Positive electrode slurry composition, positive electrode, method for manufacturing the same, and lithium secondary battery

Abstract

The present invention provides a positive electrode slurry composition, a positive electrode, a method for manufacturing the same, and a lithium secondary battery, wherein by adding a cellulose-based additive to the positive electrode slurry composition, the dispersibility of carbon nanotubes is improved, and even after the drying process during positive electrode manufacturing, carbon nanotubes remain uniformly dispersed within the positive electrode while suppressing the development that causes carbon nanotubes to accumulate in a part of the positive electrode and form a carbon nanotube film, and carbon nanotubes are selectively and uniformly coated onto the surface of the positive electrode active material during the positive electrode manufacturing process without having to perform the conventional process of coating the positive electrode active material with carbon nanotubes separately. [Solution] The present invention relates to a positive electrode slurry composition, a positive electrode, a method for producing the same, and a lithium secondary battery, wherein the positive electrode slurry composition contains a lithium nickel-based composite oxide and comprises a positive electrode active material in the form of secondary particles formed by the aggregation of a plurality of primary particles, carbon nanotubes, and a cellulose-based additive.

Description

[Technical Field]

[0001] This invention relates to a cathode slurry composition, a cathode, a method for manufacturing the same, and a lithium secondary battery. This patent is based on research conducted with funding from the government (Ministry of Trade, Industry and Energy) in 2024 and supported by the Korea Industrial Technology Development Agency (RS-2024-00419413, 2024 Industrial Innovation Human Resources Growth Support Project). [Background technology]

[0002] 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, there has been active research into using high-energy-density lithium-ion batteries as power sources or energy storage sources for hybrid and electric vehicles.

[0003] Various positive electrode active materials are being investigated to realize lithium secondary batteries suitable for such applications. Among them, lithium nickel oxides, lithium nickel manganese cobalt composite oxides, lithium nickel cobalt aluminum composite oxides, and lithium cobalt oxides are mainly used as positive electrode active materials. High-nickel positive electrode 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, but they have 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 positive electrode active materials that achieve both high energy density and long life characteristics.

[0004] There have been attempts to improve the electrical conductivity of such positive electrode active materials by uniformly coating the surface with carbon-based materials such as graphene and carbon nanotubes. However, uniformly coating the surface of the positive electrode active material with carbon-based materials requires a multi-step process, and various costs are incurred at each step, resulting in a problem of increased manufacturing costs for the positive electrode active material.

[0005] On one hand, in order to improve the conductivity of the positive electrode and reduce the content of the conductive material, attempts have been made to add carbon nanotubes as the conductive material into the positive electrode. However, it is difficult to uniformly disperse the carbon nanotubes. Even if they are uniformly dispersed, the carbon nanotubes will gather in the upper layer of the positive electrode during the drying process of the solvent during the formation of the positive electrode at a later date. Especially when applied to high-loading electrodes with a large loading amount, the above problems become prominent. In addition, since the carbon nanotubes are not selectively located on the surface of the positive electrode active material, there is a problem that the effect of improving conductivity and the effect of reducing the content of the conductive material are negligible.

Summary of the Invention

Problems to be Solved by the Invention

[0006] By adding a cellulose-based additive to the positive electrode slurry composition, the dispersibility of the carbon nanotubes is improved, and the carbon nanotubes are uniformly dispersed in the positive electrode while suppressing the phenomenon that the carbon nanotubes gather in a part of the positive electrode to form a carbon nanotube film even after passing through the drying process during the production of the positive electrode. Without separately performing the step of coating the conventional carbon nanotubes on the positive electrode active material, the positive electrode slurry composition, the positive electrode, its manufacturing method, and the lithium secondary battery in which the carbon nanotubes are selectively and uniformly coated on the surface of the positive electrode active material in the positive electrode manufacturing process are provided.

Means for Solving the Problems

[0007] One embodiment provides a positive electrode slurry composition containing a lithium nickel-based composite oxide, a positive electrode active material in the form of secondary particles in which a plurality of primary particles are aggregated, carbon nanotubes, and a cellulose-based additive.

[0008] Another embodiment provides a positive electrode including a positive electrode current collector; and a positive electrode active material layer located on the positive electrode current collector, containing a lithium nickel-based composite oxide, a positive electrode active material in the form of secondary particles in which a plurality of primary particles are aggregated, and carbon nanotubes.

[0009] Still other embodiments provide a method for manufacturing a positive electrode composition, comprising steps of manufacturing a positive electrode slurry composition containing a lithium nickel-based composite oxide, having a secondary particle form in which a plurality of primary particles are aggregated, a carbon nanotube, and a cellulose-based additive; applying and drying the positive electrode slurry composition onto a positive electrode current collector; and heat-treating.

[0010] Still other embodiments provide a lithium secondary battery including the aforementioned positive electrode, negative electrode, and electrolyte.

Advantages of the Invention

[0011] In a positive electrode slurry composition according to an embodiment, by adding a cellulose-based additive, the dispersibility of carbon nanotubes is improved, and the phenomenon that carbon nanotubes gather on a part of the positive electrode to form a carbon nanotube film even after passing through a drying process during positive electrode manufacturing can be suppressed. Without separately performing the step of coating carbon nanotubes on a positive electrode active material in the conventional art, carbon nanotubes can be selectively and uniformly coated on the surface of the positive electrode active material in the positive electrode manufacturing process.

Brief Description of the Drawings

[0012] [Figure 1] The drawing schematically shows a lithium secondary battery according to an embodiment. [Figure 2] The drawing schematically shows a lithium secondary battery according to an embodiment. [Figure 3] The drawing schematically shows a lithium secondary battery according to an embodiment. [Figure 4] The drawing schematically shows a lithium secondary battery according to an embodiment. [Figure 5] The photograph is taken by a scanning electron microscope (SEM) of a sample manufactured to confirm the dispersibility of carbon nanotubes depending on the presence or absence of a cellulose-based additive. [Figure 6] The photograph is taken by a scanning electron microscope (SEM) of the surfaces of the positive electrodes of Example 1 and Comparative Example 1. [Figure 7]These are photographs of the cross-section of the positive electrode in Example 2 and Reference Example 2, taken with a scanning electron microscope (SEM). [Figure 8] These photographs show the results of evaluating the adhesion strength of carbon nanotubes using a solution obtained by extracting the positive electrode active material from the positive electrodes manufactured in Example 1 and Reference Example 3 and adding it to N-methyl-2-pyrrolidone solvent, both before and after ultrasonic treatment with an ultrasonic device for 1 minute, 3 minutes, and 5 minutes. [Figure 9] This graph shows the life evaluation results for lithium secondary batteries manufactured in Example 1, Reference Example 3, and Comparative Examples 1-2. [Modes for carrying out the invention]

[0013] The following describes specific embodiments in detail so that they can be easily implemented by a person 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.

