KR102126706B1 - The acrylic binder for the manufacturing of cathode of lithium sulfur secondary battery - Google Patents

Abstract

The present application relates to a binder for a lithium-sulfur secondary battery positive electrode, and a composition comprising the same.
The binder of the present application makes it possible to have excellent resistance to the electrolyte of the positive electrode active material.

Description

The acrylic binder for the manufacturing of cathode of lithium sulfur secondary battery}

The present application relates to an acrylic binder for a lithium-sulfur secondary battery positive electrode, a composition thereof, a lithium-sulfur secondary battery positive electrode, and uses thereof.

As the application area of secondary batteries is expanded to electric vehicles (EVs) or energy storage devices (ESSs), lithium-ion secondary batteries are facing a limit with relatively low weight ratio energy storage density (~250Wh/kg). .

Among the next-generation secondary battery technologies capable of realizing high energy density, lithium-sulfur secondary batteries are in the spotlight because of their high commercialization potential compared to other technologies.

The lithium-sulfur secondary battery refers to a battery system using sulfur as a positive electrode active material and lithium metal as a negative electrode active material.

When discharging a lithium-sulfur secondary battery, the sulfur of the positive electrode is reduced by accepting electrons, and the lithium of the negative electrode is oxidized and ionized. The reduction reaction of sulfur is a process in which the sulfur-sulfur (SS) bond accepts two electrons and converts it into a sulfur anion form. At this time, lithium ions formed by oxidation are transferred to the anode through the electrolyte to form ionized sulfur and salt. .

Before discharge, sulfur has a cyclic S ₈ structure, and is converted to lithium polysulfide (LiS _x ) by a reduction reaction. The lithium polysulfide (LiS _x ) is reduced stepwise, and finally lithium sulfide (Li ₂ S) Becomes

The theoretical energy density through this electrochemical reaction is 2,500 Wh/kg, which is 10 times that of lithium ion batteries.

Despite the advantages of the lithium-sulfur secondary battery, there are many problems such as high solubility of lithium polysulfide, low life and output characteristics, low electrical conductivity of sulfur, and poor stability due to the use of lithium metal.

In one example, the lithium polysulfide (LiS _x ) is easily dissolved in an electrolyte, and the loss of active sulfur due to repeated charging and discharging and thus a decrease in cycle characteristics are regarded as the greatest challenge to be solved in a lithium-sulfur secondary battery. .

In order to solve the above problem, after the electrode is made of a porous body, sulfur is supported between the porous bodies to inhibit the solubility of the electrolyte, a technique of putting a substance capable of adsorbing polysulfide into the electrode or polysulfide Techniques using hydrophilic properties of have been proposed.

However, while still effectively preventing the elution of the target lithium polysulfide (LiS _x ), there is a need for continuous research on a lithium-sulfur secondary battery having excellent electrochemical performance.

Republic of Korea Patent Publication 2015-0093874

The present application effectively prevents the elution of the positive electrode active material, and provides an acrylic binder for a lithium-sulfur secondary battery positive electrode having excellent cycle characteristics.

In addition, the present application provides a composition that maintains the secondary structure of the conductive material and forms an active layer of a positive electrode for a lithium-sulfur secondary battery having excellent electrochemical performance.

Moreover, the present application provides a positive electrode for a lithium-sulfur secondary battery having an active layer containing such an acrylic binder, and a secondary battery comprising the same.

The present application relates to a binder for a lithium-sulfur secondary battery positive electrode, and a composition comprising the same.

The binder for a lithium-sulfur secondary battery positive electrode according to the present application includes a polymerization unit of a monomer that interacts with a positive electrode active material in a binder to form a positive electrode active material, specifically, lithium polysulfide (LiS _x ) formed by reduction of sulfur of the positive electrode. Elution to the electrolytic solution can be effectively prevented.

The term "acrylic binder" in the present application means a polymer that contains at least 30% by weight or more of a polymerized unit of an acrylic monomer and serves as a binder for a secondary battery. In the above, the acrylic monomer means acrylic acid, methacrylic acid or a derivative thereof.

That is, the acrylic binder of the present application is included in the active layer of the lithium-sulfur secondary battery positive electrode, and serves to bind a positive electrode active material, a conductive material, and other materials included in the active layer.

