Positive electrode and lithium secondary battery including the same

Abstract

The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the positive electrode, wherein the positive electrode has improved capacity, efficiency, lifespan, output properties, and thermal stability. The positive electrode for the lithium secondary battery includes a positive electrode current collector, and a first positive electrode active material layer and a second positive electrode active material layer sequentially stacked on the positive electrode current collector, wherein the first positive electrode active material layer and the second positive electrode active material layer include a bimodal positive electrode active material, the first positive electrode active material layer includes small-diameter particles in the form of single particles, and the second positive electrode active material layer includes small-diameter particles in the form of secondary particles.

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Korean Patent Application No. 10-2021-0012872, filed on January 29, 2021, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0004] The present invention relates to a positive electrode including a plurality of positive electrode active material layers and a lithium secondary battery including the positive electrode. Background Art

[0005] With the development of technology and the increasing demand for mobile devices, the demand for secondary batteries as energy sources is also rapidly increasing. Among these secondary batteries, lithium secondary batteries with high energy density and voltage, long cycle life and low self-discharge rate have been commercialized and widely used.

[0006] Lithium transition metal composite oxides are used as positive electrode active materials for lithium secondary batteries. Among these, lithium cobalt composite oxides, such as LiCoO2, are the most commonly used, due to their high functional voltage and excellent capacity characteristics. However, LiCoO2 exhibits unstable crystal structures after delithiation, resulting in very poor thermal properties and high cost. Therefore, LiCoO2 has limited widespread use as a power source in applications such as electric vehicles.

[0007] As a material to replace LiCoO2, lithium manganese composite metal oxides (LiMnO2, LiMn2O4, etc.), lithium iron phosphate compounds (LiFePO4, etc.) or lithium nickel composite metal oxides (LiNiO2, etc.) have been developed. Among these materials, lithium nickel composite metal oxides are more actively researched and developed. They have a high reversible capacity of about 200 mAh / g and can easily realize high-capacity batteries. However, compared with LiCoO2, LiNiO2 has poor thermal stability. In addition, LiNiO2 has the following problem, that is, when an internal short circuit occurs due to external pressure, etc. in the charging state, the positive electrode active material itself will decompose, causing the battery to rupture and burn.

[0008] Therefore, as a method to improve the lower thermal stability of LiNiO2 while maintaining its excellent reversible capacity, nickel-cobalt-manganese-based lithium composite transition metal oxides in which part of Ni is replaced by Co and Mn, nickel-cobalt-aluminum-based lithium composite transition metal oxides in which part of Ni is replaced by Co and Al, nickel-cobalt-manganese-aluminum-based composite transition metal oxides in which part of Ni is replaced by Co, Mn and Al, etc. have been developed.

[0009] At the same time, when high-capacity lithium transition metal oxides are used, the oxidation stability at high voltage will be reduced, resulting in poor stability, and a large number of cracks will be generated between the particles during long-term charge and discharge processes, which will disconnect the conductive path and lead to increased cycle resistance.

[0010] Therefore, there is a need to develop a positive electrode for lithium secondary batteries with improved capacity, efficiency, lifespan, output properties, and thermal stability. Summary of the Invention

[0011] Technical issues

[0012] One aspect of the present invention provides a positive electrode for a lithium secondary battery having improved capacity, efficiency, lifespan, output properties, and thermal stability.

[0013] Technical Solution

[0014] According to one aspect of the present invention, a positive electrode for a lithium secondary battery is provided, which includes a positive electrode collector, and a first positive electrode active material layer and a second positive electrode active material layer stacked in sequence on the positive electrode collector, wherein the first positive electrode active material layer and the second positive electrode active material layer include bimodal positive electrode active materials, the first positive electrode active material layer includes small-diameter particles in the form of single particles, and the second positive electrode active material layer includes small-diameter particles in the form of secondary particles.

[0015] According to another aspect of the present invention, a lithium secondary battery including the positive electrode is provided.

[0016] Technical Effects

[0017] The positive electrode for a lithium secondary battery of the present invention includes a double-layer positive electrode active material layer that meets specific conditions, thereby improving capacity, efficiency, lifespan, output properties and thermal stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a graph showing the capacity retention properties of half-cells manufactured using the positive electrode active materials prepared in each of Examples and Comparative Examples; and

[0019] Figure 2 is a graph showing the resistance increase properties of half cells manufactured using the positive electrode active materials prepared in each of Examples and Comparative Examples. DETAILED DESCRIPTION

[0020] It should be understood that the words or terms used in the specification and claims should not be interpreted as having the meanings defined in commonly used dictionaries. It will be further understood that based on the inventors' ability to appropriately define the meanings of these words or terms to preferably explain the principles of the present invention, these words or terms should be interpreted as having meanings consistent with their meanings in the context of the relevant technology and the technical concept of the present invention.

