KR20200055948A - Thermally expanded-reduced graphene oxide, manufacturing method, sulfur-carbon complex and lithium secondary battery comprising thereof - Google Patents

Abstract

The present invention relates to a thermally expanded reduced graphene oxide having a specific surface area of 400-1200 m^2/g, a manufacturing method therefor, a sulfur-carbon composite comprising the same, and a lithium secondary battery.

Description

Thermally expanded reduced graphene oxide, a manufacturing method thereof, and a sulfur-carbon composite and lithium secondary battery comprising the same {Thermally expanded-reduced graphene oxide, manufacturing method, sulfur-carbon complex and lithium secondary battery comprising thereof}

The present invention relates to a thermally expanded reduced graphene oxide, a method for manufacturing the same, a sulfur-carbon composite and a lithium secondary battery comprising the same.

Graphite carbon materials, including fullerene, carbon nanotubes, and graphene as nanomaterials composed only of carbon atoms, have excellent electrical properties, physical and chemical stability, and are attracting attention from academia and industry.

In particular, graphene is a material that is in the spotlight as an innovative new material due to its very high specific surface area, excellent electrical conductivity, and physical and chemical stability. Among carbon materials, graphene can be mass-produced through chemical oxidation, exfoliation, and chemical or thermal reduction processes using rich and inexpensive natural or synthetic graphite as a raw material, and its manufacturing method is disclosed. .

On the other hand, in recent years, surface functionalization through surface activation and doping has been made to supplement the insufficient properties of the electrode for secondary batteries and supercapacitors or carbon adsorbents applied to environmental adsorbents or to induce a performance improvement effect.

However, a conventional micro-particle structure is synthesized using a carbon material, and it is difficult to control uniform density, size, shape and composition through condition control, and has a complicated synthesis process. So, it is time to overcome these problems and develop a functionalized carbon structure through a simple synthesis process.

On the other hand, among the secondary batteries, the lithium-sulfur battery has a theoretical energy density of about 2600 Wh / kg, and has a high value corresponding to about 7 times that of a lithium ion battery having an energy density of about 570 Wh / kg. In addition, sulfur, which is used as a positive electrode material for a lithium-sulfur battery, has an advantage of being able to lower the manufacturing cost of the battery because of its abundant resources and low price. Due to these advantages, lithium-sulfur batteries have received high attention.

Despite the above advantages, lithium polysulfide is produced as an intermediate product during the electrochemical reaction of the lithium-sulfur battery, and thus the life of the lithium-sulfur battery is limited. Lithium polysulfide generated during the electrochemical reaction of a lithium-sulfur battery has high solubility in an organic electrolyte, and is continuously dissolved in the organic electrolyte during a discharge reaction. Accordingly, there is a problem in that the amount of the positive electrode material containing sulfur decreases and the life of the battery itself decreases. In addition, since the sulfur itself has a very low electrical conductivity, only sulfur cannot be used as a positive electrode material, and thus a technique of forming a composite by using conductive materials such as conductive carbon and a polymer or coating sulfur on them is essential. Since only sulfur cannot be used as the positive electrode active material and conductive materials other than sulfur are included together, there is a problem that the energy density of the entire cell is lowered. To solve this, while maximizing the content of sulfur in the positive electrode material, it is necessary to minimize the content of the conductive material, and evenly support the sulfur in the conductive material.

In order to solve the above problems, many studies are continuously required.

When the graphene oxide is heat-treated and expanded and reduced, the specific surface area and pore volume increase, thereby increasing the content of sulfur carried on the graphene oxide and confirming that it is possible to uniformly support sulfur.

Accordingly, an object of the present invention is to provide a thermally expanded reduced graphene oxide (TE-rGO) having a high specific surface area and pore volume and a method for manufacturing the same.

In addition, an object of the present invention is to provide a sulfur-carbon composite comprising the thermally expanded reduced graphene oxide and sulfur and a lithium secondary battery comprising the same as a positive electrode active material.

