A positive electrode active material, a method for preparing the same, and an application thereof

Abstract

The application provides a positive electrode active material and a preparation method and application thereof. The positive electrode active material comprises, from inside to outside in a radial direction, an inner layer and an outer layer. The inner layer comprises a third layer surrounding a cavity. The outer layer comprises a second layer on a side of the third layer away from the cavity and a first layer on a side of the second layer away from the third layer. The positive electrode active material is composed of primary particles. The second layer has a hollow structure comprising the primary particles. In the radial direction from inside to outside, the width of the positive electrode active material is r1, the width of the second layer is r2-r3, the width of the cavity is r4, the average hollow thickness ratio of the second layer is D1=(r2-r3) / r1, and the average hollow thickness ratio of the cavity is D2=r4 / r1. D1 and D2 satisfy 0D1≤16%, and 10%≤D2≤20%. The application helps to improve the structural stability of the positive electrode active material.

Description

TECHNICAL FIELD

[0001] The present inventors are involved in the technical field of positive electrode active materials, in particular to a positive electrode active material and a preparation method and application thereof. BACKGROUND

[0002] High-voltage use is an important development direction of positive electrode active materials (such as medium-high nickel ternary materials). With the increase of voltage, the delithiation amount of the positive electrode active material increases, and in the charging and discharging process, greater non-uniform stress is generated inside the positive electrode active material particles, which is easy to cause micro-cracks in the positive electrode active material particles, causing the collapse of the structure of the positive electrode active material, seriously damaging the structure of the positive electrode active material particles, and greatly reducing the electrochemical performance of the positive electrode active material.

[0003] Therefore, it is extremely important to improve the stability of the structure in the charging and discharging process for high-voltage polycrystalline materials. SUMMARY

[0004] The present application provides a positive electrode active material and a preparation method and application thereof, which helps to improve the structural stability of the positive electrode active material in the charging and discharging process.

[0005] The present application provides a positive electrode active material, which comprises an inner layer and an outer layer from inside to outside in the radial direction, the inner layer comprises a third circle layer surrounding a cavity, the outer layer comprises a second circle layer located on one side of the third circle layer away from the cavity, and a first circle layer located on one side of the second circle layer away from the third circle layer; the positive electrode active material is composed of primary particles, the second circle layer is a hollow structure comprising the primary particles; along the radial direction from inside to outside, the radius of the positive electrode active material is r1, the width of the second circle layer is r2-r3, the width of the cavity is r4, the average hollow thickness ratio of the second circle layer is D1=(r2-r3) / r1, and the average hollow thickness ratio of the cavity is D2=r4 / r1, D1 and D2 satisfy 0

[0006] Optionally, the cavity does not include the primary particles.

[0007] Optionally, the change rate of D50 of the positive electrode active material is less than or equal to 5% after being kept under 3.0-3.5T pressure for 30s.

[0008] Optionally, the chemical formula of the positive electrode active material is Li x Ni a Co b M c N e A fO2, wherein, 0.95≤x≤1.1, 0.5≤a<1, 0≤b≤0.2, 0≤c≤0.5, a+b+c+e=1, 0<e≤0.01, 0<f≤0.01, M includes Mn and / or Al, N includes one or more of Nb, Ti, Sr, Sb, Y, Mg, W, Mo, Zr and Al, A includes one or more of Al, W, Ti, Zr; and / or, the D50 of the positive electrode active material is 10-18 μm.

[0009] The application provides a preparation method of the positive electrode active material, comprising: 1) mixing a metal salt solution comprising nickel, cobalt and manganese, sodium hydroxide and ammonia water to obtain a mixed solution, the pH of the mixed solution is 11.0-12.3, the ammonia concentration is 2-15 g / L, the mixed solution is subjected to a first reaction to obtain a first mixed system, then the pH of the first mixed system is lowered and the ammonia concentration in the first mixed system is increased to perform a second reaction to obtain a second mixed system; 2) adjusting the pH of the second mixed system to 10.5-11.5 and the ammonia concentration to 2-8 g / L to perform a third reaction to obtain a third mixed system, then increasing the ammonia concentration in the third mixed system to perform a fourth reaction to obtain a hydroxide precursor; wherein the ratio of the time of the first reaction to the sum of the time of the first reaction, the second reaction, the third reaction and the fourth reaction is D3, the ratio of the time of the third reaction to the sum of the time of the first reaction, the second reaction, the third reaction and the fourth reaction is D4, D3 and D4 satisfy 15%<D3≤40% and 2.5%≤D4≤15%; 3) mixing the hydroxide precursor and a raw material system comprising lithium salt to obtain a mixed material; 4) performing first sintering on the mixed material at 300-650 ℃, then cooling to 20-25 ℃ to obtain the pre-lithiated lithium nickel cobalt manganese oxide; 5) performing second sintering on the pre-lithiated lithium nickel cobalt manganese oxide at 550-750 ℃, then increasing the temperature to 850-950 ℃ to perform third sintering, then performing fourth sintering at 780-880 ℃ to obtain the positive electrode active material, wherein the temperature of the fourth sintering is lower than that of the third sintering.

[0010] Optionally, the temperature of the first reaction is 50-75 DEG C; and / or, the temperature of the second reaction is 50-75 DEG C; and / or, the temperature of the third reaction is 50-75 DEG C; and / or, the temperature of the fourth reaction is 50-75 DEG C; and / or, in step 1), the first reaction and the second reaction are carried out under stirring at a rotation speed of 300-500 rpm; and / or, in step 2), the third reaction and the fourth reaction are carried out under stirring at a rotation speed of 100-400 rpm; the raw material system in step 3) further comprises a doping element, the doping element comprising one or more of Nb, Ti, Sr, Sb, Y, Mg, W, Mo, Zr and Al; and / or, the time of the first sintering is 4-15 h; and / or, the time of the second sintering is 2-4 h; and / or, the time of the third sintering is 8-15 h; and / or, the time of the fourth sintering is 3-5 h.

[0011] Optionally, the method further comprises: crushing the positive electrode active material, then mixing the positive electrode active material with a coating material, and then performing a fifth sintering at 300-700 DEG C to obtain a coated positive electrode active material, wherein the time of the fifth sintering is 5-12 h.

[0012] Optionally, step 4) comprises: heating the mixture to 300-650 DEG C at a rate of 1-5 DEG C / min for first sintering, and then cooling to 20-25 DEG C to obtain the pre-lithiated lithium nickel cobalt manganese oxide; and / or, step 5) comprises: heating the pre-lithiated lithium nickel cobalt manganese oxide to 550-750 DEG C at a rate of 1-5 DEG C / min for second sintering, then heating to 850-950 DEG C at a rate of 1-3 DEG C / min for third sintering, then cooling at a rate of 1-3 DEG C / min, and performing fourth sintering at 780-880 DEG C, to obtain the positive electrode active material, wherein the temperature of the fourth sintering is lower than the temperature of the third sintering.

[0013] The application provides a positive electrode sheet, which comprises the positive electrode active material as described above or obtained by the preparation method as described above.

[0014] The application provides a lithium ion battery, which comprises the positive electrode sheet as described above.

[0015] The present application provides a positive electrode active material and a preparation method and application thereof. The positive electrode active material has a hollow structure similar to concentric circles, i.e., a gradient hollow distribution structure from inside to outside. The hollow structure can provide a release space for stress generated in the positive electrode active material during charging and discharging. By controlling the thickness of the hollow layer (cavity and second layer) within the above range, the positive electrode active material can maintain structural stability under stress, thereby significantly enhancing the structural stability of the positive electrode active material, improving the first coulomb efficiency and cycle performance, and facilitating the full contact of the positive electrode active material with electrolyte, improving the lithium ion conduction rate, reducing impedance, and improving the rate performance. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments or prior art description will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0017] Figure 1 A cross-sectional schematic diagram of the positive electrode active material provided by the embodiments of the present application;

[0018] Figure 2 A cross-sectional SEM diagram of the hydroxide precursor of Example 1;

[0019] Figure 3 A cross-sectional SEM diagram of the positive electrode active material of Example 1;

[0020] Figure 4 A cross-sectional SEM diagram of the positive electrode active material of Comparative Example 1.

