KR102690253B1 - Thermally expanded-reduced graphene oxide, manufacturing method, sulfur-carbon complex and lithium secondary battery comprising thereof - Google Patents

Abstract

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

Description

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

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

Graphite carbon materials, including fullerenes, carbon nanotubes, and graphene, which are 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 attracting attention as a groundbreaking new material due to its very high specific surface area compared to volume, excellent electrical conductivity, and physical and chemical stability. Among carbon materials, graphene can be mass-produced using abundant and inexpensive natural or synthetic graphite through chemical oxidation, exfoliation, and chemical or thermal reduction, and its manufacturing method is disclosed. .

Meanwhile, in recent years, surface functionalization through surface activation and doping has been carried out to compensate for the insufficient properties of carbon materials applied to secondary battery electrodes, supercapacitors, or environmental adsorbents, or to induce performance improvement effects.

However, while synthesizing micro particle structures using conventional carbon materials, it is difficult to control uniform density, size, shape, and composition through condition control, and it requires a complicated synthesis process. Therefore, it is time to overcome these problems and develop functionalized carbon structures through a simple synthesis process.

Meanwhile, among secondary batteries, lithium-sulfur batteries have a theoretical energy density of about 2600 Wh/kg, which is about 7 times higher than that of lithium-ion batteries with an energy density of about 570 Wh/kg. In addition, sulfur, which is used as a cathode material for lithium-sulfur batteries, has abundant resources and is inexpensive, so it has the advantage of lowering the manufacturing cost of the battery. Because of these advantages, lithium-sulfur batteries are receiving great attention.

Despite the above advantages, there is a limit to the lifespan of the lithium-sulfur battery because lithium polysulfide is generated as an intermediate product during the electrochemical reaction of the lithium-sulfur battery. Lithium polysulfide produced during the electrochemical reaction of a lithium-sulfur battery has high solubility in the organic electrolyte solution, and is continuously dissolved in the organic electrolyte solution during the discharge reaction. Accordingly, there is a problem that the amount of cathode material containing sulfur decreases and the lifespan of the battery itself decreases. In addition, since sulfur itself has very low electrical conductivity, sulfur alone cannot be used as an anode material, so a technology for making a composite using conductive materials such as conductive carbon and polymer, or coating them with sulfur, is essential. As such, since only sulfur cannot be used as the positive electrode active material and other conductive materials other than sulfur are also included, there is a problem that the energy density of the entire cell is lowered. To solve this problem, the sulfur content in the positive electrode material must be maximized, while the content of the conductive material must be minimized, and the sulfur must be evenly supported on the conductive material.

Much research is continuously required to solve the above problems.

The present invention was completed by confirming that when graphene oxide is expanded and reduced by heat treatment, the specific surface area and pore volume increase, thereby increasing the content of sulfur supported in graphene oxide, and evenly supporting sulfur.

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

Additionally, the present invention aims to provide a sulfur-carbon composite containing the thermally expanded reduced graphene oxide and sulfur and a lithium secondary battery containing the same as a positive electrode active material.

In order to achieve the above purpose,

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 includes the steps of (a) thermally expanding graphene oxide by heat treatment at a temperature of 300 to 500°C; and

(b) reducing the thermally expanded graphene oxide by heat treatment at a temperature of 700 to 1200°C; providing a method for producing thermally expanded reduced graphene oxide comprising a.

In addition, the present invention relates to the thermally expanded reduced graphene oxide of the present invention; and

Provided is a sulfur-carbon composite containing 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 a lithium secondary battery containing an electrolyte,

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

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

Therefore, a lithium secondary battery containing a sulfur-carbon complex containing sulfur on at least a portion of the interior and surface of the thermally expanded reduced graphene oxide as a positive electrode active material can improve the reactivity of sulfur, resulting in excellent initial discharge capacity and lifespan characteristics. It can be expressed.

Figure 1 is an SEM photograph of thermally expanded reduced graphene oxide of the present invention.
Figure 2 is an SEM photo of reduced graphene oxide.
Figure 3 shows the pore volume according to the average pore size of the thermally expanded-reduced graphene oxide (TE-rGO) of Example 1 and the reduced graphene oxide of Comparative Example 1 (reduced graphene oxide (rGO)). This is a measured graph.
Figure 4 is a graph measuring the initial discharge capacity of Experimental Example 2.
Figure 5 is a graph measuring the lifespan 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) with a specific surface area of 400 to 1200 m ² /g.

