Positive electrode for rechargeable lithium battery and rechargeable lithium battery including same

Abstract

A positive electrode for a rechargeable lithium battery and a rechargeable lithium battery including the same are provided. The positive electrode includes a positive electrode active material layer, and the positive electrode active material layer includes a positive electrode active material, a conductive material, an acrylic binder, and a polyimide binder. The acrylic binder is a copolymer comprising styrene units and acrylate units.

Description

[0001] This application claims priority to Korean Patent Application No. 10-2024-0161011, filed on November 13, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This disclosure relates to a positive electrode for a rechargeable lithium battery and a rechargeable lithium battery including the positive electrode. Background Technology

[0003] The increasing prevalence of battery-powered electronic devices (such as mobile phones, laptops, and electric vehicles) has driven a rise in demand for rechargeable batteries with high energy density and capacity. Therefore, improving the performance of rechargeable lithium batteries may be advantageous.

[0004] A rechargeable lithium battery includes a positive electrode and a negative electrode as well as an electrolyte solution, and generates electrical energy from a redox reaction that occurs when lithium ions are deintercalated from the positive electrode and intercalated into the negative electrode or deintercalated from the negative electrode and intercalated into the positive electrode. Each of the positive and negative electrodes includes an active material that allows lithium ions to intercalate and deintercalate. Summary of the Invention

[0005] This disclosure describes a positive electrode for a rechargeable lithium battery that exhibits improved compaction density and bond strength.

[0006] This disclosure also describes a rechargeable lithium battery that exhibits improved lifespan characteristics.

[0007] Example embodiments of this disclosure include a positive electrode for a rechargeable lithium battery, the positive electrode including a positive electrode active material layer.

[0008] The positive electrode active material layer includes a positive electrode active material, a conductive material, an acrylic binder, and a polyimide binder, wherein the acrylic binder is or includes a copolymer comprising styrene units and acrylate units.

[0009] In an exemplary embodiment of this disclosure, the rechargeable lithium battery includes the aforementioned positive electrode, a negative electrode including a negative electrode active material, and an electrolyte solution. Attached Figure Description

[0010] The accompanying drawings are included to provide a further understanding of this disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of this disclosure and, together with the description, serve to explain the principles of this disclosure. In the drawings: Figure 1 This is a simplified conceptual diagram illustrating a rechargeable lithium battery according to an example embodiment of the present disclosure; Figures 2 to 5This is a schematic diagram illustrating a rechargeable lithium battery according to an exemplary embodiment of the present disclosure. Figure 2 A cylindrical battery is shown. Figure 3 A prismatic battery is shown, and Figure 4 and Figure 5 A pouch-type battery is shown; Figure 6 This is a cross-sectional view used to describe a positive electrode plate according to an exemplary embodiment of the present disclosure; Figure 7 It is a graph used to evaluate the interface resistance of the positive electrode manufactured according to Examples 1 and 2 of this disclosure and Comparative Example 1; and Figure 8 This is a graph used to evaluate the lifespan characteristics of rechargeable lithium batteries manufactured according to Example 1 and Example 2, as well as Comparative Example 1. Detailed Implementation

[0011] To fully understand the structure and effects of this disclosure, exemplary embodiments of the disclosure have been described with reference to the accompanying drawings. However, it should be noted that this disclosure is not limited to the following exemplary embodiments and can be implemented in various forms and with various modifications. The exemplary embodiments provided herein are intended to make this disclosure thorough and complete, and to fully convey the scope of the disclosure to those skilled in the art.

[0012] It is understood here that when a component is referred to as being "on" another component, the component may be directly on that other component, or an intermediate third component may exist between them. Furthermore, in the accompanying drawings, the thickness of components may be exaggerated for the purpose of effectively describing the technical content. The same reference numerals always refer to the same elements.

[0013] The exemplary embodiments described herein are explained with reference to sectional views and / or plan views, which serve as ideal example views of this disclosure. In the drawings, the thickness of the membranes and regions may be exaggerated for the purpose of effectively describing the technical content. Therefore, the regions presented as examples in the drawings have general properties, and the shapes of the example regions are used to illustrate the specific shapes of the device regions. Therefore, this should not be construed as limiting the scope of this disclosure. Although terms such as first, second, and third are used to describe various components in the various exemplary embodiments herein, the components should not be limited to these terms. These terms are used only to distinguish one component from another. The embodiments described and illustrated herein include complementary embodiments thereof.

[0014] The terminology used herein is not intended to limit this disclosure, but rather to describe exemplary embodiments. As used herein, the singular form includes the plural form as well. The meaning of “comprising” and / or “including” as used herein does not exclude the presence or addition of one or more other components besides those mentioned.

[0015] As used herein, the term "combination thereof" may refer to mixtures, stacks, complexes, copolymers, alloys, blends, or reaction products.

[0016] Unless otherwise defined herein, particle size may be the average particle size. Furthermore, particle size is defined as the average particle size (D50), which represents the diameter of particles at a cumulative volume of approximately 50 vol% in the particle size distribution. The average particle size (D50) can be measured by methods known to those skilled in the art, for example, by a particle size analyzer, images from a transmission electron microscope (TEM), or images from a scanning electron microscope (SEM). Alternatively, the average particle size (D50) can be measured using a measuring device employing dynamic light scattering, wherein data analysis is performed to count the number of particles in each particle size range, and the average particle size (D50) value can then be calculated. Alternatively, the average particle size (D50) can be measured using, for example, laser scattering. In measurements using laser diffraction, for example, the target particles are dispersed in a dispersion medium and introduced into a commercially available laser diffraction particle size measuring device (e.g., the MT 3000 available from Microtrac), irradiated with ultrasound at a power of 60W and approximately 28kHz. The average particle size (D50) based on the particle size distribution in the measuring device can then be calculated.

