TECHNICAL FIELD [0001] The present disclosure relates to a positive electrode active material which reduces lithium by-products and improves structural stability and includes a lithium-nickel based transition metal composite oxide in which an alkaline earth metal having oxidation number of +2 is doped, and a phosphate coated layer formed on the surface of the composite oxide, a positive electrode including the positive electrode active material, and a secondary battery including the positive electrode. BACKGROUND ART [0002] Technological developments and increased demands for mobile devices have led to a rapid increase in demands for secondary batteries as energy sources. Among various secondary batteries, lithium secondary batteries having high energy density, high voltage, long cycle life, and low self-discharge rate are commercially available and widely used. [0003] Moreover, as interest in environmental issues increases, there is growing interest in electric vehicles and hybrid electric vehicles being capable of replacing fossil fuel powered vehicles, such as gasoline vehicles and diesel vehicles, which use fossil fuel that is one of the main causes of air pollution. Accordingly, researches for using lithium secondary batteries as power sources of electric vehicles, hybrid electric vehicles, and the like are being actively conducted. [0004] In order to use lithium secondary batteries for electric vehicles, the lithium secondary batteries should have high energy density and characteristics of generating high power in a short time, and also withstand over 10 years under severe conditions. Therefore, significantly better stability than typical compact lithium secondary batteries and long-term life characteristics are necessarily required. [0005] A lithium secondary battery refers to a battery which includes an electrode assembly and a non-aqueous electrolyte containing lithium ions, wherein the electrode assembly includes a positive electrode including a positive electrode active material enabling intercalation and deintercalation of lithium ions, a negative electrode including a negative electrode active material enabling intercalation and deintercalation of lithium ions, and a microporous separator disposed between the positive electrode and the negative electrode. [0006] Examples of positive electrode active materials of lithium secondary batteries include transition metal oxides such as a lithium cobalt oxide (LiCoO2), a lithium manganese oxide (LiMn2O4), or a lithium nickel oxide (LiNiO2), and composite oxides in which transition metals contained in each aforesaid material are partially substituted with other transition metals. [0007] Among the positive electrode active materials, LiCoO2 is being widely used due to its excellent overall properties such as cycle characteristics. However, LiCoO2 has low stability and is costly due to resource limitations of cobalt as a raw material, thus disadvantageously having limited mass-utilization as power sources in the fields of electric vehicles or the like. [0008] Lithium manganese oxides such as LiMnO2 and LiMn2O4 advantageously are abundant resources and use environmentally friendly manganese, thus attracting much attention 3 as a positive electrode active material as an alternative to LiCoO2. However, these lithium manganese oxides have disadvantages such as low capacity and poor cycle characteristics. [0009] On the other hand, lithium nickel oxides such as LiNiO2 are not only cheaper than the cobalt oxides, but also higher in discharge capacity, when charged at 4.3V, wherein the reversible capacity of doped LiNiO2 approximates to about 200 mAh/g which is higher than the capacity of LiCoO2 (about 165 mAh/g). [0010] Accordingly, despite slightly low average discharge voltage and volumetric density, commercial batteries including LiNiO2 as a positive electrode active material exhibit improved energy density, and a great deal of research for developing high-capacity batteries using these nickel-based positive electrode active materials is thus being actively conducted. However, despite the advantage of high capacity, lithium nickel oxides have a limitation in practical use because lithium nickel oxides encounter several problems such as a rapid phase transition in a crystal structure caused by volumetric changes accompanying charge/discharge cycles, resultant particle fracture or pores at grain boundaries, generation of a large amount of gas during storage or cycles, and sharp decrease in surface chemical resistance when exposed to air and moisture. [0011] Accordingly, lithium transition metal oxides, in which nickel in the oxides is partially substituted with other transition metals such as manganese and cobalt, have been suggested. Such metal substituted nickel based lithium transition metal oxides advantageously have relatively excellent cycle characteristics and capacity characteristics; however, when used for a long time, cycle characteristics are drastically deteriorated, and problems such as swelling caused by gas generation in a