Electrolyte for lithium-sulfur batteries and lithium-sulfur batteries 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] This invention relates to an electrolyte for lithium-sulfur batteries and a lithium-sulfur battery containing the same.

[0002] This application claims priority based on Korean Patent Application No. 2023-0046347, filed on April 7, 2023, and all information disclosed in the specification and drawings of said application is incorporated herein. [Background technology]

[0003] A lithium-sulfur battery is a battery system that uses a sulfur-based substance having 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, is non-toxic, and has the advantage of having a low weight per atom.

[0004] As the application areas of secondary batteries expand to electric vehicles (EVs), energy storage systems (ESS), and other areas, lithium-sulfur battery technology, which can theoretically achieve a higher gravimetric energy density (~2,600 Wh / kg) compared to lithium-ion secondary batteries with a relatively lower gravimetric energy density (~250 Wh / kg), is attracting attention.

[0005] In a lithium-sulfur battery, during discharge, lithium, the negative electrode active material, releases electrons and is oxidized while being ionized into lithium cations, while the sulfur-based material, the positive electrode active material, is reduced while accepting electrons. Here, the reduction reaction of the sulfur-based material causes the SS bond to accept two electrons and is converted into a sulfur anion. The lithium cations produced by the oxidation reaction of lithium are transferred to the positive electrode via the electrolyte, where they combine with sulfur anions produced by the reduction reaction of the sulfur-based compound to form a salt. Specifically, sulfur before discharge has a cyclic S8 structure, which is converted to lithium polysulfide (Li2Sx) by the reduction reaction, and then completely converted to lithium sulfide (Li2S).

[0006] Currently, lithium-sulfur batteries still exhibit a discharge capacity that falls short of their theoretical capacity due to the accumulation of lithium sulfide on the surface of the electrodes. 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 lithium-sulfur batteries, a problem arises in which a continuous side reaction of lithium polysulfide occurs at the lithium negative electrode, corroding the negative electrode and reducing its lifespan.

[0007] Specifically, the more diluted the electrolyte used in a battery becomes, 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 continuous side reactions in the lithium negative electrode, leading to severe corrosion. Furthermore, the smaller the amount of electrolyte used, the more pronounced the shuttle effect, one of the chronic problems of lithium-sulfur batteries, becomes, resulting in loss of active material and deterioration of battery performance.

[0008] Research efforts to improve such problems have been continuously carried out. In particular, research efforts have mainly been focused on using high-concentration lithium salts so that most solvent molecules contribute to the solvation shell, reducing the activity of the solvent and thus decreasing the solubility of polysulfides. It has been reported that increasing the concentration of lithium salts lowers the volatility of the electrolyte and improves the conductivity of lithium ions, enhancing the performance of the battery under dilute electrolyte conditions. However, problems have been reported that commercialization and driving at high currents are difficult due to high prices and low ionic conductivity.

[0009] For this reason, there is an increasing need for lithium-sulfur batteries that can suppress the elution of polysulfides, improve the ionic conductivity of the electrolyte, maintain the sulfur utilization rate at the positive electrode, and be stably driven even under dilute electrolyte and high sulfur loading conditions.

Summary of the Invention

Problems to be Solved by the Invention

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

[0011] For this purpose, one aspect of the present invention aims to provide an electrolyte for a lithium-sulfur battery that can suppress the elution of lithium polysulfide into the electrolyte.

[0012] Also, one aspect of the present invention aims to provide an electrolyte for a lithium-sulfur battery that has excellent ionic conductivity while reducing the concentration of the lithium salt.

[0013] Furthermore, another aspect of the present invention aims to provide a lithium-sulfur battery with a reduced electrolyte content and an increased sulfur loading amount.

Means for Solving the Problems

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

[0015] The electrolyte for lithium-sulfur batteries according to the first embodiment comprises a lithium salt, a nitrate compound, a non-aqueous solvent, and a Lewis acidic additive. The Lewis acid additive is lithium ion (Li + A salt containing cations with an even higher Lewis acidity than ) The total concentration of the lithium salt, the nitrate compound, and the Lewis acid additive shall be 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 acid additive may be 1.5 M or less.

