TECHNICAL FIELD

This application claims the benefits of Korean Patent Application No. 10-2017-0065508, filed on May 26, 2017 and Korean Patent Application No. 10-2017-0166320, filed on Dec. 6, 2017 with the Korean Intellectual Property Office, the disclosure of which are herein incorporated by reference in their entirety.

The present invention relates to an electrolyte for a lithium-sulfur battery and the lithium-sulfur battery comprising the same.

BACKGROUND ART

Recently, electronic devices and communication devices are rapidly becoming smaller, lighter and higher performance and the need for electric vehicles has been greatly increased in relation to environmental problems. According to this, there is a growing demand for performance improvements in secondary batteries used as energy sources for these products. For the secondary battery satisfying the demand, many studies have been conducted on a lithium-sulfur battery using sulfur-based materials as a positive electrode active material.

The lithium-sulfur battery is a secondary battery that uses a sulfur-based compound including a sulfur-sulfur bond as a positive electrode active material and uses a carbon-based material, which is capable of intercalating or deintercalating alkali metal such as lithium or metal ion such as lithium ion, as a negative electrode active material.

In particular, the theoretical discharging capacity of the lithium-sulfur battery is 1,675 mAh/g and its theoretical energy density is 2,600 Wh/kg. Since the theoretical energy density of the lithium-sulfur battery is about 5 times higher than that of the lithium ion battery (about 570 Wh/kg) currently under study, the lithium-sulfur battery is a battery capable of high capacity, high energy density and long life. In addition, since sulfur, which is the main material of positive electrode active material, has a low atomic weight, is rich in resources, is easy to supply, is cheap, is non-toxic and is an environmentally friendly substance, the lithium-sulfur battery has received attention as a source of energy for medium to large-sized devices such as electric vehicles as well as portable electronic devices.

Specifically, an oxidation reaction of lithium occurs at the negative electrode of the lithium-sulfur battery and a reduction reaction of sulfur occurs at the positive electrode. Sulfur before discharging has an cyclic S8structure. During the reduction reaction (discharging), as the sulfur-sulfur bond is broken, the oxidation number of sulfur decreases, and during the oxidation reaction (charging), as the sulfur-sulfur bond is formed again, the oxidation number of S increases. Electrical energy is stored and generated using this oxidation-reaction reaction. During this electrochemical reaction, the sulfur is converted from the S8of cyclic structure to lithium polysulfide (Li2Sx, x=8, 6, 4, 2) of linear structure by the reduction reaction. When the lithium polysulfide is completely reduced, the lithium sulfide (Li2S) is finally produced. By the process of reducing to each lithium polysulfide, the discharging behavior of the lithium-sulfur battery exhibits a step-wise discharging voltage unlike lithium ion battery.

Among lithium polysulfides such as Li2S8, Li2S6, Li2S4and Li2S2which are the intermediate product of the electrochemical reaction in the lithium-sulfur battery, the lithium polysulfide (Li2Sx, usually x>4) which has a high oxidation number of sulfur is a substance with a strong polarity and is easily dissolved in an electrolyte containing hydrophilic organic solvent. The polysulfide dissolved in the electrolyte diffuses far away from the positive electrode by the concentration difference. The lithium polysulfide thus eluted from the positive electrode is out of the electrochemical reaction zone of the positive electrode, and thus the step-wise reduction to lithium sulfide (Li2S) is impossible. That is, since the lithium polysulfide which is far away from the positive electrode and exists in a dissolved state in the electrolyte cannot participate in the charging and discharging reaction of the battery, sulfur used as the positive electrode active material is lost and the loss of sulfur is a major factor in reducing capacity and life time of the lithium-sulfur battery.

Furthermore, in addition to being floated on or precipitated in the electrolyte, since the eluted lithium polysulfide reacts directly with the lithium metal as the negative electrode and lithium sulfide is stuck the surface of the lithium metal, the reaction activity is lowered and the potential characteristics are deteriorated, thus causing the problem of corrosion of the negative electrode.

