Positive electrode for lithium secondary battery, method for manufacturing the same, and lithium secondary battery comprising the same

Abstract

The present invention relates to a positive electrode for a lithium secondary battery, a manufacturing method thereof, and a lithium secondary battery comprising the same, and more particularly, to a positive electrode for a lithium secondary battery comprising a self-standing film type positive electrode material in a positive electrode active material layer and manufactured by combining the positive electrode active material layer with a positive electrode current collector through an adhesive layer, the self-standing film type positive electrode material manufactured through a dry process using the characteristic that a sulfur-carbon composite material exhibits strong self-cohesion under pressure conditions. Since the positive electrode for a lithium secondary battery of the present invention has a high sulfur content, the positive electrode for a lithium secondary battery can exhibit improved performance and lifespan, and since the positive electrode for a lithium secondary battery is manufactured through a simplified process, process efficiency can be improved in terms of cost and time.

Description

TECHNICAL FIELD

[0001] This application claims the priority benefit of Korean Patent Application No. 10-2020-0136489, filed on October 21, 2020, the entire disclosure of which is hereby incorporated by reference in its entirety.

[0002] The present application relates to a cathode for a lithium secondary battery, a manufacturing method thereof, and a lithium secondary battery including the same. BACKGROUND

[0003] Recently, as the fields of electronic devices and electric vehicles rapidly develop, the demand for secondary batteries is continuously increasing. In particular, as the trend of miniaturization and light weight of portable electronic devices is increasing, the demand for secondary batteries having high energy density capable of coping with these devices is continuously growing.

[0004] Among secondary batteries, a lithium-sulfur secondary battery is a secondary battery that uses a sulfur-based compound having a sulfur-sulfur bond as a cathode active material and uses an alkali metal such as lithium, a carbon-based material that undergoes intercalation and deintercalation of metal ions such as lithium ions, or silicon or tin that forms an alloy with lithium as a negative electrode active material. Specifically, during discharge that is a reduction reaction, as the sulfur-sulfur bond is broken, the oxidation number of sulfur decreases, and during charge that is an oxidation reaction, as the oxidation number of sulfur increases, the sulfur-sulfur bond is reformed. Through this oxidation-reduction reaction, electrical energy is stored and generated.

[0005] In particular, the theoretical energy density of sulfur used as a cathode active material in a lithium-sulfur secondary battery is 1.675 mAh / g, and thus the theoretical energy density is about 5 times that of a cathode active material used in a conventional lithium secondary battery, enabling the battery to exhibit high power and high energy density. In addition, because sulfur has the advantages of being inexpensive and resource-rich, thus being easily available and environmentally friendly, sulfur is attracting attention not only as an energy source for portable electronic devices, but also as an energy source for medium-to-large-sized devices such as electric vehicles.

[0006] Because sulfur has a conductivity of 5 × 10 -30 S / cm and is a non-conductor that does not have conductivity, there is a problem that electrons generated by an electrochemical reaction are difficult to move. Therefore, sulfur is compounded with a conductive material such as carbon that can provide an electrochemical reaction site, and a sulfur-carbon composite material manufactured thereby is used.

[0007] In order to use the sulfur-carbon composite material as a cathode active material, a method of manufacturing a cathode by a slurry process is generally used, which uses a sulfur-carbon composite material with a conductive material and a binder to prepare a slurry, and then the slurry is coated to a current collector.

[0008] However, the positive electrode manufactured through the slurry process has a problem in that the loading amount of the positive electrode active material in the positive electrode is reduced because the conductive material and the binder are used when preparing the slurry, so that the energy density is also reduced. In addition, there is a problem in that time and cost are increased because the slurry process includes complicated processes such as mixing, coating, drying, and calendering.

[0009] Therefore, there is a need to develop a technology capable of manufacturing a positive electrode for a lithium secondary battery with a high loading amount using a simple process.

[0010] Prior Art Documents

[0011] [Patent Document]

[0012] Korean Patent Application Publication No. 10-2019-0100152

[0013] Chinese Patent Application Publication No. 109873120

[0014] U.S. Patent Application Publication No. 2018-0212252 SUMMARY

[0015] [Technical Problem]

[0016] Therefore, the inventors of the present application have made various studies to solve the above problems, and as a result, it has been confirmed that when the properties of the sulfur formed on the surface of the sulfur-carbon composite material, which is melted under pressure and aggregated with the surrounding sulfur, and the flexibility of the carbon material are utilized, a self-standing film-type positive electrode material can be manufactured through a dry process of applying a pressure condition to the sulfur-carbon composite material, and the self-standing film-type positive electrode material can be bound to a current collector even with a small amount of a binder, thereby completing the present application.

[0017] Therefore, an object of the present application is to provide a positive electrode for a lithium secondary battery, which is easy to manufacture while having a high sulfur loading amount.

[0018] In addition, another object of the present application is to provide a lithium secondary battery including the positive electrode for a lithium secondary battery.

[0019] [Technical Solution]

[0020] To achieve the above object, the present application provides a positive electrode for a lithium secondary battery including: a positive electrode current collector; a binder layer sequentially formed on at least one surface of the positive electrode current collector; and a positive electrode active material layer, wherein the positive electrode active material layer is a self-standing film-type positive electrode material.

[0021] Further, the present application provides a method of manufacturing a positive electrode for a lithium secondary battery, the method including the steps of: (1) forming an adhesive layer on a positive electrode current collector; and (2) adhering a self-supporting film type positive electrode material on the adhesive layer to form a positive electrode active material layer.

[0022] Further, the present application provides a lithium secondary battery including: the positive electrode for a lithium secondary battery; a negative electrode including lithium metal or a lithium alloy; a separator located between the positive electrode and the negative electrode; and an electrolyte impregnated in the positive electrode, the negative electrode, and the separator.

