Positive electrode containing sulfur-carbon composite and lithium-ion secondary battery containing the same

Abstract

The present invention relates to a positive electrode for a lithium-sulfur battery, the positive electrode comprising a sulfur-carbon composite as a positive electrode active material. The sulfur-carbon composite includes a carbon material having high porosity and a high specific surface area, and when applied to a battery, the sulfur-carbon composite reduces the initial irreversible capacity of the battery and improves the output characteristics and life characteristics.

Description

[Technical Field]

[0001] The present invention relates to a lithium ion secondary battery having a high energy density and improved in poor initial irreversible characteristics by suppressing the elution of polysulfides, and a positive electrode for said battery.

[0002] This application claims priority based on Korean Patent Application No. 10-2021-0147387, filed on October 29, 2021, and the entire contents disclosed in the specification and drawings of that application are incorporated herein by reference. [Background technology]

[0003] Lithium-sulfur (Li-S) batteries utilize the existing catholyte system, Li2S x The problem is that the high theoretical discharge capacity (1675mAh / g) of sulfur is not fully utilized due to the catholyte type reaction resulting in the generation of polysulfide, an intermediate product in the form of sulfur, and the battery life characteristics are reduced due to battery degradation caused by polysulfide elution.

[0004] Recently, sparingly solvating electrolyte (SSE) systems have been proposed that suppress the elution of polysulfides, and the BET specific surface area is 1,500 (m 2 It was confirmed that when a carbon material with a high specific surface area (i.e., 0.01g / g or more) is used, more than 90% of the theoretical capacity can be utilized. However, the poor life and output characteristics need to be improved.

[0005] Therefore, to build a battery system with a high energy density of 400Wh / kg or more and 600Wh / L or more, a battery with a capacity of 4.0mAh / cm 2 For these reasons, an electrolyte and positive electrode active material system that can be operated even with a porosity of 60 vol% or less is required. Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide a positive electrode active material for a battery system having a high energy density of 400 Wh / kg or more and 600 Wh / L or more.

[0007] Another object of the present invention is to provide a lithium ion secondary battery containing the positive electrode active material.

[0008] It will be readily apparent that other objects and advantages of the present invention can be achieved by the means or methods recited in the claims and their combinations. [Means for solving the problem]

[0009] A first aspect of the present invention relates to a positive electrode for a lithium-sulfur battery, the positive electrode comprising a positive electrode active material including a sulfur-carbon composite. The sulfur-carbon composite comprises a porous carbon material and sulfur. The carbon material has pores with a pore diameter of less than 3 nm at a rate of 90 vol% or more relative to 100 vol% of the total pores, and pores with a pore diameter of less than 1 nm at a rate of 60 vol% or more relative to 100 vol% of the total pores. The sulfur-carbon composite has an SCP value of more than 0.8 and less than 1, as determined by the following Equation 2:

[0010] [Formula 2] SCP = Sulfur content (A) ÷ Pore volume (B) of carbon material

[0011] In the above formula 2, A is the ratio of the mass of sulfur to the mass of the carbon-sulfur composite, and B is the ratio of the pore volume in the carbon material to the total volume (apparent volume) of the carbon material.

[0012] In a second aspect of the present invention, the carbon material in the first aspect has a specific surface area (BET) of 1,600 m 2 / g or more.

[0013] In a third aspect of the present invention, in the first or second aspect, the carbon material has a circularity of 50% or more.

[0014] In a fourth aspect of the present invention, in any one of the first to third aspects, the carbon material includes activated carbon.

[0015] In a fifth aspect of the present invention, in any one of the first to fourth aspects, the porous carbon material has a pore volume of 0.8 cm 3 / g or more.

[0016] In a sixth aspect of the present invention, in any one of the first to fifth aspects, the carbon material contains 95 wt % or more of activated carbon relative to 100 wt % of the carbon material.

[0017] In a seventh aspect of the present invention, in any one of the first to sixth aspects, the positive electrode active material contains 70 wt % or more of the sulfur-carbon composite with respect to 100 wt % of the positive electrode active material.

[0018] In an eighth aspect of the present invention, in any one of the first to seventh aspects, the sulfur-carbon composite has one or more of a state in which sulfur and a carbon material are simply mixed to form a composite, a state in which the composite has a core-shell structure, and a state in which internal pores of a carbon material are filled with sulfur.

[0019] In a ninth aspect of the present invention, in any one of the first to eighth aspects, the positive electrode active material further includes a binder resin and a conductive material.

[0020] A tenth aspect of the present invention relates to a lithium-sulfur battery, comprising: The battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, the electrolyte including one or more of a cyclic ether, a linear ether, and a fluorinated ether, and the positive electrode is as described in any one of the first to ninth aspects. [Effects of the Invention]

[0021] A lithium-sulfur battery using the sulfur-carbon composite according to the present invention can reduce the initial irreversible capacity and improve the output characteristics and life characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention will be described in more detail below.

