Electrolyte for Lithium Sulfur Batteries and A Lithium Sulfur Batteries Comprising the Same

Abstract

The electrolyte for a lithium-sulfur battery according to the present invention comprises an organic solvent; a lithium salt; and an additive, wherein the additive comprises tantalum pentafluoride (TaF5). The lifespan characteristics of the lithium-sulfur battery can be improved by the electrolyte for the lithium-sulfur battery comprising tantalum pentafluoride.

Description

Technology Field

[0001] The present invention relates to a lithium-sulfur battery with improved lifespan characteristics. Background Technology

[0002] As interest in energy storage technology continues to grow, research and development in electrochemical devices are steadily increasing as application fields expand to include mobile phones, tablets, laptops, camcorders, and even electric vehicles (EVs) and hybrid electric vehicles (HEVs). Electrochemical devices are receiving the most attention in this regard, and among them, the development of rechargeable secondary batteries, such as lithium-sulfur batteries, is a focal point of interest. Recently, research and development in developing these batteries has led to the design of new electrodes and batteries to improve capacity density and specific energy.

[0003] Among such electrochemical devices, lithium-sulfur (LiS) batteries are attracting attention as next-generation rechargeable batteries capable of replacing lithium-ion batteries due to their high energy density. Lithium-sulfur is used as a positive electrode active material, and during discharge within the lithium-sulfur battery, a reduction reaction of sulfur and an oxidation reaction of lithium metal occur. At this time, sulfur forms linear-structured lithium polysulfides (Li2S2, Li2S4, Li2S6, Li2S8) from the cyclic structure of S8. These lithium-sulfur batteries are characterized by exhibiting stepwise discharge voltages until the polysulfide (PS) is completely reduced to LiS. However, these lithium polysulfides react with the electrolyte to cause side reactions and lead to battery degradation.

[0004] In addition, lithium metal can be used as the negative electrode of a lithium-sulfur battery, but as the lithium metal electrode undergoes repeated charging and discharging, problems may arise in which the lifespan characteristics deteriorate due to dendrite formation and porosity.

[0005] In addition, to achieve high energy density in lithium-sulfur batteries, the electrolyte content must be reduced; however, if the amount of electrolyte decreases, the concentration of lithium polysulfide increases, which further accelerates side reactions with the lithium metal electrode and can lead to a problem of further reduced battery life. The problem to be solved

[0006] Therefore, the objective of the present invention is to provide an electrolyte for a lithium-sulfur battery that can improve the lifespan characteristics of the lithium-sulfur battery.

[0007] In addition, another objective of the present invention is to provide a lithium-sulfur battery with high energy density and improved lifespan characteristics. means of solving the problem

[0008] In order to solve the above problem, according to one aspect of the present invention, an electrolyte for a lithium-sulfur battery or a lithium-sulfur battery of the following embodiments is provided.

[0009] The electrolyte for a lithium-sulfur battery according to the first embodiment comprises an organic solvent; a lithium salt; and an additive, wherein the additive comprises tantalum pentafluoride (TaF5).

[0010] According to the second embodiment, in the first embodiment,

[0011] The above tantalum pentafluoride is included in an amount of 0.1 to 1 weight% relative to the total weight of the electrolyte for a lithium-sulfur battery.

[0012] According to the third embodiment, in the first embodiment or the second embodiment,

[0013] The tantalum pentafluoride is included in an amount of 0.3 to 0.7 weight% relative to the total weight of the electrolyte for a lithium-sulfur battery.

[0014] According to the fourth embodiment, in any one of the first to third embodiments,

[0015] The above organic solvent includes acyclic ether or cyclic ether.

[0016] According to the fifth embodiment, in any one of the first to fourth embodiments,

[0017] The above lithium salt comprises one or more selected from LiFSI, LiTFSI, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, and LiB(C2O4)2.

[0018] According to the 6th embodiment, in any one of the 1st to 5th embodiments,

[0019] The above additive includes one or more selected from lithium nitrate (LiNO3, lithium nitrate), potassium nitrate (KNO3), cesium nitrate (CsNO3), magnesium nitrate (MgNO3), barium nitrate (BaNO3), potassium nitrite (KNO2), and cesium nitrite (CsNO2).

[0020] According to the seventh embodiment,

[0021] A lithium-sulfur battery comprising: a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, wherein the electrolyte is according to any one of the first to sixth embodiments, the positive electrode comprises a sulfur-carbon composite as a positive electrode active material, and the negative electrode comprises lithium metal as a negative electrode active material.