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

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

[0019] According to the fifth aspect, in any one of the first to fourth aspects, The Lewis acid additive may contain anions including the element fluorine.

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

[0021] According to the seventh aspect, in any one of the first to sixth aspects, The concentration of lithium salt in the electrolyte for the lithium-sulfur battery 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 electrolyte for the lithium-sulfur battery may be 0.3 M 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 electrolyte for the lithium sulfur battery may be 0.01M to 0.1M.

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

[0025] According to the 11th aspect, in any one of the first to tenth aspects, The aforementioned non-aqueous solvent includes ether-based solvents, The volume of the ether-based solvent may be 60% or more of the total volume of the non-aqueous solvent.

[0026] According to the 12th aspect, in any one of the first to 11th aspects, The aforementioned non-aqueous solvent may consist of only one type of ether-based solvent.

[0027] According to the 13th aspect, in any one of the first to 12th aspects, The non-aqueous solvent may include cyclic ethers and acyclic ethers.

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

[0029] A lithium-sulfur battery according to the 14th embodiment is: The present invention comprises an electrolyte according to any one of the first to thirteenth embodiments, 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 15th aspect, in the 14th aspect, A solid electrolyte interface (SEI) layer containing LiF may be provided on the negative electrode.

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

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

[0033] Furthermore, by forming a stable interface with a high fluorine content on the negative electrode of the lithium-sulfur battery, uniform electrodeposition of lithium is induced, suppressing the decomposition of the electrolyte. This increases the Coulomb efficiency of the lithium-sulfur battery, resulting in a longer lifespan.

[0034] The drawings accompanying this specification illustrate preferred embodiments of the present invention and are intended to further illustrate the technical idea of ​​the invention along with the content of the invention; therefore, the present invention shall not be construed as being limited only to what is shown in the drawings. [Brief explanation of the drawing]

[0035] [Figure 1] This document shows the results of evaluating the life characteristics of lithium-sulfur batteries using the electrolytes of Comparative Example 1, Example 1, and Example 2 as described herein. [Figure 2] This document presents the results of evaluating the rate performance of lithium sulfur batteries according to the charge-discharge cycles using the electrolytes of Comparative Examples 1 to 4, Example 1, and Example 2 as described herein. [Figure 3] This shows the results of the component analysis of the solid electrolyte interface (SEI) layer formed on the negative electrode of a lithium-sulfur battery using the electrolytes of Comparative Example 1 (w / o Ca(OTF)2) and Example 1 (w / Ca(OTF)2) as described herein. [Figure 4] This shows the results of the component analysis of the solid electrolyte interface (SEI) layer formed on the negative electrode of a lithium-sulfur battery using the electrolytes of Comparative Example 1 and Example 1 described herein. [Figure 5] This section shows the current measurement results obtained by the shuttle effect using the electrolytes of Comparative Example 1 and Example 1 in this specification. [Figure 6] This document shows the results of the decomposition analysis and evaluation of the electrolytes used in Comparative Example 1 and Example 1 of this specification. [Figure 7] This document presents the results of evaluating the life characteristics of lithium-sulfur batteries using the electrolytes of Comparative Example 1, Comparative Example 5, Comparative Example 6, and Examples 1 to 3 as described herein. [Figure 8] This shows the results of evaluating the specific capacity of lithium-sulfur batteries according to the charge-discharge cycles using the electrolytes of Example 4 and Comparative Example 7 in this specification. [Figure 9]This shows the charge-discharge curve for the first cycle (1st cycle) of a lithium-sulfur battery using the electrolyte of Comparative Example 8 in this specification. [Figure 10] This shows the results of evaluating the specific capacity of a lithium-sulfur battery using the electrolyte of Example 5 in this specification, according to the charge-discharge cycle. [Modes for carrying out the invention]

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

[0037] The terms and words used in this specification and in the claims are not to be interpreted in their ordinary or dictionary sense, but rather in a sense and concept corresponding to the technical idea of ​​the present invention, in accordance with the principle that inventors can appropriately define the concepts of terms themselves in order to best describe the invention.

[0038] Furthermore, throughout this specification, when a part "includes" or "has" a certain component, this does not exclude other components unless otherwise specified, but rather means that it may further include other components.

