Composite positive electrode material and preparation method thereof, positive plate and secondary battery

Abstract

The invention belongs to the technical field of battery materials, and particularly relates to a composite positive electrode material, a preparation method thereof, a positive plate and a secondary battery. The composite positive electrode material comprises an N-doped MXene carrier and a positive electrode active material grown in the N-doped MXene carrier in situ, the positive electrode active material comprises metal element doped lithium iron phosphate, and the positive electrode active material has a rice-grain-like morphology structure. In the composite positive electrode material, N-doped MXene serves as a three-dimensional conductive skeleton, pyridine nitrogen and pyrrole nitrogen provide defects due to doping of N atoms, and N-doped MXene serves as a lithium ion adsorption site to improve pseudocapacitance contribution. According to the lithium iron phosphate, grains are refined through metal doping, the b-axis and unit cell volume of the lithium iron phosphate are shortened, and Li-O bonds are increased, so that a lithium ion diffusion channel is shortened, and the diffusion rate is increased. Due to the rice-like morphology structure, the diffusion distance of lithium ions in the positive electrode active material is greatly shortened, and the rate capability is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of battery materials, and particularly relates to a composite positive electrode material and a preparation method thereof, and a positive electrode sheet and a secondary battery. BACKGROUND

[0002] Lithium iron phosphate (LiFePO4, LFP for short) is one of the mainstream positive electrode materials of lithium ion batteries. It has excellent structural and chemical stability, environmental friendliness, low cost and high energy density, and thus has been widely studied. However, the electronic conductivity (~10 -10 S / cm~10 -7 S / cm) and the ionic conductivity (~10 -12 cm 2 / s~10 - 16 cm 2 / s) of lithium iron phosphate are relatively poor, which limits the practical application of lithium iron phosphate in lithium ion batteries, and many researchers have been actively seeking effective methods to solve this problem. SUMMARY

[0003] The application aims to provide a composite positive electrode material and a preparation method thereof, and a positive electrode sheet and a secondary battery, and aims to solve the problem of low electronic conductivity and ionic conductivity of existing lithium iron phosphate to some extent.

[0004] To achieve the above application purposes, the technical solutions adopted by the application are as follows: In a first aspect, the application provides a composite positive electrode material, comprising an N-doped MXene carrier and a positive electrode active material grown in situ in the N-doped MXene carrier, the positive electrode active material comprising metal element-doped lithium iron phosphate, and the positive electrode active material having a rice grain-like morphology structure.

[0005] In some possible implementation manners, in the positive electrode active material with the rice grain-like morphology structure, the length of the short axis is less than 50 nm, and the length of the long axis is 100 nm to 300 nm.

[0006] In some possible implementation manners, in the composite positive electrode material, the mass percentage content of the N-doped MXene carrier is 0.5% to 5%.

[0007] In some possible implementation manners, the outer surface of the positive electrode active material has a carbon coating layer.

[0008] In some possible implementation manners, in the metal element-doped lithium iron phosphate, the doped metal element comprises at least one of Ti, V, Mo, Nb, Zr, Mn and Mg.

[0009] In some possible implementations, the mass percentage of the doped metal element in the lithium iron phosphate is 7000ppm to 11000ppm.

[0010] In some possible implementations, the thickness of the carbon coating layer is 1 nm to 5 nm.

[0011] In some possible implementations, the carbon content in the positive electrode active material is 1.2% to 1.5%.

[0012] In some possible implementations, the thickness of the N-doped MXene support is 20 nm to 50 nm.

[0013] In some possible implementations, the sheet width of the N-doped MXene support is 1 μm to 5 μm.

[0014] In some possible implementations, the N-doped MXene support has 1 to 3 layers.

[0015] In some possible implementations, the N element in the N-doped MXene support exists in at least one of the following forms: N atom, N-metallic bond, and CN bond.

[0016] In some possible implementations, the mass percentage of N element in the N-doped MXene support is 5% to 25%.

[0017] In some possible implementations, the N-doped MXene support contains MXene comprising Ti3C2T. x T2CT x Nb2CT x V2CT x Mo3C2T x Nb4C3T x At least one of them, wherein T x It is selected from at least one of -O, -OH or -F.

[0018] Secondly, this application provides a method for preparing a composite cathode material, comprising the following steps: Obtain N-doped MXene carriers; Phosphorus source, iron source and lithium source are dissolved in an organic solvent in a non-stoichiometric ratio, and an antioxidant and the N-doped MXene support are added to carry out a solvothermal reaction to obtain the first precursor composite material. The first precursor composite material is mixed with a doped metal source, a complexing agent and a dispersant, then ground and dried to obtain a nanoscale second precursor composite material. The second precursor composite material was calcined in an inert atmosphere to obtain a composite cathode material with a rice-grain-like morphology.

[0019] In some possible implementations, the solvothermal reaction step includes reacting in a reactor at a temperature of 100°C to 150°C for 6 to 12 hours.

[0020] In some possible implementations, the grinding process includes: grinding the particles using a sand mill until the particle size D50 is less than 200 nm.

[0021] In some possible implementations, the drying is performed using centrifugal spray drying.

[0022] In some possible implementations, the calcination treatment is carried out at a temperature of 600℃ to 720℃ for a duration of 2h to 6h.

[0023] In some possible implementations, the inert atmosphere includes at least one of nitrogen and argon.

[0024] In some possible implementations, the organic solvent includes at least one of ethanol and ethylene glycol.

[0025] In some possible implementations, the antioxidant includes at least one of ascorbic acid, L-tartaric acid, glutathione, hydroxyanisole, butylated hydroxytoluene, and tert-butylhydroquinone.

[0026] In some possible implementations, the complexing agent includes at least one of citric acid dihydrate, ethylenediaminetetraacetic acid, aminetriacetic acid, gluconic acid, oxalic acid, and diethylenetriaminepentaacetic acid.

[0027] In some possible implementations, the doped metal source includes at least one of Ti source, V source, Mo source, Nb source, Zr source, Mn source, and Mg source.

[0028] In some possible implementations, the dispersant includes at least one of polyvinylpyrrolidone, polyethylene glycol ether, Tween, Span, and fatty acid polyoxyethylene ether.

[0029] In some possible implementations, the amount of antioxidant added is 5% to 10% based on the total mass of the phosphorus source, the iron source, the lithium source, and the N-doped MXene support.

[0030] In some possible implementations, the complexing agent is added at a rate of 5% to 15% based on the first precursor composite material.

[0031] In some possible implementations, the amount of the doped metal source added is 8000ppm to 12000ppm, based on the first precursor composite material.

[0032] In some possible implementations, the amount of dispersant added is 2% to 20% based on the first precursor composite material.

[0033] In some possible implementations, the preparation of the N-doped MXene support includes the following steps: MAX ceramic phase was etched using HF to obtain multilayer MXenen; The multilayer MXene was exfoliated using dimethyl sulfoxide or tetrabutylammonium hydroxide to obtain a few-layer MXene; The few-layer MXene and N source were dissolved in dilute hydrochloric acid, filtered and dried, and then annealed in an inert atmosphere at 400℃~600℃ for 2 to 5 hours to obtain the N-doped MXene support.

[0034] In some possible implementations, the thickness of the N-doped MXene support is 20 nm to 50 nm, and the sheet diameter is 1 μm to 5 μm.

[0035] In some possible implementations, the N-doped MXene support has 1 to 3 layers.

[0036] In some possible implementations, the mass percentage of N element in the N-doped MXene support is 5% to 25%.

[0037] Thirdly, this application provides a positive electrode sheet, the positive electrode sheet comprising a current collector and a positive electrode active layer formed on at least one surface of the current collector, the positive electrode active layer comprising the above-described composite positive electrode material and / or the composite positive electrode material prepared by the above-described method.

