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

Abstract

This invention provides a positive electrode active material, a method for manufacturing the same, a positive electrode, and a lithium secondary battery that reduce manufacturing costs by unifying multiple processes, suppress structural degradation and particle cracking of the positive electrode active material to ensure structural stability, and achieve high capacity, high energy density, and long life characteristics. [Solution] The positive electrode active material comprises a lithium nickel-based composite oxide in the form of secondary particles formed by the aggregation of a plurality of primary particles, and a coating layer containing a cellulose derivative and carbon nanotubes located on the surface of the secondary particles, wherein the carbon nanotubes in the coating layer are randomly arranged on the surface of the secondary particles and connect the primary particles, the residual lithium content on the surface of the positive electrode active material is 1.5% by weight or less, and the pH of the surface of the positive electrode active material is 10 to 12.

Description

[Technical Field]

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

[0002] This patent is the result of research conducted with funding from the government (Ministry of Trade, Industry and Energy) in fiscal year 2024 and supported by the Korea Industrial Technology Development Agency (RS-2024-00419413, 2024 Industrial Innovation Human Resources Growth Support Project). [Background technology]

[0003] Lithium-ion batteries, which offer high energy density while being easily portable, are primarily used as power sources for mobile information terminals such as mobile phones, laptops, and smartphones. Recently, research has been actively conducted on using high-energy-density lithium-ion batteries as power sources or energy storage sources for hybrid and electric vehicles.

[0004] To realize lithium secondary batteries suitable for such applications, various cathode active materials are being investigated. Among these, lithium nickel oxides, lithium nickel manganese cobalt composite oxides, lithium nickel cobalt aluminum composite oxides, and lithium cobalt oxides are mainly used as cathode active materials. High-nickel cathode active materials with a nickel content of approximately 80 mol% or more can achieve high energy density and have been actively developed in recent years. However, unstable surface properties and the use of excess lithium precursors during synthesis lead to the generation of residual lithium on the surface, which causes problems such as slurry gelation and gas generation during the electrode manufacturing process. Furthermore, there are limitations accompanied by various problems such as structural degradation due to charging and discharging, surface side reactions with the electrolyte, and degradation due to particle cracking. Therefore, there is a need to develop cathode active materials that achieve high energy density and long life characteristics while effectively removing residual lithium. [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] One embodiment can reduce manufacturing costs by unifying processes through simultaneous implementation of a water washing process and a coating process, remove residual lithium to suppress slurry gelation and gas generation, and at the same time suppress structural deterioration and particle cracking of the cathode active material that may occur during repeated charge and discharge cycles through carbon nanotube coating to ensure structural stability, and at the same time realize high capacity, high energy density, and long life characteristics, and provides a cathode active material, a method for manufacturing the same, a cathode, and a lithium secondary battery.

Means for Solving the Problems

[0006] One embodiment provides a cathode active material comprising a lithium nickel-based composite oxide in the form of secondary particles aggregated from a plurality of primary particles; and a coating layer containing a cellulose derivative and carbon nanotubes located on the surface of the secondary particles, wherein the carbon nanotubes in the coating layer are randomly arranged on the surface of the secondary particles, connect between the primary particles, the content of residual lithium on the surface of the cathode active material is 1.5 wt% or less, and the pH of the surface of the cathode active material is 10 to 12.

[0007] Another embodiment provides a method for manufacturing a cathode active material, comprising steps of mixing carbon nanotubes, a cellulose-based dispersant, and a non-aqueous solvent to produce a mixed solution; adding water to the mixed solution to form a Pickering emulsion; adding and mixing a lithium nickel-based composite oxide in the form of secondary particles aggregated from primary particles to the Pickering emulsion to produce a mixture; and heat-treating the mixture after separating the lithium nickel-based composite oxide therefrom.

[0008] Yet another embodiment provides a cathode comprising a cathode current collector and a cathode active material layer located on the cathode current collector and containing the aforementioned cathode active material.

[0009] Still other embodiments provide a lithium secondary battery including the aforementioned positive electrode, negative electrode, and electrolyte.

Advantages of the Invention

[0010] The positive electrode active material for a lithium secondary battery according to one embodiment can reduce manufacturing costs by simplifying the process through simultaneous performance of a water washing process and a coating process, removing residual lithium to suppress slurry gelation and gas generation, and at the same time suppressing structural deterioration and particle cracking of the positive electrode active material that may occur during repeated charge and discharge processes by carbon nanotube coating to ensure structural stability, and can simultaneously realize high capacity, high energy density, and long life characteristics.

Brief Description of the Drawings

[0011] [Figure 1] It is a drawing schematically showing a lithium secondary battery according to one embodiment. [Figure 2] It is a drawing schematically showing a lithium secondary battery according to one embodiment. [Figure 3] It is a drawing schematically showing a lithium secondary battery according to one embodiment. [Figure 4] It is a drawing schematically showing a lithium secondary battery according to one embodiment. [Figure 5] It is a field emission scanning electron microscope (FE-SEM) photograph of the surface of the positive electrode active material particles of Example 1. [Figure 6] It is a field emission scanning electron microscope (FE-SEM) photograph of the surface of the positive electrode active material particles of Comparative Example 1. [Figure 7] It is a field emission scanning electron microscope (FE-SEM) photograph of the surface of the positive electrode active material particles of Comparative Example 2. [Figure 8] It is a field emission scanning electron microscope (FE-SEM) photograph of the surface of the positive electrode active material particles of Comparative Example 3. [Figure 9]These are graphs showing the results of high-resolution X-ray powder diffraction (HRPD) for the positive electrode active materials of Example 1, Comparative Example 1, and Comparative Example 2. [Figure 10] This is a graph of the surface analysis of the positive electrode active material of Example 1 using X-ray photoelectron spectroscopy (XPS). [Figure 11] This is a graph of the surface analysis of the positive electrode active material of Comparative Example 1 using X-ray photoelectron spectroscopy (XPS). [Figure 12] This shows the results of measuring the pH of water before and after coating the positive electrode active material produced in Example 1. [Figure 13] The following are the results of measuring the pH of the positive electrode active materials produced in Example 1, Comparative Example 1, and Comparative Example 3. [Figure 14] These are titration curve graphs obtained during the proper analysis process for measuring the surface residual lithium content of the positive electrode active materials in Example 1, Comparative Example 1, and Comparative Example 2. [Figure 15] The results for residual lithium content obtained by substituting the equivalence points determined by the titration curves of Example 1, Comparative Example 1, and Comparative Example 2 into the relevant calculation formulas are shown. [Figure 16] These are charge-discharge curve graphs for the first activation cycle (formation cycle) of Example 1, Comparative Example 1, and Comparative Example 2 at a speed of 0.1C. [Figure 17] This graph shows the life characteristics evaluation of Example 1, Comparative Example 1, and Comparative Example 2 at a speed of 1C for 200 cycles. [Figure 18] These are the electrochemical impedance spectroscopy (EIS) results for Example 1, Comparative Example 1, and Comparative Example 2 after the activation process. [Figure 19] These are the electrochemical impedance spectroscopy (EIS) results for Example 1, Comparative Example 1, and Comparative Example 2 after 50 cycles. [Figure 20] This is an X-ray nano-tomography (TXM) image of Example 1 before the charge-discharge experiment. [Figure 21]This is an X-ray nanotomography (TXM) image of Example 1 after 200 cycles. [Figure 22] This is an X-ray nano-tomography (TXM) image of Comparative Example 2 before the charge-discharge experiment. [Figure 23] This is an X-ray nano-tomography (TXM) image of Comparative Example 2 after 200 cycles. [Modes for carrying out the invention]

[0012] The following describes specific embodiments in detail so that they can be easily implemented by those with ordinary skill in the art. However, the present invention can be implemented in various different forms and is not limited to the embodiments described herein.

[0013] The terms used herein are for illustrative purposes only and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0014] Here, "these combinations" refers to mixtures of components, laminates, composites, copolymers, alloys, blends, reaction products, etc.

[0015] Here, terms such as “include,” “equip,” or “possess” should be understood as intending to specify the existence of the implemented features, figures, stages, components, or combinations thereof, and not preemptively excluding the possibility of the existence or addition of one or more other features, figures, stages, components, or combinations thereof.

[0016] In the drawings, thickness is shown enlarged to clearly represent various layers and regions, and similar parts are given the same drawing reference numerals throughout the specification. When a layer, film, region, plate, or other part is said to be "on top of" or "on" another part, this includes not only when it is "directly above" the other part, but also when there is another part in between. Conversely, when a part is said to be "directly above" another part, it means that there is no other part in between.

[0017] Furthermore, the term "layer" here includes not only the shapes formed on the entire surface when observed in a plan view, but also the shapes formed on only a portion of the surface.

[0018] The average particle size can be measured by methods known to those skilled in the art, for example, by a particle size analyzer, or by transmission electron microscope images or scanning electron microscope images. Alternatively, it can be measured using dynamic light scattering, and after performing data analysis to count the number of particles for each particle size range, the average particle size value can be calculated from there. Unless otherwise defined, the average particle size is the diameter (D) of the particle with a cumulative volume of 50% in the particle size distribution. 50 ) can mean. Also, unless otherwise defined, the average particle size is obtained by measuring the size (diameter or length of the long axis) of more than 20 random particles in a scanning electron microscope image to obtain a particle size distribution, and the diameter (D) of the particle whose cumulative volume is 50% in the particle size distribution. 50 ) may be used as the average particle size.

[0019] Here, "or" is not interpreted as having an exclusive meaning; for example, "A or B" is interpreted as including A, B, A+B, etc.

[0020] The term "metal" is interpreted as a concept that includes general metals, transition metals, and semimetals.

[0021] positive electrode active material In one embodiment, a positive electrode active material is provided comprising a lithium nickel-based composite oxide in the form of secondary particles in which a plurality of primary particles are aggregated; and a coating layer containing a cellulose derivative and carbon nanotubes located on the surface of the secondary particles, wherein the carbon nanotubes in the coating layer are randomly arranged on the surface of the secondary particles and connect the primary particles, the residual lithium content on the surface of the positive electrode active material is 1.5% by weight or less, and the pH of the surface of the positive electrode active material is 10 to 12.