The acrylic binder includes a polymerization unit of a polymerizable monomer having a polar functional group that interacts with the positive electrode active material.

The term "polymerized unit of a predetermined compound" in the present application may mean a state in which the predetermined compound is polymerized on a skeleton such as a side chain or a main chain of a polymer formed by polymerizing the predetermined compound.

It can be understood that the interaction between the polar functional group and the positive electrode active material in the present application includes all known physical or chemical interactions capable of preventing dissolution of lithium polysulfide (LiS _x ).

In one example, the interaction between the positive electrode active material and the polar functional group may be, but is not limited to, the interaction between the polar functional group and the sulfur element, that is, a dipole-dipole moment.

Through the interaction between the positive electrode active material of the lithium-sulfur secondary battery and the polar functional group in the acrylic binder, it is possible to effectively prevent the dissolution of lithium polysulfide (LiS _x ) formed by reduction of sulfur of the positive electrode into the electrolyte.

The polar functional group may be used without limitation in the present application as long as it can achieve the above-described object, for example, may be at least one selected from nitrogen-containing functional groups, alkylene oxide groups, hydroxy groups and alkoxy silyl groups. .

That is, the polar functional group according to the present application may be any one selected from the group consisting of nitrogen-containing functional groups, alkylene oxide groups, hydroxy groups, and alkoxy silyl groups.

The term "nitrogen-containing functional group" in the present application is a functional group containing nitrogen in a molecule, and examples thereof include an amine group, imine group, amide group, nitrile group, nitro group, azo group, imide group or azide group. However, it is not limited thereto.

In one example, the polymerizable monomer having the polar functional group is (meth)acrylonitrile, (meth)acrylamide, N-methylacrylamide, N,N-dimethyl (meth)acrylamide, N-butoxy methyl (Meth)acrylamide, N-vinyl pyrrolidone, N-vinyl caprolactam, 2-aminoethyl (meth)acrylate, 3-aminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylic Polymerizable monomers having nitrogen-containing functional groups such as rate or N,N-dimethylaminopropyl (meth)acrylate; Polymerizable monomers having an alkylene oxide group such as alkoxy alkylene glycol (meth)acrylic acid ester, alkoxy dialkylene glycol (meth)acrylic acid ester, or alkoxy polyethylene glycol (meth)acrylic acid ester; 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate or 8-hydroxyoxyl ( Hydroxyalkyl (meth)acrylates such as meth)acrylates, or hydroxypolyalkylene glycol (meth)acrylates such as hydroxy polyethylene glycol (meth)acrylates or hydroxypolypropylene glycol (meth)acrylates, etc. Polymerizable monomers having the same hydroxy group; Alternatively, a polymerizable monomer having an alkoxy silyl group such as 3-(trimethoxysilyl)propyl (meth)acrylate or 3-(triethoxysilyl)propyl (meth)acrylate may be exemplified, but is not limited thereto. . In the above, (meth)acrylate may mean acrylate or methacrylate.

The polymerizable monomer having such a polar functional group may be contained in the proportion of 30 to 100 parts by weight of polymerized units in the binder. In another example, the polymerizable monomer having the polar functional group is included in the binder in a proportion of 40 to 100 parts by weight, 50 to 100 parts by weight, 60 to 100 parts by weight, 70 to 100 parts by weight, or 80 to 100 parts by weight It can be.

In the present application, the term "part by weight" may mean a weight ratio between each component, unless otherwise specified.

The polar functional group in the acrylic binder of the present application interacts with the positive electrode active material of the lithium-sulfur secondary battery. As described above, the above interaction is meant to include all known physical or chemical interactions, and specifically, may be an interaction between a polar functional group and a sulfur element.

The positive electrode active material is typically included in the active layer of the positive electrode of a lithium-sulfur secondary battery, and may have, for example, a compound containing a sulfur element. The compound containing the sulfur element may be, for example, a compound containing eight sulfur atoms having a ring structure.

In a lithium-sulfur secondary battery, the compound containing the sulfur element has an elution characteristic into an electrolyte solution according to a repetitive charge/discharge mechanism, and also may cause electrochemical problems due to low electrical conductivity. It can exist in the form of a complex with a substance that can.

In one example, the positive electrode active material may be a sulfur-carbon composite.

