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

Abstract

The present invention relates to a cathode active material capable of improving the high-temperature life characteristics and high-temperature storage characteristics of a lithium secondary battery, a cathode including the cathode active material, and a lithium secondary battery. Specifically, the present invention relates to a first lithium transition metal oxide and a cathode active material having an average particle size (D 50 The present invention relates to a bimodal cathode active material comprising a second lithium transition metal oxide having a small crystal grain size, and the first lithium transition metal oxide has a larger particle strength and a smaller crystal grain size than the second lithium transition metal oxide, a cathode including the cathode active material, and a lithium secondary battery.

Description

[Technical Field]

[0001] [Cross-reference to related applications] This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0181727, filed on December 23, 2020, and all contents disclosed in the documents of this Korean patent application are incorporated herein by reference.

[0002] [Technical field] The present invention relates to a positive electrode active material for a lithium secondary battery, a positive electrode for a lithium secondary battery containing the positive electrode active material, and a lithium secondary battery. [Background technology]

[0003] As technological development and demand for mobile devices increases, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density and voltage, long cycle life, and low self-discharge rate, have become commercially available and are widely used.

[0004] Lithium-transition metal composite oxides are used as the positive electrode active material for lithium secondary batteries, and among these, lithium-cobalt composite metal oxides such as LiCoO2 are primarily used due to their high operating voltage and excellent capacity characteristics. However, LiCoO2 has very poor thermal properties due to the instability of its crystal structure caused by delithiation. In addition, the high cost of LiCoO2 limits its mass use as a power source in fields such as electric vehicles.

[0005] Lithium-manganese composite metal oxides (e.g., LiMnO2 or LiMn2O4), lithium iron phosphate compounds (e.g., LiFePO4), and lithium nickel composite metal oxides (e.g., LiNiO2) have been developed as alternatives to LiCoO2. Among these, lithium nickel composite metal oxides have been the focus of active research and development, as they offer a high reversible capacity of approximately 200 mAh / g, making it easy to realize large-capacity batteries. However, LiNiO2 has inferior thermal stability compared to LiCoO2. If an internal short circuit occurs due to external pressure during charging, the positive electrode active material itself decomposes, resulting in battery explosion and fire. Therefore, lithium transition metal oxides, in which part of the Ni is replaced with Co, Mn, or Al, have been developed to improve the low thermal stability of LiNiO2 while maintaining its excellent reversible capacity.

[0006] However, as described above, lithium transition metal oxides substituted with Co, Mn, or Al still have poor thermal stability, and when applied to batteries, there are problems with poor high-temperature life characteristics and high-temperature storage characteristics.

[0007] Therefore, there is a need for the development of a positive electrode active material that can improve the thermal stability and thereby improve the high-temperature life and storage characteristics of lithium secondary batteries. Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made to solve the above problems by providing a positive electrode active material capable of improving the high-temperature life characteristics and high-temperature storage characteristics of a lithium secondary battery, a positive electrode for a lithium secondary battery including the positive electrode active material, and a lithium secondary battery. [Means for solving the problem]

[0009] The present invention relates to a first lithium transition metal oxide and a method for producing a first lithium transition metal oxide having an average particle size (D 50) and the first lithium transition metal oxide has greater particle strength and smaller crystal grain size than the second lithium transition metal oxide.

[0010] The present invention also provides a positive electrode comprising the bimodal positive electrode active material.

[0011] The present invention also provides a lithium secondary battery comprising the positive electrode. [Effects of the Invention]

[0012] The positive electrode active material according to the present invention includes different lithium transition metal oxides having different average particle diameters, particle strengths, and crystal grain sizes. When applied to a positive electrode for a secondary battery, the positive electrode active material can improve electrode density and minimize particle cracking. When applied to a secondary battery, the positive electrode active material can improve the high-temperature life characteristics and high-temperature storage characteristics of the battery.

[0013] Therefore, the cathode active material according to the present invention can be effectively used in batteries that require high energy density, high stability, and long life, such as automobile batteries. DETAILED DESCRIPTION OF THE INVENTION

[0014] The terms and words used in this specification should not be interpreted in a limited manner based on their ordinary or dictionary meanings, but should be interpreted in a manner that is consistent with the technical idea of ​​the present invention, in accordance with the principle that the inventor can appropriately define the concept of the term in order to best explain his / her invention.

