WO2018084449A2 - Sulfur-carbon composite and lithium-sulfur battery comprising same - Google Patents

Abstract

The present invention relates to a sulfur-carbon composite and a lithium-sulfur battery comprising the same and, more particularly, to a sulfur-carbon composite and a lithium-sulfur battery comprising the same, the composite comprising: a porous carbon material; and sulfur at at least a part of the surface and the inside of the porous carbon material, wherein the porous carbon material includes ion conductive polymer-containing coating layers on the inner and the outer surface thereof. The present invention includes the ion conductive polymer coating layers on the surfaces of the porous carbon material to improve the conduction of lithium ion to the cathode, thereby enhancing the capacity and lifespan characteristics of the lithium-sulfur battery.

Description

Sulfur-carbon composites and lithium-sulfur batteries comprising the same

This application claims the benefit of priority based on Korean Patent Application No. 10-2016-0145193 filed on November 2, 2016 and Korean Patent Application No. 10-2017-0132039 filed on October 12, 2017. All content disclosed in the literature is included as part of this specification.

The present invention relates to a sulfur-carbon composite having improved ion conducting properties and a lithium-sulfur battery including the same.

Recently, with the rapid development of electronics and electric vehicles, the demand for secondary batteries is increasing. In particular, according to the trend toward miniaturization and light weight of portable electronic devices, there is a growing demand for a secondary battery having a high energy density capable of responding thereto.

The lithium-sulfur battery of the secondary battery uses a sulfur-based compound having a sulfur-sulfur bond as a positive electrode active material, and a carbon-based material or an alloy with lithium, in which insertion and deintercalation of alkali metals such as lithium or metal ions such as lithium ions occur. It is a secondary battery using silicon, tin, etc. to form as a negative electrode active material. Specifically, the electrical energy is stored by using an oxidation-reduction reaction in which the sulfur-sulfur bond breaks during discharge, which is a reduction reaction, and the oxidation number of sulfur decreases, and the sulfur-sulfur bond is formed again when the oxidation-count of sulfur increases during charging. And generate

In particular, sulfur used as a positive electrode active material in a lithium-sulfur battery has a theoretical energy density of 1,675 mAh / g, and has a theoretical energy density of about five times higher than that of a conventional positive electrode active material used in a lithium secondary battery, thereby resulting in high power and high energy density. Is a battery capable of expression. In addition, sulfur is attracting attention as an energy source for medium and large devices such as electric vehicles as well as portable electronic devices because of its low cost, rich reserves, and easy supply and environmental friendliness.

However, since sulfur has an electrical conductivity of 5 × 10 ⁻³⁰ S / cm and has no electrical conductivity, sulfur has a problem in that electrons generated by an electrochemical reaction are difficult to move. Thus it is complexed with an electrically conductive material such as carbon that can provide an electrochemical reaction site is used as a sulfur-carbon composite.

Conventional sulfur-carbon composites have a problem in that sulfur is leaked into the electrolyte during the oxidation-reduction reaction to degrade battery life, and lithium polysulfide, which is a reducing substance of sulfur, is eluted and no longer participates in the electrochemical reaction. In addition, there is a problem in that the capacity decreases when sulfur in the electrode is excessively loaded. Accordingly, various techniques have been proposed to improve the mixing quality of the conductive material and sulfur.

For example, Korean Patent Laid-Open No. 2016-0037084 discloses that by coating graphene on a carbon nanotube aggregate including sulfur, the conductivity and sulfur loading amount of the sulfur-carbon nanotube composite can be increased.

However, in order for sulfur to exhibit sufficient performance in lithium-sulfur batteries, lithium ion conductivity is required in addition to electrical conductivity. Since lithium ion conductivity is imparted through the electrolyte solution, when the sulfur-carbon composite itself exhibits lithium ion conductivity, lithium ion conductivity by the electrolyte solution is further improved and battery performance can be improved.

Accordingly, the Republic of Korea Patent Publication No. 2016-0046775 has a positive electrode coating layer made of an amphiphilic polymer on the surface of the positive electrode active part including a sulfur-carbon composite to facilitate the migration of lithium ions and to prevent the dissolution of polysulfide, thereby It is disclosed that the cycle characteristics can be improved.

The sulfur-carbon composites proposed in these patents have improved the electrical conductivity to some extent by changing the manufacturing method or composition, but the effect is not sufficient in terms of lithium ion conductivity. Therefore, further studies on sulfur-carbon composites having excellent lithium ion conductivity are needed.