[0038] Fourthly, this application provides a secondary battery, which includes the aforementioned positive electrode plate.

[0039] The composite cathode material provided in the first aspect of this application constructs a multi-level synergistic composite structure by in-situ growing metal-doped lithium iron phosphate in a rice-grain-like morphology on an N-doped MXene support. Firstly, the N-doped MXene acts as a three-dimensional conductive framework, connecting the cathode active material particles to form an electronic network structure that runs throughout the entire electrode, significantly compensating for and improving the overall electronic conductivity of the composite material. Furthermore, MXene possesses a hydrophilic surface and a layered structure, with interlayer gaps serving as channels for rapid ion migration. The doping of N atoms provides defects in pyridine and pyrrole nitrogen, introducing more defects and polar sites that can serve as lithium-ion adsorption sites, increasing pseudocapacitance contribution and enhancing its adsorption and transport capabilities for lithium ions. This not only provides an additional rapid transport path for lithium ions, aiding in improving ionic conductivity, but also increases the contact area between the electrolyte and the active material, promoting interfacial ion exchange. On the other hand, in positive electrode active materials, lithium iron phosphate (LFP) can generate defects in the crystal lattice by introducing suitable metal elements for doping, increasing the electron carrier concentration and widening the one-dimensional diffusion channels of lithium ions. Metal doping refines the grain size, shortens the b-axis and cell volume of LFP, and increases Li-O bonds, thus shortening the lithium-ion diffusion channels and increasing the diffusion rate. Furthermore, appropriate metal doping can strengthen the structure of LFP, improve the reversibility of lithium-ion insertion / extraction, and further contribute to long cycle life. Additionally, positive electrode active materials have a rice-grain-like morphology, resembling slender rice grains. This uniform particle structure typically means that the particles are very small in one or two dimensions, providing more reactive sites and significantly shortening the diffusion distance of lithium ions within the positive electrode active material. This is key to improving rate performance (fast charging and discharging).

[0040] The method for preparing the composite cathode material provided in this application introduces an N-doped MXene support into the solvothermal reaction system for preparing the precursor composite material, allowing the raw material components to adhere to the N-doped MXene support and grow in situ. An antioxidant is used to prevent the support from oxidizing. The first precursor composite material is then mixed with a doped metal source, a complexing agent, and a dispersant, followed by grinding to ensure uniform doping of the metal source into the precursor. Calcination is then performed, simultaneously achieving active material crystallization, lattice doping, and in-situ carbon coating. The complexing agent, through coordination with iron ions, effectively controls the concentration and release rate of free iron ions in the solution, thereby regulating the nucleation and growth kinetics of lithium iron phosphate crystals. It selectively adsorbs onto specific crystal faces, guiding anisotropic crystal growth and ultimately forming a regular, rice-grain-like morphology. The dispersant and complexing agent simultaneously provide a carbon source during calcination to form the carbon coating layer in situ. The preparation method of this application, through the ingenious connection and synergy of each step, achieves the precise construction of multi-level structures and the effective integration of multiple functions. It not only ensures the excellent electrochemical performance of the product, but also has the outstanding advantages of process continuity, precise control and good repeatability, making it suitable for industrial mass production.

[0041] The cathode sheet provided in this application employs a composite cathode material with a multi-level synergistic structure in its active layer. This material precisely integrates an N-doped MXene conductive network, metal-doped lithium iron phosphate, a rice-grain-like nanostructure, and an ultrathin carbon coating layer, systematically overcoming the conductivity bottleneck. The cathode sheet made using this material forms a highly efficient electron-ion dual-channel network, achieving extremely low internal resistance without requiring a large amount of additional conductive agent, thus simultaneously achieving high energy density, excellent rate performance (fast charging and discharging), and ultra-long cycle life.

[0042] The secondary battery provided in this application uses a positive electrode sheet made of the aforementioned composite positive electrode material, inheriting and demonstrating all its performance advantages. The battery internally forms a highly efficient electron-ion transport network from the material level to the electrode level, significantly reducing internal resistance and polarization effects, thereby achieving high energy density, excellent rate charge / discharge capability (especially suitable for fast charging scenarios), and ultra-long cycle life. Attached Figure Description

[0043] 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.

[0044] Figure 1 This is a schematic diagram of the structure of the composite cathode material provided in the embodiments of this application; Figure 2 This is a schematic flowchart of the preparation method of the composite cathode material provided in the embodiments of this application; Figure 3 This is an electron microscope image of the morphology of the positive electrode active material in the composite positive electrode material provided in Example 1 of this application. Detailed Implementation

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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 described in the embodiments of this application can be a mass unit known in the chemical industry, such as µg, mg, g, or kg.

[0051] 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.

[0052] The first aspect of this application provides a composite cathode material, as shown in the attached figure. Figure 1 As shown, it includes an N-doped MXene support and a positive electrode active material grown in situ in the N-doped MXene support. The positive electrode active material includes lithium iron phosphate doped with metal elements and has a rice-like morphological structure.

[0053] The composite cathode material provided in the first aspect of this application constructs a multi-level synergistic composite structure by in-situ growing metal-doped lithium iron phosphate in a rice-grain-like morphology on an N-doped MXene support. On one hand, the N-doped MXene acts as a three-dimensional conductive framework, connecting the cathode active material particles to form an electronic network structure that runs through the entire electrode, greatly compensating for and improving the overall electronic conductivity of the composite material. Furthermore, MXene possesses a hydrophilic surface and a layered structure, with interlayer gaps serving as channels for rapid ion migration; while the doping of N atoms provides defects in pyridine and pyrrole nitrogen, introducing more defects and polar sites that can serve as lithium-ion adsorption sites, increasing pseudocapacitance contribution and enhancing its adsorption and transport capabilities for lithium ions. This not only provides an additional rapid transport path for lithium ions, assisting in improving ionic conductivity, but also increases the contact area between the electrolyte and the active material, promoting interfacial ion exchange. On the other hand, in positive electrode active materials, lithium iron phosphate (LFP) can generate defects in the crystal lattice by introducing suitable metal elements for doping, increasing the electron carrier concentration and widening the one-dimensional diffusion channels of lithium ions. Metal doping refines the grain size, shortens the b-axis and cell volume of LFP, and increases Li-O bonds, thus shortening the lithium-ion diffusion channels and increasing the diffusion rate. Furthermore, appropriate metal doping can strengthen the structure of LFP, improve the reversibility of lithium-ion insertion / extraction, and further contribute to long cycle life. Additionally, positive electrode active materials have a rice-grain-like morphology, resembling slender rice grains. This uniform particle structure typically means that the particles are very small in one or two dimensions, providing more reactive sites and significantly shortening the diffusion distance of lithium ions within the positive electrode active material. This is key to improving rate performance (fast charging and discharging).

[0054] It should be noted that in the N-doped MXene support of this application embodiment, the N element can be doped at least within the MXene material. The N element can be doped on the surface of the MXene or inside the MXene, and is not specifically limited. The N-doped MXene support has a large number of two-dimensional layered structures, effectively increasing the specific surface area of ​​the composite cathode material for lithium-ion insertion / extraction, thereby improving the efficiency of lithium-ion insertion / extraction and increasing the capacity of the composite cathode material. The N-doped MXene support has excellent electronic conductivity and good mechanical strength, effectively improving the electronic conductivity and structural stability of the composite cathode material, thereby improving the cycle stability of the composite cathode material.

[0055] In some possible implementations, the positive electrode active material with a rice-grain-like morphology includes a short axis in the radial direction and a long axis in the longitudinal direction, with the long axis extending into a pointed shape at least once. It should be noted that this pointed shape can be any shape where the middle portion of the positive electrode active material along the long axis is larger than the end portion. It can be a pointed structure similar to a cone, pyramid, or wedge, or a blunt structure such as an ellipse, short circle, circle, or irregular arc.

