KR20180099572A - Preparation method of N-doped graphene, preparation method of Sulfur-graphene composite, and anode of Lithium-Sulfur battery comprising the same - Google Patents

Abstract

The present invention provides a manufacturing method of nitrogen-doped graphene, a manufacturing method of a sulfur-graphene composite using the same, an anode of a lithium-sulfur battery including the same, and a lithium-sulfur battery having the same. The manufacturing method of nitrogen-doped graphene comprises: a first step of peeling graphite by high pressure homogenization of allowing dispersion liquid containing the graphite to pass through a micro-channel of a high pressure homogenizer (HPH) including an inlet, an outlet, and the micro-channel connecting between the inlet and the outlet and having a micrometer-sized diameter to obtain graphene flakes; a second step of filtering and drying the obtained graphene flakes, then inserting the graphene flakes with an ammonium compound into a ball mill, and performing dry ball milling in an inert atmosphere to obtain grains surrounded by the ammonium compound on the surface of the graphene flakes; and a third step of heat-treating the obtained grains to obtain graphene doped with nitrogen atoms.

Description

A preparation method of a sulfur-graphene composite, a preparation method of a sulfur-containing graphene, a preparation method of a sulfur-graphene composite, and an anode of a lithium-sulfur battery comprising the same.

The present invention relates to a method for producing graphene doped with nitrogen, a method for producing a sulfur-graphene composite using the same, and a cathode for a lithium sulfur battery including the sulfur-graphene composite, and more particularly, Doped graphene which can improve the lifetime of a lithium sulphide battery by reducing the dissolution rate of lithium polysulfide as a result of increasing the amount of lithium sulfide, Lt; RTI ID = 0.0 > lithium-sulfur < / RTI >

One of the important issues to be solved for the commercialization of lithium sulfur battery is low lifetime characteristics of lithium sulfur battery by lithium polysulfide (LiPS) elution.

The reason why the lifetime characteristics of the lithium sulphate battery is poor is that the lithium polysulfide (LiPS) generated during the electrochemical reaction of the lithium sulphate battery is highly soluble in the organic electrolytic solution, Because.

To solve these problems, a method of compounding sulfur with a conductive material such as carbon, carbon nanotube (CNT), or graphene is widely used by physical and chemical methods for improving electrochemical performance.

Among them, graphene is an ideal candidate to increase the loading of sulfur. Graphene is a two-dimensional very thin sheet-like material and has a large specific surface area and is well known for its excellent electrical and mechanical properties. The nature of this graphene makes it possible to flexibly cope with mechanical stress during electrochemical cycling.

On the other hand, much research has been done to increase the surface area of graphene in order to carry a larger amount of sulfur to graphene. As a part of such studies, researches for producing graphene sheets or flakes having a thinner thickness and a representative area are actively under way, and a method of producing graphene flakes by thinly separating graphite from them by a physical method, Many methods have been proposed.

Among these methods, a method in which a dispersion containing graphite is physically peeled by passing through a high-pressure homogenization reactor has recently been utilized. However, such a simple physical exfoliation has a limitation that a surface area large enough to support a large amount of sulfur can not be obtained.

Therefore, in the manufacture of lithium sulfur batteries using graphene, there is a great need to develop a technique for extending the lifetime of lithium sulfur batteries by increasing the surface area and pore volume and increasing the amount of sulfur supported by introducing a macroporous structure It is a situation.

The present invention provides a method for manufacturing nitrogen-doped graphene having increased specific surface area and pore volume that can further increase specific surface area and pore volume by improving the structure of graphene .

The present invention also provides a method for improving the lifetime of a lithium sulfur battery by increasing the loading amount of sulfur in the lithium sulfur battery using the graphene flake obtained by the above method and consequently reducing the dissolution rate of the lithium polysulfide , A method for producing a sulfur-graphene composite, a method for producing the sulfur-graphene composite as a lithium sulfur battery positive electrode, and a lithium sulfur battery having the positive electrode.

According to an aspect of the present invention, there is provided a method of manufacturing nitrogen-doped graphene of the following embodiments.

In a first embodiment,

The dispersion containing graphite is passed through a microfluidic channel of a high pressure homogenizer (HPH) comprising an inlet portion and an outlet portion, and a microchannel connecting between the inlet portion and the outlet portion and having a micrometer-sized diameter A first step of peeling the graphite through high pressure homogenization to obtain graphene flakes;

A second step of filtering and drying the obtained graphene flake, putting it into a ball mill together with an ammonium compound, and conducting dry ball milling in an inert atmosphere to obtain grains in a state in which the ammonium compound is surrounded on the surface of the graphene flake; And

And a third step of heat treating the obtained particles to obtain graphene doped with nitrogen atoms.

