JP2024066468A - Electrolyte for lithium sulfur secondary batteries and lithium sulfur secondary battery - Google Patents

Abstract

To provide an electrolyte for lithium sulfur secondary batteries which can be utilized for manufacturing a lithium sulfur secondary battery which comprises a cathode with a large carried amount of sulfur and of which the discharge capacity in repeating a charge/discharge cycle becomes a value closer to atheoretical capacity (1672 mAhg-1).SOLUTION: An electrolyte for lithium sulfur secondary batteries contains an electrolyte which is lithium bis (fluoro-sulfonyl) imide and/or lithium hexafluorophosphate, and two or more kinds of ether-based solvents.SELECTED DRAWING: Figure 5

Description

Application for application of Article 30, Paragraph 2 of the Patent Act filed. Website address: https://confit.atlas.jp/guide/event/ecsj2023s/top Published on March 17, 2023 Name of workshop: 90th Annual Meeting of the Electrochemical Society Venue: Tohoku Institute of Technology, Yagiyama Campus (35-1 Yagiyama Kasumi-cho, Taihaku-ku, Sendai City) Date: March 27, 2023 Website address: https://confit.atlas.jp/guide/event/ecsj2023s/top Published on March 27, 2023

The present invention relates to an electrolyte for lithium-sulfur secondary batteries and lithium-sulfur secondary batteries.

Currently, lithium-ion batteries are widely used as secondary batteries for mobile phones, personal computers, digital cameras, etc., but there is a demand for further improvements in energy density and cost reductions.

In this context, lithium-sulfur secondary batteries have been attracting attention. Lithium-sulfur secondary batteries are secondary batteries that use sulfur as the positive electrode active material and lithium metal as the negative electrode active material, and charge and discharge by the movement of lithium ions (Li ⁺ ) between the positive and negative electrodes. In lithium-sulfur secondary batteries, Li ⁺ ions are dissolved from the negative electrode during discharge and react with sulfur at the positive electrode to generate Li ₂ S, and current flows to an external circuit.

Sulfur atoms have a large number of electrons per weight involved in redox reactions, and are expected to have an energy density that is theoretically more than five times higher than that of conventional lithium-ion batteries. In addition, sulfur is inexpensive and easy to obtain, making it possible to reduce material costs. For this reason, lithium-sulfur secondary batteries are being actively researched as a next-generation secondary battery.

Recently, research has been conducted on lithium-sulfur secondary batteries having a positive electrode with a large amount of sulfur supported (content). For example, the following non-aqueous electrolytes for lithium-sulfur secondary batteries are known.
A non-aqueous electrolyte solution containing LiTFSI as an electrolyte and a mixed solvent of FEC and HFE (Patent Document 1), where LiTFSI stands for lithium bis(trifluoromethanesulfonyl)imide, FEC stands for fluoroethylene carbonate, and HFE stands for hydrofluoroether.
A non-aqueous electrolyte solution containing LiTFSI as an electrolyte and a mixed solvent of tetrafluoroethylene and HFE, and a non-aqueous electrolyte solution containing lithium hexafluorophosphate (hereinafter also referred to as "LiPF ₆ ") as an electrolyte and a mixed solvent of ethylene carbonate and dimethyl carbonate (Non-Patent Documents 1 and 2).
A non-aqueous electrolyte solution containing lithium bis(fluorosulfonyl)imide (hereinafter also referred to as "LiFSI") as an electrolyte and 1,2-dimethoxyethane (hereinafter also referred to as "DME") (Non-Patent Document 3).

International Publication WO 2018/163778 A1

Progress in Natural Science: Materials International, 25(6), P612-621(2015) Electrochemistry, 85,650-655(2017) Nat. Commun. 6 6362 (2015)

However, when these nonaqueous electrolytes are used in lithium-sulfur secondary batteries having a positive electrode with a large amount of sulfur, there is a problem that the discharge capacity after repeated charge-discharge cycles is significantly lower than the theoretical capacity (1672 mAhg ⁻¹ ). In other words, the lithium-sulfur secondary batteries using these nonaqueous electrolytes do not fully utilize the theoretical capacity of sulfur, and there is room for improvement in the capacity retention rate.

One aspect of the present invention aims to provide an electrolyte for lithium-sulfur secondary batteries that can be used to manufacture lithium-sulfur secondary batteries that have a positive electrode with a high sulfur loading and whose discharge capacity after charge-discharge cycles is closer to the theoretical capacity.

In addition, conventional lithium-sulfur secondary batteries have been evaluated for battery performance such as discharge capacity at normal temperature (room temperature), and battery performance in low-temperature or high-temperature environments has not been evaluated. On the other hand, the environments in which lithium-sulfur secondary batteries are actually used may include low-temperature and high-temperature environments. Therefore, there is a demand for lithium-sulfur secondary batteries that have excellent performance even in such environments. In order to meet this demand, one aspect of the present invention also aims to provide a nonaqueous electrolyte that can be used to manufacture lithium-sulfur secondary batteries that also have excellent discharge capacity in low-temperature and high-temperature environments.

In order to solve the above problems, the present inventors have intensively studied and come up with the idea of using an electrolyte solution prepared by dissolving an electrolyte of LiFSI and/or _LiPF6 in a solvent containing two or more kinds of ether solvents. They have found that this makes it possible to manufacture a lithium-sulfur secondary battery having a positive electrode with a large amount of sulfur supported thereon and having a discharge capacity that is closer to the theoretical capacity when the charge-discharge cycle is repeated, and have completed the present invention.

That is, the present invention includes the following configurations.
An electrolyte solution for a lithium-sulfur secondary battery comprising an electrolyte and a solvent,
the electrolyte is lithium bis(fluorosulfonyl)imide and/or lithium hexafluorophosphate;
The solvent includes two or more kinds of ether solvents.

One aspect of the present invention can improve the discharge capacity of a lithium-sulfur secondary battery when the charge-discharge cycle is repeated. In particular, one aspect of the present invention can provide an electrolyte for a lithium-sulfur secondary battery that can be used to manufacture a lithium-sulfur secondary battery that has a positive electrode with a large amount of sulfur and has a discharge capacity that is closer to the theoretical capacity.

In addition, according to one aspect of the present invention, it is possible to provide an electrolyte for lithium-sulfur secondary batteries that can be used to manufacture lithium-sulfur secondary batteries that have excellent discharge capacity in low-temperature and high-temperature environments, which have not been evaluated in the past.

FIG. 1 is a schematic diagram (left side) showing an embodiment of a positive electrode active material in a lithium-sulfur secondary battery comprising an electrolyte solution containing a conventional carbonate-based solvent, and a schematic diagram (right side) showing an example of an embodiment of a positive electrode active material in a lithium-sulfur secondary battery comprising an electrolyte solution according to one embodiment of the present invention. FIG. 1 shows the results of measuring the discharge capacity and the Coulombic efficiency when the charge-discharge cycle was repeated for the lithium-sulfur secondary batteries including the electrolyte solutions for lithium-sulfur secondary batteries produced in Examples 1 to 3 and Comparative Examples 1 and 2 of the present application. FIG. 1 shows the results of measuring the discharge capacity and the Coulombic efficiency when the charge-discharge cycle was repeated for the lithium-sulfur secondary batteries including the electrolyte solutions for lithium-sulfur secondary batteries produced in Examples 4 and 5 and Comparative Examples 1 and 2 of the present application. FIG. 1 shows the results of measuring the discharge capacity and the Coulombic efficiency when the charge-discharge cycle was repeated for the lithium-sulfur secondary batteries including the electrolyte solutions for lithium-sulfur secondary batteries produced in Examples 6 to 10 and Comparative Example 1 of the present application. FIG. 1 shows the results of measuring the discharge capacity and the Coulombic efficiency when the charge-discharge cycle was repeated for the lithium-sulfur secondary batteries including the electrolyte solutions for lithium-sulfur secondary batteries produced in Examples 11 to 13 and Comparative Example 1 of the present application. FIG. 1 shows the results of measuring the discharge capacity and the Coulombic efficiency when the charge-discharge cycle was repeated for the lithium-sulfur secondary batteries including the electrolyte solutions for lithium-sulfur secondary batteries produced in Examples 13 and 14 and Comparative Example 1 of the present application. FIG. 13 shows the results of measuring the discharge capacity and the Coulombic efficiency when the charge-discharge cycle was repeated for the lithium-sulfur secondary batteries including the electrolyte solutions for lithium-sulfur secondary batteries produced in Example 13 and Comparative Examples 1 and 3 to 5 of the present application. FIG. 1 shows the results of measuring the discharge capacity and the Coulombic efficiency when the charge-discharge cycle was repeated for the lithium-sulfur secondary batteries comprising the electrolyte solutions for lithium-sulfur secondary batteries produced in Examples 13 to 16 and Comparative Example 1 of the present application. FIG. 13 shows the results of measuring the discharge capacity and Coulombic efficiency when the charge-discharge cycle was repeated for the lithium-sulfur secondary batteries comprising the electrolyte solutions for lithium-sulfur secondary batteries produced in Examples 17 to 20 of the present application. FIG. 1 shows the results of measuring the discharge capacity and Coulombic efficiency when the charge-discharge cycle was repeated for the lithium-sulfur secondary batteries comprising the electrolyte solutions for lithium-sulfur secondary batteries produced in Examples 17 and 21 to 23 of the present application. FIG. 1 shows the results of measuring the discharge capacity and Coulombic efficiency when the charge-discharge cycle was repeated for the lithium-sulfur secondary batteries comprising the electrolyte solutions for lithium-sulfur secondary batteries produced in Examples 17 and 24 to 26 of the present application. FIG. 13 is a diagram showing the results of impedance measurement for a lithium-sulfur secondary battery comprising the electrolyte for a lithium-sulfur secondary battery produced in Example 17 of the present application. FIG. 13 is a diagram showing the results of impedance measurement for a lithium-sulfur secondary battery comprising the electrolyte for a lithium-sulfur secondary battery produced in Example 19 of the present application. FIG. 13 is a diagram showing the results of XPS measurement of F1s, i.e., the 1s electron of a fluorine atom (F), for lithium-sulfur secondary batteries including the electrolyte solutions for lithium-sulfur secondary batteries produced in Examples 17 and 19 of the present application. FIG. 13 is a diagram showing the results of XPS measurement of C1s, i.e., the 1s electron of a carbon atom (C), for lithium-sulfur secondary batteries including the electrolyte solutions for lithium-sulfur secondary batteries produced in Examples 17 and 19 of the present application. FIG. 13 shows the results of measuring the discharge capacity and Coulombic efficiency of the lithium-sulfur secondary batteries comprising the electrolyte solutions for lithium-sulfur secondary batteries produced in Examples 27 to 39 of the present application when the charge-discharge cycle was repeated at a high rate (0.3/0.3C). FIG. 13 shows the results of measuring the discharge capacity and Coulombic efficiency of the lithium-sulfur secondary batteries comprising the electrolyte solutions for lithium-sulfur secondary batteries produced in Examples 30 and 31 of the present application when the charge-discharge cycle was repeated in a low-temperature environment (0° C.). FIG. 13 shows the results of measuring the discharge capacity and Coulombic efficiency of the lithium-sulfur secondary batteries comprising the electrolyte solutions for lithium-sulfur secondary batteries produced in Examples 32 to 34 of the present application when the charge-discharge cycle was repeated in a high-temperature environment (60° C.).