The thermally expanded reduced graphene oxide may be manufactured by heat treating graphene oxide to produce thermally expanded graphene oxide, and then heat treating the thermally expanded graphene oxide again to reduce it.

In the present invention, graphene oxide prepared 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 above specific surface area and pore volume, not only can a larger amount of sulfur be supported, but also internal pores and surface Sulfur can be evenly supported.

If the amount of sulfur supported in a sulfur-carbon composite in which sulfur is supported on at least part of the interior and surface of the carbon material is too small, the proportion of carbon material in the sulfur-carbon composite may increase and the energy density of the battery may decrease. In addition, if sulfur is not evenly contained, the surface of the carbon material may be covered with sulfur, which may cause a problem in which the electrical conductivity of the sulfur-carbon composite is reduced.

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

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

If 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.

Additionally, the pore volume of the thermally expanded reduced graphene oxide may be 3 to 7 cm ³ /g, and preferably 4 to 6 cm ³ /g.

If 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.

By having the twisted paper structure, it is possible to have a high specific surface area and pore volume as described above, thereby exhibiting the above-mentioned effects.

Method for manufacturing thermally expanded reduced graphene oxide

In addition, the present invention relates to a method for producing 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 thermally expanded graphene oxide by heat treatment at a temperature of 700 to 1200°C.

Step (a) is a step of thermally expanding graphene oxide by heat treatment.

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

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

Since graphene oxide in the form of a film has a stacked structure, thermally expanded reduced graphene oxide having the desired specific surface area cannot be obtained. Therefore, in the present invention, it is preferable to use graphene oxide in powder form.

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

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

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

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

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

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 and a high specific surface area cannot be obtained, and if it exceeds the above range, the yield may be reduced.

The thermally expanded reduced graphene oxide undergoes thermal expansion and reduction steps to have a twisted paper structure, and thus can exhibit 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 evenly support a large amount of sulfur 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 portion of the interior and surface of the thermally expanded reduced graphene oxide can evenly support a large amount of sulfur, thereby improving the reactivity of a lithium secondary battery using it as a positive electrode active material. By improving it, excellent initial discharge capacity and lifespan characteristics can be realized.

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

If 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.

Additionally, the pore volume of the thermally expanded reduced graphene oxide may be 3 to 7 cm ³ /g, and preferably 4 to 6 cm ³ /g.

If 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 relates to thermally expanded reduced graphene oxide; and sulfur on at least a portion of the interior and surface of the thermally expanded reduced graphene oxide.

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

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 types selected from. Preferably, inorganic sulfur (S ₈ ) can be used.

In the sulfur-carbon composite according to the present invention, the thermally expanded reduced graphene oxide and sulfur are preferably mixed at a weight ratio of 1:1 to 1:9. If the content of thermally expanded reduced graphene oxide exceeds the above range, the content of sulfur, which is an active material, decreases, causing problems in securing battery capacity. If it is less than the above range, the content of thermally expanded reduced graphene oxide is insufficient to provide electrical conductivity. Therefore, adjust appropriately within the above range.

The method of complexing the sulfur-carbon complex of the present invention is not particularly limited in the present invention, and methods commonly used in the art may be used. For example, a method of simply mixing the thermally expanded reduced graphene oxide and sulfur of the present invention and then heat treating them to form a composite may be used.

The sulfur is supported on at least part of the interior and 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 interior of the thermally expanded reduced graphene oxide refers to 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 is preferably 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 evenly supported compared to the conventional reduced graphene oxide. Therefore, the lithium secondary battery containing the sulfur-carbon composite of the present invention may have the effect of improving initial discharge capacity and lifespan characteristics.

Lithium secondary battery

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

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

By including the sulfur-carbon composite as the 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 applied to one or both sides 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 high conductivity without causing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, surface treatment of copper or stainless steel with carbon, nickel, silver, etc., aluminum-cadmium alloy, etc. can be used.

