Multi-stage coated lithium manganese iron phosphate positive electrode material as well as preparation method and application thereof

Abstract

The invention provides a multi-stage coated lithium manganese iron phosphate positive electrode material as well as a preparation method and application thereof. The lithium manganese iron phosphate positive electrode material comprises a LiMn < 1-x > FexPO4 layer, a nitrogen-doped carbon layer wrapping the surface of the LiMn < 1-x > FexPO4 layer, a three-dimensional carbon nanotube / graphene conductive network wrapping the surface of the nitrogen-doped carbon layer, and an Al2O3 layer wrapping the surface of the three-dimensional carbon nanotube / graphene conductive network. In the lithium manganese iron phosphate positive electrode material provided by the invention, the nitrogen-doped carbon layer can improve the overall conductivity of the material; the three-dimensional carbon nanotube / graphene conductive network of the middle coating layer can further construct an efficient electron transmission channel, and the conductivity is enhanced; and the Al2O3 layer of the outer coating layer specifically inhibits dissolution of manganese, so that the problem of poor high-temperature cycling stability is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion battery cathode material, and particularly relates to a multi-level coated lithium manganese iron phosphate cathode material and a preparation method and application thereof. BACKGROUND

[0002] Lithium manganese iron phosphate (LMFP) has high voltage platform (4.1V) and low cost advantage, but its application is limited by manganese dissolution, low electronic / ion conductivity and poor high-temperature cycle stability. In the prior art, the performance is mainly improved by doping, carbon coating or structure optimization, for example: Defang Nano adjusts the material morphology by polymer nanospheres to improve the compaction density and conductivity; Hunan Yunneng uses phenolic nanocellulose and SiO2 nanotube composite membrane to inhibit manganese dissolution; Xinyangfeng realizes particle grading by batch sanding slurry to optimize the electrical performance. However, the existing methods still have the following problems: high raw material cost, not fully utilizing industrial by-products; single coating layer is difficult to solve manganese dissolution and conductivity defects at the same time; high-temperature sintering process has high equipment requirement and large energy consumption.

[0003] Therefore, there is an urgent need for a multi-level coated lithium manganese iron phosphate cathode material and a preparation method and application thereof to solve the above technical problems. SUMMARY

[0004] The present application aims to provide a multi-level coated lithium manganese iron phosphate cathode material and a preparation method and application thereof, which are used to solve the technical problems of high manufacturing cost, poor conductivity and poor high-temperature cycle stability of the existing lithium manganese iron phosphate cathode material.

[0005] To solve the above technical problems, the present application provides a multi-level coated lithium manganese iron phosphate cathode material, which has a core-shell structure, and the core-shell structure comprises a core, an inner coating layer wrapped on the surface of the core, an intermediate coating layer wrapped on the surface of the inner coating layer and an outer coating layer wrapped on the surface of the intermediate coating layer. The core comprises a LiMn 1-x Fe x PO4 layer, the inner coating layer comprises a nitrogen-doped carbon layer, the intermediate coating layer comprises a three-dimensional carbon nanotube / graphene conductive network, and the outer coating layer comprises an Al2O3 layer; 0

[0006] Correspondingly, the present application also provides a preparation method of the above multi-level coated lithium manganese iron phosphate cathode material, which comprises the following steps: S10, mixing an iron source, a manganese source and a phosphorus source, adding a precipitant to perform a co-precipitation reaction, and obtaining an iron-manganese-phosphorus precursor; S20, mixing the iron-manganese-phosphorus precursor and a polydopamine solution, and obtaining a polydopamine-coated iron-manganese-phosphorus precursor after a self-polymerization reaction; S30, ball-milling the polydopamine-coated iron-manganese-phosphorus precursor, carbon nanotube and graphene dispersion liquid to form a first conductive composite structure with double-layer coated iron-manganese-phosphorus precursor; S40, mixing the first conductive composite structure with a lithium source and then performing gradient sintering treatment to obtain a second conductive composite structure with double-layer coated manganese iron lithium phosphate; S50, performing crushing treatment on the second conductive composite structure, depositing an Al2O3 layer on the second conductive composite structure to obtain a manganese iron lithium phosphate cathode material.

[0007] Preferably, in the step S10: the iron source and the manganese source are selected from an iron-manganese-containing solution obtained in a titanium white production process, and the iron-manganese-containing solution comprises ferrous sulfate and manganese sulfate.

[0008] Preferably, in the step S10: the phosphorus source comprises phosphoric acid or a phosphate, and the precipitant comprises ammonia water, and the reaction pH value during the co-precipitation reaction is 2.5-3.5.

[0009] Preferably, in the step S20: the temperature for performing the self-polymerization reaction is 40-80℃, and the time is 4-8h; and the thickness of the polydopamine layer in the polydopamine-coated iron-manganese-phosphorus precursor is 10-30nm.

[0010] Preferably, in the step S30: the first conductive composite structure comprises the iron-manganese-phosphorus precursor, a polydopamine layer coated on the surface of the iron-manganese-phosphorus precursor, and a three-dimensional carbon nanotube / graphene conductive network wrapped on the surface of the polydopamine layer.

