[DESCRIPTIONA POSITIVE ELECTRODE FOR SECONDARY BATTERY [TECHNICAL FIELD] The present invention relates to a cathode for secondary batteries. More 5 specifically, the present invention relates to a cathode for secondary batteries having long lifespan and superior storage properties and exerting superior safety based on a specific element composition. [BACKGROUND ART] Technological development and increased demand for mobile equipment have 10 led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long lifespan and low self discharge are commercially available and widely used. In addition, increased interest in environmental issues has brought about a great 15 deal of research associated with electric vehicles (EVs) and hybrid electric vehicles (HEVs) as substitutes for vehicles using fossil fuels such as gasoline vehicles and diesel vehicles which are main factors of air pollution. These electric vehicles generally use -1- nickel hydride metal (Ni-MH) secondary batteries as power sources of with electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like. However, a great deal of study associated with use of lithium secondary batteries with high energy density and discharge voltage is currently underway and some of them are commercially available. In particular, lithium secondary batteries used for electric vehicles should have high energy density, exhibit great power within a short time and be used under harsh conditions for 10 years or longer, thus requiring considerably superior stability and long lifespan, as compared to conventional small lithium secondary batteries. Conventional lithium secondary batteries generally utilize a lithium cobalt 10 composite oxide having a layered structure for a cathode and a graphite-based material for an anode. However, such lithium cobalt composite oxide is disadvantageously unsuitable for electric vehicles in terms of presence of extremely expensive cobalt as a main element and low safety. Accordingly, lithium manganese composite oxide having a spinet structure containing manganese that is cheap and has superior safety is 15 suitable for use as a cathode of lithium ion secondary batteries for electric vehicles. However, lithium manganese composite oxides cause deterioration in battery properties since manganese is released into an electrolyte due to affection of the electrolyte when stored at high temperature. Accordingly, there is a need for a solution to this phenomenon. In addition, as compared to conventional lithium cobalt -2- composite oxide or lithium nickel composite oxide, lithium manganese composite oxides have a disadvantage of low capacity per unit weight, thus having a limitation of an increase in capacity per battery weight. Lithium manganese composite oxide should be used in combination with battery design capable of solving this phenomenon 5 in order to allow the same to be practically available as a power source of electric vehicles. - In order to solve these disadvantages, layered mixed metal oxides, LiNixMnyCo7O2 (x+y+z=l) and the like are used, but they cannot secure satisfactory stability yet. Surface-treatment is attempted in order to solve this disadvantage, but 10 problems such as increase in price which is one of the most important problems in the battery market such as electric vehicles occur due to the necessity of additional processes. [DIISCLOSURL+ [TL+ CHNIICAL PROBLEM] 15 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 below, the inventors of the present invention have -3- discovered that, when a secondary battery is fabricated using a cathode comprising a cathode active material that has a specific element composition as shown in the compound of Formula I and includes a transition metal layer containing lithium, lifespan can be greatly improved without using additional processes. Based on this 5 discovery, the present invention has been completed. [TECHNICAL SOLUTION] In accordance with one aspect of the present invention, provided is a cathode for secondary batteries, comprising a compound having a transition metal layer containing lithium as at least one compound selected from the following formula 1: 10 (1-x)Li(LiyMi_y.,Maz)Oz_eAb*xLi3POa (I) wherein M is an element stable for a six-coordination structure, which is at least one selected from transition metals that belong to first and second period elements; Ma is a metal or non-metal element stable for a six-coordination structure; A is at least one selected from the group consisting of halogen, sulfur, 15 chalcogenide compounds and nitrogen; 0 A mixed transition metal precursor having a composition of Ni:Mn:Co = 15 0.53:0.27:0.2 (molar ratio) was prepared by a co-precipitation method known in the art, and lithium hydroxide and Li3PO4. were added to the mixed transition metal precursor such that conditions of x=0.01, y=0.02, z=0.0 and b=0 in Formula 1 were satisfied, followed by baking in a furnace to synthesize a cathode active material. -141- The synthesized cathode active material was mixed with NMP such that a ratio of active material : conductive material : binder became 95 : 2.5 : 2.5 (weight ratio), and the mixture was coated on an Al foil with a thickness of 20 tm to produce a cathode. The cathode was pressed such that an inner pore ratio was 25% to fabricate a coin-type battery. A Li-metal foil was used as an anode and a solution of 1M LiPF6 in a carbonate mixed solvent (EC : DMC : DEC = 1 : 2 : 1, volume ratio) was used as an electrolyte. A battery was fabricated in the same manner as in Example 1 except that a 10 cathode active material having a composition of x-0.03 was synthesized. A battery was fabricated in the same manner as in Example 1 except that a cathode active material having a composition of x=0 was synthesized. Example 3> 15 A battery was fabricated in the same manner as in Example 1 except that a cathode active material having a composition of x-0.05 was synthesized. 