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

Abstract

The present invention relates to a positive electrode active material, the positive electrode active material including a single-particle lithium transition metal composite oxide and a coating portion including cobalt formed on the single-particle lithium transition metal oxide, the coating portion including cobalt having a phase gradient from a spinel structure to a layered structure from a surface to a center of the positive electrode active material, a manufacturing method thereof, and a lithium secondary battery including the same.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0041203, filed April 1, 2022, and all contents disclosed in the documents of said Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a cathode active material including a high-nickel (High-Ni) single-particle lithium composite transition metal oxide, which improves the initial resistance characteristics of a lithium secondary battery by introducing a coating portion including cobalt having a phase gradient, prevents side reactions with an electrolyte, and suppresses gas generation, as well as a manufacturing method thereof and a lithium secondary battery including the same. [Background technology]

[0003] Recently, with the advancement of technologies such as electric vehicles, the need for high-capacity secondary batteries is increasing, and as a result, research into high-nickel (High-Ni) cathode active materials with excellent capacity characteristics is being actively conducted.

[0004] However, when the nickel content of the positive electrode active material is high, problems such as capacity reduction and resistance occur as the battery cycle progresses. To solve these problems, various coating materials have been studied.

[0005] When cobalt, a typical coating material, is applied to the surface of a high-nickel (High-Ni) cathode active material, a coating layer containing cobalt is formed on the surface. In this case, with existing coating methods, the cobalt-containing coating layer is formed in a layered structure, which makes it easy for lithium to be released from the cathode active material and increases the viscosity of the slurry used to form the cathode, resulting in problems such as an increase in the initial resistance of the battery and an increase in the amount of gas generated by the battery. Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention is an invention for solving the above problems, and an object of the present invention is to provide a cathode active material containing a high-nickel (High-Ni) single-particle lithium transition metal composite oxide, which improves the initial resistance characteristics of a lithium secondary battery, prevents side reactions with an electrolyte, and suppresses the amount of gas generation.

[0007] Another object of the present invention is to provide a method for manufacturing the cathode active material, which has a phase gradient and can introduce a coating portion containing cobalt.

[0008] Another object of the present invention is to provide a lithium secondary battery containing the above positive electrode active material, which has low initial resistance and improved storage characteristics. [Means for solving the problem]

[0009] In order to solve the above problems, the present invention provides a positive electrode active material, a method for producing a positive electrode active material, a positive electrode, and a lithium secondary battery.

[0010] (1) The present invention provides a positive electrode active material comprising: a single-particle lithium composite transition metal oxide; and a cobalt-containing coating portion formed on the single-particle lithium composite transition metal oxide, wherein the cobalt-containing coating portion has a phase gradient from a spinel structure to a layered structure from the surface toward the center of the positive electrode active material.

[0011] (2) The present invention provides a positive electrode active material according to (1) above, wherein the lithium transition metal composite oxide in the form of single particles contains 60 mol % or more of nickel relative to the total number of moles of metals other than lithium.

[0012] (3) The present invention provides a positive electrode active material according to (1) or (2), wherein the lithium composite transition metal oxide in the form of a single particle has a composition represented by the following chemical formula 1: [Chemical formula 1] Li x [Ni a Co b Mn c M1 d ]O 2-y A y In the above Chemical Formula 1, M1 is one or more selected from Y, Zr, Al, B, Ti, W, Nb, Sr, Mo, and Mg; A is one or more selected from F, Cl, Br, I and S; 0.9≦x≦1.2, 0.6≦a<1, 0≦b≦0.4, 0≦c≦0.4, 0≦d≦0.2, a+b+c+d=1, 0≦y≦0.2.

[0013] (4) The present invention provides the positive electrode active material according to any one of the above (1) to (3), wherein the coating portion containing cobalt is present locally on the surface of the positive electrode active material.

[0014] (5) In any one of the above (1) to (4), the coating portion containing cobalt has a spinel structure of Co3O4, Li 0.5 The present invention provides a positive electrode active material having a phase gradient of a spinel structure of CoO2 and a layered structure of LiCoO2.

[0015] (6) The present invention provides the positive electrode active material according to any one of (1) to (5) above, wherein the coating portion containing cobalt further contains aluminum.

[0016] (7) The present invention also provides a lithium composite transition metal oxide in the form of a single particle and an average particle size (D 50 and heat-treating the mixture.

[0017] (8) The present invention provides the method for producing a positive electrode active material according to (7), wherein the mixture contains the cobalt oxide in an amount such that the ratio (B / A) of the number of moles of cobalt contained in the cobalt oxide (B) to the total number of moles of metals other than lithium contained in the lithium composite transition metal oxide (A) is 0.1 to 5.

[0018] (9) The present invention provides a method for producing a positive electrode active material according to (7) or (8), wherein the mixture further contains an aluminum source material.

