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

Abstract

The present embodiment relates to a positive active material, a method of preparing the same, and a lithium secondary battery including the same. According to one embodiment, a positive electrode active material for a lithium secondary battery includes a metal oxide composed of single particles and a coating layer on a surface of the metal oxide and containing carbon, in which the coating layer may include an amorphous structure.

Description

Technical Field

[0001] This embodiment relates to a positive electrode active material for a lithium secondary battery, a method for preparing the same, and a lithium secondary battery including the positive electrode active material. Background Art

[0002] Recently, driven by the explosive demand for electric vehicles and the requirement for increased driving range, secondary batteries with high capacity and high energy density are being actively developed worldwide. In particular, to meet these demands, high-nickel NCM cathode materials with a high nickel content are used.

[0003] However, as the nickel content increases, the particle strength decreases, resulting in microcracks during charge and discharge. In addition, this leads to an increase in the specific surface area of the cathode material and an increase in the reaction with the electrolyte, thereby increasing gas generation. At the same time, due to the structural instability, the phenomenon of cation mixing in which unstable Ni 3+ is reduced to stable Ni 2+ and converted into stable NiO increases. Therefore, it is difficult to practically apply it as a positive electrode active material for electric vehicles or lithium-ion batteries for energy storage.

[0004] To solve this problem, a method has been proposed in which primary particles are aggregated into secondary particles, that is, a single-crystalline form in which the size of the primary particles is increased as much as possible instead of a poly-crystalline form, and the prepared cathode material is applied.

[0005] However, generally, in order to prepare a cathode material in a single-crystalline form, firing needs to be performed at a higher temperature than that of polycrystals, so the cation mixing phenomenon becomes more serious. As a result, more nickel oxides are formed on the surface of the single-crystalline particles, and these nickel oxides act as an impedance layer, resulting in problems such as an increase in the initial resistance and a decrease in the output characteristics. Summary of the Invention

[0006] Technical Problem

[0007] In this embodiment, by forming a carbon coating on the surface, it is intended to provide a positive electrode active material for a lithium secondary battery, which has a single-particle form, and the initial resistance and output characteristics are significantly improved, as well as a method for preparing the same and a lithium secondary battery including the material.

[0008] Technical Solution

[0009] The positive electrode active material for a lithium secondary battery according to an embodiment includes a metal oxide composed of single particles; and a coating containing carbon located on the surface of the metal oxide; the coating includes an amorphous structure.

[0010] A method for preparing a positive electrode active material for a lithium secondary battery according to an embodiment includes: a step of preparing a metal hydroxide containing nickel, cobalt, and manganese; a step of performing a first firing on a mixture of the metal hydroxide, a lithium raw material, and a doping raw material to obtain a lithium metal oxide; and a step of performing a second firing on the lithium metal oxide under a condition of maintaining a carbon-containing gas to form a carbon-containing coating on the surface of the lithium metal oxide.

[0011] A positive electrode for a lithium secondary battery according to an embodiment includes: a current collector; and a positive electrode active material layer located on at least one surface of the current collector and containing the positive electrode active material; the calendering density is 3.7 g / cc or more.

[0012] A lithium secondary battery according to an embodiment may include the positive electrode.

[0013] Advantages of the Invention

[0014] According to an embodiment, by flowing a carbon-containing gas through a metal oxide composed of single particles to form a coating in a gaseous state, a positive electrode active material with carbon uniformly coated on the surface of the single particles can be provided.

[0015] Thereby, while maintaining the excellent discharge capacity of the positive electrode active material composed of single particles, the high-rate output characteristics can be significantly improved.

[0016] In addition, a positive electrode active material with excellent room temperature and high temperature life, reduced initial resistance, and increased calendering density can also be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of a process for forming a coating on a positive electrode active material of a lithium secondary battery according to an embodiment.

[0018] Figure 2 Shows the charge curve of Comparative Example 1.

[0019] Figure 3 Shows the discharge curve of Comparative Example 1.

[0020] Figure 4 Shows the charge curve of Example 1.

[0021] Figure 5 Shows the discharge curve of Example 1.

[0022] Figure 6 Shows the measurement results of the average particle size (D50) of the positive electrode active materials of Comparative Example 1 and Examples 1 to 4.

[0023] Figure 7 It is the SEM analysis result of magnifying the positive electrode active material of Example 3 by 1000 times for measurement.

[0024] Figure 8 SEM analysis results of the positive electrode active material of Example 3 measured at 20,000 times magnification.

[0025] Figure 9 Image measurement results of the positive electrode active material prepared according to Example 3 using a high-resolution transmission electron microscope (HRTEM).

[0026] Figure 10 For Figure 9 The result of further magnification of the central part showing the formation of the carbon coating in Detailed Description

[0027] The terms first, second, third, etc. are used to describe various parts, components, regions, layers, and / or segments, but these parts, components, regions, layers, and / or segments should not be limited by these terms. These terms are only used to distinguish one part, component, region, layer, and / or segment from another part, component, region, layer, and / or segment. Therefore, without departing from the scope of the present invention, the first part, component, region, layer, and / or segment described below can also be described as the second part, component, region, layer, and / or segment.

