Lithium-sulfur secondary battery comprising an electrolyte containing a cyclic carbonate

Abstract

The present invention provides a lithium-sulfur secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The electrolyte contains a cyclic carbonate, which is fluoroethylene carbonate, vinylene carbonate, or a combination thereof. The content of the cyclic carbonate in the electrolyte is greater than 0 ppm and less than 1000 ppm relative to the total weight of the electrolyte. Because the lithium-sulfur secondary battery contains the specific cyclic carbonate in the electrolyte, the cycling performance of the lithium-sulfur secondary battery is improved.

Description

Technical Field

[0001] The present invention relates to a lithium-sulfur secondary battery comprising an electrolyte containing a cyclic carbonate, and more particularly to a lithium-sulfur secondary battery comprising an electrolyte containing fluoroethylene carbonate, vinylene carbonate, or a combination thereof as a cyclic carbonate.

[0002] This application claims the benefit of priority based on Korean Patent Application Nos. 10-2020-0142338 and 10-2020-0142351, filed on October 29, 2020, which are hereby incorporated by reference in their entirety. Background Art

[0003] As the application of secondary batteries expands to electric vehicles (EVs) and energy storage systems (ESSs), lithium-ion secondary batteries, with their relatively low energy storage density relative to weight (~250Wh / kg), have limited their application in these products. Alternatively, lithium-sulfur secondary batteries, which theoretically can achieve high energy storage density relative to weight (~2600Wh / kg), are attracting attention as the next-generation secondary battery technology.

[0004] Lithium-sulfur secondary batteries use sulfur-sulfur (SS) bonds as the positive electrode active material and lithium metal as the negative electrode active material. Sulfur, the main active material for the positive electrode, is abundant, non-toxic, and has a low atomic weight.

[0005] In a lithium-sulfur secondary battery, when the battery is discharged, lithium, which is the negative electrode active material, is oxidized and thereby ionized while releasing electrons, and the sulfur-based material, which is the 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 SS bond accepts two electrons and is converted into a sulfur anion form. The lithium cations generated by the oxidation reaction of lithium are transferred to the positive electrode through the electrolyte and combined with the sulfur anions generated by the reduction reaction of sulfur to form a salt. Specifically, before discharge, sulfur has a cyclic S8 structure, which is converted into lithium polysulfide (LiS) through the reduction reaction. x When the lithium polysulfide is completely reduced, lithium sulfide (Li2S) is generated.

[0006] Sulfur, as the positive electrode active material, is difficult to reactivity with electrons and lithium ions in solid form due to its low conductivity. In existing lithium-sulfur secondary batteries, in order to improve the reactivity of sulfur, Li2S is generated. xIn the form of intermediate polysulfides, a liquid phase reaction is induced and the reactivity is improved. In this case, an ether solvent such as dioxolane and dimethoxyethane with high solubility for lithium polysulfide is used as a solvent for the electrolyte. In addition, in conventional lithium-sulfur secondary batteries, in order to improve the reactivity, a cathode electrolyte type lithium-sulfur secondary battery system is constructed. In this case, due to the characteristic of lithium polysulfide being dissolved in the electrolyte, the reactivity and life characteristics of sulfur depend on the content of the electrolyte and are affected. In order to achieve high energy density, the electrolyte should be injected under low content conditions, 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 fluidity of the active material and the increase in side reactions.

[0007] In order to manufacture lithium-sulfur secondary batteries with high energy density, a battery system capable of operating high-load, low-porosity electrodes is required, and research on such a battery system continues in the related art.

[0008] Prior art literature

[0009] [Patent Document]

[0010] (Patent Document 1) Korean Patent Unexamined Publication No. 10-2019-0006923 Summary of the Invention

[0011] [Technical Issues]

[0012] An object of the present invention is to provide a lithium-sulfur secondary battery capable of improving the cycle performance of the lithium-sulfur secondary battery by adding a specific cyclic carbonate to the electrolyte of the lithium-sulfur secondary battery.

[0013] [Technical solution]

[0014] The present invention provides a lithium-sulfur secondary battery. The lithium-sulfur secondary battery comprises a positive electrode, a negative electrode, a separator and an electrolyte. The electrolyte contains a cyclic carbonate.

