A surface-coated ternary cathode material, a preparation method thereof and an application thereof

Abstract

The present invention relates to the technical field of lithium batteries, and discloses a surface-coated ternary cathode material, a preparation method and an application thereof; it includes a substrate and a composite coating layer outside the substrate, and the composite coating layer includes VGCF and LiFe2F6; in this application, by coating LiFe2F6 on the surface of the high-nickel ternary cathode material, the erosion of the electrolyte on the material is resisted, the volume change of the ternary material during charge and discharge is alleviated, and the cycle performance and safety of the battery are significantly improved; in addition, in this application, a secondary coating method is adopted during coating. First, FeF3, LiF, and VGCF are subjected to a primary coating, and then FeF2 is used to perform an impact thermal reaction on the primary-coated material for secondary coating. During the secondary coating process, LiFe2F6 is prepared, and ball milling is also carried out. While preparing LiFe2F, the coating thickness of LiFe2F can be controlled, significantly improving the product quality.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular to a surface-coated ternary cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] In the transformation of the energy pattern from high pollution and high emissions to low pollution and low emissions, energy storage facilities configured for clean energy such as wind power, solar power, and hydropower will be vigorously developed. Currently, the mainstream cathode materials of lithium-ion batteries adopted by leading global energy storage companies such as Tesla, CATL, and BYD are ternary materials. Ternary materials have the advantages of high energy density and good safety performance. However, they also have problems such as poor high and low temperature cycling performance, and serious gas generation during the charge and discharge process, resulting in battery swelling and deformation. Therefore, how to improve its high and low temperature performance and safety performance has become one of the key problems to be solved urgently.

[0003] In the prior art, the coating method is often used to process the ternary materials of lithium batteries to improve the surface safety performance of the ternary materials. For example, the publication number CN110010877B discloses a surface-coated high-nickel ternary material, a preparation method thereof, and an application thereof. The surface-coated high-nickel ternary material is a surface coating layer coated with sodium silicate and transition metal oxides on the surface of the high-nickel ternary material. This invention generates a binary coating layer composed of sodium silicate and transition metal oxides on the surface of the high-nickel ternary material. The sodium silicate with certain adhesive properties in the surface coating layer improves the interfacial stability between the metal oxide nanoparticles and the high-nickel ternary single crystal, forming an effective interfacial layer. Secondly, the amorphous sodium silicate composite layer can also reduce the erosion of the electrolyte on the high-nickel ternary material. The binary composite coating layer of sodium silicate / metal oxide effectively reduces the parasitic reaction on the surface of the high-nickel ternary single crystal and improves the long-life cycling performance of the material.

[0004] Another example is the publication number CN109244428B, which discloses a coating modification method for high-nickel ternary materials, including the following steps: preparing pyrophosphate: preparing pyrophosphate powder by a calcination method; coating pyrophosphate: mixing the prepared high-nickel ternary material powder with pyrophosphate powder, and sintering in an oxygen atmosphere to obtain a high-nickel ternary material coated with pyrophosphate; coating polymer: adding the high-nickel ternary material coated with pyrophosphate into a polymerization system for polymerization together. After polymerization, it is washed, filtered, and then vacuum dried to obtain a high-nickel ternary material coated with pyrophosphate and polymer. The coating of the polymer in this invention is carried out during the polymerization of the polymer, which can make the polymer coating more uniform. Part of the polymer coating layer is distributed in the voids where the phosphate coating layer contacts the high-nickel ternary material. This mixed coating method of ion conductor and electron conductor can strengthen the diffusion ability of ions and electrons at the interface, which is beneficial to improving the electrochemical performance of the material.

