Cathode material for lithium-sulfur batteries and lithium-sulfur batteries containing the cathode material

Abstract

The present invention relates to a sulfur-carbon composite for use in a positive electrode of a lithium-sulfur battery and a method for manufacturing the same. The carbon composite includes a porous carbon substrate and a sulfur-containing compound filled in at least one of the outer surface and the inner surfaces of the pores of the porous carbon substrate, and the surface is entirely or partially coated with an amphiphilic polythiophene.

Description

[Technical Field]

[0001] The present invention relates to a cathode material for a lithium-sulfur battery and a lithium-sulfur battery including the cathode material.

[0002] This application claims priority based on Korean Patent Application No. 10-2022-0110404, filed on August 31, 2022, the entire contents of which are incorporated herein by reference in their entirety in the specification and drawings thereof. [Background technology]

[0003] A lithium-sulfur battery is a battery system that uses a sulfur-based material with a sulfur-sulfur bond (SS bond) as the positive electrode active material and lithium metal as the negative electrode active material. Sulfur, the main material of the positive electrode active material, has the advantages of being abundant worldwide, non-toxic, and having a low weight per atom.

[0004] As the application areas of secondary batteries expand to electric vehicles (EVs) and energy storage systems (ESSs), lithium-sulfur battery technology is gaining attention due to its high theoretical energy storage density (up to 2,500Wh / kg) compared to lithium-ion secondary batteries, which have a relatively low energy storage density (up to 250Wh / kg) relative to their weight.

[0005] During discharge, lithium, the negative electrode active material, releases electrons and oxidizes, ionizing into lithium cations, while the positive electrode active material, a sulfur-based material, accepts electrons and is reduced. Through the reduction of the sulfur-based material, the S-S bond accepts two electrons and converts into sulfur anions. The lithium cations generated by the oxidation of lithium are transported to the positive electrode via the electrolyte, where they combine with sulfur anions generated by the reduction of sulfur-based compounds to form salts. Specifically, sulfur, which has a cyclic S8 structure before discharge, is converted to lithium polysulfide (Li2Sx) through a reduction reaction, and is then completely reduced to form lithium sulfide (Li2S).

[0006] As sulfur used as a positive electrode active material is a non-conductor, it is difficult for electrons generated by electrochemical reactions to move, and there are problems that need to be solved, such as a rapid decrease in battery capacity due to the elution of lithium polysulfide (LiSx) generated during the charge and discharge process.

[0007] For example, sulfur used in the positive electrode is reduced during discharge to form lithium polysulfide, which dissolves in the ether-based liquid electrolyte and is released from the positive electrode. The released lithium polysulfide passes through the separator to the negative electrode, where it reacts with lithium metal to form an unstable interface. This causes a loss of sulfur, the positive electrode active material. This series of processes is called the shuttle effect.

[0008] There is a need for a technology that can suppress the loss of energy density in batteries due to the shuttle effect described above. Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, the problem to be solved by the present invention is to solve the above-mentioned problems, The present invention aims to provide a sulfur-carbon composite whose surface is entirely or partially coated with a polymer to suppress the elution of lithium polysulfide from the positive electrode of a lithium-sulfur battery, and a method for producing the same.

[0010] In particular, the goal is to provide a lithium-sulfur battery with excellent discharge capacity and capacity retention rate by suppressing the shuttle effect and volume expansion of the positive electrode through the coating structure of the sulfur-carbon composite. [Means for solving the problem]

[0011] To solve the above problems, According to one aspect of the present invention, there is provided a sulfur-carbon composite according to the following embodiment:

[0012] The sulfur-carbon composite according to the first embodiment is A porous carbon substrate and a sulfur-containing compound contained in at least one of an outer surface of the porous carbon substrate and inner surfaces of pores, The surface is entirely or partially covered with the amphiphilic polythiophene.

[0013] According to the second embodiment, in the first embodiment, The amphiphilic polythiophene includes a 3-alkyl-substituted thiophene as a repeating unit. The 3-alkyl-substituted thiophene is a thiophene having an alkyl group substituted at the 3-position, and the alkyl group may include at least one cationic functional group and at least one anionic functional group.

[0014] According to the third embodiment, in the second embodiment, The cationic functional group may include a quaternary amine structure.

[0015] According to the fourth embodiment, in the second embodiment, The anionic functional group is —SO3 - , -COO - , -SO4 2-and their conjugate acids.

[0016] According to the fifth embodiment, in any one of the first to fourth embodiments, The amphiphilic polythiophene may include a polymer represented by the following Chemical Formula 1:

[0017] [ka]

[0018] According to the sixth embodiment, in any one of the first to fifth embodiments, The amphiphilic polythiophene may include a polymer represented by the following Chemical Formula 2:

[0019] [ka]

[0020] According to the seventh embodiment, in any one of the first to sixth embodiments, The amphiphilic polythiophene may include a polymer represented by the following Chemical Formula 3:

[0021] [ka]

[0022] According to the eighth embodiment, in any one of the first to seventh embodiments, The porous carbon substrate may include carbon black, carbon fiber, carbon nanotubes (CNT), graphene, graphene oxide (GO), reduced graphene oxide (rGO), carbon black, graphite, graphite nanofiber (GNF), carbon nanofiber (CNF), activated carbon fiber (ACF), natural graphite, artificial graphite, expanded graphite, activated carbon, fullerene, or two or more materials thereof.