[0023] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that corresponds to the technical idea of ​​the present invention, in accordance with the principle that the inventor himself can appropriately define the concept of the terms in order to best explain the invention.

[0024] Throughout this specification, when a part is said to "comprise" or "have" a certain element, this does not mean that it excludes other elements, but that it may further include other elements, unless otherwise specified.

[0025] Furthermore, the terms "about," "substantially," and the like used throughout this specification mean values ​​at or near the values ​​mentioned, when manufacturing and material tolerances inherent in the mentioned values ​​are given, and are used to prevent unscrupulous infringers from unfairly exploiting the contents of the disclosure in which precise or absolute values ​​are mentioned to aid in the understanding of the present invention.

[0026] Throughout this specification, the phrase "A and / or B" means "A or B or both."

[0027] In the present invention, the "specific surface area" is measured by the BET method, and specifically, can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77 K) using a BELSORP-mino II manufactured by BEL Japan.

[0028] As used herein, the term "polysulfide" refers to a compound that contains polysulfide ions (S x 2- , x=8, 6, 4, 2)) and Lithium polysulfide (Li2S x or LiS x - , x=8, 6, 4, 2) is a concept that includes both.

[0029] The term "composite" as used herein means a material that combines two or more materials to form physically and chemically distinct phases, thereby exhibiting more effective functions.

[0030] The term "porosity" as used herein means the ratio of the volume occupied by pores to the total volume of a structure, and is expressed in units of %, and may be used interchangeably with terms such as void ratio (porosity) and porosity.

[0031] In the present invention, "particle size D50" refers to the particle size at 50% of the volume cumulative particle size distribution of the particles to be measured. The particle size D50 can be measured using a laser diffraction method. For example, the particle powder to be measured is dispersed in a dispersion medium, introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000), and irradiated with ultrasonic waves of approximately 28 kHz at an output of 60 W. A volume cumulative particle size distribution graph is then obtained, and the particle size corresponding to 50% of the volume cumulative amount is measured.

[0032] Furthermore, throughout the specification, when a part is said to "comprise" a certain component, this does not mean that it may further include other components, unless otherwise specified.

[0033] The present invention relates to a cathode active material for an electrochemical device and a cathode including the same. In the present invention, the electrochemical device may include any device that performs an electrochemical reaction. Specific examples include all types of primary batteries, secondary batteries, fuel cells, solar cells, and capacitors such as supercapacitors. In particular, the electrochemical device may be a secondary battery, and the secondary battery may be a lithium-ion secondary battery. Examples of the lithium-ion secondary battery include a lithium-metal battery, a lithium-sulfur battery, an all-solid-state battery, and a lithium polymer battery, among which a lithium-sulfur battery is preferred.

[0034] The positive electrode active material according to the present invention includes a sulfur-carbon composite, the sulfur-carbon composite including a porous carbon material, and the porous carbon material having a BET specific surface area and a particle size within a specific range.

[0035] Lithium-sulfur batteries have attracted attention as a next-generation secondary battery due to their high discharge capacity and theoretical energy density, as well as the advantage of reducing battery manufacturing costs due to the abundant and inexpensive sulfur reserves used as a positive electrode active material, and their environmental friendliness.

[0036] Sulfur, the positive electrode active material in lithium-sulfur batteries, is a non-conductor, so to compensate for its low electrical conductivity, sulfur-carbon composites are commonly used, which are composites of sulfur with conductive carbon materials.

[0037] However, in the case of existing sulfur-carbon composites, lithium polysulfide formed during the electrochemical oxidation-reduction reaction of lithium-sulfur batteries leaks into the electrolyte, resulting in sulfur loss. This results in a rapid decrease in the amount of sulfur involved in the electrochemical reaction, making it impossible to achieve the full theoretical discharge capacity and energy density during actual operation. Furthermore, sulfur transforms into lithium sulfide (Li2S) upon full discharge, causing an approximately 80% volume expansion, reducing the void volume inside the positive electrode and making it difficult to contact the electrolyte. Furthermore, lithium polysulfide shuttles between the positive and negative electrodes, resulting in a cyclic reaction that consumes electrons without being completely reduced, resulting in reduced charge and discharge efficiency and lifespan.

[0038] Therefore, in the prior art, methods have been proposed, such as increasing the sulfur loading amount, changing the type or mixing method of the carbon material, or introducing a coating layer to suppress the elution of lithium polysulfide. However, these methods not only did not effectively improve the performance of lithium-sulfur batteries, but also caused serious problems with battery stability and were inefficient in terms of process.