[0014] 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.

[0015] Here, "these combinations" refers to mixtures of components, laminates, composites, copolymers, alloys, blends, reaction products, etc.

[0016] Here, terms such as “include,” “equip,” or “possess” should be understood as intending to specify the presence of an implemented feature, figure, stage, component, or combination thereof, and not preemptively excluding the possibility of the presence or addition of one or more other features, figures, stages, components, or combinations thereof.

[0017] 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.

[0018] 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.

[0019] 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 whose cumulative volume in the particle size distribution is 50% by volume. 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 from 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 is the diameter of the particle(D) 50 ) may be used as the average particle size.

[0020] Here, "or" is not interpreted as having an exclusive meaning; for example, "A or B" is interpreted as including A, B, A+B, etc.

[0021] The term "metal" is interpreted as a concept that includes general metals, transition metals, and semimetals.

[0022] Cathode slurry composition One embodiment provides a cathode slurry composition containing a lithium nickel-based composite oxide, a cathode active material in the form of aggregated primary particles, carbon nanotubes, and a cellulose-based additive.

[0023] Lithium nickel-based composite oxides generally have a secondary particle morphology in which multiple primary particles aggregate. As the charge-discharge cycle progresses, the repeated volume expansion of the crystal structure causes the gaps between primary particles to widen and the arrangement of primary particles to shift. This results in a large difference in the degree of contraction and expansion between the secondary particles and the positive electrode containing them. This leads to structural cracking and fracture of the secondary particles and the positive electrode, causing a disruption in 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] To address these problems, there have been attempts to uniformly coat the surface of the cathode active material with carbon-based materials such as graphene and carbon nanotubes. However, uniformly coating the surface of the cathode active material with carbon-based materials requires a multi-step process, and various costs are incurred at each step, resulting in an increase in the overall cost of manufacturing the cathode active material.

[0025] On the other hand, there have been attempts to add carbon nanotubes as a conductive material to the positive electrode to improve its conductivity and reduce the conductive material content. However, uniform dispersion of carbon nanotubes is difficult, and even if uniform dispersion is achieved, the carbon nanotubes tend to accumulate in the upper layer of the positive electrode during the drying process of the solvent used in positive electrode formation. This problem becomes particularly apparent when applied to high-loading electrodes with large loading amounts. Furthermore, because carbon nanotubes do not selectively occupy the surface of the positive electrode active material, there is a problem in that the effect of improving conductivity and reducing the conductive material content is minimal.

[0026] In a positive electrode slurry composition according to an embodiment, by adding a cellulose-based additive, the dispersibility of carbon nanotubes is improved. Subsequently, even after passing through the drying process during the production of the positive electrode, the phenomenon that the carbon nanotubes gather in a part of the positive electrode to form a carbon nanotube film is suppressed, and the carbon nanotubes are uniformly dispersed in the positive electrode. Without separately performing the step of coating the conventional carbon nanotubes on the positive electrode active material, the carbon nanotubes are selectively and uniformly coated on the surface of the positive electrode active material in the positive electrode manufacturing process. Thereby, it is possible to prevent particle cracking due to the shrinkage and expansion of the positive electrode active material caused by repeated charge and discharge, and to improve the battery life. In particular, when using carbon nanotubes compared to using a known conductive material, electron transfer in the positive electrode is smoothly performed, excellent electrical conductivity can be exhibited, and a stable conductive network can be formed through the carbon nanotubes connecting between the primary particles on the surface of the positive electrode active material.

[0027] Positive electrode active material The positive electrode active material contains a lithium nickel-based composite oxide. The positive electrode active material according to an embodiment contains a lithium nickel-based composite oxide and can achieve a high energy density.

[0028] The lithium nickel-based composite oxide is represented by, for example, the following Chemical Formula 1. [Chemical Formula 1] Li a1 Ni x1 M 1 y1 M 2 z1 O 2-b1 X b1

[0029] 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 2Each of the elements is independently 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. 1 and M 2 These elements may be different from each other.

[0030] 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。

[0031] The aforementioned lithium nickel-based composite oxide can be represented by chemical formula 2 or chemical formula 3 below as a specific example.

[0032] [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.

[0033] 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.

[0034] [Chemical formula 3] Li a3 Ni x3 Co y3 M 4 z3 M 5 w3 O 2-b3 Xb3 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 4 is 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.

[0035] In the above 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.

[0036] In the aforementioned lithium-nickel composite oxide, the nickel content relative to 100 mol% of the total metal excluding lithium 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.

[0037] 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.

[0038] The average particle size (D) of the aforementioned secondary particles 50 The particle size (D) may be 2 μm to 20 μm, for example, 3 μm to 18 μm, or 4 μm to 15 μm. Here, the average particle size (D) 50The average particle size may be obtained by randomly measuring the size (diameter or length of the major axis) of more than 20 particles from scanning electron microscope images of lithium nickel-based composite oxide particles in secondary particle form, and taking the diameter of the particle with a cumulative volume of 50 volume% from the particle size distribution as the average particle size. 50 When the above range is satisfied, high capacity and energy density can be achieved, which is advantageous for the selective positioning of carbon nanotubes on the secondary particle surface and improves the electrochemical performance of the cathode.

[0039] The average particle size (D) of the primary particles forming the secondary particles of the lithium nickel composite oxide 50 The 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 from scanning electron microscope or transmission electron microscope images of the surface of the secondary particles to obtain a particle size distribution, and then taking the size of the particle with a cumulative volume of 50 volume% from 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.

