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

Abstract

The present invention relates to an electrolyte for a lithium-sulfur secondary battery and a lithium-sulfur secondary battery containing the same. Provided is an electrolyte for a lithium-sulfur battery comprising a lithium salt and a nonaqueous solvent, wherein the nonaqueous solvent comprises an ether-based solvent and a non-solvent, the ether-based solvent comprises a linear ether and a cyclic ether, and the non-solvent comprises a compound having a specific structure.

Description

Technical Field

[0001] This application claims priority based on Korean Patent Application Nos. 10-2020-0143315 and 10-2020-0143326, filed on October 30, 2020, and Korean Patent Application No. 10-2021-0143351, filed on October 26, 2021, the entire contents of which are incorporated herein by reference.

[0002] This invention relates to an electrolyte for lithium-sulfur secondary batteries and a lithium-sulfur secondary battery containing the electrolyte. Background Technology

[0003] As the application of secondary batteries expands to electric vehicles (EVs) and energy storage devices (ESS), lithium-ion secondary batteries, with their relatively low specific energy density (~250 Wh / kg), face limitations in these applications. As an alternative, lithium-sulfur secondary batteries are gaining attention as a next-generation secondary battery technology due to their theoretically high specific energy density (~2,600 Wh / kg).

[0004] Lithium-sulfur secondary batteries refer to battery systems that use sulfur-sulfur bonds (SS bonds) as the positive electrode active material and lithium metal as the negative electrode active material. Sulfur, as the main material for the positive electrode active material, has the advantages of being abundant, non-toxic, and having a low atomic weight.

[0005] In lithium-sulfur secondary batteries, during discharge, lithium, the negative electrode active material, is oxidized and ionized while releasing electrons, while sulfur, the positive electrode active material, is reduced while accepting electrons. In this process, lithium oxidation is the release of electrons from lithium metal, converting it into lithium cations. Conversely, sulfur reduction is the acceptance of two electrons by the S8 bond, converting it into sulfide anions. The lithium cations generated by lithium oxidation are transferred to the positive electrode via the electrolyte and combine with the sulfide anions generated by sulfur reduction to form a salt. Specifically, sulfur before discharge has a cyclic S8 structure, which is converted into lithium polysulfide (LiS) through reduction. x When lithium polysulfides are completely reduced, lithium sulfide (Li₂S) is produced.

[0006] Sulfur, as a positive electrode active material, suffers from low electrical conductivity in the solid state, making it difficult to ensure reactivity with electrons and lithium ions. In existing lithium-sulfur secondary batteries, Li₂S is generated to improve the reactivity of sulfur. x Intermediate polysulfides are used to induce liquid-phase reactions and improve reactivity. In this case, ether solvents with high solubility for lithium polysulfides, such as dioxolane and dimethoxyethane, are used as electrolyte solvents.

[0007] However, when using such ether solvents, the lifespan characteristics of lithium-sulfur secondary batteries deteriorate due to various reasons. For example, the dissolution of lithium polysulfides from the positive electrode, short circuits caused by dendrite growth on the lithium negative electrode, and byproduct deposition due to electrolyte decomposition can all degrade the lifespan characteristics of lithium-sulfur secondary batteries.

[0008] In particular, when using such ether solvents, large quantities of lithium polysulfides can be dissolved, and the reactivity is high. However, due to the soluble nature of lithium polysulfides in the electrolyte, the reactivity and lifetime characteristics of sulfur are affected by the electrolyte content.

[0009] Recently, in order to develop high-energy-density lithium-sulfur secondary batteries with energy densities of over 500 Wh / kg required for aircraft and next-generation electric vehicles, it is necessary to have a large sulfur loading in the electrodes and minimize the electrolyte content.

[0010] However, due to the characteristics of ether solvents, the viscosity increases rapidly during charging and discharging as the electrolyte content decreases, which may lead to overvoltage and potential degradation.

[0011] Therefore, research is ongoing on adding non-solvents as solvents for electrolytes to prevent electrolyte decomposition and ensure excellent lifespan characteristics. However, the components and composition of electrolytes that can prevent electrolyte decomposition and improve its lifespan characteristics have not yet been clearly defined. In particular, the components and composition of electrolytes suitable for applications such as pouch cells with very low electrolyte content are still poorly understood.

[0012] [Existing technical documents]

[0013] [Patent Literature]

[0014] (Patent Document 1) Korean Patent Publication No. 10-2007-0027512 (March 9, 2007), "Electrolyte for Lithium-Sulfur Electrochemical Batteries" Summary of the Invention

[0015] Technical issues

[0016] Therefore, it has been confirmed in this invention that by including ether solvents and non-solvents in the electrolyte for lithium-sulfur secondary batteries containing lithium salts and non-aqueous solvents to prevent the decomposition of the electrolyte for lithium-sulfur secondary batteries and improve its lifespan characteristics, the above-mentioned problems are solved and the performance of lithium-sulfur secondary batteries is improved, thus completing this invention.

