CATHODE MATERIAL CONTAINING TWO TYPES OF CONDUCTIVE MATERIALS AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME FIELD OF THE INVENTION The present invention relates to a cathode mix containing two types of conductive materials and a lithium secondary battery comprising the same. More specifically, the present invention relates to a cathode mix using a mixture of a flake-like carbon material (a) and a spherical chain-like carbon material (b) in a weight ratio (a/b) of 0.01 to 1 as conductive material for a cathode, and a lithium secondary battery comprising the same. BACKGROUND OF THE INVENTION Recently, an explosive increase in the demand for portable electronic equipment has led to a rapid increase in the demand for secondary batteries. Among other things, there has been great advancement in lithium secondary batteries having high-energy density, high-discharge voltage and superior power output stability. Generally, the lithium secondary battery is comprised of a structure having an electrode assembly composed of a cathode containing a lithium transition metal oxide as a cathode active material, an anode containing a carbonaceous material as an anode active material, a porous separator, and with impregnation of the electrode assembly with a lithium electrolyte. The cathode is fabricated by coating a cathode mix containing the lithium transition metal oxide on an aluminum foil, whereas the anode is fabricated by coating an anode mix containing a carbon-based active material on a copper foil. In order to improve electrical conductivity of electrode active materials, a conductive material is usually added to the cathode mix and the anode mix. In particular, the lithium transition metal oxide used as the cathode active material is essentially low in the electrical conductivity and therefore the conductive material is inevitably added to the cathode mix. Among other things, a spherical chain-like conductive material is usually used to increase the conductivity of the cathode mix. However, such a spherical chain-like conductive material suffers from a disadvantage of difficulty to achieve a high loading density in a compression process to decrease a thickness of the cathode mix. To this end, the present invention has adopted combined use of the flake-like carbon material (a) and a spherical chain-like carbon material (b) as the conductive material for the cathode, in order to increase the loading density of the cathode mix. In this connection, Japanese Unexamined Patent Publication No. 2003-257416 discloses a technique wherein a mixture containing a Li-Co composite oxide having a mean particle size of 7-13 f^ and a Li-Co composite oxide having a mean particle size of 1-6 /^i in a specified ratio is used in a cathode active material, and a mixture containing a scale-like graphitized carbon having a mean particle size of 1-6 t®\ and carbon black having a mean particle size of 0.5 (J® or less is used in a conductive material, in order to increase an electrode density while preventing formation of irregularities on a surface of an electrode mix layer which may occur upon compression of the cathode mix. According to the disclosure of this Japanese Patent, it is described that the desired surface flattening of the cathode mix layer can be achieved with the addition of a certain cathode active material in a specified ratio. Further, this Japanese Patent proposes that the preferred ratio of the scale-like graphitized carbon and carbon black as the conductive material is specified to a range of 1:0.01 to 0.1. However, the inventors of the present invention have confirmed that such a composition ratio suffers from problems associated with deterioration of performance consistency of the cathode mix and poor high-rate discharge characteristics of the cathode mix. In particular, when a loading amount of the electrode mix coated on a current collector is increased to achieve improved capacity of the secondary battery, this may lead to further deterioration of the battery performance. Therefore, the conductive material having a composition range specified in the above Japanese Patent is undesirable. Meanwhile, secondary batteries with high-energy density employ limited amounts of conductive material and binder, in order to increase the amount of cathode active material contained in the cathode mix. When a large amount of the scale-like graphitized carbon having poor electrical conductivity as discussed before is used in a limited amount of the conductive material, this may lead to deviation of electrical conductivity within the thus-applied cathode mix, thereby causing performance inhomogeneity between battery cells. Such non-uniformity of the battery performance results in abnormal operation and malfunction of the battery in the medium/large-size device using a plurality of battery cells, thereby presenting various problems. SUMMARY OF THE INVENTION Therefore, the present invention has been made to solve the above problems and other technical problems that have yet to be resolved. As a result of a variety of extensive and intensive studies and experiments to solve the problems as described above, the inventors of the present invention have discovered that upon the use of a mixture of a flake-like carbon material (a) and a spherical chain-like carbon material (b) in a specific weight ratio as a conductive material for a cathode mix, it is possible to simultaneously improve conductivity and loading density of the cathode mix, to provide excellent discharge characteristics even with increased loading amounts of the cathode mix, and to secure performance uniformity between the battery cells according to use of the conductive material. The present invention has been completed based on these findings. