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

Abstract

The present invention relates to an electrolyte for a lithium-sulfur secondary battery capable of improving the output characteristics of the lithium-sulfur secondary battery, 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 is a mixture of an electrolyte and dilithioperoctasulfide (Li 2 S 8 ) and the first mixing energy (Gmix1) of the electrolyte and lithium sulfide (Li 2 S) and the second mixing energy (Gmix2) have a certain range.

Description

Technical Field

[0001] Cross - reference to Related Applications This application claims the benefit of priority based on Korean Patent Application No. 10 - 2022 - 0107417 filed on August 26, 2022, and all the contents disclosed in the literature of the Korean patent application are included as part of this specification.

[0002] The present invention relates to an electrolyte for a lithium - sulfur secondary battery capable of improving the output characteristics of a lithium - sulfur secondary battery and a lithium - sulfur secondary battery including the same.

Background Art

[0003] As the application areas of secondary batteries expand to electric vehicles (EVs) and energy storage systems (ESSs), etc., lithium - ion secondary batteries with relatively low weight - specific energy storage density are gradually reaching their limits. Therefore, recently, interest in various next - generation secondary batteries with high energy density has been increasing. Among them, research and development on lithium - sulfur secondary batteries, which theoretically have high weight - specific energy storage density, are being actively conducted.

[0004] Generally, such a lithium - sulfur secondary battery refers to a charge / dischargeable battery system that includes a sulfur molecule having an S - S bond (Sulfur - Sulfur Bond) (for example, S 8 ) or a sulfur - containing composite, etc., as a positive electrode active material and includes metallic lithium, etc., as a negative electrode active material. Such a lithium - sulfur secondary battery has abundant reserves worldwide and can be manufactured at relatively low cost by using sulfur having a small weight relative to the metal as a positive electrode active material, enabling the realization of a secondary battery with a very large weight - specific energy density.

[0005] As also shown in FIG. 1, in the discharging process of the lithium - sulfur secondary battery, in each electrode and electrolyte, sulfur contained in the positive electrode (for example, S 8A continuous reduction reaction of 2 and a continuous oxidation reaction of metallic lithium contained in the negative electrode occur. As such continuous oxidation / reduction reactions occur, multiple types of lithium polysulfide (LiPS) are formed in the electrolyte and are movable between the electrodes, and finally formed solid-state lithium sulfide (Li 8 →Li 2 S 8 →Li 2 S 6 →Li 2 S 4 →Li 2 S 2 →Li 2 S, and among these, S 8 , Li 2 S 2 and Li 2 S have a solid state, and the remaining lithium polysulfide (Li 2 S n ; n is 4, 6 or 8) can have a liquid state dissolved in the electrolyte.

[0006] However, in the process where sulfur, lithium polysulfide and lithium sulfide with different phases are sequentially and continuously converted, since such a conversion process is not smooth and not carried out quickly, the lithium-sulfur secondary battery has lower output characteristics compared to existing lithium-ion secondary batteries. Also, in the process of forming lithium polysulfide by the continuous oxidation / reduction reaction, a part of the lithium polysulfide dissolved in the electrolyte may cause side reactions with the negative electrode, which can be a factor in reducing the life characteristics of the lithium-sulfur secondary battery.

[0007] Such low output characteristics and life characteristics etc. are the main factors that make the application of lithium-sulfur secondary batteries difficult, and thus, the development of lithium-sulfur secondary batteries with improved output characteristics etc. has been continuously required.

Summary of the Invention

Problems to be Solved by the Invention

[0008] Therefore, the present invention provides an electrolyte for a lithium-sulfur secondary battery that can smooth the continuous conversion process from sulfur contained in the positive electrode to lithium polysulfide and lithium sulfide, and improve the output characteristics of the lithium-sulfur secondary battery.

[0009] The present invention also provides a lithium-sulfur secondary battery having improved output characteristics by including the electrolyte.

Means for Solving the Problems

[0010] The present invention is an electrolyte for a lithium-sulfur secondary battery containing a lithium salt, a non-aqueous solvent, and an additive, According to the COSMO-RS (Conductor like Screening Model for real Solvent) theory, the first mixing energy (Gmix1) between the electrolyte and dilithio peroctasulfide (Li 2 S 8 ) calculated at room temperature (20 ± 5°C) is -1.0 kcal / mol or less, and the second mixing energy (Gmix2) between the electrolyte and lithium sulfide (Li 2 S) is 4.0 kcal / mol or more. The present invention provides an electrolyte for a lithium-sulfur secondary battery.

[0011] The present invention also provides 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.

