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

Abstract

The present invention relates to an electrolyte for a lithium-sulfur secondary battery and a lithium-sulfur secondary battery including the same. By adjusting the solvent, non-solvent, and lithium salt contained in the electrolyte under specific conditions, a lithium-sulfur secondary battery with improved life characteristics can be provided.

Description

[Technical Field]

[0001] The present invention relates to an electrolyte for a lithium-sulfur secondary battery and a lithium-sulfur secondary battery including the same.

[0002] This application claims priority based on Korean Patent Application No. 10-2022-0183761, filed on December 23, 2022, the entire contents of which are incorporated herein by reference. [Background technology]

[0003] Lithium-sulfur (Li-S) batteries, which utilize conventional catholyte systems, are Li2S X However, because it relies on a liquid-phase reaction (catholyte type) that generates polysulfide, an intermediate product in the form of sulfur, it is unable to fully utilize the high theoretical discharge capacity (1675mAh / g) of sulfur, and instead has the problem of battery degradation due to the elution of polysulfide, resulting in a decrease in battery life characteristics. To solve this problem, additives such as LiNO3 are used, but the depletion of additives still makes it difficult to achieve long-life behavior for lithium-sulfur batteries.

[0004] Recently, a sparingly solvating electrolyte (SSE) system that suppresses polysulfide elution has been proposed, and it has been confirmed that normal operation is possible without charging delay even with an electrolyte that does not contain certain additives. However, there is still the issue of short lifespan, and improvement of lifespan characteristics is necessary.

[0005] Therefore, to achieve a high energy density of 400Wh / kg or 600Wh / L or more, a battery with a capacity of 4.0mAh / cm 2 As described above, there is a current need for an electrolyte system for lithium-sulfur secondary batteries that can be operated even with a porosity of 60 vol% or less and that can improve life characteristics. Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, the present invention has been made to solve the above problems, and an object of the present invention is to provide an electrolyte for a lithium-sulfur secondary battery having high energy density and improved life characteristics, and a lithium-sulfur secondary battery including the same.

[0007] It will be readily apparent that other objects and advantages of the present invention can be achieved by the means or methods and combinations thereof recited in the claims. [Means for solving the problem]

[0008] The present inventors have found that the above problems can be solved by the following electrolyte for lithium-sulfur secondary batteries and a lithium-sulfur secondary battery including the same.

[0009] According to a first aspect, a solvent, a non-solvent, and a lithium salt; The solvent and the non-solvent have an SVR factor value of 0.2 to 0.7, and the SVR factor is represented by the following formula 1: The lithium salt has a CCR factor value of 0.1 to 0.6, and the CCR factor is represented by the following formula 2: The lithium salt comprises lithium bis(fluorosulfonyl)imide (LiFSI): [Formula 1] SVR factor (Solvent Volume Ratio factor) = Volume of solvent ÷ Volume of non-solvent [Formula 2] CCR factor (Co-salt Concentration Ratio factor) = Molar concentration of LiFSI ÷ Molar concentration of all lithium salts.

[0010] According to the second aspect, in the first aspect, The present invention relates to an electrolyte for a lithium-sulfur secondary battery, wherein the lithium salt has an SVR factor value of 0.3 to 0.6.

[0011] According to the third aspect, in the first or second aspect, The present invention relates to an electrolyte for a lithium-sulfur secondary battery, wherein the lithium salt has a CCR factor value of 0.15 to 0.55.

[0012] According to a fourth aspect, in any one of the first to third aspects, The solvent and lithium salt have an MR factor value of 1.0 to 2.7, and the MR factor is represented by the following formula 3:

[0013] [Formula 3] MR factor (Molar mass ratio factor) = number of moles of solvent ÷ number of moles of total lithium salts According to a fifth aspect, in any one of the first to fourth aspects, The solubility of the solvent for the lithium salt is 0.1 M or more, The present invention relates to an electrolyte for a lithium-sulfur secondary battery, characterized in that the solubility of the non-solvent in the lithium salt is less than 0.1M.

[0014] According to a sixth aspect, in any one of the first to fifth aspects, The present invention relates to an electrolyte for a lithium-sulfur secondary battery, wherein the solvent is a linear ether, a cyclic ether, or a mixture thereof.