The sulfur-carbon composite may be formed by coating a porous carbon with a compound containing a sulfur element, or may be formed by melting the compound and mixing with carbon. At this time, the content ratio of carbon and sulfur in the sulfur-carbon composite may be, for example, a ratio of 5:95 to 50:50 based on mass, but is not limited thereto.

The carbon may be crystalline or amorphous carbon, and is not limited as long as it is conductive carbon, and may be, for example, graphite, carbon black, activated carbon fiber, inactive nanofiber, carbon nanotube, or carbon fabric.

The acrylic binder of the present application may further include a polymerized unit of an acrylic monomer, specifically an alkyl (meth)acrylate, in order to control the weight average molecular weight or glass transition temperature.

In one example, the alkyl (meth)acrylate is a (meth)acrylate having 1 to 20 carbon atoms, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (Meth)acrylate, n-butyl (meth)acrylate, t-butyl (meth)acrylate, isobutyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, 2-ethylbutyl (meth)acrylic Late, pentyl (meth)acrylate, hexyl (meth)acrylate, cyclohexyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, isononyl (meth)acrylate, decyl (Meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, octadecyl (meth)acrylate or isobornyl (meth)acrylate, etc. can be exemplified However, it is not limited thereto.

Such, alkyl (meth)acrylate, for example, may be included in the proportion of 5 to 30 parts by weight of polymerized units in the binder.

The acrylic binder according to the present application can be manufactured in various ways.

For example, the acrylic binder is a polymerizable monomer having a polar functional group that interacts with the positive electrode active material described above alone or after mixing with an alkyl (meth)acrylate in an appropriate ratio, known solution polymerization (Solution polymerization), It can be produced by applying a method such as bulk polymerization, suspension polymerization or emulsion polymerization.

In one example, when the acrylic binder is prepared by the solution polymerization method, the binder exhibits a particle size of 10 nm or less, the adhesion to the current collector may be better, and the content of the conductive material in the composition may be increased. , It can secure the electrochemical excellence.

In one example, when preparing an acrylic binder by solution polymerization, the particle size of the acrylic binder can be adjusted to a range of 10 nm or less, and through this, an appropriate peel force to the current collector and excellent dispersibility for the conductive material Can be achieved. The particle size of the acrylic binder can be measured using, for example, dynamic light scattering (DLS) equipment.

The solvent used for solution polymerization of the acrylic binder is not particularly limited, but a solvent having a boiling point of 110° C. or less may be preferable for use as a solution without further purification after solution polymerization. Examples of such a solvent include acetone, methanol, ethanol, acetonitrile, isopropanol, methyl ethyl ketone or water.

The acrylic binder of the present application may have a glass transition temperature in the range of -80°C to 50°C. In such a glass transition temperature range, proper adhesion to a current collector is secured, and a holding ability for a conductive material, resistance to an electrolyte, and the like can be advantageously secured.

The acrylic binder of the present application may also have a weight average molecular weight in the range of 5,000 to 1,000,000. In this application, the term weight average molecular weight may mean a conversion value for standard polystyrene measured by GPC (Gel Permeation Chromatograph), and unless otherwise specified, the molecular weight of any polymer means the weight average molecular weight of the polymer Can.

The acrylic binder may further include other non-acrylic monomer polymerization units in order to achieve the aforementioned glass transition temperature or weight average molecular weight. The non-acrylic monomer means a polymerizable monomer excluding the acrylic monomer, and may be, for example, a vinyl monomer.

The present application also relates to a composition for forming an active layer of a lithium-sulfur secondary battery positive electrode comprising an acrylic binder.

The composition according to the present application effectively prevents the elution of lithium polysulfide (LiS _x ), exhibits excellent cycle characteristics, and provides an active layer of a positive electrode for a lithium-sulfur secondary battery, which is secured by electrochemical characteristics due to excellent dispersion characteristics of a conductive material in the composition. Can be used to form.

In one example, the composition for forming the active layer of the lithium-sulfur secondary battery positive electrode according to the present application includes the above-described acrylic binder, positive electrode active material, and conductive material.

As described above, the acrylic binder included in the composition in the present application includes a polymerized unit of a polymerizable monomer having a polar functional group that interacts with a positive electrode active material, for example, 0.01 to 10, based on 100 parts by weight of the solid content of the composition. It may be included in the composition in a proportion by weight. Within this range, a desired binding property can be secured, the elution phenomenon of lithium polysulfide (LiS _x ) can be effectively prevented, and dispersion properties for a conductive material can be secured.