[0040] carbon nanotubes A positive electrode slurry composition according to one embodiment contains 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 composite oxide, it hinders lithium ion movement between the positive electrode active material surface and the electrolyte, reducing high capacity, high power output, and long life characteristics. However, carbon nanotubes are a 1D material and, by interconnecting primary particles on the surface of the lithium nickel composite oxide, do not cover the entire surface of the lithium nickel composite oxide compared to graphene. Therefore, they do not hinder lithium ion movement between electrolytes, maintain the shape of secondary particles, and suppress the contraction and expansion of secondary particles and the positive electrode, thereby improving battery life characteristics.

[0041] 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 are composed 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.

[0042] 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. Multi-walled carbon nanotubes are relatively less expensive than single-walled carbon nanotubes, thus ensuring price competitiveness.

[0043] 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 down and oxidize the carbon ring structure on the carbon nanotube surface, more specifically on the surface of untreated carbon nanotubes. 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. Untreated carbon nanotubes, treated carbon nanotubes, etc., can be used without limitation as the carbon nanotubes.

[0044] The average diameter of the carbon nanotubes may be between 0.8 nm and 100 nm. For example, it may be between 1 nm and 90 nm, 2 nm and 80 nm, or 3 nm and 70 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 scanning electron microscope (SEM). When the average diameter of the carbon nanotubes satisfies the above range, an appropriate amount of electrons can move and a stable conductive network can be formed.

[0045] The average aspect ratio of the carbon nanotubes may be between 100 and 500,000. For example, it may be between 200 and 300,000, 1,000 and 100,000, or 2,000 and 50,000. 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 way 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, a sufficient conductive network can be formed with only a small amount of carbon nanotubes when fabricating a cathode slurry according to one embodiment.

[0046] Cellulose-based additives A positive electrode slurry composition according to one embodiment includes a cellulose-based additive. The inclusion of the cellulose-based additive improves the dispersibility of carbon nanotubes, allowing for selective and uniform coating of carbon nanotubes on the surface of the positive electrode active material. This eliminates the need to apply the conventional manufacturing process for carbon nanotubes-coated positive electrode active materials, and the carbon nanotubes are selectively coated onto the surface of the positive electrode active material through the process of manufacturing the positive electrode. This prevents particle cracking caused by shrinkage and expansion of the positive electrode active material due to repeated charging and discharging, thereby improving battery life.

[0047] The type of cellulose-based additive is not limited as long as it contains an alkyl group and a functional group capable of hydrogen bonding. For example, the cellulose-based additive may include ethylcellulose, cellulose acetate, carboxymethylcellulose, hydroxypropylcellulose, methylcellulose, nitrocellulose, or a combination thereof.

[0048] The lithium nickel-based composite oxide is included in the cathode slurry composition in amounts of 60% to 99.9% by weight, 70% to 99.8% by weight, 80% to 99% by weight, 90% to 99% by weight, or 95% to 99% by weight, based on 100% by weight of solid content.

[0049] The carbon nanotubes are present in an amount of 0.1% to 5% by weight relative to 100% by weight of the solid content of the cathode slurry composition, for example, 0.1% to 4% by weight, 0.1% to 3% by weight, 0.1% to 2% by weight, or 0.1% to 1% by weight.

[0050] The cellulose-based additive is present in an amount of 0.01% to 5% by weight relative to 100% by weight of the solid content of the cathode slurry composition, for example, 0.05% to 4% by weight, 0.1% to 3% by weight, 0.1% to 2% by weight, or 0.1% to 1% by weight. When the content of the cellulose-based additive satisfies the above range, it can be uniformly dispersed in the solvent and can interact appropriately with carbon nanotubes.

[0051] The weight ratio of the carbon nanotubes to the cellulose-based additive may be 10:1 to 1:5, 9:1 to 1:4, 8:1 to 1:3, or 7:1 to 1:2. When the weight ratio of the carbon nanotubes to the cellulose-based additive in the positive electrode slurry composition satisfies the above range, the interaction with the carbon nanotubes is maximized, effectively dispersing the carbon nanotubes and reducing resistance inside the electrode, contributing to efficient electrode fabrication.

[0052] others The positive electrode slurry composition may further selectively include a binder, a conductive material, or a combination thereof, in addition to the positive electrode active material, the carbon nanotube, and the cellulose-based additive.

[0053] 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.

[0054] 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. 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.

[0055] In the positive electrode slurry composition, the binder content may be about 0.1% to 5% by weight per 100% by weight of the positive electrode slurry composition, and the conductive material content may be 0.1% to 5% by weight per 100% by weight of the positive electrode slurry composition.

[0056] On the other hand, the positive electrode slurry composition may contain a solvent, which may, for example, be a non-aqueous organic solvent. The solvent may be, for example, cyclic aliphatic hydrocarbons such as cyclopentane and cyclohexane; aromatic hydrocarbons such as toluene, xylene, and ethylbenzene; ketones such as acetone, ethyl methyl ketone, diisopropyl ketone, cyclohexanone, methylcyclohexane, and ethylcyclohexane; chlorinated aliphatic hydrocarbons such as methylene chloride, chloroform, and carbon tetrachloride; esters such as ethyl acetate, butyl acetate, γ-butyrolactone, and ε-caprolactone; acetonitriles such as acetonitrile and propionitrile; ethers such as tetrahydrofuran and ethylene glycol diethyl ether; alcohols such as methanol, ethanol, isopropanol, ethylene glycol, and ethylene glycol monomethyl ether; and amides such as N-methylpyrrolidone and N,N-dimethylformimide. The solvent may be a single solvent or a mixture of two or more solvents.

[0057] The positive electrode slurry composition contains a lithium nickel-based composite oxide and may further contain a lithium nickel-based composite oxide and.

[0058] The single-particle positive electrode active material may contain a lithium nickel-based composite oxide, similar to the secondary-particle positive electrode active material described above. In this case, the lithium nickel-based composite oxide in the single-particle positive electrode active material is the same as that in the secondary-particle positive electrode active material. For example, the lithium nickel-based composite oxide in the single-particle positive electrode active material can be represented by chemical formulas 1 to 3, and the nickel content may be 80 mol% or more per 100 mol% of the metal excluding lithium. The lithium nickel-based composite oxide in the single-particle positive electrode active material may have the same composition as the lithium nickel-based composite oxide in the secondary-particle positive electrode active material, and may be primary particles that have escaped from the secondary-particle positive electrode active material.

