WO2017209383A1 - Carbon-based fiber sheet and lithium-sulfur battery including same - Google Patents

Abstract

The present invention relates to a carbon-based fiber sheet and a lithium-sulfur battery including the same. According to the present invention, a carbon-based fiber sheet for a lithium-sulfur battery is doped with highly concentrated nitrogen so as to perform roles of adsorbing lithium polysulfide eluted from a cathode during charging or discharging and of preventing the diffusion thereof, such that a shuttle reaction is suppressed, thereby enabling capacity and lifespan characteristics of a lithium-sulfur battery to improve.

Description

Carbon-based fiber sheet and lithium-sulfur battery comprising the same

This application claims the benefit of priority based on Korean Patent Application No. 10-2016-0068855 filed June 2, 2016 and Korean Patent Application No. 10-2017-0031126 filed March 13, 2017. All content disclosed in the literature is included as part of this specification.

The present invention relates to a carbon-based fiber sheet having an excellent adsorption effect on lithium polysulfide and a lithium-sulfur battery including the same.

Recently, small size and light weight of electronic products, electronic devices, communication devices, etc. are rapidly progressing, and as the necessity of electric vehicles increases in relation to environmental problems, the demand for improving the performance of secondary batteries used as power sources of these products is also increasing. It is the situation. Among them, lithium secondary batteries have received considerable attention as high-performance batteries because of their high energy density and high standard electrode potential.

In particular, a lithium-sulfur (Li-S) battery is a secondary battery that uses a sulfur-based material having an SS bond (Sulfur-sulfur bond) as a positive electrode active material and uses lithium metal as a negative electrode active material. Sulfur, the main material of the positive electrode active material, is very rich in resources, has no toxicity, and has an advantage of having a low weight per atom. In addition, the theoretical discharge capacity of the lithium-sulfur battery is 1,675 mAh / g-sulfur, and the theoretical energy density is 2,600 Wh / kg, and the theoretical energy density of other battery systems currently under study (Ni-MH battery: 450 Wh / kg, Li -FeS battery: 480 Wh / kg, Li-MnO ₂ battery: 1,000 Wh / kg, Na-S battery: 800 Wh / kg) is very high compared to the most promising battery developed until now.

During the discharge reaction of the lithium-sulfur battery, an oxidation reaction of lithium occurs at a negative electrode, and a reduction reaction of sulfur occurs at a positive electrode. I have a S ₈ the structure of the cyclic sulfur prior to discharge, the reduction (discharge) the oxidation of SS bonds are broken As reduces the oxidation number of S, and as the oxidation (charge) when SS bond is formed again, increases the oxidation state of the S - Reduction reactions are used to store and generate electrical energy. During this reaction, sulfur is converted into lithium polysulfide (Lithium polysulfide, Li ₂ S _x , x = 8, 6, 4, 2) by a reduction reaction in a cyclic S ₈ , so that the lithium polysulfide is completely Reduction will eventually lead to the formation of lithium sulfide (Lithium sulfide, Li ₂ S). Discharge behavior of the lithium-sulfur battery by the process of reduction to each lithium polysulfide is characterized by showing the discharge voltage step by step unlike the lithium ion battery.

Among lithium polysulfides such as Li ₂ S ₈ , Li ₂ S ₆ , Li ₂ S ₄ , and Li ₂ S ₂ , in particular, lithium polysulfide (Li ₂ S _x , usually x> 4) having a high number of sulfur oxides can Easily melts Lithium polysulfide dissolved in the electrolyte is diffused away from the positive electrode where lithium polysulfide is formed due to the difference in concentration. The lithium polysulfide eluted from the positive electrode is lost out of the positive electrode reaction region, so that stepwise reduction to lithium sulfide (Li ₂ S) is impossible. That is, since lithium polysulfide, which is present in the dissolved state outside the positive electrode and the negative electrode, cannot participate in the charge / discharge reaction of the battery, the amount of sulfur material participating in the electrochemical reaction at the positive electrode decreases, and eventually lithium-sulfur It is a major factor causing a decrease in the charge capacity and energy of the battery.

In addition, lithium polysulfide diffused to the negative electrode, in addition to being suspended or precipitated in the electrolyte, is directly reacted with lithium and fixed in the form of Li ₂ S on the surface of the negative electrode, causing a problem of corrosion of the lithium metal negative electrode.

In order to minimize the elution of the lithium polysulfide, researches are being conducted to modify the morphology of the cathode composite which forms a composite by supporting sulfur particles in various carbon structures or metal oxides.

On the other hand, the doping of nitrogen to the carbon material has the effect of changing the surface polarity (Polarity) and the lithium polysulfide adsorbed, in particular, among the various nitrogen functional groups doped on the carbon surface (Pyrrolic group and pyridine group (Pyridinic group) ) Has been reported to have an effect on the adsorption of lithium polysulfide (Chem. Mater., 2015, 27, 2048-2055 / Adv. Funct. Mater., 2014, 24, 1243-1250). Therefore, it is time to increase the nitrogen content of the carbon material to improve the adsorption capacity of lithium polysulfide.