[0015] According to a seventh aspect, in any one of the first to sixth aspects, The present invention relates to an electrolyte for a lithium-sulfur secondary battery, wherein the non-solvent is a fluorinated ether.

[0016] According to an eighth aspect, in any one of the first to seventh aspects, The present invention relates to an electrolyte for a lithium-sulfur secondary battery, wherein the lithium salt contains lithium bis(fluorosulfonyl)imide (LiFSI) and an imide-based lithium salt other than the lithium bis(fluorosulfonyl)imide (LiFSI).

[0017] According to a ninth aspect, in any one of the first to eighth aspects, The present invention relates to an electrolyte for a lithium-sulfur secondary battery, characterized in that the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0018] According to a tenth aspect, in any one of the first to ninth aspects, The present invention relates to an electrolyte for a lithium-sulfur secondary battery, which does not contain one or more of nitric acid compounds and nitrite compounds.

[0019] According to an eleventh aspect, The present invention relates to a lithium-sulfur secondary battery comprising a negative electrode, a positive electrode, a separator, and the electrolyte according to any one of the first to tenth aspects. [Effects of the Invention]

[0020] The electrolyte for a lithium-sulfur secondary battery and the lithium-sulfur secondary battery including the same according to the present invention have the effect of improving the life characteristics by adjusting the solvent, non-solvent, and lithium salt contained in the electrolyte under specific conditions. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention will be described in more detail below.

[0022] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that corresponds to the technical idea of ​​the present invention, in accordance with the principle that the inventor can appropriately define the concept of the terms himself / herself in order to explain the invention in the best way.

[0023] Throughout this specification, when a part is said to "comprise" 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.

[0024] Throughout this specification, the phrase "A and / or B" means "A or B or both."

[0025] In the present invention, the "specific surface area" is measured by the BET method, and specifically, can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77 K) using a BELSORP-mino II manufactured by BEL Japan.

[0026] As used herein, the term "polysulfide" refers to "polysulfide ions (S x 2- , x=8, 6, 4, 2) and Lithium polysulfide (Li2S x or LiS x - , x=8, 6, 4, 2) is a concept that includes both.

[0027] The term "composite" as used herein means a substance that combines two or more materials to form physically and chemically distinct phases, thereby exhibiting more effective functions.

[0028] The term "porosity" as used herein means the ratio of the volume occupied by pores to the total volume of a structure, and is expressed in units of %, and may be used interchangeably with terms such as void ratio and porosity.

[0029] The present invention relates to an electrolyte for a lithium-sulfur secondary battery and a lithium-sulfur secondary battery including the same. The electrolyte for a lithium-sulfur secondary battery according to one aspect of the present invention includes a solvent, a non-solvent, and a lithium salt.

[0030] Lithium-sulfur secondary batteries have high discharge capacities and theoretical energy densities compared to other secondary batteries. Furthermore, sulfur, which is used as the positive electrode active material, is abundant and inexpensive, reducing battery manufacturing costs. These advantages, along with their environmental friendliness, make them promising candidates for next-generation secondary batteries.

[0031] However, the main challenge for lithium-sulfur secondary batteries is to prevent the leaching of polysulfides, and the SSE electrolyte system was developed to address this issue. However, due to the nature of the SSE electrolyte system, which induces a solid-state reaction, it is difficult to maintain the reversible reaction between the positive and negative electrode active materials, which results in a shortened battery life, which remains an unresolved issue.

[0032] Therefore, the inventors have invented a specific electrolyte system that employs an SSE electrolyte system, but excludes nitrile-based electrolyte solvents that have a fatal impact on the lifespan of lithium-sulfur batteries, and uses a solvent, non-solvent, and lithium salt that satisfy specific conditions, thereby solving the problem of lithium anode degradation and maintaining the reversible reaction of the cathode / anode active materials, thereby improving the battery lifespan.

[0033] The solvent and non-solvent have an SVR factor value, represented by the following formula 1, of 0.2 to 0.7.

[0034] [Formula 1] SVR factor (Solvent Volume Ratio factor) = Volume of solvent (A) ÷ Volume of non-solvent (B)

[0035] In the above formula 1, the volume of the solvent and the volume of the non-solvent refer to the volumes of the solvent and the non-solvent added to the electrolyte, respectively, and the SVR factor has no unit.