[0017] When the terms “about” or “substantially” are used in conjunction with numerical values ​​in this specification, it is intended that the relevant numerical value include a tolerance of ±10% around the stated value. When a range is specified, the range includes all values ​​within that range, such as increments of 0.1%.

[0018] Figure 1 This is a simplified conceptual diagram illustrating a rechargeable lithium battery according to an example embodiment of the present disclosure. (Refer to...) Figure 1 A rechargeable lithium battery may include a positive electrode 10, a negative electrode 20, a separator 30, and an electrolyte solution ELL.

[0019] The positive electrode 10 and the negative electrode 20 can be separated from each other by a diaphragm 30. The diaphragm 30 can be disposed between the positive electrode 10 and the negative electrode 20. The positive electrode 10, the negative electrode 20 and the diaphragm 30 can be in contact with the electrolyte solution ELL. The positive electrode 10, the negative electrode 20 and the diaphragm 30 can be immersed in the electrolyte solution ELL.

[0020] The electrolyte solution ELL can be or includes a medium configured to transport lithium ions between the positive electrode 10 and the negative electrode 20. In the electrolyte solution ELL, lithium ions can move toward the positive electrode 10 or the negative electrode 20 through the membrane 30.

[0021] Positive electrode 10 The positive electrode 10 for a rechargeable lithium battery may include a positive electrode current collector COL1 and a positive electrode active material layer AML1 on the positive electrode current collector COL1. The positive electrode active material layer AML1 may include a positive electrode active material, and may also include a positive electrode binder and / or conductive material.

[0022] In an example embodiment, the positive electrode 10 may also include additives that can constitute a sacrificial positive electrode.

[0023] For example, relative to 100 wt% of the positive electrode active material layer AML1, the positive electrode active material layer AML1 may contain about 90 wt% to about 99.5 wt% of positive electrode active material or about 90 wt% to about 99 wt% of positive electrode active material. Relative to 100 wt% of the positive electrode active material layer AML1, the amounts of binder and conductive material may each range from about 0.1 wt% to about 5 wt% or from about 0.5 wt% to about 5 wt%.

[0024] The binder can make the positive electrode active material particles adhere to each other and make the positive electrode active material adhere to the positive electrode current collector COL1.

[0025] Conductive materials can impart conductivity to electrodes. Any material that does not cause undesirable chemical changes and is an electronically conductive material can be used in a battery. Examples of conductive materials may include: carbon-based materials, such as at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fibers, carbon nanofibers, and carbon nanotubes; metallic materials, including at least one of copper, nickel, aluminum, silver, etc., in the form of metal powder or metal fibers; conductive polymers, such as polyphenylene derivatives; or mixtures thereof.

[0026] Al can be used as the positive electrode current collector COL1, but the exemplary embodiments of this disclosure are not limited thereto.

[0027] Passing below Figure 6 The positive electrode 10 according to an example embodiment of the present disclosure is described.

[0028] Positive electrode active material Compounds capable of reversibly inserting and deintercalating lithium (lithiation intercalation compounds) can be used as positive electrode active materials in the positive electrode active material layer AML1. For example, at least one of the composite oxides of lithium and metals such as or including at least one of cobalt, manganese, nickel, aluminum, and combinations thereof can be used.

[0029] The composite oxide can be or includes lithium transition metal composite oxides, and examples include at least one of lithium nickel oxides, lithium cobalt oxides, lithium manganese oxides, lithium iron phosphate compounds, cobalt-free lithium nickel manganese oxides, and combinations thereof.

[0030] For example, the positive electrode active material may be or include a high-nickel type positive electrode active material. Relative to 100 mol% of the metals other than lithium in the lithium transition metal composite oxide, the nickel content of the high-nickel type positive electrode active material is about 80 mol% or greater, about 85 mol% or greater, about 90 mol% or greater, about 91 mol% or greater, or about 94 mol% or greater. The high-nickel type positive electrode active material can achieve a high capacity, and thus can be applied to high-capacity and high-density rechargeable lithium batteries.

[0031] For example, the positive electrode active material may include a lithium composite oxide represented by the following Formula 1.

[0032] Formula 1: Li x M 1 y M 2 z M 3 1-y-z O 2-a X a x, a, y, and z may satisfy 0.5 ≤ x ≤ 1.8, 0 ≤ a ≤ 0.05, 0 < y ≤ 1, 0 ≤ z ≤ 1, and 0 < y + z ≤ 1.

[0033] M 1 、M 2 and M 3 may each independently include at least one element such as or including at least one of nickel (Ni), cobalt (Co), manganese (Mn), aluminum (Al), boron (B), barium (Ba), calcium (Ca), cerium (Ce), chromium (Cr), iron (Fe), molybdenum (Mo), niobium (Nb), silicon (Si), strontium (Sr), magnesium (Mg), titanium (Ti), vanadium (V), tungsten (W), zirconium (Zr), and lanthanum (La) and combinations thereof.

[0034] X may include at least one element such as or including at least one of fluorine (F), sulfur (S), phosphorus (P), and chlorine (Cl).

[0035] For example, the positive electrode active material may include lithium cobalt oxide (LiCoO2). For example, the positive electrode active material may include an NCA type positive electrode active material formed of or including at least one of Ni, Co, and Al.

[0036] negative electrode 20 The negative electrode 20 for a rechargeable lithium battery may include a negative electrode current collector COL2 and a negative electrode active material layer AML2 on the negative electrode current collector COL2. The negative electrode active material layer AML2 may include a negative electrode active material, and may also include a negative electrode binder and / or conductive material.

[0037] The negative electrode current collector COL2 may include at least one of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

[0038] The negative electrode binder enables the negative electrode active material particles to adhere to each other and also enables the negative electrode active material to adhere to the negative electrode current collector COL2. The negative electrode binder may include non-aqueous binders, aqueous binders, dry binders, or combinations thereof.

[0039] Non-aqueous adhesives may include at least one of polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene-propylene copolymer, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyimide, and combinations thereof.

