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

Abstract

The present invention relates to an electrolyte for a lithium-sulfur battery comprising a lithium salt and an organic solvent, wherein the organic solvent comprises a first solvent, a second solvent, and a third solvent, wherein the first solvent comprises a compound containing a cyano group (-CN) represented by Chemical Formula 1 or Chemical Formula 2, a linear ether containing two or more oxygen atoms, or a cyclic ether, the second solvent comprises a fluorinated ether solvent, and the third solvent comprises an ether non-solvent represented by Chemical Formula 3.

Description

Technical Field

[0001] The present invention relates to an electrolyte for a lithium-sulfur battery and a lithium-sulfur battery containing the electrolyte.

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0161255, filed on November 26, 2020, the entire contents of which are incorporated herein by reference. Background Art

[0003] As the application scope of secondary batteries expands from small portable electronic devices to medium and large electric vehicles (EVs), energy storage systems (ESS), electric ships, etc., the demand for lithium secondary batteries with high capacity, high energy density and long life is rapidly increasing.

[0004] Lithium-sulfur secondary batteries (LSBs) use sulfur-based materials with sulfur-sulfur bonds (SS bonds) as the positive electrode active material and lithium metal as the negative electrode active material. Sulfur, the primary active material in the positive electrode, has the following properties: low atomic weight; abundant resources, making it easy to supply and harvest; low cost, thus reducing battery manufacturing costs; and non-toxic, making it environmentally friendly.

[0005] In particular, lithium-sulfur secondary batteries have a theoretical discharge capacity of 1,675 mAh / g of sulfur and can theoretically achieve a high energy storage density of 2,600 Wh / kg relative to their weight. Therefore, due to their significantly higher theoretical energy density compared to other battery systems (Ni-MH batteries: 450 Wh / kg, Li-FeS batteries: 480 Wh / kg, Li-MnO2 batteries: 1,000 Wh / kg, Na-S batteries: 800 Wh / kg) and the lithium-ion batteries currently under research (250 Wh / kg), lithium-sulfur batteries are attracting significant attention in the mid-to-large-sized secondary battery market, which is currently under development.

[0006] In the case of lithium-sulfur secondary batteries, during discharge, an oxidation reaction occurs at the negative electrode where lithium gives up electrons and becomes lithium cations, while a reduction reaction occurs at the positive electrode where the sulfur-based material accepts electrons and forms sulfur anions. Through this redox reaction, sulfur is converted from the ring S8 structure before discharge to linear lithium polysulfide (Li2S x , x = 8, 6, 4, 2), and finally when the lithium polysulfide is completely reduced, lithium sulfide (Li2S) is finally produced.

[0007] In particular, lithium polysulfide (Li2S x, x>4) readily dissolves in the organic electrolyte and, due to concentration differences, gradually diffuses away from the cathode where it is generated. Consequently, as the lithium polysulfide dissolved from the cathode gradually escapes beyond the cathode reaction zone, the amount of sulfur material available for electrochemical reactions at the cathode decreases, leading to a reduction in the charge capacity of the lithium-sulfur secondary battery.

[0008] In addition, there is a problem that the dissolution of lithium polysulfide increases the viscosity of the electrolyte and reduces the ion conductivity, and lithium polysulfide directly reacts with the lithium metal negative electrode through continuous charge / discharge reactions, thereby causing lithium sulfide (Li2S) to adhere to the surface of the lithium metal, thereby reducing the reaction activity and deteriorating the potential characteristics.

[0009] In addition, attempts have been made to improve the reactivity of existing lithium-sulfur batteries by constructing lithium-sulfur battery systems with cathode-based electrolyte systems. However, this attempt has also been found to have a problem: lithium polysulfide, an intermediate product, dissolves into the electrolyte, degrading battery stability.

[0010] To solve these problems, the related technical field has studied slightly soluble electrolyte (SSE) electrolyte systems, which can reduce the solubility of lithium polysulfide in the electrolyte. However, the SSE electrolyte system based on high-concentration salts can only operate normally at high temperatures above 45°C due to its high viscosity. Therefore, research on SSE electrolyte systems that can operate batteries normally even at low temperatures is ongoing.

[0011] [Prior art literature]

[0012] [Non-patent literature]

[0013] (Non-patent document 1) Lei Cheng et al., Sparingly Solvating Electrolytes for High Energy Density Lithium-Sulfur Batteries, ACS Energy Lett. 2016, 1, 503-509. Summary of the Invention

[0014] Technical issues

[0015] In order to solve the above problems, the inventors of the present invention intend to provide a lithium-sulfur battery with improved low-temperature performance below 35°C by adding a new ether non-solvent to the SSE electrolyte system that previously could operate normally under high temperature conditions above 45°C.

[0016] Technical Solution

[0017] According to a first aspect of the present invention, the present invention provides an electrolyte for a lithium-sulfur battery, wherein the organic solvent in the lithium-sulfur battery electrolyte containing a lithium salt and an organic solvent comprises a first solvent, a second solvent, and a third solvent, wherein the first solvent comprises a compound containing a cyano group (-CN) represented by Chemical Formula 1 or Chemical Formula 2, a linear ether containing two or more oxygen atoms, or a cyclic ether, the second solvent comprises a fluorinated ether solvent, and the third solvent comprises an ether non-solvent represented by the following Chemical Formula 3.

