CATHODE MIX FOR SECONDARY BATTERY AND SECONDARY BATTERY COMPRISING THE SAME The present invention relates to a cathode active material for secondary batteries. More specifically, the present invention relates to a cathode mix for secondary batteries that contains lithium iron phosphate, coated with carbon (C), having an olivine cjstal structure, as a cathode active material, wherein a mean 10 particle diameter of primary particles in the cathode active material is 2 p, and the cathode mix contains a hydrophilic conductive material as a conductive material. [BACKGROUND ART] Carbon'materials are generally used as cathode active materials for lithium secondary batteries that are being used in rapidly increasing number. Also, the use of 15 lithium metals, sulfur compounds, silicon compounds, tin compounds and the like have been considered. Meanwhile, lithium-containing cobalt oxides (LiCo02) are generally used as cathode active materials for lithium secondary batteries. Also, the use of lithium-containing manganese oxides such as LiMnOz having a layered crystal 0 structure and LiMn204 having a spinel crystal structure, and lithium-containing nickel oxide (LiNi02) as the cathode active materials has been considered. LiCo02 is currently used owing to superior physical properties such as cycle properties, but has disadvantages of low stability, high-cost due to use of cobalt, which 5 suffers fiom natural resource limitations, and limitation of mass-use as a power source for electric automobiles. LiNi02 is unsuitable for practical application to massproduction at a reasonable cost due to many features associated with preparation methods thereof. Lithium manganese oxides such as LiMn02 and LiMn204 have a disadvantage of poor cycle properties. 10 In recent years, methods to use a lithium transition metal phosphate as a cathode active material have been researched. Lithium trarisition metal phosphates are largely divided into LixM2(P04)3 having a Nasicon structure and LiMP04 having an olivine structure, and are found to exhibit superior high-temperature stability, as compared to conventional LiCo02. To date, Li3V2(P04)3 is the most widely known 15 Nasicon structwe compound, and LiFeP04 and Li(Mn, Fe)P04 are the most widely known olivine structure compounds. Among olivine structure compounds, LiFeP04 has a high output voltage of 3.5V, a high volume density of 3.6 &m3, and a high theoretical capacity of 170 1 a g , as compared to lithium (Li), and exhibits superior high-temperature stability, 0 as compared to cobalt (Co), and utilizes cheap Fe as an ingredient, thus being highly applicable as a cathode active material for lithium secondary batteries. However, active materials used for lithium secondary batteries require high density and rate properties. Such LiFeP04 exhibits considerably low ~ id+iffu sion 5 rate and electrical conductivity. For this reason, when LiFePOl is used as a cathode active material, internal resistance of batteries disadvantageously increases. As a result, when battery circuits are closed, polarization potential increases, thus decreasing battery capacity. In order to solve these problems, Japanese Patent Application Publication No. 1 0 200 1 - 1 1 04 14 suggests incorporation of conductive materials into olivine-type metal phosphates in order to improve conductivity. However, LiFeP04 is commonly prepared using Li2C03 or LiOH as a lithium source, by solid state methods, hydrothermal methods and the like. Lithium sources ; and carbon sources added to improve conductivity disadvantageously cause a great 15 amount of Li2C03. Such Li2C03 is degraded during charging, or reacts with an electrolyte solution to produce CO2 gas, thus disadvantageously causing production of a great amount of gases during storage or cycles. As a result, disadvantageously, swelling of batteries is generated and high-temperature stability is deteriorated. - In another approach, a method in which a diffusion distance is decreased by reducing the particle size of LiFeP04 is used. In this case, great costs associated with the process for fabricating batteries are incurred due to high BET value. Such LiFeP04 has a great advantage of being low cost, but having a lower 5 density than active materials having a generally known layered structure or spinel structure due to the afore-mentioned disadvantages, thus causing a deterioration in content of active materials in the process of mixing to fabricate electrodes. In particular, when the surface of LiFeP04 is treated with carbon (C), hydrophobic hctional groups are present and further deteriorated mixing properties 10 are thus imparted. In addition, as particle size decreases, mixing properties are deteriorated. In order to reinforce these mixing properties, the amount of solvent should be increased. As the amount of solvent increases, cracks are induced in pores formed during evaporation of the solvent in the drying process, and problems such as non-uniformity of electrodes and deterioration in conductivity are caused. Such a 15 mixing problem .is encountered in the initial process of battery fabrication, thus having a great effect on all battery processes and battery characteristics. Accordingly, there is an increasing need for mixes that use LiFeP04 coated with carbon (C) as an active material, do not increase the amount of solvent, exhibit superior process properties and have a high solid content in the slurry. [TECHNICAL PROBLEM] Therefore, the present invention has been made to solve the above problems and other technical problems that have yet to be resolved. 5 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, when specific lithium iron phosphate nanoparticles coated with carbon (C) having an olivine crystal structure are used as hydrophilic conductive materials, the amount of solvekt can be reduced, the solid content of slurry can thus be increased, 10 and formation of cracks can be reduced in the process of fabricating the electrode. Based on this discovery, the present invention has been completed. [TECHNICAL SOLUTION] In accordance with one aspect of the present invention, provided is a cathode mix for secondary batteries, comprising lithium iron phosphate, coated with carbon 15 (C), having an olivine crystal structure that contains a compound represented by the following formula 1 as a cathode active material and, wherein a mean particle diameter of primary particles in the cathode active material is 2 ~ITI or less, contains a I in the slurry to be applied to a current collector in the process of fabricating electrodes. 