[0040] The waterborne adhesive may be or include at least one of the following: styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, (meth)acrylonitrile-butadiene rubber, (meth)acrylic rubber, butyl rubber, fluororubber, polyethylene oxide, polyvinylpyrrolidone, polyepoxychlorohydrin, 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.

[0041] When an aqueous binder is included as the negative electrode binder, a cellulose compound capable of imparting viscosity may be further included. The cellulose compound may include at least one of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, and their alkali metal salts. The alkali metal may include at least one of Na, K, and Li.

[0042] Dry adhesives can be or include polymeric materials capable of forming fibers. For example, dry adhesives can be or include at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, and combinations thereof.

[0043] A conductive material can impart conductivity to an electrode. Any material that does not cause an undesired chemical change and is an electron-conducting material can be used in a battery. Examples of conductive materials can include: carbon-based materials, such as at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metal-based materials, including at least one of copper, nickel, aluminum, silver, etc., in the form of metal powder or metal fiber; conductive polymers, such as polyphenylene derivatives; or mixtures thereof.

[0044] For example, the negative electrode active material layer AML2 can include from about 90 wt% to about 99 wt% of a negative electrode active material, from about 0.5 wt% to about 5 wt% of a negative electrode binder, and from about 0 wt% to about 5 wt% of a conductive material.

[0045] The conductive material, binder, and negative electrode active material can be provided in a weight ratio of 1:a:b, where the range of a can be from about 1 to about 3, and the range of b can be from about 90 to about 99.

[0046] Negative electrode active material The negative electrode active material in the negative electrode active material layer AML2 can include at least one of a material that can reversibly intercalate / deintercalate lithium ions, lithium metal, a lithium metal alloy, a material that can dope / de-dope lithium, and a transition metal oxide.

[0047] The material that can reversibly intercalate / deintercalate lithium ions can include a carbon-based negative electrode active material. For example, crystalline carbon, amorphous carbon, or a combination thereof. Examples of crystalline carbon can be graphite, such as irregular, planar, flaky, spherical, or fibrous natural graphite or artificial graphite, and examples of amorphous carbon can be or include at least one of soft carbon, hard carbon, mesophase pitch carbide, calcined coke, etc.

[0048] The lithium metal alloy includes an alloy of lithium and a metal such as or including at least one of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn.

[0049] The material that can dope / de-dope lithium can be or include a Si-based negative electrode active material or a Sn-based negative electrode active material. The Si-based negative electrode active material can be or include silicon, a silicon-carbon composite, SiO x (0 < x ≤ 2), a Si-Q alloy (where Q is or includes at least one of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element (excluding Si), a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and combinations thereof), and combinations thereof. The Sn-based negative electrode active material can be or include at least one of Sn, SnO2, Sn-based alloys, and combinations thereof.

[0050] Silicon-carbon composites can be or include composites of silicon and amorphous carbon. According to example embodiments, the silicon-carbon composite can be in the form of silicon particles and amorphous carbon coated on the surface of the silicon particles. For example, the silicon-carbon composite can include secondary particles (cores) in which primary silicon particles are assembled and an amorphous carbon coating (shell) on the surface of the secondary particles. Amorphous carbon can also be located between the primary silicon particles, and, for example, the primary silicon particles can be coated with amorphous carbon. The secondary particles can be dispersed in an amorphous carbon matrix.

[0051] Silicon-carbon composites may also include crystalline carbon. For example, a silicon-carbon composite may include a core comprising crystalline carbon and silicon particles, and an amorphous carbon coating layer on the surface of the core.

[0052] Si-based or Sn-based negative electrode active materials can be used in combination with carbon-based negative electrode active materials.

[0053] Diaphragm 30 Depending on the type of rechargeable lithium battery, the separator 30 may be present between the positive electrode 10 and the negative electrode 20. The separator 30 may include at least one of polyethylene, polypropylene, polyvinylidene fluoride, and multilayer films with two or more layers, such as mixed multilayer films such as polyethylene / polypropylene bilayer separators, polyethylene / polypropylene / polypropylene trilayer separators, polypropylene / polypropylene / polypropylene trilayer separators, etc.

[0054] The diaphragm 30 may include a porous substrate and a coating layer comprising an organic material, an inorganic material, or a combination thereof on one or both surfaces of the porous substrate.

[0055] The porous substrate may be or include a polymer membrane, which is formed of or includes any polymer or copolymer or mixture of two or more of the following: polyolefins (such as polyethylene and polypropylene), polyesters (such as polyethylene terephthalate and polybutylene terephthalate), polyacetal, polyamide, polyimide, polycarbonate, polyetherketone, polyaryletherketone, polyetherimide, polyamideimide, polybenzimidazole, polyethersulfone, polyphenylene ether, cyclic olefin copolymers, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon and polytetrafluoroethylene.

[0056] Organic materials may include polyvinylidene fluoride polymers or (meth)acrylic acid polymers.

[0057] Inorganic materials may include, but are not limited to, inorganic particles containing at least one of Al2O3, SiO2, TiO2, SnO2, CeO2, MgO, NiO, CaO, GaO, ZnO, ZrO2, Y2O3, SrTiO3, BaTiO3, Mg(OH)2, boehmite, and combinations thereof.

[0058] Organic and inorganic materials can be mixed in a coating layer, or a coating layer containing organic materials and a coating layer containing inorganic materials can be stacked together.

[0059] Electrolyte solution ELL Electrolyte solutions (ELLs) used in rechargeable lithium batteries may include non-aqueous organic solvents and lithium salts.

[0060] Non-aqueous organic solvents can serve as media for transporting ions that participate in the electrochemical reactions of a battery.

[0061] Non-aqueous organic solvents may be or include at least one of carbonate solvents, ester solvents, ether solvents, ketone solvents, alcohol solvents, aprotic solvents, and combinations thereof.

[0062] Carbonate solvents may include at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), and butyl carbonate (BC).

