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. 10-2017-0120659 filed on September 19, 2017, and all contents disclosed in the documents of the Korean patent application are incorporated as part of this specification. [3] Technical field [4] 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. [5] Background [6] 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. [7] A lithium secondary battery includes a positive electrode including a positive electrode active material capable of intercalating/detaching lithium ions, a negative electrode including a negative active material capable of intercalating/deintercalating lithium ions, and an electrode with a microporous separator interposed between the positive electrode and the negative electrode. It refers to a battery in which the assembly contains an electrolyte containing lithium ions. [8] Lithium transition metal oxide is used as a positive electrode active material of a lithium secondary battery, and lithium metal, a lithium alloy, crystalline or amorphous carbon, or carbon composite is used as the negative electrode active material. The active material is applied to the current collector with an appropriate thickness and length, or the active material itself is coated in a film shape and wound or stacked together with a separator as an insulator to form an electrode group, and then placed in a can or similar container, and then an electrolyte is injected. To manufacture a secondary battery. [9] [10] As a positive electrode active material for a lithium secondary battery that has been actively researched and developed, there is a layered lithium cobalt oxide (LiCoO 2 ). Lithium cobalt oxide (LiCoO 2 ) has a high operating voltage and excellent capacity characteristics, but has a problem in that thermal characteristics are poor due to destabilization of the crystal structure due to delithiation, and the structure becomes unstable under high voltage. In addition, as the oxidation value of Co is oxidized to 4+ during charging, there is a problem of deterioration of surface stability and lifespan due to side reactions with the electrolyte. [11] Recently, the demand for high-capacity lithium secondary batteries is gradually increasing. Unlike the ternary cathode active material , lithium cobalt oxide (LiCoO 2 ) can increase its capacity only by raising the voltage, so it is more high voltage than the existing 4.45V or less. It is necessary to secure structural stability even at 4.5V or higher, and at the same time, it is necessary to develop lithium cobalt oxide (LiCoO 2 ) that can improve surface stability by preventing side reactions with electrolyte, and improve life characteristics and high temperature/high voltage stability. Actually. [12] Detailed description of the invention Technical challenge [13] The present invention is a positive electrode active material of lithium cobalt oxide (LiCoO 2 ) having excellent structural stability , in particular, it is possible to prevent structural changes even under a high voltage of 4.5V or higher, and effectively improve surface stability to improve lifespan characteristics and stability at high temperature/high voltage. It is intended to provide the secured positive electrode active material for a lithium secondary battery and a method of manufacturing the same. [14] Means of solving the task [15] The present invention is a lithium cobalt-based oxide containing a doping element M, the particles of the lithium cobalt-based oxide contain 3,000 ppm or more of the doping element M, and the particles of the lithium cobalt-based oxide are from the center of the particle to the surface. The doping element M has a constant concentration in the bulk part corresponding to 90% of the diameter of the center side, and the doping element M is contained in a concentration higher than or equal to the bulk part in the surface part from the surface of the particle to 100 nm in the center direction. It provides a positive electrode active material for a secondary battery having a concentration gradient gradually decreasing from the surface to the center direction. [16] [17] In addition, the present invention comprises the steps of preparing a Co 3 O 4 or CoOOH precursor doped with a doping element M of 1,000 ppm or more ; Mixing the doped Co 3 O 4 or CoOOH precursor and a lithium raw material and performing a first heat treatment to prepare a lithium cobalt-based oxide containing a doping element M; And mixing the lithium cobalt-based oxide and the raw material of the doping element M and performing secondary heat treatment to prepare a lithium cobalt oxide surface-doped with the doping element M. do. [18] [19] In addition, the present invention provides a positive electrode and a lithium secondary battery including the positive electrode active material. [20] Effects of the Invention [21] The positive electrode active material for a secondary battery according to the present invention has excellent structural stability, in particular, can prevent structural changes even under a high voltage of 4.5V or higher, and effectively improve surface stability to improve lifespan characteristics and secure stability at high temperature/high voltage. . [22] Brief description of the drawing [23] 1 is a diagram schematically illustrating a concentration gradient of a doping element M according to a semi-diameter of a positive electrode active material according to an embodiment of the present invention. [24] 2 and 3 are graphs showing the ratio of the doping element M/Co by XPS analysis of the positive electrode active materials prepared according to Examples 1 and 2, respectively. [25] 4 is a graph measuring Co elution degree of positive active materials prepared according to Examples and Comparative Examples. [26] 5 is a graph for evaluating the life characteristics of a secondary battery cell manufactured using a positive electrode active material manufactured according to Examples and Comparative Examples. [27] Mode for carrying out the