Two-dimensional heterojunction coated layered oxide positive electrode material and preparation method and application thereof

Abstract

The invention discloses a two-dimensional heterojunction coated layered oxide positive electrode material and a preparation method and application thereof, and belongs to the technical field of lithium ion battery positive electrode material modification. The preparation method of the two-dimensional heterojunction coated layered oxide positive electrode material comprises the following steps: sequentially and alternately carrying out porous titanium ferrite nanosheet coating and porous reduced graphene oxide coating on a layered oxide, and sintering to obtain the two-dimensional heterojunction coated layered oxide positive electrode material, the porous titanium iron oxide nanosheet is a two-dimensional Ti < 0.6 > Fe < 0.4 > O < 2 > nanosheet with a porous structure; the layered oxide comprises ternary lithium, lithium cobalt oxide or lithium-rich manganese-based oxide. The specific porous titanium ferrite nanosheet and the porous reduced graphene oxide are combined to coat the layered oxide, so that uniform coating of the two-dimensional heterojunction on the surface of the layered oxide positive electrode is realized, and the performance difference caused by non-uniform local coating is avoided.

Description

Technical Field

[0001] The present invention belongs to the technical field of modification of cathode materials for lithium - ion batteries, and more specifically relates to a cathode material with a two - dimensional heterojunction - coated layered oxide, its preparation method and application. Background Art

[0002] With the rapid development of new energy vehicles and electronic intelligent devices, lithium - ion batteries, as efficient energy storage devices, have received extensive attention. Among them, layered oxides have become a research hotspot due to their high reversible specific capacity and wide voltage window. Taking the LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811) material as an example, its high nickel content can provide a higher specific capacity, so it can meet the requirements of high - energy - density batteries. However, there are still some problems in the practical application of layered cathode materials: for example, side reactions are likely to occur on the material surface (such as electrolyte decomposition, oxygen evolution), resulting in poor cycle stability; in addition, the layered structure makes the material prone to structural collapse during charge - discharge processes, further reducing the cycle life and rate performance of the battery.

[0003] To overcome the above problems, researchers have tried various methods to modify layered oxide cathode materials. Among them, surface coating is an effective method. However, how to achieve uniform coating of two - dimensional heterojunctions on the surface of layered oxides remains a challenge. Therefore, it is crucial to develop a low - cost technology with a uniformly distributed surface coating layer for coating cathode materials. Summary of the Invention

[0004] The purpose of the present invention is to provide a cathode material with a two - dimensional heterojunction - coated layered oxide, its preparation method and application, so as to solve the problems existing in the above - mentioned prior art and realize the preparation of a cathode material with a two - dimensional heterojunction - coated layered oxide with low cost and high coating layer uniformity.

[0005] To achieve the above purpose, the present invention provides the following solutions:

[0006] One of the technical solutions of the present invention: provides a preparation method of a cathode material with a two - dimensional heterojunction - coated layered oxide, including the following steps:

[0007] The layered oxide is successively and alternately coated with porous titanium ferrite nanosheets and porous reduced graphene oxide, and then sintered to obtain the cathode material with a two - dimensional heterojunction - coated layered oxide;

[0008] The porous titanium ferrite nanosheets are two - dimensional Ti 0.6 Fe 0.4 O2 nanosheets with a porous structure;

[0009] The layered oxide includes ternary lithium, lithium cobaltate or lithium-rich manganese-based oxide;

[0010] The number of alternations is 1 time.

[0011] Two-dimensional nanomaterials have been widely used due to their unique mechanical properties and excellent electrochemical properties. As a typical two-dimensional material, graphene has a complete conjugated π-bond system and excellent electrical conductivity, and is widely used in the design of energy storage device materials. Different from graphene or graphene oxide, porous reduced graphene oxide has partial oxygen-containing functional groups (such as epoxy groups, carboxyl groups), and the conjugated π-bond network is restored by reduction, so it has both negative charge and electrical conductivity at the same time. Titanium ferrite has good stability and safety, and is also widely used in the field of energy storage. Each of the above two-dimensional nanomaterials has its own advantages. Therefore, combining them to form a two-dimensional heterojunction coating can significantly improve the performance of the materials.