[0015] In this specification, the term "particle" refers to a micron-sized particle. Further magnification reveals distinct regions where atoms form a lattice structure in a specific direction, referred to as "crystal grains." The particle size observed by XRD is defined as the crystal grain size. Crystal grain size can be determined using the Rietveld method with XRD data. Specifically, the crystal grain size was measured by placing a sample in a groove in a general powder holder, using a glass slide to even out the surface of the sample and align its height with the edge of the holder, and then measuring the XRD data (2θ = 15°~90°, Step size = 0.02°, total scan time: 20 min) using an X-ray diffraction analyzer (Bruker, D8 Endeavor). The average crystal grain size was determined using the fundamental parameter approach built into Bruker's TOPAS program, which is based on the Rietveld method.

[0016] In this specification, particle strength was measured by placing particles on a plate, increasing the compressive force using a Micro Compression Testing Machine (Shimadzu, MCT-W500), and measuring the force at which the particles were broken, which was taken as the particle strength value. Specifically, in this specification, the average value for 20 particles was measured.

[0017] In this specification, the average particle size (D 50 The average particle size (D) can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 ) can be measured, for example, by using a laser diffraction method. More specifically, after dispersing the lithium composite transition metal oxide in a dispersion medium, the dispersion medium is introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac Mt 3000), and ultrasonic waves of about 28 kHz are irradiated at an output of 60 W. Then, the average particle size (D ) corresponding to 50% of the particle size distribution in the measuring device is measured. 50 ) can be calculated.

[0018] The present inventors have investigated the bimodal type first lithium transition metal oxide and the average particle size (D 50 In a positive electrode active material including a second lithium transition metal oxide having a small particle strength, when the particle strength of the first lithium transition metal oxide is greater than that of the second lithium transition metal oxide and the crystal grain size of the first lithium transition metal oxide is smaller than that of the second lithium transition metal oxide, the electrode density can be improved and particle cracking can be minimized when the positive electrode active material is applied to a positive electrode of a secondary battery, and the high-temperature life characteristics and high-temperature storage characteristics of the battery can be improved when the positive electrode active material is applied to a secondary battery, thereby completing the present invention.

[0019] positive electrode active material The positive electrode active material according to the present invention includes a first lithium transition metal oxide and a first lithium transition metal oxide having an average particle size (D 50 ) and the first lithium transition metal oxide is a bimodal type positive electrode active material having a higher particle strength and a smaller crystal grain size than the second lithium transition metal oxide.

[0020] The particle strength of the first lithium transition metal oxide (hereinafter referred to as large particles) contained in the bimodal type positive electrode active material is greater than that of the second lithium transition metal oxide (hereinafter referred to as small particles), which has the advantage of minimizing cracking of the large particles during electrode rolling and enabling the realization of a high-density electrode.

[0021] Furthermore, since the crystal grain size of the large particles is smaller than the crystal grain size of the small particles, when the bimodal type positive electrode active material is applied to a secondary battery, volume change due to charge and discharge can be minimized, thereby minimizing the occurrence of cracks inside the particles.

[0022] In addition, since the crystal grain size of the small particles is larger than that of the large particles, if cracks occur in the small particles during electrode rolling, the size of the generated fine powder can be controlled to be large, thereby preventing an increase in the specific surface area.

[0023] In conclusion, the positive electrode active material according to the present invention includes different lithium transition metal oxides having different average particle diameters, particle strengths, and crystal grain sizes, and therefore, when applied to a positive electrode for a secondary battery, the electrode density can be improved and particle cracking can be minimized. When applied to a secondary battery, the high-temperature life characteristics and high-temperature storage characteristics of the battery can be improved.

[0024] According to the present invention, the first lithium transition metal oxide and the second lithium transition metal oxide each independently have a composition represented by the following Chemical Formula 1.

[0025] [Chemical formula 1] Li x Ni a Co b M 1 c M 2 d O2 In the above Chemical Formula 1, M 1 is one or more selected from Mn and Al, M 2 is one or more selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; 0.9≦x≦1.2, 0.7≦a<1.0, 0 <b<0.3、0<c<0.3、0≦d≦0.1である。

[0026] The x represents the proportion of lithium atoms in the lithium transition metal oxide, and is 0.9≦x≦1.2, 1.0≦x≦1.2, or 1.0≦x≦1.15.

[0027] The a represents the ratio of nickel atoms to all transition metal atoms in the lithium transition metal oxide, and is 0.7≦a<1.0, 0.8≦a<1, or 0.85≦a<1.

[0028] The b represents the ratio of cobalt atoms to the total transition metal atoms in the lithium transition metal oxide, and is 0 <b<0.3、0<b<0.2、または0<b<0.15である。

[0029] The c is M among all transition metals in the lithium transition metal oxide. 1 This indicates the proportion of atoms in <c<0.3、0<c<0.2、または0<c<0.15である。

[0030] The d is M among all transition metals in the lithium transition metal oxide. 2 and 0≦d≦0.1, 0≦d≦0.05, or 0≦d≦0.02.

