Sulfur-carbon composite, method for manufacturing the same, positive electrode for lithium-sulfur battery including the sulfur-carbon composite, and lithium-sulfur battery

Abstract

The present invention relates to a sulfur-carbon composite, a method for manufacturing the same, a positive electrode for a lithium-sulfur battery including the sulfur-carbon composite, and a lithium-sulfur battery, the sulfur-carbon composite including: a porous carbon material; a compound having electrolyte infiltration performance; and sulfur, wherein a surface of the porous carbon material includes a coating including the compound having electrolyte infiltration performance.

Description

Technical Field

[0001] This application claims the priority and benefit of Korean Patent Application No. 10-2018-0076514, filed on Jul. 2, 2018, the entire disclosure of which is incorporated herein by reference in its entirety.

[0002] The present invention relates to a sulfur-carbon composite, a positive electrode for a lithium-sulfur battery including the sulfur-carbon composite, and a lithium-sulfur battery. Background Art

[0003] In recent years, as miniaturization, light weight, and high performance of electronic devices and communication devices are rapidly developing and environmental problems are involved, the demand for electric vehicles has increased significantly, and the demand for improving the performance and lifespan of secondary batteries used as energy sources for these products is also increasing day by day. A lithium-sulfur battery using a sulfur-based compound as a positive electrode active material is attracting attention as a secondary battery that meets this demand.

[0004] A lithium-sulfur battery is a secondary battery that uses a sulfur-based compound containing a sulfur-sulfur bond as a positive electrode active material and uses a carbon-based material capable of inserting / extracting lithium metal or lithium ions or silicon or tin that forms an alloy with lithium as a negative electrode active material.

[0005] During discharge of the lithium-sulfur battery, at the positive electrode, sulfur accepts electrons and thereby undergoes a reduction reaction, while at the negative electrode, an oxidation reaction occurs while lithium is ionized. Specifically, sulfur has a cyclic S 8 , and during the reduction reaction (discharge), as the sulfur-sulfur bond breaks, the oxidation number of sulfur decreases, and during the oxidation reaction (charge), as the sulfur-sulfur bond is reformed, the oxidation number of sulfur increases, thereby using a redox reaction to store and generate electrical energy.

[0006] In particular, the theoretical discharge capacity of a lithium-sulfur battery is 1675 mAh / g, and its theoretical energy density is 2600 Wh / kg. Since the theoretical energy density of a lithium-sulfur battery is about 5 times that of the theoretical energy density of a lithium-ion battery (about 570 Wh / kg) currently under research, a lithium-sulfur battery is a battery capable of achieving high capacity, high energy density, and long lifespan. In addition, since sulfur, which is the main material of the positive electrode active material, has a low atomic weight, is resource-rich, is easy to supply, is inexpensive, is non-toxic, and is an environmentally friendly substance, a lithium-sulfur battery can be used as an energy source for medium to large-sized devices such as electric vehicles and portable electronic devices and is therefore being widely studied.

[0007] Since sulfur used as a positive electrode active material in a lithium-sulfur battery has 5×10 -30A conductivity of S / cm means it is a non-conductor without conductivity. Thus, there are the following problems: it is difficult to transfer electrons generated by electrochemical reactions. Therefore, sulfur is combined with a conductive material (such as carbon) that can provide electrochemical reaction sites to form a sulfur-carbon composite, and then used.

[0008] Meanwhile, during the actual operation of a lithium-sulfur battery, as the number of cycles progresses, the initial capacity and cycle life decrease rapidly, and thus sufficient performance cannot be ensured. Therefore, lithium-sulfur batteries have not been commercialized. This is because of the following fact: sulfur, as the positive electrode active material, undergoes volume expansion due to reduction reactions, or polysulfide lithium, as an intermediate product during the reduction reaction, dissolves into the electrolyte solution. As a result, sulfur is lost and no longer participates in the charge / discharge reactions of the battery. Therefore, various techniques have been proposed to improve the stability and electrochemical reactivity of sulfur-carbon composites.

[0009] However, in order to maintain a low porosity of the positive electrode of a lithium-sulfur battery containing a sulfur-carbon composite as the positive electrode active material, the positive electrode is calendered. However, there are the following problems: the adhesion force between the positive electrode active material and the current collector decreases during calendering. In addition, problems such as the dissolution of polysulfide lithium and the wetting of the electrolyte occur. Therefore, there are the following problems: the performance of the battery deteriorates because the reactivity decreases significantly during initial and high-rate charge / discharge. The above problems remain unsolved.

[0010] Therefore, it is necessary to develop a sulfur-carbon composite that has excellent adhesion of the positive electrode active material to the current collector even in the case of an electrode with low porosity and can form a migration path for the electrolyte due to high electrolyte infiltration performance.

