ANODE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY AND METHOD OF MANUFACTURING SAME

Abstract

The present disclosure relates to a method of manufacturing an anode active material for a lithium secondary battery, the method including: mixing earth graphite and pitch coke with each other; preparing a raw material by adding and mixing a binder to the mixture; performing heat treatment on the raw material; graphitizing the heat-treated mixture to obtain a core part; immersing the core part in a hard carbon coating solution; and drying the coating solution in which the core part is immersed to obtain an anode active material.

Claims

1. An anode active material for a lithium secondary battery, comprising: a core part in which earth graphite and pitch coke are mixed with each other; and a hard carbon coating for coating the core part.

2. The anode active material for a lithium secondary battery of claim 1, wherein: a mixing ratio of the earth graphite to the pitch coke is 30:70 to 70:30.

3. The anode active material for a lithium secondary battery of claim 1, wherein: a thickness of the hard carbon coating part is 10 to 700 nm.

4. The anode active material for a lithium secondary battery of claim 1, wherein: a degree of graphitization of the earth graphite is 90% or more.

5. The anode active material for a lithium secondary battery of claim 1, wherein: in a method of manufacturing the anode active material for a lithium secondary battery, a volume expansion rate at a 5C-rate after 50 cycles is less than 20%.

6. The anode active material for a lithium secondary battery of claim 1, wherein: the earth graphite has a crystallite size La of 20 to 50 nm and a crystallite size Lc of 10 to 40 nm, when measured by X-ray diffraction (XRD).

7. The anode active material for a lithium secondary battery of claim 1, wherein: an average particle size D50 of the anode active material for a lithium secondary battery is 14 to 19 μm.

8. The anode active material for a lithium secondary battery of claim 1, wherein: the anode active material for a lithium secondary battery has a specific surface area (BET) of 0.9 to 1.3 m.sup.2/g.

9. The anode active material for a lithium secondary battery of claim 1, wherein: the hard carbon is one or more selected from the group consisting of a phenolic resin, a naphthalene resin, a furfuryl alcohol resin, a polyamide resin, a furan resin, a polyimide resin, an epoxy resin, a vinyl chloride resin, gum arabic, citric acid, stearic acid, sucrose, polyvinylidene fluoride, carboxymethyl cellulose (CMC), hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, an ethylene propylene diene monomer (EPDM), a sulfonated EPDM, starch, polyacrylic acid, sodium polyacrylate, polyacrylonitrile, cellulose, styrene, polyvinyl alcohol, and polyvinyl chloride.

10. A method of manufacturing an anode active material for a lithium secondary battery, the method comprising: mixing earth graphite and pitch coke with each other to prepare a mixture; preparing a raw material by adding and mixing a binder to the mixture; performing heat treatment on the raw material; graphitizing the heat-treated mixture to obtain a core part; immersing the core part in a hard carbon coating solution; and drying the coating solution in which the core part is immersed to obtain an anode active material.

11. The method of manufacturing an anode active material for a lithium secondary battery of claim 10, wherein: in the mixing of the earth graphite and the pitch coke with each other, a mixing ratio of the earth graphite to the pitch coke is 30:70 to 70:30.

12. The method of manufacturing an anode active material for a lithium secondary battery of claim 10, wherein: in the immersing of the core part in the hard carbon coating solution, a hard carbon coating is formed in an amount of 1 to 5 wt % with respect to a total weight of the core part.

13. The method of manufacturing an anode active material for a lithium secondary battery of claim 10, wherein: in the preparing of the raw material by adding and mixing the binder to the mixture, during the mixing, coal-tar pitch is further added in an amount of 10 wt % or less with respect to a total weight of the raw material.

14. The method of manufacturing an anode active material for a lithium secondary battery of claim 10, wherein: in the mixing of the earth graphite and the pitch coke with each other, a degree of graphitization of the earth graphite is 90% or more.

