Electrolyte solution for lithium secondary battery and lithium secondary battery comprising the same

Abstract

To develop electrolytes with improved stability for operation of lithium-sulfur batteries in high temperature environments.SOLUTION: An electrolyte solution for a lithium secondary battery includes a lithium salt, a nitrogen compound, and an organic solvent. As the lithium salt, it comprises lithium bis (trifluoromethanesulfonyl) imide and as the organic solvent, it comprises an ether-based solvent, which improves oxidation stability and storage stability at high temperatures.SELECTED DRAWING: Figure 3

Description

[Technical Field]

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

[0002] This application claims priority based on Korean Patent Application No. 2021-0185761 filed on December 23, 2021, and Korean Patent Application No. 2022-0148630 filed on November 9, 2022, and the contents disclosed in the specifications and drawings of those applications are incorporated into this application in their entirety. [Background technology]

[0003] As the range of applications of lithium secondary batteries expands from portable electronic devices to electric vehicles (EVs) and electric storage systems (ESSs), there is an increasing demand for lithium secondary batteries with high capacity, high energy density, and long life.

[0004] Among lithium secondary batteries, lithium-sulfur batteries are battery systems that use sulfur-based materials containing sulfur-sulfur bonds as the positive electrode active material and lithium metal, carbon-based materials in which lithium ions are inserted / extracted, or silicon or tin, which form alloys with lithium, as the negative electrode active material.

[0005] Sulfur, the main positive electrode active material in lithium-sulfur batteries, has the advantages of low atomic weight, abundant availability, low cost, non-toxicity, and environmental friendliness.

[0006] In addition, lithium-sulfur batteries undergo a conversion reaction between lithium ions and sulfur (S8+16Li + +16e -The theoretical specific capacity of the lithium secondary battery (Li-FeS battery) is 1,675mAh / g, and when lithium metal is used as the anode, it exhibits a theoretical energy density of 2,600Wh / kg. This is significantly higher than the theoretical energy densities of other battery systems currently being researched (Ni-MH battery: 450Wh / kg, Li-FeS battery: 480Wh / kg, Li-MnO2 battery: 1,000Wh / kg, Na-S battery: 800Wh / kg) and lithium-ion batteries (250Wh / kg). Therefore, it is attracting attention as a high-capacity, environmentally friendly, and low-cost lithium secondary battery among the secondary batteries developed to date.

[0007] During discharge, a lithium-sulfur battery undergoes a reduction reaction at the positive electrode, where sulfur accepts electrons, and an oxidation reaction at the negative electrode, where lithium is ionized.

[0008] During discharge, lithium-sulfur batteries contain lithium polysulfide (Li2S) at the positive electrode. x , x=2~8) are produced, some of which easily dissolve in the electrolyte, causing side reactions within the battery and accelerating battery degradation, and also causing shuttle reactions during the charging process, significantly reducing charge / discharge efficiency.In addition, in the case of lithium metal used in the anode, they continuously react with the electrolyte, accelerating the decomposition of the electrolyte's lithium salt and additives.

[0009] To solve this problem, Korean Patent Publication No. 10-2016-0037084 discloses that graphene coating on sulfur-containing carbon nanotube aggregates can block the elution of lithium polysulfide and increase the electrical conductivity and sulfur loading of the sulfur-carbon nanotube composite.

[0010] However, the problem of the lithium-sulfur battery becomes more severe at high temperatures, and electrolyte decomposition accelerates, but the prior art does not disclose any improvement to the problem at high temperatures. Summary of the Invention [Problem to be solved by the invention]

[0011] Therefore, in order to operate lithium-sulfur batteries in high-temperature environments, it is necessary to develop an electrolyte with excellent stability. [Means for solving the problem]

[0012] The present inventors have conducted extensive research to solve the above problems, and as a result have found that when lithium bis(trifluoromethanesulfonyl)imide, or lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide are contained in the lithium salt of an electrolyte for a lithium secondary battery, oxidation stability and storage ability at high temperatures are improved, and the life characteristics of a lithium secondary battery containing the same at high temperatures are improved, thereby completing the present invention.

[0013] Therefore, an object of the present invention is to provide an electrolyte for a lithium secondary battery having excellent oxidation stability and improved storage capacity at high temperatures.

[0014] Another object of the present invention is to provide a lithium secondary battery that includes the electrolyte solution for lithium secondary batteries and thereby can achieve improved life characteristics at high temperatures.

[0015] To achieve the above objectives, According to one aspect of the present invention, there is provided an electrolyte for a lithium secondary battery according to the following embodiment.

[0016] The electrolyte for a lithium secondary battery according to the first embodiment is The solution contains a lithium salt, a nitrogen compound, and an organic solvent. The lithium salt includes lithium bis(trifluoromethanesulfonyl)imide, and the organic solvent includes an ether solvent.

[0017] According to the second embodiment, in the first embodiment, The lithium bis(trifluoromethanesulfonyl)imide may be contained in an amount of 20 mol % or more based on the total number of moles of the lithium salt.

[0018] According to the third embodiment, in the first or second embodiment, The molar concentration of the lithium salt may be 0.1 to 4M.

[0019] According to a fourth embodiment, in any one of the first to third embodiments, The lithium salt may further comprise lithium bis(fluorosulfonyl)imide, and the molar concentration of the lithium bis(trifluoromethanesulfonyl)imide may be the same as or higher than the molar concentration of the lithium bis(fluorosulfonyl)imide.

[0020] According to the fifth embodiment, in any one of the first to fourth embodiments, The ether solvent may be contained in an amount of 80% by volume or more based on the total volume of the organic solvent.

[0021] According to the sixth embodiment, in any one of the first to fifth embodiments, The ether solvent may include a linear ether, a cyclic ether, or a mixture thereof.

[0022] According to the seventh embodiment, in any one of the first to sixth embodiments, The linear ether may include at least one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, ethyl methyl ether, ethyl propyl ether, ethyl tert-butyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, dimethoxypropane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, dipropylene glycol dimethylene ether, butylene glycol ether, diethylene glycol ethyl methyl ether, diethylene glycol isopropyl methyl ether, diethylene glycol butyl methyl ether, diethylene glycol tert-butyl ethyl ether, and ethylene glycol ethyl methyl ether.

[0023] According to the eighth embodiment, in any one of the first to seventh embodiments, The cyclic ether may include at least one selected from the group consisting of 2-methylfuran, 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, 2-methyl-1,3-dioxolane, 2-vinyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3-dioxolane, 2-ethyl-2-methyl-1,3-dioxolane, tetrahydropyran, 1,4-dioxane, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, 1,4-dimethoxybenzene, and isosorbide dimethyl ether.

[0024] According to the ninth embodiment, in any one of the first to eighth embodiments, The organic solvent may have a solubility of 2 g / 100 g or more of the nitrogen compound at room temperature, based on 100 g of the organic solvent.

[0025] According to the tenth embodiment, in any one of the first to ninth embodiments, The normal temperature may be in the range of 20°C to 35°C.

