ANODE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY, METHOD FOR PREPARING SAME, AND LITHIUM SECONDARY BATTERY COMPRISING SAME

Abstract

An embodiment of the present invention provides an anode active material for a lithium secondary battery, which is a porous silicon-carbon composite including a plurality of nano-silicon particles embedded in a carbon-based material and having a plurality of pores, wherein the carbon-based material includes graphite particles, soft carbon, hard carbon, or a combination thereof, and based on 100 wt % of the porous silicon-carbon composite, a weight ratio of the graphite particles to the soft carbon, the hard carbon, or a combination thereof is 1:5 to 5:1.

Claims

1. An anode active material for a lithium secondary battery, which is a porous polyacrylic silicon-carbon composite including a plurality of nano-silicon particles embedded in a carbon-based material and having a plurality of pores, wherein the carbon-based material includes graphite particles, soft carbon, hard carbon, or a combination thereof, and based on 100 wt % of the porous silicon-carbon composite, a weight ratio of the graphite particles to the soft carbon, the hard carbon, or a combination thereof is 1:5 to 5:1.

2. The anode active material of claim 1, wherein a specific surface area of the porous silicon-carbon composite is less than or equal to 20 m.sup.2/g.

3. The anode active material of claim 2, wherein a porosity of the anode active material is less than or equal to 3 volume % based on a total volume of the anode active material.

4. The anode active material of claim 3, wherein a carbon coating layer is further disposed on the surface of the anode active material, and a deposition amount of the carbon coating layer is 3 wt % to 15 wt % based on 100 wt % of the anode active material.

5. The anode active material of claim 4, wherein 30 wt % to 40 wt % of nano-silicon, and 60 wt % to 70 wt % of the carbon-based particle are included based on 100 wt % of the porous silicon-carbon composite.

6. The anode active material of claim 5, wherein an D50 particle diameter of the anode active material is 8 μm to 15 μm.

7. A method of preparing an anode active material for a lithium secondary battery, comprising preparing a porous silicon-carbon mixed powder by mixing nano-silicon particles, graphite particles, and pitch particles by dry milling; adding the porous silicon-carbon mixed powder and a binder to distilled water to prepare a mixed solution; spray-drying the mixed solution to prepare primary particles; inserting the primary particles into a mold and press-molding to produce secondary particles; heat-treating the secondary particles; and pulverizing and sieving the heat-treated secondary particles, wherein, in the preparing of the porous silicon-carbon mixed powder, the pitch particles may be included in an amount of greater than or equal to 30 wt % based on 100 wt % of the porous silicon-carbon mixed powder.

8. The method of claim 7, wherein in the preparing of the porous silicon-carbon mixed powder by mixing the nano-silicon particles, graphite particles, and pitch particles by dry milling, the weight of the pitch is greater than or equal to the weight of the graphite.

9. The method of claim 8, wherein in the preparing of the porous silicon-carbon mixed powder by mixing the nano-silicon particles, graphite particles, and pitch particles by dry milling, a weight ratio of the pitch and the graphite is 1:1 to 5:1.

10. The method of claim 7, wherein the pitch includes a combination of coal-based pitch and petroleum-based pitch, and a weight of the coal-based pitch is greater than or equal to a weight of the petroleum-based pitch.

11. The method of claim 10, wherein a weight ratio of the coal-based pitch:the petroleum-based pitch is in the range of 5:5 to 9:1 based on 100 wt % of the pitch.

12. The method of claim 7, wherein a softening point of the pitch is greater than or equal to 250° C.

13. The method of claim 12, wherein the heat-treating of the secondary particles comprises a first isothermal process in which the secondary particles are heated up to a temperature of 50° C. to 350° C. higher than the softening point of the pitch and maintained at a rate of less than or equal to 7° C./min; and a second isothermal process in which after the first isothermal process, they are heated up to a temperature range of 700° C. to 1000° C. at a rate of less than or equal to 7° C./min and then maintained.

14. The method of claim 13, wherein the first isothermal process and the second isothermal process are maintained for 1 hour to 4 hours.

15. The method of claim 7, wherein after the pulverizing and sieving of the heat-treated secondary particles, forming a carbon coating layer on the surface of the secondary particles is further included, and the forming of the carbon coating layer is performed at 750° C. to 1,000° C.

