Anode active material hybridizing carbon nano fibers for lithium secondary battery

Abstract

The present invention is to provide anode active material hybridized with carbon nano fibers for lithium secondary battery prepared by following steps comprising, i) dispersing the nano size metal catalyst to the surface of anode material selected from graphite, amorphous silicon or the complex of graphite and amorphous silicon; and ii) growing the carbon nano fiber by chemical vapor deposition method, wherein carbon nano fibers are grown in a vine form and surround the surface of anode active material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the change of structure of anode active material while carbon nano fibers are grown and hybridized with a graphite active plate.

FIG. 2A shows a process flow for preparing the carbon nano fiber on the anode active material in Examples of the present invention. FIG. 2B shows a process for preparing the carbon nano fiber on the anode active material in Comparative Examples 1 and 2 of the present invention. FIG. 2C shows a process for preparing the carbon nano fiber on the anode active material in Comparative Example 4 (U.S. Pat. No. 6,440,610 B1).

FIG. 3 is a Field Emission Scanning Electron Microscope (FE-SEM) photography of the surface of graphite in Preparation Example 1, where the carbon nano fiber is hybridized.

FIG. 3A is a photography (magnitude×1000), FIG. 3B is a photography (magnitude×5000) and FIG. 3C is a photography (magnitude×100000).

FIG. 4 is a high resolution transmission electron microscope (TEM) photography of the surface of graphite in Preparation Example 1, where the carbon nano fiber is hybridized in a vine form. FIG. 4A and FIG. 4B show carbon nano fibers on the surface of graphite and FIG. 4C shows the herringbone structure of the carbon nano fiber.

FIG. 5 is a Field Emission Scanning Electron Microscope (FE-SEM) photography of the surface of silicon in Preparation Example 4, where the carbon nano fiber is hybridized. FIG. 5A is a photography (magnitude×1000), FIG. 5B is a photography (magnitude×10000) and FIG. 5C is a photography (magnitude×50000).

FIG. 6 is a Field Emission Scanning Electron Microscope (FE-SEM) photography of the surface of silicon in Comparative Preparation Example 2, where the carbon nano fiber is not hybridized, but simply stacked. FIG. 6A is a photography (magnitude×2000) and FIG. 6B is a photography (magnitude×50000).

FIG. 7 is a graph indicating X-Ray Diffractometer (XRD) peak of the silicon powder used in Preparation Examples 3 and 4, according to the time lapse of planetary mill treatment. It shows that the crystalline degree of silicon is declined according to the planetary mill treatment.

FIG. 8 is a graph indicating X-Ray Diffractometer (XRD) peak of the complex of the silicon and the graphite powder used in Preparation Examples 5˜9, according to the time lapse of planetary mill treatment. It shows that the crystalline degree of silicon is declined according to the planetary mill treatment. The intensity of the plane (111) of silicon and graphite complexes decreases.

FIG. 9A is the diffraction pattern of transmission electron microscope (TEM) of the silicon powder used in Preparation Example 4 after planetary mill treatment. FIG. 9B is a high resolution transmission electron microscope (TEM) photograph of said silicon powder whose surface is partially amorphous

FIG. 10 is the diffraction pattern of transmission electron microscope (TEM) of the silicon powder used in Preparation Example 9 after planetary mill treatment, which shows the crystalline silicon changes to an amorphous form.

DETAILED DESCRIPTION OF THE INVENTION

The present invention affords the anode active material hybridized with the carbon nano fiber for lithium secondary battery, wherein the carbon nano fiber is grown in a vine form and surrounds the surface of anode active material selected from graphite, amorphous silicon and/or the complex of graphite and amorphous silicon. In the present invention, the metal catalyst for growing the carbon nano fiber has been prepared by co-precipitation method in an aqueous solution. Further, in order to grow the carbon nano fiber in a vine form, said metal catalyst has been uniformly dispersed on the surface of anode active material, followed by drying and heating the mixture of metal catalyst and anode active material.

By a chemical vapor deposition method, the carbon nano fiber with following structure can be prepared. The diameter of the obtained carbon nano fiber is 5˜300 nm, the aspect ratio is 10˜10000, the form of the carbon nano fiber is platelet or herringbone structure, and the thickness of the carbon nano fiber covering the surface of the active anode material is 5˜1000 nm. Because the grown carbon nano fiber surrounds anode active material in a vine form, the prevention of volume expansion of anode active material can be made in the course of inserting/emitting the lithium ion. The preferred diameter of the obtained carbon nano fiber is 5˜50 nm, the preferred aspect ratio is 10˜100, and the preferred thickness of the carbon nano fiber covering the surface of the active anode material is 15˜200 nm.

