KR20100014993A - Silicon anode composite active material for lithium secondary battery - Google Patents

Abstract

PURPOSE: A silicon-based negative active material composite is provided to improve cycle property through the coating of pyrolytic carbon and the growth of carbon nanofiber, and to improve initial efficiency by reducing specific surface area increased by the growth of carbon nanofiber. CONSTITUTION: A silicon-based negative active material composite for a lithium secondary battery is prepared by the steps of: (i) surface-coating 3~50 weight% silicon-based support powder containing silicon powder or silicon and at least one kind of metal alloy compound selected from transition metal, Groups 2, 13, 14 and 15 in the presence of a carbon supply source with amorphous pyrolytic carbon at 600~1100 °C with 5~200 nm thickness; and (ii) dispersing a transition metal catalyst on the surface of the silicon-based support powder and then growing the carbon nanofiber at 400~800 °C in the presence of carbon source in a gas-phase.

Description

Silicon anode composite active material for lithium secondary battery

The present invention relates to a silicon-based negative electrode active material composite for lithium secondary batteries, and more particularly, to a negative electrode active material composite for lithium secondary batteries prepared by growing conductive carbon nanofibers on a high-capacity silicon-based support. More specifically, the present invention relates to a silicon-based negative electrode active material composite prepared by coating pyrolytic carbon on a silicon carrier to improve cycle and initial efficiency characteristics, and then vapor-growing carbon nanofibers on a pyrolytic carbon coating film in the presence of a catalyst.

Li-ion secondary batteries have attracted attention as power sources for driving electronic devices. Although graphite is mainly used as a negative electrode material of a lithium ion secondary battery, graphite has a small capacity per unit mass of 372 mAh / g, and it is difficult to increase the capacity of a lithium ion secondary battery.

As a negative electrode material exhibiting higher capacity than graphite, for example, a material for forming an intermetallic compound with lithium such as silicon, tin, and oxides thereof is promising. However, these materials cause a change in crystal structure when absorbing and storing lithium, causing a problem of expansion of the volume. In the case of silicon, the maximum amount of lithium is absorbed and stored, which is converted to Li _4.4 Si, whereby volume expansion by charging is performed, and in this case, the volume increase rate by charging expands to about 4.12 times the volume of silicon before volume expansion. On the other hand, the volume expansion rate of graphite which is currently used as a cathode material is about 1.2 times.

Therefore, many studies for increasing the capacity of the negative electrode active material such as silicon, that is, researches for reducing the volume expansion rate through alloying of silicon have been conducted, but there was a problem in the practical use, and the main causes are Si, Sn, Al during charge and discharge. This is because metals such as alloying with lithium cause volume expansion and contraction, which causes metal micronization and deterioration in cycle characteristics.

Therefore, in order to improve cycle characteristics, it is proposed to use an amorphous alloy oxide as a negative electrode active material, as described in Y. Idota, et al: Sience, 276, 1395 (1997). In addition, an alloy having an amorphous structure was used as a negative electrode active material, as described in the 43rd Battery Debate Preliminary Proceedings, the Korean Electrochemical Society Battery Technical Committee, October 12, 2014, p. 308-309. In some cases, it is reported that the cycle characteristics are improved.

Silicon, which is known as the element that can be expected to have the highest capacity, is conventionally made of silicon based on the mechanical alloying method, although it is very difficult to conventionally amorphous silicon alone and it is difficult to amorphize the alloy whose main component is silicon. A method has been developed that can easily amorphous the material.

In the case of the amorphous material, since it is not a single structure like the crystalline material, it is known that the expansion rate due to filling is lower than that of the crystalline material and the deterioration due to charge and discharge is less than that of the crystalline material. In addition, the reason why the initial cycle performance is better than that of the crystalline material is particularly complicated by the diffusion passage of lithium ions, and the active material utilization is gradually increased as the number of cycles progresses. This is because the cycle deterioration due to this is alleviated as a result.

