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

Abstract

The present invention relates to an electrolyte for a lithium-sulfur battery, and a lithium-sulfur battery comprising the same, the electrolyte comprising: a first solvent comprising a heterocyclic compound comprising at least one double bond and simultaneously comprising an oxygen atom or a sulfur atom; a second solvent comprising at least one of an ether-based compound, an ester-based compound, an amide-based compound, and a carbonate-based compound; a lithium salt; lithium nitrate; and a borate-based lithium salt.

Description

Technical Field

[0001] This invention relates to an electrolyte for lithium-sulfur batteries and a lithium-sulfur battery containing the same, and more particularly, to an electrolyte for lithium-sulfur batteries that can improve the lifespan characteristics and lithium cycle efficiency of lithium-sulfur batteries by appropriately combining the solvent, lithium salt and additives contained in the electrolyte of lithium-sulfur batteries, and a lithium-sulfur battery containing the same.

[0002] This application claims priority based on Korean Patent Application No. 10-2021-0001823, filed on January 7, 2021, the entire contents of which are incorporated herein by reference. Background Technology

[0003] As the application of secondary batteries expands to electric vehicles (EVs) and energy storage devices (ESS), lithium-ion secondary batteries, with their relatively low gravimetric energy density (~250 Wh / kg), face limitations in these applications. In contrast, lithium-sulfur secondary batteries are attracting attention as a next-generation secondary battery technology due to their theoretically high gravimetric energy density (~2,600 Wh / kg).

[0004] Lithium-sulfur batteries are battery systems that use sulfur-sulfur alloys with SS bonds (sulfur-sulfur bonds) as the positive electrode active material and lithium metal as the negative electrode active material. Sulfur, as the main material for the positive electrode active material, has the advantages of being abundant worldwide, non-toxic, and having a low atomic weight.

[0005] In lithium-sulfur secondary batteries, during discharge, lithium, the negative electrode active material, is oxidized while releasing electrons and becoming ionized, while sulfide materials, the positive electrode active material, are reduced while accepting electrons. In this case, the lithium oxidation reaction is the process by which lithium metal releases electrons and transforms into lithium cations. Conversely, the sulfur reduction reaction is the process by which the S-S bond accepts two electrons and transforms into sulfur anions. The lithium cations generated by the lithium oxidation reaction are transferred to the positive electrode via the electrolyte and combine with the sulfur anions generated by the sulfur reduction reaction to form a salt. Specifically, sulfur before discharge has a cyclic S8 structure, which is converted into lithium polysulfide (LiS) through a reduction reaction. x When lithium polysulfides are completely reduced, lithium sulfide (Li₂S) is produced.

[0006] Sulfur, as a positive electrode active material, suffers from low electrical conductivity, making it difficult to ensure reactivity with electrons and lithium ions in the solid state. In existing lithium-sulfur secondary batteries, to improve the reactivity of sulfur, Li₂S is generated. xIntermediate polysulfides are used to induce liquid-phase reactions and improve reactivity. In this case, ether solvents with high solubility for lithium polysulfides, such as dioxolane and dimethoxyethane, are used as solvents for the electrolyte.

[0007] However, when using such ether-based solvents, the lifespan characteristics of lithium-sulfur batteries deteriorate due to various reasons. For example, the lifespan characteristics of lithium-sulfur batteries may be deteriorated by the leaching of lithium polysulfides from the positive electrode, the occurrence of short circuits due to the growth of dendrites on the lithium negative electrode, and the accumulation of byproducts from electrolyte decomposition.

[0008] In particular, when using such ether-based solvents, they can dissolve large amounts of lithium polysulfides, resulting in high reactivity. However, due to the nature of lithium polysulfides being soluble in electrolytes, the reactivity and lifetime characteristics of sulfur are affected by the electrolyte content.

[0009] In recent years, in order to develop high-energy-density lithium-sulfur secondary batteries with a capacity of over 500 Wh / kg required for aircraft and next-generation electric vehicles, it is necessary to have a large sulfur loading in the electrodes and to minimize the electrolyte content.

[0010] However, due to the characteristics of ether-based solvents, there is a problem that the viscosity increases rapidly during charging and discharging as the electrolyte content decreases, which may lead to overvoltage and battery degradation.

[0011] Therefore, research is ongoing on adding individual additives to electrolytes to prevent electrolyte decomposition and ensure excellent lifespan characteristics. However, the components and composition of electrolytes that can improve lifespan characteristics and lithium cycle efficiency have not yet been clearly determined.

[0012] [Existing Technical Documents]

[0013] [Patent Documents]

[0014] (Patent Document 1) Korean Patent Publication No. 10-2007-0027512 (March 9, 2007), "Electrolytes for Lithium-Sulfur Electrochemical Cells" Summary of the Invention

[0015] Technical issues

[0016] Therefore, in order to improve the lifespan characteristics and lithium cycle efficiency of lithium-sulfur batteries, it was found that the above problems could be solved by introducing: a first solvent comprising a heterocyclic compound containing one or more double bonds and simultaneously containing either an oxygen atom or a sulfur atom; a second solvent comprising at least one of an ether compound, an ester compound, an amide compound, and a carbonate compound; a lithium salt; lithium nitrate; and a borate lithium salt, thereby improving the performance of lithium-sulfur batteries and thus completing the present invention.

