Lithium-sulfur secondary battery comprising electrolyte containing s-o group-containing cyclic compound

Abstract

The present application provides a lithium-sulfur secondary battery, which comprises a positive electrode, a negative electrode, a separator, and an electrolyte. The electrolyte contains an S-O-based cyclic compound, which is ethylene sulfite, 1,3-propane sulfite, 1,3-propylene sulfite, ethylene sulfate, or a combination thereof. The content of the S-O-based cyclic compound in the electrolyte is more than 0 ppm and less than 1000 ppm, relative to the total weight of the electrolyte. Because the lithium-sulfur secondary battery contains a specific S-O-based cyclic compound in the electrolyte, the cycle performance of the lithium-sulfur secondary battery is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to a lithium-sulfur secondary battery comprising an electrolyte containing an S-O based cyclic compound. Specifically, the present application relates to a lithium-sulfur secondary battery comprising an electrolyte containing ethylene sulfite, 1,3-propane sultone, 1,3-propene sultone, or ethylene sulfate as an S-O based cyclic compound.

[0002] This application claims the priority benefit of Korean Patent Applications Nos. 10-2020-0142367, 10-2020-0142380, 10-2020-0142395, and 10-2020-0142412, filed on October 29, 2020, the entire contents of which are incorporated herein by reference. BACKGROUND

[0003] As the application fields of secondary batteries are expanded to electric vehicles (EVs) or energy storage systems (ESSs), lithium-ion secondary batteries having a relatively low energy storage density per weight (~250 Wh / kg) have limitations in the application of these products. Alternatively, since a lithium-sulfur secondary battery is theoretically capable of achieving a high energy storage density per weight (~2600 Wh / kg), it is attracting attention as a next-generation secondary battery technology.

[0004] A lithium-sulfur secondary battery refers to a battery system in which a sulfur-based material having a sulfur-sulfur bond (S-S bond) is used as a positive electrode active material and lithium metal is used as a negative electrode active material. Sulfur, which is the main material of the positive electrode active material, has the advantages of being very abundant in resources, non-toxic, and low in atomic weight.

[0005] In a lithium-sulfur secondary battery, when the battery is discharged, lithium as a negative electrode active material is oxidized while releasing electrons and is thereby ionized, and a sulfur-based material as a positive electrode active material is reduced while accepting electrons. In this case, the oxidation reaction of lithium is a process in which lithium metal releases electrons and is converted into a lithium cation form. In addition, the reduction reaction of sulfur is a process in which the S-S bond accepts two electrons and is converted into a sulfur anion form. The lithium cation produced by the oxidation reaction of lithium is transferred to the positive electrode through the electrolyte and combines with the sulfur anion produced by the reduction reaction of sulfur to form a salt. Specifically, sulfur has a cyclic S8 structure before discharge, which is converted into lithium polysulfide (LiS x ) by the reduction reaction. When the lithium polysulfide is completely reduced, lithium sulfide (Li2S) is generated.

[0006] Sulfur as a positive electrode active material is difficult to ensure reactivity with electrons and lithium ions in a solid state form due to its low electrical conductivity. In existing lithium-sulfur secondary batteries, in order to improve the reactivity of sulfur, Li2S xAn intermediate polysulfide in a form is used to induce a liquid phase reaction and to improve reactivity. In this case, as a solvent of an electrolyte, an ether-based solvent such as dioxolane and dimethoxyethane, which has a high solubility of lithium polysulfide, is used. In addition, in a conventional lithium-sulfur secondary battery, in order to improve reactivity, a cathode electrolyte type lithium-sulfur secondary battery system is constructed, in which case, due to the characteristics of lithium polysulfide dissolved in the electrolyte, the reactivity and the life characteristics of sulfur are affected depending on the content of the electrolyte. In order to achieve a high energy density, the electrolyte should be injected under a low content condition, but as the electrolyte content decreases, the concentration of lithium polysulfide in the electrolyte increases, making it difficult for the battery to operate normally due to the decrease in the flowability of the active material and the increase in the side reaction.

[0007] In order to manufacture a lithium-sulfur secondary battery having a high energy density, a battery system capable of operating an electrode having a high load and a low porosity is required, and research on such a battery system has been continuously conducted in the related art.

[0008] Prior Art Documents

[0009] [Patent Document]

[0010] (Patent Document 1) Korean Patent Laid-Open Publication No. 10-2019-0006923 SUMMARY

[0011] [Technical Problem]

[0012] An object of the present application is to provide a lithium-sulfur secondary battery capable of improving the cycle characteristics of a lithium-sulfur secondary battery by adding an S-O group cyclic compound to an electrolyte of a lithium-sulfur secondary battery.

[0013] [Technical Solution]

[0014] The present application provides a lithium-sulfur secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte containing an S-O group cyclic compound.

[0015] In one embodiment of the present application, the S-O group cyclic compound is ethylene sulfite, 1,3-propane sulfite, 1,3-propylene sulfite, ethylene sulfate, or a combination thereof.

[0016] In one embodiment of the present application, the content of the S-O group cyclic compound in the electrolyte is more than 0 ppm and less than 1000 ppm, with respect to the total weight of the electrolyte.

[0017] In one embodiment of the present application, the electrolyte further includes a non-aqueous solvent and a lithium salt, the non-aqueous solvent including a fluorinated linear ether.

[0018] In one embodiment of the present application, the fluorinated linear ether is selected from the group consisting of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, bis(fluoromethyl) ether, 2-fluoroethyl methyl 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, and combinations thereof.

[0019] In one embodiment of the present application, the non-aqueous solvent contains 50% by weight to 99% by weight of the fluorinated linear ether, relative to the total weight of the non-aqueous solvent.

[0020] In one embodiment of the present application, the lithium salt is selected from the group consisting of LiN(FSO2)2, LiSCN, LiN(CN)2, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiPF6, LiF, LiCl, LiBr, LiI, LiNO3, LiClO4, LiAlO4, LiAlCl4, LiSbF6, LiAsF6, LiBF2C2O4, LiBC4O8, Li(CF3)2PF4, Li(CF3)3PF3, Li(CF3)4PF2, Li(CF3)5PF, Li(CF3)6P, LiCF3SO3, LiC4F9SO3, LiCF3CF2SO3, LiCF3CF2(CF3)2CO, Li(CF3SO2)2CH, LiCF3(CF2)7SO3, LiCF3CO2, LiCH3CO2, and combinations thereof.

[0021] In one embodiment of the present application, the positive electrode comprises a positive electrode active material layer having a porosity of 30% or more and less than 70%.

[0022] In one embodiment of the present application, the positive electrode has a positive electrode active material loading of 3.0 mAh / cm 2 to 10.0 mAh / cm 2 .

[0023] In one embodiment of the present application, the positive electrode comprises a sulfur-carbon composite material as the positive electrode active material.

[0024] In one embodiment of the present application, the sulfur-carbon composite material contains 60% by weight to 90% by weight of sulfur, relative to the total weight of the sulfur-carbon composite material.