[0019] (10) The present invention provides the method for producing a positive electrode active material according to (9), wherein the mixture contains the aluminum source material in an amount of 0.01 to 0.1 parts by weight relative to 100 parts by weight of the lithium composite transition metal oxide.

[0020] (11) The present invention provides the method for producing a positive electrode active material according to any one of the above (7) to (10), wherein the heat treatment is carried out in an oxygen atmosphere.

[0021] (12) The present invention provides the method for producing a positive electrode active material according to any one of the above (7) to (11), wherein the heat treatment is carried out at a temperature of 600°C to 800°C.

[0022] (13) The present invention provides the method for producing a positive electrode active material according to any one of the above (7) to (12), wherein the heat treatment is carried out for 1 hour to 8 hours.

[0023] (14) The present invention also provides a positive electrode containing the positive electrode active material according to any one of (1) to (6) above.

[0024] (15) The present invention also provides a lithium secondary battery comprising the positive electrode according to (14), a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. [Effects of the Invention]

[0025] The positive electrode active material of the present invention includes a high-nickel (Ni)-based single-particle lithium transition metal composite oxide, and has a phase gradient from a spinel structure to a layered structure from the surface toward the center of the positive electrode active material. The introduction of a coating portion containing cobalt improves the initial resistance characteristics of a lithium secondary battery and minimizes gas generation.

[0026] Furthermore, according to the method for manufacturing a positive electrode active material of the present invention, a coating portion containing cobalt having a phase gradient can be effectively introduced. [Brief explanation of the drawings]

[0027] [Figure 1] 1 shows EPMA (Electron Probe X-ray Micro Analyzer) data of the positive electrode active material of Example 1. [Figure 2] 1 is a TEM (Transmission Electron Microscope) image of the positive electrode active material of Example 1. [Figure 3] 1 is a TEM image of the positive electrode active material of Comparative Example 1. [Figure 4] 1 is a scanning electron microscope (SEM) image of a single particle lithium transition metal composite oxide used in Examples and Comparative Examples. [Figure 5] 1 is an SEM image of the positive electrode active material of Example 1. [Figure 6] 1 is an SEM image of the positive electrode active material of Example 2. [Figure 7] 1 is an SEM image of the positive electrode active material of Comparative Example 1. [Figure 8] 1 is an SEM image of the positive electrode active material of Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention will now be described in more detail to facilitate understanding of the present invention.

[0029] The terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as having meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0030] In this specification, the terms "comprises," "includes," "has," and the like are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, but are not intended to preclude the possible presence or addition of one or more different features, numbers, steps, components, or combinations thereof.

[0031] In this specification, the term "on" refers not only to a case where one structure is formed immediately on top of another structure, but also to a case where a third structure is interposed between the structures.

[0032] In this specification, the term "single-particle positive electrode active material" refers to a positive electrode active material consisting of 10 or less primary particles, in contrast to a spherical secondary particle positive electrode active material produced by a conventional method, which is formed by aggregating tens to hundreds of primary particles. Specifically, in the present invention, the single-particle positive electrode active material may be a single particle consisting of one primary particle, or may be a secondary particle formed by aggregating several primary particles.

[0033] The term "primary particle" refers to the smallest particle unit that can be recognized when observing a positive electrode active material through a scanning electron microscope, and the term "secondary particle" refers to a secondary structure formed by aggregation of multiple primary particles.

[0034] As used herein, the term "average particle size (D 50)" means the particle diameter at the 50% point of the volume cumulative distribution of particle diameters. The average particle diameter is determined by dispersing the powder to be measured in a dispersion medium, introducing it into a commercially available laser diffraction particle size measuring device (e.g., S3500 manufactured by Microtrac), measuring the difference in the diffraction pattern depending on the particle diameter when the particles pass through a laser beam, and calculating the particle size distribution. The particle diameter at the 50% point of the volume cumulative distribution of particle diameters measured by the measuring device is then calculated. 50 can be measured.

[0035] positive electrode active material The present invention provides a positive electrode active material comprising: a single-particle lithium composite transition metal oxide; and a cobalt-containing coating portion formed on the single-particle lithium composite transition metal oxide, wherein the cobalt-containing coating portion has a phase gradient from a spinel structure to a layered structure from the surface toward the center of the positive electrode active material.

[0036] The present inventors have found that when a coating portion having a phase gradient is formed on the single-particle lithium transition metal composite oxide, specifically when a coating portion containing cobalt is formed having a phase gradient from a spinel structure to a layered structure from the surface toward the center of the positive electrode active material, not only does the electrical conductivity increase but also lithium present inside the positive electrode active material is prevented from being released, thereby improving the initial resistance characteristics and storage characteristics (especially high-temperature storage characteristics) of the lithium secondary battery, and have completed the present invention.

[0037] As used herein, the term "phase gradient" refers to a change in the crystal lattice structure, such as a layered structure or a spinel structure. Specifically, the term "phase gradient" refers to a gradual change in the crystal lattice structure as the composition of the coating changes. For example, when the composition of the coating is Li x When it comes to CoO2, this can mean that the crystal lattice structure gradually changes as the x value gradually changes from 1 to 0.5.

