JP7630154B2 - Electrolyte for lithium-sulfur secondary battery and lithium-sulfur secondary battery - Google Patents

Description

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

Lithium-ion secondary batteries are secondary batteries that have excellent performance, such as high energy density, the ability to operate under high voltages, and a long life due to low self-discharge. In recent years, there has been a demand for even higher energy density in order to promote the further spread of electric vehicles and the like.

However, because lithium-ion secondary batteries use cobalt, manganese, nickel, etc., there are resource and price constraints on achieving high energy density.

Lithium-sulfur secondary batteries, which use sulfur in the positive electrode, can achieve a much larger discharge capacity than lithium-ion secondary batteries. In addition, sulfur is an abundant resource, so it is cheaper and easier to obtain than the aforementioned cobalt, manganese, nickel, etc. For this reason, lithium-sulfur secondary batteries are attracting attention as the next generation of secondary batteries.

On the other hand, sulfur is an insulator that does not have electronic conductivity. Therefore, a method of combining sulfur with an electronically conductive substance has been proposed. Non-Patent Document 1 proposes carbon nanotubes, graphene, porous carbon, conductive polymers, polyacrylonitrile, metal oxides, and the like as examples of the electronically conductive substance. An example of the use of the electronically conductive substance is an electrode material in which sulfur is adsorbed into the nanoporosity of porous carbon, as disclosed in Patent Document 1.

Patent Document 2 discloses a binder for use in electrodes of lithium-sulfur secondary batteries, and also discloses the use of a composite of sulfur and porous carbon as a positive electrode material.

On the other hand, in lithium-sulfur secondary batteries, during discharge, lithium metal is oxidized at the negative electrode, and lithium ions dissolve in the electrolyte. The lithium ions react with sulfur at the positive electrode, and are reduced to lithium sulfide (Li ₂ S) via lithium polysulfide, which is an intermediate reaction product.

However, the lithium polysulfide easily dissolves in the electrolyte, causing self-discharge through the so-called redox shuttle, leading to rapid capacity loss and reduced charge/discharge efficiency in lithium-sulfur secondary batteries.

Patent Document 3 discloses an electrolyte for a lithium-sulfur secondary battery that uses a composite of sulfur and porous carbon as a positive electrode material, the electrolyte containing a fluorinated ether, as a technology for suppressing the elution of the lithium polysulfide.

JP 2013-118191 A JP 2016-100094 A International Publication No. 2018/163778

X. Ji, K. T. Lee and L. F. Nazar, “A highly ordered nanostructured carbon-sulfur cathode for lithium-sulfur batteries”, Nature Materials. 8 (2009) 500.

Here, when a composite material of porous carbon and sulfur is used as a positive electrode material, it can be said that the larger the amount of sulfur supported in the porous carbon, the higher the energy density of the lithium-sulfur secondary battery can be, which is preferable. From the viewpoint of supporting a large amount of sulfur, it can be said that the larger the pore size of the porous carbon, the more preferable it is.

However, the inventors discovered that when ethylene carbonate is used as a solvent for the electrolyte in a lithium-sulfur secondary battery in which the composite is used as a positive electrode material and the porous carbon has pores of 2 nm or more, a problem occurs in which charge/discharge characteristics cannot be obtained. Ethylene carbonate is a solvent for electrolytes that is commonly used in lithium-ion secondary batteries.

Patent Document 1 and Non-Patent Document 1 refer to the composite material, but make no mention of the problem between the electrolyte and the charge/discharge characteristics, and therefore cannot solve the problem.

Patent Document 2 merely discloses the use of microporous carbon as the porous carbon and the use of ethylene carbonate as a preferred solvent for the electrolyte. Patent Document 2 does not acknowledge any problems that arise when the porous carbon has pores of 2 nm or more, and therefore is unable to solve the problems.

Patent Document 3 discloses that the electrolyte of the lithium-sulfur secondary battery described therein can suppress the production of Li ₂ S ₈ among lithium polysulfides. However, as described below, the present inventors have found that the lithium-sulfur secondary battery using the electrolyte has room for improvement in terms of polarization of the charge/discharge characteristics.

Therefore, one aspect of the present invention aims to provide an electrolyte for lithium-sulfur secondary batteries that can suppress the elution of lithium polysulfide from a composite material of porous carbon and sulfur containing pores with a pore size of 2 nm or more, and can significantly improve the charge/discharge characteristics of lithium-sulfur secondary batteries.

As a result of extensive research into solving the above problems, the inventors discovered that the above problems could be solved by using an electrolyte solution that uses ethylene carbonate having a chloro group as a solvent, and thus completed the present invention.

That is, the present invention includes the following configurations:

<1> An electrolyte for a lithium-sulfur secondary battery, the lithium-sulfur secondary battery comprising a composite material of porous carbon and sulfur as a positive electrode material, the porous carbon containing pores with a pore size of 2 nm or more, and the solvent of the electrolyte containing ethylene carbonate having a chloro group.

<2> The electrolyte for a lithium-sulfur secondary battery according to <1>, wherein the total volume of the pores in the porous carbon is 0.4 cm ³ /g or more and 2.0 cm ³ /g or less.

<3> The electrolyte for a lithium-sulfur secondary battery according to <2>, wherein the total volume of the pores in the porous carbon is 0.9 cm ³ /g or more and 2.0 cm ³ /g or less.

<4> An electrolyte for a lithium-sulfur secondary battery according to any one of <1> to <3>, in which the weight of sulfur supported in the pores of the porous carbon is 30% by weight or more and 90% by weight or less of the total weight of the porous carbon and the sulfur.

<5> The electrolyte for a lithium-sulfur secondary battery according to <4>, in which the weight of sulfur supported in the pores of the porous carbon is 50% by weight or more and 90% by weight or less of the total weight of the porous carbon and the sulfur.

<6> An electrolyte for a lithium-sulfur secondary battery according to any one of <1> to <5>, in which the ratio of the amount of pores having a pore diameter of 2 nm or more to the total amount of pores in the porous carbon is 5% or more and 100% or less.

<7> An electrolyte for a lithium-sulfur secondary battery according to <6>, in which the proportion of pores having a pore diameter of 2 nm or more is 40% or more and 100% or less.

<8> An electrolyte solution for a lithium-sulfur secondary battery according to any one of <1> to <7>, in which the content of ethylene carbonate having a chloro group in the solvent is 20% by weight or more and 100% by weight or less.