[0025] In one embodiment of the present application, the non-aqueous solvent further comprises a non-fluorinated linear ether, a cyclic ether, a polyether, or a mixture thereof.

[0026] [Advantages]

[0027] In the lithium-sulfur secondary battery according to the present application, the cycle performance of the lithium-sulfur secondary battery is improved by adding a specific S-O group-containing cyclic compound to the electrolyte.

[0028] As the S-O group-containing cyclic compound, ethylene sulfite, 1,3-propane sultone, 1,3-propene sultone, or ethylene sulfate is added to the electrolyte of the lithium-sulfur secondary battery in an amount of less than 1000 ppm. It is considered that, in the related technical field, generally, when the amount of use of the electrolyte additive is 1% by weight (10000 ppm) or more, a desired effect of improving the battery performance can be obtained, and ethylene sulfite, 1,3-propane sultone, 1,3-propene sultone, or ethylene sulfate is different from ordinary electrolyte additives used in the technical field.

[0029] When ethylene sulfite, 1,3-propane sultone, 1,3-propene sultone, or ethylene sulfate is used in the same amount as ordinary electrolyte additives in the related art, the effect of improving the cycle performance of the lithium-sulfur secondary battery is not significant or almost nonexistent.

[0030] Furthermore, even when used in lithium secondary batteries other than lithium-sulfur secondary batteries, the effect of improving the cycle performance of the lithium secondary battery is not significant or almost nonexistent. DETAILED DESCRIPTION

[0031] The embodiments provided according to the present application can all be implemented by the following description. It should be understood that the following description is understood to describe preferred embodiments of the present application, and it should be understood that the present application is not necessarily limited thereto.

[0032] For the physical properties described herein, if the measurement conditions and methods are not specifically described, the physical properties are measured in accordance with the measurement conditions and methods generally used by those skilled in the art.

[0033] "Lithium secondary battery" is a more general concept than lithium-sulfur secondary battery, and includes lithium-sulfur secondary battery. However, "lithium secondary battery" in the present specification refers to an ordinary lithium secondary battery using lithium metal oxide as a positive electrode active material, and is used separately from lithium-sulfur secondary battery.

[0034] The present invention provides a lithium-sulfur secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte contains an S-O-based cyclic compound. According to one embodiment of the present invention, the S-O-based cyclic compound is ethylene sulfite (ES), 1,3-propane sulfite (PS), 1,3-propylene sulfite (PRS), ethylene sulfate (Esa), or a combination thereof. Ethylene sulfite is a compound having a structure of the following Formula 1, and 1,3-propane sulfite is a compound having a structure of the following Formula 2. 1,3-Propylene sulfite is a compound having a structure of the following Formula 3, and ethylene sulfate is a compound having a structure of the following Formula 4. The inventors of the present invention accomplished the present invention by confirming that when an S-O-based cyclic compound is added to an electrolyte of a lithium-sulfur secondary battery using a sulfur-containing material as a positive electrode active material, rather than a common lithium secondary battery using a lithium metal oxide as a positive electrode active material, the cycle performance of the battery is improved.

[0035] [Formula 1]

[0036]

[0037] [Formula 2]

[0038]

[0039] [Formula 3]

[0040]

[0041] [Formula 4]

[0042]

[0043] It is expected that ethylene sulfite, 1,3-propane sulfite, 1,3-propylene sulfite, or ethylene sulfate added to the electrolyte directly interacts with the positive electrode active material of the lithium-sulfur secondary battery, considering that there is little or no effect on improving the cycle performance of the battery in the case of a lithium secondary battery having the same composition except for the positive electrode active material.

[0044] According to one embodiment of the present application, the electrolyte contains more than 0 ppm, 50 ppm or more, 100 ppm or more, 150 ppm or more, 200 ppm or more, 250 ppm or more, 300 ppm or more, 350 ppm or more, 400 ppm or more, 450 ppm or more, or 500 ppm or more of the S-O-based cyclic compound relative to the total weight of the electrolyte, and the electrolyte contains less than 1000 ppm, 950 ppm or less, 900 ppm or less, 850 ppm or less, 800 ppm or less, 750 ppm or less, 700 ppm or less, 650 ppm or less, 600 ppm or less, 550 ppm or less, or 500 ppm or less of the S-O-based cyclic compound relative to the total weight of the electrolyte. In the present application, one feature is to add a small amount of the S-O-based cyclic compound such as less than 1000 ppm to the electrolyte. If a large amount of the S-O-based cyclic compound is added of 1000 ppm or more, the effect of improving the cycle performance can be small or no effect. In the related art, the above feature is not common in the field in view of the fact that the desired effect of improving the battery performance can be obtained when the amount of the electrolyte additive is generally 1% by weight (10000 ppm) or more.

[0045] The electrolyte constituting the lithium-sulfur secondary battery according to the present application contains a nonaqueous solvent and a lithium salt in addition to the above-described S-O-based cyclic compound. As described above, since the effect of improving the cycle performance of the battery by using the S-O-based cyclic compound can be exerted by the direct interaction between the S-O-based cyclic compound and the positive electrode active material of the lithium-sulfur secondary battery, the type of the nonaqueous solvent and the lithium salt is not particularly limited. However, if a more suitable nonaqueous solvent and a lithium salt are selected for the lithium-sulfur secondary battery, the overall cycle performance of the battery can be excellent.

[0046] According to one embodiment of the present application, the nonaqueous solvent is an ether-based solvent. The ether-based solvent can be a linear ether, a cyclic ether, a polyether, or a mixture thereof.

[0047] The linear ether can be selected from the group consisting of methyl ethyl ether, methyl propyl ether, methyl butyl ether, ethyl propyl ether, ethyl isopropyl ether, ethyl butyl ether, ethyl isobutyl ether, diethyl ether, dipropyl ether, dibutyl ether, dimethoxymethane (DMM), trimethoxyethane (TMM), dimethoxyethane (DME), diethoxyethane (DEE), dimethoxypropane (DMP), and combinations thereof, but is not limited thereto.

[0048] The cyclic ether can be selected from the group consisting of dioxolane (DOL), methyl dioxolane, alkyl, tri alkyl, tri alkane, tetrahydrofuran (THF), dihydropyran (DHP), tetrahydropyran (THP), methyltetrahydrofuran, furan, methyl furan, and combinations thereof, but are not limited thereto.

[0049] The polyether can be selected from the group consisting of: diethylene glycol dimethyl 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, butanediol ether, and combinations thereof, but is not limited thereto.

[0050] The linear ether, the cyclic ether, and the polyether can be fluorinated ether compounds. The fluorinated form of the compound can be a fluorinated linear ether, which can be selected from the group consisting of: 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, bis(fluoromethyl) ether, 2-fluoroethyl methyl 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, and combinations thereof, but is not limited thereto.