[0039] Therefore, the present invention provides a cathode containing a sulfur-carbon composite, which includes a porous carbon material whose BET specific surface area and particle size are controlled within specific ranges, in order to improve the electrochemical reactivity, stability, and electrical conductivity of the sulfur-carbon composite and thereby ensure improved capacity and life characteristics of a lithium-sulfur battery containing the sulfur-carbon composite.

[0040] Specifically, the cathode active material according to the present invention comprises a sulfur-carbon composite, which contains a porous carbon material and sulfur, and the sulfur is supported in the pores of the porous carbon material. The carbon material has pores with a diameter of less than 3 nm at a rate of 90 vol% or more and pores with a diameter of less than 1 nm at a rate of 60 vol% or more, based on 100 vol% of the total pores.

[0041] On the other hand, in the present invention, the carbon material has a BET specific surface area of ​​1,600 m 2 It is preferable that the .beta. / g is exceeded.

[0042] On the other hand, in the present invention, the carbon material has a total pore volume of 0.8 cm 3 / g or more.

[0043] The carbon material acts as a support, providing a framework for uniformly and stably immobilizing sulfur, thereby complementing the low electrical conductivity of sulfur and facilitating the smooth electrochemical reaction. In particular, the sulfur-carbon composite has a large BET specific surface area and an appropriate diameter size of the carbon material acting as a sulfur support, resulting in low irreversible capacity and high energy density despite a high sulfur loading. In other words, it has a structure that can increase the utilization rate of sulfur during electrochemical reactions.

[0044] In conventional sulfur-carbon composites, the use of carbon materials with a high specific surface area has been proposed to increase the amount of sulfur supported and improve reactivity. However, the relationship between the particle size of the carbon material and the sulfur utilization rate has not been clearly understood, making it difficult to realize high-capacity lithium-sulfur batteries.

[0045] Therefore, in the present invention, by controlling the BET specific surface area and diameter of the carbon material serving as a sulfur support within a specific range, sulfur can be uniformly dispersed on the internal and external surfaces of the carbon material, while the irreversible capacity can be reduced and the electrochemical reactivity of sulfur can be enhanced. Furthermore, the use of the carbon material improves the electrochemical reactivity, stability, and electrical conductivity of the sulfur-carbon composite, thereby improving the capacity and life characteristics of the lithium-sulfur battery and ensuring optimal charge-discharge performance even when sulfur loss or volume change occurs during charge and discharge.

[0046] In the sulfur-carbon composite of the present invention, the carbon material used as the sulfur support can generally be produced by carbonizing various carbon precursors.

[0047] The carbon material may contain a large number of pores on the surface and inside, which may vary. The carbon material has a ratio of pores with a diameter of less than 3 nm of 90 vol% or more and a ratio of pores with a diameter of less than 1 nm of 60 vol% or more. Meanwhile, in the present invention, the carbon material has a BET specific surface area of ​​1,600 m 2 For example, the BET specific surface area is preferably greater than 1,800 m / g. 2 / g or more, 2,000m 2 / g or more, or 2,500m 2 / g or more.

[0048] The carbon material may include porous and conductive carbon-based materials commonly used in the art, such as graphite, graphene, carbon blacks such as denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black, carbon nanotubes (CNTs) such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), carbon fibers such as graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs), graphites such as natural graphite, artificial graphite, and expanded graphite, carbon nanoribbons, carbon nanobelts, carbon nanorods, and activated carbon.

[0049] In the present invention, the carbon material preferably comprises spherical or pseudo-spherical activated carbon, the spherical and pseudo-spherical shapes having a circularity of 50% or more, 55% or more, or 60% or more according to the following formula 1:

[0050] When the circularity of the carbon material satisfies the above range, it exhibits excellent dispersibility during the electrode slurry preparation process, which helps improve the physical properties of the slurry and allows the preparation of a uniform electrode. Furthermore, since a uniform reaction is induced during the battery reaction process, the reversibility of the electrode active material can be improved, and the battery life characteristics can be improved.

[0051]

number

[0052] In Equation 1, A is the projected area of ​​the particle projected two-dimensionally, and P is the perimeter of the particle projected two-dimensionally. The circularity can be measured using an image analyzer based on a scanning electron microscope image of the particle. However, this is not particularly limited, and any device, instrument, and software commonly used for measuring circularity can be appropriately selected and applied. In one embodiment of the present invention, the circularity can be measured by photographing a sheet in which particles are dispersed and fixed so that they are aligned parallel to the sheet without piling up, using a scanning electron microscope (S-4800, manufactured by Hitachi High-Tech Corporation) from directly above the sheet, and analyzing the image using an Azokun (manufactured by Asahi Kasei Engineering Co., Ltd.). In this case, the circularity of 100 to 1,000 or 100 to 500 particles can be measured, and the average of the measured circularity values ​​can be used as the circularity of the sample. For example, the circularity of 300 particles can be measured, and the average of the measured circularity values ​​can be used as the circularity of the entire sample. However, the number of particles used to measure the circularity of a sample is not limited to the above range, and an ordinary technician can select an appropriate number.