[Preceding technical literature]

[Patent Documents]

Republic of Korea Patent Publication No. 2016-0037084 (2016.04.05), sulfur-carbon nanotube composite, a method for manufacturing the same, a cathode active material for a lithium-sulfur battery including the same and a lithium-sulfur battery including the same

Republic of Korea Patent Publication No. 2016-0046775 (2016.04.29), a positive electrode for a lithium-sulfur battery and a manufacturing method thereof

Accordingly, the present inventors have conducted various studies to solve the above problems. As a result, by introducing a coating layer containing an ion conductive polymer on the surface of the porous carbon material, lithium ions are more easily moved to the inside of the composite, thereby improving lithium ion conductivity. It was confirmed.

Accordingly, an object of the present invention is to provide a sulfur-carbon composite having an improved mobility of lithium ions by forming a coating layer containing an ion conductive polymer between the porous carbon material and sulfur.

Another object of the present invention is to provide a positive electrode including the sulfur-carbon composite and a lithium-sulfur battery including the same.

In order to achieve the above object, the present invention is a porous carbon material; And a sulfur-carbon composite including sulfur in at least a portion of the inside and the surface of the porous carbon material, wherein the inside and the outer surface of the porous carbon material include a coating layer including an ion conductive polymer. do.

The ion conductive polymers include polyethylene oxide, polypropylene oxide, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyolefins, polyphosphazenes, polyacrylonitrile polymethylmethacrylates, polyvinylchlorides and polysiloxanes. Characterized in that it comprises one or more selected from the group consisting of.

The ion conductive polymer is characterized in that it comprises 0.1 to 50 parts by weight based on 100 parts by weight of the porous carbon material.

The present invention also provides a sulfur-carbon composite comprising a carbon material and sulfur whose surface is coated with an ion conductive polymer.

The carbon material is at least one member selected from the group consisting of natural graphite, artificial graphite and expanded graphite.

The sulfur-carbon composite is characterized in that it further comprises a conductive material.

The present invention also provides a cathode for a lithium-sulfur battery comprising the sulfur-carbon composite.

In addition, the present invention provides a lithium-sulfur battery including the positive electrode.

The sulfur-carbon composite according to the present invention includes a coating layer containing an ion conductive polymer on the surface of the porous carbon material to effectively transfer lithium ions to the inside of the composite, thereby improving ion conductivity and reactivity with the positive electrode active material, thereby improving the lithium-sulfur battery. Capacity and lifespan characteristics can be improved.

1 is a graph showing charge and discharge characteristics of the lithium-sulfur battery coin cell manufactured by applying Example 1 and Comparative Example 1 of the present invention.

Hereinafter, the present invention will be described in more detail.

The terms or words used in this specification and claims are not to be construed as being limited to their ordinary or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best describe their invention. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that the present invention.

As used herein, the term “composite” refers to a substance in which two or more materials are combined to form physically and chemically different phases and express more effective functions.

Lithium-sulfur batteries use sulfur as a positive electrode active material and lithium metal as a negative electrode active material. During discharge of the lithium-sulfur battery, an oxidation reaction of lithium occurs at the negative electrode and a sulfur reduction reaction occurs at the positive electrode. In this case, the reduced sulfur is combined with lithium ions transferred from the negative electrode to be converted into lithium polysulfide, followed by a reaction to finally form lithium sulfide.

Lithium-sulfur batteries have a much higher theoretical energy density than conventional lithium secondary batteries, and sulfur, which is used as a positive electrode active material, is spotlighted as a next-generation battery due to its rich resources and low price, which can lower the manufacturing cost of the battery. have.

Despite these advantages, due to the low electrical conductivity and lithium ion conductivity of sulfur, the positive electrode active material, it is difficult to realize all theoretical energy density in actual driving.

In order to improve the electrical conductivity of sulfur, a method of forming a composite with a conductive material such as carbon or a polymer, or coating is used. Among various methods, sulfur-carbon composites are most commonly used as positive electrode active materials because they are effective in improving the electrical conductivity of a positive electrode, but are not yet sufficient in terms of charge and discharge capacity and efficiency. The capacity and efficiency of a lithium-sulfur battery may vary depending on the amount of lithium ions delivered to the positive electrode. Therefore, facilitating the transfer of lithium ions into the sulfur-carbon composite is important for high capacity and high efficiency of the battery.

