Cellulose nanofiber modified lithium iron phosphate / carbon composite positive electrode material and preparation method thereof

Abstract

The invention discloses a cellulose nanofiber modified lithium iron phosphate / carbon composite positive electrode material and a preparation method thereof. The preparation method comprises the following steps: step 1, preparing a metal-organic complex precursor from metal Fe ions and a nitrogen-rich base organic ligand through a one-pot method; 2, mixing the metal-organic complex precursor with a lithium source and a phosphorus source, grinding and refining, and then performing high-temperature carbonization treatment in inert gas to obtain a metal-organic complex derived carbon-coated lithium iron phosphate composite material; and 3, adding the metal-organic complex derived carbon-coated lithium iron phosphate composite material into a cellulose nanofiber suspension, uniformly mixing, carrying out suction filtration and freeze drying, and carrying out high-temperature carbonization treatment in inert gas to obtain the cellulose nanofiber modified lithium iron phosphate / carbon composite positive electrode material. According to the invention, the utilization efficiency of the cellulose nanofiber and the lithium iron phosphate is effectively improved, and good electrochemical performance is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material and a preparation method thereof. Background Art

[0002] Lithium-ion batteries have the advantages of high voltage, long cycle life, and environmental friendliness. They have been widely used in many fields such as automobiles, aerospace, and portable consumer electronic devices (such as mobile phones, laptops, etc.), playing an increasingly critical role in economic and social development and daily life.

[0003] As a key component of lithium-ion batteries, cathode materials play a decisive role in their overall performance. Therefore, structural design and performance optimization are key to improving battery performance. Olivine-type lithium iron phosphate (LIFP) has become one of the most widely used cathode materials in lithium-ion batteries due to its high theoretical specific capacity, stable voltage platform, excellent thermal stability, and environmental friendliness. However, its low intrinsic conductivity and slow lithium ion diffusion rate limit its further application.

[0004] There are many publicly available methods for preparing lithium iron phosphate, including high-temperature solid-phase methods, microwave heating, co-precipitation, hydrothermal methods, sol-gel methods, and spray drying. However, these methods are difficult to promote for industrial application due to their complex and rigorous processes, the introduction of impurities during the preparation process, the generation of large amounts of wastewater during post-treatment, and the failure to optimize the utilization of metal resources.

[0005] In summary, the existing lithium iron phosphate positive electrode material has the problem of low conductivity. Summary of the Invention

[0006] In order to overcome the problems existing in the above-mentioned prior art, the purpose of the present invention is to provide a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material and a preparation method, in which cellulose nanofibers with a high aspect ratio are compounded with lithium iron phosphate to form a composite positive electrode material having a three-dimensional carbon nanofiber conductive network and a carbon shell-coated lithium iron phosphate, so as to solve the problem of low conductivity of existing lithium iron phosphate positive electrode materials.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] A method for preparing a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material comprises the following steps:

[0009] Step 1: Metal Fe ions and nitrogen-rich base organic ligands are reacted in a one-pot process to prepare a metal-organic complex precursor;

[0010] Step 2: mixing the metal-organic complex precursor with a lithium source and a phosphorus source, grinding and refining the mixture, and then carbonizing the mixture at a high temperature in an inert gas to obtain a carbon-coated lithium iron phosphate composite material derived from the metal-organic complex;

[0011] Step 3: Add the carbon-coated lithium iron phosphate composite material derived from the metal-organic complex to the cellulose nanofiber suspension and mix evenly. After filtration and freeze-drying, the suspension is carbonized at high temperature in an inert gas to obtain a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material.

[0012] In step 1, the metal Fe ion is ferrous acetate, and the nitrogen-rich base organic ligand is an adenine molecule.

[0013] In step 1, the specific preparation process of the metal-organic complex precursor is as follows:

[0014] a) dissolving ferrous acetate in N,N-dimethylformamide (DMF) to form a uniform solution A for later use;

[0015] b) dissolving adenine molecules in DMF at 130-160°C to form a colorless, transparent solution B;

[0016] c) Pour solution A into solution B, mix and stir to react for 2 to 6 hours, and after the reaction is completed, wash and dry to obtain a metal-organic complex precursor.

[0017] In the step 1, the molar ratio of the metal Fe ion to the nitrogen-rich base organic ligand is 1:1-3.