[0022] According to the 8th embodiment, in the 7th embodiment,

[0023] The above electrolyte for the lithium-sulfur battery is included in an amount of 300% by weight or less relative to 100% by weight of the total sulfur-based material.

[0024] According to the ninth embodiment, in the seventh embodiment or the eighth embodiment,

[0025] The above sulfur-carbon composite is a sulfur-based material supported on a porous carbon material, and

[0026] With respect to a total of 100 weight% of the above sulfur-carbon composite, the sulfur-based material is included in an amount of 60 weight% or more.

[0027] According to the 10th embodiment, in any one of the 7th to 9th embodiments,

[0028] The above cathode comprises a cathode active material layer and a protective layer formed on at least one surface of the cathode active material layer, and the protective layer comprises LiF. Effects of the invention

[0029] The electrolyte for a lithium-sulfur battery according to the present invention can improve the lifespan characteristics of a lithium-sulfur battery and increase the energy density of the battery.

[0030] In addition to the above, the present invention may have various other effects, which are described in each embodiment, or effects that can be easily inferred by a person skilled in the art, etc., will be omitted. Brief explanation of the drawing

[0031] The following drawings attached to this specification illustrate preferred embodiments of the present invention and serve to further enhance understanding of the technical concept of the present invention together with the description of the invention; therefore, the present invention should not be interpreted as being limited only to the matters described in such drawings. Meanwhile, the shape, size, scale, or ratio of elements in the drawings included in this specification may be exaggerated to emphasize a clearer explanation. Figure 1 illustrates the Coulomb efficiency and capacity according to the cycle life of an example and a comparative example according to the present invention. Specific details for implementing the invention

[0032] Hereinafter, the present invention will be described in detail with reference to the drawings. Terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, and should be interpreted in a meaning and concept consistent with the technical spirit of the present invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0033] Therefore, the embodiments described in this specification and the configurations described in the drawings are merely the most preferred embodiments of the present invention and do not represent all of the technical ideas of the present invention; thus, it should be understood that various equivalents and modifications that can replace them may exist at the time of filing this application.

[0034] Furthermore, throughout the specification, when a part is described as "include, comprise," "have," or "possess" a certain component, this means that, unless specifically stated otherwise, it does not exclude other components but may include additional components.

[0035] Additionally, terms such as 'about,' 'substantially,' etc., used throughout this specification are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the said sense, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosure in which precise or absolute values ​​are mentioned to aid in understanding this invention.

[0036] Throughout this specification, the description of 'A and / or B' means 'A or B or both.'

[0037] Throughout the entire specification, unless otherwise specifically stated, temperature refers to Celsius temperature, and the unit is °C.

[0039] The present invention relates to an electrolyte for a lithium-sulfur battery and a lithium-sulfur battery comprising said electrolyte. The lithium-sulfur battery comprises a sulfur-based material as a positive electrode active material. In this specification, said sulfur-based material means comprising one or more selected from sulfur and sulfur compounds, which will be described in more detail below.

[0041] The first aspect of the present invention relates to an electrolyte for a lithium-sulfur battery.

[0042] An electrolyte for a lithium-sulfur battery according to one aspect of the present invention comprises an organic solvent; a lithium salt and an additive, wherein the additive comprises tantalum pentafluoride (TaF5). By adding the tantalum pentafluoride to the electrolyte for a lithium-sulfur battery, the energy density of the lithium-sulfur battery can be improved and the lifespan characteristics can be improved.

[0044] In one embodiment of the present invention, the tantalum pentafluoride may be included in an amount of 0.1 to 1 weight% or 0.3 to 0.7 weight% with respect to the total weight of the electrolyte for the lithium-sulfur battery.

[0045] When the above tantalum pentafluoride is included within the above range, the energy density of the lithium-sulfur battery can be further improved, and the lifespan characteristics can also be further improved.

[0047] In one embodiment of the present invention, the organic solvent may include an acyclic ether and / or a cyclic ether. The acyclic ether and the cyclic ether may be included in a weight ratio of 0.5:9.5 to 5:5. By including the acyclic ether and the cyclic ether in the above weight ratio, the improvement in energy density and lifespan characteristics of the lithium-sulfur battery due to the tantalum pentafluoride may be more clearly demonstrated.

[0048] The above-mentioned acyclic ether may include one or more selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, ethylene glycol ethyl methyl ether, 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.

