Silicon-carbon composite anode material for lithium ion batteries and a preparation method thereof

Abstract

Disclosed in the invention are a silicon-carbon composite anode material for lithium ion batteries and a preparation method thereof The material consists of a porous silicon substrate and a carbon coating layer. The preparation method of the material comprises preparing a porous silicon substrate and a carbon coating layer. The silicon-carbon composite anode material for lithium ion batteries has the advantages of high reversible capacity, good cycle performance and good rate performance. The material respectively shows reversible capacities of 1,556 mAh, 1,290 mAh, 877 mAh and 474 mAh/g at 0.2 C, 1 C, 4 C and 15 C rates; the specific capacity remains above 1,500 mAh after 40 cycles at the rate of 0.2 C and the reversible capacity retention rate is up to 90 percent.

Claims

1. A silicon-carbon composite anode material for lithium ion batteries, comprising: a porous silicon substrate having a polycrystalline structure with a particle size of 50 nm to 20 m, a pore size of 2 nm to 150 nm, a pore volume of 0.1 cm.sup.3/g to 1.5 cm.sup.3/g, and a specific surface area of 30 m.sup.2/g to 300 m.sup.2/g; and 2 wt % to 70 wt % of a carbon coating layer of amorphous carbon, having a thickness of 2 nm to 30 nm.

2. A method for producing a silicon-carbon composite anode material for lithium ion batteries of claim 1, comprising: placing materials of 1 to 3 parts by weight of mesoporous silica and 2 to 4 parts by weight of magnesium powder in a high temperature furnace; adjusting the furnace temperature to 600 to 900 C. in an atmosphere of a protective gas, and maintaining the temperature for 2 to 10 hours before allowing the materials to cool; placing the materials into 40 to 100 parts of hydrochloric acid with a concentration of 1 to 12 mol/L and stirring for 6 to 18 hours; centrifuging 3 to 5 times at 3000 to 10,000 r/min, and vacuum drying at 70 to 120 C. for 6 to 18 hours so as to obtain a porous silicon substrate; and thereafter utilizing the obtained porous silicon substrate for producing a silicon-carbon composite anode material by either: placing the porous silicon substrate in a high temperature furnace, adjusting the furnace temperature to 600 C. to 1100 C. in an atmosphere of a protective gas; carrying a gaseous carbon source or a liquid carbon source into the furnace by the protective gas, maintaining the temperature for 1 to 12 hours; forming a carbon coating layer on a surface of the porous silicon substrate after disassociation of the gaseous carbon source or the liquid carbon source so as to obtain a silicon-carbon composite anode material for lithium ion batteries, or dispersing the porous silicon substrate and a solid carbon source in a solvent to form a mixture; the mixture being dispersed homogeneously by an ultrasonic treatment and stirring; evaporating the solvent and transferring the resulting material into a high temperature furnace where the temperature is raised to 600 C. to 1100 C. in an atmosphere of a protective gas, maintaining the temperature for 1 to 12 hours; and forming a carbon coating layer on a surface of the porous silicon substrate after disassociation of the solid carbon source so as to obtain a silicon-carbon composite anode material for lithium ion batteries.

3. The method for producing a silicon-carbon composite anode material for lithium ion batteries according to claim 2 wherein the protective gas is selected from the group consisting of argon, nitrogen, helium, a gas mixture of argon and hydrogen, and a gas mixture of nitrogen and hydrogen, the content of hydrogen in the gas mixture being 2% to 20% by volume.

4. The method for producing a silicon-carbon composite anode material for lithium ion batteries according to claim 2 wherein the gaseous carbon source is selected from the group consisting of acetylene, methane, ethane, ethylene, propylene and carbon monoxide.

5. The method for producing a silicon-carbon composite anode material for lithium ion batteries according to claim 2 wherein the liquid carbon source is selected from the group consisting of benzene, toluene, xylene, ethanol, n-hexane and cyclohexane.

