Cobalt-coated lithium nickel manganese oxide positive electrode material, preparation method thereof and lithium ion battery

Abstract

According to the preparation method of the cobalt-coated lithium nickel manganese oxide positive electrode material provided by the invention, a trace cobalt coating layer is constructed on the surface of a precursor through an in-situ coprecipitation method, the interface stability is improved by utilizing the electron conduction enhancement effect of cobalt, and the bulk phase cobalt-free characteristic of the material is reserved, so that the material has a complete crystal structure; when the material is applied to a lithium ion battery, the material has relatively good specific discharge capacity and cycle performance, the first-circle discharge capacity reaches 193.52 mAh / g, the 250-circle cycle retention rate reaches 90.38%, and the electrochemical stability is remarkably enhanced. The invention also provides the cobalt-coated lithium nickel manganese oxide positive electrode material and a lithium ion battery.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to a cobalt-coated lithium nickel manganese oxide positive electrode 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 due to their high energy density and long cycle life. As its core component, positive electrode materials directly affect the performance and cost of batteries. In lithium-ion batteries (LIBs) systems, cobalt is an indispensable component of positive electrode materials. Currently, widely used positive electrode materials, such as lithium nickel manganese cobalt oxide (NMC or NCM) and lithium nickel cobalt aluminum oxide (NCA), optimize performance by combining cobalt with other metals. From a cost perspective, the cost of positive electrode materials accounts for about 50% of the total cost of battery materials, and cobalt accounts for 10% to 30% of the cost of LIBs positive electrodes, which is higher than nickel, manganese, aluminum and other components. Since cobalt plays an irreplaceable role in maintaining material structural stability, inhibiting phase change and improving cycle performance, how to reduce the amount of cobalt while maintaining or even optimizing battery performance has become a core difficulty in technical research. Summary of the invention

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

[0004] In a first aspect, the present application provides a method for preparing a cobalt-coated lithium nickel manganese oxide positive electrode material, comprising: 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 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 10-15h in an air atmosphere, and obtaining a cobalt-coated lithium nickel manganese oxide positive electrode material after cooling; In the above step description, "TM" is the total weight of transition metals or the total weight of nickel and manganese, the same below.

[0005] In one embodiment, the lithium source is Li2CO3 or LiOH.

[0006] 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.

[0007] In one embodiment, 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, then heating to 800-900°C at a heating rate of 2-5°C / min, and high temperature sintering for 12 hours.

[0008] In one embodiment, the preparation method also includes S4: dispersing the cobalt-coated lithium nickel manganese oxide positive electrode material in Tris buffer at a weight ratio of cobalt-coated lithium nickel manganese oxide positive electrode material: Tris buffer = 1:100-2:100 to obtain a first liquid, ultrasonically treating the first liquid for 0.5-1h, and then slowly adding 1%wt dopamine hydrochloride solution to the first liquid at a weight ratio of 1%wt dopamine hydrochloride solution: first liquid = 3:100-8:100 at a uniform speed, stirring at room temperature for 10-12h to obtain a second liquid, centrifuging the second liquid and collecting the first product, washing the first product and vacuum drying it, and calcining it at 400-500°C under an inert atmosphere for 2-4h to obtain PDA@cobalt-coated lithium nickel manganese oxide positive electrode material.

[0009] In one embodiment, S4 also includes adding carboxylated carbon nanotubes to the second liquid in a weight ratio of carboxylated carbon nanotubes: PDA = 1:1-1:3, maintaining stirring and ultrasonic treatment for 0.5-1h, collecting the second product after centrifugation, washing the second product and vacuum drying it, and calcining it at 400-500°C under an inert atmosphere for 2-4h to obtain PDA / CNT@cobalt-coated lithium nickel manganese oxide positive electrode material.

[0010] In a second aspect, the present application provides a cobalt-coated lithium nickel manganese oxide positive electrode material prepared according to the preparation method of the first aspect.

[0011] In one embodiment, the cobalt-coated lithium nickel manganese oxide positive electrode material is composed of primary grains and spherical secondary grains.

[0012] In one embodiment, in the PDA / CNT@cobalt-coated lithium nickel manganese oxide positive electrode material, at least a portion of the carbon nanotubes penetrate the PDA layer.

