Method for manufacturing positive electrode active material for lithium secondary battery and positive electrode active material manufactured by the same

Abstract

The present invention relates to (1) mixing a lithium composite transition metal oxide in the form of a single particle or a quasi-single particle with a cobalt-containing raw material and heat-treating the mixture to form a cobalt coating layer on the surface of the lithium composite transition metal oxide; (2) A method for producing a positive electrode active material for a lithium secondary battery, the method including the steps of mixing the lithium composite transition metal oxide on which the cobalt coating layer is formed with a boron-containing raw material and heat-treating the mixture to form a boron coating layer on the cobalt coating layer; and a positive electrode active material for a lithium secondary battery produced thereby.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0130713, filed October 1, 2021, and all contents disclosed in the documents of said Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a method for producing a positive electrode active material for a lithium secondary battery, a positive electrode active material produced thereby, and a lithium secondary battery including the positive electrode active material. [Background technology]

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

[0004] Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2 or LiMnO4, etc.), and lithium iron phosphate (LiFePO4) have been used as positive electrode active materials for lithium secondary batteries. Among these, lithium cobalt oxide has the advantages of high operating voltage and excellent capacity characteristics, but the high cost and unstable supply of cobalt, the raw material, make its commercial application in large-capacity batteries difficult. Lithium nickel oxide has poor structural stability and is difficult to achieve sufficient life characteristics. On the other hand, lithium manganese oxide has excellent stability but poor capacity characteristics. Therefore, to address the issues of lithium transition metal oxides containing only Ni, Co, or Mn, lithium composite transition metal oxides containing two or more transition metals have been developed. Among these, lithium nickel cobalt manganese oxide containing Ni, Co, and Mn is widely used in the field of electric vehicle batteries.

[0005] Conventional lithium nickel cobalt manganese oxides generally have a spherical secondary particle form, consisting of an agglomeration of tens to hundreds of primary particles. However, lithium nickel cobalt manganese oxides in this secondary particle form, consisting of an agglomeration of many primary particles, are prone to particle cracking, in which the primary particles break off during the rolling process during positive electrode production, and also suffer from internal cracks during charge and discharge. When particle cracking or cracking occurs in the positive electrode active material, the contact area with the electrolyte increases, which increases gas generation and degradation of the active material due to side reactions with the electrolyte, resulting in reduced lifespan.

[0006] Recently, there has been an increasing need for high-power, high-capacity batteries, such as those for electric vehicles, and as a result, the nickel content in the positive electrode active material is gradually increasing. When the nickel content in the positive electrode active material increases, the initial capacity characteristics improve, but the highly reactive nickel becomes more difficult to remove after repeated charging and discharging. 4+ A large amount of ions are generated, causing the structure of the positive electrode active material to collapse, which increases the rate of deterioration of the positive electrode active material and reduces the life characteristics. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been made to solve the above-mentioned problems, and aims to provide a method for manufacturing a positive electrode active material that can suppress the occurrence of particle breakage and cracks during electrode manufacturing and charge / discharge processes, contains a high content of nickel, and has excellent stability and durability.

[0008] Another object of the present invention is to provide a positive electrode active material that can be useful in realizing a lithium secondary battery that is excellent in initial charge / discharge capacity, life characteristics at high temperatures, and resistance characteristics. [Means for solving the problem]

[0009] The present invention provides (1) mixing a lithium composite transition metal oxide in a single particle or quasi-single particle form with a cobalt-containing raw material and heat-treating the mixture to form a cobalt coating layer on the surface of the lithium composite transition metal oxide; (2) mixing the lithium composite transition metal oxide on which the cobalt coating layer has been formed with a boron-containing raw material and heat-treating the mixture to form a boron coating layer on the cobalt coating layer, A method for producing a positive electrode active material for a lithium secondary battery is provided.

[0010] The present invention also provides a lithium composite transition metal oxide in the form of a single particle or quasi-single particle, a cobalt coating layer formed on the lithium composite transition metal oxide; and a boron coating layer formed on the cobalt coating layer.

[0011] The present invention also provides a positive electrode including a positive electrode active material layer containing the positive electrode active material for a lithium secondary battery according to the present invention.

[0012] The present invention also provides a lithium secondary battery including the positive electrode according to the present invention. [Effects of the Invention]

[0013] The method for producing a positive electrode active material according to the present invention involves coating a lithium composite transition metal oxide in the form of a single particle or quasi-single particle with Co and then coating it with B, thereby achieving a structural stabilization effect due to the heat treatment at a high temperature and an increase in the capacity realization rate due to the two-phase reaction.

[0014] In addition, the positive electrode active material for a lithium secondary battery according to the present invention has a high nickel content, which is advantageous for realizing a high-capacity battery, and has excellent particle strength due to its single particle or quasi-single particle form. Furthermore, the positive electrode active material for a lithium secondary battery according to the present invention has excellent surface stability and durability due to the inclusion of a Co coating layer and a B coating layer.

