Positive electrode for lithium-sulfur battery and lithium-sulfur battery comprising the same

Abstract

The present invention relates to a positive electrode for a lithium-sulfur battery comprising a sulfur-carbon composite having a plurality of island-shaped carbon coatings on the surface thereof, and a lithium-sulfur battery comprising the same. The positive electrode for a lithium-sulfur battery according to the present invention has excellent electrochemical reactivity, which results in a lithium-sulfur battery comprising the same having high capacity, high output, and long cycle life.

Description

Technical Field

[0001] This application claims the benefit of Korean Patent Application No. 10-2021-0090243, filed with the Korean Intellectual Property Office on July 9, 2021, and Korean Patent Application No. 10-2022-0083460, filed with the Korean Intellectual Property Office on July 7, 2022, the disclosure of which is incorporated herein by reference in its entirety.

[0002] This invention relates to a positive electrode for lithium-sulfur batteries and a lithium-sulfur battery containing the same. Background Technology

[0003] As the application of lithium-ion batteries expands not only to portable electronic devices and communication devices, but also to electric vehicles (EVs) and energy storage systems (ESS), the demand for high-capacity lithium-ion batteries used as their power source has also increased.

[0004] Among various lithium secondary batteries, lithium-sulfur batteries are battery systems that use sulfur-containing sulfur-sulfur bonds as positive electrode active materials and lithium metal, carbon materials with lithium ion intercalation / deintercalation, or silicon, tin, etc., which are alloyed with lithium, as negative electrode active materials.

[0005] Sulfur, as the main material of the positive electrode active material in lithium-sulfur batteries, has the advantages of low atomic weight, easy supply due to abundant resources, low price, non-toxicity, and environmental friendliness.

[0006] Furthermore, lithium-sulfur batteries exhibit a conversion reaction between lithium ions and sulfur in the positive electrode (S₈ + 16Li₂). + +16e - →8Li₂S) achieved a theoretical discharge capacity as high as 1,675 mAh / g, and when using lithium metal (theoretical capacity: 3,860 mAh / g) as the negative electrode, the theoretical energy density was 2,600 Wh / kg. This is a very high value compared to the theoretical energy density of other battery systems currently under study (Ni-MH battery: 450 Wh / kg, Li-FeS battery: 480 Wh / kg, Li-MnO₂ battery: 1,000 Wh / kg, Na-S battery: 800 Wh / kg) and the theoretical energy density of lithium-ion batteries (250 Wh / kg). Therefore, among the rechargeable batteries developed to date, lithium-sulfur batteries have attracted much attention as high-capacity, environmentally friendly, and low-cost lithium rechargeable batteries, and have been extensively studied as a next-generation battery system.

[0007] In lithium-sulfur batteries, sulfur, used as the positive electrode active material, has an electrical conductivity of 5 × 10⁻⁶. -30Sulfur is a non-conductive, non-electric material with a sulfur-to-cm ratio, thus presenting a problem of difficulty in the migration of electrons generated by electrochemical reactions. Therefore, sulfur is combined with carbon materials that can provide electrochemical reaction sites and used as a sulfur-carbon composite.

[0008] However, in addition to the lithium polysulfides (Li2S) generated during the charging and discharging process of lithium-sulfur batteries, x In addition to the dissolution problem of x = 8, 6, 4, 2), there is also the problem that the electrochemical reactivity of the sulfur-carbon composite as the positive electrode active material is reduced due to the low conductivity of sulfur as the positive electrode active material and lithium sulfide (Li2S) as its discharge product.

[0009] Therefore, despite having a high initial discharge capacity during actual operation, lithium-sulfur batteries have not yet been commercialized because their capacity and charge / discharge efficiency performance rapidly decline with cycling, and their energy density and cycle life characteristics also decrease.

[0010] To achieve lithium-sulfur batteries with commercially viable levels of energy density and cycle life, various techniques have been proposed to improve the electrochemical reactivity of sulfur-carbon composites as positive electrodes, specifically as positive electrode active materials.

[0011] As an example, Korean Patent Publication No. 2016-0046775 discloses that by providing a positive electrode coating formed of an amphiphilic polymer on a portion of the surface of the positive electrode active site containing a sulfur-carbon composite, lithium polysulfide dissolution is suppressed, thereby improving the cycle performance of lithium-sulfur batteries.

[0012] The coating provided in this patent improves the problem of lithium polysulfide dissolution to some extent; however, its effect is insufficient in maintaining the electrochemical reactivity of the cathode. Furthermore, the complex manufacturing process and expensive materials used hinder commercialization. Therefore, there remains a need to develop lithium-sulfur batteries that can achieve excellent performance and operational stability by improving the electrochemical reactivity of the cathode through a simple process.

[0013] [Existing Technical Documents]

[0014] [Patent Literature]

[0015] Korean Patent Publication No. 2016-0046775 (April 29, 2016): Cathode for lithium-sulfur battery and method of preparing the same. Summary of the Invention

[0016] Technical issues

[0017] As a result of various studies to solve the above-mentioned problems, the inventors of the present invention have confirmed that when an island-like carbon coating is introduced onto the surface of a sulfur-carbon composite, the number of electrochemical reaction sites increases, thus exhibiting improved electrochemical reactivity, and thus the present invention is complete.

[0018] Therefore, one object of the present invention is to provide a positive electrode for lithium-sulfur batteries with excellent electrochemical reactivity.

[0019] Furthermore, another object of the present invention is to provide a lithium-sulfur battery containing the positive electrode.

[0020] Technical solution

[0021] To achieve the above objectives, the present invention provides a positive electrode for lithium-sulfur batteries, comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises a sulfur-carbon composite as a positive electrode active material, and the sulfur-carbon composite has a plurality of island-shaped carbon coatings on its surface.

[0022] Furthermore, the present invention provides a lithium-sulfur battery comprising a positive electrode for the lithium-sulfur battery.

[0023] Beneficial effects

[0024] The lithium-sulfur battery cathode of the present invention comprises a sulfur-carbon composite as the cathode active material. The sulfur-carbon composite contains multiple carbon coatings distributed in a discontinuous island pattern on its surface, thereby increasing the electrochemical reaction sites and thus exhibiting excellent electrochemical reactivity of the cathode. Therefore, the capacity expression of the cathode can be maximized, thus enabling the realization of a lithium-sulfur battery with high capacity and high energy density. Attached Figure Description

[0025] Figure 1 This is a diagram illustrating, by way of example, the composition of the positive electrode active material layer according to an embodiment of the present invention.

[0026] Figure 2 The image is a scanning electron microscope image of the positive electrode according to Example 3.

[0027] Figure 3 The image is a scanning electron microscope image of the positive electrode according to Example 6.

[0028] Figure 4 It is based on the scanning electron microscope image of the positive electrode of Comparative Example 2.