[0011] [Prior Art Documents]

[0012] [Patent Documents]

[0013] Korean Patent Publication No. 10-2015-0015644 Summary of the Invention

[0014] [Technical Problem]

[0015] An object of the present invention is to provide a sulfur-carbon composite that can improve the life characteristics of a lithium-sulfur battery including a positive electrode with low porosity.

[0016] In addition, another object of the present invention is to provide a positive electrode for a lithium-sulfur battery, which includes the sulfur-carbon composite and has excellent adhesion of the positive electrode active material to the current collector when manufacturing a positive electrode with low porosity.

[0017] In addition, another object of the present invention is to provide a lithium-sulfur battery including the positive electrode for a lithium-sulfur battery.

[0018]

Technical Solution

[0019] To achieve the above object, the present invention provides a sulfur-carbon composite, which comprises a porous carbon material; a compound having electrolyte infiltration performance; and sulfur, wherein the surface of the porous carbon material comprises a coating, and the coating comprises the compound having electrolyte infiltration performance.

[0020] In addition, the present invention provides a method for preparing a sulfur-carbon composite, which comprises the following steps:

[0021] (a) Coating a porous carbon material with a compound having electrolyte infiltration performance; and

[0022] (b) Mixing and shaping the porous carbon material coated with the compound having electrolyte infiltration performance and sulfur prepared in step (a).

[0023] In addition, the present invention provides a positive electrode for a lithium-sulfur battery, which comprises a positive electrode current collector; and a positive electrode active material layer on the positive electrode current collector, wherein the positive electrode active material layer comprises the sulfur-carbon composite, a conductive material, and a binder of the present invention.

[0024] In addition, the present invention provides a lithium-sulfur battery, which comprises a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte, wherein the positive electrode is the positive electrode of the present invention.

[0025]

Advantageous Effects

[0026] The sulfur-carbon composite of the present invention makes the movement of lithium ions easy by having a coating containing a compound having electrolyte infiltration performance on the porous carbon material and thereby forming a migration path for the electrolyte, so that the performance and life characteristics of a lithium-sulfur battery comprising a positive electrode with a low porosity can be improved, and the adhesion to the positive electrode current collector can be improved. Description of the Drawings

[0027] Figure 1 A graph showing the results of the adhesion of the positive electrode active material layer to the current collector in an electrode having a porosity of 60%.

[0028] Figure 2 A graph showing the results of the adhesion of the positive electrode active material layer to the current collector in an electrode having a porosity of 65%.

[0029] Figure 3 A graph showing the results of the adhesion of the positive electrode active material layer to the current collector in an electrode having a porosity of 72%.

[0030] Figure 4A graph showing the results of the adhesion of the positive electrode active material layer to the current collector according to the porosity of the electrode.

[0031] Figure 5 A graph showing the discharge capacity of the coin-type battery according to Experimental Example 2 of the present invention.

[0032] Figure 6 and Figure 7 A graph showing the life characteristics of the coin-type battery according to Experimental Example 2 of the present invention. DETAILED DESCRIPTION

[0033] Hereinafter, the present invention will be described in more detail.

[0034] As used herein, the term "composite" refers to a material in which two or more materials are combined to exhibit a more effective function and at the same time form physically and chemically distinct phases from each other.

[0035] As used herein, the term "electrolyte impregnation performance" or "electrolyte impregnation ability" refers to the ability to carry and retain an electrolyte, and is distinguished from temporarily contacting or absorbing related substances or components in that it uniformly impregnates the electrolyte for a long time.

[0036] Among various secondary batteries, lithium-sulfur batteries are popular as next-generation batteries because of the following advantages: lithium-sulfur batteries have a high discharge capacity and theoretical energy density, and sulfur used as a positive electrode active material has abundant reserves, low cost, and environmental friendliness.

[0037] However, in lithium-sulfur batteries, sulfur is converted from a cyclic S 8 structure to a linear structure of polysulfide lithium (Li 2 S x , x = 8, 6, 4, 2) through a reduction reaction, and when the polysulfide lithium is completely reduced, lithium sulfide (Li 2 S) is finally produced. Among the polysulfide lithium that is an intermediate product of the reduction reaction of sulfur, polysulfide lithium with a high sulfur oxidation number (Li 2 S x , usually x > 4) is a strongly polar substance, which is easily dissolved in an electrolyte containing a hydrophilic organic solvent and elutes out of the reaction zone of the positive electrode, so that it can no longer participate in the electrochemical reaction. In addition, sulfur becomes lithium sulfide (Li 2 S) as a discharge product, resulting in a volume expansion of about 80%, thus there are problems that the pore volume inside the positive electrode decreases and the contact with the electrolyte becomes difficult. In addition, the electrolyte is decomposed and consumed due to the formation of a passivation layer (solid electrolyte interphase: SEI) or reaction with impurities during battery operation.

