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

Abstract

The present invention relates to an electrolyte for a lithium-sulfur secondary battery, which can reduce the resistance of the battery and improve its output characteristics, and a lithium-sulfur secondary battery including the electrolyte. The electrolyte for the lithium-sulfur secondary battery includes a lithium salt, a non-aqueous solvent, and an additive, and the first mixing energy (Gmix1) between the electrolyte and dilithio pertetrasulfide (Li2S4), the second mixing energy (Gmix2) between the electrolyte and dilithio perhexasulfide (Li2S6), and the third mixing energy (Gmix3) between the electrolyte and dilithio peroctasulfide (Li2S8) satisfy a predetermined relationship.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0139326 dated October 26, 2022, and all contents disclosed in the documents of this Korean patent application are incorporated herein by reference.

[0002] The present invention relates to an electrolyte for a lithium-sulfur secondary battery that can reduce the resistance of the lithium-sulfur secondary battery and improve the output characteristics, and a lithium-sulfur secondary battery including the same. [Background technology]

[0003] As the application areas of secondary batteries expand to electric vehicles (EVs) and energy storage systems (ESSs), lithium-ion secondary batteries, which have a relatively low energy storage density compared to their weight, are reaching their limits. Therefore, interest in various next-generation secondary batteries with high energy density has recently been growing, and among these, active research and development is being conducted on lithium-sulfur secondary batteries, which theoretically have a high energy storage density compared to their weight.

[0004] Generally, such lithium-sulfur secondary batteries refer to a rechargeable battery system that contains sulfur molecules (e.g., S8) or sulfur-containing complexes having S-S bonds (Sulfur-Sulfur Bonds) as a positive electrode active material and metallic lithium as a negative electrode active material. By using sulfur, which is abundant worldwide and has a lighter weight than metals, as a positive electrode active material, such lithium-sulfur secondary batteries can be manufactured at relatively low cost and enable the realization of secondary batteries with very high energy density relative to their weight.

[0005] As shown in FIG. 1, during the discharge process of the lithium-sulfur secondary battery, a continuous reduction reaction of sulfur (e.g., S) contained in the positive electrode and a continuous oxidation reaction of metallic lithium contained in the negative electrode occur within each electrode and electrolyte. These continuous oxidation / reduction reactions result in the formation of multiple lithium polysulfides (LiPS) in the electrolyte, which can migrate between the electrodes, and the final solid-state lithium sulfide (LiS) can accumulate on the negative electrode. For example, the reaction process of lithium polysulfides and lithium sulfide resulting from the continuous reduction reaction of sulfur can be summarized as S → LiS → LiS → LiS → LiS → LiS. Of these, S, LiS, and LiS are in the solid state, and the remaining lithium polysulfides (LiS) are in the solid state. n ; n is 4, 6 or 8) can be in a liquid state dissolved in the electrolyte.

[0006] However, because the sequential and continuous conversion of sulfur, lithium polysulfide, and lithium sulfide, which are in different phases, is not smooth and rapid, lithium-sulfur secondary batteries have relatively high electrical resistance and low output characteristics compared to existing lithium-ion secondary batteries. Furthermore, during the process of lithium polysulfide formation through the continuous oxidation / reduction reactions, some of the lithium polysulfide dissolved in the electrolyte may undergo side reactions with the anode, which can be one factor in reducing the lifespan of lithium-sulfur secondary batteries.

[0007] Such high resistance, poor output characteristics, and life characteristics are major factors that make it difficult to apply lithium-sulfur secondary batteries. Therefore, there is a continuous demand for the development of lithium-sulfur secondary batteries with lower resistance and improved output characteristics. Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, an object of the present invention is to provide an electrolyte for a lithium-sulfur secondary battery that can reduce the resistance of a lithium-sulfur secondary battery and improve its output characteristics by facilitating the continuous conversion process of lithium polysulfide.

[0009] Another object of the present invention is to provide a lithium-sulfur secondary battery that contains the electrolyte and has improved output characteristics and life characteristics. [Means for solving the problem]

[0010] The present invention provides an electrolyte for a lithium-sulfur secondary battery, comprising a lithium salt, a non-aqueous solvent, and an additive, The present invention provides an electrolyte for a lithium-sulfur secondary battery, in which a first mixing energy (Gmix1) between the electrolyte and dilithio pertetrasulfide (Li2S4), a second mixing energy (Gmix2) between the electrolyte and dilithio perhexasulfide (Li2S6), and a third mixing energy (Gmix3) between the electrolyte and dilithio peroctasulfide (Li2S8), calculated at room temperature (20±5°C) according to the COSMO-RS (Conductor-like Screening Model for real Solvent) theory, satisfy the relationships Gmix1 - Gmix2 is 0.73 kcal / mol or more, and Gmix1 - Gmix3 is 0.80 kcal / mol or more.

[0011] The present invention also provides a lithium-sulfur secondary battery comprising: a positive electrode containing sulfur as a positive electrode active material; a negative electrode containing metallic lithium; a separator interposed between the positive electrode and the negative electrode; and the electrolyte. [Effects of the Invention]

[0012] The electrolyte of the present invention optimizes the solubility difference between lithium polysulfides produced during the charge / discharge process of a lithium-sulfur secondary battery.

