A ternary positive electrode material with a core-shell structure and a preparation method thereof

Abstract

This invention discloses a ternary cathode material with a core-shell structure and its preparation method, comprising the following steps: S1: performing nanoscale wet milling on metastable lithium-rich metal oxides to adjust their particle size and obtain a slurry; S2: mixing the slurry obtained in S1 with an NCM precursor in a liquid phase, followed by drying to obtain a mixture; S3: sintering the mixture obtained in S2, followed by crushing to obtain ternary cathode material powder with a core-shell structure. The preparation method disclosed in this invention requires only one sintering step, is simple, and reduces energy consumption. Compared with traditional coatings, the lithium-containing metal oxide coating layer has high ionic conductivity. Combined with the control of the coating layer particle size, the diffusion distance of lithium ions can be shortened, effectively improving the rate performance of the NCM cathode material. By controlling the particle size of low-lithium-rich lithium iron ferrite through the milling step, the uniformity of its coating on the NCM surface is improved, maintaining good structural stability and effectively enhancing the cycle stability of the cathode.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a method for preparing a ternary cathode material with a core-shell structure. Background Technology

[0002] Nickel-cobalt-manganese ternary materials (NCM) are currently the preferred active materials for high-energy-density lithium-ion battery cathodes due to their advantages such as high specific capacity (160mAh / g-210mAh / g), high operating potential, and high rate performance. However, ternary materials suffer from poor cycle stability and safety, and are prone to spontaneous combustion or even explosion, limiting their application range. From a theoretical perspective, the problems with ternary cathode materials mainly stem from the instability of their lattice oxygen, the irreversible transformation of the surface salt-rock phase, side reactions between the highly active surface and the electrolyte, and structural damage caused by volume changes. Furthermore, with increasing nickel content, residual alkali inevitably enters the surface of high-nickel ternary cathode materials, reducing their stability in air and posing difficulties for transportation, storage, and manufacturing processes.

[0003] To address the aforementioned issues, surface modification of ternary materials through coating is the primary approach. CN201710821234.6 discloses a method for preparing carbon-coated ternary cathode materials. This method involves synthesizing a network of carbon, nitrogen, and hydrogen compounds from nitrogen-containing organic matter, followed by high-temperature treatment to synthesize nitrogen-doped carbon compounds, which are then coated onto the surface of the ternary cathode material. The excellent conductivity of the nitrogen-doped carbon material significantly improves the kinetic reaction rate of the ternary cathode material. CN202110513888.9 discloses a doping and coating method for ternary cathode materials. A saturated silicate solution is used as the base liquid to clean silicon / metal-doped ternary cathode materials. Metal salts are then added for precipitation coating and secondary sintering to obtain coated ternary cathode materials, greatly improving cycle stability. Overall, materials such as carbon, metal oxides, fluorides, and polymers have been used as coating layers for ternary cathode materials. However, the coating layers for ternary cathode materials still suffer from problems such as low ionic conductivity and poor contact with the ternary material, which limit the improvement of rate performance and cycle stability. In addition, the coating process usually involves secondary sintering and other steps, which increases the complexity of material preparation. Summary of the Invention

[0004] This invention provides a ternary cathode material with a core-shell structure and its preparation method, so as to obtain a ternary cathode material with a core-shell structure that has high rate performance and long cycle performance.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A method for preparing a ternary cathode material with a core-shell structure includes the following steps:

[0007] S1: Metastable lithium-rich metal oxides are subjected to nanoscale wet grinding to adjust their particle size and obtain a slurry.

[0008] S2: The slurry obtained in step S1 is mixed with the NCM precursor in the liquid phase and then dried to obtain a mixture.

[0009] S3: The mixture obtained in step S2 is sintered and crushed to obtain ternary cathode material powder with a core-shell structure.

[0010] Furthermore, in step S1, the grinding particle size D50 of the metastable lithium-rich metal oxide ranges from 200 to 600 nm.

[0011] Furthermore, in step S1, the metastable lithium-rich metal oxide includes any one of lithium-rich lithium iron ore oxide (Li5FeO4), lithium-rich lithium nickel ore oxide (Li2NiO2), or lithium-rich lithium cobalt ore oxide (Li6CoO4); preferably, the metastable lithium-rich metal oxide is lithium-rich lithium iron ore oxide (Li5FeO4).

