Positive electrode active material and method for manufacturing the same

Abstract

The present invention relates to a positive electrode active material including a lithium transition metal oxide in the form of a single particle divided into a surface portion and a core, and a coating portion including cobalt formed on the surface portion, the surface portion including an oxidation number gradient layer in which the oxidation number of nickel (Ni) increases toward the outermost side, and a method for manufacturing the positive electrode active material.

Description

[Technical Field]

[0001] This application claims priority under Korean Patent Application No. 10-2022-0062250 dated May 20, 2022, and all content disclosed in the said Korean Patent Application is incorporated herein by reference.

[0002] The present invention relates to a positive electrode active material and a method for producing the same. [Background technology]

[0003] Recently, with the advancement of technologies such as electric vehicles, the demand for high-capacity secondary batteries has increased, and consequently, research on cathodes using high-nickel (High Ni) cathode active materials with excellent capacity characteristics is being actively conducted.

[0004] High-nickel cathode active materials are manufactured using a co-precipitation method, and the resulting high-nickel cathode active materials have a secondary particle form in which primary particles are aggregated. However, active materials with a secondary particle form have the disadvantage that, during long-term charge-discharge processes, microscopic cracks develop in the secondary particles, causing side reactions. Furthermore, when the electrode density is increased to improve energy density, the secondary particles undergo structural collapse, resulting in a decrease in energy density and reduced lifetime characteristics due to a reduction in active material and electrolyte.

[0005] In order to solve the problems of such secondary particle-shaped high-nickel cathode active materials, recently, the development of single-particle-type nickel-based cathode active materials has been carried out. The single-particle-type nickel-based cathode active material has the advantage that particle disintegration does not occur even when increasing the electrode density due to its high energy density. However, the single-particle-type nickel-based cathode active material requires a relatively high firing temperature for its production, the R-3m layered structure cannot be properly maintained, lithium detaches from the outside of the crystal structure, and a phase change occurs to an Fm-3m rock-salt structure such as NiO, resulting in a decrease in the crystallinity of the cathode active material, an increase in the ratio of NiO on the surface of the produced single particles, an increase in resistance due to the increase in NiO, and problems such as a decrease in energy density and output. Also, when the firing temperature is low, it exists in the form of over-fired secondary particles, and there is a problem that the improvement effect on life and gas generation does not reach the level expected from single particles.

[0006] Therefore, the development of a cathode active material having a high electrode density and excellent life characteristics and output characteristics is still required.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] The problem to be solved by the present invention is to provide a cathode active material having a high electrode density and excellent life characteristics and output characteristics.

[0009] Another problem to be solved by the present invention is to provide a method for manufacturing a cathode active material having a high electrode density and excellent life characteristics and output characteristics.

Means for Solving the Problems

[0010] To solve the above problems, the present invention provides a positive electrode active material.

[0011] (1) The present invention provides a positive electrode active material comprising a lithium transition metal oxide in single-particle form which can be divided into a surface portion and a core, and a coating portion containing cobalt formed on the surface portion, wherein the surface portion includes an oxidation number gradient layer in which the oxidation number of nickel (Ni) increases toward the outermost direction.

[0012] (2) In the present invention, the surface portion of the lithium transition metal oxide in single-particle form is a positive electrode active material having a depth of 1 nm to 50 nm from the outermost part toward the center.

[0013] (3) The present invention provides a positive electrode active material in which, based on the entire surface portion and coating portion, the cobalt and nickel satisfy a Co / Ni value (mol / mol) of 0.1 to 0.8.

[0014] (4) The present invention provides a positive electrode active material in any one of (1) to (3) above, wherein the average oxidation state of nickel (Ni) up to a depth of 10 nm from the outermost part toward the center of the single-particle lithium transition metal oxide is +2.50 to +3.00, and the average oxidation state of nickel (Ni) up to a depth of 30 nm is +2.36 to +2.60.

[0015] (5) In any one of (1) to (4) above, the present invention provides a positive electrode active material in which the coating portion is formed on the outside of the surface portion in an area of ​​10% to 100% based on the total area on the outside of the surface portion.

[0016] (6) In any one of (1) to (5) above, the present invention provides a positive electrode active material in which the coating portion is located in an island shape on the outside of the surface portion.

[0017] (7) In any one of (1) to (6) above, the present invention provides a positive electrode active material in which the coating portion comprises the composition of LiCoO2.

[0018] (8) The present invention provides a positive electrode active material in any one of (1) to (7) above, wherein the surface portion further includes an oxidation number reversal layer in the outer portion, which is a region in which the oxidation number of nickel decreases in the outermost direction.

[0019] (9) The present invention provides a positive electrode active material in which the oxidation state reversal layer is contained in a region having a thickness from the outermost part of the surface to 0.1% to 50% of the total thickness of the surface, extending from the outermost part of the lithium transition metal oxide in single particle form toward the center.

[0020] (10) The present invention provides a positive electrode active material in which, in (8) or (9) above, the surface portion is divided in the thickness direction into a surface layer and an internal layer, and the internal layer has an oxidation state of nickel that increases in the outermost direction.

[0021] (11) The present invention provides a positive electrode active material in any one of (1) to (10) above, wherein the lithium transition metal oxide in single-particle form contains 50 or fewer crystal grains.

[0022] (12) The present invention provides a positive electrode active material in any one of (1) to (11) above, wherein the lithium transition metal oxide is a lithium composite transition metal oxide containing nickel (Ni), cobalt (Co), and manganese (Mn).

[0023] (13) The present invention provides a positive electrode active material in any one of (1) to (12) above, wherein the lithium transition metal oxide is a lithium composite transition metal oxide represented by the following chemical formula 1. [Chemical formula 1] Li a Ni x Co y M 1 z M 2 1-x-y-z O2 In Chemical Formula 1, M 1 is one or more selected from the group consisting of Mn and Al, and M 2 is one or more selected from the group consisting of B, Ba, Ce, Cr, F, Mg, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, S, Sr, Ta, La, and Hf, and 1.0 ≦ a ≦ 1.3, 0.6 ≦ x < 1.0, 0 ≦ y ≦ 0.4, 0 ≦ z ≦ 0.4.

