A two-dimensional heterojunction-coated layered oxide cathode material and its preparation method and application

Abstract

The present invention discloses a two-dimensional heterojunction coated layered oxide positive electrode material and its preparation method and application, which 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: the layered oxide is alternately coated with porous titanium iron oxide nanosheets and porous reduced graphene oxide, and sintered to obtain the two-dimensional heterojunction coated layered oxide positive electrode material; the porous titanium iron oxide nanosheet is a two-dimensional Ti with a porous structure. 0.6 Fe 0.4 O2 nanosheets; the layered oxide comprises ternary lithium, lithium cobalt oxide, or a lithium-rich manganese-based oxide. The present invention utilizes a combination of specific porous titanium iron oxide nanosheets and porous reduced graphene oxide to coat the layered oxide, achieving uniform coating of the layered oxide positive electrode surface with a two-dimensional heterojunction, thus avoiding performance variations caused by uneven localized coating.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion battery positive electrode material modification, and more specifically relates to a two-dimensional heterojunction-coated layered oxide positive electrode material and a preparation method and application thereof. Background Art

[0002] With the rapid development of new energy vehicles and electronic smart devices, lithium-ion batteries have attracted widespread attention as efficient energy storage devices. Among them, layered oxides have become a research hotspot due to their high reversible specific capacity and wide voltage window. 0.8 Co 0.1 Mn 0.1 Taking O2 (NCM811) material as an example, its high nickel content provides a higher specific capacity, thus meeting the needs of high-energy-density batteries. However, layered cathode materials still face some challenges in practical applications: for example, side reactions (such as electrolyte decomposition and oxygen evolution) are prone to occur on the material surface, resulting in poor cycling stability. In addition, the layered structure makes the material prone to structural collapse during charge and discharge, further reducing the battery's cycle life and rate performance.

[0003] To overcome these challenges, researchers have attempted various methods to modify layered oxide cathode materials. Surface coating is an effective approach. However, achieving uniform coating of the layered oxide surface with a two-dimensional heterojunction remains a challenge. Therefore, developing a low-cost, uniformly distributed coating technology for cathode materials is crucial. Summary of the Invention

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

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

[0006] One of the technical solutions of the present invention is to provide a method for preparing a positive electrode material of a two-dimensional heterojunction coated layered oxide, comprising the following steps:

[0007] The layered oxide is alternately coated with porous titanium iron oxide nanosheets and porous reduced graphene oxide in sequence, and sintered to obtain the positive electrode material of the two-dimensional heterojunction-coated layered oxide;

[0008] The porous titanium iron oxide nanosheet is a two-dimensional Ti with a porous structure. 0.6 Fe 0.4 O2 nanosheets;

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

[0010] The number of alternations is 1.

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

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

[0013] The present invention complements the chemical stability of porous titanium iron oxide nanosheets with the high conductivity of porous reduced graphene oxide, thereby suppressing the corrosion of the electrolyte on the positive electrode material and improving the diffusion dynamics of lithium ions. Therefore, the layered oxide positive electrode material after heterojunction coating has significantly improved in terms of interface stability and cycle stability, while maintaining good rate performance. By using nano-scale porous titanium iron oxide nanosheets, uniform coating of two-dimensional heterojunctions on the surface of the layered oxide positive electrode is achieved, avoiding performance differences caused by local uneven coating. In addition, the method is simple to operate, does not require the introduction of other coupling agents, has low cost, high production efficiency, and is suitable for large-scale industrial production.

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

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

[0016] (2) Synthetic entity H 0.42 Ti 0.6 Fe 0.4 O2: The precursor K 0.4Ti 0.6 Fe 0.4 Li 0.02 O2 is placed in a hydrochloric acid solution and the solid is collected, which is the main H 0.42 Ti 0.6 Fe 0.4 O2;

[0017] (3) Synthesis of porous titanium iron oxide nanosheets: The main H 0.42 Ti 0.6 Fe 0.4 O2 is decomposed to obtain nanosheets; hydrogen iodide is used to form pores in the nanosheets to obtain porous titanium iron oxide nanosheets.

