Positive electrode active material having a combination of large and small particles and method for preparing the same

Abstract

The present invention relates to the field of lithium-ion batteries and discloses a positive electrode active material having a combination of large and small particles, comprising large secondary spheres and small secondary spheres, where the mass ratio of the large secondary spheres is greater than 50%, the D50 particle size of the large secondary spheres is 10-20 μm, and the D50 particle size of the small secondary spheres is 2-5 μm. This invention precisely controls the doping elements and nickel content of the large and small particles, ultimately resulting in a positive electrode material with good overall performance in terms of capacity, cycle life, and DCR. The present invention also discloses a method for preparing a positive electrode active material having a combination of large and small particles. Since different elements are doped in the precursor stage, the primary particle size of the sintered material can be controlled, indirectly satisfying the requirement that the large and small particles be sintered at the same temperature during the co-sintering process, resulting in a material with better performance.

Description

[Technical Field]

[0001] The present invention relates to the field of lithium ion batteries, and in particular to a positive electrode active material having a combination of large and small particles and a method for preparing the same. [Background technology]

[0002] In industry, the pressed density of lithium-ion batteries is generally improved by combining large and small particles to form electrode plates, thereby further improving energy density. However, if large and small secondary particles with different properties are simultaneously heat-treated, the small secondary particles may be over-sintered or under-sintered, resulting in reduced performance. Therefore, in conventional sintering methods, the large and small secondary particles are heat-treated separately, and the heat-treated particles are then mixed and subjected to further heat treatment. The need for multiple heat treatment processes complicates the preparation of the positive electrode active material and increases production costs.

[0003] Chinese patent document CN114447289A discloses the co-firing of large and small particles, in which the large particles of secondary spheres contain Al and the small particles contain Mn, with the Mn content of the large particles being lower than that of the small particles. The firing conditions of the material are controlled by changing the precursor element content, so that manganese is coated on the surface of each large secondary particle, while aluminum is coated on the surface of each small secondary particle. Chinese patent document CN113823774A discloses that when the Mn concentration difference between the large and small particles is less than 2, the firing difference between the large and small particles in the material during the co-firing process can be prevented by adjusting the firing temperature according to the Mn concentration difference. Both of these patent documents describe a method in which Mn is essential to adjust the large and small particles to exhibit different properties during co-firing by using changes in Mn element. However, they are not suitable for NCA products or any other products that do not require Mn element content, and their application range is limited.

[0004] Chinese patent document CN107785550A discloses a low-temperature coating pretreatment method before co-firing, which requires complicated operations. Chinese patent document CN113394385A discloses in-situ coating of precursors with larger particle sizes and pre-oxidation of precursors with smaller particle sizes before co-firing. In both of the above patents, the need for multiple processes increases the complexity of the actual manufacturing process and makes the operation in actual manufacturing more difficult.

[0005] Furthermore, due to differences in particle size, large and small particles each have advantages and disadvantages in performance. Small particles need to improve their cycle performance, while large particles need to improve their capacity characteristics. At the same time, performance improvement requires consideration of DCR, and in the case of a combination of large and small particles, DCR must be minimized. Therefore, in a method for preparing a cathode material that combines large and small particles, it is necessary to improve the capacity and cycle performance of the material while reducing the DCR of the material. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Chinese patent document CN114447289A [Patent Document 2] Chinese patent document CN113823774A [Patent Document 3] Chinese patent document CN107785550A [Patent Document 4] Chinese patent document CN113394385A Summary of the Invention [Problem to be solved by the invention]

[0007] The technical problem to be solved by the present invention is to overcome the drawbacks and deficiencies of the background art as described above, thereby providing a method for providing a positive electrode active material with good capacity and cycle performance, and a preparation method that achieves co-firing of large particles and small particles to meet the requirements for material properties, while at the same time reducing costs and enabling simple operations. [Means for solving the problem]

[0008] In order to solve the above technical problems, the technical solutions proposed by the present invention are: A positive electrode active material having a combination of large particles and small particles, including large particle secondary spheres and small particle secondary spheres, wherein the mass ratio of the large particle secondary spheres in the positive electrode active material is greater than 50%; Large particle secondary sphere with the chemical formula Li a Ni b M c M' d O2 (wherein M is one or more of Co, Mn, and Al, M' is a doping element, and M' is selected from one or more of Zr and Sr, 0.9≦a≦1.2, 0.7≦b<1, 0 <c≦0.3、0<d≦0.1である)であり、 The chemical formula of the small secondary sphere is Li e Ni f M g M” h O2 (wherein M is one or more of Co, Mn, and Al, and M" is a doping element, and M" is selected from one or more of B, W, Mo, In, Ta, and S, and 0.9≦e≦1.2, 0.7≦f<1, 0 <g≦0.2、0<h≦0.1である)であり、 The D50 particle size of the large secondary spheres is 10 to 20 μm, and the D50 particle size of the small secondary spheres is 2 to 5 μm. It is a positive electrode active material.