15. The method of manufacturing an anode active material for a lithium secondary battery of claim 10, wherein: in the mixing of the earth graphite and the pitch coke with each other, the earth graphite has a crystallite size La of 20 to 50 nm and a crystallite size Lc of 10 to 40 nm, when measured by X-ray diffraction (XRD).

16. The method of manufacturing an anode active material for a lithium secondary battery of claim 10, wherein: in the immersing of the core part in the hard carbon coating solution, the hard carbon is one or more selected from the group consisting of a phenolic resin, a naphthalene resin, a furfuryl alcohol resin, a polyamide resin, a furan resin, a polyimide resin, an epoxy resin, a vinyl chloride resin, gum arabic, citric acid, stearic acid, sucrose, polyvinylidene fluoride, carboxymethyl cellulose (CMC), hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, an ethylene propylene diene monomer (EPDM), a sulfonated EPDM, starch, polyacrylic acid, sodium polyacrylate, polyacrylonitrile, cellulose, styrene, polyvinyl alcohol, and polyvinyl chloride.

Description

DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 illustrates pitch coke (isotropic mosaic domain) and needle coke (anisotropic flow domain).

[0035] FIG. 2 illustrates an anode piece of an exemplary embodiment of the present disclosure.

[0036] FIG. 3 illustrates an anode piece of a comparative example of the present disclosure.

MODE FOR INVENTION

[0037] The terms “first”, “second”, “third”, and the like are used to describe various parts, components, regions, layers, and/or sections, but are not limited thereto. These terms are only used to differentiate a specific part, component, region, layer, or section from another part, component, region, layer, or section. Accordingly, a first part, component, region, layer, or section which will be described hereinafter may be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention.

[0038] Terminologies used herein are to mention only a specific exemplary embodiment, and are not to limit the present invention. Singular forms used herein include plural forms as long as phrases do not clearly indicate an opposite meaning. The term “comprising” used in the present specification concretely indicates specific properties, regions, integers, steps, operations, elements, and/or components, and is not to exclude the presence or addition of other specific properties, regions, integers, steps, operations, elements, and/or components.

[0039] When any part is positioned “on” or “above” another part, it means that the part may be directly on or above the other part or another part may be interposed therebetween. In contrast, when any part is positioned “directly on” another part, it means that there is no part interposed therebetween.

[0040] In addition, unless otherwise stated, % means wt %, and 1 ppm is 0.0001 wt %.

[0041] Unless defined otherwise, all terms including technical terms and scientific terms used herein have the same meanings as understood by those skilled in the art to which the present disclosure pertains. Terms defined in a generally used dictionary are additionally interpreted as having the meaning matched to the related technical document and the currently disclosed contents and are not interpreted as ideal or very formal meanings unless otherwise defined.

[0042] Hereinafter, exemplary embodiments of the present invention will be described in detail so that those skilled in the art to which the present invention pertains may easily practice the present invention. However, the present invention may be implemented in various different forms and is not limited to exemplary embodiments described herein.

[0043] The present disclosure is to provide an anode material composition capable of improving three electrochemical properties of high discharge capacity/efficiency/output using earth graphite, which is a low-grade raw material that is cheaper than flake graphite among natural graphites. In particular, in order to implement the unique discharge capacity characteristics of the earth graphite, it is intended to secure primary structural stability by using pitch coke, which is a low-quality raw material having strong random orientation, and finally to control expansion characteristics through a hard carbon coating. In particular, considering a small crystallite size of the earth graphite, it is intended to achieve excellent performance improvement.

[0044] Hereinafter, each step will be described in detail.

[0045] An anode active material for a lithium secondary battery of an exemplary embodiment of the present invention may include: a core part in which earth graphite and pitch coke are mixed with each other; and a hard carbon coating for coating the core part. Structural stability may be secured by applying pitch coke and a hard carbon coating to earth graphite.

