Carbon-sulfur composite, method for preparing the same, and positive electrode and lithium secondary battery comprising the same

Abstract

The present invention relates to a carbon-sulfur composite, a method for preparing the same, and a positive electrode and a lithium secondary battery comprising the same. The carbon-sulfur composite comprises: a carbonized metal-organic framework (MOF); and a sulfur compound introduced into at least a portion of an outer surface and an inner portion of the carbonized metal-organic framework, wherein the carbonized metal-organic framework has a specific surface area of 2500 m 2 / g to 4000 m 2 / g, and the carbonized metal-organic framework has a pore volume of 0.1 cc / g to 10 cc / g. The present invention can effectively improve the electrochemical performance of a lithium-sulfur battery, such as initial discharge capacity or cycle retention capacity, without introducing other methods, by adjusting only the carbonization temperature.

Description

[0001] This invention patent application is a divisional application of Chinese patent application filed on September 19, 2018, with application number 201880058927.7 and invention title "Carbon-sulfur composite, preparation method thereof and lithium secondary battery comprising the same". Technical Field

[0002] This application claims priority and benefit to Korean Patent Application No. 10-2017-0125909, filed with the Korean Intellectual Property Office on September 28, 2017, the entire contents of which are incorporated herein by reference.

[0003] This invention relates to a carbon-sulfur composite, a method for preparing the same, and a lithium secondary battery comprising the same. Background Technology

[0004] Recently, there has been a growing interest in energy storage technologies. With applications expanding to power mobile phones, cameras, and laptops, and further to electric vehicles, there has been increasing research and development into electrochemical devices.

[0005] Electrochemical devices are the area that has received the most attention in these fields. Among them, the development of rechargeable and discharging secondary batteries has become a focus of attention, and the development of such batteries has progressed to the research and development of new electrode and battery design to improve capacity density and specific energy.

[0006] Among the rechargeable batteries currently in use, lithium rechargeable batteries, developed in the early 1990s, have attracted attention due to their advantages of higher operating voltage and significantly higher energy density compared to conventional batteries that use aqueous electrolytes, such as Ni-MH, Ni-Cd, and lead-sulfuric acid batteries.

[0007] In particular, lithium-sulfur (Li-S) batteries are secondary batteries that use sulfur-series materials with sulfur-sulfur bonds as the positive electrode active material and lithium metal as the negative electrode active material. Sulfur, as the main material for the positive electrode active material, has the advantages of being abundant in resources, non-toxic, and having a low atomic weight. In addition, the theoretical discharge capacity of lithium-sulfur batteries is 1675 mAh / g-sulfur, and the theoretical energy density is 2600 Wh / kg. This is very high compared with the theoretical energy densities of other battery systems currently under study (Ni-MH battery: 450 Wh / kg, Li-FeS battery: 480 Wh / kg, Li-MnO2 battery: 1000 Wh / kg, Na-S battery: 800 Wh / kg). Therefore, lithium-sulfur batteries are the most promising batteries developed to date.

[0008] During the discharge reaction of a lithium-sulfur battery, lithium oxidation occurs at the negative electrode (anode), and sulfur reduction occurs at the positive electrode (cathode). Sulfur, prior to discharge, has a cyclic S8 structure and uses a redox reaction to store and generate electrical energy. In this redox reaction, during the reduction reaction (discharge), the oxidation number of sulfur decreases as the S-S bonds break, and during the oxidation reaction (charging), the oxidation number of sulfur increases as the S-S bonds reform. During this reaction, sulfur is converted from a cyclic S8 structure to a linear lithium polysulfide (Li2S) structure via reduction. x (x = 8, 6, 4, and 2), resulting in the final formation of lithium sulfide (Li₂S) when these lithium polysulfides are completely reduced. Through the process of reduction to each type of lithium polysulfide, the discharge behavior of lithium-sulfur batteries exhibits a gradual discharge voltage that differs from that of lithium-ion batteries.

[0009] However, in such lithium-sulfur batteries, the following problems need to be addressed: the low conductivity of sulfur; the problem of lithium polysulfide dissolution and volume expansion during charging and discharging, resulting in low coulombic efficiency; and the rapid capacity reduction due to charging and discharging.

[0010] Porous carbon materials are widely used in lithium-sulfur batteries, where they provide conductivity through their combination with sulfur (i.e., the active material in lithium-sulfur batteries). The improvement of battery performance by controlling the size and volume of pores formed within these porous carbon materials has been continuously studied. Among these, metal-organic frameworks (MOFs) have the advantage that, depending on the type of organic molecules or metal atoms forming the MOF, pores of various sizes can be formed, including those with diameters of 1000 μm. 2 / g to 4000m 2 / g of porous carbon materials with high specific surface area. However, as described in Adv. Funct. Mater. 2016, 26, 8746-8756, the problem with the technique of using existing MOF materials in lithium-sulfur batteries by carbonization is that metal carbides are still required because the performance is poor in actual use in batteries.

[0011] (Non-patent literature 1) "3D Metal Carbide@Mesoporous Carbon Hybrid Architecture as a New Polysulfide Reservoir for Lithium-Sulfur Batteries", Weizhai Bao, Dawei Su, Wenxue Zhang, Xin Guo, and Guoxiu Wang*, Adv. Funct. Mater. 2016, 26, 8746-8756 Summary of the Invention

[0012] [Technical Issues]

[0013] As a result of extensive research conducted in light of the above circumstances, the inventors of this invention have confirmed the importance of carbonization temperature by adjusting the temperature of the carbonization process, thus demonstrating that battery performance depends on the carbonization temperature. It has been determined that further development of the pore structure by increasing the carbonization temperature has become an important parameter for battery performance. In view of the above, the inventors of this invention have confirmed that when MOF-5-derived mesoporous carbon is combined with sulfur as an active material and the resulting product is used in lithium-sulfur batteries, the electrochemical performance of lithium-sulfur batteries, such as initial discharge capacity or cycle retention capacity, can be improved, thus completing this invention.

