Dry process method of manufacturing an electrode for a lithium secondary battery, dry electrode manufactured by the method, and lithium secondary battery including the same

Abstract

This invention discloses a method for manufacturing an electrode for a lithium secondary battery using a conductive material comprising reduced graphene oxide and carbon nanotubes via a dry process, the dry electrode for a lithium secondary battery thereby manufactured, and a lithium secondary battery comprising the dry electrode, wherein the mechanical properties, electrochemical properties, and durability of the electrode for the lithium secondary battery can be improved.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing an electrode for a lithium secondary battery by a dry process, the dry electrode for a lithium secondary battery thereby manufactured, and a lithium secondary battery including the dry electrode, wherein conductive materials including reduced graphene oxide and carbon nanotubes can be used to improve the mechanical properties, electrochemical properties and durability of the electrode for the lithium secondary battery. Background Technology

[0002] A battery is a device constructed to store electrical energy using materials capable of electrochemical reactions at the positive and negative electrodes. A representative example of such a battery is one constructed to store electrical energy generated by the chemical potential difference during the insertion / extraction of lithium ions at the positive and negative electrodes. Lithium-ion rechargeable batteries are used not only in small electronic devices such as mobile phones and laptops, but also in large transportation vehicles such as hybrid and electric vehicles.

[0003] Lithium-ion batteries are typically constructed including a negative current collector, a negative electrode, a dielectric, a separator, a positive electrode, and a positive current collector. Currently, existing electrodes for lithium-ion batteries are manufactured using wet processes. When manufacturing electrodes using wet processes, a high-temperature drying process is required to remove the dispersion medium (e.g., solvent); however, this high-temperature process can damage the electrode active materials. Therefore, methods using dry processes to manufacture electrodes avoid any damage to the electrode active materials associated with the high-temperature drying process.

[0004] Electrodes typically consist of electrode active materials, binders, and conductive materials to improve electrode conductivity. When the conductive material includes point-contact-based zero-dimensional carbon materials (such as carbon black), the carbon particles may aggregate, making it difficult to disperse the particles throughout the electrode. Using three-dimensional carbon materials as conductive materials also presents challenges because such materials have a small specific surface area, making it difficult to achieve sufficient conductivity. This results in low contact density between electrode element materials in the fabricated electrode, leading to non-uniform conductivity. Summary of the Invention

[0005] This invention addresses the problems encountered in the prior art. On the one hand, it provides a method for manufacturing electrodes for lithium secondary batteries using a dry process, which avoids the need for any removal of the dispersion medium and any damage to the electrode active material associated with high-temperature process steps (e.g., removal of the dispersion medium and / or drying).

[0006] On the other hand, the present invention provides a method for manufacturing an electrode for a lithium secondary battery, the electrode comprising a mixture of one-dimensional carbon material and two-dimensional carbon material as a conductive material included in the electrode, and wherein the method can provide an electrode with sufficient specific surface area and good dispersion of the conductive material in the electrode.

[0007] One aspect of the present invention provides a method for manufacturing a dry electrode for a lithium secondary battery, the method comprising the steps of: preparing a conductive material comprising reduced graphene oxide (rGO) and carbon nanotubes; obtaining an electrode composition by mixing the conductive material with an electrode active material and a binder; and manufacturing a dry electrode by applying pressure to the electrode composition.

[0008] In one embodiment, the steps of preparing the conductive material may include: reacting an acid solution, graphite, and an oxidant in a reactor; synthesizing graphene oxide by adding water to the reactor for an additional reaction; and synthesizing reduced graphene oxide by pyrolyzing graphene oxide in a reducing atmosphere.

[0009] In addition, the additional reaction can be carried out under any of the following conditions: less than 1 minute at 0°C to 10°C; 90 to 150 minutes at 65°C to 75°C; and 10 to 20 minutes at 98°C to 100°C.

[0010] In one embodiment, the weight ratio of reduced graphene oxide to carbon nanotubes can be from 0.5:1.5 to 1.5:0.5.

[0011] In one embodiment, the dry electrode may include 0.1 wt% to 5 wt% of reduced graphene oxide, 0.1 wt% to 5 wt% of carbon nanotubes, and 0.1 wt% to 5 wt% of binder.

[0012] In one embodiment, the oxygen content in the reduced graphene oxide can be 0.2 wt% or lower.

[0013] In one embodiment, the carbon nanotubes may have an average length of 0.1 μm to 200 μm, an average diameter of 1 nm to 20 nm, and an oxygen content of 0 wt% to 10 wt%.

[0014] In one embodiment, the method may include attaching a dry electrode to an electrode current collector.

[0015] Another aspect of the present invention provides a dry electrode for a lithium secondary battery, comprising a conductive material, an electrode active material, and a binder, wherein the conductive material comprises reduced graphene oxide (rGO) and carbon nanotubes, and wherein the conductive material, the electrode active material, and the binder are mixed by a dry method.

[0016] Furthermore, the present invention provides a secondary battery including the aforementioned dry electrode.

[0017] Other aspects, embodiments, and objects of the present invention will become apparent and clearly understood from the various combinations and modifications of the following description, drawings, and claims. Attached Figure Description

[0018] Some aspects, embodiments, and features of the present invention will be described in various detailed ways in conjunction with certain exemplary embodiments shown in the accompanying drawings. The drawings provided herein are for illustrative purposes only and are not intended to limit the aspects, embodiments, and features of the invention or to be limited to the claims.

[0019] Figure 1 A flowchart illustrating a process for dry-processing electrodes for lithium secondary batteries according to an exemplary embodiment of the present invention;

[0020] Figure 2 The contact angle analysis results of reduced graphene oxide (e.g., rGO_1, rGO_2 and rGO_3) prepared under different additional reaction conditions according to an exemplary embodiment of the present invention are shown.

[0021] Figure 3 Raman spectra of defects in reduced graphene oxide (e.g., rGO_1, rGO_2, and rGO_3) prepared under different additional reaction conditions according to exemplary embodiments of the present invention are shown.