On the other hand, if the carbon nano tube is grown on the surface of anode active material, the enhancement of electro-conductivity can be made, while the charging/discharging properties are declined compared to those of the case applying the carbon nano fiber, according to the repetition of cycles. It has been considered that the volume expansion of anode active material can not be controlled by the carbon nano tube grown on the surface of anode active material.

Recently, graphite material has been used as anode active material instead of pure lithium metal for lithium secondary battery. Various kinds of carbon material, such as carbon nano fiber, cokes, meso-carbon, artificial graphite and/or natural graphite, have been used as anode active material. Further, crystalline graphite has been commercially used as anode active material, because it can maintain the broader voltage flatness compared to that of cokes or amorphous carbon.

Since the higher crystallinity of graphite shows the better cyclic property due to the convenience of insertion/emission of lithium ion, artificial graphite containing more than 90% of crystallinity has been prepared as anode active material by a heat treatment over 2000° C. On the other hand, natural graphite which can be easily obtained due to its high deposit in the nature has a handicap to be applied to battery owing to high non-reversible capacity and low cyclic property compared to artificial graphite. Therefore, natural graphite requires further treatments for commercial application to the battery, such as reforming the surface of natural graphite by a milling process, mixing and complexing the fine crystalline carbon material, adding various kinds of additives and oxidative treating the part of surface of graphite using an acid solution. In case that the carbon nano tube is grown on the surface of natural graphite, the electro conductivity has been increased whereas the cyclic properties have been declined by repeating charging/discharging the battery. We consider that such a carbon nano tube grown on the surface of anode active material cannot prevent the volume expansion of anode active material.

The catalyst for preparing a carbon nano fiber has been already known. For example, transition metals, such as Fe, Co, and Ni, have been used (Catal. Rev.-Sci.Eng., 42(4) pp 481-510 (2000)). In the present invention, at least 1 metal catalyst selected from Fe, Co, Ni, Cu, Mg, Mn, Ti, Sn, Si, Zr, Zn, Ge, Pb and/or In has been used. The form of catalyst can be a form of alkoxide, oxide, chloride, nitrate or carbonate.

For supporting the metal catalyst particle on the surface of anode active material, a sol-gel method, a precipitation method, a hydrothermal reaction method, a spray heating method, a spray drying method and/or a ball-mill method can be used. Further, anode active material containing metal particles can be prepared by introducing further oxidation or reduction process. However, the preferred preparation method does not require further oxidation or reduction process.

In order to grow the carbon nano fiber on the surface of anode active material, carbon sources, such as carbon monoxide, methane, acetylene and/or ethylene, can be used for a gas phase reaction under high temperature. The preferred carbon source may be carbon mono oxide or ethylene in the temperature range of 400-800° C. The growing amount of a carbon nano fiber can be 5˜200 wt % as to the amount of anode active material. The preferred amount of the carbon nano fiber shall be 5˜100 wt % as to the amount of anode active material.

Because the hybridized anode material formed from the carbon nano fiber and graphite in which the carbon nano fiber surrounds the graphite surface in a vine form does not mainly influence the change of initial particle size of anode active material, this hybridized material can be used without further milling process as anode active material for lithium secondary battery. The electrode of secondary battery can be prepared in a known method. Specifically, the electrode has been prepared with following steps comprising i) dissolving the binder (PVDF) with NMP solvent; ii) preparing the slurry containing the binder and the anode active material where their wt ratio is 15:85, respectively; and iii) coating the obtained slurry on the copper plate whose thickness is 15 micro meter. To remove the organic solvent completely, the prepared electrode shall be dried in a vacuum oven at 120˜180° C. for 12 hours. After drying the obtained electrode, the surface of the electrode is pressed using a roller in order to bind the electrode strongly to the copper plate as well as to maintain the density of electrode constantly. The shape of the electrode is coin shape with 12 mm of diameter. Further, an opposite electrode is prepared using lithium metal, while the electrolyte is prepared using 1M of LiPF₆ (EC:DEC=1:1 v/v). Through the observation using FE-SEM, the growth of carbon nano fibers has been confirmed on the carbon nano fiber/graphite hybridized anode active material. The apparatus for observation is FE-SEM model JSM-6700F made by JEOL and the standard magnitude of SEM is adjusted to ×100000 at the time of starting. Further, TEM observation has been made simultaneously to interpret the structure under 200 kV condition.

