Composite lithium manganese iron phosphate material, preparation method and application thereof

Abstract

This application relates to the field of lithium battery technology, and more particularly to a composite lithium manganese iron phosphate material, its preparation method, and its applications. The composite lithium manganese iron phosphate material of this application includes a core and a shell coating the core. The core contains lithium manganese iron phosphate, and the shell contains doped MXene, which includes MXene and a single-atom metal loaded on the MXene. The shell coating significantly improves the conductivity and ion migration rate of the core material and also inhibits manganese dissolution. The preparation method includes providing doped MXene, mixing it with lithium manganese iron phosphate, and then sintering it in an inert atmosphere. The preparation process is controllable, and the obtained composite lithium manganese iron phosphate material has stable properties. Lithium-ion batteries prepared from the composite lithium manganese iron phosphate material have low internal resistance, high discharge specific capacity, and excellent cycle performance.

Description

Technical Field

[0001] This application belongs to the field of lithium batteries, and particularly relates to composite lithium manganese iron phosphate materials, their preparation methods and applications. Background Technology

[0002] The cathode material, especially the active material, is a crucial component of lithium-ion batteries and plays a decisive role in their performance. Lithium manganese iron phosphate (LFP) is a representative cathode material, possessing advantages such as a high discharge platform and energy density, and good low-temperature performance. However, LFP has a very low electronic conductivity, only 10⁻⁶. - 13 S·cm -1 The high resistivity of lithium manganese iron phosphate (LFP) results in a relatively low discharge specific capacity and poor rate performance. Furthermore, manganese dissolves during cycling, worsening capacity retention and causing the Jameer-Taylor effect. These drawbacks limit the development of LFP cathode materials. To mitigate these shortcomings, LFP materials are often modified, such as through carbon coating, to improve conductivity and ionic conductivity. However, carbon coating often results in incomplete coating and the presence of amorphous carbon, making the modification effect and overall performance improvement of LFP cathode materials less than ideal. Summary of the Invention

[0003] The purpose of this application is to provide a composite lithium manganese iron phosphate material and its preparation method, so as to solve the technical problems of low conductivity and ionic conductivity and manganese leaching in existing lithium manganese iron phosphate cathode materials.

[0004] Another objective of this application is to provide a positive electrode and a secondary battery containing the positive electrode, so as to solve the technical problems of high internal resistance, low discharge specific capacity and poor cycle performance of secondary batteries made of existing lithium manganese iron phosphate positive electrode materials.

[0005] To achieve the aforementioned objectives, the first aspect of this application provides a composite lithium manganese iron phosphate material. This composite lithium manganese iron phosphate material comprises a core and a shell, the shell coating the core, the core being made of lithium manganese iron phosphate; the shell being made of doped MXene, the doped MXene comprising MXene and a single-atom metal, the single-atom metal being loaded onto the MXene.

[0006] The composite lithium manganese iron phosphate material provided in this application is modified by coating the core with a shell. When the core material contains lithium manganese iron phosphate and the shell material contains doped MXene, the conductivity and ion migration rate of the core material can be significantly improved, while manganese dissolution can be suppressed. Using the composite lithium manganese iron phosphate material as a cathode material, it exhibits high electronic conductivity and lithium-ion diffusion rate. Batteries containing this material have low internal resistance, high discharge specific capacity, and excellent cycle performance.

[0007] A second aspect of this application provides a method for preparing the composite lithium manganese iron phosphate material. The method for preparing the composite lithium manganese iron phosphate material includes the following steps:

[0008] Doped MXene is provided, which includes MXene and a single-atom metal, with the single-atom metal loaded on the MXene;

[0009] The composite lithium manganese iron phosphate material is obtained by first mixing MXene doped with lithium manganese iron phosphate and then sintering it in an inert atmosphere.

[0010] The method for preparing composite lithium manganese iron phosphate material provided in this application involves first preparing doped MXene, then thoroughly mixing the doped MXene with lithium manganese iron phosphate and performing a sintering process. This allows the MXene-doped shell to fully coat the lithium manganese iron phosphate core, resulting in a core-shell structured composite lithium manganese iron phosphate material. The MXene's electrochemical performance is improved by the single-atom metal, and the core is modified by the shell, resulting in high conductivity and ion migration rate in the prepared composite lithium manganese iron phosphate material, while manganese is less prone to dissolution. Furthermore, this preparation method offers controllable processes, and the resulting composite lithium manganese iron phosphate material exhibits stable properties.

[0011] A third aspect of this application provides a positive electrode sheet. The positive electrode sheet includes a current collector and a positive electrode active layer bonded to the current collector. The positive electrode active layer contains a positive electrode active material including the composite lithium manganese iron phosphate material of this application or a composite lithium manganese iron phosphate material prepared by the preparation method of this application.

[0012] The active material of the positive electrode in this application includes a composite lithium manganese iron phosphate material, so it has the advantages of lithium manganese iron phosphate in terms of energy density, low temperature performance, safety and cost, and also significantly improves conductivity and ion mobility, and inhibits the dissolution of manganese. The battery made using the positive electrode of this application has low internal resistance, high discharge specific capacity and excellent cycle performance.

[0013] A fourth aspect of this application provides a secondary battery. The secondary battery of this application includes a positive electrode and a negative electrode, the positive electrode being the positive electrode of this application.