Korean Patent Registration No. 10-529103 'Negative Active Material for Lithium Secondary Battery, Manufacturing Method and Lithium Secondary Battery' discloses a method of manufacturing a negative electrode active material for lithium secondary battery using such an amorphous silicon alloy, and more specifically, The silicon and element M powders are alloyed by a mechanical alloying method to form a SiM alloy, and the SiM alloy is heat-treated to add the powder of element X to the SiM alloy after the heat treatment, which is then alloyed by the mechanical alloying method. A negative electrode active material for a lithium secondary battery, which is manufactured by manufacturing a SiMX alloy and heat treating the SiMX alloy at a temperature lower than a processing temperature in the heat treatment, wherein M is Ni, Co, B, Cr, Cu, Fe. , At least one element selected from the group consisting of Mn, Ti, and Y, and element X is selected from the group consisting of Ag, Cu, and Au. And at least one element selected, Cu discloses a negative active material that is selected at the same time as the element M and the element X.

However, as the number of charge and discharge cycles is further increased in the case of the negative electrode active material for a lithium secondary battery manufactured by the above method, the charge and discharge capacity decreases due to deterioration of the internal silicon, and in the case of the mechanical alloy method disclosed in the patent document, crushing and compression As a result, the deterioration of the interface between microalloy structures which are not distinguished by X-ray diffraction analysis is broken and microstructures are disrupted by occlusion release of lithium.

Of course, these problems are Republic of Korea Patent Registration No. 10-529103 alloying the Si powder and the powder of the element M alloyed by mechanical alloying method and SiM alloying, after the heat treatment of the SiM alloy is added to the powder of the element X to the SiM alloy, It is solved by introducing a process of alloying this by mechanical alloying method to form a SiMX alloy and heat-treating the SiMX alloy at a temperature lower than the temperature in the heat treatment, but the negative electrode material for a lithium secondary battery that still alloys silicon is still used. It did not have sufficient charge and discharge capacity and cycle characteristics was not enough to replace the negative electrode material of the graphite lithium secondary battery.

In order to solve this problem, the inventors of the present invention, in Korea Patent Application No. 2008-25224 "Silicone anode active material for lithium secondary battery" ⅰ) at least one metal selected from transition metal, group 2, 13, 14, 15 group elements to silicon 100 parts by weight of a mixture of silicon metal alloy powdered by primary mechanical processing by mixing; Ii) 1 to 20 parts by weight of a carbon material powder compound selected from graphite powder, carbon inorganic powder or carbon powder; And iii) a mixture of silicon alloy and carbon material having an average particle size of 0.5 to 30 탆, powdered by secondary mechanical processing of 1 to 20 parts by weight of carbon nanofibers, carbon nanotubes or mixtures thereof at 400 to 1200 ° C. A negative active material for a lithium secondary battery manufactured by heat treatment for 1 to 10 hours has been developed.

However, in the present invention, while continuing to research to significantly improve the charge and discharge capacity and cycle characteristics of the anode material for lithium secondary batteries prepared by alloying with the mechanical alloying method between the silicon and metal elements, and with the known silicon and metal elements When the coating of pyrolytic carbon and vapor phase growth of carbon nanofibers were added to the SiM powder coexisting in the form of alloy, the cycle and initial efficiency characteristics and charge / discharge capacity of the lithium secondary battery anode material were remarkably improved, and the present invention was completed. will be.

The problem to be solved by the present invention is to coat the pyrolytic carbon on the silicon-based carrier powder consisting of a silicon powder or a mixed powder of silicon and alloy, and then vapor-grown carbon nanofibers in the presence of a catalyst, the silicon-based negative active material obtained as needed It is intended to develop a silicon-based negative active material composite obtained by recoating pyrolyzed carbon on the composite.

The silicon-based negative electrode active material composite according to the present invention has characteristics such as low capacity, low electrical conductivity, sudden decrease in capacity due to excessive volume expansion during charging, and deterioration of cycle performance, such as coating of pyrolytic carbon and growth of carbon nanofibers. It can be improved. On the other hand, by re-coating pyrolytic carbon on the silicon-based negative active material composite of the present invention to reduce the specific surface area increased by the growth of carbon nanofibers to improve the initial efficiency to develop a negative electrode active material for lithium secondary batteries.

An object of the present invention is to provide a silicon-based support powder composed of silicon powder or silicon and a transition metal, powder of at least one metal alloy selected from group 2, 13, 14, and 15 elements in the presence of a carbon source. Surface coating the carrier substrate surface with amorphous pyrolytic carbon at 3 ° C., 5 to 200 nm thick and 3 to 50 wt%, based on the weight of the carrier; Ii) A silicon-based negative active material for a lithium secondary battery having a maximum particle size of 45 μm or less prepared by dispersing a transition metal catalyst on a surface of a silicon-based support surface coated with pyrolytic carbon and vapor-growing carbon nanofibers at 400 to 800 ° C. in the presence of a carbon source. To provide a composite.