[0056] In some possible implementations, in positive electrode active materials with a rice-like morphology, the short axis length is less than 50 nm, and the long axis length is 100 nm to 300 nm. In this case, the short axis size (<50 nm) significantly shortens the diffusion path of lithium ions in the solid phase, fundamentally improving ionic conductivity and rate performance. While ensuring the diffusion advantage along the short axis, the moderate long axis size ensures that the particles have sufficient volume, thereby maintaining high tap density and volumetric energy density. This slender particle morphology makes it easier to achieve uniform and compact "in-situ growth" on two-dimensional N-doped MXene support sheets, maximizing the contact area between them and minimizing interfacial resistance, allowing the high conductivity of the N-doped MXene support to be fully utilized. Nanoscale high-capacity products play an important role in particle gradation. By filling the gaps between micron-sized particles with nanoscale particles to improve compaction, they can also contribute to the capacity of the graded product.

[0057] For example, in nanoparticles with a rice-like morphology, the length of the short axis can be any typical but non-limiting point value or an interval between any two points, such as 5nm, 10nm, 20nm, 30nm, 40nm, 49nm, etc., and the length of the long axis can be any typical but non-limiting point value or an interval between any two points, such as 100nm, 150nm, 200nm, 250nm, 300nm, etc.

[0058] In some possible implementations, the N-doped MXene support in the composite cathode material has a mass percentage of 0.5% to 5%. In this case, the lower limit of 0.5% ensures that the MXene content is sufficient to form a continuous, interconnected three-dimensional conductive network, effectively connecting the vast majority of lithium iron phosphate particles. The upper limit of 5% ensures the specific capacity of the composite cathode material while controlling costs, thus guaranteeing performance.

[0059] For example, in composite cathode materials, the mass percentage of N-doped MXene support can be any typical but non-limiting point value or a range between any two points, such as 0.5%, 1%, 2%, 3%, 4%, 5%.

[0060] In some possible implementations, metal-doped lithium iron phosphate (LFP) particles are doped with at least one of the following metal elements: Ti, V, Mo, Nb, Zr, Mn, and Mg. In this case, the LFP particles are doped with specific metal elements such as Ti, V, Mo, Nb, Zr, Mn, and Mg. These high-valence or appropriately sized metal ions enter the crystal lattice, effectively creating lattice defects, increasing electron carrier concentration, and even widening lithium-ion migration channels, fundamentally improving the material's electronic conductivity and ion diffusion capability.

[0061] In some possible implementations, the mass percentage of the doped metal element in metal-doped lithium iron phosphate is between 7000 ppm and 11000 ppm. In this case, it ensures that the metal element has a sufficient doping concentration to enter the crystal lattice, altering the electronic structure and improving the intrinsic conductivity. For example, the mass percentage of the doped metal element in metal-doped lithium iron phosphate can be any typical but non-limiting value, such as 7000 ppm, 8000 ppm, 9000 ppm, 10000 ppm, or 11000 ppm, or a range between any two values.

[0062] In some possible implementations, the outer surface of the positive electrode active material has a carbon coating layer. In this case, a composite positive electrode material with a specific "rice grain-like" morphology, metal-doped lithium iron phosphate, is grown in situ, and a carbon coating layer is further constructed on its outer surface. This design forms a triple synergistic mechanism of "N-doped MXene support macroscopic conductive network - carbon coating layer particle-level conductive interface - metal doping intrinsic conductivity enhancement," enabling unimpeded high-speed electron transport from the macroscopic to the microscopic level. Furthermore, the carbon coating layer can effectively isolate the active material from direct contact with the electrolyte, suppress side reactions, and improve coulombic efficiency.

[0063] In some possible implementations, the thickness of the carbon coating layer is 1 nm to 5 nm. In this case, by precisely controlling the construction of an ultrathin carbon coating layer of 1 nm to 5 nm thickness on its outer surface, this thickness ensures the formation of a continuous conductive network and a stable interface while avoiding the obstruction of ion transport caused by excessive coating thickness, thus achieving a balance between electron conduction and ion transport.

[0064] For example, the thickness of the carbon coating can be any typical but non-limiting point value such as 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, or an interval between any two point values.

[0065] In some possible implementations, the carbon content in the positive electrode active material is 1.2% to 1.5%. In this case, the carbon content in the positive electrode active material is controlled at 1.2% to 1.5%, which ensures the formation of a continuous conductive interface and protective layer while minimizing the proportion of inactive materials and guaranteeing the energy density of the battery.

[0066] For example, in the positive electrode active material, the carbon content can be any typical but non-limiting point value or a range between any two points, such as 1.2%, 1.3%, 1.4%, 1.5%.

[0067] In some possible implementations, the thickness of the N-doped MXene support is 20 nm to 50 nm. This reduces the probability of N-doped MXene support sheet peeling while simultaneously giving the N-doped MXene support structure higher mechanical strength, thus improving the stability of the composite cathode material. Furthermore, controlling the thickness of the N-doped MXene support within this range increases its surface area, thereby increasing the contact area between the N-doped MXene support and the cathode active material. This further improves the bonding stability between the cathode active material and the N-doped MXene support, further enhancing the structural stability of the composite cathode material and improving the cycle stability of the battery.

[0068] For example, the thickness of the N-doped MXene support can be any typical but non-limiting point value such as 20 nm, 30 nm, 40 nm, 50 nm, or an interval between any two point values.

[0069] In some possible implementations, the sheet width of the N-doped MXene support is 1 μm to 5 μm. In this case, the N-doped MXene support is ensured to act as a huge and continuous three-dimensional conductive network, efficiently bridging and encapsulating a large number of nanoscale active particles, thereby constructing a high-speed electronic pathway throughout the entire electrode with a low support content (0.5% to 5%).

[0070] For example, the sheet width of the N-doped MXene support can be any typical but non-limiting point value or an interval between any two point values, such as 1 μm, 2 μm, 3 μm, 4 μm, 5 μm.

[0071] In some possible implementations, the N-doped MXene support has 1 to 3 layers. In this case, the N-doped MXene support can be a single-layer structure or a few-layer structure of 2 to 3 layers. Controlling the N-doped MXene support to be a few-layer or single-layer structure improves its conductivity, further enhancing the conductivity of the composite cathode material. Furthermore, the few-layer or single-layer structure provides a relatively short ion diffusion path for the N-doped MXene support, allowing lithium ions to transport more quickly within the composite cathode material, thereby increasing the charge and discharge rate of the cathode material. In some possible implementations, the nitrogen element in the N-doped MXene support exists in at least one of the following forms: N atoms, N-metallic bonds, and CN bonds. In exemplary cases, N atoms can exist alone as dopant; N atoms can also replace C atoms in the MXene material to form N-metallic bonds, exemplarily including N-Ti, NV, N-Mo, and N-Nb; or they can form CN bonds with C. Modifying the MXene material with these N atoms further enhances the surface activity of the N-doped MXene support, improves the bonding force between the N-doped MXene support and the positive electrode active material, and promotes the composite interaction between the N-MXene material and the positive electrode active material, thereby improving the electrochemical performance of the composite positive electrode material.

[0072] In some possible implementations, the mass percentage of N element in the N-doped MXene support is 5% to 25%. In exemplary cases, the mass percentage of modified N element can be any typical but not limiting point value or a range between any two points, such as 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, and 25%.

[0073] In some possible implementations, the N-doped MXene support contains MXene, including Ti3C2T. x Ti2CT x Nb2CT x V2CT x Mo3C2T x Nb4C3T x At least one of them, wherein T xThe MXene materials are selected from at least one of -O, -OH, or -F. These MXene materials exhibit good conductivity, chemical stability, and electrochemical stability. Furthermore, the N-doped MXene support formed by modifying these MXene materials with N element possesses excellent surface activity and can form a good interfacial bond with phosphate cathode materials. Based on the excellent conductivity, chemical stability, and electrochemical stability of these MXene materials, as well as the good surface activity after forming the N-doped MXene support, the conductivity, structural stability, and electrochemical stability of the composite cathode material are further improved. The composite cathode material of the above embodiments of this application can be prepared by the methods of the following embodiments.

