FIELD OF THE INVENTION The present invention relates to a 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 Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. In recent years, applicability of secondary batteries has been realized as power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs). In the light of such trends, a great deal of research and study has been focused on secondary batteries which are capable of meeting various demands. Among other things, there has been an increased demand for lithium secondary batteries having high-energy density, high-discharge voltage and power output stability. Particularly, lithium secondary batteries for use in EVs and the like 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 time, thus necessitating remarkably superior safety and long-term service life 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 a 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, a 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 as a low capacity per unit weight, i.e., a low charge density, as compared to conventional lithium cobalt composite oxides or lithium nickel composite oxides. Thus, there is a limit to charge density of the battery and in order to enter practical use as the power source of EVs, designs of the battery to solve such disadvantages should be effected together. In order to alleviate the above-mentioned respective disadvantages, various studies and attempts to fabricate electrodes using a mixed cathode active material have been made. For example, Korean Patent Laid-open Publication No. 2003-0096214 assigned to Matsushita Electric Industrial Co., Ltd. (Japan), and Japanese Patent Laid-open Publication No. 2003-092108 disclose techniques utilizing a mixture of lithium/manganese composite oxide, and lithium/nickel/cobalt/manganese composite oxide and/or lithium/nickel/manganese composite oxide to enhance recovery and power output characteristics. These arts, however, still suffer from problems associated with inferior life characteristics 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 which ensures superior safety and can have a long-term service life at both room temperature and high temperature, even after repeated high-current charge and discharge. Another object of the present invention is to provide a lithium secondary battery comprising a cathode containing the above-mentioned cathode active material. 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 active material for a secondary battery, comprising a mixture of a lithium/manganese spinel oxide represented by Formula I below and a lithium/nickel/cobalt/manganese composite oxide represented by Formula II below, wherein at least one of two oxides has an average particle diameter of more than 15 µm: [Formula I] (Formula Removed) wherein, (Formula Removed) [Formula II] (Formula Removed) wherein, (Formula Removed) In accordance with another aspect of the present invention, there is provided a lithium secondary battery comprising the above-mentioned cathode active material-containing cathode, an anode, a separator and an electrolyte. In order to improve the life characteristics of the battery, the present nvention uses an oxide having an average particle diameter of more than 15 µm as the cathode active material. This is because an increase in the particle size of the oxide leads to inhibition in decomposition of an electrolyte and reduction in dissolution of manganese into the electrolyte. However, the manufacturing process of the oxide suffers from limitations in increasing of the oxide particle size, and an excessively larger particle size of the oxide leads to deterioration of the battery efficiency versus the weight thereof. Therefore, the average particle diameter of the oxide is preferably in the range of 15 to 30 µm. As a preferred example of the oxide particle size, mention may be made of a case where the lithium/manganese spinel oxide of Formula I has an average particle diameter of more than 15 µm, and a case where the lithium/manganese spinel oxide of Formula I and the lithium/nickel/cobalt/manganese composite oxide of Formula II have an average particle diameter of more than 15 µm, respectively. That is, when the average particle diameter of the lithium/manganese spinel oxide is more than 15 µm or the average particle diameter of both oxides is more than 15 µm, life characteristics of the battery are further improved. These facts will be illustrated and confirmed in the following Examples and Comparative Examples hereinafter. As used herein, the average particle diameter of the oxide preferably refers to a particle diameter of the oxide when large numbers of particles gather into an aggregate. The cathode active materials, i.e., individual oxide units tend to aggregate depending upon set conditions of the manufacturing process, and the resulting aggregate per se exerts desirable characteristics of active materials. Therefore, the average particle diameter of the oxide preferably means a particle diameter of such an oxide aggregate. The particle diameter of the oxide unit may vary depending upon processes for preparation of the oxides. Therefore, in a preferred embodiment, the oxide unit of the lithium/manganese spinel oxide may have a particle diameter of 0.2 to 10 urn, upon taking into consideration various characteristics of the cathode active material and morphology of the aggregate. In addition, the particle diameter of the oxides has a close relationship with the surface area of the oxide. When the oxide unit has a surface area of 0.1 to 1.0 m2/g, it is possible to exert superior characteristics of the active material. The lithium/nickel/cobalt/manganese composite oxide is a lithium oxide which simultaneously contains nickel, manganese and cobalt elements, as shown in Formula II, and significantly improves, in combination with the lithium/manganese spinel oxide, the safety and life characteristics of the cathode active material according to the present invention. The lithium/nickel/cobalt/manganese composite oxide contains each of at least 0.2 M nickel and manganese, provided that it contains cobalt. Preferred examples of the lithium/nickel/cobalt/manganese composite oxide may include, but are not limited to, Li1+zNi1/3Co1/3Mn1/3O2 and Li1 The mixing ratio of two composite oxides in the cathode active material of the present invention is preferably in the range of 90 : 10 to 10 : 90, more preferably in the range of 90 : 10 to 30 : 70 (w/w). If the content of the composite oxide (I) among two composite oxides is excessively low, the stability of the battery is lowered. Conversely, if the content of the composite oxide (II) is excessively low, it is undesirably difficult to achieve desired life characteristics. These facts "will also be illustrated and confirmed in the following Examples and Comparative Examples hereinafter. When the lithium/manganese spinel oxide of Formula I is a lithium/manganese spinel oxide of Formula III wherein a portion of manganese (Mn) is substituted with other metal elements, the life characteristics of the battery can be further improved. [Formula III] wherein, M is a metal having an oxidation number of 2 (Formula