[0029] Figure 5 and Figure 6 This is a graph showing the evaluation results of the capacity characteristics of the lithium-sulfur battery according to Experimental Example 3.

[0030] Figure 7 and Figure 8 This is a graph showing the evaluation results of the cycle life characteristics of the lithium-sulfur battery according to Experimental Example 3. Detailed Implementation

[0031] The invention will be described in more detail below.

[0032] The terms and words used in this specification and claims should not be construed as limited to their common or dictionary meanings, but should be interpreted as having meanings and concepts consistent with the technical idea of ​​the invention, based on the principle that the inventor is able to properly define the concepts of the terms in order to describe his invention in the best possible way.

[0033] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. Unless the context clearly indicates otherwise, the singular form includes the plural indicator. It should be understood that the terms "comprising" or "having" as used herein are intended to explicitly state the presence of the features, figures, steps, operations, components, portions or combinations thereof described herein, but do not preclude the possibility of the presence or addition of one or more other features, figures, steps, operations, components, portions or combinations thereof.

[0034] The term "complex" as used in this article refers to a material that combines two or more materials to exhibit more effective functions while forming phases that are physically and chemically different from each other.

[0035] The term "polysulfide" as used in this specification includes "polysulfide (S) ions". x 2- "x = 8, 6, 4, 2)" and "lithium polysulfide (Li2S)" x or LiS x - The concepts of "x = 8, 6, 4, 2" and "x = 8, 6, 4, 2" are discussed.

[0036] The term “average volume diameter (MV)” as used in this specification refers to the arithmetic particle size measured by volume, and “average particle size (D)” refers to the particle size measured by volume. 10 "Average particle size (D)" can be defined as the particle size of the 10% of the volumetric cumulative particle size distribution that can be measured using laser diffraction. 50 ")" can be defined as the particle size based on 50% of the volumetric cumulative particle size distribution that can be measured using laser diffraction.

[0037] Lithium-sulfur batteries have attracted much attention as the next generation of rechargeable batteries because, in addition to having high discharge capacity and theoretical energy density, sulfur, which is used as a positive electrode active material, also has the advantages of being abundant and inexpensive, reducing battery manufacturing costs, and being environmentally friendly.

[0038] Sulfur, as the positive electrode active material in lithium-sulfur batteries, is a non-conductor and is used in the form of a sulfur-carbon composite obtained by combining it with carbon materials, which are conductive materials, to supplement conductivity.

[0039] However, in lithium-sulfur batteries, the dissolution of lithium polysulfides generated during charge and discharge leads to the loss of sulfur participating in the electrochemical reaction. Furthermore, because lithium sulfide, as a sulfur reducing material, has very low conductivity, the electrochemical reactivity of the sulfur-carbon composite, serving as the positive electrode active material, decreases. Consequently, neither the theoretical discharge capacity nor the theoretical energy density is fully achieved in actual operation, and the degradation of battery performance and operational stability is accelerated. In addition, the conversion of sulfur to lithium sulfide causes approximately 80% volume expansion, reducing the pore volume inside the positive electrode and making contact with the electrolyte difficult, further reducing the electrochemical reactivity of the positive electrode active material.

[0040] To address this, existing technologies have proposed methods such as increasing the sulfur loading, using carbon materials with high specific surface area as sulfur carriers, or introducing coatings to suppress the dissolution of lithium polysulfides. However, the performance of lithium-sulfur batteries has not been effectively improved, and there are still serious problems that cause battery stability issues or low process efficiency.

[0041] Therefore, the present invention provides a positive electrode for lithium-sulfur batteries, which can increase the reaction sites in which electrochemical oxidation and reduction reactions can occur by forming an island-shaped discontinuous carbon coating on the surface of a sulfur-carbon composite with a carbon material with a high specific surface area, thereby improving the electrochemical reactivity of the positive electrode and realizing a lithium-sulfur battery with improved capacity, output and cycle life characteristics.

[0042] Specifically, the positive electrode for a lithium-sulfur battery according to the present invention comprises a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, and the positive electrode active material layer comprises a sulfur-carbon composite as the positive electrode active material, the sulfur-carbon composite having a plurality of island-shaped carbon coatings on its surface. In this case, "island-shaped" refers to the form in which the carbon coatings are discontinuously separated on the surface of the sulfur-carbon composite and distributed like islands, and the individual island-shaped carbon coatings are not particularly limited in shape and can have various shapes, such as spherical, cylindrical, polygonal, hemispherical, elliptical and irregular shapes.

[0043] Figure 1 The composition of the positive electrode active material layer according to one embodiment of the present invention is illustrated by way of example. Figure 1As shown, the positive electrode active material contained in the positive electrode active material layer of the present invention has a form in which discontinuous island-shaped carbon coatings 13 are distributed on the surface of the sulfur-carbon composite 11. The present invention comprises a sulfur-carbon composite having a carbon coating distributed in discontinuous islands on the surface, obtained by mixing a carbon material with a high specific surface area in powder state with a sulfur-carbon composite and dispersing it in a dispersion medium without using a separate dispersant, as the positive electrode, specifically the positive electrode active material in the positive electrode active material layer. Therefore, the positive electrode according to the present invention has a positive electrode active material with a high specific surface area contained in the positive electrode active material layer, thereby increasing the reaction sites in which electrochemical oxidation and reduction reactions can occur, thus improving the electrochemical reactivity of the positive electrode, thereby realizing a lithium-sulfur battery with improved capacity, output, and cycle life characteristics.

[0044] In this invention, the positive electrode may include a positive electrode current collector and a layer of positive electrode active material formed on one or both surfaces of the positive electrode current collector.

[0045] The positive electrode current collector supports the positive electrode active material layer and is not particularly limited, as long as it has high conductivity and will not cause chemical changes in the relevant battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, sintered carbon, copper or stainless steel surface-treated with carbon, nickel, silver, etc., and aluminum-cadmium alloys can be used as positive electrode current collectors.

[0046] Positive current collectors can enhance their bonding strength with positive active materials by forming micro-protrusions on their surface, and can be used in various forms such as membranes, sheets, foils, meshes, nets, porous bodies, foams, non-woven fabrics, etc.

[0047] The positive electrode active material layer contains a sulfur-carbon composite as the positive electrode active material, and the sulfur-carbon composite has multiple island-shaped carbon coatings on its surface.

[0048] The sulfur-carbon composite comprises a porous carbon material and sulfur on at least a portion of the inner and outer surfaces of the porous carbon material. Since sulfur alone is not conductive, it is used in combination with a conductive material, thus taking the form of a sulfur-carbon composite.

[0049] Sulfur may include at least one selected from the group consisting of elemental sulfur (S₈) and sulfur compounds. The positive electrode active material may include inorganic sulfur, Li₂S, etc. n (n≥1), disulfide compounds, organosulfur compounds, and carbon-sulfur polymers (C2S) x ) n At least one of the group consisting of x = 2.5 to 50, n ≥ 2. Preferably, the sulfur can be inorganic sulfur.

