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

Abstract

The present invention relates to an electrolyte for use in lithium-sulfur batteries, which contains a Lewis acid additive containing a cation with a higher Lewis acidity than lithium ions. This reduces the elution of polysulfides into the electrolyte, improves the ionic conductivity of the electrolyte, and improves the sulfur utilization rate, thereby providing advantageous effects for stable operation of dilute electrolyte and high-capacity lithium-sulfur batteries.

Description

[Technical Field]

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

[0002] This application claims priority based on Korean Patent Application No. 2023-0046347, filed on April 7, 2023, the entire contents of which are incorporated herein by reference in their entirety in the specification and drawings thereof. [Background technology]

[0003] A lithium-sulfur battery is a battery system that uses a sulfur-based material with a sulfur-sulfur bond (SS bond) as the positive electrode active material and lithium metal as the negative electrode active material. Sulfur, the main material of the positive electrode active material, is abundant worldwide and has the advantages of being non-toxic and having a low atomic weight.

[0004] As the application areas of secondary batteries expand to electric vehicles (EVs) and energy storage systems (ESSs), lithium-sulfur battery technology is gaining attention as it can theoretically achieve a higher weight energy density (up to 2,600Wh / kg) compared to lithium-ion secondary batteries, which have a relatively low weight energy density (up to 250Wh / kg).

[0005] During discharge, lithium, the negative electrode active material, releases electrons and is oxidized as it is ionized into lithium cations, while the positive electrode active material, a sulfur-based material, accepts electrons and is reduced. The sulfur-based material's S-type bond accepts two electrons through a reduction reaction, converting it into a sulfur anion. The lithium cations generated by the lithium oxidation reaction are transported to the positive electrode via the electrolyte and combine with sulfur anions generated by the reduction reaction of a sulfur-based compound to form a salt. Specifically, sulfur, which has a cyclic S structure before discharge, is converted to lithium polysulfide (Li2Sx) through a reduction reaction, and is then completely converted to lithium sulfide (Li2S).

[0006] At this time, the lithium sulfide formed accumulates on the surface of the electrodes of the lithium-sulfur battery, which has resulted in the lithium-sulfur battery still showing a discharge capacity less than the theoretical capacity. In particular, under conditions where the content of lithium salt in the electrolyte is reduced or the amount of electrolyte used in the battery is reduced in order to commercialize the lithium-sulfur battery, a side reaction of lithium polysulfide continues to occur in the lithium anode, causing corrosion of the anode and reducing its lifespan.

[0007] Specifically, the more dilute the electrolyte used in a battery, the more rapidly the concentration of lithium polysulfide dissolved in the electrolyte increases, eventually reaching a saturation point. In this case, the high concentration of lithium polysulfide in the electrolyte causes the lithium anode to undergo continuous side reactions, resulting in severe corrosion. Furthermore, the smaller the amount of electrolyte used, the more severe the shuttle effect, a chronic problem in lithium-sulfur batteries, becomes, resulting in loss of active material and deterioration of battery performance.

[0008] Research efforts to improve these problems have been continuously conducted, particularly focusing on using high-concentration lithium salts to ensure that most solvent molecules participate in the solvation shell, thereby reducing the solvent activity and solubility of polysulfides. It has been reported that increasing the concentration of lithium salts reduces the volatility of the electrolyte and improves the conductivity of lithium ions, thereby improving battery performance under dilute electrolyte conditions. However, problems have been reported, such as high cost and low ionic conductivity, making commercialization and high current operation difficult.

[0009] For this reason, there is currently an increasing need for lithium-sulfur batteries that can operate stably even under conditions of dilute electrolyte and high sulfur loading by improving the ionic conductivity of the electrolyte and maintaining the sulfur utilization rate in the positive electrode while suppressing the elution of polysulfides. Summary of the Invention [Problem to be solved by the invention]

[0010] Therefore, the problem to be solved by the present invention is: The objective is to solve the passivation phenomenon caused by the accumulation of lithium sulfide on the electrodes used in lithium-sulfur batteries.

[0011] Therefore, one aspect of the present invention aims to provide an electrolyte solution for a lithium-sulfur battery that can suppress the elution of lithium polysulfides into the electrolyte solution.

[0012] Another object of the present invention is to provide an electrolyte solution for a lithium-sulfur battery that has excellent ionic conductivity while having a low lithium salt concentration.

[0013] Another aspect of the present invention is to provide a lithium-sulfur battery with a reduced electrolyte content and an increased sulfur loading. [Means for solving the problem]

[0014] To solve the above problems, According to one aspect of the present invention, there is provided an electrolyte solution for a lithium-sulfur battery having the following embodiment.

[0015] The electrolyte for a lithium-sulfur battery according to the first aspect includes a lithium salt, a nitrate compound, a non-aqueous solvent, and a Lewis acidic additive, The Lewis acidic additive is a lithium ion (Li + ) is a salt containing a cation with a higher Lewis acidity than The total concentration of the lithium salt, the nitrate compound, and the Lewis acidic additive is 2M or less.

[0016] According to the second aspect, in the first aspect, The total concentration of the lithium salt, the nitrate compound, and the Lewis acidic additive may be 1.5M or less.

[0017] According to the third aspect, in the first aspect or the second aspect, The Lewis acidic additive may contain, as a cation, an aluminum (Al) ion, a magnesium (Mg) ion, a calcium (Ca) ion, a strontium (Sr) ion, a barium (Ba) ion, or two or more of these.

[0018] According to the fourth aspect, in any one of the first to third aspects, The Lewis acidic additive may include a calcium salt.

[0019] According to the fifth aspect, in any one of the first to fourth aspects, The Lewis acidic additive may include an anion containing a fluorine atom.

[0020] According to the sixth aspect, in any one of the first to fifth aspects, The Lewis acidic additive may have as an anion F - , BF4 - , PF6 - , CF3SO3 - , [(CF3SO2)2N] - , [(FSO2)2N] - Or it may contain two or more of these.