[0046] A mixing ratio of the earth graphite to the pitch coke may be 30:70 to 70:30. When the mixing ratio of the earth graphite to the pitch coke is out of the above range, structural stability of the earth graphite through the pitch coke may be not secured, such that assembly is not possible, and thus, a tap density may be 0.6 g/cc or less, which is defective. Therefore, electrochemical properties such as a discharge capacity and efficiency may be deteriorated. This is due to differences in crystallinity and structure between the two materials, and the mixing ratio is appropriately in a range of 30:70 to 70:30 in which the two materials may complement each other to achieve a tap density of 0.8 g/cc or more.

[0047] A thickness of the hard carbon coating part may be 10 to 700 nm. Specifically, the thickness of the hard carbon coating part may be 300 to 500 nm. When the thickness is too small, a coverage of the anode material may be defective, which may cause occurrence of an uncoated region, and when the thickness is too large, the hard carbon coating part becomes thick, and an agglomeration phenomenon may occur in a partial region as well as simply thickening.

[0048] A degree of graphitization of the earth graphite may be 90% or more. When the degree of graphitization is too low, there is no carbon network layer spacing and crystallite growth, which may be disadvantageous to intercalation and deintercalation of lithium ions in the manufacturing of the anode material, and may cause a stuck phenomenon in some lithium ions.

[0049] In a method of manufacturing the anode active material for a lithium secondary battery, a volume expansion rate at a 5C-rate after 50 cycles may be less than 20%. Specifically, the volume expansion rate may be less than 15%. This is because the structural stability of the earth graphite is increased through the hard carbon coating.

[0050] The earth graphite may have a crystallite size La of 20 to 50 nm and a crystallite size Lc of 10 to 40 nm, when measured by X-ray diffraction (XRD).

[0051] An average particle size D50 of the anode active material for a lithium secondary battery may be 14 to 19 um. When a particle size of the anode active material is too small or large, it is disadvantageous to intercalation and deintercalation of lithium ions in the manufacturing of the anode material and causes a stuck phenomenon in some lithium ions, such that capacity and efficiency properties may be deteriorated.

[0052] The anode active material for a lithium secondary battery may have a specific surface area (BET) of 0.9 to 1.3 m.sup.2/g. When the specific surface area is too large, an unnecessary reaction may be induced due to an increase in side reaction sites, which may cause deterioration of the performance of the anode material.

[0053] The hard carbon may be one or more selected from the group consisting of a phenolic resin, a naphthalene resin, a furfuryl alcohol resin, a polyamide resin, a furan resin, a polyimide resin, an epoxy resin, a vinyl chloride resin, gum arabic, citric acid, stearic acid, sucrose, polyvinylidene fluoride, carboxymethyl cellulose (CMC), hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, an ethylene propylene diene monomer (EPDM), a sulfonated EPDM, starch, polyacrylic acid, sodium polyacrylate, polyacrylonitrile, cellulose, styrene, polyvinyl alcohol, and polyvinyl chloride.

[0054] A method of manufacturing an anode active material for a lithium secondary battery of an exemplary embodiment of the present disclosure may include: mixing earth graphite and pitch coke with each other; preparing a raw material by adding and mixing a binder to the mixture; performing heat treatment on the raw material; graphitizing the heat-treated mixture to obtain a core part; immersing the core part in a hard carbon coating solution; and drying the coating solution in which the core part is immersed to obtain an anode active material.

[0055] In the mixing of the earth graphite and the pitch coke with each other, a mixing ratio of the earth graphite to the pitch coke may be 30:70 to 70:30. When the mixing ratio of the earth graphite to the pitch coke is out of the above range, structural stability of the earth graphite through the pitch coke may be not secured, such that assembly is not possible, and thus, a tap density may be 0.6 g/cc or less, which is defective. Therefore, electrochemical properties such as a discharge capacity and efficiency may be deteriorated. This is due to differences in crystallinity and structure between the two materials, and the mixing ratio is appropriately in a range of 30:70 to 70:30 in which the two materials may complement each other to achieve a tap density of 0.8 g/cc or more.