[0011] Preferably, the step S40 specifically comprises: S401, mixing the first conductive composite structure with a lithium source, heating to 350-450℃ under an inert atmosphere and maintaining for 1-3h to obtain a conductive network intermediate with a nitrogen-doped carbon layer; S402, heating the conductive network intermediate to 600-800℃ under an inert atmosphere and maintaining for 7-10h to obtain the second conductive composite structure.

[0012] Preferably, in the step S40: the second conductive composite structure comprises a LiMn 1-x Fe x PO4 layer, a nitrogen-doped carbon layer coated on the surface of the LiMn 1-x Fe x PO4 layer, and a three-dimensional carbon nanotube / graphene conductive network wrapped on the surface of the nitrogen-doped carbon layer.

[0013] Preferably, in the step S50: the D50 particle size of the second conductive composite structure after the crushing treatment is 1-2μm; and the thickness of the Al2O3 layer in the manganese iron lithium phosphate cathode material is 1-5nm.

[0014] Correspondingly, the application further provides application of the lithium manganese iron phosphate cathode material prepared by the preparation method of the lithium manganese iron phosphate cathode material or the multi-level coated lithium manganese iron phosphate cathode material according to any one of the above in preparation of a lithium battery.

[0015] The application has the following beneficial effects: different from the prior art, the application provides a multi-level coated lithium manganese iron phosphate cathode material, a preparation method and application thereof, the lithium manganese iron phosphate cathode material has a core-shell structure, the core-shell structure comprises a core, an inner coating layer wrapped on the surface of the core, an intermediate coating layer wrapped on the surface of the inner coating layer and an outer coating layer wrapped on the surface of the intermediate coating layer, wherein the core comprises a LiMn 1-x Fe x PO4 layer, the inner coating layer comprises a nitrogen-doped carbon layer, the intermediate coating layer comprises a three-dimensional carbon nanotube / graphene conductive network, and the outer coating layer comprises an Al2O3 layer. In the lithium manganese iron phosphate cathode material with the core-shell structure provided by the application, the core adopts a LiMn 1-x Fe x PO4 layer, the nitrogen-doped carbon layer of the inner coating layer can improve the overall conductivity of the material, the three-dimensional carbon nanotube / graphene conductive network of the intermediate coating layer can further construct an efficient electron transmission channel and enhance the conductivity, and the Al2O3 layer of the outer coating layer can specifically inhibit manganese dissolution and solve the problem of poor high-temperature cycle stability. Meanwhile, the preparation method breaks through the limitations of traditional coating technology in precision and effect, improves the electronic / ionic conductivity of the material, inhibits manganese dissolution, improves the high-temperature cycle stability, and provides a more optimal technical path for industrial production through the innovative process. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 A preparation method flowchart of the multi-level coated lithium manganese iron phosphate cathode material is provided for the embodiments of the application. Figure 2 An XRD graph of the multi-level coated lithium manganese iron phosphate cathode material is provided for the embodiment 1. Figure 3 An SEM graph of the lithium manganese iron phosphate material obtained in the embodiment 1 of the application. DETAILED DESCRIPTION

[0017] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the embodiments of the application. Obviously, the described embodiments are only a part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by a person of ordinary skill in the art without creative effort fall within the protection scope of the application.

[0018] The present application aims to overcome the above technical deficiencies, and provide a multi-level coated lithium manganese iron phosphate positive electrode material, a preparation method and application thereof, and particularly relates to a preparation method and application of a high-performance lithium manganese iron phosphate (LiMn 1-x Fe x PO4, LMFP) positive electrode material using titanium dioxide industrial by-products as raw materials and combining a multi-level coating technology.

[0019] The present application first provides a multi-level coated lithium manganese iron phosphate positive electrode material, which has a core-shell structure, and the core-shell structure comprises a core, an inner coating layer wrapped on the surface of the core, an intermediate coating layer wrapped on the surface of the inner coating layer, and an outer coating layer wrapped on the surface of the intermediate coating layer. The core comprises a LiMn 1-x Fe x PO4 layer, the inner coating layer comprises a nitrogen-doped carbon layer, the intermediate coating layer comprises a three-dimensional carbon nanotube / graphene conductive network, and the outer coating layer comprises an Al2O3 layer; and 0

[0020] The present application solves the problems of the electrical conductivity of lithium manganese iron phosphate, manganese dissolution and process cost by using a multi-level core-shell coating structure and a nanoscale modification process. The LiMn 1-x Fe x PO4 layer of the core lays a foundation for a high-voltage platform; the nitrogen-doped carbon layer of the inner coating optimizes the electron transmission path through nitrogen atom doping to improve the intrinsic conductivity; the three-dimensional carbon nanotube / graphene conductive network of the intermediate layer constructs a continuous conductive channel to further reduce the interface resistance; and the Al2O3 layer of the outer coating inhibits manganese ion dissolution through physical blocking and chemical stabilization to enhance the high-temperature cycle stability.

[0021] Referring to Figure 1 The present application also provides a preparation method of the above multi-level coated lithium manganese iron phosphate positive electrode material, which comprises the following steps: S10, mixing an iron source, a manganese source and a phosphorus source, adding a precipitant to perform a co-precipitation reaction to obtain an iron-manganese-phosphorus precursor.