[0022] Lithium nickel-based composite oxides generally have a secondary particle morphology in which multiple primary particles are aggregated. Such lithium nickel-based composite oxides have an excess of residual lithium in their crystalline structure, which leads to slurry gelation, gas generation due to side reactions with the electrolyte, and degradation of the battery life when applied to lithium secondary batteries.

[0023] Furthermore, in lithium nickel-based composite oxides, as the charge-discharge cycle progresses, repeated volume changes in the crystal structure cause the gaps between primary particles to widen and the arrangement of primary particles to shift. This leads to a large difference in the degree of contraction and expansion between the secondary particles and the positive electrode containing them. This causes structural cracks and fractures in the secondary particles and the positive electrode, leading to a disruption of the electrochemical charge transfer network, resulting in charge imbalance, an increase in the side reaction area with the electrolyte, and a deterioration of the repeated charge-discharge cycle life.

[0024] First, various methods have been proposed to remove residual lithium, the most common of which is to wash the positive electrode active material with a solvent such as ethanol or water. Ethanol washing has the advantage of preventing surface damage to the positive electrode active material, but the solubility of residual lithium is low (2.36 g / 100 g in anhydrous ethanol at 20°C), and a mixing time of up to 48 hours is required, which significantly increases the process time and cost. As an alternative, there are surface coating technologies such as phosphate treatment that convert the residual lithium on the surface to Li3PO4, but these are also time-consuming and costly. However, water washing (hereinafter the same as the water washing process) has the advantage of being quick and relatively inexpensive due to the high solubility of residual lithium, and is widely used.

[0025] On the other hand, various methods have been proposed to suppress the repeated volume changes of the crystal structure associated with the progression of charge-discharge cycles, and a well-known technique involves coating the surface of a lithium nickel composite oxide with a carbon-based material. Among these, graphene coating is known, but due to the inherent properties of graphene, when it is used as a coating, it covers the entire surface of the lithium nickel composite oxide, hindering lithium ion movement between the surface of the positive electrode active material and the electrolyte, thus reducing high capacity, high power output, and long life characteristics.

[0026] In one embodiment, the positive electrode active material removes residual lithium from the surface of the lithium nickel composite oxide through a water washing process, suppressing gelation in the positive electrode slurry and gas generation due to side reactions with the electrolyte. Simultaneously, by coating with carbon nanotubes, unlike graphene, the entire surface of the lithium nickel composite oxide is not covered, thus solving the problem of inhibited lithium ion movement between the surface of the positive electrode active material and the electrolyte. The carbon nanotubes interconnect the primary particles, preventing the alignment of the primary particles from shifting, such as the widening of gaps between primary particles due to charging and discharging, thus maintaining the morphology of the secondary particles. This suppresses the contraction and expansion of the secondary particles and the positive electrode, thereby improving the battery's lifespan characteristics.

[0027] As an example, the residual lithium content on the surface of the positive electrode active material is 1.5% by weight or less, and the pH of the surface of the positive electrode active material is 10 to 12.

[0028] The residual lithium content on the surface of the positive electrode active material may be, for example, 1.0% by weight or less, 0.5% by weight or less, or 0.3% by weight or less. The residual lithium content on the surface of the positive electrode active material can be measured by acid-base titration (Warder method). As an example of the acid-base titration method, the positive electrode active material can be dissolved in water, filtered, and the filtered solution can be titrated using a titrator to obtain a pH titration curve by adding 0.05 M HCl until the pH value is 3. The residual lithium content can then be calculated by substituting the equivalence point on the titration curve into the relevant calculation formula. When the residual lithium content on the surface of the positive electrode active material satisfies the above range, slurry gelation can be suppressed during the positive electrode manufacturing process, and the thickness of the positive electrode during the slurry coating process can be maintained uniformly. Furthermore, by mitigating side reactions with the electrolyte during the battery operation process, the generation of gases such as CO, CO2, and O2 is reduced, thereby suppressing battery expansion and the rise in internal cell pressure, and consequently ensuring long-term stability. On the other hand, if the residual lithium content on the surface of the positive electrode active material exceeds the aforementioned range, the pH of the positive electrode slurry increases, promoting crosslinking of the binder contained in the slurry, causing the slurry to gel and leading to instability in the positive electrode manufacturing process. This then promotes side reactions between the positive electrode active material and the electrolyte, increasing gas generation and generating hydrofluoric acid (HF), which in turn promotes the dissolution of the transition metal in the positive electrode, thus adversely affecting the battery's stability and lifespan.

[0029] Also, the pH of the surface of the positive electrode active material may be, for example, 10 to 11.9, or 10 to 11.85. The pH of the surface of the positive electrode active material can be measured by a pH meter measurement method. Specifically, after dissolving the positive electrode active material in water and filtering the solution, the pH meter electrode may be immersed in the solution, and after maintaining it for 200 seconds, the pH may be measured. When the pH of the surface of the positive electrode active material satisfies the above range, gelation of the slurry during the positive electrode manufacturing process is suppressed, and thereby the thickness of the positive electrode is uniformly formed. In addition, side reactions with the electrolyte during the battery driving process are alleviated, gas generation such as CO, CO2, and O2 is reduced, and thereby battery swelling and an increase in internal cell pressure can be suppressed, and as a result, long-term stability can be ensured. Also, the higher the pH value, the higher the residual lithium content on the surface of the positive electrode active material. When the pH exceeds the above range, side reactions between the positive electrode active material and the electrolyte during battery driving are promoted, the amount of gas generation increases, hydrofluoric acid (HF) is generated, and dissolution of transition metals in the positive electrode is promoted, which has an adverse effect on the stability and life of the battery.

[0030] Lithium nickel-based composite oxide The lithium nickel-based composite oxide is represented by, for example, the following Chemical Formula 1.

[0031] [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 ≦ 1.2, 0.8 ≦ x1 < 1, 0 < y1 ≦ 0.2, 0 ≦ z1 ≦ 0.2, 0.9 ≦ x1 + y1 + z1 ≦ 1.1, and 0 ≦ b1 ≦ 0.1, and M 1 and M 2 are each independently one or more elements selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu, Fe, Mg, Mn, Mo, Nb, Si, Sn, Sr, Ti, V, W, Y, Zn, and Zr, and X is one or more elements selected from F, P, and S. M 1 and M2 These elements may be different from each other.

[0032] In chemical formula 1, 0.85≦x1<1, 0 <y1≦0.15、および0≦z1≦0.15または、0.9≦x1<1、0<y1≦0.1、および0≦z1≦0.1であってもよい。

[0033] The aforementioned lithium nickel-based composite oxide is shown in Chemical Formula 2 or Chemical Formula 3 below as a specific example.

[0034] [Chemical formula 2] Li a2 Ni x2 Co y2 M 3 z2 O 2-b2 X b2 In the above chemical formula 2, 0.9 ≤ a² ≤ 1.2, 0.8 ≤ x² < 1, 0 <y2≦0.2、0≦z2≦0.2、0.9≦x2+y2+z2≦1.1、および0≦b2≦0.1であり、M 3 X is one or more elements selected from Al, B, Ba, Ca, Ce, Cr, Cu, Fe, Mg, Mn, Mo, Nb, Si, Sn, Sr, Ti, V, W, Y, Zn, and Zr, and X is F, P, S, or a combination thereof.

[0035] In the aforementioned chemical formula 2, 0.9 ≤ x² ≤ 0.99, 0.01 ≤ y² ≤ 0.1, and 0 ≤ z² ≤ 0.1 may also be used.

[0036] [Chemical formula 3] Li a3 Ni x3 Co y3 M 4 z3 M 5 w3 O 2-b3 X b3 In the above chemical formula 3, 0.9≦a3≦1.2, 0.8≦x3≦0.98, 0.01≦y3≦0.19, 0.01≦z3≦0.19, 0≦w3≦0.19, 0.9≦x3+y3+z3+w3≦1.1, and 0≦b3≦0.1, M 4is Al, Mn, or a combination thereof, M 5 X is one or more elements selected from B, Ba, Ca, Ce, Cr, Cu, Fe, Mg, Mo, Nb, Si, Sn, Sr, Ti, V, W, Y, Zn, and Zr, and X is F, P, S, or a combination thereof.

[0037] In the aforementioned chemical formula 3, 0.9 ≤ x 3 ≤ 0.98, 0.01 ≤ y 3 ≤ 0.09, 0.01 ≤ z 3 ≤ 0.09, and 0 ≤ w 3 ≤ 0.09 may also be applied.

[0038] The nickel content relative to 100 mol% of the total metal excluding lithium from the lithium-nickel composite oxide may be 80 mol% or more, for example, 85 mol% or more, 90 mol% or more, 91 mol% or more, 94 mol% or more, or 99 mol% or less. When the nickel content satisfies the above range, high capacity and high energy density can be achieved.

[0039] The lithium nickel-based composite oxide is in the form of secondary particles, which are aggregates of multiple primary particles. The secondary particles may be spherical, ellipsoidal, polyhedronal, or irregular in shape, and the primary particles may be spherical, ellipsoidal, plate-shaped, or a combination thereof.

[0040] The average particle size (D) of the secondary particles of the lithium nickel-based composite oxide 50 The average particle size (D) is 2 μm to 20 μm, and may be, for example, 4 μm to 20 μm or 6 μm to 20 μm. Here, the average particle size (D) 50 The particle size may be obtained by measuring the size (diameter or length of the major axis) of more than 20 particles randomly using scanning electron microscope images of lithium nickel-based composite oxide particles to obtain a particle size distribution, and then taking the diameter of the particle with a cumulative volume of 50% of the particle size distribution as the average particle size.