[0051] The fluorinated ether compound can be used in combination with a non-fluorinated linear ether, a non-fluorinated cyclic ether, a non-fluorinated polyether, or a combination thereof. According to one embodiment of the present application, the content of the fluorinated ether compound in the electrolyte can be 50% by weight to 99% by weight, preferably 60% by weight to 95% by weight, and more preferably 70% by weight to 90% by weight, with respect to the total weight of the solvent constituting the electrolyte. If the content of the fluorinated ether compound in the electrolyte is 50% by weight or more with respect to the total weight of the solvent constituting the electrolyte, the performance of the battery can be improved when used with a positive electrode having a positive electrode active material with low porosity and high loading amount of a lithium-sulfur secondary battery.

[0052] The lithium salt is a material that can be easily dissolved in a nonaqueous solvent, and can be selected from the group consisting of LiN(FSO2)2, LiSCN, LiN(CN)2, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiPF6, LiF, LiCl, LiBr, LiI, LiNO3, LiClO4, LiAlO4, LiAlCl4, LiSbF6, LiAsF6, LiBF2C2O4, LiBC4O8, Li(CF3)2PF4, Li(CF3)3PF3, Li(CF3)4PF2, Li(CF3)5PF, Li(CF3)6P, LiCF3SO3, LiC4F9SO3, LiCF3CF2SO3, LiCF3CF2(CF3)2CO, Li(CF3SO2)2CH, LiCF3(CF2)7SO3, LiCF3CO2, LiCH3CO2, and combinations thereof.

[0053] The concentration of the lithium salt can be 0.1 M to 8.0 M, preferably 0.5 M to 5.0 M, and more preferably 1.0 M to 3.0 M, depending 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 art of lithium secondary batteries. If the concentration of the lithium salt is less than the above range, the conductivity of the electrolyte can be reduced, whereby the performance of the electrolyte can be deteriorated. If the concentration of the lithium salt exceeds the above range, the viscosity of the electrolyte can be increased, whereby the mobility of lithium ions (Li + ) can be reduced. Therefore, it is preferable to select a suitable concentration of the lithium salt within the above range.

[0054] The positive electrode constituting the lithium-sulfur secondary battery according to the present application generally includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, a conductive material, and a binder.

[0055] The positive electrode current collector is not particularly limited as long as it has high conductivity under conditions that do not cause chemical changes in the battery, and can be, for example, stainless steel, aluminum, nickel, titanium, sintered carbon; or aluminum or stainless steel that is surface-treated with carbon, nickel, titanium, silver, or the like. In addition, the thickness of the positive electrode current collector can generally be 3 μm to 500 μm, and the adhesion to the positive electrode active material can be improved by forming fine irregularities on the surface of the current collector. For example, the positive electrode current collector can be formed in various forms such as a film, a sheet, a foil, a mesh, a porous body, a foam, or a nonwoven fabric.

[0056] The positive electrode active material is generally applied to lithium-sulfur secondary batteries, and includes, for example, elemental sulfur (S8) and sulfur-based compounds or mixtures thereof. Specifically, the sulfur-based compound can be Li2S n(n > 1), organic sulfur compounds, or sulfur-carbon compounds ((C2S x ) n : x = 2.5 ~ 50, n > 2), etc. Since elemental sulfur alone does not have electrical conductivity, it can be combined with a carbon material and used in the form of a sulfur-carbon composite.

[0057] The sulfur-carbon composite can have a particle size of 1 μm to 100 μm. If the particle size of the sulfur-carbon composite is less than 1 μm, there is a problem in that the electrical resistance between particles increases and an overvoltage is generated in the electrode of the lithium-sulfur secondary battery. If the particle size exceeds 100 μm, the surface area per unit weight decreases, thereby decreasing the wetting area with the electrolyte and the reaction site with lithium ions in the electrode, and the amount of electron transfer relative to the composite size decreases, so that the reaction can be delayed, thereby reducing the discharge capacity of the battery.

[0058] In the sulfur-carbon composite, the content of sulfur in the sulfur-carbon composite can be 60 wt% to 90 wt%, preferably 70 wt% to 80 wt%, relative to the total weight of the sulfur-carbon composite. If the content of sulfur in the sulfur-carbon composite is less than 60 wt%, there can be a problem in that the energy density of the battery decreases. If the content of sulfur in the sulfur-carbon composite exceeds 90 wt%, there can be a problem in that the electrical conductivity of the electrode decreases and the functionality of the positive active material decreases.

[0059] The carbon material (or sulfur support material) constituting the sulfur-carbon composite has porosity, and in particular, since the carbon material used as the positive active material of the present application has the characteristics of a high specific surface area (3000 m 2 / g or more) and a high porosity (pore volume per unit weight: 0.7 to 3.0 cm 3 / g), it can load a large amount of sulfur.

[0060] The carbon material can be, for example, at least one selected from the group consisting of graphite; graphene; reduced graphene oxide (rGO); carbon black such as deoiled black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal crack black; carbon nanotube (CNT) such as single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT); carbon fiber such as graphite nanofiber (GNF), carbon nanofiber (CNF), and activated carbon fiber (ACF); and activated carbon, and its shape can be in the form of a sphere, a rod, a needle, a plate, a tube, or a block.

[0061] The content of the positive electrode active material in the positive electrode active material layer can be 80 to 99% by weight, preferably 85 to 95% by weight, relative to the total weight of the positive electrode active material layer. If the content of the positive electrode active material in the positive electrode active material layer is less than 80% by weight, there can be a problem in that the energy density of the battery decreases. If the content of the positive electrode active material in the positive electrode active material layer exceeds 99% by weight, there can be a problem in that the binding force between the positive electrode active materials can decrease because of the insufficient content of the binder, and the conductivity in the electrode decreases because of the insufficient content of the conductive material.

[0062] The conductive material is used to impart conductivity to the electrode, and can be used without any particular limitation as long as it has electron conductivity and does not cause chemical changes in the resulting battery. Specific examples thereof can 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 crack black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and one or a mixture of two or more of these substances can be used.

[0063] The content of the conductive material in the positive electrode active material layer can be 0.1 to 15% by weight, preferably 0.5 to 10% by weight, relative to the total weight of the positive electrode active material layer. If the content of the conductive material in the positive electrode active material layer is less than 0.1% by weight, there can be a problem in that the conductivity in the electrode decreases because of the insufficient content of the conductive material. If the content of the conductive material in the positive electrode active material layer exceeds 15% by weight, there can be a problem in that the discharge capacity and the energy density of the battery decrease because the amount of the positive electrode active material is relatively small.

[0064] The binder is used to improve the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples thereof can include polyvinylidene fluoride (PVDF), vinylidene-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one or a mixture of two or more of these substances can be used.

[0065] The content of the binder in the positive active material layer can be 0.1 to 15% by weight, preferably 0.5 to 10% by weight, relative to the total weight of the positive active material layer. If the content of the binder in the positive active material layer is less than 0.1% by weight, there can be a problem in that the binding force between the positive active materials can be reduced due to the insufficient content of the binder. If the content of the binder in the positive active material layer exceeds 15% by weight, there can be a problem in that the discharge capacity and the energy density of the battery are reduced due to the relatively small amount of the positive active material.