[0023] [Advantageous Effects]

[0024] The positive electrode for a lithium secondary battery according to the present application can exhibit improved performance and lifespan because the content of sulfur is high and the content of non-capacity substances such as an adhesive is small. Further, the positive electrode for a lithium secondary battery according to the present application can be manufactured through a simple dry process of pressing a sulfur-carbon composite material, instead of a slurry process including a series of complicated processes including mixing, coating, drying, and calendering, thereby improving process efficiency in terms of cost and time. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is a graph showing the results of performance evaluation of lithium secondary batteries according to Example 1 and Comparative Example 1 of Experimental Example 2.

[0026] Figure 2 is a graph showing the results of performance evaluation of lithium secondary batteries according to Example 1 and 2 of Experimental Example 2. DETAILED DESCRIPTION

[0027] Hereinafter, the present application will be described in more detail.

[0028] The terms and words used in the present specification and claims should not be interpreted as being limited to the commonly used meanings or meanings in dictionaries, but should be interpreted as having meanings and concepts corresponding to the technical idea of the present application based on the principle that an inventor is able to properly define the concepts of the terms to describe his application in the best way possible.

[0029] The terms used in the present application are merely used to describe specific embodiments, and are not intended to limit the present application. Unless otherwise defined, the expressions "one", "a", and "the" as used in the specification include the plural forms. It is to be understood that the terms "include" or "have" used in the present application are intended to indicate the existence of the described features, numbers, steps, operations, components, parts, or combinations thereof, but are not intended to preclude the existence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0030] The term "composite material" as used herein refers to a material that combines two or more materials to exhibit more effective functions while forming phases that are physically and chemically different from each other.

[0031] The term "self-standing film" as used herein refers to a film that can maintain a film shape by itself without a separate carrier at room temperature and pressure.

[0032] Among secondary batteries, a lithium-sulfur secondary battery has a high theoretical discharge capacity and a theoretical energy density, and sulfur used as a positive active material is abundant in reserves, low in price, and environmentally friendly, and thus the lithium-sulfur secondary battery is attracting attention as a next-generation battery.

[0033] A conventional lithium-sulfur secondary battery positive electrode is manufactured by coating a slurry containing a positive active material, a conductive material, a binder, and the like to a positive current collector.

[0034] However, there is a disadvantage in that, because the slurry used to manufacture the positive electrode contains not only the positive active material but also other materials such as the conductive material and the binder by nature, there is a limit to the loading amount of the positive active material, and thus the energy density is reduced. In addition, in the case of the conventional method using the slurry, there is a problem in that, because a series of complicated processes such as mixing, coating, drying, and calendering are performed, not only the process is complicated, but also time and cost are consumed.

[0035] In addition, for the operation of the lithium-sulfur secondary battery, a tab for electrically connecting the positive electrode and the negative electrode to an external circuit must be provided, so that the positive electrode and the negative electrode must include a current collector capable of fixing the tab.

[0036] Therefore, the present application provides a lithium secondary battery positive electrode that is easy to manufacture while having a high sulfur loading amount compared to a conventional electrode, in which a self-standing film type positive electrode material is incorporated into a positive active material layer instead of coating a slurry to a current collector, and then the positive active material layer is bonded to the current collector by a small amount of a binder.

[0037] Specifically, the lithium secondary battery positive electrode according to the present application includes a positive current collector, a binder layer sequentially formed on at least one surface of the positive current collector, and a positive active material layer, in which the positive active material layer includes a self-standing film type positive electrode material.

[0038] In the present application, the positive current collector carries the positive active material layer described later, and serves to transfer electrons between an external wire and the positive active material layer.

[0039] The positive electrode current collector is not particularly limited as long as it has high conductivity and does not cause chemical changes in the lithium secondary battery. For example, the positive electrode current collector can be copper, stainless steel, aluminum, nickel, titanium, palladium, sintered carbon; copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc.; aluminum-cadmium alloy, etc.

[0040] The positive electrode current collector can have a fine concavo-convex structure on the surface of the positive electrode current collector, or can adopt a three-dimensional porous structure to improve the binding force with the positive electrode active material layer. Accordingly, the positive electrode current collector can be formed in various forms such as a film, a sheet, a foil, a screen, a mesh, a porous body, a foamed body, or a nonwoven fabric.

[0041] In the present application, the adhesive layer can be located between the positive electrode current collector and the positive electrode active material layer described later, so that the positive electrode active material layer is adhered to the positive electrode current collector.

[0042] The positive electrode for a lithium secondary battery according to the present application combines the positive electrode active material layer described later with the positive electrode current collector through the adhesive layer. Accordingly, since no non-capacitance material such as an adhesive is added in the positive electrode active material layer, it has the advantage that not only the sulfur loading amount in the positive electrode active material layer can be increased, but also the electrochemical reaction efficiency is excellent. Furthermore, since the adhesive layer is used only in an amount necessary to maintain the binding between the positive electrode active material layer described later and the positive electrode current collector, the amount of adhesive can be significantly reduced compared to a conventional electrode, thereby improving the electrochemical reactivity of the positive electrode for a lithium secondary battery. Furthermore, since the positive electrode active material layer described later can be directly adhered to the positive electrode current collector through the adhesive layer, it can have the advantage that no separate equipment is required in the manufacturing process and the time required for the process can be shortened.