[0012] Traditional coating methods, such as sol-gel, hydrothermal treatment and mechanical ball milling, are difficult to ensure uniformity. Atomic layer deposition (ALD) can achieve atomic-scale regulation and construct a uniform coating, but its high cost hinders its industrial application.

[0013] In the present invention, the chemical stability of porous titanium ferrite nanosheets is complementary to the high electrical conductivity of porous reduced graphene oxide, which not only inhibits the erosion of the electrolyte on the cathode material, but also improves the diffusion kinetics of lithium ions. Therefore, the layered oxide cathode material coated with the heterojunction has significantly improved interfacial stability and cycling stability, while maintaining good rate performance. The use of nanoscale porous titanium ferrite nanosheets realizes uniform coating of the two-dimensional heterojunction on the surface of the layered oxide cathode, avoiding performance differences caused by uneven local coating. And the method is simple to operate, does not require the introduction of other coupling agents, has low cost and high production efficiency, and is suitable for large-scale industrial production.

[0014] Preferably, the preparation steps of the porous titanium ferrite nanosheets include:

[0015] (1) Synthesize the precursor K 0.4 Ti 0.6 Fe 0.4 Li 0.02 O2: Take raw materials according to the corresponding molar ratio, mix them, and then perform heat treatment and calcination in sequence to obtain the precursor K 0.4 Ti 0.6 Fe 0.4 Li 0.02 O2;

[0016] (2) Synthesize the main body H 0.42 Ti 0.6 Fe 0.4 O2: Use the precursor K 0.4Ti 0.6 Fe 0.4 Li 0.02 Treat O2 in hydrochloric acid solution, collect the solid, which is the main body H 0.42 Ti 0.6 Fe 0.4 O2;

[0017] (3)Synthesize porous titanium ferrite nanosheets: Use tetrabutylammonium hydroxide solution to decompose the main body H 0.42 Ti 0.6 Fe 0.4 O2 to obtain nanosheets; Use hydrogen iodide to create pores in the nanosheets to obtain porous titanium ferrite nanosheets.

[0018] In the above step (3), I in hydrogen iodide - reacts with the nanosheets in an oxidation-reduction reaction, and I - transfers electrons to Fe 3 + , reducing it to Fe 2+ , forming I2 molecules. Fe 2+ is unstable and is oxidized to Fe2O3 particles. The formed I2 molecules wrap it and adhere to the surface of the nanosheets. After washing, the Fe2O3 particles are selectively removed from the nanosheets, thus forming nanopores to obtain two-dimensional Ti 0.6 Fe 0.4 O2 nanosheets with a porous structure.

[0019] Preferably, the temperature of the heat treatment is 800 - 1000 °C, and the time is 1 - 2 h; the temperature of the calcination is 800 - 1200 °C, and the time is 12 - 24 h.

[0020] Preferably, the concentration of the hydrochloric acid solution is 0.5 - 1 mol / L; the treatment time is 7 - 10 days.

[0021] Preferably, the concentration of the tetrabutylammonium hydroxide solution is 1 - 2 mol / L; the decomposition time is 7 - 10 days; the pore-forming time is 3 - 5 h.

[0022] In two-dimensional Ti 0.6 Fe 0.4 O2 nanosheets with a porous structure, different Fe contents will cause changes in the size of the pores formed by subsequent hydrogen iodide hole-digging. The higher the Fe content, the larger the formed pores. The larger the pores, the higher the lithium ion flux, and the lower the charge transfer resistance when assembled into a battery. If the Fe content is too low, pores cannot be formed.

[0023] Preferably, the coating of the porous titanium ferrite nanosheets includes: mixing the porous titanium ferrite nanosheets, a polar solution, and a solvent to obtain a mixed solution; placing the layered oxide powder in the mixed solution, mixing, and then drying at different temperatures to obtain the layered oxide coated with the porous titanium ferrite nanosheets.

[0024] Preferably, the coating of the porous reduced graphene oxide includes: mixing the porous reduced graphene oxide, a polar solution, and a solvent to obtain a mixed solution; placing the layered oxide coated with the porous titanium ferrite nanosheets in the mixed solution, treating it, and then sequentially removing the solvent and the polar solution to obtain the cathode material of the two-dimensional heterojunction-coated layered oxide.