[0036] The SVR factor value is 0.2 to 0.7, and according to one embodiment of the present invention, the SVR factor value may be in the range of 0.3 to 0.6 or 0.4 to 0.5. If the SVR factor value is outside the specified range, it may be difficult to effectively improve performance. In particular, if the SVR factor exceeds 0.7, excessive dissolution of lithium polysulfide in the electrolyte may occur. If the SVR factor is less than 0.2, the solubility of the lithium salt decreases, making it difficult to form an electrolyte. Even if the electrolyte is formed, the low ionic conductivity of the electrolyte may make it difficult to operate the secondary battery normally.

[0037] In the present invention, the solvent refers to a solvent having a solubility for lithium salts of 0.1 M or more, and the non-solvent refers to a solvent having a solubility for lithium salts of less than 0.1 M. Specifically, the solvent may refer to a solvent having a solubility for imide-based lithium salts such as LiTFSI, LiFSI, and LiTF of 0.1 M or more, and the non-solvent may refer to a solvent having a solubility for lithium salts of less than 0.1 M.

[0038] As long as the solvent and non-solvent satisfy the above-mentioned solubility conditions for the lithium salt and the above-mentioned SVR factor value, any solvent ordinarily used for the electrolyte of a secondary battery can be used without any restrictions.

[0039] In one embodiment of the present invention, the solvent may comprise a linear ether, a cyclic ether, or a mixture thereof.

[0040] For example, the solvent may be dimethyl ether, diethyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, ethyl methyl ether, ethyl propyl ether, ethyl tert-butyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, dimethoxypropane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, dipropylene glycol dimethyl ether, butylene glycol ether, diethylene glycol Linear ethers such as glycerol ethyl methyl ether, diethylene glycol isopropyl methyl ether, diethylene glycol butyl methyl ether, diethylene glycol tert-butyl ethyl ether, and ethylene glycol ethyl methyl ether; cyclic ethers such as dioxolane, methyl dioxolane, dimethyl dioxolane, vinyl dioxolane, methoxy dioxolane, ethyl methyl dioxolane, oxane, dioxane, trioxane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyltetrahydrofuran, dimethoxytetrahydrofuran, ethoxytetrahydrofuran, dihydropyran, tetrahydropyran, furan, and 2-methylfuran; or mixtures thereof.

[0041] In one embodiment of the present invention, the non-solvent may comprise a fluorinated ether.

[0042] For example, the non-solvent may be 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, 1H,1H,2′H-perfluorodipropyl ether, difluoromethyl 2,2,2,2-trifluoroethyl ether, fluorine-based ether compounds such as 1,2,2,2-tetrafluoroethyl trifluoromethyl ether, 1,1,2,3,3,3-hexafluoropropyl difluoromethyl ether, pentafluoroethyl 2,2,2-trifluoroethyl ether, and 1H,1H,2′H-perfluorodipropyl ether.

[0043] The lithium salt has a CCR factor value, represented by the following formula 2, of 0.1 to 0.6.

[0044] [Formula 2] CCR factor (Co-salt Concentration Ratio factor) = Molar concentration of LiFSI ÷ Molar concentration of all lithium salts

[0045] In the above formula 2, the concentration of all lithium salts means the sum of the molar concentrations of all lithium salts contained in the electrolyte, and the CCR factor has no unit.

[0046] The CCR factor value is 0.1 to 0.6, and according to one embodiment of the present invention, the CCR factor value may be in the range of 0.15 to 0.55 or 0.2 to 0.4. If the CCR factor value is outside the specified range, the degree of performance improvement becomes small. In particular, if the CCR factor exceeds 0.6, irreversible reactions inside the battery increase, and if the CCR factor is maintained below 0.1, the proportion of lithium salt is too low, making it difficult to improve the battery life.

[0047] In one embodiment of the present invention, the lithium salt may include two or more different types of lithium salts, specifically, different types of imide-based lithium salts.

[0048] In particular, the lithium salt must include lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and may include at least one other type of imide-based lithium salt.

