Binder composition for manufacturing a positive electrode of a lithium secondary battery, and a positive electrode of a lithium secondary battery manufactured using the binder composition

Abstract

A binder composition for manufacturing a positive electrode of a lithium secondary battery is provided, the binder composition includes a binder, a thickener, and a gum arabic-cysteine ​​polymer, a positive electrode is manufactured using the binder composition, and a lithium secondary battery including the positive electrode is provided. The binder composition includes the gum arabic-cysteine ​​polymer, and when applied to a lithium secondary battery, the binder composition can improve the initial discharge performance and life characteristics of the battery.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0008331 filed January 20, 2022, and Korean Patent Application No. 10-2023-0006616 filed January 17, 2023, and incorporates all contents disclosed in the documents of the relevant Korean patent applications as part of this specification.

[0002] The present invention relates to a binder composition for manufacturing a positive electrode of a lithium secondary battery, and a positive electrode of a lithium secondary battery manufactured using the binder composition. [Background technology]

[0003] The need for the development of environmentally friendly electric and hybrid vehicles and the rapid development of smart IT devices are driving a surge in demand for high-capacity, high-power batteries. Currently, commercially available lithium-ion batteries have limited energy densities due to technical issues, so the development of lithium-sulfur, lithium-selenium, or lithium-air batteries with higher energy densities is drawing attention. Among these, sulfur and oxygen, the cathode active materials in lithium-sulfur and lithium-air batteries, have similar physicochemical properties and are abundantly available, raising expectations for their commercialization.

[0004] Lithium-air batteries and lithium-sulfur batteries use lithium metal, which has high reducing power, voltage characteristics, and high reversibility, in the anode and air or sulfur in the cathode. The reaction products, Li2O2, LiOH, and Li2S, store a higher amount of lithium ions per weight and volume than the LiCoO2 used in the cathode of lithium-ion batteries. Furthermore, by using lithium metal in the anode, they can store a larger amount of charge than lithium-ion batteries that use graphite-based anodes, which have a maximum lithium storage limit of LiC6. However, despite this high theoretical energy density, actual energy densities are low, ranging from 20 to 45% of the theoretical value. Therefore, lithium-air batteries and lithium-sulfur batteries are still in the early stages of development and have not yet reached commercialization.

[0005] Specifically, in the case of a lithium-air battery, the Li2O2 and Li2O produced during charging are converted into lithium ions (Li + ) and oxygen (O2), a high overvoltage is required. Unlike lithium-ion batteries, they have an open structure that allows outside air to enter and exit, so side reactions and electrolyte volatilization are likely to occur due to the inflow of impurities from the outside air (water, carbon dioxide, etc.), resulting in a rapid decline in performance.

[0006] In lithium-sulfur batteries, the sulfur that makes up the positive electrode and the end product of the reaction, Li2S, are electrically non-conductors. For this reason, lithium-sulfur batteries use electrolytes with high dielectric constants, such as tetraethylene glycol dimethyl ether (TEGDME), which creates a shuttle mechanism in which soluble polysulfides migrate from the positive electrode to the negative electrode, where they are reduced to lower-molecular polysulfides, which then return to the positive electrode and then back to the negative electrode. As a result, insoluble Li2S and Li2S2 can accumulate on the negative electrode surface and at the separator interface. Furthermore, at the positive electrode, the intermediate reaction product, lithium polysulfide (Li2S8), has high solubility in organic electrolytes and dissolves continuously during discharge, reducing the amount of positive electrode material. This results in a rapid capacity loss with cycling. Furthermore, sulfur itself has very low electrical conductivity, so it is often combined with conductive carbon or polymers. In this case, the reduced sulfur content reduces the overall energy density of the cell.

[0007] To address these issues, various methods are being researched and developed, including the design of porous cathode structures, the development of additives to prevent overvoltage, and the formation of surface treatment layers. Among these, in the case of lithium-air batteries, methods are being considered for reducing charging overvoltage by ensuring that the discharge product Li2O2 is uniformly dispersed within a dense conductive matrix without concentrating. This allows for smooth electron transfer and maximizes the lithium ion and oxygen generation reaction rate during charging. In addition, in the case of lithium-sulfur batteries, methods are being considered for facilitating the transfer of electrons and lithium ions by uniformly dispersing the insulator Li2S within a dense conductive matrix through optimal structural and compositional design, thereby reducing charging overvoltage and suppressing the formation of lithium polysulfides at the cathode.

[0008] When manufacturing the positive electrode of a lithium-sulfur battery, binders and thickeners are used to stabilize the slurry and bind the electrode elements together. However, if only existing binders and thickeners for lithium-ion batteries are used, it is not possible to expect effects such as increased reactivity or extended lifespan by controlling the release of lithium polysulfide that occurs in lithium-sulfur batteries. While it is possible to change reactivity by adding substances with specific functional groups that can control the release of lithium polysulfide that occurs in the positive electrode during charge and discharge, the dispersion of these substances in the slurry can change the rheological properties, which can degrade the electrode's physical properties during coating and drying.