[0043] The binder layer can include at least one selected from the group consisting of styrene butadiene rubber, acrylated styrene butadiene rubber, acrylonitrile copolymer, acrylonitrile-butadiene rubber, nitrile butadiene rubber, acrylonitrile-styrene-butadiene copolymer, acrylic rubber, butyl rubber, fluoro rubber, polytetrafluoroethylene, polyethylene, polypropylene, ethylene / propylene copolymer, polybutadiene, polyethylene oxide, chlorosulfonated polyethylene, polyvinylpyrrolidone, polyvinylpyridine, polyvinyl alcohol, polyvinyl acetate, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polystyrene, latex, acrylic resin, phenol formaldehyde resin, epoxy resin, carboxymethyl cellulose, hydroxypropyl cellulose, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl cellulose, cyanoethyl sucrose, polyester, polyamide, polyether, polyimide, polycarboxylate, polycarboxylic acid, polyacrylic acid, polyacrylate, polyacrylate lithium, polymethacrylic acid, polymethacrylate, polyacrylamide, polyurethane, polyvinylidene fluoride, and poly(vinylidene fluoride)-hexafluoropropylene. Preferably, the binder layer can include at least one selected from the group consisting of styrene butadiene rubber, polytetrafluoroethylene, carboxymethyl cellulose, polyacrylic acid, polyacrylate lithium, and polyvinylidene fluoride.

[0044] The coating amount per unit area of the binder layer (i.e., the binder content per unit area of the binder layer) can be 0.1 to 20 μg / cm2 2 , preferably 0.2 to 15 μg / cm2 2 , more preferably 0.5 to 10 μg / cm2 2 , more preferably 0.5 to 5 μg / cm2 2 If the coating amount per unit area of the binder layer is less than the above range, the adhesion between the positive active material layer and the positive current collector is insufficient, so that separation can occur. In contrast, if the coating amount per unit area of the binder layer exceeds the above range, there is a problem in that the electrical conductivity is reduced and the initial discharge capacity of the battery is reduced, and as greater shrinkage occurs in the drying process of the binder layer, the adhesion is deteriorated, whereby the phenomenon of separation of the positive active material layer described later can occur.

[0045] The binder layer can be 0.002 to 0.5% by weight, preferably 0.004 to 0.3% by weight, more preferably 0.01 to 0.2% by weight, based on 100% by weight of the positive active material layer. When compared with the case of the existing positive electrode for a lithium secondary battery in which a binder is included in the positive active material layer in an amount ranging from 3 to 10% by weight based on the total weight of the positive active material layer, the content of the binder of the positive electrode for a lithium secondary battery according to the present application is very small, whereby the positive electrode exhibits better electrochemical reactivity.

[0046] The adhesive layer can have a thickness of 0.1 to 20 μm, preferably 0.5 to 10 μm. If the thickness of the adhesive layer is within the above range, the positive electrode active material layer is firmly adhered to the positive electrode current collector while not hindering the electron conduction therebetween, as described later.

[0047] The adhesive layer can include or not include a conductive material. If the adhesive layer does not include a conductive material, the electrochemical reactivity of the positive electrode for a lithium secondary battery can be improved because the current collector can be combined with only a minimum amount of adhesive.

[0048] In the present application, the positive electrode active material layer includes a self-standing film type positive electrode material.

[0049] The self-standing film type positive electrode material includes a sulfur-carbon composite material composed of 50 to 80% by weight of sulfur and 20 to 50% by weight of a porous carbon material.

[0050] The sulfur-carbon composite material refers to a form in which sulfur is supported on a porous carbon material. For example, the sulfur-carbon composite material can be in a state in which sulfur is adhered or coated on the surface of the porous carbon material. In addition, the sulfur-carbon composite material can also be in a state in which sulfur is adhered, filled, or coated in the internal pores of the porous carbon material, or a state in which sulfur penetrates and adheres to the inside of the porous carbon material.

[0051] The positive electrode material is of a self-standing film type, and the self-standing film type positive electrode material itself can be used as a positive electrode material without a process of coating it to a current collector.

[0052] Because the self-standing film type positive electrode material is prepared through a dry process using sulfur and a porous carbon material as raw materials, there is an advantage that the positive electrode for a lithium secondary battery according to the present application contains only sulfur and a porous carbon material in the positive electrode active material layer, so that when used as a positive electrode, the loading amount of sulfur as a positive electrode active material is high. In addition, the dry process can omit a series of complicated processes such as mixing, coating, drying, and calendering, which are required in a conventional slurry process, thereby reducing process costs. In addition, because the dry process does not use a slurry, the self-standing film type positive electrode material of the present application prepared through the dry process does not contain an adhesive at all, thereby being able to fundamentally eliminate the deterioration of battery performance caused by adhesive resistance.

[0053] In addition, because the self-standing film type positive electrode material of the present application manufactured through the dry process does not contain a conductive material at all, a problem of deterioration of formability caused by insufficient cohesion of the conductive material can be minimized. In addition, in a state in which the porous carbon material forms a skeleton of the positive electrode material, the self-standing film type positive electrode material is connected by sulfur formed on the surface of the porous carbon material, thereby exhibiting a form of a self-standing film.

[0054] Further, after the electrode is press-formed, the self-supporting film-type positive electrode material can have an internal adhesion of the self-supporting film-type positive electrode material of 10 gf / cm or more. The internal adhesion is due to the property of sulfur melting and gathering around during the press process. If the internal adhesion of the self-supporting film-type positive electrode material is less than 10 gf / cm, it can be difficult to form into a film form due to insufficient adhesion. Specifically, the internal adhesion can be 10 gf / cm or more, 15 gf / cm or more, 20 gf / cm or more, 25 gf / cm or more, 30 gf / cm or more, or 35 gf / cm or more. Further, the upper limit of the adhesion is 50 gf / cm or less, 60 gf / cm or less, 70 gf / cm or less, 80 gf / cm or less, 90 gf / cm or less, or 100 gf / cm or less, but is not limited thereto, and the higher the internal adhesion of the self-supporting film-type positive electrode material, the better in terms of formability, durability, and battery performance.