The positive electrode active material, as described above, has a compound containing a sulfur element, and may specifically be a sulfur-carbon composite. Further, the sulfur-carbon composite may be formed by applying a compound containing a sulfur element to porous carbon, or may be formed by melting the compound and mixing with carbon. At this time, the content ratio of carbon and sulfur in the sulfur-carbon composite may be, for example, a ratio of 5:95 to 50:50 based on mass. The type of carbon may be employed without limitation, such as graphite described above.

The positive electrode active material may be included in the composition at a ratio of 30 to 95 parts by weight compared to 100 parts by weight of the solid content of the composition, but is not limited thereto, and the range may be appropriately changed in consideration of the performance of the desired battery.

The composition of the present application includes a conductive material. In the lithium-sulfur secondary battery, in order to overcome the problem due to the electrical conductivity of low sulfur, the conductive layer must be included in the active material, but when the amount of the conductive material is too large, agglomeration of the conductive material may occur due to deterioration of dispersion characteristics. It is also possible to reduce the energy density of the entire battery.

The composition of the present application, by including an acrylic binder having excellent dispersibility with respect to the conductive material, the conductive material in an amount within a range that does not degrade the energy density of the entire battery without causing the agglomeration of the conductive material Can.

In one example, the conductive material may be included in the composition in a proportion of 2 to 70 parts by weight, 10 to 70 parts by weight, 15 to 70 parts by weight, or 18 to 70 parts by weight relative to 100 parts by weight of the composition solids.

In one example, the conductive material includes graphite such as natural graphite or artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, panel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder, and nickel powder; Conductive whiskeys such as oxidized salt and potassium titanate; Conductive metal oxides such as titanium oxide; Alternatively, conductive materials such as polyaniline, polythiophene, polyacetylene, polypyrrole, or polyphenylene derivatives may be used, but are not limited thereto.

For the conductive material, for example, a particle size of 40 nm or less and a surface area of 1,000 m 2 /g or more may be used, but is not limited thereto.

The composition of the present application may further include a non-acrylic binder in addition to the aforementioned components. The non-acrylic binder may serve to attach the positive electrode active material to the current collector, thereby performing a role of additionally providing solvent resistance to the electrolyte.

In one example, the non-acrylic binder includes a fluorine resin binder including polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE); Rubber binders including styrene-butadiene rubber, acrylonitrile-butadiene rubber or styrene-isoprene rubber; Cellulose binders including carboxymethylcellulose (CMC), starch, hydroxypropylcellulose or regenerated cellulose; Poly alcohol binders; Polyolefin binders including polyethylene or polypropylene; Polyimide binders; Polyester binders; Or a silane binder, but is not limited thereto.

The non-acrylic binder, for example, may be included in the composition in a proportion of 0 to 20 parts by weight compared to 100 parts by weight of the solid content of the composition.

The composition may also further include a solvent.

The type of the solvent can be appropriately set in consideration of the desired performance, etc., for example, N-methyl-2-pyrrolidone, methanol, ethanol, propylene carbonate, ethylene carbonate, butylene carbonate, Dimethyl carbonate, diethyl carbonate, gamma-butylo lactone, 1,2-dimethoxy ethane, tetrahydrofuran, 2-methyl tetrahydrofuran, dimethylsulfoxide, formamide, dimethylformamide, acetonitrile, Nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxy methane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, propionic acid Organic solvents such as methyl or ethyl propionate or water can be used, but water is preferred in consideration of drying temperature and environmental effects.

The proportion of the solvent contained in the composition may be appropriately selected in consideration of desired coating properties and the like.

Moreover, the composition for forming an active layer of a lithium-sulfur secondary battery positive electrode may further include various known additives.

In one example, the additive may be at least one selected from transition metal elements, group IIIA elements, group IVA elements, sulfur compounds of these elements, and alloys of sulfur of these elements.

As the transition metal element, for example, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rg, Pd, Os, Ir, Pt , Au, Hg, or the like, and the group IIIA element includes, for example, Al, Ga, In, or Ti, and the group of IVA elements may include Ge, Sn, or Pb.

The present application also relates to a positive electrode for a lithium-sulfur secondary battery.