[0009] Therefore, there is a need to research binder compositions that can solve the above problems. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Korean Patent Publication No. 10-2002-0092029 Summary of the Invention [Problem to be solved by the invention]

[0011] Therefore, the present inventors have conducted extensive research to solve the above problems, and have found that adding a gum arabic-cysteine ​​polymer to a binder composition used for manufacturing a positive electrode of a lithium secondary battery, preferably a lithium-sulfur battery, can improve the initial discharge capacity and life characteristics of the lithium-sulfur battery, thereby completing the present invention.

[0012] Therefore, an object of the present invention is to provide a binder composition for manufacturing a positive electrode of a lithium secondary battery, which can improve the initial discharge capacity and life characteristics of a lithium-sulfur battery.

[0013] Another object of the present invention is to provide a positive electrode including the binder composition and a lithium secondary battery including the same. [Means for solving the problem]

[0014] In order to achieve the above purpose, The present invention provides a binder composition for manufacturing a positive electrode of a lithium secondary battery, which comprises a binder, a thickener, and a gum arabic-cysteine ​​polymer.

[0015] The present invention also provides a battery comprising: a current collector; and a positive electrode active material layer disposed on at least one surface of the current collector; The present invention provides a positive electrode for a lithium secondary battery, wherein the positive electrode active material layer comprises the binder composition of the present invention, a positive electrode active material, and a conductive material.

[0016] The present invention also provides a lithium secondary battery comprising the positive electrode of the present invention; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte. [Effects of the Invention]

[0017] The binder composition for manufacturing a positive electrode of a lithium secondary battery according to the present invention contains a gum arabic-cysteine ​​polymer, and when the binder composition is applied to a lithium secondary battery, preferably a lithium-sulfur battery, it can improve the initial discharge capacity and life characteristics of the battery. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will now be described in more detail.

[0019] The terms and words used in this specification and claims should not be construed in a limited manner based on their ordinary or dictionary meanings, but should be construed in a manner that is consistent with the technical idea of ​​the present invention, in accordance with the principle that the inventor can appropriately define the concept of the term in order to explain his / her invention in the best possible way.

[0020] The terms used in the present invention are merely used to describe specific embodiments and are not intended to limit the present invention. A singular expression includes a plural expression unless the context clearly indicates otherwise. In the present invention, the terms "comprise" or "have" and the like specify the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the presence or additional possibility of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0021] As used herein, the term "polysulfide" means "polysulfide ions (S x 2‐ , x=1-8) and Lithium polysulfide (Li2S x or LiS x ‐ This concept includes all of the above (x=1~8).

[0022] Binder composition for manufacturing positive electrodes of lithium secondary batteries The present invention relates to a binder composition for producing a positive electrode of a lithium secondary battery, which comprises a binder, a thickener, and a gum arabic-cysteine ​​polymer.

[0023] While conventional binders and thickeners have been unable to ensure functionality such as regulating the eruption of lithium polysulfides generated in lithium secondary batteries, preferably lithium-sulfur batteries, the binder composition of the present invention contains a gum arabic-cysteine ​​polymer, which can adsorb lithium polysulfides and regulate their eruption, thereby increasing the positive electrode reactivity of lithium-sulfur batteries containing the binder composition, and improving the initial discharge capacity and life characteristics of lithium-sulfur batteries containing the positive electrode.

[0024] Gum arabic has the following chemical formula 1:

[0025] [ka]

[0026] Cysteine ​​has the structure of the following chemical formula 2:

[0027] [ka]

[0028] The gum arabic-cysteine ​​polymer is obtained by grafting gum arabic and cysteine, and may have the structure of the following chemical formula 3.

[0029] [ka]

[0030] The polymerization method is not particularly limited as long as it is one used in the art.

[0031] Gum arabic can produce stable sulfides over a relatively wide pH range. The gum arabic-cysteine ​​polymer, obtained by polymerizing gum arabic with cysteine, can adsorb lithium polysulfides generated at the positive electrode of a lithium-sulfur battery by utilizing the structural properties of gum arabic and the carboxyl group (-COOH) and amine group (-NH) of cysteine, thereby controlling the release of lithium polysulfides. Therefore, when a binder composition containing this polymer is used in the manufacture of a positive electrode for a lithium secondary battery, preferably a lithium-sulfur battery, the reactivity of the positive electrode can be increased while maintaining the slurry properties of the positive electrode active material layer. This can improve the initial discharge capacity and lifespan characteristics of lithium-sulfur batteries containing this positive electrode.

[0032] The gum arabic-cysteine ​​polymer may contain 95 to 99.9 wt% gum arabic and 0.1 to 5 wt% cysteine, based on the total weight of the polymer, or preferably 99 to 99.9 wt% gum arabic and 0.1 to 1 wt% cysteine.

[0033] If the gum arabic content is less than 95 wt % or the cysteine ​​content is more than 5 wt %, it may be difficult to express the structural characteristics of the gum arabic and maintain the slurry properties of the positive electrode active material layer. If the gum arabic content is more than 99.9 wt % or the cysteine ​​content is less than 0.1 wt %, the cysteine ​​content may be low and the adsorption effect of lithium polysulfide may be small.

