Electrolyte for lithium-sulfur secondary batteries and lithium-sulfur secondary batteries 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 of related applications This application claims priority under Korean Patent Application No. 10-2022-0107417 dated August 26, 2022, and all content disclosed in the said Korean Patent Application is incorporated herein as part of this specification.

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

[0003] As the applications of secondary batteries expand to electric vehicles (EVs) and energy storage systems (ESS), lithium-ion secondary batteries, which have a relatively low energy storage density relative to their weight, are gradually reaching their limits. Therefore, there has recently been growing interest in a variety of next-generation secondary batteries with high energy densities, and among them, research and development of lithium-sulfur secondary batteries, which theoretically have a high energy storage density relative to their weight, is being actively pursued.

[0004] Generally, such lithium-sulfur secondary batteries refer to rechargeable battery systems that include sulfur molecules with SS bonds (e.g., S8) or sulfur-containing composites as the positive electrode active material, and metallic lithium as the negative electrode active material. Such lithium-sulfur secondary batteries can be manufactured at a relatively low cost by using sulfur, which has abundant reserves worldwide and a small weight relative to metals, as the positive electrode active material, and enable the realization of secondary batteries with a very high energy density relative to weight.

[0005] As shown in Figure 1, during the discharge process of the lithium-sulfur secondary battery, a continuous reduction reaction of sulfur (e.g., S8) contained in the positive electrode and a continuous oxidation reaction of metallic lithium contained in the negative electrode occur within each electrode and electrolyte. As these continuous oxidation / reduction reactions occur, multiple types of lithium polysulfide (LiPS) are formed in the electrolyte and can move between electrodes, and the final solid lithium sulfide (Li2S) accumulates on the negative electrode. For example, the reaction process of lithium polysulfide and lithium sulfide due to the continuous reduction reaction of sulfur can be summarized as S8 → Li2S8 → Li2S6 → Li2S4 → Li2S2 → Li2S, of which S8, Li2S2, and Li2S are in a solid state, and the remaining lithium polysulfide (Li2S) n (n is 4, 6, or 8) can be in a liquid state dissolved in an electrolyte.

[0006] However, in the process of sequentially and continuously converting sulfur, lithium polysulfide, and lithium sulfide, which are in different phases, if this conversion process is not smooth and does not occur quickly, lithium-sulfur secondary batteries have lower power output characteristics compared to existing lithium-ion secondary batteries. In addition, during the process of forming lithium polysulfide through the continuous oxidation / reduction reaction, some of the lithium polysulfide dissolved in the electrolyte may undergo side reactions with the negative electrode, which can be one of the factors that reduces the lifespan characteristics of lithium-sulfur secondary batteries.

[0007] Such low power output and lifespan characteristics are considered the main reasons why lithium-sulfur secondary batteries are difficult to apply, and as a result, there is a continuing demand for the development of lithium-sulfur secondary batteries with improved power output and other characteristics. [Overview of the project] [Problems that the invention aims to solve]

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

[0009] The present invention also provides a lithium-sulfur secondary battery having improved output characteristics due to the inclusion of the electrolyte. [Means for solving the problem]

[0010] The present invention relates to an electrolyte for lithium-sulfur secondary batteries comprising a lithium salt, a non-aqueous solvent, and an additive, The present invention provides an electrolyte for lithium-sulfur secondary batteries in which, according to the COSMO-RS (Conductor-like Screening Model for real Solvent) theory, the first mixing energy (Gmix1) of the electrolyte and dilithio peroctasulfide (Li2S8), calculated at room temperature (20±5℃), is -1.0 kcal / mol or less, and the second mixing energy (Gmix2) of the electrolyte and lithium sulfide (Li2S) is 4.0 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 separation membrane interposed between the positive and negative electrodes; and the electrolyte. [Effects of the Invention]

[0012] The electrolyte of the present invention has optimized solubility for dilithioperoctasulfide (Li2S8) and lithium sulfide (Li2S), which are among the lithium polysulfide and lithium sulfide produced during the charging / discharging process of lithium-sulfur secondary batteries, and possesses a constant mixing energy when mixed with these.