[0013] By using such an electrolyte, the lithium-sulfur secondary battery was found to be able to smoothly and quickly undergo continuous reduction and conversion of lithium polysulfide during discharge, resulting in lower electrical resistance and improved output characteristics compared to previously known batteries of the same type.

[0014] In addition, the lithium-sulfur secondary battery can reduce side reactions between the lithium polysulfide and the negative electrode due to the smooth conversion process of the lithium polysulfide, which can also contribute to improving the life characteristics of the lithium-sulfur secondary battery.

[0015] Therefore, the present invention can contribute to improving the output and life characteristics, which have been the biggest obstacle to the commercialization of lithium-sulfur secondary batteries. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram showing the formation of multiple types of lithium polysulfides in the electrolyte through oxidation and reduction reactions during the charge / discharge process of a lithium-sulfur secondary battery. [Figure 2a] 1 is a graph showing the relationship between Gmix1-Gmix2 (kcal / mol) of the electrolyte and the resistance factor (mΩ) of the lithium-sulfur secondary batteries manufactured using the electrolytes of Examples 1 to 3 and Comparative Examples 1 to 3 at a state of charge (SOC) of 80%. [Figure 2b] 1 is a graph showing the relationship between Gmix1-Gmix3 (kcal / mol) of the electrolyte and the resistance factor (mΩ) of the lithium-sulfur secondary batteries manufactured using the electrolytes of Examples 1 to 3 and Comparative Examples 1 to 3 at a state of charge (SOC) of 80%. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an electrolyte and a lithium-sulfur secondary battery including the same according to specific embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0018] FIG. 1 is a schematic diagram showing the formation of multiple types of lithium polysulfides in the electrolyte through oxidation and reduction reactions during the charge / discharge process of a lithium-sulfur secondary battery.

[0019] According to one embodiment of the present invention, there is provided an electrolyte for a lithium-sulfur secondary battery, comprising a lithium salt, a non-aqueous solvent, and an additive, The present invention provides an electrolyte for a lithium-sulfur secondary battery, in which a first mixing energy (Gmix1) between the electrolyte and dilithio pertetrasulfide (Li2S4), a second mixing energy (Gmix2) between the electrolyte and dilithio perhexasulfide (Li2S6), and a third mixing energy (Gmix3) between the electrolyte and dilithio peroctasulfide (Li2S8), calculated at room temperature (20±5°C) according to the COSMO-RS (Conductor-like Screening Model for real Solvent) theory, satisfy the relationships Gmix1 - Gmix2 is 0.73 kcal / mol or more, and Gmix1 - Gmix3 is 0.80 kcal / mol or more.

[0020] The present inventors have been conducting research into the properties of electrolytes in order to develop electrolytes that can reduce the electrical resistance of lithium-sulfur secondary batteries and improve their output characteristics. In particular, the present inventors have been conducting extensive research to determine which characteristics of which conversion processes have a significant impact on the resistance and output of lithium-sulfur secondary batteries, as a continuous reduction / conversion process of S (solid) → Li2S8 (liquid) → Li2S6 (liquid) → Li2S4 (liquid) → Li2S2 (solid) → Li2S (solid) occurs during the discharge process of lithium-sulfur secondary batteries, and this process is accompanied by a phase change.

[0021] As a result of this continuous research, it was found that several types of lithium polysulfides (Li2S) existed in the electrolyte before they were converted into solids (Li2S2). n ;n is 4, 6, or 8), it was confirmed that the electrolyte properties have the greatest impact on reducing the resistance of lithium-sulfur secondary batteries.

[0022] Furthermore, as a result of further experiments, the inventors discovered that the electrical resistance of the lithium-sulfur secondary battery can be reduced and the output characteristics can be improved by controlling the electrolyte composition so that the difference between the first mixing energy (Gmix1), which defines the compatibility and solubility of the electrolyte with dilithiopertetrasulfide (Li2S4) among the various lithium polysulfides, and the second and third mixing energies (Gmix2 and Gmix3), which define the solubility of the electrolyte with dilithioperhexasulfide (Li2S6) and dilithioperoctasulfide (Li2S8), respectively, is greater than or equal to a certain level, thereby completing the invention.

[0023] This is presumably because the greater difference in solubility between the various types of lithium polysulfides allows the reduction and conversion process between the liquid lithium polysulfides, which primarily occurs in the electrolyte, to occur more smoothly, and because the first mixing energy (Gmix1) of the electrolyte with respect to dilithiopertetrasulfide (Li2S4) becomes relatively high (i.e., the solubility of the electrolyte with respect to Li2S4 becomes relatively low), the subsequent phase change process to a solid also occurs more smoothly.

[0024] Therefore, by using an electrolyte for a lithium-sulfur secondary battery that satisfies the characteristics of one embodiment, the resistance of the secondary battery during charge / discharge can be further reduced, resulting in improved output characteristics of the secondary battery. Furthermore, the conversion process between the lithium polysulfides can be more smoothly carried out, thereby suppressing side reactions between the lithium polysulfides and the negative electrode, thereby further improving the life characteristics of the lithium-sulfur secondary battery.