[0038] Despite the above advantages, due to the low amount of sulfur participating in the electrochemical reaction, combined with this loss of sulfur and electrolyte and the large change in the volume of the positive electrode, lithium-sulfur batteries not only cannot achieve the full theoretical capacity and energy density in actual driving, but also have the following problems: the deterioration of the initial capacity and cycle characteristics accelerates after a certain number of cycles.

[0039] To improve the conductivity of sulfur, methods of forming a composite with a conductive material (such as carbon or polymer) or coating a conductive material (such as carbon or polymer) have been used. Among various methods, sulfur-carbon composites are most commonly used as the positive electrode active material because they effectively improve the conductivity of the positive electrode. However, they are still insufficient in terms of charge / discharge capacity and efficiency.

[0040] In addition, lithium-sulfur batteries containing electrodes with low porosity manufactured by rolling have the following problems: lithium ions are not easily migrated and thus the reactivity is greatly reduced during initial and high-rate charge / discharge. In addition, electrodes with low porosity manufactured by rolling have the following problems: since the adhesion force between the current collector and the positive electrode active material layer is weak, the positive electrode active material layer detaches from the electrode, making it difficult to manufacture the electrode.

[0041] Therefore, by forming a coating containing a compound having electrolyte infiltration performance on the surface of a porous carbon material and thereby providing a function capable of infiltrating the electrolyte between sulfur and the carbon material, a migration path of the electrolyte is formed in the sulfur-carbon composite and thereby the movement of lithium ions is made easy. The present invention aims to provide a sulfur-carbon composite that can improve the life characteristics of a lithium-sulfur battery including a positive electrode with low porosity and has excellent adhesion to the current collector even in a positive electrode with low porosity.

[0042] That is, the present invention relates to a sulfur-carbon composite comprising a porous carbon material; a compound having electrolyte infiltration performance; and sulfur, wherein the surface of the porous carbon material comprises a coating containing the compound having electrolyte infiltration performance.

[0043] The porous carbon material of the present invention includes a coating containing a compound having electrolyte infiltration performance on its surface. The electrolyte infiltrated with the compound having electrolyte infiltration performance maintains a stable interface inside the sulfur-carbon composite, that is, between sulfur and the carbon material. Thereby, the accessibility of the electrolyte to the inside of the active material is improved, and the electrochemical reactivity of sulfur can be increased with the help of the electrolyte. In particular, even in a lithium-sulfur battery including a positive electrode having a low porosity, due to the polymer having electrolyte infiltration performance, a migration path of the electrolyte is formed in the sulfur-carbon composite to facilitate the migration of lithium ions, thereby maintaining the reactivity of the lithium-sulfur battery and improving its life characteristics. In addition, due to the coating, even in the case of a positive electrode having a low porosity, the adhesion of the positive electrode active material to the current collector can be improved, thereby solving problems such as the detachment of the positive electrode active material in the rolling process of manufacturing an electrode having a low porosity, and thus contributing to the manufacture of a positive electrode having a low porosity.

[0044] In the present invention, the low porosity means that the porosity is 68% or less.

[0045] The sulfur-carbon composite means that the porous carbon material with a coating formed thereon and sulfur are in a mixed state.

[0046] In the present invention, the compound having electrolyte infiltration performance is immiscible with the electrolyte or the solvent for preparing the positive electrode paste, and the compound may have at least one form selected from the group consisting of polymers, oligomers, and single molecular forms.

[0047] If the compound having electrolyte infiltration performance is in the form of a polymer, the polymer may include at least one selected from the group consisting of polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, poly(fluorine ethylene-propylene), poly(ethylene-tetrafluoroethylene), and polytetrafluoroethylene.

[0048] In addition, the electrolyte includes a lithium salt and a solvent. In the present invention, the solvent of the electrolyte in which the polymer has infiltration ability may include at least one selected from the group consisting of ether compounds and carbonate compounds.

[0049] The ether compound may include at least one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, polyethylene glycol methyl ethyl ether, 1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran.

[0050] The carbonate compound may be a cyclic carbonate compound or a chain carbonate compound.

[0051] The cyclic carbonate compound may include at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinyl ethylene carbonate, and their halides. Examples of such halides include, but are not limited to, fluoroethylene carbonate (FEC), etc.

[0052] In addition, the chain carbonate may include, but is not limited to, at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate.

[0053] The lithium salt can be used without limitation as long as it is commonly used in electrolytes for lithium-sulfur batteries. For example, the lithium salt may include at least one selected from the group consisting of 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, at least one selected from the group consisting of LiFSI, lithium chloroborane, lithium lower aliphatic carboxylate, etc.

[0054] In addition, the concentration of the lithium salt in the electrolyte can be 0.2 M to 2 M, specifically 0.6 M to 2 M, and more specifically 0.7 M to 1.7 M. If the concentration of the lithium salt is less than 0.2 M, the conductivity of the electrolyte may decrease and the performance of the electrolyte may deteriorate. If the concentration exceeds 2 M, the viscosity of the electrolyte may increase, reducing the mobility of lithium ions.