Removed) In the lithium/manganese spinel oxide of Formula III, manganese (Mn) is substituted with a metal (M) having an oxidation number of 2 or 3 within the predetermined range. Herein, the metal (M) may be preferably aluminum (Al), magnesium (Mg) or both of them. Since the oxidation number of the substituent metal is smaller than that of manganese (Mn), an increasing amount of the substituted metal leads to a decrease in an average value of the oxidation number and a relative increase in the oxidation number of manganese (Mn), consequently resulting in inhibition of manganese (Mn) dissolution. That is, as the amount of the substituted metal (z) in the lithium/manganese spinel oxide of Formula III increases, life characteristics are further improved. However, since an increasing amount of the substituted metal (z) is also accompanied by a decrease of initial capacity, a maximum value of z is preferably less than 0.2, which is capable of maximizing improvements of the life characteristics and minimizing reduction of the initial capacity of the battery. More preferably, the value of z is in the range of 0.01 to 0.2. Methods of preparing lithium metal composite oxides, such as lithium/manganese spinel oxides of Formula I, lithium/nickel/cobalt/manganese composite oxides of Formula II and lithium/manganese spinel oxides of Formula III wherein a portion of manganese is substituted with a certain metal, are well-known in the art and thus will not be described herein. Hereinafter, fabrication of a cathode containing a cathode active material according to the present invention will be specifically illustrated. First, the cathode active material of the present invention, and a binder and a conductive material in a content of 1 to 20% by weight relative to the active material are added to a dispersion solvent and the resulting dispersion is stirred to prepare an electrode paste. The paste is applied to a metal plate for a current collector which is then compressed and dried to fabricate a laminate electrode. The cathode current collector is generally fabricated to have a thickness of 3 to 500 um. There is no particular limit to the cathode current collector, so long as it has high conductivity without causing chemical changes in the fabricated battery. As examples of the cathode current collector, mention may be made of stainless steel, aluminum, nickel, titanium, sintered carbon, and aluminum or stainless steel which was surface-treated with carbon, nickel, titanium or silver. The current collector may be fabricated to have fine irregularities on the surface thereof so as to enhance adhesion to the cathode active material. In addition, the current collector may take various forms including films, sheets, foils, nets, porous structures, foams and non-woven fabrics. As examples of the binder that may be utilized in the present invention, mention may be made of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), cellulose, polyvinyl alcohols, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinyl pyrollidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber, fluoro rubber and various copolymers. -8- There is no particular limit to the conductive material, so long as it has suitable conductivity without causing chemical changes in the fabricated battery. As examples of conductive materials, mention may be made of conductive materials, including graphite such as natural or artificial graphite; carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black and thermal black; conductive fibers such as carbon fibers and metallic fibers; metallic powders such as carbon fluoride powder, aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives. Specific examples of commercially available conductive materials may include various acetylene black products (available from Chevron Chemical Company, Denka Singapore Private Limited and Gulf Oil Company), Ketjen Black EC series (available from Armak Company), Vulcan XC-72 (available from Cabot Company) and Super P (Timcal Co.). Where appropriate, the filler may be optionally added as an ingredient to inhibit cathode expansion. There is no particular limit to the filler, so long as it does not cause chemical changes in the fabricated battery and is a fibrous material. As examples of the filler, there may be used olefin polymers such as polyethylene and polypropylene; and fibrous materials such as glass fiber and carbon fiber. Representative examples of the dispersion solvent that can be used in the present invention may include isopropyl alcohol, N-methyl pyrrolidone (NMP) and acetone. Uniform application of the paste of electrode materials to a metal material may be carried out by conventional methods known in the art or appropriate novel methods, taking into consideration characteristics of materials to be used. For example, preferably the electrode paste is distributed onto the current collector and is then uniformly dispersed thereon using a doctor blade. Where appropriate, distribution and dispersion of the electrode paste may also be performed in a single step. Further, application of the electrode paste may be carried out by a method selected from die casting, comma coating, screen printing and the like. Alternatively, application of the electrode paste may be carried out by molding the paste on a separate substrate and then binding it to the current collector via pressing or lamination. Drying of the paste applied over the metal plate is preferably carried out in a vacuum oven at 50 to 200°C for 1 to 3 days. Further, the present invention provides a lithium secondary battery comprising the cathode fabricated as above. The lithium secondary battery of the present invention is comprised of an electrode assembly composed of the above-mentioned cathode and an anode, which are arranged opposite to each other with a separator therebetween, and a lithium salt-containing, non-aqueous electrolyte. The anode is, for example, fabricated by applying an anode active material to an anode current collector, followed by drying. If desired, the anode may further optionally include other components such as conductive material, binder and filler, as described above. The anode current collector is generally fabricated to have a thickness of 3 to 500 um. There is no particular limit to the anode current collector, so long as it has suitable conductivity without causing chemical changes in the fabricated battery. As examples of the anode current collector, mention may be made of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel having a surface treated with carbon, nickel, titanium or silver, and aluminum-cadmium alloys. Similar to the cathode current collector, the anode current collector may also be fabricated to form fine irregularities on the surface thereof so as to enhance adhesion to the anode active material. In addition, the anode current collector may take various forms including films, sheets, foils, nets, porous structures, foams and non-woven fabrics. As examples of the anode materials utilizable in the present invention, mention may be made of carbon such as non-graphitizing carbon and graphite based carbon; metal composite oxides such as LixFe2O3 (0