[0021] In this specification, it should be understood that the terms "include", "comprising" or "having" are intended to illustrate the existence of the stated features, quantities, steps, elements or combinations thereof, but do not exclude the existence or addition of one or more other features, quantities, steps, elements or combinations thereof.

[0022] In this specification, the average particle diameter (D 50 )” can be defined as the particle diameter corresponding to 50% of the volume accumulation in the particle diameter distribution curve. The average particle diameter (D 50 ) can be measured by, for example, laser diffraction. For example, the average particle diameter (D 50 ) can be measured by the following method: particles of the positive electrode active material are dispersed in a dispersion medium, which is then introduced into a commercially available laser diffraction particle size measuring device (e.g., LA-960 from HORIBA Corporation) and irradiated with ultrasonic waves of about 28 kHz to an output power of 60 W. Thereafter, the average particle diameter (D) corresponding to 50% volume accumulation in the measuring device can be calculated. 50 ).

[0023] In this specification, the term "single-particle positive electrode active material" refers to a positive electrode active material consisting of 10 or fewer primary particles, as opposed to a spherical secondary particle-shaped positive electrode active material produced by conventional methods, which is composed of tens to hundreds of primary particles aggregated. Specifically, in the present invention, a single-particle positive electrode active material may be a single particle consisting of a single primary particle or a secondary particle composed of several primary particles aggregated.

[0024] In this specification, “primary particle” refers to the smallest particle unit recognized when observing a positive electrode active material with a scanning electron microscope (SEM), and “secondary particle” refers to a secondary structure formed by aggregation of a plurality of primary particles.

[0025] In this specification, "particles" refer to particles on the micrometer scale. When magnified and observed, they can be broken down into "fine particles" with a crystal structure of several dozen nanometers. Further magnification and observation of the particles reveals subdivided regions where atoms form a lattice structure in a certain direction. These are called "grains." The particle size observed in XRD is defined as the grain size. Grain size can be quantitatively determined using the Scherrer equation from XRD data.

[0026] In this specification, overfiring means that when preparing a positive electrode active material, the firing temperature is about 50°C to 100°C higher than the generally suitable firing temperature. For example, when preparing a lithium composite transition metal oxide having a Ni:Co:Mn molar ratio of 80:10:10 as a positive electrode active material, the generally known suitable firing temperature is 750°C to 900°C, but overfiring means firing at a temperature about 50°C to 100°C higher than this suitable firing temperature.

[0027] Hereinafter, the present invention will be described in more detail.

[0028] positive electrode

[0029] The present inventors have found that when the positive electrode includes a positive electrode active material layer consisting of two layers, wherein the first positive electrode active material layer formed on the positive electrode current collector includes particles having different average particle diameters (D 50 ) in the form of secondary particles and the positive electrode active material in the form of single particles, and the second positive electrode active material layer formed on the first positive electrode active material layer includes a positive electrode active material in the form of secondary particles and having different average particle diameters (D 50 ) when two positive electrode active materials are used, a lithium secondary battery with excellent capacity, efficiency, life, output properties and thermal stability can be achieved, thereby completing the present invention.

[0030] The positive electrode of the present invention includes a positive electrode current collector, and a first positive electrode active material layer and a second positive electrode active material layer stacked in sequence on the positive electrode current collector. The first positive electrode active material layer and the second positive electrode active material layer may include a bimodal positive electrode active material. That is, the first positive electrode active material layer may include large diameter particles (first positive electrode active material) and small diameter particles (second positive electrode active material), and the second positive electrode active material layer may include large diameter particles (third positive electrode active material) and small diameter particles (fourth positive electrode active material).

[0031] The first positive electrode active material layer may include the first positive electrode active material (large diameter particle) in the form of secondary particles formed by aggregation of a plurality of primary particles and the second positive electrode active material (small diameter particle) as a single particle.

[0032] The second positive electrode active material layer may include a third positive electrode active material (large diameter particles) in the form of secondary particles formed by aggregating a plurality of primary particles, and a fourth positive electrode active material (small diameter particles) in the form of secondary particles formed by aggregating a plurality of primary particles.

[0033] That is, the present invention provides a positive electrode for a lithium secondary battery, wherein the average particle diameter (D 50 ) is larger than the average particle diameter (D 50), the average particle diameter of the third positive electrode active material (D 50 ) is larger than the average particle diameter (D 50 ). The first positive electrode active material may be the same as the third positive electrode active material.