[0015] In one embodiment of the present invention, the cyclic carbonate is fluoroethylene carbonate, vinylene carbonate, or a combination thereof.

[0016] In one embodiment of the present invention, the content of the cyclic carbonate in the electrolyte is greater than 0 ppm and less than 1000 ppm relative to the total weight of the electrolyte.

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

[0018] In one embodiment of the present invention, 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 invention, the non-aqueous solvent contains 50% 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 invention, the lithium salt is selected from the group consisting of: LiN(FSO2)2, LiSCN, Li N(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 invention, the positive electrode includes a positive electrode active material layer having a porosity of 30% or more and less than 70%.

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

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

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

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

[0026] [Beneficial Effects]

[0027] In the lithium-sulfur secondary battery according to the present invention, the cycle performance of the lithium-sulfur secondary battery is improved by adding a specific cyclic carbonate to the electrolyte.

[0028] As a specific cyclic carbonate, less than 1000 ppm of fluoroethylene carbonate or vinylene carbonate is added to the electrolyte of the lithium-sulfur secondary battery. Considering that in the relevant technical field, the desired effect of improving battery performance can be achieved when the electrolyte additive is used in an amount of 1 wt% (10000 ppm) or more, fluoroethylene carbonate or vinylene carbonate is different from common electrolyte additives used in the technical field.

[0029] When fluoroethylene carbonate or vinylene carbonate is used in the same amount as a common electrolyte additive in the related art, the effect of improving the cycle performance of the lithium-sulfur secondary battery is insignificant or almost non-existent.

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

[0031] The embodiments provided according to the present invention can all be implemented through the following description. It should be understood that the following description should be understood as describing preferred embodiments of the present invention, and it should be understood that the present invention 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 according to the measurement conditions and methods generally used by those skilled in the art.

[0033] "Lithium secondary battery" is generally a higher-level concept than lithium-sulfur secondary battery and includes lithium-sulfur secondary batteries. However, the "lithium secondary battery" in this specification refers to a common lithium secondary battery that uses lithium metal oxide as the positive electrode active material and is used separately from lithium-sulfur secondary batteries.

[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 a cyclic carbonate. According to one embodiment of the present invention, the cyclic carbonate is fluoroethylene carbonate (FEC), vinylene carbonate (VC) or a combination thereof. Fluoroethylene carbonate is a compound having a structure of the following formula 1, and vinylene carbonate is a compound having a structure of the following formula 2. The inventors of the present invention have completed the present invention by confirming that when a specific cyclic carbonate is added to the electrolyte of a lithium-sulfur secondary battery using a sulfur-containing material as a positive electrode active material instead of a conventional 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] Considering that in the case of lithium secondary batteries having the same composition except for the positive electrode active material, there is little or no effect on improving the cycle performance of the battery, it is expected that fluoroethylene carbonate or vinylene carbonate added to the electrolyte directly interacts with the positive electrode active material of the lithium-sulfur secondary battery.

[0040] According to one embodiment of the present invention, relative to the gross weight of electrolyte, electrolyte contains more than 0ppm, more than 50ppm, more than 100ppm, more than 150ppm, more than 200ppm, more than 250ppm, more than 300ppm, more than 350ppm, more than 400ppm, more than 450ppm or more cyclic carbonate of 500ppm, and relative to the gross weight of electrolyte, electrolyte contains less than 1000ppm, less than 950ppm, less than 900ppm, less than 850ppm, less than 800ppm, less than 750ppm, less than 700ppm, less than 650ppm, less than 600ppm, less than 550ppm or less 500ppm cyclic carbonate. In the present invention, a feature is that a small amount of cyclic carbonate such as less than 1000ppm is added to electrolyte. If with a large amount of added cyclic carbonate of the amount more than 1000ppm, the effect of improving cycle performance may be very little or have no effect. In the related art, considering that the desired effect of improving battery performance can only be achieved when the amount of electrolyte additives used is generally 1 wt % (10000 ppm) or more, the above-mentioned feature is not common in the art.