[0039] Furthermore, terms and phrases used throughout this specification, such as “about,” “abbreviated,” and “substantially,” are used to mean, when manufacturing and material tolerances specific to the meaning mentioned are presented, in or approximate to those values, and are used to prevent unscrupulous infringers from unfairly exploiting disclosures that mention precise or absolute values ​​in order to enhance the understanding of the invention.

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

[0041] In this specification, the term "polysulfide" refers to "polysulfide ion (Sx 2-, 1 < x ≤ 8))」and「lithium polysulfide (Li2Sx or LiSx - , 1 < x ≤ 8)」both encompass the concept.

[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 set to be 2M or less.

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

[0045] As one definition regarding acid (Acid) and base (Base), the definition by Gilbert Newton Lewis is known. According to this, substances can be classified as Lewis acid or 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, "Lewis acidity" is a measure of the tendency to accept a lone pair of electrons, which can be measured according to known methods for measuring Lewis acidity. Thus, "higher Lewis acidity" means a greater tendency to accept a lone pair of electrons, which may indicate, for example, that there are lower energy levels in the available orbitals that can accept a lone pair of electrons.

[0048] According to one embodiment of the present invention, the Lewis acid additive is lithium ion (Li + Because it is a salt containing cations with an even higher Lewis acidity than ), the electrolyte for lithium sulfur batteries contains the Lewis acid additive, and when the lithium sulfur battery is in operation, electrons (e) present in the electrolyte are reduced. - This has the effect of suppressing the decomposition of lithium salts. As a result, it is possible to provide an electrolyte that can maintain good ionic conductivity when operating a lithium-sulfur battery while keeping the lithium salt content in the electrolyte low, but the mechanism of the present invention is not limited to this in any way.

[0049] In one embodiment of the present invention, the cation of the Lewis acid additive and lithium ions (Li + The Lewis acidity between (1) and (2) can be compared, for example, by comparing the acid dissociation constants (Ka) of the conjugate bases of the cation and 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 acidity.

[0050] In one embodiment of the present invention, the cation is M + When expressed as such, the pKa value of MH, which is the conjugate base of the cation, and the lithium ion (Li + The Lewis acidity between the cation and the lithium ion can be compared by comparing it with the pKa value of the conjugate base (LiH) of the cation.

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

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

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

[0054] In another embodiment of the present invention, the Lewis acid additive may be a salt containing a known anion for the cation described above. The anion can be used without particular limitation, as long as it does not impair the purpose of the present invention when included in the electrolyte.

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

[0056] In one embodiment of the present invention, the anion of the Lewis acid additive is F - Cl - , Br - , I - , R - RO - Cyclopentadienyl (Cp), Pentamethylcyclopentadienyl, R2N -Acetylacetonate (ACAC), Hexafluoroacetylacetonate (HFAC), CF3SO2O - (-OTf), RSO2O - ROSO2O - BF4 - , BR4 - AlCl4 - PF6 - PR3F3 - AsF6 - NO3 - and SO4 - It may be one or more selected from the group consisting of, but is not limited thereto. Here, R means alkyl, cycloalkyl, aryl, alkoxy, aryloxy, haloalkyl, haloalkoxy, or polymer.

[0057] In another embodiment of the present invention, the anion of the Lewis acid additive may contain fluorine. When the Lewis acid additive contains an anion containing fluorine as an 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 acid additive contains an anion containing fluorine as an anion, fluorine is present in the electrolyte containing the Lewis acid additive, and when a lithium-sulfur battery using this is operated, a solid electrolyte interface (SEI) layer containing fluorine is formed on the surface of the negative electrode. When an SEI layer containing fluorine is formed on the surface of the negative electrode of the lithium-sulfur battery, lithium is uniformly electrodeposited on the negative electrode when the lithium-sulfur battery is operated, reducing the degradation of the negative electrode, but the present invention is not limited to this.

[0058] According to another embodiment of the present invention, the Lewis acid additive is an anion of fluoride (F - It may contain anions composed solely of fluorine, or anions containing at least one fluorine element and further containing other elements, or it may contain any of these.

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

[0060] In yet another embodiment of the present invention, the Lewis acid 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. It may contain 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 is not limited thereto.