[0049] The above cyclic ether is 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, 2-methyl-1,3-dioxolane, 2-vinyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3-dioxolane, 2-ethyl-2-methyl-1,3-dioxolane, tetrahydropyran, 1,4-dioxane, 1,2-dimethoxybenzene, 1,3-dimethoxy It may include one or more selected from the group consisting of benzene, 1,4-dimethoxybenzene and isosorbid dimethyl ether, furan, 2-methyl furan, 3-methyl furan, 2-ethyl furan, 2-butyl furan, 2,3-dimethyl furan, 2,4-dimethyl furan, 2,5-dimethyl furan, pyran, 2-methylpyran, 3-methylpyran, 4-methylpyran, benzoburane, 2-(2-nitrovinyl)furan, thiophene, 2-methylthiophene, 2-ethylthiophene, 2-propylthiophene, 2-butylthiophene, 2,3-dimethylthiophene and 2,4-dimethylthiophene and 2,5-dimethylthiophene.

[0051] In one embodiment of the present invention, the lithium salt may be used without particular limitation as long as it is a compound capable of providing lithium ions, which are generally used in lithium-sulfur batteries. For example, the lithium salt may include one or more selected from LiFSI, LiTFSI, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, and LiB(C2O4)2.

[0052] It is preferable to use the above lithium salt within the range of 0.1M to 2.0M. When the concentration of the lithium salt falls within the above range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0054] In one embodiment of the present invention, the additive may include a nitrate-based compound, and the nitrate-based compound may include one or more selected from lithium nitrate (LiNO3), potassium nitrate (KNO3), cesium nitrate (CsNO3), magnesium nitrate (MgNO3), barium nitrate (BaNO3), potassium nitrite (KNO2), and cesium nitrite (CsNO2). In this case, the additive may be included in an amount of 0.1% to 10% by weight relative to the total weight of the electrolyte. When the nitrate-based compound is included in the additive, the energy density and lifespan characteristics of the lithium-sulfur battery can be further improved.

[0055] In addition to the above-mentioned nitric acid-based compound, the above-mentioned additive may further include one or more additives for the purpose of improving the lifespan characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery, such as, for example, haloalkylene carbonate-based compounds like difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, triamide hexaphosphate, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride.

[0057] A second aspect of the present invention relates to a lithium-sulfur battery comprising the electrolyte for a lithium-sulfur battery described above.

[0058] A lithium-sulfur battery according to one aspect of the present invention comprises: a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, wherein the electrolyte is the electrolyte for a lithium-sulfur battery according to the present invention described above, the positive electrode comprises a sulfur-carbon composite as a positive electrode active material, and the negative electrode comprises lithium metal as a negative electrode active material. When the electrolyte for a lithium-sulfur battery described above is used as the electrolyte, TaF5 can react with the lithium metal of the negative electrode to form a protective layer. The protective layer will be described later.

[0059] In one embodiment of the present invention, the anode may include an anode current collector and an anode active material layer formed on at least one surface of the anode current collector. Additionally, the anode may be a freestanding type comprising only the anode active material layer.

[0060] In one embodiment of the present invention, the positive current collector supports the positive active material and is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface treated with carbon, nickel, silver, etc., and aluminum-cadmium alloy may be used.

[0061] The above positive current collector may form fine irregularities on its surface or form a perforated foil with fine holes, thereby strengthening the bonding force with the positive active material, and may be used in various forms such as film, sheet, foil, mesh, net, porous body, foam, nonwoven fabric, etc.

[0063] In one embodiment of the present invention, the positive active material layer may comprise a positive active material and a binder polymer, and may optionally further comprise a conductive material and an additive. The positive active material comprises a sulfur-carbon composite, wherein the sulfur-carbon composite is a sulfur-based material supported on a porous carbon material. Here, the sulfur-based material may comprise sulfur and / or sulfides.

[0064] The sulfur-carbon composite may be included in an amount of 50 to 95 wt%, 60 to 95 wt%, or 70 to 90 wt% with respect to a total of 100 wt% of the positive electrode active material layer. When the sulfur-carbon composite is included within the above range, the positive electrode can sufficiently undergo an electrochemical reaction, and the lithium-sulfur battery can secure sufficient energy density.

