Title of invention: cathode active material for secondary battery, manufacturing method thereof, and lithium secondary battery including the same Technical field [One] Mutual citation with related applications [2] This application claims the benefit of priority based on Korean Patent Application No. 10-2018-0129161 filed on October 26, 2018, and all contents disclosed in the documents of the Korean patent application are included as part of this specification. [3] [4] Technical field [5] The present invention relates to a cathode active material for a secondary battery, a method for manufacturing the same, and a lithium secondary battery including the same. [6] Background [7] In recent years, with the rapid spread of electronic devices using batteries such as mobile phones, notebook computers, and electric vehicles, the demand for small, lightweight, and relatively high capacity secondary batteries is rapidly increasing. In particular, lithium secondary batteries are in the spotlight as a driving power source for portable devices because they are lightweight and have high energy density. Accordingly, research and development efforts for improving the performance of lithium secondary batteries are being actively conducted. [8] Lithium secondary batteries are oxidized when lithium ions are inserted/desorbed from the positive and negative electrodes in a state in which an organic electrolyte or polymer electrolyte is charged between a positive electrode and a negative electrode made of an active material capable of intercalations and deintercalation of lithium ions. Electric energy is produced by a reduction reaction with [9] Lithium cobalt oxide (LiCoO 2 ), lithium nickel oxide (LiNiO 2 ), lithium manganese oxide (LiMnO 2 or LiMn 2 O 4 etc.), lithium iron phosphate compound (LiFePO 4 ), etc. were used as the positive electrode active material of the lithium secondary battery. . Among these, lithium cobalt oxide (LiCoO 2 ) is widely used because of its high operating voltage and excellent capacity characteristics, and has been applied as a positive electrode active material for high voltage. However, due to an increase in the price of cobalt (Co) and instability in supply, there is a limit to mass use as a power source in fields such as electric vehicles, and the need to develop a cathode active material that can replace it has emerged. [10] Accordingly, a nickel-cobalt-manganese-based lithium composite transition metal oxide (hereinafter simply referred to as'NCM-based lithium composite transition metal oxide') in which a part of cobalt (Co) is replaced with nickel (Ni) and manganese (Mn) has been developed. However, conventionally developed NCM-based lithium composite transition metal oxides are generally in the form of secondary particles in which primary particles are agglomerated, and have a large specific surface area, low particle strength, and high content of lithium by-products. There were many, and there was a problem of poor stability. In particular, in the case of an NCM-based lithium composite transition metal oxide of high-Ni, in which the content of nickel (Ni) is increased to secure a high capacity, structural and chemical stability is further deteriorated, and it is more difficult to secure thermal stability. [11] Detailed description of the invention Technical challenge [12] An object of the present invention is to provide a positive electrode active material with improved stability in a positive electrode active material of an NCM-based lithium composite transition metal oxide. In particular, in order to realize a high capacity, it is intended to provide a positive electrode active material with improved stability in a positive electrode active material of a high-Ni NCM-based lithium composite transition metal oxide containing 60 mol% or more of nickel (Ni). . [13] Specifically, the NCM-based lithium composite transition metal oxide, which reduces the specific surface area and improves the particle strength, suppresses particle cracking during rolling, reduces side reactions with the electrolyte by reducing the content of lithium by-products, and suppresses the increase in resistance. It is intended to provide a positive electrode active material. In addition, it is intended to provide a positive electrode active material of NCM-based lithium composite transition metal oxide, which can reduce the amount of gas generated when the cell is driven, and has thermal stability. [14] Means of solving the task [15] The present invention is a lithium composite transition metal oxide containing nickel (Ni), cobalt (Co) and manganese (Mn), and the lithium composite transition metal oxide is at least 2 selected from the group consisting of Zr, Al, V, Co, and Mg. At least two kinds of first dopants and at least two kinds of second dopants selected from the group consisting of Ti, Y, Sr, Nb, Ba, and Ca, and the particles of the lithium composite transition metal oxide have a crystal size of 170 It provides a positive electrode active material for a secondary battery of to 300nm. [16] In addition, the present invention is a cathode active material precursor containing nickel (Ni), cobalt (Co) and manganese (Mn), a lithium raw material and at least two or more agents selected from the group consisting of Zr, Al, V, Co, and Mg. 1 mixing the raw material of the dopant and performing primary firing; And mixing raw materials of at least two or more second dopants selected from the group consisting of Ti, Y, Sr, Nb, Ba, and Ca after the primary firing and secondary firing; including, the primary firing and It provides a method of manufacturing a cathode active material for a secondary battery that forms particles of a lithium composite transition metal oxide having a crystallite size of 170 to 300 nm through secondary firing. [17] In addition, the present invention provides a positive electrode and a lithium secondary battery including the positive electrode active material. [18] Effects of the Invention [19] According to the present invention, by reducing the specific surface area of ​​the NCM-based positive electrode active material, improving particle strength, and reducing the content of lithium by-products, side reactions with the electrolyte may be reduced. Accordingly, in the lithium secondary battery using the NCM-based positive electrode active material of the present invention, the amount of gas generated when the cell is driven may be reduced, resistance increase may be suppressed, and thermal stability may be ensured. In particular, stability may be improved even in a positive electrode active material of a high-Ni NCM-based lithium composite transition metal oxide containing 60 mol% or more of nickel (Ni) for high capacity implementation. The NCM-based positive electrode active material of the present