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a cathode mix for a lithium secondary battery, comprising a cathode active material, a conductive material and a binder, wherein a mixture of a flake-like carbon material (a) and a spherical chain-like carbon material (b) in a weight ratio (a/b) of 0.01 to 1 is used as the conductive material. Even though basic physical properties of the flake-like carbon material (a) and the spherical chain-like carbon material (b) were known in the art, it was confirmed that physical properties of the conductive material obtained by combination of Component (a) and Component (b), as also can be seen in the following Experimental Examples, exhibit significant synergistic effects above the physical properties which were generally expected to be achieved. Component (a) is a flake-like carbon material having physical properties that enable realization of a high loading density, and may be preferably selected from the group consisting of SP270, KS6, KS10, KS15, and any combination thereof. Component (b) is a spherical chain-like carbon material having high conductivity, and may be preferably selected from the group consisting of acetylene black, Denka black, Super-P, and any combination thereof. In order to exert high-discharge characteristics while achieving a maximized increase of a loading density upon compression of the cathode mix, the conductive material having an above-specified composition may be composed of Component (a) having an average particle size of 1 to 50 //m and a surface area of 10 to 500 m /g, and Component (b) having an average particle size of 10 to 200 nm and a surface area of 10 to 100 m /g. This is because it is possible to maximize the loading density increase of the cathode mix by taking advantage of slip phenomenon of a non-spherical cathode active material due to the presence of flake-like conductive material, upon compression of the cathode mix, and it is also possible to optimize discharge characteristics by a specific surface area composition. The flake-like carbon material and the spherical chain-like carbon material, used as the conductive material in the cathode mix of the present invention, are mixed in a composition ratio of 0.01 to 1, as discussed above. If the composition ratio is lower than 0.01, this leads to deterioration of loading-density increasing effects in a compression process of the cathode mix, which therefore results in an increase in stress applied to a current collector with increasing compression force or increasing numbers of compression to get the desired high loading density, consequently presenting problems such as electrode breakage. On the other hand, if the composition ratio is higher than 1, this leads to deterioration of electrical conductivity, thereby decreasing discharge characteristics and increasing the inhomogeneity of performance. More preferably, the composition ratio of carbon material is in the range of 0.7 to 1. The cathode mix is composed of the cathode active material with incorporation of above-mentioned conductive material and binder. If necessary, a filler may be further added to the cathode mix. Examples of the cathode active materials that can be used in the present invention may include, but are not limited to, layered compounds such as lithium cobalt oxide (LiCoOa) and lithium nickel oxide (LiNiC^), or compounds substituted with one or more transition metals; lithium manganese oxides such as compounds of Formula Lii+xMn2-xO4 (07; Ni-site type lithium nickel oxides of Formula LiNii.xMxO2 (M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and 0.01 (Table Removed) As can be seen from the results of Table 1, batteries of Examples 1 to 4 according to the present invention exhibited low resistance and high conductivity, and showed no breakage of the cathode current collector upon winding of the electrode assembly. In particular, the cathode of Example 4 exhibited no breakage of the cathode current collector upon winding of the electrode assembly, even though it was compressed until the loading density of the cathode mix reached to 3.857 g/cc. On the other hand, the battery of Comparative Example 1 exhibited very high resistance even with no breakage of the cathode current collector upon winding of the electrode assembly, whereas the battery of Comparative Example 2 exhibited low resistance with breakage of the cathode current collector upon winding of the electrode assembly. [Experimental Example 2] Charge/discharge capacity of the batteries fabricated in Example 3 and Comparative Example 3 was measured, and discharge efficiency and standard deviation (SD) were calculated. The results thus obtained are given in Table 2 below. Loading of the cathode mix was set to 3.153 mAh/cm2 and 3.025 mAh/cm2 for the batteries of Example 3 and