[0053] In one embodiment of the present invention, the carbon material may contain 70 wt% or more, more preferably 90 wt% or more, of the spherical or pseudo-spherical activated carbon relative to 100 wt% of the carbon material. For example, the carbon material may be composed solely of spherical or pseudo-spherical activated carbon.

[0054] Meanwhile, in one embodiment of the present invention, the spherical or pseudo-spherical activated carbon may have a primary particle diameter (D50) of 500 nm or more, preferably 1 μm or more, and the primary particle diameter (D50) may be controlled to less than 8 μm.

[0055] The sulfur-carbon composite according to the present invention is highly porous as described above, and is effective for forming a structure for enhancing the utilization rate of sulfur in an electrode by using spherical activated carbon as a carbon material, in which the proportion of pores with a diameter of less than 3 nm is 90 vol% or more and the proportion of pores with a diameter of less than 1 nm is 60 vol% or more relative to 100 vol% of the total pores.

[0056] In the present invention, the sulfur-carbon composite contains sulfur. Since sulfur alone does not have electrical conductivity, sulfur is used in combination with the above-mentioned carbon material.

[0057] The sulfur may be inorganic sulfur (S8), Li2S n (n≧1), disulfide compounds such as 2,5-dimercapto-1,3,4-thiadiazole, and 1,3,5-trithiocyanuric acid, organic sulfur compounds, and carbon-sulfur polymers (C2S x ) n , x=2.5 to 50, n≧2). Preferably, inorganic sulfur (S8) may be contained.

[0058] In one embodiment of the present invention, the cathode active material may contain 50 wt% or more, 70 wt% or more, 90 wt% or more, or 95 wt% or more of the sulfur-carbon composite having the above-described characteristics, based on 100 wt% of the cathode active material. In another embodiment of the present invention, the cathode active material may consist solely of the sulfur-carbon composite.

[0059] In the present invention, the sulfur content in the sulfur-carbon composite is such that the SCP value according to the following formula 2 is more than 0.8 and less than 1.0.

[0060] [Formula 2] SCP = Sulfur content (A) ÷ Pore volume (B) of carbon material In Equation 2, A is the ratio of the mass of sulfur to the mass of the carbon-sulfur composite (mass of sulfur / mass of sulfur-carbon composite), and B is the ratio of the pore volume in the carbon material to the total volume of the carbon material (apparent volume, the volume of carbon only + the volume of pores) (pore volume in the carbon material / apparent volume), and SCP has no unit.

[0061] In one embodiment of the present invention, the true density of the carbon is 2.0 g / cm 3 (excluding the pore volume in the carbon material), the unit volume of carbon is 0.5 cm 3 / g.

[0062] The SCP value means the content of reversibly available sulfur in a carbon material having a specific pore structure.

[0063] If the SCP value of the sulfur-carbon composite satisfies the above range, it is advantageous in terms of effective use of sulfur. If the sulfur content exceeds the above range, sulfur or sulfur compounds that cannot bond with the carbon material may aggregate or re-dissolve on the surface of the porous carbon material, making it difficult to accept electrons and unable to participate in electrochemical reactions, resulting in a loss of battery capacity.

[0064] In the sulfur-carbon composite according to the present invention, the sulfur is located on at least one of the internal and external surfaces of the pores of the carbon material. The sulfur may be present on less than 100%, preferably 1 to 95%, and more preferably 60 to 90% of the entire internal and external surfaces of the carbon material. When the sulfur is present on the surface of the carbon material within the above range, it can exhibit the greatest effects in terms of electron transfer area and electrolyte wettability. Specifically, within this range, sulfur is impregnated thinly and uniformly on the surface of the carbon material, thereby increasing the electron transfer contact area during charge and discharge. If the sulfur is located on 100% of the entire surface area of ​​the carbon material, the carbon material is completely covered with sulfur, reducing the wettability of the electrolyte and its contact with the conductive material contained in the electrode. As a result, the carbon material cannot receive electrons and is not involved in the reaction.

[0065] The sulfur-carbon composite may be prepared by simply mixing sulfur with a carbon material, or may have a core-shell structure, a coating structure, or a supported structure. The core-shell structure may be formed by coating one of sulfur and a carbon material on the other, e.g., the surface of the carbon material may be covered with sulfur, or vice versa. The supported structure may be formed by filling the interior, particularly the internal pores, of the carbon material with sulfur. The sulfur-carbon composite may have any form as long as it satisfies the above-mentioned content ratio of the sulfur-based compound to the carbon material, and is not limited by the present invention.