[0028] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. Unless clearly indicated to the contrary in the context, the singular forms used are also intended to include the plural forms. It should also be understood that the term "comprising" used in the specification can specifically refer to a certain characteristic, field, integer, step, action, element, and / or component, but does not exclude the existence or addition of other characteristics, fields, integers, steps, actions, elements, and / or components.

[0029] If a part is described as being above another part, there may be other parts directly above or between the other part. When a part is described as being directly above another part, there will be no other parts therebetween.

[0030] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. For terms defined in a dictionary, they should be interpreted as having a meaning consistent with the relevant technical literature and the content disclosed herein, and should not be interpreted in an idealized or overly formal sense.

[0031] As described above, the positive electrode material in the form of single particles has problems of initial resistance increase and output characteristic degradation. To solve this problem, the particle size of the single particles must be made very small. However, in this case, while the process cost increases, due to the residual lithium on the surface of the single particles, gel curing occurs during the preparation of the slurry for electrode plate preparation, thereby causing problems of electrode defects.

[0032] This embodiment aims to solve the above problems through a surface coating technique for the positive electrode active material in the form of single particles.

[0033] Specifically, the positive electrode active material for a lithium secondary battery according to an embodiment includes a metal oxide composed of single particles; and a carbon-containing coating located on the surface of the metal oxide; the coating includes an amorphous structure. In this embodiment,

[0034] The single particles include at least one of a single crystal structure composed of one particle and a monolith structure composed of two to three particles.

[0035] Figure 1 A schematic diagram showing the process of forming a coating on the positive electrode active material of a lithium secondary battery in this embodiment is shown.

[0036] Reference Figure 1 , in this embodiment, by flowing acetylene (C2H2) gas through single particles including a monolith structure and pyrolyzing at a high temperature, carbon can be uniformly coated on the surface of the single particles.

[0037] The currently commercialized dry coating process only forms a coating on the outer surface of single particles. However, since this embodiment uses chemical vapor deposition (CVD), a coating can be formed in a gaseous state, which can easily penetrate into minute voids to form a uniform coating.

[0038] Therefore, in this embodiment, a carbon layer can also exist at the particle boundaries of the monolith structure in which 2 to 3 particles are aggregated.

[0039] Based on the entirety of the positive electrode active material, the carbon content can be 0.6 wt% or less. More specifically, the carbon content can be in the range of more than 0 wt% and less than 0.6 wt%, 0.01 wt% to 0.50 wt%, 0.01 wt% to 0.40 wt%, or 0.01 wt% to 0.36 wt%.

[0040] In the positive electrode active material of the present embodiment, if the carbon content satisfies the above range, while maintaining excellent discharge capacity, the high-rate output characteristics can be significantly improved. In addition, the life at normal temperature and high temperature is excellent, the initial resistance can be reduced, and the calendering density can be increased.

[0041] The thickness of the coating layer can be in the range of 10 nm to 100 nm or 10 nm to 50 nm. If the thickness of the coating layer satisfies the above range, the output characteristics of single-particle positive electrode active material can be improved.

[0042] Secondly, the average particle size (D50) of the positive electrode active material can be in the range of 3 μm to 5 μm. When the average particle size (D50) satisfies the above range, even in the case of a single-particle structure, a positive electrode active material with life characteristics equivalent to or even higher than those of polycrystalline positive electrode active material can be achieved.

[0043] On the other hand, the metal oxide may include nickel, cobalt, manganese, and a doping element. The doping element may be one or more selected from the group consisting of Zr, Al, B, P, La, Ta, Ti, W, Mo, Si, Ga, Zn, Nb, Ag, Sn, Bi, Au, Y, Ge, V, Cr, and Fe. When based on 1 mole of the total of nickel, cobalt, manganese, and the doping element, the content of the doping element can be in the range of 0.0005 mole to 0.04 mole or 0.001 mole to 0.03 mole. Here, the doping element refers to the doping amount of the doping element contained in the finally obtained positive electrode active material.

[0044] In the positive electrode active material, in order to ensure the life and various electrochemical properties, the selection of the doping element is very important. In the present embodiment, the characteristics of the positive electrode active material can be improved by applying the various doping elements.

[0045] In the embodiment, the doping element may include Zr and Al.

[0046] Since Zr ions occupy the lithium sites, they act as a kind of filler, alleviating the shrinkage of the lithium ion path during charge and discharge, thereby stabilizing the layered structure. This phenomenon reduces cation mixing and increases the lithium diffusion coefficient, thereby extending the cycle life.

[0047] In addition, Al ions move to the tetragonal lattice sites, inhibiting the degradation of the layered structure into a spinel structure where lithium ion movement is relatively poor.

[0048] The content of Zr is based on 1 mole of the total of nickel, cobalt, manganese and doping elements, and is 0.001 mole to 0.01 mole, more specifically, in the range of 0.0016 mole to 0.0064 mole, 0.0017 mole to 0.0055 mole or 0.002 mole to 0.005 mole. When the doping amount of Zr satisfies the above range, the resistance increase rate at high temperature can be reduced while ensuring excellent life characteristics.

