Electrolyte for lithium-surfur secondary battery and a lithium-surfur 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. By controlling the solvent, non-solvent, and lithium salt included in the electrolyte under specific conditions, a lithium-sulfur secondary battery with improved lifespan characteristics can be provided.

Description

Technology Field

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

[0002] This application is a priority claim application for Korean Patent Application No. 10-2022-0183762 filed on December 23, 2022, and all contents disclosed in the specification of said application are incorporated into this application by reference. Background Technology

[0004] Lithium-sulfur (Li-S) batteries utilizing the existing catholyte system, Li2S X There is a problem in that the high theoretical discharge capacity (1675 mAh / g) of sulfur is not fully utilized due to reliance on a catholyte type reaction through the generation of polysulfide, an intermediate product of the form, and the battery life characteristics are degraded due to battery degradation caused by polysulfide leaching. Although LiNO3 additives are applied to solve this problem, there are still difficulties in achieving long-life behavior of lithium-sulfur batteries due to the problem of additive depletion.

[0005] Meanwhile, a sparingly solvating electrolyte (SSE) system that suppresses polysulfide leaching has recently been proposed, and it has been confirmed that normal operation is possible without charging delay even in an electrolyte without a specific additive. However, it still exhibits a low lifespan, so there is a need to improve the lifespan characteristics.

[0006] Therefore, to achieve high energy densities of 400 Wh / kg and 600 Wh / L or higher, 4.0 mAh / cm² 2 There is a need for an electrolyte system for lithium-sulfur secondary batteries that can operate even with a porosity of 60 vol% or less and can improve lifespan characteristics. The problem to be solved

[0008] Accordingly, the problem that the present invention aims to solve is to resolve the aforementioned issues and to provide an electrolyte for a lithium-sulfur secondary battery having high energy density and simultaneously improved lifespan characteristics, and a lithium-sulfur secondary battery containing the same.

[0009] Furthermore, it will be readily apparent that other objects and advantages of the present invention can be realized by means or methods described in the claims and combinations thereof. means of solving the problem

[0011] The inventors have discovered that the above problem can be solved through the following electrolyte for a lithium-sulfur secondary battery and a lithium-sulfur secondary battery containing the same.

[0012] According to the first embodiment,

[0013] It comprises a solvent, a non-solvent, and at least two types of lithium salts, and

[0014] The above lithium salt includes LiI, and

[0015] The above solvent and non-solvent have an SVR factor value of 0.2 to 0.7, and the SVR factor is represented by the following Formula 1, and

[0016] The above-mentioned solvent and lithium salt have an MR factor value of 0.75 to 1.85, and the above-mentioned MR factor value is represented by the following formula 2. This relates to an electrolyte for a lithium-sulfur secondary battery.

[0017] [Equation 1]

[0018] SVR factor (Solvent Volume Ratio factor) = Volume of solvent ÷ Volume of non-solvent

[0019] [Equation 2]

[0020] MR factor (Molar mass Ratio factor) = Moles of solvent ÷ (Moles of lithium salt excluding LiI)

[0021] According to the second embodiment, in the first embodiment,

[0022] The present invention relates to an electrolyte for a lithium-sulfur secondary battery characterized by having an SVR factor value of 0.3 to 0.65.

[0023] According to the third embodiment, in the first embodiment or the second embodiment,

[0024] The present invention relates to an electrolyte for a lithium-sulfur secondary battery characterized by having an MR factor value of 1.0 to 1.75.

[0025] According to the fourth embodiment, in any one of the first to third embodiments,

[0026] The above solvent has a solubility for lithium salt of 0.1M or higher, and

[0027] The present invention relates to an electrolyte for a lithium-sulfur secondary battery characterized in that the above-mentioned nonsolvent has a solubility of less than 0.1M for lithium salt.

[0028] According to the fifth embodiment, in any one of the first to fourth embodiments,

[0029] The present invention relates to an electrolyte for a lithium-sulfur secondary battery characterized in that the solvent comprises a linear ether, a cyclic ether, or a mixture thereof.

[0030] According to the 6th embodiment, in any one of the 1st to 5th embodiments,

[0031] The present invention relates to an electrolyte for a lithium-sulfur secondary battery characterized in that the above solvent comprises a mixture of linear ether and cyclic ether.

[0032] According to the seventh embodiment, in any one of the first to sixth embodiments,

[0033] The present invention relates to an electrolyte for a lithium-sulfur secondary battery characterized in that the above-mentioned nonsolvent comprises a fluorine-based ether.