In addition, the at least one metal selected from the transition metal, Group 2, Group 13, Group 14 and Group 15 elements is Mg, Ca, Sr, Ba, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, At least one metal selected from among Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Al, Sn, and Sb elements. .

On the other hand, the carbon nanofiber vapor phase growth step is carried out by supporting at least 0.5 to 10% by weight of at least one transition metal catalyst selected from Groups 3 to 12 on the periodic table on the surface of the metal silicon-based support, ethylene, acetylene, methane, carbon monoxide, propane, In the presence of at least one carbon source selected from propylene and ethane, carbon nanofibers are grown at a temperature of 400 to 800 ° C. on the surface of the silicon carrier by 3 to 150% by weight of the silicon carrier.

In addition, another object of the present invention is to remove the transition metal catalyst for carbon nanofiber growth in the silicon-based negative active material composite for lithium secondary battery obtained, and then the surface of the amorphous substrate pyrolytic carbon at 600 ~ 1100 ℃ in the presence of a carbon source It is to provide a lithium secondary battery silicon-based negative active material composite prepared by further comprising the step of coating 3 to 50% by weight of the secondary surface of 5 to 200 nm thickness and the weight of the composite.

Meanwhile, the carrier surface coating or the composite secondary surface coating may be a coating by pyrolytic carbon using at least one gaseous and liquid carbon source selected from ethylene, acetylene, methane, carbon monoxide, propane, propylene, and ethane; Coating by pitch of liquid and solid phases; Or an amorphous carbon coating by a ball mill or the like.

The effect of the present invention is to coat the pyrolytic carbon on the silicon-based carrier powder consisting of a silicon powder or a mixed powder of silicon and alloy, and then vapor-grown carbon nanofibers in the presence of a catalyst, and again to the silicon-based negative electrode active material composite obtained as needed It is to provide a silicon-based negative electrode active material composite obtained by recoating the pyrolysis carbon.

The silicon-based negative electrode active material composite according to the present invention has characteristics such as low capacity, low electrical conductivity, sudden decrease in capacity due to excessive volume expansion during charging, and deterioration of cycle performance, such as coating of pyrolytic carbon and growth of carbon nanofibers. It can be improved. On the other hand, by re-coating pyrolytic carbon on the silicon-based negative active material composite of the present invention to reduce the specific surface area increased by the growth of carbon nanofibers to improve the initial efficiency to provide a negative electrode active material for lithium secondary batteries.

Hereinafter, the present invention will be described in more detail.

The present invention seeks to improve the problems of low initial efficiency and cycle characteristics of silicon or silicon alloy anode materials for high capacity lithium secondary batteries, and in the transition metal, group 2, 13, 14 and 15 elements of silicon or silicon, Surface-coating with amorphous pyrolytic carbon material on the surface of the carrier using a silicon metal alloy which has been mechanically processed and powdered together with at least one selected metal, and then carbon in the presence of a carbon source using a transition metal as a catalyst It is to provide a silicon-based negative electrode active material composite in which the nanofibers are vapor-grown.

In addition, to remove the transition metal catalyst from the negative electrode active material composite obtained above to provide a silicon-based negative electrode active material composite prepared by secondary surface coating the surface of the composite with amorphous pyrolysis carbon. At this time, the silicon-based negative active material composite is characterized in that the size of the silicon-based carrier particles have a maximum particle size of 45 ㎛ or less.

In addition, the present invention is to produce a silicon-based composite by the coating of pyrolytic carbon, the growth of carbon nanofibers, the secondary coating of pyrolytic carbon on the surface of the silicon-based support powder, due to low volume, low electrical conductivity, and excessive volume expansion caused by charging Problems such as sudden drop in capacity and deterioration of cycle performance are improved by the coating of pyrolytic carbon and the growth of carbon nanofibers, and the specific surface area increased by the growth of carbon nanofibers by the secondary coating of pyrolytic carbon. There is an effect that greatly improves the efficiency and the like.