[0049] The content of Al is based on 1 mole of the total of nickel, cobalt, manganese and doping elements, and is 0.001 mole to 0.04 mole, more specifically, in the range of 0.004 mole to 0.028 mole, 0.0045 mole to 0.027 mole or 0.0055 mole to 0.025 mole. When the doping amount of Al satisfies the above range, the life and thermal stability at high temperature can be further improved.

[0050] In the metal oxide particles of this embodiment, the content of nickel is based on 1 mole of the total of nickel, cobalt, manganese and doping elements, and can be 0.85 mole or more. More specifically, the content of nickel can be in the range of 0.85 mole to 0.99 mole, or 0.87 mole to 0.99 mole.

[0051] Like this embodiment, when the content of nickel in the metal oxide is 0.85 mole or more, a positive electrode active material with high output characteristics can be realized. The positive electrode active material of this embodiment with such a composition can increase the battery capacity of the battery using it due to the increase in the energy density per unit volume, and is also very suitable for use in electric vehicles.

[0052] On the other hand, the positive electrode active material of this embodiment may also include a positive electrode active material including metal oxides in the form of secondary particles aggregated from primary particles.

[0053] That is to say, it includes a positive electrode active material composed of single particles and formed with a carbon-containing coating, and a positive electrode active material including metal oxides composed of secondary particles with an average particle size (D50) larger than that of the positive electrode active material composed of single particles and formed with a carbon-containing coating. In this way, when the positive electrode active material of the metal oxide composed of single particles and formed with a carbon-containing coating and the positive electrode active material of the metal oxide composed of secondary particles are mixed and used in a bimodal form, the binder density of the electrode can be increased, which is therefore advantageous.

[0054] Thus, in the positive electrode active material with a bimodal morphology, the mixing ratio of the positive electrode active material composed of single particles and containing a carbon-coated metal oxide to the positive electrode active material containing the metal oxide in the form of secondary particles is in the range of 30:70 to 10:90 or 25:75 to 15:85 by weight ratio (single particle: secondary particle). When using the positive electrode active material containing the metal oxide composed of single particles and secondary particles in the said weight ratio, the binder density of the electrode can be increased.

[0055] At this time, the positive electrode active material containing the metal oxide composed of single particles and secondary particles may have the same composition or different compositions. Specifically, the metal oxide constituting the secondary particles may also include nickel, cobalt, manganese, and doping elements.

[0056] For example, in the metal oxide particles constituting the secondary particle form, the content of nickel may be 0.8 moles or more based on 1 mole of the total of nickel, cobalt, and manganese. More specifically, the content of nickel may be in the range of 0.8 moles to 0.99 moles, 0.85 moles to 0.99 moles, 0.88 moles to 0.99 moles.

[0057] The metal oxide constituting the secondary particle form further contains doping elements, and the doping elements may include at least one of aluminum, zirconium, niobium, molybdenum, tungsten, titanium, cerium, magnesium, boron, phosphorus, vanadium, strontium.

[0058] The specific description and content of the said doping elements are the same as those of the positive electrode active material of the metal oxide composed of single particles described above, and are omitted here.

[0059] In addition, the average particle size (D50) of the positive electrode active material containing the metal oxide in the form of secondary particles in this embodiment may be in the range of 10 μm to 20 μm, or 12 μm to 17 μm. When the average particle size of the positive electrode active material containing the metal oxide composed of secondary particles satisfies the said range, in the positive electrode active material with a bimodal morphology, the opposing distribution of large and small particles can be appropriately positioned, thereby increasing the energy density of the lithium secondary battery.

[0060] According to another embodiment, the positive electrode active material for a lithium secondary battery includes: a step of preparing a metal hydroxide containing nickel, cobalt, and manganese; a step of performing a first firing on the mixture of the metal hydroxide, a lithium raw material substance, and a doping raw material substance to obtain a lithium metal oxide; and a step of performing a second firing on the lithium metal oxide under the condition of maintaining a carbon-containing gas to form a carbon-containing coating on the surface of the lithium metal oxide.

[0061] First, prepare a metal hydroxide containing nickel, cobalt, and manganese.

[0062] In this embodiment, an aqueous metal salt solution containing a nickel raw material substance, a cobalt raw material substance, a manganese raw material substance, and water is prepared, and then the aqueous metal salt solution is supplied to a precipitation reactor to obtain a metal hydroxide.

[0063] Next, a first firing is performed on a mixture of the metal hydroxide, a lithium raw material substance, and a doping raw material substance to obtain a lithium metal oxide. At this time, the molar ratio of lithium (Li) to all metals (Me) other than lithium (Li / Me) can be prepared in the range of 1.0 to 1.1, or in the range of 1.01 to 1.08.

[0064] The first firing can be carried out at 720 °C to 900 °C or 760 °C to 860 °C for 10 hours to 24 hours. If the temperature and time conditions of the first firing meet the above range, a cathode material in the form of single particles or monomers can be prepared.