[0041] The electrolyte comprising the lithium-sulfur secondary battery according to the present invention includes, in addition to the aforementioned cyclic carbonate, a non-aqueous solvent and a lithium salt. As described above, the use of the cyclic carbonate improves the battery's cycling performance through direct interaction with the cathode active material of the lithium-sulfur secondary battery. Therefore, the types of the non-aqueous solvent and lithium salt are not particularly limited. However, selecting a more suitable non-aqueous solvent and lithium salt for the lithium-sulfur secondary battery can improve the overall cycling performance of the battery.

[0042] According to one embodiment of the present invention, the non-aqueous solvent is an ether solvent, which can be a linear ether, a cyclic ether, a polyether or a mixture thereof.

[0043] The linear ether can be selected from: 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.

[0044] The cyclic ether may be selected from: dioxolane (DOL), methyldioxolane, Alkane, di Alkane, tri The present invention also includes, but is not limited to, alkane, tetrahydrofuran (THF), dihydropyran (DHP), tetrahydropyran (THP), methyltetrahydrofuran, furan, methylfuran, and combinations thereof.

[0045] The polyether may 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.

[0046] The linear ether, cyclic ether and polyether may be fluorinated ether compounds. The fluorinated form of the compound may be a fluorinated linear ether, and the fluorinated linear ether may 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.

[0047] Fluorinated ether compounds can be used in combination with non-fluorinated linear ethers, cyclic ethers, polyethers, or combinations thereof. According to one embodiment of the present invention, the content of the fluorinated ether compound in the electrolyte can be 50% to 99% by weight, preferably 60% to 95% by weight, and more preferably 70% to 90% by weight, relative 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 greater, relative to the total weight of the solvent constituting the electrolyte, the performance of the battery can be improved when used with a positive electrode of a lithium-sulfur secondary battery having a low porosity and a high loading of a positive electrode active material.

[0048] Lithium salts are materials that are easily soluble in non-aqueous solvents and can be selected from: 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.

[0049] The concentration of the lithium salt may be 0.1M to 8.0M, preferably 0.5M to 5.0M, more preferably 1.0M to 3.0M, depending on a variety of factors such as the exact composition of the electrolyte mixture, the solubility of the salt, the conductivity of the dissolved salt, the charging and discharging conditions of the battery, the operating temperature, and other factors known in the field of lithium secondary batteries. If the concentration of the lithium salt is less than the above range, the conductivity of the electrolyte may decrease, thereby deteriorating the performance of the electrolyte. If the concentration of the lithium salt exceeds the above range, the viscosity of the electrolyte may increase, thereby increasing the lithium ion (Li + Therefore, it is preferred to select a suitable lithium salt concentration within the above range.

[0050] The positive electrode constituting the lithium-sulfur secondary battery according to the present invention 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.

[0051] The positive electrode current collector is not particularly limited, as long as it exhibits high conductivity without causing chemical changes in the battery. Examples include stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like. The positive electrode current collector typically has a thickness of 3 to 500 μm, and fine irregularities formed on its surface can enhance adhesion to the positive electrode active material. For example, the positive electrode current collector can be formed into various forms, such as a film, sheet, foil, mesh, porous body, foam, or non-woven fabric.

[0052] The positive electrode active material is generally used in lithium-sulfur secondary batteries and includes, for example, elemental sulfur (S8) and sulfur compounds or mixtures thereof. Specifically, the sulfur compound can be Li2S n (n≥1), organic sulfur compounds or sulfur-carbon compounds ((C2S x ) n : x = 2.5 to 50, n ≥ 2), etc. Since elemental sulfur alone has no electrical conductivity, it can be combined with a carbon material and used in the form of a sulfur-carbon composite material.

[0053] The sulfur-carbon composite material can have a particle size of 1 μm to 100 μm. If the particle size of the sulfur-carbon composite material is less than 1 μm, the resistance between the particles increases, potentially leading to overvoltage in the electrodes of lithium-sulfur secondary batteries. If the particle size exceeds 100 μm, the surface area per unit weight decreases, resulting in a reduced wetted area with the electrolyte and reaction sites with lithium ions in the electrode. Furthermore, the amount of electron transfer relative to the size of the composite material decreases, potentially slowing the reaction and reducing the battery's discharge capacity.