[0061] In yet another embodiment of the present invention, the Lewis acid 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 is suitable for use as an electrolyte in the electrolyte of a lithium-sulfur battery. Examples of the lithium salt include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB 10 Cl 10This may include, but is not limited to, LiPF6, LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, (C2F5SO2)2NLi, (SO2F)2NLi, (CF3SO2)3Cli, lithium chloroborane, lithium lower aliphatic carboxylate, lithium 4-phenylborate, lithium imide, or two or more of these.

[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 collectively to nitrate-based compounds or nitrite-based compounds that can be used as additives in the electrolyte for lithium-sulfur batteries. The nitrate-based compounds have the effect of forming a stable film on the negative electrode of a material such as lithium metal, thereby improving the charge-discharge efficiency, but the mechanism of the present invention is not limited in any way to this.

[0065] In one embodiment of the present invention, the nitrate compound is not limited thereto, but may include, for example, inorganic nitrates or nitrite compounds such as lithium nitrate (LiNO3), potassium nitrate (KNO3), cesium nitrate (CsNO3), barium nitrate (Ba(NO3)2), ammonium nitrate (NH4NO3), lithium nitrite (LiNO2), potassium nitrite (KNO2), cesium nitrite (CsNO2), and ammonium nitrite (NH4NO2); organic nitrates 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 mixtures of two or more of these.

[0066] In one embodiment of the present invention, the cation of the nitrate compound is not limited thereto, but may be selected from 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 has the effect of improving ionic conductivity while lowering the overall salt content in the electrolyte.

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

[0070] Accordingly, 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 electrolyte for the lithium sulfur battery 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 electrolyte for the lithium-sulfur battery 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 electrolyte for the lithium-sulfur battery 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 electrolyte for the lithium sulfur battery may be, for example, 0.5 M to 2 M, and 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, while a low electrolyte solution can be realized by including the lithium salt, the nitrate compound, and the Lewis acid additive at the above concentrations, it is advantageous in terms of improving the lifespan of the lithium sulfur battery, but the present invention is not limited in any way to this.

[0073] In one embodiment of the present invention, the electrolyte for the lithium-sulfur battery may further contain, in addition to the lithium salt, nitrate compound, and Lewis acid additive, further salt compounds, from the standpoint of not impairing the objectives of the present invention.

[0074] In one embodiment of the present invention, if the electrolyte for the lithium-sulfur battery further contains a salt compound in addition to the lithium salt, nitrate compound, and Lewis acid additive described above, it is preferable that the overall salt concentration of the electrolyte for the lithium-sulfur battery be kept at 2.5 M or less, specifically 2 M or less.

[0075] In another embodiment of the present invention, the electrolyte for the lithium-sulfur battery may contain no salt compounds other than the lithium salt, nitrate compound, and Lewis acid additive. In this case, the total salt concentration in the electrolyte for the lithium-sulfur battery may mean the sum 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 electrolyte for the lithium-sulfur battery may be, for example, 2M or less or 1.5M or less. More specifically, the salt concentration in the electrolyte for the lithium-sulfur battery may be, for example, 0.1M to 2.5M, 0.5M to 2M, 0.5M to 1.5M, 1M to 1.25M, 1M to 1.20M, or 1.20M to 1.25M. According to one embodiment of the present invention, even when the salt concentration is within the above range, the degree to which lithium salt decomposes during battery operation is improved, resulting in excellent ionic conductivity and outstanding effects in achieving a long lifespan; however, the present invention is not limited in any way to this.

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

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

[0079] In one embodiment of the present invention, the concentration of the nitrate compound in the electrolyte for the lithium-sulfur battery may be, for example, 1 M or less or 0.5 M or less. Specifically, the concentration of the nitrate compound in the electrolyte for the lithium-sulfur battery 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 can be used without any particular limitations, as long as it is used as an electrolyte for a lithium-sulfur battery. For example, the non-aqueous solvent may include 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 can be used without any particular limitations, as long as it is one that is used as an electrolyte for lithium-sulfur batteries.

[0082] In another embodiment of the present invention, the ether-based solvent can be classified, for example, into cyclic ethers and acyclic ethers depending on its structure.

[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, ethyl hexyl ether, or a mixture of two or more of these.

[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 of these.