[0023] According to the ninth embodiment, in any one of the first to eighth embodiments, The sulfur-containing compound may include inorganic sulfur (S), lithium sulfide (LiS), lithium polysulfide (LiS, 2≦x≦8), a disulfide compound, or a mixture of two or more thereof.

[0024] According to another aspect of the present invention, there is provided a method for producing a sulfur-carbon composite according to the following embodiment.

[0025] The method for producing a sulfur-carbon composite according to the tenth embodiment includes: preparing an uncoated sulfur-carbon composite including a porous carbon substrate and a sulfur-containing compound supported on at least one of an outer surface and inner surfaces of pores of the porous carbon substrate; coating all or at least a portion of the surface of the uncoated sulfur-carbon composite with an amphiphilic thiophene monomer; and obtaining a sulfur-carbon composite whose surface is entirely or partially coated with an amphiphilic polythiophene formed through a polymerization reaction of the coated amphiphilic thiophene monomer.

[0026] According to the eleventh embodiment, in the tenth embodiment, The amphiphilic thiophene monomer may be obtained by a preparation method including the following steps:

[0027] (S10) activating the carboxylic acid group of 3-carboxyalkylthiophene; (S20) reacting the activated 3-carboxyalkylthiophene compound with a dialkylaminoalcohol compound to form a dialkylaminoalkylthiophenylalkylcarboxylate compound; and (S30) A step of obtaining the amphiphilic thiophene monomer by substituting at least one anionic functional group onto the thiophenyl alkyl carboxylate dialkylamino alkyl compound.

[0028] According to the twelfth embodiment, in the eleventh embodiment, The 3-carboxyalkylthiophene may include 2-thiopheneacetic acid.

[0029] According to the thirteenth embodiment, in the eleventh embodiment, The dialkylamino alcohol compound may include 2-dimethylaminoethanol, 2-dimethylaminobutanol, 6-dimethylaminohexanol, or a mixture of two or more thereof.

[0030] According to the fourteenth embodiment, in the eleventh embodiment, The substitution of the anionic functional group can be carried out using a cyclic sulfonic acid compound.

[0031] According to the fifteenth embodiment, in the eleventh embodiment, The substitution of the anionic functional group can be carried out using 1,3-propane sultone.

[0032] According to yet another aspect of the present invention, there is provided a positive electrode for a lithium-sulfur battery according to the following embodiment.

[0033] The positive electrode for a lithium-sulfur battery according to the sixteenth embodiment includes: The sulfur-carbon composite according to any one of the first to ninth embodiments and a binder polymer are included.

[0034] According to yet another aspect of the present invention, there is provided a lithium-sulfur battery according to the following embodiment.

[0035] The lithium-sulfur battery according to the seventeenth embodiment is The battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte solution; The positive electrode includes the positive electrode according to the sixteenth embodiment. [Effects of the Invention]

[0036] The sulfur-carbon composite according to one embodiment of the present invention has a coating structure with amphiphilic polythiophene, which can suppress the elution of lithium polysulfide when a lithium-sulfur battery containing a positive electrode made of the sulfur-carbon composite is operated. This not only improves the discharge capacity of the lithium-sulfur battery, but also improves the capacity retention rate of the battery over repeated cycles.

[0037] Furthermore, the coating structure alleviates the problem of the positive electrode expanding when the lithium-sulfur battery is in operation, thereby improving the safety of the lithium-sulfur battery.

[0038] The following drawings attached to this specification illustrate preferred embodiments of the present invention and, together with the detailed description of the invention, serve to further understand the technical concept of the present invention, and the present invention should not be interpreted as being limited only to the matters described in the drawings. [Brief explanation of the drawings]

[0039] [Figure 1] 1 is a spectrum of the result of 1H NMR of the amphiphilic thiophene monomer of Preparation Example 1-1. [Figure 2] 1 is a spectrum of the amphiphilic thiophene monomer of Preparation Example 1-2 obtained by 1H NMR. [Figure 3] 1 is a spectrum of the amphiphilic thiophene monomer of Preparation Example 2 obtained by 1H NMR. [Figure 4] 1H NMR spectrum of the amphiphilic thiophene monomer of Preparation Example 3. [Figure 5] 1 is a graph showing the results of cycle tests of lithium-sulfur coin cells using Comparative Example 1 (KB / S) and Example 1 (P(TqAZ2) 3.0 wt%). [Figure 6] 1 is a graph showing the results of cycle tests of lithium-sulfur coin cells using Comparative Example 1 (KB / S), Example 2 (P(TqAZ6) 0.1 wt%), Example 3 (P(TqAZ6) 0.2 wt%), Example 4 (P(TqAZ6) 0.5 wt%), and Example 5 (P(TqAZ6) 1.0 wt%). DETAILED DESCRIPTION OF THE INVENTION

[0040] The present invention will be described in detail below. However, the present invention is not limited to the following content, and each component may be variously modified or selectively combined as necessary. Therefore, it should be understood that the present invention includes all modifications, equivalents, and alternatives that fall within the spirit and technical scope of the present invention.

[0041] In this specification, when a certain configuration "includes" a certain component, unless otherwise specified, it does not mean that other components are excluded, but that other components may also be included.

[0042] In this specification, the phrase "A and / or B" means A or B or both.

[0043] Certain terminology used herein is for convenience only and is not limiting. For example, positional terms such as "top," "bottom," "left," "right," "front," "rear," "inside," and "outside" are used to describe relative positions and orientations of components relative to one another, or may represent positions and orientations in the drawings to which reference is made, rather than absolute positions. The terms include the terms themselves as well as words containing them, their derivatives, and words of similar meaning.