[0055] Further, the porosity of the self-supporting film-type positive electrode material can be 68% or less, 65% or less, 60% or less, 55% or less, and 45% or more, or 50% or more. If the porosity of the self-supporting film-type positive electrode material is greater than 68%, the durability of the self-supporting film-type positive electrode material can decrease. If the porosity of the self-supporting film-type positive electrode material is less than 45%, the battery can not operate normally because the space in the pores in which an electrochemical reaction occurs is narrowed.

[0056] In the present application, the sulfur can be at least one selected from the group consisting of inorganic sulfur (S8), Li2S n (n ≥ 1, n is an integer), an organic sulfur compound, and a carbon-sulfur polymer ((C2S x ) n wherein 2.5 ≤ x ≤ 50, n ≥ 2, x and n are integers.

[0057] Further, the content of sulfur can be 50 wt% or more, 55 wt% or more, or 60 wt% or more and 70 wt% or less, 75 wt% or less, 80 wt% or less, based on the total weight of the sulfur-carbon composite material. If the content of sulfur is less than 50 wt%, as the proportion of sulfur as an electrochemically active material decreases, the sulfur coating formed on the surface of the porous carbon material becomes thinner, making it difficult to properly form the sulfur-carbon composite material, or the amount of sulfur contained in the inside of the porous carbon material can decrease, thereby reducing the capacity of the battery. Further, if the content of sulfur exceeds 80 wt%, non-conductive sulfur can hinder the conductive structure of the porous carbon material, thereby hindering electrochemical activity, thereby limiting the operation of the battery.

[0058] When the content of sulfur in the sulfur-carbon composite material is 50 to 80% by weight, the sulfur is carried in a form of being suitably adhered, coated, or filled on the surface and / or in the pores of the porous carbon material, thereby enabling a self-standing positive electrode to be stably formed.

[0059] In the present application, the porous carbon material can have a structure in which pores or hollows are formed, or can be a porous carbon material having a high specific surface area, and can be any material commonly used in the art.

[0060] The porous carbon material can be, but is not limited to, at least one selected from the group consisting of graphite; graphene; carbon black selected from the group consisting of dekkaporte black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; carbon nanotube (CNT) selected from the group consisting of single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT); carbon fiber selected from the group consisting of graphite nanofiber (GNF), carbon nanofiber (CNF), and activated carbon fiber (ACF); and activated carbon. Preferably, the porous carbon material can be carbon nanotube.

[0061] The carbon nanotube can have more connection points due to the structural characteristics, which can be more advantageous in forming a self-standing film. Specifically, since the carbon nanotube has a shape having an aspect ratio greater than 1, the carbon nanotube can be advantageously connected to each other to form a self-standing film.

[0062] Further, graphene refers to a single layer form in which carbon atoms are arranged in a two-dimensional honeycomb shape, and is a material having a thin and wide cross-sectional area and excellent electrical conductivity and exhibiting excellent physical properties such as bending characteristics and high sensitivity to light. In the present application, graphene includes all reduced graphene formed by reducing graphene oxide; physically exfoliated graphene, etc. The thin film of graphene can be contained in a form of surrounding the outer surface of the carbon nanotube, and can suppress sulfur elution into the electrolyte while enhancing the conductive path during battery operation.

[0063] Further, the content of the porous carbon material can be 20% by weight or more, 25% by weight or more, 30% by weight or more, or 35% by weight or more and 40% by weight or less, 45% by weight or less, or 50% by weight or less, based on the total weight of the sulfur-carbon composite material. If the content of the porous carbon material is less than 20% by weight, the surface area and space capable of filling, adhering, or coating sulfur cannot be sufficiently provided, whereby the electrochemical availability (reactivity) of sulfur can be reduced. If the carbon material exceeds 50% by weight, the sulfur content can be relatively reduced, whereby the energy density of the battery can be excessively decreased when applied to a lithium secondary battery.

[0064] Further, the present application provides a method of manufacturing a positive electrode for a lithium secondary battery, the method including the steps of: (1) forming an adhesive layer on a positive electrode current collector; and (2) adhering a self-standing film type positive electrode material on the adhesive layer to form a positive electrode active material layer.

[0065] First, step (1) is a step of forming an adhesive layer on a positive electrode current collector, and can be performed by a method of coating a material contained in the adhesive layer on the positive electrode current collector. In this case, the positive electrode current collector and the adhesive layer are the same as described above.

[0066] The present application does not have a particular limitation on coating, and can be any known coating or application method. For example, the coating can be a method of uniform dispersion using a doctor blade or the like; and a method such as die casting, comma coating, screen printing, vacuum filtration coating, or the like.

[0067] Next, in step (2), a positive electrode active material layer is formed by adhering a self-standing film type positive electrode material on the adhesive layer, thereby manufacturing a positive electrode for a lithium secondary battery.

[0068] At this time, the self-standing film type positive electrode material can be prepared by a preparation method including the steps of: (a) mixing sulfur with a porous carbon material; (b) heat-treating the mixture prepared in step (a) to prepare a sulfur-carbon composite material; and (c) charging the sulfur-carbon composite material prepared in step (b) into a container and pressurizing it.

[0069] In general, sulfur can be shaped because its surface is melted and aggregated with surrounding sulfur under a pressure condition, but since sulfur does not have flexibility, a positive electrode material in a self-standing film form cannot be prepared by sulfur alone.

[0070] Although carbon has flexibility, since the cohesion of carbon is not sufficient, carbon cannot be shaped when pressed, so that a positive electrode material in a self-standing film form cannot be prepared.

[0071] On the other hand, the sulfur-carbon composite material is a composite material of sulfur and a porous carbon material, and since sulfur is uniformly present on the outer surface of the porous carbon material, when pressurized, the sulfur present on the outer surface of the porous carbon material is melted and exhibits a property of being aggregated with surrounding sulfur-carbon composite material, so that it can be shaped, and since the porous carbon material forming a skeleton in the sulfur-carbon composite material is flexible, a positive electrode material in a self-standing film form can also be formed by punching.

