Sulfur-carbon composite material, positive electrode for lithium secondary battery containing the same, and lithium secondary battery

Abstract

One aspect of the present invention relates to a sulfur-carbon composite material having excellent reactivity and lifespan characteristics and having an effect of reducing overvoltage, a method for producing the same, and a positive electrode for a lithium secondary battery and a lithium secondary battery including the same.

Description

Technical Field

[0001] This application claims the benefit of priority from Korean Patent Application No. 10-2019-0079364, filed on Jul. 2, 2019, and Korean Patent Application No. 10-2020-0081369, filed on Jul. 2, 2020, which are hereby incorporated by reference herein in their entirety.

[0002] One aspect of the present invention relates to a sulfur-carbon composite material, a positive electrode for a lithium secondary battery including the same, and a lithium secondary battery. Background Art

[0003] With growing interest in energy storage technology, research and development of electrochemical devices is increasing as its applications expand from power sources for mobile phones, tablets, laptops, and cameras to power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs). The field of electrochemical devices is attracting the most attention in this area. Among these, the development of rechargeable and dischargeable secondary batteries, such as lithium-sulfur secondary batteries, has become a major focus. In recent years, research and development of new electrode and cell designs has been conducted to improve capacity density and specific energy in these batteries.

[0004] Among these electrochemical devices, lithium-sulfur batteries (Li-S batteries) have a high energy density (theoretical capacity), attracting attention as next-generation secondary batteries that can replace lithium-ion batteries. In these Li-S batteries, sulfur reduction and lithium metal oxidation occur during discharge. During this process, sulfur transforms from the cyclic structure of S8 to linear lithium polysulfide (LiPS). These Li-S batteries are characterized by a step-by-step discharge voltage until the polysulfide is completely reduced to Li2S.

[0005] However, the biggest obstacle to the commercialization of lithium-sulfur batteries is the dissolution and shuttling of lithium polysulfides, which poses a serious problem of reduced capacity. Specifically, because polysulfides dissolved from the cathode have high solubility in organic electrolytes, undesirable migration of polysulfides through the electrolyte to the anode (PS shuttling) can occur. As a result, capacity is reduced due to irreversible loss of cathode active material, and battery life is shortened due to side reactions that cause sulfur particles to deposit on the lithium metal surface.

[0006] Therefore, Korean Patent Publication No. 2018-0048309 discloses that problems associated with the dissolution and shuttling phenomenon of polysulfides can be solved by applying a sulfur-carbon composite material including carbon nanotubes coated with an ion conductive polymer on the surface thereof and sulfur to a lithium-sulfur battery.

[0007] Therefore, technologies have been developed to prevent the dissolution of lithium polysulfide by wrapping the exterior of sulfur-carbon composite materials used as cathode materials for lithium-sulfur batteries, treating the surface of separators, or using protective films for negative electrodes, but the effects are not significant.

[0008] Therefore, on the other hand, in addition to simply trying to solve the dissolution problem of lithium polysulfides by using physical membranes, it is necessary to develop a technology that can solve the problem of dissolution and shuttling of lithium polysulfides.

[0009] Prior art literature

[0010] (Patent Document 1) Korean Patent Publication No. 10-2018-0048309 Summary of the Invention

[0011] [Technical Issues]

[0012] The inventors of one aspect of the present invention have conducted various studies to solve the above-mentioned problems, and as a result, have confirmed that when the following sulfur-carbon composite material is used as a positive electrode active material for a lithium secondary battery, the overvoltage of the lithium secondary battery is reduced and the reactivity and life characteristics are improved. The sulfur-carbon composite material is prepared by using a porous carbon material with a copolymer coated on the surface, and the copolymer contains a redox functional group that can catalyze the reduction reaction of lithium polysulfide and a lithium ion conductive functional group that can improve the transfer of lithium ions.

[0013] Accordingly, one aspect of the present invention provides a sulfur-carbon composite material capable of reducing overvoltage and improving reactivity and lifespan characteristics of a lithium secondary battery.

[0014] In addition, another aspect of the present invention provides a positive electrode for a lithium secondary battery including the sulfur-carbon composite material and a lithium secondary battery including the positive electrode.

[0015] [Technical solution]

[0016] To achieve the above-mentioned objectives, one aspect of the present invention provides a sulfur-carbon composite material, which comprises: a porous carbon material; and sulfur, wherein the sulfur is formed on at least a portion of the interior and surface of the porous carbon material, wherein the surface of the porous carbon material is coated with a copolymer containing redox functional groups and lithium ion conductive functional groups.

[0017] In addition, another aspect of the present invention provides a method for preparing a sulfur-carbon composite material, the method comprising the following steps: (a) coating a porous carbon material with a copolymer containing a redox functional group and a lithium ion conductive functional group; and (b) mixing and shaping the porous carbon material coated with the copolymer containing a redox functional group and a lithium ion conductive functional group prepared in step (a) with sulfur.

[0018] In another aspect, the present invention provides a positive electrode for a lithium secondary battery, comprising the sulfur-carbon composite material according to one aspect of the present invention.

[0019] In addition, another aspect of the present invention provides a lithium secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode is the positive electrode of the present invention.

[0020] [Beneficial Effects]

[0021] The sulfur-carbon composite material according to one aspect of the present invention has the effects of promoting the reduction of lithium polysulfide and promoting lithium ion transfer.

[0022] Therefore, in the case of a lithium secondary battery containing the sulfur-carbon composite material as a positive electrode active material, the reduction reaction of lithium polysulfide occurs rapidly and lithium polysulfide can be prevented from moving out of the positive electrode, thereby improving reactivity and life characteristics and reducing the occurrence of overvoltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a graph showing the results of cyclic voltammetry of the positive electrode for a lithium-sulfur battery prepared in Example 1.

[0024] Figure 2 This is a graph showing the life characteristics of the lithium-sulfur batteries of Example 1, Example 2, and Comparative Example 1.