[0059] The average particle size (D) of the single particle 50 The particle size (D) may be 1 μm to 12 μm, for example, 2 μm to 11 μm or 3 μm to 8 μm. 50 The particle size distribution may be obtained by randomly measuring the size (diameter or length of the major axis) of more than 20 particles from scanning electron microscope images of single-particle lithium nickel-based composite oxide particles, and then taking the diameter of the particle with a cumulative volume of 50% from the particle size distribution as the average particle size. 50 When the above range is met, high initial charge / discharge capacity and efficiency can be achieved, and excellent output characteristics and life characteristics can be realized.

[0060] When the secondary particle form positive electrode active material and the single particle form positive electrode active material are mixed (hereinafter the same as "mixed positive electrode active material"), the mixing ratio of the secondary particle form positive electrode active material and the single particle form positive electrode active material may be 1:9 to 9:1, for example, 2:8 to 9:1, 3:7 to 9:1, 4:6 to 9:1, 5:5 to 8:2, or 6:4 to 7:3.

[0061] When a positive electrode active material in secondary particle form and a positive electrode active material in single particle form are mixed, the average particle size (D) of the mixed positive electrode active material is 50The particle size (D) may be 1 μm to 20 μm, for example, 3 μm to 18 μm, or 4 μm to 15 μm. Here, the average particle size (D) 50 The average particle size (D) of the mixed positive electrode active material may be obtained by randomly measuring the size (diameter or length of the major axis) of more than 20 particles from scanning electron microscope images of the mixed positive electrode active material to obtain a particle size distribution, and then taking the diameter of the particle with a cumulative volume of 50 volume% from the particle size distribution as the average particle size. 50 When the above range is satisfied, high capacity and energy density can be achieved, which is advantageous for the selective positioning of carbon nanotubes on the secondary particle surface, and the electrochemical performance of the cathode is improved.

[0062] positive electrode Other embodiments provide a positive electrode comprising a positive electrode current collector; and a positive electrode active material layer located on the positive electrode current collector, containing a lithium nickel-based composite oxide and having a secondary particle form in which a plurality of primary particles are aggregated; and carbon nanotubes. The positive electrode active material layer may further include a cellulose derivative, amorphous carbon, or a combination thereof. The positive electrode according to one embodiment can achieve high capacity while suppressing structural degradation and crack formation due to charging and discharging, thereby achieving long-life characteristics.

[0063] 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 15 μm.

[0064] Cathode active material layer The positive electrode active material layer contains a lithium nickel-based composite oxide, a positive electrode active material in the form of secondary particles in which a plurality of primary particles are aggregated, and carbon nanotubes. The positive electrode active material layer may further contain a cellulose derivative, amorphous carbon, or a combination thereof.

[0065] The specific details of the positive electrode active material and carbon nanotubes are as described above.

[0066] 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, 90% to 99% by weight, or 95% to 99% 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 using known positive electrode active materials, and a relatively large amount of positive electrode active material can be included, thereby improving the energy density.

[0067] The carbon nanotubes can be uniformly dispersed within the positive electrode active material layer and simultaneously located on the surface of the positive electrode active material.

[0068] When the carbon nanotubes are evenly dispersed in the positive electrode active material and located on the surface of the positive electrode active material, the carbon nanotubes are randomly arranged on the surface of the secondary particles of the lithium nickel-based composite oxide, connecting the primary particles. For example, the carbon nanotubes may exist on the surface of the secondary particles in a three-dimensional network (net) or spiderweb-like configuration.

[0069] The carbon nanotubes are highly crystalline carbon-based materials in which carbon atoms are arranged in a hexagonal pattern and have a tubular structure, exhibiting 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 are composed 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.

[0070] The carbon nanotubes 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, carbon nanotubes adjacent to the surface of the positive electrode active material are provided spaced apart so as to have some space between them. By providing the carbon nanotubes on the surface of the positive electrode active material spaced apart so as to have 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.

[0071] The area of ​​the carbon nanotubes relative to the total area of ​​the positive electrode active material may be 30% to 80%. For example, it may be 35% to 75%, 40% to 70%, or 45% to 65%. The area of ​​the carbon nanotubes relative to the total area of ​​the positive electrode active material may be calculated quantitatively by setting a specific pixel intensity (threshold) in an image of the positive electrode active material taken with a scanning electron microscope, selecting only pixels with an intensity above the specific intensity, and considering the selected pixels as the region where the carbon nanotubes are located. When the area of ​​the carbon nanotubes relative to the total area of ​​the positive electrode active material satisfies the above range, it is possible to maintain the secondary particle morphology by not hindering the movement of lithium ions between the positive electrode active material and the electrolyte, and by preventing the alignment of primary particles from shifting, such as widening of gaps between primary particles due to charging and discharging, while simultaneously suppressing the contraction and expansion of secondary particles and the positive electrode, thereby improving the battery's lifespan characteristics. For example, if the area of ​​carbon nanotubes is less than 30%, the adhesive force that the carbon nanotubes attached to the surface of the positive electrode active material exert to maintain their particle shape weakens, reducing the improvement in battery life characteristics and potentially preventing the formation of a proper conductive network. Conversely, if the area of ​​carbon nanotubes exceeds 80%, the carbon nanotubes cover most of the surface of the positive electrode active material, hindering the smooth movement of lithium ions and degrading battery performance.

[0072] The carbon nanotubes are included in an amount of 0.1% to 5% by weight relative to 100% by weight of the positive electrode active material layer, for example, 0.1% to 4% by weight, 0.1% to 3% by weight, 0.1% to 2% by weight, or 0.1% to 1% by weight. In one embodiment, the carbon nanotubes are used in small amounts relative to the graphene content in known graphene coatings, and the content of conductive material in the positive electrode is further reduced compared to the graphene coating layer, which is advantageous for improving the energy density of the positive electrode and improving battery life characteristics.