[0038] According to the present invention, the single-particle lithium composite transition metal oxide may contain nickel in an amount of 60 mol% or more, specifically 80 mol% or more, and more specifically 85 mol% or more, based on the total moles of metals other than lithium. That is, the single-particle lithium composite transition metal oxide may be a high-nickel (High Ni)-based lithium composite transition metal oxide. In this case, the energy density of a lithium secondary battery can be improved.

[0039] According to the present invention, the lithium transition metal composite oxide in the form of a single particle may have a composition represented by the following Chemical Formula 1.

[0040] [Chemical formula 1] Li x [Ni a Co b Mn c M1 d ]O 2-y A y

[0041] In the above Chemical Formula 1, M1 is one or more selected from Y, Zr, Al, B, Ti, W, Nb, Sr, Mo, and Mg; A is one or more selected from F, Cl, Br, I and S; 0.9≦x≦1.2, 0.6≦a<1, 0≦b≦0.4, 0≦c≦0.4, 0≦d≦0.2, a+b+c+d=1, 0≦y≦0.2.

[0042] Specifically, M1 can be one or more selected from Y, Zr, and Al.

[0043] The a represents the atomic fraction of nickel among the metal elements in the lithium composite transition metal oxide, and may be 0.6≦a<1, 0.8≦a≦0.98, or 0.85≦a≦0.95.

[0044] The b represents the atomic fraction of cobalt among the metal elements in the lithium composite transition metal oxide, and may be 0≦b≦0.4, 0.01≦b≦0.2, or 0.01≦b≦0.15.

[0045] The c represents the atomic fraction of manganese among the metal elements in the lithium composite transition metal oxide, and may be 0≦b≦0.4, 0.01≦b≦0.2, or 0.01≦b≦0.15.

[0046] The d represents the atomic fraction of the M1 element among the metal elements in the lithium composite transition metal oxide, and may be 0≦d≦0.2, 0≦d≦0.1, or 0≦d≦0.05.

[0047] The lithium transition metal composite oxide in the form of a single particle has an average particle size (D 50 ) can be 1 μm to 10 μm, specifically 1 μm to 7 μm, and more specifically 1 μm to 5 μm.

[0048] According to the present invention, the cobalt-containing coating may be present locally on the surface of the positive electrode active material to improve lithium ion conductivity. That is, the coating may not cover the entire surface of the single-particle lithium composite transition metal oxide. Here, the coating may have a dot shape. Meanwhile, the coating may not be present at grain boundaries.

[0049] According to the present invention, the cobalt-containing coating has a phase gradient from a spinel structure to a layered structure from the surface toward the center of the positive electrode active material, which not only increases the conductivity but also prevents lithium present inside the positive electrode active material from being released, thereby improving the initial resistance characteristics and high-temperature storage characteristics of the lithium secondary battery.

[0050] According to the present invention, the coating portion containing cobalt has a spinel structure of Co3O4, Li, and the like, in this order from the surface to the center of the positive electrode active material.0.5 It is possible to have a phase gradient of CoO2 spinel structure and LiCoO2 layer structure. In this case, the spinel structure of Co3O4, Li 0.5 The spinel structure of CoO2 not only provides excellent electrical conductivity, but also 0.5 The CoO2 spinel structure has high lithium ion conductivity, which can reduce the initial resistance of a battery using the positive electrode active material. In addition, the Co3O4 spinel structure, which is a structure that prevents lithium from moving, is present at the outermost edge, which can further prevent lithium from being released from the positive electrode active material, thereby improving the storage characteristics of a battery using the positive electrode active material.

[0051] According to the present invention, the cobalt-containing coating portion may further include aluminum. In this case, the lithium ion conductivity of the coating portion may be improved, and when the cathode active material is applied to a battery, the initial resistance characteristics and storage characteristics of the battery may be improved. Here, the coating portion including cobalt and aluminum may be locally present on the surface of the cathode active material and have a dotted shape.

[0052] The aluminum may be included in the lithium composite transition metal oxide in an amount of 100 ppm to 1,000 ppm, 200 ppm to 700 ppm, or 400 ppm to 600 ppm. In this case, the dot-shaped coating portion is distributed in an appropriate size and amount, so that when the positive electrode active material is applied to a battery, the electrochemical characteristics of the battery can be improved.

[0053] Method for producing positive electrode active material The present invention relates to a lithium composite transition metal oxide in the form of a single particle and an average particle size (D 50 and heat-treating the mixture.

[0054] The positive electrode active material according to the present invention is manufactured by the method for manufacturing a positive electrode active material according to the present invention. That is, a lithium composite transition metal oxide in the form of a single particle and an average particle size (D 50 When a mixture containing a cobalt oxide having a particle size of 50 nm to 1000 nm is prepared and the mixture is heat-treated, a coating portion containing cobalt is formed on the lithium transition metal composite oxide in the form of a single particle, and the coating portion containing cobalt has a phase gradient (specifically, a phase gradient from a spinel structure to a layer structure from the surface toward the center of the positive electrode active material).