[0025] In the electrolyte of the above-mentioned embodiment, the first to third mixing energies are n n is 4, 6, or 8), and can be calculated based on quantum mechanical calculations according to the COSMO-RS theory using molecular structure information (see, for example, "COSMO-RS: From Quantum Chemistry to Fluid Phase Thermodynamics and Drug Design," A. Klamt, Elsevier; Amsterdam, The Netherlands, 2005, and Korean Patent Publication No. 2019-0011963). In a more specific example, the first and second mixing energies can be calculated using commercially available COSMOtherm software (COSMOlogic GmbH & Co. KG), which performs quantum mechanical calculations according to the COSMO-RS theory.

[0026] Such first to third mixing energies are obtained by mixing the electrolyte of the embodiment with the lithium polysulfide (LiS n ; n is 4, 6, or 8), and the electrolyte of one embodiment can exhibit the characteristics of a higher first mixing energy (i.e., lower solubility) when mixed with Li2S4, and low second and third mixing energies (i.e., high solubility) with respect to Li2S6 and Li2S8 by satisfying a certain composition, etc., as described below.

[0027] Therefore, in one embodiment of the electrolyte, the Gmix1-Gmix2 is 0.73 kcal / mol or more, alternatively 0.74 to 1.40 kcal / mol, or alternatively 0.80 to 1.10 kcal / mol, and the Gmix1-Gmix3 is 0.80 kcal / mol or more, alternatively 0.80 to 1.50 kcal / mol, or alternatively 0.85 to 1.20 kcal / mol, and the solubility difference between the respective lithium polysulfides can be large.

[0028] In a more specific example, the electrolyte of the embodiment has a Gmix2-Gmix3 of 0.05 kcal / mol or more, or 0.05 to 0.2 kcal / mol, or 0.06 to 0.1 kcal / mol, and the solubility of the electrolyte for Li2S6 and Li2S8 may also differ slightly.

[0029] In this way, when the difference between the first to third mixed energies exceeds a certain level, lithium polysulfide (Li2S n (n is 4, 6, or 8) When the difference in solubility between these lithium polysulfides exceeds a certain level, the reduction and conversion processes between these lithium polysulfides occur more smoothly, thereby reducing the resistance of the lithium-sulfur secondary battery and improving its output. However, if the difference between the first to third mixing energies is too large, the solubility of the electrolyte for a certain lithium polysulfide, e.g., Li2S4, may become too low, resulting in a decrease in the output characteristics of the secondary battery.

[0030] Meanwhile, the difference between the first to third mixed energies is achieved, and each lithium polysulfide (LiS n To ensure adequate solubility for α, β, β-dimethylformamide (α, β-dimethylformamide), the first mixing energy (Gmix1) can be from -3.02 kcal / mol to -0.02 kcal / mol, the second mixing energy (Gmix2) can be from -3.39 kcal / mol to -0.83 kcal / mol, and the third mixing energy (Gmix3) can be from -3.15 kcal / mol to -0.96 kcal / mol.

[0031] If the first mixing energy is too high or the second and third mixing energies are too low, the conversion process between lithium polysulfides may not be smooth or the solubility of some of the lithium polysulfides may be low, resulting in a decrease in the output characteristics of the lithium-sulfur secondary battery. Conversely, if the first mixing energy is too low or the second and third mixing energies are too high, it may be difficult to achieve the difference between the first, second, and third mixing energies, resulting in an increase in the resistance of the secondary battery.

[0032] Meanwhile, the electrolyte of one embodiment basically contains a lithium salt, a non-aqueous solvent, and an additive, and in particular contains a certain solvent composition as the non-aqueous solvent. By controlling the concentrations / contents of the lithium salt and other additives within certain ranges, the first to third mixing energies with each of the lithium polysulfides described above and the difference therebetween can be satisfied.

[0033] In order for the electrolyte to satisfy the above-mentioned mixing energy range and difference, the non-aqueous solvent is lithium polysulfide (LiS n (n is 4, 6, or 8), the first organic solvent acts as a solvent having good solubility for the lithium polysulfide, while the second organic solvent can be a non-solvent for the lithium polysulfide. In particular, the mixing energy range and the difference therebetween can be achieved by adjusting the composition of the first organic solvent and the second organic solvent included as a non-solvent.

[0034] In a specific example, the non-aqueous solvent contains 70 to 85 vol % or 70 to 80 vol % of an ether-based solvent including a dialkyl ether-based solvent and an alkylene glycol dialkyl ether-based solvent as the first organic solvent, and 15 to 30 vol % or 20 to 30 vol % of a furan-based solvent and / or a tetrahydrofuran-based solvent as the second organic solvent, which is the non-solvent, and the dialkyl ether-based solvent may be contained in an amount of 40 to 85 vol % or 40 to 80 vol % based on the volume of the entire non-aqueous solvent.

[0035] In this case, the second organic solvent, which is a furan-based or tetrahydrofuran-based solvent, and the first organic solvent, which is a dialkyl ether-based solvent or an alkylene glycol dialkyl ether-based solvent, can act as a non-solvent and a solvent, respectively, having different solubilities for the lithium polysulfide. Therefore, when the non-solvent and the solvent are within the above-mentioned optimal composition and content ranges, the differences among the first to third mixing energies are satisfied, thereby further reducing the resistance of the lithium-sulfur secondary battery and further improving the output characteristics.