[0073] The total content of the cellulose derivative, amorphous carbon, or combination thereof is 0.01% to 5% by weight per 100% by weight of the positive electrode active material layer, for example, 0.05% to 4% by weight, 0.1% to 3% by weight, 0.1% to 2% by weight, or 0.1% to 1% by weight. The cellulose derivative, amorphous carbon, or combination thereof may also be due to a cellulosic additive in the positive electrode slurry composition used when forming the positive electrode active material layer. In this case, the total content of these will be the same as the content of the cellulosic additive contained in the positive electrode slurry composition.

[0074] The cellulose derivative may be one in which cellulosic additives remain without carbonization due to heat treatment during cathode manufacturing. The type of cellulose derivative is not limited as long as it contains an alkyl group and a functional group capable of hydrogen bonding. The cellulose derivative may include, for example, ethylcellulose, cellulose acetate, carboxymethylcellulose, hydroxypropylcellulose, methylcellulose, nitrocellulose, or combinations thereof.

[0075] The amorphous carbon is carbon that does not have crystallinity or has very low crystallinity, and is distinguished from crystalline carbon or graphite-based carbon.

[0076] Examples of amorphous carbon include soft carbon, mesophase pitch carbides, and cellulose derivative carbides. Soft carbon refers to a carbon material that can be graphitized, and is easily graphitized by heat treatment at high temperatures, for example, about 2800°C. Cellulose derivative carbides are amorphous carbon substances formed by the heat treatment or carbonization process of cellulose-based additives.

[0077] An amorphous carbon according to one embodiment may include a carbide of a cellulose derivative. The type of carbide of the cellulose derivative is not limited as long as it contains an alkyl group and a functional group capable of hydrogen bonding. For example, the carbide of the cellulose derivative may include a carbide of ethylcellulose, a carbide of cellulose acetate, a carbide of carboxymethylcellulose, a carbide of hydroxypropylcellulose, a carbide of methylcellulose, a carbide of nitrocellulose, or a combination thereof.

[0078] The amorphous carbon can be positioned between the lithium nickel-based composite oxide and the carbon nanotubes. Such amorphous carbon acts as a kind of adhesive, allowing the carbon nanotubes to adhere well to the surface of the lithium nickel-based composite oxide, thereby further improving the maintenance of secondary particle morphology and the suppression of contraction and expansion between the secondary particles and the cathode while the carbon nanotubes adhere more strongly to the lithium nickel-based composite oxide.

[0079] When both the cellulose derivative and the amorphous carbon are included, the weight ratio of the cellulose derivative to the amorphous carbon may be 1:1 to 1:30, for example, 1:2 to 1:20, 1:3 to 1:15, or 1:5 to 1:10.

[0080] The positive electrode active material layer may further selectively include a binder, a conductive material, or a combination thereof, the specific contents of the binder and conductive material being as described above.

[0081] Method for manufacturing a positive electrode Further embodiments provide a method for manufacturing a positive electrode, comprising the steps of: producing a positive electrode slurry composition containing a lithium nickel-based composite oxide and a positive electrode active material in the form of aggregated primary particles, carbon nanotubes, and a cellulose-based additive; coating the positive electrode slurry composition onto a positive electrode current collector and drying it; and heat treating it.

[0082] First, a cathode slurry composition is prepared containing a lithium nickel-based composite oxide, a cathode active material in the form of secondary particles formed by the aggregation of multiple primary particles, carbon nanotubes, and a cellulose-based additive.

[0083] The specific details of the cathode active material, carbon nanotubes, and cellulose-based additives are as described above.

[0084] The production step for the positive electrode slurry composition can be carried out by adding and mixing lithium nickel-based composite oxide, carbon nanotubes, and a cellulose-based additive in a solvent; or by adding carbon nanotubes and a cellulose-based additive in a solvent to produce a pre-dispersion, and then adding the pre-dispersion to a slurry containing lithium nickel-based composite oxide.

[0085] The solvent in the pre-dispersion may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one of these alone or a mixture of two or more can be used.

[0086] The slurry manufacturing method is publicly known and therefore will be omitted.

[0087] Next, the positive electrode slurry composition is applied to the positive electrode current collector and then dried. The method for applying the positive electrode slurry composition to the positive electrode current collector is also known and will be omitted here. In the drying step, the solvent in the slurry composition can be removed by volatilization. The drying can be carried out at 60°C to 160°C, for example, at 90°C to 160°C, 100°C to 160°C, or 100°C to 140°C.

[0088] Finally, heat treatment is performed.

[0089] The heat treatment is performed at a temperature of 180°C to 300°C under a vacuum or a specific gas atmosphere, and the specific gas may include oxygen, argon, air, or nitrogen. The heat treatment partially carbonizes the cellulosic additive to form carbides, allowing carbon nanotubes to adhere more strongly to the surface of the positive electrode active material, thereby preventing particle cracking due to contraction and expansion of the positive electrode active material even during repeated charging and discharging, and further improving battery life.

[0090] Lithium-ion rechargeable battery Further embodiments provide a lithium secondary battery comprising the positive electrode, negative electrode, and electrolyte described above. The lithium secondary battery may include a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte.

[0091] 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 tab 11 and a positive electrode terminal 12, a negative electrode lead tab 21, and a negative electrode terminal 22. As shown in Figures 3 and 4, the lithium secondary battery 100 may include electrode tabs 70, namely a positive electrode tab 71 and a negative electrode tab 72, which serve as electrical pathways for guiding the current formed in the electrode assembly 40 to the outside.

[0092] The specific details of the positive electrode included in the lithium secondary battery are as described above and will therefore be omitted. In one embodiment of the lithium secondary battery, the inclusion of the aforementioned positive electrode suppresses life degradation and improves battery performance.

[0093] negative electrode A 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.

[0094] 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.

[0095] 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.

[0096] As the substance capable of reversibly inserting / desorbing the lithium ions, a carbon-based negative electrode active material can be included, 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, fired coke, and the like.

[0097] 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.

[0098] As the substance capable of doping and undoping lithium, an Si-based negative electrode active material or an Sn-based negative electrode active material can be used. As the Si-based negative electrode active material, silicon, a silicon-carbon composite, SiOx (0 < x ≦ 2), an Si-Q alloy (where Q is an element selected from alkali metals, alkaline earth metals, group 13 elements, group 14 elements (excluding Si), group 15 elements, group 16 elements, transition metals, rare earth elements, and combinations thereof, for example, Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof), or a combination thereof may be used. As the Sn-based negative electrode active material, Sn, SnO2, an Sn alloy, or a combination thereof may be used.