One embodiment of the present invention is described in detail below. However, the present invention is not limited to this, and various modifications are possible within the scope described. For example, embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. In addition, unless otherwise specified in this specification, "A to B" representing a numerical range means "greater than or equal to A and less than or equal to B."

[1. Electrolyte for Lithium-Sulfur Secondary Batteries]
An electrolyte solution for a lithium-sulfur secondary battery according to one embodiment of the present invention (hereinafter also referred to as the "electrolytic solution of the present invention") is an electrolyte solution for a lithium-sulfur secondary battery containing an electrolyte and a solvent, the electrolyte being lithium bis(fluorosulfonyl)imide and/or lithium hexafluorophosphate, and the solvent containing two or more types of ether solvents.

In a lithium-sulfur secondary battery, in order to make the discharge capacity close to the theoretical capacity when the charge-discharge cycle is repeated, it is necessary to supply a sufficient charge to the positive electrode. In a lithium-sulfur secondary battery, the charge carrier is lithium ion (Li ⁺ ). Here, in a conventional lithium-sulfur secondary battery having an electrolyte solution containing a carbonate-based solvent (e.g., Patent Document 1, Non-Patent Documents 1 and 2), as shown in the left diagram of FIG. 1, a coating (shaded area in the left diagram of FIG. 1) is formed around the positive electrode active material. As a result, the coating is formed on the positive electrode containing the positive electrode active material.

The coating is formed on the positive electrode as a result of a side reaction between the effluent such as sulfur flowing out from the positive electrode (positive electrode active material) of the conventional lithium-sulfur secondary battery and the molecules of the carbonate-based solvent. The formation of the coating inhibits the movement of Li ⁺ to the positive electrode. Therefore, the amount of Li ⁺ supplied to the positive electrode is reduced.

In addition, some of the sulfur supported on the positive electrode is consumed to form the coating. As a result, the amount of sulfur supported on the positive electrode decreases. This reduces the capacity of the positive electrode.

On the other hand, in the lithium-sulfur secondary battery equipped with the electrolyte of the present invention containing two or more kinds of ether solvents, as shown in the right diagram of Figure 1, the coating is not formed on the positive electrode, or even if the coating is formed, the coating is very thin. In that case, the movement of Li ⁺ is not hindered. As a result, it is considered that the discharge capacity can be made close to the theoretical capacity by using the electrolyte.

In addition, when the electrolytic solution of the present invention contains a highly soluble ether-based solvent described later, the concentration of the electrolyte (LiFSI and/or LiPF ₆ ) in the electrolytic solution can be made high (several moles per 1.0 L of the solvent described later). Here, when the electrolyte is dissolved in the electrolytic solution at a high concentration, as described later, the migration and dissolution of sulfur is suppressed due to the high concentration of the electrolyte, so that there is no loss of sulfur from the positive electrode active material and the discharge capacity is better maintained. As a result, it is considered that the discharge capacity of the lithium-sulfur secondary battery comprising the electrolytic solution of the present invention is closer to the theoretical capacity. For the same reason, it is considered that the lithium-sulfur secondary battery comprising the electrolytic solution of the present invention can achieve excellent discharge capacity even under low and high temperature environments.

(Electrolytes)
The electrolyte in one embodiment of the present invention is LiFSI and/or LiPF _6. Both LiFSI and LiPF ₆ dissolve to the same extent in the solvent described below and generate the same amount of Li ⁺ . Therefore, both LiFSI and LiPF ₆ can be used as the electrolyte constituting the electrolytic solution of the present invention.

The _LiPF6 has a function of generating lithium fluoride during the operation of the lithium-sulfur secondary battery, similar to the lithium salt containing fluorine as an additive described later. The lithium fluoride satisfactorily forms a solid electrolyte interface (SEI) film on the negative electrode of the lithium-sulfur secondary battery. As a result, the discharge capacity when the charge-discharge cycle is repeated can be further improved, and the discharge capacity becomes closer to the theoretical capacity.

Therefore, from the viewpoint of favorably forming the SEI film, the electrolyte preferably contains LiPF _6. In addition, the concentration of LiPF ₆ is more preferably 0.05 mol or more, further preferably 0.07 mol or more, and particularly preferably 0.1 mol or more, per 1.0 L of a solvent described below.

The higher the concentration of the electrolyte, the more Li ⁺ there will be in the electrolytic solution of the present invention, and the greater the amount of charge supplied to the positive electrode will be, so the higher the concentration of the electrolyte is, the more preferable.

When the concentration of Li ⁺ contained in the electrolyte of the present invention, in other words, the electrolyte concentration, is high, from the viewpoint of chemical equilibrium, the movement and outflow of Li ⁺ into the electrolyte is prevented, and similarly, the movement and outflow of sulfur (S) into the electrolyte is also prevented.

Therefore, with an electrolyte solution having a high electrolyte concentration, the loss of sulfur from the positive electrode active material is small, and the discharge capacity is better maintained. As a result, it is believed that the discharge capacity of a lithium-sulfur secondary battery equipped with the electrolyte solution of the present invention is closer to the theoretical capacity.

From the above viewpoints, the concentration is preferably 1.0 mol or more per 1.0 L of the solvent described below, more preferably 2.0 mol or more, and even more preferably 3.0 mol or more.

On the other hand, in the electrolyte solution of the present invention, if the electrolyte is present in an amount that exceeds its solubility in the solvent described below, the viscosity of the electrolyte solution increases. In an electrolyte solution with high viscosity, ions, which are charge carriers, are less likely to move. As a result, the amount of charge supplied to the positive electrode may decrease, and the discharge capacity may decrease when the charge-discharge cycle is repeated. Therefore, it is preferable that the concentration of the electrolyte is an amount that does not exceed its solubility in the solvent described below.

Specifically, the concentration of the electrolyte is preferably 5.0 mol or less, and more preferably 4.0 mol or less, per 1.0 L of the solvent described below.

(solvent)
In one embodiment of the present invention, the solvent contains two or more kinds of ether solvents, and preferably consists of two or more kinds of ether solvents. The solvent can dissolve a suitable amount of the electrolyte. In the solvent, the coating is not formed on the positive electrode, or even if the coating is formed, the coating is very thin.

Therefore, the lithium-sulfur secondary battery having an electrolyte solution containing the solvent can ensure that a sufficient amount of Li ^{+ ions} migrates to the positive electrode, and as a result, the discharge capacity of the lithium-sulfur secondary battery can be controlled to be close to the theoretical capacity. Therefore, the solvent can be used as a solvent constituting the electrolyte solution of the present invention.

The solvent preferably includes a highly soluble ether-based solvent and a low-solubility ether-based solvent. In this specification, a "highly soluble ether-based solvent" is a solvent in which the solubility of the electrolyte in 1.0 L of the highly soluble ether-based solvent is 1.0 mol/L or more. A "low-solubility ether-based solvent" is a solvent in which the solubility of the electrolyte in 1.0 L of the low-solubility ether-based solvent is less than 1.0 mol/L.

When the solvent contains a highly soluble ether-based solvent and a low-solubility ether-based solvent, the volume ratio of the highly soluble ether-based solvent to the low-solubility ether-based solvent is more preferably 5:5 to 7:3, even more preferably 5:5 to 6:4, and particularly preferably 5:5.

When the solvent contains a high-solubility ether-based solvent and a low-solubility ether-based solvent in the above volume ratio, the solvent can dissolve a more suitable amount of the electrolyte.

As a result, the discharge capacity of the lithium-sulfur secondary battery when the charge/discharge cycle is repeated can be improved and controlled to a value closer to the theoretical capacity. In addition, by adjusting the volume ratio to within the above-mentioned more preferable range, the coulombic efficiency can be made close to 100%, and the charge/discharge efficiency can be improved.

Examples of the highly soluble ether-based solvent include 1,2-dimethoxyethane (DME), diethoxyethane, diisopropyl ether, dioxolane, diethoxyethane, triglyme, tetraglyme, and dibutyl ether. The highly soluble ether-based solvent may be one type or a mixture of two or more types.

The low-solubility ether-based solvent may be, for example, a compound called hydrofluoroether (HFE). Specific examples of compounds called HFE include 1,1,2,2-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)propane, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1-(1,1,2,2-tetrafluoroethoxy)propane, 2-methyl-1-(1,1,2,2-tetrafluoroethoxy)propane, 1,1,1-trifluoro-2-(2,2,2-trifluoroethoxy)ethane, fluorobenzene, and difluorobenzene. The low-solubility ether-based solvent may be one type or a mixture of two or more types.

Among the specific examples of the highly soluble ether-based solvents mentioned above, DME is known to have a particularly high solubility in the electrolyte. Therefore, by including DME in the solvent, the electrolyte for lithium-sulfur secondary batteries can dissolve the electrolyte at a high concentration. Therefore, a lithium-sulfur secondary battery including the electrolyte for lithium-sulfur secondary batteries can prevent the migration and dissolution of sulfur from the sulfur-supported positive electrode to the electrolyte, and can maintain a discharge capacity closer to the theoretical capacity. Therefore, it is preferable that the highly soluble ether-based solvent is DME among the above-mentioned compounds.