[0025] Figure 3 This is a graph showing the life characteristics of the lithium-sulfur batteries of Example 3, Example 4, and Comparative Example 2.

[0026] Figure 4 This is a graph showing the life characteristics of the lithium-sulfur batteries of Example 3, Example 4, and Comparative Example 2 in the range of charge / discharge potential values from 1.8 V to 2.8 V.

[0027] Figure 5 It is a graph showing the life characteristics of the lithium-sulfur batteries of Example 1 and Comparative Examples 4 to 6.

[0028] Figure 6 This is a graph showing the initial discharge capacity of the lithium-sulfur battery of Example 5.

[0029] Figure 7 This is a graph showing the initial discharge capacity of the lithium-sulfur battery of Comparative Example 3.

[0030] Figure 8 This is a graph showing the life characteristics of the lithium-sulfur batteries of Example 5 and Comparative Example 3.

[0031] Figure 9: is a graph showing the discharge capacity decrease rates obtained by measuring the first and second discharge capacities of the lithium-sulfur batteries of Example 5 and Comparative Example 3.

[0032] Figure 10 This is a photograph of the surface of the negative electrode of the lithium-sulfur battery of Example 5 after evaluation of the life characteristics.

[0033] Figure 11 This is a photograph of the surface of the separator and the positive electrode of the lithium-sulfur battery of Example 5 after evaluation of the life characteristics. DETAILED DESCRIPTION

[0034] Hereinafter, the present invention will be described in more detail.

[0035] The term "composite material" used herein refers to a material in which two or more materials are combined to exhibit more effective functions while forming phases that are physically and chemically different from each other.

[0036] Among various secondary batteries, lithium-sulfur batteries have attracted much attention as next-generation batteries because of their high discharge capacity and theoretical energy density, and sulfur, a positive electrode active material, is abundant in resources, inexpensive, and environmentally friendly.

[0037] However, in lithium-sulfur batteries, sulfur is converted from cyclic S8 to lithium polysulfide (Li2S x , x=8, 6, 4, 2) and when lithium polysulfide is completely reduced, lithium sulfide (Li2S) is finally generated. Among the lithium polysulfides that are intermediate products of this reduction reaction of sulfur, lithium polysulfide with a high sulfur oxidation number (Li2S x , usually x>4) is a highly polar substance and is easily dissolved in an electrolyte containing a hydrophilic organic solvent, thereby dissolving out of the reaction area of the positive electrode and no longer being able to participate in the electrochemical reaction.

[0038] The lithium polysulfide dissolved from the positive electrode can move to the negative electrode and directly undergo side reactions on the surface of the lithium negative electrode, thereby causing a shuttling phenomenon, which reduces the capacity of the battery and shortens its life.

[0039] Therefore, one aspect of the present invention provides a sulfur-carbon composite material that can solve the above-mentioned problems.

[0040] Sulfur-carbon composite materials

[0041] One aspect of the present invention relates to a sulfur-carbon composite material, comprising: a porous carbon material; and sulfur formed on at least a portion of the interior and surface of the porous carbon material, wherein the surface of the porous carbon material is coated with a copolymer containing redox functional groups and lithium ion conductive functional groups.

[0042] The surface of the porous carbon material is coated with a copolymer containing redox functional groups and lithium ion conductive functional groups. The coating is applied to the inner and outer surfaces of the porous carbon material and can be regarded as coating the entire surface of the porous carbon material.

[0043] The copolymer may be a copolymer of a monomer containing a redox functional group and a monomer containing a lithium ion conductive functional group.

[0044] The redox functional group of the monomer containing the redox functional group is used to promote the reduction of lithium polysulfide dissolved from the positive electrode through redox reaction. Generally, the redox material has the property of an insulator that does not allow electrons to pass through, but has redox properties when giving or receiving electrons from the porous carbon material. Therefore, when the surface of the porous carbon material is coated with a copolymer, redox characteristics can be exhibited without generating overvoltage. Therefore, when the sulfur-carbon composite material of one aspect of the present invention is used as a positive electrode active material for a lithium secondary battery, the redox functional group plays a role in increasing kinetics and can quickly reduce lithium polysulfide, thereby increasing reactivity, and can prevent the phenomenon of lithium polysulfide dissolving from the positive electrode.

[0045] The monomer containing a redox functional group may include at least one selected from the group consisting of a naphthaleneimide compound, a perylene compound, and an imide compound, preferably a naphthaleneimide compound.

[0046] The lithium ion conductive functional groups of the monomer containing the lithium ion conductive functional groups play a role in ensuring a path for lithium ions to migrate into the interior of the sulfur-carbon composite material, i.e., into the pores of the porous carbon material. Specifically, when the sulfur-carbon composite material of one aspect of the present invention is used as a positive electrode active material for a lithium secondary battery, the reactivity with sulfur as the positive electrode active material increases with high ion conductivity, thereby improving the reactivity of the lithium secondary battery and reducing overvoltage.

[0047] The monomer containing a lithium ion conductive functional group may include at least one selected from the group consisting of an ether compound, a sulfonic acid compound, a carboxylic acid compound, and an acrylic compound, preferably an ether compound.

[0048] The copolymer containing a redox functional group and a lithium ion conductive functional group is preferably a naphthalene diimide-polyethylene oxide copolymer, but is not limited thereto.

[0049] In the copolymer, the molar ratio of the monomer containing a redox functional group to the monomer containing a lithium ion conductive functional group can be 2:8 to 8:2, specifically 2:8 or greater, 3:7 or greater, or 4:6 or greater, and 6:4 or less, 7:3 or less, or 8:2 or less. If the molar ratio of the monomer containing a redox functional group is less than this range, overvoltage may occur. If the molar ratio exceeds this range, redox activity may decrease.

