A nickel-cobalt-manganese ternary positive electrode material with a surface coating layer constructed by combining a wet-dry process and a preparation method thereof

Abstract

The present invention relates to a nickel-cobalt-manganese ternary positive electrode material and a preparation method thereof, which are combined with a wet-dry process to construct a surface coating layer in a stepwise manner, and belongs to the technical field of chemical energy storage batteries. The nickel-cobalt-manganese ternary positive electrode material is dispersed in anhydrous ethanol, and a precursor solution of the metal salt 1 is added thereto, heated and stirred to obtain a suspension; filtered, and the solid is dried to obtain a precipitate; metal salt 2 is added to the precipitate and stirred at high speed to obtain a mixture with metal salt 2 attached thereto; the mixture is calcined under an oxygen atmosphere to obtain the material. By introducing a readily soluble metal salt and a low-melting-point solid metal salt into the nickel-cobalt-manganese ternary positive electrode material in stages, taking into account the advantages of sufficient coating by the wet process and improving the efficiency of the wet process modification stage, in conjunction with subsequent dry process and primary heat treatment processes, a nickel-cobalt-manganese ternary material with a stepwise composite coating of metal oxide and lithium oxide is constructed on the surface of the material, thereby eliminating residual alkali on the surface and improving the electrochemical properties of the material.

Description

Technical Field

[0001] The present invention relates to a nickel-cobalt-manganese ternary positive electrode material with a surface coating layer constructed by a wet-dry combined cascade method and a preparation method thereof, belonging to the technical field of chemical energy storage batteries. Background Art

[0002] The current bottleneck in improving the energy density of power batteries lies in the positive electrode, with nickel-cobalt-manganese ternary materials and lithium iron phosphate materials as the primary technology approaches. The former has attracted widespread attention due to its high specific capacity, high discharge voltage, and reasonable cost. In particular, high-nickel ternary materials, with transition metal nickel content exceeding 60%, can significantly increase battery energy density by leveraging higher capacity, making them a focus of industry research. However, high-nickel ternary materials face challenges such as poor cycling performance caused by severe interfacial side reactions during electrochemical cycling, the propagation of undesirable surface phase transitions, and poor thermal stability, which have limited their further commercialization.

[0003] Constructing a coating layer on the surface of high-nickel ternary materials to protect the surface is an effective strategy. Forming a physical protective layer on the surface of the positive electrode can effectively isolate the direct contact between the positive electrode and the electrolyte, reduce the corrosive effect of interfacial side reactions and by-products on the positive electrode, and thus improve the electrochemical performance of the material. Commonly used coating media include oxides, phosphates or fast ion conductors, in order to provide chemical inertness while also stabilizing interfacial ion transport. Regarding the coating process, research reports mostly focus on wet coating and dry coating. In addition, some more sophisticated and high-cost coating processes such as atomic layer deposition (ALD) and chemical vapor deposition (CVD) have also been continuously developed, but these coating methods also have their own limitations.

[0004] Wet coating can achieve more complete "wetting" contact between the coating medium and the positive electrode material, but it often requires the use of solvents. The coating medium needs to be dissolved or dispersed in the solvent in different ways to achieve uniform contact with the positive electrode material, and the solvent is usually removed by complete evaporation or filtration. The former method has high energy consumption and low efficiency, while the latter is accompanied by the loss of some coating medium, resulting in insufficient surface coating. Therefore, improving the efficiency of the wet process is conducive to the modification of ternary materials. The dry coating method adopted in the industry is efficient and easy, but due to the insufficient solid-solid direct adsorption achieved by high-speed shear force, gaps can still exist. Therefore, impurities on the surface of the positive electrode material, especially the residual alkali formed after the material preparation, are not removed enough, and sufficient surface modification cannot be formed. How to take into account the process advantages of wet and dry methods, especially to overcome the shortcomings of the wet process that is cumbersome and time-consuming, so that the chemical state of the surface of the positive electrode material undergoes an orderly transformation and forms a favorable coating layer, is of significant significance for stabilizing the material surface and improving the cycle stability and rate performance of nickel-cobalt-manganese ternary positive electrode materials. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a nickel-cobalt-manganese ternary cathode material with a surface coating layer constructed in a tiered manner by combining a wet-dry process and a preparation method thereof. By introducing a readily soluble metal salt and a low-melting-point solid metal salt into the nickel-cobalt-manganese ternary cathode material in stages, taking into account the advantages of sufficient coating by the wet process and improving the efficiency of the wet process modification stage, and coordinating with subsequent dry processes and a single heat treatment process, a nickel-cobalt-manganese ternary material with a tiered composite coating of metal oxides and lithium oxides is constructed on the surface of the material, eliminating residual alkali on the surface, improving the electrochemical properties of the material, and solving the process problems of surface multifunctional coating layer modification, which has practical value.