[0034] According to the eighth embodiment, in any one of the first to seventh embodiments,

[0035] The present invention relates to an electrolyte for a lithium-sulfur secondary battery characterized in that the above lithium salt comprises lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and LiI.

[0036] According to the ninth embodiment, in any one of the first to eighth embodiments,

[0037] The present invention relates to an electrolyte for a lithium-sulfur secondary battery, characterized in that the content of the above LiI is 0.1 to 3.0 parts by weight per 100 parts by weight of the total electrolyte for the lithium-sulfur secondary battery.

[0038] According to the 10th embodiment, in any one of the 1st to 9th embodiments,

[0039] The present invention relates to an electrolyte for a lithium-sulfur secondary battery characterized in that the content of the above LiI is 0.35 to 15 parts by weight per 100 parts by weight of the total lithium salt.

[0040] According to the 11th embodiment, in any one of the 1st to 10th embodiments,

[0041] The present invention relates to an electrolyte for a lithium-sulfur secondary battery characterized by not containing one or more of nitric acid-based and nitrite-based compounds.

[0042] According to the 12th embodiment,

[0043] The present invention relates to a lithium-sulfur secondary battery comprising: a negative electrode; a positive electrode; a separator; and an electrolyte of any one of the first to eleventh embodiments. Effects of the invention

[0045] The electrolyte for a lithium-sulfur secondary battery according to the present invention and the lithium-sulfur secondary battery containing the same have the effect of improving lifespan characteristics by controlling the solvent, non-solvent, and lithium salt included in the electrolyte under specific conditions. Specific details for implementing the invention

[0047] The present invention will be described in more detail below.

[0048] Terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0049] Throughout this specification, when a part is described as "comprising" or "having" a certain component, unless specifically stated otherwise, this means that it does not exclude other components but may include additional components.

[0050] Throughout this specification, the description “A and / or B” means “A or B or both.”

[0051] 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 a liquid nitrogen temperature (77K) using BEL SORP-mino II from BEL Japan.

[0052] The term “polysulfide” as used in this specification refers to “polysulfide ions (S x 2- , x = 8, 6, 4, 2))” and “lithium polysulfide (Li2S x or LiS x - It is a concept that includes all of , x = 8, 6, 4, 2).

[0053] As used in this specification, the term “composite” refers to a material in which two or more materials are combined to form physically and chemically different phases, thereby exhibiting a more effective function.

[0054] The term “porosity” as used in this specification refers to the ratio of the volume occupied by pores to the total volume of a structure, and uses % as its unit; it may be used interchangeably with terms such as porosity and porosity.

[0056] The present invention relates to an electrolyte for a lithium-sulfur secondary battery and a lithium-sulfur secondary battery comprising the same.

[0057] An electrolyte for a lithium-sulfur secondary battery according to one aspect of the present invention comprises a solvent, a non-solvent, and at least two types of lithium salts, wherein the lithium salt comprises LiI, the solvent and the non-solvent have an SVR factor value of 0.2 to 0.7, and the solvent and the lithium salt have an MR factor value of 0.75 to 1.85.

[0058] Below, each component is explained in detail.

[0060] Lithium-sulfur rechargeable batteries are gaining attention as next-generation rechargeable batteries due to their advantages, including high discharge capacity and theoretical energy density among various rechargeable batteries, the ability to lower manufacturing costs as sulfur, used as the cathode active material, is abundant and inexpensive, and their eco-friendliness.

[0061] Meanwhile, suppressing polysulfide leaching is a major challenge for lithium-sulfur secondary batteries, and SSE electrolyte systems have been developed to address this. However, due to the characteristics of SSE electrolyte systems that induce solid-state reactions, it is difficult to maintain the reversible reaction between the anode and cathode active materials, and the problem of shortened battery life remains unresolved.

[0062] Accordingly, the inventors have invented a specific electrolyte system capable of improving lifespan by utilizing an SSE electrolyte system while excluding nitrile-based electrolyte solvents, which are fatal to the lifespan of lithium-sulfur batteries, and by using a solvent, non-solvent, and lithium salt that satisfy specific conditions, thereby preventing the degradation of the lithium anode or the generation of gas inside the battery and solving the difficulty of maintaining a reversible reaction between the positive and negative active materials.

[0064] The above solvent and non-solvent have an SVR factor value of 0.2 to 0.7, represented by Formula 1 below.

[0065] [Equation 1]

[0066] SVR factor (Solvent Volume Ratio factor) = Volume of solvent (A) ÷ Volume of non-solvent (B)

[0067] In the above Equation 1, the volume of the solvent and the volume of the non-solvent refer to the volumes of the solvent and non-solvent added to the electrolyte, respectively, and the SVR factor has no units.

