Positive electrode active material for secondary battery, method for producing the same, and lithium secondary battery comprising the same

Abstract

The present invention relates to a positive electrode active material for a secondary battery, the positive electrode active material being a lithium composite transition metal oxide containing nickel (Ni), cobalt (Co), and manganese (Mn), composed of single particles, and having a crystal grain size of 180 nm or more, wherein the lithium composite transition metal oxide contains nickel (Ni) in an amount of 65 mol% or more and manganese (Mn) in an amount of 5 mol% or more, based on the total amount of transition metals.

Description

TECHNICAL FIELD

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Korean Patent Application No. 10-2018-0056751, filed May 17, 2018, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. TECHNICAL FIELD

[0004] The present application relates to a positive active material for a secondary battery, a method of preparing the same, and a lithium secondary battery including the same. BACKGROUND

[0005] Recently, as electronic devices using batteries such as mobile phones, notebook computers, and electric vehicles have rapidly spread, the demand for secondary batteries having relatively high capacity and small size and light weight has rapidly increased. In particular, since lithium secondary batteries are light in weight and have high energy density, the lithium secondary batteries have attracted attention as a driving power source for portable devices. Therefore, research and development efforts to improve the performance of lithium secondary batteries have been actively conducted.

[0006] In a lithium secondary battery in a state in which an organic electrolyte solution or a polymer electrolyte solution is filled between a positive electrode and a negative electrode each of which is formed of an active material allowing lithium ions to be intercalated and deintercalated, electric energy is generated through oxidation and reduction reactions when lithium ions are intercalated into and deintercalated from the positive electrode and the negative electrode.

[0007] Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2or LiMn2O4, etc.), or lithium iron phosphate compound (LiFePO4) has been used as a positive active material for a lithium secondary battery. Among these materials, since lithium cobalt oxide (LiCoO2) is advantageous in that it has a high operating voltage and excellent capacity characteristics, lithium cobalt oxide (LiCoO2) is widely used and has been used as a positive active material for a high voltage. However, due to the price increase and unstable supply of cobalt (Co), the use of a large amount of LiCoO2as a power source for various applications such as electric vehicles is limited, and thus there is a need to develop a positive active material capable of replacing LiCoO2.

[0008] 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 portion of cobalt (Co) is substituted with nickel (Ni) and manganese (Mn) has been developed. However, since the conventional NCM-based lithium composite transition metal oxide is generally in the form of secondary particles in which primary particles are aggregated, it has a large specific surface area, a low particle strength, and a large amount of lithium by-products, and thus, it is limited in that a large amount of gas is generated during battery operation and has poor stability. In particular, for a high-Ni NCM-based lithium composite transition metal oxide in which the amount of nickel (Ni) is increased to 65 mol% or more to secure a high capacity, the structural and chemical stability is further reduced, and it is more difficult to secure thermal stability. Accordingly, there is still a need to develop a positive electrode active material that secures the stability of a high-Ni NCM-based lithium composite transition metal oxide capable of achieving a high capacity. SUMMARY

[0009] TECHNICAL PROBLEM

[0010] One aspect of the present application provides a high-Ni NCM-based lithium composite transition metal oxide positive electrode active material having improved stability, which contains nickel (Ni) in an amount of 65 mol% or more to achieve a high capacity. Specifically, the present application aims to provide a high-Ni NCM-based lithium composite transition metal oxide positive electrode active material in which particle breakage during a roll-pressing process is suppressed by reducing the specific surface area and increasing the particle strength, and side reactions with an electrolyte solution are reduced by reducing the amount of lithium by-products. In addition, the present application aims to provide a high-Ni NCM-based lithium composite transition metal oxide positive electrode active material that can reduce the amount of gas generated during battery operation and secure thermal stability.

[0011] Another aspect of the present application provides a simple method of preparing a single particle of a high-Ni NCM-based positive electrode active material having a high manganese (Mn) content and a low cobalt (Co) content.

[0012] TECHNICAL SOLUTION

[0013] According to one aspect of the present application, there is provided a positive electrode active material for a secondary battery, the positive electrode active material being a lithium composite transition metal oxide containing nickel (Ni), cobalt (Co), and manganese (Mn), consisting of single particles, and having a grain size of 180 nm or more, wherein the lithium composite transition metal oxide contains nickel (Ni) in an amount of 65 mol% or more and manganese (Mn) in an amount of 5 mol% or more, based on the total amount of transition metals.

[0014] According to another aspect of the present application, there is provided a method of preparing a positive electrode active material for a secondary battery, the method including: preparing a precursor including nickel (Ni), cobalt (Co), and manganese (Mn), wherein the amount of nickel (Ni) is 65 mol% or more and the amount of manganese (Mn) is 5 mol% or more, based on the total amount of transition metals; pre-sintering the precursor at 600°C to 800°C; and mixing the pre-sintered precursor with a lithium raw material and performing secondary sintering on the mixture at a temperature of 850°C or more, thereby forming a lithium composite transition metal oxide composed of single particles.

[0015] According to another aspect of the present application, there is provided a positive electrode including the positive electrode active material and a lithium secondary battery.

[0016] Advantageous Effects

[0017] According to the present application, by reducing the specific surface area of a high-Ni NCM-based positive electrode active material, increasing the particle strength, and reducing the amount of lithium side reactions, side reactions with an electrolyte solution can be reduced. Accordingly, for a lithium secondary battery using a high-Ni NCM-based positive electrode active material of the present application, the amount of gas generated during battery operation can be reduced, and thermal stability can be ensured.

[0018] In addition, according to the present application, single particles of a high-Ni NCM-based positive electrode active material having a high manganese (Mn) content and a low cobalt (Co) content can be easily prepared. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figures 1 to 4 is a magnified scanning electron microscope (SEM) image of the positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 and 2.

[0020] Figure 5 is a graph of heat flow versus temperature of the positive electrode active materials of Examples 1 to 3 and Comparative Examples 1 and 2 measured using a differential scanning calorimeter (Sensys evo DSC, SETARAM Instrumentation).

[0021] Figures 6 to 9 is a graph of the leakage current of a lithium secondary battery (full cell) using a positive electrode prepared according to Examples 1 and 2 and Comparative Examples 1 and 2. DETAILED DESCRIPTION

[0022] Hereinafter, the present application will be described in more detail so that the present application can be more clearly understood. In this case, it should be understood that the words or terms used in the specification and claims should not be interpreted as meanings defined in commonly used dictionaries, and it should be further understood that the meanings of the words or terms should be interpreted based on a concept of the inventor so that the present application can be best explained in accordance with the principle of the present application, in the background of the related art and the technical idea of the present application, the words or terms should be interpreted as having meanings consistent with their meanings.

