CN101924196A - A method to greatly increase the reversible capacity of graphite - Google Patents

Abstract

The invention relates to a technology for greatly improving the electrochemical performance of graphite, in particular to a method for greatly improving the reversible capacity of graphite. After chemical vapor deposition of carbon on the surface of nano-silicon sphere powder, the carbon-coated silicon powder is mixed with graphite, as follows: First, the carbon layer is uniformly coated on the surface of nano-silicon sphere powder by chemical vapor deposition, and the carbon layer The weight accounts for 5-40%; then the nano-silicon sphere powder coated with carbon layer is mixed with graphite, wherein the nano-silicon sphere powder accounts for 5-20% by weight to make the lithium-ion battery negative electrode. The present invention can greatly increase the reversible capacity of graphite by adding a small amount of nano silicon sphere powder after carbon deposition, the first reversible capacity of lithium ion battery negative electrode is 50-300% higher than that of graphite, and maintain the high coulombic efficiency and long cycle life of graphite , which solves the problems of low reversible capacity of graphite, low coulombic efficiency and poor cycle life of pure silicon powder.

Description

A method to greatly increase the reversible capacity of graphite

technical field

The invention relates to a technology for greatly improving the electrochemical performance of graphite, specifically a method for greatly improving the reversible capacity of graphite. After chemical vapor deposition of carbon on the surface of nano-silicon sphere powder, the carbon-coated silicon powder is mixed with graphite. Increase the first reversible capacity of graphite by 50-200%, and maintain the high Coulombic efficiency and long cycle life of graphite.

Background technique

Energy is an important material basis for the development of human society, but the sharp decline in the stock of mineral energy such as coal, oil, and natural gas has caused human beings to face the pressure of resource depletion, and the problem of environmental pollution is also becoming more and more serious. Therefore, energy and environmental issues have become the focus of attention of countries all over the world. Improving energy efficiency, developing and utilizing renewable energy, protecting the ecological environment, and realizing sustainable development have become the common goals and topics of governments and researchers of all countries. Strategically speaking, the development of renewable energy is fundamental to solving energy problems, so research in this area has received widespread attention. Lithium-ion batteries are an important branch of renewable energy.

Lithium-ion secondary battery negative electrode graphite material is cheap and has high Coulombic efficiency and long cycle life; however, its theoretical reversible capacity is only 372mAh/g, while the best reported reversible capacity is 350mAh/g. The theoretical reversible capacity of silicon is 4200mAh/g, and the reported experimental value is 3700mAh/g, but its coulombic efficiency and cycle life are very poor, which hinders its practical application. (Document 1, Document 2, Ryu Jh, Kim JVV, Sung YE, Oh SM. Electrochem. Solid State Lett. 7: A306 (2004)) The main problem at present is: how to increase the price of graphite anode materials , greatly improving its reversible capacity while maintaining its good Coulombic efficiency and long cycle life.

Contents of the invention

The purpose of the present invention is to provide a method for greatly improving the reversible capacity of graphite negative electrode materials, and maintain high coulombic efficiency and long cycle life of graphite, and solve the problems of low reversible capacity of existing graphite, poor coulombic efficiency and cycle life of silicon, etc. .

Technical scheme of the present invention is:

A method for greatly improving the reversible capacity of graphite. First, a carbon layer is evenly coated on the surface of nano-silicon spherical powder by chemical vapor deposition, wherein the carbon layer accounts for 5-40% by weight (preferably in the range of 10-30%) ; Then mix the nano-silicon sphere powder coated with carbon layer with graphite, wherein the nano-silicon sphere powder accounts for 5-20% by weight (the preferred range is 10-15%), and make the negative electrode of lithium-ion battery; lithium-ion battery The initial reversible capacity of the negative electrode is 50-300% higher than that of graphite, and maintains the high Coulombic efficiency and long cycle life of graphite.

The process of the chemical vapor deposition method is as follows: the nano-silicon spherical powder (spherical silicon powder with an average diameter of 20-100 nanometers) is placed in a chemical vapor phase furnace, with argon or nitrogen as the carrier gas, and the carrier gas flow rate is 100 -500ml/min; under the protection of argon or nitrogen, raise the temperature in the chemical vapor phase furnace to 600-900°C at a rate of 2-20°C/min; then feed carbon source gas (methane, acetylene, ethylene or propylene) gaseous hydrocarbons, the volume concentration of the carbon source gas is 1-20%) for chemical vapor deposition, the chemical vapor deposition time is 10 minutes-3 hours, and the pressure is normal pressure.