[0049] The imide-based lithium salt has good chemical stability and dissociation property in organic solvents, and therefore can be used as the lithium salt for lithium-sulfur secondary batteries, which can stably maintain the capacity of the lithium-sulfur battery and improve the battery life. Furthermore, when two or more imide-based lithium salts are used instead of a single lithium salt, the stability between the lithium anode and the electrolyte is improved, thereby enabling the production of a secondary battery with improved battery life.

[0050] For example, the imide-based lithium salt may include lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(perfluoroethylsulfonyl)imide (LiBETI), and the like.

[0051] Specifically, lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) may be used, which have the advantages of less side reactions with the lithium negative electrode and high solubility in ether-based solvents, and are effective in improving the battery life.

[0052] Within the ranges satisfying the CCR factor values, the molar concentrations of the total lithium salts may be in the range of 0.1 to 5.0 M, or 0.1 to 3.0 M, and the molar concentration of LiFSI may be in the range of 0.1 to 1.5 M, or 0.2 to 1.0 M. When the molar concentrations of the total lithium salts and the molar concentration of LiFSI are within the above ranges, the electrolyte has appropriate conductivity and viscosity, thereby exhibiting excellent electrolyte performance and allowing lithium ions to migrate effectively.

[0053] According to one embodiment of the present invention, the lithium salt may have an SVR factor value of 0.3 to 0.6, and the lithium salt may have a CCR factor value of 0.15 to 0.55.

[0054] The solvent and lithium salt may have an MR factor value, represented by the following formula 3, of 1.0 to 2.7.

[0055] [Formula 3] MR factor (Molar mass ratio factor) = number of moles of solvent ÷ number of moles of total lithium salts

[0056] In the formula 3, the moles of all lithium salts refer to the sum of the moles of all lithium salts contained in the electrolyte, and the moles of solvents refer to the sum of the moles of all solvents excluding non-solvents contained in the electrolyte. The MR factor has no unit.

[0057] The MR factor value may be in the range of 1.0 to 2.7, 1.15 to 2.65, or 1.5 to 2.65. When the MR factor value satisfies the specified range, dissolution of lithium polysulfide can be suppressed, the ionic conductivity of the electrolyte can be increased, and the life of the secondary battery can be improved.

[0058] For example, the calculation methods for the SVR factor, CCR factor, and MR factor will be described based on Example 3 shown in Table 1 below.

[0059] First, in Example 3, DME (solvent):TTE (non-solvent) is contained at a volume ratio of 3:7, so the SVR factor is calculated to be 3 / 7=0.43.

[0060] Next, Example 3 contains LiTFSI and LiFSI as lithium salts, with 1.25 M LiTFSI and 0.5 M LiFSI, so the CCR factor is calculated to be 0.5 / (1.25 + 0.5) = 0.29. Next, when producing 100 g of the electrolyte according to Example 3, 0.16 mol of DME (molar mass 90.12), 0.07 mol of LiTFSI (molar mass 287), and 0.028 mol of LiFSI (molar mass 187) are contained, so the MR factor is calculated to be 0.16 / (0.07 + 0.028) = 1.63.

[0061] That is, it can be confirmed that Example 3 satisfies the ranges specified in the present invention for the SVR factor, CCR factor and MR factor, and also exhibits excellent performance in the life evaluation results.

[0062] An electrolyte for a lithium-sulfur secondary battery according to one embodiment of the present invention does not contain one or more of nitric acid-based and nitrite-based compounds.

[0063] Generally, the use of nitric acid and / or nitrite compounds as electrolyte additives can form a stable coating on the lithium electrode and improve charge / discharge efficiency, but has the drawback of not being able to improve lifespan due to depletion. Therefore, the inventors of the present invention have achieved an effect of improving lifespan without containing nitric acid and / or nitrite compounds, which are commonly used as additives.

[0064] The nitric acid or nitrite compound is not particularly limited in the present invention, and examples thereof include inorganic nitric acid or nitrite compounds such as lithium nitrate (LiNO), potassium nitrate (KNO), cesium nitrate (CsNO), barium nitrate (Ba(NO)), ammonium nitrate (NHNO), lithium nitrite (LiNO), potassium nitrite (KNO), cesium nitrite (CsNO), and ammonium nitrite (NHNO); organic nitric acid or nitrite compounds such as methyl nitrate, dialkylimidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridinium nitrate, ethyl nitrate, propyl nitrate, butyl nitrate, pentyl nitrate, and octyl nitrate; organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, and dinitrotoluene; and combinations thereof. Specifically, the additive may be free of lithium nitrate (LiNO3).