[0031] The bimodal type positive electrode active material may include the first lithium transition metal oxide and the second lithium transition metal oxide in a weight ratio of 60:40 to 95:5, specifically, a weight ratio of 70:30 to 90:10. When the first lithium transition metal oxide and the second lithium transition metal oxide are included within this weight ratio range, the particle packing density can be increased, and as a result, the battery capacity can be increased when applied to a battery.

[0032] According to the present invention, the difference in particle strength between the first lithium transition metal oxide and the second lithium transition metal oxide is 50 MPa to 200 MPa, specifically 70 MPa to 200 MPa. When the difference in particle strength between the first lithium transition metal oxide and the second lithium transition metal oxide is within this range, the first lithium transition metal oxide and the second lithium transition metal oxide collide with each other during electrode rolling, causing cracks in the second lithium transition metal oxide and filling the spaces between the particles of the first lithium transition metal oxide, thereby achieving a high density electrode. Furthermore, since cracks in the first lithium transition metal oxide are minimized during electrode rolling, the rate of increase in battery resistance can be reduced and high-temperature life performance can be improved.

[0033] According to the present invention, the difference in crystal grain size between the first lithium transition metal oxide and the second lithium transition metal oxide is 50 nm to 150 nm, specifically 50 nm to 100 nm. When the difference in crystal grain size between the first lithium transition metal oxide and the second lithium transition metal oxide is within this range, the volume change that the first lithium transition metal oxide experiences during charge and discharge can be minimized, thereby reducing cracks that occur inside the particles, and the specific surface area of ​​the second lithium transition metal oxide can be minimized, thereby reducing an increase in resistance.

[0034] In particular, when the difference in particle strength between the first lithium transition metal oxide and the second lithium transition metal oxide is within the above range and the difference in crystal grain size is also within the above range, even if cracks occur, they occur in the second lithium transition metal oxide. In this case, the size of the generated fine powder is large, preventing an increase in specific surface area, minimizing particle cracks in the first lithium transition metal oxide and minimizing internal particle cracks caused by volume changes. As a result, a synergistic effect is generated in which degradation of the high-temperature life characteristics and high-temperature storage characteristics of the battery is prevented.

[0035] According to the present invention, the first lithium transition metal oxide has a particle strength of 130 MPa to 300 MPa, specifically 150 MPa to 280 MPa, more specifically 180 MPa to 280 MPa. When the particle strength of the first lithium transition metal oxide is within this range, cracking of the first lithium transition metal oxide during electrode rolling can be minimized, thereby reducing the rate of increase in battery resistance.

[0036] According to the present invention, the second lithium transition metal oxide has a particle strength of 70 MPa to 125 MPa, specifically 70 MPa to 115 MPa, more specifically 70 MPa to 115 MPa. When the particle strength of the second lithium transition metal oxide is within this range, even if the first lithium transition metal oxide and the second lithium transition metal oxide collide with each other during electrode rolling, causing cracks in the second lithium transition metal oxide, the amount of fine powder generated is small.

[0037] According to the present invention, the first lithium transition metal oxide has a crystal grain size of 80 nm to 140 nm, specifically 80 nm to 130 nm, more specifically 80 nm to 120 nm. When the crystal grain size of the first lithium transition metal oxide is within this range, the volume change that the first lithium transition metal oxide experiences during charge and discharge can be minimized, and cracks generated inside the particles can be reduced.

[0038] According to the present invention, the second lithium transition metal oxide has a crystal grain size of 150 nm to 200 nm, specifically 150 nm to 190 nm, more specifically 155 nm to 185 nm. When the crystal grain size of the second lithium transition metal oxide is within this range, even if the first lithium transition metal oxide and the second lithium transition metal oxide collide with each other during electrode rolling, causing cracks in the second lithium transition metal oxide, the size of the generated fine powder is large, thereby preventing an increase in the specific surface area.

[0039] The first lithium transition metal oxide has an average particle size (D 50The second lithium transition metal oxide has an average particle size (D 50 The average particle diameter (D) of the first lithium transition metal oxide and the second lithium transition metal oxide is 2 μm to 7 μm, specifically 3 μm to 6 μm. 50 The difference between the average particle diameters (D) of the first lithium transition metal oxide and the second lithium transition metal oxide is 1 μm to 18 μm, specifically 3 μm to 13 μm. 50 When the ratio (R) is within the above range, the second lithium transition metal oxide is appropriately distributed among the first lithium transition metal oxide, resulting in an excellent filling rate.