[0036] In addition, the furan-based or tetrahydrofuran-based second organic solvent can form a solid electrolyte interface (SEI) layer on the surface of metallic lithium, thereby suppressing the formation of lithium dendrites and suppressing electrolyte decomposition on the surface of metallic lithium in the negative electrode, thereby improving the lifespan of the secondary battery. Therefore, by optimizing the content of the second organic solvent, the lifespan of the lithium-sulfur secondary battery can be further improved.

[0037] On the other hand, if the content of the furan- or tetrahydrofuran-based second organic solvent is excessively high, the solubility of the electrolyte in a portion of the lithium polysulfide may be excessively reduced, resulting in a deterioration in the performance of the lithium-sulfur secondary battery. Also, if the content of the furan- or tetrahydrofuran-based second organic solvent is excessively low or the content range of the dialkyl ether solvent in the first organic solvent is excessively narrow, it may be difficult to satisfy the differences between the first to third mixing energies, resulting in an increase in the resistance of the lithium-sulfur secondary battery and a deterioration in its output characteristics.

[0038] In a more specific example, the second organic solvent may contain a furan-based solvent and a tetrahydrofuran-based solvent together, and in this case, the mixing volume ratio of the furan-based solvent to the tetrahydrofuran-based solvent may be 30:1 to 1:30, or 20:1 to 1:20.

[0039] In the composition of the non-aqueous solvent described above, the furan-based or tetrahydrofuran-based solvent may be a furan-based compound or a tetrahydrofuran-based compound in which an alkyl group having 1 to 4 carbon atoms is substituted or unsubstituted. Specific examples thereof include one or more selected from the group consisting of furan, 2-methylfuran, 3-methylfuran, 2-ethylfuran, 2-propylfuran, 2-butylfuran, 2,3-dimethylfuran, 2,4-dimethylfuran, 2,5-dimethylfuran, 2-methyltetrahydrofuran, and 4-methyltetrahydrofuran.

[0040] Among these, 2-methylfuran, 2-methyltetrahydrofuran, or 4-methyltetrahydrofuran, or a combination of two or more selected from these, can be preferably used, taking into consideration the differences between the first to third mixing energies described above. For example, 2-methylfuran can be used alone, or 2-methyltetrahydrofuran or 4-methyltetrahydrofuran can be combined. In such combinations, 2-methylfuran:2-methyltetrahydrofuran or 4-methyltetrahydrofuran can be mixed in a volume ratio of 30:1 to 1:30, or 20:1 to 1:20.

[0041] The dialkyl ether solvent may be an ether compound having an alkyl group having 1 to 10 carbon atoms or 1 to 5 carbon atoms bonded thereto, and specific examples thereof include at least one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, and methoxyethoxyethane.

[0042] Furthermore, the alkylene glycol dialkyl ether solvent may be a compound in which two or more alkylene glycol groups having 1 to 5 carbon atoms or 2 to 3 carbon atoms are bonded to an alkyl group having 1 to 5 carbon atoms via an ether bond (—O—). Specific examples thereof include one or more selected from the group consisting of diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, and polyethylene glycol methyl ethyl ether.

[0043] Among these, taking into consideration the differences between the first to third mixing energies described above, dimethoxyethane as the dialkyl ether solvent and diethylene glycol dimethyl ether as the alkylene glycol dialkyl ether solvent can be appropriately combined.

[0044] Meanwhile, the lithium salt is an electrolyte salt used to increase ionic conductivity, and may be any salt commonly used in the art. Specific examples of such lithium salts include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, (C2F5SO2)2NLi, (SO2F)2NLi(LiFSI), and (CF3SO2)3CLi. However, taking into consideration the electrical conductivity of the lithium-sulfur secondary battery or the aforementioned mixing energy difference, a lithium salt in the form of a sulfonate, for example, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, (C2F5SO2)2NLi, (SO2F)2NLi, or (CF3SO2)3CLi may be suitably used.

[0045] The lithium salt may be included in the electrolyte at a concentration of 0.2 to 0.8 mol %, 0.25 to 0.75 mol %, or 0.30 to 0.70 mol % so that the electrolyte can appropriately satisfy the differences between the first to third mixed energies. If the lithium salt concentration is too low, the electrical conductivity of the lithium-sulfur secondary battery may be insufficient. If the lithium salt concentration is too high, the differences between the first to third mixed energies may be difficult to satisfy, and the resistance of the lithium-sulfur secondary battery may increase.

[0046] Meanwhile, the electrolyte of the above-described embodiment further includes an additive for improving the performance of the lithium-sulfur secondary battery in addition to the non-aqueous solvent and lithium salt. Examples of such additives include one or more selected from the group consisting of lithium nitrate (LiNO), lithium thiocyanate (LiSCN), potassium nitrate (KNO), cesium nitrate (CsNO), magnesium nitrate (MgNO), barium nitrate (BaNO), lithium nitrite (LiNO), potassium nitrite (KNO), and cesium nitrite (CsNO). Among these, lithium nitrate and / or lithium thiocyanate are preferably used.