[0017] Therefore, one object of the present invention is to provide an electrolyte for lithium-sulfur batteries that improves the lifespan characteristics and lithium cycle efficiency of lithium-sulfur batteries. Another object of the present invention is to provide a lithium-sulfur battery that exhibits improved battery performance by having the above-described electrolyte.

[0018] Technical solution

[0019] To achieve the above objectives, the present invention provides an electrolyte for lithium-sulfur batteries, comprising: a first solvent comprising a heterocyclic compound, said heterocyclic compound containing one or more double bonds and simultaneously containing either an oxygen atom or a sulfur atom; a second solvent comprising at least one of an ether compound, an ester compound, an amide compound, and a carbonate compound; a lithium salt; lithium nitrate; and a borate lithium salt.

[0020] In addition, the present invention provides a lithium-sulfur battery comprising a positive electrode; a negative electrode; a separator inserted between the positive electrode and the negative electrode; and an electrolyte for the lithium-sulfur battery.

[0021] Beneficial effects

[0022] According to the present invention, the electrolyte for lithium-sulfur batteries and the lithium-sulfur battery comprising the present invention can achieve the effect of improving the life characteristics and lithium cycle efficiency of the lithium-sulfur battery having the electrolyte by introducing the following into the electrolyte for lithium-sulfur batteries: a first solvent comprising a heterocyclic compound containing one or more double bonds and simultaneously containing either an oxygen atom or a sulfur atom; a second solvent comprising at least one of an ether compound, an ester compound, an amide compound, and a carbonate compound; a lithium salt; lithium nitrate; and a borate lithium salt. Attached Figure Description

[0023] Figure 1 A graph illustrating the lifespan characteristics of lithium-sulfur batteries using the electrolytes of Examples 1 to 3 and Comparative Examples 1 and 2 of the present invention.

[0024] Figure 2 The graph illustrates the lifespan characteristics of lithium-sulfur batteries using the electrolytes of Examples 1, 4 to 8 and Comparative Example 1 of the present invention. Detailed Implementation

[0025] The present invention will be described in detail below.

[0026] All embodiments provided by the present invention can be implemented 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.

[0027] The present invention provides an electrolyte for lithium-sulfur batteries, comprising A) a first solvent containing a heterocyclic compound, said heterocyclic compound containing one or more double bonds and simultaneously containing either an oxygen atom or a sulfur atom; B) a second solvent containing at least one of an ether compound, an ester compound, an amide compound, and a carbonate compound; C) a lithium salt; D) lithium nitrate; and E) a borate lithium salt.

[0028] In the following text, each of A) a first solvent, B) a second solvent, C) a lithium salt, D) lithium nitrate and E) a borate-based lithium salt contained in the electrolyte for lithium-sulfur batteries of the present invention will be specifically described.

[0029] A) First solvent

[0030] The electrolyte for lithium-sulfur batteries according to the present invention comprises a first solvent, the first solvent comprising a heterocyclic compound containing one or more double bonds and simultaneously containing either an oxygen atom or a sulfur atom.

[0031] The first solvent comprises a heterocyclic compound containing one or more double bonds and simultaneously containing either an oxygen atom or a sulfur atom. The heterocyclic compound exhibits poor salt solubility due to the delocalization of the lone pair electrons of the heteroatoms (oxygen or sulfur atoms). In lithium-sulfur batteries using lithium-based metals as the negative electrode, during the initial discharge phase, a polymer protective film (solid electrolyte interface, SEI layer) is formed on the surface of the lithium-based metal (negative electrode) through the ring-opening reaction of the heterocyclic compound. This suppresses the formation of lithium dendrites. Furthermore, the lifespan characteristics of the lithium-sulfur battery can be improved by reducing electrolyte decomposition on the lithium-based metal surface and subsequent side reactions.

[0032] Therefore, the heterocyclic compounds of the present invention must have at least one double bond to form a polymer protective film on the surface of lithium-based metals. Furthermore, since the heterocyclic compounds of the present invention increase their affinity for other organic solvents in the electrolyte by containing oxygen or sulfur to make them polar, thereby promoting their utilization as components of the electrolyte, the heterocyclic compounds must also contain heteroatoms (oxygen or sulfur atoms).

[0033] The heterocyclic compound may be a 3- to 15-membered, preferably 3- to 7-membered, and more preferably 5- to 6-membered heterocyclic compound. Furthermore, the heterocyclic compound may be a heterocyclic compound substituted or unsubstituted with at least one of the following groups: alkyl groups having 1 to 4 carbon atoms, cycloalkyl groups having 3 to 8 carbon atoms, aryl groups having 6 to 10 carbon atoms, halogen groups, nitro (-NO2), amino (-NH2), and sulfonyl (-SO2). Additionally, the heterocyclic compound may be a polycyclic compound of at least one of a cycloalkyl group having 3 to 8 carbon atoms and an aryl group having 6 to 10 carbon atoms with a heterocyclic compound.