[0023] <Positive electrode active material>

[0024] The positive electrode active material for a secondary battery of the present application is a lithium complex transition metal oxide containing nickel (Ni), cobalt (Co), and manganese (Mn), composed of single particles, and having a grain size of 180 nm or more, wherein the amount of nickel (Ni) contained in the lithium complex transition metal oxide is 65 mol% or more, and the amount of manganese (Mn) is 5 mol% or more, based on the total amount of transition metals.

[0025] The lithium complex transition metal oxide of one embodiment of the present application can be a high-Ni NCM-based lithium complex transition metal oxide containing nickel (Ni), cobalt (Co), and manganese (Mn), wherein the amount of nickel (Ni) in the total amount of transition metals satisfies 65 mol% or more, preferably 70 mol% to less than 95 mol%, more preferably 80 mol% to 93 mol%.

[0026] In addition, the amount of manganese (Mn) in the total amount of transition metals in the lithium complex transition metal oxide can satisfy 5 mol% or more, preferably 5 mol% to 10 mol%, more preferably 5 mol% to 8 mol%.

[0027] Further, the concentration of manganese (Mn) in the particles of the lithium complex transition metal oxide can be uniform. Since the concentration of manganese (Mn) is uniformly distributed in the particles, the thermal stability of the positive electrode active material can be excellent. In the present application, the expression "the concentration of manganese (Mn) in the particles is uniform" is interpreted as meaning including the case where the difference in the concentration of manganese (Mn) throughout the particles is 1.5 mol% or less, and the case where the concentration of manganese (Mn) is the same throughout the particles.

[0028] Generally, manganese (Mn) is not favorable to form a layered structure, and functions to inhibit particle growth due to its high oxidation value, but cobalt (Co) is an element favorable to form a layered structure, and has a function to promote particle growth of an NCM-based lithium complex transition metal oxide. In other words, when the amount of manganese (Mn) increases, crystallite growth during sintering of a lithium complex transition metal oxide becomes difficult, and since cobalt (Co) promotes crystallite growth, it is difficult to form a single particle when the amount of cobalt (Co) decreases. However, according to the present application, a single particle of a cathode active material can be easily formed by pre-sintering a precursor under specific conditions even though the above-mentioned lithium complex transition metal oxide includes 65 mol% or more of nickel (Ni) and 5 mol% or more of manganese (Mn) in its composition. The preparation method of the present application will be described in detail below.

[0029] As described above, the high-Ni NCM-based cathode active material of the present application, which is a single particle and includes 65 mol% or more of nickel (Ni) and 5 mol% or more of manganese (Mn) in its composition, can ensure excellent stability while achieving high capacity. Specifically, particle breakage during roll-pressing can be inhibited by reducing the specific surface area and increasing the particle strength, and the amount of gas generated by side reactions with an electrolyte solution can be reduced by reducing the amount of lithium by-products. In addition, since the high-Ni NCM-based cathode active material of the present application can ensure excellent structural stability and chemical stability, the amount of gas generated during battery operation can be reduced, and thermal stability can be ensured.

[0030] The cathode active material of the present application is composed of single particles, which are not in the form of aggregated secondary particles, i.e., primary particles. In the present application, the expression "primary particle" means the primary structure of a single particle, and the expression "secondary particle" means an aggregate, i.e., the secondary structure, in which primary particles are aggregated by physical or chemical bonding between the primary particles, without intentionally performing the aggregation or combination process of the primary particles constituting the secondary particles.

[0031] The primary particles can have an average particle diameter (D 50 ) of 2 μm to 10 μm. The primary particles can more preferably have an average particle diameter (D 50 ) of 3 μm to 7 μm, and can most preferably have an average particle diameter (D 50 ) of 6 μm to 7 μm. For a cathode active material composed of single particles having the above-mentioned average particle diameter (D 50 ) of the primary particles, the particle strength can be increased to inhibit particle breakage during roll-pressing and increase the roll-pressing density, and since the specific surface area and lithium by-products are reduced, the amount of gas generated by side reactions with an electrolyte solution can be reduced.

[0032] In the present application, the average particle diameter (D 50) can be defined as a particle diameter at which the cumulative volume is 50% in a particle size distribution curve. For example, the average particle diameter (D 50 ) can be measured by using a laser diffraction method. For example, in a method of measuring the average particle diameter (D 50 ) of the positive electrode active material, after the particles of the positive electrode active material are dispersed in a dispersion medium, the dispersion medium is introduced into a commercially available laser diffraction particle size measuring instrument (for example, Microtrac MT 3000) and irradiated with ultrasonic waves having a frequency of about 28 kHz and an output of 60 W, and then the average particle diameter (D 50 ) at which the cumulative volume is 50% is calculated by the measuring instrument.

[0033] The positive electrode active material can have a grain size of 180 nm or more, can preferably have a grain size of 200 nm or more, and can more preferably have a grain size of 220 nm. The positive electrode active material of the embodiments of the present application, which satisfies the above grain size, can suppress particle breakage during roll-pressing, and can improve the life characteristics and stability.

[0034] In the present application, the expression "particle" means a particle body having a micrometer size, and when the particle is observed under magnification, the particle can be recognized as a "fine particle" having a crystal form of several tens of nanometer size. When the fine particle is further magnified, a divided region having a form in which atoms form a lattice structure in a predetermined direction can be recognized, and the region is referred to as a "grain", and the size of the particle observed by X-ray diffraction (XRD) is defined as the size of the grain. As for the method of measuring the grain size, the grain size can be determined by peak broadening of XRD data, and can also be quantitatively calculated according to the Scherrer equation.

[0035] The molar ratio of lithium (Li) to the total metal element (M) other than lithium (Li / M) of the lithium composite transition metal oxide of the embodiments of the present application can be 0.98 to 1.05, preferably 1.00 to 1.04, and more preferably 1.02 to 1.03. The single particle of the positive electrode active material can be easily formed by pre-sintering a precursor under specific conditions during the preparation of the positive electrode active material, and in addition, controlling the amount of added lithium raw material to satisfy Li / M of the embodiments of the present application, even though the composition of the lithium composite transition metal oxide includes 65 mol% or more of nickel (Ni) and 5 mol% or more of manganese (Mn).

[0036] In addition, the lithium composite transition metal oxide of the embodiment of the present application can further include at least one particle growth promoting element selected from the group consisting of strontium (Sr), zirconium (Zr), magnesium (Mg), yttrium (Y), and aluminum (Al), and more preferably, can further include the particle growth promoting elements of Sr and / or Zr. The amount of the particle growth promoting elements included can be 500 ppm to 2,000 ppm, preferably 800 ppm to 1,800 ppm, and more preferably 1,000 ppm to 1,500 ppm, based on the total weight of the positive electrode active material. Since the amount of the particle growth promoting elements included is within the above range, the single particle of the positive electrode active material can be easily formed even though the lithium composite transition metal oxide includes 65 mol% or more of nickel (Ni) and 5 mol% or more of manganese (Mn) in the composition thereof. Generally, manganese (Mn) is not favorable to the formation of a layered structure and functions to inhibit the particle growth due to its high oxidation value, but cobalt (Co) is an element favorable to the formation of a layered structure and has a function of promoting the particle growth of the NCM-based lithium composite transition metal oxide. However, according to one embodiment of the present application, since the surface energy of the positive electrode active material can be increased by doping the particle growth promoting elements, continuous particle growth can be achieved.