[0047] The additive may be included in an amount of 0.8 to 4.0 wt %, or 0.9 to 3.5 wt %, based on the total weight of the electrolyte. If the additive content is too low, the lithium-sulfur secondary battery may not have sufficient characteristics, and if the additive content is too high, the difference between the first, second, and third mixed energies may not be met, resulting in an increase in the resistance of the lithium-sulfur secondary battery.

[0048] As described above, the electrolyte of one embodiment includes a specific composition of a non-aqueous solvent and a specific content range of a lithium salt and an additive, thereby satisfying the differences among the first to third mixed energies described above and exhibiting optimized solubility for lithium polysulfides formed during discharge of a lithium-sulfur secondary battery. As a result, the conversion process between the lithium polysulfides during charge / discharge of the lithium-sulfur secondary battery can be facilitated, reducing the resistance of the secondary battery and further improving its output characteristics and life characteristics.

[0049] In particular, when the initial charge voltage and end-of-charge voltage profiles corresponding to changes in charge current are measured and derived for such a lithium-sulfur secondary battery, and a resistance factor corresponding to the state of charge (SOC) of the secondary battery is calculated from these profiles, the secondary battery can exhibit a low resistance factor of 400 mΩ or less, or 50 to 400 mΩ, or 100 to 350 mΩ at an SOC of 80%.

[0050] Meanwhile, the method for measuring and calculating such a resistance factor is specifically described in the International Patent Publication WO2016 / 126075 filed by the present inventors, etc.

[0051] Therefore, according to another embodiment of the present invention, there is provided a lithium-sulfur secondary battery including a positive electrode containing sulfur as a positive electrode active material, a negative electrode containing metallic lithium, a separator interposed between the positive electrode and the negative electrode, and the electrolyte of the above embodiment. Such a lithium-sulfur secondary battery includes the electrolyte of the above embodiment and can exhibit low resistance as well as improved output characteristics and life characteristics.

[0052] In the secondary battery of the other embodiment, the positive electrode may include a positive electrode active material and a binder, and may further include a conductive material in addition to the positive electrode active material and a conductive material layer formed on a positive electrode current collector.

[0053] The positive electrode current collector is not particularly limited as long as it supports the active material layer, does not induce chemical changes in the battery, and has high conductivity, and examples thereof include copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, silver, etc., and aluminum-cadmium alloys.

[0054] In addition, the positive electrode current collector may have fine irregularities on its surface to strengthen the bonding force with the active material layer, and may be in various forms such as a film, a sheet, a foil, a mesh, a net, a porous material, a foam, or a nonwoven fabric.

[0055] The positive electrode active material may contain sulfur, more specifically, elemental sulfur (S), an organic sulfur compound, a sulfur-carbon composite, or a polymer (C2S x )n:x=2.5 to 50, n≧2) can be used as the positive electrode active material. In consideration of the characteristics of the electrolyte of one embodiment, elemental sulfur (S8) can be appropriately used.

[0056] The sulfur-containing cathode active material may be included in an amount of 40 to 80 parts by weight, preferably 50 to 70 parts by weight, based on 100 parts by weight of the total cathode weight. If the content of the cathode active material is too low, the energy density of the secondary battery may decrease, while if the content is too high, the conductivity and stability of the electrode may decrease.

[0057] In addition to the positive electrode active material, the positive electrode may further include one or more additives 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.

[0058] 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, and the IIIA group elements may include Al, Ga, In, or Ti, and the IVA group elements may include Ge, Sn, or Pb.

[0059] Meanwhile, the binder is a component that helps bind the positive electrode active material and the current collector, and examples thereof include, but are not limited to, one or more selected from the group consisting of polyvinylidene fluoride (PVdF), polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVdF / HFP), polyvinyl acetate, polyvinyl alcohol, polyvinyl ether, polyethylene, polyethylene oxide, alkylated polyethylene oxide, polypropylene, polymethyl (meth)acrylate, polyethyl (meth)acrylate, polytetrafluoroethylene (PTFE), polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, polyvinylpyrrolidone, styrene-butadiene rubber, acrylonitrile-butadiene rubber, ethylene-propylene-diene monomer (EPDM) rubber, sulfonated EPDM rubber, styrene-butylene rubber, fluororubber, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, and mixtures thereof.

[0060] The binder may be added in an amount of 1 to 15 parts by weight based on 100 parts by weight of the total weight of the positive electrode. If the content of the binder is less than 1 part by weight, the adhesive strength between the active material layer and the current collector may be insufficient. If the content of the binder is more than 15 parts by weight, the adhesive strength is improved, but the content of the positive electrode active material is reduced accordingly, which may result in a smaller battery capacity.

[0061] Meanwhile, the conductive material is a component for improving electrical conductivity and is not particularly limited as long as it is an electron-conductive material that does not undergo chemical changes in the secondary battery. For example, the conductive material can be carbon black, graphite, carbon fiber, carbon nanotubes, metal powder, conductive metal oxide, or organic conductive material. Currently available conductive materials include acetylene black (e.g., products of Chevron Chemical Company or Gulf Oil Company), Ketjen Black EC (product of Armak Company), Vulcan XC-72 (product of Cabot Company), and Super P (product of MMM). Examples of conductive materials include acetylene black, carbon black, and graphite.