The present invention can be explained more concretely by following Preparation Examples, Comparative Preparation Examples, Examples and Comparative Examples. However, the scope of the present invention shall not be limited by following Examples.

EXAMPLES

Preparation Example 1

Preparation of a Negative Electrode Containing Natural Graphite Anode Material Hybridized with Carbon Nano Fibers

9 g of natural graphite, 5.09 g of nickel nitrate (Ni(NO₃)₂6H₂O), 0.5 g of ammonium bicarbonate (NH₄HCO₃) and 300 ml of water are mixed for 1 hour to prepare suspension. The removal of water content is performed by filtering the obtained suspension using a funnel filter. Then, the obtained solid content is dried using a vacuum oven at 100° C. for 24 hours. 1 g of dried graphite solid content is coated on the quartz plate. Using a horizontal quartz tube, the obtained material is heated from 100° C. to 550° C. in a heating velocity of 10° C./min with flowing helium:hydrogen mixed gas (160 ml/min:40 ml/min). The material is laid at 550° C. for 2 hours. The gas phase carbonizing reaction is carried out for 5 min by flowing ethylene:hydrogen:helium (80 ml/min:40 ml/min: 80 ml/min) mixed gas. It has been revealed that the amount of the synthesized carbon nano fiber is 23 wt %, the aspect ratio is more than 50, and the diameter of fiber is 10˜50 nm which is obtained from the FE-SEM observation. Also, through the TEM observation, the structure of the carbon nano fiber is observed as herringbone structure.

FIG. 3 and FIG. 4 show the structure of the carbon nano fiber obtained in this Example. Using the obtained anode active material, negative electrode has been prepared by spreading the slurry (anode active material:binder=85:15, weight ratio) to the copper plate.

Preparation Example 2

Preparation of a Negative Electrode Containing Natural Graphite Anode Material Hybridized with Carbon Nano Fibers

10 g of natural graphite, 0.79 g of nickel nitrate (Ni(NO₃)₂6H₂O), 0.29 g of iron nitrate (Fe(NO₃)₂9H₂O), 1.0 g of ammonium bicarbonate (NH₄HCO₃) and 300 ml of water are mixed for 1 hour to prepare suspension. The removal of water content is performed by filtering the obtained suspension using a funnel filter. Then, the obtained solid content is dried using a vacuum oven at 100° C. for 24 hours. 1 g of dried graphite solid content is coated on the quartz plate. Using a horizontal quartz tube, the obtained material is heated from 100° C. to 580° C. in a heating velocity of 10° C./min with flowing helium:hydrogen mixed gas (160 ml/min:40 ml/min). The material is laid at 580° C. for 2 hours. The gas phase carbonizing reaction is carried out for 30 min by flowing carbon monooxide:hydrogen (160 ml/min:40 ml/min) mixed gas. It has been revealed that the amount of the synthesized carbon nano fiber is 16 wt %, the aspect ratio is more than 50, and the diameter of the fiber is 20˜60 nm which is obtained from the FE-SEM observation. Also, through the TEM observation, the structure of the carbon nano fiber is observed as platelet structure.

Using the obtained anode active material, a negative electrode has been prepared by spreading the slurry (anode active material:binder=85:15, weight ratio) to the copper plate.

Preparation Example 3

Preparation of a Negative Electrode Containing Amorphous Silicon Anode Material Hybridized with Carbon Nano Fibers

