and discharging process, thereby improving the discharge capacity of the cathode material.

[0007] As a further preferred embodiment of the above technical solution, the area Q of the primary particle on the cross-section of the secondary particle is 0.030–0.120 μm. 2 When the area of ​​the primary particles on the cross-section of the secondary particles meets the above conditions, the arrangement of primary particles inside the ternary polycrystalline cathode material better inherits the precursor structure, the size of the primary particles in the cross-section tends to be uniform, which is conducive to the formation of a stable structure, relieving internal stress during charging and discharging, and slowing down the formation of microcracks.

[0008] As a further preferred embodiment of the above technical solution, the ratio of the pore area in the outer ring cross-section of the secondary particles to the pore area in the cross-section of the secondary particles is R, where 0.30 < R < 0.60; the outer ring cross-section of the secondary particles is an annular cross-section with a radius of 1 / 4 from the surface to the center of the secondary particles. The smaller the ratio of the total pore area of ​​the outer ring cross-section to the total pore area of ​​the secondary particle cross-section, the denser the primary particles are, resulting in a stable structure of the cathode material under external pressure, but this is not conducive to electrolyte wetting and lithium-ion transport. Conversely, the larger the proportion of the total pore area on the outer ring cross-section, the larger the contact area between the electrolyte and the cathode material, which is beneficial to lithium-ion transport. However, an excessively high proportion of pore area can lead to the cathode material being easily broken. Therefore, an appropriate proportion of the total pore area on the outer ring cross-section is beneficial to reducing the difficulty of lithium-ion transport while improving the capacity and stability of the cathode material.

[0009] As a further preferred embodiment of the above technical solution, the ternary polycrystalline cathode material is doped with elements X and Y, wherein element X is a metallic element with a valence state greater than or equal to positive tetravalent, and element Y is S.

[0010] As a further preferred embodiment of the above technical solution, the X element includes at least one of W, Nb, Ta, Mo, Sn, Zr and Ti.

[0011] As a further preferred embodiment of the above technical solution, the ternary polycrystalline cathode material includes a matrix and a coating layer, wherein the mass of the coating element in the coating layer accounts for 0.02-0.30% of the mass of the ternary polycrystalline cathode material; the chemical formula of the matrix is ​​Li. a Ni b Co c M d X e S f O2, wherein the element M includes at least one of Mn and Al, 1.00≤a≤1.10, 0.84≤b≤0.98, 0<c≤0.11, 0<d≤0.05, 0.15wt%≤e≤0.35wt%, 0.10wt%≤f≤0.30wt%; the elements in the coating layer include at least one of Al, Ce, Ga, B, Sr, W, Ti, Co, Mn and Mg.

[0012] Based on the same technical concept, the present invention also provides a method for preparing the ternary polycrystalline cathode material described above, comprising the following steps: S1. The cathode material precursor, lithium source, dopant containing element X and dopant containing element Y are uniformly mixed and then sintered once to obtain a sintered matrix; the element X is a metal element with a valence state greater than or equal to positive tetravalent, and the element Y is S; the melting temperature of the dopant containing element X is higher than the melting temperature of the dopant containing element Y. S2. After washing and drying the substrate, mix it evenly with the coating agent and perform secondary sintering to obtain the ternary polycrystalline cathode material.

[0013] As a further preferred embodiment of the above technical solution, the dopant containing element X is an oxide of element X, and the dopant containing element Y is a sulfate.

[0014] This invention employs co-doping with high-valence metal oxides and sulfates, utilizing the time difference in their melting processes to allow for staggered reactions. The metal oxides, due to their strong metal-oxygen bonds, have high melting points, while the sulfates, with their lower melting points, preferentially melt and occupy sites. This mitigates the impact of simultaneous doping of high-valence metal cations and anions on the charge distribution in the crystal lattice, enabling stable radial growth of primary cathode particles. Conversely, when using high-valence metal sulfates for doping, the metal ions and anions in the molten state interact simultaneously, exacerbating lattice distortion and hindering the orderly arrangement of primary particles.