[0074] Secondly, embodiments of this application provide a method for preparing a composite cathode material, as shown in the attached figure. Figure 2 As shown, it includes the following steps: S10. Obtain the N-doped MXene support; S20. Dissolve phosphorus source, iron source and lithium source in organic solvent, add antioxidant and N-doped MXene support to carry out solvothermal reaction to obtain first precursor composite material; S30. The first precursor composite material is mixed with a doped metal source, a complexing agent and a dispersant, then ground and dried to obtain a nanoscale second precursor composite material. S40. The second precursor composite material is calcined in an inert atmosphere to obtain a composite cathode material with a rice-grain-like morphology.

[0075] The method for preparing composite cathode materials provided in this application introduces an N-doped MXene support into a solvothermal reaction system for preparing a precursor composite material, allowing the raw material components to adhere to the N-doped MXene support and grow in situ. An antioxidant is used to prevent the support from oxidizing. The first precursor composite material is then mixed with a doped metal source, a complexing agent, and a dispersant, followed by grinding to ensure uniform doping of the metal source into the precursor. Calcination is then performed, simultaneously achieving active material crystallization, lattice doping, and in-situ carbon coating. The complexing agent, through coordination with iron ions, effectively controls the concentration and release rate of free iron ions in the solution, thereby regulating the nucleation and growth kinetics of lithium iron phosphate crystals. It selectively adsorbs onto specific crystal faces, guiding anisotropic crystal growth and ultimately forming a regular, rice-grain-like morphology. The dispersant and complexing agent simultaneously provide a carbon source during calcination to form the carbon coating layer in situ. The preparation method of this application, through the ingenious connection and synergy of each step, achieves the precise construction of multi-level structures and the effective integration of multiple functions. It not only ensures the excellent electrochemical performance of the product, but also has the outstanding advantages of process continuity, precise control and good repeatability, making it suitable for industrial mass production.

[0076] In step S10 above: In some possible implementations, the preparation of the N-doped MXene support includes the following steps: S11. Use HF to etch the MAX ceramic phase to obtain multilayer MXenen; S12. Use dimethyl sulfoxide or tetrabutylammonium hydroxide to exfoliate multilayer MXene to obtain few-layer MXene; S13. Dissolve the few-layer MXene and N source in dilute hydrochloric acid, filter and dry, then anneal at an inert atmosphere of 400℃~600℃ for 2 hours~5 hours to obtain the N-doped MXene support. During sintering, the N source is converted into pyrrole nitrogen and pyridine monophosphate or even graphitic nitrogen, which can provide more active sites, which is beneficial to lithium ion transport and thus improves the electrochemical performance of the material.

[0077] In some possible implementations, the thickness of the N-doped MXene support is 20 nm to 50 nm, and the sheet width is 1 μm to 5 μm.

[0078] In some possible implementations, the N-doped MXene support has 1 to 3 layers.

[0079] In some possible implementations, the mass percentage of nitrogen in the N-doped MXene support is 5% to 25%.

[0080] The effects of the above embodiments of this application have been discussed above and will not be repeated here.

[0081] In step S20 above: In some possible implementations, the iron source is one or more of the following: ferric nitrate, ferric oxide, ferrous oxalate, ferric oxide, ferrous sulfate, ferric chloride, ferric phosphate, and ferric phosphate dihydrate.

[0082] In some possible implementations, the phosphorus source is one or more of the following: phosphoric acid, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, iron phosphate, iron phosphate dihydrate, and lithium phosphate.

[0083] In some possible implementations, the lithium source is one or more of lithium carbonate, lithium phosphate, lithium fluoride, lithium acetate, lithium hydroxide, etc.

[0084] In some possible implementations, the lithium source, iron source, and phosphorus source are added in a non-stoichiometric ratio of (1.03~1.07):(0.95~0.99):1 dissolved in an organic solvent. The lithium source is added in slight excess to compensate for the inevitable lithium volatilization during subsequent high-temperature calcination, ensuring that the final product is stoichiometrically correct and highly electrochemically active LiFePO4 (LFP), avoiding the formation of impurity phases due to lithium deficiency, thereby guaranteeing the material's capacity and stability.

[0085] In some possible implementations, the organic solvent includes at least one of ethanol and ethylene glycol. These organic solvents exhibit good solubility and dispersion properties for phosphorus sources, iron sources, lithium sources, antioxidants, and N-doped MXene supports.

[0086] In some possible implementations, the solvothermal reaction step includes reacting in a reactor at 100℃~150℃ for 6h~12h. During this reaction, Li / P / Fe undergoes a redox reaction to generate lithium iron phosphate, which is then in situ supported on the surface of the N-doped MXene support. These reaction conditions ensure the synthesis of lithium iron phosphate with low crystallinity, providing more active sites for subsequent experiments and facilitating metal ion doping and complexation reactions. They also effectively protect the N-doped MXene support from high-temperature hydrothermal corrosion, maintaining its structural integrity and conductivity.

[0087] For example, the temperature of the solvothermal reaction can be any typical but non-limiting point value or a range between any two points, such as 100℃, 110℃, 120℃, 130℃, 140℃, 150℃; the reaction time can be any typical but non-limiting point value or a range between any two points, such as 6h, 7h, 8h, 9h, 10h, 11h, 12h.

[0088] In some possible implementations, antioxidants include at least one of ascorbic acid, L-tartaric acid, glutathione, hydroxyanisole, butylated hydroxytoluene, and tert-butylhydroquinone. These antioxidants can effectively reduce the high-temperature oxidation of phosphorus, iron, and lithium sources and N-doped MXene supports. In particular, they preferentially react with oxygen in the system, effectively isolating the hydrothermal environment from the oxidative degradation of MXene, thereby fully preserving its highly conductive two-dimensional structure.

[0089] In some possible implementations, the amount of N-doped MXene support added is 0.5% to 5%, based on the total mass of the phosphorus source, iron source, lithium source, and N-doped MXene support. In this case, the MXene content is sufficient to form a continuous, through-type three-dimensional conductive network, which can effectively connect the vast majority of lithium iron phosphate particles.

[0090] In some possible implementations, the amount of antioxidant added is 5% to 10% based on the total mass of the phosphorus source, iron source, lithium source, and N-doped MXene support. In this case, the N-doped MXene support can be adequately protected from oxidation and etching, laying the foundation for its subsequent function as a "conductive framework".

[0091] For example, based on the total mass of the phosphorus source, iron source, lithium source and N-doped MXene support, the amount of antioxidant added can be any typical but non-limiting point value or a range between any two point values, such as 5%, 6%, 7%, 8%, 9%, 10%.

[0092] In some embodiments, lithium source, iron source, and phosphorus source are dissolved in ethanol, ethylene glycol, or a mixture thereof in a non-stoichiometric ratio of (1.03~1.07):(0.95~0.99):1. The mixture is stirred with a magnetic stirrer for 1 to 4 hours. After stirring until homogeneous, an antioxidant and N-doped MXene support are added and stirring is continued. The mixture is then poured into a hydrothermal reactor and placed in a forced-air drying oven at 100°C~150°C for 6 to 12 hours. After the reaction is completed, the mixture is filtered and dried to obtain the first precursor composite material.

[0093] In step S30 above: In some possible implementations, the doped metal source includes at least one of Ti, V, Mo, Nb, Zr, Mn, and Mg sources. For example, the doped metal source can be a metal salt, such as a chloride or carbonate.