[0024] (14) The present invention provides a positive electrode active material which is a lithium composite transition metal oxide represented by the following Chemical Formula 2, in any one of (1) to (13) above. [Chemical Formula 2] Li a Ni b Co c Mn d M 1 e O2 In Chemical Formula 2, M 1 is one or more selected from the group consisting of Al, Zr, B, W, Mo, Cr, Nb, Mg, Hf, Ta, La, Ti, Sr, Ba, Ce, Sn, Y, Zn, F, P, and S, and 0.9 ≦ a ≦ 1.1, 0.8 ≦ b < 1, 0 < c < 0.2, 0 < d < 0.2, 0 ≦ e < 0.1, and b + c + d + e = 1.

[0025] Further, the present invention provides a method for producing the positive electrode active material to solve the above other problems.

[0026] (15) The present invention provides a method for producing a positive electrode active material, including: 1) a step of mixing lithium transition metal oxide particles in a single particle form and a cobalt (Co) source; and 2) a step of heat-treating the mixture of step 1).

[0027] (16) The present invention provides a method for producing a positive electrode active material, in (15) above, further mixing an additional metal source in step 1).

[0028] (17) The present invention provides a method for producing a positive electrode active material in which the heat treatment in step 2) is performed at 500 to 800°C, in accordance with (15) or (16) above. [Effects of the Invention]

[0029] The positive electrode active material of the present invention is a positive electrode active material comprising a lithium transition metal oxide in single-particle form, and includes a coating portion containing cobalt and a surface portion with a reduced NiO layer. As a result, it has a high electrode density and can exhibit excellent lifetime characteristics and power characteristics. [Brief explanation of the drawing]

[0030] [Figure 1] This shows the nickel balance map (Ni valence map) and Ni L3 peak graph of the single-particle type cathode active material of Example 1, measured for the region of the cathode active material of Example 1. [Figure 2] These are the nickel balance map (Ni valence map) and Ni L3 peak graph of the single-particle type cathode active material of Example 1, measured for other regions of the cathode active material of Example 1. [Modes for carrying out the invention]

[0031] The present invention will be described in more detail below to facilitate understanding of it.

[0032] The terms and words used in the description and claims of this invention should not be interpreted in a manner limited to their ordinary or dictionary meanings, but rather in a manner consistent with the technical idea of ​​this invention, in accordance with the principle that inventors may define the concepts of terms as appropriate to best describe their invention.

[0033] In this invention, the term "primary particle" refers to the smallest particle unit that can be distinguished as a single mass when a cross-section of the positive electrode active material is observed via a scanning electron microscope (SEM), and may consist of multiple crystal grains.

[0034] In this invention, the term "secondary particle" refers to a secondary structure formed by the aggregation of multiple primary particles. The average particle size of the secondary particle can be measured using a particle size analyzer.

[0035] In the present invention, the term "single-particle type" can be used instead of "single-particle form," and refers to a form that is contrasted with the form of secondary particles formed by the aggregation of several hundred primary particles manufactured by conventional methods. Furthermore, in the present invention, the terms "single-particle type positive electrode active material" or "single-particle form lithium transition metal oxide" are concepts that are contrasted with positive electrode active materials in the form of secondary particles formed by the aggregation of several hundred primary particles manufactured by conventional methods, and refer to positive electrode active materials or lithium transition metal active materials consisting of 1 to 50 particles, 1 to 40 particles, 1 to 30 particles, 1 to 20 particles, 1 to 15 particles, 1 to 10 particles, or 1 to 5 particles.

[0036] In this invention, the term "single-crystal" may be replaced with "single-crystal property," and refers to a cathode active material or lithium transition metal oxide containing 50 or fewer crystal grains, specifically 1 to 30 crystal grains. Typically, single-crystal particles represent particles in which the entire sample consists of only one crystal grain or grain region. In this invention, single-particle cathode active materials or lithium transition metal oxides in single-particle form can exhibit properties similar to those of a single-crystal particle because they contain a small number of crystal grains.

[0037] The term "single particle" refers to the smallest unit of particle recognized when observing the positive electrode active material through a scanning electron microscope, and the term "grain" or "grain region" refers to a region in the sample where atoms are arranged continuously and periodically in one direction. The grain can be analyzed using an electron backscatter diffraction (ESBD) analyzer.

[0038] In this invention, the term "average particle size (D)" is used. 50)" refers to the particle size at the 50% point of the cumulative volume distribution by particle size. The average particle size is calculated by dispersing the powder to be measured in a dispersion medium, introducing it into a commercially available laser diffraction particle size analyzer (for example, Microtrac's S3500), measuring the difference in diffraction patterns due to particle size as the particles pass through the laser beam to calculate the particle size distribution, and then calculating the particle diameter at the point where the cumulative volume distribution by particle size in the measuring device reaches 50%. 50 It can be measured.

[0039] The positive electrode active material of the present invention comprises a lithium transition metal oxide in single-particle form that can be divided into a surface portion and a core, and a coating portion containing cobalt formed on the surface portion, wherein the surface portion includes an oxidation number gradient layer in which the oxidation number of nickel (Ni) increases in the outermost direction.

[0040] The surface of the single-particle lithium transition metal oxide has a layered (R-3m) structure and has a high NiO content before the cobalt-containing coating is formed on the surface. The high firing temperature required during the production of the single-particle lithium transition metal oxide induces the formation of NiO. The NiO contained in the surface of the single-particle lithium transition metal oxide can cause an increase in resistance, a decrease in energy density and power output, and other problems.

[0041] The positive electrode active material of the present invention includes a coating portion containing cobalt formed by a process of mixing the single-particle lithium transition metal oxide and a cobalt source (raw material) and then heat-treating it, wherein the coating portion may be a layer formed by the diffusion of the cobalt from the surface toward the center of the single-particle lithium transition metal oxide during the heat-treating process. In the positive electrode active material of the present invention, the NiO layer on the surface is converted into a layered structure of nickel cobalt manganese (NCM) oxide during the process of forming the coating portion, thereby reducing and eliminating causes such as increased resistance, decreased energy density, and decreased output, and exhibiting excellent electrochemical properties.

[0042] The single-particle lithium transition metal oxide is divided into a surface portion and a core. The surface portion refers to the outer part of the single-particle lithium transition metal oxide and means a region having a predetermined thickness from the outermost part toward the center of the single-particle lithium transition metal oxide. Specifically, it means a region with a depth of 1 nm to 50 nm, specifically 5 to 30 nm, from the outermost part toward the center of the single-particle lithium transition metal oxide.

