CN108321390B - Three-dimensional flower-shaped single crystal lithium iron phosphate and preparation method thereof - Google Patents

Abstract

The invention discloses a three-dimensional flower-shaped single crystal lithium iron phosphate and a preparation method thereof. The microscopic appearance is a three-dimensional flower surrounded by nano-flaky lithium iron phosphate with a thickness of 10-100 nm, a length of 500-4000 nm and a width of 200-1000 nm. structure, and the direction perpendicular to the thickness of each piece of lithium iron phosphate is [010]. It is prepared according to the following method: urea and sodium carbonate are dissolved in ethylene glycol, phosphorus source and iron source are added, and a hydrothermal reaction is carried out after mixing evenly. After the reaction is completed, the product is filtered, washed and dried, and then the obtained precursor and lithium The three-dimensional flower-shaped single crystal lithium iron phosphate is obtained by high temperature calcination under the protection of inert gas. The invention adopts a simple hydrothermal method and a calcination method to synthesize a three-dimensional flower-like lithium iron phosphate single crystal, and has the advantages of low energy consumption, wide applicability, simple steps, easy control, easy repetition and amplification, and the like.

Description

Three-dimensional flower-like single crystal lithium iron phosphate and preparation method thereof

technical field

The invention belongs to the field of electrochemistry, and specifically relates to a three-dimensional flower-shaped single crystal lithium iron phosphate and a preparation method thereof.

Background technique

Lithium-ion battery is a common energy storage device due to its high energy density, long service life, low self-discharge rate, wide operating temperature range, no memory effect, and environmental friendliness. Applications in large-scale power supply, aviation, aerospace, national defense, energy storage and other fields have attracted much attention. Lithium-ion batteries are mainly composed of positive electrodes, negative electrodes, separators and electrolytes. Cathode materials and anode materials are important components of lithium-ion batteries, which have a decisive impact on the performance of lithium-ion batteries. Currently, the commonly used cathode materials in commerce are lithium cobalt oxide, lithium manganate and lithium iron phosphate. Because lithium iron phosphate has many advantages such as cheap, non-toxic, high theoretical specific capacity (170mAh g ^-1 ), high working voltage, good environmental compatibility, long cycle life, high temperature performance and safety performance, etc. It stands out from many cathode materials and is considered to be a new generation of lithium-ion battery cathode materials with great application potential, which can be widely used in the field of power systems.

LiFePO ₄ has an ordered and regular olivine structure, belonging to the orthorhombic system, because of its abundant raw material sources, low price, non-toxic, environmentally friendly, stable structure, high theoretical capacity (170mAh g ^-1 ), high And stable working voltage (3.4V vs. Li/Li ⁺ ), good thermal stability, excellent cycle performance, high safety advantages, is regarded as the most promising new-generation lithium-ion battery cathode material. The two phases of LiFePO ₄ and FePO ₄ are involved in the reaction of LiFePO ₄ during the charging and discharging process. The charging and discharging reactions are as follows:

Charge reaction: LiFePO ₄ -xLi ⁺ -xe ^- →xFePO ₄ +(1-x)LiFePO ₄

Discharge reaction: FePO ₄ +xLi ⁺ +xe ^- →(1-x)FePO ₄ +xLiFePO ₄

It can be seen from the charge-discharge reaction that during charging, lithium ions are deintercalated from the lattice of LiFePO ₄ to form FePO ₄ , and during discharge, lithium ions are inserted into the lattice of FePO ₄ to form LiFePO ₄ . Therefore, during the charging and discharging process, two phases of LiFePO ₄ and FePO ₄ coexist. Since FePO ₄ and LiFePO ₄ are similar in structure and volume, the volume of the material changes little during the process of lithium deintercalation, that is, the transformation of the two phases of LiFePO ₄ and FePO ₄ during the charge and discharge process does not have a serious impact on the electrochemical performance of the material. volume effect, which is also the reason for the excellent cycling stability of LiFePO ₄ .