[0037] In addition, the positive electrode active material can contain a chlorine (Cl) impurity in an amount of 20 ppm or less, preferably 10 ppm or less, and more preferably 5 ppm or less. In the case where the positive electrode active material contains a Cl impurity, the aluminum (Al) positive electrode current collector can be eroded, but since a Cl-containing raw material is not used during the preparation of the positive electrode active material according to the embodiment of the present application, the amount of the Cl impurity included can be 20 ppm or less.

[0038] Specifically, the lithium composite transition metal oxide of the embodiment of the present application can be represented by the following Formula 1.

[0039] [Formula 1]

[0040] Li 1+p [Ni 1-(x1+y1+z1) Co x1 Mn y1 M a z1 ] 1-p O2

[0041] In Formula 1, M a is at least one element selected from the group consisting of Sr, Zr, Mg, Y, and Al, and -0.02 ≤ p ≤ 0.05, 0 < x1 ≤ 0.4, 0.05 ≤ y1 ≤ 0.4, 0 ≤ z1 ≤ 0.1, and 0.05 < x1 + y1 + z1 ≤ 0.35.

[0042] In the lithium composite transition metal oxide of Formula 1, the amount of Li contained can correspond to 1 + p, i.e., 0.98 ≤ 1 + p ≤ 1.05. When 1 + p is less than 0.98, the capacity can decrease, and when 1 + p is greater than 1.05, the resistance can increase due to a large amount of Li remaining on the surface of the positive electrode active material. In consideration of the capacity characteristic improvement effect of the positive electrode active material due to the control of the amount of Li and the effect of forming single particles during the preparation of the active material, the amount of Li contained can more preferably satisfy 1.0 ≤ 1 + p ≤ 1.04.

[0043] In the lithium composite transition metal oxide of Formula 1, the amount of Ni contained can correspond to 1 - (x1 + y1 + z1), for example, 0.65 ≤ 1 - (x1 + y1 + z1) < 0.95. The amount of Ni contained can more preferably satisfy 0.70 ≤ 1 - (x1 + y1 + z1) < 0.95.

[0044] In the lithium composite transition metal oxide of Formula 1, the amount of Co contained can correspond to x1, i.e., 0 < x1 ≤ 0.4. In the case where the amount of Co in the lithium composite transition metal oxide of Formula 1 is greater than 0.4, there can be a problem in that the cost can increase. In consideration of the significant capacity characteristic improvement effect due to the inclusion of Co, the amount of Co contained can particularly satisfy 0.1 ≤ x1 ≤ 0.3.

[0045] In the lithium composite transition metal oxide of Formula 1, the amount of Mn contained corresponds to y1, i.e., 0.05 ≤ y1 ≤ 0.4. Mn can improve the stability of the positive electrode active material, and thus can improve the stability of the battery. The amount of Mn contained can particularly satisfy 0.05 ≤ y1 ≤ 0.2.

[0046] In the lithium composite transition metal oxide of Formula 1, M a may be a doping element contained in the crystal structure of the lithium composite transition metal oxide, wherein the amount of M a may correspond to z1, i.e., 0 ≤ z1 ≤ 0.1.

[0047] The positive electrode active material of the embodiments of the present application can contain the amount of residual lithium by-products of 0.5% by weight or less, preferably 0.48% by weight or less, and more preferably 0.46% by weight or less, based on the total weight of the positive electrode active material.

[0048] Due to the satisfaction of the specific surface area and / or the amount of lithium by-products described above, the side reaction with the electrolyte solution can be reduced, and the amount of gas generated during the operation of the battery can be reduced.

[0049] When the positive electrode active material is analyzed by differential scanning calorimetry (DSC) thermal analysis, a main peak having a maximum heat flow can be measured at 235°C or higher, preferably 235°C to 240°C, more preferably 236°C to 238°C. Thus, it can be confirmed that the positive electrode active material of the embodiments of the present application ensures excellent thermal stability.

[0050] Further, for the positive electrode active material of the embodiments of the present application, a coating portion can be formed on the surface of the particles of the lithium composite transition metal oxide, which comprises at least one selected from the group consisting of Al, boron (B), Zr, titanium (Ti), Mg, tantalum (Ta), niobium (Nb), molybdenum (Mo), and chromium (Cr). The coating portion can preferably include Al, B, and / or Nb, and can more preferably include Al. Due to the further formation of the coating portion, lithium by-products on the surface of the particles can be further reduced, and the amount of gas generated during battery operation can be further reduced.

[0051] <Method of preparing a positive electrode active material>

[0052] Next, a method of preparing the positive electrode active material of the present application will be described.

[0053] The positive electrode active material of the present application is prepared by a process comprising: preparing a precursor comprising nickel (Ni), cobalt (Co), and manganese (Mn), wherein the amount of nickel (Ni) is 65 mol% or more and the amount of manganese (Mn) is 5 mol% or more, based on the total amount of transition metals; pre-sintering the precursor at 600°C to 800°C; and mixing the pre-sintered precursor with a lithium raw material and performing secondary sintering on the mixture at a temperature of 850°C or higher, thereby forming a lithium composite transition metal oxide composed of single particles.

[0054] The method of preparing the positive electrode active material will be described in detail for each step.

[0055] First, a precursor comprising nickel (Ni), cobalt (Co), and manganese (Mn) is prepared, wherein the amount of nickel (Ni) is 65 mol% or more and the amount of manganese (Mn) is 5 mol% or more, based on the total amount of transition metals.

[0056] The positive electrode active material precursor can be used by purchasing a commercially available positive electrode active material precursor, or can be prepared according to a method of preparing a positive electrode active material precursor well known in the art.

[0057] For example, the precursor can be prepared by a coprecipitation reaction by adding an ammonium cation complexing agent and an alkaline compound to a transition metal solution comprising a nickel-containing raw material, a cobalt-containing raw material, and a manganese-containing raw material.

[0058] For example, the nickel-containing raw material can include a nickel-containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, or oxyhydroxide, and can specifically include Ni(OH)2, NiO, NiOOH, NiCO3·2Ni(OH)2·4H2O, NiC2O2·2H2O, Ni(NO3)2·6H2O, NiSO4, NiSO4·6H2O, a nickel fatty acid salt, a nickel halide, or a combination thereof, but the present application is not limited thereto.