[0013] By applying such an electrolyte, it was confirmed that the lithium-sulfur secondary battery undergoes a smooth and rapid continuous reduction and conversion process of sulfur (e.g., S8), lithium polysulfide, and lithium sulfide during its discharge process. As a result, it was confirmed that the lithium-sulfur secondary battery exhibits improved power characteristics compared to conventionally known batteries of the same type, particularly improved power characteristics in the 70-80% State of Charge (SOC) range, which is where existing batteries typically exhibit the lowest power characteristics.

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

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

[0016] [Figure 1] This is a schematic diagram illustrating the formation of multiple types of lithium polysulfides in the electrolyte through oxidation and reduction reactions during the charging and discharging process of a lithium-sulfur secondary battery. [Figure 2] This graph shows the changes in cell voltage and specific capacity of a lithium-sulfur secondary battery as multiple types of lithium polysulfide (Li2Sn; n is 2, 4, 6, or 8) are formed in the electrolyte through oxidation and reduction reactions during the charging and discharging process. [Figure 3] This graph shows the maximum output (kW / kg) according to the state of charge (SOC;%) of lithium-sulfur batteries manufactured using the electrolytes of Examples 1-4 and Comparative Examples 1 and 2. [Figure 4] This graph shows the maximum output (kW / kg) of lithium-sulfur batteries manufactured using the electrolytes of Examples 5-8, according to their state of charge (SOC;%).

Mode for Carrying Out the Invention

[0017] Hereinafter, referring to the attached drawings, an electrolyte according to a specific embodiment of the invention and a lithium-sulfur secondary battery including 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 the lithium-sulfur secondary battery when a plurality of lithium polysulfides (Li2S n ; n is 2, 4, 6 or 8) are formed 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 including 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 (Li2S8) 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 (Li2S) is 4.0 kcal / mol or more.

[0020] The inventors of the present invention continued research on the characteristics of an electrolyte in order to develop an electrolyte capable of improving the output characteristics and the like of a lithium-sulfur secondary battery. In particular, during the continuous reduction / conversion process of S8→Li2S8→Li2S6→Li2S4→Li2S2→Li2S that occurs during the discharge process of a lithium-sulfur battery, solid (S8)→liquid (Li2S n; when n is 4, 6, or 8) → Paying attention to the fact that the phase change process of the solid (Li2S2 and Li2S) is inevitably accompanied, it is difficult to smoothly perform the continuous conversion process to the lithium polysulfide and lithium sulfide.

[0021] Therefore, the inventors of the present invention measured the change pattern of the cell voltage and the like during the discharge process of the lithium-sulfur secondary battery as shown in FIG. 2. 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. Among them, it was confirmed that an overvoltage is applied in the circularly displayed section of FIG. 2 and the output characteristics are low. This section corresponds to the section corresponding to the state of charge (SOC) of the secondary battery of 70 to 80%.

[0022] It is predicted that in the circularly displayed section, reduction / conversion from solid sulfur (such as S8) to liquid Li2S8 is performed, while conversion and generation of solid Li2S2 and Li2S start via lithium polysulfide such as Li2S6 and Li2S4 from Li2S8. Therefore, it is predicted that the output characteristics of the lithium-sulfur secondary battery will be lowered due to the non-smooth phase change process and the application of an overvoltage.

[0023] Based on such a prediction, the inventors of the present invention completed an electrolyte of an embodiment in which the solubility of the dilithioperoxooctasulfide (Li2S8) and lithium sulfide (Li2S) and the mixing energy when mixed with them are optimized among the plurality of types of lithium polysulfide and lithium sulfide generated during the charge / discharge process of the lithium-sulfur secondary battery.

[0024] For reference, the first and second mixing energies refer to the solvation free energies when the electrolyte and Li2S8 or Li2S are mixed, and are calculated based on quantum mechanical calculations using the COSMO-RS theory based on the molecular structure information of each molecule (see, for example, "COSMO-RS: From Quantum Chemistry to Fluid Phase Thermodynamics and Drug Design", A. Klamt, Elsevier; Amsterdam, The Netherlands, 2005 and Korean Published Patent No. 2019-0011963). In more specific examples, the first and second mixing energies are calculated using commercially available COSMOtherm software (COSMOlogic GmbH & Co. KG) that performs quantum mechanical calculations using the COSMO-RS theory.