[0022] Figure 4 To illustrate the lifetime characteristics of a lithium half-cell including the dry electrodes according to Example 1 and Comparative Examples 1 to 3;

[0023] Figure 5 To illustrate the rate characteristics of a lithium half-cell including the dry electrodes according to Example 1 and Comparative Examples 1 to 3;

[0024] Figure 6 The stress-strain curves of the dry electrodes according to Example 1 and Comparative Examples 1 to 3 are shown;

[0025] Figure 7 To illustrate the lifetime characteristics of lithium half-cells including dry electrodes according to Examples 1 to 4 and Comparative Example 2;

[0026] Figure 8 To illustrate the rate characteristics of lithium half-cells including dry electrodes according to Examples 1 to 4 and Comparative Example 2;

[0027] Figure 9 The stress-strain curves of the dry electrodes according to Examples 1 to 3 and Comparative Example 2 are shown;

[0028] Figure 10 To illustrate the lifetime characteristics of lithium half-cells including dry electrodes according to Examples 1, 5 and 6 and Comparative Example 2;

[0029] Figure 11 To illustrate the rate characteristics of lithium half-cells including dry electrodes according to Examples 1, 5 and 6 and Comparative Example 2;

[0030] Figure 12The stress-strain curves of the dry electrodes according to Examples 1, 5 and 6 and Comparative Examples 1 and 2 are shown.

[0031] Figure 13 The cross-section of the dry electrode according to Example 5 and the energy dispersive X-ray spectra of its constituent elements are shown.

[0032] Figure 14 (a) Figure 14 (b) and Figure 14 (c) shows the scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) results of a cross section of the dry electrode according to Example 5. Detailed Implementation

[0033] The above and other objects, features, and advantages of the invention will become more apparent from the following aspects and embodiments, taken in conjunction with the accompanying drawings. However, the invention and the claims are not limited to the embodiments disclosed herein and can be modified into different forms according to the guidance provided herein. Exemplary aspects and embodiments are provided herein to thoroughly explain the various features of the invention and to convey the spirit of the invention to those skilled in the art.

[0034] Throughout the accompanying drawings, the same reference numerals will refer to the same or similar elements. For clarity of the invention, the dimensions of the structures are depicted as larger than their actual dimensions. It will be understood that although terms such as "first," "second," etc., may be used herein to describe various elements, these elements are not limited by these terms. These terms are used only to distinguish one element from another. For example, an element referred to as "first" below may be called "second" without departing from the scope of the invention. Similarly, a "second" element may also be called "first." As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form as well.

[0035] It will also be understood that the terms "comprise (or comprising)," "include (or including)," "have (or having)," etc., as used in this specification, specify the presence of the stated feature, integer, step, operation, element, component, or combination thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or combinations thereof. In some aspects and embodiments, these terms should be understood to encompass the terms "consisting of" and "substantially consisting of," which refer only to features, integers, numbers, steps, operations, elements, components, parts, or combinations thereof that include the stated component, or the stated component may allow a small number of other components or elements that do not materially affect the functionality of the stated feature, component, embodiment, or aspect of the invention. Therefore, some aspects and embodiments may refer to these different transitional terms, all of which constitute a part of this invention.

[0036] Furthermore, it will be understood that when an element, such as a layer, membrane, region, or sheet, refers to being "on" another element, it can be directly on the other element, or an intermediate element can exist in between. Similarly, when an element, such as a layer, membrane, region, or sheet, refers to being "below" another element, it can be directly below the other element, or an intermediate element can exist in between.

[0037] Unless otherwise stated, all figures, values, and / or expressions used herein to represent the amounts of components, reaction conditions, polymer compositions, and mixtures should be considered approximate, including the uncertainties in measurement due to various influences inherent in obtaining these values, and should therefore be understood to be modified by the term "about" in all cases. Furthermore, when this specification discloses numerical ranges, the ranges are continuous and include all values ​​from the minimum to the maximum of the range, unless otherwise stated. Additionally, when such ranges involve integer values, all integers from the minimum to the maximum are included, unless otherwise stated.

[0038] In this specification, when describing the range of a variable, it will be understood that the variable includes all values, including the endpoints described within the range. For example, the range “5” to “10” will be understood to include any subranges such as 6 to 10, 7 to 10, 6 to 9, 7 to 9, etc., as well as individual values ​​of 5, 6, 7, 8, 9, and 10, and will also be understood to include any values ​​between valid integers within the range, such as 5.5, 6.5, 7.5, 5.5 to 8.5, 6.5 to 9, etc. Similarly, the range “10% to 30%” will be understood to include subranges such as 10% to 15%, 12% to 18%, 20% to 30%, etc., as well as all integers, including 10%, 11%, 12%, 13%, etc., up to 30%, and will also be understood to include any values ​​between valid integers within the range, such as 10.5%, 15.5%, 25.5%, etc.

[0039] Furthermore, unless otherwise expressly stated, the term “about” as used or implied herein shall be understood to mean within the range of error typically found in the art (e.g., within 2 standard deviations of the mean). “About” may be understood to mean within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the value.

[0040] Figure 1 A flowchart illustrating a process for dry manufacturing of electrodes for lithium secondary batteries according to an embodiment of the present invention.

[0041] refer to Figure 1 The embodiment shown describes a method for manufacturing electrodes for lithium secondary batteries using a dry process, which may include the following steps: preparing a conductive material comprising reduced graphene oxide (rGO) and carbon nanotubes; obtaining an electrode composition by mixing the conductive material with an electrode active material and a binder; and manufacturing a dry electrode by applying pressure to the electrode composition.

[0042] Various embodiments that can be incorporated into certain steps of the method according to the invention will be described in more detail below.

[0043] Preparation of conductive materials

[0044] In some embodiments of the present invention, the method includes a conductive material comprising carbon nanotubes (i.e., one-dimensional carbon materials) and reduced graphene oxide (rGO) (i.e., two-dimensional carbon materials).

