1. A lithium secondary battery, including a positive electrode, a negative electrode, a separator and a nonaqueous electrolyte, wherein the positive electrode includes LiaNixMnyCozO2 (a+x+y+z=2, 0.9≤a≤1.1, 0≤x, 0≤y, 0≤z) as a positive electrode active material, and wherein the nonaqueous electrolyte includes (i) fluorinated cyclic carbonate expressed by Chemical Formula 1 below, (ii) propionate-based ester expressed by Chemical Formula 2 below, and (iii) non-halogenated carbonate, so that a mixture weight ratio (i : ii) thereof is 20:80 to 50:50: [Chemical Formula 1] wherein, in Chemical Formula 1, R1, R2, R3 and R4 are independently any one of F, H and methyl group, and at least one thereof is F, [Chemical Formula 2] wherein, in Chemical Formula 2, R5 is alkyl group having 1 to 5 carbons.

2. The lithium secondary battery according to claim 1, wherein the propionate-based ester expressed by Chemical Formula 2 is at least one of methyl propionate, ethyl propionate and propyl propionate.

3. The lithium secondary battery according to claim 1, wherein the content of LiaNixMnyCozO2 (a+x+y+z=2, 0.9≤a≤1.1, 0≤x, 0≤y, 0≤z) serving as a positive electrode active material is 50 wt% to 100 wt% in comparison to the 18 entire weight of the positive electrode active material.

4. The lithium secondary battery according to claim 1, wherein LiaNixMnyCozO2 (a+x+y+z=2, 0.9≤a≤1.1, 0≤x, 0≤y, 0≤z) serving as a positive electrode active material is LiaNixMnyCozO2 (a+x+y+z=2, 0.9≤a≤1.1, 0

1 FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENT RULES, 2003 COMPLETE SPECIFICATION (See Section 10 and Rule 13) Title of invention: NONAQUEOUS ELECTROLYTE LITHIUM SECONDARY BATTERY Applicant: LG CHEM, LTD. A company in Republic of Korea Having address as: 128, Yeoui-daero, Yeongdeungpo-gu, Seoul 07336, Republic of Korea The following specification particularly describes the invention and the manner in which it is to be performed. 2 TECHNICAL FIELD [001] The present application claims priority to Korean Patent Application No. 10-2014-0131949 filed on September 30, 2014 in the Republic of Korea, the disclosures of which are incorporated herein by reference. [002] The present disclosure relates to a nonaqueous electrolyte lithium secondary battery, and more particularly, to a lithium secondary battery using a high charging voltage and having improved stability. BACKGROUND ART [003] As the energy storage technique is extensively applied to cellular phones, camcorders, notebooks and electric vehicles, high energy density is demanded to the battery used as a power source of such an electric device. A lithium secondary battery is a battery capable of meeting such a requirement best and is being actively studied. [004] Lithium secondary batteries developed in the early 1990’s are made up of a negative electrode of a carbon-based material capable of intercalating and deintercalating lithium ions, a positive electrode made of lithium-containing oxide, and a nonaqueous electrolyte containing a proper amount of lithium salts dissolved in a mixed organic solvent. [005] The average discharge voltage of the lithium secondary battery is about 3.6 to 3.7 V, which is higher than those of alkali batteries, nickel-cadmium batteries or the like. For such a high operating voltage, an electrolytic composition electrochemically stable in a charge/discharge range of 0 to 4.2 V is required. For this, a mixed solvent where a cyclic carbonate compound such as ethylene carbonate and propylene carbonate and a linear carbonate compound such as dimethyl carbonate, ethylmethyl carbonate and diethyl carbonate are appropriately mixed is used as a solvent of the electrolyte. A solute of the electrolyte commonly uses a lithium salt such as LiPF6, LiBF4, LiClO4 or the like, which serves as a lithium ion source in a battery and thus enables the lithium battery to operate. [006] In particular, in recent years, due to the tendency for a light, thin, short and small design of an electronic product and the