[0050] Sulfur may be included in an amount of 60 to 90 wt%, preferably 65 to 80 wt%, relative to a total of 100 wt% positive electrode active material layer. Positive electrode active material layers with a sulfur content of less than 60 wt% have limitations in improving the energy density of the battery.

[0051] Porous carbon materials not only provide a framework that can uniformly and stably fix the sulfur, but also compensate for the low conductivity of sulfur, thereby enabling the electrochemical reaction to proceed smoothly.

[0052] As porous carbon materials, any material commonly used in the art can be used, as long as it is a carbon-based material that is conductive and has a porous structure or a high specific surface area. For example, porous carbon materials can be at least one selected from the group consisting of: graphite; graphene; carbon black, such as tandoor black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal cracking black; carbon nanotubes (CNTs), such as single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs); carbon fibers, such as graphite nanofibers (GNFs), carbon nanofibers (CNFs), or activated carbon fibers (ACFs); graphite, such as natural graphite, artificial graphite, or expanded graphite; and activated carbon.

[0053] Porous carbon materials can be prepared by carbonizing precursors made of various carbon materials, and can contain multiple non-uniform pores on the surface and inside. The average diameter of these pores ranges from 1 to 200 nm, and the porosity can range from 10% to 90% of the total volume of the porous carbon material. When the average pore diameter is smaller than the above range, the pore size is only at the molecular level, making sulfur impregnation impossible. Conversely, when the average diameter exceeds the above range, the mechanical strength of the porous carbon material weakens, which is undesirable for use in the preparation of cathodes.

[0054] The average diameter of the porous carbon material can be from 100 nm to 50 μm, preferably from 10 to 50 μm. When the average diameter of the porous carbon material is smaller than the above range, there may be problems when sulfur is loaded into the pores of the porous carbon material. Conversely, when the average diameter exceeds the above range, the sulfur-carbon composite may have reduced conductivity.

[0055] Porous carbon materials can be spherical, rod-shaped, needle-shaped, plate-shaped, tubular, or block-shaped, and can be used without restriction, as long as they are commonly used in lithium-sulfur batteries.

[0056] In the sulfur-carbon composite of the present invention, sulfur is located on at least either the interior or the surface of the porous carbon material, and in this case, sulfur can be present in less than 100%, preferably 1% to 95%, more preferably 60% to 90% of the area of ​​the entire inner and outer surfaces of the porous carbon material. When sulfur is present within the aforementioned range on the inner and outer surfaces of the porous carbon material, maximum effect can be obtained in terms of electron transport area and wettability with electrolyte. Specifically, since sulfur is thinly and uniformly impregnated within the aforementioned range on the inner and outer surfaces of the porous carbon material, it can increase the electron transport contact area during charging and discharging. When sulfur is present in 100% of the area of ​​the entire inner and outer surfaces of the porous carbon material, the porous carbon material is completely covered with sulfur, the wettability with electrolyte is reduced, and therefore electrons are not transported and become impossible to participate in electrochemical reactions.

[0057] Relative to a total of 100% by weight of the sulfur-carbon composite, the sulfur-carbon composite may contain sulfur in an amount of 65 to 90% by weight, preferably 70 to 85% by weight, and more preferably 72 to 80% by weight. When the sulfur content is less than the above range, the content of porous carbon material in the sulfur-carbon composite increases relatively, which increases the amount of binder used when preparing the cathode. This increase in binder usage ultimately increases the sheet resistance of the cathode, which causes it to act as an insulator that prevents electron migration (electron passage), and battery performance may degrade. Conversely, when the sulfur content exceeds the above range, sulfur that is not bound to the porous carbon material aggregates together or re-dissolves to the surface of the porous carbon material, making it difficult to accept electrons and thus unable to participate in electrochemical reactions, which may cause a loss of battery capacity.

[0058] Therefore, in the sulfur-carbon composite of the present invention, the weight ratio of porous carbon material to sulfur (weight% of porous carbon material: weight% of sulfur) can be from 35:65 to 10:90, preferably from 30:70 to 15:85.

[0059] Sulfur-carbon composites can be obtained by combining sulfur and porous carbon materials through simple mixing, or they can be in the form of a coating or a supported form with a core-shell structure. A core-shell coating is a form in which either sulfur or porous carbon material is coated with the other; for example, sulfur can be used to coat the surface of a porous carbon material, and vice versa. Furthermore, a supported form can be a form in which sulfur is supported within the porous carbon material. Any form can be used as the sulfur-carbon composite, as long as the content ratio of sulfur to porous carbon material is satisfied, and this form is not limited in this invention.

[0060] The method for preparing sulfur-carbon composites is not particularly limited in this invention, and methods commonly used in the art can be used. For example, a method can be used that involves simply mixing porous carbon materials with sulfur and then combining the resulting material through heat treatment.

[0061] The sulfur-carbon composite can be included in an amount of 70 to 100% by weight, preferably 85 to 98% by weight, relative to the total weight of the positive electrode active material layer. When the content of the sulfur-carbon composite is less than the above range, the relative content of other components, such as binders, increases relatively, making it difficult to achieve batteries with high capacity and high energy density. Conversely, when the content exceeds the above range, the content of binders decreases relatively, causing problems that reduce the physical properties of the positive electrode.

[0062] The carbon coating is distributed in an island-like pattern on the surface of the sulfur-carbon composite and contains reduced graphene oxide (rGO).

[0063] The specific surface area of ​​reduced graphene oxide can be 700 m². 2 / g or more, preferably 700 to 1500m 2 / g, more preferably 800 to 1000m 2 / g.

[0064] Relative to the total weight of the positive electrode active material, reduced graphene oxide can be included in an amount of 0.5 to 3% by weight, preferably 0.7 to 1.5% by weight. When the content of reduced graphene oxide is less than the above range, the amount of carbon coating is too small to achieve the desired effect. Conversely, when the content of reduced graphene oxide exceeds the above range, the carbon coating forms over an excessively wide range, causing a significant increase in initial resistance.

[0065] The average longitudinal length of the island-like structures in the carbon coating can be, for example, 10 to 100 μm, preferably 20 to 80 μm, more preferably 30 to 70 μm, but this size is not limited thereto. In this case, the method for measuring the longitudinal length in the island-like carbon coating is not particularly limited, and methods known in the field of lithium secondary batteries can be used. When the island size is too large, the surface of the sulfur-carbon composite may become excessively clogged, causing a rapid increase in initial resistance, and when the island size is too small, the effect of forming a carbon coating on the sulfur-carbon composite may be insufficient. When the island size of the carbon coating meets the above-mentioned range, it can be ensured that the effect of improving electrochemical reactivity is obtained from the formation of the carbon coating without degrading the operation or performance of the battery.