[0018] In step 2, the lithium source is lithium hydroxide and the phosphorus source is ammonium dihydrogen phosphate;

[0019] In step 2, the inert gas is nitrogen.

[0020] In step 2, the specific preparation process of the carbon-coated lithium iron phosphate composite material derived from a metal-organic complex is as follows:

[0021] a) grinding the metal-organic complex precursor of step 1 with lithium hydroxide and ammonium dihydrogen phosphate in a molar ratio of 1:(1-1.5):(1-1.5) in a mortar for 20-30 minutes;

[0022] b) drying the ground powder in an oven, taking it out and cooling it, and then carbonizing it at high temperature. After the carbonization is completed, a carbon-coated lithium iron phosphate composite material derived from a metal-organic complex is obtained. The purpose of this is to make the carbon-coated lithium iron phosphate composite material have excellent conductivity.

[0023] In the step b), the carbonization temperature is 700-800° C., the heating rate is 2-10° C. / min, and the carbonization time is 6-10 hours.

[0024] In the step 3, the mass ratio of the carbon-coated lithium iron phosphate composite material derived from the metal-organic complex to the cellulose nanofibers is 1:(0.05-0.2).

[0025] In step 3, the inert gas is nitrogen.

[0026] In step 3, the carbonization temperature is 300-800°C, the heating rate is 4-6°C / min, and the carbonization time is 2-8 hours. Under these carbonization conditions, the cellulose nanofibers are converted into a conductive carbon nanofiber network, and the material forms a hierarchical porous structure of lithium iron phosphate nanoparticles-carbon coating layer-carbon nanofiber conductive network.

[0027] In step 3, the specific preparation process of the cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material is as follows:

[0028] a) mixing a metal-organic complex-derived carbon-coated lithium iron phosphate composite material and cellulose nanofibers in deionized water and thoroughly ultrasonically dispersing them, followed by stirring on a magnetic stirrer for 4 to 6 hours to form a uniformly mixed suspension;

[0029] b) The suspension was filtered, freeze-dried for 24 hours, and carbonized at high temperature to obtain a cellulose nanofiber-modified lithium iron phosphate / carbon composite cathode material.

[0030] The present invention also provides a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material. The microstructure of this material is a "lithium iron phosphate nanoparticle-carbon shell-carbon nanofiber conductive network" structure. Nanoscale lithium iron phosphate particles are surrounded by a carbon layer of uniform thickness, and the cellulose nanofibers are interwoven to form a three-dimensional porous conductive network structure. Its network structure not only reduces the agglomeration of nanoscale lithium iron phosphate particles, but also effectively improves the embedding and extraction of lithium ions.

[0031] The present invention also provides a lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the positive electrode sheet is a positive electrode sheet prepared from the cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material.

[0032] Beneficial effects of the present invention:

[0033] The present invention is based on a carbon-coated lithium iron phosphate composite material system derived from a metal-organic complex, further introduces cellulose nanofibers as an auxiliary carbon source, uses ultrasonic dispersion for uniform mixing, and combines freeze drying and high-temperature pyrolysis to prepare a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material with a hierarchical porous structure.

[0034] The cellulose nanofiber-modified lithium iron phosphate / carbon composite cathode material has a "lithium iron phosphate nanoparticle-carbon shell-carbon nanofiber conductive network" structure. The carbon-coated lithium iron phosphate nanoparticles alleviate the stress on the material caused by lithium ion insertion and extraction, and the addition of an appropriate amount of cellulose nanofibers effectively prevents the agglomeration of lithium iron phosphate nanoparticles, effectively improving the utilization efficiency of the cellulose nanofibers and lithium iron phosphate. The three-dimensional porous conductive network structure formed by the cellulose nanofibers not only effectively shortens the transmission distance of lithium ions and electrons, providing ample storage space for the electrolyte, but also serves as a diffusion channel for lithium ions, thereby increasing the diffusion rate of lithium ions.

[0035] The cellulose nanofiber-modified lithium iron phosphate / carbon composite cathode material exhibits excellent electrochemical performance, with a discharge capacity of 158.5 mAh g at a 0.1C rate. -1 , and exhibited good redox kinetics during the cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a scanning electron microscope (SEM) image of the cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material obtained in Example 2 of the present invention.

