Binder composition for manufacturing lithium-sulfur battery positive electrode and lithium-sulfur battery positive electrode manufactured therefrom

Abstract

The present invention provides a binder composition for manufacturing a positive electrode for a lithium-sulfur battery, a positive electrode manufactured using the binder composition, and a lithium-sulfur battery comprising the positive electrode. The binder composition comprises a binder, a thickener, and gum arabic. When used in a lithium-sulfur battery, the binder composition, by including gum arabic and the thickener, can improve the initial discharge performance and cycle performance of the battery.

Description

Technical Field

[0001] The present invention relates to a binder composition for manufacturing a lithium-sulfur battery positive electrode and a lithium-sulfur battery positive electrode manufactured using the binder composition.

[0002] This application claims the benefit of priority from Korean Patent Application No. 10-2020-0162995, filed on November 27, 2020, which is hereby incorporated by reference in its entirety. Background Art

[0003] The need to develop environmentally friendly electric and hybrid vehicles, as well as the rapid growth of intelligent IT devices, is rapidly increasing demand for high-capacity, high-output batteries. Currently, commercial lithium-ion batteries have limited energy density due to technical limitations, leading to a surge in interest in the development of lithium-sulfur, lithium-selenium, and lithium-air batteries with higher energy densities. Sulfur and oxygen, the active cathode materials in lithium-sulfur and lithium-air batteries, share similar physicochemical properties, and their abundance has boosted expectations for commercialization.

[0004] In the case of lithium-air batteries or lithium-sulfur batteries that use lithium metal, which has high reduction power, voltage characteristics, and high reversibility, as a negative electrode and air or sulfur as a positive electrode, the amount of lithium ions stored per unit weight or volume in the reaction products, Li2O2, LiOH, and Li2S, is much higher than that of LiCoO2, which is used as the positive electrode of lithium-ion batteries. Moreover, because lithium-air batteries or lithium-sulfur batteries can store more charge by using lithium metal as a negative electrode than lithium-ion batteries using graphite-based negative electrodes with a maximum lithium storage limit of LiC6, they can exhibit a much higher theoretical energy density than lithium-ion batteries. However, despite the high theoretical energy density, lithium-air batteries and lithium-sulfur batteries have not yet been commercialized and are still in the early stages of development because the actual energy density is as low as 20% to 45% of the theoretical value.

[0005] Specifically, in the case of lithium-air batteries, Li2O2 and Li2O generated during charging require a high overvoltage to decompose into Li ions and O2, and unlike lithium-ion batteries, lithium-air batteries adopt an open structure that allows the ingress and egress of outside air. Due to the influx of impurities (such as water and carbon dioxide) from the outside air, side reactions and electrolyte volatilization are prone to occur, resulting in rapid performance deterioration.

[0006] Furthermore, in the case of lithium-sulfur batteries, the sulfur that forms the positive electrode and the Li2S that is the final reaction product have the properties of electrical insulators. Therefore, in lithium-sulfur batteries, tetraethylene glycol dimethyl ether (TEGDME) series electrolytes with a strong dielectric constant are used. As soluble polysulfides move from the positive electrode to the negative electrode, the polysulfides are reduced to low-order monomeric polysulfides, and a shuttling mechanism occurs in which the monomeric polysulfides return to the positive electrode and then to the negative electrode. Therefore, these insoluble Li2S and Li2S2 may accumulate on the negative electrode surface and other separator interfaces. In addition, in the positive electrode, lithium polysulfide (Li2S8), which is an intermediate reaction product, has high solubility in the organic electrolyte and is therefore continuously dissolved during the discharge reaction, while the amount of positive electrode material decreases, which causes the capacity to drop sharply with cycling. In addition, since sulfur itself has extremely low conductivity, it is used in combination with conductive carbon or polymers. However, in this case, the overall energy density of the battery decreases due to the reduction in sulfur content.

[0007] To address these issues, various approaches are being researched and developed, such as designing porous cathode structures, developing additives to prevent overvoltage, or forming surface treatment layers. In the case of lithium-air batteries, from the perspective of cathode development, a method is being considered to reduce charge overvoltage by uniformly distributing the discharge product, Li₂O₂, rather than concentrating within a dense conductive matrix, thereby maximizing the lithium ion reaction rate and oxygen generation during charging while ensuring smooth electron transfer. Furthermore, in the case of lithium-sulfur batteries, a method is being studied to uniformly disperse and distribute the insulator, Li₂S, within a dense conductive matrix by optimizing the structure / composition design, thereby promoting electron and lithium ion transfer, thereby reducing charge overvoltage and simultaneously suppressing the dissolution of lithium polysulfide in the cathode.

