Positive electrode active material, positive electrode including the positive electrode active material, and secondary battery including the positive electrode

Abstract

The present invention relates to a positive electrode active material comprising a core and a coating layer disposed on the core, wherein the core comprises Li 1+ x M y O 2+z , wherein M is at least one element selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), phosphorus (P), aluminum (Al), magnesium (Mg), calcium (Ca), zirconium (Zr), zinc (Zn), titanium (Ti), ruthenium (Ru), niobium (Nb), tungsten (W), boron (B), silicon (Si), sodium (Na), potassium (K), molybdenum (Mo) and vanadium (V), ‑0.2≤x≤0.2, 0<y≤2, and 0≤z≤2, the coating comprises carbon-based particles, wherein the carbon-based particles include a structure in which a plurality of graphene sheets are interconnected, and the D / G peak ratio during Raman spectroscopy measurement is in the range of 0.9 to 1.3; a positive electrode comprising the positive electrode active material and a secondary battery comprising the positive electrode.

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority from Korean Patent Application No. 10-2020-0111333, filed on September 1, 2020, the disclosure of which is incorporated herein by reference. Technical Field

[0004] The present invention relates to a positive electrode active material comprising a core and a coating layer disposed on the core, wherein the core comprises Li 1+x M y O 2+z , wherein M is at least one element selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), phosphorus (P), aluminum (Al), magnesium (Mg), calcium (Ca), zirconium (Zr), zinc (Zn), titanium (Ti), ruthenium (Ru), niobium (Nb), tungsten (W), boron (B), silicon (Si), sodium (Na), potassium (K), molybdenum (Mo) and vanadium (V), -0.2≤x≤0.2, 0<y≤2, and 0≤z≤2, the coating comprises carbon-based particles, wherein the carbon-based particles include a structure in which a plurality of graphene sheets are interconnected, and the D / G peak ratio during Raman spectroscopy measurement is in the range of 0.9 to 1.3; a positive electrode comprising the positive electrode active material and a secondary battery comprising the positive electrode. Background Art

[0005] In recent years, with the technological development and increasing demand for mobile devices, the demand for batteries as energy sources has increased significantly, leading to extensive research into batteries that can meet these needs. In particular, lithium secondary batteries, which offer excellent lifespan and cycle characteristics, as well as high energy density, are being actively researched as power sources for these devices.

[0006] A lithium secondary battery refers to a battery in which a non-aqueous electrolyte containing lithium ions is contained in an electrode assembly, the electrode assembly including a positive electrode, the positive electrode including a positive electrode active material capable of intercalating / deintercalating lithium ions, a negative electrode including a negative electrode active material capable of intercalating / deintercalating lithium ions, and a microporous separator disposed between the positive electrode and the negative electrode.

[0007] Lithium transition metal oxides can be used as positive electrode active materials, with cobalt, nickel, or manganese being used as the transition metal. To replace expensive cobalt, lithium transition metal oxides with high nickel or manganese content tend to be used. In particular, lithium transition metal oxides with high nickel content have the advantages of high energy density and affordability.

[0008] However, for lithium transition metal oxides with high nickel content, their electrical conductivity is lower than that of lithium transition metal oxides with high cobalt content. In addition, due to the layered structure of nickel, when the nickel content is high, the amount of lithium must also be increased. Therefore, there are problems such as an increase in the lithium component remaining on the surface of the positive electrode active material, the positive electrode active material easily reacts with moisture, and the surface structure of the positive electrode active material is destroyed in the air. In addition, since the lithium remaining on the surface easily reacts with carbon dioxide in the air to form lithium carbonate, there is a problem of a significant increase in the surface resistance of the positive electrode active material. In addition, when lithium remains in the form of LiOH, the gelation of the positive electrode slurry is caused by the reaction of the binder with the OH- of LiOH, and thus the processing performance in the positive electrode preparation is significantly reduced.

[0009] To address these issues, methods are being actively considered for forming a carbon coating on the surface of lithium transition metal oxides with high nickel content using polymers or asphalt. However, these methods require a high-temperature heat treatment process for carbonization, and in this case, the oxidation number of the metal in the high-nickel lithium transition metal oxide significantly changes due to the interaction between carbon atoms and oxygen atoms on the surface of the lithium transition metal oxide with high nickel content. As a result, during battery operation, the transition metal may dissolve, accelerating the collapse of the surface structure of the lithium transition metal oxide, significantly reducing the input / output characteristics and life characteristics of the battery.

[0010] Therefore, there is a need for a positive electrode active material that can effectively block contact with moisture, inhibit the dissolution of transition metals, minimize the collapse of the surface structure of lithium transition metal oxides, minimize the change in the oxidation number of metals in lithium transition metal oxides, and has high conductivity. Summary of the Invention

[0011] Technical issues

[0012] One aspect of the present invention provides a positive electrode active material that can minimize the collapse of the surface structure of a lithium transition metal oxide by improving the electrical conductivity of the positive electrode active material and effectively blocking the contact between moisture and the lithium transition metal oxide in the positive electrode active material, and can minimize the dissolution of a transition metal by minimizing the change in the oxidation number of the metal in the lithium transition metal oxide.

[0013] Another aspect of the present invention provides a positive electrode including the positive electrode active material, and a secondary battery including the positive electrode and having improved input / output characteristics and lifespan characteristics.

[0014] Technical Solution

[0015] According to one aspect of the present invention, there is provided a positive electrode active material comprising a core and a coating layer disposed on the core, wherein the core comprises Li1+x M y O 2+z , wherein M is at least one element selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), phosphorus (P), aluminum (Al), magnesium (Mg), calcium (Ca), zirconium (Zr), zinc (Zn), titanium (Ti), ruthenium (Ru), niobium (Nb), tungsten (W), boron (B), silicon (Si), sodium (Na), potassium (K), molybdenum (Mo) and vanadium (V), -0.2≤x≤0.2, 0<y≤2, and 0≤z≤2, the coating comprises carbon-based particles, wherein the carbon-based particles include a structure in which a plurality of graphene sheets are interconnected, and the D / G peak ratio when measured by Raman spectroscopy is in the range of 0.9 to 1.3.

[0016] According to another aspect of the present invention, a positive electrode including the positive electrode active material is provided.

[0017] According to another aspect of the present invention, a secondary battery including the positive electrode is provided.

