A method for preparing nanometer single-crystal lithium iron phosphate based on carboxylated iron phosphate

Abstract

This invention proposes a method for preparing nanocrystalline lithium iron phosphate based on carboxylated iron phosphate, comprising the following steps: S1. Iron oxide red and iron powder are mixed as an iron source, dissolved in deionized water, phosphoric acid is added, and the mixture is activated at 50-150°C. A multifunctional surface activator is then added for the first aging and crystal growth, followed by pulverization; S2. An organic solvent is added, and the mixture is aged a second time at 50-150°C. The mixture is then filtered, washed, and carboxylated iron phosphate is obtained; S3. The carboxylated iron phosphate is ball-milled and mixed with a lithium source, followed by spray drying to obtain a dry powder; S4. The dry powder is sintered to finally obtain carbon-coated nanocrystalline lithium iron phosphate. This invention facilitates the synthesis of nanocrystalline lithium iron phosphate, helps obtain nanocrystalline particles with uniform size, and promotes lithium-ion diffusion.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery technology, specifically relating to a method for preparing nano-monocrystalline lithium iron phosphate based on carboxylated iron phosphate. Background Technology

[0002] Lithium iron phosphate (LiFePO4, LFP) has become the mainstream cathode material in power batteries and energy storage due to its high safety, long cycle life, and low cost. However, its low intrinsic electronic conductivity (10⁻⁶ Ω·cm) is a significant drawback. -9 The relatively slow Li+ diffusion rate (s / cm) limits high-rate charge-discharge performance, leading to a rapid increase in electrode impedance, decreased energy efficiency, and insufficient fast-charging capability at high rates. With the increasing demand for rapid energy replenishment in electric vehicles and energy storage systems, the market is placing higher demands on the fast-charging performance of LFP batteries. To overcome the rate bottleneck of polycrystalline LFP, monocrystalline LFP has become a research hotspot. Its core advantages include: a shorter Li+ diffusion path, elimination of grain boundary impedance by nanocrystalline monocrystalline structures, and a 100-fold increase in Li+ diffusion coefficient (10⁻⁶ s / cm). -12 vs. 10 -14 cm 2 / s).

[0003] Currently, the main methods for preparing lithium iron phosphate include high-temperature carbothermal reduction, hydrothermal reduction, and solvothermal methods. However, these methods have the following problems: solid-phase methods involve high reaction temperatures and energy consumption, easily leading to particle agglomeration and larger grains, which affects rate performance, and the products are mainly polycrystalline. Traditional liquid-phase methods can obtain nanoparticles, but these easily form polycrystalline or amorphous-polycrystalline mixed structures. Therefore, defects such as cracks and dislocations are easily generated at grain boundaries, and Li+ diffusion is restricted by grain boundaries, affecting the rate performance of the material. Furthermore, structural collapse is prone to occur during cycling, shortening the battery's cycle life. Summary of the Invention

[0004] To address the problems existing in the prior art, the purpose of this invention is to propose a method for preparing nano-sized single-crystal lithium iron phosphate based on carboxylated iron phosphate. The technical solution of this invention is achieved as follows: A method for preparing nano-single-crystal lithium iron phosphate based on carboxylated iron phosphate, comprising the following steps: S1. Mix iron oxide red and iron powder as an iron source, dissolve in deionized water, add 30-85% phosphoric acid, activate at 50-150℃ for 1-4 hours, add a multifunctional surface activator, perform the first aging and crystal growth for 1-6 hours, and then pulverize. S2. Add organic solvent, age for a second time at 50-150℃ for 2-10 hours, then filter and wash to obtain carboxylated iron phosphate; S3. Carboxylated iron phosphate is ball-milled and mixed with a lithium source, and then spray-dried to obtain a dry powder; S4. The dried powder is subjected to microwave sintering in the first stage at a temperature of 350-650℃ for 3-10 hours. Then, a second stage of sintering is carried out at a temperature of 650-800℃ for 6-15 hours, and a gaseous carbon source is introduced. Nitrogen or argon inert gas is introduced throughout the sintering process, and carbon-coated nano-monocrystalline lithium iron phosphate is finally obtained.

[0005] Preferably, in step S1, the mass ratio of iron oxide red to iron powder is 1:1-10, the mass ratio of iron source to deionized water is 1:1-5, and the mass ratio of iron source to phosphoric acid is 1:2-4.