[0008] When manufacturing lithium-sulfur battery cathodes, binders and thickeners are used to stabilize the slurry and bond the electrode components. However, using only existing binders and thickeners for lithium-ion batteries does not improve reactivity or extend battery life by controlling the dissolution of lithium polysulfides generated in lithium-sulfur batteries. Reactivity can be altered by adding materials with specific functional groups that can control the dissolution of lithium polysulfides generated from the cathode during charging and discharging. However, since these specific materials are dispersed in the slurry, the physical properties of the electrode may deteriorate during coating and drying due to changes in rheological properties.

[0009] Therefore, the inventors of the present invention have completed the present invention by continuously studying a binder composition for manufacturing a positive electrode of a lithium-sulfur battery that can solve the above-mentioned problems.

[0010] [Prior art literature]

[0011] [Patent Document]

[0012] (Patent Document 1) Korean Patent Publication No. 10-2002-0092029 Summary of the Invention

[0013] Technical issues

[0014] In order to solve the above problems, the present invention provides a binder composition for manufacturing a positive electrode of a lithium-sulfur battery. By adding gum arabic and a thickener together to the binder composition for manufacturing a positive electrode of a lithium-sulfur battery, the binder composition can improve the initial discharge performance and cycle performance of the battery.

[0015] Technical Solution

[0016] According to a first aspect of the present invention, the present invention provides a binder composition for preparing a positive electrode of a lithium-sulfur battery, wherein the binder composition comprises a binder, a thickener and gum arabic.

[0017] In one embodiment of the present invention, the binder is selected from polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polybutyl acrylate, polypropyl acrylate, polyethyl acrylate, polyethylhexyl acrylate, polystyrene, ethylene propylene diene monomer rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, combinations of the above materials and copolymers of the above materials.

[0018] In one embodiment of the present invention, the thickener is selected from carboxymethyl cellulose, methyl cellulose, hydroxypropyl cellulose, methyl hydroxypropyl cellulose, ethyl hydroxyethyl cellulose, methyl ethyl hydroxyethyl cellulose, cellulose gum and combinations thereof.

[0019] In one embodiment of the present invention, the content of the binder in the binder composition is 40 wt % to 60 wt % based on the total weight of the binder composition.

[0020] In one embodiment of the present invention, the content of the thickener in the adhesive composition is 20 wt % to 35 wt % based on the total weight of the adhesive composition.

[0021] In one embodiment of the present invention, the content of the gum arabic in the adhesive composition is 30 parts by weight to 100 parts by weight relative to 100 parts by weight of the thickener.

[0022] According to a second aspect of the present invention, the present invention provides a positive electrode for a lithium-sulfur battery, which is formed by coating a slurry for manufacturing a positive electrode comprising the above-mentioned binder composition, a positive electrode active material and a conductive material on a positive electrode collector.

[0023] In one embodiment of the present invention, the content of the binder composition in the slurry for manufacturing a positive electrode is 0.01 parts by weight to 10 parts by weight relative to 100 parts by weight of solid matter in the slurry for manufacturing a positive electrode.

[0024] In one embodiment of the present invention, the content of the positive electrode active material in the slurry for manufacturing a positive electrode is 80 parts by weight to 99 parts by weight relative to 100 parts by weight of solid matter in the slurry for manufacturing a positive electrode.

[0025] In one embodiment of the present invention, the content of the conductive material in the slurry for manufacturing a positive electrode is 0.1 parts by weight to 15 parts by weight based on 100 parts by weight of solid content in the slurry for manufacturing a positive electrode.

[0026] According to a third aspect of the present invention, the present invention provides a lithium-sulfur battery, comprising the above-mentioned positive electrode, negative electrode, separator and electrolyte.

[0027] Beneficial effects

[0028] The binder composition for manufacturing a positive electrode of a lithium-sulfur battery according to the present invention contains gum arabic and a thickener, so that when applied to a lithium-sulfur battery, the initial discharge performance and cycle performance of the battery can be improved.

[0029] The adhesive composition contains a binder, a thickener, and gum arabic as three components, and when the content of the combined gum arabic does not exceed the content of the thickener, the improvement effect of the initial discharge performance and cycle performance of the battery can be more excellent. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a graph showing charge / discharge curves of battery potential with respect to specific capacity obtained by initially charging and discharging the lithium-sulfur secondary batteries according to Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0031] The embodiments provided according to the present invention can all be realized through the following description.It will be understood that the following description is understood to describe preferred embodiments of the present invention, and the present invention is not necessarily limited thereto.