[0065] Specifically, the above sulfur-based material may include one or more selected from the group consisting of inorganic sulfur (S8), Li2Sn (n≥1), disulfide compounds such as 2,5-dimercapto-1,3,4-thiadiazole, 1,3,5-trithiocyanuic acid, and organic sulfur compounds. Preferably, inorganic sulfur (S8) may be used. In addition, it may further include one or more additives selected from transition metal elements, Group IIIA elements, Group IVA elements, sulfur compounds of these elements, and alloys of these elements and sulfur. The transition metal elements are Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au, Hg, etc., the Group IIIA elements may include Al, Ga, In, Tl, etc., and the Group IVA elements may include Ge, Sn, Pb, etc.

[0066] The porous carbon material may be a crystalline or amorphous carbon material, or a conductive carbon. The porous carbon material provides a framework capable of uniformly and stably fixing sulfur-based materials and compensates for the low electrical conductivity of sulfur-based materials, thereby enabling the electrochemical reaction to proceed smoothly.

[0067] The porous carbon material can be manufactured by carbonizing various carbon-based precursors and may contain irregularities and / or pores inside, the average diameter of the pores may be 1 nm to 200 nm, and the porosity may be 10 vol% to 90 vol% of the total volume of the porous carbon material. If the average diameter of the pores satisfies the above range, the mechanical strength of the porous carbon material can be maintained.

[0068] The porous carbon material can be used without limitation as long as it is in a spherical, rod-shaped, needle-shaped, plate-shaped, tubular, or bulk type, which are commonly used in lithium-sulfur batteries. In addition, the porous carbon material may have a high specific surface area. For example, the porous carbon material may include at least one selected from the group consisting of graphite, graphene, Super P, carbon black, Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermo black, carbon fiber, carbon nanofiber, carbon nanotube (SWCNT, MWCNT), carbon nanowire, carbon nanoring, carbon fabric, and fullerene (C60).

[0069] The porous carbon material may be an aggregate in which a plurality of conductive carbon materials are aggregated. Specifically, the porous carbon material may be formed by strands of a plurality of linear conductive carbon materials intertwining with each other.

[0070] The above sulfur-carbon composite may contain a sulfur-based material in an amount of 60 wt% or more, 60 to 90 wt%, or 70 to 90 wt% with respect to a total of 100 wt%. When the sulfur-based material is included within the above range, the content of the sulfur-based material and the content of the porous carbon material are appropriately balanced, allowing only an appropriate amount of binder polymer to bind the sulfur-based material to the porous carbon material to be used. Therefore, since it is not necessary to use an excessive amount of binder polymer that can cause an increase in resistance, the battery performance of the lithium-sulfur battery can be improved.

[0071] In the above sulfur-carbon composite, the sulfur-based material may be located externally by filling at least a portion of the internal space (e.g., pores) of the porous carbon material, or by covering at least a portion of the surface of the carbon-based material together with or independently thereof. In a specific embodiment, the sulfur-based material may be located in less than 100%, 1 to 95%, or 60 to 90% of the surface of the porous carbon material. When the sulfur-based material is present within the above ranges, the wettability to the electrolyte is excellent, and the electrical conductivity is excellent.

[0072] The method for manufacturing the above sulfur-carbon composite is not particularly limited and can be manufactured using methods commonly used in the industry. For example, the sulfur-carbon composite can be manufactured by mixing sulfur with a porous carbon material and heat-treating it.

[0073] The above sulfur-based material is converted into polysulfide during the electrochemical reaction of a lithium-sulfur battery, and the converted polysulfide leaches into the electrolyte of the lithium-sulfur battery, causing the positive electrode active material to gradually decrease and the positive electrode structure to collapse. However, in the present invention, since the sulfur-based material exists inside a porous carbon material with a three-dimensional structure, even if polysulfide is generated by the electrochemical reaction, it can be suppressed from leaching into the electrolyte, and even if it is leached, the collapse of the positive electrode structure can be suppressed because the porous carbon material forms a three-dimensional structure.

[0074] The above-mentioned binder polymer retains the positive active material on the positive current collector and organically connects the positive active materials to further enhance the bonding strength between them; any binder polymer known in the industry may be used. For example, the above-mentioned binder polymer may be a fluoropolymer-based binder comprising polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE); a rubber-based binder comprising styrene butadiene rubber (SBR), acrylonitrile-butidiene rubber, or styrene-isoprene rubber; a cellulose-based binder comprising carboxyl methyl cellulose (CMC), starch, hydroxypropyl cellulose, or regenerated cellulose; a polyalcohol-based binder; a polyolefin-based binder comprising polyethylene or polypropylene; a polyimide-based binder; or a polyester-based binder. One or more mixtures or copolymers selected from the group consisting of polyacrylic binders and silane binders may be used.