16. The method of claim 7, wherein after the heat-treating of the secondary particles, a carbonization yield of the secondary particles is 60% to 95%.

17. The method of claim 7, wherein by the heat-treating of the secondary particles, the pitch is carbonized into soft carbon, and the binder is carbonized into hard carbon.

18. A lithium secondary battery, comprising a cathode; an anode; and an electrolyte, wherein the anode comprises the anode active material for a lithium secondary battery of claim 1.

Description

DESCRIPTION OF THE DRAWINGS

Mode for Invention

[0123] FIG. 1 is a graph showing a correlation between a pitch content and a specific surface area (BET).

[0124] FIG. 2 shows internal cross-sectional structures of primary particles according to Comparative Example A1 and Example A1.

[0125] FIG. 3 is a graph showing specific surface area changes according to heat-treatment conditions of secondary particles in Examples B1 to B3 and Comparative Examples B1 to B2.

[0126] FIG. 4 is a graph showing a carbonization yield after the heat-treatment according to a content increase of coal-based pitch.

[0127] FIG. 5 shows coulombic efficiency (a) and capacity retention (b) results according to the number of charges and discharges of lithium secondary battery cells of Example A1 and Comparative Example A1.

[0128] FIG. 6 shows charge and discharge curves of the lithium secondary battery cells of Example A1 (a) and Comparative Example A1 (b) at the 1.sup.st cycle.

[0129] FIG. 7 shows adsorption and desorption curves (a) of the lithium secondary battery cells of Examples C1 to C2 and Comparative Example C1 and pore distribution curves (b) thereof, which are examined in a DFT method.

[0130] Hereinafter, specific examples of the present invention will be described. However, the following examples are only specific examples of the present invention, and the present invention is not limited to the following examples.

Example A: Comparison of Characteristics Depending on Pitch Content

Example A1

(1) Preparation of Anode Active Material

[0131] Nano-silicon particles, graphite particles, and pitch particles were milled under a dry condition for 1 hour to prepare mixed powder. In the mixed powder, a weight ratio of the nano-silicon particles:the graphite particles:the pitch particles was 4:1:5.

[0132] Herein, the used pitch was coal-based pitch, and a softening point of the pitch was 250° C.

[0133] Subsequently, the mixed powder and an aqueous binder (Gum Arabic) were added to distilled water and then, well dispersed with a magnetic stirrer, and when well dispersed, the dispersion was further dispersed by using a horn-type ultrasonic wave for 1 hour, preparing a mixed solution.

[0134] Herein, in 100 wt % of the mixed solution, the mixed powder was included at a concentration of 3% to 20%.

[0135] Subsequently, the mixed solution was put in an atomizer 20,000 r.p.m at 50 mL/min for spray-drying, preparing primary particles.

[0136] Then, the primary particle powder was charged into a mold to prepare secondary particles by using a uniaxial pressure molding machine. Specifically, the primary particle powder was pressed at a temperature 50° C. higher than the softening point of the pitch with a pressure of 16 tons for 20 minutes. After the molding, air cooling was performed.

[0137] Subsequently, the secondary particles were heat-treated under an inert atmosphere and then, naturally cooled down to room temperature. Herein, the heat-treatment was performed at 300° C. for 2 hours and then, at 900° C. for 1 hour.

[0138] After the cooling, the secondary particles were pulverized with the particle diameter range of 8 μm to 15 μm based on D50 by using a jet mill. After the pulverization, the pulverized product was sieved with a #635 mesh (20 μm) to obtain a final anode active material.

(2) Manufacture of Lithium Secondary Battery Cell (Half-Cell)

[0139] The anode active material in the (1), a binder (PAA), and a conductive material (Super P) were mixed in a weight ratio of 75:24:01 of the anode active material:binder:conductive material and added to distilled water and then, uniformly mixed to prepare slurry.

[0140] The slurry was uniformly coated on a copper (Cu) current collector, pressed with a roll press, and dried to manufacture an anode. Specifically, a loading amount thereof was 4 mg/cm.sup.2 to have electrode density of 1.0 g/cc to 1.2 g/cc.

[0141] Lithium metal (Li-metal) was used as a counter electrode, and an electrolyte solution prepared by dissolving 1 mol of LiPF.sub.6 in a mixed solvent of ethylene carbonate (EC):dimethyl carbonate (DMC) in a volume ratio of 1:1 was used.