[0018] In the above step (3), I in hydrogen iodide - Redox reaction with nanosheets, I - Transfer electrons to Fe 3 + , which is reduced to Fe 2+ , forming I2 molecules. Fe 2+ Unstable, oxidized to Fe2O3 particles, the formed I2 molecules wrap them and attach to the surface of the nanosheets. After washing, the Fe2O3 particles are selectively removed from the nanosheets, thereby forming nanopores and obtaining a two-dimensional Ti with a porous structure. 0.6 Fe 0.4 O2 nanosheets.

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

[0020] Preferably, the concentration of the hydrochloric acid solution is 0.5-1 mol / L; and 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; and the pore-forming time is 3-5 hours.

[0022] Two-dimensional Ti with porous structure 0.6 Fe 0.4 The varying Fe content in the O2 nanosheets causes variations in the size of the pores formed by subsequent drilling with hydrogen iodide. Higher Fe content results in larger pores. Larger pores increase lithium ion flux and lower charge transfer resistance when assembled into a battery. However, if the Fe content is too low, no pores will form.

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

[0024] Preferably, the porous reduced graphene oxide coating includes: mixing porous reduced graphene oxide, a polar solution and a solvent to obtain a mixed solution; placing the layered oxide coated with the porous titanium iron oxide nanosheets in the mixed solution for treatment, and then removing the solvent and the polar solution in sequence to obtain a two-dimensional heterojunction-coated layered oxide positive electrode material.

[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 and the layered oxide powder is 1:200~400; the mass ratio of the porous reduced graphene oxide and the layered oxide coated with the porous titanium ferrite nanosheets is 1:200~400.

[0026] Preferably, the sintering comprises: heating the temperature to 280-350° C. at a heating rate of 4-5° C. / min under a protective atmosphere, and keeping the temperature for 3-5 hours.

[0027] The second technical solution of the present invention is to provide a positive electrode material of a two-dimensional heterojunction coated layered oxide prepared by the above preparation method.

[0028] Furthermore, the thickness of the porous reduced graphene oxide coating layer of the two-dimensional heterojunction coated layered oxide positive electrode material is 3-4 nm, and the total thickness of the porous titanium iron oxide nanosheet coating layer and the porous reduced graphene oxide coating layer is 8-9 nm.

[0029] The third technical solution of the present invention is to provide an application of the above-mentioned two-dimensional heterojunction-coated layered oxide positive electrode material 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 two-dimensional heterojunction on the surface of layered oxide positive electrode through specific porous titanium iron oxide nanosheets and porous reduced graphene oxide, avoiding the performance difference caused by local uneven coating. The reason is that the porous nanosheets are dispersed in the polar solution to form a basic solution of a certain concentration. When it is stirred or oscillated with the non-polar solution, the porous nanosheets spontaneously assemble at the interface of the two different solutions. Since the porous nanosheets have a large specific surface area and porosity, they tend to be adsorbed at the oil-water interface to minimize the interfacial energy between the immiscible liquids, thereby stably forming emulsion droplets. In the subsequent drying process, the porous nanosheets will evenly adhere to the surface of the layered oxide positive electrode to form a coating layer.

[0032] During the preparation of porous titanium iron oxide nanosheets, after hydroiodic acid treatment, the interlayer spacing of the porous titanium iron oxide nanosheets is reduced to approximately 1 nm. This 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 iron oxide nanosheets through a specific method and uses high-temperature annealing to dehydrate and condense the Ti atoms on the surface of the porous titanium iron oxide nanosheets with the oxygen-containing functional groups of porous reduced graphene oxide, forming stable Ti-OC covalent bonds. This allows the formation of a two-dimensional heterojunction on the surface of the layered oxide, achieving a more uniform coating effect than nanosheets obtained by conventional methods.

[0033] The present invention sequentially coats the layered oxide with porous titanium iron oxide nanosheets and porous reduced graphene oxide, and the coating order of the two cannot be interchanged. This is because the coating layer must present a gradient double-layer structure. The inorganic components of the inner layer act as a physical barrier to inhibit interface side reactions; while the carbon components of 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 nano-scale porous titanium iron oxide nanosheets and porous reduced graphene oxide to achieve uniform coating of the two-dimensional heterojunction on the surface of the layered oxide positive electrode, avoiding performance differences caused by local uneven coating.

[0036] (2) Structural regulation: The method described in the present invention can not only adjust the thickness of the coating layer, but also form a heterogeneous structure through multiple coatings, thereby achieving functional regulation.