Advantages of the Invention

[0012] Among lithium polysulfide and lithium sulfide generated during the charge / discharge process of the lithium-sulfur secondary battery, the electrolyte of the present invention is dilithio peroctasulfide (Li 2 S8 ) and lithium sulfide (Li 2 S) is optimized so that it has a certain mixing energy when mixed with these.

[0013] By applying such an electrolyte, in the discharge process of the lithium-sulfur secondary battery, the continuous reduction and conversion processes of sulfur (e.g., S 8 ), lithium polysulfide and lithium sulfide were confirmed to proceed smoothly and rapidly. As a result, the lithium-sulfur secondary battery can exhibit improved output characteristics compared to conventionally known batteries of the same type, particularly in the range of 70 to 80% of the state of charge (SOC) where the output characteristics are the lowest in existing batteries.

[0014] In addition, in the lithium-sulfur secondary battery, since the conversion process of the lithium polysulfide and lithium sulfide proceeds smoothly, side reactions between the lithium polysulfide and the negative electrode can be reduced, which can also contribute to the improvement of the life characteristics of the lithium-sulfur secondary battery.

[0015] Therefore, the present invention can contribute to the improvement of the output characteristics and life characteristics, which have been the most significant obstacles to the commercialization of lithium-sulfur secondary batteries.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0017] Hereinafter, with reference to the attached drawings, an electrolyte according to a specific embodiment of the invention and a lithium-sulfur secondary battery containing the same will be specifically described.

[0018] FIG. 1 is a schematic diagram showing that a plurality of lithium polysulfides are formed in an electrolyte by oxidation and reduction reactions during the charge / discharge process of a lithium-sulfur secondary battery. FIG. 2 is a graph showing the change patterns of the cell voltage and specific capacity of a lithium-sulfur secondary battery when a plurality of lithium polysulfides (Li 2 S n ; n is 2, 4, 6 or 8) are formed in the electrolyte by oxidation and reduction reactions during the charge / discharge process.

[0019] According to an embodiment of the invention, there is provided an electrolyte for a lithium-sulfur secondary battery containing a lithium salt, a non-aqueous solvent and an additive, wherein, according to the COSMO-RS (Conductor like Screening Model for real Solvent) theory, the first mixing energy (Gmix1) of the electrolyte and dilithio peroctasulfide (Li 2 S 8 ) calculated at room temperature (20 ± 5°C) is -1.0 kcal / mol or less, and the second mixing energy (Gmix2) of the electrolyte and lithium sulfide (Li 2 S) is 4.0 kcal / mol or more.

[0020] In order to develop an electrolyte capable of improving the output characteristics and the like of a lithium-sulfur secondary battery, the inventors continued their research on its characteristics. In particular, during the S 8 →Li 2 S 8 →Li 2 S 6 →Li 2 S 4 →Li 2 S 2 →Li 2 continuous reduction / conversion process of S, during the phase change process of solid (S 8 )→liquid (Li 2 S n ; n is 4, 6 or 8)→solid (Li 2 S 2 and Li 2 S) is inevitably accompanied, the inventors paid attention to the fact that the continuous conversion process to the lithium polysulfide and lithium sulfide is difficult to proceed smoothly.

[0021] Therefore, as shown in FIG. 2, the inventors measured the change pattern of the cell voltage and the like during the discharge process of the lithium-sulfur secondary battery. As a result of such measurement, it was confirmed that different discharge voltages appear at each stage due to the continuous reduction / conversion process to the plurality of types of lithium polysulfide and lithium sulfide. In particular, it was confirmed that an overvoltage is applied in the circularly displayed section of FIG. 2 and the output characteristics are lowered. The said section corresponds to the section corresponding to the state of charge (SOC) 70 to 80% of the secondary battery.

[0022] In the circularly displayed section, while the reduction / conversion from solid sulfur (S 8 etc.) to liquid Li 2 S 8 is carried out, from the Li 2 S 8 through lithium polysulfides such as Li 2 S 6 , Li 2 S 4 to solid Li 2 S 2 , Li2 It is predicted that the section where the conversion to and generation of S will start. Therefore, it is predicted that the output characteristics of the lithium-sulfur secondary battery will be low because the phase change process is not smooth and overvoltage is applied.

[0023] Based on such predictions, among the multiple types of lithium polysulfides and lithium sulfides generated during the charge / discharge process of the lithium-sulfur secondary battery, the inventors have completed an electrolyte of an embodiment in which the solubility of the dilithio octasulfide (Li 2 S 8 ) and lithium sulfide (Li 2 S) and the mixing energy during mixing with these are optimized.