[0018] Beneficial effects

[0019] According to the present invention, since the contact between the core and external moisture is effectively blocked, the structural collapse of the core can be prevented, thereby improving the input / output characteristics and life characteristics of the battery. In addition, since the coating layer containing carbon-based particles can be uniformly formed into a small thickness, the conductivity of the positive electrode active material can be significantly improved, thereby improving the input / output characteristics of the battery. In addition, since there is no separate heat treatment process during the formation of the coating layer, the oxidation number of the transition metal in the core can be prevented from excessively changing, thereby suppressing the problem of transition metal dissolution of the positive electrode active material and improving the input / output characteristics and life characteristics of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 are a schematic diagram and a transmission electron microscope (TEM) image illustrating a graphene sheet formation process of the primary carbon-based particles of Preparation Example 1;

[0021] Figure 2 TEM and STEM (scanning TEM) images of the primary carbon-based particles of Preparation Example 1;

[0022] Figure 3 is a scanning electron microscope (SEM) image of the primary carbon-based particles of Preparation Example 1;

[0023] Figure 4 (a) TEM image of the primary carbon-based particles of Preparation Example 1 and (b) TEM image of the primary carbon-based particles of Preparation Example 2.

[0024] Figure 5 is the SEM image of carbon black of sample 1;

[0025] Figure 6 The Li[Ni 0.6 Co 0.2 Mn 0.2 ]SEM image of O2;

[0026] Figure 7 is an SEM image of the positive electrode active material of Example 1 of the present invention;

[0027] Figure 8 is a SEM image of the positive electrode active material of Comparative Example 1 of the present invention; and

[0028] Figure 9 is a SEM image of the positive electrode active material of Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0029] Hereinafter, the present invention will be described in more detail so that the present invention can be more clearly understood. In this case, it should be understood that the words or terms used in the specification and claims should not be interpreted as the meanings defined in commonly used dictionaries, and it should be further understood that these words or terms should be interpreted as having the following meanings: Based on the principle that the inventor can appropriately define the meaning of words or terms to best explain the invention, consistent with their meanings in the context of the relevant technology and the technical concept of the invention.

[0030] The expression "average particle size (D 50 )” can be defined as the particle size when the cumulative volume in the particle size distribution curve is 50%. The average particle size (D 50 ) can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle sizes from submicron to several millimeters and can obtain highly reproducible and high-resolution results.

[0031] <Positive electrode active material>

[0032] The positive active material according to an embodiment of the present invention includes a core and a coating layer provided on the core, wherein the core includes Li 1+x M y O 2+z, wherein M is at least one element selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), phosphorus (P), aluminum (Al), magnesium (Mg), calcium (Ca), zirconium (Zr), zinc (Zn), titanium (Ti), ruthenium (Ru), niobium (Nb), tungsten (W), boron (B), silicon (Si), sodium (Na), potassium (K), molybdenum (Mo) and vanadium (V), -0.2≤x≤0.2, 0<y≤2, and 0≤z≤2, the coating comprises carbon-based particles, wherein the carbon-based particles include a structure in which a plurality of graphene sheets are interconnected, and the D / G peak ratio when measured by Raman spectroscopy is in the range of 0.9 to 1.3.

[0033] The core includes Li 1+x M y O 2+z , wherein M may be at least one element selected from the group consisting of Ni, Co, Mn, Fe, P, Al, Mg, Ca, Zr, Zn, Ti, Ru, Nb, W, B, Si, Na, K, Mo, and V, and x, y, and z may satisfy -0.2≤x≤0.2, 0<y≤2, and 0≤z≤2, respectively. x may preferably satisfy -0.1≤x≤0.1, and may more preferably satisfy 0≤x≤0.1. Specifically, Li 1+ x M y O 2+z Can include Li 1+x [Ni a Co b M 1 c M 2 d ]O2 or Li 1+x [Ni a Co b M 1 c M 2 d ]O2. In Li 1+x [Ni a Co b M 1 c M 2 d ]O2, M 1 It can be at least one element of Al and Mn, M 2It may be at least one element selected from the group consisting of Fe, P, Mg, Ca, Zr, Zn, Ti, Ru, Nb, W, B, Si, Na, K, Mo and V, a may satisfy 0<a<1, preferably 0.3≤a<1, more preferably 0.5≤a<1, b may satisfy 0<b<1, preferably 0<b<0.7, more preferably 0<b<0.5, c may satisfy 0<c<1, preferably 0<c<0.7, more preferably 0<c<0.5, d may satisfy 0≤d≤0.2, preferably 0≤d≤0.1. 1+x M y O 2+z It may include LiCoO2, LiNiO2, LiMnO2, Li[Ni 0.5 Co 0.3 Mn 0.2 ]O2、Li[Ni 0.6 Co 0.2 Mn 0.2 ]O2、Li[Ni 0.7 Co 0.1 Mn 0.2 ]O2、Li[Ni 0.8 Co 0.1 Mn 0.1 ]O2、Li[Ni 0.9 Co 0.05 Mn 0.05 ]O2, LiMn2O4, LiFePO4 and 0.5Li2MnO3·0.5Li[Mn 0.4 Ni 0.3 Co 0.3 ]O2. Preferably, Li 1+x M y O 2+z Can include Li[Ni 0.6 Co 0.2 Mn 0.2 ]O2、Li[Ni 0.7 Co 0.1 Mn 0.2 ]O2、Li[Ni 0.8 Co 0.1 Mn 0.1 ]O2 and Li[Ni 0.9 Co 0.05 Mn 0.05 ]O2. Since the core includes Li 1+x M y O 2+z , lithium can be fully supplied to the negative electrode, and due to Li 1+x M y O 2+zThe electrochemical activity is exhibited after the first cycle without causing a decrease in the overall performance of the battery, which can eliminate the loss of battery capacity caused by the irreversible capacity of the negative electrode.

[0034] Li 1+x M y O 2+z It may be in the form of secondary particles formed by combining or assembling primary particles, or it may be in the form of a single particle.

[0035] In Li 1+x M y O 2+z In the case of secondary particles, the energy density of the positive electrode can be improved, and because Li 1+ x M y O 2+z The contact area between the electrolyte and the lithium ions is large. 1+x M y O 2+z The movement distance in the battery is short, which can improve the capacity and output characteristics of the battery. 1+x M y O 2+z In the case of secondary particles, it is also possible to 1+x M y O 2+z The coating layer described later is uniformly formed on the surface of the primary particles and the surface of the secondary particles including the concave spaces between the primary particles. 1+x M y O 2+z In the case of a single particle, the coating layer described later can be formed on the Li 1+x M y O 2+z The surface of the composite material has a uniform thickness and shape.

[0036] The coating may be provided on the core. Specifically, the coating may cover at least a portion of the surface of the core.

[0037] The coating may include carbon-based particles.