invention can secure excellent stability, and thus can be applied to a high voltage lithium secondary battery. [20] Mode for carrying out the invention [21] Hereinafter, the present invention will be described in more detail to aid understanding of the present invention. At this time, terms or words used in the present specification and claims should not be construed as being limited to a conventional or dictionary meaning, and the inventor appropriately defines the concept of terms in order to describe his own invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of ​​the present invention based on the principle that it can be done. [22] [23] [24] The positive electrode active material for a secondary battery of the present invention is a lithium composite transition metal oxide containing nickel (Ni), cobalt (Co) and manganese (Mn), and the lithium composite transition metal oxide is composed of Zr, Al, V, Co, and Mg. At least two or more first dopants selected from the group and at least two or more second dopants selected from the group consisting of Ti, Y, Sr, Nb, Ba, and Ca, and the particles of the lithium composite transition metal oxide have a crystal size ( Crystallite size) is 170 to 300nm. [25] [26] The positive electrode active material of the present invention is an NCM-based lithium composite transition metal oxide containing nickel (Ni), cobalt (Co), and manganese (Mn). The lithium composite transition metal oxide may be a high-Ni NCM-based lithium composite transition metal oxide in which the content of nickel (Ni) is 60 mol% or more among the total metal content excluding lithium (Li). More preferably, the content of nickel (Ni) may be 65 mol% or more, and more preferably 70 mol% or more. The content of nickel (Ni) of the total metal content except for lithium (Li) of the lithium composite transition metal oxide satisfies 60 mol% or more, so that a higher capacity may be secured. Alternatively, the lithium composite transition metal oxide may be a low-Ni NCM-based lithium composite transition metal oxide in which the content of nickel (Ni) is less than 60 mol% of the total metal content excluding lithium (Li). [27] The lithium composite transition metal oxide is at least two or more first dopants selected from the group consisting of Zr, Al, V, Co, and Mg, and at least two or more agents selected from the group consisting of Ti, Y, Sr, Nb, Ba, and Ca. It contains 2 dopants. More preferably, a first dopant including Al and Zr and a second dopant including Sr and Ti may be included. When the cathode active material is synthesized by using a first dopant including two or more elements such as Al and Zr and a second dopant including two or more elements such as Sr and Ti, the lithium raw material material and the remaining lithium on the surface of the positive electrode active material and It can be effective in minimizing the secondary particle interface and reducing the specific surface area of ​​the NCM-based positive electrode active material by promoting the reactivity of the lithium source and the growth of the grain boundary by reacting, and reducing the amount of gas generated during cell operation by reducing side reactions with the electrolyte , Suppresses the increase in resistance, and can improve thermal stability. [28] The first dopant may be contained in a total content of 2,000 to 6,000 ppm. More preferably 2500 to 5500ppm, more preferably 3000 to 5000ppm may be contained. As the first dopant is contained within the above content range, there is an effect of reducing lithium diffusion resistance and securing structural stability due to stabilization of an internal structure of a positive electrode active material and improving lifespan. [29] The second dopant may be contained in a total content of 500 to 3,000 ppm. More preferably 700 to 2700 ppm, more preferably 1000 to 2500 ppm may be contained. Since the first dopant is contained within the above content range, side reactions with the electrolyte are reduced due to a decrease in reactivity due to modification on the surface of the positive electrode active material, thereby reducing gas generation. [30] More specifically, the NCM-based lithium composite transition metal oxide according to an embodiment of the present invention may be a lithium composite transition metal oxide represented by Formula 1 below. [31] [Formula 1] [32] Li p Ni 1-(x1+y1+z1+w1) Co x1 Mn y1 M a z1 M b w1 O 2+δ [33] In the above formula, M a is at least one element selected from the group consisting of Zr, Al, V, Co, and Mg, and M b is at least one element selected from the group consisting of Ti, Y, Sr, Nb, Ba, and Ca. , 1≤p≤1.5, 0 [59] Next, a method of manufacturing the positive electrode active material of the present invention will be described. [60] [61] The positive electrode active material of the present invention includes a positive electrode active material precursor containing nickel (Ni), cobalt (Co) and manganese (Mn), a lithium raw material, and at least two or more selected from the group consisting of Zr, Al, V, Co, and Mg. Mixing the raw material of the first dopant and performing primary firing; And mixing raw materials of at least two or more second dopants selected from the group consisting of Ti, Y, Sr, Nb, Ba, and Ca after the primary firing and secondary firing; including, the primary firing and Through the secondary firing, it is prepared to form particles of a lithium composite transition metal oxide having a crystallite size of 170 to 300 nm. [62] [63] The method of manufacturing the positive active material will be described in detail step by step. [64] First, a cathode active material precursor including nickel (Ni), cobalt (Co), and manganese (Mn) is prepared. [65] The cathode active material precursor may be prepared by purchasing a commercially available cathode active material precursor, or according to a method of manufacturing a cathode active material precursor well known in the art. [66] For example, the precursor may be prepared by co-precipitation reaction by adding an ammonium cation-containing complexing agent and a basic compound to a transition metal solution containing a nickel-containing raw material, cobalt-containing raw material, and manganese-containing raw material. [67] The nickel-containing raw material may be, for example, nickel-containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide or oxyhydroxide, and specifically, Ni(OH) 2 , NiO, NiOOH, NiCO 3 ㆍ 2Ni(OH) 2 ㆍ4H 2 O, NiC 2 O 2 ㆍ2H 2 O, Ni(NO 3 ) 2 ㆍ6H 2 O, NiSO 4 , NiSO 4 ㆍ6H 2 O, fatty acid nickel salt, nickel halide or their It may be a combination, but is not limited thereto. [68] The cobalt-containing raw material may be cobalt-containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide or oxyhydroxide, and specifically, Co(OH) 2 , CoOOH, Co(OCOCH 3 ) 2 ㆍ4H 2 O , Co(NO 3 ) 2 ㆍ6H 2 O, CoSO 4 , Co(SO 4 ) 2 ㆍ7H 2 O, or a combination thereof, but is not limited thereto. [69] The manganese-containing raw material may be, for example, manganese-containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, oxyhydroxide, or a combination thereof, specifically Mn 2 O 3 , MnO 2 , Mn 3 Manganese oxides such as O 4 and the like; Manganese salts such as MnCO 3 , Mn(NO 3 ) 2 , MnSO 4 , manganese acetate, manganese dicarboxylic acid, manganese citrate and manganese fatty acid; Manganese oxyhydroxide, manganese chloride, or a combination thereof may be used, but the present invention is not limited thereto. [70] The transition metal solution contains a nickel-containing raw material, cobalt-containing raw material, and manganese-containing raw material in a solvent, specifically water, or a mixed solvent of an organic solvent (eg, alcohol, etc.) that can be uniformly mixed with water It may be prepared by adding, or may be prepared by mixing an aqueous solution of a nickel-containing raw material, an aqueous solution of a cobalt-containing raw material, and a manganese-containing raw material. [71] The ammonium cation-containing complex forming agent may be, for example, NH 4 OH, (NH 4 ) 2 SO 4 , NH 4 NO 3 , NH 4 Cl, CH 3 COONH 4 , NH 4 CO 3 or a combination thereof, It is not limited thereto. Meanwhile, the ammonium cation-containing complex-forming agent may be used in the form of an aqueous solution, and in this case, water or a mixture of water and an organic solvent (specifically, alcohol, etc.) that can be uniformly mixed with water may be used as a solvent. [72] The basic compound may be a hydroxide of an alkali metal or alkaline earth metal such as NaOH, KOH or Ca(OH) 2 , a hydrate thereof, or a combination thereof. The basic compound may also be used in the form of an aqueous solution, and in this case, water or a mixture of water and an organic solvent (specifically, alcohol, etc.) that can be uniformly mixed with water may be used. [73] The basic compound is added to adjust the pH of the reaction solution, and may be added in an amount such that the pH of the metal solution is 11 to 13. [74] Meanwhile, the coprecipitation reaction may be performed at a temperature of 40°C to 70°C in an inert atmosphere such as nitrogen or argon. [75] By the above process, particles of nickel-cobalt-manganese hydroxide are produced and precipitated in the reaction solution. By controlling the concentration of the nickel-containing raw material, cobalt-containing raw material, and manganese-containing raw material, a precursor having a nickel (Ni) content of 60 mol% or more of the total metal content can be prepared. The precipitated nickel-cobalt-manganese hydroxide particles are separated according to a conventional method and dried to obtain a nickel-cobalt-manganese precursor. The precursor may be secondary particles formed by agglomeration of primary particles. [76] [77] Next, the precursor, a lithium raw material, and a raw material of at least two or more first dopants selected from the group consisting of Zr, Al, V, Co, and Mg are mixed, and then first fired. [78] As the lithium raw material, lithium-containing sulfates, nitrates, acetates, carbonates, oxalates, citrates, halides, hydroxides or oxyhydroxides may be used, and are not particularly limited as long as they can be dissolved in water. Specifically, the lithium raw material is Li 2 CO 3 , LiNO 3 , LiNO 2 , LiOH, LiOH·H 2 O, LiH, LiF, LiCl, LiBr, LiI, CH 3 COOLi, Li 2 O, Li 2 SO 4 , CH 3 COOLi, or Li 3 C 6 H 5 O 7 and the like, any one or a mixture of two or more of them may be used. [79] The raw material of the first dopant may include sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, hydroxide or oxyhydroxide containing the first dopant element. The raw material of the first dopant may be mixed so that the total content of the first dopant is contained in an amount of 2,000 to 6,000 ppm, more preferably 2,500 to 5,500 ppm, more preferably 3,000 to It can be mixed to contain 5,000 ppm. [80] In the case of a high-Ni NCM-based lithium composite transition metal oxide having a nickel (Ni) content of 60 mol% or more, the primary firing may be fired at 800 to 1,000°C, more preferably 830 to It can be fired at 980°C, more preferably 850 to 950°C. In the case of a low-Ni NCM-based lithium composite transition metal oxide in which the content of nickel (Ni) is less than 60 mol%, the primary firing may be fired at 900 to 1,100°C, more preferably 930 to 1,070°C. , More preferably, it can be fired at 950 to 1,050°C. [81] The primary sintering may be performed under an air or oxygen atmosphere, and may be performed for 15 to 35 hours. [82] [83] Next, after the first firing, the raw materials of at least two or more second dopants selected from the group consisting of Ti, Y, Sr, Nb, Ba, and Ca are mixed, followed by secondary firing. [84] The raw material of the second dopant may include sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, hydroxide or oxyhydroxide containing the second dopant element. The raw material of the second dopant may be mixed so that the total content of the second dopant is contained in an amount of 500 to 3,000 ppm with respect to the total weight of the positive electrode active material, more preferably 700 to 2,700 ppm, more preferably 1,000 to It can be mixed to contain 2,500ppm. [85] The secondary firing may be fired at 600 to 950° C., and more preferably at 600 to 950° C., in the case of a high-Ni NCM-based lithium composite transition metal oxide in which the content of nickel (Ni) is 60 mol% or more. It can be fired at 930°C, more preferably 700 to 900°C. In the case of a low-Ni NCM-based lithium composite transition metal oxide having a nickel (Ni) content of less than 60 mol%, the secondary firing may be fired at 700 to 1,050° C., more preferably 750 to 1,000° C. , More preferably, it can be fired at 800 to 950°C. [86] The secondary sintering may be performed under an air or oxygen atmosphere, and may be performed for 15 to 35 hours. [87] [88] Through the first firing and the second firing, particles of a lithium composite transition metal oxide are formed so that the crystal size is 170 nm to 300 nm. More preferably 180nm to 280nm, more preferably 190nm to 260nm can be carried out so that the sintering. [89] In addition, through the first firing and the second firing, a positive electrode active material having an average number of primary particles of 20 or less in the secondary particles on a cross-section may be formed. More preferably, a positive electrode active material having an average number of primary particles of 10 or less, more preferably 1 to 5 in the secondary particles on a cross-section can be prepared. [90] In the present invention, by doping a