[0066] As described above, the positive electrode active material of the present invention, which contains multiple island-shaped carbon coatings on the surface of the sulfur-carbon composite, exhibits an increased specific surface area and pore volume compared to a sulfur-carbon composite without carbon coatings (hereinafter referred to as a pure sulfur-carbon composite), and also contains micropores.

[0067] Specifically, the specific surface area of ​​the positive electrode active material can be 20 to 50 m². 2 / g, preferably 30 to 40m 2 / g. Furthermore, the pore volume of the positive electrode active material can be from 0.3 to 0.5 cm³. 3 / g, preferably 0.35 to 0.45cm 3 / g. In addition, the positive electrode active material may also contain pores with a size range of 30 to 70 nm.

[0068] Furthermore, since the positive electrode active material contains a carbon coating, its volume average particle size and average particle size tend to be smaller compared to pure sulfur-carbon composites.

[0069] Specifically, the volume average particle size of the positive electrode active material can be 30 to 70 μm, preferably 40 to 65 μm. Furthermore, the average particle size (D) of the positive electrode active material... 10 The particle size can be 5 to 30 μm, preferably 10 to 20 μm, and the average particle size (D) 50 The diameter can be 30 to 60 μm, preferably 40 to 55 μm.

[0070] On the other hand, when a continuous carbon coating is included on the entire surface of the sulfur-carbon composite, the final cathode active material (a sulfur-carbon composite with a carbon coating on its entire surface) has a lower specific surface area compared to the pure sulfur-carbon composite because the almost poreless surface (base surface) of the reduced graphene oxide contained in the carbon coating covers the surface.

[0071] Therefore, in the positive electrode active material of the present invention, which includes a plurality of island-shaped carbon coatings discontinuously formed on the surface of the sulfur-carbon composite, the ratio of the specific surface area of ​​the positive electrode active material according to the present invention to the specific surface area of ​​the pure sulfur-carbon composite (specific surface area of ​​the positive electrode active material according to the present invention / specific surface area of ​​the pure sulfur-carbon composite) can be 1 or more, preferably 1 to 3, and more preferably 1.5 to 2.7.

[0072] Since the positive electrode active material according to the invention contains a discontinuous carbon coating formed in an island-like manner on the surface of the sulfur-carbon composite, the average particle size is reduced, and the specific surface area and pore volume are increased. This increases the sites for electrochemical reactions, resulting in excellent electrochemical reactivity. Therefore, it can ensure the effect of improving the capacity, output and cycle life characteristics of lithium-sulfur batteries.

[0073] Reference Figure 1 In addition to the above-mentioned positive electrode active material, the positive electrode active material layer of the present invention may further include carbon bridge 15.

[0074] The carbon bridge is an aggregate of reduced graphene oxide without a carbon coating on the surface of the sulfur-carbon composite, and it acts as a bridge connecting the positive electrode active material. Furthermore, because the positive electrode active material layer of the present invention contains carbon bridges, it does not contain a separate conductive material.

[0075] Carbon bridges can have any shape, as long as they can exist between the pores in the positive electrode active material layer of the present invention, specifically between the positive electrode active materials. As an example, carbon bridges can be pleated structures, or can have at least one selected from amorphous, plate-like, sheet-like, spherical, or fibrous shapes.

[0076] In addition to the positive electrode active material, the positive electrode active material layer of the present invention may further include one or more additives selected from transition metal elements, group IIIA elements, group IVA elements, sulfur compounds of these elements, and alloys of these elements and sulfur.

[0077] Transition metal elements can be Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au, Hg, etc. Group IIIA elements can be Al, Ga, In, Tl, etc., and Group IVA elements can be Ge, Sn, Pb, etc.

[0078] Together with the above-mentioned positive electrode active material and carbon bridge or optionally the above-mentioned additives, the positive electrode active material layer may further include an adhesive for adhering the positive electrode active material well to the current collector.

[0079] The adhesive holds the positive electrode active material on the positive electrode current collector and organically connects the positive electrode active material to further increase the bonding force therebetween, and any adhesive known in the art can be used.

[0080] For example, the adhesive may be one, or a mixture or copolymer of two or more of the following: fluoropolymer adhesives, including polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE); rubber adhesives, including styrene-butadiene rubber (SBR), nitrile rubber, or styrene-isoprene rubber; cellulose adhesives, including carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, or regenerated cellulose; polyol adhesives; polyolefin adhesives, including polyethylene or polypropylene; polyimide adhesives; polyester adhesives; and silane adhesives.

[0081] The binder may be included in an amount of 0.5 to 30% by weight relative to the total weight of the positive electrode active material layer. When the binder content is less than the above range, the physical properties of the positive electrode decrease, causing the positive electrode active material in the positive electrode to detach. And when the content exceeds the above range, the proportion of positive electrode active material in the positive electrode may be relatively reduced, which may reduce the battery capacity. Therefore, it is preferable to determine an appropriate content within the above range.

[0082] There are no particular limitations on the method for manufacturing the positive electrode for lithium-sulfur batteries in this invention, and those skilled in the art can use conventional methods or various methods that are modified thereto.

[0083] As an example, a positive electrode for a lithium-sulfur battery can be prepared by preparing a positive electrode slurry composition containing the above-mentioned components, and then coating it on at least one surface of a positive electrode current collector to form a positive electrode active material layer.

[0084] The positive electrode slurry composition may include the above-mentioned positive electrode active material, binder, additives, etc., and may further include solvent.

[0085] According to one embodiment of the present invention, the positive electrode slurry composition can be prepared by the following steps: preparing a reduced graphene oxide dispersion by dispersing reduced graphene oxide in a dispersion medium; preparing a sulfur-carbon composite having a plurality of island-shaped carbon coatings on its surface by mixing the reduced graphene oxide dispersion and a sulfur-carbon composite and drying the result; and mixing the sulfur-carbon composite with carbon coatings with a binder, additives and solvent.

[0086] According to another embodiment of the present invention, a cathode slurry composition can be prepared by preparing a pre-dispersion containing reduced graphene oxide and then adding (externally adding) the pre-dispersion to a mixture containing a sulfur-carbon composite, a binder, a solvent, etc. When preparing the pre-dispersion, the reduced graphene oxide can be dispersed in the binder.

[0087] Lower alcohols, such as ethanol, can be used as dispersion media for preparing reduced graphene oxide dispersions.

[0088] As a solvent, a solvent capable of uniformly dispersing the positive electrode active material, binder, and additives is used. Water, as an aqueous solvent, is most preferred; in this case, the water can be distilled water or deionized water. However, the solvent is not limited to this, and lower alcohols that are easily miscible with water can be used as needed. Lower alcohols can be methanol, ethanol, propanol, isopropanol, butanol, etc., and preferably, these lower alcohols are miscible with water.