[0013] In a third aspect, the present application further provides a lithium-ion battery, the lithium-ion battery comprising a positive electrode and a negative electrode, the positive electrode of the lithium-ion battery being prepared from the cobalt-coated lithium nickel manganese oxide positive electrode material of the second aspect.

[0014] The cobalt-coated lithium nickel manganese oxide positive electrode material for lithium-ion batteries prepared by the method provided in the embodiment of the present application constructs a trace cobalt coating layer on the surface of the precursor by an in-situ coprecipitation method, utilizes the electronic conduction enhancement effect of cobalt to improve the interface stability, retains the cobalt-free property of the bulk phase of the material, and enables the material to have a complete crystal structure. When applied to lithium-ion batteries, it has good discharge specific capacity and cycle performance, and the electrochemical stability is significantly enhanced. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the implementation modes of the present application, the drawings required for use in the description of the implementation modes will be briefly introduced below. Obviously, the drawings described below are only some implementation modes of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0016] Figure 1 The Co-A-NM64 precursor of Example 1 of the present application, namely LiNi 0.6 Mn 0.4 SEM image of (OH)2 at a magnification of 5000 times.

[0017] Figure 2 A SEM image of Co-A-NM64 of Example 1 of the present application at a magnification of 5000 times is shown.

[0018] Figure 3 A SEM image of Co-A-NM64 of Example 1 of the present application at a magnification of 20,000 times is shown.

[0019] Figure 4 A SEM image of Co-A-NM64@PDA / CNT of Example 5 of the present application at a magnification of 20,000 times is shown.

[0020] Figure 5 The long cycle performance diagram of the battery made of the cobalt-coated lithium nickel manganese oxide positive electrode material of Example 1 of the present application is shown. DETAILED DESCRIPTION

[0021] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the implementation mode of the present application will be clearly and completely described below in conjunction with the drawings in the implementation mode of the present application. Obviously, the described implementation mode is only a part of the implementation mode of the present application, not all the implementation modes. Based on the implementation mode in the present application, all other implementation modes obtained by those skilled in the art without making creative work are within the scope of protection of the present application.

[0022] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0023] The present application embodiment provides a cobalt-coated lithium nickel manganese oxide positive electrode material prepared by the following steps: 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, exemplary, Co:TM (Ni + Mn) = 1:200 / 1:150 / 1:100 / 1:50. Molar ratio NH3:Co = 480:1-500:1, exemplary, 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, 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, wherein the lithium source can be Li2CO3 or LiOH; S3: Anneal the mixed powder at 700-1000°C in an air atmosphere for 10-15h. Preferably, heat the mixed powder to 400-600°C at a heating rate of 5-10°C / min in an air atmosphere and pre-sinter for 3-5h. 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 3h, 4h, 5h. Then, heat 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 12h and cooling, a cobalt-coated lithium nickel manganese oxide positive electrode material is obtained.

[0024] In one embodiment, the preparation method further includes S4: dispersing the cobalt-coated lithium nickel manganese oxide positive electrode material in tris (hydroxymethyl) aminomethane (Tris (hydroxymethyl) aminomethane, often referred to as Tris) buffer (pH = 8.5) at a weight ratio of cobalt-coated lithium nickel manganese oxide positive electrode material: Tris buffer = 1:100-2:100 to obtain a first liquid, ultrasonically treating the first liquid for 0.5-1h to ensure that the cobalt-coated lithium nickel manganese oxide positive electrode material is evenly dispersed, and then slowly adding 1%wt dopamine hydrochloride (DA) solution to the first liquid at a weight ratio of 1%wt dopamine hydrochloride solution: first liquid = 3:100-8:100 at a uniform speed, stirring at room temperature for 10-12h to obtain a second liquid, and controlling the stirring rate at about 300rpm. Dopamine undergoes oxidative self-polymerization in an alkaline environment to form a polydopamine (PDA) coating layer, which is coated on the cobalt-coated lithium nickel manganese oxide positive electrode material. After the second liquid was centrifuged, the first product was collected, and the first product was washed alternately with deionized water and ethanol for 3 times, vacuum dried at 60°C for 12 hours, and calcined at 400-500°C for 2-4 hours under an inert atmosphere to obtain PDA@cobalt-coated lithium nickel manganese oxide positive electrode material.