first dopant containing two or more elements such as Al and Zr during the first firing, and doping a second dopant containing two or more elements such as Sr and Ti during the second firing. The crystal size of the NCM-based positive active material may be 170 nm to 300 nm, and the undercalcination temperature for forming the average number of primary particles in the secondary particles on the cross-section to 20 or less may be lowered. Accordingly, the positive electrode active material prepared according to the present invention has a crystal size of 170 nm to 300 nm of the NCM-based positive electrode active material, and further, the average number of primary particles in the secondary particles on the cross-section satisfies 20 or less. While stability is secured, such as reduction of gas generation and suppression of resistance increase, the problem of capacity reduction due to over-calcination and increase in surface resistance of the positive electrode active material has been solved. [91] [92] [93] According to another embodiment of the present invention, a positive electrode for a secondary battery and a lithium secondary battery including the positive electrode active material are provided. [94] [95] Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including the positive electrode active material. [96] In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes to the battery, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or carbon on the surface of aluminum or stainless steel. , Nickel, titanium, silver, or the like may be used. In addition, the positive electrode current collector may generally have a thickness of 3 to 500 μm, and fine unevenness may be formed on the surface of the positive electrode current collector to increase the adhesion of the positive electrode active material. For example, it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics. [97] [98] In addition, the positive electrode active material layer may include a conductive material and a binder in addition to the positive electrode active material described above. [99] At this time, the conductive material is used to impart conductivity to the electrode, and in the battery to be configured, it can be used without particular limitation as long as it does not cause chemical changes and has electronic conductivity. Specific examples include graphite such as natural graphite and artificial graphite; Carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; Metal powders or metal fibers such as copper, nickel, aluminum, and silver; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Alternatively, a conductive polymer such as a polyphenylene derivative may be used, and one of them alone or a mixture of two or more may be used. The conductive material may be included in an amount of 1 to 30% by weight based on the total weight of the positive electrode active material layer. [100] [101] In addition, the binder serves to improve adhesion between positive electrode active material particles and adhesion between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC). ), starch, hydroxypropylcellulose, recycled cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and the like, and one of them alone or a mixture of two or more may be used. The binder may be included in an amount of 1 to 30% by weight based on the total weight of the positive electrode active material layer. [102] [103] The positive electrode may be manufactured according to a conventional positive electrode manufacturing method except for using the positive electrode active material described above. Specifically, it may be prepared by applying the above-described positive electrode active material and, optionally, a composition for forming a positive electrode active material layer including a binder and a conductive material on a positive electrode current collector, followed by drying and rolling. At this time, the types and contents of the positive electrode active material, the binder, and the conductive material are as described above. [104] The solvent may be a solvent generally used in the art, dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone or Water, and the like, and one of them alone or a mixture of two or more may be used. The amount of the solvent is sufficient to dissolve or disperse the positive electrode active material, the conductive material, and the binder in consideration of the coating thickness and production yield of the slurry, and to have a viscosity that can exhibit excellent thickness uniformity during application for the subsequent positive electrode production Do. [105] [106] Alternatively, the positive electrode may be prepared by casting the composition for forming a positive electrode active material layer on a separate support, and then laminating a film obtained by peeling from the support on a positive electrode current collector. [107] [108] According to another embodiment of the present invention, an electrochemical device including the anode is provided. The electrochemical device may specifically be a battery or a capacitor, and more specifically, a lithium secondary battery. [109] [110] The lithium secondary battery specifically includes a positive electrode, a negative electrode positioned opposite the positive electrode, a separator and an electrolyte interposed between the positive electrode and the negative electrode, and the positive electrode is as described above. In addition, the lithium secondary battery may optionally further include a battery container accommodating the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container. [111] [112] In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative active material layer disposed on the negative electrode current collector. [113] The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes to the battery, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel. Surface treatment with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, and the like may be used. In addition, the negative electrode current collector may generally have a thickness of 3 to 500 μm, and, like the positive electrode current collector, microscopic irregularities may be formed on the surface of the current collector to enhance the bonding strength of the negative electrode active material. For example, it may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwovens. [114] [115] The negative active material layer optionally includes a binder and a conductive material together with the negative active material. The negative electrode active material layer is, for example, coated on a negative electrode current collector with a negative electrode forming composition including a negative electrode active material, and optionally a binder and a conductive material, and dried, or