[0018] [Chemical Formula 1]

[0019] R1-CN

[0020] [Chemical Formula 2]

[0021] NC-R2-CN

[0022] (wherein, in Chemical Formula 1, R1 is a C1 to C10 alkyl group,

[0023] In Chemical Formula 2, R2 is a C1 to C10 alkylene group)

[0024] [Chemical Formula 3]

[0025] R3-O-R4

[0026] (wherein in Chemical Formula 3, R3 and R4 are the same as or different from each other and are each independently methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl or tert-butyl).

[0027] In one embodiment of the present invention, the first solvent may comprise one selected from the group consisting of acetonitrile, succinonitrile, pimelonitrile, glutaronitrile, adiponitrile, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, ethylene glycol ethyl methyl ether, tetrahydrofuran, 2-methyl-tetrahydrofuran, 1,3-dimethoxyethane, 1,2 ... Alkanes and combinations thereof.

[0028] In one embodiment of the present invention, the second solvent may comprise one selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1H,1H,2'H,3H-decafluorodipropyl ether, difluoromethyl 2,2,2-trifluoroethyl ether, 1,2,2,2-tetrafluoroethyl trifluoromethyl ether, 1,1,2,3,3,3-hexafluoropropyl difluoromethyl ether, 1H,1H,2'H,3H-decafluorodipropyl ether, pentafluoroethyl 2,2,2-trifluoroethyl ether, 1H,1H,2'H-perfluorodipropyl ether, bis(2,2,2-trifluoroethyl) ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether and combinations thereof.

[0029] In one embodiment of the present invention, the third solvent may include one selected from the group consisting of diisopropyl ether, ethyl tert-butyl ether, dibutyl ether, diisobutyl ether, dipropyl ether, and combinations thereof.

[0030] In one embodiment of the present invention, the organic solvent may include 15 to 45 volume ratios of the first solvent relative to 100 volume ratios of the organic solvent.

[0031] In one embodiment of the present invention, the organic solvent may include 10 to 60 volume ratios of the third solvent relative to 100 volume ratios of the organic solvent.

[0032] In one embodiment of the present invention, the organic solvent may include 25 to 45 volume ratios of the third solvent relative to 100 volume ratios of the organic solvent.

[0033] In one embodiment of the present invention, the organic solvent may include 25 to 350 volume ratios of the third solvent relative to 100 volume ratios of the second solvent.

[0034] In one embodiment of the present invention, the organic solvent may include 85 to 115 volume ratios of the third solvent relative to 100 volume ratios of the second solvent.

[0035] According to a second aspect of the present invention, the present invention provides a lithium-sulfur battery, comprising a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and the above-mentioned electrolyte.

[0036] Beneficial effects

[0037] The lithium-sulfur secondary battery according to the present invention improves the low-temperature performance by adding an ether non-solvent as a third solvent to the electrolyte, and has an effect of being superior to the conventional SSE electrolyte system in terms of discharge capacity and life characteristics of the battery even under low temperature conditions below 35°C.

[0038] In addition, due to the low density of the ether non-solvent as the third solvent, the density of the electrolyte is reduced, thereby increasing the energy density of the battery and having the effect of reducing the solubility of lithium polysulfide. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 is a graph showing the results of initial charge and discharge performance evaluation at 35° C. for Examples 1 to 6 of the present invention and Comparative Example 1.

[0040] Figure 2 : is a graph showing the evaluation results of initial charge and discharge performance of Examples 2 and 5 of the present invention and Comparative Example 1 at 25°C.

[0041] Figure 3Graph showing the evaluation results of battery life characteristics of Examples 1 to 6 of the present invention and Comparative Example 1 at 35°C.

[0042] Figure 4 It is a graph showing the evaluation results of battery life characteristics of Examples 2 and 5 of the present invention and Comparative Example 1 at 25°C. DETAILED DESCRIPTION

[0043] The embodiments provided according to the present invention can all be realized through the following description.It should be understood that the following description describes preferred embodiments of the present invention, and it should be understood that the present invention is not necessarily limited thereto.

[0044] The term "polysulfide" used in this specification includes the concept of "polysulfide ions (S x 2- , x=8, 6, 4, 2)" and "lithium polysulfide (Li2S x or LiS x - , x=8, 6, 4, 2)”.

[0045] Electrolyte for lithium-sulfur batteries

[0046] In the electrolyte for a lithium-sulfur battery containing a lithium salt and an organic solvent according to the present invention, the organic solvent comprises a first solvent, a second solvent, and a third solvent, the first solvent comprising a compound containing a cyano group (-CN) represented by Chemical Formula 1 or Chemical Formula 2, a linear ether containing two or more oxygen atoms, or a cyclic ether, the second solvent comprising a fluorinated ether solvent, and the third solvent comprising an ether non-solvent represented by the following Chemical Formula 3.