However, lithium iron phosphate itself has the defects of low electronic conductivity and small diffusion coefficient of lithium ions, which affects its high-current charge-discharge performance and limits its large-scale application. In order to overcome these shortcomings, researchers have carried out a lot of research and proposed many feasible modification methods, mainly including material nanoization, coating method and doping method. Therefore, it is a research hotspot to improve the electrochemical properties such as the rate capability of the material by modifying the lithium iron phosphate cathode material.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned technical problems, the first object of the present invention is to provide a kind of three-dimensional flower-shaped single crystal lithium iron phosphate, and the second object of the present invention is to provide the preparation method of this kind of lithium iron phosphate.

In order to achieve the above-mentioned first purpose, the technical solution of the present invention is as follows: a three-dimensional flower-shaped single crystal lithium iron phosphate, characterized in that: the microscopic morphology is composed of nanometers with a thickness of 10-100 nm, a length of 500-4000 nm, and a width of 200-1000 nm. The three-dimensional flower-like structure surrounded by sheet-like lithium iron phosphate, and the direction perpendicular to the thickness of each sheet of lithium iron phosphate is [010].

The second object of the present invention is achieved in this way: a preparation method of the three-dimensional flower-shaped single crystal lithium iron phosphate, characterized in that, prepared according to the following method: urea and sodium carbonate are dissolved in ethylene glycol, and phosphorus source is dissolved in ethylene glycol. , the iron source is added to the ethylene glycol solution, and the hydrothermal reaction is carried out after mixing evenly. After the reaction is completed, the product is filtered, washed, and dried, and then the obtained precursor is mixed with the lithium source, and is obtained by calcining at high temperature under the protection of inert gas. Three-dimensional flower-like single crystal lithium iron phosphate.

The diffusion of lithium ions in LiFePO ₄ is the most important and decisive control step in the charge-discharge process. Firstly, the lithium ion diffusion rate of LiFePO ₄ material is improved by the method of material nanometerization. Since LiFePO has one _- dimensional lithium ion diffusion channels ([010] channels), we prepared three-dimensional flower-like single-crystal lithium iron phosphate with [010] crystal orientation based on the principle of low lattice mismatch, namely single-crystal nanosieves It has a nano-scale thickness in the [010] direction, thereby effectively shortening the lithium ion diffusion path and improving the lithium ion diffusion rate. At the same time, the three-dimensional flower-like structure has a large specific surface area and a porous structure, which not only increases the contact area between the material and the electrolyte, but also promotes the penetration and infiltration of the electrolyte. In addition, the 3D flower-like structure can prevent the 2D LiFePO4 nanosheets from sticking and fully expose the 2D LiFePO4 nanosheets to the electrolyte during cycling, thereby exhibiting excellent cycling stability and excellent rate capability. . The experimental results show that at the rate of 1C, after 4000 charge-discharge cycles, the specific capacity of the three-dimensional flower-like single crystal lithium iron phosphate is stable at 164mAh g ^-1 , and the capacity retention rate is as high as 99%.

In the above scheme: the iron source is selected from Fe(NO ₃ ) ₃ , and the phosphorus source is selected from at least one of Na ₃ PO ₄ , Na ₂ HPO ₄ , and NaH ₂ PO ₄ .

In the above scheme: the lithium source is selected from at least one of Li ₃ PO ₄ , LiCO ₃ , Li ₂ C ₂ O ₄ , and LiOH.

In the above scheme: the molar ratio of urea and sodium carbonate is 1～3:1.

In the above scheme: the temperature of the hydrothermal reaction is 150-220° C., and the time is 12-24 h.

In the above scheme: the temperature of high temperature calcination is 600～760℃, and the time is 1～5h.

Beneficial effects: The three-dimensional flower-like single crystal lithium iron phosphate prepared by the present invention with exposed specific crystal planes (010) has regular morphology and uniform size. As a negative electrode material for lithium ion batteries, the unique three-dimensional flower-like structure can shorten the charging and discharging process. The transport distance of lithium ions can also prevent material agglomeration, thereby improving the electrochemical performance of electrode materials and enhancing cycling stability. The invention adopts a simple hydrothermal method and a calcination method to synthesize a three-dimensional flower-like lithium iron phosphate single crystal, and has the advantages of low energy consumption, wide applicability, simple steps, easy control, easy repetition and amplification, and the like.

