Lithium iron phosphate composite material and preparation method and application thereof

Abstract

The invention relates to the technical field of lithium battery positive electrode materials, in particular to a lithium iron phosphate composite material and a preparation method and application thereof. Comprising the following steps: adding hexagonal boron nitride into absolute ethyl alcohol for ultrasonic dispersion to obtain dispersion liquid; adding lithium iron phosphate and a carbon source into deionized water, and stirring to obtain a mixed solution; mixing the dispersion liquid and the mixed liquid, and performing ball milling and spray drying to obtain precursor powder; performing microwave carbonization treatment and transient high-energy Joule thermal field treatment on the precursor powder to obtain a lithium iron phosphate composite material; and the temperature of the transient high-energy Joule thermal field treatment is 2800-3200 DEG C. The lithium iron phosphate and the carbon source are mixed to prepare the precursor powder, the precursor powder is subjected to high-temperature heat treatment at the temperature of 2800 DEG C or above, the surface of the coated carbon source is changed into graphene under the instantaneous high-temperature condition, the hexagonal boron nitride and the graphene are subjected to a high-temperature synergistic effect by adding the hexagonal boron nitride, and the cycle life of the lithium battery is remarkably prolonged.

Description

Technical Field

[0001] This invention relates to the field of lithium battery cathode materials technology, specifically to a lithium iron phosphate composite material, its preparation method, and its application. Background Technology

[0002] With the rapid development of high-power charging and discharging devices such as electric vehicles, the performance requirements for lithium-ion batteries are increasing. Lithium iron phosphate (LFP), as a cathode material for lithium-ion batteries, has attracted much attention due to its low cost, abundant raw materials, low toxicity, good high-temperature electrochemical performance, and high theoretical capacity. However, its low electronic conductivity and ion diffusion coefficient lead to high internal resistance, limiting its application in high-power devices. To address this issue, researchers have developed various methods for preparing carbon-coated LFP materials, such as high-temperature solid-state methods, sol-gel methods, and hydrothermal methods. These methods improve the conductivity and structural stability of LFP by coating its surface with a layer of carbon material.

[0003] However, simply coating the surface of lithium iron phosphate with a layer of carbon material as the positive electrode of a lithium battery will cause the lithium iron phosphate particles to agglomerate and grow during use, resulting in a low cycle life of the lithium battery. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for preparing lithium iron phosphate composite materials. The method involves mixing lithium iron phosphate and a carbon source to prepare a precursor powder, which is then subjected to high-temperature heat treatment at temperatures above 2800°C. This causes the surface of the coated carbon source to transform into graphene under instantaneous high-temperature conditions. Furthermore, the addition of hexagonal boron nitride enables a high-temperature synergistic effect with the graphene, thereby preventing the aggregation and growth of lithium iron phosphate and significantly improving the cycle life of the lithium battery.

[0005] Therefore, the present invention provides the following technical solution: In a first aspect, the present invention provides a method for preparing a lithium iron phosphate composite material in an optional embodiment, comprising the following steps: Hexagonal boron nitride was added to anhydrous ethanol and ultrasonically dispersed to obtain a dispersion. Lithium iron phosphate and a carbon source were added to deionized water and stirred to obtain a mixture. The dispersion and the mixture were mixed, then ball-milled and spray-dried to obtain the precursor powder. The precursor powder was subjected to microwave carbonization and transient high-energy Joule thermal field treatment to obtain the lithium iron phosphate composite material. The temperature of the transient high-energy Joule thermal field treatment is 2800-3200℃.

[0006] In this invention, instantaneous high temperature (above 2800°C) causes the hydroxyl groups (-OH) at the edge of hexagonal boron nitride to undergo a dehydration reaction with the carboxyl groups (-COOH) or epoxy groups (-O-) at the edge of graphene, forming COB covalent bonds. This firmly binds graphene and hexagonal boron nitride together, thereby forming a stable three-dimensional conductive network on the surface of lithium iron phosphate particles. This suppresses the aggregation and growth of lithium iron phosphate particles and improves the cycle life of the battery.