[0021] Explanation of reference signs:

[0022] 10-first layer, 20-second layer, 30-third layer, 40-cavity, 1-circle 1, 2-circle 2, 3-circle 3, 4-circle 4. DETAILED DESCRIPTION

[0023] In order to make those skilled in the art better understand the technical solutions of the present application, the present application will be further described in detail. The following specific embodiments are only used to describe the principles and characteristics of the present application, and the examples are used to explain the present application, but not to limit the scope of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0024] In the prior art, greater uneven stress is generated inside the positive electrode active material particles during the charging and discharging process, which easily leads to micro-cracks in the positive electrode active material particles, causes the collapse of the structure of the positive electrode active material, severely damages the structure of the positive electrode active material particles, and greatly reduces the electrochemical performance of the positive electrode active material.

[0025] In order to overcome the defects in the prior art, the embodiments of the present application provide a positive electrode active material, which comprises, in the radial direction from inside to outside, an inner layer and an outer layer in sequence, the inner layer comprises a third layer surrounding a cavity, the outer layer comprises a second layer located on the side of the third layer away from the cavity and a first layer located on the side of the second layer away from the third layer; the positive electrode active material is composed of primary particles, the second layer is a hollow structure comprising primary particles; in the radial direction from inside to outside, the radius of the positive electrode active material is r1, the width of the second layer is r2-r3, the width of the cavity is r4, the average hollow thickness ratio of the second layer is D1=(r2-r3) / r1, the average hollow thickness ratio of the cavity is D2=r4 / r1, and D1 and D2 satisfy 0

[0026] According to the research and analysis of the inventor, as shown in Figure 1 The positive electrode active material comprises, in the radial direction from inside to outside, a cavity (40), a third layer (30) surrounding the cavity (40), a second layer (20) located on the side of the third layer (30) away from the cavity (40), and a first layer (10) located on the side of the second layer (20) away from the third layer (30), wherein the positive electrode active material is composed of primary particles, and the second layer (20) is a hollow structure comprising primary particles; the cavity (40) and the second layer (20) can serve as hollow structure regions in the positive electrode active material, so that the positive electrode active material can be regarded as having a hollow structure similar to concentric circles, i.e., a structure with a gradient hollow distribution from inside to outside. This hollow structure can provide a release space for the stress generated in the positive electrode active material during the charging and discharging process, and by controlling the thickness of the hollow layer (cavity (40) and second layer (20)) within the above range, the structure of the positive electrode active material can be kept stable under the action of stress, the structural stability of the positive electrode active material is significantly enhanced, and the first coulombic efficiency and cycle performance are improved. In addition, this structure of the positive electrode active material is also beneficial to the full contact of the positive electrode active material with the electrolyte, improves the conduction rate of lithium ions, helps to reduce the impedance, and improves the rate performance.

[0027] Therefore, the positive electrode active material of the present invention has strong structural stability and can maintain high structural stability during charging and discharging, which helps to improve the first coulombic efficiency and cycle performance of the battery. In addition, the lithium-ion conduction rate of the above positive electrode active material is high, which helps to reduce impedance and improve rate performance.

[0028] The positive electrode active material in this invention can be a near-spherical or spherical particle, such as... Figure 1 As shown, circle 1 (1) is located on the outer edge of the first layer (10), circle 2 (2) is located on the outer edge of the second layer (20), circle 3 (3) is located on the outer edge of the third layer (30), and circle 4 (4) is located on the outer edge of the cavity (40). The diameters of circles 1, 2, 3 and 4 are d1, d2, d3 and d4 respectively. The average hollow thickness ratio D1 of the second layer can also be expressed as (d2-d3) / d1, and the average hollow thickness ratio D2 of the cavity can also be expressed as d4 / d1.

[0029] For example, the average hollow thickness percentage D1 of the second concentric ring can be a range of 1%, 5%, 10%, 15%, 16%, or any two of these.

[0030] For example, the average hollow thickness percentage D2 of the cavity can be a range of 10%, 12%, 14%, 16%, 18%, 20%, or any combination thereof.

[0031] The aforementioned positive electrode active material can be composed of primary particles.

[0032] In some embodiments, the cavity of the inner layer of the positive electrode active material does not contain primary particles, that is, the cavity is a hollow structure, which helps to provide space for the stress generated by the positive electrode active material during charging and discharging, thereby improving the structural stability of the positive electrode active material.

[0033] In some embodiments, the second layer of the outer layer of the above-mentioned positive electrode active material includes primary particles, i.e., as shown in the figure. Figure 1 The second ring (20) between circles 2 and 3 shown contains connecting pillars to connect the first ring (10) and the third ring (30), which provides stress relief space, improves lithium ion transport efficiency, and maintains the stability of the positive electrode active material structure.

[0034] Furthermore, in some embodiments, as can be seen from the cross-sectional view of the positive electrode active material particles, the support structure in the second layer (hollow layer) divides the second layer (hollow layer) into multiple chambers, the number of which is n, where n≥1, which can improve the stress resistance performance of the positive electrode active material and thus improve its structural stability.

[0035] The change rate (ΔD50) of D50 of the positive electrode active material after maintaining under 3.0-3.5T pressure for 30s is less than or equal to 5%, which indicates that the positive electrode active material of the embodiment has good particle strength even if the structure has a hollow.

[0036] The change rate (ΔD50) of D50 of the positive electrode active material after maintaining under 3.0-3.5T pressure for 30s is less than or equal to 5%, which indicates that the positive electrode active material of the embodiment has good particle strength even if the structure has a hollow. 压前 -D50 压后 ) / D50 压前 , that is, first calculate the difference between D50 of the positive electrode active material before the pressure is applied (D50 压前 ) and D50 of the positive electrode active material after the pressure is applied (D50 压后 ), and then calculate the ratio of the difference to D50 压前 , to obtain the change rate (ΔD50) of D50.

[0037] In some embodiments, the D50 of the positive electrode active material is 10-18μm. Within the range of D50, the positive electrode active material has better structural stability.

[0038] The positive electrode material can be a polycrystalline ternary positive electrode material.

[0039] In some embodiments, the chemical formula of the positive electrode material is Li x Ni a Co b M c N e A f O2, wherein 0.95≤x≤1.1, 0.5≤a<1, 0≤b≤0.2, 0≤c≤0.5, a+b+c+e=1, 0<e≤0.01, 0<f≤0.01, M includes Mn and / or Al, N includes one or more of Nb, Ti, Sr, Sb, Y, Mg, W, Mo, Zr and Al, and A includes one or more of Al, W, Ti and Zr.

[0040] The application also provides a preparation method of the positive electrode active material, comprising: 1) mixing a metal salt solution comprising nickel, cobalt and manganese, sodium hydroxide and ammonia water to obtain a mixed solution, wherein the pH of the mixed solution is 11.0-12.3 and the ammonia concentration is 2-15 g / L, and the mixed solution is subjected to a first reaction to obtain a first mixed system, then the pH of the first mixed system is lowered and the ammonia concentration in the first mixed system is increased, and a second reaction is performed to obtain a second mixed system; 2) adjusting the pH of the second mixed system to 10.5-11.5 and the ammonia concentration to 2-8 g / L, and performing a third reaction to obtain a third mixed system, then increasing the ammonia concentration in the third mixed system, and performing a fourth reaction to obtain a hydroxide precursor; wherein the ratio of the time of the first reaction to the sum of the times of the first reaction, the second reaction, the third reaction and the fourth reaction is D3, the ratio of the time of the third reaction to the sum of the times of the first reaction, the second reaction, the third reaction and the fourth reaction is D4, and D3 and D4 satisfy 15% < D3 ≤ 40% and 2.5% ≤ D4 ≤ 15%; 3) mixing the hydroxide precursor and a raw material system comprising a lithium salt to obtain a mixture; 4) performing a first sintering on the mixture at 300-650 ℃, and then cooling to 20-25 ℃ to obtain a pre-lithiated lithium nickel cobalt manganese oxide; 5) performing a second sintering on the pre-lithiated lithium nickel cobalt manganese oxide at 550-750 ℃, then increasing the temperature to 850-950 ℃ to perform a third sintering, and then performing a fourth sintering at 780-880 ℃ to obtain the positive electrode active material, wherein the temperature of the fourth sintering is lower than that of the third sintering.