Instruction drawings

Figure 1 is an XRD analysis chart, all X-ray powder diffraction peaks can be indexed to be lithium iron phosphate crystals.

FIG. 2 is an FE-SEM photograph of Example 1. FIG.

Figure 3 is a high-power transmission of a three-dimensional flower-like lithium iron phosphate single crystal.

FIG. 4 is a rate performance diagram of the three-dimensional flower-shaped lithium iron phosphate single crystal of Example 1. FIG.

Detailed ways

The present invention will be further elaborated below in conjunction with the accompanying drawings and specific embodiments:

Example 1

Dissolve urea and sodium carbonate in 30ml ethylene glycol solution, add ferric nitrate and sodium dihydrogen phosphate, dissolve in ethylene glycol solution, mix and stir, the concentration of urea in the obtained mixed solution is 0.10mol/L, and carbonic acid The concentration of sodium is 0.10 mol/L, the concentration of ferric nitrate is 0.15 mol/L, and the concentration of sodium dihydrogen phosphate is 0.12 mol/L. The mixed solution was transferred into the reaction kettle, the reaction kettle was sealed, and the hydrothermal reaction was carried out in a blast drying oven at 150 °C for a reaction time of 24 hours.

The obtained precursor and lithium phosphate (2mmol lithium phosphate) were mixed uniformly and then calcined in a tube furnace under an argon atmosphere. The calcination temperature was 700°C and the calcination time was 2h.

Example 2

Dissolve urea and sodium carbonate in 30ml ethylene glycol solution, add ferric nitrate and sodium monohydrogen phosphate, dissolve in ethylene glycol solution, mix and stir, the concentration of urea in the obtained mixed solution is 0.10mol/L, and the carbonic acid The concentration of sodium is 0.30 mol/L, the concentration of ferric nitrate is 0.15 mol/L, and the concentration of sodium monohydrogen phosphate is 0.12 mol/L. The mixed solution was transferred into the reaction kettle, the reaction kettle was sealed, and the hydrothermal reaction was carried out in a blast drying oven at 220°C for 12 hours. The precipitate obtained by the reaction was washed by centrifugation, and then dried to obtain the precursor.

The obtained precursor and lithium carbonate (3 mmol lithium carbonate) were mixed uniformly and then calcined in a tube furnace under an argon atmosphere. The calcination temperature was 760 °C and the calcination time was 1 h.

Example 3

(1) urea, sodium carbonate are dissolved in 30ml ethylene glycol solution, add ferric nitrate, sodium phosphate, and be dissolved in ethylene glycol solution, mix, stir, the concentration of urea in the gained mixed solution is 0.10mol/L, The concentration of sodium carbonate is 0.20 mol/L, the concentration of ferric nitrate is 0.15 mol/L, and the concentration of sodium phosphate is 0.12 mol/L. The mixed solution was transferred into the reaction kettle, the reaction kettle was sealed, and the hydrothermal reaction was carried out in a blast drying oven at 180°C for 18 hours. The precipitate obtained by the reaction was washed by centrifugation, and then dried to obtain the precursor.

The obtained precursor and lithium oxalate were mixed uniformly and then calcined in a tube furnace under an argon atmosphere. The calcination temperature was 600 °C and the calcination time was 5 h. The product was three-dimensional flower-shaped single crystal lithium iron phosphate.

Example 4

(1) urea, sodium carbonate are dissolved in 30ml ethylene glycol solution, add ferric nitrate, sodium dihydrogen phosphate, and be dissolved in ethylene glycol solution, mix, stir, the concentration of urea in gained mixed solution is 0.10mol/ L, the concentration of sodium carbonate is 0.15mol/L, the concentration of ferric nitrate is 0.15mol/L, and the concentration of sodium dihydrogen phosphate is 0.12mol/L. The mixed solution was transferred to the reaction kettle, the reaction kettle was sealed, and the hydrothermal reaction was carried out in a blast drying oven at 180°C for a reaction time of 18h.