The positive electrode of the present application includes a current collector and an active layer. The active layer is formed using the composition for forming the active layer described above, and includes an acrylic binder.

That is, the positive electrode for a lithium-sulfur secondary battery of the present application includes a current collector; And an active layer formed on the current collector and including an acrylic binder. In addition, the acrylic binder includes a polymerization unit of a polymerizable monomer having a polar functional group that interacts with the positive electrode active material.

In the present application, the current collector may be selected without particular limitation as long as it is generally used in a positive electrode for a lithium-sulfur secondary battery.

For the current collector, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum may be used, and if necessary, surface treatment using carbon, nickel, titanium, or silver may be applied to the surface of the stainless steel or the like. It may have been performed.

If necessary, fine irregularities or the like may be formed on the surface of the current collector, and such irregularities may help improve adhesion to the active layer. When the surface of the current collector is roughened, the method is not particularly limited, and for example, a known method such as a mechanical polishing method, an electropolishing method or a chemical polishing method can be applied.

The current collector may have various forms, such as a film, sheet, foil, net, porous body, foam, or nonwoven fabric.

The thickness of the current collector is not particularly limited, and may be set to an appropriate range in consideration of the mechanical strength of the positive electrode, productivity, and capacity of the battery.

The positive electrode for a lithium-sulfur secondary battery has an active layer formed on the current collector. The active layer can be formed using the above-described composition.

In one example, the active layer is coated with a composition for forming an active layer containing an acrylic binder, a positive electrode active material, a conductive material, and other additives by using a known coating method on a current collector, followed by a drying process, etc. It may be formed on the current collector.

The coating step, for example, a bar coating method, a screen coating method, a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method or a known coating method including a extrusion method may be applied without limitation.

The coating amount on the current collector of the active layer composition of the present application is also not particularly limited, and may be adjusted, for example, in a range in which an active layer having a desired thickness is finally formed.

In addition, a known process required for the production of the positive electrode before or after the process of forming the active layer, for example, a rolling or drying process, may be performed as necessary.

The thickness of the active layer may be, for example, in the range of 1 to 200 μm, 20 to 200 μm, or 30 to 200 μm, but is not limited thereto, and the thickness range may be changed in consideration of desired performance and the like. .

The present application also relates to a lithium-sulfur secondary battery comprising a positive electrode for such a lithium-sulfur secondary battery.

In detail, the lithium-sulfur secondary battery includes a negative electrode comprising a lithium metal or a lithium alloy as a negative electrode active material; An anode comprising the current collector and the active layer described above; A separator positioned between the anode and the cathode; And an electrolyte impregnated in the negative electrode, the positive electrode and the separator, and containing a lithium salt and an organic solvent.

The negative active material is a lithium alloy is a metal alloy selected from the group consisting of lithium and Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al and Sn, but is not limited thereto. .

The separator positioned between the positive electrode and the negative electrode separates or insulates the positive electrode and the negative electrode from each other, and enables lithium ion transport between the positive electrode and the negative electrode, and may be made of a porous non-conductive or insulating material. The separator may be an independent member such as a film, or may be a coating layer added to the anode and/or cathode.

The material of the separator includes, for example, polyolefin such as polyethylene and polypropylene, glass fiber filter paper, and ceramic materials, but is not limited thereto, and the thickness is about 5 to about 50 μm, specifically about 5 to about 25 μm. Can be

The electrolyte impregnated in the negative electrode, positive electrode and separator includes a lithium salt and an organic solvent.

The concentration of the lithium salt is from about 0.2 to 2.0, 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 conditions for charging and discharging the cell, the working temperature and other factors known in the lithium battery art. It may be M. Examples of lithium salts for use in the present application include LiSCN, LiBr, LiI, LiPF ₆ , LiBF ₄ , LiSO ₃ CF ₃ , LiClO ₄ , LiSO ₃ CH ₃ , LiB(Ph) ₄ , LiC(SO ₂ CF ₃ ) ₃ And LiN (SO ₂ CF ₃ ) ₂ It may include one or more selected from the group consisting of.

The organic solvent may use a single solvent or a mixed organic solvent of two or more. When two or more mixed organic solvents are used, it is preferable to select and use one or more solvents from two or more groups of the weak polar solvent group, the strong polar solvent group, and the lithium metal protective solvent group.