[0072] Therefore, the method of manufacturing a positive electrode for a lithium secondary battery of the present application as described above further includes a process of preparing a self-standing film type positive electrode material by a dry process using the properties of the sulfur-carbon composite material during pressurization.

[0073] Hereinafter, each step of the method of producing the self-standing film type positive electrode material will be described in detail.

[0074] First, in step (a), a mixture of sulfur and a porous carbon material can be formed, which is a raw material for producing a sulfur-carbon composite material. In this case, the types and suitable weight ranges of the sulfur and the porous carbon material are the same as described above.

[0075] Next, in step (b), by heat-treating the mixture formed in step (a), a sulfur-carbon composite material can be produced.

[0076] When the mixture of sulfur and the porous carbon material is heated, the sulfur becomes liquid, and the liquid sulfur enters the inside of the porous carbon material or is coated or adhered on the surface thereof, so that a sulfur-carbon composite material in which sulfur is supported or filled and / or coated on the porous carbon material can be formed. For example, if the porous carbon material is a carbon nanotube, the liquid sulfur can be sucked into the carbon nanotube by capillary action, and the sulfur can be supported in the carbon nanotube.

[0077] The heat treatment can be performed at a temperature above the melting point of sulfur. For example, the heat treatment temperature can be 130°C or higher, 140°C or higher, or 150°C or higher and 160°C or lower, 165°C or lower, or 170°C or lower. If the heat treatment temperature is lower than 130°C, it can be difficult to form a composite material supported or coated on the porous carbon material because the sulfur does not melt. If the heat treatment temperature is higher than 170°C, a sulfur-carbon composite material can be produced, but volatilization of sulfur can occur, which can cause loss of sulfur and deterioration of the production equipment.

[0078] Further, as long as the heat treatment time is an appropriate time sufficient to melt the sulfur by heat treatment and support it on the porous carbon material, the heat treatment time is acceptable, and the heat treatment time can be 25 minutes or more or 30 minutes or more and 40 minutes or less, 45 minutes or less, or 50 minutes or less.

[0079] Next, in step (c), the sulfur-carbon composite material formed in step (b) can be packed in a container, and then pressurized, to produce a positive electrode material in a self-standing film form.

[0080] As described above, the sulfur-carbon composite material has a characteristic of exhibiting strong self-cohesion in a pressurized state. Specifically, the sulfur on the surface of the sulfur-carbon composite material partially melts in a pressurized state to provide connectivity between the composite materials, thereby exhibiting strong self-cohesion. Therefore, when a pressure is applied to the sulfur-carbon composite material in a particulate state, cohesion is generated between the particles, and further, the carbon material serves as a skeleton and has flexibility as such, thereby allowing a self-standing film to be formed.

[0081] The pressure during the pressurization can be a pressure sufficient to form a self-standing film by sufficiently generating cohesion between the sulfur-carbon composite. For example, the pressure during the pressurization can be 0.8 MPa or more, 0.9 MPa or more, or 1 MPa or more and 5 MPa or less, 8 MPa or less, 10 MPa or less, 13 MPa or less, or 15 MPa or less. If the pressure during the pressurization is less than 0.8 MPa, a self-standing film can not be formed due to weak cohesion between the sulfur-carbon composite. If the pressure during the pressurization exceeds 15 MPa, the porosity of the positive electrode material is too low, and the structure of a positive electrode including the positive electrode material can collapse.

[0082] Further, the present application provides a lithium secondary battery including the positive electrode for a lithium secondary battery.

[0083] The lithium secondary battery according to the present application includes: a positive electrode; a negative electrode; and an electrolyte between the positive electrode and the negative electrode, wherein the positive electrode includes the positive electrode for a lithium secondary battery according to the present application.

[0084] The positive electrode is as described above, and is characterized by including a self-standing film type positive electrode material in a positive electrode active material layer, and the positive electrode active material layer is joined to a positive electrode current collector through an adhesive layer.

[0085] In particular, the positive electrode of the present application includes a self-standing film type positive electrode material prepared by a dry process that does not require an adhesive or a conductive material in a positive electrode active material layer, thereby being capable of loading a greater amount of sulfur than a conventional electrode. Accordingly, in the present application, the loading amount of sulfur in the positive electrode (i.e., the mass of sulfur per unit area of the positive electrode active material layer in the positive electrode) can be 3.0 to 5.0 mAh / cm 2 Therefore, since the positive electrode has a high sulfur loading amount, the lithium secondary battery including the positive electrode according to the present application can exhibit excellent discharge capacity and life characteristics.

[0086] The negative electrode can be a negative electrode in which a negative electrode active material layer including a negative electrode active material is formed on at least one surface of a negative electrode current collector, or can be a separate negative electrode active material layer (e.g., a lithium metal plate, a lithium metal thin film, a lithium foil).

[0087] The negative electrode current collector serves to support the negative electrode active material layer, and is the same as described with respect to the positive electrode current collector.

[0088] The negative electrode active material layer includes a negative electrode active material, and can further include a conductive material, an adhesive, etc.

[0089] The negative electrode active material can include: a material capable of reversibly intercalating or deintercalating lithium (Li + ); a material capable of reacting with lithium ions to reversibly form a lithium-containing compound; lithium metal; or a lithium alloy.

[0090] materials capable of reversibly intercalating or deintercalating lithium ions (Li + ) can be, for example, crystalline carbon, amorphous carbon, or a mixture thereof. Materials capable of reacting with lithium ions (Li + ) to reversibly form lithium-containing compounds can be, for example, tin oxide, titanium nitrate, or silicon. Lithium alloys can be, for example, lithium (Li) and an alloy of 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).

