Positive electrode active material, method for preparing same, and positive electrode and lithium secondary battery comprising same

Abstract

The positive electrode active material according to the present invention contains lithium nickel-based transition metal oxide particles in which the molar ratio of nickel in all transition metals is 80 mol% or more, and a coating layer formed on the surfaces of the lithium nickel-based transition metal oxide particles, the lithium nickel-based transition metal oxide particles are in the form of single particles consisting of single nodules or quasi-single particles as a composite of at most 30 nodules, the coating layer contains cobalt and aluminum, and the aluminum content is 0.1 mol% to 0.5 mol% with respect to 100 mol of the lithium nickel-based transition metal oxide.

Description

Technical Field

[0001] This application claims the benefit of Korean Patent Application No. 10-2022-0182376, filed with the Korean Intellectual Property Office on Dec. 22, 2022, the disclosure of which is incorporated herein by reference in its entirety.

[0002] The present invention relates to a positive electrode active material, a method for preparing the same, a positive electrode and a lithium secondary battery including the same, and more particularly, to a single-particle form positive electrode active material including a coating layer containing cobalt and a certain amount of aluminum, thereby having a lower initial resistance and excellent high-temperature life, a method for preparing the same, a positive electrode and a lithium secondary battery including the same. Background Art

[0003] A lithium secondary battery generally includes a positive electrode, a negative electrode, a separator, and an electrolyte, and the positive electrode and the negative electrode include active materials capable of intercalating / deintercalating lithium ions.

[0004] Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2, LiMnO4, etc.), lithium iron phosphate compound (LiFePO4), etc. have been used as positive electrode active materials for lithium secondary batteries. Among those listed above, lithium cobalt oxide has a high working voltage and excellent capacity characteristics, but it is difficult to commercially apply lithium cobalt oxide to large-capacity batteries because cobalt, which is a raw material of lithium cobalt oxide, is expensive and its supply is unstable. Lithium nickel oxide has poor structural stability, so it is difficult to provide sufficient life. At the same time, lithium manganese oxide has excellent stability but poor capacity. Therefore, lithium composite transition metal oxides containing at least two transition metals have been developed to compensate for the limitations of lithium transition metal oxides containing only Ni, Co, or Mn. In particular, lithium nickel cobalt manganese oxide containing Ni, Co, and Mn has been widely used in the field of batteries for electric vehicles.

[0005] Conventional lithium nickel cobalt manganese oxides are generally in the form of spherical secondary particles agglomerated from dozens to hundreds of primary particles. However, when applying lithium nickel cobalt manganese oxides in the form of secondary particles agglomerated from a large number of primary particles, shedding (i.e., fragmentation of particles) of primary particles easily occurs during the rolling process of manufacturing the positive electrode, and cracks are generated inside the particles during the charge / discharge process. When fragmentation or cracking of the positive electrode active material particles occurs, the contact area with the electrolyte increases, thereby increasing gas generation caused by side reactions with the electrolyte and deterioration of the active material, and thus, the life characteristics deteriorate.

[0006] To solve the above problems, a technique has been proposed to prepare a positive electrode active material in the form of single particles (instead of secondary particles) by increasing the firing temperature during the preparation of lithium nickel cobalt manganese oxide. Since the contact area of the positive electrode active material in the form of single particles with the electrolyte is smaller than that of the conventional positive electrode active material in the form of secondary particles, side reactions with the electrolyte occur less frequently. Also, because single particles have excellent particle strength, less particle breakage occurs during electrode manufacturing. Therefore, when using the positive electrode active material in the form of single particles, there are benefits such as reduced gas generation and excellent life characteristics. However, conventional positive electrode active materials in the form of single particles require a high firing temperature during synthesis. As a result, the crystal structure on the particle surface changes from a layered structure to a rock salt structure. The non-conductive rock salt structure surface hinders the movement of lithium ions during charging and discharging, thereby reducing the battery life. Summary of the Invention

[0007] Technical problem

[0008] To solve the above tasks, one aspect of the present invention provides a positive electrode active material that includes a coating layer containing cobalt and a certain amount of aluminum, thereby suppressing its deterioration during charge and discharge.

[0009] Another aspect of the present invention provides a positive electrode and a lithium secondary battery that include the positive electrode active material and thus have improved initial resistance and high-temperature life.

[0010] Technical solution

[0011] According to one aspect of the present invention, there is provided a positive electrode active material including lithium nickel-based transition metal oxide particles in which the molar ratio of nickel in all transition metals is 80 mol% or more, and a coating layer formed on the surface of the lithium nickel-based transition metal oxide particles. The lithium nickel-based transition metal oxide particles are in the form of single particles composed of a single nodule or quasi-single particles as a composite of at most 30 nodules. The coating layer contains cobalt and aluminum, and the aluminum content is 0.1 mol% to 0.5 mol% relative to 100 mol of the lithium nickel-based transition metal oxide.

[0012] The cobalt content can be 0.5 mol% to 5.0 mol% relative to 100 mol of the lithium nickel-based transition metal oxide.

[0013] In the lithium nickel-based transition metal oxide particles, the molar ratio of nickel in all transition metals can be 90 mol% or more.

[0014] The coating layer can be formed to be discontinuously distributed on the surface of the lithium nickel-based transition metal oxide particles.

[0015] The positive electrode active material may have a peak in the range of 2θ = 37° to 38° on the XRD pattern.

[0016] The positive electrode active material may have a peak in the range of 2θ = 45° to 46° on the XRD pattern.

[0017] The average particle diameter of the nodules of the positive electrode active material may be 1.0 μm to 9.0 μm.

[0018] The D of the positive electrode active material 50 may be 4.0 μm to 9.0 μm.

[0019] According to another aspect of the present invention, there is provided a method for preparing a positive electrode active material, comprising: (A) mixing a transition metal precursor containing nickel, cobalt, and manganese with a lithium raw material, and subjecting the mixture to a first firing to form a lithium nickel-based transition metal oxide, and (B) mixing the lithium nickel-based transition metal oxide with cobalt and aluminum, and subjecting the mixture to a second firing to form a positive electrode active material including a coating layer, wherein, in the lithium nickel-based transition metal oxide, the molar ratio of nickel in all transition metals is 80 mol% or more, the lithium nickel-based transition metal oxide is a single particle composed of a single nodule or a quasi-single particle as a composite of at most 30 nodules, and in step (B), the molar ratio of aluminum to the lithium nickel-based transition metal oxide (aluminum / lithium nickel-based transition metal oxide) is 0.001 to 0.005.

[0020] Step (B) may be carried out at 660°C to 720°C.

[0021] The coating layer may be formed to be discontinuously distributed on the surface of the lithium nickel-based transition metal oxide particles.

[0022] Step (A) may be carried out at 800°C to 890°C.

[0023] No additional washing process may be carried out between step (A) and step (B).

[0024] According to another aspect of the present invention, there are provided a positive electrode containing the above positive electrode active material and a lithium secondary battery containing the positive electrode.