<9> The electrolyte for a lithium-sulfur secondary battery according to <8>, in which the content of ethylene carbonate having a chloro group in the solvent is 30% by weight or more and 100% by weight or less.

<10> An electrolyte solution for a lithium-sulfur secondary battery according to any one of <1> to <9>, wherein the solvent contains one or more selected from vinylene carbonate, hydrofluoroether, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and fluoroethylene carbonate.

<11> A lithium-sulfur secondary battery comprising the electrolyte for a lithium-sulfur secondary battery according to any one of <1> to <10>, a composite material of porous carbon and sulfur is used as a positive electrode material, and the porous carbon contains pores with a pore diameter of 2 nm or more.

According to one aspect of the present invention, even when a composite material of porous carbon containing pores with a pore size of 2 nm or more and sulfur is used as a positive electrode material, it is possible to effectively suppress the elution of lithium polysulfide from the composite material. In addition, according to one aspect of the present invention, it is possible to provide a lithium-sulfur secondary battery with excellent charge/discharge characteristics.

FIG. 1 is a diagram showing the results of a charge-discharge test of the lithium-sulfur secondary batteries prepared in the examples and comparative examples. 1 is a graph showing the results of a cycle characteristic test of the lithium-sulfur secondary batteries prepared in the examples and comparative examples. 1 is a graph showing the results of a cycle characteristic test of the lithium-sulfur secondary batteries prepared in the examples and comparative examples. 1 is a graph showing the results of a cycle characteristic test of the lithium-sulfur secondary batteries prepared in the examples.

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 "A or more, B or less."

[1. Electrolyte for Lithium-Sulfur Secondary Batteries]
An electrolyte for a lithium-sulfur secondary battery according to one embodiment of the present invention is an electrolyte for a lithium-sulfur secondary battery, the lithium-sulfur secondary battery comprising a composite material of porous carbon and sulfur as a positive electrode material, the porous carbon including pores having a pore size of 2 nm or more, and a solvent for the electrolyte including ethylene carbonate having a chloro group.

(1-1. Positive electrode materials for lithium-sulfur secondary batteries)
The lithium-sulfur secondary battery using the electrolyte for lithium-sulfur secondary batteries according to one embodiment of the present invention includes a composite material of porous carbon and sulfur as a positive electrode material. In this specification, the positive electrode material is a material that is applied to or filled in a current collector to produce a positive electrode, and contains a positive electrode active material, a conductive assistant, a binder, and the like.

The sulfur is a positive electrode active material. The porous carbon is not particularly limited, but may be preferably an activated carbon obtained by carbonizing a resin, a fossil resource material, or the like, and then activating it with an alkali such as potassium hydroxide. The activated carbon is preferred because it has a large specific surface area and pore volume, and has a structure suitable for supporting a large amount of sulfur. In addition, porous carbon in which the pore structure of the carbon is controlled by a template method is also preferably used. The porous carbon may be one produced based on a known method, or may be a commercially available product.

The "composite material of porous carbon and sulfur" is porous carbon in which sulfur is supported in the pores of the porous carbon. In other words, the composite material of porous carbon and sulfur also includes porous carbon on which polysulfides are adsorbed. The adsorption may be physical adsorption or chemical adsorption.

The porous carbon contains pores with a pore diameter of 2 nm or more. The shape of the pores in the porous carbon can be slit-like, wide-open, connected, etc., but the shape is not particularly limited. The types of pores in the porous carbon include micropores with a pore diameter of less than 2 nm, mesopores with a pore diameter of 2 nm or more but less than 50 nm, and macropores with a pore diameter of 50 nm or more.

As mentioned above, Patent Document 2 discloses the use of microporous carbon as the porous carbon and the use of ethylene carbonate as a preferred solvent for the electrolyte.

The microporous carbon disclosed in Patent Document 2 is a porous carbon in which the pores are micropores. In this case, ethylene carbonate cannot enter the micropores, so the sulfur supported in the micropores does not dissolve into the electrolyte as lithium polysulfide.

However, because micropores have a very small pore size, the amount of sulfur that can be supported is naturally small. Therefore, it is necessary to support sulfur in mesopores, which have a larger pore size than micropores, and to reduce the amount of sulfur that dissolves into the electrolyte.

The porous carbon contains pores with a pore size of 2 nm or more, so it can support more sulfur than porous carbon that has only micropores. This allows the lithium-sulfur secondary battery to have a high energy density.

Moreover, as described below, the electrolyte for a lithium-sulfur secondary battery according to one embodiment of the present invention contains ethylene carbonate having a chloro group as a solvent, and therefore can prevent the sulfur from dissolving into the electrolyte as lithium polysulfide. Therefore, the electrolyte for a lithium-sulfur secondary battery according to one embodiment of the present invention can provide a lithium-sulfur secondary battery with excellent charge and discharge characteristics.

The total volume of the pores in the porous carbon is preferably 0.4 cm ³ /g to 2.0 cm ³ /g, more preferably 0.9 cm ³ /g to 2.0 cm ³ /g, and even more preferably 1.2 ^{cm 3} /g to 2.0 cm ³ /g, from the viewpoint of supporting a large amount of sulfur.

When the total volume of the pores in the porous carbon is within the above range, it is possible to sufficiently support sulfur in the porous carbon, thereby further improving the energy density of the lithium-sulfur secondary battery.

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

The weight of sulfur supported in the pores of the porous carbon is preferably 30% by weight or more and 90% by weight or less, more preferably 50% by weight or more and 90% by weight or less, and even more preferably 70% by weight or more and 90% by weight or less, based on the total weight of the porous carbon and the sulfur.

In one embodiment of the present invention, the weight of sulfur supported in the pores of the porous carbon may be 70% by weight or less of the total weight of the porous carbon and the weight of the sulfur.

When the weight of the sulfur is within the above range, the amount of sulfur supported in the porous carbon can be increased sufficiently, thereby improving the charge/discharge characteristics of the lithium-sulfur secondary battery. Although a larger amount of sulfur is preferable, the upper limit is about 90% by weight, taking into account the support capacity of the porous carbon. The amount of sulfur supported can be confirmed by the method described later in the examples.