[0022] Specifically, the step S10 further comprises: The titanium dioxide production by-products are used as the iron source and the manganese source to generate the iron-manganese-phosphorus precursor (Fe x Mn 1-x PO4·2H2O, 0

[0023] Specifically, the iron source and the manganese source are selected from an iron-manganese-containing solution obtained in a titanium dioxide production process, and the iron-manganese-containing solution comprises ferrous sulfate and manganese sulfate; the phosphorus source comprises phosphoric acid or a phosphate salt, and the precipitant comprises ammonia water.

[0024] Specifically, the present application uses titanium dioxide production by-product ferrous sulfate and manganese sulfate mixed solution as iron and manganese sources, and prepares iron-manganese-phosphorus precursor by ammonia co-precipitation method under the condition of pH value of 2.5-3.5, which not only realizes high-value utilization of industrial waste and greatly reduces raw material cost, but also precisely controls reaction conditions to avoid the introduction of impurities and ensures the purity of the precursor, thereby providing a high-quality basis for the subsequent synthesis of high-performance multi-level coated manganese iron lithium phosphate positive electrode material.

[0025] In one embodiment, the chemical formula of the iron-manganese-phosphorus precursor is Fe 0.5 Mn 0.5 PO4·2H2O.

[0026] In S20, the iron-manganese-phosphorus precursor is mixed with a polydopamine solution, and after self-polymerization reaction, a polydopamine-coated iron-manganese-phosphorus precursor is obtained.

[0027] Specifically, S20 further includes: The iron-manganese-phosphorus precursor is mixed with a polydopamine (PDA) solution, and a 10-30 nm PDA layer is formed on the surface of the particles through self-polymerization reaction, thereby obtaining a polydopamine-coated iron-manganese-phosphorus precursor (PDA@Fe x Mn 1-x PO4·2H2O), so as to enhance the ion diffusion rate; wherein the self-polymerization reaction is carried out at a temperature of 40-80°C for 4-8h.

[0028] Specifically, the process uses the high ion affinity and nanoscale thickness advantages of the PDA layer to significantly enhance the ion diffusion rate while not damaging the structure of the precursor through controllable self-polymerization reaction under mild conditions, thereby laying a foundation for improving the overall ion conductivity of the material in the subsequent multi-level coating structure; at the same time, the method is simple and easy to operate, and the conditions are mild, without the need for complex equipment, which is suitable for industrial production processes and effectively solves the problem of difficult performance improvement and process feasibility in traditional methods.

[0029] In S30, the polydopamine-coated iron-manganese-phosphorus precursor, carbon nanotube, and graphene dispersion liquid are subjected to ball milling composite treatment to form a first conductive composite structure of double-layer coated iron-manganese-phosphorus precursor.

[0030] Specifically, S30 further includes: The PDA-coated precursor is ball milled with carbon nanotubes (CNT) and graphene dispersion liquid to form a three-dimensional conductive network, and the ball milling speed is 600-1000 r / min (preferably 800 r / min), and the time is 3-5h (preferably 4h); the first conductive composite structure (three-dimensional carbon nanotube / graphene conductive network@PDA@Fe x Mn 1-xPO4·2H2O) includes an iron-manganese-phosphorus precursor, a polydopamine layer coated on the surface of the iron-manganese-phosphorus precursor, and a three-dimensional carbon nanotube / graphene conductive network wrapped on the surface of the polydopamine layer.

[0031] Specifically, this process achieves efficient compounding of carbon nanotubes and graphene outside the polydopamine layer by optimizing ball milling parameters, constructing a continuous and dense electron transmission channel, significantly improving the electronic conductivity of the material; the polydopamine layer serves as an intermediate transition layer, enhancing the interfacial bonding between the carbon material and the precursor, further reducing the interfacial resistance; at the same time, the ball milling composite process is controllable and low-cost, providing a feasible solution for the large-scale preparation of highly conductive lithium manganese iron phosphate positive electrode materials, effectively solving the problems of insufficient conductivity improvement and poor interface compatibility in traditional methods.

[0032] S40, mixing the first conductive composite structure with a lithium source and performing a gradient sintering process to obtain a second conductive composite structure coated with a double layer of lithium manganese iron phosphate.

[0033] Specifically, step S40 further includes: S401, mixing the first conductive composite structure with a lithium source in a molar ratio of 1: (1.01-1.1, preferably 1.05), heating the mixture to 350-450° C. (e.g., 400° C.) at a rate of 2° C. / min under an inert atmosphere (e.g., a nitrogen atmosphere), and maintaining the temperature for 1-3 hours (e.g., 2 hours) to obtain a conductive network intermediate having a nitrogen-doped carbon layer (three-dimensional carbon nanotube / graphene conductive network@nitrogen-doped carbon layer@Fe x Mn 1-x PO4·2H2O); S402, heating the conductive network intermediate to 600-800°C (e.g., 750°C) under an inert atmosphere (e.g., argon atmosphere) and keeping the temperature for 7-10 hours (e.g., 8 hours) to promote the LiMn x Fe 1-x PO4 (0<x<1) lattice is formed and the three-dimensional carbon nanotube / graphene conductive network is stabilized to obtain the second conductive composite structure (three-dimensional carbon nanotube / graphene conductive network@nitrogen-doped carbon layer@LiMn 1-x Fe x PO4).