cast the negative electrode composition on a separate support. , It can also be produced by laminating a film obtained by peeling from this support on a negative electrode current collector. [116] [117] As the negative active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; Metal compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; SiO β (0 <β <2), SnO 2Metal oxides capable of doping and undoping lithium such as vanadium oxide and lithium vanadium oxide; Or a composite including the metal compound and a carbonaceous material, such as a Si-C composite or a Sn-C composite, and any one or a mixture of two or more of them may be used. In addition, a metal lithium thin film may be used as the negative electrode active material. In addition, as the carbon material, both low crystalline carbon and high crystalline carbon may be used. As low crystalline carbon, soft carbon and hard carbon are typical, and high crystalline carbon is amorphous, plate-like, scale-like, spherical or fibrous natural or artificial graphite, Kish graphite (Kish). graphite), pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches, and petroleum or coal tar pitch High-temperature calcined carbon such as derived cokes) is typical. [118] In addition, the binder and the conductive material may be the same as described above for the positive electrode. [119] [120] On the other hand, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ions, and can be used without particular limitation as long as it is used as a separator in a general lithium secondary battery. On the other hand, it is preferable to have low resistance and excellent electrolyte-moisturizing ability. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer, or these A stacked structure of two or more layers of may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of a high melting point glass fiber, polyethylene terephthalate fiber, or the like may be used. In addition, in order to secure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material may be used, and optionally, a single layer or a multilayer structure may be used. [121] [122] In addition, electrolytes used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc. that can be used in the manufacture of lithium secondary batteries, limited to these. It does not become. [123] [124] Specifically, the electrolyte may include an organic solvent and a lithium salt. [125] The organic solvent may be used without particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of a battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; Ether solvents such as dibutyl ether or tetrahydrofuran; Ketone solvents such as cyclohexanone; Aromatic hydrocarbon solvents such as benzene and fluorobenzene; Dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), propylene carbonate, Carbonate-based solvents such as PC); Alcohol solvents such as ethyl alcohol and isopropyl alcohol; Nitriles such as R-CN (R is a C2 to C20 linear, branched or cyclic hydrocarbon group, and may contain a double bonded aromatic ring or an ether bond); Amides such as dimethylformamide; Dioxolanes such as 1,3-dioxolane; Alternatively, sulfolanes or the like may be used. Among them, carbonate-based solvents are preferable, and cyclic carbonates having high ionic conductivity and high dielectric constant that can increase the charging/discharging performance of the battery (for example, Ethylene carbonate or propylene carbonate, etc.), and a low-viscosity linear carbonate-based compound (eg, ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) are more preferable. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the electrolyte may exhibit excellent performance. [126] [127] The lithium salt may be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salt is LiPF 6 , LiClO 4 , LiAsF 6 , LiBF 4 , LiSbF 6 , LiAl0 4 , LiAlCl 4 , LiCF 3 SO 3 , LiC 4 F 9 SO 3 , LiN(C 2 F 5 SO 3 ) 2 , LiN(C 2 F 5 SO 2 ) 2 , LiN(CF 3 SO 2 ) 2 . LiCl, LiI, or LiB(C 2 O 4 ) 2 or the like may be used. The concentration of the lithium salt is preferably used within the range of 0.1 to 2.0M. When the concentration of the lithium salt is within the above range, since the electrolyte has an appropriate conductivity and viscosity, excellent electrolyte performance can be exhibited, and lithium ions can move effectively. [128] [129] In addition to the electrolyte constituents, the electrolyte includes, for example, haloalkylene carbonate-based compounds such as difluoroethylene carbonate, pyridine, and tri- Ethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imida One or more additives such as zolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol, or aluminum trichloride may be further included. In this case, the additive may be included in an amount of 0.1 to 5% by weight based on the total weight of the electrolyte. [130] [131] As described above, since the lithium secondary battery including the positive electrode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, portable devices such as mobile phones, notebook computers, digital cameras, and hybrid electric vehicles ( It is useful in electric vehicle fields such as hybrid electric vehicle, HEV). [132] [133] Accordingly, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same are provided. [134] The battery module or battery pack may include a power tool; Electric vehicles including electric vehicles (EV), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEV); Alternatively, it may be used as a power supply for any one or more medium and large-sized devices among systems for power storage. [135] [136] Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein. [137] [138] Example 1 [139] In a batch-type 5L reactor set at 60°C, NiSO 4 , CoSO 4 , and MnSO 4 were mixed in water in an amount such that the molar ratio of nickel: cobalt: manganese was 60:20:20, and the concentration of 2.4M A precursor formation solution of was prepared. [140] After putting 1 liter of deionized water into the co-precipitation reactor (capacity 5L), nitrogen gas was purged into the reactor at a rate of 2 liters/minute to remove dissolved oxygen in the water, and the inside of the reactor was made into a non-oxidizing atmosphere. Thereafter, 10 ml of a 25% aqueous NaOH solution was added, followed by stirring at a stirring speed of 1200 rpm at a temperature of 60° C., and a pH of 12.0 was maintained. [141] Since each added to the precursor to form a solution at a rate of 180ml / hr, and, NaOH aqueous solution and NH 4 to 18 hours co-precipitation reaction and introduced with the OH aqueous solution of nickel-cobalt-manganese-containing hydroxide (Ni 0 . 60 Co 0 . 20 Mn 0 . 