[0025] In one embodiment, S4 also includes adding carboxylated carbon nanotubes (CNT-COOH) to the second liquid, maintaining stirring and ultrasonically treating in an 80W 40kHz ultrasonic machine for 0.5-1h, collecting the second product after centrifugation, washing the second product alternately with deionized water and ethanol three times, vacuum drying it at 60°C for 12h, and calcining it at 400-500°C under an inert atmosphere for 2-4h to obtain PDA / CNT@cobalt-coated lithium nickel manganese oxide positive electrode material.

[0026] The cobalt-coated lithium nickel manganese oxide positive electrode material for lithium-ion batteries prepared by the method provided in the embodiment of the present application constructs a trace cobalt coating layer on the surface of the precursor by an in-situ coprecipitation method, utilizes the electronic conduction enhancement effect of cobalt to improve the interface stability, retains the cobalt-free property of the bulk phase of the material, and enables the material to have a complete crystal structure, good discharge specific capacity and cycle performance, and significantly enhanced electrochemical stability.

[0027] The technical scheme of the present invention is further described below by specific implementation. The models and manufacturers of the various instruments used in the following examples and comparative examples are: Vacuum drying oven, DZF-250, Beijing Yongguangming 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 testing system, CT2001A, Wuhan Blue Electric Electronics Co., Ltd.; electrochemical workstation, V54829, Ivium (Netherlands).

[0028] Embodiment 1:

[0029] Take Co(NO3)2·6H2O, 0.1596g and NM64 precursor (LiNi 0.6 Mn 0.4 5g of (OH)2), first add 0.1596g of Co(NO3)2·6H2O into 20mL of analytical pure ammonia water and stir until completely dissolved, then add NM64 precursor, stir for 1.5 hours and filter, wash with deionized water and anhydrous ethanol three times respectively, first put it in a 120℃ oven to dry for 2-3 hours, then transfer it to a vacuum drying oven at 80℃ and dry it for 12 hours to obtain Co-A-NM64 precursor, please refer to Figure 1 , Figure 1 A SEM image of the Co-A-NM64 precursor at a magnification of 5000 times is shown.

[0030] Take 5g of Co-A-NM64 precursor and 2.127g of Li2CO3 at a molar ratio of TM (Ni+Mn):Li=1:1.05, grind for 1 h until the mixture is uniform, and obtain a mixed powder of the precursor and the lithium source; Put the mixed powder of precursor and lithium source into the furnace and set the calcination program under air atmosphere: 1) Heating from room temperature to 500 °C at a heating rate of 5 °C / min and pre-sintering for 300 min; 2) Heating from 500°C to 850°C at a heating rate of 2°C / min and sintering for 720 min; 3) End the process and cool naturally to room temperature to obtain the cobalt-coated lithium nickel manganese oxide positive electrode material Co-LiNi 0.6 Mn 0.4 O2, denoted as Co-A-NM64; The sintered materials were stored in a vacuum drying oven at 80 °C.

[0031] The obtained Co-A-NM64 was tested by electron scanning electron microscopy (SEM). The SEM image obtained by the test is shown in Figure 2 and Figure 3 , see Figure 2-3 , showing that Co-A-NM64 has a spherical secondary grain morphology composed of primary grains and has a typical polycrystalline structure.

[0032] Embodiment 2:

[0033] The only difference between this embodiment and embodiment 1 is that 0.0798Co(NO3)2·6H2O and 5g of NM64 precursor are used, and other conditions and parameters are exactly the same as those in embodiment 1.

[0034] Embodiment 3:

[0035] The only difference between this embodiment and embodiment 1 is that 0.3192 g of Co(NO3)2·6H2O and 5 g of NM64 precursor are used, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0036] Embodiment 4:

[0037] The difference between this embodiment and embodiment 1 is that 1 g of sintered Co-A-NM64 material is dispersed in 100 mL Tris buffer (pH = 8.5), ultrasonically treated for 30 minutes to ensure uniform dispersion of particles, and 5 mL of 1% mass fraction dopamine hydrochloride (DA) solution is slowly and uniformly added to the above solution within 2 hours, and the stirring rate is controlled at 300 rpm to avoid particle agglomeration; then stirred continuously for 12 hours at room temperature (25 ° C), centrifuged at 8000 rpm for 10 minutes, and the product is washed alternately with deionized water and ethanol 3 times to remove unreacted DA monomer. Finally, vacuum dried at 60 ° C for 12 hours, and then calcined at 400 ° C for 4 hours under inert gas protection to obtain PDA @ cobalt-coated lithium nickel manganese oxide positive electrode material, recorded as Co-A-NM64@PDA.