[0034] When the first positive electrode active material layer is present on the positive electrode current collector, and the second positive electrode active material layer is present on the first positive electrode active material layer, the presence of the buffer layer slows heat transfer, thereby improving thermal stability compared to the case where only the first or second positive electrode active material layer is present on the positive electrode current collector. Furthermore, by appropriately adjusting the thickness ratio of the first and second positive electrode active material layers, as well as the proportions of the active material, conductive material, and binder included in each layer, the resistance and output of a battery including this positive electrode can be further improved. However, when the second positive electrode active material layer is present on the positive electrode current collector, and the first positive electrode active material layer is present on the second positive electrode active material layer, the positive electrode active material particles on the surface of the electrode are easily cracked during rolling, resulting in disadvantages in terms of battery life and resistance.

[0035] There are no particular limitations on the positive electrode current collector, as long as it is conductive and does not cause chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. can be used. In addition, the thickness of the positive electrode current collector is generally 3μm to 500μm, and fine concavo-convex structures can be formed on the surface of the current collector to improve the adhesion of the positive electrode active material. For example, the positive electrode current collector can take various forms, such as a film, sheet, foil, mesh, porous body, foam, non-woven fabric, etc.

[0036] According to the present invention, the thickness ratio of the first positive electrode active material layer to the second positive electrode active material layer may be 3:7 to 7:3, particularly 5:5 to 6:4. When the thickness ratio of the first positive electrode active material layer to the second positive electrode active material layer is within the above range, cracking of the positive electrode active material particles on the positive electrode surface can be prevented, thereby improving electrical resistance and thermal stability.

[0037] According to the present invention, the first positive electrode active material, the second positive electrode active material, the third positive electrode active material, and the fourth positive electrode active material may each independently have a composition represented by Formula 1 below.

[0038] [Formula 1]

[0039] Li x [Ni a Co b M 1 c M 2 d ]O2

[0040] In Formula 1 above, M 1 is one or more selected from Mn and Al, and M 2 is one or more selected from B, Mg, Ca, Ti, V, Cr, Fe, Zn, Ga, Y, Zr, Nb, Mo, Ta, and W, and 0.9 ≤ x ≤ 1.1, 0.7 ≤ a < 1, 0 < b < 0.3, 0 < c < 0.3, 0 ≤ d ≤ 0.1, and a + b + c + d = 1.

[0041] a represents the atomic fraction of nickel among the metal elements other than lithium in the active material, and a can satisfy 0.7 ≤ a < 1, 0.8 ≤ a < 1, or 0.85 ≤ a ≤ 0.95.

[0042] b represents the atomic fraction of cobalt among the metal elements other than lithium in the active material, and b can satisfy 0 < b < 0.3, 0 < b < 0.2, 0.01 ≤ b < 0.2, or 0.02 ≤ b ≤ 0.1.

[0043] c represents the atomic fraction of M 1 element among the metal elements other than lithium in the active material, and c can satisfy 0 < c < 0.3, 0 < c < 0.2, 0.01 ≤ c < 0.2, or 0.02 ≤ c ≤ 0.1.

[0044] d represents the atomic fraction of M 2 element among the metal elements other than lithium in the active material, and d can satisfy 0 ≤ d ≤ 0.1 or 0 ≤ d ≤ 0.05.

[0045] According to the present invention, the average particle diameter (D 50 ) of the first positive electrode active material can be 5 μm to 20 μm, particularly 8 μm to 18 μm, more particularly 10 μm to 16 μm, while the average particle diameter (D 50 ) of the second positive electrode active material can be 3 μm to 10 μm, particularly 4 μm to 10 μm, more particularly 4 μm to 8 μm. When the average particle diameter (D 50 ) of the first positive electrode active material and the average particle diameter (D 50 ) of the second positive electrode active material simultaneously satisfy the above ranges, there are advantages that the energy density of the positive electrode is large, and the accumulation between the positive electrode active materials is promoted during the rolling process.

[0046] According to the present invention, the average particle diameter (D 50 ) of the third positive electrode active material can be 5 μm to 20 μm, particularly 8 μm to 18 μm, more particularly 10 μm to 16 μm, while the average particle diameter (D 50) may be 3 μm to 10 μm, particularly 4 μm to 10 μm, and more particularly 4 μm to 8 μm. When the average particle diameter (D 50 ) and the average particle diameter of the fourth positive electrode active material (D 50 ) When the above ranges are satisfied at the same time, there are advantages in that the energy density of the positive electrode is large and the stacking of the positive electrode active materials is promoted during the rolling process.