[0032] Regarding the physical properties described herein, when measurement conditions and methods are not specifically described, the physical properties are measured according to measurement conditions and methods generally used by those skilled in the art.

[0033] Adhesive composition

[0034] The present invention provides a binder composition for preparing a positive electrode for a lithium-sulfur battery, comprising a binder, a thickener, and gum arabic. Existing binders and thickeners for lithium secondary batteries cannot effectively control the dissolution of lithium polysulfides in lithium-sulfur batteries. However, the binder composition according to the present invention, by additionally including gum arabic, provides a binder composition more suitable for improving the performance of lithium-sulfur batteries.

[0035] Binder is a material for improving the adhesion between the components in the positive electrode and the adhesion between the positive electrode active material and the positive electrode collector, and is not particularly limited as long as it is commonly used in the relevant technical field. The binder can be used as an emulsion binder so that it can be evenly dispersed in the slurry for making the positive electrode. The emulsion binder can be polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polybutyl acrylate, polypropyl acrylate, polyethyl acrylate, polyethylhexyl acrylate, polystyrene, ethylene propylene diene monomer rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber or its various copolymers, and one of these binders or a mixture of two or more thereof can be used. Here, the copolymer includes not only a block copolymer formed by combining each polymer, but also a random copolymer formed by mixing and combining the monomers of each polymer. For example, in this specification, the copolymer of polyethylene and polypropylene is interpreted as including the concept of ethylene-propylene copolymer.

[0036] According to one embodiment of the present invention, the binder content in the binder composition is 40% to 60% by weight, preferably 45% to 60% by weight, and more preferably 45% to 55% by weight, based on the total weight of the binder composition. If the binder content in the binder composition is less than 40% by weight, the binding force between the positive electrode components and the binding force between the positive electrode active material and the positive electrode current collector are reduced. If the binder content in the binder composition exceeds 60% by weight, the performance improvement effect achieved by adding a thickener and gum arabic cannot be expected for lithium-sulfur batteries.

[0037] The thickener is basically used to control the viscosity, and in view of the physical properties of the slurry used to make the positive electrode and the physical properties of the lithium-sulfur battery, a material suitable for use with the binder and gum arabic can be selected as the thickener. The thickener can be a cellulose polymer, and the cellulose polymer can be selected from carboxymethyl cellulose (CMC), methyl cellulose (MC), hydroxypropyl cellulose (HPC), methyl hydroxypropyl cellulose (MHPC), ethyl hydroxyethyl cellulose (EHEC), methyl ethyl hydroxyethyl cellulose (MEHEC), cellulose gum and a combination thereof. According to one embodiment of the present invention, carboxymethyl cellulose is used as the thickener. The cellulose polymer can be in a lithiated form. Because the cellulose polymer contains functional groups such as hydroxyl or carboxyl groups, it can be lithiated by replacing the hydrogen of the functional group with lithium. When the cellulose polymer is lithiated, an additional lithium source can be guaranteed, thereby helping to improve the performance of the lithium-sulfur battery.

[0038] According to one embodiment of the present invention, the content of the thickener in the binder composition is 20% to 35% by weight, preferably 25% to 35% by weight, based on the total weight of the binder composition. If the content of the thickener in the binder composition is less than 20% by weight, the viscosity of the slurry used to manufacture the positive electrode is reduced, and therefore, it is not easy to uniformly disperse the components of the positive electrode and ensure the function. If the content of the thickener in the binder composition exceeds 35% by weight, the fluidity of the slurry used to manufacture the positive electrode is reduced, and therefore, it is not easy to uniformly disperse the components of the positive electrode and ensure the function.

[0039] In the binder composition, gum is used to ensure additional functions, such as controlling the dissolution of lithium polysulfide in lithium-sulfur batteries. Although there are many types of gums, such as xanthan gum and guar gum, gum arabic exhibits more special functions when used with the above-mentioned binders and thickeners. Gum arabic is obtained by drying or desalting the secretions of the gum arabic tree (Acaciasenegal Willdenow) of the Leguminosae family or other plants belonging to the same genus, and its main component is a polysaccharide. Specifically, because a stable emulsion can be obtained over a relatively wide pH range, when used with the above-mentioned binders and thickeners, sufficient functionality related to the performance of lithium-sulfur batteries can be ensured.