[0050] The number average molecular weight (Mn) of the copolymer may be 500 to 200,000, specifically 500 or more, 1,000 or more, or 1,500 or more and 100,000 or less, 150,000 or less, or 200,000 or less.

[0051] In addition, the copolymer can be included in an amount of 0.25 wt% to 5 wt% relative to the total weight of the porous carbon material, specifically, the copolymer can be included in an amount of more than 0.25 wt%, more than 0.3 wt% or more than 0.5 wt%, and the copolymer can be included in an amount of less than 1 wt%, less than 3 wt% or less than 5 wt%.

[0052] If the copolymer is included in an amount less than 0.25 wt%, the performance of the redox functional group is deteriorated, thereby possibly failing to promote the reduction of lithium polysulfide. If the copolymer exceeds 5 wt%, overvoltage may be generated and reactivity may be reduced.

[0053] Furthermore, because the surface of the porous carbon material is coated with the copolymer, it can also function as a protective film as the electrolyte decomposes.

[0054] The porous carbon material provides a skeleton capable of uniformly and stably fixing sulfur as a positive electrode active material, and supplements the conductivity of sulfur to enable electrochemical reactions to proceed smoothly.

[0055] Porous carbon materials can generally be produced by carbonizing precursors of various carbon materials. Porous carbon materials can include non-uniform pores, with an average pore diameter ranging from 1 nm to 200 nm, and a porosity ranging from 10% to 90% of the total pore volume. If the average pore diameter is smaller than this range, the pore size is only at the molecular level, and impregnation with sulfur is impossible. Conversely, if the average pore diameter exceeds this range, the mechanical strength of the porous carbon material is weakened, which is not preferred for electrode manufacturing.

[0056] The shape of the porous carbon material is in the form of spheres, rods, needles, plates, tubes, or blocks, and may be used without limitation as long as it is generally used in lithium-sulfur batteries.

[0057] The porous carbon material may have a porous structure or a high specific surface area, and may be any of the porous carbon materials conventionally used in the art. For example, the porous carbon material may be, but is not limited to, at least one selected from the following: graphite; graphene; carbon black, such as danka black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; carbon nanotubes (CNTs), such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); carbon fibers, such as graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs); and natural graphite, artificial graphite, expanded graphite, and activated carbon. Preferably, the porous carbon material may be a carbon nanotube.

[0058] Sulfur may include at least one selected from the following: sulfur (S8), Li2S n (n≥1), organic sulfur compounds and carbon-sulfur polymers (C2S x ) n , x = 2.5 to 50, n ≥ 2).

[0059] In addition, the diameter of the sulfur-carbon composite material can be 5 μm to 100 μm, specifically 5 μm or more, 10 μm or more or 15 μm or more and 60 μm or less, 70 μm or less or 100 μm or less. At this time, the diameter of the sulfur-carbon composite material refers to the length of the longest axis in the cross section of the particle. If the diameter of the sulfur-carbon composite material is less than 5 μm, the porosity of the positive electrode containing the sulfur-carbon composite material is reduced, and the reactivity of the positive electrode is reduced. If the diameter of the sulfur-carbon composite material exceeds 100 μm, a short circuit occurs due to the unevenness of the positive electrode containing the sulfur-carbon composite material, and problems such as increased porosity may occur. In addition, because the electrolyte needs to enter the inner end of the sulfur-carbon composite material and then react with it, the output characteristics may be poor.

[0060] In addition, the porous carbon material coated with a copolymer containing a redox functional group and a lithium ion conductive functional group on the surface may be included in an amount of 10 wt % to 50 wt % based on the total weight of the sulfur-carbon composite material, and sulfur may be included in an amount of 50 wt % to 90 wt %.

[0061] Preparation method of sulfur-carbon composite material

[0062] In addition, another aspect of the present invention relates to a method for preparing a sulfur-carbon composite material, the method comprising the following steps:

[0063] (a) coating a porous carbon material with a copolymer containing redox functional groups and lithium ion conductive functional groups; and

[0064] (b) The porous carbon material coated with the copolymer prepared in step (a) is mixed with sulfur and molded.

[0065] Step (a) is a step of coating the porous carbon material with a copolymer containing a redox functional group and a lithium ion conductive functional group.

[0066] The copolymer and the porous carbon material are the same as described above.

[0067] The coating is not particularly limited as long as it is a coating method used in the art for forming a coating layer, and can be formed by wet coating, drop casting, dip coating, doctor blade coating, spray coating, Meyer rod coating, or vacuum filtration.

[0068] When coating, if solvent is needed, water can be used; or organic solvents such as ethanol, acetone, ethyl isoalcohol (IPA), tetrahydrofuran (THF), methylene chloride (MC), dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and dimethylacetamide (DMAc). Among them, THF or a compound with similar properties is preferably used as a solvent.

[0069] In addition, when the surface of the porous carbon material is coated with a copolymer containing a redox functional group and a lithium ion conductive functional group, the interface between the porous carbon material and the coating containing the copolymer is bonded by the reaction between the copolymer and the carbon (bonded by π-π interaction). At this time, the reaction can be carried out at 15°C to 100°C for 1 to 24 hours. Specifically, the reaction temperature can be above 15°C, above 20°C, above 30°C, or above 40°C and below 70°C, below 80°C, below 90°C, or below 100°C. Through the above reaction, electrons can be transferred to the redox functional group and the lithium ion conductive functional group to reduce lithium polysulfide and ensure the movement path of lithium ions.

[0070] Step (b) is a step of preparing a sulfur-carbon composite material by mixing and shaping the porous carbon material coated with the copolymer containing redox functional groups and lithium ion conductive functional groups prepared in the above step (a) with sulfur.

[0071] The process of mixing and shaping sulfur may be any method as long as it is a method known in the art.