[Preceding technical literature]

[Patent Documents]

Korean Laid-Open Patent Publication No. 2016-0032862, (2016.03.25), a method for preparing nitrogen-doped graphene and nitrogen-doped graphene prepared therefrom

[Non-Patent Documents]

Jia-Jia Chen et al., Conductive Lewis Base Matrix to Recover the Missing Link of Li ₂ S ₈ during the Sulfur Redox Cycle in Li-S Battery, Chem. Mater., 2015, 27, 2048-2055

Jiangxuan Son et al., Nitrogen-Doped Mesoporous Carbon Promoted Chemical Adsorption of Sulfur and Fabrication of High-Areal-Capacity Sulfur Cathode with Exceptional Cycling Stability for Lithium-Sulfur Batteries, Adv. Funct. Mater., 2014, 24, 1243-1250

As described above, the lithium-sulfur battery has a problem in that the capacity and life characteristics of the battery decrease as the charge / discharge cycle progresses due to lithium polysulfide eluted from the positive electrode.

Accordingly, the present inventors have conducted various studies to solve the above problems. As a result, by increasing the concentration of nitrogen doped to the carbon-based fiber sheet, lithium polysulfide eluted from the positive electrode is effectively suppressed, thereby improving the capacity and life of the battery. It was confirmed.

Accordingly, an object of the present invention is to provide a carbon-based fiber sheet doped with a high concentration of nitrogen.

Another object of the present invention to provide a method for producing the carbon-based fiber sheet.

In addition, another object of the present invention to provide a lithium-sulfur battery comprising the carbon-based fiber sheet.

In order to achieve the above object, the present invention provides a carbon-based fiber sheet doped with a high concentration of nitrogen.

In addition, the present invention provides a method for producing a carbon-based fiber sheet doped with a high concentration of nitrogen.

In addition, the present invention provides a lithium-sulfur battery including a carbon-based fiber sheet doped with a high concentration of nitrogen between the positive electrode and the separator.

The carbon-based fiber sheet for lithium-sulfur batteries according to the present invention serves to prevent diffusion by adsorbing lithium polysulfide eluted from the positive electrode during charging and discharging, thereby inhibiting the shuttle reaction, and thus capacity and life characteristics of the lithium-sulfur battery. Can improve.

1 is a cross-sectional view of a lithium-sulfur battery to which the carbon-based fiber sheet of the present invention is applied.

2 is a scanning electron microscope image of carbon fiber felt stabilized by heat treatment of polyacrylonitrile according to the preparation example of the present invention.

3 is a scanning electron microscope image of a carbon-based fiber sheet prepared by carbonizing a carbon fiber felt according to the preparation example of the present invention.

4 is a graph showing the initial discharge capacity of the lithium-sulfur battery according to the Examples and Comparative Examples of the present invention.

5 is a graph showing cycle life characteristics of a lithium-sulfur battery according to Examples and Comparative Examples of the present invention.

6 is a graph showing the charging and discharging efficiency of the lithium-sulfur battery according to the Examples and Comparative Examples of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. These drawings may be implemented in various different forms as an embodiment for explaining the invention, it is not limited to this specification. In the drawings, parts irrelevant to the description are omitted in order to clearly describe the present invention, and similar reference numerals are used for similar parts throughout the specification. In addition, the size and relative size of the components shown in the drawings are not related to the actual scale, may be reduced or exaggerated for clarity of description.

The present invention is intended to prevent diffusion by adsorbing lithium polysulfide eluted from the positive electrode through a carbon-based fiber sheet between the positive electrode and the separator of the lithium-sulfur battery. Hereinafter, each configuration will be described in detail.

Carbon fiber sheet

The carbon-based fiber sheet proposed in the present invention means a plurality of fiber structures in which carbon-based fibers form a three-dimensional porous network structure. The fiber structure includes carbon-based fibers forming irregular aggregates in the form of plates, or carbon-based fibers forming regular aggregates in three dimensions through weaving.

In this case, the carbon-based fiber may be one or more selected from the group consisting of carbon fibers, carbon nanofibers, graphite fibers, and graphene fibers, but is not limited thereto. Preferably carbon fibers are used, which can be crystalline or amorphous.

The carbon-based fiber sheet may be in the form of one or more fabrics selected from the group consisting of felt, mat, paper, and cloth, preferably using felt.

The term 'felt' refers to the formation of irregular aggregates in the form of a plate made of a fiber produced by spinning a carbon or graphite material.

The 'mat' and 'paper' refer to the formation of a thinner aggregate of carbon fiber fibers using an organic binder.

The term 'cloth' means that the carbon fibers produced through the spinning and the like form a three-dimensionally regular aggregate through weaving.

The carbon-based fiber sheet has a three-dimensional network structure formed by combining carbon-based fibers regularly or irregularly, and excellent in rigidity, and have a plurality of pores without being easily deformed, thereby allowing the movement of the electrolyte to be smoothly performed. . The carbon-based fiber sheet has a complex micropores, mesopores, macropores, etc. in the structure, the control of these pores may vary depending on the manufacturing method.