battery and low chemical stability have not been sufficiently solved. Therefore, it is necessary to develop improved techniques to solve problems of high temperature stability while using a lithium nickel based positive electrode active material suitable for a high-capacity battery. [0012] Moreover, lithium nickel based positive electrode active materials basically generate a large amount of lithium by-products (Li2CO3 and LiOH) on the surface thereof. These lithium by-products form a resistive film, and react with a solvent (for example, PVDF) to cause gelation of slurry during the preparation of positive electrode active material slurry, and also generate gas in a battery to cause swelling, thereby significantly reducing life characteristics of the battery. [0013] Therefore, various attempts have been made to solve aforementioned problems through surface stabilization using surface treatment, doping or the like or improvement of structural stability, but efficient methods have not yet been developed. [0014] Based on the above-described background, while conducting research on a method for improving life characteristics of a battery by improving structural stability and by reducing lithium by-products and thus preventing swelling and resistive film formation caused by the by-products, the present inventors found that lithium by-products were significantly reduced on the surface of a positive electrode active material, and life characteristics of a battery including the positive electrode active material significantly increased, wherein the positive electrode active material is prepared in such a way that an alkaline earth metal having oxidation number of +2 was doped into a lithium-nickel based transition metal composite oxide and a phosphate coated 4 layer was formed on the surface of the composite oxide; and finally completed the present invention. DISCLOSURE OF THE INVENTION TECHNICAL PROBLEM [0015] An object of the present disclosure is to provide a positive electrode active material which reduces lithium by-products and improves structural stability and includes a lithium-nickel based transition metal composite oxide and a phosphate coated layer formed on the surface of the composite oxide. [0016] Another object of the present disclosure is to provide a method of preparing the positive electrode active material. [0017] Still another object of the present disclosure is to provide a positive electrode for a secondary battery, in which a current collector is coated with positive electrode active material slurry containing the positive electrode active material. [0018] Furthermore, even another object of the present disclosure is to provide a secondary battery having excellent life characteristics, the secondary battery including the positive electrode for a secondary battery, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte. TECHNICAL SOLUTION [0019] To solve the above-described problems, an aspect of the present disclosure provides a positive electrode active material including: a lithium-nickel based transition metal composite oxide having a layered structure, which is expressed by Chemical Formula 1 below; and a phosphate coated layer formed on the surface of the composite oxide. [0020] [Chemical Formula 1] [0021] LixNiaMbAwO2-yDy [0022] where, 1.0≤x≤1.2, 0.5≤a≤1, 0f; and M' is NiaMbAw, where M, A, a, b and w are the same as mentioned above. [0054] A phosphate precursor as a raw material of the phosphate may be at least one selected from the group consisting of (NH4)2HPO4, (NH4)2H2PO4, (NH4)3PO4·3H2O, H3PO4, and P2O5, and may be preferably (NH4)2HPO4. 8 [0055] Another aspect of the present disclosure provides a method of preparing the positive electrode active material having reduced lithium by-products and improved structural stability. [0056] The method of preparing the positive electrode active material according to an embodiment of the present disclosure includes: mixing an alkaline earth metal precursor having oxidation number of +2 into a mixed solution of a transition metal precursor and a lithium precursor, and then sintering the resultant solution to prepare the lithium-nickel based transition metal composite oxide expressed by Chemical Formula 1 above (step 1); and adding a phosphate precursor into the lithium-nickel based transition metal composite oxide, and then sintering the resultant mixture to form a phosphate coated layer on the outer surface of the composite oxide (step 2). [0057] Step 1 is a step for preparing a lithium-nickel based transition metal composite oxide in which the alkaline earth metal expressed by Chemical Formula 1 above is doped, although not particularly limited, the composite oxide may be prepared by methods generally known in the art, and may be prepared by, for example, a solid phase reaction method, a coprecipitation method, a sol-gel method, or a hydrothermal synthesis method. [0058] Specifically, the lithium-nickel based transition metal composite oxide may be prepared in such a way that a nickel precursor forming the nickel based