[0045] Carbon nanotubes possess excellent mechanical strength and, due to their linear structure, offer improved electrical conductivity compared to zero-dimensional carbon conductive materials. Therefore, carbon nanotubes hold promise as a next-generation conductive material for lithium-ion batteries. However, carbon nanotubes can aggregate into bundles, and this aggregation is due to their sp... 2This is due to the strong van der Waals bonds generated by π-π interactions in carbon materials. Therefore, when carbon nanotubes are used as the conductive material in a dry process for manufacturing electrodes, they may not be sufficiently dispersed in the electrode and may hinder the movement of lithium ions.

[0046] This invention overcomes the aggregation problem by providing a dry process method. The method includes a mixture of carbon nanotubes and reduced graphene oxide as a conductive material, which makes the conductive material easier to disperse in the electrode and provides sufficient specific surface area.

[0047] In some embodiments, depending on the number of walls, the carbon nanotubes according to the present invention can be classified into single-walled carbon nanotubes, double-walled carbon nanotubes, multi-walled carbon nanotubes, rope-like carbon nanotubes, etc. In some specific embodiments, the method uses single-walled carbon nanotubes (SWCNTs).

[0048] In embodiments, the size and composition of carbon nanotubes can vary within a range known in the art. In some non-limiting embodiments, for example, the average length of the carbon nanotubes can be from 0.1 μm to 200 μm, the average diameter can be from 1 nm to 20 nm, and the oxygen content can be from 0 wt% to 10 wt%.

[0049] In embodiments, the reduced graphene oxide according to the invention can be provided or obtained from commercial sources or prepared using known methods. In one embodiment, reduced graphene oxide (rGO) can be prepared by chemically exfoliating graphite to form graphene oxide (GO) followed by thermal reduction.

[0050] In some further embodiments, the preparation of reduced graphene oxide (rGO) may include the following steps: placing an acid solution, graphite, and an oxidant into a reactor (e.g., mixing, reacting, contacting, etc.) under conditions that allow for the formation of a first reaction product; synthesizing graphene oxide by placing water and the first reaction product into a reactor (e.g., mixing, reacting, contacting, etc.) under conditions that allow for the formation of graphene oxide; and synthesizing reduced graphene oxide by pyrolyzing graphene oxide under a reducing atmosphere.

[0051] In some specific embodiments, an acid solution, such as a strong acid (e.g., concentrated sulfuric acid), is placed in the reactor, and graphite and an oxidizing agent are added to the acid solution. In some embodiments, the oxidizing agent may include sodium nitrate (NaNO3), potassium permanganate (KMnO4), or other known oxidizing agents. In some further embodiments, the oxidizing agent includes potassium permanganate (KMnO4).

[0052] Not limited to the mechanism, but only used to illustrate an exemplary embodiment of the present invention, an oxidant (e.g., potassium permanganate) is added to an acid solution (e.g., sulfuric acid) in a reactor to react and form a reactive oxide (e.g., manganese heptaoxide (Mn2O7)), which can oxidize graphite, resulting in chemical exfoliation.

[0053] In the embodiments, the conditions under which graphite can undergo oxidation are not particularly limited, for example, the following reaction conditions may be included: a temperature and duration ranging from about 35°C to 45°C and about 2 to 24 hours.

[0054] After oxidation and formation of the first reaction product, water can be added (i.e., added, mixed, reacted, or contacted) to the first reaction product in the reactor.

[0055] Not limited to the mechanism, but only used to illustrate an exemplary embodiment of the present invention, after water is added to the reactor, a concentrated strong acid (e.g., sulfuric acid) is converted into a strongly acidic aqueous solution, and the main oxidant inducing the graphite oxidation reaction changes from a reactive oxide (e.g., manganese heptaoxide) to a reagent that oxidizes graphite (e.g., permanganate ions). In the illustrative embodiment, the oxidation reaction of graphite by permanganate ions affects the surface properties of graphene oxide, as well as the degree of defect formation and oxygen content in the crystal structure of reduced graphene oxide prepared from graphene oxide. Therefore, this illustrative embodiment shows that the hydrophilicity and electrical conductivity of reduced graphene oxide can be improved by controlling the conditions of this additional reaction.

[0056] In one embodiment, the additional reaction can be carried out at a variety of temperatures and durations. In some embodiments, the conditions may include: less than 1 minute at a temperature of 0°C to 10°C, 90 to 150 minutes at a temperature of 65°C to 75°C, and / or 10 to 20 minutes at a temperature of 98°C to 100°C. According to these different embodiments (e.g., when the additional oxidation reaction of graphite is carried out for less than 1 minute at 0°C to 10°C, 90 to 150 minutes at 65°C to 75°C, or 10 to 20 minutes at 98°C to 100°C, or various combinations and / or variations thereof), the surface properties of graphene oxide can be controlled, and the hydrophilicity and electrical conductivity of the reduced graphene oxide synthesized from graphene oxide can be improved.

[0057] According to embodiments of the present invention, after graphene oxide is synthesized as described above or in other ways herein, the graphene oxide can be placed in a furnace and pyrolyzed under a reducing atmosphere, under conditions for the synthesis of reduced graphene oxide (rGO). In embodiments, the pyrolysis temperature and time are not particularly limited and can be varied over a wide range. For example, in some non-limiting embodiments, pyrolysis can be carried out at 1,100°C for 1 hour. In some embodiments, a reducing atmosphere can be achieved by introducing a reducing gas atmosphere (e.g., a mixture of nitrogen and hydrogen in an appropriate ratio (e.g., about 9:1)) into the furnace.

[0058] In one embodiment, the oxygen content in the reduced graphene oxide obtained after pyrolysis can be 0.2 wt% or lower. In some embodiments, the contact angle with water can be 150° or lower. In some embodiments, based on the characteristic results of the Raman spectrum of the reduced graphene oxide, the peak intensity of the D band (I... D ) and the peak intensity of the G-band (I G The ratio of (I) D / I G The value can range from 1.65 to 1.80.