[0075] The content of the binder polymer may be included in an amount of 0.5 to 30 wt% relative to the total 100 wt% of the positive electrode active material layer. When the content of the binder polymer satisfies this range, the physical properties of the positive electrode are improved, preventing the detachment of the active material and / or conductive material within the positive electrode, and the ratio of the active material and / or conductive material in the positive electrode is appropriately controlled to ensure battery capacity.

[0076] The above conductive material is a material that electrically connects the electrolyte and the positive active material and serves as a path for electrons to move from the current collector to the positive active material, and can be used without limitation as long as it has conductivity. For example, the above conductive material may be carbon black such as Super-P, Denka Black, Acetylene Black, Ketjen Black, Channel Black, Furnace Black, Lamp Black, Thermo Black, Carbon Black; carbon derivatives such as carbon nanotubes, graphene, and fullerene; conductive fibers such as carbon fibers or metal fibers; metal powders such as fluorocarbon, aluminum, and nickel powder; or conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole, used alone or in combination. The content of the above conductive material may be 0.01 to 30 wt%, 0.01 to 10 wt%, or 0.01 to 5 wt% based on 100 wt% of the total positive active material layer. When the above conductive material is included within the above range, it enables electrons from the current collector to move more effectively to the positive active material.

[0077] The method of forming a positive active material layer on the above current collector is based on known coating methods and is not particularly limited. For example, bar coating, screen coating, doctor blade method, dip method, reverse roll method, direct roll method, gravure method, or extrusion method may be applied as coating methods. The amount of positive active material layer applied on the above current collector is not particularly limited and is adjusted considering the final desired thickness of the positive active material layer. In addition, known processes required for manufacturing an electrode, such as rolling or drying processes, may be performed before or after the process of forming the positive active material layer.

[0079] In one embodiment of the present invention, the cathode may be a freestanding type consisting only of a cathode active material layer without including a current collector. Additionally, the cathode may include a cathode current collector and a cathode active material layer formed on at least one surface of the cathode current collector.

[0080] In one embodiment of the present invention, the negative electrode current collector supports the negative electrode active material and is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface treated with carbon, nickel, silver, etc., and aluminum-cadmium alloy may be used.

[0081] The above-mentioned negative current collector may form fine irregularities on its surface or form a perforated foil with fine holes, thereby strengthening the bonding force with the negative active material, and may be used in various forms such as film, sheet, foil, mesh, net, porous body, foam, nonwoven fabric, etc.

[0082] The thickness of the above-mentioned negative current collector may be 1 μm to 300 μm, but is not particularly limited and can be set to an appropriate range considering the mechanical strength of the electrode, productivity, or capacity of the battery.

[0084] In one embodiment of the present invention, the negative electrode active material layer comprises lithium metal. Specifically, the negative electrode active material layer may comprise at least one of lithium metal or lithium alloy. Specifically, the lithium metal may be a single-phase lithium metal, for example, a lithium metal thin film, and the lithium alloy may be an alloy of lithium with one or more 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, or Al, but is not limited thereto.

[0085] When the above materials are used as negative electrode active materials, the energy density may be higher compared to cases where lithium is mixed with other materials (e.g., carbon and / or silicon) and used as a negative electrode active material. For example, when lithium is mixed with other materials and used as a negative electrode active material, the negative electrode active material is coated onto a negative electrode current collector; in this case, the energy density is lowered because the content of the negative electrode active material is reduced due to the use of the current collector.

[0086] The thickness of the above negative electrode active material layer can be 1 μm to 200 μm, 5 μm to 100 μm, 10 μm to 80 μm, or 20 μm to 50 μm. If the thickness of the negative electrode active material layer falls within the above range, the growth of lithium dendrites can be suppressed and sufficient battery capacity can be secured.

[0088] In one embodiment of the present invention, the cathode may be a current collector-free type consisting only of a cathode active material layer without including a current collector, or a cathode active material layer formed on at least one surface of a cathode current collector.

[0089] The above-mentioned negative current collector may be a current collector used in the technical field of lithium secondary batteries and lithium-sulfur batteries, and is not particularly limited as long as it has high conductivity without causing side reactions in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy may be used.

[0090] The thickness of the above-mentioned negative current collector may be 1 μm to 300 μm, but is not particularly limited and can be set to an appropriate range considering the mechanical strength of the electrode, productivity, or capacity of the battery.

[0092] In one embodiment of the present invention, the cathode comprises a cathode active material layer and a protective layer formed on at least one surface of the cathode active material layer, and the protective layer may comprise LiF.