[0062] In the lithium-sulfur secondary battery according to the other embodiment, a filler may be selectively added to the positive electrode as a component for suppressing expansion of the sulfur-containing positive electrode active material, etc. Such a filler is not particularly limited as long as it can suppress expansion of the electrode without inducing chemical changes in the battery. For example, an olefin polymer such as polyethylene or polypropylene; a fibrous material such as glass fiber or carbon fiber; etc. may be used.

[0063] The positive electrode may be fabricated by dispersing and mixing the positive electrode active material, conductive material, and binder in a dispersion medium (solvent) to form a slurry, which is then applied to a positive electrode current collector, dried, and rolled. The dispersion medium may be, but is not limited to, N-methyl-2-pyrrolidone (NMP), dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), ethanol, isopropanol, water, or a mixture thereof.

[0064] In the secondary battery according to the other embodiments described above, the negative electrode may include metallic lithium, for example, a lithium metal or alloy layer formed on a negative electrode current collector.

[0065] The negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery, and may be selected from the group consisting of copper, aluminum, stainless steel, zinc, titanium, silver, palladium, nickel, iron, chromium, alloys thereof, and combinations thereof. The stainless steel may be surface-treated with carbon, nickel, titanium, or silver, and the alloy may be an aluminum-cadmium alloy. Alternatively, calcined carbon, a non-conductive polymer or a conductive polymer surface-treated with a conductive material, etc. may also be used. Generally, a copper sheet is used as the negative electrode current collector.

[0066] The metallic lithium may be lithium metal or an alloy. The lithium alloy includes an element capable of alloying with lithium, specifically, an alloy of lithium with one or more elements selected from the group consisting of Si, Sn, C, Pt, Ir, Ni, Cu, Ti, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Sb, Pb, In, Zn, Ba, Ra, Ge, and Al. The metallic lithium may be in the form of a sheet or foil, and may be in the form of lithium or a lithium alloy deposited or coated on a current collector by a dry process, or in the form of particulate metal or alloy deposited or coated by a wet process.

[0067] A conventional separator may be interposed between the positive electrode and the negative electrode. The separator is a physical separator that physically separates the electrodes. Any conventional separator may be used without any particular limitations. In particular, a separator that has low resistance to ion migration of the electrolyte and excellent electrolyte humidification capability is preferred. The separator separates or insulates the positive electrode and the negative electrode from each other while allowing lithium ions to be transported between the positive electrode and the negative electrode. The separator may be porous and made of a non-conductive or insulating material. The separator may be an independent member such as a film, or a coating layer attached to the positive electrode and / or the negative electrode.

[0068] Examples of polyolefin-based porous membranes that can be used as the separation membrane include membranes formed from polyolefin-based polymers such as polyethylene (e.g., high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene), polypropylene, polybutylene, and polypentene, either alone or in combination. Examples of nonwoven fabrics that can be used as the separator include nonwoven fabrics made of polymers such as polyphenylene oxide, polyimide, polyamide, polycarbonate, polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polyphenylene sulfide, polyacetal, polyethersulfone, polyetheretherketone, and polyester, either alone or in combination. Such nonwoven fabrics are in the form of fibers that form a porous web, and include spunbond or meltblown fabrics made of long fibers.

[0069] The thickness of the separator is not particularly limited, but is preferably in the range of 1 to 100 μm, and more preferably in the range of 5 to 50 μm. If the thickness of the separator is less than 1 μm, it will not be able to maintain its mechanical properties, and if it exceeds 100 μm, it will act as a resistance layer, resulting in reduced battery performance. The pore size and porosity of the separator are not particularly limited, but the pore size is preferably 0.1 to 50 μm and the porosity is preferably 10 to 95%. If the pore size of the separator is less than 0.1 μm or the porosity is less than 10%, the separator will act as a resistance layer, and if the pore size is more than 50 μm or the porosity is more than 95%, it will not be able to maintain its mechanical properties.

[0070] Another embodiment of a lithium-sulfur secondary battery including the above-described electrolyte, positive electrode, negative electrode, and separator can be manufactured by placing the positive electrode and the negative electrode opposite each other, interposing a separator therebetween, and then injecting an electrolyte.

[0071] The lithium-sulfur secondary battery can be used not only as a battery cell used as a power source for small devices, but also as a unit battery of a battery module that is a power source for medium- to large-sized devices. In this regard, a battery module including two or more lithium-sulfur secondary batteries electrically connected (in series or parallel) can be provided.

[0072] The number of lithium-sulfur secondary batteries included in the battery module can be adjusted in various ways depending on the application and capacity of the battery module. Furthermore, a battery pack in which the battery modules are electrically connected can be provided using conventional techniques in the art. The battery modules and battery packs can be used as power sources for one or more medium- to large-sized devices, including, but not limited to, power tools; electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric trucks; electric commercial vehicles; or power storage systems.

[0073] Below, preferred examples are presented to help understand the invention. However, the following examples are merely illustrative of the invention, and it will be obvious to those skilled in the art that various changes and modifications are possible within the scope of the invention and technical idea. It is natural that such changes and modifications also fall within the scope of the appended claims.