[0017] Therefore, one object of the present invention is to provide an electrolyte for lithium-sulfur secondary batteries that prevents electrolyte decomposition and improves its lifespan characteristics. Furthermore, another object of the present invention is to provide a lithium-sulfur secondary battery with improved performance by comprising the above-described electrolyte.

[0018] Technical solution

[0019] To achieve the above objectives, the present invention provides an electrolyte for lithium-sulfur secondary batteries containing lithium salts and non-aqueous solvents, wherein the non-aqueous solvents comprise ether solvents and non-solvents, the ether solvents comprise linear ethers and cyclic ethers, and the non-solvents comprise compounds represented by the following chemical formula 1:

[0020] [Chemical Formula 1]

[0021]

[0022] Rf may be the same or different from each other, and each independently represents an alkyl group having 1 to 3 carbon atoms that has been substituted with fluorine.

[0023] Furthermore, the present invention provides an electrolyte for a lithium-sulfur secondary battery, wherein the lithium salt is at least one selected from the group consisting of: LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 LiB(Ph)4, LiC4BO8, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiSO3CH3, LiSO3CF3, LiSCN, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(SO2F)2, lithium chloroborane, lower aliphatic carboxylic acids, lithium tetraphenylborate, and lithium imide.

[0024] Furthermore, the present invention provides an electrolyte for lithium-sulfur secondary batteries, wherein the linear ether is selected from the group consisting of 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, butanediol ether, diethylene glycol 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.

[0025] Furthermore, the present invention provides an electrolyte for lithium-sulfur secondary batteries, wherein the cyclic ether is selected from dioxolane, methyldioxolane, dimethyldioxolane, vinyldioxolane, methoxydioxolane, ethylmethyldioxolane, etc. Alkane, di Alkane, trialkyl The group consisting of alkanes, tetrahydrofuran, methyltetrahydrofuran, dimethyltetrahydrofuran, dimethoxytetrahydrofuran, ethoxytetrahydrofuran, dihydropyran, tetrahydropyran, furan, and 2-methylfuran.

[0026] Furthermore, the present invention provides an electrolyte for lithium-sulfur secondary batteries, wherein the fluorinated alkyl group of chemical formula 1 is a fluorinated C1 to C3 alkyl group substituted with 1 to 7 fluorine atoms.

[0027] Furthermore, the present invention provides an electrolyte for lithium-sulfur secondary batteries, wherein the fluorinated alkyl group of chemical formula 1 is a fluorinated C1 to C2 alkyl group substituted with 1 to 5 fluorine atoms.

[0028] Furthermore, the present invention provides an electrolyte for lithium-sulfur secondary batteries, wherein the fluorinated alkyl group of chemical formula 1 contains difluoromethyl or trifluoromethyl at the end.

[0029] Furthermore, the present invention provides an electrolyte for lithium-sulfur secondary batteries, wherein the content of the non-solvent is 5% to 20% by volume relative to the total volume of the non-aqueous solvent.

[0030] Furthermore, the present invention provides an electrolyte for lithium-sulfur secondary batteries, wherein the content of the linear ether is 50% to 90% by volume relative to the total volume of the non-aqueous solvent, and the content of the cyclic ether is 10% to 30% by volume relative to the total volume of the non-aqueous solvent.

[0031] Furthermore, the present invention provides an electrolyte for lithium-sulfur secondary batteries, wherein the volume ratio of the ether solvent to the non-solvent is 95:5 to 80:20.

[0032] Furthermore, the present invention provides an electrolyte for lithium-sulfur secondary batteries, wherein the volume ratio of the linear ether to the cyclic ether is 9:1 to 5:5.

[0033] Furthermore, the present invention provides an electrolyte for lithium-sulfur secondary batteries, wherein the linear ether is dimethoxyethane.

[0034] Furthermore, the present invention provides an electrolyte for lithium-sulfur secondary batteries, wherein the cyclic ether is 2-methylfuran.

[0035] Furthermore, the present invention provides an electrolyte for lithium-sulfur secondary batteries, wherein the compound represented by chemical formula 1 is tris(2,2-difluoroethyl) orthoformate or tris(2,2,2-trifluoroethyl) orthoformate.

[0036] In addition, the present invention provides a lithium-sulfur secondary battery, the secondary battery comprising the above-mentioned electrolyte, positive electrode, negative electrode and separator.

[0037] Beneficial effects

[0038] The electrolyte for lithium-sulfur secondary batteries according to the present invention comprises lithium salts and non-aqueous solvents, wherein the non-aqueous solvents comprise ether solvents and non-solvents, wherein the ether solvents comprise linear ethers and cyclic ethers, and wherein the non-solvents comprise compounds having specific structures, thereby having the effect of preventing electrolyte decomposition and improving lifespan characteristics during operation of the lithium-sulfur secondary battery. Detailed Implementation

[0039] All embodiments provided by the present invention can be implemented through the following description. It should be understood that the following description describes preferred embodiments of the present invention, and it should be understood that the present invention is not necessarily limited thereto.