[0037] (3) Performance improvement: The chemical stability of porous titanium iron oxide nanosheets complements the high conductivity of porous reduced graphene oxide, which not only inhibits the corrosion of the electrolyte on the positive electrode material, but also improves the diffusion kinetics of lithium ions. As a result, the layered oxide positive electrode material after heterojunction coating has significantly improved the interface stability and cycle stability, while maintaining good rate performance.

[0038] (4) Simple process: The method of the present invention is easy to operate, does not require the introduction of other coupling agents, has low cost, high production efficiency, and is suitable for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Schematic diagram of the process for preparing the positive electrode material of the two-dimensional heterojunction coated layered oxide according to the present invention;

[0040] Figure 2 This is a physical picture of the positive electrode material of the two-dimensional heterojunction coated layered oxide according to the present invention;

[0041] Figure 3 This is an optical microscope image of the positive electrode material obtained in Example 1;

[0042] Figure 4 This is the SEM image of the positive electrode material obtained in Comparative Example 1;

[0043] Figure 5 This is the SEM image of the positive electrode material obtained in Comparative Example 2;

[0044] Figure 6 This is the SEM image of the positive electrode material obtained in Comparative Example 3;

[0045] Figure 7 This is the SEM image of the positive electrode material obtained in Comparative Example 4;

[0046] Figure 8 This is the SEM image of the positive electrode material obtained in Comparative Example 5;

[0047] Figure 9 This is an SEM image of the NCM811 positive electrode material described in Comparative Example 6 that has not undergone any coating treatment;

[0048] Figure 10 This is a SEM image of the positive electrode material obtained in Example 1;

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

[0050] Figure 12 The graph is a comparison of the cycle performance of the positive electrode materials obtained in Example 1 and Comparative Example 6 at 1C. DETAILED DESCRIPTION

[0051] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0052] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value within any stated value or stated range, and each smaller range between any other stated value or intermediate value within the stated range, is also encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0053] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice 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 associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0054] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be illustrative only.

[0055] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0056] It should be pointed out that the matters not described in detail in the present invention are conventional operating means in this field 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) Synthesis of precursor K 0.4 Ti 0.6 Fe 0.4 Li 0.02 O2: Weigh and mix K2CO3, TiO2, Fe2O3, and Li2CO3 in a molar ratio of 0.2:0.6:0.2:0.01. Grind the mixture in a mortar for 30 minutes. Then, heat it in an air muffle furnace at 900°C for 1 hour to decarburize the mixture. Grind the cooled powder for 30 minutes, then heat it in air at 1000°C for 24 hours and cool it.

[0060] (2) Synthesis of the main H through the protonation process 0.42 Ti 0.6 Fe 0.4 O2: the precursor K 0.4 Ti 0.6 Fe 0.4 Li 0.02 O2 dissolved in 1 mol / L The mixture was stirred for 7 days and the solid product H was collected by filtration. 0.42 Ti 0.6 Fe 0.4 O2 and washed repeatedly with deionized water.

[0061] (3) Peeling off nanosheets: 0.42 Ti 0.6 Fe 0.4 O2 was placed in a 1.5 mol / L TBAOH solution and stirred for 7 days to decompose and obtain titanium iron oxide nanosheets (Ti 0.6 Fe 0.4 O2). It was then washed with deionized water to remove residual TBAOH.