[0006] To achieve the above objectives, the technical solutions of the present invention are as follows.

[0007] A method for preparing a nickel-cobalt-manganese ternary positive electrode material by combining a wet-dry method and a stepwise construction of a surface coating layer, the method comprising the following steps:

[0008] (1) preparing a precursor solution of metal salt 1;

[0009] (2) dispersing the nickel-cobalt-manganese ternary cathode material in anhydrous ethanol, adding the precursor solution of the metal salt 1 thereto, heating and stirring to obtain a suspension;

[0010] (3) filtering the suspension and drying the solid to obtain a precipitate, wherein the precipitate is a nickel-cobalt-manganese ternary positive electrode material with a metal salt 1 precursor attached to the surface;

[0011] (4) adding metal salt 2 to the precipitate and stirring at high speed, and using shear force to make the metal salt 2 adhere to the surface of the precipitate to obtain a mixture with the metal salt 2 attached;

[0012] (5) calcining the mixture in an oxygen atmosphere to obtain a nickel-cobalt-manganese ternary positive electrode material having a surface coating layer constructed by a wet-dry process combined with a stepwise process;

[0013] Wherein, the metal salt 1 is an acetate, oxalate or nitrate of a soluble metal element;

[0014] The metal salt 2 is an acetate, oxalate, nitrate or hydroxide of a low-melting-point metal element, and the melting point of the metal salt 2 is less than 600° C.

[0015] The amount of metal salt 1 added is 0.1-0.5 mol% of the nickel-cobalt-manganese ternary positive electrode material;

[0016] The amount of metal salt 2 added is 0.1-0.5 mol% of the nickel-cobalt-manganese ternary positive electrode material;

[0017] In step (2), the stirring temperature is 60°C to 80°C, the stirring rate is 300 rpm to 500 rpm, and the stirring time is 5 min to 10 min;

[0018] In step (4), the stirring rate is 1000 rpm to 1500 rpm, and the stirring time is 10 min to 30 min;

[0019] In step (5), the calcination temperature is 400°C to 600°C, and the calcination time is 3h to 5h.

[0020] Preferably, the soluble metal element is Al, Sc, Ti, V, Y, Zr, Nb, Mo, Sn, La or Ce.

[0021] Preferably, the low melting point metal element is Mg, Al, Sc, Ti, V, Y, Zr, Nb, Mo, Sn, La or Ce.

[0022] Preferably, in step (1), the metal salt 1 is dissolved in anhydrous ethanol and / or deionized water and stirred to dissolve to obtain a precursor solution of the metal salt 1.

[0023] Preferably, in step (2), the chemical formula of the nickel-cobalt-manganese ternary material is LiNi x Co y Mn 1-x-y O2, where 0.6≤x<1, 0<y≤0.2, 0<(1-xy)≤0.2.

[0024] Preferably, in step (2), the ratio of the nickel-cobalt-manganese ternary material to anhydrous ethanol is 0.1 g to 1 g: 1 mL.

[0025] A nickel-cobalt-manganese ternary positive electrode material with a surface coating layer constructed in a cascaded manner by combining a wet-dry method. The material is prepared by the above method, and the surface is compositely coated with metal oxide and lithium oxide in a cascaded manner.

[0026] A lithium-ion secondary battery, wherein the positive electrode material of the battery is a nickel-cobalt-manganese ternary positive electrode material with a surface coating layer constructed by a wet-dry combined cascade method as described in the present invention.

[0027] Beneficial effects

[0028] (1) The present invention provides a nickel-cobalt-manganese ternary positive electrode material and a preparation method thereof, wherein a metal salt source with specific physicochemical properties is introduced into the surface of the ternary positive electrode material particles in stages through a wet-dry process. The soluble metal salt can be uniformly coated on the material surface by wet short-term contact and subsequent filtration, and has good contact with the nano-residual alkali layer on the material surface. In the subsequent dry process, the shear force generated by high-speed stirring is used to disperse the same or another solid metal salt on the material surface over a large area. Finally, during the calcination process, the metal salt in contact with the residual alkali is directly converted into metal lithium oxide, and the rest is converted into metal oxide, that is, a tiered composite coating layer of metal oxide and lithium oxide is constructed on the material surface.