The obtained precursor and lithium hydroxide (5mmol lithium hydroxide) were mixed uniformly and then calcined in a tube furnace under an argon atmosphere. The calcination temperature was 720°C and the calcination time was 1.5h. The product was three-dimensional flower-shaped single crystal phosphoric acid. Lithium iron.

The three-dimensional flower-shaped lithium iron phosphate single crystal prepared in Example 1-4 was detected, and Figure 1-4 was obtained:

1 is an XRD analysis diagram, and all X-ray powder diffraction peaks can be indexed to be lithium iron phosphate crystals, indicating that the synthesis of Examples 1-4 is high-purity lithium iron phosphate nanomaterials.

Fig. 2 is the FE-SEM photograph of embodiment 1, can see from the photograph that three-dimensional flower-shaped lithium iron phosphate can be prepared on a large scale.

Figure 3 shows the high-power transmission of the three-dimensional flower-like lithium iron phosphate single crystal. It can be seen from Figure 3 that the lattice of the three-dimensional flower-like lithium iron phosphate is continuous, which proves that the three-dimensional flower-like lithium iron phosphate is a single crystal structure.

Figure 4 is a rate performance diagram of the three-dimensional flower-shaped lithium iron phosphate single crystal of Example 1. It can be seen from the figure that the Coulombic efficiency of the three-dimensional flower-shaped lithium iron phosphate single crystal can still maintain more than 97% after 6000 cycles.

Table 1 shows the discharge capacity and cycle performance of the three-dimensional flower-shaped lithium iron phosphate single crystal obtained in Example 1-4 and the lithium battery cathode material of the comparative material.

Table I

As can be seen from Table 1, the performance of the three-dimensional flower-shaped single crystal lithium iron phosphate prepared by the present invention is obviously better than that of the sheet-shaped single crystal lithium iron phosphate and lithium iron phosphate nanoparticles.

The present invention is not limited to the above-mentioned specific embodiments, and it should be understood that those skilled in the art can make many modifications and changes according to the concept of the present invention without creative labor. In a word, all technical solutions that those skilled in the art can obtain through logical analysis, reasoning or limited experiments on the basis of the prior art according to the conception of the present invention, all should be within the protection scope determined by the claims.

Claims (4)

1. A three-dimensional flower-like single crystal lithium iron phosphate is characterized in that: the microcosmic appearance is a three-dimensional flower-shaped structure surrounded by nano flaky lithium iron phosphate with the thickness of 10-100 nm, the length of 500-4000 nm and the width of 200-1000 nm, and the direction of each piece of lithium iron phosphate perpendicular to the thickness is [010]](ii) a The three-dimensional flower-shaped single crystal lithium iron phosphate is prepared by the following method: dissolving urea and sodium carbonate in ethylene glycol, adding a phosphorus source and an iron source into an ethylene glycol solution, uniformly mixing, carrying out a solvothermal reaction, filtering, washing and drying a product after the reaction is finished, then mixing the obtained precursor with a lithium source, and carrying out high-temperature calcination under the protection of inert gas to obtain three-dimensional flower-shaped monocrystal lithium iron phosphate; the iron source is Fe (NO)₃)₃The phosphorus source is Na₃PO₄、Na₂HPO₄、NaH₂PO₄At least one of; the temperature of the solvothermal reaction is 150 ℃ and the time is 24 h.

2. The method for preparing the three-dimensional flower-like single crystal lithium iron phosphate according to claim 1, which is characterized in that: the lithium source is selected from Li₃PO₄、LiCO₃、Li₂C₂O₄And LiOH.

3. The method for preparing the three-dimensional flower-like single crystal lithium iron phosphate according to claim 2, which is characterized in that: the mol ratio of urea to sodium carbonate is 1-3: 1.

4. the method for preparing the three-dimensional flower-like single crystal lithium iron phosphate according to claim 3, which is characterized in that: the high-temperature calcination temperature is 600-760 ℃, and the time is 1-5 h.

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