In order to minimize the elution of the lithium polysulfide, various methods are being studied, which include a method of adding an additive having a property of adsorbing sulfur to the positive electrode complex or the electrolyte, a method of surface-treating the surface of the positive electrode active material with a substance containing a specific functional group, a method of using a composite containing sulfur in a carbon material or a metal oxide as a positive electrode active material or the like.

As an example, Korean Patent Application Publication No. 2015-0032670 discloses that by adding a nitrogen-containing additive, a sulfur-containing additive or an organic peroxide to the electrolyte, the elution of the lithium polysulfide can be suppressed and thus the problem of performance reduction of the battery can be improved.

In addition, Korean Patent Application Publication No. 2016-0046775 discloses that a positive electrode coating layer made of an amphipathic polymer is provided on the surface of the positive electrode active part including the sulfur-carbon composite to prevent the lithium polysulfide from eluting into the electrolyte, thereby improving the capacity and cycle characteristics of the battery.

These patents inhibit the elution of the lithium polysulfide to some extent through the additive or the coating layer, but the effect is not sufficient. In addition, there are disadvantages that when using the additives, a problem of deterioration of electrical conductivity or promotion of side reactions of the battery occurs, and when forming the coating layer, the sulfur is lost in the surface treatment process and it takes much time and cost. Therefore, there is further need to develop a lithium-sulfur battery which can effectively suppress the reduction of the capacity and life time of the lithium-sulfur battery due to the elution of lithium polysulfide in the lithium-sulfur battery.

PRIOR ART LITERATURE

Patent Literature

Korean Patent Application Publication No. 2015-0032670 (2015 Mar. 27), ELECTROCHEMICAL CELLS COMPRISING ELECTROLYTE ADDITIVES AND IONOMER ARTICLES, AND METHODS FOR MAKING AND USING THE SAME

Korean Patent Application Publication No. 2016-0046775 (2016 Apr. 29), CATHODE FOR LITHIUM-SULFUR BATTERY AND METHOD OF PREPARING THE SAME

DISCLOSURE

Technical Problem

As a result of various studies to solve the above problem, the inventors of the present invention have identified that when the electrolyte for the lithium-sulfur battery includes an alkali metal ion-containing polymer as an additive, the conductivity of the lithium ion is improved to improve the stability, performance and life of the battery, thereby completing the present invention.

Accordingly, it is an aspect of the present invention to provide an electrolyte for the lithium-sulfur battery having excellent performance and life characteristics.

In addition, another aspect of the present invention is to provide the lithium-sulfur battery comprising the electrolyte.

Technical Solution

According to an aspect of the present invention, the present invention provides an electrolyte for the lithium-sulfur battery, comprising a lithium salt, an organic solvent and an additive, wherein the additive includes an alkali metal ion-containing polymer (also referred to as an alkali metal salt-type ionomer).

The alkali metal ion-containing polymer may comprise at least one ionic group selected from the group consisting of carboxylate group, sulfonate group, sulfonyl group, sulfate group, sulfinate group, phosphate group and phosphonate group.

The alkali metal ion-containing polymer may be represented by the following Formula 1:

wherein R1to R3, X, Y, M and n are as described in the specification.

The alkali metal ion-containing polymer may comprise at least one selected from the group consisting of lithium poly(acrylate), lithium poly(methacrylate), lithium poly(styrene sulfonate), lithium poly(2-acrylamido-2-methyl-1-propanesulfonate) and lithium poly(vinyl sulfonate).

A number average molecular weight of the alkali metal ion-containing polymer may be in a range of from 1,000 to 10,000.

The alkali metal ion-containing polymer may be included in the electrolyte in an amount of from 0.1 to 0.5% by weight, based on a total electrolyte for the lithium-sulfur battery.

According to another aspect of the present invention, the present invention also provides a lithium-sulfur battery comprising the electrolyte.

Advantageous Effects

When the electrolyte for the lithium-sulfur battery according to the present invention includes an ionomer including an alkali metal salt as an additive, the migration characteristics of lithium ions can be improved to improve the capacity and life characteristics of the lithium-sulfur battery.