transition metal composite oxide and a transition metal (excluding nickel) precursor are respectively dissolved in a solvent, and thereafter coprecipitated to prepare a transition metal composite hydroxide. A lithium precursor is added to the transition metal composite hydroxide to prepare a mixed solution, and thereafter an alkaline earth metal precursor having oxidation number of +2 is mixed and sintered. [0059] The transition metal composite hydroxide may be a compound expressed by Me(OH1-x)2 (0≤x≤0.5), where Me refers to a transition metal which is expressed by NiaMb in Chemical Formula 1 above. [0060] Furthermore, as described above, the nickel precursor, the transition metal (excluding nickel) precursor, and the alkaline earth metal precursor having oxidation number of +2 may be preferably used as being adjusted such that the content of nickel is 70 mol% or more based on the total amount of metallic components excluding lithium. [0061] The sintering in step 1 may be, but is not limited to, heat treatment for 20 to 30 hours at 700 to 900 ℃. [0062] The transition metal precursor and the lithium precursor may be, but is not particularly limited to, in the form of salts of each metal, such as nitrate, sulfate, carbonate, hydroxide, acetate, oxalate, and chloride. [0063] Furthermore, the alkaline earth metal precursor having oxidation number of +2 may be an alkaline earth metal salt, and specifically may be SrCO3. [0064] Step 2 is a step for preparing a positive electrode active material having reduced lithium by-products and improved structural stability, by forming a phosphate coated layer on the outer surface of the lithium-nickel based transition metal composite oxide prepared in step 1, in which an alkaline earth metal is doped, and may be performed by adding a phosphate precursor 9 into the lithium-nickel based transition metal composite oxide and then sintering the phosphate precursor added lithium-nickel based transition metal composite oxide. [0065] The sintering in step 2 may be heat treatment for 10 hours or less at 100 to 700 ℃, and specifically the heat treatment may be performed for a time in the range of 1 minute to 10 hours. [0066] The phosphate precursor may be the same as aforementioned precursors, or may be included therein. [0067] Still another aspect of the present disclosure provides a positive electrode for a secondary battery, in which a current collector is coated with positive electrode slurry containing the positive electrode active material. [0068] The positive electrode according to an embodiment of the present disclosure may be prepared by coating positive electrode active material slurry containing the positive electrode active material onto a positive electrode current collector, and then drying and rolling the slurry coated current collector. [0069] The positive electrode current collector generally may be used with a thickness of 3 to 500 ㎛. Although not particularly limited, any material which has a high conductivity without causing any chemical change in the battery, for example, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel which is surface-treated with carbon, nickel, titanium, silver, or the like may be used as the positive electrode current collector. [0070] The positive electrode active material slurry may be prepared by adding and mixing additives such as a binder, a conducting agent, a filler, and a dispersant into the positive electrode active material. [0071] The binder is a component for aiding a bond between the positive electrode active material and the conducting agent and a bond for the current collector, and generally may be added up to 1 to 30 wt% based on the total amount of the positive electrode active material. Although not particularly limited, a typical binder known in the art may be used as the binder. For example, the binder may be one or a mixture of two or more selected from the group consisting of vinylidenefluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride, polyacrylonitrile, polymethylmethacrylate, polyvinylalcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butylene rubber (SBR), and fluororubber. [0072] The conducting agent generally may be added up to 0.05 to 5 wt% based on the total amount of the positive electrode active material. Although not particularly limited, any material which has conductivity without causing side reactions with other elements in the battery may be used as the conducting agent. For example, graphite such as natural graphite or artificial graphite; carbon black such as carbon black (super-p), acetylene black, ketjenblack, channel black, furnace black, lamp black, or thermo-black; conductive fibers such as carbon fibers or metal fibers; metal powder such as fluorocarbon, aluminum, and nickel powder; conductive 10 whisker such as zinc oxide or potassium titanate; conductive metal oxide such as titanium oxide; conductive materials such as polyphenylene derivatives may be used as the conducting agent. [0073] The filler is a component for preventing swelling of the positive