[0009] In the positive electrode active material having a combination of large particles and small particles as described above, the mass ratio of large secondary spherical particles to small secondary spherical particles is preferably (7:3) to (9:1).

[0010] Preferably, f and b satisfy f>b and fb<0.1. Within this range, increasing the Ni content in the small particles is more beneficial to the DCR of the co-fired secondary spheres that combine large and small particles.

[0011] Preferably, the positive electrode active material is obtained by co-firing a precursor of large particle secondary spheres and a precursor of small particle secondary spheres with doping elements M' and M" added during the preparation process of the precursors.

[0012] Based on the general inventive concept, the present invention further provides a method for preparing a cathode active material having a combination of large particles and small particles, the method comprising the steps of: (1) mixing metal salt solutions corresponding to the large secondary spheres and the small secondary spheres, respectively, with metal salt solutions corresponding to a precipitant, a complexing agent, and a doping element to carry out a coprecipitation reaction, and then filtering and drying the mixture to prepare large secondary sphere precursors containing the doping element M' and small secondary sphere precursors containing the doping element M", respectively; (2) mixing the large particle secondary sphere precursor and the small particle secondary sphere precursor with a lithium source to obtain a mixture, and then calcining the mixture to obtain a positive electrode active material having a combination of large particles and small particles; Includes:

[0013] In the above preparation method, preferably, in step (1), both of the metal salt solutions corresponding to the large particle secondary spheres and the small particle secondary spheres contain a nickel salt and an M salt, the precipitating agent contains one or more of sodium hydroxide and potassium hydroxide, and the complexing agent contains one or more of ammonia solution, ammonium sulfate, and ammonium bicarbonate.

[0014] Preferably, in step (1), the conditions for the coprecipitation reaction are that all solutions are added to the reaction vessel at a rate of 0.1 to 10 L / h, pH 9 to 12, temperature 40 to 80°C, and stirring speed 300 to 600 rpm during the reaction process.

[0015] Preferably, in step (2), the lithium source is selected from one or more of lithium carbonate, lithium hydroxide, and lithium nitrate, and the molar ratio of the metal elements in the lithium source to the total metal elements in the precursors of the large and small secondary spheres is (0.9-1.2):1.

[0016] Preferably, in step (2), the firing temperature is 500 to 1000°C.

[0017] More preferably, the firing step specifically comprises the following steps: S1: placing the mixture in an oxygen atmosphere for primary firing, first firing the mixture at 400-550°C for 1-3 hours, then heating it at 1-5°C / min to 600-800°C for 8-20 hours, and then cooling it and passing it through a 300 mesh sieve to obtain a primary fired product; S2: The primary fired material is washed, filtered, and dried, placed in an oxygen atmosphere, and subjected to secondary firing at a firing temperature of 300 to 700°C for a holding time of 3 to 20 hours. After cooling, the material is passed through a 300-mesh sieve to obtain a positive electrode active material having a combination of large and small particles. Includes:

[0018] Doping elements that have different effects on the precursors of large and small secondary spheres can alleviate the temperature difference during firing. The large-particle precursor is doped with an element that increases the size of the fired primary particles, while the small-particle precursor is doped with an element that decreases the size of the fired primary particles. This method allows two precursors with different nickel contents to be co-fired, since the primary particles contained in the final fired secondary spheres have the same or similar sizes.

[0019] The size of the primary particles in the secondary spheres provides a direct way to determine whether the sintering temperature is appropriate, and the material will only perform at its best if its primary particles have the appropriate size. Typically, large and small secondary spheres have the same primary particle size only if the actual sintering temperature of the large particles is higher than that of the small particles. A primary particle size that is too large can affect the Li-ion migration path, while a primary particle size that is too small can result in incomplete crystallinity during sintering. Doping the large and small precursors with different elements improves the material's performance and simplifies the sintering process. The doping elements in the precursors promote a more uniform distribution of the internal elements in the material, which contributes to more uniform performance and reduces polarization during the charge / discharge process.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. In the present invention, the large and small secondary spheres are doped with elements beneficial to compensate for their respective shortcomings, the precursors of the small particles are doped with elements beneficial to improving the cycling performance of the small particles, and the precursors of the large particles are doped with elements capable of improving capacity. These work together to balance capacity and cycling performance, further increasing the Ni content of the small secondary spheres. Although the proportion of small particles is not high, this increase in Ni content has a significant effect on reducing the overall DCR of the material. In summary, in this invention, the doping elements and nickel content of the large and small particles are carefully adjusted, ultimately resulting in a positive electrode material that exhibits high overall performance in terms of capacity, cycling, and DCR.