Accordingly, in the present invention, a carbon having a coating layer made of an ion conductive polymer to impart lithium ion conductivity to the sulfur-carbon composite to secure reactivity between the sulfur-carbon composite and the electrolyte and to improve the capacity and efficiency characteristics of the lithium-sulfur battery It provides a sulfur-carbon composite comprising a material.

Sulfur-carbon composite according to an embodiment of the present invention is a porous carbon material; And a sulfur-carbon composite including sulfur in at least some of the inner and outer surfaces of the porous carbon material, wherein the inner and outer surfaces of the porous carbon material include a coating layer comprising an ion conductive polymer.

According to one embodiment of the invention, the porous carbon material comprises a coating layer comprising an ion conductive polymer on the inner and outer surfaces. The ion conductive polymer has a high ion conductivity and a high reactivity with sulfur as a cathode active material by securing a migration path of lithium ions to the inside of the sulfur-carbon composite, that is, into the pores of the porous carbon material. The characteristic can be improved at the same time.

The ion conductive polymer is polyethylene oxide (poly (ethylene oxide); PEO), polypropylene oxide (poly (propylene oxide); PPO), polyvinylidene fluoride (poly (vinylidene fluoride); PVDF), polyvinylidene fluoride Hexafluoropropylene (poly (vinylidene fluoride-co-hexafluoropropylene); PVdF-HFP), polyolefin (poly) (PO), polyphosphazene, polyacrylonitrile (poly (acrylonitrile); PAN) , Polymethyl methacrylate (poly (methylmethacrylate); PMMA), polyvinyl chloride (poly (vinyl chloride); PVC) and may include one or more selected from the group consisting of polysiloxane (polysiloxane). Preferably, the ion conductive polymer may include at least one member selected from the group consisting of polyethylene oxide, polypropylene oxide, polyvinylidene fluoride, and polyvinylidene fluoride-hexafluoropropylene.

The weight average molecular weight of the ion conductive polymer is not particularly limited and may be used without limitation as long as it is commonly used in the art. For example, the weight average molecular weight of the ion conductive polymer may be 20,000 to 50,000,000 g / mol.

The ion conductive polymer may be used in an amount of 0.1 to 50 parts by weight, preferably 1 to 25 parts by weight, based on 100 parts by weight of the porous carbon material. If the ion conductive polymer is less than the above range, the coating layer may not be sufficiently formed on the porous carbon material, and thus, the desired ion conductivity improvement effect may not be obtained. It may adversely affect.

The porous carbon material provides a skeleton in which sulfur, which is a cathode active material, can be uniformly and stably immobilized, and complements the electrical conductivity of sulfur so that the electrochemical reaction can proceed smoothly.

The porous carbon material may generally be prepared by carbonizing precursors of various carbon materials. The porous carbon material may include non-uniform pores therein and the average diameter of the pores may range from 1 to 200 nm, and the porosity or porosity may range from 10 to 90% of the total volume of the porous. If the average diameter of the pores is less than the above range, the pore size is only molecular level, so that impregnation of sulfur is impossible. On the contrary, if the average diameter exceeds the above range, the mechanical strength of the porous carbon is weakened, and thus it is preferable to apply to the electrode manufacturing process. Not.

The form of the porous carbon material may be used without limitation as long as it is conventionally used in lithium-sulfur batteries in spherical shape, rod shape, needle shape, plate shape, tubular shape or bulk shape.

The porous carbon material may have any porous structure or specific surface area as long as it is commonly used in the art. For example, the porous carbon material includes graphite; Graphene; Carbon blacks such as denka black, acetylene black, ketjen black, channel black, furnace black, lamp black and summer black; Carbon nanotubes (CNT) such as single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT); Carbon fibers such as graphite nanofibers (GNF), carbon nanofibers (CNF), and activated carbon fibers (ACF); And it may be one or more selected from the group consisting of activated carbon, but is not limited thereto.

The sulfur is from the group consisting of inorganic sulfur (S ₈ ), Li ₂ S _n (n ≥ 1), an organic sulfur compound and a carbon-sulfur polymer [(C ₂ S _x ) _n , x = 2.5 to 50, n ≥ 2] It may be one or more selected. Preferably inorganic sulfur (S ₈ ) can be used.