[0024] For reference, the first and second mixing energies refer to the solvation free energy during the mixing of the electrolyte and Li 2 S 8 or Li 2 S, and are calculated based on quantum mechanical calculations by the COSMO-RS theory based on each molecular structure information (for example, refer to "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 are calculated by utilizing commercialized COSMOtherm software (COSMOlogic GmbH & Co. KG) that performs quantum mechanical calculations by the COSMO-RS theory.

[0025] Such first and second mixing energies can reflect the solubility of the electrolyte of the embodiment and Li 2 S 8 or Li 2 S. The electrolyte of an embodiment satisfies a specific composition described later, etc., and thus Li 2 S 8has a lower first mixing energy (i.e., higher solubility) when mixed, and Li 2 S can exhibit the characteristic of having a high second mixing energy (i.e., low solubility).

[0026] By applying the electrolyte of such an embodiment to a lithium-sulfur secondary battery, due to the low mixing energy and high solubility for the said Li 2 S 8 the reduction / conversion process is carried out rapidly and smoothly. At the same time, due to the high mixing energy and low solubility for Li 2 S, the conversion from lithium polysulfide containing the said Li 2 S 8 to solid-state Li 2 S 2 、Li 2 S is carried out very smoothly.

[0027] As a result, during the discharge process of the said lithium-sulfur secondary battery, the continuous reduction and conversion processes of sulfur (e.g., S 8 )、lithium polysulfide and lithium sulfide are carried out smoothly and rapidly. It has been confirmed that the lithium-sulfur secondary battery containing the electrolyte of the said embodiment can show improved output characteristics compared with the conventionally known ones, especially improved output characteristics in the SOC 70 - 80% range where the output characteristics of existing batteries are low.

[0028] In addition, since the conversion processes of the said lithium polysulfide and lithium sulfide in the lithium-sulfur secondary battery are carried out smoothly, side reactions between the said lithium polysulfide and the negative electrode can be reduced, which can also contribute to the improvement of the life characteristics of the lithium-sulfur secondary battery.

[0029] In the electrolyte of the above-described embodiment, the first mixing energy (Gmix1) may be -2.50 kcal / mol to -1.05 kcal / mol, or -2.00 kcal / mol to -1.10 kcal / mol, and the second mixing energy (Gmix2) may be 5.50 kcal / mol to 9.00 kcal / mol, or 6.20 kcal / mol to 8.80 kcal / mol.

[0030] In case the first mixing energy becomes excessively high or the second mixing energy becomes excessively low, the conversion process of the lithium polysulfide and lithium sulfide may not be smooth, and the output characteristics of the lithium-sulfur secondary battery may not be sufficiently improved. Conversely, if the first mixing energy becomes excessively low or the second mixing energy becomes excessively high, the stability of the electrolyte of the above-described embodiment may decrease, and as a result, the life characteristics of the lithium-sulfur secondary battery may decrease.

[0031] In addition, the electrolyte of the above-described embodiment can also exhibit optimized mixing energy and solubility with respect to the remaining lithium polysulfide (Li 2 S n ; n is 2, 4 or 6) generated additionally during the discharge process of the lithium-sulfur secondary battery. As a result, the continuous reduction / conversion process during the discharge process is smoothly performed, the output characteristics of the secondary battery are further improved, while side reactions between the lithium polysulfide and the negative electrode are suppressed, and the life characteristics of the secondary battery can be improved.

[0032] For example, the mixing energy of the electrolyte and Li 2 S 2 can be 1.5 kcal / mol or more and can have a value 2.5 kcal / mol or more smaller than the second mixing energy (Gmix2). In a specific example, the mixing energy may be 2.00 kcal / mol to 6.50 kcal / mol, or 3.00 kcal / mol to 5.00 kcal / mol.

[0033] In addition, the electrolyte and Li 2 S4 or Li 2 S 6 The mixing energy with is greater than the first mixing energy (Gmix1) by 0.05 kcal / mol or more and may be 0.01 kcal / mol or less. In a more specific example, the electrolyte and Li 2 S 4 The mixing energy with is -1.50 kcal / mol to 0.01 kcal / mol, or -0.90 kcal / mol to 0.01 kcal / mol, and the electrolyte and Li 2 S 6 The mixing energy with may be -2.40 kcal / mol to -0.90 kcal / mol, or -2.00 kcal / mol to -1.00 kcal / mol.