[0056] In the immersing of the core part in the hard carbon coating solution, a hard carbon coating may be formed in an amount of 1 to 5 wt % with respect to a total weight of the core part. When the amount of the hard carbon coating is less than 1 wt %, a coverage of the anode material may be defective, which may cause occurrence of an uncoated region, and when the amount of the hard carbon coating exceeds 5 wt %, the hard carbon coating part becomes thick, and an agglomeration phenomenon may occur in a partial region as well as simply thickening.

[0057] In addition, a content of the coating solution may be controlled according to the type of the coating solution used. For example, in a case of a phenolic resin, since a coverage is excellent, a core material coverage is excellent even when only 1 wt % of the phenolic resin is added, but in a case of a furan resin, 3 wt % of the furan resin may be added.

[0058] In the preparing of the raw material by adding and mixing the binder to the mixture, during the mixing, coal-tar pitch may be further added in an amount of 10 wt % or less with respect to a total weight of the raw material. The coal-tar pitch is a material having viscoelasticity unlike powder pitch. The coal-tar pitch may be added to increase efficiency of kneading, and when the amount of coal-tar pitch added is too much, stuck may occur during the manufacturing process, and when the amount of coal-tar pitch added is too small, assembly efficiency may be reduced. Whether or not coke is added and the amount of coke added may be controlled depending on properties of coke to be added.

[0059] In the mixing of the earth graphite and the pitch coke with each other, a degree of graphitization of the earth graphite may be 90% or more. When the degree of graphitization is too low, there is no carbon network layer spacing and crystallite growth, which may be disadvantageous to intercalation and deintercalation of lithium ions in the manufacturing of the anode material, and may cause a stuck phenomenon in some lithium ions.

[0060] In the mixing of the earth graphite and the pitch coke with each other, the earth graphite may have a crystallite size La of 20 to 50 nm and a crystallite size Lc of 10 to 40 nm, when measured by X-ray diffraction (XRD).

[0061] In the immersing of the core part in the hard carbon coating solution, the hard carbon may be one or more selected from the group consisting of a phenolic resin, a naphthalene resin, a furfuryl alcohol resin, a polyamide resin, a furan resin, a polyimide resin, an epoxy resin, a vinyl chloride resin, gum arabic, citric acid, stearic acid, sucrose, polyvinylidene fluoride, carboxymethyl cellulose (CMC), hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, an ethylene propylene diene monomer (EPDM), a sulfonated EPDM, starch, polyacrylic acid, sodium polyacrylate, polyacrylonitrile, cellulose, styrene, polyvinyl alcohol, and polyvinyl chloride.

[0062] Hereinafter, Examples of the present invention will be described in detail so that those skilled in the art to which the present invention pertains may easily practice the present invention. However, the present invention may be implemented in various different forms and is not limited to Examples described herein.

[0063] Evaluation of Performance of Core Part Before Coating

[0064] Manufacture of Core Part

[0065] Generally used flake graphite and earth graphite products, and pitch coke were used. The earth graphite was pulverized using a jet mill to have a particle size, a tap density, and a specific surface area similar to those of the flake graphite. Table 1 shows specifications of the used raw materials. A coefficient of thermal expansion (CTE) of the pith coke as coal-based calcined pitch coke was 40×10.sup.−71° C.

TABLE-US-00001 TABLE 1 Tap Particle size density BET Classification D10 D50 D90 (g/cc) (m.sup.2/g) Earth graphite 3.00 5.10 13.10 0.51 2.89 Flake 3.20 4.70 12.50 0.49 3.00 graphite Pitch coke 7.32 8.93 19.83 0.58 1.85

[0066] The pitch coke (isotropic mosaic domain) has a porous structure and tends to have a random orientation in comparison to needle coke (anisotropic flow domain). The pitch coke has the effect of improving output, and is mixed with the earth graphite to make the structure dense.