6. The method for producing a silicon-carbon composite anode material for lithium ion batteries according to claim 2 wherein the solid carbon source is selected from the group consisting of polyvinyl chloride, polyvinylidene fluoride, polyacrylonitrile, polyvinyl alcohol, polystyrene, phenolic resins, epoxy resins, coal tar pitch, petroleum pitch, sucrose and glucose, wherein the molecular weight of polyvinyl chloride is in a range of 50,000 to 120,000, the molecular weight of polyvinylidene fluoride is in a range of 250,000 to 1,000,000, the molecular weight of polyacrylonitrile is in a range of 30,000 to 200,000, the molecular weight of the polyvinyl alcohol is in a range of 20,000 to 300,000, the molecular weight of polystyrene is in a range of 50,000 to 200,000, the molecular weight of phenolic resin is in a range of 500 to 10,000, the molecular weight of epoxy resin is in a range of 300 to 8000.

7. The method for producing a silicon-carbon composite anode material for lithium ion batteries according to claim 2 wherein the solvent is selected from the group consisting of water, ethanol, ethyl ether, acetone, tetrahydrofuran, benzene, toluene, xylene, dimethyl formamide and N-methyl pyrrolidone.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the SEM image (a) and the TEM image (b) of the porous silicon substrate obtained in Example 1.

(2) FIG. 2 shows the pore size distribution curve of the porous silicon substrate obtained in Example 1.

(3) FIG. 3 shows the TEM image of the silicon-carbon composite anode material for lithium ion batteries obtained in Example 1.

(4) FIG. 4 shows the charge-discharge curve at the first, the second and the tenth cycle of the silicon-carbon composite anode material for lithium ion batteries obtained in Example 1.

(5) FIG. 5 shows the capacity versus cycle number curve of the first 40 cycles of the lithium ion battery assembled from the silicon-carbon composite anode material for lithium ion batteries obtained in Example 1.

(6) FIG. 6 shows the capacity versus cycle number curve at various rates of the lithium ion battery assembled from the silicon-carbon composite anode material for lithium ion batteries obtained in Example 1.

(7) FIG. 7 shows the TEM image of the silicon-carbon composite anode material for lithium ion batteries obtained in Example 2.

(8) FIG. 8 shows the SEM image of the porous silicon substrate obtained in Example 3.

(9) FIG. 9 shows the capacity versus cycle number curve of the first 40 cycles of the lithium ion battery assembled from the silicon-carbon composite material obtained in Comparative Example 1 which does not have a porous structure.

(10) FIG. 10 shows the capacity versus cycle number curve of the first 40 cycles of the lithium ion battery assembled from the porous silicon substrate obtained in Comparative Example 2 which does not have a carbon coating layer.

DETAILED DESCRIPTION OF THE INVENTION

(11) The following examples further illustrate the present invention, but the present invention is not limited to the following Examples.

(12) Assembly of the lithium ion battery and the test method are described as follows.

(13) The silicon-carbon composite anode material for lithium ion batteries of the present invention, 20 wt % of the binder (a N-methylpyrrolidone solution of polyvinylidene fluoride with a concentration of 2 wt %, or a styrene-butadiene rubber-sodium carboxymethyl cellulose emulsion) and 20 wt % of a conductive agent (SuperP conductive carbon black) were mixed and stirred uniformly, then the material was coated on a copper foil which is then placed in a drying oven at 60 C. to 80 C. The copper foil was punched into an electrode with a 12-16 mm diameter puncher. The electrode is placed in a vacuum oven and dried at 0 C. to 120 C. for 8 to 12 hours, then transferred into an argon-filled glove box. A lithium plate is used as a counter electrode, ENTEK PE porous membrane is used as a separator, 1 mol/L of lithium hexafluorophosphate in a mixed solution of ethylene carbonate with dimethyl carbonate (volume ratio 1:1) is used as the electrolyte. Thus a button battery CR2016 is assembled. Constant current charge-discharge performance test is carried out on a LAND battery test system (Wuhan Jinnuo Electronics Co., Ltd.). The charge-discharge cut-off voltage with respect to Li/Li.sup.+ is 0.01 to 1.2 V, the charge-discharge rate is 0.05 C to15 C, wherein the current density at the rate of 0.2 C is 300 mA/g.