[0034] On the other hand, the electrolyte of one embodiment basically includes a lithium salt, a non-aqueous solvent, and an additive. In particular, it includes a specific solvent composition as the non-aqueous solvent, and controls the concentration / content of the lithium salt and other additives within a certain range, so as to satisfy the mixing energy with the above-mentioned lithium polysulfide and lithium sulfide.

[0035] First, in order for the electrolyte to satisfy the above-mentioned mixing energy range, the non-aqueous solvent includes a furan-based compound or a tetrahydrofuran-based compound and a chain ether-based compound in a volume ratio (v / v) of 1:1 to 1:5, or 1:1.04 to 1:4.50, and does not include a dioxolane-based compound.

[0036] In a more specific example, the non-aqueous solvent can include 15 to 50 volume %, or 20 to 49 volume % of one or more of the furan-based compound or the tetrahydrofuran-based compound, and 50 to 85 volume %, or 51 to 80 volume % of one or more of the chain ether-based compound, and does not include additional solvents such as the dioxolane-based compound.

[0037] At this time, the furan-based compound or tetrahydrofuran-based compound and the chain ether-based compound can each act as a solvent and a non-solvent having different solubilities with respect to the lithium polysulfide. Therefore, by containing such a solvent and a non-solvent in an optimal ratio, the above-described first and second mixing energies are satisfied, and the output characteristics of the lithium-sulfur battery can be further improved.

[0038] However, when the content of the furan-based compound or tetrahydrofuran-based compound becomes excessively large, or when a solvent such as the dioxolane-based compound is included, the first mixing energy of the electrolyte becomes large, or the second mixing energy becomes small, and the improvement of the output characteristics of the lithium-sulfur secondary battery may not be successfully performed. Conversely, when the content of the furan-based compound or tetrahydrofuran-based compound becomes excessively small, the formation of the protective film due to the reaction between such a solvent and the metallic lithium of the negative electrode is not sufficient, and the life characteristics of the lithium-sulfur secondary battery may not be sufficient.

[0039] This is because the furan-based compound or tetrahydrofuran-based compound can suppress the generation of lithium dendrites by forming a SEI layer (solid electrolyte interface) on the surface of the metallic lithium, and can suppress the decomposition of the electrolyte solution on the surface of the metallic lithium of the negative electrode.

[0040] In the composition of the non-aqueous solvent described above, as the furan-based compound or tetrahydrofuran-based compound, a furan-based compound or tetrahydrofuran-based compound in which an alkyl group having 1 to 4 carbon atoms is substituted or unsubstituted can be used. 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.

[0041] Among them, in consideration of the fulfillment of the above-described first and second mixing energies, 2-methylfuran, 2-methyltetrahydrofuran, 4-methyltetrahydrofuran, or a combination of two or more selected from these can be preferably used. For example, the 2-methylfuran may be combined with 2-methyltetrahydrofuran or 4-methyltetrahydrofuran. In this case, 2-methylfuran:2-methyltetrahydrofuran or 4-methyltetrahydrofuran can be mixed at a volume ratio of 30:1 to 1:30.

[0042] In addition, as the chain ether compound, a compound in which one or more alkyl groups or alkylene glycol groups having 1 to 10 carbon atoms or 1 to 5 carbon atoms are bonded to an alkyl group having 1 to 5 carbon atoms via an ether bond (-O-) can be used. Specific examples thereof include dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, 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 one or more selected from the group consisting of polyethylene glycol methyl ethyl ether.

[0043] Among them, in consideration of the fulfillment of the above-described first and second mixed energies, etc., dimethoxyethane, diethylene glycol dimethyl ether, or triethylene glycol dimethyl ether, or a combination of two or more selected from these can be preferably used. For example, the dimethoxyethane and ethylene glycol dimethyl ether or triethylene glycol dimethyl ether may be combined, and in this case, dimethoxyethane:ethylene glycol dimethyl ether or triethylene glycol dimethyl ether can be mixed at a volume ratio of 10:1 to 1:10, or 5:1 to 1:1.

[0044] On the other hand, as the lithium salt, it is an electrolyte salt used to increase ionic conductivity, and any one commonly used in the art can be used without limitation. Specific examples of such lithium salts include LiCl, LiBr, LiI, LiClO 4 , LiBF 4 , LiB 10Cl 10 、LiPF 6 、LiCF 3 SO 3 、LiCF 3 CO 2 、LiC 4 BO 8 、LiAsF 6 、LiSbF 6 、LiAlCl 4 、CH 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 and (CF 3 SO 2 ) 3 One or more selected from the group consisting of CLi. However, considering the electrical conductivity of the lithium-sulfur battery or the charging of the mixed energy described above, lithium salts in sulfonate form, for example, CH 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 or (CF 3 SO 2 ) 3 CLi can be used appropriately.