[0054] In the sulfur-carbon composite material, the sulfur content of the sulfur-carbon composite material may be 60% to 90% by weight, preferably 70% to 80% by weight, relative to the total weight of the sulfur-carbon composite material. If the sulfur content of the sulfur-carbon composite material is less than 60% by weight, the energy density of the battery may be reduced. If the sulfur content of the sulfur-carbon composite material exceeds 90% by weight, the conductivity of the electrode may be reduced, and the functionality of the positive electrode active material may be reduced.

[0055] The carbon material (or sulfur support material) constituting the sulfur-carbon composite material has porosity, especially since the carbon material used as the positive electrode active material of the present invention has a high specific surface area (3000 m 2 / g or more) and high porosity (pore volume per unit weight: 0.7 to 3.0 cm 3 / g), so it can load a large amount of sulfur.

[0056] The carbon material may be, for example, at least one selected from the group consisting of graphite; graphene; reduced graphene oxide (rGO); carbon black such as deco black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; carbon nanotubes (CNTs) such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); carbon fibers such as graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs); and activated carbon, and may be in the form of spheres, rods, needles, plates, tubes, or blocks.

[0057] The content of the positive electrode active material in the positive electrode active material layer may 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, the energy density of the battery may be reduced. If the content of the positive electrode active material in the positive electrode active material layer exceeds 99% by weight, the following problems may occur: the binding force between the positive electrode active materials may be reduced due to insufficient binder content; and the conductivity in the electrode may be reduced due to insufficient conductive material content.

[0058] 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 constructed battery. Specific examples thereof may 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, 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 of these substances or a mixture of two or more thereof may be used.

[0059] The content of the conductive material in the positive electrode active material layer may 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 may be a problem of reduced conductivity in the electrode due to 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 may be a problem of reduced discharge capacity and energy density of the battery due to the relatively small amount of positive electrode active material.

[0060] Binders are 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 may include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber or various copolymers thereof, and one of these substances or a mixture of two or more thereof may be used.

[0061] The content of the binder in the positive electrode active material layer may 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 binder in the positive electrode active material layer is less than 0.1% by weight, there may be a problem of reduced bonding strength between the positive electrode active materials due to insufficient binder content. If the content of the binder in the positive electrode active material layer exceeds 15% by weight, there may be a problem of reduced discharge capacity and energy density of the battery due to the relatively small amount of positive electrode active material.

[0062] The positive electrode active material, binder, 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 electrode current collector, followed by drying and rolling 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.

[0063] The positive electrode of a lithium-sulfur secondary battery according to the present invention can have a lower porosity than the positive electrode of a conventional lithium-sulfur secondary battery in the related art. Porosity is the ratio of the pore volume of the positive electrode to the total volume, typically expressed as a percentage. If the porosity of the positive electrode of a conventional lithium-sulfur secondary battery is low, the material may not readily migrate with electrolyte penetration, and thus the battery's performance may not be properly maintained. Even so, increasing the porosity of the positive electrode of a lithium-sulfur secondary battery can be undesirable because the volume of the positive electrode increases to accommodate the same amount of positive electrode active material. Using the electrolyte according to the present invention in a lithium-sulfur secondary battery is more preferable because it allows for adequate battery performance in both high-porosity and low-porosity positive electrodes. According to one embodiment of the present invention, the porosity of the positive electrode active material layer in the positive electrode is 30% or greater and less than 70%, preferably 50% to 65%, and more preferably 55% to 65%. A porosity of less than 70% is lower than the porosity of the positive electrode active material layer of a typical lithium-sulfur secondary battery in the related art. If the performance of the battery can be properly achieved at the relevant porosity, there is an advantage in that the volume of the positive electrode can be reduced for the same amount of positive electrode active material loaded. The porosity can be measured by methods commonly used in the relevant technical field. The thickness of the positive electrode active material layer is measured by an instrument for measuring material thickness (TESA, u-hite), and then the porosity is calculated using the true density of the positive electrode active material layer measured by an instrument for measuring the true density of the material (Microtrac, BELPycno).

