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

Abstract

To provide a positive electrode active material having high energy density, high operation voltage, and high conductivity, and a lithium secondary battery having high energy density, high operation voltage, and a high low-temperature characteristic.SOLUTION: The present invention relates to a positive electrode active material for a lithium secondary battery, and a lithium secondary battery including the same. More specifically, the positive electrode active material includes a first particle containing a compound expressed by Chemical Formula 1 and having a first average particle diameter, and a second particle containing a compound expressed by Chemical Formula 2 and having a second average particle diameter that is smaller than the first average particle diameter. The content amount of the first particle is more than that of the second particle.SELECTED DRAWING: Figure 6a

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for a lithium secondary battery, a positive electrode containing the same, and a lithium secondary battery containing the same, and more particularly to a positive electrode active material containing an olivine-based lithium compound, a positive electrode containing the same, and a lithium secondary battery containing the same. [Background technology]

[0002] Recently, with the rapid spread of battery-powered electronic devices such as mobile phones, laptops, and electric vehicles, the demand for high-energy-density, high-capacity secondary batteries is rapidly increasing. As a result, research and development efforts to improve the performance of lithium secondary batteries are being actively conducted.

[0003] A lithium secondary battery is a battery that includes a cathode and an anode, each containing an active material capable of intercalating and deintercalating lithium ions, and an electrolyte. Electrical energy is produced through oxidation and reduction reactions that occur when lithium ions are intercalated and deintercalated at the cathode and anode.

[0004] In order to provide a positive electrode active material having a high energy density, a high operating voltage, and a high electrical conductivity, a technique using a mixture of different lithium transition metal oxides as a positive electrode material has been attempted. Summary of the Invention [Problem to be solved by the invention]

[0005] The problem to be solved by the present invention is to provide a positive electrode active material having high energy density, high operating voltage, and high conductivity.

[0006] Another problem to be solved by the present invention is to provide a lithium secondary battery having high energy density, high operating voltage, and excellent low-temperature characteristics. [Means for solving the problem]

[0007] The positive electrode active material according to the concept of the present invention may include a compound of Chemical Formula 1 below, a first particle having a first average particle size, a compound of Chemical Formula 2 below, and a second particle having a second average particle size. The content of the first particles may be greater than the content of the second particles. [Chemical Formula 1] Li a1 Mn z1 Fe x1 A y1 PO 4-c1 (In Chemical Formula 1, 0.8 < a1 ≤ 1.2, 0.3 ≤ x1 ≤ 0.7, 0 ≤ y1 ≤ 0.05, 0.3 ≤ z1 ≤ 0.7, 0 < c1 ≤ 0.05, and x1 + y1 + z1 = 1, and A is at least one element selected from the group consisting of Ti, Mg, V, and Nb), [Chemical Formula 2] Li a2 Co z2 D x2 O c2 (In Chemical Formula 2, 1 < a2 ≤ 1.5, 0.9 ≤ z2 ≤ 1.0, 0 ≤ x2 ≤ 0.05, and 2 < c2 ≤ 2.3, and D may be at least one element selected from the group consisting of Al, Ti, and Mg).

[0008] The positive electrode for a lithium secondary battery according to another concept of the present invention may include a positive electrode current collector and a positive electrode active material layer on the positive electrode current collector. The positive electrode active material layer may include a positive electrode active material, a conductive material, and a binder.

[0009] The lithium secondary battery according to another concept of the present invention may include a positive electrode, a negative electrode current collector, a negative electrode including a negative electrode active material layer on the negative electrode current collector, and a separator between the positive electrode and the negative electrode.

Advantages of the Invention

[0010] The positive electrode active material according to the present invention may include first particles having an olivine structure and second particles having a single crystal structure of several micrometers in size. This may improve the composite density, capacity, high-voltage characteristics, and energy density of the positive electrode active material. The positive electrode active material layer according to the present invention may be smoothly attached to the positive electrode current collector with a relatively small amount of binder. The lithium secondary battery according to the present invention may have a relatively high average voltage. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a conceptual diagram illustrating a lithium secondary battery according to an embodiment of the present invention; [Figure 2] 1 is a schematic diagram showing a lithium secondary battery according to an embodiment, which can be said to have a cylindrical battery shape. [Figure 3] 1 is a schematic diagram showing a lithium secondary battery according to an embodiment, which can be said to have a prismatic battery shape. [Figure 4] 1 is a schematic diagram showing a lithium secondary battery according to an embodiment, which can be said to have a pouch-type battery configuration. [Figure 5] 1 is a schematic diagram showing a lithium secondary battery according to an embodiment, which can be said to have a pouch-type battery configuration. [Figure 6a] 2 is an enlarged view of a positive electrode active material layer of a lithium secondary battery according to an embodiment of the present invention; [Figure 6b] 2 is an enlarged view of a positive electrode active material layer of a lithium secondary battery according to an embodiment of the present invention; [Figure 7a] 1 is an SEM image of the positive electrode active material of Comparative Example 1-2 of the present invention. [Figure 7b] 1 is an SEM image of the positive electrode active material of Comparative Example 2-1 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] In this specification, when a component is referred to as being on another component, it means that the component may be directly formed on the other component, or that a third component may be interposed between them. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various forms and may undergo various modifications. The description of the present embodiments is provided solely to ensure complete disclosure of the present invention and to fully convey the scope of the invention to those skilled in the art.

[0013] In the drawings, the thickness of components is exaggerated for the sake of clarity. The same reference numerals denote the same components throughout the specification.

[0014] Unless otherwise specified herein, the singular can also include the plural. Furthermore, unless otherwise specified, "A or B" can mean "including A, including B, or including A and B." As used herein, "comprises" and / or "comprising" does not exclude the presence or addition of one or more other elements to the referenced element.

