Positive electrode active material, method for preparing same, positive electrode, and rechargeable lithium battery

Abstract

Disclosed embodiments include a positive electrode active material, a method of preparing the same, a positive electrode, and a rechargeable lithium battery. The positive electrode active material includes a particle including a lithium transition metal composite oxide and a coating layer positioned on a surface of the particle and including porous graphene.

Description

Technical Field

[0001] A positive electrode active material, a method for preparing the same, a positive electrode including the same, and a rechargeable lithium battery are disclosed. Background Art

[0002] Portable information devices (such as exemplified by cellular phones, laptop computers, smart phones, etc.) or electric vehicles generally use rechargeable lithium batteries having a high energy density and being easy to carry as a driving power source. Research is underway to use rechargeable lithium batteries having a high energy density as a driving power source or a power storage source for hybrid vehicles or electric vehicles.

[0003] Various positive electrode active materials can constitute rechargeable lithium batteries for the above applications. Among such positive electrode active materials, lithium nickel-based oxides, lithium nickel manganese cobalt composite oxides, lithium nickel cobalt aluminum composite oxides, and lithium cobalt oxides can be used. However, the demand for large-sized, high-capacity, or high-energy density rechargeable lithium batteries continues to increase, thus requiring more development of positive electrode active materials. Summary of the Invention

[0004] Example embodiments include a positive electrode active material, a method for preparing the positive electrode active material, a positive electrode including the positive electrode active material, and a rechargeable lithium battery, the positive electrode active material being configured to improve electrode performance by promoting lithium movement and to improve the energy density per volume by increasing the electrode density.

[0005] Example embodiments include a positive electrode active material having particles including a lithium transition metal composite oxide and a coating layer positioned on the surface of the particles and including porous graphene.

[0006] In some example embodiments, a method for preparing a positive electrode active material includes the steps of: preparing an etching solution in which an etchant is added to a first solvent; adding graphene oxide powder to the etching solution and stirring the solution to prepare a mixed solution; filtering and drying the mixed solution and subjecting the resultant to a heat treatment to obtain porous graphene; adding the porous graphene to a second solvent and dispersing the porous graphene in the second solvent to prepare a dispersion; adding particles including a lithium transition metal composite oxide to the dispersion and mixing the particles in the dispersion.

[0007] In some example embodiments, a positive electrode includes: a positive electrode current collector; and a positive electrode active material layer on the positive electrode current collector, wherein the positive electrode active material layer includes the aforementioned positive electrode active material and a binder.

[0008] Example embodiments include a rechargeable lithium battery including the aforementioned positive electrode, a negative electrode, and an electrolyte.

[0009] The positive electrode active material according to some example embodiments improves lithium mobility by smoothly facilitating lithium movement, improves energy density by increasing electrode density, and can also ensure economic efficiency by minimizing production costs. A rechargeable lithium battery using the positive electrode active material can facilitate the movement of lithium ions, exhibit high rate capability, and improve electrode performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figures 1 to 4 is a schematic diagram schematically showing a rechargeable lithium battery according to some example embodiments.

[0011] Figure 5 Shows images of the positive electrode active material particles prepared in Comparative Example 1 taken with a scanning electron microscope (SEM) at a magnification of ×7000 and at a magnification of ×30,000.

[0012] Figure 6 Shows images of the positive electrode active material particles prepared in Comparative Example 2 taken with a SEM at a magnification of ×7000 and at a magnification of ×30,000.

[0013] Figure 7 Shows images of the positive electrode active material particles prepared in Example 1 taken with a SEM at a magnification of ×7000 and at a magnification of ×30,000.

[0014] Figure 8 Shows an image of the surface of the positive electrode active material prepared in Comparative Example 1 taken with a SEM.

[0015] Figure 9 Shows an image of the surface of the positive electrode active material prepared in Comparative Example 2 taken with a SEM.

[0016] Figure 10 Shows an image of the surface of the positive electrode active material prepared in Example 1 taken with a SEM.

[0017] Figure 11 Shows the results of Raman spectroscopic analysis of the positive electrode active material prepared in Example 1.

[0018] Figure 12 Shows the results of Raman spectroscopic analysis of the positive electrode active material prepared in Example 1 and Comparative Examples 1 and 2.

[0019] Figure 13 Shows the results of evaluating the rate discharge capabilities of the rechargeable lithium battery cells manufactured in Example 1 and Comparative Examples 1 and 2.

[0020] Figure 14 Shows the cyclic voltammetry (CV) curve of the positive electrode fabricated in Comparative Example 1.

[0021] Figure 15 Shows the cyclic voltammetry (CV) curve of the positive electrode fabricated in Comparative Example 2.

[0022] Figure 16 Shows the cyclic voltammetry (CV) curve of the positive electrode fabricated in Example 1.

[0023] Figure 17 Is a cyclic voltammetry (CV) curve showing the peak current during the reduction process of the positive electrodes fabricated in Example 1 and Comparative Examples 1 and 2 with respect to the square root of the scan rate.

[0024] Figure 18 Is a cyclic voltammetry (CV) curve showing the peak current during the oxidation process of the positive electrodes prepared in Example 1 and Comparative Examples 1 and 2 with respect to the square root of the scan rate.

[0025] Figure 19 Shows the initial impedance measurement results of the rechargeable lithium battery cells fabricated in Example 1 and Comparative Examples 1 and 2.

[0026] Figure 20 Shows the impedance measurement results of the symmetric battery cells of the rechargeable lithium battery cells fabricated in Example 1 and Comparative Examples 1 and 2.

[0027] Figure 21 Shows the impedance measurement results of the rechargeable lithium battery cells fabricated in Example 1 and Comparative Examples 1 and 2 after 50 cycles.

[0028] Figure 22 Shows the impedance resistance results of the rechargeable lithium battery cells fabricated in Example 1 and Comparative Example 2.

[0029] Figure 23 Shows the results of X-ray photoelectron spectroscopy (XPS) analysis of graphene oxide of the reference example and each graphene used in Example 1 and Comparative Example 2.

[0030] Figure 24 Shows the results of Figure 23 X-ray photoelectron spectroscopy (XPS) analysis of the C1s peak in.

[0031] Figure 25 Shows the results of Figure 23 X-ray photoelectron spectroscopy (XPS) analysis of the O1s peak in.

[0032] Figure 26The results of FT-IR analysis of graphene oxide of the reference example and each graphene used in Example 1 and Comparative Example 2 are shown.

[0033] Figure 27 The results of conductivity analysis of graphene oxide of the reference example and each graphene used in Example 1 and Comparative Example 2 are shown.

[0034] Figure 28 The results of specific surface area and pore size analysis of graphene oxide of the reference example and each graphene used in Example 1 and Comparative Example 2 are shown.

[0035] Figure 29 The evaluation results of the rate discharge capacity according to the change in the stirring time during the preparation process of the porous graphene are shown.

[0036] Figure 30 The evaluation results of the rate discharge capacity according to the change in the concentration of the etching solution during the preparation process of the porous graphene are shown. Detailed Description

[0037] Hereinafter, exemplary embodiments will be described in detail so that those of ordinary skill in the art can easily implement the exemplary embodiments. However, the present disclosure may be embodied in many different forms and is not to be construed as limited to the exemplary embodiments set forth herein.

[0038] The terms used herein are for the purpose of describing only the exemplary embodiments and are not intended to limit the present disclosure. Unless the context clearly indicates otherwise, a singular expression includes a plural expression.

[0039] As used herein, "a combination (or combinations thereof)" refers to a mixture of components, a laminate, a composite, a copolymer, an alloy, a blend, a reaction product, etc.

[0040] Here, it should be understood that terms such as "comprising," "including," or "having" are intended to indicate the presence of the embodied features, numbers, steps, elements, or combinations thereof, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, elements, or combinations thereof.

[0041] In the drawings, for clarity, the thicknesses of layers, films, panels, regions, etc. are exaggerated, and throughout the specification, the same reference numerals denote the same elements. It will be understood that when an element such as a layer, film, region, or substrate is referred to as being "on" another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being "directly on" another element, no intervening elements are present.

[0042] In addition, the "layer" here includes not only the shape formed on the entire surface when viewed from a plan view, but also the shape formed on a partial surface.

[0043] The average particle diameter can be measured by methods well-known to those skilled in the art (e.g., by a particle size analyzer, or by transmission electron microscope images or scanning electron microscope images). Optionally, it can be obtained by measuring using the dynamic light scattering method, performing data analysis, counting the number of particles in each particle size range, and calculating therefrom the average particle diameter value. Unless otherwise defined, the average particle diameter may refer to the diameter (D 50 ) of the particles having a cumulative volume of 50% by volume in the particle size distribution. As used herein, when no other definition is provided, the average particle diameter refers to the diameter (D 50 ) of the particles having a cumulative volume of 50% by volume in the particle size distribution obtained by randomly measuring the sizes (diameter or major axis length) of about 20 particles in a scanning electron microscope image.