[0064] The positive electrode of the lithium-sulfur secondary battery according to the present invention can have a higher positive electrode active material loading than the positive electrode of the conventional 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 invention, because the porosity of the positive electrode can be reduced, as described above, a high positive electrode active material loading can be maintained even at a relatively small volume. According to one embodiment of the present invention, the positive electrode active material loading of the positive electrode is 3.0 mAh / cm 2 Up to 10.0 mAh / cm 2 , preferably 3.5mAh / cm 2 Up to 7.0mAh / cm 2 , more preferably 4.0 mAh / cm 2 Up to 7.0mAh / cm 2Theoretically, if the loading amount of the positive electrode active material is increased, it may help improve the performance of the battery. However, there are limitations in increasing the loading amount of the positive electrode active material due to the increase in electrode volume and the problem of the difference between the theoretical discharge capacity and the actual discharge capacity. The theoretical discharge capacity (mAh) of the positive electrode active material loaded on the positive electrode is divided by the surface area (cm2) of the positive electrode active material layer in contact with the positive electrode current collector. 2 ), calculate the loading amount of the positive electrode active material. For example, in the case of sulfur, it has a theoretical specific discharge capacity of 1675 mAh / g, and the theoretical discharge capacity of sulfur can be calculated by multiplying the theoretical specific discharge capacity by the mass (g) of sulfur loaded on the positive electrode.

[0065] The negative electrode constituting the lithium-sulfur secondary battery according to the present invention includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector.

[0066] The negative electrode active material layer comprises a negative electrode active material, a binder and a conductive material. The negative electrode active material can be: a material capable of reversibly inserting or removing lithium ions (Li + ) materials; materials capable of reacting with lithium ions to reversibly form lithium-containing compounds; lithium metal or lithium alloy. + ) can be, for example, crystalline carbon, amorphous carbon or a mixture thereof. + ) to reversibly form a lithium-containing compound may be, for example, tin oxide, titanium nitrate, or silicon. The lithium alloy may be, for example, an alloy of 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).

[0067] The binder, conductive material and negative electrode current collector can be selected with reference to the configuration of the positive electrode, but are not necessarily limited thereto. In addition, the method of forming the negative electrode active material layer on the negative electrode current collector is based on a coating method known in the positive electrode and is not particularly limited.

[0068] The separator used in the lithium-sulfur secondary battery according to the present invention is a physical separator having the function of physically separating the electrodes. The separator can be used without particular limitation as long as it can be used as a conventional separator. In particular, a separator having excellent electrolyte moisture retention while exhibiting low resistance to ion migration of the electrolyte is preferred. The separator can allow lithium ions to be transferred 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 can be used, for example, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer and an ethylene / methacrylate copolymer, and a non-woven fabric made of a high melting point glass fiber, etc. can be used. Among them, a porous polymer film is preferably used.

[0069] If a polymer film is used for both the buffer layer and the separator, the electrolyte impregnation capacity and ion conductivity are reduced, and the effects of reducing overvoltage and improving capacity characteristics become negligible. Conversely, if a non-woven fabric material is used for both the buffer layer and the separator, mechanical rigidity cannot be guaranteed, leading to battery short circuits. However, if a membrane-type separator is used together with a polymer non-woven fabric buffer layer, the buffer layer improves battery performance while also ensuring mechanical strength.

[0070] According to one embodiment of the present invention, an ethylene homopolymer (polyethylene) polymer film is used as a separator and a polyimide nonwoven fabric is used as a buffer layer. In this case, preferably, the polyethylene polymer film has a thickness of 10 μm to 25 μm and a porosity of 40% to 50%.

[0071] The lithium-sulfur secondary battery of the present invention can be manufactured by placing 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 invention can be manufactured by laminating the electrode assemblies, impregnating the electrode assemblies with the electrolyte, placing the resulting product into a battery case, and then sealing the battery case.

[0072] Hereinafter, in order to facilitate understanding of the present invention, preferred examples will be described. However, the following examples are provided to facilitate understanding of the present invention, but the present invention is not limited thereto.

[0073] Preferred Implementation

[0074] Example

[0075] Example 1

[0076] To a solvent obtained by mixing dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) in a volume ratio of 3:7, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, LiN(CF3SO2)2) at a concentration of 1.0 M was added and mixed, and then 500 ppm of fluoroethylene carbonate was added to the mixture to prepare an electrolyte for a lithium-sulfur secondary battery.