[0065] Meanwhile, the electrolyte may further contain other additives for the purpose of improving charge / discharge characteristics, flame retardancy, etc. Examples of the additives include pyridine, triethyl phosphite, 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, aluminum trichloride, fluoroethylene carbonate (FEC), propene sultone (PRS), vinylene carbonate (VC), etc.

[0066] The method for producing the electrolyte for the lithium-sulfur secondary battery of the present invention is not particularly limited, and any known method commonly used in the art can be used.

[0067] In one embodiment of the present invention, there is provided a lithium-sulfur secondary battery comprising a negative electrode, a positive electrode, a separator, and the aforementioned electrolyte.

[0068] The negative electrode includes a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector, and the negative electrode active material layer includes a negative electrode active material, a conductive material, and a binder.

[0069] Specifically, the negative electrode may be fabricated by coating one or both sides of a long sheet-shaped negative electrode current collector with a negative electrode slurry prepared by dispersing a negative electrode active material, a conductive material, and a binder in a solvent, drying the coated negative electrode current collector to remove the solvent, and then rolling the coated negative electrode current collector. Meanwhile, a negative electrode including a non-coated portion may be fabricated by not coating a portion of the negative electrode current collector, for example, one end of the negative electrode current collector, with the negative electrode slurry during coating.

[0070] The negative electrode active material is lithium (Li +Substances that can be reversibly inserted (intercalated) or desorbed (deintercalated), substances that can react with lithium ions to reversibly form lithium-containing compounds, and lithium metal may be included. The substances that can reversibly insert or desorb the lithium ions can be, for example, crystalline carbon, amorphous carbon, or a mixture thereof. Specifically, artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, soft carbon, hard carbon, etc. can be cited as examples, but it is not limited thereto. The substances that can react with the lithium ions to reversibly form lithium-containing compounds can be, for example, tin oxide, titanium nitrate, or silicon-based compounds. The lithium alloy can be, for example, an alloy of lithium (Li) and a metal selected from sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn). Preferably, the negative electrode active material can be lithium metal, specifically, in the form of a lithium metal thin film or lithium metal powder. The silicon-based negative electrode active material can be Si, Si-Me alloy (where Me is one or more selected from the group consisting of Al, Sn, Mg, Cu, Fe, Pb, Zn, Mn, Cr, Ti, and Ni), SiO y (where 0 < y < 2), a Si-C composite, or a combination thereof, and preferably, SiO y (where 0 < y < 2). Since the silicon-based negative electrode active material has a high theoretical capacity, when the silicon-based negative electrode active material is included, the capacity characteristics can be improved.

[0071] The negative electrode current collector may be a negative electrode current collector commonly used in the art, such as copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or aluminum-cadmium alloy. The negative electrode current collector typically has a thickness of 3 to 500 μm, and the surface of the current collector may be provided with fine irregularities to enhance the binding strength of the negative electrode active material. The negative electrode current collector may be in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0072] The conductive material is used to impart conductivity to the negative electrode. Any material that exhibits electronic conductivity without undergoing chemical changes in the resulting battery can be used without any particular limitations. Specific examples include graphite, such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black, carbon fiber, and carbon nanotubes; metal powders or fibers, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. The conductive material is typically present in an amount of 1 to 30 wt%, 1 to 20 wt%, or 1 to 10 wt% of the total weight of the negative electrode active material layer.

[0073] The binder improves adhesion between negative electrode material particles and between the negative electrode active material and the negative electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination. The binder may be included in an amount of 1 to 30 wt%, 1 to 20 wt%, or 1 to 10 wt% of the total weight of the negative electrode material layer.

[0074] The positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material and may further include a conductive material, a binder, an additive, etc.

[0075] The positive electrode current collector is for supporting the positive electrode active material and is the same as that described for the negative electrode current collector, and can be made of, for example, copper, stainless steel, aluminum, nickel, titanium, palladium, baked carbon, copper or stainless steel whose surface is treated with carbon, nickel, silver, or the like, or an aluminum-cadmium alloy.