[0142] The anode, the lithium metal, and the electrolyte solution were used according to a common manufacturing method to manufacture a CR 2032 half coin cell.

Example A2

(1) Preparation of Anode Active Material

[0143] An anode active material was prepared according to the same method as the (1) of Example A1 except that the weight ratio of nano-silicon particles:graphite particles:pitch particles in the mixed powder was changed into 4:2:4.

(2) Manufacture of Lithium Secondary Battery Cell

[0144] A lithium secondary battery cell was manufactured in the same manner as in the (2) of Example 1 using the anode active material of (1).

Example A3

(1) Preparation of Anode Active Material

[0145] An anode active material was prepared according to the same method as the (1) of Example A1 except that the weight ratio of nano-silicon particles:graphite particles:pitch particles in the mixed powder was changed into 4:3:3.

(2) Manufacture of Lithium Secondary Battery Cell

[0146] A lithium secondary battery cell was manufactured in the same manner as in the (2) of Example 1 using the anode active material of (1).

Comparative Example A1

(1) Preparation of Anode Active Material

[0147] An anode active material was prepared according to the same method as the (1) of Example A1 except that the weight ratio of nano-silicon particles:graphite particles:pitch particles in the mixed powder was changed into 4:5:1.

(2) Manufacture of Lithium Secondary Battery Cell

[0148] A lithium secondary battery cell was manufactured in the same manner as in the (2) of Example 1 using the anode active material of (1).

Comparative Example A2

(1) Preparation of Anode Active Material

[0149] An anode active material was prepared according to the same method as the (1) of Example A1 except that the weight ratio of nano-silicon particles:graphite particles:pitch particles in the mixed powder was changed into 4:4:2.

(2) Manufacture of Lithium Secondary Battery Cell

[0150] A lithium secondary battery cell was manufactured in the same manner as in the (2) of Example 1 using the anode active material of the (1).

[0151] Subsequently, a specific surface area depending on a pitch content in Example A and Comparative Example A was measured, and the results are shown in Tables 1 and 2.

[0152] Air permeability, specific surface area, and tap density were measured in the following methods.

TPV (Total Pore Volume), Vt-Plot, Fmicro, APD Measurement Method

[0153] A total pore volume (TPV) was calculated as a relative pressure measured at a single point under a relative pressure of 0.95.

[0154] A Vt-plot is a volume of micro pores obtained by a t-plot.

[0155] Fmicro was calculated as a fraction of Vt-plot and TPV (=V(t-plot)/TPV).

[0156] APD (average pore diameter) was calculated by TPV (total pore volume), that is, a correlation equation between surface area and volume, when the pores were assumed to have a cylinder shape.

Measurement of Specific Surface Area

[0157] A BET method (a surface area and porosity analyzer, ASAP2020, Micromeritics Instrument Corp.) was used to measure a specific surface area.

Measurement of Tap Density

[0158] According to ASTM-B527, 10 g of powder was put in a 50 mL container and 3000 cycles tapped at 284 cycles/min to measure packing density.

Measurement of Carbonization Yield

[0159] A carbonization yield was measured by using TG-DTA, and a yield according to a temperature-increasing profile was measured at 900° C. A difference of the carbonization yield depending on a temperature-increasing rate, a step-temperature during the heating, step-holding time, and holding time at the final temperature was examined.

TABLE-US-00001 TABLE 1 Pitch content BET TPV Vt-plot Fmicro APD (wt %) (m.sup.2/g) (cm.sup.3/g) (cm.sup.3/g) (%) (nm) Comparative 10 21.9 0.044 0.0015 3.44 8.06 Example A1 secondary particle Comparative 19.43 0.038 0.0012 3.20 7.76 Example A1 primary particle Example A1 50 5.8 0.006 0.0011 18.58 4.24 secondary particle Example A1 11.57 0.012 0.0016 12.52 4.29 primary particle

TABLE-US-00002 TABLE 2 Carbon- Tap ization Particle diameter (μm) density yield D.sub.1 D.sub.10 D.sub.50 D.sub.90 D.sub.max (g/cc) (%) Comparative 2.7 5.1 11.7 20.8 ≤36.0 0.51 96.01 Example A1 Example A1 2.2 4.2 10.7 19.3 ≤36.0 0.87 86.80

[0160] Referring to Table 1, pore structures of the primary particles and the secondary particles according to the example and the comparative example were identified.