[0044] As used herein, "alkyl group" refers to a substituted or unsubstituted hydrocarbon, which may independently be a straight or branched chain.

[0045] The term "substituted or unsubstituted" means that the group is substituted with one or more substituents selected from the group consisting of deuterium, halogen, nitrile, nitro, hydroxy, -COOH, alkoxy, alkyl, cycloalkyl, alkenyl, cycloalkenyl, aryl, heterocyclic, containing one or more N, O, S, or P atoms, and heteroaryl.

[0046] According to one embodiment of the present invention, there is provided a sulfur-carbon composite that can be used as a positive electrode material for lithium-sulfur secondary batteries.

[0047] The sulfur-carbon composite includes a porous carbon substrate and a sulfur-containing compound contained in at least one of the outer surface and the inner surfaces of the pores of the porous carbon substrate, and has a structure in which the entire or part of the surface is covered with an amphiphilic polythiophene.

[0048] As used herein, the term "amphiphilic" refers to a property of containing both a non-polar hydrophobic portion and a polar hydrophilic portion. For example, an amphiphilic compound refers to a compound having a structure containing both a non-polar hydrophobic portion and a polar hydrophilic portion. Thus, the term "amphiphilic polythiophene" refers to a polythiophene having a structure containing both a non-polar hydrophobic portion and a polar hydrophilic portion.

[0049] In this specification, the polythiophene refers to a polymer containing thiophene as a repeating unit. The thiophene may be substituted with an alkyl group at any position, and the alkyl group may be unsubstituted or substituted.

[0050] In one embodiment of the present invention, the amphiphilic polythiophene may include a 3-alkyl-substituted thiophene as a repeating unit, and the alkyl group may include at least one cationic functional group and at least one anionic functional group.

[0051] In this specification, the cationic functional group refers to a functional group that exhibits a cation in an aqueous solvent. In one embodiment of the present invention, the cationic functional group may include, for example, a quaternary amine structure.

[0052] As used herein, the anionic functional group refers to a functional group that exhibits an anion in an aqueous solvent. For example, a compound containing the anionic functional group may have a hydrogen atom (H) that contains an anion (-) in an aqueous solvent such as water. + ) may have the property of dissociating cations such as

[0053] In one embodiment of the present invention, the anionic functional group is -SO3 - , -COO - , -SO4 2- or may contain two or more of these functional groups.

[0054] According to one embodiment of the present invention, the amphiphilic polythiophene has a structure in which the nonpolar portion where the thiophene repeat unit is located provides excellent bonding strength with the porous carbon substrate, and a polar portion where the cationic and anionic functional groups are located can impart hydrophilicity to the surface of the sulfur-carbon composite.

[0055] According to one embodiment of the present invention, the anionic functional group contained in the amphiphilic polythiophene can provide anions to the surface of the sulfur-carbon composite coated with the amphiphilic polythiophene, thereby improving the electrical conductivity of the sulfur-carbon composite, but the effects of the present invention are not limited thereto.

[0056] In one embodiment of the present invention, the amphiphilic polythiophene may include a 3-alkyl-substituted thiophene as a repeating unit. In this case, the 3-alkyl-substituted thiophene may have a structure in which an alkyl group is substituted at the 3-position of the thiophene. In this case, the alkyl group substituted at the 3-position may have a structure having, for example, 1 to 80 carbon atoms, 1 to 50 carbon atoms, 1 to 40 carbon atoms, 1 to 30 carbon atoms, 1 to 20 carbon atoms, or 1 to 15 carbon atoms.

[0057] In another embodiment of the present invention, the amphiphilic polythiophene may include one having at least one structure selected from the following Chemical Formula 1 to Chemical Formula 3.

[0058] [ka]

[0059] [ka]

[0060] [ka]

[0061] In one embodiment of the present invention, in the chemical formula of the polythiophene, n is not particularly limited and may be selected, for example, within a range such that the weight average molecular weight of the polythiophene is 200 g / mol to 5,000,000 g / mol, but the present invention is not limited thereto.

[0062] In one embodiment of the present invention, the structure of the sulfur-carbon composite can be divided into a coating structure coated with the amphiphilic polythiophene and a core structure including a porous carbon substrate and a sulfur-containing compound.

[0063] The porous carbon substrate constituting the core structure may have the functions of improving the electrical conductivity of the sulfur-carbon composite and supporting a sulfur-containing compound to improve sulfur utilization efficiency, but the mechanism of the present invention is not limited thereto.

[0064] The porous carbon substrate is a material containing a large number of micropores, and the sulfur-containing compound is supported on at least one of the outer surface of the porous carbon substrate and the inner surface of the pores.

[0065] In one embodiment of the present invention, the porous carbon substrate has a large number of micropores on the outer surface and inside thereof, and the average diameter (D 50 ) can be, for example, 5 nm to 100 nm, specifically 5 nm to 80 nm, 5 nm to 60 nm, or 5 nm to 50 nm.

[0066] The porous carbon substrate is not particularly limited in material as long as it contains a large number of micropores as described above and is capable of supporting the sulfur-containing compound.