EXAMPLE 1

(14) Preparation of mesoporous silica: 4.0 g of ethylene oxide/propylene oxide block copolymer (tradename: Pluronic P123) was dissolved in a mixed solution of 30.0 g of water and 120.0 g of hydrochloric acid (2 mol/L), after being stirred homogeneously, 8.4 g of tetraethyl orthosilicate (TEOS) was added therein. The mixture was then stirred at 35 C. for 24 hours and transferred into a hydrothermal reaction kettle where the temperature was kept at 100 C. for 24 hour. After being cooled, the mixture was centrifugalized at 4000 r/min, dried at 95 C., and then calcined at 550 C. in an air atmosphere for 2 hours. Thus, mesoporous silica was obtained.

(15) (1) 0.3 g of mesoporous silica and 0.3 g of magnesium powder were placed in a high temperature furnace. In a gas mixture of argon and hydrogen (the content of hydrogen was 5% by volume), the temperature was raised to 650 C., and temperature was kept for 7 hours before it was allowed to cool. Then the materials were put into 25 ml of hydrochloric acid (2 mol/L) and stirred for 12 hours. After 4 times of centrifugation at 4000 r/min, and vacuum drying at 80 C. for 12 hours, a porous silicon substrate was obtained;

(16) (2) The porous silicon substrate was placed in a high temperature furnace, where the temperature was raised to 900 C. in an atmosphere of argon. Then acetylene was carried into the furnace by argon (the volume ratio of argon and acetylene was 5:1 and the total flow rate was 300 ml/min), the temperature was kept for 4 hours. After the disassociation of acetylene, a carbon coating layer was formed on the surface of the porous silicon substrate, thus a silicon-carbon composite anode material for lithium ion batteries was obtained.

(17) The morphology and structure of the porous silicon substrate was shown in FIG. 1. The particles were approximate cylindrical with a length of approximately 600 nm and a diameter of approximately 400 nm, and the substrate shows a porous structure. The pore size distribution curve shown in FIG. 2 indicates that the pore size was about 40 nm, the pore volume was 0.56 cm.sup.3/g, the specific surface area was 78.5 m.sup.2/g. FIG. 3 shows a TEM image of the interface of the porous silicon substrate with the carbon coating layer. Silicon crystal plane (111) can be seen from FIG. 3, and the plane spacing was 0.31 nm; the carbon coating layer consists of amorphous carbon, and its thickness was approximately 7 nm. The carbon coating layer accounted for 40.0 wt %. It can be seen from the electron diffraction image in FIG. 3 that the silicon was of a polycrystalline structure. The polycrystalline diffraction ring having the smallest diameter in the image corresponds to (111) crystal plane of the silicon.

(18) The silicon-carbon composite anode material for lithium ion batteries thus prepared was assembled into a lithium ion battery, on which charge-discharge tests were carried out. FIG. 4 shows the charge-discharge curve of the first three cycles. FIG. 5 shows the capacity versus cycle number of the first 40 cycles. The initial charge-discharge coulombic efficiency was 72.0%. The reversible capacity after 40 cycles at the rate of 0.2 C was 1509 mAh/g, the capacity retention rate was 90.1%. Tests were carried out at rates of 0.05 C, 0.2 C, 0.5 C, 1 C, 4 C, 8 C and 15 C, the silicon-carbon composite anode material for lithium ion batteries of the present invention showed reversible capacity of 1583 mAh/g, 1556 mAh/g, 1370 mAh/g, 1290 mAh/g, 877 mAh/g, 598 mAh/g and 474 mAh/g, as can be seen in FIG. 6. The electrochemical properties of this material were better than the traditional silicon-carbon composite material.

EXAMPLE 2

(19) Preparation of mesoporous silica: 3.0 g of Pluronic P123 was dissolved in a mixed solution of 22.5 g of water, 3.0 g of 1-butanol and 90.0 g of hydrochloric acid (2 mol/L), after being stirred homogeneously, 6.3 g of TEOS was added therein. The mixture was then stirred at 35 C. for 24 hours and transferred into a hydrothermal reaction kettle where the temperature was kept at 100 C. for 24 hour. After being cooled, the mixture was centrifugalized at 4000 r/min, dried at 100 C., and then calcined at 600 C. in an air atmosphere for 2 hours. Thus, mesoporous silica was obtained.

