The present invention relates to a cathode active material for a lithium secondary battery. 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 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 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 secondary 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, HEVs and the like, 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, Japanese Patent Laid-open Publication Nos. 2002-110253 and 2004-134245 disclose techniques utilizing a mixture of lithium/manganese composite oxide, and lithivim/nickel/cobalt/manganese composite oxide and/or lithium/nickel/cobalt/manganese composite oxide to enhance recovery output and the like. These arts, however, still suffer from problems associated with a poor cycle life 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 manganese spinel oxide having a substitution of a manganese (Mn) site with a certain metal element and a lithium/nickel/cobalt/manganese composite oxide, whereby the battery can secure safety and, due to alleviation of disadvantages of the lithium/manganese oxide, 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 the above-mentioned cathode active material, upon fabrication of the cathode. 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 lithium 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: [Formula I] wherein, M is a metal having an oxidation number of 2 to 3; 0 < x < 0.2; and 0 (Table Removed)As can be seen from Table 1, in the composite oxide mixtures of the cathode active materials, substitution of a manganese (Mn) site of the lithium/manganese spinel oxide with aluminum (Al) or magnesium (Mg) has led to significant improvements in life characteristics of the battery. In addition, the higher substitution amounts (y-value) of metal ions have led to further improvements in life characteristics. However, as will be seen in Comparative Examples 2 and 3 hereinafter, it was confirmed that when the substitution amount, i.e., the y value, exceeds 0.2, the initial capacity of the battery is decreased. [Comparative Example 2] A lithium secondary battery was assembled in the same manner as in Example 1, except that a cathode active material was prepared using a substituted lithium/manganese spinel oxide of LiuJMnnAlojCU. The thus-fabricated lithium secondary battery was subjected to charge/discharge cycling in a voltage range of 3.0 to 4.2 V, and the initial capacity of the battery was measured and compared with the secondary battery of Example 1. The results have confirmed 14% decrease of battery capacity, relative to the initial capacity of the secondary battery of Example 1. [Comparative Example 3] A lithium secondary battery was assembled in the same manner as in Example 1, except that a cathode active material was prepared using a lithium/manganese spinel oxide of Lii+xMnuMgojO/j. The thus-fabricated lithium secondary battery was subjected to charge/discharge cycling in a voltage range of 3.0 to 4.2 V, and the initial capacity of the battery was measured and compared with the secondary battery of Example 1. The results have confirmed 24% decrease of battery capacity, relative to the initial capacity of the secondary battery of Example 1. [Example 7] A lithium secondary battery was assembled in the same manner as in Example 1, except that a cathode active material was prepared using a substituted lithium/manganese spinel oxide of Lii+xMni.9Alo.iO4 and a lithium/nickel/cobalt/manganese composite oxide of Lii+zNii/sCoi/sMni/sOa in a weight ratio of 90 : 10. The thus-fabricated lithium secondary battery was subjected to charge/discharge cycling in a voltage range of 3.0 to 4.2 V and life characteristics of the battery were measured. The results are given in Table 2 below. [Example 8] A lithium secondary battery was assembled in the same manner as in Example 1, except that a cathode active material was prepared using a substituted lithium/manganese spinel oxide of Lii+xMni.pAlo.iCU and a lithium/nickel/cobalt/manganese composite oxide of Lii+zNii/sCoi/aMni/sOa in a weight ratio of 70 : 30. The thus-fabricated lithium secondary battery was subjected