[0065] Subsequently, a second firing is performed on the lithium metal oxide in the presence of a carbon-containing gas to form a carbon-containing coating on the surface of the lithium metal oxide.

[0066] The carbon-containing gas may include at least one of acetylene (C2H2) gas, methane (CH4) gas, ethylene (C2H4) gas, and propane (C3H8) gas.

[0067] The second firing process can be carried out at 560 °C to 680 °C, 580 °C to 660 °C, or 600 °C to 640 °C for 0.5 hours to 2 hours. When the temperature and time conditions of the second firing meet the above range, it has the advantage that a carbon coating is easily formed.

[0068] Here, the retention time of the carbon-containing gas can be carried out in the range of 0.5 minutes to 10 minutes, or 1 minute to 4 minutes. In the coating formation process, when the retention time of the carbon-containing gas meets the above range, it has the advantage of an optimal carbon coating thickness.

[0069] In another embodiment, a cathode is provided that includes a current collector and a cathode active material layer located on one side of the current collector and containing the cathode active material prepared according to the previous embodiment.

[0070] The calendering density of the cathode is 3.7 g / cc or more, and more specifically, it is a cathode for a lithium secondary battery in the range of 3.7 g / cc to 4.0 g / cc.

[0071] When the calendering density meets this range, the energy density of the lithium secondary battery can be significantly increased. Therefore, when the cathode of this embodiment is applied to an electric vehicle, the driving distance can be significantly increased.

[0072] The characteristics of the positive electrode active material constituting the positive electrode active material layer are the same as those described above. Therefore, the specific description of the positive electrode active material is omitted.

[0073] As the current collector, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or a material obtained by surface treatment such as carbon, nickel, titanium, silver, etc. on the surface of aluminum or stainless steel can be used.

[0074] Meanwhile, the positive electrode active material layer may include a binder and a conductive agent.

[0075] At this time, the binder serves to enhance the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), polyvinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinyl alcohol, polyacrylonitrile, sodium carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene - propylene - diene rubber (EPDM rubber), sulfonated EPDM, styrene - butadiene rubber (SBR), fluororubber or various copolymers thereof, etc. One of them can be used alone or two or more of them can be used in combination, but it is not limited thereto. The binder may account for 1 to 30% by weight of the total weight of the positive electrode active material layer.

[0076] In addition, the conductive material is used to impart conductivity to the electrode. In the assembled battery, as long as it has electronic conductivity and does not cause chemical changes, it can be used without special restrictions. 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, summer black, carbon fiber, etc.; metal powders or metal fibers such as copper, nickel, aluminum, silver, etc.; conductive whiskers such as zinc oxide, potassium titanate, etc.; conductive metal oxides such as titanium oxide, etc.; or conductive polymers such as polystyrene derivatives, etc. One of these materials can be used alone or two or more of them can be used in combination, but it is not limited thereto. The conductive material is usually contained in an amount of 1 to 30% by weight based on the total weight of the positive electrode active material layer.

[0077] When using the positive electrode active material, the positive electrode can be prepared according to a conventional positive electrode preparation method.

[0078] Specifically, the positive electrode is prepared by coating the positive electrode active material and a composition for forming a positive electrode active material layer that selectively contains a binder, a conductive material, or a solvent as needed on a positive electrode current collector, and then drying and rolling. At this time, the types and contents of the positive electrode active material, the binder, and the conductive material are as described above.

[0079] The solvent may be a solvent commonly used in the relevant technical field, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, etc. These solvents can be used alone or in combination of two or more. The amount of the solvent should consider the coating thickness of the slurry and the preparation yield to dissolve or disperse the positive electrode active material, the conductive agent, and the binder, and make the viscosity required for excellent thickness uniformity of the coating for subsequent positive electrode preparation.

[0080] In addition, another method is to cast the composition for preparing the positive electrode active material layer on an independent support, then peel it off from the support to obtain a thin film, and laminate it onto the positive electrode current collector to prepare the positive electrode.

[0081] In another embodiment, a lithium secondary battery including the positive electrode is provided.

[0082] The lithium secondary battery specifically includes a positive electrode, a negative electrode opposite to the positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, where the positive electrode is as described above. In addition, the lithium secondary battery may optionally include a battery container for accommodating the electrode assembly including the positive electrode, the negative electrode, and the separator, and a sealing member for sealing the battery container.

[0083] In this lithium secondary battery, the negative electrode may include a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0084] The negative electrode current collector is not particularly limited to a certain material, 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, materials with surface treatment of carbon, nickel, titanium, silver, etc. on the surface of copper or stainless steel, or aluminum-cadmium alloy, etc. can be used. In addition, the negative electrode current collector generally has a thickness of 3 to 500 microns. Similar to the positive electrode current collector, minute irregularities can be formed on the surface of the current collector to enhance the binding force of the negative electrode active material. For example, various forms such as films, sheets, foils, meshes, porous bodies, foams, non-woven fabrics, etc. can be adopted.