[0021] According to the seventh aspect, in any one of the first to sixth aspects, The lithium salt concentration in the lithium-sulfur battery electrolyte may be 2M or less.

[0022] According to the eighth aspect, in any one of the first to seventh aspects, The concentration of the Lewis acid additive in the lithium-sulfur battery electrolyte may be 0.3M or less.

[0023] According to the ninth aspect, in any one of the first to eighth aspects, The concentration of the Lewis acid additive in the lithium-sulfur battery electrolyte may be 0.01M to 0.1M.

[0024] According to the tenth aspect, in any one of the first to ninth aspects, The non-aqueous solvent may include an ether solvent.

[0025] According to the eleventh aspect, in any one of the first to tenth aspects, the non-aqueous solvent includes an ether-based solvent, The volume of the ether solvent may be 60% by volume or more based on the total volume of the non-aqueous solvent.

[0026] According to the twelfth aspect, in any one of the first to eleventh aspects, The non-aqueous solvent may consist of only one ether solvent.

[0027] According to the thirteenth aspect, in any one of the first to twelfth aspects, The non-aqueous solvent may include cyclic ethers and acyclic ethers.

[0028] According to another aspect of the present invention, there is provided a lithium-sulfur battery having the following configuration.

[0029] A lithium-sulfur battery according to a fourteenth aspect includes: The battery includes the electrolytic solution according to any one of the first to thirteenth aspects, a positive electrode containing inorganic sulfur (S8), a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

[0030] According to the fifteenth aspect, in the fourteenth aspect, The negative electrode may be provided with a solid electrolyte interface (SEI) layer containing LiF.

[0031] According to the 16th aspect, in any one of the 13th aspect to the 15th aspect, The El / S (electrolyte / sulfur) ratio may be 7 μl / mg or less. [Effects of the Invention]

[0032] The electrolyte for a lithium-sulfur battery according to one aspect of the present invention comprises: This solves the passivation phenomenon caused by the accumulation of lithium sulfide on the negative electrode of a lithium-sulfur battery, thereby improving the operating stability of the lithium-sulfur battery.

[0033] In addition, by forming a stable interface with a high fluorine content on the negative electrode of a lithium-sulfur battery, it induces uniform lithium electrodeposition and suppresses decomposition of the electrolyte, thereby increasing the coulombic efficiency of the lithium-sulfur battery and achieving a longer lifespan.

[0034] The drawings attached to this specification illustrate preferred embodiments of the present invention and serve to further understand the technical concepts of the present invention as well as the content of the invention, and therefore the present invention should not be interpreted as being limited to only the matters depicted in the drawings. [Brief explanation of the drawings]

[0035] [Figure 1] 1 shows the results of evaluating the life characteristics of lithium-sulfur batteries using the electrolyte solutions of Comparative Example 1, Example 1, and Example 2 in this specification. [Figure 2] 1 shows the results of evaluating the rate performance according to the charge-discharge cycle of lithium-sulfur batteries using the electrolyte solutions of Comparative Examples 1 to 4, Example 1, and Example 2 in this specification. [Figure 3] 1 shows the results of component analysis of the solid electrolyte interface (SEI) layer formed on the negative electrode of a lithium-sulfur battery using the electrolyte solutions of Comparative Example 1 (w / o Ca(OTF)2) and Example 1 (w / Ca(OTF)2) in this specification. [Figure 4] 1 shows the results of component analysis of the solid electrolyte interface (SEI) layer formed on the negative electrode of a lithium-sulfur battery using the electrolyte solutions of Comparative Example 1 and Example 1 in this specification. [Figure 5] 1 shows the results of measuring current due to the shuttle effect using the electrolyte solutions of Comparative Example 1 and Example 1 in this specification. [Figure 6] 1 shows the results of carrying out decomposition analysis evaluation of the electrolyte solutions of Comparative Example 1 and Example 1 in the present specification. [Figure 7] 1 shows the results of evaluating the life characteristics of lithium-sulfur batteries using the electrolyte solutions of Comparative Examples 1, 5, and 6 and Examples 1 to 3 in this specification. [Figure 8] 1 shows the results of evaluating the specific capacity according to the charge-discharge cycle of lithium-sulfur batteries using the electrolyte solutions of Example 4 and Comparative Example 7 in this specification. [Figure 9]1 shows charge-discharge curves in the first cycle of charge-discharge of a lithium-sulfur battery using the electrolyte solution of Comparative Example 8 in the present specification. [Figure 10] 1 shows the results of evaluating the specific capacity according to the charge-discharge cycle of a lithium-sulfur battery using the electrolyte solution of Example 5 in this specification. DETAILED DESCRIPTION OF THE INVENTION

[0036] The present invention will be described in detail below.

[0037] The terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as having meanings and concepts corresponding to the technical ideas of the present invention, in accordance with the principle that the inventor can appropriately define the concepts of terms himself / herself in order to best explain the invention.

[0038] Furthermore, throughout this specification, when a part is described as "comprising" or "having" a certain component, this does not mean that other components are excluded, but that other components may also be included, unless otherwise specified.

[0039] Furthermore, terms and phrases used throughout this specification such as "about," "approximately," and "substantially" are used to mean at or near a numerical value when manufacturing and material tolerances inherent in the stated meaning are given, and are used to prevent unscrupulous infringers from unfairly taking advantage of the contents of the disclosure in which precise or absolute numerical values ​​are stated to further the understanding of the present invention.

[0040] Furthermore, throughout this specification, the phrase "A and / or B" means "A or B or both."

[0041] The term "polysulfide" as used herein means "polysulfide ions (Sx 2-, "1 < x ≤ 8))" and "lithium polysulfide (Li2Sx or LiSx - , 1 < x ≤ 8)" is a concept that encompasses both.

[0042] The present invention relates to an electrolyte for a lithium-sulfur battery and a lithium-sulfur battery containing the same.