20 (OH) 2 to form particles of a). The hydroxide particles were separated, washed, and dried in an oven at 120° C. to prepare a cathode active material precursor. The positive electrode active material precursor thus prepared was in the form of secondary particles in which primary particles were aggregated. [142] The positive electrode active material precursor and lithium raw material LiOH prepared as described above were put into Henschel mixer 700L so that the final Li/M (Ni,Co,Mn) molar ratio was 1.02, and the first dopant raw material Al(OH) 2 , ZrO 2 was added so that the final total content of the first dopant element was 3500 ppm, and mixed at 300 rpm for 20 minutes. The mixed powder was put in an alumina crucible having a size of 330mmx330mm, and the mixture was first fired at 930°C for 15 hours under an oxygen (O 2 ) atmosphere. [143] Thereafter, the second dopant raw materials SrCO 3 and TiO 2 were added to the Henschel mixer (700L) with the first calcined material so that the final total content of the second dopant element was 2000 ppm, and mixed at 300 rpm for 20 minutes . The mixed powder was placed in an alumina crucible having a size of 330mmx330mm, and the secondary sintering was performed at 830°C for 15 hours in an oxygen (O 2 ) atmosphere to form a lithium composite transition metal oxide. [144] 300 g of the lithium composite transition metal oxide thus prepared was added to 300 mL of ultrapure water, stirred for 30 minutes, washed with water, and filtered for 20 minutes. After drying the filtered lithium composite transition metal oxide at 130° C. for 10 hours in a vacuum oven, sieving was performed to prepare a positive electrode active material. [145] [146] Example 2 [147] In a batch-type 5L reactor set at 60℃, NiSO 4 , CoSO 4 , MnSO 4 are mixed in water in an amount such that the molar ratio of nickel: cobalt: manganese is 50:30:20, and a concentration of 2.4M A precursor formation solution of was prepared. [148] After putting 1 liter of deionized water into the co-precipitation reactor (capacity 5L), nitrogen gas was purged into the reactor at a rate of 2 liters/minute to remove dissolved oxygen in the water, and the inside of the reactor was made into a non-oxidizing atmosphere. Thereafter, 10 ml of a 25% aqueous NaOH solution was added, followed by stirring at a stirring speed of 1200 rpm at a temperature of 60° C., and a pH of 12.0 was maintained. [149] After input, each of the precursor to form a solution at a rate of 180ml / hr, and, NaOH aqueous solution and a NH 4 OH solution to 18 hours co-precipitation reaction and introduced with the nickel-cobalt-manganese-containing hydroxide (Ni 0 . 50 Co 0 . 30 Mn 0 . 20 (OH) 2 to form particles of a). The hydroxide particles were separated, washed, and dried in an oven at 120° C. to prepare a cathode active material precursor. The positive electrode active material precursor thus prepared was in the form of secondary particles in which primary particles were aggregated. [150] The positive electrode active material precursor and lithium raw material LiOH prepared as described above were put into Henschel mixer 700L so that the final Li/M (Ni,Co,Mn) molar ratio was 1.02, and the first dopant raw material Al(OH) 2 , ZrO 2 was added so that the final total content of the first dopant element was 3500 ppm, and mixed at 300 rpm for 20 minutes. The mixed powder was put in an alumina crucible having a size of 330mmx330mm, and the mixture was first fired at 970°C for 15 hours under an oxygen (O 2 ) atmosphere. [151] Thereafter, the second dopant raw materials SrCO 3 and TiO 2 were added to the Henschel mixer (700L) with the first calcined material so that the final total content of the second dopant element was 2000 ppm, and mixed at 300 rpm for 20 minutes. . The mixed powder was put in an alumina crucible having a size of 330mmx330mm, and the secondary sintering was performed at 870°C for 15 hours in an oxygen (O 2 ) atmosphere to form a lithium composite transition metal oxide. [152] The lithium composite transition metal oxide thus prepared was dried in a vacuum oven at 130° C. for 10 hours and then sieved to prepare a positive electrode active material. [153] [154] Example 3 [155] Co(OH) 2 and MgO as the first dopant raw material are mixed so that the final total content of the first dopant element is 5000 ppm, and Nb 2 O 5 and Y 2 O 3 as the second dopant raw material are used as the final total second dopant element. A positive electrode active material was prepared in the same manner as in Example 1, except for mixing so that the content was 1500 ppm. [156] [157] Example 4 [158] Co(OH) 2 and MgO as the first dopant raw material are mixed so that the final total content of the first dopant element is 5000 ppm, and Nb 2 O 5 and Y 2 O 3 as the second dopant raw material are used as the final total second dopant element. A positive electrode active material was prepared in the same manner as in Example 2, except for mixing so that the content was 1500 ppm. [159] [160] Example 5 [161] A positive electrode active material was prepared in the same manner as in Example 1, except that the first firing temperature was set to 890°C and the second firing temperature was set to 800°C. [162] [163] Example 6 [164] A positive electrode active material was prepared in the same manner as in Example 2, except that the first firing temperature was set to 930°C and the second firing temperature was set to 850°C. [165] [166] Comparative Example 1 [167] A positive electrode active material was manufactured in the same manner as in Example 1, except that the first dopant raw material and the second dopant raw material were not mixed. [168] [169] Comparative Example 2 [170] A positive electrode active material was manufactured in the same manner as in Example 2, except that the first dopant raw material and the second dopant raw material were not mixed. [171] [172] Comparative Example 3 [173] Example 1 except that ZrO 2 as the first dopant raw material was mixed so that the final first dopant element content was 3500 ppm, and SrCO 3 as the second dopant raw material was mixed so that the final second dopant element content was 2000 ppm. In the same manner as, a positive active material was prepared. [174] [175] Comparative Example 4 [176] Example 2, except that ZrO 2 as the first dopant raw material was mixed so that the final first dopant element content was 3500 ppm, and SrCO 3 as the second dopant raw material was mixed so that the final second dopant element content was 2000 ppm. In the same manner as, a positive active material was prepared. [177] [178] [Experimental Example 1: Specific surface area and crystal size of positive electrode active material] [179] The specific surface area and crystal size of the positive electrode active materials prepared in Examples 1 to 6 and Comparative Examples 1 to 4 were measured. The specific surface area was measured using a gas adsorption analyzer (BELSORP mini II), and the crystallite size