[0025] Such first and second mixing energies can reflect the solubility of the electrolyte of the embodiment and Li2S8 or Li2S. The electrolyte of the embodiment can exhibit characteristics such as a lower first mixing energy (i.e., higher solubility) when mixed with Li2S8 and a higher second mixing energy (i.e., lower solubility) when mixed with Li2S, by satisfying specific compositions described later.

[0026] By applying such an electrolyte embodiment to a lithium-sulfur secondary battery, the reduction / conversion process for Li2S8 is carried out quickly and smoothly due to its low mixing energy and high solubility, and at the same time, the conversion from lithium polysulfide containing Li2S8 to solid-state Li2S2 and Li2S is carried out very smoothly due to its high mixing energy and low solubility for Li2S.

[0027] As a result, it was confirmed that during the discharge process of the lithium-sulfur secondary battery, the continuous reduction and conversion processes of sulfur (e.g., S8), lithium polysulfide, and lithium sulfide proceeded smoothly and rapidly, and that the lithium-sulfur secondary battery containing the electrolyte of the above embodiment could exhibit improved output characteristics compared to conventionally known batteries, particularly improved output characteristics in the SOC 70-80% range, where existing batteries exhibit low output characteristics.

[0028] In addition, the lithium-sulfur secondary battery can reduce side reactions between the lithium polysulfide and the negative electrode by ensuring a smooth conversion process between the lithium polysulfide and lithium sulfide, which can also contribute to improving the lifespan characteristics of the lithium-sulfur secondary battery.

[0029] In the electrolyte of the embodiment described above, 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] If the first mixing energy becomes excessively high or the second mixing energy becomes excessively low, the conversion process between lithium polysulfide and lithium sulfide may not proceed smoothly, 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 in the embodiment may decrease, and as a result, the life characteristics of the lithium-sulfur secondary battery may decrease.

[0031] Furthermore, the electrolyte in the above embodiment is the remaining lithium polysulfide (Li2S) that is additionally generated during the discharge process of the lithium-sulfur secondary battery. nOptimized mixing energy and solubility can be observed even for n (where n is 2, 4, or 6), resulting in a smooth continuous reduction / conversion process during the discharge process, further improving the output characteristics of the secondary battery, while suppressing side reactions between lithium polysulfide and the negative electrode, thereby improving the lifespan characteristics of the secondary battery.

[0032] For example, the mixing energy of the electrolyte and Li2S2 is 1.5 kcal / mol or more, and can be 2.5 kcal / mol or more less than the second mixing energy (Gmix2). In a specific example, the mixing energy may be between 2.00 kcal / mol and 6.50 kcal / mol, or between 3.00 kcal / mol and 5.00 kcal / mol.

[0033] Furthermore, the mixing energy of the electrolyte with Li2S4 or Li2S6 may be 0.05 kcal / mol or more greater than the first mixing energy (Gmix1), and may be 0.01 kcal / mol or less. In a more specific example, the mixing energy of the electrolyte with Li2S4 may be -1.50 kcal / mol to 0.01 kcal / mol, or -0.90 kcal / mol to 0.01 kcal / mol, and the mixing energy of the electrolyte with Li2S6 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 comprises a lithium salt, a non-aqueous solvent, and additives, and in particular, includes a specific solvent composition as the non-aqueous solvent. By controlling the concentration / content of the lithium salt and other additives within a certain range, the mixing energy with the lithium polysulfide and lithium sulfide described above can be satisfied.

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

[0036] In a more specific example, the non-aqueous solvent may contain 15-50% by volume, or 20-49% by volume, of one or more furan compounds or tetrahydrofuran compounds, and 50-85% by volume, or 51-80% by volume, of one or more chain ether compounds, and may not contain additional solvents such as the dioxolane compounds.

[0037] In this case, the furan compound or tetrahydrofuran compound and the chain ether compound can act as a solvent and a non-solvent, respectively, with different solubility in relation to the lithium polysulfide. Therefore, by including such solvents and non-solvents in optimal ratios, the first and second mixed energies described above are satisfied, and the output characteristics of the lithium-sulfur battery can be further improved.

[0038] However, if the content of the furan compound or tetrahydrofuran compound becomes excessively large, or if a solvent such as the dioxolane compound is included, the first mixing energy of the electrolyte may increase or the second mixing energy may decrease, which may prevent the improvement of the output characteristics of the lithium-sulfur secondary battery. Conversely, if the content of the furan compound or tetrahydrofuran compound becomes excessively small, the formation of a protective film by the reaction between such a solvent and the metallic lithium phase of the negative electrode may be insufficient, which may result in an unsatisfactory lifespan for the lithium-sulfur secondary battery.