[0093] The above protective layer may be formed by TaF5 included in the electrolyte reacting with the lithium metal of the negative electrode. The above protective layer can suppress porosity of the negative electrode by allowing lithium to be uniformly electrodeposited on the negative electrode. In addition, the lithium-sulfur battery including the above negative electrode can have improved Coulomb efficiency and cycle life.

[0094] The above protective layer may be formed on at least a portion or the entire surface of the negative electrode active material layer. For example, the protective layer may be formed to cover an area of ​​90% or more of the surface of the negative electrode active material layer.

[0095] The above protective layer can be formed with a uniform thickness, or with a thickness of 5 to 500 nm. When the thickness of the protective layer falls within the above range, it is possible to prevent the resistance of the cathode from rising excessively while appropriately suppressing porosity of the cathode and suppressing the formation of dendrites on the surface of the cathode.

[0097] In one embodiment of the present invention, the separator may be a porous polymer substrate or a porous coating layer formed on at least one surface of the porous polymer substrate.

[0098] The above separator separates the negative and positive electrodes and provides a pathway for the movement of lithium ions. It can be used without any particular restrictions as long as it is typically used as a separator in a lithium-sulfur battery, and it is particularly desirable that it has low resistance to the movement of electrolyte ions and excellent electrolyte moisture retention capacity.

[0099] The material of the above porous substrate is not specifically limited in the present invention, and any porous substrate commonly used in electrochemical devices can be used. For example, the porous substrate is a polyolefin such as polyethylene and polypropylene, a polyester such as polyethyleneterephthalate and polybutyleneterephthalate, a polyamide, a polyacetal, a polycarbonate, a polyimide, a polyetheretherketone, a polyethersulfone, a polyphenyleneoxide, a polyphenylenesulfide, a polyethylenenaphthalate, a polytetrafluoroethylene, a polyvinylidene fluoride, a polyvinyl chloride, a polyacrylonitrile, a cellulose, a nylon, It may include one or more materials selected from the group consisting of poly(p-phenylene benzobisoxazole) and polyarylate.

[0100] The thickness of the porous substrate is not particularly limited, but may be 1 to 100 μm, preferably 5 to 50 μm. Although the thickness range of the porous substrate is not limited to the aforementioned range, if the thickness is excessively thin compared to the aforementioned lower limit, the mechanical properties may deteriorate and the separator may be easily damaged during battery use.

[0101] The average diameter and porosity of the pores present in the porous substrate are also not particularly limited, but may be 0.001 to 50 μm and 10 to 95 volume%, respectively.

[0102] In one embodiment of the present invention, the inorganic particles and binder included in the porous coating layer may be used without particular limitation as long as they are typically used in the porous coating layer of a separation membrane, and the method of manufacturing the same is also not particularly limited.

[0104] In a lithium-sulfur battery according to one embodiment of the present invention, the electrolyte for the lithium-sulfur battery may be included in an amount of 300% by weight or less with respect to 100% by weight of the total sulfur-based material. Specifically, the content (E) of the electrolyte for the lithium-sulfur battery and the content (S) of the sulfur-based material may satisfy the following formula.

[0106] [ceremony]

[0107] (E / S) ×100 ≤ 300 (%)

[0109] When a lithium-sulfur battery contains a large amount of electrolyte, the overall volume of the battery increases or the content of the cathode active material decreases, resulting in a lower energy density. Therefore, to improve the energy density of the lithium-sulfur battery, the electrolyte content must be reduced and the cathode active material content increased. However, conventional lithium-sulfur batteries had a problem in that it was difficult to operate the battery when the electrolyte content was low.

[0110] The lithium-sulfur battery according to the present invention can operate smoothly even under low electrolyte conditions in which the electrolyte is included in an amount of 300% by weight or less relative to 100% by weight of the total sulfur-based material.

[0112] The present invention will be described in more detail below through examples, but the following examples are intended to illustrate the invention and the scope of the invention is not limited thereto.

[0114] Example 1

[0115] <Manufacture of Electrolyte for Lithium-Sulfur Batteries>

[0116] Dimethylfuran (2MeF) and dimethyl ether (DME) were mixed in a weight ratio of 2:8, and LiFSI was added to make the mixture 0.75 M to prepare a mixture. Subsequently, LiNO3 and TaF5 were added to the mixture to prepare an electrolyte. At this time, LiNO3 was set to 5 wt% and TaF5 to 0.1 wt% of the total weight of the electrolyte.