[0043] Furthermore, the core refers to the interior of the lithium transition metal oxide in single-particle form, excluding the surface portion.

[0044] The surface portion has a layered (R-3m) structure and includes an oxidation number gradient layer having a gradient in which the oxidation number of nickel (Ni) gradually increases from the center outward from the center of the single-particle lithium transition metal oxide.

[0045] The oxidation number gradient layer refers to a region in which the oxidation number of nickel (Ni) increases from the center outward in the single-particle lithium transition metal oxide, and may be included in part or all of the surface portion, specifically, in part. That is, the single-particle transition metal oxide contained in the positive electrode active material according to an example of the present invention may have a surface portion in which the oxidation number of nickel increases in the outward direction in a certain region, and in other cases, the single-particle transition metal oxide contained in the positive electrode active material according to an example of the present invention may have a surface portion in which the oxidation number of nickel increases in the outward direction.

[0046] The nickel (Ni) contained in the surface portion can have an average oxidation state of +2.36 to +3.00, and more specifically, the average oxidation state of the nickel contained in the surface portion can be +2.36 to +2.95, +2.36 to +2.91, +2.37 to +2.95, and more specifically, +2.37 to +2.91. The average oxidation state of the nickel contained in the surface portion can change depending on the amount of cobalt coating that forms the coating portion, and when the above range is satisfied, an appropriate amount of coating portion is formed on the surface, and the NiO degradation layer of lithium transition metal oxide is sufficiently converted into a layered structure of nickel cobalt manganese (NCM) oxide, thereby preventing problems of cation mixing and structural instability due to the collapse of the layered structure of the NiO degradation layer. As a result, the surface portion can include a layered structure of nickel cobalt manganese oxide converted from the NiO layer.

[0047] The average oxidation state of nickel (Ni) in the lithium transition metal oxide in single-particle form, from the outermost layer toward the center to a depth of 10 nm, can be +2.50 to +3.00, specifically +2.50 to +2.95, +2.50 to +2.90, +2.50 to +2.88, +2.52 to +2.95, +2.52 to +2.90, +2.52 to +2.88, and more specifically, +2.54 to +2.86. Furthermore, the average oxidation number of nickel (Ni) from the outermost part toward the center of the single-particle lithium transition metal oxide to a depth of 30 nm can be +2.36 to +2.60, specifically +2.36 to +2.58, +2.36 to +2.55, +2.36 to +2.52, +2.36 to +2.50, +2.37 to +2.58, +2.37 to +2.55, +2.37 to +2.52, +2.37 to +2.50, and more specifically, +2.27 to +2.48. When the average oxidation number of nickel satisfies the ranges of average oxidation numbers up to 10 nm and up to 30 nm, the nickel can exhibit an appropriate oxidation number gradient on the surface, and an appropriate reduction effect on the NiO degradation layer of the single-particle lithium transition metal oxide can be obtained.

[0048] The coating portion may be formed on the outside of the surface portion, that is, on the outermost surface of the single-particle lithium transition metal oxide, or it may be formed on a part or all of the outside of the surface portion, and it may be formed covering 10% to 100% (area %) of the total area outside the surface portion. Specifically, the coating portion may be formed on a part of the outside of the surface portion, or it may be formed covering 30% to 90% of the total area outside the surface portion.

[0049] The coating portion may be formed in an island-like manner on the outside of the surface portion. The island-like manner means a form that is discontinuously formed on the outside of the surface portion, that is, the coating portion may be partially dispersed and distributed on the outermost surface of the single-particle lithium transition metal oxide.

[0050] The coating portion may contain a LiCoO2 composition, and specifically, the coating portion may include island-shaped LiCoO2 on the outside of the surface portion.

[0051] Based on the entire surface and coating portion, the cobalt and nickel can satisfy a Co / Ni value (mol / mol) of 0.10 to 0.80, specifically 0.15 to 0.80, 0.20 to 0.80, 0.10 to 0.75, 0.15 to 0.75, and more specifically 0.20 to 0.75. If the Co / Ni value is too small compared to the above range, the cobalt will diffuse excessively into the interior of the single-particle lithium transition metal oxide particles, resulting in a low cobalt concentration in the surface layer. This makes it difficult to achieve the effect of forming a cobalt-containing coating portion, and during the process of excessive cobalt diffusion into the particles, a NiO degradation layer may be re-formed on the surface of the particles. Furthermore, if the Co / Ni value is too large compared to the above range, the coated cobalt may be Co3O4 or lithium cobalt oxide (Li x CO y O z) may exist on the particle surface as another oxide in a different form, in which case the NiO degradation layer of the single-particle lithium transition metal oxide may not be sufficiently converted to a layered structure of nickel-cobalt-manganese (NCM) oxide, and the unwanted amount of coating formed on the surface of the lithium transition metal oxide may act as resistance or cause a decrease in capacity.

[0052] On the other hand, in a positive electrode active material according to one embodiment of the present invention, the surface portion may include an oxidation number reversal layer in the outermost part, which is a region in which the oxidation number of nickel decreases in the outermost direction. The surface portion may show a tendency for the oxidation number of nickel to increase in the outermost direction, and the outer portion of the surface portion may include a region in which the oxidation number of nickel decreases, and the outer portion of the surface portion in which the oxidation number of nickel decreases represents the oxidation number reversal layer.

[0053] The surface portion of the single-particle lithium transition metal oxide can be divided into a surface layer and an internal layer in the direction of its thickness. The thickness direction of the surface portion refers to the direction from the outermost part to the center of the single-particle lithium transition metal oxide, and the surface portion of the single-particle lithium transition metal oxide can be divided into an outer surface layer and an inner internal layer according to the depth from the outermost part to the center.

[0054] If sufficient heat treatment is performed during the process of forming the coating portion on the outside of the surface portion, a region can be formed on the outer part of the surface portion in which the nickel content and oxidation number increase in the outermost direction, and then the oxidation number of the nickel content begins to decrease. The portion in which the oxidation number of nickel increases in the outermost direction can be shown as the inner layer, and the portion in which a region in which the oxidation number of nickel decreases in the outermost direction can be formed can be shown as the surface layer.

[0055] The inversion layer may be included in the surface layer, and when the inversion layer is included in the surface layer, the internal layer of the surface portion may have a gradient in which the oxidation number of nickel increases in the outermost direction, and the inversion layer of the surface portion may have a gradient in which the oxidation number of nickel decreases in the outermost direction.