[0091] Preferably, the negative electrode active material can be lithium metal, in particular, can be in the form of a lithium metal thin film or lithium metal powder.

[0092] The method of forming the negative electrode active material layer is not particularly limited, and a method of forming a layer or film commonly used in the art can be used. For example, a method such as compression, coating, and deposition can be used. In addition, the negative electrode of the present application also includes a case where a thin film of metal lithium is formed on a metal plate by initial charging after assembling the battery in the case where there is no lithium thin film in the current collector.

[0093] The conductive material is a material that electrically connects the negative electrode active material and the electrolyte to serve as a path through which electrons provided by the current collector move to the negative electrode active material, and can be used without limitation as long as it has electrical conductivity.

[0094] For example, as the conductive material, the following substances can be used alone or in combination: graphite, such as natural graphite or artificial graphite; carbon black, such as Super-P, deo black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black; carbon derivatives, such as carbon nanotubes and fullerenes; conductive fibers, such as carbon fibers and metal fibers; fluorinated carbon; metal powders, such as aluminum powder and nickel powder; or conductive polymers, such as polyaniline, polythiophene, polyacetylene, and polypyrrole.

[0095] The binder is the same as described for the binder layer.

[0096] The electrolyte contains lithium ions, and functions to cause electrochemical oxidation or reduction reactions between the positive electrode and the negative electrode by lithium ions.

[0097] As the electrolyte, all electrolytes commonly used in lithium secondary batteries can be used.

[0098] For example, in the electrolyte, lithium salts capable of being contained as electrolytes can be used without limitation, as long as they are lithium salts commonly used in electrolytes for lithium secondary batteries, for example, the anion of the lithium salt can be any one selected from the group consisting of F - , Cl - , Br- - - - - - - - - - - - - - - - - - - - - - - - - Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2 (lithium bis(perfluoroethylsulfonyl)imide, BETI), LiN(CF3SO2)2 (lithium bis(trifluoromethylsulfonyl)imide, LiTFSI), LiN(C a 2a+1 b 2b+1 (wherein a and b are natural numbers, preferably 1≤a≤20 and 1≤b≤20), lithium poly[4,4'-(hexafluoroisopropylidene)diphenyloxy]sulfonylimide (LiPHFIPSI), LiCl, LiI, LiB(C2O4)2, LiNO3, and the like. Among them, lithium sulfonimide compounds containing a sulfonimide group, such as LiTFSI, BETI, or LiPHFIPSI, can be more preferable.

[0099] ​​​​​​​​​​​​​​​​​​​​​​​​​​​In the electrolyte used in the present application, as the organic solvent contained in the electrolyte, an organic solvent commonly used in electrolytes for lithium secondary batteries can be used without any limitation. Generally, the organic solvent can be any one or a mixture of two or more selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate, tetraethylene glycol dimethyl ether (TEGDME), dioxolane (DOL), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite, and tetrahydrofuran. In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates among the carbonate-based organic solvents, are high-viscosity organic solvents, and can be preferably used because the dielectric constant of the cyclic carbonates is high, whereby lithium salts are well dissociated in the electrolyte. When a linear carbonate having a low viscosity and a low dielectric constant, such as dimethyl carbonate and diethyl carbonate, is mixed with these cyclic carbonates in an appropriate ratio, an electrolyte having a high conductivity can be prepared, which can be more preferably used.

[0100] In addition to the above-described lithium salt and organic solvent, the electrolyte can further contain a nitric acid-based or nitrous acid-based compound as an additive.

[0101] In the present application, the nitric acid-based or nitrous acid-based compound is not particularly limited, but can be at least one selected from the group consisting of inorganic nitric acid or nitrous acid-based compounds such as lithium nitrate (LiNO3), potassium nitrate (KNO3), cesium nitrate (CsNO3), barium nitrate (Ba(NO3)2), ammonium nitrate (NH4NO3), lithium nitrite (LiNO2), potassium nitrite (KNO2), cesium nitrite (CsNO2), and ammonium nitrite (NH4NO2); organic nitric acid or nitrous acid-based compounds such as methyl nitrate, dialkylimidazole guanidine nitrate, imidazole nitrate pyridine nitrate ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite, and octyl nitrite; organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, dinitrotoluene, and combinations of the above materials, preferably lithium nitrate.

[0102] A separator can be additionally included between the positive electrode and the negative electrode.

[0103] The separator can be made of a porous, non-conductive, or insulating material that separates or insulates the positive and negative electrodes from each other and allows lithium ions to transport between the positive and negative electrodes. The separator can be used without particular limitation, as long as it is used as a separator in a conventional lithium-ion secondary battery. The separator can be a separate component such as a membrane, or it can be a coating added to the positive and / or negative electrodes.

[0104] Preferably, the diaphragm exhibits excellent wettability of the electrolyte while having low resistance to ion migration.

[0105] The membrane can be made of a porous substrate, and can be used as long as the porous substrate is the same as that commonly used in secondary batteries. Porous polymer membranes can be used alone or by laminating them, for example, nonwoven fabrics or polyolefin porous membranes made of high-melting-point glass fibers, polyethylene terephthalate fibers, etc., but are not limited to these.

[0106] In this invention, the material of the porous substrate is not particularly limited, and any material can be used, as long as it is a porous substrate commonly used in electrochemical devices. For example, the porous substrate may include at least one material selected from the following: polyolefins (such as polyethylene and polypropylene), polyesters (such as polyethylene terephthalate and polybutylene terephthalate), polyamides, polyacetals, polycarbonates, polyimides, polyetheretherketones, polyethersulfones, polyphenylene ethers, polyphenylene sulfides, polyethylene naphthalate, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, cellulose, nylon, poly(p-phenylenebenzobismuth submersible) (azole) and polyarylate.