[0025] Beneficial effect

[0026] The positive electrode active material of the present invention contains a lithium nickel-based transition metal oxide in the form of single particles and / or quasi-single particles having excellent particle strength, and thus is less likely to have particle breakage or cracking caused by rolling during electrode manufacturing. Therefore, gas generation and deterioration of the active material caused by side reactions with the electrolyte are less, thereby providing excellent high-temperature life.

[0027] In addition, the positive electrode active material of the present invention forms a coating layer with a high cobalt content on the surface of the lithium nickel-based transition metal oxide by mixing the lithium nickel-based transition metal oxide, cobalt, and a certain amount of aluminum at one time, which can inhibit the deterioration of the positive electrode active material during the charge and discharge process of the battery, thereby providing excellent high-temperature life. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a scanning electron microscope (SEM) image of the positive electrode active material powder prepared in Example 1 of the present invention;

[0029] Figure 2 is a scanning electron microscope (SEM) image of the positive electrode active material powder prepared in Example 2 of the present invention;

[0030] Figures 3 to 6 is an SEM image of the positive electrode active material powder prepared in Comparative Examples 1 to 4 of the present invention;

[0031] Figure 7 is an electron energy loss spectroscopy (EELS) image of the positive electrode active material prepared in Example 1 of the present invention;

[0032] Figures 8 to 11 is an EELS image of the positive electrode active material prepared in Comparative Examples 1 to 4 of the present invention;

[0033] Figure 12 is a graph showing the X-ray diffraction (XRD) analysis results of the positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 to 4 of the present invention;

[0034] Figure 13 is a graph showing the capacity retention rate varying with the number of cycles of the batteries containing the positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 to 5 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0035] It will be understood that the words or terms used in this specification and the claims of the present invention should not be construed as being limited to the meanings defined in a commonly used dictionary. It will also be understood that based on the principle that the inventor can appropriately define the meanings of the words or terms to best explain the present invention, the words or terms should be construed as having meanings consistent with their meanings in the context of the prior art and the technical concept of the present invention.

[0036] As used herein, "single particle" refers to a particle composed of a single nodule. As used herein, "quasi-single particle" refers to a particle that is a composite formed by at most 30 nodules.

[0037] As used herein, "nodule" refers to the particulate unit that constitutes a single particle and a quasi-single particle, and the nodule can be a single crystal without any crystalline grain boundaries, or can be a polycrystal that does not have grain boundaries when observed using a scanning electron microscope (SEM) at a magnification of 5000 to 20000 times. As used herein, the average particle size of the nodule can be determined as the arithmetic mean of the particle sizes of each nodule measured using a scanning electron microscope (SEM).

[0038] As used herein, "secondary particle" refers to a particle formed by the aggregation of dozens to hundreds of primary particles. More specifically, the secondary particle is an aggregate of 40 or more primary particles.

[0039] As used herein, when describing "particle", it can include any one or all of a single particle, a quasi-single particle, a primary particle, a nodule, and a secondary particle.

[0040] As used herein, "D 50 " refers to the particle size corresponding to 50 in the volume cumulative particle size distribution of the positive electrode active material. D 50 can be measured using the laser diffraction method. For example, the average particle size D50 can be measured as follows: The positive electrode active material powder is dispersed in a dispersion medium, the dispersion medium is introduced into a commercially available laser diffraction particle size measuring instrument (e.g., Microtrac MT 3000), and irradiated with ultrasonic waves having a frequency of about 28 kHz and an output of 60 W to obtain a volume cumulative particle size distribution diagram, and the particle size at 50% of the volume cumulative is calculated.

[0041] As used herein, the "specific surface area" is measured by the BET method. Specifically, it can be calculated using BELSORP-mino II of BEL JAPAN Inc. based on the nitrogen adsorption amount at liquid nitrogen temperature (77K).

[0042] Hereinafter, the present invention will be described in more detail.

[0043] Positive electrode active material

[0044] The positive electrode active material of the present invention contains lithium nickel-based transition metal oxide particles in which the molar ratio of Ni in all transition metals is 80 mol% or more, preferably 90 mol% or more. When the molar ratio of Ni satisfies the above range, excellent capacity can be achieved.

[0045] The lithium nickel-based transition metal oxide particles are in the form of single particles composed of a core and / or quasi-single particles that are composites of up to 30 cores, preferably 2 to 20 cores, more preferably 2 to 10 cores. Since the lithium nickel-based transition metal oxides in the form of single particles and / or quasi-single particles have higher particle strength compared to the lithium nickel-based transition metal oxides in the form of secondary particles formed by agglomeration of dozens to hundreds of primary particles in the conventional case, the particles are less likely to break during calendering.

[0046] In addition, since the sub-parts (i.e., cores) of the constituent particles of the lithium nickel-based transition metal oxides in the form of single particles or quasi-single particles of the present invention are fewer, the changes caused by the volume expansion / contraction of the primary particles during the charge and discharge processes are very small, and thus the generation of internal cracking in the particles is significantly reduced.

[0047] The positive electrode active material of the present invention includes a coating layer formed on the surface of the lithium nickel-based transition metal oxide particles. The coating layer contains cobalt and aluminum.

[0048] The coating layer containing cobalt and aluminum suppresses the deterioration of the positive electrode active material during the charge and discharge processes of the battery, thereby improving the high-temperature life.

[0049] In particular, the inventors of the present invention have found that when the positive electrode active material with an aluminum content in the coating layer of 0.1 mol% to 0.5 mol% relative to 100 moles of the lithium nickel-based transition metal oxide is applied, the diffusion of cobalt into the particles at a high firing temperature is suppressed, thereby forming a coating layer with a high cobalt content, and thus the high-temperature life characteristics are further improved. Preferably, relative to 100 moles of the lithium nickel-based transition metal oxide, the aluminum content in the coating layer can be 0.2 mol% to 0.5 mol%.

[0050] When the aluminum content in the coating layer is less than 0.1 mol% relative to 100 moles of the lithium nickel-based transition metal oxide, the diffusion of cobalt into the particles at a high firing temperature cannot be suppressed, resulting in an insignificant effect of improving the high-temperature life characteristics, and when the aluminum content is greater than 0.5 mol%, due to the presence of a large amount of aluminum in the coating layer, the calendering density is excessively reduced and the initial resistance increases.

[0051] Relative to 100 moles of the lithium nickel-based transition metal oxide, the cobalt content in the coating layer can be 0.5 mol% to 5.0 mol%, preferably 1.0 mol% to 4.0 mol%, more preferably 1.5 mol% to 2.5 mol%. When the cobalt content in the coating layer is less than 0.5 mol% relative to 100 moles of the lithium nickel-based transition metal oxide, the high-temperature life is poor due to insufficient cobalt coating amount, and when the cobalt content is greater than 5.0 mol%, the amount of nickel that contributes to the capacity among all transition metals may decrease, which may lead to a reduction in the charge capacity.

[0052] When both the aluminum content and the cobalt content in the coating layer satisfy the above ranges, it is possible to achieve the effect of minimizing the reduction in charge capacity while improving the high-temperature life characteristics.