The second embodiment, in the first embodiment,

Wherein the high pressure homogenization is performed by applying a pressure of 500 to 3,000 bar to the dispersion.

The third embodiment is, in the first embodiment or the second embodiment,

Wherein the ammonium compound is ammonium chloride.

The fourth embodiment is, in any one of the first to third embodiments,

The second step is to prepare nitrogen-doped graphene by stirring inert gas and air into the ball mill and stirring the mixture at 200 to 700 rpm for 10 to 30 hours using a ball having a diameter of 0.1 to 5 mm ≪ / RTI >

The fifth embodiment is, in any one of the first through fourth embodiments,

Further comprising the step of sorting the particles obtained in the second step before the third step using a mesh.

The sixth embodiment is, in any one of the first through fifth embodiments,

The heat treatment in the third step is carried out by heating the obtained particles in an alumina boat and heating the mixture in a tube furnace under an inert atmosphere at a temperature of 700 to 1000 DEG C for 0.5 to 5 hours and a temperature rise of 8 to 15 DEG C / To a method for producing doped graphene.

According to an aspect of the present invention, there is provided a method of manufacturing a sulfur-graphene composite of the following embodiment.

A seventh embodiment relates to a method of manufacturing a sulfur-graphene composite in which sulfur is supported on nitrogen-doped graphene produced by the method of any one of Embodiments 1 to 6 to composite.

According to an aspect of the present invention, there is provided a lithium sulfur battery anode of the following embodiment.

An eighth embodiment relates to a lithium sulfur battery anode comprising the sulfur-graphene composite produced by the seventh embodiment.

According to an aspect of the present invention, there is provided a lithium sulfur battery of the following embodiments.

The ninth embodiment relates to a lithium sulfur battery having a positive electrode manufactured by the eighth embodiment.

According to one embodiment of the present invention, the specific surface area and the pore volume can be drastically increased compared to conventional graphenes, and when combined with sulfur, the sulfur supporting performance is significantly increased, and nitrogen atoms are introduced into graphene The dissolution of lithium polysulfide during the electrochemical reaction of the lithium sulfur battery can be suppressed, the initial capacity can be improved, and the life of the battery can be prolonged.

FIGS. 1A to 1C are views illustrating a process of fabricating nitrogen-doped graphene according to an embodiment of the present invention.
FIG. 2 is a SEM photograph showing the surface state of graphene flakes in each step of preparing nitrogen-doped graphene according to Example 1 of the present invention. FIG.
3 is a result of XPS analysis of nitrogen-doped graphene prepared according to Example 1 of the present invention.
4A and 4B are SEM and TEM photographs of nitrogen doped graphene prepared according to Example 1 of the present invention, respectively.
FIG. 5A shows nitrogen adsorption / desorption isotherm of nitrogen-doped graphene prepared according to Example 1 of the present invention by pressure. FIG.
5B shows the pore volume of the nitrogen-doped graphene prepared according to Example 1 of the present invention by the pore diameter.
FIG. 5C shows nitrogen adsorption / desorption isomers of nitrogen-doped graphene prepared according to Comparative Example 1 by pressure.
5D shows the pore volume of the nitrogen-doped graphene prepared according to Comparative Example 1 by pore diameter.
FIG. 6 shows adsorption / desorption isomers of nitrogen in the graphene nanoflake after the nitrogen (N) doping process and the graphene nanoflake after the nitrogen (N) doping process in Comparative Example 3. FIG.
FIG. 7 shows the pore volume of the graphene nanoflake after the nitrogen (N) doping process and the graphene nanoflake after the nitrogen (N) doping process of Comparative Example 3. FIG.
8 shows charging / discharging characteristics of the lithium sulfur battery produced according to Example 2, Comparative Example 2 and Comparative Example 4. Fig.

Hereinafter, the present invention will be described in detail. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims and their equivalents. .

According to one aspect of the present invention, there is provided a process for producing nitrogen-doped graphene using graphene flakes obtained by stripping graphite through high-pressure homogenization.

The method of manufacturing the graphene flake according to an embodiment of the present invention includes three steps.