to charge/discharge cycling in a voltage range of 3.0 to 4.2 V and life characteristics of the battery were measured. The results are given in Table 2 below. [Example 9] A lithium secondary battery was assembled in the same manner as in Example 1, except that a cathode active material was prepared using a substituted lithium/manganese spinel oxide of Lii+xMni.9Alo.iO4 and a lithium/nickel/cobalt/manganese composite oxide of Lii+zNii/aCoi/gMni/aOa in a weight ratio of 50 : 50 (1 : 1). The thus-fabricated lithium secondary battery was subjected to charge/discharge cycling in a voltage range of 3.0 to 4.2 V and life characteristics of the battery were measured. The results are given in Table 2 below. [Example 10] A lithium secondary battery was assembled in the same manner as in Example 1, except that a cathode active material was prepared using a substituted lithium/manganese spinel oxide of Lii+xMni.9Alo.iO4 and a lithium/nickel/cobalt/manganese composite oxide of Lii+zNii/sCoi/gMni/aOa in a weight ratio of 30 : 70. The thus-fabricated lithium secondary battery was subjected to charge/discharge cycling in a voltage range of 3.0 to 4.2 V and life characteristics of the battery were measured. The results are given in Table 2 below. [Example 11] A lithium secondary battery was assembled in the same manner as in Example 1, except that a cathode active material was prepared using a substituted lithium/manganese spinel oxide of Lii+xMni.9Alo.i04 and a lithium/nickel/cobalt/manganese composite oxide of Lii+zNii/sCoi/sMni/sCh in a weight ratio of 10 : 90. The thus-fabricated lithium secondary battery was subjected to charge/discharge cycling in a voltage range of 3.0 to 4.2 V and life characteristics of the battery were measured. The results are given in Table 2 below. [Comparative Example 4] A lithium secondary battery was assembled in the same manner as in Example 1, except that a cathode active material was prepared using only a substituted lithium/manganese spinel oxide of Lii+xMni.9Alo.iO4. The thus-fabricated lithium secondary battery was subjected to charge/discharge cycling in a voltage range of 3.0 to 4.2 V and life characteristics of the battery were measured. The results are given in Table 2 below. [Comparative Example 5] A lithium secondary battery was assembled in the same manner as in Example 1, except that a cathode active material was prepared using only a lithium/nickel/cobalt/manganese composite oxide of LiuJsfii/sCot/sMni/sOa. The thus-fabricated lithium secondary battery was subjected to charge/discharge cycling in a voltage range of 3.0 to 4.2 V and life characteristics of the battery were measured. The results are given in Table 2 below. (Table Removed) As can be seen from Table 2, life characteristics of the battery began to improve when more than 10% lithium/nickel/cobalt/manganese composite oxide was added to the Al-substituted lithium/manganese spinel oxide, and it could be confirmed that the thus-obtained life characteristics are similar to life characteristics achieved upon addition of more than 30% lithium/nickel/cobalt/manganese composite oxide. However, an excessively high content of the lithium/nickel/cobalt/manganese composite oxide may result in relatively low safety of the battery and therefore it is preferred to use the lithium/nickel/cobalt/manganese composite oxide in an amount of less than 90%. [Example 12] A lithium secondary battery was assembled in the same manner as in Example 1, except that a cathode active material was prepared using a substituted lithium/manganese spinel oxide of Lii+xMni.9Alo.iO4 and a lithium/nickel/cobalt/manganese composite oxide of Lii+zNio.4Mno.4Co0.2O2 in a weight ratio of 90 : 10. The thus-fabricated lithium secondary battery was subjected to charge/discharge cycling in a voltage range of 3.0 to 4.2 V and life characteristics of the battery were measured. The results are given in Table 3 below. [Example 13] A lithium secondary battery was assembled in the same manner as in Example 1, except that a cathode active material was prepared using a substituted lithium/manganese spinel oxide of Lin-xMni.9Alo.iO4 