[0015] Furthermore, a lithium secondary battery containing the positive electrode active material has a high initial capacity, a low rate of increase in resistance due to charging and discharging, and a high capacity retention rate. [Brief explanation of the drawings]

[0016] [Figure 1] 1 shows the results of evaluating the initial charge / discharge capacity of lithium secondary batteries to which the positive electrode active materials of Example 1 and Comparative Examples 1 to 4 are applied. [Figure 2] 1 is a graph showing the differential capacity at the time of initial charge of lithium secondary batteries to which the positive electrode active materials of Example 1 and Comparative Example 1 are applied. [Figure 3] 1 shows the results of evaluating the life characteristics at high temperatures of lithium secondary batteries to which the positive electrode active materials of Example 1 and Comparative Examples 1 to 4 are applied. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention will now be described in more detail to facilitate understanding of the present invention.

[0018] 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.

[0019] In the present invention, "primary particles" refers to particle units that do not appear to have grain boundaries when observed under a scanning electron microscope at a magnification of 5,000 to 20,000 times. "Average particle size of primary particles" refers to the arithmetic mean value calculated after measuring the particle sizes of primary particles observed under a scanning electron microscope image.

[0020] In the present invention, "secondary particles" are particles formed by agglomeration of a plurality of primary particles. In order to distinguish them from conventional secondary particles formed by agglomeration of tens to hundreds of primary particles, secondary particles formed by agglomeration of 30 or less primary particles are referred to as pseudo-single particles.

[0021] In the present invention, the "average particle size D 50 " means the particle size at 50% of the volume cumulative particle size distribution of the lithium composite transition metal oxide powder or the positive electrode active material powder, and in the case where the lithium composite transition metal oxide is a secondary particle, means the average particle size of the secondary particle. 50 can be measured using a laser diffraction method. For example, after dispersing a lithium composite transition metal oxide powder or a positive electrode active material powder in a dispersion medium, the dispersion is introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000), and ultrasonic waves of about 28 kHz are irradiated at an output of 60 W. A volume cumulative particle size distribution graph is obtained, and the particle diameter corresponding to 50% of the volume cumulative amount is determined.

[0022] positive electrode active material Hereinafter, a method for producing a positive electrode active material according to the present invention and the positive electrode active material produced thereby will be described.

[0023] Compared with conventional lithium composite transition metal oxide particles in the form of secondary particles, lithium composite transition metal oxide particles in the form of single particles or quasi-single particles have relatively large primary particle sizes and fewer interfaces between primary particles that serve as diffusion paths for lithium ions, resulting in poor lithium mobility, and because they are manufactured at relatively high sintering temperatures, a rocksalt phase forms on the particle surfaces, resulting in high surface resistance. Therefore, when using lithium composite transition metal oxide particles in the form of single particles or quasi-single particles, there are problems such as higher resistance and poorer output characteristics compared to when using conventional lithium composite transition metal oxide particles in the form of secondary particles.

[0024] Therefore, the present inventors discovered that the above problem could be solved by sequential (two-step) coating with Co and B, and thus completed the present invention. Specifically, they confirmed that the structure of the lithium composite transition metal oxide can be stabilized by performing heat treatment at a high temperature during Co coating, and that the capacity realization rate can be increased by a two-phase reaction at the end of charging during B coating.

[0025] The method for producing a positive electrode active material for a lithium secondary battery of the present invention includes the following steps (1) and (2).

[0026] (1) Mixing a lithium composite transition metal oxide in the form of a single particle or a quasi-single particle with a cobalt-containing raw material and heat-treating the mixture to form a cobalt coating layer on the surface of the lithium composite transition metal oxide. (2) mixing the lithium composite transition metal oxide on which the cobalt coating layer has been formed with a boron-containing raw material and heat-treating the mixture to form a boron coating layer on the cobalt coating layer;

[0027] In the present invention, the single particle is a particle consisting of one primary particle, and the quasi-single particle is a particle consisting of a secondary particle formed by agglomeration of 30 or less primary particles.

[0028] In one embodiment of the present invention, in step (1), the cobalt-containing raw material may be mixed in an amount of 1 wt % to 5 wt % relative to the total content of the lithium composite transition metal oxide, and preferably in an amount of 2 wt % to 4 wt %. The inclusion of 5 wt % or less of the cobalt-containing raw material is preferred in that it ensures a predetermined or higher Ni content relative to the total content of the lithium composite transition metal oxide and reduces the amount of expensive cobalt raw material used, thereby reducing costs.

[0029] The cobalt-containing raw material may be one or more selected from Co3O4, Co(OH)2, Co2O3, Co3(PO4)2, CoF3, Co(OCOCH3)2·4H2O, Co(NO3)·6H2O, Co(SO4)2·7H2O and CoC2O4, preferably one or more selected from Co3O4 and Co(OH)2, more preferably Co(OH)2.

