Positive electrode active material, method for manufacturing the same, positive electrode containing the same, and lithium secondary battery

Abstract

The present invention relates to a positive electrode active material and a method for producing the same, which include a lithium composite transition metal oxide in the form of secondary particles aggregated from a plurality of grains, and having an oriented structure in which the long axes of the grains are arranged from the center to the surface direction of the secondary particles in at least a part of the secondary particles, and a coating layer formed on the surface of the secondary particles and containing Co element, wherein the lithium composite transition metal oxide contains nickel and cobalt, and when the ratio of the number of moles of cobalt to the number of moles of nickel inside the grains is C1, the ratio of the number of moles of cobalt to the number of moles of nickel at the grain boundary which is the interface between the grains is C2, and the ratio of the number of moles of cobalt to the number of moles of nickel in the coating layer is C3, C1 < C2 < C3 is satisfied.

Description

Technical Field

[0001] The present invention relates to a positive electrode active material, a method for manufacturing the same, a positive electrode including the same, and a lithium secondary battery. More specifically, the present invention relates to a positive electrode active material capable of realizing excellent high-temperature life characteristics with a small amount of gas generation at high temperatures, a method for manufacturing the same, a positive electrode including the positive electrode active material, and a lithium secondary battery.

Background Art

[0002] Recently, with the increasing development and demand for technologies for mobile devices and electric vehicles, the demand for secondary batteries as an energy source has been rapidly increasing. Among such secondary batteries, lithium secondary batteries having a high energy density, a high voltage, a long cycle life, and a low self-discharge rate have been commercialized and widely used.

[0003] As positive electrode active materials for lithium secondary batteries, lithium transition metal oxides such as lithium cobalt oxide such as LiCoO2, lithium nickel oxide such as LiNiO2, lithium manganese oxide such as LiMnO2 or LiMn2O4, and lithium iron phosphate oxide such as LiFePO4 have been developed. Recently, lithium composite transition metal oxides containing two or more transition metals such as Li[Ni a Co b Mn c O2, Li[Ni a Co b Al c O2, Li[Ni a Co[[ID=3l]] b Mn c [[ID=३4]]Al d O2 have been developed and used.

[0004] [[ID=३9]] Among these, lithium composite transition metal oxides containing two or more transition metals are usually manufactured in the form of spherical secondary particles in which dozens to hundreds of primary particles are aggregated, and physical properties such as lithium ion mobility and electrolyte impregnation properties change according to the orientation form of the primary particles and the shape (aspect ratio) of the primary particles. Therefore, research has been attempted to control the particle structure of the positive electrode active material particles to improve the performance of the positive electrode active material.

[0005] Korean Registered Patent Publication No. 10-1611784 (Patent Document 1) discloses a positive electrode active material in which the length of the a-axis of the primary particles is longer than the length of the c-axis, the a-axis orientation of the primary particles is directed toward the center of the secondary particles, and the primary particles grow adjacent to each other along a predetermined path.

[0006] As described above, in the case of a positive electrode active material in which the primary particles are radially arranged toward the center of the secondary particles, the lithium migration path within the secondary particles is shortened, facilitating the insertion and desorption of lithium ions, and the effect of reducing the electrical resistance can be obtained. However, a positive electrode active material in which the primary particles are radially arranged is more likely to crack during rolling than a positive electrode active material in which the primary particles are arranged without orientation. As a result, side reactions with the electrolyte increase, accelerating cell degradation and increasing gas generation. Such problems are particularly prominent during high-temperature exposure.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] The present invention is for solving the above problems, and by including an oriented structure in which the long axis of the grains is arranged from the center to the surface direction of the secondary particles in at least a part of the secondary particles, it is excellent in lithium mobility and reduction characteristics, and is formed such that the molar ratio of Co / Ni increases in the order of the inside of the grains, the grain boundary (grain boundary) between the grains, and the coating layer on the surface of the secondary particles. Even during high-temperature exposure, a positive electrode active material and a method for manufacturing the same that can realize excellent life characteristics and gas generation characteristics are provided.

[0009] In addition, the present invention provides a positive electrode and a lithium secondary battery including the above positive electrode active material.

Means for Solving the Problem

[0010] According to one embodiment, the present invention is in the form of secondary particles in which a plurality of grains are aggregated, and has an oriented structure in which the long axes of the grains are arranged from the center to the surface direction of the secondary particles in at least a part of the secondary particles. A lithium composite transition metal oxide, and a positive electrode active material including a coating layer containing Co element formed on the surface of the secondary particles, wherein the lithium composite transition metal oxide contains nickel and cobalt, and the ratio of the number of moles of cobalt to the number of moles of nickel inside the grain is C1, the ratio of the number of moles of cobalt to the number of moles of nickel at the grain boundary which is the interface between the grains is C2, and the ratio of the number of moles of cobalt to the number of moles of nickel in the coating layer is C3. When C3, a positive electrode active material satisfying C1 < C2 < C3 is provided.

[0011] According to another embodiment, the present invention includes a step of preparing a lithium composite transition metal oxide in the form of secondary particles in which a plurality of grains are aggregated, and having an oriented structure in which the long axes of the grains are arranged from the center to the surface direction of the secondary particles in at least a part of the secondary particles, and mixing the lithium composite transition metal oxide with a coating solution containing cobalt element, followed by a first coating step of performing a first heat treatment, and a second coating step of dry-mixing the first-coated lithium composite transition metal oxide and a coating raw material containing cobalt element, followed by a second heat treatment. A method for manufacturing a positive electrode active material is provided.

[0012] According to still another embodiment, the present invention provides a positive electrode including the positive electrode active material according to the present invention, and a lithium secondary battery including the positive electrode.

Advantages of the Invention

[0013] The positive electrode active material according to the present invention includes an oriented structure in which the long axes of the grains are arranged so that they radiate from the center of the secondary particles toward the surface, resulting in a short lithium diffusion path within the particles, thereby enabling excellent lithium mobility and resistance characteristics.