[0038] The carbon-based particles may include a structure in which a plurality of graphene sheets are connected. In the present invention, the term "graphene sheet" refers to a carbonaceous structure having a thickness of less than 20 nm, flexibility, and film form. Specifically, in the carbon-based particles, at least two graphene sheets may be directly connected to each other or may be indirectly connected. Since the carbon-based particles include a structure in which a plurality of graphene sheets are connected, the surface of the core may be smoothly covered by the carbon-based particles. That is, since most of the surface of the core is covered by the carbon-based particles, the contact between the core and external moisture is effectively blocked, thereby preventing the structural collapse of the core. The carbon-based particles may include graphene sheets with different planar directions.

[0039] The carbon-based particles can be in the form of secondary particles formed by connecting multiple graphene sheets. Specifically, multiple graphene sheets can be interconnected to form secondary particles in the form of long chains. More specifically, the chain-like secondary particles can partially include regions of aggregated graphene sheets. Due to the unique chain-like connection structure of the secondary particles, the carbon-based particles have excellent electrical and thermal conductivity.

[0040] The carbon-based particles may also include a connecting portion connected to at least a portion of a graphene sheet in the plurality of graphene sheets. In the present invention, during the preparation process of the carbon-based particles, the carbonaceous material, such as carbon black, is continuously oxidized and broken to form graphene sheets, and a portion may also exist that maintains its original shape without breaking. In this case, the portion that maintains its shape may correspond to the connecting portion. Therefore, the connecting portion may have a non-graphene shape, and the expression "non-graphene shape" may refer to a block having a thickness greater than that of the graphene sheet, as distinct from the aforementioned graphene sheet.

[0041] A portion of each of the plurality of graphene sheets may be directly connected to one another. Alternatively, at least a portion of the plurality of graphene sheets may be connected to one another via the connecting portion. Specifically, at least a portion of each of the plurality of graphene sheets may be connected to the connecting portion. The carbon-based particles may include two connection methods.

[0042] The carbon-based particles are formed while the primary carbon-based particles are coated on the core. The primary carbon-based particles can be modified by oxidation to form nearly spherical particles of carbon black, such as acetylene black, furnace black, thermal black, channel black, and lamp black. Figure 1As shown, the structure of carbon black can be changed by oxidation treatment to form primary carbon-based particles including a plurality of graphene sheets, and the primary carbon-based particles can be coated on a core to become carbon-based particles. In the case where the carbon black is in the form of secondary particles, carbon-based particles in the form of secondary particles can be formed in which particles including a plurality of graphene sheets are aggregated.

[0043] The graphene sheets may have an average thickness of 10 nm or less, particularly 0.34 nm to 10 nm, and more particularly 0.34 nm to 5 nm. When the average thickness of the graphene sheets falls within the above range, the carbon-based particles exhibit excellent electrical conductivity due to the flexibility unique to the graphene sheets and improved surface contact due to the graphene sheets. The graphene sheets may have a shape in which 10 or fewer graphene layers are stacked. The average thickness of the graphene sheets can be determined by transmission electron microscopy (TEM) analysis and may correspond to the average value obtained by measuring the thickness of 100 graphene sheets.

[0044] The graphene sheet may have a lateral size of 10 nm to 500 nm, particularly 10 nm to 300 nm or less, more particularly 10 nm to 100 nm, for example, 50 nm to 90 nm. The lateral size of the graphene sheet can be controlled according to the degree of heat treatment. For example, the lateral size of the graphene sheet can be controlled by further performing a separate heat treatment in an inert atmosphere after the oxidation treatment. When the lateral size of the graphene sheet satisfies the above range, the ions in the electrolyte can diffuse smoothly in the electrode. Therefore, the fast charging characteristics of the battery can be improved and the rate capability can also be improved. In addition, since the graphene sheet can effectively cover the surface of the core, the contact between the core and external moisture is effectively blocked, thereby preventing the structural collapse of the core. In addition, the side reactions caused by lithium on the core surface can be suppressed. Therefore, the input / output characteristics and life characteristics of the battery can be improved. The lateral size of a graphene sheet refers to the average value of the sizes of 100 graphene sheets observed by a scanning electron microscope (SEM) or TEM, and herein, the expression "size" refers to the longest length of a line assumed to run from one point to another point on a graphene sheet.

[0045] The coating layer may have a thickness of 1 nm to 500 nm, particularly 10 nm to 300 nm, more particularly 10 nm to 100 nm. In the case where the thickness satisfies the above range, the coating layer may be formed by 1+x M y O 2+z By minimizing the change in the oxidation number of the transition metal in the battery, the dissolution of the transition metal can be suppressed without inhibiting the diffusion of lithium ions intercalation and deintercalation. Therefore, the input / output characteristics and life characteristics of the battery can be improved.

[0046] The weight ratio of the core to the coating layer may be in the range of 99.9:0.1 to 90:10, particularly 99.9:0.1 to 95:5, more particularly 99.9:0.1 to 99:1. 1+x M y O 2+z The change in the oxidation number of the transition metal is minimized without inhibiting the diffusion of intercalation and deintercalation to suppress the dissolution of the transition metal lithium ions. Therefore, the input / output characteristics and life characteristics of the battery can be improved.

[0047] In the Raman spectrum measurement of the positive electrode active material, the D / G peak ratio may be in the range of 0.9 to 1.3, particularly 0.9 to 1.1, and more particularly 0.9 to 1.0. In the Raman spectrum, 1590 cm -1 The G peak near the carbon is due to the sp 2 Key E 2g Vibration mode, when the sp 2 When the key is defective, 1350cm -1 A D peak appears near the nucleus. When the D / G peak ratio is met, it indicates that the carbon-based particles with a high degree of graphitization are coated on the surface of the core by a strong shear force. Therefore, when carbon-based particles are used, the capacity and electrical characteristics of the battery can be improved due to the high conductivity of the carbon-based particles.

[0048] The positive electrode active material may have a 2m 2 / g to 8m 2 / g, for example 2m 2 / g to 5m 2 / g specific surface area. The specific surface area of the positive electrode active material is less than 2m 2 When the specific surface area of the positive electrode active material is greater than 8m 2 / g, the side reaction of the electrolyte may occur excessively. Therefore, when the specific surface area of the positive electrode active material is 2m 2 / g to 8m 2 When the content of MgO2 is within the range of 1:1 / g, the side reaction of the electrolyte can be suppressed while maintaining the energy density.