[0077] 90 parts by weight of a sulfur-carbon composite material (weight ratio of S:C = 75:25) as a positive electrode active material (a pore volume of 1.8 cm in the sulfur-carbon composite material was used) was used. 3 / g activated carbon), 5 parts by weight of Deco Black as a conductive material, and 5 parts by weight of styrene-butadiene rubber / carboxymethyl cellulose (SBR:CMC=7:3) as a binder were mixed to prepare a positive electrode slurry composition. The prepared slurry composition was coated on an aluminum foil current collector, dried at 50°C for 12 hours, and compressed using a calendaring device to prepare a positive electrode (at this time, the loading capacity was 4.0 mAh / cm 2 , and the porosity of the positive electrode active material layer in the positive electrode is 65%).

[0078] The prepared positive electrode and a 60μm-thick lithium metal negative electrode were placed facing each other, with a polyethylene (PE) separator inserted between them. The prepared electrolyte was then injected to produce a coin-cell lithium-sulfur secondary battery. Meanwhile, in the manufacture of the lithium-sulfur secondary battery, the positive electrode was punched into a 14-diameter circular electrode, the polyethylene separator was punched into a 19-diameter separator, and the lithium metal was punched into a 16-diameter negative electrode.

[0079] Comparative Example 1

[0080] A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that fluoroethylene carbonate was not added when preparing the electrolyte.

[0081] Comparative Example 2

[0082] A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that 5000 ppm of fluoroethylene carbonate was added when preparing the electrolyte.

[0083] Comparative Example 3

[0084] 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 manufacturing the positive electrode.

[0085] 90 parts by weight of LiNi as the positive electrode active material 0.6 Co 0.2 Mn 0.2O2 (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 positive electrode slurry composition. The prepared slurry composition was coated on an aluminum foil current collector, dried at 50°C for 12 hours, and compressed using a calendaring device to prepare a positive electrode (at this time, the loading capacity was 3.0 mAh / cm 2 , and the porosity of the positive electrode active material layer in the positive electrode is 30%).

[0086] Comparative Example 4

[0087] A lithium secondary battery was prepared in the same manner as in Comparative Example 3, except that fluoroethylene carbonate was not added when preparing the electrolyte.

[0088] Comparative Example 5

[0089] A lithium secondary battery was prepared in the same manner as in Comparative Example 3, except that 5000 ppm of fluoroethylene carbonate was added when preparing the electrolyte.

[0090] Comparative Example 6

[0091] 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 in preparing the electrolyte.

[0092] Lithium hexafluorophosphate (LiPF6) at a concentration of 1.0 M was added to a solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 and mixed, and then 500 ppm of fluoroethylene carbonate was added to the mixture to prepare an electrolyte for a lithium secondary battery.

[0093] Comparative Example 7

[0094] A lithium secondary battery was prepared in the same manner as in Comparative Example 6, except that fluoroethylene carbonate was not added when preparing the electrolyte.

[0095] Comparative Example 8

[0096] A lithium secondary battery was prepared in the same manner as in Comparative Example 6, except that 5000 ppm of fluoroethylene carbonate was added when preparing the electrolyte.

[0097] Example 2

[0098] A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that vinylene carbonate was added instead of fluoroethylene carbonate when preparing the electrolyte.

[0099] Comparative Example 9

[0100] A lithium-sulfur secondary battery was prepared in the same manner as in Example 2, except that vinylene carbonate was not added when preparing the electrolyte.

[0101] Comparative Example 10

[0102] A lithium-sulfur secondary battery was prepared in the same manner as in Example 2, except that 5000 ppm of vinylene carbonate was added when preparing the electrolyte.

[0103] Comparative Example 11

[0104] A lithium secondary battery was prepared in the same manner as in Example 2, except that the positive electrode was manufactured by the following method when manufacturing the positive electrode.

[0105] 90 parts by weight of LiNi as the positive electrode active material 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 positive electrode slurry composition. The prepared slurry composition was coated on an aluminum foil current collector, dried at 50°C for 12 hours, and compressed using a calendaring device to prepare a positive electrode (at this time, the loading capacity was 3.0 mAh / cm 2 , and the porosity of the positive electrode active material layer in the positive electrode is 30%).

[0106] Comparative Example 12

[0107] A lithium secondary battery was prepared in the same manner as in Comparative Example 11, except that vinylene carbonate was not added when preparing the electrolyte.

