Positive electrode active materials for lithium secondary batteries, their preparation methods, and lithium secondary batteries containing them.

Abstract

This invention provides a positive electrode active material for lithium-ion secondary batteries and a method for preparing the same. The positive electrode active material comprises a lithium transition metal oxide, on which a coating derived from wrinkled graphene is formed. The wrinkled graphene has a thickness of 0.1 to 10 nm. In the positive electrode active material, the thin and uniform coating derived from wrinkled graphene effectively imparts conductivity to the lithium transition metal oxide. When the positive electrode active material is applied to a lithium-ion secondary battery, the performance of the lithium-ion secondary battery, especially its discharge capacity at high C-rates, is improved.

Description

Technical Field

[0001] This invention relates to a positive electrode active material for lithium secondary batteries, a method for preparing the same, and a lithium secondary battery comprising the same. Specifically, this invention relates to a positive electrode active material for lithium secondary batteries, a method for preparing the same, and a lithium secondary battery comprising the same, wherein the positive electrode active material comprises a lithium transition metal oxide, and a coating derived from wrinkled graphene is formed on the lithium transition metal oxide.

[0002] This application claims priority based on Korean Patent Application No. 10-2021-0001809, filed on January 7, 2021, the entire contents of which are incorporated herein by reference. Background Technology

[0003] With the technological advancements and increasing demands of mobile devices, the need for secondary batteries as energy sources has grown rapidly. Among these secondary batteries, lithium-ion batteries, characterized by high energy density and voltage, long cycle life, and low self-discharge rate, have been commercialized and are widely used.

[0004] Lithium transition metal oxides are used as positive electrode active materials in lithium-ion secondary batteries, with lithium cobalt oxide (LiCoO2) being the primary choice due to its high operating voltage and excellent capacity performance. However, LiCoO2 suffers from very poor thermal properties due to crystal structure instability caused by delithiation, and its high cost limits its widespread use as a power source in fields such as electric vehicles. As alternatives to LiCoO2, lithium manganese oxides (LiMnO2, LiMn2O4, etc.), lithium iron phosphate compounds (LiFePO4, etc.), and lithium nickel oxides (LiNiO2, etc.) have been developed. Among these, lithium nickel oxides, with their high reversible capacity of approximately 200 mAh / g, have been actively researched and developed, facilitating the realization of large-capacity batteries. However, LiNiO2 suffers from poorer thermal stability compared to LiCoO2, and when an internal short circuit occurs due to external pressure during charging, the positive electrode active material itself decomposes, leading to battery rupture and fire. Furthermore, compared to LiCoO2, LiNiO2 has lower electrical conductivity and is more susceptible to moisture.

[0005] To address these issues, carbon coatings using lithium transition metal oxide pitch have been studied; however, the high-temperature heat treatment process for carbonization is absolutely necessary. In this case, the carbon elements on the surface react with the oxygen on the surface of the lithium transition metal oxide and are reduced. Furthermore, as the oxidation number of the lithium transition metal oxide changes drastically, the performance of the positive electrode active material deteriorates.

[0006] Therefore, the field has been investigating ways to improve the performance of lithium secondary batteries by improving the composition or structure of positive electrode active materials containing lithium transition metal oxides.

[0007] Existing technical documents

[0008] [Patent Literature]

[0009] (Patent Document 1) Korean Patent Application Publication No. 10-2017-0119973 Summary of the Invention

[0010] Technical issues

[0011] The present invention provides a positive electrode active material for lithium secondary batteries, a method for preparing the same, and a lithium secondary battery containing the same. The positive electrode active material improves the functionality of the positive electrode active material and improves the battery performance when applied to a lithium secondary battery by thinly and uniformly coating wrinkled graphene on a lithium transition metal oxide.

[0012] Technical solution

[0013] According to a first aspect of the present invention, the present invention provides a positive electrode active material for lithium secondary batteries, the positive electrode active material comprising a lithium transition metal oxide, wherein a coating derived from wrinkled graphene is formed on the lithium transition metal oxide.

[0014] In one embodiment of the invention, the wrinkled graphene has a thickness of 0.1 to 10 nm.

[0015] In one embodiment of the invention, the wrinkled graphene has a thickness exceeding 400 μm. 2 / g BET specific surface area.

[0016] In one embodiment of the invention, the wrinkled graphene contains 0.1 to 3% by weight of oxygen based on the total weight of the wrinkled graphene.

[0017] In one embodiment of the invention, the wrinkled graphene has a size of 50 to 500 nm.