[0025] Preferably, the boiling point of the polar solution is higher than that of the solvent; the polar solution independently includes acetone, acetonitrile, or ethanol; the solvent includes n-hexane; the concentration of the mixture obtained by mixing the porous titanium ferrite nanosheets and the polar solution is independently 0.1~2 mg / mL; the volume ratio of the mixture obtained by mixing the porous titanium ferrite nanosheets and the polar solution to the solvent is 10:1~8; the volume ratio of the mixture obtained by mixing the porous reduced graphene oxide and the polar solution to the solvent is 10:1~8; the mass ratio of the porous titanium ferrite nanosheets to the layered oxide powder is 1:200~400; the mass ratio of the porous reduced graphene oxide to the layered oxide coated with the porous titanium ferrite nanosheets is 1:200~400.

[0026] Preferably, the sintering includes: under a protective atmosphere, heating at a heating rate of 4~5 °C / min to 280~350 °C, and holding for 3~5 h.

[0027] The second technical solution of the present invention: provides a cathode material of the two-dimensional heterojunction-coated layered oxide prepared by the above preparation method.

[0028] Furthermore, the thickness of the coating layer of the porous reduced graphene oxide of the cathode material of the two-dimensional heterojunction-coated layered oxide is 3~4 nm, and the total thickness of the coating layers of the porous titanium ferrite nanosheets and the porous reduced graphene oxide is 8~9 nm.

[0029] The third technical solution of the present invention: provides an application of the above cathode material of the two-dimensional heterojunction-coated layered oxide in the preparation of lithium-ion batteries.

[0030] The technical mechanism of the present invention is:

[0031] The present invention realizes the uniform coating of a two-dimensional heterojunction on the surface of a layered oxide cathode through specific porous titanium ferrite nanosheets and porous reduced graphene oxide, avoiding performance differences caused by uneven local coating. The reason is as follows: The porous nanosheets are dispersed in a polar solution to form a basic solution with a certain concentration. When it is stirred or oscillated with a non-polar solution, the porous nanosheets spontaneously assemble at the interface of the two different solutions. Due to the large specific surface area and porosity of the porous nanosheets, they tend to adsorb at the oil-water interface to minimize the interfacial energy between immiscible liquids, thus stably forming emulsion droplets. During the subsequent drying process, the porous nanosheets will uniformly adhere to the surface of the layered oxide cathode to form a coating layer.

[0032] During the preparation of the porous titanium ferrite nanosheets, after treatment with hydroiodic acid, the interlayer spacing of the porous titanium ferrite nanosheets is reduced to about 1 nm, which is attributed to the removal of Fe atoms and the formation of nanopores, which will facilitate the passage of lithium ions. The present invention prepares porous titanium ferrite nanosheets through a specific method, and uses high-temperature annealing to dehydrate and condense the Ti atoms on the surface of the porous titanium ferrite nanosheets with the oxygen-containing functional groups of the porous reduced graphene oxide to form stable Ti-O-C covalent bonds, so as to form a two-dimensional heterojunction on the surface of the layered oxide, achieving a uniform coating effect compared with the nanosheets obtained by conventional methods.

[0033] The present invention sequentially coats the layered oxide with porous titanium ferrite nanosheets and porous reduced graphene oxide, and the coating order of the two cannot be reversed. This is because to make the coating layer present a gradient double-layer structure, the inorganic component in the inner layer acts as a physical barrier to inhibit interfacial side reactions; while the carbon component in the outer layer can accelerate charge transfer and act as a conductive agent.

[0034] The present invention discloses the following technical effects:

[0035] (1) Uniform coating: The present invention uses nanoscale porous titanium ferrite nanosheets and porous reduced graphene oxide to realize the uniform coating of a two-dimensional heterojunction on the surface of a layered oxide cathode, avoiding performance differences caused by uneven local coating.

[0036] (2) Structure regulation: The method of the present invention can not only adjust the thickness of the coating layer, but also form a heterostructure through multiple coatings, so as to realize functional regulation.