[0015] As used herein, "combinations thereof" can mean mixtures, laminates, composites, copolymers, alloys, blends, reaction products, and the like of compositions.

[0016] Unless otherwise defined herein, particle size refers to the average particle size. Furthermore, particle size refers to the average particle size (D50), which refers to the diameter of particles with a cumulative volume of 50% in a particle size distribution. The average particle size (D50) can be measured by methods well known to those skilled in the art, such as using a particle size analyzer or a transmission electron microscope (TEM) or scanning electron microscope (SEM) image. Alternatively, the average particle size (D50) can be measured using a measuring device that uses dynamic light scattering, and data analysis can be performed to count the number of particles in each particle size range, after which the average particle size (D50) can be calculated. Alternatively, the average particle size (D50) can be measured using a laser diffraction method. More specifically, when measuring by the laser diffraction method, the particles to be measured are dispersed in a dispersion medium, then introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT3000), and irradiated with ultrasonic waves of approximately 28 kHz at an output of 60 W. The average particle size (D50) based on 50% of the particle size distribution in the measuring device can then be calculated.

[0017] 1 is a schematic diagram illustrating a lithium secondary battery according to an embodiment of the present invention. Referring to FIG. 1, the lithium secondary battery may include a positive electrode 10, a negative electrode 20, a separator 30, and an electrolyte solution ELL.

[0018] The positive electrode 10 and the negative electrode 20 may be separated from each other by a separator 30. The separator 30 may be disposed between the positive electrode 10 and the negative electrode 20. The positive electrode 10, the negative electrode 20, and the separator 30 may be in contact with an electrolyte solution ELL. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated within the electrolyte solution ELL.

[0019] The electrolyte ELL can be a medium for transferring lithium ions between the positive electrode 10 and the negative electrode 20. In the electrolyte ELL, the lithium ions can pass through the separator 30 and move toward the positive electrode 10 or the negative electrode 20.

[0020] positive electrode 10 The positive electrode 10 for a lithium secondary battery may include a current collector COL1 and a positive electrode active material layer AML1 formed on the current collector COL1. The positive electrode active material layer AML1 includes a positive electrode active material and may further include a binder and / or a conductive material. A detailed description of the positive electrode active material layer AML1 according to an embodiment of the present invention will be provided below with reference to FIG. 6. The current collector COL1 may be made of, but is not limited to, aluminum.

[0021] negative electrode 20 The lithium secondary battery positive electrode 20 may include a current collector COL2 and a negative electrode active material layer AML2 formed on the current collector COL2. The negative electrode active material layer AML2 includes a negative electrode active material and may further include a binder and / or a conductive material.

[0022] For example, the negative electrode active material layer AML2 may contain 90% to 99% by weight of the negative electrode active material, 0.5% to 5% by weight of the binder, and 0% to 5% by weight of the conductive material.

[0023] The binder serves to effectively bond the negative electrode active material particles to each other and to effectively attach the negative electrode active material to the current collector COL 2. As the binder, a non-aqueous binder, an aqueous binder, a dry binder, or a combination thereof can be used.

[0024] Non-aqueous binders include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene propylene copolymer, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or combinations thereof.

[0025] The water-based binder may be selected from styrene-butadiene rubber, (meth)acrylate styrene-butadiene rubber, (meth)acrylonitrile-butadiene rubber, (meth)acrylic rubber, butyl rubber, fluororubber, polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, poly(meth)acrylonitrile, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyether resin, (meth)acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0026] When an aqueous binder is used as the negative electrode binder, it may further contain a cellulose-based compound to impart viscosity. The cellulose-based compound may be a mixture of one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or alkali metal salts thereof. The alkali metal may be sodium, potassium, or lithium.

[0027] The dry binder is a fiberizable polymeric material, such as polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, ethylene oxide, or combinations thereof.

[0028] The conductive material is used to impart conductivity to the electrode and may be any electron-conductive material that does not undergo chemical changes in the battery that is constructed. Specific examples include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metal-based materials in the form of metal powder or metal fiber, including copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; and mixtures thereof.

[0029] As the current collector COL2, those selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrates coated with conductive metals, and combinations thereof can be used.

[0030] negative electrode active material The negative electrode active material in the negative electrode active material layer AML2 includes substances that can reversibly insert / desorb lithium ions, lithium metal, alloys of lithium metal, substances that can be doped or undoped with lithium, or transition metal oxides.

[0031] Substances that can reversibly insert / desorb lithium ions include carbon-based negative electrode active materials, which may include, for example, crystalline carbon, amorphous carbon, or combinations thereof. Examples of crystalline carbon include graphite such as amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite, and examples of amorphous carbon include soft carbon or hard carbon, mesophase pitch carbide, calcined coke, etc.

[0032] As the alloy of the lithium metal, an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn can be used.

[0033] As substances that can be doped or undoped with lithium, Si-based negative electrode active materials or Sn-based negative electrode active materials can be used. The Si-based negative electrode active material can be silicon, a silicon-carbon composite, SiOx (0 < x < 2), a Si-Q alloy (Q is selected from alkali metals, alkaline earth metals, group 13 elements, group 14 elements (excluding Si), group 15 elements, group 16 elements, transition metals, rare earth elements, and combinations thereof), or combinations thereof. The Sn-based negative electrode active material can be Sn, SnO2, Sn-based alloys, or combinations thereof.

[0034] The silicon-carbon composite may be a composite of silicon and amorphous carbon. According to one embodiment, the silicon-carbon composite may be in the form of silicon particles and the surfaces of the silicon particles coated with amorphous carbon. For example, the composite may include secondary particles (cores) formed by combining primary silicon particles and amorphous carbon coating layers (shells) located on the surfaces of the secondary particles. Amorphous carbon may also be located between the primary silicon particles, e.g., the primary silicon particles may be coated with amorphous carbon. The secondary particles may be dispersed in an amorphous carbon matrix.