[0014] Because the secondary battery of this application includes the positive electrode sheet mentioned above, it has high energy density, good low-temperature performance, high safety, high discharge specific capacity, low internal resistance, and excellent cycle performance. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 These are scanning electron microscope (SEM) images of the composite lithium manganese iron phosphate material provided in Example A1 of this application;

[0017] Figure 2 This is a scanning transmission electron microscope (STEM) image of the composite lithium manganese iron phosphate material provided in Example A1 of this application;

[0018] Figure 3 These are the charge-discharge curves of the batteries provided in Example B1 and Comparative Example B1 of this application at a 0.1C rate;

[0019] Figure 4 This is a schematic diagram of the preparation method of the composite lithium manganese iron phosphate material according to the embodiments of this application. Detailed Implementation

[0020] To make the technical problems, technical solutions, and beneficial effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0021] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0022] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0023] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0024] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms "a" and "the" as used in the embodiments of this application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0025] The weights of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the mass in the embodiments of this application can be a well-known unit of mass in the chemical industry, such as μg, mg, g, or kg.

[0026] The terms "first" and "second" are used for descriptive purposes only, to distinguish objects, such as substances, from one another, and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. For example, without departing from the scope of the embodiments of this application, "first XX" may also be referred to as "second XX," and similarly, "second XX" may also be referred to as "first XX." Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of that feature.

[0027] In a first aspect, embodiments of this application provide a composite lithium manganese iron phosphate material. The composite lithium manganese iron phosphate material of this application includes a core and a shell, the shell coating the core, the core being made of lithium manganese iron phosphate; the shell being made of doped MXene, the doped MXene comprising MXene and a single-atom metal, the single-atom metal being loaded onto the MXene.

[0028] The composite lithium manganese iron phosphate material provided in this application modifies the core material by coating the core with a shell. When the core material contains lithium manganese iron phosphate and the shell material contains doped MXene, the conductivity and ion migration rate of the core material can be significantly improved, and the dissolution of manganese can be suppressed. Using the composite lithium manganese iron phosphate material as a cathode material results in high electronic conductivity and lithium-ion diffusion rate. Batteries containing this material have low internal resistance, high discharge specific capacity, and excellent cycle performance.

[0029] Specifically, doped MXene includes both MXene and single-atom metals. MXene is a term for a two-dimensional inorganic material composed of transition metal carbides, nitrides, or carbonitrides several atomic layers thick. It can be monolayer or multilayer. The structure of MXene can be understood as nanosheets, and this composition and structure endow it with excellent conductivity and structural stability. The inventors have found that loading single-atom metals onto MXene materials can effectively improve the electrochemical properties of MXene and modulate its electronic structure. The single-atom metal can be located between adjacent layers of the MXene material or on its surface. Here, each atom of the single-atom metal can be understood as an elemental metal, not a compound. In a core-shell structure, the single-atom metal can be located on the inner surface of the shell near the core, within the shell, or on the outer surface of the shell away from the core.

[0030] Therefore, coating the surface of lithium manganese iron phosphate core with a shell containing doped MXene can significantly improve the conductivity and ion migration rate of lithium manganese iron phosphate. The single-atom metal loaded with MXene can also exchange electrons with the manganese and iron atoms in the core, modulate their electronic energy bands, and thus suppress the dissolution of manganese.

[0031] In some embodiments, the monatomic metal may include at least one of zinc, gold, silver, copper, iron, cobalt, nickel, magnesium, calcium, tin, bismuth, platinum, palladium, tungsten, molybdenum, vanadium, and aluminum. The inventors have found that these monatomic metals significantly improve the electronic structure and electrochemical performance of MXene and effectively inhibit manganese leaching.

[0032] In some embodiments, the percentage of single-atom metal in the total mass of doped MXene can be from 0.01% to 3.5%. By controlling the percentage of single-atom metal in the total mass of doped MXene within these proportions, MXene can be more stably and uniformly loaded with single-atom metal, improving the performance enhancement effect of single-atom metal on MXene, and making it less likely to form metal particles that affect the doping effect. Furthermore, it enhances the electron exchange between nitrogen atom metal and manganese and iron atoms in the core, suppressing manganese dissolution. In exemplary embodiments, the mass percentage can be, but is not limited to, 0.01%, 0.1%, 0.5%, 1%, 2%, 3%, and 3.5%.

[0033] MXene, as a two-dimensional material, can be a single layer or two or more layers. In some embodiments, the number of MXene layers can be 1 to 100. Studies have shown that MXene layers with these numbers of layers exhibit significantly improved electrochemical performance after being loaded with single-atom metals, and are effective in modifying the core when used as shell materials. In exemplary examples, the number of layers can include, but is not limited to, 1, 5, 10, 30, 50, 80, and 100 layers.

[0034] In some embodiments, when MXene is a single layer, i.e., a monolayer, a single-atom metal can be bonded to the MXene surface. When MXene has two or more layers, the single-atom metal can be bonded to at least one location, such as on the MXene surface or between adjacent layers of MXene, for example: ① bonded only to the MXene surface, ② bonded only between adjacent layers of MXene (at least one adjacent layer), ③ bonded to both the MXene surface and between adjacent layers. All of these loading and bonding methods can enhance the performance improvement effect of the single-atom metal on MXene, improving the modification effect of the doped MXene shell on the core.