[0039] This is because the furan-based compound or tetrahydrofuran-based compound can suppress the formation of lithium dendrites by forming an SEI layer (solid electrolyte interface) on the surface of the metallic lithium phase, thereby suppressing electrolyte decomposition on the surface of the metallic lithium phase of the negative electrode.

[0040] In the composition of the non-aqueous solvent described above, the furan compound or tetrahydrofuran compound can be a furan compound or tetrahydrofuran compound having 1 to 4 C1 alkyl groups, with or without substitution. Specific examples 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] In particular, considering the satisfaction of the first and second mixing energies mentioned above, 2-methylfuran, 2-methyltetrahydrofuran, or 4-methyltetrahydrofuran, or combinations of two or more selected from these, can be suitably used. For example, 2-methylfuran may be combined with 2-methyltetrahydrofuran or 4-methyltetrahydrofuran, and in this case, 2-methylfuran:2-methyltetrahydrofuran or 4-methyltetrahydrofuran can be mixed in a volume ratio of 30:1 to 1:30.

[0042] Furthermore, as the chain-like ether compound, one or more alkyl groups or alkylene glycol groups having 1 to 10 or 1 to 5 carbon atoms are bonded to an alkyl group having 1 to 5 carbon atoms via an ether bond (-O-). Specific examples include 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.

[0043] In particular, considering the satisfaction of the first and second mixing energies mentioned above, dimethoxyethane, diethylene glycol dimethyl ether, or triethylene glycol dimethyl ether, or combinations of two or more selected from these, can be suitably used. For example, dimethoxyethane may be combined with ethylene glycol dimethyl ether or triethylene glycol dimethyl ether, in which case the dimethoxyethane:ethylene glycol dimethyl ether or triethylene glycol dimethyl ether can be mixed in a volume ratio of 10:1 to 1:10, or 5:1 to 1:1.

[0044] On the other hand, the lithium salt can be any electrolyte salt used to increase ionic conductivity that is commonly used in the industry, without any limitations. Specific examples of such lithium salts include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB 10 Cl 10One or more selected from the group consisting of LiPF6, LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, (C2F5SO2)2NLi, (SO2F)2NLi, and (CF3SO2)3CLi are examples. However, considering the electrical conductivity of the lithium-sulfur battery or the sufficiency of the mixed energy as described above, lithium salts in sulfonate form, such as CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, (C2F5SO2)2NLi, (SO2F)2NLi, or (CF3SO2)3CLi, can be used appropriately.

[0045] The lithium salt is included in the electrolyte at a concentration of 0.2 to 0.8 mol%, 0.23 to 0.75 mol%, or 0.25 to 0.70 mol%, such that the electrolyte adequately satisfies the first and second mixing energies. If the concentration of lithium salt is excessively low, the electrical conductivity of the lithium-sulfur secondary battery may be insufficient, and if the concentration of lithium salt is 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, in addition to a non-aqueous solvent and a lithium salt, additives for improving the characteristics of the lithium-sulfur secondary battery. Examples of such additives include one or more selected from the group consisting of lithium nitrate (LiNO3), potassium nitrate (KNO3), cesium nitrate (CsNO3), magnesium nitrate (Mg(NO3)2), barium nitrate (Ba(NO3)2), lithium nitrite (LiNO2), potassium nitrite (KNO2), and cesium nitrite (CsNO2), among which lithium nitrate can be preferably used.

[0047] Furthermore, the additive is included in the electrolyte in an amount of 0.8-4.0% by weight, 0.9-3.5% by weight, 0.8-3.0% by weight, or 0.9-2.0% by weight. If the content of the additive is excessively low, the characteristics of the lithium-sulfur secondary battery may not be sufficient, and if the content of the additive is excessively high, the first or second mixed energy of the electrolyte may not be able to satisfy the above range, making it difficult to achieve the output characteristics of the lithium-sulfur secondary battery.