[0059] When rGO is characterized by Raman spectroscopy, the G band is typically assigned to appear at approximately 1580 cm⁻¹. -1 The peak at that location is commonly found in sp. 2 In carbon materials where bonded carbon forms a hexagonal lattice as the basic structural unit, such as graphite and carbon nanotubes, the D-band typically appears at approximately 1350 cm⁻¹. -1 The peak at that point is related to defects in the crystal structure, indicating the presence of amorphous carbon with poor crystallinity.

[0060] In one embodiment, the weight ratio of reduced graphene oxide to carbon nanotubes in the conductive material can be from 0.5:1.5 to 1.5:0.5. Without being limited by any underlying mechanism, when the weight ratio of carbon nanotubes (based on a 0.5 weight ratio of reduced graphene oxide) exceeds 1.5, the discharge capacity and rate characteristics of the dry electrode including such conductive materials may decrease. Furthermore, when the weight ratio of carbon nanotubes (based on a 1.5 weight ratio of reduced graphene oxide) is less than 0.5, the discharge capacity, capacity retention, rate characteristics, mechanical properties, etc., of the dry electrode including such conductive materials may decrease.

[0061] Obtaining the electrode composition

[0062] According to various aspects and embodiments of the present invention, an electrode composition can be prepared by mixing a conductive material obtained by the methods described herein with an electrode active material and a binder. The process of mixing the conductive material, the electrode active material, and the binder is not limited to any particular embodiment and generally includes a dry mixing process (i.e., without adding a separate mixing solvent). For example, in a non-limiting embodiment, mixing can be carried out using a mixer such as a small mill, a planetary mixer, a ball mill, a homogenizing mixer, etc.

[0063] In some embodiments, where the mixing process includes a binder capable of forming microfibers (such as PTFE), the electrode composition can be a fibrillated electrode composition.

[0064] In one embodiment, the electrode active material includes a positive electrode active material or a negative electrode active material, depending on whether the electrode manufactured using the electrode composition is a positive electrode or a negative electrode.

[0065] In some embodiments, the positive electrode active material can be an oxide active material or a sulfide active material.

[0066] Non-limiting examples of oxide active materials include rock salt layer-type active materials such as LiCoO2, LiMnO2, LiNiO2, LiVO2, Li 1+x Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 1-(x+y) Co x Mn y O2, etc., spinel-type active materials such as LiMn2O4, Li(Ni 0.5 Mn 1.5 )O4, etc., inverse spinel-type active materials such as LiNiVO4, LiCoVO4, etc., olivine-type active materials such as LiFePO4, LiMnPO4, LiCoPO4, LiNiPO4, etc., silicon-containing active materials such as Li2FeSiO4, Li2MnSiO4, etc., rock salt layer-type active materials in which a part of the transition metal is replaced by a different metal, for example LiNi 0.8 Co (0.2-x) Al x O2 (0 < x < 0.2), spinel-type active materials in which a part of the transition metal is replaced by a different metal, for example Li 1+x Mn 2-x-y M y O4 (where M is at least one of Al, Mg, Co, Fe, Ni, and Zn, 0 < x + y < 2), lithium titanate, for example Li4Ti5O 12 etc.

[0067] In some embodiments, the sulfide active material may be copper Chevrel (e.g., copper-containing molybdenum sulfide), iron sulfide, cobalt sulfide, nickel sulfide, etc.

[0068] In some embodiments, the negative electrode active material may include carbon-based negative electrode active materials, non-carbon-based negative electrode active materials, etc.

[0069] In some further embodiments, the carbon-based anode active material may include graphite, such as medium carbon microspheres (MCMB), highly oriented pyrolytic graphite (HOPG), etc.; and amorphous carbon, such as hard carbon and soft carbon. In some further embodiments, the non-carbon-based anode active material may include metals, metal oxides, etc., including at least one of In, Al, Si, Sn, and combinations thereof.

[0070] Non-limiting examples of adhesives may include butadiene rubber, nitrile rubber, hydrogenated nitrile rubber, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), etc. In some preferred embodiments, the adhesive includes PTFE.

[0071] Manufacturing dry electrodes

[0072] In one embodiment, a dry electrode can be manufactured by applying pressure to the electrode composition obtained above.

[0073] The method of applying pressure to the electrode composition is not particularly limited. For example, a self-supporting dry electrode can be manufactured by heating the electrode composition to a specific temperature and then applying shear force using a device such as a roller mill or other process. Therefore, the force can be applied repeatedly (e.g., calendering process) to obtain the desired self-supporting film thickness.

[0074] The term "dry electrode" as used herein should be understood as an electrode manufactured using a dry process, rather than an electrode that contains no liquid material. Specifically, a dry electrode can be understood as an electrode in which the electrode components are dispersed by simple mechanical mixing without the use of a dispersing solvent during the manufacturing process.

[0075] In one embodiment, the dry electrode may include 0.1 wt% to 5 wt% of reduced graphene oxide and 0.1 wt% to 5 wt% of carbon nanotubes. In some embodiments, the dry electrode may include 0.1 wt% to 5 wt% of a binder. The remainder of the dry electrode may be electrode active material.

[0076] In one embodiment, a dry electrode for a lithium secondary battery may be provided, comprising a conductive material (including reduced graphene oxide (rGO) and carbon nanotubes), an electrode active material, and a binder; wherein the conductive material, the electrode active material, and the binder are mixed in a dry manner.

[0077] In further embodiments, the method of attaching the dry electrode to the electrode current collector is not particularly limited and can be performed by methods known in the art or other methods described herein. For example, in some embodiments, the dry electrode and the electrode current collector can be stacked and then attached to each other by rolling at a specific temperature (e.g., 70°C to 80°C).

[0078] In some embodiments, the electrode current collector can be a positive current collector or a negative current collector, depending on whether the adjacent dry electrode includes a positive active material or a negative active material.

[0079] In some embodiments, the positive current collector may be a conductive plate-like substrate. In some further embodiments, the positive current collector may be provided in the form of a sheet, foil, or film. According to the present invention, the positive current collector may include at least one of indium (In), copper (Cu), magnesium (Mg), aluminum (Al), stainless steel, iron, and / or combinations thereof. In some embodiments, the positive current collector may include aluminum, such as aluminum foil.