[0012] Furthermore, in step S2, the ratio of Ni, Co, and Mn in the NCM precursor is 1:1:1, 4:2:4, 5:2:3, 6:2:2, 6:1:3, or 8:1:1;

[0013] The molar ratio of lithium ions to NCM precursor in the lithium-rich metal oxide is 1 to 1.5:1.

[0014] Furthermore, in step S1, the wet grinding conditions are as follows: grinding is performed using a low-temperature planetary ball mill, the grinding media is zirconia balls, the grinding time is 1.5 to 5.5 hours, the grinding speed is 500 to 1000 rpm, the grinding atmosphere is an inert atmosphere, the grinding temperature is 5 to 10°C, and the ball milling solvent is one of ethanol, isopropanol, and N-methylpyrrolidone, with a solid content of 30 to 50%. This grinding process can also be performed using a horizontal sand mill to grind metastable lithium-rich metal oxides.

[0015] Furthermore, in step S3, the sintering is a gradient sintering including stage I and stage II;

[0016] The sintering conditions for stage I are: sintering at 300-500℃ for 2-4 hours, with a sintering atmosphere of carbon dioxide;

[0017] The sintering conditions for stage II are: sintering at 600–950°C for 8–12 hours, and the sintering atmosphere for stage II is oxygen or air.

[0018] The heating rate for gradient sintering is 5–10 °C / min.

[0019] Furthermore, in step S2, the slurry and NCM precursor are mixed using a high-speed shear disperser with a rotation speed of 20-40 Hz and a shear dispersion time of 30-60 min.

[0020] Furthermore, in step S2, the drying method is closed-loop spray drying, and the spray particle size D50 is 10-16 μm.

[0021] Furthermore, in step S3, the equipment used in the crushing process includes one or more of the following: jaw crusher, double roller mill, mechanical pulverizer, and air jet mill.

[0022] A ternary cathode material with a core-shell structure, prepared by the preparation method disclosed in this invention.

[0023] The beneficial effects of this invention are:

[0024] The method for preparing a core-shell structured ternary cathode material disclosed in this invention requires only one sintering step, which is simple and reduces energy consumption in the material production process. Furthermore, the lithium-containing metal oxide coating layer exhibits higher ionic conductivity compared to traditional carbon, metal oxide, fluoride, and polymer coating layers. Controlling the particle size of the coating layer helps shorten the lithium-ion diffusion distance, effectively improving the rate performance of the NCM cathode material. In addition, controlling the particle size of low-lithium-rich lithium iron phosphate through a grinding step improves its coating uniformity on the NCM surface, allowing the in-situ formed lithium-containing metal oxide coating layer to be tightly bonded to the NCM through chemical bonds. This isolates the NCM from direct contact with the electrolyte to reduce side reactions while adapting to the volume changes of the NCM during charge and discharge, maintaining good structural stability and effectively improving the cycle stability of the cathode. The capacity retention rate of this material exceeds 90% after 1000 cycles. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] Example 1:

[0027] S1: Weigh 1000g of lithium iron ferrite (Li5FeO4) and place it into the grinding jar of a low-temperature planetary ball mill. After adding ethanol solution (30% solid content), the mixture is purged with nitrogen to obtain a pretreated slurry. Subsequently, the pretreated slurry is ball-milled at 1000rpm using a low-temperature planetary ball mill. Zirconia balls are used as the grinding media during the ball milling process. The atmosphere inside the grinding jar is nitrogen. The temperature during the ball milling process is controlled at 7℃±2℃, and the ball milling time is 3 hours. The particle size D50 of Li5FeO4 is controlled at 400nm to obtain the slurry.

[0028] S2: Weigh the NCM622 precursor and place it in Li5FeO4 slurry. The molar ratio of Li5FeO4 slurry to NCM622 precursor is 1:4 (the molar ratio of lithium ions to NCM622 precursor is 1.25:1). Then, use a high-speed shear machine to mix at 1000 rpm for 60 min to obtain a homogeneous slurry. Then, use a closed-loop drying equipment to spray dry the homogeneous slurry to obtain a mixture. The inlet temperature of the closed-loop drying equipment is 180±20℃, the outlet temperature is 90±10℃, and the spray particle size D50 is controlled at 10μm.

[0029] S3: The mixture is placed in a tube furnace for sintering. First, the temperature is raised to 450°C at a heating rate of 5°C / min and held for 4 hours in a carbon dioxide atmosphere (Stage I sintering) to convert lithium-rich lithium ferrite into lithium carbonate and lithium ferrite (Li5FeO4+2CO2→2LiCO3+LiFeO2). Then, the temperature is raised to 850°C at a heating rate of 5°C / min and held for 10 hours (Stage II sintering). After the sintered mixture is cooled, it is crushed using a roller mill and a mechanical pulverizer to obtain lithium ferrite-coated NCM622 material (LiFeO2@NCM622), thus obtaining a ternary cathode material with a core-shell structure.