[0044] Here, "or" should not be construed in an exclusive sense. For example, "A or B" is construed as "including A, including B, or including both A and B".

[0045] "Metal" is construed to include the concepts of ordinary metals, transition metals, and metalloids (semi-metals).

[0046] When the terms "about" or "substantially" are used in combination with a numerical value in this specification, it means that the relevant numerical value includes a tolerance of ±10% around the stated numerical value. When a range is specified, the range includes all values therebetween such as with an increment of 0.1%.

[0047] Positive electrode active material In some exemplary embodiments, the positive electrode active material includes: particles including a lithium transition metal composite oxide; and a coating layer located on the surface of the particles and including porous graphene.

[0048] Among the main conductive materials used in the positive electrode, a conductive material of carbon black having a 0D (zero-dimensional) type or small spherical shape structure can be used. These conductive materials are usually too short to contact and electrically connect the positive electrode active material, such that a large amount of conductive material may be required. Additionally, spherical conductive materials generally have strong incompressibility, which makes it difficult to ensure the volumetric energy density for stacking a large amount of positive electrode active material inside the battery, resulting in high costs for forming a high-density electrode. To increase the volumetric energy density, graphene can be used as the conductive material, but in the case of graphene, it may be challenging for lithium to move in a direction perpendicular to the plane (fundamental plane). To solve this problem, porous graphene having many pores formed on the surface of graphene can be used, and the movement of lithium ions can be smoothly promoted.

[0049] Accordingly, some example embodiments include a positive electrode active material. By introducing a coating layer having porous graphene on the surface of particles including a lithium transition metal composite oxide, the movement of lithium proceeds smoothly, thereby ensuring improved lithium mobility and electrode performance.

[0050] Particles including lithium transition metal composite oxide The positive electrode active material includes particles containing a lithium transition metal composite oxide. As an example, the lithium transition metal composite oxide can be or include a lithium nickel-based composite oxide and can have a layered structure.

[0051] Based on the total amount of elements other than lithium and oxygen, the nickel content in the lithium nickel-based composite oxide can be greater than or equal to about 30 mol%, for example, greater than or equal to about 40 mol%, greater than or equal to about 50 mol%, greater than or equal to about 60 mol%, greater than or equal to about 70 mol%, greater than or equal to about 80 mol%, or greater than or equal to about 90 mol%, and can be less than or equal to about 99.9 mol% or less than or equal to about 99 mol%. For example, the nickel content in the lithium nickel-based composite oxide can be higher than the content of other respective elements such as cobalt, manganese, and aluminum. When the nickel content satisfies any of the above ranges, the positive electrode active material can achieve a high capacity and exhibit improved or desired battery performance.

[0052] The positive electrode active material can include a lithium nickel-based composite oxide represented by Chemical Formula 1.

[0053] [Chemical Formula 1] Li a1 Ni x1 M 1 y1 M 2 z1 O 2-b1 X b1 In Chemical Formula 1, 0.9 ≤ a1 ≤ 1.8, 0.3 ≤ x1 ≤ 1, 0 ≤ y1 ≤ 0.7, 0 ≤ z1 ≤ 0.7, 0.9 ≤ x1 + y1 + z1 ≤ 1.1, and 0 ≤ b1 ≤ 0.1, M 1 and M 2 are independently or include Al, B, Ba, Ca, Ce, Co, Cr, Fe, Mg, Mn, Mo, Nb, Si, Sr, Ti, V, W, Zr, or a combination thereof, and X is F, P, S, or a combination thereof.

[0054] In Chemical Formula 1, 0.4 ≤ x1 ≤ 1, 0 ≤ y1 ≤ 0.6, and 0 ≤ z1 ≤ 0.6; 0.5 ≤ x1 ≤ 1, 0 ≤ y1 ≤ 0.5, and 0 ≤ z1 ≤ 0.5; 0.6 ≤ x1 ≤ 1, 0 ≤ y1 ≤ 0.4, and 0 ≤ z1 ≤ 0.4; 0.7 ≤ x1 ≤ 1, 0 ≤ y1 ≤ 0.3, and 0 ≤ z1 ≤ 0.3; 0.8 ≤ x1 ≤ 1, 0 ≤ y1 ≤ 0.2, and 0 ≤ z1 ≤ 0.2; or 0.9 ≤ x1 < 1, 0 < y1 ≤ 0.1, and 0 ≤ z1 ≤ 0.1.

[0055] The positive electrode active material may include, for example, a lithium nickel-based composite oxide represented by Chemical Formula 2.

[0056] [Chemical Formula 2] Li a2 Ni x2 Co y2 M 3 z2 O 2-b2 X b2 In Chemical Formula 2, 0.9 ≤ a2 ≤ 1.8, 0.3 ≤ x2 < 1, 0 < y2 ≤ 0.7, 0 ≤ z2 ≤ 0.7, 0.9 ≤ x2 + y2 + z2 ≤ 1.1, and 0 ≤ b2 ≤ 0.1, M 3 is or includes Al, B, Ba, Ca, Ce, Cr, Fe, Mg, Mn, Mo, Nb, Si, Sr, Ti, V, W, Zr, or a combination thereof, and X is F, P, S, or a combination thereof.

[0057] In Chemical Formula 2, 0.3 ≤ x2 ≤ 0.99, 0.01 ≤ y2 ≤ 0.7, and 0 ≤ z2 ≤ 0.7; 0.4 ≤ x2 ≤ 0.99, 0.01 ≤ y2 ≤ 0.6, and 0 ≤ z2 ≤ 0.6; 0.5 ≤ x2 ≤ 0.99, 0.01 ≤ y2 ≤ 0.5, and 0 ≤ z2 ≤ 0.5; 0.6 ≤ x2 ≤ 0.99, 0.01 ≤ y2 ≤ 0.4, and 0 ≤ z2 ≤ 0.4; 0.7 ≤ x2 ≤ 0.99, 0.01 ≤ y2 ≤ 0.3, and 0 ≤ z2 ≤ 0.3; 0.8 ≤ x2 ≤ 0.99, 0.01 ≤ y2 ≤ 0.2, and 0 ≤ z2 ≤ 0.2; or 0.9 ≤ x2 ≤ 0.99, 0.01 ≤ y2 ≤ 0.1, and 0 ≤ z2 ≤ 0.1.

[0058] The positive electrode active material may include, for example, a lithium nickel-based composite oxide represented by Chemical Formula 2A.

[0059] [Chemical Formula 2A] Li a2 Ni x2 Co y2 Mn z2 O 2 In Chemical Formula 2A, 0.9 ≤ a2 ≤ 1.8, 0.3 ≤ x2 < 1, 0 < y2 ≤ 0.7, 0 < z2 ≤ 0.7, 0.9 ≤ x2 + y2 + z2 ≤ 1.1.

[0060] In Chemical Formula 2A, a2 + x2 + y2 + z2 = 2.

[0061] The positive electrode active material may include, for example, a lithium nickel-based composite oxide represented by Chemical Formula 3.

[0062] [Chemical Formula 3] Li a3 Ni x3 Co y3 M 4 z3 M 5 w3 O 2-b3 X b3 In Chemical Formula 3, 0.9 ≤ a3 ≤ 1.8, 0.3 ≤ x3 ≤ 0.98, 0.01 ≤ y3 ≤ 0.69, 0.01 ≤ z3 ≤ 0.69, 0 ≤ w3 ≤ 0.69, 0.9 ≤ x3 + y3 + z3 + w3 ≤ 1.1, and 0 ≤ b3 ≤ 0.1, M 4 is or includes Al, Mn, or a combination thereof, M 5 is B, Ba, Ca, Ce, Cr, Fe, Mg, Mo, Nb, Si, Sr, Ti, V, W, Zr, or a combination thereof, and X is F, P, S, or a combination thereof.