[0040] This invention provides an electrolyte for lithium-sulfur secondary batteries comprising a lithium salt and a non-aqueous solvent, wherein the non-aqueous solvent comprises an ether solvent and a non-solvent, the ether solvent comprising linear ethers and cyclic ethers, and the non-solvent comprising a compound represented by the following chemical formula 1:

[0041] [Chemical Formula 1]

[0042]

[0043] Rf may be the same or different from each other, and each independently represents a fluorinated C1 to C3 alkyl group.

[0044] The electrolyte for lithium-sulfur secondary batteries of the present invention comprises a lithium salt and a non-aqueous solvent, wherein the lithium salt can be a material that is readily soluble in a non-aqueous organic solvent, and is optionally selected from LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB. 10 Cl 10 The group consisting of LiB(Ph)4, LiC4BO8, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiSO3CH3, LiSO3CF3, LiSCN, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(SO2F)2, lithium chloroborane, lower aliphatic carboxylic acids, lithium tetraphenylborate, and lithium imino, with LiN(CF3SO2)2 being preferred.

[0045] The concentration of the lithium salt can be from 0.2 to 2 M, preferably from 0.5 to 1.8 M, and more preferably from 0.6 to 1.7 M. This concentration depends on various factors, such as the exact composition of the electrolyte mixture, the solubility of the salt, the conductivity of the dissolved salt, the charging and discharging conditions of the battery, the operating temperature, and other factors known in the field of lithium batteries. If the concentration of the lithium salt is below 0.2 M, the conductivity of the electrolyte may decrease, potentially degrading the performance of the electrolyte. If the concentration of the lithium salt exceeds 2 M, the viscosity of the electrolyte may increase, potentially reducing the lithium-ion concentration. + ) migration rate.

[0046] The non-aqueous solvents include ether solvents and non-solvents, and the ether solvents include linear ethers and cyclic ethers.

[0047] The linear ether may be selected from the group consisting of 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, butanediol ether, diethylene glycol 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, and preferably may be dimethoxyethane.

[0048] The cyclic ether may be selected from dioxolane, methyldioxolane, dimethyldioxolane, vinyldioxolane, methoxydioxolane, ethylmethyldioxolane, etc. Alkane, di Alkane, trialkyl It is the group consisting of alkane, tetrahydrofuran, methyltetrahydrofuran, dimethyltetrahydrofuran, dimethoxytetrahydrofuran, ethoxytetrahydrofuran, dihydropyran, tetrahydropyran, furan and 2-methylfuran, and preferably 2-methylfuran.

[0049] Furthermore, the non-solvent comprises a compound represented by the following chemical formula 1:

[0050] [Chemical Formula 1]

[0051]

[0052] Wherein Rf may be the same or different from each other, and each independently represents a fluorinated C1 to C3 fluorinated alkyl group.

[0053] The compound represented by Formula 1 has a high flash point above 150°C and a low viscosity below 5 cP, and does not solubilize lithium ions. Specifically, the compound represented by Formula 1 exhibits the effect of reducing flammability and viscosity. This prevents electrolyte degradation and improves lithium ion mobility, ensuring high stability even during long-term battery operation.

[0054] The fluorinated alkyl group of Formula 1 may be a fluorinated C1 to C3 alkyl group substituted with 1 to 7 fluorine atoms, preferably a fluorinated C1 to C2 alkyl group substituted with 1 to 5 fluorine atoms, and more preferably a fluorinated alkyl group containing difluoromethyl or trifluoromethyl at the end of the fluorinated alkyl group.

[0055] Furthermore, the compound represented by Formula 1 can be a fluorine-substituted trialkyl orthoformate compound, and the three alkyl groups in the trialkyl orthoformate compound can each be an alkyl group having 1 to 3 carbon atoms, preferably an alkyl group having 1 to 2 carbon atoms. Additionally, the alkyl groups can each be a fluorinated alkyl group substituted with 1 to 7 fluorine atoms, preferably a fluorinated alkyl group substituted with 1 to 5 fluorine atoms, more preferably an alkyl group terminally substituted with difluoromethyl or trifluoromethyl. Specifically, the compound represented by Formula 1 can be tris(2,2-difluoroethyl) orthoformate or tris(2,2,2-trifluoroethyl) orthoformate.

[0056] Furthermore, the electrolyte for lithium-sulfur secondary batteries of the present invention contains ether solvents and non-solvents as non-aqueous solvents, thereby exhibiting the effect of preventing electrolyte decomposition and improving the lifespan characteristics of the lithium-sulfur secondary battery. In particular, the electrolyte contains linear ethers and cyclic ethers as ether solvents, and a compound represented by Chemical Formula 1 as a non-solvent, thereby exhibiting the effect of significantly improving the lifespan characteristics of the battery.

[0057] In this case, the content of the non-solvent relative to the total volume of the non-aqueous solvent can be from 5 vol% to 20 vol%, preferably from 5 vol% to 10 vol%. Furthermore, the volume ratio of the ether solvent to the non-solvent can be from 99:1 to 50:50, preferably from 95:5 to 70:30, and more preferably from 95:5 to 80:20.