[0007] Preferably, the ultrasonic dispersion power is 250-350W; and / or, the ultrasonic dispersion time is 20-40 min. The mass concentration of the dispersion is 0.4-0.6 mg / mL; and / or, the mass ratio of lithium iron phosphate to carbon source is 4-10:1; and / or, the carbon source is selected from one or more of biomass waste, polydopamine, polypyrrole, chitosan, carbon black, sucrose, glucose, starch, citric acid, polyaniline, polyvinyl alcohol, polyacrylic acid, phenolic resin, vitamins, or nano-carbon materials. The mass of the hexagonal boron nitride is 2-5% of the mass of lithium iron phosphate; and / or, the thickness of the hexagonal boron nitride is 1-5 layers, and the sheet diameter is 50-200 nm.

[0008] Preferably, the ball milling time is 4-6 hours; and / or the ball milling speed is 250-350 r / min. The spray drying temperature is 150-200℃; and / or the precursor powder particle size is 10-50 μm. The microwave carbonization power is 800-1050 W; and / or the microwave carbonization time is 2-4 minutes. The pulse current of the transient high-energy Joule thermal field treatment is 6 × 10⁻⁶. 3 -2.5×10 6 A / cm2; and / or, the processing time of the transient high-energy Joule thermal field treatment is 40-240ms.

[0009] Secondly, in an optional embodiment, the present invention provides a lithium iron phosphate composite material, which is prepared by the above-described preparation method.

[0010] Thirdly, in an optional embodiment, the present invention provides an application of the above-mentioned lithium iron phosphate composite material in lithium battery cathode materials.

[0011] Compared with the prior art, the present invention has one of the following beneficial effects: 1. This invention prepares a precursor powder by mixing lithium iron phosphate and a carbon source, and then heat-treats it at a high temperature of over 2800°C. This causes the surface of the coated carbon source to transform into graphene under instantaneous high temperature conditions. Furthermore, by adding hexagonal boron nitride, a high-temperature synergistic effect is achieved between the carbon source and the graphene, thereby avoiding the aggregation and growth of lithium iron phosphate and significantly improving the cycle life of lithium batteries. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the fully automatic ramp-type pulse Joule heat treatment device for transient high-energy Joule thermal field treatment in Embodiment 1 of the present invention. Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0014] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0015] In the following embodiments, the transient high-energy Joule thermal field treatment employs, as follows: Figure 1 The fully automated inclined pulse Joule heat treatment device shown achieves this, utilizing a high-current applied instantaneously to raise the temperature of the heating element to over 3000℃ instantly. An automatic feeder places the material at the top of the inclined heating device. The material's descent speed is controlled by adjusting the inclination angle α of the inclined heating, and the heating time is controlled by adjusting the length L of the inclined slope. This descent method ensures uniform heating. The high-current instantaneous heating of the heating element transforms the surface carbon-containing raw material into graphene.

[0016] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0017] Example 1: This example provides a method for preparing lithium iron phosphate composite materials, including the following steps: 5g of hexagonal boron nitride (average thickness of 2 layers, average flake diameter of 80nm) was dispersed in 10L of anhydrous ethanol and ultrasonically dispersed (power 300W, time 30min) to obtain a dispersion with a mass concentration of 0.5g / L. 166.67g of lithium iron phosphate and 16.67g of glucose were added to deionized water and stirred to obtain a mixture. The dispersion and mixture were mixed and then transferred to a ball mill jar and ball-milled for 4 hours (300 r / min). The slurry after ball milling was spray-dried (at 180°C) to obtain precursor powder. The precursor powder was placed in a microwave reactor, the power was set to 1000W, and it was rapidly heated for 2 minutes, causing the mixture to carbonize and form a porous, multidimensional heterostructure. Using methods such as... Figure 1 The fully automatic inclined pulse Joule heat treatment device shown uses a screw conveyor with a feeding speed of 20 g / min, a vibrating feeding frequency of 50 Hz, and applies a pulse current with a pulse current density of 6 × 10⁻⁶.3 A / cm², setting the material heating time to 40ms, so that the surface temperature of the material reaches 3000℃, to obtain lithium iron phosphate composite material.