[0067] Each of the earth graphite having the above specifications or artificial graphite was mixed with coal-based calcined pitch coke at the ratio shown in Table 2. A petroleum pitch binder having a softening point of 200° C. was added to the mixture in an amount of 20 wt % with respect to the total weight of the mixture, and mixing was performed at room temperature and a rotation speed of 30 rpm using a V-mixer for 1 hour, thereby preparing an anode raw material. The mixed raw material was added to a horizontal kneader, and heat treatment was performed at a speed of 45 rpm and 550° C. for a total of 5 hours. After 2 hours of the heat treatment, when the temperature reached 300° C., coal-tar was additionally added in an amount of 10 wt % with respect to the total weight of the raw material. Subsequently, when the temperature reached 550° C. and the kneading was completed, the raw material was rested for 1 hour. Subsequently, the kneaded raw material was naturally cooled for 1 hour.

[0068] Thereafter, graphitization was performed in an induction heating furnace at 3,000° C. for about 3 hours to manufacture a core part.

[0069] Manufacture of Anode

[0070] 97 wt % of the manufactured core part as an anode material, 2 wt % of a binder containing carboxymethyl cellulose and styrene butadiene rubber, and 1 wt % of a carbon black conductive material were mixed in a distilled water solvent to prepare an anode active material slurry.

[0071] After the anode material slurry was applied to a copper (Cu) current collector, drying was performed at 100° C. for 10 minutes, and compression was performed in a roll press. Thereafter, vacuum drying was performed in a vacuum oven at 100° C. for 12 hours to manufacture an anode. After the vacuum drying, an electrode density of the anode was set to 1.5 to 1.7 g/cc.

[0072] Manufacture of Lithium Secondary Battery

[0073] The manufactured anode and a lithium (Li)-metal as a cathode were used, a solution obtained by dissolving 1 mol of a LiPF.sub.6 solution in a mixed solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1:1 was used as an electrolyte. A 2032 coin type half coin cell was manufactured by a common manufacturing method using the respective components.

[0074] Table 2 shows evaluation results of mixing ratios and properties and electrochemical properties of the anode material.

TABLE-US-00002 TABLE 2 Discharge Anode Tap density BET capacity Efficiency material (g/cc) (m.sup.2/g) (mAh/g) (%) Earth 0.5 2.8 342 93 graphite:pitch coke = 20:80 Earth 0.5 3.1 338 92 graphite:pitch coke = 30:70 Earth 0.6 4.7 325 90 graphite:pitch coke = 70:30 Earth 0.5 5.2 321 91 graphite:pitch coke = 80:20 Flake 0.7 2.9 347 92 graphite:pitch coke = 20:80 Flake 0.7 2.5 349 91 graphite:pitch coke = 30:70 Flake 0.8 4.1 351 93 graphite:pitch coke = 70:30 Flake 0.8 4.6 353 92 graphite:pitch coke = 80:20

[0075] Generally, in the case of the anode material using earth graphite, the tap density was lower than that of the flake graphite based on the same ratio, and the BET was high, and thus, it was determined that the assembly was not performed properly. This is because a large amount of side reaction sites are randomly scattered, and it is determined that the random direction of pitch coke has a limitation in stably maintaining the structure. In addition, it could be appreciated that the performance was deteriorated also in the discharge capacity (third charge and discharge cycle) and efficiency (efficiency in the first cycle=discharge capacity/charge capacity).

[0076] Evaluation of Performance of Anode Active Material after Coating

[0077] The core part in which the materials were mixed at the ratio shown in Table 2 was coated with soft carbon (coal-based pitch) or hard carbon (furan resin), and electrochemical properties thereof were evaluated. The used coating materials, contents, coating layer thicknesses, and evaluation results of electrochemical properties are shown in Table 3.