[0041] The steps 1) and 2) in the above preparation method can be understood as a process of preparing the hydroxide precursor by using the precipitation method, which is specifically divided into the step 1) - nucleation stage and the step 2) - growth stage; the steps 3), 4) and 5) can be understood as a process of preparing the positive electrode active material by using the above hydroxide precursor.

[0042] In the nucleation stage, the sodium hydroxide is used as a precipitant and the ammonia water is used as a complexing agent, the pH of the mixed solution is controlled to be 11.0-12.3 and the ammonia concentration is controlled to be 2-15 g / L by adjusting the flow rates of the sodium hydroxide and the ammonia water, then the first reaction is performed, after the first reaction is completed, the pH of the first mixed system is lowered and the ammonia concentration in the first mixed system is increased, and the second reaction is performed. By adjusting the pH in the second reaction stage to be lower than that in the first reaction stage and the ammonia concentration in the second reaction stage to be higher than that in the first reaction stage, an inner layer structure with loose inside and dense outside (the inner core of the precursor) can be formed, which lays a foundation for forming the structure of the above positive electrode active material.

[0043] Exemplarily, the pH of the nucleation stage can be 11.0, 11.5, 12.0, 12.3, or a range consisting of any two of them, and the ammonia concentration can be 2 g / L, 5 g / L, 10 g / L, 15 g / L, or a range consisting of any two of them.

[0044] In the growth stage, the pH of the second mixed system is controlled to be 10.5-11.5 and the ammonia concentration is controlled to be 2-8 g / L by adjusting the flow rates of sodium hydroxide and ammonia water, a third reaction is carried out, and after the third reaction is completed, the ammonia concentration in the third mixed system is increased, and a fourth reaction is carried out. By adjusting the ammonia concentration in the fourth reaction stage to be higher than that in the third reaction stage, an outer layer structure with a loose inner layer and a dense outer layer can be grown and formed on the above-mentioned inner layer structure, laying a foundation for forming the structure of the above-mentioned positive electrode active material.

[0045] Exemplarily, the pH of the growth stage can be 10.5, 11.0, 11.5, or a range consisting of any two of them, and the ammonia concentration can be 2 g / L, 5 g / L, 6 g / L, 8 g / L, or a range consisting of any two of them.

[0046] By controlling the time of the above-mentioned first reaction and third reaction to satisfy 15% < D3 ≤ 40%, 2.5% ≤ D4 ≤ 15%, it is helpful to form a positive electrode active material with a cavity and a second layer thickness satisfying 0 < D1 ≤ 16%, 10% ≤ D2 ≤ 20%.

[0047] By controlling the precipitation reaction process of the above-mentioned step 1) and step 2), a nickel-cobalt-manganese hydroxide precursor with a specific structure (loose inside and tight outside, loose first and tight later in the growth stage) is obtained, which is conducive to forming a positive electrode active material with a specific hollow structure in the sintering process.

[0048] In step 3), the above-mentioned hydroxide precursor and a raw material system including lithium salt are thoroughly mixed to obtain a mixed material.

[0049] In step 4), the above-mentioned mixed material is subjected to first sintering at 300-650°C, and then cooled to 20-25°C to obtain a pre-lithiated lithium nickel cobalt manganese oxide. Exemplarily, the temperature of the above-mentioned first sintering can be 300°C, 400°C, 500°C, 600°C, 650°C, or a range consisting of any two of them.

[0050] After the first sintering is completed, the material can be cooled by natural cooling to room temperature, for example, 20-25°C, to obtain a pre-lithiated lithium nickel cobalt manganese oxide.

[0051] In step 5), the pre-lithiated lithium nickel cobalt manganese oxide (secondary ball) is subjected to a second sintering at 550-750°C, and then a third sintering at 850-950°C, and then a fourth sintering at 780-880°C, wherein the temperature of the fourth sintering is lower than that of the third sintering. In step 5), the sintering temperature (holding platform temperature) is controlled to control the diffusion speed of the lithium salt inside the pre-lithiated lithium nickel cobalt manganese oxide (secondary ball), so that the lithium salt is deposited orderly inside the precursor and in-situ growth is achieved during the sintering process (high temperature section), forming a positive electrode active material with a hollow structure having an inner layer (inner ring layer) and an outer layer (outer ring layer).

[0052] Exemplarily, the temperature of the second sintering can be 550°C, 600°C, 650°C, 700°C, 750°C, or a range formed by any two of them, the temperature of the third sintering can be 850°C, 900°C, 950°C, or a range formed by any two of them, and the temperature of the fourth sintering can be 780°C, 800°C, 850°C, 880°C, or a range formed by any two of them.

[0053] The positive electrode active material with the aforementioned similar concentric hollow structure can be prepared by the above preparation method, which has strong structural stability, helps to improve the first coulombic efficiency and cycle performance of the battery, and in addition, has a high lithium ion conduction rate, which helps to reduce the impedance and improve the rate performance.

[0054] The embodiment of the present application is not limited to the raw materials for providing nickel, cobalt and manganese in the metal salt solution containing nickel, cobalt and manganese, and for example, nickel sulfate, cobalt sulfate and manganese sulfate can be used as raw materials.

[0055] In some embodiments, the metal salt solution containing nickel, cobalt and manganese (mixed metal salt solution) can be prepared by the following process: using water (e.g. pure water) as a solvent, and using nickel sulfate, cobalt sulfate and manganese sulfate as raw materials to prepare.

[0056] In step 1), the metal salt solution containing nickel, cobalt and manganese (mixed metal salt solution), NaOH solution and ammonia solution can also be simultaneously added to the reaction kettle through the mass flow meter.

[0057] Generally, the feeding speed of the mixed metal salt solution during the nucleation stage can be controlled to be 70-90 mL / min, for example, 70 mL / min, 80 mL / min, 90 mL / min, or a range formed by any two of them.

[0058] The temperature of the first reaction can be 50-75°C; and / or, the temperature of the second reaction can be 50-75°C; and / or, the temperature of the third reaction can be 50-75°C; and / or, the temperature of the fourth reaction can be 50-75°C. By controlling the temperature of the first reaction, the second reaction, the third reaction and the fourth reaction to be within the above range, it is helpful to avoid abnormality in morphology, primary particles, structure, etc. during the precursor synthesis process, further ensuring that the positive electrode material with a similar concentric circle hollow structure of the present application can be synthesized.

[0059] In addition, in step 1), the first reaction and the second reaction can be carried out under stirring at a rotation speed of 300-500 rpm. It is helpful to improve the above structure of the positive electrode active material, improve its structural stability and lithium ion conduction rate, and further improve the first coulombic efficiency, cycle performance, rate performance of the battery, and reduce the impedance. Illustratively, the above rotation speed can be 300 rpm, 400 rpm, 500 rpm, or a range consisting of any two of them.

[0060] In step 2), the third reaction and the fourth reaction can be carried out under stirring at a rotation speed of 100-400 rpm. It is helpful to improve the above structure of the positive electrode active material, improve its structural stability and lithium ion conduction rate, and further improve the first coulombic efficiency, cycle performance, rate performance of the battery, and reduce the impedance. Illustratively, the above rotation speed can be 100 rpm, 200 rpm, 300 rpm, 400 rpm, 500 rpm, or a range consisting of any two of them.

[0061] Generally, the growth stage can control the feeding speed of the mixed metal salt solution to be 90-110 mL / min, for example, 90 mL / min, 100 mL / min, 110 mL / min, or a range consisting of any two of them.

[0062] The lithium salt in step 3) can include lithium salts with low melting points, for example, can include one or more of lithium hydroxide, lithium nitrate, lithium acetate.

[0063] In addition, the raw material system in step 3) can also include a doping element (dopant), which can include one or more of Nb, Ti, Sr, Sb, Y, Mg, W, Mo, Zr and Al. After adding the above-mentioned doping elements, a lithium nickel cobalt manganese metal oxide positive electrode active material containing element doping can be prepared, further improving the stability of the positive electrode active material.

[0064] The step 4) further comprises: heating the mixture to 300-650℃ at a rate of 1-5℃ / min for first sintering, and then cooling to 20-25℃ to obtain the pre-lithiated lithium nickel cobalt manganese oxide. Limiting the heating rate within the above range helps to improve the above structure of the positive electrode active material, improve its structural stability and lithium ion conduction rate, and further improve the first coulombic efficiency, cycle performance, rate performance of the battery, and reduce the impedance. Illustratively, the above heating rate can be 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, or a range formed by any two of them.