[0094] In some possible implementations, based on the first precursor composite material, the amount of doped metal source added is 8000ppm to 12000ppm. In this case, the lower limit (8000ppm) ensures that there is sufficient dopant concentration to effectively enter the crystal lattice, significantly change the electronic structure, and achieve a substantial improvement in intrinsic conductivity. The upper limit (12000ppm) prevents excessive doping from causing excessive lattice distortion, generating impurity phases, or blocking lithium-ion migration channels, achieving the best balance between improving conductivity and maintaining structural stability.

[0095] For example, based on the first precursor composite material, the amount of doped metal source added can be any typical but non-limiting point value or an interval between any two point values, such as 8000ppm, 9000ppm, 10000ppm, 11000ppm, 12000ppm.

[0096] In some possible implementations, the complexing agent includes at least one of citric acid dihydrate, ethylenediaminetetraacetic acid, aminetriacetic acid, gluconic acid, oxalic acid, and diethylenetriaminepentaacetic acid. In this case, these complexing agents can effectively control the concentration and release rate of free iron ions in the solution through coordination with iron ions, thereby regulating the nucleation and growth kinetics of lithium iron phosphate crystals. They can also selectively adsorb onto specific crystal faces, guiding anisotropic crystal growth and ultimately forming a regular, rice-grain-like morphological structure.

[0097] In some possible implementations, based on the first precursor composite material, the amount of complexing agent added is 5% to 15%. In this case, the lower limit (5%) ensures that there are enough molecules to encapsulate and complex each doped metal ion, preventing hydrolysis or segregation during subsequent grinding and drying, and achieving uniform dispersion at the molecular level. The upper limit (15%) avoids excessive organic matter leading to an excessively thick carbon layer or the introduction of impurities during subsequent calcination.

[0098] For example, based on the first precursor composite material, the amount of complexing agent added can be any typical but non-limiting point value or an interval between any two point values, such as 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%.

[0099] In some possible implementations, the dispersant includes at least one of polyvinylpyrrolidone, polyethylene glycol ether, Tween, Span, and fatty acid polyoxyethylene ether. In this case, these dispersants can effectively prevent the hard agglomeration of nanoscale precursor particles during grinding and subsequent drying through steric hindrance, thereby maintaining their high specific surface area and reactivity.

[0100] In some possible implementations, based on the first precursor composite material, the amount of dispersant added is 2% to 20%. In this case, the lower limit (2%) ensures the formation of a sufficient adsorption layer on the particle surface, providing effective steric hindrance and preventing soft / hard agglomeration of nanoparticles during grinding and drying. The upper limit (20%), while ensuring excellent dispersion, avoids unnecessary burden on the subsequent calcination process and the final carbon content and structure due to excessive organic matter.

[0101] For example, based on the first precursor composite material, the amount of dispersant added can be any typical but non-limiting point value or an interval between any two point values, such as 2%, 5%, 8%, 10%, 12%, 15%, 16%, 18%, 20%.

[0102] In some possible implementations, the grinding process includes sand milling until the particle size D50 is less than 200 nm. In this case, ensuring that the precursor composite particles reach and stabilize at the nanoscale directly guarantees the extremely short lithium-ion diffusion path in the final product, which is the physical basis for obtaining excellent rate performance.

[0103] For example, the grinding process includes: performing a sand milling process using a sand mill until the particle size D50 can be any typical but non-limiting point value or an interval between any two point values, such as 10nm, 50nm, 80nm, 100nm, 120nm, 150nm, 180nm, 190nm.

[0104] In some possible implementations, centrifugal spray drying is employed. This process can instantly atomize the slurry into micron-sized droplets and dry them rapidly, greatly avoiding the severe hard agglomeration of nanoparticles caused by capillary forces during conventional drying processes, and effectively maintaining the nanoscale and high specific surface area obtained after sand milling.

[0105] In some embodiments, the first precursor composite material and the dopant element source are mixed in a certain ratio, and then a complexing agent and a dispersant are added. The mixture is then milled using a sand mill with a particle size D50 < 200 nm. After milling, the material is dried using a centrifugal spray dryer to obtain the second precursor composite material.

[0106] In step S40 above: In some possible implementations, the inert atmosphere includes at least one of nitrogen and argon.

[0107] In some possible implementations, the calcination temperature is 600℃~720℃, and the duration is 2h~6h. In this case, the lower temperature limit (600℃) ensures that lithium iron phosphate fully crystallizes into a pure phase with high electrochemical activity, while simultaneously allowing doped metal atoms to effectively diffuse into the crystal lattice. The upper temperature limit (720℃) prevents excessive temperature from causing lithium iron phosphate decomposition, excessive grain growth (which would sacrifice the advantages of nanostructure), and excessive graphitization or loss of the carbon coating layer. The calcination time (2h~6h) ensures an optimal balance between crystallinity, doping uniformity, and carbonization degree; too short a time results in incomplete reaction, while too long a time is uneconomical and may cause side effects.

[0108] For example, the calcination temperature can be any typical but non-limiting point value or an interval between any two points, such as 600℃, 610℃, 620℃, 650℃, 680℃, 700℃, 720℃, etc., and the duration can be any typical but non-limiting point value or an interval between any two points, such as 2h, 3h, 4h, 5h, 6h.

[0109] Thirdly, embodiments of this application provide a positive electrode sheet, which includes a current collector and a positive electrode active layer formed on at least one surface of the current collector. The positive electrode active layer includes the composite positive electrode material described above and / or the composite positive electrode material prepared by the method described above.

[0110] The cathode sheet provided in this application uses a composite cathode material with a multi-level synergistic structure in its active layer. This material precisely integrates an N-doped MXene conductive network, metal-doped lithium iron phosphate, a rice-grain-like nanostructure, and an ultrathin carbon coating layer, systematically overcoming the conductivity bottleneck. The cathode sheet made using this material forms a highly efficient electron-ion dual-channel network, achieving extremely low internal resistance without requiring a large amount of additional conductive agent, thus simultaneously achieving high energy density, excellent rate performance (fast charging and discharging), and ultra-long cycle life.

[0111] In some possible implementations, the preparation of the positive electrode active layer includes the following steps: mixing the above-mentioned composite positive electrode material, conductive agent and binder to form an electrode slurry, coating the electrode slurry onto the current collector, and then preparing the positive electrode sheet through steps such as drying, rolling and die cutting.

[0112] In some possible implementations, the mass percentage of composite cathode material in the cathode active layer of the cathode sheet is 90% to 95%. Specifically, the mass percentage of composite cathode material in the cathode active material layer can be 90%, 91%, 92%, 93%, 94%, 95%, etc.

[0113] In some possible implementations, the current collector of the positive electrode includes, but is not limited to, any one of copper foil or aluminum foil.

[0114] In some possible implementations, the binder content in the positive electrode active material layer is 2wt% to 5wt%. In specific embodiments, the binder content can be typical but not limited to 2wt%, 3wt%, 4wt%, 5wt%, etc.

[0115] In some possible implementations, the binder includes one or more of the following: polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives.

[0116] In some possible implementations, the conductive agent content in the positive electrode active material layer is 0.7wt% to 5wt%. In specific embodiments, the conductive agent content can be a typical but not limited content such as 3wt%, 4wt%, or 5wt%.

[0117] In some possible implementations, the conductive agent includes graphite, carbon black, acetylene black, graphene, carbon fiber, and C. 60 And one or more of carbon nanotubes.

[0118] Fourthly, embodiments of this application provide a secondary battery, which includes the aforementioned positive electrode sheet.

[0119] The secondary battery provided in this application embodiment uses the aforementioned composite cathode material as its cathode sheet, inheriting and exhibiting all of its performance advantages. The battery internally forms a highly efficient electron-ion transport network from the material level to the electrode level, significantly reducing internal resistance and polarization effects, thereby achieving high energy density, excellent rate charge / discharge capability (especially suitable for fast charging scenarios), and ultra-long cycle life in terms of overall performance.