[0029] (2) The two-step composite process of the present invention can, on the one hand, precisely cover the surface impurity phase by wet infiltration, reduce the surface residual alkali amount of the ternary material, improve the air stability of the material, and inhibit the gas production behavior of the material under high charge state in the cycle; on the other hand, the constructed coating layer can accelerate the interfacial ion transport and inhibit the side reaction between the positive electrode material and the electrolyte, thereby improving the electrochemical performance and safety performance of the ternary material.

[0030] (3) From a process perspective, the method of the present invention takes into account the problems of defects in coating uniformity and adhesion strength in traditional single wet or dry modification, thereby improving the efficiency of the modification work. In addition, the method has no strict restrictions on the target modified matrix material, and the electrode material can be further expanded to other positive or negative electrode materials. It is highly compatible with current industrial equipment, simple and easy to use, and has industrialization potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The following are scanning electron microscope (SEM) images and energy dispersive spectrum (EDS) images of the materials described in Comparative Example 1 and Example 1.

[0032] Figure 2 This is a comparison chart of the electrochemical cycle performance test results of the materials described in Comparative Example 1 and Example 1.

[0033] Figure 3 This is a comparison chart of the electrochemical rate performance test results of the materials described in Comparative Example 1 and Example 1.

[0034] Figure 4 This is a comparison chart of the surface residual alkali measurement amounts of the materials described in Comparative Examples 1 to 4 and Examples 1 to 4. DETAILED DESCRIPTION

[0035] The present invention will be described in detail below with reference to the accompanying drawings.

[0036] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and Examples.It should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.In addition, the endpoints and any values ​​of the scope disclosed in this article are not limited to this accurate range or value, and these ranges or values ​​should be interpreted as comprising 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 a separate point value, and between the separate point value, one or more new numerical ranges can be combined with each other, and these numerical ranges should be considered as specifically disclosed in this article.

[0037] In the following examples and comparative examples, the material characterization and analysis methods used are as follows:

[0038] (1) Scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS) test: Scanning electron microscope, instrument model is FEIQuanta, Netherlands; Energy dispersive spectrometer, OXFORD IE 350, UK.

[0039] (2) X-ray diffraction (XRD) test: X-ray diffractometer, instrument model: Rigaku Ultima IV, Japan.

[0040] (3) Determination of residual alkali content of materials: pH titrator, instrument model: Thermo Scientific Orion StarT910, USA.

[0041] (4) Assembly and testing of CR2025 button cells: The positive electrode material (the final product prepared in the example), acetylene black, and polyvinylidene fluoride (PVdF) were prepared into a slurry at a mass ratio of 8:1:1 and coated on aluminum foil. The dried aluminum foil loaded with the slurry was cut into small discs with a diameter of approximately 1 cm using a cutting machine for use as the positive electrode. A metal lithium sheet was used as the negative electrode, Celgard 2500 was used as the separator, and a 1M carbonate solution was used as the electrolyte (wherein the solvent was a mixed solution of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in a volume ratio of 1:1:1, and the solute was LiPF6). CR2025 button cells were assembled in an argon atmosphere glove box. The charge and discharge current density was 1C = 200 mA / g, and the charge and discharge tester used was a Land CT2100A, China.

[0042] Comparative Example 1

[0043] To synthesize the ternary material LiNi 0.8 Co 0.1 Mn 0.1 Taking O2 as an example, the nickel-cobalt-manganese ternary positive electrode material is obtained by a high-temperature solid-phase reaction between a transition metal hydroxide precursor and a lithium salt.

[0044] (1) Weigh 420.56 g, 56.22 g, and 33.80 g of nickel sulfate hexahydrate, cobalt sulfate monohydrate, and manganese sulfate heptahydrate, respectively, at a molar ratio of Ni:Co:Mn=8:1:1, and prepare 1 L of a metal salt solution with a total metal ion concentration of 2 mol / L; (2) Weigh 160 g of NaOH powder and add deionized water to prepare 1 L of a 4 mol / L NaOH solution. Take 50 mL of a 30% ammonia solution and add deionized water to prepare 1 L of ammonia solution; (3) Add 1 L of deionized water as the reaction base liquid to the coprecipitation reactor. Use a peristaltic pump to continuously and slowly pump the prepared metal salt solution, NaOH solution, and ammonia solution into the reactor in an Ar atmosphere at a rate of 200 mL / h. Control the base liquid pH to 11, the temperature to 55°C, and the stirring rate to 600 r / min. After the feeding is completed, the reactor is aged for 12 hours. After aging, the material was filtered, washed repeatedly with deionized water until neutral, and dried in an oven at 80°C for 10 h to obtain the precursor Ni 0.8 Co 0.1 Mn 0.1 (OH)2. Take a certain amount of precursor and LiOH·H2O in a molar ratio of 1:1.05, disperse and mix them in anhydrous ethanol until the ethanol is completely volatilized, transfer them to a tube furnace in an O2 atmosphere, pre-calculate at 550°C for 5h, then heat to 750°C and keep it for 15h, and then cool naturally to room temperature to obtain LiNi 0.8 Co 0.1 Mn 0.1 O2 high nickel positive electrode material.