[0055] As in the present invention, the average particle size (D 50 When a cobalt oxide having an average particle size (D) of 50 nm to 1000 nm is used as a raw material, a coating portion containing cobalt having a phase gradient from a spinel structure to a layer structure from the surface to the center of the positive electrode active material can be formed. 50 When cobalt oxide with an average particle size (D) of less than 50 nm is used as the raw material for the coating, the diffusion is too fast and the coating is not formed, and the cobalt diffuses into the lithium composite transition metal oxide. 50 When cobalt oxide having a particle size exceeding 1000 nm is used as the raw material for the coating, there is a problem that the cobalt oxide remains in a solidified state.

[0056] According to the present invention, the cobalt oxide has an average particle size (D 50 ) may be 50 nm to 1000 nm, specifically 50 nm to 300 nm, and more specifically 50 nm to 200 nm. In this case, the coating portion may not be formed on the grain boundary, but may be formed locally on the surface of the positive electrode active material.

[0057] The cobalt oxide may be at least one selected from Co3O4, Co2O3, and CoO, and specifically may be Co3O4.

[0058] According to the present invention, the mixture may contain the cobalt oxide in an amount such that the ratio (B / A) of the number of moles of cobalt (B) contained in the cobalt oxide to the total number of moles of metals other than lithium (A) contained in the lithium composite transition metal oxide is 0.1 to 5, specifically 0.5 to 4, and more specifically 1 to 2.5. In this case, the formed coating portion can be controlled to have a desired phase, morphology, etc., and as a result, the resistance characteristics and high-temperature storage characteristics of a battery including the manufactured positive electrode active material can be further improved.

[0059] According to the present invention, the mixture may further include an aluminum raw material. In this case, aluminum may enable more precise control of the shape of the coating portion, eliminate cation mixing, and improve lithium ion conductivity. As a result, the structural stability and lithium ion conductivity of the resulting cathode active material may be improved, and when the cathode active material is applied to a battery, the initial resistance characteristics and storage characteristics of the battery may be further improved. For reference, when the mixture further includes an aluminum raw material, more dot-shaped coating portions may be formed.

[0060] According to the present invention, the mixture may include the aluminum source material in an amount of 0.01 to 0.1 parts by weight, specifically 0.02 to 0.07 parts by weight, and more specifically 0.04 to 0.06 parts by weight, based on 100 parts by weight of the lithium composite transition metal oxide. In this case, the dot-shaped coating portions may be distributed in an appropriate size and amount, thereby improving the structural stability of the prepared cathode active material. When the cathode active material is applied to a battery, the resistance characteristics and high-temperature storage stability of the battery may be further improved.

[0061] The aluminum source material may be at least one selected from Al(OH)2, Al2O3, AlCl3, Al(NO)3, and AlSO4, specifically at least one selected from Al(OH)2, Al2O3, and Al(NO)3, more specifically Al(OH)2.

[0062] According to the present invention, the heat treatment can be carried out in an oxygen atmosphere to prevent the lithium transition metal oxide from deteriorating into a rock salt structure.

[0063] According to the present invention, the heat treatment can be carried out at a temperature of 600°C to 800°C, specifically 650°C to 750°C, and more specifically 680°C to 730°C to form a coating portion having a desired phase.

[0064] According to the present invention, the heat treatment can be performed for 1 hour to 10 hours, specifically 2 hours to 8 hours, and more specifically 3 hours to 6 hours in order to control interdiffusion between the lithium transition metal oxide and the coating portion.

[0065] positive electrode The present invention provides a positive electrode containing the positive electrode active material.

[0066] According to an embodiment of the present invention, the positive electrode may include 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 may include the positive electrode active material.

[0067] According to one embodiment of the present invention, the positive electrode current collector may include a highly conductive metal, and is not particularly limited as long as it is easily adhered to the positive electrode active material layer and is non-reactive within the voltage range of the battery. Examples of the positive electrode current collector include stainless steel, aluminum, nickel, titanium, calcined carbon, and aluminum or stainless steel whose surfaces are surface-treated with carbon, nickel, titanium, silver, etc. The positive electrode current collector typically has a thickness of 3 μm to 500 μm, and the surface of the current collector may be micro-irregularized to enhance adhesion of the positive electrode active material. The positive electrode current collector may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0068] According to one embodiment of the present invention, the positive electrode active material layer may optionally include a conductive material and a binder in addition to the positive electrode active material. The positive electrode active material may be included in an amount of 80 wt % to 99 wt %, more specifically, 85 wt % to 98.5 wt %, based on the total weight of the positive electrode active material layer. Within this range, excellent capacity characteristics can be exhibited.