[0043] The electrolyte for a lithium-sulfur battery according to one aspect of the present invention contains a lithium salt, a nitrate compound, a non-aqueous solvent, and a Lewis acidic additive. In particular, the total concentration of the lithium salt, the nitrate compound, and the Lewis acidic additive is to be 2 M or less.

[0044] The "Lewis acidic additive" is to be a salt containing a cation having a higher Lewis acidity than lithium ion (Li + ).

[0045] As one definition regarding acid and base, the definition by Gilbert Newton Lewis is known. According to this, a substance can be classified as a Lewis acid or a Lewis base. Specifically, the "Lewis base" can be defined as a substance that donates a non-bonding electron pair to a Lewis acid, and the "Lewis acid" can be defined as a substance that receives a non-bonding electron pair from the Lewis base.

[0046] More specifically, the Lewis acid collectively refers to chemical species containing vacant orbitals that can receive an electron pair from a Lewis base to form a Lewis adduct, and the Lewis base collectively refers to chemical species containing filled orbitals that contain an electron pair capable of forming a Lewis adduct through a chemical bond with a Lewis acid.

[0047] In this specification, the "Lewis acidity" is a measure of the tendency to accept an unshared electron pair and can be measured according to a known method for measuring Lewis acidity. Thus, the "higher Lewis acidity" means a greater tendency to accept an unshared electron pair, and can indicate, for example, that the energy level of the vacant orbital that can accept the unshared electron pair is lower.

[0048] According to one embodiment of the present invention, the Lewis acidic additive is a lithium ion (Li + ), the lithium-sulfur battery electrolyte contains the Lewis acid additive, and thus the electrons (e - This has the advantage of suppressing the decomposition of the lithium salt due to oxidation. Therefore, it is possible to provide an electrolyte solution that can maintain good ionic conductivity during operation of a lithium-sulfur battery while reducing the content of lithium salt in the electrolyte solution, but the mechanism of the present invention is not limited thereto.

[0049] In one embodiment of the present invention, the cation of the Lewis acidic additive and the lithium ion (Li + The comparison of Lewis acidity between the cation and the lithium ion can be performed, for example, by comparing the acid dissociation constants (Ka) of the conjugate bases of the cation and the lithium ion. The method of comparing acidity by comparing acid dissociation constants is already known, and the smaller the pKa (-logKa) value, the stronger the acid.

[0050] In one embodiment of the invention, the cation is M + When expressed as , the pKa value of the cation's conjugate base, MH, and the lithium ion (Li + A comparison of Lewis acidity between the cation and lithium ion can be made by comparing the pKa value of the conjugate base (LiH) of the cation.

[0051] Specifically, in one embodiment of the present invention, the Lewis acidic additive may include a cation having a conjugate base with a pKa value greater than the pKa value of LiH, which is the conjugate base of the lithium ion.

[0052] In one embodiment of the present invention, the Lewis acidic additive may contain, as a cation, an aluminum (Al) ion, a magnesium (Mg) ion, a calcium (Ca) ion, a strontium (Sr) ion, a barium (Ba) ion, or two or more of these.

[0053] In one embodiment of the present invention, the Lewis acidic additive may include a calcium ion as a cation. Specifically, the Lewis acidic additive may include a calcium salt.

[0054] In another embodiment of the present invention, the Lewis acidic additive may be a salt containing a known anion for the above-mentioned cation. The anion may be any anion that does not impair the objective of the present invention when contained in the electrolyte.

[0055] In one embodiment of the present invention, the anion of the Lewis acidic additive may include, for example, but is not limited to, a halide, an alkyl, an alkoxide, an aryl, an aryloxide, an alkylate, a cyclopentadienyl, an acetylacetonate, an amide, a sulfonate, a sulfate, a borate, an aluminate, an aluminoxide, a phosphate, an arsenate, an imide, or two or more thereof.

[0056] In one embodiment of the invention, the anion of the Lewis acidic additive is F - , Cl - , Br - , I - , R - , R.O. - , cyclopentadienyl (Cp), pentamethylcyclopentadienyl, R2N -, acetylacetonate (acac), hexafluoroacetylacetonate (hfac), CF3SO2O - (-OTf), RSO2O - , ROSO2O - , BF4 - , BR4 - , AlCl4 - , PF6 - , PR3F3 - , AsF6 - , NO3 - and SO4 - The group may be, but is not limited to, one or more selected from the group consisting of: wherein R represents alkyl, cycloalkyl, aryl, alkoxy, aryloxy, haloalkyl, haloalkoxy, or a polymer.

[0057] In another embodiment of the present invention, the anion of the Lewis acidic additive may include elemental fluorine. When the Lewis acidic additive contains an anion containing elemental fluorine as the anion, it has the effect of rapidly forming a solid electrolyte interface (SEI) layer on the surface of the negative electrode. In particular, when the Lewis acidic additive contains an anion containing elemental fluorine as the anion, elemental fluorine is present in the electrolyte containing the Lewis acidic additive, and a solid electrolyte interface (SEI) layer containing elemental fluorine can be formed on the surface of the negative electrode during operation of a lithium-sulfur battery using the same. When an SEI layer containing elemental fluorine is formed on the surface of the negative electrode of the lithium-sulfur battery, lithium can be uniformly electrodeposited on the negative electrode during operation, thereby reducing negative electrode degradation, but the present invention is in no way limited thereto.

[0058] According to another embodiment of the present invention, the Lewis acidic additive has fluoride (F - ), or may contain an anion containing at least one fluorine element and further containing a different element, or may contain both of these.

[0059] In another embodiment of the present invention, the Lewis acidic additive may be, for example, F as the anion. - , BF4 - , PF6 - , CF3SO3 - (OTF - ), [(CF3SO2)2N] - (TFSI - ), [(FSO2)2N] - (FSI - ) or two or more of these, but the present invention is not limited thereto.