[0108] Comparative Example 13

[0109] A lithium secondary battery was prepared in the same manner as in Comparative Example 11, except that 5000 ppm of vinylene carbonate was added when preparing the electrolyte.

[0110] Comparative Example 14

[0111] 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 in preparing the electrolyte.

[0112] Lithium hexafluorophosphate (LiPF6) having a concentration of 1.0 M was added to a solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 and mixed, and then 500 ppm of vinylene carbonate was added to the mixture to prepare an electrolyte for a lithium secondary battery.

[0113] Comparative Example 15

[0114] A lithium secondary battery was prepared in the same manner as in Comparative Example 14, except that vinylene carbonate was not added when preparing the electrolyte.

[0115] Comparative Example 16

[0116] A lithium secondary battery was prepared in the same manner as in Comparative Example 14, except that 5000 ppm of vinylene carbonate was added when preparing the electrolyte.

[0117] Experimental Example: Evaluation of Cycling Performance of Manufactured Batteries

[0118] Experimental Example 1: Evaluation of Fluoroethylene Carbonate

[0119] 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 battery. Taking into account the optimal conditions depending on the battery type, the charging and discharging voltage ranges of the lithium-sulfur secondary battery and the lithium secondary battery were set to 1.0-3.6V and 2.7-4.4V, respectively, and the cycle performance of the battery was evaluated under a temperature condition of 25°C. The cycle performance of the battery was evaluated by the number of cycles at which it exhibited a discharge capacity of 80% or more relative to the initial discharge capacity, and when the number of relevant cycles was exceeded, the discharge capacity decreased to less than 80% relative to the initial discharge capacity. The evaluation results are shown in Table 1 below.

[0120] Table 1:

[0121] Number of cycles Example 1 75 Comparative Example 1 50 Comparative Example 2 44 Comparative Example 3 20 Comparative Example 4 20 Comparative Example 5 21 Comparative Example 6 49 Comparative Example 7 50 Comparative Example 8 54

[0122] Table 1 above confirms that when 500 ppm of fluoroethylene carbonate was added to the electrolyte of a lithium-sulfur secondary battery (Example 1), the cycle performance was significantly improved compared to the case where no fluoroethylene carbonate was added (Comparative Example 1). However, when 5000 ppm of fluoroethylene carbonate was added to the electrolyte of a lithium-sulfur secondary battery (Comparative Example 2), no effect of adding fluoroethylene carbonate was observed. In contrast, the cycle performance was reduced compared to the case where no fluoroethylene carbonate was added (Comparative Example 1).

[0123] Furthermore, in lithium secondary batteries using lithium metal oxide (NCM 622) as the positive electrode active material rather than a sulfur-carbon composite, ether solvents decompose in the electrolyte containing ether solvents as the battery cycles, resulting in generally low cycling performance. Unlike lithium-sulfur secondary batteries, in lithium secondary batteries, cycling performance did not significantly improve when compared to the case where 500 ppm of fluoroethylene carbonate was added to the electrolyte (Comparative Example 3), the case where no fluoroethylene carbonate was added (Comparative Example 4), and the case where 5000 ppm of fluoroethylene carbonate was added (Comparative Example 5).

[0124] In order to confirm whether the results of Comparative Examples 3 to 5 are due to the problem of the solvent of the electrolyte, other experiments were performed using an electrolyte containing a carbonate solvent suitable for a lithium secondary battery using lithium metal oxide (NCM 622) as a positive electrode active material. In an electrolyte containing a carbonate solvent, even if circulated, the carbonate solvent is hardly decomposed, so that 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 solvent (Comparative Examples 6 to 8), compared with the case of adding 500ppm of fluoroethylene carbonate to the electrolyte (Comparative Example 6), the case of not adding fluoroethylene carbonate (Comparative Example 7) and the case of adding 5000ppm of fluoroethylene carbonate (Comparative Example 8), the cycle performance of the lithium secondary battery is not significantly improved, as in the case of a lithium secondary battery using an electrolyte containing an ether solvent (Comparative Examples 3 to 5).