electrode, and whether or not to use the filler may be determined as needed. Although not particularly limited, any fibrous material which does not cause any chemical change in the battery, for example, olefin polymer such as polyethylene or polypropylene, and fibrous materials such as glass fibers or carbon fibers may be used as the filler. [0074] The dispersant (dispersion solution) may be, for example, but is not particularly limited to, isopropyl alcohol, N-methylpyrrolidone (NMP), or acetone. [0075] The coating may be performed by a method generally known in the art. For example, the coating may be performed in such a way that the positive electrode active material slurry is distributed on the top surface of one side of the positive electrode current collector, and then uniformly dispersed using doctor blade or the like. In addition, the coating may be performed by various methods such as die casting, comma coating, and screen printing. [0076] Although not particularly limited, the drying may be performed for a day or less in a vacuum oven at 50 to 200 ℃. [0077] Furthermore, even another aspect of the present disclosure provides a lithium secondary battery including the positive electrode for a secondary battery, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte. [0078] The lithium secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, the positive electrode including a positive electrode active material having reduced lithium by-products and improved structural stability by doping an alkaline earth metal having oxidation number of +2 into a lithium-nickel based transition metal composite oxide and forming a phosphate coated layer on the surface of the composite oxide. [0079] Furthermore, the lithium secondary battery has a capacity retention ratio of 90% or more compared to the initial capacity, after 55 cycles under 1.0C charge and 1.0C discharge condition at 45 ℃. [0080] Although not particularly limited, the negative electrode may be prepared by coating negative electrode active material slurry containing a negative electrode active material onto the top surface of one side of a negative electrode current collector and then drying the slurry coated current collector. The negative electrode active material slurry may include additives such as a binder, a conducting agent, a filler, and a dispersant, in addition to the negative electrode active material. [0081] The negative electrode current collector may be the same as the aforementioned positive electrode current collectors, or may be included therein. [0082] Examples of the negative electrode active material may include, but are not particularly limited to, carbon materials, lithium metal, silicon, or tin which are generally known in the art, in which lithium ions are able to be intercalated and deintercalated. Preferably, carbon 11 materials may be used, and low crystalline carbon, highly crystalline carbon, and the like may be all used as the carbon material. Representative examples of the low crystalline carbon include soft carbon and hard carbon, and representative examples of the highly crystalline carbon include natural carbon, Kish graphite, pyrolytic carbon, mesophase pitch based carbon fibers, meso-carbon microbeads, mesophase pitches, and high temperature baked carbon such as petroleum or coal tar pitch derived coke. [0083] The additives such as a binder, a conducting agent, a filler, and a dispersant, which are used for the negative electrode, may be the same as the aforementioned additives used for preparation of the positive electrode, or may be included therein. [0084] The separator may be an insulating thin film having high ionic permeability and mechanical strength, and generally may have a pore diameter of 0.01 to 10 ㎛ and a thickness of 5 to 300 ㎛. Although not limited, a porous polymer film, for example, made from polyolefin-based polymers such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer may be used alone or in a laminated form as the separator. Alternatively, general porous non-woven fabrics such as non-woven fabrics made from glass fibers having a high melting point, polyethyleneterephthalate fibers, or the like. [0085] Furthermore, the electrolyte may include, but is not limited to, an organic solvent and a lithium salt which are generally used in an electrolyte. [0086] An anion of the lithium salt may be, for example, at least one selected from the group consisting of F-, Cl-, I-, NO3- , N(CN)2-, BF4-, ClO4-, PF6-, (CF3)2PF4-, (CF3)3PF3-, (CF3)4PF2-, (CF3)5PF-, (CF3)6P-, CF3SO3-, CF3CF2SO3-, (CF3SO2)2N-, (FSO2)2N-, CF3CF2(CF3)2CO-, (CF3SO2)2CH-, (SF5)3C-, (CF3SO2)3C-, CF3(CF2)7SO3-, CF3CO2-, CH3CO2-, SCN- and (CF3CF2SO2)2N-. [0087] Representative examples of the organic solvent may include at least one selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite, and tetrahydrofuran. [0088] Particularly, among the carbonate based organic solvents, ethylene carbonate and propylene carbonate which are cyclic carbonate may be preferably used because they have high permittivity as a highly