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

[0073] The heating temperature may be any temperature that melts the sulfur, and specifically may be 120°C or higher, 130°C or higher, 140°C or higher, or 150°C or higher and 160°C or lower, 170°C or lower, or 180°C or lower. If the heating temperature is lower than 120°C, the sulfur is not sufficiently melted, and the structure of the sulfur-carbon composite material may not be properly formed. If the heating temperature exceeds 180°C, it may be difficult to achieve the desired effect because the applied compound is not retained.

[0074] Through the above step (b), a sulfur-carbon composite material can be prepared, and the porous carbon material is coated with a copolymer containing redox functional groups and lithium ion conductive functional groups on the surface.

[0075] Positive electrode for lithium secondary battery

[0076] Furthermore, another aspect of the present invention relates to a positive electrode for a lithium secondary battery comprising the sulfur-carbon composite material according to one aspect of the present invention.

[0077] Specifically, another aspect of the present invention relates to a positive electrode for a lithium secondary battery, which comprises a positive electrode collector and a positive electrode active material layer formed on the positive electrode collector, wherein the positive electrode active material layer comprises the sulfur-carbon composite material of one aspect of the present invention, a conductive material and a binder.

[0078] The sulfur-carbon composite material is a positive electrode active material, so the positive electrode according to another aspect of the present invention may be a positive electrode for a lithium-sulfur battery.

[0079] In order to allow electrons to move smoothly in the positive electrode, a positive electrode active material and a conductive material may be included. In order to improve the binding force between the positive electrode active materials or between the positive electrode active material and the current collector, a binder may be included.

[0080] The positive electrode current collector can generally be made into 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. For example, a conductive metal such as stainless steel, aluminum, copper or titanium can be used as the positive electrode current collector, and preferably an aluminum current collector can be used. The positive electrode current collector can be formed in various forms such as a film, a sheet, a foil, a mesh, a porous body, a foam or a non-woven fabric.

[0081] The conductive material may be a carbon material such as carbon black, acetylene black, and Ketjen black; or a conductive polymer such as polyaniline, polythiophene, polyacetylene, and polypyrrole, and may preferably be included in an amount of 5% to 20% by weight based on the total weight of the positive electrode active material layer. If the content of the conductive material is less than 5% by weight, the effect of improving conductivity by using the conductive material is not significant. On the other hand, if the content of the conductive material exceeds 20% by weight, the content of the positive electrode active material becomes relatively small, and thus the capacity characteristics may be degraded.

[0082] In addition, the binder can be poly (vinyl acetate), polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, alkylated polyethylene oxide, cross-linked polyethylene oxide, polyvinyl ether, poly (methyl methacrylate), polyvinylidene fluoride, polyhexafluoropropylene and polyvinylidene fluoride copolymer (product name: Kynar), poly (ethyl acrylate), polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, polyvinyl pyridine, polystyrene, ethylene - co - vinyl acetate, cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, pullulan, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, carboxymethyl cellulose, styrene butadiene rubber, acrylonitrile - styrene - butadiene copolymer, polyimide and the above materials derivatives, blends and copolymers etc. In addition, based on the total weight of the positive electrode active material layer, the binder can be preferably included in an amount of 5 wt % to 20 wt %. If the content of the binder is less than 5 wt %, the effect of improving the bonding force between the positive electrode active materials or between the positive electrode active material and the current collector depending on the use of the binder is insufficient. On the other hand, if the content of the binder exceeds 20% by weight, the content of the positive electrode active material becomes relatively small, and thus there is a possibility that capacity characteristics may deteriorate.

[0083] The positive electrode as described above can be manufactured by a conventional method, specifically, by applying a composition for forming a positive electrode active material layer in a slurry state onto a current collector, followed by drying and optionally rolling. The composition is prepared by mixing a positive electrode active material, a conductive material and a binder in an organic solvent.

[0084] At this time, the organic solvent may be a solvent that can uniformly disperse the positive electrode active material, the binder, and the conductive material and evaporates easily. Specifically, the organic solvent may include acetonitrile, methanol, ethanol, tetrahydrofuran, isopropyl alcohol, and the like.

[0085] The loading amount of positive electrode active material can be 3mg / cm 2 Above or 5mg / cm 2 Above and 6mg / cm 2 Below or 8mg / cm 2 The following, but not limited to.

[0086] Even at the aforementioned loading levels, i.e., low and high loadings, a lithium secondary battery comprising the positive electrode can exhibit excellent reactivity and lifespan characteristics. This is because the copolymer containing redox functional groups and lithium ion conductive functional groups is coated on the surface of the porous carbon material of the sulfur-carbon composite material.

[0087] lithium secondary batteries

[0088] One aspect of the present invention relates to a lithium secondary battery comprising: a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, wherein the positive electrode is the positive electrode according to another aspect of the present invention.

[0089] The lithium secondary battery may preferably be a lithium-sulfur battery.

[0090] The negative electrode may be composed of a current collector and a negative electrode active material layer formed on one or both surfaces of the current collector. Alternatively, the negative electrode may be a lithium metal plate.

[0091] The current collector is used to support 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 while having excellent conductivity. For example, the following can be used: copper; stainless steel; aluminum; nickel; titanium; palladium; sintered carbon; or copper or stainless steel whose surface is treated with carbon, nickel, silver, etc.; or aluminum-cadmium alloy, etc.

[0092] The negative electrode current collector enhances binding force with the negative electrode active material by having fine irregularities on its surface, and may be formed in various forms such as a film, sheet, foil, screen, mesh, porous body, foam, or nonwoven fabric.

[0093] The negative electrode active material may 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] The material capable of reversibly intercalating or deintercalating lithium ions may be, for example, crystalline carbon, amorphous carbon, or a mixture thereof.

[0095] The material capable of reacting with lithium ions to reversibly form a lithium-containing compound may be, for example, tin oxide, titanium nitrate, or silicon.