Furthermore, when the low-solubility ether-based solvent is combined with DME among the above-mentioned compounds, the formation of the coating on the positive electrode in the lithium-sulfur secondary battery electrolyte can be more easily prevented. Therefore, in a lithium-sulfur secondary battery including the lithium-sulfur secondary battery electrolyte, the inhibition of the movement of Li ⁺ to the positive electrode is less. As a result, a higher discharge capacity closer to the theoretical capacity can be obtained. From this viewpoint, the low-solubility ether-based solvent is preferably 1,1,2,2-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)propane.

In view of the above, from the viewpoint of further improving the discharge capacity when the charge/discharge cycle of the lithium-sulfur secondary battery is repeated and bringing it closer to the theoretical capacity, it is more preferable that the solvent contains DME and HFE, and it is particularly preferable that the solvent contains DME and 1,1,2,2-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)propane.

(Additive)
The electrolytic solution of the present invention may further include an additive a in addition to the electrolyte and the solvent. The additive a includes a lithium salt containing a halogen. Examples of the halogen include fluorine and chlorine. As the lithium salt containing a halogen, a lithium salt containing fluorine is particularly preferred.

When the electrolyte of the present invention contains the additive a, a lithium halide is generated from the lithium salt containing the halogen during operation of a lithium-sulfur secondary battery equipped with the electrolyte of the present invention. The lithium halide satisfactorily forms an SEI film on the negative electrode of the lithium-sulfur secondary battery. As a result, the discharge capacity can be further improved when charge/discharge cycles are repeated, and the discharge capacity can be made closer to the theoretical capacity. For example, when the lithium salt containing the halogen is a lithium salt containing fluorine, lithium fluoride is generated as the lithium halide.

From the viewpoint of improving the discharge capacity, the content of the lithium salt containing the halogen is preferably 0.05 mol or more, and more preferably 0.07 mol or more, per 1.0 L of the solvent. Also, from the viewpoint, the content is preferably 1 mol or less, more preferably 0.7 mol or less, even more preferably 0.5 mol or less, and particularly preferably 0.2 mol or less, per 1.0 L of the solvent.

The halogen-containing lithium salt is not particularly limited. For example, lithium difluoro(oxalato)borate (hereinafter referred to as "LiDFOB"), lithium fluoride salts such as _LiPF6 , and lithium perchlorate ( _LiClO4 ) and other chlorine-containing lithium salts can be used. The halogen-containing lithium salts may be used alone or in combination of two or more.

The electrolytic solution may further contain, in addition to the electrolyte and the solvent, an additive b containing one or more compounds selected from the group consisting of a compound represented by the following structural formula (1) and a compound represented by the following structural formula (2). Hereinafter, the compound represented by structural formula (1) will be referred to as compound (1), and the compound represented by structural formula (2) will be referred to as compound (2).

(In structural formula (1), ^R1 and ^R2 are each independently a group selected from the group consisting of hydrogen, halogen, alkyl, and alkoxy.)

(In structural formula (2), R ¹ and R ² are each independently a group selected from the group consisting of hydrogen, halogen, alkyl, and alkoxy.)
R ¹ and R ² in the structural formula (1) and the structural formula (2) are preferably groups selected from the group consisting of hydrogen, fluorine and chlorine. Compound (1) is more preferably vinylene carbonate (VC). Compound (2) is more preferably fluoroethylene carbonate (FEC) and/or chloroethylene carbonate (CLEC). In other words, it is more preferable that the additive b contains one or more compounds selected from the group consisting of VC, FEC and CLEC.

It is known that repeated charge-discharge cycles of a lithium-sulfur secondary battery can cause a rapid drop in discharge capacity due to the elution of polysulfides from the positive electrode. It is also known that under high-rate and/or high-temperature conditions, the polysulfides are more likely to elute from the positive electrode, which can easily cause the aforementioned rapid drop in discharge capacity.

On the other hand, in a lithium-sulfur secondary battery comprising the electrolyte of the present invention, during operation, a reductive decomposition product of one or more compounds selected from the group consisting of compound (1) and compound (2) is generated near the positive electrode, and a coating containing the reductive decomposition product is formed on the surface of the positive electrode. At this time, the coating prevents the elution of polysulfides from the positive electrode. Therefore, a sudden decrease in discharge capacity is less likely to occur.

The electrolyte of the present invention contains the additive b, and thus the cycle characteristics of a lithium-sulfur secondary battery containing the electrolyte of the present invention can be improved, particularly the cycle characteristics under high-rate and/or high-temperature conditions. In other words, a lithium-sulfur secondary battery containing the electrolyte of the present invention containing the additive b can maintain its discharge capacity in a good range for a long period of time (many cycles) when the charge-discharge cycle is repeated.

The electrolyte solution of the present invention contains at least a specific amount of one or more compounds selected from the group consisting of compound (1) and compound (2), which allows the coating to be well formed on the surface of the positive electrode, and the cycle characteristics of a lithium-sulfur secondary battery containing the electrolyte solution of the present invention to be further improved. From this perspective, the content of one or more compounds selected from the group consisting of compound (1) and compound (2) is preferably 1 wt % or more, more preferably 3 wt % or more, and even more preferably 5 wt % or more, when the total weight of the electrolyte solution of the present invention is taken as 100 wt %.

On the other hand, by not making the content of one or more compounds selected from the group consisting of compound (1) and compound (2) excessive, it is possible to prevent the coating from becoming thicker than necessary and the movement of Li ⁺ to the positive electrode from being hindered. As a result, it is possible to adjust the discharge capacity to a range closer to the theoretical capacity while improving the cycle characteristics of the lithium-sulfur secondary battery. From this viewpoint, the content of one or more compounds selected from the group consisting of compound (1) and compound (2) is preferably 10 wt % or less, more preferably 5 wt % or less, and even more preferably 3 wt % or less, when the weight of the entire electrolyte solution of the present invention is 100 wt %.

The electrolyte of the present invention may contain, as an additive other than the additive a and additive b, an additive that may be generally contained in an electrolyte for a lithium-sulfur secondary battery in an amount that does not impair the effects of the present invention.

2. Lithium-sulfur secondary battery
A lithium-sulfur secondary battery according to one embodiment of the present invention includes the electrolyte of the present invention. More specifically, the lithium-sulfur secondary battery includes a battery element in which a structure in which a positive electrode and a negative electrode face each other with a separator interposed therebetween are impregnated with the electrolyte, and the battery element is sealed in an exterior material. Since the lithium-sulfur secondary battery includes the electrolyte of the present invention, the discharge capacity is improved when charge-discharge cycles are repeated, and the discharge capacity becomes closer to the theoretical capacity.

(Positive electrode)
The positive electrode includes a positive electrode material for lithium-sulfur secondary batteries. More specifically, the positive electrode includes a positive electrode mixture for lithium-sulfur secondary batteries (hereinafter, simply referred to as a "positive electrode mixture") that includes the positive electrode material for lithium-sulfur secondary batteries and is supported on a current collector.

The positive electrode material for the lithium-sulfur secondary battery may be a positive electrode material generally used in lithium-sulfur secondary batteries, and is not particularly limited. The positive electrode material for the lithium-sulfur secondary battery may be, for example, a positive electrode material including a positive electrode active material in which sulfur and a carbonaceous material are composited. The "positive electrode active material in which sulfur and a carbonaceous material are composited" is a carbonaceous material on which sulfur is supported.

Examples of the carbonaceous material include activated carbon, carbon nanotubes, and graphene. The carbonaceous material may be one type of substance or a mixture of two or more types of substances.

Sulfur may be supported on the carbonaceous material by physical adsorption or chemical adsorption. Sulfur may be supported on the carbonaceous material, for example, by mixing the carbonaceous material and sulfur in a mortar or the like, and then heating the resulting mixture in a muffle furnace. Commercially available sulfur flowers, etc., can be used as the sulfur.

The carbonaceous material preferably has pores. In this case, the sulfur diffuses and is adsorbed inside the pores. Therefore, the carbonaceous material having the pores can more strongly support the sulfur.

As described below, the "positive electrode active material in which sulfur and a carbonaceous material are composited" preferably carries 60% or more by weight of sulfur relative to the weight of the positive electrode active material. As a method for adjusting the weight ratio (wt%) of sulfur in the positive electrode active material to a desired value, for example, in the above-mentioned method for carrying sulfur on a carbonaceous material, the following (A) and (B) can be used.

(A) The amount of sulfur used is adjusted to be 1 to 4 weight percent greater than the desired weight percentage of sulfur in the positive electrode active material.

(B) The heat treatment conditions are as shown in Production Examples 1 and 2 described below.

In this specification, the form of sulfur is not particularly limited. For example, one or more sulfurs selected from the group consisting of α-sulfur (orthorhombic sulfur), β-sulfur (monoclinic sulfur), γ-sulfur (monoclinic sulfur), rubber sulfur, and plastic sulfur can be used.

When the carbonaceous material has pores, the larger the specific surface area and the volume of the pores, the more suitable the carbonaceous material is for supporting a large amount of sulfur. Therefore, from the viewpoint of having a suitable structure for supporting a large amount of sulfur, it is preferable to use activated carbon obtained by carbonizing resin, fossil resource materials, etc., and then activating them with an alkali such as potassium hydroxide, etc., as the carbonaceous material.

From the viewpoint of supporting a large amount of sulfur, the specific surface area of the carbonaceous material is preferably 1500 cm ² g ^-1 to 2900 cm ² g ^-1 , and more preferably 2200 cm ² g ^-1 to 2900 cm ² g ^-1 .

The specific surface area is a value measured by the BET method. The specific surface area can be measured, for example, using Quantachrome's AUTOSORB iQ.

Furthermore, when the carbonaceous material has pores, the volume of the pores is preferably 0.6 cc·g ⁻¹ to 1.2 cc·g ⁻¹ and more preferably 0.9 cc·g ⁻¹ to 1.2 cc·g ⁻¹ from the viewpoint of supporting a large amount of sulfur.

The pore volume can be measured, for example, using Quantachrome's AUTOSORB iQ.

The "positive electrode active material in which sulfur and a carbonaceous material are composited" preferably carries 60% or more by weight of sulfur relative to the total weight of the positive electrode active material. In other words, it is preferable that the ratio of the weight of sulfur to the total weight of the sulfur and the carbonaceous material constituting the positive electrode active material is 60% or more by weight.