[0063] Ester solvents may include at least one of the following: methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, propylene propionate, decanoic acid lactone, mevalonate lactone, caprolactone, etc.

[0064] Ether solvents may include at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, tetrahydrofuran, etc. Additionally, ketone solvents may include cyclohexanone, etc. Alcohol solvents may include ethanol, isopropanol, etc. Aprotic solvents may include: nitriles, such as R-CN (wherein R is a C2 to C20 straight-chain, branched, or cyclic hydrocarbon group, and may include double bonds, aromatic rings, or ether groups); amides, such as dimethylformamide; dioxolane, such as 1,3-dioxolane, 1,4-dioxolane; sulfolane, etc.

[0065] Non-aqueous organic solvents can be used alone or in combination of two or more solvents.

[0066] Furthermore, when using carbonate solvents, cyclic carbonates and chain carbonates can be mixed, and the cyclic carbonates and chain carbonates can be mixed in a volume ratio ranging from about 1:1 to about 1:9.

[0067] Lithium salts dissolved in organic solvents are configured to supply lithium ions in batteries, enabling rechargeable lithium batteries to operate and improving lithium ion transport between the positive and negative electrodes. Typical examples of lithium salts may include LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiCl, LiI, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide, LiFSI), LiC4F9SO3, LiN(C x F 2x+1 SO2)(C y F 2y+1 At least one of the following: (SO2) (where x and y are integers in the range of 1 to 20), lithium trifluoromethane sulfonate, lithium tetrafluoroethane sulfonate, lithium difluoro(oxalate)borate (LiDFOB), lithium difluorobis(oxalate)phosphate (LiDFBOP), and lithium bis(oxalate)borate (LiBOB).

[0068] Rechargeable lithium batteries Rechargeable lithium batteries can be classified according to their shape as cylindrical batteries, prismatic batteries, pouch batteries, or coin-shaped batteries, etc. Figures 2 to 5 This is a schematic diagram illustrating a rechargeable lithium battery according to an exemplary embodiment of the present disclosure, and Figure 2 A cylindrical battery is shown. Figure 3 A prismatic battery is shown. Figure 4 and Figure 5 A pouch-type battery is shown. (See reference) Figures 2 to 5 The rechargeable lithium battery 100 may include an electrode assembly 40 and a housing 50. The electrode assembly 40 includes a separator 30 between a positive electrode 10 and a negative electrode 20, and the electrode assembly 40 is housed within the housing 50. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated with an electrolyte solution (not shown). Figure 2 As shown, the rechargeable lithium battery 100 may include a sealing member 60 of the sealed housing 50. Furthermore, in Figure 3 In this context, the rechargeable lithium battery 100 may include a positive electrode lead connector 11, a positive electrode terminal 12 connected to the positive electrode lead connector 11, a negative electrode lead connector 21, and a negative electrode terminal 22 connected to the negative electrode lead connector 21. For example... Figure 4 and Figure 5 As shown, the rechargeable lithium battery 100 may include Figure 5The electrode terminal 70 shown, or for example Figure 4 The positive electrode terminal 71 and negative electrode terminal 72 shown herein form a conductive path for guiding the current generated in the electrode assembly 40 to the outside of the rechargeable lithium battery 100.

[0069] As a non-limiting example, the rechargeable lithium battery according to exemplary embodiments of this disclosure can be applied to, for example, vehicles, mobile phones and / or various types of electronic devices.

[0070] The positive electrode according to an exemplary embodiment of the present disclosure is described in detail below. Figure 6 This is a cross-sectional view showing a positive electrode (positive electrode plate) 10 according to an exemplary embodiment of the present disclosure.

[0071] The binders included in the positive electrode active material layer AML1 may include acrylic binders and polyimide binders.

[0072] Polyimides exhibit desirable or improved adhesive properties and provide stability at high temperatures, thus making them suitable as binders for the positive electrodes of rechargeable lithium-ion batteries that require high-temperature stability. However, polyimides typically possess rigid mechanical properties, and when polyamic acid, a precursor compound for polyimides, is converted to polyimides, it undergoes a dehydration reaction, becoming a shrinking polyimide, which causes the positive electrode to bend.

[0073] Typically, a positive electrode is manufactured by the following steps: dispersing a binder, a positive electrode active material, and a conductive material in an organic solvent to prepare a positive electrode active material slurry; applying the positive electrode active material slurry onto a current collector to form a positive electrode active material layer; and allowing the positive electrode active material layer to dry.

[0074] In this scenario, during the drying process, the polyimide precursor compound (polyamic acid) used as a binder undergoes a dehydration reaction, releasing water molecules and thus transforming into polyimide. The dehydration reaction caused by the release of water molecules creates voids, leading to a corresponding shrinkage of the polyimide. This shrinkage of the polyimide also causes shrinkage of the positive electrode active material layer, and the difference in volume shrinkage between the interface between the positive electrode active material layer and the current collector and the surface of the positive electrode active material layer results in bending of the positive electrode.

[0075] In this situation, the coating process is not applicable to the back side of the positive electrode, making double-sided coating impossible. This, in turn, leads to a relative reduction in the amount of positive electrode active material used, resulting in a decrease in both the compaction density of the positive electrode and the battery energy density. Furthermore, forcing the curved portion of the positive electrode to flatten for double-sided coating may cause cracks to form on the positive electrode surface, potentially degrading the electrode's electrochemical properties. Additionally, applying only a small amount of positive electrode active material slurry to one side of the current collector to mitigate positive electrode curvature may result in a significant reduction in battery capacity.

[0076] To address these limitations, this disclosure uses a highly flexible acrylic binder blended with a polyimide binder as the positive electrode binder.

