Electrolyte for lithium-sulfur battery and lithium-sulfur battery including the same

Abstract

The electrolyte for a lithium-sulfur battery according to the present invention includes an organic solvent, a lithium salt, and an additive, the additive including tantalum pentafluoride (TaF5). By including tantalum pentafluoride in the electrolyte for a lithium-sulfur battery, the life characteristics of the lithium-sulfur battery can be improved.

Description

[Technical Field]

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

[0002] This application claims priority based on Korean Patent Application No. 10-2022-0185078, filed on December 26, 2022, the entire contents of which are incorporated herein by reference in their entirety in the specification and drawings thereof. [Background technology]

[0003] As interest in energy storage technology continues to grow, its applications are expanding to include mobile phones, tablets, laptops, and video cameras, as well as electric vehicles (EVs) and hybrid electric vehicles (HEVs). This has led to gradual progress in research and development efforts on electrochemical devices. From this perspective, electrochemical devices are the field that has attracted the most attention. In particular, the development of secondary batteries, such as rechargeable lithium-sulfur batteries, has been the focus of attention. Recently, the development of such batteries has led to research and development efforts on new electrode and battery designs to improve capacity density and specific energy.

[0004] Among these electrochemical devices, lithium-sulfur (LiS) batteries have attracted attention as next-generation secondary batteries with high energy density that can replace lithium-ion batteries. Lithium-sulfur is used as a cathode active material. During discharge in a lithium-sulfur battery, a reduction reaction of sulfur and an oxidation reaction of lithium metal occur. During this reaction, sulfur converts from a cyclic structure (S8) to a linear structure (Li2S2, Li2S4, Li2S6, Li2S8). These lithium-sulfur batteries exhibit a gradual discharge voltage until polysulfide (PS) is completely reduced to LiS. However, these lithium polysulfides react with the electrolyte, causing side reactions and battery degradation.

[0005] Furthermore, although lithium metal can be used for the negative electrode of a lithium-sulfur battery, there is a risk that the lithium metal electrode will develop dendrites and become porous as it is repeatedly charged and discharged, resulting in a reduction in its lifespan.

[0006] Furthermore, in order to achieve a high energy density of a lithium-sulfur battery, the content of the electrolyte must be reduced. However, if the amount of electrolyte is reduced, the concentration of lithium polysulfide increases, which may further promote side reactions with the lithium metal electrode, further shortening the battery life. Summary of the Invention [Problem to be solved by the invention]

[0007] Therefore, an object of the present invention is to provide an electrolyte for a lithium-sulfur battery that can improve the life characteristics of the lithium-sulfur battery.

[0008] Another object of the present invention is to provide a lithium-sulfur battery that has a high energy density and improved life characteristics. [Means for solving the problem]

[0009] In order to solve the above problems, according to one aspect of the present invention, there is provided an electrolyte for a lithium-sulfur battery or a lithium-sulfur battery having the following aspects.

[0010] The electrolyte for a lithium-sulfur battery according to the first embodiment includes an organic solvent, a lithium salt, and an additive, and the additive includes tantalum pentafluoride (TaF5).

[0011] According to the second aspect, in the first aspect, The tantalum pentafluoride is contained in an amount of 0.1 to 1% by weight based on the total weight of the electrolyte for lithium-sulfur batteries.

[0012] According to the third aspect, in the first aspect or the second aspect, The tantalum pentafluoride is contained in an amount of 0.3 to 0.7% by weight based on the total weight of the electrolyte for lithium-sulfur batteries.

[0013] According to a fourth aspect, in any one of the first to third aspects, The organic solvent includes an acyclic ether or a cyclic ether.

[0014] According to the fifth aspect, in any one of the first to fourth aspects, The lithium salt includes 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.

[0015] According to the sixth aspect, in any one of the first to fifth aspects, The additive includes at least one selected from the group consisting of lithium nitrate (LiNO3), potassium nitrate (KNO3), cesium nitrate (CsNO3), magnesium nitrate (MgNO3), barium nitrate (BaNO3), potassium nitrite (KNO2), and cesium nitrite (CsNO2).

[0016] According to a seventh aspect, A lithium-sulfur battery includes a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and an electrolyte, wherein the electrolyte is as defined in any one of the first to sixth aspects, the positive electrode includes a sulfur-carbon composite as a positive electrode active material, and the negative electrode includes lithium metal as a negative electrode active material.