When the weight ratio of the sulfur is 60% or more by weight, the amount of sulfur supported is large, making it possible to provide a lithium-sulfur battery that meets the recent demand for increased sulfur content in the positive electrode material.

The weight percentage of the sulfur is preferably 70% by weight or more, and the higher the better, but the upper limit can be determined by factors such as the carrying capacity of the carbonaceous material. For example, if the carbonaceous material is activated carbon, the upper limit is approximately 60 to 66% by weight. The amount of sulfur carried can be confirmed by the method described later in the Examples.

Hereinafter, the "positive electrode active material in which sulfur and a carbonaceous material are composited" in one embodiment of the present invention is also referred to as a "carbonaceous material carrying sulfur" in comparison with a carbonaceous material not carrying sulfur. Herein, in a lithium-sulfur secondary battery, the battery performance may be reduced by the sulfur being dissolved as polysulfide due to the expansion of the positive electrode or the like during operation. When the positive electrode material for the lithium-sulfur secondary battery contains a carbonaceous material not carrying sulfur as the "other component", even if the sulfur is dissolved as polysulfide, the carbonaceous material can efficiently adsorb the polysulfide. As a result, it is possible to more easily provide a lithium-sulfur secondary battery that prevents the above-mentioned deterioration of battery performance and exhibits excellent cycle characteristics, etc.

From the above viewpoint, the content of the carbonaceous material carrying sulfur is preferably 50% by weight or more and 98% by weight or less, and more preferably 60% by weight or more and 90% by weight or less, when the total weight of the carbonaceous material carrying sulfur and the carbonaceous material not carrying sulfur is 100% by weight. In addition, the content of the carbonaceous material not carrying sulfur is preferably 2% by weight or more and 50% by weight or less, and more preferably 10% by weight or more and 40% by weight or less, when the total weight of the carbonaceous material carrying sulfur and the carbonaceous material not carrying sulfur is 100% by weight.

The "carbonaceous material not carrying sulfur" means a carbonaceous material that does not carry (adsorb) sulfur. Therefore, in order to prepare the "carbonaceous material not carrying sulfur", it is not necessary to go through a step of carrying (adsorbing) sulfur on the positive electrode active material, as in the preparation of the "carbonaceous material carrying sulfur". As the "carbonaceous material not carrying sulfur", for example, a commercially available carbonaceous material can be used as it is.

The "carbonaceous material not carrying sulfur" may be the same type of carbonaceous material as the carbonaceous material constituting the "carbonaceous material carrying sulfur", or may be a different type of carbonaceous material. On the other hand, from the viewpoint of adsorbing the eluted polysulfides, it is preferable that the carbonaceous material constituting the "carbonaceous material not carrying sulfur" is the same type of carbonaceous material. For example, activated carbon obtained by carbonizing the above-mentioned resin, fossil resource material, etc., and then activating it with an alkali can be preferably used as the "carbonaceous material not carrying sulfur".

When the positive electrode material for lithium-sulfur secondary batteries includes the "carbonaceous material not carrying sulfur," the positive electrode material for lithium-sulfur secondary batteries is a mixture containing the "carbonaceous material carrying sulfur" and the "carbonaceous material not carrying sulfur." The mixture is prepared by mixing the constituent materials of the positive electrode material for lithium-sulfur secondary batteries so that the "carbonaceous material carrying sulfur" and the "carbonaceous material not carrying sulfur" are homogeneous. As a mixing method, for example, a mortar or the like is used to mix the constituent materials of the positive electrode mixture, such as the conductive assistant and the aqueous binder, which are other components of the electrode mixture described below, in addition to the "carbonaceous material carrying sulfur" and the "carbonaceous material not carrying sulfur," and the mixture obtained is further stirred using a rotation/revolution mixer to prepare the electrode mixture containing the positive electrode material for lithium-sulfur secondary batteries.

The positive electrode mixture may contain other components in addition to the positive electrode material for lithium-sulfur secondary batteries. Examples of the other components include a conductive assistant and a water-based binder.

Any electronically conductive material that does not adversely affect battery performance can be used as the conductive additive. Usually, carbon black such as acetylene black or ketjen black is used, but conductive materials such as natural graphite (scale graphite, flake graphite, earthy graphite, etc.), artificial graphite, carbon whiskers, carbon fiber powder, metal (copper, nickel, aluminum, silver, gold, etc.) powder, metal fiber, conductive ceramic material, etc. may also be used. These may be used alone or as a mixture of two or more types.

There are no particular limitations on the aqueous binder, so long as it can bind the positive electrode active material particles and the conductive assistant, etc. As the aqueous binder, for example, one or more of the following can be used: an aqueous dispersion of styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), alginate, etc.

It is preferable that the positive electrode mixture contains 90% by weight or more and 95% by weight or less of the positive electrode material and 5% by weight or more and 10% by weight or less of the other components based on the total weight of the positive electrode mixture.

The positive electrode mixture can be prepared by a conventional method. For example, the positive electrode mixture can be prepared by homogeneously mixing the positive electrode material and the other components.

The positive electrode for the lithium-sulfur secondary battery can be produced by a conventional method. For example, a coating liquid (slurry) for producing a positive electrode containing the positive electrode mixture and a solvent is prepared, and the coating liquid is applied or filled (coated) on a current collector, dried, pressure-molded, and further vacuum-dried.

As the current collector, any electronic conductor that does not adversely affect the constructed battery can be used. Examples include aluminum, titanium, stainless steel, nickel, baked carbon, conductive polymers, conductive glass, etc. For the purpose of improving adhesion, conductivity, oxidation resistance, etc., a current collector whose surface has been treated with carbon, nickel, titanium, silver, etc. may also be used.

The shape of the collector may be any of foil, film, sheet, net, etc. Among them, a collector having a three-dimensional structure, such as a honeycomb shape, is preferred because it allows for a larger amount of the coating liquid to be filled and allows for a larger capacity.

(Negative electrode)
The negative electrode can be prepared by supporting a negative electrode mixture for a lithium-sulfur secondary battery, which contains metallic lithium as an active material, the conductive assistant, and an aqueous binder, on a current collector, for example, in the same manner as in the method for preparing the positive electrode for a lithium-sulfur secondary battery. The current collector can be the same as the current collector in the positive electrode for a lithium-sulfur secondary battery.

(Production method)
The lithium-sulfur secondary battery according to one embodiment of the present invention can be manufactured by assembling a positive electrode, a negative electrode, the electrolyte solution of the present invention, a separator, a case, and the like by a conventionally known method.

〔summary〕
The present invention includes the following aspects.
<1> An electrolyte solution for a lithium-sulfur secondary battery, comprising an electrolyte and a solvent,
the electrolyte is lithium bis(fluorosulfonyl)imide and/or lithium hexafluorophosphate;
The solvent includes two or more kinds of ether solvents.
<2> The electrolyte solution for a lithium-sulfur secondary battery according to <1>, wherein a concentration of the electrolyte is 4.0 mol or less per 1.0 L of the solvent.
<3> The solvent contains a high-solubility ether-based solvent and a low-solubility ether-based solvent,
The electrolyte solution for a lithium-sulfur secondary battery according to item 1 or 2, wherein a volume ratio of the high-solubility ether-based solvent to the low-solubility ether-based solvent is 5:5 to 7:3.
<4> The electrolyte solution for a lithium-sulfur secondary battery according to any one of <1> to <3>, further comprising, in addition to the electrolyte and the solvent, an additive a including a lithium salt including a halogen.
<5> The electrolyte solution for a lithium-sulfur secondary battery according to any one of <1> to <4>, further comprising, in addition to the electrolyte and the solvent, an additive b containing one or more compounds selected from the group consisting of a compound represented by the following structural formula (1) and a compound represented by the following structural formula (2):

(In structural formula (1), ^R1 and ^R2 are each independently a group selected from the group consisting of hydrogen, halogen, alkyl, and alkoxy.)

(In structural formula (2), R ¹ and R ² are each independently a group selected from the group consisting of hydrogen, halogen, alkyl, and alkoxy.)
<6> The electrolyte solution for a lithium-sulfur secondary battery according to <5>, wherein the additive b includes at least one compound selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, and chloroethylene carbonate.
<7> The electrolyte solution for a lithium-sulfur secondary battery according to any one of <1> to <6>, wherein the solvent contains 1,2-dimethoxyethane and a hydrofluoroether.
<8> A lithium-sulfur secondary battery comprising the electrolyte for a lithium-sulfur secondary battery according to any one of <1> to <7>.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[Measuring method]
By the methods described below, the physical properties of the positive electrode materials for lithium-sulfur secondary batteries produced in the Production Examples, and the performances of the lithium-sulfur secondary batteries produced in Examples 1 to 34 and Comparative Examples 1 to 5 were measured and calculated.

<Sulfur carrying capacity>
The positive electrode material for lithium-sulfur secondary batteries was placed in an alumina cell, and thermogravimetric analysis (TGA) was performed using a DTG-60AH manufactured by Shimadzu Corporation. The measurement was performed under the conditions of measurement gas Ar, gas flow rate 50 ml/min, temperature rise rate 5° C./min, and upper limit temperature 600° C. From the obtained TGA results, the ratio of the weight of sulfur supported by the positive electrode material for lithium-sulfur secondary batteries to the total weight of the positive electrode material for lithium-sulfur secondary batteries (sulfur supported amount, unit: wt%) was calculated.

<Charge/discharge test (cycle characteristic test)>
A constant current charge/discharge test A was performed using a BTS2400W (manufactured by Nagano Corporation) with the positive electrodes of the lithium-sulfur secondary batteries produced in Examples 1 to 16 and Comparative Examples 1 to 5 described below as working electrodes. A constant current charge/discharge test B was performed using an ABE1028-0505S (manufactured by Electrofield Corporation) with the positive electrodes of the lithium-sulfur secondary batteries produced in Examples 17 to 26 as working electrodes. The following descriptions in this section are common to both the constant current charge/discharge test A and the constant current charge/discharge test B.