[0026] According to the eleventh embodiment, in any one of the first to tenth embodiments, The organic solvent may not include a carbonate-based solvent.

[0027] According to the twelfth embodiment, in any one of the first to eleventh embodiments, The carbonate solvent may be dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinyl ethylene carbonate, halides thereof, or a mixture of two or more thereof.

[0028] According to the thirteenth embodiment, in any one of the first to twelfth embodiments, The nitrogen compound may include a nitrate compound or a nitrite compound.

[0029] According to the 14th embodiment, in any one of the 1st to 13th embodiments, The nitrogen compound may be contained in an amount of 2 wt % to 10 wt % based on the total weight of the electrolyte solution for the lithium secondary battery.

[0030] According to the fifteenth embodiment, in any one of the first to fourteenth embodiments, When the electrolyte solution for a lithium secondary battery is maintained at a temperature of 45° C. or higher, 90% by weight or more of the initial weight of the lithium bis(trifluoromethanesulfonyl)imide may remain.

[0031] According to the 16th embodiment, in any one of the 1st to 11th embodiments, The electrolyte solution for a lithium secondary battery may be one in which 90 wt % to 98 wt % of the initial weight of the lithium bis(trifluoromethanesulfonyl)imide remains when kept at a temperature of 45° C. or higher for 4 weeks.

[0032] According to the seventeenth embodiment, in any one of the first to sixteenth embodiments, The holding temperature may be 45°C to 65°C.

[0033] The lithium secondary battery according to the eighteenth embodiment includes: A lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, and the electrolyte may be any of the electrolytes described in any one of the first to seventeenth embodiments.

[0034] According to the 19th embodiment, in the 18th embodiment, The positive electrode may include a sulfur-containing compound as a positive electrode active material.

[0035] According to the 20th embodiment, in the 18th or 19th embodiment, The sulfur-containing compound may include inorganic sulfur (S), lithium polysulfide (LiS, 1≦n≦8), carbon-sulfur polymer (CS, 2.5≦x≦50, 2≦m), or a mixture of two or more thereof.

[0036] According to the 21st embodiment, in any one of the 18th to 20th embodiments, The negative electrode may contain lithium metal, a lithium alloy, or a mixture thereof as a negative electrode active material.

[0037] According to the 22nd embodiment, in any one of the 18th to 21st embodiments, The lithium secondary battery may be a coin-type battery or a pouch-type battery. [Effects of the Invention]

[0038] The electrolyte solution for lithium secondary batteries of the present invention exhibits the effects of high oxidation stability and excellent storage stability at high temperatures.

[0039] Furthermore, the lithium secondary battery containing the electrolyte solution for lithium secondary batteries of the present invention has an excellent life characteristic at high temperatures. [Brief explanation of the drawings]

[0040] [Figure 1] 1 shows photographs of the oxidation stability of the electrolyte solutions for lithium-sulfur batteries of Examples 1 to 4 and Comparative Example 1. [Figure 2] 1 is a graph showing the capacity retention rate of a lithium-sulfur battery measured after storage at high temperatures. [Figure 3] 1 is a graph showing the measured life characteristics of a coin cell type lithium-sulfur battery at high temperature (45° C.). [Figure 4] 1 is a graph showing the measurement of the life characteristics of a pouch cell type lithium-sulfur battery at high temperature (45° C.). [Figure 5] 1 shows photographs evaluating the solubility of nitrogen compounds depending on the type of organic solvent in Experimental Example 6 herein (left: Example 4, right: Comparative Example 2). DETAILED DESCRIPTION OF THE INVENTION

[0041] The present invention will now be described in more detail.

[0042] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that corresponds to the technical idea of ​​the present invention, in accordance with the principle that the inventor himself can appropriately define the concept of the term in order to explain the invention in the best way.

[0043] The terms used in this specification are merely used to describe exemplary embodiments and do not limit the present invention. The singular expressions include the plural expressions unless the context clearly indicates otherwise. Furthermore, in this specification, the terms "comprise" or "have" specify the presence of a feature, number, step, operation, component, part, or combination thereof described in the specification, but do not exclude the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0044] The term "composite" as used herein means a material in which two or more materials are combined to form physically and chemically distinct phases, thereby exhibiting more effective functions.

[0045] As used herein, the term "polysulfide" means "polysulfide (S x 2- , x - =8, 6, 4, 2) and Lithium Polysulfide (Li2S x or Li2S x - It is a concept that includes all of the following:

[0046] Lithium secondary batteries, especially lithium-sulfur batteries, have a problem in that the lithium anode and the electrolyte continuously react with each other during charging and discharging, accelerating the decomposition of lithium salts. As the temperature increases, the decomposition of lithium salts accelerates, resulting in poor stability.

[0047] Lithium bis(fluorosulfonyl)imide, which has traditionally been used as a lithium salt, has stability issues because it cannot effectively suppress the decomposition of the lithium salt in the electrolyte, especially since the decomposition accelerates in high-temperature environments above 45°C.

[0048] One aspect of the present invention aims to solve the above problems.

[0049] Electrolyte for lithium secondary batteries Therefore, the present invention relates to an electrolyte for lithium secondary batteries that can solve the above problems.

[0050] The electrolyte solution for a lithium secondary battery of the present invention comprises: The solution contains a lithium salt, a nitrogen compound, and an organic solvent. the lithium salt includes lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), The organic solvent includes an ether solvent.

[0051] The lithium bis(trifluoromethanesulfonyl)imide has excellent oxidation stability and high-temperature stability, and can suppress decomposition even when the electrolyte continuously reacts with lithium metal, which is the negative electrode of a lithium secondary battery. Therefore, when the lithium bis(trifluoromethanesulfonyl)imide is included as the lithium salt in the present invention, an electrolyte for a lithium secondary battery having excellent oxidation stability and storage stability, particularly at high temperatures, can be provided, but the mechanism of the present invention is not limited thereto.

[0052] In the present invention, high-temperature storage stability means that when the electrolyte is applied to a lithium secondary battery, the lithium salt is not decomposed and at least a portion of the content remains even at a temperature of 45° C. or higher. For example, when the electrolyte is applied to a lithium secondary battery, the lithium salt applied to the electrolyte maintains a content of 90 wt % or more of the initial weight at a temperature of 45° C. or higher.

[0053] In one embodiment of the present invention, the lithium bis(trifluoromethanesulfonyl)imide may be included in an amount of, for example, 20 mol% or more based on the total moles of the lithium salt. For example, the amount may be 20 mol% to 100 mol%, 25 mol% to 95 mol%, 30 mol% to 90 mol%, 40 mol% to 80 mol%, 45 mol% to 75 mol%, 50 mol% to 75 mol%, or 60 mol% to 75 mol% based on the total moles of the lithium salt. When the amount of lithium bis(trifluoromethanesulfonyl)imide is within the above-mentioned range, advantageous effects can be obtained in terms of exhibiting the above-mentioned effects of lithium bis(trifluoromethanesulfonyl)imide, but the present invention is not limited thereto.