[0055] The electrolyte impregnation ability of the polymer having electrolyte impregnation performance for an electrolyte containing an ether compound or a carbonate compound as a solvent can be 200% or more, preferably 260% to 600%. At this time, the electrolyte impregnation ability can be measured by preparing a film using the compound having electrolyte impregnation performance and then calculating the weight difference before and after impregnation in the electrolyte. Specifically, a film of the compound is prepared by dissolving the compound having electrolyte impregnation performance in a suitable solvent (preferably a mixed solvent of ethanol and acetone), casting, and then drying. The prepared film of the compound is impregnated at 25 °C for 48 hours to saturate the coated compound with the electrolyte, and the weight at this time is designated as the weight after impregnation.

[0056] Based on the total weight of the sulfur-carbon composite, it may contain 0.5 wt% to 5 wt%, preferably 1 wt% to 3 wt% of the compound having electrolyte impregnation performance. If the content of the compound is less than 0.5 wt%, the formation of the coating on the porous carbon material is insufficient, so that the life characteristics of the lithium-sulfur battery including a positive electrode having a low porosity cannot be improved, and the adhesion of the positive electrode active material to the current collector in the positive electrode having a low porosity will not increase. If the content of the compound exceeds 5 wt%, the functions as a positive electrode active material and the battery performance may be adversely affected.

[0057] The porous carbon material provides a framework in which sulfur as a positive electrode active material can be uniformly and stably fixed, and compensates for the conductivity of sulfur so that the electrochemical reaction can proceed smoothly.

[0058] The porous carbon material can generally be manufactured by carbonizing precursors of various carbon materials. The porous carbon material may contain uneven pores therein, the average diameter of the pores is in the range of 1 nm to 200 nm, and the porosity may be in the range of 10% to 90% of the total volume of the porous carbon material. If the average diameter of the pores is less than the above range, the pore diameter is only at the molecular level and it is impossible for sulfur to infiltrate. On the contrary, if the average diameter of the pores exceeds the above range, the mechanical strength of the porous carbon will be weakened, which is not preferable for application to the manufacturing process of the electrode.

[0059] The shape of the porous carbon material is in the form of spheres, rods, needles, plates, tubes, and blocks, and can be used without limitation as long as it is commonly used in lithium-sulfur batteries.

[0060] The porous carbon material may have a porous structure or a high specific surface area, and can be any of those commonly used in the art. For example, the porous carbon material may be, but is not limited to, selected from the group consisting of graphite; graphene; carbon black, such as DENKA carbon black (Denka black), acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black; carbon nanotubes (CNT), such as single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT); carbon fibers, such as graphite nanofibers (GNF), carbon nanofibers (CNF), and activated carbon fibers (ACF); natural graphite, artificial graphite, expanded graphite, and activated carbon. Preferably, the porous carbon material may be carbon nanotubes.

[0061] Sulfur may contain at least one selected from the group consisting of inorganic sulfur (S 8 );Li 2 S n (n≥1); disulfide compounds, such as 2,5-dimercapto-1,3,4-thiadiazole and trithiocyanuric acid; organic sulfur compounds and carbon-sulfur polymers ((C 2 S x ) n : x = 2.5 to 50, n≥2), etc. Preferably, inorganic sulfur (S 8 ) can be used.

[0062] In the sulfur-carbon composite according to the present invention, the weight ratio of the above-mentioned porous carbon material containing a coating to sulfur can be 1:9 to 5:5, preferably 2:8 to 3:7. If the sulfur is less than the above range, then as the content of the porous carbon material increases, the amount of the binder required for preparing the slurry for the positive electrode will increase. The increase in the amount of the binder will ultimately increase the sheet resistance of the electrode and act as an insulator blocking the passage of electrons, which may reduce the battery performance. On the contrary, if the sulfur exceeds the above weight ratio range, then due to sulfur agglomerating together, it may be difficult for sulfur to directly participate in the electrode reaction.

[0063] In addition, in the present invention, relative to the total weight of the sulfur-carbon composite, the sulfur-carbon composite contains 50 wt% to 90 wt% of sulfur, 6 wt% to 45 wt% of the porous carbon material, and 0.5 wt% to 5 wt% of a compound having electrolyte infiltration performance. If the content in the sulfur-carbon composite falls within the above range, then the life characteristics of the lithium-sulfur battery including the above positive electrode with low porosity can be improved, and the adhesion of the positive electrode active material to the current collector can be increased.