[0053] Meanwhile, the positive electrode active material of the present invention may include a coating layer formed to be discontinuously distributed on the surface of the lithium nickel-based transition metal oxide particles. The discontinuously distributed coating layer means that a thick coating layer that maximally suppresses cobalt doping on the surface of the positive electrode active material is formed, and this coating layer can maximize the effect of protecting the surface of the positive electrode active material. In contrast, the case where the coating layer has a layered structure continuously present on the particle surface means that a thin coating layer is formed, and cobalt doping on the surface of the positive electrode active material may be locally formed, thereby reducing the effect of protecting the surface of the positive electrode active material.

[0054] Meanwhile, the positive electrode active material of the present invention may have peaks in the range of 2θ of 37° to 38° and / or 45° to 46° in the XRD pattern. The measurement conditions are based on Cu-Kα radiation, and the range of 2θ from 35° to 48° is measured at 40 kV to obtain an X-ray diffraction analysis pattern. On the surface of the lithium nickel-based transition metal oxide, cobalt can react with residual Li to form a LiCoO2 phase. By having peaks in this 2θ range, a coated form of the LiCoO2 phase is provided on the surface of the lithium nickel-based transition metal oxide to suppress the deterioration of the positive electrode active material during the charge and discharge process of the battery. Therefore, excellent high-temperature life can be achieved.

[0055] Meanwhile, the positive electrode active material of the present invention may include a lithium nickel-based oxide, specifically, a lithium nickel-based oxide having a composition shown in Formula 1 below.

[0056] [Formula 1]

[0057] Li a Ni b Co c M 1 d M 2 e O2

[0058] In Formula 1, M 1 is Mn, Al, or a combination thereof, preferably Mn or a combination of Mn and Al, and M 2 is at least one selected from the group consisting of Ba, Ca, Zr, Ti, Mg, Ta, Nb, and Mo. Although not necessarily including the M 2 element, when an appropriate amount of the M 2 element is included, it can be used to promote grain growth or enhance the stability of the crystal structure during the firing process.

[0059] A represents the molar ratio of lithium in the lithium nickel-based oxide and can satisfy 0.80 ≤ a ≤ 1.20, 0.90 ≤ a ≤ 1.10, or 0.95 ≤ a ≤ 1.15. When the molar ratio of lithium satisfies the above range, a stable layered crystal structure can be formed.

[0060] b represents the molar ratio of nickel among all metals other than lithium in the nickel-based oxide and can satisfy 0.80 ≤ b < 1, 0.85 ≤ b < 1, or 0.90 ≤ b < 1. When the molar ratio of nickel satisfies the above range, excellent capacity can be achieved. In particular, when the molar ratio of nickel is above 0.90, further improved capacity characteristics can be achieved.

[0061] c represents the molar ratio of cobalt among all metals other than lithium in the lithium nickel-based oxide and can satisfy 0 < c < 0.45, 0 < c < 0.40, 0 < c < 0.20, or 0 < c < 0.18.

[0062] d represents the molar ratio of M among all metals other than lithium in the lithium nickel-based oxide 1 and can satisfy 0 < d < 0.45, 0 < d < 0.40, 0 < d < 0.20, or 0 < d < 0.18.

[0063] e represents the molar ratio of M among all metals other than lithium in the lithium nickel-based oxide 2 and can satisfy 0 ≤ e ≤ 0.20, 0 ≤ e ≤ 0.15, or 0 ≤ e ≤ 0.10.

[0064] The average agglomerate particle size of the positive electrode active material of the present invention can be 1.0 μm to 9.0 μm, 2.0 μm to 6.0 μm, or 3.0 μm to 5.0 μm. When the average particle size of the agglomerates of the positive electrode active material of the present invention satisfies the above range, high energy density and low initial resistance can be achieved. When the average particle size of the agglomerates of the positive electrode active material is less than 1.0 μm, the specific surface area of the entire positive electrode active material increases, so the side reaction with the electrolyte may increase. When the average particle size of the agglomerates is greater than 9.0 μm, the lithium mobility in the positive electrode active material decreases, so the output characteristics of the battery may deteriorate.

[0065] The D of the positive electrode active material of the present invention 50 can be 4.0 μm to 9.0 μm, 5.0 μm to 8.0 μm, or 6.0 μm to 8.0 μm. When the D of the positive electrode active material of the present invention 50 satisfies the above range, high energy density and low initial resistance can be achieved. When the D of the positive electrode active material is less than 4.0 μm, it is difficult to achieve a high rolling density, so the energy density may decrease. When D 50 is less than 4.0 μm, it is difficult to achieve a high rolling density, so the energy density may decrease. When D 50When it is greater than 9.0 μm, the lithium mobility in the positive electrode active material decreases, and thus the initial resistance of the lithium secondary battery containing the positive electrode active material may increase.

[0066] Preparation method of positive electrode active material

[0067] Next, a method for preparing the positive electrode active material of the present invention will be described.

[0068] When preparing a positive electrode active material in which the molar ratio of Ni in all transition metals is 80 mol% or more, due to the high firing temperature, the completion level of the surface structure of the positive electrode active material may be low, and the concentration of residual lithium may be high. In addition, when the particle size of the positive electrode active material is large, the initial resistance is high. Therefore, a technique is required that is designed to improve the completion level of the surface structure, reduce the concentration of residual lithium, and reduce the initial resistance by optimizing the coating conditions through a coating layer formed on the surface of the lithium nickel-based transition metal oxide particles.

[0069] The method for preparing the positive electrode active material of the present invention includes: (A) mixing a transition metal precursor containing nickel, cobalt, and manganese with a lithium raw material, and subjecting the mixture to a first firing to form a lithium nickel-based transition metal oxide, and (B) mixing the lithium nickel-based transition metal oxide with cobalt and aluminum, and subjecting the mixture to a second firing to form a positive electrode active material containing a coating layer.

[0070] In addition, the lithium nickel-based transition metal oxide is a single particle composed of a single nodule or a quasi-single particle that is a composite of at most 30 nodules, in which the molar ratio of Ni in all transition metals is 80 mol% or more. Since the above description also applies, redundant descriptions will be skipped.

[0071] Each step of the method for preparing the positive electrode active material will be described in detail below.

[0072] First, a transition metal precursor containing nickel, cobalt, and manganese is mixed with a lithium raw material, and the mixture is subjected to a first firing to form a lithium nickel-based transition metal oxide (step (A)).

[0073] In this case, the positive electrode active material precursor can be purchased and used as a commercially available precursor (such as nickel cobalt manganese hydroxide), or can be prepared according to a precursor preparation method known in the art (such as the co-precipitation method).

[0074] For example, a transition metal-containing solution containing nickel (Ni), cobalt (Co), and M 1 cations is prepared, and then the transition metal-containing solution is co-precipitated by adding a complexing agent containing ammonium cations and an alkaline aqueous solution to prepare a positive electrode active material precursor.

[0075] The solution containing transition metals may include a nickel-containing raw material, a cobalt-containing raw material, and a raw material containing M 1 The raw material containing M 1 may be a manganese-containing raw material and / or an aluminum-containing raw material.