[0038] Embodiment 5:

[0039] The difference between this embodiment and embodiment 1 is that 1 g of the sintered Co-A-NM64 material is taken, dispersed in 100 mL of Tris buffer (pH = 8.5), and ultrasonically treated for 30 minutes to ensure uniform dispersion of the particles. 5 mL of 1% mass fraction dopamine hydrochloride (DA) solution is slowly and uniformly added to the above solution within 2 hours, and the stirring rate is controlled at 300 rpm to avoid particle agglomeration; then stirring is continued for 12 hours at room temperature (25°C), 0.05 g of carboxylated carbon nanotubes (CNT-COOH) is added to the solution, stirring is maintained and ultrasonically treated in an 80W 40kHz ultrasonic machine for 60 minutes, centrifuged at 8000 rpm for 10 minutes, and the product is washed alternately with deionized water and ethanol three times, vacuum dried at 60°C for 12 hours, and then calcined at 400°C for 4 hours under inert gas protection to obtain Co-A-NM64@PDA / CNT. The obtained PDA / CNT@cobalt-coated lithium nickel manganese oxide positive electrode material was recorded as Co-A-NM64@PDA / CNT and subjected to electron scanning electron microscopy (SEM) test. The SEM image obtained by the test is shown in Figure 4 , Figure 4 It shows that the carbon nanotubes are uniformly attached to the surface of the material, and it can be seen that some of the carbon nanotubes penetrate the PDA layer.

[0040] Comparative Example 1: The difference between this embodiment and embodiment 1 is that the NM64 precursor (LiNi0.6 Mn 0.4 (OH)2) 5g, 5g NM64 precursor was added into 20mL analytical pure ammonia water, stirred for 1.5 h and then filtered, washed with deionized water and anhydrous ethanol three times respectively, first placed in a 120 ℃ oven to dry for 2-3 h, and then transferred to a vacuum drying oven at 80 ℃ to dry for 12 h to obtain A-NM64; Co(NO3)2·6H2O, A-NM64 and Li2CO3 were taken in a molar ratio of Co:TM(Ni+Mn):Li=0.01:1:1.05 and ground for 1 h until they were uniformly mixed to obtain a mixed powder. The calcination parameters of the mixed powder were exactly the same as those in Example 1.

[0041] Comparative Example 2: The difference between this embodiment and embodiment 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 mixed evenly to obtain a mixed powder, and the calcination parameters of the mixed powder are exactly the same as those in embodiment 1.

[0042] Comparative Example 3: The only difference between this embodiment and embodiment 1 is that NM64 precursor and Li2CO3 are taken in a molar ratio of TM (Ni+Mn):Li=1:1.05, ground for 1 h until mixed evenly to obtain a mixed powder, and the calcination parameters of the mixed powder are exactly the same as those in embodiment 1.

[0043] Comparative Example 4: The difference between this embodiment and embodiment 1 is that the amount of analytical pure ammonia water is changed to 5 ml, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0044] Comparative Example 5: The only difference between this embodiment and embodiment 4 is that the Co-A-NM64@PDA material obtained in embodiment 4 is dispersed uniformly in ethanol, then ultrasonically treated in an 80W 40kHz ultrasonic machine for 120s, collected by centrifugation and then dried.

[0045] Comparative Example 6: The only difference between this embodiment and embodiment 5 is that the Co-A-NM64@PDA / CNT material obtained in embodiment 5 is evenly dispersed in ethanol, then ultrasonically treated for 120 s in an 80 W 40 kHz ultrasonic machine, collected by centrifugation, and then dried.