[0045] Example 1

[0046] First, niobium oxalate was used to prepare a niobium-containing precursor coating solution: 0.11 g of niobium oxalate salt was dissolved in 10 mL of deionized water and stirred thoroughly to dissolve it; 10 g of LiNi 0.8 Co 0.1 Mn 0.1 The O2 high-nickel ternary material is dispersed in 10 mL of anhydrous ethanol, to which the prepared niobium precursor coating solution is added, and the mixture is rapidly stirred at 500 rpm at 60 ° C for 5 minutes, and then filtered and dried to obtain the precipitate, which is the ternary positive electrode material with niobium salt attached to the surface. The precipitate is transferred to a high-speed stirrer, and 0.15 g of niobium hydroxide is further added to it. The stirring rate of the high-speed stirrer is controlled to 1500 rpm and stirred for 10 minutes to allow the solid niobium hydroxide to adhere well to the surface of the high-nickel material. The mixture is then transferred to a tube furnace and calcined at 500 ° C for 5 hours in a pure oxygen atmosphere to obtain a modified nickel-cobalt-manganese ternary positive electrode material with surface niobium oxide and lithium niobate cascaded composite coating.

[0047] The morphology and surface element analysis of the material prepared in Example 1 were performed using SEM and EDS techniques. The results are as follows: Figure 1 As shown, the surface of the coated material is smooth and a coating layer containing Nb is formed; the materials prepared in Comparative Example 1 and Example 1 are electrochemically tested using a constant current charge and discharge technique, and the test results are shown in FIG. Figure 2 As shown in FIG1 , the unmodified material directly prepared in Comparative Example 1 has a first-week discharge capacity of 192.4 mAh / g at a 1C rate, and a 50-week cycle retention rate of 83.5%; while the modified material in Example 1 has a first-week discharge capacity of 190.8 mAh / g at a 1C rate, and a 50-week cycle retention rate of 95.3%, which is significantly higher than that of the unmodified material. The rate performance test results are shown in FIG1 . Figure 3 As shown in Figure 1, the modified material in Example 1 can still release a higher specific capacity than the unmodified material at a high rate. The electrochemical test results show that the ternary cathode material prepared in Example 1 has good cycle stability and rate performance. Figure 4 As shown, the results show that the process in this embodiment significantly reduces the residual alkali content on the surface of the material by combining with the surface lithium impurities, improves the air storage stability, and the improved surface state is beneficial to the subsequent pulping.

[0048] Example 2

[0049] First, yttrium acetate was used to prepare the precursor coating solution containing yttrium: 0.078g of yttrium acetate was dissolved in 10mL of anhydrous ethanol and stirred thoroughly to dissolve it; 10g of LiNi 0.8 Co 0.1 Mn 0.1 The O2 high-nickel ternary material is dispersed in 10 mL of anhydrous ethanol, to which the prepared yttrium-containing precursor coating solution is added, and the precipitate is rapidly stirred at 500 rpm at 80 ° C for 5 minutes and then filtered and dried to obtain a ternary cathode material with yttrium salt attached to the surface. The precipitate is transferred to a high-speed stirrer, and 0.132 g of yttrium acetate is continued to be added thereto. The stirring rate of the high-speed stirrer is controlled to 1000 rpm and stirred for 30 minutes so that the solid yttrium acetate particles are well attached to the surface of the high-nickel material. The mixture is then transferred to a tube furnace and calcined at 600 ° C for 5 hours in a pure oxygen atmosphere to obtain a modified nickel-cobalt-manganese ternary cathode material with surface yttrium oxide and lithium yttrium oxide layered composite coating.