In order to achieve the above object,

The present invention provides thermally expanded-reduced graphene oxide (TE-rGO) having a specific surface area of 400 to 1200 m ² / g.

In addition, the present invention comprises the steps of (a) thermally expanding the graphene oxide to a temperature of 300 to 500 ° C; And

(b) reducing the heat-expanded graphene oxide by heat treatment at a temperature of 700 to 1200 ° C.

In addition, the present invention is the thermally expanded reduced graphene oxide of the present invention; And

It provides a sulfur-carbon composite comprising; sulfur on at least a portion of the interior and surface of the thermally expanded reduced graphene oxide.

In addition, the present invention is an anode; cathode; A separator interposed between the anode and the cathode; And lithium secondary battery comprising an electrolyte,

The positive electrode provides a lithium secondary battery comprising the sulfur-carbon composite of the present invention.

The thermally expanded reduced-reduced graphene oxide (TE-rGO) of the present invention has a high specific surface area and pore volume, and thus can support a large amount of sulfur and evenly support sulfur. .

Therefore, a lithium secondary battery comprising a sulfur-carbon composite containing sulfur in at least a portion of the inside and the surface of the thermally expanded reduced graphene oxide as a positive electrode active material can improve the reactivity of sulfur, thereby providing excellent initial discharge capacity and life characteristics. Can be represented.

1 is a SEM photograph of the thermally expanded reduced graphene oxide of the present invention.
2 is a SEM photograph of reduced graphene oxide.
Figure 3 shows the pore volume according to the average pore size of the reduced graphene oxide of the thermally expanded reduced graphene oxide (Thermally expanded-reduced graphene oxide, TE-rGO) and Comparative Example 1 (reduced graphene oxide, rGO) of Example 1 It is a measured graph.
4 is a graph measuring the initial discharge capacity of Experimental Example 2.
5 is a graph measuring the life characteristics of the battery of Experimental Example 2.

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

Thermally expanded reduced graphene oxide

The present invention relates to thermally expanded-reduced graphene oxide (TE-rGO) having a specific surface area of 400 to 1200 m ² / g.

The thermally expanded reduced graphene oxide may be prepared by heat-treating graphene oxide to prepare thermally expanded graphene oxide, and then heat-reducing the thermally expanded graphene oxide to reduce it.

In the present invention, the graphene oxide produced through the thermal expansion and reduction steps as described above is defined as thermally expanded-reduced graphene oxide (TE-rGO).

In the present invention, the thermally expanded reduced graphene oxide can be used as a carrier capable of supporting sulfur, and by having the specific surface area and pore volume, it can support a larger amount of sulfur, as well as internal pores and surfaces You can evenly support the sulfur.

If the sulfur-carrying amount is too small in the sulfur-carbon composite in which sulfur is supported on at least a part of the inside and the surface of the carbon material, the energy density of the battery may decrease due to an increase in the proportion of the carbon material in the sulfur-carbon composite. In addition, when the sulfur is not evenly supported, the surface of the carbon material is covered with sulfur, which may cause a problem in that the electrical conductivity of the sulfur-carbon composite decreases.

However, the sulfur-carbon composite in which sulfur is supported on at least a part of the inside and the surface of the thermally expanded reduced graphene oxide can evenly support a large amount of sulfur, thereby reactivity of the lithium secondary battery using it as a positive electrode active material. By improving it, excellent initial discharge capacity and life characteristics can be realized.

Specifically, the specific surface area of the thermally expanded reduced graphene oxide is 400 to 1200 m ² / g, preferably 600 to 1200 m ² / g, and more preferably 800 to 1200 m ² / g.

When the specific surface area is 400 to 1200 m ² / g, a large amount of sulfur can be evenly supported on the thermally expanded reduced graphene oxide.

In addition, the pore volume of the thermally expanded reduced graphene oxide is 3 to 7 cm ³ / g, preferably 4 to 6 cm ³ / g.

When the pore volume is 3 to 7 cm ² / g, a large amount of sulfur can be evenly supported on the thermally expanded reduced graphene oxide.