[0064] In the sulfur-carbon composite according to the present invention, sulfur is located on at least one of the inner surface and the outer surface of the pores of the porous carbon material. At this time, sulfur may be present in an area of less than 100%, preferably 1% to 95%, more preferably 60% to 90% of the entire inner surface and outer surface of the porous carbon material. When sulfur is on the surface of the porous carbon material within the above range, it can show the greatest effect in terms of electron transfer area and wettability of the electrolyte. Specifically, since sulfur is thinly and uniformly infiltrated on the surface of the porous carbon material within the above range, the electron transfer contact area can be increased during the charge / discharge process. If sulfur is located in the 100% area of the entire surface of the porous carbon material, then the porous carbon material is completely covered by sulfur, whereby the wettability of the electrolyte deteriorates and the contact with the conductive material contained in the electrode decreases, thus preventing the transmission of electrons and its inability to participate in the reaction.

[0065] The present invention also relates to a method for preparing a sulfur-carbon composite.

[0066] The method for preparing a sulfur-carbon composite according to the present invention comprises the following steps:

[0067] (a) Coating a porous carbon material with a compound having electrolyte infiltration performance; and

[0068] (b) Mixing and molding the porous carbon material coated with the compound having electrolyte infiltration performance prepared in step (a) and sulfur.

[0069] Step (a) is a step of forming a coating of a compound having electrolyte infiltration performance on the surface of the porous carbon material, and can be carried out by adding a porous conductive material to a solution in which a compound having electrolyte infiltration performance is dissolved, stirring the solution, and then filtering and drying the solution, or can be carried out by any method known in the art.

[0070] The solvent of the solution in which the compound having electrolyte infiltration performance is dissolved is not particularly limited as long as it can dissolve the polymer, but it is preferably a mixed solvent of ethanol and acetone.

[0071] Step (b) can be carried out by mixing the porous carbon material coated with the compound having electrolyte infiltration performance with sulfur, and heating and molding the resulting mixture, or can be carried out by any method known in the art.

[0072] The mixing is used to increase the degree of mixing between the above materials and can be carried out by using a stirring device commonly used in the art. At this time, the mixing time and mixing rate can also be selectively controlled according to the content and conditions of the raw materials.

[0073] The heating temperature can be the temperature at which sulfur melts, specifically 120°C to 180°C, preferably 150°C to 180°C. If the heating temperature is lower than 120°C, then sulfur cannot be sufficiently melted and thus may not properly form the sulfur-carbon composite structure. If the heating temperature exceeds 180°C, it is difficult to obtain the desired effect because the coated compound will not remain. In addition, the heating time can be controlled according to the sulfur content.

[0074] Through step (b), a sulfur-carbon composite can be prepared in which the surface of the porous carbon material contains a coating of a compound having electrolyte impregnation performance.

[0075] The sulfur-carbon composite of the present invention is a sulfur-carbon composite in which a coating of a compound having electrolyte impregnation performance is formed on the surface of a porous carbon material, wherein the sulfur-carbon composite is prepared by mixing the porous carbon material containing the coating and sulfur. Therefore, compared with the case of coating a compound having electrolyte impregnation performance after manufacturing the sulfur-carbon composite, the sulfur-carbon composite of the present invention can be coated with a higher concentration of a polymer having electrolyte impregnation performance, and thus, a lithium-sulfur battery containing the sulfur-carbon composite can exhibit the effect of not generating overvoltage. In addition, in the case of coating a compound having electrolyte impregnation performance after manufacturing the sulfur-carbon composite, the sulfur infiltrated inside can be eluted to the outside by a solution in which the compound having electrolyte impregnation performance is dissolved. However, the sulfur-carbon composite of the present invention has stability without causing the above problems.

[0076] In addition, the present invention relates to a positive electrode for a lithium-sulfur battery, which includes a positive electrode current collector; and a positive electrode active material layer on the positive electrode current collector, wherein the positive electrode active material layer contains the sulfur-carbon composite of the present invention, a conductive material, and a binder.

[0077] The positive electrode has a low porosity, and the porosity of the positive electrode can be 68% or less, preferably 50% to 68%. By having the porosity as described above, the energy density per unit volume can be increased and the minimum infiltration amount of the electrolyte can be reduced, thereby increasing the energy density per unit mass.

[0078] By using the sulfur-carbon composite of the present invention, the adhesion force of the positive electrode active material layer to the positive electrode current collector can be 2 gf / cm or more, preferably 2 gf / cm to 15 gf / cm.

[0079] Generally, when manufacturing a positive electrode with low porosity, the adhesion between the positive electrode current collector and the positive electrode active material layer is poor, making it difficult to manufacture the electrode and the activity of the positive electrode is not excellent. However, in the present invention, by using a sulfur-carbon composite containing a coating containing a compound having electrolyte infiltration performance on the surface of a porous carbon material, even in a positive electrode with low porosity, the adhesion between the positive electrode current collector and the positive electrode active material layer can be increased, thereby improving the characteristics of the lithium-sulfur battery. If a sulfur-carbon composite without a compound having electrolyte infiltration performance is used, the above effects cannot be achieved because the adhesion of the positive electrode active material layer to the positive electrode current collector is very poor in a positive electrode with low porosity.