[0041] The average particle size (D) of the primary particles constituting the secondary particles of the lithium nickel composite oxide 50The particle size may be 6 μm or less, for example, 100 nm to 4 μm, 100 nm to 2 μm, 200 nm to 800 nm, or 300 nm to 700 nm. The average particle size of the primary particles may be obtained by measuring the size (diameter or length of the major axis) of more than 20 primary particles randomly using scanning electron microscope or transmission electron microscope images of the surface of the secondary particles to obtain a particle size distribution, and taking the size of the particle with a cumulative volume of 50 volume% in the particle size distribution as the average particle size. When the size of the primary particles satisfies the above range, the positive electrode active material can achieve high initial charge / discharge capacity and efficiency, and excellent output characteristics and lifetime characteristics.

[0042] coating layer A coating layer according to one embodiment includes a cellulose derivative and carbon nanotubes.

[0043] The cellulose derivative is not limited in type as long as it contains an alkyl group and a functional group capable of hydrogen bonding. For example, the cellulose derivative may include ethylcellulose, cellulose acetate, carboxymethylcellulose, hydroxypropylcellulose, methylcellulose, nitrocellulose, or combinations thereof.

[0044] The cellulose derivative can be located between the lithium nickel composite oxide and the carbon nanotube. For example, the cellulose derivative can be located between the secondary particles of the lithium nickel composite oxide and the carbon nanotube, and more specifically, between the primary particles located on the surface of the secondary particles and the carbon nanotube. For example, the primary particles located on the surface of the secondary particles and the carbon nanotube can adhere to each other via the cellulose derivative. Such a cellulose derivative can play a role in maintaining the shape of the secondary particles and suppressing the shrinkage and expansion of the positive electrode by enabling the carbon nanotube to adhere to the surface of the lithium nickel composite oxide.

[0045] The cellulose derivative in the coating layer is present in an amount of 0.005% to 50% by weight relative to 100% by weight of the positive electrode active material, for example, 0.05% to 40% by weight, 0.1% to 30% by weight, or 0.2% to 20% by weight. Alternatively, the cellulose derivative may be present in an amount of 0.5% to 90% by weight relative to 100% by weight of the coating layer, for example, 10% to 85% by weight, 20% to 80% by weight, or 30% to 70% by weight. The content (by weight) of the cellulose derivative can be measured using thermal decomposition analysis, such as thermogravimetric analysis (TGA) or differential scanning calorimetry (DSC). TGA analysis is performed in an air atmosphere, for example, by heating from room temperature to approximately 800°C at a rate of 10°C / min. By measuring the mass change of the sample according to temperature, the thermal decomposition behavior of cellulose derivatives can be classified. Cellulose derivatives, such as ethyl cellulose, exhibit thermal decomposition behavior at around 240°C, so the content can be calculated based on the mass loss in the relevant range (e.g., 200°C to 350°C). In addition to the above thermal decomposition analysis, the content of cellulose derivatives can also be calculated using solvent extraction methods. For example, by immersing the positive electrode active material in a specific solvent that can dissolve cellulose derivatives (e.g., acetonitrile, ethanol, methanol, toluene, acetone, ethyl acetate, xylene, etc.), cellulose derivatives in the coating layer can be selectively dissolved and removed. The content of cellulose derivatives in the coating layer can be calculated by measuring the mass loss of the solids remaining after dissolution or the solid content of the cellulose derivative dissolved in the solution. If it is difficult to directly separate the coating layer and analyze its content, the thickness and volume ratio of the coating layer can be evaluated through cross-sectional analysis using an electron microscope (TEM, SEM) or focused ion beam (FIB), and the content of the cellulose derivative on a 100% wt basis of the coating layer can be corrected by converting this to a ratio relative to the total mass of the positive electrode active material.Furthermore, by performing differential scanning calorimetry (DSC) in parallel, the temperature at which the pyrolysis peak occurs can be clearly distinguished, thereby matching the mass loss interval in TGA with the pyrolysis temperature of the cellulose derivative and improving measurement accuracy. As an example, the content of the above-mentioned cellulose derivative may be measured by thermogravimetric analysis (TGA).

[0046] The coating layer may further contain amorphous carbon. Amorphous carbon is distinguished from crystalline carbon or graphite-based carbon as carbon that does not have crystallinity or has very low crystallinity. The amorphous carbon may include carbides of cellulose derivatives. Carbohydrates of cellulose derivatives are amorphous carbon materials formed when cellulose derivatives are heat-treated or undergo a carbonization process depending on the heat treatment temperature described later. The type of carbide of cellulose derivative is not limited as long as it contains alkyl groups and functional groups capable of hydrogen bonding. For example, the carbide of cellulose derivative may include carbides of ethylcellulose, cellulose acetate, carboxymethylcellulose, hydroxypropylcellulose, methylcellulose, nitrocellulose, or combinations thereof.

[0047] The amorphous carbon can be located between the lithium nickel composite oxide and the carbon nanotube, together with the cellulose derivative. For example, the amorphous carbon can be located between the secondary particles of the lithium nickel composite oxide and the carbon nanotube, and more specifically, between the primary particles located on the surface of the secondary particles and the carbon nanotube. For example, the primary particles located on the surface of the secondary particles and the carbon nanotube can be bonded together via the amorphous carbon. The amorphous carbon, located between the lithium nickel composite oxide and the carbon nanotube together with the cellulose derivative, acts as a kind of adhesive that allows the carbon nanotube to adhere well to the surface of the lithium nickel composite oxide, further improving the maintenance of the secondary particle morphology and the suppression of shrinkage and expansion between the secondary particles and the positive electrode while the carbon nanotube is more strongly bonded to the lithium nickel composite oxide.

[0048] One embodiment of the coating layer includes carbon nanotubes. When graphene coating is performed using known graphene coating techniques, graphene is a 2D material and, by covering the entire surface of the lithium nickel-based composite oxide, it hinders lithium ion movement between the surface of the positive electrode active material and the electrolyte, reducing high capacity, high power, and long life characteristics. However, carbon nanotubes are a 1D material and, unlike graphene, do not cover the entire surface of the lithium nickel-based composite oxide by interconnecting primary particles on the surface of the lithium nickel-based composite oxide. Therefore, they do not hinder lithium ion movement between the electrolyte, thus maintaining the shape of secondary particles and suppressing the contraction and expansion of secondary particles and the positive electrode, thereby improving battery life characteristics.

[0049] The carbon nanotubes are randomly arranged on the surface of secondary particles of lithium nickel-based composite oxide, and the carbon nanotubes connecting the primary particles may exist on the surface of the secondary particles in a three-dimensional network (net) or spiderweb-like form.

[0050] As an example, carbon nanotubes are provided in a form that interconnects primary particles constituting secondary particles on the surface of the secondary particles, and amorphous carbon can be located between the carbon nanotubes and secondary particles and act as a kind of adhesive. For example, primary particles located on the surface of secondary particles and the carbon nanotubes can be bonded together via amorphous carbon.

[0051] The carbon nanotubes are highly crystalline carbon-based materials in which carbon atoms are arranged in a hexagonal pattern to form a tube, and they exhibit excellent electrical conductivity and lithium-ion conductivity. Therefore, they can maintain a uniform current and voltage distribution within the positive electrode between charge and discharge cycles, significantly improving cycle characteristics and enhancing the battery's output characteristics through improved conductivity. Furthermore, because the carbon nanotubes consist of carbon atoms bonded together by strong covalent bonds, they possess excellent tensile strength and high resistance to fracture, thus significantly improving the safety of the battery.

[0052] The carbon nanotubes within the coating layer are provided independently in the form of a single fiber or a bundle of fibers, and are connected via some point contacts or point surfaces. Furthermore, adjacent carbon nanotubes within the coating layer are provided separated so as to have some space between them. By providing the carbon nanotubes within the coating layer with some space between them, lithium ions can move freely between the surface of the positive electrode active material and the electrolyte, enabling the realization of high capacity, high power output, and long life characteristics.

[0053] The area occupied by the carbon nanotubes relative to the total surface area of ​​the secondary particles of the positive electrode active material may be 1% to 80%, for example, 35% to 75%, 40% to 70%, or 45% to 65%. The area of ​​the carbon nanotubes relative to the total surface area of ​​the secondary particles can be measured, for example, by measuring the area of ​​the secondary particles from a scanning electron microscope image of the surface of the secondary particles of the positive electrode active material using an image program such as Image J, measuring the area of ​​the region where the carbon nanotubes are present, and calculating the ratio of the latter to electrons. For more accurate measurements, analysis can be performed under the same brightness and contrast settings, using threshold adjustment and binary conversion to maintain a clear distinction between the carbon nanotubes and the background. When the area of ​​the carbon nanotubes is within the specified range relative to the total area of ​​the coating layer, it is possible to maintain the secondary particle morphology without hindering the movement of lithium ions between the positive electrode active material and the electrolyte, and without degrading the contact characteristics of the primary particles, such as the widening of gaps between primary particles due to charging and discharging. At the same time, it is possible to improve the battery's lifespan characteristics by suppressing the contraction and expansion of the secondary particles and the positive electrode.

[0054] The carbon nanotube may include single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), or a combination thereof. In one embodiment, the carbon nanotube may be a multi-walled carbon nanotube. When a multi-walled carbon nanotube is used as the carbon nanotube, the cost is usually lower compared to when a single-walled carbon nanotube is used.

[0055] The carbon nanotubes may include untreated (bare) carbon nanotubes, treated carbon nanotubes, or combinations thereof. Untreated carbon nanotubes have surface functional groups (such as CO, CH, and C-OOH) that are normally present on the carbon nanotube surface, and such functional groups are generated during the synthesis process when producing carbon nanotubes. Treated carbon nanotubes have additional functional groups attached to them by using an acid such as nitric acid to break the carbon ring structure on the carbon nanotube surface, more specifically the surface of untreated carbon nanotubes, and oxidize it. Such additional functional groups include carboxylic acids, amines, and polyethylene glycols, and examples of surface-treated carbon nanotubes with such additional functional groups attached include COOH-functionalized CNTs, amine-functionalized CNTs, and polyethylene glycol-functionalized CNTs. In one embodiment, the carbon nanotubes in the coating layer can be unrestricted and include untreated carbon nanotubes, treated carbon nanotubes, etc.