[0035] In some embodiments, the general formula for MXene is M n+1 X n In this context, M is a transition metal element, X is carbon or nitrogen, and n is 1 to 3. Studies have shown that when MXene constitutes the components in the composite lithium manganese iron phosphate material of this application, the electrochemical performance of MXene doping is significantly improved, and it also exhibits good core modification effects when used as a shell material. In exemplary examples, n can be 1, 2, or 3. Furthermore, in some embodiments, the transition metal element may include at least one of titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, and tungsten, which will further improve the performance of MXene doping and enhance the coating modification effect.

[0036] In some embodiments, MXene doping can account for 0.5% to 5% of the total mass of the composite lithium manganese iron phosphate material. Studies have shown that controlling the mass percentage of MXene doping is beneficial for improving the shell's coating effect on the core, increasing the conductivity and ion migration rate of the composite lithium manganese iron phosphate material, and enhancing the suppression of manganese dissolution. Furthermore, it avoids excessively high shell content from affecting electrolyte penetration and the content of active materials, thus preventing a negative impact on material performance. In exemplary examples, the mass percentage can include, but is not limited to, 0.5%, 1%, 2%, 3%, 4%, and 5%.

[0037] In some embodiments, the shell thickness can be 1–200 nm. In some embodiments, the core particle size can be 0.1–10 μm. Studies have shown that controlling the shell thickness or core particle size to these parameters is beneficial for improving the electrical conductivity and ion migration rate of the core material, enhancing the electron exchange between the single-atom metal and the core atoms, and improving the coating modification effect. In exemplary examples, the shell thickness can be, but is not limited to, 1 nm, 30 nm, 50 nm, 100 nm, 150 nm, and 200 nm. The core particle size can be, but is not limited to, 0.1 μm, 0.5 μm, 1 μm, 2 μm, 5 μm, 8 μm, and 10 μm.

[0038] Secondly, this application also provides a method for preparing the aforementioned composite lithium manganese iron phosphate material. The method for preparing the composite lithium manganese iron phosphate material in this application is as follows: Figure 4As shown, it includes the following steps:

[0039] S01. Provide doped MXene, which includes MXene and a single-atom metal, with the single-atom metal loaded on the MXene;

[0040] S02. The mixture including doped MXene and lithium manganese iron phosphate is first mixed and then sintered in an inert atmosphere to obtain a composite lithium manganese iron phosphate material.

[0041] The method for preparing composite lithium manganese iron phosphate material provided in this application involves first preparing doped MXene, then thoroughly mixing the doped MXene with lithium manganese iron phosphate and performing a sintering process. This allows the MXene-doped shell to fully coat the lithium manganese iron phosphate core, resulting in a core-shell structured composite lithium manganese iron phosphate material. The MXene's electrochemical performance is improved by the single-atom metal, and the core is modified by the shell, resulting in high conductivity and ion migration rate in the prepared composite lithium manganese iron phosphate material, while manganese is less prone to leaching. Furthermore, this preparation method is process-controllable, and the resulting composite lithium manganese iron phosphate material exhibits stable properties.

[0042] Step S01

[0043] This step involves first stripping away the layered MXene, then mixing it with a metal salt, and finally reducing the filter residue separated from the solid-liquid mixture. This process effectively loads single-atom metals onto the surface and / or interlayer of MXene, improving the loading effect and enhancing the electrochemical performance of MXene. Step S01 may include the following steps:

[0044] S011. Provides stripped MXene;

[0045] S012. After a second mixing treatment of the metal salt solution and MXene, a solid-liquid separation treatment is performed, and then the filter residue is reduced in a reducing atmosphere to obtain doped MXene.

[0046] In step S011, the MXene to be exfoliated can be prepared by forming a dispersion of MXene raw material in a solvent (water or organic solvent), exfoliating it by sufficient ultrasonic vibration (e.g., vibration for 1 to 100 hours), then performing solid-liquid separation (e.g., centrifugation or filtration) and drying (e.g., vacuum drying or freeze drying) to obtain nanosheets with a two-dimensional structure, i.e., the exfoliated MXene. The types of MXene raw materials can include the types of MXene materials in the composite lithium manganese iron phosphate materials provided in the above embodiments.

[0047] Step S012 is a step of further doping the stripped MXene with a single-atom metal to obtain doped MXene. The second mixing process can be to thoroughly mix the metal salt solution with the stripped MXene until uniformly dispersed. The second mixing process can be performed according to a mass ratio of metal salt to MXene of 1:0.12 to 1:19.63 to further enhance the electrochemical performance enhancement effect of the single-atom metal on the MXene in the obtained doped MXene. Furthermore, since losses occur during subsequent metal salt reduction treatment in actual preparation, the inventors, through research, have controlled the mass ratio of the metal salt solution to MXene within the above-mentioned range, thereby obtaining doped MXene with a single-atom metal mass ratio within the desired range. In the examples, the mass ratio of metal salt to MXene can include, but is not limited to, 1:19.63, 1:10, 1:5, 1:1.17, 1:0.77, 1:0.41, and 1:0.12. Furthermore, the concentration of the metal salt solution can be from 0.01 to 1 mol / L to further improve the properties of the doped MXene. In the example, the concentration of the metal salt solution can be, but is not limited to, 0.01 mol / L, 0.05 mol / L, 0.1 mol / L, 0.5 mol / L, and 1 mol / L. The metal ions contained in the metal salt solution can include at least one of zinc, gold, silver, copper, iron, cobalt, nickel, magnesium, calcium, tin, bismuth, platinum, palladium, tungsten, molybdenum, vanadium, or aluminum ions to enhance the effect of the single-atom metal contained in the doped MXene on improving its electrochemical performance.