[0045] The lithium salt is contained in the electrolyte at a concentration of 0.2 to 0.8 mol%, or 0.23 to 0.75 mol%, or 0.25 to 0.70 mol% so that the above-described electrolyte can appropriately satisfy the first and second mixing energies. In the unlikely event that the concentration of the lithium salt becomes excessively low, the electrical conductivity of the lithium-sulfur secondary battery is insufficient. When the concentration of the lithium salt becomes excessively high, the second mixing energy may become low or the first mixing energy may become high, resulting in insufficient output characteristics of the lithium-sulfur secondary battery.

[0046] On the other hand, the electrolyte of the above-described embodiment further includes an additive for improving the characteristics of the lithium-sulfur secondary battery in addition to the non-aqueous solvent and the lithium salt. Examples of such additives include lithium nitrate (LiNO 3 ), potassium nitrate (KNO 3 ), cesium nitrate (CsNO 3 ), magnesium nitrate (Mg(NO 3 ) 2 ), barium nitrate (Ba(NO 3 ) 2 ), lithium nitrite (LiNO 2 ), potassium nitrite (KNO 2 ), and cesium nitrite (CsNO 2 ), and one or more selected from the group consisting of these can be mentioned. Among them, lithium nitrate can be preferably used.

[0047] Also, the additive is contained in the electrolyte at a content of 0.8 to 4.0 wt%, 0.9 to 3.5 wt%, 0.8 to 3.0 wt%, or 0.9 to 2.0 wt%. When the content of the additive becomes excessively low, the characteristics of the lithium-sulfur secondary battery may be insufficient. When the content of the additive becomes excessively high, the first or second mixing energy of the electrolyte cannot satisfy the above-described range, and it is difficult to realize the output characteristics of the lithium-sulfur secondary battery.

[0048] As described above, the electrolyte of one embodiment contains a non-aqueous solvent with a specific composition, together with a lithium salt and an additive within a certain content range, thereby satisfying the first and second mixed energies described above, and can exhibit optimized solubility with respect to lithium polysulfide and lithium sulfide formed during the discharge of the lithium-sulfur secondary battery. As a result, the formation of the lithium polysulfide and lithium sulfide can be smoothed during the discharge of the lithium-sulfur secondary battery, and the application of overvoltage during such a formation process can be reduced. As a result, the output characteristics of the lithium-sulfur secondary battery can be improved.

[0049] In particular, such a lithium-sulfur secondary battery can exhibit improved output characteristics, for example, 1.2 kW / kg or more, or 1.3 kW / kg or more, or an output characteristic of 1.3 to 3.0 kW / kg in the SOC 70 to 80% range where overvoltage was applied in existing batteries and particularly low output characteristics were shown.

[0050] Further, according to another embodiment of the 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 one embodiment. Such a lithium-sulfur secondary battery can exhibit improved output characteristics and life characteristics by including the electrolyte of one embodiment.

[0051] In the secondary battery of the other embodiment, the positive electrode may include a positive electrode active material and a binder, and in addition, may further include a conductive material. Further, the positive electrode may be one in which an active material layer containing the positive electrode active material and the binder is formed on a positive electrode current collector.

[0052] At this time, the positive electrode current collector is not particularly limited as long as it supports the active material layer and has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, fired carbon, those with a surface treatment of carbon, nickel, silver, etc. on the surface of copper or stainless steel, aluminum-cadmium alloy, etc. can be used.

[0053] In addition, the positive electrode current collector can form fine irregularities on its surface to strengthen the bonding force with the active material layer, and various forms such as films, sheets, foils, meshes, nets, porous bodies, foams, non-woven bodies, etc. can be used.

[0054] The positive electrode active material can contain sulfur. More specifically, elemental sulfur (Elemental sulfur, S 8 ), organic sulfur compounds, sulfur-carbon composites or polymers ((C 2 S x ): x = 2.5 to 50, n ≥ 2), etc. can be used as the positive electrode active material. Considering the characteristics of the electrolyte in one embodiment, the elemental sulfur (Elemental sulfur, S n ) can be appropriately used. 8 )

[0055] Such a positive electrode active material containing sulfur is contained 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 weight of the positive electrode. When the content of the positive electrode active material is low, the energy density of the secondary battery decreases, and when its content is excessively large, the conductivity and stability of the electrode may decrease.

[0056] In addition, the positive electrode can further contain one or more additives selected from transition metal elements, Group IIIA elements, Group IVA elements, sulfur compounds of these elements, and alloys of these elements and sulfur in addition to the positive electrode active material.