[0059] The cobalt-containing raw material can include a cobalt-containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, or oxyhydroxide, and can specifically include Co(OH)2, CoOOH, Co(OCOCH3)2·4H2O, Co(NO3)2·6H2O, Co(SO4)2, Co(SO4)2·7H2O, or a combination thereof, but the present application is not limited thereto.

[0060] For example, the manganese-containing raw material can include a manganese-containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, oxyhydroxide, or a combination thereof, and can specifically include a manganese oxide such as Mn2O3, MnO2, and Mn3O4, a manganese salt such as MnCO3, Mn(NO3)2, MnSO4, a manganese acetate, a manganese dicarboxylate, a manganese citrate, and a manganese fatty acid salt, a manganese oxyhydroxide, a manganese chloride, or a combination thereof, but the present application is not limited thereto.

[0061] The transition metal solution can be prepared by adding the nickel-containing raw material, the cobalt-containing raw material, and the manganese-containing raw material to a solvent, particularly water or a mixture of water and an organic solvent (e.g., an alcohol or the like) that can be uniformly mixed with water, or can be prepared by mixing an aqueous solution of the nickel-containing raw material, an aqueous solution of the cobalt-containing raw material, and the manganese-containing raw material.

[0062] For example, the ammonium cation-containing complexing agent can include NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4, NH4CO3, or a combination thereof, but the present application is not limited thereto. The ammonium cation-containing complexing agent can be used in the form of an aqueous solution, in which case water or a mixture of water and an organic solvent (e.g., an alcohol or the like) that can be uniformly mixed with water can be used as the solvent.

[0063] The basic compound can include a hydroxide of an alkali metal or an alkaline earth metal such as NaOH, KOH, or Ca(OH)2, or a hydrate thereof, or a combination thereof. The basic compound can be used in the form of an aqueous solution, in which case water or a mixture of water and an organic solvent (e.g., an alcohol or the like) that can be uniformly mixed with water can be used as the solvent.

[0064] The basic compound is added to adjust the pH of the reaction solution, and the amount of the added basic compound can be such that the pH of the metal solution is 11 to 13.

[0065] The co-precipitation reaction can be performed in an inert atmosphere such as nitrogen or argon at a temperature range of 40°C to 70°C.

[0066] The particles of the nickel cobalt manganese hydroxide are formed by the above process and precipitated in the reaction solution. The concentrations of the nickel-containing raw material, the cobalt-containing raw material, and the manganese-containing raw material can be adjusted to prepare a precursor in which the amount of nickel (Ni) is 65 mol% or more and the amount of manganese (Mn) is 5 mol% or more, based on the total amount of transition metals. The precipitated nickel cobalt manganese hydroxide particles can be separated according to a conventional method and dried, thereby preparing a nickel cobalt manganese precursor. The precursor can be secondary particles formed by aggregation of primary particles, and the average particle diameter (D 50 ) of the precursor secondary particles can be 3 μm to 6 μm, preferably 3 μm to 5 μm, and more preferably 3 μm to 4 μm.

[0067] Next, the precursor is pre-sintered at 600°C to 800°C. The precursor can be more preferably pre-sintered at 630°C to 780°C, for example, pre-sintered at 650°C to 750°C. Since the precursor is pre-sintered at the above temperature range, gas in the secondary particles can be removed, and the binding force between metal ions and oxygen can be ensured. Since the stable binding force between metal ions and oxygen is ensured, thermal energy can be used for particle growth during subsequent sintering. Thus, since the precursor is pre-sintered, single particles are easily formed during secondary sintering after mixing with a lithium raw material. In the case where the pre-sintering temperature is lower than 600°C, the metal ions and oxygen can not be sufficiently bound to each other, and in the case where the pre-sintering temperature is higher than 800°C separation of the metal oxide layer can occur.

[0068] The pre-sintering can be performed for 4 hours to 8 hours, and can be more preferably performed for 5 hours to 7 hours. The pre-sintering can be performed in an air atmosphere or an oxygen atmosphere.

[0069] Next, the pre-sintered precursor is mixed with a lithium raw material, and the mixture is subjected to secondary sintering at a temperature of 850°C or more, thereby forming a lithium composite transition metal oxide composed of single particles. By subjecting the pre-sintered precursor to secondary sintering at a temperature of 850°C or more with a lithium raw material, single particles can be formed, specifically, single particles in which the average particle diameter (D 50 ) of the primary particles is 2 μm to 10 μm can be formed.

[0070] As the lithium source, a lithium-containing sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, hydroxide, or oxyhydroxide can be used, and these materials are not particularly limited as long as they are soluble in water. Specifically, the lithium source can include Li2CO3, LiNO3, LiNO2, LiOH, LiOH-H2O, LiH, LiF, LiCl, LiBr, LiI, CH3COOLi, Li2O, Li2SO4, CH3COOLi, or Li3C6H5O7, and any one of these or a mixture of two or more of these can be used.

[0071] The lithium source can be mixed such that the molar ratio of lithium (Li) to the total metal elements (M) other than lithium of the lithium complex transition metal oxide (Li / M) is in the range of 0.98 to 1.05. The lithium source can be mixed such that Li / M is more preferably 1.00 to 1.04, for example, 1.02 to 1.03. Since the amount of the added lithium source is adjusted such that Li / M satisfies the above range, a single crystal of the positive electrode active material can be easily formed even if the composition of the lithium complex transition metal oxide includes 65 mol% or more of nickel (Ni) and 5 mol% or more of manganese (Mn).

[0072] The secondary sintering can be performed at a temperature of 850°C or higher, preferably 880°C to 980°C, more preferably 870°C to 900°C. The secondary sintering can be performed for 6 hours to 24 hours, preferably 7 hours to 20 hours, more preferably 8 hours to 12 hours. The secondary sintering can be performed in an oxygen atmosphere or an air atmosphere, and can more preferably be performed in an oxygen atmosphere.

[0073] During the secondary sintering, the sintering can be performed after further mixing a particle growth promoter containing at least one particle growth promoting element selected from the group consisting of Sr, Zr, Mg, Y, and Al, more preferably a particle growth promoter containing the particle growth promoting elements Sr and / or Zr. The particle growth promoter can be mixed such that the amount of the contained particle growth promoting elements can be 500 ppm to 2,000 ppm, preferably 800 ppm to 1,800 ppm, more preferably 1,000 ppm to 1,500 ppm, based on the total weight of the positive electrode active material. Since the particle growth promoter in the above range is further mixed, a single particle of the positive electrode active material can be easily formed even if the composition of the lithium complex transition metal oxide includes 65 mol% or more of nickel (Ni) and 5 mol% or more of manganese (Mn).

[0074] The secondary sintering can be performed such that the average particle diameter (D 50 ) of the primary particles of the single particle of the NCM-based positive electrode active material prepared is 2 μm to 10 μm. The sintering can be more preferably performed such that the average particle diameter (D 50) is 3 μm to 7 μm, for example, 6 μm to 7 μm.

