Cobalt-coated lithium nickel manganese oxide cathode material, its preparation method and lithium-ion battery

Abstract

The present application provides a preparation method of a cobalt-coated lithium nickel manganese oxide cathode material. By means of an in-situ coprecipitation method, a trace amount of cobalt coating layer is constructed on the surface of the precursor. The interfacial stability is improved by using the electron conduction enhancement effect of cobalt, and the cobalt-free property of the material bulk is retained, so that the material has a complete crystal structure. When applied to a lithium-ion battery, it has good discharge specific capacity and cycling performance. The first-cycle discharge capacity reaches 193.52 mAh / g, and the 250-cycle retention rate reaches 90.38%. The electrochemical stability is significantly enhanced. The present application also provides a cobalt-coated lithium nickel manganese oxide cathode material and a lithium-ion battery.

Description

Technical Field

[0001] This application relates to the technical field of lithium-ion batteries, and specifically relates to a cobalt-coated lithium nickel manganese oxide cathode material, a preparation method thereof, and a lithium-ion battery. Background Art

[0002] Lithium-ion batteries are widely used in consumer electronics, electric vehicles, and energy storage fields due to their advantages such as high energy density and long cycle life. As the core component, the cathode material directly affects the performance and cost of the battery. In the lithium-ion battery (LIBs) system, cobalt is an indispensable component of the cathode material. Currently widely used cathode materials, such as lithium nickel manganese cobalt oxide (NMC or NCM), lithium nickel cobalt aluminum oxide (NCA), optimize their performance by combining cobalt with other metals. Analyzed from the cost dimension, the cost of the cathode material accounts for about 50% of the total cost of battery materials, while cobalt accounts for 10% - 30% of the cost of the LIBs cathode, which is higher than the costs of components such as nickel, manganese, and aluminum. Since cobalt plays an irreplaceable role in maintaining the structural stability of the material, inhibiting phase transformation, and improving the cycle performance, how to reduce the cobalt usage while maintaining or even optimizing the battery performance has become the core difficulty in technological research. Summary of the Invention

[0003] The purpose of this application is to propose a cobalt-coated lithium nickel manganese oxide cathode material, a preparation method thereof, and a lithium-ion battery to improve at least one of the above technical problems. This application realizes the above purpose through the following technical solutions.

[0004] In the first aspect, this application provides a preparation method of a cobalt-coated lithium nickel manganese oxide cathode material, including:

[0005] S1: Weigh Co(NO3)2·6H2O and LiNi 0.6 Mn 0.4 (OH)2 according to the molar ratio Co:TM (Ni + Mn) = 1:200 - 1:50. Dissolve Co(NO3)2·6H2O in analytical pure ammonia water and stir until completely dissolved, then add LiNi 0.6 Mn 0.4 (OH)2. Stir at room temperature for 1 - 2 h, then filter, wash, and dry to obtain the Co-A-NM64 precursor;

[0006] S2: Weigh the Co-A-NM64 precursor and the lithium source according to the molar ratio TM (Ni + Mn):Li = 1:1.05 - 1:1.1, grind for 0.5 - 1 h to mix evenly, and obtain a mixed powder;

[0007] S3: Anneal the mixed powder in an air atmosphere at 700 - 1000 °C for 10 - 15 h, and cool to obtain the cobalt-coated lithium nickel manganese oxide cathode material;

[0008] In the above step description, "TM" is the total weight of transition metals or the total weight of nickel and manganese, and the same applies hereinafter.

[0009] In one embodiment, the lithium source is Li2CO3 or LiOH.

[0010] In one embodiment, step S1 includes dissolving Co(NO3)2·6H2O in analytical pure ammonia water at a molar ratio of NH3:Co = 480:1 - 500:1 and stirring until completely dissolved.