[0005] The lithium deintercalation / insertion reaction of the high-nickel ternary material occurs at the interface between the electrode and the electrolyte. Coating on the surface of the ternary material can significantly improve the corrosion resistance of the material and reduce the parasitic reaction between the material and the electrolyte. More common coating layer materials include oxides, fluorides, lithium ion conductors, etc. Due to the large amount of lithium deintercalation / insertion in the high-nickel ternary material, the material exhibits a certain volume change. After long-term cycling, interface separation occurs between some coating layer materials and the high-nickel ternary material, resulting in the failure of the protective effect of the coating layer. Coating the surface of the ternary material with LiFe2F6 can resist the erosion of the electrolyte on the material and also alleviate the volume change of the material during charge and discharge, thereby effectively improving the long-life cycle performance and safety of the battery. Summary of the Invention

[0006] In order to overcome the problems of poor cycle performance and low safety of ternary cathode materials in the prior art, the present application provides a surface-coated ternary cathode material, a preparation method and its application. By coating LiFe2F6 on the surface of the high-nickel ternary cathode material, the erosion of the electrolyte on the material is reduced, and at the same time, the volume change of the ternary material during charge and discharge is alleviated, significantly improving the cycle performance and safety of the battery.

[0007] The specific technical solution of the present invention is as follows:

[0008] A surface-coated ternary cathode material, comprising a substrate and a composite coating layer outside the substrate, and the composite coating layer comprises VGCF and LiFe2F6.

[0009] The present application uses LiFe2F6 as the coating layer of the high-nickel ternary cathode material, significantly improving the cycle performance and safety of the cathode material; LiFe2F6 has a triple rutile structure and has the ability to deintercalate / insert lithium ions by itself. Its theoretical capacity can reach 237 mAh / g. After coating on the surface of the high-nickel ternary material substrate, it improves the conduction of lithium ions. At the same time, the volume deformation of LiFe2F6 after delithiation is extremely small. After using LiFe2F6 as the coating layer of the high-nickel ternary material, the LiFe2F6 coating layer can form a strong outer shell. When the volume of the ternary material substrate expands, the LiFe2F6 outer shell can play a role of applying tightening, which can well inhibit the volume expansion of the substrate after charge and discharge, and significantly improve the cycle performance of the lithium battery.

[0010] Preferably, the chemical formula of the substrate is Li 1.02 Ni 1-x-y Co x Mn y M zO2, where M is selected from one of Al, Mg, Ti, and Si, and (1 - xy), x, y, and z are the molar ratios of Ni, Co, Mn, and M respectively, where: 0.6 ≤ (1 - x - y) < 1, 0 < x < 0.4, 0 < y < 0.4, 0 < z ≤ 0.02.

[0011] A preparation method of a surface-coated ternary cathode material, characterized in that the preparation steps include:

[0012] (1) According to the above molar ratios, nickel salt, cobalt salt, and manganese salt are configured into solution A, sodium hydroxide and ammonia water are configured into solution B, solution A and solution B are mixed and then heated and stirred evenly, aged, filtered, washed, and dried to obtain Ni 1-x-y Co x Mn y (OH)2 substrate precursor;

[0013] (2) According to the above molar ratios, take the Ni 1-x-y Co x Mn y (OH)2 high-nickel ternary cathode material precursor, add it to a lithium source and a salt solution of surface gradient doping elements to obtain a mixed raw material; the molar ratios of Li, Ni, Co, Mn, and M in the mixed raw material are 1.02: (1 - x - y): x: y: z; the mixed raw material is stirred evenly, heated to 60 - 80 °C, and dried; the dried material is sintered at high temperature in an oxygen atmosphere, cooled, crushed, and sieved to obtain Li 1.02 Ni 1-x-y Co x Mn y M z O2 substrate;

[0014] (3) Disperse FeF3, LiF, and VGCF in an adhesive to make a primary slurry, spray the primary slurry on the surface of the Li 1.02 Ni1 - x - yCoxMn y M z O2 substrate obtained in step (2) and perform heat treatment to make a primary coated substrate;

[0015] (4) Perform ball milling and heat treatment on the primary coated substrate obtained in step (3) and FeF2 to obtain a surface-coated ternary cathode material of LiFe2F6.