In the first step, graphite flakes are obtained by peeling the graphite through high pressure homogenization,

In a second step, the obtained graphene flakes are ball-milled with an ammonium compound under an inert atmosphere to form an ammonium compound on the graphene flake surface,

In a third step, the graphene flakes surrounding the ammonium compound thus obtained are heat-treated to decompose the salt and allow the nitrogen atoms to be doped.

First, the manufacturing step of the graphene flakes will be described.

As shown in FIG. 1A, when a dispersion containing graphite is passed through a micro flow path of a high pressure homogenizer and graphene is grafted on the graphite by a high shear force applied while passing through the micro flow path, Thereby obtaining a graphene flake.

The microfluidic channel of the high pressure homogenizer may have a diameter of about 50 to 300 mu m, and the pressure applied to the dispersion may be adjusted to about 500 to 3000 bar.

In one embodiment of the present invention, a dispersion medium commonly used in water, N-methyl-2-pyrrolidone (NMP), acetone,

Further, in one embodiment of the present invention, the graphite may be graphite of a general shape, or expanded graphite having a wide interlayer spacing may be used to facilitate the separation of graphene. Especially, expanded graphite can be used as the expanding graphite. The expandable graphite can have an expansion degree of about 0.01 to 0.5 g / cm ³ and a BET (surface area) of about 5 to 50 m ³ / g.

Figure 1 (b) shows that graphite flakes are formed through the high pressure homogenization to form graphene flakes.

Next, the graphene flakes obtained by the above process are pulverized through filtration and drying, and the powder is put into a ball mill, specifically a planetary ball mill, and subjected to dry ball milling in an inert atmosphere.

The ball milling may be performed by placing the inert gas and air into the planetary ball mill and stirring the mixture at 200 to 700 rpm for 10 to 30 hours using a ball having a ball size of 0.1 to 5 mm.

In one embodiment of the present invention, the ammonium compound is selected from the group consisting of ammonium chloride, ammonium nitrate, ammonium sulfate, ammonium persulfate, ammonium perchlorate, (Bi) carbonate, (b) ammonium (b) fluoride, ammonium phosphate, ammonium oxalate, ammonium hydroxide, ammonium formate ), And (middle) ammonium (chromium) chromate, but the present invention is not limited thereto. Compounds containing an ammonium group can be used without limitation.

The particles obtained in the above step are in the form of particles in the state of the graphene flakes surrounding the ammonium compound (see FIG. 1C), and the particles thus obtained can be sorted by using a mesh to use only particles of uniform size have.

Next, the resulting grains, that is, graphene flakes surrounding the ammonium compound on the surface thereof are heat-treated to decompose the salt to obtain graphene doped with nitrogen atoms.

The heat treatment can be performed in a tube furnace under an inert atmosphere at a temperature of 700 to 1000 DEG C, for 0.5 to 5 hours, and at a temperature of 8 to 15 DEG C / min, on the alumina boat.

The graphene produced by such a process is characterized in that it has a large specific surface area and a large pore volume because nitrogen atoms are doped on the surface and a large amount of pores and wrinkles are formed on the surface.

Therefore, a sulfur-graphene composite can be formed by supporting sulfur in such graphene, and the sulfur-graphene composite thus formed increases the amount of sulfur supported. Therefore, when used as a positive electrode of a lithium sulfur battery, elution of lithium polysulfide It is expected that the lifetime of the lithium sulfur battery can be improved by reducing the speed.

According to one embodiment of the present invention, the sulfur-graphene composite can be prepared by impregnating a sulfur or sulfur-based compound into the outer surface and interior of the carbon. The impregnation may be performed by mixing nitrogen-doped graphene with a sulfur or sulfur-based compound powder and then heating to impregnate the molten sulfur or sulfur-based compound with carbon. In the mixing, the dry ball mill method, A mill method or a dry dynomill method can be used.

The positive electrode according to an embodiment of the present invention may include a conductive material for allowing electrons to move smoothly in the positive electrode together with the sulfur-graphene composite as a positive electrode active material, and a positive electrode active material, And a binder for increasing the binding force of the binder.