[0011] In one embodiment, step S3 includes heating the mixed powder in an air atmosphere at a heating rate of 5 - 10 °C / min to 400 - 600 °C, pre-sintering for 3 - 5 h, and then heating to 800 - 900 °C at a heating rate of 2 - 5 °C / min and performing high-temperature sintering for 12 h.

[0012] In one embodiment, the preparation method further includes S4: dispersing the cobalt-coated lithium nickel manganese oxide cathode material in Tris buffer solution at a weight ratio of cobalt-coated lithium nickel manganese oxide cathode material:Tris buffer solution = 1:100 - 2:100 to obtain a first liquid, ultrasonically treating the first liquid for 0.5 - 1 h, and then uniformly and slowly dropping a 1%wt dopamine hydrochloride solution into the first liquid at a weight ratio of 1%wt dopamine hydrochloride solution:first liquid = 3:100 - 8:100, stirring at room temperature for 10 - 12 h to obtain a second liquid, centrifuging the second liquid to collect a first product, washing the first product and then drying it under vacuum, and calcining it at 400 - 500 °C in an inert atmosphere for 2 - 4 h to obtain PDA@cobalt-coated lithium nickel manganese oxide cathode material.

[0013] In one embodiment, S4 further includes adding carboxylated carbon nanotubes to the second liquid at a weight ratio of carboxylated carbon nanotubes:PDA = 1:1 - 1:3, maintaining stirring and ultrasonically treating for 0.5 - 1 h, centrifuging to collect a second product, washing the second product and then drying it under vacuum, and calcining it at 400 - 500 °C in an inert atmosphere for 2 - 4 h to obtain PDA / CNT@cobalt-coated lithium nickel manganese oxide cathode material.

[0014] In a second aspect, the present application provides a cobalt-coated lithium nickel manganese oxide cathode material prepared according to the preparation method of the first aspect.

[0015] In one embodiment, the cobalt-coated lithium nickel manganese oxide cathode material has a secondary grain morphology in which primary grains form a sphere.

[0016] In one embodiment, in the PDA / CNT@cobalt-coated lithium nickel manganese oxide cathode material, at least part of the carbon nanotubes penetrate the PDA layer.

[0017] In a third aspect, the present application also provides a lithium-ion battery, which includes a positive electrode and a negative electrode. The positive electrode of the lithium-ion battery is prepared from the cobalt-coated lithium nickel manganese oxide positive electrode material of the second aspect.

[0018] The cobalt-coated lithium nickel manganese oxide positive electrode material for lithium-ion batteries prepared by the method provided in the embodiments of the present application constructs a trace cobalt coating layer on the surface of the precursor through an in-situ co-precipitation method, improves the interface stability by using the electron conduction enhancement effect of cobalt, retains the cobalt-free property of the material bulk phase, and enables the material to have a complete crystal structure. When applied to lithium-ion batteries, it has good discharge specific capacity and cycling performance, and the electrochemical stability is significantly enhanced. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without creative efforts.

[0020] Figure 1 Shows the SEM image of the Co-A-NM64 precursor of Example 1 of the present application, namely LiNi 0.6 Mn 0.4 (OH)2 at a magnification of 5000 times.

[0021] Figure 2 Shows the SEM image of Co-A-NM64 of Example 1 of the present application at a magnification of 5000 times.

[0022] Figure 3 Shows the SEM image of Co-A-NM64 of Example 1 of the present application at a magnification of 20000 times.

[0023] Figure 4 Shows the SEM image of Co-A-NM64@PDA / CNT of Example 5 of the present application at a magnification of 20000 times.

[0024] Figure 5 Shows the long-term cycling performance graph of the battery prepared from the cobalt-coated lithium nickel manganese oxide positive electrode material of Example 1 of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] To enable those skilled in the art to better understand the solution of this application, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of this application. Obviously, the described embodiments are only a part of the embodiments of this application, rather than all the embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative efforts fall within the scope of protection of this application.