[0056] The average diameter of the carbon nanotubes may be between 1 nm and 50 nm, for example, between 1 nm and 40 nm, 1 nm and 30 nm, 1 nm and 20 nm, or 1 nm and 10 nm. The average diameter of the carbon nanotubes may also be the average of the diameters of the widest parts measured by observing 10 or more carbon nanotubes using a transmission electron microscope (TEM). When the average diameter of the carbon nanotubes satisfies the above range, the elastic properties of the carbon nanotubes can be utilized to enhance the mechanical properties of the coating layer.

[0057] The average aspect ratio of the carbon nanotubes may be between 100 and 50,000, for example, between 500 and 40,000, between 5,000 and 30,000, or between 10,000 and 25,000. The average aspect ratio of the carbon nanotubes is the ratio of the average length to the average diameter (average length / average diameter ratio). The average aspect ratio of the carbon nanotubes can be calculated by measuring the average diameter and average length and dividing the average length by the average diameter. Here, the average diameter can be measured in the same manner as the measurement method described above, and the average length may be the average value obtained by observing 10 or more carbon nanotubes using a scanning electron microscope (SEM) and measuring their lengths. When the aspect ratio of the carbon nanotubes satisfies the above range, the ratio of carbon nanotubes covering the positive electrode active material per unit weight can be increased, and an effect of generally coating the surface of the positive electrode active material can be provided.

[0058] The carbon nanotubes are present in an amount of 0.01% to 10% by weight relative to 100% by weight of the positive electrode active material, for example, 0.05% to 10% by weight, 0.05% to 5% by weight, or 0.1% to 5% by weight. The carbon nanotubes are also present in an amount of 10% to 99.5% by weight relative to 100% by weight of the coating layer, for example, 15% to 90% by weight, 20% to 80% by weight, or 30% to 70% by weight. The carbon nanotube content (by weight) can be measured using pyrolysis analysis, such as thermogravimetric analysis (TGA) or differential scanning calorimetry (DSC). TGA analysis is performed in an air atmosphere, for example, by heating from room temperature to approximately 800°C at a rate of 10°C / min. By measuring the change in mass of the sample according to temperature, the thermal decomposition behavior of carbon nanotubes can be classified. Since carbon nanotubes, such as multi-walled carbon nanotubes, exhibit thermal decomposition behavior at around 600°C, the content can be calculated based on the mass loss in the relevant range (e.g., 450°C to 700°C). In addition to the above thermal decomposition analysis, the carbon nanotube content can also be calculated by separating the coating layer or by using solvent extraction methods that allow for selective elution. For example, after dissolving and removing cellulose derivatives in the coating layer, the relative weight ratio of carbon nanotubes in the coating layer can be calculated by measuring the mass of the remaining solids. If it is difficult to directly separate and analyze the coating layer, the thickness and volume ratio of the coating layer can be evaluated through cross-sectional analysis using an electron microscope (TEM, SEM) or focused ion beam (FIB), and this can be converted to a ratio to the total mass of the positive electrode active material to correct the carbon nanotube content based on 100 wt% of the coating layer. Furthermore, by performing differential scanning calorimetry (DSC) in parallel, the temperature at which the pyrolysis peak occurs can be clearly distinguished, and by comparing this with the mass loss interval obtained by TGA, the accuracy of the analysis can be improved.For example, the carbon nanotube content mentioned above may have been measured by thermogravimetric analysis (TGA).

[0059] In one embodiment, carbon nanotubes are used in small amounts relative to the graphene content during known graphene coating, which further reduces the content of conductive material within the positive electrode relative to the graphene coating layer, which is advantageous for improving the energy density of the positive electrode while also improving battery life characteristics.

[0060] In one embodiment, the positive electrode active material includes the aforementioned lithium nickel-based composite oxide and the aforementioned coating layer, thereby preventing the gaps between primary particles from widening even during repeated charging and discharging, and maintaining the secondary particle morphology and the positive electrode morphology.

[0061] The thickness of the coating layer may be 1 nm to 300 nm, for example, 30 nm to 200 nm or 50 nm to 200 nm. The thickness of the coating layer may be measured by, for example, TOF-SIMS, XPS, or EDS analysis, and the range of the coating layer thickness may be measured by TEM-EDS line profile. When the thickness of the coating layer satisfies the range, the mechanical properties of the coating layer containing carbon nanotubes are enhanced, improving the lifetime characteristics of the positive electrode active material. If the thickness of the coating layer exceeds the range, the coating layer may act as a resistive element that hinders the movement of lithium ions to the positive electrode active material, potentially reducing the capacity.

[0062] Method for manufacturing positive electrode active material One embodiment provides a method for producing a positive electrode active material, comprising the steps of: mixing carbon nanotubes, a cellulose-based dispersant, and a non-aqueous solvent to produce a mixed solution; adding water to the mixed solution to form a pickering emulsion; adding and mixing lithium nickel-based composite oxide in the form of aggregated primary particles into the pickering emulsion to produce a mixture; and separating the lithium nickel-based composite oxide from the mixture and then heat-treating it.

[0063] First, a mixed solution is prepared by mixing carbon nanotubes, a cellulose-based dispersant, and a non-aqueous solvent.

[0064] Since the carbon nanotubes are the same as those described for the positive electrode active material, they will be omitted below.

[0065] The type of cellulose-based dispersant is not limited as long as it contains an alkyl group and a functional group capable of hydrogen bonding. For example, the cellulose-based dispersant may include ethylcellulose, cellulose acetate, carboxymethylcellulose, hydroxypropylcellulose, methylcellulose, nitrocellulose, or a combination thereof, and may also include ethylcellulose as an example. By including the cellulose-based dispersant, carbon nanotubes can be uniformly dispersed in the solvent without aggregation. The weight ratio of the carbon nanotubes to the cellulose-based dispersant may be 10:90 to 90:10, 15:85 to 88:12, or 15:85 to 80:20. When the weight ratio of the carbon nanotubes to the cellulose-based dispersant satisfies the above range, aggregation of carbon nanotubes can be minimized, and the carbon nanotubes can be uniformly coated onto the surface of the positive electrode active material.

[0066] The non-aqueous solvent can be a solvent that does not mix with water, and may include, for example, hexane, heptane, toluene, cyclohexane, isooctane, pentane, or combinations thereof.

[0067] The concentration of carbon nanotubes in the mixed solution may be 0.14 mg / mL to 0.5 mg / mL, and the weight ratio of carbon nanotubes to cellulose-based dispersant may be 1:1 to 1:5. When the concentration of carbon nanotubes in the mixed solution satisfies the above range, and the weight ratio of carbon nanotubes to cellulose-based dispersant satisfies this range, aggregation of carbon nanotubes is minimized, and a mixed solution in which carbon nanotubes are uniformly dispersed can be produced, and the carbon nanotubes can be uniformly coated onto the surface of the positive electrode active material.

[0068] By mixing the cellulose-based dispersant with the solvent to produce a mixed solution, the carbon nanotubes can be well dispersed in the solvent by preventing aggregation due to van der Waals forces through π-□ interaction (where □ = CH, OH, or π) between the carbon nanotubes and the cellulose-based dispersant.

[0069] Next, water is added to the mixed solution to form a pickering emulsion.

[0070] The step of forming the Pickering emulsion may be carried out by adding water to the mixed solution and then using a method involving vortexing, sonication, or a combination thereof.

[0071] Since the non-aqueous solvent in the mixed solution is immiscible with water, when water is added to the mixed solution, an emulsion is formed, and the carbon nanotubes and cellulose-based dispersant can act as stabilizers at the interface between the non-aqueous solvent and water. Specifically, the cellulose-based dispersant forms hydrogen bonds at the interface, and the carbon nanotubes physically adsorb to the interface, providing stability to the emulsion. The positions of the internal and external solvents in the droplet of the Pickering emulsion are determined by the amount of solvent, with a relatively small amount of solvent constituting the internal solvent of the droplet and a relatively large amount of solvent constituting the external solvent. Depending on the types of internal and external solvents in the droplet, differences arise in the characteristics of carbon nanotube dispersion, droplet size within the Pickering emulsion, dispersion characteristics of the dispersant, and uniformity of the coating.

[0072] The weight ratio of the non-aqueous solvent to the added water in the mixed solution may be 10:1 to 2:1, or for example, 8:1 to 2:1, 6:1 to 2:1, or 4:1 to 2:1. In this case, the non-aqueous solvent may be located as the external solvent of the pickering emulsion droplet, and the water may be located as the internal solvent of the pickering emulsion droplet. By having the water located as the internal solvent of the pickering emulsion droplet, when the lithium nickel-based composite oxide described later is introduced into the droplet, a water washing process can be performed simultaneously with the coating process using the water located in the internal droplet, thereby removing residual lithium.

[0073] In other words, the pickering emulsion contains droplets, the carbon nanotubes and the cellulose-based dispersant are located at the interface of the droplets, the water is located inside the droplets, and the non-aqueous solvent may be located outside the droplets.

[0074] The average size of the Pickering emulsion droplets may be 0.5 μm to 30 μm, for example, 1 μm to 25 μm, 2 μm to 20 μm, or 3 μm to 15 μm. The average size of the Pickering emulsion droplets can be measured by optical microscopy analysis, and the average size of the Pickering emulsion droplets varies depending on the volume ratio of the non-aqueous solvent to the water. When the average size of the Pickering emulsion droplets satisfies the above range, carbon nanotubes can be easily coated onto the surface of lithium nickel-based composite oxide particles, and at the same time, lithium nickel-based composite oxide can be inserted and washed with water.

[0075] Since the method for forming the aforementioned pickering emulsion is well known, the specific details will be omitted.

[0076] Next, lithium nickel-based composite oxide in the form of secondary particles, which are aggregates of primary particles, is added to the pickering emulsion and mixed to produce a mixture.

[0077] The lithium nickel-based composite oxide in the form of secondary particles, which is the aggregated form of the primary particles, is the same as that described for the positive electrode active material, and will therefore be omitted below.

[0078] The mixing method after adding the lithium nickel-based composite oxide can be carried out by known methods, such as stirring or vortexing. The mixing speed may be 100 rpm to 2,000 rpm.