[0065] In addition, the time (holding time) of the first sintering can be 4-15h, for example, 4h, 5h, 6h, 10h, 11h, 14h, 15h, or a range formed by any two of them.

[0066] The step 5) further comprises: heating the pre-lithiated lithium nickel cobalt manganese oxide to 550-750℃ at a rate of 1-5℃ / min for second sintering, then heating to 850-950℃ at a rate of 1-3℃ / min for third sintering, then cooling at a rate of 1-3℃ / min, and performing fourth sintering at 780-880℃ to obtain the positive electrode active material, wherein the temperature of the fourth sintering is lower than that of the third sintering.

[0067] After the fourth sintering is completed, the material can be naturally cooled to room temperature to obtain the positive electrode active material.

[0068] In addition, the time (holding time) of the second sintering can be 2-4h, for example, 2h, 3h, 4h, or a range formed by any two of them, the time (holding time) of the third sintering can be 8-15h, for example, 8h, 10h, 12h, 14h, 15h, or a range formed by any two of them, and the time (holding time) of the fourth sintering can be 3-5h, for example, 3h, 4h, 5h, or a range formed by any two of them. By controlling the sintering temperature (holding platform temperature) while controlling the holding time within the above corresponding range, the diffusion speed of the lithium salt inside the pre-lithiated lithium nickel cobalt manganese oxide (secondary ball) can be controlled, so that the lithium salt is deposited orderly inside the precursor and grows in situ during the sintering process (high temperature section), forming a positive electrode active material with both inner layer (inner ring layer) and outer layer (outer ring layer) hollow structures.

[0069] The first sintering, the second sintering, the third sintering, the fourth sintering, and the fifth sintering of the embodiments of the present application can all be performed in an oxygen or air atmosphere.

[0070] In some embodiments, the preparation method further comprises: crushing (fragmentation treatment) the positive electrode active material, then mixing the positive electrode active material with the coating material, and then performing a fifth sintering at 300-700℃ to obtain the coated positive electrode active material, wherein the time (holding time) of the fifth sintering is 5-12h. A coating layer with a certain thickness is formed on the surface and inside of the positive electrode active material, so that the stability of the bulk phase and the inner and outer surfaces is improved.

[0071] Further, in the process of the fifth sintering, the mixture can be heated at a rate of 1-10℃ / min to 300-700℃ for the fifth sintering.

[0072] Understandably, after the completion of the fifth sintering, the sintered material can be naturally cooled to room temperature, and then sieved to obtain the positive electrode active material.

[0073] In some embodiments, the positive electrode active material with a certain hollow structure is obtained by high-temperature solid-phase sintering using the aforementioned special-structure hydroxide precursor and low-melting-point lithium salt, and the hollow structure is similar to a concentric circle. In addition, the bulk phase is doped and the surface is coated (for example, wet coating), a coating layer with a certain thickness is formed on the surface and inside of the positive electrode active material, which protects the internal structure and the inner and outer interfaces of the positive electrode active material, so that the stability of the bulk phase and the inner and outer surfaces is improved, which not only accelerates the transmission efficiency of lithium ions, but also maintains the integrity of the structure of the positive electrode active material in long-term cycling.

[0074] Based on the same inventive concept, the embodiments of the present application also provide a positive electrode sheet, which comprises the positive electrode active material or the positive electrode active material obtained by the aforementioned preparation method.

[0075] The positive electrode sheet of the embodiments of the present application specifically comprises a positive electrode current collector and a positive electrode active layer formed by the aforementioned positive electrode active material arranged on the surface of the positive electrode current collector.

[0076] In the preparation of the positive electrode sheet, for example, the aforementioned positive electrode active material of the present application, a conductive agent and a binder can be dispersed in an appropriate amount of N-methyl pyrrolidone (NMP) solvent, and stirred sufficiently to form a uniform positive electrode slurry; the positive electrode slurry is uniformly coated on the positive electrode current collector, and then dried, rolled and cut to obtain the positive electrode sheet. For example, the positive electrode active layer comprises 70-99wt% of the positive electrode active material, 0.5-15wt% of the conductive agent and 0.5-15wt% of the binder in terms of mass percentage, and further comprises 80-98wt% of the positive electrode active material, 1-10wt% of the conductive agent and 1-10wt% of the binder.

[0077] The material of the above positive current collector can be at least one of an aluminum foil and a nickel foil; the conductive agent can be at least one of carbon black, acetylene black, graphene, ketjen black, and carbon fiber; and the binder can be at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, an oxirane-containing polymer, polyvinylpyrrolidone, and polyurethane.

[0078] The battery prepared by using the positive electrode sheet has excellent coulomb efficiency, cycle performance, rate performance, and low impedance.

[0079] Based on the same inventive concept, the embodiment of the present application provides a lithium ion battery comprising the above positive electrode sheet. It is conceivable that the lithium ion battery of the embodiment of the present application comprises a negative electrode sheet, an electrolyte, and a separator in addition to the above positive electrode sheet.

[0080] The embodiment of the present application is not strictly limited to the negative active material in the negative electrode sheet, and can be at least one of the negative active materials commonly used in the lithium ion battery, such as graphite, hard carbon, soft carbon, mesocarbon microbeads, silicon-based negative materials (mainly including silicon monoxide and silicon-carbon negative electrodes), tin-based negative materials (mainly including tin and tin alloy), and the like.

[0081] The embodiment of the present application is not strictly limited to the selection of the electrolyte, and can comprise one or more of the solvents commonly used in the electrolyte of the lithium ion battery, and the electrolyte lithium salt commonly used in the lithium ion electrolyte, for example: the solvent can be ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, methyl ethyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, gamma-butyrolactone, and the like; and the lithium salt can be one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0082] The embodiment of the present application is not strictly limited to the material selection of the separator, and can be one of the separator materials commonly used in the lithium ion battery, such as a polypropylene separator (PP), a polyethylene separator (PE), a polypropylene / polyethylene double-layer composite film (PP / PE), a polyimide electrospun separator (PI), a polypropylene / polyethylene / polypropylene three-layer composite film (PP / PE / PP), a cellulose non-woven fabric separator, and a ceramic-coated separator.

[0083] In the preparation of lithium ion battery, the positive electrode sheet, the separator and the negative electrode sheet are wound or laminated to obtain a bare battery cell, and the bare battery cell is packaged into an aluminum-plastic film bag which is pre-punched and formed. After the packaged battery is dried at 85℃, the electrolyte is injected into the dried battery, and the battery is completed after standing, formation and secondary sealing. The lithium ion battery has excellent coulomb efficiency, cycle performance, rate performance and low impedance.

[0084] The application will be further described below by specific examples and comparative examples. Unless otherwise specified, the reagents, materials and instruments used in the following are conventional reagents, conventional materials and conventional instruments, which are commercially available. The reagents and materials involved can also be synthesized by conventional synthesis methods.