[0048] Solid-liquid separation can be achieved through centrifugation or filtration. Centrifugation can be performed at speeds of 6000–10000 r / min for 5–10 min. The filter residue can be dried (e.g., vacuum drying or freeze-drying), and then reduced in a reducing atmosphere to reduce the metal ions in the metal salt to single-atom metals, thus producing doped MXene. The reducing atmosphere can be a mixture of hydrogen and other non-reactive gases, such as a 5% / 95% hydrogen-argon mixture. Reduction can be achieved through high-temperature calcination, for example, at temperatures of 300–550°C for 1–12 h, to fully reduce the metal salt. In exemplary cases, the reduction temperatures can include, but are not limited to, 300°C, 350°C, 400°C, 500°C, and 550°C; the times can include, but are not limited to, 1 h, 3 h, 5 h, 8 h, 10 h, and 12 h. In some embodiments, the length or width of the doped MXene can be independently 0.3–5 μm. Controlling the length or width of the doped MXene can further stabilize its properties and also effectively coat the core in subsequent preparation steps, improving the coating modification effect. In the example, the length and width of the doped MXene can be, but are not limited to, 0.3 μm, 0.5 μm, 1 μm, 3 μm, and 5 μm.

[0049] Step S02

[0050] This step can produce a core-shell structured composite lithium manganese iron phosphate material by combining doped MXene and lithium manganese iron phosphate. The first mixing treatment can involve thoroughly mixing and uniformly dispersing the doped MXene and lithium manganese iron phosphate, for example, by ball milling. These materials, along with an organic solvent, can be added to a ball mill jar, and the ball milling can be performed under an inert gas atmosphere (e.g., nitrogen or argon). The doped MXene and lithium manganese iron phosphate can be mixed at a mass ratio of 0.5:95.5 to 5:95. These mass ratios can improve the coating and modification effect of the doped MXene on the core in the resulting composite lithium manganese iron phosphate material. In exemplary cases, the mass ratio of doped MXene to lithium manganese iron phosphate can include, but is not limited to, 0.5:95.5, 1:99, 2:98, 3:97, 4:96, and 5:95. The preparation method of lithium manganese iron phosphate can involve ball milling a lithium source, phosphorus source, iron source, manganese source, and organic solvent together until uniform, drying, and then calcining at high temperature in an inert atmosphere to obtain lithium manganese iron phosphate particles. In addition, doped MXene and lithium manganese iron phosphate are two essential materials in the first mixing process, and other raw materials that can be used for the core, shell and other materials that help with modification can also be added according to performance design.

[0051] Before sintering, the product of the first mixing treatment can be dried to remove solvents, for example, by vacuum drying or freeze drying. Sintering can be carried out in an inert atmosphere, such as nitrogen or argon, at a temperature of 500–900°C for 6–18 hours. By performing a first mixing treatment followed by sintering, a stable core-shell structure can be obtained, ensuring the effectiveness of the coating modification. In the examples, the sintering temperature can be, but is not limited to, 500°C, 600°C, 700°C, 800°C, and 900°C; the time can be, but is not limited to, 6 hours, 8 hours, 10 hours, 12 hours, 14 hours, 16 hours, and 18 hours.

[0052] Thirdly, embodiments of this application also provide a positive electrode sheet. The positive electrode sheet of this application includes a current collector and a positive electrode active layer bonded to the current collector. The positive electrode active layer contains a positive electrode active material including the composite lithium manganese iron phosphate material described in the above embodiments or a composite lithium manganese iron phosphate material prepared by the preparation method described in the above embodiments.

[0053] The active material of the positive electrode in this application embodiment includes a composite lithium manganese iron phosphate material. Therefore, it has the advantages of lithium manganese iron phosphate in terms of energy density, low temperature performance, safety, and cost, and also significantly improves conductivity and ion mobility. It also inhibits the dissolution of manganese. The battery made using the positive electrode in this application embodiment has low internal resistance, high discharge specific capacity, and excellent cycle performance.

[0054] In one embodiment, the mass content of the composite lithium manganese iron phosphate material in the positive electrode active layer described in the above application embodiments can be designed according to battery performance requirements. The positive electrode active layer may include, in addition to the composite lithium manganese iron phosphate material, a binder and a conductive agent. The binder can be a commonly used electrode binder, such as one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives. In this application embodiment, the conductive agent can be a commonly used conductive agent, such as one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60, and carbon nanotubes. Of course, the positive electrode active layer may also contain other electrode active materials, such as one or more of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxide phosphate, lithium fluorinated vanadium phosphate, lithium titanate, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide.

[0055] In the embodiments, the positive electrode preparation process can be as follows: mixing positive electrode active material, conductive agent and binder to obtain electrode slurry, coating the electrode slurry on positive electrode current collector, and preparing positive electrode sheet through steps such as drying, rolling and die cutting.

[0056] Fourthly, embodiments of this application also provide a secondary battery. The secondary battery of this application includes a positive electrode and a negative electrode, the positive electrode being the same as the positive electrode described in the above embodiments.

[0057] The secondary battery in this application embodiment includes the positive electrode sheet of the above embodiment, therefore it has high energy density, good low-temperature performance, high safety, high discharge specific capacity, low internal resistance, and excellent cycle performance.

[0058] The following description is based on specific embodiments.

