CN102299334A - Carbon coated LiFePO4 porous anode and preparation method thereof - Google Patents

Abstract

The invention relates to a carbon-coated _LiFePO4 porous positive electrode and a preparation method thereof. The carbon-coated LiFePO ₄ porous positive electrode is obtained by performing high-temperature heat treatment on the traditional LiFePO ₄ positive electrode to carbonize the gelatin. LiFePO ₄ and acetylene black are evenly distributed, and the pole piece has a porous structure. Gelatin-based carbon is evenly distributed in LiFePO ₄ , acetylene black and Between the current collectors, the structure is stable. It is used in the positive electrode of lithium-ion batteries, which is conducive to the infiltration of electrolyte, improves the conductivity of the pole piece, and then improves the electrochemical performance of lithium-ion batteries, especially the rate performance. The preparation method is simple and the cost is low.

Description

A carbon-coated LiFePO₄ porous positive electrode and its preparation method

Technical field:

The invention relates to a carbon-coated _LiFePO4 porous positive electrode and a preparation method thereof, belonging to the technical field of lithium ion batteries.

Background technique:

Lithium-ion batteries have the advantages of large specific energy, high working voltage, no memory effect and environmental friendliness. They are not only widely used in small electrical appliances such as mobile phones, cameras, and notebooks, but also in large electric vehicles, satellites, and fighter jets Apps in devices are also popular. In lithium-ion batteries, since the specific capacity and cycle performance of the carbon negative electrode material can reach a high level (300mAh/g), but the specific capacity of the positive electrode material is low (130mAh/g), and it needs an additional burden on the irreversible energy of the negative electrode. Therefore, the research and improvement of cathode materials has always been a key issue in the research of lithium-ion batteries. LiFePO ₄ is currently one of the main positive electrode materials for lithium-ion batteries due to ^its high safety, good cycle performance, and environmental ^friendliness . 10 ^-14 ～10 ^-16 cm ² s ^-1 ), resulting in poor rate performance is one of the main reasons that limit its industrial application, especially in large electric equipment.

In recent years, in order to improve the rate performance of lithium iron phosphate, researchers have made great efforts, such as reducing the particle size, carbon coating or preparing LiFePO ₄ /C composite materials. CN101913588A, CN101393982, CN101777648A, CN101428781, US7344659, etc. reduce the particle size of LiFePO ₄ powder and synthesize nano-scale LiFePO ₄ , which greatly improves the rate performance of LiFePO ₄ . As the size of the material decreases, the specific surface area generally increases. This can reduce the current density of the electrode and reduce the polarization of the electrode; at the same time, it can provide more channels for the migration of lithium ions, shorten the migration path, and reduce the diffusion resistance. However, in practical applications, reducing the particle size cannot compensate for its negative effects, such as reduced cycle performance, reduced tap density, and increased production costs. CN101800311A, CN101339991, CN101567441, US11993925, US11267107, US12105895, etc. synthesize carbon-coated _LiFePO by adding different carbon sources in the synthesis, which can not only improve the electrical conductivity, but also control the grain growth during the _LiFePO synthesis process, making the particle size smaller small, thereby improving its rate performance. However, the method of doping the conductive agent and carbon coating not only has a limited degree of increasing the rate, but also leads to a decrease in the tap density of LiFePO ₄ and greatly increases the processing cost.

Invention content:

The purpose of the present invention is to propose a carbon-coated LiFePO ₄ porous positive electrode and its preparation method. The carbon-coated LiFePO 4 porous positive electrode is obtained by performing high-temperature heat treatment on the traditional LiFePO ₄ positive electrode to carbonize the gelatin in place, and the _distribution of LiFePO ₄ and acetylene black Uniform, the pole piece has a porous structure, and the gelatin-based carbon is evenly distributed between LiFePO ₄ and acetylene black and the current collector, and the structure is stable. The positive electrode of the lithium-ion battery is conducive to the infiltration of the electrolyte, improves the electrical conductivity of the pole piece, and then improves the electrochemical performance of the lithium-ion battery, especially the rate performance.