viscous organic solvent and thus the lithium salt in the electrolyte easily dissociates thereby. If linear carbonate having low viscosity and low permittivity, such as dimethyl carbonate and diethyl carbonate, are mixed into the cyclic carbonate at a proper ratio, then an electrolytic solution having a high electric conductivity may be produced and thus may be more preferably used. [0089] Furthermore, the electrolyte may, if necessary, further include pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinoneimine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy 12 ethanol, aluminum trichloride, or the like in order to improve charge/discharge characteristics and fire retardant properties. In some cases, a halogen-containing solvent such as carbon tetrachloride, trifluoro ethylene may be further included to impart nonflammability, carbon dioxide gas may be further included to improve high temperature storage characteristics, and fluoro-ethylene carbonate (FEC), propene sultone (PRS), and fluoro propylene carbonate (FPC) may be further included. [0090] The lithium secondary battery of the present disclosure may be manufactured in such a way that a separator is disposed between a positive electrode and a negative electrode to form an electrode assembly, the electrode assembly is put into a cylindrical battery case or a prismatic battery case, and then an electrolyte is injected into the cell. Alternatively, the lithium secondary battery may also be manufactured in such a way that the electrode assembly is stacked, thereafter the stacked assembly is impregnated into an electrolyte and the resultant assembly is put into a battery case, and then the battery case is sealed. [0091] A battery case generally used in the art may be selected for the battery case used in the present disclosure. The shape of the battery case according to the use of a battery may be, but is not limited to, a cylinder shape using a can, a square shape, a pouch shape, or a coin shape. [0092] The lithium secondary battery according to the present disclosure may not only be used for a battery cell which is used as a power supply of a small-sized device but also preferably be used as a single cell for a middle/large sized battery module including a plurality of battery cells. Preferred examples of the middle/large device may include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and electric power storage systems. [0093] Hereinafter, the present disclosure will be described in detail with reference to Examples in order to concretely describe the present disclosure. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided to more completely describe the concept of the invention to those of ordinary skill in the art. [0094] Example 1 [0095] Ni0.78Mn0.11Co0.11OOH was prepared as a transition metal precursor, and LiOH was mixed into the transition metal precursor at a molar ratio of Li/transition metal = 1 to prepare a mixture. 0.2 wt% of SrCO3, based on the weight of the mixture, was added into the mixture and mixed, and then sintered for 24 hours at 800 ℃ to prepare Sr-doped lithium-nickel based transition metal composite oxide powder. 0.5 wt% of (NH4)2HPO4 powder, based on the weight of the composite oxide, was mixed with the composite oxide powder, and the resultant mixture was heat-treated at 500 ℃ and then sieved (No. 400) to obtain positive electrode active material powder. [0096] Example 2 [0097] Positive electrode active material powder was prepared using the same method as in 13 Example 1 except that 1.0 wt% of (NH4)2HPO4 powder was used. [0098] Comparative Example 1 [0099] Ni0.78Mn0.11Co0.11OOH was prepared as a transition metal precursor, and LiOH was mixed into the transition metal precursor at a molar ratio of Li/transition metal = 1, and then the resultant mixture was sintered for 24 hours at 800 ℃ to prepare positive electrode active material powder. [00100] Comparative Example 2 [00101] Ni0.78Mn0.11Co0.11OOH was prepared as a transition metal precursor, and LiOH was mixed into the transition metal precursor at a molar ratio of Li/transition metal = 1 to prepare a mixture. 0.2 wt% of SrCO3, based on the weight of the mixture, was added into the mixture and mixed, and then sintered for 24 hours at 800 ℃ to prepare positive electrode active material powder. [00102] Comparative Example 3 [00103] Ni0.78Mn0.11Co0.11OOH was prepared as a transition metal precursor, and LiOH was mixed into the transition metal precursor at a molar ratio of Li/transition metal = 1, and then the resultant mixture was sintered for 24 hours at 800 ℃ to prepare lithium-nickel based transition metal composite oxide powder. 