[0014] Furthermore, the positive electrode active material according to the present invention is formed such that the ratio of molars of Co / Ni increases in the order of the interior of the grain, the grain boundaries (interfaces between grains), and the coating layer on the surface of the secondary particles. This minimizes side reactions with the electrolyte even if particle cracking occurs due to rolling, thereby achieving superior high-temperature characteristics compared to conventional positive electrode active materials with oriented structures.

[0015] The positive electrode active material according to the present invention is obtained by wet coating a lithium composite transition metal oxide having an oriented structure with a Co-containing coating solution, and then dry coating it by mixing it with a Co-coating material, thereby increasing the ratio of Co / Ni moles in the coating layer in the order of the interior of the grains of the positive electrode active material, the grain boundaries, and the surface of the secondary particles. [Brief explanation of the drawing]

[0016] [Figure 1] This is a cross-sectional SEM image of a lithium composite transition metal oxide produced by Production Example 1. [Figure 2] This is a cross-sectional SEM image of a lithium composite transition metal oxide produced by Manufacturing Example 2. [Modes for carrying out the invention]

[0017] The terms and words used in this specification and in the claims should not be interpreted in a manner limited to their ordinary or dictionary meanings, but rather should be interpreted in a manner consistent with the technical idea of ​​the present invention, in accordance with the principle that inventors may define the concepts of terms as appropriate to best describe their invention.

[0018] In the present invention, "grain" refers to the smallest particle unit that, when observed with a scanning electron microscope at a field of view of 5,000x to 20,000x, does not appear to have grain boundaries and can be distinguished as a single mass. It may consist of one crystallite or multiple crystallites. In the present invention, the average grain size can be measured by measuring the size of each particle distinguished by cross-sectional SEM data of the positive electrode active material particles and then calculating the arithmetic mean of these sizes.

[0019] In this invention, "secondary particle" means a secondary structure formed by the aggregation of multiple grains.

[0020] In the present invention, "D 50 " refers to the particle size at the 50% reference level of the volume cumulative particle size distribution of the positive electrode active material powder. The average particle size D 50 This can be measured using the laser diffraction method. For example, after dispersing the positive electrode active material powder in a dispersion medium, the particle size can be measured by introducing it into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000), irradiating it with ultrasound at approximately 28 kHz with an output of 60 W, obtaining a volume-cumulative particle size distribution graph, and then determining the particle size corresponding to 50% of the volume-cumulative amount.

[0021] In the present invention, "oriented structure" means a structure in which the long axes of the grains are aligned from the center of the secondary particle toward the surface. Here, "the long axes of the grains are aligned from the center of the secondary particle toward the surface" means that the angle between the shortest line segment connecting the center and surface of the secondary particle while passing through the grains and the long axis of the grains is in the range of -15° to 15°.

[0022] In this invention, "grain aspect ratio" means the ratio of the major axis length to the minor axis length of the grain, and "average aspect ratio" means the arithmetic mean of the aspect ratios of the grains in that region.

[0023] The present invention will be described in detail below.

[0024] <Cathode active material> The positive electrode active material according to the present invention comprises (1) a lithium composite transition metal oxide and (2) a coating layer formed on the surface of secondary particles of the lithium composite transition metal oxide and containing the element Co.

[0025] The lithium composite transition metal oxide is in the form of secondary particles in which multiple grains are aggregated, and at least a portion of the secondary particles includes an oriented structure in which the long axes of the grains are aligned from the center of the secondary particle toward the surface. Here, "the long axes of the grains are aligned from the center of the secondary particle toward the surface" means that the angle between the long axis of the grain and the shortest line segment connecting the center and surface of the secondary particle while passing through the grain is in the range of -15° to 15°. The long axis of the grain means the line segment having the longest length among the straight lines connecting two points on the surface of the grain while passing through the center of the grain. Within the secondary particles of the positive electrode active material, the interfaces between grains become diffusion pathways for lithium ions. When grains are aligned so as to be directed from the center of the secondary particle toward the surface, the diffusion pathway for lithium ions within the secondary particle is shortened, increasing lithium mobility, which in turn improves the power and / or resistance characteristics.

[0026] On the other hand, in grains where the long axes of the grains are aligned from the center of the secondary particles toward the surface, the angle between the long axis and the a-axis direction of the crystal structure can be within -15° to 15°, preferably within -10° to 10°. Since lithium ions move in the a-axis direction within the grain, when the angle between the long axis of the grain and the a-axis direction of the crystal structure is -15° to 15°, the insertion and removal of lithium ions becomes easier, thereby improving the output and / or resistance characteristics.

[0027] On the other hand, grains in which the long axis of the grains is aligned from the center of the secondary particles toward the surface can have an aspect ratio of 1.5 to 15, preferably 2 to 15, and more preferably 4 to 15. When the aspect ratio of the grains satisfies the above range, during charging and discharging, the contraction and expansion of the grains mainly occur in the direction of the short axis, which is perpendicular to the direction of alignment, and the occurrence of cracks caused by uneven contraction and expansion of the grains can be minimized.

[0028] On the other hand, the lithium composite transition metal oxide can have a core-shell structure, but is not limited to this, comprising a core portion in which grains are aggregated without any special orientation, and a shell portion in which the long axes of the grains are aligned from the center of the secondary particles toward the surface.

[0029] The core portion is a region in which grains aggregate disorderly without any particular orientation, and is formed in the center of the secondary particle. The core portion can be a portion formed as a seed during a coprecipitation reaction for the formation of a precursor for the positive electrode active material, and can be, for example, a region at a distance of (1 / 3)R from the center of the secondary particle, or a region at a distance of (1 / 4)R from the center of the secondary particle, where R is the radius of the secondary particle.

[0030] The grains within the core can be nearly spherical in shape, and their aspect ratio can be 0.7 to 1.3, preferably 0.8 to 1.2.