[0015] As a further preferred embodiment of the above technical solution, the single sintering is a multi-stage sintering process. First, the temperature is raised to 450-550℃ at a rate of 5-10℃ / min and held for 8-12 hours. Then, the temperature is raised to 750-800℃ at a rate of 1-3℃ / min and held for 1-5 hours. Traditional staged sintering processes have limited low-temperature sintering time and a relatively low heating rate, resulting in insufficient reaction between the precursor and lithium source. After high-temperature sintering, defects such as dislocations and vacancies are easily formed in the cathode material lattice, which is detrimental to its structural stability. In the preparation process of this invention, the temperature is first rapidly increased to a low-temperature platform and sintered for a long time. The thermal hysteresis effect of the temperature rise promotes the full reaction between the lithium source and the precursor. At the same time, it causes some sulfate to melt and penetrate along the surface of the precursor, occupying part of the pores, which can alleviate the accumulation of lattice stress caused by rapid temperature rise. Then, the temperature is slowly increased to a high-temperature platform and sintered for a short time. This promotes the full diffusion of lithium ions and metal ions in high-valence metal oxides in the precursor. The high-valence metal ions dope into the lattice, promoting the directional growth of primary particles. The sulfate uniformly distributed at the grain boundaries uses its occupancy to help regulate the size of primary particles, thereby controlling the aspect ratio. Moreover, the high-temperature sintering time is short, and the growth of primary particles inside the cathode material is less affected than that of primary particles on the surface. The cathode material can maintain structural stability, which is beneficial to the cycle stability and capacity improvement of the cathode material.

[0016] As a further preferred embodiment of the above technical solution, in the washing and drying process of S2, the washing solid-liquid ratio is (1-3):1, the washing temperature is 4-15℃, the washing time is 3-10 min, the drying temperature is 140-170℃, and the drying time is 7-13 h.

[0017] As a further preferred embodiment of the above technical solution, the temperature of the secondary sintering is 250~550℃, the holding time is 2~12h, and the heating rate is 1℃ / min.

[0018] As a further preferred embodiment of the above technical solution, the dopant containing element X includes at least one of SnO2, WO3, Nb2O5, Ta2O5, MoO3, ZrO2, and TiO2; the dopant Y includes at least one of Li2SO4, K2SO4, Al2(SO4)3, Na2SO4, and MgSO4; and the coating agent includes at least one of aluminum oxide, cerium oxide, gallium oxide, tungsten oxide, boric acid, strontium carbonate, titanium oxide, cobalt tetroxide, manganese oxide, and magnesium oxide.

[0019] The present invention has the following beneficial effects: (1) In the ternary polycrystalline cathode material of the present invention, the growth arrangement of primary particles in secondary particles has a special form, which can relieve the internal stress of cathode material, improve the cycle stability and cell safety of cathode material, and at the same time facilitate the contact and wetting of electrolyte and cathode material, thereby improving the discharge capacity of cathode material.

[0020] (2) The preparation method of the ternary polycrystalline cathode material of the present invention promotes the stable and uniform directional growth of the cathode material in the radial direction by controlling the dopant and the temperature, and controls the primary particle size so that the cathode material obtains the expected structural morphology; and the preparation process is simple and suitable for large-scale industrial production. Attached Figure Description

[0021] Figure 1 The images show the cross-sectional imaging (left) and primary particle identification image (right) of the ternary polycrystalline cathode material obtained in Example 1 of this invention. Figure 2 The images show the cross-sectional imaging (left) and primary particle identification image (right) of the ternary polycrystalline cathode material obtained in Comparative Example 1 of this invention. Figure 3 The images show the cross-sectional imaging (left) and primary particle identification image (right) of the ternary polycrystalline cathode material obtained in Comparative Example 2 of this invention. Figure 4 The images show the cross-sectional imaging (left) and primary particle identification image (right) of the ternary polycrystalline cathode material obtained in Comparative Example 3 of this invention. Figure 5 The images show the cross-sectional imaging (left) and primary particle identification image (right) of the ternary polycrystalline cathode material obtained in Comparative Example 4 of this invention. Detailed Implementation

[0022] The following detailed description is based on embodiments of the present invention, but the present invention can be implemented in many different ways as defined and covered by the claims.