The conductive material may be a carbon-based material such as denka black, carbon black, acetylene black, Ketjen black, Vapor Grown Carbon Fiber (VGCF); Or a conductive polymer such as polyaniline, polythiophene, polyacetylene, and polypyrrole. The conductive material may be used in an amount of 0.1 to 20 parts by weight, more preferably 1 to 17 parts by weight, and more preferably 2 to 15 parts by weight based on 100 parts by weight of the cathode active material. When the content of the conductive material satisfies the above range, there is an effect of improving the conductivity through formation of a suitable conductive network, and the problem of reducing the overvoltage during charging and discharging and deteriorating the capacity characteristics can be prevented.

Examples of the binder include styrene-butadiene rubber (SBR) / carboxymethyl cellulose (CMC), poly (vinyl acetate), polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, alkylated polyethylene oxide, crosslinked polyethylene oxide , A copolymer of polyvinyl ether, poly (methyl methacrylate), polyvinylidene fluoride, polyhexafluoropropylene and polyvinylidene fluoride (trade name: Kynar), poly (ethyl acrylate), polytetrafluoro Ethylene, polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, polystyrene, polyacrylic acid, derivatives thereof, blends, copolymers and the like can be used. The content of the binder may be 0.1 to 20 parts by weight, specifically 1 to 17 parts by weight, more specifically 2 to 15 parts by weight based on 100 parts by weight of the cathode active material. When the content of the binder satisfies the above range, the adhesion between the positive electrode active material or between the positive electrode active material and the current collector is greatly improved, and the problem that the capacity characteristic is lowered can also be prevented. In addition, polysulfide dissolution inhibition by polysulfide and specific functional groups of the polymer chains used as binders can also be expected.

The positive electrode may be prepared according to a conventional method. Specifically, a composition for forming a positive electrode active material layer, which is prepared by mixing a positive electrode active material, a conductive material and a binder, which are the sulfur-graphene composite, on a dispersion medium, Followed by drying and optionally rolling.

At this time, it is preferable that the cathode active material, the binder, and the conductive material can be uniformly dispersed in the dispersion medium and easily evaporated. Specific examples include acetonitrile, methanol, ethanol, tetrahydrofuran, water, isopropyl alcohol, N-methyl-2-pyrrolidone (NMP) and the like.

The lithium selenium battery according to one aspect of the present invention includes a positive electrode and a negative electrode disposed opposite to each other, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode is the above-described positive electrode.

The negative electrode may include a material capable of reversibly intercalating or deintercalating lithium ions as a negative electrode active material, a material capable of reacting with lithium ions to reversibly form a lithium-containing compound, a lithium metal, and a lithium alloy .

As the material capable of reversibly intercalating / deintercalating lithium ions, any carbonaceous anode active material commonly used in the lithium sulfur battery can be used as the carbonaceous material, and specific examples thereof include crystalline carbon, amorphous carbon, Can be used. Typical examples of the material capable of reacting with the lithium ion to form a lithium-containing compound reversibly include tin oxide (SnO ₂ ), titanium nitrate, silicon (Si), and the like, but are not limited thereto. The lithium metal alloy may specifically be an alloy of lithium and a metal of Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, or Cd.

In addition, the negative electrode may further include a binder in addition to the negative active material.

The binder acts as a paste for the anode active material, mutual adhesion between the active materials, adhesion between the active material and the current collector, buffering effect on expansion and contraction of the active material, and the like. Specifically, the binder is the same as described above.

The negative electrode may further include a negative electrode collector for supporting the negative electrode active material layer including the negative electrode active material and the binder.

The negative electrode current collector may be selected from the group consisting of Cu, Al, stainless steel, Ti, Ag, Pd, Ni, alloys thereof, and combinations thereof. The stainless steel may be surface-treated with C, Ni, Ti or Ag, and an Al-Cd alloy may be used as the alloy. In addition, fired carbon, a nonconductive polymer surface-treated with a conductive material, or a conductive polymer may be used.

According to an embodiment of the present invention, the cathode may be a thin film of lithium metal.

In addition, in the lithium sulfur battery, the separation membrane is a physical separator having a function of physically separating an electrode, and can be used without any particular limitation as long as it is used as a separation membrane in a lithium sulfur battery. Particularly, It is preferable that the electrolytic solution has a small resistance and excellent ability to humidify the electrolyte.

More specifically, the separator may be a porous polymer film such as a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer The polymer film may be used alone or as a laminate thereof, or may be a nonwoven fabric made of a conventional porous nonwoven fabric, for example, glass fiber of high melting point, polyethylene terephthalate fiber or the like, but is not limited thereto.