[0068] The above SVR factor value is 0.2 to 0.7, and according to one embodiment of the present invention, the SVR factor value may have a range of 0.3 to 0.65 or 0.4 to 0.5. If the above SVR factor value falls outside the presented range, it is difficult to achieve effective performance improvement. In particular, if the SVR factor exceeds 0.7, it may cause a problem where lithium polysulfide dissolves excessively in the electrolyte, and if the SVR factor is less than 0.2, the solubility of the lithium salt is low, so the electrolyte is not formed, or even if it is formed, normal operation of the secondary battery is difficult due to the low ionic conductivity of the electrolyte.

[0069] In the present invention, the solvent refers to one having a solubility of 0.1 M or more for a lithium salt, and the non-solvent refers to one having a solubility of less than 0.1 M for a lithium salt. Specifically, the solvent may refer to one having a solubility of 0.1 M or more for imide-based lithium salts such as LiTFSI, LiFSI, and LiTF, and the non-solvent may refer to one having a solubility of less than 0.1 M for a lithium salt.

[0070] If the above solvent and non-solvent satisfy the solubility conditions for the aforementioned lithium salt and the above SVR factor value, those commonly used in secondary battery electrolytes can be used without limitation.

[0071] In one embodiment of the present invention, the solvent may include a linear ether, a cyclic ether, or a mixture thereof.

[0072] For example, the solvent is a linear ether-based solvent such as dimethyl ether, diethyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, ethylmethyl ether, ethylpropyl 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 dimethylene ether, butylene glycol ether, diethylene glycol ethylmethyl ether, diethylene glycol isopropylmethyl ether, diethylene glycol butylmethyl ether, diethylene glycol tert-butylethyl ether, ethylene glycol ethylmethyl ether, etc.; Cyclic ethers such as dioxolane, methyldioxolane, dimethyldioxolane, vinyldioxolane, methoxydioxolane, ethylmethyldioxolane, oxane, dioxane, trioxane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyltetrahydrofuran, dimethoxytetrahydrofuran, ethoxytetrahydrofuran, dihydropyran, tetrahydropyran, furan, 2-methylfuran, etc.; or a mixture thereof may be included.

[0073] Meanwhile, the solvent may include a mixture of the linear ether and the cyclic ether. Specific examples may include dimethoxyethane and tetrahydrofuran, dimethoxyethane and 2-methyltetrahydrofuran, dimethoxyethane and tetrahydropyran, ethylene glycol diethyl ether and tetrahydrofuran, or ethylene glycol diethyl ether and methyltetrahydrofuran. Including a mixture of linear ether and cyclic ether as described above is advantageous in terms of stabilizing the cathode and suppressing gas generation.

[0074] In one embodiment of the present invention, the nonsolvent may include a fluorinated ether. For example, the nonsolvent 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 It may include fluorine-based ether compounds such as 2,2,2-trifluoroethyl ether (Difluoromethyl 2,2,2-trifluoroethyl ether), 1,2,2,2-tetrafluoroethyl trifluoromethyl ether (1,2,2,2-Tetrafluoroethyl trifluoromethyl ether), 1,1,2,3,3,3-hexafluoropropyl difluoromethyl ether (1,1,2,3,3,3-Hexafluoropropyl difluoromethyl ether), pentafluoroethyl 2,2,2-trifluoroethyl ether (Pentafluoroethyl 2,2,2-trifluoroethyl ether), and 1H,1H,2'H-perfluorodipropyl ether (1H,1H,2'H-Perfluorodipropyl ether).

[0076] In the present invention, the electrolyte for a lithium-sulfur secondary battery comprises at least two types of lithium salts, and the lithium salts comprise LiI. That is, the lithium salts may comprise LiI and at least one other type of lithium salt.

[0077] Lithium salts that may be included other than the above LiI may be included without limitation as long as they are compounds capable of providing lithium ions used in lithium secondary batteries. The above lithium salts include imide-based lithium salts, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, and LiCH3CO 2, LiCF3SO3, LiC4F9SO3, LiCl, LiBr, or LiB(C2O4)2, etc., may be used.

[0078] Due to the chemical stability and the characteristic of dissociating well in organic solvents, the above-mentioned imide-based lithium salt can maintain the capacity of a lithium-sulfur secondary battery and improve battery life when used as a lithium salt. 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 enhanced, making it possible to manufacture a secondary battery with an improved battery life.

[0079] For example, the above imide-based lithium salt may include lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(perfluoroethylsulfonyl)imide (LiBETI), etc.

[0080] The lithium salt included in the electrolyte for a lithium-sulfur secondary battery according to one embodiment of the present invention may include three different types of lithium salts.