[0023] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0024] In the following embodiments and comparative examples, the major axis, minor axis, area, aspect ratio of the primary particles in the cathode material cross-section, and the pore area of ​​the cathode material cross-section were obtained by the following methods: The testing method for the major axis, minor axis, area, and aspect ratio of primary particles in the cross-section of ternary polycrystalline cathode materials is as follows: Cathode material particles are cut using a focused ion beam (FIB) system to obtain samples with observable particle cross-sections. Images of the cross-sections are then captured under the following conditions: ion beam acceleration voltage of 5.00 kV, magnification of 20000x, and shooting modes of secondary electrons and secondary electron combinations. Image analysis software is then used to identify and statistically analyze the major axis, minor axis, and area of ​​all primary particles within the selected area for 10 cathode material particles from the captured images. The aspect ratio of the primary particles in the cross-section is obtained by calculating the ratio of the major axis to the minor axis. The average values ​​of the major axis, minor axis, area, and aspect ratio of each primary particle are then statistically calculated. Finally, the median values ​​of the major axis, minor axis, area, and aspect ratio of the 10 samples are calculated to obtain the major axis L, minor axis N, area Q, and aspect ratio K of the primary particles in the cross-section of the ternary polycrystalline cathode material.

[0025] The test method for the cross-sectional porosity of ternary polycrystalline cathode materials is as follows: Cathode material particles are cut using a focused ion beam (FIB) system to obtain samples with observable particle cross-sections, and images of the cross-sections are captured. Specific capturing conditions are: ion beam acceleration voltage of 5.00 kV, magnification of 20000x, and capturing modes of secondary electrons and secondary electron combinations. Image analysis software is then used to extract the particle porosity of 10 cathode material particles within the selected regions from the captured images. A Python script is used to calculate the pore area of ​​the outer 1 / 4 cross-section from the particle surface to the center of the sphere and the pore area of ​​the secondary particle cross-section, where R = pore area of ​​the outer 1 / 4 cross-section from the particle surface to the center of the sphere / pore area of ​​the secondary particle cross-section. The median R of the 10 cathode material samples is then calculated to obtain the ratio of the pore area of ​​the outer 1 / 4 cross-section from the particle surface to the center of the sphere to the pore area of ​​the secondary particle cross-section.

[0026] Example 1: like Figure 1 As shown, the ternary polycrystalline cathode material of this embodiment consists of secondary particles formed by the agglomeration of primary particles. The primary particles grow and are arranged in a radial pattern along the secondary particles, and the primary particles on the cross-section of the secondary particles are strip-shaped, with a major axis L and a minor axis N, where L = 240 nm and N = 136 nm. The aspect ratio of the primary particles is K, where K = 1.75. The area Q of the primary particles on the cross-section of the secondary particles is 0.042 μm. 2 The ratio of the pore area in the outer layer cross-section of the secondary particle to the pore area in the cross-section of the secondary particle is R, where R = 0.43. The outer layer cross-section of the secondary particle is an annular cross-section extending from the surface of the secondary particle to a radius of 1 / 4 of the circle's radius. The ternary polycrystalline cathode material includes a substrate and a coating layer; the substrate is Li.1.05 Ni 0.94 Co 0.04 Mn 0.02 W 0.25wt% S 0.20wt% O 2.00 The coating layer consists of cerium oxide and boric acid. The mass of cerium in the coating layer accounts for 0.05% of the mass of the ternary polycrystalline cathode material, and the mass of boron accounts for 0.12% of the mass of the ternary polycrystalline cathode material.

[0027] The preparation method of the ternary polycrystalline cathode material in this embodiment includes the following steps: S1, Ni 0.94 Co 0.04 Mn 0.02 (OH)2, lithium hydroxide, tungsten oxide, and lithium sulfate were added to a high-speed mixer in a certain proportion and mixed evenly to obtain a primary mixture. The total molar ratio of Ni, Mn, and Co in the cathode precursor to lithium hydroxide was 1:1.05. The W and S elements in tungsten oxide and lithium sulfate accounted for 0.25 wt% and 0.20 wt% of the cathode material matrix, respectively. The high-speed mixer operated at 1000 rpm for 35 min. The mixture was placed in an oxygen atmosphere furnace and heated to 500℃ at a heating rate of 8℃ / min, held at that temperature for 10 h, and then heated to 780℃ at a heating rate of 2℃ / min, held at that temperature for 3 h, and then allowed to cool naturally to room temperature. The mixture was then passed through a 300-mesh sieve to obtain a primary matrix. S2. The sintered matrix was dispersed in deionized water and washed for 5 minutes. The solid-liquid ratio was 1:1, and the temperature of the deionized water was controlled at 5°C. After washing, the sample was placed in a vacuum oven and vacuum dried at 150°C for 8 hours. Then, it was naturally cooled to room temperature and sieved through a 300-mesh sieve to obtain the washed material. The washed material was then mixed with cerium oxide and boric acid in a high-speed mixer in a certain proportion to obtain a secondary mixture. The high-speed mixer was run at 1000 rpm for 35 minutes. The mixture was placed in an oxygen atmosphere furnace and heated to 330°C at a heating rate of 1°C / min and held at that temperature for 4 hours. Then, it was naturally cooled to room temperature and sieved through a 300-mesh sieve to obtain the ternary polycrystalline cathode material of this embodiment.