[0120] In some possible implementations, the negative electrode active material of the secondary battery includes, but is not limited to, carbon materials such as graphite, soft carbon (e.g., coke), and hard carbon, or nitrides, tin-based oxides, tin alloys, and nano-anode materials. The current collector includes, but is not limited to, any one of copper foil and aluminum foil.

[0121] In some possible implementations, the steps for making the negative electrode sheet include: mixing the negative electrode active material with conductive agents such as conductive carbon black, binders such as carboxymethyl cellulose and styrene-butadiene rubber, and solvents such as water in a mass ratio of (80~99):(1~5):(2~10):100 to make a positive electrode mixed slurry, then degassing under vacuum, discharging the material, coating it on a coating machine, and obtaining the negative electrode sheet after rolling, slitting, and die-cutting.

[0122] In some possible implementations, the membrane is capable of blocking electrons while allowing ions to pass through. Exemplary membranes include, but are not limited to, at least one material selected from polypropylene fibers, polyacrylonitrile fibers, polyvinyl formal fibers, poly(ethylene glycol terephthalate), polyethylene terephthalate, polyamide fibers, and poly(p-phenylene terephthalamide).

[0123] In some possible implementations, the electrolyte comprises at least one soluble metal salt. In some specific embodiments, the metal salt includes LiClO4, LiBF4, LiPF6, LiAsF6, LiCF3SO3, LiTDI, Li[(CF3SO2)2N], Li[(FSO2)2N], Li[(C m F 2m+1 SO2)(C n F 2n+1 At least one of SO2(N)[m, n], where m and n are natural numbers. These electrolytic salts can ensure high ionic conductivity of the electrolyte and do not undergo harmful side reactions with electrode materials, electrolyte, diaphragm, etc., and have good chemical stability.

[0124] In some possible implementations, the secondary battery includes at least one of a battery cell, a battery module, and a battery pack.

[0125] In some possible implementations, the battery cell types include lithium-ion batteries, as well as novel batteries such as lithium metal batteries.

[0126] In some possible implementations, the battery cells of this application can be assembled into a battery module. The battery module can contain multiple battery cells, the specific number of which can be adjusted according to the application and capacity of the battery module. The battery module may also include a housing with a receiving space in which multiple battery cells are received.

[0127] In one possible implementation, battery cells and / or battery modules can also be assembled into a battery pack, and the number of battery cells or battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0128] To enable those skilled in the art to clearly understand the above-described implementation details and operations of this application, and to demonstrate the significant improvement in the performance of the composite cathode material, its preparation method, cathode sheet, and secondary battery in the embodiments of this application, the following examples illustrate the above technical solutions.

[0129] Example 1 A composite cathode material comprises multiple nanoparticles with a rice-grain-like morphology (short axis 18nm~25nm, long axis 210nm~250nm). The nanoparticles include an N-doped MXene support (approximately 2% content, thickness 20nm~50nm, sheet diameter 1μm~5μm, 1-3 layers, N element mass percentage in the MXene support 5%~25%, MXene being Ti3C2T). x ) and positive electrode active material (LFP) grown in situ in N-doped MXene support.

[0130] Its preparation includes the following steps: 1. Disperse 156.42g of Li2CO3, 460.12g of NH4H2PO4 and 555.30g of FeC2O4 together in a 65% ethanol solution. After stirring for 1 hour, add 6g of N-doped MXene support and 60g of ascorbic acid and continue stirring for 2 hours to obtain a mixed solution.

[0131] 2. Transfer the mixed solution to a stainless steel reactor, place it in a forced-air drying oven, react at 120℃ for 8 hours, filter and dry to obtain the first precursor composite material.

[0132] 3. Take 500g of the first precursor composite material and dissolve it together with 8g TiO2, 0.5g ammonium metavanadate, 30g polyvinylpyrrolidone (PVP), and 50g citric acid dihydrate in deionized water. Mix the materials using a sand mill with a particle size D50 < 200nm. After the particle size is qualified, transfer the mixture to a centrifugal spray dryer to obtain the second precursor composite material.

[0133] 4. The second precursor composite material is placed in a tube furnace, and the sintering temperature is set to 650℃, the holding time is 4h, and the heating rate is 3℃ / min. After calcination, a solid powder is obtained, which is the metal-doped high-capacity rice-grain-like nano LFP@N-doped MXene carrier composite cathode material.

[0134] A secondary battery is prepared by dissolving the composite positive electrode material, polyvinylidene fluoride (PVDF), and conductive carbon black (SP) in an 8:1:1 ratio in N-methylpyrrolidone (NMP), stirring evenly, coating it onto carbon-coated aluminum foil, drying it, and cutting it into small circular pieces with a diameter of 1.4 cm, i.e., positive electrode pieces, and assembling them into a coin cell.

[0135] Example 2 A composite cathode material comprises multiple nanoparticles with a rice-grain-like morphology (short axis approximately 15 nm–25 nm, long axis 200 nm–240 nm). The nanoparticles include an N-doped MXene support (content approximately 1.8%–4.5%, thickness 20 nm–50 nm, sheet diameter 1 μm–5 μm, number of layers 1–3, N element mass percentage in the MXene support 5%–25%, MXene being Ti2CT). x ) and positive electrode active material (LFP) grown in situ in N-doped MXene support.

[0136] Its preparation includes the following steps: 1. Disperse 158.50g of Li2CO3, 470.12g of NH4H2PO4 and 555.30g of FeC2O4 together in a 65% ethanol solution. After stirring for 1 hour, add 6g of N-doped MXene support and 60g of ascorbic acid and continue stirring for 2 hours to obtain a mixed solution.

[0137] 2. Transfer the mixed solution to a stainless steel reactor, place it in a forced-air drying oven, react at 110℃ for 8 hours, filter and dry to obtain the first precursor composite material.

[0138] 3. Take 500g of the first precursor composite material, dissolve it together with 7g TiO2, 0.8g ammonium metavanadate and 60g polyvinylpyrrolidone (PVP) in deionized water, mix them using a sand mill, set the sand milling particle size D50 < 150nm, and after the particle size is qualified, transfer it to a centrifugal spray dryer to obtain the second precursor composite material.

[0139] 4. The second precursor composite material is placed in a tube furnace, and the sintering temperature is set to 680℃, the holding time is 4h, and the heating rate is 3℃ / min. After calcination, a solid powder is obtained, which is the metal-doped high-capacity rice-grain-like nano LFP@N-doped MXene carrier composite cathode material.

[0140] A secondary battery, the preparation method of which is the same as in Example 1.

[0141] Example 3 A composite cathode material comprises multiple nanoparticles with a rice-grain-like morphology (short axis 10nm~20nm, long axis 200nm~230nm). The nanoparticles include an N-doped MXene support (content 1.6%~1.9%, thickness 20nm~50nm, sheet diameter 1μm~5μm, number of layers 1-3, N element mass percentage in the MXene support 5%~25%, MXene being V2CT). x ) and positive electrode active material (LFP) grown in situ in N-doped MXene support.

[0142] Its preparation includes the following steps: 1. Disperse 161.50g of Li2CO3, 479.12g of NH4H2PO4 and 557.30g of FeC2O4 together in a 65% ethanol solution. After stirring for 1 hour, add 6g of N-doped MXene support and 60g of L-tartaric acid and continue stirring for 2.5 hours to obtain a mixed solution.

[0143] 2. Transfer the mixed solution to a stainless steel reactor, place it in a forced-air drying oven, react at 130°C for 8 hours, filter and dry to obtain the first precursor composite material.

[0144] 3. Take 500g of the first precursor composite material and dissolve it together with 10g TiO2, 0.3g ammonium metavanadate, 35g oxalic acid and 45g citric acid in deionized water. Mix them using a sand mill and set the sand milling particle size D50 < 150nm. After the particle size is qualified, transfer it to a centrifugal spray dryer to obtain the second precursor composite material.