The charging mode was the C.C. method ("C.C." is an abbreviation for constant current), and the discharging mode was the C.C. mode. The set current density was 167.2 mA/g (a current density of 1672 mA/g is defined as 1 C. Hereinafter, 167.2 mA/g is referred to as 0.1 C). The cutoff voltage had a lower limit of 1.0 V and an upper limit of 3.4 V. The test was performed in an environment of 25°C. That is, the charge/discharge rate was set to 0.1/0.1 C, charging and discharging were performed in cycles, and the charge capacity and discharge capacity in a specific charge/discharge cycle were measured.

The charge capacity and discharge capacity were based on the weight of sulfur and the unit was defined as mA·h (g sulfur) ⁻¹ .

Using the measured charge capacity and discharge capacity values in a specific charge/discharge cycle, the capacity retention rate and the coulombic efficiency were calculated based on the following formulas (1) and (2).
Capacity retention rate (%)={(discharge capacity [mAhg ⁻¹ ])/(theoretical capacity=1672 mAhg ⁻¹ )}×100 (1)
Coulombic efficiency = {(discharge capacity [mAhg ^-1 ]) / (charge capacity [mAhg ^-1 ])} × 100 (2)
<High-rate charge/discharge test (cycle characteristic test)>
The lithium-sulfur secondary batteries manufactured in Examples 27 to 29 described later were used, and the set current density was changed to 501.6 mA/g, i.e., the charge/discharge rate was changed to 0.3/0.3 C. Other than these, charging and discharging were performed in the same manner as described in the <Charge/Discharge Test (Cycle Characteristics Test)> section above, and the charge capacity and discharge capacity in a specific charge/discharge cycle were measured. In addition, the capacity retention rate was calculated based on the charge capacity and discharge capacity in the same manner as described in the above section.

<Charge/discharge test at low temperature (cycle characteristic test)>
The lithium-sulfur secondary batteries manufactured in Examples 30 and 31 described later were used, and the temperature of the environment in which the test was performed was changed from 25° C. to 0° C. Other than these, charging and discharging were performed in the same manner as described in the above section <Charge and Discharge Test (Cycle Characteristics Test)>, and the charge capacity and discharge capacity in a specific charge and discharge cycle were measured. In addition, based on the charge capacity and discharge capacity, the capacity retention rate and the coulombic efficiency were calculated in the same manner as described in the above section.

<High temperature and low rate charge/discharge test (cycle characteristic test)>
The lithium-sulfur secondary batteries manufactured in Examples 32 and 33 described later were used, and the temperature of the environment in which the test was performed was changed from 25° C. to 60° C. Other than these, charging and discharging were performed in the same manner as described in the above section <Charge and Discharge Test (Cycle Characteristics Test)>, and the charge capacity and discharge capacity in a specific charge and discharge cycle were measured. In addition, based on the charge capacity and discharge capacity, the capacity retention rate and the coulombic efficiency were calculated in the same manner as described in the above section.

<High-temperature and high-rate charge/discharge test (cycle characteristic test)>
The lithium-sulfur secondary battery manufactured in Example 34 described later was used, and the temperature of the environment in which the test was performed was changed from 25° C. to 60° C., and the set current density was changed to 501.6 mA/g, that is, the charge/discharge rate was changed to 0.3/0.3 C. Other than these, charging and discharging were performed in the same manner as described in the <Charge/Discharge Test (Cycle Characteristics Test)> section above, and the charge capacity and discharge capacity in a specific charge/discharge cycle were measured. In addition, the capacity retention rate and the coulombic efficiency were calculated based on the charge capacity and discharge capacity by the same method as described in the above section.

[Impedance measurement]
At the same time as the charge/discharge tests using the lithium-sulfur secondary batteries produced in Examples 17 and 19, impedance measurements were performed at the first, fifth, and tenth cycles under the following measurement conditions using a measuring device manufactured by Bio-Logic Science Instruments Itd., product name: VMP-300.

(Measurement condition)
・Frequency range: 1MHz to 1mHz
Amplitude: 10mV
Charging: 3.4V
[Measurement by X-ray photoelectron spectroscopy (XPS)]
Except for using the lithium-sulfur secondary batteries 17' and 19' manufactured in Examples 17 and 19, charging and discharging were performed in the same manner as described in the section <Charge and Discharge Test (Cycle Characteristics Test)>. The charging and discharging was performed until 5 cycles of charging were completed. Then, the positive electrodes were taken out from the lithium-sulfur secondary batteries 17' and 19' after the charging. Next, the positive electrodes were washed with a mixed solution obtained by mixing the DME and 1,1,2,2-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)propane (hereinafter referred to as "HFE1") at a volume ratio of 5:5. Specifically, the washing was an operation of putting the positive electrode and 0.5 mL of the mixed solution into a screw tube bottle (volume: 10 mL), stirring, and then taking out the positive electrode from the screw tube bottle. The washing operation was performed twice in succession. The positive electrode that had undergone the second washing operation was left at room temperature (25°C) for 12 hours to dry naturally. The positive electrode was subjected to XPS measurement under the following measurement conditions using a scanning X-ray photoelectron spectrometer (manufactured by ULVAC-PHI, product name PHI 5000 VersaProbe 3).

(Measurement condition)
X-ray source: monochromatic ALKα radiation (1486.6 eV)
・X-ray beam diameter: 100 μm
Photoelectron take-off angle: 45° to the normal of the sample
Neutralization: Yes [Production Example 1: Production of positive electrode]
(Preparation of positive electrode material for lithium-sulfur secondary battery)
0.34 g of activated carbon and 0.66 g of sulfur flowers (manufactured by Wako Pure Chemical Co., Ltd., purity over 99%) were mixed in an agate mortar to obtain a mixture. The mixture was transferred to a heat-resistant metal container, which was then placed in a muffle furnace and subjected to a heat treatment in the air.

The activated carbon had a pore volume of 1.2 cm ³ /g and a specific surface area of 2863 cm ² /g. The ratio of the number of macropores:mesopores:micropores in the pores of the activated carbon was 0:5:95.

Specifically, the heat-resistant metal container containing the mixture was placed inside the muffle furnace, and the temperature inside was raised to 155°C and maintained at this temperature for 5 hours to melt the sulfur flowers. Next, the temperature inside was raised to 300°C at a rate of 5°C/min, and maintained at this temperature for 2 hours. The inside was then thoroughly air-cooled, and the heat-resistant metal container was removed from the inside. Activated carbon carrying sulfur, that is, a positive electrode active material in which sulfur and a carbonaceous material are composited, was obtained from the heat-resistant metal container. The obtained positive electrode active material in which sulfur and a carbonaceous material are composited was used as a positive electrode material for lithium-sulfur secondary batteries.

The weight ratio of sulfur carried by the positive electrode material for lithium-sulfur secondary batteries (sulfur carrying amount) was measured using the method described above. As a result, the sulfur carrying amount was 64% by weight relative to the total weight of the positive electrode material for lithium-sulfur secondary batteries.

(Preparation of positive electrode mixture and positive electrode for lithium-sulfur secondary battery)
The positive electrode material for lithium-sulfur secondary batteries, activated carbon not carrying sulfur, carbon nanotubes as a conductive assistant, and a water-based binder (polypeptide-based binder) as a binder were weighed out in a weight ratio of 75:19:1:5 to prepare a positive electrode mixture for lithium-sulfur secondary batteries. The activated carbon not carrying sulfur was the same activated carbon as that used in the preparation of the positive electrode material for lithium-sulfur secondary batteries.

The measured materials were mixed in an agate mortar, and the resulting mixture was transferred to an ointment case. The mixture was then stirred for a total of 25 minutes at 2000 rpm using a planetary centrifugal mixer to obtain a positive electrode mixture for lithium-sulfur secondary batteries.

Next, the positive electrode mixture for lithium-sulfur secondary batteries was filled into a 3D aluminum current collector (manufactured by Sumitomo Electric Industries, Ltd.) so as to be 5 mg/ ^cm2 per unit area, and dried for 1 hour at 40 ° C. using a hot plate. The dried current collector was rolled using a roll press machine, and then cut into a φ12 mm size using a punch to obtain a molded body, which was then further dried for 12 hours using a vacuum bell jar at 50 ° C. to produce a positive electrode.

[Production Example 2: Production of large-scale positive electrode]
A positive electrode mixture for a lithium-sulfur secondary battery was obtained by the same method as in Production Example 1. Next, a large-scale positive electrode was produced by the same method as in Production Example 1 except for the following points.
The positive electrode mixture for lithium-sulfur secondary batteries was filled into a 3D aluminum current collector (manufactured by Sumitomo Electric Industries, Ltd.) so as to have a density of 7.5 mg/ ^cm2 per unit area;
The shape of the dried current collector cut out was changed from φ12 mm to a rectangle of 23 mm length x 25 mm width.

[Example 1]
(Preparation of electrolyte for lithium-sulfur secondary battery)
DME and HFE1 were mixed in a volume ratio of 5:5 to obtain a mixed solvent (1). 2.0 mol of LiFSI was dissolved in 1.0 L of the mixed solvent (1) to prepare an electrolyte solution. This electrolyte solution is designated as electrolyte solution (1) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
The shape of the cell can be a cylindrical metal can type, a coin type, a laminate type, etc., but in this test, a two-electrode flat cell similar to a coin type cell was used. A lithium-sulfur battery was fabricated in an argon atmosphere with a dew point of -70°C or lower by the following procedure using the positive electrode produced in Production Example 1, the electrolyte solution for lithium-sulfur secondary batteries (1), lithium foil (manufactured by Honjo Metals Co., Ltd.) as the negative electrode, and a polyethylene-based microporous film as the separator.

That is, the lithium foil was punched out with a punch to a size of φ13 mm to prepare a negative electrode. The polyethylene-based microporous film was punched out with a punch to a size of φ16 mm to prepare a separator. Furthermore, the positive electrode was punched out with a punch to a size of φ12 mm. The negative electrode, the separator, and the punched positive electrode were stacked in this order in a two-electrode flat cell, and 80 μL of the electrolyte solution (1) for lithium-sulfur secondary batteries was added to prepare a lithium-sulfur secondary battery. This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (1).

The lithium-sulfur secondary battery (1) was subjected to 50 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 2.

[Example 2]
(Preparation of electrolyte for lithium-sulfur secondary battery)
Except for changing the amount of LiFSI used to 3.0 mol, an electrolyte solution was prepared in the same manner as in Example 1. This electrolyte solution was designated as electrolyte solution (2) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (2) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (2).

