LITHIUM MANGANESE-BASED OXIDE AND CATHODE ACTIVE MATERIAL INCLUDING THE SAME [TECHNICAL FIELD] 5 The present invention relates to a litliium manganese-based oxide and a cathode active material including the same and, more particularly, to a lithium manganese (Mn)-based oxide including Mn as an essential transition metal and having a layered crystal structure, in which tlie amount of Mn is greater tlia~i that of other transition ~netal(s), the lithium manganese-based oxide exhibits flat level section 10 characteristics in which release of oxygen occurs together with lithiutn deintercalation during first charging in a high voltage range of 4.4 V or higher, and a transition metal layer including Mn atidlor an oxygen layer are substituted or doped with a pillar element. [BACKGROUND ART) 15 As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, litliium secondary batteries, \vIiich exhibit high energy density and operating potential, have long cycle lifespan, and have a low self-discharge rate, are conlmercially available and widely used. In addition, as interest in enviro~unental problenls is increasing, research into electric vehicles and hybrid electric vehicles that can replace vehicles using fossil fiiels, 5 such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is actively underway. As a power source of electric vehicles, hybrid electric vehicles, and the like, a nickel nletal-hydride secondary battery is nlainly used. However, research into litllium secondary batteries having high energy density and high discharge voltage is actively underway and some lithium secondary batteries are 10 conlmercially available. In a lithium ion secondary battery used in conventional small batteries, generally, a cathode is formed of a lithium cobalt composite oxide having a layered st~ucture and an anode is formed of a graphite-based material. I-Iowever, a lithium cobalt co~nposite oxide is unsuitable for use in batteries for electric vehicles because 15 Co, which is a main component of the lithium cobalt composite oxide, is very expensive and the lithiurn cobalt composite oxide is not safe. Thus, litttiu~n manganese composite oxides, which are inexpensive, have high safety, consist of Mn, and have a spinel structure, may be suitable for use in a cathode of lithiunl ion batteries for electric vehicles. In general, spinel-structure lithium manganese-based oxides have high thennal safety, are itlexpensive, and are easy to synthesize, wllilc having low capacity, deteriorated lifespan characteristics due to side reaction, poor high-temperature characteristics, and low conductivity. 5 To address these problems, use of lithiutn manganese conlposite oxides having a spinel structure, some ~netale leme~~otsf which are substituted, has been tried. For example, Korean Patent Application Publication No. 2002-65191 discloses a spinelstructured lithium manganese composite oxide with high thermal safety. However, a battery including the lithium manganese composite oxide exhibits low capacity and 10 deteriorated high-temperature storage characteristics and cycle lifespan. To complenlent the low capacity problems of spinel and secure excellent thermal stability of manganese-based active materials, use of lithium manganese composite oxides having a layered structure has been tried. However, such lithium manganese composite oxides have an unstable st~ucture, undergo phase transition 15 during charge and discharge, and exhibit rapidly reduced capacity and deteriorated lifespan characteristics. In addition, when such lithium manganese composite oxides are stored at high temperature, Mn is eluted to an electrolyte by the impact of the electrolyte and thus battery characteristics are deteriorated and therefore there is a need to develop imnprovements to address these problems. In addition, such lithium manganese cotnposite oxides have lower capacity per unit weight than existing lithiunl cobalt conlposite oxides or co~lventional lithium nickel composite oxides and thus there are limitations in increasing capacity per unit battery weight. Thus, batteries that address 5 these problems need to be designed to enable practical use thereof as a power source of electric vehicles. Tllerefore, there is an urgent need to develop techtiology for enhancing stmch~rals tability of a cathode active material at higll voltage without deterioration of battery characteristics. 10 [DISCLOSURE] [TECHNICAL PROBLEM] Therefore, the present invention has been made to solve the above problems and other techtkal proble~nsth at have yet to be resolved. That is, it is an object of the present invention to provide a lithium manganese 15 (Mn)-based oxide it1 which a transition metal layer, an oxygen layel; or the like is substituted or doped with a pillar element and thus probletns due to collapse of a layered structure, sucl~ as elution of Mn to a surface of the cathode active material during battery charge and discharge and the like, are addressed, whereby safety and lifespan characteristics of a batte~yin cluding the cathode active material nlap be enhanced. -4- [TECHNICAL SOLUTION] In accordance with one aspect of the present invention, provided is a lithium nianganese (Mn)-based oxide as a cathode active niaterial that includes Mn as an essential transition metal and has a layered crystal structure, in which the ariiount of Mn 5 is greater than that of other transition metal(s), the lithium manganese-based oxide exhibits flat level section characteristics in which release of oxygen occurs together with lithium deintercalation during first charging it1 a high voltage range of 4.4 V or l~iglier, and a tra~isition metal layer including Mn and/or an oxygen layer are substituted or doped with a pillar element. 