[0028] Example 2: The ternary polycrystalline cathode material of this embodiment consists of secondary particles formed by the agglomeration of primary particles. The primary particles grow and are arranged radially along the secondary particles, and the primary particles on the cross-section of the secondary particles are strip-shaped, with a major axis L and a minor axis N, where L = 263 nm and N = 137 nm. The aspect ratio of the primary particles is K, where K = 1.66. The area Q of the primary particles on the cross-section of the secondary particles is 0.083 μm. 2The ratio of the pore area in the outer layer cross-section of the secondary particle to the pore area in the cross-section of the secondary particle is R, where R = 0.43. The outer layer cross-section of the secondary particle is an annular cross-section extending from the surface of the secondary particle to a radius of 1 / 4 of the circle's radius. The ternary polycrystalline cathode material includes a substrate and a coating layer; the substrate is Li. 1.05 Ni 0.94 Co 0.04 Mn 0.02 W 0.35wt% S 0.15wt% O 2.00 The coating layer consists of cerium oxide, aluminum oxide, and boric acid. The mass of cerium in the coating layer accounts for 0.05% of the mass of the ternary polycrystalline cathode material, the mass of aluminum accounts for 0.05% of the mass of the ternary polycrystalline cathode material, and the mass of boron accounts for 0.10% of the mass of the ternary polycrystalline cathode material.

[0029] The preparation method of the ternary polycrystalline cathode material in this embodiment includes the following steps: S1, Ni 0.94 Co 0.04 Mn 0.02 (OH)2, lithium hydroxide, tungsten oxide, and lithium sulfate were added to a high-speed mixer in a certain proportion and mixed evenly to obtain a primary mixture. The total molar ratio of Ni, Mn, and Co in the cathode precursor to lithium hydroxide was 1:1.05. The W and S elements in tungsten oxide and lithium sulfate accounted for 0.35 wt% and 0.15 wt% of the cathode material matrix, respectively. The high-speed mixer operated at 1000 rpm for 35 min. The mixture was placed in an oxygen atmosphere furnace and heated to 550℃ at a heating rate of 5℃ / min, held at that temperature for 10 h, and then heated to 790℃ at a heating rate of 3℃ / min, held at that temperature for 5 h, and then allowed to cool naturally to room temperature. The mixture was then passed through a 300-mesh sieve to obtain a primary matrix. S2. The sintered matrix was dispersed in deionized water and washed for 5 minutes. The solid-liquid ratio was 1:1, and the temperature of the deionized water was controlled at 5°C. After washing, the sample was placed in a vacuum oven and vacuum dried at 150°C for 8 hours. Then, it was naturally cooled to room temperature and sieved through a 300-mesh sieve to obtain the washed material. The washed material was then mixed with cerium oxide, alumina, and boric acid in a high-speed mixer in a certain proportion to obtain a secondary mixture. The high-speed mixer was run at 1000 rpm for 35 minutes. The mixture was placed in an oxygen atmosphere furnace and heated to 300°C at a heating rate of 1°C / min and held at that temperature for 8 hours. Then, it was naturally cooled to room temperature and sieved through a 300-mesh sieve to obtain the ternary polycrystalline cathode material of this embodiment.

[0030] Example 3: The ternary polycrystalline cathode material of this embodiment consists of secondary particles formed by the agglomeration of primary particles. The primary particles grow and are arranged radially along the secondary particles, and the primary particles on the cross-section of the secondary particles are strip-shaped, with a major axis L and a minor axis N, where L = 286 nm and N = 147 nm. The aspect ratio of the primary particles is K, where K = 1.68. The area Q of the primary particles on the cross-section of the secondary particles is 0.042 μm. 2 The ratio of the pore area in the outer layer cross-section of the secondary particle to the pore area in the cross-section of the secondary particle is R, where R = 0.31; the outer layer cross-section of the secondary particle is an annular cross-section extending from the surface of the secondary particle to a radius of 1 / 4 of the circle's radius. The ternary polycrystalline cathode material includes a substrate and a coating layer; the substrate is Li. 1.05 Ni 0.94 Co 0.04 Mn 0.02 W 0.20wt% S 0.30wt% O 2.00 The coating layer consists of aluminum oxide and boric acid. In the coating layer, the mass of aluminum accounts for 0.10% of the mass of the ternary polycrystalline cathode material, and the mass of boron accounts for 0.15% of the mass of the ternary polycrystalline cathode material.