[0018] In one embodiment of the present invention, the lithium transition metal oxide is selected from: LiCoO2, LiNiO2, LiMnO2, Li2MnO3, LiMn2O4, Li(Ni a Co b Mn c )O2(0 <a<1,0<b<1,0<c<1,a+b+c=1)、LiNi 1-y Co y O2(0 <y<1)、LiCo 1-y Mn yO2, LiNi 1-y Mn y O2(0 < y < 1), Li(Ni a Co b Mn c )O4(0 < a < 2, 0 < b < 2, 0 < c < 2, a + b + c = 2), LiMn 2-z Ni z O4(0 < z < 2), LiMn 2-z Co z O4(0 < z < 2) and their combinations.

[0019] In one embodiment of the present invention, the coating derived from wrinkled graphene has an I D / I G value of 0.1 to 0.2.

[0020] In one embodiment of the present invention, the coating derived from wrinkled graphene has a thickness of 1 to 500 nm.

[0021] In one embodiment of the present invention, the lithium transition metal oxide on which the coating derived from wrinkled graphene is formed has a conductivity of 1.0×10 -2 to 1.0×10 S / cm.

[0022] In one embodiment of the present invention, the lithium transition metal oxide on which the coating derived from wrinkled graphene is formed has a BET specific surface area of 2 to 10 m 2 / g.

[0023] According to the second aspect of the present invention, the present invention provides a method for preparing a positive electrode active material for a lithium secondary battery as described above, the method comprising the step of coating wrinkled graphene on a lithium transition metal oxide to form a coating derived from wrinkled graphene on the lithium transition metal oxide.

[0024] In one embodiment of the present invention, the I D / I G value of the coating derived from wrinkled graphene is 30% to 70% of the I D / I G value of the wrinkled graphene.

[0025] In one embodiment of the present invention, the wrinkled graphene is coated on the lithium transition metal oxide by mechanical fusion method.

[0026] Beneficial effects

[0027] The positive electrode active material for lithium secondary batteries according to the present invention has the following structure: a thin and uniform coating derived from wrinkled graphene is formed on the lithium transition metal oxide by coating a thin layer of wrinkled graphene onto the lithium transition metal oxide.

[0028] In positive electrode active materials, thin and uniform coatings derived from wrinkled graphene effectively impart conductivity to lithium transition metal oxides.

[0029] When positive electrode active materials are applied to lithium secondary batteries, the performance of lithium secondary batteries, especially the discharge capacity at high C-rate, is improved. Attached Figure Description

[0030] Figure 1 is a SEM image of an example wrinkled graphene;

[0031] Figure 1a The image is observed at a magnification of (×100,000), and

[0032] Figure 1b The image is observed at a magnification of 10,000.

[0033] Figure 2 This is a SEM image of NCM 622 without carbon-based coating.

[0034] Figure 3 The image is a SEM image of the positive electrode active material according to Example 1.

[0035] Figure 4 The image is based on the SEM image of the positive electrode active material of Comparative Example 1.

[0036] Figure 5 The image is based on the SEM image of the positive electrode active material of Comparative Example 2.

[0037] Figure 6 The image is based on the SEM image of the positive electrode active material of Comparative Example 3.

[0038] Figure 7 The image is based on the SEM image of the positive electrode active material of Comparative Example 4. Detailed Implementation

[0039] All embodiments provided by the present invention can be implemented through the following description. It should be understood that the following description describes preferred embodiments of the present invention, and the present invention is not necessarily limited thereto.

[0040] When the conditions and methods for measuring the properties described in this specification are not specifically described, the property shall be measured using the measurement conditions and methods commonly used by those skilled in the art.

[0041] Positive electrode active materials and their preparation methods

[0042] The present invention provides a positive electrode active material for lithium secondary batteries, the positive electrode active material comprising a lithium transition metal oxide, wherein a coating derived from wrinkled graphene is formed on the lithium transition metal oxide.

[0043] "Wrinkled graphene" is a raw material used before being coated onto lithium transition metal oxide, and has a structure in which thin sheets of graphene are wrinkled in a zigzag pattern. Because wrinkled graphene can exist in the coating in a collapsed state depending on the coating method, in this specification, the coating formed using wrinkled graphene as a raw material is described as a "coating derived from wrinkled graphene".

[0044] Graphene is typically prepared in a top-down manner. Specifically, graphene sheets are prepared by chemically exfoliating low-cost graphite in a strong acid solvent using an intercalation mechanism, followed by mechanically exfoliating it using shear force in a solvent with a dispersant. However, when graphene prepared in this way is made into a powder, a recombination problem occurs, where graphene particles overlap and re-agglomerate. Therefore, graphene prepared only in a dispersed solution retains its original state and may exist separately from each other. Furthermore, even when graphene oxide prepared in this way is thermally or chemically reduced, surface damage still occurs, leading to quality degradation.

[0045] The wrinkled graphene according to the present invention is prepared in a bottom-up rather than a top-down manner. Specifically, wrinkled graphene is prepared by self-growth in a single-step thermal plasma method using methane gas as a raw material without the use of other substrates or catalysts, as shown in Equation 1 below, ultimately having a zigzag wrinkled structure with small growth units.