In the sulfur-carbon composite according to the present invention, the weight ratio of the porous carbon material and the sulfur including the coating layer described above may be 1: 9 to 5: 5, preferably 2: 8 to 3: 7. If less than the weight ratio range, as the content of the porous carbonaceous material increases, the amount of binder added required in preparing the positive electrode slurry increases. The increase in the amount of binder added may eventually increase the sheet resistance of the electrode and serve as an insulator that prevents electron pass, thereby degrading cell performance. On the contrary, when the weight ratio exceeds the range, sulfur may be agglomerated among them, and it may be difficult to directly participate in an electrode reaction because it is difficult to receive electrons.

In addition, in the present invention, the sulfur-carbon composite may include a sulfur: porous carbon material: ion conductive polymer in a weight ratio of 50 to 90: 6 to 45: 0.01 to 15. When the composition ratio in the sulfur-carbon composite falls within the above range, the lithium ion migration characteristics and reactivity improvement effects may be secured.

In addition, in the sulfur-carbon composite according to one embodiment of the present invention, the sulfur is located on the surface as well as inside the pores of the porous carbon material, wherein less than 100% of the entire outer surface of the porous carbon material, preferably 1 to 95% More preferably in the 60-90% region. When the sulfur is in the above range on the surface of the porous carbon material can exhibit the maximum effect in terms of the electron transfer area and the wettability of the electrolyte. Specifically, since the sulfur is thinly and evenly impregnated on the surface of the porous carbon material in the above range region, the electron transfer contact area may be increased during the charge and discharge process. If the sulfur is located at 100% of the surface of the porous carbon material, the porous carbon material is completely covered with sulfur, so that the wettability of the electrolyte is inferior and the contact with the conductive material included in the electrode is poor, thereby preventing the electron transfer and thus participating in the reaction. It becomes impossible.

In addition, the sulfur-carbon composite according to another embodiment of the present invention includes a carbon material and sulfur whose surface is coated with an ion conductive polymer.

According to another embodiment of the present invention, in a sulfur-carbon composite in which carbon material and sulfur are simply mixed, lithium ions are transferred to the inside of the composite by introducing a coating layer containing an ion conductive polymer on the inner and outer surfaces of the carbon material. It can promote smoothness and can improve battery performance and lifespan more.

The ion conductive polymer and sulfur are as described in the embodiment of the present invention.

The carbon material serves to impart electrical conductivity to sulfur and promote uniform distribution. The carbon material may include at least one selected from the group consisting of natural graphite, artificial graphite, and expanded graphite. Preferably, the carbon material may be expanded graphite.

The expanded graphite is usually prepared from a graphite or partially graphite starting material selected from the group consisting of natural graphite, pyrolytic graphite, kish graphite, compressed expanded graphite, partially oxidized graphite and graphite fibers. The starting material expands after reaction with the intercal material to provide an intercalation compound. The insert may be halogen, SO ₃ , NO ₃ , alkali metal or other compound. Preferably, the interlayer compound is obtained by treating the starting material, preferably graphite, with an oxidizing agent with a strong acid such as cyclohexane or concentrated nitric acid. In this case, an organic acid such as formic acid or acetic acid may be used instead of the strong acid. The intercalation compound prepared through reaction with the intercalation material, ie intercalated graphite, is washed and / or dried. The intercalating compound can be used directly or purchased commercially available products. For example, the insertion compound can be obtained from Enges Naturgraphit GmbH, Germany, LUH GmbH, Germany, and Technograft GmbH, Germany. Can be.

When the intercalation compound is rapidly heated to 200 to about 1000 ° C., a reaction occurs by thermal decomposition of the intercalation material such as N- or S-compound, and the crystal layer of graphite is exfoliated to release the gas decomposition product. The heat treatment can be carried out, for example, through an expansion oven, a plasma oven or a microwave. The volume of the expanded graphite can reach up to 280 times the volume of the starting material. In this case, the volume change may vary depending on the particle size of the graphite used, the type of starting material (for example, natural graphite or artificial graphite), heating form, speed, and the like.

The production of the expanded graphite is known to those skilled in the art, and for example, the production method disclosed in EP 1,491,497 can be used.

The sulfur-carbon composite may further include a conductive material.

The conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, the conductive material may be carbon black such as super-P, denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, or summer black; Carbon derivatives such as carbon nanotubes and fullerenes; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Or conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole may be used alone or in combination.

The present invention also provides a method for producing the sulfur-carbon composite.

The method for producing the sulfur-carbon composite according to the present invention is not particularly limited and may be prepared by methods commonly known in the art.