[0063] In Chemical Formula 3, 0.4 ≤ x3 ≤ 0.98, 0.01 ≤ y3 ≤ 0.59, 0.01 ≤ z3 ≤ 0.59, and 0 ≤ w3 ≤ 0.59; 0.5 ≤ x3 ≤ 0.98, 0.01 ≤ y3 ≤ 0.49, 0.01 ≤ z3 ≤ 0.49, and 0 ≤ w3 ≤ 0.49; 0.6 ≤ x3 ≤ 0.98, 0.01 ≤ y3 ≤ 0.39, 0.01 ≤ z3 ≤ 0.39, and 0 ≤ w3 ≤ 0.39; 0.7 ≤ x3 ≤ 0.98, 0.01 ≤ y3 ≤ 0.29, 0.01 ≤ z3 ≤ 0.29, and 0 ≤ w3 ≤ 0.29; 0.8 ≤ x3 ≤ 0.98, 0.01 ≤ y3 ≤ 0.19, 0.01 ≤ z3 ≤ 0.19, and 0 ≤ w3 ≤ 0.19; or 0.9 ≤ x3 ≤ 0.98, 0.01 ≤ y3 ≤ 0.09, 0.01 ≤ z3 ≤ 0.09, and 0 ≤ w3 ≤ 0.09.

[0064] Particles including the lithium transition metal composite oxide may be in a polycrystalline form and may be in the form of secondary particles formed by aggregating at least two primary particles.

[0065] The particle size (D 50 ) (i.e., the average particle size of the secondary particles) of the particles including the lithium transition metal composite oxide can be greater than or equal to about 7 μm, greater than or equal to about 9 μm, greater than or equal to about 9.5 μm, or greater than or equal to about 10 μm, and can be less than or equal to about 25 μm, less than or equal to about 23 μm, or less than or equal to about 20 μm. The average particle size refers to the diameter (D 50 ) of the particles with a cumulative volume of 50% by volume in the particle size distribution obtained by randomly measuring the diameters of about 20 particles in the scanning electron microscope image of the positive electrode active material.

[0066] The particles including the lithium transition metal composite oxide can consist essentially of the lithium transition metal composite oxide. Here, "consist essentially of" can mean containing 95 wt% or more.

[0067] Coating layer The positive electrode active material includes a coating layer located on the surface of the particles including the lithium transition metal composite oxide, and the coating layer includes porous graphene in which a plurality of pores are formed in graphene.

[0068] In conventional graphene, it is difficult for lithium to move in the direction perpendicular to the plane, which reduces lithium mobility. Therefore, porous graphene is used as a material for the coating layer. By using porous graphene, the smooth movement of lithium is promoted even in the direction perpendicular to the plane of graphene in the coating layer, thereby improving the electrode performance.

[0069] When porous graphene is used as the coating layer material for the positive electrode active material, the electrical connection distance between the particles including the lithium transition metal composite oxide of the positive electrode active material and the porous graphene can be shorter than the electrical connection distance when simply adding and dispersing the lithium transition metal composite oxide and the conductive material as the positive electrode active material. The shorter electrical connection distance is beneficial for forming a high-density electrode. In addition, by applying porous graphene as the coating layer material, the amount of the conductive material used in the manufacturing process of the positive electrode can be reduced or omitted, which can be beneficial for improving the energy density per volume of the electrode.

[0070] Based on the total weight of the positive electrode active material, the content of the porous graphene can be, for example, greater than or equal to about 0.05 wt%, greater than or equal to about 0.08 wt%, or greater than or equal to about 0.1 wt%, and less than or equal to about 3 wt%, less than or equal to about 2 wt%, less than or equal to about 1 wt%, less than or equal to about 0.5 wt%, or less than or equal to about 0.35 wt%. In any range within the above ranges, the desired improvement in electrode performance and energy density per volume can be effectively achieved.

[0071] In some example embodiments, based on the total weight of the coating layer including porous graphene, the content of porous graphene can be greater than or equal to about 60 wt%, for example, greater than or equal to about 80 wt%, greater than or equal to about 90 wt%, or greater than or equal to about 95 wt%. Additionally, based on the total weight of the coating layer, the content of porous graphene can be less than or equal to about 100 wt%, for example, less than or equal to about 99.9 wt%. Within any of the above ranges, the electrode performance can be effectively improved by promoting lithium mobility, and it can be more advantageous in terms of improving the energy density per volume.

[0072] The average diameter of the pores of the porous graphene can be, for example, greater than or equal to about 10 nm, greater than or equal to about 20 nm, greater than or equal to about 30 nm, or greater than or equal to about 35 nm, and less than or equal to about 1 μm, less than or equal to about 95 nm, less than or equal to about 90 nm, or less than or equal to about 85 nm. In an example, the average distance between the pores of the porous graphene can be, for example, about 30 nm to about 5 μm, where the average distance between the pores represents the shortest distance between any two pores and can be measured by the SEM image measurement method. Within this range, the porous graphene can smoothly promote lithium mobility, and the average diameter of the pores can be measured using various methods (such as the Barret-Joyner-Halenda (BJH) measurement method or the SEM image measurement method).

[0073] The Brunauer-Emmett-Teller (BET) specific surface area of the porous graphene can be, for example, greater than or equal to about 15 m 2 / g, greater than or equal to about 18 m 2 / g, or greater than or equal to about 20 m 2 / g, less than or equal to about 30 m 2 / g, less than or equal to about 28 m 2 / g, or less than or equal to about 25 m 2 / g. Within any of the above ranges, the improved or desired high-rate capability and electrode performance of the positive electrode active material can be effectively achieved. For example, the BET specific surface area can be measured using a gas adsorption method with a BET (Brunauer-Emmett-Teller) device.

[0074] The coating layer can be formed by stacking sheet-like porous graphene on the surface of particles including lithium transition metal composite oxide. Thus, the coating layer can include multiple porous graphene layers, for example, 1 to 30 porous graphene layers.

[0075] The thickness of the coating layer can be, for example, greater than or equal to about 0.3 nm, greater than or equal to about 0.6 nm, greater than or equal to about 0.9 nm, or greater than or equal to about 1.2 nm, and less than or equal to about 30 nm, less than or equal to about 28 nm, less than or equal to about 25 nm, or less than or equal to about 20 nm. Within any of the above ranges, a high density of electrodes and improved or desired electrode performance can be effectively achieved. The coating layer on the particle surface can be formed continuously or discontinuously. The thickness of the coating layer can be measured by calculating the average value of the coating layer thickness from the SEM-EDS image of the cross-section of the positive electrode active material.

[0076] The positive electrode active material can satisfy the following Equation 1. Within this range, a high electrode density and improved or desired electrode performance can be effectively achieved.

[0077] [Equation 1] 1.02 ≤ I D / I G ≤ 1.3 In Equation 1, I D represents the peak intensity near 1350 cm -1 in Raman spectroscopy analysis, and I G represents the peak intensity near 1580 cm -1 in Raman spectroscopy analysis.

[0078] Method for preparing positive electrode active material In some exemplary embodiments, a method for preparing a positive electrode active material includes: preparing an etching solution in which an etchant is added to a first solvent; adding graphene oxide powder to the etching solution and stirring the etching solution to prepare a mixed solution; filtering and drying the mixed solution, and performing heat treatment on the obtained product to obtain porous graphene; adding the porous graphene to a second solvent and dispersing the porous graphene in the second solvent to prepare a dispersion; adding particles including a lithium transition metal composite oxide to the dispersion and mixing the particles in the dispersion.

[0079] Prepare an etching solution for surface treatment to form defects or pores in graphene. The etching solution can be prepared by adding an etchant to a first solvent. The first solvent can be used without limitation as long as the first solvent is a material that can dissolve the above etchant. For example, an aqueous solvent can be used as the first solvent, and the aqueous solvent can be or include distilled water, an alcohol solvent, or a combination thereof.

[0080] An etchant is a treatment agent for forming porous graphene through surface treatment and can be an alkaline compound. Examples of the etchant include KOH, NaOH, or a combination thereof. When such an etchant is used, chemical surface treatment can be performed on the graphene used as a material for a coating layer to form grooves or pores without direct mechanical (or physical) treatment, thereby effectively manufacturing porous graphene.

[0081] The concentration of the etching solution can be greater than or equal to about 1 M, greater than or equal to about 2 M, or greater than or equal to about 3 M, and less than or equal to about 15 M, less than or equal to about 13 M, less than or equal to about 12 M, less than or equal to about 10 M, or less than or equal to about 8 M. Here, the concentration of the etching solution refers to the concentration of the etchant, and within the above concentration range, improved or desired discharge characteristics can be effectively ensured within a C-rate range of about 0.02 C to about 5 C. On the other hand, when the etching solution has a concentration of about 3 M to about 8 M, improved or desired discharge characteristics can be ensured even at a high C-rate of greater than or equal to about 5 C.

[0082] Subsequently, graphene oxide powder is added to the etching solution and then stirred to prepare a mixed solution. During this process, grooves or pores can be formed in the graphene oxide.