The thermally expanded reduced graphene oxide of the present invention may have a crumpled paper structure by being manufactured through thermal expansion and reduction steps.

As it has the warped paper structure, it can have a high specific surface area and pore volume as described above, and thus can exhibit the above-described effect.

Method for manufacturing thermally expanded reduced graphene oxide

In addition, the present invention relates to a method of manufacturing thermally expanded reduced graphene oxide (Thermally expanded-reduced graphene oxide, TE-rGO),

(a) thermally expanding graphene oxide by heat treatment at a temperature of 300 to 500 ° C; And

(b) reducing the heat-expanded graphene oxide by heat treatment at a temperature of 700 to 1200 ° C.

The step (a) is a step of thermal expansion by heat treatment of graphene oxide.

As the heat treatment is performed, the oxygen functional group of the graphene oxide is easily removed so that the thermal expansion of the graphene oxide can easily occur. When thermal expansion of graphene oxide occurs, an oxygen functional group of graphene oxide may be removed by thermal shock to have an expanded crumpled paper structure.

The graphene oxide of step (a) may be in powder form.

Since the graphene oxide in the film form has a stacked structure, it is impossible to obtain a thermally expanded reduced graphene oxide having a desired specific surface area. Therefore, in the present invention, it is preferable to use a graphene oxide in powder form.

In addition, the heat treatment may be performed for 5 to 30 minutes at a temperature of 300 to 500 ° C, and preferably for 5 to 15 minutes at a temperature of 350 to 450 ° C.

If the heat treatment temperature and time are less than the above range, thermal expansion of graphene oxide does not occur sufficiently, so that a high specific surface area cannot be obtained, and if it exceeds the above range, yield may deteriorate.

The step (b) is a step of reducing the heat-expanded graphene oxide prepared in step (a) by heat treatment.

As the additional heat treatment is performed in step (b), a reduction process of the thermally expanded graphene oxide occurs, and thus a thermally expanded reduced graphene oxide having a crumpled paper structure is finally obtained.

In addition, the heat treatment may be performed for 1 to 5 hours at a temperature of 700 to 1200 ° C, and preferably for 2 to 4 hours at a temperature of 800 to 1000 ° C.

If the heat treatment temperature and time are less than the above range, thermal expansion of the thermally expanded graphene oxide does not occur sufficiently, so that a high specific surface area cannot be obtained, and if it exceeds the above range, yield may deteriorate.

The thermally expanded reduced graphene oxide has a twisted paper structure through thermal expansion and reduction steps, and thus can exhibit a high specific surface area and pore volume.

The thermally expanded reduced graphene oxide produced by the production method of the present invention can be used as a carrier capable of supporting sulfur, and can have a large amount of sulfur evenly as it has a high specific surface area and pore volume. Therefore, the sulfur-carbon composite in which sulfur is supported on at least a part of the inside and the surface of the thermally expanded reduced graphene oxide can evenly support a large amount of sulfur, thereby reactivity of the lithium secondary battery using it as a positive electrode active material. By improving it, excellent initial discharge capacity and life characteristics can be realized.

More specifically, the specific surface area of the thermally expanded reduced graphene oxide is 400 to 1200 m ² / g, preferably 600 to 1200 m ² / g, and more preferably 800 to 1200 m ² / g.

When the specific surface area is 400 to 1200 m ² / g, a large amount of sulfur can be evenly supported on the thermally expanded reduced graphene oxide.

In addition, the pore volume of the thermally expanded reduced graphene oxide is 3 to 7 cm ³ / g, preferably 4 to 6 cm ³ / g.

When the pore volume is 3 to 7 cm ² / g, a large amount of sulfur can be evenly supported on the thermally expanded reduced graphene oxide.

Sulfur-carbon complex

The present invention is a thermally expanded reduced graphene oxide; And sulfur on at least a portion of the inside and the surface of the thermally expanded reduced graphene oxide.

The thermally expanded reduced graphene oxide is the same as described above, and may be manufactured by the above-described manufacturing method.