[0054] In another embodiment of the present invention, the lithium salt may further include other types of lithium salts in addition to lithium bis(trifluoromethanesulfonyl)imide.

[0055] In an embodiment of the present invention, the lithium salt other than lithium bis(trifluoromethanesulfonyl)imide may include, for example, lithium bis(fluorosulfonyl)imide.

[0056] In one embodiment of the present invention, the electrolyte for the lithium secondary battery can achieve the above-mentioned effects even when it further contains lithium bis(trifluoromethanesulfonyl)imide and lithium bis(trifluorosulfonyl)imide as the lithium salt. When lithium bis(trifluoromethanesulfonyl)imide is further contained, it may be advantageous for the molar concentration of lithium bis(trifluoromethanesulfonyl)imide to be equal to or higher than the molar concentration of lithium bis(fluorosulfonyl)imide to achieve the above-mentioned effects. More specifically, it is preferable that the molar concentration of lithium bis(trifluoromethanesulfonyl)imide be higher than the molar concentration of lithium bis(fluorosulfonyl)imide, but the present invention is not limited thereto.

[0057] In one embodiment of the present invention, the lithium bis(trifluoromethanesulfonyl)imide and the lithium bis(fluorosulfonyl)imide may be included in a molar ratio of, for example, 1:5 to 5:1, 1:3 to 3:1, 1:2 to 2:1, 1:1.5 to 1.5:1, 1:1 to 1.5:1, 1:1 to 2:1, 1:1 to 3:1, 1.5:1 to 5:1, or 2:1 to 3:1. When the lithium bis(trifluoromethanesulfonyl)imide and the lithium bis(fluorosulfonyl)imide are included in the above molar ratio, advantageous effects can be achieved in terms of the stability of the electrolyte, but the present invention is not limited thereto.

[0058] In one embodiment of the present invention, the concentration of the lithium salt may be appropriately determined taking into consideration ionic conductivity, solubility, etc. For example, it may be 0.1 to 4 M, preferably 0.5 to 2 M. When the concentration of the lithium salt is within the above range, it is advantageous to ensure ionic conductivity suitable for battery operation, or to exhibit an appropriate viscosity of the electrolyte, thereby improving the mobility of lithium ions and suppressing the decomposition reaction of the lithium salt itself, but the present invention is not limited thereto.

[0059] The nitrogen compound, in addition to the lithium salt, is dissolved in the electrolyte of the lithium secondary battery to provide ions, thereby improving the electrical conductivity of the lithium secondary battery, and also improves the life characteristics of the battery when the electrolyte for the lithium secondary battery is used in a lithium-sulfur battery. Specifically, the nitrogen compound may, but is not limited to, inhibit the reduction reaction of polysulfides that occurs during the charge and discharge process of the lithium-sulfur battery, thereby preventing irreversible consumption of polysulfides, thereby improving the performance of the lithium-sulfur battery.

[0060] In an embodiment of the present invention, the nitrogen compound may be any compound capable of forming a stable coating on a lithium metal electrode serving as the negative electrode of a lithium secondary battery, specifically a lithium-sulfur battery, and improving charge / discharge efficiency, and may be, for example, a nitrate compound, a nitrite compound, or a mixture thereof.

[0061] In one embodiment of the present invention, the nitrogen compound may be selected from the group consisting of inorganic nitric acid or nitrite compounds such as lithium nitrate (LiNO), potassium nitrate (KNO), cesium nitrate (CsNO), barium nitrate (Ba(NO)), ammonium nitrate (NHNO), lithium nitrite (LiNO), potassium nitrite (KNO), cesium nitrite (CsNO), and ammonium nitrite (NHNO); organic nitric acid or nitrite compounds such as methyl nitrate, dialkylimidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridinium nitrate, ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite, and octyl nitrite; organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, and dinitrotoluene; and combinations thereof, and preferably includes lithium nitrate.

[0062] In one embodiment of the present invention, the nitrogen compound may be included in an amount of, for example, 1 wt % to 10 wt %, 2 wt % to 10 wt %, or 3 wt % to 10 wt %, specifically 3 wt % to 8 wt %, 3 wt % to 6 wt %, or 3 wt % to 5 wt %, based on the total weight of the electrolyte for the lithium secondary battery, but is not limited thereto. When the nitrogen compound is included in the above amounts, the nitrogen compound may exhibit more advantageous effects in terms of improving the electrical conductivity of the electrolyte and inhibiting the reduction of polysulfides when used in a lithium-sulfur battery, but the present invention is not limited thereto.

[0063] The organic solvent is a medium in which ions involved in the electrochemical reaction of the lithium secondary battery can move, and this is because the organic solvent dissolves the lithium salt and / or the nitrogen compound.

[0064] In the present invention, the organic solvent includes an ether solvent.

[0065] In one embodiment of the present invention, the organic solvent may contain the ether-based solvent in an amount of 80% by volume or more, for example, 85% to 100% by volume, 90% to 100% by volume, 95% to 100% by volume, 98% to 100% by volume, 90% to 98% by volume, or 90% to 95% by volume, based on the total volume of the organic solvent. When the amount of the ether-based solvent is within the above range, based on the total volume of the organic solvent, advantageous effects can be achieved in terms of the solubility of the lithium salt and the nitrogen compound, but the present invention is not limited thereto.

[0066] In an embodiment of the present invention, the ether solvent may include a linear ether, a cyclic ether, or a mixture thereof.

[0067] In one embodiment of the present invention, the linear ether may include at least one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, ethyl methyl ether, ethyl propyl ether, ethyl tert-butyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, dimethoxypropane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, dipropylene glycol dimethylene ether, butylene glycol ether, diethylene glycol ethyl methyl ether, diethylene glycol isopropyl methyl ether, diethylene glycol butyl methyl ether, diethylene glycol tert-butyl ethyl ether, and ethylene glycol ethyl methyl ether. Preferably, the linear ether may include at least one selected from the group consisting of dimethyl ether, dimethoxyethane, diethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether, and more preferably, dimethoxyethane.

[0068] In one embodiment of the present invention, the cyclic ether may include at least one selected from the group consisting of 2-methylfuran, 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, 2-methyl-1,3-dioxolane, 2-vinyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3-dioxolane, 2-ethyl-2-methyl-1,3-dioxolane, tetrahydropyran, 1,4-dioxane, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, 1,4-dimethoxybenzene, and isosorbide dimethyl ether. Preferably, it may contain one or more selected from the group consisting of 2-methylfuran, 1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, and 2,5-dimethyltetrahydrofuran, and more preferably it may contain 2-methylfuran.

[0069] In an embodiment of the present invention, the organic solvent may include dimethoxyethane and 2-methylfuran.

[0070] The organic solvent may contain a linear ether and a cyclic ether in a volume ratio of 95:5 to 5:95, preferably 95:5 to 50:50, and most preferably 90:10 to 70:30. In the present invention, the volume ratio corresponds to the ratio of "volume % of linear ether:volume % of cyclic ether" in the ether-based solvent.

