The present invention relates to a cathode active material for a secondary battery and a battery thereof. FIELD OF THE INVENTION The present invention relates to a non-aqueous electrolyte-based high power lithium secondary battery having a long-term service life and superior safety at both room temperature and high temperature, even after repeated high-current charge and discharge. BACKGROUND OF THE INVENTION Recently, strict control and regulation of vehicle emissions, in many countries including the USA and Europe, has accelerated development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) using internal combustion engines and batteries as power sources, thereby partially entering commercialization. Currently, batteries that can be utilized in EVs or HEVs are high power batteries and for example, Ni-MH secondary batteries, safety of which was verified, are commercially available. In addition, development of lithium secondary batteries having output density and energy density superior to Ni-MH secondary batteries is also actively underway. However, lithium secondary batteries for use in EVs require not only high energy density and capability to exert large power output within a short period of time, but also a long-term service life of more than 10 years even under severe conditions in which high current charge/discharge cycles are repeated within a short term, thus necessitating remarkably superior safety and long-term service life properties compared to conventional small-size lithium secondary batteries. Lithium ion batteries that have been used in conventional small size batteries generally employ a layered structure of lithium cobalt composite oxide as a cathode material and graphite-based material as an anode material. However, the main constitutional element of the lithium cobalt composite oxide, cobalt, is very expensive and is not suitable for use in electric vehicles due to safety concerns. Therefore, as the cathode material of lithium ion batteries for EVs, lithium manganese composite oxide having a spinel structure made up of manganese is ideal in terms of both cost and safety. However, the lithium manganese composite oxide, upon high-temperature and high current charge/discharge, undergoes elution of manganese ions into an electrolyte due to the influence of the electrolyte, thus resulting in degradation of battery properties and performance. Thus, there is a need for measures to prevent such problems. In addition, the lithium manganese composite oxide has drawbacks such low charge density as compared to conventional lithium cobalt composite oxide or lithium nickel composite oxide. Thus, there is a limit to charge density of the battery and in order to enter practical use as the powder source of EVs, HEVs, etc., specific designs of the battery to overcome such disadvantages should be effected together. In order to alleviate the above-mentioned respective disadvantages, various studies and attempts to prepare electrodes using a mixed cathode active material have been made. For example, Japanese Patent Publication Laid-open Nos. 2002-110253 and 2003-168430 disclose techniques utilizing a mixture of lithium manganese oxide and/or lithium cobalt oxide, and lithium nickel-manganese-cobalt composite oxide (B) to enhance recovery output. These techniques, however, suffer from problems associated with inferior cycle life span of the lithium manganese oxide and limited improvement of safety. SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to solve the above problems, and other technical problems that have yet to be resolved. Specifically, an object of the present invention is to provide a cathode active material for a secondary battery, comprising a mixture of a lithium manganese-metal composite oxide (A) having a spinel structure and composed of a particular metal element composition and a lithium nickel-manganese-cobalt composite oxide (B) having a layered structure and composed of a particular metal element composition, such that the cathode active material has superior safety and a long-term service life at both room temperature and high temperature due to improved properties of lithium manganese oxide, even after repeated high current charge and discharge. Another object of the present invention is to provide a lithium secondary battery comprising the above-mentioned cathode active material. Such a lithium secondary battery may be preferably used as high power, large capacity batteries, in particular, for electric vehicles (EVs) and hybrid electric vehicles (HEVs). In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a cathode active material for a secondary battery, comprising a lithium manganese-metal composite oxide (A) having a spinel structure and represented by the following General Formula 1 and a lithium nickel-manganese-cobalt composite oxide (B) having a layered structure and represented by the following General Formula 2: General Formula 1 wherein, 0 < x < 0.2; 02 were mixed in a weight ratio of 90:10. 