[0079] In the step of adding and mixing the lithium nickel-based composite oxide, the lithium nickel-based composite oxide is inserted into the pickering emulsion droplets, and the lithium nickel-based composite oxide inserted into the droplets can be washed by the water located inside the droplets. This washing can remove residual lithium from the surface of the lithium nickel-based composite oxide.

[0080] Afterward, the lithium nickel-based composite oxide is separated from the mixture and then heat-treated.

[0081] The process of separating the lithium nickel-based composite oxide from the aforementioned mixture can be carried out by vacuum filtration or centrifuge, and can be understood as a process of removing the liquid phase from the mixture to separate the solid components. In this case, carbon nanotubes and a cellulosic dispersant are present on the surface of the separated lithium nickel-based composite oxide.

[0082] In other words, the process of separating lithium nickel-based composite oxides from a mixture can be understood as the process of separating the lithium nickel-based composite oxides located inside the droplets from the mixture, and the carbon nanotubes and cellulose-based dispersant located at the interface of the droplets. For example, it may involve separating an intermediate in which carbon nanotubes and cellulose-based dispersant are attached to the surface of lithium nickel-based composite oxides in the form of secondary particles.

[0083] The separation step followed by the heat treatment step may further include a drying step. In this drying step, the internal solvent of droplets present on the lithium nickel composite oxide surface can be removed. This step can also effectively remove the external solvent of droplets remaining on the surface during the separation process. The drying step is performed separately from the heat treatment step described later, but it can also be performed together with the heat treatment step.

[0084] The heat treatment may be performed in conjunction with the drying step to remove the internal solvent of droplets and the external solvent of droplets remaining on the surface of the separated lithium nickel composite oxide. The heat treatment step is compatible with the heat-treatable conditions of the lithium nickel composite oxide and the cellulosic dispersant. When the drying step is performed together with the heat treatment step, the heat treatment may be performed at a temperature of 60°C to 800°C under vacuum or a specific gas atmosphere, for example, at temperatures of 60°C to 600°C, 60°C to 400°C, 60°C to 200°C, 60°C to 180°C, 600°C to 160°C, 60°C to 140°C, 80°C to 140°C, 100°C to 140°C, or 100°C to 120°C. When the drying step and the heat treatment step are performed separately, drying can be performed at a temperature of 60°C to 150°C, and heat treatment can be performed at a temperature of 150°C to 800°C under vacuum or a specific atmosphere. The specified gas can be oxygen, argon, air, nitrogen, etc., and at the heat treatment temperature and atmosphere, the water internal solvent and the non-aqueous solvent external solvent of the droplets remaining on the surface of the lithium nickel composite oxide are removed, and the cellulose-based dispersant can be positioned between the lithium nickel composite oxide and the carbon nanotubes, playing a role in adhering the carbon nanotubes to the surface of the lithium nickel composite oxide. If the heat treatment temperature is set to a temperature that carbonizes the cellulose-based dispersant, a portion of the cellulose-based dispersant will be carbonized. Even if the cellulose-based dispersant is carbonized, it can still be positioned between the surface of the lithium nickel composite oxide and the carbon nanotubes, acting as a kind of adhesive in the form of amorphous carbon carbides, thereby allowing the carbon nanotubes to adhere more strongly to the lithium nickel composite oxide.

[0085] As described above, the positive electrode active material produced by the aforementioned method has carbon nanotubes randomly arranged on the surface of the secondary particles of the lithium nickel composite oxide, connecting the primary particles and preventing the primary particle arrangement from shifting even with repeated charging and discharging. This maintains the shape of the secondary particles and the positive electrode, preventing a decrease in charge-discharge cycle life characteristics. At the same time, the residual lithium content on the surface of the positive electrode active material is 1.5% by weight or less, and the pH of the surface of the positive electrode active material is 10 to 12, which removes residual lithium and suppresses gas generation due to side reactions with the electrolyte and life degradation.

[0086] positive electrode A positive electrode according to one embodiment includes a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector and containing the aforementioned positive electrode active material. The positive electrode according to one embodiment can achieve high capacity while suppressing structural degradation and crack generation due to charging and discharging, thereby achieving long-life characteristics.

[0087] positive electrode current collector The positive electrode current collector is not particularly limited as long as it is conductive without causing a chemical change to the lithium secondary battery, and may be an aluminum foil or stainless steel foil with a thickness of 10 μm to 20 μm.

[0088] Cathode active material layer The positive electrode active material layer includes the aforementioned positive electrode active material.

[0089] The specific details regarding the positive electrode active material are as described above.

[0090] The content of the positive electrode active material may be 60% to 99.9% by weight, 70% to 99.8% by weight, 80% to 99% by weight, or 90% to 98% by weight, based on 100% by weight of the positive electrode active material layer. When the aforementioned positive electrode active material is included, a similar effect can be achieved using a relatively small amount of conductive material compared to when using known positive electrode active materials, allowing for a relatively larger amount of positive electrode active material to be included, thereby enabling an improvement in energy density.

[0091] The positive electrode active material layer may further selectively include a binder, a conductive material, or a combination thereof.

[0092] The binder plays a role in ensuring that the positive electrode active material particles adhere well to each other and that the positive electrode active material adheres well to the current collector. Typical examples of binders include, but are not limited to, polyvinyl alcohol, carboxymethylcellulose, hydroxypropylcellulose, diacetylcellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, epoxy resin, (meth)acrylic resin, polyester resin, and nylon.

[0093] The conductive material is used to impart conductivity to the electrodes, and any electronically conductive material that does not undergo chemical changes in the battery that is constructed can be used. Examples of conductive materials include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjenblack, carbon fibers, carbon nanofibers, and carbon nanotubes; metallic materials containing copper, nickel, aluminum, silver, etc., in the form of metal powder or metal fiber; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0094] The binder content in the positive electrode active material layer may be about 0.1% to 5% by weight relative to 100% by weight of the positive electrode active material layer, and the conductive material content may be 0.1% to 5% by weight relative to 100% by weight of the positive electrode active material layer.

[0095] Lithium-ion battery A lithium secondary battery according to one embodiment includes the positive electrode, negative electrode, and electrolyte described above. The lithium secondary battery may include the positive electrode, negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte.

[0096] Lithium secondary batteries can be classified into cylindrical, prismatic, pouch-type, coin-type, and other types depending on their form. Figures 1 to 4 are schematic diagrams showing a lithium secondary battery according to one embodiment, with Figure 1 being cylindrical, Figure 2 being prismatic, and Figures 3 and 4 being pouch-type batteries. Referring to Figures 1 to 4, the lithium secondary battery 100 may include an electrode assembly 40 with a separator 30 interposed between a positive electrode 10 and a negative electrode 20, and a case 50 in which the electrode assembly 40 is housed. The positive electrode 10, negative electrode 20, and separator 30 are impregnated with an electrolyte (not shown). The lithium secondary battery 100 may include a sealing member 60 that seals the case 50, as shown in Figure 1. Also, in Figure 2, the lithium secondary battery 100 may include a positive electrode lead tap 11 and a positive electrode terminal 12, a negative electrode lead tap 21 and a negative electrode terminal 22. As shown in Figures 3 and 4, the lithium secondary battery 100 may include electrode taps 70, namely a positive electrode tap 71 and a negative electrode tap 72, which serve as electrical pathways for inducing the current formed in the electrode assembly 40 to the outside.

[0097] negative electrode The negative electrode for a lithium secondary battery includes a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector. The negative electrode active material layer comprises a negative electrode active material and may further optionally include a binder, a conductive material, or a combination thereof.

[0098] The negative electrode current collector is not particularly limited as long as it is conductive without causing a chemical change to the lithium secondary battery, and may be a copper foil with a thickness of 10 μm to 15 μm.

[0099] The negative electrode active material includes a substance capable of reversibly inserting / de-inserting lithium ions, lithium metal, an alloy of lithium metal, a substance that can be doped and de-doped with lithium, or a transition metal oxide.

[0100] As the substance capable of reversibly inserting / desorbing the lithium ions, a carbon-based negative electrode active material can be used, which can include, for example, crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon include graphite such as amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon include soft carbon or hard carbon, mesophase pitch carbide, calcined coke, and the like.

[0101] As the alloy of the lithium metal, an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al and Sn is used.

[0102] As the substance capable of doping and undoping with lithium, a Si-based negative electrode active material or a Sn-based negative electrode active material can be used. As the Si-based negative electrode active material, it may be silicon, a silicon-carbon composite, SiOx (0 < x ≦ 2), a Si-Q alloy (where Q is an element selected from alkali metals, alkaline earth metals, group 13 elements, group 14 elements (excluding Si), group 15 elements, group 16 elements, transition metals, rare earth elements, and combinations thereof, for example, Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof), or a combination thereof. As the Sn-based negative electrode active material, it may be Sn, SnO2, a Sn alloy, or a combination thereof.

[0103] The silicon-carbon composite may be a composite of silicon and amorphous carbon. The average particle size (D of the silicon-carbon composite particles 50The particle size may be, for example, 0.5 μm to 20 μm. According to one embodiment, the silicon-carbon composite may be in the form of silicon particles and amorphous carbon coated on the surface of the silicon particles. For example, it may include secondary particles (core) formed by granulating primary silicon particles and an amorphous carbon coating layer (shell) located on the surface of these secondary particles. The amorphous carbon may also be located between the primary silicon particles, for example, the primary silicon particles may be coated with amorphous carbon. The secondary particles may be dispersed in the amorphous carbon matrix.

[0104] The silicon-carbon composite may further contain crystalline carbon. For example, the silicon-carbon composite may include a core containing crystalline carbon and silicon particles and an amorphous carbon coating layer located on the surface of this core. The crystalline carbon may be artificial graphite, natural graphite, or a combination thereof. The amorphous carbon may be soft carbon or hard carbon, mesophase pitch carbide, calcined coke, and the like.