[0076] The nickel-containing raw material may be, for example, nickel acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, hydroxyoxide, etc., specifically Ni(OH)2, NiO, NiOOH, NiCO3·2Ni(OH)2·4H2O, NiC2O2·2H2O, Ni(NO3)2·6H2O, NiSO4, NiSO4·6H2O, nickel fatty acid salt, nickel halide, or a combination thereof, but not limited thereto.

[0077] The cobalt-containing raw material may be, for example, cobalt acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, hydroxyoxide, etc., specifically Co(OH)2, CoOOH, Co(OCOCH3)2·4H2O, Co(NO3)2·6H2O, CoSO4, Co(SO4)2·7H2O, or a combination thereof, but not limited thereto.

[0078] The manganese-containing raw material may be, for example, manganese acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, hydroxyoxide, or a combination thereof, specifically: manganese oxides, such as Mn2O3, MnO2, and Mn3O4; manganese salts, such as MnCO3, Mn(NO3)2, and MnSO4, manganese acetate, manganese dicarboxylate, manganese citrate, or manganese fatty acid salt; manganese hydroxyoxide, manganese chloride, or a combination thereof, but not limited thereto.

[0079] The aluminum-containing raw material may be, for example, Al2O3, Al(OH)3, Al(NO3)3, Al2(SO4)3, (HO)2AlCH3CO2, HOAl(CH3CO2)2, Al(CH3CO2)3, aluminum halide, or a combination thereof.

[0080] The solution containing transition metals can be prepared by adding a nickel-containing raw material, a cobalt-containing raw material, and a raw material containing M 1 to a solvent. The solvent is specifically water, or a solvent mixture of water and an organic solvent that can be uniformly mixed with water (such as alcohol, etc.), or by mixing an aqueous solution of the nickel-containing raw material, an aqueous solution of the cobalt-containing raw material, and an aqueous solution of the raw material containing M 1 to prepare.

[0081] The complexing agent containing an ammonium cation may be, for example, NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4, NH4CO3, or a combination thereof, but is not limited thereto. Meanwhile, the complexing agent containing an ammonium cation may be used in the form of an aqueous solution. In this case, water or a mixture of water and an organic solvent that can be uniformly mixed with water (specifically, alcohols, etc.) may be used as the solvent.

[0082] The basic compound may be a hydroxide of an alkali metal or an alkaline earth metal, such as NaOH, KOH, or Ca(OH)2, its hydrate, or a combination thereof. The basic compound may also be used in the form of an aqueous solution. In this case, water or a mixture of water and an organic solvent that can be uniformly mixed with water (specifically, alcohols, etc.) may be used as the solvent.

[0083] A basic compound is added to adjust the pH of the reaction solution, and the addition amount may be such that the pH of the metal solution becomes 8 to 12.

[0084] Coprecipitation may be carried out in an inert atmosphere (such as a nitrogen or argon atmosphere) at a temperature of 35°C to 80°C.

[0085] The precursor particles of the positive electrode active material formed from nickel-cobalt-M 1 hydroxide are produced by the above process and precipitate in the reaction solution. By adjusting the concentrations of the nickel-containing raw material, the cobalt-containing raw material, and the raw material containing M 1 a positive electrode active material precursor with a nickel (Ni) content of more than 80 mol% of the total metal content can be prepared. The precipitated positive electrode active material precursor particles can be separated and dried by a conventional method to prepare the positive electrode active material precursor.

[0086] Thereafter, the positive electrode active material precursor can be mixed with a lithium raw material.

[0087] As the lithium raw material, a lithium-containing sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, hydroxide, or hydroxyoxide can be used, and the lithium raw material is not particularly limited as long as it can be dissolved in water. Specifically, the lithium raw material may be Li2CO3, LiNO3, LiNO2, LiOH, LiOH·H2O, LiH, LiF, LiCl, LiBr, LiI, CH3COOLi, Li2O, Li2SO4, CH3COOLi, Li3C6H5O7, etc., and these materials can be used alone or in combination of two or more of them.

[0088] The positive electrode active material precursor and the lithium raw material may be mixed, for example, at a molar ratio of about 1:1, about 1:1.05, about 1:1.10, about 1:1.15, or about 1:1.20.

[0089] Thereafter, the mixture can be subjected to a first firing. The first firing can be carried out in an air or oxygen atmosphere. The first firing can be carried out at a temperature of 800°C to 890°C, 800°C to 870°C, or 800°C to 860°C. The first firing can be carried out for 4 to 12 hours, 6 to 12 hours, or 8 to 12 hours.

[0090] Next, a lithium nickel-based transition metal oxide is mixed with cobalt and aluminum, and the mixture is subjected to a second firing to form a positive electrode active material including a coating layer (step (B)).

[0091] For example, the surface of lithium nickel-based transition metal oxide particles is coated with a cobalt compound and an aluminum compound.

[0092] For example, the cobalt compound can be one or more of Co(OH)2, CoOOH, Co(OCOCH3)2·4H2O, Co(NO3)2·6H2O, CoSO4, or Co(SO4)2·7H2O.

[0093] For example, the aluminum compound can be one or more of Al2O3, Al(OH)3, Al(NO3)3, Al2(SO4)3, (HO)2AlCH3CO2, HOAl(CH3CO2)2, Al(CH3CO2)3, or aluminum halide.

[0094] Thus, the coating layer contains both cobalt and aluminum, so the high-temperature life can be improved. When only coated with cobalt, due to the deterioration of the positive electrode active material during the charge and discharge process of the battery, the life characteristics may be reduced, which is caused by the diffusion of cobalt into the positive electrode active material at high temperature.

[0095] The molar ratio of cobalt to lithium nickel-based transition metal oxide (cobalt / lithium nickel-based transition metal oxide) is 0.01 or more, preferably 0.01 to 0.04, more preferably 0.02 to 0.03.

[0096] The molar ratio of aluminum to lithium nickel-based transition metal oxide (aluminum / lithium nickel-based transition metal oxide) is 0.001 to 0.005, preferably 0.0015 to 0.005, more preferably 0.002 to 0.004. When the above range is satisfied, the diffusion of cobalt into the particles at a high firing temperature is suppressed. Therefore, a coating layer with a high cobalt content is formed, resulting in further improved high-temperature life characteristics.

[0097] When the aluminum content in the coating layer is less than 0.1 mol% relative to 100 moles of lithium nickel-based transition metal oxide, the diffusion of cobalt into the particles at a high firing temperature cannot be suppressed, resulting in an insignificant effect of improving the high-temperature life characteristics. And when the aluminum content is greater than 0.5 mol%, due to the large amount of aluminum present in the coating layer, the calendering density is excessively reduced and the initial resistance increases.

[0098] The secondary firing can be carried out at a temperature of 660 °C to 720 °C, preferably 670 °C to 710 °C, more preferably 690 °C to 710 °C.