[0161] Specifically, in the comparative example including more graphite than pitch, the specific surface area tended to rather increase after manufacturing the secondary particles.

[0162] The reason is that when the pitch was more included according to a composition ratio, plasticity increased, but when the graphite was dominantly present, elasticity predominantly worked, providing a restoring force against pressure during the compression and thus increased the specific surface area.

[0163] Surely, the graphite is expected to show different particle contact behaviors depending on a particle size due to during the compression, but a relative content of the pitch turned out to be dominantly important to secure a high-density compression structure.

[0164] Table 2 shows carbonization yield, tap density, and the like of the anode active materials according to the example and the comparative example.

[0165] As shown in Table 2, the larger the pitch content, the smaller the carbonization yield, but the larger the tap density.

[0166] This is contrary to the result shown in FIG. 1 showing that the larger the pitch content, the smaller the specific surface area, which will be described later.

[0167] In addition, this is also shown in FIG. 1.

[0168] FIG. 1 is a graph showing a correlation between the pitch content and the specific surface area (BET).

[0169] Composite spheres defined as the primary particles and the secondary particles (after the press) manufactured by molding exhibited that the specific surfaces overall tended to decrease, as the pitch was increased.

[0170] However, when the decrease tendency and changes before and after the press were examined, the primary particles exhibited a tendency to decrease in the specific surface area at greater than or equal to 30% of the pitch input amount, but the secondary particles exhibited a tendency to linearly decrease in the specific surface area, as the pitch amount was increased.

[0171] Specifically, when the pitch amount was small, but the graphite amount was large (Comparative Example 1), the primary particles exhibited a tendency to rather increase in the specific surface area after the press. However, when the pitch weight was 30% or more, the specific surface area was remarkably reduced after the press.

[0172] As aforementioned, when the pitch amount was large, plasticity increased. Specifically, when the graphite was dominantly present, the specific surface area was increased due to the phenomenon that elasticity predominantly acts and works as a restoring force against the pressure during the compression. Accordingly, in order to secure the high density compress structure, when the pitch content was equal to or larger than that of the graphite particles, the specific surface area-reducing effect turned out to be dominant.

[0173] FIG. 2 shows an internal cross-section structure of the primary particles according to Comparative Example A1 and Example A1.

[0174] As shown in FIG. 2, Comparative Example A1 in which the graphite was predominantly present had no relatively complete bond among the components but exhibited a lot of spaces such as pores (voids).

[0175] However, as the pitch content was increased, the specific surface area, which is a space between the particles, tended to decrease.

[0176] Specifically, as shown in Example A1 of FIG. 2, when 50 wt % of the pitch was input, a region where the pitch distinguished from the graphite was agglomerated increased

[0177] In other words, in order to secure uniform dispersity and increase contact property between the particles based on a cross-section structure of the porous silicon-carbon composite, it is important to derive an optimumal ratio of the composition.

[0178] FIG. 5 shows coulombic efficiency (a) and capacity retention (b) results of the lithium secondary battery cells according to Example A1 and Comparative Example A1 depending on the number of charge and discharge.

[0179] Methods of measuring the coulombic efficiency and the capacity retention depending on the number of charge and discharge are as follows.

Measurement of Coulombic Efficiency and Capacity Retention

[0180] Each final active material obtained in the examples and the comparative examples was applied to a half-cell and evaluated.

[0181] Specifically, the cells were operated under charge conditions of 0.5 C, 0.005 V, and 0.005 C cut-off and discharge conditions of 0.5 C and 1.5 V cut-off, and the coulombic efficiency and capacity retention thereof were measured and shown in FIG. 6.

[0182] Specifically, FIG. 5 is a graph showing electrochemical property changes of the lithium secondary battery cells according to an input amount of the pitch. More specifically, as shown in (a) of FIG. 5, Comparative Example A1 including a small pitch amount of 10 wt % exhibited sharply decreased efficiency within a 20th to 30th charge and discharge section, compared with Example A1.

[0183] As described above, when the pitch content was too small, the number of points where an electrical contact path was disconnected due to expansion and contraction was increased, rapidly deteriorating efficiency.

