Lithium-sulfur batteries with high energy density

Abstract

A lithium-sulfur battery having high energy density is disclosed, which uses an SSE (sparingly solvating electrolyte) electrolyte system (discharge capacity: up to 1,600 mAh / gs) instead of the conventional catholyte electrolyte system (discharge capacity: up to 1,200 mAh / gs) but does not use a nitrile-based solvent, thereby utilizing more than 80% of the theoretical discharge capacity of sulfur (1,675 mAh / g), and also realizes excellent life performance by using a positive electrode carbon material with a high specific surface area. The lithium-sulfur battery includes an electrolyte including a first solvent including a fluorine-based ether compound, a second solvent including a glyme-based compound, and a lithium salt; and a positive electrode including sulfur and a carbon material as active materials; and the carbon material includes two or more types of carbon materials having different average pore diameters.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0052496 filed on April 28, 2022, and all contents disclosed in the documents of that Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a high-energy-density lithium-sulfur battery that can utilize 80% or more of the theoretical discharge capacity of sulfur and achieve excellent life performance. More specifically, the present invention relates to a high-energy-density lithium-sulfur battery that uses an SSE (sparingly solvating electrolyte) electrolyte system (discharge capacity: up to 1,600 mAh / gs) instead of the conventional catholyte electrolyte system (discharge capacity: up to 1,200 mAh / gs) but does not use a nitrile-based solvent, thereby utilizing 80% or more of the theoretical discharge capacity of sulfur (1,675 mAh / g). Furthermore, the present invention relates to a high-energy-density lithium-sulfur battery that can achieve excellent life performance by using a cathode carbon material with a high specific surface area. [Background technology]

[0003] As interest in energy storage technology grows, its application fields expand to include mobile phones, tablets, laptops, and camcorders, as well as the energy sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs), and research and development into electrochemical devices is gradually increasing. Electrochemical devices are the field that has received the most attention in this regard, and the development of secondary batteries such as rechargeable lithium-sulfur batteries has been a focus of attention. Recently, the development of such batteries has led to research and development into new electrode and battery designs to improve capacity density and specific energy.

[0004] These electrochemical devices, especially lithium-sulfur batteries (Li-S batteries), have a high energy density (theoretical capacity) and are attracting attention as next-generation secondary batteries that can replace lithium-ion batteries. In these lithium-sulfur batteries, a sulfur reduction reaction and a lithium metal oxidation reaction occur during discharge, during which sulfur converts from a ring-type S8 to a linear-type lithium polysulfide (LiPS). These lithium-sulfur batteries are characterized by a gradual discharge voltage until the polysulfide is completely reduced to Li2S.

[0005] However, the biggest obstacle to the commercialization of lithium-sulfur batteries is their lifespan, as their charge / discharge efficiency decreases during the charge / discharge process.The causes of this deterioration in the lifespan of lithium-sulfur batteries include electrolyte side reactions (accumulation of by-products due to electrolyte decomposition), lithium metal instability (dendrite growth on the lithium anode, causing short circuits), and accumulation of by-products on the cathode (elution of lithium polysulfides from the cathode).

[0006] In batteries that use sulfur-based compounds as the positive electrode active material and alkali metals such as lithium as the negative electrode active material, lithium polysulfide leaching and shuttle phenomenon occurs during charging and discharging. The lithium polysulfide is transferred to the negative electrode, reducing the capacity of the lithium-sulfur battery, resulting in a significant problem of reduced lifespan and reduced reactivity. Polysulfides leached from the positive electrode have high solubility in organic electrolytes, which can cause unwanted migration to the negative electrode via the electrolyte (PS shuttling). This results in a reduction in capacity due to irreversible loss of the positive electrode active material and a reduction in battery life due to the deposition of sulfur particles on the lithium metal surface due to side reactions.

[0007] Meanwhile, the behavior of such lithium-sulfur batteries can vary greatly depending on the electrolyte. An electrolyte in which sulfur in the positive electrode is dissolved into the electrolyte in the form of lithium polysulfide (LiPS) is called catholyte, while an electrolyte in which sulfur is hardly dissolved in the form of lithium polysulfide is called SSE (sparingly solvating electrolyte). Conventional lithium-sulfur batteries that utilize catholyte systems are called Li2S x Because it relies on a liquid-phase reaction through the production of intermediate polysulfides (catholyte type), it is unable to fully utilize the high theoretical discharge capacity of sulfur (1,675mAh / g), and instead has the problem of battery degradation due to the elution of polysulfides, resulting in a rapid reduction in battery life.