In the present invention, at 600 to 1100 ° C using silicon or a silicon metal alloy which is mechanically processed and powdered together with at least one metal selected from transition metals, group 2, group 13, group 14 and group 15 elements to silicon or silicon. The amorphous pyrolytic carbon is coated on the surface of the support substrate under one or more carbon sources selected from ethylene, acetylene, methane, carbon monoxide, propane, propylene and ethane.

A negative electrode active material is prepared by dispersing a transition metal catalyst on a surface of silicon-based silicon coated with pyrolytic carbon and vapor-growing carbon nanofibers in the presence of a carbon source at 400 to 800 ° C. Thereafter, the anode active material composite material was secondarily coated with amorphous pyrolytic carbon on the surface of the support substrate under one or more carbon sources selected from ethylene, acetylene, methane, carbon monoxide, propane, propylene, and ethane at 600 to 1100 ° C. on the surface of the composite material. To prepare.

In the composite material coated with pyrolytic carbon, the coating amount of pyrolytic carbon is 3 to 50% by weight based on the weight of the support substrate, and the growth amount of carbon nanofibers is 3 to 150 based on the weight of the support substrate. Weight percent.

At this time, if the coating amount of pyrolytic carbon is less than 3% by weight based on the weight of the support substrate, the effect of improving the binding property between the surface of the support and the carbon nanofibers grown on the support surface is insufficient, and the cycling characteristics of the anode active material composite are deteriorated. There is. In addition, when the amount of pyrolytic carbon coating is 50% by weight or more based on the weight of the support substrate, the Si content which can react with Li decreases as the amount of pyrolytic carbon increases, thereby reducing the charge and discharge capacity.

On the other hand, if the growth amount of carbon nanofibers is less than 3% by weight based on the weight of the carrier substrate, the electron inhalation effect of the carbon nanofibers and the effect of wrapping the carriers to prevent expansion are insufficient, and the cycling characteristics of the anode active material composite are not sufficient. If the growth amount of the fiber is more than 150% by weight based on the weight of the support substrate, there is a problem that the initial efficiency and the charge and discharge capacity is lowered due to the increase in the specific surface area by the carbon nanofibers and the coating amount of the active material per unit volume.

In the composite material coated with pyrolytic carbon, the coating amount of pyrolytic carbon is 3 to 50% by weight based on the weight of the composite, to provide a composite anode active material for a silicon-based lithium secondary battery. In addition, the maximum particle size of the silicon-based support substrate powder is characterized in that less than 45 ㎛. At this time, the particle size of the powder is advantageous to make the particle size as small as possible in order to maximize the disorder of the expansion direction for each powder to prevent the expansion of the powder by the charging of lithium ions, the maximum particle diameter is more than 45 ㎛ As the swollen paper expands and charges and discharges repeatedly, binding between powders and binding between the powder and the current collector are reduced, thereby greatly reducing cycling characteristics.

In addition, the carbon source is characterized in that the organic compound having a gas phase or liquid carbon organic compound structure and the organic material structure can be destroyed by the temperature rise. As an example, it may be selected from ethylene, acetylene, methane, carbon monoxide, propane, propylene, ethane and the like.

In addition, pyrolytic carbon is amorphous and has a coating thickness of 5 to 200 nm. At this time, if the coating thickness of the pyrolytic carbon is less than 5 nm, the effect of improving the binding property between the carbon nanofibers grown on the carrier surface and the carrier surface is not sufficient, and there is a problem in that the cycling characteristics are deteriorated. There is a problem that the charge and discharge capacity is lowered because the Si content that can react with Li decreases with increasing pyrolysis carbon content.

In addition, the transition metal is characterized by impregnating 0.5 to 10% by weight relative to the weight of the carrier with at least one selected from Groups 3 to 12 of the periodic table. At this time, if the impregnation amount of the transition metal is less than 0.5% by weight based on the weight of the carrier, the growth of the carbon nanofibers is unevenly grown or hardly grown, and when the amount exceeds 10% by weight, the growth of the carbon nanofibers is excessively large. Therefore, there is a problem that the initial efficiency and the charge and discharge capacity is lowered due to the increase in the specific surface area and the coating amount of the active material per unit volume.

The present invention will be described in more detail with reference to the following examples. However, these examples do not limit the scope of the present invention.