The preparation method of a carbon-coated _LiFePO4 porous positive electrode provided by the present invention is to carry out high-temperature heat treatment to the traditional _LiFePO4 positive electrode (including current collector, _LiFePO4 particle, conductive agent and gelatin binder) with gelatin as binder A method for in situ carbonization of gelatin yields a carbon-coated _LiFePO porous cathode. The specific steps and conditions are:

Gradually raise the temperature of the _LiFePO4 positive pole piece with gelatin as the binder to 200-350°C under the protection of nitrogen, and perform pre-carbonization for 1.5-3 hours; then gradually raise the temperature to 400-600°C, and keep it for 1-3 hours for carbonization; Then it was naturally cooled to room temperature under the protection of nitrogen to obtain a carbon-coated _LiFePO porous cathode.

In the above method of the present invention, the optimum temperature for pre-carbonization is 300°C.

In the above method of the present invention, the gradual temperature increase rate is 2-10° C./min.

The preparation method of the above-mentioned LiFePO ₄ positive electrode sheet using gelatin as a binding agent of the present invention is a prior art. LiFePO ₄ , acetylene black and gelatin solution (mass concentration 1 to 20%) are mixed with LiFePO ₄ , acetylene black and gelatin at 70 ～85:0～9:6～20 mass ratio mixing, in which acetylene black is an optional component, after wet grinding, the mixed viscous material is coated on the current collector aluminum foil, and vacuum dried at 60～120℃ 2 ~ 24h to get.

The present invention has the advantages that the selected binder is gelatin, which is a natural and environmentally friendly water-soluble biomacromolecule with low price, wide sources, good cohesiveness and dispersibility, and enables the active material and acetylene black to be evenly distributed , under the protection of nitrogen, the pole piece is subjected to high-temperature heat treatment, and the insulating gelatin is converted into gelatin-based carbon with certain conductivity, which is evenly distributed between LiFePO ₄ and acetylene black and the current collector, which is conducive to the transportation of electrons and reduces the impedance of the pole piece. . At the same time, due to the decomposition and shrinkage of gelatin, a porous structure is formed, which increases the contact area between the electrolyte and the active material, which is conducive to the infiltration of the electrolyte, thereby facilitating the transportation of lithium ions. The invention controls and optimizes the distribution of LiFePO ₄ and acetylene black through the gelatin binder, achieves the effects of pore formation and electrical conductivity improvement through the carbonization of the gelatin binder, and has the advantages of not changing the tap density, simple process, and low cost.

The effect of the present invention: the carbon-coated LiFePO ₄ porous positive electrode prepared by the present invention is used in lithium-ion batteries, which improves the electron and ion conductivity of the pole piece. The test results show that the electrochemical performance is greatly improved, especially the rate performance. At the same time, the preparation method of the present invention is simple, the cost is low, and it is suitable for industrial production, and opens up a new way for improving the rate performance of the LiFePO ₄ positive electrode.

Description of drawings:

Fig. 1 is the scanning electron microscope picture of comparative example 1 gained pole piece

Fig. 2 is the scanning electron micrograph of embodiment 1 gained pole piece

Fig. 3 is the discharge curve of the pole piece obtained in embodiment 1 at 0.5C and 5C rate

Fig. 4 is the bending photo of the pole piece obtained in embodiment 2

Detailed ways:

The present invention will be further described below through examples, but the protection scope of the present invention is not limited to the examples listed.

Comparative example 1:

LiFePO ₄ and acetylene black were pre-dried at 120°C under vacuum for 6 hours, gelatin was dissolved in deionized water at 60°C to prepare a 2wt.% gelatin solution, and LiFePO ₄ , acetylene black, and gelatin were mixed in a mass ratio of 85:9:6, After wet milling for 1 hour, the mixed viscous material was coated on an aluminum foil, and vacuum-dried at 60° C. for 12 hours to obtain a pole piece. Its SEM image is shown in Figure 1. Due to the good cohesiveness and dispersion of gelatin, LiFePO ₄ and acetylene black are uniformly distributed.