[0022] 2. In the preparation method of the present invention, the size of the primary particles of the fired material can be controlled by doping different elements in the precursor stage, and the requirement that the large particles and small particles have the same firing temperature in the co-firing process is indirectly met, so the material obtained by co-firing large particles and small particles can exhibit better performance. Therefore, the co-firing of large particles and small particles can meet the material property requirements without the need for any specific elements, which has a wide range of applications and can reduce costs with simple operation. [Brief explanation of the drawings]

[0023] In order to more clearly describe the technical solutions provided in the embodiments of the present invention or the prior art, the accompanying drawings for describing the embodiments or the prior art are briefly described below. Obviously, the drawings in the following description are only some embodiments of the present invention, and those skilled in the art can derive other drawings from these drawings without creative efforts. [Figure 1] FIG. 1 is an SEM image of the positive electrode active material obtained by co-firing large particles and small particles in the first embodiment. [Figure 2] FIG. 2 is an SEM image of the positive electrode active material obtained by co-firing large particles and small particles in the second embodiment. [Figure 3] FIG. 3 is an SEM image of the positive electrode active material obtained by co-firing large particles and small particles in Comparative Example 1. [Figure 4] FIG. 4 is an SEM image of the positive electrode active material obtained by co-firing large particles and small particles in Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0024] example In order to better understand the present invention, the present invention will be described more comprehensively and in detail in conjunction with the drawings and preferred embodiments, however, the protection scope of the present invention is not limited to the specific examples described below.

[0025] 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 only intended to describe specific embodiments and are not intended to limit the scope of protection of the present invention.

[0026] Unless otherwise specified, the various raw materials, reagents, instruments and equipment used in the present invention are either commercially available or can be prepared by conventional methods. [Example]

[0027] Example 1 A positive electrode active material having a combination of large particles and small particles, including large particle secondary spheres and small particle secondary spheres, wherein the chemical formula of the large particle secondary spheres is Li 1.05 Ni 0.9 Co 0.08 Al 0.02 Zr 0.002 O2, and the chemical formula of the small particle secondary sphere is Li 1.05 Ni 0.92 Co 0.06 Al 0.02 B 0.0005 The positive electrode active material, O, was prepared by the following steps: (1) Nickel sulfate solution and cobalt sulfate solution were mixed, and sodium aluminate solution dissolved in sodium hydroxide solution, zirconium sulfate solution, 0.2 mol / L ammonia solution, and 5.5 mol / L sodium hydroxide solution were added. The mixture was fed into a reactor at a rate of 2.0 L / h and reacted. During the reaction process, the pH was 11, the temperature was 60°C, and the stirring speed was 400 rpm. The metal molar ratio of Ni to Co to Al was 0.9:0.08:0.02, and the molar mass of Zr was 0.2 mol%. After aging, centrifugation, and drying, large-particle precursors with a D50 particle size of 16 μm were obtained. The small-particle precursors were synthesized in the same manner as the large-particle precursors, except that the metal molar ratio of Ni to Co to Al was 0.91:0.07:0.02 during the preparation process. Instead of adding zirconium sulfate solution, boric acid solution was added. The molar mass of element B was 0.05 mol%. The reaction was stopped when the D50 particle size reached 2.9 μm, and the small particle precursor was obtained by post-treatment. (2) Large particle precursor Ni doped with 0.2 mol% elemental Zr 0.9 Co 0.08 Al 0.02 Small particle precursor Ni doped with (OH)2 and 0.05 mol% element B 0.92 Co 0.06 Al 0.02 (OH)2 was weighed in a mass ratio of 7.5:2.5, and the two precursors were mixed with lithium hydroxide in a high-speed mixer in a metal molar ratio of 1:1.05. The mixture was then placed in an oxygen atmosphere furnace for primary firing. The mixture was first fired at 400°C for 3 hours, then heated to 710°C for 12 hours at a heating rate of 5°C / min, and allowed to cool naturally to obtain a matrix material. The fired sample was passed through a 300-mesh sieve to obtain the primary fired material, which was then washed with water. The prepared material was stirred with an electric stirrer during washing, washed with deionized water in a 1:1 ratio for 30 minutes, filtered, and the filter cake was dried in a vacuum drying oven for 10 hours. After washing and drying, the sample was subjected to secondary firing, then naturally cooled in an oxygen atmosphere furnace at 650°C for 6 hours, and passed through a 300-mesh sieve to obtain a high-density cathode material.

[0028] As shown in Figure 1, different elements were doped into the large and small secondary spherical particles in the precursor, and after co-firing, the large and small particles had similar primary particle sizes.