[0056] In this specification, the "oxidation inversion layer" may be included in the surface layer when the surface is divided into an outer surface layer and an inner layer according to the depth from the outermost part toward the center of the single-particle lithium transition metal oxide, and may be included in a form in which a part is included in the surface layer and the remaining part is exposed on the outer surface of the lithium transition metal oxide.

[0057] Accordingly, a positive electrode active material according to one embodiment of the present invention is a positive electrode active material comprising a single-particle lithium transition metal oxide having a layered (R-3m) structure divided into a surface portion and a core, wherein the single-particle lithium transition metal oxide comprises a cobalt (Co) coating formed on the outside of the surface portion, the surface portion comprises an oxidation number gradient layer in which the oxidation number of nickel (Ni) increases in the outermost direction, and the surface portion may further comprise an oxidation number reversal layer which is a region in which the nickel content and oxidation number decrease in the outermost direction.

[0058] The "surface layer" can mean a region having a thickness of 0.1% to 50% of the total thickness of the surface portion, specifically, a region having a thickness of 1% to 30%, and more specifically, a region having a thickness of 10% to 20%, extending from the outermost edge toward the center. Thus, the "internal layer" refers to the remaining region inside the surface layer of the surface portion. The inversion layer may be included in all or part of the surface portion, specifically, in part of the surface layer of the surface portion.

[0059] The single-particle lithium transition metal oxide can be a lithium composite transition metal oxide containing nickel (Ni), cobalt (Co), and manganese (Mn).

[0060] Specifically, the lithium transition metal oxide in single-particle form can be a lithium composite transition metal oxide represented by the following chemical formula 1.

[0061] [Chemical formula 1] Li a Ni x Co y M 1 z M 2 1-x-y-z O2

[0062] In the above chemical formula 1, M 1 M is one or more selected from the group consisting of Mn and Al, 2 x is one or more elements selected from the group consisting of B, Ba, Ce, Cr, F, Mg, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, S, Sr, Ta, La, and Hf, and satisfies the following conditions: 0.9 ≤ a ≤ 1.3, 0.6 ≤ x < 1.0, 0 ≤ y ≤ 0.4, and 0 ≤ z ≤ 0.4.

[0063] The above a represents the molar ratio of lithium in the lithium transition metal oxide, and can be 1.0 ≤ a ≤ 1.3, specifically 1.0 ≤ a ≤ 1.25, and more specifically 1.0 ≤ a ≤ 1.20.

[0064] The aforementioned x represents the molar ratio of nickel to the total transition metals, and can be 0.6 ≤ x < 1.0, specifically 0.6 ≤ x ≤ 0.99 or 0.70 ≤ x ≤ 0.99, and more specifically 0.8 ≤ x ≤ 0.95. When the nickel content satisfies the above range, excellent capacity characteristics can be achieved.

[0065] The aforementioned y represents the molar ratio of cobalt among the total transition metals, and can be 0 ≤ y ≤ 0.40, specifically 0 ≤ y ≤ 0.35, and more specifically 0.01 ≤ y ≤ 0.30.

[0066] The aforementioned z is element M of the entire transition metals. 1 This shows the molar ratio, which can be expressed as 0≦z≦0.40, specifically 0≦z<0.35, and more specifically 0.01≦z≦0.30.

[0067] The aforementioned 1-xyz is M of the entire transition metals. 2 This shows the molar ratio, which can be expressed as 0 ≤ 1-xyz ≤ 0.4, specifically 0 ≤ 1-xyz ≤ 0.35, and more specifically 0 ≤ 1-xyz ≤ 0.30.

[0068] Furthermore, the lithium transition metal oxide can be a positive electrode active material which is a lithium composite transition metal oxide represented by the following chemical formula 2.

[0069] [Chemical formula 2] Li a Ni b Co c Mn d M 1 e O2

[0070] In the above chemical formula 2, M 1 is one or more elements selected from the group consisting of Al, Zr, B, W, Mo, Cr, Nb, Mg, Hf, Ta, La, Ti, Sr, Ba, Ce, Sn, Y, Zn, F, P, and S, and 0.9 ≤ a ≤ 1.1, 0.8 ≤ b < 1, 0 <c<0.2、0<d<0.2、0≦e<0.1、b+c+d+e=1である。

[0071] Furthermore, the lithium transition metal oxide can be a lithium composite transition metal oxide represented by the following chemical formula 3.

[0072] [Chemical formula 3] Li g Ni h Co i Mn j O2

[0073] In the above chemical formula 3, 0.9 ≤ g ≤ 1.1, 0.8 ≤ h < 1, 0 <i<0.2、0<j<0.2、h+i+j=1である。

[0074] The positive electrode active material has an average particle size (D) of 1 to 50 μm, taking into consideration the specific surface area and density of the positive electrode composite material. 50) can have, specifically, an average particle size (D) of 2 to 20 μm. 50 ) can have. A positive electrode active material according to an example of the present invention has an average particle size (D 50 The positive electrode active material can be a single-particle type formed by the aggregation of particles having a diameter of 0.1 μm to 10 μm. When the average particle size of the particles satisfies the above range, advantages can be obtained in terms of rolling ratio, electrode voids, etc., but when the average particle size is too small or too large compared to the above range, performance may deteriorate in terms of electrode capacity, lifetime characteristics, resistance, etc.

[0075] The present invention also provides a method for producing the positive electrode active material.

[0076] The positive electrode active material can be produced by a manufacturing method comprising: 1) mixing lithium transition metal oxide particles in single particle form with a cobalt source; and 2) heat-treating the mixture from step 1).

[0077] In step 1) above, the cobalt source can be used in an amount of 0.1 mol% to 10 mol%, specifically 0.5 mol% to 5 mol%, and more specifically 1 mol% to 3 mol%, based on the positive electrode active material. If the amount of cobalt source used is too small compared to the above range, a sufficient coating cannot be formed on the outside of the surface of the lithium transition metal oxide particles. If the amount of cobalt source used is too large, an excessive coating may be formed, or unreacted cobalt compounds may remain on the surface of the particles, leading to drawbacks such as increased resistance and decreased capacity.