[0085] Example 1

[0086] The embodiment provides a preparation method of a positive electrode active material, comprising:

[0087] 1) taking pure water as a solvent, nickel sulfate, cobalt sulfate and manganese sulfate as raw materials, and preparing a metal salt solution (mixed metal salt solution) containing nickel, cobalt and manganese with a metal ion concentration of 2 mol / L according to Ni 2+ :Co 2+ :Mn 2+ (molar ratio) = 6:2:2, an NaOH solution with an NaOH concentration of 2 mol / L and an ammonia water solution with an ammonia concentration of 4.3 mol / L are prepared at the same time; the mixed metal salt solution, the NaOH solution and the ammonia water solution are simultaneously added to a reaction kettle for mixing through a mass flow meter, a mixed solution is obtained, the feeding speed of the mixed metal salt solution is controlled to be 80 mL / min, the pH of the mixed solution is controlled to be 11.8 and the ammonia concentration is controlled to be 2.5 g / L by adjusting the flow of sodium hydroxide and ammonia water, the mixed solution is subjected to a first reaction, a first mixed system is obtained, after the first reaction is performed for 30 h, the pH of the first mixed system is adjusted to be 11.6 and the ammonia concentration is adjusted to be 3.5 g / L, a second reaction is performed, the time of the second reaction is 30 h, a second mixed system is obtained; wherein the above first reaction and the second reaction are performed at a rotating speed of 400 rpm and a temperature of 55℃;

[0088] 2) the feeding speed of the mixed metal salt solution is adjusted to be 100 mL / min, the pH of the second mixed system is adjusted to be 10.8 and the ammonia concentration is adjusted to be 3 g / L, a third reaction is performed, a third mixed system is obtained, after the third reaction is performed for 10 h, the pH of the third mixed system is increased to be 10.9 and the ammonia concentration is increased to be 4 g / L, a fourth reaction is performed, the time of the fourth reaction is 40 h, and a hydroxide precursor (Ni 0.6 Co 0.2 Mn 0.2(OH)2 precursor); wherein, the ratio of the time of the first reaction to the sum of the times of the first reaction, the second reaction, the third reaction, and the fourth reaction is D3 = 27.27%, the ratio of the time of the third reaction to the sum of the times of the first reaction, the second reaction, the third reaction, and the fourth reaction is D4 = 9.09%, the third reaction and the fourth reaction are performed at a rotation speed of 200 rpm and a temperature of 55°C;

[0089] 3) The hydroxide precursor, lithium hydroxide, and zirconium oxide are mixed in a molar ratio of 1:1.08:0.002 to obtain a mixture;

[0090] 4) The mixture is heated to 500°C at a rate of 3°C / min under an oxygen atmosphere for the first sintering, and then cooled to room temperature, to obtain a pre-lithiated lithium nickel cobalt manganese oxide;

[0091] 5) The pre-lithiated lithium nickel cobalt manganese oxide is heated to 600°C at a rate of 3°C / min under an oxygen atmosphere for the second sintering, and then heated to 900°C at a rate of 2°C / min for the third sintering, and then cooled to 800°C at a rate of 2°C / min for the fourth sintering, and then naturally cooled to room temperature, to obtain a positive electrode active material (doped modified positive electrode active material);

[0092] 6) The positive electrode active material is crushed, and then the crushed positive electrode active material and aluminum oxide are mixed in a molar ratio of 1:0.0005, and then the mixture is heated to 550°C at a rate of 3°C / min under an oxygen atmosphere for the fifth sintering, and then naturally cooled to room temperature, and then sieved to obtain a positive electrode active material with a hollow structure.

[0093] It is detected that the D50 of the positive electrode active material is 13.0 μm, the average hollow thickness ratio D1 of the second circle layer is 10%, the average hollow thickness ratio D2 of the cavity is 13%, and the ΔD50 is 2.38% after applying a pressure of 3.5T.

[0094] Example 2

[0095] This example is basically the same as Example 1, except that:

[0096] The time of the first reaction is 42h, the time of the second reaction is 18h, the time of the third reaction is 10h, and the time of the fourth reaction is 50h, i.e., the ratio of the time of the first reaction to the sum of the times of the first reaction, the second reaction, the third reaction, and the fourth reaction is D3 = 35%, and the ratio of the time of the third reaction to the sum of the times of the first reaction, the second reaction, the third reaction, and the fourth reaction is D4 = 8.33%; other conditions remain unchanged.

[0097] The particle size of the hydroxide precursor of Example 2 was 16.5 μm, the D50 of the positive electrode active material was 16.0 μm, the average hollow thickness ratio D1 of the second circle layer was 6%, the average hollow thickness ratio D2 of the cavity was 17%, and the ΔD50 was 3.84% after applying a pressure of 3.5T.

[0098] Example 3

[0099] This example is basically the same as Example 1, except that:

[0100] The pH of the mixed solution in step 1) was 11.3, and the ammonia concentration was 2 g / L. After the first reaction was completed, the pH of the first mixed system was adjusted to 11, and the ammonia concentration was adjusted to 3 g / L.

[0101] The pH of the second mixed system in step 2) was 11.5, and the ammonia concentration was 6 g / L. After the third reaction was completed, the ammonia concentration of the third mixed system was adjusted to 8 g / L. Other conditions remained unchanged.

[0102] The D50 of the positive electrode active material was 13.0 μm, the average hollow thickness ratio D1 of the second circle layer was 7.5%, the average hollow thickness ratio D2 of the cavity was 10.6%, and the ΔD50 was 1.02% after applying a pressure of 3.5T.

[0103] Example 4

[0104] This example is basically the same as Example 1, except that:

[0105] The pH of the mixed solution in step 1) was 12.3, and the ammonia concentration was 14 g / L. The pH of the first mixed system was adjusted to 12.2, and the ammonia concentration was adjusted to 15 g / L.

[0106] The pH of the second mixed system in step 2) was 10.5, and the ammonia concentration was 2 g / L. After the third reaction was completed, the ammonia concentration of the third mixed system was adjusted to 3 g / L. Other conditions remained unchanged.

[0107] The D50 of the positive electrode active material was 13.0 μm, the average hollow thickness ratio D1 of the second circle layer was 14.7%, the average hollow thickness ratio D2 of the cavity was 18.4%, and the ΔD50 was 4.31% after applying a pressure of 3.5T.

[0108] Example 5

[0109] This example is basically the same as Example 1, except that:

[0110] The first reaction time is 16.5 h, the second reaction time is 43.5 h, the third reaction time is 16.5 h, and the fourth reaction time is 33.5 h, that is, the ratio of the first reaction time to the sum of the first reaction, second reaction, third reaction and fourth reaction times is D3=15%, and the ratio of the third reaction time to the sum of the first reaction, second reaction, third reaction and fourth reaction times is D4=15%; other conditions remain unchanged.

[0111] It is detected that the D50 of the positive electrode active material is 13.0 μm, the average hollow thickness ratio D1 of the second circle layer is 15.3%, the average hollow thickness ratio D2 of the cavity is 11.2%, and the ΔD50 after applying a pressure of 3.5T is 3.31%.

[0112] Example 6

[0113] This example is basically the same as Example 1, except that:

[0114] The first reaction time is 44 h, the second reaction time is 16 h, the third reaction time is 3 h, and the fourth reaction time is 47 h, that is, the ratio of the first reaction time to the sum of the first reaction, second reaction, third reaction and fourth reaction times is D3=40%, and the ratio of the third reaction time to the sum of the first reaction, second reaction, third reaction and fourth reaction times is D4=2.7%; other conditions remain unchanged.

[0115] It is detected that the D50 of the positive electrode active material is 13.0 μm, the average hollow thickness ratio D1 of the second circle layer is 2.8%, the average hollow thickness ratio D2 of the cavity is 19.6%, and the ΔD50 after applying a pressure of 3.5T is 2.89%.

[0116] Example 7

[0117] This example is basically the same as Example 1, except that:

[0118] Step 4) comprises: heating the above mixture to 300℃ at a rate of 1℃ / min under an air atmosphere for first sintering, and the holding time is 15 h, and then cooling to room temperature to obtain a pre-lithiated lithium nickel cobalt manganese oxide;

[0119] Step 5) comprises: heating the above pre-lithiated lithium nickel cobalt manganese oxide to 550℃ at a rate of 1℃ / min under an air atmosphere for second sintering, and the holding time is 4 h, then heating to 950℃ at a rate of 3℃ / min for third sintering, and the holding time is 8 h, then cooling to 880℃ at a rate of 1℃ / min for fourth sintering, and the holding time is 3 h, and then naturally cooling to room temperature to obtain a positive electrode active material (doped modified positive electrode active material);

[0120] Step 6) comprises: crushing the above positive electrode active material, then mixing the crushed positive electrode active material and aluminum oxide at a molar ratio of 1:0.0005, uniformly mixing, then under an oxygen atmosphere, heating the mixed material to 700℃ at a rate of 10℃ / min for fifth sintering, after 5h of heat preservation, naturally cooling to room temperature, then sieving to obtain a positive electrode active material with a hollow structure.

[0121] It is detected that the D50 of the positive electrode active material is 13.0μm, the average hollow thickness ratio D1 of the second circle layer is 8.95%, the average hollow thickness ratio D2 of the cavity is 11.69%, and the ΔD50 is 3.78% after applying a pressure of 3.5T.