[0046] [Formula 1]

[0047]

[0048] Because the wrinkled graphene prepared in this way does not clump together even when it is in powder form rather than solution form, its dispersibility is greatly improved even when used as powder itself. Furthermore, because the unique sp2 structure of high-quality graphene is well developed without surface defects, its conductivity is also excellent.

[0049] According to one embodiment of the invention, the wrinkled graphene has a thickness of 0.1 to 10 nm, preferably 0.5 to 5 nm, more preferably 1 to 3 nm. The wrinkled graphene can easily maintain its zigzag structure because its thickness is as thin as described above.

[0050] According to one embodiment of the present invention, the wrinkled graphene has a thickness of over 400 μm. 2 / g, preferably 400 to 2,000m 2 / g, more preferably 400 to 1,600m 2 / g BET specific surface area. Wrinkled graphene has a relatively large BET specific surface area because it has a thin zigzag wrinkled structure.

[0051] According to one embodiment of the invention, the wrinkled graphene contains 0.1 to 3% by weight, preferably 0.1 to 1.5% by weight, and more preferably 0.1 to 1% by weight of oxygen, based on the total weight of the wrinkled graphene. Because the wrinkled graphene is prepared as described above in a single-step thermal plasma process using methane gas as a raw material without the use of other substrates or catalysts, the wrinkled graphene has a lower oxygen content compared to plate-like graphene prepared by conventional oxidation processes.

[0052] According to one embodiment of the invention, the wrinkled graphene has a size of 50 to 500 nm, preferably 100 to 300 nm, and more preferably 100 to 200 nm. In this case, the size refers to the length of the longest straight-line distance from any point on the wrinkled graphene sheet to another point. Because the wrinkled graphene is instantaneously prepared as described above in a single-step thermal plasma process using methane gas as a raw material without the use of other substrates or catalysts, sheet-like graphene with relatively small dimensions of short growth units is observed as a zigzag wrinkled structure. Figure 1a and Figure 1b The SEM image of the example wrinkled graphene in the image confirms the structure.

[0053] According to one embodiment of the invention, the wrinkled graphene has an I content of 0.3 to 0.5, preferably 0.3 to 0.4. D / I G value. I D / I G The value represents the ratio of the relative intensities of the D-band peak and the G-band peak in the Raman spectrum, where each band is caused by the sp3 and sp2 bond structure of carbon atoms, indicating that the I... D / I G The smaller the value, the better the layered structure of graphene with sp2 bonds, exhibiting excellent crystallinity and no defects. The Ig value of wrinkled graphene... D / I G The value is slightly greater than that of plate-shaped graphene. D / I G The value is also much smaller than the I of carbon black. D / I G value.

[0054] According to one embodiment of the present invention, the crumpled graphene has a conductivity of 1.0×10 2 to 1.0×10 3 S / cm, preferably 1.0×10 2 to 5.0×10 2 S / cm, more preferably 1.0×10 2 to 3.0×10 2 S / cm. Carbon materials such as graphene and carbon black basically have high conductivity and contribute to improving the conductivity of lithium transition metal oxides with low conductivity. However, the conductivity of the carbon materials is reflected as it is in the lithium transition metal oxides. Therefore, in order to effectively improve the conductivity, it is important to uniformly coat the carbon materials on the lithium transition metal oxides.

[0055] The lithium transition metal oxide serves as the main positive electrode active material for exchanging electrons in the positive electrode of a lithium secondary battery. According to one embodiment of the present invention, the transition metal in the lithium transition metal oxide has the form of Li 1+x M y O 2+Z (0≤x≤5, 0<y≤2, 0≤z≤2), where M is selected from Ni, Co, Mn, Fe, P, Al, Mg, Ca, Zr, Zn, Ti, Ru, Nb, W, B, Si, Na, K, Mo, V, and combinations thereof, and there is no particular limitation within the above range. More specifically, the lithium transition metal oxide is selected from: LiCoO2, LiNiO2, LiMnO2, Li2MnO3, LiMn2O4, Li(Ni a Co b Mn c )O2(0<a<1, 0<b<1, 0<c<1, a + b + c = 1), LiNi 1-y Co y O2(0<y<1), LiCo 1-y Mn y O2, LiNi 1-y Mn y O2(0<y<1), Li(Ni a Co b Mn c )O4(0<a<2, 0<b<2, 0<c<2, a + b + c = 2), LiMn 2-z Ni z O4(0<z<2), LiMn 2-z Co z O4(0<z<2), and combinations thereof.