[0030] In one embodiment of the present invention, in step (2), the boron-containing raw material may be mixed in an amount of 0.03 wt % to 0.08 wt %, preferably 0.04 wt % to 0.06 wt %, based on the total content of the lithium composite transition metal oxide on which the cobalt coating layer is formed. The boron-containing raw material may be mixed in an amount of 0.08 wt % or less, which is preferable in that an increase in resistance due to the boron coating layer can be minimized.

[0031] The boron-containing raw materials include H3BO3, B2O3, B4C, BF3, (C3H7O)3B, (C6H5O)3B, [CH3(CH2)3O]3B, C 13 H 19 It can be one or more selected from O3, C6H5B(OH)2 and B2F4, preferably one or more selected from H3BO3 and B2O3, more preferably H3BO3.

[0032] In one embodiment of the present invention, the heat treatment in step (1) can be carried out at 600° C. to 800° C., preferably 650° C. to 750° C. Furthermore, the heat treatment in step (1) is preferably carried out for 3 hours to 10 hours.

[0033] In one embodiment of the present invention, the heat treatment in step (2) can be carried out at 250 to 400°C, preferably 280 to 350°C. The heat treatment temperature in step (2) is preferably in the above range, since it is highly reactive to form the Li-BO phase. The heat treatment in step (2) is preferably carried out for 3 to 10 hours.

[0034] In one embodiment of the present invention, the mixing in steps (1) and (2) can be dry mixing without a solvent, respectively.

[0035] In one embodiment of the present invention, the lithium composite transition metal oxide can be represented by the following Chemical Formula 1.

[0036] [Chemical Formula 1] Li a Ni 1-x-y Co x M1 y M2 w O2

[0037] In Chemical Formula 1, 1.0 ≤ a ≤ 1.5, 0 ≤ x ≤ 0.2, 0 ≤ y ≤ 0.2, 0 ≤ w ≤ 0.1, and 0 < x + y ≤ 0.4. M1 is one or more selected from the group consisting of Mn and Al. M2 is one or more selected from the group consisting of Ba, Ca, Zr, Ti, Mg, Ta, Nb, and Mo.

[0038] Specifically, x, y, and w in Chemical Formula 1 can be 0.025 ≤ x ≤ 0.15, 0.025 ≤ y ≤ 0.15, 0 ≤ w ≤ 0.05, and 0.05 ≤ x + y ≤ 0.2, and more specifically, 0.025 ≤ x ≤ 0.10, 0.025 ≤ y ≤ 0.10, 0 ≤ w ≤ 0.05, and 0.05 ≤ x + y ≤ 0.15.

[0039] In particular, the lithium composite transition metal oxide can be a super-high nickel substance with a nickel content of 90 mol% or more, that is, in Chemical Formula 1, 0 < x + y ≤ 0.1. Here, x, y, and w can be 0 ≤ x ≤ 0.08, 0 ≤ y ≤ 0.08, and 0 ≤ w ≤ 0.05, respectively.

[0040] In addition, the positive electrode active material of the present invention includes a lithium composite transition metal oxide in the form of a single particle or quasi-single particle, a cobalt coating layer formed on the lithium composite transition metal oxide, and a boron coating layer formed on the cobalt coating layer, and may be manufactured by the manufacturing method described above.

[0041] In one embodiment of the present invention, the single particle is a particle consisting of one primary particle, and the quasi-single particle is a particle consisting of a secondary particle formed by agglomeration of 30 or less primary particles, and the average particle size (D 50 The average particle size (D) of the secondary particles can be 1 μm to 5 μm, preferably 1 μm to 3 μm. 50 The average particle size (D) of the secondary particles can be 3 μm to 7 μm, preferably 3 μm to 5 μm. 50 ) is in the above range, it is preferable in terms of resistance, particle cracking and rolling density.

[0042] In one embodiment of the present invention, the molar concentration of Co in the cobalt coating layer may be 10,000 ppm to 40,000 ppm, preferably 15,000 ppm to 35,000 ppm, based on the total positive electrode active material.

[0043] In one embodiment of the present invention, the molar concentration of B in the boron coating layer may be 300 ppm to 700 ppm, preferably 400 ppm to 600 ppm, based on the total positive electrode active material.

[0044] In one embodiment of the present invention, the average particle size (D 50 ) can be 3 μm to 10 μm, preferably 3 μm to 5 μm.

[0045] Meanwhile, the lithium composite transition metal oxide may be prepared by mixing a precursor and a lithium source material and then calcining the mixture.

[0046] Here, the precursor may be a commercially available precursor or may be prepared by a precursor preparation method well known in the art.

[0047] For example, the precursor can be prepared by adding an aqueous solution of a transition metal, an ammonium cation complexing agent, and a basic compound to a reactor and carrying out a coprecipitation reaction while stirring.

[0048] The transition metal aqueous solution can be prepared by dissolving a transition metal-containing raw material in a solvent such as water, for example, by dissolving a nickel-containing raw material, a cobalt-containing raw material, a manganese-containing raw material, and / or an aluminum-containing raw material in water.