[0040] positive electrode The present invention also provides a positive electrode for a lithium secondary battery, comprising the bimodal positive electrode active material. Specifically, the positive electrode for a lithium secondary battery comprises a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, the positive electrode active material layer comprising the positive electrode active material according to the present invention.

[0041] Here, since the positive electrode active material is the same as that described above, a detailed description thereof will be omitted, and only other components will be described in detail below.

[0042] The positive electrode current collector may include a highly conductive metal, and may be any material that is easily adhered to the positive electrode active material layer and is non-reactive within the battery voltage range. Examples of the positive electrode current collector include stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. The positive electrode current collector typically has a thickness of 3 μm to 500 μm, and the surface of the current collector may be micro-irregularized to enhance adhesion of the positive electrode active material. The positive electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0043] The positive electrode active material layer may optionally contain a conductive material, a binder, and a dispersant in addition to the positive electrode active material, as needed.

[0044] In this case, the positive electrode active material may be included in an amount of 80 wt % to 99 wt %, more specifically 85 wt % to 98.5 wt %, based on the total weight of the positive electrode active material layer, and excellent capacity characteristics may be exhibited when included in this content range.

[0045] The conductive material is used to impart conductivity to the electrode and can be any material that provides electronic conductivity without causing chemical changes in the battery. Specific examples include graphite, such as natural graphite or artificial graphite; carbon-based materials, such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powder or metal fiber, such as copper, nickel, aluminum, and silver; conductive tubes, such as carbon nanotubes; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials can be used alone or in combination. The conductive material can be included in an amount of 0.1 wt % to 15 wt % based on the total weight of the positive electrode active material layer.

[0046] The binder serves to improve adhesion between positive electrode active material particles and between the positive electrode active material and the current collector. Specific examples of the binder include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, polymethyl methacrylate, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, polymers in which hydrogen is substituted with Li, Na, or Ca, or various copolymers thereof, and the like. These may be used alone or in combination. The binder may be included in an amount of 0.1 wt % to 15 wt % based on the total weight of the positive electrode active material layer.

[0047] The dispersant may include a water-based dispersant or an organic dispersant such as N-methyl-2-pyrrolidone.

[0048] The positive electrode may be fabricated by a conventional method for fabricating a positive electrode, except for using the positive electrode active material. Specifically, the positive electrode may be fabricated by dissolving or dispersing the positive electrode active material and, optionally, a binder, a conductive material, and a dispersant in a solvent to form a positive electrode active material layer, which is prepared by coating the composition on a positive electrode current collector, followed by drying and rolling.

[0049] The solvent is a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), dimethyl formamide (DMF), acetone, or water, and may be used alone or in combination. The amount of the solvent used is determined in consideration of the coating thickness of the slurry and the production yield, and is sufficient to provide a viscosity that can dissolve or disperse the positive electrode active material, conductive material, binder, and dispersant and exhibit excellent thickness uniformity when applied to subsequently manufacture a positive electrode.

[0050] Alternatively, the positive electrode may be manufactured by casting the composition for forming a positive electrode active material layer on a support, peeling the composition from the support, and laminating the resulting film on a positive electrode current collector.

[0051] secondary battery The present invention also provides a lithium secondary battery comprising the positive electrode.

[0052] Specifically, the lithium secondary battery includes a positive electrode, a negative electrode facing the positive electrode, and a separator and an electrolyte interposed between the positive electrode and the negative electrode. Since the positive electrode is the same as that described above, a detailed description thereof will be omitted and only other components will be described in detail below.

[0053] Meanwhile, the lithium secondary battery may optionally further include a battery container that houses an electrode assembly including the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0054] In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0055] The negative electrode current collector is not particularly limited as long as it has high conductivity and does not cause chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surfaces treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys can be used. The negative electrode current collector typically has a thickness of 3 μm to 500 μm. Similar to the positive electrode current collector, the current collector surface can be formed with fine irregularities to strengthen 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, net, porous material, foam, or nonwoven fabric.

[0056] The negative electrode active material layer may optionally include a binder and a conductive material in addition to the negative electrode active material.

[0057] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; and SiO βExamples of the negative electrode active material include metal oxides capable of doping and dedoping lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide (0<β<2); or composites containing the metallic compounds and carbonaceous materials, such as Si-C composites and Sn-C composites. A mixture of two or more of these materials can be used. A thin film of metallic lithium can also be used as the negative electrode active material. The carbon material can be either low-crystalline carbon or high-crystalline carbon. Typical low-crystalline carbons include soft carbon and hard carbon, while typical high-crystalline carbons include amorphous, plate-like, scaly, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-calcined carbons such as petroleum or coal tar pitch-derived cokes.