[0034] When the heterocyclic compound is substituted with an alkyl group having 1 to 4 carbon atoms, it is preferred because it stabilizes the free radical and can suppress side reactions between electrolytes. Furthermore, when substituted with a halogen group or a nitro group, it is preferred because a functional protective film can be formed on the surface of the lithium metal. In this case, the formed functional protective film is stable as a compressible protective film, can uniformly deposit the lithium metal, and has the advantage of suppressing side reactions between polysulfides and lithium metal.

[0035] Specific examples of the heterocyclic compound may be furan, 2-methylfuran, 3-methylfuran, 2-ethylfuran, 2-propylfuran, 2-butylfuran, 2,3-dimethylfuran, 2,4-dimethylfuran, 2,5-dimethylfuran, pyran, 2-methylpyran, 3-methylpyran, 4-methylpyran, benzofuran, 2-(2-nitrovinyl)furan, thiophene, 2-methylthiophene, 2-ethylthiophene, 2-propylthiophene, 2-butylthiophene, 2,3-dimethylthiophene, 2,4-dimethylthiophene, 2,5-dimethylthiophene, etc., preferably 2-methylfuran.

[0036] The content of the first solvent containing such heterocyclic compounds can be from 5% to 50% of the total volume of organic solvents (i.e., the first solvent + the second solvent) contained in the electrolyte for lithium-sulfur batteries of the present invention, preferably from 10% to 30% of the volume, and more preferably from 15% to 20% of the volume (the remainder corresponding to the second solvent). If the content of the first solvent is less than the above range, the following problems may exist: the ability to reduce the leaching of polysulfides is reduced, thus failing to suppress the increase in electrolyte resistance, or the protective film not being fully formed on the surface of the lithium-based metal. In addition, if the content of the first solvent exceeds the above range, there is a concern that the battery capacity and lifespan may be reduced due to the increased surface resistance of the electrolyte and the lithium-based metal. Therefore, it is preferable that the content of the first solvent meets the above range.

[0037] B) Second solvent

[0038] The electrolyte for lithium-sulfur batteries according to the present invention comprises a second solvent, the second solvent comprising at least one of an ether compound, an ester compound, an amide compound, and a carbonate compound.

[0039] The second solvent comprises at least one of ether compounds, ester compounds, amide compounds, and carbonate compounds. The second solvent is used not only to dissolve the lithium salt to give the electrolyte lithium-ion conductivity, but also to dissolve sulfur, which serves as the positive electrode active material, thereby facilitating its electrochemical reaction with lithium. The carbonate compound can be a linear carbonate compound or a cyclic carbonate compound.

[0040] Specific examples of the ether compounds may be, but are not limited to, at least one selected from the group consisting of: dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, 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, preferably dimethoxyethane.

[0041] Additionally, the ester compound may be, but is not limited to, at least one selected from the group consisting of: methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone. Furthermore, the amide compound may be a conventional amide compound used in the art.

[0042] In addition, the linear carbonate compound may be, but is not limited to, at least one of the following: dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl ethyl carbonate (EMC), methyl propyl carbonate (MPC), and ethyl propyl carbonate (EPC).

[0043] In addition, the cyclic carbonate compound may be, but is not limited to, at least one selected from the group consisting of: ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentene carbonate, 2,3-pentene carbonate, vinylene carbonate, vinyl ethylene carbonate and its halides (fluoroethylene carbonate (FEC), etc.).

[0044] On the other hand, if the second solvent is included in a smaller than appropriate amount, there are concerns that the lithium salt cannot dissolve sufficiently, thus reducing lithium-ion conductivity, and that sulfur, as an active material, exceeds its soluble concentration, leading to precipitation problems. If the second solvent is included in an excessive amount, there may be problems such as excessive leaching of sulfur, as a positive electrode active material, causing severe shuttling between lithium polysulfides and the lithium anode, and reducing lifetime.

[0045] On the other hand, the organic solvent comprising the first solvent and the second solvent may be contained in an amount of 70 to 97% by weight, preferably 75 to 96% by weight, and more preferably 90 to 96% by weight relative to the total weight of the electrolyte for lithium-sulfur batteries of the present invention. If the organic solvent is contained in an amount of less than 70% by weight relative to the total weight of the electrolyte for lithium-sulfur batteries, there may be problems such as increased viscosity of the electrolyte and decreased ionic conductivity, or incomplete dissolution of the lithium salt or additives in the electrolyte. If the organic solvent is contained in an amount of more than 97% by weight, there may be problems such as decreased ionic conductivity as the concentration of lithium salt in the electrolyte decreases. Therefore, it is preferable that the contents of the first solvent and the second solvent satisfy the above-mentioned ranges.

[0046] Furthermore, the volume ratio of the first solvent to the second solvent can be from 1:2.5 to 1:6, preferably from 1:4 to 1:6, and more preferably from 1:4 to 1:5.5. If the volume ratio of the first solvent to the second solvent is less than the above range, the lithium salt may not dissolve sufficiently, resulting in reduced lithium-ion conductivity, and sulfur, as an active material, may precipitate beyond its soluble concentration. If the volume ratio of the first solvent to the second solvent exceeds the above range, excessive leaching of sulfur, as a positive electrode active material, may occur, causing severe shuttling between lithium polysulfides and the lithium anode, and reducing its lifespan. Therefore, it is preferable that the volume ratio of the first solvent to the second solvent meets the above range.