[0047] [Chemical Formula 1]

[0048] R1-CN

[0049] [Chemical Formula 2]

[0050] NC-R2-CN

[0051] (wherein in Chemical Formula 1, R1 is a C1 to C10 alkyl group,

[0052] In Chemical Formula 2, R2 is a C1 to C10 alkylene group)

[0053] [Chemical Formula 3]

[0054] R3-O-R4

[0055] (wherein in Chemical Formula 3, R3 and R4 are the same as or different from each other and are each independently methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl or tert-butyl).

[0056] The first solvent may include a compound containing a cyano group (—CN) represented by Chemical Formula 1 or Chemical Formula 2, a linear ether containing two or more oxygen atoms, or a cyclic ether.

[0057] The first solvent may be a compound containing a cyano group (-CN) represented by the above Chemical Formula 1 or Chemical Formula 2, and may preferably include one selected from the group consisting of acetonitrile, succinonitrile, pimelonitrile, glutaronitrile, adiponitrile, and combinations thereof. Specifically, if acetonitrile containing a cyano group is included as the first solvent, it can promote the solid-state reaction of sulfur without forming lithium polysulfide due to its high polarity and the property of promoting the formation of S3- radicals.

[0058] The first solvent may be a linear ether containing two or more oxygen atoms, and may preferably include one selected from the group consisting of 1,2-dimethoxyethane, diethylene glycol dimethyl ether, ethylene glycol ethyl methyl ether, and combinations thereof.

[0059] The first solvent may be a cyclic ether, and may preferably comprise one selected from the group consisting of tetrahydrofuran, 2-methyl-tetrahydrofuran, 1,3-dihydrofuran, Alkanes and combinations thereof.

[0060] The first solvent may comprise one selected from the group consisting of acetonitrile, succinonitrile, pimelonitrile, glutaronitrile, adiponitrile, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, ethylene glycol ethyl methyl ether, tetrahydrofuran, 2-methyl-tetrahydrofuran, 1,3-dimethoxyethane, Alkanes and combinations thereof, and may preferably be acetonitrile.

[0061] The first solvent contained in the organic solvent may be at a volume ratio of 15 or more, 16 or more, 17 or more, 18 or more, 19 or more, 20 or more, 21 or more, 22 or more, 23 or more, 24 or more, 25 or more, 26 or more, 27 or more, or 28 or more, and at a volume ratio of 45 or less, 44 or less, 43 or less, 42 or less, 41 or less, 40 or less, 39 or less, 38 or less, 37 or less, 36 or less, 35 or less, 34 or less, 33 or less, or 32 or less relative to 100 volume ratios of the organic solvent. If the volume ratio is less than 15 volume ratio, there may be a problem in that the reactivity of sulfur decreases due to the difficulty in forming S3- radicals, making it difficult to ensure high performance. In contrast, if the volume ratio exceeds 45 volume ratio, there may be a problem in that acetonitrile may cause a chemical side reaction with the lithium negative electrode, thereby possibly deteriorating the performance of the battery itself.

[0062] The second solvent may include a fluorinated ether solvent.

[0063] If the second solvent is a fluorinated ether solvent, there is no particular limitation on its type, and it may include one selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), 1H,1H,2'H,3H-decafluorodipropyl ether, difluoromethyl 2,2,2-trifluoroethyl ether, 1,2,2,2-tetrafluoroethyl trifluoromethyl ether, 1,1,2,3,3,3-hexafluoropropyl difluoromethyl ether, 1H,1H,2'H,3H-decafluorodipropyl ether, pentafluoroethyl 2,2,2-trifluoroethyl ether, 1H,1H,2'H-perfluorodipropyl ether, bis(2,2,2-trifluoroethyl) ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, and combinations thereof, and may preferably be 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0064] The second solvent contained in the organic solvent may be 10 volume ratios or more, 12 volume ratios or more, 14 volume ratios or more, 16 volume ratios or more, 18 volume ratios or more, 20 volume ratios or more, 22 volume ratios or more, 24 volume ratios or more, 25 volume ratios or more, 26 volume ratios or more, 28 volume ratios or more, 30 volume ratios or more, 32 volume ratios or more, or 34 volume ratios or more, and 85 volume ratios or less, 84 volume ratios or less, 82 volume ratios or less, 80 volume ratios or less, 78 volume ratios or less. The volume ratio is less than 10 volume ratio, 76 volume ratio, 74 volume ratio, 72 volume ratio, 70 volume ratio, 68 volume ratio, 66 volume ratio, 64 volume ratio, 62 volume ratio, 60 volume ratio, 58 volume ratio, 56 volume ratio, 54 volume ratio, 52 volume ratio, 50 volume ratio, 48 volume ratio, 46 ​​volume ratio, 45 volume ratio, 44 ​​volume ratio, 42 volume ratio, 40 volume ratio, 38 volume ratio or 36 volume ratio. If the volume ratio is less than 10 volume ratio, the problem may arise that the viscosity of the electrolyte increases due to the small amount of the second solvent for controlling the viscosity, thereby greatly reducing the wettability of the counter electrode and reducing the ionic conductivity of the entire electrolyte. On the other hand, if the volume ratio exceeds 85 volume ratio, the problem may arise that the ratio of the first solvent complex represented by acetonitrile decreases rapidly, resulting in a decrease in ionic conductivity.