[0056] Wrinkled graphene is coated onto a lithium transition metal oxide. As a coating method, methods commonly used in the art can be used, but coating methods capable of forming mechanochemical bonds to increase the adhesion between the lithium transition metal oxide and the coating can be used. According to one embodiment of the invention, the mixture is coated onto the lithium transition metal oxide by a mechanical melting method capable of applying high shear forces. When wrinkled graphene is coated onto the lithium transition metal oxide by mechanical melting, the zigzag structure of the wrinkled graphene partially or completely collapses, thus allowing it to be coated thinly and uniformly on the surface of the lithium transition metal oxide.

[0057] Wrinkled graphene is coated onto lithium transition metal oxide to form a coating derived from wrinkled graphene on lithium transition metal oxide.

[0058] According to one embodiment of the invention, the coating derived from wrinkled graphene has a thickness of 1 to 500 nm, preferably 1 to 300 nm, more preferably 1 to 100 nm. In particular, the structure of the wrinkled graphene can be collapsed by a coating method such as mechanical melting, so that the wrinkled graphene can be thinly and uniformly coated on the surface of a lithium transition metal oxide.

[0059] According to one embodiment of the invention, the coating derived from wrinkled graphene has an I content of 0.1 to 0.2, preferably 0.15 to 0.2. D / I G Value. As mentioned above, considering that the pre-coated wrinkled graphene has an I value of 0.3 to 0.5. D / I G Value, I D / I G The value is reduced by a significant percentage through coating. According to one embodiment of the invention, the coating derived from wrinkled graphene... D / I G The value of I in wrinkled graphene D / I G The value is 30% to 70%, preferably 35% to 65%, and more preferably 40% to 60%. Because I D / I G The value is due to the sp2 and sp3 bond structure of carbon atoms, so I D / I G The decrease in the value refers to the zigzag-shaped, wrinkled graphene coating transcending simple physical adhesion and adhering tightly to the surface of the active material in a planar manner. At this point, as the stress of the zigzag structure is released, the sp2 structure on the plane develops. This may mean that, at this point, due to the elimination of structural defects in the carbon atoms inside the wrinkled graphene, a carbon coating that is mechanically and chemically stronger and more robust is formed.

[0060] According to one embodiment of the invention, a lithium transition metal oxide having a coating derived from wrinkled graphene formed thereon has a density of 1.0 × 10⁻⁶. -2 Up to 1.0 × 10⁻⁶ S / cm, preferably 1.0 × 10⁻⁶. -2 Up to 1.0×10 -1 S / cm, more preferably 5.0×10 -2 Up to 1.0×10 -1 The conductivity is measured in S / cm. Because the thin and uniform coating derived from wrinkled graphene effectively imparts conductivity to the lithium transition metal oxide, it exhibits high conductivity compared to coatings with other carbon-based materials.

[0061] According to one embodiment of the invention, a lithium transition metal oxide having a coating derived from wrinkled graphene formed thereon has a thickness of 2 to 10 μm. 2 / g, preferably 2 to 7.5m 2 / g, more preferably 2 to 5m 2 The BET specific surface area is [value missing] / g. As mentioned above, the wrinkled graphene material itself has a BET specific surface area exceeding 400m² / g. 2 The graphene has a high BET specific surface area of ​​ / g, but during the coating process, the zigzag structure of the wrinkled graphene partially or completely collapses, thus adhering tightly to the surface of the active material in a planar manner. Therefore, the specific surface area after coating is relatively significantly reduced compared to before coating.

[0062] The positive electrode active material for lithium secondary batteries according to the present invention effectively imparts conductivity to lithium transition metal oxides by forming a thin and uniform graphene coating on the lithium transition metal oxide using wrinkled graphene as a raw material. When this positive electrode active material is applied to lithium secondary batteries, the performance of the lithium secondary batteries, especially the discharge capacity at high C rates, is improved.

[0063] Lithium secondary batteries

[0064] This invention provides a lithium secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte. In the lithium secondary battery, the positive and negative electrodes are placed facing each other, and the separator is located between the positive and negative electrodes. The electrode assembly of the positive electrode, negative electrode, and separator is housed in a battery container filled with an electrolyte.

[0065] The positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing the aforementioned positive electrode active material.

[0066] In the positive electrode, there are no particular restrictions on the positive current collector, as long as it is conductive and does not cause chemical changes to the battery. Materials used include, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with surface treatments such as carbon, nickel, titanium, or silver. Furthermore, the positive current collector can typically have a thickness from 3 μm to 500 μm, and fine irregularities can be formed on its surface to improve the adhesion of the positive electrode active material. For example, it can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0067] The positive electrode active material layer may include conductive materials, binders, and the aforementioned positive electrode active material.

[0068] Conductive materials are used to impart conductivity to electrodes and can be used without any particular restrictions, provided that they do not cause chemical changes in the battery and possess electronic conductivity. Specific examples may include: graphite such as natural and artificial graphite; carbon materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermally cracked carbon black, single-walled or multi-walled carbon nanotubes, carbon fibers, graphene, activated carbon, and activated carbon fibers; metal powders or fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives, wherein one or a mixture of two or more may be used. The content of conductive material is typically 1% to 30% by weight, based on the total weight of the positive electrode active material layer.