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

In the sulfur-carbon composite according to the present invention, the reduced thermally expanded graphene oxide and sulfur are preferably mixed in a weight ratio of 1: 1 to 1: 9. When the content of the thermally expanded reduced graphene oxide exceeds the above range, the content of sulfur as an active material is lowered, resulting in problems in securing battery capacity, and when it is less than the above range, the content of the thermally expanded reduced graphene oxide is insufficient to impart electrical conductivity. Therefore, it is appropriately adjusted within the above range.

The method for complexing the sulfur-carbon composite of the present invention is not particularly limited in the present invention, and a method commonly used in the art may be used. As an example, a method of simple mixing of the thermally expanded reduced graphene oxide and sulfur of the present invention and then heat treatment may be used.

The sulfur is supported on at least a portion of the inside and the surface of the thermally expanded reduced graphene oxide, and a larger amount of sulfur is supported on the inside than on the surface.

In the present invention, the inside of the thermally expanded reduced graphene oxide means the pores of the thermally expanded reduced graphene oxide.

The diameter of the sulfur-carbon composite of the present invention is not particularly limited in the present invention, and may vary, but may preferably be 0.1 to 20 μm, more preferably 1 to 10 μm. When the above range is satisfied, a high loading electrode can be manufactured.

Since the sulfur-carbon composite uses the thermally expanded reduced graphene oxide of the present invention described above as a carbon material, a large amount of sulfur can be supported evenly compared to the conventional reduced graphene oxide. Therefore, the lithium secondary battery including the sulfur-carbon composite of the present invention may have an effect of improving initial discharge capacity and life characteristics.

Lithium secondary battery

The present invention is an anode; cathode; A separator interposed between the anode and the cathode; And lithium secondary battery comprising an electrolyte,

The positive electrode includes the sulfur-carbon composite of the present invention as a positive electrode active material.

As the sulfur-carbon composite is included as a positive electrode active material, the lithium secondary battery of the present invention may be a lithium-sulfur battery.

The positive electrode may include a positive electrode current collector and a positive electrode active material layer coated on one or both surfaces of the positive electrode current collector.

The positive electrode current collector supports the positive electrode active material, and is not particularly limited as long as it has a high conductivity without causing a chemical change in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface treated with carbon, nickel, silver, or the like, aluminum-cadmium alloy, or the like may be used.

The positive electrode current collector may form fine irregularities on its surface to enhance the bonding force with the positive electrode active material, and may use various forms such as a film, sheet, foil, mesh, net, porous body, foam, and nonwoven fabric.

The positive electrode active material layer may include a positive electrode active material, a binder, and a conductive material.

The positive electrode active material includes the sulfur-carbon composite of the present invention described above.

As described above, the carbon material of the sulfur-carbon composite is a thermally expanded reduced graphene oxide of the present invention, and has a higher specific surface area and pore volume, so it can evenly support a larger amount of sulfur. Therefore, in the present invention, the loading amount of sulfur of the positive electrode may be 2 to 15 mg / cm ² , and preferably 6 to 10 mg / cm ² . As described above, the lithium secondary battery including the positive electrode may exhibit an effect of initial discharge capacity and lifetime characteristics as it has a high loading amount.

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

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

The conductive material is intended to improve electrical conductivity, and is not particularly limited as long as it is an electronic conductive material that does not cause a chemical change in a lithium secondary battery.

In general, carbon black, graphite, carbon fiber, carbon nanotubes, metal powders, conductive metal oxides, and organic conductive materials can be used. As a commercially available product, acetylene black series (Chevron Chemical) Company (Chevron Chemical Company or Gulf Oil Company, etc.), Ketjen Black EC series (Armak Company), Vulcan XC-72 (Cavity Company) (Manufactured by Cabot Company) and Super P (manufactured by MMM). Examples include acetylene black, carbon black, and graphite.

In addition, the positive electrode active material may include a binder having a function of maintaining the positive electrode active material in the positive electrode current collector and connecting between the active materials. Examples of the binder include, for example, polyvinylidene fluoride-hexafluorofluoropropylene (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, poly Various types of binders such as polymethyl methacrylate, styrene-butadiene rubber (SBR), and carboxyl methyl cellulose (CMC) can be used.