[0083] The stirring can be carried out for greater than or equal to about 12 hours, greater than or equal to about 14 hours, or greater than or equal to about 15 hours, and less than or equal to about 72 hours, less than or equal to about 48 hours, less than or equal to about 45 hours, less than or equal to about 40 hours, or less than or equal to about 30 hours. When a positive electrode active material prepared by satisfying the above conditions is used, improved or desired discharge characteristics can be effectively ensured within a C-rate range of about 0.02 C to about 5 C. On the other hand, when a positive electrode active material prepared with a stirring time of about 15 hours to about 40 hours is used, improved or desired discharge characteristics can be ensured at a high C-rate of greater than or equal to about 5 C.

[0084] After filtering and drying the mixed solution, heat treatment is performed on the resulting product. Through filtration and drying, porous graphene oxide is obtained, and the porous graphene oxide is reduced by heat treatment to obtain porous graphene. On the other hand, the above description can equally apply to porous graphene.

[0085] The heat treatment can be carried out at a temperature greater than or equal to about 600 °C, greater than or equal to about 650 °C, greater than or equal to about 700 °C, greater than or equal to about 750 °C, or greater than or equal to about 800 °C, and less than or equal to about 1000 °C, less than or equal to about 970 °C, less than or equal to about 950 °C, less than or equal to about 940 °C, less than or equal to about 910 °C, or less than or equal to about 900 °C. Within any of the above ranges, graphene oxide can not only be effectively reduced and thus reduce or inhibit the decrease in conductivity caused by oxygen (O), but also effectively form porous graphene.

[0086] The heat treatment discussed above can be carried out for a period of time greater than or equal to about 30 minutes, greater than or equal to about 40 minutes, greater than or equal to about 50 minutes, or greater than or equal to about 1 hour, and less than or equal to about 5 hours, less than or equal to about 3 hours, or less than or equal to about 2 hours. Within any of the above ranges, the performance of the fabricated electrode can be effectively improved.

[0087] The heat treatment can be carried out in a reducing atmosphere. The reducing atmosphere can include, for example, nitrogen as the main component and hydrogen in the range of about 0 vol% to about 10 vol%.

[0088] The porous graphene is added to a second solvent and dispersed therein to prepare a dispersion. The second solvent can be or include an organic solvent, for example, at least one of dimethylformamide (DMF), dimethyl sulfoxide (DMSO), dimethylacetamide (DMAc), dimethyl carbonate (DMC), 2,4,6-trimethylphenol (TMP), N-methyl-2-pyrrolidone (NMP), or a combination thereof.

[0089] In the dispersion, the content of the porous graphene can be about 0.5 wt% to about 5 wt%.

[0090] The dispersion can be carried out by ultrasonic treatment (for example, by using a tip sonotrode device as an ultrasonic dispersion device) to uniformly disperse the porous graphene.

[0091] In order to form a coating layer on the particles including the lithium transition metal composite oxide, the particles including the lithium transition metal composite oxide are added to the dispersion, and then mixing is carried out.

[0092] The mixing process of the particles of the lithium transition metal composite oxide and the dispersion can be carried out at a temperature greater than or equal to about 10 °C, greater than or equal to about 15 °C, or greater than or equal to about 20 °C and less than or equal to about 40 °C, less than or equal to about 35 °C, or less than or equal to about 30 °C, and is typically carried out at room temperature. In another example, the mixing process of the particles of the lithium transition metal composite oxide and the dispersion can be carried out for a period of time greater than or equal to about 30 seconds, greater than or equal to about 40 seconds, greater than or equal to about 50 seconds, greater than or equal to about 1 minute, less than or equal to about 10 minutes, less than or equal to about 8 minutes, less than or equal to about 6 minutes, less than or equal to about 5 minutes, or less than or equal to about 2 minutes under the above temperature conditions. Under the above conditions, the coating layer containing porous graphene can be formed substantially uniformly on the surface of the particles of the lithium transition metal composite oxide.

[0093] After the mixing process of the particles of the lithium transition metal composite oxide and the dispersion liquid, a cleaning process (referred to as a washing process) using a third solvent that substantially does not damage the positive electrode active material can be further included. The third solvent can be any solvent that substantially does not damage the positive electrode active material (for example, an organic solvent, such as an alcohol solvent exemplified by ethanol), without particular limitation.

[0094] After the cleaning, a centrifugation process can be further carried out to collect the porous graphene from ethanol (referred to as a separation process here). The centrifugation process can be carried out at a rotational speed of about 1,000 rpm to about 5,000 rpm. In order to substantially completely dry the remaining solvent after the separation process, a drying process can be further carried out, and this drying process can be carried out at a temperature of about 70 °C to about 100 °C in a vacuum atmosphere.

[0095] Positive electrode In some exemplary embodiments, the positive electrode includes a positive electrode current collector and a positive electrode active material layer on the positive electrode current collector, wherein the positive electrode active material layer includes the aforementioned positive electrode active material and a binder. The positive electrode active material layer can optionally further include a conductive material.

[0096] According to some example embodiments, the electrode density of the finally compressed positive electrode can be greater than or equal to about 3.5 g / cc, greater than or equal to about 3.6 g / cc, greater than or equal to about 3.8 g / cc, or greater than or equal to about 4.0 g / cc, and less than or equal to about 4.7 g / cc, less than or equal to about 4.6 g / cc, less than or equal to about 4.5 g / cc, or less than or equal to about 4.4 g / cc. When applying the positive electrode active material according to some example embodiments, it is beneficial to achieve such a loading level and positive electrode density, and a positive electrode that satisfies the loading level and positive electrode density within the above range is suitable for achieving a high-capacity, high-energy density rechargeable lithium battery.

[0097] Binder The binder improves the binding properties between the positive electrode active material particles and between the positive electrode active material particles and the current collector. Examples of the binder can include, for example, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, (meth)acrylic esterified styrene-butadiene rubber, epoxy resin, (meth)acrylic resin, polyester resin, and nylon, but are not limited thereto.

[0098] Based on 100 wt% of the positive electrode active material layer, the content of the binder can be from about 0.2 wt% to about 5 wt%.

[0099] Conductive material A conductive material is included to provide electrode conductivity, and any electrically conductive material can be used as the conductive material unless the electrically conductive material causes a chemical change in the battery. Examples of the conductive material can include: carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, Denka black, carbon fiber, carbon nanofiber, carbon nanotube, etc.; metal-based materials including metal powders or metal fibers such as copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0100] According to some example embodiments, the positive electrode active material layer may optionally include a conductive material. By using the positive electrode active material including the aforementioned porous graphene coating layer, the amount of the conductive material required during the manufacturing process can be substantially reduced or omitted. Therefore, when the positive electrode active material layer includes a conductive material, based on 100 wt% of the positive electrode active material layer, the content of the conductive material may be less than or equal to about 5 wt%, less than or equal to about 3 wt%, less than or equal to about 1 wt%, less than or equal to about 0.5 wt%, or less than or equal to about 0.1 wt%. Optionally, the positive electrode active material layer may not include a conductive material. By using the aforementioned porous graphene to form the coating layer of the positive electrode active material, the use of the conductive material itself can be reduced or omitted, the cost can be reduced, and a better energy density per volume can be achieved.

[0101] The positive electrode current collector may include Al, but is not limited thereto.

[0102] Rechargeable lithium battery Some example embodiments include a rechargeable lithium battery, which includes the aforementioned positive electrode, negative electrode, and electrolyte.

[0103] The rechargeable lithium battery may be classified into a cylindrical shape, a prismatic shape, a pouch shape, a coin shape, etc. according to its shape. Figures 1 to 4 is a schematic diagram showing a rechargeable lithium battery according to some example embodiments, wherein, Figure 1 is a cylindrical battery, Figure 2 is a prismatic battery, Figure 3 and Figure 4 is a pouch-shaped battery. Referring to Figures 1 to 4 , the rechargeable lithium battery 100 includes: an electrode assembly 40 having a separator 30 disposed between the positive electrode 10 and the negative electrode 20; and a housing 50 in which the electrode assembly 40 is accommodated. The positive electrode 10, negative electrode 20, and separator 30 may be impregnated with an electrolyte (not shown). The rechargeable lithium battery 100 may include a sealing member 60 for sealing the housing 50, as shown in Figure 1 . Additionally, in Figure 2 , the rechargeable lithium battery 100 may include a positive electrode lead tab 11, a positive electrode terminal 12, a negative electrode lead tab 21, and a negative electrode terminal 22. As shown in Figure 3 and Figure 4 , the rechargeable lithium battery 100 includes the electrode tab 70 shown in Figure 4 (i.e., the positive electrode tab 71 and the negative electrode tab 72 shown in Figure 3 that constitute a circuit path for guiding the current formed in the electrode assembly 40 to the outside).