The proportion of the amount of pores having a pore diameter of 2 nm or more is preferably 5% or more, more preferably 20% or more, and particularly preferably 40% or more, relative to the total amount of pores possessed by the porous carbon. In addition, the higher the proportion of the amount of pores having a pore diameter of 2 nm or more, the more preferable, and the proportion is preferably 40% or less rather than 20% or less, more preferably 60% or less, even more preferably 80% or less, and particularly preferably 100% or less, relative to the total amount of pores possessed by the porous carbon.

In this specification, the "amount of pores" may mean either the number of pores or the volume of the pores. That is, the ratio of the amount of pores having a pore diameter of 2 nm or more to the total amount of pores in the porous carbon may be the ratio of the number of pores having a pore diameter of 2 nm or more to the total number of pores in the porous carbon. Alternatively, the ratio of the amount of pores having a pore diameter of 2 nm or more to the total amount of pores in the porous carbon may be the ratio of the volume of pores having a pore diameter of 2 nm or more to the total volume of pores in the porous carbon.

By setting the ratio to 5% or more, the amount of sulfur carried can be increased significantly compared to porous carbon with only micropores, making it easier to provide a lithium-sulfur secondary battery with high energy density.

In addition, if the proportion of macropores in the pores is high, the density of the electrode tends to decrease. Therefore, it is more preferable that the pores having a pore diameter of 2 nm or more are mesopores.

The proportion of the number of pores with a pore diameter of 2 nm or more and the pore diameter of the pores can be confirmed by measuring the specific surface area of the porous carbon by the BET method and measuring the pore distribution and pore volume by quenched solid phase density function method (QSDFT). Quantachrome's AUTOSORB iQ can be used as a measuring device.

The specific surface area, pore distribution, and pore volume of the porous carbon can be measured by measuring the nitrogen adsorption isotherm at liquid nitrogen temperature for a sample that has been vacuum degassed at 200°C for 3 hours.

The volume ratio of pores with a pore diameter of 2 nm or more can be confirmed using the pore distribution diagram obtained from the above-mentioned AUTOSORB iQ.

The loading of sulfur into the pores can be achieved, for example, by mixing the porous carbon with sulfur in a mortar or the like, and then heating the resulting mixture in a muffle furnace or the like. Commercially available sulfur flowers or the like can be used as the sulfur.

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.

From the viewpoint of supporting a large amount of sulfur, the specific surface area of the porous carbon is preferably 750 m ² /g to 3300 m ² /g, and more preferably 1200 m ² /g to 3300 m ² /g.

The conductive assistant, which is the material for the positive electrode, can be any electronically conductive material that does not adversely affect battery performance and is not particularly limited. Carbon black such as acetylene black and ketjen black is usually used as the conductive assistant, but other conductive assistants may also be used. For example, 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, carbon nanotubes, etc. may be used. These may be used alone or as a mixture of two or more types.

As the binder, for example, an aqueous binder can be used. There is no particular limitation on the aqueous binder as long as it can bind the composite material of porous carbon and sulfur with the conductive assistant, etc. For example, one or more binders selected from the group consisting of styrene-butadiene rubber (SBR) as a rubber-based binder, carboxymethyl cellulose (CMC) as a dispersant, alginate as an aqueous binder, and polyglutamate can be used.

The positive electrode material can be prepared, for example, by homogeneously mixing the porous carbon carrying sulfur, the conductive additive, and the binder. One method of mixing is to mix the porous carbon carrying sulfur and the conductive additive in a mortar or the like, and then further stir the resulting mixture using a rotation/revolution mixer.

(1-2. Electrolyte for Lithium-Sulfur Secondary Batteries)
The electrolyte for a lithium-sulfur secondary battery (hereinafter also simply referred to as the electrolyte) contains a solvent and an electrolyte containing lithium.

The electrolyte solvent for the lithium-sulfur secondary battery (hereinafter also simply referred to as electrolyte solvent) contains ethylene carbonate having a chloro group. The number of chloro groups in the ethylene carbonate having a chloro group is not particularly limited and may be 1 to 4.

The bonding position of the chloro group on the ethylene carbonate is not particularly limited, and may be present at any position. That is, for example, all carbons in ethylene carbonate may have one or two chloro groups, or only one carbon may have a chloro group. Also, the chloro group may not be bonded directly to the carbon of ethylene carbonate, but may be bonded to the carbon as a chloromethylene group, for example.

Examples of ethylene carbonates having a chloro group include chloroethylene carbonate (4-chloro-1,3-dioxolan-2-one), 3-chloropropylene carbonate, and 4,4,5,5-tetrachloro-1,3-dioxolan-2-one. The electrolyte solvent may contain only one type of ethylene carbonate containing a chloro group, or two or more types of ethylene carbonates may be contained.

By including ethylene carbonate having a chloro group in the solvent of the electrolyte, even if the composite material of porous carbon and sulfur contains pores with a pore size of 2 nm or more, it is possible to prevent sulfur from eluting from the pores into the electrolyte as lithium polysulfide.

As shown in Example 1 below, when an electrolyte solution containing ethylene carbonate having a chloro group as a solvent was used, a charge/discharge curve was obtained that is believed to indicate that a coating was formed in the pores of the composite material (sulfur-loaded activated carbon).

From this, it is believed that in the electrolyte for a lithium-sulfur secondary battery according to one embodiment of the present invention, first, ethylene carbonate having a chloro group forms a coating in the pores. Next, the coating is believed to suppress contact between the sulfur and the electrolyte and suppress the elution of lithium polysulfide from the pores. As a result, it is believed that the loss of sulfur contained in the composite material can be prevented and sulfur can be made to effectively act as a positive electrode active material, thereby providing excellent charge and discharge characteristics to the lithium-sulfur secondary battery.

In addition, in the examples described below, it has been shown that the use of ethylene carbonate having a chloro group as the solvent can provide lithium-sulfur secondary batteries with superior charge/discharge characteristics than the use of fluoroethylene carbonate as the solvent.

The content of the ethylene carbonate having a chloro group in the solvent of the electrolyte is preferably 20% by weight or more and 100% by weight or less, more preferably 30% by weight or more and 100% by weight or less, and even more preferably 50% by weight or more and 100% by weight or less, when the weight of the solvent is 100% by weight.

When the content is 20% by weight or more, when used as a solvent for the electrolyte, the coating can be formed to a degree that sufficiently suppresses the elution of lithium polysulfide, which is preferable for improving the discharge capacity of a lithium-sulfur secondary battery.