[0006] Preferably, in step S1, the multifunctional surfactant is one or more of peroxy polyacrylic acid, peroxy phthalic acid, tartaric acid, citric acid, polyethylene glycol phosphate, polypropylene glycol phosphate, per-N-dodecyl-N or N-dimethylamine oxide, and the amount added is 1-10% of the mass of the iron source.

[0007] Preferably, in step S2, the organic solvent is one or more of ethanol, isopropanol, N-methylpyrrolidone, ethylene glycol, formic acid, acetic acid, propionic acid or oleic acid, and the amount added is 1-5% of the mass of the iron source.

[0008] Preferably, in step S3, the lithium source is one or more of LiOH·H2O, Li2CO3, and lithium acetate.

[0009] Preferably, in step S4, the gaseous carbon source is one or more of methane, ethane, propane, and propylene.

[0010] Preferably, in step S1, the activation method is ultrasonic dispersion.

[0011] The beneficial effects of this invention are: 1. By using a composite iron source of low-cost iron red and iron powder, the problems of incomplete oxidation or excessive reduction caused by using a single iron source are avoided. 2. Carboxyl surfactants help Fe 2+ Sufficient oxidation yields FePO4, which can be used to regulate the amorphization and surface carboxyl functionalization of FePO4. Furthermore, secondary aging with organic solvents can control the carboxyl density on the particle surface, induce the growth of amorphous structures, and enhance the mixing uniformity with the lithium source, thus facilitating the synthesis of amorphous carboxylated iron phosphate. 3. Using carboxylated iron phosphate as a precursor and employing microwave-assisted sintering is beneficial for the synthesis of nanocrystalline lithium iron phosphate. The instantaneous high temperature of microwaves promotes the rapid decomposition of the precursor, forming a large number of crystal nuclei; while short-time sintering restricts the growth of crystal nuclei, which helps to obtain nanocrystalline single crystals with uniform particle size (50-200nm). The rapid heating of microwaves reduces Li vacancy defects in the lithium iron phosphate crystal structure, improves crystallinity, and is conducive to lithium ion diffusion. Attached Figure Description

[0012] Figure 1 The XRD results are for nano-sized single-crystal lithium iron phosphate in Examples 1 and 2. Figure 2 The above are the SEM results of nano-sized single-crystal lithium iron phosphate in Example 1. Figure 3 The above are the SEM results of nano-sized single-crystal lithium iron phosphate in Example 2. Figure 4 The charge-discharge curves at 0.1C for the nano-monocrystalline lithium iron phosphate cathode material in Example 3 are shown. Detailed Implementation

[0013] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example 1

[0014] A method for preparing nano-sized single-crystal lithium iron phosphate based on carboxylated iron phosphate includes the following steps: S1. According to the mass ratio, mix 1 part of iron oxide red with 3.5 parts of iron powder as an iron source, dissolve it in 9 parts of deionized water, add 9 parts of 80% phosphoric acid, activate at 60℃ for 4 hours, add 0.45 parts of peroxy polyacrylic acid, carry out the first aging and crystal growth for 6 hours, and then pulverize. S2. Add 0.2 parts of isopropanol organic solvent, age for a second time at 50°C for 10 hours, then filter and wash to obtain carboxyphosphate iron; S3. Nine parts of carboxylated ferric phosphate and two parts of lithium hydroxide were ball-milled and then spray-dried to obtain a dry powder. S4. The dried powder is subjected to microwave sintering in the first stage at a temperature of 350°C for 6 hours. Then, a second stage of sintering is carried out at a temperature of 720°C for 8 hours, and methane gas is introduced. Nitrogen inert gas is introduced throughout the sintering process, and finally carbon-coated nano-monocrystalline lithium iron phosphate is obtained.

[0015] like Figure 1The XRD results of the sintered material are shown, aligned one-to-one with the LiFePO4 card, indicating that it is a pure-phase lithium iron phosphate material; Figure 2 The image shows the microstructure under an electron microscope, exhibiting a distinct single-crystal structure. Example 2

[0016] S1. According to the mass ratio, mix 1 part of iron oxide red and 6 parts of iron powder as an iron source, dissolve in 14 parts of deionized water, add 14 parts of 85% phosphoric acid, activate at 150℃ for 3 hours, add 0.07 parts of peroxyphthalic acid, carry out the first aging and crystal growth for 1 hour, and then pulverize. S2. Add 0.3 parts of N-methylpyrrolidone organic reagent, age for a second time at 75°C for 5 hours, then filter and wash to obtain carboxyphosphate iron; S3. Mix 6 parts of carboxylated ferric phosphate with 1.2 parts of Li2CO3 by ball milling, and then spray dry to obtain dry powder; S4. The dried powder is subjected to microwave sintering in the first stage at a temperature of 450℃ for 10 hours. Then, a second stage of sintering is carried out at a temperature of 750℃ for 5 hours, and ethane gas is introduced. Argon inert gas is introduced throughout the sintering process, and finally carbon-coated nano-monocrystalline lithium iron phosphate is obtained.