The advantages of the present invention are:

1. The method of the present invention is simple, and the carbon can be evenly coated on the surface of the nano-silicon sphere powder by one-step chemical vapor deposition.

2. The present invention can accurately control the carbon content coated on the surface of the nano-silicon sphere powder by precisely controlling the conditions of the chemical vapor deposition.

3. The present invention coats a thin layer of carbon on the surface of nano-silicon spherical powder by precisely controlling the conditions of chemical vapor deposition. The weight percentage of carbon is precisely controllable between 5-40%. The conductivity can also limit the volume expansion of silicon when lithium ions are intercalated, so it is beneficial to improve the first-time efficiency and cycle stability of silicon-containing composite negative electrodes.

4. The present invention adds a small amount (5-20wt%) of nano-silicon sphere powder coated with a carbon layer, which can greatly increase the reversible capacity of graphite, which can increase the reversible capacity of graphite by 50-300%, and keep graphite relatively high. Good coulombic efficiency and long cycle life, the first coulombic efficiency is about 90%, the capacity retention rate of 20 cycles is greater than 80%, and the capacity retention rate of 40 cycles is about 60%.

Description of drawings

Figure 1. Scanning electron micrographs of nano silicon sphere powder coated with carbon layer.

Figure 2. The cycle curve of carbon nano-silicon sphere powder mixed with graphite to make negative electrode.

Detailed ways

The present invention is described in detail below by way of examples.

Example 1.

Put silicon sphere powder with an average diameter of 20 nm in a vertical reaction furnace, use argon as the carrier gas (flow rate of 300ml/min), raise the temperature to 700°C at a rate of 10°C/min, and feed acetylene gas, Chemical vapor deposition was carried out at this temperature, the deposition time was 15 minutes, and the volume concentration of acetylene was 5%. After the chemical vapor deposition is over, turn off the acetylene gas, and drop to room temperature under the protection of argon. Transmission electron microscopy and thermogravimetric analysis show that the carbon layer is evenly coated on the surface of the nano-silicon sphere powder. Under this condition, the carbon content of the coating is 10wt%. The ratio is uniformly mixed to make a lithium-ion battery negative electrode material. The scanning electron microscope photo is shown in Figure 1. The first reversible capacity of lithium-ion battery anode material is 600mAh/g, the coulombic efficiency is 90%, and the capacity retention rate after 40 cycles is 61%

Example 2.

Put silicon spherical powder with an average diameter of 60 nanometers in a vertical reaction furnace, use nitrogen as a carrier gas (flow rate of 300ml/min), raise the temperature to 700°C at a rate of 5°C/min, feed acetylene gas, and Chemical vapor deposition was carried out at this temperature, the deposition time was 45 minutes, and the volume concentration of acetylene was 5%. After the chemical vapor deposition is over, turn off the acetylene gas, and drop to room temperature under the protection of argon. Transmission electron microscopy and thermogravimetric analysis show that the carbon layer is uniformly coated on the surface of the nano-silicon sphere powder. Under this condition, the carbon content of the coating is 25wt%. Mix evenly to make lithium-ion battery negative electrode material, and its charge-discharge cycle curve is shown in Figure 2. The first reversible capacity of the lithium-ion battery anode material is 670mAh/g, the coulombic efficiency is 91%, and the capacity retention rate after 40 cycles is 62%.

Example 3.

Put silicon spherical powder with an average diameter of 50 nm in a vertical reaction furnace, use nitrogen as a carrier gas (flow rate of 300ml/min), raise the temperature to 800°C at a rate of 20°C/min, feed ethylene gas, and Chemical vapor deposition was carried out at this temperature, the deposition time was 30 minutes, and the volume concentration of ethylene was 2%. After the chemical vapor deposition is over, turn off the ethylene gas, and drop to room temperature under the protection of argon. Transmission electron microscopy and thermogravimetric analysis show that the carbon layer is uniformly coated on the surface of the nano-silicon sphere powder. Under this condition, the carbon content of the coating is 18wt%. Mix evenly to make lithium ion battery negative electrode material. The first reversible capacity of the lithium-ion battery anode material is 700mAh/g, the coulombic efficiency is 92%, and the capacity retention rate after 20 cycles is 81%.

Example 4.