invention [28] Hereinafter, the present invention will be described in more detail to aid understanding of the present invention. At this time, the terms or words used in the specification and claims should not be interpreted as being limited to a conventional or dictionary meaning, and the inventor appropriately defines the concept of the term 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. [29] [30] The positive electrode active material for a secondary battery of the present invention comprises the steps of preparing a Co 3 O 4 or CoOOH precursor doped with 1,000 ppm or more of a doping element M ; Mixing the doped Co 3 O 4 or CoOOH precursor and a lithium raw material and performing a first heat treatment to prepare a lithium cobalt-based oxide containing a doping element M; And mixing the lithium cobalt-based oxide and the raw material of the doping element M and performing secondary heat treatment to prepare a lithium cobalt oxide surface-doped with the doping element M. [31] The positive electrode active material of the present invention prepared as described above is a lithium cobalt-based oxide containing a doping element M, the lithium cobalt-based oxide particles contain 3,000 ppm or more of the doping element M, and the lithium cobalt-based oxide particles, The doping element M has a constant concentration in the bulk part corresponding to 90% of the half diameter from the center of the particle to the surface, and the doping element M is the bulk part at the surface part from the surface of the particle to 100 nm in the center direction. It is contained in a higher or equal concentration, and has a concentration gradient gradually decreasing from the surface toward the center. [32] The positive electrode active material of the present invention can prevent structural change of lithium cobalt-based oxide by containing a high content of doping element at a uniform concentration in the particle bulk part, and to have a concentration gradient with a higher content of the doping element on the surface of the particle. It contains, and can effectively improve the surface stability. [33] [34] The method of manufacturing the positive electrode active material for a lithium secondary battery of the present invention will be described in detail step by step below. [35] [36] [37] The cathode active material for a secondary battery of the present invention is prepared using a precursor doped with a high content of 1,000 ppm or more of a doping element M. [38] The precursor doped with 1,000 ppm or more of the doping element M may be prepared by doping the precursor by co-precipitation reaction with the raw material of the doping element M when forming the precursor. In the precursor co-precipitation step, the precursor doping by adding the raw material of the doping element M together, the doping element M can be doped at a uniform concentration in the precursor, and the precursor can be doped with a high content. [39] [40] For the precursor doping, first, a precursor formation solution including a cobalt-containing starting material and a source material of the doping element M is prepared. [41] As the cobalt-containing starting material, a sulfate, halide, acetate, sulfide, hydroxide, oxide or oxyhydroxide containing cobalt may be used, and it is not particularly limited as long as it is soluble in water. For example, the cobalt-containing starting material is Co(SO 4 ) 2 ㆍ7H 2 O, CoCl 2 , Co(OH) 2 , Co(OCOCH 3 ) 2 ㆍ4H 2 O or Co(NO 3 ) 2 ㆍ6H 2 O and the like may be mentioned, and any one or a mixture of two or more of them may be used. [42] The raw material of the doping element M may include sulfate, nitrate, acetate, halide, hydroxide, or oxyhydroxide containing the doping element M, and any one or a mixture of two or more of them may be used. The doping element M may be at least one or more selected from the group consisting of Al, Ti, Zr, Mg, Nb, Ba, Ca, and Ta, and more preferably, the doping element M may be Al, Ti, or Mg. [43] The precursor formation solution may be prepared by adding the cobalt-containing starting material and the raw material of the doping element M to a solvent, specifically water, or a mixture of water and an organic solvent (specifically, alcohol, etc.) that can be uniformly mixed with water. Alternatively, a solution containing a starting material containing cobalt and a solution containing a raw material of the doping element M may be prepared and then mixed and used. [44] The raw material of the doping element M may be added in an amount of 0.1 to 1.0% by weight based on the total content of the starting material containing cobalt and the raw material of the doping element M, more preferably 0.1 to 0.5% by weight, most preferably 0.2 to 0.35% by weight can be added. [45] [46] Next, the precursor-forming solution may be subjected to a co-precipitation reaction, and a Co 3 O 4 or CoOOH precursor doped with 1,000 ppm or more of a doping element M may be formed. [47] Specifically, the precursor-forming solution may be added to a reactor, and a chelating agent and a basic aqueous solution may be added to prepare a Co 3 O 4 or CoOOH precursor doped with a doping element M through a co-precipitation reaction . [48] The chelating agent may include NH 4 OH, (NH 4 ) 2 SO 4 , NH 4 NO 3 , NH 4 Cl, CH 3 COONH 4 , or NH 4 CO 3 , and one of them alone or two Mixtures of the above can be used. In addition, the chelating 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 the solvent. [49] The basic compound may be a hydroxide of an alkali metal or alkaline earth metal such as NaOH, KOH, or Ca(OH) 2 , or a hydrate thereof, and one of them alone or a mixture of two or more may be used. 