[0030] Example 2:

[0031] The only difference between this embodiment and Embodiment 1 is that in this embodiment, the ball milling time in step S1 is 5 hours, and the grinding particle size D50 of lithium iron ferrite is controlled at 300nm.

[0032] Example 3:

[0033] The only difference between this embodiment and Embodiment 1 is that in this embodiment, the ball milling time in step S1 is 2 hours, and the D50 of lithium iron ferrite is controlled at 550nm.

[0034] Example 4:

[0035] The only difference between this embodiment and Embodiment 1 is that in this embodiment, the ball milling time in step S1 is 5.5 hours, and the D50 of lithium iron ferrite is controlled at 200nm.

[0036] Example 5:

[0037] The only difference between this embodiment and Embodiment 1 is that in this embodiment, the ball milling time in step S1 is 1.5 hours, and the D50 of lithium iron ferrite is controlled at 600nm.

[0038] Example 6:

[0039] The only difference between this embodiment and Embodiment 1 is that, in this embodiment, the molar ratio of Li5FeO4 to NCM622 precursor in step S2 is 1.2:4 (the molar ratio of lithium ions to NCM622 precursor is 1.5:1).

[0040] Example 7:

[0041] The only difference between this embodiment and Embodiment 1 is that, in this embodiment, the molar ratio of Li5FeO4 to NCM622 precursor in step S2 is 1:5 (the molar ratio of lithium ions to NCM622 precursor is 1.0:1).

[0042] Example 8:

[0043] The only difference between this embodiment and embodiment 1 is that in this embodiment, the NCM precursor in step S2 is NCM811 precursor material, and in step S3, the sintering atmosphere in stage II is oxygen, and the sintering temperature is 750°C.

[0044] Example 9:

[0045] The only difference between this embodiment and embodiment 1 is that, in this embodiment, the NCM precursor in step S2 is NCM613 precursor material.

[0046] Example 10:

[0047] The only difference between this embodiment and Embodiment 1 is that, in this embodiment, the sintering temperature of stage II in step S3 is 950°C.

[0048] Example 11:

[0049] The only difference between this embodiment and Embodiment 1 is that, in this embodiment, the sintering temperature of stage II in step S3 is 750°C.

[0050] Comparative Example 1

[0051] The only difference between this comparative example and Example 1 is that the preparation steps in this comparative example do not include the step of controlling the particle size of lithium iron ferrite in S1.

[0052] Comparative Example 2

[0053] The only difference between this comparative example and Example 1 is that battery-grade lithium carbonate is used as the lithium source, and the ratio of lithium carbonate to NCM precursor is 1.1:1.

[0054] Test example:

[0055] (1) Test Method: Lithium-ion battery cathodes prepared using the ternary materials obtained in Examples 1-11 and Comparative Examples 1-2 were tested. Specifically, the ternary materials were mixed uniformly with conductive carbon black and PVDF in NMP at a mass ratio of 80:10:10, and then coated, baked, and rolled to prepare the lithium-ion battery cathode. A commercial lithium-ion battery electrolyte (1M LiPF6 inEC / DMC (vol. 1:1)) was used to assemble button cells, and constant current charge-discharge tests were performed. The counter electrode was lithium metal, and the separator was Celgard 2400. The constant current charge-discharge current density was 1C = 172 mA / g, and the voltage range during the test was set to 2.5–4.2V vs. Li / Li. + The test environment was 25℃ and 45% RH.

[0056] (2) The test results are shown in Table 1:

[0057] Table 1: Electrochemical performance test results of LiFeO2@ternary cathode

[0058]