(20) (1) 0.4 g of mesoporous silica and 0.4 g of magnesium powder were placed in a high temperature furnace. In an atmosphere of argon, the temperature was raised to 700 C., and temperature was kept for 6 hours before it was allowed to cool. Then the materials were put into 30 ml of hydrochloric acid (2 mol/L) and stirred for 12 hours. After 4 times of centrifugation at 4000 r/min, and vacuum drying at 80 C. for 12 hours, a porous silicon substrate was obtained;

(21) (2) The porous silicon substrate was placed in a high temperature furnace, where the temperature was raised to 800 C. in an atmosphere of nitrogen. Then toluene was carried into the furnace by nitrogen (the flow rate of nitrogen was 800 ml/min), the temperature was kept for 2 hours. After the disassociation of toluene, a carbon coating layer was formed on the surface of the porous silicon substrate, thus a silicon-carbon composite anode material for lithium ion batteries was obtained.

(22) The porous silicon substrate was of a polycrystalline structure, the average particle size was 2.4 m, the average pore size was 35 nm, the pore volume was 0.61 cm.sup.3/g, the specific surface area was 73.3 m.sup.2/g. FIG. 7 shows a TEM image of the silicon-carbon composite anode material for lithium ion batteries. It can be seen from FIG. 7(a) that the material has a porous structure. FIG. 7(b) shows the interface of the porous silicon substrate with the carbon coating layer. Crystal plane (111) can be seen from FIG. 7(b), and the plane spacing was 0.31 nm. The carbon coating layer consists of amorphous carbon and has a thickness of 5 nm. The carbon coating layer accounted for 25.6 wt %.

(23) The silicon-carbon composite anode material for lithium ion batteries thus prepared was assembled into a lithium ion battery, on which charge-discharge tests were carried out. The initial charge-discharge coulombic efficiency was 75.2%. The reversible capacity after 40 cycles was 1325 mAh/g, and the capacity retention rate was 73.7%.

EXAMPLE 3

(24) Preparation of mesoporous silica: 4.0 g of Pluronic P123 was dissolved in a mixed solution of 30.0 g of water, 4.0 g of 1-butanol and 120.0 g of hydrochloric acid (2 mol/L), after being stirred homogeneously, 8.4 g of TEOS was added therein. The mixture was then stirred at 35 C. for 24 hours and transferred into a hydrothermal reaction kettle where the temperature was kept at 100 C. for 24 hour. After being cooled, the mixture was centrifugalized at 4000 r/min, dried at 100 C., and then calcined at 600 C. in an air atmosphere for 2 hours. Thus, mesoporous silica was obtained.

(25) (1) 0.4 g of mesoporous silica and 0.4 g of magnesium powder were placed in a high temperature furnace. In a gas mixture of argon and hydrogen (the content of hydrogen was 5% by volume), the temperature was raised to 750 C., and temperature was kept for 7 hours before it was allowed to cool. Then the materials were put into 30 ml of hydrochloric acid (2 mol/L) and stirred for 12 hours. After 4 times of centrifugation at 4000 r/min, and vacuum drying at 80 C. for 12 hours, a porous silicon substrate was obtained;

(26) (2) The porous silicon substrate was placed in a high temperature furnace, where the temperature was raised to 900 C. in an atmosphere of argon. Then acetylene was carried into the furnace by argon (the volume ratio of argon and acetylene was 4:1 and the total flow rate was 250 ml/min), the temperature was kept for 3 hours. After the disassociation of acetylene, a carbon coating layer was formed on the surface of the porous silicon substrate, thus a silicon-carbon composite anode material for lithium ion batteries was obtained.

(27) The porous silicon substrate was of a polycrystalline structure, the average particle size was 2.5 m, the average pore size was 32 nm, the pore volume was 0.64 cm.sup.3/g, the specific surface area was 73.0 m.sup.2/g. FIG. 8 shows the morphology. The silicon-carbon composite anode material for lithium ion batteries comprised 34.6 wt % of the carbon coating layer which consists of amorphous carbon and has a thickness of 6 nm.