Weak polar solvents are defined as solvents having a dielectric constant less than 15 that can dissolve sulfur elements in aryl compounds, bicyclic ethers, and acyclic carbonates, and strong polar solvents are bicyclic carbonates, sulfoxide compounds, lactone compounds, Among the ketone compounds, ester compounds, sulfate compounds, and sulfite compounds, a dielectric constant capable of dissolving lithium polysulfide is defined as a solvent greater than 15, and the lithium metal protective solvent is a saturated ether compound, an unsaturated ether compound, N, O , S or a combination thereof, it is defined as a solvent having a charge and discharge cycle efficiency of 50% or more to form a stable Solid Electrolyte Interface (SEI) on a lithium metal such as a heterocyclic compound.

Specific examples of weak polar solvents include xylene, dimethoxyethane, 2-methyltetrahydrofuran, diethyl carbonate, dimethyl carbonate, toluene, dimethyl ether, diethyl ether, diglyme or tetraglyme.

Specific examples of strong polar solvents include hexamethyl phosphoric triamide, γ-butyrolactone, acetonitrile, ethylene carbonate, propylene carbonate, N-methylpyrrolidone, 3-methyl-2-oxazolidone , Dimethyl formamide, sulfolane, dimethyl acetamide, dimethyl sulfoxide, dimethyl sulfate, ethylene glycol diacetate, dimethyl sulfite, or ethylene glycol sulfite.

Specific examples of the lithium protective solvent include tetrahydrofuran, ethylene oxide, dioxolane, 3,5-dimethyl isoxazole, furan, 2-methyl furan, 1,4-oxane, 4-methyldioxolane, and the like.

In addition, the present application provides a battery module including the lithium-sulfur secondary battery as a unit cell.

The battery module may be specifically used as a power source for electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, or power storage devices.

The present application can provide an acrylic binder included in the active layer of the positive electrode of a lithium-sulfur secondary battery capable of effectively preventing the elution phenomenon of the positive electrode active material and ultimately securing excellent cycle characteristics, and a composition comprising the same.

In addition, the present application is excellent in dispersion characteristics, it is possible to provide a composition for forming an active layer of a lithium-sulfur secondary battery positive electrode that may include an appropriate amount of a conductive material and a positive electrode comprising an active layer formed therefrom.

Hereinafter, with reference to the embodiments of the present application, the following examples are intended to illustrate the present application, and the scope of rights of the present application is not limited by the following examples. Self-evident

The physical properties presented in the examples and comparative examples were evaluated in the following manner.

[One. Binder conversion rate measurement method]

· Analytical Instruments

-Gas chromatography (PerkinElmer)

· Analysis conditions

-Solvent: Tetrahydrofuran

-Initial temperature: 50 to 3 minutes, Ramp: 200 to 30/min

-Injection volume: 0.5 μl

· Analysis procedure

The reaction was diluted in a solvent at a concentration of 20 mg/mL and 5 mg/mL of toluene was added as a standard, and then gas chromatography was measured. The conversion is calculated by changing the ratio of the monomer peak size to the toluene peak.

Conversion rate (%) = (A _ini -A- _fin )/A _ini x 100

A _ini : area ratio relative to the toluene peak of the monomer peak at the start of the reaction

A _fin : Area ratio relative to the toluene peak of the monomer peak at the end of the reaction

[2. Molecular weight evaluation of binder]

The weight average molecular weight (Mw) and molecular weight distribution (PDI) were measured under the following conditions using GPC, and the measurement results were fantastic using standard polystyrene of the Agilent system for the production of a calibration curve.

<Measurement conditions>

Meter: Agilent GPC (Agilent 1200 series, U.S.)

Column: 2 PL Mixed B connections

Column temperature: 40℃

Eluent: Tetrahydrofuran or N,N-dimethylformaldehyde

Flow rate: 1.0 mL/min

Concentration: ~1mg/mL (100μl injection)

[3. Formation of positive electrode active layer]

A carbon-sulfur composite was obtained through a wet ball milling process of a mixture having a carbon powder:sulfur weight ratio of 10:90. The carbon-sulfur composite 75.0 mass%: Super-P (conductive material) 20.0 mass%: After preparing a slurry by adding a composition having a binder 5.0 mass% composition to water as a solvent, an aluminum current collector phase having a thickness of about 20 μm on A positive electrode having a loading amount of 2.0 mAh/cm ² was prepared by coating.