[0099] When the secondary firing temperature is lower than the above range, the diffusion of cobalt into the positive electrode active material is suppressed. Therefore, cobalt may react with the residual Li on the surface to form LiCoO2. However, compared with the case where the secondary firing is carried out within the above range, a coating layer with a low cobalt content is formed, so the effect of improving the high-temperature life characteristics may not be significant.

[0100] The secondary firing can be carried out for 3 to 12 hours, preferably 4 to 8 hours, more preferably 5 to 7 hours.

[0101] When the lithium nickel-based transition metal oxide is mixed with cobalt and aluminum at the above firing temperature and the mixture is subjected to secondary firing within the above firing time range, a discontinuous distribution of cobalt and aluminum coating layer can be formed on the surface of the lithium nickel-based transition metal oxide particles.

[0102] Another washing process may not be carried out between step (A) and step (B). Generally, the lithium by-products present on the surface of the positive electrode active material are washed with water. This is because when lithium by-products are present, side reactions with the electrolyte will occur when the positive electrode active material is applied to the battery, and gas generation increases during high-temperature storage. In contrast, in the preparation method of one aspect of the present invention without an additional water washing process, lithium by-products are present on the particle surface, and these lithium by-products react with cobalt to form a coating layer of LiCoO2 phase on the particle surface, thereby achieving excellent high-temperature life characteristics.

[0103] Positive electrode

[0104] The positive electrode of the present invention contains the positive electrode active material of the present invention described above. Specifically, the positive electrode contains a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and the positive electrode active material layer contains the positive electrode active material of the present invention. Since the positive electrode active material has been described above, its detailed description will be skipped, and only other components will be described below.

[0105] The positive electrode current collector may include a highly conductive metal, without particular limitation, as long as the positive electrode active material layer can easily adhere thereto and the positive electrode current collector has no reactivity within the voltage range of the battery. For example, as the positive electrode current collector, stainless steel, aluminum, nickel, titanium, heat-treated carbon, aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, etc. can be used. In addition, the thickness of the positive electrode current collector can generally be 3 μm to 500 μm, and fine unevenness can be formed on the surface of the current collector to improve the adhesion of the positive electrode active material. For example, the positive electrode current collector can be used in various forms, such as film, sheet, foil, net, porous body, foam body, and non-woven fabric body.

[0106] When necessary, in addition to the positive electrode active material, the positive electrode active material layer may optionally contain a conductive material and a binder.

[0107] In this case, relative to the total weight of the positive electrode active material layer, the content of the positive electrode active material can be 80% by weight to 99% by weight, more specifically 90% by weight to 98% by weight.

[0108] The conductive material is used to impart conductivity to the electrode, and any conductive material can be used without particular limitation as long as it has electron conductivity without causing chemical changes in the formed battery. Specific examples thereof may include: graphite, such as natural graphite or artificial graphite; carbon materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, and carbon fiber; metal powders or metal fibers, such as copper, nickel, aluminum, and silver; conductive tubes, such as carbon nanotubes; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and any one of them or a mixture of two or more of them can be used. Relative to the total weight of the positive electrode active material layer, the content of the conductive material can be 0.01% by weight to 10% by weight, preferably 0.1% by weight to 9% by weight, and more preferably 0.1% by weight to 5% by weight.

[0109] Binders are used to improve the binding between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples thereof may include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, polymethyl methacrylate, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, polyacrylic acid, and polymers obtained by substituting hydrogen in the above materials with Li, Na, or Ca, or various copolymers, and any one of them or a mixture of two or more thereof may be used. The content of the binder may be 1% by weight to 30% by weight, preferably 1% by weight to 20% by weight, more preferably 1% by weight to 10% by weight, based on the total weight of the positive electrode active material layer.

[0110] In addition to using the above positive electrode active material, the positive electrode can be prepared according to a conventional positive electrode preparation method. Specifically, a positive electrode slurry composition is prepared by dissolving or dispersing the positive electrode active material in a solvent, optionally dissolving or dispersing a binder, a conductive material, and a dispersant as necessary, and the positive electrode slurry composition is applied to a positive electrode current collector and then dried and calendered to prepare the positive electrode.

[0111] The solvent may be a solvent commonly used in the art, may be dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), dimethylformamide (DMF), acetone, water, etc., and any one of them or a mixture of two or more thereof may be used. If the solvent can dissolve or disperse the positive electrode active material, the conductive material, the binder, and the dispersant in consideration of the application thickness of the slurry and the preparation yield, and thereafter has a viscosity that can exhibit excellent thickness uniformity when used for preparing the positive electrode, the amount of the solvent is sufficient.

[0112] In addition, in another method, the positive electrode can be prepared by casting the positive electrode slurry composition on a separate carrier and then laminating the film separated from the carrier on the positive electrode current collector.

[0113] Lithium secondary battery

[0114] Next, the lithium secondary battery of the present invention will be described.

[0115] Specifically, the lithium secondary battery includes a positive electrode, a negative electrode disposed to face the positive electrode, a separator inserted between the positive electrode and the negative electrode, and an electrolyte. The positive electrode is the same as described above, and thus its detailed description will be omitted. Hereinafter, only the remaining components will be described in detail.

[0116] In addition, the lithium secondary battery may optionally further include a battery case for accommodating an electrode assembly composed of a positive electrode, a negative electrode, and a separator, and a sealing member for sealing the battery case.

[0117] In a 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.

[0118] The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, heat-treated carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., an aluminum-cadmium alloy, etc. can be used. In addition, the thickness of the negative electrode current collector can generally be 3 μm to 500 μm, and the same as in the case of the positive electrode current collector, fine irregularities can be formed on the surface of the negative electrode current collector to improve the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam body, and a non-woven fabric body.

[0119] In addition to the negative electrode active material, the negative electrode active material layer may optionally further include a binder and a conductive material.

[0120] Compounds capable of reversibly inserting and extracting lithium can be used as the negative electrode active material. Specific examples thereof may 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 alloys, Sn alloys, or Al alloys; metal oxides capable of doping and de-doping lithium such as SiO β (0 < β < 2), SnO2, vanadium oxides, and lithium vanadium oxides; or composites containing metal compounds and carbonaceous materials such as Si-C composites or Sn-C composites, and any one of them or a mixture of two or more of them can be used. In addition, as the negative electrode active material, a lithium metal thin film can be used. Further, as the carbon material, low-crystalline carbon, high-crystalline carbon, etc. can be used. Typical examples of low-crystalline carbon can be soft carbon and hard carbon, and typical examples of high-crystalline carbon can be natural graphite or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase carbon microspheres, mesophase pitch, and high-temperature heat-treated carbon such as coke derived from petroleum or coal tar pitch in an irregular shape, a planar shape, a flaky shape, a spherical shape, or a fibrous shape.

[0121] Relative to the total weight of the negative electrode active material layer, the content of the negative electrode active material can be 80% by weight to 99% by weight, 82% by weight to 99% by weight, or 84% by weight to 99% by weight.