[0081] Specifically, the lithium salt included in the electrolyte for a lithium-sulfur secondary battery according to one embodiment of the present invention may include lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and LiI. When including the above combination of lithium salts, there is an advantage of reducing side reactions on the negative electrode and increasing the reversibility of sulfur.

[0082] At this time, the content of LiI may be 0.1 to 3.0 parts by weight or 0.1 to 2 parts by weight per 100 parts by weight of the total electrolyte for a lithium-sulfur secondary battery. Alternatively, the content of LiI may be 0.35 to 15 parts by weight per 100 parts by weight of the total lithium salt. When included within the above content range, the reversibility of sulfur can be increased, thereby improving battery life.

[0083] The molar concentration of the total lithium salt may be in the range of 0.1 to 5.0 M or 0.1 to 3.0 M. When included within the above range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0085] The above solvent and lithium salt have an MR factor value of 0.75 to 1.85, represented by Formula 2 below.

[0086] [Equation 2]

[0087] MR factor (Molar mass Ratio factor) = Moles of solvent ÷ (Moles of lithium salt excluding LiI)

[0088] In Equation 2 above, the number of moles of lithium salts excluding LiI refers to the total sum of moles of lithium salts excluding LiI among all lithium salts included in the electrolyte, and the number of moles of solvent refers to the total sum of moles of all solvents excluding non-solvents included in the electrolyte. The above MR factor has no units.

[0089] The above MR factor value is 0.75 to 1.85, and according to one embodiment of the present invention, the above MR factor value may have a range of 1.0 to 1.75 or 1.0 to 1.7. When the above MR factor value satisfies the presented range, the dissolution of lithium polysulfide can be suppressed and the ionic conductivity of the electrolyte can be increased, thereby improving the lifespan of the secondary battery.

[0090] For example, a method for calculating the SVR factor and MR factor based on Example 1 described in Table 1 below will be explained.

[0091] First, since Example 1 contains 2MeTHF (solvent 1) : EGDEE (solvent 2) : TTE (non-solvent) in a volume ratio of 2:1:7, the SVR factor is calculated as (2+1) / 7 = 0.43.

[0092] Next, Example 1 includes LiTFSI, LiFSI, and LiI as lithium salts, but LiI is excluded when calculating the total number of moles of lithium salts for the MR factor calculation. Based on the preparation of 1 kg of electrolyte, 0.66 moles of 2 MeTHF (molar mass: 86.134), 0.73 moles of EGDEE (molar mass: 118.18), 0.71 moles of LiTFSI (molar mass 287), and 0.23 moles of LiFSI (molar mass 187) are included, so the MR factor is calculated as (0.66+0.73) / (0.71+0.23) = 1.48.

[0093] That is, it can be confirmed that the above Example 1 satisfies all the ranges of the SVR factor and MR factor presented in the present invention, and also shows excellent performance in the life evaluation results.

[0095] An electrolyte for a lithium-sulfur secondary battery according to one embodiment of the present invention does not include one or more of nitric acid-based and nitrite-based compounds.

[0096] Generally, the above-mentioned nitric acid and / or nitrite compounds are used as electrolyte additives to form a stable film on the lithium electrode and improve charge / discharge efficiency; however, there was a problem in that lifespan was not improved due to depletion. Accordingly, the inventors of the present invention have achieved an effect that improves lifespan without including nitric acid and / or nitrite compounds that are commonly used as additives.

[0097] The above nitric acid or nitrite compounds are not specifically limited in the present invention, but include 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); and organic nitric acid or nitrite compounds such as methyl nitrate, dialkyl imidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridinium nitrate, ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite, and octyl nitrite. It may be one or more organic nitro compounds selected from nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, dinitrotoluene, or combinations thereof. Specifically, the additive may not contain lithium nitrate (LiNO3).

[0099] Meanwhile, the above electrolyte may further include other additives for the purpose of improving charge / discharge characteristics, flame retardancy, etc. Examples of the above additives include pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphate trialamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, fluoroethylene carbonate (FEC), propene sulfone (PRS), vinylene carbonate (VC), etc.

[0101] The method for manufacturing the electrolyte for a lithium-sulfur secondary battery according to the present invention is not specifically limited in the present invention and can be manufactured by conventional methods known in the art.

[0103] In one embodiment of the present invention, a lithium-sulfur secondary battery comprising a negative electrode; a positive electrode; a separator; and the above-described electrolyte is provided.

[0104] The above cathode comprises a cathode current collector; and a cathode active material layer formed on at least one side surface of the cathode current collector; and the cathode active material layer comprises a cathode active material, a conductive material, and a binder.