[0050] The morphology and surface element analysis of the material prepared in Example 2 were performed using SEM and EDS techniques. The results showed that the surface of the coated material was smooth and a coating layer containing Y was formed. The materials prepared in Comparative Example 1 and Example 2 were electrochemically tested using constant current charge and discharge technology. The unmodified material directly prepared in Comparative Example 1 had a discharge capacity of 192.4 mAh / g in the first week at a rate of 1C, and a cycle retention rate of 83.5% after 50 weeks. The modified material in Example 2 had a discharge capacity of 191.2 mAh / g in the first week at a rate of 1C, and a cycle retention rate of 94.4% after 50 weeks, which was significantly higher than that of the unmodified material. The rate performance test showed that the modified material in Example 2 was still able to release a higher specific capacity than the unmodified material at a high rate. The electrochemical test results showed that the ternary positive electrode material prepared in Example 2 had good cycle stability and rate performance. The results of determining the residual alkali content on the surface of the material are shown in the figure. Figure 4 As shown, the results show that the process in this embodiment significantly reduces the residual alkali content on the surface of the material by combining with the surface lithium impurities, improves the air storage stability, and the improved surface state is beneficial to the subsequent pulping.

[0051] Example 3

[0052] First, yttrium acetate was used to prepare the precursor coating solution containing yttrium: 0.3g of yttrium acetate was dissolved in 10mL of anhydrous ethanol and stirred thoroughly to dissolve it; 10g of LiNi 0.8 Co 0.1 Mn 0.1 The O2 high-nickel ternary material is dispersed in 40 mL of anhydrous ethanol, to which the prepared yttrium-containing precursor coating solution is added, and the mixture is rapidly stirred at 500 rpm for 5 minutes at 80 ° C and then filtered and dried to obtain the precipitate, which is the ternary positive electrode material with yttrium salt attached to the surface. The precipitate is transferred to a high-speed stirrer, and 0.058 g of magnesium oxalate is added thereto. The stirring rate of the high-speed stirrer is controlled to 1000 rpm and stirred for 30 minutes so that the solid magnesium oxalate particles are well attached to the surface of the high-nickel material. The mixture is then transferred to a tubular furnace and calcined at 600 ° C for 3 hours in a pure oxygen atmosphere to obtain a modified nickel-cobalt-manganese ternary positive electrode material with surface magnesium oxide and lithium yttrium oxide layered composite coating.

[0053] The morphology and surface element analysis of the material prepared in Example 3 were performed using SEM and EDS techniques. The results showed that the surface of the coated material was smooth, and a coating layer containing Y and Mg was formed. The constant current charge and discharge technology was used to perform electrochemical tests on the materials prepared in Comparative Example 1 and Example 3. The unmodified material directly prepared in Comparative Example 1 had a discharge capacity of 192.4 mAh / g in the first week at a rate of 1C, and a cycle retention rate of 83.5% after 50 cycles. The modified material in Example 3 had a discharge capacity of 190.2 mAh / g in the first week at a rate of 1C, and a cycle retention rate of 94.4% after 50 cycles, which were significantly higher than those of the unmodified material. The rate performance test showed that the modified material in Example 3 was still able to release a higher specific capacity than the unmodified material at a high rate. The electrochemical test results showed that the ternary positive electrode material prepared in Example 3 had good cycle stability and rate performance. The results of determining the residual alkali content on the surface of the material are shown in the figure. Figure 4 As shown, the results show that the process in this embodiment significantly reduces the residual alkali content on the surface of the material by combining with the surface lithium impurities, improves the air storage stability, and the improved surface state is beneficial to the subsequent pulping.

[0054] Example 4

[0055] First, cerium nitrate was used to prepare a cerium-containing precursor coating solution: 0.089g of cerium nitrate was dissolved in 2mL of deionized water and stirred thoroughly to dissolve it; 10g of LiNi 0.8 Co 0.1 Mn 0.1 The O2 high-nickel ternary material is dispersed in 8 mL of anhydrous ethanol, to which the prepared cerium-containing precursor coating solution is added, and the mixture is rapidly stirred at 400 rpm at 70 ° C for 10 minutes, and then filtered and dried to obtain the precipitate, which is the ternary positive electrode material with cerium salt attached to the surface. The precipitate is transferred to a high-speed stirrer, and 0.04 g of aluminum hydroxide is added to it. The stirring rate of the high-speed stirrer is controlled to 1200 rpm and stirred for 20 minutes to allow the solid aluminum hydroxide to adhere well to the surface of the high-nickel material. The mixture is then transferred to a tube furnace and calcined at 400 ° C for 4 hours in a pure oxygen atmosphere to obtain a modified nickel-cobalt-manganese ternary positive electrode material with a surface cerium oxide and lithium aluminate composite coating.