[0105] When the silicon-carbon composite contains silicon and amorphous carbon, the silicon content may be 10% to 50% by weight per 100% by weight of the silicon-carbon composite, and the amorphous carbon content may be 50% to 90% by weight. Furthermore, when the composite contains silicon, amorphous carbon, and crystalline carbon, the silicon content may be 10% to 50% by weight per 100% by weight of the silicon-carbon composite, the crystalline carbon content may be 10% to 70% by weight, and the amorphous carbon content may be 20% to 40% by weight.

[0106] Furthermore, the thickness of the amorphous carbon coating layer may be 5 nm to 100 nm. The average particle size (D) of the silicon particles (primary particles) 50) may be 10 nm to 1 μm, or 10 nm to 200 nm. The silicon particles can exist alone as silicon, in the form of a silicon alloy, or in an oxidized form. The oxidized form of silicon is represented by SiOx (0 < x ≦ 2). At this time, the atomic content ratio of Si:O indicating the degree of oxidation may be 99:1 to 33:67. In this specification, unless otherwise defined, the average particle size (D 50 ) means the diameter of the particles with a cumulative volume of 50% by volume in the particle size distribution.

[0107] The Si-based negative electrode active material or the Sn-based negative electrode active material can be used by mixing with a carbon-based negative electrode active material. When the Si-based negative electrode active material or the Sn-based negative electrode active material and the carbon-based negative electrode active material are mixed and used, the mixing ratio may be 1:99 to 90:10 by weight ratio.

[0108] The negative electrode active material is contained at 90% to 99.8% by weight, or 94% to 99% by weight based on 100% by weight of the negative electrode active material layer.

[0109] The binder serves to make the negative electrode active material particles adhere well to each other and make the negative electrode active material adhere well to the current collector. As the binder, a non-aqueous binder, an aqueous binder, a dry binder, or a combination thereof can be used.

[0110] Examples of the non-aqueous binder include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene-propylene copolymer, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

[0111] The aqueous binder may be selected from styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, (meth)acrylonitrile-butadiene rubber, (meth)acrylic rubber, butyl rubber, fluororubber, polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, poly(meth)acrylonitrile, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, (meth)acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0112] When an aqueous binder is used as the negative electrode binder, it may further contain a cellulosic compound that can impart viscosity. This cellulosic compound can be a mixture of one or more carboxymethylcellulose, hydroxypropylmethylcellulose, methylcellulose, or alkali metal salts thereof. The alkali metal can be Na, K, or Li.

[0113] The dry binder is a polymeric substance that can be formed into fibers, and may be, for example, polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, or a combination thereof.

[0114] The conductive material is used to impart conductivity to the electrodes, and any electronically conductive material that does not undergo chemical changes can be used in the battery that is constructed. Specific examples include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjenblack, carbon fibers, carbon nanofibers, and carbon nanotubes; metallic materials containing copper, nickel, aluminum, silver, etc., in the form of metal powders or metal fibers; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0115] The content of the binder may be 0.1% to 5% by weight relative to 100% by weight of the negative electrode active material layer, and the content of the conductive material may be 0.1% to 5% by weight relative to 100% by weight of the negative electrode active material layer.

[0116] electrolyte The electrolyte for lithium secondary batteries may, for example, be an electrolyte solution, which includes a non-aqueous organic solvent and a lithium salt.

[0117] The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move. The non-aqueous organic solvent may be a carbonate-based, ester-based, ether-based, ketone-based, or alcohol-based solvent, an aprotic solvent, or a combination thereof.

[0118] As carbonate-based solvents, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC) can be used. As ester-based solvents, methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, decanolide, mevalonolactone, valerolactone, and caprolactone can be used. As ether-based solvents, dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, and tetrahydrofuran can be used. Furthermore, ketone solvents such as cyclohexanone can be used. As alcoholic solvents, ethyl alcohol and isopropyl alcohol can be used, and as aprotic solvents, nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon with 2 to 20 carbon atoms, and can include double bonds, aromatic rings, or ether groups); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane and 1,4-dioxolane; and sulfolanes can be used.

[0119] Non-aqueous organic solvents can be used alone or in mixtures of two or more, and the mixing ratio when using a mixture of two or more can be appropriately adjusted according to the desired battery performance, which is widely understood by those working in this field.

[0120] When using carbonate-based solvents, cyclic carbonates and linear carbonates can be mixed, and the cyclic carbonates and linear carbonates can be mixed in a volume ratio of 1:1 to 1:9.

[0121] Non-aqueous organic solvents may further include aromatic hydrocarbon organic solvents. For example, carbonate solvents and aromatic hydrocarbon organic solvents can be mixed and used in a volume ratio of 1:1 to 30:1.

[0122] The electrolyte may further contain vinyl ethyl carbonate, vinylene carbonate, or ethylene carbonate compounds to improve battery life.

[0123] Typical examples of the aforementioned ethylene carbonate compounds include fluoroethylene carbonate, difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, and cyanoethylene carbonate.

[0124] Lithium salts are substances that dissolve in organic solvents and act as a source of lithium ions in batteries, enabling the operation of basic lithium secondary batteries and facilitating the movement of lithium ions between the positive and negative electrodes. Typical examples of lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiCl, LiI, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide; LiFSI), LiC4F9SO3, and LiN(C x F 2x+1 SO2)(C y F 2y+1 It may contain one or more selected from SO2) (where x and y are integers from 1 to 20), lithium trifluoromethanesulfonate, lithium tetrafluoroethanesulfonate, lithium difluorobis(oxalate)phosphate (LiDFBOP), and lithium bis(oxalate) borate (LiBOB).

[0125] The lithium salt concentration is preferably used within the range of 0.1 M to 2.0 M. When the lithium salt concentration falls within this range, the electrolyte has appropriate ionic conductivity and viscosity, resulting in excellent performance and effective lithium ion movement.

[0126] Separator Depending on the type of lithium secondary battery, a separator may be present between the positive and negative electrodes. Such separators can be made of polyethylene, polypropylene, polyvinylidene fluoride, or multilayer films of two or more layers of these materials. Mixed multilayer films such as polyethylene / polypropylene two-layer separators, polyethylene / polypropylene / polyethylene three-layer separators, and polypropylene / polyethylene / polypropylene three-layer separators can also be used.

[0127] The separator may include a porous substrate and a coating layer comprising organic, inorganic, or a combination thereof located on one or both sides of the porous substrate.

[0128] The porous substrate may be a polymer film formed from one polymer selected from polyethylene, polyolefins such as polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacetal, polyamide, polyimide, polycarbonate, polyetherketone, polyaryletherketone, polyetherimide, polyamideimide, polybenzimidazole, polyethersulfone, polyphenylene oxide, cyclic olefin copolymer, polyphenylene sulfide, polyethylene naphthalate, glass fiber, and polytetrafluoroethylene (e.g., TEFLON®), or from a copolymer or mixture of two or more of these polymers.

[0129] The porous substrate can have a thickness of approximately 1 μm to 40 μm, for example, 1 μm to 30 μm, 1 μm to 20 μm, 5 μm to 15 μm, or 10 μm to 15 μm.

[0130] The organic material may include a (meth)acrylic copolymer comprising a first structural unit derived from (meth)acrylamide, and a second structural unit comprising at least one of a structural unit derived from (meth)acrylic acid or (meth)acrylate and a structural unit derived from (meth)acrylamide sulfonic acid or a salt thereof.

[0131] The inorganic material may include, but is not limited to, inorganic particles selected from Al2O3, SiO2, TiO2, SnO2, CeO2, MgO, NiO, CaO, GaO, ZnO, ZrO2, Y2O3, SrTiO3, BaTiO3, Mg(OH)2, boehmite, and combinations thereof. The average particle size (D) of the inorganic particles is 50 The wavelength range may be 1 nm to 2000 nm, for example, 100 nm to 1000 nm or 100 nm to 700 nm.

[0132] The organic and inorganic materials can exist mixed together in a single coating layer, or in a form where a coating layer containing organic materials and a coating layer containing inorganic materials are stacked on top of each other.

[0133] The thickness of the coating layer may be 0.5 μm to 20 μm, for example, 1 μm to 10 μm, or 1 μm to 5 μm.

[0134] Examples and comparative examples of the present invention are described below. However, the following examples are merely illustrative examples of the present invention, and the present invention is not limited to the following examples.

[0135] Example 1 (1) Manufacturing of positive electrode active material Average particle size of secondary particles (D 50 The size of the secondary particles is approximately 16 μm, and the average particle size (D) of the primary particles that make up the secondary particles is approximately 16 μm. 50 ) is a lithium nickel-based composite oxide (LiNi 0.91 Co 0.08 Al0.01 Prepare O2.

[0136] A mixed solution is prepared by adding a mixture of multi-walled carbon nanotubes with an average diameter of approximately 6 nm and an average aspect ratio of approximately 15,000, and ethyl cellulose as a cellulose-based dispersant in a weight ratio of 1:5, to heptane as a non-aqueous solvent that is not mixed with water as described later.

[0137] Water was added to the aforementioned mixed solution to form a Pickering emulsion by vortexing and sonication. At this time, the weight ratio of heptane to added water in the mixed solution was 8:1, and the average size of the Pickering emulsion droplets was approximately 3 μm. Ethyl cellulose and carbon nanotubes were located at the interface of the Pickering emulsion droplets as cellulosic dispersants, water was located inside the droplets, and heptane was located outside the droplets as a non-aqueous solvent.

[0138] Subsequently, the lithium nickel-based composite oxide was added and mixed to produce a mixture. The lithium nickel-based composite oxide was mixed to a total of 100% by weight of carbon nanotubes and ethyl cellulose, with the mixture consisting of 89% by weight of lithium nickel-based composite oxide, 2% by weight of carbon nanotubes, and 9% by weight of ethyl cellulose. The mixing was performed by a vortex method. The lithium nickel-based composite oxide was inserted into the droplet and washed with water located in the internal droplet. The washing process with water removed some of the residual lithium present on the surface of the lithium nickel-based composite oxide.