[0016] The present application also provides a method for preparing a surface-coated ternary cathode material. The preparation steps include the preparation of a ternary material precursor, the preparation of a ternary material substrate, the primary coating of the substrate, and the secondary coating of the substrate. The above method performs two coatings on the substrate respectively. First, FeF3, LiF, and VGCF are coated on the surface of the substrate using an adhesive to form a primary-coated substrate, and then the primary-coated substrate and FeF2 are subjected to ball milling and heating treatment. The present application does not directly coat LiFe2F6 on the surface of the substrate. When coating, it is necessary to control the thickness of the coating layer on the substrate surface. Therefore, after coating, the coated ternary material substrate needs to be ball milled according to the thickness of the coating layer. Usually, other harder polishing materials are used to polish the ternary material during the ball milling process. During this process, impurities will be introduced into the ternary cathode material. Therefore, the present application uses a secondary coating method to coat the ternary material substrate. When FeF2 impacts and heats FeF3 and LiF, FeF3 and LiF will first form an ordered triple rutile configuration, and then under the continuous impact of FeF2, Fe 2+ will occupy a part of the Fe in the triple rutile 3+ positions to form a mixed LiFe2F6 triple rutile configuration of Fe 2+ and Fe 3+ In the coating process of the present application, FeF3, LiF, and VGCF are first coated once, and then FeF2 is used to perform an impact thermal reaction on the primary-coated material for secondary coating. During the secondary coating process, LiFe2F6 is prepared and a ball milling process is also carried out. While preparing LiFe2F6, the coating thickness of LiFe2F6 can be controlled. This method does not introduce other impurities and the product quality is high.

[0017] Preferably, in the step (1), the heating temperature is 40 - 60°C and the stirring speed is 800 - 1000 rpm.

[0018] Preferably, in the step (2), the Li 1.02 Ni 1-x-y Co x Mn y M z O2 substrate is spherical-like with a particle size D50 of 5 - 12 μm.

[0019] Preferably, the process conditions for high-temperature sintering in the step (2) are: first raise the temperature to 400 - 550°C and sinter for 3 - 6 h, and then continue to raise the temperature to 650 - 850°C and sinter for 10 - 20 h.

[0020] Preferably, in the step (3), the heat treatment temperature is 300-500°C and the time is 3-6 h. By heat-treating the substrate coated with the primary slurry (300-500°C), carboxymethyl cellulose is gradually converted into carbon, and the organic components on the substrate surface are converted into inorganic components, increasing the conductivity of the coating layer to electrons and ions. After heat treatment, FeF3, LiF, VGCF, and the carbon structure form a dense coating layer shell, which is coated outside the ternary material substrate.

[0021] Preferably, in the step (3), the mass ratio of FeF3, LiF, VGCF, and the adhesive is 5-10:8-16:2-5:6-12, and the adhesive is an aqueous solution of sodium carboxymethyl cellulose (CMC).

[0022] Preferably, in the step (4), the ball milling speed is 200-500 rpm, the ball milling time is 12 h, and the heating temperature is 150°C.

[0023] Application of a surface-coated ternary cathode material in a lithium battery, using the above surface-coated ternary cathode material to prepare a lithium-ion battery.

[0024] Compared with the prior art, the present application has the following technical effects:

[0025] (1) By coating LiFe2F6 on the surface of the high-nickel ternary cathode material, it can resist the erosion of the electrolyte by the material, and can also alleviate the volume change of LiFe2F6 during charge and discharge, significantly improving the cycle performance and safety of the battery;

[0026] (2) In the present application, a secondary coating method is adopted during coating. First, FeF3, LiF, and VGCF are coated once, and then FeF2 is used to perform an impact thermal reaction on the once-coated material for secondary coating. During the secondary coating process, LiFe2F6 is prepared, and ball milling is also carried out. While preparing LiFe2F6, the coating thickness of LiFe2F6 can be controlled, significantly improving the product quality. Specific Embodiments

[0027] The present invention will be further described below in conjunction with embodiments.