[0078] In step 1), a process of further mixing an additional metal source may be carried out. The additional metal source may be used together with the cobalt source and, through subsequent steps, be coated onto the surface of the single-particle lithium transition metal oxide. The coating formed by the additional metal source may be mixed with and coated together with the coating formed by the cobalt source on the outside of the surface of the single-particle lithium transition metal oxide particles, or it may be formed by another coating. The additional metal source may be coated onto the surface of the single-particle lithium transition metal oxide to form a metal oxide, lithium metal oxide, cobalt metal oxide, or lithium cobalt metal oxide.

[0079] The mixing in step 1) above can be a dry mixing, for example, by mixing a powdered cobalt source material with a lithium transition metal oxide in single-particle form without a solvent. Such dry mixing can be a simple mixing process, and the simplification of the process can offer advantages in cost reduction and quality stabilization.

[0080] 2) In the step of heat-treating the mixture from step 1), cobalt diffuses from the surface of the single-particle lithium transition metal oxide toward the center, forming a coating. When the coating is formed, discontinuous island-like coatings can be formed on the surface of the lithium transition metal oxide. In the method for producing a positive electrode active material according to one embodiment of the present invention, since the single-particle lithium transition metal oxide particles and the cobalt (Co) source are dry-mixed and then heat-treated, the coating can be formed in an island-like manner.

[0081] The heat treatment in step 2) can be carried out at 500°C to 800°C, specifically at 600°C to 800°C, and more specifically at 650°C to 750°C. When the heat treatment temperature is within the range, the cobalt present on the surface of the single-particle lithium transition metal oxide during the heating process for the heat treatment, specifically the cobalt that was formed as an island on the surface of the single-particle lithium transition metal oxide and existed as the LiCoO2 phase, penetrates to an appropriate depth into the interior of the single-particle lithium transition metal oxide, and the NiO layer, which is the degraded layer, can be appropriately transformed into a layered structure of nickel cobalt manganese (NCM) oxide. As a result, the surface portion of the lithium transition metal oxide has a layered (R-3m) structure, and the surface portion includes an oxidation number gradient layer in which the oxidation number of nickel increases towards the outermost direction, thereby exhibiting excellent effects in cell performance such as charge / discharge capacity, initial efficiency, and initial resistance. If the heat treatment temperature in step 2) is low, the thickness of the coating portion will increase, resulting in the formation of an excessive amount of coating, making it difficult to realize the advantages of forming the coating portion as described above. If the heat treatment temperature in step 2) is high, the cobalt may be deeply doped into the lithium transition metal oxide, and the coating portion may not be properly formed on the surface.

[0082] The heat treatment in step 2) can be carried out for 2 to 12 hours, specifically 2 to 9 hours, and more specifically 2 to 6 hours. When the heat treatment is carried out within the above time range, excellent productivity can be achieved and uniform firing can be performed.

[0083] The cobalt source can be a cobalt-containing oxide, hydroxide, oxyhydroxide, carbonate, sulfate, halide, sulfide, acetate, nitrate, carboxylate, or a combination thereof, such as Co(OH)2, CoOOH, Co(OCOCH3)2·4H2O, Co(NO3)2·6H2O, or Co(SO4)2·7H2O, and one or more of these can be used as a mixture. Specifically, Co(OH)2 can be used.

[0084] The aforementioned additional metal source is, for example, an oxide, hydroxide, oxyhydroxide, carbonate, sulfate, halide, sulfide, acetate, nitrate, carboxylate, or combination thereof, containing one or more elements selected from the group consisting of Al, Ti, W, B, F, P, Mg, Fe, Cr, V, Cu, Ca, Zn, Zr, Nb, Mo, Sr, Sb, Bi, Si, Cr, Hf, Ta, La, Ba, Ce, Sn, Y, and S. Specifically, these include ZnO, Al2O3, Al(OH)3, AlSO4, AlCl3, Al-isopropoxide, AlNO3, TiO2, WO3, AlF, H2BO3, HBO2, H3BO3, H2B4O7, B2O3, C6H5B(OH)2, (C6H5O)3B, (CH3(CH2)3O)3B, C3H9B3O6, (C3H7O3)B, Li3WO4, (NH4) 10 W 12 O 41 Examples include 5H2O and NH4H2PO4, but are not limited to these.

[0085] The additional metal source can be used in an amount such that the additional metal is present in a concentration of 100 ppm to 50,000 ppm, specifically 200 ppm to 10,000 ppm, based on the total number of moles of metal in the positive electrode active material. When the additional metal is present within this range, it can be expected that side reactions with the electrolyte will be effectively suppressed, and the electrochemical properties can be further improved.

[0086] In one example of the present invention, the lithium transition metal oxide particles in single-particle form can be produced by mixing a transition metal oxide precursor and a lithium raw material, performing primary calcination, crushing the calcined product produced by primary calcination, and then performing secondary calcination.

[0087] According to yet another embodiment of the present invention, a positive electrode containing the above-mentioned positive electrode active material is provided.

[0088] 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 containing the positive electrode active material described above.

[0089] The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with surface treatment using carbon, nickel, titanium, silver, etc., can be used. The positive electrode current collector can also typically have a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesion of the positive electrode active material. For example, it can be used in various forms such as film, sheet, foil, mesh, porous material, foam, nonwoven fabric.

[0090] The positive electrode active material layer may include a conductive material and a binder, along with the positive electrode active material described above.

[0091] In this case, the conductive material is used to impart conductivity to the electrodes and can be used without particular limitations as long as it does not cause chemical changes in the battery 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 whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these alone or a mixture of two or more can be used. The conductive material can usually be included in an amount of 1 to 30% by weight relative to the total weight of the positive electrode active material layer.

[0092] 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), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, of which one or more can be used. The binder may be present in an amount of 1 to 30% by weight relative to the total weight of the positive electrode active material layer.

[0093] The positive electrode can be manufactured by a conventional method for manufacturing a positive electrode, except for using the positive electrode active material described above. Specifically, it can be manufactured by applying a composition for forming a positive electrode active material layer, which is prepared by mixing or dispersing the positive electrode active material and, selectively, a binder and a conductive material in a solvent, onto a positive electrode current collector, followed by drying and rolling. Here, the types and contents of the positive electrode active material, binder, and conductive material are as described above.

[0094] The solvent can be any solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one or more of these can be used individually or in mixtures of two or more. The amount of solvent used should be sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, and to have a viscosity that allows for excellent thickness uniformity when applied for the manufacture of the positive electrode, taking into consideration the coating thickness of the slurry and the manufacturing yield.