[0057] Examples of the transition metal element 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. Examples of the Group IIIA element include Al, Ga, In, and Ti. Examples of the Group IVA element include Ge, Sn, and Pb.

[0058] On the other hand, the binder is a component that helps bind the positive electrode active material and the current collector. For example, one or more selected from the group consisting of polyvinylidene fluoride (PVdF), polyvinylidene fluoride - hexafluoropropylene 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, polyvinyl pyridine, polyvinyl pyrrolidone, styrene - butadiene rubber, acrylonitrile - butadiene rubber, ethylene - propylene - diene monomer (EPDM) rubber, sulfonated EPDM rubber, styrene - butylene rubber, fluorine rubber, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, and mixtures thereof can be used, but it is not necessarily limited thereto.

[0059] The binder is usually 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 adhesion between the active material layer and the current collector becomes insufficient. If it exceeds 15 parts by weight, the adhesion improves, but the content of the positive electrode active material decreases accordingly, which may lead to a decrease in the battery capacity.

[0060] On the one hand, the conductive material is a component for improving electrical conductivity and is not particularly limited as long as it is an electron-conductive substance that does not cause a chemical change in the secondary battery. For example, as the conductive material, carbon black, graphite, carbon fiber, carbon nanotube, metal powder, conductive metal oxide, or organic conductive material can be used. Currently, commercially available products as conductive materials include acetylene black-based (products manufactured by Chevron Chemical Company or Gulf Oil Company, etc.), Ketjen Black EC-based (products manufactured by Armak Company), Vulcan XC-72 (products manufactured by Cabot Company), and Super P (products manufactured by MMM), etc. For example, acetylene black, carbon black, graphite, etc. can be mentioned.

[0061] Also, in the lithium-sulfur secondary battery of the other embodiment, a filler may be selectively added to the positive electrode as a component for suppressing expansion of a positive electrode active material containing sulfur, etc. Such a filler is not particularly limited as long as it can suppress the expansion of the electrode without inducing a chemical change in the battery. For example, olefin-based polymers such as polyethylene and polypropylene; fibrous substances such as glass fiber and carbon fiber; etc. can be used.

[0062] The positive electrode can be manufactured by dispersing and mixing a positive electrode active material, a conductive material, and a binder in a dispersion medium (solvent) to form a slurry, applying this onto a positive electrode current collector, and then drying and rolling. As the dispersion medium, NMP (N-methyl-2-pyrrolidone), DMF (Dimethyl formamide), DMSO (Dimethyl sulfoxide), ethanol, isopropanol, water, and mixtures thereof can be used, but it is not limited thereto.

[0063] In the secondary battery of the above-described other embodiments, the negative electrode may contain metallic lithium, and for example, may include a lithium metal or alloy layer formed on a negative electrode current collector.

[0064] Such a negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery, and can 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. As the alloy, an aluminum-cadmium alloy can be used. In addition, fired carbon, a non-conductive polymer or a conductive polymer surface-treated with a conductive material may also be used. Generally, a copper thin plate is applied as the negative electrode current collector.

[0065] The metallic lithium may be lithium metal or an alloy. At this time, the lithium alloy contains an element that can be alloyed with lithium. Specifically, it may be an alloy of lithium and one or more 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. In some cases, it may be in the form where lithium or a lithium alloy is deposited or coated on the current collector by a dry process, or in the form where particulate metal and alloy are deposited or coated by a wet process or the like.

[0066] A normal separator membrane can be interposed between the positive electrode and the negative electrode. The separator membrane is a physical separator membrane having a function of physically separating the electrodes, and can be used without particular limitation as long as it is a separator membrane normally used. In particular, those having low resistance to the movement of electrolyte ions and excellent electrolyte moisture retention ability are preferable. Further, the separator membrane enables the transport of lithium ions between the positive electrode and the negative electrode while separating or insulating the positive electrode and the negative electrode from each other. Such a separator membrane may be made of a porous and non-conductive or insulating material. The separator membrane may be an independent member such as a film, or a coating layer added to the positive electrode and / or the negative electrode.

[0067] Examples of the polyolefin-based porous membrane used as the separator membrane include membranes formed of polyolefin-based polymers such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, ultra-high molecular weight polyethylene, polypropylene, polybutylene, and polypentene, either alone or as a mixture of these. Examples of the non-woven fabric used as the separator membrane include non-woven fabrics formed 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 as a mixture of these. Such non-woven fabrics are in the form of fibers forming a porous web and include spunbond or meltblown forms composed of long fibers.