[0105] Specifically, the cathode can be manufactured by applying a cathode slurry, prepared by dispersing a cathode active material, a conductive material, and a binder in a solvent, to one or both sides of a long sheet-shaped cathode current collector, removing the solvent from the cathode slurry through a drying process, and then rolling. Meanwhile, a cathode including an uncoated portion can be manufactured by not applying the cathode slurry to a portion of the cathode current collector, for example, one end of the cathode current collector, when applying the cathode slurry.

[0106] The above negative electrode active material is lithium (Li +The material may include a material capable of reversibly intercalating or deintercalating lithium ions, a material capable of reacting with lithium ions to reversibly form a lithium-containing compound, a lithium metal, or a lithium alloy. The material capable of reversibly intercalating or deintercalating lithium ions may be, for example, crystalline carbon, amorphous carbon, or a mixture thereof; specifically, examples include artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, soft carbon, hard carbon, etc., but are not limited thereto. The material capable of reacting with lithium ions to reversibly form a lithium-containing compound may be, for example, tin oxide, titanium nitrate, or a silicon-based compound. The above lithium alloy may be, for example, an alloy of a metal selected from the group consisting of lithium (Li), 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 above negative electrode active material may be lithium metal, specifically, in the form of a lithium metal thin film or lithium metal powder. The above silicon-based negative electrode active material may be Si, Si-Me alloy (wherein Me is one or more selected from the group consisting of Al, Sn, Mg, Cu, Fe, Pb, Zn, Mn, Cr, Ti, and Ni), SiO₂ y (here, 0 <y<2), Si-C 복합체 또는 이들의 조합일 수 있으며, 바람직하게는 SiO y (here, 0 <y<2)일 수 있다. 실리콘계 음극 활물질은 높은 이론 용량을 가지기 때문에 실리콘계 음극 활물질을 포함할 경우, 용량 특성을 향상시킬 수 있다.

[0107] As the above-mentioned negative current collector, negative current collectors generally used in the relevant technical field may be used, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy may be used. The above-mentioned negative current collector may typically have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to strengthen the bonding strength of the negative active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0108] The above conductive material is used to impart conductivity to the negative electrode, and in the battery being constructed, it may be used without special limitations as long as it has electronic conductivity without causing chemical changes. 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, thermal black, carbon fiber, carbon nanotube; metal powder or metal fiber such as copper, nickel, aluminum, or silver; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and one of these alone or a mixture of two or more may be used. The above conductive material may typically be included in an amount of 1 to 30 wt%, 1 to 20 wt%, or 1 to 10 wt% based on the total weight of the negative electrode active material layer.

[0109] The above binder serves to improve adhesion between negative electrode active material particles and adhesion 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, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these alone or a mixture of two or more may be used. The above binder may be included in an amount of 1 to 30 wt%, 1 to 20 wt%, or 1 to 10 wt% based on the total weight of the negative electrode active material layer.

[0111] The above positive electrode comprises a positive current collector; and a positive active material layer formed on at least one side surface of the positive current collector; wherein the positive active material layer comprises a positive active material and may further comprise a conductive material, a binder, an additive, etc.

[0112] The above positive current collector is intended to support the positive active material and is as described in the description of the current collector supporting the negative electrode. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface treated with carbon, nickel, silver, etc., and aluminum-cadmium alloy may be used.

[0113] The above-mentioned positive active material may include at least one selected from the group consisting of sulfur, particularly elemental sulfur (S8) and sulfur compounds. The above-mentioned positive active material may include inorganic sulfur, Li2Sn (n≥1), disulfide compounds, organic sulfur compounds, and carbon-sulfur polymers ((C2S xIt may include one or more selected from the group consisting of )n, x = 2.5 ~ 50, n≥2). Specifically, the positive electrode active material may include inorganic sulfur.

[0114] Since the sulfur included in the above-mentioned positive electrode active material does not possess electrical conductivity on its own, it is used together with a conductive material such as a carbon material. Therefore, sulfur is included in the form of a sulfur-carbon composite, and preferably, the positive electrode active material may be a sulfur-carbon composite.

[0115] The carbon included in the above sulfur-carbon composite is a porous carbon material that provides a framework capable of uniformly and stably fixing sulfur and compensates for the low electrical conductivity of sulfur, thereby enabling the electrochemical reaction to proceed smoothly.