[0026] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0027] An embodiment of this application provides a cobalt-coated lithium nickel manganese oxide cathode material, which is prepared by the following steps:

[0028] S1: Weigh Co(NO3)2·6H2O and LiNi 0.6 Mn 0.4 (OH)2 according to the molar ratio Co:TM (Ni + Mn) = 1:200 - 1:50. Exemplarily, Co:TM (Ni + Mn) = 1:200 / 1:150 / 1:100 / 1:50. According to the molar ratio NH3:Co = 480:1 - 500:1. Exemplarily, NH3:Co 2+ = 480:1 / 490:1 / 500:1. Dissolve Co(NO3)2·6H2O in analytical pure ammonia water and stir until completely dissolved, then add LiNi 0.6 Mn 0.4 (OH)2. Stir at room temperature for 1 - 2 h, then filter, wash and dry to obtain the Co-A-NM64 precursor;

[0029] S2: Weigh the Co-A-NM64 precursor and the lithium source according to the molar ratio TM (Ni + Mn):Li = 1:1.05 - 1:1.1, grind for 0.5 - 1 h and mix evenly to obtain a mixed powder, where the lithium source can be Li2CO3 or LiOH;

[0030] S3: Anneal the mixed powder at 700 - 1000 °C for 10 - 15 h in an air atmosphere. Preferably, heat the mixed powder to 400 - 600 °C at a heating rate of 5 - 10 °C / min in an air atmosphere for pre-sintering for 3 - 5 h. Exemplarily, the heating rate can be 5 °C / min, 7 °C / min, 8 °C / min, the pre-sintering temperature can be 400 °C, 500 °C, 600 °C, and the pre-sintering time can be 3 h, 4 h, 5 h. Then heat it to 800 - 900 °C at a heating rate of 2 / 3 / 4 / 5 °C / min. Exemplarily, the sintering temperature can be 800 °C, 850 °C, 900 °C. After high-temperature sintering for 12 h and cooling, a lithium cobalt oxide-coated lithium nickel manganese oxide cathode material is obtained.

[0031] In one embodiment, the preparation method further includes S4: Disperse the lithium cobalt oxide-coated lithium nickel manganese oxide cathode material in a tris(hydroxymethyl)aminomethane (Tris) buffer solution (pH = 8.5) according to the weight ratio of lithium cobalt oxide-coated lithium nickel manganese oxide cathode material:Tris buffer solution = 1:100 - 2:100 to obtain a first liquid. Ultrasonically treat the first liquid for 0.5 - 1 h to ensure uniform dispersion of the lithium cobalt oxide-coated lithium nickel manganese oxide cathode material. Then, slowly and uniformly drop a 1%wt dopamine hydrochloride (DA) solution into the first liquid according to the weight ratio of 1%wt dopamine hydrochloride solution:first liquid = 3:100 - 8:100, and stir at room temperature for 10 - 12 h to obtain a second liquid. The stirring rate is controlled at about 300 rpm. Dopamine undergoes oxidative self-polymerization in an alkaline environment to form a polydopamine (PDA) coating layer, which is coated on the lithium cobalt oxide-coated lithium nickel manganese oxide cathode material. Centrifuge the second liquid and collect the first product. Wash the first product alternately with deionized water and ethanol 3 times, then vacuum dry it at 60 °C for 12 h, and calcine it at 400 - 500 °C for 2 - 4 h in an inert atmosphere to obtain PDA@lithium cobalt oxide-coated lithium nickel manganese oxide cathode material.

[0032] In one embodiment, S4 further includes adding carboxylated carbon nanotubes (CNT-COOH) to the second liquid, maintaining stirring, and ultrasonically treating it for 0.5 - 1 h in an 80W 40kHz ultrasonic machine. Centrifuge and collect the second product. Wash the second product alternately with deionized water and ethanol 3 times, then vacuum dry it at 60 °C for 12 h, and calcine it at 400 - 500 °C for 2 - 4 h in an inert atmosphere to obtain PDA / CNT@lithium cobalt oxide-coated lithium nickel manganese oxide cathode material.