[0008] Meanwhile, the recent development of SSE (sparingly solvating electrolyte) electrolyte systems that can suppress polysulfide elution has enabled the utilization of over 90% of the theoretical discharge capacity of sulfur, but the problem of a shorter battery life due to the continuous decrease in sulfur content in the positive electrode has been a problem.Moreover, most SSE electrolyte systems rely on nitrile-based solvents, which react with the lithium anode, causing deterioration of the lithium anode and generating gas inside the lithium-sulfur battery, which are fatal drawbacks to the battery life.

[0009] Therefore, various researches are being conducted in the industry on lithium-sulfur batteries that do not allow sulfur, the active material of the positive electrode, to dissolve into the electrolyte (researches such as adding LiPS adsorbent materials to the positive electrode composite or modifying the separator membrane made of conventional PE, etc.), and in particular, research is being conducted on electrolytes in which sulfur promotes a solid-to-solid reaction with Li2S, the final discharge product, but no significant results have been achieved yet. Therefore, there is a need to develop an innovative battery that uses an SSE electrolyte system but has excellent lifespan performance due to a higher sulfur content in the positive electrode than conventional batteries, and that has a high energy density without deterioration of the lithium anode or generation of gas inside the battery (*High energy density means approximately 400Wh / kg or more or 600Wh / L or more, and to build this, a battery with a capacity of 4.0mAh / cm2 or more is required). 2 Therefore, an electrolyte and positive electrode active material system that can operate even with a porosity of 60% or less is required. Summary of the Invention [Problem to be solved by the invention]

[0010] Therefore, the object of the present invention is to provide a lithium-sulfur battery with high energy density that uses an SSE (sparingly solvating electrolyte) electrolyte system (discharge capacity: up to 1,600 mAh / gs) instead of the conventional catholyte electrolyte system (discharge capacity: up to 1,200 mAh / gs), but does not use a nitrile-based solvent, thereby utilizing more than 80% of the theoretical discharge capacity of sulfur (1,675 mAh / g).In addition, by using a cathode carbon material with a high specific surface area, it is possible to achieve long life performance. [Means for solving the problem]

[0011] To achieve the above object, the present invention provides a lithium-sulfur battery comprising: an electrolyte including a first solvent containing a fluorinated ether compound, a second solvent containing a glyme compound, and a lithium salt; and a positive electrode including sulfur and a carbon material as active materials, wherein the carbon material includes two or more types of carbon materials having different average pore sizes. [Effects of the Invention]

[0012] The lithium-sulfur battery with high energy density according to the present invention uses an SSE (sparingly solvating electrolyte) electrolyte system (discharge capacity: up to 1,600 mAh / gs) instead of the conventional catholyte electrolyte system (discharge capacity: up to 1,200 mAh / gs), but does not use a nitrile-based solvent, thereby enabling the utilization of more than 80% of the theoretical discharge capacity of sulfur (1,675 mAh / g). Furthermore, the use of a positive electrode carbon material with a high specific surface area also offers the advantage of achieving excellent life performance. Furthermore, the lithium-sulfur battery with high energy density according to the present invention has a discharge capacity of 4.0 mAh / cm. 2 As described above, by providing an electrolyte and a positive electrode active material system that can be driven even with a porosity of 60% or less, there is an advantage in that the energy density can be maintained at a high level of about 400 Wh / kg or more or 600 Wh / L or more. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a graph showing the life performance of lithium-sulfur batteries manufactured according to an example and a comparative example of the present invention. [Figure 2] 1 is a graph showing the initial discharge capacity of lithium-sulfur batteries manufactured according to an example and a comparative example of the present invention. [Figure 3] 1 is a graph showing discharge capacity according to charge-discharge cycles of lithium-sulfur batteries manufactured according to an example and a comparative example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will be described in detail below.

[0015] The lithium-sulfur battery according to the present invention includes an electrolyte including a first solvent containing a fluorinated ether compound, a second solvent containing a glyme compound, and a lithium salt; and a positive electrode including sulfur and a carbon material as active materials, wherein the carbon material includes two or more types of carbon materials having different average pore sizes.

[0016] In recent years, the development of SSE (sparingly solvating electrolyte) electrolyte systems that can suppress polysulfide elution has enabled the utilization of over 90% of the theoretical discharge capacity of sulfur. However, the problem of shortened battery life due to the continuous decrease in sulfur content in the positive electrode remains. In addition, due to the characteristics of SSE electrolyte systems that rely on nitrile-based solvents, the lithium anode deteriorates due to reactions with the lithium anode, and gas is generated inside the lithium-sulfur battery, shortening the battery life.