[0017] According to the eighth aspect, in the seventh aspect, The lithium-sulfur battery electrolyte is contained in an amount of 300% by weight or less relative to 100% by weight of the total sulfur-based material.

[0018] According to the ninth aspect, in the seventh or eighth aspect, The sulfur-carbon composite is a composite in which a sulfur-based material is supported on a porous carbon material, The sulfur-based material is contained in an amount of 60% by weight or more relative to 100% by weight of the total sulfur-carbon composite.

[0019] According to the tenth aspect, in any one of the seventh to ninth aspects, The negative electrode includes a negative electrode active material layer and a protective layer formed on at least one surface of the negative electrode active material layer, and the protective layer includes LiF. [Effects of the Invention]

[0020] The electrolyte for a lithium-sulfur battery according to the present invention can improve the life characteristics of a lithium-sulfur battery and increase the energy density of the battery.

[0021] In addition to these, the present invention can have various other effects, which may be described in the sections for each embodiment, or the description of effects that can be easily inferred by a person of ordinary skill in the art may be omitted.

[0022] The drawings attached to this specification are intended to illustrate preferred embodiments of the present invention and serve to further understand the technical concepts of the present invention as well as the contents of the invention, and therefore the present invention should not be interpreted as being limited to the matters depicted in the drawings. Meanwhile, the shape, size, scale, or ratio of elements in the drawings attached to this specification may be exaggerated to emphasize a clearer description. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 10 is a graph showing the coulombic efficiency and capacity as a function of cycle life for an example according to the present invention and a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention will be described in detail below with reference to the accompanying drawings. The terms and words used in this specification and claims are not to be construed as being limited to their ordinary or dictionary meanings, but rather are to be construed as meanings and concepts corresponding to the technical ideas of the present invention, in accordance with the principle that the inventor can appropriately define the concepts of terms himself in order to best describe the invention.

[0025] Therefore, it should be understood that the embodiments described in this specification and the configurations shown in the drawings are merely preferred embodiments of the present invention and do not represent the entire technical idea of ​​the present invention, and that there may be various equivalent and modified embodiments that can be substituted for them at the time of this application.

[0026] Furthermore, throughout this specification, when a part is said to "include," "comprise," "have," or "have" a certain element, this does not mean that other elements are excluded, and that other elements may also be included, unless otherwise specified.

[0027] Furthermore, terms and phrases used throughout this specification such as "about," "approximately," and "substantially" are used to mean at or near a numerical value when given manufacturing and material tolerances inherent in the stated meaning, and are used to prevent unscrupulous infringers from unfairly exploiting the contents of the disclosure in which precise or absolute numerical values ​​are stated to aid in the understanding of the invention.

[0028] Furthermore, throughout this specification, the phrase "A and / or B" means "A or B or both."

[0029] Additionally, throughout this specification, unless otherwise specified, temperatures are in degrees Celsius and are expressed in °C.

[0030] The present invention relates to an electrolyte for a lithium-sulfur battery and a lithium-sulfur battery including the electrolyte. The lithium-sulfur battery includes a sulfur-based material as a positive electrode active material. In this specification, the sulfur-based material refers to a material including at least one selected from sulfur and sulfur compounds, which will be described in more detail below.

[0031] A first embodiment of the present invention relates to an electrolyte for a lithium-sulfur battery.

[0032] According to an embodiment of the present invention, an electrolyte for a lithium-sulfur battery includes an organic solvent, a lithium salt, and an additive, the additive including tantalum pentafluoride (TaF5). By adding the tantalum pentafluoride to the electrolyte for a lithium-sulfur battery, the energy density and life characteristics of the lithium-sulfur battery can be improved.

[0033] In one embodiment of the present invention, the tantalum pentafluoride may be contained in an amount of 0.1 to 1 wt %, or 0.3 to 0.7 wt %, based on the total weight of the electrolyte for lithium-sulfur batteries.

[0034] When the tantalum pentafluoride is contained within the above range, the energy density of the lithium-sulfur battery can be further improved, and the life characteristics can also be further improved.

[0035] 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 this weight ratio, the energy density and life characteristics of the lithium-sulfur battery using the tantalum pentafluoride can be further improved.

[0036] The 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.