[0035] The silicon-carbon composite may further comprise crystalline carbon. For example, the silicon-carbon composite may comprise a core comprising crystalline carbon and silicon particles and an amorphous carbon coating layer disposed on the core.

[0036] The Si-based negative electrode active material or the Sn-based negative electrode active material can be used in combination with a carbon-based negative electrode active material.

[0037] Separator 30 Depending on the type of lithium secondary battery, a separator 30 may be present between the positive electrode 10 and the negative electrode 20. Such separator 30 may be made of polyethylene, polypropylene, polyvinylidene fluoride, or a multilayer film of two or more layers thereof, but it goes without saying that mixed multilayer films such as a polyethylene / polypropylene two-layer separator, a polyethylene / polypropylene / polyethylene three-layer separator, or a polypropylene / polyethylene / polypropylene three-layer separator may also be used.

[0038] Separator 30 may include a porous substrate and a coating layer comprising an organic material, an inorganic material, or a combination thereof located on one or both sides of the porous substrate.

[0039] The porous substrate may be a polymer membrane made of any one polymer selected from polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacetal, polyamide, polyimide, polycarbonate, polyether ketone, polyaryl ether ketone, polyetherimide, polyamide imide, polybenzimidazole, polyethersulfone, polyphenylene oxide, cyclic olefin copolymers, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon, and polytetrafluoroethylene, or a copolymer or mixture of two or more of these.

[0040] The organic material may include a polyvinylidene fluoride polymer or a (meth)acrylic polymer.

[0041] The inorganic materials may include, but are not limited to, inorganic particles selected from Al2O3, SiO2, TiO2, SnO2, CeO2, MgO, NiO, CaO, GaO, ZnO, ZrO2, Y2O3, SrTiO3, BaTiO3, Mg(OH)2, boehmite, and combinations thereof.

[0042] The organic material and the inorganic material may be mixed in one coating layer, or a coating layer containing an organic material and a coating layer containing an inorganic material may be stacked.

[0043] Electrolyte ELL The electrolyte ELL for lithium secondary batteries contains a non-aqueous organic solvent and a lithium salt.

[0044] The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can migrate.

[0045] The non-aqueous organic solvent can be a carbonate-based, ester-based, ether-based, ketone-based, or alcohol-based solvent, a non-quantum solvent, or a combination thereof.

[0046] Examples of carbonate solvents that can be used include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl pyrrolyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC).

[0047] Examples of ester solvents that can be used include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, decanolide, mevalonolactone, valerolactone, and caprolactone.

[0048] Examples of ether solvents that can be used include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, and tetrahydrofuran. Examples of ketone solvents that can be used include cyclohexanone. Examples of alcohol solvents that can be used include ethyl alcohol and isopropyl alcohol. Examples of non-quantum solvents that can be used include nitriles such as R-CN (R is a hydrocarbon group having 2 to 20 carbon atoms and having a linear, branched, or cyclic structure, and may contain a double bond, an aromatic ring, or an ether group), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane and 1,4-dioxolane, and sulfolanes.

[0049] The non-aqueous organic solvents may be used alone or in combination of two or more.

[0050] When a carbonate-based solvent is used, a cyclic carbonate and a chain carbonate may be mixed together, and the cyclic carbonate and the chain cyclic carbonate may be mixed in a volume ratio of 1:1 to 1:9.

[0051] Lithium salts are dissolved in organic solvents and act as a source of lithium ions in the battery, enabling basic lithium secondary battery operation and facilitating the movement of lithium ions between the positive and negative electrodes. Representative examples of lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiCl, LiI, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide (LiFSI), LiC4F9SO3, LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2) (x and y are integers from 1 to 20), lithium trifluoromethanesulfonate, lithium tetrafluoroethanesulfonate, lithium difluorobis(oxalato)phosphate (LiDFBOP), and lithium bis(oxalato)borate (LiBOB).

[0052] Lithium secondary battery Lithium secondary batteries may be classified into cylindrical, prismatic, pouch, and coin types depending on their shape. FIGS. 2 to 5 are schematic diagrams showing lithium secondary batteries according to embodiments, with FIG. 2 illustrating a cylindrical battery, FIG. 3 illustrating a prismatic battery, and FIGS. 4 and 5 illustrating pouch battery types. Referring to FIGS. 2 to 4, a lithium secondary battery 100 may include an electrode assembly 40 having a separator 30 interposed between a positive electrode 10 and a negative electrode 20, and a housing 50 in which the electrode assembly 40 is embedded. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated with an electrolyte (not shown). The lithium secondary battery 100 may include a sealing member 60 that seals the housing 50, as shown in FIG. 2. Also, in FIG. 3, the lithium secondary battery 100 may include a positive electrode lead tab 11, a positive electrode terminal 12, a negative electrode lead tab 21, and a negative electrode terminal 22. As shown in FIGS. 4 and 5, the lithium secondary battery 100 may include electrode tabs 70, i.e., a positive electrode tab 71 and a negative electrode tab 72, which serve as electrical paths for conducting the current generated in the positive electrode assembly 40 to the outside.

[0053] The lithium secondary battery according to an embodiment of the present invention may be applied to automobiles, mobile phones, and / or various types of electrical devices, but the present invention is not limited thereto.