[0046] Performance Testing: The positive electrode materials, conductive agent (superconducting carbon), and binder (preferably polyvinylidene fluoride PVDF) obtained in each embodiment and comparative example were weighed and dissolved in 1-methyl-2-pyrrolidone NMP according to a mass ratio of 8:1:1, and stirred for 4-8 h to obtain Co-A-NM64 positive electrode slurry. The positive electrode slurry was evenly coated on the aluminum foil current collector using a coating machine, vacuum dried for 12 h, and sliced ​​to obtain a positive electrode sheet with a diameter of 12 mm. The surface loading of the electrode sheet was about 4-7 mg cm -2 In a glove box filled with argon (H2O < 0.5ppm, O2 < 0.5ppm), the obtained positive electrode sheet was combined with a commercially available lithium sheet and a commercially available PP separator, and a commercial electrolyte (3240) was added and the battery casing was assembled to obtain a battery.

[0047] Battery performance test method: Cyclic performance test: Using the Wuhan Blue Electric test system in China, the constant current constant voltage charge and discharge (CCCV) curve of the battery was tested at a constant temperature of 30°C. The battery was charged to 4.45V at a current density of 180 mA / g, and then the voltage was maintained at 4.45V until the current density dropped below 9 mAh / g. It was best to discharge the battery to 2.7V at a current density of 180 mAh / g, which was recorded as the first cycle. The first reversible capacity D1 and the first coulombic efficiency were recorded. Repeat the above charge and discharge process, record the reversible capacity Dn of the nth cycle, and calculate the capacity retention rate of the nth cycle.

[0048] After the assembled battery was left to stand for 12 h, the electrochemical performance test was performed with the voltage window set to 2.7-4.45 V. The test results are shown in Table 1.

[0049] Table 1:

[0050] 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 higher first-cycle charge / discharge specific capacity and a significantly increased first-cycle coulomb efficiency under the test conditions of 2.7 V-4.45 V and 1 C. For example, the first-cycle specific capacity of Example 1 is 193.52 mAh / g, and the coulomb efficiency is 87.00%. In addition, please refer to Figure 5 ,Depend on Figure 5 It can be seen that the specific capacity of Example 1 is as high as 174.9 mAh / g after 250 cycles, and the capacity retention rate is 90.38%.

[0051] The cobalt coating process of Example 1 adopts an in-situ coprecipitation method to modify the precursor. Before calcination, cobalt nitrate is coprecipitated with the NM64 precursor to evenly distribute the cobalt element on the surface of the precursor and reduce the proportion of nickel elements on the surface, reduce the cation mixing on the surface, and then calcine to form a stable cobalt coating layer. This coating process enhances the stability of the material interface, inhibits surface reconstruction and transition metal TM dissolution during charging and discharging, and surface cobalt coating can improve reaction kinetics by accelerating ion and electron transport, thereby achieving a high first-cycle discharge capacity (193.52 mAh / g), high coulomb efficiency (87%) and excellent cycle retention rate (90.38% for 250 cycles). Example 2 uses a low cobalt ratio, and Example 3 uses a high cobalt ratio. Although both have high cycle retention rates (91.36% and 93.34%), the first cycle capacity (172.87 and 168.92 mAh / g) and Coulomb efficiency (81.63% and 80.28%) are lower than those of Example 1. This shows that although too little cobalt provides a certain effect of stabilizing the interface, too little cobalt fails to effectively promote ion conduction, and too much cobalt may result in too little nickel on the surface, which may form a special phase and hinder the diffusion of lithium ions.

[0052] In Comparative Example 1, cobalt nitrate was mixed evenly with A-NM64 and Li2CO3 and then calcined directly. Although the nickel element on the surface was still reduced in the first step, there was no in-situ cobalt coating layer, which may lead to uneven distribution of cobalt elements and fail to well inhibit surface reconstruction during charge and discharge. Although the first cycle capacity (187.63 mAh / g) and coulomb efficiency (86.95%) were close to those in Example 1, the cycle retention rate was only 78.20%, which was significantly reduced. In Comparative Example 2, the in-situ coprecipitation step was not performed, 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 brought into play, the presence of bulk cobalt also triggered a more serious phase change in the inherent charge and discharge process of cobalt-rich materials, reducing the cycle stability, and the first cycle discharge specific capacity (181.34 mAh / g), the first cycle coulomb efficiency (81.75%) and the cycle retention rate (68.31%) were low. The experimental results show that the coprecipitation method plays a key role in the in-situ construction of a stable and uniform cobalt coating.