[0062] (4) Pore formation: Ti 0.6 Fe 0.4 O2 nanosheets were dispersed in water to prepare TiO2 with a concentration of 1 mol / L. 0.6 Fe 0.4 O2 nanosheet solution; use a peristaltic pump to 0.6 Fe 0.4 The O2 nanosheet solution was slowly added dropwise to a 2 mol / L hydroiodic acid solution and stirred for 5 hours. The resulting precipitate was centrifuged and repeatedly washed with ethanol to remove the generated iodine. Washing was continued until both the supernatant and the precipitate turned white. Finally, the solution was freeze-dried at -30°C for 12 hours 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] Porous reduced graphene oxide (POGO) was prepared using natural graphite powder as the starting material using the Hummers method. Briefly, graphite powder (1 g) and potassium nitrate (1.2 g) were added to concentrated sulfuric acid (50 mL) and stirred vigorously at room temperature for 10 minutes. Potassium permanganate (6 g) was then slowly added to the reaction system to avoid increasing the suspension temperature. The oxidation process was carried out at room temperature for 12 hours. The resulting slurry was then poured into 250 mL of deionized water to terminate the reaction. The prepared GO sample was exfoliated by ultrasonication for 2 hours and then washed three times with deionized water to remove graphite powder and unexfoliated GO agglomerates. A 30 wt% H₂O₂ aqueous solution (10 mL) was mixed with a 2 mg / mL GO dispersion in water (100 mL) and heated at 100°C with stirring for 4 hours. The resulting solution was purified by centrifugation and washed with deionized water to remove residual H₂O₂. The solution was then dried using a freeze dryer to obtain black, porous GO powder. Ascorbic acid solution (5 mL) was mixed with 2 mg / mL graphene oxide dispersed aqueous solution (100 mL), stirred and heated at 80 °C for 4 h, then washed three times with deionized water, and finally freeze-dried to obtain porous reduced graphene oxide nanosheets.

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

[0066] Example 1

[0067] This embodiment provides a preparation method for a positive electrode material comprising a two-dimensional heterojunction coated layered oxide, and the specific steps are as follows:

[0068] (1) 5 mg of porous titanium iron oxide nanosheets were dispersed in 10 mL of acetonitrile to form a base solution with a concentration of 0.5 mg / mL, followed by water bath sonication for 1 h;

[0069] (2) Add 2 mL of n-hexane to the mixed solution of step (1) and shake for 3 minutes to form droplets;

[0070] (3) Add 2.5 g of NCM811 cathode powder to the droplets prepared in step (2), followed by water bath sonication (30 °C) for 30 min;

[0071] (4) The mixed solution obtained in step (3) was first vacuum dried at 70°C for 10 minutes to remove n-hexane, and then vacuum dried at 85°C for 1 hour to remove acetonitrile, thereby obtaining a porous titanium iron oxide nanosheet-coated NCM811 positive electrode material;

[0072] (5) The porous titanium iron oxide nanosheets in step (1) were replaced with an equal amount of porous reduced graphene oxide, and the NCM811 cathode powder in step (3) was replaced with an equal amount of NCM811 cathode material coated with porous titanium iron oxide nanosheets, and steps (1) to (4) were repeated; finally, the dried powder was heated from room temperature to 320°C at a rate of 5°C / min under an argon atmosphere, kept warm for 3.5 h, and cooled to obtain a two-dimensional heterojunction-coated layered oxide cathode material.

[0073] Example 2

[0074] The difference from Example 1 is that the "NCM811 positive electrode powder" is replaced with "lithium cobalt oxide positive electrode powder" of equal mass, and the rest is the same as Example 1.

[0075] Example 3

[0076] The difference from Example 1 is that the "NCM811 positive electrode powder" is replaced with an equal mass of "lithium-rich manganese-based oxide positive electrode powder", and the rest is the same as Example 1.

[0077] Comparative Example 1

[0078] The difference from Example 1 is that the coating of the porous reduced graphene oxide is omitted, and the rest is the same as Example 1.

[0079] Specifically:

[0080] (1) 5 mg of porous titanium iron oxide nanosheets were dispersed in 10 mL of acetonitrile to form a base solution with a concentration of 0.5 mg / mL, followed by water bath sonication for 1 h;

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

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

[0083] (4) The mixed solution obtained in step (3) was first vacuum dried at 70°C for 10 minutes to remove n-hexane, and then vacuum dried at 85°C for 1 hour to remove acetonitrile; finally, the dried powder was heated from room temperature to 320°C at a rate of 5°C / min under an argon atmosphere, kept at this temperature for 3.5 hours, and cooled to obtain a positive electrode material of porous titanium iron oxide nanosheets coated with layered oxide.

[0084] Comparative Example 2

[0085] The difference from Example 1 is that the coating of the porous titanium iron oxide nanosheets is omitted, and the rest is the same as Example 1.