[0096] The lithium alloy may 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 is also included between the positive and negative electrodes. The separator separates or insulates the positive and negative electrodes from each other while enabling lithium ion transport between them. The separator can be made of a porous, non-conductive or insulating material. The separator can be a standalone component such as a membrane or a coating added to the positive and / or negative electrodes.

[0098] Materials constituting the separator include, but are not limited to, for example: polyolefins such as polyethylene and polypropylene; glass fiber filter paper; and ceramic materials, and the thickness of the separator may be approximately 5 μm or more or 10 μm or more and 25 μm or less or 50 μm or less.

[0099] The electrolyte is a non-aqueous electrolyte containing a lithium salt and is composed of a lithium salt and an electrolyte. As the electrolyte, a non-aqueous organic solvent, an organic solid electrolyte, and an inorganic solid electrolyte can be used.

[0100] As the lithium salt, lithium salts conventionally used in the electrolyte of lithium secondary batteries, preferably lithium sulfur batteries, can be used without limitation. The lithium salt may include, for example, at least one selected from the following: LiSCN, LiBr, LiI, LiPF6, LiBF4, LiB 10 Cl 10 , LiSO3CF3, LiCl, LiClO4, LiSO3CH3, LiB(Ph)4, LiC(SO2CF3)3, LiN(SO2CF3)2, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiFSI, lithium chloroborane, lower aliphatic carboxylic acid lithium, etc.

[0101] In addition, the concentration of the lithium salt in the electrolyte may be 0.2 to 2 M, specifically 0.2 M or more, 0.6 M or more, or 0.7 M or more and 1.5 M or less, 1.7 M or less, or 2 M or less. If the concentration of the lithium salt is less than 0.2 M, the conductivity of the electrolyte may decrease, thereby deteriorating the performance of the electrolyte. If the concentration of the lithium salt exceeds 2 M, the viscosity of the electrolyte may increase, thereby reducing the mobility of lithium ions.

[0102] The non-aqueous organic solvent should dissolve the lithium salt well, and the non-aqueous organic solvent of another aspect of the present invention may include, for example, aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, 4- Methyl-1,3-dioxane, diethyl ether, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate and ethyl propionate, and these organic solvents can be used alone or as a mixture of two or more solvents thereof.

[0103] As the organic solid electrolyte, for example, polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate polymers, polyalginate-lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and polymers containing ion dissociative groups can be used.

[0104] As an inorganic solid electrolyte, for example, Li nitrides, halides, sulfates, etc. can be used, such as Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, LiSiO4, LiSiO4-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH, Li3PO4-Li2S-SiS2.

[0105] To improve charge / discharge characteristics, flame retardancy, etc., the electrolyte of another aspect of the present invention may further include, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. In some cases, to impart non-flammability, halogen-containing solvents such as carbon tetrachloride or trifluoroethylene may be added, and to improve storage characteristics at high temperatures, carbon dioxide gas may be further included. Fluoroethylene carbonate (FEC), propylene sultone (PRS), fluoropropylene carbonate (FPC), etc. may also be included.

[0106] As the electrolyte, a liquid electrolyte or a solid electrolyte membrane can be used. When a liquid electrolyte is used, a membrane made of porous glass, plastic, ceramic or polymer is also included to serve as a physical membrane that has the function of physically separating the electrodes.

[0107] Another aspect of the present invention relates to a lithium secondary battery, preferably a lithium-sulfur battery, comprising the sulfur-carbon composite material of one aspect of the present invention as a positive electrode active material. Because the surface of the porous carbon material of the sulfur-carbon composite material is coated with a copolymer containing redox functional groups and lithium ion conductive functional groups, the redox functional groups enhance kinetics, thereby rapidly reducing lithium polysulfide to increase reactivity and prevent lithium polysulfide from dissolving from the positive electrode, thereby improving the reactivity and lifespan of the battery. Furthermore, the lithium ion conductive functional groups ensure a path for the migration of lithium ions, thereby increasing reactivity and reducing overvoltage.

[0108] Example

[0109] Hereinafter, in order to help understand the present invention, preferred embodiments are provided, but the following examples are only intended to illustrate the present invention. It will be apparent to those skilled in the art that various changes and variants can be made within the scope and spirit of the present invention, and it is apparent that such changes and variants fall within the scope of the appended claims.

[0110] <Preparation of lithium-sulfur batteries>

[0111] Example 1

[0112] By making polyetheramine (product name: Jeff amine TM ) and naphthalene anhydride in a molar ratio of 1:1 to prepare naphthalene diimide-polyethylene oxide (Nap-PEO).

[0113] A solution was prepared by dissolving 1% by weight of naphthalene diimide-polyethylene oxide (Nap-PEO) in a dichloromethane solvent relative to the total weight of the carbon nanotubes. After adding 0.6 g of carbon nanotubes to the solution, the mixture was stirred and dried at 25°C for 12 hours to coat the surface of the carbon nanotubes with Nap-PEO. In this case, the weight of Nap-PEO refers to the weight of Nap-PEO relative to the total weight of the carbon nanotubes coated with Nap-PEO.

[0114] 0.6 g of the Nap-PEO-coated carbon nanotubes prepared above was uniformly mixed with 1.4 g of sulfur, and then heat-treated at 155° C. for 30 minutes to produce a sulfur-carbon composite material containing sulfur:Nap-PEO-coated carbon nanotubes at a weight ratio of 70:30.

[0115] The sulfur-carbon composite material, the conductive material and the binder were mixed with deionized water (DIW) in a weight ratio of sulfur-carbon composite material: conductive material: binder of 90:5:5 to prepare a slurry, which was then coated on an aluminum foil current collector with a thickness of 20 μm to prepare an electrode. The weight of the sulfur-carbon composite material was set to 20 wt % based on the total weight of deionized water. In addition, carbon black was used as a conductive material, and styrene-butadiene rubber and carboxymethyl cellulose were used as binders. Subsequently, the positive electrode was prepared by drying in an oven at 50°C overnight. At this time, the loading amount of the positive electrode active material was 4 mg / cm 2 (low load).