[0033] The lithium cobalt-coated nickel manganese lithium oxide cathode material prepared by the method provided in the embodiments of the present application constructs a trace cobalt coating layer on the surface of the precursor through an in-situ coprecipitation method, utilizes the electron conduction enhancement effect of cobalt to improve the interface stability, retains the cobalt-free property of the material bulk phase, enables the material to have a complete crystal structure, has good discharge specific capacity and cycling performance, and significantly enhances the electrochemical stability.

[0034] The technical solutions of the present invention will be further described below through specific embodiments. The models and manufacturers of the instruments used in the following examples and comparative examples are:

[0035] Vacuum drying oven, DZF-250, Beijing Yongguang Medical Instrument Co., Ltd.; Tube furnace, Hefei Kejing Material Technology Co., Ltd.; Coating machine, AFA-1, Hefei Kejing Material Technology Co., Ltd.; Glove box, MB-Labstar, Braun Inert Gas (Shanghai) Co., Ltd.; Battery test system, CT2001A, Wuhan Blue Electronic Co., Ltd.; Electrochemical workstation, V54829, Ivium (Netherlands).

[0036] Example 1:

[0037] Take 0.1596 g of Co(NO3)2·6H2O and 5 g of NM64 precursor (LiNi 0.6 Mn 0.4 (OH)2). First, add 0.1596 g of Co(NO3)2·6H2O to 20 mL of analytical pure ammonia water and stir until completely dissolved, then add the NM64 precursor. After stirring for 1.5 h, filter by suction, wash three times with deionized water and anhydrous ethanol respectively, first place it in an oven at 120 °C for 2 - 3 h, and then transfer it to a vacuum drying oven at 80 °C for 12 h to obtain the Co-A-NM64 precursor. Please refer to Figure 1 , Figure 1 which shows the SEM image of the Co-A-NM64 precursor with a magnification of 5000 times.

[0038] Take 5 g of Co-A-NM64 precursor and 2.127 g of Li2CO3 according to the molar ratio TM (Ni + Mn):Li = 1:1.05, and grind for 1 h until evenly mixed to obtain a mixed powder of the precursor and the lithium source;

[0039] Put the mixed powder of the precursor and the lithium source into the furnace and set the calcination program under an air atmosphere:

[0040] 1) Heat from room temperature to 500 °C at a heating rate of 5 °C / min and pre-sinter for 300 min;

[0041] 2) Heat from 500 °C to 850 °C at a heating rate of 2 °C / min and sinter for 720 min;

[0042] 3) End the program and naturally cool to room temperature to obtain the cobalt-coated lithium nickel manganese oxide cathode material Co-LiNi 0.6 Mn 0.4 O2, denoted as Co-A-NM64;

[0043] The sintered material is stored in a vacuum drying oven at 80 °C.

[0044] The obtained Co-A-NM64 was tested by scanning electron microscopy (SEM). The scanning electron microscope images obtained from the test are shown in Figure 2 and Figure 3 , please refer to Figure 2-3 , showing that Co-A-NM64 has a spherical secondary grain morphology composed of primary grains and has a typical polycrystalline structure.

[0045] Example 2:

[0046] The difference between this example and Example 1 is only that 0.0798 Co(NO3)2·6H2O and 5 g of NM64 precursor are taken, and other conditions and parameters are exactly the same as those in Example 1.

[0047] Example 3:

[0048] The difference between this example and Example 1 is only that 0.3192 g of Co(NO3)2·6H2O and 5 g of NM64 precursor are taken, and other conditions and parameters are exactly the same as those in Example 1.