[0034] The gum arabic-cysteine ​​polymer may be included in an amount of 10 to 65 wt %, preferably 15 to 60 wt %, and most preferably 15 to 30 wt %, based on the total weight of the binder composition. If the gum arabic-cysteine ​​polymer is included in an amount less than 10 wt %, it may be difficult to achieve effects such as regulating the release of lithium polysulfide, while if it is included in an amount greater than 65 wt %, an overvoltage may occur during initial discharge of a lithium-sulfur battery using the polymer.

[0035] If the gum arabic-cysteine ​​polymer is used as a binder in the binder composition, the adhesive strength is so low that it is not possible to form a positive electrode active material layer containing the binder composition on a current collector, making it impossible to manufacture a positive electrode.

[0036] The binder is a material used to improve the adhesive strength between the components of the positive electrode and the adhesive strength between the positive electrode active material and the positive electrode current collector, and is not particularly limited as long as it is a material commonly used in the relevant technical field.

[0037] The binder may be an emulsion-type binder to ensure uniform dispersion within the slurry for preparing the positive electrode. Examples of the emulsion-type binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polybutyl acrylate, polypropyl acrylate, polyethyl acrylate, polyethylhexyl acrylate, polystyrene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, a mixture containing one or more of the above, and a copolymer containing one or more of the above. Here, the copolymer includes not only a block copolymer in which each polymer is bonded, but also a random copolymer in which the monomers of each polymer are mixed and bonded. For example, in this specification, a copolymer of polyethylene and polypropylene is understood to include an ethylene-propylene copolymer.

[0038] The binder may be included in an amount of 20 to 60 wt %, preferably 25 to 60 wt %, and most preferably 40 to 60 wt %, based on the total weight of the binder composition. If the binder is included in an amount less than 20 wt %, the adhesive strength between the positive electrode components and the adhesive strength between the positive electrode active material and the positive electrode current collector will be reduced. If the binder is included in an amount more than 60 wt %, the performance improvement effects of the thickener and gum arabic-cysteine ​​polymer additive will not be expected.

[0039] The thickener is primarily used to adjust viscosity. A suitable material for use with the binder and gum arabic-cysteine ​​polymer can be selected based on the physical properties of the slurry for preparing the positive electrode and the physical properties of the lithium-sulfur battery. The thickener may be a cellulose-based polymer, which may be one or more selected from the group consisting of carboxymethyl cellulose (CMC), methyl cellulose (MC), hydroxypropyl cellulose (HPC), methyl hydroxypropyl cellulose (MHPC), ethyl hydroxyethyl cellulose (EHEC), methyl ethyl hydroxyethyl cellulose (MEHEC), and cellulose gum. The thickener may be in a lithiated form. In the present invention, carboxymethyl cellulose is preferably used as the thickener, and the cellulose-based polymer may be in a lithiated form. The cellulose-based polymer contains functional groups such as hydroxyl groups or carboxyl groups, and thus can be lithiated by substituting lithium for the hydrogen of the functional groups. When the cellulose-based polymer is lithiated, an additional lithium source can be secured, which is useful for improving the performance of lithium-sulfur batteries.

[0040] The thickener may be included in an amount of 15 to 35 wt %, preferably 15 to 30 wt %, based on the total weight of the binder composition. If the thickener is included in an amount less than 15 wt %, the viscosity of the slurry for preparing the positive electrode is low, making it difficult to uniformly disperse the positive electrode components and thereby ensure functionality. If the thickener is included in an amount greater than 35 wt %, the fluidity of the slurry for preparing the positive electrode is low, making it difficult to uniformly disperse the positive electrode components and thereby ensure functionality.

[0041] Positive electrodes for lithium secondary batteries The present invention relates to a battery comprising: a current collector; and a positive electrode active material layer disposed on at least one surface of the current collector; The positive electrode active material layer relates to a positive electrode for a lithium secondary battery, characterized in that it contains the binder composition of the present invention described above, a positive electrode active material, and a conductive material.

[0042] The binder composition in the positive electrode active material layer may be adjusted to maximize battery performance based on its basic function of adhering the components of the positive electrode.

[0043] The binder composition may be included in an amount of 3 to 20 wt%, preferably 3 to 15 wt%, and most preferably 3 to 10 wt%, based on 100 wt% of the total base solids content in the positive electrode active material layer. Here, the base solids content refers to the solid components of the positive electrode active material, conductive agent, and binder composition, excluding the solvent, from the positive electrode slurry composition used to prepare the positive electrode active material layer. Because functionality is complemented by the thickener and gum arabic-cysteine ​​polymer, improved adhesion and battery performance can be expected even with a small amount of binder composition. If the binder composition is included in an amount exceeding 20 wt%, the content of the positive electrode active material is relatively reduced, which is undesirable in terms of improving battery performance.

[0044] Furthermore, the gum arabic-cysteine ​​polymer in the binder composition may be present in an amount of 0.5 wt % or more and less than 10 wt %, preferably 1 to 5 wt %, based on 100 wt % of the total base solids in the positive electrode active material layer. If the gum arabic-cysteine ​​polymer is present in an amount less than 0.5 wt %, the adsorption effect of lithium polysulfide is small, and improvement in positive electrode reactivity cannot be expected. If the gum arabic-cysteine ​​polymer is present in an amount greater than 10 wt %, the content of the positive electrode active material is relatively reduced, and an overvoltage may occur during initial discharge of a lithium-sulfur battery containing the polymer.