[0028] Example 1:

[0029] (1) Prepare an aqueous solution A by mixing nickel salt (NiSO4·6H2O), cobalt salt (CoSO4·7H2O), and manganese salt (MnSO4·H2O) in a ratio of Ni:Co:Mn = 0.8:0.1:0.1. Prepare a mixed solution B of sodium hydroxide and ammonia water as a precipitant and complexing agent. Use a peristaltic pump to pump solutions A and B into a reaction kettle, control the pH value of the mixed solution in the reaction kettle to 11.5, the temperature to 50 °C, and the stirring speed to 900 r / min. Synthesize the precursor of the high-nickel ternary cathode material by the coprecipitation method, and then obtain the Ni 0.8 Co 0.1 Mn 0.1 (OH)2 precursor of the high-nickel ternary cathode material;

[0030] (2) Take the Ni 0.8 Co 0.1 Mn 0.1 (OH)2 prepared in step (1) above, and add it to an aqueous solution of LiOH in the corresponding amount according to the molar ratio, so that the molar ratio of Li, Ni, Co, and Mn in the mixed raw materials satisfies 1.02:0.8:0.1:0.1. Stir evenly, then heat to control the temperature at 80 °C to evaporate the water, and dry the obtained material; then carry out high-temperature sintering in an oxygen atmosphere. The sintering process is carried out under a pure oxygen atmosphere with stage temperature control. Heat at a heating rate of 3 °C / min to 500 °C, hold for 5 h, and then heat to 750 °C at a heating rate of 3 °C / min and calcine for 15 h. Cool with the furnace, crush, and screen to obtain the chemical formula Li 1.02 Ni 0.8 Co 0.1 Mn 0.1 Mg 0.01 O2 substrate.

[0031] (3) Mix and stir FeF3 (Alfa, 97%), LiF (Aladdin, 99.99%), VGCF (dried in a 60 °C vacuum oven), and CMC aqueous solution (0.005%) according to a mass ratio of 8:12:4:9 to make a primary slurry. Spray the primary slurry on the surface of the Li 1.02 Ni 0.8 Co 0.1 Mn 0.1 Mg 0.01 O2 substrate and carry out heat treatment to make a primary coated substrate. The heat treatment temperature is 400 °C and the heating time is 5 h;

[0032] (4) Use a ball mill to conduct ball milling and heat treatment on the primary coated substrate prepared in step (3) and FeF₂. The ball mill tank is cleaned with stainless steel balls before use, dried at 120 °C, the ball milling speed is 500 rpm, the ball milling time is 12 h, the heating temperature is 150 °C, and after ball milling and screening, a ternary cathode material with a surface coated with LiFe₂F₆ is obtained.

[0033] Example 2:

[0034] (1) Prepare an aqueous solution A by mixing nickel salt (NiSO₄·6H₂O), cobalt salt (CoSO₄·7H₂O), and manganese salt (MnSO₄·H₂O) in a ratio of Ni:Co:Mn = 0.8:0.1:0.1. Prepare a mixed solution B of sodium hydroxide and ammonia water as a precipitating agent and complexing agent. Use a peristaltic pump to pump solutions A and B into a reaction kettle, control the pH value of the mixed solution in the reaction kettle to 11.5, the temperature to 40 °C, and the stirring speed to 800 r / min. Synthesize a high-nickel ternary cathode material precursor by coprecipitation method, and then obtain a Ni 0.8 Co 0.1 Mn 0.1 (OH)₂ high-nickel ternary cathode material precursor through aging, filtration, washing, and drying;