[0067] In one embodiment of the present invention, the porous carbon substrate may include, for example, carbon black, carbon fiber, carbon nanotube (CNT), graphene, graphene oxide (GO), reduced graphene oxide (rGO), carbon black, graphite, graphite nanofiber (GNF), carbon nanofiber (CNF), activated carbon fiber (ACF), natural graphite, artificial graphite, expanded graphite, activated carbon, fullerene, or two or more materials thereof.

[0068] In one embodiment of the present invention, when the material of the porous carbon substrate includes carbon nanotubes, the carbon nanotubes may include single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), or both.

[0069] In another embodiment of the present invention, when the material of the porous carbon substrate includes carbon nanotubes, the carbon nanotubes may include entangled carbon nanotubes formed as secondary structures by aggregating a plurality of carbon nanotubes as primary structures.

[0070] According to one embodiment of the present invention, the entangled carbon nanotubes may have an improved porosity compared to the primary structure carbon nanotubes due to an interstitial volume formed by the entanglement of primary structure carbon nanotubes.

[0071] In one embodiment of the present invention, the size of the porous carbon substrate may be, but is not limited to, 10 μm to 100 μm, specifically 20 μm to 50 μm. When the size of the porous carbon substrate is within the above range, advantageous effects can be achieved in terms of controlling the solid content when preparing a slurry for forming an electrode active material layer, and in terms of electrode properties, such as adhesive strength, and battery performance (output, capacity, etc.).

[0072] In one embodiment of the present invention, the pore volume of the porous carbon substrate is, for example, 1 cm 3 / g~5cm 3 / g, specifically 1cm 3 / g~4cm 3 The pore volume may be a value calculated or measured through, for example, N adsorption isotherm analysis obtained based on the adsorption of liquid nitrogen.

[0073] In one embodiment of the present invention, the BET specific surface area of the porous carbon substrate is, but is not limited to, for example, 150 m 2 / g~2,000m 2 / g, specifically 250m 2 / g~700m 2The BET specific surface area may be / g. The BET specific surface area is measured by the BET method and may represent a value measured by a known method for measuring a BET specific surface area. For example, the BET specific surface area may be a value calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77 K) using a BELSORP-mini II manufactured by BEL Japan.

[0074] In one embodiment of the present invention, the sulfur-containing compound may be, for example, but is not limited to, inorganic sulfur (S), lithium sulfide (LiS), lithium polysulfide (LiS, 2≦x≦8), a disulfide compound, or a mixture of two or more thereof.

[0075] In one embodiment of the present invention, the porous carbon substrate and the sulfur-containing compound may be mixed in a weight ratio of, for example, 1:9 to 9:1, more specifically, 2:8 to 8:2, more specifically, 3:7 to 7:3, or 4:6 to 2:8.

[0076] In one embodiment of the present invention, the core structure of the sulfur-carbon composite may be formed by mixing the porous carbon substrate with a sulfur-containing compound and then heat-treating the mixture at a temperature of, for example, 130°C to 200°C, specifically, 130°C to 180°C, or 150°C to 160°C.

[0077] As described above, according to one embodiment of the present invention, there is provided a sulfur-carbon composite in which the sulfur-containing compound is contained in at least one of the outer surface and the inner surfaces of the pores of the porous carbon substrate, and the entire or partial surface is coated with the amphiphilic polythiophene.

[0078] In one embodiment of the present invention, the sulfur-carbon composite has an average particle size (D 50 ).

[0079] In one embodiment of the present invention, the total content of the amphiphilic polythiophene, based on the total weight of the sulfur-carbon composite, may be, but is not limited to, 0.1 wt% to 10 wt%. Specifically, the total content of the polythiophene, based on the total weight of the sulfur-carbon composite, may be 0.1 wt% to 5 wt%, specifically 0.1 wt% to 1 wt%, 0.2 wt% to 0.8 wt%, 0.3 wt% to 0.7 wt%, 0.3 wt% to 0.6 wt%, 0.4 wt% to 0.6 wt%, or 0.5 wt%. The amphiphilic polythiophene content within the above range is advantageous in terms of improving the lifespan of a lithium-sulfur battery using the sulfur-carbon composite, but the present invention is not limited thereto.

[0080] Another embodiment of the present invention provides a method for producing the above-mentioned sulfur-carbon composite.

[0081] According to another embodiment of the present invention, a method for preparing a sulfur-carbon composite includes the steps of: preparing an uncoated sulfur-carbon composite including a porous carbon substrate and a sulfur-containing compound supported on at least one of an outer surface of the porous carbon substrate and inner surfaces of pores of the porous carbon substrate; coating all or at least a portion of the surface of the uncoated sulfur-carbon composite with an amphiphilic thiophene monomer; and obtaining a sulfur-carbon composite whose surface is entirely or partially coated with an amphiphilic polythiophene formed by polymerization of the coated amphiphilic thiophene monomer.

[0082] In the production method, the porous carbon substrate, the sulfur-containing compound, and the amphiphilic polythiophene are described above.

[0083] First, the step of preparing the uncoated sulfur-carbon composite includes mixing the porous carbon substrate with the sulfur-containing compound. In one embodiment of the present invention, the porous carbon substrate and the sulfur-containing compound may be mixed in a weight ratio of, for example, 1:9 to 9:1. Specifically, the porous carbon substrate and the sulfur-containing compound may be mixed in a weight ratio of 2:8 to 8:2, more specifically, 3:7 to 7:3, or 4:6 to 2:8.

[0084] In one embodiment of the present invention, the method may further include the step of pulverizing the porous carbon substrate and the sulfur-containing compound after mixing them together. The pulverization may improve the mixing uniformity of the porous carbon substrate and the sulfur-containing compound, but the present invention is not limited thereto.