[0145] 4. The second precursor composite material is placed in a tube furnace, and the sintering temperature is set to 650℃, the holding time is 4h, and the heating rate is 3℃ / min. After calcination, a solid powder is obtained, which is the metal-doped high-capacity rice-grain-like nano LFP@N-doped MXene carrier composite cathode material.

[0146] A secondary battery, the preparation method of which is the same as in Example 1.

[0147] Comparative Example 1 [Non-MXene-free] A composite cathode material, the preparation of which includes the following steps: 1. Disperse 161.50g of Li2CO3, 479.12g of NH4H2PO4 and 557.30g of FeC2O4 together in a 65% ethanol solution, stir for 1 hour, then add 60g of L-tartaric acid and continue stirring for 2.5 hours to obtain a mixed solution.

[0148] 2. Transfer the mixed solution to a stainless steel reactor, place it in a forced-air drying oven, react at 130°C for 8 hours, filter and dry to obtain the first precursor composite material.

[0149] 3. Take 500g of the first precursor composite material and dissolve it in deionized water with 10g TiO2, 0.3g ammonium metavanadate, 35g oxalic acid and 45g citric acid. Mix them using a sand mill with the sand milling particle size D50 < 150nm. After the particle size is qualified, transfer it to a centrifugal spray dryer to obtain the second precursor composite material.

[0150] 4. The second precursor composite material is placed in a tube furnace, and the sintering temperature is set to 650℃, the holding time is 4h, and the heating rate is 3℃ / min. After calcination, a solid powder is obtained, which is the composite cathode material.

[0151] A secondary battery, the preparation method of which is the same as in Example 1.

[0152] Comparative Example 2 [No complexing agent, no rice grain morphology formed] A composite cathode material, the preparation of which includes the following steps: 1. Disperse 161.50g of Li2CO3, 479.12g of NH4H2PO4 and 557.30g of FeC2O4 together in a 65% ethanol solution. After stirring for 1 hour, add 6g of N-doped MXene support and 60g of L-tartaric acid and continue stirring for 2.5 hours to obtain a mixed solution.

[0153] 2. Transfer the mixed solution to a stainless steel reactor, place it in a forced-air drying oven, react at 130°C for 8 hours, filter and dry to obtain the first precursor composite material.

[0154] 3. Take 500g of the first precursor composite material, dissolve 10g of TiO2 and 0.3g of ammonium metavanadate in deionized water, mix them using a sand mill, set the sand milling particle size D50 < 150nm, and after the particle size is qualified, transfer it to a centrifugal spray dryer to obtain the second precursor composite material.

[0155] 4. The second precursor composite material is placed in a tube furnace, and the sintering temperature is set to 650℃, the holding time is 4h, and the heating rate is 3℃ / min. After calcination, a solid powder is obtained, which is the composite cathode material.

[0156] A secondary battery, the preparation method of which is the same as in Example 1.

[0157] Comparative Example 3 [No Metal Ion Doping] A composite cathode material, the preparation of which includes the following steps: 1. Disperse 161.50g of Li2CO3, 479.12g of NH4H2PO4 and 557.30g of FeC2O4 together in a 65% ethanol solution. After stirring for 1 hour, add 6g of N-doped MXene support and 60g of L-tartaric acid and continue stirring for 2.5 hours to obtain a mixed solution.

[0158] 2. Transfer the mixed solution to a stainless steel reactor, place it in a forced-air drying oven, react at 130°C for 8 hours, filter and dry to obtain the first precursor composite material.

[0159] 3. Take 500g of the first precursor composite material, dissolve it in deionized water with 35g of oxalic acid and 45g of citric acid, mix it with a sand mill, set the sand milling particle size D50 < 150nm, and after the particle size is qualified, transfer it to a centrifugal spray dryer to obtain the second precursor composite material.

[0160] 4. The second precursor composite material is placed in a tube furnace, and the sintering temperature is set to 650℃, the holding time is 4h, and the heating rate is 3℃ / min. After calcination, a solid powder is obtained, which is the composite cathode material.

[0161] A secondary battery, the preparation method of which is the same as in Example 1.

[0162] To verify the progressiveness of the embodiments of this application, the composite cathode materials prepared in the above embodiments and comparative examples were subjected to the following performance tests: 1. The morphology of the composite cathode material prepared in Example 1 was observed using a scanning electron microscope, as shown in the attached figure. Figure 3 As shown, the positive electrode active material forms a nanoscale, rice-grain-like morphological structure, uniformly loaded in the N-doped MXene support.

[0163] 2. Compacted density test: The test is conducted using a universal testing machine: Weigh 1g~1.0005g of sample and put it into the mold, and set a pressure of 3 tons for testing.

[0164] 3. Specific Surface Area Test: The BET specific surface area analyzer was used for testing. 0.5g of sample was degassed in a vacuum or inert gas environment to remove impurities adsorbed on the surface. Inert gas (most commonly nitrogen) was adsorbed onto the sample surface at an extremely low temperature (usually liquid nitrogen temperature, -196°C). The amount of gas adsorbed by the sample under different pressures was measured to obtain adsorption isotherms. The BET equation was used to analyze the adsorption isotherms to calculate the monolayer saturated adsorption capacity of the sample, and thus the specific surface area value was obtained.

[0165] 4. Carbon content test: The carbon-sulfur analyzer was used for testing: 0.1g of sample + 0.5g of co-solvent were added to the crucible and the test was carried out in an oxygen atmosphere.

[0166] 5. Test the unit cell parameters: Use a Bruker XRD in Germany to test the samples, and set the test parameters: 10° / min, 10° - 80°, and the unit cell parameters and bond lengths can be obtained through refinement with GSAS-2.

[0167] The above test results are shown in Table 1 below:

[0168] It can be seen from the test results in Table 1 above that the composite cathode materials prepared in the embodiments of this application all exhibit a higher specific surface area. The tap density of the composite cathode materials prepared in the embodiments of this application is comparable to the carbon material content. In Comparative Example 1, there is no N-MXene carrier, in Comparative Example 2, the rice grain morphology is not formed, and in Comparative Example 3, there is no metal ion doping to refine the grains. Therefore, the particle size of the composite materials prepared is higher than that of the examples, and thus the tap density will be slightly higher than that of the examples.

[0169] In addition, it can be seen from the test results of the unit cell parameters that in Examples 1 - 3, the morphology of this composite material is similar to slender rice grains, with the short axis a < 50 nm and the long axis b: 100 < b < 300 nm. The overall shape is similar to a rice grain with at least one end being pointed. The size in the short axis direction of the rice grain-shaped particles is very small, which greatly shortens the diffusion distance of lithium ions inside the solid particles and is beneficial to the improvement of electrical performance. In addition, this composite material further refines the grains through metal doping, shortens the b-axis and unit cell volume of LFP, and increases the Li - O bond length.

[0170] Perform the following performance tests on the secondary batteries prepared in the above-mentioned examples and comparative examples respectively: 6. 0.1C charge and discharge capacity test: Place the prepared coin cell in a Blue Energy test device, set the 0.1C charge and discharge process steps, conduct the charge and discharge test, and directly read the data such as charge / discharge specific capacity and efficiency in the test software after the test.

[0171] 7. 1C charge and discharge capacity test: Place the prepared coin cell in a Blue Energy test device, set the 1C charge and discharge process steps, conduct the charge and discharge test, and directly read the data such as charge / discharge specific capacity and efficiency in the test software after the test.