0.5 wt% of (NH4)2HPO4 powder, based on the weight of the composite oxide, was mixed with the composite oxide powder, and the resultant mixture was heat-treated at 500 ℃ and then sieved (No. 400) to obtain positive electrode active material powder. [00104] Example 1-1 [00105] The positive electrode active material powder prepared in Example 1 was mixed into NMP so that the weight ratio of positive electrode active material powder : conducting agent : binder is 95 : 2.5 : 2.5, to prepare positive electrode active material slurry. The positive electrode active material slurry was coated with a thickness of 200 ㎛ onto an aluminum foil having a thickness of 20 ㎛, and then the coated aluminum foil was subjected to roll pressing and drying to prepare a positive electrode. [00106] The positive electrode was blanked into a coin shape, and a coin-shaped cell was manufactured using the positive electrode, a Li metal as a negative electrode, and a carbonate electrolytic solution in which 1 mole of LiPF6 was dissolved as an electrolyte. [00107] Example 2-1 [00108] A cell was manufactured using the same method as in Example 1-1 except that the positive electrode active material powder prepared in Example 2 was used instead of the positive electrode active material powder prepared in Example 1. 14 [00109] Comparative Example 1-1 [00110] A cell was manufactured using the same method as in Example 1-1 except that the positive electrode active material powder prepared in Comparative Example 1 was used instead of the positive electrode active material powder prepared in Example 1. [00111] Comparative Example 2-1 [00112] A cell was manufactured using the same method as in Example 1-1 except that the positive electrode active material powder prepared in Comparative Example 2 was used instead of the positive electrode active material powder prepared in Example 1. [00113] Comparative Example 3-1 [00114] A cell was manufactured using the same method as in Example 1-1 except that the positive electrode active material powder prepared in Comparative Example 3 was used instead of the positive electrode active material powder prepared in Example 1. [00115] Experimental Example 1 [00116] In order to comparatively analyze the amount of unreacted residual lithium by-products (Li2CO3 and LiOH) on the surface of each positive electrode active material powder prepared in Examples 1 and 2 and Comparative Examples 1 to 3, the amount of lithium by-products present on the surface of each positive electrode active material powder was measured using a pH titration method. [00117] Prior to the pH titration, 5 g of each positive electrode active material powder in Examples 1 and 2 and Comparative Examples 1 to 3 was added into 25 ml of water and stirred, and then decanted to separately collect about 20 ml of a transparent solution from the powder. 25 ml of water was again added to the powder and decanted with stirring to collect a transparent solution. By repetitively performing soaking and decanting in this way, 100 ml of a transparent solution containing a water soluble base was collected, and then 0.1 M HCl solution was added dropwise into the transparent solution with stirring to perform pH titration. The titration experiment was terminated when the pH value reached 3 or less, and flow rate was properly adjusted in such a range that titration took about 20 to 30 minutes. The content of the water soluble base was determined by the amount of the acid used until pH reached a value less than 5, and the content of basic impurities on the surface of the powder was calculated therefrom. The results are shown in Table 1 below. [00118] [Table 1] Item Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Li2CO3 (wt%) 0.110 0.071 0.118 0.170 0.101 LiOH (wt%) 0.166 0.151 0.305 0.211 0.210 Total lithium by-products 0.276 0.222 0.423 0.381 0.311 15 (wt%) [00119] As shown in Table 1, it was found that both Li2CO3 and LiOH by-products were significantly reduced in the positive electrode active materials in Examples 1 and 2, in which an alkaline earth metal having oxidation number of +2 was doped and a phosphate coated layer was included according to the present disclosure, compared with the positive electrode active materials in Comparative Examples 1 to 3. [00120] Specifically, it was found that the amount of Li2CO3 and LiOH by-products was significantly reduced in the positive electrode active materials in Examples 1 and 2 according to the present disclosure, compared with the positive electrode active material in Comparative Example 1, in which an alkaline earth metal having oxidation number of +2, i.e., Sr was not doped and a phosphate coated layer was not included. [00121] Furthermore, it was found that the amount of lithium by-products was significantly reduced in the positive electrode active materials in Examples 1 and 2 according to the present disclosure, even compared with the positive electrode active material in Comparative Example 2, in which Sr was doped but a phosphate coated layer was not included, and the positive electrode active material in Comparative Example 3, in which a phosphate coated layer was included but Sr was not doped. These results mean that the positive electrode active material according to the present disclosure may more effectively reduce lithium by-products by being doped with an alkaline earth metal having oxidation number of +2 and including a phosphate coated layer. [00122] Therefore, the positive