CN102838102A - Preparation method of lithium iron phosphate monocrystalline nanorods - Google Patents

Abstract

The invention relates to a preparation method of lithium iron phosphate single crystal nanorods, which is mainly characterized in that ethylene glycol and water constitute a mixed solvent required for solvothermal reaction, and the volume ratio of ethylene glycol and water in the mixed solvent is designed to be 3:1 ~1:3, and polyethylene glycol was introduced to affect the formation of crystal nuclei and crystal growth, realizing the solvothermal synthesis of lithium iron phosphate single crystal nanorods. First, the antioxidant ascorbic acid is fully dissolved in a mixed solvent of water and ethylene glycol, and then dissolved in phosphoric acid and ferrous sulfate hexahydrate in sequence. Then the water and ethylene glycol solution that are dissolved with lithium hydroxide are added dropwise to the previous solution containing phosphoric acid, sulfur, ferrous sulfate and ascorbic acid. Then introduce an appropriate amount of polyethylene glycol, mix well, seal it in the reactor system, and perform a solvothermal reaction at a high temperature and high pressure of 160-240°C to obtain lithium iron phosphate single crystal nanorods. The product of the invention has stable quality, high purity, good particle dispersibility, is beneficial to the diffusion of lithium ions, improves the electrochemical performance of lithium ion batteries, and has simple preparation process, easy control, no pollution, low cost and easy large-scale production.

Description

A kind of preparation method of lithium iron phosphate single crystal nanorod

technical field

The invention relates to a preparation method of lithium iron phosphate single crystal nanorods, belonging to the fields of inorganic non-metallic materials and energy storage battery materials.

Background technique

Lithium-ion battery, as a high-performance rechargeable green power source, has been widely used in various portable electronic products and communication tools in recent years, and has been gradually developed as a power source for electric vehicles, thereby promoting its development towards safety, environmental protection, Development in the direction of low cost and high specific energy. Among them, the development of new electrode materials, especially positive electrode materials, is extremely critical. At present, the anode materials of lithium-ion batteries that have been extensively studied are concentrated on transition metal oxides of lithium, such as layered LiMO ₂ (M=Co, Ni, Mn) and spinel LiMn ₂ O ₄ . However, as cathode materials, they have their own disadvantages. LICoO ₂ has high cost, poor resources, and high toxicity; lithium nickelate (LiNiO ₂ ) is difficult to prepare and has poor thermal stability; LIMn ₂ O ₄ has low capacity and poor cycle stability, especially high temperature performance. Difference. In order to solve the defects of the above materials, a lot of research has been done in the battery industry. While various modifications have been made to the above cathode materials to improve their performance, the development of new cathode materials has always been the focus of attention. The study found that the lithium iron phosphate material has a moderate working voltage (3.4V), a good platform, a theoretical capacity of 170mAh/g, excellent cycle performance, and low cost. Its high energy density and high safety performance make it suitable for use in power lithium-ion batteries It has outstanding application prospects, but its disadvantages are its poor conductivity and slow diffusion rate of lithium ions, which is greatly related to the microscopic morphology of lithium iron phosphate cathode materials. At present, the specific mechanism and process of lithium iron phosphate deintercalation in the academic circle are still in the stage of hypothesizing, which is difficult to verify through experiments. At present, the industrialized lithium iron phosphate cathode materials are basically synthesized by high-temperature solid-state method, and the microscopic morphology is spherical. According to the literature search, the microscopic morphology of lithium iron phosphate prepared in the laboratory is still difficult to control, and it is concentrated in the diamond-shaped block. And spherical, such as the positive electrode material prepared by Kang Byoungwoo et al. high temperature solid phase method (nature. 2009,458,190.), or the lithium iron phosphate prepared by Yang SF et al. by hydrothermal method (ELECTROCHEMISTRY COMMUNICATIONS. 2001,3,505.) are all Spherical, none of the above is conducive to improving the energy density of lithium-ion batteries, and makes it difficult to study the dynamic changes in the crystal structure of positive electrode materials through in-situ TEM and other observation methods.

Contents of the invention

Aiming at the deficiencies in the prior art, the object of the present invention is to provide a method for preparing lithium iron phosphate single crystal nanorods with simple process and easy control.