[0048] As described above, the electrolyte of one embodiment, by including a lithium salt and additives in a certain content range along with a non-aqueous solvent of a specific composition, can satisfy the first and second mixed energies described above and exhibit optimized solubility for lithium polysulfide and lithium sulfide formed during the discharge of a lithium-sulfur secondary battery. As a result, the generation of lithium polysulfide and lithium sulfide during the discharge of a lithium-sulfur secondary battery can be facilitated, and the application of overvoltage during such generation can be reduced. Consequently, the output characteristics of the lithium-sulfur secondary battery can be improved.

[0049] In particular, such lithium-sulfur secondary batteries can exhibit improved output characteristics in the 70-80% SOC range, where existing batteries previously showed particularly low output characteristics when overvoltage was applied. For example, they can exhibit output characteristics of 1.2 kW / kg or more, or 1.3 kW / kg or more, or 1.3-3.0 kW / kg.

[0050] Furthermore, according to another embodiment of the invention, a lithium-sulfur secondary battery is provided comprising a positive electrode containing sulfur as a positive electrode active material; a negative electrode containing metallic lithium; a separation membrane interposed between the positive and negative electrodes; and the electrolyte of the first embodiment. Such a lithium-sulfur secondary battery can exhibit improved output characteristics and life characteristics due to the inclusion of the electrolyte of the first embodiment.

[0051] In the secondary battery of the other embodiment described above, the positive electrode may include a positive electrode active material and a binder, and may further include a conductive material. Alternatively, the positive electrode may have an active material layer including the positive electrode active material and binder formed on a positive electrode current collector.

[0052] In this case, 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, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, silver, etc., and aluminum-cadmium alloys can be used.

[0053] Furthermore, the positive electrode current collector can have fine irregularities formed on its surface to strengthen its bonding force with the active material layer, and can be used in a variety of forms such as films, sheets, foils, meshes, nets, porous materials, foams, and nonwoven fabrics.

[0054] The positive electrode active material may contain sulfur, more specifically, elemental sulfur (S8), organosulfur compounds, sulfur-carbon composites or polymers (C2S). x ) n A positive electrode active material can be used with values ​​such as x = 2.5 to 50, n ≥ 2. The elemental sulfur (S8) can be appropriately used, taking into consideration the characteristics of the electrolyte in one embodiment.

[0055] Such sulfur-containing positive electrode active material is present in an amount of 40 to 80 parts by weight, preferably 50 to 70 parts by weight, per 100 parts by weight of the total weight of the positive electrode. If the content of the positive electrode active material is too low, the energy density of the secondary battery decreases, and if the content is too high, the conductivity and stability of the electrode may decrease.

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

[0057] The transition metal elements include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au, or Hg, the group IIIA elements include Al, Ga, In, Ti, and the group IVA elements include Ge, Sn, Pb.

[0058] On the other hand, the binder is a component that assists in the bonding of the positive electrode active material and the current collector, and may be, but is not necessarily 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, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, and mixtures thereof.

[0059] The binder is typically added in amounts of 1 to 15 parts by weight, based on 100 parts by weight of the total weight of the positive electrode. If the binder content is less than 1 part by weight, the adhesion between the active material layer and the current collector will be insufficient. If it exceeds 15 parts by weight, the adhesion will improve, but the amount of positive electrode active material will decrease, which may reduce the battery capacity.

[0060] On the other hand, the conductive material is a component for improving electrical conductivity and is not particularly limited as long as it is an electronically conductive substance that does not undergo chemical changes in a 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, commercially available conductive materials include acetylene black (products from Chevron Chemical Company or Gulf Oil Company, etc.), Ketjen Black EC (products from Armak Company), Vulcan XC-72 (products from Cabot Company), and Super P (products from MMM). Examples include acetylene black, carbon black, and graphite.

[0061] Furthermore, in the lithium-sulfur secondary battery of the other embodiment described above, a filler may be selectively added to the positive electrode as a component that suppresses the expansion of the positive electrode active material containing sulfur. 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, and for example, olefin polymers such as polyethylene and polypropylene; fibrous materials such as glass fibers and carbon fibers; 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 create a slurry, applying this slurry to a positive electrode current collector, and then drying and rolling it. The dispersion medium can be, but is not limited to, NMP (N-methyl-2-pyrrolidone), DMF (Dimethyl formamide), DMSO (Dimethyl sulfoxide), ethanol, isopropanol, water, and mixtures thereof.