[0029] Example 2 A positive electrode active material having a combination of large particles and small particles, including large particle secondary spheres and small particle secondary spheres, wherein the chemical formula of the large particle secondary spheres is Li 1.03 Ni 0.9 Co 0.05 Mn 0.05 Sr 0.0013 O2, and the chemical formula of the small particle secondary sphere is Li 1.03 Ni 0.91 Co 0.04 Mn 0.05 B 0.001 The positive electrode active material, O, was prepared by the following steps: (1) Nickel sulfate solution, cobalt sulfate solution, manganese sulfate solution, strontium hydroxide solution, 0.2 mol / L ammonia solution, and 6.0 mol / L sodium hydroxide solution were fed into a reactor at a rate of 2.0 L / h and reacted. During the reaction process, the pH was 11, the temperature was 60 °C, and the stirring speed was 350 rpm. The metal molar ratio of Ni to Co to Mn was 0.9:0.05:0.05, and the molar mass of Sr was 0.13 mol%. After aging, centrifugation, and drying, large-particle precursors with a D50 particle size of 14.5 μm were obtained. The small-particle precursors were synthesized in the same manner as the large-particle precursors, except that the metal molar ratio of Ni to Co to Mn was 0.91:0.04:0.05 during the preparation process, boric acid solution was used instead of strontium hydroxide solution, and element B had a molar mass of 0.1 mol%. The reaction was stopped when the D50 particle size reached 3.1 μm, and the small particle precursor was obtained by post-treatment. (2) Large particle precursor Ni doped with 0.13 mol% elemental Sr 0.9 Co 0.05 Mn 0.05 Small particle precursor Ni doped with (OH)2 and 0.05 mol% element B 0.91 Co 0.04 Mn 0.05(OH)2 was weighed in a mass ratio of 7:3, and the two precursors were mixed with lithium hydroxide in a high-speed mixer in a metal molar ratio of 1:1.03. The mixture was then placed in an oxygen atmosphere furnace for primary firing. The mixture was first fired at 400°C for 3 hours, then heated to 710°C for 12 hours at a heating rate of 5°C / min, and allowed to cool naturally to obtain a matrix material. The fired sample was passed through a 300-mesh sieve to obtain the primary fired material, which was then washed with water. The prepared material was stirred with an electric stirrer and washed with deionized water in a 1:1 ratio for 30 minutes, then filtered. The filter cake was placed in a vacuum drying oven and dried for 10 hours. After washing and drying, the sample was subjected to secondary firing, then naturally cooled in an oxygen atmosphere furnace at 500°C for 6 hours. The cooled sample was then passed through a 300-mesh sieve to obtain a high-density cathode material.

[0030] As shown in Figure 2, different elements were doped into the large and small secondary spherical particles in the precursor, and after co-firing, the large and small particles had similar primary particle sizes.

[0031] Example 3 A positive electrode active material having a combination of large particles and small particles, including large particle secondary spheres and small particle secondary spheres, wherein the chemical formula of the large particle secondary spheres is Li 1.03 Ni 0.9 Co 0.07 Al 0.03 Sr 0.001 O2, and the chemical formula of the small particle secondary sphere is Li 1.03 Ni 0.91 Co 0.06 Al 0.03 W 0.0005 The positive electrode active material, O, was prepared by the following steps: (1) Nickel sulfate solution and cobalt sulfate solution were mixed, and sodium aluminate solution dissolved in sodium hydroxide solution, strontium hydroxide solution, 0.2 mol / L ammonia solution, and 5.5 mol / L sodium hydroxide solution were added. The mixture was fed into a reactor at a rate of 2 L / h to react. During the reaction, the pH was 11.8, the temperature was 60°C, and the stirring speed was 400 rpm. The metal molar ratio of Ni to Co to Al was 0.9:0.07:0.03, and the molar mass of strontium was 0.1 mol%. After aging, centrifugation, and drying, a large particle precursor with a D50 particle size of 14.5 μm was obtained. The small particle precursor was synthesized in the same manner as the large particle precursor, except that the metal molar ratio of Ni to Co to Al was 0.91:0.06:0.03 during the preparation process, no strontium-containing solution was added, but instead a sodium tungstate solution was added, and the element W had a molar mass of 0.05 mol%. The reaction was stopped when the D50 particle size reached 3.2 μm, and the small particle precursor was obtained by post-treatment. (2) Large particle precursor Ni doped with 0.1 mol% elemental Sr 0.9 Co 0.07 Al 0.03 Small particle precursor Ni doped with (OH)2 and 0.05 mol% elemental W 0.91 Co 0.06 Al 0.03 (OH)2 was weighed out in a mass ratio of 7:3, and the two precursors were mixed with lithium hydroxide in a high-speed mixer in a metal molar ratio of 1:1.03. The mixture was then placed in an oxygen atmosphere furnace for primary firing. The mixture was first fired at 400°C for 3 hours, then heated to 715°C for 12 hours at a heating rate of 5°C / min, and allowed to cool naturally to obtain a matrix material. The fired sample was passed through a 300-mesh sieve to obtain the primary fired material, which was then washed with water. The prepared material was stirred with an electric stirrer during washing, washed with deionized water in a 1:1 ratio for 30 minutes, filtered, and the filter cake was placed in a vacuum drying oven and dried for 10 hours. After washing and drying, the sample was subjected to secondary firing, then naturally cooled in an oxygen atmosphere furnace at 630°C for 6 hours, and passed through a 300-mesh sieve to obtain a high-density cathode material.