[0058] If the content of the non-solvent and the volume ratio of the ether solvent to the non-solvent are less than the above range, the effect of improving lifetime characteristics is insufficient. If the content of the non-solvent and the volume ratio of the ether solvent to the non-solvent exceed the above range, there may be a problem of being unable to discharge at a high current density. Therefore, it is preferable that the content of the non-solvent and the volume ratio of the ether solvent to the non-solvent satisfy the above range.

[0059] Furthermore, relative to the total volume of the non-aqueous solvent, the content of the linear ether can be from 50 vol% to 90 vol%, preferably 60 vol% to 80 vol%, and more preferably 60 vol% to 75 vol%. Furthermore, relative to the total volume of the non-aqueous solvent, the content of the cyclic ether can be from 10 vol% to 30 vol%, preferably 15 vol% to 25 vol%.

[0060] The volume ratio of the linear ether to the cyclic ether can be 9:1 to 1:9, preferably 8:2 to 2:8, and more preferably 7:3 to 3:7.

[0061] Furthermore, the volume ratio of the linear ether to the non-solvent can be 20:1 to 1:1, preferably 15:1 to 5:1, and more preferably 15:1 to 3:1.

[0062] Furthermore, the volume ratio of the cyclic ether to the non-solvent can be 1:10 to 10:1, preferably 1:5 to 5:1, and more preferably 1:4 to 1:1.

[0063] If the content of the linear ether, the content of the cyclic ether, the volume ratio of the linear ether to the cyclic ether, the volume ratio of the linear ether to the non-solvent, and the volume ratio of the cyclic ether to the non-solvent exceed the above ranges, the effect of improving battery life characteristics may be insufficient, and the expected effect may not be achieved. Therefore, it is preferable that the content of the linear ether, the content of the cyclic ether, the volume ratio of the linear ether to the cyclic ether, the volume ratio of the linear ether to the non-solvent, and the volume ratio of the cyclic ether to the non-solvent satisfy the above ranges.

[0064] The electrolyte for lithium-sulfur secondary batteries of the present invention may further contain nitric acid or nitrite compounds as additives. The nitric acid or nitrite compounds have the effect of forming a stable film on the lithium electrode and improving charging / discharging efficiency. The nitric acid or nitrite compounds may be, but are not limited to, at least one selected from the group consisting of: inorganic nitric acid or nitrite compounds, such as lithium nitrate (LiNO3), potassium nitrate (KNO3), cesium nitrate (CsNO3), barium nitrate (Ba(NO3)2), ammonium nitrate (NH4NO3), lithium nitrite (LiNO2), potassium nitrite (KNO2), cesium nitrite (CsNO2), and ammonium nitrite (NH4NO2); organic nitric acid or nitrite compounds, such as methyl nitrate and dialkylimidazolium nitrate. guanidine nitrate, imidazole nitrate Pyridine Nitrate Ethyl nitrite, propyl nitrite, butyl nitrite, amyl nitrite, and octyl nitrite; organic nitro compounds, such as nitromethane, nitrobenzene, nitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, and dinitrotoluene, and combinations thereof. Lithium nitrate (LiNO3) is preferred.

[0065] In addition, the electrolyte may contain other additives to improve charging / discharging characteristics, flame retardancy, etc. Examples of such additives may include pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol dimethyl ether, hexamethylphosphoryltriamine, nitrobenzene derivatives, sulfur, quinone imine dyes, and N-substituted compounds. Zyzolidinediones, N,N-substituted imidazolidinedions, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, aluminum trichloride, fluoroethylene carbonate (FEC), propylene sulpholactone (PRS), vinylene carbonate (VC), etc.

[0066] The method for preparing the electrolyte for lithium-sulfur secondary batteries of the present invention is not particularly limited in the present invention, and the electrolyte can be prepared by conventional methods known in the art.

[0067] Furthermore, the present invention provides a lithium-sulfur secondary battery comprising an electrolyte for lithium-sulfur secondary batteries.

[0068] The lithium-sulfur secondary battery may include a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte, wherein the electrolyte may include the electrolyte for lithium-sulfur secondary batteries according to the present invention.

[0069] The positive electrode may include a positive electrode current collector and a layer of positive electrode active material coated on one or both sides of the positive electrode current collector.

[0070] The positive electrode current collector supports the positive electrode active material layer and is not particularly limited, as long as it has high conductivity and does not cause chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, sintered carbon; copper or stainless steel with surface treatments of carbon, nickel, silver, etc.; aluminum-cadmium alloys, etc., can be used as the positive electrode current collector.

[0071] The positive current collector can enhance its bonding strength with the positive active material layer by having fine irregularities on its surface, and can be formed in various forms such as film, sheet, foil, mesh, net, porous body, foam or nonwoven fabric.

[0072] The positive electrode active material layer may include a positive electrode active material, as well as optional conductive materials and adhesives.