[0018] Example 2: This example provides a method for preparing lithium iron phosphate composite materials, including the following steps: 5g of hexagonal boron nitride (average thickness of 2 layers, average flake diameter of 80nm) was dispersed in 10L of anhydrous ethanol and ultrasonically dispersed (power 300W, time 30min) to obtain a dispersion with a mass concentration of 0.5g / L. 166.67g of lithium iron phosphate and 33.33g of polydopamine were added to deionized water and stirred to obtain a mixture. The dispersion and mixture were mixed and then transferred to a ball mill jar and ball-milled for 4 hours (300 r / min). The slurry after ball milling was spray-dried (at 180°C) to obtain precursor powder. The precursor powder was placed in a microwave reactor, the power was set to 1000W, and it was rapidly heated for 2 minutes, causing the mixture to carbonize and form a porous, multidimensional heterostructure. Using methods such as... Figure 1 The fully automatic inclined pulse Joule heat treatment device shown uses a screw conveyor to feed material at a speed of 25 g / min, a vibrating feeding frequency of 60 Hz, and applies a pulse current with a pulse current density of 1.2 × 10⁻⁶. 5 A / cm², setting the material heating time to 180ms, so that the surface temperature of the material reaches 3200℃, to obtain lithium iron phosphate composite material.

[0019] Example 3: This example provides a method for preparing lithium iron phosphate composite materials, including the following steps: 5g of hexagonal boron nitride (average thickness of 2 layers, average flake diameter of 80nm) was dispersed in 10L of anhydrous ethanol and ultrasonically dispersed (power 300W, time 30min) to obtain a dispersion with a mass concentration of 0.5g / L. 166.67g of lithium iron phosphate and 20.83g of citric acid were added to deionized water and stirred to obtain a mixture. The dispersion and mixture were mixed and then transferred to a ball mill jar and ball-milled for 4 hours (300 r / min). The slurry after ball milling was spray-dried (at 180°C) to obtain precursor powder. The precursor powder was placed in a microwave reactor, the power was set to 800W, and it was rapidly heated for 4 minutes, causing the mixture to carbonize and form a porous, multidimensional heterostructure. Using methods such as... Figure 1 The fully automatic inclined pulse Joule heat treatment device shown uses a screw conveyor with a feeding speed of 18 g / min and a vibration feeding frequency of 45 Hz. It employs a two-stage pulse process: Stage 1: Current density is set to 1 × 10⁻⁶. 4 A / cm2 By controlling the length and angle of the ramp and setting the heating time to 100ms, pre-graphitization is achieved; Second stage: the current density is increased to 5×10 4 A / cm 2 By controlling the length and angle of the slope and setting the heating time to 120ms, the densification process is completed, and the surface temperature of the material reaches 3000℃, thus obtaining lithium iron phosphate composite material.

[0020] Example 4: This example provides a method for preparing a lithium iron phosphate composite material, including the following steps: 5g of hexagonal boron nitride (average thickness of 2 layers, average flake diameter of 80nm) was dispersed in 10L of anhydrous ethanol and ultrasonically dispersed (power 300W, time 30min) to obtain a dispersion with a mass concentration of 0.5g / L. 166.67g of lithium iron phosphate and 41.67g of phenolic resin were added to deionized water and stirred to obtain a mixture. The dispersion and mixture were mixed and then transferred to a ball mill jar and ball-milled for 4 hours (300 r / min). The slurry after ball milling was spray-dried (at 180°C) to obtain precursor powder. The precursor powder was placed in a microwave reactor, the power was set to 1050W, and it was rapidly heated for 2 minutes, causing the mixture to carbonize and form a porous, multidimensional heterostructure. Using methods such as... Figure 1 The fully automatic inclined pulse Joule heat treatment device shown uses a screw conveyor with a feeding speed of 22 g / min, a vibrating feeding frequency of 55 Hz, and applies a pulse current with a pulse current density of 9 × 10⁻⁶. 4 A / cm², setting the material heating time to 220ms, so that the surface temperature of the material reaches 2800℃, thus obtaining lithium iron phosphate composite material.