(28) The silicon-carbon composite anode material for lithium ion batteries thus prepared was assembled into a lithium ion battery, on which charge-discharge tests were carried out. The initial charge-discharge coulombic efficiency was 72.2%. The reversible capacity after 40 cycles was 1570 mAh/g, and the capacity retention rate was 84.8%.

EXAMPLE 4

(29) Preparation of mesoporous silica: 2.0 g of Pluronic P123 was dissolved in a mixed solution of 15 g of water and 60.0 g of hydrochloric acid (2 mol/L), after being stirred homogeneously, 4.2 g of TEOS was added therein. The mixture was then stirred at 35 C. for 24 hours and transferred into a hydrothermal reaction kettle where the temperature was kept at 100 C. for 24 hour. After being cooled, the mixture was centrifugalized at 5000 r/min, dried at 90 C., and then calcined at 650 C. in an air atmosphere for 2 hours. Thus, mesoporous silica was obtained.

(30) (1) 0.35 g of mesoporous silica and 0.35 g of magnesium powder were placed in a high temperature furnace. In an atmosphere of argon, the temperature was raised to 700 C., and temperature was kept for 6 hours before it was allowed to cool. Then the materials were put into 30 ml of hydrochloric acid (2 mol/L) and stirred for 12 hours. After 4 times of centrifugation at 5000 r/min, and vacuum drying at 80 C. for 12 hours, a porous silicon substrate was obtained;

(31) (2) The porous silicon substrate was placed in a high temperature furnace, where the temperature was raised to 770 C. in an atmosphere of nitrogen. Then toluene was carried into the furnace by nitrogen (the flow rate of nitrogen was 1000 ml/min), the temperature was kept for 1 hour. After the disassociation of toluene, a carbon coating layer was formed on the surface of the porous silicon substrate, thus a silicon-carbon composite anode material for lithium ion batteries was obtained.

(32) The porous silicon substrate was of a polycrystalline structure, the average particle size was 700 nm, the average pore size was 23 nm, the pore volume was 0.42 cm.sup.3/g, the specific surface area was 78.1 m.sup.2/g. The silicon-carbon composite anode material for lithium ion batteries comprised 18.3 wt % of the carbon coating layer which consists of amorphous carbon and has a thickness of 4 nm.

(33) The silicon-carbon composite anode material for lithium ion batteries thus prepared was assembled into a lithium ion battery, on which charge-discharge tests were carried out. The initial charge-discharge coulombic efficiency was 76.5%. The reversible capacity after 40 cycles was 1825 mAh/g, and the capacity retention rate was 83.6%.

EXAMPLE 5

(34) Preparation of mesoporous silica: 3.5 g of Pluronic P123 was dissolved in a mixed solution of 26.3 g of water and 105.0 g of hydrochloric acid (2 mol/L), after being stirred homogeneously, 7.4 g of TEOS was added therein. The mixture was then stirred at 35 C. for 24 hours and transferred into a hydrothermal reaction kettle where the temperature was kept at 100 C. for 24 hour. After being cooled, the mixture was centrifugalized at 5000 r/min, dried at 80 C., and then calcined at 600 C. in an air atmosphere for 2 hours. Thus, mesoporous silica was obtained.

(35) (1) 0.3 g of mesoporous silica and 0.3 g of magnesium powder were placed in a high temperature furnace. In a gas mixture of argon and hydrogen (the content of hydrogen was 10% by volume), the temperature was raised to 700 C., and temperature was kept for 7 hours before it was allowed to cool. Then the materials were put into 25 ml of hydrochloric acid (2 mol/L) and stirred for 12 hours. After 4 times of centrifugation at 5000 r/min, and vacuum drying at 80 C. for 12 hours, a porous silicon substrate was obtained;

(36) (2) 0.2 g of the porous silicon substrate and 0.7 g of polyvinyl chloride were dispersed in 15 ml of tetrahydrofuran. The mixture were dispersed homogeneously by an ultrasonic treatment and stirring. Then tetrahydrofuran was evaporated, the material was transferred into a high temperature furnace where the temperature was raised to 900 C. in an atmosphere of argon, the temperature was kept for 2 hours. After the disassociation of polyvinyl chloride, a carbon coating layer was formed on the surface of the porous silicon substrate, thus a silicon-carbon composite anode material for lithium ion batteries was obtained.