In addition, according to an embodiment of the present invention, the separator may be a porous substrate such as the porous polymer film, porous nonwoven fabric, or the like alone, and may further include a porous material including inorganic particles and a binder polymer on at least one surface of the porous substrate It may be a form having a coating layer.

The lithium sulfur battery may further include an electrolyte immersed in the separation membrane. The electrolyte may include an organic solvent and a lithium salt.

The organic solvent may specifically be a polar solvent such as an aryl compound, a bicyclic ether, a cyclic carbonate, a sulfoxide compound, a lactone compound, a ketone compound, an ester compound, a sulfate compound or a sulfite compound.

More specifically, the organic solvent is selected from the group consisting of 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane, Dioxolane (DOL), 1,4-dioxane, tetrahydrofuran , Dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), ethyl propyl carbonate, dipropyl carbonate, butyl ethyl carbonate, ethyl propanoate (EP), toluene, xylene, dimethyl ether (DME), diethyl ether, ethylene glycol ethyl methyl ether (EGEME), triethylene glycol monomethyl ether TEGME), diglyme, tetraglyme, hexamethyl phosphoric triamide, gamma butyrolactone (GBL), acetonitrile, propionitrile, ethylene carbonate (EC), propylene carbonate - Me Dimethyl sulfoxide (SL), methyl sulfolane, dimethylacetamide, dimethyl sulfoxide, dimethyl sulfate, ethylene glycol (dimethyl sulfoxide), dimethyl sulfoxide Diacetate, dimethylsulfite, or ethylene glycol sulfite.

Among the above-mentioned organic solvents, a mixed solvent of ethylene glycol ethyl methyl ether / dioxolane / dimethyl ether may be more preferable.

In addition, the lithium salt can be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, LiN (F ₅ SO ₂₎ with the lithium salt _{2 (lithium bis (fluorosulfonyl) imide} , LiFSI), LiNO 3, LiPF 6, LiClO 4, LiAsF 6, LiBF 4, LiSbF 6, LiAl0 4, LiAlCl 4, _{LiCF 3 SO 3, LiC 4 F} 9 SO 3, LiN (C 2 F 5 SO 3) 2, LiN (C 2 F 5 SO 2) 2 (Lithium bis (perfluoroethylsulfonyl) imide, BETI), LiN (CF 3 SO 2 _{) 2 (Lithium bis (Trifluoromethanesulfonyl)} imide, LiTFSI), LiN (C a F 2a + 1 SO 2) (C b F 2b + 1 SO 2) ( However, a and b is a natural number, preferably 1≤a≤ 20 and 1? B? 20), lithium poly [4,4 '- (hexafluoroisopropylidene) diphenoxy] sulfonylimide (lithium poly [4,4'- (hexafluoroisopropylidene) diphenoxy] sulfonylimide , LiPHFIPSI), LiCl, LiI, LiB (C ₂ O ₄ ) _2, etc. Among them, a sulfonyl group-containing imide lithium compound such as LiFSI, LiTFSI, BETI or LiPHFIPSI may be more preferable

The lithium salt may be contained in an amount of 1 to 60% by weight, more preferably 10 to 35% by weight based on the total weight of the electrolyte. When the content of the lithium salt satisfies this range, the conductivity and viscosity of the electrolyte are appropriately controlled to improve the electrolyte performance and improve the mobility of the lithium ion.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to examples. However, the embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.

Example 1

A graphene flake was prepared by using a high-pressure homogenizer (Nano Disperser (NH500)) as a dispersion containing thermally expanding graphite (TIMCAL, BNB90) in distilled water as a dispersion medium. Specifically, a high pressure of about 1,600 bar was applied to the dispersion by means of a high-pressure homogenizer, and a fine channel having a diameter of 175 mu m was passed through to obtain a sample from the outlet of the high-pressure homogenizer.

The sampled sample was filtered and dried to recover the powder.

Then, 20 cc of 1 mm diameter zirconia balls were filled in an 80 cc jar of a planetary ball mill apparatus (Pulvereisette 6, Fritsch), 0.7 g of the obtained graphene powder and 10 g of ammonium chloride were filled in the glove box, I closed the jar lid and sealed it.

Subsequently, the reaction was carried out under stirring at 450 rpm for 20 hours, and the reaction was carried out in an atmosphere of argon atmosphere. Thereafter, after the end of the time, the polarized powder was collected using a mesh.