[0031] The preparation method of the ternary polycrystalline cathode material in this embodiment includes the following steps: S1, Ni 0.94 Co 0.04 Mn 0.02 (OH)2, lithium hydroxide, tungsten oxide, and lithium sulfate were added to a high-speed mixer in a certain proportion and mixed evenly to obtain a primary mixture. The total molar ratio of Ni, Mn, and Co in the positive electrode precursor to lithium hydroxide was 1:1.05. The W and S elements in tungsten oxide and lithium sulfate accounted for 0.20 wt% and 0.30 wt% of the positive electrode matrix, respectively. The high-speed mixer operated at 1000 rpm for 35 min. The mixture was placed in an oxygen atmosphere furnace and heated to 550℃ at a heating rate of 10℃ / min, held at that temperature for 8 h, and then heated to 800℃ at a heating rate of 3℃ / min, held at that temperature for 3 h. Afterward, it was naturally cooled to room temperature and passed through a 300-mesh sieve to obtain a primary matrix. S2. Disperse the sintered matrix in deionized water and wash for 5 minutes. The solid-liquid ratio during washing is 1:1. The temperature of the deionized water is controlled at 5°C. After washing, place the sample in a vacuum oven and vacuum dry at 150°C for 8 hours. Then, allow it to cool naturally to room temperature and sieve through a 300-mesh sieve to obtain the washed material. Add the washed material, alumina, and boric acid to a high-speed mixer in a certain proportion and mix to obtain a secondary mixture. The high-speed mixer operates at 1000 rpm for 35 minutes. Place the mixture in an oxygen atmosphere furnace and heat it to 350°C at a heating rate of 1°C / min. Hold the temperature for 4 hours and sinter. Then, allow it to cool naturally to room temperature and sieve through a 300-mesh sieve to obtain the ternary polycrystalline cathode material of this embodiment.

[0032] Example 4: The ternary polycrystalline cathode material of this embodiment consists of secondary particles formed by the agglomeration of primary particles. The primary particles grow and are arranged radially along the secondary particles, and the primary particles on the cross-section of the secondary particles are strip-shaped, with a major axis L and a minor axis N, where L = 247 nm and N = 104 nm. The aspect ratio of the primary particles is K, where K = 1.73. The area Q of the primary particles on the cross-section of the secondary particles is 0.049 μm. 2 The ratio of the pore area in the outer layer cross-section of the secondary particle to the pore area in the cross-section of the secondary particle is R, where R = 0.40; the outer layer cross-section of the secondary particle is an annular cross-section extending from the surface of the secondary particle to the center at a distance of 1 / 4 of the radius. The ternary polycrystalline cathode material includes a substrate and a coating layer; the substrate is Li. 1.05 Ni 0.94 Co 0.04 Mn 0.02 Zr 0.30wt% S 0.25wt% O 2.00 The coating layer consists of aluminum oxide, titanium oxide, and boric acid. In the coating layer, the mass of aluminum accounts for 0.05% of the mass of the ternary polycrystalline cathode material, the mass of titanium accounts for 0.10% of the mass of the ternary polycrystalline cathode material, and the mass of boron accounts for 0.20% of the mass of the ternary polycrystalline cathode material.