[0049] Meanwhile, the transition metal-containing raw material may be acetate, carbonate, nitrate, sulfate, halide, sulfide, or oxide of the transition metal.

[0050] Specifically, the nickel-containing source material may be, for example, NiO, NiCO3·2Ni(OH)2·4H2O, NiC2O2·2H2O, Ni(NO3)2·6H2O, NiSO4, NiSO4·6H2O, nickel halides, or combinations thereof.

[0051] The cobalt-containing source material can be, for example, CoSO4, Co(OCOCH3)2·4H2O, Co(NO3)2·6H2O, CoSO4·7H2O, or a combination thereof.

[0052] The manganese-containing source material can be, for example, Mn2O3, MnO2, Mn3O4, MnCO3, Mn(NO3)2, MnSO4·H2O, manganese acetate, manganese halide, or a combination thereof.

[0053] The aluminum-containing source material may be, for example, Al2O3, Al(OH)3, Al(NO3)3, Al2(SO4)3, (HO)2AlCH3CO2, HOAl(CH3CO2)2, Al(CH3CO2)3, aluminum halide, or a combination thereof. However, in the case of Al, it may be added together with the lithium source material in the calcination step described below, without being added to the transition metal aqueous solution.

[0054] Here, the amount of each transition metal-containing raw material to be added may be determined in consideration of the molar ratio of the transition metal in the final cathode active material to be produced.

[0055] The ammonium cation complexing agent may include at least one compound selected from the group consisting of NHOH, (NH)SO, NHNO, NHCl, CHCOONH, and NH(CO), and may be added to the reactor in the form of a solution in which the compound is dissolved in a solvent. The solvent may be water or a mixture of water and an organic solvent (e.g., alcohol) that is uniformly miscible with water.

[0056] The basic compound may be at least one compound selected from the group consisting of NaOH, KOH, and Ca(OH)2, and may be added to the reactor in the form of a solution in which the compound is dissolved in a solvent, which may be water or a mixture of water and an organic solvent (e.g., alcohol) that is uniformly miscible with water.

[0057] As described above, when the aqueous transition metal solution, the ammonium cation complexing agent, and the basic compound are charged into a reactor and stirred, the transition metal in the aqueous transition metal solution is coprecipitated to produce precursor particles in the form of transition metal hydroxide.

[0058] Here, the aqueous transition metal solution, the ammonium cation complex-forming agent, and the basic compound are added in amounts such that the pH of the reaction solution falls within a desired range.

[0059] Once the precursor particles are formed by the above method, they are separated from the reaction solution to obtain the precursor. For example, the reaction solution can be filtered to separate the precursor from the reaction solution, and the separated precursor can then be washed with water and dried to obtain the precursor. Here, steps such as pulverization and / or classification can be performed as necessary.

[0060] Next, the precursor is mixed with a lithium source material and then calcined to produce a lithium composite transition metal oxide, where, if necessary, a metal-containing source material M2 can be mixed and calcined.

[0061] The lithium source material may be a lithium-containing sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, hydroxide, or oxyhydroxide, such as Li2CO3, LiNO3, LiNO2, LiOH, LiOH·H2O, LiH, LiF, LiCl, LiBr, LiI, CH3COOLi, Li2O, Li2SO4, CH3COOLi, Li3C6H5O7, or a mixture thereof.

[0062] Meanwhile, the lithium source material and the precursor may be mixed so that the molar ratio of Li:total metals in the precursor is 1:1 to 1.2:1, preferably 1:1 to 1.1:1. When the mixing ratio of the lithium source material and the metals in the precursor satisfies this range, the layered crystal structure of the lithium composite transition metal oxide is well developed, and a positive electrode active material with excellent capacity characteristics and structural stability can be produced.

[0063] Meanwhile, the calcination is performed at a temperature that allows the formation of monoparticles or quasi-monoparticles. To form monoparticles or quasi-monoparticles, the calcination must be performed at a temperature higher than that used in the preparation of conventional lithium composite transition metal oxides in the form of secondary particles. For example, when the precursor composition is the same, the calcination temperature must be about 30°C to 100°C higher than that used in the preparation of conventional lithium composite transition metal oxides in the form of secondary particles. The calcination temperature for the formation of monoparticles or quasi-monoparticles may vary depending on the metal composition of the precursor. For example, when a high-nickel (Ni) NCM-based lithium composite transition metal oxide having a nickel (Ni) content of 80 mol% or more is formed as monoparticles or quasi-monoparticles, the calcination temperature may be about 800°C to 950°C, preferably about 850°C to 920°C. When the calcination temperature satisfies the above range, a monoparticle or quasi-monoparticle-form positive electrode active material with excellent electrochemical properties may be prepared. If the calcination temperature is less than 800°C, a secondary particle-shaped positive electrode active material is produced, and if the temperature exceeds 950°C, excessive calcination occurs, the layered crystal structure is not properly formed, and the electrochemical properties are deteriorated.