The solvent of the electrolytic solution may contain a solvent other than ethylene carbonate having a chloro group as a co-solvent. Such a co-solvent may be one or more selected from the group consisting of carbonates other than ethylene carbonate, ethers, sulfur compounds, halogenated hydrocarbons, phosphate ester compounds, sulfolane hydrocarbons, and ionic liquids.

The carbonates other than ethylene carbonate are preferably cyclic carbonates such as fluoroethylene carbonate (FEC) and vinylene carbonate (VC); or chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC).

The ethers are preferably cyclic ethers such as dioxolane (DOL) and tetrahydrofuran; or chain ethers such as dimethoxyethane (DME), triglyme (G3), tetraglyme (G4), and hydrofluoroethers.

Among the co-solvents, one or more selected from vinylene carbonate, hydrofluoroether, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and fluoroethylene carbonate are particularly preferred from the viewpoint of improving the charge/discharge characteristics of lithium-sulfur secondary batteries.

An example of the hydrofluoroether is 1,1,2,2-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)-propane.

Examples of the lithium-containing electrolyte include LiPF ₆ , LiBF ₄ , lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium bis(fluorosulfonyl)imide (LiFSI). Among them, LiPF ₆ , LiFSI, and LiTFSI are preferred, and LiTFSI is more preferred. The lithium-containing electrolyte may be used alone or in combination of two or more.

2. Lithium-sulfur secondary battery
A lithium-sulfur secondary battery according to an embodiment of the present invention includes an electrolyte for a lithium-sulfur secondary battery according to an embodiment of the present invention, and uses a composite material of porous carbon and sulfur as a positive electrode material, and the porous carbon includes pores having a pore diameter of 2 nm or more.

According to this configuration, as shown in the examples described below, the amount of sulfur supported in the pores of the porous carbon contained in the positive electrode material can be increased, and the sulfur can be prevented from being eluted as lithium polysulfide by the electrolyte, thereby providing a lithium-sulfur secondary battery that exhibits a high discharge capacity.

The electrolyte and the sulfur-supported porous carbon have been described above. The electrolyte can be prepared, for example, by mixing an electrolyte containing lithium with a solvent.

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

The positive electrode can be prepared, for example, by applying or filling a positive electrode slurry onto a current collector, drying it, press-molding it, and then vacuum drying it.

The positive electrode slurry can be prepared, for example, by adding a conductive assistant to a binder solution prepared by dissolving the binder in pure water, and then adding and mixing the composite material of porous carbon and sulfur.

The weight ratio of the composite material, the conductive additive, and the binder in the positive electrode slurry is preferably 80:10:10 to 97:0:3, and more preferably 85:5:10 to 95:0:5, from the viewpoint of uniform application or filling to the current collector.

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 current collector may be any of foil, film, sheet, net, etc. Among them, a current collector having a three-dimensional structure such as a honeycomb structure is preferred because it allows for a larger amount of the positive electrode material for the lithium-sulfur secondary battery to be filled, enabling a larger capacity. An example of such a current collector is a 3D aluminum current collector.

The negative electrode can be prepared, for example, by applying or filling a negative electrode slurry onto a current collector, drying it, press-molding it, and then vacuum drying it.

The negative electrode slurry can be prepared, for example, by adding a conductive assistant to a binder solution prepared by dissolving a binder in pure water, and then adding and mixing the negative electrode active material. Metallic lithium can be used as the negative electrode active material.

The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. The technical scope of the present invention also includes embodiments obtained by appropriately combining the technical means disclosed in different embodiments.

<Charge/discharge test>
A constant current charge/discharge test was performed using a BTS2400W (manufactured by Nagano Co., Ltd.) with the positive electrode of the lithium-sulfur secondary battery prepared in the examples and comparative examples described below as the working electrode. 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 167.2mA/g (a current density of 1672mA/g is defined as 1C. Hereinafter, 167.2mA/g is shown as 0.1C). The cutoff voltage had a lower limit of 1.0V and an upper limit of 3.0V. The test was performed in an environment of 25°C.

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

<Cycle characteristic test>
The charge/discharge rate was set to 0.1/0.1 C, and the charge/discharge cycles were 1 to 100. The higher the discharge capacity in each cycle, the better the lithium-sulfur secondary battery.

The coulombic efficiency and capacity retention rate were calculated by the following formulas.
Coulombic efficiency (%) = (100th cycle coulombic efficiency / 2nd cycle coulombic efficiency) x 100
Capacity retention rate (%) = (100th cycle discharge capacity/2nd cycle discharge capacity) x 100
Example 1
(Preparation of sulfur-loaded activated carbon)
Activated carbon having mesopores with a pore size of 2 nm to 4 nm and sulfur flowers (manufactured by Wako Pure Chemical Co., Ltd., purity over 99%) were mixed in an agate mortar to give a mixture with a weight ratio of 35:65. The mixture was transferred to a heat-resistant metal container and placed in a muffle furnace to perform a heat treatment in the air.

The activated carbon had a pore volume of 1.6 cm ³ /g and a specific surface area of 1307 m ² /g. The ratio of the number of macropores:mesopores:micropores in the activated carbon was 0:40:60.

Specifically, after the metal container was placed in a furnace, the temperature inside the furnace was raised to 155°C and maintained at this temperature for 5 hours to melt the sulfur. The sulfur was adsorbed into the pores of the activated carbon by capillary action. The furnace was then thoroughly air-cooled to room temperature, and the metal container was removed from the furnace to obtain activated carbon carrying sulfur. This activated carbon corresponds to the "composite material of porous carbon and sulfur" in one embodiment of the present invention.

The amount of sulfur supported in the activated carbon (weight of sulfur supported in the pores of the porous carbon) was measured by the following method. That is, the activated carbon supporting sulfur was placed in an alumina cell, and thermogravimetric analysis (TGA) was performed using a Shimadzu DTG-60AH. The measurement was performed under the conditions of Ar as the measurement gas, a gas flow rate of 50 ml/min, a heating rate of 5°C/min, and an upper limit temperature of 600°C. The amount of sulfur supported was 65% by weight based on the total weight of the activated carbon supporting sulfur.

(Preparation of Binder Solution)
As a binder, styrene-butadiene rubber (SBR), which is a rubber-based binder, and carboxyl methyl cellulose (CMC) as a dispersant were dissolved in pure water to prepare a binder solution with a carboxyl methyl cellulose (CMC) concentration of 1 to 2% by weight.