[0122] Example 8

[0123] This example is basically the same as Example 1, with the difference being:

[0124] Step 4) comprises: under an air atmosphere, heating the above mixed material to 650℃ at a rate of 5℃ / min for first sintering, with a heat preservation time of 4h, then cooling to room temperature to obtain a pre-lithiated lithium nickel cobalt manganese oxide;

[0125] Step 5) comprises: under an air atmosphere, heating the above pre-lithiated lithium nickel cobalt manganese oxide to 750℃ at a rate of 5℃ / min for second sintering, with a heat preservation time of 2h, then heating to 850℃ at a rate of 1℃ / min for third sintering, with a heat preservation time of 15h, then cooling to 780℃ at a rate of 3℃ / min for fourth sintering, with a heat preservation time of 5h, then naturally cooling to room temperature to obtain a positive electrode active material (doped modified positive electrode active material);

[0126] 6) crushing the above positive electrode active material, then mixing the crushed positive electrode active material and aluminum oxide at a molar ratio of 1:0.0005, uniformly mixing, then under an oxygen atmosphere, heating the mixed material to 300℃ at a rate of 1℃ / min for fifth sintering, after 12h of heat preservation, naturally cooling to room temperature, then sieving to obtain a positive electrode active material with a hollow structure.

[0127] It is detected that the D50 of the positive electrode active material is 13.0μm, the average hollow thickness ratio D1 of the second circle layer is 12.42%, the average hollow thickness ratio D2 of the cavity is 15.43%, and the ΔD50 is 1.88% after applying a pressure of 3.5T.

[0128] Example 9

[0129] This example is basically the same as Example 1, with the difference being:

[0130] The time of the first reaction is 36 h, the time of the second reaction is 29 h, the time of the third reaction is 8 h, and the time of the fourth reaction is 57 h, that is, the ratio of the time of the first reaction to the sum of the times of the first reaction, the second reaction, the third reaction and the fourth reaction is D3 = 27.69%, and the ratio of the time of the third reaction to the sum of the times of the first reaction, the second reaction, the third reaction and the fourth reaction is D4 = 6.15%; other conditions remain unchanged.

[0131] It is detected that the particle size of the hydroxide precursor of Example 9 is 18.5 μm, the D50 of the positive electrode active material is 16.0 μm, the average hollow thickness ratio D1 of the second circle layer is 8.73%, the average hollow thickness ratio D2 of the cavity is 12.24%, and the ΔD50 is 2.73% after applying a pressure of 3.5 T.

[0132] Example 10

[0133] This example is basically the same as Example 1, except that:

[0134] The time of the first reaction is 27 h, the time of the second reaction is 28 h, the time of the third reaction is 8 h, and the time of the fourth reaction is 37 h, that is, the ratio of the time of the first reaction to the sum of the times of the first reaction, the second reaction, the third reaction and the fourth reaction is D3 = 27%, and the ratio of the time of the third reaction to the sum of the times of the first reaction, the second reaction, the third reaction and the fourth reaction is D4 = 8%; other conditions remain unchanged.

[0135] It is detected that the particle size of the hydroxide precursor of Example 10 is 10.5 μm, the D50 of the positive electrode active material is 10.0 μm, the average hollow thickness ratio D1 of the second circle layer is 9.86%, the average hollow thickness ratio D2 of the cavity is 15.73%, and the ΔD50 is 1.86% after applying a pressure of 3.5 T.

[0136] Example 11

[0137] This example is basically the same as Example 1, except that:

[0138] Step 1) adjusting Ni 2+ :Co 2+ :Mn 2+ (molar ratio) = 5:2:3 to obtain a hydroxide precursor (Ni 0.5 Co 0.2 Mn 0.3 (OH)2precursor) with a particle size of 13.5 μm; other conditions remain unchanged.

[0139] The particle size of the hydroxide precursor of Example 11 is 13.5 μm, the D50 of the positive electrode active material is 13.0 μm, the average hollow thickness ratio D1 of the second circle layer is 8.46%, the average hollow thickness ratio D2 of the cavity is 12.73%, and the ΔD50 is 1.77% after applying a pressure of 3.5T.

[0140] Example 12

[0141] This example is basically the same as Example 1, except that:

[0142] Step 1) adjusting Ni 2+ :Co 2+ :Mn 2+ (molar ratio) = 9:0.5:0.5, to obtain a hydroxide precursor (Ni 0.9 Co 0.05 Mn 0.05 (OH)2precursor) with a particle size of 13.5 μm, and other conditions remain unchanged.

[0143] The particle size of the hydroxide precursor of Example 12 is 13.5 μm, the D50 of the positive electrode active material is 13.0 μm, the average hollow thickness ratio D1 of the second circle layer is 11.26%, the average hollow thickness ratio D2 of the cavity is 14.77%, and the ΔD50 is 2.63% after applying a pressure of 3.5T.

[0144] Example 13

[0145] This example is basically the same as Example 1, except that:

[0146] In step 3), replace zirconium oxide with titanium dioxide, and other conditions remain unchanged.

[0147] Example 14

[0148] In step 6), replace aluminum oxide with tungsten oxide, and other conditions remain unchanged.

[0149] Comparative Example 1

[0150] This comparative example is basically the same as Example 1, except that:

[0151] Step 2) includes adjusting the feed rate of the mixed metal salt solution to 100 mL / min, and adjusting the pH of the second mixing system to 10.8 and the ammonia concentration to 10 g / L, and performing a third reaction for 50 h to obtain a hydroxide precursor (Ni 0.6 Co 0.2 Mn 0.2 (OH)2precursor) with a particle size of 13.5 μm;

[0152] Step 4) and Step 5) comprise: under an oxygen atmosphere, the above mixture is heated to 900°C at a rate of 3°C / min for first sintering, and the holding time is 10h, and then naturally cooled to room temperature to obtain the positive electrode active material (doped modified positive electrode active material); other conditions remain unchanged.

[0153] It is detected that the particle size of the hydroxide precursor of Comparative Example 1 is 13.5μm, the D50 of the positive electrode active material is 13.0μm, the average hollow thickness ratio D1 of the second circle layer is 0%, the average hollow thickness ratio D2 of the cavity is 21.2%, and the ΔD50 after applying a pressure of 3.5T is 6.93%.

[0154] Comparative Example 2

[0155] This comparative example is basically the same as Example 1, and the difference is that:

[0156] Step 1) comprises: taking pure water as the solvent, taking nickel sulfate, cobalt sulfate and manganese sulfate as raw materials, and preparing a metal salt solution (mixed metal salt solution) containing nickel, cobalt and manganese with a metal ion concentration of 2mol / L according to Ni 2+ :Co 2+ :Mn 2+ (molar ratio) = 6:2:2; at the same time, an NaOH solution with a NaOH concentration of 2mol / L and an ammonia water solution with an ammonia concentration of 4.3mol / L are prepared; the mixed metal salt solution, the NaOH solution and the ammonia water solution are simultaneously added to the reaction kettle for mixing through a mass flow meter, a mixed solution is obtained, the feeding speed of the mixed metal salt solution is controlled to be 80mL / min, the pH of the mixed solution is controlled to be 11.8 and the ammonia concentration is controlled to be 3.5g / L by adjusting the flow rates of sodium hydroxide and ammonia water, the mixed solution is subjected to first reaction, and the first reaction time is 60h; wherein, the above first reaction is carried out at a rotation speed of 400rpm and a temperature of 55°C; a first reaction system is obtained; then the first reaction system is subjected to reactions according to Steps 2) to 6) in Example 1 to obtain the positive electrode active material.

[0157] It is detected that the particle size of the hydroxide precursor of Comparative Example 2 is 13.5μm, the D50 of the positive electrode active material is 13.0μm, the average hollow thickness ratio D1 of the second circle layer is 9.3%, the average hollow thickness ratio D2 of the cavity is 4.7%, and the ΔD50 after applying a pressure of 3.5T is 2.15%.