[0032] Example 4 A positive electrode active material having a combination of large particles and small particles, including large particle secondary spheres and small particle secondary spheres, wherein the chemical formula of the large particle secondary spheres is Li 1.03 Ni 0.9 Co 0.07 Al 0.03 Zr 0.0015 O2, and the chemical formula of the small particle secondary sphere is Li 1.03 Ni 0.92 Co 0.05 Al 0.03 W 0.001 Ti 0.001 The positive electrode active material, O, was prepared by the following steps: (1) Nickel sulfate solution and cobalt sulfate solution were mixed, and sodium aluminate solution dissolved in sodium hydroxide solution, zirconium sulfate solution, 0.2 mol / L ammonia solution, and 5.0 mol / L sodium hydroxide solution were added. The mixture was fed into a reactor at a rate of 2.2 L / h to react. During the reaction, the pH was 11, the temperature was 55°C, and the stirring speed was 400 rpm. The metal molar ratio of Ni to Co to Al was 0.9:0.07:0.03, and the molar mass of zirconium was 0.15 mol%. After aging, centrifugation, and drying, a large particle precursor with a D50 particle size of 14.5 μm was obtained. The small-particle precursor was synthesized in the same manner as the large-particle precursor, except that the molar ratio of Ni to Co to Al was 0.91:0.06:0.03, and the zirconium sulfate solution was not added. Instead, sodium tungstate solution and titanic acid solution were added simultaneously. The molar mass of element W was 0.1 mol %, and the molar mass of element Ti was 0.1 mol %. The reaction was stopped when the D50 particle size reached 3.2 μm, and the small-particle precursor was obtained by post-treatment. (2) Large particle precursor Ni doped with 0.15 mol% elemental Zr 0.9 Co 0.07 Al 0.03 (OH)2 and small particle precursor Ni doped with 0.1 mol% elemental W and 0.1 mol% Ti 0.92 Co 0.05 Al 0.03(OH)2 was weighed in a mass ratio of 7:3, and the two precursors were mixed with lithium hydroxide in a high-speed mixer in a metal molar ratio of 1:1.03. The mixture was then placed in an oxygen atmosphere furnace for primary firing. The mixture was first fired at 400°C for 3 hours, then heated to 710°C for 12 hours at a heating rate of 5°C / min, and allowed to cool naturally to obtain a matrix material. The fired sample was passed through a 300-mesh sieve to obtain the primary fired material, which was then washed with water. The prepared material was stirred with an electric stirrer during washing, washed with deionized water in a 1:1 ratio for 30 minutes, filtered, and the filter cake was placed in a vacuum drying oven and dried for 10 hours. After washing and drying, the sample was subjected to secondary firing, then naturally cooled in an oxygen atmosphere furnace at 600°C for 6 hours, and passed through a 300-mesh sieve to obtain a high-density cathode material.

[0033] Example 5 A positive electrode active material having a combination of large particles and small particles, including large particle secondary spheres and small particle secondary spheres, wherein the chemical formula of the large particle secondary spheres is Li 1.03 Ni 0.9 Co 0.07 Al 0.03 Zr 0.0015 O2, and the chemical formula of the small particle secondary sphere is Li 1.03 Ni 0.925 Co 0.045 Al 0.03 W 0.0015 Mg 0.001 The positive electrode active material, O, was prepared by the following steps: (1) Nickel sulfate solution and cobalt sulfate solution were mixed, and sodium aluminate solution dissolved in sodium hydroxide solution, zirconium sulfate solution, 0.2 mol / L ammonia solution, and 5.5 mol / L sodium hydroxide solution were added. The mixture was fed into a reactor at a rate of 2.5 L / h to react. During the reaction, the pH was 11.5, the temperature was 55°C, and the stirring speed was 400 rpm. The metal molar ratio of Ni to Co to Al was 0.9:0.07:0.03, and the molar mass of zirconium was 0.15 mol% of the total metal molar mass. After aging, centrifugation, and drying, a large particle precursor with a D50 particle size of 14.5 μm was obtained. The small particle precursor was synthesized in the same manner as the large particle precursor, except that the molar ratio of Ni to Co to Al was 0.91:0.07:0.02, magnesium sulfate solution and sodium tungstate solution were added, and the molar mass of element W was 0.15 mol% and the molar mass of element Mg was 0.1 mol%. The reaction was stopped when the D50 particle size reached 3.2 μm, and the small particle precursor was obtained by post-treatment. (2) Large particle precursor Ni doped with 0.15 mol% elemental Zr 0.9 Co 0.07 Al 0.03 Small particle precursor Ni doped with (OH)2 and 0.15 mol% elemental W and 0.1 mol% Mg 0.925 Co 0.045 Al 0.03 (OH)2 was weighed out in a mass ratio of 7:3, and the two precursors were mixed with lithium hydroxide in a high-speed mixer in a metal molar ratio of 1:1.03. The mixture was then placed in an oxygen atmosphere furnace for primary firing. The mixture was first fired at 400°C for 3 hours, then heated to 710°C for 12 hours at a heating rate of 5°C / min, and allowed to cool naturally to obtain a matrix material. The fired sample was passed through a 300-mesh sieve to obtain the primary fired material, which was then washed with water. The prepared material was stirred with an electric stirrer during washing, washed with deionized water in a 1:1 ratio for 30 minutes, filtered, and the filter cake was placed in a vacuum drying oven and dried for 10 hours. After washing and drying, the sample was subjected to secondary firing, then naturally cooled in an oxygen atmosphere furnace at 550°C for 6 hours, and passed through a 300-mesh sieve to obtain a high-density cathode material.