[0037] The cyclic ethers include 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-dimethoxybenzene, 1,4-dimethoxybenzene, and isosorbide dimethyl ether. ether), furan, 2-methylfuran, 3-methylfuran, 2-ethylfuran, 2-butylfuran, 2,3-dimethylfuran, 2,4-dimethylfuran, 2,5-dimethylfuran, pyran, 2-methylpyran, 3-methylpyran, 4-methylpyran, benzofuran, 2-(2-nitrovinyl)furan, thiophene, 2-methylthiophene, 2-ethylthiophene, 2-propylthiophene, 2-butylthiophene, 2,3-dimethylthiophene, 2,4-dimethylthiophene, and 2,5-dimethylthiophene.

[0038] In one embodiment of the present invention, the lithium salt may be any compound capable of providing lithium ions generally used in lithium-sulfur batteries, and may include at least one selected from the group consisting of LiFSI, LiTFSI, LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCFSO, LiC4F9SO, LiN(C2F5SO3), LiN(C2F5SO2), LiN(CF3SO2), LiCl, LiI, and LiB(C2O4)2.

[0039] The concentration of the lithium salt may be in the range of 0.1 M to 2.0 M. When the concentration of the lithium salt falls within this range, the electrolyte has appropriate conductivity and viscosity, and therefore can exhibit excellent electrolyte performance and allow lithium ions to migrate effectively.

[0040] In one embodiment of the present invention, the additive may include a nitrate-based compound, which may include at least one selected from the group consisting of lithium nitrate (LiNO3), potassium nitrate (KNO3), cesium nitrate (CsNO3), magnesium nitrate (MgNO3), barium nitrate (BaNO3), potassium nitrite (KNO2), and cesium nitrite (CsNO2). The additive may be included in an amount of 0.1 wt % to 10 wt % based on the total weight of the electrolyte. When the additive includes the nitrate-based compound, the energy density and life characteristics of the lithium-sulfur battery may be further improved.

[0041] In addition to the nitric acid-based compound, the additive may further include one or more additives such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphate, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, and aluminum trichloride, with the aim of improving the life characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery.

[0042] A second embodiment of the present invention relates to a lithium-sulfur battery including the lithium-sulfur battery electrolyte described above.

[0043] A lithium-sulfur battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and an electrolyte, the electrolyte being the lithium-sulfur battery electrolyte according to the present invention described above. The positive electrode includes a sulfur-carbon composite as a positive electrode active material, and the negative electrode includes lithium metal as a negative electrode active material. When the lithium-sulfur battery electrolyte described above is used as the electrolyte, TaF can react with the lithium metal in the negative electrode to form a protective layer. The protective layer will be described later.

[0044] In one embodiment of the present invention, the positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector. The positive electrode may be a free-standing type including only the positive electrode active material layer.

[0045] In one embodiment of the present invention, the positive electrode current collector is not particularly limited as long as it supports the positive electrode active material, does not cause chemical changes in the battery, and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, silver, or the like, aluminum-cadmium alloy, etc. can be used.

[0046] The positive electrode current collector may have minute irregularities formed on its surface or may be formed as a perforated foil with minute holes formed therein, thereby strengthening the bonding force with the positive electrode active material, and a wide variety of forms such as a film, sheet, foil, mesh, net, porous body, foam, or nonwoven fabric body can be used.

[0047] In one embodiment of the present invention, the positive electrode active material layer may include a positive electrode active material and a binder polymer, and may optionally further include a conductive material and an additive. The positive electrode active material may include a sulfur-carbon composite, which is a sulfur-based material supported on a porous carbon material. Here, the sulfur-based material may include sulfur and / or a sulfide.

[0048] The sulfur-carbon composite may be contained in an amount of 50 to 95 wt%, 60 to 95 wt%, or 70 to 90 wt%, based on a total of 100 wt% of the positive electrode active material layer. When the sulfur-carbon composite is contained in the amount within the above range, the positive electrode can induce a sufficient electrochemical reaction, and the lithium-sulfur battery can ensure a sufficient energy density.