[0047] The first positive electrode active material may have a grain size of 100 nm to 150 nm, the second positive electrode active material may have a grain size of 200 nm to 250 nm, the third positive electrode active material may have a grain size of 100 nm to 150 nm, and the fourth positive electrode active material may have a grain size of 70 nm to 100 nm.

[0048] When the grain sizes of the first and third positive electrode active materials are within the above ranges, there may be fewer cracks in the positive electrode active material particles, and when the grain size of the second positive electrode active material is within the above range, not only may there be fewer cracks in the positive electrode active material particles, but the BET specific surface area is also small, so that side reactions rarely occur.

[0049] The BET specific surface area of the first positive electrode active material may be 0.3 m 2 / g to 0.7m 2 / g, the BET specific surface area of the second positive electrode active material can be 0.2 m 2 / g to 0.4m 2 / g. In addition, the BET specific surface area of the third positive electrode active material may be 0.3 m 2 / g to 0.7m 2 / g, the BET specific surface area of the fourth positive electrode active material may be 0.7 m 2 / g to 1.2m 2 / g.

[0050] The BET specific surface area can be measured by nitrogen adsorption at liquid nitrogen temperature (77K) using BELSORP-mini II (Mictrotrac-BEL), and when the BET specific surface area of each positive electrode active material is within the above range, side reactions between the positive electrode active material and the electrolyte can be prevented.

[0051] According to the present invention, the first positive electrode active material layer may include the first positive electrode active material and the second positive electrode active material in a weight ratio of 85:15 to 55:45, specifically 85:15 to 60:40, and more specifically 80:20 to 70:30. In this case, the capacity characteristics and rolling density characteristics of the battery may be advantageous.

[0052] The sum of the content of the first positive electrode active material and the content of the second positive electrode active material may be 80 wt % to 99 wt %, more particularly 85 wt % to 98 wt %, based on the total weight of the first positive electrode active material layer. When included within the above content range, excellent capacity characteristics may be exhibited.

[0053] According to the present invention, the second positive electrode active material layer may include the third positive electrode active material and the fourth positive electrode active material in a weight ratio of 85:15 to 55:45, specifically 85:15 to 60:40, and more specifically 80:20 to 70:30. In this case, the capacity characteristics and rolling density characteristics of the battery may be advantageous.

[0054] The sum of the content of the third positive electrode active material and the content of the fourth positive electrode active material may be 80 wt % to 99 wt %, more particularly 85 wt % to 98 wt %, based on the total weight of the second positive electrode active material layer. When included within the above content range, excellent capacity characteristics may be exhibited.

[0055] The first positive electrode active material layer and the second positive electrode active material layer may each independently include a conductive material and a binder, as well as a positive electrode active material.

[0056] Conductive materials are used to impart conductivity to the electrode. According to the present invention, the first positive electrode active material layer may further include a dot-shaped conductive material, and the second positive electrode active material layer may further include a dot-shaped conductive material and a linear conductive material. Specific examples of dot-shaped conductive materials include furnace black, acetylene black, and lamp black, while examples of linear conductive materials include low-BET CNTs and SWCNTs. When the second positive electrode active material layer further includes a dot-shaped conductive material and a linear conductive material, the conductive network between the positive electrode active material particles is improved, thereby enhancing electronic conductivity. This facilitates the movement of lithium ions, which can improve output and battery life.

[0057] The conductive material may be included in an amount of 1 wt % to 30 wt % based on the total weight of each of the first and second positive electrode active material layers.

[0058] The role of the binder is to improve the bonding between the positive active material particles and the adhesion between the positive active material and the current collector. Specific examples thereof may include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber or its various copolymers, and any one or a mixture of two or more thereof can be used. Based on the total weight of the positive active material layer of each of the first positive active material layer and the second positive active material layer, the content of the binder may be 1% by weight to 30% by weight.

[0059] The positive electrode can be manufactured according to a conventional method for manufacturing a positive electrode. Specifically, the positive electrode can be manufactured by coating a composition for forming a positive electrode active material layer on a positive electrode current collector, followed by drying and roll pressing. The composition is prepared by dissolving or dispersing a positive electrode active material and optionally a binder and a conductive material in a solvent. In this case, the types and contents of the positive electrode active material, binder, and conductive material are as described above.