[0073] The positive electrode active material can be at least one selected from the group consisting of: elemental sulfur (S8); Li2S n (n≥1), organic sulfur compounds and carbon-sulfur polymers (C2S) x ) n (x = 2.5–50, n ≥ 2). Inorganic sulfur (S8) is preferred.

[0074] In addition to the positive electrode active material, the positive electrode may also contain at least one additive selected from the following: transition metal elements, group IIIA elements, group IVA elements, sulfur compounds of these elements, and alloys of these elements with sulfur.

[0075] 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 group IIIA elements may include Al, Ga, In, Ti, etc.; and the group IVA elements may include Ge, Sn, Pb, etc.

[0076] The conductive material is intended to improve conductivity and is not particularly limited, as long as it is a conductive material that will not cause chemical changes in the lithium secondary battery. Commonly used materials include carbon black, graphite, carbon fiber, carbon nanotubes, metal powders, conductive metal oxides, and organic conductive materials. Currently marketed products as conductive materials include acetylene black series (products from Chevron Chemical Company or Gulf Oil Company), Ketjen Black EC series (products from Armak), Vulcan XC-72 (products from Cabot Company), and Super P (products from MMM). For example, acetylene black, carbon black, and graphite can be used.

[0077] Furthermore, the positive electrode active material layer may also include an adhesive, which functions to hold the positive electrode active material on the positive electrode current collector and to connect the active materials. As the adhesive, various types of adhesives can be used, such as polyvinylidene fluoride-hexafluoropropylene (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), etc.

[0078] As the positive electrode, a positive electrode with a high sulfur loading can be used. The sulfur loading can be 3.0 mAh / cm³. 2 Above, preferably 4.0mAh / cm 2 Above, or more preferably 5.0mAh / cm 2 above.

[0079] The negative electrode may include a negative electrode current collector and a negative electrode active material located on the negative electrode current collector. Alternatively, the negative electrode may be a lithium metal plate.

[0080] The negative electrode current collector is used to support the negative electrode active material, and there are no particular limitations, as long as it has excellent conductivity and is electrochemically stable within the voltage range of the lithium secondary battery. For example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon; copper or stainless steel with surface treatments of carbon, nickel, titanium, silver, etc.; aluminum-cadmium alloys, etc., can be used as negative electrode current collectors.

[0081] The negative electrode current collector can enhance its bonding force with the negative electrode active material by having fine irregularities on its surface, and can be formed in various forms such as film, sheet, foil, mesh, net, porous body, foam or nonwoven fabric.

[0082] The negative electrode active material may contain lithium ions that can be reversibly intercalated or deintercalated (Li ions). + Materials capable of reversibly forming lithium-containing compounds through reaction with lithium ions, or lithium metal or lithium alloys. The reversible insertion and extraction of lithium ions (Li...) + The material can be, for example, crystalline carbon, amorphous carbon, or a mixture thereof. The material capable of reacting with lithium ions (Li...) + The material that reversibly reacts to form a lithium-containing compound can be, for example, tin oxide, titanium nitrate, or silicon. The lithium alloy can be, for example, an alloy of lithium (Li) with a metal selected from the group consisting of: 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 film or lithium metal powder.

[0083] There are no particular limitations on the method for forming the negative electrode active material; methods commonly used in the art for forming layers or films can be used. For example, methods such as compression, coating, and deposition can be used. Furthermore, the negative electrode of the present invention also includes cases where, after assembling the battery without a lithium film in the current collector, a lithium metal film is formed on a metal plate through initial charging.

[0084] The separator is used to physically separate the positive and negative electrodes in the lithium-sulfur secondary battery of the present invention, and can be used without any particular limitation, as long as it is generally used as a separator in a lithium-sulfur secondary battery. In particular, the separator is preferred when it has low resistance to ion migration in the electrolyte and excellent electrolyte impregnation ability.

[0085] The diaphragm can be made of a porous substrate, and the porous substrate can be any porous substrate conventionally used in electrochemical devices. For example, polyolefin porous membranes or nonwoven fabrics can be used as the porous substrate, but the porous substrate is not particularly limited thereto.

[0086] Examples of the polyolefin porous membrane may include membranes formed from polyolefin polymers such as polyethylene (e.g., high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene), polypropylene, polybutene, and polypentene, alone or in mixtures thereof.

[0087] In addition to the polyolefin-based nonwoven fabrics mentioned above, the nonwoven fabrics may also comprise, for example, nonwoven fabrics formed from polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene ether, polyphenylene sulfide, polyethylene naphthalate alone or mixtures thereof. The structure of the nonwoven fabric may be a spunbond nonwoven fabric or a meltblown nonwoven fabric composed of long fibers.

[0088] There is no particular limitation on the thickness of the porous substrate, but it can be from 1 to 100 μm, preferably from 5 to 50 μm.

[0089] There are no particular limitations on the size and porosity of the pores present in the porous substrate, but they can be from 0.001 μm to 50 μm and from 10% to 95%, respectively.

[0090] The electrolyte contains lithium ions, which are used to induce electrochemical oxidation or reduction reactions at the positive and negative electrodes, as described above.

