Preparation method of F-doped carbon-coated MOF-derived lithium iron phosphate material

Abstract

The invention provides a preparation method of an F-doped carbon-coated MOF-derived lithium iron phosphate material. The preparation method comprises the following steps: uniformly mixing and dispersing a trivalent iron salt, a fluorine-containing ligand, a surfactant and a solvent, and carrying out solvothermal reaction to obtain fluorine-containing Fe-MOF; the preparation method comprises the following steps: uniformly mixing fluorine-containing Fe-MOF with a phosphorus source and a lithium source, grinding, and carrying out solid-phase sintering to obtain the F-doped carbon-coated MOF-derived lithium iron phosphate material (LFP (at) FC). The F-doped functional porous carbon has relatively high adsorption capacity, meanwhile, the Li < + > diffusion barrier is reduced, additional active sites are further endowed, the electronic conductivity of the material is improved, and a diffusion channel is accelerated. The LFP (at) FC positive electrode shows excellent rate capability and long cycle stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion battery material preparation, and in particular relates to a lithium iron phosphate positive electrode material and a preparation method thereof. Background Art

[0002] With the growing demand for efficient energy storage devices, lithium-ion batteries (LIBs), owing to their high energy density and long cycle life, have become one of the most sought-after energy storage systems. Cathode material technology is a key prerequisite for the development of high-performance LIBs. Among currently commercialized LIB cathode materials, olivine-type LFP has attracted considerable attention due to its low cost, excellent environmental compatibility, outstanding safety performance, and thermal stability. However, LFP's inherent low electronic conductivity and slow lithium ion diffusion kinetics severely limit its rate performance. Common modification strategies include particle nano-sizing, metal cation doping, and composites with conductive carbon. However, the first two methods sacrifice material tap density and require complex processes, making scale-up difficult. Therefore, constructing carbon-based LFP composites is the most effective approach to overcome these challenges. Metal-organic frameworks (MOFs) are crystalline porous materials formed by the coordination of organic ligands and metal nodes. They possess properties such as ultrahigh porosity, large surface area, abundant active sites, and tunable functionality. In recent years, MOFs, as periodically porous precursors, have been used to synthesize MOF-derived carbon-based cathode materials in situ. This strategy has attracted extensive research attention due to its unique structural advantages. For example, calcining MOF precursors in an inert atmosphere results in carbon materials that retain their original morphology. The resulting porous carbon matrix acts as an electronically conductive network, promoting the uniform dispersion of electroactive components (such as LiFePO₄) and optimizing lithium-ion diffusion kinetics. Furthermore, by selecting specific MOF precursors (e.g., ligands containing elements such as O, S, B, F, and Cl), heteroatom doping can be precisely introduced into the carbon matrix. Such doping not only increases the material's defect density and active sites but also significantly enhances the carbon matrix's intrinsic electronic conductivity, thereby enhancing the electrochemical performance of the cathode material. However, despite these advances, the practical application of MOF-derived carbon-based cathode materials remains in its infancy, primarily due to the lack of efficient synthesis strategies and the difficulty in precisely controlling their structure and properties. Summary of the Invention

[0003] In view of the problems existing in the prior art, the purpose of the present invention is to provide a method for preparing lithium iron phosphate materials. The MOF precursor integrates carbon source and iron source in one step, which simplifies the process and is suitable for large-scale production. + The adsorption and diffusion of ions are enhanced, thereby providing abundant active sites, improving electronic conductivity, and accelerating diffusion channels. This improves specific capacity, rate capacity, and cycle performance.