[0066] The positive active material, the binder, the conductive material, etc. are dispersed in a dispersion medium (solvent) and mixed to form a slurry, and the slurry can be coated on a positive current collector, followed by drying and calendering to prepare a positive electrode. The dispersion medium can be, but is not limited to, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol, isopropanol, water, or a mixture thereof.

[0067] The positive electrode of the lithium-sulfur secondary battery according to the present application can have a lower porosity than the positive electrode of the general lithium-sulfur secondary battery in the related art. Here, the porosity is the ratio of the pore volume to the total volume of the positive electrode, which is generally expressed in percentage. If the porosity of the positive electrode of the general lithium-sulfur secondary battery is low, it can not easily move the material with the penetration of the electrolyte, so it can not normally exhibit the performance of the battery. Even so, if the porosity of the positive electrode of the lithium-sulfur secondary battery is increased, since the volume of the positive electrode is increased in order to load the same amount of the positive active material, this can be undesirable. If the electrolyte according to the present application is applied to the lithium-sulfur secondary battery, this can be more preferable because the performance of the battery can be properly achieved in the positive electrode having a high porosity as well as in the positive electrode having a low porosity. According to one embodiment of the present application, the porosity of the positive active material layer in the positive electrode is 30% or more and less than 70%, preferably 50 to 65%, more preferably 55 to 65%. The porosity of less than 70% is lower than the porosity of the positive active material layer of the positive electrode of the typical lithium-sulfur secondary battery in the related art. If the performance of the battery can be properly achieved at the related porosity, there is an advantage in that the volume of the positive electrode can be reduced for loading the same amount of the positive active material. The porosity can be measured by a method commonly used in the related art field. The thickness of the positive active material layer is measured by an apparatus for measuring the thickness of a material (TESA, u-hite), and then the porosity is calculated using the true density of the positive active material layer measured by an apparatus for measuring the true density of a material (Microtrac, BEL Pycno).

[0068] The positive electrode of the lithium-sulfur secondary battery according to the present application can have a higher positive electrode active material loading than the positive electrode of a general lithium-sulfur secondary battery in the related art. Generally, if the loading of the positive electrode active material increases, the volume of the positive electrode inevitably increases. However, in the case of the lithium-sulfur secondary battery according to the present application, because the porosity of the positive electrode can be reduced as described above, a high positive electrode active material loading can be maintained even with a relatively small volume. According to one embodiment of the present application, the positive electrode active material loading of the positive electrode is 3.0 mAh / cm 2 to 10.0 mAh / cm 2 , preferably 3.5 mAh / cm 2 to 7.0 mAh / cm 2 , more preferably 4.0 mAh / cm 2 to 7.0 mAh / cm 2 . In theory, if the loading of the positive electrode active material increases, it can help improve the performance of the battery, but there is a limit in increasing the loading of the positive electrode active material due to problems caused by the increase in the volume of the electrode and the difference between the theoretical discharge capacity and the actual discharge capacity. The loading of the positive electrode active material is calculated by dividing the theoretical discharge capacity (mAh) of the positive electrode active material loaded on the positive electrode by the area (cm 2 ) of the surface of the positive electrode active material layer in contact with the positive electrode current collector. For example, in the case of sulfur, it has a theoretical specific discharge capacity of 1675 mAh / g, and by multiplying the theoretical specific discharge capacity by the mass (g) of the sulfur loaded on the positive electrode, the theoretical discharge capacity of the sulfur can be calculated.

[0069] The negative electrode constituting the lithium-sulfur secondary battery according to the present application includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector.

[0070] The negative electrode active material layer includes a negative electrode active material, a binder, and a conductive material. The negative electrode active material can be a material capable of reversibly intercalating or deintercalating lithium ions (Li + ), a material capable of reacting with lithium ions to reversibly form a lithium-containing compound, lithium metal, or a lithium alloy. The material capable of reversibly intercalating or deintercalating lithium ions (Li + ) can be, for example, crystalline carbon, amorphous carbon, or a mixture thereof. The material capable of reacting with lithium ions (Li + ) to reversibly form a lithium-containing compound can be, for example, tin oxide, titanium nitrate, or silicon. The lithium alloy can be, for example, lithium (Li) and 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).

[0071] The binder, the conductive material, and the negative current collector can be selected with reference to the above-described configuration of the positive electrode, but are not necessarily limited thereto. Furthermore, the method of forming the negative active material layer on the negative current collector is based on a coating method as known in the positive electrode, and is not particularly limited.

[0072] The separator for the lithium-sulfur secondary battery according to the present application is a physical separator having a function of physically separating electrodes. The separator can be used without particular limitation as long as it can be used as a conventional separator. In particular, a separator that exhibits a low resistance to ion migration of an electrolyte while having excellent electrolyte retention capability is preferable. The separator can transport lithium ions between the positive electrode and the negative electrode while separating or insulating the positive electrode and the negative electrode from each other. Such a separator can be made of a porous, non-conductive or insulating material having a porosity of 30 to 50%. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, and the like, can be used, and a non-woven fabric made of a high-melting glass fiber or the like can be used. Among them, a porous polymer film is preferably used.

[0073] If a polymer film is used for both the buffer layer and the separator, the impregnation amount of the electrolyte and the ion conduction properties decrease, and the effects of reducing the overvoltage and improving the capacity characteristics become insignificant. In contrast, if a non-woven fabric material is used for both the buffer layer and the separator, the mechanical rigidity cannot be ensured, and thus the problem of battery short circuit occurs. However, if a film-type separator and a polymer non-woven fabric buffer layer are used together, the mechanical strength can be ensured while having the effect of improving the battery performance due to the adoption of the buffer layer.

[0074] According to one embodiment of the present application, an ethylene homopolymer (polyethylene) polymer film is used as the separator, and a polyimide non-woven fabric is used as the buffer layer. In this case, it is preferable that the polyethylene polymer film has a thickness of 10 to 25 μm and a porosity of 40 to 50%.

[0075] The lithium-sulfur secondary battery of the present application can be manufactured by disposing a separator between the positive electrode and the negative electrode to form an electrode assembly, inserting the electrode assembly into a cylindrical battery case or a rectangular battery case, and then injecting an electrolyte. Alternatively, the lithium-sulfur secondary battery of the present application can be manufactured by laminating an electrode assembly, impregnating the electrode assembly with an electrolyte, placing the resulting product in a battery case, and then sealing it.

[0076] Hereinafter, preferred examples will be described in order to facilitate the understanding of the present application. However, the following examples are provided in order to facilitate the understanding of the present application, but the present application is not limited thereto.

[0077] Preferred Embodiments

[0078] Example

[0079] Example 1

[0080] To a solvent obtained by mixing dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) at a volume ratio of 3:7, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, LiN(CF3SO2)2) was added at a concentration of 1.0 M and mixed, and then 500 ppm of ethylene sulfite was added to the mixture to prepare an electrolyte for a lithium-sulfur secondary battery.