[0053] Comparative Example 3 uses the original cobalt-free binary material, and the first cycle capacity (168.51 mAh / g) and cycle retention rate (86.42%) are significantly reduced, 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 insufficient ammonia water was present in the precursor synthesis step, resulting in insufficient coprecipitation reaction and uneven surface coating structure. The first cycle capacity (172.94 mAh / g) and retention rate (81.39%) are both lower than those in Example 1. The above results show that the modification process of the precursor has a great influence on the final performance of the material.

[0054] Example 4 introduces a polydopamine (PDA) layer on the surface of the cobalt-coated lithium-ion battery positive electrode material, and significantly improves the cycle life through its unique dual mechanism 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 layer, inhibit the dissolution of cobalt ions under high pressure (the dissolution amount can be reduced by more than 50%) and the migration of transition metals (reducing the amount of Mn deposition on the negative electrode surface). At the same time, its rich amino (-NH2) and phenolic hydroxyl (-OH) functional groups can form coordination bonds with cobalt, stabilize lattice oxygen and inhibit lattice distortion caused by oxygen vacancies. In 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 interfacial side reactions and reducing electrolyte decomposition, and the flexible structure of PDA can dynamically repair SEI cracks caused by charge and discharge volume expansion. In the end, its cycle performance was significantly improved, achieving multi-dimensional protection for high energy density positive electrode materials.

[0055] The Co-A-NM64@PDA / CNT material in Example 5 is due to the addition of CNT (carbon nanotubes), and the carboxylated CNT can effectively combine with the functional groups of PDA. At the same time, some CNTs penetrate the PDA layer in ultrasound, effectively fixing it on the cobalt coating layer, thereby having better mechanical stability. Therefore, the performance of Co-A-NM64@PDA / CNT after ultrasonic treatment does not change much. Therefore, CNT and PDA produce an effective synergistic effect. In addition, CNT has good electrical conductivity, especially after calcination. It is added between PDA and the cobalt coating layer, and between the cobalt coating layer and the outside world to establish an effective fast conductive network, achieving a significant improvement in the first-cycle coulomb efficiency. Comparative Examples 5 and 6 simulate the situation where two positive electrode materials are destroyed by external forces during actual use. The calcined Co-A-NM64@PDA material is brittle, and the coating layer is easily separated from the main body, causing it to fail, which in turn leads to reduced performance.

[0056] In addition, the description of the terms "some embodiments", "other embodiments", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In the present application, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine the different embodiments or examples described in the present application and the features of the different embodiments or examples without contradiction.

[0057] The above implementation modes are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned implementation modes, a person skilled in the art should understand that the technical solutions described in the aforementioned implementation modes can still be modified, or some of the technical features can be replaced by equivalents. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various implementation modes of the present application, and should all be included in 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.

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. The preparation method according to claim 1, characterized in that: The preparation method also includes S4: dispersing the cobalt-coated lithium nickel manganese oxide positive electrode material 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, ultrasonically treating the first liquid for 0.5-1h, and then slowly and uniformly adding 1%wt dopamine hydrochloride solution to the first liquid at a weight ratio of 1%wt dopamine hydrochloride solution: the first liquid = 3:100-8:100, stirring at room temperature for 10-12h to obtain a second liquid, centrifuging the second liquid and collecting a first product, washing the first product and vacuum drying it, and calcining it at 400-500°C for 2-4h under an inert atmosphere to obtain PDA@cobalt-coated lithium nickel manganese oxide positive electrode material.

6. The preparation method according to claim 5, characterized in that: S4 also includes adding carboxylated carbon nanotubes to the second liquid in a weight ratio of carboxylated carbon nanotubes: PDA = 1:1-1:3, maintaining stirring and ultrasonic treatment for 0.5-1h, collecting the second product after centrifugation, washing the second product and vacuum drying it, and calcining it at 400-500°C under an inert atmosphere for 2-4h to obtain PDA / CNT@cobalt-coated lithium nickel manganese oxide positive electrode material.

7. 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 6.

8. The cobalt-coated lithium nickel manganese oxide positive electrode material according to claim 7, characterized in that: The cobalt-coated lithium nickel manganese oxide positive electrode material is a spherical secondary grain morphology composed of primary grains.

9. The cobalt-coated lithium nickel manganese oxide positive electrode material according to claim 7, 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.

10. 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 7 to 9.

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