[0004] The present invention achieves the above-mentioned purpose through the following technical solutions: A method for preparing a F-doped carbon-coated MOF-derived lithium iron phosphate material comprises the following steps: Step (1): after uniformly dispersing the trivalent iron salt, the fluorine-containing ligand, the surfactant, and the solvent, a fluorine-containing iron-based metal organic framework material (fluorine-containing Fe-MOF) is obtained by solvent thermal reaction; Step (2): uniformly mixing the fluorine-containing Fe-MOF material in step 1 with a phosphorus source, a lithium source, and a doping source, grinding the material, solid-phase sintering the material, and cooling the material to obtain a F-doped carbon-coated MOF-derived lithium iron phosphate material; In step (1), optionally, the trivalent iron salt includes one of FeCl3·6H2O, Fe(NO3)3·9H2O, and Fe2(SO4)3, preferably FeCl3·6H2O; the fluorine-containing ligand includes one of tetrafluoroterephthalic acid (H2TFBDC) and pentafluoropyridine carboxylic acid (H2PFPA), preferably H2TFBDC; the surfactant includes one of polyvinylpyrrolidone (PVP), triblock copolymer (F127), polyethylene glycol (PEG-4000, PEG-6000), and sorbitan monooleate polyoxyethylene ether (Tween 80), preferably PVP; and the solvent includes one of DMF, ethanol, and a DMF / ethanol mixed solvent.

[0005] In step (1), optionally, the solvent thermal reaction is carried out in a high-pressure reactor at 120-160° C. for 6-24 h.

[0006] In step (1), optionally, the molar ratio of the trivalent iron salt to the fluorine-containing ligand is 1:1 to 1:3.

[0007] In step (2), optionally, the phosphorus source includes one of ammonium dihydrogen phosphate (NH4H2PO4) and diammonium hydrogen phosphate ((NH4)2HPO4), preferably NH4H2PO4; the lithium source includes one of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium acetate, preferably lithium carbonate.

[0008] In step (2), optionally, the solid phase sintering is to sinter the ground precursor in a tube furnace under an inert nitrogen atmosphere at 1-5°C min -1 The sample was annealed at a heating rate of 650-800 °C for 6-15 h and then naturally cooled to room temperature.

[0009] In step (2), optionally, the molar ratio of Li, Fe, P, and F in the lithium source, fluorine-containing Fe-MOF, and phosphorus source is 1~1.5:1:1~1.5:0.01~0.1.

[0010] The present invention also provides a F-doped carbon-coated MOF-derived lithium iron phosphate material, which is prepared using the preparation method of the F-doped carbon-coated MOF-derived lithium iron phosphate material.

[0011] Beneficial effects of the present invention: The three-dimensional ordered pores of the fluorinated Fe-MOF precursor are transformed into a porous skeleton of LiFePO4 after annealing, increasing the specific surface area and shortening the lithium ion diffusion path. The MOF-derived nanoscale LiFePO4 particles are evenly dispersed, inhibiting particle agglomeration. The MOF organic ligands are carbonized into an amorphous carbon layer, providing a continuous conductive network that coats the LiFePO4 particles and improves the electronic conductivity of the material. - Doping substituted part O 2- , causing the lattice to shrink slightly, the lithium ion diffusion coefficient is improved, and high-rate charge and discharge and long cycle life are achieved.

[0012] The first discharge capacity of the button cell assembled with LFP@FC-1 at 0.1C reached 169 mAh g -1 Above, after 500 cycles at 1C, it still has 160mAh·g -1 The above discharge capacity.

[0013] In addition, the fluorine-containing Fe-MOF serves as an iron source, a carbon source, and a fluorine source at the same time, which reduces the cost of raw materials and is suitable for large-scale production. It can be used in power batteries and large-scale energy storage systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 These are the constant current charge and discharge curves of LFP, LFP@C, LFP@FC, and LFP@FC-1 in the present invention at 0.1C.

[0015] Figure 2 These are the long cycle curves of LFP, LFP@C, LFP@FC, and LFP@FC-1 in the present invention at 1C.

[0016] Figure 3 These are the rate curves of LFP, LFP@C, LFP@FC, and LFP@FC-1 in the present invention at 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 10C, and 15C. DETAILED DESCRIPTION

[0017] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to form one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed in the present invention.

[0018] In order to better understand the present invention, the content of the present invention is further explained below in conjunction with the embodiments and related drawings, but the content of the present invention is not limited to the following embodiments.