Method for producing a sulfur-carbon composite according to an embodiment of the present invention comprises the steps of forming a coating layer containing an ion conductive polymer on the inner and outer surfaces of the porous carbon material; Mixing the porous conductive material, sulfur, and an organic solvent in which the coating layer is formed; And melting the sulfur by heating the mixture to be supported on at least a portion of the inner and outer surfaces of the porous carbon material to form a sulfur-carbon composite.

According to another aspect of the present invention, a method of manufacturing a sulfur-carbon composite includes forming a coating layer including an ion conductive polymer on a surface of a carbon material, and mixing the carbon material and sulfur on which the coating layer is formed to form a sulfur-carbon composite. It may include the step.

Forming the coating layer of the ion conductive polymer on the porous conductive material or the carbon material may be carried out by adding a porous conductive material or carbon material to the solution in which the ion conductive polymer is dissolved, stirring, filtering, and drying. However, any method known in the art may be used.

The mixing is to increase the degree of mixing between the above-described materials can be carried out using a stirring device commonly used in the art. In this case, the mixing time and speed may also be selectively adjusted according to the content and conditions of the raw materials.

The heating temperature may be any temperature at which sulfur is melted, and may be specifically 120 to 180 ° C., preferably 150 to 180 ° C. When the heating temperature is less than 120 ℃ sulfur may not be sufficiently melted structure of the sulfur-carbon composite is not properly formed, if it exceeds 180 ℃ coated polymer does not remain difficult to obtain the desired effect. In addition, the heating time may be adjusted according to the content of sulfur.

Through the above-described manufacturing method, a sulfur-carbon composite in which a coating layer including an ion conductive polymer is formed between the porous carbon material or the carbon material and the sulfur may be manufactured, and the sulfur-carbon composite may be formed of the porous carbon material or the carbon material. By coating the surface with an ion conductive polymer, lithium ions can be easily moved into the composite. This increases the reactivity with the electrolyte when introduced into the battery as a positive electrode active material, which may exhibit an effect of improving the capacity and life of the battery.

The present invention also provides a cathode for a lithium-sulfur battery comprising the sulfur-carbon composite. The sulfur-carbon composite may be included as a cathode active material in a cathode.

The positive electrode may further include one or more additives selected from transition metal elements, group IIIA elements, group IVA elements, sulfur compounds of these elements, and alloys of these elements and sulfur, in addition to the positive electrode active material.

The transition metal element may be Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au or Hg and the like, the Group IIIA element may include Al, Ga, In, Ti, and the like, and the Group IVA element may include Ge, Sn, Pb and the like.

The positive electrode may further include a positive electrode active material, or optionally an additive, an electrically conductive conductive material for allowing electrons to move smoothly in the positive electrode, and a binder for attaching the positive electrode active material to the current collector.

The conductive material is not particularly limited as long as it has conductivity without causing chemical changes to the battery, but is not limited to super-P, denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, Carbon black such as summer black and carbon black; Carbon derivatives such as carbon nanotubes and fullerenes; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Or conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole may be used alone or in combination.

The conductive material may be added in an amount of 0.01 to 30 wt% based on the total weight of the mixture including the cathode active material.

The binder has a function of maintaining the positive electrode active material in the positive electrode current collector and organically connecting the positive electrode active materials, such as polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, Regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene rubber (EPDM), sulfonated-EPDM, styrene-butadiene rubber, fluorine rubber, various of these And copolymers.

The binder may be added in an amount of 0.5 to 30 wt% based on the total weight of the mixture including the cathode active material. If the content of the binder is less than 0.5% by weight, the physical properties of the positive electrode may be degraded to cause the active material and the conductive material to fall off. If the content is more than 30% by weight, the ratio of the active material and the conductive material to the positive electrode is relatively decreased, thereby reducing battery capacity. can do.

Looking at the method of manufacturing the positive electrode of the present invention in detail, first, the binder is dissolved in a solvent for preparing a slurry, and then the conductive material is dispersed. As a solvent for preparing the slurry, a positive electrode active material, a binder, and a conductive material can be uniformly dispersed, and it is preferable to use one that is easily evaporated. Typically, acetonitrile, methanol, ethanol, tetrahydrofuran, water, iso Propyl alcohol and the like. Next, the positive electrode active material, or optionally together with an additive, is uniformly dispersed again in a solvent in which the conductive material is dispersed to prepare a positive electrode slurry. The amount of solvent, positive electrode active material, or optionally additives included in the slurry does not have a particularly important meaning in the present application, and it is sufficient only to have an appropriate viscosity to facilitate the coating of the slurry.