[0014] Therefore, one aspect of the present invention provides a carbon-sulfur composite and a method for preparing the same, wherein the carbon-sulfur composite can improve battery performance simply by adjusting the carbonization temperature without introducing other methods.

[0015] [Technical Solution]

[0016] According to one aspect of the present invention, a carbon-sulfur composite is provided, the carbon-sulfur composite comprising: a carbonized metal-organic framework (MOF); and a sulfur compound, the sulfur compound being introduced into at least a portion of the outer surface and interior of the carbonized metal-organic framework, wherein the carbonized metal-organic framework has a diameter of 2000 m. 2 / g to 3500m 2 The specific surface area is 2.2 cc / g, and the carbonized metal-organic framework has a pore volume of more than 2.2 cc / g.

[0017] According to another aspect of the present invention, a method for preparing a carbon-sulfur complex is provided, the method comprising the steps of: (a) preparing a carbonized metal-organic framework (MOF) by carbonizing a metal-organic framework (MOF) at a temperature above 950°C; and (b) preparing a carbon-sulfur complex by mixing the carbonized metal-organic framework (MOF) in step (a) with a sulfur compound.

[0018] According to another aspect of the invention, a positive electrode comprising the carbon-sulfur complex is provided.

[0019] According to another aspect of the present invention, a lithium secondary battery is provided, comprising: the positive electrode; the negative electrode; and an electrolyte.

[0020] [Beneficial Effects]

[0021] This invention can effectively improve the electrochemical performance of lithium-sulfur batteries, such as initial discharge capacity or cycle retention capacity, simply by adjusting the carbonization temperature without introducing other methods. Attached Figure Description

[0022] Figure 1 This is a graph showing the N2 adsorption / desorption isotherms of the sulfur-carbon complex according to embodiments and comparative examples of the present invention.

[0023] Figure 2 This is a graph showing the results of quenched solid density functional theory analysis of sulfur-carbon composites according to embodiments and comparative examples of the present invention.

[0024] Figure 3 This is a graph showing the N2 adsorption / desorption isotherms of a sulfur-carbon complex according to another comparative example of the invention.

[0025] Figure 4 SEM images of sulfur-carbon composites according to embodiments and comparative examples of the present invention are shown.

[0026] Figure 5 This is a graph showing the initial charge and discharge performance of lithium-sulfur batteries manufactured from the sulfur-carbon composites of the embodiments and comparative examples of the present invention.

[0027] Figure 6 This is a graph showing the initial charge and discharge performance of a lithium-sulfur battery made from a sulfur-carbon composite of another comparative example of the present invention.

[0028] Figure 7 This is a graph showing the initial charge and discharge performance of a lithium-sulfur battery made from a sulfur-carbon composite of another comparative example of the present invention.

[0029] Figure 8This is a graph showing the charging and discharging efficiency of lithium-sulfur batteries with sulfur-carbon composites according to embodiments and comparative examples of the present invention. Detailed Implementation

[0030] The invention will now be described in detail with reference to the accompanying drawings, enabling those skilled in the art to readily implement it. However, the invention can be embodied in various different forms and is not limited to this specification.

[0031] In the accompanying drawings, parts not relevant to the description are omitted for clarity, and similar reference numerals are used for similar elements throughout the specification. Furthermore, the dimensions and relative sizes of the components shown in the drawings are not actual scale and may be reduced or exaggerated for clarity of description.

[0032] The carbon-sulfur composite of the present invention comprises: a carbonized metal-organic framework (MOF); and a sulfur compound, said sulfur compound being introduced into at least a portion of the outer surface and interior of said carbonized metal-organic framework.

[0033] Carbonized metal-organic framework

[0034] The carbon-sulfur complex of the present invention comprises a carbonized metal-organic framework (MOF).

[0035] Metal-organic frameworks (MOFs) are porous materials that form one-dimensional, two-dimensional, or three-dimensional skeletons by crosslinking the coordination bonds of inorganic nodes (metal ions or metal oxide clusters) with multi-coordination organic linkers. They are also known as "porous coordination polymers" or "porous organic-inorganic hybrid materials." MOFs possess coordination vacancies at the metal center and well-defined pores, and have been used in adsorbents, gas storage materials, sensors, membranes, functional thin films, drug delivery materials, catalysts, catalyst supports, etc., to capture guest molecules or individual molecules. Recently, MOFs have been actively studied.

[0036] The following methods are commonly used to prepare such metal-organic frameworks: solvothermal methods, in which metal and organic ligand precursors are dissolved in a suitable solvent and the result is reacted under high temperature and high pressure; gas-phase diffusion methods, in which different solvents are diffused and permeated to reduce the solubility of the solvent in which the precursor is dissolved; and layer diffusion methods, in which a layer is formed between two solutions containing different precursors to produce diffusion between the two layers.

[0037] The metal-organic framework (MOF) used in this invention may contain structural units represented by the following chemical formula 1.