50 g of crystalline silicon and 500 g of metal sphere having 10 mm diameter are laid on 500 ml of metal bowl in argon atmosphere. Using the planetary mill, crystalline silicon is milled with rotation at 200 rpm. The milling time is 3 hours (FIG. 7). 10 g of milled partially amorphous silicon powder, 0.99 g of cobalt nitrate (Co(NO₃)₃9H₂O), 2.2 g of ammonium bicarbonate (NH₄HCO₃) and 300 ml of water are mixed for 1 hour to prepare suspension. The removal of water content is performed by filtering the obtained suspension using a funnel filter. Then, the obtained solid content is dried using a vacuum oven at 100° C. for 24 hours. 1 g of dried graphite solid content is coated on the quartz plate. Using a horizontal quartz tube, the obtained material is heated from 100° C. to 550° C. in a heating velocity of 10° C./min with flowing helium:hydrogen mixed gas (160 ml/min:40 ml/min). The material is laid at 550° C. for 2 hours. The gas phase carbonizing reaction is carried out for 10 min by flowing ethylene:hydrogen:helium (80 ml/min:40 ml/min:80 ml/min) mixed gas. It has been revealed that the amount of the synthesized carbon nano fiber is 15 wt %, the aspect ratio is more than 50, and the diameter of the fiber is 10˜20 nm which is obtained from the FE-SEM observation. Also, through the TEM observation, the structure of the carbon nano fiber is observed as herringbone structure.

Using the obtained anode active material, a negative electrode has been prepared by spreading the slurry (anode active material:binder=85:15, weight ratio) to the copper plate.

Preparation Example 4

Preparation of a Negative Electrode Containing Amorphous Silicon Anode Material Hybridized with Carbon Nano Fibers

Anode active material and carbon nano fibers are prepared as the same manner with Preparation Example 3 except that the milling time using planetary mill is changed from 3 hours to 6 hours (FIG. 7 and FIG. 9).

It has been revealed that the amount of the synthesized carbon nano fiber is 31 wt %, the aspect ratio is more than 50, and the diameter of the fiber is 10˜20 nm which is obtained from the FE-SEM observation. Also, through the TEM observation, the structure of the carbon nano fiber is observed as herringbone structure.

FIG. 5 shows the structure of the carbon nano fiber obtained in this Example. Using the obtained anode active material, a negative electrode has been prepared by spreading the slurry (anode active material:binder=85:15, weight ratio) to the copper plate.

Preparation Example 5

Preparation of a Negative Electrode Containing Natural Graphite and Amorphous Silicon Complex Anode Material Hybridized with Carbon Nano Fibers

43.5 g of crystalline silicon, 6.5 g of natural graphite and 500 g of metal sphere having 10 mm diameter are laid on 500 ml of metal bowl in argon atmosphere. Using the planetary mill, crystalline silicon is milled with rotation at 200 rpm. The milling time is 1 hour (FIG. 8). 10 g of milled amorphous silicon/graphite complex powder, 0.99 g of cobalt nitrate (Co(NO₃)₃9H₂O), 2.2 g of ammonium bicarbonate (NH₄HCO₃) and 300 ml of water are mixed for 1 hour to prepare suspension. The removal of water content is performed by filtering the obtained suspension using a funnel filter. Then, the obtained solid content is dried using a vacuum oven at 100° C. for 24 hours. 1 g of dried graphite solid content is coated on the quartz plate. Using a horizontal quartz tube, the obtained material is heated from 300° C. to 550° C. in a heating velocity of 10° C./min with flowing helium:hydrogen mixed gas (160 ml/min:40 ml/min). The material is laid at 550° C. for 2 hours. The gas phase carbonizing reaction is carried out for 10 min by flowing ethylene:hydrogen:helium (80 ml/min:40 ml/min:80 ml/min) mixed gas. It has been revealed that the amount of the synthesized carbon nano fiber is 12 wt %, the aspect ratio is more than 50, and the diameter of the fiber is 10˜20 nm which is obtained from the FE-SEM observation. Also, through the TEM observation, the structure of the carbon nano fiber is observed as herringbone structure.

Using the obtained anode active material, a negative electrode has been prepared by spreading the slurry (anode active material:binder=85:15, weight ratio) to the copper plate.

Preparation Example 6

Preparation of a Negative Electrode Containing Natural Graphite and Amorphous Silicon Complex Anode Material Hybridized with Carbon Nano Fibers

Anode active material and carbon nano fibers are prepared as the same manner with Preparation Example 5 except that the milling time using planetary mill is changed from 1 hour to 8 hours (FIG. 8).

It has been revealed that the amount of the synthesized carbon nano fiber is 21 wt %, the aspect ratio is more than 50, and the diameter of the fiber is 10˜20 nm which is obtained from the FE-SEM observation. Also, through the TEM observation, the structure of the carbon nano fiber is observed as herringbone structure.