[0085] The negative electrode active material layer may selectively include a negative electrode active material, a binder, and a conductive agent. As an example, the negative electrode active material layer can be prepared by coating a composition for forming the negative electrode active material layer including the negative electrode active material and optionally including a binder and a conductive agent on the negative electrode current collector and drying, or by casting the composition for forming the negative electrode on an independent support, and then laminating the obtained thin film peeled off from the support onto the negative electrode current collector.

[0086] The negative electrode active material can be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples include carbonaceous materials such as synthetic graphite, natural graphite, graphitized carbon fiber, amorphous carbon, etc.; metal compounds capable of alloying with lithium such as silicon, aluminum, tin, lead, zinc, bismuth, indium, magnesium, gallium, cadmium, silicon alloys, tin alloys or aluminum alloys; metal oxides capable of lithium insertion and deintercalation such as SiO β (0 < β < 2), SnO2, vanadium oxides, lithium vanadium oxides, etc.; or composites including the metal compounds and carbonaceous materials such as Si-C composites or Sn-C composites, etc. Any single material or a mixture of two or more of these can be used. In addition, as the negative electrode active material, a thin film of metallic lithium can also be used. In addition, as the carbon material, low-crystalline carbon and high-crystalline carbon, etc. can be used. As the low-crystalline carbon, soft carbon and hard carbon are representative; as the high-crystalline carbon, it includes amorphous, flaky, needle-shaped, spherical or fibrous natural graphite or synthetic graphite, Kish graphite, pyrolytic carbon, liquid crystal pitch-based carbon fiber, mesoporous carbon microspheres, liquid crystal pitch, and high-temperature calcined carbon such as petroleum and coal tar-based coke, etc.

[0087] The binder and the conductive agent can be the same as those described in the above positive electrode description.

[0088] Next, depending on the type of the lithium secondary battery, a separator may exist between the positive electrode and the negative electrode.

[0089] These separators can be made of polyethylene, polypropylene, polyvinylidene fluoride or a multi-layer film of two or more layers thereof, and hybrid multi-layer films such as polyethylene / polypropylene two-layer separator, polyethylene / polypropylene / polyethylene three-layer separator, polypropylene / polyethylene / polypropylene three-layer separator, etc. can also be used.

[0090] In addition, in the lithium secondary battery, as the electrolyte, an organic solvent electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc. that can be used in the preparation of the lithium secondary battery can be used, and it is not limited thereto.

[0091] Specifically, the organic liquid electrolyte can include an organic solvent and a lithium salt.

[0092] The organic solvent can be used without special restrictions as long as it can serve as a medium for the migration of ions in the battery electrochemical reaction. Specifically, the following substances can be used as the organic solvent. Ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, ε-caprolactone, etc.; ether solvents such as dibutyl ether or tetrahydrofuran, etc.; ketone solvents such as cyclohexanone, etc.; aromatic hydrocarbon solvents such as benzene, fluorobenzene, etc.; carbonate solvents such as dimethylcarbonate (DMC), diethyl carbonate (DEC), methylethylcarbonate (MEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), etc.; alcohol solvents such as ethanol, isopropanol, etc.; nitriles such as R-CN (where R is a straight-chain, branched-chain or cyclic hydrocarbon group with 2 to 20 carbon atoms, which may contain double bonds, aromatic rings or ether bonds), etc.; amide solvents such as dimethylformamide, etc.; dioxolane solvents such as 1,3-dioxolane, etc.; or sulfolane solvents such as sulfolane, etc. Among them, carbonate solvents are preferred, and more preferably a mixture of a cyclic carbonate (such as ethylene carbonate or propylene carbonate, etc.) with high ionic conductivity and high dielectric constant, which can improve the charge and discharge performance of the battery, and a low-viscosity chain carbonate compound (such as methylethylcarbonate, dimethylcarbonate or diethyl carbonate, etc.). In this case, when the cyclic carbonate and the chain carbonate are mixed and used at a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte can be excellent.

[0093] Regarding the lithium salt, any compound that can provide lithium ions for use in a lithium secondary battery can be used without particular limitation. Specifically, the lithium salt includes 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 used in the range of 0.1 to 2.0 M. When the concentration of the lithium salt is within this range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance, and lithium ions can migrate effectively.

[0094] As described above, the lithium secondary battery containing the positive electrode active material according to the present invention is very useful in portable devices such as mobile phones, laptop computers, digital cameras, etc., and in the field of electric vehicles such as hybrid electric vehicles (HEV), etc., because it exhibits excellent discharge capacity, output characteristics, and capacity retention rate.

[0095] Embodiments of the invention

[0096] The embodiments of the present invention will be described in detail below. However, this is provided only as an example, and the present invention is not limited thereto, and the present invention is only defined by the scope of the following claims.

[0097] Comparative Example 1 - Single-particle positive electrode active material

[0098] (1) Preparation of the precursor

[0099] The precursor was prepared by a general co-precipitation method.

[0100] Specifically, NiSO4·6H2O was used as the nickel raw material, CoSO4·7H2O was used as the cobalt raw material, and MnSO4·H2O was used as the manganese raw material. These raw materials were dissolved in distilled water to prepare a metal salt solution.