[0017] Therefore, the present applicant has developed an SSE electrolyte system that can utilize 80% or more, preferably 90 to 100%, and more preferably 94 to 100% of the theoretical discharge capacity, while at the same time providing excellent life performance due to a higher sulfur content in the positive electrode than conventional systems. Nitrile-based electrolyte solvents, which are fatal to the life of lithium-sulfur batteries, are eliminated, and a stable ether-based electrolyte solvent is used, preventing deterioration of the lithium negative electrode or generation of gas inside the battery (i.e., improved life), and achieving a discharge capacity of 4.0 mAh / cm. 2As described above, we have developed a lithium-sulfur battery with a high energy density of approximately 400 Wh / kg or more or 600 Wh / L or more by using an electrolyte and positive electrode active material system that can operate even with a porosity of 60% or less. Meanwhile, the "sulfur utilization" specifically refers to the ratio of the discharge capacity (mAh) per weight (gram) of elemental sulfur contained in the positive electrode of the battery to the theoretical capacity per weight of sulfur, which is 1,675 mAh / g (sulfur). For example, if the discharge capacity per weight of elemental sulfur present in the positive electrode of a lithium-sulfur battery is 1,600 mAh / g (sulfur), the sulfur utilization is 95.5% (1,600 / 1,675).

[0018] That is, the applicant has sought solutions to address the problems of the prior art, which was unable to maximize battery performance despite using an SSE electrolyte system. As a result, the applicant has confirmed a synergistic effect between an electrolyte containing a first solvent including a fluorinated ether compound, a second solvent including a glyme compound, and a lithium salt, and a positive electrode containing sulfur and a carbon material as an active material. As a result, the applicant has invented a lithium-sulfur battery that can utilize 80% or more, preferably 90 to 100%, and more preferably 94 to 100% of the theoretical discharge capacity of sulfur, and that uses a positive electrode carbon material with a high specific surface area to maintain a high sulfur content in the positive electrode, enabling long-life operation at room temperature and having a high energy density of about 400 Wh / kg or more or 600 Wh / L or more.

[0019] Hereinafter, each of A) the first solvent containing a fluorine-based ether compound, B) the second solvent containing a glyme-based compound, C) the lithium salt, and D) the positive electrode contained in the lithium-sulfur battery of the present invention will be specifically described.

[0020] A) First Solvent The first solvent is an electrolyte solvent containing a fluorine-based ether compound, and it dissolves polysulfides and inhibits solvent decomposition, thereby improving the coulombic efficiency (CE) of the battery and ultimately improving the battery's lifespan. More specifically, the first solvent containing a fluorine-based ether compound has excellent structural stability compared to general organic solvents containing alkanes due to fluorine substitution, and therefore is highly stable. Therefore, when this first solvent is used in an electrolyte for a lithium-sulfur battery, the stability of the electrolyte can be significantly improved, thereby improving the lifespan of the lithium-sulfur battery.

[0021] Examples of the fluorine-based ether compound include one or more hydrofluoroether-based (HFE type) compounds selected from the group consisting of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), bis(fluoromethyl)ether, 2-fluoromethyl ether, bis(2,2,2-trifluoroethyl)ether, propyl 1,1,2,2-tetrafluoroethyl ether, isopropyl 1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl isobutyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, 1H,1H,2′H,3H-decafluorodipropyl ether, and 1H,1H,2′H-perfluorodipropyl ether.

[0022] B) Second Solvent The second solvent is an electrolyte solvent containing a glyme-based compound (but not containing fluorine), and not only dissolves lithium salt to allow the electrolyte to have lithium ion conductivity, but also dissolves sulfur, a positive electrode active material, to facilitate the electrochemical reaction with lithium.

[0023] Specific examples of the glyme-based compound include one or more selected from the group consisting of dimethoxyethane, diethoxyethane, methoxyethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, and polyethylene glycol methyl ethyl ether, but are not limited to these. Of these, the use of dimethoxyethane may be preferred.

[0024] C) lithium salts The lithium salt is an electrolyte salt used to increase ionic conductivity, and any salt commonly used in the art may be used without limitation. Specific examples of the lithium salt include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, (C2F5SO2)2NLi, (SO2F)2NLi, (CF3SO2)3CLi, lithium chloroborane, lithium lower aliphatic carboxylates having 4 or less carbon atoms, lithium tetraphenylborate, and lithium imide.

[0025] The concentration of the lithium salt can be determined taking into consideration ionic conductivity and the like, and may be, for example, 0.1 to 2 M, preferably 0.5 to 1 M, and more preferably 0.5 to 0.75 M. If the concentration of the lithium salt is below this range, it may be difficult to ensure ionic conductivity suitable for battery operation, whereas if the concentration exceeds this range, the viscosity of the electrolyte may increase, reducing the mobility of lithium ions, or the decomposition reaction of the lithium salt itself may increase, resulting in a decrease in battery performance.