[0158] Comparative Example 3

[0159] This comparative example is basically the same as Example 1, and the difference is that:

[0160] Step 1) comprises: taking pure water as the solvent, taking nickel sulfate, cobalt sulfate and manganese sulfate as raw materials, and preparing a metal salt solution (mixed metal salt solution) containing nickel, cobalt and manganese with a metal ion concentration of 2mol / L according to Ni2+ :Co 2+ :Mn 2+ (molar ratio) = 6:2:2, a metal salt solution (mixed metal salt solution) including nickel, cobalt, and manganese was prepared with a metal ion concentration of 2 mol / L, and a NaOH solution with a NaOH concentration of 2 mol / L and an ammonia solution with an ammonia concentration of 4.3 mol / L were prepared; the mixed metal salt solution, the NaOH solution, and the ammonia solution were simultaneously added to a reactor through a mass flow meter for mixing to obtain a mixed solution, the feeding speed of the mixed metal salt solution was controlled to be 80 mL / min, the pH of the mixed solution was controlled to be 11.3 and the ammonia concentration was controlled to be 3.5 g / L by adjusting the flow rates of the sodium hydroxide and the ammonia water, and the mixed solution was subjected to a first reaction, the time of the first reaction was 60 h; wherein the first reaction was carried out at a rotation speed of 400 rpm and a temperature of 55°C; a first reaction system was obtained;

[0161] Step 2) includes adjusting the feeding speed of the mixed metal salt solution to be 100 mL / min, adjusting the pH of the second mixed system to be 10.8 and the ammonia concentration to be 10 g / L, and performing a third reaction, the time of the third reaction being 50 h, to obtain a hydroxide precursor (Ni 0.6 Co 0.2 Mn 0.2 (OH)2precursor) with a particle size of 13.5 μm; other conditions remain unchanged.

[0162] It was detected that the particle size of the hydroxide precursor of Comparative Example 3 was 13.5 μm, the D50 of the positive electrode active material was 13.0 μm, the average hollow thickness ratio D1 of the second circle layer was 0%, the average hollow thickness ratio D2 of the cavity was 0.76%, and the ΔD50 was 0.97% after applying a pressure of 3.5T.

[0163] Comparative Example 4

[0164] This comparative example is basically the same as Example 1, the difference is that:

[0165] the time of the first reaction was 30 h, the time of the second reaction was 30 h, the time of the third reaction was 30 h, and the time of the fourth reaction was 10 h, that is, the ratio of the time of the first reaction to the sum of the times of the first reaction, the second reaction, the third reaction, and the fourth reaction was D3 = 27.27%, and the ratio of the time of the third reaction to the sum of the times of the first reaction, the second reaction, the third reaction, and the fourth reaction was D4 = 30% (greater than 15%); other conditions remain unchanged.

[0166] The particle size of the hydroxide precursor of Comparative Example 4 was 13.5 μm, the D50 of the positive electrode active material was 13.0 μm, the average hollow thickness ratio D1 of the second circle layer was 36.81%, the average hollow thickness ratio D2 of the cavity was 12.41%, and the ΔD50 was 7.46% after applying a pressure of 3.5 T.

[0167] Comparative Example 5

[0168] This comparative example is basically the same as Example 1, except that:

[0169] The time of the first reaction was 12 h, the time of the second reaction was 48 h, the time of the third reaction was 10 h, and the time of the fourth reaction was 40 h, i.e., the ratio of the time of the first reaction to the sum of the times of the first reaction, the second reaction, the third reaction, and the fourth reaction was D3 = 10.9 (less than 15%), and the ratio of the time of the third reaction to the sum of the times of the first reaction, the second reaction, the third reaction, and the fourth reaction was D4 = 9.09%; other conditions remained unchanged.

[0170] The particle size of the hydroxide precursor of Comparative Example 5 was 13.5 μm, the D50 of the positive electrode active material was 13.0 μm, the average hollow thickness ratio D1 of the second circle layer was 9.79%, the average hollow thickness ratio D2 of the cavity was 22.42%, and the ΔD50 was 6.98% after applying a pressure of 3.5 T.

[0171] Comparative Example 6

[0172] This comparative example is basically the same as Example 1, except that:

[0173] The time of the first reaction was 12 h, the time of the second reaction was 48 h, the time of the third reaction was 10 h, and the time of the fourth reaction was 40 h, i.e., the ratio of the time of the first reaction to the sum of the times of the first reaction, the second reaction, the third reaction, and the fourth reaction was D3 = 10.9 (less than 15%), and the ratio of the time of the third reaction to the sum of the times of the first reaction, the second reaction, the third reaction, and the fourth reaction was D4 = 9.09%; other conditions remained unchanged.

[0174] The particle size of the hydroxide precursor of Comparative Example 6 was 13.5 μm, the D50 of the positive electrode active material was 13.0 μm, the average hollow thickness ratio D1 of the second circle layer was 10.27%, the average hollow thickness ratio D2 of the cavity was 5.78%, and the ΔD50 was 3.79% after applying a pressure of 3.5 T.

[0175] Test Example 1

[0176] The following parameters of the positive electrode active materials of the above examples and comparative examples were detected:

[0177] 1) Particle size of hydroxide precursor, D50 of positive electrode active material: tested by Malvern, MasterSize 3000 particle size instrument.

[0178] 2) Average hollow thickness ratio D1 of the second circle layer, average hollow thickness ratio D2 of the cavity: D1 = (r2-r3) / r1, D2 = r4 / r1, r1 is the D50 of the positive electrode particles, r2, r3 and r4 are measured as follows, taking r2 as an example, taking a cross-sectional SEM image of the positive electrode particles, as shown in Figure 3 , randomly taking a point on the outer edge of the second circle layer (20) through the center point of the particle to the other end of the outer edge of the circle layer as a sample point, taking at least 10 points, and the software automatically calculates the average particle size, which is the value of r2, r3 and r4 are measured according to the above method.

[0179] 3) ΔD50: ΔD50 = (D50 压前 -D50 压后 ) / D50 压前 , D50 压前 represents the D50 of the positive electrode active material, and D50 压后 represents the D50 result after 3.5T pressure test.

[0180] 3) Scanning electron microscope (SEM) images of the hydroxide precursor and the positive electrode active material of Example 1, and the scanning electron microscope (SEM) images of the positive electrode active material of Comparative Example 1: respectively seen Figure 2 、 Figure 3 、 Figure 4 .

[0181] Test results

[0182] Table 1

[0183]

[0184]

[0185] Data analysis: the average hollow thickness ratio D1 of the second circle layer and the average hollow thickness ratio D2 of the cavity of the positive electrode active material of each of the above examples satisfy 0 < D1 ≤ 16%, 10% ≤ D2 ≤ 20%, and ΔD50 is less than or equal to 5%. Compared with the examples, the outer layer of the positive electrode active material of Comparative Example 1 does not have a hollow structure, and its structure stability is poor, and ΔD50 is 6.93%; the average hollow thickness of the outer layer of the positive electrode active material of Comparative Example 4 is too large, which is not conducive to maintaining the structure stability, and ΔD50 is 7.46%; the average hollow thickness of the inner layer of the positive electrode active material of Comparative Example 5 is too large, which is not conducive to maintaining the structure stability, and ΔD50 is 6.98%.

[0186] Test Example 2

[0187] After the positive electrode active materials of the examples and comparative examples were made into positive electrode sheets, the negative electrode sheets, electrolyte and separator were assembled into 800 mAh soft package batteries according to the following method. The method included:

[0188] The positive electrode active material, conductive agent Super-P and binder PVDF were mixed in a mass ratio of 90:5:5, and an appropriate amount of NMP solution was added to form a slurry, then the mixed slurry was coated on the positive electrode current collector with an aluminum foil, dried and baked in a vacuum oven at 120°C for 12h to obtain a positive electrode sheet, and finally the positive electrode sheet, negative electrode sheet (artificial graphite), separator (PP / PE / PP), electrolyte (1.0M LiPF6 solution of EC / DMC (1:1 by volume)) were assembled to obtain an 800 mAh soft package battery.