[0074] Examples 1 to 3 and Comparative Examples 1 to 3: Production of electrolyte for lithium-sulfur secondary battery A non-aqueous solvent was mixed according to the composition shown in Table 1 below, and then a lithium salt and an additive were dissolved in the concentrations and contents shown in Table 1 to prepare the electrolytes of the examples and comparative examples.

[0075] [Table 1]

[0076] Test Example 1: Calculation of the first to third mixing energies (Gmix1 to Gmix3) when mixing an electrolyte with lithium polysulfide Using the commercialized COSMOtherm software (COSMOlogic GmbH & Co. KG) based on the COSMO-RS theory, the electrolytes of the examples and comparative examples and lithium polysulfide (LiSn The mixing energy when mixing with each of the above (n is 4, 6 or 8) was calculated, and the calculation results are summarized in Table 2 below.

[0077] [Table 2]

[0078] Referring to Table 2, it was confirmed that the electrolytes of Examples 1 to 3 contain a non-aqueous solvent of a specific composition and a lithium salt and additive at a certain concentration and content, thereby satisfying the characteristics that Gmix1-Gmix2 is 0.73 kcal / mol or more and Gmix1-Gmix3 is 0.80 kcal / mol or more.

[0079] In contrast, it was confirmed that the electrolytes of Comparative Examples 1 to 3 had relatively low values ​​for Gmix1-Gmix2 and Gmix1-Gmix3 due to the different composition of the non-aqueous solvent from that of the Examples.

[0080] Test Example 2: Fabrication of Lithium-Sulfur Secondary Battery and Evaluation of Resistance Factor electrolyte The electrolyte used was that of the above example or comparative example.

[0081] Cathode manufacturing The positive electrode active material was a sulfur-carbon composite (S:C = 70:30 weight ratio) of 95 parts by weight (the content of sulfur alone was set to 67.5% by weight based on the total weight of the positive electrode, and the carbon material had a pore volume of 1.8 cm). 3 A positive electrode slurry composition was prepared by mixing 5 parts by weight of activated carbon (used in an amount of 1 / g) and 5 parts by weight of styrene butadiene rubber / carboxymethyl cellulose (SBR:CMC=7:3) as a binder. The resulting slurry composition was then coated on a current collector (Al foil), dried at 50°C for 12 hours, and pressed using a roll press to prepare a positive electrode (the porosity of the electrode was 65%).

[0082] Lithium-sulfur secondary battery manufacturing The cathode and a 150 μm-thick lithium metal anode were placed facing each other, and a polyethylene (PE) separator was placed between them, followed by injection of the electrolyte to fabricate a coin cell-type lithium-sulfur secondary battery. The cathode was punched out at 15 phi, the polyethylene separator was punched out at 19 phi, and the lithium metal was punched out at 16 phi.

[0083] The resistance factor of the lithium-sulfur secondary batteries manufactured using the electrolytes of the Examples or Comparative Examples was measured and calculated by the method described in International Patent Publication WO2016 / 126075.

[0084] More specifically, for the lithium-sulfur secondary battery, first, a plurality of initial charge voltage data and a plurality of end-of-charge voltage data corresponding to changes in the magnitude of the charge current for each temperature and state of charge (SOC) of the secondary battery were measured and stored in a memory. Next, an end-of-charge IV profile was determined from the plurality of end-of-charge voltage data, and an intersection point where the end-of-charge IV profile meets a boundary line corresponding to a preset upper limit charge current or upper limit charge voltage as an upper limit charge condition was determined. Furthermore, an initial charge IV profile was determined from the plurality of initial charge voltage data, and a first derivative value of the initial charge IV profile calculated based on the current value at the intersection point was determined. The determined first derivative value was then determined and calculated as a resistance factor corresponding to the temperature and state of charge of the secondary battery.

[0085] At this time, a current greater than 2ea was used, and the resistance factor at the maximum current (Imax) was calculated using the following formula 1.

[0086]

number

[0087] The resistance factors at an SOC of 80% for the batteries fabricated using the electrolytes of each Example and Comparative Example are summarized in Table 3 below. Figure 2a shows the relationship between the Gmix1-Gmix2 (kcal / mol) of the electrolyte and the resistance factor (mΩ) of the secondary battery at an SOC of 80% for the lithium-sulfur secondary batteries fabricated using the electrolytes of Examples 1-3 and Comparative Examples 1-3. Figure 2b shows the relationship between the Gmix1-Gmix3 (kcal / mol) of the electrolyte and the resistance factor (mΩ) of the secondary battery at an SOC of 80% for the lithium-sulfur secondary batteries fabricated using the electrolytes of Examples 1-3 and Comparative Examples 1-3.

[0088] [Table 3]

[0089] 2a and 2b, it was confirmed that the lithium-sulfur secondary batteries fabricated using the electrolytes of the Examples exhibited a lower resistance factor at an SOC of 80% than the Comparative Examples. In particular, the Examples exhibited higher values ​​than the Comparative Examples, with Gmix1-Gmix2 being 0.73 kcal / mol or more and Gmix1-Gmix3 being 0.80 kcal / mol or more, demonstrating a lower resistance factor and superior output.