Cut the pole piece into a disc with a diameter of 12mm as the positive electrode, and use 1mol/L lithium hexafluorophosphate dissolved in ethylene carbonate/diethyl carbonate (volume ratio 1:1) as the electrolyte, in a glove box under the protection of argon, The lithium sheet was used as the negative electrode to assemble it into a CR2025 button battery and the electrochemical performance test was carried out. The test results show that the specific discharge capacity at 0.5C is 130mAh/g. The discharge specific capacity at 5C is 90mAh/g.

Comparative example 2:

LiFePO ₄ and acetylene black were pre-dried at 120°C under vacuum for 6 hours, gelatin was dissolved in deionized water at 60°C to prepare a 3wt.% gelatin solution, and LiFePO ₄ , acetylene black, and gelatin were mixed in a mass ratio of 85:5:10, After wet milling for 1 hour, the mixed viscous material was coated on an aluminum foil, and vacuum-dried at 80° C. for 16 hours to obtain a pole piece. The battery was assembled by the method described in Comparative Example 1, and the electrochemical test results showed that the specific discharge capacity at 0.5C was 109.5mAh/g, and the specific discharge capacity at 5C was 78.4mAh/g.

Comparative example 3:

LiFePO ₄ and acetylene black were pre-dried at 120°C under vacuum for 6 hours, gelatin was dissolved in deionized water at 60°C to prepare a 4wt.% gelatin solution, and LiFePO ₄ , acetylene black, and gelatin were mixed in a mass ratio of 78:2:20, After wet milling for 1 hour, the mixed viscous material was coated on an aluminum foil, and vacuum-dried at 120° C. for 8 hours to obtain a pole piece. The battery was assembled by the method described in Comparative Example 1, and the electrochemical test results showed that the discharge specific capacity at 0.5C was 96.6mAh/g, and the discharge specific capacity at 5C was 29.3mAh/g.

Comparative example 4:

LiFePO ₄ was pre-dried under vacuum at 120°C for 6 hours, gelatin was dissolved in deionized water at 60°C to prepare a 10wt.% gelatin solution, LiFePO ₄ and gelatin were mixed at a mass ratio of 80:20, wet milled for 1 hour, and the mixed The viscous material was coated on an aluminum foil, and vacuum-dried at 60° C. for 24 hours to obtain a pole piece. The battery was assembled by the method described in Comparative Example 1, and the test results showed that the discharge specific capacity was 94.7mAh/g at 0.5C. The discharge specific capacity at 5C is 24.8mAh/g.

Example 1:

Put the electrode piece prepared in Comparative Example 1 in a tube furnace under the protection of nitrogen to raise the temperature to 300°C at a heating rate of 3°C/min and keep it for 1.5h for pre-carbonization; then raise the temperature at a heating rate of 5°C/min To 600 ℃, and keep 1h, carry out carbonization to obtain pole pieces. Its SEM image is shown in Figure 2. After carbonization, LiFePO ₄ and acetylene black are still uniformly distributed. Due to the decomposition and shrinkage of gelatin at high temperature, holes of different sizes are formed on the pole piece. These holes will facilitate the infiltration of electrolyte. , to increase the contact area between the electrolyte and the active material, which is conducive to the transportation of lithium ions.

The battery was assembled by the method described in Comparative Example 1, and the electrochemical test results showed that the discharge specific capacity was 145mAh/g at 0.5C, and 108mAh/g at 5C. The discharge platform at 0.5C and 5C is shown in Figure 3, the discharge platform is stable, and the discharge platform voltage can still reach 3.3V at 5C. The porous structure is conducive to the infiltration of the electrolyte and thus the transportation of lithium ions, ensuring the integrity of the pole piece. During the carbonization process, the gelatin used as a binder is converted into gelatin-based carbon, which greatly reduces LiFePO ₄ , acetylene black and current collectors. The impedance of the transport of electrons between them greatly improves the rate performance.

Example 2:

The pole piece prepared in Comparative Example 1 was placed in a tube furnace under the protection of nitrogen, and the temperature was raised to 330°C at a heating rate of 3°C/min and kept for 2h for pre-carbonization; then the temperature was raised to 330°C at a heating rate of 6°C/min 500°C, and keep it for 2 hours, and carbonize to obtain pole pieces. The macro-bending picture is shown in Figure 4. Under the bending of tweezers, the pole piece can maintain good strength and toughness without damage and slag dropping, which ensures the stability of the pole piece and avoids the loss of active materials. .