[0045] The positive electrode active material is elemental sulfur (S8), Li2S n (n≧1, n is an integer), organic sulfur compounds, carbon-sulfur polymers [(C2Sx ) n , 2.5≦x≦50, n≧2, x and n are integers] and sulfur-carbon composites, preferably sulfur-carbon composites.

[0046] The sulfur-carbon composite contains sulfur at least partially in a porous carbon material and in the interior and exterior surfaces of the porous carbon material.

[0047] The sulfur-carbon composite includes a porous carbon material that provides a framework to which the sulfur can be uniformly and stably fixed and compensates for the low electrical conductivity of sulfur to facilitate smooth electrochemical reactions.

[0048] The porous carbon material can generally be prepared by carbonizing various carbon precursors. The porous carbon material contains variable pores, with an average pore diameter ranging from 1 to 200 nm and a porosity ranging from 10 to 90% of the total volume of the porous carbon material. If the average pore diameter is less than this range, the pore size is merely at the molecular level, making sulfur impregnation impossible. Conversely, if the average pore diameter exceeds this range, the mechanical strength of the porous carbon material is weakened, making it unsuitable for use in electrode manufacturing processes.

[0049] The shape of the porous carbon material may be spherical, rod-like, needle-like, plate-like, tubular or bulk-like, and any shape commonly used in lithium secondary batteries may be used without limitation.

[0050] The porous carbon material may be any material commonly used in the art that has a porous structure or a high specific surface area. For example, the porous carbon material may be at least one selected from the group consisting of graphite, graphene, carbon black (e.g., denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black), carbon nanotubes (CNTs) (e.g., single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs)), carbon fibers (e.g., graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs), graphite (e.g., natural graphite, artificial graphite, and expanded graphite), and activated carbon. Preferably, the porous carbon material may be carbon nanotubes.

[0051] In the sulfur-carbon composite according to the present invention, the sulfur is located in at least one of the interior and exterior surfaces of the porous carbon material. For example, the sulfur may be present in an area of ​​less than 100%, preferably 1 to 95%, and more preferably 40 to 96% of the entire interior and exterior surfaces of the porous carbon material. When the sulfur is present in the interior and exterior surfaces of the porous carbon material within the above range, it exhibits the greatest effects in terms of electron transfer area and wettability with the electrolyte. Specifically, the sulfur is thinly and uniformly impregnated in the interior and exterior surfaces of the porous carbon material within the above range, thereby increasing the electron transfer contact area during charge and discharge. If the sulfur is located in 100% of the entire interior and exterior surfaces of the porous carbon material, the porous carbon material will be completely covered with sulfur, reducing wettability and contact with the electrolyte, preventing electron transfer and preventing participation in the electrochemical reaction.

[0052] The sulfur-carbon composite may contain 65 to 90 wt%, preferably 70 to 85 wt%, and more preferably 72 to 80 wt%, of sulfur, based on 100 wt% of the sulfur-carbon composite. If the sulfur content is below the above range, the porous carbon content in the sulfur-carbon composite increases relatively, resulting in an increased specific surface area and an increased binder content during cathode fabrication. This increased binder usage ultimately increases the surface resistance of the cathode and acts as an insulator, preventing electron transfer, thereby reducing battery performance. Conversely, if the sulfur content exceeds the above range, sulfur that cannot bond with the porous carbon material may aggregate or re-emerge on the surface of the porous carbon material, making it difficult for the sulfur to receive electrons and therefore unable to participate in the electrochemical reaction, resulting in a loss of battery capacity.

[0053] The method for preparing the sulfur-carbon composite of the present invention is not particularly limited, and may be a method commonly used in the art. For example, sulfur and a porous carbon material may be simply mixed and then heat-treated to prepare a composite.

[0054] In addition to the above-described composition, the positive electrode active material may further include one or more additives selected from a transition metal element, a Group IIIA element, a Group IVA element, sulfur compounds of these elements, and alloys of these elements with sulfur.

[0055] The transition metal elements may include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au, or Hg, the IIIA group elements may include Al, Ga, In, or Tl, and the IVA group elements may include Ge, Sn, or Pb.

[0056] In the positive electrode for a lithium secondary battery according to the present invention, the positive electrode active material may be included in an amount of 50 to 95 wt %, preferably 70 to 95 wt %, and more preferably 85 to 95 wt %, based on 100 wt % of the total base solids content included in the positive electrode active material layer. If the content of the positive electrode active material is less than this range, it may be difficult to fully demonstrate the electrochemical reaction of the electrode. Conversely, if the content exceeds this range, there may be a problem of deterioration in the physical properties of the electrode, which will be described later.

[0057] The conductive material electrically connects the electrolyte and the positive electrode active material and serves as a path for electrons to move from a current collector to the positive electrode active material. Any conductive material may be used.

[0058] For example, the conductive material may be carbon black such as Super-P, Denka Black, acetylene black, Ketjen Black, channel black, furnace black, lamp black, thermal black, or carbon black; carbon derivatives such as carbon nanotubes or fullerenes; conductive fibers such as carbon fibers or metal fibers; metal powders such as carbon fluoride, aluminum, or nickel powder; or conductive polymers such as polyaniline, polythiophene, polyacetylene, or polypyrrole, which may be used alone or in combination.