[0033] The preparation method of the ternary polycrystalline cathode material in this embodiment includes the following steps: S1, Ni 0.94 Co 0.04 Mn 0.02(OH)2, lithium hydroxide, zirconium oxide, and lithium sulfate were added to a high-speed mixer in a certain proportion and mixed evenly to obtain a primary mixture. The total molar ratio of Ni, Mn, and Co in the cathode precursor to lithium hydroxide was 1:1.05. The Zr and S elements in zirconium oxide and lithium sulfate accounted for 0.30 wt% and 0.25 wt% of the cathode material matrix, respectively. The high-speed mixer operated at 1000 rpm for 35 min. The mixture was placed in an oxygen atmosphere furnace and heated to 550℃ at a heating rate of 5℃ / min, held at that temperature for 10 h, and then heated to 780℃ at a heating rate of 3℃ / min, held at that temperature for 5 h, and then allowed to cool naturally to room temperature. The mixture was then passed through a 300-mesh sieve to obtain a primary matrix. S2. The sintered matrix was dispersed in deionized water and washed for 5 minutes. The solid-liquid ratio was 1:1, and the temperature of the deionized water was controlled at 5°C. After washing, the sample was placed in a vacuum oven and vacuum dried at 150°C for 8 hours. Then, it was naturally cooled to room temperature and sieved through a 300-mesh sieve to obtain the washed material. The washed material was then mixed with alumina, titanium dioxide, and boric acid in a high-speed mixer in a certain proportion to obtain a secondary mixture. The high-speed mixer was run at 1000 rpm for 35 minutes. The mixture was placed in an oxygen atmosphere furnace and heated to 300°C at a heating rate of 1°C / min and held at that temperature for 8 hours. Then, it was naturally cooled to room temperature and sieved through a 300-mesh sieve to obtain the ternary polycrystalline cathode material of this embodiment.

[0034] Comparative Example 1: like Figure 2 As shown, the ternary polycrystalline cathode material in this comparative example consists of secondary particles formed by the agglomeration of primary particles. The primary particles on the cross-section of the secondary particles are strip-shaped, with a major axis L and a minor axis N, where L = 326 nm and N = 169 nm. The aspect ratio of the primary particles is K, where K = 1.53. The area Q of the primary particles on the cross-section of the secondary particles is 0.056 μm. 2 The ratio of the pore area in the outer layer cross-section of the secondary particle to the pore area in the cross-section of the secondary particle is R, where R = 0.41; the outer layer cross-section of the secondary particle is an annular cross-section extending from the surface of the secondary particle to a radius of 1 / 4 of the circle's radius. The ternary polycrystalline cathode material includes a substrate and a coating layer; the substrate is Li. 1.05 Ni 0.94 Co 0.04 Mn 0.02 Al 0.25wt% S 0.20 wt% O 2.00 The coating layer is the same as in Example 1.

[0035] The preparation method of the ternary polycrystalline cathode material in this comparative example differs from that in Example 1 only in that the dopant tungsten oxide in S1 is replaced with aluminum oxide, while all other conditions remain the same as in Example 1.

[0036] Comparative Example 2: like Figure 3 As shown, the ternary polycrystalline cathode material in this comparative example consists of secondary particles formed by the agglomeration of primary particles. The primary particles on the cross-section of the secondary particles are strip-shaped, with a major axis L and a minor axis N, where L = 178 nm and N = 101 nm. The aspect ratio of the primary particles is K, where K = 1.64. The area Q of the primary particles on the cross-section of the secondary particles is 0.020 μm. 2 The ratio of the pore area in the outer layer cross-section of the secondary particle to the pore area in the cross-section of the secondary particle is R, where R = 0.43. The outer layer cross-section of the secondary particle is an annular cross-section extending from the surface of the secondary particle to a radius of 1 / 4 of the circle's radius. The ternary polycrystalline cathode material includes a substrate and a coating layer; the substrate is Li. 1.05 Ni 0.94 Co 0.04 Mn 0.02 W 0.25wt% S 0.20wt% O 2.00 The coating layer is the same as in Example 1.

[0037] The preparation method of the ternary polycrystalline cathode material in this comparative example differs from that in Example 1 in that the primary mixture obtained in S1 is sintered using a traditional one-stage sintering process. Specifically, the primary mixture is placed in an oxygen atmosphere furnace and heated to 500°C at a heating rate of 2°C / min, held at that temperature for 2 hours, then heated to 760°C at a heating rate of 3°C / min, held at that temperature for 11 hours, and then naturally cooled to room temperature.

[0038] Comparative Example 3: like Figure 4 As shown, the ternary polycrystalline cathode material in this comparative example consists of secondary particles formed by the agglomeration of primary particles. The primary particles on the cross-section of the secondary particles are strip-shaped, with a major axis L and a minor axis N, where L = 406 nm and N = 163 nm. The aspect ratio of the primary particles is K, where K = 1.70. The area Q of the primary particles on the cross-section of the secondary particles is 0.055 μm. 2 The ratio of the pore area in the outer layer cross-section of the secondary particle to the pore area in the cross-section of the secondary particle is R, where R = 0.22. The outer layer cross-section of the secondary particle is an annular cross-section extending from the surface of the secondary particle to a radius of 1 / 4 of the circle's radius. The ternary polycrystalline cathode material includes a substrate and a coating layer; the substrate is Li. 1.05 Ni 0.94 Co 0.04 Mn 0.02 W 0.25wt% O 2.00 The coating layer is the same as in Example 1.