(Preparation Example 1) Preparation of silicon based negative electrode active material composite for lithium secondary battery through pyrolytic carbon coating and carbon nanofiber vapor phase growth

10 g of crystalline silicon powder (silicon made in Japan, purity of 99.0% or more, average particle size of 10 μm) was put in a horizontal position, and heated to 900 ° C. (10 ° C./min) while supplying helium gas (200 ml / min), The surface of the silicon carrier was coated with pyrolytic carbon while supplying methane (160 ml / min) gas and hydrogen (40 ml / min) gas at 900 ° C. for 30 minutes. It was naturally cooled while supplying helium (200 ml / min) gas to room temperature, and the coating amount of pyrolytic carbon (weight _output / weight _input * 100 [%]) was 6% by weight based on the weight of the carrier. Surfaces were observed with a scanning electron microscope (JEOL, Japan) and a transmission electron microscope (JEOL, Japan) to confirm the coating of pyrolytic carbon.

To 5 g of the surface-coated silicon carrier powder obtained above, 5 wt% of Fe catalyst for carbon nanofiber vapor phase growth was dried using iron nitrate (Fe [NO ₃ ] ₃ -9H ₂ O), and then horizontally Into a temperature of 600 ° C. (10 ° C./min) while supplying helium gas (200 ml / min), and then dipping carbon monoxide (160 ml / min) and hydrogen (40 ml / min) at 600 ° C. for 30 minutes. Carbon nanofibers were grown on the surface of the retardation substrate. It was naturally cooled while supplying helium (200 ml / min) to room temperature, and the growth amount of carbon nanofibers was 99% by weight based on the weight of the carrier. The island diameter of the prepared carbon nanofibers was observed in a nonuniform form of 20-200 nm.

7 g of binder (SBR, Japan) and 3 wt% of thickener (CMC, Japan) were added to 1 g of the negative electrode active material composite thus prepared to prepare a slurry, and then coated on a current collector (Cu foil, thickness of 18 μm). After drying (45 ° C./1 hour) and vacuum drying (105 ° C./12 hour), the electrode was manufactured in the form of a disk having a diameter of 12 mm after pressing the coating electrode on a roller.

Preparation Example 2 Preparation of Silicon-Based Negative Electrode Active Material Composite for Lithium Secondary Battery through Pyrolytic Carbon Coating and Carbon Nanofiber Gas Phase Growth (removal of Transition Metal Catalyst)

In order to remove the Fe catalyst impregnated in 5 g of the silicon-based negative active material composite prepared in Preparation Example 1, the solution was immersed in a 10% hydrochloric acid solution, stirred for 24 hours, filtered, dried (80 ° C./6 hours), and vacuum dried (105). C / 12 hours). After adding 7 wt% binder (SBR, Japan) and 3 wt% thickener (CMC, Japan) to 1 g of the silicon-based negative active material composite from which the Fe catalyst was removed, a slurry was prepared, and a current collector (Cu foil, thickness of 18 μm) was prepared. After coating, drying (45 ° C./1 hour) and vacuum drying (105 ° C./12 hour), the electrode was manufactured in the form of a disk having a diameter of 12 mm after pressing the coating electrode on a roller.

Preparation Example 3 Preparation of Silicon-Based Negative Electrode Active Material Composite for Lithium Secondary Battery by Pyrolytic Carbon Coating and Carbon Nanofiber Vapor Phase Growth (Secondary Surface Coating)

5 g of the silicon-based negative active material composite prepared in Preparation Example 2 was subjected to secondary coating of pyrolytic carbon on the surface of the silicon-based negative active material composite under the same conditions as in Preparation Example 1. The coating amount of pyrolytic carbon (weight _output / weight _input * 100 [%]) was 6% by weight of the composite weight. 7 g of binder (SBR, Japan) and 3 wt% of thickener (CMC, Japan) were added to 1 g of the silicon-based negative electrode active material composite to prepare a slurry, and the coating was applied to a current collector (Cu foil, thickness of 18 μm). After drying (45 ° C./1 hour) and vacuum drying (105 ° C./12 hour), the electrode was manufactured in the form of a disk having a diameter of 12 mm after pressing the coating electrode with a roller.