[0040] According to one embodiment of the present invention, the content of gum arabic in the adhesive composition is 10% to 30% by weight, preferably 15% to 30% by weight, and more preferably 15% to 25% by weight, based on the total weight of the adhesive composition. If the content of gum arabic in the adhesive composition is less than 10% by weight, it is difficult to ensure additional functions, such as controlling the dissolution of lithium polysulfide. If the content of gum arabic in the adhesive composition exceeds 30% by weight, it is not preferred because overvoltage may occur during the initial discharge of a lithium-sulfur battery using it.

[0041] The content of gum arabic in the adhesive composition is 30 to 100 parts by weight, preferably 45 to 85 parts by weight, and more preferably 60 to 70 parts by weight, relative to 100 parts by weight of the thickener. As described above, when gum arabic is used together with the thickener, sufficient functionality can be ensured, and it is more advantageous to ensure that the functionality is adjusted within the above range.

[0042] Positive electrode and lithium-sulfur battery containing the same

[0043] The present invention provides a positive electrode for a lithium-sulfur battery, prepared from the aforementioned binder composition. The positive electrode according to the present invention is prepared by applying a slurry for preparing the positive electrode onto one or both surfaces of a positive electrode current collector, followed by drying and roll-pressing. The slurry for preparing the positive electrode contains a positive electrode active material, a conductive material, and the aforementioned binder composition. The layer formed by applying, drying, and rolling the slurry onto the positive electrode current collector is the layer of the battery containing the positive electrode active material and can be referred to as a positive electrode active material layer.

[0044] The binder composition is the same as described above. In the slurry for preparing the positive electrode, the binder composition can be adjusted in the direction of maximizing the performance of the battery based on the basic function of combining the positive electrode components. According to one embodiment of the present invention, the content of the binder composition in the slurry for manufacturing the positive electrode is 0.01 to 10 parts by weight, preferably 1 to 8 parts by weight, and more preferably 2 to 5 parts by weight, relative to 100 parts by weight of the solids in the slurry for preparing the positive electrode. Here, the solids in the slurry refer to the solid components of the positive electrode active material, the conductive material and the binder composition, excluding the solvent used when preparing the slurry. Since the functionality is supplemented by the thickener and gum arabic, even if a small amount of the binder composition is used, it is possible to expect an improvement in adhesion and battery performance. If the content of the binder composition in the slurry for manufacturing the positive electrode exceeds 10 parts by weight, it is not preferred in terms of improving battery performance because the content of the positive electrode active material is relatively reduced.

[0045] The positive electrode current collector is used to carry the positive electrode active material and is not particularly limited as long as it is generally made to have a thickness of 3 to 500 μm, has excellent conductivity and is electrochemically stable within the voltage range of the lithium secondary battery. For example, the positive electrode current collector can be any one metal selected from the following: copper, aluminum, stainless steel, titanium, silver, palladium, nickel, alloys thereof, and combinations thereof. The stainless steel can be surface-treated with carbon, nickel, titanium or silver, and an aluminum-cadmium alloy can preferably be used as the alloy. In addition, calcined carbon, a non-conductive polymer surface-treated with a conductive material, or a conductive polymer can be used.

[0046] The positive electrode current collector can enhance the binding force with the positive electrode active material by having fine concavoconvexity on its surface, and can be formed into various forms such as a film, sheet, foil, screen, mesh, porous body, foam, or nonwoven fabric.

[0047] The positive electrode active material is used to substantially perform the performance of the battery by exchanging the electrons of the battery, and the positive electrode active material in the lithium-sulfur battery contains elemental sulfur. Specifically, the positive electrode active material contains elemental sulfur (S8), sulfur compounds or mixtures thereof. The sulfur compounds include Li2S n (n≥1), organic sulfur compounds or sulfur-carbon compounds (C2S x ) n : x = 2.5 to 50, n ≥ 2). Since elemental sulfur alone does not have electrical conductivity, it can be combined with a carbon material and used in the form of a sulfur-carbon composite material.

[0048] The sulfur-carbon composite material may have a particle size of 1 μm to 100 μm. If the particle size of the sulfur-carbon composite material is less than 1 μm, the resistance between particles increases, leading to overvoltage in the electrodes of lithium-sulfur batteries. If the particle size exceeds 100 μm, the surface area per unit weight decreases, resulting in a decrease in the electrolyte wetted area and reaction sites with lithium ions in the electrode. Furthermore, the amount of electron transfer decreases relative to the size of the composite material, potentially delaying the reaction and reducing the battery's discharge capacity.