[0080] The positive electrode current collector generally has a thickness of 3 μm to 500 μm and is not particularly limited as long as it has high conductivity and does not cause chemical changes in the battery. Specifically, a conductive material such as stainless steel, aluminum, copper, or titanium can be used as the positive electrode current collector. In addition, the positive electrode current collector can be formed in various forms, such as a film, sheet, foil, net, porous body, foam, or non-woven fabric.

[0081] The conductive material is not particularly limited as long as it has conductivity and does not cause chemical changes in the battery, and carbon black materials such as Super-P, DENKA carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, and carbon black; carbon derivatives such as carbon nanotubes or fullerenes; conductive fibers such as carbon fibers or metal fibers; fluorinated carbon; metal powders such as aluminum powder and nickel powder; and conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole can be used alone or in combination.

[0082] Based on the total weight of the mixture containing the positive electrode active material, the content of the conductive material can be 0.01% by weight to 30% by weight.

[0083] The binder is a component that maintains the positive electrode active material on the positive electrode current collector and has the function of organically connecting between the positive electrode active materials. The binder can be, for example, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, various copolymers thereof, etc.

[0084] Based on the total weight of the mixture containing the positive electrode active material, the content of the binder can be from 0.5% by weight to 30% by weight. If the content of the binder is less than 0.5% by weight, the physical properties of the positive electrode may deteriorate and the active material and the conductive material in the positive electrode may become detached. If the content of the binder exceeds 30% by weight, the ratio of the active material and the conductive material in the positive electrode may be relatively reduced, thereby reducing the capacity of the battery.

[0085] The positive electrode active material layer is formed by forming the positive electrode active material in the form of a slurry, in which the binder is dissolved in the solvent for preparing the slurry, and then the conductive material is dispersed therein. The solvent for preparing the slurry is preferably a solvent that can uniformly disperse the positive electrode active material, the binder, and the conductive material and is easy to evaporate. The solvent for preparing the slurry can generally be acetonitrile, methanol, ethanol, tetrahydrofuran, water, isopropyl alcohol, etc. Next, a slurry for the positive electrode is prepared by uniformly dispersing the sulfur-carbon composite containing the coating, optionally together with an additive, into the solvent in which the conductive material is dispersed. The amount of the solvent, the sulfur-carbon composite, or the optional additive contained in the slurry is not particularly important in the present application and is sufficient only when the slurry has an appropriate viscosity to facilitate the coating of the slurry.

[0086] Thereafter, the positive electrode current collector can be coated with the slurry and then calendered to manufacture a positive electrode for a lithium-sulfur battery. The slurry can be coated on the current collector with an appropriate thickness according to the viscosity of the slurry and the thickness of the positive electrode to be formed.

[0087] After coating the slurry, the slurry can be further dried before calendering, and the drying can be carried out at a temperature of about 80 °C.

[0088] After drying, the positive electrode is prepared by calendering, and the calendering can be carried out by a method known in the art, such as by a roll pressing method.

[0089] In addition, the present invention relates to a lithium-sulfur battery, which includes a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte, wherein the positive electrode is the positive electrode of the present invention as described above.

[0090] The negative electrode can be composed of a current collector and a negative electrode active material layer formed on one or both sides thereof. In addition, the negative electrode can be a lithium metal plate.

[0091] The current collector is used to carry the negative electrode active material and is not particularly limited as long as it is electrochemically stable within the voltage range of the lithium secondary battery and has excellent conductivity at the same time. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, sintered carbon; copper or stainless steel surface-treated with carbon, nickel, silver, etc.; aluminum cadmium alloy, etc. can be used as the current collector.

[0092] The negative electrode current collector can enhance the adhesion to the negative electrode active material by having fine irregularities formed on its surface, and can be formed in various forms such as a film, sheet, foil, mesh, net, porous body, foam, or non-woven fabric.

[0093] The negative electrode active material can include a material capable of reversibly intercalating or deintercalating lithium ions, a material capable of reacting with lithium ions to reversibly form a lithium-containing compound, or lithium metal or a lithium alloy.

[0094] Examples of the material capable of reversibly intercalating or deintercalating lithium ions can be crystalline carbon, amorphous carbon, or a mixture thereof.

[0095] Examples of the material capable of reacting with lithium ions to reversibly form a lithium-containing compound can be tin oxide, titanium nitrate, or silicon.

[0096] The lithium alloy can be, for example, an alloy of lithium (Li) and a metal selected from the group consisting of sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

[0097] A separator can be further included between the above-mentioned positive electrode and negative electrode. The separator separates or insulates the positive electrode and the negative electrode from each other and allows lithium ions to be transmitted between the positive electrode and the negative electrode. The separator can be made of a porous non-conductive or insulating material. Such a separator can be an independent member, such as a film, or a coating added to the positive electrode and / or the negative electrode.