[0034] Specifically, the second conductive composite structure includes LiMn 1-x Fe x PO4 layer, coated on LiMn 1-x Fe x A nitrogen-doped carbon layer on the surface of the PO4 layer and a three-dimensional carbon nanotube / graphene conductive network wrapped on the surface of the nitrogen-doped carbon layer.

[0035] Specifically, the step is achieved by a two-stage temperature control sintering process to realize the synergistic regulation of nitrogen-doped carbon layer construction and lattice optimization under an inert atmosphere. In the low-temperature section (350-450℃): after mixing the lithium source with the first conductive composite structure, the polydopamine (PDA) layer is pyrolyzed into a nitrogen-doped carbon layer through slow heating (2℃ / min) and heat treatment, and the nitrogen atom doping significantly improves the intrinsic conductivity of the carbon layer, while the excess lithium source compensates for the lithium evaporation during the sintering process, ensuring that the core LiMn 1-x Fe x PO4 stoichiometric ratio precision; In the high-temperature section (600-800℃): under an argon atmosphere, the olivine lattice is fully developed, and the three-dimensional carbon nanotube / graphene conductive network is stabilized simultaneously, avoiding the collapse or agglomeration of the carbon structure at high temperatures, forming a "nitrogen-doped carbon layer-three-dimensional carbon network" dual conductive channel, which greatly reduces the interface resistance. This process breaks through the energy consumption bottleneck of traditional single high-temperature sintering by controlling the temperature in stages, and can realize lattice completion and conductive network stabilization at 750℃, which is 50-100℃ lower than the traditional process. The synergistic effect of nitrogen-doped carbon and three-dimensional carbon network improves the electronic conductivity of the material by 2-3 orders of magnitude, effectively solving the problems of high energy consumption and poor stability of the conductive network in the prior art.

[0036] S50, after the second conductive composite structure is crushed, an Al2O3 layer is deposited on the second conductive composite structure to obtain a lithium manganese iron phosphate positive electrode material.

[0037] Specifically, the S50 step further comprises: The second conductive composite structure (sintered product) is air-flow crushed to D50=1-2μm, and an Al2O3 layer of 1-5nm (preferably 2nm) is deposited on the surface by atomic layer deposition (ALD), further inhibiting manganese dissolution.

[0038] Specifically, the present application precisely controls the particle size of the sintered product to D50=1-2μm by air-flow crushing, and then uniformly coats a 1-5nm Al2O3 layer on the surface using atomic layer deposition (ALD) technology. Not only does it increase the specific surface area and optimize the interface contact by refining the particle size, but it also forms a dense Al2O3 physical barrier with atomic-level deposition accuracy through the ALD technology, effectively blocking the corrosion of the electrolyte on the active material and inhibiting the dissolution of manganese ions. At the same time, the low-temperature process avoids damaging the internal three-dimensional conductive network, taking into account the improvement of high-temperature cycle stability and conductivity of the material, and solving the contradiction between manganese dissolution inhibition and conductivity maintenance in the prior art.

[0039] Correspondingly, the application further provides application of the lithium manganese iron phosphate cathode material prepared by the preparation method of the lithium manganese iron phosphate cathode material of the multi-stage coated lithium manganese iron phosphate cathode material according to any one of the above in preparation of a lithium battery.

[0040] The technical solutions of the application will be further described in detail with specific embodiments.

[0041] Embodiment 1: Embodiment 1 of the application first provides a multi-stage coated lithium manganese iron phosphate cathode material, the lithium manganese iron phosphate cathode material is a core-shell structure (Al2O3@three-dimensional carbon nanotube / graphene conductive network@nitrogen-doped carbon@LiMn 0.5 Fe 0.5 PO4), the core-shell structure comprises a core, an inner coating layer wrapped on the surface of the core, an intermediate coating layer wrapped on the surface of the inner coating layer and an outer coating layer wrapped on the surface of the intermediate coating layer; The core comprises a LiMn 0.5 Fe 0.5 PO4 layer, the inner coating layer comprises a nitrogen-doped carbon layer, the intermediate coating layer comprises a three-dimensional carbon nanotube / graphene conductive network, and the outer coating layer comprises an Al2O3 layer.

[0042] Embodiment 1 of the application further provides a preparation method of the multi-stage coated lithium manganese iron phosphate cathode material, and the specific steps comprise the following: Step one, raw material pretreatment: 1000g of a titanium white powder by-product (a mixed solution of ferrous sulfate and manganese sulfate, Fe:Mn molar ratio 1:1) is taken, ammonia water is added to adjust the pH to 3.0, and stirring reaction is carried out for 4h, and after centrifugation, washing and drying, an iron-manganese-phosphorus precursor Fe 0.5 Mn 0.5 PO4·2H2O is obtained.

[0043] Step two, inner layer coating: the iron-manganese-phosphorus precursor Fe 0.5 Mn 0.5 PO4·2H2O is mixed with a 0.5wt% polydopamine (PDA) solution, and self-polymerization reaction is carried out at 60℃ for 6h to form a polydopamine coated iron-manganese-phosphorus precursor (PDA@Fe 0.5 Mn 0.5 PO4·2H2O), wherein the PDA layer is about 15nm.