[0049] When a simple mixing method (e.g., using an acoustic mixer, a paint stirrer, or a blade mixer) is used during coating formation, sufficient shearing force may not be provided to rearrange the carbon in the coating, and the coating may not uniformly coat the core, thereby failing to obtain a range of D / G peak ratios and a range of specific surface areas. In the present invention, since a strong shearing force can be applied by a mechanofusion method to form a coating to the extent that rearrangement of the carbon structure occurs, a range of low-level D / G peak ratios (high degree of graphitization) and a range of specific surface areas can be obtained, and the D / G peak ratio can be significantly changed before and after coating.

[0050] The positive electrode active material may have a 1.0×10 -3 s / cm to 1.0×10 s / cm, especially 1.0×10 -3 s / cm to 1.0×10 -1 s / cm, more particularly 1.0×10 -2 s / cm to 1.0×10 -1 s / cm. In the preparation process of the positive electrode active material of the present invention, the carbon-based particles are arranged on the core by a mechanical fusion method that can apply a strong shear force. In this case, since a coating that is dense enough to rearrange the carbon structure in the carbon-based particles is formed, the above-mentioned powder conductivity can be obtained. When the above range is met, it can be obtained by making Li 1+x M y O 2+z This minimizes changes in the oxidation number of transition metals in the battery while suppressing the diffusion of lithium insertion and deinsertion, thereby inhibiting transition metal dissolution. This improves the battery's input / output characteristics and lifespan. Powder conductivity can be measured using a four-probe powder resistance measurement method.

[0051] <Positive electrode>

[0052] A positive electrode according to another embodiment of the present invention may include the positive electrode active material of the above embodiment. The description of the positive electrode active material is the same as above and thus omitted.

[0053] The positive electrode may include a current collector and a positive electrode active material layer disposed on the current collector and containing a positive electrode active material. In addition, the positive electrode active material layer may further include a binder.

[0054] The current collector is not particularly limited, as long as it has conductivity and does not cause adverse chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, aluminum, or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. can be used as the current collector. Specifically, transition metals such as copper and nickel that are well absorbed by carbon can be used as the current collector. The positive electrode active material layer can be provided on one or both surfaces of the current collector.

[0055] The binder may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylate, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and a material having its hydrogen replaced by lithium (Li), sodium (Na) or calcium (Ca), or may include various copolymers thereof.

[0056] Secondary batteries

[0057] A secondary battery according to another embodiment of the present invention may include a negative electrode, a positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode is the positive electrode described in the above embodiment. Therefore, the description of the positive electrode will be omitted.

[0058] The negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on one surface or both surfaces of the negative electrode current collector.

[0059] The negative electrode current collector is not particularly limited, as long as it is conductive and does not cause adverse chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, aluminum, or stainless steel surface-treated with one of carbon, nickel, titanium, and silver can be used as the negative electrode current collector. Specifically, transition metals such as copper and nickel, which are well absorbed by carbon, can be used as the current collector.

[0060] The negative electrode active material layer may include a negative electrode active material, a negative electrode conductor, and a negative electrode binder.

[0061] The negative electrode active material may include graphite-based active material particles or silicon-based active material particles. At least one selected from artificial graphite, natural graphite, graphitized carbon fiber, and graphitized intermediate carbon microbeads may be used as the graphite-based active material particles, and when artificial graphite is used, the rate performance may be improved. x At least one of the group consisting of (0<x<2), Si-C composite and Si-Y alloy (wherein Y is an element selected from alkali metals, alkaline earth metals, transition metals, Group 13 elements, Group 14 elements, rare earth elements and combinations thereof) can be used as silicon-based active material particles, especially when Si and SiO are used. x (0<x<2).

[0062] The negative electrode binder may include a material selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylate, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and a material having its hydrogen replaced by lithium (Li), sodium (Na), or calcium (Ca), or may include various copolymers thereof.

[0063] There is no particular limitation on the negative electrode conductive agent as long as it has conductivity and does not cause adverse chemical changes in the battery. For example, conductive materials such as graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbon powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; or polyphenylene derivatives.

[0064] Separator separates the negative electrode and the positive electrode and provides a path for the movement of lithium ions, wherein any separator can be used as a separator without particular limitation, as long as it is commonly used in secondary batteries, in particular, a separator having high water retention capacity for the electrolyte and low resistance to electrolyte ion transfer can be used. Specifically, porous polymer film, for example, a porous polymer film prepared by a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer or a laminated structure with two or more layers can be used as a separator. In addition, typical porous nonwoven fabrics can be used, for example, a nonwoven fabric formed by high melting point glass fiber or polyethylene terephthalate fiber. In addition, in order to ensure heat resistance or mechanical strength, a coating separator including a ceramic component or a polymer material can be used, and a separator with a single layer or multilayer structure can be selectively used.

[0065] The electrolyte may include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten-type inorganic electrolyte that can be used to prepare a lithium secondary battery, but the present invention is not limited thereto.

[0066] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0067] As the nonaqueous organic solvent, for example, an aprotic solvent such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate can be used.

[0068] Especially, in carbonate organic solvent, owing to the high dielectric constant as high viscosity organic solvent, ethylene carbonate and propylene carbonate as cyclic carbonate can dissociate lithium salt well, therefore can preferably use cyclic carbonate.By using with suitable ratio mixing of the linear carbonate of low viscosity, low dielectric constant such as above-mentioned cyclic carbonate and dimethyl carbonate, diethyl carbonate, can obtain having the electrolyte of high conductivity, therefore can more preferably use cyclic carbonate.

[0069] Lithium salts can be used as metal salts. Lithium salts are materials that are easily soluble in non-aqueous organic solvents, for example, selected from F - 、Cl - , I - 、NO3 - 、N(CN)2 - 、BF4 - 、ClO4 - PF6 - 、(CF3)2PF4 - 、(CF3)3PF3 - 、(CF3)4PF2 - 、(CF3)5PF - 、(CF3)6P - CF3SO3 - CF3CF2SO3 - 、(CF3SO2)2N - 、(FSO2)2N - CF3CF2(CF3)2CO - 、(CF3SO2)2CH - 、(SF5)3C - 、(CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - 、CH3CO2 - 、SCN - , and (CF3CF2SO2)2N- At least one of the constituted group can be used as an anion of the lithium salt.

[0070] In order to improve the life characteristics of the battery, suppress the reduction in battery capacity, and increase the discharge capacity of the battery, in addition to the electrolyte components, at least one additive, for example, a halogenated alkylene carbonate-based compound such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol or aluminum trichloride may be further added to the electrolyte.