[0037] Figure 2 This is a transmission electron microscope (TEM) image of the cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material obtained in Example 2 of the present invention.

[0038] Figure 3 1 is the X-ray diffraction (XRD) pattern of the cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material obtained in Examples 1-3 of the present invention.

[0039] Figure 4 This is a graph showing the first charge and discharge performance of the cellulose nanofiber-modified lithium iron phosphate / carbon composite cathode material obtained in Examples 1-3 of the present invention at a rate of 0.1C, with a voltage window of 2.5 to 4.2V.

[0040] Figure 5 This is a graph showing the first charge and discharge performance of the cellulose nanofiber-modified lithium iron phosphate / carbon composite cathode material obtained in Example 2 of the present invention at different rates, with a voltage window of 2.5 to 4.2 V.

[0041] Figure 6This is a cycling performance diagram of the cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material obtained in Example 2 of the present invention and the carbon-coated lithium iron phosphate composite material derived from the metal-organic complex obtained in Comparative Example 1 at a 1C rate. DETAILED DESCRIPTION

[0042] The present invention will be described in further detail below with reference to the accompanying drawings.

[0043] Example 1:

[0044] This embodiment provides a method for preparing a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material, which is prepared by the following steps:

[0045] Step 1: Dissolve 2 mmol of ferrous acetate in 40 mL of dimethylformamide (DMF) and ultrasonically disperse to form a transparent, homogeneous solution A. Place 4 mmol of adenine in a round-bottom flask containing 160 mL of DMF and heat to 140°C on a magnetic stirrer. Stir and dissolve to form a colorless, transparent solution B. Pour solution A into solution B under vigorous stirring (at 2000 rpm) and continue the reaction at 140°C for 4 hours. After the reaction is complete, cool to room temperature, wash by centrifugation, and dry to obtain the metal-organic complex precursor.

[0046] Step 2: 0.40 g of the metal-organic complex precursor obtained in Step 1 was ground with 0.30 g of ammonium dihydrogen phosphate and 0.07 g of lithium hydroxide in a mortar for 20 minutes until uniformly mixed. The ground powder was then dried in an oven at 60°C for 2 hours. After cooling to room temperature, the mixture was placed in an alumina crucible and placed in a tube furnace. Under a nitrogen atmosphere, the mixture was heated at a rate of 5°C / min to 700°C and held at this temperature for 8 hours to obtain a carbon-coated lithium iron phosphate composite material derived from the metal-organic complex.

[0047] Step 3: Weigh 0.60g of cellulose nanofibers (mass fraction: 13.45%) and place them in deionized water. Ultrasonic dispersion is performed to form a cellulose nanofiber suspension. Weigh 1.60g of the carbon-coated lithium iron phosphate composite material derived from the metal-organic complex obtained in step 2 and disperse it in the above suspension. Mix and stir on a magnetic stirrer for 4 hours. After mixing, filter and freeze for 2 hours. Then, freeze-dry in a freeze dryer for 24 hours. Then, heat at a rate of 5°C / min under an inert gas nitrogen atmosphere to 350°C for 2 hours, then continue to heat to 700°C for 8 hours to obtain a cellulose nanofiber-modified lithium iron phosphate / carbon composite cathode material.

[0048] The composite positive electrode material obtained in this embodiment has a hierarchical porous structure of "lithium iron phosphate nanoparticles-carbon shell-carbon nanofiber conductive network".

[0049] Example 2:

[0050] This embodiment provides a method for preparing a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material, which is prepared by the following steps:

[0051] Step 1: The basic operation of preparing the metal-organic complex precursor in step 1 of this embodiment is basically the same as step 1 of embodiment 1, and will not be repeated here.

[0052] Step 2: The basic operation of preparing the carbon-coated lithium iron phosphate composite material derived from the metal-organic complex in step 2 of this embodiment is basically the same as that in step 2 of embodiment 1, and will not be repeated here.

[0053] Step 3: Weigh 1.20g of cellulose nanofibers (mass fraction: 13.45%) and place them in deionized water. Ultrasonic dispersion is performed to form a cellulose nanofiber suspension. Weigh 1.60g of the carbon-coated lithium iron phosphate composite material derived from the metal-organic complex obtained in step 2 and disperse it in the above suspension. Mix and stir on a magnetic stirrer for 4 hours. After mixing, filter and freeze for 2 hours. Then, freeze-dry in a freeze dryer for 24 hours. Then, heat at a rate of 5°C / min under an inert gas nitrogen atmosphere to 350°C for 2 hours, then continue to heat to 700°C for 8 hours to obtain a cellulose nanofiber-modified lithium iron phosphate / carbon composite cathode material.