In the case of using particles such as carbon black, fixing in the battery is difficult, and even if fixed, movement occurs easily during driving of the battery, and thus uniform contact with the electrolyte is difficult. In addition, in the case of the carbon-based fiber, it is excellent in chemical resistance, abrasion resistance, heat resistance, light weight, stability in a wide voltage range, etc., and thus is suitable for application to batteries. In comparison, materials other than carbon, such as metal, are inferior in thermal, chemical, mechanical, and electrical and electronic properties to carbon, and it is difficult to satisfy all of the basic properties required for battery application such as electrolyte damping and resistance. Therefore, the use of a carbon-based fiber sheet in terms of form and material may be most preferred.

In particular, the carbon-based fiber sheet in the present invention uses a nitrogen doped. The doping means that a part of the carbon atoms constituting the carbon-based fiber is substituted with a nitrogen atom.

The study of nitrogen-doped carbon materials has already become one of the issues in the field of international carbon materials. The main reason is that the nitrogen atom has one more valence electron than the carbon atom, and the nitrogen atom doping provides a six-membered ring structure. After entering, it forms a functional group containing nitrogen such as pyridine, pyrrole, graphite nitrogen and pyridine oxide, thereby improving the surface chemical activity of the carbon material and controlling the electronic structure.

In the lithium-sulfur battery, there is a conventional technique using a nitrogen-doped carbon material to adsorb lithium polysulfide, but the nitrogen doping concentration is low at 0.01 to 3% by weight, and thus satisfactory lithium polysulfide adsorption effect cannot be obtained. .

In the present invention, by doping at a high concentration of at least 5% by weight or more with nitrogen instead of carbon in the carbon-based fiber sheet, lithium polysulfide adsorption capacity is improved to exhibit improved electrochemical properties.

Carbon-based fiber sheet according to the present invention is nitrogen doped at least 5% by weight, preferably 5 to 30% by weight, more preferably 10 to 25% by weight based on the total weight, through a conventional nitrogen doping process It is characterized by a high concentration of nitrogen doped compared to the prepared carbon material. If the doping concentration of nitrogen is less than the above range it is not possible to obtain the effect of improving lithium polysulfide adsorption capacity, on the contrary, if it exceeds the above range, capacity reduction and battery characteristics may be reduced.

As described later, the carbon-based fiber sheet of the present invention is interposed in a lithium-sulfur battery, specifically, between the positive electrode and the separator. Therefore, it is necessary to control the parameters so as to increase the adsorption capacity of lithium polysulfide without affecting the function of the anode and the separator and not affecting the flow of the electrolyte (ie lithium ion transfer, etc.) existing between them. .

The parameter may include the diameter of the fibers constituting the carbon-based fiber sheet, the size of the pores in the sheet, the porosity, the specific surface area and the bulk density, and by limiting them, the effect may be maximized.

First, in order to increase the polysulfide adsorption effect capacity and increase the lithium ion transfer effect without inhibiting the lithium-sulfur battery performance, it is made of a porous sheet. Therefore, the carbon-based fiber sheet is made of a carbon-based fiber, it is necessary to limit the diameter of the carbon-based fiber, the porosity, porosity and bulk density of the carbon-based fiber sheet, thereby maximizing the effects described above Can be.

Specifically, the diameter of the individual fibers constituting the carbon-based fiber sheet according to the present invention may be 0.01 to 100 ㎛, preferably 0.01 to 50 ㎛, more preferably 0.05 to 10 ㎛, carbon having such characteristics The fiber sheet is preferable to ensure excellent lithium polysulfide adsorption capacity. If the diameter of the carbon-based fiber is less than 0.01 μm, a problem arises that the structure is not maintained and the structure collapses during heat treatment for carbonization. On the contrary, if the diameter of the carbon-based fiber exceeds 100 μm, the thickness of the carbon-based fiber sheet is too thick. The problem of lowering the energy density of the battery is rather reduced, so it is suitably used within the above range.

In addition, the carbon-based fiber sheet according to the present invention may have a thickness of 0.1 to 100 ㎛, preferably 0.1 to 50 ㎛. If the thickness of the carbon-based fiber sheet is less than the above range it is difficult to secure the lithium polysulfide adsorption effect, on the contrary, if the carbon-based fiber sheet exceeds the above range may act as a resistance may cause problems in battery performance. Usually, considering that the thickness of the positive electrode active material is 100 μm, it is most preferable that it is 0.1 to 50 μm.

In addition, the carbon-based fiber sheet according to the present invention may have a pore size of 0.1 to 10 ㎛, porosity of 5 to 90%, specific surface area of 5 to 500 m 2 / g. Preferably, the pore size is 0.5 to 5 μm, the porosity is 30 to 90%, the specific surface area is 10 to 200 m 2 / g, and such pore characteristics are not applied as a resistance when applied to lithium-sulfur batteries. Therefore, the electrochemical property is desirable to secure an improved effect. If the porosity of the carbon-based fiber sheet is less than 0.1 μm, the electrolyte may be difficult to penetrate into the sheet, and thus the battery may not operate. This causes a problem that lithium polysulfide adsorption does not occur evenly, so it is suitably used within the above range.