[0035] (2) Take the Ni 0.8 Co 0.1 Mn 0.1 (OH)₂ prepared in step (1) above, add the corresponding amount of LiOH and water solution according to the molar ratio, so that the molar ratio of Li, Ni, Co, and Mn in the mixed raw materials satisfies 1.02:0.8:0.1:0.1, stir evenly, then heat to control the temperature at 80 °C to evaporate the water, and dry the obtained material; then carry out high-temperature sintering in an oxygen atmosphere. The sintering process is carried out under a pure oxygen atmosphere with stage temperature control. Heat up to 500 °C at a heating rate of 3 °C / min, hold for 5 h, and then heat up to 750 °C at a heating rate of 3 °C / min and calcine for 15 h, cool with the furnace, crush, and screen to obtain a substrate with the chemical formula Li 1.02 Ni 0.8 Co 0.1 Mn 0.1 Mg 0.01 O₂.

[0036] (3) Mix and stir FeF₃ (Alfa, 97%), LiF (Aladdin, 99.99%), VGCF (dried in a 60 °C vacuum oven), and CMC aqueous solution (0.005%) according to a mass ratio of 5:8:2:6 to prepare a primary slurry, and spray the primary slurry on the Li 1.02 Ni 0.8 Co 0.1 Mn0.1 Mg 0.01 Perform heat treatment on the surface of the O2 substrate to make a primary coated substrate. The heat treatment temperature is 300 °C and the heat treatment time is 3 h;

[0037] (4) Use a ball mill to perform ball milling and heat treatment on the primary coated substrate prepared in step (3) and FeF2. The ball mill tank is cleaned with stainless steel balls before use, dried at 120 °C, the ball milling speed is 500 rpm, the ball milling time is 12 h, the heating temperature is 150 °C, and after screening after ball milling, a ternary cathode material coated with LiFe2F6 on the surface is obtained.

[0038] Example 3:

[0039] (1) Prepare an aqueous solution A of nickel salt (NiSO4·6H2O), cobalt salt (CoSO4·7H2O), and manganese salt (MnSO4·H2O) in a ratio of Ni:Co:Mn = 0.8:0.1:0.1. Prepare a mixed solution B of sodium hydroxide and ammonia water as a precipitating agent and a complexing agent. Use a peristaltic pump to pump solutions A and B into the reaction kettle, control the pH value of the mixed solution in the reaction kettle to 11.5, the temperature to 40 °C, and the stirring speed to 1000 r / min. Synthesize a high-nickel ternary cathode material precursor by coprecipitation method, and then obtain Ni 0.8 Co 0.1 Mn 0.1 (OH)2 high-nickel ternary cathode material precursor through aging, filtration, washing, and drying;

[0040] (2) Take the Ni 0.8 Co 0.1 Mn 0.1 (OH)2 prepared in the above step (1), add it to an aqueous solution of LiOH in the corresponding amount according to the molar ratio, so that the molar ratio of Li, Ni, Co, and Mn in the mixed raw materials satisfies 1.02:0.8:0.1:0.1, stir evenly, then heat to control the temperature at 80 °C to evaporate the water, and dry the obtained material; then perform high-temperature sintering in an oxygen atmosphere. The sintering process is carried out under a pure oxygen atmosphere with stage temperature control. Heat up to 500 °C at a heating rate of 3 °C / min, hold for 5 h, and then heat up to 750 °C at a heating rate of 3 °C / min and calcine for 15 h, cool with the furnace, crush, and screen to obtain a substrate with the chemical formula Li 1.02 Ni 0.8 Co 0.1 Mn 0.1 Mg 0.01 O2.