(Preparation of slurry for positive electrode of lithium-sulfur secondary battery)
The binder solution was added with acetylene black, which is a conductive additive, and then with the activated carbon carrying sulfur, to prepare a positive electrode slurry. The weight ratio of the activated carbon carrying sulfur, acetylene black, dispersant (CMC), and binder (SBR) was 90:5:3:2.

(Preparation of positive electrode for lithium-sulfur secondary battery)
A positive electrode for a two-electrode flat cell was prepared as a positive electrode for a lithium-sulfur secondary battery.

The positive electrode for the two-electrode flat cell was prepared by the following method. First, the positive electrode slurry was applied to an aluminum foil (current collector) so that the weight of the activated carbon carrying sulfur was 1.3 mg/ ^cm2 per unit area of the current collector, and the slurry was dried at 40°C for 1 hour using an oven under atmospheric pressure. Next, the dried current collector was rolled using a roll press machine, and then punched out to a size with a diameter of 12 mm to obtain a molded body. The molded body was further vacuum-dried for 12 hours using a bell jar at 50°C to prepare a positive electrode for the two-electrode flat cell.

(Preparation of negative electrode for lithium-sulfur secondary battery)
As a negative electrode for a lithium-sulfur secondary battery, a two-electrode flat cell negative electrode was prepared.

The negative electrode for the two-electrode flat cell was prepared by punching out a 20 μm thick lithium foil to a diameter of 16 mm in an air atmosphere with a dew point of -40°C or lower.

(Preparation of electrolyte for lithium-sulfur secondary battery)
Lithium bis(trifluoromethanesulfonyl)imide (hereinafter referred to as LiTFSI) was used as the lithium-containing electrolyte, and a mixed solvent of 4-chloro-1,3-dioxolan-2-one (chloroethylene carbonate) (hereinafter referred to as ClEC) and 1,1,2,2-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)-propane (hereinafter referred to as HFE) in a volume ratio of 50:50 was used as the non-aqueous solvent.

The method for preparing the electrolyte solution will now be described in more detail. ClEC and HFE were mixed in a volume ratio of 50:50 to prepare a mixed solvent. LiTFSI was then dissolved in the mixed solvent to give a LiTFSI/ClEC:HFE ratio of 1.0 mol/L, and this solution was used as the electrolyte solution for the lithium-sulfur secondary battery.

(Preparation of Lithium-Sulfur Secondary Battery)
In an air atmosphere having a dew point of −40° C. or lower, a lithium-sulfur secondary battery was produced by the following procedure using the positive electrode produced by the above (Preparation of positive electrode for lithium-sulfur secondary battery), the negative electrode produced by the above (Preparation of negative electrode for lithium-sulfur secondary battery), the electrolyte prepared by the above (Preparation of electrolyte for lithium-sulfur secondary battery), and a polypropylene-based microporous film as a separator.

A two-electrode flat cell (coated electrodes) was used as the cell for the lithium-sulfur secondary battery. First, the positive electrode, separator, and negative electrode were stacked in this order inside the cell. Next, the electrolyte was injected into the cell to create a lithium-sulfur secondary battery.

The lithium-sulfur secondary battery was subjected to the charge/discharge test and cycle characteristic test described above.

Example 2
(Preparation of sulfur-loaded activated carbon)
An activated carbon carrying sulfur was obtained in the same manner as in Example 1, except that the activated carbon had mesopores with a pore size of 2 nm or more and 3 nm or less.

The activated carbon had a pore volume of 1.3 cm ³ /g and a specific surface area of 2924 m ² /g. The ratio of the number of macropores:mesopores:micropores in the activated carbon was 0:5:95.

The amount of sulfur supported on the activated carbon was measured in the same manner as in Example 1. The amount of sulfur supported was 61% by weight based on the total weight of the activated carbon supporting sulfur.

(Preparation of Binder Solution)
As the binder, polyglutamic acid salt, which is a water-based binder, was dissolved in pure water to prepare a binder solution with a polyglutamic acid salt concentration of 5% by weight.

(Preparation of slurry for positive electrode of lithium-sulfur secondary battery)
The carbon nanotubes, which are a conductive additive, and the activated carbon carrying sulfur were then added to the binder solution to prepare a positive electrode slurry. The weight ratio of the activated carbon carrying sulfur, the carbon nanotubes, and the binder was 94:1:5.

(Preparation of Lithium-Sulfur Secondary Battery)
A positive electrode for a two-electrode flat cell was produced in the same manner as in Example 1, except that the weight of the sulfur-supported activated carbon was applied to an aluminum foil ( ^current collector) so as to be 1.1 to 1.3 mg/cm2 per unit area of the current collector.

A lithium-sulfur secondary battery was fabricated in the same manner as in Example 1 in all other respects.

Example 3
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 2, except that the electrolyte solution was prepared as follows.

ClEC and 4-fluoro-1,3-dioxolane-2-one (fluoroethylene carbonate) (hereinafter referred to as FEC) were mixed in a volume ratio of 50:50 to prepare a mixed solvent. LiTFSI was then dissolved in the mixed solvent to give a LiTFSI/ClEC:FEC ratio of 1.0 mol/L, and the resulting solution was used as the electrolyte for the lithium-sulfur secondary battery.

Example 4
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 2, except that the electrolyte solution was prepared as follows.

ClEC and the VC were mixed in a volume ratio of 50:50 to prepare a mixed solvent. Next, LiTFSI was dissolved in the mixed solvent to give a LiTFSI/ClEC:VC ratio of 1.0 mol/L, and the resulting solution was used as the electrolyte for the lithium-sulfur secondary battery.

The amount of sulfur loaded in the sulfur-loaded activated carbon was 66% by weight, based on the total weight of the sulfur-loaded activated carbon.

Example 5
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 2, except that the electrolyte solution was prepared as follows.

LiTFSI was dissolved in ClEC to give a LiTFSI/ClEC ratio of 1.0 mol/L, which was used as the electrolyte for the lithium-sulfur secondary battery.

The amount of sulfur loaded in the sulfur-loaded activated carbon was 63% by weight, based on the total weight of the sulfur-loaded activated carbon.

Example 6
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 2, except that the electrolyte solution was prepared as follows.

LiTFSI was dissolved in ClEC to prepare a solution with a LiTFSI/ClEC ratio of 1.0 mol/L. Next, 10% vinylene carbonate (VC) was added to the 100% solution to prepare an electrolyte for a lithium-sulfur secondary battery.