[0049] The sulfur-based material may include at least one selected from the group consisting of inorganic sulfur (S8), Li2Sn (n≧1), disulfide compounds such as 2,5-dimercapto-1,3,4-thiadiazole, and 1,3,5-trithiocyanuic acid, and organic sulfur compounds. Preferably, inorganic sulfur (S8) can be used. The sulfur-based material may further include at least one additive selected from the group consisting of transition metal elements, Group IIIA elements, Group IVA elements, sulfur compounds of these elements, and alloys of these elements with sulfur. The transition metal elements may include 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 IIIA group elements may include Al, Ga, In, Tl, etc., and the IVA group elements may include Ge, Sn, Pb, etc.

[0050] The porous carbon material may be a crystalline or amorphous carbon material, and may be conductive carbon. The porous carbon material provides a skeleton for uniformly and stably immobilizing the sulfur-based material, thereby compensating for the low electrical conductivity of the sulfur-based material and facilitating the electrochemical reaction.

[0051] The porous carbon material can be prepared by carbonizing a variety of carbonaceous precursors, and may have internal irregularities and / or pores, with the average pore diameter being 1 nm to 200 nm and the porosity being 10 vol% to 90 vol% of the total volume of the porous carbon material. When the average pore diameter is within this range, the mechanical strength of the porous carbon material can be maintained.

[0052] The porous carbon material may be any shape commonly used in lithium-sulfur batteries, such as spherical, rod-shaped, needle-shaped, plate-shaped, tubular, or bulk. The porous carbon material may have a high specific surface area. For example, the porous carbon material may include at least one material selected from the group consisting of graphite, graphene, Super P, carbon black, denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, carbon nanofiber, carbon nanotubes (SWCNT, MWCNT), carbon nanowires, carbon nanorings, carbon fabrics, and fullerene (C60).

[0053] The porous carbon material may be an aggregate of a plurality of conductive carbon materials. Specifically, the porous carbon material may be formed by entangling a plurality of linear conductive carbon materials with each other.

[0054] The sulfur-carbon composite may contain 60 wt% or more, 60 to 90 wt%, or 70 to 90 wt% of the sulfur-based material, based on a total weight of 100 wt%. When the sulfur-based material is contained within the above range, the sulfur-based material content and the porous carbon material content are appropriate, allowing for the use of an appropriate amount of binder polymer to bind the sulfur-based material to the porous carbon material. Therefore, excessive use of binder polymer, which can increase resistance, is not necessary, thereby improving the performance of lithium-sulfur batteries.

[0055] In the sulfur-carbon composite, the sulfur-based substance can be located externally by filling at least a portion of the internal space (e.g., pores) of the porous carbon material, or, in addition to or independently, by coating at least a portion of the surface of the carbon-based material. In a specific embodiment, the sulfur-based substance can 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 substance is present in the above range, the composite has excellent wettability with an electrolyte and excellent electrical conductivity.

[0056] The method for preparing the sulfur-carbon composite is not particularly limited, and may be a method commonly used in the art. For example, the sulfur-carbon composite may be prepared by mixing sulfur with a porous carbon material and heat-treating the mixture.

[0057] The sulfur-based material is converted to polysulfide during the electrochemical reaction of a lithium-sulfur battery, but the converted polysulfide dissolves in the electrolyte of the lithium-sulfur battery, causing a gradual decrease in the positive electrode active material and resulting in the collapse of the positive electrode structure. However, in the present invention, since the sulfur-based material is present inside a three-dimensional porous carbon material, even if polysulfide is generated by the electrochemical reaction, it can be prevented from dissolving into the electrolyte. Even if polysulfide dissolves, the three-dimensional structure of the porous carbon material can prevent the collapse of the positive electrode structure.

[0058] The binder polymer holds the positive electrode active material on the positive electrode current collector and organically connects the positive electrode active materials to each other to further enhance the binding strength between them. Any binder polymer known in the art can be used. For example, the binder polymer can be one or a mixture or copolymer of two or more selected from the group consisting of fluororesin-based binders including polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE), rubber-based binders including styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, and styrene-isophrene rubber, cellulose-based binders including carboxyl methyl cellulose (CMC), starch, hydroxypropyl cellulose, and regenerated cellulose, polyalcohol-based binders, polyolefin-based binders including polyethylene and polypropylene, polyimide-based binders, polyester-based binders, polyacrylic binders, and silane-based binders.

[0059] The content of the binder polymer may be 0.5 to 30 wt% based on a total of 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 active material and / or conductive material from falling off from the positive electrode, and the proportion of the active material and / or conductive material in the positive electrode is appropriately controlled, ensuring battery capacity.