[0058] The negative electrode active material may be included in an amount of 80 wt % to 99 wt % based on the total weight of the negative electrode active material layer.

[0059] The binder is a component that aids in bonding between the conductive material, active material, and current collector, and is typically added in an amount of 0.1 to 10 wt % based on the total weight of the negative electrode active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0060] The conductive material is a component for further improving the conductivity of the negative electrode active material and may be included in an amount of 10 wt % or less, specifically 5 wt % or less, based on the total weight of the negative electrode active material layer. The conductive material may be any material that is conductive without causing chemical changes in the battery, and may include, for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fibers such as carbon fiber or metal fiber; metal powders such as carbon fluoride, aluminum, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives.

[0061] The negative electrode active material layer may be prepared by coating a negative electrode active material layer-forming composition, which is prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent, on a negative electrode current collector and drying the coating. Alternatively, the negative electrode active material layer-forming composition may be separately cast on a support, and then peeled off from the support to obtain a film, which may then be laminated on the negative electrode current collector.

[0062] Meanwhile, in the lithium secondary battery, the separator separates the anode and cathode and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without limitation. In particular, a separator that exhibits low resistance to electrolyte ion movement and excellent humidification ability for the electrolyte 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 used in a single-layer or multi-layer structure.

[0063] In addition, 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 manufacturing lithium secondary batteries, but are not limited to these.

[0064] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0065] 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. Specific 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 suitable solvents include carbonate-based solvents such as ethylene carbonate (PC); alcohol-based solvents such as ethanol and isopropyl alcohol; nitriles such as R-CN (where R is a hydrocarbon group having 2 to 20 carbon atoms, linear, branched, or cyclic structure, and may contain a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constants, which can enhance 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. In this case, the cyclic carbonate and chain carbonate should be mixed in a volume ratio of about 1:1 to about 1:9 to achieve excellent electrolyte performance.

[0066] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the anion of the lithium salt may be F. - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - The lithium salt may be at least one selected from the group consisting of LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2) 2. LiCl, LiI, or LiB(C2O4)2 can be used. The concentration of the lithium salt is preferably in the range of 0.1 to 2.0 M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, thereby exhibiting excellent electrolyte performance and allowing lithium ions to migrate effectively.

[0067] In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds (e.g., difluoroethylene carbonate), pyridine, triethyl phosphite, triethanolamine, 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-methoxyethanol, or aluminum trichloride, for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity. In this case, the additives may be contained in an amount of 0.1 to 5 wt % based on the total weight of the electrolyte.

[0068] As described above, the lithium secondary battery including the cathode active material according to the present invention exhibits excellent discharge capacity, output characteristics, and life characteristics, especially excellent high-temperature life characteristics and high-temperature storage characteristics, 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).

[0069] As a result, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same can be provided.

[0070] The battery module or battery pack may be used as a power source for one or more medium- to large-sized devices in a power tool; an electric vehicle (EV), a hybrid electric vehicle, and an electric vehicle (PHEV), including a plug-in hybrid electric vehicle; or a power storage system.

[0071] The shape of the lithium secondary battery of the present invention is not particularly limited, but may be a cylindrical shape using a can, a square shape, a pouch shape, or a coin shape.

[0072] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for a small device, but also as a unit battery for a medium- to large-sized battery module including a number of battery cells.

[0073] Examples of the medium- to large-sized devices include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0074] Hereinafter, the present invention will be described in detail with reference to examples. However, the examples according to the present invention can be modified in several different forms, and the scope of the present invention should not be construed as being limited to the examples described below. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0075] Examples and Comparative Examples Example 1 Ni 0.8 Co 0.1 Mn 0.1 A precursor having a composition represented by (OH)2 was mixed with LiOH·H2O so that the molar ratio of (Ni+Co+Mn):Li was 1:1.02, and the mixture was heat-treated at 750°C for 27 hours in an oxygen atmosphere to produce a first lithium transition metal oxide with a particle strength of 185 MPa and a crystal size of 107 nm. 1.0 Ni 0.8 Co 0.1 Mn 0.1 O2, and the primary particles are aggregated into secondary particles. The average particle size of the secondary particles (D 50 ) was 13 μm.