[0076] The positive electrode active material may contain at least one selected from the group consisting of sulfur, particularly elemental sulfur (S), and sulfur compounds. The positive electrode active material may include inorganic sulfur, Li2Sn (n≧1), disulfide compounds, organic sulfur compounds, and carbon-sulfur polymers (C2S x)n, x=2.5 to 50, n≧2) Specifically, the positive electrode active material may contain inorganic sulfur.

[0077] The sulfur contained in the positive electrode active material does not have electrical conductivity by itself, and therefore is used together with a conductive material such as a carbon material. Therefore, the sulfur is contained in the form of a sulfur-carbon composite, and preferably, the positive electrode active material may be a sulfur-carbon composite.

[0078] The carbon contained in the sulfur-carbon composite is a porous carbon material that provides a framework for uniformly and stably immobilizing sulfur and compensates for the low electrical conductivity of sulfur, thereby facilitating the smooth progress of electrochemical reactions.

[0079] The porous carbon material can generally be prepared by carbonizing various carbonaceous precursors. The porous carbon material may contain non-uniform pores within the material, with the average pore diameter ranging from 1 to 200 nm and the porosity ranging from 10 to 90% of the total volume of the porous carbon material. If the average pore diameter is smaller than this range, the pore size is merely at the molecular level, making impregnation with sulfur impossible. Conversely, if the average pore diameter exceeds this range, the mechanical strength of the porous carbon material decreases, making it unsuitable for use in electrode manufacturing processes.

[0080] The shape of the porous carbon material may be spherical, rod-like, needle-like, plate-like, tubular or bulk, and any shape commonly used in lithium-sulfur batteries may be used without limitation.

[0081] The porous carbon material may have a porous structure or a large specific surface area, and may be any material commonly used in the art. Examples of the porous carbon material include graphite, graphene, carbon blacks such as denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; carbon nanotubes (CNTs) such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); carbon fibers such as graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs); graphites such as natural graphite, artificial graphite, and expanded graphite; and activated carbon. Preferably, the porous carbon material is carbon nanotubes.

[0082] The sulfur-carbon composite may contain 60 to 90 parts by weight, preferably 65 to 85 parts by weight, and more preferably 70 to 80 parts by weight of sulfur per 100 parts by weight of the sulfur-carbon composite. If the sulfur content is below this range, the content of the porous carbon material in the sulfur-carbon composite increases relatively, thereby increasing its specific surface area and increasing the amount of binder used relative to the sulfur content during cathode preparation. This increased binder usage ultimately increases the surface resistance of the cathode, acting as an insulator that blocks electron passage and degrades battery performance. Conversely, if the sulfur content exceeds this range, sulfur that cannot bond with the porous carbon material aggregates or re-leaches onto the surface of the porous carbon material, making it difficult to accept electrons and unable to participate in the electrochemical reaction, resulting in a decrease in battery capacity.

[0083] Furthermore, in the sulfur-carbon composite, sulfur is present on at least one of the inner and outer surfaces of the porous carbon material. In this case, sulfur may be present on less than 100%, 1 to 95%, or 60 to 90% of the area of ​​the entire inner and outer surfaces of the porous carbon material. When sulfur is present on the inner and outer surfaces of the porous carbon material within this range, it can exhibit the maximum effect in terms of electron transfer area and wettability with the electrolyte. Specifically, sulfur is impregnated thinly and uniformly on the inner and outer surfaces of the porous carbon material within this range, thereby increasing the electron transfer contact area during charge and discharge. When sulfur is present on 100% of the entire inner and outer surfaces of the porous carbon material, the carbon material is completely covered with sulfur, resulting in poor wettability with the electrolyte and poor contact with the contained electrically conductive material. As a result, the carbon material cannot accept electrons from the electrode and cannot participate in the electrochemical reaction.

[0084] The method for preparing the sulfur-carbon composite is not particularly limited in the present invention, and a method commonly used in the art can be used, for example, by simply mixing sulfur and a porous carbon material and then heat-treating the mixture to form a composite.

[0085] In addition to the above, the positive electrode active material may further contain one or more additives selected from a transition metal element, a Group IIIA element, a Group IVA element, a sulfur compound of these elements, an alloy of these elements with sulfur, and the above components.