[0037] (3) Performance improvement: Through the complementarity of the chemical stability of the porous titanium ferrite nanosheets and the high conductivity of the porous reduced graphene oxide, the erosion of the electrolyte on the cathode material is inhibited, and the diffusion kinetics of lithium ions is improved. Therefore, the layered oxide cathode material after heterojunction coating has significantly improved interfacial stability and cycle stability, while maintaining good rate performance.

[0038] (4) Simple process: The method described in the present invention is easy to operate, without the need to introduce other coupling agents, and has low cost and high production efficiency, being suitable for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic flow chart of the preparation method of the cathode material of the two-dimensional heterojunction-coated layered oxide described in the present invention;

[0040] Figure 2 It is a physical diagram of the cathode material of the two-dimensional heterojunction-coated layered oxide described in the present invention;

[0041] Figure 3 It is an optical microscope image of the cathode material obtained in Example 1;

[0042] Figure 4 It is an SEM image of the cathode material obtained in Comparative Example 1;

[0043] Figure 5 It is an SEM image of the cathode material obtained in Comparative Example 2;

[0044] Figure 6 It is an SEM image of the cathode material obtained in Comparative Example 3;

[0045] Figure 7 It is an SEM image of the cathode material obtained in Comparative Example 4;

[0046] Figure 8 It is an SEM image of the cathode material obtained in Comparative Example 5;

[0047] Figure 9 It is an SEM image of the uncoated NCM811 cathode material described in Comparative Example 6;

[0048] Figure 10 It is an SEM image of the cathode material obtained in Example 1;

[0049] Figure 11 It is a TEM image of the cathode material obtained in Example 1;

[0050] Figure 12 It is a comparative curve graph of the cycling performance of the cathode materials obtained in Example 1 and Comparative Example 6 at 1C. DETAILED DESCRIPTION OF THE INVENTION

[0051] Now, various exemplary embodiments of the present invention will be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, characteristics, and implementation schemes of the present invention.

[0052] It should be understood that the terms described in the present invention are only for describing specific embodiments and are not intended to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0053] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0054] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific embodiments of the present invention specification, which are obvious to those skilled in the art. Other embodiments obtained from the specification of the present invention are obvious to those skilled in the art. The specification and examples of the present invention are merely exemplary.

[0055] Regarding the use of "comprising", "including", "having", "containing", etc. herein, they are all open-ended terms, meaning including but not limited to.

[0056] It should be noted that the aspects not detailedly described in the present invention are all conventional operating means in the art and are not the focus of the present invention.

[0057] Unless otherwise specified, the raw materials used in the following examples and comparative examples of the present invention are all commercially available products, and the sources of the commercially available products do not affect the technical effects of the present invention.

[0058] The preparation steps of the porous titanium iron oxide nanosheets used in the following examples and comparative examples of the present invention are as follows:

[0059] (1) Synthesize the precursor K 0.4 Ti 0.6 Fe 0.4 Li 0.02 O2: Weigh the raw materials according to the molar ratio of K2CO3, TiO2, Fe2O3 and Li2CO3 of 0.2:0.6:0.2:0.01 and mix them. Place the mixture in a mortar and grind it for 30 min, then heat it in a muffle furnace at 900 °C for 1 h in an air environment to decarbonize the mixture. Grind the cooled powder for 30 min, then heat it in air at 1000 °C for 24 h and cool it.

[0060] (2) Synthesize the host H through a protonation process 0.42 Ti 0.6 Fe 0.4 O2: Dissolve the precursor K 0.4 Ti 0.6 Fe 0.4 Li 0.02 O2 in 1 mol / L HCl solution and stir for 7 days. Collect the solid product H by filtration 0.42 Ti 0.6 Fe 0.4 O2, and wash it repeatedly with deionized water

[0061] (3)Exfoliate the nanosheets: Place H 0.42 Ti 0.6 Fe 0.4 O2 in 1.5 mol / L TBAOH solution and stir for 7 days to decompose it to obtain titanium iron oxide nanosheets (Ti 0.6 Fe 0.4 O2). Then wash it with deionized water to remove the residual TBAOH