[0054] The composite positive electrode material obtained in this embodiment has a hierarchical porous structure of "lithium iron phosphate nanoparticles-carbon shell-carbon nanofiber conductive network", has good electrochemical performance, and exhibits good redox kinetic characteristics during the cycle process.

[0055] Example 3:

[0056] This embodiment provides a method for preparing a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material, which is prepared by the following steps:

[0057] Step 1: The basic operation of preparing the metal-organic complex precursor in step 1 of this embodiment is basically the same as step 1 of embodiment 1, and will not be repeated here.

[0058] Step 2: The basic operation of preparing the carbon-coated lithium iron phosphate composite material derived from the metal-organic complex in step 2 of this embodiment is basically the same as that in step 2 of embodiment 1, and will not be repeated here.

[0059] Step 3: Weigh 2.40g of cellulose nanofibers (mass fraction: 13.45%) and place them in deionized water. Ultrasonic dispersion is performed to form a cellulose nanofiber suspension. Weigh 1.60g of the carbon-coated lithium iron phosphate composite material derived from the metal-organic complex obtained in step 2 and disperse it in the above suspension. Mix and stir on a magnetic stirrer for 4 hours. After mixing, filter and freeze for 2 hours. Then, freeze-dry in a freeze dryer for 24 hours. Then, heat at a rate of 5°C / min to 350°C and keep warm for 2 hours under an inert gas nitrogen atmosphere. Then continue to heat to 700°C for 8 hours to obtain a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material.

[0060] The composite positive electrode material obtained in this embodiment has a hierarchical porous structure of "lithium iron phosphate nanoparticles-carbon shell-carbon nanofiber conductive network".

[0061] Comparative Example 1:

[0062] This comparative example provides a method for preparing a carbon-coated lithium iron phosphate composite material derived from a metal-organic complex, which is prepared by the following steps:

[0063] Step 1: The basic operation of preparing the metal-organic complex precursor in step 1 of this comparative example is basically the same as step 1 of embodiment 1, and will not be repeated here.

[0064] Step 2: 0.40 g of the metal-organic complex precursor obtained in Step 1 was ground with 0.30 g of ammonium dihydrogen phosphate and 0.07 g of lithium hydroxide in a mortar for 20 minutes until uniformly mixed. The ground powder was then dried in an oven at 60°C for 2 hours. After cooling to room temperature, the mixture was placed in an alumina crucible and placed in a tube furnace. Under a nitrogen atmosphere, the mixture was heated at a rate of 5°C / min to 700°C and held at this temperature for 8 hours to obtain a carbon-coated lithium iron phosphate composite material derived from the metal-organic complex.

[0065] Comparative Example 2:

[0066] This comparative example provides a method for preparing a carbon-coated lithium iron phosphate composite material derived from a metal-organic complex, which is prepared by the following steps:

[0067] Step 1: The basic operation of preparing the metal-organic complex precursor in step 1 of this embodiment is basically the same as step 1 of embodiment 1, and will not be repeated here.

[0068] Step 2: 0.40 g of the metal-organic complex precursor obtained in Step 1 was ground with 0.30 g of ammonium dihydrogen phosphate and 0.07 g of lithium hydroxide in a mortar for 20 minutes until uniformly mixed. The ground powder was then dried in an oven at 60°C for 2 hours. After cooling to room temperature, the mixture was placed in an alumina crucible and placed in a tube furnace. Under a nitrogen atmosphere, the mixture was heated at a rate of 5°C / min to 800°C and held at this temperature for 8 hours to obtain a carbon-coated lithium iron phosphate composite material derived from the metal-organic complex.