[0089] The solvent can be contained at a concentration that promotes the coating of the positive electrode slurry composition, and the specific content varies depending on the coating method and equipment.

[0090] As needed, and for purposes such as improving functionality, the cathode slurry composition may further include materials commonly used in the art. For example, the cathode slurry composition may include viscosity modifiers, fluidizing agents, fillers, etc.

[0091] The method of coating the positive electrode slurry composition is not particularly limited in this invention, and can be, for example, a doctor blade, die casting, comma coating, and screen printing. Alternatively, the positive electrode slurry can be formed on a separate substrate and then coated onto the positive electrode current collector using a pressing or laminating method.

[0092] Following the above coating, a drying process for solvent removal can be performed. The drying process is carried out at a temperature and time sufficient to remove the solvent, and these conditions are not particularly limited in this invention, as they can vary depending on the type of solvent. Examples include irradiation by warm air, hot air, or low-humidity air, vacuum, (far)infrared radiation, electron beams, etc. The drying speed is typically adjusted within a range that prevents the positive electrode active material layer from cracking due to stress concentration or from peeling off from the positive electrode current collector, thereby removing the solvent as quickly as possible.

[0093] Furthermore, the present invention provides a lithium-sulfur battery comprising a positive electrode for lithium-sulfur batteries.

[0094] A lithium-sulfur battery comprises a positive electrode; a negative electrode; an electrolyte therebetween; and as the positive electrode, it includes a positive electrode for a lithium-sulfur battery according to the present invention.

[0095] The positive electrode is based on the present invention and follows the above description.

[0096] The negative electrode may comprise a negative electrode current collector and a layer of negative electrode active material coated on one or both surfaces of the negative electrode current collector. Alternatively, the negative electrode may be a lithium metal plate.

[0097] The negative current collector is used to support the negative active material layer, as described in the positive current collector.

[0098] The negative electrode active material layer may contain a negative electrode active material, and optionally may further contain conductive materials, adhesives, etc., wherein the conductive materials and adhesives follow the description above.

[0099] The negative electrode active material can be capable of reversibly inserting or deintercalating lithium ions (Li). + Materials that can reversibly form lithium-containing compounds by reacting with lithium ions, lithium metal or lithium alloys.

[0100] Capable of reversibly inserting or de-inserting lithium ions (Li) + The material can be, for example, crystalline carbon, amorphous carbon, or a mixture thereof. It can react with lithium ions (Li... + Materials that reversibly form lithium-containing compounds through a reaction can be, for example, tin oxide, titanium nitrate, or silicon. Lithium alloys can be, for example, alloys of lithium (Li) with metals selected from the group consisting of: sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

[0101] Preferably, the negative electrode active material can be lithium metal, specifically, it can be in the form of a lithium metal thin film or lithium metal powder.

[0102] The method for preparing the negative electrode is not particularly limited, and methods commonly used in the art for forming layers or films can be used. For example, methods such as pressing, coating, and deposition can be used. Furthermore, the negative electrode of the present invention also includes the case where the battery is assembled without a lithium film on the current collector, and then a lithium metal film is formed on a metal plate through initial charging.

[0103] An electrolyte is disposed between the positive and negative electrodes to allow electrochemical oxidation or reduction reactions to occur at the positive and negative electrodes, and the electrolyte contains a lithium salt and a non-aqueous organic solvent.

[0104] Lithium salts can be used without restriction, as long as they are commonly used in lithium secondary batteries.

[0105] Specific examples of lithium salts can be at least one selected from the group consisting of: LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 LiPF6, LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, LiSO3CH3, LiSO3CF3, LiSCN, LiC(CF3SO2)3, LiN(CF3SO3)2 (lithium bis(trifluoromethanesulfonyl)imide; LiTFSI), LiN(C2F5SO2)2, LiN(SO2F)2 (lithium bis(fluorosulfonyl)imide; LiFSI), lithium chloroborane, lower aliphatic carboxylic acids, lithium tetraphenylborate, and lithium imide.

[0106] The concentration of lithium salts can range from 0.2 to 4 M, specifically 0.6 to 2 M, and more specifically 0.7 to 1.7 M, depending on various factors such as the composition of the electrolyte, the solubility of the lithium salt, the conductivity of the dissolved lithium salt, the charging and discharging conditions of the battery, the operating temperature, and other factors known in the field of lithium secondary batteries. When using lithium salt concentrations below 0.2 M, the conductivity of the electrolyte may decrease, leading to a decline in electrolyte performance, and when using lithium salt concentrations above 4 M, the viscosity of the electrolyte may increase, causing a decrease in lithium-ion mobility.

[0107] As a non-aqueous organic solvent, non-aqueous organic solvents commonly used in electrolytes for lithium secondary batteries can be used without restriction. For example, the organic solvent can be a single ether, ester, amide, straight-chain carbonate, cyclic carbonate, etc., or a mixture of two or more. Among them, ether compounds are usually included.

[0108] As an example, ether compounds may be at least one selected from the group consisting of: dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, polyethylene glycol methyl ethyl ether, 1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran, but are not limited thereto.

[0109] The ester in the above-mentioned organic solvent may be any one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone and ε-caprolactone, or a mixture of two or more thereof, but is not limited thereto.

[0110] The linear carbonate compound may specifically be selected from any one of the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate and ethyl propyl carbonate, or a mixture of two or more thereof, but the linear carbonate compound is not limited thereto.

[0111] Furthermore, the cyclic carbonate compound may specifically be any one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentene carbonate, 2,3-pentene carbonate, vinylene carbonate, vinyl ethylene carbonate, and their halides, or a mixture of two or more thereof. Its halides may be, for example, fluoroethylene carbonate (FEC), but are not limited thereto.

[0112] In addition to the organic solvents described above, the solvent may also include at least one selected from the group consisting of: N-methylpyrrolidone, dimethyl sulfoxide, sulfolane, furan, 2-methylfuran, 3-methylfuran, 2-ethylfuran, 2-propylfuran, 2-butylfuran, 2,3-dimethylfuran, 2,4-dimethylfuran, 2,5-dimethylfuran, pyran, 2-methylpyran, 3-methylpyran, 4-methylpyran, benzofuran, 2-(2-nitrovinyl)furan, thiophene, 2-methylthiophene, 2-ethylthiophene, 2-propylthiophene, 2-butylthiophene, 2,3-dimethylthiophene, 2,4-dimethylthiophene, and 2,5-dimethylthiophene.

[0113] In addition to the above-mentioned components, the electrolyte may further include nitrate compounds commonly used in the art. Examples include lithium nitrate (LiNO3), potassium nitrate (KNO3), cesium nitrate (CsNO3), magnesium nitrate (Mg(NO3)2), barium nitrate (Ba(NO3)2), lithium nitrite (LiNO2), potassium nitrite (KNO2), and cesium nitrite (CsNO2).

