Positive electrode active material and its manufacturing method, positive electrode including the same, and lithium secondary battery

Abstract

To reduce cation mixing and promote lithium ion diffusion in a positive electrode active material containing a lithium nickel-manganese composite oxide, thereby improving capacity, efficiency and rate characteristics, and enhancing structural stability to improve life characteristics.SOLUTION: The present invention relates to a method for manufacturing a positive electrode active material, which includes mixing a nickel-manganese composite hydroxide and a lithium raw material, performing a primary heat treatment at 200°C to 350°C, and a secondary heat treatment at 800°C to 1000°C, and the positive electrode active material, as well as a positive electrode and a lithium secondary battery including the same.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a positive electrode active material, a method for producing the same, a positive electrode containing the same, and a lithium secondary battery.

Background Art

[0002] Lithium secondary batteries having a high energy density and being easy to carry are mainly used as driving power sources for mobile information terminals such as mobile phones, notebook computers, and smartphones. Recently, research has been actively conducted on using lithium secondary batteries having a high energy density as driving power sources or power storage power sources for hybrid vehicles and electric vehicles.

[0003] In order to realize a lithium secondary battery suitable for such applications, various positive electrode active materials have been studied. Among these, lithium nickel-based oxides, lithium nickel manganese cobalt composite oxides, lithium nickel cobalt aluminum composite oxides, lithium cobalt oxides, etc. are mainly used as positive electrode active materials. By the way, recently, while the demand for large-sized, high-capacity, or high-energy density lithium secondary batteries is rapidly increasing, the supply amount of positive electrode active materials containing rare metal cobalt is expected to be extremely insufficient. That is, since cobalt is expensive and the remaining reserves are not large, it is necessary to develop a positive electrode active material that excludes cobalt or reduces its content.

Summary of the Invention

Problems to be Solved by the Invention

[0004] In a positive electrode active material containing a lithium nickel-manganese-based composite oxide, cation mixing is reduced to promote lithium ion diffusion, capacity, efficiency, and rate characteristics are improved, structural stability is strengthened, and life characteristics are improved.

Means for Solving the Problems

[0005] In one embodiment, a method for manufacturing a positive electrode active material is provided, which includes mixing a nickel-manganese-based composite hydroxide and a lithium raw material, performing a first heat treatment at 200°C to 350°C, and performing a second heat treatment at 800°C to 1000°C.

[0006] In another embodiment, a positive electrode active material including a layered lithium nickel-manganese-based composite oxide represented by Chemical Formula 1 is provided.

[0007] [Chemical Formula 1] Li a1 Ni x1 Mn y1 M 1 z1 M 2 w1 O 2-b1 X b1 In the above Chemical Formula 1, 0.9 ≤ a1 ≤ 1.2, 0.7 ≤ x1 ≤ 0.8, 0.2 ≤ y1 ≤ 0.3, 0 ≤ z1 ≤ 0.05, 0 ≤ w1 ≤ 0.05, 0.9 ≤ x1 + y1 + z1 + w1 ≤ 1.1, and 0 ≤ b1 ≤ 0.1, M 1 is Co, and M 2 is one or more elements selected from Al, B, Ba, Ca, Ce, Cr, Cu, Fe, Mg, Mo, Nb, Si, Sn, Sr, Ti, V, W, and Zr, and X is one or more elements selected from F, P, and S.

[0008] In another embodiment, a positive electrode for a lithium secondary battery including the positive electrode active material is provided.

[0009] In another embodiment, a lithium secondary battery including the positive electrode, a negative electrode, and an electrolyte is provided.

Advantages of the Invention

[0010] The positive electrode active material according to one embodiment maximizes the capacity while minimizing the production cost, reduces cation mixing, promotes the diffusion of lithium ions, improves charge and discharge efficiency and high rate characteristics, ensures structural stability, and can improve the high temperature life characteristics.

Brief Description of the Drawings

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DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, specific embodiments will be described in detail so that those with ordinary knowledge in this technical field can easily implement them. However, the present invention can be implemented in various different forms and is not limited to the embodiments described herein.

[0013] The terms used herein are merely used to explain exemplary embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0014] Here, "these combinations" means a mixture of components, a laminate, a composite, a copolymer, an alloy, a blend, a reaction product, etc.

[0015] Here, terms such as "comprising", "including" or "having" are intended to specify the presence of implemented features, numbers, steps, components or combinations thereof, and should be understood not to preclude in advance the presence or addition possibility of one or more other features, numbers, steps, components, or combinations thereof.

[0016] For the purpose of clearly showing various layers and regions in the drawings, the thickness is enlarged, and the same drawing reference numerals are assigned to similar parts throughout the specification. When a part such as a layer, a film, a region, a plate, etc. is "on" or "above" another part, this includes not only the case where it is "directly above" the other part, but also the case where there are other parts in between. Conversely, when a part is "directly above" another part, it means that there are no other parts in between.

[0017] Also, here, "layer" includes not only the shape formed on the entire surface when observed in a plan view, but also the shape formed on a part of the surface.