Claims

1. An electrolyte for a lithium-sulfur secondary battery, comprising a lithium salt, a non-aqueous solvent, and an additive, The electrochemical properties of the electrolyte and dilithiopertetrasulfide (Li) calculated at room temperature (20±5°C) according to the COSMO-RS (Conductor-like Screening Model for real solvent) theory were ２ S ４ ) and the first mixing energy (Gmix1) with the electrolyte and dilithioperhexasulfide (dilithio perhexasulfide; Li ２ S ６ ) and the second mixing energy (Gmix2) with the electrolyte and dilithioperoctasulfide (dilithio peroctasulfide; Li ２ S ８ a third mixing energy (Gmix3) of the lithium-sulfur secondary battery electrolyte with the sulfur-containing electrolyte (Sb) satisfies the relationship that Gmix1-Gmix2 is 0.73 kcal / mol or more, and Gmix1-Gmix3 is 0.80 kcal / mol or more.

2. 2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein Gmix1-Gmix2 is 0.74 kcal / mol to 1.40 kcal / mol, and Gmix1-Gmix3 is 0.80 kcal / mol to 1.50 kcal / mol.

3. 2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein Gmix2-Gmix3 is 0.05 kcal / mol or more.

4. The first mixing energy (Gmix1) is −3.02 kcal / mol to −0.02 kcal / mol; The second mixing energy (Gmix2) is −3.39 kcal / mol to −0.83 kcal / mol; 2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the third mixing energy (Gmix3) is −3.15 kcal / mol to −0.96 kcal / mol.

5. The non-aqueous solvent is lithium polysulfide (Li ２ S ｎ n is 4, 6, or 8), comprising a first organic solvent and a second organic solvent having different solubilities for the solvent; The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the second organic solvent is a non-solvent for the lithium polysulfide.

6. The non-aqueous solvent contains 70% by volume to 85% by volume of an ether-based solvent including a dialkyl ether-based solvent and an alkylene glycol dialkyl ether-based solvent as the first organic solvent, The second organic solvent contains 15% by volume to 30% by volume of a furan-based solvent or a tetrahydrofuran-based solvent, 6. The electrolyte for a lithium-sulfur secondary battery according to claim 5, wherein the dialkyl ether solvent is contained in an amount of 40% by volume to 85% by volume based on the total volume of the non-aqueous solvent.

7. 7. The electrolyte for a lithium-sulfur secondary battery according to claim 6, wherein the furan-based solvent or tetrahydrofuran-based solvent comprises at least one solvent selected from the group consisting of furan, 2-methylfuran, 3-methylfuran, 2-ethylfuran, 2-propylfuran, 2-butylfuran, 2,3-dimethylfuran, 2,4-dimethylfuran, 2,5-dimethylfuran, 2-methyltetrahydrofuran, and 4-methyltetrahydrofuran.

8. 7. The electrolyte for a lithium-sulfur secondary battery according to claim 6, wherein the dialkyl ether solvent comprises at least one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, and methoxyethoxyethane.

9. 7. The electrolyte for a lithium-sulfur secondary battery according to claim 6, wherein the alkylene glycol dialkyl ether solvent comprises at least one selected from the group consisting of diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, and polyethylene glycol methyl ethyl ether.

10. 2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the lithium salt is dissolved in the non-aqueous solvent at a concentration of 0.2 mol % to 0.8 mol %.

11. The lithium salts include LiCl, LiBr, LiI, and LiClO. ４ , LiBF ４ , LiB １０ Cl １０ , LiPF ６ , LiCF ３ SO ３ , LiCF ３ CO ２ , LiC ４ BO ８ , LiAsF ６ , LiSbF ６ , LiAlCl ４ , C.H. ３ SO ３ Li, CF ３ SO ３ Li, (CF ３ SO ２ ) ２ NLi, (C ２ F ５ SO ２ ) ２ NLi, (SO ２ F) ２ NLi(LiFSI) and (CF ３ SO ２ ) ３ 2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, comprising one or more selected from the group consisting of CLi.

12. 2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the additive is contained in an amount of 0.8 wt % to 4.0 wt % based on the total weight of the electrolyte.

13. The additive is lithium nitrate (LiNO ３ ), lithium thiocyanate (LiSCN), potassium nitrate (KNO ３ ), cesium nitrate (CsNO ３ ), magnesium nitrate (MgNO ３ ), barium nitrate (BaNO ３ ), lithium nitrite (LiNO ２ ), potassium nitrite (KNO ２ ) and cesium nitrite (CsNO ２ 2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, comprising one or more selected from the group consisting of:

14. a positive electrode containing sulfur as a positive electrode active material; a negative electrode comprising metallic lithium; a separator interposed between the positive electrode and the negative electrode; and A lithium-sulfur secondary battery comprising the electrolyte according to any one of claims 1 to 13.

15. The positive electrode active material is elemental sulfur (S ８ 15. The lithium-sulfur secondary battery of claim 14, comprising:

16. 15. The lithium-sulfur secondary battery of claim 14, wherein when initial-charge voltage and end-of-charge voltage profiles corresponding to changes in charging current are measured and derived for the lithium-sulfur secondary battery, and a resistance factor corresponding to the state of charge (SOC) of the secondary battery is calculated from the profiles, the secondary battery exhibits a resistance factor of 400 mΩ or less at an SOC of 80%.

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