[0019] Example 1 A method for preparing a F-doped carbon-coated MOF-derived lithium iron phosphate material comprises the following steps: Step 1: A fluorinated Fe-MOF polyhedron precursor was synthesized using a solvothermal method. 1.2 g of tetrafluoroterephthalic acid (H2TFBDC), 1.2 g of FeCl3·6H2O, and 0.4 g of polyvinylpyrrolidone (PVP, K30, Mw = 40,000) were added to 50 mL of N,N-dimethylformamide (DMF) and ultrasonically dispersed. The mixture was then transferred to a 100 mL polytetrafluoroethylene-lined stainless steel autoclave and reacted at 145°C for 12 h. After cooling to room temperature, the resulting precipitate was purified by centrifugation with ethanol and DMF several times and dried in a vacuum oven at 60°C overnight to obtain the fluorinated Fe-MOF precursor.

[0020] Step 2: LFP@FC was synthesized by solid phase sintering method. 0.81g ammonium dihydrogen phosphate (NH4H2PO4), 0.26g lithium carbonate (Li2CO3) and 1.2g fluorinated Fe-MOF were uniformly mixed in a mortar for 30min. Then, the ground precursor was heated in a tube furnace under an inert nitrogen atmosphere at 2℃ min -1 The LFP@FC was obtained by annealing at 700℃ for 10 h and then naturally cooling to room temperature.

[0021] Example 2 A method for preparing a F-doped carbon-coated MOF-derived lithium iron phosphate material comprises the following steps: Step 1: A Fe-MOF polyhedral precursor was synthesized using a solvothermal method. 0.8 g of terephthalic acid (H2BDC), 1.2 g of FeCl3·6H2O, and 0.4 g of polyvinylpyrrolidone (PVP, K30, Mw = 40,000) were added to 50 mL of N,N-dimethylformamide (DMF) and ultrasonically dispersed. The mixture was then transferred to a 100 mL polytetrafluoroethylene-lined stainless steel autoclave and reacted at 145°C for 12 h. After cooling to room temperature, the resulting precipitate was purified by centrifugation with ethanol and DMF several times and dried in a vacuum oven at 60°C overnight to obtain the Fe-MOF precursor.

[0022] Step 2: LFP@FC was synthesized by solid phase sintering method. 0.81g ammonium dihydrogen phosphate (NH4H2PO4), 0.26g lithium carbonate (Li2CO3) and 1.15g Fe-MOF were uniformly mixed in a mortar for 30min. Then, the ground precursor was heated in a tube furnace under an inert nitrogen atmosphere at 2℃ min -1 The LFP@C was obtained by annealing at 700 °C for 10 h and then cooling to room temperature naturally.

[0023] Example 3 Using a precursor preparation method similar to that used for LFP@FC, the fluorine-containing Fe-MOF was annealed in air to produce carbon-free iron oxide. Subsequently, ammonium dihydrogen phosphate, lithium carbonate, and ferric oxide were uniformly mixed in a stoichiometric ratio and annealed at 700°C for 10 h under a nitrogen atmosphere to prepare LFP.

[0024] Example 4 LFP@FC-1 was obtained by using a precursor preparation method similar to that of LFP@FC, except that tetrafluoroterephthalic acid was replaced by pentafluoropyridinecarboxylic acid.

[0025] To verify the electrochemical performance of the lithium iron phosphate cathode materials obtained in Examples 1-3, the materials were mixed in N-methylpyrrolidone at a mass ratio of LiFePO4:Super P:PVDF = 8:1:1 to form a slurry. The slurry was coated onto aluminum foil and vacuum-dried at 100°C for 12 hours. The slurry was then punched to obtain circular electrode sheets. These circular cathode sheets were then assembled with a separator, electrolyte, and lithium metal sheet into a button cell. Constant current charge-discharge cycling tests were conducted within the 2.0-3.75V voltage window at room temperature.