[0095] Alternatively, the positive electrode can also be manufactured by casting the positive electrode active material layer forming composition onto another support, peeling it off the support, and then laminating the resulting film onto the positive electrode current collector.

[0096] According to yet another example of the present invention, an electrochemical element including the positive electrode is provided. Specifically, the electrochemical element may be a battery, a capacitor, and more specifically, a lithium secondary battery.

[0097] Specifically, the lithium secondary battery includes a positive electrode, a negative electrode positioned opposite the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode is as described above. The lithium secondary battery may also selectively further include a battery container for housing the electrode assembly of the positive electrode, negative electrode, and separator, and a sealing member for sealing the battery container.

[0098] In the lithium secondary battery described above, the negative electrode includes a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0099] 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 alloys can be used. The negative electrode current collector can usually have a thickness of 3 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.

[0100] The negative electrode active material layer selectively includes a binder and a conductive material together with the negative electrode active material.

[0101] 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. x(0 < x < 2), metal oxides such as SnO2, vanadium oxides, and lithium vanadium oxides that can be doped and undoped with lithium; or composites containing the metallic compound and a carbonaceous material, such as Si-C composites or Sn-C composites, etc. Any one or a mixture of two or more of these can be used. Also, a thin film of metallic lithium may be used as the negative electrode active material. Also, as the carbon material, both low-crystalline carbon and high-crystalline carbon can be used. Representative examples of low-crystalline carbon are soft carbon and hard carbon, and representative examples of high-crystalline carbon are amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbons such as petroleum or coal tar pitch derived cokes.

[0102] Also, the binder and conductive material are as described above for the positive electrode.

[0103] As an example, the negative electrode active material layer is produced by applying and drying a negative electrode forming composition obtained by dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent on a negative electrode current collector, or by casting the negative electrode forming composition on another support and then laminating a film obtained by peeling off this support on the negative electrode current collector.

[0104] 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. Any separator commonly used in lithium secondary batteries can be used without particular limitations, but those with low resistance to ion movement of the electrolyte and excellent moisture-absorbing capacity for the electrolyte 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. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, coated separators containing ceramic components or polymeric substances may be used to ensure heat resistance or mechanical strength, and can be selectively used as single-layer or multi-layer structures.

[0105] 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.

[0106] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0107] The organic solvent can be used without particular limitations as long as it serves as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the 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); alcohol solvents such as ethanol and isopropyl alcohol; nitriles such as R-CN (where R is a C2-C20 linear, branched, or cyclic hydrocarbon group, and may 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. In this case, mixing the cyclic carbonate and linear carbonate in a volume ratio of about 1:1 to about 1:9 allows the electrolyte to exhibit excellent performance.

[0108] The lithium salt can be any compound capable of providing lithium ions for use in a lithium secondary battery, without any particular limitations. Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, 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 within 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.

[0109] In addition to the components of the electrolyte, the electrolyte may also contain one or more additives, such as haloalkylene carbonate compounds including difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for purposes such as improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity. In this case, the additive may be present in an amount of 0.1 to 5% by weight relative to the total weight of the electrolyte.

[0110] As described above, the lithium secondary battery containing the positive electrode active material according to the present invention exhibits excellent discharge capacity, output characteristics, and capacity retention rate stably, making it useful in portable devices such as mobile phones, notebook computers, and digital cameras, as well as in the field of electric vehicles such as hybrid electric vehicles (HEVs).

[0111] Therefore, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same are provided.

[0112] The aforementioned battery module or battery pack can be used as a power source for one or more medium-to-large devices, including power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0113] The external shape of the lithium secondary battery of the present invention is not particularly limited, but it can be cylindrical, rectangular, pouch-type, or coin-type, using a can.

[0114] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for small devices, but also preferably as a unit battery in medium- and large-sized battery modules containing a large number of battery cells. [Examples]

[0115] Hereinafter, embodiments of the present invention will be described in detail so that they can be easily implemented by a person with ordinary skill in the art to which the present invention pertains. However, the present invention can be realized in various different forms and is not limited to the embodiments described herein.

[0116] Manufacturing Example 1 Cathode active material precursor [Composition: Ni 0.95 Co 0.03 Mn 0.02 (OH)2, average particle size (D 50 (3.5 μm) and LiOH as a lithium raw material are mixed in a molar ratio of 1:1.05, and the mixture is subjected to primary calcination at a temperature of 850°C for 9 hours under an oxygen atmosphere to produce a calcined product. The calcined product was crushed, and then subjected to secondary calcination at 750°C in an oxygen atmosphere for 9 hours to produce lithium transition metal oxide in single-particle form.

[0117] Example 1 The single-particle lithium transition metal oxide produced in Production Example 1 and the powdered cobalt source Co(OH)2 (manufactured by HUAYOU COBALT) were mixed in a ratio of 98 mol% and 2 mol%, respectively.

[0118] The mixture was heat-treated at 700°C for 5 hours to obtain a cake-like cathode active material, which was then pulverized to produce a powder-like single-particle cathode active material.

[0119] Example 2 A powder-type single-particle cathode active material was produced in the same manner as in Example 1, except that the mixture was heat-treated at a temperature of 740°C.

[0120] Example 3 A powder-type single-particle cathode active material was produced in the same manner as in Example 1, except that the mixture was heat-treated at a temperature of 660°C.

[0121] Comparative Example 1 The lithium transition metal oxide in single-particle form produced in the above-mentioned Production Example 1 was used as a single-particle type cathode active material.

[0122] Comparative Example 2 A powder-type single-particle cathode active material was produced in the same manner as in Example 1, except that the mixture was heat-treated at a temperature of 400°C.

[0123] Comparative Example 3 A powder-type single-particle cathode active material was produced in the same manner as in Example 1, except that the mixture was heat-treated at a temperature of 900°C.

[0124] Comparative Example 4 The single-particle lithium transition metal oxide produced in Production Example 1 and the powdered cobalt source Co(OH)2 (manufactured by HUAYOU COBALT) were mixed in a ratio of 99.98 mol% and 0.02 mol%, respectively.

[0125] The mixture was heat-treated at 750°C for 5 hours to obtain a cake-like cathode active material, which was then pulverized to produce a powder-like single-particle cathode active material.