[0026] In the electrolyte containing the first solvent, the second solvent, and the lithium salt, the molar ratio of the lithium salt to the second solvent to the first solvent may be 1:0.5-3:4.1-15. In one embodiment of the present invention, the molar ratio of the lithium salt to the second solvent to the first solvent may be 1:2:4.1-13, 1:2.5:4.5-10, or 1:3:5-10. The electrolyte contained in the lithium-sulfur battery of the present invention may contain a first solvent containing a fluorinated ether compound at a higher content ratio than the second solvent containing a glyme-based compound. When the first solvent containing a fluorinated ether compound is contained at a higher content ratio than the second solvent containing a glyme-based compound, this is advantageous in that it suppresses polysulfide formation, enables the realization of a battery capacity close to the theoretical capacity of sulfur, and suppresses the decrease in battery capacity during battery use. Therefore, it is preferable to contain the first solvent containing a fluorinated ether compound at a higher content ratio than the second solvent containing a glyme-based compound.

[0027] D) positive electrode The lithium-sulfur battery according to the present invention includes a positive electrode containing sulfur and a carbon material as active materials, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte containing A) a first solvent containing a fluorinated ether compound, B) a second solvent containing a glyme-based compound, and C) a lithium salt. The positive electrode will now be described in detail.

[0028] The positive electrode included in the lithium-sulfur battery of the present invention includes a positive electrode active material, a binder, a conductive material, etc. The positive electrode active material may include elemental sulfur (S), a sulfur-based compound, or a mixture thereof. Specifically, the sulfur-based compound is Li2S n (n≧1), organic sulfur compounds or sulfur-carbon complexes (C2S x ) n (x=2.5 to 50, n≧2) However, since sulfur substances alone are not electrically conductive, they must be used in combination with a conductive material, and therefore, the positive electrode active material preferably contains a sulfur-carbon composite.

[0029] The sulfur-carbon composite may have a particle size of 1 to 100 μm. If the particle size of the sulfur-carbon composite is less than 1 μm, the resistance between particles increases, which may cause overvoltage in the electrode of the lithium-sulfur battery. If the particle size exceeds 100 μm, the surface area per unit weight decreases, which reduces the wetting area with the electrolyte in the electrode and the reaction site with lithium ions. This reduces the amount of electron transfer relative to the size of the composite, which may slow down the reaction and reduce the discharge capacity of the battery.

[0030] The sulfur (S) may be contained in an amount of 60 to 80 wt %, preferably 65 to 80 wt %, and more preferably 65 to 75 wt %, based on the total weight of the positive electrode. The sulfur content in the positive electrode of a typical lithium-sulfur battery is about 40 to 60 wt % based on the total weight of the positive electrode. Despite using a significantly higher sulfur content in the present invention, the battery exhibits a high initial discharge capacity under the electrolyte conditions of the present invention. If the sulfur content is less than 60 wt % based on the total weight of the positive electrode, the battery energy density may decrease. If the sulfur content exceeds 80 wt %, the conductivity within the electrode may decrease, resulting in poor electrode stability.

[0031] The carbon material (or sulfur-carrier) constituting the sulfur-carbon composite has porosity. In particular, the carbon material used as the positive electrode active material of the present invention is characterized by a high specific surface area (1,000 to 4,000 m) so that the sulfur content in the positive electrode can be maintained at a high level. 2 / g, preferably 1,500 to 3,500 m 2 / g). To achieve this, the carbonaceous material constituting the sulfur-carbon composite must contain two or more carbonaceous materials with different average pore sizes. Preferably, the carbonaceous material constituting the sulfur-carbon composite contains a first carbonaceous material with an average pore size of less than 2 nm and a second carbonaceous material with an average pore size of 2 nm or more. More preferably, the carbonaceous material constituting the sulfur-carbon composite contains a first carbonaceous material with an average pore size of 0.01 to 1.50 nm and a second carbonaceous material with an average pore size of 2 to 20 nm. If the carbonaceous material constituting the sulfur-carbon composite of the present invention does not contain a carbonaceous material with such an average pore size, sulfur will not be sufficiently supported on the carbonaceous material, resulting in a decrease in the sulfur content in the positive electrode and a consequent decrease in the lifespan.

[0032] The first carbon material having an average pore size of less than 2 nm (that is, having micropores of less than 2 nm) is preferably activated carbon.

[0033] The second carbon material having an average pore diameter of 2 nm or more (i.e., having mesopores of 2 nm or more) may be one selected from the group consisting of reduced graphene oxide (rGO); carbon black such as denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; carbon nanotubes (CNT) such as single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT); graphite; graphene; and carbon fibers such as graphite nanofibers (GNF), carbon nanofibers (CNF), and activated carbon fibers (ACF).