[0026] Test results: like Figure 1 As shown in Figure 2, the first discharge specific capacities of LFP, LFP@C, LFP@FC, and LFP@FC-1 at 0.1C are 129.61 mAh g -1 、151.79 mAh g -1 、169.26 mAh g -1 、169.69 mAh g -1 .like Figure 2 As shown in the figure, the button cells assembled with LFP, LFP@C, LFP@FC, and LFP@FC-1 at 1C still have a capacity of 100.09 mAh g after 500 cycles. -1 、140.50mAh g -1 、159.89 mAh g -1 、163.94 mAh g -1The discharge specific capacity and capacity retention rates are 77.22%, 92.56%, 94.46% and 96.61% respectively. Figure 3 As shown in the figure, the rate performance of each embodiment was tested at 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 10C, and 15C. The discharge specific capacity of the button cells assembled with LFP, LFP@C, LFP@FC, and LFP@FC-1 at a rate of 15C was 22.6 mAh g -1 , 59 mAh g -1 , 82 mAh g -1 、85.6 mAh g -1 .

[0027] The experimental results show that the fluorine-containing Fe-MOF precursor provides a porous skeleton and a continuous conductive network, and the F-doped porous carbon matrix provides additional active sites, which improves the electronic conductivity and accelerates the Li + diffusion pathway, which is conducive to excellent rate capability and stable cycling performance.

[0028] The above embodiments are merely examples for clear description and are not intended to limit the implementation manner.

Claims

1. A method for preparing a F-doped carbon-coated MOF-derived lithium iron phosphate material, characterized in that: The following steps are involved: Step (1): After the trivalent iron salt, the fluorine-containing ligand, the surfactant and the solvent are uniformly dispersed, a fluorine-containing iron-based metal organic framework material, i.e., the fluorine-containing Fe-MOF, is obtained by solvent thermal reaction; Step (2): The fluorine-containing Fe-MOF material in step (1) is uniformly mixed with a phosphorus source and a lithium source, ground, solid-phase sintered, and cooled to obtain a F-doped carbon-coated MOF-derived lithium iron phosphate material.

2. The method for preparing a F-doped carbon-coated MOF-derived lithium iron phosphate material according to claim 1, characterized in that: The trivalent iron salt in step (1) includes one of FeCl3·6H2O, Fe(NO3)3·9H2O, and Fe2(SO4)3.

3. The method for preparing a F-doped carbon-coated MOF-derived lithium iron phosphate material according to claim 1, characterized in that: The fluorine-containing ligand is one of tetrafluoroterephthalic acid H2TFBDC and pentafluoropyridine carboxylic acid H2PFPA.

4. The method for preparing a F-doped carbon-coated MOF-derived lithium iron phosphate material according to claim 1, characterized in that: The surfactant includes one of polyvinyl pyrrolidone, triblock copolymer, polyethylene glycol, and sorbitan monooleate polyoxyethylene ether.

5. The method for preparing a F-doped carbon-coated MOF-derived lithium iron phosphate material according to claim 1, characterized in that: The solvent includes one of DMF or ethanol or a combination of the two.

6. The method for preparing a F-doped carbon-coated MOF-derived lithium iron phosphate material according to claim 1, characterized in that: The solvent thermal reaction in step (1) is carried out in a high pressure reactor at 120-160° C. for 6-24 h.

7. The method for preparing a F-doped carbon-coated MOF-derived lithium iron phosphate material according to claim 1, characterized in that: In step (1), the molar ratio of the trivalent iron salt to the fluorine-containing ligand is 1:1-3.

8. The method for preparing a F-doped carbon-coated MOF-derived lithium iron phosphate material according to claim 1, characterized in that: The phosphorus source in step (2) includes one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate; the lithium source includes one of lithium carbonate, lithium hydroxide, lithium nitrate and lithium acetate.

9. The method for preparing a F-doped carbon-coated MOF-derived lithium iron phosphate material according to claim 1, characterized in that: The solid phase sintering in step (2) is to sinter the ground precursor in a tube furnace under an inert nitrogen atmosphere at 1-5°C min -1 The sample was annealed at a heating rate of 650-800 °C for 6-15 h and then naturally cooled to room temperature.

10. The method for preparing a F-doped carbon-coated MOF-derived lithium iron phosphate material according to claim 1, characterized in that: In step (2), the molar ratio of Li, Fe, P, and F in the lithium source, fluorine-containing Fe-MOF, and phosphorus source is 1-1.5:1:1-1.5:0.01-0.1.

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