[0099] The silicon-carbon composite may be a composite of silicon and amorphous carbon. The average particle size (D 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 coating 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.

[0100] 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.

[0101] 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.

[0102] 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 may be 10 nm to 200 nm. The silicon particles can exist alone as silicon, exist in the form of a silicon alloy, or can also exist 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.

[0103] The Si-based negative electrode active material or the Sn-based negative electrode active material can be used in mixture with a carbon-based negative electrode active material. When the Si-based negative electrode active material or the Sn-based negative electrode active material and the carbon-based negative electrode active material are used in mixture, the mixing ratio may be 1:99 to 90:10 by weight ratio.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] 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 it is constructed from can be used. 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.

[0111] 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.

[0112] 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.

[0113] The non-aqueous organic solvent acts 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.

[0114] 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.

[0115] Non-aqueous organic solvents can be used alone or in combination of two or more, and the mixing ratio when using two or more can be appropriately adjusted according to the desired battery performance, which is widely understood by those working in this field.

[0116] 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.

[0117] 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.

[0118] The electrolyte may further contain vinyl ethyl carbonate, vinylene carbonate, or ethylene carbonate compounds to improve battery life.

[0119] 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.

[0120] 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 (LiDFDOP), and lithium bis(oxalate) borate (LiBOB).

[0121] 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.

[0122] 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.

[0123] 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.

[0124] The porous substrate may be a polymer film formed from any 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.

[0125] 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.

[0126] 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.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] 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.

[0131] Example 1 (1) Manufacturing of the positive electrode LiRing 0.91 Co 0.08 Al 0.01 It contains O2 lithium nickel-based composite oxide, and the average particle size of the secondary particles (D 50 The size of the secondary particles is approximately 15 μm, and the average particle size (D) of the primary particles that make up the secondary particles is approximately 15 μm.50 A cathode slurry composition was prepared by adding a cathode active material of approximately 700 nm, multi-walled carbon nanotubes with an average aspect ratio of approximately 15,000 and an average diameter of approximately 5 nm, ethyl cellulose as a cellulose-based additive, and polyvinylidene fluoride as a binder in a weight ratio of 98.5:0.25:0.25:1 to N-methyl-2-pyrrolidone solvent and mixing with a centrifugal mixer.

[0132] At this time, the positive electrode slurry composition was adjusted so that the solid content was approximately 70%.

[0133] Subsequently, a cathode slurry with a solid content of 70% was applied onto aluminum foil using a doctor blade and dried in a convection oven at 120°C for 20 minutes to remove N-methyl-2-pyrrolidone.

[0134] Subsequently, the electrodes from which the solvent had been removed were heat-treated at 250°C using a tube furnace. The heating rate was 5°C / min, the temperature was maintained for 10 minutes, and the treatment was carried out under an argon atmosphere.

[0135] After heat treatment, the electrode was rolled to a slurry density of 3 g / cc to produce a positive electrode in which a positive electrode active material layer was formed on aluminum foil. At this time, the positive electrode active material in the positive electrode active material layer was 98.5% by weight, carbon nanotubes were 0.25% by weight, amorphous carbon was 0.225% by weight, cellulose derivative was 0.025% by weight, and the binder was 1% by weight, and the loading level of the positive electrode active material layer was 5 mg / cm³ 2 It was to that extent.

[0136] (2) Manufacturing of lithium secondary batteries An electrode assembly was manufactured by interposing glass fiber filter paper as a separator between lithium metal as the positive and negative electrodes. After inserting the assembly into a case, an electrolyte was injected to produce a 2032 coin cell lithium secondary battery. The electrolyte used was a solvent prepared by mixing EC (ethylene carbonate):MEC (ethyl methyl carbonate):DMC (dimethyl carbonate) in a volume ratio of 3:3:4, in which 3% by weight of VC (vinylene carbonate) and 1M LiPF6 were dissolved.

[0137] Example 2 In Example 1, the loading level of the positive electrode active material layer was 25 mg / cm². 2 The positive electrode and lithium secondary battery were manufactured in substantially the same manner as in Example 1, except that the positive electrode was manufactured in such a manner.

[0138] Comparative Example 1 Except for the fact that, instead of mixing the positive electrode active material, carbon nanotubes, cellulose-based additive, and binder in a weight ratio of 98.5:0.25:0.25:1 as in Example 1, the same positive electrode active material, Super P as the conductive material, and polyvinylidene fluoride binder were mixed in a weight ratio of 98.75:0.25:1 to produce the positive electrode slurry composition, the same as in Example 1, but with a difference in weight ratio. Otherwise, the positive electrode and lithium secondary battery were manufactured in substantially the same manner as in Example 1.

[0139] Comparative Example 2 In Comparative Example 1, drying was performed during the manufacturing of the positive electrode, but the positive electrode and lithium secondary battery were manufactured in substantially the same manner as in Comparative Example 1, except that heat treatment was not performed after drying.

[0140] Reference example 1 The cathode and lithium secondary battery were manufactured in substantially the same manner as in Example 1, except that a cellulose-based additive was not added during the preparation of the cathode slurry composition in Example 1, and the cathode active material, carbon nanotubes, and binder were mixed in a weight ratio of 98.75:0.25:1.

[0141] Reference example 2 The loading level of the positive electrode active material layer in Reference Example 1 is 25 mg / cm³. 2The positive electrode and lithium secondary battery were manufactured in substantially the same manner as in Reference Example 1, except that the positive electrode was manufactured in such a manner.

[0142] Reference example 3 In Example 1, drying was performed during the production of the positive electrode, but the positive electrode and lithium secondary battery were manufactured in substantially the same manner as in Example 1, except that heat treatment was not performed after drying. At this time, the positive electrode active material in the positive electrode active material layer was 98.5% by weight, carbon nanotubes were 0.25% by weight, cellulose derivatives were 0.25% by weight, and the binder was 1% by weight.