[0078] The coating materials were applied at the ratios shown in Table 3. Coating of the soft carbon was performed by dry coating, and coating of the hard carbon was performed by wet coating. After the core part was subjected to coating, heat treatment was performed in a carbonization furnace at 1,200° C. for 1 hour.

[0079] Thereafter, an anode and a 2032 coin type lithium secondary half coin cell were manufactured using the manufactured anode active material as described above, and electrochemical properties thereof were evaluated.

[0080] The results of the electrochemical evaluation are shown in Table 3, and the particle size (D50) and the specific surface area are shown in Table 4.

TABLE-US-00003 TABLE 3 Coating Coating material layer Discharge Effi- Anode Coating content thickness capacity ciency material material wt % (nm) (mAh/g) (%) Earth Soft carbon 3 1050 332 92 graphite:pitch (coal-based coke = 30:70 pitch) Earth Soft carbon 3 890 299 90 graphite:pitch (coal-based coke = 70:30 pitch) Flake Soft carbon 3 1070 345 91 graphite:pitch (coal-based coke = 30:70 pitch) Flake Soft carbon 3 750 347 93 graphite:pitch (coal-based coke = 70:30 pitch) Earth Furan resin 3 280 345 92 graphite:pitch coke = 30:70 Earth Furan resin 3 550 343 90 graphite:pitch coke = 70:30 Flake Furan resin 3 400 347 91 graphite:pitch coke = 30:70 Flake Furan resin 3 700 350 93 graphite:pitch coke = 70:30

TABLE-US-00004 TABLE 4 Coating Specific material surface content D50 area Anode material Coating material wt % (um) (m.sup.2/g) Earth graphite:pitch Soft carbon (coal- 3 8.9 1.5 coke = 20:80 based pitch) Earth graphite:pitch Soft carbon (coal- 3 8.2 1.0 coke = 30:70 based pitch) Earth graphite:pitch Soft carbon (coal- 3 6.2 1.1 coke = 70:30 based pitch) Earth graphite:pitch Soft carbon (coal- 3 5.8 0.9 coke = 80:20 based pitch) Flake graphite:pitch Soft carbon (coal- 3 8.1 1.6 coke = 20:80 based pitch) Flake graphite:pitch Soft carbon (coal- 3 8.2 1.2 coke = 30:70 based pitch) Flake graphite:pitch Soft carbon (coal- 3 5.6 1.0 coke = 70:30 based pitch) Flake graphite:pitch Soft carbon (coal- 3 5.7 2.0 coke = 80:20 based pitch) Earth graphite:pitch Furan resin 8.5 1.7 coke = 20:80 Earth graphite:pitch Furan resin 3 8.2 1.3 coke = 30:70 Earth graphite:pitch Furan resin 3 6.7 1.1 coke = 70:30 Earth graphite:pitch Furan resin 3 5.8 1.4 coke = 80:20 Flake graphite:pitch Furan resin 3 8.4 1.8 coke = 20:80 Flake graphite:pitch Furan resin 3 8.0 1.2 coke = 30:70 Flake graphite:pitch Furan resin 3 6.6 0.8 coke = 70:30 Flake graphite:pitch Furan resin 3 5.4 1.5 coke = 80:20

[0081] A coating content was represented by a weight ratio with respect to the total weight of the anode material (core part).

[0082] The coating material was coated even in an amount of 1 wt % or less, but it was confirmed that in the case of the soft carbon coating, the coverage of the core part was defective, and in the case of the hard carbon coating, the performance was excellent in a coating content of 0.5 wt %, but lifespan characteristics were rapidly deteriorated. That is, as a result of measuring a capacity retention rate based on a 2C-rate, it could be observed that in the case where the hard carbon coating was performed in an amount of 0.5 wt %, the capacity retention rate was reduced to 70% immediately after 10 cycles. A similar result was obtained even in the case where the hard carbon coating was performed in an amount of 1 wt %.