[0116] The porous carbon material can generally be manufactured by carbonizing various carbonaceous precursors. The porous carbon material may contain non-uniform pores internally, the average diameter of the pores may be in the range of 1 to 200 nm, and the porosity may be in the range of 10 to 90% of the total volume of the porous carbon material. If the average diameter of the pores is smaller than the above range, the pore size is only at the molecular level, making sulfur impregnation impossible. Conversely, if the average diameter of the pores exceeds the above range, the mechanical strength of the porous carbon material is weakened, making it undesirable for application in the electrode manufacturing process.

[0117] The shape of the above porous carbon material is spherical, rod, needle, plate, tube, or bulk, and can be used without limitation as long as it is commonly used in lithium-sulfur batteries.

[0118] The porous carbon material may have a porous structure or a high specific surface area, and may be one commonly used in the industry. For example, the porous carbon material may include graphite; graphene; carbon black such as Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black; carbon nanotubes (CNT) such as single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT); carbon fibers such as graphite nanofibers (GNF), carbon nanofibers (CNF), and activated carbon fibers (ACF); graphite such as natural graphite, artificial graphite, and expanded graphite; activated carbon, etc. Preferably, the porous carbon material may be a carbon nanotube.

[0119] The above sulfur-carbon composite may contain a sulfur content of 60 to 90 parts by weight, preferably 65 to 85 parts by weight, and more preferably 70 to 80 parts by weight, per 100 parts by weight of the sulfur-carbon composite. If the sulfur content is lower than the above range, the content of porous carbon material within the sulfur-carbon composite increases relatively, thereby increasing its specific surface area and consequently increasing the amount of binder used relative to the sulfur content during the manufacture of the anode. This increase in the amount of binder used ultimately increases the sheet resistance of the anode and acts as an insulator that blocks the passage of electrons, thereby degrading the performance of the battery. Conversely, if the sulfur content exceeds the above range, sulfur that cannot bond with the porous carbon material aggregates or re-leaches to the surface of the porous carbon material, making it difficult to accept electrons and unable to participate in electrochemical reactions, resulting in a decrease in battery capacity.

[0120] In addition, in the above sulfur-carbon composite, sulfur is located on at least one of the inner and outer surfaces of the aforementioned porous carbon material, and in this case, sulfur may be present in an area of ​​less than 100%, 1 to 95%, or 60 to 90% of the total inner and outer surfaces of the porous carbon material. When such sulfur is present on the inner and outer surfaces of the porous carbon material within the above range, it can exhibit maximum effects in terms of electron transfer area and wettability with the electrolyte. Specifically, since sulfur is thinly and uniformly impregnated on the inner and outer surfaces of the porous carbon material within the above range, the electron transfer contact area can be increased during the charge-discharge process. When sulfur is located on 100% of the total area of ​​the 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 included electrically conductive material, so it cannot receive electrons from the electrode and cannot participate in the electrochemical reaction.

[0121] The method for manufacturing the above sulfur-carbon composite is not particularly limited in the present invention, and methods commonly used in the art may be used. For example, a method of simply mixing sulfur and a porous carbon material and then heat-treating them to form a composite may be used.

[0122] In addition to the above, the positive electrode active material may further include transition metal elements, Group IIIA elements, Group IVA elements, sulfur compounds of these elements, alloys of these elements and sulfur, and one or more additives selected from the aforementioned components.

[0123] The above 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., and group IIIA elements may include Al, Ga, In, Tl, etc., and group IVA elements may include Ge, Sn, Pb, etc.

[0124] The sulfur may be included in an amount of 40 to 95 weight%, preferably 50 to 90 weight%, and more preferably 60 to 85 weight%, relative to 100 weight% of the positive active material layer constituting the positive electrode. In one embodiment of the present invention, when a sulfur-carbon composite is used as the positive active material, the sulfur-carbon composite may be included in an amount of 90 weight% to 97 weight% relative to 100 weight% of the positive active material layer. If the content of the positive active material is less than the above range, it is difficult to sufficiently exhibit the electrochemical reaction of the positive electrode. Conversely, if the content exceeds the above range, the content of the conductive material and binder described below is relatively insufficient, causing the resistance of the positive electrode to increase and the physical properties of the positive electrode to deteriorate.

[0125] The above positive active material layer may optionally further include a conductive material that enables electrons to move smoothly within the positive electrode (specifically, the positive active material) and a binder that effectively attaches the positive active material to the current collector.

[0126] The above conductive material is a substance that electrically connects the electrolyte and the positive active material, serving as a pathway for electrons to move from the current collector to the positive active material. The above conductive material may be used without limitation as long as it possesses electrical conductivity.

[0127] For example, the conductive material may be used alone or in combination with 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 fullerenes; electrically conductive fibers, such as carbon fibers and metal fibers; fluorocarbon; metal powders, such as aluminum and nickel powders; or electrically conductive polymers, such as polyaniline, polythiophene, polyacetylene, and polypyrrole.