[0184] Such a phenomenon that the electrical contact path was disconnected also was confirmed through (b) of FIG. 5. Specifically, as shown in the (b) of FIG. 5, in Comparative Example A1, as the number of points where the electrical contact path is disconnected is increased in a section of 20 to 30 charges/discharges, capacity retention also sharply decreased.

[0185] FIG. 6 shows charge and discharge curves at the 1.sup.st cycle of the lithium secondary battery cells according to Example A1(a) and Comparative Example A1 (b).

[0186] As shown in FIG. 6, Example A1 exhibited initial efficiency of 86.5%, while Comparative Example A1 exhibited initial efficiency of 86.3%, which exhibited a similar tendency but had a different slope of initial discharge profile, and accordingly, a composite material including more graphite exhibited larger resistance during the discharge.

[0187] This is also due to a pitch content difference in Example A1(a) and Comparative Example A1 (b).

Example B: Comparison of Characteristics According to Conditions of Heat-Treatment of Secondary Particles

Example B1

(1) Preparation of Anode Active Material

[0188] Compared with the (1) of Example A1, the weight ratio of the nano-silicon particles:graphite particles:pitch particles of the mixed powder was changed into 3:3:4. Herein, the pitch was coal-based pitch, and a softening point of the pitch was 250° C.

[0189] Subsequently, an anode active material was prepared according to the same method as the (1) of Example A1 except that the first isothermal process was performed by increasing the temperature up to 600° C. at 5° C./min and maintaining it for 1 hour, and the second isothermal process was performed by increasing the temperature up to 900° C. at 5° C./min and maintaining it for 1 hour in the heat-treatment of the secondary particles.

(2) Manufacture of Lithium Secondary Battery Cell

[0190] A lithium secondary battery cell was manufactured in the same manner as in the (2) of Example 1 using the anode active material of (1).

Example B2

(1) Preparation of Anode Active Material

[0191] Compared with the (1) of Example B1, the pitch was coal-based pitch, and a softening point of the pitch was 250° C.

[0192] Subsequently, an anode active material was prepared according to the same method as above except that the first isothermal process was performed by increasing a temperature up to 400° C. at 5° C./min and then, maintaining it for 2 hours, and the second isothermal process was performed by increasing up to 900° C. at 5° C./min and maintaining it for 1 hour in the heat-treatment of the secondary particles.

(2) Manufacture of Lithium Secondary Battery Cell

[0193] A lithium secondary battery cell was manufactured in the same manner as in the (2) of Example 1 using the anode active material of (1).

Comparative Example B1

(1) Preparation of Anode Active Material

[0194] An anode active material was prepared according to the same method as the (1) of Example B1 except that the first isothermal process was performed by increasing a temperature up to 600° C. at 10° C./min and maintaining it for 2 hours, and the second isothermal process was performed by increasing the temperature up to 900° C. at 10° C./min and maintaining it for 1 hour in the heat-treatment of the secondary particles.

(2) Manufacture of Lithium Secondary Battery Cell

[0195] A lithium secondary battery cell was manufactured in the same manner as in the (2) of Example 1 using the anode active material of (1).

Comparative Example B2

(1) Preparation of Anode Active Material

[0196] An anode active material was prepared according to the same method as the (1) of Example B1 except that the first isothermal process was performed by increasing a temperature up to 400° C. at 10° C./min and maintaining it for 1 hour, and the second isothermal process was performed by increasing the temperature up to 900° C. at 10° C./min and maintaining it for 1 hour in the heat-treatment of the secondary particles.

(2) Manufacture of Lithium Secondary Battery Cell

[0197] A lithium secondary battery cell was manufactured in the same manner as in the (2) of Example 1 using the anode active material of (1).

Comparative Example B3

(1) Preparation of Anode Active Material

[0198] An anode active material was prepared according to the same method as the (1) of Example B1 except that the heat-treatment of the secondary particles was performed by increasing a temperature up to 900° C. at 10° C./min without the isothermal processes.

(2) Manufacture of Lithium Secondary Battery Cell

[0199] A lithium secondary battery cell was manufactured in the same manner as in the (2) of Example 1 using the anode active material of (1).

[0200] Subsequently, carbonization yields of Examples B1 to B3 and Comparative Examples B1 to B2 were measured, and the results are shown in Table 3.

[0201] Specific surface areas of the anode active materials were measured in the same method as above, and the carbonization yields were measured in the following method.