[0044] Step three, outer layer coating: the polydopamine coated iron-manganese-phosphorus precursor is ball milled with 1wt% carbon nanotubes (CNT) and 0.5wt% graphene dispersion liquid, the ball milling speed is 800r / min, and the time is 4h, to form a first conductive composite structure of double-layer coated iron-manganese-phosphorus precursor (three-dimensional carbon nanotube / graphene conductive network@PDA@Fe 0.5 Mn 0.5PO4·2H2O).

[0045] Step four, low-temperature sintering: the first conductive composite structure is mixed with Li2CO3 at a molar ratio of 1:1.05, and is heated to 400℃ at a rate of 2℃ / min under a nitrogen atmosphere, and is kept for 2h, to obtain a conductive network intermediate (three-dimensional carbon nanotube / graphene conductive network@nitrogen-doped carbon layer@Fe 0.5 Mn 0.5 PO4·2H2O).

[0046] Step five, high-temperature sintering: the conductive network intermediate is heated to 750℃ under an argon atmosphere and is kept for 8h, to promote LiMn 0.5 Fe 0.5 PO4 lattice formation, and to stabilize the three-dimensional carbon nanotube / graphene conductive network, to obtain a second conductive composite structure (three-dimensional carbon nanotube / graphene conductive network@nitrogen-doped carbon layer@LiMn 0.5 Fe 0.5 PO4).

[0047] Step six, post-processing: the second conductive composite structure is pulverized by airflow to D 50 =1.1μm, and a 2nm Al2O3 layer is deposited on the surface by atomic layer deposition (ALD), to obtain a multi-level coated lithium manganese iron phosphate positive electrode material (Al2O3@three-dimensional carbon nanotube / graphene conductive network@nitrogen-doped carbon@LiMn 0.5 Fe 0.5 PO4).

[0048] Please refer to Figure 2 , Figure 2 the XRD (X-ray diffraction) pattern of the multi-level coated lithium manganese iron phosphate positive electrode material provided in this embodiment 1; Figure 2 The XRD pattern of the multi-level coated lithium manganese iron phosphate positive electrode material provided in this embodiment 1 shows typical olivine-type LiMn 1-x Fe x PO4 characteristic diffraction peaks, and the main peak crystal face diffraction peak is sharp and has no impurity peaks, proving that the core material has high crystallinity, and that Fe / Mn elements are uniformly solid-solved to form a single solid-solution phase. The nitrogen-doped carbon layer and the Al2O3 coating layer are amorphous structures, and have no obvious characteristic peaks in the XRD pattern, but a slight broadening of the substrate peak can be observed, indicating the uniform existence of the amorphous coating layer.

[0049] Please refer to Figure 3 , Figure 3 the SEM (scanning electron microscope) pattern of the lithium manganese iron phosphate material obtained in this embodiment 1; Figure 3The SEM image confirms that the manganese iron phosphate lithium positive electrode material has uniform spherical particle morphology (D50=1-2 μm), a continuous three-dimensional carbon nanotube / graphene conductive network is successfully constructed on the surface, and an Al2O3 layer of 1-5 nm is uniformly coated by the ALD technology, and each coating layer is closely combined with the core LiMn 0.5 Fe 0.5 PO4 interface without delamination, directly verifying the controllability and integrity of the multi-level coating structure, and providing a microstructure basis for the high conductivity, manganese dissolution inhibition and high temperature cycle stability of the material.

[0050] Example 2: The embodiment 2 of the present application provides a multi-level coated manganese iron phosphate lithium positive electrode material (Al2O3@three-dimensional carbon nanotube / graphene conductive network@nitrogen-doped carbon@LiMn 0.5 Fe 0.5 PO4) and a preparation method thereof, the preparation method of the manganese iron phosphate lithium positive electrode material is substantially the same as that of the embodiment 1 of the present application, and the difference is only that: Step two, inner layer coating: mixing the iron-manganese-phosphorus precursor Fe 0.5 Mn 0.5 PO4·2H2O with 0.5wt% polydopamine (PDA) solution, and performing self-polymerization reaction at 60°C for 8h to form a polydopamine coated iron-manganese-phosphorus precursor (PDA@Fe 0.5 Mn 0.5 PO4·2H2O), wherein the PDA layer is about 20nm.

[0051] Example 3: The embodiment 3 of the present application provides a multi-level coated manganese iron phosphate lithium positive electrode material (Al2O3@three-dimensional carbon nanotube / graphene conductive network@nitrogen-doped carbon@LiMn 0.5 Fe 0.5 PO4) and a preparation method thereof, the preparation method of the manganese iron phosphate lithium positive electrode material is substantially the same as that of the embodiment 1 of the present application, and the difference is only that: Step three, outer layer coating: ball-milling the polydopamine coated iron-manganese-phosphorus precursor with 1.5wt% carbon nanotubes (CNT) and 0.5wt% graphene dispersion liquid (i.e. the mass ratio of CNT to graphene is 3:1), the ball-milling rotation speed is 800r / min, and the time is 4h, to form a first conductive composite structure of double-coated iron-manganese-phosphorus precursor (three-dimensional carbon nanotube / graphene conductive network@PDA@Fe 0.5 Mn 0.5 PO4·2H2O).