[0107] There is no particular limitation on the thickness of the porous substrate, but it can be from 1 μm to 100 μm, preferably from 5 μm to 50 μm. Although the thickness range of the porous substrate is not limited to the above range, if the thickness is much thinner than the lower limit mentioned above, the mechanical properties will deteriorate, and the separator may be easily damaged during battery use.

[0108] There are no particular limitations on the average diameter and porosity of the pores present in the porous substrate, but they can be from 0.001 μm to 50 μm and from 10% to 95%, respectively.

[0109] The shape of the lithium secondary battery described above is not particularly limited. For example, it can be a rolled type, a stacked type, a stacked-folded type (including a stacked-Z-folded type) or a laminated-stacked type, preferably a stacked-folded type.

[0110] The negative electrode, separator, and positive electrode are stacked sequentially as described above, and an electrolyte is injected to prepare an electrode assembly. The electrode assembly is then placed in a battery case and sealed with a cover plate and gasket to manufacture a lithium secondary battery.

[0111] In this case, the lithium secondary battery can be classified into various batteries such as a lithium-sulfur secondary battery, a lithium-air battery, a lithium oxide battery, and a lithium all-solid-state battery according to the material of the used positive electrode / negative electrode, can be classified into a cylindrical, a rectangular, a coin-shaped, a pouch type according to the type, and can be classified into a block type and a thin film type according to the size. The structure and the preparation method of these batteries are well known in the art, and thus a detailed description thereof is omitted.

[0112] In the present application, since the lithium secondary battery includes a self-standing film type positive electrode material including a sulfur-carbon composite material as a positive electrode, the lithium secondary battery can be a lithium-sulfur secondary battery. The lithium-sulfur secondary battery can include lithium metal as a negative electrode active material. During discharge of the lithium-sulfur secondary battery, oxidation reaction of lithium occurs at the negative electrode, and reduction reaction of sulfur occurs at the positive electrode. At this time, the reduced sulfur is converted into lithium polysulfide by combining with lithium ions removed from the negative electrode, and finally accompanies the reaction of forming lithium sulfide.

[0113] Further, the present application provides a battery module including the lithium secondary battery as a unit cell.

[0114] The battery module can be used as a power source for a medium-large sized device requiring high temperature stability, long cycle characteristics, high capacity characteristics, etc.

[0115] Examples of the medium-large sized device can include an electric tool powered by a motor driven by a battery, an electric vehicle including an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), etc., an electric two-wheeled vehicle including an electric bicycle (E-bike) and an electric scooter (E-scooter), an electric golf cart, and a power storage system, but are not limited thereto.

[0116] Preferred Embodiment

[0117] Hereinafter, preferred embodiments of the present application will be described to help understanding of the present application. However, it will be apparent to those skilled in the art that the following embodiments are used to illustrate the present application, and various modifications and variations can be made within the scope and spirit of the present application, and such variations and modifications are naturally within the scope of the appended claims.

[0118] Preparation Example 1: Preparation of a sulfur-carbon composite material

[0119] Sulfur (S) and carbon nanotube (CNT) were uniformly mixed in a weight ratio of 65:35 in a solid state, and then ball-milled at 100 rpm for 1 hour to prepare a mixture.

[0120] The mixture was heat-treated at 155°C for 35 minutes to carry the sulfur into the pores of the CNT and coat on the surface thereof to prepare a sulfur-carbon composite (S-CNT). In this case, CNT having a specific surface area of 350 m 2 / g was used.

[0121] Examples and Comparative Examples

[0122] [Example 1]

[0123] (1) Production of a positive electrode for a lithium secondary battery

[0124] The sulfur-carbon composite (S-CNT) of Preparation Example 1 was charged into a mold, and then a hydraulic press was used to pressurize at a pressure of 1 MPa to prepare a self-standing film-type positive electrode material.

[0125] Polyacrylic acid lithium was coated on an aluminum current collector having a thickness of 20 μm at a coating amount of 0.5 μg / cm 2

[0126] The self-standing film-type positive electrode material was adhered to the aluminum current collector on which the binder layer was formed to prepare a positive electrode for a lithium secondary battery.

[0127] (2) Production of a lithium secondary battery

[0128] The positive electrode for a lithium secondary battery and the negative electrode prepared in the above (1) were placed to face each other, and a porous polyethylene separator having a thickness of 20 μm and a porosity of 45% was interposed therebetween to prepare an electrode assembly, the electrode assembly was put into a case, and 70 μl of an electrolyte was injected into the case to prepare a lithium secondary battery.

[0129] At this time, a lithium metal thin film having a thickness of 150 μm was used as the negative electrode, and a mixed solution in which 1 M of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 0.1 M of lithium nitrate (LiNO3) were dissolved in an organic solvent in which tetraethylene glycol dimethyl ether (TEGDME) / dioxolane (DOL) / dimethoxyethane (DME) were mixed at a volume ratio of 1:1:1 was used as the electrolyte.

[0130] [Example 2]

[0131] A positive electrode for a lithium secondary battery was produced in the same manner as in Example 1, except that the coating amount of polyacrylic acid lithium per unit area was 5 μg / cm 2

[0132] [Example 3]

[0133] A positive electrode for a lithium secondary battery was produced in the same manner as in Example 1, except that the coating amount of polyacrylic acid lithium per unit area was 10 μg / cm 2 ​​A positive electrode for a lithium secondary battery was produced in the same manner as in Example 1 except that the coating amount per unit area of the lithium polyacrylate was 0.05 μg / cm

[0134] [Example 4]

[0135] A positive electrode for a lithium secondary battery was produced in the same manner as in Example 1 except that the coating amount per unit area of the lithium polyacrylate was 30 μg / cm 2 A positive electrode for a lithium secondary battery was produced in the same manner as in Example 1 except that the coating amount per unit area of the lithium polyacrylate was 0.05 μg / cm

[0136] [Example 5]

[0137] A positive electrode for a lithium secondary battery was produced in the same manner as in Example 1 except that the coating amount per unit area of the lithium polyacrylate was 30 μg / cm 2 A positive electrode for a lithium secondary battery was produced in the same manner as in Example 1 except that the coating amount per unit area of the lithium polyacrylate was 0.05 μg / cm

[0138] [Comparative Example 1]

[0139] A positive electrode for a lithium secondary battery was produced in the same manner as in Example 1 except that the sulfur-carbon composite material (S-CNT) of Preparation Example 1, a conductive material, and a binder were mixed in a weight ratio of 90:5:5 to prepare a slurry, the slurry was then coated on an aluminum current collector having a thickness of 20 μm, and dried and calendered to produce the positive electrode.