[0060] The solvent may be a commonly used solvent in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, water, or the like. Any one thereof or a mixture of two or more thereof may be used. If the solvent can dissolve and disperse the positive electrode active material, the binder, and the conductive material, and has a viscosity that can exhibit excellent thickness uniformity in the coating for manufacturing the positive electrode, taking into account the coating thickness and production yield of the slurry, then the amount of solvent used is sufficient.

[0061] Furthermore, in another method, the positive electrode may be manufactured by casting a composition for forming a positive electrode active material layer on a separate support and then laminating the film obtained by peeling off the support on a positive electrode current collector.

[0062] lithium secondary batteries

[0063] Furthermore, the present invention can provide a lithium secondary battery including the positive electrode.

[0064] Specifically, a lithium secondary battery includes a positive electrode, a negative electrode facing the positive electrode, a separator interposed between the positive and negative electrodes, and an electrolyte. The positive electrode is the same as described above, so a detailed description thereof will be omitted. Below, only the remaining components will be described in detail.

[0065] In addition, the lithium secondary battery may optionally further include a battery case for accommodating an electrode assembly consisting of a positive electrode, a negative electrode, and a separator, and a sealing member for sealing the battery case.

[0066] In a 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.

[0067] 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 surface-treated with one of carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys can be used. Furthermore, the thickness of the negative electrode current collector is generally 3 μm to 500 μm, and as with the positive electrode current collector, fine irregularities may be formed on the surface of the negative electrode current collector to improve the adhesion of the negative electrode active material. For example, the negative electrode current collector can take various forms, such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabrics.

[0068] The negative electrode active material layer selectively includes a binder and a conductive material in addition to the negative electrode active material.

[0069] As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium can be used. Specific examples of the negative electrode active material may include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; metal materials that can be alloyed with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; metal oxides that can be doped and dedoped with lithium, such as SiO β (0<β<2), SnO2, vanadium oxide and lithium vanadium oxide; or a composite material including a metal material and a carbonaceous material, such as a Si-C composite material or a Sn-C composite material, and any one of them or a mixture of two or more thereof can be used. In addition, a metallic lithium film can also be used as a negative electrode active material. In addition, low-crystalline carbon and high-crystalline carbon, etc. can be used as carbon materials. Representative examples of low-crystalline carbon may include soft carbon and hard carbon, and representative examples of high-crystalline carbon may include irregular, planar, flaky, spherical or fibrous natural graphite or artificial graphite, Kish graphite, pyrolytic carbon, medium-phase pitch-based carbon fibers, medium-phase carbon microbeads, medium-phase pitch and high-temperature sintered carbon, such as coke derived from petroleum or coal tar pitch.

[0070] The negative electrode active material layer may include an amount of 80 wt % to 99 wt % based on the total weight of the negative electrode active material layer.

[0071] The binder is a component used to help the conductive material, active material and current collector bond together, and can generally be added in an amount of 0.1% to 10% by weight based on the total weight of the negative electrode active material layer. Examples of binders may include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluororubber, various copolymers thereof, and the like.

[0072] The conductive material is a component used to further improve the conductivity of the negative electrode active material and can be added in an amount of 10% by weight or less, preferably 5% by weight or less, based on the total weight of the negative electrode active material layer. The conductive material is not particularly limited as long as it has conductivity and does not cause chemical changes in the battery. For example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as fluorocarbon powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives, etc. can be used.

[0073] The negative electrode active material layer can be prepared by applying a negative electrode mixed material on the negative electrode current collector and then drying it. The negative electrode mixed material is prepared by dissolving or dispersing the negative electrode active material and optionally a binder and a conductive material in a solvent. Alternatively, the negative electrode active material layer can be prepared by casting the negative electrode mixed material on a separate support and then laminating the film peeled from the support on the negative electrode current collector.

[0074] Meanwhile, in lithium secondary batteries, the diaphragm is used to separate the negative electrode and the positive electrode and to provide a path for lithium ions to move. Any diaphragm can be used without particular limitation, as long as it is a diaphragm commonly used in lithium secondary batteries. In particular, it is preferred that the diaphragm has a high moisture retention capacity for the electrolyte and a low resistance to the movement of electrolyte ions. Specifically, a porous polymer film can be used, for example, 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 and an ethylene / methacrylate copolymer, or a laminated structure having two or more layers thereof. In addition, a common porous non-woven fabric can also be used, for example, a non-woven fabric formed of glass fiber or polyethylene terephthalate fiber with a high melting point. In addition, a coated diaphragm including a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and a single layer or multilayer structure can be selectively used.

[0075] In addition, the electrolyte used in the present invention can be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc., which can be used to manufacture lithium secondary batteries, but are not limited thereto.