Using the obtained anode active material, a negative electrode has been prepared by spreading the slurry (anode active material:binder=85:15, weight ratio) to the copper plate.

Preparation Example 7

Preparation of a Negative Electrode Containing Natural Graphite and Amorphous Silicon Complex Anode Material Hybridized with Carbon Nano Fibers

Anode active material and carbon nano fibers are prepared as the same manner with Preparation Example 5 except that the milling time using a planetary mill is changed from 1 hour to 13 hours (FIG. 8).

It has been revealed that the amount of the synthesized carbon nano fiber is 35 wt %, the aspect ratio is more than 50, and the diameter of the fiber is 10˜20 nm which is obtained from the FE-SEM observation. Also, through the TEM observation, the structure of the carbon nano fiber is observed as herringbone structure.

Using the obtained anode active material, a negative electrode has been prepared by spreading the slurry (anode active material:binder=85:15, weight ratio) to the copper plate.

Preparation Example 8

Preparation of a Negative Electrode Containing Natural Graphite and Amorphous Silicon Complex Anode Material Hybridized with Carbon Nano Fibers

Anode active material and carbon nano fibers are prepared as the same manner with Preparation Example 5 except that the milling time using a planetary mill is changed from 1 hour to 18 hours (FIG. 8).

It has been revealed that the amount of the synthesized carbon nano fiber is 39 wt %, the aspect ratio is more than 50, and the diameter of the fiber is 10˜20 nm which is obtained from the FE-SEM observation. Also, through the TEM observation, the structure of the carbon nano fiber is observed as herringbone structure.

Using the obtained anode active material, a negative electrode has been prepared by spreading the slurry (anode active material:binder=85:15, weight ratio) to the copper plate.

Preparation Example 9

Preparation of a Negative Electrode Containing Natural Graphite and Amorphous Silicon Complex Anode Material Hybridized with Carbon Nano Fibers

Anode active material and carbon nano fibers are prepared as the same manner with Preparation Example 5 except that the milling time using planetary mill is changed from 1 hour to 25 hours (FIG. 8 and FIG. 10)

It has been revealed that the amount of the synthesized carbon nano fiber is 50 wt %, the aspect ratio is more than 50, and the diameter of the fiber is 10˜20 nm which is obtained from the FE-SEM observation. Also, through the TEM observation, the structure of the carbon nano fiber is observed as herringbone structure.

Using the obtained anode active material, a negative electrode has been prepared by spreading the slurry (anode active material:binder=85:15, weight ratio) to the copper plate.

Comparative Preparation Example 1

Preparation of a Negative Electrode Containing Crystalline Silicon/Graphite Anode Material Hybridized with Carbon Nano Fibers

43.5 g of crystalline silicon powder screened by 320 mesh sieve and 6.5 g of natural graphite powder are laid on 500 ml of plastic bowl, and mixed in dried ball-mill method for 1 hour.

10 g of silicon/graphite mixed powder, 0.99 g of cobalt nitrate (Co(NO₃)₃9H₂O), 2.2 g of ammonium bicarbonate (NH₄HCO₃) and 300 ml of water are mixed for 1 hour to prepare suspension. The removal of water content is performed by filtering the obtained suspension using a funnel filter. Then, the obtained solid content is dried using a vacuum oven at 100° C. for 24 hours. 1 g of dried graphite solid content is coated on the quartz plate. Using a horizontal quartz tube, the obtained material is heated from 300° C. to 550° C. in a heating velocity of 10° C./min with flowing helium:hydrogen mixed gas (160 ml/min:40 ml/min). The material is laid at 550° C. for 2 hours. The gas phase carbonizing reaction is carried out for 10 min by flowing ethylene:hydrogen:helium (80 ml/min:40 ml/min:80 ml/min) mixed gas (FIG. 2B). It has been revealed that the amount of the synthesized carbon nano fiber is 12 wt %, the aspect ratio is more than 50, and the diameter of the fiber is 10˜80 nm which is obtained from the FE-SEM observation. The growth of the carbon nano fiber mainly occurs on the surface of graphite active material, whereas only the small amount of growth of the carbon nano fiber is observed on the surface of silicon.

Using the obtained anode active material, a negative electrode has been prepared by spreading the slurry (anode active material:binder=85:15, weight ratio) to the copper plate.