[0056] The morphology and surface element analysis of the material prepared in Example 4 were performed using SEM and EDS techniques. The results showed that the surface of the coated material was smooth, and a coating layer containing Ce and Al was formed. The materials prepared in Comparative Example 1 and Example 4 were electrochemically tested using constant current charge and discharge technology. The unmodified material directly prepared in Comparative Example 1 had a discharge capacity of 192.4 mAh / g in the first week at a rate of 1C, and a cycle retention rate of 83.5% after 50 weeks. The modified material in Example 4 had a discharge capacity of 191.5 mAh / g in the first week at a rate of 1C, and a cycle retention rate of 93.4% after 50 weeks, which was significantly higher than that of the unmodified material. The rate performance test showed that the modified material in Example 4 was still able to release a higher specific capacity than that of the unmodified material at a high rate. The electrochemical test results showed that the ternary positive electrode material prepared in Example 4 had good cycle stability and rate performance. The results of determining the residual alkali content on the surface of the material are shown in the figure. Figure 4 As shown, the results show that the process in this embodiment significantly reduces the residual alkali content on the surface of the material by combining with the surface lithium impurities, improves the air storage stability, and the improved surface state is beneficial to the subsequent pulping.

[0057] Comparative Example 2

[0058] First, cerium hydroxide was used to prepare a cerium-containing precursor coating solution: 0.02g of cerium hydroxide salt was added to 10mL of deionized water and stirred thoroughly; 10g of LiNi 0.8 Co 0.1 Mn 0.1 The O2 high-nickel ternary material was dispersed in 10 mL of anhydrous ethanol, to which the prepared cerium-containing precursor coating solution was added, and the mixture was rapidly stirred at 500 rpm for 5 minutes at 60°C, and then the precipitate was filtered and dried. The precipitate was transferred to a high-speed stirrer, and 0.55 g of niobium oxalate was added thereto. The stirring rate of the high-speed stirrer was controlled to 1500 rpm and stirred for 10 minutes to allow the solid niobium oxalate to adhere well to the surface of the high-nickel material. The mixture was then transferred to a tube furnace and calcined at 500°C for 5 hours in a pure oxygen atmosphere to obtain a modified nickel-cobalt-manganese ternary positive electrode material with surface niobium oxide and a small amount of lithium niobate as a composite coating.

[0059] The morphology and surface element analysis of the material prepared in Comparative Example 2 were performed using SEM and EDS techniques. The results showed that the surface of the coated material was not smooth, indicating that the metal salt that could not be fully dispersed by the wet method could not be uniformly coated, and only an incomplete coating layer containing Ce and Nb was formed. The Ce signal was weak, proving that the salt that could not be well dissolved and dispersed could not fully participate in the wet coating process, resulting in losses. The materials prepared in Comparative Example 1 and Comparative Example 2 were electrochemically tested using constant current charge and discharge technology. The unmodified material directly prepared in Comparative Example 1 was not smooth at 1C. At the 1C rate, the first-week discharge capacity is 192.4 mAh / g, and the 50-week cycle retention rate is 83.5%; while the modified material in Comparative Example 2 is cycled at a 1C rate, the first-week discharge capacity is 189.2 mAh / g, and the 50-week cycle retention rate is 79.5%, which is lower than that of the unmodified material. From the rate performance test, it can be seen that the modified material in Comparative Example 2 can release a capacity close to that of the unmodified material at a high rate. The electrochemical test results show that the ternary positive electrode material prepared in Comparative Example 2 failed to effectively improve the cycle stability and rate performance. The results of measuring the residual alkali content on the surface of the material are as follows: Figure 4 As shown, the results show that the process in this embodiment cannot fully combine with the surface lithium impurities, so the residual alkali content on the surface of the material is not significantly reduced, proving that for metal salts that cannot be completely dispersed in the wet process (cerium hydroxide is difficult to dissolve in deionized water to achieve full dispersion), the modification effect of the wet treatment step is limited, and the uneven dispersion leads to increased local impedance, which is not conducive to the material capacity. This proves the necessity of screening the metal salts used in the wet step.