The positive electrode current collector can strengthen the bonding force with the positive electrode active material by forming fine irregularities on its surface, and can be used in various forms such as film, sheet, foil, mesh, net, porous material, foam, and non-woven 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 the thermally expanded reduced graphene oxide of the present invention, and has a high specific surface area and pore volume, so it can evenly support a larger amount of sulfur. Therefore, in the present invention, the sulfur loading amount of the anode may be 2 to 15 mg/cm ² , and preferably 6 to 10 mg/cm ² . With such a high loading amount, the lithium secondary battery including the positive electrode can exhibit effects on initial discharge capacity and lifespan characteristics.

In addition to the positive electrode active material, 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.

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, the group IIIA elements include Al, Ga, In, Ti, etc., and the group IVA elements may include Ge, Sn, Pb, etc.

The conductive material is intended to improve electrical conductivity, and there is no particular limitation as long as it is an electronically conductive material that does not cause chemical changes in a lithium secondary battery.

In general, carbon black, graphite, carbon fiber, carbon nanotubes, metal powder, conductive metal oxide, organic conductive materials, etc. can be used. Products currently sold as conductive materials include acetylene black series (Chevron Chemical) Company (Chevron Chemical Company or Gulf Oil Company products, etc.), Ketjen Black EC series (Armak Company product), Vulcan XC-72 (Cabot Company) (Cabot Company) and Super P (MMM). Examples include acetylene black, carbon black, and graphite.

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

The positive electrode as described above can be manufactured according to a conventional method. Specifically, a composition for forming a positive active material layer in a slurry state prepared by mixing a positive active material, a conductive material, and a binder in an organic solvent is applied and dried on a current collector. , Optionally, it can be manufactured by compression molding on a current collector to improve electrode density. At this time, it is preferable to use an organic solvent that can uniformly disperse the positive electrode active material, binder, and conductive material and that evaporates easily. Specifically, acetonitrile, methanol, ethanol, tetrahydrofuran, water, isopropyl alcohol, etc. are mentioned.

The composition for forming the positive electrode active material layer can be coated on the positive electrode current collector using a common method known in the art, for example, dipping method, spray method, or roll court method. A variety of methods can be used, such as gravure printing, bar court, die coating, comma coating, or a combination thereof.

The positive active material layer that has gone through this coating process undergoes a subsequent drying process to achieve evaporation of the solvent or dispersion medium, density of the coating film, and adhesion between the coating film and the current collector. At this time, drying is carried out 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. The current collector may be a negative electrode 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 may include copper, aluminum, stainless steel, zinc, titanium, silver, palladium, nickel, iron, chromium, alloys thereof, and these. It may be selected from the group consisting of a combination of. The stainless steel may be surface treated with carbon, nickel, titanium, or silver, and the alloy may be an aluminum-cadmium alloy, and 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 may be used, such as films, sheets, foils, nets, porous materials, foams, and non-woven fabrics with or without fine irregularities formed on the surface.

In addition, the negative electrode current collector is used in a thickness range of 3 to 500 ㎛. If the thickness of the negative electrode current collector is less than 3 ㎛, the current collecting effect is reduced. On the other hand, if the thickness is more than 500 ㎛, there is a problem that processability is reduced when the cell is folded and assembled.

The lithium-based metal may be lithium or a lithium alloy. At this time, the lithium alloy includes elements that can be alloyed 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 types 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, lithium or lithium alloy may be deposited or coated on a current collector by a dry process, or metals and alloys on particles may be deposited or coated by a wet process, etc. It may be in a given form.

A conventional separator may be interposed between the anode and the cathode. The separator is a physical separator that has the function of physically separating electrodes, and can be used without particular limitations as long as it is used as a normal separator. In particular, it is desirable to have low resistance to ion movement in the electrolyte solution and excellent electrolyte moisturizing ability.

Additionally, the separator separates or insulates the positive and negative electrodes from each other and enables the transport of lithium ions between the positive and negative electrodes. These separators are porous and may be made of non-conductive or insulating materials. 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 polyolefin-based porous membranes 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, individually or together. Examples include membranes formed from mixed polymers.

Examples of nonwoven fabrics that can be used as the separator include polyphenyleneoxide, polyimide, polyamide, polycarbonate, polyethyleneterephthalate, and polyethylenenaphthalate. , polybutyleneterephthalate, polyphenylenesulfide, polyacetal, polyethersulfone, polyetheretherketone, polyester, etc., individually or Nonwoven fabrics made of polymers mixed with these are possible, and such nonwoven fabrics are in the form of fibers that form a porous web, and include spunbond or meltblown forms made of long fibers.