[0065] The third solvent may include an ether-based non-solvent represented by Chemical Formula 3 above.

[0066] The ether non-solvent can be added to the electrolyte of a lithium-sulfur battery to improve the cycle life of the battery including the SSE electrolyte system. Specifically, when the high viscosity and low lithium mobility of the complex formed by the first solvent represented by acetonitrile and the lithium salt represented by LiTFSI in the SSE electrolyte system lead to a shortened cycle life, the addition of the ether non-solvent can improve viscosity and reactivity.

[0067] The third solvent may comprise one selected from the group consisting of diisopropyl ether, ethyl tert-butyl ether, dibutyl ether, diisobutyl ether, di-n-propyl ether and combinations thereof, and preferably, “if R3 and R4 are different from each other and each corresponds to an ethyl group or a tert-butyl group”, the third solvent may be ethyl tert-butyl ether, and “if R3 and R4 correspond to an n-butyl group”, the third solvent may be dibutyl ether.

[0068] The third solvent contained in the organic solvent may have a volume ratio of 10 or more, 12 or more, 14 or more, 16 or more, 18 or more, 20 or more, 22 or more, 24 or more, 25 or more, 26 or more, 28 or more, 30 or more, 32 or more, or 34 or more, and a volume ratio of 60 or less, 58 or less, 56 or less, 54 or less, 52 or less, 50 or less, 48 ​​or less, 46 or less, 45 or less, 44 or less, 42 or less, 40 or less, 38 or less, or 36 or less relative to 100 volume ratios of the organic solvent. If the volume ratio is less than 10 volume ratio, there may be a problem in that the density and viscosity of the electrolyte increase, thereby reducing the reactivity of the battery. In contrast, if the volume ratio exceeds 60 volume ratios, the behavior of the SSE electrolyte system is no longer exhibited, and an overcharge phenomenon may occur due to the shuttle effect of lithium polysulfides.

[0069] The third solvent contained in the organic solvent may be 25 volume ratio or more, 30 volume ratio or more, 35 volume ratio or more, 40 volume ratio or more, 45 volume ratio or more, 50 volume ratio or more, 55 volume ratio or more, 60 volume ratio or more, 65 volume ratio or more, 70 volume ratio or more, 75 volume ratio or more, 80 volume ratio or more, 85 volume ratio or more, 90 volume ratio or more or 95 volume ratio or more relative to 100 volume ratio of the second solvent, and 350 volume ratio or less, 335 volume ratio or less, 350 volume ratio or less, 350 volume ratio or less, 350 volume ratio or less, 350 volume ratio or less, 350 volume ratio or less, 350 volume ratio or less, 350 volume ratio or less, 350 volume ratio or less, 350 volume ratio or less, 350 volume ratio or less, 350 volume ratio or less, 350 volume ratio or more ... In some embodiments, the present invention relates to an electrolyte system having an electrolyte density of less than 25 volume ratio, an electrolyte density of less than 35 volume ratio, an electrolyte density of less than 10 ...

[0070] The electrolyte for the lithium-sulfur battery of the present invention may contain a lithium salt. The lithium salt is a good material soluble in an organic solvent and can be selected from: LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiB(Ph)4, LiC4BO8, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiSO3CH3, LiSO3CF3, LiSCN, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(SO2F)2, lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenylborate and lithium imide, and preferably LiN(CF3SO2)2(LITFSI).

[0071] The concentration of the lithium salt may be 0.1 to 5.0 M, preferably 0.2 to 3.0 M, more specifically 0.5 to 2.5 M, depending on various factors such as the exact composition of the mixture contained in the electrolyte, the solubility of the salt, the conductivity of the dissolved salt, the charging and discharging conditions of the battery, the operating temperature, and other factors known in the field of lithium batteries. If the concentration of the lithium salt is less than 0.1 M, the conductivity of the electrolyte may be reduced, which may deteriorate the performance of the electrolyte. If the concentration of the lithium salt is greater than 5.0 M, the viscosity of the electrolyte may increase, which may reduce the lithium ion (Li + )'s migration rate.

[0072] In addition to the above components, the electrolyte for the lithium-sulfur battery of the present invention may further include additives commonly used in the art. For example, the electrolyte may include one 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), cesium nitrite (CsNO2), and combinations thereof.

[0073] The preparation method of the electrolyte for lithium-sulfur batteries according to the present invention is not particularly limited in the present invention and can be a conventional method known in the art.

[0074] lithium-sulfur batteries

[0075] The lithium-sulfur battery according to the present invention comprises a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the electrolyte comprises the electrolyte for the lithium-sulfur battery according to the present invention.

[0076] The positive electrode may include a positive electrode current collector and a positive electrode active material layer coated on one surface or both surfaces of the positive electrode current collector.

[0077] The positive electrode current collector supports the positive electrode active material and is not particularly limited as long as it has high conductivity and does not cause chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, sintered carbon; copper or stainless steel surface-treated with carbon, nickel, silver, etc.; aluminum-cadmium alloy, etc. can be used as the positive electrode current collector.

[0078] The positive electrode current collector may have enhanced binding force with the positive electrode active material by having fine irregularities on its surface, and may be formed into various forms such as a film, sheet, foil, screen, mesh, porous body, foamed body, or non-woven fabric body.