[0049] Example 4:

[0050] The difference between this example and Example 1 is only that 1 g of the sintered Co-A-NM64 material is taken, dispersed in 100 mL of Tris buffer (pH = 8.5), ultrasonically treated for 30 minutes to ensure uniform dispersion of the particles. Then, 5 mL of 1% mass fraction dopamine hydrochloride (DA) solution is slowly added dropwise to the above solution within 2 h, and the stirring rate is controlled at 300 rpm to avoid particle agglomeration. Then, it is continuously stirred at room temperature (25 °C) for 12 hours, centrifuged at 8000 rpm for 10 minutes, and the product is washed alternately with deionized water and ethanol 3 times to remove unreacted DA monomers. Finally, it is vacuum dried at 60 °C for 12 hours, and then calcined at 400 °C for 4 h under inert gas protection to obtain the PDA @ cobalt-coated lithium nickel manganese oxide cathode material, denoted as Co-A-NM64@PDA.

[0051] Example 5:

[0052] The difference between this example and Example 1 is only that 1 g of the sintered Co-A-NM64 material is taken and dispersed in 100 mL of Tris buffer solution (pH = 8.5), and ultrasonic treatment is carried out for 30 minutes to ensure uniform dispersion of the particles. 5 mL of 1% mass fraction dopamine hydrochloride (DA) solution is slowly added dropwise to the above solution at a constant speed within 2 h, and the stirring rate is controlled at 300 rpm to avoid particle agglomeration; then, stirring is continued at room temperature (25 °C) for 12 hours, 0.05 g of carboxylated carbon nanotubes (CNT-COOH) is added to the solution, stirring is maintained, and ultrasonic treatment is carried out in an 80 W 40 kHz ultrasonic machine for 60 min. The product is centrifuged at 8000 rpm for 10 minutes, washed alternately with deionized water and ethanol three times, dried in vacuo at 60 °C for 12 hours, and then calcined at 400 °C for 4 h under inert gas protection to obtain Co-A-NM64@PDA / CNT. The obtained PDA / CNT@lithium cobalt nickel manganate cathode material was denoted as Co-A-NM64@PDA / CNT and was subjected to scanning electron microscopy (SEM) test. The scanning electron microscope images obtained from the test are shown in Figure 4 , Figure 4 showing that carbon nanotubes are uniformly attached to the surface of the material, and it can be seen that some carbon nanotubes penetrate the PDA layer.

[0053] Comparative Example 1:

[0054] The difference between this example and Example 1 is that 5 g of NM64 precursor (LiNi 0.6 Mn 0.4 (OH)2) is taken. 5 g of NM64 precursor is added to 20 mL of analytical pure ammonia water, stirred for 1.5 h, and then filtered by suction. After washing three times with deionized water and absolute ethanol respectively, it is first dried in an oven at 120 °C for 2 - 3 h, and then transferred to a vacuum drying oven at 80 °C and dried for 12 h to obtain A-NM64;

[0055] Co(NO3)2·6H2O, A-NM64 and Li2CO3 are taken in a molar ratio of Co:TM (Ni + Mn):Li = 0.01:1:1.05, ground for 1 h until evenly mixed to obtain a mixed powder, and the calcination parameters of the mixed powder are exactly the same as those in Example 1.

[0056] Comparative Example 2:

[0057] The difference between this example and Example 1 is that Co(NO3)2·6H2O, NM64 precursor and Li2CO3 are taken in a molar ratio of Co:TM (Ni + Mn):Li = 0.01:1:1.05, ground for 1 h until evenly mixed to obtain a mixed powder, and the calcination parameters of the mixed powder are exactly the same as those in Example 1.

[0058] Comparative Example 3:

[0059] The difference between this example and Example 1 is only that the NM64 precursor and Li2CO3 are taken in a molar ratio of TM (Ni+Mn):Li = 1:1.05, ground for 1 h until evenly mixed to obtain a mixed powder, and the calcination parameters of the mixed powder are exactly the same as those in Example 1.

[0060] Comparative Example 4:

[0061] The difference between this example and Example 1 is only that the amount of analytical pure ammonia water is changed to 5 ml, and other conditions and parameters are exactly the same as those in Example 1.