[0069] Adhesives are used to improve the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples may include polyvinylidene fluoride (PVDF), PVDF-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers thereof, wherein one or a mixture of two or more can be used. Based on the total weight of the positive electrode active material layer, the adhesive content can be from 1% to 30% by weight.

[0070] In addition to using the aforementioned positive electrode active material, the positive electrode can be prepared according to conventional positive electrode preparation methods. Specifically, it can be prepared by coating a positive electrode active material layer forming composition comprising the aforementioned positive electrode active material and optionally a binder and conductive material onto a positive electrode current collector, followed by drying and rolling. In this case, the type and content of the positive electrode active material, binder, and conductive material are as described above.

[0071] The solvent can be one commonly used in the art and can include dimethyl sulfoxide (DMSO), isopropanol, N-methylpyrrolidone (NMP), acetone, water, etc., wherein one or a mixture of two or more can be used alone. Considering the coating thickness and production yield of the slurry, the amount of solvent used is sufficient to achieve a viscosity that can dissolve or disperse the positive electrode active material, conductive material, and binder, and exhibit excellent thickness uniformity when subsequently used to prepare the positive electrode.

[0072] As another method, the positive electrode can be prepared by casting a composition for forming a positive electrode active material layer onto a separate carrier and then stacking the film obtained by peeling off the carrier onto the positive electrode current collector.

[0073] The negative electrode includes a negative electrode current collector and a layer of negative electrode active material disposed on the negative electrode current collector.

[0074] The negative electrode active material layer optionally includes a binder and a conductive material as well as the negative electrode active material.

[0075] As anode active materials, compounds capable of reversibly inserting or de-intercalating lithium can be used. Specific examples include: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; (semi-)metallic materials capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; and (semi-)metal oxides capable of doping or de-doping lithium, such as SiO₂. β (0<β<2), SnO2, vanadium oxide and lithium vanadium oxide; or composite materials containing (semi-)metallic materials and carbonaceous materials such as Si-C composite materials and Sn-C composite materials, wherein any one or a mixture of two or more can be used. In addition, lithium metal thin films can be used as negative electrode active materials. Furthermore, as carbonaceous materials, low-crystallinity carbon, high-crystallinity carbon, etc. can be used. Representative examples of low-crystallinity carbon include soft carbon and hard carbon, and representative examples of high-crystallinity carbon include high-temperature calcined carbon such as irregular, plate-shaped, flake-shaped, spherical or fibrous natural or artificial graphite, condensed graphite, pyrolytic carbon, mesophase pitch carbon fibers, mesophase carbon microspheres, mesophase pitch and coke derived from petroleum or coal tar pitch.

[0076] The binder, conductive material, and negative electrode current collector can be selected with reference to the composition of the positive electrode described above, but are not limited thereto. Furthermore, the method for forming the negative electrode active material layer on the negative electrode current collector can include known coating methods as those used in the positive electrode, and is not particularly limited thereto.

[0077] In lithium-ion secondary batteries, the separator separates the negative and positive electrodes and provides a migration channel for lithium ions. Separators commonly used in lithium-ion secondary batteries can be used without any particular limitations. In particular, separators with low resistance to electrolyte ion migration and excellent electrolyte absorption capacity are preferred. Specifically, porous polymer membranes can be used, such as porous polymer membranes made from polyolefin polymers like ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminates of two or more layers thereof. Furthermore, conventional porous nonwoven fabrics can be used, such as nonwoven fabrics made from high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Additionally, separators coated with ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and single-layer or multi-layer structures can be selectively used.

[0078] The electrolyte used in this invention may include, but is not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., which can be used when preparing lithium secondary batteries.

[0079] Specifically, the electrolyte may contain organic solvents and lithium salts.

[0080] Organic solvents can be used without any particular restrictions, as long as they can serve as the medium through which ions participating in the electrochemical reactions of the battery pass. Specifically, organic solvents that can be used include: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group from C2 to C20, and may contain double bonds, aromatic rings, or ether bonds); amides such as dimethylformamide; dioxolane such as 1,3-dioxolane; sulfolane, etc. Carbonate solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate, propylene carbonate, etc.) with high ionic conductivity and high dielectric constant, which enhance the charging and discharging performance of the battery, with low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, etc.) are even more preferred. In this case, when a mixture of cyclic carbonates and linear carbonates is used in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte may be excellent.

[0081] Lithium salts can be used without any particular restrictions, as long as they are compounds capable of providing lithium ions used in lithium secondary batteries. Specifically, lithium salts that can be used include LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, and LiB(C2O4)2. Lithium salts are preferably used in a concentration range of 0.1 to 2.0 M. When the concentration of the lithium salt is within this range, the electrolyte exhibits suitable conductivity and viscosity, thus demonstrating excellent electrolyte performance, and lithium ions can migrate efficiently.