was calculated by measuring XRD (Bruker D4 Endeavor). [180] [Table 1] Specific surface area (m 2 /g) Crystal size (nm) Example 1 0.45 255 Example 2 0.55 251 Example 3 0.67 234 Example 4 0.61 223 Example 5 0.57 210 Example 6 0.60 190 Comparative Example 1 0.15 312 Comparative Example 2 0.21 308 Comparative Example 3 0.92 155 Comparative Example 4 0.87 167 [181] Referring to Table 1, the positive electrode active materials prepared in Examples 1 to 6 had a specific surface area of ​​0.7 m 2 /g or less, and the specific surface area was reduced compared to the positive electrode active materials of Comparative Examples 3 to 4. In addition, the positive electrode active materials of Examples 1 to 6 had a crystal size of 170 nm to 300 nm, and the crystal size was increased than that of the positive electrode active materials of Comparative Examples 3 to 4. On the other hand, in the case of Comparative Examples 1 to 2 in which the first and second dopants were not doped at all, there was no energy consumption required for dopant migration compared to Examples 1 to 2 in which the dopant was present, so that the driving force of crystallite growth was increased. It was found that the crystal size exceeded 300 nm. [182] [183] [ Experimental Example 2: Observation of cathode active material] [184] After etching the positive electrode active material prepared in Examples 1 to 6 and Comparative Examples 1 to 4 into a cross section using a FIB (Focused Ion Beam) equipment, 20 or more secondary particles using a scanning electron microscope (FE-SEM) The cross section was observed. At this time, the total number of primary particles relative to the total number of secondary particles was calculated as follows to determine the average number of primary particles in the secondary particles on the cross section. The results are shown in Table 2. [185] [Table 2] Average number of primary particles in secondary particles on the cross section Example 1 4.3 Example 2 4.9 Example 3 11 Example 4 14.7 Example 5 7.9 Example 6 7 Comparative Example 1 5.1 Comparative Example 2 3.5 Comparative Example 3 28.4 Comparative Example 4 27 [186] Referring to Table 2, the positive electrode active materials prepared in Examples 1 to 6 of the present invention have an average number of primary particles of 20 or less in the secondary particles on the cross section, but each dopant element is doped with only one type of comparative example. In the case of 3 to 4, the average number of primary particles in the secondary particles on the cross section exceeded 20. The positive electrode active materials prepared as in Examples 1 to 6 were doped with at least two specific first and second dopants during the first firing and the second firing, respectively, so that the average primary in the secondary particles on the cross-section was lowered, It can be seen that it is formed so that the number of particles is 20 or less. On the other hand, in the case of Comparative Examples 1 and 2 in which the first and second dopants were not doped at all, the average in the secondary particles, which is about the level of Examples 1 and 2, was not an element that would interfere with the growth of the primary particles compared to the case where the dopant was added. It seems that the number of primary particles was less than 20. [187] [188] [Experimental Example 3: Thermal Stability Evaluation] [189] In order to evaluate the thermal stability of the positive electrode active material prepared in Examples 1 to 2 and 5 and Comparative Example 3, heat flow according to temperature was measured using a differential scanning calorimeter (SETARAM's SENSYS Evo). Specifically, the lithium secondary battery prepared as in Preparation Example using the positive electrode active material of Examples 1 to 2 and 5 and Comparative Example 3 was decomposed in a state of 100% SOC charge, and the positive electrode and a new electrolyte solution were added to the DSC measurement cell. The measurement was carried out while increasing the temperature from room temperature to 400°C at 10°C per minute. The results are shown in Table 3. [190] [Table 3] DSC main peak (℃) Maximum heat flow (W/g) Example 1 320 1.36 Example 2 299 1.5 Example 5 301 1.58 Comparative Example 3 288 1.71 [191] Referring to Table 3, in the case of Examples 1 to 2 and 5 of the present invention, it was confirmed that the main peak with the maximum heat flow was found at a relatively high temperature of 299°C or higher, and the maximum heat flow was significantly reduced compared to Comparative Example 3. I can. Through this, it can be seen that the thermal stability is remarkably improved in the case of the embodiment of the present invention. [192] [193] [Experimental Example 4: Evaluation of high temperature storage properties] [194] The cathode active material, the carbon black conductive material, and the PVdF binder prepared in Examples 1 to 6 and Comparative Examples 1 to 3 were mixed in a weight ratio of 96:2:2 in an N-methylpyrrolidone solvent, and a cathode mixture (Viscosity: 5000 mPa·s) was prepared, coated on one surface of an aluminum current collector, dried at 130° C., and rolled to prepare a positive electrode. [195] Lithium metal was used as the negative electrode. [196] An electrode assembly was manufactured by interposing a porous polyethylene separator between the positive electrode and the negative electrode prepared as described above, and after placing the electrode assembly in the case, an electrolyte was injected into the case to prepare a lithium secondary battery. At this time, the electrolyte is 1.0M lithium hexafluorophosphate (LiPF 6 ) dissolved in an organic solvent consisting of ethylene carbonate/ethylmethyl carbonate/diethyl carbonate/(EC/EMC/DEC mixing volume ratio = 3/4/3 ). Was prepared. [197] The lithium secondary battery half cell prepared as described above was charged in CCCV mode to 0.5C and 4.4V (termination current 1/20C). Two positive electrodes and two polyethylene separators were alternately stacked on the lower plate of the coin cell. Then, after injecting the electrolyte, the coin cell, which was then covered with a gasket, was put into an aluminum pouch and sealed with vacuum. After that, the gas generated by storing at 60° C. for 2 weeks was measured using a gas chromatograph-mass spectrometer (GC-MS). The results are shown in Table 4 below. [198] [Table 4] High-temperature storage gas generation (µl/g) Example 1 1320 Example 2 1200 Example 3 1580 Example 4 1610 Example 5 1550 Example 6 1480 Comparative Example 1 2440 Comparative Example 2 2615 Comparative Example 3 2430 Comparative Example 4 2190 [199] Referring to Table 4, the amount of high-temperature storage gas generated in the positive electrode active materials prepared in Examples 1 to 6 was significantly