[0104] Negative electrode The negative electrode may include a current collector and a negative electrode active material layer on the current collector, and the negative electrode active material layer may include a negative electrode active material, and may further include a binder, a conductive material, or a combination thereof.

[0105] Negative electrode active material The negative electrode active material may include a material that reversibly intercalates / deintercalates lithium ions, lithium metal, a lithium metal alloy, a material capable of doping / de-doping lithium, or a transition metal oxide.

[0106] The material that reversibly intercalates / deintercalates lithium ions may include, for example, crystalline carbon, amorphous carbon, or a combination thereof as a carbon-based negative electrode active material. The crystalline carbon may be natural graphite or artificial graphite having an irregular, flaky, lamellar, spherical, or fibrous shape. The amorphous carbon may be or include soft carbon, hard carbon, mesophase pitch carbonized product, calcined coke, etc.

[0107] The lithium metal alloy includes an alloy of lithium and at least one metal such as Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn.

[0108] The material capable of doping / de-doping lithium may be a Si-based negative electrode active material or a Sn-based negative electrode active material. The Si-based negative electrode active material may include silicon, a silicon-carbon composite, SiO x (0 < x < 2), a Si-Q alloy (where Q is or includes at least one element such as an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element (excluding Si), a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and a combination thereof, for example, Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, and a combination thereof) or a combination thereof. The Sn-based negative electrode active material may be or include Sn, SnO 2 , a Sn alloy, or a combination thereof.

[0109] The silicon-carbon composite may be or include a composite of silicon and amorphous carbon. The average particle size (D 50It can be, for example, from about 0.5 μm to about 20 μm. According to some exemplary embodiments, the silicon-carbon composite can be in the form of silicon particles and amorphous carbon coated on the surface of the silicon particles. For example, the silicon-carbon composite can include secondary particles (cores) in which silicon primary particles are assembled and an amorphous carbon coating layer (shells) on the surface of the secondary particles. Amorphous carbon can also be present between the silicon primary particles. For example, the silicon primary particles can be coated with amorphous carbon. The secondary particles can be dispersed in an amorphous carbon matrix.

[0110] The silicon-carbon composite can also include crystalline carbon. For example, the silicon-carbon composite can include a core containing crystalline carbon and silicon particles and an amorphous carbon coating layer on the surface of the core. The crystalline carbon can be or include artificial graphite, natural graphite, or a combination thereof. The amorphous carbon can include soft carbon or hard carbon, mesophase pitch carbonized products, and calcined coke.

[0111] When the silicon-carbon composite includes silicon and amorphous carbon, based on 100 wt% of the silicon-carbon composite, the silicon content can be from about 10 wt% to about 50 wt%, and the content of the amorphous carbon can be from about 50 wt% to about 90 wt%. Additionally, when the composite includes silicon, amorphous carbon, and crystalline carbon, based on 100 wt% of the silicon-carbon composite, the silicon content can be from about 10 wt% to about 50 wt%, the content of the crystalline carbon can be from about 10 wt% to about 70 wt%, and the content of the amorphous carbon can be from about 20 wt% to about 40 wt%.

[0112] Additionally, the thickness of the amorphous carbon coating layer can be from about 5 nm to about 100 nm. The average particle size (D 50 ) of the silicon particles (primary particles) can be from about 10 nm to about 1 μm, or from about 10 nm to about 200 nm. The silicon particles can exist as elemental silicon, in the form of a silicon alloy, or in an oxidized form of silicon. The oxidized form of silicon can be represented by SiO x (0 < x < 2). At this time, the atomic content ratio (Si:O) representing the degree of oxidation can be from about 99:1 to about 33:67. As used herein, when no other definition is provided, the average particle size (D 50 ) represents the particle size of the particles with a cumulative volume of about 50 volume% in the particle distribution.

[0113] The Si-based negative electrode active material or the Sn-based negative electrode active material can be mixed with the carbon-based negative electrode active material. When the Si-based negative electrode active material or the Sn-based negative electrode active material is used in combination with the carbon-based negative electrode active material, the mixing ratio can be a weight ratio of from about 1:99 to about 90:10.

[0114] Binder The binder is configured to sufficiently adhere the negative electrode active material particles to each other and to sufficiently adhere the negative electrode active material to the current collector. The binder can be or include a non-aqueous binder, an aqueous binder, a dry binder, or a combination thereof.

[0115] The non-aqueous binder can include at least one of polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene-propylene copolymer, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

[0116] The aqueous binder can include at least one of styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, (meth)acrylonitrile-butadiene rubber, (meth)acrylic rubber, butyl rubber, fluororubber, polyethylene oxide, polyvinylpyrrolidone, epichlorohydrin rubber, polyphosphazene, poly(meth)acrylonitrile, ethylene-propylene-diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, (meth)acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, or a combination thereof.

[0117] When using an aqueous binder as the negative electrode binder, a cellulose compound capable of imparting viscosity can also be included. As the cellulose compound, one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or their alkali metal salts can be used in combination. The alkali metal can be or include Na, K, or Li.

[0118] The dry binder can be a polymeric material capable of becoming fibrous and can be or include, for example, polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, or a combination thereof.

[0119] Conductive material A conductive material can be included to provide electrode conductivity, and any electrically conductive material can be used as the conductive material, unless the electrically conductive material causes a chemical change in the battery. Examples of the conductive material include: carbonaceous materials such as at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanofiber, carbon nanotube, etc.; metallic materials including at least one of metal powders or metal fibers such as copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0120] Based on 100 wt% of the negative electrode active material layer, the content of the negative electrode active material can be about 95 wt% to about 99.5 wt%, and based on 100 wt% of the negative electrode active material layer, the content of the binder can be about 0.5 wt% to about 5 wt%. For example, the negative electrode active material layer can include about 90 wt% to about 99 wt% of the negative electrode active material, about 0.5 wt% to about 5 wt% of the binder, and about 0.5 wt% to about 5 wt% of the conductive material.

[0121] Current collector The negative electrode current collector can include, for example, indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), germanium (Ge), lithium (Li), or an alloy thereof, and can be in the form of a foil, sheet, or foam. The thickness of the negative electrode current collector can be, for example, about 1 μm to about 20 μm, about 5 μm to about 15 μm, or about 7 μm to about 10 μm.

[0122] Electrolyte For example, the electrolyte for a rechargeable lithium battery can be or include an electrolyte solution, which can include a non-aqueous organic solvent and a lithium salt.

[0123] The non-aqueous organic solvent can constitute a medium for transporting ions participating in the electrochemical reaction of the battery. The non-aqueous organic solvent can be or include a carbonate solvent, an ester solvent, an ether solvent, a ketone solvent, an alcohol solvent, an aprotic solvent, or a combination thereof.

[0124] The carbonate solvent can include at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. The ester solvent can include at least one of methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, decanolide, mevalonolactone, valerolactone, caprolactone, etc. The ether solvent can include at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, tetrahydrofuran, etc. In addition, the ketone solvent can include cyclohexanone, etc. The alcohol solvent can include ethanol, isopropyl alcohol, etc., and the aprotic solvent can include: nitriles, such as R-CN (wherein R is a C2 to C20 straight-chain, branched-chain, or cyclic hydrocarbon group, and includes double bonds, aromatic rings, ether groups, etc.); amides, such as dimethylformamide; dioxolanes, such as 1,3-dioxolane, 1,4-dioxolane, etc.; sulfolane; etc.

[0125] The non-aqueous organic solvent can be used alone or as a mixture of two or more solvents, and when two or more non-aqueous organic solvents are used as a mixture, the mixing ratio can be appropriately adjusted according to the desired battery performance, which is generally known to those skilled in the art.

[0126] When using a carbonate solvent, a cyclic carbonate and a chain carbonate can be used in combination, and the cyclic carbonate and the chain carbonate can be mixed in a volume ratio of about 1:1 to about 1:9.

[0127] The non-aqueous organic solvent may also include an aromatic hydrocarbon organic solvent. For example, a carbonate solvent and an aromatic hydrocarbon organic solvent can be mixed and used in a volume ratio of about 1:1 to about 30:1.

[0128] The electrolyte solution may also include vinylene carbonate, vinylene carbonate or ethylene carbonate compounds to improve the battery cycle life.

[0129] Examples of the ethylene carbonate compounds may include at least one of fluoroethylene carbonate, difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate and cyanoethylene carbonate.