[0126] Comparative Example 5 The single-particle lithium transition metal oxide produced in Production Example 1 and the powdered cobalt source Co(OH)2 (manufactured by HUAYOU COBALT) were mixed in a ratio of 99.98 mol% and 0.02 mol%, respectively.

[0127] The mixture was heat-treated at 700°C for 5 hours to obtain a cake-like cathode active material, which was then pulverized to produce a powder-like single-particle cathode active material.

[0128] Experimental Example 1 1) EELS measurement and confirmation of Ni oxidation state The cathode active material powder was prepared into a thin film sample with a thickness of 100-200 nm using an FEI Helios G4 UX FIB system. Then, the Ni L3 energy loss spectrum of the sample was measured using an FEI Titan G2 80-200 ChemiSTEM system and a Gatan Continuum S EELS system. 2+ and Ni 3+ Using a reference spectrum, nonlinear least-square fitting was performed to measure the average Ni oxidation number on the surface of the positive electrode active material from the outermost layer toward the center, to depths of 5 nm, 10 nm, and 30 nm. 2+ In the reference spectrum, the Ni L3 peak is formed where the energy loss is lower. 3+In the reference spectrum, the Ni L3 peak is formed at the higher energy loss. The results are shown in Table 1, Figure 1, and Figure 2.

[0129] 2) Co / Ni measurement of the surface Using a Thermofisher K-alpha XPS system, the Co / Ni content ratio on the surface of the cathode active material powder to a depth of 10 nm was measured by electron spectroscopy for chemical analysis (ESCA), and the results are shown in Table 1.

[0130] [Table 1]

[0131] Referring to Table 1, the EELS measurement results for Examples 1, 2, and 3 showed that the oxidation state of Ni in the surface up to a depth of 10 nm was higher than that of Ni in the surface up to a depth of 30 nm, moving from the outermost part toward the center of the positive electrode active material. This confirmed that the oxidation state of Ni in the surface of the positive electrode active materials of Examples 1, 2, and 3 increased toward the outermost part. Furthermore, in Examples 1, 2, and 3, the overall Ni balance map confirmed that the oxidation state of Ni gradually increased toward the surface, confirming that the NiO layer on these surfaces was suppressed and reduced.

[0132] On the other hand, in the case of Comparative Examples 1 to 4, it was confirmed that the oxidation state of Ni contained in the surface up to a depth of 30 nm from the outermost part toward the center of the positive electrode active material was at the same level as the oxidation state of Ni contained in the surface up to a depth of 10 nm. Furthermore, it was confirmed that the oxidation state of Ni contained in the surface up to a depth of 30 nm from the outermost part toward the center of the positive electrode active material was lower than in the examples, and that the NiO layer on the surface was relatively abundantly distributed.

[0133] Furthermore, measurements of the Co / Ni values ​​on the surface revealed that Examples 1, 2, and 3 showed a Co / Ni ratio in the range of 0.1 to 0.8, while Comparative Examples 1 and 3 showed a Co / Ni ratio of less than 0.1.

[0134] Figures 1 and 2 show the nickel balance map (Ni valence map) and Ni L3 peak graph of the single-particle type cathode active material of Example 1, respectively. Figures 1 and 2 show measurements taken in different regions of the single-particle type cathode active material of Example 1.

[0135] Figure 1 shows the nickel balance map (Ni valence map) of the single-particle cathode active material of Example 1 on the left, and the Ni L3 peak graph on the right. In Figure 1, index 1 shows the Ni L3 peak graph at a depth of 10 nm, index 2 shows the graph at a depth of 30 nm, and index 3 shows the graph at a depth of 70 nm.

[0136] Looking at the nickel balance map (Ni valence map) on the left side of Figure 1, we can see that indices 3, 2, and 1 show darker colors toward the surface, confirming that in the single-particle type cathode active material of Example 1, the oxidation number of nickel contained in the surface gradually increases toward the outermost surface, thus confirming that the Co / Ni ratio increases toward the outermost surface due to the coating. Referring to the Ni L3 peak graph on the right side of Figure 1, in Example 1, the graph at a depth of 70 nm and index 3 shows that the Ni L3 peak is formed toward the side with lower energy loss, and Ni 2+ It shows a state similar to this, but at index 2 with a depth of 30 nm, a Ni L3 peak is formed on the side with higher energy loss, and at index 1 with a depth of 10 nm, a Ni L3 peak is formed on the side with even higher energy loss, with Ni moving towards the surface. 3+ It can be confirmed that the state has changed to one close to that state. This confirms that the single-particle type positive electrode active material of Example 1 contains an oxidation number gradient layer in which the oxidation number of nickel contained in the surface increases.

[0137] Figure 2 also shows the nickel balance map (left) and Ni L3 peak graph (right) obtained at different positions than those shown in Figure 1 for the single-particle type cathode active material of Example 1. In Figure 2, index 1 shows the Ni L3 peak graph at a depth of 5 nm, index 2 shows the graph at a depth of 10 nm, and index 3 shows the graph at a depth of 30 nm.

[0138] Looking at the nickel balance map (Ni valence map) on the left side of Figure 2, we can see that the color remains dark in index 3 and two directions, but becomes bright again at index 1. This confirms that the single-particle type cathode active material of Example 1 may contain a region with a low nickel oxidation state in its outermost part.

[0139] Referring to the Ni L3 peak graph on the right side of Figure 2, in Example 2, the graph at a depth of 30 nm and index 3 shows that the Ni L3 peak is formed on the side with higher energy loss. 3+ It shows a state close to this, and at index 2 with a depth of 10 nm, a Ni L3 peak is formed on the side with even higher energy loss, and further Ni 3+ It showed a state close to this, but at index 1 with a depth of 5 nm, the Ni L3 peak was formed on the side with lower energy loss, and Ni formed on the outermost surface. 2+ We were able to confirm that a region containing nickel in a state close to that of nickel was formed.

[0140] This confirmed that the single-particle type positive electrode active material of Example 1 includes an oxidation number gradient layer on its surface where the oxidation number of nickel increases, and that a portion of the outer part of the surface may include an oxidation number reversal layer, which is a region where the oxidation number of nickel decreases.