[0064] The firing may be carried out for 5 to 35 hours in an oxygen atmosphere. In this specification, the term "oxygen atmosphere" refers to an atmosphere containing sufficient oxygen for firing, including the air atmosphere. In particular, firing is preferably carried out in an atmosphere having a higher oxygen partial pressure than the air atmosphere.

[0065] Furthermore, when preparing the lithium composite transition metal oxide of the present invention, it is preferable not to perform a water washing process after the calcination. Conventionally, when preparing a high-nickel (Ni) NCM-based lithium composite transition metal oxide having a nickel (Ni) content of 80 mol% or more, a water washing process has been performed after calcination to reduce the content of lithium by-products. However, research by the present inventors has shown that performing a water washing process when preparing a lithium composite transition metal oxide in the form of a single particle or quasi-single particle can deteriorate the surface properties of the lithium composite transition metal oxide and increase its resistance. Therefore, when preparing the lithium composite transition metal oxide of the present invention, it is preferable not to perform water washing, but to consume the remaining lithium on the surface of the lithium composite transition metal oxide during the process of forming a coating layer. In this way, when preparing a positive electrode active material without washing the lithium composite transition metal oxide with water, an increase in resistance due to surface defects can be suppressed.

[0066] positive electrode Next, the positive electrode according to the present invention will be described.

[0067] The positive electrode according to the present invention includes a positive electrode active material layer including the positive electrode active material according to the present invention. Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including the positive electrode active material.

[0068] The positive electrode current collector in the positive electrode is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. can be used. The positive electrode current collector typically has a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the positive electrode current collector to enhance the adhesive strength of the positive electrode active material. The positive electrode current collector can be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0069] The positive electrode active material layer may contain a conductive material and a binder in addition to the positive electrode active material.

[0070] The conductive material is used to impart conductivity to the electrode. Any material that does not cause chemical changes in the resulting battery and has electronic conductivity can be used without particular limitations. Specific examples include graphite, such as natural graphite or artificial graphite; carbon black, such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; carbon-based materials, such as carbon fibers and carbon nanotubes; metal powders or fibers, such as copper, nickel, aluminum, and silver; 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 can be used alone or in combination. The conductive material is typically present in an amount of 1 to 30 wt %, preferably 1 to 20 wt %, and more preferably 1 to 10 wt %, based on the total weight of the positive electrode active material layer.

[0071] The binder improves adhesion between positive electrode active material particles and between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination. The binder may be included in an amount of 1 to 30 wt %, preferably 1 to 20 wt %, and more preferably 1 to 10 wt %, based on the total weight of the positive electrode active material layer.

[0072] The positive electrode may be manufactured by a conventional method for manufacturing a positive electrode. For example, the positive electrode may be manufactured by mixing a positive electrode active material, a binder, and / or a conductive material in a solvent to prepare a positive electrode slurry, applying the positive electrode slurry to a positive electrode current collector, and then drying and rolling the slurry. Here, the types and contents of the positive electrode active material, binder, and conductive material are as described above.

[0073] The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and may be used alone or in combination. The amount of solvent used is determined in consideration of the coating thickness of the slurry and the manufacturing yield, and is sufficient as long as the solvent has a viscosity that can dissolve or disperse the positive electrode active material, conductive material, and binder and provide excellent thickness uniformity during subsequent coating for manufacturing a positive electrode.

[0074] Alternatively, the positive electrode can be produced by casting the positive electrode slurry on a separate support, peeling it off from the support, and laminating the resulting film onto a positive electrode current collector.

[0075] Lithium secondary battery Next, the lithium secondary battery according to the present invention will be described.

[0076] The lithium secondary battery of the present invention includes the positive electrode according to the present invention. Specifically, the lithium secondary battery includes a positive electrode, a negative electrode facing the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, the positive electrode being as described above. The lithium secondary battery may also optionally further include a battery container that houses the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0077] In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0078] The negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surfaces treated with carbon, nickel, titanium, silver, or the like, and aluminum-cadmium alloys can be used. The negative electrode current collector typically has a thickness of 3 to 500 μm. As with the positive electrode current collector, the surface of the current collector can be formed with fine irregularities to enhance the binding strength of the negative electrode active material. The negative electrode current collector can be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0079] The negative electrode active material layer includes a negative electrode active material, and optionally a binder and a conductive material.

[0080] 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. β (0<β<2), metal oxides that can be doped and dedoped with lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the above metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of these may be used.

[0081] The negative electrode active material may be a thin film of metallic lithium. The carbon material may be either low-crystalline or high-crystalline. Typical examples of low-crystalline carbon include soft carbon and hard carbon. Typical 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-calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0082] The conductive material is used to impart conductivity to the electrode. Any material that does not cause chemical changes in the battery and has electronic conductivity can be used without particular limitations. Specific examples include graphite, such as natural graphite or artificial graphite; carbon black, such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; carbon-based materials, such as carbon fibers and carbon nanotubes; metal powders or fibers, such as copper, nickel, aluminum, and silver; 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 can be used alone or in combination. The conductive material is typically present in an amount of 1 to 30 wt %, preferably 1 to 20 wt %, and more preferably 1 to 10 wt %, based on the total weight of the negative electrode active material layer.