[0066] The present invention also provides a method for producing the sulfur-carbon composite.

[0067] The method for producing the sulfur-carbon composite according to the present invention is not particularly limited, and any method commonly known in the art, which includes (S1) a step of mixing a carbon material and sulfur, and then (S2) a step of compounding, may be employed.

[0068] The mixing in step (S1) is intended to enhance the degree of mixing of sulfur and the carbon material and may be carried out using a stirrer commonly used in the art. Here, the mixing time and speed may be selectively adjusted depending on the content and conditions of the raw materials.

[0069] The compounding method in step (S2) is not particularly limited in the present invention and may be a method commonly used in the art. For example, methods commonly used in the art, such as dry compounding or wet compounding such as spray coating, may be used. For example, the mixture of sulfur and carbon material obtained after mixing may be pulverized using a ball mill, and then placed in an oven at 120°C to 160°C for 20 minutes to 1 hour, allowing the molten sulfur to uniformly coat the inner and outer surfaces of the carbon material.

[0070] The sulfur-carbon composite prepared by the above-described method has a structure with a high specific surface area and a high sulfur loading, which improves sulfur utilization. This not only improves the electrochemical reactivity of sulfur but also improves the accessibility and contact of the electrolyte, thereby improving the capacity and life characteristics of lithium-sulfur batteries.

[0071] Another embodiment of the present invention relates to a positive electrode containing the sulfur-carbon composite. The positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, a conductive material, and a binder resin. The positive electrode active material may contain 70 wt% or more, preferably 85 wt% or more, of the positive electrode active material relative to 100 wt% of the positive electrode active material layer. In the present invention, the positive electrode active material includes the sulfur-carbon composite described above. In addition to the sulfur-carbon composite, the positive electrode active material may further contain one or more additives selected from a transition metal element, a Group IIIA element, a Group IVA element, sulfur compounds of these elements, and alloys of these elements with sulfur. In one embodiment of the present invention, the positive electrode active material may contain 70 wt% or more, preferably 80 wt% or more, more preferably 90 wt% or more of the sulfur-carbon composite relative to 100 wt% of the positive electrode active material. In one embodiment of the present invention, the positive electrode active material may consist solely of the sulfur-carbon composite.

[0072] In a specific embodiment of the present invention, the positive electrode active material layer may include a lithium transition metal composite oxide represented by the following Chemical Formula 1:

[0073] [Chemical formula 1] Li a Ni b Co c M 1 d M 2 e O2 In the above Chemical Formula 1, M 1 may be Mn, Al or a combination thereof, preferably Mn, or Mn and Al.

[0074] Said M2 is at least one selected from the group consisting of Zr, W, Y, Ba, Ca, Ti, Mg, Ta and Nb, preferably at least one selected from the group consisting of Zr, Y, Mg and Ti, more preferably Zr, Y or a combination thereof. 2 Although the elements are not necessarily contained, when contained in an appropriate amount, they can be useful in promoting grain growth during firing and improving the stability of the crystal structure.

[0075] Meanwhile, various positive electrode current collectors used in the art may be used as the positive electrode current collector. For example, the positive electrode current collector may be made of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like. The positive electrode current collector may typically have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the positive electrode current collector to enhance the adhesive strength of the positive electrode active material. The positive electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0076] The conductive material is used to impart conductivity to the electrode and can be any material that exhibits electronic conductivity without causing chemical changes in the resulting battery. Specific examples include graphite, such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, and carbon nanotubes; metal powders or fibers, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. The conductive material is typically present in an amount of 1 to 30 wt%, preferably 1 to 20 wt%, and more preferably 1 to 10 wt%, based on the total weight of the positive electrode active material layer.

[0077] The binder improves adhesion between positive electrode active material particles and between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These binders may be used alone or in combination. The binder may be present in an amount of 1 to 30 wt%, preferably 1 to 20 wt%, and more preferably 1 to 10 wt%, based on the total weight of the positive electrode active material layer.

[0078] The positive electrode can be manufactured by conventional methods known in the art.

[0079] For example, to describe in detail a method for preparing a positive electrode of the present invention, first, the binder is dissolved in a solvent to prepare a slurry, and then the conductive material is dispersed therein. The solvent used to prepare the slurry is preferably one that can uniformly disperse the positive electrode active material, binder, and conductive material and that is easily evaporated. Representative examples include acetonitrile, methanol, ethanol, tetrahydrofuran, water, and isopropyl alcohol. Next, the positive electrode active material, along with optional additives, is uniformly dispersed in the solvent in which the conductive material was dispersed to prepare a positive electrode slurry. The amounts of the solvent, positive electrode active material, and optional additives contained in the slurry are not particularly important herein; it is sufficient for the slurry to have an appropriate viscosity to facilitate coating.