The amount of sulfur loaded in the sulfur-loaded activated carbon was 66% by weight, based on the total weight of the sulfur-loaded activated carbon.

Example 7
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 2, except that the electrolyte solution was prepared as follows.

ClEC and HFE were mixed in a volume ratio of 90:10 to prepare a mixed solvent. LiTFSI was then dissolved in the mixed solvent to obtain a LiTFSI/ClEC:HFE ratio of 1.0 mol/L to prepare a solution. VC was then added at 10 volume% to the 100 volume% solution to prepare an electrolyte for a lithium-sulfur secondary battery.

The amount of sulfur loaded in the sulfur-loaded activated carbon was 66% by weight, based on the total weight of the sulfur-loaded activated carbon.

Example 8
ClEC and HFE were mixed in a volume ratio of 70:30 to prepare a mixed solvent. LiTFSI was then dissolved in the mixed solvent to prepare a 1.0 mol/L LiTFSI/ClEC:HFE solution. Furthermore, VC was added at 10 volume% to the 100 volume% solution to prepare an electrolyte for a lithium-sulfur secondary battery. A lithium-sulfur secondary battery was prepared in the same manner as in Example 7 except for the above.

The amount of sulfur loaded in the sulfur-loaded activated carbon was 66% by weight, based on the total weight of the sulfur-loaded activated carbon.

Example 9
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 7, except that ClEC and HFE were mixed in a volume ratio of 50:50 to form a mixed solvent.

The amount of sulfur loaded in the sulfur-loaded activated carbon was 63% by weight, based on the total weight of the sulfur-loaded activated carbon.

Example 10
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 7, except that ClEC and HFE were mixed in a volume ratio of 30:70 to form a mixed solvent.

The amount of sulfur loaded in the sulfur-loaded activated carbon was 63% by weight, based on the total weight of the sulfur-loaded activated carbon.

Example 11
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 2, except that the electrolyte, positive electrode, negative electrode, and electrolyte were prepared as follows: The amount of sulfur supported in the sulfur-supported activated carbon was 66% by weight based on the total weight of the sulfur-supported activated carbon.

The positive electrode for the laminated cell was prepared by the following method.

First, the slurry for the positive electrode prepared by the same method as in Example 1 was filled into a 3D aluminum current collector (Celmet, manufactured by Sumitomo Electric Industries, Ltd.) so that the weight of the activated carbon carrying sulfur was 11 mg/cm ² or more and 13 mg/cm ² or less per unit area of the current collector. Next, the 3D aluminum current collector was dried at 40° C. for 1 hour using an oven under atmospheric pressure. Next, the dried current collector was rolled using a roll press machine and then cut into a size of 24 mm x 24 mm using scissors to obtain a molded body. The molded body was further vacuum-dried for 12 hours using a bell jar at 50° C. to produce a positive electrode for a laminate cell.

The negative electrode for the laminated cell was prepared by cutting a 30 μm thick lithium foil to a size of 30 mm x 35 mm using scissors in an air atmosphere with a dew point of -40°C or lower.

The electrolyte for the lithium-sulfur secondary battery was prepared in the same manner as in Example 6.

Example 12
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 11, except that ClEC and HFE were mixed in a volume ratio of 90:10 to prepare a mixed solvent. The amount of sulfur supported in the sulfur-supported activated carbon was 66 wt % based on the total weight of the sulfur-supported activated carbon.

Example 13
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 11, except that ClEC and HFE were mixed in a volume ratio of 70:30 to prepare a mixed solvent. The amount of sulfur supported in the sulfur-supported activated carbon was 64 wt % based on the total weight of the sulfur-supported activated carbon.

Example 14
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 11, except that ClEC and HFE were mixed in a volume ratio of 50:50 to prepare a mixed solvent. The amount of sulfur supported in the sulfur-supported activated carbon was 65 wt % based on the total weight of the sulfur-supported activated carbon.

Example 15
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 11, except that in the preparation of the electrolyte, ClEC and dimethyl carbonate (hereinafter referred to as DMC) were mixed in a volume ratio of 70:30 to form a mixed solvent. The amount of sulfur supported in the sulfur-supported activated carbon was 67% by weight, based on the total weight of the sulfur-supported activated carbon.

Example 16
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 11, except that in the preparation of the electrolyte, ClEC and DMC were mixed in a volume ratio of 50:50 to prepare a mixed solvent. The amount of sulfur supported in the sulfur-supported activated carbon was 64 wt % based on the total weight of the sulfur-supported activated carbon.

Example 17
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 11, except that in the preparation of the electrolyte, ClEC and ethyl methyl carbonate (hereinafter referred to as EMC) were mixed in a volume ratio of 70:30 to prepare a mixed solvent. The amount of sulfur supported in the sulfur-supported activated carbon was 65% by weight based on the total weight of the sulfur-supported activated carbon.

Example 18
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 11, except that in the preparation of the electrolyte, ClEC and diethyl carbonate (hereinafter referred to as DEC) were mixed in a volume ratio of 70:30 to prepare a mixed solvent. The amount of sulfur supported in the sulfur-supported activated carbon was 67% by weight, based on the total weight of the sulfur-supported activated carbon.

Comparative Example 1
In preparing the electrolyte solution, lithium hexafluorophosphate (hereinafter referred to as _LiPF6 ) was used as the lithium-containing electrolyte. In addition, a mixed solvent (commercially available from Kishida Chemical Co., Ltd.) in which ethylene carbonate (hereinafter referred to as EC) and DMC were mixed in a volume ratio of 50:50 was used as the non-aqueous solvent.

Otherwise, the lithium-sulfur secondary battery was fabricated in the same manner as in Example 1.

Comparative Example 2
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 1, except that in the preparation of the electrolyte, FEC was used instead of ClEC as the non-aqueous solvent.

Comparative Example 3
In preparing the electrolyte, a mixed solvent of EC and DMC in a volume ratio of 50:50 (a commercially available product manufactured by Kishida Chemical Co., Ltd.) was used as the non-aqueous solvent. A lithium-sulfur secondary battery was produced in the same manner as in Example 2.

Comparative Example 4
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 2, except that in preparing the electrolyte solution, a mixed solvent of FEC and HFE in a volume ratio of 50:50 was used as the non-aqueous solvent.

Comparative Example 5
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 11, except that the electrolyte solution was prepared as follows.