The positive electrode as described above can be prepared according to a conventional method, specifically, a positive electrode active material, a conductive material and a binder are mixed on an organic solvent to prepare a slurry-forming positive electrode active material layer-forming composition on a current collector and dried. , Optionally it can be produced by compression molding on the current collector to improve the electrode density. At this time, as the organic solvent, 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. Specific examples include acetonitrile, methanol, ethanol, tetrahydrofuran, water, and isopropyl alcohol.

The composition for forming the positive electrode active material layer may be coated on a positive electrode current collector using a conventional method known in the art, for example, dipping, spray, or roll court method. , A gravure printing method, a bar court method, a die coating method, a comma coating method, or a mixture method thereof.

The positive electrode active material layer that has undergone such a coating process is then evaporated from a solvent or dispersion medium through a drying process, denseness of the coating film, and adhesion between the coating film and the current collector. At this time, drying is performed according to a conventional method, and is not particularly limited.

The negative electrode is a lithium-based metal, and may further include a current collector on one side of the lithium-based metal. A negative electrode current collector may be used as the current collector.

The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and copper, aluminum, stainless steel, zinc, titanium, silver, palladium, nickel, iron, chromium, alloys thereof, and these It can be selected from the group consisting of. The stainless steel may be surface-treated with carbon, nickel, titanium, or silver, and an aluminum-cadmium alloy may be used as the alloy. In addition, calcined carbon, a non-conductive polymer surface-treated with a conductive material, or a conductive polymer, etc. You can also use Generally, a thin copper plate is used as the negative electrode current collector.

In addition, various forms such as fine / uneven film / sheet, foil, net, porous body, foam, nonwoven fabric with fine irregularities formed on the surface can be used.

In addition, the negative electrode current collector is applied in a thickness range of 3 to 500 μm. If the thickness of the negative electrode current collector is less than 3 μm, the current collecting effect is deteriorated. On the other hand, when the thickness exceeds 500 μm, when the cells are assembled by folding, there is a problem that workability is deteriorated.

The lithium-based metal may be lithium or a lithium alloy. At this time, the lithium alloy contains an element capable of alloying with lithium, specifically, lithium and Si, Sn, C, Pt, Ir, Ni, Cu, Ti, Na, K, Rb, Cs, Fr, Be, Mg, Ca, It may be an alloy with one or more selected from the group consisting of Sr, Sb, Pb, In, Zn, Ba, Ra, Ge and Al.

The lithium-based metal may be in the form of a sheet or foil, and in some cases, a lithium or lithium alloy is deposited or coated on a current collector by a dry process, or a metal or alloy on a particle is deposited or coated by a wet process or the like. Can be in the form of

A conventional separator may be interposed between the anode and the cathode. The separator is a physical separator having a function of physically separating an electrode, and can be used without particular limitation as long as it is used as a normal separator. In particular, it is preferable that the electrolyte has excellent resistance to ion migration of the electrolyte and excellent moisture absorption ability.

In addition, the separator enables the transport of lithium ions between the positive electrode and the negative electrode while separating or insulating the positive electrode and the negative electrode from each other. The separator may be made of a porous, non-conductive or insulating material. The separator may be an independent member such as a film, or may be a coating layer added to the anode and / or cathode.

Examples of the polyolefin-based porous membrane that can be used as the separation membrane include polyolefin-based polymers such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene, polypropylene, polybutylene, and polypentene, respectively. And a film formed of a mixed polymer.

Examples of the non-woven fabric that can be used as the separator, polyphenylene oxide (polyphenyleneoxide), polyimide (polyimide), polyamide (polyamide), polycarbonate (polycarbonate), polyethylene terephthalate (polyethyleneterephthalate), polyethylene naphthalate (polyethylenenaphthalate) , Polybutylene terephthalate, polyphenylenesulfide, polyacetal, polyethersulfone, polyetheretherketone, polyester, or the like, respectively, or Non-woven fabrics formed of polymers mixed with them are possible, and these non-woven fabrics are in the form of fibers forming a porous web, and include spunbond or meltblown forms composed of long fibers.