(37) The porous silicon substrate was of a polycrystalline structure, the average particle size was 650 nm, the average pore size was 24 nm, the pore volume was 0.43 cm.sup.3/g, the specific surface area was 77.8 m.sup.2/g. The silicon-carbon composite anode material for lithium ion batteries comprised 31.4 wt % of the carbon coating layer which consists of amorphous carbon and has a thickness of 6 nm.

(38) The silicon-carbon composite anode material for lithium ion batteries thus prepared was assembled into a lithium ion battery, on which charge-discharge tests were carried out. The initial charge-discharge coulombic efficiency was 74.1%. The initial lithium intercalation capacity was 1855 mAh/g, the initial lithium deintercalation capacity was 1374 mAh/g.

EXAMPLE 6

(39) Preparation of mesoporous silica: 2.0 g of Pluronic P123 was dissolved in a mixed solution of 15.0 g of water, 2.0 g of 1-butanol and 60.0 g of hydrochloric acid (2 mol/L), after being stirred homogeneously, 4.2 g of TEOS was added therein. The mixture was then stirred at 35 C. for 24 hours and transferred into a hydrothermal reaction kettle where the temperature was kept at 100 C. for 24 hour. After being cooled, the mixture was centrifugalized at 6000 r/min, dried at 100 C., and then calcined at 550 C. in an air atmosphere for 2 hours. Thus, mesoporous silica was obtained.

(40) (1) 0.35 g of mesoporous silica and 0.35 g of magnesium powder were placed in a high temperature furnace. In an atmosphere of argon, the temperature was raised to 650 C., and temperature was kept for 7 hours before it was allowed to cool. Then the materials were put into 30 ml of hydrochloric acid (2 mol/L) and stirred for 12 hours. After 4 times of centrifugation at 6000 r/min, and vacuum drying at 80 C. for 12 hours, a porous silicon substrate was obtained;

(41) (2) 0.2 g of the porous silicon substrate and 0.4 g of polyacrylonitrile were dispersed in 10 ml of dimethyl formamide. The mixture were dispersed homogeneously by an ultrasonic treatment and stirring. Then dimethyl formamide was evaporated, the material was transferred into a high temperature furnace where the temperature was raised to 900 C. in an atmosphere of nitrogen, the temperature was kept for 2 hours. After the disassociation of polyacrylonitrile, a carbon coating layer was formed on the surface of the porous silicon substrate, thus a silicon-carbon composite anode material for lithium ion batteries was obtained.

(42) The porous silicon substrate was of a polycrystalline structure, the average particle size was 2.5 m, the average pore size was 34 nm, the pore volume was 0.66 cm.sup.3/g, the specific surface area was 72.8 m.sup.2/g. The silicon-carbon composite anode material for lithium ion batteries comprised 20.9 wt % of the carbon coating layer which consists of amorphous carbon and has a thickness of 4 nm.

(43) The silicon-carbon composite anode material for lithium ion batteries thus prepared was assembled into a lithium ion battery, on which charge-discharge tests were carried out. The initial charge-discharge coulombic efficiency was 64.0%. The initial lithium intercalation capacity was 1242 mAh/g, the initial lithium deintercalation capacity was 795 mAh/g.

EXAMPLE 7

(44) Preparation of mesoporous silica: 3.0 g of Pluronic P123 was dissolved in a mixed solution of 22.5 g of water, 3.0 g of 1-butanol and 135.0 g of hydrochloric acid (2 mol/L), after being stirred homogeneously, 9.5 g of TEOS was added therein. The mixture was then stirred at 35 C. for 24 hours and transferred into a hydrothermal reaction kettle where the temperature was kept at 100 C. for 24 hour. After being cooled, the mixture was centrifugalized at 5000 r/min, dried at 80 C., and then calcined at 650 C. in an air atmosphere for 2 hours. Thus, mesoporous silica was obtained.