1 wherein M is at least one selected fiom Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, I 5 Nb, Zr, Ce, In, Zn and Y, A is at least one selected from F, S and N, and O 15 An electrode was fabricated in the same manner as in Example 1 except that acetylene black was used as a conductive material. v An electrode was fabricated in the same manner as in Example 1 except that the cathode mix was coated on a current collector to a thickness of 300 p. An electrode was fabricated in the same manner as in Example 1 except that 5 the cathode mix was coated on a current collector to a thickness of 350 w. An electrode was fabricated in the same manner as in Example 1 except that the cathode mix was coated on a current collector to a thickness of 400 p. An electrode was fabricated in the same manner as in Comparative Example 1 except that the cathode mix was coated on a current collector to a thickness of 300 p. An electrode was fabricated in the same' manner as in Comparative Example 1 except that the cathode mix was coated on a current collector to a thickness of 350 p. An electrode was fabricated in the same manner as in Comparative Example 1 except that the cathode mix was coated on a current collector to a thickness of 400 p~. (Test Example I> In order to compare the amount of solid in the process for fabricating 5 electrodes, the amount of used NMP was normalized and the results are shown in the following Table 1. For comparison, electrodes were fabricated in the same manner as . in Comparative Example 1 using LiMn204 and LiNilI3Mnll3ColnO2a s active materials and were tested. The results are shown in the following Table 1. 10 As can be seen from~able1, the slurries of Examples exhibited an about 30% Sluny of Example Slun;y of Comparative Example Reference (LiMnzOd) Reference (L~N~II~M~II~COII~O~) decrease in used NMP amount, as compared to slurries of Comparative Examples. Amount of used Normalized NMP (%) 64 100 80 67 This means that the process can be improved to about 30% or more. In addition, the i) slurries have a high solid content, as compared to solid contents of commonly used cathode active materials, thus exhibiting superior processability. (Test Example 2> The electrodes of Examples 1 to 4 and Comparative Examples 1 to 4 were pressed in the form of a coin and a coin-type battery was fabricated using Li metal as an anode and a carbonate electrolyte solution, in which one mole of LiPF6 was dissolved, as an electrolyte solution. The obtained battery was subjected to 0.1C charge and discharge (twice), 0.5C charge and discharge (twice), 1 .OC charge and discharge (twice), 2.OC charge and discharge (twice) and then 1 C charge and discharge. 2.OCIO.lC discharge capacity ratio (rate property) measured in the test and 50t h/ 1s t discharge capacity ratio upon 1C charge and discharge (cycle property) are shown in the following Table 2. (Table 2> Cycle property (5 0 9 1 ", %) 99.3 96.5 Example 1 Comparative Example 1 Rate capability (2.0C/O.lC, %) 95.2 93.1 As can be seen from Table 2, the battery in which the electrode of Example is used exhibited superior electrochemical properties. In particular, as electrode thickness increases, the effect is remarkable. The reason for this is that electrode cracks become more serious since a greater amount of solvent should be evaporated as the thickness increases. In the secondary battery, electrode thickness is a considerably important element that improves capacity of batteries and has a great 98.8 97.5 97.7 94.2 92.6 89.5 Example 2 Example 3 Example 4 Comparative Example 2 Comparative Example 3 Comparative Example 4 effect on applicability of a specific active material. 94.3 92.7 90.6 90.2 86.3 82.6 From these results, although it may be thought that there is no great difference in cycle properties, when taking into consideration the fact that the battery will be used generally for vehicles and po.wer storage batteries that should be used 2,000 to 5,000 times or more, difference in cycle properties will further increase. As apparent from the fore-going, the cathode mix for secondary batteries according to the present invention can reduce a solvent content, thus advantageously exhibiting a high solid content in the slurry, minimizing cracks in the process of fabricating electrodes, and improving processability. 5 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. [CLAIMS] (claim 1] A cathode mix for secondary batteries, comprising lithium iron phosphate, coated with carbon (C), having an olivine crystd structure that contains a compound represented by the following formula 1 as a cathode active material, 5 wherein a mean particle diameter of primary particles in the cathode active material is 2 pm or less, and the cathode mix contains a hydrophilic conductive material as a conductive material: wherein M is at least one selected from Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn and Y; A is at least one selected from F, S and N; and [claim 2] The cathode mix for secondary batteries according to claim 1, 15 wherein a mean particle diameter of primary particles is 50 to 1000 nrn. - 4b [Claim 3] The cathode mix for secondary batteries according to claim 1, wherein the hydrophilic conductive material has a hydrophilic hctional group content not less than 0.1 % by weight and lower than 20% by weight. [Claim 4] The cathode mix for secondary batteries according to claim 1, wherein the hydrophilic conductive material has a hydrophilic functional group content not less than 0.2% by weight and lower than 5% by weight. [claim 5] The cathode mix for secondary batteries according to claim 1, wherein the hydrophilic conductive material has a mean particle diameter of 300 nm or less. [claim 6] A cathode for secondary batteries in which the cathode mix for secondary batteries according to any one of claims 1 to 5 is applied to a current collector. [Claim 7] A lithium secondary battery comprising the cathode for secondary batteries according to claim 6. [Claim 8] The lithium secondary battery according to claim 7, wherein the secondary battery has a 2.OClO.lC discharge capacity ratio of 90% or more and 50t h1 1 st cycle discharge capacity under 1 C charge and discharge conditions of 95% or more. i [claim 9] A battery module comprishg the lithium secondary battery according to claim 7 as a unit battery. [claim 101 A battery pack comprising the battery module according to claim 9 as a unit battery. [claim 11] The battery pack according to claim 10, wherein the battery pack is used as a power source of medium and large devices. [claim 12] The battery pack according to claim 11, wherein the medium and large devices- are electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in . . hybrid electric vehicles (PHEVs) or power storage systems. Dated this July 22,2013