[0071] According to another embodiment of the present invention, a battery module including a secondary battery as a unit cell and a battery pack including the battery module are provided. Since the battery module and the battery pack include secondary batteries having high capacity, high rate capability, and high cycle characteristics, the battery module and the battery pack can be used as a power source for medium and large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0072] <Method for Preparing Positive Electrode Active Material>

[0073] According to another embodiment of the present invention, a method for preparing a positive electrode active material includes the following steps: preparing primary carbon-based particles; and forming a coating layer by coating the primary carbon-based particles on a core, wherein forming the coating layer including the carbon-based particles by coating the primary carbon-based particles on the core includes using a mechanical fusion method after mixing the primary carbon-based particles and the core, the core including Li 1+x M y O 2+z , wherein M is at least one element selected from Ni, Co, Mn, Fe, P, Al, Mg, Ca, Zr, Zn, Ti, Ru, Nb, W, B, Si, Na, K, Mo, and V, 0≤x≤5, 0<y≤2, 0≤z≤2, and the carbon-based particles may include a structure in which a plurality of graphene sheets are interconnected. The positive electrode active material of the above embodiment can be formed by the above-described preparation method. The core, coating, and carbon-based particles ultimately contained in the positive electrode active material are the same as those of the above embodiment.

[0074] The preparation of primary carbon-based particles includes the following steps: preparing a carbonaceous material, and modifying the carbonaceous material by oxidation treatment, wherein modifying the carbonaceous material by oxidation treatment may include at least one of the following: a) performing a first heat treatment on the carbonaceous material at a temperature of 200°C to 800°C in at least one of an oxygen atmosphere and an air atmosphere; b) reacting the carbonaceous material with acidic steam at 120°C to 300°C.

[0075] The preparation of primary carbon-based particles may include the following steps: preparing a carbonaceous material, and modifying the carbonaceous material through an oxidation treatment.

[0076] In the preparation of the carbonaceous material, the carbonaceous material may be carbon black. Specifically, the carbonaceous material may be at least one selected from the group consisting of acetylene black, furnace black, thermal black, channel black, and lamp black. More specifically, the carbonaceous material may be acetylene black produced at a maximum temperature to substantially have an excellent degree of graphitization.

[0077] The preparation of the carbonaceous material may include pyrolysis of acetylene gas, and carbon black, such as acetylene black, may be formed by pyrolysis. The acetylene gas may be high-purity acetylene gas, specifically acetylene gas with a purity of 95% or more, such as 98% or more.

[0078] The pyrolysis of acetylene gas can be performed at a temperature of 1,500°C or higher, particularly 1,500°C to 2,200°C, and more particularly 1,500°C to 2,000°C. When the temperature satisfies the above range, the degree of graphitization of the prepared carbonaceous material can be high, and the degree of graphitization of the primary carbon-based particles prepared therefrom can also be high. Therefore, the electrical conductivity of the primary carbon-based particles can be improved.

[0079] The carbonaceous material may be carbon black, but acetylene black is particularly preferred for the following reasons. The graphene sheets contained in the carbon-based particles included in the coating layer of the positive electrode active material of the present invention can be formed by modifying the surface of the carbonaceous material through an oxidation treatment. Acetylene black formed through pyrolysis can have a highly graphitized surface. Therefore, compared to other oxidized carbon blacks that inevitably contain some oxygen functional groups on their surfaces, acetylene black can smoothly form a graphene sheet structure when subjected to an oxidation treatment.

[0080] Pyrolysis can be performed by introducing acetylene gas into the reaction furnace after adjusting the internal temperature of the reaction furnace to the above temperature range and immediately performing pyrolysis. In addition, air, oxygen, and H2O can be further added during this process to control the density of the primary carbon-based particles and oxygen functional groups, and to control the connection structure in the primary carbon-based particles.

[0081] Modification of the carbonaceous material by oxidation treatment may include at least one of: a) subjecting the carbonaceous material to a first heat treatment at a temperature of 200°C to 800°C in at least one oxygen atmosphere or air atmosphere (step a); and b) reacting the carbonaceous material with acidic steam at 120°C to 300°C (step b).

[0082] In step a, at least one of an oxygen atmosphere and an air atmosphere can be formed by introducing oxygen and / or air into a reactor containing the carbonaceous material. Specifically, the graphene sheet structure can be formed in the reactor through an oxidation process by setting an appropriate flow rate and flow rate of oxygen or air during the first heat treatment, a reaction temperature, and a reaction time. In addition, the conditions of the oxidation process can vary depending on the density of the carbonaceous material and the amount of oxygen functional groups.

[0083] In step a, the first heat treatment can be performed by controlling the temperature of the reactor in the reactor containing the carbonaceous material. The first heat treatment can be performed at a heat treatment temperature of 200°C to 800°C, specifically at a heat treatment temperature of 200°C to 450°C. When the heat treatment temperature satisfies the above range, excessive oxidation of the carbonaceous material can be prevented, and a graphite sheet having a desired size can be formed. The first heat treatment can be performed for 1 hour to 50 hours.

[0084] In step b, the carbonaceous material may react with acidic vapor to be oxidized to form graphene. Specifically, the acidic vapor may be vapor derived from an acidic solution such as HCl or HNO3. The temperature of the acidic vapor reacting with the carbonaceous material may be in the range of 120°C to 300°C.

[0085] After the carbonaceous material is modified by oxidation treatment, a second heat treatment process may be further performed in an inert atmosphere to increase the size of the graphene sheets formed. Specifically, the method for preparing a positive electrode active material may further include performing a second heat treatment on the carbonaceous material modified by oxidation treatment at a temperature of 500°C or higher in an inert atmosphere after modifying the carbonaceous material by oxidation treatment and before forming a coating layer comprising carbon-based particles by coating primary carbon-based particles on the core. In this case, the inert atmosphere may be formed by a vacuum or any one gas selected from helium, argon, and nitrogen. The second heat treatment temperature may be 500°C or higher, for example, 600°C to 1,600°C.

[0086] The mechanism of forming primary carbon-based particles by the step of preparing primary carbon-based particles in the present invention can be described as follows. During the preparation of primary carbon-based particles, spherical or chain carbon black is subjected to oxidation treatment, wherein the average diameter of the spherical primary particles is 50nm or less, and the primary particles share a structure, for example, acetylene black under specific conditions. In this case, oxidants such as oxygen and acidic vapor penetrate and oxidation reactions occur from defective portions such as grain boundaries or dislocations present in the unit microstructure of the carbon black. When the oxidation treatment is carried out for a predetermined time within the temperature range described in the preparation method, the oxidant penetrates into the internal microstructure of the carbon black and causes oxidation. In this case, in order to alleviate the structural stress of the microstructure of the primary particles whose radius of curvature is greater than the radius of curvature of the surface of the spherical primary particles, oxidation reactions occur rapidly in the primary particles. Therefore, the internal carbon atoms are oxidized into gases such as CO, CO2, CH4, and the primary particles become hollow type. Most of the structural stress remaining in the spherical primary particles is also released, while the surface structure of the hollow primary particles is also destroyed by the continuous oxidation treatment, resulting in the appearance of graphene sheets. Therefore, as the average diameter of the carbon black primary particles decreases, the internal density of the particles decreases, and the amount of oxygen functional groups in the primary particles increases, becoming greater than that on the surface. Furthermore, step a is preferable to step b in terms of further accelerating the modification process.