[0076] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0077] Any organic solvent can be used without particular limitation, as long as it can serve as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, as an organic solvent, ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone and ε-caprolactone can be used; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC) and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (wherein R is a linear, branched or cyclic C2 to C20 hydrocarbon group, and may include a double bond aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolane. Among these solvents, carbonate solvents are preferred, and more preferably a mixture of a cyclic carbonate (such as ethylene carbonate or propylene carbonate) having high ionic conductivity and a high dielectric constant and a linear carbonate compound (such as ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) having a low viscosity, which can improve the charge / discharge properties of the battery. In this case, when the cyclic carbonate and the linear carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the properties of the electrolyte solution are excellent.

[0078] Any compound can be used as a lithium salt without particular limitation, as long as it can provide lithium ions for lithium secondary batteries. Specifically, as lithium salts, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO2, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. can be used. The concentration of the lithium salt can be 0.1M to 2.0M. When the concentration of the lithium salt is within the above range, the electrolyte has suitable conductivity and viscosity, thereby exhibiting excellent properties, and lithium ions can be effectively moved.

[0079] In order to improve the life properties of the battery, suppress the decrease in battery capacity, and improve the discharge capacity of the battery, the electrolyte may further include one or more additives, for example, halogenated alkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol or aluminum chloride, etc. In this case, the content of the additives may be 0.1 wt % to 5 wt % based on the total weight of the electrolyte.

[0080] Lithium secondary batteries comprising the positive electrode active material of the present invention exhibit excellent capacity, efficiency, life and output properties and are therefore very useful in portable devices such as mobile phones, notebook computers and digital cameras, as well as in the field of electric vehicles such as hybrid electric vehicles (HEVs).

[0081] Therefore, according to the present invention, a battery module including the lithium secondary battery as a unit cell, and a battery pack including the battery module can be provided.

[0082] The battery module or battery pack can serve as a power source for one or more medium-sized and large-sized devices, for example, power tools, electric vehicles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), or power storage systems.

[0083] The external shape of the lithium secondary battery of the present invention is not particularly limited, and may be cylindrical, square, pouch, or coin-shaped.

[0084] The lithium secondary battery of the present invention can be used as a battery cell for a power source of small devices, and can also be preferably used as a unit cell of a medium or large battery module including a plurality of battery cells.

[0085] Below, the embodiment of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention can be implemented in many different forms and is not limited to the embodiment described herein.

[0086] Examples and Comparative Examples

[0087] The particle shape, composition, average particle diameter (D 50 ), grain size and BET specific surface area are shown in Table 1 below.

[0088] [Table 1]

[0089]

[0090] Example 1

[0091] The positive electrode active materials A and B shown in the table above were mixed in a weight ratio of 8:2. The mixed positive electrode active materials A and B, carbon black, and PVdF binder were then mixed in an NMP solvent at a weight ratio of 97.5:1.0:1.5 to prepare a first positive electrode slurry. The first positive electrode slurry was coated on one surface of an aluminum current collector (thickness: 12 μm) and dried at 130°C to form a first positive electrode active material layer on the aluminum current collector.

[0092] A second positive electrode slurry was prepared by mixing the positive electrode active materials C and D shown in the table above at a weight ratio of 8:2. The mixed positive electrode active materials C and D, a conductive material comprising carbon black and SWCNTs at a weight ratio of 90:10, and a PVdF binder at a weight ratio of 97.5:1.0:1.5 were then mixed in an NMP solvent. The second positive electrode slurry was applied onto the first positive electrode active material layer, dried at 130°C, and then roll-pressed to produce a positive electrode.

[0093] At this time, the thickness ratio of the first positive electrode active material layer to the second positive electrode active material layer was 5:5.

[0094] Example 2

[0095] A positive electrode was manufactured in the same manner as in Example 1, except that the coating amounts of the first positive electrode slurry and the second electrode slurry were adjusted so that the thickness ratio of the first positive electrode active material layer to the second positive electrode active material layer was 6:4.

[0096] Example 3

[0097] The positive electrode was manufactured in the same manner as in Example 1, except that the positive electrode active material A and the positive electrode active material B were mixed in a weight ratio of 7:3, and the coating amounts of the first positive electrode slurry and the second positive electrode slurry were adjusted so that the thickness ratio of the first positive electrode active material layer and the second positive electrode active material layer was 6:4.