[0034] Comparative Example 1 The positive electrode active material was prepared by the following steps: Large particle precursor Ni 0.9 Co 0.08 Al 0.02 (OH)2 and small particle precursor Ni 0.91 Co 0.07 Al 0.02 (OH)2 was weighed in a mass ratio of 7.5:2.5. The large particles had a D50 particle size of 14.5 μm, and the small particles had a D50 particle size of 3.5 μm. The two precursors were mixed with lithium hydroxide in a high-speed mixer at a metal molar ratio of 1:1.04, and the mixture was placed in an oxygen atmosphere furnace for primary firing. The mixture was first fired at 400°C for 3 hours, then heated to 720°C for 12 hours at a heating rate of 5°C / min, and allowed to cool to obtain the matrix material. The fired sample was passed through a 300-mesh sieve to obtain the primary fired material, which was then washed with water. The prepared material was stirred with an electric stirrer during washing, washed with deionized water in a 1:1 ratio for 30 minutes, then filtered. The filter cake was then placed in a vacuum drying oven and dried for 10 hours. After washing and drying, the sample was subjected to secondary firing, then naturally cooled in an oxygen atmosphere furnace at 600°C for 6 hours, and the cooled sample was passed through a 300 mesh sieve to obtain a positive electrode material with high compaction density.

[0035] As shown in FIG. 3, the primary particle diameters of the undoped large particles and small particles after firing were significantly different.

[0036] Comparative Example 2 The positive electrode active material was prepared by the following steps: Large particle precursor Ni 0.9 Co 0.05 Mn 0.05 (OH)2 and small particle precursor Ni 0.91 Co 0.04 Mn 0.05(OH)2 was weighed at a mass ratio of 7:3. The large particles had a D50 particle size of 14.5 μm, and the small particles had a D50 particle size of 3.1 μm. The two precursors were mixed with lithium hydroxide in a high-speed mixer at a metal molar ratio of 1:1.03, and the mixture was placed in an oxygen atmosphere furnace for primary firing. The mixture was first fired at 400°C for 3 hours, then heated to 750°C for 12 hours at a heating rate of 5°C / min, and allowed to cool to obtain the matrix material. The fired sample was passed through a 300-mesh sieve to obtain the primary fired material, which was then washed with water. The prepared material was stirred with an electric stirrer during washing, washed with deionized water in a 1:1 ratio for 30 minutes, then filtered. The filter cake was then placed in a vacuum drying oven and dried for 10 hours. After washing and drying, the sample was subjected to secondary firing, then naturally cooled in an oxygen atmosphere furnace at 500°C for 6 hours, and the cooled sample was passed through a 300 mesh sieve to obtain a positive electrode material with high compaction density.

[0037] As shown in FIG. 4, the undoped large and small particles after calcination had significantly different primary particle sizes.

[0038] Comparative Example 3 The positive electrode active material was prepared by the following steps: (1) Nickel sulfate solution, cobalt sulfate solution, manganese sulfate solution, zirconium sulfate solution, 0.15 mol / L ammonia solution, and 6.0 mol / L sodium hydroxide solution were fed into a reactor at a specific rate to react. The metal molar ratio was 0.9:0.05:0.05, and the molar mass of the zirconium solution was 0.15 mol%. After aging, centrifugation, and drying, a large-particle precursor with a D50 particle size of 14.0 μm was obtained. The small-particle precursor was synthesized in the same manner as the large-particle precursor, except that the metal molar ratio was 0.91:0.04:0.05 and 0.15 mol% zirconium solution was added. The reaction was stopped when the D50 particle size reached 3.3 μm, and the small-particle precursor was obtained by post-treatment. (2) Large particle precursor Ni doped with 0.15 mol% elemental Zr 0.9 Co 0.05 Mn 0.05 Small particle precursor Ni doped with (OH)2 and 0.15 mol% elemental Zr0.91 Co 0.04 Mn 0.05 (OH)2 was weighed in a mass ratio of 7:3, and the two precursors were mixed with lithium hydroxide in a high-speed mixer in a metal molar ratio of 1:1.05. The mixture was then placed in an oxygen atmosphere furnace for primary firing. The mixture was first fired at 400°C for 3 hours, then heated to 745°C for 12 hours at a heating rate of 5°C / min, and allowed to cool naturally to obtain a matrix material. The fired sample was passed through a 300-mesh sieve to obtain the primary fired material, which was then washed with water. The prepared material was stirred with an electric stirrer and washed with deionized water in a 1:1 ratio for 30 minutes, then filtered. The filter cake was placed in a vacuum drying oven and dried for 10 hours. After washing and drying, the sample was subjected to secondary firing, then naturally cooled in an oxygen atmosphere furnace at 500°C for 6 hours. The cooled sample was then passed through a 300-mesh sieve to obtain a high-density cathode material.