The slurry thus prepared is applied to a current collector and vacuum dried to form a positive electrode. The slurry may be coated on the current collector in an appropriate thickness depending on the viscosity of the slurry and the thickness of the positive electrode to be formed.

The current collector can be generally made to a thickness of 3 to 500 ㎛, and is not particularly limited as long as it has a high conductivity without causing chemical changes in the battery. Specifically, a conductive material such as stainless steel, aluminum, copper, titanium, or the like may be used, and more specifically, a carbon-coated aluminum current collector may be used. The use of an aluminum substrate coated with carbon has an advantage in that the adhesion to the active material is excellent, the contact resistance is low, and the corrosion of polysulfide of aluminum is prevented, compared with the non-carbon coated aluminum substrate. In addition, the current collector may be in various forms such as a film, sheet, foil, net, porous body, foam or nonwoven fabric.

In addition, the present invention is a positive electrode comprising a sulfur-carbon composite described above; cathode; And it provides a lithium-sulfur battery comprising an electrolyte interposed between the positive electrode and the negative electrode.

The anode is according to the present invention and follows the foregoing.

The negative electrode may include a current collector and a negative electrode active material layer formed on one or both surfaces thereof. The negative electrode active material may be a material capable of reversibly intercalating or deintercalating lithium ions, a material capable of reacting with lithium ions to reversibly form a lithium-containing compound, lithium metal or a lithium alloy.

Materials capable of reversibly intercalating or deintercalating the lithium ions may be, for example, crystalline carbon, amorphous carbon or mixtures thereof.

The material capable of reacting with the lithium ions to reversibly form a lithium-containing compound may be, for example, tin oxide, titanium nitrate, or silicon.

The lithium alloy may be, for example, an alloy of lithium with a metal selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al, and Sn.

The separator may be additionally included between the positive electrode and the negative electrode. The separator may be made of a porous non-conductive or insulating material to separate or insulate the positive electrode and the negative electrode from each other, and to enable lithium ion transport between the positive electrode and the negative electrode. The separator may be an independent member such as a film, or may be a coating layer added to the anode and / or the cathode.

The material constituting the separator includes, for example, polyolefins such as polyethylene and polypropylene, glass fiber filter paper, and ceramic materials, but is not limited thereto, and the thickness thereof is about 5 to about 50 μm, preferably about 5 to about 25 May be μm.

The electrolyte is located between the positive electrode and the negative electrode and includes a lithium salt and an electrolyte solvent.

The concentration of the lithium salt is 0.2 to 2 M, depending on several factors such as the exact composition of the electrolyte solvent mixture, the solubility of the salt, the conductivity of the dissolved salt, the charging and discharging conditions of the cell, the operating temperature and other factors known in the lithium battery art, Specifically, it may be 0.6 to 2 M, more specifically 0.7 to 1.7 M. When the concentration of the lithium salt is less than 0.2 M, the conductivity of the electrolyte may be lowered, and thus the performance of the electrolyte may be lowered. When the lithium salt is used, the viscosity of the electrolyte may be increased to reduce the mobility of lithium ions.

The lithium salt may be used without limitation as long as it is conventionally used in a lithium-sulfur battery electrolyte. For example, LiSCN, LiBr, LiI, LiPF ₆ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiSO ₃ CF ₃ , LiCl, LiClO ₄ , LiSO ₃ CH ₃ , LiB (Ph) ₄ , LiC (SO ₂ CF ₃ ) _{3 , LiN (SO 2 CF 3)} 2, LiCF 3 CO 2, LiAsF 6, LiSbF 6, LiAlCl 4, may be included are one or more from LiFSI, chloro group consisting of borane lithium, lower aliphatic carboxylic acid lithium or the like.

The electrolyte solvent is a non-aqueous organic solvent, a single solvent may be used, or two or more mixed organic solvents may be used. When using two or more mixed organic solvents, it is preferable to use one or more solvents selected from two or more groups of a weak polar solvent group, a strong polar solvent group, and a lithium metal protective solvent group.