[0049] The sulfur-carbon composite material may contain sulfur in an amount of 60 to 90% by weight, preferably 70 to 80% by weight, based on the total weight of the sulfur-carbon composite material. If the sulfur content of the sulfur-carbon composite material is less than 60% by weight, the energy density of the battery may be reduced. If the sulfur content of the sulfur-carbon composite material exceeds 90% by weight, the conductivity of the electrode may be reduced, and the functionality of the positive electrode active material may be degraded.

[0050] The carbon material (or sulfur carrier) constituting the sulfur-carbon composite material has porosity. In particular, the carbon material used as the positive electrode active material of the present invention has a high specific surface area (3,000 m 2 / g or more) and high porosity (pore volume per unit weight: 0.7 to 3.0 cm 3 / g) characteristics, so it can carry a large amount of sulfur.

[0051] The carbon material may be, but is not limited to, at least one selected from the group consisting of graphite, graphene, reduced graphene oxide (rGO), 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 activated carbon. The porous carbon material may be in the form of spheres, rods, needles, plates, tubes, or blocks.

[0052] According to one embodiment of the present invention, the content of the positive electrode active material in the slurry for preparing the positive electrode is 80 to 99 parts by weight, preferably 85 to 95 parts by weight, relative to 100 parts by weight of the solid matter in the slurry for preparing the positive electrode. If the content of the positive electrode active material in the slurry for preparing the positive electrode is less than 80 parts by weight, there may be a problem of reduced energy density of the battery. If the content of the positive electrode active material in the slurry for preparing the positive electrode exceeds 99 parts by weight, there is a problem that the bonding force between the positive electrode active materials may be reduced due to insufficient content of the binder and the conductivity in the electrode may be reduced due to insufficient content of the conductive material.

[0053] The conductive material is used to impart conductivity to the electrode and can be used without any particular limitation, as long as it has electron conductivity and does not cause chemical changes in the constructed battery. Specific examples of the conductive material include: graphite, such as natural graphite or artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives. One of these materials or a mixture of two or more thereof may be used.

[0054] According to one embodiment of the present invention, the content of the conductive material in the slurry for manufacturing the positive electrode is 0.1 to 15 parts by weight, preferably 1 to 10 parts by weight, and more preferably 2 to 8 parts by weight, relative to 100 parts by weight of the solid matter in the slurry for preparing the positive electrode. If the content of the conductive material in the slurry for manufacturing the positive electrode is less than 0.1% by weight, there may be a problem of reduced conductivity in the electrode due to insufficient content of the conductive material. If the content of the conductive material in the slurry for manufacturing the positive electrode exceeds 15% by weight, there may be a problem of reduced discharge capacity and energy density of the battery due to a relative reduction in the amount of the positive electrode active material.

[0055] The present invention provides a lithium-sulfur battery comprising a negative electrode, a separator, an electrolyte, and the positive electrode described above. The lithium-sulfur battery is manufactured by placing a battery assembly formed by sequentially stacking a positive electrode, a separator, and a negative electrode in a battery case and injecting the electrolyte.

[0056] The negative electrode may include a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, and the negative electrode active material layer may include a negative electrode active material, a binder, and a conductive material.

[0057] The negative electrode active material may include a material capable of reversibly inserting or extracting lithium ions (Li + ) materials, materials capable of reacting with lithium ions to reversibly form lithium-containing compounds, lithium metal or lithium alloys. + ) can be, for example, crystalline carbon, amorphous carbon or a mixture thereof. + ) to reversibly form a lithium-containing compound may be, for example, tin oxide, titanium nitrate, or silicon. 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).

[0058] The binder, conductive material, and negative electrode current collector can be selected with reference to the above-mentioned positive electrode composition, but are not necessarily limited thereto. In addition, as in the positive electrode, the method of forming the negative electrode active material layer on the negative electrode current collector is based on a known coating method and is not particularly limited. If lithium metal or the like is used as the negative electrode active material, the negative electrode can be constructed without a binder, conductive material, or negative electrode current collector.

[0059] The separator is used to physically separate the two electrodes in the lithium-sulfur battery of the present invention and can be used without any particular limitation, as long as it is commonly used as a separator in lithium-sulfur batteries. In particular, separators having low resistance to electrolyte ion migration and excellent electrolyte impregnation capacity are preferred. The separator can be made of a porous substrate. As the porous substrate, any porous substrate commonly used in electrochemical devices can be used, and for example, polyolefin-based porous films or nonwoven fabrics can be used, but are not particularly limited thereto.

[0060] Examples of the polyolefin-based porous film may include a film formed of each of polyethylene such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene; and polyolefin-based polymers such as polypropylene, polybutene, and polypentene alone or by mixing them.