[0031] Next, the shell portion is a region formed on the outside of the core portion in which grains are arranged in an oriented structure. The shell portion can be a part formed as particles grow during a coprecipitation reaction for the formation of a precursor for the positive electrode active material, and can be, for example, a region from (1 / 3)R to R of the secondary particle, or a region from (1 / 4)R to R of the secondary particle, where R is the radius of the secondary particle from the center of the secondary particle.

[0032] The grain within the shell portion can be rod-shaped, and its aspect ratio can preferably be 2 to 15, more preferably 4 to 15.

[0033] On the other hand, in the positive electrode active material of the present invention, the average grain size can be 0.05 μm to 4 μm, preferably 0.1 μm to 3 μm, and more preferably 0.1 μm to 2 μm. If the average grain size is too large, a rock salt phase may be formed, which may reduce the resistance characteristics and lifetime characteristics. If the average grain size is too small, the contact area with the electrolyte may increase, which may lead to rapid degradation.

[0034] The lithium composite transition metal oxide comprises nickel and cobalt, and may have a composition represented by, for example, the following [Chemical Formula 1].

[0035] [Chemical formula 1] Li X [Ni a Co b M 1 c M 2 d ]O 2-y A y

[0036] In the above [Chemical Formula 1], the M 1 can be one or more elements selected from the group consisting of Mn and Al, for example, Mn or a combination of Mn and Al.

[0037] Said M 2 This can be one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo.

[0038] Furthermore, A can be one or more elements selected from the group consisting of F, Cl, Br, I, At, and S.

[0039] The x represents the ratio of the number of moles of Li to the total number of moles of transition metals, and can be 0.98 ≦ x ≦ 1.20, preferably 0.99 ≦ x ≦ 1.10, more preferably 1.0 ≦ x ≦ 1.10.

[0040] The a represents the ratio of the number of moles of Ni to the total number of moles of the remaining metals other than lithium, and can be 0 < a < 1, preferably 0.3 ≦ a < 1, more preferably 0.6 ≦ a < 1, still more preferably 0.8 ≦ a < 1, still more preferably 0.85 ≦ a < 1. <s

[0041] <s? The b represents the ratio of the number of moles of Co to the total number of moles of the remaining metals other than lithium, and can be 0 < b < 1, preferably 0 < b < 0.7, more preferably 0 < b < 0.4, still more preferably 0 < b < 0.2, still more preferably 0 < b ≦ 0.1.

[0042] The c represents the ratio of the number of moles of M to the total number of moles of the remaining metals other than lithium 1 and can be 0 < c < 1, preferably 0 < c < 0.7, more preferably 0 < c < 0.4, still more preferably 0 < c < 0.2, still more preferably 0 < c ≦ 0.1.

[0043] The d represents the ratio of the number of moles of M to the total number of moles of the remaining metals other than lithium 2 and can be 0 ≦ d ≦ 0.2, preferably 0 ≦ d ≦ 0.15, more preferably 0 ≦ d ≦ 0.10.

[0044] The y represents the ratio of the number of moles of element A substituted at the oxygen site, and can be 0 ≦ y ≦ 0.2, preferably 0 ≦ y ≦ 0.15, more preferably 0 ≦ y ≦ 0.10.

[0045] On the other hand, the lithium composite transition metal oxide includes a coating layer containing the element Co in at least a portion of the surface of the secondary particles. When a coating layer is formed on the surface of the lithium composite transition metal oxide, the coating layer suppresses contact between the lithium composite transition metal oxide and the electrolyte, reducing side reactions with the electrolyte, thereby improving the lifetime characteristics. In particular, when the coating layer contains the element Co, the effect of reducing initial resistance is better compared to when it contains other coating elements.

[0046] The shape and area of ​​the coating layer are not particularly limited. For example, the coating layer may be a continuous film surrounding the entire surface of the secondary particles, or it may be particulate, discontinuously distributed on the surface of the secondary particles. Furthermore, the area of ​​the coating layer can be 10% to 100%, 10% to 80%, or 20% to 70% of the total surface area of ​​the secondary particles.

[0047] The coating layer may be an oxide containing lithium and cobalt, and may have a composition represented by the following [Chemical Formula 2].

[0048] [Chemical formula 2] Li z Co 1-w M 3 w O2

[0049] In the above [Chemical Formula 2], the M 3 This can be one or more selected from the group consisting of Ni, Mn, Al, W, Cu, Fe, V, Cr, Ti, Zr, Zn, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and can satisfy 0.8 ≤ z ≤ 1.2 and 0 ≤ w ≤ 0.2. Preferably, the M 3 z can be one or more elements selected from the group consisting of Ni, Mn, Al, Ti, Zr, and Mg, and can satisfy the conditions 0.8 ≤ z ≤ 1.1 and 0 ≤ w ≤ 0.1.

[0050] When the coating layer satisfies the composition as described above, the effects of improving the initial resistance characteristics and high-temperature life can be obtained.

[0051] On the other hand, for the positive electrode active material according to the present invention, when the ratio of the number of moles of cobalt to the number of moles of nickel (Co / Ni) inside the grain is C1, the ratio of the number of moles of cobalt to the number of moles of nickel at the grain boundary (Grain boundary) which is the interface between the grains is C2, and the ratio of the number of moles of cobalt to the number of moles of nickel in the coating layer is C3, it satisfies C1 < C2 < C3. As in the present invention, when the molar ratio of Co / Ni is higher at the grain boundary than inside the grain, the effect of improving the strength of the particles can be obtained by Co present at the grain boundary, whereby cracking of the particles due to rolling is suppressed. Also, as in the present invention, in the case of secondary particles in which the grains are arranged with orientation, compared to secondary particles without grain orientation, more side reactions of the electrolytic solution and surface deterioration occur on the surface of the secondary particles. However, as in the present invention, when the ratio of the number of moles of Co / Ni is formed to be the highest on the surface, the effect of alleviating the side reaction and deterioration of the electrolytic solution on the surface of the secondary particles can be obtained. That is, when the ratio of the number of moles of Co / Ni satisfies the above conditions, cracking of the particles and side reactions with the electrolytic solution on the surface of the secondary particles can be minimized, and thereby, excellent high-temperature characteristics can be realized compared to the conventional positive electrode active material having an oriented structure.