[0091] Electrolyte injection can be performed at an appropriate stage during the manufacturing process of the electrochemical device, depending on the manufacturing process of the final product and the required physical properties. That is, it can be performed before the assembly of the electrochemical device or in the final stage of the assembly of the electrochemical device.

[0092] In addition to the conventional winding process, the lithium-sulfur secondary battery according to the present invention can also be manufactured by laminating, stacking and folding the separator and the electrode.

[0093] There are no particular limitations on the shape of the lithium-sulfur secondary battery; it can be various shapes such as cylindrical, laminated, and coin-shaped.

[0094] Methods of implementing the invention

[0095] The following preferred embodiments are provided to help understand the present invention, but the following embodiments are provided only to make the present invention easier to understand, and the present invention is not limited thereto.

[0096] Example

[0097] Preparation of electrolyte for lithium-sulfur secondary batteries

[0098] Example 1-1

[0099] An electrolyte for lithium-sulfur secondary batteries was prepared by dissolving 0.75 M (mol / L) lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 5 wt% lithium nitrate (LiNO3) in a mixed solvent obtained by mixing 75 vol% dimethoxyethane (DME), 20 vol% 2-methylfuran (2-MeF), and 5 vol% tri(2,2,2-trifluoroethyl) orthoformate (TFEO) relative to the total volume of the non-aqueous solvent.

[0100] Examples 1-2

[0101] The electrolyte for lithium-sulfur secondary batteries was prepared in the same manner as in Examples 1-1, except that the amount of dimethoxyethane (DME) was 70% by volume and the amount of tris(2,2,2-trifluoroethyl) orthoformate (TFEO) was 10% by volume.

[0102] Examples 1-3

[0103] The electrolyte for lithium-sulfur secondary batteries was prepared in the same manner as in Examples 1-1, except that the amount of dimethoxyethane (DME) was 60% by volume and the amount of tris(2,2,2-trifluoroethyl) orthoformate (TFEO) was 20% by volume.

[0104] Example 2-1

[0105] The electrolyte for lithium-sulfur secondary batteries was prepared in the same manner as in Examples 1-1, except that tris(2,2-difluoroethyl) orthoformate (TDOF) was used instead of tris(2,2,2-trifluoroethyl) orthoformate (TFEO).

[0106] Example 2-2

[0107] The electrolyte for lithium-sulfur secondary batteries was prepared in the same manner as in Example 2-1, except that the amount of dimethoxyethane (DME) was 70% by volume and the amount of tris(2,2-difluoroethyl) orthoformate (TDOF) was 10% by volume.

[0108] Example 2-3

[0109] The electrolyte for lithium-sulfur secondary batteries was prepared in the same manner as in Example 2-1, except that the amount of dimethoxyethane (DME) was 60% by volume and the amount of tris(2,2-difluoroethyl) orthoformate (TDOF) was 20% by volume.

[0110] Comparative Example 1-1

[0111] The electrolyte for lithium-sulfur secondary batteries was prepared in the same manner as in Examples 1-1, except that the amount of dimethoxyethane (DME) was 77% by volume and the amount of tris(2,2,2-trifluoroethyl) orthoformate (TFEO) was 3% by volume.

[0112] Comparative Examples 1-2

[0113] The electrolyte for lithium-sulfur secondary batteries was prepared in the same manner as in Examples 1-1, except that the amount of dimethoxyethane (DME) was 58% by volume and the amount of tris(2,2,2-trifluoroethyl) orthoformate (TFEO) was 22% by volume.

[0114] Comparative Examples 1-3

[0115] The electrolyte for lithium-sulfur secondary batteries was prepared in the same manner as in Examples 1-1, except that the amount of dimethoxyethane (DME) was 80% by volume and tris(2,2,2-trifluoroethyl) orthoformate (TFEO) was not used.

[0116] Comparative Examples 1-4

[0117] The electrolyte for lithium-sulfur secondary batteries was prepared in the same manner as in Examples 1-1, except that the amount of dimethoxyethane (DME) was 90% by volume, the amount of tri(2,2,2-trifluoroethyl) orthoformate (TFEO) was 10% by volume, and 2-methylfuran (2-MeF) was not used.

[0118] Comparative Examples 1-5

[0119] The electrolyte for lithium-sulfur secondary batteries was prepared in the same manner as in Examples 1-1, except that the amount of 2-methylfuran (2-MeF) was 90% by volume, the amount of tris(2,2,2-trifluoroethyl) orthoformate (TFEO) was 10% by volume, and dimethoxyethane (DME) was not used.

[0120] Comparative Example 2-1

[0121] The electrolyte for lithium-sulfur secondary batteries was prepared in the same manner as in Example 2-1, except that the amount of dimethoxyethane (DME) was 77% by volume and the amount of tris(2,2-difluoroethyl) orthoformate (TDOF) was 3% by volume.

[0122] Comparative Example 2-2

[0123] The electrolyte for lithium-sulfur secondary batteries was prepared in the same manner as in Example 2-1, except that the amount of dimethoxyethane (DME) was 58% by volume and the amount of tris(2,2-difluoroethyl) orthoformate (TDOF) was 22% by volume.