[0018] The average particle size can be measured by methods widely known to those skilled in the art. For example, it can be measured using a particle size analyzer, or it can also be measured from a transmission electron microscope image or a scanning electron microscope image. As another method, it can be measured using the dynamic light scattering method, data analysis is performed to count the number of particles for each particle size range, and then based on this, the average particle size value can be obtained by calculation. Unless otherwise defined, the average particle size means the diameter D of the particle with a cumulative volume of 50% by volume in the particle size distribution. 50 It can also mean. Unless otherwise defined, the average particle size is obtained by measuring the sizes (diameter or major axis length) of more than 20 randomly selected particles from a scanning electron microscope image to obtain a particle size distribution, and taking the diameter D of the particle with a cumulative volume of 50% by volume in the said particle size distribution 50 as the average particle size.

[0019] Here, "or" is not construed in an exclusive sense. For example, "A or B" is construed to include A, B, A + B, etc.

[0020] "Metal" is construed as a concept including common metals, transition metals, and metalloids.

[0021] Method for manufacturing a positive electrode active material In one embodiment, a method for manufacturing a cathode active material is provided, which includes mixing a nickel-manganese composite hydroxide and a lithium raw material, performing a first heat treatment at 200°C to 350°C, and performing a second heat treatment at 800°C to 1000°C.

[0022] Recently, the price of the rare metal cobalt has soared, and there is a demand for the development of cathode active materials that exclude cobalt or reduce its content. Among these, cathode active materials with an olivine crystal structure such as lithium iron phosphate (LFP), lithium manganese phosphate (LMP), lithium manganese iron phosphate (LMFP), or a spinel crystal structure such as lithium manganese phosphate (LMO) have a limited ability to achieve high capacity because the amount of lithium that can be utilized within the structure is small. Layered nickel-manganese-based cathode active materials have a high amount of lithium within the structure, are excellent in terms of capacity and efficiency characteristics, and are suitable as materials for high-capacity batteries. However, when cobalt, which plays a crucial role in the layered structure, is removed, there is a problem that the structural stability decreases, the resistance increases, and it becomes difficult to ensure long-life characteristics. Cobalt is known to play an important role in reducing the cation disorder that occurs within the structure. When such cobalt is replaced with Mn, 4+ Ni is reduced by Mn, and Ni 2+ ions increase, resulting in a significant increase in cation disorder. When the cation disorder becomes severe, Ni 2+ ions occupy the lithium sites, hindering the diffusion of lithium ions. As a result, the discharge capacity decreases and the rate characteristics deteriorate compared to materials with less severe cation disorder.

[0023] Normally, it is known that the decomposition reaction of the transition metal hydroxide of the cathode active material precursor and the intercalation (or lithiation) reaction of lithium occur at the same temperature during the synthesis of the cathode active material. In one embodiment, when mixing and heat-treating the transition metal hydroxide and the lithium raw material, a method is proposed in which pre-heat treatment is performed in a temperature range where the intercalation reaction of lithium occurs but the decomposition reaction of the transition metal hydroxide does not occur, and then final heat treatment is performed at a high temperature without continuously firing at the same temperature. According to the synthesis method according to one embodiment, the cation disorder of the layered lithium nickel-manganese-based composite oxide decreases, the diffusion of lithium ions is promoted, the capacity, efficiency, and high-rate characteristics are improved, the voids within the particles decrease, the structural stability is ensured, and the high-temperature life characteristics can be improved.

[0024] The primary heat treatment can be said to be a pre-heat treatment or an intermediate heat treatment, and is characterized in that it is carried out in a temperature range where the reaction of intercalating lithium occurs while the nickel-manganese composite hydroxide is not decomposed. The primary heat treatment is carried out in a temperature range of 200°C to 350°C, and for example, it may be carried out at 220°C to 350°C, 220°C to 300°C, or 250°C to 300°C. When the secondary heat treatment is subsequently carried out after the primary heat treatment in the above temperature range to synthesize the layered nickel-manganese-based positive electrode active material, cation mixing is reduced, voids in the particles are reduced, structural stability is ensured, and the capacity, efficiency, rate characteristics, and life characteristics of the lithium secondary battery can all be improved.

[0025] The primary heat treatment may be carried out in an oxygen atmosphere, and may be carried out for 0.5 hour to 5 hours, or 1 hour to 4 hours, or 1 hour to 3 hours. When the primary heat treatment is carried out within the above time range, cation mixing can be reduced, voids in the particles can be reduced, and structural stability can be achieved. For example, when the primary heat treatment time is very long, cation mixing may not be reduced or structural stability may not be improved.

[0026] The secondary heat treatment can be expressed as this heat treatment or the final heat treatment, and is carried out at 800°C to 1000°C, and for example, it may be carried out at 800°C to 950°C, 820°C to 900°C, or 820°C to 880°C. The secondary heat treatment may also be carried out in an oxygen atmosphere for 4 hours to 12 hours, or 4 hours to 8 hours, or 4 hours to 6 hours. By adjusting the conditions such as the temperature and time of the secondary heat treatment within the above range, a positive electrode active material with reduced cation mixing, reduced voids in the structure, and improved structural stability can be produced.