In addition, when the porosity is higher than the above-mentioned range, mechanical properties of the carbon-based fiber sheet may be degraded, thereby causing problems in battery assembly or driving. On the contrary, when the porosity is lower than the above-mentioned range, the flow of electrolyte may be delayed. Since the differential pressure inside the battery may increase and battery performance may decrease due to overvoltage, it is suitably used within the above range. In addition, as a parameter related to the density of the carbon-based fiber sheet, a bulk density of 0.05 to 0.2 g / cm 3 and preferably 0.1 to 0.15 g / cm 3 based on 3 mm thickness is used. When the bulk density is higher than the above-mentioned range, the fluid pressure of the electrolyte may be interrupted to increase the pressure difference in the battery. On the contrary, when the bulk density is lower than the above-mentioned range, the residence time of the electrolyte in the carbon-based fiber sheet is sufficiently increased. Can't keep up

The carbon-based fiber sheet of the present invention may further include a catalyst material in order to enhance the adsorption effect of lithium polysulfide.

The catalyst material may be a carbon-based conductive material and / or metal particles.

For example, the carbon-based conductive material may be carbon paper, carbon fiber, carbon black, acetylene black, activated carbon, fullerene, carbon nanotube, carbon nanowire, carbon nano-horn, carbon nano ring (carbon nano ring) and one kind selected from the group consisting of a combination thereof are possible.

The metal particles are Na, Al, Mg, Li, Ti, Zr, Cr, Mn, Co, Cu, Zn, Ru, Pd, Rd, Pt, Ag, Au, W, Ti, Zr, Ni, Cu, and Fe At least one selected from the group consisting of is possible. They may use particles of several nanometers to several hundred microns for the catalytic effect of the electrochemical reaction, preferably those having a nanoscale particle size.

The catalyst material is used at a level that does not prevent the flow of the electrolyte, and may be used in an amount of 10% by weight or less in the carbon-based fiber sheet.

Various methods may be used to prepare the carbon-based fiber sheet according to the present invention as described above.

For example, after the carbon-based fiber sheet is prepared, doping may be performed by depositing nitrogen atoms on the carbon-based fiber sheet through a process such as chemical vapor deposition. However, this method adversely affects the lithium polysulfide adsorption capacity since the doped nitrogen is not uniformly doped throughout the sheet.

Preferably, the carbon-based fiber sheet of the present invention can be produced through a spinning process, in particular using a precursor containing a high nitrogen content to be doped with a high concentration of nitrogen.

 Specifically, the carbon-based fiber sheet doped with a high concentration of nitrogen of the present invention

i) preparing a spinning solution comprising a carbon-based precursor;

ii) spinning the spinning solution to produce a sheet in the form of a fibrous web;

iii) stabilizing the sheet through heat treatment in the presence of oxygen; And

iv) carbonizing the sheet in an inert atmosphere.

Hereinafter, each step will be described in detail.

First, a carbon-based precursor is dissolved in a solvent to prepare a spinning solution (step i)).

In this case, the carbon-based precursor is a nitrogen-containing precursor including nitrogen in the molecular structure, and specifically includes 5 to 20% of nitrogen atoms. If out of the range it is not easy to control the doping concentration of nitrogen in the carbon-based fiber sheet after carbonization.

The carbon-based precursor may be polyacrylonitrile (PAN), polyaniline (Polyaniline (PANI), polypyrrole (PPY), polyimide (PI), polybenzimidazole (Polybenzimidazole: PBI), polypyrrolidone ( Polypyrrolidone (Ppy), Polyamide (PA), Polyamide-imide (PAI), Polyaramide, Melamine, Melamine-formaldehyde and Fluorine mica At least one selected from the group consisting of), according to one embodiment it is preferable to apply polyacrylonitrile (PAN).

For example, when the spinning solution is prepared, the carbon-based precursor may be dissolved as it is or dissolved in a solvent. Specifically, the method of dissolving in a solvent is advantageous, wherein the carbon-based precursor is preferably included in 3 to 20 parts by weight based on 100 parts by weight of the solvent. If the content is less than 3 parts by weight, a kind of agglomerates called beads are formed for each structure due to low viscosity during electrospinning, and uniform fibers of a certain thickness cannot be produced. The thickness of the fiber sheet produced and produced is nonuniform.

For example, organic solvents for the preparation of spinning solutions include ethanol, methanol, propanol, butanol, butanol, isopropylalcohol (IPA), dimethylformamide (DMF) and acetone. (Acetone), tetrahydrofuran (THF), toluene, dimethylacetamide (DMAC) and the like can be used. The solvent is used in accordance with the hydrophilicity or hydrophobicity of the polymer material, and in the case of a polymer having hydrophilicity, distilled water (H ₂ O) as well as an organic solvent may be used.

For example, the spinning solution preferably has a viscosity of 1,000 to 30,000 cP. When the spinning solution has a high viscosity, the fiber orientation may be slowed down even when a high voltage is applied during electrospinning. Because it can be induced. The spinning solution may be prepared by selecting a polymer material prepared from an appropriate molecular weight and a chemical component among known materials used as the carbon precursor material to have the above-described viscosity.