The lithium-sulfur secondary battery (2) was subjected to 50 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 2.

[Example 3]
(Preparation of electrolyte for lithium-sulfur secondary battery)
Except for changing the amount of LiFSI used to 4.0 mol, an electrolyte solution was prepared in the same manner as in Example 1. This electrolyte solution was designated as electrolyte solution (3) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (3) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (3).

The lithium-sulfur secondary battery (3) was subjected to 30 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 2.

[Example 4]
(Preparation of electrolyte for lithium-sulfur secondary battery)
A mixed solvent (1) was prepared in the same manner as in Example 1. 1.0 mol of LiFSI and 1.0 mol of LiDFOB were dissolved in 1.0 L of the mixed solvent (1) to prepare an electrolyte solution. This electrolyte solution is designated as an electrolyte solution (4) for lithium-sulfur secondary batteries. LiDFOB corresponds to the additive a described above.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (4) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (4).

The lithium-sulfur secondary battery (4) was subjected to 50 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 3.

[Example 5]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution was prepared in the same manner as in Example 4, except that the amount of LiFSI used was changed to 1.5 mol. This electrolyte solution was designated as electrolyte solution (5) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (5) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (5).

The lithium-sulfur secondary battery (5) was subjected to 50 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 3.

[Example 6]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution was prepared in the same manner as in Example 5, except that the amount of LiDFOB used was changed to 0.2 mol. This electrolyte solution was designated as electrolyte solution (6) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (6) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (6).

The lithium-sulfur secondary battery (6) was subjected to 100 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 4.

[Example 7]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution was prepared in the same manner as in Example 6, except that the amount of LiFSI used was changed to 2.0 mol. This electrolyte solution was designated as electrolyte solution (7) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (7) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (7).

The lithium-sulfur secondary battery (7) was subjected to 100 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 4.

[Example 8]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution was prepared in the same manner as in Example 6, except that the amount of LiFSI used was changed to 2.5 mol. This electrolyte solution was designated as electrolyte solution (8) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (8) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (8).

The lithium-sulfur secondary battery (8) was subjected to 100 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 4.

[Example 9]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution was prepared in the same manner as in Example 6, except that the amount of LiFSI used was changed to 2.75 mol. This electrolyte solution was designated as electrolyte solution (9) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (9) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (9).

The lithium-sulfur secondary battery (9) was subjected to 100 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 4.

[Example 10]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution was prepared in the same manner as in Example 6, except that the amount of LiFSI used was changed to 3.0 mol. This electrolyte solution was designated as electrolyte solution (10) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (10) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (10).

The lithium-sulfur secondary battery (10) was subjected to 100 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 4.

[Example 11]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution was prepared in the same manner as in Example 4, except that the amount of LiFSI used was changed to 2.8 mol and the amount of LiDFOB used was changed to 0.1 mol. This electrolyte solution was designated as electrolyte solution (11) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (11) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (11).

The lithium-sulfur secondary battery (11) was subjected to 100 charge/discharge cycles using the above-mentioned charge/discharge test (cycle characteristic test) method, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 5.

[Example 12]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution was prepared in the same manner as in Example 11, except that the amount of LiFSI used was changed to 2.9 mol. This electrolyte solution was designated as electrolyte solution (12) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (12) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (12).

The lithium-sulfur secondary battery (12) was subjected to 100 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 5.

[Example 13]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution was prepared in the same manner as in Example 11, except that the amount of LiFSI used was changed to 3.0 mol. This electrolyte solution was designated as electrolyte solution (13) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (13) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (13).

The lithium-sulfur secondary battery (13) was subjected to 100 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figures 5 to 8.

[Example 14]
A mixed solvent (1) was prepared in the same manner as in Example 1. 3.0 mol of LiFSI and 0.1 mol of LiPF ₆ were dissolved in 1.0 L of the mixed solvent (1) to prepare an electrolyte solution. This electrolyte solution is designated as an electrolyte solution (14) for lithium-sulfur secondary batteries. LiPF ₆ corresponds to the additive a described above.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (14) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (14).

The lithium-sulfur secondary battery (14) was subjected to 100 charge/discharge cycles using the above-mentioned charge/discharge test (cycle characteristic test), and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figures 6 and 8.

[Example 15]
DME and HFE1 were mixed in a volume ratio of 6:4 to obtain a mixed solvent (2). 3.0 mol of LiFSI and 0.1 mol of LiDFOB were dissolved in 1.0 L of the mixed solvent (2) to prepare an electrolyte solution. This electrolyte solution is designated as an electrolyte solution (15) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (15) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (15).

The lithium-sulfur secondary battery (15) was subjected to 100 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 8.

[Example 16]
DME and HFE1 were mixed in a volume ratio of 7:3 to obtain a mixed solvent (3). 3.0 mol of LiFSI and 0.1 mol of LiDFOB were dissolved in 1.0 L of the mixed solvent (3) to prepare an electrolyte solution. This electrolyte solution is designated as an electrolyte solution (16) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (15) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as a lithium-sulfur secondary battery (16).

The lithium-sulfur secondary battery (16) was subjected to 100 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 8.

[Comparative Example 1]
FEC and HFE1 were mixed in a volume ratio of 5:5 to obtain a mixed solvent (4). 1.0 mol of LiTFSI was dissolved in 1.0 L of the mixed solvent (4) to prepare an electrolyte solution. This electrolyte solution is designated as comparative lithium-sulfur secondary battery electrolyte solution (1).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the comparative lithium-sulfur secondary battery electrolyte (1) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery was designated as the comparative lithium-sulfur secondary battery (1).

The comparative lithium-sulfur secondary battery (1) was subjected to 100 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figures 2 to 8.

[Comparative Example 2]
2.0 mol of LiFSI was dissolved in 1.0 L of DME to prepare an electrolyte solution, which is designated as comparative electrolyte solution (2) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the comparative lithium-sulfur secondary battery electrolyte (2) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery was designated as the comparative lithium-sulfur secondary battery (2).

A charge/discharge cycle was performed on the comparative lithium-sulfur secondary battery (2) using the method of the charge/discharge test (cycle characteristic test) described above. As a result, a short circuit occurred before the 10th cycle was reached. The discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figures 2 and 3.

[Comparative Example 3]
A mixed solvent (1) was prepared in the same manner as in Example 1. 2.0 mol of LiTFSI and 0.1 mol of LiDFOB were dissolved in 1.0 L of the mixed solvent (1) to prepare an electrolyte solution. This electrolyte solution was designated as comparative lithium-sulfur secondary battery electrolyte solution (3).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the comparative lithium-sulfur secondary battery electrolyte (3) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery was designated as the comparative lithium-sulfur secondary battery (3).

The comparative lithium-sulfur secondary battery (3) was subjected to 100 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 7.

[Comparative Example 4]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution was prepared in the same manner as in Comparative Example 3, except that the amount of LiTFSI used was changed to 2.5 mol. This electrolyte solution was designated as comparative electrolyte solution (4) for lithium-sulfur secondary batteries.

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the comparative lithium-sulfur secondary battery electrolyte (4) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery was designated as the comparative lithium-sulfur secondary battery (4).

A charge/discharge cycle was performed on the comparative lithium-sulfur secondary battery (4) using the method of the charge/discharge test (cycle characteristic test) described above. As a result, a short circuit occurred after 10 cycles and before reaching 30 cycles. The discharge capacity and charge capacity at each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 7.

[Comparative Example 5]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution was prepared in the same manner as in Comparative Example 3, except that the amount of LiTFSI used was changed to 2.75 mol. This electrolyte solution was designated as comparative electrolyte solution for lithium-sulfur secondary batteries (5).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the comparative lithium-sulfur secondary battery electrolyte (5) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is designated as the comparative lithium-sulfur secondary battery (5).

A charge/discharge cycle was performed on the comparative lithium-sulfur secondary battery (5) using the method of the charge/discharge test (cycle characteristic test) described above. As a result, a short circuit occurred before the 10th cycle was reached. The discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 1 and Figure 7.

[Example 17]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution for a lithium-sulfur secondary battery was prepared in the same manner as in Example 13. This electrolyte solution was designated as an electrolyte solution for a lithium-sulfur secondary battery (17).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 1 except for the following points. This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (17).
- Using the lithium-sulfur secondary battery electrolyte (17) instead of the lithium-sulfur secondary battery electrolyte (1);
Instead of the positive electrode produced in Production Example 1, the large-scale positive electrode produced in Production Example 2 was used;
Instead of the lithium foil punched out to a size of φ13 mm, two sheets of the lithium foil punched out to a size of 30 mm length x 30 mm width were used as the negative electrode, and a structure was used in which both sides of the positive electrode were sandwiched between the two lithium foils (negative electrode).

The lithium-sulfur secondary battery (17) was subjected to 50 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 2 and Figures 9 to 11. At the same time, the impedance measurement described above was also performed. The results are shown in Figure 12.

(Manufacturing another lithium-sulfur secondary battery)
Another lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the electrolyte solution for lithium-sulfur secondary batteries (17) was used instead of the electrolyte solution for lithium-sulfur secondary batteries (1). The other lithium-sulfur secondary battery was designated as a lithium-sulfur secondary battery (17').

The lithium-sulfur secondary battery (17') was subjected to the above-mentioned XPS measurement. The results are shown in Figures 14 and 15.

[Example 18]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution was prepared in the same manner as in Example 13, and then vinylene carbonate (VC) was added to the electrolyte solution as an additive b so that the content of VC was 1% by weight to obtain an electrolyte solution for a lithium-sulfur secondary battery. The electrolyte solution for a lithium-sulfur secondary battery was designated as electrolyte solution for a lithium-sulfur secondary battery (18).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 17, except that the lithium-sulfur secondary battery electrolyte (18) was used instead of the lithium-sulfur secondary battery electrolyte (17). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (18).

The lithium-sulfur secondary battery (18) was subjected to 50 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 2 and Figure 9.