Comparative Preparation Example 2

Preparation of a Negative Electrode Containing Crystalline Silicon Anode Material Hybridized with Carbon Nano Fibers

10 g of crystalline silicon powder screened by 320 mesh sieve, 0.99 g of cobalt nitrate (Co(NO₃)₃9H₂O), 2.2 g of ammonium bicarbonate (NH₄HCO₃) and 300 ml of water are mixed for 1 hour to prepare suspension. The removal of water content is performed by filtering the obtained suspension using a funnel filter. Then, the obtained solid content is dried using a vacuum oven at 100° C. for 24 hours. 1 g of dried graphite solid content is coated on the quartz plate. Using a horizontal quartz tube, the obtained material is heated from 300° C. to 550° C. in a heating velocity of 10° C./min with flowing helium:hydrogen mixed gas (160 ml/min:40 ml/min). The material is laid at 550° C. for 2 hours. The gas phase carbonizing reaction is carried out for 10 min by flowing ethylene:hydrogen:helium (80 ml/min:40 ml/min:80 ml/min) mixed gas. It has been revealed that the amount of the synthesized carbon nano fiber is 28 wt %, the aspect ratio is more than 50, and the diameter of the fiber is 10˜30 nm which is obtained from the FE-SEM observation. The growth of carbon nano fiber mainly occurs only on the corner part of silicon powder, but not on the plane part of silicon powder (FIG. 6A). Further, silicon powder and the carbon nano fiber are easily separated without hybridization (FIG. 6B).

Using the obtained anode active material, a negative electrode has been prepared by spreading the slurry (anode active material:binder=85:15, weight ratio) to the copper plate.

Comparative Preparation Example 3

Preparation of a Negative Electrode Containing Only Natural Graphite

Using only natural graphite as anode active material, a negative electrode has been prepared by spreading the slurry (natural graphite:binder=85:15, weight ratio) to the copper plate.

Comparative Preparation Example 4

Preparation of a Negative Electrode Containing Natural Graphite Anode Material and Carbon Nano Material

Carbon nano material is prepared according to the method disclosed in U.S. Pat. No. 6,440,610 B1. Then, a negative electrode has been prepared by spreading the slurry (anode active material prepared by the method disclosed in U.S. Pat. No. 6,440,610 B1, natural graphite:binder=85:15, weight ratio) to the copper plate.

The following description is a method for preparing carbon nano material in U.S. Pat. No. 6,440,610 B1. “After dissolving 20 g of nickel nitrate into water, the solution was mixed with 200 g of natural graphite. Graphite material on a surface layer on which particles of nickel nitrate were formed was obtained by spray drying the mixture. A resulting graphite material on which nickel oxides were formed was obtained by carbonizing the obtained graphite material at a temperature of 800° C., and oxidizing the carbide in air at a temperature of 400° C. for about 4 hours. The obtained resulting graphite material was passed through a reduction process in which hydrogen was used for about 20 hours at a temperature of 500° C., obtaining natural graphite powder on a surface layer of which Ni particles were formed. A vapor growing fiber was grown on Ni catalysts in a vapor deposition method by putting the obtained powder into a ceramic boat and injecting acetylene gas into the boat at a temperature of about 600° C. After a reaction for about 30 minutes, acetylene gas was substituted with argon, and vapor growing fibers were slowly cooled to the ordinary temperature” (FIG. 2C).

We have conducted an experiment according to the method described above. However, the desirable carbon nano fiber having following conditions, such as diameter (5˜300 nm), aspect ratio (10˜10000), and thickness of carbon nano fibers (5˜1000 nm), cannot be obtained. What we can obtain is only fiber type carbon polymeric material. However, we have measured the yield of the carbon nano tube using an analytical apparatus. In any event, the amount of the carbon nano tube in the carbon polymeric material is less than 5 wt % of the total carbon polymeric material.

Using the carbon nano material obtained in this Example, a negative electrode has been prepared by spreading the slurry (the obtained carbon nano material:binder=85:15, weight ratio) to the copper plate.