[0039] Comparative Example 4 A positive electrode active material having a combination of large particles and small particles, including large particle secondary spheres and small particle secondary spheres, wherein the chemical formula of the large particle secondary spheres is Li 1.04 Ni 0.9 Co 0.08 Al 0.02 O2, and the chemical formula of the small particle secondary sphere is Li 1.04 Ni 0.91 Co 0.07 Al 0.02 A positive electrode active material, O (further doped with 0.1 mol % elemental W), was prepared by the following steps: (1) Nickel sulfate solution and cobalt sulfate solution were mixed, and sodium aluminate solution, sodium tungstate solution, 0.2 mol / L ammonia solution, and 5.5 mol / L sodium hydroxide solution were added to the reactor at a specific rate to react. The metal molar ratio was 0.91:0.07:0.02, and the molar mass of W was 0.1 mol%. After aging, centrifugation, and drying, a small particle precursor with a D50 particle size of 3 μm was obtained. The large particle precursor was synthesized in the same manner as the small particle precursor, except that the metal molar ratio was 0.90:0.08:0.02. During the preparation process, sodium tungstate solution was not added. The reaction was stopped when the D50 particle size reached 14.5 μm, and the large particle precursor was obtained by post-processing. (2) Large particle precursor Ni 0.9 Co 0.08 Al 0.02 Small particle precursor Ni doped with (OH)2 and 0.1 mol% elemental W 0.91 Co 0.07 Al 0.02 (OH)2 was weighed in a mass ratio of 7.5:2.5, and the two precursors were mixed with lithium hydroxide in a high-speed mixer in a metal molar ratio of 1:1.04. The mixture was then placed in an oxygen atmosphere furnace for primary firing. The mixture was first fired at 400°C for 3 hours, then heated to 710°C for 12 hours at a heating rate of 5°C / min, and allowed to cool naturally to obtain a matrix material. The fired sample was passed through a 300-mesh sieve to obtain the primary fired material, which was then washed with water. The prepared material was stirred with an electric stirrer during washing, washed with deionized water in a 1:1 ratio for 30 minutes, filtered, and the filter cake was placed in a vacuum drying oven and dried for 10 hours. After washing and drying, the sample was subjected to secondary firing, then naturally cooled in an oxygen atmosphere furnace at 600°C for 6 hours, and passed through a 300-mesh sieve to obtain a high-density cathode material.

[0040] Comparative Example 5 The positive electrode active material was prepared by the following steps: Large particle precursor Ni 0.9 Co 0.05 Mn 0.05 (OH)2 and small particle precursor Ni0.90 Co 0.05 Mn 0.05 (OH)2 was weighed at a mass ratio of 7:3. The large particles had a D50 particle size of 14.0 μm, and the small particles had a D50 particle size of 3.0 μm. The two precursors were mixed with lithium hydroxide in a high-speed mixer at a metal molar ratio of 1:1.03, and the mixture was placed in an oxygen atmosphere furnace for primary firing. The mixture was first fired at 400°C for 3 hours, then heated to 750°C for 12 hours at a heating rate of 5°C / min, and allowed to cool to obtain the matrix material. The fired sample was passed through a 300-mesh sieve to obtain the primary fired material, which was then washed with water. The prepared material was stirred with an electric stirrer during washing, washed with deionized water in a 1:1 ratio for 30 minutes, then filtered. The filter cake was then placed in a vacuum drying oven and dried for 10 hours. After washing and drying, the sample was subjected to secondary firing, then naturally cooled in an oxygen atmosphere furnace at 500°C for 6 hours, and the cooled sample was passed through a 300 mesh sieve to obtain a positive electrode material with high compaction density.