[0071] In an embodiment of the present invention, the organic solvent may have excellent solubility for the nitrogen compound at room temperature. As described above, in order for the nitrogen compound to effectively exhibit its effects, the organic solvent must be able to sufficiently dissolve the nitrogen compound.

[0072] In one embodiment of the present invention, the organic solvent may have a solubility of the nitrogen compound of 2 g / 100 g or more, for example, 2 g / 100 g to 20 g / 100 g, 3 g / 100 g to 20 g / 100 g, 3 g / 100 g to 15 g / 100 g, 5 g / 100 g to 15 g / 100 g, 5 g / 100 g to 10 g / 100 g, or 7 g / 100 g to 10 g / 100 g, based on 100 g of the organic solvent at room temperature. When the organic solvent has the above solubility of the nitrogen compound, it can reduce the amount of the nitrogen compound and provide an advantageous effect in terms of improving the performance of the lithium secondary battery due to the nitrogen compound, but the present invention is not limited thereto.

[0073] In one embodiment of the present invention, the "room temperature" may refer to, for example, a temperature range of 20°C to 35°C, specifically, a temperature range of 25°C to 30°C.

[0074] In an embodiment of the present invention, the organic solvent may be an ether-based solvent or another organic solvent capable of dissolving the nitrogen compound. For example, organic solvents used in conventional electrolytes for lithium secondary batteries include esters, amides, linear carbonates, and cyclic carbonates in addition to the ether-based solvents. In an embodiment of the present invention, the organic solvent may further include the organic solvents used in conventional electrolytes for lithium secondary batteries in addition to the ether-based solvents. However, in terms of the solubility of the nitrogen compound, it is preferable that the organic solvent in the lithium secondary battery electrolyte does not include the carbonate-based solvent.

[0075] In one embodiment of the present invention, the ester may be, for example, any one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone, or a mixture of two or more thereof, but is not limited thereto.

[0076] In one embodiment of the present invention, the linear carbonate may be, for example, any one selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate, or a mixture of two or more thereof, but is not limited thereto.

[0077] In one embodiment of the present invention, the cyclic carbonate may be any one or a mixture of two or more selected from the group consisting of ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, and halides thereof, and examples of halides thereof include, but are not limited to, fluoroethylene carbonate.

[0078] In another embodiment of the present invention, the organic solvent may not include a carbonate solvent because the carbonate solvent cannot dissolve the nitrogen compound or has low solubility in the carbonate solvent.

[0079] In one embodiment of the present invention, the organic solvent may contain a small amount of carbonate-based solvent so that the carbonate-based solvent does not affect the solubility of the nitrogen compound. For example, when the organic solvent contains the carbonate-based solvent, the content of the carbonate solvent may be 3 wt % or less, 2 wt % or less, 1 wt % or less, 0.5 wt % or less, or 0 wt % (i.e., none) based on the total weight of the electrolyte solution for lithium secondary batteries.

[0080] The electrolyte solution for a lithium secondary battery of the present invention may specifically be an electrolyte solution for a lithium-sulfur battery.

[0081] The electrolyte solution for a lithium secondary battery of the present invention contains the lithium salt, and therefore has excellent oxidation stability and can inhibit decomposition of the lithium salt, thereby exhibiting excellent stability, particularly at high temperatures.

[0082] More specifically, the lithium salt hardly decomposes even when stored at high temperatures of 60°C or higher, and lithium-sulfur batteries containing this material can exhibit improved life characteristics at temperatures of 45°C or higher.

[0083] In one embodiment of the present invention, the electrolyte for a lithium secondary battery may be such that, for example, when maintained at high temperatures, the lithium bis(trifluoromethanesulfonyl)imide is not decomposed at all or only a small amount is decomposed, thereby maintaining a weight of 90 wt % or more of the initial weight.

[0084] Specifically, the electrolyte solution for a lithium secondary battery may maintain a weight of 90% by weight or more, for example, 90 to 100% by weight, 90 to 98% by weight, or 93 to 96% by weight, of the initial weight of the lithium bis(trifluoromethanesulfonyl)imide when kept at a high temperature, for example, 45°C or higher, specifically, 45 to 65°C, 50 to 60°C, or 55 to 60°C, for 2 weeks or more, for example, 2 to 12 weeks, 2 to 10 weeks, 3 to 8 weeks, 3 to 6 weeks, or for example, 4 to 5 weeks.

[0085] In an embodiment of the present invention, the content of lithium bis(trifluoromethanesulfonyl)imide in the electrolyte for the lithium secondary battery before and after storage may be measured by a known content analysis method such as nuclear magnetic resonance analysis (NMR), ion chromatography, or immediate constituent analysis of the electrolyte.

[0086] Lithium secondary battery The present invention also relates to a lithium secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the electrolyte is the same as the above-described electrolyte of the present invention.

[0087] positive electrode The positive electrode may include a positive electrode current collector and a positive electrode active material layer coated on one or both surfaces of the positive electrode current collector.

[0088] The positive electrode current collector is not particularly limited as long as it supports the positive electrode active material, does not induce chemical changes in the battery, and has high conductivity, and may be made of, for example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, silver, or the like, or an aluminum-cadmium alloy.

[0089] The positive electrode current collector may have fine irregularities on its surface to strengthen the bonding force with the positive electrode active material, and may be in various forms such as a film, sheet, foil, mesh, net, porous material, foam, or nonwoven fabric.

[0090] The positive electrode active material layer includes a positive electrode active material, and may further include a conductive material, a binder, an additive, and the like.

[0091] The positive electrode active material includes a porous carbon material and a sulfur-carbon composite containing sulfur in at least a portion of the interior and exterior surfaces of the porous carbon material. Since sulfur contained in the positive electrode active material does not have electrical conductivity by itself, it is used in a composite with a conductive material such as a carbon material. Thus, the sulfur is contained in the form of a sulfur-carbon composite.

[0092] Therefore, the lithium secondary battery of the present invention can be a lithium-sulfur battery.

[0093] The sulfur may include at least one selected from the group consisting of elemental sulfur (S) and sulfur compounds. The positive electrode active material may include inorganic sulfur, Li2Sn (n≧1), disulfide compounds, organic sulfur compounds, and carbon-sulfur polymers (C2S x ) n , x=2.5 to 50, n≧2). Preferably, the sulfur is inorganic sulfur.

[0094] The sulfur-carbon composite includes a porous carbon material that not only provides a framework in which the sulfur can be uniformly and stably immobilized, but also compensates for the low electrical conductivity of sulfur to facilitate electrochemical reactions.

[0095] Typically, the porous carbon material can be prepared by carbonizing various carbon precursors. The porous carbon material contains pores with varying internal sizes, with the average pore diameter ranging from 1 to 200 nm and the porosity ranging from 10 to 90 vol% of the total volume of the porous carbon material. If the average pore diameter is less than this range, the pore size is merely at the molecular level, making sulfur impregnation impossible. Conversely, if the average pore diameter exceeds this range, the mechanical strength of the porous carbon material is weakened, making it unsuitable for use in electrode manufacturing processes.