[0052] Example 4: Embodiment 4 of the present application provides a multi-level coated lithium manganese iron phosphate positive electrode material (Al2O3@ three-dimensional carbon nanotube / graphene conductive network@ nitrogen-doped carbon@ LiMn 0.5 Fe 0.5 PO4) and a preparation method thereof. The preparation method of the lithium manganese iron phosphate positive electrode material is substantially the same as that of Embodiment 1 of the present application, and the only difference is that: Step five, high-temperature sintering: the conductive network intermediate is heated to 700℃ under an argon atmosphere and kept for 8h, so as to promote the formation of LiMn 0.5 Fe 0.5 PO4 lattice and stabilize the three-dimensional carbon nanotube / graphene conductive network, and then a second conductive composite structure (three-dimensional carbon nanotube / graphene conductive network@ nitrogen-doped carbon layer@ LiMn 0.5 Fe 0.5 PO4) is obtained.

[0053] Embodiment 5: Embodiment 5 of the present application provides a multi-level coated lithium manganese iron phosphate positive electrode material (Al2O3@ three-dimensional carbon nanotube / graphene conductive network@ nitrogen-doped carbon@ LiMn 0.5 Fe 0.5 PO4) and a preparation method thereof. The preparation method of the lithium manganese iron phosphate positive electrode material is substantially the same as that of Embodiment 1 of the present application, and the only difference is that: Step six, post-treatment: the second conductive composite structure is pulverized by airflow to D 50 =1.1μm, an Al2O3 layer of 3nm is deposited on the surface by atomic layer deposition (ALD), and a multi-level coated lithium manganese iron phosphate positive electrode material (Al2O3@ three-dimensional carbon nanotube / graphene conductive network@ nitrogen-doped carbon@ LiMn 0.5 Fe 0.5 PO4) is obtained.

[0054] Comparative Example 1: Comparative Example 1 provides a single carbon-coated lithium manganese iron phosphate positive electrode material and a preparation method thereof. The specific steps of the above preparation method include the following: Step one, raw material pretreatment: 1000g of titanium white by-product (a mixed solution of ferrous sulfate and manganese sulfate, Fe:Mn molar ratio 1:1) is taken, ammonia water is added to adjust the pH to 3.0, and stirring reaction is carried out for 4h. After centrifugation, washing and drying, an iron-manganese phosphate precursor Fe 0.5 Mn 0.5 PO4·2H2O is obtained.

[0055] Step two, coating: the iron-manganese phosphate precursor Fe 0.5 Mn 0.5PO4·2H2O mixed with glucose carbon source, carbonized at 800℃ for 4h, to form single carbon-coated iron-manganese-phosphorus precursor (single C source @Fe 0.5 Mn 0.5 PO4·2H2O).

[0056] Step three, high-temperature sintering: the single carbon-coated iron-manganese-phosphorus precursor is heated to 750℃ under argon atmosphere and kept for 8h to promote LiMn 0.5 Fe 0.5 PO4 lattice formation, to obtain single carbon-coated lithium manganese iron phosphate positive electrode material (single C source @LiMn 0.5 Fe 0.5 PO4).

[0057] Comparative Example 2: Comparative Example 2 provides a multi-stage coated lithium manganese iron phosphate positive electrode material (Al2O3@three-dimensional carbon nanotube / graphene conductive network@nitrogen-doped carbon@LiMn 0.5 Fe 0.5 PO4) and a preparation method thereof, the preparation method of the lithium manganese iron phosphate positive electrode material thereof is substantially the same as the preparation method of Example 1 of the present application, and the only difference is that: Step one, raw material pretreatment: a mixed solution of 1000g of pure ferrous sulfate and manganese sulfate (without using titanium dioxide by-product, Fe:Mn molar ratio 1:1) is prepared, ammonia water is added to adjust the pH to 3.0, and stirred for 4h, and then centrifuged, washed and dried to obtain an iron-manganese-phosphorus precursor Fe 0.5 Mn 0.5 PO4·2H2O.

[0058] Specifically, the lithium manganese iron phosphate positive electrode materials prepared in Examples 1-5 and Comparative Examples 1-2 above are prepared into positive electrode sheets, which are combined with lithium metal sheets, polypropylene separators and lithium hexafluorophosphate electrolyte to prepare lithium ion batteries, and then the electrochemical performance of the lithium ion batteries is tested. The charge and discharge conditions for the test are as follows: first, the battery is charged at a constant current of 0.1C to a voltage of 4.5V at a temperature of 25℃, and then further charged at a constant voltage of 4.5V until the current is 0.05C, and then discharged at a constant current of 0.1C to a voltage of 2.5V, which is one charge and discharge cycle at a rate of 0.1C. Then, the second and third charge and discharge cycles are carried out at rates of 0.2C and 0.5C, respectively. After the battery is fully activated, the charge and discharge cycle is carried out at a rate of 1C for 200 times, and the discharge capacity after 200 cycles is detected to obtain the capacity retention rate.