increase of power consumption, a lithium secondary 3 battery used as an energy source of such an electronic device has larger capacity. Large capacity of a lithium secondary battery may be obtained by using a high-capacity active material, increasing a capacity by raising a charging voltage or utilizing an inner space more efficiently. However, if a nickel-based ternary-system positive electrode material spotlighted in these days is used to raise a charging voltage, decomposition reaction may abruptly occur at the electrolyte, which may deteriorate battery performance such as life cycle. This phenomenon may become worse depending on service environment, and if the service temperature increases, the deterioration of battery performance becomes rapidly increasing. In addition, an electrode density tends to gradually increase in line with the tendency of maximizing utilization of a battery inner space, and this demands an electrolyte with low viscosity and high ion conductivity. However, an electrolyte having such characteristics generally has bad stability against oxidation reaction. DISCLOSURE Technical Problem [007] The present disclosure is designed to solve the problems of the related art, and therefore the present disclosure is directed to providing a lithium secondary battery, which includes a nickel-based ternary-system positive electrode material and uses a high charging voltage to ensure improved stability. Technical Solution [008] In one aspect of the present disclosure, there is provided a lithium secondary battery, including a positive electrode, a negative electrode, a separator and a nonaqueous electrolyte, wherein the positive electrode includes LiaNixMnyCozO2 (a+x+y+z=2, 0.9≤a≤1.1, 0≤x, 0≤y, 0≤z) as a positive electrode active material, and wherein the nonaqueous electrolyte includes (i) fluorinated cyclic carbonate expressed by Chemical Formula 1 below, (ii) propionate-based ester expressed by Chemical Formula 2 below, and (iii) non-halogenated carbonate, so that a mixture weight ratio (i : ii) thereof is 20:80 to 50:50: 4 [009] [Chemical Formula 1] wherein, in Chemical Formula 1, R1, R2, R3 and R4 are independently any one of F, H and methyl group, and at least one thereof is F, [0010] [Chemical Formula 2] wherein, in Chemical Formula 2, R5 is alkyl group having 1 to 5 carbons. [0011] The propionate-based ester expressed by Chemical Formula 2 may employ methyl propionate, ethyl propionate or propyl propionate, without being specially limited thereto. [0012] In addition, the content of LiaNixMnyCozO2 (a+x+y+z=2, 0.9≤a≤1.1, 0≤x, 0≤y, 0≤z) serving as a positive electrode active material may be 50 to 100 wt% in comparison to the entire weight of the positive electrode active material. [0013] Further, LiaNixMnyCozO2 (a+x+y+z=2, 0.9≤a≤1.1, 0≤x, 0≤y, 0≤z) serving as a positive electrode active material may be LiaNixMnyCozO2 (a+x+y+z=2, 0.9≤a≤1.1, 00.85), the capacity of LiMnO2 per volume is small and thus has no competitiveness in comparison to LiCoO2. In addition, in LiMnO2, the entire mol fraction of Ni including Ni2+ and Ni3+ may be 0.4 to 0.7. If the content of Ni is smaller than 0.4, high capacity is hardly expected, and if the content of Ni is greater than 0.7, stability may be greatly deteriorated. The mol fraction of manganese is 0.05 to 0.4, and the mol fraction of cobalt is 0.1 to 0.4. [0035] In addition, if the mol fraction of Ni2+ with respect to the content of Ni is too high, an amount of mixed cations increases, thereby locally forming a rock-salt structure with no electrochemical reaction, which may disturb charging and discharging and accordingly reduce discharge capacity. Meanwhile, if the mol fraction of Ni2+ is too low, structural instability increases, which may deteriorate cycle stability. Therefore, the mol fraction of Ni2+ with respect to the entire amount of Ni may be 0.05 to 0.4. [0036] In addition, the positive electrode according to an embodiment of the present disclosure may further include any one selected from