Next, the spinning solution is spun to prepare a sheet in the form of a fibrous web (step ii)).

At this time, the spinning method and apparatus may be appropriately performed by those skilled in the art by selecting any known methods and apparatus.

For example, the distance between the spinning nozzle and the current collector is fixed at 10 to 20 cm, the voltage is set at 10 to 60 kV, and the supply rate of the spinning solution is adjusted to 0.5 to 100 ml per hour to be electrospinning. Do. Electrospinning is highly influenced by the voltage applied between the electrodes. At a voltage lower than the above range, fiberization does not occur easily, and beads are easily formed in the final sheet, and at a voltage exceeding the above range, current is easily flowed while the insulated equipment is shorted. Preferably, the fibers are effectively formed by spinning at intervals of 15 to 20 cm at a voltage of 20 to 40 kV. In addition, the feed rate of the solution in electrospinning is related to the production efficiency of the product. In consideration of the time required for sheet production within the above range, it can be appropriately selected.

In one embodiment, the spinning solution containing the carbon precursor may be electrospun to prepare a sheet in the form of a fibrous web. The sheet may be felt, mat, paper and cloth as described above, preferably felt. In another embodiment, a carbon fiber felt prepared from a commercially available carbon precursor containing nitrogen may be purchased and used, wherein the carbon fiber felt may be applied regardless of the method.

For example, when electrospinning is performed with a spinning solution containing polyacrylonitrile, a modified acrylic containing 5-15% of a copolymer as well as a homopolymer having a molecular weight of 150,000 or more may be used. . In this case, itaconic acid or methylacrylate may be used as the composition of the copolymer.

Next, the sheet is stabilized by heat treatment in the presence of oxygen (step iii)).

In this case, the sheet may be stabilized while heating at a temperature of 100 to 400 ° C., more preferably at a temperature of 200 to 350 ° C., at a rate of 5 to 20 ° C./min for 1 to 10 hours. Such heat treatment conditions are preferable to ensure pore size and porosity for polysulfide adsorption, so that stabilization conditions can be set within the above range.

In particular, in the present invention, as the carbon polymer is brought into contact with air and stabilized through heat treatment in the presence of oxygen, the carbon polymer is converted into a more stable chemical structure, and thus, the carbon polymer contains a large amount of nitrogen, thereby maximizing the doping effect of nitrogen. have. At this time, the flow rate of oxygen is preferably 50 ~ 200 ㎖ / min, there may be a problem of insufficient oxygen when stabilizing when less than, there is a problem, when there is a problem that the structure of the carbon-based fiber sheet collapses by oxidation.

In the stabilization step, if the temperature increase rate is less than the range, it takes too long, and if it exceeds the range, the stabilization reaction may occur rapidly, causing problems. In addition, when the temperature is less than the range takes a long time to stabilize, if the temperature exceeds the range there is a problem that the sheet is oxidized.

For example, when polyacrylonitrile is applied as a nitrogen-containing carbon precursor, since the polyacrylonitrile should not be pyrolyzed in this stabilization process, the stabilization process temperature is performed at a temperature lower than that of the polyacrylonitrile used. Through the stabilization process the polyacrylonitrile may be stabilized (incompatible).

The sheet is then carbonized in an inert atmosphere (step iv)).

At this time, the carbonization process is fired in an inert gas atmosphere such as helium (He), nitrogen (N ₂ ), argon (Ar), neon (Ne) or xenon (Xe). If oxygen or air is present, oxidation occurs and it is difficult to manufacture a sheet consisting solely of carbon / nitrogen. Therefore, carbonization may be performed in an inert atmosphere, and the carbonization process may proceed according to conventional conditions.

For example, the carbonization step may be performed at 600 to 1500 ° C., but firing while heating at a rate of 1 to 10 ° C./min is preferable to secure pore size and porosity for lithium polysulfide adsorption. Can be set.

Lithium-sulfur battery

The carbon-based fiber sheet doped with a high concentration of nitrogen presented in one embodiment of the above is preferably applied between an anode and a separator of a lithium-sulfur battery.

More specifically, as shown in FIG. 1, a lithium-sulfur battery including a positive electrode 100, a negative electrode 200, a separator 300, and an electrolyte 400 impregnated therein, includes the positive electrode 100 and the separator ( It is possible to assemble through the carbon-based fiber sheet 500 between the 300). The carbon-based fiber sheet 500 transmits lithium ions 10 and adsorbs the lithium polysulfide 20. As a result, the initial discharge capacity may be increased, and overvoltage characteristics may be improved. In addition, it exhibits excellent discharge capacity retention and maintains high capacity even after a long cycle.

Although the carbon-based fiber sheet 500 may be considered to be interposed between the separator 300 and the cathode 200, it does not play a large role in preventing direct adsorption of lithium polysulfide, and is in contact with the anode 100. Most advantageous. In addition, when interposed between the separator 300 and the negative electrode 200 there is a problem that the final thickness of the battery increases.