[0139] Subsequently, the lithium nickel-based composite oxide was separated from the mixture by vacuum filtration, and the cathode active material was produced by heat treatment at 110°C for 2 hours under a vacuum atmosphere. A coating layer was formed on the surface of the lithium nickel-based composite oxide, which was in the form of secondary particles formed by the aggregation of primary particles, with carbon nanotubes randomly arranged and carbon nanotubes connecting the primary particles. Ethyl cellulose was positioned between the lithium nickel-based composite oxide and the carbon nanotubes.

[0140] Here, the carbon nanotube content in the coating layer was 17% by weight based on 100% by weight of the coating layer, and the ethyl cellulose content was 83% by weight. In addition, the residual lithium content on the surface of the positive electrode active material was 0.23% by weight, and the pH was 11.84.

[0141] (2) Manufacturing of the positive electrode A positive electrode active material composition was prepared by mixing the aforementioned positive electrode active material, polyvinylidene fluoride as a binder, and carbon nanotubes as a conductive material in a weight ratio of 90.2:5:4.8 (positive electrode active material:binder:conductive material). This was dispersed in N-methylpyrrolidone solvent to produce a positive electrode active material slurry, which was then coated onto a 17 μm thick Al foil, dried, and rolled to produce a positive electrode.

[0142] (3) Manufacturing of lithium secondary batteries Lithium metal was used as the negative electrode.

[0143] An electrode assembly was manufactured by interposing a glass fiber separator between the positive and negative electrodes, inserting it into a case, and then injecting an electrolyte to produce a coin half-cell. As the electrolyte, a solution of 1M LiPF6 was used in a solvent containing 3% vinylene carbonate (VC) in a volume ratio of EC (ethylene carbonate):EMC (ethyl methyl carbonate):DMC (dimethyl carbonate) of 3:3:4.

[0144] Comparative Example 1 In Example 1, the lithium nickel composite oxide was placed in a pickering emulsion, and the washing and coating steps were omitted. 0.91 Co 0.08 Al 0.01 A positive electrode and lithium secondary battery were manufactured in the same manner as in Example 1, except that O2 was used as the positive electrode active material.

[0145] Comparative Example 2 The positive electrode and lithium secondary battery were manufactured in the same manner as in Example 1, except that a coating layer was not formed on the surface of the lithium nickel-based composite oxide during the manufacturing of the positive electrode active material, and only a water washing process was performed. Specifically, the lithium nickel-based composite oxide (LiNi) used in Example 1 was used. 0.91 Co 0.08 Al 0.01 O2) and water were mixed in a 1:1 weight ratio, washed at a speed of 400 rpm for 1 minute, filtered, and dried at 110°C for 2 hours to be used as the positive electrode active material.

[0146] Comparative Example 3 Except for adding acetonitrile, which is unsuitable for the water washing process, instead of water during the preparation of the mixed solution for the production of the positive electrode active material in Example 1, the positive electrode active material, positive electrode, and lithium secondary battery were manufactured in the same manner as in Example 1.

[0147] Evaluation Example 1. Surface Analysis of Cathode Active Material The positive electrode active materials produced in Example 1 and Comparative Examples 1-3 were photographed using a field emission scanning electron microscope (FE-SEM) and compared. Figure 5 is an FE-SEM image of the particle surface of the positive electrode active material of Example 1. Figure 6 is an FE-SEM image of the particle surface of the positive electrode active material of Comparative Example 1, Figure 7 is an FE-SEM image of the particle surface of the positive electrode active material of Comparative Example 2, and Figure 8 is an FE-SEM image of the particle surface of the positive electrode active material of Comparative Example 3. Referring to Figures 5-8, it can be confirmed that the positive electrode active materials of Example 1 and Comparative Example 3 are coated with carbon nanotubes that are randomly arranged on the surface and connect the primary particles, while in the case of Comparative Examples 1-2, it can be confirmed that the surface of the positive electrode active material is not coated with carbon nanotubes.

[0148] This confirmed that when a cellulosic dispersant such as ethylcellulose is added, it is positioned between the lithium nickel composite oxide and the carbon nanotubes, and the cellulosic dispersant can play a role in attaching the carbon nanotubes to the surface of the lithium nickel composite oxide.

[0149] Evaluation Example 2. Evaluation of Residual Lithium The residual lithium on the surface of the positive electrode active materials produced in Example 1 and Comparative Examples 1-3 was evaluated as follows.

[0150] (1) HRPD assessment High-resolution X-ray powder diffraction (HRPD) analysis using a synchrotron accelerator was performed on the cathode active materials produced in Example 1 and Comparative Examples 1-2. The peaks corresponding to residual lithium present on the surface of the cathode active materials were measured in the range of 15° to 135.5° with a step size of 0.005°, and the results are shown in Figure 9.

[0151] Referring to Figure 9, in the X-ray diffraction pattern of Comparative Example 1, which did not undergo a water washing process, a Li2CO3 peak is observed between 22° and 32°. Such a peak does not appear in Example 1 and Comparative Example 2, which underwent a water washing process, indicating that residual lithium was removed during the manufacturing process of Example 1 and Comparative Example 2.

[0152] (2) XPS evaluation To investigate the surface chemical composition of the cathode active materials prepared in Example 1 and Comparative Example 1, X-ray photoelectron spectroscopy (XPS) was performed using Thermo Fisher Scientific K-Alpha+XPS. Data fitting was performed using Thermo Scientific Advantage software, which was corrected to match the impurity carbon peak with a binding energy of 284.8 eV. The results are shown in Figures 10-11.

[0153] Figure 10 is the XPS graph of the positive electrode active material according to Example 1, and Figure 11 is the XPS graph of the positive electrode active material according to Comparative Example 1. Referring to Figures 10 and 11, the O1s XPS spectra of Example 1 and Comparative Example 1 are shown, and peaks are observed at 531.6 eV (Li2CO3), 530.2 eV (LiOH), and 529 eV (lattice oxygen), respectively. The XPS analysis results confirmed that the intensity of the 531.6 eV peak in Example 1 was significantly reduced compared to Comparative Example 1, which demonstrates that residual lithium was effectively removed in Example 1.

[0154] (3) pH evaluation In Example 1, the pH of the water used before and after the coating process was measured using a pH meter. First, the pH of the water was measured using a pH meter before adding water to the heptane solvent in which carbon nanotubes were dispersed. Then, after the carbon nanotube coating process using Pickering emulsion was completed, the pH of the filtered water was measured again using a pH meter to evaluate the pH change of the water during the coating process, and the results are shown in Figure 12.

[0155] Referring to Figure 12, the pH of the water before being introduced into the Pickering emulsion coating process was 6.91, but the pH of the filtered water after the coating process was completed increased to 12.53. This indicates that the surface-residual lithium, which mainly existed in the form of LiOH, was dissolved in the water and removed during the coating process, confirming that the surface-residual lithium was effectively removed throughout the coating process.

[0156] Furthermore, 2 g of the positive electrode active material produced in Example 1, Comparative Example 1, and Comparative Example 3 was added to 10 mL of water and stirred at a speed of 400 rpm for 10 minutes, after which the pH was evaluated using a pH meter. The results are shown in Figure 13.

[0157] Referring to Figure 13, regarding the pH measurement results of the positive electrode active material, the pH of the positive electrode active material after 10 minutes rose to approximately 12.55 and 13.15 for Comparative Example 1, which did not perform a water washing step, and 13.15 for Comparative Example 3, which used an acetonitrile solvent unsuitable for the water washing step, respectively, while the pH of Example 1 was measured to be low at approximately 11.84. This suggests that residual lithium was effectively removed during the coating process in Example 1, where an aqueous solvent was used, resulting in a relatively low pH for the positive electrode active material produced in Example 1.

[0158] (4) Titration evaluation The residual lithium content on the surface of the positive electrode active materials prepared in Example 1, Comparative Example 1, and Comparative Example 2 was measured using a titration method. Each positive electrode active material was dissolved in water and filtered. The filtered solution was then titrated using a titrator to obtain a pH titration curve by adding 0.05 M HCl until the pH value reached 3. The residual lithium content was calculated by substituting the equivalence points in the obtained titration curves into the relevant calculation formulas, and the results are shown in Figures 14 and 15.

[0159] In Figure 14, Comparative Example 1 required approximately 25 mL of HCl to reach pH 3, while Example 1 and Comparative Example 2 each reached the same pH with only 5 mL. This indicates that in Comparative Example 1, which did not undergo a water washing process, more lithium dissolved in the positive electrode active material, raising the pH, and requiring additional HCl to lower it, suggesting a very high concentration of residual lithium on the surface. On the other hand, the positive electrode active materials of Example 1 and Comparative Example 2 showed much lower residual lithium concentrations, and significantly less HCl was required to reach the same pH. Figure 15 quantitatively illustrates these results.

[0160] Referring to Figure 15, Comparative Example 1 contained 1.52 wt% residual lithium, while Example 1 and Comparative Example 2 showed residual lithium levels of 0.27 wt% and 0.23 wt%, respectively. Since the positive electrode active material of Example 1, in which the water washing and coating processes were performed simultaneously, showed a similar residual lithium content to the positive electrode active material of Comparative Example 2, in which only the water washing process was performed without the coating process, it was confirmed that the water washing process was carried out by the water present inside the droplets of the Pickering emulsion in the positive electrode active material produced by Example 1 effectively removed residual lithium.

[0161] Evaluation Example 3 The charge-discharge cycle (formation cycle) of the activation process step performed in Example 1, Comparative Example 1, and Comparative Example 2 was in the voltage range of 3 to 4.3V (Li / Li + The test was performed using C / 10 current density (1C = 180mAg) as the standard. -1 The results are shown in Figure 16.

[0162] In the first charge-discharge cycle, Comparative Examples 1 and 2 showed discharge capacities of 214 mAh / g and 215 mAh / g, respectively, while Example 1 showed a discharge capacity of 213 mAh / g. These results suggest that the positive electrode active material exhibits similar initial capacities. An overshoot phenomenon was observed at 3.7V during charging, which is a phenomenon that typically occurs after cleaning or coating with high-nickel positive electrode active materials.