[0054] 6a and 6b are enlarged views of a positive electrode active material layer of a lithium secondary battery according to an embodiment of the present invention. Referring to FIGS. 6a and 6b, as described above, the positive electrode active material layer AML1 (see FIG. 1) may include first particles PTC1, second particles PTC2, a conductive material CDM, and a binder BND. The plurality of first particles PTC1 and the plurality of second particles PTC2 may constitute a positive electrode active material according to an embodiment of the present invention.

[0055] The contents of the positive electrode active materials PTC1 and PTC2 in the positive electrode active material layer AML1 may be 90 wt % to 99.5 wt % relative to 100 wt % of the positive electrode active material layer AML1, and the contents of the binder BND and the conductive material CDM may be 0.5 wt % to 5 wt % relative to 100 wt % of the positive electrode active material layer AML1.

[0056] The binder BND may bind the first particles PTC1, the second particles PTC2, and the conductive material CDM to one another. As an example, the binder BND may include at least one selected from the group consisting of, but not limited to, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, (meth)acrylic styrene-butadiene rubber, epoxy resin, (meth)acrylic resin, polyester resin, and nylon.

[0057] The conductive material CDM can be used to improve the conductivity of the positive electrode active material layer AML1. Any conductive material that does not cause a chemical change in the positive electrode active material layer AML1 can be used as the conductive material CDM without limitation. For example, as the conductive material CDM, carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, carbon nanotube, metal-based materials containing copper, nickel, aluminum, silver, etc. in the form of metal powder or metal fiber, conductive polymers such as polyphenylene derivatives, or mixtures thereof can be included.

[0058] 1st particle PTC1 The first particle PTC1 may contain an olivine-based lithium compound represented by the following Chemical Formula 1. [Chemical Formula 1] [[ID=IO]]Li a1 Mn z1 Fe x1 A y1 PO 4-c1 In Chemical Formula 1, 0.8 < a1 ≤ 1.2, 0.3 ≤ x1 ≤ 0.7, 0 ≤ y1 ≤ 0.05, 0.3 ≤ z1 ≤ 0.7, 0 < c1 ≤ 0.05, and x1 + y1 + z1 = 1 may hold. A may be at least one element selected from the group consisting of Ti, Mg, V, and Nb. A may be a dopant doped into the first particle PTC1.

[0059] When doping the first particle PTC1 with Ti, it has the effect of controlling the growth of the first primary particles NNP of the first particle PTC1, and as a result, the size of the first primary particles NNP of the first particle PTC1 can be made smaller. When doping the first particle PTC1 with Ti, the size of the first primary particles NNP can be grown uniformly, and as a result, the low-temperature characteristics can be improved.

[0060] Referring to FIG. 6a, the first particle PTC1 may have a single particle shape. In this specification, a single particle may refer to a single particle without an internal grain boundary. A single particle may refer to a morphological phase in which particles exist as an independent phase that is not aggregated with each other, a monolith structure, a single body structure, or a non-aggregated particle. For example, a single particle may be a single crystal. Alternatively, a single particle may be a particle containing several crystals. A single particle may be in a singly separated form. Alternatively, a single particle may be in a form in which 2 to 100 first primary particles NNP are attached to each other.

[0061] As an example, the average particle size may refer to the diameter of particles that make up 50% by volume of the cumulative volume in the particle size distribution (D50).

[0062] The average particle size (D50) of the first particles PTC1 may be a value measured using a particle size analyzer. The average particle size (D50) of the single-particle first particles PTC1 may be 0.5 μm to 2.5 μm. The average particle size (D50) of the first particles PTC1 according to this embodiment may be smaller than the average particle size (D50) of the second particles PTC2, which will be described later.

[0063] The first particle PTC1 according to this embodiment may include at least one primary particle (or single particle). The size of the at least one first primary particle constituting one first particle PTC1 according to this embodiment may be 100 nm to 200 nm when measured using a scanning electron microscope (SEM). In one embodiment, the size of the first primary particle may refer to the diameter measured by randomly selecting about 30 first primary particles from an electron microscope image of the positive electrode active material. The size of the first primary particles may be uniform.

[0064] For example, the first particles PTC1 may include a first coating layer on their surfaces. The coating layer may cover the entire surface of the first particles PTC1 or may cover only a portion of the surface of the first particles PTC1. For example, the coating layer may include carbon and / or a carbon-containing compound. The first coating layer may further include at least one selected from the group consisting of a titanium-containing compound, a magnesium-containing compound, and a vanadium-containing compound. The metal-containing compound, such as a titanium-containing compound, a magnesium-containing compound, or a vanadium-containing compound, may be, for example, a metal oxide, a metal hydroxide, a metal carbonate, a composite thereof, or a mixture thereof. The metal-containing compound may further include other metals or non-metal elements. For example, the metal-containing compound may further include lithium. The coating layer may improve the structural stability and electrical conductivity of the first particles PTC1.

[0065] 6b is an enlarged view of a positive electrode active material layer of a lithium secondary battery according to another embodiment of the present invention. Referring to FIG. 6b, in another embodiment, the first particle PTC1 may be polycrystalline and include a secondary particle formed by agglomeration of at least two or more first primary particles NNP. In other words, one first particle PTC1 may include a plurality of first primary particles NNP agglomerated together. The first particle PTC1, which is made up of a plurality of first primary particles NNP, may have a spherical or elliptical shape.

[0066] For example, the first particle PTC1 may include a coating layer on its surface. The coating layer may cover the entire surface of the first particle PTC1 or may cover only a portion of the surface of the first particle PTC1. The coating layer may be present inside the first particle PTC1. The coating layer may be formed by coating along the interface between the internal particles of the first particle PTC1. For example, the coating layer may include carbon and / or a carbon-containing compound. The coating layer may improve the structural stability of the first particle PTC1, and a uniform coating layer may be formed on the surface of the first particle PTC1. Further including a coating layer may improve electrical conductivity.