[0027] As an example, the time of the first heat treatment may be shorter than that of the second heat treatment. For example, the ratio of the first heat treatment time to the second heat treatment time may be 1:2 to 1:5. By appropriately adjusting the first and second heat treatment times, a cathode active material with a reduced cation disorder degree and improved structural stability can be produced.

[0028] In the nickel-manganese composite hydroxide, the nickel content relative to the total metal may be, for example, 70 mol% to 80 mol%, for example, 72 mol% to 78 mol%, or 73 mol% to 77 mol%. Further, the manganese content relative to the total metal in the nickel-manganese composite hydroxide may be 20 mol% to 30 mol%, for example, 23 mol% to 27 mol%.

[0029] As an example, the nickel-manganese composite hydroxide is represented by the following Chemical Formula 11. [Chemical Formula 11] Ni x11 Mn y11 M 1 z11 M 2 w11 (OH)2

[0030] In Chemical Formula 11 above, 0.7 ≤ x11 ≤ 0.8, 0.2 ≤ y11 ≤ 0.3, 0 ≤ z11 ≤ 0.05, 0 ≤ w11 ≤ 0.05, and 0.9 ≤ x11 + y11 + z11 + w11 ≤ 1.1, and M 1 is Co, and M 2 is one or more elements selected from Al, B, Ba, Ca, Ce, Cr, Cu, Fe, Mg, Mo, Nb, Si, Sn, Sr, Ti, V, W, and Zr.

[0031] In Chemical Formula 11 above, for example, 0.73 ≤ x11 ≤ 0.77 and 0.23 ≤ y11 ≤ 0.27 may be satisfied, 0 ≤ z11 ≤ 0.01, or z11 = 0 may also be satisfied.

[0032] The nickel-manganese composite hydroxide is in a particle form, and its average particle size D50 It may be 8 μm to 20 μm, for example, 8 μm to 18 μm, 8 μm to 16 μm, 8 μm to 14 μm, or 8 μm to 12 μm. Further, the nickel-manganese composite hydroxide may be in the form of secondary particles formed by aggregation of a plurality of primary particles.

[0033] In the manufacturing method, the molar ratio of lithium of the lithium raw material to the total metal of the nickel-manganese composite hydroxide may be, for example, 1.0 to 1.2, 1.01 to 1.15, or 1.03 or 1.1.

[0034] Positive electrode active material In one embodiment, a cathode active material manufactured through the method described above is provided.

[0035] In one example, a cathode active material including a layered lithium nickel-manganese composite oxide represented by the following Chemical Formula 1 is provided. [Chemical Formula 1] Li a1 Ni x1 Mn y1 M 1 z1 M 2 w1 O 2-b1 X b1

[0036] In Chemical Formula 1 above, 0.9 ≤ a1 ≤ 1.2, 0.7 ≤ x1 ≤ 0.8, 0.2 ≤ y1 ≤ 0.3, 0 ≤ z1 ≤ 0.05, 0 ≤ w1 ≤ 0.05, 0.9 ≤ x1 + y1 + z1 + w1 ≤ 1.1, and 0 ≤ b1 ≤ 0.1, and M 1 is Co, and M 2 is one or more elements selected from Al, B, Ba, Ca, Ce, Cr, Cu, Fe, Mg, Mo, Nb, Si, Sn, Sr, Ti, V, W, and Zr, and X is one or more elements selected from F, P, and S.

[0037] In the above Chemical Formula 1, 0.72 ≦ x1 ≦ 0.78 and 0.22 ≦ y1 ≦ 0.28, or 0.73 ≦ x1 ≦ 0.77 and 0.23 ≦ y1 ≦ 0.27 may be satisfied. Also, 0 ≦ z1 ≦ 0.01 or z1 = 0 may be satisfied. Further, 1 ≦ a1 ≦ 1.15, or 1.03 ≦ a1 ≦ 1.1 may be satisfied.

[0038] The positive electrode active material may be in the form of secondary particles formed by aggregation of a plurality of primary particles, and the average particle size D of the positive electrode active material 50 may be 8 μm to 20 μm, for example, 8 μm to 18 μm, 8 μm to 16 μm, 8 μm to 14 μm, or 8 μm to 12 μm.

[0039] The cation mixing of the positive electrode active material according to one embodiment may be 3.5% or less, for example, 3.2% or less, 3% or less, or 2.6% or less, and for example, may be 1% to 3.5%.

[0040] Further, the porosity inside the particles of the positive electrode active material may be 5% by volume or less with respect to 100% by volume of the positive electrode active material, for example, 4% by volume or less or 1% to 5% by volume. Here, the porosity may be measured, for example, through transmission X-ray microscopy analysis (TXM).

[0041] Positive electrode In one embodiment, a positive electrode for a lithium secondary battery including the above-described positive electrode active material is provided. For example, in one embodiment, a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector are included, and the positive electrode active material layer provides a positive electrode including the above-described positive electrode active material. The positive electrode active material layer may further include other types of positive electrode active materials in addition to the above-described positive electrode active material. Also, the positive electrode active material layer may selectively further include a binder, a conductive material, or a combination thereof.