[0080] The slurry thus prepared is applied to a current collector and dried under vacuum to form a positive electrode. The slurry can be coated on the current collector to an appropriate thickness depending on the viscosity of the slurry and the thickness of the positive electrode to be formed.

[0081] The coating may be performed by a method commonly known in the art, for example, by dispensing the positive electrode active material slurry onto one side of the positive electrode current collector and then uniformly dispersing it using a doctor blade, etc. Alternatively, the coating may be performed by methods such as die casting, comma coating, screen printing, etc.

[0082] The drying is not particularly limited, but can be performed in a vacuum oven at 50°C to 200°C within one day.

[0083] The present invention also provides a lithium-sulfur battery including an electrode assembly including a positive electrode having the sulfur-carbon composite and an electrolyte. The electrode assembly includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

[0084] The electrode assembly may be stacked with a separator interposed between the negative electrode and the positive electrode to form a stack or stack / fold structure, or may be wound up to form a jelly roll structure. When forming a jelly roll structure, a separator may be further disposed on the outside to prevent contact between the negative electrode and the positive electrode.

[0085] The negative electrode includes a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector, and the negative electrode active material layer includes a negative electrode active material, a conductive material, and a binder.

[0086] Next, the negative electrode will be described in more detail.

[0087] The negative electrode may include a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector, and the negative electrode active material layer may include a negative electrode active material, a conductive material, and a binder.

[0088] Specifically, the negative electrode may be fabricated by coating one or both sides of a long sheet-shaped negative electrode current collector with a negative electrode slurry prepared by dispersing a negative electrode active material, a conductive material, and a binder in a solvent such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and then drying the coated negative electrode current collector to remove the solvent. The coated negative electrode may also include a non-coated portion on a portion of the negative electrode current collector, e.g., one end of the negative electrode current collector, where the negative electrode slurry is not applied.

[0089] The negative electrode active material is a lithium ion (Li +Substances that can be reversibly inserted (intercalated, occluded) or deinserted (deintercalated, desorbed, released), substances that can react with lithium ions to reversibly form lithium-containing compounds, or lithium metal or lithium alloys may be included. The substances that can reversibly insert or deinsert the lithium ions can be, for example, crystalline carbon, amorphous carbon, or a mixture thereof, and specifically, artificial graphite, natural graphite, graphitized carbon fibers, amorphous carbon, soft carbon, hard carbon, etc. can be cited as examples, but are not limited thereto. The substances that can react with the lithium ions to reversibly form lithium-containing compounds can be, for example, tin oxide, titanium nitrate, or silicon-based compounds. The lithium alloy can be, for example, an alloy of lithium (Li) and a metal 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). Preferably, the negative electrode active material can be lithium metal, and specifically, it can be in the form of a lithium metal thin film or lithium metal powder. The silicon-based negative electrode active material can be Si, Si-Me alloy (where Me is one or more selected from the group consisting of Al, Sn, Mg, Cu, Fe, Pb, Zn, Mn, Cr, Ti, and Ni), SiO y (where 0 < y < 2), Si-C composite, or a combination thereof, and preferably, SiO y (where 0 < y < 2). Since the silicon-based negative electrode active material has a high theoretical capacity, when a silicon-based negative electrode active material is included, the capacity characteristics can be improved.

[0090] The negative electrode current collector may be a negative electrode current collector commonly used in the art, such as copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy. The negative electrode current collector typically has a thickness of 3 μm to 500 μm, and, like the positive electrode current collector, the surface of the current collector may be provided with fine irregularities to enhance the binding strength of the negative electrode active material. The negative electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0091] The conductive material is used to impart conductivity to the negative electrode. Any material that exhibits electronic conductivity without causing chemical changes in the resulting battery can be used without any particular limitations. Specific examples include graphite, such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, and carbon nanotubes; metal powders or fibers, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. The conductive material is typically present in an amount of 1 to 30 wt %, preferably 1 to 20 wt %, and more preferably 1 to 10 wt %, based on the total weight of the negative electrode active material layer.

[0092] The binder improves adhesion between negative electrode active material particles and between the negative electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination. The binder may be included in an amount of 1 to 30 wt%, preferably 1 to 20 wt%, and more preferably 1 to 10 wt%, based on the total weight of the negative electrode active material layer.

[0093] Meanwhile, the electrode assembly further includes a separator, which is disposed within the electrode assembly so as to be interposed between the negative electrode and the positive electrode. The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used as a separator in lithium secondary batteries can be used without any particular limitation. Specifically, the separator may be a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof. Alternatively, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of a high-melting point glass fiber or polyethylene terephthalate fiber, may be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material may be used.