Comparative Preparation Example 5

Preparation of a Negative Electrode Containing Natural Graphite Anode Material Hybridized with Carbon Nano Tubes

10 g of natural graphite, 3.65 g of iron nitrate (Fe(NO₃)₂9H₂O), 7.3 g of ammonium bicarbonate (NH₄HCO₃) and 300 ml of water are mixed for 1 hour to prepare suspension. The removal of water content is performed by filtering the obtained suspension using a funnel filter. Then, the obtained solid is dried using a vacuum oven at 100° C. for 24 hours. 1 g of dried graphite solid is coated on the quartz plate. Using a horizontal quartz tube, the obtained material is heated from 300° C. to 680° C. in a heating velocity of 10° C./min with flowing helium:hydrogen mixed gas (160 ml/min:40 ml/min). The material is laid at 550° C. for 2 hours. The gas phase carbonizing reaction is carried out for 30 min by flowing carbon monooxide:hydrogen (160 ml/min:40 ml/min) mixed gas. It has been revealed that the amount of the synthesized carbon nano material is 5 wt %, the aspect ratio is more than 50, and the diameter of the fiber is 20˜40 nm which is obtained from the FE-SEM observation. Also, through the TEM observation, the structure of the carbon nano material is observed as carbon nano tubes.

Using the obtained anode active material, a negative electrode has been prepared by spreading the slurry (anode active material:binder=85:15, weight ratio) to the copper plate.

Table 1 shows the composition of anode active material and the amount of grown carbon nano fibers in Preparation Examples and Comparative Preparation Examples.

TABLE 1
	Amount of	Amount of		Amorphous	Amount of
	graphite	silicon	Milling time	degree	grown carbon
Sample	(wt part)	(wt part)	(hr)	(%)	nano fiber (%)
Prep. Exp. 1	100	0	0	—	23
Prep. Exp. 2	100	0	0	—	16
Prep. Exp. 3	0	100	3	15	15
Prep. Exp. 4	0	100	6	59	31
Prep. Exp. 5	13	87	1	2	12
Prep. Exp. 6	13	87	8	71	21
Prep. Exp. 7	13	87	13	86	35
Prep. Exp. 8	13	87	18	89	39
Prep. Exp. 9	13	87	25	89	50
Com. Prep. Exp. 1	13	87	0	0	12
Com. Prep. Exp. 2	0	100	0	0	28
Com. Prep. Exp. 3	100	0	0	—	0
Com. Prep. Exp. 4	100	0	0	—	<5
Com. Prep. Exp. 5	100	0	0	—	5

In this table, ‘Amorphous degree’ is measured by the comparison of main peak strength in crystalline silicon d₍₁₁₁₎ plate through XRD. It is calculated by the following equation;

(Main peak strength in crystalline silicon d₍₁₁₁₎ plate before milling−main peak strength in silicon d₍₁₁₁₎ plate for preparation)/main peak strength in crystalline silicon d₍₁₁₁₎ plate before milling×100(%)

‘The amount of the grown carbon nano fiber’ is calculated by the following equation;

(Weight of hybridized anode active material−weight of anode active material before reaction)/weight of anode active material before reaction×100(%)

Example 1˜9

Charging/Discharging Test of Anode in Secondary Battery

The charging/discharging capacity has been measured using the anode prepared in Preparation Examples 1˜9.

Using the assembled half cell, charging/discharging cycles (12 min cycle, 1 hr cycle, 10 hrs cycle) have been carried out 30 times. The maintenance of charging/discharging capacity has been measured in each cycle. Table 2 shows the maintenance of charging/discharging capacity.

Comparative Example 1˜5

Charging/Discharging Test of Anode in Secondary Battery

The charging/discharging capacity has been measured using the anode prepared in Comparative Preparation Examples 1˜5. In Comparative Preparation Examples 1˜2, the anode is prepared by hybridizing grown carbon nano fibers randomly. In Comparative Preparation Examples 3, the anode is prepared by natural graphite. In Comparative Preparation Examples 4, the anode is prepared by hybridizing grown carbon nano fibers randomly. In Comparative Preparation Examples 5, the anode is prepared by hybridizing grown carbon nano tubes.

Using the assembled half cell, charging/discharging cycles (12 min cycle, 1 hr cycle, 10 hrs cycle) have been carried out 30 times. The maintenance of charging/discharging capacity has been measured in each cycle. Table 2 shows the maintenance of charging/discharging capacity.