[0042] Binder The binder serves to make the positive electrode active material particles adhere well to each other and also to make the positive electrode active material adhere well to the current collector. Representative examples of the binder include, but are not limited to, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, epoxy resin, (meth)acrylic resin, polyester resin, nylon, etc.

[0043] Conductive material The conductive material is used to impart conductivity to the electrode, and in the battery being constructed, any electron conductive material that does not cause a chemical change can be used. Examples of the conductive material include carbon-based substances such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, carbon nanotube; metal-based substances in the form of metal powder or metal fiber containing copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0044] The contents of the binder and the conductive material may each be 0.5 wt% to 5 wt% based on 100 wt% of the positive electrode active material layer.

[0045] As the positive electrode current collector, Al, SUS, etc. may be used, but it is not limited thereto.

[0046] Lithium secondary battery In one embodiment, a lithium secondary battery including the above-described positive electrode, negative electrode, and electrolyte is provided. As an example, the lithium secondary battery may include a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode, and an electrolyte solution. In another embodiment, an all-solid-state secondary battery including the positive electrode, the negative electrode, and a solid electrolyte layer positioned between the positive electrode and the negative electrode is provided. Hereinafter, for convenience, the configuration of a lithium ion battery using an electrolyte solution will be described in detail.

[0047] Lithium secondary batteries can be classified into cylindrical, prismatic, pouch, coin, etc. depending on their form. FIGS. 1 to 4 are schematic views showing a lithium secondary battery according to an embodiment. It can be said that FIG. 1 is circular, FIG. 2 is prismatic, and FIGS. 3 and 4 are pouch-type battery forms. Referring to FIGS. 1 to 4, the lithium secondary battery 100 may include an electrode assembly 40 in which a separator 30 is interposed between a positive electrode 10 and a negative electrode 20, and a case 50 in which the electrode assembly 40 is incorporated. The positive electrode 10, negative electrode 20, and separator 30 may be impregnated with an electrolyte solution (not shown). The lithium secondary battery 100 may include a sealing member 60 that seals the case 50 as shown in FIG. 1. Further, as shown in FIG. 2, 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. 3 and 4, the lithium secondary battery 100 may include electrode tabs 70, that is, a positive electrode tab 71 and a negative electrode tab 72, which serve as an electrical path for guiding the current formed by the electrode assembly 40 to the outside.

[0048] Negative electrode The negative electrode may include a current collector and a negative electrode active material layer positioned on the current collector, and includes a negative electrode active material, and may further include a binder, a conductive material, or a combination thereof.

[0049] Negative electrode active material The negative electrode active material includes a material capable of reversibly intercalating / deintercalating lithium ions, lithium metal, an alloy of lithium metal, a material capable of doping and undoping lithium, or a transition metal oxide.

[0050] As the substance capable of reversibly intercalating / deintercalating the lithium ions, a carbon-based negative electrode active material may be used, which may include, for example, crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon include graphite such as amorphous, plate-shaped, flaky, spherical, or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon include soft carbon or hard carbon, mesophase pitch carbide, calcined coke, and the like.

[0051] 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 may be used.

[0052] As the substance capable of doping and undoping lithium, an Si-based negative electrode active material or an Sn-based negative electrode active material may be used. As the Si-based negative electrode active material, silicon, a silicon-carbon composite, SiOx (0 < x < 2), an Si-Q alloy (where Q is an element 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, for example, Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof), or a combination thereof may be used. As the Sn-based negative electrode active material, Sn, SnO2, an Sn alloy, or a combination thereof may be used.

[0053] The silicon-carbon composite may be a composite of silicon and amorphous carbon. The average particle size D of the silicon-carbon composite particles 50It may be, for example, from 0.5 μm to 20 μm. According to one embodiment, the silicon-carbon composite may be in a form in which silicon particles and amorphous carbon are coated on the surface of the silicon particles. For example, it may include secondary particles (cores) assembled from primary silicon particles and an amorphous carbon coating layer (shell) located on the surface of the secondary particles. The amorphous carbon may also be located between the primary silicon particles. For example, the primary silicon particles may be coated with amorphous carbon. The secondary particles may be present dispersed in an amorphous carbon matrix.

[0054] The silicon-carbon composite may further include crystalline carbon. For example, the silicon-carbon composite may include a core containing crystalline carbon and silicon particles and an amorphous carbon coating layer located on the surface of the core. The crystalline carbon may be artificial graphite, natural graphite, or a combination thereof. Examples of the amorphous carbon include soft carbon, hard carbon, mesophase pitch carbide, and calcined coke.

[0055] When the silicon-carbon composite contains silicon and amorphous carbon, the content of silicon may be from 10% by weight to 50% by weight based on 100% by weight of the silicon-carbon composite, and the content of amorphous carbon may be from 50% by weight to 90% by weight. Also, when the composite contains silicon, amorphous carbon, and crystalline carbon, the content of silicon may be from 10% by weight to 50% by weight based on 100% by weight of the silicon-carbon composite, the content of crystalline carbon may be from 10% by weight to 70% by weight, and the content of amorphous carbon may be from 20% by weight to 40% by weight.