[0082] To improve battery life characteristics, suppress battery capacity decline, and improve battery discharge capacity, in addition to the electrolyte component, the electrolyte may further contain one or more additives, such as alkylene carbonate halides like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (condensed) glycol dimethyl ethers, hexamethylphosphoryltriamine, nitrobenzene derivatives, sulfur, quinone imine dyes, and N-substituted compounds. The additives include azole ketones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, and aluminum trichloride. In this case, the content of the additives can be from 0.1% to 5% by weight, based on the total weight of the electrolyte.

[0083] As described above, lithium secondary batteries containing the positive electrode active material according to the present invention stably exhibit excellent discharge capacity, output performance and capacity retention, and thus can be used in portable devices such as mobile phones, laptop computers and digital cameras, as well as electric vehicles such as hybrid electric vehicles (HEVs).

[0084] Therefore, according to another embodiment of the present invention, the present invention provides a battery module comprising the lithium secondary battery as a unit battery and a battery pack comprising the battery module.

[0085] Battery modules or battery packs can be used as a power source for any one or more medium to large-sized devices, including: power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles and plug-in hybrid electric vehicles (PHEVs); or energy storage systems.

[0086] Preferred implementation scheme

[0087] Preferred examples will be given below to aid in understanding the invention, but these examples are not intended to limit the invention, but rather to provide a better understanding of it.

[0088] Example

[0089] Example 1

[0090] By using wrinkled graphene (manufacturer: Nanointegris Technologies, Inc.; product: PureWaveGraphene; average thickness: 2.4 nm; BET specific surface area: over 400 m²), 2 / g; Size: 150-200nm; Oxygen content: 1%) with LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM 622) was mixed at a weight ratio of 1:99, and the mixture was then placed in a mechanical fusion apparatus (manufacturer: Hosokawa Micron Corporation; product: Nobilta NOB-130) and driven at 3,000 rpm for 10 minutes to prepare a positive electrode active material coated with wrinkled graphene.

[0091] Comparative Example 1

[0092] Except that a simple mechanical mixing device (manufacturer: Red Devil, Inc.; product: Classic Shaker 1400; conditions: 60 Hz, 60 minutes) was used instead of a mechanical fusion device when coating wrinkled graphene, the positive electrode active material coated with wrinkled graphene was prepared in the same manner as in Example 1.

[0093] Comparative Example 2

[0094] In addition to using plate-shaped graphene (manufacturer: KNANO Graphene Technology Corporation Limited; product: Graphene Powder; average thickness: 100nm; BET specific surface area: 51m²), 2 A positive electrode active material coated with plate-like graphene was prepared in the same manner as in Example 1, except that the graphene was replaced by a pleated graphene (g; size: 7 μm; oxygen content: less than 2%).

[0095] Comparative Example 3

[0096] Except that a simple mechanical mixing device (manufacturer: Red Devil, Inc.; product: Classic Shaker 1400; conditions: 60 Hz, 60 minutes) was used instead of a mechanical fusion device when coating plate graphene, the positive electrode active material coated with plate graphene was prepared in the same manner as in Comparative Example 2.

[0097] Comparative Example 4

[0098] Except that carbon black (manufacturer: Imerys; product: Super C65) was used instead of wrinkled graphene, the positive electrode active material coated with carbon black was prepared in the same manner as in Example 1.

[0099] Comparative Example 5

[0100] Except that a simple mechanical mixing device (manufacturer: Red Devil, Inc.; product: Classic Shaker 1400; conditions: 60 Hz, 60 minutes) was used instead of a mechanical fusion device when coating carbon black, the graphene-coated positive electrode active material was prepared in the same manner as in Comparative Example 4.

[0101] Experimental Example

[0102] Experimental Example 1: Comparison of SEM images of the prepared positive electrode active materials

[0103] NCM 622 without carbon-based material (graphene or carbon black) coating, as well as the positive electrode active materials according to Example 1 and Comparative Examples 1 to 4, were photographed using a scanning electron microscope (SEM) (magnification: ×50,000). The captured SEM images are shown below. Figure 2 (NCM 622) Figure 3 (Example 1) Figure 4 (Comparative Example 1) Figure 5 (Comparative Example 2) Figure 6 (Comparative Example 3) and Figure 7 (Comparative Example 4)

[0104] When based on the SEM image of NCM 622 without carbon coating ( Figure 2 When evaluating coated carbonaceous materials, firstly, in the case of wrinkled graphene, it can be confirmed that when coated using a mechanical mixing device, the structure of the wrinkled graphene remains unchanged, thus failing to form a uniform coating. Figure 4 However, when coated using a mechanical fusion device, the structure of the wrinkled graphene collapses, resulting in a thin and uniform coating. Figure 3 ).