[0060] Comparative Example 3

[0061] First, niobium oxalate was used to prepare a niobium-containing precursor coating solution: 0.11 g of niobium oxalate salt was dissolved in 10 mL of deionized water and stirred thoroughly to dissolve it; 10 g of LiNi 0.8 Co 0.1 Mn 0.1 The O2 high-nickel ternary material is dispersed in 10 mL of anhydrous ethanol, to which the prepared niobium-containing precursor coating solution is added. The mixture is rapidly stirred at 500 rpm at 60°C for 5 minutes, and then filtered and dried to obtain the precipitate, which is the ternary positive electrode material with niobium salt attached to the surface. The precipitate is transferred to a high-speed stirrer, and 0.15 g of niobium hydroxide is further added to it. The stirring rate of the high-speed stirrer is controlled to 1500 rpm and stirred for 10 minutes to allow the solid niobium hydroxide to adhere well to the surface of the high-nickel material. The mixture is then transferred to a tube furnace and calcined at 300°C for 2 hours in a pure oxygen atmosphere to obtain a surface-modified nickel-cobalt-manganese ternary positive electrode material.

[0062] The morphology and surface element analysis of the material prepared in Comparative Example 3 were performed using SEM and EDS techniques. The results showed that the surface of the coated material was smooth and a coating layer containing Nb was formed. The materials prepared in Comparative Example 1 and Comparative Example 3 were electrochemically tested using constant current charge and discharge technology. The unmodified material directly prepared in Comparative Example 1 had a discharge capacity of 192.4 mAh / g in the first week at a rate of 1C, and a cycle retention rate of 83.5% after 50 weeks. The modified material in Comparative Example 3 had a discharge capacity of 185 mAh / g in the first week at a rate of 1C, and a cycle retention rate of 75.9% after 50 weeks, which was lower than that of the unmodified material. The rate performance test also showed that the discharge capacity of the modified material in Comparative Example 3 at a high rate was lower than that of the unmodified material. The electrochemical test results showed that the ternary positive electrode material prepared in Comparative Example 3 failed to effectively improve the cycle stability and rate performance. The results of determining the residual alkali content on the surface of the material are shown in the figure. Figure 4 As shown, the results show that the process under this embodiment is not sufficiently combined with the surface lithium impurities, so the degree of reduction of the residual alkali content on the surface of the material is limited. The calcination temperature and duration adopted in this comparative example are outside the range described in the patent. The calcination conditions cannot provide sufficient energy to drive the exogenous coating material to undergo chemical combination, resulting in poor crystallization of the coating layer and increased particle impedance. Therefore, the electrochemical properties of the material cannot be improved, which also proves the necessity of optimizing the subsequent calcination treatment conditions.

[0063] Comparative Example 4

[0064] Compared with Example 4, the preparation of the cerium-containing precursor coating solution was omitted, and the two salts with the same amount as in Example 4 were directly introduced into the dry stirring process section, that is, 10g LiNi 0.8 Co 0.1 Mn 0.1 The O2 high-nickel ternary material was placed in a high-speed blender, to which 0.089g of cerium nitrate and 0.04g of aluminum hydroxide were added. The stirring rate of the high-speed blender was controlled at 1200rpm and stirred for 20 minutes to allow the solid salt to adhere well to the surface of the high-nickel material. The mixture was then transferred to a tube furnace and calcined at 400°C for 4 hours in a pure oxygen atmosphere to obtain a surface-modified modified nickel-cobalt-manganese ternary cathode material.

[0065] The morphology and surface element analysis of the material prepared in Example 4 were performed using SEM and EDS techniques. The results showed that the surface of the coated material was uneven, with attached particles and local clusters, and Ce and Al signals were observed. The materials prepared in Comparative Examples 1 and 4 were electrochemically tested using constant current charge and discharge technology. The unmodified material directly prepared in Comparative Example 1 had a first-week discharge capacity of 192.4 mAh / g at a rate of 1C, and a 50-week cycle retention rate of 83.5%. The modified material in Comparative Example 4 had a first-week discharge capacity of 184.2 mAh / g at a rate of 1C, and a 50-week cycle retention rate of 71.9%, which was lower than that of the unmodified material. From the rate performance test, it can be seen that the modified material in Comparative Example 4 has a lower capacity release at high rates than the unmodified material. The electrochemical test results show that the ternary positive electrode material prepared in Comparative Example 4 failed to effectively improve the cycle stability and rate performance. The results of determining the residual alkali content on the surface of the material are as follows: Figure 4 As shown, the results show that the process under this embodiment does not fully combine with the surface lithium impurities, so the degree of reduction of the residual alkali content on the surface of the material is limited. The process adopted in this comparative example cancels the wet treatment process in the front section, so it cannot take advantage of the full contact and uniform coating of the metal salt with the main body of the ternary material under the wet process, so it fails to form a favorable coating layer, and the excessive introduction of the solid-phase coating material causes the impedance of the material particles to increase, so it cannot improve the electrochemical properties of the material, which also proves the technical significance of the front-stage wet process to the overall modification scheme.