[0114] The electrolyte can be injected at an appropriate stage of the battery manufacturing process, depending on the manufacturing process of the final product and the required physical properties. In other words, the electrolyte can be injected before battery assembly or at the final stage of battery assembly.

[0115] A membrane may be further included between the positive and negative electrodes.

[0116] A separator separates or insulates the positive and negative electrodes from each other and allows lithium ions to transport between them. It can be formed from porous, non-conductive, or insulating materials, and its use is not particularly limited, as long as it is typically used as a separator in lithium-ion secondary batteries. Such a separator can be a stand-alone component, such as a membrane, or it can be a coating added to the positive and / or negative electrodes.

[0117] The preferred membrane has excellent water-holding capacity for the electrolyte and low resistance to ion migration of the electrolyte.

[0118] The separator can be formed from a porous substrate, and any porous substrate can be used as long as it is commonly used in secondary batteries. It can be used alone as a porous polymer membrane or as a laminate thereof. For example, nonwoven fabrics or polyolefin porous membranes made of high-melting-point glass fibers, polyethylene terephthalate, etc., can be used, but are not limited to these.

[0119] The materials used for the porous substrate in this invention are not particularly limited, as long as the porous substrate is commonly used in electrochemical devices. For example, the porous substrate can be at least one selected from the group consisting of: polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyamides, polyacetals, polycarbonates, polyimides, polyetheretherketones, polyethersulfones, polyphenylene ethers, polyphenylene sulfides, polyethylene naphthalate, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, cellulose, nylon, and poly(p-phenylenebenzobismuth submersible) (azole) and polyarylate.

[0120] There is no particular limitation on the thickness of the porous substrate, but it can be from 1 to 100 μm, preferably from 5 to 50 μm. The thickness range of the porous substrate is not limited to the above range, but when the thickness is too thin than the lower limit mentioned above, the mechanical properties decrease, and the separator may be easily damaged during battery use.

[0121] There are no particular limitations on the average diameter and porosity of the pores present in the porous substrate, but they can be from 0.001 to 50 μm and from 10% to 95%, respectively.

[0122] The lithium-sulfur batteries of the present invention can be classified according to shape as cylindrical, prismatic, coin-shaped, pouch-shaped, etc., and according to size as block-shaped and thin-film-shaped. Since the structure and manufacturing methods of these batteries are widely known in the art, their detailed description will be omitted.

[0123] Furthermore, the present invention provides a battery module that includes a lithium-sulfur battery as a unit cell.

[0124] Battery modules can be used as power sources for medium to large-sized devices that require high-temperature stability, long cycle performance, and high capacity.

[0125] Medium and large-sized devices can be power tools that receive power from a battery-powered motor to operate; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), etc.; electric two-wheeled vehicles, including electric bicycles (E-bikes) and electric scooters (E-scooters); electric golf carts; and power storage systems, but are not limited to these.

[0126] Preferred Implementation

[0127] In the following description, preferred embodiments are given to facilitate understanding of the invention, but these embodiments are merely intended to illustrate the invention. It will be apparent to those skilled in the art that various changes and variations can be made within the scope and spirit of the invention, and that such changes and variations fall within the scope of the appended claims.

[0128] Examples and Comparative Examples

[0129] [Example 1]

[0130] With a specific surface area of ​​1000m 2 / g of reduced graphene oxide and binder (styrene-butadiene rubber (SBR) / carboxymethyl cellulose (CMC), SBR:CMC = 7:3 (weight ratio)) were mixed in a weight ratio of 1:1 to prepare a pre-dispersion, and then a positive electrode slurry composition was prepared in which sulfur-carbon composite (sulfur:porous carbon material = 75:25 (weight ratio)), binder and reduced graphene oxide were mixed in a weight ratio (weight %) of 94:5:1.

[0131] The above-prepared positive electrode slurry composition was coated onto an aluminum foil with a thickness of 20 μm until the thickness reached 250 μm, and then dried at 50°C for 12 hours to prepare the positive electrode.

[0132] The prepared positive electrode and a lithium metal negative electrode with a thickness of 40 μm are placed facing each other, and then a separator is inserted between them. Electrolyte (70 μl) is injected into them to manufacture a lithium-sulfur battery.

[0133] In this case, a polyethylene membrane with a thickness of 16 μm and a porosity of 46% was used, and a mixture solution obtained by dissolving 0.75 M LiFSI and 3% by weight lithium nitrate in an organic solvent formed by mixing 2-methylfuran and dimethoxyethane in a volume ratio of 33:77 was used as the electrolyte.

[0134] [Example 2]

[0135] In addition to using a specific surface area of ​​1000m 2 After preparing a predispersant by mixing / g of reduced graphene oxide and binder (styrene-butadiene rubber (SBR) / carboxymethyl cellulose (CMC), SBR:CMC = 7:3 (weight ratio)) at a weight ratio of 1:1, a lithium-sulfur battery was manufactured in the same manner as in Example 1, except that a positive electrode slurry composition in which sulfur-carbon composite (sulfur: porous carbon material = 75:25 (weight ratio)), binder and reduced graphene oxide were mixed at a weight ratio (weight %) of 93.5:5:1.5.

[0136] [Example 3]

[0137] In addition to using a specific surface area of ​​1000m 2After preparing a predispersant by mixing / g of reduced graphene oxide and binder (styrene-butadiene rubber (SBR) / carboxymethyl cellulose (CMC), SBR:CMC = 7:3 (weight ratio)) in a 1:1 weight ratio, a lithium-sulfur battery was manufactured in the same manner as in Example 1, except that a positive electrode slurry composition in which sulfur-carbon composite (sulfur: porous carbon material = 75:25 (weight ratio)), binder and reduced graphene oxide were mixed in a 92:5:3 weight ratio (wt%).

[0138] [Example 4]

[0139] With a specific surface area of ​​1000m 2 / g of reduced graphene oxide was dispersed in ethanol to prepare a reduced graphene oxide dispersion (reduced graphene oxide: ethanol = 1:99 (weight ratio)).

[0140] After mixing a sulfur-carbon composite (sulfur:porous carbon material = 75:25 (weight ratio)) into a dispersion of reduced graphene oxide, the result was dried to prepare a sulfur-carbon composite with a carbon coating containing reduced graphene oxide on its surface. The content of reduced graphene oxide in the coated sulfur-carbon composite was 1% by weight.

[0141] The above-prepared sulfur-carbon composite, with 95% by weight as the positive electrode active material, and 5% by weight as a binder, styrene-butadiene rubber / carboxymethyl cellulose (SBR:CMC = 7:3, by weight) were mixed to prepare a positive electrode slurry composition.