[0189] The first coulombic efficiency (initial efficiency), rate performance, impedance and room temperature cycle performance of each soft package battery were tested, and the specific detection method was as follows:

[0190] First coulombic efficiency (initial efficiency): at room temperature 25°C, the battery was charged at 0.1C rate to 4.35V to obtain the charge specific capacity C 0.1 , and discharged at 0.1C rate to 2.8V to obtain the discharge specific capacity D 0.1 , calculate D 0.1 / C 0.1 , and the first coulombic efficiency was obtained;

[0191] Rate performance: at room temperature 25°C, the battery was charged at 0.05C rate to 4.35V, and discharged at the same rate to 2.8V to obtain the discharge specific capacity D 0.05 at 0.05C, and then charged at 0.33C rate to 4.35V, and discharged at the same rate to 2.8V to obtain the discharge specific capacity D 0.33 at 0.33C, calculate D 0.33 / D 0.05 to obtain the rate performance;

[0192] Impedance: at room temperature 25°C, the battery was charged at 1C rate to 4.35V, and then discharged at 1C rate to 50% SOC, and the voltage at this time was recorded as V1 after 1h standing, and then discharged at 5C rate for 10s, and the voltage at this time was recorded as V2, and the impedance was calculated as (V1-V2) / 4;

[0193] Room temperature cycle performance: at room temperature 25°C, the battery was charged at 1C rate to 4.35V, and then discharged at 1C rate to 2.8V to obtain the discharge specific capacity D1 of the first cycle, and after 100 cycles according to this system, the discharge specific capacity D 100 of the 100th cycle was obtained, and D1-D was calculated.100 D1 to get the cycle retention rate.

[0194] Detection results

[0195] Table 2

[0196]

[0197] It can be seen that the positive active material of the embodiment of the application helps to improve the first coulomb efficiency and the cycle performance, in addition, the structure of the positive active material is also conducive to the full contact of the positive active material and the electrolyte, improves the conduction rate of lithium ions, helps to reduce the impedance, and improves the rate performance.

[0198] Compared with the examples, the comparative examples 1 and 3 do not have the outer hollow structure, the average hollow thickness of the inner layer of the comparative examples 2 and 6 is small, the stress may be released unevenly in the charging and discharging process, leading to the damage of the structure and poor electrical performance; the average hollow thickness of the outer layer of the comparative example 4 is too large, the average hollow thickness of the inner layer of the comparative example 5 is too large, the cycle performance is poor, the hollow layer thickness is too large, in the charging and discharging process, too much electrolyte may seep in to cause more side reactions to occur, causing serious irreversible phase change, and then also leading to poor rate and impedance.

[0199] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the application, but not to limit them; although the application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement to part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the application.

Claims

1. A positive electrode active material, characterized in that, The positive electrode active material includes an inner layer and an outer layer in the radial direction from the inside to the outside. The inner layer includes a third ring layer that surrounds a cavity. The outer layer includes a second ring layer located on the side of the third ring layer away from the cavity and a first ring layer located on the side of the second ring layer away from the third ring layer. The positive electrode active material is composed of primary particles, and the second layer is a hollow structure including the primary particles; Along the radial direction from the inside out, the radius of the positive electrode active material is r1, the width of the second layer is r2-r3, the width of the cavity is r4, the average hollow thickness percentage of the second layer is D1=(r2-r3) / r1, the average hollow thickness percentage of the cavity is D2=r4 / r1, and D1 and D2 satisfy 2.8%≤D1≤16% and 10%≤D2≤20%. The change rate of D50 of the positive electrode active material after being held under a pressure of 3.0~3.5T for 30s is less than or equal to 5%.

2. The positive electrode active material according to claim 1, characterized in that, The cavity does not contain the primary particles.

3. The positive electrode active material according to claim 1 or 2, characterized in that, The chemical formula of the positive electrode active material is Li x Ni a Co b M c N e A f O2, wherein 0.95≤x≤1.1, 0.5≤a<1, 0≤b≤0.2, 0≤c≤0.5, a+b+c+e=1, 0<e≤0.01, 0<f≤0.01, M includes Mn and / or Al, N includes one or more of Nb, Ti, Sr, Sb, Y, Mg, W, Mo, Zr and Al, and A includes one or more of Al, W, Ti and Zr; And / or, the D50 of the positive electrode active material is 10~18μm.

4. A method for preparing the positive electrode active material according to any one of claims 1-3, characterized in that, include: 1) A metal salt solution including nickel, cobalt, and manganese, sodium hydroxide, and ammonia are mixed to obtain a mixed solution with a pH of 11.0 to 12.3 and an ammonia concentration of 2 to 15 g / L. The mixed solution is subjected to a first reaction to obtain a first mixed system. Then, the pH of the first mixed system is decreased and the ammonia concentration in the first mixed system is increased to carry out a second reaction to obtain a second mixed system. 2) Adjust the pH of the second mixture to 10.5–11.5 and the ammonia concentration to 2–8 g / L, carry out the third reaction to obtain the third mixture, then increase the ammonia concentration in the third mixture to carry out the fourth reaction to obtain the hydroxide precursor; wherein, the ratio of the time of the first reaction to the sum of the times of the first reaction, the second reaction, the third reaction, and the fourth reaction is D3, and the ratio of the time of the third reaction to the sum of the times of the first reaction, the second reaction, the third reaction, and the fourth reaction is D4, and D3 and D4 satisfy 15% < D3 ≤ 40% and 2.5% ≤ D4 ≤ 15%; 3) The hydroxide precursor and the raw material system including lithium salt are mixed to obtain a mixture; 4) The mixture is subjected to a first sintering at 300~650℃, and then cooled to 20~25℃ to obtain pre-lithiated lithium nickel cobalt manganese oxide; 5) The pre-lithiated lithium nickel cobalt manganese oxide is subjected to a second sintering at 550~750℃, then heated to 850~950℃ for a third sintering, and then subjected to a fourth sintering at 780-880℃ to obtain the positive electrode active material, wherein the temperature of the fourth sintering is lower than the temperature of the third sintering.

5. The preparation method according to claim 4, characterized in that, The temperature of the first reaction is 50~75℃; And / or, the temperature of the second reaction is 50~75℃; And / or, the temperature of the third reaction is 50~75°C; And / or, the temperature of the fourth reaction is 50~75℃; And / or, in step 1), the first reaction and the second reaction are carried out under stirring at a speed of 300 rpm to 500 rpm; And / or, in step 2), the third reaction and the fourth reaction are carried out under stirring at a speed of 100 rpm to 400 rpm; The raw material system in step 3) also includes doping elements, which include one or more of Nb, Ti, Sr, Sb, Y, Mg, W, Mo, Zr and Al; And / or, the first sintering time is 4~15h; And / or, the second sintering time is 2~4 hours; And / or, the third sintering time is 8~15h; And / or, the fourth sintering time is 3~5h.

6. The preparation method according to claim 4 or 5, characterized in that, Also includes: The positive electrode active material is pulverized, then mixed with the coating material, and then subjected to a fifth sintering at 300~700℃ to obtain the coated positive electrode active material. The fifth sintering time is 5-12 hours.

7. The preparation method according to claim 4 or 5, characterized in that, Step 4) includes: heating the mixture to 300-650°C at a rate of 1-5°C / min for a first sintering, and then cooling it to 20-25°C to obtain the pre-lithiated lithium nickel cobalt manganese oxide; And / or, step 5) includes: heating the pre-lithiated lithium nickel cobalt manganese oxide to 550-750°C at a rate of 1-5°C / min for a second sintering, then heating it to 850-950°C at a rate of 1-3°C / min for a third sintering, then cooling it down at a rate of 1-3°C / min and performing a fourth sintering at 780-880°C to obtain the positive electrode active material, wherein the temperature of the fourth sintering is lower than the temperature of the third sintering.

8. The preparation method according to claim 6, characterized in that, Step 4) includes: heating the mixture to 300-650°C at a rate of 1-5°C / min for a first sintering, and then cooling it to 20-25°C to obtain the pre-lithiated lithium nickel cobalt manganese oxide; And / or, step 5) includes: heating the pre-lithiated lithium nickel cobalt manganese oxide to 550-750°C at a rate of 1-5°C / min for a second sintering, then heating it to 850-950°C at a rate of 1-3°C / min for a third sintering, then cooling it down at a rate of 1-3°C / min and performing a fourth sintering at 780-880°C to obtain the positive electrode active material, wherein the temperature of the fourth sintering is lower than the temperature of the third sintering.

9. A positive electrode plate, characterized in that, The positive electrode sheet comprises the positive electrode active material according to any one of claims 1-3 or the positive electrode active material obtained by the preparation method according to any one of claims 4-8.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the positive electrode sheet as described in claim 9.

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