[0076] Ni 0.8 Co 0.1 Mn 0.1A precursor having a composition represented by (OH)2 was mixed with LiOH·H2O so that the molar ratio of (Ni+Co+Mn):Li was 1:1.02, and the mixture was heat-treated at 780°C for 25 hours in an oxygen atmosphere to produce a second lithium transition metal oxide with a particle strength of 111 MPa and a crystal size of 160 nm. 1.0 Ni 0.8 Co 0.1 Mn 0.1 O2, and the primary particles are aggregated into secondary particles. The average particle size of the secondary particles (D 50 ) was 3 μm.

[0077] The first lithium transition metal oxide and the second lithium transition metal oxide were mixed in a weight ratio of 75:25 to prepare a bimodal type positive electrode active material.

[0078] Example 2 Ni 0.8 Co 0.1 Mn 0.1 A precursor having a composition represented by (OH)2 was mixed with LiOH·H2O so that the molar ratio of (Ni+Co+Mn):Li was 1:1.02, and the mixture was heat-treated at 750°C for 32 hours in an oxygen atmosphere to produce a first lithium transition metal oxide having a particle strength of 271 MPa and a crystal size of 110 nm. 1.0 Ni 0.8 Co 0.1 Mn 0.1 O2, and the primary particles are aggregated into secondary particles. The average particle size of the secondary particles (D 50 ) was 13 μm.

[0079] Ni 0.8 Co 0.1 Mn 0.1 A precursor having a composition represented by (OH)2 was mixed with LiOH·H2O so that the molar ratio of (Ni+Co+Mn):Li was 1:1.02, and the mixture was heat-treated at 780°C for 21 hours in an oxygen atmosphere to produce a second lithium transition metal oxide with a particle strength of 73 MPa and a crystal size of 165 nm. 1.0Ni 0.8 Co 0.1 Mn 0.1 O2, and the primary particles are aggregated into secondary particles. The average particle size of the secondary particles (D 50 ) was 3 μm.

[0080] The first lithium transition metal oxide and the second lithium transition metal oxide were mixed in a weight ratio of 75:25 to prepare a bimodal type positive electrode active material.

[0081] Example 3 Ni 0.8 Co 0.1 Mn 0.1 A precursor represented by (OH)2 was mixed with LiOH·H2O so that the molar ratio of (Ni+Co+Mn):Li was 1:1.02, and the mixture was heat-treated at 720°C for 28 hours in an oxygen atmosphere to produce a first lithium transition metal oxide with a particle strength of 205 MPa and a crystal size of 83 nm. 1.0 Ni 0.8 Co 0.1 Mn 0.1 O2, and the primary particles are aggregated into secondary particles. The average particle size of the secondary particles (D 50 ) was 13 μm.

[0082] Ni 0.8 Co 0.1 Mn 0.1 A precursor represented by (OH)2 was mixed with LiOH·H2O so that the molar ratio of (Ni+Co+Mn):Li was 1:1.02, and the mixture was heat-treated at 790°C for 24 hours in an oxygen atmosphere to produce a second lithium transition metal oxide with a particle strength of 108 MPa and a crystal size of 181 nm. 1.0 Ni 0.8 Co 0.1 Mn 0.1 O2, and the primary particles are aggregated into secondary particles. The average particle size of the secondary particles (D 50 ) was 3 μm.

[0083] The first lithium transition metal oxide and the second lithium transition metal oxide were mixed in a weight ratio of 75:25 to prepare a bimodal type positive electrode active material.

[0084] Comparative Example 1 Ni 0.8 Co 0.1 Mn 0.1 A precursor having a composition represented by (OH)2 was mixed with LiOH·H2O so that the molar ratio of (Ni+Co+Mn):Li was 1:1.02, and the mixture was heat-treated at 780°C for 27 hours in an oxygen atmosphere to produce a first lithium transition metal oxide with a particle strength of 193 MPa and a crystal size of 161 nm. 1.0 Ni 0.8 Co 0.1 Mn 0.1 O2, and the primary particles are aggregated into secondary particles. The average particle size of the secondary particles (D 50 ) was 13 μm.

[0085] Ni 0.8 Co 0.1 Mn 0.1 A precursor having a composition represented by (OH)2 was mixed with LiOH·H2O so that the molar ratio of (Ni+Co+Mn):Li was 1:1.02, and the mixture was heat-treated at 750°C for 25 hours in an oxygen atmosphere to produce a second lithium transition metal oxide having a particle strength of 115 MPa and a crystal size of 132 nm. 1.0 Ni 0.8 Co 0.1 Mn 0.1 O2, and the primary particles are aggregated into secondary particles. The average particle size of the secondary particles (D 50 ) was 3 μm.

[0086] The first lithium transition metal oxide and the second lithium transition metal oxide were mixed in a weight ratio of 75:25 to prepare a bimodal type positive electrode active material.