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. At this time, the concentration of the basic aqueous solution may be 2M to 10M. [50] The co-precipitation reaction for preparing the positive electrode active material precursor may be performed under a pH of 10 to 12. When the pH is out of the above range, there is a concern that the size of the positive electrode active material precursor to be prepared may be changed or particles may be split. More specifically, it may be carried out under the conditions of pH 11 to pH 12. The pH adjustment as described above can be controlled through the addition of a basic aqueous solution. [51] [52] The coprecipitation reaction for preparing the positive electrode active material precursor may be performed in a temperature range of 30° C. to 80° C. in an inert atmosphere such as nitrogen. In order to increase the reaction speed during the reaction, a stirring process may be selectively performed, and the stirring speed may be 100 rpm to 2000 rpm. [53] [54] As a result of the co-precipitation reaction, Co 3 O 4 or CoOOH precursor doped with the doping element M is precipitated. The content of the doping element M doped in the precursor may be 1,000 ppm or more, more preferably 3,000 to 6,000 ppm. By doping the precursor as described above, the doping element M can be doped in a high content. In addition, the prepared precursor may be uniformly doped with the doping element M without a concentration gradient from the center of the cathode active material precursor particle to the surface. [55] For the precipitated Co 3 O 4 or CoOOH precursor, after separation according to a conventional method, a drying process may be selectively performed, and the drying process may be performed at 110°C to 400°C for 15 to 30 hours. [56] [57] [58] Next, the Co 3 O 4 or CoOOH precursor doped with 1,000 ppm or more of the doping element M and a lithium raw material are mixed and subjected to primary heat treatment to prepare lithium cobalt oxide. [59] [60] 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 or the like, any one or a mixture of two or more of them may be used. [61] In addition, the amount of the lithium raw material used may be determined according to the content of lithium in the finally produced lithium cobalt-based oxide and metal elements (Co, etc.) excluding lithium, and specifically, the final produced lithium cobalt-based oxide, It may be used in an amount such that the molar ratio of lithium and the metal elements excluding lithium (the molar ratio of lithium/metal elements) is 0.98 to 1.1. [62] [63] Meanwhile, when the precursor and lithium raw material are mixed, a sintering agent may be optionally further added. Specifically, the sintering agent is a compound containing ammonium ions such as NH 4 F, NH 4 NO 3 , or (NH 4 ) 2 SO 4 ; Metal oxides such as B 2 O 3 or Bi 2 O 3 ; Or a metal halide such as NiCl 2 or CaCl 2, and any one or a mixture of two or more of them may be used. The sintering agent may be used in an amount of 0.01 to 0.2 moles per 1 mole of the precursor. If the content of the sintering agent is too low, such as less than 0.01 mol, the effect of improving the sintering properties of the positive electrode active material precursor may be insignificant, and if the content of the sintering agent is too high, exceeding 0.2 mol, the performance as a positive electrode active material decreases due to an excessive amount of sintering agent. And there is a concern that the initial capacity of the battery may decrease during charging and discharging. [64] [65] In addition, when mixing the precursor and the lithium raw material, a moisture remover may be optionally further added. Specifically, the moisture removing agent may include citric acid, tartaric acid, glycolic acid or maleic acid, and any one or a mixture of two or more of them may be used. The moisture remover may be used in an amount of 0.01 to 0.2 moles per 1 mole of the precursor. [66] [67] The first heat treatment may be performed at 900°C to 1,100°C, and more preferably at 1,000 to 1,050°C. If the primary heat treatment temperature is less than 900°C, there is a risk of a decrease in discharge capacity per unit weight, a decrease in cycle characteristics, and a decrease in operating voltage due to the remaining unreacted raw materials. If it exceeds 1,100°C, the capacity decreases due to an increase in particle size. And deterioration of rate characteristics may occur. [68] The first heat treatment may be performed for 5 to 30 hours in an oxidizing atmosphere such as air or oxygen. [69] [70] In the lithium cobalt-based oxide prepared by using a precursor-doped precursor as described above, mixing it with a lithium raw material, and then first heat treatment, the precursor-doped doping element M has a constant concentration in the particles of the lithium cobalt-based oxide. I can. [71] [72] [73] Next, the lithium cobalt-based oxide and the raw material of the doping element M are mixed and subjected to secondary heat treatment to prepare a lithium cobalt-based oxide surface-doped with the doping element M. [74] The raw material of the doping element M may include sulfate, nitrate, acetate, halide, hydroxide, or oxyhydroxide containing the doping element M, and any one or a mixture of two or more of them may be used. The doping element M may be at least one or more selected from the group consisting of Al, Ti, Zr, Mg, Nb, Ba, Ca, and Ta, and more preferably, the doping element M may be Al, Ti, or Mg. The doping element M mixed during the secondary heat treatment refers to the same doping element as the doping element M used during precursor doping. [75] The raw material of the doping element M may be mixed in an amount of 0.05 to 0.5 parts by weight based on 100 parts by weight of lithium cobalt-based oxide, and more preferably 0.1 to 0.3 parts by weight. When the amount of the raw material of the doping element M is less than 0.05 parts by weight during the secondary heat treatment, it may be difficult to secure the surface stability due to insufficient doping element content on the surface of the positive electrode active material. There is a fear of increased resistance. [76] [77] Meanwhile, when mixing the raw material of the lithium cobalt-based oxide and the doping element M, a cobalt-containing