[0083] The binder improves adhesion between negative electrode active material particles and between the negative electrode active material and the negative electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination. The binder may be included in an amount of 1 to 30 wt %, preferably 1 to 20 wt %, and more preferably 1 to 10 wt %, based on the total weight of the negative electrode active material layer.

[0084] For example, the negative electrode active material layer may be manufactured by coating a negative electrode slurry containing a negative electrode active material, and optionally a binder and a conductive material, on a negative electrode current collector and drying the coating, or by casting the negative electrode slurry on a separate support, peeling the negative electrode slurry from the support, and laminating the resulting film on the negative electrode current collector.

[0085] Meanwhile, in the lithium secondary battery, 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 limitations. In particular, a separator that exhibits low resistance to ion migration and excellent humidification ability for the electrolyte solution 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.

[0086] Furthermore, examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the production of lithium secondary batteries, but are not limited to these.

[0087] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0088] The organic solvent may be any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move. 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), 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 C2-C20 linear, branched, or cyclic hydrocarbon group that 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.

[0089] The lithium salt can be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, examples of the lithium salt include LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(C, F, SO), LiN(C, F, SO), LiN(CF, SO), LiCl, LiI, and LiB(C, O) . The lithium salt concentration is preferably within a range of 0.1 to 5.0 M, and more preferably 0.1 to 3.0 M. When the lithium salt concentration is within this range, the electrolyte exhibits appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0090] In addition to the electrolyte components, the electrolyte may further contain additives for purposes such as improving battery life characteristics, suppressing battery capacity loss, and improving battery discharge capacity. Examples of additives include, but are not limited to, haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethyl alcohol amine, cyclic ethers, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethyl alcohol, and aluminum trichloride, which may be used alone or in combination. The additives may be present in an amount of 0.1 to 10 wt %, preferably 0.1 to 5 wt %, based on the total weight of the electrolyte.

[0091] As described above, the lithium secondary battery including the cathode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and is therefore useful in portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0092] Therefore, according to another 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.

[0093] The battery module or battery pack may be used as a power source for one or more medium- to large-sized devices, including power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0094] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the preferred embodiments so that those skilled in the art can easily carry out the present invention.

[0095] [Examples and Comparative Examples: Production of Positive Electrode Active Material] Comparative Example 1 A coprecipitation reactor (20 L capacity) was charged with 4 L of distilled water and, while maintaining the temperature at 50°C, 100 mL of a 28 wt% aqueous ammonia solution was added. A 3.2 mol / L transition metal solution (NiSO4, CoSO4, MnSO4, and Al(OH)3 mixed at a molar ratio of nickel:cobalt:manganese:aluminum of 85:10:4:1) was then added at 300 mL / hr, followed by 42 mL / hr of a 28 wt% aqueous ammonia solution. A 40 wt% sodium hydroxide solution was also added to maintain the pH of the reaction solution at 11.0. The coprecipitation reaction was carried out for 20 hours to form precursor particles. The precursor particles were separated, washed, and then dried in an oven at 130°C to produce the precursor.

[0096] The precursor synthesized by the coprecipitation reaction and LiOH were added to a Henschel mixer (700 L) in an amount such that the molar ratio of (Ni + Co + Mn + Al):Li was 1:1.03, and mixed for 20 minutes at a speed of 400 rpm at the center. The mixed powder was placed in an alumina crucible of 330 mm x 330 mm and heat-treated at 890 °C for 12 hours in an oxygen (O2) atmosphere to obtain the lithium composite transition metal oxide Li[Ni 0.85 Co 0.10 Mn 0.04 Al 0.01 ]O2 was produced.

[0097] The produced lithium transition metal composite oxide has a single particle and / or pseudo-single particle form, and the average particle size of the primary particles (D 50 ) is 1.0 μm, and the average particle size of secondary particles (D 50 ) was 3.5 μm.

[0098] Next, the prepared lithium composite transition metal oxide was dry-mixed with 3 wt% Co(OH)2 based on the total weight of the lithium composite transition metal oxide. The mixture was heat-treated in an air atmosphere at 700°C for 5 hours to prepare a cathode active material having a cobalt coating layer formed thereon. Here, the molar concentration of Co in the cobalt coating layer was 30,000 ppm relative to the total cathode active material, and the average particle size (D 50 ) was 3.8 μm.

[0099] Comparative Example 2 The lithium composite transition metal oxide Li[Ni 0.85 Co 0.10 Mn 0.04 Al 0.01 ]O2 was produced.