[0087] Comparative Example 2 Ni 0.8 Co 0.1 Mn0.1 A precursor having a composition represented by (OH)2 was mixed with LiOH·H2O so that the molar ratio of (Ni+Co+Mn):Li was 1:1.02, and the mixture was heat-treated at 760°C for 23 hours in an oxygen atmosphere to produce a first lithium transition metal oxide having a particle strength of 110 MPa and a crystal size of 123 nm. 1.0 Ni 0.8 Co 0.1 Mn 0.1 O2, and the primary particles are aggregated into secondary particles. The average particle size of the secondary particles (D 50 ) was 13 μm.

[0088] Ni 0.8 Co 0.1 Mn 0.1 A precursor having a composition represented by (OH)2 was mixed with LiOH·H2O so that the molar ratio of (Ni+Co+Mn):Li was 1:1.02, and the mixture was heat-treated at 790°C for 29 hours in an oxygen atmosphere to produce a second lithium transition metal oxide with a particle strength of 151 MPa and a crystal size of 171 nm. 1.0 Ni 0.8 Co 0.1 Mn 0.1 O2, and the primary particles are aggregated into secondary particles. The average particle size of the secondary particles (D 50 ) was 13 μm.

[0089] The first lithium transition metal oxide and the second lithium transition metal oxide were mixed in a weight ratio of 75:25 to prepare a bimodal type positive electrode active material.

[0090] Comparative Example 3 Ni 0.8 Co 0.1 Mn 0.1 A precursor having a composition represented by (OH)2 was mixed with LiOH·H2O so that the molar ratio of (Ni+Co+Mn):Li was 1:1.02, and the mixture was heat-treated at 775°C for 21 hours in an oxygen atmosphere to produce a first lithium transition metal oxide with a particle strength of 95 MPa and a crystal size of 153 nm.1.0 Ni 0.8 Co 0.1 Mn 0.1 O2, and the primary particles are aggregated into secondary particles. The average particle size of the secondary particles (D 50 ) was 13 μm.

[0091] Ni 0.8 Co 0.1 Mn 0.1 A precursor having a composition represented by (OH)2 was mixed with LiOH·H2O so that the molar ratio of (Ni+Co+Mn):Li was 1:1.02, and the mixture was heat-treated at 730°C for 29 hours in an oxygen atmosphere to produce a second lithium transition metal oxide with a particle strength of 158 MPa and a crystal size of 118 nm. 1.0 Ni 0.8 Co 0.1 Mn 0.1 O2, and the primary particles are aggregated into secondary particles. The average particle size of the secondary particles (D 50 ) was 3 μm.

[0092] The first lithium transition metal oxide and the second lithium transition metal oxide were mixed in a weight ratio of 75:25 to prepare a bimodal type positive electrode active material.

[0093] The particle strength and crystal size of the first lithium transition metal oxide and the second lithium transition metal oxide used in Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 1 below.

[0094] [Table 1]

[0095] Experimental example Experimental Example 1: Evaluation of changes in average particle size of first lithium transition metal oxide due to electrode rolling The bimodal cathode active materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3, respectively, carbon black conductive material, and PVdF binder were mixed in a weight ratio of 95:2:3 in N-methyl-2-pyrrolidone (NMP) solvent to prepare cathode slurries. The cathode slurries were applied to one side of an aluminum current collector and dried to prepare pre-rolled electrodes.

[0096] The electrode before rolling has an electrode density of 3.15 g / cm 2 After rolling, an electrode was produced.

[0097] The pre-rolled electrode and the post-rolled electrode were baked in an air atmosphere at 600°C for 3 hours to remove the conductive material and binder from the electrode, and then only the positive electrode active material particles were collected and the particle size distribution was measured to determine the average particle size (D 50 ) are shown in Table 2 below.

[0098] [Table 2]

[0099] The first lithium transition metal oxides contained in the bimodal cathode active materials of Examples 1 to 3 have greater particle strength than the second lithium transition metal oxides contained in the bimodal cathode active materials of Examples 1 to 3. Therefore, it can be confirmed that almost no particle cracking occurs during electrode rolling, and the change in average particle size of the particles before and after electrode rolling is smaller than in Comparative Examples 2 and 3.

[0100] Experimental Example 2: Evaluation of high-temperature battery life and storage performance The bimodal cathode active materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3, respectively, carbon black conductive material, and PVdF binder were mixed in a weight ratio of 95:2:3 in N-methyl-2-pyrrolidone (NMP) solvent to prepare cathode slurries. The cathode slurries were applied to one side of an aluminum current collector, dried, and then rolled to prepare cathodes.