[0075] Next, to remove lithium by-products present on the surface of the lithium composite transition metal oxide, a process of washing the lithium composite transition metal oxide can be further performed.

[0076] For the lithium composite transition metal oxide containing a high concentration of nickel, since it is structurally unstable compared to the lithium composite transition metal oxide containing a low nickel content, more lithium by-products, for example, unreacted lithium hydroxide or lithium carbonate, are generated during the preparation process. In the case where a large amount of lithium by-products is present in the positive electrode active material, the lithium by-products react with the electrolyte solution to generate gas and cause a swelling phenomenon, and thus the high temperature stability is significantly reduced. Therefore, the washing process of removing lithium by-products from the lithium composite transition metal oxide containing a high concentration of nickel can be further performed.

[0077] For example, the washing process can be performed by a method of adding the lithium composite transition metal oxide in ultrapure water and stirring. In this case, the washing temperature can be 20°C or less, for example, 10°C to 20°C, and the washing time can be about 10 minutes to about 1 hour. When the washing temperature and the washing time each satisfy the above range, the lithium by-products can be effectively removed.

[0078] Next, by selectively mixing the lithium composite transition metal oxide with a coating raw material including at least one selected from the group consisting of Al, B, Zr, Ti, Mg, Ta, Nb, Mo, and Cr and performing a heat treatment, a coating portion can be formed. The coating raw material can preferably include Al, B, and / or Nb, and can more preferably include Al. In the case where the coating raw material is Al, for example, Al(OH)3, Al2O3, AlPO4, AlCl3, and Al2(SO4)3 can be used.

[0079] The heat treatment can be performed in a temperature range of 250°C to 350°C, and more preferably can be performed in a temperature range of 270°C to 300°C. The heat treatment can be performed for 4 hours to 6 hours.

[0080] Since the coating portion is further formed, the lithium by-products on the surface of the particles can be further reduced, and the amount of gas generated during the operation of the battery can be further reduced.

[0081] <Positive electrode and secondary battery>

[0082] According to another embodiment of the present application, a positive electrode for a secondary battery including the above-described positive electrode active material and a lithium secondary battery are provided.

[0083] Specifically, the positive electrode includes a positive electrode current collector, and a positive electrode active material layer disposed on the positive electrode current collector and including the positive electrode active material.

[0084] In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity and does not cause an adverse chemical change in the battery, and, for example, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, or silver can be used. In addition, the thickness of the positive electrode current collector can generally be 3 μm to 500 μm, and the adhesion of the positive electrode active material can be improved by forming minute irregularities on the surface of the current collector. For example, various shapes of positive electrode current collectors, such as films, sheets, foils, nets, porous bodies, foam bodies, and nonwoven cloth bodies, can be used.

[0085] In addition, the positive electrode active material layer can contain a conductive agent and a binder in addition to the above-described positive electrode active material.

[0086] In this case, a conductive material is used to provide conductivity to the electrode, and any conductive agent can be used without particular limitation as long as it has suitable electronic conductivity and does not cause an adverse chemical change in the battery. Specific examples of the conductive agent can be 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 crack carbon black, and carbon fibers; powders or fibers of metals such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium dioxide; or conductive polymers, such as polyphenylene derivatives, and any one of these or a mixture of two or more of these can be used. The amount of the conductive agent contained can generally be 1 to 30% by weight based on the total weight of the positive electrode active material layer.

[0087] In addition, the binder improves the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples of the binder can be polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one of these or a mixture of two or more of these can be used. The amount of the binder contained can generally be 1 to 30% by weight based on the total weight of the positive electrode active material layer.

[0088] The positive electrode can be prepared according to a conventional method of preparing a positive electrode, except that the above-described positive electrode active material is used. Specifically, a positive electrode active material layer-forming composition containing the above-described positive electrode active material, and optionally a binder and a conductive agent, is coated on a positive electrode current collector, and then the coated positive electrode current collector can be dried and rolled to prepare the positive electrode. In this case, the positive electrode active material, the binder, and the conductive agent are the same as those described above.

[0089] The solvent can be a solvent commonly used in the art. The solvent can include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methyl pyrrolidone (NMP), acetone, or water, and any one of these or a mixture of two or more of these can be used. The amount of the solvent used can be sufficient in consideration of the coating thickness of the slurry and the manufacturing yield, if the solvent can dissolve or disperse the positive electrode active material, the conductive agent, and the binder, and has a viscosity that can provide excellent thickness uniformity during the subsequent coating in the preparation of the positive electrode.

[0090] In addition, as another method, the positive electrode can be prepared by casting the positive electrode active material layer-forming composition on a separate support, and then laminating the film separated from the support on the positive electrode current collector.

[0091] According to another embodiment of the present application, an electrochemical device including the positive electrode is provided. Specifically, the electrochemical device can be a battery or a capacitor, and for example, can be a lithium secondary battery.

[0092] Specifically, the lithium secondary battery includes the positive electrode, a negative electrode disposed opposite the positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode is as described above. In addition, the lithium secondary battery can also optionally include a battery container that houses the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0093] 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.

[0094] The negative electrode current collector can have no particular limitation as long as it has high conductivity and does not cause an adverse chemical change in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or copper or stainless steel surface-treated with one of carbon, nickel, titanium, or silver, and an aluminum cadmium alloy can be used. In addition, the thickness of the negative electrode current collector can generally be 3 μm to 500 μm, and similar to the positive electrode current collector, the adhesion of the negative electrode active material can be improved by forming minute irregularities on the surface of the current collector. For example, various shapes of the negative electrode current collector, such as a film, a sheet, a foil, a mesh, a porous body, a foam body, and a nonwoven fabric body, etc. can be used.

[0095] In addition to the negative electrode active material, the negative electrode active material layer selectively includes a binder and a conductive agent. The negative electrode active material layer can be prepared by: coating a negative electrode forming composition in the form of a slurry containing a binder, a conductive agent, and a negative electrode active material onto a negative electrode current collector and drying the coated negative electrode current collector; or casting the negative electrode forming composition onto a separate support and then pressing the film layer, separated from the support, onto the negative electrode current collector.