[0096] In one embodiment of the present invention, the "average pore diameter" can be measured by any known method for measuring the pore diameter of a porous material in the art, and the measurement method is not particularly limited. For example, the pore diameter can be measured by a scanning electron microscope (SEM), a field emission electron microscope, or a laser diffraction method. Measurement using the laser diffraction method can be performed using, for example, a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000).

[0097] In one embodiment of the present invention, the term "porosity" refers to the ratio of the volume occupied by pores to the total volume of a structure, and is expressed in units of vol%. It may be used interchangeably with terms such as porosity, porosity, etc. In the present invention, the measurement of the porosity is not particularly limited, and according to one embodiment of the present invention, it may be measured by, for example, Brunauer-Emmett-Teller (BET) measurement using nitrogen gas or mercury penetration (Hg porosimeter) and ASTM D2873.

[0098] The shape of the porous carbon material may be spherical, rod-like, needle-like, plate-like, tubular or bulk, and any shape commonly used in lithium-sulfur batteries may be used without limitation.

[0099] The porous carbon material may be any material commonly used in the art, having a porous structure or a high specific surface area. For example, the porous carbon material may be at least one selected from the group consisting of graphite, graphene, carbon black (e.g., denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black), carbon nanotubes (CNTs) (e.g., single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs)), carbon fibers (e.g., graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs), graphite (e.g., natural graphite, artificial graphite, and expanded graphite), and activated carbon. Preferably, the porous carbon material is carbon nanotubes.

[0100] In the sulfur-carbon composite according to the present invention, the sulfur is located on at least one of the inner and outer surfaces of the porous carbon material. For example, the sulfur may be present on less than 100% of the total inner and outer surfaces of the porous carbon material, preferably 1 to 95%, and more preferably 40 to 96% of the total inner and outer surfaces of the porous carbon material. When the sulfur is present on the inner and outer surfaces of the porous carbon material within the above range, it can exhibit the maximum effect in terms of electron transfer area and wettability with the electrolyte. Specifically, the sulfur is impregnated thinly and uniformly on the inner and outer surfaces of the porous carbon material within the above range, thereby increasing the contact area for electron transfer during charge and discharge. If the sulfur is located on 100% of the total inner and outer surfaces of the porous carbon material, the porous carbon material will be completely covered with sulfur, reducing wettability and contact with the electrolyte, preventing it from receiving electrons and participating in the electrochemical reaction.

[0101] The sulfur-carbon composite may contain sulfur in an amount of, for example, 65 wt % or more, specifically, 65 to 90 wt %, 70 to 85 wt %, or 72 to 80 wt %, based on 100 wt % of the sulfur-carbon composite. When the sulfur content is within the above range, advantageous effects can be achieved in terms of improving battery performance and ensuring capacity, but the present invention is not limited thereto.

[0102] The method for preparing the sulfur-carbon composite of the present invention is not particularly limited, and may be a method commonly used in the art. For example, the sulfur and the porous carbon material may be simply mixed and then heat-treated to prepare the composite.

[0103] In addition to the above-described components, the positive electrode active material may further include at least one selected from a transition metal element, a Group IIIA element, a Group IVA element, sulfur compounds of these elements, and alloys of these elements with sulfur.

[0104] The transition metal elements may include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au, or Hg, the IIIA elements may include Al, Ga, In, or Ti, and the IVA elements may include Ge, Sn, or Pb.

[0105] In the positive electrode for a lithium secondary battery according to the present invention, the positive electrode active material may be present in an amount of, for example, 80 wt % or more, specifically 80 wt % to 100 wt %, more specifically 85 wt % to 98 wt %, or 80 wt % to 95 wt %, based on the total weight of the positive electrode active material layer. The content of the positive electrode active material may have a lower limit of 70 wt % or more or 85 wt % or more, and an upper limit of 99 wt % or less or 90 wt % or less, based on 100 wt % of the total weight of the positive electrode active material layer. The content of the positive electrode active material may be determined by combining the lower and upper limits. If the content of the positive electrode active material is below the above ranges, the relative contents of auxiliary materials such as conductive materials and binders increase, while the content of the positive electrode active material decreases, making it difficult to realize a battery with high capacity and high energy density. Conversely, if the content exceeds the above ranges, the content of the conductive materials or binders, described below, is relatively insufficient, resulting in poor physical properties of the electrode.

[0106] The conductive material electrically connects the electrolyte and the positive electrode active material and serves as a path for electrons to move from a current collector to the positive electrode active material. The conductive material is a component of an electrode that is physically distinct from the carbon contained in the sulfur-carbon composite and has electrical conductivity. Any conductive material may be used without limitation.

[0107] For example, the conductive material may be carbon black such as Super-P, Denka Black, Acetylene Black, Ketjen Black, Channel Black, Furnace Black, Lamp Black, Thermal Black, or Carbon Black; carbon derivatives such as carbon nanotubes or fullerenes; conductive fibers such as carbon fibers or metal fibers; metal powders such as carbon fluoride, aluminum, or nickel powder; or conductive polymers such as polyaniline, polythiophene, polyacethylene, or polypyrrole, which may be used alone or in combination.

[0108] The content of the conductive material may be 1 to 10 wt % based on the total weight of the positive electrode active material. If the content of the conductive material is below this range, electron transfer between the positive electrode active material and the current collector is impaired, resulting in a decrease in voltage and capacity. Conversely, if the content exceeds this range, the proportion of the positive electrode active material decreases relatively, resulting in a decrease in the total energy (charge) of the battery. Therefore, it is preferable to determine the titration content within the above range.

[0109] The binder supports the positive electrode active material on the positive electrode current collector and organically connects the positive electrode active materials to each other to further enhance the binding strength therebetween. Any binder known in the art may be used.

[0110] For example, the binder may be one or a mixture or copolymer of two or more selected from the group consisting of: fluororesin-based binders including polyvinylidene fluoride (PVdF), polyvinylidene fluoride-based polymers containing at least one vinylidene fluoride repeating unit, polytetrafluoroethylene (PTFE), or a mixture of two or more thereof; rubber-based binders including styrene butadiene rubber (SBR), acrylonitrile butadiene rubber, and styrene-isoprene rubber; acrylic-based binders; cellulose-based binders including carboxyl methyl cellulose (CMC), starch, hydroxypropyl cellulose, and regenerated cellulose; polyalcohol-based binders; polyolefin-based binders including polyethylene and polypropylene; polyimide-based binders; polyester-based binders; and silane-based binders.

[0111] The content of the binder may be 1 to 10 wt % based on the total weight of the positive electrode active material layer. If the content of the binder is less than this range, the physical properties of the positive electrode may be reduced, causing the positive electrode active material and conductive material to fall off. If the content of the binder exceeds this range, the ratio of the positive electrode active material to the conductive material in the positive electrode may be relatively reduced, causing a decrease in battery capacity. Therefore, it is preferable to determine the content of the binder within the above range.