[0124] Comparative Examples 2-3

[0125] The electrolyte for lithium-sulfur secondary batteries was prepared in the same manner as in Example 2-1, except that the amount of dimethoxyethane (DME) was 80% by volume and tris(2,2-difluoroethyl) orthoformate (TDOF) was not used.

[0126] Comparative Examples 2-4

[0127] The electrolyte for lithium-sulfur secondary batteries was prepared in the same manner as in Example 2-1, except that the amount of dimethoxyethane (DME) was 90% by volume, the amount of tris(2,2-difluoroethyl) orthoformate (TDOF) was 10% by volume, and 2-methylfuran (2-MeF) was not used.

[0128] Comparative Examples 2-5

[0129] The electrolyte for lithium-sulfur secondary batteries was prepared in the same manner as in Example 2-1, except that the amount of 2-methylfuran (2-MeF) was 90% by volume, the amount of tris(2,2-difluoroethyl) orthoformate (TDOF) was 10% by volume, and dimethoxyethane (DME) was not used.

[0130] The contents of linear ethers, cyclic ethers, and compounds represented by chemical formula 1 in the electrolytes for lithium-sulfur secondary batteries prepared in Examples 1-1 to 1-3, Examples 2-1 to 2-3, Comparative Examples 1-1 to 1-5, and Comparative Examples 2-1 to 2-5 are shown in Table 1 below:

[0131] Table 1:

[0132]

[0133] *DME: Dimethoxyethane

[0134] **2-MeF: 2-Methylfuran**

[0135] ***TFEO: Tris(2,2,2-trifluoroethyl) orthoformate

[0136] ****TDOF: Tris(2,2-difluoroethyl) orthoformate

[0137] Experimental Example

[0138] Evaluation of the lifespan characteristics of lithium-sulfur secondary batteries

[0139] Sulfur was mixed with conductive materials and binders in acetonitrile using a ball mill to prepare a slurry for the positive electrode active material layer. Carbon black was used as the conductive material, and a mixture of SBR and CMC binders was used as the binder, with a mixing ratio of sulfur:conductive material:binder of 72:24:4 by weight. The slurry for the positive electrode active material layer was then coated onto an aluminum current collector to achieve a loading of 5.0 mAh / cm². 2 The material is then dried to produce a positive electrode with a porosity of 68%. Additionally, a 45 μm thick layer of lithium metal is used as the negative electrode.

[0140] The positive and negative electrodes prepared by the above method are positioned facing each other, and then a polyethylene diaphragm with a thickness of 20 μm and a porosity of 45% is inserted between the positive and negative electrodes.

[0141] Subsequently, the electrolytes according to Examples 1-1 to 1-3, Examples 2-1 to 2-3, Comparative Examples 1-1 to 1-5, and Comparative Examples 2-1 to 2-5 were injected into the casing to manufacture lithium-sulfur secondary batteries.

[0142] The lithium-sulfur secondary battery prepared by the above method was repeatedly discharged and charged 2.5 times at a current density of 0.1C, and then repeatedly discharged and charged 3 times at a current density of 0.2C. Subsequently, the battery's lifespan characteristics were confirmed by measuring the cycle life at which the capacity retention rate of the lithium-sulfur secondary battery reached 80% during 300 cycles at a current density of 0.5C. The results obtained are shown in Table 2.

[0143] Table 2:

[0144] Example 1-1 228 Examples 1-2 235 Examples 1-3 219 Example 2-1 258 Example 2-2 284 Example 2-3 235 Comparative Example 1-1 198 Comparative Examples 1-2 177 Comparative Examples 1-3 194 Comparative Examples 1-4 132 Comparative Examples 1-5 - Comparative Example 2-1 201 Comparative Example 2-2 189 Comparative Examples 2-3 194 Comparative Examples 2-4 158 Comparative Examples 2-5 -

[0145] However, the electrolytes of Comparative Examples 1-5 and 2-5 do not contain linear ethers and cannot adequately dissolve lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium nitrate (LiNO3), making it difficult to operate the battery and thus impossible to measure its life characteristics.

[0146] As shown in Table 2 above, when lithium-sulfur secondary batteries using the electrolytes according to Examples 1-1 to 1-3 and Examples 2-1 to 2-3 are used, the battery life characteristics are confirmed to be excellent compared to lithium-sulfur secondary batteries using the electrolytes according to Comparative Examples 1-1 to 1-2 and Comparative Examples 2-1 to 2-2.

[0147] Specifically, the lithium-sulfur secondary battery according to the present invention uses an electrolyte for lithium-sulfur secondary batteries according to Examples 1-1 to 1-3 and Examples 2-1 to 2-3, wherein the electrolyte has a content of 5 vol% to 20 vol% of a compound represented by Chemical Formula 1 (tris(2,2,2-trifluoroethyl) orthoformate or tris(2,2-difluoroethyl) orthoformate) in a non-aqueous solvent, thereby forming a stable SEI (solid-electrolyte interphase) film on the lithium anode to suppress the reaction between lithium and polysulfides and the decomposition of the electrolyte, thereby exhibiting the effect of improving the battery life characteristics.