[0056] Also, the thickness of the amorphous carbon coating layer may be from 5 nm to 100 nm. The average particle size D of the silicon particles (primary particles) 50may be 10 nm to 1 μm, or may also be 10 nm to 200 nm. The silicon particles may exist alone as silicon, or may exist in the form of a silicon alloy, or may exist in an oxidized form. The oxidized form of silicon can be represented by SiOx (0 < x < 2). At this time, the atomic content ratio of Si:O indicating the degree of oxidation may be 99:1 to 33:67. In this specification, unless otherwise defined, the average particle size D 50 means the diameter of the particle with a cumulative volume of 50% by volume in the particle size distribution.

[0057] The Si-based negative electrode active material or the Sn-based negative electrode active material may be used in mixture with a carbon-based negative electrode active material. When the Si-based negative electrode active material or the Sn-based negative electrode active material and the carbon-based negative electrode active material are used in mixture, the mixing ratio may be 1:99 to 90:10 by weight.

[0058] Binder The binder serves to make the negative electrode active material particles adhere well to each other and also make the negative electrode active material adhere well to the current collector. As the binder, a non-aqueous binder, an aqueous binder, a dry binder, or a combination thereof may be used.

[0059] Examples of the non-aqueous binder include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene-propylene copolymer, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

[0060] The aqueous binder may be selected from styrene-butadiene rubber, (meth)acrylated 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, polyester resin, (meth)acrylic resin, phenol resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0061] When using an aqueous binder as the negative electrode binder, it may further contain a cellulose-based compound capable of imparting viscosity. As this cellulose-based compound, one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or alkali metal salts thereof may be mixed and used. As the alkali metal, Na, K, or Li may be used.

[0062] The dry binder is a polymer substance capable of being fibrillated, and may be, for example, polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, or combinations thereof.

[0063] Conductive material The conductive material is used to impart conductivity to the electrode, and in the battery being configured, any electron conductive material that does not cause a chemical change can be used. Specific examples include carbon-based substances such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metal-based substances in the form of metal powder or metal fiber containing copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0064] The content of the negative electrode active material may be 95% by weight to 99.5% by weight based on 100% by weight of the negative electrode active material layer, and the content of the binder may be 0.5% by weight to 5% by weight based on 100% by weight of the negative electrode active material layer. For example, the negative electrode active material layer may contain 90% by weight to 99% by weight of the negative electrode active material, 0.5% by weight to 5% by weight of the binder, and 0.5% by weight to 5% by weight of the conductive material.

[0065] Current collector The negative electrode current collector may contain, for example, indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), germanium (Ge), lithium (Li), or an alloy thereof, and may be in the form of a foil, sheet, or foam. The thickness of the negative electrode current collector may be, for example, 1 μm to 20 μm, or may be 5 μm to 15 μm, or 7 μm to 10 μm.

[0066] Electrolyte The electrolyte for the lithium secondary battery may be, for example, an electrolytic solution, which may contain a non-aqueous organic solvent and a lithium salt.

[0067] The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move. The non-aqueous organic solvent may be a carbonate-based, ester-based, ether-based, ketone-based, or alcohol-based solvent, an aprotic solvent, or a combination thereof.

[0068] As carbonate solvents, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. may be used. As ester solvents, methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, decanolide, mevalonolactone, valerolactone, caprolactone, etc. may be used. As ether solvents, dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, tetrahydrofuran, etc. may be used. Further, as ketone solvents, cyclohexanone, etc. may be used. As alcohol solvents, ethyl alcohol, isopropyl alcohol, etc. may be used, and as aprotic solvents, nitriles such as R-CN (R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may contain a double bond, a benzene ring, or an ether group), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane and 1,4-dioxolane, sulfolane, etc. may be used.

[0069] The non-aqueous organic solvent may be used alone or in a mixture of two or more. When used in a mixture of two or more, the mixing ratio can be appropriately adjusted according to the intended battery performance, which should be widely understood by those skilled in the art.

[0070] When using a carbonate solvent, a cyclic carbonate and a chain carbonate may be mixed and used, and the cyclic carbonate and the chain carbonate may be mixed at a volume ratio of 1:1 to 1:9.

[0071] The non-aqueous organic solvent may further contain an aromatic hydrocarbon-based organic solvent. For example, the carbonate-based solvent and the aromatic hydrocarbon-based organic solvent may be mixed and used at a volume ratio of 1:1 to 30:1.

[0072] The electrolytic solution may further contain vinyl ethyl carbonate, vinylene carbonate or an ethylene carbonate-based compound in order to improve the battery life.

[0073] Typical examples of the ethylene carbonate-based compound include fluoroethylene carbonate, difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate and the like.