[0047] C) lithium salts

[0048] The electrolyte for lithium-sulfur batteries according to the present invention contains a lithium salt as an electrolyte salt for improving ion conductivity.

[0049] Examples of the lithium salt may be, but are not limited to, at least one selected from the group consisting of: LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10LiB(Ph)4, LiPF6, LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (C2F5SO2)2NLi, (SO2F)2NLi, (CF3SO2)3CLi and lower aliphatic carboxylic acids having 4 or fewer carbon atoms, preferably, may contain LiFSI ((SO2F)2NLi) as an essential component.

[0050] The concentration of the lithium salt can be determined taking into account factors such as ionic conductivity, and can be, for example, from 0.2M to 2M, preferably from 0.5M to 1M. If the concentration of the lithium salt is less than the above range, it may be difficult to ensure suitable ionic conductivity for battery operation. If the concentration of the lithium salt exceeds the above range, the lithium ion mobility decreases as the electrolyte viscosity increases, or the decomposition reaction of the lithium salt itself increases, thus potentially degrading battery performance. Therefore, it is preferable that the concentration of the lithium salt meets the above range.

[0051] D) Lithium nitrate

[0052] Furthermore, the electrolyte for lithium-sulfur batteries according to the present invention comprises lithium nitrate (LiNO3). However, if desired, the electrolyte may further comprise at least one selected from the group consisting of: lanthanum nitrate (La(NO3)3), 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).

[0053] The lithium nitrate can be contained in an amount of 1 to 7% by weight, preferably 2 to 6% by weight, and more preferably 3 to 5% by weight, relative to the total weight of the electrolyte for lithium-sulfur batteries. If the lithium nitrate content is less than 1% by weight relative to the total weight of the electrolyte for lithium-sulfur batteries, the coulombic efficiency may be significantly reduced. If the lithium nitrate content exceeds 7% by weight, the viscosity of the electrolyte may increase, making the battery difficult to operate. Therefore, it is preferable that the lithium nitrate content meets the above-mentioned range.

[0054] E) Borate lithium salts

[0055] The electrolyte for lithium-sulfur batteries according to the present invention comprises a borate-based lithium salt as an additive. The borate-based lithium salt may be at least one selected from the group consisting of: lithium tetrafluoroborate (LiBF4), lithium bis(oxalate-borate)borate (LiBOB), lithium difluorooxalate-borate (LiFOB), and lithium bis(2-methyl-2-fluoro-malonic acid)borate, preferably lithium difluorooxalate-borate (LiFOB, LiDFOB).

[0056] The content of the borate-based lithium salt relative to the total weight of the electrolyte for lithium-sulfur batteries can be from 0.01 wt% to 5.0 wt%, preferably from 0.05 wt% to 4.0 wt%, and more preferably from 0.1 wt% to 3.0 wt%. If the content of the borate-based lithium salt is less than the above range, there may be a problem that the protective film cannot be sufficiently formed on the surface of the lithium-based metal. If the content of the borate-based lithium salt exceeds the above range, the capacity and life of the battery may be reduced due to the increased surface resistance of the lithium-based metal. Therefore, it is preferable that the content of the borate-based lithium salt meets the above range.

[0057] Furthermore, the weight ratio of the borate-based lithium salt to lithium nitrate can be from 1:1 to 1:30, preferably from 1:2 to 1:30, and more preferably from 1:3 to 1:30. If the weight ratio of the borate-based lithium salt to lithium nitrate is less than the above range, the coulombic efficiency may decrease rapidly. If the weight ratio of the borate-based lithium salt to lithium nitrate exceeds the above range, there may be a problem that the protective film cannot be sufficiently formed on the surface of the lithium-based metal. Therefore, it is preferable that the weight ratio of the borate-based lithium salt to lithium nitrate meets the above range.

[0058] On the other hand, the total content of the borate-based lithium salt and lithium nitrate relative to the total weight of the electrolyte for lithium-sulfur batteries can be from 2% to 8% by weight, preferably from 3% to 7% by weight, and more preferably from 3% to 5% by weight. If the total content of the borate-based lithium salt and lithium nitrate is less than the above range, there may be problems such as insufficient formation of a protective film on the surface of the lithium-based metal and a rapid decrease in the coulombic efficiency of the battery. If the total content of the borate-based lithium salt and lithium nitrate exceeds the above range, the viscosity of the electrolyte may increase, thereby making the battery difficult to operate. Therefore, it is preferable that the total content of the borate-based lithium salt and lithium nitrate meets the above range.

[0059] Next, a lithium-sulfur battery according to the present invention will be described. The lithium-sulfur battery includes a positive electrode, a negative electrode, a separator inserted between the positive electrode and the negative electrode, and an electrolyte for the lithium-sulfur battery.

[0060] As described above, the electrolyte for the lithium-sulfur battery comprises A) a first solvent, B) a second solvent, C) a lithium salt, D) lithium nitrate, and E) a borate-based lithium salt, as described in detail above. Furthermore, the lithium-sulfur battery can be any lithium-sulfur battery commonly used in the art, among which a lithium-sulfur battery is perhaps the most desirable.