[0079] The positive electrode active material layer may include a positive electrode active material, a binder, and a conductive material.

[0080] The positive electrode active material may include at least one selected from the group consisting of elemental sulfur (S8), organic sulfur compounds, Li2S n (n≥1) and carbon-sulfur polymers ((C2S x ) n :x=2.5~50,n≥2).

[0081] Since sulfur contained in the positive electrode active material has no conductivity alone, it is used in combination with a conductive material such as a carbon material. Therefore, sulfur is contained in the form of a sulfur-carbon composite material. Preferably, the positive electrode active material can be a sulfur-carbon composite material.

[0082] The carbon in the sulfur-carbon composite material is a porous carbon material and provides a framework capable of uniformly and stably fixing sulfur, and supplements the low electrical conductivity of sulfur to enable electrochemical reactions to proceed smoothly.

[0083] The porous carbon material can generally be produced by carbonizing various carbon material precursors. The porous carbon material may contain non-uniform pores, with an average pore diameter ranging from 1 to 200 nm, and a porosity ranging from 10 to 90% of the total volume of the porous carbon material. If the average pore diameter is smaller than this range, the pore size is only at the molecular level, making sulfur impregnation impossible. Conversely, if the average pore diameter exceeds this range, the mechanical strength of the porous carbon material is weakened, which is not preferred for electrode manufacturing.

[0084] The porous carbon material may be in the form of spheres, rods, needles, plates, tubes, or blocks, and any material commonly used in lithium-sulfur batteries may be used without limitation.

[0085] The porous carbon material may have a porous structure or a high specific surface area, and may be any porous carbon material conventionally used in the art. For example, the porous carbon material may be, but is not limited to, at least one selected from the following: graphite; graphene; carbon black such as denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; carbon nanotubes (CNTs) such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); carbon fibers such as graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs); graphites such as natural graphite, artificial graphite, and expanded graphite, as well as activated carbon, and preferably carbon nanotubes (CNTs).

[0086] In the present invention, there is no particular limitation on the method for preparing the sulfur-carbon composite material, and commonly used methods in the art can be used.

[0087] In addition to the above-mentioned positive electrode active material, the positive electrode may further include at least one additive selected from the group consisting of transition metal elements, Group IIIA elements, Group IVA elements, sulfur compounds of these elements, and alloys of these elements with sulfur.

[0088] The transition metal elements may include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au, Hg, etc., the Group IIIA elements may include Al, Ga, In, Ti, etc., and the Group IVA elements may include Ge, Sn, Pb, etc.

[0089] The conductive material is a material that serves as a path for electrons to transfer from a current collector to a positive electrode active material by electrically connecting the electrolyte and the positive electrode active material, and can be used without limitation as long as it has conductivity.

[0090] For example, as the conductive material, the following can be used alone or in combination: graphite such as natural graphite or artificial graphite; carbon black such as Super P, Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black and thermal black; carbon derivatives such as carbon nanotubes and fullerenes; conductive fibers such as carbon fibers and metal fibers; carbon fluorides; metal powders such as aluminum powder and nickel powder, or conductive polymers such as polyaniline, polythiophene, polyacetylene and polypyrrole.

[0091] The binder maintains the positive electrode active material in the positive electrode current collector and organically connects the positive electrode active materials to further increase the binding force therebetween. Any binder known in the art can be used as the binder.

[0092] For example, the adhesive can be a fluororesin adhesive, including polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE); a rubber adhesive, including styrene-butadiene rubber (SBR), nitrile rubber and styrene-isoprene rubber; a cellulose adhesive, including carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose and regenerated cellulose; a polyol adhesive; a polyolefin adhesive, including polyethylene and polypropylene; a polyimide adhesive; a polyester adhesive; and a silane adhesive, or a mixture or copolymer of two or more thereof.

[0093] The present invention is not particularly limited to the method for preparing the positive electrode, and methods commonly used in the art can be used. For example, the positive electrode can be prepared by preparing a positive electrode slurry composition and then applying it to at least one surface of a positive electrode current collector.

[0094] The slurry composition for a positive electrode includes the positive electrode active material, the conductive material, and the binder as described above, and may further include a solvent other than the above solvents.

[0095] As a solvent, a solvent that can uniformly disperse the positive electrode active material, the conductive material and the binder is used. Such a solvent is an aqueous solvent, and most preferably water, in which case the water can be distilled water or deionized water. However, it is not necessarily limited to this, and if necessary, a lower alcohol that can be easily mixed with water can be used. Examples of lower alcohols include methanol, ethanol, propanol, isopropanol and butanol, and preferably, they can be used in combination with water.

[0096] The sulfur loading in the cathode can be 1 to 10 mAh / cm 2 , preferably 3 to 6 mAh / cm 2 .

[0097] The negative electrode may include a negative electrode current collector and a negative electrode active material layer coated on one surface or both surfaces of the negative electrode current collector. Alternatively, the negative electrode may be a lithium metal plate.