[0059] Further, the electrochemical performance test results of the lithium ion batteries prepared in Examples 1-5 and Comparative Examples 1-2 of the present application are shown in Table 1 as follows: Table 1 Electrochemical performance test results of lithium ion batteries prepared in each group

[0060] Specifically, as can be seen from Table 1, the lithium ion batteries prepared from Examples 1 to 5 have better 1C discharge capacity and better 200-cycle capacity retention rate than the lithium ion battery prepared from Comparative Example 1, which indicates that the lithium ion battery prepared from the multi-stage coated lithium manganese iron phosphate cathode material provided by the application has more excellent electrochemical performance than the lithium ion battery prepared from the single carbon-coated lithium manganese iron phosphate cathode material.

[0061] Specifically, as can be seen from Table 1, the lithium ion batteries prepared from Examples 1 to 5 have better 1C discharge capacity and better 200-cycle capacity retention rate than the lithium ion battery prepared from Comparative Example 1, which indicates that the lithium ion battery prepared from the multi-stage coated lithium manganese iron phosphate cathode material provided by the application has more excellent electrochemical performance than the lithium ion battery prepared from the single carbon-coated lithium manganese iron phosphate cathode material.

[0062] The synergistic effect of the multi-stage coating technology, the gradient sintering process and the atomic layer deposition (ALD) surface modification in the preparation of the lithium manganese iron phosphate (LMFP) cathode material is verified by systematic experiments, and the influence mechanism of the key parameters on the material performance is revealed: the synergistic optimization of the multi-stage coating and the gradient sintering (Example 1) uses a 15nm polydopamine (PDA) inner layer coating combined with a CNT / graphene three-dimensional conductive network, which significantly improves the ion / electron transport efficiency; the gradient sintering process (400℃ nitrogen pre-carbonization→750℃ argon lattice reconstruction) synchronously optimizes the carbon layer crystallinity and lattice stability; the ALD deposition of a 2nm Al2O3 layer suppresses manganese dissolution, achieving a 1C discharge capacity of 150mAh / g, a 200-cycle capacity retention rate of 96%, and comprehensive performance reaching the industry-leading level.

[0063] The parameter adjustment for preparing the lithium manganese iron phosphate positive material in the technical solution has the following differentiated effects on performance: in Example 2, the 20 nm PDA layer enhances the ion diffusion capacity, and the high-temperature cycle retention rate at 45°C is increased to 97.5%, but the capacity is slightly attenuated (148 mAh / g), indicating that the high-temperature stability and energy density requirements need to be balanced according to the application scenario; in Example 3, the 3:1 CNT / graphene ratio increases the compaction density to 2.4 g / cm3, but the excess CNTs may hinder lithium ion diffusion (capacity retention rate 95.8%), and the synergistic mechanism of the conductive network and ion transmission needs to be further explored. In Example 4, the temperature in the high-temperature sintering step is reduced to 700°C, which can reduce energy consumption by 10%, but the lattice integrity is limited, resulting in a decrease in capacity (142 mAh / g), which is suitable for cost-sensitive large-scale production scenarios. In Example 5, the 3 nm Al2O3 layer increases the cycle retention rate to 98% (25°C), verifying the inhibitory effect of surface passivation on manganese dissolution, but the thickness of the Al2O3 layer needs to be controlled to avoid increased lithium ion migration resistance.

[0064] Specifically, single carbon coating (when only glucose carbon source is used to prepare single carbon coated lithium manganese iron phosphate positive material in Comparative Example 1, the capacity retention rate drops to 85%, proving that multi-level coating (PDA+CNT / graphene+Al2O3) is a necessary technical path to solve the contradiction between conductivity and manganese dissolution.

[0065] In Comparative Example 2, the lithium battery performance using traditional pure ferrous sulfate / manganese raw materials is close to that of Example 1 (capacity retention rate 95%), but the raw material cost increases by 35%, highlighting the economic and environmental advantages of titanium dioxide by-product (reducing waste disposal pressure by more than 30%).

[0066] Compared with the prior art, the multi-level coated lithium manganese iron phosphate positive material and the preparation method thereof provided by the present application have the following advantages: I. Raw material innovation: resource utilization of titanium dioxide by-product realizes the dual goals of "cost reduction" and "green manufacturing"; II. Structural design innovation: multi-level coating (ion / electron synergistic transmission) + gradient sintering (lattice / carbon layer synergistic stabilization) + ALD (surface passivation) form a three-in-one technical solution; III. Industrialization potential: process parameters (such as ALD thickness, sintering temperature) can be flexibly adapted to the differentiated needs of power batteries (high energy density / long cycle life) and energy storage systems (low cost / high environmental friendliness).