[0074] The lithium salt is a substance that dissolves in the organic solvent, acts as a source of lithium ions in the battery to enable the operation of a basic lithium secondary battery, and plays a role in promoting the movement of lithium ions between the positive electrode and the negative electrode. Typical examples of the lithium salt 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)(where x and y are integers from 1 to 20), lithium trifluoromethanesulfonate, lithium tetrafluoroethanesulfonate, lithium difluorobis(oxalate)phosphate (LiDFOB), lithium bis(oxalate)borate (LiBOB), and may contain one or more selected therefrom.

[0075] The concentration of the lithium salt is preferably used within the range of 0.1 M to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate ionic conductivity and viscosity, so it can exhibit excellent performance and lithium ions can effectively move.

[0076] Separator Depending on the type of lithium secondary battery, a separator may be present between the positive electrode and the negative electrode. As such a separator, polyethylene, polypropylene, polyvinylidene fluoride, or a multilayer film of two or more layers thereof may be used, and it goes without saying that a mixed multilayer film such as a two-layer separator of polyethylene / polypropylene, a three-layer separator of polyethylene / polypropylene / polyethylene, or a three-layer separator of polypropylene / polyethylene / polypropylene may be used.

[0077] The separator may include a porous substrate and a coating layer located on one or both sides of the porous substrate and containing an organic substance, an inorganic substance, or a combination thereof.

[0078] The porous substrate may be a polymer selected from any one of polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacetal, polyamide, polyimide, polycarbonate, polyether ketone, polyaryl ether ketone, polyether imide, polyamide imide, polybenzimidazole, polyether sulfone, polyphenylene oxide, cyclic olefin copolymer, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon (registered trademark), and polytetrafluoroethylene, or a polymer film formed of a copolymer or mixture of two or more of these.

[0079] The porous substrate may have a thickness of about 1 μm to 40 μm, for example, a thickness of 1 μm to 30 μm, 1 μm to 20 μm, 5 μm to 15 μm, or 10 μm to 15 μm.

[0080] The organic substance may include a (meth)acrylic copolymer containing a first structural unit derived from (meth)acrylamide, and a second structural unit containing at least one of a structural unit derived from (meth)acrylic acid or (meth)acrylate and a structural unit derived from (meth)acrylamidosulfonic acid or a salt thereof.

[0081] The inorganic substance may include, but is 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. The average particle size D of the inorganic particles 50 may be from 1 nm to 2000 nm, for example, from 100 nm to 1000 nm, or from 100 nm to 700 nm.

[0082] The organic substance and the inorganic substance may be mixed and present in one coating layer, or may be present in a form in which a coating layer containing the organic substance and a coating layer containing the inorganic substance are laminated.

[0083] The thickness of each coating layer may be from 0.5 μm to 20 μm, for example, from 1 μm to 10 μm, or from 1 μm to 5 μm.

[0084] Hereinafter, examples and comparative examples of the present invention will be described. The following examples are merely illustrative of the present invention, and the present invention is not limited to the following examples.

[0085] Comparative Example 1 1. Manufacture of positive electrode active material Average particle size D 50 is about 10 μm of Ni 0.75 Mn 0.25(OH)2 and LiOH were mixed so that the ratio of Li / (Ni+Mn) became 1.077, and after mixing, the temperature was raised in an oxygen atmosphere for 6 hours and heat-treated at 850 °C for 8 hours. After cooling to room temperature, the composition was Li 1.037 (Ni 0.75 Mn 0.25 ) 0.963 O2, and a positive electrode active material in the form of secondary particles with an average particle size D 50 of about 10 μm was produced. Fig. 5 shows the heat treatment temperature profile by the one-step synthesis method of Comparative Example 1.

[0086] 2. Manufacture of lithium secondary battery 98.5 wt% of the produced positive electrode active material, 1.0 wt% of polyvinylidene fluoride binder, and 0.5 wt% of carbon nanotube conductive material were mixed to produce a positive electrode active material layer slurry, which was coated on an aluminum foil current collector, dried and rolled to produce a positive electrode.

[0087] 97.5 wt% of graphite negative electrode active material, 1.5 wt% of carboxymethyl cellulose and 1 wt% of styrene butadiene rubber were mixed in an aqueous solvent to produce a negative electrode active material layer slurry. The negative electrode active material layer slurry was coated on a copper foil current collector, dried and rolled to produce a negative electrode.

[0088] Using a polytetrafluoroethylene separator, an electrolyte solution in which 1M LiPF6 was dissolved in a solvent in which ethylene carbonate and dimethyl carbonate were mixed at a volume ratio of 3:7 was used to manufacture a lithium secondary battery by a normal method.

[0089] Example 1 In the production of the positive electrode active material, except that Ni 0.75 Mn 0.25 (OH)2 and LiOH were mixed, then the temperature was raised for about 2 hours and primary heat-treated at 200 °C for 2 hours, the temperature was raised for 4 hours and secondary heat-treated at 850 °C for 6 hours, and then cooled to room temperature to produce the positive electrode active material, the positive electrode active material and the lithium secondary battery were produced in a method substantially the same as that of Comparative Example 1.

[0090] Example 2 The positive electrode active material and the lithium secondary battery were produced in substantially the same manner as in Example 1, except that the primary heat treatment temperature was changed to 250°C. The heat treatment temperature profile by the two-step synthesis method of Example 2 is shown in FIG. 6.