the group consisting of LiMn2-zNizO4 (0 [0054] A 1M LiPF6 solution having a composition where fluoroethylene carbonate (FEC) : propylene carbonate (PC) : methyl propionate (MP) is 30:10:60 (weight ratio) was prepared as an electrolyte. [0055] Li (NiaCobMnc)O2 (a=0.8, b=0.1 c=0.1) serving as a positive electrode active material, polyvinylidene fluoride (PVdF) serving as a binder, and carbon serving as a conductive material were mixed at a weight ratio of 93:4:4, and then dispersed in N-methyl-2-pyrrolidone to prepare positive electrode slurry. The slurry was coated to an aluminum current collector with a loading amount of 0.02 g/cm2, then dried and roll-pressed to prepare a positive electrode. [0056] In addition, natural graphite serving as a negative electrode active material, styrene-butadiene rubber serving as a binder, and carboxymethyl cellulose serving as a viscosity agent were mixed at a 96:2:2 of weight ratio, and then dispersed in water to prepare negative electrode slurry. The slurry was coated to a copper current collector, then dried and roll-pressed to prepare a negative electrode. [0057] After that, a coin-type battery was prepared in a common way by using the prepared positive electrode and the prepared negative electrode together with a PE separator, and then the prepared electrolyte was injected thereto to completely manufacture a battery. Example 2 [0058] A battery was manufactured in the same way as Example 1, except that ethyl propionate (EP) was used instead of methyl propionate (MP), when the nonaqueous 14 electrolyte was prepared. Example 3 [0059] A battery was manufactured in the same way as Example 1, except that a composition where fluoroethylene carbonate (FEC) : methyl propionate (MP) : dimethyl carbonate (DMC) was 20:70:10 (weight ratio) was used, when the nonaqueous electrolyte was prepared. Example 4 [0060] A 1M LiPF6 solution having a composition where fluoroethylene carbonate (FEC) : ethylene carbonate (EC) : methyl propionate (MP) = 14.5:19.2:66.3 (weight ratio) was prepared as an electrolyte. [0061] Li (NiaCobMnc)O2 (a=0.8, b=0.1 c=0.1) serving as a positive electrode active material, polyvinylidene fluoride (PVdF) serving as a binder, and carbon serving as a conductive material were mixed at a weight ratio of 93:4:4, and then dispersed in N-methyl-2-pyrrolidone to prepare positive electrode slurry. The slurry was coated to an aluminum current collector with a loading amount of 0.02 g/cm2, then dried and roll-pressed to prepare a positive electrode. [0062] In addition, natural graphite serving as a negative electrode active material, styrene-butadiene rubber serving as a binder, and carboxymethyl cellulose serving as a viscosity agent were mixed at a 96:2:2 of weight ratio, and then dispersed in water to prepare negative electrode slurry. The slurry was coated to a copper current collector, then dried and roll-pressed to prepare a negative electrode. [0063] After that, a cylindrical battery was prepared in a common way by using the prepared positive electrode and the prepared negative electrode together with a PE separator, and then the prepared electrolyte was injected thereto to completely manufacture a battery. Comparative Example 1 [0064] A battery was manufactured in the same way as Example 1, except that a composition 15 where ethylene carbonate (EC) : propylene carbonate (PC) : methyl propionate (MP) = 30:10:60 (weight ratio) was used, when the nonaqueous electrolyte was prepared. Comparative Example 2 [0065] A battery was manufactured in the same way as Example 1, except that a composition where fluoroethylene carbonate (FEC) : propylene carbonate (PC) : methyl butyrate (MB) = 30:10:60 (weight ratio) was used, when the nonaqueous electrolyte was prepared. Comparative Example 3 [0066] A battery was manufactured in the same way as Example 1, except that a composition where fluoroethylene carbonate (FEC) : methyl propionate (MP) : dimethyl carbonate (DMC) = 9 : 81 :10 (weight ratio) was used, when the