[0034] The first carbon material having micropores is preferably contained in a higher content ratio than the second carbon material having mesopores. Thus, the weight ratio of the first carbon material to the second carbon material is 80 to 99:20 to 1, preferably 90 to 99:10 to 1, and more preferably 97 to 99:3 to 1. If the weight ratio of the first carbon material to the second carbon material is outside of 80 to 99:20 to 1, there is a possibility that a problem of a decrease in discharge capacity may occur due to a decrease in sulfur reactivity.

[0035] Furthermore, when the sulfur-carbon composite contains only the first carbon material, a problem of low discharge voltage may occur. When the sulfur-carbon composite contains only the second carbon material, a problem of low discharge capacity may occur.

[0036] The cathode active material containing sulfur and a carbon material may be included in an amount of 80 to 99 parts by weight, preferably 90 to 95 parts by weight, based on 100 parts by weight of the total cathode weight. If the content of the cathode active material is less than 80 parts by weight based on 100 parts by weight of the total cathode weight, the energy density of the battery may decrease, whereas if it exceeds 99 parts by weight, the conductivity within the electrode may decrease, resulting in a decrease in the stability of the electrode.

[0037] The binder is a component that aids in binding the positive electrode active material to the conductive material and the like and in binding to the current collector. Examples of the binder include, but are not limited to, one or more selected from the group consisting of polyvinylidene fluoride (PVdF), polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVdF / HFP), polyvinyl acetate, polyvinyl alcohol, polyvinyl ether, polyethylene, polyethylene oxide, alkylated polyethylene oxide, polypropylene, polymethyl (meth)acrylate, polyethyl (meth)acrylate, polytetrafluoroethylene (PTFE), polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, polyvinylpyrrolidone, styrene-butadiene rubber, acrylonitrile-butadiene rubber, ethylene-propylene-diene monomer (EPDM) rubber, sulfonated EPDM rubber, styrene-butylene rubber, fluororubber, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, and mixtures thereof.

[0038] The binder is typically added in an amount of 1 to 50 parts by weight, preferably 3 to 15 parts by weight, based on 100 parts by weight of the total weight of the positive electrode. If the content of the binder is less than 1 part by weight, the adhesive strength between the positive electrode active material and the current collector may be insufficient. If the content of the binder is more than 50 parts by weight, the adhesive strength is improved, but the content of the positive electrode active material is reduced accordingly, which may result in a lower battery capacity.

[0039] The conductive material contained in the positive electrode is not particularly limited as long as it does not cause side reactions in the internal environment of the battery, does not cause chemical changes in the battery, and has excellent electrical conductivity. Representative examples include graphite or conductive carbon, and include, but are not limited to, graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, denka black, thermal black, channel black, furnace black, and lamp black; carbon-based materials having a graphene or graphite crystal structure; carbon nanotubes; conductive fibers such as carbon fibers and metal fibers; carbon fluoride; metal powders such as aluminum powder and nickel powder; conductive whiskey such as zinc oxide and potassium titanate; conductive oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives. These may be used alone or in combination of two or more.

[0040] The conductive material may be added in an amount of typically 0.5 to 10 parts by weight, preferably 0.5 to 5 parts by weight, based on 100 parts by weight of the total positive electrode, but may not be included in the positive electrode of the present invention. If the content of the conductive material exceeds 10 parts by weight, the amount of the positive electrode active material will be relatively small, which may result in a decrease in capacity and energy density. The method for incorporating the conductive material into the positive electrode is not particularly limited, and conventional methods known in the art, such as coating the positive electrode active material, can be used. If necessary, adding a conductive second coating layer to the positive electrode active material can replace the addition of the conductive material.

[0041] A filler may be selectively added to the positive electrode of the present invention as a component for suppressing its expansion. Such a filler is not particularly limited as long as it can suppress the expansion of the electrode without causing chemical changes in the battery. For example, olefin polymers such as polyethylene and polypropylene; fibrous materials such as glass fiber and carbon fiber; etc. may be used.

[0042] The positive electrode can be manufactured by dispersing and mixing a positive electrode active material, a binder, a conductive material, etc. in a dispersion medium (solvent) to prepare a slurry, applying the slurry to a positive electrode current collector, and then drying and rolling the slurry. The dispersion medium may be, but is not limited to, NMP (N-methyl-2-pyrrolidone), DMF (dimethyl formamide), DMSO (dimethyl sulfoxide), ethanol, isopropanol, water, or a mixture thereof.

[0043] The positive electrode current collector may be made of, but is not limited to, platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), ruthenium (Ru), nickel (Ni), stainless steel (STS), aluminum (Al), molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W), ITO (In-doped SnO), FTO (F-doped SnO), alloys thereof, or aluminum (Al) or stainless steel surface-treated with carbon (C), nickel (Ni), titanium (Ti), or silver (Ag). The positive electrode current collector may be in the form of a foil, film, sheet, punched, porous, foam, or the like.