[0052] The positive electrode active material is D 50 can be 2 μm to 25 μm, preferably 2 μm to 20 μm, more preferably 4 μm to 18 μm. When D of the positive electrode active material 50 satisfies the above range, it is possible to prevent the positive electrode active material particles from cracking in the rolling process or the processability from decreasing during the production of the slurry.

[0053] <Method for manufacturing positive electrode active material> Next, the method for manufacturing the positive electrode active material according to the present invention will be described.

[0054] The method for producing a positive electrode active material according to the present invention includes the steps of: (1) preparing a lithium composite transition metal oxide in the form of secondary particles in which a plurality of grains are aggregated, and in at least a portion of the secondary particles having an oriented structure in which the long axes of the grains are aligned from the center of the secondary particles toward the surface; (2) a first coating step of mixing the lithium composite transition metal oxide with a coating solution containing a cobalt element and then performing a first heat treatment; and (3) a second coating step of dry mixing the first coated lithium composite transition metal oxide with a coating raw material containing a cobalt element and then performing a second heat treatment.

[0055] (1) Preparation step of lithium complex transition metal oxide First, a lithium composite transition metal oxide is prepared in which at least a portion of the secondary particles have an oriented structure in which the long axes of the grains are aligned from the center of the secondary particles toward the surface. The specific specifications of the lithium composite transition metal oxide are as described above.

[0056] As described above, lithium composite transition metal oxides containing an oriented structure may be prepared by purchasing commercially available products, or they may be manufactured using a lithium composite transition metal oxide manufacturing method that is well known in the industry.

[0057] For example, the lithium composite transition metal oxide can be produced by introducing a metal solution containing nickel and cobalt elements, an ammonium cation complex forming agent, and a basic compound into a reactor, causing a coprecipitation reaction to produce a precursor for the positive electrode active material, mixing the precursor with a lithium raw material, and then calcining the mixture.

[0058] The metal solution can be produced by dissolving nickel and cobalt raw materials in a solvent such as water. The nickel raw material can be nickel acetate, carbonate, nitrate, sulfate, halide, sulfide, or oxide, and is not limited to NiO, NiCO3·2Ni(OH)2·4H2O, NiC2O2·2H2O, Ni(NO3)2·6H2O, NiSO4, NiSO4·6H2O, nickel halides, or combinations thereof. The cobalt raw material can be cobalt metal acetate, carbonate, nitrate, sulfate, halide, sulfide, or oxide, and is not limited to CoSO4, Co(OCOCH3)2·4H2O, Co(NO3)2·6H2O, CoSO4·7H2O, or combinations thereof.

[0059] The ammonium cation complex-forming agent can be at least one selected from the group consisting of NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4, and (NH4)2CO3, and the compound can be introduced into the reactor in solution in a solvent. Here, the solvent can be water, or a mixture of water and an organic solvent that is homogeneously miscible with water (specifically, an alcohol, etc.).

[0060] Furthermore, the basic compound can be at least one selected from the group consisting of NaOH, KOH, and Ca(OH)2, and the compound can be introduced into the reactor in solution in a solvent. Here, the solvent can be water, or a mixture of water and an organic solvent that can be homogeneously mixed with water (specifically, an alcohol, etc.).

[0061] On the other hand, in order to produce a positive electrode active material having an oriented structure as in the present invention, a positive electrode active material precursor having an oriented structure in which primary particles are arranged from the center toward the surface of secondary particles must be used. The aggregation morphology of the primary particles of the positive electrode active material precursor is affected by the pH, stirring speed, and reaction temperature during the coprecipitation reaction. Therefore, by appropriately adjusting the pH, stirring speed, and reaction temperature during the production of the positive electrode active material precursor, a positive electrode active material precursor having an oriented structure can be formed, and a lithium composite transition metal oxide having an oriented structure can be produced using this precursor.

[0062] On the other hand, when producing the lithium composite transition metal oxide of the present invention, M 1 Elements and M 2 The element may be introduced in a coprecipitation reaction step for producing a precursor for the positive electrode active material, or it may be introduced when mixing with the lithium raw material. Also, M 1 Elements and M 2 When using two or more elements, each M 1 Elements and M 2 The timing of element addition may be the same or different. For example, M 1 If the material contains both Mn and Al as elements, the Mn can be added in the precursor coprecipitation step, and the Al can be added in the mixing step with the lithium raw material.

[0063] M 1 Elements and M 2 The timing of element addition can be adjusted as appropriate, taking into account the final composition of the cathode active material to be manufactured. For example, when manufacturing a cathode active material with a Ni content exceeding 80 atm%, it is more preferable to add Al during the mixing step with the lithium raw material rather than during the coprecipitation reaction. This is because adding Al during the coprecipitation step can adversely affect the growth of the crystal structure.

[0064] The lithium raw material and the positive electrode active material precursor can be mixed such that the molar ratio of Li to total transition metals in the precursor is 1:1 to 1.2:1, preferably 1:1 to 1.1:1. When the mixing ratio of the lithium raw material and the transition metals in the positive electrode active material precursor satisfies the above range, the crystal structure of the positive electrode active material develops well, and a positive electrode active material with excellent capacity characteristics and structural stability can be produced.

[0065] Examples of the lithium raw material include lithium-containing carbonates (e.g., lithium carbonate), hydrates (e.g., lithium hydroxide hydrate (LiOH·H2O)), hydroxides (e.g., lithium hydroxide), nitrates (e.g., lithium nitrate (LiNO3)), and chlorides (e.g., lithium chloride (LiCl)). One of these alone or a mixture of two or more can be used.