[0122] The binder is a component that aids in binding the conductive agent, active material, and current collector, and the addition amount is generally 0.1 wt% to 10 wt% relative to the total weight of the negative electrode active material layer. Examples of the binder may include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber, nitrile rubber, fluororubber, various copolymers thereof, etc.

[0123] The conductive agent is a component used to further improve the conductivity of the negative electrode active material, and the content of the conductive agent can be 1 wt% to 30 wt%, 1 wt% to 20 wt%, or 1 wt% to 10 wt% relative to the total weight of the negative electrode active material layer. The conductive agent is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, the following conductive materials can be used: graphite, such as natural graphite and artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black; conductive fibers, such as carbon fibers and metal fibers; carbon fluorides; metal powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives, etc.

[0124] The negative electrode active material layer can be prepared by dissolving or dispersing the negative electrode active material and optionally the binder and conductive agent in a solvent to prepare a negative electrode slurry composition, applying the negative electrode slurry composition on the negative electrode current collector, and drying the resulting product, or can be prepared by casting the negative electrode slurry composition on a separate carrier and then laminating the film separated from the carrier on the negative electrode current collector.

[0125] Meanwhile, in a lithium secondary battery, the separator separates the negative electrode from the positive electrode and provides a migration path for lithium ions. Any separator can be used without particular limitation as long as it is generally used as a separator in a lithium secondary battery. In particular, a separator having a high water retention capacity for the electrolyte and a low resistance to the migration of electrolyte ions is preferred. Specifically, a porous polymer membrane can be used, such as a porous polymer membrane prepared using polyolefin polymers (such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer), or a laminated structure having two or more layers can be used. In addition, a conventional porous non-woven fabric can be used. For example, a non-woven fabric formed of glass fibers having a high melting point, polyethylene terephthalate fibers, etc. can be used. In addition, a coated separator containing a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and a separator having a single-layer structure or a multi-layer structure can be selectively used.

[0126] In addition, the electrolyte used in the present invention can be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc., all of which can be used to prepare lithium secondary batteries, but are not limited thereto.

[0127] Specifically, the electrolyte may contain an organic solvent and a lithium salt.

[0128] Any organic solvent can be used without particular limitation as long as it can serve as a medium through which the ions participating in the electrochemical reaction of the battery can move. Specifically, as the organic solvent, the following can be used: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (where R is a linear, branched, or cyclic C2 to C20 hydrocarbon group and may contain double bonds, aromatic rings, or ether bonds); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes. Among these solvents, carbonate solvents are preferred, and a mixture of a cyclic carbonate having a high ionic conductivity and a high dielectric constant (e.g., ethylene carbonate or propylene carbonate) and a linear carbonate compound having a low viscosity (e.g., methyl ethyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred, as this mixture can improve the charge-discharge performance of the battery.

[0129] Any compound can be used as the lithium salt without particular limitation as long as it can provide lithium ions used in lithium secondary batteries. Specifically, the anion of the lithium salt can be selected from F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2- , CH3CO2 - , SCN - and at least one selected from the group consisting of (CF3CF2SO2)2N - As the lithium salt, 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. The lithium salt can be used in a concentration range of 0.1 M to 4.0 M, preferably 0.5 M to 3.0 M, more preferably 1.0 M to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, thereby exhibiting excellent performance, and lithium ions can move effectively.

[0130] In addition to the above electrolyte components, one or more additives can be further included in the electrolyte, such as halogenated alkylene carbonate compounds (such as ethylene difluorocarbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol ethers, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, and aluminum trichloride, for the purposes of enhancing the life characteristics of the battery, suppressing the decrease in battery capacity, enhancing the discharge capacity of the battery, etc. In this case, the content of the additive can be 0.1% by weight to 10.0% by weight based on the total weight of the electrolyte.

[0131] The lithium secondary battery including the positive electrode active material of the present invention as described above stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate. Therefore, it can be used in portable devices such as mobile phones, laptop computers, and digital cameras, and can also be used in the field of electric vehicles such as hybrid electric vehicles (HEV).

[0132] Therefore, according to another embodiment of the present invention, there are provided a battery module including the lithium secondary battery as a unit cell and a battery pack including the battery module.

[0133] The battery module or the battery pack can be used as a power source for one or more medium and large-sized devices such as power tools, electric vehicles (such as electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (PHEV)), or power storage systems.

[0134] Hereinafter, embodiments of the present invention will be described in detail in a manner that can be easily implemented by those skilled in the art to which the present invention pertains. However, the present invention can be implemented in various different forms and is not limited to the embodiments described herein.

[0135] Examples and comparative examples

[0136] Example 1

[0137] A transition metal precursor with a molar ratio of Ni:Co:Mn of 95:3:2 is mixed with a lithium raw material (LiOH) such that the molar ratio of transition metal (Ni + Co + Mn):Li is 1:1.05, and then the mixture is subjected to a first firing at 800 °C for 15 hours to prepare LiNi 0.95 Co 0.03 Mn 0.02 O2.

[0138] After that, the first-fired product is mixed with Al2O3 and Co3O4 such that the molar ratio of the first-fired product:Al:Co is 100:0.2:2, and then the mixture is subjected to a second firing at 700 °C for 6 hours to prepare a positive electrode active material LiNi 0.928 Co 0.05 Mn 0.02 Al 0.002 O2.

[0139] The SEM image of the prepared positive electrode active material powder is shown in Figure 1 The EELS image of the prepared positive electrode active material powder is shown in Figure 7 shown.

[0140] Example 2

[0141] A positive electrode active material powder LiNi with a cobalt and aluminum coating layer is prepared in the same manner as in Example 1, except that the second firing is carried out such that the molar ratio of the first-fired product:Al:Co is 100:0.4:2 0.926 Co 0.05 Mn 0.02 Al 0.004 O2.

[0142] The SEM image of the prepared positive electrode active material powder is shown in Figure 2 shown.

[0143] Comparative example 1

[0144] A transition metal precursor with a molar ratio of Ni:Co:Mn of 93:5:2 was mixed with a lithium raw material (LiOH) such that the molar ratio of transition metal (Ni + Co + Mn):Li was 1:1.05, and then the mixture was subjected to a first firing at 800 °C for 15 hours to prepare LiNi 0.93 Co 0.05 Mn 0.02 O2, and the positive electrode active material powder was prepared in the same manner as in Example 1 except that no coating layer was formed due to not performing a second firing.

[0145] The SEM image of the prepared positive electrode active material powder is shown in Figure 3 The EELS image of the prepared positive electrode active material powder is shown in Figure 8

[0146] Comparative example 2

[0147] The positive electrode active material powder LiNi was prepared in the same manner as in Example 1 except that the first fired product was only mixed with Co3O4 such that the molar ratio of the first fired product:Co was 100:2, and then a second firing was carried out at 700 °C for 6 hours to form a cobalt-only coating layer. 0.93 Co 0.05 Mn 0.02 O2.