[0067] The coating layer may further include at least one selected from the group consisting of a titanium-containing compound, a magnesium-containing compound, and a vanadium-containing compound. Metal-containing compounds such as titanium-containing compounds, magnesium-containing compounds, and vanadium-containing compounds may be, for example, metal oxides, metal hydroxides, metal carbonates, composites thereof, or mixtures thereof. The metal-containing compounds may further include other metals or non-metal elements. For example, the metal-containing compounds may further include lithium.

[0068] As an example, the first particle PTC1 may further include a grain boundary coating layer on the surface of each of the first primary particles NNP. The grain boundary coating layer may be present inside the first particle PTC1. The grain boundary coating layer may be formed by coating along the interface between the first primary particles NNP inside the first particle PTC1. In other words, the grain boundary coating layer may refer to a material coated on the grain boundaries inside the first particle PTC1.

[0069] The grain boundary coating layer may contain carbon and / or a carbon-containing compound. The grain boundary coating layer may further contain at least one selected from the group consisting of a titanium-containing compound, a magnesium-containing compound, and a vanadium-containing compound.

[0070] The interior of the first particle PTC1 described above may refer to the entire interior of the first particle PTC1 excluding the surface of the first particle PTC1. For example, the interior of the first particle PTC1 may refer to the region from a depth of about 10 nm from the surface of the first particle PTC1 to the entire inside, or from a depth of 10 nm to a depth of about 2 μm.

[0071] The first particles PTC1 may further include a grain boundary coating portion, which may enhance structural stability and form a uniform coating layer on the surface of the first particles PTC1, and may further improve the electrical conductivity of the first particles PTC1.

[0072] The first particles PTC1 may further contain carbon derived from the coating layer and / or the grain boundary coating layer. The carbon element content in the first particles PTC1 may be 0.5 to 10 wt %, 1 to 3 wt %, or 1.5 to 2.5 wt %. For example, the carbon content of the secondary particle-like first particles PTC1 shown in Figure 6b may be higher than the carbon content of the single particle-like first particles PTC1 shown in Figure 6a.

[0073] The first particles PTC1 may have a spherical shape formed by agglomeration of a plurality of first primary particles NNP. The first particles PTC1 may exhibit the following properties due to the first primary particles NNP being closely agglomerated to one another: The above-described coating layer and / or grain boundary coating layer of the first particles are well maintained, thereby increasing electrical conductivity and improving low-temperature characteristics. The electrode plate binding strength is increased, thereby reducing the capacity of the binder. The first particles PTC1 may have a spherical or elliptical shape.

[0074] As an example, the average particle size may be measured using a particle size analyzer. The average particle size may refer to the diameter of particles with a cumulative volume of 50% (D50) in the particle size distribution. The average particle size (D50) of the first particles PTC1 of the present invention measured using this method may be 3 μm to 10 μm.

[0075] The first particles PTC1 according to this embodiment may include at least one primary particle (or single particle). The size of the at least one first primary particle NNP constituting one first particle PTC1 according to this embodiment may be 50 nm to 150 nm.

[0076] In one embodiment, the size of the first primary particles NNP may refer to the diameter measured by randomly selecting about 30 first primary particles NNP from an electron microscope photograph of the positive electrode active material. The size of the first primary particles NNP may be uniform.

[0077] The porosity of the first particles PTCl may be about 20% to 40%. The Span value of the first particles PTCl analyzed by a particle size analyzer may be 0.3 to 0.75.

[0078] When the first particle PTC1 is in the form of secondary particles, since the first average particle size is large, a relatively small amount of binder BND may be required to attach the first particles to the current collector COL1 (see FIG. 1). For example, the content of the binder BND may be from 0.5% to 3% by weight based on 100% by weight of the positive electrode active material layer AML1.

[0079] The low-temperature characteristics of the lithium secondary battery including the positive electrode active material of the present invention can be improved. As an example, the capacity at -20°C (capacity at -20°C / initial capacity) in terms of the initial capacity ratio of the lithium secondary battery can be 40% or more. For example, the capacity at -20°C (capacity at -20°C / initial capacity) in terms of the initial capacity ratio of the lithium secondary battery of the present invention can be 40% to 100%, 50% to 100%, or 96% to 99%.

[0080] The operating voltage of the lithium secondary battery including the positive electrode active material of the present invention can be improved. As an example, the operating voltage range of the lithium secondary battery of the present invention can be 3V to 5V. For example, the operating voltage range can be 3V to 4.5V, or 3.5V to 4V.

[0081] The life characteristics of the lithium secondary battery including the positive electrode active material of the present invention can be improved. As an example, the lithium secondary battery of the present invention can have a capacity retention rate of 98% or more after 50 charge-discharge cycles at a constant current of 0.1C at the above-mentioned voltage. For example, the capacity retention rate can be 98% to 100%, or 99.8% to 100%.

[0082] 2nd particle PTC2 The second particle PTC2 may contain a layered lithium compound represented by the following Chemical Formula 2. [Chemical Formula 2] Li a2 Co z2 D x2 O c2 In the Chemical Formula 2, 1 < a2 ≤ 1.5, 0.9 ≤ z2 ≤ 1.0, 0 ≤ x2 ≤ 0.05, and 2 < c2 ≤ 2.3 can be satisfied. D may be at least one element selected from the group consisting of Al, Ti, and Mg. D may be a dopant doped into the second particles PTC2. The content of Ti in the second particles may be less than the content of Mg in the second particles.