[0141] Experimental Example 2 Manufacturing of positive electrodes Using the positive electrode active material produced in Example 1, carbon black (Denka Black, manufactured by Denka Corporation) as a conductive material and PVdF (KF1300, manufactured by Kureha Corporation) as a binder were added to a solvent (N-methylpyrrolidone (NMP), manufactured by Oi Chemical Co., Ltd.) in a weight ratio of 95:3:2 (positive electrode active material: conductive material: binder) to produce a composition for forming a positive electrode active material layer.

[0142] The manufactured positive electrode active material layer forming composition was coated onto one surface of a 20 μm thick aluminum foil current collector and dried at 135°C for 3 hours to form a positive electrode active material layer. Next, the positive electrode active material layer was rolled using a roll persing method, and after rolling, a positive electrode with a porosity of 20% in the positive electrode active material layer was manufactured.

[0143] Instead of the positive electrode active material produced in Example 1, positive electrodes were manufactured in the same manner as described above, using the positive electrode active materials of Examples 2 and 3 and Comparative Examples 1 to 5, respectively.

[0144] A coin-shaped half-cell was manufactured by using lithium metal as the negative electrode along with the aforementioned positive electrode.

[0145] Evaluation of electrochemical properties The electrochemical properties of the half-cells manufactured as described above were evaluated as follows.

[0146] Each manufactured coin half-cell was charged at 25°C with a constant current (CC) of 0.2C until it reached 4.25V. Then, it was charged again with a constant voltage (CV) of 4.25V until the charging current reached 0.05mAh, and the charging capacity was measured. Next, after being left for 20 minutes, it was discharged with a constant current of 0.2C until it reached 2.5V, and the discharge capacity of the first cycle was measured. The charge / discharge efficiency of the first cycle was evaluated.

[0147] The batteries were fully charged using the same method, and a discharge current of 0.2C was applied for 10 seconds. The initial resistance (DCIR) was measured by dividing the voltage difference between immediately before and 10 seconds after the current was applied by the current. The results are shown in Table 2.

[0148] [Table 2]

[0149] Referring to Table 2 above, it can be confirmed that the positive electrode active materials in Examples 1 to 3, which include an oxidation state gradient layer on the surface where the oxidation state of nickel increases towards the outermost direction, exhibit higher discharge capacity and efficiency, as well as lower initial resistance, compared to the positive electrode active materials in Comparative Examples 1 to 5, which do not have an oxidation state gradient layer on the surface. As a result, it can be confirmed that the coating formed on the outside of the surface can improve cation mixing and structural instability caused by the collapse of the layered structure by making the NiO degradation layer an NCM structure with a relatively high cobalt concentration, thereby solving problems such as increased resistance, decreased capacity, and decreased output.

Claims

1. Lithium transition metal oxides in single-particle form that can be divided into a surface and a core, The surface portion includes a coating portion containing cobalt formed on the surface portion, The surface portion includes an oxidation number gradient layer in which the oxidation number of nickel (Ni) increases in the outermost direction. The nickel contained in the aforementioned surface portion has an average oxidation state of +2.36 to +3.

00. The lithium transition metal oxide is a positive electrode active material represented by the following chemical formula 1. [Chemical formula 1] Li a Ni x Co y M 1 z M 2 1-x-y-z O 2 In the above chemical formula 1, M1 is one or more selected from the group consisting of Mn and Al, and M2 is one or more selected from the group consisting of B, Ba, Ce, Cr, F, Mg, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, S, Sr, Ta, La, and Hf, and 1.0 ≤ a ≤ 1.3, 0.6 ≤ x < 1.0, 0 ≤ y ≤ 0.4, and 0 ≤ z ≤ 0.

4.

2. The positive electrode active material according to claim 1, wherein the surface portion is a region with a depth of 1 nm to 50 nm extending from the outermost part of the single-particle lithium transition metal oxide toward the center.

3. The positive electrode active material according to claim 1, wherein, based on the entire surface portion and coating portion, the cobalt and nickel satisfy a Co / Ni value (mol / mol) of 0.1 to 0.

8.

4. The positive electrode active material according to claim 1, wherein the average oxidation state of nickel in the single-particle lithium transition metal oxide is +2.50 to +3.00 from the outermost part toward the center, and the average oxidation state of nickel up to a depth of 30 nm is +2.36 to +2.

60.

5. The positive electrode active material according to claim 1, wherein the coating portion is formed on the outside of the surface portion to cover 10% to 100% of the total surface area on the outside of the surface portion.

6. The positive electrode active material according to claim 1, wherein the coating portion is located in an island-like manner on the outside of the surface portion.

7. The aforementioned coating portion is LiCoO ２ A positive electrode active material according to claim 1, comprising the composition of the above.

8. The positive electrode active material according to claim 1, wherein the surface portion further includes an oxidation number reversal layer in the outer portion, which is a region in which the oxidation number of nickel decreases in the outermost direction.

9. The positive electrode active material according to claim 8, wherein the oxidation state reversal layer is included in a region of the single-particle lithium transition metal oxide that extends from the outermost part toward the center, and has a thickness from the outermost part of the surface to 0.1% to 50% of the total thickness of the surface.

10. The positive electrode active material according to claim 8, wherein the surface portion is divided into a surface layer and an inner layer in the thickness direction, and the oxidation state of nickel in the inner layer increases towards the outermost direction.

11. The lithium transition metal oxide in single-particle form comprises 50 or fewer crystal grains, as described in claim 1.

12. The positive electrode active material according to claim 1, wherein the lithium transition metal oxide is a lithium composite transition metal oxide represented by the following chemical formula 2. [Chemical formula 2] Li ａ Ni ｂ Co ｃ Mn ｄ M １ ｅ O ２ In the chemical formula (2), M １ is one or more selected from the group consisting of Al, Zr, B, W, Mo, Cr, Nb, Mg, Hf, Ta, La, Ti, Sr, Ba, Ce, Sn, Y, Zn, F, P, and S, 0.9 ≦ a ≦ 1.1, 0.8 ≦ b < 1, 0 < c < 0.2, 0 < d < 0.2, 0 ≦ e < 0.1, and b + c + d + e = 1.

13. 1) A step of mixing lithium transition metal oxide particles in single-particle form and a cobalt (Co) source, 2) A method for producing a positive electrode active material according to claim 1, comprising the step of heat-treating the mixture from step 1).

14. The method for producing a positive electrode active material according to claim 13, wherein in step 1), an additional metal source is further mixed.

15. The method for producing a positive electrode active material according to claim 13, wherein the heat treatment in step 2) is performed at 500 to 800°C.

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