[0101] After preparing the co-precipitation reactor, to prevent the oxidation of metal ions during co-precipitation, the reactor was purged with nitrogen, and the reactor temperature was maintained at 50°C.

[0102] NH4OH as a complexing agent was added to the co-precipitation reactor, and the pH was adjusted using NaOH. According to the co-precipitation process, the obtained precipitate was filtered, washed with distilled water, and then dried in an oven at 100°C for 24 hours to prepare the positive electrode active material precursor.

[0103] The composition of the prepared precursor was (Ni 0.88 Co0.07 Mn 0.05 )(OH)2, with an average particle size (D50) of about 4 μm.

[0104] (2) Preparation of the positive electrode active material

[0105] The precursor prepared in the above (1) is uniformly mixed with LiOH·H2O (Tris Electrochemistry, battery grade), ZrO2 (Aldrich, 4N), and Al(OH)3 (Aldrich, 4N) to obtain a mixture.

[0106] At this time, the molar ratio of lithium (Li) to all metals (Me) other than lithium (Li / Me) is designed to be slightly higher than 1, and 0.003 moles of Zr and 0.01 moles of Al are added for doping.

[0107] Then, the mixture is fired in a box-type firing furnace with an oxygen flow rate of 1000 ml / min at 850 °C for 24 hours to obtain a positive electrode active material with a composition of LiNi 0.87 Co 0.07 Mn 0.05 Al 0.01 O2. At this time, since the addition amount of Zr is extremely small, it is not indicated in the composition. The obtained single particles are crushed by using a jet mill to obtain a positive electrode active material in the form of single particles with an average particle size (D50) of about 4 microns.

[0108] Example 1 - Preparation of a positive electrode active material with a carbon coating

[0109] The single-particle type positive electrode active material prepared in Comparative Example 1 is placed in an alumina sheet with a size of 10 cm × 5 cm (10 g) and placed in the middle of a quartz tube with a diameter of 20 cm and a length of 50 cm, and then placed in the middle of a firing furnace. A thermocouple is passed through the quartz tube so that the tip of the thermocouple contacts the quartz surface to measure the temperature. The outlet of the quartz tube installed in the firing furnace is connected to a pipe, and the initial vacuum degree in the pipe is maintained at 100 mTorr by a rotary pump. Subsequently, the temperature of the firing furnace is raised to 620 °C, and acetylene (C2H2) gas is introduced into the inlet part of the quartz tube so that the acetylene pressure inside the quartz tube reaches 380 Torr. At this time, the acetylene gas flows in the quartz tube for 1 minute.

[0110] After that, the vacuum state is maintained again, and the temperature of the firing furnace is lowered to room temperature to prepare a positive electrode active material with a carbon coating.

[0111] Comparative Examples 2 to 3, Examples 2 to 12, and Reference Examples 1 to 6

[0112] When preparing single-particle cathode active materials in oxide form, except for adjusting the firing temperature, the composition of the single-particle cathode active material, the firing temperature during carbon coating formation, the retention time of acetylene gas in the quartz tube, and the carbon coating amount according to Table 1 below, single-particle cathode active materials or coated cathode active materials were prepared using the same method as in Example 1.

[0113]

Table 1

[0114]

[0115]

[0116] Experimental Example 1 - Measurement of Life Characteristics

[0117] (1) Fabrication of Coin-Type Half-Cell

[0118] Using the cathode active material prepared as described above, a CR2032 coin cell was fabricated and subjected to electrochemical tests. The results are listed in Table 2 below.

[0119] Specifically, the cathode active material, a conductive agent (carbon black), and a polyvinylidene fluoride binder (trade name: KF1120) were mixed at a weight ratio of 96.5:1.5:2, and the mixture was added to an N-methyl-2-pyrrolidone solvent to make the solid content about 30 wt%, thereby preparing a cathode active material slurry.

[0120] The slurry was coated on an aluminum foil (Al foil, thickness: 15 μm) serving as the cathode current collector with a scraper, dried, and calendered to fabricate the cathode. The loading amount of the cathode was about 15 mg / cm 2 . Using the above cathode, a lithium metal anode (thickness 300 μm, MTI), an electrolyte, and a polypropylene separator, a 2032 coin-type half-cell was fabricated by a conventional method. The preparation method of the above electrolyte is as follows: 1 M LiPF6 was dissolved in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate, and ethyl methyl carbonate (EMC) (the mixing ratio was EC:DMC:EMC = 3:4:3 by volume%), and then 3 wt% of vinylene carbonate (VC) was added thereto.

[0121] (2) Evaluation of Charge-Discharge Characteristics

[0122] After the coin-type half-cell prepared in (1) was aged at room temperature (25 °C) for 10 hours, a charge-discharge test was conducted.

[0123] The capacity evaluation was based on a reference capacity of 200 mAh / g, and the charge-discharge conditions were constant current (CC) / constant voltage (CV) from 3.4 V to 4.3 V, with a 1 / 20C cut-off. The initial capacity was measured by 0.1C charge / 0.1C discharge.