[0098] Comparative Example 1

[0099] Positive electrode active materials C and D shown in the table above were mixed in a weight ratio of 8:2. The mixed positive electrode active materials C and D, carbon black and SWCNTs in a weight ratio of 90:10, a conductive material, and a PVdF binder in a weight ratio of 97.5:1.0:1.5 were mixed in an NMP solvent to prepare a positive electrode slurry. The positive electrode slurry was coated on one surface of an aluminum current collector (thickness: 12 μm), dried at 130°C, and then roll-pressed to produce a positive electrode.

[0100] Comparative Example 2

[0101] Positive electrode active material A and positive electrode active material B shown in the table above were mixed in a weight ratio of 8:2. The mixed positive electrode active material A and positive electrode active material B, carbon black, and PVdF binder were then mixed in an NMP solvent in a weight ratio of 97.5:1.0:1.5 to prepare a positive electrode slurry. The positive electrode slurry was coated on one surface of an aluminum current collector (thickness: 12 μm), dried at 130°C, and then roll-pressed to produce a positive electrode.

[0102] Comparative Example 3

[0103] A positive electrode was manufactured in the same manner as in Example 1, except that the second positive electrode slurry was coated on the aluminum current collector to form a second positive electrode active material layer, and the first positive electrode slurry was coated on the second positive electrode active material layer to form a first positive electrode active material layer.

[0104] Experimental example

[0105] Experimental Example 1: Evaluation of Half-Cell Characteristics

[0106] The positive electrodes manufactured in Examples 1 to 3 and Comparative Examples 1 to 3 were used to manufacture half cells, and each half cell was evaluated for initial charge and discharge capacity, initial efficiency, life characteristics, resistance characteristics, and output properties.

[0107] First, a separator was inserted between the positive electrode and the Li metal disk negative electrode produced in each of Examples 1 to 3 and Comparative Examples 1 to 3 to produce an electrode assembly. This electrode assembly was then placed in a battery case. An electrolyte solution was then injected into the battery case to produce a lithium secondary battery. A 1M LiPF6 solution dissolved in an organic solvent of EC / EMC (5 / 5, vol%) was used as the electrolyte solution to produce a half-cell.

[0108] The half-cell manufactured as described above was charged at a constant current of 0.2C to a voltage of 4.25V at 25°C, and then discharged at a constant current of 0.2C to a voltage of 3.0V. The initial charge capacity and initial discharge capacity values are shown in Table 2 below, and the ratio of the initial discharge capacity to the initial charge capacity is set as the initial efficiency (@0.2C), as shown in Table 2 below. The initial efficiency (@1.0C) and initial efficiency (@2.0C) were obtained in the same manner as above, except that the C rate was adjusted to 1.0C and 2.0C during the charge and discharge processes, and are shown in Table 2 below.

[0109] [Table 2]

[0110]

[0111] In addition, the charge and discharge cycles were repeated 30 times at a constant current of 0.3C in the range of 2.5V to 4.25V at 45°C to measure the capacity of the half-cell, and in particular, the ratio of the capacity of the Nth cycle to the capacity of the first cycle was set as the capacity retention rate (%), as shown in FIG. Figure 1 In addition, the ratio of the DCIR obtained in the Nth discharge cycle to the DCIR obtained in the first discharge cycle is set as the resistance increase rate ΔDCIR (%), as shown in Figure 2 shown.

[0112] Finally, the half-cells manufactured above were discharged at ΔSOC 30 (SOC 35% to SOC 20%) under -10°C (low temperature) and 25°C (room temperature) conditions, and the changes in voltage values were confirmed by IR drops at 0.4C for 1350 seconds, as shown in Table 3 below.

[0113] [Table 3]

[0114]

[0115] Refer to Table 2 and Table 3 above and Figure 1 and Figure 2 It was confirmed that the batteries including the positive electrodes of Examples 1 to 3 were superior in capacity, efficiency, capacity retention, and low-temperature and room-temperature output properties compared to the batteries including the positive electrodes of Comparative Examples 1 to 3. In the case of the batteries including the positive electrodes of Comparative Examples 1 and 2, since the positive electrode active material layer had a single-layer structure, the path through which lithium ions could migrate was very small. Therefore, compared to the batteries of Examples 1 and 2, there was a problem of inferior capacity, efficiency, capacity retention, and low-temperature and room-temperature output properties.

[0116] In the case of the battery including the positive electrode of Comparative Example 3, positive electrode active material particles present on the surface of the positive electrode were severely cracked, so that there was a problem in that the resistance of the battery increased.

[0117] Experimental Example 2: Thermal Stability Evaluation (DSC)

[0118] Half cells were fabricated using the positive electrodes fabricated in Examples 1 to 3 and Comparative Examples 1 to 3, and the thermal stability of each half cell was evaluated.