The weak polar solvent is defined as a solvent having a dielectric constant of less than 15 which is capable of dissolving elemental sulfur among aryl compounds, bicyclic ethers, and acyclic carbonates, and strong polar solvents include acyclic carbonates, sulfoxide compounds, and lactone compounds. In the ketone compound, ester compound, sulfate compound, sulfite compound, a dielectric constant capable of dissolving lithium polysulfide is defined as greater than 15, and the lithium metal protective solvent is a saturated ether compound, unsaturated ether compound, N, It is defined as a solvent having a charge / discharge cycle efficiency of 50% or more that forms a stable solid interface (SEI) on a lithium metal such as a heterocyclic compound including O, S, or a combination thereof.

Specific examples of the weak polar solvent include xylene, dimethoxyethane, 2-methyltetrahydrofuran, diethyl carbonate, dimethyl carbonate, toluene, dimethyl ether, diethyl ether, diglyme or tetraglyme. .

Specific examples of the strong polar solvent include hexamethyl phosphoric triamide, γ-butyrolactone, acetonitrile, ethylene carbonate, propylene carbonate, N-methylpyrrolidone, 3-methyl-2-oxazoli Don, dimethyl formamide, sulfolane, dimethyl acetamide, dimethyl sulfoxide, dimethyl sulfate, ethylene glycol diacetate, dimethyl sulfite, or ethylene glycol sulfite.

Specific examples of the lithium protective solvent include tetrahydrofuran, ethylene oxide, dioxolane, 3,5-dimethylisoxazole, furan, 2-methylfuran, 1,4-oxane or 4-methyldioxolane.

The electrolyte may include one or more selected from the group consisting of a liquid electrolyte, a gel polymer electrolyte and a solid polymer electrolyte. It may be preferably an electrolyte in a liquid state.

In addition, the present invention provides a battery module including the lithium-sulfur battery as a unit cell.

The battery module may be used as a power source for medium and large devices requiring high temperature stability, long cycle characteristics, and high capacity characteristics.

Examples of the medium-to-large device include a power tool that is driven by an electric motor; Electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like; Electric motorcycles including electric bicycles (E-bikes) and electric scooters (E-scooters); Electric golf carts; Power storage systems and the like, but is not limited thereto.

Hereinafter, preferred examples are provided to aid the understanding of the present invention, but the following examples are merely for exemplifying the present invention, and it will be apparent to those skilled in the art that various changes and modifications can be made within the scope and spirit of the present invention. It is natural that such variations and modifications fall within the scope of the appended claims.

Example  And Comparative example

Example 1

1 g of carbon nanotubes were added to a solution containing polyethylene oxide, stirred at 80 ° C. for 12 hours, washed, and dried to form a polyethylene oxide coating layer on the surface of the porous carbon material. At this time, the weight ratio of polyethylene oxide: carbon of the porous carbon material was 2: 8. In this case, carbon refers to the total weight of carbon nanotubes.

1.25 g of the polyethylene oxide-coated porous carbon material and 3 g of sulfur were evenly mixed, followed by heat treatment at 155 ° C. for 30 minutes, and sulfur: porous carbon material: polyethylene oxide = 70.6: 23.5: 5.9 Carbon composites were prepared.

The slurry was prepared in a weight ratio of sulfur-carbon composite: conductive material: binder = 90: 5: 5 using the prepared sulfur-carbon composite, and then coated on a current collector of 20 μm thick aluminum foil to prepare an electrode. In this case, carbon black was used as the conductive material, and styrene butadiene rubber and carboxymethyl cellulose were used as the binder.

Example 2

An electrode was manufactured in the same manner as in Example 1, except that expanded graphite was used instead of carbon nanotubes.

Comparative Example 1

An electrode was manufactured in the same manner as in Example 1, except that carbon nanotubes without a polyethylene oxide coating layer were used.

Comparative Example 2

The carbon nanotubes, sulfur and polyethylene glycol were simultaneously mixed in a 25: 75: 6.25 weight ratio. This mixture was carried out in the same manner as in Example 1 to prepare an electrode.

Experimental Example  One. Charging and discharging  Property evaluation

An electrode prepared in Examples and Comparative Examples was used as a positive electrode, polyethylene was used as a separator, and a lithium-sulfur battery coin cell was manufactured using a lithium foil having a thickness of 150 μm as a negative electrode. In this case, the coin cell was an electrolyte prepared by dissolving 1 M LiFSI, 1% LiNO ₃ in an organic solvent consisting of diethylene glycol dimethyl ether and 1,3-dioxolane (DECDME: DOL = 6: 4 (volume ratio)). .