The preparation method of the lithium iron phosphate single crystal nanorod of the present invention adopts a solvothermal synthesis method, comprising the following steps:

1) Mix ethylene glycol and deionized water at a volume ratio of 3:1~1:3 to obtain a mixed solvent of ethylene glycol and water;

2) dissolving ascorbic acid in the mixed solvent of ethylene glycol and water in step 1), and stirring until fully dissolved to obtain ascorbic acid solution;

3) According to the molar ratio of P and Fe being 1:1, measure and weigh phosphoric acid and ferrous sulfate hexahydrate, and dissolve phosphoric acid and ferrous sulfate hexahydrate in the ascorbic acid solution prepared in step 2). The target mass percentage of lithium iron phosphate prepared is 10-30%, the concentrations of phosphoric acid and hexahydrate ferrous sulfate are both 0.05-0.5mol/L, and fully stirred to obtain a solution containing phosphoric acid, ferrous sulfate and ascorbic acid;

4) According to the amount of phosphoric acid weighed in step 3), the molar ratio of Li and P is 3:1, measure and weigh lithium hydroxide, and dissolve lithium hydroxide in the mixed solvent of ethylene glycol and water in step 1) In, make the concentration of lithium hydroxide be 0.15～1.5mol/L, fully stir, obtain lithium hydroxide solution;

5) While stirring, add the lithium hydroxide solution in step 4) dropwise to the solution containing phosphoric acid, ferrous sulfate and ascorbic acid prepared in step 3) to form a suspension containing precipitates;

6) Under stirring, add polyethylene glycol to the suspension in step 5), the mass percentage of polyethylene glycol and the prepared target lithium iron phosphate is 10~100%; then transfer it to a high-pressure reactor, and use Step 1) Adjust the prepared mixed solvent so that its volume accounts for 2/3~4/5 of the volume of the inner tank of the reactor, continue to stir for at least 5 minutes, seal it, and keep it warm at 160~240°C for 4~36 hours, then drop After reaching room temperature, the reaction product was taken out, filtered, washed with deionized water, absolute ethanol or acetone in sequence, and dried at a temperature of 40°C to 100°C to obtain lithium iron phosphate single crystal nanorods.

In the present invention, the purity of said phosphoric acid, ferrous sulfate hexahydrate, lithium hydroxide, ascorbic acid, polyethylene glycol, ethylene glycol, dehydrated alcohol and acetone is not less than chemically pure.

The radial size of the lithium iron phosphate single crystal nanorod prepared by the method of the invention is 50-200 nanometers, and the length is 0.5-10 microns.

the

The present invention uses the mixed solvent of ethylene glycol and water as the reaction solvent, and by designing the volume ratio of ethylene glycol and water in the mixed solvent, combined with the surface modification effect of polyethylene glycol, regulates the nucleation and growth process in the heat treatment process, and realizes lithium iron phosphate Solvothermal Synthesis of Single Crystal Nanorods. The purpose of dehydration with absolute ethanol and acetone and drying at no higher than 100 ^o C is to obtain lithium iron phosphate single crystal nanorods with good dispersion.

The product prepared by the method of the invention has stable quality, high purity and good particle dispersibility, is beneficial to the diffusion of lithium ions, and improves the high-current charging and discharging performance of the lithium ion battery. The preparation process of the invention is simple, easy to control, non-polluting, low in cost and easy for large-scale production.

Description of drawings

Fig. 1 X-ray diffraction (XRD) pattern of lithium iron phosphate single crystal nanorods synthesized by the present invention;

Figure 2 Scanning electron microscope (SEM) photo of lithium iron phosphate single crystal nanorods synthesized by the present invention;

Figure 3 High-resolution transmission electron microscope (HRTEM) photo of lithium iron phosphate single crystal nanorods of the present invention;

Figure 4. The charge and discharge curves of lithium iron phosphate single crystal nanorods synthesized in the present invention as positive electrode materials after carbon-coating heat treatment.

Detailed ways

Below in conjunction with embodiment further illustrate the present invention.

Example 1

The preparation method comprises the following steps:

1) Measure 20ml of ethylene glycol and 20ml of deionized water, and mix them to prepare a mixed solvent of ethylene glycol and water.

2) Measure and weigh 0.1g of ascorbic acid and dissolve it in the mixed solvent of ethylene glycol and water prepared in step 1), and stir for 30 minutes to fully dissolve it.