[0119] First, a separator was inserted between the positive electrode and the Li metal disk negative electrode produced in each of Examples 1 to 3 and Comparative Examples 1 to 3 to produce an electrode assembly. This electrode assembly was then placed in a battery case. An electrolyte solution was then injected into the battery case to produce a lithium secondary battery. A 1M LiPF6 solution dissolved in an organic solvent of EC / DMC / EMC (3 / 4 / 3, vol%) was used as the electrolyte solution to produce a half-cell.

[0120] The half-cells manufactured as described above were charged with a constant current of 0.1C and disassembled at an SOC of 100%. Thereafter, the positive electrode obtained from each half-cell and a new electrolyte solution were introduced into a cell for DSC measurement using a differential scanning calorimeter (Setaram, high pressure (HP)-DSC) while increasing the temperature from room temperature to 500°C at a rate of 10°C / min. The temperature at which the maximum heat flow peak occurred is shown in Table 4 below.

[0121] [Table 4]

[0122] DSC main peak (℃) Example 1 230 Example 2 233 Example 3 241 Comparative Example 1 221 Comparative Example 2 224 Comparative Example 3 226

[0123] Referring to Table 4 above, it can be confirmed that the batteries including the positive electrodes of Examples 1 to 3 have less heat transfer due to the double-layer buffer effect, and thus have excellent thermal stability, compared to the batteries including the positive electrodes of Comparative Examples 1 and 2. In addition, in the batteries including the positive electrodes of Examples 1 to 3, the positive electrode active material particles present on the positive electrode surface did not crack as much as in the case of the battery including the positive electrode of Comparative Example 3 (the positive electrode active material particles were hardly cracked), and thus it can be confirmed that the thermal stability is excellent.

Claims

1. A positive electrode for a lithium secondary battery, comprising a positive electrode current collector, and a first positive electrode active material layer and a second positive electrode active material layer stacked in sequence on the positive electrode current collector, wherein: The first positive electrode active material layer and the second positive electrode active material layer comprise a bimodal positive electrode active material; The first positive electrode active material layer comprises small-diameter particles in the form of single particles and large-diameter particles in the form of secondary particles as the positive electrode active material; and The second positive electrode active material layer comprises small-diameter particles in the form of secondary particles and large-diameter particles in the form of secondary particles as the positive electrode active material.

2. The positive electrode according to claim 1, wherein The thickness ratio of the first positive electrode active material layer to the second positive electrode active material layer is 3:7 to 7:

3.

3. The positive electrode according to claim 1, wherein The bimodal positive electrode active materials included in the first positive electrode active material layer and the second positive electrode active material layer each independently have a composition represented by the following Formula 1: [Formula 1] Li x [Ni a Co b M 1 c M 2 d ]O2 Wherein, in the above Formula 1, M 1 is one or more selected from Mn and Al, M 2 is one or more selected from B, Mg, Ca, Ti, V, Cr, Fe, Zn, Ga, Y, Zr, Nb, Mo, Ta and W, and 0.9 ≤ x ≤ 1.1, 0.7 ≤ a < 1, 0 < b < 0.3, 0 < c < 0.3, 0 ≤ d ≤ 0.1, and a + b + c + d = 1.

4. The positive electrode according to claim 3, wherein 0.8 ≤ a < 1, 0 < b < 0.2, 0 < c < 0.2, and 0 ≤ d ≤ 0.

1.

5. The positive electrode according to claim 1, wherein The average particle diameter D of the large-diameter particles included in the first positive electrode active material layer is 50 is 5 μm to 20 μm, and the average particle diameter D of the small-diameter particles included in the first positive electrode active material layer is 50 3μm to 10μm.

6. The positive electrode according to claim 1, wherein The average particle diameter D of the large-diameter particles included in the second positive electrode active material layer is 50 is 5 μm to 20 μm, and the average particle diameter D of the small-diameter particles included in the second positive electrode active material layer is 50 3μm to 10μm.

7. The positive electrode according to claim 1, wherein The first positive electrode active material layer comprises large-diameter particles and small-diameter particles with a weight ratio of 85:15 to 55:

45.

8. The positive electrode according to claim 1, wherein The second positive electrode active material layer comprises large-diameter particles and small-diameter particles with a weight ratio of 85:15 to 55:

45.

9. The positive electrode according to claim 1, wherein The first positive electrode active material layer further comprises a dot-shaped conductive material.

10. The positive electrode according to claim 1, wherein The second positive electrode active material layer further comprises a dot-shaped conductive material and a linear conductive material.

11. A lithium secondary battery, comprising the positive electrode according to claim 1.

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