[0061] In the following description of the lithium-sulfur battery according to the present invention, the positive electrode, negative electrode, and separator will be described in more detail.

[0062] As described above, the positive electrode in the lithium-sulfur battery of the present invention comprises a positive electrode active material, a binder, and a conductive material.

[0063] The positive electrode active material can be a positive electrode active material applicable to conventional lithium-sulfur batteries, such as elemental sulfur (S8), sulfide compounds, or mixtures thereof. Specifically, the sulfide compound can be Li2S. n (n≥1), organic sulfur compounds or carbon-sulfur complexes (C2S) x ) n (x = 2.5–50, n ≥ 2). Furthermore, the positive electrode active material may comprise a sulfur-carbon composite, and since sulfur materials alone are not conductive, they can be used in combination with conductive materials. The carbon material (or carbon source) constituting the sulfur-carbon composite may have a porous structure or a high specific surface area, and any carbon material can be used, provided it is commonly used in the art. For example, the porous carbon material may be, but is not limited to, at least one selected from the group consisting of: graphite; graphene; carbon black, such as Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking 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); and activated carbon, and the porous carbon material may be spherical, rod-shaped, needle-shaped, plate-shaped, tubular, or block-shaped, and can be used without restriction, as long as it is commonly used in lithium-sulfur batteries.

[0064] Furthermore, pores are formed in the carbon material, and the porosity of these pores is 40% to 90%, preferably 60% to 80%. If the porosity is less than 40%, the pores may act as resistive components and cause problems because lithium ions cannot be properly transported. If the porosity exceeds 90%, a decrease in mechanical strength may occur. Additionally, the pore size of the carbon material is 10 nm to 5 μm, preferably 50 nm to 5 μm. If the pore size is less than 10 nm, lithium ion transport may be impaired. If the pore size exceeds 5 μm, battery short circuits and safety issues may occur due to contact between electrodes.

[0065] The adhesive is a component that assists in the bonding between the positive electrode active material and the conductive material, as well as the bonding with the current collector. For example, it may be, but is not limited to, at least one of the following: polyvinylidene fluoride (PVdF), polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVdF / HFP), polyvinyl acetate, polyvinyl alcohol, polyvinyl ether, polyethylene, polyethylene oxide, alkylated polyethylene oxide, polypropylene, poly(meth)acrylate, poly(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-butene rubber, fluororubber, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, and mixtures thereof.

[0066] The binder is typically added in an amount of 1 to 50 parts by weight, preferably 3 to 15 parts by weight, relative to the total weight of 100 parts by weight of the positive electrode. If the binder content is less than 1 part by weight, the adhesive strength between the positive electrode active material and the current collector may be insufficient. If the binder content is greater than 50 parts by weight, the adhesive strength is improved, but the content of the positive electrode active material may be reduced, thereby decreasing the battery capacity.

[0067] There are no particular restrictions on the conductive material contained in the positive electrode, as long as it does not cause side reactions in the internal environment of the lithium-sulfur battery, has excellent conductivity, and does not cause chemical changes in the battery. The conductive material can typically be graphite or conductive carbon, and can be, for example, but not limited to, one of the following: graphite, such as natural or artificial graphite; carbon black, such as carbon black, acetylene black, Ketjen black, Danka black, thermally cracked carbon black, channel black, furnace black, lamp black, and summer black; carbon-based materials with a crystal structure of graphene or graphite; conductive fibers, such as carbon fibers and metal fibers; fluorocarbons; metal powders, such as aluminum and nickel powders; conductive whiskers, such as zinc oxide and potassium titanate; conductive oxides, such as titanium oxides; conductive polymers, such as polyphenylene derivatives; and mixtures of two or more thereof.

[0068] The conductive material is typically added in an amount of 0.5 to 50 parts by weight, preferably 1 to 30 parts by weight, relative to the total weight of 100 parts by weight of the positive electrode. If the content of the conductive material is too low, i.e., less than 0.5 parts by weight, it is difficult to achieve the effect of improving conductivity, or the electrochemical characteristics of the battery may deteriorate. If the content of the conductive material exceeds 50 parts by weight, i.e., if it is excessive, the amount of positive electrode active material is relatively small, thereby potentially reducing capacity and energy density. There are no particular limitations on the method of introducing the conductive material into the positive electrode, and conventional methods known in the relevant art can be used, such as coating onto the positive electrode active material. Furthermore, if desired, adding a conductive second coating to the positive electrode active material can replace the addition of the conductive material as described above.

[0069] Alternatively, fillers can be selectively added to the positive electrode of the present invention as a component for suppressing positive electrode expansion. There are no particular limitations on such fillers, as long as they can suppress electrode expansion without causing chemical changes in the battery. Examples include olefin polymers, such as polyethylene and polypropylene; and fibrous materials, such as glass fibers and carbon fibers.

[0070] The positive electrode active material, binder, conductive material, etc., are dispersed and mixed in a dispersion medium (solvent) to form a slurry. The slurry can be applied to a positive electrode current collector, and then dried and calendered to prepare the positive electrode. The dispersion medium can be, but is not limited to, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol, isopropanol, water, or mixtures thereof.