[0060] In yet another embodiment of the present invention, the Lewis acidic additive is, for example, AlF3, Al(BF4)3, Al(PF6)3, Al(CF3SO3)3, Al[(CF3SO2)2N]3, Al[(FSO2)2N]3, MgF2, Mg(BF4)2, Mg(PF6)2, Mg(CF3SO3)2, Mg[(CF3SO2)2N]2, Mg[(FSO2)2N]2, CaF 2, Ca(BF4)2, Ca(PF6)2, Ca(CF3SO3)2, Ca[(CF3SO2)2N]2, Ca[(FSO2)2N]2, SrF2, Sr(BF4)2, Sr(PF6)2, Sr(CF3SO3)2, Sr[(CF3SO2)2N]2, Sr[(FSO2)2N]2, or two or more of these, but are not limited thereto.

[0061] In yet another embodiment of the present invention, the Lewis acidic additive may include Ca(CF3SO3)2.

[0062] In one embodiment of the present invention, the lithium salt can be used without limitation as long as it can be used as an electrolyte in the electrolyte solution of a lithium-sulfur battery. The lithium salt can be, for example, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10, LiPF6, LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, (C2F5SO2)2NLi, (SO2F)2NLi, (CF3SO2)3CLi, lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenylborate, lithium imide, or two or more of these, but are not limited thereto.

[0063] In one embodiment of the present invention, the lithium salt may include (CF3SO2)2NLi(LiFSI).

[0064] In one embodiment of the present invention, the nitrate compound refers to a nitric acid-based compound or a nitrite-based compound that can be used as an additive in an electrolyte for a lithium-sulfur battery. The nitric acid-based compound or the nitrite-based compound has the effect of forming a stable film on a negative electrode made of a material such as lithium metal, thereby improving charge / discharge efficiency, but the mechanism of the present invention is not limited thereto.

[0065] In one embodiment of the present invention, the nitrate compound may be, but is not limited to, inorganic nitric acid or nitrite compounds such as lithium nitrate (LiNO), potassium nitrate (KNO), cesium nitrate (CsNO), barium nitrate (Ba(NO)), ammonium nitrate (NHNO), lithium nitrite (LiNO), potassium nitrite (KNO), cesium nitrite (CsNO), and ammonium nitrite (NHNO); organic nitric acid or nitrite compounds such as methyl nitrate, dialkylimidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridinium nitrate, ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite, and octyl nitrite; organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, and dinitrotoluene, or a mixture of two or more thereof.

[0066] In one embodiment of the present invention, the cation of the nitrate compound may be selected from, but is not limited to, alkali metals such as lithium, sodium, potassium, rubidium, and cesium.

[0067] In other embodiments of the present invention, the nitrate compound may include lithium nitrate (LiNO3).

[0068] As described above, according to one embodiment of the present invention, the Lewis acid additive can reduce the total salt content in the electrolyte while improving ionic conductivity.

[0069] According to one aspect of the present invention, the electrolyte for a lithium-sulfur battery includes a lithium salt as a salt, a nitrate compound, and a Lewis acid additive.

[0070] Thus, according to one aspect of the present invention, the total concentration of the lithium salt, the nitrate compound, and the Lewis acid additive in the lithium-sulfur battery electrolyte is 2M or less.

[0071] Specifically, according to one embodiment of the present invention, the total concentration of the lithium salt, the nitrate compound, and the Lewis acid additive in the lithium-sulfur battery electrolyte may be, for example, 1.8 M or less, 1.75 M or less, 1.70 M or less, 1.65 M or less, 1.60 M or less, 1.55 M or less, 1.50 M or less, 1.45 M or less, 1.40 M or less, 1.35 M or less, 1.30 M or less, 1.25 M or less, or 1.20 M or less. More specifically, the total concentration of the lithium salt, the nitrate compound, and the Lewis acid additive in the lithium-sulfur battery electrolyte may be, for example, 0.5 M or more, 0.8 M or more, or 1 M or more.

[0072] In one embodiment of the present invention, the total concentration of the lithium salt, the nitrate compound, and the Lewis acid additive in the lithium-sulfur battery electrolyte may be, for example, 0.5 M to 2 M, and more specifically, 0.8 M to 1.8 M, 1 M to 1.5 M, 1 M to 1.3 M, 1 M to 1.25 M, or 1.20 M to 1.25 M. According to one aspect of the present invention, the lithium salt, the nitrate compound, and the Lewis acid additive are contained in the above concentrations to achieve a low-electrolyte electrolyte solution while exhibiting an advantageous effect in terms of improving the life of a lithium-sulfur battery, but the present invention is not limited thereto.

[0073] In one embodiment of the present invention, the lithium-sulfur battery electrolyte may further include, in addition to the lithium salt, the nitrate compound, and the Lewis acid additive, an additional salt compound, as long as the object of the present invention is not impaired.

[0074] In one embodiment of the present invention, when the lithium-sulfur battery electrolyte solution further contains an additional salt compound in addition to the lithium salt, nitrate compound, and Lewis acid additive, the total salt concentration of the lithium-sulfur battery electrolyte solution may be preferably maintained at 2.5M or less, specifically, 2M or less.

[0075] In another embodiment of the present invention, the lithium-sulfur battery electrolyte may contain no salt compound other than the lithium salt, the nitrate compound, and the Lewis acid additive. Therefore, in this case, the total salt concentration in the lithium-sulfur battery electrolyte may refer to the total concentration of the lithium salt, the nitrate compound, and the Lewis acid additive.

[0076] Specifically, according to one embodiment of the present invention, the total salt concentration in the lithium-sulfur battery electrolyte may be, for example, 2 M or less or 1.5 M or less. More specifically, the salt concentration in the lithium-sulfur battery electrolyte may be, for example, 0.1 M to 2.5 M, 0.5 M to 2 M, 0.5 M to 1.5 M, 1 M to 1.25 M, 1 M to 1.20 M, or 1.20 M to 1.25 M. According to one embodiment of the present invention, even when the salt concentration is within the above range, the degree of decomposition of the lithium salt during battery operation is improved, resulting in excellent ionic conductivity and an outstanding effect in realizing a long battery life, but the present invention is by no means limited thereto.