[0083] 6a and 6b, the second particles PTC2 may have a single particle shape. In this specification, a single particle may refer to a single particle without an internal particle boundary. A single particle may refer to a morphological phase in which particles exist as an independent phase without being aggregated with each other, a monolith structure, a single body structure, or a non-aggregated particle. In one example, the single particle of the second particles PTC2 may be a single crystal. In one example, the single particle may have a form in which a plurality of second primary particles MMP are attached to each other. Alternatively, the single particle may have a form in which 2 to 100 second primary particles MMP are attached to each other.

[0084] As an example, the average particle size may refer to the diameter of particles with a cumulative volume of 50% (D50) in a particle size distribution. The average particle size (D50) of the second particles PTC2 may be a value measured using a particle size analyzer. Accordingly, the average particle size (D50) of the second particles PTC2 may be 3 μm to 6 μm.

[0085] The size of the second primary particles MMP of the second particles PTC2 may be larger than the size of the first primary particles NNP of the first particles PTC1 described with reference to FIG. 6b.

[0086] As an example, the second particles PTC2 may include a second coating layer on their surfaces. The coating layer may cover the entire surfaces of the second particles PTC2 or may cover only a portion of the surfaces of the second particles PTC2. The coating layer may include at least one selected from the group consisting of a cobalt-containing compound, a titanium-containing compound, and an aluminum-containing compound.

[0087] 6a and 6b, the cathode active material according to the embodiment of the present invention will be described in more detail. The cathode active material according to the present invention may include first particles PTC1 and second particles PTC2.

[0088] The first particles PTC1 have an olivine structure and are highly stable and chemically stable. The compound LiCoO2 that makes up the second particles PTC2 has a layered structure, which provides high electrical conductivity and a fast lithium ion diffusion rate. Furthermore, since it can be synthesized as relatively large single-crystal particles measuring several to several tens of micrometers, it has the advantage of providing a large amount of energy per unit volume.

[0089] However, if the upper limit voltage is increased to further increase the capacity of LiCoO2, the compound that makes up the second particles PTC2, a phase transition may occur, deteriorating the lifespan characteristics. Also, the elution of cobalt (Co) during high-voltage charging or the local formation of a cobalt oxide (CoO2) layer on the surface of the electrode may also cause a decrease in lifespan.

[0090] The present invention is characterized by a positive electrode active material that contains both the first particles PTC1 and the second particles PTC2.

[0091] Various embodiments of the present invention provide a cathode active material that combines second particles PTC2 with a layered structure and first particles PTC1 with an olivine structure, thereby improving energy density and minimizing surface reactivity to provide high voltage stability.

[0092] The second particles PTC2 may be surface-treated to be stable under high voltage conditions, and the second particles PTC2 may be surface-coated with Al, Ti, etc. to increase the surface stability.

[0093] The second particles PTC2 may be doped with a metal element to ensure surface and structural stability, and the metal element may be one or more elements selected from Mg, K, Na, Ca, Si, Ti, Zr, Sn, Y, Sr, Mo, and Mn.

[0094] The second particles PTC2 may be monoparticulate or monocrystalline. The second particles PCT2 are a high-voltage material and may achieve a higher capacity than the first particles PCT1. The cathode active material according to this embodiment may have improved capacity and operating voltage compared to a typical LFP battery by mixing the first particles PTC1 and the second particles PTC2 in an appropriate ratio.

[0095] In the present invention, the electrical conductivity and energy density can be improved by using the second particles PTC2 as single particles.

[0096] The cathode active material of the present invention can improve the composite density, capacity, and energy density by mixing the second particles PTC2, each having a size of several microns, with the first particles PTC1, each having a size of several microns. In one embodiment, the composite density of the cathode active material of the present invention can be 2.5 g / cc to 3.5 g / cc. The low-temperature characteristics of a lithium secondary battery including the cathode active material of the present invention can be improved.

[0097] When the first particles PTC1 are single particles (or primary particles), the size of the first primary particles NNP is very small, so a large amount of binder BND may be required to attach the first particles PTC1 to the current collector COL1 (see FIG. 1). By additionally including second particles PTC2 with a larger average particle size, the positive electrode active material layer AML1 can be smoothly attached to the current collector COL1.

[0098] The first particles PTC1 are formed in a secondary particle state, which improves the electrode plate binding strength and allows for a reduction in the amount of binder BND. When the first particles PTC1 are formed in a secondary particle state, the carbon coating is well maintained, which increases conductivity and improves low-temperature characteristics. As a result, the capacity and life characteristics can be increased.

[0099] The mixture ratio of the first particles PTC1 and the second particles PTC2 in the positive electrode active material may be 95:5 to 60:40, or 90:10 to 70:30. The content of the first particles PTC1 in the positive electrode active material may be greater than the content of the second particles PTC2.

[0100] Comparative Example 1-1: Production of single particle-shaped first particles Mn 0.6 Fe 0.4 PO4 manganese iron phosphate precursor, lithium carbonate, and titanium oxide were mixed in a molar ratio of 1:1.03:0.004. 10 wt% glucose was added to the mixture. The mixture was wet-pulverized using a ball mill. The mixture was evaporated to dryness in a heating oven tray and then placed in a vacuum oven at 120°C for 4 hours. The dried mixture was calcined at 750°C for 10 hours under a nitrogen atmosphere. The calcined product was pulverized to obtain single-particle primary particles. The average size of the primary particles PTC1 was approximately 200nm to 300nm.

[0101] Comparative Example 1-2: Production of primary particles in the form of secondary particles Mn 0.6 Fe 0.4 PO4 manganese iron phosphate precursor, lithium carbonate, and titanium oxide were mixed in a molar ratio of 1:1.03:0.004. 10 wt% glucose was added to the mixture. The slurry mixture was spray-dried at a spray pressure of 0.5 MPa and a temperature of 230°C, followed by evaporation. The dried mixture was calcined at 750°C for 10 hours in a nitrogen atmosphere to obtain primary particles in the form of secondary particles. The average size of the primary particles within the primary particles was approximately 100 nm to 200 nm.