[0085] In one embodiment of the present invention, the step of preparing the uncoated sulfur-carbon composite may further include a step of heat-treating the porous carbon substrate and the sulfur-containing compound after mixing (or mixing and pulverizing) them. The heat treatment may be performed at a temperature of, for example, 130°C to 200°C, specifically, 130°C to 180°C, or 150°C to 160°C. The heat treatment may improve the mechanical strength of the sulfur-carbon composite, but the present invention is not limited thereto.

[0086] Next, the entire or at least a part of the surface of the prepared uncoated sulfur-carbon composite is coated with the amphiphilic thiophene monomer.

[0087] In an embodiment of the present invention, the method for preparing the sulfur-carbon composite may further include the step of preparing an amphiphilic thiophene monomer, including the steps of:

[0088] (S10) activating the carboxylic acid group of 3-carboxyalkylthiophene; (S20) reacting the activated 3-carboxyalkylthiophene compound with a dialkylaminoalcohol compound to form a dialkylaminoalkylthiophenylalkylcarboxylate compound; and (S30) A step of obtaining the amphiphilic thiophene monomer by substituting at least one anionic functional group onto the thiophenyl alkyl carboxylate dialkylamino alkyl compound.

[0089] In one embodiment of the present invention, the coating is performed by contacting the sulfur-carbon composite with the amphiphilic thiophene monomer. For example, the sulfur-carbon composite can be coated by immersing the sulfur-carbon composite in a dispersion of the amphiphilic thiophene monomer and then stirring the mixture.

[0090] In one embodiment of the present invention, the dispersion in which the amphiphilic thiophene monomer is dispersed may be, for example, 0.1 wt % to 10 wt %, 0.1 wt % to 5 wt %, 0.1 wt % to 1 wt %, 0.1 wt % to 0.8 wt %, 0.1 wt % to 0.5 wt %, 0.2 wt % to 0.5 wt %, 0.3 wt % to 0.5 wt %, or 0.4 wt % to 0.5 wt %, but the present invention is not limited thereto.

[0091] In one embodiment of the present invention, the amount of the coated amphiphilic thiophene monomer may be 0.1 wt% to 10 wt%, 0.1 wt% to 5 wt%, 0.1 wt% to 3 wt%, 0.1 wt% to 1 wt%, or 0.3 wt% to 0.8 wt% based on the total weight of the coated sulfur-carbon composite. The amount of the coated amphiphilic thiophene monomer may be calculated based on the increased weight of the coated sulfur-carbon composite relative to the weight of the uncoated sulfur-carbon composite.

[0092] The coated thiophene monomer is then polymerized.

[0093] In one embodiment of the present invention, the polymerization reaction of the amphiphilic thiophene monomer may be an in-situ reaction.

[0094] Through the polymerization reaction, all or part of the surface of the sulfur-carbon composite can be covered with the amphiphilic polythiophene.

[0095] In one embodiment of the present invention, the polymerization reaction may be carried out in the presence of a polymerization initiator, such as, but not limited to, ammonium persulfate.

[0096] In one embodiment of the present invention, the polymerization reaction is preferably carried out at a temperature at which the amphiphilic thiophene monomer is not decomposed, for example, 60°C to 100°C, for example, 70°C, but the present invention is not limited thereto.

[0097] In one embodiment of the present invention, in the process of preparing the amphiphilic thiophene monomer, the 3-carboxyalkylthiophene may include 2-thiopheneacetic acid.

[0098] In one embodiment of the present invention, in the process of preparing the amphiphilic thiophene monomer, the dialkylamino alcohol compound may include 2-dimethylaminoethanol, 2-dimethylaminobutanol, 6-dimethylaminohexanol, or a mixture of two or more thereof.

[0099] In one embodiment of the present invention, in the process of preparing the amphiphilic thiophene monomer, the substitution of the anionic functional group may be performed using a cyclic sulfonic acid compound. Specifically, the substitution of the anionic functional group may be performed, for example, by a ring-opening reaction of the cyclic sulfonic acid compound. In this case, the amphiphilic thiophene monomer substituted with the anionic functional group through the ring-opening reaction of the cyclic sulfonic acid compound may include a sulfonic acid group as the anionic functional group.

[0100] In one embodiment of the present invention, in the process of preparing the amphiphilic thiophene monomer, the substitution of the anionic functional group may be performed using 1,3-propane sultone.

[0101] By the above method, a sulfur-carbon composite can be prepared, which includes a porous carbon substrate and a sulfur-containing compound contained in at least one of the outer surface and the inner surfaces of the pores of the porous carbon substrate, and has a structure in which the entire or part of the surface is covered with the amphiphilic polythiophene.

[0102] According to yet another embodiment of the present invention, there is provided a positive electrode for a lithium-sulfur battery, comprising the above-described sulfur-carbon composite and a binder polymer.

[0103] In one embodiment of the present invention, the binder polymer may be any binder that can be used for the positive electrode of a lithium-sulfur battery, including, but not limited to, PVDF (polyvinylidene fluoride), specifically, PVDF dispersed in NMP (N-methyl-2-pyrrolidone).

[0104] In another embodiment of the present invention, the binder polymer may include, but is not limited to, an aqueous binder such as SBR (styrene butadiene rubber), specifically, an aqueous binder dispersed in an aqueous solvent such as water.