[0038] [Chemical Formula 1]

[0039] [M x (L)y ]

[0040] (In chemical formula 1,

[0041] M is a metal selected from one or more of the following: copper (Cu), zinc (Zn), iron (Fe), nickel (Ni), chromium (Cr), scandium (Sc), cobalt (Co), titanium (Ti), manganese (Mn), vanadium (V), aluminum (Al), magnesium (Mg), gallium (Ga), and indium (In).

[0042] L is an organometallic ligand selected from one or more of the following types: 1,4-phthalic acid (BDC), 1,3,5-benzenetricarboxylic acid (BTC), 1,1'-biphenyl-3,3',5,5'-tetracarboxylic acid (BPTC), and 2-(N,N,N',N'-tetra(4-carboxyphenyl)-biphenyl-4,4'-diamine (TCBTDA).

[0043] x is an integer from 2 to 6, and y is an integer from 2 to 12.

[0044] In this invention, a metal-organic framework is carbonized, and the carbonized metal-organic framework (MOF) is used. The method of carbonizing the metal-organic framework is not particularly limited, as long as it is a method that can contain carbon in the metal-organic framework. Preferably, the metal-organic framework (MOF) can be carbonized at a temperature of 950°C or higher.

[0045] In the carbonized metal-organic framework (MOF) prepared as described above, elements other than carbon can be removed from the MOF through a carbonization process.

[0046] The carbonized metal-organic framework of the present invention prepared as described above can have a density of 1000 m 2 / g to 4000m 2 / g, preferably 1500m 2 / g to 3000m 2 / g, optimal value 2000m 2 / g to 2500m 2 Specific surface area per g. Specific surface area range greater than 4000 m². 2 The problem with / g is that it results in more processing time and cost compared to the processing time and cost required to achieve the specific surface area, and this range is less than 1000m. 2 The problem with / g is that it may not be able to adequately load sulfur.

[0047] Furthermore, the carbonized metal-organic frameworks described above can have pore volumes ranging from 0.1 cc / g to 10 cc / g, preferably from 2.2 cc / g to 3.0 cc / g, and most preferably from 2.2 cc / g to 2.5 cc / g. The problem with pore volumes less than 0.1 cc / g is insufficient space for sulfur loading, while the problem with pore volumes greater than 10 cc / g is a reduction in specific surface area. Pore volumes can be measured using conventional methods used in the art, and preferably using the Barrett-Joyner-Halenda (BJH) method, density functional theory (DFT), etc.

[0048] carbon-sulfur complex

[0049] The carbon-sulfur complex of the present invention comprises a sulfur compound introduced into at least a portion of the outer surface and interior of a carbonized metal-organic framework.

[0050] As sulfur compounds, various sulfur compounds used in lithium-sulfur batteries can be used, including elemental sulfur (S₈), sulfur series compounds, or mixtures thereof. Sulfur series compounds can specifically be selected from the following: dissolved solid Li₂S n (n≥1) positive electrode electrolyte, organic sulfur compounds and carbon-sulfur polymers [(C2S x ) n (x = 2.5 to 50, n ≥ 2).

[0051] Due to the various sizes of pores within the framework and the three-dimensionally interconnected and regularly arranged pores, the carbon-sulfur composite can be loaded with sulfur at a high content. Therefore, even when soluble polysulfides are generated by electrochemical reactions but located within the carbon-sulfur composite, the three-dimensional entangled structure is maintained, and the phenomenon of electrode structure destruction can be suppressed, even in the event of polysulfide dissolution. As a result, lithium-sulfur batteries containing sulfur-carbon composites exhibit the advantage of high capacity even under high loads. The sulfur loading of the carbon-sulfur composite according to the invention can be 1 mg / cm³. 2 Up to 20 mg / cm 2 .

[0052] In carbon-sulfur composites, the weight ratio of carbonized metal-organic frameworks to sulfur compounds can be from 9:1 to 1:9, and preferably from 5:5 to 1:9. When the content of sulfur or sulfur compounds is less than the above range, the content of the carbon-sulfur composite increases, and with the increase of carbon content, the amount of binder added needs to be increased during slurry preparation. The increase in the amount of binder ultimately increases the sheet resistance of the electrode and acts as an insulator to prevent electron migration, which may reduce battery performance. When the content of sulfur or sulfur compounds is greater than the above range, sulfur or sulfur compounds that are not bound to the composite aggregate or re-dissolve onto the composite surface, making electron acceptance difficult and direct participation in electrode reactions may become difficult.

[0053] Methods for preparing carbon-sulfur complexes containing carbonized metal-organic frameworks

[0054] The carbon-sulfur complex of the present invention is prepared by: (a) preparing a carbonized metal-organic framework (MOF) by carbonizing a metal-organic framework (MOF) at a temperature above 950°C; and (b) preparing the carbon-sulfur complex by mixing the carbonized metal-organic framework (MOF) in (a) with a sulfur compound.

[0055] First, the method for preparing carbon-sulfur composites according to the present invention includes (a) preparing carbonized metal-organic frameworks (MOFs) by carbonizing them at temperatures above 950°C.

[0056] In step (a), the metal-organic framework (MOF) can be carbonized at temperatures above 950°C, preferably between 950°C and 2000°C, and more preferably between 950°C and 1500°C.

[0057] The method of carbonizing metal-organic frameworks is not particularly limited, as long as it is a method that can contain carbon in the metal-organic framework, and preferably, the metal-organic framework can be carbonized in an atmosphere such as argon or nitrogen.