Meanwhile, the cathode 100 of the present invention may include elemental sulfur (S ₈ ), a sulfur-based compound, or a mixture thereof as a cathode active material, and since the sulfur material alone is not electrically conductive, Apply in combination. Specifically, the sulfur-based compound may be Li ₂ S _n (n ≧ 1), an organic sulfur compound, or a carbon-sulfur polymer ((C ₂ S _x ) _n : x = 2.5 to 50, n ≧ 2).

The conductive material may be porous. Therefore, the conductive material may be used without limitation as long as it has porosity and conductivity, and for example, a carbon-based material having porosity may be used. Such carbon-based materials may be carbon black, graphite, graphene, activated carbon, carbon fiber, carbon nanotubes (CNT), and the like. Moreover, metallic fibers, such as a metal mesh; Metallic powders such as copper, silver, nickel and aluminum; Or organic conductive materials, such as a polyphenylene derivative, can also be used. The conductive materials may be used alone or in combination.

The negative electrode 200 is a negative electrode active material, a material capable of reversibly intercalating or deintercalating lithium ions (Li ⁺ ), a material capable of reacting with lithium ions to form a reversible lithium-containing compound, lithium Metals or lithium alloys can be used. The material capable of reversibly occluding or releasing the lithium ions (Li ⁺ ) may be, for example, crystalline carbon, amorphous carbon or a mixture thereof. The material capable of reacting with the lithium ions (Li ⁺ ) to form a lithium-containing compound reversibly may be, for example, tin oxide, titanium nitrate or silicon. The lithium alloy is, for example, lithium (Li) and sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium ( It may be an alloy of a metal selected from the group consisting of Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), silicon (Si) and tin (Sn).

In addition, in the process of charging and discharging the lithium-sulfur battery, sulfur used as the positive electrode active material may be changed into an inert material and adhered to the surface of the lithium negative electrode. As described above, inactive sulfur refers to sulfur in which sulfur is no longer able to participate in the electrochemical reaction of the anode through various electrochemical or chemical reactions, and the inert sulfur formed on the surface of the lithium cathode is a protective layer of the lithium cathode. It also has the advantage of acting as).

A conventional separator 300 may be interposed between the anode 100 and the cathode 200. The separator 300 is a physical separator having a function of physically separating an electrode, and may be used without particular limitation as long as it is used as a conventional separator, and particularly, has a low resistance to ion movement of the electrolyte 400 and an electrolyte 400. ) It is preferable that the moisture content is excellent.

In addition, the separator 300 enables transport of lithium ions between the anode 100 and the cathode 200 while separating or insulating the anode 100 and the cathode 200 from each other. The separator 300 may be made of a porous and nonconductive or insulating material. The separator 300 may be an independent member such as a film or a coating layer added to the anode and / or the cathode.

Specifically, a porous polymer film made of a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer may be used alone. It may be used as a lamination or or a conventional porous non-woven fabric, for example, a non-woven fabric made of glass fibers, polyethylene terephthalate fibers of high melting point, etc. may be used, but is not limited thereto.

The electrolyte 400 impregnated in the positive electrode 100, the negative electrode 200, and the separator 300 is a non-aqueous electrolyte containing lithium salt, and is composed of lithium salt and electrolyte solution. In addition, an organic solid electrolyte and an inorganic solid electrolyte are used. Etc. are used.

Lithium salt of the present invention is a good material to be dissolved in a non-aqueous organic solvent, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiB (Ph) ₄ , LiPF ₆ , LiCF ₃ SO ₃ , _{LiCF 3 CO 2, LiAsF 6,} LiSbF 6, LiAlCl 4, LiSO 3 CH 3, LiSO 3 CF 3, LiSCN, LiC (CF 3 SO 2) 3, LiN (CF 3 SO 2) 2, LiNO 3, chloroborane lithium And one or more from the group consisting of lower aliphatic lithium carbonate, lithium tetraphenyl carbonate, lithium imide.

The concentration of the lithium salt is 0.2-4M, depending on several factors such as the exact composition of the electrolyte mixture, the solubility of the salt, the conductivity of the dissolved salt, the charging and discharging conditions of the cell, the operating temperature and other factors known in the lithium battery art, 0.3 to 2M, more specifically 0.3 to 1.5M. If the amount is less than 0.2M, the conductivity of the electrolyte may be lowered, and thus the performance of the electrolyte may be lowered. If it is used more than 4M, the viscosity of the electrolyte may be increased to reduce the mobility of lithium ions (Li ⁺ ).

The non-aqueous organic solvent should dissolve lithium salts 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, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethylsulfoxide, 1, 3-dioxolane, 4-methyl-1,3-dioxene, diethyl ether, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxy methane , Aprotic organic solvents such as dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate and ethyl propionate It may be used, the organic solvent may be a mixture of one or more organic solvents.

Examples of the organic solid electrolyte include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, poly etchation lysine, polyester sulfides, polyvinyl alcohol, polyvinylidene fluoride, and ionic dissociation. Polymers containing groups and the like can be used.

Examples of the inorganic solid electrolyte of the present invention 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 , Nitrides, halides, sulfates and the like of Li, such as Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , and the like, may be used.