[0087] Similar to the carbon-based particles of the above embodiment, the primary carbon-based particles have a structure in which a plurality of graphene sheets are interconnected, and have the same physical properties, such as the lateral size and thickness of the graphene sheets in the carbon-based particles of the above embodiment.

[0088] The primary carbon-based particles may have a 200 m 2 / g or greater, especially 300m 2 / g to 1,100m 2 / g specific surface area (m 2 / g) / g, more particularly 500m 2 / g to 900m 2 / g. Meeting this specific surface area range means the graphene sheets within the primary carbon-based particles are large in area, ensuring electrode conductivity even with a low conductive content. Furthermore, since contact between the core and external moisture is effectively prevented, structural collapse of the core is prevented. Consequently, the initial charge capacity of the battery can be increased.

[0089] In the primary carbon-based particles, the oxygen content of the primary carbon-based particles can be 1 weight % or more, particularly 1 weight % to 10 weight %, more particularly 1 weight % to 5 weight %. When the oxygen content of the primary carbon-based particles meets the above range, since the primary carbon-based particles can be smoothly dispersed in the dispersion when the coating is formed by a wet process, the coating can be formed more uniformly. The oxygen content can be measured by elemental analysis of carbon (C), hydrogen (H), oxygen (O) and nitrogen (N).

[0090] The oxygen content can be controlled during the oxidation treatment of the carbon black. Specifically, oxygen-containing functional groups can be formed on the surface of the primary carbon-based particles through the oxidation treatment. The oxygen-containing functional groups can be at least one selected from the group consisting of carboxyl groups, hydroxyl groups, and carbonyl groups. After the oxidation treatment, the oxygen content can be further controlled by heat treating the primary carbon-based particles in an inert atmosphere.

[0091] In this respect, the primary carbon-based particles differ from typical graphene. That is, since typical graphene is produced by grinding particles such as artificial graphite, the oxygen content may not be as high as 1% by weight or more, but may be very low or even zero. Furthermore, according to the method for producing typical graphene formed by grinding, graphene is formed one by one (graphene sheets), but according to the present invention, the primary carbon-based particles formed include a structure in which multiple graphene sheets are connected.

[0092] The primary carbon-based particles can have a higher degree of graphitization than carbon black before oxidation treatment. Specifically, since the high structural stress caused by the surface tension of carbon black can be partially eliminated when forming graphene sheets, the degree of graphitization of the prepared primary carbon-based particles can be increased.

[0093] The primary carbon-based particles may have a value calculated by the following Equation 1 of 0.12 or less, specifically 0 to 0.1, and more specifically 0 to 0.07.

[0094] [Equation 1]

[0095]

[0096] In Equation 1, a is the specific surface area (m2) of the primary carbon-based particles measured by the nitrogen adsorption BET method. 2 / g), b is the iodine adsorption value (mg / g) of primary carbon-based particles. In the case of including a pore structure inside or between primary carbon-based particles, small-sized nitrogen (N2) molecules may be adsorbed in the pores in large quantities. In contrast, since iodine (I2) is a relatively large molecule, it is difficult to enter the pores compared with nitrogen, so the iodine adsorption value is not large. That is, when there is a pore structure, the value according to equation 1 increases. In other words, in primary carbon-based particles, the value according to equation 1 is 0.12 or less, which means that primary carbon-based particles do not include micropores. That is, in the absence of micropores, since the adsorption of iodine is similar to that of nitrogen, the value of equation 1 decreases. This means that the surface of primary carbon-based particles is a free surface (free surface). Specifically, most of carbon black is modified into a hollow structure by oxidation treatment, and the structure is destroyed by continuous oxidation treatment to form graphene sheets. In this case, the graphene sheets can be formed to open outward without forming a pore structure.

[0097] The step of forming the coating layer including carbon-based particles by coating the primary carbon-based particles on the core includes using a mechanical fusion method after mixing the primary carbon-based particles and the core.

[0098] Specifically, the mechanical fusion method can be carried out using the Nobilta equipment of Hosokawa Micron Corporation. After the mixture of primary carbon-based particles and core is placed in a container, the container is rotated to move the mixture to the inner wall of the container by centrifugal force. Afterwards, due to the strong shear force applied by the arm head approaching the inner wall of the container with a small gap, the primary carbon-based particles are firmly coated on the core by interaction, thereby forming a coating comprising carbon-based particles. Between the particle surfaces of the mixture, the primary carbon-based particles can be converted into the above-mentioned carbon-based particles.

[0099] The shear force may be in the range of 1 m / s to 500 m / s, for example, 10 m / s to 100 m / s.

[0100] In the case of using an acoustic mixer different from the present invention, since the acoustic mixer uses simple vibration, the primary carbon-based particles are relatively unevenly arranged on the surface of the core rather than forming a coating containing carbon-based particles, which is different from the positive electrode active material of the present invention. In contrast, since the mechanofusion method corresponds to a method capable of applying high mechanical shear force, the mechanofusion method is used during the preparation of the positive electrode active material to significantly improve conductivity by uniform coating in the present invention.

[0101] Because the mechanofusion method applies strong shear forces through an arm with a small gap, it is difficult to control particle breakage. However, after extensive research, the researchers of the present invention have confirmed that by using primary carbon-based particles with multiple graphene sheets, particle breakage can be suppressed. As a result, a uniform coating can be formed by mechanofusion while suppressing particle breakage.

[0102] Hereinafter, embodiments of the present invention will be described in detail in a manner that can be easily implemented by those skilled in the art. However, the present invention can be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

[0103] Preparation Example 1: Preparation of primary carbon-based particles

[0104] (1) Formation of carbonaceous material (acetylene black)

[0105] Acetylene black is produced by pyrolyzing 98% pure acetylene gas by instantaneously injecting it into a reactor with an internal temperature of 2,000°C.