[0143] Evaluation Example 1: Evaluation of carbon nanotube dispersibility To evaluate the dispersibility of carbon nanotubes with the addition of cellulose-based additives, slurry compositions were prepared in the same manner as in Example 1 and Reference Example 1, except that the cathode active material was not added during the preparation of the cathode slurry composition. These slurry compositions were then coated onto aluminum foil, dried, and heat-treated to prepare samples. The prepared samples were photographed with a scanning electron microscope (SEM) to evaluate the dispersibility of carbon nanotubes.

[0144] Figure 5 shows scanning electron microscope images of samples prepared to confirm the dispersibility of carbon nanotubes with and without cellulose-based additives. The left side of Figure 5 shows the sample with cellulose-based additives added, and the right side shows the sample without cellulose-based additives.

[0145] Referring to Figure 5, in the sample with a cellulose-based additive (upper left of Figure 5) and the sample without a cellulose-based additive (upper right of Figure 5), areas where carbon nanotubes are observed to be concentrated on the surface can be seen. When this area is magnified, it can be confirmed that in the sample with a cellulose-based additive (lower left of Figure 5), the carbon nanotubes are relatively uniformly dispersed and evenly distributed across the entire surface of the aluminum foil, whereas in the sample from Reference Example 1 (lower right of Figure 5), the carbon nanotubes are not uniformly dispersed but concentrated on the surface of the aluminum foil. This confirms that the dispersibility of carbon nanotubes is improved when a cellulose-based additive is added, and therefore, it can be predicted that when a cellulose-based additive is added during the production of the cathode slurry composition, the carbon nanotubes will be uniformly dispersed on the surface of the cathode.

[0146] Evaluation Example 2. Evaluation of the cathode surface and cross-section The surfaces of the positive electrodes in Example 1 and Reference Example 1 were photographed using a scanning electron microscope (SEM) to evaluate the presence of carbon nanotubes on the surface of the positive electrode active material. Cross-sections of the positive electrodes in Example 2 and Reference Example 2 were also photographed using a scanning electron microscope (SEM) to evaluate the presence of carbon nanotubes on the surface of the positive electrode active material.

[0147] Figure 6 shows scanning electron microscope images of the surfaces of the positive electrodes of Example 1 and Reference Example 1. The two upper images in Figure 6 are scanning electron microscope images of the surface of the positive electrode of Example 1 taken at different magnifications, while the two lower images in Figure 6 are scanning electron microscope images of the surface of the positive electrode of Reference Example 1 taken at different magnifications.

[0148] Referring to Figure 6, in Example 1 (Figure 6 a) and b)), which was manufactured using a cathode slurry composition with added cellulose-based additives, it can be confirmed that carbon nanotubes are selectively attached to the surface of the cathode active material. In Reference Example 1 (Figure 6 c) and d)), which did not have added cellulose-based additives, it can be confirmed that carbon nanotubes are not selectively attached to the surface of the cathode active material. Comparing magnified images of the surface of the secondary particles, it can be confirmed that in Example 1 (Figure 6 a)), a large amount of carbon nanotubes are attached to the surface of the cathode active material, while in Reference Example 1 (Figure 6 c)), only a small amount of carbon nanotubes are attached. This confirms that cellulose-based additives can play a role in the selective coating of carbon nanotubes.

[0149] Figure 7 shows photographs of the cross-sections of the positive electrodes of Example 2 and Reference Example 2, taken with a scanning electron microscope at different magnifications. The two upper images in Figure 7 are photographs of the cross-sections of the positive electrode of Example 2, taken with a scanning electron microscope at different magnifications, while the two lower images in Figure 7 are photographs of the cross-sections of the positive electrode of Reference Example 2, taken with a scanning electron microscope at different magnifications. Referring to Figure 7, in Example 2, which was manufactured using a cathode slurry composition with added cellulose-based additives, it was possible to observe that carbon nanotubes exist in a form selectively coated on the surface of the cathode active material (Figure 7a), and the phenomenon of carbon nanotubes binding to the binder and floating up in the upper layer of the electrode was mitigated, so a shape in which carbon nanotubes accumulate in the upper layer of the electrode was not observed (Figure 7b). In contrast, in Reference Example 2, in which no cellulose-based additives were added, the coating of carbon nanotubes did not progress, and it was not possible to observe that carbon nanotubes exist in a form selectively coated on the surface of the cathode active material (Figure 7c), and a shape in which carbon nanotubes accumulate in the upper layer of the electrode was observed (Figure 7d). This confirmed that even under high loading, the cellulose-based additive plays a role in selectively coating carbon nanotubes and can increase the degree of dispersion of carbon nanotubes.

[0150] Evaluation Example 3. Evaluation of the adhesion strength of carbon nanotubes by heat treatment. The cathode active material was extracted from the cathodes manufactured in Example 1 and Reference Example 3 by scraping using a tool such as a scraper or knife. The adhesion strength of carbon nanotubes was evaluated using a solution obtained by adding these materials to N-methyl-2-pyrrolidone solvent, both before and after ultrasonic treatment with an ultrasonic device for 1 minute, 3 minutes, and 5 minutes.

[0151] Figure 8 is a photograph showing the results of evaluating the adhesion strength of carbon nanotubes attached to cathode active materials extracted from the cathodes manufactured in Example 1 and Reference Example 3. From left to right in Figure 8 are the tests without sonication (After 30 min rest), with 1 minute of sonication (1 min sonication), with 3 minutes of sonication (3 min sonication), and with 5 minutes of sonication (5 min sonication). In each test, the sample on the left is the solution obtained by adding the cathode active material extracted from the cathode of Reference Example 3 to N-methyl-2-pyrrolidone solvent, and the sample on the right is the solution obtained by adding the cathode active material extracted from the cathode of Example 1 to N-methyl-2-pyrrolidone solvent.

[0152] Referring to Figure 8, it can be seen that the carbon nanotubes in the positive electrode active material of Example 1, which underwent heat treatment, do not separate well even when subjected to the same amount of ultrasonic treatment as the positive electrode active material of Reference Example 3, which did not undergo heat treatment. In Figure 8, it can be seen that in the case of Reference Example 3, the carbon nanotubes separate over time in the ultrasonic treatment machine, and the solution gradually turns black. This confirms that by performing additional heat treatment after drying during positive electrode manufacturing, the carbon nanotubes adhere even more strongly to the surface of the positive electrode active material.