[0085] In one embodiment of the present invention, the ester solvent may be, for example, 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 of these, but is not limited thereto.

[0086] In one embodiment of the present invention, the carbonate-based 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, and halides thereof, or a mixture of two or more of these. Examples of these halides include, but are not limited to, fluoroethylene carbonate.

[0087] In other embodiments of the present invention, the non-aqueous solvent may substantially contain the carbonate-based solvent because the carbonate-based solvent is insoluble in or exhibits low solubility of the nitrate compound.

[0088] In one embodiment of the present invention, the non-aqueous solvent may contain a very small amount of carbonate-based solvent so as not to affect the solubility of the nitrate compound. For example, if the non-aqueous solvent contains the carbonate-based solvent, the content of the carbonate-based solvent may be 3% by weight or less, 2% by weight or less, 1% by weight or less, 0.5% by weight or less, or 0% by weight (i.e., not present at all), based on the total weight of the electrolyte for the lithium sulfur battery.

[0089] In one embodiment of the present invention, as described above, the Lewis acid additive can suppress the elution of lithium polysulfide, thus having the advantage of enabling a superior lithium-sulfur battery without the need to separately use a non-solvent for lithium polysulfide. From this perspective, the non-aqueous solvent may include an ether-based solvent.

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

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

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

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

[0094] In other embodiments of the present invention, the non-aqueous solvent may include a cyclic ether and an acyclic ether. For example, the non-aqueous solvent may include a mixture of 2-methylfuran (2-MeF) as a cyclic ether and diethoxyethane (DME) as an acyclic ether.

[0095] In 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 included 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, using the electrolyte composition for lithium-sulfur batteries 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 achieving the effect of improving the driving stability of lithium-sulfur batteries. However, the present invention is not limited to this.

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

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

[0099] Here, the positive electrode may contain inorganic sulfur (S8) as the positive electrode active material. Specifically, the positive electrode may contain a sulfur-containing compound in addition to inorganic sulfur as the positive electrode active material, in which case the sulfur-containing compound is Li2S n (n≧1), disulfide compounds, organosulfur compounds and carbon-sulfur polymers ((C2S x ) n It may include one or more elements selected from the group consisting of x = 2.5 to 50, n ≥ 2.

[0100] In one embodiment of the present invention, the positive electrode, negative electrode, and separator can be used without limitation as long as they are suitable for use in a lithium-sulfur battery, and further description of these components is omitted in this specification. Furthermore, the shape of the lithium-sulfur battery is not particularly limited, and a wide variety of shapes such as cylindrical, stacked, and coin-shaped can be adopted.

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

[0103] According to one embodiment of the present invention, the lithium-sulfur battery can be made to have a low electrolyte and high capacity by including the above-mentioned electrolyte.

[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 in any way to this. In particular, since one of the goals of the lithium-sulfur battery is to achieve low electrolyte and high capacity, it goes without saying that when using the electrolyte described above, it is possible to drive a battery with an El / S ratio even higher than the El / S ratio described above, and it also means that it can show excellent performance even at the El / S ratio described above, so the El / S ratio is not limited in any way to this.

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

[0106] The present invention will be explained in detail below with reference to examples to deepen understanding of the present invention.

[0107] Experimental Example 1 [Manufacturing of electrolyte for lithium-sulfur batteries] To confirm that including Lewis acid additives in the electrolyte for lithium-sulfur batteries can improve their performance, we prepared electrolytes with the compositions shown in Table 1.

[0108] Specifically, comparative examples 2 to 4 were prepared to evaluate the performance of electrolytes 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 non-solvents for lithium polysulfide.

[0109] [Table 1]

[0110] Evaluation Example 1. Evaluation of solubility in lithium polysulfide To evaluate the performance of Lewis acid additives in suppressing the elution of lithium polysulfide, the following experiment was conducted.

[0111] Using the electrolytes from Comparative Example 1 and Example 1, the electrolyte was added dropwise at room temperature (23°C) until the same amount of lithium polysulfide (Li2S8) was completely dissolved. When no more lithium polysulfide in its initial solid state was 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, a larger volume of electrolyte was added in Example 1 than in Comparative Example 1, and as a result, the final concentration of the experimental solution was found to be even lower.