[0094] Another embodiment of the present invention relates to an electrochemical device including the electrode assembly. The electrochemical device includes a battery case containing the electrode assembly and an electrolyte, and the battery case may be of any suitable type commonly used in the art, such as a pouch type or a metal can type, without any particular limitation.

[0095] The electrolyte used in the present invention may be any of various electrolytes that can be used in lithium secondary batteries, such as organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes, and the type thereof is not particularly limited.

[0096] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0097] The organic solvent can be used without any particular limitation as long as it can function as a medium through which ions involved in the electrochemical reaction of the battery can move. Specific examples of the organic solvent include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; carbonate-based solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; and R-CN (where R is C2 to C6). 20 Nitriles such as dimethylformamide, dioxolanes such as 1,3-dioxolane, or sulfolanes may be used.

[0098] On the other hand, in one embodiment of the present invention, the non-aqueous solvent of the electrolyte preferably contains an ether-based solvent from the viewpoint of improving the charge / discharge performance of the battery. Examples of such ether solvents include cyclic ethers (e.g., 1,3-dioxolane, tetrahydrofuran, tetrahydropyran, etc.), chain ether compounds (e.g., 1,2-dimethoxyethane, etc.), low-viscosity fluorinated ethers (e.g., 1H,1H,2′H,3H-decafluorodipropyl ether, difluoromethyl 2,2,2-trifluoroethyl ether, 1,2,2,2-tetrafluoroethyl trifluoromethyl ether, 1,1,2,3,3,3-hexafluoropropyl di ... ether, 1H,1H,2′H,3H-Decafluorodipropyl ether, Pentafluoroethyl 2,2,2-trifluoroethyl ether, 1H,1H,2′H-Perfluorodipropyl ether, and a mixture of one or more of these may be contained as the non-aqueous solvent.

[0099] The lithium salt can be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, examples of the lithium salt 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. The lithium salt can be used at a concentration of 0.1 to 5.0 M, preferably 0.1 to 3.0 M. When the lithium salt is used within this concentration range, the electrolyte has appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0100] In addition to the electrolyte components, the electrolyte may further contain additives for purposes such as improving battery life characteristics, suppressing battery capacity loss, and improving battery discharge capacity. Examples of additives include, but are not limited to, haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, and aluminum trichloride, which may be used alone or in combination. The additives may be present in an amount of 0.1 to 10 wt %, preferably 0.1 to 5 wt %, based on the total weight of the electrolyte.

[0101] The shape of the lithium-sulfur battery is not particularly limited, and various shapes such as a cylindrical type, a laminated type, and a coin type can be used.

[0102] The present invention also provides a battery module including the lithium-sulfur battery as a unit cell, which can be used as a power source for medium- to large-sized devices that require high-temperature stability, long cycle characteristics, and high capacity characteristics.

[0103] Examples of the medium- to large-sized devices include, but are not limited to, power tools powered by battery-powered motors; electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric two-wheeled vehicles including electric bicycles (E-bikes) and electric scooters; electric golf carts; and power storage systems.

[0104] Preferred examples are presented below to aid in understanding the present invention. However, the following examples are merely illustrative of the present invention, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope of the scope and technical idea of ​​the present invention. It is to be understood that such changes and modifications also fall within the scope of the appended claims.

[0105] Examples and Comparative Examples [Production of sulfur-carbon composite] Activated carbon and sulfur (S8) were uniformly mixed in the weight ratios shown in Table 2 below, pulverized in a ball mill, and then placed in an oven at 155°C for 30 minutes to prepare sulfur-carbon composites. In Table 2 below, SCP is a value calculated based on Equation 2 above. Table 1 shows the characteristics of the carbon materials used in each example and comparative example.

[0106] [Battery manufacturing] A positive electrode slurry composition was prepared by mixing 90 wt % of the prepared sulfur-carbon composite as a positive electrode active material, 5 wt % of Denka black as a conductive material, and 5 wt % of styrene-butadiene rubber / carboxymethyl cellulose (SBR:CMC=7:3 weight ratio) as a binder in a solvent.

[0107] The prepared positive electrode slurry composition was applied to a thickness of 350 μm on an aluminum current collector having a thickness of 20 μm, dried at 50° C. for 12 hours, and pressed with a roll press to prepare a positive electrode.

[0108] A 35 μm-thick lithium metal thin film was used as the negative electrode together with the positive electrode, and a mixed solution of 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in an organic solvent consisting of dimethyl ether and 1,2,2,2-tetrafluoroethyl trifluoromethyl ether (DME:TTE=1:1 (volume ratio)) was used as the electrolyte.