[0061] In addition to the above-mentioned polyolefin non-woven fabrics, the non-woven fabric may be formed, for example, from any polymer selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, polyethylene naphthalene, etc., alone or as a mixture thereof. The non-woven fabric may be a spunbond non-woven fabric or a meltblown non-woven fabric composed of long fibers.

[0062] The thickness of the porous substrate is not particularly limited and may be 1 to 100 μm or 5 to 50 μm. The size and porosity of the pores present in the porous substrate are also not particularly limited, but may be 0.001 to 50 μm and 10% to 95%, respectively.

[0063] The electrolyte contains lithium ions and is intended to cause electrochemical oxidation or reduction reactions in the positive and negative electrodes by these lithium ions. The electrolyte can be a non-aqueous electrolyte or a solid electrolyte that does not react with lithium metal, but is preferably a non-aqueous electrolyte and contains an electrolyte salt and an organic solvent.

[0064] The electrolyte salt contained in the non-aqueous electrolyte solution is a lithium salt. The lithium salt can be used without limitation as long as it is commonly used in electrolyte solutions for lithium secondary batteries. The lithium salt can be selected from LiN(FSO2)2, LiSCN, LiN(CN)2, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiPF6, LiF, LiCl, LiBr, LiI, LiNO3, LiClO4, LiAlO4, LiAlCl4, LiSbF6, LiAsF6, LiBF2C2O4, LiBC4O8, Li(CF3)2PF4, Li(CF3)3PF3, Li(CF3)4PF2, Li(CF3)5PF, Li(CF3)6P, LiCF3SO3, LiC4F9SO3, LiCF3CF2SO3, LiCF3CF2(CF3)2CO, Li(CF3SO2)2CH, LiCF3(CF2)7SO3, LiCF3CO2, LiCH3CO2 and combinations thereof, but is not limited thereto.

[0065] The concentration of the lithium salt may be 0.1M to 8.0M, preferably 0.5M to 5.0M, more preferably 1.0 to 3.0M, depending on various factors such as the exact composition of the electrolyte mixture, the solubility of the salt, the conductivity of the dissolved salt, the charging and discharging conditions of the battery, the operating temperature, and other factors known in the field of lithium secondary batteries. If the concentration of the lithium salt is below the above range, the conductivity of the electrolyte may be reduced, and thus the performance of the battery may be deteriorated. If the concentration of the lithium salt exceeds the above range, the viscosity of the electrolyte may increase, and thus the lithium ion (Li + Therefore, it is preferred to select an appropriate lithium salt concentration within the above range.

[0066] The organic solvent contained in the non-aqueous electrolyte can be any organic solvent commonly used in lithium-sulfur battery electrolytes without limitation. According to one embodiment of the present invention, the organic solvent can be a carbonate solvent, an ester solvent, an ether solvent, a ketone solvent, an alcohol solvent, or an aprotic solvent. Ether solvents are generally used.

[0067] Examples of the carbonate solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), ethylmethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC) or butylene carbonate (BC).

[0068] Examples of ester solvents include methyl acetate, ethyl acetate, n-propyl acetate, 1,1-dimethylethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanoic acid lactone, valerolactone, mevalonolactone, caprolactone, and the like.

[0069] The ether solvent can specifically be dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, polyethylene glycol methyl ethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran or polyethylene glycol dimethyl ether, etc.

[0070] Examples of the ketone solvent include cyclohexanone, etc. Examples of the alcohol solvent include ethanol or isopropyl alcohol, etc.

[0071] Examples of the aprotic solvent may specifically include: nitriles, such as acetonitrile; amides, such as dimethylformamide; dioxolanes, such as 1,3-dioxolane (DOL); or sulfolane.

[0072] The non-aqueous organic solvent may be used alone or in combination of two or more. The mixing ratio when used in combination of two or more can be appropriately adjusted depending on the desired battery performance.

[0073] The electrolyte may further contain LiNO 3. If the electrolyte contains LiNO 3, the inhibition effect on the shuttle can be improved. The electrolyte may contain LiNO 3 in an amount of 0.1 to 10 wt % based on the total weight of the electrolyte.

[0074] The electrolyte may include at least one selected from a liquid electrolyte, a gel polymer electrolyte, and a solid polymer electrolyte. The electrolyte may preferably be a liquid electrolyte.

[0075] Depending on the manufacturing process and required performance of the final product, the injection of the non-aqueous electrolyte can be performed at an appropriate stage of the electrochemical device manufacturing process. That is, the non-aqueous electrolyte can be applied before assembling the electrochemical device or at the final stage of assembling the electrochemical device.