[0106] (2) Preparation of primary carbon-based particles

[0107] Subsequently, the internal temperature of the reaction furnace containing acetylene black was set to 250°C, and then an oxidation treatment was performed for 30 hours while introducing oxygen. As a result, primary carbon-based particles having a secondary particle structure including a shape in which a plurality of graphene sheets having a lateral size of about 41 nm were connected to each other were obtained. (See Figure 2 and Figure 3 )

[0108] The Raman spectrum D / G ratio of the primary carbon-based particles was 1.42. The Raman spectrum D / G ratio was measured by analyzing a Raman spectrum obtained using an Ar ion laser with a wavelength of 514.5 nm using a Raman spectrometer (NRS-2000B, Jasco).

[0109] Preparation Example 2: Preparation of primary carbon-based particles

[0110] The primary carbon-based particles obtained in the preparation process of the primary carbon-based particles of Preparation Example 1 were subjected to an additional heat treatment at 900° C. for 1 hour in an inert atmosphere to obtain primary carbon-based particles having a secondary particle structure including a shape in which a plurality of graphene sheets having a lateral size of about 65 nm are connected to each other. Figure 4 As shown, it can be understood that the primary carbon-based particles of Preparation Example 1 ( Figure 4 (a)) are converted into the primary carbon-based particles of Preparation Example 2 by heat treatment ( Figure 4 (b)).

[0111] The Raman spectrum D / G ratio of the primary carbon-based particles is 1.27.

[0112] Sample 1: Preparation of carbon black

[0113] Carbon black (acetylene black) was prepared in the form of secondary particles in which primary particles were aggregated. The average particle size of the primary particles of the prepared carbon black was 12 nm. (Denka Company Limited, SAB (Small Acetylene Black)) (See Figure 5 )

[0114] The Raman spectrum D / G ratio of carbon black is 1.68.

[0115] Sample 2: Preparation of graphene

[0116] Graphene powder (KNANOGraphene Technology Corporation Limited) with an average thickness of 100 nm and an average diameter of 7 μm was prepared. The Raman spectrum D / G ratio of the graphene was 0.22.

[0117] [Table 1]

[0118]

[0119] a is the specific surface area of carbon-based particles measured by nitrogen adsorption BET method (m 2 / g), b is the iodine adsorption value of the carbon-based particles (mg / g).

[0120] 1) Nitrogen adsorption specific surface area (m 2 / g): measured using a BET meter (BEL-SORP-MAX, Nippon Bell) at 200°C for 8 hours and N2 absorption / desorption at 77K.

[0121] 2) Iodine adsorption value (mg / g): measured according to ASTM D1510.

[0122] 3) Oxygen content (wt%): The amounts of C, H, and N elements were measured by an element analyzer (CHN-coder MT-5, Yanako), and the oxygen content (differential) was calculated to reflect the residual ash amount.

[0123] Example 1: Preparation of positive electrode active material

[0124] Li[Ni] in the form of secondary particles formed by combining or aggregating primary particles were mixed at a weight ratio of 99:1.0.6 Co 0.2 Mn 0.2 ]O2(average particle size(D 50 ) is 9.0 μm, see Figure 6 ) and the primary carbon-based particles of Preparation Example 1, a mechanical fusion device was used at 3,000 rpm for 10 minutes to form Li[Ni 0.6 Co 0.2 Mn 0.2 ]O2 formed a coating layer containing carbon-based particles (Nobilta NOB-130, Hosokawa Micron) to prepare a positive electrode active material (see Figure 7 ).

[0125] Example 2: Preparation of positive electrode active material

[0126] A positive electrode active material was prepared by forming a coating layer in the same manner as in Example 1, except that the primary carbon-based particles of Preparation Example 2 were used instead of the primary carbon-based particles used in Example 1.

[0127] Comparative Example 1: Preparation of positive electrode active material

[0128] Li[Ni] in the form of secondary particles formed by combining or aggregating primary particles were mixed at a weight ratio of 99:1. 0.6 Co 0.2 Mn 0.2 ]O2(average particle size(D 50 ) is 9.0 μm) and the primary carbon-based particles of Preparation Example 1, and then mixed 3 times (6 minutes in total) at a speed of 1,500 rpm using an acoustic mixer (Lab RAM-II, Resodyn) for 2 minutes. 0.6 Co 0.2 Mn 0.2 ]O2 is provided with primary carbon-based particles on its surface to form the positive electrode active material (see Figure 8 ).

[0129] Comparative Example 2: Preparation of positive electrode active material

[0130] A positive electrode active material was prepared by forming a coating layer in the same manner as in Example 1, except that the carbon black of Sample 1 was used instead of the primary carbon-based particles used in Example 1 (see Figure 9 ).

[0131] Comparative Example 3: Preparation of positive electrode active material

[0132] A positive electrode active material was prepared by forming a coating layer in the same manner as in Comparative Example 1, except that the carbon black of Sample 1 was used instead of the primary carbon-based particles used in Comparative Example 1.

[0133] Comparative Example 4: Preparation of positive electrode active material

[0134] A positive electrode active material was prepared by forming a coating layer in the same manner as in Comparative Example 1, except that the graphene of Sample 2 was used instead of the primary carbon-based particles used in Comparative Example 1.

[0135] [Table 2]

[0136]

[0137] After measuring the size of 100 graphene sheets in the coating using TEM (JEOL, JEM-2010F), the lateral size (nm) of the graphene sheet was determined from the average value of the sizes. 2 The specific surface area (D / G) was measured by the BET method. Specifically, the specific surface area was calculated from the amount of nitrogen adsorbed at liquid nitrogen temperature (77 K) using a BELSORP-miniII, manufactured by Bell Japan Inc. The D / G peak ratio of the positive electrode active material was measured using a Raman spectrometer (NRS-2000B, Jasco) using an Ar ion laser with a wavelength of 514.5 nm.

[0138] 5 g of the prepared positive electrode active material was placed in a dedicated holder and then pressed with a force of 30 kN. The powder conductivity (S / cm) of the positive electrode active material was measured using a 4-probe powder resistance measurement electrode for powder only (Powder Resistance System (MCP-PD51), Mitsubishi Chemical).

[0139] Experimental Example 1: SEM image evaluation

[0140] Figure 7 、 Figure 8 、 Figure 9 The SEM images of the positive electrode active materials of Example 1, Comparative Example 1, and Comparative Example 2 are respectively.

[0141] like Figure 7 As shown, it can be understood that the carbon-based particles are uniformly arranged on the Li[Ni 0.6 Co 0.2 Mn 0.2 ]O2 and the entire surface of Li[Ni 0.6 Co 0.2 Mn 0.2 ]O2 primary particles to form a coating. Specifically, for Example 1, since the graphene sheet shape of the carbon-based particles completely collapsed, the positive electrode active material was observed in the form of carbon-based particles smoothly coating the surface while the graphene sheets were restacked and rearranged.