[0041] [Table 1]

[0042] Table 1 shows a comparison of the performance of the positive electrode materials in Examples 1 to 5 and Comparative Examples 1 to 5. From the above results, it can be seen that in the Comparative Examples, the temperature required for firing the large and small particles during the co-firing process could not be accurately controlled, resulting in a suboptimal overall performance of the fired secondary spheres. In the Examples, the primary particle size of the material can be controlled by doping different elements at the precursor stage, and the requirement that the large and small particles have the same firing temperature is indirectly met during the co-firing process. Therefore, all of the materials obtained by co-firing suitable large and small particles exhibited high performance. In Example 2, the large and small particles were doped with different elements, resulting in improved capacity and cycle performance compared to Comparative Example 2. Similar results were observed in Example 3 and Comparative Example 3. Thus, it is clear that Examples 1 to 5 achieved more significant performance improvements compared to the Comparative Examples. This is because the large and small particles were simultaneously doped with suitable elements, further improving performance. Furthermore, since the large and small particles in Comparative Example 5 had the same Ni content, the DCR of this example was higher than the other test results.

Claims

1. A positive electrode active material having a combination of large particles and small particles, comprising large particle secondary spheres and small particle secondary spheres, wherein the mass ratio of the large particle secondary spheres in the positive electrode active material is greater than 50%; The chemical formula of the large particle secondary sphere is Li ａ Ni ｂ M ｃ M' ｄ O ２ wherein M is one or more of Co, Mn, and Al; M′ is a doping element; and M′ is selected from one or more of Zr and Sr; and 0.9≦a≦1.2, 0.7≦b<1, 0<c≦0.3, 0<d≦0.1; The chemical formula of the small particle secondary sphere is Li ｅ Ni ｆ M ｇ M” ｈ O ２ wherein M is one or more of Co, Mn, Al, and M″ is a doping element selected from one or more of B, W, Mo, In, Ta, S, and 0.9≦e≦1.2, 0.7≦f<1, 0<g≦0.2, 0<h≦0.1; The large secondary spheres have a D50 particle size of 10 to 20 μm, and the small secondary spheres have a D50 particle size of 2 to 5 μm.

2. 2. The positive electrode active material having a combination of large particles and small particles according to claim 1, wherein the mass ratio of the large secondary spheres to the small secondary spheres is from 7:3 to 9:

1.

3. 2. The positive electrode active material having a combination of large particles and small particles according to claim 1, wherein f and b satisfy the conditions: f>b and f-b<0.

1.

4. 3. A positive electrode active material having a combination of large and small particles according to claim 1 or 2, obtained by mixing precursors of the large secondary spheres and precursors of the small secondary spheres, then adding doping elements M' and M" during the preparation process of the precursors, and co-firing the mixture.

5. (1) mixing metal salt solutions corresponding to the large secondary spheres and the small secondary spheres, respectively, with metal salt solutions corresponding to a precipitant, a complexing agent, and a doping element to carry out a coprecipitation reaction, and then filtering and drying the mixture to prepare large secondary sphere precursors containing the doping element M' and small secondary sphere precursors containing the doping element M", respectively; (2) mixing the large particle secondary sphere precursor and the small particle secondary sphere precursor with a lithium source to obtain a mixture, and then calcining the mixture to obtain a positive electrode active material having a combination of large particles and small particles; 3. A method for preparing a positive electrode active material having a combination of large particles and small particles according to claim 1 or 2, comprising:

6. 6. The method according to claim 5, wherein in step (1), the metal salt solutions corresponding to the large particle secondary spheres and the small particle secondary spheres all contain a nickel salt and an M salt, the precipitating agent comprises one or more of sodium hydroxide and potassium hydroxide, and the complexing agent comprises one or more of ammonia solution, ammonium sulfate, and ammonium bicarbonate.

7. 6. The preparation method according to claim 5, wherein in step (1), the conditions of the coprecipitation reaction are: adding all the solutions to the reaction vessel at a rate of 0.1-10 L / h, pH 9-12, temperature 40-80°C, and stirring speed 300-600 rpm during the reaction process.

8. 6. The method of claim 5, wherein in step (2), the lithium source is selected from one or more of lithium carbonate, lithium hydroxide, and lithium nitrate, and the molar ratio of the metal elements in the lithium source to the total metal elements in the precursors of the large and small secondary spheres is (0.9-1.2):

1.

9. The preparation method according to claim 5, wherein in step (2), the calcination has a temperature of 500 to 1000°C.

10. Specifically, the firing step includes the following steps: S1: placing the mixture in an oxygen atmosphere for primary firing, first firing the mixture at 400-550°C for 1-3 hours, then heating it at 1-5°C / min to 600-800°C for 8-20 hours, and then cooling it down and passing it through a 300 mesh sieve to obtain a primary fired product; S2: The primary fired product is washed, filtered, and dried, placed in an oxygen atmosphere, and subjected to secondary firing at a firing temperature of 300 to 700°C for a holding time of 3 to 20 hours. After cooling, the product is passed through a 300-mesh sieve to obtain a positive electrode active material having a combination of large and small particles. Including, 10. The preparation method according to claim 9.

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