[0076] In addition to the usual winding process, the lithium-sulfur battery according to the present invention can also be manufactured by laminating, stacking and folding the separator and electrodes. The shape of the lithium-sulfur battery is not particularly limited and can be various shapes such as cylindrical, laminated and coin-shaped.

[0077] Hereinafter, preferred embodiments are provided to help understand the present invention, but the following embodiments are provided only to make the present invention easier to understand, and the present invention is not limited thereto.

[0078] Preferred Implementation

[0079] Example

[0080] Example 1

[0081] 1. Manufacturing of positive electrode

[0082] A binder composition was prepared by mixing butyl acrylate-styrene copolymer (a product from LG Chem) with lithiated carboxymethyl cellulose (a product from GL Chem (GBLi-1000)) and gum arabic (a product from Daejung Chemicals & Metals). In addition, sulfur (a product from Sigma-Aldrich) and carbon nanotubes (CNTs) were mixed in a weight ratio of 75:25 using a ball mill and heat-treated at 155°C to prepare a positive active material of a sulfur-carbon composite material. Super P was prepared as a conductive material. The above-mentioned positive active material, conductive material and binder composition (binder, thickener and gum arabic) were added to water as a solvent and mixed by a bead milling method to prepare a slurry for preparing a positive electrode. At this time, the mixing ratio was such that the weight ratio of positive active material: conductive material: binder: thickener: gum arabic was 90:5:2.5:1.5:1. The prepared slurry for manufacturing a positive electrode was coated on an aluminum foil current collector and then dried at 50° C. for 2 hours to prepare a positive electrode (energy density of the positive electrode: 5.5 mAh / cm 2 ).

[0083] 2. Manufacturing of lithium-sulfur batteries

[0084] Lithium-sulfur batteries (CR-2032 coin cells) were assembled by preparing the negative electrode, separator, and electrolyte as follows and the positive electrode prepared by the above method.

[0085] (1) Negative electrode

[0086] Lithium foil was used as the negative electrode.

[0087] (2) Diaphragm

[0088] A polyethylene film was used as a separator.

[0089] (3) Electrolytes

[0090] As the electrolyte, an electrolyte prepared by mixing LiTFSI at a concentration of 0.1 mol in a mixed solvent of dioxolane (DOL) and dimethyl ether (DME) and adding LiNO 3 in an amount of 1 wt % compared to the electrolytic solution was used.

[0091] Comparative Example 1

[0092] A lithium-sulfur battery was manufactured in the same manner as in Example 1 except that gum arabic was not used in preparing the binder composition, and only the emulsion-type binder and lithiated carboxymethyl cellulose were mixed and used, and thus the mixing ratio in the slurry for manufacturing the positive electrode was adjusted so that the positive electrode active material:conductive material:binder:thickener was 91:5:2.5:1.5.

[0093] Comparative Example 2

[0094] A lithium-sulfur battery was manufactured in the same manner as in Example 1 except that, in the preparation of the binder composition, lithiated carboxymethyl cellulose was not used, and only an emulsion-type binder and gum arabic were mixed and used, and thus the mixing ratio in the slurry for manufacturing the positive electrode was adjusted so that the positive electrode active material:conductive material:binder:gum arabic was 91.5:5:2.5:1.

[0095] Comparative Example 3

[0096] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that guar gum was used instead of gum arabic in preparing the binder composition.

[0097] Comparative Example 4

[0098] A lithium-sulfur battery was manufactured in the same manner as in Example 1, except that xanthan gum was used instead of gum arabic in preparing the adhesive composition.

[0099] Comparative Example 5

[0100] A lithium-sulfur battery was manufactured in the same manner as in Example 1 except that the mixing ratio in the slurry for manufacturing the positive electrode was adjusted so that positive electrode active material:conductive material:binder:thickener:gum arabic was 89:5:2.5:1.5:2.

[0101] Experimental Example: Evaluation of Initial Discharge Performance and Cycle Performance of Manufactured Batteries

[0102] For the lithium-sulfur battery manufactured in Example 1 and the lithium-sulfur battery manufactured in Comparative Examples 1 to 5, the cycle performance of the battery was evaluated by charging at a rate of 0.3C and discharging at a rate of 0.5C after charging and discharging at a voltage range of 1.8V to 2.5V under a temperature condition of 25°C. The cycle performance of the battery was evaluated as the number of cycles showing a discharge capacity of 80% or more based on the initial discharge capacity, and when the number of cycles was exceeded, the discharge capacity dropped to less than 80% based on the initial discharge capacity. Representatively, a charge / discharge curve graph of the battery potential relative to the specific capacity obtained by initially charging and discharging the lithium-sulfur batteries prepared in Example 1 and Comparative Example 1 was prepared and shown below. Figure 1 The evaluation results of the initial discharge performance and cycle performance of all lithium-sulfur batteries are shown in Table 1 below.