[0077] In one embodiment of the present invention, the concentration of the lithium salt in the lithium-sulfur battery electrolyte may be, for example, 2 M or less. Specifically, the concentration of the lithium salt in the lithium-sulfur battery electrolyte may be, for example, 1.8 M or less, 1.5 M or less, or 1 M or less. Specifically, within the above-mentioned upper limit range, the concentration of the lithium salt in the lithium-sulfur battery electrolyte may be, for example, 0.05 M or more, 0.1 M or more, 0.5 M or more, or 0.6 M or more. More specifically, the concentration of the lithium salt in the lithium-sulfur battery electrolyte may be, for example, 0.05 M to 2 M, 0.1 M to 1.75 M, 0.5 M to 1.5 M, 0.5 M to 1.25 M, 0.5 M to 1.0 M, 0.5 M to 0.8 M, 0.5 M to 0.75 M, 0.6 M to 0.8 M, or 0.75 M to 1.0 M.

[0078] In one embodiment of the present invention, the concentration of the Lewis acid additive in the lithium-sulfur battery electrolyte may be, for example, 0.3 M or less, 0.2 M or less, or 0.1 M or less. Specifically, the concentration of the Lewis acid additive in the lithium-sulfur battery electrolyte may be 0.01 M to 0.3 M, 0.01 M to 0.2 M, 0.01 M to 0.1 M, 0.03 M to 0.2 M, 0.04 M to 0.1 M, 0.04 M to 0.075 M, or 0.05 M to 0.075 M.

[0079] In one embodiment of the present invention, the concentration of the nitrate compound in the lithium-sulfur battery electrolyte may be, for example, 1 M or less, or 0.5 M or less. Specifically, the concentration of the nitrate compound in the lithium-sulfur battery electrolyte may be 0.05 M to 1 M, 0.1 M to 0.8 M, 0.2 M to 0.6 M, 0.3 M to 0.5 M, or 0.4 M to 0.5 M.

[0080] In one embodiment of the present invention, the non-aqueous solvent may be any solvent that is used in electrolytes for lithium-sulfur batteries, including, for example, an ether-based solvent, an ester-based solvent, an amide-based solvent, a carbonate-based solvent, or two or more of these solvents.

[0081] In one embodiment of the present invention, the ether-based solvent may be any ether-based solvent that is used in electrolytes for lithium-sulfur batteries.

[0082] In another embodiment of the present invention, the ether solvents can be classified into cyclic ethers and acyclic ethers, for example, depending on their structures.

[0083] In one embodiment of the present invention, the acyclic ether may be, for example, dimethoxyethane, diethoxyethane, ethylene glycol ethyl methyl ether, 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, polyethylene glycol methyl ethyl ether, glycol diethyl ether, polyethylene glycol methyl ethyl ether, bis(2,2,2-trifluoroethyl)ether, methyl propyl ether, ethyl propyl ether, dipropyl ether, methyl t-butyl ether, methyl hexyl ether, ethyl t-butyl ether, ethylhexyl ether, or a mixture of two or more thereof.

[0084] In one embodiment of the present invention, the cyclic ether may be, for example, 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, or a mixture of two or more thereof.

[0085] In one embodiment of the present invention, the ester-based solvent may be, for example, any 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, or a mixture of two or more thereof, but is not limited thereto.

[0086] In one embodiment of the present invention, the carbonate solvent may be, for example, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, or a halide thereof, or a mixture of two or more thereof. Examples of the halide thereof include, but are not limited to, fluoroethylene carbonate.

[0087] In another embodiment of the present invention, the non-aqueous solvent may be substantially free of the carbonate-based solvent because the carbonate-based solvent cannot dissolve the nitric acid compound or exhibits low solubility in the carbonate-based solvent.

[0088] In one embodiment of the present invention, the non-aqueous solvent may contain a very small amount of a carbonate-based solvent to the extent that it does not affect the solubility of the nitrate compound. For example, when the non-aqueous solvent contains the carbonate-based solvent, the content of the carbonate-based solvent may be 3 wt % or less, 2 wt % or less, 1 wt % or less, 0.5 wt % or less, or 0 wt % (i.e., none) based on the total weight of the lithium-sulfur battery electrolyte.

[0089] In one embodiment of the present invention, as described above, the Lewis acid additive can suppress the leaching of lithium polysulfide, thereby providing an excellent lithium-sulfur battery without the need for a separate non-solvent for lithium polysulfide. In this regard, the non-aqueous solvent may include an ether-based solvent.

[0090] In one embodiment of the present invention, the non-aqueous solvent may contain an ether-based solvent in a majority amount based on the total volume of the non-aqueous solvent. Specifically, the volume of the ether-based solvent may be 50% by volume or more, more specifically 60% by volume or more, more specifically 70% by volume to 100% by volume, 80% by volume to 100% by volume, 85% by volume to 100% by volume, 90% by volume to 100% by volume, or 95% by volume to 100% by volume, based on the total volume of the non-aqueous solvent.

[0091] In yet another embodiment of the present invention, the non-aqueous solvent may consist of only one ether solvent.

[0092] According to one embodiment of the present invention, the non-aqueous solvent may be composed solely of the acyclic ether. For example, the non-aqueous solvent may contain 100% by volume of the acyclic ether based on the total volume.

[0093] More specifically, in one embodiment of the present invention, the non-aqueous solvent may contain diethoxyethane (DME) as an ether-based solvent, and the volume of DME may be 60% by volume or more, more specifically, 70% by volume to 100% by volume, 80% by volume to 100% by volume, 85% by volume to 100% by volume, 90% by volume to 100% by volume, or 95% by volume to 100% by volume, based on the total volume of the non-aqueous solvent.