[0096] Compounds capable of reversibly inserting and deintercalating lithium can be used as anode active materials. Specific examples of anode active materials can be: carbonaceous materials, such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; metallic materials that can be alloyed with lithium, such as silicon (Si), aluminum (Al), tin (Sn), lead (Pb), zinc (Zn), bismuth (Bi), indium (In), magnesium (Mg), gallium (Ga), cadmium (Cd), Si alloys, Sn alloys, or Al alloys; and lithium-doped or undoped metal oxides, such as SiO₂. β (0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing metallic and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of them can be used. Additionally, thin films of metallic lithium can be used as the negative electrode active material. Furthermore, both low-crystallinity carbon and high-crystallinity carbon can be used as carbon materials. Typical examples of low-crystallinity carbon can be soft carbon and hard carbon, while typical examples of high-crystallinity carbon can be amorphous, plate-like, sheet-like, spherical, or fibrous natural or artificial graphite, condensed graphite, pyrolytic carbon, mesophase pitch-like carbon fibers, mesophase carbon microspheres, mesophase pitch, and high-temperature sintered carbon such as coke derived from petroleum or coal tar pitch.

[0097] In addition, the adhesive and conductive agent can be the same as those described above for the positive electrode.

[0098] In lithium-ion secondary batteries, a separator separates the negative and positive electrodes and provides a pathway for lithium ion movement. Any separator can be used without specific limitations, as long as it is generally suitable for lithium-ion secondary batteries. Specifically, a separator with high moisture retention capacity for the electrolyte and low resistance to electrolyte ion transfer can be used. Specifically, porous polymer membranes can be used, such as porous polymer membranes prepared from polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene-butene copolymers, ethylene-hexene copolymers, or ethylene-methacrylate copolymers, or laminated structures having two or more layers. Alternatively, typical porous nonwoven fabrics can be used, such as nonwoven fabrics formed from high-melting-point glass fibers or polyethylene terephthalate fibers. Furthermore, coated separators containing ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and separators with single-layer or multi-layer structures can be selectively used.

[0099] In addition, the electrolyte used in the present application can include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten-type inorganic electrolyte, which can be used to manufacture a lithium secondary battery, but the present application is not limited thereto.

[0100] Specifically, the electrolyte can include an organic solvent and a lithium salt.

[0101] Any organic solvent can be used as the organic solvent without specific limitation, as long as it can be used as a medium through which ions participating in an electrochemical reaction of a battery move. Specifically, as the organic solvent, an ester-based solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; an ether-based solvent such as dibutyl ether or tetrahydrofuran; a ketone-based solvent such as cyclohexanone, etc.; an aromatic hydrocarbon-based solvent such as benzene or fluorobenzene; a carbonate-based solvent such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methylethyl carbonate (MEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), or propylene carbonate (PC); an alcohol-based solvent such as ethanol and isopropanol; a nitrile such as R-CN (wherein R is a linear, branched, or cyclic C2 to C20 hydrocarbon group, and can include a double-bonded aromatic ring and an ether bond); an amide such as dimethylformamide; dioxolane such as 1,3-dioxolane; or a sulfolane can be used. Among these solvents, a carbonate-based solvent such as a mixture of a cyclic carbonate having high ionic conductivity and high dielectric constant (e.g., ethylene carbonate or propylene carbonate) which can improve the charge / discharge performance of a battery, and a low-viscosity linear carbonate compound (e.g., ethylmethyl carbonate, dimethyl carbonate, or diethyl carbonate) can be used. In this case, when the cyclic carbonate and the linear carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte solution can be excellent.

[0102] A lithium salt can be used without specific limitation, as long as it is a compound capable of providing lithium ions for a lithium secondary battery. Specifically, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2 can be used as the lithium salt. The lithium salt can be used at a concentration ranging from 0.1 M to 2.0 M. In the case of including a lithium salt at a concentration within the above range, since the electrolyte has appropriate conductivity and viscosity, excellent electrolyte performance can be obtained, and lithium ions can move effectively.

[0103] To improve the life characteristics of the battery, suppress the decrease in the battery capacity, and improve the discharge capacity of the battery, at least one additive, such as a halogenated alkylene carbonate compound (e.g., difluoroethylene carbonate, etc.), pyridine, triethyl phosphite, triethanolamine, a cyclic ether, ethylenediamine, n-glyme, triamide hexaphosphate, a nitrobenzene derivative, sulfur, a quinonimine dye, an N-substituted oxazolidinone, an N,N-substituted imidazolidine, an ethylene glycol dialkyl ether, an ammonium salt, a pyrrole, 2-methoxyethanol, or aluminum trichloride, can be added to the electrolyte in addition to the electrolyte components. In this case, the amount of the additive contained can be 0.1 to 5% by weight, based on the total weight of the electrolyte.

[0104] As described above, since the lithium secondary battery including the positive electrode active material of the present application stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, the lithium secondary battery is suitable for portable devices (e.g., mobile phones, notebook computers, and digital cameras) and electric vehicles (e.g., hybrid electric vehicles (HEV)).

[0105] Therefore, according to another embodiment of the present application, there are provided a battery module including the above-described lithium secondary battery as a unit cell, and a battery pack including the battery module.

[0106] The battery module or the battery pack can be used as a power source for at least one of large- and medium-sized devices, such as a power tool, an electric vehicle including an electric vehicle (EV), a hybrid electric vehicle (HEV), and a plug-in hybrid electric vehicle (PHEV), or an energy storage system.

[0107] Hereinafter, embodiments of the present application will be described in detail in such a manner that those skilled in the art can easily implement the present application. However, the present application can be implemented in various different forms, and should not be construed as being limited to the embodiments mentioned herein.

[0108] Example 1

[0109] In a 5L batch reactor set at 60°C, NiSO4, CoSO4, and MnSO4 were mixed in water in an amount such that the molar ratio of nickel: cobalt: manganese was 82:12:6 to prepare a precursor forming solution having a concentration of 2.4M.

[0110] 1L of deionized water was put into a co-precipitation reactor (capacity 5L), and then the reactor was purged with nitrogen at a rate of 2L / min to remove dissolved oxygen in the water and create a non-oxidizing atmosphere in the reactor. Thereafter, 10ml of a 25% NaOH aqueous solution was added, and then stirring was performed at a speed of 1,200rpm and a temperature of 60°C to maintain a pH of 12.0.

[0111] Subsequently, a co-precipitation reaction was performed for 18 hours while adding the precursor forming solution with an aqueous NaOH solution and an aqueous NH4OH solution at a rate of 180 ml / hr, thereby forming nickel cobalt manganese-containing hydroxide (Ni 0.82 Co 0.12 Mn 0.06 (OH)2) particles. The hydroxide particles were separated, washed, and then dried in a furnace at 120°C to prepare a positive electrode active material precursor (D 50 = 4 pm). The positive electrode active material precursor thus prepared was in the form of secondary particles in which primary particles were aggregated.

[0112] The positive electrode active material precursor thus prepared was pre-sintered at 700°C for 5 hours in an air atmosphere. Thereafter, the pre-sintered positive electrode active material precursor and a lithium raw material LiOH were added to a Henschel mixer (700L) such that the final molar ratio of Li / M(Ni,Co,Mn) was 1.02, and mixed at a center speed of 300 rpm for 20 minutes. The powder of the mixture was put into an alumina crucible having a size of 330 mm x 330 mm and secondary sintered at 880°C for 10 hours under an oxygen (O2) atmosphere, thereby forming a lithium composite transition metal oxide.