[0062] Comparative Example 5:

[0063] The difference between this example and Example 4 is only that the Co-A-NM64@PDA material obtained in Example 4 is evenly dispersed in ethanol, then ultrasonically treated for 120 s in an 80W 40kHz ultrasonic machine, centrifuged and collected, and then dried.

[0064] Comparative Example 6:

[0065] The difference between this example and Example 5 is only that the Co-A-NM64@PDA / CNT material obtained in Example 5 is evenly dispersed in ethanol, then ultrasonically treated for 120 s in an 80W 40kHz ultrasonic machine, centrifuged and collected, and then dried.

[0066] Performance test:

[0067] Weigh the cathode materials, conductive agents (superconducting carbon), and binders (preferably polyvinylidene fluoride PVDF) obtained in each example and comparative example according to a mass ratio of 8:1:1, dissolve them in 1-methyl-2-pyrrolidone NMP, and stir for 4-8 h to obtain a Co-A-NM64 cathode slurry. Use a coater to evenly coat the cathode slurry on an aluminum foil current collector, vacuum dry for 12 h, and cut into cathode sheets with a diameter of 12 mm. The areal loading of the cathode sheets is about 4-7 mg cm -2 . Inside a glove box filled with argon (H2O < 0.5 ppm, O2 < 0.5 ppm), combine the obtained cathode sheets with commercially available lithium sheets and commercially available pp separators, add a commercial electrolyte (3240), and assemble the battery case to obtain a battery.

[0068] Battery performance test method:

[0069] Cycling performance test: Using the Wuhan Blue Electric test system in China, the constant current constant voltage charge-discharge (CCCV) curve of the battery was tested in a constant temperature environment of 30 °C. It was charged to 4.45 V at a current density of 180 mA / g, and then the voltage of 4.45 V was maintained until the current density decreased below 9 mAh / g. Preferably, it was discharged to 2.7 V at a current density of 180 mAh / g, which was recorded as the first cycle. The first-cycle reversible capacity D1 and the first Coulombic efficiency were recorded. The aforementioned charge-discharge process was repeated, the reversible capacity Dn of the nth cycle was recorded, and the capacity retention rate of the nth cycle was calculated.

[0070] After the assembled battery was left standing for 12 h, electrochemical performance tests were carried out, and the voltage window was set to 2.7 - 4.45 V. The test results are shown in Table 1.

[0071] Table 1:

[0072]

[0073] As shown in Table 1, the positive electrode material prepared by the lithium-ion battery positive electrode material preparation process of the present invention has a high first-cycle charge / discharge specific capacity and a significantly increased first-cycle Coulombic efficiency when subjected to electrochemical tests under the test conditions of a voltage of 2.7 V - 4.45 V and 1 C. For example, the first-cycle specific capacity of Example 1 was 193.52 mAh / g, and the Coulombic efficiency reached 87.00%. In addition, please refer to Figure 5 , it can be seen from Figure 5 that the specific capacity of Example 1 was as high as 174.9 mAh / g after 250 cycles, and the capacity retention rate was 90.38%.

[0074] The cobalt coating process of Example 1 uses the in-situ coprecipitation method for precursor modification. Before calcination, cobalt nitrate is coprecipitated with the NM64 precursor, so that cobalt elements are evenly distributed on the surface of the precursor and the proportion of surface nickel elements is reduced, reducing the cation mixing on the surface. Then, calcination is carried out to form a stable cobalt coating layer. This coating process enhances the interface stability of the material, inhibits surface reconstruction and transition metal TM dissolution during charge and discharge. The surface cobalt coating can improve the reaction kinetics by accelerating ion and electron transport, thus achieving a high first-cycle discharge capacity (193.52 mAh / g), a high Coulomb efficiency (87%), and an excellent cycle retention rate (90.38% after 250 cycles). Example 2 uses a low cobalt ratio, and Example 3 uses a high cobalt ratio. Although both have relatively high cycle retention rates (91.36%, 93.34%), the first-cycle capacities (172.87, 168.92 mAh / g) and Coulomb efficiencies (81.63%, 80.28%) are lower than those of Example 1. This shows that although too little cobalt provides a certain role in stabilizing the interface, too little cobalt fails to effectively promote ion conduction, and too much may lead to too little surface nickel element, possibly forming special phases that hinder lithium-ion diffusion.