[0060] The conductive material electrically connects the electrolyte and the positive electrode active material and serves as a path for electrons to move from the current collector to the positive electrode active material. Any conductive material can be used. For example, the conductive material can be carbon black, such as Super P, Denka Black, acetylene black, Ketjen Black, channel black, or furnace black; carbon derivatives, such as carbon nanotubes, graphene, or fullerene; conductive fibers, such as carbon fiber or metal fiber; metal powders, such as carbon fluoride, aluminum, or nickel powder; or conductive polymers, such as polyaniline, polythiophene, polyacetylene, or polypyrrole, either alone or in combination. The content of the conductive material can 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 electrode active material layer. The conductive material content within the above ranges facilitates electron movement from the current collector to the positive electrode active material.

[0061] The method for forming the positive electrode active material layer on the current collector may be any known coating method, and is not particularly limited. For example, bar coating, screen coating, doctor blade coating, dipping, reverse roll coating, direct roll coating, gravure coating, or extrusion may be used. The amount of the positive electrode active material layer applied to the current collector is not particularly limited, and is adjusted in consideration of the final desired thickness of the positive electrode active material layer. Furthermore, known processes required for electrode production, such as rolling and drying, may be performed before or after the step of forming the positive electrode active material layer.

[0062] In one embodiment of the present invention, the negative electrode may be a free-standing type negative electrode including only a negative electrode active material layer without including a current collector, and may include a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector.

[0063] In one embodiment of the present invention, the negative electrode current collector can be made of any material, including, but not limited to, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, silver, or the like, and aluminum-cadmium alloys, as long as the negative electrode current collector supports the negative electrode active material, does not cause chemical changes in the battery, and has high conductivity.

[0064] The negative electrode current collector may have minute irregularities formed on its surface or may be formed as a perforated foil with minute holes formed therein, thereby strengthening the bonding force with the negative electrode active material, and a wide variety of forms such as a film, sheet, foil, mesh, net, porous body, foam, or nonwoven fabric body can be used.

[0065] The thickness of the negative electrode current collector may be 1 μm to 300 μm, but is not particularly limited thereto, and can be set within an appropriate range in consideration of the mechanical strength of the electrode, productivity, battery capacity, and the like.

[0066] In one embodiment of the present invention, the negative electrode active material layer includes lithium metal. Specifically, the negative electrode active material layer may include at least one of lithium metal and a 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 and one or more of Si, Sn, C, Pt, Ir, Ni, Cu, Ti, Na, KRb, Cs, Fr, Be, Mg, Ca, Sr, Sb, Pb, In, Zn, Ba, Ra, Ge, or Al, but is not limited thereto.

[0067] When the material is used as a negative electrode active material, the energy density is higher than when 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 used by coating it on a negative electrode current collector. In this case, the content of the negative electrode active material is reduced due to the use of the current collector, resulting in a lower energy density.

[0068] The thickness of the negative electrode active material layer may be 1 μm to 200 μm, 5 μm to 100 μm, 10 μm to 80 μm, or 20 μm to 50 μm. When 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 ensured.

[0069] In one embodiment of the present invention, the negative electrode may be a current collector-free type negative electrode that does not include a current collector and is composed only of a negative electrode active material layer, or may be a negative electrode current collector having a negative electrode active material layer formed on at least one surface thereof.

[0070] The negative electrode current collector may be a current collector used in the technical fields of lithium secondary batteries and lithium-sulfur batteries, and is not particularly limited as long as it does not cause side reactions in the battery and has high conductivity, such as copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, and aluminum-cadmium alloy.

[0071] The thickness of the negative electrode current collector may be 1 μm to 300 μm, but is not particularly limited thereto, and can be set within an appropriate range in consideration of the mechanical strength of the electrode, productivity, battery capacity, and the like.

[0072] In one embodiment of the present invention, the negative electrode includes a negative electrode active material layer and a protective layer formed on at least one surface of the negative electrode active material layer, and the protective layer may include LiF.

[0073] The protective layer may be formed by a reaction between TaF5 contained in the electrolyte and lithium metal in the anode. The protective layer may allow lithium to be uniformly deposited on the anode and prevent the anode from becoming porous. Furthermore, a lithium-sulfur battery including the anode may have improved coulombic efficiency and cycle life.