[0069] According to one embodiment of the present invention, the conductive material is used to impart conductivity to the electrode. Any conductive material can be used without particular limitations as long as it does not cause chemical changes in the resulting 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 powder or metal fiber, 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; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. The conductive material may be present in an amount of 0.1 wt % to 15 wt % based on the total weight of the positive electrode active material layer.

[0070] According to an embodiment of the present invention, the binder improves adhesion between positive electrode active material particles and between the positive electrode active material and the current collector. Specific examples of the binder include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, polymethyl methacrylate, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, polymers in which hydrogen is substituted with Li, Na, or Ca, or various copolymers thereof. These may be used alone or in combination. The binder may be contained in an amount of 0.1% by weight to 15% by weight based on the total weight of the positive electrode active material layer.

[0071] According to one embodiment of the present invention, the positive electrode can be manufactured by a conventional method for manufacturing a positive electrode, except for using the positive electrode active material. Specifically, the positive electrode can be manufactured by coating a positive electrode active material layer-forming composition, which is prepared by dissolving or dispersing the positive electrode active material and, optionally, a binder, a conductive material, and a dispersant in a solvent, on a positive electrode current collector, followed by drying and rolling, or by casting the positive electrode active material layer-forming composition on a separate support, peeling it from the support, and laminating the resulting film on a positive electrode current collector.

[0072] According to an embodiment of the present invention, the solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), dimethylformamide (DMF), acetone, or water, and may be used alone or in combination. The amount of the solvent used may 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 provide a viscosity that allows excellent thickness uniformity during subsequent coating for manufacturing a positive electrode.

[0073] Lithium secondary battery The present invention provides a lithium secondary battery including the positive electrode.

[0074] According to an embodiment of the present invention, the lithium secondary battery may include the positive electrode, the negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. The lithium secondary battery may further include a battery container that houses the electrode assembly including the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0075] According to an embodiment of the present invention, the negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0076] According to one embodiment of the present invention, the negative electrode current collector may be made of any material that does not cause chemical changes in the battery and has high conductivity. Examples of such materials include copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surfaces that have been surface-treated with carbon, nickel, titanium, silver, or the like, and aluminum-cadmium alloys. The negative electrode current collector may typically have a thickness of 3 μm to 500 μm. Similar to the positive electrode current collector, the current collector may have a surface with fine irregularities to enhance the binding strength of the negative electrode active material. The negative electrode current collector may be made in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0077] According to an embodiment of the present invention, the negative electrode active material layer may optionally include a binder and a conductive material in addition to the negative electrode active material.

[0078] According to an embodiment of the present invention, the negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples thereof 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, and Al alloys; and SiO βExamples of the negative electrode active material include metal oxides capable of doping and dedoping lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide (0<β<2); and composites containing the metallic compounds and carbonaceous materials, such as Si-C composites and Sn-C composites. These may be used alone or in combination. A thin film of metallic lithium may also be used as the negative electrode active material. The carbon material may be either low-crystalline carbon or high-crystalline carbon. Representative examples of low-crystalline carbon include soft carbon and hard carbon, while representative examples of high-crystalline carbon include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature fired carbon such as petroleum or coal tar pitch-derived cokes. The negative electrode active material may be included in an amount of 80 wt% to 99 wt% of the total weight of the negative electrode active material layer.

[0079] According to one embodiment of the present invention, the binder in the negative electrode active material layer is a component that helps bind the conductive material, active material, and current collector, and is typically added in an amount of 0.1 to 10 wt % based on the total weight of the negative electrode active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0080] According to one embodiment of the present invention, the conductive material in the negative electrode active material layer is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 10 wt % or less, preferably 5 wt % or less, based on the total weight of the negative electrode active material layer. The conductive material may be any conductive material that does not cause chemical changes in the battery and has conductivity. Examples of such conductive materials include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; 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.

[0081] According to one embodiment of the present invention, the negative electrode may be fabricated by coating a negative electrode active material layer-forming composition, prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent, on a negative electrode current collector and drying the coating. Alternatively, the negative electrode active material layer-forming composition may be fabricated by casting the negative electrode active material layer-forming composition on a separate support, peeling it off from the support, and laminating the resulting film on the negative electrode current collector.

[0082] According to one embodiment of the present invention, the separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without particular limitation. In particular, a separator that exhibits low resistance to electrolyte ion movement and excellent electrolyte humidification is preferred. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material can also be used, and can be selectively used in a single-layer or multi-layer structure.

[0083] According to an embodiment of the present invention, the electrolyte may be, but is not limited to, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc., which can be used in manufacturing a lithium secondary battery. As a specific example, the electrolyte may include an organic solvent and a lithium salt.

[0084] According to an embodiment of the present invention, the organic solvent may be any solvent capable of acting as a medium through which ions involved in the electrochemical reaction of the battery can migrate. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of solvents that can be used include carbonate-based solvents such as ethylene carbonate (PC), alcohol-based solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (where R is a hydrocarbon group having 2 to 20 carbon atoms and having a linear, branched, or cyclic structure, and may contain a double-bonded aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred.