[0077] According to exemplary embodiments of this disclosure, the positive electrode active material slurry physically compensates for the volume shrinkage caused by water release during the dehydration reaction of the polyimide precursor compound using an acrylic binder. This avoids or prevents bending of the positive electrode 10 and enhances the adhesion strength at the interface between the positive electrode active material layer and the current collector. Furthermore, examples of this disclosure can improve the lifespan characteristics of rechargeable lithium batteries by using an aromatic polyimide exhibiting high antioxidant properties as a polyimide binder. Therefore, when a polyimide binder and an acrylic binder are mixed and used as the positive electrode binder, a rechargeable lithium battery exhibiting effectively improved lifespan characteristics can be provided.

[0078] For example, the amount of acrylic binder in the positive electrode active material layer AML1 can be from about 0.1 wt% to about 5 wt%. The amount of polyimide binder in the positive electrode active material layer AML1 can be from about 0.1 wt% to about 5 wt%. The weight ratio of acrylic binder to polyimide binder in the positive electrode active material layer AML1 can be in the range of about 1:9 to about 4:6.

[0079] When the amount of acrylic binder is less than about 0.1 wt%, it is difficult to reduce or prevent the bending of the positive electrode plate during manufacturing. When the amount of acrylic binder is greater than about 5 wt%, the proportion of polyimide in the positive electrode plate decreases, making it impossible to control the drastic volume changes of the positive electrode active material due to repeated charging and discharging. Therefore, when the amounts of acrylic binder and polyimide binder meet the above-mentioned ranges, the positive electrode 10 can have improved compaction density, thus the rechargeable lithium battery can have improved battery life characteristics.

[0080] acrylic adhesive Acrylic adhesives can be or include copolymers comprising styrene units and acrylate units. For example, the copolymer may include styrene monomers or repeating units derived from styrene monomers, and acrylate monomers or repeating units derived from acrylate monomers. The copolymer may be or include random copolymers, alternating copolymers, or block copolymers.

[0081] The copolymer can exhibit high flexibility provided by including acrylate units, thus enabling desired or improved adhesive properties between substrates. Therefore, the adhesive strength between the positive electrode active material layer AML1 and the positive electrode current collector COL1 can be improved, and the flexural strength and compaction density of the positive electrode 10 can be improved. For example, the adhesive strength between the positive electrode current collector COL1 and the positive electrode active material layer AML1 can be in the range of about 1.5 gf / mm to about 5.0 gf / mm. For example, the flexural strength of the positive electrode 10 can be in the range of about 0.7 N to about 2.0 N. For example, the positive electrode can have a compaction density of about 3.5 g / cc or greater.

[0082] Furthermore, the positive electrode 10 according to the embodiment may also have improved interface resistance. For example, the positive electrode 10 may have an interface resistance of approximately 0.02 Ωcm. 2 To approximately 0.05Ωcm 2 The interface resistance is within a certain range. The rechargeable lithium battery including the positive electrode 10 according to the embodiment can have improved lifetime retention. For example, the rechargeable lithium battery can have a lifetime retention in the range of about 85% or greater.

[0083] For example, the molar ratio of styrene units to acrylate units in the copolymer can be from about 25:75 to about 70:30. For example, the molar ratio can be from about 25:75 to about 40:60. When the above ranges are met, the positive electrode 10 can have improved compaction density, and therefore the rechargeable lithium battery can have improved battery life characteristics.

[0084] In an example embodiment, the copolymer may include repeating units derived from styrene monomers represented by Formula 2 below.

[0085] Formula 2: .

[0086] The acrylate monomer may be or include at least one of the following: for example, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, n-pentyl acrylate, n-hexyl acrylate, n-heptyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, n-nonyl acrylate, n-decyl acrylate, isodecyl acrylate, lauryl acrylate, stearate acrylate, and methacrylate compounds thereof (wherein the substituent attached to the acrylic group is alkyl); for example, 2-methoxyethyl acrylate, 2-ethoxyethyl acrylate, 2-methoxyethoxyethyl acrylate, 2-ethoxyethoxyethyl acrylate, 2-acetoxyethyl acrylate, 2-ethyl... Acyloxyethoxyethyl esters and methacrylate compounds of these compounds (wherein the substituents attached to the acrylic group are alkoxy groups); for example, 1-hydroxymethyl acrylate, 2-hydroxyethyl acrylate, 3-hydroxypropyl acrylate, 2-hydroxyethoxyethyl acrylate, acrylic acid, 1-carboxymethyl acrylate, 2-carboxyethyl acrylate, 3-carboxypropyl acrylate, acrylamide, N,N-dimethylacrylamide, N-hydroxymethylacrylamide, N-butoxymethylacrylamide, 1-aminomethyl acrylate, 2-aminomethyl acrylate, 3-aminopropyl acrylate, acrylonitrile and methacrylate compounds of these compounds (wherein the substituents attached to the acrylic group have a hydroxyl, carboxyl, amide, amino, or nitrile group at the end).

[0087] In the example embodiment, the acrylate monomer may be or include 2-ethylhexyl acrylate represented by Formula 3 below.

[0088] Formula 3: .

[0089] In the example embodiments, the copolymer may also include repeating units derived from acrylic monomers represented by Formula 4 below.

[0090] Formula 4: .

[0091] In an example embodiment, the copolymer may be or include poly[styrene-co-(2-ethylhexyl acrylate)] obtained by polymerizing styrene monomer and 2-ethylhexyl acrylate.

[0092] In an example embodiment, the copolymer may be or include poly[styrene-co-(2-ethylhexyl acrylate)-co-acrylic acid] obtained by polymerizing styrene monomer, 2-ethylhexyl acrylate and acrylic acid.

[0093] Copolymers can be prepared using free radical polymerization, reversible addition-fragmentation chain transfer (RAFT) polymerization, or atom transfer radical polymerization (ATRP). For example, copolymers can be prepared by free radical polymerization using N-methyl-2-pyrrolidone (NMP) as a solvent.