[0075] In Comparative Example 1, cobalt nitrate was directly calcined after being mixed evenly with A-NM64 and Li2CO3. Although the surface nickel element was still reduced in the first step, no in-situ cobalt coating layer was generated, which may lead to uneven distribution of cobalt elements and fail to effectively inhibit surface reconstruction during charge and discharge. Although the first-cycle capacity (187.63 mAh / g) and Coulomb efficiency (86.95%) are close to those of Example 1, the cycle retention rate is only 78.20%, showing a significant decrease. In Comparative Example 2, the in-situ coprecipitation step was not carried out, and cobalt was only doped into the bulk phase of the material in the traditional way of grinding. Although the advantage of cobalt in improving the reaction kinetics was exerted, the existence of bulk cobalt also triggered more serious phase changes during the charge and discharge of the cobalt-rich material, reducing the cycle stability, with a low first-cycle discharge specific capacity (181.34 mAh / g), first-cycle Coulomb efficiency (81.75%), and cycle retention rate (68.31%). The experimental results show that the coprecipitation method plays a key role in in-situ constructing a stable and uniform cobalt coating layer.

[0076] Comparative Example 3 uses the original cobalt-free binary material, and both the first-cycle capacity (168.51 mAh / g) and the cycle retention rate (86.42%) decrease significantly, proving the importance of cobalt in promoting reaction kinetics and maintaining structural stability. In Comparative Example 4, the amount of ammonia water was reduced, and the ammonia water was insufficient in the precursor synthesis step, resulting in incomplete coprecipitation reaction and non-uniform surface coating layer structure. Both the first-cycle capacity (172.94 mAh / g) and the retention rate (81.39%) are lower than those of Example 1. The above results show that the modification process of the precursor has a great impact on the final performance of the material.

[0077] Example 4 A polydopamine (PDA) layer was introduced onto the surface of the cobalt-coated lithium-ion battery cathode material, significantly enhancing the cycle life through its unique dual mechanisms of physical barrier and chemical regulation. As a dense physical barrier, the PDA layer can isolate the direct contact between the electrolyte and the cobalt coating, inhibiting the dissolution of cobalt ions under high voltage (the dissolution amount can be reduced by more than 50%) and the migration of transition metals (reducing the Mn deposition amount on the negative electrode surface). At the same time, its abundant amino (-NH2) and phenolic hydroxyl (-OH) functional groups can form coordination bonds with cobalt, stabilizing the lattice oxygen and inhibiting the lattice distortion caused by oxygen vacancies. During the electrochemical process, PDA preferentially reacts with the electrolyte to form a stable solid-liquid electrolyte interface (SEI) film rich in LiF / Li3PO4, significantly reducing the interfacial side reactions and electrolyte decomposition. Moreover, the flexible structure of PDA can dynamically repair the SEI cracks caused by the volume expansion during charge and discharge. Eventually, its cycle performance is significantly improved, achieving multi-dimensional protection for the high-energy density cathode material.

[0078] For the Co-A-NM64@PDA / CNT material in Example 5, due to the addition of CNT (carbon nanotube), carboxylated CNT can effectively combine with the functional groups of PDA. At the same time, some CNTs penetrate the PDA layer during ultrasonic treatment and are effectively fixed on the cobalt coating, resulting in better mechanical stability. Therefore, the performance of Co-A-NM64@PDA / CNT after ultrasonic treatment changes little. Thus, CNT and PDA have an effective synergistic effect. In addition, CNT has good electrical conductivity, especially after calcination. It is added between PDA and the cobalt coating, and an effective fast conductive network is also established between the cobalt coating and the outside, significantly improving the first-cycle Coulombic efficiency. Comparative Example 5 and Comparative Example 6 simulated the situation where two cathode materials were damaged by external forces during actual use. The calcined Co-A-NM64@PDA material is brittle, and the coating layer is prone to detachment from the main body, leading to its failure and subsequent performance degradation.