[0081] A slurry composition for a positive electrode was prepared by mixing 90 parts by weight of a sulfur-carbon composite material (weight ratio of S:C = 75:25) as a positive electrode active material (in the sulfur-carbon composite material, activated carbon having a pore volume of 1.8 cm 3 / g was used), 5 parts by weight of deo black as a conductive material, and 5 parts by weight of butadiene-styrene rubber / carboxymethyl cellulose (SBR:CMC = 7:3) as a binder. The prepared slurry composition was coated on an aluminum foil current collector, dried at 50°C for 12 hours, and compressed with a calender device to prepare a positive electrode (at this time, the loading amount was 4.0 mAh / cm 2 , and the porosity of the positive electrode active material layer in the positive electrode was 65%).

[0082] A lithium-sulfur secondary battery of a coin cell type was manufactured by placing the prepared positive electrode and a lithium metal negative electrode having a thickness of 60 μm facing each other, interposing a polyethylene (PE) separator therebetween, and injecting the prepared electrolyte. On the other hand, in the manufacture of the lithium-sulfur secondary battery, the positive electrode was punched into a circular electrode of φ 14 to be used, the polyethylene separator was punched into a separator of φ 19 to be used, and the lithium metal was punched into a negative electrode of φ 16 to be used.

[0083] Comparative Example 1

[0084] A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that ethylene sulfite was not added when the electrolyte was manufactured.

[0085] Comparative Example 2

[0086] A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that 5000 ppm of ethylene sulfite was added when the electrolyte was manufactured.

[0087] Comparative Example 3

[0088] A lithium secondary battery was prepared in the same manner as in Example 1, except that the positive electrode was manufactured by the following method when the positive electrode was manufactured.

[0089] LiNi0.8Co0.1Mn0.1O2(NCM 811) as a positive electrode active material, 5 parts by weight of Super-P as a conductive material, and 5 parts by weight of polyvinylidene fluoride (PVDF) as a binder were mixed to prepare a slurry composition for a positive electrode. The prepared slurry composition was coated on an aluminum foil current collector, dried at 50°C for 12 hours, and compressed with a calender device to prepare a positive electrode (at this time, the loading amount was 3.0 mAh / cm2, and the porosity of the positive electrode active material layer in the positive electrode was 30%). 0.6 Co 0.2 Mn 0.2 LiNi0.8Co0.1Mn0.1O2(NCM 811) as a positive electrode active material, 5 parts by weight of Super-P as a conductive material, and 5 parts by weight of polyvinylidene fluoride (PVDF) as a binder were mixed to prepare a slurry composition for a positive electrode. The prepared slurry composition was coated on an aluminum foil current collector, dried at 50°C for 12 hours, and compressed with a calender device to prepare a positive electrode (at this time, the loading amount was 3.0 mAh / cm2, and the porosity of the positive electrode active material layer in the positive electrode was 30%). 2

[0090] Comparative Example 4

[0091] A lithium secondary battery was prepared in the same manner as in Comparative Example 3, except that ethylene sulfite was not added when preparing the electrolyte.

[0092] Comparative Example 5

[0093] A lithium secondary battery was prepared in the same manner as in Comparative Example 3, except that 5000 ppm of ethylene sulfite was added when preparing the electrolyte.

[0094] Comparative Example 6

[0095] A lithium secondary battery was prepared in the same manner as in Comparative Example 3, except that the electrolyte was prepared by the following method when preparing the electrolyte.

[0096] To a solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 3:7, lithium hexafluorophosphate (LiPF6) having a concentration of 1.0 M was added and mixed, and then 500 ppm of ethylene sulfite was added to the mixture to prepare an electrolyte for a lithium secondary battery.

[0097] Comparative Example 7

[0098] A lithium secondary battery was prepared in the same manner as in Comparative Example 6, except that ethylene sulfite was not added when preparing the electrolyte.

[0099] Comparative Example 8

[0100] A lithium secondary battery was prepared in the same manner as in Comparative Example 6, except that 5000 ppm of ethylene sulfite was added when preparing the electrolyte.

[0101] Example 2

[0102] A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that 1,3-propane sultone was added instead of ethylene sulfite when preparing the electrolyte.

[0103] ​Comparative Example 9

[0104] A lithium-sulfur secondary battery was prepared in the same manner as in Example 2, except that 1,3-propane sultone was not added when preparing the electrolyte.

[0105] Comparative Example 10

[0106] A lithium-sulfur secondary battery was prepared in the same manner as in Example 2, except that 5000 ppm of 1,3-propane sultone was added when preparing the electrolyte.

[0107] Comparative Example 11

[0108] A lithium secondary battery was prepared in the same manner as in Example 2, except that the cathode was prepared by the following method when manufacturing the cathode.

[0109] 90 parts by weight of LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM 622) as a cathode active material, 5 parts by weight of Super-P as a conductive material, and 5 parts by weight of polyvinylidene fluoride (PVDF) as a binder were mixed to prepare a slurry composition for a cathode. The prepared slurry composition was coated on an aluminum foil current collector, dried at 50°C for 12 hours, and compressed with a calender device to prepare a cathode (at this time, the loading amount was 3.0 mAh / cm 2 , and the porosity of the cathode active material layer in the cathode was 30%).

[0110] Comparative Example 12

[0111] A lithium secondary battery was prepared in the same manner as in Comparative Example 11, except that 1,3-propane sultone was not added when preparing the electrolyte.

[0112] Comparative Example 13

[0113] A lithium secondary battery was prepared in the same manner as in Comparative Example 11, except that 5000 ppm of 1,3-propane sultone was added when preparing the electrolyte.

[0114] Comparative Example 14

[0115] A lithium secondary battery was prepared in the same manner as in Comparative Example 11, except that the electrolyte was prepared by the following method when preparing the electrolyte.

[0116] To a solvent obtained by mixing ethylene carbonate (EC) and ethylmethyl carbonate (EMC) at a volume ratio of 3:7, lithium hexafluorophosphate (LiPF6) having a concentration of 1.0 M was added and mixed, and then 500 ppm of 1,3-propane sultone was added to the mixture to prepare an electrolyte for a lithium secondary battery.

[0117] Comparative Example 15

[0118] A lithium secondary battery was prepared in the same manner as in Comparative Example 14, except that 1,3-propane sultone was not added when preparing the electrolyte.

[0119] Comparative Example 16

[0120] A lithium secondary battery was prepared in the same manner as in Comparative Example 14, except that 5000 ppm of 1,3-propane sultone was added when preparing the electrolyte.

[0121] Example 3

[0122] A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that 1,3-propene sultone was added when preparing the electrolyte instead of ethylene sulfite.

[0123] Comparative Example 17

[0124] A lithium-sulfur secondary battery was prepared in the same manner as in Example 3, except that 1,3-propene sultone was not added when preparing the electrolyte.

[0125] Comparative Example 18

[0126] A lithium-sulfur secondary battery was prepared in the same manner as in Example 3, except that 5000 ppm of 1,3-propene sultone was added when preparing the electrolyte.