3) According to the molar ratio of P and Fe being 1:1, measure and weigh 0.4612g of phosphoric acid and 1.1121g of ferrous sulfate hexahydrate, dissolve them in the ascorbic acid solution prepared in step 2), and stir for 10 minutes to obtain For the solution of iron and ascorbic acid, ascorbic acid accounts for 15.8% of the target lithium iron phosphate mass, the concentration of phosphoric acid is 0.1mol/L, and the concentration of ferrous sulfate hexahydrate is 0.1mol/L.

4) According to the amount of phosphoric acid taken in step 3), the molar ratio of Li and P is 3:1, and 0.5035g of lithium hydroxide is weighed, dissolved in the mixed solvent of ethylene glycol and water prepared in step 1) , stirred for 30 minutes to obtain a clear lithium hydroxide solution, the concentration of lithium hydroxide being 0.3mol/L.

5) Under stirring, add the lithium hydroxide solution prepared in step 4) dropwise to the solution containing phosphoric acid, ferrous sulfate and ascorbic acid prepared in step 3) to form a suspension containing precipitates. After mixing is complete, stir for 30 minutes.

6) Under stirring, add 0.0631g of polyethylene glycol to the suspension prepared in step 5), then transfer it to the high-pressure reactor, and adjust it with the mixed solvent prepared in step 1) so that its volume occupies the volume of the reactor 4/5 of the gallbladder volume, continue stirring for 10 minutes. Airtight, heat treatment at 200°C for 6 hours. Then, cool down to room temperature, take out the reaction product, filter, wash with deionized water and acetone in sequence, and dry at 80° C. to obtain lithium iron phosphate single crystal nanorods.

Its X-ray diffraction (XRD) pattern is shown in Fig. 1. It can be seen from the figure that the crystal phase is pure, free of impurities, and has a complete crystal structure.

The microscopic morphology is single, and the structural size is relatively concentrated. It is a single crystal nanorod with a radial size of 50-100 nanometers and a length of 0.5-5 microns (see Figure 2 and Figure 3).

Electrochemical performance of lithium iron phosphate single crystal nanorods for lithium ion batteries:

Pour the lithium iron phosphate single crystal nanorods prepared in this example into 20ml of deionized water dissolved with 0.15g of ascorbic acid, mix and stir for 30 minutes, dry in an oven at 80°C for 12 hours, take it out for annealing treatment, and sinter under nitrogen protection at 600°C After 6 hours, the lithium iron phosphate cathode material powder was obtained after grinding. The lithium iron phosphate cathode material powder, acetylene black and polyvinylidene fluoride are mixed according to the mass ratio (75:15:10) and fully stirred to disperse evenly. Spread it on aluminum foil, dry it in a vacuum oven at 100°C for 12 hours, and then cut it into a positive electrode sheet. Use metal lithium sheet as the negative electrode, cel-gard 2000 microporous membrane as the diaphragm, 1 mol/L LiPF (99.9%, the solvent is a mixture of ethylene carbonate and dimethyl carbonate with a volume ratio of 1:1) as the electrolyte , in a glove box with a water content less than 0.1×10 ^-6 , put the positive electrode sheet, diaphragm, negative electrode sheet, foam nickel and negative electrode cover from bottom to top into the positive electrode shell and press them to make a button battery, and carry out Charge and discharge test. The lithium-ion battery assembled with the synthesized lithium iron phosphate single crystal nanorods carbon-coated and heat-treated as the positive electrode material has good cycle stability, long life, and excellent rate performance. It is charged at a current of 170mA/g (current density=1C). In the discharge test, an excellent capacity of 141 mAh/g was obtained. Fig. 4 is the charge-discharge curve of lithium iron phosphate single crystal nanorods synthesized in the present invention after carbon-coated heat treatment as the positive electrode material assembly of the lithium ion battery.

Example 2

1) Measure 40ml of ethylene glycol and 40ml of deionized water, and mix them to prepare a mixed solvent of ethylene glycol and water.

2) Measure and weigh 0.063g of ascorbic acid and dissolve it in the mixed solvent of ethylene glycol and water prepared in step 1), and stir for 30 minutes to fully dissolve it.