[0098] The negative electrode current collector is used to support the negative electrode active material layer and is not particularly limited as long as it has high conductivity and does not cause chemical changes in the battery. It can be selected from copper, aluminum, stainless steel, zinc, titanium, silver, palladium, nickel, iron, chromium, and alloys and combinations thereof. Stainless steel can be surface-treated with carbon, nickel, titanium, or silver, and the alloy can be an aluminum-cadmium alloy. In addition, sintered carbon, non-conductive polymers surface-treated with a conductive material, or conductive polymers can be used.

[0099] The negative electrode current collector may be in various shapes, such as a film, sheet, foil, net, porous body, foamed body, non-woven fabric body, etc., with or without fine irregularities on the surface.

[0100] The negative electrode active material layer may further include, in addition to the negative electrode active material, a conductive material, a binder, etc. In this case, the conductive material and the binder are as described above.

[0101] The negative electrode active material may include a material capable of reversibly inserting and deinserting lithium (Li + ) material, a material capable of reacting with lithium ions to reversibly form a lithium-containing compound, lithium metal, or a lithium alloy.

[0102] Able to reversibly insert or deinsert lithium ions (Li + ) can be, for example, crystalline carbon, amorphous carbon or a mixture thereof. + ) to reversibly form a lithium-containing compound may be, for example, tin oxide, titanium nitrate, or silicon. The lithium alloy may be, for example, an alloy of lithium (Li) and a metal selected from the group consisting of sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

[0103] Preferably, the negative electrode active material may be lithium metal, specifically in the form of a lithium metal film or lithium metal powder.

[0104] There are no particular limitations on the method for forming the negative electrode active material, and methods commonly used in the art for forming layers or films may be used. For example, methods such as compression, coating, and deposition may be used. Furthermore, the negative electrode of the present invention also includes the case where a metallic lithium thin film is formed on the metal plate during initial charging after assembling a battery in which no lithium thin film is present on the current collector.

[0105] The electrolyte is used to cause electrochemical oxidation or reduction reactions in the positive electrode and the negative electrode therethrough, and is as described above.

[0106] The electrolyte injection can be performed at an appropriate step in the lithium-sulfur battery manufacturing process, depending on the manufacturing process and the desired physical properties of the final product. That is, it can be performed before the lithium-sulfur battery is assembled or at the final stage of assembly.

[0107] A conventional separator may be interposed between the positive electrode and the negative electrode. The separator is a physical separator having the function of physically separating the electrodes and can be used without particular limitation as long as it is used as a conventional separator. In particular, a separator having low resistance to ion migration in the electrolyte and excellent electrolyte impregnation ability is preferred.

[0108] The separator is capable of transporting lithium ions between the positive electrode and the negative electrode while isolating or insulating the positive electrode from each other. The separator can be made of a porous, non-conductive, or insulating material. The separator can be used without any particular limitation, as long as it is commonly used as a separator in lithium-sulfur batteries. The separator can be a separate component such as a membrane, or it can be a coating added to the positive electrode and / or the negative electrode.

[0109] The separator can be made of a porous substrate. Any porous substrate can be used, as long as it is a porous substrate commonly used in lithium-sulfur batteries. A porous polymer film can be used alone or in the form of a laminate. For example, non-woven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, or polyolefin porous films can be used, but are not limited thereto.

[0110] In the present invention, there is no particular limitation on the material of the porous substrate, and any material can be used as long as it is a porous substrate commonly used in lithium-sulfur batteries. For example, the porous substrate may include at least one material selected from the group consisting of polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyamides, polyacetals, polycarbonates, polyimides, polyetheretherketones, polyethersulfones, polyphenylene oxides, polyphenylene sulfides, polyethylene naphthalate, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, cellulose, nylon, poly(p-phenylene benzobis[theta]yl)benzene ... azoles), and polyarylates.

[0111] The thickness of the porous substrate is not particularly limited, but may be 1 to 100 μm, preferably 5 to 50 μm. Although the thickness range of the porous substrate is not particularly limited to the above range, if the thickness is excessively thinner than the above lower limit, the mechanical properties deteriorate, and the separator may be easily damaged during battery use.

[0112] The average size and porosity of the pores present in the porous substrate are also not particularly limited, but may be 0.1 to 50 μm and 10 to 95%, respectively.

[0113] In the case of the lithium-sulfur battery according to the present invention, in addition to the winding process as a general process, a lamination or stacking and folding process of the separator and the electrode may be performed.

[0114] There is no particular limitation on the shape of the lithium-sulfur battery, and it may have various shapes such as a cylindrical type, a stacked type, and a coin type.

[0115] Hereinafter, preferred embodiments are provided to help understand the present invention. However, the following embodiments are provided only to make the present invention easier to understand, and the present invention is not limited thereto.

[0116] Preferred Implementation

[0117] Example: Fabrication of Lithium-Sulfur Batteries

[0118] Preparation of electrolyte for lithium-sulfur batteries: Preparation Examples 1 to 7

[0119] [Preparation Example 1]

[0120] Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) with a concentration of 2.3 M was dissolved in an organic solvent to prepare an electrolyte for a lithium-sulfur battery.

[0121] A solvent prepared by mixing acetonitrile (ACN) as a first solvent, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) as a second solvent, and ethyl tert-butyl ether (EtBE) as a third solvent at a volume ratio of 30:52.5:17.5 was used as the organic solvent.