[0058] When the metal-organic framework (MOF) is carbonized above 950°C as in step (a), carbon with a further developed pore structure is prepared, and the use of this carbon in lithium-sulfur batteries has the advantages of improving initial capacity and improving cycle retention capacity.

[0059] The carbonized metal-organic framework of the present invention prepared as described above can have a density of 1000 m 2 / g to 4000m 2 / g, preferably 1500m 2 / g to 3000m 2 / g, optimal value 2000m 2 / g to 2500m 2Specific surface area per g. Specific surface area range greater than 4000 m². 2 The problem with / g is that it results in more processing time and cost compared to the processing time and cost required to achieve the specific surface area, and this range is less than 1000m. 2 The problem with / g is that it may not be able to adequately load sulfur.

[0060] Furthermore, the carbonized metal-organic frameworks described above can have pore volumes ranging from 0.1 cc / g to 10 cc / g, preferably from 2.2 cc / g to 3.0 cc / g, and most preferably from 2.2 cc / g to 2.5 cc / g. The problem with pore volumes less than 0.1 cc / g is insufficient space for sulfur loading, while the problem with pore volumes greater than 10 cc / g is a reduction in specific surface area. Pore volumes can be measured using conventional methods used in the art, and preferably using the Barrett-Joyner-Halenda (BJH) method, density functional theory (DFT), etc.

[0061] The other properties of the metal-organic framework (MOF) used in step (a) are the same as described above.

[0062] Subsequently, the method for preparing carbon-sulfur complexes according to the present invention includes (b) preparing carbon-sulfur complexes by mixing a metal-organic framework (MOF) carbonized in (a) with a sulfur compound.

[0063] In step (b), the method of mixing sulfur compounds is not particularly limited in this invention, and known methods can be used.

[0064] As an example of a method for mixing sulfur compounds, carbonized metal-organic frameworks and sulfur compound powders can be uniformly mixed, the mixture heated, and the molten sulfur compound can be impregnated into the carbonized metal-organic frameworks.

[0065] Here, sulfur compounds can be mixed by flowing into nearby carbonized metal-organic frameworks due to capillary action.

[0066] The heating temperature can be from 115°C to 180°C, more specifically from 150°C to 160°C. According to one embodiment, sulfur can also be uniformly coated around the carbonized metal-organic framework, rather than coated in the gaps between the carbonized metal-organic framework.

[0067] The heating time can be adjusted according to the content of sulfur compounds and the content of carbonized metal-organic frameworks. For example, the heating time can be more than 10 seconds or more than 30 seconds and less than 2 hours, less than 1 hour, less than 30 minutes, or less than 10 minutes.

[0068] When the melting temperature is below 115°C, sulfur compound particles will not melt, and sulfur compounds may not be properly impregnated into the carbonized metal-organic framework.

[0069] Sulfur compounds can be impregnated by dissolving them in an organic solvent and then growing them by adding a carbonized metal-organic framework.

[0070] The organic solvent may be one or a mixture of two or more of the following: ethanol, toluene, benzene, N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), acetone, chloroform, dimethylformamide, cyclohexane, tetrahydrofuran, and dichloromethane.

[0071] Impregnation with sulfur compounds can be performed by mixing carbonized metal-organic frameworks with sulfur compound powder and then impregnating them using ball milling.

[0072] The mixing method can be carried out by introducing the powder into a powder mixer for a certain period of time. Here, the mixing time can be more than 10 minutes or more than 30 minutes and less than 10 hours, less than 5 hours or less than 2 hours.

[0073] When a carbonized metal-organic framework (MOF) and a sulfur compound are mixed, the weight ratio of the carbonized MOF to the sulfur compound can be from 9:1 to 1:9, preferably from 5:5 to 1:9. When the content of sulfur or sulfur compound is less than the above range, the content of the carbon-sulfur complex increases, and with the increase of carbon content, the amount of binder added needs to be increased when preparing the slurry. The increase in the amount of binder ultimately increases the sheet resistance of the electrode and acts as an insulator to prevent electron migration, which may reduce battery performance. When the content of sulfur or sulfur compound is greater than the above range, sulfur or sulfur compound that is not bound to the complex aggregates or re-dissolves onto the surface of the complex, making electron acceptance difficult and direct participation in the electrode reaction may become difficult.

[0074] positive electrode

[0075] The carbon-sulfur composite provided in this invention is used as the positive electrode active material for lithium secondary batteries, preferably as the positive electrode active material for lithium-sulfur batteries.

[0076] Lithium-sulfur batteries contain sulfur as the positive electrode active material, but this presents the problem of lithium polysulfides dissolving during charging and discharging. In the carbon-sulfur composite according to the invention, sulfur can be uniformly distributed within the framework, and due to the presence of pores of various sizes and a three-dimensionally interconnected and regularly arranged pore structure, sulfur can be loaded at a high content. Therefore, the three-dimensionally entangled structure can be maintained even in the event of polysulfide dissolution, and phenomena that damage the positive electrode structure can be suppressed. As a result, lithium-sulfur batteries containing the carbon-sulfur composite have the advantage of achieving high capacity even under high loads.

[0077] A positive electrode is prepared by coating a positive electrode current collector with a composition for forming a positive electrode active material layer and drying the result. The composition for forming the positive electrode active material layer can be prepared by mixing the aforementioned carbon-sulfur composite, conductive material, binder, and solvent.

[0078] Specifically, to further enhance the conductivity of the prepared carbon-sulfur composite, a conductive material can be added to the cathode composition. The conductive material facilitates the smooth migration of electrons within the cathode, and there are no particular limitations on the material, as long as it possesses excellent conductivity, provides a large surface area, and does not cause chemical changes in the battery. However, carbon-based materials are preferred.