(45) (1) 0.45 g of mesoporous silica and 0.45 g of magnesium powder were placed in a high temperature furnace. In an atmosphere of argon, the temperature was raised to 750 C., and temperature was kept for 6 hours before it was allowed to cool. Then the materials were put into 30 ml of hydrochloric acid (2 mol/L) and stirred for 12 hours. After 4 times of centrifugation at 5000 r/min, and vacuum drying at 80 C. for 12 hours, a porous silicon substrate was obtained;

(46) (2) 0.3 g of the porous silicon substrate and 0.95 g of polyvinyl chloride were dispersed in 10 ml of tetrahydrofuran. The mixture were dispersed homogeneously by an ultrasonic treatment and stirring. Then tetrahydrofuran was evaporated, the material was transferred into a high temperature furnace where the temperature was raised to 900 C. in an atmosphere of argon, the temperature was kept for 4 hours. After the disassociation of polyvinyl chloride, a carbon coating layer was formed on the surface of the porous silicon substrate, thus a silicon-carbon composite anode material for lithium ion batteries was obtained.

(47) The porous silicon substrate was of a polycrystalline structure, the average particle size was 2.6 m, the average pore size was 33 nm, the pore volume was 0.65 cm.sup.3/g, the specific surface area was 72.9 m.sup.2/g. The silicon-carbon composite anode material for lithium ion batteries comprised 29.3 wt % of the carbon coating layer which consists of amorphous carbon and has a thickness of 6 nm.

(48) The silicon-carbon composite anode material for lithium ion batteries thus prepared was assembled into a lithium ion battery, on which charge-discharge tests were carried out. The initial charge-discharge coulombic efficiency was 67.2%. The initial lithium intercalation capacity was 1291 mAh/g, the initial lithium deintercalation capacity was 867 mAh/g.

EXAMPLE 8

(49) Preparation of mesoporous silica: 4.0 g of Pluronic P123 was dissolved in a mixed solution of 30.0 g of water and 120.0 g of hydrochloric acid (2 mol/L), after being stirred homogeneously, 8.4 g of TEOS was added therein. The mixture was then stirred at 35 C. for 24 hours and transferred into a hydrothermal reaction kettle where the temperature was kept at 100 C. for 24 hour. After being cooled, the mixture was centrifugalized at 5000 r/min, dried at 80 C., and then calcined at 550 C. in an air atmosphere for 2 hours. Thus, mesoporous silica was obtained.

(50) (1) 0.35 g of mesoporous silica and 0.4 g of magnesium powder were placed in a high temperature furnace. In a gas mixture of argon and hydrogen (the content of hydrogen is 10% by volume), the temperature was raised to 700 C., and temperature was kept for 7 hours before it was allowed to cool. Then the materials were put into 30 ml of hydrochloric acid (2 mol/L) and stirred for 12 hours. After 4 times of centrifugation at 5000 r/min, and vacuum drying at 80 C. for 12 hours, a porous silicon substrate was obtained;

(51) (2) 0.25 g of the porous silicon substrate and 0.5 g of polyacrylonitrile were dispersed in 15 ml of dimethyl formamide. The mixture were dispersed homogeneously by an ultrasonic treatment and stirring. Then dimethyl formamide was evaporated, the material was transferred into a high temperature furnace where the temperature was raised to 900 C. in an atmosphere of nitrogen, the temperature was kept for 4 hours. After the disassociation of polyacrylonitrile, a carbon coating layer was formed on the surface of the porous silicon substrate, thus a silicon-carbon composite anode material for lithium ion batteries was obtained.

(52) The porous silicon substrate was of a polycrystalline structure, the average particle size was 600 nm, the average pore size was 24 nm, the pore volume was 0.44 cm.sup.3/g, the specific surface area was 77.7 m.sup.2/g. The silicon-carbon composite anode material for lithium ion batteries comprised 21.3 wt % of the carbon coating layer which consists of amorphous carbon and has a thickness of 4 nm.

