CATHODE ACTIVE MATEMAH, FOR SECONDARY BATTERY The present invention relates to a cathode active material for secondary 5 batteries. More specific:ally, the present invention relates to a cathode active material for secondary batteries having long lifespan and superior storage propelties and exei-ting 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 secondaty balteries, lithium secondary batteries having 11if:lr energy density and voltage, long lifespan and low self-discharge are commercially available - aiid widely used. 111 addition, increased interest in enviroamental Issues has brought about a great 15 deal of research associated witli electric vehicles (EVs) and hybrid electric vehicles (HEVs) as substitutes for vehicles using fossil fiaels such as gasoline vehicles and diesel vehicles which are main factors of air pollution. Tliese electric vehicles generally use nickel hydride metal mi-MH) secondary batteries as power sources of electric vehicles (EVs), hybrid electric vehicles (HEVs) and the lilte. H-lowever, a great deal of study associated with use of lithium secondasy batteries with high energy density and discharge voltage is currently underway and some of them are commercially available. 5 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 arid 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 ail anode. However, such lithium cobalt composite oxide is disadva~itageously unsuitable for electric vehicles in terms of presence of ektremely expelisive cobalt as a main element and low safety. Accordingly, lithium manganese composite oxide having a spinel structure containing manganese that is cheap and has superior safety is 15 suitable h r use as a cathode of lithium ion secondary batteries for electric vehicles. However, lithium manganese con~posite 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 coiiventional lithiuni cobalt coinposite oxide or lithium nickel composite oxide, lithium manganese composite oxides have a disadva~itage 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 meis11 oxides, LiNi,Mn,Co,O2 (x+ytz=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. ~TECH- NIGALPR OBLEM] 15 Therefore, the present invention has been made to solve the above problems and other technical problems t1ia.t have yet to be resolved. As a result of a variety of extensive and intensive studies and experiments to solve the problems a.s described above, the inventors of the present invention have discovered that, when a cathode is produced using a cathode active material that ha.s a specific element composition as shown in the compound of Formula 1 and includes a transition metal layer containing lithium, and a secondary battery is fabricated based on the cathode, lifespan can be greatly improved without using additional processes. 5 Based on this discovery, the present invention has been completed. In accordance with one aspect of the present invention, provided is a cathode active material for secondary batteries comprising a compound having a transition metal layer containing lithium as at least one coinpound selected from tlie following Fosniula 10 1: wliesein M is ari element stable for a six-coordination structure, which is at least one selected from transition metals that belong to first and second period elements; -4xy is satisfied iai a case of 3x-y. l'lie cathode active material according to the preseiit invention exhibits improved cycle propelties and storage properties of an active material, tluough stabilization of crystal structure, based on the strong bonding force of P contained in the elernent composition and variation in oxidation number by Li (lithium) present in tlie 5 transition metal layer. When the content of P is excessively high, it inhibits crystallization of the cathode active material arid it may be difficult to improve ~serformance 01' (Ire active material. As defined above, the content is preferably lower than 0.1, more preferably O A battery was fabricated in the same manner as in Example 1 excep"chal a cathode active material having a composition of x=O was synthesized. 5 A battery was fabricated in the same inanner as in Example 1 except that a cathode active material having a composition of x=0.05 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=0.005 was synthesized. The batteries fabricated in Examples I to 4 and Comparative Example 1 were charged and discharged at O.IC, capacities thereof were measured, and deterioration in caps-city .with cycles was measured under charge and discharge conditions of 0.5C. 'The results thus obtained are shown in the following Table 1. 15 [TABLE 11 cycle capacity (%) contain P as a different element other than a transition metal, they did not exhibit a great Ex. 1 -- Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 difference in capacity. As the content of P increased, the capacity there01 slightly decreased, but was not significant. As can be seen from Table 1 above, although the batteries of Examples 1 to 4 167 165 166 162 168 5 On the other hand, the batteries (Examples 1 to 4) using a cathode active 98.0 93.2. - 98.3 98.1 97.2 inaterial containing P exhibited a considerably low capacity deterioration with an increase in cycles, as compared to the battery (Comparative Example 1) using a cathode active material containing no P. Specifically, for the 30'" cycle capacity to the 1'' cycle capacity, the batteries of Examples 1 to 4 exhibited at least 4% or higher capacity, as 10 compared to the battery of Comparative Exaniple 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 increases. The batteries fabricated in Example 1 and Comparative Example 1 were charged and discharged 5 and 25 cycles at 0.5C9 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 5 the end stage of discharge, and in particular, a re~narltable 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 thc c11d stage of discharge is the most impostant factor that rapidly deteriorates the power of batteries for electric, vehicles or hybrid electric vehicles and the Iactor is more important than a 10 decreased capacity that can be measured in general batteries. In this regard, the cathode active material of the present invention can considerably redlnce 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 25''' cycle is greater 15 than thatat the 5"' 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 conditions of 10 years or longer, thus making this difference considerably great. Accordingly, small difference in small conventional batteries fu~ther increases in batteries for vehicles, and difference in cycle properties, variation in charge and discharge profiles and the like are more irnpo~tanth an the small difference in capacity. Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those slcilled in the art will appreciate that various 5 modifications, additions and substitutions are possible, without departing fiom the scope and spirit of the invention as disclosed in the accompanying claims. As apparent from the afore-going, the cathode active material according to the present invention can improve lifespan properties based on a specific elenient 10 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 1 A cathode active material for seconldary batteries comprising a compound having a transition metal layer containing lithium as at least one compound selected from the following Formula 1 : wherein M is an element stable for a six-coordination stmcture, ~vlrich is at least one selected from transition metals that belong to the first and second period elements; -4xy is satisfied in a case of 3x-y -- [claim 2 The cathode active material according to claim 1, wherein the content of P satisfies the condition of 0