[0066] The firing can be carried out at an appropriate temperature, taking into account the composition of the lithium composite transition metal oxide, for example, 600°C to 1000°C, preferably 700°C to 900°C. The firing time can be, for example, 5 to 30 hours, preferably 8 to 15 hours, but is not limited thereto.

[0067] (2) First coating step Once a lithium composite transition metal oxide containing an oriented structure is prepared, a first coating step is performed in which the lithium composite transition metal oxide is mixed with a coating solution containing a cobalt element, followed by a first heat treatment.

[0068] The aforementioned coating solution containing the cobalt element can be formed by dissolving one or more elements selected from the group consisting of Co(NO3)2, Co(NO3)2·6H2O, CoCl2, CoSO4, Co(OCOCH3)2·4H2O, and Co(OH)2 in a solvent such as water or ethanol.

[0069] As described above, when the first coating is performed by a wet coating method in which a lithium composite transition metal oxide coating solution is mixed and then heat-treated, the cobalt element contained in the coating solution penetrates not only the surface of the secondary particles of the lithium composite transition metal oxide but also to the interfaces (grain boundaries) between grains, making it possible to form a higher concentration of cobalt at the grain boundaries than inside the grains.

[0070] On the other hand, the first heat treatment can be performed at 300°C to 800°C, preferably 400°C to 700°C, and more preferably 500°C to 650°C. When the first heat treatment temperature is within the above range, the cobalt elements contained in the coating solution concentrate at the grain boundaries, and a higher ratio of moles of cobalt to moles of nickel at the grain boundaries can be formed compared to the inside of the grains. If the first heat treatment temperature is too low, the coating will not be performed smoothly, and if the first heat treatment temperature is too high, the cobalt will diffuse into the inside of the grains and will not concentrate at the grain boundaries.

[0071] (3) Second coating step Next, a second coating step is performed in which the lithium composite transition metal oxide and the coating material containing cobalt, which have undergone the first coating, are dry-mixed and then subjected to a second heat treatment.

[0072] The coating material containing the cobalt element can be one or more selected from the group consisting of Co(OH)2, CoO, Co2O3, Co3O4, CoO(OH), and Co(OCOCH3)2.

[0073] As described above, when coating is performed using a dry method, unlike wet coating, the cobalt element does not sufficiently penetrate the interface between grains and remains on the surface of the secondary particles, which helps in the formation of a coating layer on the surface of the secondary particles. Therefore, the cobalt element introduced during the first coating and the cobalt element introduced during the second coating accumulate on the surface of the secondary particles, resulting in a higher ratio of moles of cobalt to moles of nickel at the surface of the secondary particles than at the grain boundaries.

[0074] On the other hand, the second heat treatment step can be carried out at 300°C to 800°C, preferably 400°C to 700°C, and more preferably 500°C to 700°C. When the second heat treatment temperature is within the above range, a higher cobalt concentration can be formed on the surface of the secondary particles than at the grain boundaries. If the second heat treatment temperature is too low, the coating will not be carried out smoothly, and if the second heat treatment temperature is too high, the cobalt will diffuse into the grain boundaries and / or into the grain, making it difficult to form a high cobalt concentration on the surface of the secondary particles.

[0075] <Positive electrode> Next, the positive electrode according to the present invention will be described.

[0076] The positive electrode includes the positive electrode active material according to the present invention. Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and the positive electrode active material layer includes the positive electrode active material according to the present invention.

[0077] Since the positive electrode active material is as described above, a detailed explanation will be omitted, and only the remaining components will be described in detail below.

[0078] The positive electrode current collector is not particularly limited as long as it contains a highly conductive metal, allows for easy adhesion of the positive electrode active material layer, and is unreactive within the battery voltage range. Examples of materials that can be used for the positive electrode current collector include stainless steel, aluminum, nickel, titanium, heat-treated carbon, or aluminum or stainless steel with surface treatments such as carbon, nickel, titanium, or silver. Furthermore, the positive electrode current collector can typically have a thickness of 3 to 500 μm, and fine irregularities can be formed on its surface to enhance the adhesion of the positive electrode active material. It can be used in various forms, such as films, sheets, foils, meshes, porous materials, foams, and nonwoven fabrics.

[0079] The positive electrode active material layer may, as necessary, selectively include a conductive material and a binder together with the positive electrode active material.

[0080] Here, the positive electrode active material can be included in an amount of 80 to 99% by weight, more specifically 85 to 98.5% by weight, relative to the total weight of the positive electrode active material layer, and when included within this range, it can exhibit excellent capacity characteristics.

[0081] The conductive material is used to impart conductivity to the electrodes and can be used without particular limitations in the battery it is configured in, as long as it does not cause chemical changes and has electronic conductivity. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive tubes such as carbon nanotubes; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. Of these, one or more can be used. The conductive material may be included in an amount of 0.1 to 15% by weight relative to the total weight of the positive electrode active material layer.

[0082] The binder plays a role in improving the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, polymethyl methacrylate, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and polymers in which the hydrogen atoms of these materials are substituted with Li, Na, or Ca, or various copolymers thereof. One of these materials alone or a mixture of two or more materials can be used. The binder can be present in an amount of 0.1 to 15% by weight relative to the total weight of the positive electrode active material layer.

[0083] The positive electrode can be manufactured by a conventional method for manufacturing a positive electrode, except that the positive electrode active material is used. Specifically, it can be manufactured by applying a positive electrode slurry composition, prepared by dissolving or dispersing the positive electrode active material and, if necessary, selectively, a binder, a conductive material, and a dispersant in a solvent, onto a positive electrode current collector, followed by drying and rolling.