[0079] As carbon-based materials, materials selected from the following can be used: graphite materials, such as natural graphite, artificial graphite, expanded graphite, or graphene; activated carbon materials; carbon black materials, such as channel black, furnace black, thermal cracking carbon black, contact carbon black, lamp black, or acetylene black; carbon fiber materials; carbon nanostructures, such as carbon nanotubes (CNTs) or fullerenes; and combinations of the above materials.

[0080] In addition to carbon-based materials, conductive materials can also be used depending on the purpose: metal fibers, such as metal mesh; metal powders, such as copper (Cu), silver (Ag), nickel (Ni), and aluminum (Al); or organic conductive materials, such as polyphenylene derivatives. Conductive materials can be used alone or as mixtures.

[0081] In addition, to provide adhesion of the current collector to the positive electrode active material, a binder may also be included in the positive electrode composition. The binder needs to be well soluble in the solvent, have appropriate electrolyte impregnation properties, and form a conductive network well with the positive electrode active material and the conductive material.

[0082] The adhesives that can be used in this invention can be any adhesives known in the art, specifically, can be a mixture or copolymer of one or more of the following: fluoropolymer adhesives, including polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE); rubber adhesives, including styrene-butadiene rubber, acrylonitrile-butadiene 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, but not limited thereto.

[0083] Based on the total weight of the positive electrode for lithium-sulfur batteries, the content of binder resin can range from 0.5% by weight to 30% by weight, but is not limited to this. When the content of binder resin is less than 0.5% by weight, the physical properties of the positive electrode may deteriorate, leading to the shedding of the positive electrode active and conductive materials. Furthermore, when the content exceeds 30% by weight, the proportion of active and conductive materials in the positive electrode is relatively reduced, thereby decreasing the battery capacity.

[0084] The solvent used to prepare the slurry-state positive electrode composition for lithium-sulfur batteries needs to be easily dried, and preferably, it should maintain the positive electrode active material and conductive material in a dispersed state without dissolving them while effectively dissolving the binder. When the solvent dissolves the positive electrode active material, sulfur settles in the slurry due to its high specific gravity (D = 2.07), and concentrates on the current collector during coating, causing problems with the conductive network and potentially affecting battery operation.

[0085] As a solvent according to the present invention, water or an organic solvent can be used, and as an organic solvent, an organic solvent including one or more solvents selected from the following: dimethylformamide, isopropanol, acetonitrile, methanol, ethanol and tetrahydrofuran can be used.

[0086] For mixing the cathode composition, conventional mixers such as paste mixers, high-shear mixers, and homogenizers can be used to stir using conventional methods.

[0087] A positive electrode for lithium-sulfur batteries can be formed by coating a positive electrode composition onto a current collector and then vacuum drying the result. The slurry can be coated onto the current collector to an appropriate thickness according to the viscosity of the slurry and the thickness of the positive electrode to be formed. Preferably, the coating thickness can be appropriately selected in the range of 10 μm to 300 μm.

[0088] Here, the method of coating the slurry is not limited, and the following methods can be used for preparation: doctor blade coating, dip coating, gravure coating, die-cut coating, spin coating, comma coating, bar coating, reverse roller coating, screen coating, cover coating, etc.

[0089] The positive electrode current collector is not particularly limited, as long as it can be fabricated to typically have a thickness of 3 μm to 500 μm and possess high conductivity without causing chemical changes in the battery. For example, conductive metals such as stainless steel, aluminum, copper, or titanium can be used, and preferably, aluminum current collectors can be used. Such positive electrode current collectors can take various forms, such as films, sheets, foils, meshes, porous bodies, foams, or nonwoven fabrics.

[0090] Lithium secondary batteries

[0091] As one embodiment of the present invention, a lithium secondary battery may include: the above-mentioned positive electrode; a negative electrode, the negative electrode comprising lithium metal or lithium alloy as a negative electrode active material; a separator disposed between the positive electrode and the negative electrode; and an electrolyte impregnated in the negative electrode, the positive electrode and the separator, and comprising a lithium salt and an organic solvent.

[0092] The negative electrode can use the following materials as the negative electrode active material: materials capable of reversibly inserting or de-intercalating lithium ions (Li). + Materials capable of reversibly forming lithium-containing compounds by reacting with lithium ions; lithium metal; or lithium alloys. Examples of materials capable of reversibly inserting or deintercalating lithium ions may include crystalline carbon, amorphous carbon, or mixtures thereof. Examples of materials capable of reversibly forming lithium-containing compounds by reacting with lithium ions may include tin oxide, titanium nitrate, or silicon. Examples of lithium alloys may include alloys of lithium with metals selected from the following: Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al, and Sn.

[0093] Furthermore, during the charging and discharging of lithium-sulfur batteries, sulfur, used as the positive electrode active material, can be transformed into an inactive material and adhere to the surface of the lithium negative electrode. Inactive sulfur refers to sulfur that has undergone various electrochemical or chemical reactions and can no longer participate in the electrochemical reactions of the positive electrode. The inactive sulfur formed on the lithium negative electrode surface has the advantage of acting as a protective layer for the lithium negative electrode. Therefore, lithium metal and inactive sulfur such as lithium sulfide formed on it can be used as the negative electrode.

[0094] In addition to the negative electrode active material, the negative electrode of the present invention may further include a pretreatment layer formed of a lithium ion conducting material and a lithium metal protective layer formed on the pretreatment layer.