[0069] Performance Testing

[0070] (1) Preparation of the positive electrode: The cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material provided in Example 1-3 and the metal-organic complex-derived carbon-coated lithium iron phosphate composite material provided in Comparative Example 1-2 were respectively used as positive electrode active materials, and were weighed with conductive carbon black and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1, and then fully ground in an agate mortar to achieve uniform mixing of the materials. Then, the mixture was mixed and dissolved with N-methylpyrrolidone (NMP) solvent and stirred for 12 hours to obtain a uniform black slurry. Subsequently, the viscous slurry was evenly coated on aluminum foil, placed in a vacuum oven and evacuated to -0.1 MPa, dried at 120°C for 12 hours, cooled to room temperature, and then taken out. Then, a manual slicer was used to punch and cut into circular pole pieces with a diameter of 10 mm for standby use.

[0071] (2) Assembly of button batteries

[0072] The prepared positive electrode sheet, negative electrode sheet (metal lithium sheet), separator (Celgard 2500PP separator) and electrolyte (1M LiPF6 / EC+DEC+EMC (1:1:1, vol%)) were assembled into a CR2032 button battery in an argon-filled glove box. The battery was left to stand for 12 hours before electrochemical performance testing.

[0073] (3) Battery performance test

[0074] The rested battery was tested using a CT3002A test system. The test was performed using a constant current charge / discharge method, with parameters such as current density, number of cycles, and voltage range (2.5-4.2V) set.

[0075] Test Example 1:

[0076] The cellulose nanofiber-modified lithium iron phosphate / carbon composite cathode material prepared in Example 2 was subjected to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) tests, and the images obtained were as follows: Figure 1 and 2 shown.

[0077] Depend on Figure 1It can be observed that the cellulose nanofibers interweave to form a three-dimensional porous network structure, further reducing the agglomeration of nanoscale lithium iron phosphate particles. This structure also facilitates the penetration of electrolyte and the rapid diffusion of lithium ions, making the reaction between the electrode material and electrolyte smoother at the interface, thereby improving the material's structure and cycle stability.

[0078] Figure 2 It can be observed that the lithium iron phosphate particles are surrounded by a carbon skeleton, and the surface of the particles is coated with amorphous carbon of about 4 nm with a uniform thickness, which is conducive to stabilizing the structure and accelerating the electrochemical reaction kinetics.

[0079] Figure 3 The X-ray diffraction (XRD) test spectra of the cellulose nanofiber-modified lithium iron phosphate / carbon composite cathode materials prepared in Examples 1-3 of the present invention are given. The results show that the main characteristic peaks displayed by Examples 1-3 are consistent with those of pure orthorhombic lithium iron phosphate (JCPDS 81-1173), all of which are olivine structures of the Pnma orthorhombic system, which means that the materials have good crystallinity. In addition, for the cellulose nanofiber-modified lithium iron phosphate / carbon composite cathode materials, the addition of cellulose nanofibers does not cause other impurity reflection peaks, proving the successful preparation of the cellulose nanofiber-modified lithium iron phosphate / carbon composite cathode materials.

[0080] Figure 4 This is the first charge and discharge curve of the cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material obtained in Examples 1-3 of the present invention at a rate of 0.1C. All materials have a flat voltage platform near 3.4V, which corresponds to the insertion and extraction of lithium ions during the electrochemical reaction.

[0081] Figure 5 The first cycle charge and discharge curves of the cellulose nanofiber modified lithium iron phosphate / carbon composite cathode material obtained in Example 2 of the present invention at different rates are shown in the figure. As can be seen from the figure, the discharge specific capacity of the cellulose nanofiber modified lithium iron phosphate / carbon composite cathode material gradually decreases with the increase of the rate. In addition, the curves in the figure all show Fe 2+ / Fe 3+ The voltage platform is flat, and the charge and discharge platforms are relatively flat. As the current density increases, the voltage difference between the charge and discharge platforms of the electrode increases slightly, indicating good redox reaction kinetics.

[0082] Figure 6This is a cycling performance graph of the cellulose nanofiber-modified lithium iron phosphate / carbon composite cathode material obtained in Example 2 of the present invention and the carbon-coated lithium iron phosphate composite material derived from the metal-organic complex obtained in Comparative Example 1 at 25°C and 1C. As can be seen from the graph, the first discharge specific capacities of Example 2 and Comparative Example 1 are 146.8 and 138.7 mAh g, respectively. -1 , Coulombic efficiencies are 99.68% and 98.91% respectively, and the capacity retention rates after 200 cycles are 87.08% and 72.46% respectively.