[0068] The thickness of the separation membrane is not particularly limited, but a range of 1 to 100 μm is preferred, and more preferably a range of 5 to 50 μm. When the thickness of the separation membrane is less than 1 μm, mechanical properties cannot be maintained. When it exceeds 100 μm, the separation membrane acts as a resistance layer and the performance of the battery deteriorates. The pore size and porosity of the separation membrane 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 separation membrane is less than 0.1 μm or the porosity is less than 10%, the separation membrane acts as a resistance layer. When the pore size exceeds 50 μm or the porosity exceeds 95%, mechanical properties cannot be maintained.

[0069] A lithium-sulfur secondary battery of another embodiment including the electrolyte, positive electrode, negative electrode, and separation membrane as described above can be manufactured by a process of facing the positive electrode to the negative electrode, interposing the separation membrane therebetween, and then injecting the electrolyte.

[0070] On the other hand, the lithium-sulfur secondary battery can be suitably used not only as a power source for small devices but also as a unit cell of a battery module that is a power source for medium and large devices. In this regard, a battery module including two or more lithium-sulfur secondary batteries electrically connected (in series or in parallel) can be provided.

[0071] Of course, the number of lithium-sulfur secondary batteries included in the battery module can be adjusted in various ways in consideration of the use and capacity of the battery module. Furthermore, a battery pack in which the battery modules are electrically connected can also be provided by the ordinary technology in this field. The battery module and the battery pack can be used as a power source for one or more medium to large-sized devices, including a power tool; an electric vehicle (EV), a hybrid electric vehicle (HEV), and a plug-in hybrid electric vehicle (PHEV); an electric truck; an electric commercial vehicle; or a power storage system, but are not necessarily limited thereto.

[0072] Hereinafter, preferred embodiments will be presented for the understanding of the invention. However, it is obvious to those skilled in the art that the following embodiments are merely illustrative of the invention and various changes and modifications are possible within the scope of the invention and the scope of the technical idea. It is natural that such changes and modifications belong to the appended claims.

[0073] Examples 1 to 8, Comparative Examples 1 and 2: Production of Electrolytes for Lithium-Sulfur Secondary Batteries After mixing non-aqueous solvents according to the composition in Table 1 below, lithium salts and additives were dissolved at the concentrations and contents in Table 1 to produce electrolytes for each example and comparative example.

[0074]

Table 1

[0075] Test Example 1: Calculation of Mixing Energy during Mixing of Electrolyte with Lithium Sulfide or Lithium Polysulfide Using the commercialized COSMOtherm software (COSMOlogic GmbH & Co. KG) based on the COSMO-RS theory, at room temperature (25 °C), the electrolyte of the examples or comparative examples was mixed with lithium polysulfide (Li 2 S n ; n is 2, 4, 6 or 8) or lithium sulfide (Li 2 S), and the mixing energy at the time of mixing was calculated respectively, and the calculation results are summarized in Table 2 below:

[0076]

Table 2

[0077] Referring to Table 2 above, the electrolytes of Examples 1 to 8 contain a non-aqueous solvent with a specific composition, and by containing a lithium salt and an additive at a certain concentration and content, the first mixing energy (Gmix1) with respect to Li 2 S 8 is -1.0 kcal / mol or less, and the second mixing energy (Gmix2) with respect to Li 2 S is 4.0 kcal / mol or more, and it was confirmed that the characteristics are satisfied.

[0078] On the other hand, the electrolytes of Comparative Examples 1 and 2 contain dioxolane compounds such as 2-methoxy-1,3-dioxolane in the non-aqueous solvent, or the volume ratio of the furan-based compound or tetrahydrofuran-based compound to the chain ether-based compound exceeds a certain range, and the concentration and content of the lithium salt and the additive are excessively large, so it was confirmed that the ranges of the first and second mixing energies cannot be satisfied.

[0079] Test Example 2: Manufacture and output characteristic evaluation of lithium-sulfur battery Electrolyte The electrolyte of the above examples or comparative examples was used.

[0080] Manufacture of positive electrode As the positive electrode active material, 95 parts by weight of a sulfur-carbon composite (weight ratio of S:C = 70:30) was used (the sulfur content alone was set to 67.5% by weight based on the total weight of the positive electrode, and the carbon material used was activated carbon with a pore volume of 1.8 cm 3 / g). 5 parts by weight of styrene-butadiene rubber / carboxymethyl cellulose (SBR:CMC = 7:3) was mixed as a binder to produce a positive electrode slurry composition. After that, the produced slurry composition was coated on a current collector (Al Foil), dried at 50 °C for 12 hours, and pressed with a roll press device to produce a positive electrode (at this time, the porosity of the electrode was set to 65%).