[0124] (3) Determination of initial resistance characteristics at room temperature

[0125] The initial resistance at room temperature (DC internal resistance: DC-IR) is measured by charging the battery to 4.3V (SOC100) at 100% at 25°C, then applying a discharge current of 0.1C, measuring the voltage value (the voltage value after the drop) after 60 seconds, and substituting it into Ohm's law for calculation.

[0126] (4) Output characteristic determination

[0127] The output characteristic is calculated by dividing the discharge capacity at 1C, 2C, and 4C by the charge capacity charged at 0.5C and converting it to a percentage (%).

[0128] (5) Cycle life characteristic determination

[0129] The cycle life characteristic is measured 50 times under the conditions of charging at 0.3C / discharging at 0.3C at room temperature (25°C) and high temperature (45°C).

[0130] (6) Determination of calendered density of the electrode plate

[0131] The electrode plate density is determined by measuring the weight per unit volume of the electrode plate after coating the positive electrode paste, drying, and calendering.

[0132] (7) Average particle size (D50) determination

[0133] The average particle size (D50) is measured using a particle size distribution analyzer (PSA, particle size analyzer) from Microtrac.

[0134] (8) Carbon content determination

[0135] The total carbon content in the positive electrode active material is quantified using a C / S (Eltra Corporation, carbon / sulfur analyzer).

[0136]

Table 2

[0137]

[0138]

[0139] Referring to Table 2, when the carbon coating process is carried out as described in Examples 1 to 12 and the carbon content in the positive electrode active material is maintained within a certain range, it can be confirmed that the output characteristics are very excellent without a significant decrease in the discharge capacity. Especially in terms of the high-rate output characteristics, which are the disadvantages of the single-particle structure positive electrode active material, the effect is very excellent. In addition, it can be understood that the room temperature and high temperature life of Examples 1 to 12 have not decreased significantly compared to before coating and remain good.

[0140] At the same time, it is also possible to confirm that one of the greatest advantages of the single-particle structure, namely the calendering density, has not decreased significantly and remains good.

[0141] However, as shown in Reference Examples 1 to 6, when the carbon content exceeds a certain value, due to a significant increase in the initial resistance, the discharge capacity, efficiency, high-rate output characteristics, etc. will decrease significantly. In addition, not only does the average particle size (D50) increase significantly, but the calendering density is also significantly lower than that of the Examples, resulting in a significant decline in battery characteristics.

[0142] Experimental Example 2 - Comparison of Charge-Discharge Curves

[0143] Figure 2 Shows the charge curve of Comparative Example 1, Figure 3 Shows the discharge curve of Comparative Example 1, Figure 4 Shows the charge curve of Example 1, Figure 5 Shows the discharge curve of Example 1.

[0144] By comparing them with each other Figures 2 to 5 , when the carbon content of the positive electrode active material is 0.03% by weight, it can be seen that the charge-discharge curves hardly change before and after coating, and the charge-discharge process proceeds smoothly. This is also evident in Examples 2, 3, and 4.

[0145] Experimental Example 3 - Comparison of Charge-Discharge Curves

[0146] Figure 6 Shows the measurement results of the average particle size (D50) of the positive electrode active material composed of single particles in the state where the positive electrode active materials of Comparative Example 1 and Examples 1 to 4 are completed by classification after being crushed by a jet mill.

[0147] As shown in Table 2, the single-particle positive electrode active material showing good output results in the Examples only brings a slightly increased particle size effect compared to Comparative Example 1, and the change is not significant.

[0148] Experimental Example 4 - SEM Analysis and Structure Analysis

[0149] Figure 7 Is the SEM analysis result of the positive electrode active material of Example 3 at a magnification of 1000 times, Figure 8 Is the SEM analysis result of the positive electrode active material of Example 3 at a magnification of 20000 times.

[0150] Referring to the SEM image at a low magnification Figure 7 , it can be seen that there is no subdivision or agglomeration, and the particle size distribution is very uniform. Therefore, even if the surface is coated with a carbon layer, the surface can be well formed without damaging the particles.

[0151] Reference SEM images at high magnification Figure 8 , it can be seen that the surface of a single particle remains nearly spherical, further confirming the good coating of the carbon layer.

[0152] Figure 9 are the results of image measurement of the positive electrode active material prepared according to Example 3 using a high-resolution transmission electron microscope (HRTEM) device. Figure 10 are for Figure 9 the results of magnifying the central part where the carbon coating layer shown in white is formed at a higher magnification.

[0153] Referring to Figure 9 , it can be seen that in the positive electrode active material with a monolith structure in which 2 to 3 single crystal particles are aggregated, carbon coating layers ranging from 10 nm to 100 nm are well filled in the gaps therein. In addition, referring to Figure 10 , at the points where the particles are in contact with each other, the particles are coated with a carbon layer, and through this, it can be confirmed that the carbon layer has penetrated well into the interior of the particles.

[0154] In addition, the single crystal particles exhibit a crystal structure, while the coating layer exists in an amorphous state. From these results, it can be seen that coating carbon by the CVD method is a very effective method capable of uniformly coating the surface and interior of single particles.