[0100] Next, the prepared lithium composite transition metal oxide was dry-mixed with 0.05 wt % of H3BO3 based on the total weight of the lithium composite transition metal oxide. The mixture was heat-treated in an air atmosphere at 320°C for 5 hours to prepare a cathode active material having a boron coating layer formed thereon. Here, the molar concentration of B in the boron coating layer was 460 ppm relative to the total cathode active material, and the average particle size (D 50 ) was 4.0 μm.

[0101] Comparative Example 3 The lithium composite transition metal oxide Li[Ni 0.85 Co 0.10 Mn 0.04 Al 0.01 ]O2 was produced.

[0102] Next, the prepared lithium composite transition metal oxide was dry-mixed with 0.3 wt% WO3 based on the total weight of the lithium composite transition metal oxide, and the mixture was heat-treated in an air atmosphere at 400°C for 10 hours to form a tungsten coating layer.

[0103] Next, the lithium composite transition metal oxide having the tungsten coating layer formed thereon was dry-mixed with 0.2 wt % of H3BO3 based on the total weight of the lithium composite transition metal oxide having the tungsten coating layer formed thereon. The mixture was heat-treated in an air atmosphere at 320°C for 8 hours to produce a cathode active material having a boron coating layer formed on the tungsten coating layer. Here, the molar concentrations of W in the tungsten coating layer and B in the boron coating layer were 400 ppm and 500 ppm, respectively, based on the total cathode active material. The average particle size (D 50 ) was 3.5 μm.

[0104] Comparative Example 4 The lithium composite transition metal oxide Li[Ni 0.85 Co 0.10 Mn 0.04 Al 0.01 ]O2 was produced.

[0105] Next, the prepared lithium composite transition metal oxide was dry-mixed with 3 wt% Co(OH)2 and 0.25 wt% H3BO3 based on the total weight of the lithium composite transition metal oxide. The mixture was heat-treated in an air atmosphere at 650°C for 10 hours to prepare a cathode active material having a coating layer containing both cobalt and boron. The molar concentrations of Co and B in the coating layer were 20,000 ppm and 500 ppm, respectively, based on the total weight of the cathode active material. The average particle size (D 50 ) was 4.0 μm.

[0106] Example 1 The lithium composite transition metal oxide Li[Ni 0.85 Co 0.10 Mn 0.04 Al 0.01 ]O2 was produced.

[0107] Next, the prepared lithium composite transition metal oxide was dry-mixed with 3 wt% Co(OH)2 based on the total weight of the lithium composite transition metal oxide, and the mixture was heat-treated in an air atmosphere at 700°C for 5 hours to form a cobalt coating layer.

[0108] Next, the lithium composite transition metal oxide having the cobalt coating layer formed thereon was dry-mixed with 0.05 wt % of H3BO3 based on the total weight of the lithium composite transition metal oxide having the cobalt coating layer formed thereon. The mixture was heat-treated in an air atmosphere at 320°C for 5 hours to produce a cathode active material having a boron coating layer formed on the cobalt coating layer. Here, the molar concentrations of Co in the cobalt coating layer and B in the boron coating layer were 30,000 ppm and 500 ppm, respectively, based on the total cathode active material. The average particle size (D 50 ) was 4.1 μm.

[0109] [Experimental example: Evaluation of initial capacity and life characteristics] (1) Manufacture of lithium secondary batteries The positive electrode active materials prepared in Example 1 and Comparative Examples 1 to 4, conductive material (carbon black), and PVDF binder were mixed in N-methylpyrrolidone at a weight ratio of 95:2:3 to prepare a positive electrode slurry. The positive electrode slurry was applied to one side of an aluminum current collector, dried at 100°C, and rolled to prepare a positive electrode.

[0110] An electrode assembly was fabricated by interposing a porous polyethylene separator between the positive electrode and the lithium metal negative electrode, and then placed inside a battery case. An electrolyte solution was then injected into the case to fabricate a half-cell lithium secondary battery. The electrolyte solution was prepared by dissolving 1M LiPF6 in a mixed organic solvent of ethylene carbonate / dimethyl carbonate / diethyl carbonate in a volume ratio of 3:4:3.

[0111] (2) Evaluation of initial charge / discharge The lithium secondary batteries employing the positive electrode active materials of Example 1 and Comparative Examples 1 to 4 were charged at a rate of 0.1 C in CCCV mode at 25° C. to 4.25 V. Then, they were discharged at a constant current of 0.1 C to 2.5 V, and the initial charge / discharge capacity was measured. The results are shown in FIG. 1 and Table 1 below.

[0112] FIG. 2 shows a graph of differential capacity (dQ / dV) versus voltage during the initial charge for the lithium secondary batteries using the positive electrode active materials of Example 1 and Comparative Example 1.

[0113] (3) Evaluation of life characteristics The lithium secondary batteries employing the positive electrode active materials of Example 1 and Comparative Examples 1 to 4 were charged at 45° C. in CCCV mode at a rate of 0.5 C to 4.25 V, and then discharged at a constant current of 1.0 C to 2.5 V. After 50 charge / discharge cycles, the capacity retention rate and the resistance increase rate were measured, and the results are shown in FIG. 3 and Table 1 below.