[0116] A coin cell was prepared using polyethylene as a separator and a 45 μm-thick lithium foil as a negative electrode. At this time, an electrolyte prepared by dissolving 1 M LiTFSI and 3 wt% LiNO3 in an organic solvent consisting of DOL / DME solvent (volume ratio of 1:1) was used to prepare the coin cell as a lithium-sulfur battery.

[0117] Example 2

[0118] A solution was prepared by dissolving 2 wt% of naphthalene diimide-polyethylene oxide (Nap-PEO) in a dichloromethane solvent relative to the total weight of the carbon nanotubes. After adding 0.6 g of the carbon nanotubes to the solution, the mixture was stirred and dried at 25°C for 12 hours to form a Nap-PEO coating on the surface of the carbon nanotubes.

[0119] The subsequent processes were performed in the same manner as in Example 1 to manufacture a lithium-sulfur battery coin cell.

[0120] At this time, the loading amount of the positive electrode active material is 4 mg / cm 2 (low load).

[0121] Example 3

[0122] In addition to the positive electrode active material loading of 5.5 mg / cm 2 A lithium-sulfur battery coin cell was manufactured in the same manner as in Example 1 except that (high load) .

[0123] Example 4

[0124] In addition to the positive electrode active material loading of 5.5 mg / cm 2 A lithium-sulfur battery coin cell was manufactured in the same manner as in Example 2 except that (high load) was used.

[0125] Example 5

[0126] A pouch-type lithium-sulfur battery was manufactured in the same manner as in Example 1, except that the lithium-sulfur battery was manufactured as a pouch-type battery instead of a coin-type battery. At this time, the loading amount of the positive electrode active material was 5.5 mg / cm 2 (High load).

[0127] Comparative Example 1

[0128] After 0.6 g of carbon nanotubes and 1.4 g of sulfur were uniformly mixed, the mixture was heat-treated at 155° C. for 30 minutes to prepare a sulfur-carbon composite material containing sulfur and carbon nanotubes at a weight ratio of sulfur:carbon nanotubes of 70:30.

[0129] The subsequent processes were performed in the same manner as in Example 1 to manufacture a lithium-sulfur battery coin cell.

[0130] At this time, the loading amount of the positive electrode active material is 4 mg / cm 2 (low load).

[0131] Comparative Example 2

[0132] In addition to the positive electrode active material loading of 5.5 mg / cm 2 A lithium-sulfur battery coin cell was manufactured in the same manner as in Comparative Example 1 except for (high load).

[0133] Comparative Example 3

[0134] A lithium-sulfur battery of a pouch type was manufactured in the same manner as in Comparative Example 2, except that the lithium-sulfur battery was manufactured as a pouch type battery instead of a coin cell.

[0135] At this time, the loading amount of the positive electrode active material was 5.5 mg / cm 2 (High load).

[0136] Comparative Example 4

[0137] A lithium-sulfur battery coin cell was manufactured in the same manner as in Example 1, except that naphthalene diimide was used instead of naphthalene diimide-polyethylene oxide (Nap-PEO) when forming the coating layer of the carbon nanotube.

[0138] Comparative Example 5

[0139] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that polyethylene oxide was used instead of naphthalene diimide-polyethylene oxide (Nap-PEO) in forming the coating layer of the carbon nanotube.

[0140] Comparative Example 6

[0141] A lithium-sulfur battery coin cell was manufactured in the same manner as in Example 1, except that naphthalene diimide-polyethylene oxide (Nap-PEO) was not used when forming the coating layer of the carbon nanotubes and Nap-PEO was added when preparing the positive electrode slurry. The amount of Nap-PEO added was 1 wt % relative to the total weight of the carbon nanotubes contained in the sulfur-carbon composite material.

[0142] Experimental Example 1. Cyclic Voltammetry Measurement of Positive Electrode for Lithium-Sulfur Batteries

[0143] Cyclic voltammetry was performed to observe the potential region where the carbon nanotubes coated with Nap-PEO prepared in Example 1 became reactive. The current values measured at the working electrode were recorded when the scan rate was varied from 20 mV / s to 200 mV / s and a voltage of 1.6 V to 3.6 V was applied. The results are shown in FIG. Figure 1 middle.

[0144] From the above results, it was confirmed that the carbon nanotubes coated with Nap-PEO exhibited reactivity in the range of 1.8 V to 2.8 V.

[0145] Experimental Example 2. Measurement of Life Characteristics of Lithium-Sulfur Batteries (Coin Cells)

[0146] The life characteristics of the lithium-sulfur batteries of the coin cells manufactured in Examples 1 to 4 and Comparative Examples 1 to 2 were evaluated.

[0147] Life characteristics were measured by performing charge / discharge at 0.1C / 0.1C during the initial 3 cycles, then performing charge / discharge at 0.2C / 0.2C for 3 cycles, and then repeating charge / discharge at 0.3C / 0.5C using a meter.

[0148] 2-1. Measurement of Lifespan Characteristics of Lithium-Sulfur Batteries with Low-Load Cathode

[0149] The life characteristics of the lithium-sulfur batteries manufactured in Examples 1 to 2 and Comparative Example 1 were measured. The lithium-sulfur batteries of Example 1, Example 2 and Comparative Example 1 were composed of a positive electrode active material with a loading of 4 mg / cm 2 Lithium-sulfur battery with low-load cathode.

[0150] In the results of Examples 1 to 2 and Comparative Example 1 ( Figure 2 ), the lithium-sulfur batteries of Example 1, Example 2, and Comparative Example 1 maintained the discharge capacity even after cycling, thereby showing excellent results in terms of life characteristics.

[0151] However, compared with Examples 1 and 2, Comparative Example 1 showed lower results in terms of reactivity.