[0059] The content of the conductive material may be 1 to 10 wt % based on 100 wt % of the total base solids contained in the positive electrode active material layer. If the content of the conductive material is below this range, electron transfer between the positive electrode active material and the current collector is difficult, resulting in a decrease in voltage and capacity. Conversely, if the content exceeds this range, the proportion of the positive electrode active material decreases relatively, resulting in a decrease in the total energy (charge amount) of the battery. Therefore, it is preferable to determine the appropriate content within the above range.

[0060] The positive electrode current collector is not particularly limited as long as it supports the positive electrode active material, does not cause chemical changes in the battery, and has high conductivity, and examples thereof include copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, silver, etc., and aluminum-cadmium alloys.

[0061] The positive electrode current collector may have fine irregularities on its surface to strengthen the bonding force with the positive electrode active material, and may be in various forms such as a film, a sheet, a foil, a mesh, a net, a porous body, a foam, or a nonwoven fabric.

[0062] In the present invention, the method for manufacturing the positive electrode for the lithium secondary battery is not particularly limited, and any method known to those skilled in the art or various methods modified therefrom may be used.

[0063] For example, the positive electrode for the lithium-sulfur battery may be manufactured by preparing a positive electrode slurry composition including the above-described composition, and then coating the same on at least one surface of the positive electrode current collector to form the positive electrode active material layer.

[0064] The positive electrode slurry composition includes the positive electrode active material layer composition described above, which includes a positive electrode active material, a binder composition, and a conductive material, and may further include additives and a solvent.

[0065] The solvent is one that can uniformly disperse the positive electrode active material, binder composition, and conductive material. The solvent is most preferably an aqueous solvent, such as water, including distilled water and deionized water. However, the solvent is not limited thereto, and if necessary, a lower alcohol that is easily miscible with water can be used. Examples of the lower alcohol include methanol, ethanol, propanol, isopropanol, and butanol, and these can be preferably mixed with water.

[0066] The content of the solvent may be at a level that allows easy coating, and the specific content varies depending on the coating method and device.

[0067] The positive electrode slurry composition may further include, as needed, substances commonly used in the relevant technical field for the purpose of improving its performance, etc. For example, viscosity modifiers, fluidizing agents, fillers, etc.

[0068] The method for applying the positive electrode slurry composition is not particularly limited in the present invention, and examples thereof include doctor blade, die casting, comma coating, screen printing, etc. Alternatively, the positive electrode slurry may be applied to a positive electrode current collector by a pressing or lamination method after being separately formed on a substrate.

[0069] After the coating, a drying process may be performed to remove the solvent. The drying process is performed at a temperature and time sufficient to sufficiently remove the solvent. The conditions may vary depending on the type of solvent, and are not particularly limited in the present invention. Examples include drying with warm air, hot air, or low-humidity air, vacuum drying, and drying using (far) infrared rays or electron beams. The drying speed is typically adjusted to remove the solvent as quickly as possible, within a range that does not cause cracks in the positive electrode active material layer due to stress concentration or peeling of the positive electrode active material layer from the positive electrode current collector.

[0070] Furthermore, after the drying, the current collector can be pressed to increase the density of the positive electrode active material in the positive electrode. Examples of pressing methods include die pressing and roll pressing.

[0071] The positive electrode fabricated using the above-described composition and fabrication method, specifically the positive electrode active material layer, has a porosity of 50 to 80%, preferably 60 to 75%. If the positive electrode porosity is less than 50%, the filling level of the positive electrode slurry composition (including the positive electrode active material, additives, conductive material, and binder) is too high, resulting in insufficient electrolyte for ionic and / or electrical conduction between the positive electrode active material and the positive electrode active material, which can lead to reduced battery output and cycle characteristics and severe overvoltage and discharge capacity loss. Conversely, if the positive electrode porosity is too high (greater than 80%), the physical and electrical connection with the current collector is weakened, reducing adhesion and making reactions more difficult. Furthermore, electrolyte may fill the pores inside the positive electrode, reducing the battery's energy density. Therefore, the porosity should be appropriately adjusted within this range.

[0072] Lithium secondary battery The present invention also relates to a lithium secondary battery comprising the above-described positive electrode of the present invention; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte.

[0073] The lithium secondary battery of the present invention is preferably a lithium-sulfur battery.

[0074] The negative electrode may include a negative electrode current collector and a negative electrode active material layer coated on one or both sides of the negative electrode current collector, or may be a lithium metal plate.

[0075] The negative electrode current collector is for supporting the negative electrode active material layer, and is the same as that described for the positive electrode current collector.

[0076] The negative electrode active material layer may include a conductive material, a binder, etc., in addition to the negative electrode active material. In this case, the conductive material and the binder are the same as those described above.

[0077] The negative electrode active material is a lithium ion (Li +The lithium ion-containing compound 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, lithium metal, or a lithium alloy.