[0102] Comparative Example 2-1: Production of single particle-shaped second particles Lithium carbonate, Co3O4 (D50: 4.5 μm), and aluminum hydroxide Al(OH)3 were mixed so that the molar ratio of Li:Co:Al was 1.04:0.995:0.005 to obtain a first mixture. The first mixture was heated to 1088°C at a heating rate of 6°C / min, and then subjected to a primary heat treatment at this temperature in an air atmosphere for 15 hours and crushed to obtain Li 1.04 Co 0.98 Al 0.020 Small particles with an average particle size (D50) of approximately 4 μm and a layered structure of O2 were produced, where the molar ratio of lithium to transition metals (Li / Me) was 1.040. The transition metals were cobalt and aluminum.

[0103] Then, titanium oxide and cobalt hydroxide (Co(OH)2) were added, and a secondary heat treatment was carried out at about 900°C to obtain a single crystal positive electrode active material.

[0104] Comparative Example 2-2: Production of single particle-shaped second particles The doping amount of Al was increased in the second particles of Comparative Example 2-1.

[0105] Comparative Example 2-3: Production of single particle-shaped second particles The amount of Ti coating was increased in the second particles of Comparative Example 2-2.

[0106] Example 1-1: Preparation of a mixture of first particles and second particles The first particles of Comparative Example 1-2 and the second particles of Comparative Example 2 were mixed in a mass ratio of 95:5 to prepare a positive electrode active material.

[0107] Example 1-2: Preparation of a mixture of first particles and second particles The first particles of Comparative Example 1-2 and the second particles of Comparative Example 2 were mixed in a mass ratio of 90:10 to prepare a positive electrode active material.

[0108] Examples 1-3: Preparation of a mixture of first particles and second particles The first particles of Comparative Example 1-2 and the second particles of Comparative Example 2 were mixed in a mass ratio of 85:15 to prepare a positive electrode active material.

[0109] Examples 1-4: Preparation of a mixture of first particles and second particles The first particles of Comparative Example 1-2 and the second particles of Comparative Example 2 were mixed in a mass ratio of 80:20 to prepare a positive electrode active material.

[0110] Examples 1-5: Preparation of a mixture of first particles and second particles The first particles of Comparative Example 1-2 and the second particles of Comparative Example 2 were mixed in a mass ratio of 70:30 to prepare a positive electrode active material.

[0111] Examples 1-6: Preparation of a mixture of first particles and second particles The first particles of Comparative Example 1-2 and the second particles of Comparative Example 2 were mixed in a mass ratio of 60:40 to prepare a positive electrode active material.

[0112] The produced positive electrode active materials are summarized in Table 1 below.

[0113] [Table 1]

[0114] Cathode manufacturing 95% by weight of the final positive electrode active material, 3% by weight of polyvinylidene fluoride binder, and 2% by weight of carbon black conductive material were mixed in N-methylpyrrolidone solvent to prepare a positive electrode active material slurry. The positive electrode active material slurry was applied to an aluminum current collector, dried, and then rolled to prepare a positive electrode.

[0115] Anode manufacturing Graphite, a binder, and a conductive material were mixed in N-methylpyrrolidone solvent to prepare a negative electrode active material slurry, which was then coated on a copper current collector, dried, and rolled to prepare a negative electrode.

[0116] Lithium secondary battery manufacturing A 2032-type coin half-cell was fabricated using the prepared positive electrode and a lithium metal counter electrode as the counter electrode. A separator (approximately 16 μm thick) made of porous polyethylene (PE) film was placed between the positive electrode and the lithium metal counter electrode, and an electrolyte was injected to fabricate a lithium secondary battery. The electrolyte used was a mixture of 1.3 M LiPF6 with a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethylene carbonate (DMC) in a volume ratio of 3:4:3.

[0117] Evaluation example 1: Analysis of the surface of the positive electrode active material An SEM image of the first particles prepared in Comparative Example 1-2 is shown in Figure 7a. An SEM image of the second particles prepared in Comparative Example 2-1 is shown in Figure 7b. Referring to Figure 7a, it can be seen that the first particles according to the present invention are spherical, assembled particles of several micrometers in size. Referring to Figure 7b, it can be seen that the second particles according to the present invention are single particles of several micrometers in size. It can be seen that the second particles are either single crystals or have a morphology in which multiple single particles are attached to each other.

[0118] Evaluation example 2: Evaluation of battery characteristics The characteristics of secondary batteries fabricated using the positive electrode active materials of the above Examples and Comparative Examples were evaluated.

[0119] The lithium secondary battery was initially charged at a constant current (0.2C) and a constant voltage (4.25V), rested for 10 minutes, and then discharged to 2.5V at a constant current (0.1C). It was then charged and discharged 50 times at 0.2C / 0.2C. A new coin cell was also fabricated to evaluate the -20°C capacity, and it was charged and discharged once at a constant current (0.2C) and a constant voltage (4.25V). The 0.2C capacity of the charged cell was measured at 20°C. The battery characteristics evaluation results are shown in Table 2 below.

[0120] [Table 2]

[0121] Referring to Table 2, it can be seen that the secondary batteries according to Examples 1-1 to 1-6 of the present invention have improved average voltages and capacities compared to the secondary batteries according to Comparative Examples 1-1 and 1-2. It can be seen that the secondary batteries according to Examples 1-1 to 1-6 of the present invention have improved −20°C capacities compared to the secondary batteries according to Comparative Examples 2-1 to 2-3. It can also be seen that the secondary batteries according to Examples 1-1 to 1-4 have significantly better −20°C capacities compared to the secondary batteries according to Examples 1-5 and 1-6.