In the electrolyte, LiPF ₆ was used as the lithium-containing electrolyte. In addition, a mixed solvent (commercially available from Kishida Chemical Co., Ltd.) in which EC and DMC were mixed in a volume ratio of 50:50 was used as the non-aqueous solvent. Next, a solution was prepared in which LiPF ₆ was dissolved in the mixed solvent to give a LiPF ₆ /EC:DMC ratio of 1.0 mol/L. Furthermore, 10 volume% of vinylene carbonate (VC) was added to the 100 volume% solution to prepare an electrolyte for a lithium-sulfur secondary battery.

Comparative Example 6
A lithium-sulfur secondary battery was fabricated in the same manner as in Example 11, except that the electrolyte solution was prepared as follows.

In the electrolyte, a mixed solvent of FEC and HFE mixed in a volume ratio of 50:50 was used as the non-aqueous solvent. Next, LiTFSI was dissolved in the mixed solvent to obtain a LiTFSI/FEC:HFE ratio of 1.0 mol/L to prepare a solution. Next, 10 volume% of vinylene carbonate (VC) was added to the 100 volume% solution to prepare an electrolyte for a lithium-sulfur secondary battery.

〔result〕
FIG. 1 is a graph showing the results of a charge-discharge test of the lithium-sulfur secondary batteries prepared in Example 1 and Comparative Examples 1 and 2.

From Figure 1, it can be seen that the lithium-sulfur secondary batteries prepared in Example 1 and Comparative Example 2 were able to be charged and discharged more than 100 times, whereas the lithium-sulfur secondary battery prepared in Comparative Example 1 only exhibited the discharge characteristics of the first cycle and could not be recharged.

Ethylene carbonate (EC) cannot penetrate the micropores (pore diameter less than 2 nm) of activated carbon, but can penetrate the mesopores. As mentioned above, the ratio of the number of pores in the activated carbon used in Example 1, Comparative Example 1, and Comparative Example 2 was macropores:mesopores:micropores = 0:40:60. The result of Comparative Example 1 is thought to be due to the fact that the solvent EC penetrated into the mesopores, causing sulfur to dissolve as lithium polysulfide, and current generation ceased.

Also, from Figure 1, it can be seen that a characteristic discharge occurs at around 1.8 to 2.3 V in the first cycle in Example 1 and Comparative Example 1. This shows that a coating is formed in the pores of the activated carbon carrying sulfur in the positive electrode.

The lithium-sulfur secondary batteries prepared in Example 1 and Comparative Example 2 were able to perform charge/discharge cycles of 100 or more, unlike Comparative Example 1, because the formation of the coating prevented the solvent from entering the pores.

On the other hand, it can be seen that the lithium-sulfur secondary battery prepared in Example 1 has a much smaller polarization in the charge/discharge characteristics than the lithium-sulfur secondary battery prepared in Comparative Example 2. In other words, it has become clear that by using ClEC as the solvent for the electrolyte, the resistance of the lithium-sulfur secondary battery can be significantly reduced compared to when FEC is used.

Therefore, it can be said that the lithium-sulfur secondary battery prepared in Example 1 exhibits better charge-discharge characteristics than the lithium-sulfur secondary battery prepared in Comparative Example 1.

Figure 2 and Table 1 show the results of the lithium-sulfur secondary batteries prepared in Example 1 and Comparative Examples 1 and 2 subjected to the cycle characteristics test described above.

As shown in Figure 2 and Table 1, the lithium-sulfur secondary battery prepared in Comparative Example 1 could not operate. As mentioned above, this result is thought to be due to the fact that EC used as a solvent in the electrolyte penetrated into the mesopores, causing sulfur to dissolve as lithium polysulfide.

The lithium-sulfur secondary battery prepared in Example 1 exhibited a higher discharge capacity in the second cycle than the lithium-sulfur secondary battery prepared in Comparative Example 2, and also exhibited higher capacity retention and average coulombic efficiency.

Furthermore, Figure 2 shows that the lithium-sulfur secondary battery prepared in Example 1 exhibits a higher discharge capacity than the lithium-sulfur secondary battery prepared in Comparative Example 2, even when the number of cycles is increased, and the difference becomes larger as the number of cycles increases.

In other words, it can be seen that the use of ClEC as a solvent for the electrolyte can improve the discharge capacity of a lithium-sulfur secondary battery compared to the use of EC or FEC as the solvent.

Figure 3 shows the results of cycle characteristic tests of the lithium-sulfur secondary batteries prepared in Example 2 and Comparative Examples 3 and 4. Table 2 shows the results of measuring the discharge capacity in the second cycle of the lithium-sulfur secondary batteries prepared in Examples 2 and 3 and Comparative Examples 3 and 4.

As shown in Figure 3 and Table 2, the lithium-sulfur secondary battery prepared in Comparative Example 3 was able to operate for up to 20 cycles, but the discharge capacity was low and it shorted out after the 20th cycle, making it unable to operate.

The lithium-sulfur secondary battery prepared in Comparative Example 3 uses EC as a solvent for the electrolyte, but the proportion of micropores in the total pores of the activated carbon used is greater than in the lithium-sulfur secondary battery prepared in Comparative Example 1 described above. In other words, the proportion of activated carbon into which EC cannot penetrate is greater. Therefore, unlike the lithium-sulfur secondary battery prepared in Comparative Example 1, it is believed that the battery was able to operate for up to 20 cycles.

However, it is believed that the lithium-sulfur secondary battery prepared in Comparative Example 3 short-circuited after the 20th cycle because EC penetrated into the mesopores, causing the sulfur supported in the mesopores to dissolve.

From Table 2, it can be seen that the lithium-sulfur secondary batteries prepared in Examples 2 and 3 have a higher discharge capacity at the second cycle than the lithium-sulfur secondary batteries prepared in Comparative Examples 3 and 4. Also, from Figure 3, it can be seen that the lithium-sulfur secondary battery prepared in Example 2 exhibits a higher discharge capacity than the lithium-sulfur secondary batteries prepared in Comparative Examples 3 and 4, even when the number of cycles is increased.

The lithium-sulfur secondary batteries prepared in Examples 2 and 3 are believed to have been able to retain a large amount of sulfur in the mesopores due to the use of ClEC as the electrolyte solvent. Therefore, it is believed that the lithium-sulfur secondary batteries prepared in Examples 2 and 3 were able to exhibit excellent discharge capacity, unlike the lithium-sulfur secondary battery prepared in Comparative Example 3.