[0140] At this time, vapor-grown carbon fiber (VGCF) was used as the conductive material, and styrene butadiene rubber (SBR) was used as the binder.

[0141] [Comparative Example 2]

[0142] A positive electrode for a lithium secondary battery was produced in the same manner as in Example 1 except that the self-standing film-type positive electrode material produced in Example 1 was used as the positive electrode.

[0143] Experimental Example 1: Evaluation of Physical Properties of Positive Electrode

[0144] For the positive electrodes produced in the examples, the adhesion of the binder layer was measured.

[0145] Specifically, the adhesion was measured using an adhesion measuring device (AMETEK, LS1), and the results obtained are shown in Table 1.

[0146] Table 1:

[0147]

[0148] As shown in Table 1 above, it was confirmed that in the cases of Examples 1 to 3, the adhesion of the binder layer increased as the coating amount per unit area of the binder layer increased.

[0149] Further, it can be seen from Examples 4 and 5 that if the coating amount per unit area of the adhesive layer exceeds a certain range, the adhesion between the self-supporting film-type positive electrode material as the positive electrode active material layer and the positive electrode current collector decreases.

[0150] Experimental Example 2: Evaluation of battery performance

[0151] For the batteries manufactured in Examples 1 to 3 and Comparative Examples 1 and 2, the capacity (mAh) was measured by discharging at 0.1C and charging at 0.1C at 25°C, and the capacity and charge efficiency were measured by repeating the charge / discharge, and the results are shown in Table 2 and Figure 1 and 2 .

[0152] Table 2:

[0153]

[0154] Referring to Table 2 and Figure 1 , it can be seen that Examples 1 and 2 containing the self-supporting film-type positive electrode material prepared by the dry process and having a small amount of the adhesive layer show higher initial discharge capacity compared to Comparative Example 1 prepared using a slurry as a wet process, and also have excellent life characteristics.

[0155] Further, it can be seen from Table 2 and Figure 2 that as the coating amount per unit area of the adhesive layer increases, the high-rate deterioration accelerates.

Claims

1. A positive electrode for a lithium secondary battery, the positive electrode for the lithium secondary battery comprising: Positive current collector; An adhesive layer, said adhesive layer being sequentially formed on at least one surface of the positive current collector; and Positive electrode active material layer, The adhesive layer has a coating weight of 0.5 μg / cm² per unit area. 2 Up to 5 μg / cm 2 , in, The positive electrode active material layer is a self-standing membrane type positive electrode material containing sulfur and porous carbon materials.

2. The positive electrode for a lithium secondary battery according to claim 1, wherein, The adhesive layer comprises at least one selected from the following: styrene-butadiene rubber, polytetrafluoroethylene, carboxymethyl cellulose, polyacrylic acid, lithium polyacrylate, and polyvinylidene fluoride.

3. The positive electrode for a lithium secondary battery according to claim 1, wherein, The adhesive layer has a thickness of 0.1 μm to 20 μm.

4. The positive electrode for a lithium secondary battery according to claim 1, wherein, The self-standing membrane cathode material is composed of 50% to 80% sulfur and 20% to 50% porous carbon material.

5. The positive electrode for a lithium secondary battery according to claim 1, wherein, The self-standing membrane cathode material has an internal adhesion force of more than 10 gf / cm.

6. The positive electrode for a lithium secondary battery according to claim 1, wherein, The self-standing membrane cathode material has a porosity of less than 68%.

7. A method for manufacturing a positive electrode for a lithium secondary battery, the method comprising the following steps: (1) Forming an adhesive layer on the positive current collector; and (2) Adhere the self-standing film-type positive electrode material onto the adhesive layer to form a positive electrode active material layer. The positive electrode active material layer is a self-standing film type positive electrode material containing sulfur and porous carbon materials. The adhesive layer is applied at a rate of 0.5 μg / cm² per unit area. 2 Up to 5 μg / cm 2 .

8. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 7 further includes the step of preparing the self-standing film-type positive electrode material by the following preparation method. The preparation method includes the following steps: (a) Mixing sulfur with porous carbon materials; (b) Heat-treating the mixture prepared in step (a) to prepare a sulfur-carbon composite material; and (c) The sulfur-carbon composite material prepared in step (b) is placed into a container and pressurized.

9. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 8, wherein, The heat treatment is carried out at a temperature of 130°C to 170°C.

10. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 8, wherein, The pressurization is carried out under pressure conditions ranging from 0.8 MPa to 15 MPa.

11. A lithium secondary battery, the lithium secondary battery comprising: The positive electrode for a lithium secondary battery as described in claim 1; A negative electrode, wherein the negative electrode comprises lithium metal or a lithium alloy; A diaphragm, the diaphragm being located between the positive electrode and the negative electrode; and An electrolyte, which is permeated in the positive electrode, the negative electrode and the membrane.

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

13. The lithium secondary battery according to claim 11, wherein, The positive electrode has a capacity of 3.0 mAh / cm³. 2 Up to 5.0 mAh / cm 2 Sulfur loading.

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