[0079] In addition, the descriptions such as "some embodiments", "other embodiments", etc. mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present application, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in the present application and the features of different embodiments or examples.

[0080] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit it; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various embodiments of the present application, and should all be included within the protection scope of the present application.

Claims

1. A method for preparing a cobalt-coated lithium nickel manganese oxide positive electrode material, characterized in that: include: S1: Weigh Co(NO3)2·6H2O and LiNi at a molar ratio of Co:TM(Ni+Mn) = 1:200-1:50 0.6 Mn 0.4 (OH)2, dissolve Co(NO3)2·6H2O in analytical pure ammonia water and stir until completely dissolved, then add LiNi 0.6 Mn 0.4 (OH)2, stirred at room temperature for 1-2h, filtered, washed and dried to obtain Co-A-NM64 precursor; S2: Weigh the Co-A-NM64 precursor and the lithium source at a molar ratio of TM (Ni+Mn):Li = 1:1.05-1:1.1, grind for 0.5-1h and mix evenly to obtain a mixed powder; S3: annealing the mixed powder at 700-1000° C. for 10-15 h in an air atmosphere, and obtaining the cobalt-coated lithium nickel manganese oxide positive electrode material after cooling; S4: The cobalt-coated lithium nickel manganese oxide positive electrode material is dispersed in Tris buffer at a weight ratio of the cobalt-coated lithium nickel manganese oxide positive electrode material: Tris buffer = 1:100-2:100 to obtain a first liquid, the first liquid is ultrasonically treated for 0.5-1h, and then 1%wt dopamine hydrochloride solution is slowly and uniformly added to the first liquid at a weight ratio of 1%wt dopamine hydrochloride solution: the first liquid = 3:100-8:100, and stirred at room temperature for 10-12h to obtain a second liquid, carboxylated carbon nanotubes are added to the second liquid at a weight ratio of carboxylated carbon nanotubes: PDA = 1:1-1:3, stirring is maintained and ultrasonic treatment is performed for 0.5-1h, the second product is collected after centrifugation, the second product is washed and vacuum dried, and calcined at 400-500°C under an inert atmosphere for 2-4h to obtain PDA / CNT@cobalt-coated lithium nickel manganese oxide positive electrode material.

2. The preparation method according to claim 1, characterized in that: The lithium source is Li2CO3 or LiOH.

3. The preparation method according to claim 1, characterized in that: Step S1 comprises a molar ratio of NH3:Co 2+ =480:1-500:1, dissolve Co(NO3)2·6H2O in analytical pure ammonia water and stir until completely dissolved.

4. The preparation method according to claim 1, characterized in that: Step S3 includes heating the mixed powder to 400-600°C at a heating rate of 5-10°C / min in an air atmosphere, pre-sintering for 3-5 hours, and then heating to 800-900°C at a heating rate of 2-5°C / min, and high-temperature sintering for 12 hours.

5. A cobalt-coated lithium nickel manganese oxide positive electrode material, characterized in that: Prepared according to the preparation method according to any one of claims 1 to 4.

6. The cobalt-coated lithium nickel manganese oxide positive electrode material according to claim 5, characterized in that: In the PDA / CNT@cobalt-coated lithium nickel manganese oxide positive electrode material, at least part of the carbon nanotubes penetrate the PDA layer.

7. A lithium ion battery, characterized in that: The lithium-ion battery comprises a positive electrode and a negative electrode, and the positive electrode of the lithium-ion battery is prepared from the cobalt-coated lithium nickel manganese oxide positive electrode material according to any one of claims 5 to 6.

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