[0094] In another embodiment of the present invention, the non-aqueous solvent may comprise a cyclic ether and an acyclic ether, for example, a mixture of 2-methylfuran (2-MeF) as a cyclic ether and diethoxyethane (DME) as an acyclic ether.

[0095] According to one embodiment of the present invention, when the non-aqueous solvent contains a cyclic ether and an acyclic ether, the cyclic ether and the acyclic ether may be contained in a volume ratio of, for example, 5:1 to 1:5, 4:1 to 1:4, 3:1 to 1:3, 2:1 to 1:2, or 1:1, but the present invention is not limited thereto.

[0096] As described above, the use of the composition of the lithium-sulfur battery electrolyte according to one aspect of the present invention can suppress the elution of lithium polysulfide and improve the passivation phenomenon caused by the accumulation of lithium sulfide on the negative electrode, thereby improving the driving stability of the lithium-sulfur battery, but the present invention is not limited thereto.

[0097] According to another aspect of the present invention, there is provided a lithium-sulfur battery including the above-described lithium-sulfur battery electrolyte.

[0098] The lithium-sulfur battery includes the lithium-sulfur battery electrolyte having the above-described composition, a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

[0099] Here, the positive electrode may contain inorganic sulfur (S8) as a positive electrode active material. Specifically, the positive electrode may contain a sulfur-containing compound in addition to inorganic sulfur as a positive electrode active material, and in this case, the sulfur-containing compound is Li2S n (n≧1), disulfide compounds, organic sulfur compounds and carbon-sulfur polymers (C2S x ) n , x=2.5 to 50, n≧2).

[0100] In one embodiment of the present invention, the positive electrode, the negative electrode, and the separator may be any material suitable for use in a lithium-sulfur battery, and a description of these materials will be omitted herein. The shape of the lithium-sulfur battery is not particularly limited, and various shapes, such as a cylindrical shape, a laminated shape, and a coin shape, may be used.

[0101] As described above, the lithium-sulfur battery includes an electrolyte solution containing a Lewis acid additive. According to one embodiment of the present invention, the Lewis acid additive may include fluorine as an anion. During initial charging of the lithium-sulfur battery, a solid electrolyte interface (SEI) layer may be formed on the negative electrode. When the Lewis acid additive includes fluorine as an anion, according to one embodiment of the present invention, the lithium-sulfur battery may have a fluorine-containing SEI layer formed on the negative electrode during initial charging.

[0102] Specifically, according to one embodiment of the present invention, the lithium-sulfur battery may have an SEI layer containing LiF on the negative electrode during initial charging.

[0103] According to one embodiment of the present invention, the lithium-sulfur battery includes the above-mentioned electrolyte solution, thereby realizing a low-electrolyte, high-capacity lithium-sulfur battery.

[0104] For example, the lithium-sulfur battery can achieve the effect of stable operation even when the El / S (electrolyte / sulfur) ratio is, for example, 7 μl / mg or less, specifically 5 μl / mg or less, but the present invention is not limited thereto. In particular, since the lithium-sulfur battery aims to achieve low electrolyte and high capacity, when the above-mentioned electrolyte is used, it goes without saying that the battery can be operated with an El / S ratio greater than the above-mentioned El / S ratio. This means that excellent performance can be exhibited even at the above-mentioned El / S ratio, so the El / S ratio is not limited thereto.

[0105] According to one embodiment of the present invention, the lithium-sulfur battery may have an El / S (electrolyte / sulfur) ratio of, for example, 2 to 7 μl / mg, 3 to 5 μl / mg, 4 to 5 μl / mg, 2 to 5 μl / mg, 2 to 3 μl / mg, or 2 to 2.5 μl / mg, but the present invention is not limited thereto.

[0106] The present invention will now be described in detail with reference to examples to help deepen understanding of the present invention.

[0107] Experimental Example 1 [Manufacturing electrolyte for lithium-sulfur batteries] To confirm that the performance of lithium-sulfur batteries can be improved by adding a Lewis acid additive to the electrolyte solution for lithium-sulfur batteries, an electrolyte solution with the composition shown in Table 1 was prepared.

[0108] Specifically, the electrolyte solutions of Comparative Examples 2 to 4 were prepared to evaluate the performance of electrolyte solutions containing 2-methylfuran (2-MeF), bis(2,2,2-trifluoroethyl) ether (BTFE), or 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) as a non-solvent for lithium polysulfide.

[0109] [Table 1]

[0110] Evaluation example 1. Evaluation of solubility in lithium polysulfide In order to evaluate the ability of a Lewis acidic additive to inhibit the elution of lithium polysulfide, the following experiment was carried out.

[0111] Using the electrolytes of Comparative Example 1 and Example 1, the same amount of lithium polysulfide (LiS) was added dropwise at room temperature of 23° C. until it was completely dissolved. When the initial solid-state lithium polysulfide was no longer observed, the concentration of the experimental solution was measured, and the results are shown in Table 2 below.

[0112] [Table 2]

[0113] As can be seen from Table 2 above, in order to dissolve the same amount of lithium polysulfide, the volume of the electrolyte in Example 1 was greater than the volume of the electrolyte in Comparative Example 1, and as a result, the final concentration of the experimental solution was lower.

[0114] Through this, it was confirmed that the Lewis acid additive suppresses the dissolution of lithium polysulfide in the electrolyte, thereby suppressing the elution of lithium polysulfide into the electrolyte.

[0115] [Manufacturing lithium-sulfur batteries] A lithium-sulfur battery was manufactured using the electrolyte solution prepared above as follows.

[0116] A positive electrode slurry was prepared by coating an aluminum foil with a sulfur-carbon composite (S / C: 70 / 30) and polyacrylic acid (PAA) binder and then drying it to prepare a positive electrode (the sulfur content was 45 wt% based on the total weight of the positive electrode). A 35 μm-thick lithium metal thin film was used as the negative electrode. Porous polyethylene was used as the separator. A pouch cell was fabricated by inserting a separator between the positive and negative electrodes prepared as described above and injecting the prepared electrolyte.