3) According to the molar ratio of P and Fe being 1:1, measure and weigh 0.4612g of phosphoric acid and 1.1121g of ferrous sulfate hexahydrate, dissolve them in the ascorbic acid solution prepared in step 2), and stir for 10 minutes to obtain For the solution of iron and ascorbic acid, ascorbic acid accounts for 10% of the target lithium iron phosphate mass, the concentration of phosphoric acid is 0.05mol/L, and the concentration of hexahydrate ferrous sulfate is 0.05mol/L.

4) According to the amount of phosphoric acid taken in step 3), the molar ratio of Li and P is 3:1, and 0.5035g of lithium hydroxide is weighed, dissolved in the mixed solvent of ethylene glycol and water prepared in step 1) , stirred for 30 minutes to obtain a clear lithium hydroxide solution, the concentration of lithium hydroxide being 0.15mol/L.

5) Under stirring, add the lithium hydroxide solution prepared in step 4) dropwise to the solution containing phosphoric acid, ferrous sulfate and ascorbic acid prepared in step 3) to form a suspension containing precipitates. After mixing is complete, stir for 30 minutes.

6) Under stirring, add 0.316g of polyethylene glycol to the suspension prepared in step 5), then transfer it to the high-pressure reactor, and adjust it with the mixed solvent prepared in step 1) so that its volume occupies the volume of the reactor 4/5 of the gallbladder volume, continue stirring for 10 minutes. Airtight, heat treatment at 240°C for 6 hours. Then, cool down to room temperature, take out the reaction product, filter, wash with deionized water and acetone in sequence, and dry at 80° C. to obtain lithium iron phosphate single crystal nanorods. 100-200 nm in radial dimension and 1-5 µm in length

Example 3

1) Measure 20ml of ethylene glycol and 20ml of deionized water, and mix them to prepare a mixed solvent of ethylene glycol and water.

2) Measure and weigh 0.189g of ascorbic acid and dissolve it in the mixed solvent of ethylene glycol and water prepared in step 1), and stir for 30 minutes to fully dissolve it.

3) According to the molar ratio of P and Fe being 1:1, measure and weigh 0.4612g of phosphoric acid and 1.1121g of ferrous sulfate hexahydrate, dissolve them in the ascorbic acid solution prepared in step 2), and stir for 10 minutes to obtain A solution of iron and ascorbic acid, ascorbic acid accounts for 30% of the target lithium iron phosphate mass, the concentration of phosphoric acid is 0.1mol/L, and the concentration of hexahydrate ferrous sulfate is 0.1mol/L.

4) According to the amount of phosphoric acid taken in step 3), the molar ratio of Li and P is 3:1, and 0.5035g of lithium hydroxide is weighed, dissolved in the mixed solvent of ethylene glycol and water prepared in step 1) , stirred for 30 minutes to obtain a clear lithium hydroxide solution, the concentration of lithium hydroxide being 0.3mol/L.

5) Under stirring, add the lithium hydroxide solution prepared in step 4) dropwise to the solution containing phosphoric acid, ferrous sulfate and ascorbic acid prepared in step 3) to form a suspension containing precipitates. After mixing is complete, stir for 30 minutes.

6) Under stirring, add 0.631g of polyethylene glycol to the suspension prepared in step 5), then transfer it to the high-pressure reactor, and adjust it with the mixed solvent prepared in step 1) so that its volume occupies the volume of the reactor 4/5 of the gallbladder volume, continue stirring for 10 minutes. Airtight, heat treatment at 160°C for 36 hours. Then, cool down to room temperature, take out the reaction product, filter, wash with deionized water and acetone in turn, and dry at 80°C to obtain lithium iron phosphate single crystal nanorods with a radial size of 100-200 nanometers and a length of 5- 10 microns.

Example 4

1) Measure 20ml of ethylene glycol and 20ml of deionized water, and mix the two to prepare a mixed solvent of ethylene glycol and water.

2) Measure and weigh 0.5g of ascorbic acid and dissolve it in the mixed solvent of ethylene glycol and water prepared in step 1), and stir for 30 minutes to fully dissolve it.