[0122] [Preparation Example 2]

[0123] An electrolyte for a lithium-sulfur battery was prepared in the same manner as in Preparation Example 1, except that when preparing the organic solvent, the first solvent, the second solvent, and the third solvent were mixed in a volume ratio of 30:35:35.

[0124] [Preparation Example 3]

[0125] An electrolyte for a lithium-sulfur battery was prepared in the same manner as in Preparation Example 1, except that when preparing the organic solvent, the first solvent, the second solvent, and the third solvent were mixed in a volume ratio of 30:17.5:52.5.

[0126] [Preparation Example 4]

[0127] An electrolyte for a lithium-sulfur battery was prepared in the same manner as in Preparation Example 1, except that diisopropyl ether (DiPE) was used as the third solvent instead of ethyl tert-butyl ether (EtBE) when preparing the organic solvent.

[0128] [Preparation Example 5]

[0129] An electrolyte for a lithium-sulfur battery was prepared in the same manner as in Preparation Example 2, except that diisopropyl ether (DiPE) was used as the third solvent instead of ethyl tert-butyl ether (EtBE) when preparing the organic solvent.

[0130] [Preparation Example 6]

[0131] An electrolyte for a lithium-sulfur battery was prepared in the same manner as in Preparation Example 3, except that diisopropyl ether (DiPE) was used as the third solvent instead of ethyl tert-butyl ether (EtBE) when preparing the organic solvent.

[0132] [Preparation Example 7]

[0133] An electrolyte for a lithium-sulfur battery was prepared in the same manner as in Preparation Example 1, except that the third solvent was not used, and acetonitrile (ACN) as the first solvent and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) as the second solvent were mixed in a volume ratio of 30:70 to obtain an organic solvent.

[0134] Table 1:

[0135]

[0136] *The mixing volume ratio refers to the volume ratio of "first solvent:second solvent:third solvent".

[0137] *ACN: acetonitrile

[0138] TTE: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether

[0139] EtBE: ethyl tert-butyl ether

[0140] DiPE: diisopropyl ether

[0141] Preparation of Lithium-Sulfur Batteries: Examples 1 to 6 and Comparative Example 1

[0142] [Example 1]

[0143] Water was used as a solvent, and a sulfur-carbon composite, a conductive material, and a binder were mixed in a ratio of 90:10:10 to prepare a positive electrode active material slurry. The sulfur-carbon composite was prepared by mixing sulfur and carbon nanotubes (CNTs) in a weight ratio of 7:3, followed by melt diffusion at 155°C. Furthermore, Denka Black was used as the conductive material, and a binder in the form of a mixture of SBR and CMC was used as the binder.

[0144] The positive electrode active material slurry was coated on one surface of the aluminum current collector and then dried to prepare a positive electrode with a loading capacity of 5 mAh / cm 2 positive electrode.

[0145] In addition, lithium metal with a thickness of 50 μm was used as the negative electrode.

[0146] After the prepared positive and negative electrodes were placed facing each other, a polyethylene separator with a thickness of 20 μm and a porosity of 45% was inserted between the positive and negative electrodes. Thereafter, the lithium-sulfur battery electrolyte of Preparation Example 1 was injected into the case to manufacture a lithium-sulfur battery.

[0147] [Examples 2 to 6]

[0148] A lithium-sulfur battery was prepared in the same manner as in Example 1, except that the electrolytes of Preparation Examples 2 to 6 were used as the electrolytes for the lithium-sulfur battery.

[0149] [Comparative Example 1]

[0150] A lithium-sulfur battery was prepared in the same manner as in Example 1, except that the electrolyte of Preparation Example 7 was used as the electrolyte for the lithium-sulfur battery.

[0151] Experimental Example 1: Initial charge and discharge performance evaluation

[0152] The initial charge and discharge performances of the lithium-sulfur batteries manufactured by the above Examples 1 to 6 and Comparative Example 1 were evaluated.

[0153] Specifically, when the operating temperature of the battery was 35°C and 25°C, the initial discharge capacity and nominal voltage of the first cycle were measured by performing 3 cycles of 0.1C charge / 0.1C discharge in the voltage range of 1.0 to 3.3V. The results are shown in Table 2 below. In addition, the evaluation results at an operating temperature of 35°C are shown in Figure 1 The evaluation results at 25°C are shown in Figure 2 .

[0154] Table 2:

[0155]

[0156] According to Table 2 and Figure 1 and 2 , it was confirmed that Examples 1 to 6 in which "a portion of TTE as the second solvent was replaced by a third solvent which was an ether non-solvent" still had a capacity of 1300 mAh / g even at 35°C 硫 The above excellent initial discharge capacity and high nominal voltage of 2.014V or more are at a low temperature of 35°C relative to the operating temperature of conventional slightly soluble electrolyte (SSE) electrolyte systems, and it is found that the energy density of the battery is also increased.

[0157] In particular, in the case of Examples 2 and 5, which include electrolytes obtained not only by adding the first solvent but also by mixing the second solvent and the third solvent in a volume ratio of 1:1, it was confirmed that the initial discharge capacity and the nominal voltage were superior to those of other Examples in which the solvent type was the same but only the mixing volume ratio was different.