[0127] Comparative Example 19

[0128] A lithium secondary battery was prepared in the same manner as in Example 3, except that the positive electrode was manufactured by the following method when manufacturing the positive electrode.

[0129] 90 parts by weight of LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM 622) as a positive electrode active material, 5 parts by weight of Super-P as a conductive material, and 5 parts by weight of polyvinylidene fluoride (PVDF) as a binder were mixed to prepare a slurry composition for a positive electrode. The prepared slurry composition was coated on an aluminum foil current collector, dried at 50°C for 12 hours, and compressed with a calender device to prepare a positive electrode (at this time, the loading amount was 3.0 mAh / cm 2 , and the porosity of the positive electrode active material layer in the positive electrode was 30%).

[0130] Comparative Example 20

[0131] A lithium secondary battery was prepared in the same manner as in Comparative Example 19, except that 1,3-propene sultone was not added when preparing the electrolyte.

[0132] Comparative Example 21

[0133] A lithium secondary battery was prepared in the same manner as in Comparative Example 19, except that 5000 ppm of 1,3-propene sultone was added when preparing the electrolyte.

[0134] Comparative Example 22

[0135] A lithium secondary battery was prepared in the same manner as in Comparative Example 19, except that the electrolyte was prepared by the following method when preparing the electrolyte.

[0136] To a solvent obtained by mixing ethylene carbonate (EC) and ethylmethyl carbonate (EMC) at a volume ratio of 3:7, lithium hexafluorophosphate (LiPF6) having a concentration of 1.0 M was added and mixed, and then 500 ppm of 1,3-propene sultone was added to the mixture to prepare an electrolyte for a lithium secondary battery.

[0137] Comparative Example 23

[0138] A lithium secondary battery was prepared in the same manner as in Comparative Example 22, except that 1,3-propene sultone was not added when preparing the electrolyte.

[0139] Comparative Example 24

[0140] A lithium secondary battery was prepared in the same manner as in Comparative Example 22, except that 5000 ppm of 1,3-propene sultone was added when preparing the electrolyte.

[0141] Example 4

[0142] A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that ethylene sulfate was added instead of ethylene sulfite when preparing the electrolyte.

[0143] Comparative Example 25

[0144] A lithium-sulfur secondary battery was prepared in the same manner as in Example 4, except that ethylene sulfate was not added when preparing the electrolyte.

[0145] Comparative Example 26

[0146] A lithium-sulfur secondary battery was prepared in the same manner as in Example 4, except that 5000 ppm of ethylene sulfate was added when preparing the electrolyte.

[0147] Comparative Example 27

[0148] A lithium secondary battery was prepared in the same manner as in Example 4, except that the cathode was manufactured by the following method when manufacturing the cathode.

[0149] LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM 622), 5 parts by weight of Super-P as a conductive material, and 5 parts by weight of polyvinylidene fluoride (PVDF) as a binder were mixed to prepare a slurry composition for a positive electrode. The prepared slurry composition was coated on an aluminum foil current collector, dried at 50°C for 12 hours, and compressed with a calender device to prepare a positive electrode (at this time, the loading amount was 3.0 mAh / cm 2 , and the porosity of the positive electrode active material layer in the positive electrode was 30%).

[0150] Comparative Example 28

[0151] A lithium secondary battery was prepared in the same manner as in Comparative Example 27, except that no ethylene sulfate was added when preparing the electrolyte.

[0152] Comparative Example 29

[0153] A lithium secondary battery was prepared in the same manner as in Comparative Example 27, except that 5000 ppm of ethylene sulfate was added when preparing the electrolyte.

[0154] Comparative Example 30

[0155] A lithium secondary battery was prepared in the same manner as in Comparative Example 27, except that the electrolyte was prepared by the following method when preparing the electrolyte.

[0156] To a solvent obtained by mixing ethylene carbonate (EC) and ethylmethyl carbonate (EMC) at a volume ratio of 3:7, lithium hexafluorophosphate (LiPF6) having a concentration of 1.0 M was added and mixed, and then 500 ppm of ethylene sulfate was added to the mixture to prepare an electrolyte for a lithium secondary battery.

[0157] Comparative Example 31

[0158] A lithium secondary battery was prepared in the same manner as in Comparative Example 30, except that no ethylene sulfate was added when preparing the electrolyte.

[0159] Comparative Example 32

[0160] A lithium secondary battery was prepared in the same manner as in Comparative Example 30, except that 5000 ppm of ethylene sulfate was added when preparing the electrolyte.

[0161] Experimental Example: Evaluation of cycle characteristics of manufactured batteries

[0162] Experimental Example 1: Evaluation of ethylene sulfite

[0163] The lithium-sulfur secondary battery manufactured in Example 1 and the lithium secondary batteries manufactured in Comparative Examples 1 to 8 were charged and discharged at a rate of 0.3C to evaluate the cycle performance of the batteries. Considering the optimum conditions depending on the battery type, the charge and discharge voltage ranges of the lithium-sulfur secondary battery and the lithium secondary batteries were set to 1.0-3.6V and 2.7-4.4V, respectively, and the cycle performance of the batteries was evaluated under temperature conditions of 25°C. The cycle performance of the batteries was evaluated in terms of the number of cycles at which the batteries exhibited a discharge capacity of 80% or more with respect to the initial discharge capacity, and when the number of cycles exceeded the relevant cycle, the discharge capacity decreased to less than 80% with respect to the initial discharge capacity. The evaluation results are shown in Table 1 below.

[0164] Table 1:

[0165] Cycle number Example 1 74 Comparative Example 1 50 Comparative Example 2 36 Comparative Example 3 20 Comparative Example 4 21 Comparative Example 5 23 Comparative Example 6 51 Comparative Example 7 50 Cycle number 47

[0166] It was confirmed from Table 1 above that when 500 ppm of ethylene sulfite was added to the electrolyte of the lithium-sulfur secondary battery (Example 1), the cycle performance was significantly improved compared to the case where ethylene sulfite was not added (Comparative Example 1). However, it was confirmed that when 5000 ppm of ethylene sulfite was added to the electrolyte of the lithium-sulfur secondary battery (Comparative Example 2), the effect of adding ethylene sulfite was not observed, and on the contrary, the cycle performance decreased compared to the case where ethylene sulfite was not added (Comparative Example 1).

[0167] In addition, in the case of a lithium secondary battery using a lithium metal oxide (NCM 622) instead of a sulfur-carbon composite material as a positive active material, an ether-based solvent is decomposed in an electrolyte containing the ether-based solvent as the battery cycles, and thus the cycle performance, which is generally measured, is low. Unlike the lithium-sulfur secondary battery, in the case of the lithium secondary battery, the cycle performance was not significantly improved compared to the case where 500 ppm of ethylene sulfite was added to the electrolyte (Comparative Example 3), the case where ethylene sulfite was not added (Comparative Example 4), and the case where 5000 ppm of ethylene sulfite was added (Comparative Example 5).