Preparation Example 1 Preparation of Silicon-Based Composite Anode Material (No Pyrolytic Carbon Coating and Carbon Nanofiber Growth)

After mixing 1 g of crystalline silicon powder and 1 g of a conductive agent (Ketjen Black, Japan), a mixture was prepared using a table-type ball mill. As the conditions of the ball mill, 2 g of the mixture, 20 g of zirconia ball and 10 ml of ethanol were added thereto, and the mixture was treated for 24 hours at 250 rpm, and then filtered, dried (80 ° C./6 hours) and vacuum dried (105 ° C./12 hours). . A slurry was prepared by adding 7 wt% binder (SBR, Japan) and 3 wt% thickener (CMC, Japan) to 1 g of the silicon-conductive agent (Ketjen Black) mixture thus prepared, followed by current collector (Cu foil, 18 μm thick), dried (45 ° C./1 hour) and vacuum dried (105 ° C./12 hour), the electrode was manufactured in the form of a disk with a diameter of 12 mm after pressing the coating electrode on a roller. .

(Comparative Example 2) Preparation of silicon composite anode material (only pyrolytic carbon coating, no carbon nanofiber growth)

In Preparation Example 1, 1 g of the silicon powder coated with pyrolysis carbon of the carbon nanofiber vapor phase growth step and 1 g of a conductive agent (Ketjen Black, Japan) were mixed, and then prepared using a table-type ball mill. As the conditions of the ball mill, 2 g of the mixture, 20 g of zirconia ball and 10 ml of ethanol were added thereto, and the mixture was treated for 24 hours at 250 rpm, and then filtered, dried (80 ° C./6 hours) and vacuum dried (105 ° C./12 hours). . A slurry was prepared by adding 7 wt% binder (SBR, Japan) and 3 wt% thickener (CMC, Japan) to 1 g of the silicon-conductive agent (Ketjen Black) mixture thus prepared, followed by current collector (Cu foil, 18 μm thick), dried (45 ° C./1 hour) and vacuum dried (105 ° C./12 hour), the electrode was manufactured in the form of a disk with a diameter of 12 mm after pressing the coating electrode on a roller. .

(Comparative Example 3) Preparation of silicon composite anode material (growth of carbon nanofiber without pyrolytic carbon)

5 g of a Fe catalyst using iron nitrate (Fe [NO ₃ ] ₃ -9H ₂ O) was dried on a carrier of 5 g of crystalline silicon powder, dried, put in a horizontal furnace, and helium gas (200 ml / min) was added thereto. After the temperature was raised to 600 ° C. (10 ° C./min) while supplying, carbon nanofibers were grown on the surface of silicon while supplying carbon monoxide (160 ml / min) and hydrogen (40 ml / min) at 600 ° C. for 30 minutes. It was naturally cooled while supplying helium (200 ml / min) to room temperature, and the growth amount of carbon nanofibers was 98% by weight based on the weight of silicon. The island diameter of the prepared carbon nanofibers was observed in a nonuniform form of 20-150 nm in size. The slurry was prepared by adding 7% by weight of binder (SBR, Japan) and 3% by weight of thickener (CMC, Japan) to 1 g of the silicon-carbon nanofiber composite thus prepared, followed by current collector (Cu foil, thickness 18). After coating, drying (45 ° C./1 hour) and vacuum drying (105 ° C./12 hour), the electrode was manufactured in the form of a disk having a diameter of 12 mm after pressing the coating electrode on a roller.

(Evaluation results)

Table 1 summarizes the manufacturing conditions of Preparation Examples 1-3 and Comparative Examples 1-3.

1 is a representative view showing a silicon-based negative active material composite model coated with pyrolytic carbon of the present invention.

FIG. 2 shows SEM (a) and TEM (b) photographs of Preparation Example 1 to show the conductivity by coating pyrolytic carbon on the silicon carrier before the carbon nanofiber vapor phase growth, so that the SEM observation (a) was easy, and the TEM observation ( b) The coating thickness of the resulting pyrolytic carbon was 20 to 30 nm.

3 is a SEM observation photograph of Preparation Example 1 (a), Preparation Example 3 (b), Preparation Comparative Example 1 (c) and Preparation Comparative Example 3 (d). Preparation Examples 1 and 3 and Comparative Example 3 were almost similar in SEM observation, but Comparative Example 1 was found that the conductivity is not good because the Ketjenblack is a separate silicon and a conductive agent. In addition, Preparation Examples 1 and 2 and Comparative Examples 1 and 2 showed almost similar surface shapes as a result of SEM observation.