[0041] (3) Mix and stir FeF3 (Alfa, 97%), LiF (Aladdin, 99.99%), VGCF (dried in a vacuum oven at 60 °C), and an aqueous CMC solution (0.005%) according to a mass ratio of 10:16:5:12 to prepare a primary slurry. Spray the primary slurry onto the surface of the Li 1.02 Ni 0.8 Co 0.1 Mn 0.1 Mg 0.01 O2 substrate and heat it to prepare a primary coated substrate. The heat treatment temperature is 500 °C and the heat treatment time is 6 h;

[0042] (4) Use a ball mill to perform ball milling and heat treatment on the primary coated substrate prepared in step (3) and FeF2. The ball mill tank is cleaned with stainless steel balls before use, dried at 120 °C, the ball milling speed is 500 rpm, the ball milling time is 12 h, the heating temperature is 150 °C, and after screening, a ternary cathode material coated with LiFe2F6 on the surface is obtained.

[0043] Example 4:

[0044] (1) Prepare an aqueous solution A by mixing nickel salt (NiSO4·6H2O), cobalt salt (CoSO4·7H2O), and manganese salt (MnSO4·H2O) in a ratio of Ni:Co:Mn = 0.9:0.5:0.5. Prepare a mixed solution B of sodium hydroxide and ammonia water as a precipitant and complexing agent. Use a peristaltic pump to pump solutions A and B into a reaction kettle, control the pH value of the mixed solution in the reaction kettle to 11.5, the temperature to 40 °C, and the stirring speed to 1000 r / min. Synthesize a high-nickel ternary cathode material precursor by coprecipitation method, and then obtain Ni 0.9 Co 0.05 Mn 0.05 (OH)2 high-nickel ternary cathode material precursor through aging, filtration, washing, and drying;

[0045] (2) Take the Ni 0.99 Co 0.05 Mn 0.05 (OH)2 prepared in the above step (1), add the corresponding amount of aqueous LiOH solution according to the molar ratio, so that the molar ratio of Li, Ni, Co, and Mn in the mixed raw materials satisfies 1.02:0.9:0.05:0.05, stir evenly, then heat to control the temperature at 80 °C to evaporate water, and dry the obtained material; then perform high-temperature sintering in an oxygen atmosphere. The sintering process is carried out under a pure oxygen atmosphere with stage temperature control. Heat it to 500 °C at a heating rate of 3 °C / min, hold for 5 h, and then heat it to 750 °C at a heating rate of 3 °C / min and calcine for 15 h, cool with the furnace, crush, and screen to obtain the chemical formula Li1.02 Ni 0.9 Co 0.05 Mn 0.05 Mg 0.01 substrate of O2.

[0046] (3) Mix and stir FeF3 (Alfa, 97%), LiF (Aladdin, 99.99%), VGCF (dried in a vacuum oven at 60 °C), and CMC aqueous solution (0.005%) according to a mass ratio of 8:12:4:9 to prepare a primary slurry. Spray the primary slurry on the Li 1.02 Ni 0.9 Co 0.05 Mn 0.05 Mg 0.01 O2 substrate surface and perform heat treatment to obtain a primary coated substrate. The heat treatment temperature is 400 °C and the heat treatment time is 4 h;

[0047] (4) Use a ball mill to perform ball milling and heat treatment on the primary coated substrate prepared in step (3) and FeF2. The ball mill tank is cleaned with stainless steel balls before use, dried at 120 °C, the ball milling speed is 500 rpm, the ball milling time is 12 h, the heating temperature is 150 °C, and after screening, a ternary cathode material with LiFe2F6 coated on the surface is obtained.;

[0048] Comparative Example 1: (No coating layer on the substrate surface)

[0049] Compared with Example 1, Comparative Example 1 has no steps (3) and (4), and the other conditions are the same as those in Example 1.