[0059] Examples 1-5 illustrate the importance of controlling the particle size of lithium-rich lithium iron ferrite. Increasing the ball milling time can reduce the particle size of lithium-rich lithium iron ferrite (from 600 nm to 200 nm), improving its coating uniformity on the NCM surface, thereby improving cycle stability (capacity retention rate increases from 94.0% to 98.2%). Simultaneously, reducing the particle size of the coating layer helps shorten the lithium-ion diffusion distance, improving the rate performance of the cathode (3C / 0.1C capacity retention rate increases from 90.0% to 93.2%). Conversely, in Comparative Example 1, lithium-rich lithium iron ferrite was not subjected to nano-milling, making it difficult for it to uniformly adhere to the surface of the ternary material particles, resulting in poor electrochemical performance of the prepared ternary material. In Comparative Example 2, conventional lithium carbonate was used as the lithium source, which does not generate solid products as a coating layer during sintering, resulting in insufficient protection of the NCM surface and poor cycle stability. Examples 1 and 6... 7 indicates that increasing the amount of lithium-rich lithium iron ferrite can provide sufficient lithium source to ensure the full lithiation of the NCM precursor during sintering, which is beneficial to improving the specific capacity of the cathode. However, it will lead to an increase in the coating thickness, which in turn increases the diffusion distance of lithium ions and reduces the rate performance. Examples 1, 8, and 9 prepared Li5FeO4@NCM622, Li5FeO4@NCM811, and Li5FeO4@NCM613 samples, respectively, indicating that the preparation method provided by the present invention can be applied to NCM cathode materials with different element ratios. Examples 1, 10, and 11 show that as the sintering temperature increases (from 750℃ to 950℃), the electrochemical performance of Li5FeO4@NCM622 does not change significantly, indicating that the presence of the coating can prevent the agglomeration and growth of crystal particles during high-temperature sintering, which is beneficial to maintaining good electrochemical performance while enhancing the crystallinity of NCM at high temperatures.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a ternary cathode material with a core-shell structure, characterized in that, Includes the following steps: S1: Metastable lithium-rich metal oxides are subjected to nanoscale wet grinding to adjust their particle size and obtain a slurry. S2: The slurry obtained in step S1 is mixed with the NCM precursor in the liquid phase and then dried to obtain a mixture. S3: The mixture obtained in step S2 is sintered and crushed to obtain ternary cathode material powder with a core-shell structure; In step S1, the grinding particle size D50 of the metastable lithium-rich metal oxide ranges from 200 to 600 nm; the metastable lithium-rich metal oxide includes any one of lithium iron phosphate, lithium nickel oxide, and lithium cobalt oxide. In step S3, the sintering is a gradient sintering including stage I and stage II; The sintering conditions for stage I are: sintering at 300~500℃ for 2~4 hours, with a sintering atmosphere of carbon dioxide; The sintering conditions for stage II are: sintering at 600~950℃ for 8~12 hours, and the sintering atmosphere for stage II is oxygen or air.

2. The method for preparing a ternary cathode material with a core-shell structure according to claim 1, characterized in that, In step S2, the ratio of Ni, Co, and Mn in the NCM precursor is 1:1:1, 4:2:4, 5:2:3, 6:2:2, 6:1:3, or 8:1:1; The molar ratio of lithium ions to NCM precursor in the lithium-rich metal oxide is 1~1.5:

1.

3. The method for preparing a ternary cathode material with a core-shell structure according to claim 1, characterized in that, In step S1, the conditions for wet grinding are as follows: grinding is performed using a low-temperature planetary ball mill, the grinding media is zirconia balls, the grinding time is 1.5 to 5.5 hours, the grinding speed is 500 to 1000 rpm, the grinding atmosphere is an inert atmosphere, the grinding temperature is 5 to 10°C, and the ball milling solvent is one of ethanol, isopropanol, and N-methylpyrrolidone, with a solid content of 30 to 50%.

4. The method for preparing a ternary cathode material with a core-shell structure according to claim 1, characterized in that, In step S3, the heating rate of the gradient sintering is 5~10℃ / min.

5. The method for preparing a ternary cathode material with a core-shell structure according to claim 1, characterized in that, In step S2, the slurry and NCM precursor are mixed using a high-speed shear disperser with a rotation speed of 20-40 Hz and a shear dispersion time of 30-60 min.

6. The method for preparing a ternary cathode material with a core-shell structure according to claim 1, characterized in that, In step S2, the drying method is closed-loop spray drying, and the spray particle size D50 is 10~16μm.

7. The method for preparing a ternary cathode material with a core-shell structure according to claim 1, characterized in that, In step S3, the equipment used in the crushing process includes one or more of the following: jaw crusher, double roll mill, and air jet mill.

8. The method for preparing a ternary cathode material with a core-shell structure according to claim 1, characterized in that, In step S3, the equipment used in the crushing process is a mechanical crusher.

9. A ternary cathode material with a core-shell structure, characterized in that, A ternary cathode material with a core-shell structure prepared by the preparation method according to any one of claims 1-8.

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