[0081] Manufacture of Lithium-Sulfur Battery The manufactured positive electrode and a lithium metal negative electrode with a thickness of 150 μm were positioned to face each other, and after interposing a polyethylene (PE) separator between them, the electrolyte was injected to manufacture a coin cell type lithium-sulfur battery. On the other hand, in the manufacture of the battery, the positive electrode was punched out and used at 15 phi, the polyethylene separator was punched out and used at 19 phi, and the lithium metal was punched out and used at 16 phi.

[0082] The resistance of the lithium-sulfur battery was measured by the voltage drop due to 5C10-second discharge, and based on this, the maximum output corresponding to the state of charge (SOC) was converted. The results of such conversion are shown in FIGS. 3 and 4, and the maximum output at SOC 70% is also shown in Table 3 below.

[0083]

Table 3

[0084] Referring to Table 3, FIGS. 3 and 4, it was confirmed that the lithium-sulfur secondary battery manufactured using the electrolyte of the example showed excellent output characteristics even at SOC 70% where the overvoltage was most greatly applied, and showed high output characteristics at the overall SOC.

[0085] In contrast, it was confirmed that the lithium-sulfur secondary battery of the comparative example could not show substantial output due to excessive overvoltage at SOC 70%.

Claims

1. An electrolyte for a lithium-sulfur secondary battery, comprising a lithium salt, a non-aqueous solvent, and an additive, According to the COSMO-RS (Conductor like Screening Model for real Solvent) theory, the first mixing energy (Gmix1) of the electrolyte and dilithio peroctasulfide (Li ２ S ８ ) calculated at room temperature (20 ± 5 °C) is -1.0 kcal / mol or less, and the second mixing energy (Gmix2) of the electrolyte and lithium sulfide (Li ２ S) is 4.0 kcal / mol or more. An electrolyte for a lithium-sulfur secondary battery.

2. wherein the first mixing energy (Gmix1) is -2.50 kcal / mol to -1.05 kcal / mol, and the second mixing energy (Gmix2) is 5.50 kcal / mol to 9.00 kcal / mol. The electrolyte for a lithium-sulfur secondary battery according to Claim 1.

3. The mixing energy of the electrolyte and lithium persulfide (Li ２ S ２ ) is 1.5 kcal / mol or more and has a value 2.5 kcal / mol or less smaller than the second mixing energy (Gmix2). The electrolyte for a lithium-sulfur secondary battery according to claim 1.

4. The electrolyte and the mixed energy with dilithio pertetrasulfide (Li ２ S ４ ) or dilithio perhexasulfide (Li ２ S ６ ) has a value greater than the first mixed energy (Gmix1) by 0.05 kcal / mol or more and is 0.01 kcal / mol or less. The electrolyte for a lithium-sulfur secondary battery according to claim 1.

5. The non-aqueous solvent contains a furan-based compound or a tetrahydrofuran-based compound and a chain ether-based compound in a volume ratio (v / v) of 1:1 to 1:5, and does not contain a dioxolane-based compound. The electrolyte for a lithium-sulfur secondary battery according to Claim 1.

6. The furan-based compound or the tetrahydrofuran-based compound contains 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. The electrolyte for a lithium-sulfur secondary battery according to Claim 5.

7. The chain ether compound-containing electrolyte for a lithium-sulfur secondary battery according to claim 5 contains one or more selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, 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.

8. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the lithium salt is contained at a concentration of 0.2 to 0.8 mol%.

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

10. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the additive is contained at a content of 0.8 to 4.0% by weight.

11. The additive is lithium nitrate (LiNO ３ ), potassium nitrate (KNO ３ ), cesium nitrate (CsNO ３ ), magnesium nitrate (Mg(NO ３ )) ２ ), barium nitrate (Ba(NO ３ )) ２ ), lithium nitrite (LiNO ２ ), potassium nitrite (KNO ２ ), and cesium nitrite (CsNO ２ ), and the electrolyte for a lithium-sulfur secondary battery according to claim 1, comprising one or more selected from the group consisting of these substances.

12. 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 A lithium-sulfur secondary battery containing the electrolyte according to claim 1.

13. The positive electrode active material contains sulfur element (Elemental sulfur, S ８ ), and the lithium-sulfur secondary battery according to claim 12.

14. The lithium-sulfur secondary battery according to claim 12, wherein the positive electrode contains a positive electrode active material and a binder.

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