[0085] According to one embodiment of the present invention, the lithium salt may be any compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the anion of the lithium salt may be 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 may be at least one selected from the group consisting of: LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. The concentration of the lithium salt is preferably within a range of 0.1M to 2.0M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, thereby exhibiting excellent electrolyte performance and allowing lithium ions to migrate effectively.

[0086] According to one embodiment of the present invention, in addition to the electrolyte components, the electrolyte may further include one or more additives, such as haloalkylene carbonate compounds (e.g., difluoroethylene carbonate), pyridine, triethyl phosphite, triethyl alcohol amine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethyl alcohol, or aluminum trichloride, for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity. Here, the additives may be included in an amount of 0.1 wt % to 5 wt % based on the total weight of the electrolyte.

[0087] A lithium secondary battery including the positive electrode active material according to the present invention stably exhibits excellent capacity characteristics, output characteristics, and life characteristics, and is therefore useful in portable devices such as mobile phones, notebook computers, and digital cameras, and in the field of electric vehicles such as hybrid electric vehicles (HEVs) and electric vehicles (EVs).

[0088] The external shape of the lithium secondary battery of the present invention is not particularly limited, and may be a cylindrical shape using a can, a square shape, a pouch shape, a coin shape, or the like.

[0089] The lithium secondary battery according to the present invention can be used as a battery cell used as a power source for a small device, and can also be preferably used as a unit battery in a medium- to large-sized battery module including a large number of battery cells.

[0090] Therefore, according to one embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell, and a battery pack including the same.

[0091] According to one embodiment of the present invention, the battery module or battery pack may be used as a power source for one or more medium- to large-sized devices, such as a power tool; an electric vehicle (EV), a hybrid electric vehicle, and an electric vehicle (PHEV), including a plug-in hybrid electric vehicle; or a power storage system.

[0092] While the present invention may be embodied in various different forms, it is to be understood that the invention is not limited to the specific embodiments set forth herein, and that the invention may be embodied in various different forms without departing from the spirit or scope of the present invention. [Example]

[0093] Examples and Comparative Examples Example 1 Single particle lithium transition metal oxide (composition: LiNi 0.86 Co 0.05 Mn 0.09 O2; average particle size (D 50 ): 4 μm) (LG Chemical Co., Ltd.) and average particle size (D 50 A mixture was prepared by uniformly mixing cobalt oxide (Co3O4) having a particle size of 200 nm with cobalt oxide (Co3O4) in an amount such that the ratio of the number of moles of cobalt contained in the cobalt oxide to the total number of moles of metals other than lithium contained in the lithium composite transition metal oxide was 2. The mixture was heat-treated in an oxygen atmosphere at a temperature of 680°C for 5 hours to prepare a positive electrode active material in which a coating portion containing cobalt was formed on the single-particle lithium composite transition metal oxide.

[0094] Example 2 Single particle lithium transition metal oxide (composition: LiNi 0.86 Co 0.05 Mn 0.09 O2; average particle size (D 50 ): 4 μm) (LG Chemical Co., Ltd.) and average particle size (D 50A mixture was prepared by uniformly mixing cobalt oxide (Co3O4) having a particle size of 200 nm and aluminum hydroxide (Al(OH)2) (manufactured by KC Corporation) (the cobalt oxide was used in an amount such that the ratio of the number of moles of cobalt contained in the cobalt oxide to the total number of moles of metals other than lithium contained in the lithium composite transition metal oxide was 2, and the aluminum hydroxide was mixed in an amount of 0.05 parts by weight per 100 parts by weight of the lithium composite transition metal oxide). The mixture was heat-treated in an oxygen atmosphere at a temperature of 700°C for 5 hours to prepare a cathode active material in which a coating portion containing cobalt and aluminum was formed on the single-particle lithium composite transition metal oxide.

[0095] Comparative Example 1 Single particle lithium transition metal oxide (composition: LiNi 0.86 Co 0.05 Mn 0.09 O2; average particle size (D 50 A mixture was prepared by uniformly mixing cobalt hydroxide (Co(OH)2) (manufactured by Huayou) in amounts such that the ratio of the number of moles of cobalt contained in the cobalt hydroxide to the total number of moles of metals other than lithium contained in the lithium composite transition metal oxide was 2. The mixture was heat-treated in an oxygen atmosphere at a temperature of 680°C for 5 hours to prepare a positive electrode active material in which a coating layer containing cobalt was formed on the single particle lithium composite transition metal oxide.