[0113] 300 g of the lithium composite transition metal oxide thus prepared was added to 300 mL of ultrapure water and stirred for 30 minutes to wash the lithium composite transition metal oxide, and the washed lithium composite transition metal oxide was filtered for 20 minutes. The filtered lithium composite transition metal oxide was dried in a vacuum furnace at 130°C for 10 hours, and then sieved, to prepare a positive electrode active material. The positive electrode active material thus prepared contained 5 ppm of chlorine (Cl) as an impurity.

[0114] Example 2

[0115] A positive electrode active material was prepared in the same manner as in Example 1, except that, during the secondary sintering, sintering was performed after further mixing SrCO3 such that the concentration was 1,200 ppm based on the total weight of the positive electrode active material.

[0116] Example 3

[0117] A positive electrode active material was prepared in the same manner as in Example 2, except that the pre-sintering temperature was set to 600°C.

[0118] Example 4

[0119] A positive electrode active material was prepared in the same manner as in Example 1, except that the pre-sintering temperature was set to 780°C.

[0120] Example 5

[0121] The positive electrode active material was prepared in the same manner as in Example 1, except that lithium raw material LiOH was mixed during the secondary sintering so that the final Li / M(Ni,Co,Mn) molar ratio was 1.01.

[0122] Example 6

[0123] The positive electrode active material was prepared in the same manner as in Example 1, except that lithium raw material LiOH was mixed during the secondary sintering so that the final Li / M(Ni,Co,Mn) molar ratio was 1.04.

[0124] Comparative Example 1

[0125] The positive electrode active material was prepared in the same manner as in Example 1, except that the positive electrode active material precursor without pre-sintering was mixed with lithium raw material, and then the mixture was sintered at 880°C for 10 hours under an oxygen (O2) atmosphere, thereby forming a lithium composite transition metal oxide.

[0126] Comparative Example 2

[0127] The positive electrode active material was prepared in the same manner as in Example 1, except that the positive electrode active material precursor without pre-sintering was mixed with lithium raw material, and then the mixture was sintered at 770°C for 10 hours under an oxygen (O2) atmosphere, thereby forming a lithium composite transition metal oxide.

[0128] Comparative Example 3

[0129] The positive electrode active material was prepared in the same manner as in Example 1, except that the pre-sintering temperature was set to 400°C.

[0130] Comparative Example 4

[0131] The positive electrode active material was prepared in the same manner as in Example 1, except that the pre-sintering temperature was set to 1,000°C.

[0132] Experimental Example 1: Observation of Positive Electrode Active Material

[0133] Figures 1 to 4 Enlarged scanning electron microscope (SEM) images of the positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 and 2 are shown.

[0134] Referring to Figures 1 to 4 , the positive electrode active materials prepared in Examples 1 and 2 of the present application formed a primary structure of single particles, but it could be confirmed that the precursors without pre-sintering of Comparative Examples 1 and 2 were in the form of secondary particles in which the primary particles were aggregated with each other. Referring to Figure 3 (Comparative Example 1), the primary particles were apparently grown, but did not form a single crystal, and were still in the form of aggregated secondary particles.

[0135] Experimental Example 2: Specific surface area, primary particle diameter, and grain size of positive electrode active material

[0136] The specific surface area, primary particle diameter, and grain size of the positive electrode active materials prepared in Examples 1 to 6 and Comparative Examples 1 to 4 were measured. The primary particle diameters of Examples 1 to 6 were measured using a laser diffraction method, the primary particle diameters of Comparative Examples 1 to 4 were measured using SEM images, the grain sizes were measured by XRD, and their values were calculated.

[0137] Table 1

[0138] Single particles present Primary particles (pm) Grain size (nm) Example 1 ○ D 50 6.5]]> 220 Example 2 ○ D 50 7.0]]> 250 Example 3 ○ D 50 6.2]]> 230 Example 4 ○ D 50 6.6]]> 230 Example 5 ○ D 50 6.2]]> 215 Example 6 ○ D 50 6.0]]> 215 Comparative Example 1 × 1 to 4 180 Comparative Example 2 × 0.5 to 1 150 Comparative Example 3 × 1 to 4 190 Comparative Example 4 × 2 to 5 220

[0139] Referring to Table 1, the positive electrode active materials prepared in Examples 1 to 6 were in the form of single particles, but the positive electrode active materials prepared in Comparative Examples 1 to 4 were not in the form of single particles, but in the form of aggregated secondary particles. In addition, the grain sizes of the positive electrode active materials of Examples 1 to 6 were greater than the grain sizes of the positive electrode active materials of Comparative Examples 1 to 3.

[0140] Experimental Example 3: Lithium by-product measurement

[0141] After dispersing 5 g of each of the positive electrode active materials prepared in Examples 1 to 6 and Comparative Examples 1 to 4 in 100 mL of water, the change in pH was measured while titrating the solution with 0.1 M HC1 to obtain a pH titration curve. Using the pH titration curve, the residual amount of LiOH and the residual amount of Li2CO3 in each of the positive electrode active materials were calculated, the sum of these values was evaluated as the total residual amount of lithium by-products, and is listed in Table 2 below.

[0142] Table 2

[0143] Residual amount of lithium by-product (wt%) Example 1 0.44 Example 2 0.40 Example 3 0.45 Example 4 0.46 Example 5 0.38 Example 6 0.35 Comparative Example 1 0.6 Comparative Example 2 0.7 Comparative Example 3 0.65 Comparative Example 4 0.7

[0144] Referring to Table 2, the amount of lithium by-products of the positive electrode active materials prepared in Examples 1 to 3 was 0.5 wt% or less, in which the amount of lithium by-products was reduced compared to Comparative Examples 1 to 4.

[0145] Experimental Example 4: Thermal stability evaluation

[0146] The heat flow of the positive electrode active materials of Examples 1 to 3 and Comparative Examples 1 and 2 was measured with respect to temperature using a differential scanning calorimeter (Sensys evo DSC, SETARAM Instrumentation). Specifically, 16 mg of each of the positive electrode active materials of Examples 1 to 3 and Comparative Examples 1 and 2 was put into a pressure-resistant pan for DSC measurement, and then 20 μL of an electrolyte solution (EVPS) was injected. The temperature range for DSC analysis was set to 25°C to 400°C, and the heating rate was set to 10°C / min. DSC measurement was performed more than three times for each positive electrode active material to calculate an average value. The measurement results are presented in Table 3 and Figure 5

[0147] Table 3

[0148] Main peak measurement temperature (°C) Example 1 235 Example 2 236 Example 3 235 Comparative Example 1 225 Comparative Example 2 220

[0149] Referring to Table 3 and Figure 5 , for the positive electrode active materials of Examples 1 to 3, the main peak appeared at 235°C or more, and for the positive electrode active materials of Comparative Examples 1 and 2, it could be confirmed that the main peak appeared at a relatively low temperature of about 220°C to about 225°C. This indicates that the thermal stability of the positive electrode active materials of Examples 1 to 3 is better than that of the positive electrode active materials of Comparative Examples 1 and 2.