[0083] As a result of capturing images with a transmission electron microscope (TEM), it could be appreciated that in the case of the furan resin, a thinner coating film was formed. It was confirmed that in the case of the soft carbon, the discharge capacity was reduced in both two samples, and in particular, it was observed that in the case of the earth graphite, the capacity was further reduced. As a result of performing coating using a furan resin, which was a kind of hard carbon, an increase in discharge capacity was observed, but in the case of the flake graphite, the discharge capacity was reduced. It seems that a relatively large crystallite size is based on structural heterogeneity.

[0084] Evaluation of Performance According to Hard Carbon Coating Type

[0085] Based on the results of Table 3, the electrochemical performance was evaluated using a phenolic resin as a precursor in order to confirm the lifespan characteristics of the hard carbon coating, that is, the expansion inhibition performance. The content of the phenolic resin was set to 1 wt % with respect to the total weight of the core part, the coating was performed by wet coating, and carbonization was performed after the coating, thereby manufacturing an anode active material. An anode and a lithium secondary half cell were manufactured using the anode active material in the same manner as those of Examples.

[0086] The evaluation results of the electrochemical properties are shown in Table 5, and the results are obtained by measuring the capacity retention rate after 50 cycles at each C-rate.

[0087] The D50 and the specific surface area are shown in Table 6.

TABLE-US-00005 TABLE 5 Coating Anode material material wt % 0.5C 1C 3C 5C Earth graphite:pitch Phenolic 1 100 99.6 98.5 97.5 coke = 30:70 resin Earth graphite:pitch Phenolic 1 100 99.9 97.8 96.4 coke = 70:30 resin Flake graphite:pitch Phenolic 1 99 99.5 97.4 92.4 coke = 30:70 resin Flake graphite:pitch Phenolic 1 100 99.6 96.2 91.5 coke = 70:30 resin

TABLE-US-00006 TABLE 6 Specific surface Coating D50 area Anode material material wt % (um) (m.sup.2/g) Earth graphite:pitch Phenolic 1 8.8 1.4 coke = 20:80 resin Earth graphite:pitch Phenolic 1 8.3 1.1 coke = 30:70 resin Earth graphite:pitch Phenolic 1 6.0 1.2 coke = 70:30 resin Earth graphite:pitch Phenolic 1 5.7 1.0 coke = 80:20 resin Flake graphite:pitch Phenolic 1 8.4 1.5 coke = 20:80 resin Flake graphite:pitch Phenolic 1 8.1 1.3 coke = 30:70 resin Flake graphite:pitch Phenolic 1 5.8 1.1 coke = 70:30 resin Flake graphite:pitch Phenolic 1 8.8 1.4 coke = 80:20 resin

[0088] It could be confirmed that in the case of using earth graphite, the capacity retention rate was maintained even at a 5C-rate in comparison to the case of using flake graphite.

[0089] In addition, views obtained by capturing images of an electrode piece after 50 cycles at a 5C-rate in a case where a ratio of earth graphite:pitch coke is 30:70 (FIG. 2) and in a case where a ratio of flake graphite:pitch coke is 30:70 (FIG. 3) with a scanning electron microscope (SEM) are illustrated in FIGS. 2 and 3. The electrode piece was cut using a focused ion beam.

[0090] The electrode was manufactured by setting all thicknesses of the active materials on the initial electrode (copper plate), and it could be confirmed that in the case of using flake graphite, the anode active material was expanded by about 20% or more after 50 cycles in comparison to the case of using flake graphite.

[0091] The present invention is not limited to the exemplary embodiments, but may be manufactured in various different forms, and it will be apparent to those skilled in the art to which the present invention pertains that various modifications and alterations may be made without departing from the spirit or essential feature of the present invention. Therefore, it is to be understood that the exemplary embodiments described hereinabove are illustrative rather than being restrictive in all aspects.