The battery was assembled by the method described in Comparative Example 1, and the electrochemical test results showed that the discharge specific capacity was 142mAh/g at 0.5C. The discharge specific capacity at 5C is 102mAh/g.

Example 3:

The pole piece prepared in Comparative Example 1 was placed in a tube furnace under the protection of nitrogen, and the temperature was raised to 350°C at a heating rate of 3°C/min and kept for 2h for pre-carbonization; then the temperature was raised to 350°C at a heating rate of 5°C/min 400°C, and kept for 3h, carbonized to obtain pole pieces.

The battery was assembled by the method described in Comparative Example 1, and the electrochemical test results showed that the discharge specific capacity was 146mAh/g at 0.5C. The discharge specific capacity at 5C is 99mAh/g.

Example 4:

Put the pole piece prepared in Comparative Example 2 in a tube furnace under the protection of nitrogen to raise the temperature to 300°C at a heating rate of 3°C/min and keep it for 1.5h for pre-carbonization; then raise the temperature at a heating rate of 4°C/min to 600°C, and kept for 1.5h, and carbonized to obtain pole pieces.

The battery was assembled by the method described in Comparative Example 1, and the electrochemical test results showed that the discharge specific capacity was 143.3mAh/g at 0.5C. The discharge specific capacity at 5C is 107.8mAh/g.

Example 5:

Put the pole piece prepared in Comparative Example 3 in a tube furnace under the protection of nitrogen to raise the temperature to 330°C at a heating rate of 3°C/min and keep it for 1.5h for pre-carbonization; then raise the temperature at a heating rate of 3°C/min To 600 ℃, and keep 1h, carry out carbonization to obtain pole pieces.

The battery was assembled by the method described in Comparative Example 1, and the electrochemical test results showed that the discharge specific capacity was 151.2mAh/g at 0.5C. The discharge specific capacity at 5C is 115.6mAh/g.

Embodiment 6:

Put the pole piece prepared in Comparative Example 4 in a tube furnace under the protection of nitrogen to raise the temperature to 330°C at a heating rate of 3°C/min and keep it for 1.5h for pre-carbonization; then raise the temperature at a heating rate of 8°C/min To 600 ℃, and keep 1h, carry out carbonization to obtain pole pieces.

The battery was assembled by the method described in Comparative Example 1, and the electrochemical test results showed that the discharge specific capacity was 149.1mAh/g at 0.5C. The discharge specific capacity at 5C is 110.6mAh/g.

Claims (4)

1. A carbon-coated _LiFePO4 porous positive pole and a preparation method thereof, by carrying out high-temperature heat treatment to the traditional _LiFePO4 positive pole with gelatin as a binder to carbonize the gelatin site to obtain a carbon-coated _LiFePO4 porous positive pole; specific steps and condition as: Gradually raise the temperature of the _LiFePO4 positive pole piece with gelatin as the binder to 200-350°C under the protection of nitrogen, and perform pre-carbonization for 1.5-3 hours; then gradually raise the temperature to 400-600°C, and keep it for 1-3 hours for carbonization; Then it was naturally cooled to room temperature under the protection of nitrogen to obtain a carbon-coated _LiFePO porous cathode. 2. The method for preparing a carbon-coated LiFePO ₄ porous positive electrode according to claim 1, characterized in that: the pre-carbonization temperature is 300°C. 3. The method for preparing a carbon-coated LiFePO ₄ porous positive electrode according to claim 1, characterized in that: the gradual temperature increase rate is 2-10°C/min. 4. The method for preparing the carbon-coated LiFePO ₄ porous positive electrode according to claim 1, 2 or 3, characterized in that: the LiFePO ₄ positive electrode sheet using gelatin as a binder is prepared by the following method: LiFePO ₄ , Acetylene black and gelatin solution with a mass concentration of 1-20% are mixed according to the mass ratio of LiFePO ₄ , acetylene black, and gelatin 70-85:0-9:6-20, wherein acetylene black is an optional component. After wet grinding, the The mixed viscous material is coated on the current collector aluminum foil, and vacuum-dried at 60-120° C. for 2-24 hours.

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