[0044] The negative electrode may be a lithium-based metal and may further include a current collector on one side of the lithium-based metal. The current collector may be a negative electrode current collector. The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and may be selected from the group consisting of copper, aluminum, stainless steel, zinc, titanium, silver, palladium, nickel, iron, chromium, alloys thereof, and combinations thereof. The stainless steel may be surface-treated with carbon, nickel, titanium, or silver. The alloy may be an aluminum-cadmium alloy, or may be calcined carbon, a non-conductive polymer, or a conductive polymer surface-treated with a conductive material. A copper thin plate is generally used as the negative electrode current collector.

[0045] The negative electrode current collector may be in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric, with or without fine irregularities on the surface. The negative electrode current collector has a thickness of 3 to 50 μm. If the thickness of the negative electrode current collector is less than 3 μm, the current collection effect decreases, while if the thickness exceeds 50 μm, the processability decreases when the cell is folded and assembled.

[0046] The lithium-based metal may be lithium or a lithium alloy. In this case, the lithium alloy contains an element that can be alloyed with lithium, and specifically may be an alloy of lithium with one or more elements selected from the group consisting of Si, Sn, C, Pt, Ir, Ni, Cu, Ti, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Sb, Pb, In, Zn, Ba, Ra, Ge, and Al.

[0047] The lithium-based metal may be in the form of a sheet or foil, and in some cases, may be in the form of lithium or a lithium alloy deposited or coated on a current collector by a dry process, or may be in the form of metal or alloy deposited or coated on particles by a wet process, etc.

[0048] A conventional separator may be interposed between the positive electrode and the negative electrode. The separator is a physical separator having a function of physically separating the electrodes, and any separator commonly used may be used without particular limitation. In particular, a separator having low resistance to ion migration of the electrolyte and excellent humidifying ability for the electrolyte is preferred.

[0049] The separator separates or insulates the positive electrode and the negative electrode from each other while allowing lithium ions to be transported between them. The separator may be made of a porous, non-conductive, or insulating material. The separator may be an independent member such as a film, or a coating layer attached to the positive electrode and / or the negative electrode.

[0050] Examples of polyolefin-based porous membranes that can be used as the separation membrane include membranes formed from polyolefin-based polymers such as polyethylene (e.g., high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene), polypropylene, polybutylene, and polypentene, either alone or in combination. Examples of nonwoven fabrics that can be used as the separator include nonwoven fabrics made of polymers such as polyphenylene oxide, polyimide, polyamide, polycarbonate, polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polyphenylene sulfide, polyacetal, polyethersulfone, polyetheretherketone, and polyester, either alone or in combination. Such nonwoven fabrics may be spunbond or meltblown, consisting of long fibers, as the fiber form that forms the porous web.

[0051] The thickness of the separator is not particularly limited, but is preferably in the range of 1 to 100 μm, and more preferably in the range of 5 to 50 μm. If the thickness of the separator is less than 1 μm, it will not be able to maintain its mechanical properties, and if it exceeds 100 μm, it will act as a resistance layer, resulting in reduced battery performance. The pore size and porosity of the separator are not particularly limited, but the pore size is preferably 0.1 to 50 μm and the porosity is preferably 10 to 95%. If the pore size of the separator is less than 0.1 μm or the porosity is less than 10%, it will act as a resistance layer, and if the pore size is more than 50 μm or the porosity is more than 95%, it will not be able to maintain its mechanical properties.

[0052] The lithium-sulfur battery of the present invention, including the above-described electrolyte, positive electrode, negative electrode, and separator, can be manufactured by placing the positive electrode and the negative electrode opposite each other, interposing a separator therebetween, and then injecting the electrolyte.

[0053] Meanwhile, the lithium-sulfur battery according to the present invention can be applied not only to battery cells used as power sources for small devices, but also particularly suitably used as a unit battery of a battery module, which is a power source for medium- to large-sized devices. In this regard, the present invention also provides a battery module including two or more lithium-sulfur batteries electrically connected (in series or parallel). The number of lithium-sulfur batteries included in the battery module can be adjusted in various ways, taking into account the application and capacity of the battery module. Furthermore, the present invention also provides a battery pack in which the battery modules are electrically connected according to conventional techniques in the art. The battery module and battery pack can be used as a power source for one or more medium- to large-sized devices, including, but not limited to, power tools; electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric trucks; electric commercial vehicles; and power storage systems.

[0054] Preferred examples will be described 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. Naturally, these changes and modifications also fall within the scope of the appended claims.

[0055] [Example 1] Lithium-sulfur battery manufacturing Electrolyte production First, LiTFSI (concentration: 0.65 M), dimethoxyethane (second solvent), and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE, first solvent) were mixed in a molar ratio of 1:2:9 at room temperature to prepare a lithium-sulfur battery electrolyte (SSE).