Then, the powder was placed in an alumina boat, and heat treatment was performed in a tube furnace in an Ar atmosphere. The heat treatment was performed at 800 ° C for 1 hour, and the temperature was raised at a rate of 10 ° C / min.

SEM and TEM images were taken to analyze the powder characteristics in the course of the production, and the surface morphology was confirmed. The results of the XPS analysis were shown in FIGS. 2 and 3, respectively.

FIG. 2 is a SEM photograph showing the surface state of graphene flakes in each step of preparing nitrogen-doped graphene according to Example 1 of the present invention. FIG.

3 is a result of XPS analysis of nitrogen-doped graphene prepared according to Example 1 of the present invention.

As shown in FIG. 2, the surface state of the graphene flake in each step of preparing nitrogen-doped graphene according to Example 1 was confirmed to be formed with a large amount of pores and wrinkles on the surface as the process progresses.

Further, as shown in FIG. 3, nitrogen-doped graphene prepared according to Example 1 of the present invention showed two N-state peaks at 398.4 eV (pyridinic N) and 400.6 eV (graphitic N) And it was confirmed that about 3% of the doped nitrogen was formed.

4A and 4B are SEM photographs and TEM photographs of nitrogen-doped graphene prepared according to Example 1 of the present invention, respectively. As shown in the figure, it was confirmed that the graphene flakes prepared according to Example 1 of the present invention exhibited a morphology having a large amount of pores and wrinkles.

Comparative Example 1

The graphene flakes were prepared by using high pressure homogenizer in dispersion containing distilled water and thermally expanding graphite. Specifically, a high pressure of about 1600 bar was applied to the dispersion by a high-pressure homogenizer to pass a microfluidic channel, and a sample was taken from the outlet of the high-pressure homogenizer.

The sampled sample was filtered and dried to recover the powder.

Then, the powder was placed in an alumina boat, and heat treatment was performed in a tube furnace in an Ar atmosphere. The heat treatment was performed at 800 ° C for 1 hour, and the temperature was raised at a rate of 10 ° C / min.

Comparative Example 1 was different from Example 1 in that the graphene flakes obtained by using the high-pressure homogenizer of the thermally expanding graphite were subjected to a heat treatment immediately without adding a nitrogen-doping step of ball milling with addition of ammonium chloride.

Example 2 (Preparation of positive electrode and lithium sulfur battery)

6 parts by weight of the nitrogen-doped graphene prepared in Example 1 and 14 parts by weight of sulfur were subjected to molar mixing and melt-diffusion at 155 DEG C for 30 minutes to prepare a sulfur-graphene composite . At this time, the optimum temperature for causing the sulfur to melt and flow into the pores of the carbon host, graphene, was 155 ° C. It was carried out in a heat treatment process in an oven with the contact with outside air being blocked.

Specifically, at room temperature, orthorhombic sulfur in the form of S8 was converted to monoclinic sulfur at 95.5 ° C and melted at 118.7 ° C. However, liquid sulfur coexisted with cyclo-S8 and catena-S8 forms near 155 ° C. The S8 form forms a linear sulfenyl diradical and is melt-diffused into the carbon host.

The mixture of sulfur-graphene composite material: Denka Black: SBR: CMC in a weight ratio of 90: 5: 3.5: 1.5 was used as a conductive material, and SBR / CMC was used as a binder. To prepare an aqueous slurry having a solid content of 20%.

The prepared slurry was applied to an aluminum foil current collector and dried to prepare a positive electrode (energy density of the positive electrode: 3 mAh / cm 2).

A lithium metal thin film was prepared as a cathode.

After the prepared positive electrode and negative electrode were positioned to face each other, a polyethylene separator was interposed between the positive electrode and the negative electrode.

Thereafter, an electrolyte was injected into the case to prepare a lithium sulfur battery. At this time, the electrolyte is an organic solvent ethylene glycol methyl ether / dioxolane / dimethyl ether (EGEME / DOL / DME) ( 4/2/4 volume ratio) mixed at a 0.4 molar concentration of lithium salt LiFSI in, and the electrolytic solution LiNO ₃ And 4% by weight based on the electrolyte.

Comparative Example 2 (Preparation of positive electrode and lithium sulfur battery)

A sulfur-graphene composite, a positive electrode using the same, and a lithium-sulfur battery including such a positive electrode were prepared in the same manner as in Example 2, except that the graphene flakes prepared in Comparative Example 1 were used.