[0080] The thickness of the positive electrode current collector is not particularly limited and can vary within a typical range, for example, from 1 μm to 500 μm.

[0081] In some embodiments, the negative electrode current collector may be a conductive plate-like substrate. In some further embodiments, the negative electrode current collector may be provided in the form of a sheet, film, or foil. In embodiments, the negative electrode current collector may include a material that does not react with lithium. In some further embodiments, the negative electrode current collector may include at least one of Ni, Cu, SUS (stainless steel), and / or combinations thereof.

[0082] The thickness of the negative electrode current collector is not particularly limited and can vary within a typical range, for example, from 1 μm to 500 μm.

[0083] The dry electrode manufactured according to the various methods and embodiments described herein is characterized in that it can be manufactured without the use of solvents.

[0084] In one aspect, the present invention provides a secondary battery comprising a dry electrode according to the aspects and embodiments described herein. The secondary battery includes a positive current collector, a positive electrode, a separator, an electrolyte impregnated in the separator, a negative electrode, and a negative current collector, wherein the positive current collector, the positive electrode, the negative electrode, and the negative current collector all conform to the above aspects and embodiments.

[0085] In some embodiments, the separator is used to separate the negative and positive electrodes from each other and to provide a channel through which lithium ions can move. According to these embodiments, any separator commonly used in lithium secondary batteries can be used. In some further embodiments, the separator has low resistance to the movement of ions through the electrolyte and / or excellent electrolyte wetting ability. In some further embodiments, the separator may comprise a porous polymer membrane, such as a porous polymer membrane made of a polyolefin polymer (e.g., ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, ethylene / methacrylate copolymer, etc.), or a stacked structure made of two or more layers thereof. In some embodiments, the separator may comprise a typical porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. In some embodiments, the separator may comprise a coated separator containing a ceramic component or polymer material. In such embodiments, the coated separator provides heat resistance or mechanical strength and may optionally be used in a single-layer or multi-layer structure.

[0086] In the embodiments, the electrolyte used in this invention may include, but is not limited to, organic liquid electrolyte, inorganic liquid electrolyte, solid polymer electrolyte, gel polymer electrolyte, solid inorganic electrolyte, molten inorganic electrolyte, etc., any of which can be used to manufacture lithium secondary batteries.

[0087] In some further embodiments, the electrolyte may include an organic solvent and a lithium salt.

[0088] In the embodiments, any organic solvent may be used, as long as it serves as a medium through which the ions involved in the electrochemical reactions of the battery can move. Some non-limiting examples of organic solvents may include: ester solvents, such as methyl acetate, ethyl acetate, γ-butyrolactone, ε-caprolactone, etc.; ether solvents, such as dibutyl ether or tetrahydrofuran; ketone solvents, such as cyclohexanone, etc.; aromatic hydrocarbon solvents, such as benzene, fluorobenzene, etc.; carbonate solvents, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), etc.; alcohol solvents, such as ethanol, isopropanol, etc.; nitriles, such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, which may include double-bonded aromatic rings or ether bonds), etc.; amides, such as dimethylformamide, etc.; dioxolane compounds, such as 1,3-dioxolane, etc.; and sulfolane. In some embodiments, the organic solvent includes a carbonate solvent; in some further embodiments, the organic solvent may include a mixture of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant, which can improve the charge-discharge performance of the battery, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate). In embodiments including such mixtures, using a mixture of cyclic carbonates and linear carbonates in a volume ratio of about 1:1 to about 1:9 yields a dielectric with excellent performance.

[0089] Any lithium salt comprising a compound capable of providing lithium ions can be used in the lithium secondary battery according to the present invention. Some non-limiting examples of lithium salts may include LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, and LiB(C2O4)2. In embodiments, the concentration of the lithium salt is in the range of 0.1 to 2.0 M, which generally provides an electrolyte with suitable conductivity and viscosity, exhibiting excellent electrolyte performance and enabling efficient movement of lithium ions.

[0090] A better understanding of the invention can be obtained through the following illustrative embodiments and comparative examples. However, these embodiments should not be considered as limiting the spirit of the invention.

[0091] Preparation Example 1—rGO-1

[0092] 138 ml of sulfuric acid, 18 g of potassium permanganate, and 3 g of graphite were placed in a reactor and oxidized at 35 °C for 2 hours. The oxidation reaction was terminated by adding a mixture of 276 ml of distilled water cooled to 0 °C and 30 ml of 30% hydrogen peroxide solution, thus synthesizing graphene oxide.

[0093] Subsequently, the synthesized graphene oxide was placed in a furnace and heated to 1100°C, and then pyrolyzed for 1 hour in a reducing atmosphere of nitrogen and hydrogen (ratio 90:10) to obtain the reduced graphene oxide (rGO-1) according to Preparation Example 1 of this invention.

[0094] Preparation Example 2—rGO-2

[0095] Reduced graphene oxide (rGO-2) according to Preparation Example 2 was prepared in the same manner as in Preparation Example 1, except that after an oxidation reaction was carried out at 35°C for 2 hours, distilled water and hydrogen peroxide were added to a reactor, and the mixture in the reactor was then heated to 70°C and stirred for about 2 hours to carry out an additional reaction to synthesize graphene oxide.

[0096] Preparation Example 3—rGO-3

[0097] The reduced graphene oxide (rGO-3) of Preparation Example 3 was prepared in the same manner as in Preparation Example 1, except that after an oxidation reaction was carried out at 35°C for 2 hours, distilled water and hydrogen peroxide were added to a reactor, and the mixture in the reactor was then heated to 98°C and stirred for about 15 minutes to carry out an additional reaction to synthesize graphene oxide.

[0098] Test Example 1

[0099] To investigate the properties of the synthesized reduced graphene oxide, the water contact angle of rGO prepared according to Examples 1 to 3 was measured, and the results are shown in... Figure 2 Furthermore, the elemental composition of the synthesized rGO was analyzed using XPS (X-ray photoelectron spectroscopy), and the results are shown in Table 1 below. Additionally, Raman spectroscopy was used to analyze defects in the crystal structure of the synthesized rGO, and the results are shown in... Figure 3 .