[0095] A separator positioned between the positive and negative electrodes isolates or insulates them from each other, allowing lithium ions to migrate between them. This separator can be formed of a porous, non-conductive, or insulating material. As an insulator with high ion permeability and mechanical strength, such a separator can be a standalone component such as a thin film or membrane, or a coating added to the positive and / or negative electrodes. Alternatively, when a solid electrolyte, such as a polymer, is used as the electrolyte, the solid electrolyte can also be used as the separator.

[0096] The diaphragm preferably has a pore size typically from 0.01 μm to 10 μm and a thickness typically from 5 μm to 300 μm, and glass electrolytes, polymer electrolytes, ceramic electrolytes, etc., can be used as such diaphragms. For example, sheets, nonwoven fabrics, kraft paper, etc., made of chemically resistant and hydrophobic olefin polymers (such as polypropylene), glass fibers, or polyethylene are used. Typical commercially available examples may include the Celgard series (Celgard... R 2400, 2300 (Hoechest Celanese products), polypropylene diaphragms (Ube Industries or Pall RAI products), polyethylene series (Tonen or Entek), etc.

[0097] Solid electrolyte membranes may contain less than 20% by weight of non-aqueous organic solvents, and in this case, suitable gelling agents may also be included to reduce the flowability of the organic solvents. Typical examples of such gelling agents may include polyethylene oxide, polyvinylidene fluoride, polyacrylonitrile, etc.

[0098] The electrolyte impregnated into the negative electrode, positive electrode, and membrane is a non-aqueous electrolyte containing lithium salt, formed by lithium salt and electrolyte, and non-aqueous organic solvents, organic solid electrolytes, inorganic solid electrolytes, etc. are used as electrolytes.

[0099] As materials readily soluble in non-aqueous organic solvents, the lithium salts of the present invention may include one or more selected from the following: LiSCN, LiCl, LiBr, LiI, LiPF6, LiBF4, LiSbF6, LiAsF6, LiB 10 Cl 10 LiCH3SO3, LiCF3SO3, LiCF3CO2, LiClO4, LiAlCl4, Li(Ph)4, LiC(CF3SO2)3, LiN(FSO2)2, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(SFO2)2, LiN(CF3CF2SO2)2, lithium chloroborane, lower aliphatic carboxylic acids, lithium tetraphenylborate, lithium imino, and combinations thereof.

[0100] Depending on various factors such as the precise composition of the electrolyte solvent mixture, the solubility of the salt, the conductivity of the dissolved salt, the charging and discharging conditions of the battery, the operating temperature, and other factors known in the field of lithium batteries, the concentration of lithium salts can range from 0.2M to 2M, specifically from 0.6M to 2M, and more specifically from 0.7M to 1.7M. When used in amounts less than 0.2M, the conductivity of the electrolyte may decrease, leading to a decline in electrolyte performance, while when used in amounts greater than 2M, the viscosity of the electrolyte increases, resulting in a decrease in lithium ion concentration (Li). + The migration rate of ) decreases.

[0101] Non-aqueous organic solvents need to dissolve lithium salts well, and examples of non-aqueous organic solvents of the present invention may include aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butyl carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, 4-methyl -1,3-dioxane, diethyl ether, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate or ethyl propionate, and the organic solvent can be used alone or as a mixture of two or more organic solvents.

[0102] As organic solid electrolytes, for example, polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate polymers, polyalginate lysine, polyester sulfides, polyvinyl alcohol, polyvinylidene fluoride, polymers containing ion-dissociating groups, etc., can be used.

[0103] As an inorganic solid electrolyte, it can be used in the form of nitrides, halides, sulfates, etc., such as Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, LiSiO4, LiSiO4-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH or Li3PO4-Li2S-SiS2.

[0104] To improve charge / discharge performance and flame retardancy, the electrolyte of this invention may be supplemented with substances such as pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (condensed) glycol dimethyl ethers, hexamethylphosphoric triamine, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, aluminum trichloride, etc. In some cases, to impart non-flammability, halogenated solvents such as carbon tetrachloride and trifluoroethylene may be included. To improve high-temperature storage performance, carbon dioxide gas may be included. Furthermore, fluoroethylene carbonate (FEC), propylene sulfonate lactone (PRS), fluoropropylene carbonate (FPC), etc., may also be included.

[0105] Electrolytes can be used as liquid electrolytes or as solid electrolyte membranes. When used as liquid electrolytes, they also include a membrane formed of porous glass, plastic, ceramic, or polymer as a physical membrane that functions to physically separate the electrodes.

[0106] Preferred implementation scheme

[0107] Preferred embodiments are provided below to illustrate the invention; however, these embodiments are for illustrative purposes only, and it will be apparent to those skilled in the art that various modifications and variations can be made within the scope and concept of the invention, and such modifications and variations also fall within the scope of the appended claims.

[0108] [Example]

[0109] Preparation of sulfur-carbon complex

[0110] [Example 1]

[0111] 0.8g Zn(NO3)2 . 6H₂O and 0.149 g of 1,4-phthalic acid (H₂BDC) (molar ratio = 3:1) were introduced into a 50 mL container containing 30 mL of DMF. The reaction solution was heated in an oven at 120 °C for 1 day. The crystalline product was washed twice with DMF and MC. The crystalline product was washed several times with anhydrous DMF and anhydrous MC. The product was dried overnight in a vacuum oven at 150 °C to prepare a metal-organic framework (MOF-5) (0.28 g, yield = 91%).