[Example 19]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution was prepared in the same manner as in Example 13, and then fluoroethylene carbonate (FEC) was added as an additive b to the electrolyte solution so that the content of the FEC was 1% by weight to obtain an electrolyte solution for a lithium-sulfur secondary battery. The electrolyte solution for a lithium-sulfur secondary battery was designated as electrolyte solution for a lithium-sulfur secondary battery (19).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 17, except that the lithium-sulfur secondary battery electrolyte (19) was used instead of the lithium-sulfur secondary battery electrolyte (17). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (19).

The lithium-sulfur secondary battery (19) was subjected to 50 charge/discharge cycles using the charge/discharge test (cycle characteristic test) method described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 2 and Figure 9. At the same time, the impedance measurement described above was also performed. The results are shown in Figure 13.

(Manufacturing another lithium-sulfur secondary battery)
Another lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the electrolyte solution for lithium-sulfur secondary batteries (19) was used instead of the electrolyte solution for lithium-sulfur secondary batteries (1). The other lithium-sulfur secondary battery was designated as lithium-sulfur secondary battery (19').

The lithium-sulfur secondary battery (19') was subjected to the above-mentioned XPS measurement. The results are shown in Figures 14 and 15.

[Example 20]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution was prepared in the same manner as in Example 13, and then chloroethylene carbonate (CLEC) was added to the electrolyte solution so that the content of the CLEC was 1% by weight to obtain an electrolyte solution for a lithium-sulfur secondary battery. The electrolyte solution for a lithium-sulfur secondary battery is designated as electrolyte solution for a lithium-sulfur secondary battery (20).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 17, except that the lithium-sulfur secondary battery electrolyte (20) was used instead of the lithium-sulfur secondary battery electrolyte (17). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (20).

The lithium-sulfur secondary battery (20) was subjected to 50 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 2 and Figure 9.

[Example 21]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution for a lithium-sulfur secondary battery was obtained in the same manner as in Example 18, except that VC was added to an electrolyte solution prepared in the same manner as in Example 13 so that the content of VC was 3 wt %. This electrolyte solution for a lithium-sulfur secondary battery was designated as electrolyte solution for a lithium-sulfur secondary battery (21).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 17, except that the lithium-sulfur secondary battery electrolyte (21) was used instead of the lithium-sulfur secondary battery electrolyte (17). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (21).

The lithium-sulfur secondary battery (21) was subjected to 50 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 2 and Figure 10.

[Example 22]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution for a lithium-sulfur secondary battery was obtained in the same manner as in Example 19, except that FEC was added to an electrolyte solution prepared in the same manner as in Example 13 so that the content of the FEC was 3 wt %. The electrolyte solution for a lithium-sulfur secondary battery was designated as electrolyte solution for a lithium-sulfur secondary battery (22).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 17, except that the lithium-sulfur secondary battery electrolyte (22) was used instead of the lithium-sulfur secondary battery electrolyte (17). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (22).

The lithium-sulfur secondary battery (22) was subjected to 50 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 2 and Figure 10.

[Example 23]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution for a lithium-sulfur secondary battery was obtained in the same manner as in Example 20, except that CLEC was added to an electrolyte solution prepared in the same manner as in Example 13 so that the content of the CLEC was 3 wt %. This electrolyte solution for a lithium-sulfur secondary battery was designated as electrolyte solution for a lithium-sulfur secondary battery (23).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 17, except that the lithium-sulfur secondary battery electrolyte (23) was used instead of the lithium-sulfur secondary battery electrolyte (17). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (23).

The lithium-sulfur secondary battery (23) was subjected to 50 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 2 and Figure 10.

[Example 24]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution for a lithium-sulfur secondary battery was obtained in the same manner as in Example 18, except that VC was added to an electrolyte solution prepared in the same manner as in Example 13 so that the content of VC was 5 wt %. This electrolyte solution for a lithium-sulfur secondary battery was designated as electrolyte solution for a lithium-sulfur secondary battery (24).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 17, except that the lithium-sulfur secondary battery electrolyte (24) was used instead of the lithium-sulfur secondary battery electrolyte (17). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (24).

The lithium-sulfur secondary battery (24) was subjected to 50 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 2 and Figure 11.

[Example 25]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution for a lithium-sulfur secondary battery was obtained in the same manner as in Example 19, except that FEC was added to an electrolyte solution prepared in the same manner as in Example 13 so that the content of the FEC was 5% by weight. This electrolyte solution for a lithium-sulfur secondary battery was designated as electrolyte solution for a lithium-sulfur secondary battery (25).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 17, except that the lithium-sulfur secondary battery electrolyte (25) was used instead of the lithium-sulfur secondary battery electrolyte (17). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (25).

The lithium-sulfur secondary battery (25) was subjected to 50 charge/discharge cycles using the charge/discharge test (cycle characteristic test) method described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 2 and Figure 11.

[Example 26]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution for a lithium-sulfur secondary battery was obtained in the same manner as in Example 20, except that CLEC was added to an electrolyte solution prepared in the same manner as in Example 13 so that the content of the CLEC was 5 wt %. This electrolyte solution for a lithium-sulfur secondary battery is designated as electrolyte solution for a lithium-sulfur secondary battery (26).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 17, except that the lithium-sulfur secondary battery electrolyte (26) was used instead of the lithium-sulfur secondary battery electrolyte (17). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (26).

The lithium-sulfur secondary battery (26) was subjected to 50 charge/discharge cycles using the method of the charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 2 and Figure 11.

[Example 27]
(Preparation of electrolyte for lithium-sulfur secondary battery, manufacturing of lithium-sulfur secondary battery)
The lithium-sulfur secondary battery was obtained by preparing an electrolyte solution for a lithium-sulfur secondary battery and fabricating a lithium-sulfur secondary battery using the electrolyte solution for a lithium-sulfur secondary battery in the same manner as in Example 17. The lithium-sulfur secondary battery was designated as lithium-sulfur secondary battery (27).

The lithium-sulfur secondary battery (27) was subjected to 50 charge/discharge cycles using the method of the high-rate charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured and the capacity retention rate was calculated. The results are shown in Table 2 and Figure 16.

[Example 28]
(Preparation of electrolyte for lithium-sulfur secondary battery, manufacturing of lithium-sulfur secondary battery)
The lithium-sulfur secondary battery was obtained by preparing an electrolyte solution for a lithium-sulfur secondary battery and fabricating a lithium-sulfur secondary battery using the electrolyte solution for a lithium-sulfur secondary battery in the same manner as in Example 19. The lithium-sulfur secondary battery was designated as lithium-sulfur secondary battery (28).

The lithium-sulfur secondary battery (28) was subjected to 50 charge/discharge cycles using the method of the high-rate charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured and the capacity retention rate was calculated. The results are shown in Table 2 and Figure 16.

[Example 29]
(Preparation of electrolyte for lithium-sulfur secondary battery, manufacturing of lithium-sulfur secondary battery)
The lithium-sulfur secondary battery was obtained by preparing an electrolyte solution for a lithium-sulfur secondary battery and fabricating a lithium-sulfur secondary battery using the electrolyte solution for a lithium-sulfur secondary battery in the same manner as in Example 20. The lithium-sulfur secondary battery was designated as lithium-sulfur secondary battery (29).

The lithium-sulfur secondary battery (29) was subjected to 50 charge/discharge cycles using the method of the high-rate charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured and the capacity retention rate was calculated. The results are shown in Table 2 and Figure 16.

[Example 30]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution for a lithium-sulfur secondary battery was prepared in the same manner as in Example 17. This electrolyte solution was designated as an electrolyte solution for a lithium-sulfur secondary battery (30).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (30) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (30).

The lithium-sulfur secondary battery (30) was subjected to 100 charge/discharge cycles using the method of the low-temperature charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 2 and Figure 17.

[Example 31]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution for a lithium-sulfur secondary battery was prepared in the same manner as in Example 19. This electrolyte solution was designated as an electrolyte solution for a lithium-sulfur secondary battery (31).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (31) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (31).

The lithium-sulfur secondary battery (31) was subjected to 100 charge/discharge cycles using the method of the low-temperature charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 2 and Figure 17.

[Example 32]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution for a lithium-sulfur secondary battery was prepared in the same manner as in Example 17. This electrolyte solution is designated as an electrolyte solution for a lithium-sulfur secondary battery (32).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (32) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (32).

The lithium-sulfur secondary battery (32) was subjected to 30 charge/discharge cycles using the method of the high-temperature, low-rate charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 2 and Figure 18.

[Example 33]
(Preparation of electrolyte for lithium-sulfur secondary battery)
An electrolyte solution for lithium-sulfur secondary batteries was prepared in the same manner as in Example 25. This electrolyte solution was designated as an electrolyte solution for lithium-sulfur secondary batteries (33).

(Manufacturing lithium-sulfur secondary batteries)
A lithium-sulfur secondary battery was produced in the same manner as in Example 1, except that the lithium-sulfur secondary battery electrolyte (33) was used instead of the lithium-sulfur secondary battery electrolyte (1). This lithium-sulfur secondary battery is referred to as lithium-sulfur secondary battery (33).

The lithium-sulfur secondary battery (33) was subjected to 30 charge/discharge cycles using the method of the high-temperature, low-rate charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 2 and Figure 18.

[Example 34]
(Preparation of electrolyte for lithium-sulfur secondary battery, manufacturing of lithium-sulfur secondary battery)
The lithium-sulfur secondary battery was obtained by preparing an electrolyte solution for a lithium-sulfur secondary battery and fabricating a lithium-sulfur secondary battery using the electrolyte solution for a lithium-sulfur secondary battery in the same manner as in Example 33. The lithium-sulfur secondary battery was designated as lithium-sulfur secondary battery (34).

The lithium-sulfur secondary battery (34) was subjected to 30 charge/discharge cycles using the method of the high-temperature and high-rate charge/discharge test (cycle characteristic test) described above, and the discharge capacity and charge capacity in each cycle were measured, and the capacity retention rate and coulombic efficiency were calculated. The results are shown in Table 2 and Figure 18.