[0021] Example 5: This example provides a method for preparing lithium iron phosphate composite materials, including the following steps: 5g of hexagonal boron nitride (average thickness of 2 layers, average flake diameter of 80nm) was dispersed in 10L of anhydrous ethanol and ultrasonically dispersed (power 300W, time 30min) to obtain a dispersion with a mass concentration of 0.5g / L. 166.67g of lithium iron phosphate and 27.78g of polyacrylonitrile (PAN) were added to deionized water and stirred to obtain a mixture. The dispersion and mixture were mixed and then transferred to a ball mill jar and ball-milled for 4 hours (300 r / min). The slurry after ball milling was spray-dried (at 180°C) to obtain precursor powder. The precursor powder was placed in a microwave reactor, the power was set to 800W, and it was rapidly heated for 4 minutes, causing the mixture to carbonize and form a porous, multidimensional heterostructure. Using methods such as... Figure 1 The fully automatic inclined pulse Joule heat treatment device shown uses a screw conveyor to feed material at a speed of 15 g / min, a vibrating feeding frequency of 70 Hz, and applies a pulse current with a pulse current density of 2.5 × 10⁻⁶. 6 A / cm², setting the material heating time to 50ms, so that the surface temperature of the material reaches 2900℃, thus obtaining lithium iron phosphate composite material.

[0022] Comparative Example 1: This comparative example provides a method for preparing a lithium iron phosphate composite material, including the following steps: 166.67g of lithium iron phosphate and 16.67g of glucose were added to deionized water and stirred to obtain a mixture. The mixture was then transferred to a ball mill jar and ball-milled for 4 hours (300 r / min). The slurry after ball milling was spray-dried (at 180℃) to obtain precursor powder. The precursor powder was placed in a microwave reactor, the power was set to 1000W, and it was rapidly heated for 2 minutes, causing the mixture to carbonize and form a porous, multidimensional heterostructure. Using methods such as... Figure 1 The fully automatic inclined pulse Joule heat treatment device shown uses a screw conveyor with a feeding speed of 20 g / min, a vibrating feeding frequency of 50 Hz, and applies a pulse current with a pulse current density of 6 × 10⁻⁶. 3 A / cm², setting the material heating time to 40ms, so that the surface temperature of the material reaches 3000℃, to obtain lithium iron phosphate composite material.

[0023] Comparative Example 2: This comparative example provides a method for preparing a lithium iron phosphate composite material, including the following steps: 5g of hexagonal boron nitride (average thickness of 2 layers, average flake diameter of 80nm) was dispersed in 10L of anhydrous ethanol and ultrasonically dispersed (power 300W, time 30min) to obtain a dispersion with a mass concentration of 0.5g / L. 166.67g of lithium iron phosphate and 16.67g of glucose were added to deionized water and stirred to obtain a mixture. The dispersion and mixture were mixed and then transferred to a ball mill jar and ball-milled for 4 hours (300 r / min). The slurry after ball milling was spray-dried (at 180°C) to obtain precursor powder. The precursor powder was placed in a microwave reactor, the power was set to 1000W, and it was rapidly heated for 2 minutes, causing the mixture to carbonize and form a porous, multidimensional heterostructure. Using methods such as... Figure 1 The fully automatic inclined pulse Joule heat treatment device shown uses a screw conveyor with a feeding speed of 20 g / min, a vibrating feeding frequency of 50 Hz, and applies a pulse current with a pulse current density of 6 × 10⁻⁶. 3 A / cm², setting the material heating time to 40ms, so that the surface temperature of the material reaches 2500℃, to obtain lithium iron phosphate composite material.