[0125] Experimental Example 2: Evaluation of vinylene carbonate

[0126] 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 0.3C to evaluate the cycle performance of the battery. Taking into account the optimal conditions depending on the battery type, the charging and discharging voltage ranges of the lithium-sulfur secondary battery and the lithium secondary battery were set to 1.0-3.6V and 2.7-4.4V, respectively, and the cycle performance of the battery was evaluated under a temperature condition of 25°C. The cycle performance of the battery was evaluated by the number of cycles at which it exhibited a discharge capacity of 80% or more relative to the initial discharge capacity, and when the number of relevant cycles was exceeded, the discharge capacity decreased to less than 80% relative to the initial discharge capacity. The evaluation results are shown in Table 2 below.

[0127] Table 2:

[0128] Number of cycles Example 2 70 Comparative Example 9 50 Comparative Example 10 43 Comparative Example 11 20 Comparative Example 12 22 Comparative Example 13 19 Comparative Example 14 48 Comparative Example 15 50 Comparative Example 16 52

[0129] Table 2 above confirms that when 500 ppm of vinylene carbonate was added to the electrolyte of a lithium-sulfur secondary battery (Example 2), the cycle performance was significantly improved compared to the case where no vinylene carbonate was added (Comparative Example 9). However, when 5000 ppm of vinylene carbonate was added to the electrolyte of a lithium-sulfur secondary battery (Comparative Example 10), no effect of vinylene carbonate addition was observed. In contrast, the cycle performance was reduced compared to the case where no vinylene carbonate was added (Comparative Example 9).

[0130] Furthermore, in lithium secondary batteries using lithium metal oxide (NCM 622) as the positive electrode active material rather than a sulfur-carbon composite material, ether solvents decompose in the electrolyte containing ether solvents as the battery cycles, resulting in generally low cycling performance. Unlike lithium-sulfur secondary batteries, in lithium secondary batteries, cycling performance did not significantly improve when compared to the case where 500 ppm of vinylene carbonate was added to the electrolyte (Comparative Example 11), the case where no vinylene carbonate was added (Comparative Example 12), and the case where 5000 ppm of vinylene carbonate was added (Comparative Example 13).

[0131] In order to confirm whether the results of Comparative Examples 11 to 13 are due to the problem of the solvent of the electrolyte, other experiments were performed using an electrolyte containing a carbonate solvent suitable for a lithium secondary battery using lithium metal oxide (NCM 622) as a positive electrode active material. In an electrolyte containing a carbonate solvent, the carbonate solvent is hardly decomposed even if circulated, so that 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 solvent (Comparative Examples 14 to 16), compared with the case of adding 500ppm of vinylene carbonate to the electrolyte (Comparative Example 14), the case of not adding vinylene carbonate (Comparative Example 15) and the case of adding 5000ppm of vinylene carbonate (Comparative Example 16), the cycle performance of the lithium secondary battery is not significantly improved, as in the case of a lithium secondary battery using an electrolyte containing an ether solvent (Comparative Examples 11 to 13).

[0132] All simple modifications and variations of the present invention are within the scope of the present invention, and the specific scope of protection of the present invention is set forth 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 a non-aqueous solvent and a cyclic carbonate, wherein the cyclic carbonate is fluoroethylene carbonate, vinylene carbonate or a combination thereof, wherein the content of the cyclic carbonate in the electrolyte is greater than 0 ppm and less than 1000 ppm relative to the total weight of the electrolyte, The non-aqueous solvent is an ether solvent, and the ether solvent contains fluorinated linear ether. 2 . The lithium-sulfur secondary battery according to claim 1 , wherein the electrolyte further comprises a lithium salt.

3. 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. 4 . The lithium-sulfur secondary battery according to claim 1 , wherein the non-aqueous solvent comprises 50 wt % to 99 wt % of the fluorinated linear ether relative to the total weight of the non-aqueous solvent.

5. The lithium-sulfur secondary battery according to claim 2, 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. 6 . 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%.

7. The lithium-sulfur secondary battery according to claim 1, wherein the positive electrode has a capacitance of 3.0 mAh / cm 2 Up to 10.0 mAh / cm 2 of positive electrode active material loading. 8 . 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. 9 . The lithium-sulfur secondary battery according to claim 8 , wherein the sulfur-carbon composite material comprises 60 wt % to 90 wt % of sulfur relative to the total weight of the sulfur-carbon composite material. 10 . 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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