3) According to the molar ratio of P and Fe being 1:1, measure and weigh 2.306g of phosphoric acid and 5.5605g of ferrous sulfate hexahydrate, dissolve them in the ascorbic acid solution prepared in step 2), and stir for 10 minutes to obtain For the solution of iron and ascorbic acid, ascorbic acid accounts for 15.8% of the target lithium iron phosphate mass, the concentration of phosphoric acid is 0.5mol/L, and the concentration of ferrous sulfate hexahydrate is 0.5mol/L.

4) According to the amount of phosphoric acid taken in step 3), the molar ratio of Li and P is 3:1, and 2.5175g of lithium hydroxide is weighed, dissolved in the mixed solvent of ethylene glycol and water prepared in step 1) , stirred for 30 minutes to obtain a clear lithium hydroxide solution, the concentration of lithium hydroxide being 1.5mol/L.

5) Under stirring, add the lithium hydroxide solution prepared in step 4) dropwise to the solution containing phosphoric acid, ferrous sulfate and ascorbic acid prepared in step 3) to form a suspension containing precipitates. After mixing is complete, stir for 30 minutes.

6) Under stirring, add 0.0631g of polyethylene glycol to the suspension prepared in step 5), then transfer it to the high-pressure reactor, and adjust it with the mixed solvent prepared in step 1) so that its volume occupies the volume of the reactor 2/3 of the gallbladder volume, continue stirring for 10 minutes. Airtight, heat treatment at 200°C for 24 hours. Then, cool down to room temperature, take out the reaction product, filter, wash with deionized water and acetone in turn, and dry at 80°C to obtain lithium iron phosphate single crystal nanorods with a radial size of 50-100 nanometers and a length of 2- 8 microns.

Claims (3)

1. a preparation method of lithium iron phosphate single crystal nano rod, it is characterized in that comprising the following steps: 1) Mix ethylene glycol and deionized water at a volume ratio of 3:1~1:3 to obtain a mixed solvent of ethylene glycol and water; 2) dissolving ascorbic acid in the mixed solvent of ethylene glycol and water in step 1), and stirring until fully dissolved to obtain ascorbic acid solution; 3) According to the molar ratio of P and Fe being 1:1, measure and weigh phosphoric acid and ferrous sulfate hexahydrate, and dissolve phosphoric acid and ferrous sulfate hexahydrate in the ascorbic acid solution prepared in step 2). The target mass percentage of lithium iron phosphate prepared is 10-30%, the concentrations of phosphoric acid and hexahydrate ferrous sulfate are both 0.05-0.5mol/L, and fully stirred to obtain a solution containing phosphoric acid, ferrous sulfate and ascorbic acid; 4) According to the amount of phosphoric acid weighed in step 3), the molar ratio of Li and P is 3:1, measure and weigh lithium hydroxide, and dissolve lithium hydroxide in the mixed solvent of ethylene glycol and water in step 1) In, make the concentration of lithium hydroxide be 0.15～1.5mol/L, fully stir, obtain lithium hydroxide solution; 5) Under stirring, add the lithium hydroxide solution in step 4) dropwise to the solution containing phosphoric acid, ferrous sulfate and ascorbic acid prepared in step 3) to form a suspension containing precipitates; 6) Under stirring, add polyethylene glycol to the suspension in step 5), the mass percentage of polyethylene glycol and the prepared target lithium iron phosphate is 10~100%; then transfer it to a high-pressure reactor, and use Step 1) Adjust the prepared mixed solvent so that its volume accounts for 2/3~4/5 of the volume of the inner tank of the reactor, continue to stir for at least 5 minutes, seal it, and keep it warm at 160~240°C for 4~36 hours, then drop After reaching room temperature, the reaction product was taken out, filtered, washed with deionized water, absolute ethanol or acetone in sequence, and dried at a temperature of 40°C to 100°C to obtain lithium iron phosphate single crystal nanorods. 2. the preparation method of lithium iron phosphate single crystal nanorod according to claim 1 is characterized in that said phosphoric acid, ferrous sulfate hexahydrate, lithium hydroxide, ascorbic acid, polyethylene glycol, ethylene glycol, anhydrous The purity of ethanol and acetone is not less than chemically pure. 3. The preparation method of lithium iron phosphate single crystal nanorod according to claim 1, characterized in that the prepared lithium iron phosphate single crystal nanorod has a radial dimension of 50-200 nanometers and a length of 0.5-10 microns.

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