[0039] The preparation method of the ternary polycrystalline cathode material in this comparative example differs from that in Example 1 only in that tungsten oxide is added as a dopant in S1, while all other conditions remain the same as in Example 1.

[0040] Comparative Example 4: like Figure 5 As shown, the ternary polycrystalline cathode material in this comparative example consists of secondary particles formed by the agglomeration of primary particles. The primary particles on the cross-section of the secondary particles are strip-shaped, with a major axis L and a minor axis N, where L = 257 nm and N = 186 nm. The aspect ratio of the primary particles is K, where K = 1.38. The area Q of the primary particles on the cross-section of the secondary particles is 0.049 μm. 2 The ratio of the pore area in the outer layer cross-section of the secondary particle to the pore area in the cross-section of the secondary particle is R, where R = 0.37. The outer layer cross-section of the secondary particle is an annular cross-section extending from the surface of the secondary particle to a radius of 1 / 4 of the circle's radius. The ternary polycrystalline cathode material includes a substrate and a coating layer; the substrate is Li. 1.05 Ni 0.92 Co 0.04 Mn 0.02 Zr 0.30wt% O 2.00 The coating layer is the same as in Example 4.

[0041] The preparation method of the ternary polycrystalline cathode material in this comparative example differs from that in Example 4 only in that zirconium sulfate is added as a dopant in S1, wherein the Zr element accounts for 0.30 wt% of the cathode material matrix. All other conditions are consistent with those in Example 1.

[0042] The ternary polycrystalline positive electrode materials of Examples 1-4 and Comparative Examples 1-4 were respectively mixed with conductive carbon black (SP) and polyvinylidene fluoride (PVDF) in a mass ratio of 92.5:5:2.5 with solvent NMP to prepare slurries, which were then coated onto an aluminum foil substrate. The coated positive electrode sheets were dried in a forced-air drying oven, cut, and rolled to obtain the positive electrode sheets. The negative electrode of the battery is a lithium metal sheet, and the electrolyte consists of 1 mol / L LiPF6 solute and EC / DEC mixed solvent in a volume ratio of 1:2. The positive electrode sheet, negative electrode sheet, electrolyte, and separator were assembled into a CR2032 coin cell in a glove box for electrical performance testing. The test voltage was 3.0-4.3V. The initial discharge capacity was measured at 25℃ and 0.1C rate. Room temperature cycling tests measured the cycle retention rate at 25℃ and 1C / 1C charge / discharge rate; high temperature cycling tests measured the cycle retention rate at 45℃ and 0.5C / 0.5C charge / discharge rate. The test results are shown in Table 1. Table 1: Performance test results of coin cells made from the cathode materials of each embodiment and comparative example

[0043] As can be seen from Table 1, compared with Comparative Examples 1-4, Examples 1-4 show advantages in the first-cycle discharge capacity and cycle performance of the ternary cathode materials obtained in the coin cell test. In Comparative Example 1, the cathode material doped with aluminum oxide, where Al has a low valence, and the primary particles in the cross-section are irregularly arranged in all directions when doped into the crystal lattice (e.g., ...). Figure 2 Intergranular non-directional forces are detrimental to the structural stability of cathode materials, reducing cycle performance, complicating grain boundary profiles, increasing the difficulty of lithium-ion transport, and negatively impacting capacity. In Comparative Example 2, the short low-temperature sintering time resulted in incomplete lithiation reactions between the lithium source and precursor. After prolonged high-temperature sintering, significant differences in primary particle size were observed (e.g., ...). Figure 3 During charging and discharging, lithium ions cannot be uniformly inserted and extracted, resulting in defects such as dislocations and vacancies within the particles. This leads to poor bulk structure, susceptibility to cracking, and negatively impacts capacity and cycle life. In Comparative Example 3, the cathode material is only doped with tungsten oxide. Although the aspect ratio of the primary particles in the cathode material cross-section is similar to that of Example 1, the primary particles are more densely packed (e.g., ...). Figure 4 As shown in the example, the pore area ratio at the outer 1 / 4 layer is lower than in the previous example, resulting in a reduced electrolyte wetting area in the cathode material, increasing the difficulty of lithium-ion transport and negatively impacting capacity. In Comparative Example 4, the cathode material is only doped with zirconium sulfate. The thermal hysteresis effect at low temperatures causes some zirconium sulfate to melt and penetrate into the cathode material, and the high-valence metal ions Zr change with temperature. 4+ and S 2- Simultaneous doping affects the local charge distribution in its lattice, causing the primary particle growth direction to become randomized (e.g., Figure 5 As shown in the figure, the lithium-ion transport path is tortuous, which is not conducive to capacity. Furthermore, the intergranular anisotropic forces and the sulfate occupancy at grain boundaries reduce the aspect ratio of primary particles, which is detrimental to the cycle performance of the cathode material.