[0059] 1. Composite lithium manganese iron phosphate materials and their preparation methods

[0060] Example A1

[0061] This embodiment provides a composite lithium manganese iron phosphate material, comprising a core and a shell. The shell encapsulates the core, which is lithium manganese iron phosphate. The shell is doped with MXene, which includes MXene (Ti3C2) nanosheets and single-atom Ag atoms supported on the nanosheets. The single-atom Ag atoms account for 0.15% of the total mass of the doped MXene, and the doped MXene accounts for 1.9% of the total mass of the composite lithium manganese iron phosphate material.

[0062] The preparation method of the composite lithium manganese iron phosphate material in this embodiment includes the following steps:

[0063] S1: Provides stripped MXene

[0064] 10g of MXene raw material—Ti3C2 powder—was weighed using a balance and dispersed in 100mL of ethanol solution. The dispersion was ultrasonically dispersed for 5h under ice bath conditions. After centrifugation, the dispersion was washed 5 times with water and ethanol. The washed solid was placed in a vacuum drying oven and dried at 80℃ for 8h to obtain the exfoliated MXene.

[0065] S2: Preparation of doped MXene

[0066] 1g of exfoliated MXene was dispersed in a 0.05M silver nitrate solution and magnetically stirred for 3 hours. Solid-liquid separation was performed using a centrifuge. The centrifuged solid was then vacuum dried at 70℃ for 10 hours. The dried solid was placed in a tube furnace and calcined at 500℃ for 6 hours after passing a 5% / 95% hydrogen-argon mixed gas through it. This yielded silver single-atom modified MXene (Ti3C2) nanosheets, i.e., doped MXene, labeled Ag-SAc@MXene.

[0067] S3: Preparation of composite lithium manganese iron phosphate materials

[0068] 0.1g of Ag-SAc@MXene, 5g of lithium manganese iron phosphate powder, and 30mL of anhydrous ethanol were added to a ball mill jar. Under N2 protection, the mixture was ball milled at 300r / min for 5h. After that, it was placed in a vacuum drying oven and dried at 70℃ for 8h. Then, it was transferred to a tube furnace, under N2 protection, and calcined at 600℃ for 3h. The product is the composite lithium manganese iron phosphate material doped with MXene and labeled as LMFP / Ag-SAs@MXene.

[0069] Example A2

[0070] This embodiment provides a composite lithium manganese iron phosphate material, which differs from Embodiment A1 in that the MXene doping includes Ti2C MXene nanosheets and single-atom Ni supported on the nanosheets. The single-atom Ni accounts for 0.26% of the total mass of the doped MXene, and the doped MXene accounts for 1.9% of the total mass of the composite lithium manganese iron phosphate material.

[0071] The preparation method of composite lithium manganese iron phosphate materials includes the following steps:

[0072] S1: Provides stripped MXene

[0073] 10g of MXene raw material—Ti2C powder—was weighed using a balance and dispersed in 100mL of N-methylpyrrolidone (NMP) solution. The dispersion was ultrasonically dispersed for 8h under ice bath conditions. After centrifugation, the dispersion was washed 5 times with water and ethanol. The washed solid was placed in a vacuum drying oven and dried at 70℃ for 10h to obtain the exfoliated MXene.

[0074] S2: Preparation of doped MXene

[0075] 1g of exfoliated MXene was dispersed in a 0.1M nickel chloride solution and magnetically stirred for 5h. Solid-liquid separation was performed using a centrifuge. The centrifuged solid was then vacuum dried at 80℃ for 8h. The dried solid was placed in a tube furnace and calcined at 450℃ for 3h after passing through a 5% / 95% hydrogen-argon mixed gas. This yielded nickel single-atom modified MXene (Ti2C) nanosheets, i.e., doped MXene, labeled Ni-SAc@MXene.

[0076] S3: Preparation of composite lithium manganese iron phosphate materials

[0077] 0.6g of Ni-SAc@MXene and 2.6g of lithium manganese iron phosphate powder with the same composition as in Example A1, along with 30mL of anhydrous ethanol, were simultaneously added to a ball mill jar. Under N2 protection, the mixture was ball-milled at 350r / min for 6 hours. After that, it was placed in a vacuum drying oven and dried at 80℃ for 6 hours. Then, it was transferred to a tube furnace, under N2 protection, and calcined at 700℃ for 5 hours. The product is the composite lithium manganese iron phosphate material doped with MXene and labeled as LMFP / Ni-SAs@MXene.

[0078] Example A3

[0079] This embodiment provides a composite lithium manganese iron phosphate material, which differs from Embodiment A1 in that the MXene doping includes Nb3C2 MXene nanosheets and single-atom Fe supported on the nanosheets. The single-atom Fe accounts for 0.35% of the total mass of the doped MXene, and the doped MXene accounts for 1.9% of the total mass of the composite lithium manganese iron phosphate material.

[0080] The preparation method of composite lithium manganese iron phosphate materials includes the following steps:

[0081] S1: Provides stripped MXene

[0082] 10g of MXene raw material—Nb3C2 powder—was weighed using a balance and dispersed in 100mL of N,N-dimethylformamide (DMF) solution. The dispersion was ultrasonically dispersed for 6h under ice bath conditions. After centrifugation, the dispersion was washed 5 times with water and ethanol. The washed solid was placed in a vacuum drying oven and dried at 80℃ for 12h to obtain the exfoliated MXene.