1.25 g of the resulting mixture was added to 25 mL of triple distilled water, which was then subjected to ultra-sonication for 30 min, followed by measuring pH using a pH meter. The results are shown in Table 1 below. Examples 2 through 5 Experiments were repeated using the same procedure as in Example 1, except that the mixing ratio of Lii.iMni.g5Alo.os04 to LiNio.4Mno.4Coo.2O2 was varied, and the pH of the thus-obtained product was measured. The results are shown in Table 1 below. Comparative Example 1 An experiment was repeated using the same procedure as in Example 1, except that the mixing ratio (by weight) of Lii.iMni.g5Alo.o504 to LiNio.4Mno.4Coo.2O2 was 100:0, and the pH of the thus-obtained product was measured. The results are shown in Table 1 below. Comparative Example 2 An experiment was repeated using the same procedure as in Example 1, except that the mixing ratio (by weight) of. Lii.iMni.85Alo.osO4 to LiNio.4Mno.4Coo.2O2 was 0:100, and the pH of the thus-obtained product was measured. The results are shown in Table 1 below. [Table 1] Mixing weight ratio of pH (Table Removed) Example 6 In the same manner as in Example 1, Lii.iMni.g5Alo.o504 and LiNio.4Mno.4Coo.2O2 were mixed in a weight ratio of 90:10 to prepare a cathode active material. The cathode active material, carbon black and, as a binder, polyvinylidene fluoride (PVDF), in a weight ratio of 85:10:5 were mixed in an organic solvent, NMP, to prepare a slurry. The resulting slurry was applied to both sides of aluminum foil having a thickness of 20 ujn and dried to prepare a cathode. A button type battery was assembled using the thus-prepared cathode, a lithium metal as an anode, a porous polyethylene film as a separator, and a 1M LiPFe EC/EMC solution as an electrolyte. In order to evaluate high-temperature service life of the thus-prepared battery, the battery was subjected to 50 charge/discharge cycles at a current density of 0.2 C and a temperature of 50°C. Discharge capacity retention rate of the battery was calculated according to the following Equation 1. The results are shown in Table 2 below. Equation 1 Discharge capacity retention rate (%) = (discharge capacity after 100 charge/discharge cycles / discharge capacity after 1 charge/discharge cycle) x 100 * Note: 100 charge/discharge cycles is set to find optimal condition for relative comparison. Examples 7 through 1Q Batteries were assembled using the same procedure as in Example 6, except that the mixing ratio (by weight) of Lii.iMni.8sAlo.o5O4 to LiNio.4Mno.4Coo.2O2 was controlled as listed in Table 2 below, and high temperature service life of the batteries was evaluated. The results are shown in Table 2 below and FIG. 3, respectively. Comparative Example 3 A battery was assembled using the same procedure as in Example 6, except that the mixing ratio (by weight) of Lii.iMni.8sAlo.osO4 to LiNio,4Mno.4Coo.2O2 was 100:0, and high temperature service life of the battery was evaluated. The results are shown in Table 2 below and FIG. 3, respectively. Comparative Example 4 A battery was assembled using the same procedure as in Example 6, except that the mixing ratio (by weight) of Lii.iMni.8sAlo.o5O4 to LiNio.4Mno.4Coo.2O2 was 0:100, and high temperature service life of the battery was evaluated. The results are shown in Table 2 below and FIG. 3, respectively. [Table 2] (Table Removed) As can be seen from Tables 1 and 2, upon mixing the lithium manganese-metal composite oxide (A) of Lin-xMn2.x.yMy04 and the lithium nickel-manganese-cobalt composite oxide (B) of Lii.aNibMncCoi-b.cO2, a pH of the cathode active material increases as the mixing ratio of the composite oxide (B) increases. It is believed that such an increase of pH inhibits elution of manganese ions from the spinel structure composite oxide (A) into the electrolyte, thereby leading to increased service life of the battery at high temperatures. In addition, it can be seen that due to admixing with the lithium nickel-manganese-cobalt composite oxide (B), it is also possible to solve problems associated with low charge density, disadvantageous^ exhibited by the spinel structure lithium manganese-metal composite oxide (A). Although the cathode active material of Comparative Example 4 exhibits superior discharge capacity retention, this active material suffers from safety problems, as will be seen from Comparative Example 6. Example 11 A lithium manganese-metal composite oxide (A) of Lii.iMni.gsMgo.osO4 and lithium nickel-manganese-cobalt composite oxide (B) of LiNii/sMni/jCoj/aOawere used in a weight ratio of 90:10 to prepare a cathode active material. The cathode active material, carbon black and as a binder, PVDF in a weight ratio of 85:10:5 were mixed in an organic solvent, NMP to prepare a slurry. The resulting slurry was applied to both sides of aluminum foil having a thickness of 20 (M and dried to prepare a cathode. Spherical artificial graphite powder having high crystallinity and average particle size of 12 um and the binder, PVDF were mixed in a weight ratio of 90:10, and then admixed in NMP to prepare a slurry. The resulting slurry was applied to copper foil having a thickness