[0100] refer to Figure 2 The contact angles observed for rGO_1 were 146°, rGO_2 was 141°, and rGO_3 was 139°. That is, the rGO prepared in Example 3 had the smallest contact angle, while the rGO prepared in Example 1 had the largest. Therefore, it is predicted that the rGO prepared in Example 2 has a better affinity for the electrode active material than that prepared in Example 1, and the rGO prepared in Example 3 has a better affinity for the electrode active material than that prepared in Example 2. This indicates that all prepared rGOs exhibit good dispersibility in the electrode.

[0101] [Table 1]

[0102] sample Carbon (wt%) Hydrogen (wt%) Oxygen (wt%) rGO_1 98.2 0.5 0 rGO_2 98.4 0.4 0 rGO_3 98.2 0.4 0.2

[0103] Referring to Table 1, the oxygen content of rGO prepared in each of Preparation Examples 1 and 2 is lower than that of rGO prepared in Preparation Example 3. Generally, conductivity decreases with increasing oxygen content in rGO. Therefore, considering the oxygen content, the conductivity of rGO prepared in each of Preparation Examples 1 and 2 is determined to be higher than that of Preparation Example 3.

[0104] In addition, refer to Figure 3 The I of rGO_1 was measured. D / I G The value is 1.75 ± 0.09, I of rGO_2 D / I G The value is 1.73 ± 0.07, I of rGO_3 D / I G The value was 1.68 ± 0.09. Based on the preparation examples 1 to 3, the I... D / I G The values ​​are similar and within the error range. Generally, conductivity decreases with increasing number of defects in the crystal structure. Therefore, from the perspective of defects, the conductivity of rGO prepared in Examples 1 to 3 is determined to be similar.

[0105] Example

[0106] Prepare the following for later use: LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811) was used as the positive electrode active material; PTFE was used as the binder; single-walled carbon nanotubes (SWCNTs), rGO prepared according to Preparation Examples 1 to 3, and Super-P were used as conductive materials. The positive electrode active material, binder, and conductive material were mixed to prepare the example compositions shown in Table 2 below, and 10g of electrode composition was obtained. Pressure was then applied to it to manufacture the positive electrode.

[0107] [Table 2]

[0108]

[0109] Test Example 2—Example 1 and Comparative Examples 1 to 3

[0110] To test the performance of the electrodes manufactured according to the manufacturing method of the present invention, cell batteries were manufactured using the electrodes of the examples shown in Table 2, and the life characteristics of their lithium half-cells were evaluated.

[0111] Specifically, using the electrodes of each of Examples 1 and Comparative Examples 1 to 3 as positive electrodes and lithium metal as the counter electrode, a CR2032 button-type lithium half-cell was assembled.

[0112] Subsequently, 50 charge-discharge cycles were performed at 25°C at a rate of 0.33C (1C = 200 mAh / g) to compare its lifetime characteristics. The results are shown in Table 3 below. Figure 4 . Figure 4 This is a graph showing the lifetime characteristics of a lithium half-cell including the dry electrodes according to Example 1 and Comparative Examples 1 to 3.

[0113] [Table 3]

[0114]

[0115] refer to Figure 4 As shown in Table 3, the mixture of rGO and SWCNT in a 1:1 ratio exhibits superior capacity and lifetime characteristics compared to using conductive materials such as Super-P, SWCNT, and rGO-1 alone. In particular, considering the very poor performance of rGO alone (Comparative Example 3), the improvement in dry electrode performance appears to be due to the excellent conductivity of SWCNT. In Example 1, even though the amount of expensive SWCNT was reduced by half compared to Comparative Example 2, the performance was still actually improved, which is likely due to the introduction of the relatively inexpensive rGO-1.

[0116] Next, in order to determine the rate characteristics of the button cell manufactured in the same manner as described above, charging and discharging were performed while changing the rate. Figure 5 The rate characteristics of lithium half-cells using dry electrodes according to Example 1 and Comparative Examples 1 to 3 are shown in Table 4. The results are summarized and presented in Table 4.

[0117] [Table 4]

[0118]

[0119] refer to Figure 5 As shown in Table 4, compared with Comparative Examples 1 to 3 (where conductive materials such as Super-P, SWCNT, and rGO-1 were used alone without mixing), Example 1 (where rGO-1 and SWCNT were used in a 1:1 mixture) exhibited superior rate performance in all 1C discharge capacity, 0.1C discharge capacity, and 1C / 0.1C rate.

[0120] Next, in order to confirm the variation of the mechanical properties of the electrode samples manufactured according to the examples and comparative examples with the type of conductive material, the tensile strength of the dry electrodes according to Example 1 and Comparative Examples 1 to 3 was measured.

[0121] Specifically, the tensile strength of the dry electrode samples according to Example 1 and Comparative Examples 1 to 3 was measured using a universal testing machine (UTM) according to ASTM D638, and the results are shown in... Figure 6 and Table 5 below. Figure 6 The stress-strain curves (as a measure of ductility) of the dry electrodes according to Example 1 and Comparative Examples 1 to 3 are shown.

[0122] [Table 5]

[0123]

[0124]

[0125] refer to Figure 6 As shown in Table 5, in Comparative Example 1 using Super-P as the conductive material, both tensile strength and ductility were very poor. Therefore, it was confirmed that SWCNT has the effect of improving electrode tensile strength, and rGO has the effect of improving electrode ductility.

[0126] Test Examples 3—Examples 1 to 4 and Comparative Examples 1 and 2

[0127] Using the electrodes of each of Examples 1 to 4 and Comparative Examples 1 and 2 as the positive electrode and lithium metal as the counter electrode, the CR2032 button-type lithium half-cell was assembled in the same manner as in Test Example 2.