material may be optionally further added. As the cobalt-containing material, a cobalt-containing starting material used during precursor doping may be used, for example, Co(SO 4 ) 2 ㆍ7H 2 O, CoSO 4 , CoCl 2 , Co(OH) 2 , Co(OCOCH 3 ) 2 ㆍ4H 2 O or Co(NO 3 ) 2 ㆍ6H 2 O, and the like, and any one or a mixture of two or more of them may be used. When the cobalt-containing material is added together, lithium cobalt oxide can be further formed on the surface, surface doping in which the doping element M is substituted at the cobalt position can be better formed, and a Li-deficient structure is formed on the surface. Kinetically advantageous active materials may be formed. [78] [79] The secondary heat treatment may be performed at 800°C to 950°C, more preferably at 850 to 900°C. If the secondary heat treatment temperature is less than 800°C, a coating layer may be formed on the surface that is not doped in lithium cobalt oxide, and the crystallinity is low, which is disadvantageous to life characteristics, and there is a risk of Co elution, and exceeds 950°C. When the doping element M is diffused into the interior, the doping element is not in a rich state on the surface portion, and there is a concern that surface stabilization may be deteriorated. [80] The secondary heat treatment may be performed for 3 to 15 hours in an oxidizing atmosphere such as air or oxygen. [81] [82] The lithium cobalt-based oxide prepared by secondary heat treatment by inputting the raw material of the doping element M as described above is in the bulk portion corresponding to 90% of the radius from the center of the lithium cobalt-based oxide particle to the surface. The doping element M has a constant concentration, and the doping element M is contained in a concentration higher than or equal to the bulk portion in the surface portion from the surface of the lithium cobalt-based oxide particle to the center direction to 100 nm, and from the surface to the center direction It may have a concentration gradient that gradually decreases. [83] [84] Meanwhile, a coating layer including an inorganic oxide may be further formed on the surface of the prepared lithium cobal-based oxide particle. [85] The coating layer is Mg, Ti, Fe, Cu, Ca, Ba, Sn, Sb, Na, Z, Si, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Sc, Ce, Pr, Nd , Gd, Dy, Yb, Er, Co, Al, Ga, and may contain at least one oxide selected from the group consisting of B, mixing a coating material containing an element forming the coating layer, and tertiary heat treatment Thus, a coating layer can be formed. In this case, the third heat treatment temperature when forming the coating layer may be about 300 to 600°C. [86] [87] The positive electrode active material for a lithium secondary battery of the present invention prepared as described above will be described in detail below. [88] [89] [90] The positive electrode active material according to an embodiment of the present invention prepared as described above may be a lithium cobalt-based oxide represented by Formula 1 below. [91] [Formula 1] [92] Li a Co (1-x) M x O 2 [93] In Formula 1, 0.95≤a≤1.05, 0 [112] According to another embodiment of the present invention, a positive electrode for a lithium secondary battery and a lithium secondary battery including the positive electrode active material are provided. [113] [114] 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. [115] 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. [116] [117] 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. [118] In this case, the conductive material is used to impart conductivity to the electrode, and in the battery to be configured, it may 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 powder 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. [119] [120] In addition, the binder serves to improve adhesion between the 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. [121] [122] 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, the positive electrode active material and optionally, a composition for forming a positive electrode active material layer including a binder, a conductive material, and a solvent may be coated on a positive electrode current collector, followed by drying and rolling. At this time, the types and contents of the positive electrode active material, binder, and conductive material are as described above. [123] 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 determined by dissolving or dispersing the positive electrode active material, the conductive material, and the binder in consideration of the coating thickness of the composition for forming the positive electrode active material layer and the production yield, and then exhibiting excellent thickness uniformity when applying for the positive electrode production. It is enough to have it. [124] [125] 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. [126] [127] According to another embodiment of the present invention, an electrochemical device including the anode is provided. The electrochemical device may be specifically a battery or a capacitor, and more specifically, a lithium secondary battery. [128] [129] Specifically, the lithium secondary battery 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. [130] [131] In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. [132] The negative electrode current collector is not particularly limited as long as it has high conductivity without causing a chemical change in 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 nonwoven fabrics. [133] [134] The negative active material layer optionally includes a binder, a conductive material, and a solvent together with the negative active material. The negative 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 the negative electrode forming composition is cast 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. [135] [136] 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; Metal oxides capable of doping and undoping lithium such as SiOx (0