[0128] The conductive material may be included in an amount of 0.01 to 30 weight% relative to 100 weight% of the total positive active material layer constituting the positive electrode. If the content of the conductive material is lower than the above range, it is difficult for electrons to move between the positive active material and the current collector, resulting in a decrease in voltage and capacity. Conversely, if the content exceeds the above range, the relative proportion of the positive active material decreases, which may reduce the total energy (charge amount) of the battery. Therefore, it is desirable to determine the content of the conductive material to be an appropriate amount within the aforementioned range.

[0129] The binder serves to hold the positive active material in the positive current collector and organically connect the positive active materials to increase the bonding strength between them, and any binder known in the art may be used.

[0130] For example, the binder comprises a fluoropolymer-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 carboxymethylcellulose (CMC), starch, hydroxypropylcellulose 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.

[0131] The content of the binder may be 0.5 to 30 weight percent based on 100 weight percent of the total positive active material layer constituting the positive electrode. If the binder content is less than 0.5 weight percent, the physical properties of the positive electrode may deteriorate, causing the positive active material and the conductive material to delaminate. If the content exceeds the above range, the ratio of the positive active material to the conductive material within the positive electrode may decrease relatively, which may reduce the capacity of the battery. Therefore, it is desirable to determine the binder content to an appropriate amount within the aforementioned range.

[0133] The above separator separates the negative electrode and the positive electrode and provides a pathway for the movement of lithium ions; any separator typically used in lithium secondary batteries can be used without any special restrictions. Specifically, the separator may be a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer like ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof. Alternatively, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, may be used. Furthermore, a coated separator containing ceramic components or polymer materials may be used to ensure heat resistance or mechanical strength.

[0135] The shape of the above lithium-sulfur battery is not particularly limited and can be made in various shapes such as cylindrical, stacked, or coin-shaped.

[0136] In addition, the present invention provides a battery module comprising the lithium-sulfur battery as a unit cell. The battery module can be used as a power source for medium to large-sized devices requiring high-temperature stability, long cycle characteristics, and high capacity characteristics.

[0137] Examples of the above medium-to-large devices include, but are not limited to, power tools powered by electric 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.

[0139] Hereinafter, the present invention will be described in detail with reference to examples to specifically explain the invention. However, the embodiments according to the present invention may be modified in various different forms, and the scope of the present invention should not be interpreted as being limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the invention to those with average knowledge in the art.

[0141] Examples 1 to 12 and Comparative Examples 1 to 5

[0142] Electrolyte manufacturing

[0143] An electrolyte containing a solvent, a non-solvent, and a lithium salt was prepared to meet the SVR factor, MR factor, and LiI content listed in Table 1 below.

[0144] At this time, 2-methyl tetrahydrofuran (2MeTHF), ethylene glycol diethyl ether (EGDEE), tetrahydrofuran (THF), tetrahydropyran (THP), dimethyl ether (DME), or a mixture of two or more of these were used as solvents, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) was used as a non-solvent, and lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and LiI were used as lithium salts.

[0146] Anode manufacturing

[0147] 90 parts by weight of a sulfur-carbon composite (S : C = 75 : 25 weight ratio) as the cathode active material (the sulfur content alone was set to 67.5 wt% relative to the total cathode weight, and the carbon material has a pore volume of 1.8 cm 3Activated carbon with a mAh / g content was used), 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 were mixed to prepare an anode slurry composition. Subsequently, the prepared slurry composition was coated onto a current collector (Al Foil), dried at 50°C for 12 hours, and compressed using a roll press to manufacture an anode (at this time, the loading amount was 3.5 mAh / cm²). 2 The porosity of the electrode was set to 65%.

[0149] Lithium-sulfur battery manufacturing

[0150] A coin cell type lithium-sulfur battery was manufactured by positioning the above-manufactured positive electrode and a 150 μm thick lithium metal negative electrode facing each other, interposing a polyethylene (PE) separator between them, and then injecting the above-manufactured electrolyte. Meanwhile, in manufacturing the battery, the positive electrode was used by punching it into a 14 phi circular electrode, the polyethylene separator was used by punching it into a 19 phi electrode, and the lithium metal was used by punching it into a 16 phi electrode. In addition, the battery was manufactured using an SSE (sparing solvating electrolyte) electrolyte system.