[0074] The protective layer may be formed on at least a part of or the entire surface of the negative electrode active material layer, for example, the protective layer may be formed to cover 90% or more of the surface area of ​​the negative electrode active material layer.

[0075] The protective layer may be formed to a uniform thickness of 5 to 500 nm. When the protective layer has a thickness within this range, the negative electrode can be prevented from becoming porous while preventing the resistance of the negative electrode from increasing excessively, and dendrite formation on the surface of the negative electrode can be suppressed.

[0076] In one embodiment of the present invention, the separator may be a porous polymer substrate, or may be a porous polymer substrate having a porous coating layer formed on at least one surface thereof.

[0077] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator that is generally used as a separator in a lithium-sulfur battery can be used without any particular restrictions. In particular, it is preferable that the separator has low resistance to the movement of ions in the electrolyte and excellent moisture-absorbing ability for the electrolyte.

[0078] The material of the porous substrate is not particularly limited in the present invention, and any porous substrate commonly used in electrochemical devices can be used. For example, the porous substrate can be made of polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyamides, polyacetals, polycarbonates, polyimides, polyether ether ketones, polyether sulfones, polyphenylene oxides, polyphenylene sulfides, polyethylene naphthalate, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, etc. The material may include one or more materials selected from the group consisting of poly(p-phenylene benzobisoxazole), polyacrylonitrile, cellulose, nylon, poly(p-phenylene benzobisoxazole), and polyarylate.

[0079] 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 of the porous substrate is not limited to the above range, if the thickness is smaller than the lower limit, the mechanical properties may be reduced, and the separator may be easily damaged during use of the battery.

[0080] The average diameter and porosity of the pores present in the porous substrate are not particularly limited, but may be 0.001 to 50 μm and 10 to 95% by volume, respectively.

[0081] In one embodiment of the present invention, the inorganic particles and binder contained in the porous coating layer may be any inorganic particles and binder that are commonly used in porous coating layers of separators, and the manufacturing method thereof is also not particularly limited.

[0082] In the lithium-sulfur battery according to an embodiment of the present invention, the lithium-sulfur battery electrolyte may be included in an amount of 300 wt% or less based on 100 wt% of the total sulfur-based material. Specifically, the content of the lithium-sulfur battery electrolyte (E) and the content of the sulfur-based material (S) may satisfy the following formula:

[0083] [Formula 1] (E / S)×100≦300(%)

[0084] When a lithium-sulfur battery contains a large amount of electrolyte, the overall volume of the battery increases and the content of the positive electrode active material decreases, resulting in a lower energy density of the lithium-sulfur battery. Therefore, to improve the energy density of a lithium-sulfur battery, the content of the electrolyte must be reduced and the content of the positive electrode active material must be increased. However, conventional lithium-sulfur batteries have a problem in that the battery is difficult to operate when the content of the electrolyte is low.

[0085] The lithium-sulfur battery according to the present invention can be operated smoothly even under low electrolyte conditions where the electrolyte is contained in an amount of 300% by weight or less relative to a total of 100% by weight of sulfur-based materials.

[0086] The present invention will be described in more detail below with reference to examples. However, the following examples are merely for the purpose of illustrating the present invention, and the scope of the present invention is not limited to the following examples.

[0087] Example 1 <Production of electrolytes for lithium-sulfur batteries> Dimethylfuran (2MeF) and dimethyl ether (DME) were mixed in a weight ratio of 2:8, and LiFSI was added to the mixture to a concentration of 0.75M to prepare a mixed solution. LiNO3 and TaF5 were then added to the mixed solution to prepare an electrolyte. At this time, the LiNO3 and TaF5 contents were adjusted to 5 wt% and 0.1 wt%, respectively, based on the total weight of the electrolyte.

[0088] <Production of positive electrodes> Sulfur (S8) and multi-walled carbon nanotubes (MWCNTs) were uniformly mixed in a weight ratio of 7:3, and then heat-treated in an oven at 150°C for 30 minutes to impregnate the carbon with sulfur, producing a sulfur-carbon composite.

[0089] The sulfur-carbon composite and polyacrylic acid as a binder polymer were mixed in water to prepare a cathode slurry, with the weight ratio of the sulfur-carbon composite to the binder polymer being 96:4.