[0118] Manufacturing of the anode

[0119] Sulfur (S8) and multi-walled carbon nanotubes (MWCNT) were mixed in a weight ratio of 7:3 and uniformly mixed. Then, the sulfur was impregnated into the carbon by heat treatment in an oven at 150°C for 30 minutes to prepare a sulfur-carbon composite.

[0120] An anode slurry was prepared by adding the sulfur-carbon composite obtained above and polyacrylic acid as a binder polymer to water and mixing them. At this time, the weight ratio of the sulfur-carbon composite to the binder polymer was set to 96:4.

[0121] The above slurry was coated onto a 20㎛ thick aluminum foil using a Matisse coating device, dried at 50℃ for 24 hours, and subsequently rolled to manufacture an anode. At this time, the slurry loading was 3.3mAh / cm² 2 It was made so that the porosity during rolling was 72 vol%.

[0123] Manufacture of Lithium-Sulfur Batteries

[0124] A lithium metal thin film (Ganfeng) with a thickness of 60 μm was prepared as the cathode.

[0125] A lithium-sulfur battery was manufactured by positioning the anode and cathode prepared above so as to face each other, interposing a polyethylene separator with a thickness of 16 μm and a porosity of 46 vol% between them, and then injecting the electrolyte to a weight of 270 wt% relative to the weight of sulfur.

[0126] Example 2

[0127] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that TaF5 was added to make up 0.5 wt% of the total electrolyte.

[0128] Example 3

[0129] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that TaF5 was added to make up 1% by weight of the total electrolyte.

[0130] Comparative Example 1

[0131] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that TaF5 was not added.

[0132] Comparative Example 2

[0133] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that TaF5 was added to make up 1.2 wt% of the total electrolyte.

[0134] Comparative Example 3

[0135] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that AgF was added instead of TaF5 to make up 0.5 wt% of the total electrolyte.

[0136] Comparative Example 4

[0137] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that AlF3 was added instead of TaF5 to make up 0.5 wt% of the total electrolyte.

[0138] Comparative Example 5

[0139] Manufacturing of the anode

[0140] LiNi 0.8 Co 0.1 Mn 0.1An anode slurry was prepared by adding O2, Denka Black, and styrene butadiene rubber / carboxymethyl cellulose (SBR:CMC = 7:3 by weight ratio) in a weight ratio of 96:2:2 to a solvent and mixing. The slurry was coated onto a 20㎛ thick aluminum foil using a Matisse coating device, dried at 50°C for 24 hours, and subsequently rolled to manufacture an anode. At this time, the slurry loading was 3.3 mAh / cm² 2 It was made so that the porosity during rolling was 30 vol%.

[0142] Manufacture of Lithium-ion Batteries

[0143] A lithium-ion battery was manufactured in the same manner as in Example 1, except that the anode prepared above was used and TaF5 was not used in the electrolyte.

[0145] Comparative Example 6

[0146] A lithium-ion battery was manufactured in the same manner as Comparative Example 5, except that TaF5 was added to make up 0.5 wt% of the total electrolyte.

[0148] Experimental Example 1

[0149] After fabricating pouch cells using the lithium-sulfur batteries prepared in Examples 1 to 3 and Comparative Examples 1 to 6, the cells were subjected to a stabilization process by charging and discharging three times at a constant current of 0.1C at 25°C and then charging and discharging three times at 0.2C. Subsequently, the lifespan characteristics of the cells were measured by charging at 0.2C and discharging at 0.3C. At this time, the lower limit of discharge was set to 1.8V and the upper limit of charge to 2.5V, and the energy density was calculated based on the third cycle of charging and discharging at 0.1C. Additionally, the number of cycles at which the capacity reaches 80% of the first capacity when discharged at 0.3C was calculated as the lifespan cycle of the corresponding electrolyte.

[0150] The calculated energy density and lifetime cycle are shown in Table 1 below.

[0151] Capacity (mAh / g) Energy density (Wh / kg) Life cycle Example 1 1165 396 120 Example 2 1180 401 178 Example 3 1168 397 150 Comparative Example 1 1168 397 88 Comparative Example 2 1163 395 69 Comparative Example 3 - - 52 Comparative Example 4 - - 86 Comparative Example 5 - - 96 Comparative Example 6 - - 97

[0153] It was confirmed that the lithium-sulfur batteries manufactured in Examples 1 to 3, which contain 0.1 to 1.0 wt% of TaF5 relative to the total weight of the electrolyte, have sufficient capacity even after 120 cycles or more. On the other hand, it was confirmed that Comparative Example 1, which does not contain TaF5, and Comparative Example 2, which contains an excessive amount of TaF5, showed significantly inferior lifespan characteristics compared to Examples 1 to 3.