[Results]

As shown in Table 1 and Figures 1 to 7, the lithium-sulfur secondary batteries produced in Examples 1 to 5 and 7 to 16 did not short-circuit until at least 30 cycles, and the capacity retention rate at the 10th cycle was higher than that of the lithium-sulfur secondary batteries produced in Comparative Examples 1 and 3. The lithium-sulfur secondary battery produced in Example 6 did not short-circuit until at least 50 cycles, and the capacity retention rate at the 10th cycle was higher than that of the lithium-sulfur secondary battery produced in Comparative Example 1. The lithium-sulfur secondary batteries produced in Comparative Examples 2 and 5 were not suitable as lithium-sulfur secondary batteries because they short-circuited before reaching 10 cycles. The lithium-sulfur secondary battery produced in Comparative Example 4 had a high capacity retention rate at 10 cycles of 98.8%, but was not suitable as a lithium-sulfur secondary battery because they short-circuited before reaching 30 cycles.

The lithium-sulfur secondary batteries produced in Examples 1 to 14 and 16 have a higher capacity retention rate at the 30th cycle than the lithium-sulfur secondary batteries produced in Comparative Examples 1 and 3.

In addition, the lithium-sulfur secondary batteries prepared in Examples 1, 4, 5, 7 to 10, 13, and 14 have a higher capacity retention rate at the 50th cycle than the lithium-sulfur secondary batteries prepared in Comparative Examples 1 and 3.

Furthermore, the lithium-sulfur secondary batteries prepared in Examples 2, 6, and 11 have a higher capacity retention rate at the 50th cycle than the lithium-sulfur secondary battery prepared in Comparative Example 3.

In addition, the lithium-sulfur secondary batteries produced in Examples 7 to 9, 13, and 14 have a higher capacity retention rate at the 100th cycle than the lithium-sulfur secondary battery produced in Comparative Example 1.

Therefore, it was found that the lithium-sulfur secondary battery equipped with the nonaqueous electrolyte of the present invention has an improved discharge capacity when subjected to repeated charge-discharge cycles, and is closer to the theoretical capacity, compared to a lithium-sulfur secondary battery equipped with a conventional nonaqueous electrolyte.

In addition, from a comparison between Examples 7 to 13 and Examples 1 to 3, it was found that when the electrolyte concentration in the electrolyte solution of the present invention is about the same, the discharge capacity can be further improved by further containing a lithium salt containing a halogen such as LiDFOB as an additive a. Furthermore, from Example 14, it was found that the electrolyte solution containing LiPF ₆ as additive a can further improve the discharge capacity, similar to the electrolyte solution containing a lithium salt containing a halogen.

In addition, referring to FIG. 7, the lithium-sulfur secondary batteries prepared in Examples 15 and 16 have lower Coulombic efficiency and lower charge/discharge efficiency compared to the lithium-sulfur secondary battery prepared in Example 13. Also, the capacity retention rate at 30 cycles is higher in Example 13 than in Examples 15 and 16.

Therefore, when the electrolyte of the present invention is a mixed solvent of a high-solubility ether-based solvent and a low-solubility ether-based solvent, it is found that a volume ratio of the high-solubility ether-based solvent to the low-solubility ether-based solvent of 5:5 is particularly preferable.

As shown in Table 2 and Figures 9 to 11, the lithium-sulfur secondary batteries prepared in Examples 17 to 26 did not short-circuit for at least 30 cycles, and their capacity retention rate remained within a good range for up to about 25 cycles.

The lithium-sulfur secondary batteries prepared in Examples 18 to 26 using one or more compounds selected from the group consisting of compound (1) and compound (2) as additive b tend to maintain their capacity retention rate in a good range for a longer period (number of cycles) compared to the lithium-sulfur secondary battery prepared in Example 17.

As shown in Figures 12 and 13, the lithium-sulfur secondary battery prepared in Example 19, in which FEC corresponding to compound (2) was used as additive b, exhibited a smaller increase in resistance during repeated charge-discharge cycles than the lithium-sulfur secondary battery prepared in Example 17.

From the descriptions in Figures 14 and 15, it can be seen that in the lithium-sulfur secondary battery produced in Example 19, in which FEC corresponding to compound (2) was used as additive b, a coating containing a reduction product of FEC was formed on the positive electrode surface when the charge-discharge cycle was repeated.

Therefore, it can be said that the lithium-sulfur secondary battery having the nonaqueous electrolyte of the present invention can maintain the discharge capacity in a good range for a long period of time when the charge-discharge cycle is repeated, compared with the lithium-sulfur secondary battery having a conventional nonaqueous electrolyte. In addition, in the lithium-sulfur secondary battery having the nonaqueous electrolyte of the present invention containing one or more compounds selected from the group consisting of compound (1) and compound (2) as additive b, the increase in resistance when the charge-discharge cycle is repeated is reduced due to the coating formed on the positive electrode surface. Therefore, it was found that the lithium-sulfur secondary battery containing additive b can maintain the discharge capacity in a good range for a longer period of time, and has superior cycle characteristics.

As shown in Table 2 and FIG. 16, the lithium-sulfur secondary batteries prepared in Examples 27 to 29 did not short-circuit and maintained their capacity retention rate in a good range for at least 30 cycles, even when repeatedly charged and discharged at a high rate (0.3/0.3C). Furthermore, the lithium-sulfur secondary batteries prepared in Examples 28 and 29 using compound (2) as additive b maintained their capacity retention rate in a good range for a longer period (number of cycles) compared to the lithium-sulfur secondary battery prepared in Example 27.

Therefore, it can be said that the lithium-sulfur secondary battery comprising the nonaqueous electrolyte of the present invention can maintain the discharge capacity in a good range for a long period of time when the charge-discharge cycle is repeated, even when the charge-discharge cycle is repeated at a high rate, compared to a lithium-sulfur secondary battery comprising a conventional nonaqueous electrolyte. In addition, it was found that the lithium-sulfur secondary battery comprising the nonaqueous electrolyte of the present invention containing one or more compounds selected from the group consisting of compound (1) and compound (2) as additive b can maintain the discharge capacity in a good range for a longer period of time, even when the charge-discharge cycle is repeated at a high rate, and has superior cycle characteristics.

As shown in Table 2 and Figure 17, the lithium-sulfur secondary batteries prepared in Examples 30 and 31 did not short circuit even after 100 charge-discharge cycles in a low-temperature (0°C) environment, and their capacity retention rate remained within a good range.

Therefore, it can be said that the lithium-sulfur secondary battery equipped with the nonaqueous electrolyte of the present invention can maintain the discharge capacity in a good range for a long period of time, even when the charge-discharge cycle is repeated in a low-temperature environment, compared to the lithium-sulfur secondary battery equipped with a conventional nonaqueous electrolyte.

As shown in Table 2 and FIG. 18, the lithium-sulfur secondary batteries prepared in Examples 32 to 34 did not short-circuit for up to about 20 cycles even when repeatedly charged and discharged in a high-temperature (60°C) environment, and their capacity retention rate was maintained within a good range. Furthermore, the lithium-sulfur secondary batteries prepared in Examples 33 and 34 using compound (2) as additive b maintained their capacity retention rate within a good range for a longer period (number of cycles) compared to the lithium-sulfur secondary battery prepared in Example 32.

Therefore, it can be said that a lithium-sulfur secondary battery having the nonaqueous electrolyte of the present invention can maintain the discharge capacity in a good range for a long period of time, even when charge-discharge cycles are repeated in a high-temperature environment, compared to a lithium-sulfur secondary battery having a conventional nonaqueous electrolyte. In addition, a lithium-sulfur secondary battery having the nonaqueous electrolyte of the present invention containing one or more compounds selected from the group consisting of compound (1) and compound (2) as additive b can maintain the discharge capacity in a good range for a longer period of time, even when charge-discharge cycles are repeated in a high-temperature environment, and can be said to have superior cycle characteristics.

From the above, it has been found that the nonaqueous electrolyte of the present invention has the effect of being usable in the manufacture of lithium-sulfur secondary batteries that have a positive electrode with a high sulfur loading and a discharge capacity that is closer to the theoretical capacity.

In addition, the nonaqueous electrolyte of the present invention can be used to manufacture lithium-sulfur secondary batteries that have excellent discharge capacity even in low-temperature and high-temperature environments.

Furthermore, the nonaqueous electrolyte of the present invention, which contains one or more compounds selected from the group consisting of compound (1) and compound (2) as additive b, can maintain the discharge capacity in a good range for a long period of time and can be used to manufacture a lithium-sulfur secondary battery with excellent cycle characteristics.

The electrolyte for a lithium-sulfur secondary battery according to one embodiment of the present invention can be used to manufacture a lithium-sulfur secondary battery having a positive electrode with a high sulfur loading and a discharge capacity that is closer to the theoretical capacity (1672 mAhg ⁻¹ ) after repeated charge-discharge cycles.

Claims (8)

An electrolyte solution for a lithium-sulfur secondary battery comprising an electrolyte and a solvent,
the electrolyte is lithium bis(fluorosulfonyl)imide and/or lithium hexafluorophosphate;
The solvent includes two or more kinds of ether solvents. The electrolyte solution for lithium-sulfur secondary batteries according to claim 1, wherein the concentration of the electrolyte is 4.0 mol or less per 1.0 L of the solvent. the solvent comprises a high-solubility ether-based solvent and a low-solubility ether-based solvent,
2. The electrolyte for lithium-sulfur secondary batteries according to claim 1, wherein a volume ratio of the highly soluble ether-based solvent to the poorly soluble ether-based solvent is 5:5 to 7:3. The electrolyte for a lithium-sulfur secondary battery according to claim 1, further comprising an additive a containing a lithium salt containing a halogen, in addition to the electrolyte and the solvent. The electrolyte for a lithium-sulfur secondary battery according to any one of claims 1 to 4, further comprising an additive b containing one or more compounds selected from the group consisting of a compound represented by the following structural formula (1) and a compound represented by the following structural formula (2), in addition to the electrolyte and the solvent.

(In structural formula (1), R ¹ and R ² are each independently a group selected from the group consisting of hydrogen, halogen, alkyl, and alkoxy.)
(In structural formula (2), R ¹ and R ² are each independently a group selected from the group consisting of hydrogen, halogen, alkyl, and alkoxy.) The electrolyte for a lithium-sulfur secondary battery according to claim 5, wherein the additive b includes one or more compounds selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, and chloroethylene carbonate. The electrolyte for lithium-sulfur secondary batteries according to claim 1, wherein the solvent contains 1,2-dimethoxyethane and hydrofluoroether. A lithium-sulfur secondary battery comprising the electrolyte for lithium-sulfur secondary batteries according to claim 1.

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