[0130] The lithium salt dissolved in the organic solvent supplies lithium ions in the battery, realizes the basic operation of the rechargeable lithium battery, and improves the transport of lithium ions between the positive electrode and the negative electrode. Examples of the lithium salt may include LiPF 6 , LiBF 4 , LiSbF 6 , LiAsF 6 , LiClO 4 , LiAlO 2 , LiAlCl 4 , LiPO 2 F 2 , LiCl, LiI, LiN(SO 3 C 2 F 5 ), 2 , Li(FSO 2 ), 2 N (lithium bis(fluorosulfonyl)imide; LiFSI), LiC 4 F 9 SO 3 , LiN(C x F 2x+1 SO 2 )(C y F 2y+1 SO 2(where x and y are integers from 1 to 20), at least one of lithium trifluoromethanesulfonate, lithium tetrafluoroethanesulfonate, lithium difluoro bis(oxalato)phosphate (LiDFOB), and lithium bis(oxalato)borate (LiBOB).

[0131] The concentration of the lithium salt can be in the range of about 0.1 M to about 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte solution has appropriate ionic conductivity and viscosity, and thus improved or desired performance can be achieved, and lithium ions can move effectively.

[0132] Separator Depending on the type of rechargeable lithium battery, a separator can be present between the positive electrode and the negative electrode. The separator can include at least polyethylene, polypropylene, polyvinylidene fluoride, or a multilayer film of two or more layers thereof and a mixed multilayer film, such as at least one of a polyethylene / polypropylene two-layer separator, a polyethylene / polypropylene / polyethylene three-layer separator, a polypropylene / polyethylene / polypropylene three-layer separator, etc.

[0133] The separator can include a porous substrate and a coating layer on one or both surfaces of the porous substrate, the coating layer including at least an organic material, an inorganic material, or a combination thereof.

[0134] The porous substrate can be or include a polymer film formed of or including any one of the following polymers or a copolymer or mixture of two or more of them: such as polyolefins, which are or include at least one of polyethylene and polypropylene; polyesters, such as polyethylene terephthalate and polybutylene terephthalate; polyacetals; polyamides; polyimides; polycarbonates; polyether ketones; polyarylether ketones; polyether ketones; polyetherimides; polyamideimides; polybenzimidazoles; polyethersulfones; polyphenylene ethers; cycloolefin copolymers; polyphenylene sulfides; polyethylene naphthalate; glass fibers; TEFLON; and polytetrafluoroethylene.

[0135] The porous substrate can have a thickness of about 1 μm to about 40 μm, for example, about 1 μm to about 30 μm, about 1 μm to about 20 μm, about 5 μm to about 15 μm, or about 10 μm to about 15 μm.

[0136] The organic material can include a (meth)acrylic copolymer, the (meth)acrylic copolymer including: a first structural unit derived from (meth)acrylamide; and a second structural unit including at least one of a structural unit derived from (meth)acrylic acid or (meth)acrylate and a structural unit derived from (meth)acrylamidosulfonic acid or its salt.

[0137] The inorganic material can include inorganic particles, such as Al2 O 3 , SiO 2 , TiO 2 , SnO 2 , CeO 2 , MgO, NiO, CaO, GaO, ZnO, ZrO 2 , Y 2 O 3 , SrTiO 3 , BaTiO 3 , Mg(OH) 2 , boehmite, and at least one of combinations thereof, but not limited thereto. The average particle size (D 50 ) of the inorganic particles may be from about 1 nm to about 2000 nm, for example, may be from about 100 nm to about 1000 nm, or from about 100 nm to about 700 nm.

[0138] The organic material and the inorganic material may be mixed in one coating layer, or a coating layer including the organic material and a coating layer including the inorganic material may be stacked.

[0139] The thickness of the coating layer may be from about 0.5 μm to about 20 μm, for example, may be from about 1 μm to about 10 μm, or from about 1 μm to about 5 μm.

[0140] Examples and comparative examples of the present disclosure are described below. However, the following examples are only examples of the present disclosure, and the present disclosure is not limited to the following examples.

[0141] Example 1 1. Preparation of positive electrode active material (1)Preparation of porous graphene The etchant KOH was dissolved in a solvent of distilled water to prepare a 5 M KOH solution as an etching solution. Subsequently, 4 g of graphene oxide powder was added to the etching solution, and then mixed by rotation for 24 hours for surface treatment. The surface-treated graphene was filtered and dried, and then heat-treated at 800 °C for 1 hour in an atmosphere of 95 vol% nitrogen and 5 vol% hydrogen to produce porous graphene. The BET specific surface area of the obtained porous graphene was measured using a BET (Brunauer - Emmett - Teller) apparatus, and the BET specific surface area results were in the range of 15 m 2 / g to 30 m 2 / g.

[0142] (2)Preparation of particles including lithium transition metal composite oxide As a metal raw material, nickel sulfate (NiSO 4 ·6H 2O), cobalt sulfate (CoSO 4 ·7H 2 O), and manganese sulfate (MnSO 4 ·H 2 O) are dissolved in distilled water as a solvent to prepare a mixed solution. To form a complex compound, ammonia water (NH 4 OH) and sodium hydroxide (NaOH) as a precipitating agent are prepared.

[0143] After adding the diluted ammonia water solution to the continuous reactor, the mixed solution of the metal raw materials is continuously added thereto, and sodium hydroxide is added thereto to maintain the pH inside the reactor. When the reaction proceeds slowly for about 78 hours and becomes stable, the product overflowing therefrom is collected, and then washed and dried to obtain the final precursor. Thus, nickel-based hydroxide (Ni 0.95 Co 0.04 Mn 0.01 (OH) 2 ) in the form of secondary particles in which primary particles are aggregated is obtained, and then washed and dried.

[0144] The nickel-based hydroxide is mixed with LiOH so that the molar ratio of lithium to the total metal of the nickel-based hydroxide is 1.04, and then heat-treated in an oxygen atmosphere at about 750 °C for 15 hours to obtain lithium nickel-based composite oxide particles (LiNi 0.95 Co 0.04 Mn 0.01 O 2 ). The obtained lithium nickel-based composite oxide is in the form of secondary particles in which primary particles are aggregated and the average particle size is about 9.5 μm.

[0145] (3) Coating of porous graphene The obtained porous graphene is added to the DMF solution at a concentration of 1 wt%, and then ultrasonic dispersion treatment is carried out by using a tip ultrasonic probe device to prepare a dispersion containing porous graphene. 3 g of the obtained dispersion and 3 g of lithium nickel-based composite oxide particles (LiNi 0.95 Co 0.04 Mn 0.01 O 2 ) are mixed to prepare a mixed solution, and the mixed solution is added to a conical tube, and then mixed by vortex rotation at room temperature for 2 minutes, and washed with ethanol in an amount 10 times the amount of the lithium nickel-based composite oxide particles. Subsequently, a positive electrode active material having a porous graphene coating layer formed on the surface of the lithium nickel-based composite oxide particles is manufactured by separating at 3000 rpm with a centrifuge and drying in a vacuum atmosphere at 80 °C.

[0146] Images of the prepared positive electrode active material particles were taken by SEM at magnifications of ×7000 and ×30,000. Additionally, the images are shown in Figure 7 and an image of the surface of the positive electrode active material was taken by SEM, and the image is shown in Figure 10 .

[0147] Referring to Figure 7 and Figure 10 , on the surface of the positive electrode active material of Example 1, a porous graphene shape in which a plurality of pores with a pore distance of 30 nm to 5 μm are formed was observed. Therefore, it was confirmed that the positive electrode active material of Example 1 has a porous graphene coating layer formed on the lithium nickel-based composite oxide particles.

[0148] 2. Manufacture of rechargeable lithium battery cell A positive electrode was manufactured by mixing 99.5 wt% of the positive electrode active material and 0.5 wt% of the polyvinylidene fluoride binder without a conductive material to prepare a positive electrode slurry, coating the positive electrode slurry on an aluminum foil current collector, drying the positive electrode slurry, and pressing the positive electrode slurry with a press to a density of 4.0 g / cc.

[0149] A negative electrode slurry was prepared by mixing 97.5 wt% of the graphite negative electrode active material, 1.5 wt% of carboxymethyl cellulose, and 1 wt% of styrene-butadiene rubber in an aqueous solvent. The negative electrode slurry was coated on a copper foil current collector, dried and pressed, thereby manufacturing a negative electrode.

[0150] The positive electrode and the negative electrode were used together with a polytetrafluoroethylene separator and an electrolyte solution, thereby manufacturing a rechargeable lithium battery cell by a conventional method. The electrolyte solution was prepared by dissolving 1 M LiPF 6 in a mixed solvent of ethylene carbonate and dimethyl carbonate with a volume ratio of 3:7.

[0151] Comparative Example 1 Except for using lithium nickel-based composite oxide particles (LiNi 0.95 Co 0.04 Mn 0.01 O 2 ) themselves as the positive electrode active material without using porous graphene, the positive electrode active material was prepared in substantially the same manner as in Example 1. Images of the prepared positive electrode active material were taken by SEM at magnifications of ×7000 and ×30,000, and the images are shown in Figure 5 and an image of the surface of the positive electrode active material was taken by SEM, and the image is shown in Figure 8 .