Table 2 shows the negative electrode mixture density, charge / discharge capacity and efficiency in one cycle. In the manufacturing method of the evaluation cell, the disk-shaped electrode was manufactured by using a CR2032 type coin cell in a glove box of argon atmosphere, and as a counter electrode, lithium ribbon (0.3 mm thick), PE separator (18 μm thick), 1M LiPF ₆ / (EC: DEC, 1: 1 vol%) (Ube, Japan) electrolyte solution was used. The manufactured coin cell was left at room temperature for 24 hours for voltage stabilization, and the capacity, initial efficiency, and cycle characteristics were evaluated at a current density of 100 mA / g in a constant current manner in a range of 0 to 1.5 V.

Preparation Examples 1, 2, 3 and Comparative Example 3 had similar electrode densities of 0.98 to 1.01 g / cc, and Comparative Examples 1 and 2 containing Ketjenblack as a conductive agent had relatively low electrode densities (0.79 to 0.81). g / cc). In addition, the discharge capacity in one cycle is 1115 to 1325 mAh / g in the case of the silicon-carbon nanofiber composite (Manufacture Examples 1, 2, 3 and Comparative Comparative Example 3), and the silicon-conductor mixed product (Manufacture Comparative Example 1, 2,709 ~ 811 mAh / g) was very high, the battery characteristics of the silicon-carbon nanofiber composite was excellent.

In addition, in terms of initial efficiency, Comparative Examples 1 to 3 exhibited a very low initial efficiency of 50 to 65%. The silicon-carbon nanofiber composite of Comparative Example 3 was prepared in Comparative Examples 1 or 2 in which a conductive agent was mixed. Appeared to be slightly superior. Here, it can be seen that silicon coated only with pyrolytic carbon exhibited very low characteristics in capacity and initial efficiency, and silicon-carbon nanofiber composite without pyrolytic carbon coating had high initial discharge capacity but low efficiency.

In contrast, Example 1 coated with pyrolytic carbon showed a very high initial efficiency of 78.3% compared to Preparation Comparative Example 3, which is the same silicon-carbon nanofiber composite, showing that the effect of the coating of pyrolytic carbon is great. Also in Preparation Example 2 with the catalyst removed, the initial efficiency was similar to Preparation Example 1. In Preparation Example 3, a very high initial efficiency (84.7%) was shown because the specific surface area produced by carbon nanofibers was reduced by the coating of pyrolytic carbon in the final product.

4 shows charge and discharge curves up to three cycles of Production Examples 1-3 and Comparative Examples 1-3. The preparation examples tended to increase slightly in 2 cycles and showed almost perfect agreement up to 3 cycles. On the other hand, Comparative Examples 1 and 2 showed low initial discharge capacity and cycle characteristics, and Comparative Example 3, which is a silicon-carbon nanofiber composite, also showed that the discharge capacity decreased and the cycle characteristics were not good.

5 shows cycle characteristics of Preparation Examples 1-3 and Comparative Examples 1-3. In Examples 1 to 3, the capacity was gradually decreased, whereas in Comparative Examples 1 and 2, the capacity was drastically decreased from 2 cycles, and 10 cycles showed very low capacity of 150 mAh / g or less. In addition, in Comparative Example 3 of the silicon-carbon nanofiber composite, the reduction degree of the discharge capacity was greater than that of the production examples 1 to 3, and from 8 cycles, the discharge capacity was lower than that of the production example 2.

Table 3 shows the discharge capacities up to 20 cycles and the storage capacity retention ratios at 20 cycles of Preparation Examples 1 to 3 and Comparative Examples 1-3. Preparation Examples 1 to 3 exhibited an excellent retention property of 71% or more, while Comparative Examples 1 and 2 showed less than 11%, and Comparative Example 3 also showed less than 50%.

As described above, Examples 1 to 3 of the silicon-carbon nanofiber composite coated with pyrolytic carbon had excellent initial capacity and cycle characteristics compared to those of Comparative Examples 1 to 3, and prepared carbon nanofibers in which silicon was grown. The results distinguished from Example 3 are also shown.

FIG. 6 illustrates a process step-by-step model of a silicon-carbon nanofiber composite coated with pyrolytic carbon.