[0150] Experimental Example 5: Leakage Current Evaluation

[0151] Each of the positive electrode active materials prepared in Examples 1 to 6 and Comparative Examples 1 to 4, a carbon black conductive agent, and a PVdF binder were mixed in an N-methylpyrrolidone solvent at a weight ratio of 96:2:2 to prepare a positive electrode material mixture (viscosity: 5,000 mPa-s), one surface of an aluminum current collector was coated with the positive electrode material mixture, dried at 130°C, and then rolled to prepare a positive electrode.

[0152] Lithium metal was used as a negative electrode.

[0153] An electrode assembly was prepared by disposing a porous polyethylene separator between the positive electrode and the negative electrode prepared as described above, the electrode assembly was disposed in a case, and then an electrolyte solution was injected into the case, thereby preparing each lithium secondary battery. In this case, the electrolyte solution was prepared by dissolving 1.0 M lithium hexafluorophosphate (LiPF6) in an organic solvent composed of ethylene carbonate / ethyl methyl carbonate / diethyl carbonate (a mixed volume ratio of EC / EMC / DEC = 3 / 4 / 3).

[0154] ​Charging and discharging tests were performed on each lithium secondary battery half-cell prepared as described above. Specifically, after charging and discharging each half-cell at 0.2C / 0.2C and 50°C, each half-cell was charged at 0.2C to a voltage of 4.7V in a constant current / constant voltage (CCCV) mode with a termination set to 130 hours, and then the average leakage current was obtained by dividing the capacity of 130 hours by the time, the results of which are presented in Table 4 below and Figures 6 to 9

[0155] Table 4

[0156]

[0157] Referring to Table 4 and Figures 6 to 9 In the case of using the positive electrode active materials of Examples 1 to 6, it can be confirmed that the charging and discharging capacities are excellent and that the leakage current hardly occurs. In contrast, in the case of using the positive electrode active materials of Comparative Examples 1 to 3, it can be seen that the amount of leakage current during 130 hours is significantly increased. For Comparative Example 4, since the pre-sintering temperature is too high, phase separation of Ni, Co, and Mn occurs, and thus it seems that the capacity obtained is less.

[0158] Experimental Example 6: Evaluation of High Temperature Storage Characteristics

[0159] Each positive electrode active material prepared in Examples 1 to 6 and Comparative Examples 1 to 3 was charged to a voltage of 4.4V (termination current of 1 / 20C) at 0.5C in a CCCV mode with each lithium secondary battery prepared in Experimental Example 5. Two positive electrodes and two polyethylene separators charged thus were alternately stacked on a coin cell base plate. Thereafter, the coin cell prepared by injecting an electrolyte solution and then covering with a gasket was put into an aluminum bag and vacuum-sealed. Thereafter, the amount of gas generated during storage for 2 weeks at 60°C was measured using a gas chromatograph-mass spectrometer (GC-MS). The results are presented in Table 6 below.

[0160] Table 6

[0161]

[0162] Referring to Table 6, for the positive electrode active materials prepared in Examples 1 to 6, the amount of gas generated during high temperature storage was significantly reduced compared to the positive electrode active materials prepared in Comparative Examples 1 to 3.​

Claims

1. A positive electrode active material for secondary batteries, wherein the positive electrode active material is a lithium composite transition metal oxide containing nickel (Ni), cobalt (Co), and manganese (Mn), composed of single particles, and having a grain size of 200 nm or larger. in, Based on the total amount of transition metals, the lithium composite transition metal oxide contains more than 80 mol% nickel (Ni) and 5 mol% to 10 mol% manganese (Mn). Wherein, the single particle is the average particle size D 50 Primary particles ranging from 6 μm to 7 μm in size, and The single particle is formed by a method including the following steps: Pre-sintering of precursors containing nickel (Ni), cobalt (Co), and manganese (Mn) at 600°C to 800°C; and The pre-sintered precursor is mixed with lithium raw material and the mixture is sintered again at a temperature of 850°C to 980°C.

2. The positive electrode active material for secondary batteries as described in claim 1, wherein, The amount of chlorine (Cl) impurities in the positive electrode active material is less than 20 ppm.

3. The positive electrode active material for secondary batteries as described in claim 1, wherein, Based on the total weight of the positive electrode active material, the amount of residual lithium byproducts in the positive electrode active material is less than 0.5% by weight.

4. The positive electrode active material for secondary batteries as described in claim 1, wherein, When the positive electrode active material was thermally analyzed by differential scanning calorimetry (DSC), the main peak with the maximum heat flux was measured above 235°C.

5. A method for preparing a positive electrode active material for secondary batteries, the method comprising: A precursor containing nickel (Ni), cobalt (Co) and manganese (Mn) was prepared, wherein, based on the total amount of transition metals, the amount of nickel (Ni) was more than 80 mol% and the amount of manganese (Mn) was 5 mol% to 10 mol%. The precursor is pre-sintered at 600°C to 800°C; and The pre-sintered precursor is mixed with lithium raw material and the mixture is sintered again at a temperature of 850°C to 980°C to form a lithium composite transition metal oxide composed of single particles with a grain size of more than 200 nm. Wherein, the single particle is the average particle size D 50 Primary particles ranging from 6μm to 7μm.

6. The method of claim 5, wherein, The pre-sintering process takes 4 to 8 hours.

7. The method of claim 5, wherein, The precursor is secondary particles formed by the aggregation of primary particles, and the average particle size D of the secondary particles is... 50 The size ranges from 3μm to 6μm.

8. The method of claim 5, wherein, The secondary sintering is carried out at 880°C to 980°C.

9. The method of claim 5, wherein, The lithium raw materials are mixed in such a way that the molar ratio of lithium (Li) to all metal elements M other than lithium in the lithium composite transition metal oxide, Li / M, is 0.98 to 1.

05.

10. The method of claim 5, wherein, During the secondary sintering, the sintering is carried out after further mixing of a particle growth promoter, which contains at least one particle growth promoting element selected from the group consisting of strontium (Sr), zirconium (Zr), magnesium (Mg), yttrium (Y) and aluminum (Al).

11. The method of claim 10, wherein, The particle growth promoter is mixed in such a way that the content of the particle growth promoting element is between 500 ppm and 2,000 ppm based on the total weight of the positive electrode active material.

12. A positive electrode for a secondary battery, said positive electrode comprising the positive electrode active material according to any one of claims 1 to 4.

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