[0105] In another embodiment of the present invention, the positive electrode for a lithium-sulfur battery may further include, in addition to the sulfur-carbon composite and the binder polymer, a conductive material, an additive, etc. Here, specific types of the conductive material and the additive may be conventional ones, and therefore, description thereof will be omitted.

[0106] In yet another embodiment of the present invention, the positive electrode for a lithium-sulfur battery may include a positive electrode current collector, and a positive electrode active material layer in which the sulfur-carbon composite as the positive electrode active material is coated on one or both surfaces of the current collector together with the binder polymer.

[0107] In this case, the positive electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity.

[0108] In one embodiment of the present invention, the positive electrode including the sulfur-carbon composite may exhibit excellent effects in terms of initial capacity and cycle stability, but the effects of the present invention are not limited thereto.

[0109] According to another embodiment of the present invention, there is provided a lithium-sulfur battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. The positive electrode includes the above-described positive electrode.

[0110] In one embodiment of the present invention, the anode and separator may be any material usable in a lithium-sulfur battery without limitation, as long as the anode and separator do not impair the object of the present invention. For example, lithium metal may be used as the anode.

[0111] In one embodiment of the present invention, the separator may be any material that can be commonly used as a separator for lithium-sulfur batteries. The separator may include a porous polyolefin substrate, and optionally, may further include inorganic particles on at least one surface of the porous polyolefin substrate. The separator may also optionally include a binder to bind the inorganic particles.

[0112] In another embodiment of the present invention, the separator may be a film-type electrolyte membrane containing a solid electrolyte, and may further contain a binder for binding the solid electrolyte, if necessary. The solid electrolyte may be any solid electrolyte that can be commonly used in lithium-sulfur batteries, such as a polymer-based solid electrolyte, an inorganic-based solid electrolyte, or a mixture thereof, without limitation.

[0113] In one embodiment of the present invention, the electrolyte solution includes those commonly used in lithium-sulfur batteries, and may include a lithium salt and a non-aqueous solvent.

[0114] The lithium salt may be any salt that can be commonly used in the electrolyte of a lithium-sulfur battery. Examples of the lithium salt include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB 10 Cl10 , LiPF6, LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, (C2F5SO2)2NLi, (SO2F)2NLi, (CF3SO2)3CLi, lithium chloroborane, lithium lower aliphatic carboxylates, lithium tetraphenylborate, lithium imide, or two or more thereof.

[0115] The non-aqueous solvent may be any solvent that can be commonly used in electrolytes for lithium-sulfur batteries, including, but not limited to, cyclic carbonate solvents, linear carbonate solvents, ester solvents, ketone solvents, or mixtures of two or more thereof.

[0116] In one embodiment of the present invention, the electrolyte may include (CF3SO2)2NLi as a lithium salt and a dioxolane (DOL) / dimethoxyethane (DME) binary system as a non-aqueous solvent. For example, the electrolyte may further include a conventional additive such as LiNO3.

[0117] In one embodiment of the present invention, the lithium-sulfur battery may have an outer shape, such as a coin shape, a cylindrical shape, a pouch shape, or a rectangular shape, but the outer shape of the battery is not particularly limited. Furthermore, the lithium-sulfur battery may be used not only as a battery cell used as a power source for a small device, but also as a unit battery in a medium- to large-sized battery module including a plurality of battery cells, and the usage form is not particularly limited.

[0118] In one embodiment of the present invention, a lithium-sulfur battery using a cathode including the sulfur-carbon composite can effectively alleviate the problem of capacity reduction due to repeated cycles by suppressing the elution of lithium polysulfide during battery operation and mitigating the shuttle effect, but the effects of the present invention are not limited thereto.

[0119] In one embodiment of the present invention, the lithium-sulfur battery may exhibit an effect of improving energy density by increasing the sulfur loading amount in the positive electrode, but the effect of the present invention is not limited thereto.

[0120] Hereinafter, a method for preparing a sulfur-carbon composite and a method for preparing a lithium-sulfur battery using the same according to an embodiment of the present invention will be described in detail with reference to examples. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0121] Preparation Example 1-1. Preparation of amphiphilic thiophene monomer (TqAZ2) The amphiphilic thiophene monomer was prepared according to the following reaction scheme:

[0122] [ka]

[0123] First, 3-thiopheneacetic acid was reacted with SOCl2 in benzene at 90°C to activate the carboxyl group. The activated thiophene monomer was dissolved in methylene chloride, and then an equivalent amount of triethyleneamine (TEA) and 2-dimethylaminoethanol (DMAE) was added. The thiophene monomer was reacted with DMAE at 30°C to synthesize TAA-DMAE. TAA-DMAE was dissolved in THF and reacted with an excess amount of propane sultone at 50°C to synthesize the amphiphilic thiophene monomer (TqAZ2).

[0124] The acquisition of TqAZ2 was confirmed through 1H NMR, and the 1H NMR results are shown in Figure 1.

[0125] Preparation Example 1-2. Preparation of amphiphilic polythiophene (P(TqAZ2)) An amphiphilic polythiophene was prepared according to the following reaction scheme:

[0126] [ka]

[0127] TqAZ2 from Preparation Example 1-1 was dissolved in water and polymerized at 70°C using ammonium persulfate as an oxidizing agent to synthesize amphiphilic polythiophene (P(TqAZ2)). It was confirmed that the synthesized P(TqAZ2) was insoluble in water.