[0098] The materials used to form the separator include, but are not limited to, polyolefins such as polyethylene and polypropylene; glass fiber filter paper and ceramic materials, and the thickness of the separator can be about 5 μm to about 50 μm, preferably about 5 μm to about 25 μm.

[0099] The electrolyte is located between the positive electrode and the negative electrode, and the lithium-sulfur battery of the present invention uses the above-mentioned electrolyte.

[0100] In addition to the general winding process, a lamination (stacking) and folding process of the separator and the electrode can also be performed on the lithium-sulfur battery according to the present invention.

[0101] The shape of the lithium-sulfur battery is not particularly limited and can be various shapes such as cylindrical, laminated, and coin-shaped.

[0102] Hereinafter, preferred embodiments of the present invention will be described to facilitate the understanding of the present invention. However, it will be obvious to those skilled in the art that the following embodiments illustrate the present invention and various changes and modifications can be made within the scope and gist of the present invention. It will be obvious that these changes and modifications are intended to fall within the scope of the appended claims.

[0103] <Positive electrode for lithium-sulfur battery>

[0104] Example 1

[0105] 0.5 g of carbon nanotubes was added to a solution prepared by dissolving poly(vinylidene fluoride - hexafluoropropylene) (Sigma Aldrich), a polymer having electrolyte impregnation performance, in a mixed solvent of acetone and ethanol in an amount of 1.25% based on the total weight of the sulfur - carbon composite, and then the resulting solution was stirred at 25 °C and dried for 12 hours to form a poly(vinylidene fluoride - hexafluoropropylene) coating on the surface of the porous carbon material. At this time, the weight ratio of poly(vinylidene fluoride - hexafluoropropylene) : porous carbon material was 1:20. The porous carbon material refers to the total weight of the carbon nanotubes.

[0106] 0.5 g of the above - prepared porous carbon material coated with poly(vinylidene fluoride - hexafluoropropylene) and 2 g of sulfur were uniformly mixed, and then heat - treated at 155 °C for 30 minutes to obtain a sulfur - carbon composite, which contains sulfur, porous carbon material and polymer in a weight ratio of sulfur : porous carbon material : polymer of 75:23.75:1.25.

[0107] The sulfur - carbon composite, conductive material and binder thus prepared were mixed in a weight ratio of sulfur - carbon composite : conductive material : binder of 90:5:5 to prepare a slurry for forming a positive electrode active material layer, and then it was coated on an aluminum foil current collector with a thickness of 20 μm and dried at a temperature of 80 °C to manufacture a positive electrode for a lithium - sulfur battery.

[0108] At this time, carbon black was used as the conductive material, and styrene - butadiene rubber and carboxymethyl cellulose were used as the binder.

[0109] Example 2

[0110] A positive electrode for a lithium - sulfur battery was prepared in the same manner as in Example 1, except that polyvinylidene fluoride (Sigma Aldrich) was used as the polymer having electrolyte impregnation performance.

[0111] Comparative Example 1

[0112] A positive electrode for a lithium - sulfur battery was prepared in the same manner as in Example 1, except that uncoated carbon nanotubes were used.

[0113] Comparative Example 2

[0114] 1 g of carbon nanotubes and 3 g of sulfur were uniformly mixed, and then heat - treated at 155 °C for 30 minutes to prepare a sulfur - carbon composite.

[0115] Subsequently, 0.5 g of carbon nanotubes was added to a solution prepared by dissolving poly(vinylidene fluoride - hexafluoropropylene) (Sigma - Aldrich), which is a polymer having electrolyte impregnation performance, in a mixed solvent of acetone and ethanol in an amount of 1.25% based on the total weight of the sulfur - carbon composite. Then, the resulting solution was stirred at 25°C and dried for 12 hours to form a polymer coating on the surface of the porous carbon material.

[0116] The sulfur - carbon composite, conductive material, and binder thus prepared were mixed at a weight ratio of sulfur - carbon composite:conductive material:binder of 90:5:5 to prepare a slurry for forming a positive electrode active material layer, which was then coated on an aluminum foil current collector with a thickness of 20 μm and dried at a temperature of 80°C to fabricate a positive electrode for a lithium - sulfur battery.

[0117] At this time, carbon black was used as the conductive material, and styrene - butadiene rubber and carboxymethyl cellulose were used as the binder.

[0118] Experimental Example 1: Measurement of adhesion force of positive electrode for lithium-sulfur battery

[0119] The positive electrodes for lithium - sulfur batteries prepared in Example 1, Example 2, and Comparative Example 1 were calendered with different calendering gaps to fabricate positive electrodes for lithium - sulfur batteries having porosities of 60%, 65%, and 72% respectively.