[0067] To sum up, different from the prior art, the application provides a multi-level coated lithium manganese iron phosphate positive electrode material and a preparation method and application thereof. 1-x Fe x PO4layer, the inner coating layer comprises a nitrogen-doped carbon layer, the intermediate coating layer comprises a three-dimensional carbon nanotube / graphene conductive network, and the outer coating layer comprises an Al2O3layer. 1-x Fe x PO4layer, the inner coating layer comprises a nitrogen-doped carbon layer, the intermediate coating layer comprises a three-dimensional carbon nanotube / graphene conductive network, and the outer coating layer comprises an Al2O3layer. 1-x Fe x PO4layer, the inner coating layer comprises a nitrogen-doped carbon layer, the intermediate coating layer comprises a three-dimensional carbon nanotube / graphene conductive network, and the outer coating layer comprises an Al2O3layer. The preparation method breaks through the limitations of traditional coating technology in precision and effect, improves the electronic / ion conductivity of the material, suppresses manganese dissolution, improves the high-temperature cycle stability, and provides a more optimal technical path for industrial production through the innovative process.

[0068] It should be noted that the above embodiments all belong to the same inventive concept, and the description of each embodiment has its own emphasis. If the description is not exhaustive in individual embodiments, the description in other embodiments can be referred to.

[0069] The above embodiments only express the implementation of the application, and the description is more specific and detailed, but it should not be understood as limiting the scope of the patent. It should be noted that for ordinary skilled persons in the art, without departing from the inventive concept, a number of modifications and improvements can be made, which are all within the protection scope of the application. Therefore, the protection scope of the patent of the application should be subject to the appended claims.

Claims

1. A multi-stage coated lithium manganese iron phosphate positive electrode material, characterized in that: The lithium manganese iron phosphate positive electrode material is a core-shell structure, which includes a core, an inner coating layer wrapped around the surface of the core, an intermediate coating layer wrapped around the surface of the inner coating layer, and an outer coating layer wrapped around the surface of the intermediate coating layer; Wherein, the core comprises LiMn 1-x Fe x PO4 layer, the inner coating layer includes a nitrogen-doped carbon layer, the middle coating layer includes a three-dimensional carbon nanotube / graphene conductive network, and the outer coating layer includes an Al2O3 layer; 0<x<1.

2. A method for preparing the multi-stage coated lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The method comprises the following steps: S10, mixing an iron source, a manganese source, and a phosphorus source, and then adding a precipitant to perform a co-precipitation reaction to obtain an iron-manganese-phosphorus precursor; S20, mixing the iron-manganese-phosphorus precursor with a polydopamine solution to obtain a polydopamine-coated iron-manganese-phosphorus precursor through a self-polymerization reaction; S30, ball milling the polydopamine-coated iron-manganese-phosphorus precursor, carbon nanotubes, and graphene dispersion to form a first conductive composite structure with a double layer coating the iron-manganese-phosphorus precursor; S40, mixing the first conductive composite structure with a lithium source and performing a gradient sintering process to obtain a second conductive composite structure coated with a double-layer lithium manganese iron phosphate; S50, crushing the second conductive composite structure, and then depositing an Al2O3 layer on the second conductive composite structure to obtain the lithium manganese iron phosphate positive electrode material.

3. The method for preparing the multi-stage coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: In the step S10: the iron source and the manganese source are selected from an iron-manganese solution obtained in a titanium dioxide production process, and the iron-manganese solution includes ferrous sulfate and manganese sulfate.

4. The method for preparing a multi-stage coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: In the step S10, the phosphorus source includes phosphoric acid or phosphate, the precipitant includes aqueous ammonia, and the reaction pH value during the coprecipitation reaction is 2.5-3.

5.

5. The method for preparing the multi-stage coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: In the step S20, the temperature for the self-polymerization reaction is 40-80° C. and the time is 4-8 hours; the thickness of the polydopamine layer in the polydopamine-coated iron-manganese-phosphorus precursor is 10-30 nm.

6. The method for preparing the multi-stage coated lithium manganese iron phosphate positive electrode material according to claim 5, characterized in that: In the step S30: the first conductive composite structure includes the iron-manganese-phosphorus precursor, the polydopamine layer coated on the surface of the iron-manganese-phosphorus precursor, and the three-dimensional carbon nanotube / graphene conductive network wrapped on the surface of the polydopamine layer.

7. The method for preparing a multi-stage coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: The S40 step specifically includes: S401, mixing the first conductive composite structure with a lithium source, heating the mixture to 350-450° C. in an inert atmosphere, and maintaining the temperature for 1-3 hours to obtain a conductive network intermediate having the nitrogen-doped carbon layer; S402 , heating the conductive network intermediate to 600-800° C. in an inert atmosphere and maintaining the temperature for 7-10 hours to obtain the second conductive composite structure.

8. The method for preparing a multi-stage coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: In the step S40: the second conductive composite structure includes the LiMn 1-x Fe x PO4 layer, coated on the LiMn 1-x Fe x The nitrogen-doped carbon layer on the surface of the PO4 layer and the three-dimensional carbon nanotube / graphene conductive network wrapped on the surface of the nitrogen-doped carbon layer.

9. The method for preparing a multi-stage coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: In the step S50, the D50 particle size of the second conductive composite structure after the pulverization process is 1-2 μm; and the thickness of the Al2O3 layer in the lithium manganese iron phosphate positive electrode material is 1-5 nm.

10. Use of the lithium iron manganese phosphate positive electrode material according to claim 1 or the lithium iron manganese phosphate positive electrode material prepared by the preparation method of the multi-stage coated lithium iron manganese phosphate positive electrode material according to any one of claims 2 to 9 in the preparation of a lithium battery.

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