[0142] The above-prepared positive electrode slurry composition was coated onto an aluminum foil with a thickness of 20 μm until the thickness reached 250 μm, and then dried at 50°C for 12 hours to prepare the positive electrode.

[0143] The prepared positive electrode and a lithium metal negative electrode with a thickness of 40 μm are placed facing each other, and then a separator is inserted between them. Electrolyte (70 μl) is injected into them to manufacture a lithium-sulfur battery.

[0144] In this case, a polyethylene membrane with a thickness of 16 μm and a porosity of 46% was used, and a mixture solution obtained by dissolving 0.75 M LiFSI and 3% by weight lithium nitrate in an organic solvent formed by mixing 2-methylfuran and dimethoxyethane in a volume ratio of 33:77 was used as the electrolyte.

[0145] [Example 5]

[0146] With a specific surface area of ​​1000m 2 / g of reduced graphene oxide was dispersed in ethanol to prepare a reduced graphene oxide dispersion (reduced graphene oxide: ethanol = 1:99 (weight ratio)).

[0147] After mixing a sulfur-carbon composite (sulfur:porous carbon material = 75:25 (weight ratio)) into a dispersion of reduced graphene oxide, the result was dried to prepare a sulfur-carbon composite with a carbon coating containing reduced graphene oxide on its surface. The content of reduced graphene oxide in the coated sulfur-carbon composite was 1.5% by weight.

[0148] In addition to using the sulfur-carbon composite prepared above as the positive electrode active material, lithium-sulfur batteries were manufactured in the same manner as in Example 4.

[0149] [Example 6]

[0150] With a specific surface area of ​​1000m 2 / g of reduced graphene oxide was dispersed in ethanol to prepare a reduced graphene oxide dispersion (reduced graphene oxide: ethanol = 1:99 (weight ratio)).

[0151] After mixing a sulfur-carbon composite (sulfur:porous carbon material = 75:25 (weight ratio)) into a dispersion of reduced graphene oxide, the result was dried to prepare a sulfur-carbon composite with a carbon coating containing reduced graphene oxide on its surface. The content of reduced graphene oxide in the coated sulfur-carbon composite was 3% by weight.

[0152] In addition to using the sulfur-carbon composite prepared above as the positive electrode active material, lithium-sulfur batteries were manufactured in the same manner as in Example 4.

[0153] [Comparative Example 1]

[0154] A positive electrode slurry composition was prepared by mixing 95% by weight of a sulfur-carbon composite (sulfur: porous carbon material = 75:25 by weight) as the positive electrode active material and 5% by weight of styrene-butadiene rubber / carboxymethyl cellulose (SBR:CMC = 7:3 by weight) as a binder.

[0155] The above-prepared positive electrode slurry composition was coated onto an aluminum foil with a thickness of 20 μm until the thickness reached 250 μm, and then dried at 50°C for 12 hours to prepare the positive electrode.

[0156] The prepared positive electrode and a lithium metal negative electrode with a thickness of 40 μm are placed facing each other, and then a separator is inserted between them. Electrolyte (70 μl) is injected into them to manufacture a lithium-sulfur battery.

[0157] In this case, a polyethylene membrane with a thickness of 16 μm and a porosity of 46% was used, and a mixture solution obtained by dissolving 0.75 M LiFSI and 3% by weight lithium nitrate in an organic solvent formed by mixing 2-methylfuran and dimethoxyethane in a volume ratio of 33:77 was used as the electrolyte.

[0158] [Comparative Example 2]

[0159] Reduced graphene oxide (specific surface area: 100 m²) 2 After dispersing the sulfur-carbon composite (sulfur:porous carbon material = 75:25 by weight) in ethanol, a carbon coating containing reduced graphene oxide was prepared with the carbon coating continuously formed on the entire surface.

[0160] In addition to using the sulfur-carbon composite prepared above as the positive electrode active material, a lithium-sulfur battery was manufactured in the same manner as in Comparative Example 1.

[0161] [Comparative Example 3]

[0162] With a specific surface area of ​​1000m 2 / g of reduced graphene oxide was dispersed in ethanol to prepare a reduced graphene oxide dispersion (reduced graphene oxide: ethanol = 1:99 (weight ratio)).

[0163] After mixing a sulfur-carbon composite (sulfur:porous carbon material = 75:25 (weight ratio)) into a dispersion of reduced graphene oxide, the result was dried to prepare a sulfur-carbon composite with a carbon coating containing reduced graphene oxide on its surface. The content of reduced graphene oxide in the coated sulfur-carbon composite was 5% by weight.

[0164] In addition to using the sulfur-carbon composite prepared above as the positive electrode active material, a lithium-sulfur battery was manufactured in the same manner as in Comparative Example 1.

[0165] Experimental Example 1

[0166] The specific surface area, pore volume, pore size, and average particle size of the positive electrode active materials used in the examples and comparative examples were measured.

[0167] Specifically, nitrogen adsorption and desorption were measured using a specific surface area measuring device (model name: BELSORP-MINI, manufacturer: BEL Japan). Isothermal adsorption and desorption curves were obtained, and the specific surface area was calculated using the BET (Brunauer-Emmett-Teller) equation. Furthermore, pore volume and pore size distribution were measured using BJH (Barrett Joyner Halenda) calculations. Additionally, the volume average particle size (MV) and average particle size (D) were measured using a particle size distribution measuring device.10 ) and average particle size (D 50 The results obtained here are shown in Tables 1 and 2.

[0168] Table 1

[0169] Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 <![CDATA[Specific surface area (m 2 / g)]]> 29.12 36.52 38.20 13.97 12.81 11.52 <![CDATA[Pore volume (cm 3 / g)]]> 0.33 0.45 0.45 0.11 0.10 0.08 Hole size (nm) 30-70 30-70 30-70 80-100 - -

[0170] Table 2

[0171]

[0172] As shown in Table 1 above, it can be confirmed that, compared with the positive electrode active material of the comparative example which contains a pure sulfur-carbon composite without forming a carbon coating, the positive electrode active material of the embodiment has an improved specific surface area and pore volume, and a reduced pore size because it contains a discontinuous carbon coating.

[0173] Furthermore, as can be seen from Table 2, compared with the positive electrode active material of the comparative example, the positive electrode active material according to the embodiment has a reduced volume average particle size and average particle size.

[0174] Experimental Example 2

[0175] The positive electrodes prepared in Examples 3 and 6 and Comparative Example 2 were observed using a scanning electron microscope (SEM). In this case, a Hitachi S-4800 was used as the scanning electron microscope, and the results obtained are shown in... Figures 2 to 4 middle.

[0176] Reference Figure 2 and 3 Since the positive electrodes according to Examples 3 and 6 contain positive electrode active materials, wherein a discontinuous island-like carbon coating is formed on the surface of the sulfur-carbon composite, it can be confirmed that the pores of the surface covered with reduced graphene oxide and the porous carbon material contained in the sulfur-carbon composite are visible, and carbon bridges (regions indicated by dashed lines) exist therebetween.