[0084] The solvent can be any solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), dimethylformamide (DMF), acetone, or water, and one of these alone or a mixture of two or more can be used. The amount of solvent used should be sufficient to dissolve or disperse the positive electrode active material, conductive material, binder, and dispersant, taking into consideration the coating thickness of the slurry and the manufacturing yield, and to have a viscosity that allows for excellent thickness uniformity when applied for the manufacture of the positive electrode.

[0085] Alternatively, the positive electrode can also be manufactured by casting the positive electrode slurry composition onto another support, peeling it off the support, and then laminating the resulting film onto the positive electrode current collector.

[0086] <Electrochemical elements> The present invention makes it possible to manufacture an electrochemical element including the positive electrode. Specifically, the electrochemical element may be a battery, a capacitor, and more specifically, a lithium secondary battery.

[0087] The lithium secondary battery can specifically include a positive electrode, a negative electrode positioned opposite the positive electrode, and a separator and electrolyte interposed between the positive and negative electrodes. As the positive electrode is as described above, a detailed explanation will be omitted, and only the remaining components will be described in detail below.

[0088] Furthermore, the lithium secondary battery may selectively further include a battery container for housing the electrode assembly comprising the positive electrode, negative electrode, and separator, and a sealing member for sealing the battery container.

[0089] In the lithium secondary battery, the negative electrode may include a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0090] The negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel with surface treatment using carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy can be used. The negative electrode current collector can usually have a thickness of 3 μm to 500 μm, and, similar to the positive electrode current collector, fine irregularities can be formed on the surface of the current collector to strengthen the bonding force of the negative electrode active material. For example, it can be used in various forms such as film, sheet, foil, mesh, porous material, foam, and nonwoven fabric.

[0091] The negative electrode active material layer may selectively include a binder and a conductive material together with the negative electrode active material.

[0092] As the negative electrode active material, compounds capable of reversible intercalation and deintercalation of lithium can be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic 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; and SiO2. βExamples include metallic oxides that can be doped and dedoped with lithium, such as (0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the metallic compound and carbonaceous material, such as Si-C composites or Sn-C composites, and any one or more mixtures thereof can be used. A metallic lithium thin film can also be used as the negative electrode active material. Furthermore, both low-crystallinity carbon and high-crystallinity carbon can be used as the carbon material. Typical examples of low-crystalline carbon include soft carbon and hard carbon, while typical examples of high-crystalline carbon include amorphous, plate-like, flaky, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0093] The anode active material can be present in an amount of 80% to 99% by weight relative to the total weight of the anode active material layer.

[0094] The binder is a component that helps to bond the conductive material, active material, and current collector, and is usually added in an amount of 0.1% to 10% by weight relative to the total weight of the negative electrode active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0095] The conductive material is a component for further improving the conductivity of the negative electrode active material and can be added in an amount of 10% by weight or less, preferably 5% by weight or less, relative to the total weight of the negative electrode active material layer. Such a conductive material is not particularly limited as long as it does not cause a chemical change in the battery and is conductive, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; carbon fluoride; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives can be used.

[0096] The negative electrode active material layer can be manufactured by coating a negative electrode slurry composition, which is prepared by dissolving or dispersing a negative electrode active material and, selectively, a binder and a conductive material in a solvent, onto a negative electrode current collector and then drying it, or by casting the negative electrode slurry composition onto another support, peeling it off the support, and then laminating the resulting film onto the negative electrode current collector.

[0097] On the other hand, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Generally, any separator used in lithium secondary batteries can be used without particular limitations, but those with low resistance to ion movement of the electrolyte and excellent electrolyte moisture absorption capacity are particularly preferred. Specifically, porous polymer films, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof can be used. Ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. Furthermore, coated separators containing ceramic components or polymeric substances can be used to ensure heat resistance or mechanical strength, and can be selectively used as single-layer or multi-layer structures.

[0098] Furthermore, the electrolytes used in the present invention include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0099] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0100] The aforementioned organic solvent can be used without particular limitations, as long as it can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the aforementioned organic solvents include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (propylene Carbonate solvents such as carbonate (PC); alcoholic solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, and can include a double-bonded aromatic ring or ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes can be used. Among these, carbonate solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of the battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred.

[0101] The lithium salt can be used without particular limitations as long as it is a compound that can provide lithium ions for use in lithium secondary batteries. Specifically, the anion of the lithium salt is 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 (CF3CF2SO2)2N - The lithium salt can be selected from the group consisting of LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO2, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. The concentration of the lithium salt is preferably in the range of 0.1 to 2.0 M. When the concentration of the lithium salt falls within this range, the electrolyte can exhibit excellent electrolyte performance due to having appropriate conductivity and viscosity, and lithium ions can move effectively.

[0102] As described above, the lithium secondary battery containing the positive electrode active material according to the present invention exhibits excellent capacity characteristics and lifespan characteristics, and can be usefully used in various fields such as portable devices like mobile phones, notebook computers, and digital cameras, as well as electric vehicles.

[0103] The present invention will be described in detail below with reference to examples. However, the examples of the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the examples detailed below. The examples of the present invention are provided to give a more complete explanation of the present invention to a person of average skill in the art.

[0104] Manufacturing Example 1 - Production of oriented lithium transition metal oxides Transition metal aqueous solutions were prepared by mixing NiSO4, CoSO4, and MnSO4 in distilled water in amounts such that the molar ratio of nickel:cobalt:manganese was 8:1:1.

[0105] Next, after adding deionized water to the reactor, nitrogen gas was purged into the reactor to remove dissolved oxygen from the water, and NaOH was added to maintain a pH of 11 inside the reactor.

[0106] Next, the transition metal aqueous solution, NaOH aqueous solution, and NH4OH aqueous solution were added to the reactor, and a coprecipitation reaction was carried out for 30 hours under the conditions of a reaction temperature of 50°C, pH 11, and stirring speed of 400 rpm, to obtain the average particle size (D 50 The precursor Ni for positive electrode active material has a diameter of 12 μm. 0.8 Co 0.1 Mn 0.1 (OH)2 was prepared. Here, the molar ratio of transition metal ions to NH4OH was 1:1.10.