[0066] In summary, the invention includes but is not limited to the above embodiments. Any equivalent replacement or partial improvement made under the spirit and principle of the present invention shall be deemed to be within the scope of protection of the present invention.

Claims

1. A method for preparing a nickel-cobalt-manganese ternary cathode material by combining a wet-dry process with a stepwise construction of a surface coating layer, characterized by: The method comprises the following steps: (1) preparing a precursor solution of metal salt 1; (2) dispersing the nickel-cobalt-manganese ternary cathode material in anhydrous ethanol, adding the precursor solution of the metal salt 1 thereto, heating and stirring to obtain a suspension; (3) filtering the suspension and drying the solid to obtain a precipitate; (4) adding metal salt 2 to the precipitate and stirring at high speed to obtain a mixture with metal salt 2 attached thereto; (5) calcining the mixture in an oxygen atmosphere to obtain a nickel-cobalt-manganese ternary positive electrode material having a surface coating layer constructed by a wet-dry process combined with a stepwise process; Wherein, the metal salt 1 is an acetate, oxalate or nitrate of a soluble metal element; The metal salt 2 is an acetate, oxalate, nitrate or hydroxide of a low-melting-point metal element, and the melting point of the metal salt 2 is less than 600° C. The amount of metal salt 1 added is 0.1-0.5 mol% of the nickel-cobalt-manganese ternary positive electrode material; The amount of metal salt 2 added is 0.1-0.5 mol% of the nickel-cobalt-manganese ternary positive electrode material; In step (2), the stirring temperature is 60°C to 80°C, the stirring rate is 300 rpm to 500 rpm, and the stirring time is 5 min to 10 min; In step (4), the stirring rate is 1000 rpm to 1500 rpm, and the stirring time is 10 min to 30 min; In step (5), the calcination temperature is 400°C to 600°C, and the calcination time is 3h to 5h.

2. The method for preparing a nickel-cobalt-manganese ternary cathode material by combining a wet-dry process and a stepwise construction of a surface coating layer according to claim 1, characterized in that: The soluble metal element is Al, Sc, Ti, V, Y, Zr, Nb, Mo, Sn, La or Ce.

3. The method for preparing a nickel-cobalt-manganese ternary cathode material by combining a wet-dry process and a stepwise construction of a surface coating layer according to claim 1, characterized in that: The low melting point metal element is Mg, Al, Sc, Ti, V, Y, Zr, Nb, Mo, Sn, La or Ce.

4. The method for preparing a nickel-cobalt-manganese ternary cathode material by combining a wet-dry process and a stepwise construction of a surface coating layer according to claim 1, characterized in that: In step (1), the metal salt 1 is dissolved in anhydrous ethanol and / or deionized water and stirred to dissolve, thereby obtaining a precursor solution of the metal salt 1.

5. The method for preparing a nickel-cobalt-manganese ternary cathode material by combining a wet-dry process and a stepwise construction of a surface coating layer according to claim 1, characterized in that: In step (2), the chemical formula of the nickel-cobalt-manganese ternary positive electrode material is LiNi x Co y Mn 1-x- y O2, where 0.6≤x<1, 0<y≤0.2, 0<(1-xy)≤0.

2.

6. The method for preparing a nickel-cobalt-manganese ternary cathode material by combining a wet-dry process and a stepwise construction of a surface coating layer according to claim 1, characterized in that: In step (2), the ratio of the nickel-cobalt-manganese ternary positive electrode material to anhydrous ethanol is 0.1 g to 1 g: 1 mL.

7. A nickel-cobalt-manganese ternary cathode material with a surface coating layer constructed by a wet-dry process, characterized in that: The material is prepared by the method according to any one of claims 1 to 6, and the surface is coated with metal oxide and lithium oxide in a stepwise composite manner.

8. A lithium-ion secondary battery, characterized in that: The positive electrode material of the battery is a nickel-cobalt-manganese ternary positive electrode material with a surface coating layer constructed by a wet-dry combined cascade method as described in claim 7.

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