[0152] 2-2. Measurement of Lifespan Characteristics of Lithium-Sulfur Batteries with High-Loaded Positive Electrodes

[0153] The life characteristics of the lithium-sulfur batteries manufactured in Examples 3 to 4 and Comparative Example 2 were measured. The lithium-sulfur batteries of Examples 3, 4 and Comparative Example 2 contained a positive electrode active material with a loading of 5.5 mg / cm 2 Lithium-sulfur battery with high-load cathode.

[0154] In the results of Examples 3 to 4 and Comparative Example 2 ( Figure 3 ), Comparative Example 2 failed to maintain the discharge capacity as the cycle progressed, showing unstable results, and also showed poor results in terms of life characteristics.

[0155] Example 3 showed a result that the life characteristics deteriorated after 50 cycles, but Example 4 showed an excellent result in terms of life characteristics by maintaining the discharge capacity even when the cycles progressed.

[0156] Specifically, Comparative Example 2, which included a sulfur-carbon composite material without coating the surface of the porous carbon material, exhibited very poor lifespan characteristics in the presence of a high-load electrode, showing unstable reactivity. However, Examples 3 and 4, which included sulfur-carbon composite materials coated on the surface of the porous carbon material with a copolymer containing redox functional groups and lithium-ion conductive functional groups, exhibited excellent lifespan characteristics. In particular, Example 4, which included 2% by weight of the copolymer relative to the total weight of the porous carbon material, exhibited superior lifespan characteristics compared to Example 3, which included 1% by weight of the copolymer.

[0157] 2-3. Measurement of Lifespan Characteristics of Lithium-Sulfur Batteries with High-Loaded Cathodes

[0158] The life characteristics of the lithium-sulfur batteries manufactured in Examples 3 to 4 and Comparative Example 2 were measured. The lithium-sulfur batteries of Examples 3, 4 and Comparative Example 2 contained a positive electrode active material with a loading of 5.5 mg / cm 2 At this time, the life characteristics were measured while setting the charge / discharge potential value to 1.8 V to 2.8 V (the 1.8 to 2.8 V is the range where reactivity appears in the measurement by cyclic voltammetry).

[0159] In the results of Examples 3 to 4 and Comparative Example 2 ( Figure 4 ), Comparative Example 2 showed poor reactivity results in this range. On the other hand, Examples 3 and 4 showed excellent results in terms of reactivity and life characteristics.

[0160] That is, Comparative Example 2, which comprises a sulfur-carbon composite material in which the surface of the porous carbon material is not coated, shows poor reactivity results in the reactivity interval, while Examples 3 and 4, which comprise a sulfur-carbon composite material in which the surface of the porous carbon material is coated with a copolymer containing a redox functional group and a lithium ion conductive functional group, show excellent results in terms of life characteristics and reactivity.

[0161] 2-4. Measurement of Lifespan Characteristics of Lithium-Sulfur Batteries Depending on the Type of Coating Material of Porous Carbon Material and the Type of Copolymer Containing Redox Functional Groups and Lithium Ion Conductive Functional Groups

[0162] The life characteristics of the lithium-sulfur batteries of the coin cells manufactured in Example 1 and Comparative Examples 4 to 6 were evaluated.

[0163] Life characteristics were measured by performing charge / discharge at 0.1C / 0.1C during the initial 2.5 cycles, then performing charge / discharge at 0.2C / 0.2C for 3 cycles, and then repeating charge / discharge at 0.3C / 0.5C using a meter.

[0164] (1) Measurement of life characteristics depending on the type of coating material of porous carbon material

[0165] The discharge capacity measurement results of Example 1 and Comparative Examples 4 and 5 ( Figure 5 ), Example 1 shows excellent results in discharge capacity compared with Comparative Examples 4 and 5.

[0166] That is, it was found that compared with Comparative Example 4 (porous carbon material coated with a material containing redox functional groups) and Comparative Example 5 (porous carbon material coated with a material containing lithium ion conductive functional groups), Example 1, which contains a copolymer containing redox functional groups and lithium ion conductive functional groups as a coating material for the porous carbon material, has a smaller capacity decrease rate.

[0167] (2) Measurement of life characteristics depending on the inclusion type of the copolymer containing a redox functional group and a lithium ion conductive functional group

[0168] The discharge capacity measurement results of Example 1 and Comparative Example 6 ( Figure 5 ), Example 1 showed excellent results in terms of discharge capacity compared to Comparative Example 6.

[0169] That is, it was confirmed that Example 1, in which a copolymer containing redox functional groups and lithium ion conductive functional groups was coated on the surface of the porous carbon material of the sulfur-carbon composite material, had a smaller capacity decrease rate than Comparative Example 6, in which the copolymer was contained in the positive electrode in the form of a simple mixture with the sulfur-carbon composite material.

[0170] Experimental Example 3. Measurement of Charge / Discharge Characteristics of Lithium-Sulfur Batteries (Pouch-Type Batteries)

[0171] The discharge capacity and life characteristics of the lithium-sulfur batteries manufactured in Example 5 and Comparative Example 3 were measured. The lithium-sulfur batteries of Example 5 and Comparative Example 3 contained a positive electrode active material with a loading of 5.5 mg / cm 2 At this time, the life characteristics were measured while setting the charge / discharge potential value to 1.8 V to 2.8 V (the 1.8 to 2.8 V is the range where reactivity appears in the measurement by cyclic voltammetry).

[0172] The life characteristics were measured by performing charge / discharge at 0.1C / 0.1C during the initial 3 cycles, then performing charge / discharge at 0.2C / 0.2C for 3 cycles, and then repeating charge / discharge at 0.3C / 0.5C using a meter.

[0173] The measurement results of the initial discharge capacity of Example 5 and Comparative Example 3 ( Figure 6 and Figure 7 ), Example 5 shows excellent results in terms of initial discharge capacity compared to Comparative Example 3.