[0062] (4)Pore formation: Disperse the Ti 0.6 Fe 0.4 O2 nanosheets in water to prepare a Ti 0.6 Fe 0.4 O2 nanosheet solution with a concentration of 1 mol / L; Use a peristaltic pump to slowly drip the above Ti 0.6 Fe 0.4 O2 nanosheet solution into a 2 mol / L hydroiodic acid solution, stir for 5 h, obtain a precipitate by centrifugation, and wash the obtained precipitate repeatedly with ethanol to remove the generated iodine. During the washing process, continue washing until both the supernatant and the precipitate become white. Finally, freeze-dry at -30 °C for 12 h to obtain porous titanium iron oxide nanosheets

[0063] The preparation steps of the porous reduced graphene oxide used in the following examples and comparative examples of the present invention are as follows

[0064] Using natural graphite powder as raw material, porous reduced graphene oxide was prepared by the Hummers method. Briefly, graphite powder (1 g) and potassium nitrate (1.2 g) were added to concentrated sulfuric acid (50 mL), and vigorously stirred at room temperature for 10 min. Then, potassium permanganate (6 g) was slowly added to the reaction system to avoid increasing the suspension temperature. The oxidation process was carried out at room temperature for 12 h. Subsequently, 250 mL of deionized water was poured into the resulting slurry to terminate the reaction. The prepared graphite oxide sample was ultrasonically exfoliated for 2 h, and then washed three times with deionized water to remove graphite powder and unexfoliated graphite oxide agglomerates. An aqueous solution of 30 wt% H2O2 (10 mL) was mixed with an aqueous solution of 2 mg / mL graphene oxide dispersion (100 mL), and stirred and heated at 100 °C for 4 h. After the prepared solution was centrifugally purified, it was washed with deionized water to remove residual H2O2. Then, the solution was dried with a freeze dryer to obtain black porous graphene oxide powder. An ascorbic acid solution (5 mL) was mixed with an aqueous solution of 2 mg / mL graphene oxide dispersion (100 mL), and stirred and heated at 80 °C for 4 h. Subsequently, it was washed three times with deionized water, and finally freeze-dried to obtain porous reduced graphene oxide nanosheets.

[0065] Unless otherwise specified, the room temperature involved in the present invention is calculated as 25 ± 5 °C.

[0066] Example 1

[0067] This example provides the preparation of a cathode material with a two-dimensional heterojunction-coated layered oxide, and the specific steps are as follows:

[0068] (1) 5 mg of porous titanium ferrite nanosheets were dispersed in 10 mL of acetonitrile, and the resulting basic solution concentration was 0.5 mg / mL. Subsequently, it was ultrasonically treated in a water bath for 1 h;

[0069] (2) 2 mL of n-hexane was added to the mixed solution in step (1), and oscillated for 3 min to form droplets;

[0070] (3) 2.5 g of NCM811 cathode powder was added to the droplets prepared in step (2), and then ultrasonically treated in a water bath (30 °C) for 30 min;

[0071] (4) The mixed solution obtained in step (3) was first vacuum dried at 70 °C for 10 min to remove n-hexane, and then vacuum dried at 85 °C for 1 h to remove acetonitrile, obtaining a NCM811 cathode material coated with porous titanium ferrite nanosheets;

[0072] (5) Replace the porous titanium ferrite nanosheets in step (1) with an equal amount of porous reduced graphene oxide, and replace the NCM811 cathode powder in step (3) with an equal amount of NCM811 cathode material coated with porous titanium ferrite nanosheets, and repeat steps (1) to (4); finally, heat the dried powder from room temperature to 320 °C at a rate of 5 °C / min under an argon atmosphere, hold for 3.5 h, and cool to obtain a cathode material with a two-dimensional heterojunction-coated layered oxide.

[0073] Example 2

[0074] The difference from Example 1 is: replace the "NCM811 cathode powder" with an equal mass of "lithium cobalt oxide cathode powder", and the others are the same as Example 1.

[0075] Example 3

[0076] The difference from Example 1 is: replace the "NCM811 cathode powder" with an equal mass of "lithium-rich manganese-based oxide cathode powder", and the others are the same as Example 1.