The manufactured coin cell was measured for a capacity from 1.5 to 2.7 V using a charge and discharge measuring device. Specifically, charging and discharging efficiency was measured by repeating 30 cycles of charging at 0.1 C rate CC / CV and discharging at 0.1 C rate CC (CC: Constant Current, CV: Constant Voltage). The results obtained at this time are shown in Table 1 and FIG.

	Initial charge and discharge capacity (mAh / g)	Charge and discharge efficiency (%) after 30 times
Example 1	1220	100.2
Example 2	1100	99.7
Comparative Example 1	1120	99.3
Comparative Example 2	1150	99.5

Through Table 1, it can be seen that the Example is superior to the initial charge and discharge capacity and after 30 times the charge and discharge efficiency compared to the comparative example. In particular, as shown in FIG. 1, the initial capacity of Comparative Example 1 without a coating layer was 1,120 mAh / g, but when the sulfur-carbon composite of Example 1 was used as the positive electrode active material, the initial capacity was 1,220 mAh / g. It can be seen that the charge and discharge efficiency is also excellent after 30 times. In addition, compared to the initial capacity of the conventional sulfur-carbon composite containing expanded graphite without a separate ion conductive polymer coating layer is 1,000 mAh / g level, in Example 2, improved charge and discharge capacity by having an ion conductive polymer coating layer And it can be seen that the efficiency. Through this, it can be confirmed that the sulfur-carbon composite according to the present invention is effective in improving initial charge and discharge capacity and efficiency.

The sulfur-carbon composite of the present invention includes the ion conductive polymer coating layer on the surface of the porous carbon material, thereby improving lithium ion conductivity to the positive electrode, thereby enabling high capacity, high stability, and long life of the lithium-sulfur battery.

Claims (14)

1. Porous carbon material; And

   In the sulfur-carbon composite comprising sulfur in at least a portion of the inside and the surface of the porous carbon material,

   Sulfur-carbon composite, wherein the inner and outer surfaces of the porous carbon material comprises a coating layer containing an ion conductive polymer.
2. The method of claim 1,

   The ion conductive polymers include polyethylene oxide, polypropylene oxide, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyolefins, polyphosphazenes, polyacrylonitrile polymethylmethacrylates, polyvinylchlorides and polysiloxanes. Sulfur-carbon composite, characterized in that it comprises one or more selected from the group consisting of.
3. The method of claim 1,

   The ion-conducting polymer is sulfur-carbon composite, characterized in that contained in 0.1 to 50 parts by weight based on 100 parts by weight of the porous carbon material.
4. The method of claim 1,

   The porous carbon material is sulfur, carbon composites, characterized in that at least one selected from the group consisting of graphite, graphene, carbon black, carbon nanotubes, carbon fibers and activated carbon.
5. The method of claim 1,

   Sulfur-carbon composite, characterized in that the average diameter of the pores of the porous carbon material is 1 to 200 nm.
6. The method of claim 1,

   The porosity of the porous carbon material is sulfur-carbon composite, characterized in that 10 to 90% of the total volume of the porous carbon material.
7. The method of claim 1,

   The sulfur is from the group consisting of inorganic sulfur (S ₈ ), Li ₂ S _n (n ≥ 1), an organic sulfur compound and a carbon-sulfur polymer [(C ₂ S _x ) _n , x = 2.5 to 50, n ≥ 2] Sulfur-carbon composite, characterized in that at least one selected.
8. The method of claim 1,

   Sulfur-carbon composite, characterized in that the weight ratio of the porous carbon material and sulfur is 1: 9 to 5: 5.
9. Sulfur-carbon composite, characterized in that the surface comprises a carbon material and sulfur coated with an ion conductive polymer.
10. The method of claim 9,

    The ion conductive polymer may be polyethylene oxide, polypropylene oxide, polyvinylidene fluoride, copolymer of polyhexafluoropropylene-polyvinylidene fluoride, polyolefin, polyphosphazene, polyacrylonitrile polymethyl methacrylate, poly Sulfur-carbon composite, characterized in that it comprises one or more selected from the group consisting of vinyl chloride and polysiloxane.
11. The method of claim 9,

    The carbon material is sulfur-carbon composite, characterized in that at least one selected from the group consisting of natural graphite, artificial graphite and expanded graphite.
12. The method of claim 9,

    The sulfur-carbon composite is characterized in that it further comprises a conductive material.
13. A cathode for a lithium-sulfur battery comprising the sulfur-carbon composite according to claim 1.
14. A lithium-sulfur battery comprising the positive electrode according to claim 13.

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