FIG. 1 is a representative view of a silicon-based negative electrode active material composite of the present invention prepared by coating pyrolytic carbon on the surface of silicon particles, growing carbon nanofibers, removing a catalyst, and recoating the surface of the composite with pyrolytic carbon. to be

FIG. 2A is a SEM photograph after coating pyrolytic carbon on the surface of the silicon carrier particles before the carbon nanofiber vapor phase growth in Preparation Example 1, and FIG. 2B is the silicon carrier particles before the carbon nanofiber vapor phase growth in Preparation Example 1 TEM picture of silicon particles coated with pyrolytic carbon on the surface.

3a is a SEM photograph of a composite prepared by coating pyrolytic carbon on silicon particles and growing carbon nanofibers as Preparation Example 1, and FIG. 3b is coating pyrolytic carbon on silicon particles and preparing carbon nanofibers as Preparation Example 3. SEM image of a composite prepared by growing a catalyst after removing the catalyst and recoating the surface of the composite with pyrolytic carbon. FIG. 3C is a SEM photograph of a composite prepared by ball milling silicon particles and a ketjen black, which is a conductive agent as Comparative Example 1. 3d is a SEM photograph of a composite prepared by growing carbon nanofibers on silicon particles as Comparative Example 3.

4 is a view showing charge and discharge curves up to three cycles of Production Examples 1-3 and Comparative Examples 1-3.

5 is a view showing the cycle life up to 20 cycles of Preparation Examples 1-3 and Comparative Examples 1-3. (a) Manufacturing Example 1, (b) Manufacturing Example 2, (c) Manufacturing Example 3, (d) Manufacturing Comparative Example 1, (e) Manufacturing Comparative Example 2, and (f) Manufacturing Comparative Example 3

FIG. 6 is a schematic diagram showing a process-specific model of a silicon-carbon nanofiber composite coated with pyrolytic carbon.

Claims (6)

Iii) a silicon-based support powder composed of silicon powder or at least one metal alloy mixed powder selected from silicon and transition metal, group 2, 13, 14, and 15 elements at 600 to 1100 ° C. in the presence of a carbon source. Surface coating the amorphous pyrolytic carbon with a thickness of 5 to 200 nm and a weight of 3 to 50% by weight of the support; Ii) A silicon-based negative active material for a lithium secondary battery having a maximum particle size of 45 μm or less prepared by dispersing a transition metal catalyst on a surface of a silicon-based support surface coated with pyrolytic carbon and vapor-growing carbon nanofibers at 400 to 800 ° C. in the presence of a carbon source. Composite According to claim 1, wherein the transition metal, at least one metal selected from Group 2, Group 13, Group 14, Group 15 elements is Mg, Ca, Sr, Ba, Ti, V, Cr, Mn, Fe, Co, Ni At least one selected from among Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Al, Sn, and Sb elements Silicon negative electrode active material composite material for a lithium secondary battery characterized in that the metal The method of claim 1, wherein the carbon nanofiber vapor phase growth step is carried out by supporting one or more transition metal catalyst selected from Groups 3 to 12 on the periodic table on the surface of the metal silicon-based support, and then ethylene, acetylene, methane Lithium secondary, characterized in that carbon nanofibers are grown 3 to 150% by weight relative to the weight of the silicon carrier at 400 to 800 ° C. in the presence of at least one carbon source selected from carbon monoxide, propane, propylene and ethane. Battery-based silicon negative electrode active material composite After removing the transition metal catalyst for growing carbon nanofibers in the silicon-based negative electrode active material composite for lithium secondary battery according to claim 1, the surface of the composite substrate is 5 to 200 nm thick with amorphous pyrolytic carbon at 600 to 1100 ° C. in the presence of a carbon source. 3 to 50% by weight of the secondary lithium secondary battery silicon-based negative electrode active material composite prepared by the step of surface coating 5. The carrier surface coating or composite secondary surface coating of claim 1 or 4, wherein the carrier surface coating or composite secondary surface coating is formed by pyrolytic carbon using at least one gaseous and liquid carbon source selected from ethylene, acetylene, methane, carbon monoxide, propane, propylene, ethane. coating; Coating by pitch of liquid and solid phases; Or silicon-based negative active material composite for lithium secondary battery, characterized in that it is amorphous carbon coating by a ball mill or the like.        Lithium secondary battery using the silicon-based negative electrode active material composite of claim 1 or 4

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