The thickness of the separator is not particularly limited, but is preferably in the range of 1 to 100 μm, more preferably in the range of 5 to 50 μm. When the thickness of the separator is less than 1 μm, mechanical properties cannot be maintained, and when it exceeds 100 μm, the separator acts as a resistive layer, thereby deteriorating the performance of the battery.

The pore size and porosity of the separator are not particularly limited, but the pore size is 0.1 to 50 μm, and the porosity is preferably 10 to 95%. If the pore size of the separator is less than 0.1 μm or the porosity is less than 10%, the separator acts as a resistive layer. When the pore size exceeds 50 μm or the porosity exceeds 95%, mechanical properties cannot be maintained. .

The electrolyte is a non-aqueous electrolyte containing a lithium salt, which is composed of a lithium salt and an electrolyte, and a non-aqueous organic solvent, an organic solid electrolyte and an inorganic solid electrolyte are used as the electrolyte.

The lithium salt may be used without limitation as long as it is commonly used in the electrolyte solution for lithium-sulfur batteries. 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.

In addition, the concentration of the lithium salt in the electrolyte may be 0.2 to 2 M, specifically 0.6 to 2 M, and more specifically 0.7 to 1.7 M. When the concentration of the lithium salt is used less than 0.2 M, the conductivity of the electrolyte may be lowered and the performance of the electrolyte may be deteriorated. When the concentration is more than 2 M, the viscosity of the electrolyte may increase and mobility of lithium ions may be reduced.

The non-aqueous organic solvent should dissolve a lithium salt well, and as the non-aqueous organic solvent of the present invention, for example, N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, di Ethyl carbonate, ethylmethyl carbonate, gamma-butyrolactone, 1,2-dimethoxy ethane, 1,2-diethoxy ethane, tetrahydroxyfran, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1, 3-dioxolane, 4-methyl-1,3-dioxene, diethyl ether, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trime Non-proton such as methoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl pyropionate, ethyl propionate An organic solvent may be used, and the organic solvent may be a mixture of one or more organic solvents.

Examples of the organic solid electrolyte include, for example, polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphoric acid ester polymers, poly agitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and ionic dissociation. Polymers containing groups and the like can be used.

As the inorganic solid electrolyte, for example, Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Li ₄ Li nitrides such as SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li2 _S -SiS ₂ , halides, sulfates and the like can be used.

In the electrolyte of the present invention, for the purpose of improving charge / discharge characteristics, flame retardance, etc., for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme (glyme), hexaphosphate triamide, nitro Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxy ethanol, aluminum trichloride, etc. may be added. . In some cases, in order to impart non-flammability, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further included, or carbon dioxide gas may be further included to improve high temperature storage characteristics, and FEC (Fluoro-ethylene) carbonate), PRS (Propene sultone), FPC (Fluoro-propylene carbonate), and the like.

The electrolyte may be used as a liquid electrolyte, or may be used in the form of a solid electrolyte separator. When used as a liquid electrolyte, a physical separator having a function of physically separating electrodes further includes a separator made of porous glass, plastic, ceramic, or polymer.

Hereinafter, a preferred embodiment is provided to help the understanding of the present invention, but the following examples are only illustrative of the present invention, and it is apparent to those skilled in the art that various changes and modifications are possible within the scope and technical scope of the present invention. It is natural that such modifications and corrections fall within the scope of the appended claims.

Example 1 Preparation of thermally expanded reduced graphene oxide

Graphene oxide (graphene oxide, GO, product name SE2430, manufactured by sixth element) was heat-treated for 10 minutes at a temperature of 400 ° C. in an inert atmosphere to prepare thermally expanded graphene oxide.

The thermally expanded graphene oxide was heat-treated for 3 hours at a temperature of 900 ° C. in an inert atmosphere to prepare thermally expanded-reduced graphene oxide (TE-rGO).