[0155] The present invention is not limited to the above-described embodiments, but can be implemented in various different forms, and those skilled in the art to which the present invention pertains should understand that the present invention can be implemented in other specific forms without changing the technical idea or basic characteristics of the present invention. Therefore, it should be understood that the above-described embodiments are exemplary in all aspects and not restrictive.

Claims

1. A positive electrode active material for a lithium secondary battery, comprising: A metal oxide composed of single particles; And A carbon-containing coating located on the surface of the metal oxide; The coating includes an amorphous structure.

2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein The single particles include at least one of a single crystal structure composed of one particle and an integral structure composed of two to three particles.

3. The positive electrode active material for a lithium secondary battery according to claim 2, wherein The metal oxide includes a carbon layer present at the grain boundaries of the integral structure where two to three particles are aggregated.

4. The positive electrode active material for a lithium secondary battery according to claim 1, wherein Based on the total amount of the positive electrode active material, the content of carbon is 0.6 wt% or less.

5. The positive electrode active material for a lithium secondary battery according to claim 1, wherein The thickness of the coating is in the range of 10 nm to 100 nm.

6. The positive electrode active material for a lithium secondary battery according to claim 1, wherein The average particle size (D50) of the positive electrode active material is in the range of 3.8 μm to 4.5 μm.

7. The positive electrode active material for a lithium secondary battery according to claim 1, wherein The metal oxide includes nickel, cobalt, manganese, and a doping element.

8. The positive electrode active material for a lithium secondary battery according to claim 7, wherein The doping element includes one or more selected from the group consisting of Zr, Al, B, P, La, Ta, Ti, W, Mo, Si, Ga, Zn, Nb, Ag, Sn, Bi, Au, Y, Ge, V, Cr, and Fe.

9. The positive electrode active material for a lithium secondary battery according to claim 7, wherein The content of the doping element is in the range of 0.0005 mol to 0.04 mol based on 1 mol of the total of nickel, cobalt, manganese, and the doping element.

10. The positive electrode active material for a lithium secondary battery according to claim 7, wherein The doping element includes Zr and Al.

11. The positive electrode active material for a lithium secondary battery according to claim 10, wherein The content of Zr is in the range of 0.001 mol to 0.01 mol based on 1 mol of the total of nickel, cobalt, manganese, and the doping element.

12. The positive electrode active material for a lithium secondary battery according to claim 10, wherein The content of Al is in the range of 0.001 mol to 0.04 mol based on 1 mol of the total of nickel, cobalt, manganese, and the doping element.

13. The positive electrode active material for a lithium secondary battery according to claim 7, wherein In the metal oxide, the content of nickel is 0.85 mol or more based on 1 mol of the total of nickel, cobalt, and manganese.

14. The positive electrode active material for a lithium secondary battery according to claim 1, wherein The positive electrode active material further includes a positive electrode active material containing a metal oxide composed of secondary particles aggregated from primary particles.

15. The positive electrode active material for a lithium secondary battery according to claim 14, wherein The average particle diameter (D50) of the positive electrode active material containing the metal oxide composed of secondary particles is greater than the average particle diameter (D50) of the positive electrode active material containing the metal oxide composed of single particles and having a carbon-containing coating formed thereon.

16. The positive electrode active material for a lithium secondary battery according to claim 15, wherein, The average particle diameter (D50) of the positive electrode active material containing the metal oxide composed of secondary particles is in the range of 10 μm to 20 μm.

17. A method for preparing a positive electrode active material for a lithium secondary battery, comprising: A step of preparing a metal hydroxide containing nickel, cobalt, and manganese; A step of performing a first firing on a mixture of the metal hydroxide, a lithium raw material, and a doping raw material to obtain a lithium metal oxide; And A step of performing a second firing on the lithium metal oxide under conditions of maintaining a carbon-containing gas to form a carbon-containing coating on the surface of the lithium metal oxide.

18. The method for preparing a positive electrode active material for a lithium secondary battery according to claim 17, wherein, In the step of forming the coating, the carbon-containing gas includes at least one of acetylene (C2H2) gas, methane (CH4) gas, ethylene (C2H4) gas, and propane (C3H8) gas.

19. The method for preparing a positive electrode active material for a lithium secondary battery according to claim 17, wherein, The second firing step is performed in the range of 560 °C to 680 °C.

20. The method for preparing a positive electrode active material for a lithium secondary battery according to claim 17, wherein, In the step of forming the coating, the retention time of the carbon-containing gas is in the range of 0.5 minutes to 10 minutes.

21. The method for preparing a positive electrode active material for a lithium secondary battery according to claim 17, wherein, The first firing is performed in the range of 720 °C to 900 °C.

22. A positive electrode for a lithium secondary battery, comprising: A current collector; And A positive electrode active material layer located on at least one surface of the current collector and containing the positive electrode active material according to any one of claims 1 to 16; The calendering density is 3.7 g / cc or more.

23. A lithium secondary battery, comprising: The positive electrode according to claim 22.

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