[0078] The lithium ion (Li + The material capable of reversibly inserting or de-inserting lithium ions (Li) may be, for example, crystalline carbon, amorphous carbon, or a mixture thereof. + The material capable of reacting with lithium (Li) to reversibly form a lithium-containing compound is, for example, tin oxide, titanium nitride, or silicone. 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).

[0079] Preferably, the negative electrode active material may be lithium metal, specifically in the form of a lithium metal thin film or lithium metal powder.

[0080] A separator may be interposed between the positive electrode and the negative electrode.

[0081] The separator separates or insulates the positive electrode and the negative electrode from each other and allows lithium ions to be transported between the positive electrode and the negative electrode. The separator may be made of a porous, non-conductive, or insulating material, and may be any material commonly used as a separator in lithium secondary batteries. The separator may be an independent member such as a film, or may be a coating layer attached to the positive electrode and / or the negative electrode.

[0082] The separation membrane is preferably one that has low resistance to the ion migration of the electrolyte and has excellent moisture-absorbing ability for the electrolyte.

[0083] The separator may be made of a porous substrate, and the porous substrate may be any porous substrate commonly used in secondary batteries. A porous polymer film may be used alone or in combination. For example, a nonwoven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, or the like, or a polyolefin-based porous membrane may be used, but is not limited thereto.

[0084] The material of the porous substrate is not particularly limited in the present invention, and any porous substrate commonly used in electrochemical devices can be used. For example, the porous substrate can be made of polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyamides, polyacetals, polycarbonates, polyimides, polyetheretherketones, polyethersulfones, polyphenylene oxides, polyphenylene sulfides, polyethylene naphthalenes, polytetrafluoroethylenes, polyvinylidene fluoride, polyvinyl chloride, etc. The material may include one or more materials selected from the group consisting of poly(p-phenylene benzobisoxazole), polyacrylonitrile, cellulose, nylon, poly(p-phenylene benzobisoxazole), and polyarylate.

[0085] The thickness of the porous substrate is not particularly limited, but is 1 to 100 μm, preferably 5 to 50 μm. Although the thickness of the porous substrate is not limited to the above range, if the thickness is less than the lower limit, the mechanical properties may be reduced, and the separator may be easily damaged during use of the battery.

[0086] The average diameter and porosity of the pores present in the porous substrate are not particularly limited, but are 0.001 to 50 μm and 10 to 95%, respectively.

[0087] The electrolyte contains lithium ions, and serves as a medium for causing an electrochemical oxidation or reduction reaction between the positive electrode and the negative electrode.

[0088] The electrolyte may be a non-aqueous electrolyte solution or a solid electrolyte that does not react with lithium metal, but is preferably a non-aqueous electrolyte that includes an electrolyte salt and an organic solvent.

[0089] The electrolyte salt contained in the non-aqueous electrolyte solution is a lithium salt. The lithium salt may be any salt commonly used in electrolyte solutions for lithium secondary batteries. For example, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, LiN(SO2F)2, lithium chloroborane, lithium lower aliphatic carboxylate, lithium 4-phenylborate, lithium imide, etc. may be used.

[0090] The concentration of the lithium salt is 0.2 to 2 M, specifically 0.4 to 2 M, more specifically 0.4 to 1.7 M, depending on several factors such as the exact composition of the electrolyte solvent mixture, the solubility of the salt, the conductivity of the dissolved salt, the charge and discharge conditions of the battery, the operating temperature, and other factors known in the lithium battery field. If the lithium salt concentration is less than 0.2 M, the conductivity of the electrolyte may be reduced, resulting in a decrease in electrolyte performance, while if the concentration is more than 2 M, the viscosity of the electrolyte may increase, resulting in a decrease in lithium ion mobility.

[0091] The organic solvent contained in the non-aqueous electrolyte may be any organic solvent commonly used in electrolytes for lithium secondary batteries, and may include, without limitation, ethers, esters, amides, linear carbonates, cyclic carbonates, etc., which may be used alone or in combination of two or more. Among these, ether-based compounds may be included.

[0092] The ether-based compound may include acyclic ethers and cyclic ethers.

[0093] For example, the acyclic ether may be at least one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, ethylene glycol ethyl methyl ether, 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, and polyethylene glycol methyl ethyl ether, but is not limited thereto.

[0094] For example, the cyclic ether may be at least one selected from the group consisting of 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, 2-methyl-1,3-dioxolane, 2-vinyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3-dioxolane, 2-ethyl-2-methyl-1,3-dioxolane, tetrahydropyran, 1,4-dioxane, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, 1,4-dimethoxybenzene, and isosorbide dimethyl ether, but is not limited thereto.

[0095] Among the organic solvents, the ester may be any one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone, or a mixture of two or more thereof, but is not limited thereto.

[0096] Specific examples of the linear carbonate compound include, but are not limited to, any one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate, or a mixture of two or more thereof.

[0097] Specific examples of the cyclic carbonate compound include any one or a mixture of two or more selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, and halides thereof. Examples of halides thereof include, but are not limited to, fluoroethylene carbonate (FEC).

[0098] In addition to the electrolyte salt and organic solvent, the electrolyte may further include a nitric acid or nitrite-based compound as an additive, which forms a stable coating on the lithium metal electrode, which serves as the negative electrode, thereby improving charge / discharge efficiency.