	TABLE 2
		Maintenance
	Amount of	capacity after
	grown carbon	discharging (%)	Structure of carbon
Example No.	nano fiber (%)	0.1 C	1 C	5 C	nano fiber
Exp. 1	23	98	93	84	herringbone
Exp. 2	16	99	96	87	platelet
Exp. 3	15	98	94	86	herringbone
Exp. 4	31	99	95	85	herringbone
Exp. 5	12	98	92	84	herringbone
Exp. 6	21	99	94	84	herringbone
Exp. 7	35	99	94	88	herringbone
Exp. 8	39	99	96	87	herringbone
Exp. 9	50	99	88	83	herringbone
Com. Exp. 1	12	98	62	72	random herringbone
Com. Exp. 2	28	98	61	49	random herringbone
Com. Exp. 3	0	98	75	30	natural graphite
Com. Exp. 4	0	98	75	30	random carbon
					fiber
Com. Exp. 5	5	99	92	68	carbon nano
					tube(CNT)

Claims

1. Anode active material hybridized with carbon nano fibers for lithium secondary battery prepared by following steps comprising, i) dispersing the nano size metal catalyst to the surface of anode material selected from graphite, amorphous silicon and/or the complex of graphite and amorphous silicon; andii) growing the carbon nano fiber by chemical vapor deposition method, wherein carbon nano fibers are grown in a vine form and surround the surface of anode active material.

2. The anode active material hybridized with carbon nano fibers for lithium secondary battery according to claim 1, wherein said amorphous silicon is prepared by pre-treatment using mechanical friction energy in an inert atmosphere.

3. The anode active material hybridized with carbon nano fibers for lithium secondary battery according to claim 2, wherein the complex of graphite and amorphous silicon is prepared by the weight ratio of 1˜50 wt % of graphite and 50˜99 wt % of amorphous silicon.

4. The anode active material hybridized with carbon nano fibers for lithium secondary battery according to claim 1, wherein the structure of the carbon nano fiber is platelet or herringbone structure hybridized with anode active material.

5. The anode active material hybridized with carbon nano fibers for lithium secondary battery according to claim 1, wherein the grown amount of carbon nano fibers is 1˜200 wt part as to 100 wt part of anode active material, the diameter of carbon nano fibers is 5˜300 nm, the aspect ratio is 10˜10000, the thickness of carbon nano fibers on the active anode material is 5˜1000 nm.

6. The anode active material hybridized with carbon nano fibers for lithium secondary battery according to claim 5, wherein the grown amount of carbon nano fibers is 5˜100 wt part as to 100 wt part of anode active material, the diameter of carbon nano fibers is 5˜100 nm, the aspect ratio is 10˜1000, the thickness of carbon nano fibers on the active anode material is 10˜500 nm.

7. The anode active material hybridized with carbon nano fibers for lithium secondary battery according to claim 6, wherein the grown amount of carbon nano fibers is 10˜80 wt part as to 100 wt part of anode active material, the diameter of carbon nano fibers is 5˜50 nm, the aspect ratio is 10˜100, the thickness of carbon nano fibers on the active anode material is 15˜200 nm.

8. The anode active material hybridized with carbon nano fibers for lithium secondary battery according to claim 1, wherein said carbon nano fiber is prepared by chemical vapor deposition method using a carbon source selected from carbon monoxide, methane, acetylene or ethylene in the presence of metal catalyst and said metal catalyst comprised at least one selected from the group consisting of Fe, Co, Ni, Cu, Mg, Mn, Ti, Sn, Si, Zr, Zn, Ge, Pb and In, which is in the form of alkoxide, oxide, chloride, nitrate or carbonate.

9. The anode active material hybridized with carbon nano fibers for lithium secondary battery according to claim 8, wherein said catalyst can be prepared in the form of a supported catalyst using a sol-gel method, a precipitation method, a hydrothermal method, a spray heating method, a spray drying method or a ball-mill method

10. The anode active material hybridized with carbon nano fibers for lithium secondary battery according to claim 1, wherein said carbon nano fiber is prepared by following steps comprising i) heating the anode active material particles selected from graphite, amorphous silicon and/or the complex of graphite and amorphous silicon using mixed gas of helium and hydrogen (3˜5 L/min: 1 L/min) at 300˜650° C.; andii) growing the carbon nano fiber by vapor deposition using a carbon source selected from carbon monoxide, methane, acetylene or ethylene in the presence of catalyst composition made by nickel nitrate and ammonium bicarbonate in mixed gas of helium and hydrogen at 400˜800° C.

11. A lithium secondary battery prepared by anode active material of claim 1.