[0090] The slurry was coated onto a 20 μm thick aluminum foil using a Matisse coating device, dried at 50° C. for 24 hours, and then rolled to prepare a cathode. At this time, the loading of the slurry was 3.3 mAh / cm. 2 During rolling, the porosity was adjusted to 72 vol%.

[0091] <Manufacturing lithium-sulfur batteries> A lithium metal thin film (manufactured by Gan Fong Lithium Co., Ltd.) with a thickness of 60 μm was prepared as the negative electrode.

[0092] The prepared positive and negative electrodes were positioned facing each other, and a polyethylene separator with a thickness of 16 μm and a porosity of 46 vol% was sandwiched between them. The electrolyte was then injected to a weight ratio of 270 wt % relative to the weight of sulfur to prepare a lithium-sulfur battery.

[0093] Example 2 A lithium-sulfur battery was fabricated in the same manner as in Example 1, except that TaF5 was added to the electrolyte in an amount of 0.5 wt % of the total electrolyte.

[0094] Example 3 A lithium-sulfur battery was fabricated in the same manner as in Example 1, except that TaF5 was added to account for 1 wt % of the total electrolyte.

[0095] Comparative Example 1 A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that TaF5 was not added.

[0096] Comparative Example 2 A lithium-sulfur battery was fabricated in the same manner as in Example 1, except that TaF5 was added to the electrolyte in an amount of 1.2 wt % of the total electrolyte.

[0097] Comparative Example 3 A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that AgF was added in place of TaF5 so as to account for 0.5 wt % of the total electrolyte.

[0098] Comparative Example 4 A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that AlF3 was added in place of TaF5 so as to account for 0.5 wt% of the total electrolyte.

[0099] Comparative Example 5 <Production of positive electrodes> LiNi 0.8 Co 0.1 Mn 0.1 O2, Denka black, and styrene butadiene rubber / carboxymethyl cellulose (SBR:CMC = 7:3 weight ratio) were mixed in a solvent at a weight ratio of 96:2:2 to prepare a cathode slurry. The slurry was coated onto a 20 μm thick aluminum foil using a Matisse coating device, dried at 50°C for 24 hours, and then rolled to prepare a cathode. The loading of the slurry was 3.3 mAh / cm. 2During rolling, the porosity was adjusted to 30 vol%.

[0100] <Lithium-ion battery manufacturing> A lithium ion battery was fabricated in the same manner as in Example 1, except that the prepared positive electrode was used as the positive electrode and TaF5 was not used as the electrolyte.

[0101] Comparative Example 6 A lithium ion battery was manufactured in the same manner as in Comparative Example 5, except that TaF5 was added to the electrolyte in an amount of 0.5 wt % of the total electrolyte.

[0102] Experimental Example 1 Pouch cells were fabricated using the lithium-sulfur batteries produced in Examples 1 to 3 and Comparative Examples 1 to 6, and then charged and discharged three times at a constant current of 0.1 C at 25°C. After a stabilization process by charging and discharging three times at 0.2 C, the life characteristics of the cells were measured by charging at 0.2 C and discharging at 0.3 C. The lower limit of discharge was 1.8 V and the upper limit of charge was 2.5 V. Energy density was calculated based on the third cycle of charging and discharging at 0.1 C. In addition, the number of cycles at which the capacity reached 80% of the initial capacity when discharging at 0.3 C was calculated as the life cycle of the electrolyte.

[0103] The calculated energy density and life cycle are shown in Table 1 below.

[0104] [Table 1]

[0105] It was confirmed that the lithium-sulfur batteries manufactured in Examples 1 to 3, which contained 0.1 to 1.0 wt % of TaF5 relative to the total weight of the electrolyte, had sufficient capacity even after 120 cycles or more. In contrast, it was confirmed that Comparative Example 1, which did not contain TaF5, and Comparative Example 2, which contained excessive TaF5, had significantly inferior life characteristics compared to Examples 1 to 3.

[0106] It was also confirmed that Comparative Examples 3 and 4, in which an additive other than TaF5 was added, were also significantly inferior to Examples 1 to 3 in terms of life characteristics.