[0153] Evaluation Example 4. Lithium-ion battery life evaluation The lifespan evaluation of the lithium secondary batteries manufactured in Example 1, Reference Example 3, and Comparative Examples 1-2 was performed as follows.

[0154] The battery was charged at 25°C with a constant current of 0.1C until it reached 4.3V. Then, the constant voltage of 4.3V was maintained, and charging continued until the current dropped to 0.05C. This charge-discharge process was then repeated twice, discharging at 0.1C until the voltage reached 3V.

[0155] Under the same conditions as above, only the applied current was changed to 1C, and the cycle was repeated 100 times to evaluate the lifespan. The results are shown in Figure 9.

[0156] Referring to Figure 9, it was confirmed that Comparative Example 2 (38.09%), which used Super P, showed a faster rate of capacity reduction with increasing cycles compared to Reference Example 3, which used carbon nanotubes, assuming no heat treatment was performed. It is predicted that when Super P is used as a conductive material, the conductive network cannot be maintained as the cycles progress, and a faster rate of capacity reduction will be observed with increasing cycles, even though the content of carbon nanotubes and Super P are the same. In other words, it can be confirmed that when using Super P as a conductive material, a higher content of Super P than that added to Comparative Example 2 is required to maintain a conductive network at the same level as when using carbon nanotubes as a conductive material. Specifically, in the case of Reference Example 3, which used carbon nanotubes, it was confirmed that the life characteristics improved by more than 25% based on 100 cycles, even though the Super P content was the same as that of Comparative Example 2. It is predicted that this is because, unlike Super P, carbon nanotubes can form a conductive network even in small amounts.

[0157] Even assuming heat treatment was performed, Comparative Example 1 (41.25%) using Super P showed a faster rate of volume reduction as the cycle progressed compared to both Example 1 and Reference Example 3. This is presumed to be because the Super P content was too low, causing rapid degradation, and because ethyl cellulose was not added as a cellulose-based additive, the effect of strengthening the adhesion of the conductive material through heat treatment was not achieved.

[0158] On the other hand, in Example 1 (81.61%), which underwent heat treatment, it was confirmed that the lifespan characteristics were improved by about 15% compared to Reference Example 3 (66.68%), which did not undergo heat treatment. This is presumably because, when heat treatment is performed assuming the use of carbon nanotubes as the conductive material, the carbon nanotubes adhere more strongly to the surface of the positive electrode active material, and the conductive network is maintained even as the cycle progresses.

[0159] Although preferred embodiments have been described in detail above, the scope of the present invention is not limited thereto. Various modifications and improvements by those skilled in the art, utilizing the basic concepts defined in the claims, also fall within the scope of the present invention. [Explanation of Symbols]

[0160] 100: Lithium-ion rechargeable battery 10: Positive electrode 11: Positive lead tab 12: Positive terminal 20: Negative electrode 21: Negative lead tab 22: Negative terminal 30: Separator 40: Electrode Assembly 50: Case 60: Sealing member 70: Electrode Tab 71: Positive Tab 72: Negative electrode tab

Claims

1. A positive electrode active material containing a lithium nickel-based composite oxide and having a secondary particle form in which multiple primary particles are aggregated, carbon nanotubes, and cellulose-based additives, Cathode slurry composition.

2. The lithium nickel-based composite oxide is represented by chemical formula 1, The positive electrode slurry composition 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 slurry composition 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 slurry composition according to claim 1.

5. The carbon nanotubes include single-walled carbon nanotubes, multi-walled carbon nanotubes, or combinations thereof. The positive electrode slurry composition according to claim 1.

6. The average diameter of the carbon nanotubes is 0.8 nm to 100 nm. The average aspect ratio of the carbon nanotubes is 100 to 500,000. The positive electrode slurry composition according to claim 1.

7. The aforementioned cellulosic additive includes ethylcellulose, cellulose acetate, carboxymethylcellulose, hydroxypropylcellulose, methylcellulose, nitrocellulose, or a combination thereof. The positive electrode slurry composition according to claim 1.

8. The lithium nickel-based composite oxide is present in an amount of 90% to 99% by weight relative to 100% by weight of the solid content of the positive electrode slurry composition. The carbon nanotubes are present in an amount of 0.1% to 5% by weight relative to 100% by weight of the solid content of the cathode slurry composition. The cellulose-based additive is present in an amount of 0.01% to 5% by weight relative to 100% by weight of the solid content of the positive electrode slurry composition. The positive electrode slurry composition according to claim 1.

9. The weight ratio of the carbon nanotube to the cellulose-based additive is 10:1 to 1:

5. The positive electrode slurry composition according to claim 1.

10. The positive electrode slurry composition further comprises a binder, a conductive material, or a combination thereof. The positive electrode slurry composition according to claim 1.

11. A positive electrode current collector; a positive electrode active material layer located on the positive electrode current collector, containing a lithium nickel-based composite oxide and having a secondary particle form in which a plurality of primary particles are aggregated, and containing carbon nanotubes; The positive electrode active material layer further comprises a cellulose derivative, amorphous carbon, or a combination thereof. Positive electrode.

12. A step of producing a cathode slurry composition comprising a lithium nickel-based composite oxide, a cathode active material in the form of secondary particles formed by the aggregation of multiple primary particles, carbon nanotubes, and a cellulose-based additive; The steps of applying the positive electrode slurry composition onto the positive electrode current collector and drying it; and The step of performing a heat treatment; A method for manufacturing a positive electrode.

13. The manufacturing step of the cathode slurry composition is to be a method of adding and mixing lithium nickel-based composite oxide, carbon nanotubes, and cellulosic additives in a solvent; or to be a method of adding carbon nanotubes and cellulosic additives in a solvent to produce a predispersion, and then adding the predispersion to a slurry containing lithium nickel-based composite oxide. A method for manufacturing a positive electrode according to claim 12.

14. The drying is carried out at a temperature of 60°C to 160°C. The heat treatment is carried out at 180°C to 300°C for 10 to 60 minutes under vacuum or a specific gas atmosphere. The specified gas includes oxygen, argon, air, or nitrogen. A method for manufacturing a positive electrode according to claim 12.

15. The positive electrode, negative electrode, and electrolyte according to claim 11, Lithium-ion rechargeable battery.

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