[0152] In addition, a rechargeable lithium battery cell was manufactured in the same manner as in Example 1, except that: 98 wt% of lithium nickel composite oxide particles (LiNi 0.95 Co 0.04 Mn 0.01 O 2 ), 1 wt% of Denka black as a conductive material, and 1 wt% of polyvinylidene fluoride binder were mixed to prepare a positive electrode paste to manufacture a positive electrode having a density of 3.2 g / cc.

[0153] Comparative Example 2 The positive electrode active material and the rechargeable lithium battery cell were manufactured in substantially the same manner as in Example 1, except that: graphene reduced by heat-treating graphene oxide at 800 °C for 1 hour was coated on the surface of the lithium nickel composite oxide particles (LiNi 0.95 Co 0.04 Mn 0.01 O 2 ), instead of porous graphene. Thus, the positive electrode active material paste has 99.5 wt% of the positive electrode active material, and the positive electrode manufactured therefrom has a density of 4.0 g / cc.

[0154] In addition, images of the positive electrode active material prepared according to Comparative Example 2 were taken by SEM at a magnification of ×7000 and at a magnification of ×30,000, and the images are shown in Figure 6 and images of the surface of the positive electrode active material were taken by SEM, and the images are shown in Figure 9 .

[0155] Referring to Figure 6 and Figure 9 , in the positive electrode active material of Comparative Example 2, different from Example 1, there are no pores in the graphene coating layer on the surface of the lithium nickel composite oxide particles.

[0156] Evaluation Example 1: Raman spectrum Figure 11 shows the results of Raman spectroscopic analysis of the positive electrode active material manufactured in Example 1. Referring to Figure 11 , the positive electrode active material exhibits a D band peak at 1350 cm -1 and a G band peak at 1590 cm -1 from the porous graphene. In addition, the positive electrode active material exhibits an E g band peak at 475 cm -1 to 530 cm -1 due to the lithium nickel composite oxide particles, and at 550 cm -1 to 560 cm -1Exhibits A at this location 1g Peak. Therefore, the positive electrode active material of Example 1 exhibits porous graphene coated on the surface of lithium nickel-based composite oxide particles.

[0157] In addition, the Raman spectrum analysis results of the positive electrode active materials of Example 1, Comparative Example 1, and Comparative Example 2 are shown in Figure 12 . Specifically, in the Raman spectrum analysis, the peak intensity (I -1 ) near 1350 cm D , the peak intensity (I -1 ) near 1580 cm G , and the peak intensity ratio (I D / I G ) are shown in Figure 12 . Among them, the positive electrode active material of Example 1 exhibits an I D / I G ratio of 1.02, which is higher than the respective I D / I G of the positive electrode active materials of Comparative Example 1 and Comparative Example 2.

[0158] Evaluation Example 2: Evaluation of rate discharge capacity Each of the rechargeable lithium battery cells of Example 1, Comparative Example 1, and Comparative Example 2 was charged and discharged at a charging C rate of 0.2C and a cut-off C rate of 0.05C in the range of 2.75V to 4.3V to evaluate the discharge capacity at each rate of 0.2C, 0.5C, 1C, 2C, 5C, and 10C, and the results are shown in Figure 13 .

[0159] Referring to Figure 13 , Example 1 exhibits a high discharge capacity at almost all C rates. In contrast, Comparative Example 2 exhibits a significantly deteriorated discharge capacity at higher rates of 2C or higher.

[0160] Evaluation Example 3: Cyclic voltammetry (CV) analysis of positive electrode The cyclic voltammetry (CV) curves of the positive electrodes of Example 1, Comparative Example 1, and Comparative Example 2 are shown in Figure 16 , Figure 14 and Figure 15 respectively.

[0161] Measurement was carried out using a three-electrode electrochemical cell that uses a counter electrode, a positive electrode as the working electrode, and lithium metal as the reference electrode, and cyclic scanning was performed from 2.75V to 4.3V and from 4.3V to 2.75V at scanning rates of 0.15mV / s, 0.30mV / s, 0.50mV / s, 0.75mV / s, and 1.050mV / s.

[0162] Reference Figures 14 to 16 In Example 1 where no conductive material is used in addition to the positive electrode active material, a redox reaction occurs stably at the same voltage compared to Comparative Example 1 where a conductive material is used in addition to the positive electrode active material.

[0163] In addition, as a result of analyzing the CV curves of the positive electrodes of Analytical Example 1, Comparative Example 1, and Comparative Example 2, the peak current during reduction is plotted against the square root of the scan rate as Figure 17 in the graph, and the peak current during oxidation is plotted against the square root of the scan rate as Figure 18 in the graph.

[0164] Reference Figure 17 and Figure 18 As represented by the Randle - Sevcik equation, in Example 1, the peak current proportional to the diffusion of Li + is improved to a level equal to or higher than that in Comparative Example 1 and Comparative Example 2 at a specific scan rate.

[0165] Therefore, in Example 1 where a coating layer including porous graphene is formed on the surface of lithium nickel composite oxide particles by applying porous graphene as a coating layer material for the positive electrode active material, less or no conductive material is used in the process of manufacturing the positive electrode, and sufficient electrode performance is achieved even when the amount of the conductive material is small or no conductive material is used.

[0166] Evaluation Example 4: EIS measurement results The electrochemical impedance spectra (EIS) of the rechargeable lithium battery cells according to Example 1, Comparative Example 1, and Comparative Example 2 are measured using a potentiostat device. The EIS results are shown in Figures 19 to 21 where charging is carried out at a charging C - rate of 0.2C and a cut - off C - rate of 0.05C in the range of 2.75V to 4.3V.

[0167] Figure 19 Shows the measurement results of the initial impedance of the rechargeable lithium battery cells manufactured in Example 1, Comparative Example 1, and Comparative Example 2 at room temperature (25°C). Figure 20 Shows the impedance measurement results of the symmetric battery cells of the rechargeable lithium battery cells manufactured in Example 1, Comparative Example 1, and Comparative Example 2. Figure 21 Shows the impedance measurement results of the rechargeable lithium battery cells manufactured in Example 1, Comparative Example 1, and Comparative Example 2 after 50 cycles.

[0168] Reference Figures 19 to 21Regarding the impedance measurement results, the impedance resistance results of the rechargeable lithium battery cells of Example 1 and Comparative Example 2 are summarized in Figure 22 . In other words, the impedance resistance results of Example 1 and Comparative Example 2 are provided in (a), (b), and (c) respectively. (a) comparatively shows Figure 19 the initial impedance results, (b) comparatively shows Figure 20 the impedance results of the symmetric battery cells, and (c) comparatively shows Figure 21 the impedance results after 50 cycles.

[0169] Referring to Figures 19 to 21 , since the diameter of the semicircle in the EIS diagram represents the interfacial resistance between the positive electrode and the negative electrode, the rechargeable lithium battery cell of Example 1 is confirmed to have the greatest improvement or the desired resistance reduction or suppression effect.

[0170] In addition, referring to Figure 22 , Example 1, in which porous graphene is used as the coating layer material of the positive electrode active material, shows an improved or desired resistance reduction or suppression effect compared with Comparative Example 2.

[0171] Evaluation Example 5: XPS measurement results Graphene oxide was prepared as a reference example, Figure 23 and the results of XPS analysis of the graphene oxide of the reference example and the respective graphenes used in Example 1 and Comparative Example 2 are shown in Figure 24 . The results of XPS analysis of the C1s peak in Figure 23 are shown in Figure 25 , and the results of XPS analysis of the O1s peak in Figure 23 are shown in

[0172] Referring to Figures 23 to 25 , compared with the graphene oxide of the reference example, the porous graphene used in Example 1 and the graphene used in Comparative Example 2 have an increased carbon (C) content and a decreased oxygen (O) content. Therefore, the porous graphene used in Example 1 and the graphene used in Comparative Example 2 are more reduced than the graphene oxide used in the reference example.

[0173] Evaluation Example 6: FT-IR analysis results Figure 26 The results of FT-IR analysis of the graphene oxide of the reference example and the respective graphenes used in Example 1 and Comparative Example 2 are shown in Figure 26 . Referring to

[0174] Evaluation Example 7: Conductivity evaluation The conductivity change according to density of graphene oxide of the measurement reference example and each graphene of Example 1 and Comparative Example 2 is shown in Figure 27 . Referring to Figure 27 , compared with the graphene oxide of the reference example, the graphene of Example 1 exhibits improved or desired conductivity.