[0128] The lifetime characteristics were compared by performing 50 charge-discharge cycles at 25°C and a rate of 0.33C (1C = 200 mAh / g). The results are shown in Table 6 below. Figure 7 . Figure 7 This is a graph showing the lifetime characteristics of lithium half-cells using the dry electrodes of Examples 1 to 4 and Comparative Example 2.

[0129] [Table 6]

[0130]

[0131] refer to Figure 7 As shown in Table 6, when the mixing ratio of SWCNT and rGO was adjusted, the dry electrode using the mixture of SWCNT and rGO exhibited improved charge / discharge capacity in all cases compared to the electrode using SWCNT alone. In particular, in Example 3, improved performance was still observed even though the SWCNT content was reduced by 75% compared to Comparative Example 2.

[0132] The results show that using a mixture of SWCNT and rGO as a conductive material can improve performance while reducing the total amount of SWCNT.

[0133] Next, in order to determine the rate characteristics of the button cell manufactured in the same manner as described above, charging and discharging were performed while changing the rate. Figure 8 The following is a rate characteristic diagram of lithium half-cells using dry electrodes according to Examples 1 to 4 and Comparative Example 2. The results are summarized and shown in Table 7 below.

[0134] [Table 7]

[0135]

[0136] refer to Figure 8 As shown in Table 7, when the mixing ratio of SWCNT and rGO was adjusted, the dry electrode using the mixture of SWCNT and rGO exhibited improved lifetime characteristics in all cases compared to the electrode using SWCNT alone. Furthermore, for rate performance, the highest performance was observed when the ratio of rGO to SWCNT was 1:1. In particular, the rate performance was further improved when the total content of rGO and SWCNT was reduced to 1% (Example 4). This indicates that both the mixing ratio and the total content of rGO and SWCNT have an impact on rate performance.

[0137] Next, to confirm the variation of the mechanical properties of the electrode samples manufactured according to the examples and comparative examples with the type of conductive material, the tensile strength of the dry electrodes of Examples 1 to 3 and Comparative Examples 1 and 2 was measured in the same manner as in Test Example 2. The results are shown in Figure 9 and Table 8 below. Figure 9 The stress-strain curves of the dry electrodes according to Examples 1 to 3 and Comparative Examples 1 and 2 are shown.

[0138] [Table 8]

[0139] Grouping Tensile strength (MPa) Example 1 <![CDATA[75.3x 10 -2 ]]> Example 2 <![CDATA[46.0x 10 -2 ]]> Example 3 <![CDATA[83.4x 10 -2 ]]> Comparative Example 1 <![CDATA[9.3x 10 -2 ]]> Comparative Example 2 <![CDATA[76.8x 10 -2 ]]>

[0140] refer to Figure 9 As shown in Table 8, in Comparative Example 1 using Super-P as the conductive material, both tensile strength and ductility were very poor. On the other hand, Comparative Example 2 using SWCNT was confirmed to have excellent tensile strength. Therefore, the data indicate that the presence of SWCNT has the effect of improving the electrode tensile strength, and the presence of rGO has the effect of improving the electrode ductility.

[0141] Test Examples 4—Examples 1, 5 and 6 and Comparative Examples 1 and 2

[0142] Using the electrodes of each of Examples 1, 5 and 6 and Comparative Examples 1 and 2 as the positive electrode and lithium metal as the counter electrode, the CR2032 button-type lithium half-cell was assembled in the same manner as in Test Example 2.

[0143] The lifetime characteristics were compared by performing 50 charge-discharge cycles at 25°C and a rate of 0.33C (1C = 200 mAh / g). The results are shown in Table 9 below. Figure 10 . Figure 10 This is a graph showing the lifetime characteristics of lithium half-cells using dry electrodes according to Examples 1, 5 and 6 and Comparative Example 2.

[0144] [Table 9]

[0145]

[0146] refer to Figure 10 As shown in Table 9, there were no significant differences in charge / discharge capacity and lifetime characteristics among Examples 1, 5, and 6 (which were doped with one of rGO-1, rGO-2, or rGO-3, the characteristics of which were controlled by changing the additional reaction conditions), and the absolute capacity was slightly improved compared to Comparative Example 2, which used SWCNT alone as the conductive material.

[0147] Next, in order to determine the rate characteristics of the button cell manufactured in the same manner as described above, charging and discharging were performed while changing the rate. Figure 11 The following is a rate characteristic diagram of lithium half-cells using dry electrodes according to Examples 1, 5 and 6 and Comparative Example 2. The results are summarized and shown in Table 10 below.

[0148] [Table 10]

[0149]

[0150] refer to Figure 11 As shown in Table 10, the rate performance of rGO synthesized under different conditions varies considerably. Specifically, rGO-2 exhibits improved rate performance compared to dry electrodes prepared using rGO-1 and rGO-3. This is consistent with the prediction based on observations in Test Example 1, indicating that rGO-2 and rGO-3 have enhanced hydrophilicity compared to rGO-1, and that their electrical conductivity will be higher due to the lower content of oxygen-containing functional groups in rGO-2 compared to rGO-3. The data appear to suggest that improvements, even approaching optimal values, have been achieved in both hydrophilicity and electrical conductivity, thus paving the way for improved dispersion of conductive materials and enhanced battery performance.

[0151] Next, to confirm the variation of the mechanical properties of the electrode samples manufactured according to the examples and comparative examples with the type of conductive material, the tensile strength of the dry electrodes according to Examples 1, 5, and 6, and Comparative Examples 1 and 2, was measured in the same manner as in Test Example 2. The results are shown in... Figure 12 and Table 11 below. Figure 12 The stress-strain curves of the dry electrodes according to Examples 1, 5 and 6 and Comparative Examples 1 and 2 are shown.

[0152] [Table 11]

[0153] Grouping Tensile strength (MPa) Example 1 <![CDATA[75.3x 10 -2 ]]> Example 5 <![CDATA[35.7x 10 -2 ]]> Example 6 <![CDATA[48.4x 10 -2 ]]> Comparative Example 1 <![CDATA[9.3x 10 -2 ]]> Comparative Example 2 <![CDATA[76.8x 10 -2 ]]>

[0154] refer to Figure 12 As shown in Table 11, the tensile strength increases or decreases with the type of rGO, but it is still superior to conventional Super-P based dry electrodes.