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

The pore size and porosity of the separator are not particularly limited, but it is preferable that the pore size is 0.1 to 50 ㎛ and the porosity is 10 to 95%. If the pore size of the separator is less than 0.1 ㎛ or the porosity is less than 10%, the separator acts as a resistance layer, and if the pore size is greater than 50 ㎛ or the porosity is greater than 95%, the mechanical properties cannot be maintained. .

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

The lithium salt may be used without limitation as long as it is commonly used in electrolytes 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 , LiFSI, lithium chloroborane, lithium lower aliphatic carboxylate, etc. may be included.

Additionally, the concentration of lithium salt in the electrolyte solution may be 0.2 to 2 M, specifically 0.6 to 2 M, and more specifically 0.7 to 1.7 M. If the lithium salt is used at a concentration of less than 0.2 M, the conductivity of the electrolyte may decrease and electrolyte performance may deteriorate, and if it is used at a concentration exceeding 2 M, the viscosity of the electrolyte may increase and the mobility of lithium ions may decrease.

The non-aqueous organic solvent must dissolve the lithium salt well, and non-aqueous organic solvents of the present invention include, for example, N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, Ethyl carbonate, ethylmethyl carbonate, gamma-butyrolactone, 1,2-dimethoxy ethane, 1,2-diethoxy ethane, tetrahydroxy franc (franc), 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 Aprotic substances such as toxin methane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl pyropionate, ethyl propionate, etc. An organic solvent may be used, and the organic solvent may be one or a mixture of two or more organic solvents.

The organic solid electrolyte includes, for example, polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphoric acid ester polymers, poly agitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, ionic dissociation. A polymer containing a group, etc. may be used.

Examples of the inorganic solid electrolyte include 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 Nitride, halide, sulfate, etc. of Li such as SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li2 _S -SiS ₂ may be used.

The electrolyte of the present invention includes, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphoric acid triamide, and nitroamine for the purpose of improving charge/discharge characteristics, flame retardancy, etc. Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol, aluminum trichloride, etc. may be added. . In some cases, halogen-containing solvents such as carbon tetrachloride and trifluoroethylene may be further included to provide incombustibility, and carbon dioxide gas may be further included to improve high-temperature preservation characteristics, and FEC (Fluoro-ethylene carbonate), PRS (Propene sultone), FPC (Fluoro-propylene carbonate), etc. may be further included.

The electrolyte can be used as a liquid electrolyte or in the form of a solid electrolyte separator. When used as a liquid electrolyte, it further includes a physical separator that has the function of physically separating electrodes and is made of porous glass, plastic, ceramic, or polymer.

Hereinafter, preferred embodiments are presented to aid understanding of the present invention. However, the following examples are merely illustrative of the present invention, and it is clear to those skilled in the art that various changes and modifications are possible within the scope and spirit of the present invention. It is natural that such variations and modifications fall within the scope of the attached patent claims.

Example 1. Preparation of thermally expanded reduced graphene oxide

Graphene oxide (GO, product name SE2430, Sixth Element) was heat-treated at 400°C for 10 minutes in an inert atmosphere to prepare thermally expanded graphene oxide.

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

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

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

In addition to the TE-rGO prepared in Example 1, reduced graphene oxide (SE1231, Sixth Element, FIG. 2) was used as Comparative Example 1 to determine the specific surface area, pore volume, and average of TE-rGO and rGO. 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 using a pore distribution analyzer (Bell Japan Inc, Belsorp-II mini) using a nitrogen gas adsorption distribution method. It was measured using the 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 higher specific surface area, pore volume, and average pore size compared to rGO of Comparative Example 1.

This shows that the TE-rGO of the present invention has a distorted paper structure as thermal expansion and reduction steps are performed, and as a result, it has a higher specific surface area, pore volume, and average pore size than rGO.

In addition, by showing the above specific surface area and pore volume, it can be seen that TE-rGO can evenly support a larger amount of sulfur than rGO.

<Lithium-sulfur battery manufacturing>

Example 2.

TE-rGO and sulfur prepared in Example 1 were mixed at a weight ratio of 3:7 and then reacted at a temperature of 155°C for 35 minutes to produce sulfur-carbon with sulfur supported on the interior (pores) and surface of TE-rGO. A composite was prepared.