[0105] Conversely, in the case of sheet-like graphene, it can be confirmed that when coated using a mechanical mixing device, the sheet-like graphene aggregates and clumps together, thus failing to form a uniform coating. Figure 6 Furthermore, even when using a mechanical fusion device, the sheet-like graphene remained as is on the surface of NCM 622, thus failing to form a uniform coating. Figure 5Furthermore, even in the case of carbon black, it can be confirmed that even when using a mechanical fusion device, carbon black will accumulate and cover the surface of NCM 622, thus failing to form a uniform coating. Figure 7 ).

[0106] Experimental Example 2: Electrical conductivity, BET specific surface area, and It of carbon-based materials D / I G Comparison of values

[0107] For the wrinkled graphene used in Examples 1 and 1, the sheet-like graphene used in Comparative Examples 2 and 3, and the carbon black used in Comparative Examples 4 and 5, the conductivity, BET specific surface area, and Ig were measured before and after coating. D / I G The values ​​are shown in Table 1 below. Electrical conductivity and BET specific surface area are measurements for the entire material, while I... D / I G The values ​​are measurements for carbonaceous materials. A 5g sample was placed in a holder and then rolled under a force of 30kN. Conductivity was measured using a conductivity measuring device with a 4-pin probe (manufacturer: Mitsubishi Chemical Corporation; product: MCP-PD51). BET specific surface area was measured using a BET specific surface area measuring device (manufacturer: Nippon Bell Corporation; product: BEL_SORP_MAX) by degassing at 200°C for 8 hours and performing N2 adsorption / desorption at 77K. D / I G The values ​​were measured using an Ar ion laser at a wavelength of 514.5 nm by Raman spectrometry analysis with a Raman spectrometer (manufacturer: Jasco, Inc.; product: NRS-2000B).

[0108] [Table 1]

[0109]

[0110] *I D / I G Value change rate (%) = I after coating D / I G Value / I before coating D / I G value × 100

[0111] According to Table 1 above, unlike sheet-like graphene and carbon black, it can be confirmed that when using wrinkled graphene, both electrical conductivity and specific surface area remain high. Furthermore, when using wrinkled graphene, the surface is coated sufficiently smoothly using a mechanical fusion device instead of a mechanical mixing device, thereby significantly reducing the BET specific surface area compared to the raw material and increasing electrical conductivity by approximately 7 times. (See Table 1 above for details.) D / I G As can be confirmed, it is determined that when wrinkled graphene is coated using a mechanical fusion device, the zigzag structure of the wrinkled graphene collapses, thus adhering tightly to the surface. Furthermore, with the release of structural stress, the structural defects of the sp2 bonds on the plane are alleviated, and I... D / I G The value is significantly reduced, thereby significantly improving conductivity while forming a thin and uniform coating.

[0112] Experimental Example 3: Evaluation of the C-rate characteristics of lithium secondary batteries using the positive electrode active materials of Example 1 and Comparative Examples 1 to 5

[0113] A positive electrode active material slurry was prepared by mixing the various positive electrode active materials from Examples 1 and Comparative Examples 1 to 5 with the conductive material carbon black and the binder PVDF in an N-methylpyrrolidone solvent at a weight ratio of 97.5:1:1.5 (positive electrode active material: conductive material: binder). The slurry was then coated onto one surface of an aluminum current collector (loading: 10-12 mg / cm²). 2 It is then dried at 130°C and then rolled to produce the positive electrode.

[0114] Lithium metal is used as the negative electrode, and a porous polyethylene separator is inserted between the positive and negative electrodes to prepare the electrode assembly. The electrode assembly is placed inside the battery case, and then the electrolyte is injected into the case to manufacture a lithium secondary battery. In this case, the electrolyte is prepared by dissolving 1.0 M lithium hexafluorophosphate (LiPF6) in an organic solvent composed of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (EC / DMC / EMC mixed volume ratio = 3 / 4 / 3).

[0115] A coin-shaped half-cell was fabricated using the method described above. It was then charged to 4.3V at 0.1C (CC / CV) at 25°C, followed by charging at 0.005C until cutoff, and finally discharged at 0.1C (CC) to 3.0V for initial charge and discharge capacity measurements and battery verification. Subsequently, the fabricated battery was charged to 4.25V at a constant current of 0.2C at 25°C and then charged at 0.05C until cutoff. The initial charge and discharge capacity were then measured by discharging to 2.5V at a constant current of 0.2C.

[0116] Subsequently, it was charged to 4.25V at a constant current of 0.2C and then charged at 0.05C until cutoff, and finally discharged to 2.5V at a constant current of 2.0C. Two cycles were performed, using charge and discharge as one cycle. Afterwards, the ratio of the battery's 2.0C discharge capacity to its 0.2C discharge capacity was measured and is shown in Table 2 below. The ratio (%) of the 2.0C discharge capacity to the 0.2C discharge capacity is calculated as "2.0C discharge capacity / 0.2C discharge capacity × 100".