[0117] The electrolyte solutions used were those of Comparative Examples 1 to 6 and Examples 1 to 3 prepared above, and were injected in amounts according to the target El / S ratio (unit: μl / mg).

[0118] Evaluation example 2: Battery life evaluation As described above, batteries were manufactured using the electrolyte solutions of Comparative Example 1, Example 1, and Example 2 so that the El / S ratio was 5 μl / mg.

[0119] The fabricated lithium-sulfur batteries were then discharged at 0.3 C in constant current (CC) mode at 25°C to 1.8 V, and then charged at a constant current of 0.3 C to 2.7 V, and the discharge capacities were measured and compared. The discharge capacities were measured based on the sulfur content (mAh / g (weight of sulfur)).

[0120] The measurement results are shown in Figure 1.

[0121] 1, it was confirmed that the inclusion of a Lewis acid additive in the electrolyte ensured the battery's operational stability under conditions of a dilute electrolyte and high sulfur loading (El / S: 5 μl / mg). In particular, it was confirmed that the electrolyte of Example 1 operated stably up to 100 cycles.

[0122] Evaluation example 3: Evaluation of battery rate performance As described above, batteries were manufactured using the electrolyte solutions of Comparative Examples 1 to 4, Example 1 and Example 2 so that the El / S ratio was 4 μl / mg.

[0123] The charge-discharge characteristics were evaluated in the same manner as in Evaluation Example 2, and the results are shown in FIG.

[0124] The electrolyte solutions containing a non-solvent for lithium polysulfide (Comparative Examples 2 to 4) are expected to reduce the amount of lithium polysulfide eluted into the electrolyte solution by introducing a non-solvent. However, according to the results of FIG. 2, the capacity of the electrolyte was 800 mAh / g. s It was confirmed that the maximum current density at which the above discharge capacity could be obtained was inferior to that of Examples 1 and 2. In particular, it was confirmed that it was half the level of that of Example 2. In contrast, in the case of Examples 1 and 2, it was confirmed that the introduction of a Lewis acidic additive reduced the elution of lithium polysulfides and improved the mobility of lithium ions.

[0125] Evaluation example 4: Evaluation of the components of the SEI layer of the negative electrode The lithium-sulfur batteries prepared in Evaluation Example 2 using Comparative Example 1 and Example 1 were driven for 5 cycles, and then the negative electrodes were removed. Then, the components of the SEI layer formed on the negative electrodes were analyzed using molecular dynamics (MD) and X-ray photoelectron spectroscopy (XPS).

[0126] The results of the component analysis are shown in Figures 3 and 4, respectively.

[0127] According to the results in Figure 3, when introducing Lewis acidic additives, lithium ions (Li + ) and FSI - It was confirmed that the coordination number of FSI increases. - The LiF component in the SEI layer, generated by the decomposition of FSI, reduces the consumption of bulk lithium and electrolyte, improving the battery's life performance. - The coordination number of FSI increased, whereas, according to the results in Fig. 4, - Since the amount of decomposition itself is reduced, it was confirmed that the introduction of Lewis acidic additives forms a stable SEI layer, but by preventing excessive decomposition of the electrolyte, battery performance can be improved.

[0128] Evaluation example 5. Evaluation of decomposition current of electrolyte The decomposition currents of the electrolyte solutions of Comparative Example 1 and Example 1 prepared above were evaluated by the following method.

[0129] First, the voltage was fixed at 2.3 V during the charging process of the lithium-sulfur batteries using Comparative Example 1 and Example 1 prepared in Evaluation Example 2, and then the current generated during shuttle of lithium polysulfide to the negative electrode was measured. The results are shown in FIG. 5.

[0130] Next, lithium-nickel batteries were prepared using the electrolytes of Comparative Example 1 and Example 1 prepared above, and 1.6 M lithium polysulfide (LiPS) was added to the electrolyte to create conditions similar to those of the lithium-sulfur battery during discharge. The fabricated lithium-nickel batteries were scanned over a voltage range from the open circuit voltage (OCV) to 0.05 V at a scan rate of 1 mV / s, and the voltage-current graph obtained by linear sweep voltammetry (LSV, an electrochemical measurement method in which the electrode potential is linearly swept at a constant rate to measure the current-potential curve) is shown in Figure 6.

[0131] 5 and 6, it was confirmed that the shuttle current of Example 1 was significantly lower than that of Comparative Example 1 due to the Lewis acidic additive. In addition, it was confirmed that the reduction reaction of LiPS to LiS was significantly reduced in Example 1 containing the Lewis acidic additive.

[0132] Through this, it was confirmed that the Lewis acid additive can improve the amount of cathode active material lost due to the shuttle effect when the lithium-sulfur battery is in operation, and improve the electrolyte decomposition phenomenon that is problematic when the battery is in operation with a low electrolyte, thereby achieving a long life for the lithium-sulfur battery.

[0133] Evaluation example 6: Battery life evaluation In order to evaluate the effect on the battery life when the content of the Lewis acid additive was kept constant and the concentration of the lithium salt was varied, the battery life was evaluated in the same manner as in Evaluation Example 2 using the electrolyte solutions of Comparative Example 1, Comparative Example 5, Comparative Example 6, and Examples 1 to 3, and the results are shown in FIG. 7.

[0134] 7, when using the Lewis acid additive according to one embodiment of the present invention, Examples 1 to 3, in which the total concentration of the lithium salt, nitrate compound, and acidic additive was 2M or less, exhibited superior performance compared to Comparative Example 1, which did not contain an acidic additive. However, Comparative Examples 5 and 6, even though they contained a Lewis acid additive, had a total concentration of the lithium salt, nitrate compound, and acidic additive of 2M or more, and therefore exhibited inferior life characteristics compared to Comparative Example 1, which did not contain an acidic additive.