[0109] Specifically, the prepared positive and negative electrodes were arranged face to face, and a polyethylene separator having a thickness of 20 μm and a porosity of 45 vol% was interposed between them. Then, 70 μL of the prepared electrolyte was injected to prepare a lithium-sulfur battery.

[0110] [Table 1]

[0111] [Table 2]

[0112] [Table 3]

[0113] Experimental example 1. Evaluation of the physical properties of carbon materials The specific surface area, total pore volume, and average pore diameter of the carbon materials used in the Production Examples were measured. Specifically, for each carbon material used in the Production Examples, the nitrogen adsorption and desorption amounts were measured under vacuum using a specific surface area analyzer (model name: BELSORP-MINI, manufacturer: BEL Japan Inc.). This resulted in an isothermal adsorption / desorption curve, and the specific surface area, total pore volume, pore volume (vol%) with a diameter of less than 3 nm, pore volume (vol%) with a diameter of less than 1 nm, and pore volume were calculated using the BET (Brunaure-Emmett-Teller) method.

[0114] Experimental Example 2: Evaluation of cycle characteristics After three cycles of activation at a 0.1C / 0.1C rate in the range of 1.0 to 3.6V under a constant temperature condition of 25°C, cycle evaluation was performed under 0.3C / 0.5C charge / discharge conditions.

[0115] Experimental Example 3: Method for measuring particle size Using a particle size analyzer (model: Bluewave, manufacturer: Microtrac), the dry method was used to measure D 50 When the carbon material was aggregated into secondary particles, the primary particle size was observed and measured using a scanning electron microscope (model name: SEM, manufacturer: JEOL).

[0116] As shown in Tables 1 and 2, in Examples 1 to 3, the BET of the carbon material was 1,600 m 2 / g, the ratio of pores with a diameter of less than 3 nm to the total pore volume of the carbon material was 90 vol% or more, the ratio of pores with a diameter of less than 1 nm to the total pore volume of the carbon material was 60 vol% or more, the SCP was more than 0.8 and less than 1, and the circularity was 50% or more. As a result, as shown in Table 3, the remaining capacity was able to be maintained at 80% or more even after 120 cycles.

[0117] On the other hand, when the pore diameter of the carbon material did not satisfy the range according to the present invention (Comparative Examples 1, 2, and 5), when the SCP value did not satisfy the range according to the present invention (Comparative Examples 1, 3, 4, 5, 6, and 7), and when the circularity did not satisfy the range according to the present invention (Comparative Examples 2 and 5), the remaining capacity was 80% or less before 100 cycles. In particular, in Comparative Examples 3 and 6, where the SCP value was 1 or more, the remaining capacity was 80% or less even before 10 cycles.

Claims

1. a positive electrode active material including a sulfur-carbon composite; The sulfur-carbon composite comprises a porous carbon material and sulfur; the porous carbon material comprises activated carbon; The porous carbon material has a ratio of pores having a pore diameter of less than 3 nm of 90 vol% or more relative to 100 vol% of the total pores, and a ratio of pores having a pore diameter of less than 1 nm of 60 vol% or more relative to 100 vol% of the total pores, The sulfur-carbon composite has an SCP value of more than 0.8 and less than 1 as determined by the following formula 2: [Formula 2] SCP = Sulfur content (A) ÷ Pore volume (B) of carbon material In the formula 2, A is the ratio of the mass of sulfur to the mass of the carbon-sulfur composite, and B is the ratio of the pore volume in the carbon material to the total volume (apparent volume) of the carbon material. The ratio of the mass of sulfur to the mass of the carbon-sulfur composite is 60 mass% or more, The sulfur-carbon composite is a composite material obtained by simply mixing sulfur and a carbon material, and the sulfur is located on the inner and outer surfaces of pores of the carbon material.

2. The porous carbon material has a specific surface area (BET) of 1,600 m ２ 2. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the positive electrode has a Cr content greater than 1.0%.

3. 2. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the porous carbon material has a circularity of 50% or more.

4. The porous carbon material has a pore volume of 0.8 cm ３ 2. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the Sr content is 1 / g or more.

5. 2. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the porous carbon material contains 95 wt % or more of activated carbon relative to 100 wt % of the carbon material.

6. 2. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the positive electrode active material contains 70 wt % or more of the sulfur-carbon composite with respect to 100 wt % of the positive electrode active material.

7. The positive electrode for a lithium-sulfur battery according to claim 1 , wherein the positive electrode further comprises a binder resin and a conductive material.

8. 10. A lithium-sulfur battery comprising: a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, wherein the electrolyte contains one or more of a cyclic ether, a linear ether, and a fluorinated ether, and the positive electrode is the positive electrode for a lithium-sulfur battery according to claim 1.

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