[0112] The obtained metal-organic framework was placed in an oven and then carbonized at 1000 °C for 6 hours. Using the carbonized metal-organic framework, sulfur was mixed with the carbonized metal-organic framework at a weight ratio of 7:3, and the result was heat-treated at 155 °C for 30 minutes to prepare a sulfur-carbon composite.

[0113] [Comparative Example 1]

[0114] The sulfur-carbon complex was prepared in the same manner as in Example 1, except that activated carbon was used instead of carbonized metal-organic frameworks to prepare the sulfur-carbon complex.

[0115] [Comparative Example 2]

[0116] In addition to the Ti3C2T prepared using the method described in "Adv.Funct.Mater.2016,26,8746-8756", x The sulfur-carbon complex was prepared in the same manner as in Example 1, except that the Meso-C / S complex replaced the carbonized metal-organic framework.

[0117] [Comparative Example 3]

[0118] The sulfur-carbon composite was prepared in the same manner as in Example 1, except that the metal-organic framework was placed in an oven and then carbonized at 900°C.

[0119] Experimental Example 1: Evaluation of the performance of carbonized metal-organic frameworks

[0120] (Surface photographic analysis)

[0121] SEM images (HITACHI S-4800) were taken for the sulfur-carbon composite (MOF-5(1000)) prepared in Example 1 and the sulfur-carbon composite (MOF-5(900)) prepared in Comparative Example 3, and the results are shown below. Figure 4 a (Example 1) and Figure 4 In b (Comparative Example 3).

[0122] (N2 adsorption / desorption isotherm analysis)

[0123] For each of the carbonized metal-organic frameworks (MOF-5(1000)) prepared in Example 1 and the carbonized metal-organic frameworks (MOF-5(900)) prepared in Comparative Example 3, N2 adsorption / desorption isotherms were measured (using an AUTOSORB-iQ-MP instrument manufactured by Quantachrome Instruments) under vacuum for 2 hours at room temperature, and the results are presented in... Figure 1 The N2 adsorption / desorption isotherms of the activated carbon prepared in Comparative Example 1 are shown in the figure. Figure 3 middle.

[0124] pass Figure 1It can be seen that, compared with the carbonized metal-organic framework (MOF-5(900)) prepared in Comparative Example 3, the carbonized metal-organic framework (MOF-5(1000)) prepared in Example 1 has a relatively larger specific surface area. Furthermore, through... Figure 3 It can be seen that, compared with the activated carbon prepared in Comparative Example 1, the carbonized metal-organic frameworks prepared in Example 1 and Comparative Example 3 have relatively smaller specific surface areas.

[0125] (Analysis of density functional theory for quenched solids)

[0126] For the carbonized metal-organic framework (MOF-5(1000)) prepared in Example 1 and the carbonized metal-organic framework (MOF-5(900)) prepared in Comparative Example 3, the pore size distribution is shown using quenched solid density functional theory (QSDFT) (slit / cylindrical / spherical aperture) (AUTOSORB-iQ-MP instrument manufactured by Quantachrome Instruments). Figure 2 middle.

[0127] pass Figure 2 It can be seen that, compared with the carbonized metal-organic framework (MOF-5(900)) prepared in Comparative Example 3, the carbonized metal-organic framework (MOF-5(1000)) prepared in Example 1 contains more intermediate-sized pores.

[0128] Based on the results, BET, which depends on the carbonization temperature, was obtained, and the results are shown in Table 1.

[0129] [Table 1]

[0130] carbonization temperature <![CDATA[Specific surface area [m 2 / g]]]> Pore ​​volume [cc / g] Example 1 MOF-5 (1000℃) 2178 2.36 Comparative Example 3 MOF-5 (900℃) 1947 1.844 Comparative Example 1 Activated carbon 3890 2.17

[0131] Experiment Example 2: Evaluation of Battery Performance

[0132] Using the prepared sulfur-carbon composite, a slurry was prepared with a sulfur-carbon composite:conductive material:binder weight ratio of 90:5:5. This slurry was then coated onto a 20 μm thick aluminum foil current collector to fabricate an electrode. Here, carbon black was used as the conductive material, styrene-butadiene rubber and carboxymethyl cellulose were used as the binder, and the loading was 3 mAh / cm³. 2 .

[0133] (Battery performance evaluation)

[0134] For lithium-sulfur batteries manufactured using each of the sulfur-carbon composites prepared in Example 1 (MOF-5(1000)) and Comparative Examples 1 to 3 (MOF-5(900)), changes in charge and discharge performance were tested using a charge and discharge measurement device. Using the obtained batteries, an initial discharge / charge cycle of 2.5 cycles was performed at 0.1C / 0.1C, followed by 3 cycles at 0.2C / 0.2C, and then repeated for 10 cycles at 0.5C / 0.3C and 3 cycles at 0.2C / 0.2C. The results were measured and are shown below. Figures 5 to 8 middle.

[0135] Figure 5 This is a graph showing the initial charge and discharge performance of the lithium-sulfur batteries manufactured in Example 1 and Comparative Example 3. Figure 6 This is a graph showing the initial charge and discharge performance of the lithium-sulfur battery manufactured in Comparative Example 1. Figure 7 This is a graph showing the initial charge and discharge performance of the lithium-sulfur battery manufactured in Comparative Example 2.