[0056] Cathode manufacturing A cathode slurry composition was prepared by mixing 90 parts by weight of a sulfur-carbon composite (S:C = 70:30 weight ratio) as a cathode active material (the content of sulfur alone was set to 63 wt% of the total cathode weight, and the carbon material contained in the sulfur-carbon composite was a mixture of activated carbon (first carbon material) with an average pore diameter of less than 2 nm and reduced graphene oxide (rGO, second carbon material) with an average pore diameter of 2 nm or more, in a weight ratio of 99:1). Five parts by weight of Denka Black as a conductive material and five parts by weight of styrene butadiene rubber / carboxymethyl cellulose (SBR:CMC = 7:3) as a binder were then coated onto a current collector (Al foil), dried at 50°C for 12 hours, and pressed using a roll press to prepare a cathode (the sulfur loading was 3.0 mAh / cm). 2 (We decided to do so.)

[0057] Lithium-sulfur battery manufacturing The prepared positive electrode and a 150 μm-thick lithium metal negative electrode were placed facing each other, a polyethylene (PE) separator was interposed between them, and the prepared electrolyte was injected to fabricate a coin cell-type lithium-sulfur battery. The positive electrode was punched out to a circular electrode of 14 phi, the polyethylene separator was punched out to 19 phi, and the lithium metal was punched out to 16 phi. The battery was also fabricated using a sparing solvating electrolyte (SSE) electrolyte system.

[0058] [Comparative Example 1] Lithium-sulfur battery manufacturing A coin-cell type lithium-sulfur battery was fabricated in the same manner as in Example 1, except that only activated carbon (first carbon material) with an average pore diameter of less than 2 nm was used as the carbon material contained in the sulfur-carbon composite (i.e., no second carbon material was used).

[0059] Comparative Example 2 Lithium-sulfur battery manufacturing A coin-cell lithium-sulfur battery was fabricated in the same manner as in Example 1, except that only reduced graphene oxide (rGO, second carbon material) with an average pore diameter of 2 nm or more was used as the carbon material contained in the sulfur-carbon composite (i.e., the first carbon material was not used).

[0060] [Experimental Example 1] Evaluation of the life performance of lithium-sulfur batteries The lithium-sulfur batteries prepared in Example 1 and Comparative Examples 1 and 2 were charged and discharged for a total of 15 cycles (3 cycles each) at 0.1 C, 0.2 C, 0.3 C, 0.5 C, and 1.0 C, and then charged and discharged at 0.3 C to evaluate the battery life characteristics. The voltage range used was 1.0 to 3.6 V, and the evaluation temperature was 25°C.

[0061] FIG. 1 is a graph showing the lifespan performance of lithium-sulfur batteries prepared according to an example and comparative examples of the present invention, where FIG. 1a corresponds to Example 1, FIG. 1b corresponds to Comparative Example 1, and FIG. 1c corresponds to Comparative Example 2. As a result of evaluating the lifespan performance of the lithium-sulfur batteries prepared in Example 1 and Comparative Examples 1 and 2, as described above, it was confirmed that the lithium-sulfur battery of Example 1, in which a sulfur-carbon composite was used that contained both a first carbon material having an average pore diameter of less than 2 nm and a second carbon material having an average pore diameter of 2 nm or more, exhibited superior lifespan performance, as shown in FIG. 1, compared with the lithium-sulfur battery of Comparative Example 1, in which a sulfur-carbon composite was used that contained only the first carbon material having an average pore diameter of less than 2 nm, and the lithium-sulfur battery of Comparative Example 2, in which a sulfur-carbon composite was used that contained only the second carbon material having an average pore diameter of 2 nm or more.

[0062] [Experimental Example 2] Evaluation of the discharge capacity of lithium-sulfur batteries The lithium-sulfur batteries prepared in Example 1 and Comparative Examples 1 and 2 were charged and discharged at a rate of 0.1 C for the first 3 cycles, 0.2 C for the subsequent 6 cycles, 0.3 C for the subsequent 9 cycles, 0.5 C for the subsequent 12 cycles, and 1.0 C for the subsequent 16 cycles, to evaluate the discharge capacity of the batteries. The voltage range used was 1.0 to 3.6 V, and the evaluation temperature was 25°C.

[0063] Figure 2 is a graph showing the initial discharge capacity of lithium-sulfur batteries prepared according to an embodiment of the present invention and comparative examples, where Figure 2a corresponds to Example 1, Figure 2b corresponds to Comparative Example 1, and Figure 2c corresponds to Comparative Example 2. Figure 3 is a graph showing the discharge capacity of lithium-sulfur batteries prepared according to an embodiment of the present invention and comparative examples over charge and discharge cycles, where Figure 3a corresponds to Example 1, Figure 3b corresponds to Comparative Example 1, and Figure 3c corresponds to Comparative Example 2.