[0083] S2: Preparation of doped MXene

[0084] 1g of exfoliated MXene was dispersed in a 0.1M ferric nitrate solution and magnetically stirred for 6 hours. Solid-liquid separation was performed using a centrifuge. The centrifuged solid was then vacuum dried at 70℃ for 12 hours. The dried solid was placed in a tube furnace and calcined at 400℃ for 1 hour after passing a 5% / 95% hydrogen-argon mixed gas through it. Iron single-atom modified MXene (Nb3C2) nanosheets, i.e., doped MXene, were obtained and labeled Fe-SAc@MXene.

[0085] S3: Preparation of composite lithium manganese iron phosphate materials

[0086] 0.5g of Fe-SAc@MXene and 2.1g of lithium manganese iron phosphate powder with the same composition as in Example A1, along with 50mL of anhydrous ethanol, were simultaneously added to a ball mill jar. Under Ar protection, the mixture was ball-milled at 400r / min for 3h, then placed in a vacuum drying oven and dried at 80℃ for 5h. After drying, the mixture was transferred to a tube furnace and calcined at 600℃ for 6h under N2 protection. The product is the MXene-doped composite lithium manganese iron phosphate material, labeled as LMFP / Fe-SAs@MXene.

[0087] Example A4

[0088] This embodiment provides a composite lithium manganese iron phosphate material, which differs from Embodiment A3 in that single-atom Cu is loaded on the nanosheet.

[0089] The preparation method differs from Example A3 in that the ferric nitrate solution in step S2 is replaced with a copper nitrate solution, and other steps are adjusted accordingly. The final product from step S3 is labeled LMFP / Cu-SAs@MXene.

[0090] Example A5

[0091] This embodiment provides a composite lithium manganese iron phosphate material, which differs from Embodiment A3 in that the nanosheets are loaded with single-atom Mg.

[0092] The preparation method differs from that of Example A3 in that the ferric nitrate solution in step S2 is replaced with a magnesium nitrate solution, and other steps are adjusted accordingly. The final product from step S3 is labeled LMFP / Mg-SAs@MXene.

[0093] Example A6

[0094] This embodiment provides a composite lithium manganese iron phosphate material, which is the same as in Embodiment A1.

[0095] The difference between the preparation method and Example A1 is that the calcination temperature in step S3 is 450°C.

[0096] Example A7

[0097] This embodiment provides a composite lithium manganese iron phosphate material, which is the same as in Embodiment A1.

[0098] The difference between the preparation method and Example A1 is that the calcination temperature in step S3 is 950°C.

[0099] Example A8

[0100] This embodiment provides a composite lithium manganese iron phosphate material, which is the same as in Embodiment A1.

[0101] The difference between the preparation method and Example A1 is that the calcination temperature in step S2 is 250°C.

[0102] Example A9

[0103] This embodiment provides a composite lithium manganese iron phosphate material, which is the same as in Embodiment A1.

[0104] The difference between the preparation method and Example A1 is that the calcination temperature in step S2 is 600℃.

[0105] Example A10

[0106] This embodiment provides a composite lithium manganese iron phosphate material, which differs from embodiment A1 in that the single-atom Ag loaded on the MXene nanosheet accounts for 4% of the total mass of the doped MXene.

[0107] Example A11

[0108] This embodiment provides a composite lithium manganese iron phosphate material, which differs from Embodiment A1 in that the MXene doping accounts for 0.2% of the total mass of the composite lithium manganese iron phosphate material.

[0109] Example A12

[0110] This embodiment provides a composite lithium manganese iron phosphate material, which differs from embodiment A1 in that the MXene doping accounts for 6% of the total mass of the composite lithium manganese iron phosphate material.

[0111] Comparative Example A1

[0112] Lithium manganese iron phosphate materials

[0113] The lithium manganese iron phosphate material provided in this comparative example has the same composition as the lithium manganese iron phosphate materials in Examples A1 to A12.

[0114] Comparative Example A2

[0115] Graphene-coated lithium manganese iron phosphate material

[0116] The graphene-coated lithium manganese iron phosphate material provided in this comparative example differs from that in Example A1 only in that the shell layer is graphene.

[0117] The preparation method differs from that in Example A1 in that steps S1 and S2 are omitted, and step S3 is performed by replacing 0.1g of Ag-SAc@MXene with 0.1g of graphene.

[0118] Comparative Example A3

[0119] MXene-coated lithium manganese iron phosphate material

[0120] The only difference between the MXene-coated lithium manganese iron phosphate material provided in this comparative example and Example A1 is that the shell does not include Ag, but only includes Ti3C2 MXene nanosheets.

[0121] The preparation method differs from that in Example A1 in that step S2 is omitted, and step S3 replaces 0.1g of Ag-SAc@MXene with 0.1g of peeled MXene (Ti3C2).

[0122] 2. Lithium-ion batteries:

[0123] 2.1 Using the composite lithium manganese iron phosphate materials of Examples A1 to A12 and the various lithium manganese iron phosphate materials provided in Comparative Examples A1 to A3 as the positive electrode active materials of lithium-ion batteries Examples B1 to B12 and Comparative Examples B1 to B3, respectively, the positive electrode sheets were prepared according to the following methods:

[0124] (1) Preparation of slurry: 2.33 kg of positive electrode active material, 0.012 kg of superconducting carbon black (SP) and 0.048 kg of binder polyvinylidene fluoride (PVDF) were added to an agate ball mill jar, and then 1.6 kg of solvent N-methylpyrrolidone (NMP) was added. The slurry was prepared by ball milling at 360 r / min for 4 h.