[0086] Examples of the transition metal elements include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au, and Hg. Examples of the IIIA group elements include Al, Ga, In, and Tl. Examples of the IVA group elements include Ge, Sn, and Pb.

[0087] The sulfur may be contained in an amount of 40 to 95 wt %, preferably 50 to 90 wt %, and more preferably 60 to 85 wt %, based on 100 wt % of the positive electrode active material layer constituting the positive electrode. In one embodiment of the present invention, when a sulfur-carbon composite is used as the positive electrode active material, the sulfur-carbon composite may be contained in an amount of 90 to 97 wt %, based on 100 wt % of the positive electrode active material layer. If the content of the positive electrode active material is less than the above range, it becomes difficult to fully exert the electrochemical reaction of the positive electrode. Conversely, if the content exceeds the above range, the contents of the conductive material and binder described below become relatively insufficient, resulting in increased positive electrode resistance and reduced physical properties of the positive electrode.

[0088] The positive electrode active material layer may optionally further include a conductive material that allows electrons to move smoothly within the positive electrode (specifically, the positive electrode active material) and a binder that reliably attaches the positive electrode active material to the current collector.

[0089] 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 ceramic material can be used without limitation as long as it has electrical conductivity.

[0090] For example, the conductive material may be graphite such as natural graphite or artificial graphite; carbon black such as Super-P, Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; carbon derivatives such as carbon nanotubes and fullerene; conductive fibers such as carbon fibers and metal fibers; metal powders such as fluorocarbon, aluminum, and nickel powder; or electrically conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole, which may be used alone or in combination.

[0091] The conductive material may be contained in an amount of 0.01 to 30 wt % relative to 100 wt % of the total positive electrode active material layer constituting the positive electrode. If the content of the conductive material is below this range, electron transfer between the positive electrode active material and the current collector becomes difficult, resulting in a decrease in voltage and capacity. Conversely, if the content exceeds this range, the proportion of the positive electrode active material decreases relatively, which may result in a decrease in the total energy (charge amount) of the battery. Therefore, it is desirable to determine the content of the conductive material appropriately within this range.

[0092] The binder serves to hold the positive electrode active material on the positive electrode current collector, organically connect the positive electrode active material, and enhance the bonding strength therebetween, and any binder known in the art may be used.

[0093] For example, the binder may include a fluororesin-based binder including polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE); a rubber-based binder including styrene butadiene rubber (SBR), acrylonitrile-butadiene rubber, and styrene-isoprene rubber; a cellulose-based binder including carboxyl methyl cellulose (CMC), starch, hydroxypropyl cellulose, and regenerated cellulose; a polyalcohol-based binder; a polyolefin-based binder including polyethylene and polypropylene; a polyimide-based binder; a polyester-based binder; and a silane-based binder; or a mixture or copolymer of two or more of these.

[0094] The content of the binder may be 0.5 to 30 wt % relative to 100 wt % of the total positive electrode active material layer constituting the positive electrode. If the content of the binder is less than 0.5 wt %, the physical properties of the positive electrode may be reduced, and the positive electrode active material and the conductive material may peel off. If the content exceeds the above range, the ratio of the positive electrode active material to the conductive material in the positive electrode may relatively decrease, resulting in a decrease in battery capacity. Therefore, it is preferable to determine the content of the binder to an appropriate content within the above range.

[0095] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without particular limitations. Specifically, the separator can be a porous polymer film, such as a porous polymer film made of a polyolefin polymer, such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. Furthermore, to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymeric material can also be used.

[0096] The shape of the lithium-sulfur battery is not particularly limited, and various shapes such as a cylindrical type, a laminated type, and a coin type can be adopted.

[0097] The present invention also provides a module including the lithium-sulfur battery as a unit cell. The battery module can be used as a power source for medium- to large-sized devices that require high-temperature stability, long cycle characteristics, and high capacity.

[0098] Examples of the medium- to large-sized devices include, but are not limited to, power tools powered by battery-powered motors; electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric two-wheeled vehicles including electric bicycles (E-bikes) and electric scooters; electric golf carts; and power storage systems.

[0099] Hereinafter, the present invention will be described in detail with reference to examples. However, the examples according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the examples detailed below. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0100] Examples 1 to 7 and Comparative Examples 1 to 8 Electrolyte production An electrolyte containing a solvent, a non-solvent and a lithium salt was prepared so as to meet the SVR factor, CCR factor and MR factor shown in Table 1 below.