[0063] In the secondary batteries of the other embodiments described above, the negative electrode may include metallic lithium, and for example, it may include a lithium metal or alloy layer formed on the 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, and as the alloy, an aluminum-cadmium alloy can be used, as well as calcined carbon, non-conductive polymers or conductive polymers surface-treated with conductive materials. Generally, a thin copper sheet is used as the negative electrode current collector.

[0065] The metallic lithium phase may be lithium metal or an alloy. In this case, the lithium alloy contains elements that can be alloyed with lithium, and specifically, it may be 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 phase may be in the form of a sheet or foil, or, depending on the case, lithium or a lithium alloy may be deposited or coated onto a current collector by a dry process, or particulate metals and alloys may be deposited or coated by a wet process or the like.

[0066] A conventional separation membrane can be interposed between the positive and negative electrodes. The separation membrane is a physical separation membrane that has the function of physically separating the electrodes, and any conventional separation membrane can be used without special limitations. Particularly preferred is one that has low resistance to the movement of electrolyte ions and excellent electrolyte moisture absorption capacity. The separation membrane also separates or insulates the positive and negative electrodes from each other while enabling the transport of lithium ions between them. Such a separation membrane may be made of a porous, non-conductive, or insulating material. The separation membrane may be an independent component such as a film, or a coating layer added to the positive and / or negative electrodes.

[0067] Examples of polyolefin-based porous membranes used as the separation membrane include membranes formed from polyethylene such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene, as well as polyolefin polymers such as polypropylene, polybutylene, and polypentene, either individually or as mixtures thereof. Examples of nonwoven fabrics used as the separation membrane include nonwoven fabrics formed from polymers such as polyphenylene oxide, polyimide, polyamide, polycarbonate, polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polyphenylene sulfide, polyacetal, polyethersulfone, polyetheretherketone, and polyester, either individually or in mixtures thereof. Such nonwoven fabrics are fibrous forms that form 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 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 separation membrane is less than 1 μm, the mechanical properties cannot be maintained, and if it exceeds 100 μm, the separation membrane acts as a resistive layer, reducing the performance of the battery. 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 resistive layer, and if the pore size exceeds 50 μm or the porosity exceeds 95%, the mechanical properties cannot be maintained.

[0069] Lithium-sulfur secondary batteries of other embodiments, including the electrolyte, positive electrode, negative electrode, and separator membrane described above, can be manufactured by a process of placing the positive electrode facing the negative electrode, interposing a separator membrane between them, and then injecting the electrolyte.

[0070] On the other hand, the lithium-sulfur secondary battery is not only suitable for use as a battery cell in a small device, but is also particularly suitable for use as a unit battery in a battery module that powers a medium-to-large device. In this respect, a battery module can be provided that includes two or more lithium-sulfur secondary batteries that are electrically connected (in series or parallel).

[0071] The number of lithium-sulfur secondary batteries contained in the battery module can, of course, be adjusted in various ways, taking into account the application and capacity of the battery module. Furthermore, battery packs in which the battery modules are electrically connected using conventional techniques in this field can also be provided. The battery modules and battery packs can be used as power sources for one or more medium-to-large devices, including 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, but are not necessarily limited to these uses.

[0072] The following examples illustrate the invention, but these examples are merely illustrative, 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 the technical concept, and that such changes and modifications naturally fall within the scope of the claims.

[0073] Examples 1-8, Comparative Examples 1 and 2: Production of electrolytes for lithium-sulfur secondary batteries After mixing a non-aqueous solvent according to the composition shown in Table 1 below, the lithium salt and additives were dissolved at the concentrations and contents shown in Table 1 to prepare the electrolytes for each example and comparative example.

[0074] [Table 1] *DME:1,2-dimethoxyethane;LiFSI:(SO2F)2NLi;Li Triflate:CF3SO3Li

[0075] Test Example 1: Calculation of mixing energy when an electrolyte is mixed with lithium sulfide or lithium polysulfide. Using the commercially available 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 (Li₂S n ; n is 2, 4, 6 or 8) or lithium sulfide (Li₂S), 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) for Li₂S₈ is -1.0 kcal / mol or less, and the second mixing energy (Gmix2) for Li₂S is 4.0 kcal / mol or more. It was confirmed that the characteristics are satisfied.