[0174] In addition, the results of measuring the life characteristics of Example 5 and Comparative Example 3 ( Figure 8 ), Comparative Example 3 showed poor results in terms of life characteristics and reactivity compared to Example 5.

[0175] That is, compared with Comparative Example 3 including a sulfur-carbon composite material in which the surface of the porous carbon material is not coated, Example 5 including a sulfur-carbon composite material in which the surface of the porous carbon material is coated with a copolymer containing redox functional groups and lithium ion conductive functional groups shows better results in terms of life characteristics and reactivity, and also shows improved overvoltage results.

[0176] On the other hand, the initial first discharge capacity and the second discharge capacity of Example 5 and Comparative Example 3 were measured to measure the discharge capacity decrease rate ( Figure 9 ).

[0177] In the above results, Example 5 was measured to have a smaller discharge capacity decrease rate than Comparative Example 3. This confirmed that in the case of Example 5, which included a sulfur-carbon composite material in which a copolymer containing a redox functional group and a lithium ion conductive functional group was coated on the surface of a porous carbon material, the capacity decrease rate was smaller than that of Comparative Example 3, which included a sulfur-carbon composite material in which the surface of the porous carbon material was not coated.

[0178] In addition, after evaluating the life characteristics of the lithium-sulfur battery of Example 5, the pouch-type battery was disassembled to observe the negative electrode ( Figure 10) and the separator and cathode ( Figure 11 left and right) surfaces.

[0179] The negative electrode of the lithium-sulfur battery of Example 5 was observed to have a clean surface. It can be seen that the copolymer (i.e., the redox functional groups contained in the copolymer) coated on the porous carbon material of the sulfur-carbon composite material contained in the positive electrode of the lithium-sulfur battery of Example 5 promoted the reduction reaction of lithium polysulfide, thereby preventing the lithium polysulfide from migrating from the positive electrode, resulting in a clean surface of the negative electrode. Specifically, the phrase "preventing the migration of lithium polysulfide from the positive electrode" can mean that the reduction reaction of lithium polysulfide is promoted and the lithium polysulfide disappears quickly, thereby reducing the possibility of migration from the positive electrode.

[0180] Furthermore, when the positive electrode and separator of the lithium-sulfur battery in the form of a pouch-type battery of Example 5 were separated, the positive electrode was observed to be black. Due to the nature of the pouch-type battery, since the separator and the positive electrode are stacked and separated under pressure, a portion of the positive electrode surface is transferred to the separator, and the interior of the positive electrode is observed to be black. If the lithium polysulfide formed in the positive electrode remains inside the positive electrode, a yellow color is observed. However, in the case of the above-mentioned positive electrode, because the redox functional groups contained in the copolymer coated on the porous carbon material of the sulfur-carbon composite material promote the reduction reaction of the lithium polysulfide, thereby eliminating the lithium polysulfide, the positive electrode appears black, not yellow.

[0181] Thus, it was confirmed that the lithium-sulfur battery of the present invention, which includes a sulfur-carbon composite material containing a porous carbon material whose surface is coated with a copolymer containing redox functional groups and lithium ion conductive functional groups, and sulfur as a positive electrode active material, can improve the reactivity and life characteristics of the battery by promoting the reduction reaction of lithium polysulfide and thereby suppressing the migration of lithium polysulfide from the positive electrode, and can reduce overvoltage and improve reactivity by ensuring the migration path of lithium ions.

Claims

1. A sulfur-carbon composite material, comprising: porous carbon materials; and sulfur formed on at least a portion of the interior and surface of the porous carbon material, The surface of the porous carbon material is coated with a copolymer containing redox functional groups and lithium ion conductive functional groups, Wherein, based on the total weight of the porous carbon material, the content of the copolymer is 0.25 wt % to 5 wt %, The copolymer is a copolymer of a monomer containing a redox functional group and a monomer containing a lithium ion conductive functional group, The molar ratio of the monomer containing a redox functional group to the monomer containing a lithium ion conductive functional group is 2:8 to 8:

2. 2 . The sulfur-carbon composite material according to claim 1 , wherein the monomer containing a redox functional group comprises at least one selected from the group consisting of a naphthaleneimide compound, a perylene compound, and an imide compound. 3 . The sulfur-carbon composite material according to claim 1 , wherein the monomer containing a lithium ion conductive functional group comprises at least one selected from the group consisting of an ether compound, a sulfonic acid compound, a carboxylic acid compound, and an acrylic compound. The sulfur-carbon composite material according to claim 1 , wherein the copolymer comprises naphthalene diimide-polyethylene oxide.

5. The sulfur-carbon composite material according to claim 1 , wherein the content of the porous carbon material coated with the copolymer containing a redox functional group and a lithium ion conductive functional group on the surface is 10 wt % to 50 wt %, and the content of sulfur is 50 wt % to 90 wt %, based on the total weight of the sulfur-carbon composite material.

6. A method for preparing the sulfur-carbon composite material according to claim 1, comprising the following steps: (a) coating a porous carbon material with a copolymer containing redox functional groups and lithium ion conductive functional groups; and (b) mixing the porous carbon material coated with the copolymer containing redox functional groups and lithium ion conductive functional groups prepared in step (a) with sulfur and shaping the resultant. 7 . A positive electrode for a lithium secondary battery, comprising the sulfur-carbon composite material according to claim 1 .

8. The positive electrode for a lithium secondary battery according to claim 7, wherein the positive electrode has a 2 Up to 8 mg / cm 2 of positive electrode active material loading. 9 . The positive electrode for a lithium secondary battery according to claim 7 , wherein the positive electrode for a lithium secondary battery is a positive electrode for a lithium-sulfur battery.

10. A lithium secondary battery comprising: a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte. The positive electrode is the positive electrode according to claim 7. 11 . The lithium secondary battery according to claim 10 , wherein the lithium secondary battery is a lithium-sulfur battery.

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