[0114] Through this process, we confirmed that the Lewis acid additive suppresses the dissolution of lithium polysulfide in the electrolyte, thereby preventing the elution of lithium polysulfide into the electrolyte.

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

[0116] A positive electrode slurry was prepared by coating aluminum foil with a sulfur-carbon composite (S / C:70 / 30) and polyacrylic acid (PAA) binder, and then drying it (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 separator was interposed between the positive and negative electrodes prepared as described above, and the electrolyte prepared above was injected to create a pouch cell.

[0117] As the electrolyte, the electrolytes of Comparative Examples 1 to 6 and Examples 1 to 3 prepared above were used, and the electrolyte was injected according to the desired El / S ratio (unit: μl / mg).

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

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

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

[0121] As shown in Figure 1, it was confirmed that the inclusion of a Lewis acid additive in the electrolyte ensures stable battery operation under conditions of dilute electrolyte and high sulfur loading (El / S: 5 μl / mg). In particular, the electrolyte of Example 1 was confirmed to operate stably for up to 100 cycles.

[0122] Evaluation Example 3. Evaluation of Battery Rate Performance As described above, batteries were manufactured using the electrolytes from Comparative Examples 1 to 4, Example 1, and Example 2, respectively, so that the El / S ratio was 4 μl / mg.

[0123] The charge and discharge characteristics were evaluated using the same method as in Evaluation Example 2, and the results are shown in Figure 2.

[0124] While electrolytes containing a non-solvent for lithium polysulfide (Comparative Examples 2-4) are expected to reduce the amount of lithium polysulfide eluted into the electrolyte by introducing a non-solvent, the results in Figure 2 show that 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 to be half the level of that of Example 2. In contrast, it was confirmed that in Examples 1 and 2, the introduction of Lewis acid additives could reduce the elution of lithium polysulfide and improve the mobility of lithium ions.

[0125] Evaluation Example 4. Evaluation of the components of the SEI layer of the negative electrode. After five cycles of operation of the lithium-sulfur batteries using Comparative Example 1 and Example 1 prepared in Evaluation Example 2, the negative electrode was removed, and the components of the SEI layer formed on the negative electrode 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 acid additives, lithium ions (Li + ) and FSI - It was confirmed that the coordination number increases. FSI - The LiF component of the SEI layer, generated by the decomposition of FSI, reduces the consumption of bulk lithium and electrolyte, thereby improving battery life performance. - While the coordination number of increased, according to the results in Figure 4, FSI - Since the amount of decomposition itself is reduced, it was confirmed that the introduction of Lewis acid additives forms a stable SEI layer, and that the battery performance can be improved by preventing excessive decomposition of the electrolyte.

[0128] Evaluation Example 5. Evaluation of the decomposition current of the electrolyte. The decomposition current of the electrolytes of Comparative Example 1 and Example 1, which were prepared above, was evaluated using the following method.

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

[0130] Next, lithium-nickel batteries were prepared using the electrolytes of Comparative Example 1 and Example 1 prepared above, and 1.6M lithium polysulfide (LiPS) was injected into the electrolyte to create conditions approximately identical to those of the electrolyte during discharge of a lithium-sulfur battery. Figure 6 shows the voltage-current graph obtained by linear sweep voltammetry (LSV, an electrochemical measurement method that measures the current-potential curve by linearly sweeping the electrode potential at a constant speed) using the manufactured lithium-nickel battery, while scanning the voltage range from the open-circuit voltage (OCV) to 0.05V at a scanning speed of 1mV / s.

[0131] As shown in Figures 5 and 6, it was confirmed that the shuttle current in Example 1 was significantly lower than in Comparative Example 1 due to the presence of the Lewis acid additive. Furthermore, it was confirmed that in Example 1, which contained the Lewis acid additive, the reaction in which LiPS is reduced to Li2S was greatly reduced.

[0132] Through this process, we confirmed that Lewis acid additives can improve the amount of positive electrode active material lost due to the shuttle effect during operation of lithium-sulfur batteries, and improve the electrolyte decomposition phenomenon, which is a concern when operating with low electrolyte levels, thereby enabling a longer lifespan for lithium-sulfur batteries.