Put silicon sphere powder with an average diameter of 50 nanometers in a vertical reaction furnace, use nitrogen as a carrier gas (flow rate of 300ml/min), raise the temperature to 800°C at a rate of 15°C/min, feed ethylene gas, and Chemical vapor deposition was carried out at this temperature, the deposition time was 1 hour, and the volume concentration of ethylene was 20%. After the chemical vapor deposition is over, turn off the ethylene gas, and drop to room temperature under the protection of argon. Transmission electron microscopy and thermogravimetric analysis show that the carbon layer is uniformly coated on the surface of the nano-silicon sphere powder. Under this condition, the carbon content of the coating is 30wt%. Mix evenly to make lithium ion battery negative electrode material. The first reversible capacity of the lithium-ion battery anode material is 1200mAh/g, the coulombic efficiency is 90.6%, and the capacity retention rate after 40 cycles is 58%.

Example 5

Put silicon sphere powder with an average diameter of 30 nm in a vertical reaction furnace, use argon as a carrier gas (flow rate of 200ml/min), raise the temperature to 600°C at a rate of 15°C/min, and feed acetylene gas, Chemical vapor deposition was carried out at this temperature, the deposition time was 30 minutes, and the volume concentration of acetylene was 10%. After the chemical vapor deposition is over, turn off the acetylene gas, and drop to room temperature under the protection of argon. Transmission electron microscopy and thermogravimetric analysis show that the carbon layer is uniformly coated on the surface of the nano-silicon sphere powder. Under this condition, the carbon content of the coating is 17wt%. Mix evenly to make lithium ion battery negative electrode material. The first reversible capacity of the lithium-ion battery anode material is 950mAh/g, the coulombic efficiency is 90.1%, and the capacity retention rate after 20 cycles is 80%.

Example 6

Put silicon sphere powder with an average diameter of 100 nanometers in a vertical reaction furnace, use argon as a carrier gas (flow rate of 400ml/min), raise the temperature to 900°C at a rate of 20°C/min, and feed propylene gas, Chemical vapor deposition was carried out at this temperature, the deposition time was 3 hours, and the volume concentration of propyne was 10%. After the chemical vapor deposition is over, turn off the propylene gas, and drop to room temperature under the protection of argon. Transmission electron microscopy and thermogravimetric analysis show that the carbon layer is uniformly coated on the surface of the nano-silicon sphere powder. Under this condition, the carbon content of the coating is 40wt%. Mix evenly to make lithium ion battery negative electrode material. The first reversible capacity of the lithium-ion battery anode material is 450mAh/g, the coulombic efficiency is 89.9%, and the capacity retention rate after 40 cycles is 60%.

The results of the examples show that the present invention can precisely control the amount of carbon and crystallinity deposited on the surface of the nano-silicon sphere powder by optimizing the heating rate, time, and carbon source concentration of the chemical vapor deposition. After the carbon nano-silicon sphere powder is mixed with graphite, it can The reversible capacity of graphite is greatly improved, and the high Coulombic efficiency and long cycle life of graphite are maintained.

Claims (6)

1. A method for greatly improving the reversible capacity of graphite, characterized in that: at first, uniformly coat the carbon layer on the surface of the nano-silicon spherical powder by chemical vapor deposition, wherein the carbon layer weight accounts for 5-40%; Nano-silicon sphere powder covered with carbon layer is mixed with graphite, and the weight of nano-silicon sphere powder accounts for 5-20% to make the negative electrode of lithium-ion battery; the first reversible capacity of the negative electrode of lithium-ion battery is 50-300% higher than that of graphite. And maintain the high coulombic efficiency and long cycle life of graphite. 2. according to the method for greatly improving graphite reversible capacity according to claim 1, it is characterized in that, the method for described chemical vapor deposition, the process is as follows: nano-silicon spherical powder is placed in chemical vapor phase furnace, with argon or nitrogen As carrier gas, the carrier gas flow rate is 100-500ml/min; under the protection of argon or nitrogen, the temperature in the chemical vapor phase furnace is raised to 600-900°C; then carbon source gas is introduced for chemical vapor deposition, and the chemical vapor deposition time is For 10 minutes-3 hours, thereby depositing a carbon layer on the surface of the nano silicon sphere powder. 3. The method for greatly increasing the reversible capacity of graphite according to claim 2, characterized in that: said nano-silicon spherical powder is spherical silicon powder with an average diameter of 20-100 nanometers. 4. The method for greatly increasing the reversible capacity of graphite according to claim 2, characterized in that: the chemical vapor phase furnace is a vertical reaction furnace. 5. The method for greatly increasing the reversible capacity of graphite according to claim 2, characterized in that: during the process of raising the temperature in the chemical vapor phase furnace to 600-900°C, the rate of temperature rise is 2-20°C/min. 6. according to the method for greatly improving graphite reversible capacity according to claim 2, it is characterized in that: described carbon source gas is gaseous hydrocarbons such as methane, acetylene, ethylene or propylene; The volume concentration of described carbon source gas is 1-20%.

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