[0086] Specifically:

[0087] (1) Disperse 5 mg of porous reduced graphene oxide into 10 mL of acetonitrile to form a base solution with a concentration of 0.5 mg / mL, followed by water bath sonication for 1 h;

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

[0089] (3) Add 2.5 g of NCM811 cathode powder to the droplets prepared in step (2), followed by water bath sonication for 30 min;

[0090] (4) The mixed solution obtained in step (3) was first vacuum dried at 70°C for 10 minutes to remove n-hexane, and then vacuum dried at 85°C for 1 hour to remove acetonitrile; finally, the dried powder was heated from room temperature to 320°C at a rate of 5°C / min under an argon atmosphere, kept at this temperature for 3.5 hours, and cooled to obtain a positive electrode material of porous reduced graphene oxide coated with layered oxide.

[0091] Comparative Example 3

[0092] The difference from Example 1 is that the amount of NCM811 positive electrode powder is adjusted to 5 g, and the rest is the same as Example 1.

[0093] Comparative Example 4

[0094] The difference from Example 1 is that the amount of NCM811 positive electrode powder is adjusted to 1 g, and the rest is the same as Example 1.

[0095] Comparative Example 5

[0096] The difference from Example 1 is that the amount of NCM811 positive electrode powder is adjusted to 0.5 g, and the rest is the same as Example 1.

[0097] Comparative Example 6

[0098] This comparative example is the NCM811 positive electrode material that has not undergone any coating treatment.

[0099] Comparative Example 7

[0100] This comparative example is a lithium cobalt oxide positive electrode material that has not undergone any coating treatment.

[0101] Comparative Example 8

[0102] This comparative example is a lithium-rich manganese-based oxide positive electrode material that has not undergone any coating treatment.

[0103] Comparative Example 9

[0104] The difference from Example 1 is that the coating order of the porous titanium iron oxide nanosheets and the porous reduced graphene oxide is exchanged, and the rest is the same as Example 1.

[0105] Specifically:

[0106] (1) Disperse 5 mg of porous reduced graphene oxide into 10 mL of acetonitrile to form a base solution with a concentration of 0.5 mg / mL, followed by water bath sonication for 1 h;

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

[0108] (3) Add 2.5 g of NCM811 cathode powder to the droplets prepared in step (2), followed by water bath sonication (30 °C) for 30 min;

[0109] (4) The mixed solution obtained in step (3) was first vacuum dried at 70°C for 10 minutes to remove n-hexane, and then vacuum dried at 85°C for 1 hour to remove acetonitrile, thereby obtaining a porous titanium iron oxide nanosheet-coated NCM811 positive electrode material;

[0110] (5) The porous reduced graphene oxide in step (1) is replaced with an equal amount of porous titanium iron oxide nanosheets, and the NCM811 positive electrode powder in step (3) is replaced with an equal amount of porous reduced graphene oxide-coated NCM811 positive electrode material, and steps (1) to (4) are repeated; finally, the dried powder is heated from room temperature to 320°C at a rate of 5°C / min under an argon atmosphere, kept warm for 3.5 hours, and cooled to obtain a two-dimensional heterojunction-coated layered oxide positive electrode material.

[0111] Comparative Example 10

[0112] The difference from Example 1 is that the porous Ti 0.6 Fe 0.4 O2 nanosheets replaced by porous Ti 0.8 Fe 0.1 O2 nanosheets, other aspects are the same as in Example 1.

[0113] The electrochemical properties of the cathode materials obtained in the following examples and comparative examples were tested:

[0114] Testing Method: The positive electrode materials obtained from each Example and Comparative Example were mixed with conductive carbon black (Super-P) and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1. N-methylpyrrolidone (NMP) was added as a solvent (the amount of solvent required was sufficient to ensure uniform mixing of the raw materials) and stirred thoroughly to form a slurry. The slurry was coated onto aluminum foil (coating thickness: 90 μm), then vacuum-dried at 70°C for 12 hours. Round electrode pieces with a diameter of 14 mm were punched out and used as the positive electrode of a lithium-ion battery. A metal lithium sheet was used as the negative electrode. CR2032 button-type lithium-ion batteries were assembled in an argon-filled glove box and electrochemical performance was tested at room temperature (2.8-4.3 V, 1C). The results are shown in Table 1.