[0107] On the other hand, when the lithium-ion battery according to Comparative Example 5, which does not contain TaF5, is compared with the lithium-ion battery according to Comparative Example 6, which contains 0.5 wt% of TaF5 based on the total electrolyte weight, it is confirmed that there is almost no change in the life characteristics of the lithium-ion battery depending on whether or not TaF5 is contained. In contrast, when the lithium-sulfur battery according to Comparative Example 2, which does not contain TaF5, is compared with the lithium-sulfur battery according to Example 2, which contains 0.5 wt% of TaF5 based on the total electrolyte weight, it is confirmed that a significant improvement in the life characteristics can be obtained depending on whether or not TaF5 is contained.

[0108] Figure 1 shows the coulombic efficiency and capacity as a function of cycle life for lithium-sulfur batteries according to Examples 1 to 3 of the present invention and Comparative Examples 1 and 2. It can be seen that the coulombic efficiency and capacity remain constant as the cycles progress in Examples 1 to 3, but the coulombic efficiency and capacity rapidly decrease in the lithium-sulfur batteries of Comparative Examples 1 and 2. In particular, although Comparative Example 2 has a TaF5 content 0.2 wt% higher than Example 3, a significant difference was observed in the changes in coulombic efficiency and capacity, confirming the critical significance of the TaF5 content range according to the present invention.

[0109] As described above, although the present invention has been described using limited embodiments and drawings, the present invention is not limited thereby in any way, and it goes without saying that a person having ordinary knowledge in the technical field to which the present invention pertains can make various modifications and variations within the technical spirit of the present invention and the equivalent scope of the appended claims.

Claims

1. 1. An electrolyte for a lithium-sulfur battery, comprising an organic solvent, a lithium salt, and an additive, The additive is tantalum pentafluoride (TaF ５ 1. An electrolyte for a lithium-sulfur battery, comprising:

2. 2. The electrolyte for a lithium-sulfur battery according to claim 1, wherein the tantalum pentafluoride is contained in an amount of 0.1 wt % or more and 1 wt % or less based on the total weight of the electrolyte for a lithium-sulfur battery.

3. 2. The electrolyte for a lithium-sulfur battery according to claim 1, wherein the tantalum pentafluoride is contained in an amount of 0.3 wt % or more and 0.7 wt % or less based on the total weight of the electrolyte for a lithium-sulfur battery.

4. The electrolyte for a lithium-sulfur battery according to claim 1 , wherein the organic solvent comprises an acyclic ether or a cyclic ether.

5. The lithium salts include LiFSI, LiTFSI, and LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiSbF ６ , LiAlO ４ , LiAlCl ４ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiN(C ２ F ５ SO ３ ) ２ , LiN(C ２ F ５ SO ２ ) ２ , LiN(CF ３ SO ２ ) ２ , LiCl, LiI and LiB(C ２ O ４ ) ２ The electrolyte for a lithium-sulfur battery according to claim 1, comprising one or more selected from the group consisting of:

6. The additive is lithium nitrate (LiNO ３ , lithium nitrate), potassium nitrate (KNO ３ ), cesium nitrate (CsNO ３ ), magnesium nitrate (MgNO ３ ), barium nitrate (BaNO ３ ), potassium nitrite (KNO ２ ) and cesium nitrite (CsNO ２ 2. The electrolyte for a lithium-sulfur battery according to claim 1, comprising one or more selected from the group consisting of:

7. A lithium-sulfur battery including a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and an electrolyte, The electrolyte is the electrolyte according to any one of claims 1 to 6, the positive electrode contains a sulfur-carbon composite as a positive electrode active material, The lithium-sulfur battery, wherein the negative electrode contains lithium metal as a negative electrode active material.

8. The sulfur-carbon composite is a composite in which a sulfur-based material is supported on a porous carbon material, 8. The lithium-sulfur battery according to claim 7, wherein the electrolyte is contained in an amount of 300% by weight or less relative to a total of 100% by weight of the sulfur-based material.

9. The sulfur-carbon composite is a composite in which a sulfur-based material is supported on a porous carbon material, 8. The lithium-sulfur battery according to claim 7, wherein the sulfur-carbon composite contains the sulfur-based material in an amount of 60% by weight or more relative to 100% by weight of the total sulfur-carbon composite.

10. 8. The lithium-sulfur battery according to claim 7, wherein the negative electrode includes a negative electrode active material layer and a protective layer formed on at least one surface of the negative electrode active material layer, the protective layer including LiF.

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