[0168] To confirm whether the results of Comparative Examples 3 to 5 were caused by a problem of the solvent of the electrolyte, other experiments were performed using an electrolyte containing a carbonate-based solvent suitable for a lithium secondary battery using a lithium metal oxide (NCM 622) as a positive electrode active material. In the electrolyte containing the carbonate-based solvent, the carbonate-based solvent was hardly decomposed even when cycling was performed, and thus the cycle performance was generally similar to that of the lithium-sulfur secondary battery. However, even in the case of the lithium secondary battery using the electrolyte containing the carbonate-based solvent (Comparative Examples 6 to 8), the cycle performance of the lithium secondary battery was not significantly improved as in the case of the lithium secondary battery using the electrolyte containing the ether-based solvent (Comparative Examples 3 to 5) compared to the case where 500 ppm of ethylene sulfite was added to the electrolyte (Comparative Example 6), the case where no ethylene sulfite was added (Comparative Example 7), and the case where 5000 ppm of ethylene sulfite was added (Comparative Example 8).

[0169] Experimental Example 2: Evaluation of 1,3-propane sultone

[0170] The lithium-sulfur secondary battery manufactured in Example 2 and the lithium secondary batteries manufactured in Comparative Examples 9 to 16 were charged and discharged at a rate of 0.3 C to evaluate the cycle performance of the batteries. Considering the optimum conditions depending on the battery type, the charge and discharge voltage ranges of the lithium-sulfur secondary battery and the lithium secondary batteries were set to 1.0 ~ 3.6 V and 2.7 ~ 4.4 V, respectively, and the cycle performance of the batteries was evaluated under temperature conditions of 25°C. The cycle performance of the batteries was evaluated in terms of the number of cycles at which the discharge capacity showed 80% or more with respect to the initial discharge capacity, and when the number of cycles exceeded the relevant cycle, the discharge capacity decreased to less than 80% with respect to the initial discharge capacity. The evaluation results are shown in Table 2 below.

[0171] Table 2:

[0172] Example 2 Comparative Example 9 73 Comparative Example 10 50 Comparative Example 11 43 Comparative Example 12 20 Comparative Example 13 22 Comparative Example 14 25 Comparative Example 15 49 Cycle number 50 Example 3 47

[0173] It was confirmed from Table 2 above that when 500 ppm of 1,3-propane sultone was added to the electrolyte of the lithium-sulfur secondary battery (Example 2), the cycle performance was significantly improved compared to the case where no 1,3-propane sultone was added (Comparative Example 9). However, it was confirmed that when 5000 ppm of 1,3-propane sultone was added to the electrolyte of the lithium-sulfur secondary battery (Comparative Example 10), the effect of adding 1,3-propane sultone was not observed, and on the contrary, the cycle performance decreased compared to the case where no 1,3-propane sultone was added (Comparative Example 9).

[0174] Further, in the case of a lithium secondary battery using lithium metal oxide (NCM 622) instead of a sulfur-carbon composite as a positive electrode active material, as the battery cycles, an ether-based solvent is decomposed in an electrolyte containing the ether-based solvent, whereby the cycle performance, which is generally measured, is low. Unlike in the case of a lithium-sulfur secondary battery, in the case of a lithium secondary battery, the cycle performance was not significantly improved in the case where 500 ppm of 1,3-propane sultone was added to the electrolyte (Comparative Example 11), in the case where 1,3-propane sultone was not added (Comparative Example 12), and in the case where 5000 ppm of 1,3-propane sultone was added (Comparative Example 13).

[0175] To confirm whether the results of Comparative Examples 11 to 13 were due to a problem with the solvent of the electrolyte, other experiments were performed using an electrolyte containing a carbonate-based solvent suitable for use in a lithium secondary battery using lithium metal oxide (NCM 622) as a positive electrode active material. In the electrolyte containing the carbonate-based solvent, even when cycling was performed, the carbonate-based solvent was hardly decomposed, whereby the cycle performance was generally similar to that of a lithium-sulfur secondary battery. However, even in the case of a lithium secondary battery using an electrolyte containing a carbonate-based solvent (Comparative Examples 14 to 16), the cycle performance of the lithium secondary battery was not significantly improved in the case where 500 ppm of 1,3-propane sultone was added to the electrolyte (Comparative Example 14), in the case where 1,3-propane sultone was not added (Comparative Example 15), and in the case where 5000 ppm of 1,3-propane sultone was added (Comparative Example 16), as in the case of a lithium secondary battery using an electrolyte containing an ether-based solvent (Comparative Examples 11 to 13).

[0176] Experimental Example 3: Evaluation of 1,3-propane sultone

[0177] The lithium-sulfur secondary batteries manufactured in Example 3 and the lithium secondary batteries manufactured in Comparative Examples 17 to 24 were subjected to charge and discharge at a rate of 0.3C to evaluate the cycle performance of the batteries. In consideration of the optimum conditions depending on the type of battery, the charge and discharge voltage ranges of the lithium-sulfur secondary batteries and the lithium secondary batteries were set to 1.0 to 3.6 V and 2.7 to 4.4 V, respectively, and the cycle performance of the batteries was evaluated under temperature conditions of 25°C. The cycle performance of the batteries was evaluated in terms of the number of cycles at which the discharge capacity exhibited 80% or more with respect to the initial discharge capacity, and when the number of cycles exceeded the relevant cycle, the discharge capacity decreased to less than 80% with respect to the initial discharge capacity. The evaluation results are shown in Table 3 below.

[0178] Table 3:

[0179] Comparative Example 17 Comparative Example 18 71 Comparative Example 19 50 Comparative Example 20 48 Comparative Example 21 20 Comparative Example 22 19 Comparative Example 23 18 Cycle number 50 Example 4 50 Comparative Example 25 48

[0180] It was confirmed from Table 3 above that when 500 ppm of 1,3-propene sultone was added to the electrolyte of the lithium-sulfur secondary battery (Example 3), the cycle performance was significantly improved compared to the case where 1,3-propene sultone was not added (Comparative Example 17). However, it was confirmed that when 5000 ppm of 1,3-propene sultone was added to the electrolyte of the lithium-sulfur secondary battery (Comparative Example 18), the effect of adding 1,3-propene sultone was not observed, and on the contrary, the cycle performance was decreased compared to the case where 1,3-propene sultone was not added (Comparative Example 17).

[0181] In addition, in the case of a lithium secondary battery using a lithium metal oxide (NCM 622) instead of a sulfur-carbon composite as a positive active material, as the battery cycles proceed, ether-based solvents are decomposed in an electrolyte containing ether-based solvents, and thus the cycle performance, which is generally measured, is low. Unlike in the case of a lithium-sulfur secondary battery, in the case of a lithium secondary battery, the cycle performance was not significantly improved compared to the case where 500 ppm of 1,3-propene sultone was added to the electrolyte (Comparative Example 19), the case where 1,3-propene sultone was not added (Comparative Example 20), and the case where 5000 ppm of 1,3-propene sultone was added (Comparative Example 21).