Comparative Example 3, respectively. In addition, the discharge efficiency and standard deviation (SD) were calculated from the charge and discharge capacity measured with varying C-rates in 20 batteries. Table 2 below shows the discharge efficiency as an average value of the discharge capacity to the charge capacity. The discharge efficiency of each battery for such an average value was given with its standard deviation (SD). (Table Removed) As can be seen from the results of Table 2, batteries of Example 3 according to the present invention exhibited high discharge efficiency and low standard deviation (SD) at both of low-rate discharge and high-rate discharge, even with increased loading amounts of the cathode mix, as compared to batteries of Comparative Example 3. In particular, since batteries of Example 3 exhibited low standard deviation (SD), it was confirmed that such batteries can be easily applied without causing adverse side effects on battery performance and safety when it is desired to fabricate a large-capacity battery pack by multiple combination of lithium secondary batteries according to the present invention. [Experimental Example 3] Cathodes fabricated in Example 3 and Comparative Example 2 were compressed to a desired density of a cathode mix, and a distance between press rolls was measured. The results thus obtained are given in Table 3 below. The same pressure of 8 MPa was applied to the press rolls. The roll-to-roll distance was set on the basis of a relative value "0", indicating that getting closer to a positive value means an increase in the roll-to-roll distance and getting closer to a negative value means a decrease in the roll-to-roll distance. That is, when the cathode mix was compressed to a desired density under the same pressure, loading-density increasing effects were compared by measuring the compression degree in terms of the roll-to-roll distance. (Table Removed) As can be seen from the results of Table 3, the cathodes of Example 3 according to the present invention could easily realize the desired density of cathode mix even under low compression stress. That is, when the cathode mix was compressed to the desired density under the same pressure, the cathodes of Example 3 exhibited a small distance between the press rolls, as compared to the electrodes of Comparative Example 2, indicating that less amounts of compression stress are applied to a cathode current collector of the present invention. As a result, low compression stress applied upon compression of the electrode leads to less breakage of the cathode current collector, and it is thereby possible to realize a high loading density. INDUSTRIAL APPLICABILITY As apparent from the above description, a cathode mix according to the present invention and a lithium secondary battery comprising the same can achieve simultaneous improvements in both of conductivity and loading density of the cathode mix, provide excellent discharge characteristics even with increased loading amounts of the cathode mix, and secure performance uniformity between the battery cells via use of the conductive material according to the present invention. Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. WHAT IS CLAIMED IS: 1. A cathode mix comprising a cathode active material, a conductive material and a binder, wherein a mixture of a flake-like carbon material (a) and a spherical chain-like carbon material (b) in a weight ratio (a/b) of 0.01 to 1 is used as the conductive material. 2. The cathode mix according to claim 1, wherein Component (a) is selected from the group consisting of SP270, KS6, KS10, KS15, and any combination thereof. 3. The cathode mix according to claim 1, wherein Component (b) is selected from the group consisting of acetylene black, Denka black, Super-P, and any combination thereof. 4. The cathode mix according to claim 1, wherein Component (a) has an average particle size of 1 to 50 µm and a surface area of 10 to 500 m2/g. 5. The cathode mix according to claim 1, wherein Component (b) has an average particle size of 10 to 200 nm and a surface area of 10 to 100 m2/g. 6. The cathode mix according to claim 1, wherein the ratio of a/b is in the range of 0.7 to 1. 7. A lithium secondary battery comprising a cathode to which the cathode mix of claim 1 is applied. 8. A medium/large-size battery pack comprising a plurality of lithium secondary batteries of claim 7 as a unit cell.

Documents

Application Documents

#	Name	Date
1	1481-del-2007-gpa.pdf	2011-08-21
1	1481-DEL-2007_EXAMREPORT.pdf	2016-06-30
2	1481-del-2007-abstract.pdf	2011-08-21
2	1481-del-2007-form-5.pdf	2011-08-21
3	1481-del-2007-claims.pdf	2011-08-21
3	1481-del-2007-form-3.pdf	2011-08-21
4	1481-del-2007-correspondence-others.pdf	2011-08-21
4	1481-del-2007-form-2.pdf	2011-08-21
5	1481-del-2007-form-1.pdf	2011-08-21
5	1481-del-2007-description (complete).pdf	2011-08-21
6	1481-del-2007-description (complete).pdf	2011-08-21
6	1481-del-2007-form-1.pdf	2011-08-21
7	1481-del-2007-correspondence-others.pdf	2011-08-21
7	1481-del-2007-form-2.pdf	2011-08-21
8	1481-del-2007-claims.pdf	2011-08-21
8	1481-del-2007-form-3.pdf	2011-08-21
9	1481-del-2007-abstract.pdf	2011-08-21
9	1481-del-2007-form-5.pdf	2011-08-21
10	1481-DEL-2007_EXAMREPORT.pdf	2016-06-30
10	1481-del-2007-gpa.pdf	2011-08-21