As a result of observing the TE-rGO using SEM, it was confirmed that it has a crumple paper structure (FIG. 1).

Experimental Example 1. Measurement of specific surface area, pore volume and average pore size of thermally expanded reduced graphene oxide

With the TE-rGO prepared in Example 1, the reduced graphene oxide (reduced graphene oxide, SE1231, sixth element company, FIG. 2) was used as Comparative Example 1, and the specific surface area, pore volume, and average of TE-rGO and rGO. The pore size was measured.

The specific surface area, pore volume, and average pore size of TE-rGO of Example 1 and rGO of Comparative Example 1 were measured for nitrogen gas adsorption and distribution using a Porosimetry analyzer (Bell Japan Inc, Belsorp-II mini). It was measured by BET (Brunauer-Emmett-Teller; BET) 6-point method, and the results are shown in Table 1 below.

Specific surface area (m ² / g) Pore volume (cm ³ / g) Average pore size (nm) Example 1 (TE-rGO) 921 5.13 22.28 Comparative Example 1 (rGO) 185 0.61 12.74

TE-rGO of Example 1 of the present invention showed a higher specific surface area, pore volume, and average pore size compared to rGO of Comparative Example 1.

It can be seen that the TE-rGO of the present invention has a distorted paper structure as it performs thermal expansion and reduction steps, and thus has a higher specific surface area, pore volume, and average pore size compared to rGO.

In addition, by indicating the specific surface area and pore volume, it can be seen that TE-rGO can evenly support a large amount of sulfur compared to rGO.

<Lithium-sulfur battery manufacturing>

Example 2.

After mixing the TE-rGO and sulfur prepared in Example 1 in a weight ratio of 3: 7, reacting for 35 minutes at a temperature of 155 ° C. and sulfur-carbon supported by sulfur on the inside (pore) and surface of TE-rGO. Composites were prepared.

The positive electrode active material slurry was prepared by mixing the sulfur-carbon composite, conductive material, and binder in acetonitrile using a ball mill. In this case, the sulfur-carbon composite was used as a positive electrode active material, carbon black was used as a conductive material, and polyethylene oxide (molecular weight 5,000,000 g / mol) was used as a binder, and the mixing ratio was sulfur-carbon composite as a weight ratio: conductive material: The binder was brought to 90: 5: 5. The positive electrode active material slurry was coated on an aluminum current collector and dried to prepare a positive electrode.

At this time, the loading amount of the positive electrode active material was 5 mAh / cm ² or less, and the loading amount of sulfur was 6.7 mg / cm ² .

A lithium metal thin film having a thickness of 40 μm was used as a negative electrode.

After placing the prepared positive electrode and negative electrode so as to face each other, and placing a polyethylene separator therebetween, an electrolyte solution was injected to prepare a coin-type lithium-sulfur battery.

As the electrolyte, a mixed solution of DOL (1,3-dioxolane): DEGDME (diethylene glycol dimethyl ether) = 4: 6 (v / v) in which 1% LiFSI 1 wt% LiNO ₃ was dissolved was used.

Comparative Example 2.

A lithium-sulfur battery of Comparative Example 2 was prepared in the same manner as in Example 2, except that rGO of Comparative Example 1 was used instead of TE-rGO of Example 1.

Experimental Example 2. Measurement of initial discharge capacity and life characteristics of lithium-sulfur batteries

2-1. Initial discharge capacity measurement

The lithium-sulfur batteries prepared in Example 2 and Comparative Example 2 were tested for charge / discharge characteristic changes using a charge / discharge measurement device. The obtained battery was examined for initial capacity under 0.1C / 0.1C charge / discharge conditions, and the results are shown in FIG. 4.

The initial discharge capacity of the lithium-sulfur battery of Example 2 exceeded 1200 mAh / g, and the initial discharge capacity of the lithium-sulfur battery of Comparative Example 2 was less than 1100 mAh / g.