[0155] Test Example 5

[0156] To confirm the distribution of the conductive material in the dry electrode, the dry electrode according to Example 5 was analyzed using energy-dispersive X-ray spectroscopy, and the results are shown in... Figure 13 .

[0157] refer to Figure 13 It was confirmed that the conductive material was uniformly distributed across the entire surface of the active material in the dry electrode of Example 5. When using a one-dimensional carbon material like SWCNT alone as the conductive material, agglomeration may occur, making it difficult to disperse in the electrode. However, in this invention, rGO minimizes agglomeration, confirming a uniform distribution of the conductive material on the surface of the active material.

[0158] Here, Ni-K can refer to the Ni element contained in the positive electrode active material, CK can refer to the C element present in the electrode material, and FK can represent the F element contained in the binder.

[0159] Furthermore, the cross-section of the dry electrode according to Example 5 was analyzed using scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), as shown in... Figure 14 (b) is shown in the figure. For reference, to facilitate analysis of the cross-section of the dry electrode according to Example 5, Figure 14 (a) and Figure 14 (c) shows the SEM-EDS results for the respective shapes of rGO-2 and SWCNT.

[0160] refer to Figure 14 In (b) of this paper, fibrous PTFE binder and SWCNTs were observed across the entire cross-section of the dry electrode according to Example 5, but no shape similar to rGO was observed. This indicates that rGO has superior dispersibility compared to SWCNTs, meaning that rGO is uniformly distributed in the electrode without agglomeration. This uniform distribution can mitigate agglomeration between SWCNTs and promote additional conductive paths.

[0161] As can be seen from the above data, the present invention provides a method for manufacturing a dry electrode for a lithium secondary battery. The method includes the following steps: dry mixing a conductive material, an electrode active material, and a binder without using a separate dispersion medium to obtain an electrode composition; and then manufacturing an electrode for a lithium secondary battery by applying pressure to the electrode composition. This method avoids the process step of removing the dispersion medium and prevents damage to the electrode active material caused by any high-temperature drying process.

[0162] Furthermore, using a mixture of one-dimensional carbon materials such as carbon nanotubes and two-dimensional carbon materials such as reduced graphene oxide (rGO) as conductive materials in the electrode enables the fabrication of electrodes for lithium secondary batteries with improved dispersion of conductive materials in the electrode, ease of fabrication, and sufficient specific surface area.

[0163] Furthermore, when graphene oxide is synthesized by introducing water into a reactor and carrying out additional reactions, the hydrophilicity and electrical conductivity of the reduced graphene oxide can be improved by controlling the additional reaction conditions.

[0164] The effects and advantages associated with this invention are not limited to those described above. It should be understood that the effects and advantages of this invention include all effects and advantages that can be derived or inferred from the description of this invention.

[0165] As described above, those skilled in the art should understand that various modifications and alterations can be made by changing, deleting, or adding components without departing from the scope and spirit of the invention as claimed, and such modifications and alterations should also be considered to be included within the scope of the invention.

Claims

1. A method for manufacturing a dry electrode for a lithium secondary battery, the method comprising the following steps: Prepare conductive materials comprising reduced graphene oxide (rGO) and carbon nanotubes; The conductive material is mixed with an electrode active material and a binder to obtain an electrode composition; as well as The dry electrode is manufactured by applying pressure to the electrode composition.

2. The method according to claim 1, wherein the step of preparing the conductive material comprises the following steps: The acid solution, graphite, and oxidant react in the reactor; Graphene oxide is synthesized by adding water to the reactor for an additional reaction; and Reduced graphene oxide is synthesized by pyrolyzing the graphene oxide under a reducing atmosphere.

3. The method of claim 2, wherein the additional reaction is carried out under any of the following conditions: Perform the test at 0℃ to 10℃ for less than 1 minute; Exposed to 65°C to 75°C for 90 to 150 minutes; and Perform at 98℃ to 100℃ for 10 to 20 minutes.

4. The method according to claim 1, wherein the weight ratio of the reduced graphene oxide to the carbon nanotubes is from 0.5:1.5 to 1.5:0.

5.

5. The method of claim 1, wherein the dry electrode comprises 0.1 wt% to 5 wt% of the reduced graphene oxide, 0.1 wt% to 5 wt% of the carbon nanotubes, and 0.1 wt% to 5 wt% of the binder.

6. The method according to claim 1, wherein the oxygen content in the reduced graphene oxide is 0.2 wt% or less.

7. The method according to claim 1, wherein the carbon nanotubes have an average length of 0.1 μm to 200 μm, an average diameter of 1 nm to 20 nm, and an oxygen content of 0 wt% to 10 wt%.

8. The method of claim 1, further comprising the step of: The dry electrode is attached to the electrode current collector.

9. A dry electrode for a lithium secondary battery, the dry electrode comprising: Conductive materials including reduced graphene oxide (rGO) and carbon nanotubes; Electrode active materials; as well as Adhesive, The conductive material, the electrode active material, and the binder are mixed using a dry method.

10. The dry electrode according to claim 9, comprising the reduced graphene oxide and the carbon nanotubes in a weight ratio ranging from 0.5:1.5 to 1.5:0.

5.

11. The dry electrode of claim 9, wherein the dry electrode comprises 0.1 wt% to 5 wt% of the reduced graphene oxide, 0.1 wt% to 5 wt% of the carbon nanotubes, and 0.1 wt% to 5 wt% of the binder.

12. The dry electrode according to claim 9, wherein the oxygen content in the reduced graphene oxide is 0.2 wt% or less.

13. The dry electrode according to claim 9, wherein the carbon nanotubes have an average length of 0.1 μm to 200 μm, an average diameter of 1 nm to 20 nm, and an oxygen content of 0 wt% to 10 wt%.

14. A lithium secondary battery comprising the dry electrode according to claim 9.

15. A lithium secondary battery comprising a dry electrode prepared according to claim 1.

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