[0158] In addition, even under the operating condition of 25°C below 35°C, compared with Comparative Example 1, Examples 2 and 5 of "the third solvent including the ether non-solvent" have an initial discharge capacity of 1130 mAh / g. 硫 The above and the nominal voltage increase and show better effect.

[0159] Experimental Example 2: Battery Life Characteristics Evaluation

[0160] For the lithium-sulfur batteries prepared in Examples 1 to 6 and Comparative Example 1, the life characteristics of the batteries were evaluated by repeating charge / discharge cycles.

[0161] Specifically, after three cycles of 0.1C charge / 0.1C discharge in the voltage range of 1.0 to 3.3 V, the battery life characteristics were evaluated while repeating 0.1C charge / 0.3C discharge. The results of the battery life characteristics evaluation at a battery operating temperature of 35°C are shown in FIG. Figure 3 The results obtained at a working temperature of 25°C are shown in Figure 4 In addition, the discharge capacity after the 50th cycle was measured when the battery operating temperature was 35°C and 25°C, and the results are shown in Table 3 below.

[0162] Table 3:

[0163]

[0164] First, refer to Figure 3 As shown in Table 3, Examples 1 to 6 in which a portion of TTE as the second solvent was replaced by a third solvent which was an ether non-solvent were confirmed to have excellent battery life characteristics at 35°C, which is a low temperature relative to the operating temperature of a conventional SSE electrolyte system, compared to Comparative Example 1 which did not include an ether non-solvent at all.

[0165] In addition, refer to Figure 4 As shown in Table 3, Examples 2 and 5, which contain a third solvent comprising an ether non-solvent, demonstrate superior battery life characteristics even at an operating temperature of 25°C, compared to Comparative Example 1, which contains no ether non-solvent at all. In particular, in Comparative Example 1, the battery discharge capacity decreases dramatically after 30 cycles under low-temperature operation at 25°C.

[0166] All simple modifications and variations of the present invention fall within the scope of the present invention, and the specific protection scope of the present invention will become apparent from the appended claims.

Claims

1. An electrolyte for a lithium-sulfur battery comprising a lithium salt and an organic solvent, wherein the organic solvent comprises a first solvent, a second solvent, and a third solvent, The first solvent comprises a compound containing a cyano group -CN represented by Chemical Formula 1 or Chemical Formula 2, The second solvent comprises a fluorinated ether solvent, The third solvent comprises an ether non-solvent represented by the following Chemical Formula 3, [Chemical Formula 1] R1-CN [Chemical Formula 2] NC-R2-CN Wherein in Chemical Formula 1, R1 is a C1 to C10 alkyl group, In Chemical Formula 2, R2 is a C1 to C10 alkylene group, [Chemical Formula 3] R3-O-R4 In Chemical Formula 3, R3 and R4 are the same as or different from each other, and are each independently methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl or tert-butyl. 2 . The electrolyte for lithium-sulfur batteries according to claim 1 , wherein the first solvent is selected from the group consisting of acetonitrile, succinonitrile, pimelonitrile, glutaronitrile, adiponitrile, and combinations thereof.

3. The electrolyte for a lithium-sulfur battery according to claim 1, wherein the second solvent is selected from the group consisting of: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1H,1H,2'H,3H-decafluorodipropyl ether, difluoromethyl 2,2,2-trifluoroethyl ether, 1,2,2,2-tetrafluoroethyl trifluoromethyl ether, 1,1,2,3,3,3-hexafluoropropyl difluoromethyl ether, pentafluoroethyl 2,2,2-trifluoroethyl ether, 1H,1H,2'H-perfluorodipropyl ether, bis(2,2,2-trifluoroethyl) ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, and combinations thereof.

4. The electrolyte for lithium-sulfur batteries according to claim 1, wherein the third solvent is selected from the group consisting of diisopropyl ether, ethyl tert-butyl ether, dibutyl ether, diisobutyl ether, dipropyl ether, and combinations thereof. 5 . The electrolyte for a lithium-sulfur battery according to claim 1 , wherein the organic solvent contains 15 to 45 volume ratios of the first solvent relative to 100 volume ratios of the organic solvent. 6 . The electrolyte for a lithium-sulfur battery according to claim 1 , wherein the organic solvent comprises 10 to 60 volume ratios of the third solvent relative to 100 volume ratios of the organic solvent. 7 . The electrolyte for a lithium-sulfur battery according to claim 1 , wherein the organic solvent comprises 25 to 45 volume ratios of the third solvent relative to 100 volume ratios of the organic solvent. 8 . The electrolyte for a lithium-sulfur battery according to claim 1 , wherein the organic solvent comprises 25 to 350 volume ratios of the third solvent relative to 100 volume ratios of the second solvent. 9 . The electrolyte for a lithium-sulfur battery according to claim 1 , wherein the organic solvent comprises 85 to 115 volume ratios of the third solvent relative to 100 volume ratios of the second solvent.

10. A lithium-sulfur battery, comprising: positive electrode, negative electrode, a separator between the positive and negative electrodes, and The electrolyte according to any one of claims 1 to 9.

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