nonaqueous electrolyte was prepared. Comparative Example 4 [0067] A battery was manufactured in the same way as Example 1, except that a composition where fluoroethylene carbonate (FEC) : methyl propionate (MP) : dimethyl carbonate (DMC) = 54:36:10 (weight ratio) was used, when the nonaqueous electrolyte was prepared. Comparative Example 5 [0068] A battery was manufactured in the same way as Example 1, except that a composition where fluoroethylene carbonate (FEC) : ethylene carbonate (EC) : methyl propionate (MP)= 14.5:19.2:66.3 (weight ratio) was used, when the nonaqueous electrolyte was prepared. Experimental Example 1: Evaluation of life performance [0069] The batteries (with a battery capacity of 4.3 mAh) prepared according to Examples 1 to 3 and Comparative Examples 1 to 4 were charged at 60C with a constant current of 0.7C up to 4.3V, and then charged with a constant voltage of 4.3V. The charging was terminated if the charging current becomes 0.215 mA. After that, the batteries were left along for 10 minutes and discharged with a constant current of 0.5C down to 3.0V. A discharge capacity was measured while repeating the charging/discharging processes. The measurement results are shown in Fig. 1. [0070] In addition, for the batteries (with a battery capacity of 4.3 mAh) prepared according to Example 4 and Comparative Example 5, a discharge capacity was measured in the same 16 way, The measurement results are shown in Fig. 2. Experimental Example 2: Evaluation of high-temperature storage performance [0071] The batteries (with a battery capacity of 4.3 mAh) prepared according to Examples 1 to 3 and Comparative Examples 1 to 4 were charged at 60C with a constant current of 0.7C up to 4.3V, and then charged with a constant voltage of 4.3V. The charging was terminated if the charging current becomes 0.215 mA. After that, the batteries were left along at 60C for 1 week. After that, the batteries were discharged with a constant current of 0.5C down to 3.0V, and a residual capacity was checked. In addition, the batteries were charged in the same way, and then left alone for 10 minutes and discharged, to measure a recovery capacity. This is expressed as a percentage in comparison to the initial capacity. The results are shown in Table 1 below. Table 1 Residual capacity (%) Recovery capacity (%) Example 1 82.4 93.3 Example 2 80.5 90.3 Example 3 81.1 92.1 Comparative Example 1 65.6 70.3 Comparative Example 2 67.4 73.5 Comparative Example 3 55.3 66.1 Comparative Example 4 34.8 50.6 17 WE CLAIM: 1. A lithium secondary battery, including a positive electrode, a negative electrode, a separator and a nonaqueous electrolyte, wherein the positive electrode includes LiaNixMnyCozO2 (a+x+y+z=2, 0.9≤a≤1.1, 0≤x, 0≤y, 0≤z) as a positive electrode active material, and wherein the nonaqueous electrolyte includes (i) fluorinated cyclic carbonate expressed by Chemical Formula 1 below, (ii) propionate-based ester expressed by Chemical Formula 2 below, and (iii) non-halogenated carbonate, so that a mixture weight ratio (i : ii) thereof is 20:80 to 50:50: [Chemical Formula 1] wherein, in Chemical Formula 1, R1, R2, R3 and R4 are independently any one of F, H and methyl group, and at least one thereof is F, [Chemical Formula 2] wherein, in Chemical Formula 2, R5 is alkyl group having 1 to 5 carbons. 2. The lithium secondary battery according to claim 1, wherein the propionate-based ester expressed by Chemical Formula 2 is at least one of methyl propionate, ethyl propionate and propyl propionate. 3. The lithium secondary battery according to claim 1, wherein the content of LiaNixMnyCozO2 (a+x+y+z=2, 0.9≤a≤1.1, 0≤x, 0≤y, 0≤z) serving as a positive electrode active material is 50 wt% to 100 wt% in comparison to the 18 entire weight of the positive electrode active material. 4. The lithium secondary battery according to claim 1, wherein LiaNixMnyCozO2 (a+x+y+z=2, 0.9≤a≤1.1, 0≤x, 0≤y, 0≤z) serving as a positive electrode active material is LiaNixMnyCozO2 (a+x+y+z=2, 0.9≤a≤1.1, 0