[0177] On the other hand, through Figure 4 It can be seen that when a positive electrode active material, as in Comparative Example 2, is included in which a continuous carbon coating is formed on the entire surface of the sulfur-carbon composite, the surface pores of the sulfur-carbon composite are completely covered by the reduced graphene oxide contained in the carbon coating.

[0178] Experimental Example 3

[0179] For the batteries manufactured in the examples and comparative examples, capacity and cycle life characteristics were evaluated using charging and discharging measurement devices.

[0180] Specifically, the battery was discharged to 1.8V at 0.1C and charged to 2.5V at 0.1C at 25°C, while its capacity performance was evaluated. The results obtained are shown in... Figure 5 and 6 middle.

[0181] Furthermore, the battery was repeatedly discharged to 1.8V and charged to 2.5V at a current density of 0.1C three times at 25°C, followed by three charge-discharge cycles at a current density of 0.2C within the same voltage range, and then discharged at a current density of 0.3C and charged at a current density of 0.2C. Simultaneously, the battery's cycle life characteristics were evaluated. The results obtained are shown in Table 3 and... Figure 7 and 8 middle.

[0182] Table 3

[0183] Discharge capacity (mAh / g) Example 1 1145 Example 2 1162 Example 3 1178 Example 4 1162 Example 5 1183 Example 6 1169 Comparative Example 1 1050 Comparative Example 2 1038 Comparative Example 3 1045

[0184] pass Figure 4 and 5 As can be confirmed by Table 3, the battery according to the embodiment has superior capacity performance compared to the battery of the comparative example.

[0185] Specifically, refer to Figure 5 The initial discharge capacities of the batteries in Examples 2 and 5 were 1162 mAh / g and 1183 mAh / g, respectively, while the initial discharge capacity of the battery in Comparative Example 1 was 1050 mAh / g. Therefore, it can be seen that the batteries in Examples 2 and 5 have excellent capacity and output performance due to the improved discharge capacity.

[0186] In addition, through Figure 6 It can be confirmed that when the carbon coating exceeds 3% by weight (Comparative Example 3), the specific surface area of ​​the positive electrode active material decreases, and therefore the reactivity may decrease compared to when no carbon coating is formed (Comparative Example 1).

[0187] like Figure 7 As shown in Table 3 above, it can be confirmed that, compared with the battery according to Comparative Example 1, the batteries according to Examples 1 to 6 not only exhibit improved discharge capacity, but also have excellent performance in maintaining the improved discharge capacity.

[0188] In addition, such as Figure 8 As shown, when a positive electrode active material containing a continuous carbon coating formed on the entire surface of the sulfur-carbon composite, as in Comparative Example 2, is used, it can be seen that it has an initial discharge capacity similar to that of Comparative Example 1. However, the porous structure of the porous carbon material contained in the sulfur-carbon composite cannot be utilized, causing the discharge capacity to deteriorate rapidly with cycling and resulting in poor cycle life characteristics.

[0189] [Figure Labels]

[0190] 11: Sulfur-carbon complex

[0191] 13: Carbon coating

[0192] 15: Carbon Bridge

Claims

1. A positive electrode for a lithium-sulfur battery, the positive electrode for a lithium-sulfur battery comprising: a positive electrode current collector; and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises a sulfur-carbon composite as a positive electrode active material, the sulfur-carbon composite having a plurality of island-shaped carbon coatings on a surface thereof, wherein the carbon coating comprises reduced graphene oxide, wherein a content of the reduced graphene oxide is 0.5 to 3% by weight with respect to a total weight of the positive electrode active material.

2. The positive electrode for lithium-sulfur batteries according to claim 1, wherein the specific surface area of the reduced graphene oxide is 700 m2 / g or more. 2 / g or more.

3. The positive electrode for lithium-sulfur batteries according to claim 2, wherein the specific surface area of the reduced graphene oxide is 700 m 2 / g to 1500 m 2 / g.

4. The positive electrode for lithium-sulfur batteries according to claim 1, wherein the specific surface area of the positive electrode active material is 20 m 2 / g to 50 m 2 / g.

5. The positive electrode for lithium-sulfur batteries according to claim 1, wherein the pore volume of the positive electrode active material is 0.3 cm 3 / g to 0.5 cm 3 / g. 6.The positive electrode for a lithium-sulfur battery according to claim 1, wherein a pore size in the positive electrode active material is 30 to 70 nm. 7.The positive electrode for a lithium-sulfur battery according to claim 1, wherein a ratio of a specific surface area of the positive electrode active material to a specific surface area of a sulfur-carbon composite not comprising a carbon coating is 1 to 3. 8.The positive electrode for a lithium-sulfur battery according to claim 1, wherein the positive electrode active material layer further comprises a carbon bridge connecting the positive electrode active material. 9.The positive electrode for a lithium-sulfur battery according to claim 8, wherein the carbon bridge is a crumpled structure or has a fiber shape. 10.A method of preparing a positive electrode for a lithium-sulfur battery, the method comprising the steps of: preparing a reduced graphene oxide dispersion by dispersing reduced graphene oxide in a dispersion medium; preparing a sulfur-carbon composite having a plurality of island-shaped carbon coatings on a surface thereof by mixing the reduced graphene oxide dispersion and the sulfur-carbon composite and drying the mixture; preparing a positive electrode slurry composition comprising the sulfur-carbon composite having a plurality of island-shaped carbon coatings; and coating the positive electrode slurry composition on at least one surface of a positive electrode current collector, wherein a content of the reduced graphene oxide is 0.5 to 3% by weight with respect to a total weight of the positive electrode active material. 11.The method of preparing a positive electrode for a lithium-sulfur battery according to claim 10, wherein the dispersion medium comprises a lower alcohol. 12.A method of preparing a positive electrode for a lithium-sulfur battery, the method comprising the steps of: preparing a pre-dispersion comprising reduced graphene oxide; preparing a positive electrode slurry composition by adding the pre-dispersion to a mixture comprising a sulfur-carbon composite and a binder; and coating the positive electrode slurry composition on at least one surface of a positive electrode current collector, wherein the positive electrode for a lithium-sulfur battery comprises a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises a sulfur-carbon composite as a positive electrode active material, the sulfur-carbon composite having a plurality of island-shaped carbon coatings on a surface thereof, wherein the carbon coating comprises reduced graphene oxide, wherein the reduced graphene oxide has a specific surface area of 700 m 2 / g or more, wherein a content of the reduced graphene oxide is 0.5 to 3% by weight with respect to a total weight of the positive electrode active material. 13.A lithium-sulfur battery, the lithium-sulfur battery comprising: the positive electrode for a lithium-sulfur battery according to claim 1; a negative electrode; and an electrolyte.

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