[0101] In this case, tetrahydrofuran (THF) or dimethyl ether (DME) was used as the solvent, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) or 1,1,2,2-tetrafluoroethyl isobutyl ether (TFIBE) was used as the non-solvent, and lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) were used as the lithium salt.

[0102] Cathode manufacturing 90 parts by weight of sulfur-carbon composite (S:C = 75:25 weight ratio) as the positive electrode active material (the content of sulfur alone was set to 67.5% by weight of the total weight of the positive electrode, and the carbon material had a pore volume of 1.8 cm 3 A positive electrode slurry composition was prepared by mixing 5 parts by weight of activated carbon (used in an amount of 1.5 mAh / cm), 5 parts by weight of Denka Black as a conductive material, and 5 parts by weight of styrene butadiene rubber / carboxymethyl cellulose (SBR:CMC=7:3 (weight ratio)) as a binder. The positive electrode slurry composition was then coated on a current collector (aluminum foil), dried at 50°C for 12 hours, and pressed with a roll press to prepare a positive electrode (at this time, the loading was 3.5 mAh / cm). 2 and the porosity of the positive electrode was set to 65%.

[0103] Lithium-sulfur battery manufacturing The prepared positive electrode and a 150 μm-thick lithium metal negative electrode were positioned 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 into a circular electrode of 14 phi, the polyethylene separator was punched into 19 phi, and the lithium metal was punched into 16 phi. The battery was also fabricated using a sparing solvating electrolyte (SSE) electrolyte system.

[0104] [Table 1]

[0105] Lifespan assessment To confirm the lifespan characteristics of the lithium-sulfur secondary batteries prepared in Examples 1 to 7 and Comparative Examples 1 to 8, they were charged and discharged at a current density of 0.1 C 2.5 times, and then charged at a current density of 0.2 C and discharged at a current density of 0.5 C 150 times. The lifespan cycle at which the capacity retention rate of the lithium-sulfur secondary battery reached 80% was measured, and the lifespan characteristics of the batteries were confirmed.

Claims

1. a solvent, a non-solvent, and a lithium salt; The solvent and the non-solvent have an SVR factor value of 0.2 or more and 0.7 or less, and the SVR factor is represented by the following formula 1: The lithium salt has a CCR factor value of 0.1 or more and 0.6 or less, and the CCR factor is represented by the following formula 2: An electrolyte for a lithium-sulfur secondary battery, wherein the lithium salt comprises lithium bis(fluorosulfonyl)imide (LiFSI): [Formula 1] SVR factor (Solvent Volume Ratio factor) = volume of solvent ÷ volume of non-solvent [Formula 2] CCR factor (Co-salt Concentration Ratio factor) = Molar concentration of LiFSI ÷ Molar concentration of all lithium salts.

2. 2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the solvent and the non-solvent have an SVR factor value of 0.3 or more and 0.6 or less.

3. 2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the lithium salt has a CCR factor value of 0.15 or more and 0.55 or less.

4. 2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the solvent and the lithium salt have an MR factor value of 1.0 or more and 2.7 or less, and the MR factor is represented by the following formula 3: [Formula 3] MR factor (Molar mass ratio factor) = number of moles of solvent ÷ number of moles of total lithium salts.

5. The solubility of the solvent for the lithium salt is 0.1 M or more; 2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the non-solvent has a solubility for the lithium salt of less than 0.1 M.

6. 6. The electrolyte for a lithium-sulfur secondary battery according to claim 5, wherein the solvent comprises a linear ether, a cyclic ether, or a mixture thereof.

7. The electrolyte for a lithium-sulfur secondary battery according to claim 5 , wherein the non-solvent comprises a fluorinated ether.

8. 2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the lithium salt comprises lithium bis(fluorosulfonyl)imide (LiFSI) and an imide-based lithium salt other than lithium bis(fluorosulfonyl)imide (LiFSI).

9. 2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the lithium salt comprises lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

10. 2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, which does not contain at least one of nitric acid compounds and nitrite compounds.

11. A lithium-sulfur secondary battery comprising a negative electrode, a positive electrode, a separator, and the electrolyte according to any one of claims 1 to 10.

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