Comparative Example 3 (Preparation of N-doped graphene nanoflotlet)

In an 80 cc jar of a planetary ball mill apparatus (Pulvereisette 6, Fritsch), 20 cc of 1 mm diameter zirconia balls were filled, 0.7 g of graphene nanoplatelets (XG sciences, XGNP) and 10 g of ammonium chloride were filled , The jar lid was closed and sealed in the glove box for Ar filling.

Subsequently, the reaction was carried out under stirring at 450 rpm for 20 hours, and the reaction was carried out in an atmosphere of argon atmosphere. Thereafter, after the end of the time, the polarized powder was collected using a mesh.

Then, the powder was placed in an alumina boat, and heat treatment was performed in a tube furnace in an Ar atmosphere. The heat treatment was performed at 800 ° C for 1 hour, and the temperature was raised at a rate of 10 ° C / min.

Comparative Example 4 (Preparation of positive electrode and lithium sulfur battery)

Graphene composite, a positive electrode using the same, and lithium sulfur having such an anode were obtained in the same manner as in Example 2, except that the N-doped graphene nanoflotette prepared according to Comparative Example 3 was used. A battery was prepared.

Evaluation results

(1) Elemental analysis

Elemental analysis (EA) of the nitrogen-doped graphene prepared according to Example 1 of the present invention was performed, and the results are shown in Table 1 below.

At this time, the element analysis method is as follows.

1) Measure approximately 0.1 g of sample accurately and place in corning tube.

2) Dissolve aqua regia (hydrochloric acid: nitric acid = 3: 1 (volume ratio)) into the coating tube containing the sample.

3) The corning tube containing the sample is boiled in water to evaporate the acid.

4) If the acid remains below 1 mL, dilute to 25 mL of ultrapure water and add the Internal Standard (Internal Standard). (Internal standard substance: Sc)

5) Filter the effluent and measure it.

C H N O Example 1 92.9 0.46 2.93 1.05

(2) Measurement of specific surface area, total pore volume and mean pore diameter

To evaluate the characteristics and performance of the nitrogen-doped graphene prepared in Example 1 and the graphene flake prepared according to Comparative Example 1, the adsorption / desorption isomers of nitrogen per pressure and the pore volume of each pore diameter were measured 5a to 5d.

FIG. 6 shows the adsorption / desorption isotherm of the nitrogen of the graphene nanoflake after the nitrogen (N) doping process and the graphene nanoflotlet after the nitrogen (N) doping process of Comparative Example 3. FIG. 7 shows the pore volume of the graphene nanoflake after the nitrogen (N) doping process and the graphene nanoflake after the nitrogen (N) doping process of Comparative Example 3. FIG.

The nitrogen doped graphene prepared in Example 1, the graphene flake prepared according to Comparative Example 1, the graphene nanoflite before the nitrogen (N) doping treatment of Comparative Example 3, and the nitrogen (N) doping After the treatment, the specific surface area, total pore volume, and average pore diameter of the graphene nanoflake tablet were measured. The results are shown in Table 2 below.

The adsorption / desorption isotherm of nitrogen in the specific surface area, total pore volume, average pore diameter and pressure was measured by a nitrogen gas adsorption / distribution method using a porosimetry analyzer (Bell Japan Inc, Belsorp-II mini) BET (Brunauer-Emmett-Teller) 6-point method.

Specific surface area (m ² / g) Total pore volume
_{(p / p 0 = 0.990)} (cm 3 / g) Average pore diameter (nm) Comparative Example 1
(Graphene flake) 64.78 0.18 11.32 Example 1
(Nitrogen doped graphene) 293.67 0.93 12.70 Comparative Example 3
(Graphene nanopletlet before nitrogen doping) 18.4 0.09 19.39 Comparative Example 3
(Graphene nanoflotetlet after nitrogen doping) 101 0.12 4.73

Referring to Table 2, the graphene flakes obtained by peeling off by applying the high pressure homogenization process in Comparative Example 1 are graphenes having about 40 layers, and compared with the graphene flakes of Comparative Example 1, In the case of nitrogen doped graphene obtained by treating ammonium chloride, the specific surface area and the pore volume were increased 4 times or more.

On the other hand, in Comparative Example 3, the graphene nanoflotette before the nitrogen doping treatment was graphen having about 140 layers, and the graphene nanoflake obtained after the nitrogen doping treatment had an increased specific surface area and pore volume Respectively.