[4. Lithium-sulfur secondary battery manufacturing]

A positive electrode prepared according to the above-described method of the present application was used, and a lithium foil having a thickness of about 150 μm was used as the negative electrode, and a polyolefin membrane (Celgard® 2400) was used as a separator. Lithium-sulfur using electrolyte mixed with 1M LiN(CF ₃ SO ₂ ) ₂ ) and 0.1LiNO ₃ dissolved in TEGDME (Tetraethylene glycol dimethyl ether), DOL (1,3-dioxolane), and DME (dimethoxyethane) Manufacturing of the secondary battery was completed.

[5. Evaluation of cycle characteristics]

Equipment: 100mA class charge/discharger

Charging: 0.1C, constant current/constant voltage mode

Discharge: 0.1C, constant current mode (1.5V)

Cycle temperature: 25℃

[Resin Preparation Example 1]-Preparation of acrylic binder (A1)

Into a 250 mL round-bottom flask, 7.5 g of polyethylene oxide methyl ether methacrylate, 6.0 g of N-vinyl-2-pyrrolidone, 1.5 g of acrylonitrile, and 60 g of water were added and the inlet was sealed. . Oxygen was removed through bubbling nitrogen for 30 minutes, the reaction flask was immersed in an oil bath heated to 60° C., and 0.03 g of V-50 (Wako Chemical) was administered, and the reaction was initiated. When the conversion rate was 87% after 24 hours, the reaction was terminated to obtain an acrylic binder having a weight average molecular weight of about 300,000.

[Resin Preparation Examples 2 to 4]-Preparation of acrylic binders (A2, A3, A4)

An acrylic binder was prepared in the same manner as in Production Example 1, except that the types and contents of the monomers used during polymerization were adjusted as shown in Table 1 below.

A1 A2 A3 A4 PEOMA (parts by weight) 50 50 50 50 VP (parts by weight) 40 25 40 DMAA (parts by weight) 40 25 AN (parts by weight) 10 10 MMA (parts by weight) 10 Mw 300,000 350,000 500,000 400,000 PEOMA: Poly(ethylene oxide) methyl ether methacrylate
VP: N-vinyl-2-pyrrolidone
DMAA: N,N-dimethylacrylamide
AN: acrylonitrile
MMA: Methyl methacrylate

[Example 1]-Preparation of lithium-sulfur secondary battery

A lithium-sulfur secondary battery was manufactured using an anode having an active layer comprising an acrylic binder (A1) prepared according to Preparation Example 1. After charging/discharging was evaluated for 100 cycles between 1.5V and 2.8V at 0.1C/0.1C, the capacity retention rate was measured by calculating the remaining capacity in the second cycle and the remaining capacity in the 100th cycle compared to the initial capacity. Table 2 shows the results.

[Examples 2 to 4]-Preparation of lithium-sulfur secondary battery

Battery in the same manner as in Example 1, except that a lithium-sulfur secondary battery was prepared using an anode having an active layer comprising an acrylic binder (A2, A3, A4) prepared according to Preparation Examples 2 to 4 above. It was prepared, and the capacity retention rate was evaluated and shown in Table 2.

[Comparative Examples 1 to 2]-Preparation of lithium-sulfur secondary battery

Except for using a polyvinylidene fluoride (PVDF) binder or a 1:1 mixture of styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) as an anode binder instead of an acrylic binder (A1, A2, A3, A4) Then, a battery was prepared in the same manner as in Example 1, and capacity retention was evaluated, and the results are shown in Table 2.

Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 bookbinder A1 A2 A3 A4 PVDF SBR+CMC Capacity retention rate (%) 86 85 88 88 75 80

As shown in Table 2, in the case of the lithium-sulfur secondary battery according to the embodiment, sulfur is included in the active layer by including an acrylic binder containing a polymerization unit of a polymerizable monomer having a polar functional group that interacts with the positive electrode active material, sulfur, in the active layer. It was able to suppress the phenomenon of elution, and thus it was found that the capacity retention rate was high according to the cycle progression.