[0099] The nitric acid or nitrite compound may be any one selected from the group consisting of inorganic nitric acid or nitrite compounds such as lithium nitrate (LiNO), potassium nitrate (KNO), cesium nitrate (CsNO), barium nitrate (Ba(NO)), ammonium nitrate (NHNO), lithium nitrite (LiNO), potassium nitrite (KNO), cesium nitrite (CsNO), and ammonium nitrite (NHNO); organic nitric acid or nitrite compounds such as methyl nitrate, dialkylimidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridinium nitrate, ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite, and octyl nitrite; and organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, and dinitrotoluene, and combinations thereof. Preferably, lithium nitrate is used.

[0100] The electrolyte injection can be performed at an appropriate stage during the manufacturing process of the electrochemical device depending on the manufacturing process and required properties of the final product, i.e., before assembling the electrochemical device or at the final stage of assembling the electrochemical device.

[0101] The lithium secondary battery according to the present invention can be manufactured by laminating (stacking) the separator and the electrode and by folding the same in addition to the general winding process.

[0102] The shape of the lithium secondary battery is not particularly limited, and may be various shapes such as a cylindrical shape, a stacked shape, or a coin shape.

[0103] Hereinafter, preferred examples will be presented to aid in understanding the present invention. However, the following examples are merely illustrative of the present invention, and it will be obvious to those skilled in the art that various changes and modifications are possible within the scope of the scope and technical idea of ​​the present invention, and it is natural that such changes and modifications also fall within the scope of the accompanying claims.

[0104] <Lithium-sulfur battery manufacturing> Example 1 . Gum arabic (Dejong Chemical Industry Co., Ltd.) and cysteine ​​were mixed in a weight ratio of 99:1 to prepare a gum arabic-cysteine ​​polymer.

[0105] A binder composition was prepared by mixing a butyl acrylate-styrene copolymer (LG Chemicals) as a binder, a lithiated carboxymethyl cellulose (Delchem ​​GBLi-1000) as a thickener, and the gum arabic-cysteine ​​polymer.

[0106] In addition, sulfur (Sigma-Aldrich product) was mixed with carbon nanotubes (CNTs) in a weight ratio of 75:25 using a ball mill, and then heat-treated at 155°C to prepare a sulfur-carbon composite cathode active material.

[0107] Denka black was prepared as the conductive material.

[0108] The sulfur-carbon composite, conductive material, and binder composition (binder, thickener, and gum arabic-cysteine ​​polymer) were added to water as a solvent and mixed by bead milling to prepare a slurry for preparing a positive electrode. The mixture ratio by weight of the positive electrode active material: conductive material: binder: thickener: gum arabic-cysteine ​​polymer was 90:5:2.5:1.5:1. The prepared slurry for preparing a positive electrode was applied to an aluminum foil current collector and dried at 50°C for 2 hours to prepare a positive electrode (energy density of the positive electrode: 5.5 mAh / cm). 2 ).

[0109] A 100 μm-thick lithium foil was used as the negative electrode, and a 20 μm-thick polyethylene membrane was used as the separator. The electrolyte was a mixture of dioxolane (DOL) and dimethyl ether (DME) (1:1 v / v) with 1 M LiTFSI, and 1 wt% LiNO3 was added to the electrolyte. A lithium-sulfur battery (CR-2032 coin cell) was fabricated using this electrolyte.

[0110] Example 2 . A lithium-sulfur battery was fabricated in the same manner as in Example 1, except that the weight ratio of the positive electrode active material, conductive material, binder, thickener, and gum arabic-cysteine ​​polymer was 86:5:2.5:1.5:5.

[0111] Example 3 . A lithium-sulfur battery was fabricated in the same manner as in Example 1, except that the weight ratio of the positive electrode active material, conductive material, binder, thickener, and gum arabic-cysteine ​​polymer was 80:5:2.5:1.5:10.

[0112] Example 4 . A lithium-sulfur battery was fabricated in the same manner as in Example 3, except that gum arabic (Dajeong Chemical Industry Co., Ltd.) and cysteine ​​were mixed in a weight ratio of 95:5 to prepare a gum arabic-cysteine ​​polymer.

[0113] Comparative Example 1 . A lithium-sulfur battery was fabricated in the same manner as in Example 1, except that the gum arabic-cysteine ​​polymer was not used and the positive electrode active material:conductive material:binder:thickener were used in a weight ratio of 91:5:2.5:1.5.

[0114] Comparative Example 2 . A lithium-sulfur battery was fabricated in the same manner as in Example 1, except that gum arabic was used instead of the gum arabic-cysteine ​​polymer.

[0115] Experimental Example 1: Characterization of Lithium-Sulfur Batteries The initial discharge capacity and life characteristics of the lithium-sulfur batteries of Examples 1 to 4 and Comparative Examples 1 and 2 were measured.

[0116] Each of the lithium-sulfur batteries was discharged / charged three times at 0.1 C, discharged / charged three times at 0.3 C, and then charged and discharged at 0.5 C within a voltage range of 1.8 to 2.5 V.

[0117] The cycle performance of the battery was evaluated by the number of cycles at which the capacity became 1000 mAh / gS or less.