TABLE-US-00003 TABLE 3 Specific surface area Carbonization yield (BET) (@900° C.) (m.sup.2/g) (%) Example B1 9.9 86.2 Example B2 10.22 94.4 Comparative Example B1 9.1 64.2 Comparative Example B2 9.39 90.3 Comparative Example B3 10.24 57.5

[0202] Specifically, the specific surface areas of the anode active materials according to Examples B1 to B2 and Comparative Examples B1 to B3 were measured, and the results are shown in Table 3 and FIG. 3.

[0203] FIG. 3 is a graph showing specific surface area changes of the secondary particles according to Examples B1 to B2 and Comparative Examples B1 to B3 according to conditions of the heat-treatment.

[0204] As shown in Table 3 and FIG. 3, Comparative Example B3 in which the heat-treatment was performed up to 700° C. without the isothermal processes exhibited a high specific surface area and a very low carbonization yield. On the other hand, when the isothermal processes were performed as in the examples, a low specific surface area and an excellent carbonization yield were obtained.

[0205] Accordingly, the examples in which the isothermal processes were performed to pyrolyze the pitch as slowly as possible around the softening point exhibited a clear tendency difference from a sample (Comparative Example B3) linearly heated up to a target temperature at a constant rate.

[0206] On the other hand, even when the isothermal processes were performed, different results were obtained depending on a temperature and a rate.

[0207] Specifically, the slower the heating rate up to a temperature around the softening point of the pitch, the better the carbonization yield. More specifically, the carbonization yields of Examples B1 and B2 in which the first isothermal process was performed at 5° C./min were higher than those of Comparative Examples B1 and B2.

[0208] The carbonization yield of Comparative Example B2 in which the temperature of the first isothermal process was maintained around the softening point among Comparative Examples B1 and B2 having a fast heating rate was more excellent. Specifically, under the same heating rate condition, when low molecular weight volatiles decomposable during the heat-treatment were removed for a long time, a higher yield was obtained.

[0209] In other words, a final carbonization yield of the anode active material turned out to be closely related to conditions of the heat-treatment of the secondary particles.

Example C: Comparison of Characteristics According to Amount of Carbon Deposited on Surface of the Si—C Composite

Example C1

(1) Preparation of Anode Active Material

[0210] The anode active material according to the (1) of Example A1 was put in a horizontal furnace, and 200 sccm of argon (Ar) gas and 400 sccm of CH.sub.4 were injected thereinto at 760° C. for 1 hour to conduct a CVD deposition experiment. The above two processes provided a deposition amount of about 2%. Accordingly, an anode active material was prepared according to the same method as the (1) of Example A1 except that a carbon component was deposited on the surface thereof.

(2) Manufacture of Lithium Secondary Battery Cell

[0211] A lithium secondary battery cell was manufactured in the same manner as in the (2) of Example A1 using the anode active material of the (1).

Example C2

(1) Preparation of Anode Active Material

[0212] The anode active material according to the (1) of Example A1 was put in a horizontal furnace, and 200 sccm of argon (Ar) gas and 400 sccm of CH.sub.4 were injected thereinto at 1,000° C. for 1 hour to conduct the CVD deposition experiment. The above two processes provided a deposition amount of about 7%. Accordingly, an anode active material was prepared according to the same method as the (1) of Example A1 except that a carbon component was deposited on the surface thereof the anode active material.

Comparative Example C1

(1) Preparation of Anode Active Material

[0213] The anode active material according to the (1) of Comparative Example A1 was put in a horizontal furnace, and 200 sccm of argon (Ar) gas and 400 sccm of CH.sub.4 were injected thereinto at 760° C. for 1 hour to conduct the CVD deposition experiment. The above processes provided a deposition amount of about 7%. Accordingly, an anode active material was prepared according to the same method as the (1) of Example A1 except that a carbon component was deposited on the surface thereof.

(2) Manufacture of Lithium Secondary Battery Cell

[0214] A lithium secondary battery cell was manufactured in the same manner as in the (2) of Example C1 using the anode active material of (1).

[0215] Specific surface areas and pores of the lithium secondary battery cells according to the example and the comparative example were measured, and the results are shown in Table 4 and FIG. 7.