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

The battery pack including the lithium-sulfur battery is an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), power It can be used as various power supplies such as storage devices.

The positive electrode 100 and the negative electrode 200 are interposed between the positive electrode plate and the negative electrode plate cut to a predetermined size with a separator 300 cut to a predetermined size corresponding to the positive electrode plate and the negative electrode plate, and the positive electrode 100 and the separator The stacked electrode assembly may be manufactured by stacking the carbon-based fiber sheet 500 between the 400.

Alternatively, the positive electrode 100 and the negative electrode 200 face each other with the separator 300 sheet interposed therebetween, and the carbon-based fiber sheet 500 is interposed between the positive electrode 100 and the separator 400, and at least two positive electrode plates and negative electrode plates are provided. Or two or more of the unit cells in which the two or more positive and negative plates are stacked with the separator interposed therebetween, and wound the separator sheet, or the size of the electrode plate or unit cell. By stacking the separator sheet, a stack-and-fold type electrode assembly can be manufactured.

Hereinafter, the present invention will be described in detail with reference to Examples. However, embodiments according to the present invention can be modified in many different forms, the scope of the present invention should not be construed as limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

< Production Example >

The polyacrylonitrile (PAN) prepared by the electrospinning of 50 μm thick felt was heat-treated for 2 hours while heating up at 200 ° C./min at 200 ° C. in an air atmosphere. Thereafter, carbonization was carried out for 2 hours while the temperature was raised at 700 ° C./min at 5 ° C./min to prepare a carbon-based fiber sheet.

< Test Example  1> Analysis

(1) nitrogen content analysis

Elemental analysis was performed to determine the nitrogen content of the carbon-based fiber sheet prepared in the preparation example. As a result, it was confirmed that nitrogen contained 16% by weight.

(2) property analysis

2 is a scanning electron microscope image of carbon fiber felt stabilized by heat treatment of polyacrylonitrile according to the preparation example of the present invention. 3 is a scanning electron microscope image of a carbon-based fiber sheet prepared by carbonizing a carbon fiber felt according to the preparation example of the present invention.

2 and 3, it was confirmed that even after the stabilization and carbonization process, the carbon-based fiber constituting the carbon-based fiber sheet maintains not only the original fiber shape but also the overall structure.

In addition, as a result of analysis through a scanning electron microscope (Scanning Electron Microscope, SEM) and BET (Brnauer, Emett & Teller) measurement, it was confirmed that the carbon-based fiber sheet of the present invention has the following physical properties.

Fiber diameter: 1 μm

* Pore Size: 4 ㎛

* Specific surface area: 60 ㎡ / g

Bulk Density: 0.001 g / cm ³

* Sheet porosity: 50%

* Sheet Thickness: 14 ㎛

< Example  1>

A lithium-sulfur battery was manufactured by using the carbon-based fiber sheet prepared in the preparation example.

First, a positive electrode mixture of 80 wt% sulfur / carbon composite, 10 wt% carbon black (carbon material), and 10 wt% PVDF (binder) as a conductive material was added to NMP (N-methyl-2-pyrrolidone) as a solvent. To prepare a positive electrode slurry. The positive electrode slurry was coated on a 20 μm thick aluminum current collector and dried to prepare a positive electrode for a lithium-sulfur battery having a thickness of 150 μm.

Lithium foil having a thickness of about 150 μm was used as the cathode, and 20 μm thick polyethylene was used as the separator.

After stacking in the order of positive electrode, carbon-based fiber sheet, separator and negative electrode, mixed solvent of dimethoxyethane, dioxolane and diglyme (14:65:21 volume ratio) in which 1M LiN (SO ₂ CF ₃ ) ₂ was dissolved in electrolyte solution Was injected to produce a lithium-sulfur battery.

< Comparative example  1>

A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that the carbon-based fiber sheet was not included.

< Comparative example  2>

Ni-doped carbon particles were added to the positive electrode active material composition to prepare a lithium-sulfur battery.

Specifically, 75% by weight of sulfur / carbon composite, 10% by weight carbon black, 10% by weight PVDF and 5% by weight of N-doped carbon particles as an additive to NMP (N-methyl-2-pyrrolidone) by adding A positive electrode slurry was prepared. The positive electrode slurry was coated on a 20 μm thick aluminum current collector and then dried to prepare a positive electrode having a thickness of 150 μm.

Lithium foil having a thickness of about 150 μm was used as the negative electrode, and 20 μm thick polyethylene was used as the separator.

After stacking in the order of the positive electrode, the separator and the negative electrode, a mixed solvent of dimethoxyethane, dioxolane, and diglyme (14:65:21 volume ratio) in which 1M LiN (SO ₂ CF ₃ ) ₂ was dissolved was injected into an electrolyte solution. A sulfur battery was produced.

< Test Example  2>

After charging and discharging the lithium-sulfur batteries of Examples and Comparative Examples prepared above for 10 hours at a rate of 0.1 C, data on initial discharge capacity, discharge cycle characteristics, and charge and discharge efficiency were recorded. 4 to 6 are shown.