[0122] Evaluation Example 3: Evaluation of active materials The pellet density (PD) of the positive electrode active materials of Examples 1-1, 1-2, 1-3, and 1-4 and Comparative Examples 1-1 and 1-2 was measured, and the results are shown in Table 3.

[0123] [Table 3]

[0124] Referring to Table 3, it can be seen that the positive electrode active materials according to Examples 1-1 to 1-6 of the present invention have a larger average mixture density than the positive electrode active materials according to Comparative Examples 1-1 and 1-2.

[0125] Evaluation example 4: Evaluation of battery characteristics of single-plate cells A single-plate cell was fabricated using a graphite anode. Table 4 shows the battery performance of the single-plate cell.

[0126] [Table 4]

[0127] Energy density can be evaluated by positive electrode utilization or volumetric capacity. Positive electrode utilization can be defined as the capacity of a fabricated secondary battery per mass of positive electrode active material. Volumetric capacity can be defined as the capacity of a fabricated secondary battery per volume of active material.

[0128] Referring to Table 4, it can be seen that the energy density of the batteries including the positive electrode active materials of Examples 1-1 to 1-6 is improved compared to the batteries including the positive electrode active materials of Comparative Examples 1-1 to 1-3.

[0129] The above is a specific embodiment for carrying out the present invention. The present invention includes not only the above-described embodiment but also embodiments that can be easily modified or changed. The present invention also includes techniques that can be easily implemented by modifying the embodiments. Therefore, the scope of the present invention should not be limited to the above-described embodiment, but should be determined not only by the claims below but also by equivalents to the claims of the present invention.

Claims

1. First particles comprising a compound represented by the following Chemical Formula 1 and having a first average particle size: and second particles having a second average particle size, the second particles comprising a compound represented by Chemical Formula 2: [Chemical formula 1] Li a1 Mn z1 Fe x1 A y1 2O 4-c1 (In Chemical Formula 1, 0.8<a1≦1.2, 0.3≦x1≦0.7, 0≦y1≦0.05, 0.3≦z1≦0.7, 0<c1≦0.05, and x1+y1+z1=1, A is at least one element selected from the group consisting of Ti, Mg, V, and Nb). [Chemical formula 2] Li a2 Co z2 D x2 O c2 (In Chemical Formula 2, 1<a2≦1.5, 0.9≦z2≦1.0, 0≦x2≦0.05, and 2<c2≦2.3, and D is at least one element selected from the group consisting of Al, Ti, and Mg. The positive electrode active material has a content of the first particles greater than a content of the second particles.

2. The positive electrode active material of claim 1 , wherein the first particles account for 60 to 95 wt % of the total weight of the first particles and the second particles.

3. the first particles include at least one first primary particle; The positive electrode active material of claim 1 , wherein the at least one first primary particle has a size of 500 nm to 200 nm.

4. The positive electrode active material according to claim 3 , wherein the size of the at least one first primary particle is smaller than the second average particle size.

5. the first particles include a first coating layer comprising carbon; 2. The positive electrode active material of claim 1, wherein the carbon content in the first particles is 1.5 to 2.5 wt%.

6. the second particles include a second coating layer; The positive electrode active material of claim 1 , wherein the second coating layer comprises Ti, Al, or a combination thereof.

7. the second particles have a single particle shape; The positive electrode active material of claim 1 , wherein the second average particle size is 3 μm to 6 μm.

8. The positive electrode active material of claim 1 , wherein the second particles have a Ti content lower than that of the first particles.

9. The D contains Ti and Mg, The positive electrode active material of claim 1 , wherein the content of Ti in the second particles is less than the content of Mg in the second particles.

10. the first particles are in the form of secondary particles, The positive electrode active material of claim 1 , wherein the first average particle size is 3 μm to 10 μm.

11. The positive electrode active material of claim 10 , wherein the first particles have a porosity of 20% to 40%.

12. The positive electrode active material of claim 10 , wherein the first particles have a Span value of 0.3 to 0.75 as analyzed by a particle size analyzer.

13. The positive electrode active material according to claim 10 , wherein the first average particle size is larger than the second average particle size.

14. the first particles have a single particle shape; The positive electrode active material of claim 1 , wherein the first average particle size is 0.5 μm to 2.5 μm.

15. a positive electrode current collector; a positive electrode active material layer on the positive electrode current collector, The positive electrode for a lithium secondary battery, wherein the positive electrode active material layer comprises the positive electrode active material according to claim 1 , a conductive material, and a binder.

16. 16. The positive electrode for a lithium secondary battery according to claim 15, wherein the content of the binder is 0.5 to 5 parts by weight based on 100 parts by weight of the positive electrode active material layer.

17. 16. The positive electrode for a lithium secondary battery according to claim 15, wherein the binder comprises at least one selected from the group consisting of polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, (meth)acrylate styrene-butadiene rubber, epoxy resin, (meth)acrylic resin, polyester resin, and nylon.

18. 16. The positive electrode for a lithium secondary battery according to claim 15, wherein the conductive material is present in an amount of 0.5 to 5 parts by weight based on 100 parts by weight of the positive electrode active material layer.

19. 16. The positive electrode for a lithium secondary battery according to claim 15, wherein the conductive material comprises a carbon-based material such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, or carbon nanotube; a metal-based material in the form of a metal powder or metal fiber containing copper, nickel, aluminum, silver, or the like; a conductive polymer such as a polyphenylene derivative; or a mixture thereof.

20. The positive electrode according to claim 15 ; a negative electrode including a negative electrode current collector and a negative electrode active material layer on the negative electrode current collector; a separator between the positive electrode and the negative electrode.

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