[0050] Comparative Example 2: (Coated with LiFe2F6)

[0051] Compared with Example 1, in Comparative Example 2: directly use LiFe2F6 to coat the substrate in step (3), mix and stir LiFe2F6, VGCF (dried in a vacuum oven at 60 °C), and CMC aqueous solution (0.005%) according to a mass ratio of 18:4:9 to prepare a slurry, and spray the slurry on the Li 1.02 Ni 0.8 Co 0.1 Mn 0.1 Mg 0.01 O2 substrate surface and perform heat treatment to obtain a LiFe2F6-coated Li 1.02 Ni 0.8 Co 0.1 Mn 0.1 Mg 0.01 O2 substrate, the heat treatment temperature is 400 °C and the heat treatment time is 4 h; For the LiFe2F6-coated Li 1.02Ni 0.8 Co 0.1 Mn 0.1 Mg 0.01 The O2 substrate was subjected to ball milling. During the ball milling process, zirconia was used, the ball milling speed was 1200 rpm, and the ball milling time was 24 h; the other conditions were the same as those in Example 1.

[0052] Comparative Example 3:

[0053] Compared with Example 1, in Comparative Example 3: the heat treatment temperature in step (3) was 200 °C, and the other conditions were the same as those in Example 1.

[0054] Comparative Example 4

[0055] Compared with Example 1, in Comparative Example 4: the heat treatment temperature in step (3) was 700 °C, and the other conditions were the same as those in Example 1.

[0056] The ternary cathode materials prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were made into lithium batteries. The preparation steps included: fully dry milling the active component, conductive agent, and binder in an agate mortar for about 10 min and then wet milling for 10 min with NMP to obtain a uniformly textured slurry. The ratio of ternary material: conductive agent (SP): PVDF was 8.5:0.5:0.1; the aluminum foil (thickness 0.016 mm) was laid flat on a casting machine, fixed after cleaning with alcohol, and the slurry was coated on the aluminum foil into a pole piece with a doctor blade (0.022 mm); the coated pole piece was vacuum baked in a vacuum oven at 120 °C for 6 h; the pole piece was cut into circular pieces with a diameter of 14 mm with a cutting machine to obtain the battery positive electrode piece; the battery positive electrode piece, lithium metal negative electrode, 2025-type battery case, pp separator, and 5V high-voltage electrolyte were assembled into a lithium battery.

[0057] The electrochemical performance of the materials was tested using a lithium battery test system. The first charge and discharge of the battery were tested at a 0.1C rate in a constant temperature environment of 25 °C, and the test voltage range was 2.8 V - 4.3 V; the cycle life test was carried out with charge and discharge cycling at a 1C rate, and the test voltage range was 2.8 V - 4.3 V. The capacity retention rate at a 1C rate refers to the percentage of the discharge specific capacity after cycling n times at a 1C rate to the first discharge specific capacity at a 1C rate. The electrochemical performance test results are shown in Table 1.

[0058] Table 1 Electrochemical Performance Test Results

[0059]

[0060] As can be seen from Table 1, when comparing Examples 1-3 with Comparative Example 1, it can be seen that after the substrate surface is coated with LiFe2F6, its initial discharge capacity increases, and the discharge capacity retention rate after 500 cycles at 1C is high. For the uncoated substrate, the discharge efficiency retention rate significantly decreases after 500 cycles at a 1C rate. This indicates that after the substrate surface is coated with LiFe2F6, it can effectively reduce the erosion of the electrolyte on the internal substrate, and significantly improve the cycling performance and safety performance of the lithium battery.

[0061] When comparing Examples 1-3 with Comparative Example 2, it can be seen that after using the secondary coating method provided in this application, the electrochemical performance of the lithium battery is better than that of directly using LiFe2F6 to coat the substrate. This indicates that the cathode material prepared by the secondary coating method provided in this application has better quality.

[0062] When comparing Examples 1-3 with Comparative Examples 3 and 4, in step (3) of the preparation method of the surface-coated ternary cathode material, the heat treatment process parameters are crucial. Too high heat treatment temperature will cause the structure of the matrix material to be damaged, thus affecting the performance of the product, and too low temperature will cause the formed coating layer to react incompletely and not be dense enough to play the role of a protective layer.