[0077] Comparative Example 1

[0078] The difference from Example 1 is: omit the coating of porous reduced graphene oxide, and the others are the same as Example 1.

[0079] Specifically:

[0080] (1) Disperse 5 mg of porous titanium ferrite nanosheets into 10 mL of acetonitrile, and the concentration of the resulting basic solution is 0.5 mg / mL, and then perform water bath sonication for 1 h;

[0081] (2) Add 2 mL of n-hexane to the mixed solution in step (1), and shake for 3 min to form droplets;

[0082] (3) Add 2.5 g of NCM811 cathode powder to the droplets prepared in step (2), and then perform water bath sonication for 30 min;

[0083] (4) First, vacuum dry the mixed solution obtained in step (3) at 70 °C for 10 min to remove n-hexane, and then vacuum dry at 85 °C for 1 h to remove acetonitrile; finally, heat the dried powder from room temperature to 320 °C at a rate of 5 °C / min under an argon atmosphere, hold for 3.5 h, and cool to obtain a cathode material with porous titanium ferrite nanosheets coated on a layered oxide.

[0084] Comparative Example 2

[0085] The difference from Example 1 is: omit the coating of porous titanium ferrite nanosheets, and the others are the same as Example 1.

[0086] Specifically:

[0087] (1) Disperse 5 mg of porous reduced graphene oxide into 10 mL of acetonitrile. The concentration of the resulting basic solution is 0.5 mg / mL, and then ultrasonicate it in a water bath for 1 h;

[0088] (2) Add 2 mL of n-hexane to the mixed solution in step (1), and oscillate for 3 min to form droplets;

[0089] (3) Add 2.5 g of NCM811 cathode powder to the droplets prepared in step (2), and then ultrasonicate it in a water bath for 30 min;

[0090] (4) First, vacuum dry the mixed solution obtained in step (3) at 70 °C for 10 min to remove n-hexane, and then vacuum dry it at 85 °C for 1 h to remove acetonitrile; finally, heat the dried powder from room temperature to 320 °C at a rate of 5 °C / min under an argon atmosphere, hold for 3.5 h, and cool to obtain a cathode material of porous reduced graphene oxide-coated layered oxide.

[0091] Comparative Example 3

[0092] The difference from Example 1 is that the amount of NCM811 cathode powder is adjusted to 5 g, and the others are the same as in Example 1.

[0093] Comparative Example 4

[0094] The difference from Example 1 is that the amount of NCM811 cathode powder is adjusted to 1 g, and the others are the same as in Example 1.

[0095] Comparative Example 5

[0096] The difference from Example 1 is that the amount of NCM811 cathode powder is adjusted to 0.5 g, and the others are the same as in Example 1.

[0097] Comparative Example 6

[0098] This comparative example is the NCM811 cathode material without any coating treatment.

[0099] Comparative Example 7

[0100] This comparative example is the lithium cobalt oxide cathode material without any coating treatment.

[0101] Comparative Example 8

[0102] This comparative example is the lithium-rich manganese-based oxide cathode material without any coating treatment.

[0103] Comparative Example 9

[0104] The difference from Example 1 is that the coating order of porous titanium ferrite nanosheets and porous reduced graphene oxide is reversed, and the others are the same as in Example 1.

[0105] Specifically:

[0106] (1) Disperse 5 mg of porous reduced graphene oxide into 10 mL of acetonitrile. The concentration of the resulting basic solution is 0.5 mg / mL, and then ultrasonicate it in a water bath for 1 h;

[0107] (2) Add 2 mL of n-hexane to the mixed solution in step (1), and oscillate for 3 min to form droplets;

[0108] (3) Add 2.5 g of NCM811 cathode powder to the droplets prepared in step (2), and then ultrasonicate it in a water bath (at 30 °C) for 30 min;

[0109] (4) First, vacuum dry the mixed solution obtained in step (3) at 70 °C for 10 min to remove n-hexane, and then vacuum dry it at 85 °C for 1 h to remove acetonitrile, obtaining the NCM811 cathode material coated with porous titanium ferrite nanosheets;

[0110] (5) Replace the porous reduced graphene oxide in step (1) with an equal amount of porous titanium ferrite nanosheets, and replace the NCM811 cathode powder in step (3) with an equal amount of NCM811 cathode material coated with porous reduced graphene oxide, and repeat steps (1) to (4); finally, heat the dried powder in an argon atmosphere from room temperature to 320 °C at a rate of 5 °C / min, hold for 3.5 h, and after cooling, obtain the cathode material of two-dimensional heterojunction-coated layered oxide.