[0094] polyimide adhesive Polyimide binders can include, for example, aromatic polyimides. Aromatic polyimides are polymers composed of aromatic compounds directly linked by imide bonds. Due to the conjugated structure formed by aryl groups linked by imide bonds, aromatic polyimides possess a rigid and stable molecular structure, and also exhibit relatively high levels of thermal, mechanical, and chemical properties due to the strong intermolecular forces of the imide bonds. The desired or improved antioxidant properties of aromatic polyimides can enable rechargeable lithium-ion batteries incorporating aromatic polyimides to have improved lifespan characteristics.

[0095] Aromatic polyimides can be synthesized by polymerizing dianhydrides and diamines. Using dianhydrides and diamines, polyamic acids (PAAs) can be synthesized as precursors for the formation of polyimides. Examples of dianhydrides that can be used include, for example, 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), pyromellitic dianhydride (PMDA), or 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride (DSDA). Examples of diamines that can be used include, for example, 3,3'-diaminodiphenyl sulfone (3,3'-DDSO2), 4,4'-diaminodiphenyl sulfone (4,4'-DDSO2), 4,4'-methylenediphenylamine (4,4'-MDA), 4,4'-oxodiphenylamine (4,4'-ODA), or 1,4-phenylenediamine (p-PDA).

[0096] As an example, polyimide adhesives may include compounds represented by Formula 5 or Formula 6 below.

[0097] Formula 5: .

[0098] Formula 6: .

[0099] The present disclosure is described in more detail below with reference to examples and comparative examples. However, the examples are merely illustrative and the scope of the disclosure is not limited to the examples below.

[0100] Example 1 An acrylic binder and a polyimide binder for use as a positive electrode are prepared. The acrylic binder is a copolymer comprising styrene units and acrylate units, and the polyimide binder includes aromatic polyimides.

[0101] The copolymer can be synthesized through the following process. In a flask, styrene monomers, acrylate monomers, and other monomers (water-soluble monomers) as needed can be mixed, and the mixture can be stirred in an aqueous solution or organic solvent containing sodium hydroxide and / or ammonia, as required. The atmosphere in the flask can be purged with nitrogen, and the mixture can be heated to a temperature of 60°C. When the temperature of the mixture reaches 60°C, a reaction initiator can be added to initiate polymerization. The mixture can be stirred and polymerized to synthesize the copolymer. For example, in the following steps, the copolymer can be used in a solution state.

[0102] For example, as reaction initiators, azo compounds such as 2,2'-azobis(2-methyl-N-2-hydroxyethylpropionamide) or 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropamidinium] tetrahydrate can be used; or persulfates such as ammonium persulfate or potassium persulfate.

[0103] Synthetic acrylic adhesives can be obtained by removing solvents such as water, but aqueous dispersions of acrylic adhesives can also be used as is. For example, acrylic adhesives and polyimide adhesives can be prepared by the following methods.

[0104] Preparation Example 1: Synthesis of Copolymers Styrene and 2-ethylhexyl acrylate as monomers, N-methyl-2-pyrrolidone (NMP) as solvent, and 2,2'-azobis(2-methyl-N-2-hydroxyethylpropionamide) as initiator were placed in a flask and polymerized at 85°C for 4 hours or longer. The amount of styrene was approximately 25 wt% and the amount of 2-ethylhexyl acrylate was approximately 75 wt% relative to the total weight of the monomers. Subsequently, the reaction solution was concentrated by heating and vacuum distillation to remove unreacted monomers, and ammonia and deionized water were added to adjust the solid content of the copolymer solution to 5 wt% and the pH to 7.5, thereby obtaining a poly[styrene-co-(2-ethylhexyl acrylate)] copolymer. The obtained copolymer had a weight-average molecular weight of 900,000 and a molecular weight distribution of 3.1. Furthermore, the obtained 5 wt% copolymer solution had a viscosity of 4200 mPa·s, as measured by a Type B viscometer (25°C, 30 rpm). There are almost no unreacted monomers to remove, so the amount of raw materials used is taken as the amount of units derived from each monomer in the copolymer.

[0105] Preparation Example 2: Synthesis of Aromatic Polyimides 4,4'-O-diphenylamine (4,4'-ODA) was mixed in N,N-dimethylacetamide (DMAc) for 1 hour under a nitrogen atmosphere, and an equimolar amount of pyromellitic dianhydride (PMDA) was added to induce the reaction for 24 hours to form polyamic acid. The reaction was carried out at room temperature. Once a highly viscous yellow solution was formed, N,N-dimethylacetamide (DMAc) was added to dilute the solution to approximately 30 wt%, and then the diluted solution was added dropwise to toluene under vigorous stirring to precipitate the polyamic acid. Subsequently, the resulting filaments were filtered, washed with toluene, and then dried in a vacuum oven at 60 °C for 24 hours. The remaining solvent and byproducts were removed by gradually heating from 100 °C to 200 °C, and the resulting product was then heat-treated at 250 °C to 300 °C to convert the amide bonds to imide bonds, thereby obtaining the compound represented by Formula 5 below.

[0106] Formula 5: .

[0107] Manufacturing of positive electrode A slurry of positive electrode active material was prepared by mixing lithium nickel cobalt aluminum oxide (LiNiCoAlO2, NCA) as the positive electrode active material, poly[styrene-co-(2-ethylhexyl acrylate)] as an acrylic binder, a compound represented by Formula 5 above as a polyimide binder, and Ketjen black as a conductive material in a weight ratio of 91:4:4:1 and dispersing it in N-methyl-2-pyrrolidone (NMP) as a solvent using a PD mixer.

[0108] The prepared positive electrode active material slurry was applied to one side of an aluminum current collector with a thickness of 10 μm using a reverse roller coating machine to achieve 30 mg / cm². 2 Up to 60 mg / cm 2 The loading level (L / L) was determined, and then dried at 110°C. The slurry was pressed using a roller press to prepare the positive electrode.