[0017] like Figure 1 The XRD results of the sintered material are shown, aligned one-to-one with the LiFePO4 card, indicating that it is a pure-phase lithium iron phosphate material; Figure 3 The image shows the microstructure under an electron microscope, exhibiting a distinct single-crystal structure. Example 3

[0018] S1. According to the mass ratio, mix 1 part of iron oxide red and 1 part of iron powder as an iron source, dissolve in 5 parts of deionized water, add 4.4 parts of 65% phosphoric acid, activate at 100℃ for 3 hours, add 0.1 parts of tartaric acid, carry out the first aging and crystal growth for 3 hours, and then pulverize. S2. Add 0.06 parts of formic acid organic solvent, age for a second time at 150℃ for 2 hours, then filter and wash to obtain carboxylated iron phosphate; S3. Mix 4 parts of carboxylated ferric phosphate with 2.2 parts of lithium acetate by ball milling, and then spray dry to obtain a dry powder; S4. The dried powder is subjected to microwave sintering in the first stage at a temperature of 650°C for 3 hours. Then, a second stage of sintering is carried out at a temperature of 800°C for 5 hours, and propylene gas is introduced. Argon inert gas is introduced throughout the sintering process, and finally carbon-coated nano-monocrystalline lithium iron phosphate is obtained.

[0019] like Figure 4 The figure shows the charge and discharge test curves of a half-cell assembled with lithium metal using sintered material as the positive electrode material. It can exhibit a high specific capacity at a 0.1C rate, with a discharge specific capacity of up to 161 mAh / g.

[0020] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing nano-single-crystal lithium iron phosphate based on carboxylated iron phosphate, characterized in that, Includes the following steps: S1. Mix iron oxide red and iron powder as an iron source, dissolve in deionized water, add 30-85% phosphoric acid, activate at 50-150℃ for 1-4 hours, add a multifunctional surface activator, age for the first time for 1-6 hours, and then pulverize. S2. Add organic solvent, age for a second time at 50-150℃ for 2-10 hours, then filter and wash to obtain carboxylated iron phosphate; S3. Carboxylated iron phosphate is mixed with a lithium source by planetary ball milling, and then spray-dried to obtain a dry powder; S4. The dried powder is subjected to microwave sintering in the first stage at a temperature of 350-650℃ for 3-10 hours. Then, a second stage of sintering is carried out at a temperature of 650-800℃ for 6-15 hours, and a gaseous carbon source is introduced. Nitrogen or argon inert gas is introduced throughout the sintering process, and carbon-coated nano-monocrystalline lithium iron phosphate is finally obtained.

2. The preparation method according to claim 1, characterized in that, In step S1, the mass ratio of iron oxide red to iron powder is 1:1-10, the mass ratio of iron source to deionized water is 1:1-5, and the mass ratio of iron source to phosphoric acid is 1:2-4.

3. The preparation method according to claim 1, characterized in that, In step S1, the multifunctional surfactant is one or more of the following: peroxy polyacrylic acid, peroxy phthalic acid, tartaric acid, citric acid, polyethylene glycol phosphate, polypropylene glycol phosphate, per-N-dodecyl-N or N-dimethylamine oxide, and the amount added is 1-10% of the mass of the iron source.

4. The preparation method according to claim 1, characterized in that, In step S2, the organic solvent is one or more of ethanol, isopropanol, N-methylpyrrolidone, ethylene glycol, formic acid, acetic acid, propionic acid or oleic acid, and the amount added is 1-5% of the mass of the iron source.

5. The preparation method according to claim 1, characterized in that, In step S3, the lithium source is one or more of LiOH·H2O, Li2CO3, and lithium acetate.

6. The preparation method according to claim 1, characterized in that, In step S4, the gaseous carbon source is one or more of methane, ethane, propane, and propylene.

7. The preparation method according to claim 1, characterized in that, In step S1, the activation method is ultrasonic dispersion.

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