[0148] The SEM image of the prepared positive electrode active material powder is shown in Figure 4 The EELS image of the prepared positive electrode active material powder is shown in Figure 9

[0149] Comparative example 3

[0150] The positive electrode active material powder LiNi was prepared in the same manner as in Example 1 except that the first fired product was only mixed with Co3O4 such that the molar ratio of the first fired product:Co was 100:2, and then a second firing was carried out at 650 °C for 6 hours to form a cobalt-only coating layer. 0.93 Co 0.05 Mn 0.02 O2.

[0151] The SEM image of the prepared positive electrode active material powder is shown in Figure 5 The EELS image of the prepared positive electrode active material powder is shown in Figure 10

[0152] Comparative example 4

[0153] ​​​In addition to performing secondary firing such that the molar ratio of Al:Co in the primary fired product is 100:0.05:2, a positive electrode active material powder LiNi having a cobalt and aluminum coating layer formed thereon was prepared in the same manner as in Example 1 0.925 Co 0.05 Mn 0.02 Al 0.005 O2.

[0154] The SEM image of the obtained positive electrode active material powder is shown in Figure 6 The EELS image of the obtained positive electrode active material powder is shown in Figure 11

[0155] Comparative example 5

[0156] In addition to performing secondary firing such that the molar ratio of Al:Co in the primary fired product is 100:0.6:2, a positive electrode active material powder LiNi having a cobalt and aluminum coating layer formed thereon was prepared in the same manner as in Example 1 0.924 Co 0.05 Mn 0.02 Al 0.006 O2.

[0157] Experimental example 1: Observation of the surface of the positive electrode active material

[0158] Using a scanning electron microscope device, SEM images of the positive electrode active material powders obtained in Example 1 and 2 and Comparative Examples 1 to 4 were obtained. These SEM images are shown in Figures 1 to 6

[0159] The surfaces of the positive electrode active material powders obtained in Example 1 and Comparative Examples 1 to 4 were analyzed by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). The obtained surface analysis images are each shown in Figures 7 to 11

[0160] Figure 1 , Figure 2 and Figure 7 show that in the positive electrode active materials obtained in Example 1 and 2, the cobalt and aluminum coating layers are formed to be discontinuously distributed on the surfaces of the lithium nickel-based transition metal oxide particles.

[0161] Figures 3 to 6 and Figures 8 to 11 ​​​It is shown that the positive electrode active material obtained in Comparative Example 1 has a clean surface without a coating layer, the positive electrode active material obtained in Comparative Example 2 includes a uniformly layered cobalt coating layer on the surface, the positive electrode active material obtained in Comparative Example 3 has a discontinuously distributed cobalt coating layer on the surface due to a coating temperature lower than that in Comparative Example 2, and the positive electrode active material obtained in Comparative Example 4 has a uniformly layered cobalt coating layer on the surface due to almost no formation of an aluminum coating layer.

[0162] Experimental example 2: XRD analysis

[0163] For comparative analysis of the coating layers contained in the positive electrode active materials obtained in Examples 1 and 2 and Comparative Examples 1 to 5 respectively, XRD analysis was performed on each positive electrode active material using a diffractometer from Rigaku, and the results are shown in Figure 12 . The measurement conditions were based on Cu-Kα radiation, and the 2θ range from 35° to 48° was measured at 40 kV to obtain an X-ray diffraction analysis pattern. In the final analysis data, the presence or absence of peaks in the 2θ ranges of 37° to 38° and 45° to 46° indicates the coating form with a LiCoO2 (hereinafter referred to as LCO) phase on the surface of the lithium nickel-based transition metal oxide particles in each positive electrode active material.

[0164] Figure 12 It is shown that, as the XRD analysis result of the positive electrode active material obtained in Example 1, it is found that the coating layer exists in the LCO phase due to the presence of peaks in the 2θ ranges of 37° to 38° and 45° to 46°.

[0165] In the positive electrode active materials obtained in Comparative Examples 1, 2, and 4, it was found that there were no peaks in the 2θ ranges of 37° to 38° and 45° to 46°. In contrast, in the positive electrode active material obtained in Comparative Example 3, it was found that peaks existed in the 2θ ranges of 37° to 38° and 45° to 46°, but the intensity was lower than the peaks in Examples 1 and 2.

[0166] Experimental example 3: Measurement of calendering density

[0167] The positive electrode active materials obtained in Examples 1 and 2 and Comparative Examples 1 to 5 were calendered under the condition of 9 tons, and then the calendered density was measured. The measurement results are shown in Table 1 below. Specifically, 3 g of the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 to 5 were divided into small portions using a densitometer (Carve Pellet Press), each portion was placed in a cylindrical mold, and then the mold containing the positive electrode active material was pressurized with a pressure of 9 tons. After that, a vernier caliper was used to measure the height of the pressurized mold to obtain the calendered density.

[0168] [Table 1]

[0169] Calendering density (g / cc) Example 1 3.54 Example 2 3.53 Comparative example 1 3.56 Comparative example 2 3.58 Comparative example 3 3.56 Comparative example 4 3.57 Comparative example 5 3.48

[0170] As shown in Table 1 above, the calendering density of the positive electrode active materials prepared in Examples 1 and 2 and Comparative Example 5 is lower than that of the positive electrode active materials prepared in Comparative Examples 1 to 4, and the calendering density of the positive electrode active material of Comparative Example 5 is lower than that of the positive electrode active materials of Examples 1 and 2. This indicates that a cobalt coating layer and an aluminum coating layer are formed on the surfaces of the positive electrode active materials of Examples 1 and 2 and Comparative Example 5 where the molar ratio of aluminum to the primary firing product (aluminum / primary firing product) is 0.2, 0.4, and 0.6, respectively. Therefore, as the surface properties of the positive electrode active material change, the calendering density decreases.

[0171] Experimental example 4: Measurement of the initial resistance value

[0172] The initial resistance values of lithium secondary battery coin half-cells manufactured using the positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 to 5, respectively, were measured. The measurement results are shown in Table 2 below. The initial resistance value was calculated as the voltage change rate when the SOC was set to 50% at 0.3C in the second cycle and then a current of 2.5C was applied for 10 seconds with respect to the first cycle discharge capacity measured at 0.3C.

[0173] Specifically, the lithium secondary battery coin half-cells were manufactured as follows.

[0174] The positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 to 5, a conductive material (carbon black, Denka), and a PVDF binder were mixed in a weight ratio of 96:1:3 in N-methylpyrrolidone to prepare a positive electrode paste. The positive electrode paste was applied to one surface of an aluminum current collector, dried at 130°C, and then calendered to prepare a positive electrode.

[0175] The negative electrode active material, binder, and conductive material were mixed in a weight percentage of 97.0:1.5:1.5 in an N-methylpyrrolidone solvent to prepare a negative electrode mixture material. In this case, artificial graphite was used as the negative electrode active material, carboxymethyl cellulose (CMC) was used as the binder, and carbon black was used as the conductive material. The prepared negative electrode mixture material was applied to a copper current collector, dried at 110°C, and calendered to prepare a negative electrode.