[0128] The acquisition of P(TqAZ2) was confirmed through 1H NMR, and the 1H NMR results are shown in Figure 2.

[0129] Preparation Example 2: Preparation of amphiphilic thiophene monomer (TqAZ4) The amphiphilic thiophene monomer was prepared according to the following reaction scheme:

[0130] [ka]

[0131] The carboxyl group was activated by reacting 3-thiopheneacetic acid with SOCl2 in benzene at 90°C. The activated thiophene monomer was dissolved in methylene chloride, and then an equivalent amount of triethyleneamine (TEA) and 2-dimethylaminobutanol (DMAB) was added. The thiophene monomer was reacted with DMAB at 30°C to synthesize TAA-DMAB. TAA-DMAB was dissolved in THF and reacted with an excess amount of propane sultone at 50°C to synthesize an amphiphilic thiophene monomer (TqAZ4).

[0132] The acquisition of TqAZ4 was confirmed through 1H NMR, and the 1H NMR results are shown in Figure 3.

[0133] Preparation Example 3: Preparation of amphiphilic thiophene monomer (TqAZ6) The amphiphilic thiophene monomer was prepared according to the following reaction scheme:

[0134] [ka]

[0135] The carboxyl group was activated by reacting 3-thiopheneacetic acid with SOCl2 in benzene at 90°C. The activated thiophene monomer was dissolved in methylene chloride, and then an equivalent amount of triethyleneamine (TEA) and 2-dimethylaminohexanol (DMAH) was added. The thiophene monomer was reacted with DMAH at 30°C to synthesize TAA-DMAH. TAA-DMAH was dissolved in THF and reacted with an excess amount of propane sultone at 50°C to synthesize the amphiphilic thiophene monomer (TqAZ6).

[0136] The acquisition of TqAZ6 was confirmed through 1H NMR, and the 1H NMR results are shown in Figure 4.

[0137] [Production of sulfur-carbon composite] The sulfur-carbon composites of Comparative Example 1 and Examples 1 to 5 were produced according to the following methods.

[0138] Comparative example 1.KB-S Sulfur (S8) and Ketjen black (KB) were mixed in a mass ratio of 7:3, ground in a mortar grinder, and heat-treated at 155°C for 30 minutes to prepare an uncoated sulfur-carbon composite (KB-S) in which sulfur was supported on the outer surface and inner surface of the pores of the Ketjen black.

[0139] Example 1. P(TqAZ2) 3.0 wt% The uncoated sulfur-carbon composite prepared in Comparative Example 1 and a 3.0 wt % aqueous solution of the amphiphilic thiophene monomer (TqAZ2) synthesized in Preparation Example 1-1 were added to water, and the mixture was sonicated for 10 hours and stirred for 16 hours to disperse the mixture. The resulting mixture was then filtered to obtain a sulfur-carbon composite coated with the amphiphilic thiophene monomer.

[0140] 0.2 g of the sulfur-carbon composite coated with the amphiphilic thiophene monomer was placed in 2 mL of water, and ammonium persulfate was added. The amphiphilic thiophene monomer was then in-situ polymerized at 70°C for 24 hours, followed by filtration and drying to obtain the amphiphilic polythiophene-coated sulfur-carbon composite of Preparation Example 1-2.

[0141] Example 2. P(TqAZ6) 0.1 wt% An amphiphilic polythiophene-coated sulfur-carbon composite was obtained in the same manner as in Example 1, except that the amphiphilic thiophene monomer (TqAZ6) from Preparation Example 3 was used and the amount of the amphiphilic thiophene monomer was 0.1 wt%.

[0142] Example 3. P(TqAZ6) 0.2 wt% An amphiphilic polythiophene-coated sulfur-carbon composite was obtained in the same manner as in Example 1, except that the amphiphilic thiophene monomer (TqAZ6) from Preparation Example 3 was used and dissolved at a concentration of 0.2 wt%.

[0143] Example 4. P(TqAZ6) 0.5 wt% An amphiphilic polythiophene-coated sulfur-carbon composite was obtained in the same manner as in Example 1, except that the amphiphilic thiophene monomer (TqAZ6) from Preparation Example 3 was used and dissolved at a concentration of 0.5 wt%.

[0144] Example 5. P(TqAZ6) 1.0 wt% An amphiphilic polythiophene-coated sulfur-carbon composite was obtained in the same manner as in Example 1, except that the amphiphilic thiophene monomer (TqAZ6) from Preparation Example 3 was used and dissolved at 1.0 wt%.

[0145] [Evaluation of the performance of lithium-sulfur coin cells] To evaluate the battery performance when the sulfur-carbon composite prepared as above was applied to the cathode, a lithium-sulfur coin cell was fabricated as follows.

[0146] The prepared sulfur-carbon composite was mixed with polyacrylic acid (PAA) binder (MW 450,000) and carbon conductive material (Super P) in a ratio of 85:10:5, and then 0.5 wt% of PVA dispersant (MW 9,500) was added. The mixture was mixed in a mixer (Thinky) until the solids concentration in the aqueous phase reached 18 wt% to prepare a cathode slurry. The prepared cathode slurry was coated onto aluminum foil to a thickness of 400 μm using a doctor blade and dried at 50°C for 14 hours.

[0147] The fabricated cathode and lithium metal anode were prepared, and a Celgard 2400 separator was sandwiched between the cathode and anode. The electrode assembly was then immersed in a liquid electrolyte of DOL / DME (1 / 1 v / v), 1M LiTFSI (with 5 wt% LiNO3) to fabricate a lithium-sulfur coin cell.