A positive electrode active material slurry was prepared by mixing the sulfur-carbon composite, conductive material, and binder in acetonitrile using a ball mill. At this time, the sulfur-carbon composite was used as the positive electrode active material, carbon black was used as the conductive material, and polyethylene oxide (molecular weight 5,000,000 g/mol) was used as the binder, and the mixing ratio was sulfur-carbon composite: conductive material: The binder was made to be 90:5:5. The positive electrode active material slurry was applied to 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 with a thickness of 40 μm was used as the cathode.

A coin-type lithium-sulfur battery was manufactured by placing the prepared positive electrode and the negative electrode face to face, placing a polyethylene separator between them, and then injecting an electrolyte solution.

The electrolyte was a mixture of 1 M LiFSI and 1 wt% of LiNO ₃ dissolved in DOL (1,3-dioxolane):DEGDME (diethylene glycol dimethyl ether)=4:6 (v/v).

Comparative Example 2.

A lithium-sulfur battery of Comparative Example 2 was manufactured in the same manner as 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 battery

2-1. Initial discharge capacity measurement

For the lithium-sulfur batteries manufactured in Example 2 and Comparative Example 2, changes in charge and discharge characteristics were tested using a charge and discharge measuring device. The initial capacity of the obtained battery was examined under 0.1C/0.1C charge/discharge conditions, and the results are shown in Figure 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, which is 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. Accordingly, a large amount of sulfur can be evenly supported, contributing to improving the reactivity of sulfur, resulting in a very high initial discharge capacity.

On the other hand, in the lithium-sulfur battery of Comparative Example 2, the carbon material of the sulfur-carbon composite, which is the positive electrode active material, is rGO of Comparative Example 1, and the rGO has a much smaller ratio than the TE-rGO of Example 1 as measured in Experimental Example 1. Due to its surface area and pore volume, it was unable to evenly support as much sulfur as TE-rGO, resulting in a lower initial discharge capacity than Example 2.

Therefore, the lithium-sulfur battery of the present invention uses TE-rGO, which has a very high specific surface area and pore volume, as the carbon material of the sulfur-carbon composite, which is the positive electrode active material, and can evenly support a larger amount of sulfur, thereby increasing the reactivity of sulfur. Depending on the condition, a high initial discharge capacity can be achieved.

2-2. Measurement of lifetime characteristics

For the lithium-sulfur battery prepared in Example 2 and Comparative Example 2, charge/discharge was performed at 0.1C/0.1C for the first 3 cycles and at 0.2C/0.2C for the next 3 cycles using a charge/discharge measuring device. /discharged, and then charged/discharged at 0.3C/0.5C and repeated 50 cycles of charge/discharge to measure the lifespan characteristics, and the results are shown in Figure 5.

The lithium-sulfur battery of Example 2 had a discharge capacity of about 800 mAh/g at a high rate of 0.5 C, and maintained the capacity for 50 cycles.

On the other hand, the lithium-sulfur battery of Comparative Example 2 had a discharge capacity of less than 600 mAh/g at a high rate of 0.5 C, and the battery deteriorated after about 40 cycles and failed to maintain capacity.

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

Claims (13)

A positive electrode active material containing a sulfur-carbon complex,
The sulfur-carbon composite is thermally expanded reduced graphene oxide; And sulfur on at least a portion of the interior and surface of the thermally expanded reduced graphene oxide,
The thermally expanded reduced graphene oxide is a positive electrode active material having a specific surface area of 400 to 1200 m ² /g and a pore volume of 3 to 7 cm ³ /g. delete The positive electrode active material of 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;
(b) reducing the thermally expanded graphene oxide by heat treatment at a temperature of 700 to 1200°C; and
(c) mixing the thermally expanded reduced graphene oxide with sulfur to produce a sulfur-carbon composite; The method of manufacturing a positive electrode active material according to claim 4, wherein the graphene oxide in step (a) is in powder form. The method of claim 4, wherein the thermal expansion in 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. delete delete delete anode; cathode; A separator interposed between the anode and the cathode; and a lithium secondary battery containing an electrolyte,
The positive electrode is a lithium secondary battery comprising the positive electrode active material of any one of claims 1 and 3. The lithium secondary battery according to claim 11, wherein the sulfur loading 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.