[0109] Manufacturing of negative electrode A mixture of artificial graphite and silicon nanoparticles in a weight ratio of 93:7 (as the negative electrode active material), styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed in a weight ratio of 97:1:2 and dispersed in distilled water to prepare a negative electrode active material slurry.

[0110] The negative electrode active material slurry was applied to a 10 μm thick copper current collector, dried at 100 °C, and then pressed using a roller press to prepare the negative electrode.

[0111] Manufacturing of rechargeable lithium batteries The positive and negative electrodes were cut into squares of 29mm×29mm and 27mm×27mm, respectively. Nickel and aluminum leads were soldered on, and then a porous polyethylene separator was placed. Nine positive electrode layers and ten negative electrode layers were alternately stacked to prepare an electrode stack. Subsequently, with the leads extended, the electrode stack was stored in an aluminum laminate, injected with an electrolyte solution, and sealed under reduced pressure to manufacture a rechargeable lithium battery. As the electrolyte solution, 1M LiPF6 and 1wt% vinylene carbonate were dissolved in a solvent in which ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 20:40:40. The injection amount was 1.25g. The designed capacity of the manufactured battery was 500mAh.

[0112] Example 2 The rechargeable lithium battery was manufactured in the same manner as in Example 1, except that styrene, 2-ethylhexyl acrylate and acrylic acid were used as monomers, and about 25 wt% of styrene, about 70 wt% of 2-ethylhexyl acrylate and about 5 wt% of acrylic acid were used to synthesize poly[styrene-co-(2-ethylhexyl acrylate)-co-acrylic acid] copolymer as acrylic binder.

[0113] Example 3 The rechargeable lithium battery was manufactured in the same manner as in Example 1, except that about 70 wt% styrene and about 30 wt% 2-ethylhexyl acrylate were used.

[0114] Comparison Example 1 The rechargeable lithium battery is manufactured in the same manner as in Example 1, except that acrylic binder and polyimide binder are not included, polyvinylidene fluoride (PVdF) is used as binder, and the positive electrode active material, polyvinylidene fluoride (PVdF) and conductive material are provided in a weight ratio of 91:8:1.

[0115] Comparison Example 2 The rechargeable lithium battery is manufactured in the same manner as in Example 1, except that a polyimide binder is used instead of an acrylic binder, and the positive electrode active material, the polyimide binder, and the conductive material are provided in a weight ratio of 91:8:1.

[0116] Evaluation Example 1: Evaluation of Bending Strength The bending strength of the positive electrodes according to Examples 1 to 3, and Comparative Examples 1 and 2, was evaluated. The bending strength was measured according to ASTM D790 using the following method. Positive electrode plates prepared according to the examples and comparative examples were cut to dimensions of 15 mm width and 20 mm length to prepare samples. The samples were placed between the first and second points at a 10 mm interval using the three-point bending method, and the center of the sample (the third point) was pressed with a probe at a constant rate in the thickness direction to perform the bending property test. The force applied while moving the probe at a rate of 5 mm / min in the thickness direction was measured. The measurement results are shown in Table 1 below.

[0117] Evaluation Example 2: Evaluation of Heat Resistance Damage The positive electrode plates according to Examples 1 to 3, as well as Comparative Examples 1 and 2, were cut into 50 mm wide and 50 mm long dimensions and placed in ovens heated to 200°C, 230°C, and 250°C. After 10 minutes, the samples were removed from the ovens and the damage was observed. When damaged, the sample was marked with "O"; when undamaged, the sample was marked with "X". The results are shown in Table 1 below.

[0118] Evaluation Example 3: Evaluation of Adhesion Strength The positive electrode plates from Examples 1 to 3, as well as Comparative Examples 1 and 2, were cut to dimensions of 25 mm wide and 100 mm long. This was achieved using adhesive tape (Celotape from Nichiban Corporation). ® Samples for evaluating the adhesion strength of the positive electrode were prepared by side-attaching the positive electrode active material layer to a glass substrate using No. 405 as the adhesive surface. The samples were mounted on a peel tester (Instron 3400 series), and the stress was measured when the current collector was peeled off at a rate of 300 mm / min at a 180° angle. The measurement was performed five times, and the average value was calculated as the peel strength. The adhesion strength (i.e., peel strength) was calculated according to Equation 1 below, and the results are shown in Table 1 below.

[0119] Equation 1: Adhesive strength (gf / mm) = Measured force (gf) / Length of adhesive area (mm).

[0120] Evaluation Example 4: Evaluation of Compacted Density The positive electrode plates according to Examples 1 to 3, as well as Comparative Examples 1 and 2, were cut into 25 mm wide and 25 mm long dimensions to prepare specimens, and the compaction density was measured using a compaction density measuring device (HPRM-1000, Hantech, South Korea) under a force of 2,000 kgf. The results are shown in Table 1 below.

[0121] Table 1:

[0122] Evaluation Example 5: Evaluation of Interface Resistance The interfacial resistance of the positive electrode plates according to Examples 1 to 3, and Comparative Examples 1 and 2, was measured at 25°C using an electrode resistance measurement system (Hioki, RM2610). In the electrode resistance measurement system (Hioki, RM2610), a probe was placed on the positive electrode with the positive electrode active material layer facing the probe. A constant current was passed through the surface of the positive electrode active material layer, and the interfacial resistance between the positive electrode active material layer and the positive electrode current collector was measured. The results are shown in Table 2 below.

[0123] Table 2:

[0124] Evaluation Example 6: Evaluation of the lifespan characteristics of rechargeable lithium batteries The rechargeable lithium batteries manufactured according to Examples 1 to 3, and Comparative Examples 1 and 2, were subjected to 100 charge-discharge cycles under the conditions of "charge at 25°C and 1.0C (CC / CV, 4.5V 0.02C cutoff) / discharge at 1.0C (CC, 3.0V cutoff)", and then the lifetime retention rate was determined. The lifetime retention rate was calculated according to Equation 2 below. The results are shown in Table 3 below.