[0148] [Evaluation of discharge capacity after repeated cycles] The lithium-sulfur coin cell prepared as described above was tested at 25°C using a battery cycle tester (WBS3000 Battery Cycler, WonA Tech) at 1.8 to 2.8 V (vs Li / Li + A charge-discharge cycle test was performed by repeatedly charging and discharging at a cut-off voltage and a current density of 0.5C.

[0149] The evaluation results of the discharge capacity after repeated cycles are shown in FIGS.

[0150] 5 and 6, the graphs show Comparative Example 1 (KB / S), Example 1 (P(TqAZ2) 3.0 wt%), Example 2 (P(TqAZ6) 0.1 wt%), Example 3 (P(TqAZ6) 0.2 wt%), Example 4 (P(TqAZ6) 0.5 wt%), and Example 5 (P(TqAZ6) 1.0 wt%).

[0151] Referring to FIG. 5, it can be seen that the capacity retention rate of the lithium-sulfur battery using the sulfur-carbon composite of Example 1 is improved with repeated cycles, compared with Comparative Example 1.

[0152] 6, it can be seen that the capacity retention rates of the lithium-sulfur batteries using the sulfur-carbon composites of Examples 2 to 5 were improved compared to Comparative Example 1. Among them, it can be seen that the capacity retention rate of Example 4 was the best.

Claims

1. A porous carbon substrate and a sulfur-containing compound contained in at least one of an outer surface of the porous carbon substrate and inner surfaces of pores, A sulfur-carbon composite whose surface is wholly or partially covered with an amphiphilic polythiophene.

2. 2. The sulfur-carbon composite according to claim 1, wherein the amphiphilic polythiophene comprises a 3-alkyl-substituted thiophene as a repeating unit, and the alkyl group of the 3-alkyl-substituted thiophene comprises at least one cationic functional group and at least one anionic functional group as a substituent.

3. 3. The sulfur-carbon composite according to claim 2, wherein the cationic functional group comprises a quaternary amine structure.

4. The anionic functional group is —SO ３ － , -COO － , -SO ４ ２－ or two or more functional groups selected from these.

5. 2. The sulfur-carbon composite according to claim 1, wherein the amphiphilic polythiophene comprises a polymer represented by the following Chemical Formula 1: 【Chemical 1】

6. 2. The sulfur-carbon composite according to claim 1, wherein the amphiphilic polythiophene comprises a polymer represented by the following Chemical Formula 2: 【Chemistry 2】

7. 2. The sulfur-carbon composite according to claim 1, wherein the amphiphilic polythiophene comprises a polymer represented by the following Chemical Formula 3: 【Chemistry 3】

8. 2. The sulfur-carbon composite according to claim 1, wherein the porous carbon substrate comprises carbon black, carbon fiber, carbon nanotube, graphene, graphene oxide, reduced graphene oxide, carbon black, graphite, graphite nanofiber, carbon nanofiber, activated carbon fiber, natural graphite, artificial graphite, expanded graphite, activated carbon, fullerene, or two or more materials thereof.

9. The sulfur-containing compound is inorganic sulfur (S ８ ), lithium sulfide (Li ２ S), lithium polysulfide (Li ２ 2. The sulfur-carbon composite according to claim 1, comprising a sulfur-carbon complex, ...

10. preparing an uncoated sulfur-carbon composite including a porous carbon substrate and a sulfur-containing compound supported on at least one of an outer surface of the porous carbon substrate and inner surfaces of pores of the porous carbon substrate; coating all or at least a portion of the surface of the uncoated sulfur-carbon composite with an amphiphilic thiophene monomer; and obtaining a sulfur-carbon composite whose surface is entirely or partially coated with an amphiphilic polythiophene formed by a polymerization reaction of the coated amphiphilic thiophene monomer.

11. The amphiphilic thiophene monomer can be prepared by the following steps: (S10) activating the carboxylic acid group of 3-carboxyalkylthiophene; (S20) reacting the activated 3-carboxyalkylthiophene compound with a dialkylaminoalcohol compound to form a dialkylaminoalkylthiophenylalkylcarboxylate compound; and (S30) substituting at least one anionic functional group on the thiophenyl alkyl carboxylate dialkylamino alkyl compound to obtain the amphiphilic thiophene monomer; The method for producing a sulfur-carbon composite according to claim 10, wherein the sulfur-carbon composite is obtained by a production method comprising the steps of:

12. 12. The method for producing a sulfur-carbon composite according to claim 11, wherein the 3-carboxyalkylthiophene includes 2-thiopheneacetic acid.

13. 12. The method for producing a sulfur-carbon composite according to claim 11, wherein the dialkylaminoalcohol compound comprises 2-dimethylaminoethanol, 2-dimethylaminobutanol, 6-dimethylaminohexanol, or a mixture of two or more thereof.

14. The method for producing a sulfur-carbon composite according to claim 11, wherein the substitution of the anionic functional group is performed using a cyclic sulfonic acid compound.

15. The method for producing a sulfur-carbon composite according to claim 11, wherein the substitution of the anionic functional group is performed using 1,3-propane sultone.

16. A positive electrode for a lithium-sulfur battery, comprising the sulfur-carbon composite according to any one of claims 1 to 9 and a binder polymer.

17. The battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte solution; 17. A lithium-sulfur battery, wherein the positive electrode comprises the positive electrode of claim 16.

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