[0101] Then, the negative electrode active material (graphite), carbon black conductive material, and PVdF binder were mixed in water (H2O) at a weight ratio of 95:1:4 to prepare a negative electrode slurry. The negative electrode slurry was applied to one side of a copper current collector, dried, and then rolled to prepare a negative electrode.

[0102] The electrode assembly was fabricated by interposing a separator between the positive and negative electrodes, and then placed inside a battery case. An electrolyte solution was then injected to fabricate a lithium secondary battery. The electrolyte solution was prepared by dissolving 2 wt % vinylene carbonate (VC) and 1 M LiPF6 in an organic solvent containing ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume ratio of 3:3:4.

[0103] The secondary batteries were then charged to 4.2 V at a constant current of 0.5 C at 45°C. They were then discharged to 2.5 V at a constant current of 0.5 C. This charge and discharge behavior constitutes one cycle, and after 100 cycles, the ratio of the 100th cycle capacity to the 1st cycle capacity was determined as the capacity retention, which is shown in Table 3 below. The resistance increase rate was also determined as the ratio of the DCIR value, calculated by dividing the voltage drop (ΔV) for 60 seconds in the 1st discharge cycle by the current, to the DCIR value, calculated by dividing the voltage drop (ΔV) for 60 seconds in the 100th discharge cycle by the current, which is shown in Table 3 below.

[0104] Furthermore, an initial baseline performance test (RPT) was performed on each of the secondary batteries, and the amount of gas generated during the initial baseline performance test was measured using gas chromatography (GC Agilent 7890b). The results are shown in Table 3 below.

[0105] [Table 3]

[0106] Referring to Table 3, it can be seen that the secondary batteries including the bimodal cathode active materials of Examples 1 to 3 minimize cracking of the first lithium transition metal oxide (large particles) and volume change due to charge and discharge, increase the size of fine powder generated by cracking of the second lithium transition metal oxide (small particles), suppress an increase in specific surface area, and minimize reactivity with the electrolyte. As a result, they have excellent capacity retention at high temperatures, a significantly low rate of resistance increase, and low gas generation.

[0107] In contrast, in Comparative Examples 1 and 3, the crystal size of the first lithium transition metal oxide contained in the bimodal cathode active material was larger than the crystal size of the second lithium transition metal oxide, which caused a large volume change during the charge / discharge process, resulting in many cracks inside the particles, resulting in a large resistance increase rate and poor life performance.Furthermore, in Comparative Examples 2 and 3, the particle strength of the first lithium transition metal oxide contained in the bimodal cathode active material was smaller than the particle strength of the second lithium transition metal oxide, resulting in cracks of the large particles when they collided with the small particles during the electrode rolling process, resulting in a large increase in resistance.

Claims

1. The first lithium transition metal oxide and the average particle size (D ５０ ) a second lithium transition metal oxide having a small The first lithium transition metal oxide has a higher particle strength and a smaller crystal grain size than the second lithium transition metal oxide, The second lithium transition metal oxide has a crystal grain size of 150 nm to 190 nm.

2. The bimodal cathode active material according to claim 1 , wherein the first lithium transition metal oxide and the second lithium transition metal oxide each independently have a composition represented by the following Chemical Formula 1: [Chemical formula 1] Li ｘ Ni ａ Co ｂ M １ ｃ M ２ ｄ O ２ In the above Chemical Formula 1, M １ is one or more selected from Mn and Al, M ２ is one or more selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; 0.9≦x≦1.2, 0.7≦a<1.0, 0<b<0.3, 0<c<0.3, 0≦d≦0.

1.

3. 3. The bimodal cathode active material of claim 1, wherein a weight ratio of the first lithium transition metal oxide to the second lithium transition metal oxide is 60:40 to 95:

5.

4. The bimodal cathode active material according to claim 1 , wherein a difference in particle strength between the first lithium transition metal oxide and the second lithium transition metal oxide is 50 MPa to 200 MPa.

5. 5. The bimodal cathode active material according to claim 1, wherein the difference in crystal grain size between the first lithium transition metal oxide and the second lithium transition metal oxide is 50 nm to 150 nm.

6. The bimodal type positive electrode active material according to claim 1 , wherein the first lithium transition metal oxide has a particle strength of 130 MPa to 300 MPa.

7. The bimodal cathode active material according to claim 1 , wherein the second lithium transition metal oxide has a particle strength of 70 MPa to 125 MPa.

8. The bimodal type positive electrode active material according to claim 1 , wherein the first lithium transition metal oxide has a crystal grain size of 80 nm to 140 nm.

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

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

Images