However, it was confirmed that the nitrogen doped graphene of Example 1 exhibited a remarkably improved value in terms of specific surface area and pore volume, as compared with the graphene nanoplettelet obtained after the nitrogen doping treatment of Comparative Example 3.

(3) Charging / discharging characteristic measurement

A cathode of a CR2032 coin cell was produced by using the positive electrodes of Example 2, Comparative Example 2 and Comparative Example 4. The charge / discharge characteristics of the cell were measured. The results are shown in FIG.

In this case, lithium metal having a diameter of 15 mm was used as a cathode, polypropylene (PE) having a diameter of 18 mm was used as a separator, 1,3-dioxolane (DOL): dimethyl ether (DME) 1: 1 (v / v% ), it was used as the electrolyte containing 1M LiTFSi, 0.5M LiNO _3.

In the charge / discharge characteristics measurement, the charge / discharge voltage range was 1.5 to 2.6 V, and the rate was 0.1 C for discharge and 0.1 C for charge.

Referring to FIG. 7, when the charge / discharge characteristics of the sulfur and graphene composite electrode having the loading of about ² mAh / cm ² were evaluated, the cell having the anode of Comparative Example 2 was polarized at an initial discharge curve, mAh / g, and a battery having a positive electrode of Comparative Example 4 showed a discharge capacity as low as about 1,000 mAh / g.

On the contrary, the anode prepared by complexing nitrogen with sulfur-doped graphene [293 m 2 / g specific surface area, 0.93 cm 3 / g pore volume] prepared by ball milling ammonium chloride with graphene flake (GF) Showed an increased discharge capacity at an initial capacity of about 1,140 mAh / g.

From the above experimental results, it can be seen from the above results that when graphene flakes are formed by forming a large amount of pores and wrinkles in the graphene flakes according to the method of the present invention, and a composite in which sulfur is supported using the graphene flakes is used as the anode of the lithium sulfur battery, The specific surface area and pore volume of the pin flakes can be drastically increased as compared with conventional graphenes, so that the sulfur bearing performance is significantly increased. Also, since nitrogen atoms are introduced into the graphene, lithium polysulfide It is expected that the lifetime of the battery can be prolonged.

As described above, preferred embodiments of the present invention have been disclosed in the present specification and drawings, and although specific terms have been used, they have been used only in a general sense to easily describe the technical contents of the present invention and to facilitate understanding of the invention , And are not intended to limit the scope of the present invention. It is to be understood by those skilled in the art that other modifications based on the technical idea of the present invention are possible in addition to the embodiments disclosed herein.

Claims (9)

The dispersion containing the graphite is passed through a microfluidic channel of a high pressure homogenizer (HPH) comprising an inlet, an outlet, and a microchannel having a micrometer-sized diameter connecting the inlet and outlet, A first step of peeling the graphite through high pressure homogenization to obtain graphene flakes;
A second step of filtering and drying the obtained graphene flake, putting it into a ball mill together with an ammonium compound, and conducting dry ball milling in an inert atmosphere to obtain grains in a state in which the ammonium compound is surrounded on the surface of the graphene flake; And
And a third step of heat treating the obtained grains to obtain graphene doped with nitrogen atoms. The method according to claim 1,
Wherein the high-pressure homogenization is performed by applying a pressure of 500 to 3,000 bar to the dispersion. The method according to claim 1,
Wherein the ammonium compound is ammonium chloride. The method according to claim 1,
The second step is to prepare nitrogen-doped graphene by stirring inert gas and air into the ball mill and stirring the mixture at 200 to 700 rpm for 10 to 30 hours using a ball having a diameter of 0.1 to 5 mm Way. The method according to claim 1,
Further comprising the step of sorting the particles obtained in the second step using a mesh before the third step. The method according to claim 1,
The heat treatment in the third step is carried out in a tube furnace under an inert atmosphere at a temperature of 700 to 1,000 DEG C, for 0.5 to 5 hours, and at a temperature raising condition of 8 to 15 DEG C / min, A method for producing graphene doped with nitrogen. A process for producing a sulfur-graphene composite, comprising the steps of: (a) preparing a sulfur-graphene composite by mixing sulfur with nitrogen in a nitrogen-doped graphene produced by the process of any one of claims 1 to 6; A lithium sulfur battery anode comprising a sulfur-graphene composite produced by the method of claim 7. A positive electrode of claim 8; cathode; And a separator disposed between the anode and the cathode.

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