[0152] Composition (volume ratio) of solvent and non-solvent SVR factor MR factor LiI content (parts by weight of LiI per 100 parts by weight of total electrolyte) Life assessment (cycle) Example 1 2MeTHF:EGDEE:TTE(2:1:7) 0.43 1.48 1 247 Example 2 2MeTHF:EGDEE:TTE(1.5:1.5:7) 0.43 1.33 1 215 Example 3 2MeTHF:EGDEE:TTE(1:2:7) 0.43 1.11 1 237 Example 4 2MeTHF:EGDEE:TTE(1:2:7) 0.43 1.04 2 307 Example 5 THP:EGDEE:TTE(1:2:7) 0.43 1.2 2 264 Example 6 DME:TTE (3:7) 0.43 1.64 0.1 96 Example 7 DME:TTE (3:7) 0.43 1.52 1 122 Example 8 THF:DME:TTE(1.5:1.5:7) 0.43 1.74 1 137 Example 9 THF:DME:TTE(1.5:1.5:7) 0.43 1.61 2 143 Example 10 THP:DME:TTE(0.5:2.5:7) 0.43 1.54 1 146 Example 11 2MeTHF:DME:TTE(0.5:2.5:7) 0.43 1.53 1 145 Example 12 2MeTHF:DME:EGDEE:TTE(1.5:1:1.5:6) 0.67 1.62 1 147 Comparative Example 1 DME:TTE(3:7) 0.43 1.65 - 85 Comparative Example 2 THF:TTE(2:8) 0.25 1.9 - 79 Comparative Example 3 DME:TTE(3:7) 0.43 2.58 1 45 Comparative Example 4 EGDEE:TTE(3:7) 0.43 1.43 - 28 Comparative Example 5 EGDEE:TTE(3:7) 0.43 0.71 - 7

[0154] Lifespan assessment

[0155] In order to confirm the life characteristics of the lithium-sulfur secondary battery prepared in the above examples and comparative examples, the battery life characteristics were confirmed by measuring the life cycle at the point when the capacity retention rate of the lithium-sulfur secondary battery was 80%, while proceeding with 150 cycles of discharging and charging at a current density of 0.1 C, charging at a current density of 0.2 C, and discharging at a current density of 0.5 C.

Claims

Claim 1 An electrolyte for a lithium-sulfur secondary battery comprising a solvent, a non-solvent, and at least two types of lithium salts, wherein the lithium salt comprises LiI, the solvent and non-solvent have an SVR factor value of 0.2 to 0.7, the SVR factor is represented by the following Equation 1, the solvent and lithium salt have an MR factor value of 0.75 to 1.85, the MR factor value is represented by the following Equation 2, and wherein the solubility of the solvent for the lithium salt is 0.1 M or higher, and the solubility of the non-solvent for the lithium salt is less than 0.1 M: [Equation 1] SVR factor (Solvent Volume Ratio factor) = Volume of solvent ÷ Volume of non-solvent [Equation 2] MR factor (Molar mass Ratio factor) = Moles of solvent ÷ (Moles of lithium salt excluding LiI) Claim 2 An electrolyte for a lithium-sulfur secondary battery according to claim 1, characterized in that the SVR factor value is 0.3 to 0.

65. Claim 3 An electrolyte for a lithium-sulfur secondary battery according to claim 1, characterized in that the MR factor value is 1.0 to 1.

75. Claim 4 delete Claim 5 An electrolyte for a lithium-sulfur secondary battery according to claim 1, characterized in that the solvent comprises a linear ether, a cyclic ether, or a mixture thereof. Claim 6 An electrolyte for a lithium-sulfur secondary battery according to claim 1, characterized in that the solvent comprises a mixture of linear ether and cyclic ether. Claim 7 An electrolyte for a lithium-sulfur secondary battery according to claim 1, characterized in that the non-solvent comprises a fluorine-based ether. Claim 8 An electrolyte for a lithium-sulfur secondary battery according to claim 1, characterized in that the lithium salt comprises lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and LiI. Claim 9 The lithium-sulfur secondary battery electrolyte according to claim 1, characterized in that the content of LiI is 0.1 to 3.0 parts by weight per 100 parts by weight of the total lithium-sulfur secondary battery electrolyte. Claim 10 An electrolyte for a lithium-sulfur secondary battery according to claim 8, characterized in that the content of LiI is 0.35 to 15 parts by weight per 100 parts by weight of the total lithium salt. Claim 11 The electrolyte for a lithium-sulfur secondary battery according to claim 1, characterized in that the electrolyte for the lithium-sulfur secondary battery does not contain one or more of nitric acid-based and nitrite-based compounds. Claim 12 A lithium-sulfur secondary battery comprising: a negative electrode; a positive electrode; a separator; and an electrolyte according to any one of claims 1 to 3 and 5 to 11.

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