[0135] Through this, it was confirmed that the acidic additive is effective in improving the lifespan of a lithium-sulfur battery under the condition that the total concentration of the lithium salt, nitrate compound, and acidic additive is 2M or less.

[0136] Experimental Example 2 [Manufacturing electrolyte for lithium-sulfur batteries] To confirm whether the addition of a Lewis acid additive to a lithium-sulfur battery electrolyte can improve the performance of the lithium-sulfur battery according to the specific composition, electrolytes with the compositions shown in Table 3 below were prepared.

[0137] [Table 3]

[0138] [Manufacturing lithium-sulfur batteries] Using an electrolyte solution having the composition shown in Table 3, a lithium-sulfur battery was manufactured as follows.

[0139] A positive electrode slurry was prepared by coating an aluminum foil with a sulfur-carbon composite (S / C: 70 / 30) and polyacrylic acid (PAA) binder and then drying it to prepare a positive electrode (the sulfur content was 45 wt% based on the total weight of the positive electrode). A 35 μm-thick lithium metal thin film was used as the negative electrode. Porous polyethylene was used as the separator. A pouch cell was fabricated by inserting a separator between the positive and negative electrodes prepared as described above and injecting the prepared electrolyte.

[0140] The electrolyte solution used was the electrolyte solution of Example 4, Example 5, Comparative Example 7, or Comparative Example 8 produced above, and was injected so that the El / S ratio (unit: μl / mg) was 2.4.

[0141] The fabricated lithium-sulfur batteries were then discharged at 0.3 C to 1.8 V in constant current (CC) mode at 25°C, and then charged at a constant current of 0.3 C to 2.7 V to evaluate their charge-discharge characteristics. The results are shown in Figures 8 to 10. Each capacity was measured based on the sulfur content (mAh / g (weight of sulfur)).

[0142] The results of evaluating the specific capacity as a function of charge-discharge cycles are shown in Figure 8. As confirmed by the results described above, it was confirmed that the inclusion of a Lewis acidic additive dramatically improved the problem of capacity reduction that occurs with repeated charge-discharge cycles.

[0143] 9 shows the charge-discharge curves for one charge-discharge cycle of a lithium-sulfur battery using the electrolyte of Comparative Example 8. It was confirmed from FIG. 9 that even if the electrolyte contains a Lewis acid additive, the absence of a nitrate compound additive causes a delay in charging, making it impossible to proceed with subsequent cycles normally.

[0144] Figure 10 shows the results of evaluating the specific capacity as a function of charge / discharge cycles of the lithium-sulfur battery using the electrolyte of Example 5. Figure 10 shows that when an excessive amount of Lewis acid additive is used, a rapid increase in discharge and voltage occurs at the beginning of the cycle, making it impossible to proceed with subsequent cycles.

Claims

1. a lithium salt, a nitrate compound, a non-aqueous solvent, and a Lewis acidic additive; The Lewis acidic additive is a lithium ion (Li ＋ ) is a salt containing a cation having a higher Lewis acidity than The electrolyte for a lithium-sulfur battery, wherein the total concentration of the lithium salt, the nitrate compound, and the Lewis acid additive is 2M or less.

2. 2. The electrolyte solution for a lithium-sulfur battery according to claim 1, wherein the total concentration of the lithium salt, the nitrate compound, and the Lewis acidic additive is 1.5 M or less.

3. 2. The electrolyte solution for lithium-sulfur batteries according to claim 1, wherein the Lewis acidic additive contains, as a cation, an aluminum (Al) ion, a magnesium (Mg) ion, a calcium (Ca) ion, a strontium (Sr) ion, a barium (Ba) ion, or two or more of these.

4. 2. The electrolyte for a lithium-sulfur battery according to claim 1, wherein the Lewis acidic additive comprises a calcium salt.

5. 2. The electrolyte for a lithium-sulfur battery according to claim 1, wherein the Lewis acidic additive comprises an anion containing a fluorine element.

6. The Lewis acidic additive may contain, as an anion, F － , B.F. ４ － , P.F. ６ － , C.F. ３ SO ３ － , [(CF ３ SO ２ ) ２ N] － , [(FSO ２ ) ２ N] － The electrolyte solution for a lithium-sulfur battery according to claim 1, comprising two or more of these.

7. 2. The electrolyte solution for lithium-sulfur batteries according to claim 1, wherein the concentration of the lithium salt in the electrolyte solution for lithium-sulfur batteries is 2M or less.

8. 2. The lithium-sulfur battery electrolyte according to claim 1, wherein the concentration of the Lewis acid additive in the lithium-sulfur battery electrolyte is 0.3 M or less.

9. 2. The lithium-sulfur battery electrolyte according to claim 1, wherein the concentration of the Lewis acid additive in the lithium-sulfur battery electrolyte is 0.01 M to 0.1 M.

10. The electrolyte solution for a lithium-sulfur battery according to claim 1 , wherein the non-aqueous solvent includes an ether-based solvent.

11. the non-aqueous solvent includes an ether-based solvent, 2. The electrolyte solution for a lithium-sulfur battery according to claim 1, wherein the volume of the ether-based solvent is 60% by volume or more based on the total volume of the non-aqueous solvent.

12. 2. The electrolyte solution for a lithium-sulfur battery according to claim 1, wherein the non-aqueous solvent is composed of only one kind of ether solvent.

13. The electrolyte solution for a lithium-sulfur battery according to claim 1 , wherein the non-aqueous solvent comprises a cyclic ether and an acyclic ether.

14. The electrolyte solution according to any one of claims 1 to 13; Inorganic sulfur (S ８ a positive electrode comprising a negative electrode; a separator interposed between the positive electrode and the negative electrode; Lithium-sulfur batteries, including:

15. 15. The lithium-sulfur battery of claim 14, wherein the negative electrode is overlain by a solid electrolyte interface (SEI) layer containing LiF.

16. 15. The lithium-sulfur battery of claim 14, wherein the El / S (electrolyte / sulfur) ratio is 7 μl / mg or less.

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