[0133] Evaluation Example 6. Battery Life Evaluation To evaluate the effect on battery life when the content of Lewis acid additive is kept the same but the concentration of lithium salt is varied, the battery life was evaluated using the electrolytes of Comparative Example 1, Comparative Example 5, Comparative Example 6, and Examples 1 to 3, following the same method as in Evaluation Example 2, and the results are shown in Figure 7.

[0134] As shown in Figure 7, when using a Lewis acid additive according to one embodiment of the present invention, Examples 1 to 3, in which the total concentration of lithium salt, nitrate compound, and acid additive is 2M or less, show superior performance compared to Comparative Example 1, which does not contain an acid additive. In contrast, Comparative Examples 5 and 6, even though they contain a Lewis acid additive, were found to have inferior lifetime characteristics compared to Comparative Example 1, which does not contain an acid additive, because the total concentration of lithium salt, nitrate compound, and acid additive is 2M or more.

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

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

[0137] [Table 3]

[0138] [Manufacturing of lithium-sulfur batteries] A lithium-sulfur battery was manufactured using an electrolyte having the composition shown in Table 3, as described below.

[0139] A positive electrode slurry was prepared by coating aluminum foil with a sulfur-carbon composite (S / C:70 / 30) and polyacrylic acid (PAA) binder, and then drying it (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 separator was interposed between the positive and negative electrodes prepared as described above, and the electrolyte prepared above was injected to create a pouch cell.

[0140] As the electrolyte, the electrolytes prepared in Example 4, Example 5, Comparative Example 7, or Comparative Example 8 described above were used, and the electrolytes were injected so that the El / S ratio (unit: μl / mg) was 2.4.

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

[0142] Figure 8 shows the results of evaluating the specific capacity according to the charge-discharge cycle. As confirmed by the results described above, it was confirmed that including Lewis acid additives dramatically improves the problem of capacity degradation associated with repeated charge-discharge cycles.

[0143] Figure 9 shows the charge-discharge curve for one cycle of a lithium-sulfur battery using the electrolyte of Comparative Example 8. As shown in Figure 9, it was confirmed that even if Lewis acid additives are included in the electrolyte composition, if nitrate compound additives are not included, the charging delay prevents subsequent cycles from proceeding normally.

[0144] Figure 10 shows the results of evaluating the specific capacity of a lithium-sulfur battery using the electrolyte of Example 5 according to the charge-discharge cycle. As shown in Figure 10, it was confirmed that when an excessive amount of Lewis acid additive is used, the rapid increase in discharge and voltage at the beginning of the cycle prevents subsequent cycles from proceeding.

Claims

1. It comprises a lithium salt, a nitrate compound, a non-aqueous solvent, and a Lewis acid additive. The Lewis acid additive contains lithium ions (Li ＋ A salt containing cations with an even higher Lewis acidity than ) An electrolyte for a lithium-sulfur battery, wherein the total concentration of the lithium salt, the nitrate compound, and the Lewis acid additive is 2 M or less.

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

3. The electrolyte for lithium sulfur batteries according to claim 1, wherein the Lewis acid additive contains, as cations, aluminum (Al) ions, magnesium (Mg) ions, calcium (Ca) ions, strontium (Sr) ions, barium (Ba) ions, or two or more of these.

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

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

6. The Lewis acid additive has, as an anion, F － , BF ４ － , PF ６ － , CF ３ SO ３ － , [(CF ３ SO ２ ) ２ N] － , [(FSO ２ ) ２ N] － The electrolyte for a lithium-sulfur battery according to claim 1, comprising one or more of these.

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

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

11. The aforementioned non-aqueous solvent includes ether-based solvents, The electrolyte 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. The electrolyte for a lithium sulfur battery according to claim 1, wherein the non-aqueous solvent consists of only one type of ether-based solvent.

13. The electrolyte 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 according to any one of claims 1 to 13, Inorganic sulfur (S ８ A positive electrode containing ) and The negative electrode and, A separator interposed between the positive electrode and the negative electrode, Lithium sulfur batteries, including those containing lithium sulfur.

15. The lithium sulfur battery according to claim 14, wherein a solid electrolyte interface (SEI) layer containing LiF is provided on the negative electrode.

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

Images