In the lithium-sulfur battery of Example 2, the carbon material of the sulfur-carbon composite as the positive electrode active material is TE-rGO of Example 1, and the TE-rGO has a high specific surface area and pore volume as measured in Experimental Example 1. Therefore, it can be seen that a large amount of sulfur can be evenly supported, contributing to the improvement of the reactivity of sulfur, so that the initial discharge capacity is very high.

On the other hand, in the lithium-sulfur battery of Comparative Example 2, the carbon material of the sulfur-carbon composite, which is a positive electrode active material, is rGO of Comparative Example 1, and the rGO is much smaller than that of TE-rGO of Example 1 as measured in Experimental Example 1. As it had a surface area and pore volume, it could not support sulfur equal to TE-rGO evenly, so it showed an initial discharge capacity lower than that of Example 2.

Therefore, the lithium-sulfur battery of the present invention is a carbon material of the sulfur-carbon composite, which is a positive electrode active material, and can use a TE-rGO having a very high specific surface area and a high pore volume to support a larger amount of sulfur evenly, thereby increasing the reactivity of sulfur. High initial discharge capacity can be exhibited according to the application.

2-2. Life characteristics measurement

For the lithium-sulfur batteries prepared in Example 2 and Comparative Example 2, a charge / discharge measuring device was used to charge / discharge 0.1C / 0.1C during the initial 3 cycles, and then charge 0.2C / 0.2C during the 3 cycles / Discharge, and then charged / discharged at 0.3C / 0.5C, the charging / discharging of 50 cycles was repeated to measure the life characteristics, and the results are shown in FIG. 5.

In the lithium-sulfur battery of Example 2, the discharge capacity was measured to be about 800 mAh / g in a high rate section of 0.5 C, and the capacity was maintained for 50 cycles.

On the other hand, in the lithium-sulfur battery of Comparative Example 2, the discharge capacity was less than 600 mAh / g in a high rate section of 0.5 C, and the battery was degraded after about 40 cycles, and the capacity was not maintained.

Therefore, the lithium-sulfur battery of the present invention increases the reactivity of sulfur by evenly supporting a larger amount of sulfur by using TE-rGO having a very high specific surface area and pore volume as the carbon material of the sulfur-carbon composite, which is a positive electrode active material. It can exhibit improved life characteristics.

Claims (13)

Thermally expanded reduced graphene oxide having a specific surface area of 400 to 1200 m ² / g. The thermally expanded reduced graphene oxide according to claim 1, wherein the thermally expanded reduced graphene oxide has a pore volume of 3 to 7 cm ³ / g. The thermally expanded reduced graphene oxide according to claim 1, wherein the thermally expanded reduced graphene oxide has a crumpled paper structure. (a) thermally expanding graphene oxide by heat treatment at a temperature of 300 to 500 ° C; And
(B) a step of reducing the heat-expanded graphene oxide by heat treatment at a temperature of 700 to 1200 ℃; thermally expanded reducing graphene oxide manufacturing method comprising a. [6] The method of claim 4, wherein the graphene oxide in the step (a) is in the form of a powder. The method of claim 4, wherein the thermal expansion of step (a) is performed for 5 to 30 minutes. The method of claim 4, wherein the reduction in step (b) is performed for 1 to 5 hours. The method of claim 4, wherein the thermally expanded reduced graphene oxide has a specific surface area of 400 to 1200 m ² / g. The method of claim 4, wherein the thermally expanded reduced graphene oxide has a pore volume of 3 to 7 cm ³ / g. The thermally expanded reduced graphene oxide of any one of claims 1 to 3; And
Sulfur-carbon composite comprising; sulfur on at least a portion of the interior and surface of the thermally expanded reduced graphene oxide. anode; cathode; A separator interposed between the anode and the cathode; And lithium secondary battery comprising an electrolyte,
The positive electrode is a lithium secondary battery comprising the sulfur-carbon composite of claim 10. The lithium secondary battery according to claim 11, wherein the loading amount of sulfur of the positive electrode is 2 to 15 mg / cm ² . The lithium secondary battery according to claim 11, wherein the lithium secondary battery is a lithium-sulfur battery.

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