[0112] In the present invention, the method for manufacturing the positive electrode for the lithium secondary battery is not particularly limited, and any method known to those skilled in the art or various modified methods thereof may be used.

[0113] In one example, the positive electrode for the lithium secondary battery may be manufactured by preparing a positive electrode slurry composition including the above-described composition, and then applying the same to at least one surface of the positive electrode current collector to form the positive electrode active material layer.

[0114] The positive electrode slurry composition may include the positive electrode active material described above, and may further include a binder, a conductive material, and a solvent.

[0115] The solvent used is one that can uniformly disperse the positive electrode active material. The most preferred solvent is water, which may be distilled water or deionized water. However, the solvent is not limited thereto, and if necessary, a lower alcohol that is easily mixed with water may be used. Examples of the lower alcohol include methanol, ethanol, propanol, isopropanol, and butanol. Preferably, these may be mixed with water.

[0116] The content of the solvent is set to a level that allows easy coating, and the specific content varies depending on the coating method and device.

[0117] The positive electrode slurry composition may further contain, as needed, substances commonly used in the art for the purpose of improving its performance, etc. Examples thereof include a viscosity modifier, a fluidizing agent, and a filler.

[0118] The method for applying the positive electrode slurry composition is not particularly limited in the present invention, and examples thereof include doctor blade, die casting, comma coating, screen printing, etc. Alternatively, the positive electrode slurry may be applied to a positive electrode current collector by molding on a separate substrate and then pressing or lamination.

[0119] After the coating, a drying process may be performed to remove the solvent. The drying process is performed at a temperature and time sufficient to sufficiently remove the solvent. The drying conditions vary depending on the type of solvent, and are not particularly limited in the present invention. Examples include drying with warm air, hot air, or low-humidity air, vacuum drying, and drying by irradiation with (far) infrared rays or electron beams. The drying rate is typically adjusted to remove the solvent as quickly as possible, but within a range that does not cause cracks in the positive electrode active material layer due to stress concentration or peel off the positive electrode active material layer from the positive electrode current collector.

[0120] Additionally, the density of the positive electrode active material in the positive electrode can be increased by pressing the current collector after drying. Examples of pressing methods include die pressing and roll pressing.

[0121] The positive electrode fabricated using the above-described composition and fabrication method, specifically the positive electrode active material layer, may have a porosity of 50 to 80 vol%, specifically 60 to 75 vol%. If the porosity of the positive electrode is less than 50 vol%, the filling level of the positive electrode slurry composition (including the positive electrode active material, conductive material, and binder) is too high, resulting in insufficient electrolyte for ionic and / or electrical conduction between the positive electrode active material and the positive electrode active material, which can lead to reduced battery output and cycle characteristics and significant overvoltage and discharge capacity reduction. Conversely, if the porosity of the positive electrode is too high (exceeding 80 vol%), the physical and electrical connection with the current collector may be weakened, resulting in reduced adhesion and poor reactivity. Furthermore, the increased porosity may result in electrolyte filling, resulting in reduced battery energy density. Therefore, the porosity should be appropriately adjusted within this range.

[0122] negative electrode The negative electrode may include a negative electrode current collector and a negative electrode active material layer coated on one or both sides of the negative electrode current collector, or may be a lithium metal plate.

[0123] The negative electrode current collector is for supporting the negative electrode active material layer, and is the same as that described for the positive electrode current collector.

[0124] The negative electrode active material layer may include, in addition to the negative electrode active material, a conductive material, a binder, etc. In this case, the conductive material and the binder may be the same as those described above.

[0125] The negative electrode active material is lithium (Li + The lithium ion-containing compound may include a material capable of reversibly intercalating or deintercalating lithium ions, a material capable of reacting with lithium ions to reversibly form a lithium-containing compound, lithium metal, or a lithium alloy.

[0126] The lithium ion (Li + The material capable of reversibly inserting or desorbing lithium ions (Li) can be, for example, crystalline carbon, amorphous carbon, or a mixture thereof. + The material capable of reversibly forming a lithium-containing compound by reacting with lithium (Li) may be, for example, tin oxide, titanium nitride, 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).

[0127] Preferably, the negative electrode active material may be lithium metal, specifically, in the form of a lithium metal thin film or lithium metal powder.

[0128] Separation membrane The separator separates or insulates the positive electrode and the negative electrode from each other and allows lithium ions to be transported between the positive electrode and the negative electrode. The separator is made of a porous non-conductive or insulating material and can be any material commonly used as a separator in lithium secondary batteries. The separator may be an independent member such as a film, or a coating layer applied to the positive electrode and / or the negative electrode.

[0129] The separation membrane preferably has low resistance to the ion migration of the electrolyte and has excellent electrolyte impregnation ability.

[0130] The separator is made of a porous substrate, and the porous substrate may be any porous substrate commonly used in secondary batteries, including a porous polymer film alone or in combination with other porous polymer films, such as, but not limited to, a nonwoven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, or a polyolefin-based porous membrane.

[0131] The material of the porous substrate is not particularly limited in the present invention, and any porous substrate commonly used in electrochemical devices can be used. For example, the porous substrate can be made of polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyamides, polyacetals, polycarbonates, polyimides, polyetheretherketones, polyethersulfones, polyphenylene oxides, polyphenylene sulfides, polyethylene naphthalenes, polytetrafluoroethylenes, polyvinylidene fluoride, polyvinyl chloride, and the like. The material may include one or more materials selected from the group consisting of poly(p-phenylene benzobisoxazole), polyacrylonitrile, cellulose, nylon, poly(p-phenylene benzobisoxazole), and polyarylate.

[0132] The thickness of the porous substrate is not particularly limited, but may be 1 to 100 μm, preferably 5 to 50 μm. Although the thickness of the porous substrate is not limited to the above range, if the thickness is thinner than the above lower limit, the mechanical properties may be reduced, and the separator may be easily damaged during use of the battery.

[0133] The average diameter and porosity of the pores present in the porous substrate are not particularly limited, but may be 0.001 to 50 μm and 10 to 95 vol%, respectively.

[0134] The lithium secondary battery according to the present invention can be manufactured by a conventional winding process, as well as by lamination, stacking, and folding processes of the separator and electrodes.

[0135] The shape of the lithium secondary battery is not particularly limited, and may be various shapes such as a cylindrical shape, a stacked shape, a coin shape, a pouch shape, and the like.

[0136] The present invention will be described in detail below with reference to examples to aid in understanding the present invention. However, the examples according to the present invention can be modified in various ways, and the scope of the present invention is not limited to the following examples. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0137] <Production of electrolyte for lithium-sulfur batteries> Examples 1 to 4 and Comparative Example 1 An electrolyte for a lithium-sulfur battery was prepared according to the composition shown in Table 1 below.

[0138] [Table 1]

[0139] The nitrogen compound is contained in an amount of 5% by weight based on the total weight of the electrolyte for lithium-sulfur batteries, and the 2-MeF is 2-methylfuran, and the DME is dimethoxyethane.