10 In general, when lithium manganese-based oxides including Mn in an amount of 50 mol% based on a total amount of transition metals are used, high capacity may be obtained only wit11 charging to at least 4.4 V. In particular, such litliiutn manganesebased oxides have flat level section characteristics it1 w11ich release of oxygen occurs together wit11 lithium deititercalation during first charging in a high voltage range of 4.4 15 V or higher and thus undergo phase transition during charging and discharging due to unstable structure thereof and exhibit rapidly reduced capacity and deteriorated lifespan characteristics. By contrast, in the litl~iunm~a nganese-based oxide according to the present invention, interlayer interaction of the cathode active material occurs by the substitt~ted or doped pillar elenietlt present in the transition metal layer, the oxygen layer, or the like and thus structural collapse thereof during charging and discharging is prevented, which enables resolutioti of probletns such as deterioration of battery characteristics due to elntiotl of Mn to an electrolyte by the impact of the electrolyte when existing cathode 5 active materials are stored at high temperature. In a specific embodiment, the lithium manganese-based oxide may be a compound represented by Formula 1 below: wherein O 10 A cathode and a coin cell were mailufactured in the same manner as in Example 2, except that tl~ep illar material was not added in the process of prepasiug the lithium matlganese eonlposite oxide of Example 2. A cathode and a coil] cell were n~a~~ufacturiend t he same lnatuler as in 15 Exanlple 3, except that the pillar material was not added in the process of preparing the lithium manganese conlposite oxide of Exanlple 3. Discharge capacities of the cells manufactured according to Exa~nples 1 and 2 and Comparative Example 1 were measured by performing an initial cycle under the following conditions: voltage of 2.5 V to 4.65 V and current of 0.1 C rate and results are shown in Table 1 below and FIG. 1. 5 (Table l> 1'' discharge capacity An experiment for lifespan characteristics of the cells of Examples 1 and 2 and Cotnparative Example 1 was impletilented under the following conditions: voltage of 3.0 V to 4.4. V and current of 0.5 C rate. In this regard, the lifespan characteristics 10 were evaluated as retention rate with respect to initial capacity after 30 cycles and results are shown in Table 2 below and FIG. 2. Exa~nple1 198 mAb/g (Table 2> Example 2 204 mAh/g Comparative Example 1 213 mAldg Lifespan characteristics Example 1 101% Example 2 93% Colnparative Example 1 91% Discharge capacities of the cells of Exan~ple 3 and Cotuparative Exanlple 2 were measured by performing an initial cycle under the following conditions: voltage of 3.0 V to 4.4 V and current of 0.1 C rate. Results are shown in Table 3 below and FIG. 4. An experiment for lifespan characteristics of the cells of Exanlple 3 and 1 "discharge capacity Conlparative Example 2 was implenlented under the following conditions: voltage of 3.0 V to 4.4. V and current of 0.5 C rate. In this regard, the lifespan characteristics Exan~ple3 165 mAh/g 10 were evaluated as retention rate with respect to initial capacity after 30 cycles and Cotnparative Example 2 167 mAlv'g results are shown in Table 4 below and FIG. 4. confirtned that, although the cells of Examples 1 to 3 exhibit a reduction in initial Lifespan characteristics 15 capacity due to the pillar material added in the process of preparing the cathode active According to the results shown in Tables 1 to 4 and FIGS. 1 to 4, it can be Example 3 95% Coniparative Example 2 96% material, the cells of Examples 1 to 3 exhibit superior lifespan and capacity characteristics to the cells of Comparative Exatnples 1 atid 2. Although the preferred etnbodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various 5 modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. [CLAIMS] [Claim 11 A lithium manganese (Mn)-based oxide comprising Mn as an essential transition metal and having a layered crystal st~~~ctnre, \vl~ereina n a~nounot f Mn is greater than that of otlier transition metal(s), 5 the lithium manganese-based oxide exhibits flat level section characteristics in which release of oxygen occurs together with lithium deintercalation during first charging in a high voltage range of 4.4 V or higl~er,a nd at least one of a transition metal layer including Mn and an oxygen layer is substituted or doped with a pillar element. 10 [Claim 21 The litliit~mm anganese-based oxide according to claim 1, wherein the lithium manganese-based oxide is represented by Forn~ula1 below: wl~ereinO