[0148] It was also confirmed that, when using the electrolytes according to Examples 1-1 to 1-3 and Examples 2-1 to 2-3, the lithium-sulfur secondary batteries exhibited superior lifespan characteristics compared to lithium-sulfur secondary batteries using the electrolytes according to Comparative Examples 1-3 to 1-5 and Comparative Examples 2-3 to 2-5.

[0149] Specifically, it was confirmed that the lithium-sulfur secondary battery according to the present invention uses the electrolyte for lithium-sulfur secondary batteries according to Examples 1-1 to 1-3 and Examples 2-1 to 2-3, the electrolyte containing each of linear ethers, cyclic ethers and compounds represented by Chemical Formula 1 as non-aqueous solvents, and therefore has excellent battery life characteristics compared with Comparative Examples 1-3 and 2-3 which do not contain the compounds represented by Chemical Formula 1, Comparative Examples 1-4 and 2-4 which do not contain the cyclic ethers, and Comparative Examples 1-5 and 2-5 which do not contain the linear ethers.

[0150] All simple modifications and variations of this invention fall within the scope of this invention, and the specific scope of protection of this invention will become apparent from the appended claims.

Claims

1. An electrolyte for lithium-sulfur secondary batteries, comprising a lithium salt and a non-aqueous solvent, The non-aqueous solvents include ether solvents and non-solvents. The ether solvents include linear ethers and cyclic ethers, and The non-solvent comprises a compound represented by the following chemical formula 1: [Chemical Formula 1] Wherein Rf may be the same or different from each other, and each independently represents a fluorinated C1 to C3 alkyl group, and The content of the non-aqueous solvent is from 5% to 20% by volume relative to the total volume of the non-aqueous solvent.

2. The electrolyte for lithium-sulfur secondary batteries according to claim 1, wherein the lithium salt is at least one selected from the group consisting of LiCI, LiBr, Lil, LiCI04, LiBF4, LiB 10 Cl 10 , LiB(Ph)4, LiC4BO8, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCI4, LiSO3CH3, LiSO3CF3, LiSCN, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(SO2F)2, chloroborane lithium, lithium lower aliphatic carboxylate, lithium tetraphenylborate, and lithium imide.

3. The electrolyte for lithium-sulfur secondary batteries according to claim 1, wherein the linear ether is selected from the group consisting of 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, butanediol ether, diethylene glycol 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.

4. The electrolyte for lithium-sulfur secondary batteries according to claim 1, wherein the cyclic ether is selected from dioxolane, methyldioxolane, dimethyldioxolane, vinyldioxolane, methoxydioxolane, ethylmethyldioxolane, ... Alkane, di Alkane, trialkyl The group consisting of alkanes, tetrahydrofuran, methyltetrahydrofuran, dimethyltetrahydrofuran, dimethoxytetrahydrofuran, ethoxytetrahydrofuran, dihydropyran, tetrahydropyran, furan, and 2-methylfuran.

5. The electrolyte for lithium-sulfur secondary batteries according to claim 1, wherein the fluorinated alkyl group of chemical formula 1 is a fluorinated C1 to C3 alkyl group substituted with 1 to 7 fluorine atoms.

6. The electrolyte for lithium-sulfur secondary batteries according to claim 1, wherein the fluorinated alkyl group of chemical formula 1 is a fluorinated C1 to C2 alkyl group substituted with 1 to 5 fluorine atoms.

7. The electrolyte for lithium-sulfur secondary batteries according to claim 1, wherein the fluorinated alkyl group of chemical formula 1 contains difluoromethyl or trifluoromethyl at the end.

8. The electrolyte for lithium-sulfur secondary batteries according to claim 1, wherein the content of the linear ether is 50% to 90% by volume relative to the total volume of the non-aqueous solvent, and the content of the cyclic ether is 10% to 30% by volume relative to the total volume of the non-aqueous solvent.

9. The electrolyte for lithium-sulfur secondary batteries according to claim 1, wherein the volume ratio of the ether solvent to the non-solvent is 95:5 to 80:

20.

10. The electrolyte for lithium-sulfur secondary batteries according to claim 1, wherein the volume ratio of the linear ether to the cyclic ether is 9:1 to 5:

5.

11. The electrolyte for lithium-sulfur secondary batteries according to claim 1, wherein the linear ether is dimethoxyethane.

12. The electrolyte for lithium-sulfur secondary batteries according to claim 1, wherein the cyclic ether is 2-methylfuran.

13. The electrolyte for lithium-sulfur secondary batteries according to claim 1, wherein the compound represented by chemical formula 1 is tris(2,2-difluoroethyl) orthoformate or tris(2,2,2-trifluoroethyl) orthoformate.

14. A lithium-sulfur secondary battery comprising the electrolyte according to any one of claims 1 to 13; a positive electrode; a negative electrode; and a separator.

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