[0140] Experimental Example 1: Evaluation of the oxidation stability of electrolytes for lithium-sulfur batteries The oxidation stability of the electrolyte solutions for the lithium-sulfur batteries prepared in Examples 1 to 4 and Comparative Example 1 was evaluated.

[0141] The oxidation stability was evaluated by exposing the electrolyte solutions for lithium-sulfur batteries prepared in Examples 1 to 4 and Comparative Example 1 to air (20% oxygen) at 25°C and 1 atm, and then storing the vials with the lids closed. After 24 hours, the degree of browning was visually observed and evaluated, and the results are shown in FIG. 1.

[0142] Referring to FIG. 1, the electrolyte solutions for lithium-sulfur batteries prepared in Examples 1 to 4 and Comparative Example 1 all showed browning due to oxidation by oxygen in the air present inside the vial. However, it was confirmed that the degree of browning was less as the concentration of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) increased.

[0143] Therefore, it can be seen that the higher the concentration of lithium bis(trifluoromethanesulfonyl)imide as the lithium salt of the electrolyte, the better the oxidation stability.

[0144] Experimental Example 2: Evaluation of high temperature (60°C) stability of electrolyte for lithium-sulfur batteries The electrolyte solutions for the lithium-sulfur batteries prepared in Examples 2 and 3 and Comparative Example 1 were sealed and stored in a high-temperature chamber at 60°C for 4 weeks, and the amount of lithium salt remaining in the electrolyte solution was quantitatively analyzed to evaluate its high-temperature stability.

[0145] Quantitative analysis of the lithium salt was performed by isotope NMR analysis and ion chromatography (IC) analysis after extracting the organic solvent with THF, and the results are shown in Table 2. In Table 2, the weight of the remaining lithium salt relative to the initial weight of the lithium salt was calculated as a percentage (%).

[0146] [Table 2]

[0147] In the results shown in Table 2, lithium bis(fluorosulfonyl)imide (LiFSI) was completely decomposed, with no residue remaining, whereas lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) remained at up to 98%.

[0148] Therefore, it was confirmed that lithium bis(trifluoromethanesulfonyl)imide was not decomposed even at high temperatures (60°C) and was very stable.

[0149] Experimental Example 3: Evaluation of capacity retention rate of lithium-sulfur batteries after high-temperature (60°C) storage A positive electrode slurry composition was prepared by mixing 95 wt% of sulfur-carbon composite (S:C = 75:25 (weight ratio)) as a positive electrode active material and 5 wt% of LiPAA (Lithium Polyacrylate) as a binder. The positive electrode slurry composition was applied to an aluminum current collector and dried to prepare a positive electrode. The loading of the prepared positive electrode was 3.5-4.5 mAh / cm. 2 It was.

[0150] Lithium metal was used for the negative electrode.

[0151] The positive electrode and the negative electrode were positioned face to face, and a polyethylene separator having a thickness of 16 μm and a porosity of 46 vol% was interposed therebetween. Then, the electrolytes prepared in Examples 2 and 3 and Comparative Example 1 were respectively injected to prepare a pouch cell type lithium-sulfur battery.

[0152] A pouch cell lithium-sulfur battery is one in which the electrolyte is not exposed to air.

[0153] Specifically, the electrolyte solutions used were the electrolyte solutions prepared in Examples 2 and 3 and Comparative Example 1, which were stored for 1 week, 2 weeks, 3 weeks, and 4 weeks, and the capacity retention rates of pouch cell-type lithium-sulfur batteries filled with the electrolyte solutions were evaluated. This was repeated for 4 weeks.

[0154] The capacity retention rate was calculated based on the fourth discharge capacity after four cycles of 0.1C discharge / charge before storage, and the fourth discharge capacity after four cycles of 0.1C discharge / charge after storage. The results are shown in Figure 2.

[0155] The results of FIG. 2 show that Comparative Example 1, which does not contain lithium bis(trifluoromethanesulfonyl)imide as a lithium salt, exhibits a significant decrease in capacity retention rate as the high-temperature storage period of the electrolyte increases.

[0156] Example 2, in which the molar concentrations of lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide were the same, and Example 3, in which the molar concentration of lithium bis(trifluoromethanesulfonyl)imide was higher than that of lithium bis(fluorosulfonyl)imide, showed results that showed that the capacity was maintained even when the electrolyte was stored at high temperature for a long period of time.

[0157] In Experimental Example 2, when the electrolyte solution was sealed and stored in a high-temperature chamber at 60°C for 4 weeks, most of the lithium bis(trifluoromethanesulfonyl)imide remained, but all of the lithium bis(fluorosulfonyl)imide was decomposed.

[0158] Therefore, it is possible to predict from the results of Experimental Example 3 that lithium bis(fluorosulfonyl)imide was decomposed over time, and as a result, Examples 2 and 3, which had higher concentrations of lithium bis(trifluoromethanesulfonyl)imide, showed excellent capacity retention.

[0159] From the above results, it can be seen that the electrolyte for lithium-sulfur batteries of the present invention has excellent storage stability at high temperatures (60°C), and therefore can improve the capacity retention rate of lithium-sulfur batteries containing the electrolyte.

[0160] Experimental Example 4: High-temperature (45°C) performance evaluation of coin cell lithium-sulfur batteries Coin cell-type lithium-sulfur batteries were fabricated by injecting the electrolytes for lithium-sulfur batteries of Examples 1 to 4 and Comparative Example 1 in the same manner as in Experimental Example 3, except that a copper current collector was used as the negative electrode by coating lithium metal thereon.

[0161] In coin cell lithium-sulfur batteries, the electrolyte is exposed to air and oxidizes.

[0162] The life characteristics of the coin cell-type lithium-sulfur batteries into which the electrolyte solutions for lithium-sulfur batteries of Examples 1 to 4 and Comparative Example 1 were injected were evaluated using a charge / discharge measuring device (LAND CT-2001A, manufactured by Wuhan Co., Ltd.).

[0163] Specifically, the battery was discharged at a current density of 0.1 C at 45°C until the voltage reached 1.8 V, and then charged at a constant current until the voltage reached 2.5 V, repeating this cycle 2.5 times. After that, the battery was discharged and charged three times at a current density of 0.2 C, and then cycled at a current density of 0.5 C to measure the lifespan characteristics. The results are shown in Figure 3.

[0164] The results in FIG. 3 show that Comparative Example 1, which does not contain lithium bis(trifluoromethanesulfonyl)imide as a lithium salt, exhibits poor life characteristics at 45°C.

[0165] Example 1 contained lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide as lithium salts, but the molar concentration of lithium bis(trifluoromethanesulfonyl)imide was lower than that of lithium bis(fluorosulfonyl)imide, and thus showed poor life characteristics similar to Comparative Example 1.

[0166] In Example 2, the molar concentrations of lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide were the same, and the results showed that the life characteristics were improved under the condition of 45°C.