[0071] The positive electrode current collector can be, but is not necessarily limited to, platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), ruthenium (Ru), nickel (Ni), stainless steel (STS), aluminum (Al), molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W), ITO (In-doped SnO2), FTO (F-doped SnO2), or alloys thereof, or aluminum (Al) or stainless steel whose surface has been treated with carbon (C), nickel (Ni), titanium (Ti), or silver (Ag). The positive electrode current collector can be in the form of foil, film, sheet, perforated, porous, foamed, etc.

[0072] The negative electrode is a lithium-based metal, and a current collector may be further included on one side of the lithium-based metal. The current collector can be a negative electrode current collector. There are no particular limitations on the negative electrode current collector, as long as it has high conductivity without causing chemical changes in the battery, and it can be selected from the group consisting of copper, aluminum, stainless steel, zinc, titanium, silver, palladium, nickel, iron, chromium, and their alloys and combinations. The stainless steel can be surface-treated with carbon, nickel, titanium, or silver, and the alloy can be an aluminum-cadmium alloy. Alternatively, sintered carbon, or a non-conductive polymer surface-treated with a conductive material or conductive polymer can be used. Typically, a thin copper foil is used as the negative electrode current collector.

[0073] Furthermore, the negative electrode current collector can be of various shapes, such as films, sheets, foils, meshes, porous bodies, foams, nonwoven fabrics, etc., with or without micro-uneven surfaces. Additionally, the thickness of the negative electrode current collector is in the range of 3 to 500 μm. If the thickness of the negative electrode current collector is less than 3 μm, the current collection efficiency decreases. On the other hand, if the thickness exceeds 500 μm, there is a problem of reduced processability when folding and assembling a single cell.

[0074] The lithium-based metal can be lithium or a lithium alloy. In this case, the lithium alloy contains an element capable of alloying with lithium. Specifically, the lithium alloy can be an alloy of lithium with at least one 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.

[0075] The lithium-based metal can be in the form of sheets or foils. In some cases, it can be in the form of lithium or lithium alloys deposited or coated on a current collector by a dry process, or it can be in the form of particulate metals and alloys deposited or coated by a wet process.

[0076] A conventional diaphragm can be inserted between the positive and negative electrodes. The diaphragm is a physical diaphragm that functions to physically separate the electrodes and can be used without particular limitations, as long as it is used as a conventional diaphragm. In particular, a diaphragm with low resistance to ion migration in the electrolyte and excellent electrolyte impregnation ability is preferred.

[0077] Furthermore, the separator is capable of transporting lithium ions between the positive and negative electrodes while separating or insulating them from each other. The separator can be made of a porous, non-conductive, or insulating material. The separator can be a standalone component, such as a membrane, or a coating added to the positive and / or negative electrodes.

[0078] Examples of polyolefin-based porous membranes that can be used as the diaphragm can be membranes formed solely from any polymer selected from: polyethylene, such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene; and polyolefin polymers, such as polypropylene, polybutene, and polypentene, or membranes formed from mixtures of their polymers. Examples of nonwoven fabrics that can be used as the diaphragm are nonwoven fabrics formed solely from the following polymers or mixtures thereof: polyphenylene ether, polyimide, polyamide, polycarbonate, polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polyphenylene sulfide, polyacetal, polyethersulfone, polyetheretherketone, polyester, etc. Such nonwoven fabrics include nonwoven fabrics in fibrous form to form a porous web, i.e., spunbond or meltblown nonwoven fabrics composed of long fibers.

[0079] There is no particular limitation on the thickness of the separator, but it is preferably in the range of 1 to 100 μm, more preferably 5 to 50 μm. If the thickness of the separator is less than 1 μm, mechanical properties cannot be maintained. If the thickness of the separator exceeds 100 μm, the separator acts as a resistive layer, thereby degrading the performance of the battery. There is no particular limitation on the pore size and porosity of the separator, but it is preferably 0.1 to 50 μm in pore size and 10% to 95% in porosity. If the pore size of the separator is less than 0.1 μm or the porosity is less than 10%, the separator acts as a resistive layer. If the pore size of the separator is greater than 50 μm or the porosity is greater than 95%, mechanical properties cannot be maintained.

[0080] The lithium-sulfur battery of the present invention, comprising the positive electrode, negative electrode, separator, and electrolyte as described above, can be manufactured by a process of placing the positive electrode facing the negative electrode, inserting the separator therebetween, and then injecting the electrolyte for lithium secondary batteries according to the present invention.

[0081] On the other hand, the lithium-sulfur battery according to the present invention is suitable not only as a battery cell for use as a power source for small devices, but also particularly suitable as a unit battery for use as a power source for medium to large devices. In this regard, the present invention also provides a battery module in which at least two lithium-sulfur batteries are electrically connected (in series or in parallel). Obviously, the number of lithium-sulfur batteries contained in the battery module can be adjusted in various ways considering the intended use and capacity of the battery module. Furthermore, the present invention provides a battery pack in which battery modules are electrically connected according to conventional techniques in the art. The battery modules and battery packs can be used as a power source for at least one medium to large device selected from: power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric trucks; electric commercial vehicles; or energy storage systems, but the present invention is not limited thereto.