[0111] Comparative Example 10

[0112] The difference from Example 1 is: replace the porous Ti 0.6 Fe 0.4 O2 nanosheets with porous Ti 0.8 Fe 0.1 O2 nanosheets, and the others are the same as Example 1.

[0113] Test the electrochemical performance of the cathode materials obtained in the following examples and comparative examples:

[0114] Test method: Mix the cathode materials obtained in each example and comparative example with conductive carbon black (Super-P) and polyvinylidene fluoride (PVDF) according to a mass ratio of 8:1:1, and add N-methylpyrrolidone (NMP) as a solvent (the amount of the solvent only needs to ensure that each raw material can be mixed evenly), and stir evenly to make a slurry. Coat the slurry on an aluminum foil (the coating thickness is 90 μm), then vacuum dry it at 70 °C for 12 h, punch it into a circular pole piece with a diameter of 14 mm, use it as the positive electrode of a lithium-ion battery, use a lithium metal sheet as the negative electrode, assemble it into a CR2032 type button lithium-ion battery in an argon atmosphere glove box, and conduct electrochemical performance tests at room temperature (the test conditions are 2.8~4.3 V, 1C). The results are shown in Table 1.

[0115] Table 1 Electrochemical Performance Test Results

[0116]

[0117] As can be seen from Table 1, the charge transfer resistance of Example 1 (27.34 Ω) decreased by 77.1% compared to Comparative Example 6 (119.40 Ω), proving that the two-dimensional heterojunction coating layer with a nanoporous structure significantly improved the lithium-ion transport efficiency and brought a lower charge transfer resistance. For Comparative Examples 4 (231.70 Ω) and 5 (355.80 Ω), the diffusion kinetics deteriorated due to the too thick coating layer, resulting in a significant reduction in the specific discharge capacity of the first cycle. Therefore, a porous nanosheet coating layer with an appropriate thickness can effectively improve the diffusion kinetics of lithium-ion cathode materials.

[0118] Compared with Comparative Example 2, Example 1 was closer to Example 4 in terms of charge transfer resistance and specific discharge capacity of the first cycle, but the capacity retention rate after 200 cycles was 83.52%, which was 13.4 percentage points higher than 70.12% of Comparative Example 2. The reason for this situation is that the porous titanium ferrite nanosheets contained in Example 1 have higher stability under oxidation conditions, and their surface passivation ability can inhibit the continuous erosion of the electrolyte on the positive electrode, thus prolonging the cycle life.

[0119] Compared with Comparative Examples 7 and 8, the capacity retention rates of Examples 2 and 3 were greatly improved after 200 cycles, which verified the feasibility of the two-dimensional heterojunction coating layer on other layered oxides.

[0120] Compared with Comparative Example 9, Example 1 was better than Comparative Example 9 in terms of charge transfer resistance and specific discharge capacity of the first cycle. The reason for this situation is that the porous reduced graphene oxide cannot play the role of accelerating charge transfer when it is in the inner layer.

[0121] Compared with Comparative Example 10, the charge transfer resistance of Example 1 was 27.34 Ω, which was much lower than 125.6 Ω of Comparative Example 10. The reason for this situation is that the Fe content of the porous titanium ferrite in Example 1 was higher than that of Comparative Example 10, and the formed pores were larger. Therefore, the lithium-ion flux was higher, and the charge transfer resistance during battery assembly was lower.

[0122] The various embodiments in this specification are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. For the same or similar parts between the embodiments, reference can be made to each other.