First, Figure 4 is a graph showing the initial discharge capacity, the lithium-sulfur battery of Example 1 with a carbon-based fiber sheet containing a high concentration of nitrogen, the initial discharge capacity was significantly increased compared to the lithium-sulfur battery of the comparative example, It was confirmed that overvoltage was also improved.

In particular, as shown in Figure 4, in Comparative Example 2 containing nitrogen-doped carbon particles in the positive electrode mixture, the initial discharge capacity slightly increased compared to Comparative Example 1, but the addition of nitrogen doped carbon particles with low electrical conductivity As a result, the overvoltage increased in the discharge process.

 5 is a graph showing the discharge cycle characteristics, the lithium-sulfur battery of Example 1 showed an excellent discharge capacity retention rate and maintained a capacity of about 740 mAh / g even after 50 cycles. In addition, in Comparative Example 2 including nitrogen-doped carbon particles as an additive through Figure 5 it was confirmed that the discharge capacity retention rate is sharply lowered.

 6 is a graph showing charge and discharge efficiency. The lithium-sulfur battery according to Example 1 compared to Comparative Examples 1 and 2 maintained an efficiency of 99% or more even after 36 cycles.

Therefore, it can be seen from the results of FIGS. 4 to 6 that the battery capacity and lifespan characteristics can be improved when the porous carbon sheet containing the high concentration of nitrogen according to the present invention is applied between the anode and the separator.

[Description of the code]

10. Lithium Ion

20. Lithium Polysulfide

100. Anode

200. Cathode

300. Membrane

400. Electrolyte

500. Carbon Based Fiber Sheet

Claims (16)

1. Carbon-based fiber sheet for lithium-sulfur battery doped with high concentration of nitrogen.
2. The method of claim 1,

   The nitrogen is a carbon-based fiber sheet for a lithium-sulfur battery that is doped at a doping concentration of 5 to 30% by weight in the carbon-based fiber sheet.
3. The method of claim 1,

   The carbon-based fiber is at least one carbon-based fiber sheet for a lithium-sulfur battery selected from the group consisting of carbon fibers, carbon nanofibers, graphite fibers and graphene fibers.
4. The method of claim 1,

   The carbon-based fiber sheet is a carbon-based fiber sheet for a lithium-sulfur battery that is in the form of one or more fabrics selected from the group consisting of felt, mat, paper and cloth.
5. The method of claim 1,

   The carbon-based fiber sheet is a carbon-based fiber sheet for lithium-sulfur battery having a thickness of 0.1 to 100 ㎛.
6. The method of claim 1,

   The carbon-based fiber sheet is a carbon-based fiber sheet for a lithium-sulfur battery made of carbon-based fibers having a diameter of 0.01 to 100 ㎛.
7. The method of claim 1,

   The carbon-based fiber sheet is a carbon-based fiber sheet for a lithium-sulfur battery having a pore size of 0.1 to 10 ㎛, porosity of 5 to 90%.
8. The method of claim 1,

   The carbon-based fiber sheet has a specific surface area of 5 to 500 m 2 / g, bulk density of 0.05 to 0.2 g / cm ³ carbon-based fiber sheet for a lithium-sulfur battery.
9. i) preparing a spinning solution comprising a carbon-based precursor;

   ii) spinning the spinning solution to produce a sheet in the form of a fibrous web;

   iii) stabilizing the sheet through heat treatment in the presence of oxygen; And

   iv) carbonizing the sheet in an inert atmosphere;

   The carbon-based precursor is a method for producing a carbon-based fiber sheet for lithium-sulfur battery containing a nitrogen element in the molecular structure.
10. The method of claim 9,

    The carbon-based precursor of step i) is made of polyacrylonitrile, polyaniline, polypyrrole, polyimide, polybenzimidazole, polypyrrolidone, polyamide, polyamideimide, polyaramid, melamine, melamine-formaldehyde and fluorine mica Method for producing a carbon-based fiber sheet for lithium-sulfur battery, characterized in that at least one member selected from the group.
11. The method of claim 9,

    The carbon precursor is a method for producing a carbon-based fiber sheet for a lithium-sulfur battery containing 5 to 20% of the nitrogen atoms in the molecular structure.
12. The method of claim 9,

    The heat treatment of step iii) is a method for producing a carbon-based fiber sheet for lithium-sulfur battery, characterized in that carried out at 100 to 400 ℃.
13. The method of claim 9,

    The heat treatment of step iii) is a method for producing a carbon-based fiber sheet for a lithium-sulfur battery performed in an oxidizing atmosphere.
14. The method of claim 9,

    The carbonization of the step iv) is carried out at 600 to 1500 ℃ carbon-based fiber sheet for a lithium-sulfur battery.
15. The method of claim 9,

    The carbonization of step iv) is a method for producing a carbon-based fiber sheet for a lithium-sulfur battery performed in an inert gas atmosphere.
16. anode; cathode; A separator interposed therebetween; And in the lithium-sulfur battery comprising an electrolyte impregnated therein,

    A lithium-sulfur battery, wherein the carbon-based fiber sheet according to any one of claims 1 to 8 is interposed between the anode and the separator.

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