[0082] The best mode for implementing an invention

[0083] Preferred embodiments are provided below to aid in understanding the invention, but the following embodiments are merely illustrative of the invention. It will be apparent to those skilled in the art that various changes and variations can be made within the scope and spirit of the invention, and such changes and variations are all within the scope of the appended claims.

[0084] Example

[0085] Preparation of electrolyte for lithium-sulfur secondary batteries

[0086] Example 1

[0087] An electrolyte for lithium-sulfur batteries was prepared by adding 3.0 wt% lithium nitrate (LiNO3) and 0.1 wt% lithium difluorooxalatoborate (LiDFOB) relative to the total weight of the electrolyte to an organic solvent obtained by mixing 2-methylfuran (first solvent) and 1,2-dimethoxyethane (second solvent) in a volume ratio (volume / volume) of 1:4, and dissolving lithium bis(fluorosulfonyl)imide (LiFSI) to a concentration of 0.75 M (mol / L).

[0088] Example 2

[0089] The electrolyte for lithium-sulfur batteries was prepared in the same manner as in Example 1, except that 0.5% by weight of lithium difluorooxalate borate (LiDFOB) was used.

[0090] Example 3

[0091] The electrolyte for lithium-sulfur batteries was prepared in the same manner as in Example 1, except that 1.0% by weight of lithium difluorooxalate borate (LiDFOB) was used.

[0092] Example 4

[0093] The electrolyte for lithium-sulfur batteries was prepared in the same manner as in Example 1, except that an organic solvent obtained by mixing 2-methylfuran (first solvent) and 1,2-dimethoxyethane (second solvent) in a volume ratio (volume / volume) of 1:4.5 was used as the organic solvent.

[0094] Example 5

[0095] The electrolyte for lithium-sulfur batteries was prepared in the same manner as in Example 1, except that an organic solvent obtained by mixing 2-methylfuran (first solvent) and 1,2-dimethoxyethane (second solvent) in a volume ratio (volume / volume) of 1:5 was used as the organic solvent.

[0096] Example 6

[0097] The electrolyte for lithium-sulfur batteries was prepared in the same manner as in Example 1, except that an organic solvent obtained by mixing 2-methylfuran (first solvent) and 1,2-dimethoxyethane (second solvent) in a volume ratio (volume / volume) of 1:5.5 was used as the organic solvent.

[0098] Example 7

[0099] The electrolyte for lithium-sulfur batteries was prepared in the same manner as in Example 1, except that an organic solvent obtained by mixing 2-methylfuran (first solvent) and 1,2-dimethoxyethane (second solvent) in a volume ratio (volume / volume) of 1:6 was used as the organic solvent.

[0100] Example 8

[0101] The electrolyte for lithium-sulfur batteries was prepared in the same manner as in Example 1, except that an organic solvent obtained by mixing 2-methylfuran (first solvent) and 1,2-dimethoxyethane (second solvent) in a volume ratio (volume / volume) of 1:2.5 was used as the organic solvent.

[0102] Comparative Example 1

[0103] The electrolyte for lithium-sulfur batteries was prepared in the same manner as in Example 1, except that lithium difluorooxalate borate (LiDFOB) was not added.

[0104] Comparative Example 2

[0105] The electrolyte for lithium-sulfur batteries was prepared in the same manner as in Example 1, except that an organic solvent obtained by mixing dioxolane (first solvent) and 1,2-dimethoxyethane (second solvent) in a volume ratio (volume / volume) of 1:2 was used as the organic solvent.

[0106] The contents of the first solvent, the second solvent, and the borate-based lithium salt in the electrolytes for lithium-sulfur batteries of Examples 1 to 8 and Comparative Examples 1 and 2 are shown in Table 1 below.

[0107] Table 1:

[0108]

[0109] Experimental Example 1: Lifetime Characteristics of Lithium-Sulfur Batteries

[0110] Sulfur was mixed with a conductive material and a binder in acetonitrile to prepare a slurry for the positive electrode active material. Carbon 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, such that the mixing ratio was sulfur:conductive material:binder weight ratio of 72:24:4. The slurry for the positive electrode active material was prepared at 4.1 mAh / cm³. 2 A loading of lithium metal was applied to an aluminum current collector and then dried to prepare a positive electrode with a porosity of 70%. Additionally, a 45 μm thick layer of lithium metal was used as the negative electrode.

[0111] After positioning the positive and negative electrodes prepared by the above method so that they face each other, a polyethylene diaphragm with a thickness of 20 μm and a porosity of 45% is inserted between the positive and negative electrodes.

[0112] Subsequently, the electrolytes according to Examples 1 to 8 and Comparative Examples 1 and 2 were injected into the casing to prepare lithium-sulfur batteries.