[0123] The foregoing description of the disclosed embodiments enables those skilled in the art to practice or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A preparation method of a cathode material of a two-dimensional heterojunction-coated layered oxide, characterized in that It includes the following steps: Successively and alternately coating the layered oxide with porous titanium ferrite nanosheets and porous reduced graphene oxide, and sintering to obtain the cathode material of the two-dimensional heterojunction-coated layered oxide; The porous titanium ferrite nanosheet is a two-dimensional Ti 0.6 Fe 0.4 O2 nanosheet; The layered oxide includes ternary lithium, lithium cobaltate or lithium-rich manganese-based oxide; The number of alternations is 1 time.

2. The preparation method according to claim 1, characterized in that, The preparation steps of the porous titanium ferrite nanosheets include: (1) Synthetic precursor K 0.4 Ti 0.6 Fe 0.4 Li 0.02 O2: Take raw materials according to the corresponding molar ratio, mix them, and then carry out heat treatment and calcination in sequence to obtain precursor K 0.4 Ti 0.6 Fe 0.4 Li 0.02 O2; (2) Synthesis of the main body H 0.42 Ti 0.6 Fe 0.4 O2: Treat the precursor K 0.4 Ti 0.6 Fe 0.4 Li 0.02 O2 in a hydrochloric acid solution, collect the solid, which is the main body H 0.42 Ti 0.6 Fe 0.4 O2; (3) Synthesize porous titanium ferrite nanosheets: Use tetrabutylammonium hydroxide solution to decompose the main body H 0.42 Ti 0.6 Fe 0.4 O2 to obtain nanosheets; Use hydrogen iodide to create pores in the nanosheets to obtain porous titanium ferrite nanosheets.

3. The preparation method according to claim 2, wherein, The temperature of the heat treatment is 800-1000°C, and the time is 1-2 h; the temperature of the calcination is 800-1200°C, and the time is 12-24 h; the concentration of the hydrochloric acid solution is 0.5-1 mol / L; the time of the treatment is 7-10 days.

4. The preparation method according to claim 2, wherein The concentration of the tetrabutylammonium hydroxide solution is 1-2 mol / L; the time of the decomposition is 7-10 days; the time of the pore formation is 3-5 h.

5. The preparation method according to claim 1, characterized in that, The porous titanium ferrite nanosheet coating includes: mixing the porous titanium ferrite nanosheets, a polar solution and a solvent to obtain a mixed solution; placing the layered oxide powder in the mixed solution, and successively removing the solvent and the polar solution after treatment to obtain the layered oxide coated with porous titanium ferrite nanosheets.

6. The preparation method according to claim 1, characterized in that, The porous reduced graphene oxide coating includes: mixing the porous reduced graphene oxide, a polar solution and a solvent to obtain a mixed solution; placing the layered oxide coated with porous titanium ferrite nanosheets in the mixed solution, and successively removing the solvent and the polar solution after treatment to obtain the cathode material of the two-dimensional heterojunction-coated layered oxide.

7. The preparation method according to claim 5 or 6, characterized in that, The boiling point of the polar solution is higher than that of the solvent; the polar solution independently includes acetone, acetonitrile or ethanol; the solvent includes n-hexane; the concentration of the mixture obtained by mixing the porous titanium ferrite nanosheets and the polar solution is independently 0.1-2 mg / mL; the volume ratio of the mixture obtained by mixing the porous titanium ferrite nanosheets and the polar solution to the solvent is 10:1-8; the volume ratio of the mixture obtained by mixing the porous reduced graphene oxide and the polar solution to the solvent is 10:1-8; the mass ratio of the porous titanium ferrite nanosheets to the layered oxide powder is 1:200-400; the mass ratio of the porous reduced graphene oxide to the layered oxide coated with porous titanium ferrite nanosheets is 1:200-400.

8. The preparation method according to claim 1, wherein, The sintering includes: under a protective atmosphere, heating to 280-350°C at a heating rate of 4-5°C / min, and holding for 3-5 h.

9. A cathode material of a two-dimensional heterojunction-coated layered oxide, characterized in that, The cathode material of the two-dimensional heterojunction-coated layered oxide is prepared by the preparation method described in any one of claims 1-8.

10. Application of a cathode material of a two-dimensional heterojunction-coated layered oxide in the preparation of a lithium-ion battery, characterized in that, Using the cathode material of the two-dimensional heterojunction-coated layered oxide described in claim 9 as the cathode material to prepare a lithium-ion battery.

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