[0136] When reference Figure 5 As the carbonization temperature of the metal-organic framework (MOF) increased to 1000°C, carbon with a further developed pore structure was prepared in the lithium-sulfur battery using the sulfur-carbon composite of Example 1. The carbon was used in the lithium-sulfur battery, and it was observed that Example 1 had improved initial capacity and improved cycle retention capacity compared to Comparative Example 3, which had a weaker developed pore structure.

[0137] On the other hand, when referring to Figure 6 It was observed that, under high discharge overvoltage, the lithium-sulfur battery fabricated using the sulfur-carbon composite of Comparative Example 1 did not exhibit its initial discharge capacity. This indicates that, like the BET and pore volume of carbon, the pore distribution also has a significant impact on battery performance. In other words, this is due to the fact that micropores are predominantly distributed in activated carbon.

[0138] Additionally, when referring to Figure 7 It can be seen that the sulfur-carbon complex of Comparative Example 2 was prepared in a similar manner to that in Example 1, but the initial capacity of the sulfur-carbon complex of Comparative Example 2 was about 1000 mAh / g, which was lower than the initial capacity of Example 1.

[0139] Figure 8 This is a graph showing the charging and discharging efficiencies of the lithium-sulfur batteries manufactured in Example 1 and Comparative Example 3. (Refer to...) Figure 8 It can be seen that the lithium-sulfur battery made using the sulfur-carbon composite of Example 1 has a higher discharge retention capacity during cycling.

Claims

1. A carbon-sulfur complex, said carbon-sulfur complex comprising: Carbonized metal-organic frameworks (MOFs); and A sulfur compound, said sulfur compound being introduced into at least a portion of the outer surface and interior of the carbonized metal-organic framework, The carbonized metal-organic framework described herein has a diameter of 2500 m. 2 / g to 3000 m 2 Specific surface area / g, and The carbonized metal-organic framework has a pore volume of 2.2 cc / g to 3.0 cc / g. The carbonized metal-organic framework (MOF) is prepared by carbonizing the metal-organic framework (MOF) at a temperature above 950°C.

2. The carbon-sulfur complex according to claim 1, wherein the metal-organic framework (MOF) comprises structural units represented by the following chemical formula 1: [Chemical Formula 1] [M x (L) y ] In chemical formula 1, M is a metal selected from one or more of the following: copper (Cu), zinc (Zn), iron (Fe), nickel (Ni), chromium (Cr), scandium (Sc), cobalt (Co), titanium (Ti), manganese (Mn), vanadium (V), aluminum (Al), magnesium (Mg), gallium (Ga), and indium (In); L is an organometallic ligand selected from one or more of the following types: 1,4-phthalic acid (BDC), 1,3,5-benzenetricarboxylic acid (BTC), 1,1'-biphenyl-3,3',5,5'-tetracarboxylic acid (BPTC), and 2-(N,N,N',N'-tetra(4-carboxyphenyl)-biphenyl-4,4'-diamine (TCBTDA); and x is an integer from 2 to 6, and y is an integer from 2 to 12.

3. The carbon-sulfur complex according to claim 1, comprising the carbonized metal-organic framework (MOF) and the sulfur compound in a weight ratio of 9:1 to 1:

9.

4. A method for preparing the carbon-sulfur composite of claim 1, the method comprising the following steps: (a) Preparation of carbonized metal-organic frameworks (MOFs) by carbonizing them above 950 °C; and (b) A carbon-sulfur complex is prepared by mixing the metal-organic framework (MOF) carbonized in step (a) with a sulfur compound.

5. The method for preparing a carbon-sulfur complex according to claim 4, wherein, In step (a), the metal-organic framework (MOF) is carbonized at 950°C to 2000°C.

6. The method for preparing a carbon-sulfur complex according to claim 4, wherein, In step (a), the metal-organic framework (MOF) is carbonized at 950°C to 1500°C.

7. The method for preparing a carbon-sulfur complex according to claim 4, wherein, In step (a), the carbonized metal-organic framework has a diameter of 2000 m. 2 / g to 3500 m 2 Specific surface area per g and pore volume of over 2.2 cc / g.

8. The method for preparing a carbon-sulfur complex according to claim 4, wherein the metal-organic framework (MOF) comprises structural units represented by the following chemical formula 1: [Chemical Formula 1] [M x (L) y ] In chemical formula 1, M is a metal selected from one or more of the following: copper (Cu), zinc (Zn), iron (Fe), nickel (Ni), chromium (Cr), scandium (Sc), cobalt (Co), titanium (Ti), manganese (Mn), vanadium (V), aluminum (Al), magnesium (Mg), gallium (Ga), and indium (In); L is an organometallic ligand selected from one or more of the following types: 1,4-phthalic acid (BDC), 1,3,5-benzenetricarboxylic acid (BTC), 1,1'-biphenyl-3,3',5,5'-tetracarboxylic acid (BPTC), and 2-(N,N,N',N'-tetra(4-carboxyphenyl)-biphenyl-4,4'-diamine (TCBTDA); and x is an integer from 2 to 6, and y is an integer from 2 to 12.

9. The method for preparing a carbon-sulfur complex according to claim 4, wherein, In step (b), the carbonized metal-organic framework (MOF) and the sulfur compound are mixed in a weight ratio of 9:1 to 1:

9.

10. A positive electrode comprising a carbon-sulfur complex according to any one of claims 1 to 3.

11. The positive electrode according to claim 10, wherein the positive electrode is used in a lithium-sulfur battery.

12. A lithium secondary battery, the lithium secondary battery comprising: The positive electrode according to claim 10; Negative electrode; and Electrolytes.

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