[0113] The lithium-sulfur batteries manufactured using the above method were subjected to 2.5 cycles as follows: 0.1C discharge in CC mode at 25°C until the open-circuit voltage (OCV) reached 1.8V, followed by 0.1C charging until it reached 2.5V again. After battery stabilization cycling, 0.3C charge / 0.5C discharge cycles were performed within a voltage range between 1.8V and 2.5V to evaluate the cycle life at 80% retention of the initial high-rate capacity. The results are shown in Table 2 below. Figures 1 to 2 middle.

[0114] Table 2:

[0115] Number of cycles (based on 80% capacity retention) Example 1 160 Example 2 127 Example 3 133 Example 4 283 Example 5 219 Example 6 208 Example 7 166 Example 8 110 Comparative Example 1 97 Comparative Example 2 40

[0116] Figure 1 and 2 A graph illustrating the lifespan characteristics of lithium-sulfur batteries comprising electrolytes according to embodiments and comparative examples of the present invention. (See figure) Figure 1 and 2 As shown in Table 2 above, it was confirmed that compared with the case of using an electrolyte without borate-based lithium salt (Comparative Example 1) and the case of using a heterocyclic compound without double bonds as the first solvent (Comparative Example 2), the lithium-sulfur battery using the electrolyte of the present invention, which contains a first solvent (containing a heterocyclic compound containing one or more double bonds and simultaneously containing either an oxygen atom or a sulfur atom (2-methylfuran)) and a borate-based lithium salt (LiDFOB), has a significantly higher cycle life based on 80% capacity retention of the battery.

[0117] Experimental Example 2: Evaluation of Lithium Cycle Efficiency of Lithium-Sulfur Batteries

[0118] A symmetrical single cell of the 2032 coin cell type (CR 2032) was manufactured using a lithium metal electrode (working electrode and counter electrode) with a thickness of 20 μm, a polyethylene separator with a thickness of 20 μm and a porosity of 45%, and electrolytes according to Examples 1 to 8 and Comparative Examples 1 and 2.

[0119] For the lithium-sulfur battery in the form of a symmetrical single cell prepared by the above method, the lithium cycle efficiency was measured at a 1C depth of discharge (DOD) of 83%, and the results are shown in Table 3.

[0120] 1C DOD 83% refers to a charge / discharge quantity corresponding to 83% of 16.6μm of 20μm Li, and indicates a current density of 3.7mA / cm². 2 This is the rate at which the capacity can be charged and discharged for 1 hour.

[0121] Table 3:

[0122] Lithium cycle efficiency (%) Example 1 99.2 Example 2 98.9 Example 3 98.6 Example 4 99.2 Example 5 98.9 Example 6 98.6 Example 7 96.7 Example 8 96.1 Comparative Example 1 95.4 Comparative Example 2 97.5

[0123] As shown in Table 3 above, it was confirmed that compared with the case of using an electrolyte without borate-based lithium salt (Comparative Example 1) and the case of using a heterocyclic compound without double bonds as the first solvent (Comparative Example 2), the lithium-sulfur battery using the electrolyte of the present invention, which contains a first solvent (containing a heterocyclic compound containing one or more double bonds and simultaneously containing either an oxygen atom or a sulfur atom (2-methylfuran)) and a borate-based lithium salt (LiDFOB), has improved stability between the lithium-sulfur battery electrolyte and the lithium anode, and therefore has excellent lithium cycle efficiency.

[0124] All simple variations and modifications of this invention fall within its scope, and the specific scope of protection of this invention will become apparent from the appended claims.

Claims

1. An electrolyte for lithium-sulfur batteries, comprising: The first solvent is 2-methylfuran; The second solvent is dimethoxyethane; The lithium salt is (SO2F)2NLi; Lithium nitrate; and Borate lithium salts, The volume ratio of the first solvent to the second solvent is from 1:4.5 to 1:5.

5. The content of the borate-based lithium salt is from 0.01% to 5.0% by weight relative to the total weight of the electrolyte for lithium-sulfur batteries, and The borate-based lithium salt is selected from at least one of the group consisting of lithium bis(oxalate)borate, lithium difluorooxalateborate, and lithium bis(2-methyl-2-fluoro-malonic acid)borate.

2. The electrolyte for lithium-sulfur batteries according to claim 1, wherein the weight ratio of the borate-based lithium salt to the lithium nitrate is 1:1 to 1:

30.

3. The electrolyte for lithium-sulfur batteries according to claim 1, wherein the concentration of the lithium salt is 0.2 to 2.0 M.

4. The electrolyte for lithium-sulfur batteries according to claim 1, wherein the electrolyte for lithium-sulfur batteries further comprises at least one selected from the group consisting of: lanthanum nitrate, potassium nitrate, cesium nitrate, magnesium nitrate, barium nitrate, lithium nitrite, potassium nitrite, and cesium nitrite.

5. The electrolyte for lithium-sulfur batteries according to claim 1, wherein the electrolyte for lithium-sulfur batteries comprises 2-methylfuran as a first solvent, dimethoxyethane as a second solvent, (SO2F)2NLi as a lithium salt, lithium difluorooxalate borate and lithium nitrate as borate-based lithium salts.

6. A lithium-sulfur battery, comprising positive electrode; negative electrode; A diaphragm inserted between the positive and negative electrodes; and The electrolyte for lithium-sulfur batteries according to claim 1.

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