[0125] (2) Coating of slurry: Adjust the scale of the scraper of the coating machine, and evenly coat the slurry after ball milling onto the aluminum foil. Place the coated electrode in a vacuum drying oven at 130℃ and bake for 3 hours.

[0126] (3) Rolling and stamping: Place the aluminum foil coated with slurry flat in the middle of the roller and roll it to press the electrode sheet; then press the rolled electrode sheet with the front side tightly against the perforated area and stamp it in sequence; the compaction density of the electrode sheet is controlled at 2.0 g / cm³. 3 ~2.4g / cm 3 The diameter is 14mm and the thickness is 0.05mm~0.10mm; the punched electrode is placed in a vacuum drying oven at 130℃ and baked for 3 hours;

[0127] In this context, the composite lithium manganese iron phosphate material in Example A1 above is used as the positive electrode active material in Battery Example B1, the composite lithium manganese iron phosphate material in Example A2 is used as the positive electrode active material in Battery Example B2, and so on. The lithium manganese iron phosphate material in Comparative Example A1 is used as the positive electrode active material in Comparative Example B1, and so on.

[0128] 2.2 Assemble the button cell battery. In the glove box, assemble the battery in the following order: negative electrode shell, spring, steel sheet, lithium sheet, separator, positive electrode sheet and positive electrode shell. During the process, inject 10 μL of electrolyte. Then, use a sealing machine to seal the button cell battery to obtain battery examples B1 to B12 and battery comparative examples B1 to B3.

[0129] 3. Relevant performance tests and results analysis

[0130] 1. Relevant Tests for Lithium Manganese Iron Phosphate Materials

[0131] The lithium manganese iron phosphate cathode materials coated with single-atom modified MXene nanosheets provided in Examples A1 to A12 were characterized by SEM and TEM, respectively. The SEM image of Example A1 is shown below. Figure 1 As shown. By Figure 1 As can be seen, the outer layer of the lithium manganese iron phosphate material prepared in Example A1 is uniformly coated with single-atom metal-modified MXene nanosheets with a thickness of 1-200 nm. This morphology can effectively improve the conductivity and lithium-ion migration rate of the material, while inhibiting the dissolution of manganese in lithium manganese iron phosphate during cycling, thereby improving the cycle stability of the battery. Figure 2 The image shown is a high-resolution spherical aberration electron microscope (STEM-HRTEM) image of Example A1. Many independent, dispersed white bright spots can be clearly seen in the field of view. These bright spots are single-atom Ag atoms loaded on MXene nanosheets.

[0132] The relevant data and some process parameters of the preparation methods for the composite lithium manganese iron phosphate materials provided in Examples A1 to A12 are summarized in Table 1 below.

[0133] Table 1

[0134]

[0135] 2. Examples and comparative tests of lithium-ion batteries:

[0136] The electrochemical performance test results of the lithium secondary batteries of Examples B1 to B12 and Comparative Examples B1 to B3 are shown in Table 2 below. The resistivity in Table 2 refers to the resistivity of the positive electrode, D. li is the lithium-ion diffusion coefficient of the positive electrode.

[0137] Table 2

[0138]

[0139]

[0140] As can be seen from Tables 1 and 2, the single-atom modified MXene nanosheet-coated lithium manganese iron phosphate materials provided in Examples A1 to A12, compared with the various lithium manganese iron phosphate materials in Comparative Examples A1 to A3, exhibit better conductivity and lithium-ion diffusion coefficient after being fabricated into positive electrodes. Among them, the positive electrode resistivity of the battery made from the material in Example A1 is the lowest, at 6.9 Ω·cm, indicating the highest conductivity. After being assembled into a lithium-ion battery, the... Figure 3It can be seen that the initial discharge specific capacity of Example B1 at 0.1C can reach a maximum of 150.4 mAh / g, which is much higher than that of Comparative Example B1 (120.1 mAh / g). Furthermore, the cycle stability of the battery in Example B1 is greatly improved, with a capacity retention rate of 98.9% after 200 cycles, significantly better than the retention rate of Comparative Example B1 (80.1%). In addition, the structural stability of lithium manganese iron phosphate coated with single-atom modified MXene nanosheets is also significantly improved. After 200 cycles, the lowest detected manganese dissolution is only 56 ppm (Example B1), far lower than... Comparative Example B1 (356 ppm) highlights the superior performance of the composite lithium manganese iron phosphate in the embodiments of this application. Compared with graphene-coated lithium manganese iron phosphate and MXene-coated lithium manganese iron phosphate, the same conclusion can be drawn. The resistivity of Example B12 is slightly higher than that of Comparative Example B2. This is because the coating material of Comparative Example B2 is graphene, which has high conductivity. Therefore, after coating the lithium manganese iron phosphate core, the resistivity of the prepared material can be effectively reduced. However, Example B12 also has better test results than Comparative Example B2 in terms of lithium ion diffusion rate, cycle capacity retention, and Mn dissolution.

[0141] Compared with Example A1, Examples A6 and A7 adjusted the calcination temperature in step S3. It can be seen that the relevant performance decreased to a certain extent. This is because too low a temperature will result in too low crystallinity of lithium manganese iron phosphate, while too high a temperature will cause MXene to undergo a phase transition. Therefore, the calcination temperature in step S3 is within a certain range, which is beneficial to improving the performance of the composite lithium manganese iron phosphate material in the examples.