[0163] Evaluation Example 4 The lifetime characteristics of Example 1, Comparative Example 1, and Comparative Example 2 were evaluated in the voltage range of 3 to 4.3V (Li / Li + The battery was charged and discharged at a rate of 1C (reference), and the results are shown in Figure 17.

[0164] After 200 cycles in a lithium-metal half-cell at a rate of 1C, the capacity retention rates for Example 1, Comparative Example 1, and Comparative Example 2 were 82.8%, 65.7%, and 48.5%, respectively. The decrease in capacity retention rate observed in Comparative Example 2 was attributed to the increased fragility of the cathode active material surface after the water washing process, which promoted side reactions at the cathode-electrolyte interface, resulting in the formation of the NiO phase. This indicates that the unstable chemical properties of the M surface cause degradation during electrochemical cycling, leading to a decrease in capacity. However, Example 1, which underwent the water washing process, showed a superior capacity retention rate compared to Comparative Example 2, which underwent only a simple water washing process without a coating process, and a higher capacity retention rate than Comparative Example 1, which did not undergo the water washing process. This suggests that the carbon nanotube coating layer formed on the cathode active material in Example 1 contributed to improving the electrochemical performance. The specific evaluation of the electrochemical performance improvement effect of carbon nanotubes is confirmed through evaluation examples 5 and 6, which demonstrate that Example 1 can maintain a high capacity retention rate and stable performance.

[0165] Evaluation Example 5 Electrochemical impedance spectroscopy (EIS) measurements were performed at 25°C in the frequency range from 10 mHz to 100 kHz for Example 1, Comparative Example 1, and Comparative Example 2. The resistance changes after the activation process and after the 50th cycle were compared and analyzed to evaluate the effect of carbon nanotube coating on the cathode active material.

[0166] Figure 18 shows the electrochemical impedance spectroscopy (EIS) results for Example 1, Comparative Example 1, and Comparative Example 2 after the activation cycle, and Figure 19 shows the electrochemical impedance spectroscopy (EIS) results for Example 1, Comparative Example 1, and Comparative Example 2 after 50 cycles. In Figures 18 and 19, "Z'" represents the real part of the impedance, and "-Z''" represents the negative imaginary part of the impedance. Using this, the resistance values ​​were derived through a fitting program and are shown in Table 1. Here, Rf is the film resistance of the electrode surface, representing the resistance generated in the coating layer at the electrolyte-electrode interface, and Rct is the charge transfer resistance, meaning the electron transfer resistance during the electrode reaction process.

[0167] [Table 1]

[0168] Referring to Figures 18 and 19 and Table 1, Example 1 exhibited lower charge transfer resistance (Rct) during long-term cycles compared to Comparative Example 2. After 50 cycles, the charge transfer resistance of Example 1 increased gradually from 40.56 Ω to 218.78 Ω, while that of Comparative Example 2 increased significantly from 37.41 Ω to 253.60 Ω. These results demonstrate that carbon nanotube coatings with excellent electrical conductivity effectively reduce charge transfer resistance (Rct), smooth current flow, and improve conductivity within the electrode. Furthermore, they demonstrate that they mitigate the degradation of the positive electrode active material during long-term cycles and contribute to improved electrochemical performance.

[0169] Evaluation Example 6 X-ray nanotomography (TXM) images from Example 1 and Comparative Example 2 were analyzed via a synchrotron radiation accelerator. To obtain each tomography image, 900 projection images were collected with an exposure time of 0.4 seconds. The collected images were reconstructed using a filtered backprojection algorithm in Octopus software (TESCAN), and the results are shown in Figures 20-23.

[0170] Figures 20 and 21 are cross-sectional images of Example 1, and Figures 22 and 23 are cross-sectional images of Comparative Example 2. Figures 20 and 22 are cross-sectional images before the charge-discharge experiment, and Figures 21 and 23 are cross-sectional images after 200 cycles.

[0171] Referring to Figures 21 and 23, it was confirmed that after 200 cycles, less crack formation occurred in the cross-section of Example 1 compared to Comparative Example 2. This demonstrates that the carbon nanotube coating layer can improve the electrochemical performance of the electrode by suppressing crack formation and preventing the newly formed surface from undergoing side reactions with the electrolyte.

[0172] Although preferred embodiments of the present invention have been described above, the present invention is not limited thereto, and can be implemented in various ways within the scope of the claims, the detailed description of the invention, and the attached drawings, and these also naturally fall within the scope of the present invention. [Explanation of Symbols]

[0173] 100: Lithium-ion rechargeable battery 10: Positive electrode 11: Positive lead tap 12: Positive terminal 20: Negative electrode 21: Negative lead tap 22: Negative terminal 30: Separator 40: Electrode Assembly 50: Case 60: Sealing member 70: Electrode Tap 71: Positive Tap 72: Negative Tap

Claims

1. It is a positive electrode active material, Lithium nickel-based composite oxides in the form of secondary particles formed by the aggregation of multiple primary particles; and A coating layer containing a cellulose derivative and carbon nanotubes located on the surface of the secondary particles; Includes, The carbon nanotubes within the coating layer are randomly arranged on the surface of the secondary particles and connect the primary particles. The residual lithium content on the surface of the positive electrode active material is 1.5% by weight or less. The pH of the surface of the positive electrode active material is 10 to 12. Cathode active material.

2. The lithium nickel-based composite oxide is represented by chemical formula 1, The positive electrode active material according to claim 1. [Chemical formula 1] Li a1 Ni x1 M 1 y1 M 2 z1 O 2-b1 X b1 (In the chemical formula (1), 0.9 ≦ a1 ≦ 1.2, 0.8 ≦ x1 < 1, 0 < y1 ≦ 0.2, 0 ≦ z1 ≦ 0.2, 0.9 ≦ x1 + y1 + z1 ≦ 1.1, and 0 ≦ b1 ≦ 0.1, M 1 and M 2 are each independently one or more elements selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu, Fe, Mg, Mn, Mo, Nb, Si, Sn, Sr, Ti, V, W, Y, Zn, and Zr, and X is one or more elements selected from F, P, and S.)

3. The nickel content relative to 100 mol% of the metal obtained by removing lithium from the aforementioned lithium-nickel composite oxide is 80 mol% or more. The positive electrode active material according to claim 1.

4. The average particle size (D) of the secondary particles 50 ) are 2 μm to 20 μm, The average particle size (D) of the primary particles 50 ) is 6 μm or less. The positive electrode active material according to claim 1.

5. The cellulose derivative is located between the lithium nickel-based composite oxide and the carbon nanotube, The positive electrode active material according to claim 1.

6. The cellulose derivative includes ethylcellulose, cellulose acetate, carboxymethylcellulose, hydroxypropylcellulose, methylcellulose, nitrocellulose, or a combination thereof. The positive electrode active material according to claim 1.

7. The cellulose derivative is included in an amount of 0.005% to 50% by weight relative to 100% by weight of the positive electrode active material. It is contained in an amount of 0.5% to 90% by weight relative to 100% by weight of the coating layer. The positive electrode active material according to claim 1.

8. The coating layer further contains amorphous carbon, The amorphous carbon includes carbides of cellulose derivatives, The cellulose derivative carbides include ethylcellulose carbides, cellulose acetate carbides, carboxymethylcellulose carbides, hydroxypropylcellulose carbides, methylcellulose carbides, nitrocellulose carbides, or combinations thereof. The positive electrode active material according to claim 1.

9. The amorphous carbon is located between the lithium nickel-based composite oxide and the carbon nanotube. The positive electrode active material according to claim 8.

10. The carbon nanotubes include single-walled carbon nanotubes, multi-walled carbon nanotubes, or combinations thereof. The positive electrode active material according to claim 1.

11. The average diameter of the carbon nanotubes is 1 nm to 50 nm. The average aspect ratio of the carbon nanotubes is 100 to 50,000. The positive electrode active material according to claim 1.

12. The carbon nanotubes are present in an amount of 0.01% to 10% by weight relative to 100% by weight of the positive electrode active material. The coating layer is contained in an amount of 10% to 99.5% by weight relative to 100% by weight. The positive electrode active material according to claim 1.

13. A step of preparing a mixed solution by mixing carbon nanotubes, a cellulose-based dispersant, and a non-aqueous solvent; The step of adding water to the mixed solution to form a Pickering emulsion; A step of adding and mixing lithium nickel-based composite oxide in the form of secondary particles, which are aggregated primary particles, into the Pickering emulsion to produce a mixture; and The process includes separating the lithium nickel-based composite oxide from the mixture and then heat-treating it. A method for manufacturing a positive electrode active material.

14. The cellulose-based dispersant includes ethylcellulose, cellulose acetate, carboxymethylcellulose, hydroxypropylcellulose, methylcellulose, nitrocellulose, or a combination thereof. A method for producing a positive electrode active material according to claim 13.

15. The non-aqueous solvent includes hexane, heptane, toluene, cyclohexane, isooctane, pentane, or a combination thereof. A method for producing a positive electrode active material according to claim 13.

16. The Pickering emulsion contains droplets, The carbon nanotubes and the cellulose-based dispersant are located at the interface of the droplet. Inside the aforementioned droplet, the water is located, Outside the aforementioned droplet, the non-aqueous solvent is located. A method for producing a positive electrode active material according to claim 13.

17. In the step of adding and mixing the lithium nickel-based composite oxide to produce a mixture, the lithium nickel-based composite oxide is inserted into the Pickering emulsion droplets, and the lithium nickel-based composite oxide inserted into the droplets is washed by the water located inside the droplets. A method for producing a positive electrode active material according to claim 13.

18. The heat treatment is carried out at a temperature of 60°C to 800°C under a vacuum or a specific gas atmosphere. The specified gas includes oxygen, argon, air, or nitrogen. A method for producing a positive electrode active material according to claim 13.

19. A positive electrode current collector, and a positive electrode active material layer located on the positive electrode current collector, comprising the positive electrode active material described in any one of claims 1 to 12. Positive electrode.

20. The positive electrode, negative electrode, and electrolyte described in claim 19, Lithium-ion rechargeable battery.

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