[0154] In addition, it was confirmed that Comparative Examples 3 and 4, which had additives other than TaF5 added, also showed significantly inferior lifespan characteristics compared to Examples 1 to 3.

[0155] Meanwhile, when comparing the lithium-ion battery according to Comparative Example 5, which does not contain TaF5, with the lithium-ion battery according to Comparative Example 6, which contains 0.5 wt% of TaF5 relative to the total weight of the electrolyte, it was confirmed that there was almost no change in the lifespan characteristics of the lithium-ion battery depending on whether TaF5 was included. On the other hand, in the case of the lithium-sulfur battery, when comparing the lithium-sulfur battery according to Comparative Example 2, which does not contain TaF5, with the lithium-sulfur battery according to Example 2, which contains 0.5 wt% of TaF5 relative to the total weight of the electrolyte, it was confirmed that there was a significant improvement in the lifespan characteristics depending on whether TaF5 was included.

[0157] FIG. 1 illustrates the Coulomb efficiency and capacity according to the cycle life of lithium-sulfur batteries according to Examples 1 to 3 and Comparative Examples 1 to 2 of the present invention. It was confirmed that the Coulomb efficiency and capacity of Examples 1 to 3 remained constant even as the cycle progressed, whereas it was confirmed that the Coulomb efficiency and capacity of the lithium-sulfur batteries of Comparative Examples 1 to 2 decreased rapidly. In particular, Comparative Example 2 contains 0.2 wt% more TaF5 than Example 3, and a significant difference in the change of Coulomb efficiency and capacity was confirmed, thereby confirming the critical significance of the TaF5 content range according to the present invention.

[0159] As described above, although the present invention has been explained by limited embodiments and drawings, the present invention is not limited thereto, and it is obvious that various modifications and variations are possible within the scope of the technical spirit of the present invention and the equivalent scope of the claims described below by those skilled in the art to which the present invention belongs.

Claims

Claim 1 An electrolyte for a lithium-sulfur battery comprising an organic solvent; a lithium salt; and an additive, wherein the additive comprises tantalum pentafluoride (TaF5), and wherein the tantalum pentafluoride is included in an amount of 0.1 to 1 weight% with respect to the total weight of the electrolyte for the lithium-sulfur battery. Claim 2 delete Claim 3 The lithium-sulfur battery electrolyte according to claim 1, characterized in that the tantalum pentafluoride is included in an amount of 0.3 to 0.7 weight% with respect to the total weight of the lithium-sulfur battery electrolyte. Claim 4 An electrolyte for a lithium-sulfur battery according to claim 1, characterized in that the organic solvent comprises an acyclic ether or a cyclic ether. Claim 5 The electrolyte for a lithium-sulfur battery according to claim 1, wherein the lithium salt comprises one or more selected from LiFSI, LiTFSI, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, and LiB(C2O4)2. Claim 6 The electrolyte for a lithium-sulfur battery according to claim 1, characterized in that the additive comprises one or more selected from lithium nitrate (LiNO3, lithium nitrate), potassium nitrate (KNO3), cesium nitrate (CsNO3), magnesium nitrate (MgNO3), barium nitrate (BaNO3), potassium nitrite (KNO2), and cesium nitrite (CsNO2). Claim 7 A lithium-sulfur battery comprising: a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, wherein the electrolyte is an electrolyte according to any one of claims 1, 3 to 6, the positive electrode comprises a sulfur-carbon composite as a positive electrode active material, and the negative electrode comprises a lithium metal as a negative electrode active material. Claim 8 A lithium-sulfur battery according to claim 7, wherein the sulfur-carbon composite comprises a sulfur-based material supported on a porous carbon material, and the electrolyte comprises 300% by weight or less of the total 100% by weight of the sulfur-based material. Claim 9 A lithium-sulfur battery according to claim 7, wherein the sulfur-carbon composite comprises a sulfur-based material supported on a porous carbon material, and wherein the sulfur-based material comprises 60 weight% or more of the total 100 weight% of the sulfur-carbon composite. Claim 10 A lithium-sulfur battery according to claim 7, wherein the cathode comprises a cathode active material layer and a protective layer formed on at least one surface of the cathode active material layer, and the protective layer comprises LiF.

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