[0117] [Table 2]

[0118] Positive electrode active material for lithium secondary batteries 2.0C discharge capacity ratio (%) Example 1 94.8 Comparative Example 1 89.6 Comparative Example 2 91.5 Comparative Example 3 78.4 Comparative Example 4 74.9 Comparative Example 5 75.3

[0119] According to Table 2 above, when using wrinkled graphene and sheet graphene, it can be confirmed that the discharge capacity is improved at a high C-rate of 2.0C when coated using a mechanical fusion device compared to coating using a mechanical mixing device. Furthermore, when using wrinkled graphene, excellent discharge capacity is generally exhibited at a high C-rate of 2.0C; in particular, when coated using a mechanical fusion device, the discharge capacity at a high C-rate of 2.0C is almost not degraded even compared to the discharge capacity at a low C-rate of 0.2C.

[0120] All simple variations and modifications of this invention should fall within the scope of protection of this invention, and the specific scope of protection of this invention will become apparent from the appended claims.

Claims

1. A positive electrode active material for lithium secondary batteries, said positive electrode active material comprising a lithium transition metal oxide, wherein a coating derived from wrinkled graphene is formed on said lithium transition metal oxide, wherein, The wrinkled graphene has a thickness of 0.1 nm to 10 nm. In this case, the zigzag structure of the wrinkled graphene may partially or completely collapse. The lithium transition metal oxide on which the coating derived from wrinkled graphene is formed has a density of 1.0 × 10⁻⁶. -2 Conductivity from S / cm to 1.0×10 S / cm.

2. The positive electrode active material for lithium secondary batteries according to claim 1, wherein, The wrinkled graphene has a thickness of over 400 μm. 2 / g BET specific surface area.

3. The positive electrode active material for lithium secondary batteries according to claim 1, wherein, Based on the total weight of the wrinkled graphene, the wrinkled graphene contains 0.1% to 3% oxygen by weight.

4. The positive electrode active material for lithium secondary batteries according to claim 1, wherein, The wrinkled graphene has a size ranging from 50 nm to 500 nm.

5. The positive electrode active material for lithium secondary batteries according to claim 1, wherein, The lithium transition metal oxide is selected from: LiCoO2, LiNiO2, LiMnO2, Li2MnO3, LiMn2O4, Li(Ni a Co b Mn c O2, where 0 <a<1,0<b<1,0<c<1,a+b+c=1, LiNi 1-y Co y O2, where 0 <y<1, LiCo 1-y Mn y O2, LiNi 1-y Mn y O2, where 0 <y<1, Li(Ni a Co b Mn c )O4, of which 0 <a<2,0<b<2,0<c<2,a+b+c=2, LiMn 2-z Ni z O4, where 0 <z<2, LiMn 2-z Co z O4, where 0 < z < 2, and Their combination.

6. The positive electrode active material for lithium secondary batteries according to claim 1, wherein, The coating derived from wrinkled graphene has an I0.1 to 0.

2. D / I G value.

7. The positive electrode active material for lithium secondary batteries according to claim 1, wherein, The coating, derived from pleated graphene, has a thickness of 1 nm to 500 nm.

8. The positive electrode active material for lithium secondary batteries according to claim 1, wherein, The lithium transition metal oxide on which the coating derived from wrinkled graphene is formed has a 2 μm 2 / g to 10 m 2 / g BET specific surface area.

9. A method for preparing the positive electrode active material for a lithium secondary battery according to claim 1, the method comprising the step of coating wrinkled graphene onto a lithium transition metal oxide to form a coating derived from wrinkled graphene on the lithium transition metal oxide, wherein, The wrinkled graphene has a thickness of 0.1 nm to 10 nm.

10. The method for preparing a positive electrode active material for lithium secondary batteries according to claim 9, wherein, The wrinkled graphene has a thickness of over 400 μm. 2 / g BET specific surface area.

11. The method for preparing a positive electrode active material for lithium secondary batteries according to claim 9, wherein, Based on the total weight of the wrinkled graphene, the wrinkled graphene contains 0.1% to 3% oxygen by weight.

12. The method for preparing a positive electrode active material for lithium secondary batteries according to claim 9, wherein, The wrinkled graphene has a size of 50 nm to 500 nm.

13. The method for preparing positive electrode active material for lithium secondary batteries according to claim 9, wherein, The coating derived from wrinkled graphene I D / I G The value is I of the wrinkled graphene D / I G Values ​​range from 30% to 70%.

14. The method for preparing a positive electrode active material for lithium secondary batteries according to claim 9, wherein, The wrinkled graphene is coated onto the lithium transition metal oxide by mechanical fusion.

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