[0107] The aforementioned cathode active material precursor and LiOH were mixed so that the molar ratio of Li to transition metal (Ni+Co+Mn) was 1.02:1. After firing at 730°C for 20 hours, the mixture was washed with water and dried to produce a lithium composite transition metal oxide.

[0108] Figure 1 shows a scanning electron microscope (SEM) image of a cross-section of the lithium composite transition metal oxide manufactured as described above. Referring to Figure 1, it can be confirmed that the lithium composite transition metal oxide manufactured by the above method has the form of secondary particles in which multiple grains are aggregated, and that the long axes of the grains are aligned in the direction from the center of the secondary particle toward the surface.

[0109] Manufacturing Example 2 - Production of non-oriented lithium transition metal oxides Lithium composite transition metal oxide was produced in the same manner as in Production Example 1, except that, during the production of the cathode active material precursor, transition metal aqueous solution, NaOH aqueous solution, and NH4OH aqueous solution were added to achieve a molar ratio of transition metal to NH4OH of 1:0.8, a coprecipitation reaction was carried out for 30 hours under conditions of reaction temperature of 60°C, pH 11, and stirring speed of 300 rpm, and calcination was performed at 750°C.

[0110] Figure 2 shows a scanning electron microscope (SEM) image of a cross-section of the lithium composite transition metal oxide manufactured as described above. Referring to Figure 2, it can be seen that the lithium composite transition metal oxide manufactured by the above method is in the form of secondary particles in which multiple grains are aggregated, and the grains are arranged disorderly without any particular orientation.

[0111] Examples A Co-containing coating solution was prepared by dissolving Co(NO3)2·6H2O in deionized water (DI water).

[0112] The lithium composite transition metal oxide produced by Production Example 1 was added to the coating solution, stirred at 500 rpm for 3 hours, and then subjected to a first heat treatment at 600°C (first coating step).

[0113] Next, the lithium composite transition metal oxide and Co(OH)2 were dry-mixed in a weight ratio of 1:0.002, and a second heat treatment was performed at 680°C (second coating step) to produce the positive electrode active material.

[0114] Comparative Example 1 The cathode active material was prepared in the same manner as in the examples, except that the lithium composite transition metal oxide prepared by Preparation Example 2 was used instead of the lithium composite transition metal oxide prepared by Preparation Example 1.

[0115] Comparative Example 2 The cathode active material was manufactured in the same manner as in the examples, except that only the first coating step was performed and the second coating step was omitted.

[0116] Comparative Example 3 The cathode active material was manufactured in the same manner as in the examples, except that the first coating step was omitted and only the second coating step was performed.

[0117] Comparative Example 4 A lithium composite transition metal oxide produced by Production Example 1 and Co(OH)2 were dry-mixed in a weight ratio of 1:0.002, and a coating layer was formed by heat treatment at 680°C.

[0118] Subsequently, Co(NO3)2·6H2O was dissolved in deionized water (DI water) to produce a Co-containing coating solution. The lithium composite transition metal oxide on which the coating layer was formed was then added to the Co-containing coating solution, stirred at 500 rpm for 3 hours, and then heat-treated at 600°C to produce a positive electrode active material.

[0119] Experimental Example 1: Measurement of Co concentration Using EDX (Energy Dispersive X-ray Spectroscopy), the molar ratio of Co / Ni in the interior of the grains, at grain boundaries, and on the surface of secondary particles of the positive electrode active materials produced in Examples and Comparative Examples 1-4 was measured. The measurement results are shown in Table 1 below.

[0120] <Manufacturing of lithium-ion secondary batteries> The positive electrode active materials prepared in Examples and Comparative Examples 1-4 were mixed with a conductive material (Denka Black) and a binder (PVDF) in an N-methyl-2-pyrrolidone (NMP) solvent in a weight ratio of 96:2:2 to produce a positive electrode slurry. The positive electrode slurry was applied to one surface of an aluminum current collector, dried, and then rolled to produce a positive electrode.

[0121] Next, a negative electrode slurry was prepared by mixing a negative electrode active material (natural graphite), a conductive material (carbon black), and a binder (PVDF) in an N-methylpyrrolidone solvent in a weight ratio of 96:2:2. The negative electrode slurry composition was applied to a copper current collector, dried, and then rolled to produce a negative electrode.

[0122] After manufacturing an electrode assembly with a separator interposed between the positive and negative electrodes, the assembly was placed inside a battery case, and then an electrolyte was injected to manufacture a lithium secondary battery. The electrolyte used was a solution of 1.0 M LiPF6 dissolved in an organic solvent, which was a mixture of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7.

[0123] Experimental Example 2: High-Temperature Lifetime Characteristics Each lithium secondary battery manufactured as described above was charged to 4.2V at 45°C with a constant current of 1.0C, and then discharged to 2.5V with a constant current of 0.5C. This constituted one charge-discharge cycle. After 50 charge-discharge cycles, the capacity retention rate was measured. The measurement results are shown in [Table 1] below.

[0124] Experimental Example 3: Gas Generation During High-Temperature Storage After charging each of the lithium secondary batteries manufactured as described above to SOC 100, they were stored at 60°C for 8 weeks. Then, they were perforated in a vacuum chamber to expel the gas from inside the battery and collected inside the vacuum chamber. The amount of gas generated in the chamber was analyzed using a gas chromatography-flame ionization detector (GC-FID). The ratio of the gas generation amount of lithium secondary batteries using the positive electrode active materials of Comparative Examples 1 to 4 is shown in [Table 1] below, when the gas generation amount of the lithium secondary battery using the positive electrode active material of the example is set to 100%.