As described above, the use of ClEC as the solvent reduces the polarization of the charge/discharge characteristics compared to the use of FEC as the solvent without ClEC. This is thought to be the reason why the lithium-sulfur secondary battery prepared in Example 2 exhibited a higher discharge capacity than the lithium-sulfur secondary battery prepared in Comparative Example 4.

As shown in Example 3, the lithium-sulfur secondary battery using ClEC and FEC in combination as the solvent had a better second cycle discharge capacity than the lithium-sulfur secondary battery prepared in Comparative Example 4, which did not use ClEC as a solvent. This also shows the usefulness of using ClEC as the solvent.

Table 3 shows the results of cycle characteristic tests of the lithium-sulfur secondary batteries prepared in Examples 2, 4, and 5.

As shown in Example 5 of Table 3, it was found that even when ClEC was used alone as the electrolyte solvent, the lithium-sulfur secondary battery exhibited a high discharge capacity.

Table 4 and Figure 4 show the results of the cycle characteristic test for the lithium-sulfur secondary batteries prepared in Examples 6 to 10.

As shown in Table 4 and Figure 4, all of the lithium-sulfur secondary batteries prepared in Examples 6 to 10 exhibited high discharge capacities. As shown in Example 10, it was found that the lithium-sulfur secondary batteries exhibited high discharge capacities even when the proportion of ClEC in the electrolyte solvent was 30 volume % or less (the volume ratio of ClEC to HFE to VC was 30:70:10).

In other words, the ClEC content in the solvent in the electrolyte does not necessarily need to be higher than that of the co-solvent, and it is possible to obtain a high discharge capacity with a low content.

Table 5 shows the results of the cycle characteristic tests for Examples 11 to 18 and Comparative Examples 5 and 6, including the discharge capacity at the second and fifth cycles and the capacity retention rate.

As can be seen from Table 5, the second cycle discharge capacity of the lithium-sulfur secondary batteries prepared in each of the Examples was significantly higher than that of the lithium-sulfur secondary battery prepared in Comparative Example 5. In other words, it was found that even when applied to a laminate cell, the discharge capacity can be significantly improved by using ClEC as a solvent for the electrolyte compared to using EC.

The second cycle discharge capacity of the lithium-sulfur secondary batteries prepared in Examples 11 to 14, 16, and 17 was significantly higher than that of the lithium-sulfur secondary battery prepared in Comparative Example 6. In other words, it was found that even when applied to a laminate cell, the discharge capacity can be improved by using ClEC as the electrolyte solvent compared to the case of using FEC.

The second cycle discharge capacity of the lithium-sulfur secondary batteries prepared in Examples 15 and 18 is comparable to that of the lithium-sulfur secondary battery prepared in Comparative Example 6. However, in light of the results in Table 5, which show that the capacity retention rate (5th cycle discharge capacity/2nd cycle discharge capacity x 100) is greater when ClEC is used as the electrolyte solvent than when FEC is used, it can be said that ClEC is useful as an electrolyte solvent.

The above results show that the electrolyte for the lithium-sulfur secondary battery according to one embodiment of the present invention can prevent sulfur from eluting from the activated carbon even when the activated carbon has a high ratio of mesopores among its pores. It has also become clear that this can impart excellent charge/discharge characteristics and cycle characteristics to the lithium-sulfur secondary battery.

The lithium-sulfur secondary battery electrolyte and lithium-sulfur secondary battery according to one embodiment of the present invention can be used in a wide range of fields, including mobile terminal devices such as mobile phones and personal digital assistants (PDAs), small electronic devices such as notebook computers, video cameras and digital cameras, home appliances, mobile devices (vehicles) such as electric bicycles, electric automobiles (electric vehicles), hybrid automobiles and trains, power generation devices such as thermal power generation, wind power generation, hydroelectric power generation, nuclear power generation, geothermal power generation and solar power generation, and natural energy storage systems.

Claims (10)

An electrolyte for a lithium-sulfur secondary battery,
The lithium-sulfur secondary battery comprises a composite material of porous carbon and sulfur as a positive electrode material,
The porous carbon includes pores having a pore size of 2 nm or more,
a solvent for the electrolytic solution contains ethylene carbonate having a chloro group, and the content of the ethylene carbonate having a chloro group in the solvent is 20% by weight or more and 100% by weight or less;
The solvent of the electrolyte may contain a co-solvent other than ethylene carbonate.
Electrolyte for lithium-sulfur secondary batteries. 2. The electrolyte for a lithium-sulfur secondary battery according to claim 1, wherein the total volume of the pores in the porous carbon is 0.4 cm ³ /g or more and 2.0 cm ³ /g or less. 3. The electrolyte for a lithium-sulfur secondary battery according to claim 2, wherein the total volume of the pores in the porous carbon is 0.9 cm ³ /g or more and 2.0 cm ³ /g or less. The electrolyte for a lithium-sulfur secondary battery according to any one of claims 1 to 3, wherein the weight of sulfur supported in the pores of the porous carbon is 30% by weight or more and 90% by weight or less of the total weight of the porous carbon and the sulfur. The electrolyte for a lithium-sulfur secondary battery according to claim 4, wherein the weight of the sulfur supported in the pores of the porous carbon is 50% by weight or more and 90% by weight or less of the total weight of the porous carbon and the sulfur. The electrolyte for a lithium-sulfur secondary battery according to any one of claims 1 to 5, wherein the ratio of the amount of pores having a pore diameter of 2 nm or more to the total amount of pores possessed by the porous carbon is 5% or more and 100% or less. The electrolyte for a lithium-sulfur secondary battery according to claim 6, wherein the proportion of pores having a pore diameter of 2 nm or more is 40% or more and 100% or less. The electrolyte for a lithium-sulfur secondary battery according to any one of claims 1 to 7 , wherein the content of ethylene carbonate having a chloro group in the solvent is 30% by weight or more and 100% by weight or less. The electrolyte for a lithium-sulfur secondary battery according to any one of claims 1 to 8 , wherein the solvent contains at least one selected from the group consisting of vinylene carbonate, hydrofluoroether, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and fluoroethylene carbonate. The present invention relates to an electrolyte for a lithium- sulfur secondary battery,
A lithium-sulfur secondary battery using a composite material of porous carbon and sulfur as a positive electrode material, the porous carbon containing pores having a pore size of 2 nm or more.

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