reduced compared to the positive electrode active materials prepared in Comparative Examples 1 to 4. In the case of Comparative Examples 1 and 2, in which the first and second dopants were not doped at all, the amount of gas generated was remarkably high because there was no dopant to serve as a surface and structure stabilization. In the case of 4, since sufficient undercalcination was not performed compared to Examples 1 to 6, the BET was relatively high, and the amount of gas generated was remarkably high due to the increase in the reaction surface area. [200] [201] [ Experimental Example 5: Room temperature resistance evaluation] [202] Voltage drop measured by carrying out constant current discharge for 10 seconds at 25° C. and SOC 10% at 1.0 C for a lithium secondary battery prepared as in Experimental Example 4 using the positive electrode active materials of Examples 1 to 6 and Comparative Examples 1 to 4 The resistance was calculated and compared, and the results are shown in Table 5 below. [203] [Table 5] 25℃ resistance (Ω) Example 1 22.4 Example 2 23 Example 3 26.6 Example 4 24.1 Example 5 20.5 Example 6 21 Comparative Example 1 45.7 Comparative Example 2 48.5 Comparative Example 3 25.8 Comparative Example 4 27.2 [204] Referring to Table 5, in Examples 1 to 6 of the present invention, room temperature resistance was improved compared to Comparative Examples 1 to 4. In particular, compared to Comparative Examples 1 to 2 in which the first and second dopants were not doped at all, the room temperature resistance characteristics of Examples 1 to 6 were significantly improved, which is the lithium ion diffusion resistance due to the addition of the dopant in Comparative Examples 1 to 2 It is thought that this is because it cannot have an improvement effect. Claims [Claim 1] It is a lithium composite transition metal oxide containing nickel (Ni), cobalt (Co) and manganese (Mn), and the lithium composite transition metal oxide is at least two or more agents selected from the group consisting of Zr, Al, V, Co, and Mg. 1 dopant and at least two or more second dopants selected from the group consisting of Ti, Y, Sr, Nb, Ba, and Ca, and the particles of the lithium composite transition metal oxide have a crystallite size of 170 to 300 nm. Positive active material for secondary batteries. [Claim 2] The method of claim 1, wherein the particles of the lithium composite transition metal oxide are etched into a cross section using a FIB (Focused Ion Beam) equipment, and then 20 or more secondary particles are cross-sectioned using a scanning electron microscope (FE-SEM). When observed, the positive electrode active material for secondary batteries having an average number of primary particles of 20 or less in the secondary particles on the cross section. [Claim 3] The cathode active material for a secondary battery according to claim 1, wherein the first dopant is contained in a total content of 2,000 to 6,000 ppm. [Claim 4] The cathode active material for a secondary battery according to claim 1, wherein the second dopant is contained in a total content of 500 to 3,000 ppm. [Claim 5] The cathode active material for a secondary battery according to claim 1, wherein the particles of the lithium composite transition metal oxide have a specific surface area of ​​0.2 to 0.7 m 2 /g. [Claim 6] The cathode active material for a secondary battery according to claim 2, wherein the particles of the lithium composite transition metal oxide have an average number of primary particles in the secondary particles on a cross section of 10 or less. [Claim 7] The cathode active material for a secondary battery according to claim 1, wherein the lithium composite transition metal oxide has a nickel (Ni) content of 60 mol% or more among a total metal content excluding lithium (Li). [Claim 8] The positive electrode active material of claim 1, wherein the lithium composite transition metal oxide has a nickel (Ni) content of less than 60 mol% of a total metal content excluding lithium (Li). [Claim 9] A positive electrode active material precursor containing nickel (Ni), cobalt (Co) and manganese (Mn), a lithium raw material, and a raw material of at least two or more first dopants selected from the group consisting of Zr, Al, V, Co, and Mg Mixing and first firing; And mixing raw materials of at least two or more second dopants selected from the group consisting of Ti, Y, Sr, Nb, Ba, and Ca after the primary firing and secondary firing; including, the primary firing and A method of manufacturing a positive electrode active material for a secondary battery to form particles of a lithium composite transition metal oxide having a crystallite size of 170 to 300 nm through secondary firing. [Claim 10] The method of claim 9, wherein when the positive electrode active material precursor has a nickel (Ni) content of 60 mol% or more of the total metal content, the primary firing is fired at 800 to 1,000°C. [Claim 11] The method of claim 9, wherein when the positive electrode active material precursor has a nickel (Ni) content of less than 60 mol% of the total metal content, the primary firing is fired at 900 to 1,100°C. [Claim 12] The method of claim 9, wherein when the positive electrode active material precursor has a nickel (Ni) content of 60 mol% or more of the total metal content, the secondary firing is fired at 600 to 950°C. [Claim 13] The method of claim 9, wherein when the positive electrode active material precursor has a nickel (Ni) content of less than 60 mol% of the total metal content, the secondary firing is fired at 700 to 1,050°C. [Claim 14] The method of claim 9, wherein the particles of the lithium composite transition metal oxide formed through the primary firing and the secondary firing are etched into a cross section using a Focused Ion Beam (FIB) equipment, and then a scanning electron microscope (FE-SEM). When the cross-section of 20 or more secondary particles is observed using, the average number of primary particles in the secondary particles on the cross-section is 20 or less. [Claim 15] The method of claim 9, wherein the raw material of the first dopant is mixed so that the total content of the first dopant is contained in an amount of 2,000 to 6,000 ppm with respect to the total weight of the positive electrode active material. [Claim 16] The method of claim 9, wherein the raw material of the second dopant is mixed so that the total content of the second dopant is contained in an amount of 500 to 3,000 ppm with respect to the total weight of the positive electrode active material. [Claim 17] A positive electrode for a secondary battery comprising the positive electrode active material according to any one of claims 1 to 8. [Claim 18] A lithium secondary battery comprising the positive electrode according to claim 17.