[0167] Example 3 contained lithium bis(trifluoromethanesulfonyl)imide at a molar concentration higher than that of lithium bis(fluorosulfonyl)imide, and Example 4 contained only lithium bis(trifluoromethanesulfonyl)imide. Both Examples 3 and 4 showed improved life characteristics at 45°C, and showed better results than Example 2.

[0168] From the above results, it was found that when lithium bis(trifluoromethanesulfonyl)imide is included as a lithium salt or when the molar concentration of lithium bis(trifluoromethanesulfonyl)imide is higher than that of lithium bis(fluorosulfonyl)imide, improved life characteristics can be obtained at high temperatures (45°C).

[0169] Experimental Example 5: High-temperature (45°C) performance evaluation of a pouch cell-type lithium-sulfur battery In the same manner as in Experimental Example 3, the electrolyte solutions for the lithium-sulfur batteries prepared in Examples 2 to 4 and Comparative Example 1 were injected into pouch cell-type lithium-sulfur batteries, respectively.

[0170] A pouch cell lithium-sulfur battery is one in which the electrolyte is not exposed to air.

[0171] The life characteristics were evaluated under the same conditions as in Experimental Example 4, and the results are shown in FIG.

[0172] The results in Figure 4 show the same tendency as the results in Figure 3. However, since the electrolyte in the pouch cell lithium-sulfur battery is not exposed to air and therefore does not oxidize, it exhibits superior performance at high temperatures compared to the coin cell lithium-sulfur battery.

[0173] Experimental Example 6: Evaluation of solubility of nitrogen compounds by organic type The solubility of nitrogen compounds in ether-based solvents and carbonate-based solvents was evaluated using lithium nitrate (LiNO3) as follows.

[0174] As an electrolyte solution using an ether-based solvent, the electrolyte solution prepared in Example 4 was prepared. Next, as Comparative Example 2, an electrolyte solution was prepared in the same manner as in Example 4, except that the organic solvent was changed from 2-MeF:DME (2:8 (v / v)) to ethyl carbonate (EC):dimethyl carbonate (DMC) (1:2 (v / v)).

[0175] A photograph of the produced electrolyte is shown in FIG. 5 (left: Example 4, right: Comparative Example 2). In addition, when preparing Example 4 and Comparative Example 2, lithium salt was dissolved in an organic solvent prepared at room temperature (25°C), and the amount of lithium nitrate added was measured at the point where lithium nitrate was not dissolved but precipitated. The measured lithium nitrate content was expressed as solubility in 100 g of organic solvent, and the results are shown in Table 3 below.

[0176] [Table 3]

[0177] From the results of FIG. 5 and Table 3, it was confirmed that when a nitrogen compound is contained in an electrolyte for a lithium secondary battery, it is preferable that the organic solvent contains an ether-based solvent in order to effectively use the nitrogen compound.

Claims

1. The solution contains a lithium salt, a nitrogen compound, and an organic solvent. the lithium salt comprises lithium bis(trifluoromethanesulfonyl)imide; 1. An electrolytic solution for a lithium secondary battery, wherein the organic solvent comprises an ether-based solvent.

2. 2. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the lithium bis(trifluoromethanesulfonyl)imide is contained in an amount of 20 mol % or more based on the total number of moles of the lithium salt.

3. 2. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the molar concentration of the lithium salt is 0.1M to 4M.

4. the lithium salt further comprises lithium bis(fluorosulfonyl)imide; 2. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the molar concentration of the lithium bis(trifluoromethanesulfonyl)imide is the same as or higher than the molar concentration of the lithium bis(fluorosulfonyl)imide.

5. 2. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the ether solvent is contained in an amount of 80% by volume or more based on the total volume of the organic solvent.

6. 2. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the ether-based solvent comprises a linear ether, a cyclic ether, or a mixture thereof.

7. 7. The electrolyte solution for a lithium secondary battery according to claim 6, wherein the linear ether comprises at least one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, ethyl methyl ether, ethyl propyl ether, ethyl tert-butyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, dimethoxypropane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, dipropylene glycol dimethylene ether, butylene glycol ether, diethylene glycol ethyl methyl ether, diethylene glycol isopropyl methyl ether, diethylene glycol butyl methyl ether, diethylene glycol tert-butyl ethyl ether, and ethylene glycol ethyl methyl ether.

8. The cyclic ethers include 2-methylfuran, 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, 2-methyl-1,3-dioxolane, 2-vinyl 7. The electrolyte solution for a lithium secondary battery according to claim 6, comprising at least one selected from the group consisting of 1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3-dioxolane, 2-ethyl-2-methyl-1,3-dioxolane, tetrahydropyran, 1,4-dioxane, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, 1,4-dimethoxybenzene, and isosorbide dimethyl ether.

9. 2. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the organic solvent exhibits a solubility of the nitrogen compound of 2 g / 100 g or more based on 100 g of the organic solvent at room temperature.

10. 10. The electrolyte for a lithium secondary battery according to claim 9, wherein the room temperature is in the range of 20°C to 35°C.

11. 2. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the organic solvent does not contain a carbonate-based solvent.

12. 12. The electrolyte solution for a lithium secondary battery according to claim 11, wherein the carbonate solvent is dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, a halide thereof, or a mixture of two or more thereof.

13. 2. The electrolyte for a lithium secondary battery according to claim 1, wherein the nitrogen compound comprises a nitrate compound or a nitrite compound.

14. 2. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the nitrogen compound is contained in an amount of 2 wt % to 10 wt % based on the total weight of the electrolyte solution for a lithium secondary battery.

15. 2. The electrolyte solution for a lithium secondary battery according to claim 1, wherein, when the electrolyte solution for a lithium secondary battery is maintained at a temperature of 45° C. or higher, 90 wt % or more of the initial weight of the lithium bis(trifluoromethanesulfonyl)imide remains.

16. 16. The electrolyte solution for a lithium secondary battery according to claim 15, wherein when the electrolyte solution for a lithium secondary battery is kept at a temperature of 45° C. or higher for 4 weeks, 90 wt % to 98 wt % of the initial weight of the lithium bis(trifluoromethanesulfonyl)imide remains.

17. 16. The electrolyte for a lithium secondary battery according to claim 15, wherein the holding temperature is 45°C to 65°C.

18. A lithium secondary battery comprising: a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte solution, A lithium secondary battery, wherein the electrolyte solution is the electrolyte solution according to any one of claims 1 to 17.

19. 19. The lithium secondary battery according to claim 18, wherein the positive electrode contains a sulfur-containing compound as a positive electrode active material.

20. The sulfur-containing compound is inorganic sulfur (S ８ ), lithium polysulfide (Li ２ Sn, 1≦n≦8), carbon-sulfur polymer (C ２ 20. The lithium secondary battery according to claim 19, comprising: Sx)m, 2.5≦x≦50, 2≦m), or a mixture of two or more thereof.

21. 20. The lithium secondary battery according to claim 18, wherein the negative electrode contains lithium metal, a lithium alloy, or a mixture thereof as a negative electrode active material.

22. 20. The lithium secondary battery according to claim 18, wherein the lithium secondary battery is a coin-type battery or a pouch-type battery.

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