TABLE-US-00004 TABLE 4 BET change according to CVD loading amount (m.sup.2/g) No treatment CVD-2% CVD-7% 21.9 20.45 — (Comparative (Comparative Example A1) Example C1) 5.8 (Example A1) 5.29 (Example C1) 4.11 (Example C2)

[0216] As shown in Table 4, Examples C1 and C2 and Comparative Example C1 in which an amorphous carbon layer was further formed by introducing CH.sub.4 gas exhibited changes of the specific surface areas and pores.

[0217] The characteristics are also shown in FIG. 7.

[0218] FIG. 7 shows adsorption and desorption curves (a) and pore distribution curves (b), which are examined in the DFT method, of the lithium secondary battery cells according to Examples C1 to C2 and Comparative Example C1.

[0219] In other words, as shown in Table 4 and FIG. 7, the deposition amount of the carbon layer was changed depending on a temperature in the CVD deposition process. Specifically, when the deposition experiment was conducted at 760° C., the deposition amount was 2%, and when at 1000° C., the deposition amount was 7%. Accordingly, it is important to set an appropriate temperature section for the CVD deposition.

[0220] Specifically, the specific surface area examined in the BET method decreased, as the deposition amount was increased. This is because external pores and defect structures were coated and filled during the CVD deposition.

[0221] In addition, as shown in FIG. 7, when pore structure changes were examined in the DFT method, as the deposition amount was increased, micropores of less than or equal to 2 nm significantly decreased, but at the deposition amount of 2%, pores in a meso region slightly decreased but increased again at the deposition amount of 7%.

[0222] Accordingly, as pores of the micro regions were more present, a BET value easily increased, and accordingly, the CVD deposition was effective in reducing the BET value.

Experimental Example: Comparison of Co-Carbonization Characteristics of Powders According to Mixing of Petroleum-Based Pitch and Coal-Based Pitch

[0223] Specifically, the following experimental examples were disclosed to examine a co-carbonization behavior according to a heat-treatment by preparing powder with pitch alone. In other words, as shown in Table 5, the powder was prepared by using one type of pitch or a mixture of the pitch and then, examined with respect to a weight change by using TG/DTA.

Measurement of Yield Change

[0224] In Table 5 below, a yield change means a yield change according to holding time at a final target temperature of 900° C. Specifically, each different yield change was obtained according to an isothermal treatment at 900° C. and a pitch composition.

TABLE-US-00005 TABLE 5 Petroleum- Pitch mixing Final based pitch ratio (coal- carbonization Yield content based:petro- yield change (wt %) leum-based) (wt %) @900° C. Experimental 0 100:0  75.41 2.72 Example 1 Experimental 10 90:10 78.07 1.85 Example 2 Experimental 20 80:20 79.79 2.30 Example 3 Experimental 30 70:30 79.65 2.83 Example 4 Experimental 40 60:40 79.99 2.76 Example 5 Experimental 50 50:50 79.20 3.30 Example 6 Comparative 100  0:100 67.55 2.13 Experimental Example 1

[0225] As shown in Table 5, when petroleum-based pitch or coal-based pitch alone was used, a final carbonization yield was higher, compared with when the petroleum-based pitch or the coal-based pitch was used as a mixture.

[0226] In addition, even though the pitch alone was used, when the coal-based pitch alone was used, a final yield was similar to that of another example embodiment, but Comparative Example C1 of using the petroleum-based pitch alone exhibited a lower final yield.

[0227] This is also confirmed through FIG. 4.

[0228] FIG. 4 is a graph showing a carbonization yield after the heat-treatment according to a content increase of the coal-based pitch.

[0229] Specifically, as shown in FIG. 4 and Table 5, compared with when the pitch was used alone, a better carbonization yield brought about a smaller specific surface area, when used as a mixture. Accordingly, a yield-improving effect due to the co-carbonization of the coal-based pitch and the petroleum-based pitch was confirmed.

[0230] In addition, as shown in FIG. 4, in a fraction of 50% or more of the coal-based pitch, the carbonization yield was in good agreement with a quadratic trend line, exhibiting a fitness level of R2=0.9937. This indicates a degree of agreement with a theoretical quadratic trend line, and the closer to 1, the more consistent with the theoretical trend.

[0231] While this invention has been described in connection with what is presently considered to be practical example embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. Therefore, the aforementioned embodiments should be understood to be exemplary but not limiting the present invention in any way.