[0091] Example 3 The positive electrode active material and the lithium secondary battery were produced in substantially the same manner as in Example 1, except that the primary heat treatment temperature was changed to 300°C.

[0092] Example 4 The positive electrode active material and the lithium secondary battery were produced in substantially the same manner as in Example 1, except that the primary heat treatment temperature was changed to 350°C.

[0093] Comparative Example 2 The positive electrode active material and the lithium secondary battery were produced in substantially the same manner as in Example 1, except that the primary heat treatment temperature was changed to 400°C.

[0094] Evaluation Example 1: Cation mixing X-ray diffraction analysis (XRD) was performed on the positive electrode active materials produced in Examples 1 to 4 and Comparative Examples 1 and 2, and cation mixing, which is the ratio (%) occupied by nickel ions (Ni 2+ ) mixed in the lithium site, was measured through Rietveld analysis and shown in Table 1 below.

[0095]

Table 1

[0096] Referring to Table 1, it can be seen that in the case of Examples 1 to 4 to which the two-step synthesis method according to one embodiment is applied, cation mixing in the positive electrode active material is reduced, through which the diffusion of lithium ions becomes easy, and it is expected that charge / discharge efficiency, rate characteristics, etc. are improved.

[0097] Evaluation Example 2: Analysis of internal pores SEM images of the cross-sections of the positive electrode active materials produced in Comparative Example 1 and Example 2, cut by FIB, are shown at the bottom of FIG. 7 and the bottom of FIG. 8, respectively, and images of the pores (voids) inside the particles analyzed by TXM are shown at the top of FIG. 7 and the top of FIG. 8, respectively. Comparing FIG. 7 and FIG. 8, it can be confirmed that in Example 2, the internal voids in the secondary particles and the internal pores in the primary particles decreased compared to Comparative Example 1, and the structural stability increased. Also, the porosity inside the positive electrode active materials of Comparative Example 1 and Example 2 was analyzed through TXM and shown in FIG. 9. The porosity shown in FIG. 9 means the volume percentage occupied by pores with respect to 100% by volume of the entire positive electrode active material particles. Referring to FIG. 9, it can be seen that the porosity decreased to 3.51% by volume in Example 1 compared to 6.62% by volume in Comparative Example 1. Thus, it can be understood that by applying the two-step synthesis method according to one embodiment, the pores in the positive electrode active material decreased and the structural stability improved, and thus the life characteristics are expected to be improved.

[0098] Evaluation Example 3: Initial charge-discharge capacity, efficiency, life characteristics, and rate characteristics For the lithium secondary batteries produced in Comparative Example 1 and Examples 2 and 3, after charging at a constant current of 0.2C to 4.5V and then at a constant voltage of 0.05C to 4.5V at 25°C, and then discharging at 0.2C to 3.0V, the initial charge and discharge were performed. Table 2 below shows the initial charge capacity, the initial discharge capacity, and the initial charge-discharge efficiency, which is the ratio of the latter to the former.

[0099] Next, a cycle of charging at 0.5C and discharging at 0.5C in a voltage range of 2.8V to 4.7V at 45°C was repeated 100 times, and the ratio of the 100th discharge capacity to the initial discharge capacity was shown as the life in Table 2 below.

[0100] Separately from this, the lithium secondary batteries produced in Comparative Example 1 and Example 2 were subjected to a rate evaluation of charging at 0.2C to 4.5V at 25°C and discharging at 0.1C, 0.5C, 1C, 2C, 5C, and 0.5C, and the results are shown in FIG. 10.

[0101]

Table 2

[0102] Referring to Table 2 above, in the case of Examples 2 and 3, it can be seen that the initial discharge capacity increases and the charge-discharge efficiency and life characteristics are improved compared with Comparative Example 1. In the case of Examples 2 and 3, it is understood that the cracks inside the secondary particles of the positive electrode active material decrease, the internal voids decrease, and the high-temperature life performance is improved.

[0103] Also, referring to FIG. 10, in the case of Example 2, it can be seen that the characteristics at high rates of 2C and 5C are even more excellent compared with Comparative Example 1. In the case of Example 2, it is understood that the diffusion of lithium ions is promoted by the decrease in the cationic disorder degree in the positive electrode active material, and thus the rate-determining characteristics are improved.

[0104] Evaluation Example 4: DTG-DSC Ni 0.75 Mn 0.25 (OH)2 sample and Ni 0.75 Mn 0.25 (OH)2 and LiOH have been subjected to DTG-DSC analysis for each of the mixed samples, and the results are shown in FIG. 11. The upper graph in FIG. 11 is the result for the single sample of the transition metal hydroxide, and it is confirmed that decomposition occurs at a temperature exceeding 300°C. The lower graph in FIG. 11 is the result for the mixed sample of the transition metal hydroxide and the lithium raw material, and it can be seen that decomposition occurs at a temperature lower than 300°C. FIG. 12 is an enlarged view of the graph in FIG. 11 in the region of approximately 200 to 350°C, and a graph corresponding to the intercalation of lithium is also added. Referring to FIG. 12, it is confirmed that although the intercalation of lithium occurs at about 250°C, the decomposition of the transition metal precursor does not occur. After performing the primary heat treatment in such a temperature range and then performing the secondary heat treatment at a high temperature, it is understood that the cationic disorder degree decreases, the internal voids in the particles decrease, and the structural and chemical stability is improved.