[0175] Therefore, in the process of manufacturing the porous graphene of Example 1, reduction of graphene oxide is included, a coating layer including porous graphene is formed, and thus the conductivity is improved.

[0176] Evaluation Example 8: Evaluation of specific surface area and pore size The specific surface area and pore size of the graphene oxide of the reference example and each graphene used in Example 1 and Comparative Example 2 were analyzed, and the results are shown in Figure 28 . Here, the specific surface area is measured by using physical adsorption and chemisorption phenomena and the Brunauer-Emmett-Teller (BET) method. In other words, by measuring the weight of the prepared active material, nitrogen is adsorbed on the surface of the active material to measure the amount of adsorbed nitrogen, and then the specific surface area is obtained by using the BET calculation formula. In addition, the pore size is obtained by measuring the average pore radius by the Barret-Joyner-Halenda (BJH) method.

[0177] Referring to Figure 28 , the specific surface area of the porous graphene used in Example 1 is about 23 m 2 / g, the specific surface area of the graphene oxide of the reference example is about 12 m 2 / g, and the specific surface area of the graphene used in Comparative Example 2 is about 36 m 2 / g. Therefore, the specific surface area of the porous graphene used in Example 1 is larger than that of the graphene oxide, but smaller than that of the graphene used in Comparative Example 2.

[0178] In addition, the average pore radius of the porous graphene used in Example 1 (that is, the value obtained by doubling the average pore radius represents the average diameter of the pore) is about 18.5 nm, the average pore radius of the graphene oxide of the reference example is about 3 nm, and the average pore radius of the graphene used in Comparative Example 2 is about 4 nm. Therefore, the porous graphene used in Example 1 has the largest pore size.

[0179] Evaluation Example 9: Evaluation of rate discharge capacity according to stirring time change The positive electrode active material and the rechargeable lithium battery cell are manufactured in substantially the same manner as in Example 1, except that: the stirring time and the concentration of the etching solution in the manufacturing process of the porous graphene of Example 1 are changed, and then the rate discharge capabilities of the positive electrode active material and the rechargeable lithium battery cell are evaluated in substantially the same manner as in Evaluation Example 2, and the results are shown in Figure 29 in.

[0180] Referring to Figure 29 , regardless of the stirring time in the manufacturing process of the porous graphene, an improved or desired discharge capacity was ensured at a C-rate of less than 5C. On the contrary, at a high rate of 5C or higher, an improved or desired discharge capacity was ensured only when stirring for 15 to 40 hours.

[0181] Evaluation Example 10: Evaluation of rate discharge capacity according to etching solution concentration change The positive electrode active material and the rechargeable lithium battery cell are manufactured in substantially the same manner as in Example 1, except that: the concentration of the etching solution in the manufacturing process of the porous graphene of Example 1 is changed, and then the rate discharge capabilities of the positive electrode active material and the rechargeable lithium battery cell are evaluated in substantially the same manner as in Evaluation Example 2, and the results are shown in Figure 30 in.

[0182] Referring to Figure 30 , regardless of the concentration in the manufacturing process of the porous graphene, an improved or desired discharge capacity was ensured at a C-rate of less than 5C. On the contrary, at a high rate of 5C or higher, an improved or desired discharge capacity was ensured when the etching solution had a concentration of 3M to 8M.

[0183] Although the present disclosure has been described in connection with the content of embodiments that are currently considered to be practical, it will be understood that the invention is not limited to the disclosed embodiments. On the contrary, the present disclosure is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

[0184] <Description of Reference Numerals> 100: Rechargeable lithium battery 10: Positive electrode 11: Positive electrode lead tab 12: Positive electrode terminal 20: Negative electrode 21: Negative electrode lead tab 22: Negative electrode terminal 30: Separator 40: Electrode assembly 50: Housing 60: Sealing member 70: Electrode terminal 71: Positive electrode terminal 72: Negative electrode terminal.

Claims

1. A positive electrode active material, comprising: particles, including lithium transition metal composite oxides; as well as A coating layer is on the surface of the particle and includes porous graphene.

2. The positive electrode active material according to claim 1, wherein The average diameter of pores of the porous graphene is 10 nm to 1 μm.

3. The positive electrode active material according to claim 1, wherein The porous graphene may be present in an amount of 0.05 wt % to 3 wt % based on the total weight of the positive electrode active material.

4. The positive electrode active material according to claim 1, wherein The content of the porous graphene is 60 wt % to 100 wt % based on the total weight of the coating layer.

5. The positive electrode active material according to claim 1, wherein The average distance between pores of the porous graphene is 30 nm to 5 μm.

6. The positive electrode active material according to claim 1, wherein The BET specific surface area of ​​the porous graphene is 15 m 2 / g to 30m 2 / g.

7. The positive electrode active material according to claim 1, wherein The lithium transition metal composite oxide includes a lithium nickel composite oxide.

8. The positive electrode active material according to claim 1, wherein The lithium transition metal composite oxide is represented by Chemical Formula 1: [Chemical formula 1] Li a1 Ni x1 M 1 y1 M 2 z1 O 2-b1 X b1 Wherein, in chemical formula 1, 0.9≤a1≤1.8, 0.3≤x1≤1, 0≤y1≤0.7, 0≤z1≤0.7, 0.9≤x1+y1+z1≤1.1, 0≤b1≤0.1, M 1 and M 2 Independently includes at least one of Al, B, Ba, Ca, Ce, Co, Cr, Fe, Mg, Mn, Mo, Nb, Si, Sr, Ti, V, W and Zr, and X includes at least one of F, P and S.

9. The positive electrode active material according to claim 1, wherein The lithium transition metal composite oxide is represented by Chemical Formula 2: [Chemical formula 2] Li a2 Ni x2 Co y2 M 3 z2 O 2-b2 X b2 Wherein, in chemical formula 2, 0.9≤a2≤1.8, 0.3≤x2<1, 0 <y2≤0.7,0≤z2≤0.7,0.9≤x2+y2+z2≤1.1,0≤b2≤0.1,M 3 Includes at least one of Al, B, Ba, Ca, Ce, Cr, Fe, Mg, Mn, Mo, Nb, Si, Sr, Ti, V, W and Zr, and X includes at least one of F, P and S.

10. The positive electrode active material according to claim 1, wherein The lithium transition metal composite oxide has a layered structure.

11. The positive electrode active material according to claim 1, wherein The coating layer has an average thickness of 0.3 nm to 30 nm.

12. The positive electrode active material according to claim 1, wherein: The particles including the lithium transition metal composite oxide are in the form of secondary particles formed by agglomeration of a plurality of primary particles, and The particles of the positive electrode active material have an average particle size D of 7 μm to 25 μm. 50 .

13. The positive electrode active material according to claim 1, wherein Equation 1 satisfies: [Equation 1] Where, in equation 1, I D Indicates 1350cm in Raman spectroscopy analysis -1 The peak intensity near I G Indicates 1580cm in Raman spectroscopy analysis -1 The peak intensity near .

14. A method for preparing a positive electrode active material, the method comprising the following steps: preparing an etching solution in which an etchant is added to a first solvent; adding graphene oxide powder to the etching solution and stirring the graphene oxide powder to prepare a mixed solution; filtering and drying the mixed solution, and heat-treating the mixed solution to obtain porous graphene; adding the porous graphene to a second solvent and dispersing the porous graphene to prepare a dispersion; as well as Particles including a lithium transition metal composite oxide are added to the dispersion, and the particles and the dispersion are mixed.

15. The method according to claim 14, wherein: The heat treatment is performed in a reducing atmosphere.

16. The method according to claim 14, wherein: The heat treatment is performed at a temperature of 600°C to 1,000°C.

17. The method according to claim 16, wherein: The heat treatment is performed for 30 minutes to 5 hours.

18. The method according to claim 14, wherein: The etchant includes a basic compound.

19. The method according to claim 14, wherein: The etchant includes at least one of KOH and NaOH.

20. The method according to claim 14, wherein: The concentration of the etching solution is 1M to 15M.

21. The method according to claim 14, wherein: The stirring is performed for 12 to 72 hours.

22. A positive electrode, comprising: positive electrode current collector; as well as A positive electrode active material layer, on the positive electrode current collector, Wherein, the positive electrode active material layer includes the positive electrode active material according to claim 1.

23. The positive electrode according to claim 22, wherein The positive electrode has an electrode density of 3.5 g / cc to 4.7 g / cc.

24. The positive electrode according to claim 22, wherein The positive electrode active material layer does not include a conductive material.

25. A rechargeable lithium battery, comprising: The positive electrode according to claim 22; Negative electrode; as well as Electrolyte.

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