of 10 um and dried, followed by roll pressing to a thickness of 60 um so as to prepare an anode. A stacked type lithium battery, as shown in FIG. 1, was fabricated using the thus-prepared cathode and anode, and a 1M LiPFg EC/EMC solution as an electrolyte. When the lithium metal is used as the anode, it is difficult to confirm high power output higher than 5 C, due to slow diffusion rate of the lithium metal. Therefore, using carbon as the anode, life span performance of the battery upon charge/discharge with high current pulse was tested. In order to evaluate the life span performance of the battery upon charge/discharge with high current pulse, a large number of charge/discharge were repeated with current of 50A at room temperature and 40 to 60% DOD (Depth Of Discharge). High current charge/discharge discharge capacity retention rate of the battery was calculated according to the following Equation 2. The results are shown in Table 3 below and FIG. 4, respectively: Equation High current charge/discharge discharge capacity retention rate (%) = (discharge capacity at initial current density of 1 C / discharge capacity at current density of 1 C after 50 A cycle charge/discharge) x 100 In addition, the battery safety was tested. The results are shown in Table 4 below. Safety testing of the battery was carried out by overcharging the battery to SOC (State Of Charge) of 200 or 20 volts by high current of 32 A and confirming the presence of battery firing. Examples 12 through 15 Batteries were assembled using the same procedure as in Example 11, except that a mixing ratio (by weight) of Lii.iMni.85Mgo.os04 to LiNii/3Mni/3Coi/3O2 was varied. High current pulse charge/discharge life span performance (high load discharge rate) of the batteries was evaluated and safety testing of the batteries was also carried out. The results are shown in the following Tables 3 and 4, and FIG. 4, respectively. Comparative Example 5 A battery was assembled using the same procedure as in Example 11, except that the mixing ratio (by weight) of Lii.iMni.gjMgo.osC^ to LiNii/aMnj/sCoi^ was 100:0. High current pulse charge/discharge life span performance (high load discharge rate) of the battery was evaluated and safety testing of the battery was carried out. The results are shown in the following Tables 3 and 4, and FIG. 4, respectively. Comparative Example 6 A battery was assembled using the same procedure as in Example 11, except '> that the mixing ratio (by weight) of Lii.iMni.85Mgo.osO4 to LiNii/sMni^Coj/sOa was 0:100. High current pulse charge/discharge life span performance (high load discharge rate) of the battery was evaluated and safety testing of the battery was carried out. The results are shown in the following Tables 3 and 4, and FIG. 4, respectively. [Table 3] (Table Removed) As can be seen from Table 3, upon mixing the lithium manganese-metal composite oxide (A) and the lithium nickel-manganese-cobalt composite oxide (B), a high current charge/discharge life span of the battery increases as the mixing ratio of the lithium nickel-manganese-cobalt composite oxide (B) increases. This is believed due to that, even though high current charge/discharge of the lithium secondary battery results in elevated temperature of the battery, a mixed electrode of the spinel structure lithium manganese-metal composite oxide (A) and layered structure lithium nickel-manganese-cobalt composite oxide (B) has a high pH and basically stable structure at high temperature, thereby having effects on increase of battery life span, as also shown in preceding examples. [Table 4] (Table Removed) From the results of Table 4, it can be seen that in the weight mixing ratio of the lithium manganese-metal composite oxide (A) and lithium nickel-manganese-cobalt composite oxide (B), battery safety can be secured when the proportion of the composite oxide (A) exceeds 50%. This effect is believed due to safety of the spinel structure lithium manganese-metal composite oxide (A). Although, the cathode active material of Comparative Example 5 exhibited superior safety due to no fire in experiments, it suffers from a problem associated with low discharge capacity retention rate after charge/discharge cycles, as can be seen from Table 3. Example 16 A battery was assembled using the same procedure as in Example 6, except that Lii.o8Mni.87Alo.osO4 was used as the lithium manganese-metal composite oxide (A), instead of Li]jMnj j5Alo.o$O4, and high temperature service life of the battery was evaluated. The results are shown in Table 5 below. Comparative Example 7 A battery was assembled using the same procedure as in Example 16, except that Li].ogMni.92O4 was used as the lithium manganese-metal composite oxide (A), instead of Lii.ogMni^Alo.osO^ and high temperature service life of the battery was evaluated. The results are shown in Table 5. [Table 5] Discharge capacity retention rate Capacity /gram (Table Removed) As can be seen from Table 5, due to substitution of Mn with other metal M, the lithium manganese-metal composite oxide (A) of Lii+xMn2-x-yMyC>4 [provided, 0