Claims

1. A method for preparing a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material, characterized in that: The following steps are involved: Step 1: Metal Fe ions and nitrogen-rich base organic ligands are reacted in a one-pot process to prepare a metal-organic complex precursor; Step 2: mixing the metal-organic complex precursor with a lithium source and a phosphorus source, grinding and refining the mixture, and then carbonizing the mixture at a high temperature in an inert gas to obtain a carbon-coated lithium iron phosphate composite material derived from the metal-organic complex; Step 3: Add the carbon-coated lithium iron phosphate composite material derived from the metal-organic complex to the cellulose nanofiber suspension and mix evenly. After filtration and freeze-drying, the suspension is carbonized at high temperature in an inert gas to obtain a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material.

2. The method for preparing a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material according to claim 1, characterized in that: In step 1, the metal Fe ion is ferrous acetate, and the nitrogen-rich base organic ligand is an adenine molecule.

3. The method for preparing a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material according to claim 2, characterized in that: In step 1, the specific preparation process of the metal-organic complex precursor is as follows: a) dissolving ferrous acetate in N,N-dimethylformamide (DMF) to form a uniform solution A for later use; b) dissolving adenine molecules in DMF at 130-160°C to form a colorless, transparent solution B; c) Pour solution A into solution B, mix and stir to react for 2 to 6 hours, and after the reaction is completed, wash and dry to obtain a metal-organic complex precursor.

4. The method for preparing a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material according to claim 3, characterized in that: In the step 1, the molar ratio of the metal Fe ion to the nitrogen-rich base organic ligand is 1:1-3.

5. The method for preparing a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material according to claim 1, characterized in that: In step 2, the lithium source is lithium hydroxide and the phosphorus source is ammonium dihydrogen phosphate; In step 2, the inert gas is nitrogen.

6. The method for preparing a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material according to claim 5, characterized in that: In step 2, the specific preparation process of the carbon-coated lithium iron phosphate composite material derived from a metal-organic complex is as follows: a) grinding the metal-organic complex precursor of step 1 with lithium hydroxide and ammonium dihydrogen phosphate in a molar ratio of 1:(1-1.5):(1-1.5) in a mortar for 20-30 minutes; b) drying the ground powder in an oven, cooling it, and then carbonizing it at high temperature to obtain a carbon-coated lithium iron phosphate composite material derived from a metal-organic complex; In the step b), the carbonization temperature is 700-800° C., the heating rate is 2-10° C. / min, and the carbonization time is 6-10 hours.

7. The method for preparing a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material according to claim 1, characterized in that: In step 3, the mass ratio of the carbon-coated lithium iron phosphate composite material derived from the metal-organic complex to the cellulose nanofibers is 1:(0.05-0.2); In step 3, the inert gas is nitrogen; In step 3, the carbonization temperature is 300-800° C., the heating rate is 4-6° C. / min, and the carbonization time is 2-8 hours.

8. The method for preparing a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material according to claim 7, characterized in that: In step 3, the specific preparation process of the cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material is as follows: a) mixing a metal-organic complex-derived carbon-coated lithium iron phosphate composite material and cellulose nanofibers in deionized water and thoroughly ultrasonically dispersing them, followed by stirring on a magnetic stirrer for 4 to 6 hours to form a uniformly mixed suspension; b) The suspension was filtered, freeze-dried for 24 hours, and carbonized at high temperature to obtain a cellulose nanofiber-modified lithium iron phosphate / carbon composite cathode material.

9. A cellulose nanofiber-modified lithium iron phosphate / carbon composite cathode material prepared by the method according to any one of claims 1 to 8, characterized in that: The positive electrode material is a "lithium iron phosphate nanoparticle-carbon shell-carbon nanofiber conductive network" structure. Nano-scale lithium iron phosphate particles are surrounded by a carbon layer of uniform thickness, and cellulose nanofibers are interwoven to form a three-dimensional porous conductive network structure. Its network structure not only reduces the agglomeration of nano-scale lithium iron phosphate particles, but also effectively improves the insertion and removal of lithium ions.

10. A lithium-ion battery comprising a positive electrode, a negative electrode, an electrolyte and a separator, wherein: The positive electrode sheet is prepared from a cellulose nanofiber-modified lithium iron phosphate / carbon composite positive electrode material prepared by the method according to any one of claims 1 to 8.

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