[0105] Although the preferred embodiments have been described in detail above, the scope of the rights of the present invention is not limited thereto, and various modifications and improvements made by those skilled in the art using the basic concepts defined in the following claims also fall within the scope of the rights of the present invention.

Explanation of Signs

[0106] 100: Lithium secondary battery 10: Positive electrode 11: Positive electrode lead tab 12: Positive electrode terminal 20: Negative electrode 21: Negative electrode lead tab 22: Negative electrode terminal 30: Separator 40: Electrode assembly 50: Case 60: Sealing member 70: Electrode tab 71: Positive electrode tab 72: Negative electrode tab

Claims

1. A method for manufacturing a positive electrode active material, comprising: mixing a nickel-manganese composite hydroxide and a lithium source, performing a first heat treatment at 200°C to 350°C, and performing a second heat treatment at 800°C to 1000°C.

2. The method of claim 1 , wherein the first heat treatment temperature is 220° C. to 300° C.

3. 2. The method of claim 1, wherein the first heat treatment is performed in an oxygen atmosphere for 0.5 to 5 hours.

4. 2. The method of claim 1, wherein the second heat treatment is performed in an oxygen atmosphere for 4 to 12 hours.

5. The method for producing a positive electrode active material according to claim 1 , wherein a time of the first heat treatment is shorter than a time of the second heat treatment.

6. 2. The method of claim 1, wherein the nickel content of the nickel-manganese composite hydroxide is 70 mol % to 80 mol % based on the total metal content.

7. The method for producing a positive electrode active material according to claim 1 , wherein the nickel-manganese composite hydroxide is represented by chemical formula 11. [Chemical formula 11] Ni x11 Mn y11 M 1 z11 M 2 w11 (OH) 2 In the above formula 11, 0.7≦x11≦0.8, 0.2≦y11≦0.3, 0≦z11≦0.05, 0≦w11≦0.05, and 0.9≦x11+y11+z11+w11≦1.1; M 1 is Co, M 2 is one or more elements selected from Al, B, Ba, Ca, Ce, Cr, Cu, Fe, Mg, Mo, Nb, Si, Sn, Sr, Ti, V, W, and Zr.

8. The nickel-manganese composite hydroxide is in the form of particles, and has an average particle diameter D 50 The method for producing a positive electrode active material according to claim 1 , wherein the thickness of the first electrode is 8 μm to 20 μm.

9. 2. The method for producing a positive electrode active material according to claim 1, wherein a molar ratio of lithium in the lithium raw material to the total metal of the nickel-manganese composite hydroxide is 1.0 to 1.

2.

10. A positive electrode active material comprising a layered lithium nickel-manganese composite oxide represented by chemical formula 1. [Chemical formula 1] Li a1 Ni x1 Mn y1 M 1 z1 M 2 w1 O 2-b1 X b1 In the above Chemical Formula 1, 0.9≦a1≦1.2, 0.7≦x1≦0.8, 0.2≦y1≦0.3, 0≦z1≦0.05, 0≦w1≦0.05, 0.9≦x1+y1+z1+w1≦1.1, and 0≦b1≦0.1; M 1 is Co, M 2 is one or more elements selected from Al, B, Ba, Ca, Ce, Cr, Cu, Fe, Mg, Mo, Nb, Si, Sn, Sr, Ti, V, W, and Zr, and X is one or more elements selected from F, P, and S.

11. 11. The positive electrode active material according to claim 10, wherein, in Chemical Formula 1, 0.73≦x1≦0.77 and 0.23≦y1≦0.

27.

12. The positive electrode active material according to claim 10 , wherein in Chemical Formula 1, 0≦z1≦0.

01.

13. The positive electrode active material according to claim 10 , wherein in Chemical Formula 1, 1.03≦a1≦1.

1.

14. The positive electrode active material according to claim 10 , wherein the positive electrode active material is in the form of secondary particles formed by agglomeration of a plurality of primary particles.

15. The average particle diameter D of the positive electrode active material 50 The positive electrode active material according to claim 10, wherein the thickness of the first electrode is 8 μm to 20 μm.

16. 11. The cathode active material of claim 10, wherein the cathode active material has a cation loading of 3.5% or less.

17. The positive electrode active material of claim 10 , wherein the positive electrode active material has an intraparticle porosity of 5 vol % or less as measured by a transmission X-ray microscope analysis.

18. A positive electrode for a lithium secondary battery, comprising the positive electrode active material produced by the method according to any one of claims 1 to 9, or the positive electrode active material according to any one of claims 10 to 17.

19. The positive electrode according to claim 18 ; A negative electrode; and an electrolyte.

20. The positive electrode according to claim 18 ; A negative electrode; a solid electrolyte layer located between the positive electrode and the negative electrode.

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