[0103] Table 1:

[0104] Initial discharge capacity Number of cycles Example 1 1130 75 Comparative Example 1 1085 60 Comparative Example 2 1120 30 Comparative Example 3 1100 20 Comparative Example 4 1105 20 Comparative Example 5 1100 55

[0105] According to Table 1, it has been determined that when gum arabic is used together with adhesive and thickener as adhesive composition, not only initial discharge capacity is improved, but also cycle performance is improved. Specifically, in the adhesive composition comprising gum arabic, adhesive and thickener, if gum arabic (Comparative Example 1) or thickener (Comparative Example 2) is not included, initial discharge performance and cycle performance are reduced. In addition, in the adhesive composition, if gum arabic is replaced by guar gum (Comparative Example 3) or xanthan gum (Comparative Example 4), initial discharge performance and cycle performance are similarly deteriorated. Even if the adhesive composition comprising gum arabic, adhesive and thickener is used, when the consumption of gum arabic exceeds thickener, initial discharge performance and cycle performance are also almost not improved.

[0106] All simple modifications and variations of the present invention are within the scope of the present invention, and the detailed scope of protection of the present invention will become apparent from the appended claims.

Claims

1. A binder composition for manufacturing a positive electrode of a lithium-sulfur battery, the binder composition comprising a binder, a thickener and gum arabic, in, The content of the binder in the binder composition is 40 wt % to 60 wt % based on the total weight of the binder composition. Wherein, based on the total weight of the adhesive composition, the content of the thickener in the adhesive composition is 20 wt % to 35 wt %, Wherein, based on the total weight of the adhesive composition, the content of the gum arabic in the adhesive composition is 10 wt % to 30 wt %, Wherein, the content of gum arabic in the adhesive composition is 30 parts by weight to 100 parts by weight based on 100 parts by weight of the thickener.

2. The binder composition for producing a positive electrode of a lithium-sulfur battery according to claim 1, wherein The adhesive is selected from polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polybutyl acrylate, polypropyl acrylate, polyethyl acrylate, polyethylhexyl acrylate, polystyrene, EPDM rubber, sulfonated EPDM, styrene-butadiene rubber, fluororubber, combinations of the above materials and copolymers of the above materials.

3. The binder composition for producing a positive electrode of a lithium-sulfur battery according to claim 1, wherein The binder is regenerated cellulose.

4. The binder composition for manufacturing a positive electrode of a lithium-sulfur battery according to claim 1, wherein The thickener is selected from carboxymethyl cellulose, methyl cellulose, hydroxypropyl cellulose, methyl hydroxypropyl cellulose, ethyl hydroxyethyl cellulose, methyl ethyl hydroxyethyl cellulose and combinations thereof.

5. The binder composition for manufacturing a positive electrode of a lithium-sulfur battery according to claim 1, wherein The thickener is cellulose gum.

6. The binder composition for manufacturing a positive electrode of a lithium-sulfur battery according to claim 1, wherein The thickener is in lithiated form.

7. A positive electrode for a lithium-sulfur battery, the positive electrode for a lithium-sulfur battery being formed by coating a slurry for manufacturing a positive electrode on a positive electrode current collector, The slurry for manufacturing a positive electrode comprises the binder composition according to claim 1 , a positive electrode active material, and a conductive material.

8. The positive electrode for lithium-sulfur batteries according to claim 7, wherein The content of the binder composition in the slurry for manufacturing a positive electrode is 0.01 parts by weight to 10 parts by weight relative to 100 parts by weight of solid matter in the slurry for manufacturing a positive electrode.

9. The positive electrode for lithium-sulfur batteries according to claim 7, wherein The content of the positive electrode active material in the slurry for manufacturing a positive electrode is 80 parts by weight to 99 parts by weight relative to 100 parts by weight of solid matter in the slurry for manufacturing a positive electrode.

10. The positive electrode for lithium-sulfur batteries according to claim 7, wherein The content of the conductive material in the slurry for manufacturing a positive electrode is 0.1 parts by weight to 15 parts by weight relative to 100 parts by weight of solid matter in the slurry for manufacturing a positive electrode.

11. A lithium-sulfur battery comprising the positive electrode according to claim 7, a negative electrode, a separator and an electrolyte.

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