15 A battery was fabricated in the same manner as in Example 1 except that a cathode active material having a composition of x=0.005 was synthesized. A battery was fabricated in the same manner as in Example 1 except that a 5 cathode active material was synthesized by adding aluminum hydroxide such that a part (z=0.02) of the transition metal was substituted by Al, followed by baking. A battery was fabricated in the same manner as in Example 1 except that a cathode active material having a composition of x=0 was synthesized, and mixed with 10 NMP such that active material: lithium phosphate: conductive material: binder was 90 : 5 : 2.5 : 2.5 (weight ratio), and the cathode active material was coated on an Al foil with a thickness of 20 μm to fabricate a cathode. The batteries fabricated in Examples 1 to 4 and Comparative Examples I and 2 15 were charged and discharged at 0.1 C, capacities thereof were measured, and deterioration in capacity with cycles was measured under charge and discharge conditions of 0.5C. The results thus obtained are shown in the following Table 1. [TABLE 1] -16- Discharge capacity (mAh/g) 30" cycle capacity/1" cycle capacity (%) Ex. 1 166 97.4 Comp. Ex. 1 165 93.0 Ex. 2 166 97.8 Ex. 3 160 98.2 Ex. 4 168 97.3 Ex. 5 165 97.5 Comp. Ex. 2 153 78 As can be seen from Table 1 above, although the batteries of Examples 1 to 4 contained Li3POa not directly contributing to charge and discharge, they did not exhibit a great difference in initial capacity. As the content of Li3POa increased, the capacity thereof slightly decreased, but was not significant. 5 On the other hand, the batteries (Examples 1 to 5) using a cathode active material containing Li3POa exhibited a considerably low capacity deterioration with an increase in cycles, as compared to the batteries (Comparative Examples I to 2) using a cathode active material containing no Li3POa. Specifically, for the 30t" cycle capacity to the 1st cycle capacity, the batteries of Examples 1 to 5 exhibited at least 4% or higher 10 capacity, as compared to the battery of Comparative Example 1. This difference reached several tens of % at 300 cycles or more, and as described above, batteries for vehicles are charged 1000 cycles or more and under these conditions, the difference increased. Furthermore, the battery of Comparative Example 2 exhibited 15% or more lower cycle properties, as compared to the batteries of Examples 1 to 5. This is due to the fact that battery capacity is decreased, cycle characteristics are rapidly deteriorated and electrical conductivity was lowered by separately adding Li3PO4 to the cathode active material. The battery of Comparative Example 2 was greatly distinguished with batteries of Examples 1 to 5 fabricated using a cathode active material synthesized by mixing other precursors with Li3POa, followed by baking. 10 The batteries fabricated in Example 1 and Comparative Example 1 were charged and discharged 5 and 25 cycles at 0.5C, and discharge profiles at these cycles are shown in Fig. 1. As can be seen from Fig. 1, the battery of Example 1 exhibited deterioration at 15 the end stage of discharge, and in particular, a remarkable decrease in voltage drop, as compared to the battery of Comparative Example 1. This means that deterioration is. decreased due to structural change of the cathode. Such deterioration at the end stage of discharge is the most important factor that rapidly deteriorates the power of batteries -18- for electric vehicles or hybrid electric vehicles and the factor is more important than a decreased capacity that can be measured in general batteries. In this regard, the cathode active material of the present invention can considerably reduce deterioration at the end stage of discharge. As can be seen from Fig. 1, such a phenomenon becomes serious, as the number of cycles increases. That is, the deterioration difference at the end stage of discharge at the 25th eye le is greater than that at the 5th cycle. Batteries for vehicles require 3600 cycles or more of charge and discharge although they are charged and discharged only once a day under product guarantee 10 conditions of 10 years or longer, thus making this difference considerably great. Accordingly, small difference in small conventional batteries further increases in batteries for vehicles, and difference in cycle properties, variation in charge and discharge profiles and the like are more important than the small difference in capacity. Although the preferred embodiments of the present invention have been 1-5 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. (INDUSTRIAL APPLICABILITY] -19- As apparent from the afore-going, the cathode according to the present invention can improve lifespan properties based on a specific element composition, and in particular, is thus preferably useful for devices requiring use for a long period of time due to superior cycle properties. CLAIMS [Claim 1I A cathode for secondary batteries comprising a compound having a transition metal layer containing lithium as at least. one compound selected from the following formula 1: 5(- x1)Li(LiyMi_.y_M1az)Oz_bAhxLi3pO4 (1) wherein M is an element stable for a six-coordination structure, which is at least one selected from transition metals that belong to first and second period elements; Ma is a metal or non-metal element stable for a six-coordination structure; A is at least one selected from the group consisting of halogen, sulfur, 10 chalcogenide compounds and nitrogen; 0