[0078] In contrast, the electrolytes of Comparative Examples 1 and 2 contain a dioxolane-based compound such as 2-methoxy-1,3-dioxolane in the non-aqueous solvent, or the volume ratio of a furan-based compound or a tetrahydrofuran-based compound to a 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) (the content of sulfur alone is set to 67.5% by weight based on the total weight of the positive electrode, and the carbon material has a pore volume of 1.8 cm 3A cathode slurry composition was prepared by mixing 5 parts by weight of styrene-butadiene rubber / carboxymethylcellulose (SBR:CMC=7:3) as a binder with activated carbon (using 1 / g of 1

[0081] Manufacturing of lithium-sulfur batteries The manufactured positive electrode and a lithium metal negative electrode with a thickness of 150 μm were positioned facing each other, a polyethylene (PE) separation membrane was interposed between them, and then the electrolyte was injected to manufacture a coin cell type lithium-sulfur battery. In the manufacture of the battery, the positive electrode was punched out at 15 phi, the polyethylene separation membrane at 19 phi, and the lithium metal at 16 phi.

[0082] The resistance of the aforementioned lithium-sulfur battery was measured by the voltage drop after 5C 10 seconds of discharge, and the maximum output corresponding to the state of charge (SOC) was calculated based on this. The results of this calculation are shown in Figures 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, Figures 3 and 4, it was confirmed that the lithium-sulfur secondary battery manufactured using the electrolyte of the example exhibited excellent output characteristics even at SOC 70%, where the overvoltage was highest, and showed high output characteristics across the overall SOC.

[0085] In contrast, the comparative lithium-sulfur secondary battery was found to be unable to exhibit effective output due to excessive overvoltage at a State of Charge (SOC) of 70%.

Claims

1. An electrolyte for lithium-sulfur secondary batteries comprising a lithium salt, a non-aqueous solvent, and an additive, The aforementioned electrolyte and dilithio peroctasulfide (Li) were calculated at room temperature (20±5℃) using the COSMO-RS (Conductor-like Screening Model for real Solvent) theory. ２ S ８ The first mixing energy (Gmix1) of the electrolyte and lithium sulfide (Li ２ The second mixing energy (Gmix2) with S) is 4.0 kcal / mol or more. The lithium salt is one or more selected from the group consisting of LiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₄, LiCF₃SO₃, LiCF₃CO₂, LiC₄BO₄, LiAsF₄, LiSbF₄, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi, (C₂F₅SO₂)₂NLi, (SO₂F)₂NLi, and (CF₃SO₂)₃CLi. The aforementioned non-aqueous solvent contains a furan compound or a tetrahydrofuran compound and a linear ether compound in a volume ratio of 1:1 to 1:5 (v / v), and does not contain a dioxolane compound. The aforementioned additive is one or more selected from the group consisting of lithium nitrate (LiNO₃), potassium nitrate (KNO₃), cesium nitrate (CsNO₃), magnesium nitrate (Mg(NO₃)₂), barium nitrate (Ba(NO₃)₂), lithium nitrite (LiNO₂), potassium nitrite (KNO₂), and cesium nitrite (CsNO₂). The lithium salt is contained in an electrolyte for lithium-sulfur secondary batteries at a concentration of 0.2 to 0.8 mol%.

2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, 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.

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

4. The electrolyte and dilithio pertetrasulfide (Li ２ S ４ ), or the mixing energy with dilithio perhexasulfide (Li ２ S ６ ) has a value greater than the first mixing 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 electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the non-aqueous solvent contains a furan compound or a tetrahydrofuran compound and a linear ether compound in a volume ratio (v / v) of 1:1.04 to 1:4.

50.

6. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the furan compound or tetrahydrofuran compound comprises 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.

7. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the chain-like ether compound comprises 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 additive is included in a content of 0.8 to 4.0% by weight.

9. A positive electrode containing sulfur as the positive electrode active material; A negative electrode containing metallic lithium; A separation membrane interposed between the positive and negative electrodes; and A lithium-sulfur secondary battery comprising the electrolyte described in claim 1.

10. The positive electrode active material is an element of sulfur (S ８ A lithium-sulfur secondary battery according to claim 9, comprising )

11. The lithium-sulfur secondary battery according to claim 9, wherein the positive electrode comprises a positive electrode active material and a binder.

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