[0142] Compared with Example A1, Examples A8 and A9 adjusted the calcination temperature in step S2. It can be seen that the relevant performance decreased to a certain extent. This is because if the temperature is too low, it will be difficult to reduce metal ions, and if the temperature is too high, it will cause the single-atom metal to agglomerate, affecting the doping modification effect of the single-atom metal on MXene. Therefore, the calcination temperature in step S2 is beneficial to improve the performance of the composite lithium manganese iron phosphate material in the examples within a certain range.

[0143] Compared to Example A1, Example A10 adjusted the proportion of Ag in the total mass of doped MXene. It can be seen that the relevant performance decreased to a certain extent because the proportion of metal mass was too high, forming some metal particles, which affected the proportion of MXene doped in the form of single-atom metal, thus affecting the performance. Therefore, a certain range of single-atom metal in the total mass of doped MXene is beneficial to improving the performance of the composite lithium manganese iron phosphate material in the example.

[0144] Compared to Example A1, Examples A11 and A12 adjusted the proportion of MXene doping in the total mass of the composite lithium manganese iron phosphate material. It can be seen that the relevant performance decreased to some extent. This is because too low an MXene doping content resulted in an insignificant coating effect, leading to low material conductivity and increased Mn content, among other negative effects. Conversely, too high a content resulted in an excessively thick coating layer, hindering electrolyte penetration and reducing the proportion of active material, also impacting product performance. Therefore, maintaining an appropriate range of MXene doping within the total mass of the composite lithium manganese iron phosphate material is beneficial for improving the performance of the composite lithium manganese iron phosphate material in these examples.

[0145] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A composite lithium manganese iron phosphate material, characterized in that, It includes a core and a shell, the shell covering the core, the core being made of lithium manganese iron phosphate; the shell being made of doped MXene, the doped MXene comprising MXene and single-atom metal, the single-atom metal being loaded on the surface and / or between the layers of the MXene in an independently dispersed elemental form.

2. The composite lithium manganese iron phosphate material according to claim 1, characterized in that, The single-atom metal includes at least one of zinc, gold, silver, copper, iron, cobalt, nickel, magnesium, calcium, tin, bismuth, platinum, palladium, tungsten, molybdenum, vanadium, and aluminum; and / or The single-atom metal accounts for 0.01% to 3.5% of the total mass of the doped MXene; and / or When the MXene is a single layer, the single-atom metal is bonded to the surface of the MXene; and / or When the MXene has two or more layers, the single-atom metal is bonded to at least one location on the MXene surface or between adjacent layers of the MXene; and / or The general formula for MXene is M n+1 X n Where M is a transition metal element, X is carbon or nitrogen element, and n is 1 to 3.

3. The composite lithium manganese iron phosphate material according to claim 2, characterized in that, The number of layers in the MXene is no more than 100; and / or The transition metal element includes at least one of titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, and tungsten.

4. The composite lithium manganese iron phosphate material according to any one of claims 1 to 3, characterized in that, The doped MXene accounts for 0.5% to 5% of the total mass of the composite lithium manganese iron phosphate material; and / or The shell thickness is 1 ~ 200 nm; and / or The particle size of the nuclei is 0.1 ~ 10 μm.

5. A method for preparing a composite lithium manganese iron phosphate material, comprising the following steps: A doped MXene is provided, the doped MXene comprising MXene and a single-atom metal, the single-atom metal being loaded on the MXene; The doped MXene and lithium manganese iron phosphate are first mixed and then sintered in an inert atmosphere to obtain a composite lithium manganese iron phosphate material.

6. The preparation method according to claim 5, characterized in that, The length or width of the doped MXene is independently 0.3 ~ 5 μm; and / or The mass ratio of doped MXene to lithium manganese iron phosphate is 0.5:95.5 to 5:95; and / or The first mixing process includes ball milling; and / or The sintering temperature is 500 ~ 900 ℃.

7. The preparation method according to claim 5 or 6, characterized in that, The provision of doped MXene includes the following steps: Provide the stripped MXene; After a second mixing treatment of the metal salt solution and the MXene, a solid-liquid separation treatment is performed, and then the filter residue is reduced in a reducing atmosphere to obtain the doped MXene.

8. The preparation method according to claim 7, characterized in that, The concentration of the metal salt solution is 0.01 ~ 1 mol / L; and / or The metal salt solution contains a metal salt in a mass ratio of 1:0.12 to 1:19.63 to the MXene; and / or The reduction treatment is performed at a temperature of 300 ~ 550 ℃.

9. The preparation method according to claim 8, characterized in that, The metal ions contained in the metal salt solution include at least one of zinc, gold, silver, copper, iron, cobalt, nickel, magnesium, calcium, tin, bismuth, platinum, palladium, tungsten, molybdenum, vanadium, or aluminum ions.

10. A positive electrode sheet, comprising a current collector and a positive electrode active layer bonded to the current collector, characterized in that, The positive electrode active layer contains a positive electrode active material including the composite lithium manganese iron phosphate material according to any one of claims 1 to 4 or the composite lithium manganese iron phosphate material prepared by the preparation method according to any one of claims 5 to 9.

11. A secondary battery, comprising a positive electrode and a negative electrode, characterized in that, The positive electrode includes the positive electrode as described in claim 10.

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