[0118] The measurement results of the initial discharge capacity and life characteristics are shown in Table 1 below.

[0119] [Table 1]

[0120] As shown in Table 1, the lithium-sulfur batteries of Examples 1 to 4 containing gum arabic-cysteine ​​polymer exhibited better initial discharge capacity than the lithium-sulfur battery of Comparative Example 1, which did not contain gum arabic, and Comparative Example 2, which contained gum arabic. Furthermore, Examples 1 to 3 also exhibited better life characteristics than Comparative Examples 1 and 2. Example 3, which contained 10 wt% of the gum arabic-cysteine ​​polymer based on 100 wt% of the total base solids in the positive electrode active material layer, exhibited a lower initial discharge capacity than Examples 1 and 2. Example 4, which contained 10 wt% of the gum arabic-cysteine ​​polymer based on 100 wt% of the total base solids in the positive electrode active material layer and polymerized gum arabic and cysteine ​​at a weight ratio of 95:5, exhibited the lowest initial discharge capacity among the Examples, and the number of cycles was similar to that of Comparative Example 1.

[0121] Comparative Example 2 contained only gum arabic instead of the gum arabic-cysteine ​​polymer. Since the gum arabic does not contain cysteine, the carboxyl and amine groups of cysteine ​​were not effective in adsorbing lithium polysulfides. As a result, the initial discharge capacity and the number of cycles were both smaller than those of Examples 1 to 4, which contained the gum arabic-cysteine ​​polymer.

[0122] Therefore, it was found that when a binder composition containing a gum arabic-cysteine ​​polymer is used and applied to the positive electrode of a lithium-sulfur battery, it adsorbs lithium polysulfide, increasing the reactivity of the positive electrode and improving the initial discharge capacity of the lithium-sulfur battery containing the same. Furthermore, it was found that when the gum arabic-cysteine ​​polymer is included in an amount of 1 wt% to 10 wt%, based on 100 wt% of the total base solids contained in the positive electrode active material layer, not only the initial discharge capacity but also the life characteristics of the lithium-sulfur battery can be improved.

Claims

1. 1. A binder composition comprising a binder, a thickener, and a gum arabic-cysteine ​​polymer, The gum arabic-cysteine ​​polymer contains 95% to 99.9% by weight of gum arabic and 0.1% to 5% by weight of cysteine ​​based on the total weight of the polymer; The binder composition for manufacturing a positive electrode of a lithium secondary battery comprises 20% to 60% by weight of a binder, 15% to 35% by weight of a thickener, and 10% to 65% by weight of a gum arabic-cysteine ​​polymer, based on the total weight of the composition.

2. 2. The binder composition of claim 1, wherein the gum arabic-cysteine ​​polymer comprises 99% to 99.9% by weight of gum arabic and 0.1% to 1% by weight of cysteine, based on the total weight of the polymer.

3. 2. The binder composition of claim 1, wherein the binder composition comprises 40 to 60 wt % of a binder, 15 to 30 wt % of a thickener, and 15 to 30 wt % of a gum arabic-cysteine ​​polymer, based on the total weight of the composition.

4. The binder composition for manufacturing a positive electrode of a lithium secondary battery according to claim 1, wherein the thickener is in a lithiated form.

5. 2. The binder composition of claim 1, wherein the binder is selected from the group consisting of polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polybutyl acrylate, polypropyl acrylate, polyethyl acrylate, polyethylhexyl acrylate, polystyrene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, a mixture containing at least one thereof, and a copolymer obtained by polymerizing at least one thereof.

6. 2. The binder composition for manufacturing a positive electrode of a lithium secondary battery according to claim 1, wherein the thickener is at least one selected from the group consisting of carboxymethyl cellulose, methyl cellulose, hydroxypropyl cellulose, methyl hydroxypropyl cellulose, ethyl hydroxyethyl cellulose, methyl ethyl hydroxyethyl cellulose, and cellulose gum.

7. a current collector; and a positive electrode active material layer disposed on at least one surface of the current collector; A positive electrode for a lithium secondary battery, wherein the positive electrode active material layer comprises the binder composition according to claim 1 , a positive electrode active material, and a conductive material.

8. 8. The positive electrode for a lithium secondary battery according to claim 7, wherein the binder composition is contained in an amount of 3 to 20 wt % based on 100 wt % of the total base solid content contained in the positive electrode active material layer.

9. 8. The positive electrode for a lithium secondary battery according to claim 7, wherein the gum arabic-cysteine ​​polymer is contained in an amount of 0.5 wt % or more and less than 10 wt % based on 100 wt % of the total base solid content contained in the positive electrode active material layer.

10. The positive electrode active material is elemental sulfur (S ８ ), Li ２ S ｎ (n≧1, n is an integer), organic sulfur compounds, carbon-sulfur polymers [(C ２ S ｘ ) ｎ 8. The positive electrode for a lithium secondary battery according to claim 7, comprising at least one selected from the group consisting of a sulfur-carbon composite, ...

11. A lithium secondary battery comprising: the positive electrode according to claim 7; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte.

12. 12. The lithium secondary battery according to claim 11, wherein the lithium secondary battery is a lithium-sulfur battery.

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