[0120] The adhesion forces of the positive electrodes for lithium - sulfur batteries of Example 1, Example 2, and Comparative Example 1 having porosities of 60%, 65%, and 72% were measured. The adhesion force of the positive electrode was measured by calculating the force when pulling the electrode in the 90 - degree direction using a peel - test device to cause the positive electrode active material layer to fall off.

[0121] The adhesion force refers to the adhesion force of the positive electrode active material layer to the positive electrode current collector.

[0122] The results are shown in Table 1 and Figures 1 to 4 in.

[0123] Table 1:

[0124] (Unit: gf / cm)

[0125] Example 1 Example 2 Comparative Example 1 60％ 2.95 3 1.75 65％ 3.25 3 1.52 72％ 1.15 0.82 0.95

[0126] Therefore, in the case of low porosities of 60% and 65% with porosities less than or equal to 68%, the adhesion forces of the positive electrode active material layers of Example 1 and Example 2 containing the sulfur - carbon composite of the present invention to the positive electrode current collector are superior to those of Comparative Example 1. In the case of a porosity of 65%, the result is about 2 times that of Comparative Example 1.

[0127] On the other hand, in the case of a porosity of 72% which is higher than 68%, Comparative Example 1 and Example 1 showed similar values, and Example 2 was not superior to Comparative Example 1.

[0128] From the above results, it was confirmed that the sulfur-carbon composite of the present invention can increase the adhesion between the positive electrode current collector and the positive electrode active material layer in the positive electrode having a low porosity.

[0129] Experimental Example 2: Evaluation of charge / discharge characteristics of lithium-sulfur battery

[0130] Positive electrodes of Example 1 and Example 2 having a porosity of 68%, Comparative Example 1 and Comparative Example 2 were used. A polyethylene was used as a separator and a lithium foil having a thickness of 150 μm was used as a negative electrode to fabricate a lithium-sulfur battery coin-type cell. At this time, an electrolytic solution prepared by dissolving 1M LiFSI and 1% LiNO 3 in an organic solvent of diethylene glycol dimethyl ether was used to fabricate the coin-type cell.

[0131] The capacity of the fabricated coin-type cell was measured from 1.8 V to 2.5 V using a charge-discharge measurement device (LAND CT-2001A, Wuhan, China). Specifically, initial charge / discharge was performed at 0.1C / 0.1C, and then the charge / discharge cycle performed at 0.3C / 0.5C was repeated 120 times to measure the discharge capacity.

[0132] As a result, the capacity of the coin-type cell of Example 1 was superior to that of Comparative Example 1 ( Figure 5 ).

[0133] Furthermore, it was confirmed that even during cycling, the coin-type cells of Example 1 and Example 2 containing the sulfur-carbon composite of the present invention maintained a constant discharge capacity, but the discharge capacities of the coin-type cells of Comparative Example 1 and Comparative Example 2 gradually decreased during cycling ( Figure 6 and Figure 7 ).

[0134] Thus, it was confirmed that the lithium-sulfur battery containing the sulfur-carbon composite of the present invention has excellent performance, particularly excellent life characteristics.

Claims

1. A positive electrode for a lithium-sulfur battery, which comprises a positive electrode current collector; and a positive electrode active material layer on the positive electrode current collector, wherein the positive electrode active material layer comprises a sulfur-carbon composite, a conductive material, and a binder, The sulfur-carbon composite comprises a porous carbon material; a compound having electrolyte infiltration performance; and sulfur, wherein the surface of the porous carbon material comprises a coating, and the coating comprises the compound having electrolyte infiltration performance, The positive electrode has a porosity of 60% to 68%, The adhesion of the positive electrode active material layer to the positive electrode current collector is 2 gf / cm or more and 15 gf / cm or less, Based on the total weight of the sulfur-carbon composite, it contains 0.5% by weight to 5% by weight of the compound having electrolyte infiltration performance.

2. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the sulfur-carbon composite is in a state in which the porous carbon material having the coating formed thereon and sulfur are mixed.

3. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the compound having electrolyte infiltration performance has at least one form selected from the group consisting of polymers, oligomers, and monomolecular forms.

4. The positive electrode for a lithium-sulfur battery according to claim 3, wherein the polymer comprises at least one selected from the group consisting of polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, poly(fluorovinylidene fluoride-propylene), poly(ethylene-tetrafluoroethylene), and polytetrafluoroethylene.

5. The positive electrode for a lithium-sulfur battery according to claim 4, wherein the electrolyte comprises at least one selected from the group consisting of ether compounds and carbonate compounds as a solvent.

6. The positive electrode for a lithium-sulfur battery according to claim 5, wherein the compound has an electrolyte infiltration performance of 200% or more.

7. A lithium-sulfur battery, which comprises a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte, wherein the positive electrode is the positive electrode according to any one of claims 1 to 6.

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