[0125] Experimental Example 4: Initial Resistance After charging each of the lithium secondary batteries manufactured as described above to SOC 100, they were discharged for 10 seconds, and the change in voltage was measured. The initial resistance was then calculated by dividing the measured change in voltage by the discharge current. The measurement results are shown in [Table 1] below.

[0126] [Table 1]

[0127] As shown in Table 1 above, in the positive electrode active materials of Examples and Comparative Example 1, which were manufactured by sequentially performing wet coating and dry coating, the molar ratio of Co / Ni increased as it moved from the interior of the grain to the grain boundaries and the coating layer on the surface of the secondary particles. In contrast, the positive electrode active material of Comparative Example 2, which was wet-coated only, showed the same level of Co / Ni molar ratio at the grain boundaries and on the surface of the secondary particles, and the positive electrode active material of Comparative Example 3, which was dry-coated only, showed the same level of Co / Ni molar ratio at the grain boundaries and inside the grain. On the other hand, in the case of the positive electrode active material of Comparative Example 4, which was dry-coated first and then wet-coated, the molar ratio of Co / Ni at the grain boundaries was higher than the Co / Ni concentration in the coating layer.

[0128] Furthermore, referring to Table 1, it can be confirmed that the lithium secondary battery using the positive electrode active material of the example exhibits superior high-temperature life characteristics and initial resistance characteristics, less gas generation after high-temperature storage, and excellent high-temperature characteristics compared to the lithium secondary batteries using the positive electrode active materials of Comparative Examples 1 to 4.

Claims

1. A lithium composite transition metal oxide having a form of secondary particles in which multiple grains are aggregated, and having an oriented structure in which the long axes of the grains are aligned from the center of the secondary particle toward the surface in at least a portion of the secondary particles, and a positive electrode active material having a coating layer formed on the surface of the secondary particles and containing the element Co, The lithium composite transition metal oxide comprises nickel and cobalt, When C1 is the ratio of moles of cobalt to moles of nickel within the grain, C2 is the ratio of moles of cobalt to moles of nickel at the grain boundary (the interface between the grains), and C3 is the ratio of moles of cobalt to moles of nickel in the coating layer, the following conditions must be met: C1 < C2 < C3. The lithium composite transition metal oxide has the composition represented by the following [Chemical Formula 1] as a positive electrode active material: [Chemical formula 1] Li X [Nia Co b M 1 c M 2 d ] O 2-y A y In the above [Chemical Formula 1], The aforementioned M1 is one or more elements selected from the group consisting of Mn and Al. The aforementioned M2 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo. A is one or more elements selected from the group consisting of F, Cl, Br, I, At, and S. 0.98 ≤ x ≤ 1.20, 0.8 < a < 1, 0 < b < 1, 0 < c < 1, 0 ≤ d ≤ 0.2, 0 ≤ y ≤ 0.

2.

2. The positive electrode active material according to claim 1, wherein the grains, whose long axes are arranged from the center of the secondary particles toward the surface, have an angle between their long axes and the a-axis direction of the crystal structure of -15° to 15°.

3. The positive electrode active material according to claim 1, wherein the grains, whose long axes are arranged from the center of the secondary particles toward the surface, have an aspect ratio of 1.5 to 15.

4. The lithium composite transition metal oxide is The core consists of grains that are collectively aggregated in a disordered manner, The positive electrode active material according to claim 1, comprising a shell portion formed on the outside of the core portion, wherein grains are arranged in an oriented structure.

5. The positive electrode active material according to claim 4, wherein the grains in the core portion have an aspect ratio of 0.8 to 1.

2.

6. The positive electrode active material according to claim 4, wherein the grains within the shell portion have an aspect ratio of 1.5 to 15.

7. The coating layer has a composition represented by the following [Chemical Formula 2], and is the positive electrode active material according to claim 1: [Chemical formula 2] Li ｚ Co １－ｗ M ３ ｗ O ２ In the above [Chemical Formula 2], the M ３ z is one or more elements selected from the group consisting of Ni, Mn, Al, W, Cu, Fe, V, Cr, Ti, Zr, Zn, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and satisfies 0.8 ≤ z ≤ 1.2 and 0 ≤ w ≤ 0.

2.

8. The positive electrode active material according to claim 1, wherein the average particle size of the grains is 0.05 μm to 4 μm.

9. The positive electrode active material is D ５０ The positive electrode active material according to claim 1, wherein the diameter is 2 μm to 20 μm.

10. The steps include preparing a lithium composite transition metal oxide, which is in the form of secondary particles in which multiple grains are aggregated, and which includes an oriented structure in which the long axes of the grains are aligned from the center of the secondary particle toward the surface in at least a portion of the secondary particles, A first coating step involves mixing the lithium composite transition metal oxide with a coating solution containing cobalt, followed by a first heat treatment. A method for producing a positive electrode active material, comprising a second coating step of dry mixing the first coated lithium composite transition metal oxide and a coating raw material containing a cobalt element, followed by a second heat treatment.

11. The coating solution containing the cobalt element is Co(NO ３ ), Co(NO ２ ), Co(NO ３ ), Co(NO ２ )・6H ２ O, CoCl ２ , CoSO ４ , Co(OCOCH ３ ), Co(OCOCH ２ )・4H ２ O, and Co(OH) ２ , and is formed by dissolving one or more selected from the group consisting of in water. The method for producing a positive electrode active material according to claim 10.

12. The method for producing a positive electrode active material according to claim 10, wherein the first heat treatment step is performed at 300°C to 800°C.

13. The coating material containing the aforementioned cobalt element is Co(OH) ２ CoO, Co ２ O ３ Co ３ O ４ , CoO(OH) and Co(OCOCH) ３ ) ２ A method for producing a positive electrode active material according to claim 10, wherein the active material is one or more selected from the group consisting of the following:

14. The method for producing a positive electrode active material according to claim 10, wherein the second heat treatment step is performed at 300°C to 800°C.

15. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 9.

16. A lithium secondary battery comprising the positive electrode described in claim 15.

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