[0064] First, the initial discharge capacities of the lithium-sulfur batteries fabricated in Example 1 and Comparative Examples 1 and 2 were evaluated. As a result, it was confirmed that the lithium-sulfur battery of Example 1, in which the sulfur-carbon composite contained both the first carbon material having an average pore diameter of less than 2 nm and the second carbon material having an average pore diameter of 2 nm or more, had a relatively superior initial discharge capacity to the lithium-sulfur battery of Comparative Example 1, in which the sulfur-carbon composite contained only the first carbon material having an average pore diameter of less than 2 nm, and the lithium-sulfur battery of Comparative Example 2, in which the sulfur-carbon composite contained only the second carbon material having an average pore diameter of 2 nm or more, as shown in FIG. 2 .

[0065] Next, the discharge capacities of the lithium-sulfur batteries fabricated in Example 1 and Comparative Examples 1 and 2 were evaluated through charge-discharge cycles. As a result, the discharge capacity of the lithium-sulfur battery of Example 1, in which the sulfur-carbon composite contained both the first carbon material having an average pore size of less than 2 nm and the second carbon material having an average pore size of 2 nm or more, was found to be superior overall to the discharge capacities of the lithium-sulfur battery of Comparative Example 1, in which the sulfur-carbon composite contained only the first carbon material having an average pore size of less than 2 nm, and the lithium-sulfur battery of Comparative Example 2, in which the sulfur-carbon composite contained only the second carbon material having an average pore size of 2 nm or more, as shown in Figure 3. In particular, the difference became more pronounced as the charge-discharge cycles increased, and the battery still maintained a discharge capacity of nearly 1,600 mAh / g even after 14 cycles.

[0066] As described above, the lithium-sulfur battery of the present invention achieves long-life operation while utilizing more than 80% of the theoretical discharge capacity of sulfur, a level not previously achieved by conventional lithium-sulfur batteries employing an SSE electrolyte system. Therefore, it can be seen that in lithium-sulfur batteries employing an SSE electrolyte system, the carbon material used as the positive electrode active material must include two or more types of carbon materials having different average pore sizes, and preferably, the carbon material used as the positive electrode active material must include both a first carbon material having an average pore size of less than 2 nm and a second carbon material having an average pore size of 2 nm or more, in order to maximize battery performance.

Claims

1. a first solvent containing a fluorinated ether compound, a second solvent containing a glyme compound, and an electrolyte containing a lithium salt; and a positive electrode comprising sulfur and a carbon material as active materials; The carbon material comprises a mixture of two or more carbon materials having different average pore sizes, The lithium-sulfur battery, wherein the carbonaceous material comprises a first carbonaceous material having an average pore size of less than 2 nm and a second carbonaceous material having an average pore size of 2 nm to 20 nm.

2. 2. The lithium-sulfur battery of claim 1, wherein the carbon material comprises a first carbon material having an average pore size of 0.01 nm to 1.50 nm.

3. 2. The lithium-sulfur battery of claim 1, wherein the first carbon material is activated carbon having an average pore size of less than 2 nm.

4. 2. The lithium-sulfur battery of claim 1, wherein the second carbon material is selected from the group consisting of reduced graphene oxide, carbon black, carbon nanotubes (CNTs), graphite, graphene, and carbon fiber, each having an average pore size of 2 nm to 20 nm.

5. 5. The lithium-sulfur battery according to claim 1, wherein the weight ratio of the first carbon material to the second carbon material is 80-99:20-1.

6. 2. The lithium-sulfur battery according to claim 1, wherein the utilization rate of the sulfur contained in the positive electrode is 80% or more of the theoretical discharge capacity.

7. 2. The lithium-sulfur battery according to claim 1, wherein the positive electrode active material comprises a sulfur-carbon composite.

8. 2. The lithium-sulfur battery of claim 1, wherein the sulfur is contained in an amount of 60 to 80% by weight based on the total weight of the positive electrode.

9. 2. The lithium-sulfur battery of claim 1, wherein the electrolyte does not contain a nitrile-based solvent.

10. 2. The lithium-sulfur battery of claim 1, wherein the molar ratio of the lithium salt, the second solvent, and the first solvent is 1:0.5-3:4.1-15.

11. 2. The lithium-sulfur battery according to claim 1, wherein the utilization rate of the sulfur contained in the positive electrode is 90% to 100% of the theoretical discharge capacity.

12. 2. The lithium-sulfur battery according to claim 1, wherein the energy density of the lithium-sulfur battery is 400 Wh / kg or more or 600 Wh / L or more.

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