[0182] In order to confirm whether the results of Comparative Examples 19 to 21 were caused by a problem of the solvent of the electrolyte, other experiments were performed using an electrolyte containing a carbonate-based solvent suitable for a lithium secondary battery using a lithium metal oxide (NCM 622) as a positive active material. In the electrolyte containing a carbonate-based solvent, the carbonate-based solvent is hardly decomposed even when cycling is performed, and thus the cycle performance is generally similar to that of a lithium-sulfur secondary battery. However, even in the case of a lithium secondary battery using an electrolyte containing a carbonate-based solvent (Comparative Examples 22 to 24), the cycle performance of the lithium secondary battery was not significantly improved compared to the case where 500 ppm of 1,3-propene sultone was added to the electrolyte (Comparative Example 22), the case where 1,3-propene sultone was not added (Comparative Example 23), and the case where 5000 ppm of 1,3-propene sultone was added (Comparative Example 24), as in the case of a lithium secondary battery using an electrolyte containing an ether-based solvent (Comparative Examples 19 to 21).

[0183] Experimental Example 4: Evaluation of ethylene sulfate

[0184] The lithium-sulfur secondary batteries manufactured in Example 4 and the lithium secondary batteries manufactured in Comparative Examples 25 to 32 were charged and discharged at a rate of 0.3C to evaluate the cycle performance of the batteries. Considering the optimum conditions depending on the battery type, the charge and discharge voltage ranges of the lithium-sulfur secondary batteries and the lithium secondary batteries were set to 1.0 ~ 3.6 V and 2.7 ~ 4.4 V, respectively, and the cycle performance of the batteries was evaluated under temperature conditions of 25°C. The cycle performance of the batteries was evaluated in terms of the number of cycles at which the batteries exhibited a discharge capacity of 80% or more with respect to the initial discharge capacity, and when the number of cycles exceeded the relevant cycle, the discharge capacity decreased to less than 80% with respect to the initial discharge capacity. The evaluation results are shown in Table 4 below.

[0185] Table 4:

[0186] Comparative Example 26 Comparative Example 27 73 Comparative Example 28 50 Comparative Example 29 40 Comparative Example 30 20 Comparative Example 31 20 Comparative Example 32 17 ​ 48 ​ 50 ​ 45

[0187] It was confirmed from Table 4 above that when 500 ppm of ethylene sulfate was added to the electrolyte of the lithium-sulfur secondary battery (Example 4), the cycle performance was significantly improved compared to the case where ethylene sulfate was not added (Comparative Example 25). However, it has been confirmed that when 5000 ppm of ethylene sulfate was added to the electrolyte of the lithium-sulfur secondary battery (Comparative Example 26), the effect of adding ethylene sulfate was not observed, and on the contrary, the cycle performance decreased compared to the case where ethylene sulfate was not added (Comparative Example 25).

[0188] In addition, in the case of a lithium secondary battery using a lithium metal oxide (NCM 622) instead of a sulfur-carbon composite material as a positive electrode active material, an ether-based solvent is decomposed in an electrolyte containing the ether-based solvent as the battery cycles, and thus the cycle performance, which is generally measured, is low. Unlike the lithium-sulfur secondary battery, in the case of the lithium secondary battery, the cycle performance was not significantly improved compared to the case where 500 ppm of ethylene sulfate was added to the electrolyte (Comparative Example 27), the case where ethylene sulfate was not added (Comparative Example 28), and the case where 5000 ppm of ethylene sulfate was added (Comparative Example 29).

[0189] To confirm whether the results of Comparative Examples 27 to 29 were caused by a problem of the solvent of the electrolyte, other experiments were performed using an electrolyte containing a carbonate-based solvent suitable for a lithium secondary battery using lithium metal oxide (NCM 622) as a positive electrode active material. In the electrolyte containing the carbonate-based solvent, the carbonate-based solvent was hardly decomposed even when cycling was performed, and thus the cycle performance was generally similar to that of a lithium-sulfur secondary battery. However, even in the case of the lithium secondary battery using the electrolyte containing the carbonate-based solvent (Comparative Examples 30 to 32), the cycle performance of the lithium secondary battery was not significantly improved as in the case of the lithium secondary battery using the electrolyte containing the ether-based solvent (Comparative Examples 27 to 29) compared to the case where 500 ppm of ethylene sulfate was added to the electrolyte (Comparative Example 30), the case where ethylene sulfate was not added (Comparative Example 31), and the case where 5000 ppm of ethylene sulfate was added (Comparative Example 32).

[0190] All simple variations and changes of the present application fall within the scope of the present application, and the specific scope of protection of the present application is clarified by the claims.

Claims

1. A lithium-sulfur secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte contains an S-O-based cyclic compound, and the S-O-based cyclic compound is ethylene sulfite, 1,3-propane sulfite, 1,3-propene sulfite, ethylene sulfate, or a combination thereof, wherein the electrolyte further contains a non-aqueous solvent and a lithium salt, wherein the content of the S-O-based cyclic compound in the electrolyte is more than 0 ppm and less than 1000 ppm relative to the total weight of the electrolyte, wherein the non-aqueous solvent comprises 50 to 99 weight percent of a fluorinated linear ether relative to the total weight of the non-aqueous solvent.

2. The lithium-sulfur secondary battery according to claim 1, wherein the fluorinated linear ether is selected from the group consisting of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, bis(fluoromethyl) ether, 2-fluoroethyl methyl 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, and combinations thereof.

3. The lithium-sulfur secondary battery according to claim 1, wherein the lithium salt is selected from the group consisting of LiN(FSO2)2, LiSCN, LiN(CN)2, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiPF6, LiF, LiCl, LiBr, LiI, LiNO3, LiClO4, LiAlO4, LiAlCl4, LiSbF6, LiAsF6, LiBF2C2O4, LiBC4O8, Li(CF3)2PF4, Li(CF3)3PF3, Li(CF3)4PF2, Li(CF3)5PF, Li(CF3)6P, LiCF3SO3, LiC4F9SO3, LiCF3CF2SO3, LiCF3CF2(CF3)2CO, Li(CF3SO2)2CH, LiCF3(CF2)7SO3, LiCF3CO2, LiCH3CO2, and combinations thereof.

4. The lithium-sulfur secondary battery according to claim 1, wherein the positive electrode comprises a positive electrode active material layer having a porosity of 30% or more and less than 70%.

5. The lithium-sulfur secondary battery according to claim 1, wherein the positive electrode has a positive electrode active material loading of 3.0 mAh / cm 2 to 10.0 mAh / cm 2 2.

6. The lithium-sulfur secondary battery according to claim 1, wherein the positive electrode comprises a sulfur-carbon composite material as a positive electrode active material.

7. The lithium-sulfur secondary battery according to claim 6, wherein the sulfur-carbon composite material comprises 60 to 90 weight percent of sulfur relative to the total weight of the sulfur-carbon composite material.

8. The lithium-sulfur secondary battery according to claim 1, wherein the non-aqueous solvent further comprises a non-fluorinated linear ether, a cyclic ether, a polyether, or a mixture thereof.

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