[0172] The above test results are shown in Table 2 below:

[0173] As shown in Table 2 above, the composite material prepared in this application, through its rice-grain-like morphology and metal doping to refine the grains, shortens the b-axis and cell volume of the LFP, and increases the Li-O bond length. This shortens the lithium-ion diffusion channel and increases the diffusion rate. Furthermore, the N-doped MXene support can act as a conductive agent, improving the electronic conductivity of the LFP, and its pyridine nitrogen and pyrrole nitrogen deposits provide defects that can serve as lithium-ion adsorption sites. Therefore, this rice-grain-like composite material exhibits excellent electrochemical performance in the tests, significantly improving the charge-discharge capacitance and efficiency of the secondary battery.

[0174] In Comparative Example 1, the lack of conductive agent N-doped MXene support resulted in reduced ionic and electronic conductivity. Furthermore, the absence of pyrrole N and pyridine N to provide more active sites led to poorer electrical performance. In Comparative Example 2, the lack of a complexing agent prevented the formation of a rice-grain-like morphology, resulting in larger primary particle size and deteriorated performance. Comparative Example 3, lacking metal element doping, lacked the grain-refining effect of metal ions; the b-axis and cell volume remained unchanged, leading to increased compaction and worse electrical performance. The slightly higher 0.1C charging capacity of Comparative Examples 1-3 may be due to the low dopant content in the composite cathode material. Under equal doping conditions, some ions can enter the crystal lattice while others cannot. With low doping, the lithium iron phosphate inversion phenomenon is reduced, and no doping element replaces lithium or iron sites, resulting in more lithium ions being extracted during the first charge, thus leading to a higher initial charge. Additionally, errors in the testing system may also contribute to the difference in initial charge capacity.

[0175] 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 cathode material, characterized in that, The composite positive electrode material comprises an N-doped MXene carrier and a positive electrode active material in-situ grown on the N-doped MXene carrier, the positive electrode active material comprises metal element-doped lithium iron phosphate, and the positive electrode active material has a rice grain-like morphology structure.

2. The composite cathode material of claim 1, wherein, In the positive electrode active material with the rice grain-like morphology structure, a short axis in a radial direction and a long axis in a length direction are included, and at least in an extension direction of the long axis, a tip end is formed; And / or, in the positive electrode active material with the rice grain-like morphology structure, the length of the short axis is less than 50 nm, and the length of the long axis is 100 nm to 300 nm; And / or, in the composite positive electrode material, the mass percentage content of the N-doped MXene carrier is 0.5% to 5%; And / or, the outer surface of the positive electrode active material has a carbon coating layer.

3. The composite cathode material of claim 2, wherein, In the metal element-doped lithium iron phosphate, the doped metal element comprises at least one of Ti, V, Mo, Nb, Zr, Mn and Mg; And / or, in the metal element-doped lithium iron phosphate, the mass percentage content of the doped metal element is 7000 ppm to 11000 ppm; And / or, the thickness of the carbon coating layer is 1 nm to 5 nm; And / or, in the positive electrode active material, the carbon content is 1.2% to 1.5%.

4. The composite cathode material according to any one of claims 1 to 3, characterized in that, The thickness of the N-doped MXene carrier is 20 nm to 50 nm; And / or, the flake diameter width of the N-doped MXene carrier is 1 μm to 5 μm; And / or, the number of layers of the N-doped MXene carrier is 1 to 3 layers; And / or, in the N-doped MXene carrier, the N element exists in at least one of the forms of N atoms, N-metal bonds and C-N bonds; And / or, in the N-doped MXene carrier, the mass percentage of the N element is 5% to 25%; and / or, in the N-doped MXene support, the MXene comprises at least one of Ti3C2T x , Ti2CT x , Nb2CT x , V2CT x , Mo3C2T x , Nb4C3T x , wherein T x is selected from at least one of -O, -OH, or -F.

5. A method for preparing a composite cathode material, characterized by, The method comprises the following steps: An N-doped MXene carrier is obtained; A phosphorus source, an iron source and a lithium source are dissolved in an organic solvent, an antioxidant and the N-doped MXene carrier are added, and a solvothermal reaction is performed to obtain a first precursor composite material; The first precursor composite material is mixed with a doped metal source, a complexing agent and a dispersant, and then subjected to a grinding treatment, and a nanoscale second precursor composite material is obtained after drying; In an inert atmosphere, the second precursor composite material is subjected to a calcination treatment to obtain a composite positive electrode material with a rice grain-like morphology.

6. The method of claim 5, wherein the lithium transition metal composite cathode material is prepared by the steps of: mixing the lithium transition metal composite cathode material with a solvent to form a slurry; and coating the slurry on a current collector. The solvothermal reaction step comprises: performing a reaction in a reaction kettle at a temperature of 100°C to 150°C for 6 h to 12 h; And / or, the grinding treatment step comprises: using a sand mill for sand milling treatment until the particle size D50 of the particles is less than 200 nm; And / or, the drying adopts centrifugal spray drying; And / or, the calcination treatment is performed at a temperature of 600°C to 720°C for 2 h to 6 h; And / or, the inert atmosphere comprises at least one of nitrogen and argon.

7. The method for preparing a composite cathode material according to claim 5 or 6, wherein the lithium transition metal oxide is LiMn2O4. The organic solvent comprises at least one of ethanol and ethylene glycol; And / or, the antioxidant comprises at least one of ascorbic acid, L-tartaric acid, glutathione, hydroxyanisole, dibutylhydroxytoluene and tert-butyl hydroquinone. And / or, the complexing agent comprises at least one of dicitrate, ethylenediaminetetraacetic acid, amine triacetic acid, gluconic acid, oxalic acid, diethylene triamine pentaacetic acid; And / or, the doping metal source comprises at least one of Ti source, V source, Mo source, Nb source, Zr source, Mn source, Mg source; And / or, the dispersing agent comprises at least one of polyvinylpyrrolidone, polyethylene glycol ether, Tween, Span, fatty acid polyoxyethylene ether.

8. The method for preparing the composite cathode material as described in claim 7, characterized in that, The N-doped MXene carrier is added in an amount of 0.5% to 5% based on the total mass of the phosphorus source, the iron source, the lithium source and the N-doped MXene carrier; And / or, the antioxidant is added in an amount of 5% to 10% based on the total mass of the phosphorus source, the iron source, the lithium source and the N-doped MXene carrier; And / or, the complexing agent is added in an amount of 5% to 15% based on the first precursor composite material; And / or, the doping metal source is added in an amount of 8000ppm to 12000ppm based on the first precursor composite material; And / or, the dispersing agent is added in an amount of 2% to 20% based on the first precursor composite material.

9. The method of claim 5, 6 or 8, wherein the composite cathode material is prepared by the steps of: mixing a lithium transition metal oxide and a lithium metal oxide to form a mixture; and heating the mixture to form the composite cathode material. The preparation of the N-doped MXene carrier comprises the following steps: Etching the MAX ceramic phase using HF to obtain multi-layer MXenen; Using dimethyl sulfoxide or tetrabutylammonium hydroxide to exfoliate the multi-layer MXene to obtain few-layer MXene; Dissolving the few-layer MXene and N source in dilute hydrochloric acid, drying by suction filtration, and then performing annealing treatment at a temperature of 400°C to 600°C in an inert atmosphere for 2 to 5 hours to obtain the N-doped MXene carrier; And / or, the thickness of the N-doped MXene carrier is 20nm to 50nm, and the flake diameter width is 1μm to 5μm; And / or, the number of layers of the N-doped MXene carrier is 1 to 3 layers; And / or, the mass fraction of N element in the N-doped MXene carrier is 5% to 25%.

10. A positive electrode sheet characterized by comprising: The positive electrode sheet comprises a current collector and a positive electrode active layer formed on at least one surface of the current collector, and the positive electrode active layer comprises the composite positive electrode material according to any one of claims 1 to 4 and / or the composite positive electrode material prepared by the method according to any one of claims 5 to 9.

11. A secondary battery characterized by comprising: The secondary battery comprises the positive electrode sheet according to claim 10.

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