Claims (22)

It is an acrylic binder for a lithium-sulfur secondary battery positive electrode comprising a polymerization unit of a polymerizable monomer having an alkylene oxide group, which is a polar functional group that interacts with a positive electrode active material,
In addition to the alkylene oxide group, further comprising a polymerization unit of a polymerizable monomer having at least one polar functional group selected from nitrogen-containing functional groups, hydroxy groups and alkoxy silyl groups,
The polymerizable monomer having an alkylene oxide group is an alkoxy alkylene glycol (meth)acrylic acid ester, an alkoxy dialkylene glycol (meth) acrylic acid ester or an alkoxy polyethylene glycol (meth)acrylic acid ester,
Acrylic binder for a lithium-sulfur secondary battery positive electrode having a particle size of 10 nm or less. It is an acrylic binder for a lithium-sulfur secondary battery positive electrode comprising a polymerization unit of a polymerizable monomer having a polar functional group that interacts with a positive electrode active material,
The acrylic binder is a polymerization unit of a polymerizable monomer having an alkylene oxide group which is a polar functional group; And a polymerization unit of a polymerizable monomer having a nitrogen-containing functional group which is a polar functional group,
The polymerizable monomer having an alkylene oxide group is an alkoxy alkylene glycol (meth)acrylic acid ester, an alkoxy dialkylene glycol (meth) acrylic acid ester or an alkoxy polyethylene glycol (meth)acrylic acid ester,
Acrylic binder for a lithium-sulfur secondary battery positive electrode having a particle size of 10 nm or less. delete The method according to claim 1 or 2,
The polymerizable monomer having an alkylene oxide group is a poly(ethylene oxide) methyl ether methacrylate acrylic binder for a lithium-sulfur secondary battery positive electrode. delete The method according to claim 1 or 2,
The positive electrode active material is an acrylic binder for a lithium-sulfur secondary battery positive electrode having a compound containing a sulfur element. The method of claim 6,
The positive electrode active material is a sulfur-carbon composite, an acrylic binder for a lithium-sulfur secondary battery positive electrode. The method according to claim 1 or 2,
The interaction is an acrylic binder for a lithium-sulfur secondary battery positive electrode, which is an interaction between a polar functional group and a sulfur element. The method according to claim 1 or 2,
Acrylic binder for a lithium-sulfur secondary battery positive electrode further comprising a polymerization unit of an alkyl (meth)acrylate. The method of claim 9,
The polymerization unit of the polymerizable monomer having the polar functional group is included in a proportion of 30 to 100 parts by weight,
Acrylic binder for a lithium-sulfur secondary battery positive electrode comprising a polymerization unit of the alkyl (meth)acrylate in a proportion of 5 to 30 parts by weight. delete The method according to claim 1 or 2,
Acrylic binder for a lithium-sulfur secondary battery positive electrode having a glass transition temperature in the range of -80°C to 50°C. The method according to claim 1 or 2,
Acrylic binder for a lithium-sulfur secondary battery positive electrode having a weight average molecular weight in the range of 5,000 to 3,000,000. A composition for forming an active layer of a lithium-sulfur secondary battery positive electrode comprising the acrylic binder according to claim 1, a positive electrode active material, and a conductive material. The method of claim 14,
Acrylic binder is a composition for forming an active layer of a lithium-sulfur secondary battery positive electrode contained in a proportion of 0.01 to 10 parts by weight relative to 100 parts by weight of the solid content of the composition. The method of claim 15,
The positive electrode active material is a composition for forming an active layer of a lithium-sulfur secondary battery positive electrode, which is a sulfur-carbon composite. The method of claim 14,
The positive electrode active material is a composition for forming an active layer of a lithium-sulfur secondary battery positive electrode that is included in a proportion of 30 to 95 parts by weight based on 100 parts by weight of the solid content of the composition. The method of claim 14,
A composition for forming an active layer of a lithium-sulfur secondary battery positive electrode further comprising a non-acrylic binder. The method of claim 14,
The conductive material is a composition for forming an active layer of a lithium-sulfur secondary battery positive electrode contained in a proportion of 2 to 70 parts by weight compared to 100 parts by weight of solids of the composition. Current collector;
A positive electrode for a lithium-sulfur secondary battery formed on the current collector and having an active layer comprising the acrylic binder according to claim 1. The method of claim 20,
The thickness of the active layer is a positive electrode for a lithium-sulfur secondary battery in the range of 1 to 200㎛. A lithium-sulfur secondary battery comprising the positive electrode of claim 20.