[0096] Comparative Example 2 Single particle lithium transition metal oxide (composition: LiNi 0.86 Co 0.05 Mn 0.09 O2; average particle size (D 50A mixture was prepared by uniformly mixing cobalt hydroxide (Co(OH)2) (manufactured by Huayou), aluminum hydroxide (Al(OH)2) (manufactured by KC Chemical), and aluminum hydroxide (Al(OH)2) (manufactured by KC Chemical). (The cobalt hydroxide was used in an amount such that the ratio of the number of moles of cobalt contained in the cobalt hydroxide to the total number of moles of metals other than lithium contained in the lithium composite transition metal oxide was 2, and the aluminum hydroxide was mixed in an amount of 0.05 parts by weight per 100 parts by weight of the lithium composite transition metal oxide.) The mixture was heat-treated in an oxygen atmosphere at a temperature of 700°C for 5 hours to prepare a cathode active material in which a coating layer containing cobalt and aluminum was formed on the single-particle lithium composite transition metal oxide.

[0097] Comparative Example 3 Average particle size (D 50 Instead of cobalt oxide (Co3O4) with an average particle size (D 50 A positive electrode active material was prepared in the same manner as in Example 2, except that cobalt oxide (Co3O4) having a particle size of 30 nm was used.

[0098] Comparative Example 4 Average particle size (D 50 Instead of cobalt oxide (Co3O4) with an average particle size (D 50 A positive electrode active material was prepared in the same manner as in Example 2, except that cobalt oxide (Co3O4) having a particle size of 1.5 μm was used.

[0099] Experimental Example Experimental Example 1: Positive electrode active material analysis To confirm how the cobalt-containing coating was formed on the surface of the positive electrode active material prepared in Example 1, an EPMA (Electron Probe X-ray Micro Analyzer) was used to analyze the surface, and the EPMA data is shown in FIG. 1.

[0100] In addition, the coating portions or coating layers of the positive electrode active materials prepared in Example 1 and Comparative Example 1 were observed using a transmission electron microscope (TEM). The TEM image of Example 1 is shown in FIG. 2, and the TEM image of Comparative Example 1 is shown in FIG. 3.

[0101] Referring to FIG. 1, it can be seen that in Example 1 of the present invention, a coating portion containing cobalt is locally formed on the surface of the positive electrode active material.

[0102] 2 and 3, in Example 1, the positive electrode active material has a spinel structure of Co3O4 from the surface to the center, and Li 0.5 It can be seen that a coating layer with a phase gradient of a CoO2 spinel structure and a LiCoO2 layer structure was formed. On the other hand, in the case of Comparative Example 1, which did not use cobalt oxide, it can be seen that a coating layer with a LiCoO2 layer structure was formed on the surface of the positive electrode active material due to the low melting point and high reactivity of Co(OH)2.

[0103] On the other hand, the average particle size (D 50 In the case of Comparative Examples 3 and 4, which do not use cobalt oxide with a particle size of 50 nm to 1000 nm, an appropriate coating portion is not formed, which may affect the performance of the battery due to the difference in reactivity depending on the particle size of the cobalt oxide.

[0104] Experimental Example 2: Confirmation of SEM image of positive electrode active material The single particle form of lithium composite transition metal oxide (composition: LiNi 0.86 Co 0.05 Mn 0.09 O2; average particle size (D 50 :4 μm) (manufactured by LG Chemical Co., Ltd.) was photographed using a SEM (Scanning Electron Microscope), and the SEM image is shown in FIG. 4.

[0105] The positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 and 2 were photographed using an SEM. The SEM image of the positive electrode active material of Example 1 is shown in FIG. 5, the SEM image of the positive electrode active material of Example 2 is shown in FIG. 6, the SEM image of the positive electrode active material of Comparative Example 1 is shown in FIG. 7, and the SEM image of the positive electrode active material of Comparative Example 2 is shown in FIG. 8.

[0106] 4 to 8, it can be seen that the positive electrode active material according to the present invention has a coating portion present in the form of dots locally on the surface of the positive electrode active material, whereas the positive electrode active materials of Comparative Examples 1 and 2 do not have this.

[0107] Experimental Example 3: Evaluation of battery characteristics The positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 and 2, carbon black conductive material, and polyvinylidene fluoride (PVDF) binder were mixed in an N-methylpyrrolidone (NMP) solvent at a ratio of 96:2:2 to prepare positive electrode slurry. The positive electrode slurry was applied to one side of an aluminum current collector, dried at 150°C, and rolled to prepare positive electrodes.

[0108] A lithium metal electrode was used as the negative electrode, and a porous polyethylene separator was interposed between the positive and negative electrodes to prepare an electrode assembly. The electrode assembly was then placed inside a battery case, and an electrolyte solution was injected into the case to prepare a half-cell. The electrolyte solution was prepared by dissolving 1.0 M LiPF6 in an organic solvent mixture of ethylene carbonate (EC): ethyl methyl carbonate (EMC): diethyl carbonate (DEC) in a volume ratio of 3:4:3.