[0024] Comparative Example 3: This comparative example provides a method for preparing a lithium iron phosphate composite material, including the following steps: 5g of hexagonal boron nitride (average thickness of 2 layers, average flake diameter of 80nm) was dispersed in 10L of anhydrous ethanol and ultrasonically dispersed (power 300W, time 30min) to obtain a dispersion with a mass concentration of 0.5g / L. 166.67g of lithium iron phosphate and 16.67g of glucose were added to deionized water and stirred to obtain a mixture. The dispersion and mixture were mixed and then transferred to a ball mill jar and ball-milled for 4 hours (300 r / min). The slurry after ball milling was spray-dried (at 180°C) to obtain precursor powder. The precursor powder was placed in a microwave reactor, the power was set to 1000W, and it was rapidly heated for 2 minutes, causing the mixture to carbonize and form a porous, multidimensional heterostructure. Using methods such as... Figure 1 The fully automatic inclined pulse Joule heat treatment device shown uses a screw conveyor with a feeding speed of 20 g / min, a vibrating feeding frequency of 50 Hz, and applies a pulse current with a pulse current density of 6 × 10⁻⁶. 3 A / cm², setting the material heating time to 40ms, so that the surface temperature of the material reaches 3500℃, thus obtaining lithium iron phosphate composite material.

[0025] Comparative Example 4: This comparative example provides a method for preparing a lithium iron phosphate composite material, including the following steps: 5g of hexagonal boron nitride (average thickness of 2 layers, average flake diameter of 80nm) was dispersed in 10L of anhydrous ethanol and ultrasonically dispersed (power 300W, time 30min) to obtain a dispersion with a mass concentration of 0.5g / L. 166.67g of lithium iron phosphate and 16.67g of glucose were added to deionized water and stirred to obtain a mixture. The dispersion and mixture were mixed and then transferred to a ball mill jar for ball milling for 4 hours (300 r / min). The slurry after ball milling was spray dried (at 180°C) to obtain the lithium iron phosphate composite material.

[0026] Experimental Example: The lithium iron phosphate composite materials (100 wt%) prepared in Examples 1-5 and Comparative Examples 1-4 were mixed with conductive agent Super P (5 wt%) and binder PVDF (10 wt%). N-methylpyrrolidone (NMP) was added to adjust the slurry viscosity to 2000-3000 mPa·s, and the mixture was stirred for 12 hours to form a uniform dispersion. The slurry was coated onto an aluminum foil current collector with a thickness of 12 μm, resulting in a coating thickness of 50 μm. The coating was then vacuum dried at 80°C for 12 hours, followed by a second drying at 120°C for 2 hours to completely remove the solvent. The dried electrode was then cold-pressed using a roller press, with the compaction density controlled at 2.0-2.2 g / cm³. 3 Cut them into circular electrode sheets with a diameter of 14mm for later use.

[0027] Assemble the CR2032 coin cell in the following order within an argon glove box (H2O < 0.1 ppm, O2 < 0.1 ppm): Negative electrode: Lithium metal sheet with a diameter of 16mm (purity 99.9%). Membrane: Celgard 2400 polypropylene microporous membrane (25μm thick); Positive electrode: Lithium iron phosphate electrode sheet with a diameter of 14mm; Electrolyte: 1M LiPF6 is dissolved in a 1:1 volume ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC) mixed solvent, with 5% fluoroethylene carbonate (FEC) added as a film-forming additive.

[0028] After the battery is packaged, it is left to stand at 25°C for 24 hours, and then charged at a rate of 0.05C to 3.65V. It is then charged at a constant voltage until the current drops to 0.02C, thus completing the formation and activation of the SEI film.

[0029] Cyclic life test method: The Xinwei CT-4008 battery testing system was used to conduct cyclic testing in a 25℃ constant temperature chamber. The specific parameters are as follows: Charge and discharge regime: Charging: 1C constant current charging to 3.65V, then switch to constant voltage charging until the current is ≤0.05C; Discharge: 1C constant current discharge to 2.5V.

[0030] Cycle life: When the capacity retention rate is less than 80% for three consecutive cycles, the total number of cycles is recorded as the cycle life of the battery.

[0031] Record the discharge capacity every 10 cycles and calculate the capacity retention rate (relative to the capacity at the 10th cycle). The results are shown in Table 1.