and a lithium/nickel/cobalt/manganese composite oxide of Lin-zNio.4Mno.4Coo.2O2 in a weight ratio of 70 : 30. The thus-fabricated lithium secondary battery was subjected to charge/discharge cycling in a voltage range of 3.0 to 4.2 V and life characteristics of the battery were measured. The results are given in Table 3 below. [Example 14] A lithium secondary battery was assembled in the same manner as in Example 1, except that a cathode active material was prepared using a substituted lithium/manganese spinel oxide of Lin-xMnisAlo.iCU and a lithium/nickel/cobalt/manganese composite oxide of Lii+zNio.4Mno.4Coo.2O2 in a weight ratio of 50 : 50 (1 : 1). The thus-fabricated lithium secondary battery was subjected to charge/discharge cycling in a voltage range of 3.0 to 4.2 V and life characteristics of the battery were measured. The results are given in Table 3 below. [Example 15] A lithium secondary battery was assembled in the same manner as in Example 1, except that a cathode active material was prepared using a substituted lithium/manganese spinel oxide of Lii+xMni.9Alo.iO4 and a lithium/nickel/cobalt/manganese composite oxide of Lii+zNio.4Mno.4Coo.202 in a weight ratio of 30 : 70. The thus-fabricated lithium secondary battery was subjected to charge/discharge cycling in a voltage range of 3.0 to 4.2 V and life characteristics of the battery were measured. The results are given in Table 3 below. [Example 16] A lithium secondary battery was assembled in the same manner as in Example 1, except that a cathode active material was prepared using a substituted lithium/manganese spinel oxide of Lii4.xMni.9Alo.iO4 and a lithium/nickel/cobalt/manganese composite oxide of Lii+zNio^Mno^Coo^Oa in a weight ratio of 10 : 90. The thus-fabricated lithium secondary battery was subjected to charge/discharge cycling in a voltage range of 3.0 to 4.2 V and life characteristics of the battery were measured. The results are given in Table 3 below. [Comparative Example 6] A lithium secondary battery was assembled in the same manner as in Example 1, except that a cathode active material was prepared using only a lithium/nickel/cobalt/manganese composite oxide of Lii+zNio.4Mno.4Coo,2O2. The thus-fabricated lithium secondary battery was subjected to charge/discharge cycling in a voltage range of 3.0 to 4.2 V and life characteristics of the battery were measured. The results are given in Table 3 below, (Table Removed)As can be seen from Table 3, life characteristics of the battery began to improve when more than 10% lithium/nickel/cobalt/manganese composite oxide was added to the Al-substituted lithium/manganese spinel, and it could be confirmed that the thus-obtained life characteristics are similar to life characteristics achieved -upon addition of more than 30% lithium/nickel/cobalt/manganese composite oxide. However, an excessively high content of the lithium/nickel/cobalt/manganese composite oxide may result in relatively low safety of the battery and therefore it is preferred to use the lithium/nickel/cobalt/manganese composite oxide in an amount of less than 90%. INDUSTRIAL APPLICABILITY As apparent from the above description, a lithium secondary battery using a mixture of a manganese spinel oxide having a substitution of a manganese (Mn) site with a certain metal element and a certain lithium/nickel/cobalt/manganese composite oxide, according to the present invention, as a cathode active material, can secure safety of the battery and improve a service life thereof, even under high current, short period charge/discharge cycle conditions. Although the preferred embodiments of the present invention have been 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. We Claim: 1. A cathode active material for a lithium secondary battery, comprising a mixture of a lithium/manganese spinel oxide represented by Formula I and a lithium/nickel/cobalt/manganese oxide represented by Formula II wherein the mixing ratio of the lithium/manganese spinel oxide:lithium/nickel/cobalt/manganese oxide is in the range of 10:90 to 90:10 (w/w): (Formula Removed) wherein, M is Al, Mg or both of them; 0≤x≤0.2; 0 < y ≤ 0.2; 0≤z≤0.1; 0.2≤b≤0.7; 0.2 ≤ c ≤ 0.7; and b+c