[0044] The above are merely preferred embodiments of the present invention, and the scope of protection of the present invention is not limited to the above embodiments. For those skilled in the art, improvements and modifications obtained without departing from the inventive concept should also be considered within the scope of protection of the present invention.

[0045] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A ternary polycrystalline cathode material, characterized in that, The ternary polycrystalline cathode material consists of secondary particles formed by the agglomeration of primary particles. The primary particles grow and are arranged radially along the secondary particles, and the primary particles on the cross-section of the secondary particles are strip-shaped, having a major axis L and a minor axis N, where 220nm < L < 350nm, 100nm < N < 200nm, and the aspect ratio K of the primary particles is 1.55 < K < 1.

90. The area Q of the primary particles on the cross-section of the secondary particles is 0.030–0.120 μm. 2 The ratio of the pore area in the outer ring cross-section of the secondary particle to the pore area in the cross-section of the secondary particle is R, where 0.30 < R < 0.60; the outer ring cross-section of the secondary particle is an annular cross-section with a radius of 1 / 4 from the surface to the center of the secondary particle; the ternary polycrystalline cathode material is doped with elements X and Y, where element X is a metallic element with a valence greater than or equal to tetravalent, and element Y is S; element X includes at least one of W, Nb, Ta, Mo, Sn, Zr, and Ti.

2. The ternary polycrystalline cathode material according to claim 1, characterized in that, The ternary polycrystalline cathode material comprises a matrix and a coating layer, wherein the mass of the coating element in the coating layer accounts for 0.02–0.30% of the mass of the ternary polycrystalline cathode material; the chemical formula of the matrix is ​​Li. a Ni b Co c M d X e S f O2, wherein the element M includes at least one of Mn and Al, 1.00≤a≤1.10, 0.84≤b≤0.98, 0<c≤0.11, 0<d≤0.05, 0.15wt%≤e≤0.35wt%, 0.10wt%≤f≤0.30wt%; the elements in the coating layer include at least one of Al, Ce, Ga, B, Sr, W, Ti, Co, Mn and Mg.

3. A method for preparing the ternary polycrystalline cathode material according to claim 1 or 2, characterized in that, Includes the following steps: S1. The cathode material precursor, lithium source, dopant containing element X and dopant containing element Y are uniformly mixed and then sintered once to obtain a sintered matrix; the element X is a metal element with a valence state greater than or equal to positive tetravalent, and the element Y is S; the melting temperature of the dopant containing element X is higher than the melting temperature of the dopant containing element Y. S2. After washing and drying the substrate, mix it evenly with the coating agent and perform secondary sintering to obtain the ternary polycrystalline cathode material.

4. The method for preparing the ternary polycrystalline cathode material according to claim 3, characterized in that, The first sintering is a multi-stage sintering process. First, the temperature is raised to 450-550℃ at a rate of 5-10℃ / min and held for 8-12 hours. Then, the temperature is raised to 750-800℃ at a rate of 1-3℃ / min and held for 1-5 hours.

5. The method for preparing the ternary polycrystalline cathode material according to claim 3, characterized in that, The secondary sintering temperature is 250~550℃, the holding time is 2~12h, and the heating rate is 1℃ / min.

6. The method for preparing the ternary polycrystalline cathode material according to any one of claims 3-5, characterized in that, The dopant containing element X includes at least one of SnO2, WO3, Nb2O5, Ta2O5, MoO3, ZrO2, and TiO2; the dopant containing element Y includes at least one of Li2SO4, K2SO4, Al2(SO4)3, Na2SO4, and MgSO4; the coating agent includes at least one of aluminum oxide, cerium oxide, gallium oxide, tungsten oxide, boric acid, strontium carbonate, titanium oxide, cobalt tetroxide, manganese oxide, and magnesium oxide.

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