[0032] As shown in Table 1, the cycle life of Comparative Example 1 was shortened by 39% compared to Example 1, and the capacity retention rate after 1000 cycles decreased by 13.8%. The cycle life of Comparative Example 2 was shortened by 24% compared to Example 1, and the capacity retention rate after 1000 cycles decreased by 8.9%. The cycle life of Comparative Example 3 was shortened by 34% compared to Example 1, and the capacity retention rate after 1000 cycles decreased by 13.1%. The cycle life of Comparative Example 4 was shortened by 56% compared to Example 1, and the capacity retention rate after 1000 cycles decreased by 20.2%. Therefore, it is evident that preparing a precursor powder by mixing lithium iron phosphate and a carbon source, subjecting it to high-temperature heat treatment above 2800°C, transforming the coated carbon source surface into graphene under instantaneous high temperature conditions, and then adding hexagonal boron nitride to achieve a high-temperature synergistic effect with graphene can significantly improve the cycle life of lithium batteries.

[0033] Although the principles of the present invention have been described in detail above with reference to preferred embodiments, those skilled in the art should understand that the above embodiments are merely illustrative explanations of the implementation of the present invention and are not intended to limit the scope of the present invention. The details in the embodiments do not constitute a limitation on the scope of the present invention. Any obvious changes, such as equivalent transformations or simple substitutions, based on the technical solutions of the present invention without departing from the spirit and scope of the present invention fall within the protection scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate composite material, characterized in that, Includes the following steps: Hexagonal boron nitride was added to anhydrous ethanol and ultrasonically dispersed to obtain a dispersion. Lithium iron phosphate and a carbon source were added to deionized water and stirred to obtain a mixture. The dispersion and the mixture were mixed, then ball-milled and spray-dried to obtain the precursor powder. The precursor powder was subjected to microwave carbonization and transient high-energy Joule thermal field treatment to obtain the lithium iron phosphate composite material. The temperature of the transient high-energy Joule thermal field treatment is 2800-3200℃.

2. The method for preparing the lithium iron phosphate composite material according to claim 1, characterized in that, The power of the ultrasonic dispersion is 250-350W; and / or, The ultrasonic dispersion time is 20-40 minutes.

3. The method for preparing the lithium iron phosphate composite material according to claim 1, characterized in that, The dispersion has a mass concentration of 0.4-0.6 mg / mL; and / or, The mass ratio of lithium iron phosphate to carbon source is 4-10:1; and / or, The carbon source is selected from one or more of the following: biomass waste, polydopamine, polypyrrole, chitosan, carbon black, sucrose, glucose, starch, citric acid, polyaniline, polyvinyl alcohol, polyacrylic acid, phenolic resin, vitamins, or nano-carbon materials.

4. The method for preparing the lithium iron phosphate composite material according to claim 1, characterized in that, The mass of the hexagonal boron nitride is 2-5% of the mass of lithium iron phosphate; and / or, The thickness of the hexagonal boron nitride is 1-5 layers, and the sheet diameter is 50-200 nm.

5. The method for preparing the lithium iron phosphate composite material according to claim 1, characterized in that, The ball milling time is 4-6 hours; and / or, The ball mill rotates at a speed of 250-350 r / min.

6. The method for preparing the lithium iron phosphate composite material according to claim 1, characterized in that, The spray drying temperature is 150-200℃; and / or, The precursor powder has a particle size of 10-50 μm.

7. The method for preparing the lithium iron phosphate composite material according to claim 1, characterized in that, The microwave carbonization power is 800-1050W; and / or, The microwave carbonization time is 2-4 minutes.

8. The method for preparing the lithium iron phosphate composite material according to claim 1, characterized in that, The pulse current for the transient high-energy Joule thermal field treatment is 6 × 10⁻⁶. 3 -2.5×10 6 A / cm2; and / or, The processing time for the transient high-energy Joule thermal field treatment is 40-240 ms.

9. A lithium iron phosphate composite material, characterized in that, It is prepared by the preparation method described in any one of claims 1-8.

10. The application of the lithium iron phosphate composite material according to claim 9 in the cathode material of lithium batteries.

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