[0114] [Table 1]

[0115] From the results of FIGS. 1 and 3 and Table 1, it can be seen that the battery of Example 1, which uses a positive electrode active material in which a Co coating layer and a B coating layer are formed in sequence, has a higher initial charge / discharge capacity, a higher capacity retention rate in a high-temperature environment, and a lower resistance increase rate than the batteries of Comparative Examples 1 to 4, which use positive electrode active materials in which only a Co coating layer or a B coating layer is formed alone (Comparative Examples 1 and 2), a W coating layer and a B coating layer are formed in sequence (Comparative Example 3), or a Co and B composite coating layer is formed (Comparative Example 4).

[0116] This confirms that forming a coating layer with a combination of Co and B on a lithium composite transition metal oxide in the form of a single particle or quasi-single particle is most effective in improving surface stability and durability, and that coating Co and B sequentially in two separate steps is far more effective in improving the resistance increase rate than coating them simultaneously in a single step.

[0117] 2, it can be seen that when B is further coated after Co coating (Example 1), the differential capacity (dQ / dV) is higher in the vicinity of 4.15 to 4.2 V compared to when Co is solely coated (Comparative Example 1). This is attributed to the effect of the two-phase reaction at the end of charging caused by the additional coating of B.

Claims

1. (1) mixing a lithium composite transition metal oxide in the form of a single particle or quasi-single particle with a cobalt-containing raw material and heat-treating the mixture to form a cobalt coating layer on the surface of the lithium composite transition metal oxide; (2) mixing the lithium composite transition metal oxide on which the cobalt coating layer has been formed with a boron-containing raw material and heat-treating the mixture to form a boron coating layer on the cobalt coating layer; In the step (2), the boron-containing raw material is mixed in an amount of 0.03 wt % to 0.08 wt % based on the total content of the lithium composite transition metal oxide on which the cobalt coating layer is formed; The single particle is a particle consisting of one primary particle, The pseudo-single particle is a particle consisting of a secondary particle formed by agglomeration of 30 or less primary particles, The lithium composite transition metal oxide is represented by the following chemical formula 1: Method for producing a positive electrode active material for a lithium secondary battery: [Chemical formula 1] Li a Ni 1-x-y Co x M1 y M2 w O 2 In the above Chemical Formula 1, 1.0≦a≦1.5, 0≦x≦0.2, 0≦y≦0.2, 0≦w≦0.1, 0<x+y≦0.4, M1 is one or more selected from the group consisting of Mn and Al; M2 is one or more elements selected from the group consisting of Ba, Ca, Zr, Ti, Mg, Ta, Nb, and Mo.

2. 2. The method of claim 1, wherein in step (1), the cobalt-containing raw material is mixed in an amount of 1 wt % to 5 wt % based on the total content of the lithium composite transition metal oxide.

3. 2. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein the heat treatment in step (1) is carried out at 600°C to 800°C.

4. 2. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein the heat treatment in step (2) is carried out at 250°C to 400°C.

5. 2. The method of claim 1, wherein x, y, and w in Formula 1 satisfy the following relationships: 0.025≦x≦0.15, 0.025≦y≦0.15, 0≦w≦0.05, and 0.05≦x+y≦0.2, respectively.

6. a lithium composite transition metal oxide in a single particle or quasi-single particle form; a cobalt coating layer formed on the lithium composite transition metal oxide; a boron coating layer formed on the cobalt coating layer, the molar concentration of boron in the boron coating layer is 300 ppm to 700 ppm relative to the total amount of the positive electrode active material; The single particle is a particle consisting of one primary particle, The pseudo-single particle is a particle consisting of a secondary particle formed by agglomeration of 30 or less primary particles, The lithium composite transition metal oxide is a positive electrode active material for a lithium secondary battery represented by the following Chemical Formula 1: [Chemical formula 1] Li a Ni 1-x-y Co x M1 y M2 w O 2 In the formula 1, 1.0≦a≦1.5, 0≦x≦0.2, 0≦y≦0.2, 0≦w≦0.1, and 0<x+y≦0.4; M1 is one or more selected from the group consisting of Mn and Al; M2 is one or more elements selected from the group consisting of Ba, Ca, Zr, Ti, Mg, Ta, Nb, and Mo.

7. 7. The positive electrode active material for a lithium secondary battery according to claim 6, wherein the primary particles have an average particle size of 1 μm to 5 μm.

8. 7. The positive electrode active material for a lithium secondary battery according to claim 6, wherein the average particle size of the positive electrode active material is 3 μm to 10 μm.

9. A positive electrode comprising a positive electrode active material layer containing the positive electrode active material for a lithium secondary battery according to any one of claims 6 to 8.

10. A lithium secondary battery comprising the positive electrode according to claim 9.

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