[0063] The above are only the preferred embodiments of the present invention, and do not impose any limitations on the present invention. Any simple modifications, changes, and equivalent transformations made to the above embodiments according to the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A surface-coated ternary cathode material, characterized in that, It includes a substrate and a composite coating layer outside the substrate. The composite coating layer includes VGCF and LiFe₂F₆. The chemical formula of the substrate is Li 1.02 Ni 1-x-y Co x Mn y M z O₂, where M is selected from one of Al, Mg, Ti, and Si. (1 - x - y), x, y, and z are the molar ratios of Ni, Co, Mn, and M respectively, where: 0.6 ≤ (1 - x - y) < 1, 0 < x < 0.4, 0 < y < 0.4, 0 < z ≤ 0.

02. LiFe₂F₆ is made by subjecting the primary coated substrate and FeF₂ to ball milling and heat treatment. The primary coated substrate is made by coating FeF₃, LiF, VGCF, and an adhesive on the surface of the substrate.

2. The preparation method of the surface-coated ternary cathode material according to claim 1, characterized in that, The preparation steps include: (1) Prepare solution A by mixing nickel salt, cobalt salt, and manganese salt, and prepare solution B by mixing sodium hydroxide and ammonia water. After mixing solution A and solution B, heat and stir evenly, age, filter, wash, and dry to obtain the Ni 1-x-y Co x Mn y (OH)2 substrate precursor; (2) According to the above molar ratio, take the Ni prepared in step (1) 1-x-y Co x Mn y (OH)2 high-nickel ternary cathode material precursor, and add it to a lithium source and a salt solution of surface gradient doping elements to obtain a mixed raw material; the molar ratio of Li, Ni, Co, Mn, and M in the mixed raw material is 1.02:(1 - x - y):x:y:z; stir the mixed raw material evenly, heat it to 60-80 °C, and dry it; sinter the dried material at high temperature in an oxygen atmosphere, cool it, crush it, and screen it to obtain Li 1.02 Ni 1-x-y Co x Mn y M z O2 substrate; (3) Disperse FeF3, LiF, and VGCF in an adhesive to make a primary slurry, and spray the primary slurry on the surface of the Li 1.02 Ni 1-x-y Co x Mn y M z O2 substrate and conduct a heat treatment to make a primary coated substrate; (4) Performing ball milling and heat treatment on the primary coated substrate prepared in step (3) and FeF2 to obtain a ternary cathode material with a surface coated with LiFe2F6.

3. The preparation method of a surface-coated ternary cathode material according to claim 2, characterized in that, In step (1), the heating temperature is 40 - 60 °C and the stirring speed is 800 - 1000 rpm.

4. The preparation method of a surface-coated ternary cathode material according to claim 2, characterized in that, In step (2), Li 1.02 Ni 1-x-y Co x Mn y M z The O2 substrate is spherical-like.

5. The preparation method of a surface-coated ternary cathode material according to claim 2, characterized in that, In step (2), the process conditions for high-temperature sintering are: first heating to 400 - 550 °C and sintering for 3 - 6 h, then continuing to heat to 650 - 850 °C and sintering for 10 - 20 h.

6. The preparation method of a surface-coated ternary cathode material according to claim 2, characterized in that, In step (3), the heating treatment temperature is 300 - 500 °C and the time is 3 - 6 h.

7. The preparation method of a surface-coated ternary cathode material according to claim 2, characterized in that, In step (3), the mass ratio of FeF3, LiF, VGCF, and the adhesive is 5 - 10: 8 - 16: 2 - 5: 6 - 12, and the adhesive is an aqueous solution of sodium carboxymethyl cellulose.

8. The preparation method of a surface-coated ternary cathode material according to claim 2, characterized in that, In step (4), the ball milling speed is 200 - 500 rpm, the ball milling time is 12 h, and the heating temperature is 150 °C.

9. Application of a surface-coated ternary cathode material in a lithium battery, characterized in that, A lithium-ion battery is prepared using the surface-coated ternary cathode material described in claim 1.

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