[0125] Equation 2: Lifetime retention rate (%) = (Discharge capacity after 100 cycles / Initial discharge capacity) × 100.

[0126] Table 3:

[0127] Overall evaluation Refer to Tables 1 and 2 above, and Figure 7 As can be seen, the positive electrode (Example 1 to Example 3) according to the concept of this disclosure is superior to the positive electrode of the comparative example in terms of flexural strength, adhesive strength, compaction density and interfacial resistance.

[0128] Refer to Table 3 above and Figure 8 As can be seen, the rechargeable lithium batteries according to the present disclosure (Examples 1 to 3) exhibit better lifespan characteristics at room temperature compared to the rechargeable lithium batteries of the comparative examples.

[0129] The positive electrode for a rechargeable lithium battery according to the example embodiment can exhibit improved compaction density and enhanced adhesion strength between the positive electrode active material layer and the positive electrode current collector.

[0130] The rechargeable lithium battery according to the example embodiment can exhibit improved lifespan characteristics.

[0131] Although exemplary embodiments of the present disclosure have been described above with reference to the accompanying drawings, the present disclosure may be applied in other specific forms without altering its technical concept or essential characteristics. Therefore, the above exemplary embodiments are to be considered illustrative rather than restrictive in all respects.

Claims

1. A positive electrode for a rechargeable lithium battery, the positive electrode comprising a positive electrode active material layer, in, The positive electrode active material layer comprises: a positive electrode active material; a conductive material; an acrylic binder; and a polyimide binder, and The acrylic binder comprises a copolymer comprising styrene units and acrylate units.

2. The positive electrode for a rechargeable lithium battery according to claim 1, wherein, The polyimide binder comprises an aromatic polyimide.

3. The positive electrode for a rechargeable lithium battery according to claim 1, wherein, The copolymer comprises poly[styrene-co-(2-ethylhexyl acrylate)].

4. The positive electrode for a rechargeable lithium battery according to claim 1, wherein, The copolymer comprises poly[styrene-co-(2-ethylhexyl acrylate)-co-acrylic acid].

5. The positive electrode for a rechargeable lithium battery according to claim 1, wherein, The weight ratio of the acrylic binder to the polyimide binder in the positive electrode active material layer is in the range of 1:9 to 4:

6.

6. The positive electrode for a rechargeable lithium battery according to claim 1, wherein, The molar ratio of the styrene units to the acrylate units in the copolymer is in the range of 25:75 to 70:

30.

7. The positive electrode for a rechargeable lithium battery according to claim 1, wherein, The positive electrode active material layer contains 90 wt% to 99 wt% of the positive electrode active material.

8. The positive electrode for a rechargeable lithium battery according to claim 1, wherein, The positive electrode active material layer contains 0.1 wt% to 5 wt% of the conductive material.

9. The positive electrode for a rechargeable lithium battery according to claim 1, wherein, The positive electrode active material layer contains 0.1 wt% to 5 wt% of the acrylic binder.

10. The positive electrode for a rechargeable lithium battery according to claim 1, wherein, The positive electrode active material layer contains 0.1 wt% to 5 wt% of the polyimide binder.

11. The positive electrode for a rechargeable lithium battery according to claim 1, wherein, The conductive material comprises at least one of the following: Carbonaceous materials, including at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanofiber and carbon nanotube; Metallic materials, including at least one of copper, nickel, aluminum and silver, and having the form of metal powder or metal fiber; Conductive polymers, including polyphenylene derivatives; and Mixtures thereof.

12. The positive electrode for a rechargeable lithium battery according to claim 1, wherein, The positive electrode active material comprises a lithium composite oxide represented by Formula 1: Formula 1: Li x M 1 y M 2 z M 3 1-y-z O 2-a X a x, a, y and z satisfy 0.5 ≤ x ≤ 1.8, 0 ≤ a ≤ 0.05, 0 < y ≤ 1, 0 ≤ z ≤ 1 and 0 < y + z ≤ 1, M 1 M 2 and M 3 Each independently includes at least one of the following: nickel, cobalt, manganese, aluminum, boron, barium, calcium, cerium, chromium, iron, molybdenum, niobium, silicon, strontium, magnesium, titanium, vanadium, tungsten, zirconium, lanthanum, and combinations thereof, and X comprises at least one of fluorine, sulfur, phosphorus and chlorine.

13. The positive electrode for a rechargeable lithium battery according to claim 12, wherein, The positive electrode active material comprises an NCA-type positive electrode active material, the NCA-type positive electrode active material comprising at least one of Ni, Co and Al.

14. The positive electrode for a rechargeable lithium battery according to claim 1, wherein, The positive electrode has a tap density of 3.5 g / cc or greater.

15. The positive electrode for a rechargeable lithium battery according to claim 1, the positive electrode further comprising a positive electrode current collector, in, The adhesion strength between the positive electrode current collector and the positive electrode active material layer is in the range of 1.5 gf / mm to 5.0 gf / mm.

16. The positive electrode for a rechargeable lithium battery according to claim 1, wherein, The bending strength of the positive electrode is in the range of 0.7 N to 2.0 N.

17. A rechargeable lithium battery, the rechargeable lithium battery comprising: The positive electrode according to any one of claims 1 to 16; A negative electrode, comprising a negative electrode active material; And An electrolyte solution.

18. The rechargeable lithium battery according to claim 17, wherein, The negative electrode active material comprises at least one of carbonaceous negative electrode active materials, Si-based negative electrode active materials, Sn-based negative electrode active materials and combinations thereof.

19. The rechargeable lithium battery according to claim 17, wherein, The positive electrode has a strength of 0.02 Ωcm. 2 up to 0.05Ωcm 2 The interface resistance within the specified range.

20. The rechargeable lithium battery according to claim 17, the rechargeable lithium battery having a life retention rate in the range of 85% or greater.

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