[0176] A separator was inserted between the positive electrode and the negative electrode prepared by the above method to manufacture an electrode assembly. The electrode assembly was placed in a battery case, and then an electrolyte was injected into the battery case to manufacture a coin half-cell. The electrolyte was prepared by dissolving LiPF6 at a concentration of 1M in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) mixed in a volume ratio of 1:1:1 and adding 5 wt% of vinylene carbonate (VC).

[0177] [Table 2]

[0178] Initial resistance (Ω) Example 1 1.50 Example 2 1.50 Comparative example 1 1.54 Comparative example 2 1.43 Comparative example 3 1.48 Comparative example 4 1.48 Comparative example 5 1.55

[0179] Table 2 above shows that the initial resistance values of the positive electrode active materials prepared in Examples 1 and 2 are lower than those of Comparative Example 1 in which no coating layer is formed on the surface, but their initial resistance values are higher than those of Comparative Examples 2 to 4. It was found that the initial resistance value of the positive electrode active material prepared in Comparative Example 5 is significantly higher than those of the positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 to 4. This is considered to be due to the decrease in the lithium mobility in the positive electrode active material when aluminum increases on the surface of the positive electrode active material, resulting in this initial resistance value.

[0180] Experimental example 5: Evaluation of high-temperature life

[0181] The lithium secondary battery coin half-cells containing the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 to 5 manufactured by the above method were charged and discharged 50 times (hereinafter set as one cycle: charged to 4.25 V at 0.5C in CC-CV mode at 45 °C, and discharged to 2.5 V at 1.0C), and then the capacity retention rate (%) was measured to evaluate the high-temperature life characteristics. The measurement results are shown in Figure 13 .

[0182] Referring to Figure 13 , it was found that under the condition of 50 cycles, the capacity retention rates of the lithium secondary batteries of Examples 1 and 2 and Comparative Example 5 are respectively above 90.0%, while the capacity retention rates of the lithium secondary batteries of Comparative Examples 1 to 4 are respectively 85.0%, 86.6%, 88.3% and 88.8%.

[0183] In the positive electrode active materials of Examples 1 and 2, both a cobalt coating layer and an aluminum coating layer are formed on the surface. Aluminum inhibits the diffusion of cobalt into the positive electrode active material particles, so a coating layer with a high cobalt content can be formed. That is, it was found that compared with Comparative Examples 1 to 4, the lithium secondary batteries containing the positive electrode active materials of Examples 1 and 2 have higher capacity retention rates and longer high-temperature lives.

[0184] It was found that in the positive electrode active material of Comparative Example 1, no coating layer was formed on the surface. Therefore, the capacity retention rate of the lithium secondary battery was significantly lower, indicating the worst high-temperature life characteristics. It was found that the positive electrode active material of Comparative Example 2 had a cobalt-only coating layer formed on the surface. Therefore, due to the relatively high coating temperature, cobalt was absorbed from the surface of the positive electrode active material and diffused into the positive electrode active material, indicating that the improvement in high-temperature life characteristics was not significant. It was found that the positive electrode active material of Comparative Example 3 had a cobalt-only coating layer formed on the surface, but at a coating temperature lower than that of Comparative Example 2, the phenomenon of cobalt being absorbed from the surface of the positive electrode active material and diffused into the positive electrode active material was reduced. Therefore, the capacity retention rate was higher, and the improvement in high-temperature life characteristics was greater than that of Comparative Example 2. It can be seen that in the positive electrode active material of Comparative Example 4, almost no aluminum coating layer was formed on the surface. Therefore, the capacity retention rate of the positive electrode active material of Comparative Example 4 was almost similar to that of Comparative Example 3.

[0185] Meanwhile, it was found that in the positive electrode active material of Comparative Example 5, similar to the positive electrode active materials of Examples 1 and 2, a cobalt coating layer and an aluminum coating layer were formed on the surface, and aluminum inhibited the diffusion of cobalt into the positive electrode active material particles, thereby forming a cobalt-rich coating layer. Therefore, compared with Comparative Examples 1 to 4, the positive electrode active material of Comparative Example 5 had a higher capacity retention rate and a longer high-temperature life. However, as described above, in the positive electrode active material of Comparative Example 5, the calendering density was excessively reduced and the initial resistance value increased.

Claims

1. A positive electrode active material, comprising: Lithium nickel transition metal oxide particles, wherein a molar ratio of nickel in all transition metals of 80 mol% or more; and a coating layer formed on the surface of the lithium nickel-based transition metal oxide particles, wherein the lithium nickel-based transition metal oxide particles are in the form of single particles composed of a single nodule or quasi-single particles that are a composite of at most 30 nodules, the coating layer contains cobalt and aluminum, and the aluminum content is 0.1 mol% to 0.5 mol% relative to 100 moles of the lithium nickel-based transition metal oxide.

2. The positive electrode active material according to claim 1, wherein The cobalt content is 0.5 mol% to 5.0 mol% relative to 100 moles of the lithium nickel-based transition metal oxide.

3. The positive electrode active material according to claim 1, wherein In the lithium nickel-based transition metal oxide particles, the molar ratio of nickel in all transition metals is 90 mol% or more.

4. The positive electrode active material according to claim 1, wherein The coating layer is formed to be discontinuously distributed on the surface of the lithium nickel-based transition metal oxide particles.

5. The positive electrode active material according to claim 1, wherein The positive electrode active material has a peak in the range of 2θ = 37° to 38° in the XRD pattern.

6. The positive electrode active material according to claim 1, wherein The positive electrode active material has a peak in the range of 2θ = 45° to 46° in the XRD pattern.

7. The positive electrode active material according to claim 1, wherein, The average particle size of the nodules of the positive electrode active material is 1.0 μm to 9.0 μm.

8. The positive electrode active material according to claim 1, wherein, The D of the positive electrode active material 50 is from 4.0 μm to 9.0 μm.

9. A method for preparing a positive electrode active material, the method comprising: (A) mixing a transition metal precursor containing nickel, cobalt, and manganese with a lithium raw material, and subjecting the mixture to a first firing to form a lithium nickel-based transition metal oxide; and (B) mixing the lithium nickel-based transition metal oxide with cobalt and aluminum, and subjecting the mixture to a second firing to form a positive electrode active material containing a coating layer, wherein, in the lithium nickel-based transition metal oxide, the molar ratio of nickel in all transition metals is 80 mol% or more, the lithium nickel-based transition metal oxide is a single particle composed of a single nodule or a quasi-single particle that is a composite of at most 30 nodules, and in step (B), the molar ratio of aluminum to the lithium nickel-based transition metal oxide, i.e., aluminum / lithium nickel-based transition metal oxide, is 0.001 to 0.

005.

10. The method of claim 9, wherein: Step (B) is carried out at 660°C to 720°C.

11. The method of claim 9, wherein: The coating layer is formed to be discontinuously distributed on the surface of the lithium nickel-based transition metal oxide particles.

12. The method according to claim 9, wherein, Step (A) is carried out at 800°C to 890°C.

13. The method of claim 9, wherein: No additional washing process is carried out between step (A) and step (B).

14. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 8.

15. A lithium secondary battery comprising the positive electrode according to claim 14.

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