[0142] On the contrary, Figure 8 As shown, the difference between the positive electrode active material of Comparative Example 1 and the positive electrode active material of Example 1 is that only in Li[Ni 0.6 Co 0.2 Mn 0.2 ] Primary carbon-based particles are locally present on a portion of the surface of O2. In addition, Figure 9 As shown, in the positive electrode active material of Comparative Example 2, the carbon black particles aggregated with each other without 0.6 Co 0.2 Mn 0.2 ]Changes the carbon structure at the interface between O2 primary particles.

[0143] Experimental Example 2: Battery Performance Evaluation (Output / Discharge Characteristics Evaluation)

[0144] After preparing the battery in the following manner, the prepared secondary battery was charged with constant current / constant voltage (CC / CV) at 0.1C to 4.3V, cut-off charge was performed at 0.005C, and the first charge capacity was measured by constant current (CC) discharge at 0.1C to 3.0V, and the results are listed in Table 3 below.

[0145] Method for preparing the battery:

[0146] (1) Preparation of positive electrode

[0147] Positive electrodes were prepared including the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 to 4. PVdF was used as a binder, and carbon black was used as a conductive agent.

[0148] The positive electrode active material, binder, and conductive agent were mixed in a weight ratio of 97.5:1.5:1 in NMP as a solvent to prepare a positive electrode slurry. The positive electrode slurry was coated on a 20 μm thick positive electrode collector (Al) and dried at 130°C to prepare a positive electrode.

[0149] (2) Preparation of batteries

[0150] SiO2 and artificial graphite were mixed as negative electrode active materials in a weight ratio of 1:9, carbon black as a negative electrode conductive agent, styrene-butadiene rubber (SBR) as a negative electrode binder, and carboxymethyl cellulose (CMC) in a weight ratio of 96.5:2:1:0.5 in distilled water to prepare a negative electrode slurry. A 10 μm thick negative electrode current collector (Cu) was coated with the prepared slurry and dried at 100°C to prepare a negative electrode.

[0151] Thereafter, after preparing a single cell by combining the above-prepared negative electrode and positive electrode with a 15 μm thick polyethylene-based separator disposed therebetween, an electrolyte (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 1 / 2 (volume ratio)), lithium hexafluorophosphate (1 M LiPF6)) was injected into the single cell to prepare a lithium secondary battery.

[0152] The battery prepared above was charged at 25° C. at a constant current of 0.2 C to 4.25 V, and the charge was cut off at 0.05 C. Subsequently, the battery was discharged at a constant current of 0.2 C to a voltage of 2.5 V to measure the initial charge and discharge capacity.

[0153] The battery was then charged at a constant current of 0.2C to 4.25V, then cut off at 0.05C and discharged at a constant current of 2.0C to 2.5V. This charge-discharge cycle was considered one, and two cycles were performed. The battery's discharge capacity at 2.0C was then measured relative to its discharge capacity at 0.2C. The results are shown in Table 3 below.

[0154] Experimental Example 3: Battery Performance Evaluation (Life Characteristics Evaluation)

[0155] Each of the prepared batteries was subjected to a charge / discharge setting of 0.33C / 0.33C in the voltage range of 4.25V to 2.8V at 45°C for one cycle. After a total of 100 cycles, the discharge capacity after 100 cycles was evaluated based on 100% of the discharge capacity after 1 cycle, and the results are presented in Table 3 below.

[0156] [Table 3]

[0157] 2.0C discharge capacity / 0.2C discharge capacity Capacity retention (%) Example 1 92.9 94.8 Example 2 93.7 96.2 Comparative Example 1 84.5 90.4 Comparative Example 2 75.3 82.1 Comparative Example 3 74.9 79.6 Comparative Example 4 71.4 75.3

[0158] Referring to Table 3, it can be confirmed that when the positive electrode active materials of Examples 1 and 2 are used, output / discharge characteristics are better than the case where the positive electrode active materials of Comparative Examples 1 to 4 are used, and life characteristics can be improved.

Claims

1. A positive electrode active material comprising a core and a coating disposed on the core, wherein the core comprises Li 1+x M y O 2+z , wherein M is at least one element selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), phosphorus (P), aluminum (Al), magnesium (Mg), calcium (Ca), zirconium (Zr), zinc (Zn), titanium (Ti), ruthenium (Ru), niobium (Nb), tungsten (W), boron (B), silicon (Si), sodium (Na), potassium (K), molybdenum (Mo) and vanadium (V), -0.2≤x≤0.2, 0<y≤2, and 0≤z≤2, The coating comprises carbon-based particles, wherein the carbon-based particles comprise a structure in which a plurality of graphene sheets are interconnected, and The positive electrode active material has a D / G peak ratio in a Raman spectrum measurement in the range of 0.9 to 1.

3.

2. The positive electrode active material according to claim 1, wherein The Li 1+x M y O 2+z Including Li 1+x [Ni a Co b M 1 c M 2 d ]O2, Among them, M 1 is at least one element selected from Al and Mn, M 2 is at least one element selected from the group consisting of Fe, P, Mg, Ca, Zr, Zn, Ti, Ru, Nb, W, B, Si, Na, K, Mo and V, and 0<a<1, 0<b<1, 0<c<1, and 0≤d≤0.

2. 3 . The cathode active material according to claim 1 , wherein a lateral dimension of the graphene sheet is 10 nm to 500 nm.

4. The positive electrode active material according to claim 1 , further comprising a connection portion connected to at least a portion of a graphene sheet among the plurality of graphene sheets, The connecting portion has a non-graphene shape. 5 . The positive electrode active material according to claim 4 , wherein at least a portion of each of the plurality of graphene sheets is connected to the connecting portion. The positive electrode active material according to claim 1 , wherein the average thickness of the graphene sheet is 0.34 nm to 10 nm. The positive electrode active material according to claim 1 , wherein the coating layer has a thickness of 1 nm to 500 nm.

8. The positive electrode active material according to claim 1, wherein the powder conductivity of the positive electrode active material is 1.0×10 -3 s / cm to 1.0×10 s / cm. 9 . The cathode active material according to claim 1 , wherein a weight ratio of the core to the coating layer is in a range of 99.9:0.1 to 90:

10.

10. The positive electrode active material according to claim 1, wherein the specific surface area of the positive electrode active material is 2 m 2 / g to 8m 2 / g range. A positive electrode comprising the positive electrode active material according to claim 1 . 12 . A secondary battery comprising the positive electrode according to claim 11 .

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