[0109] Each half-cell thus prepared was charged at 0.1 C in CC-CV mode at 25°C up to 4.3 V, and then discharged at a constant current of 0.1 C down to 3.0 V, while measuring the initial charge capacity and initial discharge capacity, and the direct current internal resistance (DCIR) was calculated and shown in Table 1. For reference, the DCIR value was calculated by dividing the difference between the voltage at 60 seconds and the initial voltage while discharging at a constant current of 0.1 C by the applied current.

[0110] [Table 1] [Table 1]

[0111] The half-cells prepared as described above were stored at 60°C for 12 weeks, and the volume change of the battery was measured every one or two weeks. The results are shown in Table 2. If the reactivity increases during high-temperature storage, the amount of gas generated increases, and the volume change of the battery becomes large. Therefore, if the volume change of the battery is large, it can be evaluated that the high-temperature storage characteristics are poor.

[0112] [Table 2] [Table 2]

[0113] As shown in Tables 1 and 2, it can be confirmed that the batteries containing the positive electrode active materials of Examples 1 and 2 not only have low initial resistance but also have a small volume change rate during high-temperature storage.

[0114] On the other hand, batteries containing the cathode active materials of Comparative Examples 1 and 2, in which the cobalt-containing coating layer is present in a layered structure, are found to have problems such as high initial resistance and high gas generation due to increased side reactions during high-temperature storage caused by lithium detachment. Batteries containing the cathode active materials of Comparative Examples 3 and 4, in which an appropriate coating is not formed, are also found to have problems such as high initial resistance and / or high gas generation.

[0115] As a result, it can be seen that the positive electrode active material of the present invention includes a coating portion having a phase gradient on the single particle lithium composite transition metal oxide, specifically, a coating portion having a phase gradient from a spinel structure to a layered structure from the surface to the center of the positive electrode active material, thereby improving the initial resistance characteristics and storage characteristics of a lithium secondary battery. In addition, according to the method for manufacturing the positive electrode active material of the present invention, the average particle diameter (D 50It can be seen that the positive electrode active material according to the present invention can be effectively manufactured by using cobalt oxide having a thickness of 50 nm to 1000 nm as a coating material.

Claims

1. a lithium composite transition metal oxide in the form of a single particle; a coating portion including cobalt formed on the single particle lithium composite transition metal oxide, the cobalt-containing coating portion has a phase gradient from a spinel structure to a layer structure from the surface to the center of the positive electrode active material, The lithium transition metal oxide in the form of a single particle contains nickel in an amount of 60 mol% or more relative to the total number of moles of metals other than lithium, The positive electrode active material, wherein the cobalt-containing coating portion has a phase gradient of a Co 3 O 4 spinel structure, a Li 0.5 CoO 2 spinel structure, and a LiCoO 2 layered structure in this order from the surface toward the center of the positive electrode active material.

2. The positive electrode active material according to claim 1 , wherein the single particle lithium composite transition metal oxide has a composition represented by the following Chemical Formula 1: [Chemical formula 1] Li ｘ [Ni ａ Co ｂ Mn ｃ M1 ｄ ]O ２－ｙ A ｙ In the above Chemical Formula 1, M1 is one or more selected from Y, Zr, Al, B, Ti, W, Nb, Sr, Mo, and Mg; A is one or more selected from F, Cl, Br, I and S; 0.9≦x≦1.2, 0.6≦a<1, 0≦b≦0.4, 0≦c≦0.4, 0≦d≦0.2, a+b+c+d=1, 0≦y≦0.

2.

3. The positive electrode active material according to claim 1 , wherein the coating portion containing cobalt is present locally on the surface of the positive electrode active material.

4. The positive electrode active material of claim 1 , wherein the cobalt-containing coating further contains aluminum.

5. Single particle form lithium composite transition metal oxide and average particle size (D ５０ preparing a mixture containing cobalt oxide having a thickness of 50 nm to 1000 nm; and heat-treating the mixture.

6. 6. The method for producing a positive electrode active material according to claim 5, wherein the mixture contains the cobalt oxide in an amount such that a ratio (B / A) of the number of moles of cobalt contained in the cobalt oxide (B) to the total number of moles of metals other than lithium contained in the lithium composite transition metal oxide (A) is 0.1 to 5.

7. The method of claim 5 , wherein the mixture further comprises an aluminum source material.

8. 8. The method of claim 7, wherein the mixture contains the aluminum source material in an amount of 0.01 to 0.1 parts by weight based on 100 parts by weight of the lithium composite transition metal oxide.

9. The method for producing a positive electrode active material according to claim 5 , wherein the heat treatment is carried out in an oxygen atmosphere.

10. The method for producing a positive electrode active material according to claim 5, wherein the heat treatment is carried out at a temperature of 600°C to 800°C.

11. The method for producing a positive electrode active material according to claim 5, wherein the heat treatment is carried out for 1 hour to 8 hours.

12. A positive electrode comprising the positive electrode active material according to claim 1 .

13. The positive electrode according to claim 12; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte.

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