(53) The silicon-carbon composite anode material for lithium ion batteries thus prepared was assembled into a lithium ion battery, on which charge-discharge tests were carried out. The initial charge-discharge coulombic efficiency was 72.0%. The initial lithium intercalation capacity was 1263 mAh/g, the initial lithium deintercalation capacity was 910 mAh/g.

COMPARATIVE EXAMPLE 1

(54) 0.15 g of nano silicon powder (particle size: 50 to 150 nm) and 0.45 g of polyvinyl chloride were dispersed in 10 ml of tetrahydrofuran. The mixture were dispersed homogeneously by an ultrasonic treatment and stirring. Then tetrahydrofuran was evaporated, the material was transferred into a high temperature furnace where the temperature was raised to 900 C. in a gas mixture of nitrogen and hydrogen (the content of hydrogen is 5% by volume), the temperature was kept for 2 hours. After the disassociation of polyvinyl chloride and cooling, a silicon-carbon composite material free of pores was obtained. The carbon coating layer accounted for 28.8 wt %, it consisted of amorphous carbon and had a thickness of 6 nm.

(55) The silicon-carbon composite material thus prepared was assembled into a lithium ion battery, on which charge-discharge tests were carried out. FIG. 9 shows the capacity versus cycle number curve of the first 40 cycles. The initial charge-discharge coulombic efficiency was 78.0%. The initial reversible capacity was 1194 mAh/g, the reversible capacity after 40 cycles was 186 mAh/g, thus the capacity retention rate was only 15.6%.

COMPARATIVE EXAMPLE 2

(56) Preparation of mesoporous silica: 2.0 g of Pluronic P123 was dissolved in a mixed solution of 15.0 g of water, 2.0 g of 1-butanol and 60.0 g of hydrochloric acid (2 mol/L), after being stirred homogeneously, 4.2 g of TEOS was added therein. The mixture was then stirred at 35 C. for 24 hours and transferred into a hydrothermal reaction kettle where the temperature was kept at 100 C. for 24 hour. After being cooled, the mixture was centrifugalized at 5000 r/min, dried at 90 C., and then calcined at 650 C. in an air atmosphere for 2 hours. Thus, mesoporous silica was obtained.

(57) (1) 0.35 g of mesoporous silica and 0.35 g of magnesium powder were placed in a high temperature furnace. In a gas mixture of argon and hydrogen (the content of hydrogen is 5% by volume), the temperature was raised to 700 C., and temperature was kept for 6 hours before it was allowed to cool. Then the materials were put into 30 ml of hydrochloric acid (2 mol/L) and stirred for 12 hours. After 4 times of centrifugation at 5000 r/min, and vacuum drying at 80 C. for 12 hours, a porous silicon substrate was obtained.

(58) The porous silicon substrate was of a polycrystalline structure, the average particle size was 2.5 m, the average pore size was 34 nm, the pore volume was 0.66 cm.sup.3/g, the specific surface area was 72.8 m.sup.2/g. No carbon coating layer was formed.

(59) The silicon-carbon composite material thus prepared was assembled into a lithium ion battery, on which charge-discharge tests were carried out. FIG. 10 shows the capacity versus cycle number curve of the first 40 cycles. The initial charge-discharge coulombic efficiency was 81.1%. The initial reversible capacity was 2837 mAh/g, the reversible capacity after 40 cycles was 1554 mAh/g, thus the capacity retention rate was only 54.8%.

(60) Compared with the silicon-carbon composite material of Comparative Example 1 which does not have a porous structure, it can be seen that the silicon-carbon composite material for lithium ion batteries of the present invention having a porous structure and a carbon coating layer has a better cycle performance. This results from the uniform distribution of the porous structure, which can effectively alleviate the volume effect during the lithium intercalation and deintercalation process and improve the stability of the electrode structure.

(61) Compared with the porous silicon material of Comparative Example 2 which does not have a carbon coating layer, it can be seen that the silicon-carbon composite material for lithium ion batteries of the present invention having a porous structure and a carbon coating layer has a better cycle performance This results from the carbon coating layer which improves conductivity and maintains an electrode conductive network.