[0115] Table 1 Electrochemical performance test results

[0116]

[0117] As shown in Table 1, the charge transfer impedance of Example 1 (27.34Ω) is 77.1% lower than that of Comparative Example 6 (119.40Ω), demonstrating that the nanoporous two-dimensional heterojunction coating significantly improves lithium-ion transport efficiency and results in lower charge transfer impedance. However, in Comparative Examples 4 (231.70Ω) and 5 (355.80Ω), the overly thick coating layer degrades diffusion kinetics, significantly reducing the first-cycle discharge capacity. Therefore, a porous nanosheet coating of appropriate thickness can effectively improve the diffusion kinetics of lithium-ion cathode materials.

[0118] Compared with Comparative Example 2, Example 1 is closer to Example 4 in terms of charge transfer impedance and first-cycle discharge specific capacity. However, the capacity retention rate after 200 cycles is 83.52%, 13.4 percentage points higher than the 70.12% in Comparative Example 2. This is because the porous titanium iron oxide nanosheets contained in Example 1 are more stable under oxidizing conditions, and their surface passivation ability can inhibit the continuous corrosion of the electrolyte on the positive electrode, thereby extending the cycle life.

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

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

[0121] Compared with Comparative Example 10, the charge transfer impedance of Example 1 is 27.34Ω, which is much lower than 125.6Ω of Comparative Example 10. The reason for this is that the Fe content of the porous titanium ferrite in Example 1 is higher than that in Comparative Example 10, and the pores formed are larger, so the lithium ion flux is higher and the charge transfer impedance when assembled into a battery is lower.

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

[0123] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one 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. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a positive electrode material of a two-dimensional heterojunction coated layered oxide, characterized in that: The steps include: The layered oxide is alternately coated with porous titanium iron oxide nanosheets and porous reduced graphene oxide in sequence, and sintered to obtain the positive electrode material of the two-dimensional heterojunction-coated layered oxide; The porous titanium iron oxide nanosheet is a two-dimensional Ti 0.6 Fe 0.4 O2 nanosheets; The layered oxide includes ternary lithium, lithium cobaltate or lithium-rich manganese-based oxide; The number of alternations is 1.

2. The preparation method according to claim 1, characterized in that The preparation steps of the porous titanium iron oxide nanosheets include: (1) Synthesis of precursor K 0.4 Ti 0.6 Fe 0.4 Li 0.02 O2: Take the raw materials according to the corresponding molar ratio and mix them, then heat treat and calcine them in sequence to obtain the precursor K 0.4 Ti 0.6 Fe 0.4 Li 0.02 O2; (2) Synthetic entity H 0.42 Ti 0.6 Fe 0.4 O2: The precursor K 0.4 Ti 0.6 Fe 0.4 Li 0.02 O2 is placed in a hydrochloric acid solution and the solid is collected, which is the main H 0.42 Ti 0.6 Fe 0.4 O2; (3) Synthesis of porous titanium iron oxide nanosheets: The main H 0.42 Ti 0.6 Fe 0.4 O2 is decomposed to obtain nanosheets; hydrogen iodide is used to form pores in the nanosheets to obtain porous titanium iron oxide nanosheets.

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

4. The preparation method according to claim 2, characterized in that The concentration of the tetrabutylammonium hydroxide solution is 1-2 mol / L; the decomposition time is 7-10 days; and the pore creation time is 3-5 hours.

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

6. The preparation method according to claim 1, characterized in that The porous reduced graphene oxide coating comprises: mixing porous reduced graphene oxide, a polar solution and a solvent to obtain a mixed solution; placing the layered oxide coated with the porous titanium iron oxide nanosheets in the mixed solution for treatment, and then removing the solvent and the polar solution in sequence to obtain a positive electrode material of a 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 and the layered oxide powder is 1:200~400; the mass ratio of the porous reduced graphene oxide and the layered oxide coated with the porous titanium ferrite nanosheets is 1:200~400.

8. The preparation method according to claim 1, characterized in that The sintering comprises: heating to 280-350° C. at a heating rate of 4-5° C. / min under a protective atmosphere, and keeping the temperature for 3-5 hours.

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

10. Application of a two-dimensional heterojunction-coated layered oxide cathode material in the preparation of lithium-ion batteries, characterized in that: A lithium-ion battery is prepared using the two-dimensional heterojunction-coated layered oxide positive electrode material according to claim 9 as the positive electrode material.

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