electrode active material according to the present disclosure has low content of lithium by-products (LiOH and Li2CO3) as basic impurities, and thus may minimize swelling resulting from gas generation caused by reactions with an electrolytic solution during the operation of a battery using the positive electrode active material and have structural stability, thereby being capable of improving life characteristics of the battery. [00123] Experimental Example 2 [00124] Initial capacity characteristics of each cell manufactured in Examples 1-1 and 2-1 and Comparative Examples 1-1, 2-1, and 3-1, were comparatively analyzed. [00125] The each cell was charged up to 4.24 V at 25 ℃ under 0.1C CC/CV condition, then discharged to 3.0 V under 0.1C CC condition to measure charge capacity and discharge capacity, and charge/discharge efficiency and discharge rate characteristics were analyzed therethrough. Also, the ratio of discharge capacity at 2.0C to discharge capacity at 0.1C (discharge rate) was measured. The results are shown in Table 2 below. [00126] [Table 2] Item Example 1-1 Example 2-1 Comparative Example 1-1 Comparative Example 2-1 Comparative Example 3-1 Charge capacity (mAh/g) 216 215 214 217 215 Discharge capacity (mAh/g) 188 188 188 188 187 16 Charge/Discharge Efficiency (%) 87.2 87.3 87.2 86.7 87.0 Discharge rate (%, 2.0C/0.1C) 88.9 88.9 89.1 89.0 88.8 [00127] As shown in Table 2, it was found that the cells in Examples 1-1 and 2-1, which include the positive electrode active material according to the present disclosure, had excellent initial capacity characteristics of the same level without deterioration, compared with the secondary cells in Comparative Examples 1-1, 2-1, and 3-1, which include a typical lithium-nickel based composite oxide as a positive electrode active material. [00128] Experimental Example 3 [00129] Life characteristics of each cell in Examples 1-1 and 2-1 and Comparative Examples 1-1, 2-1, and 3-1 were comparatively analyzed. [00130] Each cell was repetitively charged and discharged over 100 cycles under 1.0C charge and 1.0C discharge condition, and capacity degradation according to the number of repetitions was measured. The results are shown in FIG. 1. [00131] As shown in FIG. 1, it was found that the cells in Examples 1-1 and 2-1, in which an alkaline earth metal having oxidation number of +2, i.e., Sr was doped and a phosphate coated layer was included according to the present disclosure, had better retention ratio during 100 charge/discharge cycles, compared with the cells in Comparative Examples 1-1, 2-1, and 3-1. [00132] Particularly, the cell in Example 1-1, after 55 charge/discharge cycles, showed higher capacity by about 10 % or more, compared with the cells in Comparative Examples 1-1, 2-1, and 3-1, and after 100 charge/discharge cycles, showed higher capacity by about 20 % or more, compared with the cells in Comparative Examples 1-1 and 2-1, and by about 15 % or more, compared with the cells in Comparative Examples 3-1. That is, as the number of charge/discharge cycles increases, differences in cell capacity between the cells in Comparative Examples 1-1, 2-1, and 3-1 and the cells in Examples 1-1 and 2-1 become still greater. Therefore, it was confirmed that the cells in Examples 1-1 and 2-1 according to the present disclosure have remarkably higher capacity retention ratios, and thus have significantly better life characteristics. [00133] These results mean that the positive electrode active material according to the present disclosure includes a lithium-nickel based transition metal composite oxide in which an alkaline earth metal having oxidation number of +2 is doped and a phosphate coated layer formed on the outer surface of the composite oxide, and thus the alkaline earth metal having oxidation number of +2 acts as a kind of pillar in crystal lattices of the composite oxide, thereby improving structural stability of the positive electrode active material and reducing a natural loss of lithium cations and thus reducing the formation of lithium by-products caused by the natural loss of lithium cations, and at the same time, the phosphate coated layer formed on the outer surface of the composite oxide reacts with lithium by-products present on the outer surface of the composite oxide to reduce the lithium by-products, thereby preventing swelling and resistive film formation caused by the by-products, in turn improving storage characteristics and life 17 characteristics of the battery including the positive electrode active material. [00134] While this invention has been particularly shown and described with reference to preferred embodiments thereof and drawings, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 18 WE CLAIM: 1. A positive electrode active material, comprising: a lithium-nickel based transition metal composite oxide having a layered structure, which is expressed by Chemical Formula 1 below; and a phosphate coated layer formed on the surface of the composite oxide, [Chemical Formula 1] LixNiaMbAwO2-yDy where, 1.0≤x≤1.2, 0.5≤a≤1, 0