Documents

Application Documents

#	Name	Date
1	1108-DELNP-2008-PCT-308.pdf	2011-08-21
1	251795-PATENT CERTIFICATE-030412.pdf	2024-07-31
2	1108-DELNP-2008-PCT-304.pdf	2011-08-21
2	1108-DELNP-2008-RELEVANT DOCUMENTS [23-08-2023(online)].pdf	2023-08-23
3	1108-delnp-2008-pct-301.pdf	2011-08-21
3	1108-DELNP-2008-ASSIGNMENT WITH VERIFIED COPY [24-11-2022(online)].pdf	2022-11-24
4	1108-delnp-2008-pct-237.pdf	2011-08-21
4	1108-DELNP-2008-FORM-16 [24-11-2022(online)].pdf	2022-11-24
5	1108-DELNP-2008-POWER OF AUTHORITY [24-11-2022(online)].pdf	2022-11-24
5	1108-delnp-2008-pct-210.pdf	2011-08-21
6	1108-DELNP-2008-RELEVANT DOCUMENTS [15-09-2022(online)].pdf	2022-09-15
6	1108-delnp-2008-pct-101.pdf	2011-08-21
7	1108-DELNP-2008-RELEVANT DOCUMENTS [29-09-2021(online)].pdf	2021-09-29
7	1108-delnp-2008-gpa.pdf	2011-08-21
8	1108-DELNP-2008-RELEVANT DOCUMENTS [27-09-2021(online)].pdf	2021-09-27
8	1108-delnp-2008-form-5.pdf	2011-08-21
9	1108-delnp-2008-form-3.pdf	2011-08-21
9	1108-DELNP-2008-RELEVANT DOCUMENTS [21-02-2020(online)].pdf	2020-02-21
10	1108-delnp-2008-form-2.pdf	2011-08-21
10	1108-DELNP-2008-RELEVANT DOCUMENTS [28-03-2019(online)].pdf	2019-03-28
11	1108-delnp-2008-form-1.pdf	2011-08-21
11	1108-DELNP-2008-RELEVANT DOCUMENTS [31-03-2018(online)].pdf	2018-03-31
12	1108-delnp-2008-description (complete).pdf	2011-08-21
12	1108-DELNP-2008-FORM 4 [09-08-2017(online)].pdf	2017-08-09
13	1108-delnp-2008-correspondence-others.pdf	2011-08-21
13	Form 27 [31-03-2017(online)].pdf	2017-03-31
14	1108-delnp-2008-claims.pdf	2011-08-21
14	1108-DELNP-2008_EXAMREPORT.pdf	2016-06-30
15	1108-delnp-2008-Abstract.pdf	2012-04-04
15	1108-DELNP-2008-Petition-137-(25-08-2011).pdf	2011-08-25
16	1108-DELNP-2008-Claims-(25-08-2011).pdf	2011-08-25
16	1108-DELNP-2008-GPA-(25-08-2011).pdf	2011-08-25
17	1108-DELNP-2008-Form-3-(25-08-2011).pdf	2011-08-25
17	1108-DELNP-2008-Correspondence Others-(25-08-2011).pdf	2011-08-25
18	1108-DELNP-2008-Description (Complete)-(25-08-2011).pdf	2011-08-25
18	1108-DELNP-2008-Form-2-(25-08-2011).pdf	2011-08-25
19	1108-DELNP-2008-Form-1-(25-08-2011).pdf	2011-08-25
20	1108-DELNP-2008-Description (Complete)-(25-08-2011).pdf	2011-08-25
20	1108-DELNP-2008-Form-2-(25-08-2011).pdf	2011-08-25
21	1108-DELNP-2008-Correspondence Others-(25-08-2011).pdf	2011-08-25
21	1108-DELNP-2008-Form-3-(25-08-2011).pdf	2011-08-25
22	1108-DELNP-2008-Claims-(25-08-2011).pdf	2011-08-25
22	1108-DELNP-2008-GPA-(25-08-2011).pdf	2011-08-25
23	1108-delnp-2008-Abstract.pdf	2012-04-04
23	1108-DELNP-2008-Petition-137-(25-08-2011).pdf	2011-08-25
24	1108-DELNP-2008_EXAMREPORT.pdf	2016-06-30
24	1108-delnp-2008-claims.pdf	2011-08-21
25	Form 27 [31-03-2017(online)].pdf	2017-03-31
25	1108-delnp-2008-correspondence-others.pdf	2011-08-21
26	1108-delnp-2008-description (complete).pdf	2011-08-21
26	1108-DELNP-2008-FORM 4 [09-08-2017(online)].pdf	2017-08-09
27	1108-delnp-2008-form-1.pdf	2011-08-21
27	1108-DELNP-2008-RELEVANT DOCUMENTS [31-03-2018(online)].pdf	2018-03-31
28	1108-delnp-2008-form-2.pdf	2011-08-21
28	1108-DELNP-2008-RELEVANT DOCUMENTS [28-03-2019(online)].pdf	2019-03-28
29	1108-delnp-2008-form-3.pdf	2011-08-21
29	1108-DELNP-2008-RELEVANT DOCUMENTS [21-02-2020(online)].pdf	2020-02-21
30	1108-delnp-2008-form-5.pdf	2011-08-21
30	1108-DELNP-2008-RELEVANT DOCUMENTS [27-09-2021(online)].pdf	2021-09-27
31	1108-DELNP-2008-RELEVANT DOCUMENTS [29-09-2021(online)].pdf	2021-09-29
31	1108-delnp-2008-gpa.pdf	2011-08-21
32	1108-DELNP-2008-RELEVANT DOCUMENTS [15-09-2022(online)].pdf	2022-09-15
32	1108-delnp-2008-pct-101.pdf	2011-08-21
33	1108-DELNP-2008-POWER OF AUTHORITY [24-11-2022(online)].pdf	2022-11-24
33	1108-delnp-2008-pct-210.pdf	2011-08-21
34	1108-delnp-2008-pct-237.pdf	2011-08-21
34	1108-DELNP-2008-FORM-16 [24-11-2022(online)].pdf	2022-11-24
35	1108-delnp-2008-pct-301.pdf	2011-08-21
35	1108-DELNP-2008-ASSIGNMENT WITH VERIFIED COPY [24-11-2022(online)].pdf	2022-11-24
36	1108-DELNP-2008-RELEVANT DOCUMENTS [23-08-2023(online)].pdf	2023-08-23
36	1108-DELNP-2008-PCT-304.pdf	2011-08-21
37	1108-DELNP-2008-PCT-308.pdf	2011-08-21
37	251795-PATENT CERTIFICATE-030412.pdf	2024-07-31