KR20190120740A - The acrylic binder for the manufacturing of cathode of lithium sulfur secondary battery - Google Patents

Abstract

The present invention relates to a binder for a lithium-sulfur secondary battery positive electrode, and a composition comprising the same. The binder of the present invention can have excellent resistance to an electrolyte of the positive electrode active material. The binder comprises a polymerized unit of a polymerizable monomer having an alkylene oxide group which is a polar functional group interacting with 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 a use thereof.

As the application area of secondary batteries is extended to electric vehicles (EVs) and energy storage devices (ESSs), lithium-ion secondary batteries are facing limitations with relatively low weight-to-weight energy storage densities (~ 250 Wh / kg). .

Among the next-generation secondary battery technologies capable of realizing a high energy density, lithium-sulfur secondary batteries are spotlighted for 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 using lithium metal as a negative electrode active material.

In the lithium-sulfur secondary battery, upon discharge, sulfur of the positive electrode receives electrons and is reduced, and lithium of the negative electrode is oxidized and ionized. The reduction of sulfur is a process in which a sulfur-sulfur (SS) bond accepts two electrons and converts them into a sulfur anion form, whereby the oxidized lithium ions are transferred to the anode through an electrolyte to form ionized sulfur and salts. .

The sulfur before discharge has a cyclic S ₈ structure, and is converted to lithium poly sulfide (LiS _x ) by a reduction reaction. The lithium poly sulfide (LiS _x ) is reduced in stages, 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 a lithium ion battery.

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

In one example, the lithium polysulfide (LiS _x ) is easily dissolved in the electrolyte, the loss of active sulfur due to repeated charging and discharging and the degradation of the cycle characteristics is considered as the biggest challenge to be solved in the lithium-sulfur secondary battery. .

In order to solve the above problem, the electrode is made of a porous body, and then sulfur is supported between the porous bodies to inhibit the solubility of the electrolyte, a technique of introducing a substance capable of adsorbing polysulfide to the electrode, or polysulfide The technique using the hydrophilic characteristic of is proposed.

However, there is still a need for continuous research on a lithium-sulfur secondary battery which effectively prevents elution of a desired lithium polysulfide (LiS _x ) and has excellent electrochemical performance.

Republic of Korea Patent Publication 2015-0093874

The present application effectively prevents dissolution 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 maintains the secondary structure of the conductive material, and provides a composition for forming an active layer of the positive electrode for a lithium-sulfur secondary battery having excellent electrochemical performance.

Furthermore, the present application provides a positive electrode for a lithium-sulfur secondary battery having an active layer including such an acrylic binder and a secondary battery including 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 polymer unit of a monomer that interacts with a positive electrode active material in the binder, and thus, a lithium polysulfide (LiS _x ) formed by reducing sulfur of the positive electrode active material, specifically, the positive electrode. Elution to electrolyte solution can be prevented effectively.

The term "acrylic binder" of the present application means a polymer containing at least 30% by weight or more of polymerized units of an acrylic monomer and serving as a binder of a secondary battery. In the above, the acrylic monomer means acrylic acid, methacrylic acid or derivatives 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 material included in the positive electrode active material, the conductive material, and other active layers.

The acrylic binder includes polymerized units of a polymerizable monomer having a polar functional group interacting with the positive electrode active material.

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

In the present application, the interaction between the polar functional group and the positive electrode active material may be understood to include all known physical or chemical interactions capable of preventing the dissolution of lithium poly sulfide (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, an interaction between the polar functional group and the sulfur element, that is, 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 dissolution of lithium poly sulfide (LiS _x ) formed by reducing 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 object, for example, may be one or more selected from nitrogen-containing functional group, alkylene oxide group, hydroxy group and alkoxy silyl group. .

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

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

In one example, the polymerizable monomer having a polar functional group may be (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 acrylate or N, N-dimethylaminopropyl (meth) acrylate; Polymerizable monomers having alkylene oxide groups such as alkoxy alkylene glycol (meth) acrylic acid esters, alkoxy dialkylene glycol (meth) acrylic acid esters or alkoxy polyethylene glycol (meth) acrylic acid esters; 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 hydroxyl group; Or a polymerizable monomer having an alkoxy silyl group such as 3- (trimethoxysilyl) propyl (meth) acrylate or 3- (triethoxysilyl) propyl (meth) acrylate, but is not limited thereto. . In the above (meth) acrylate may mean acrylate or methacrylate.

Such a polymerizable monomer having a polar functional group may be included in the binder at a ratio of 30 to 100 parts by weight of a polymerized unit. In another example, the polymerizable monomer having the polar functional group may be included in the binder in a polymer unit ratio 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. There may be.

In the present application, the term "parts by weight" may mean a weight ratio between each component unless specifically described otherwise.

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 interaction is meant to include all known physical or chemical interactions, and in particular, may be an interaction between a polar functional group and an elemental sulfur.

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 elemental sulfur. The compound containing the elemental sulfur may be, for example, a compound containing eight sulfur atoms in a ring structure.

In the lithium-sulfur secondary battery, the compound containing elemental sulfur has an eluting property to the electrolyte according to the repetitive charging and discharging mechanism, and may also cause electrochemical problems due to low electrical conductivity, thereby improving such characteristics. It may be present in the form of a composite with a substance that can be present.

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

The sulfur-carbon composite may be formed by applying a compound containing elemental sulfur to porous carbon, or may be formed by melting the compound and mixing it with carbon. In this case, the content ratio of carbon and sulfur of 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, inert nanofibers, carbon nanotubes, or carbon fabrics.

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

In one example, the alkyl (meth) acrylate is a (meth) acrylate having 1 to 20 carbon atoms, such as 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 Latex, 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 and the like can be exemplified. But It is not limited.

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

The acrylic binder according to the present application can be prepared 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 in combination with an alkyl (meth) acrylate in an appropriate ratio, and then known solution polymerization, It can be prepared by applying a bulk polymerization (Bulk poylmerization), suspension polymerization (suspendion polymerization) or emulsion polymerization (emulsion polymerization) method.

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

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

The solvent used for the solution polymerization of the acrylic binder is not particularly limited, but a solvent having a boiling point of 110 ° C. or lower may be preferable in order to use the solution as it is without further purification after solution polymerization. Such solvents include, for example, 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. Appropriate adhesion with the current collector is secured within the glass transition temperature range, and the holding ability to the conductive material, the resistance to the 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. As used herein, the term weight average molecular weight may mean a conversion value for standard polystyrene measured by gel permeation chromatography (GPC), and unless otherwise specified, the molecular weight of any polymer may mean the weight average molecular weight of the polymer. Can be.

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

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

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

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

As described above, the acrylic binder included in the composition includes a polymerized unit of a polymerizable monomer having a polar functional group that interacts with the positive electrode active material, for example, from 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 parts by weight. Within this range, it is possible to secure the desired binding characteristics, to effectively prevent the dissolution of lithium polysulfide (LiS _x ), and to secure dispersion characteristics for the conductive material.

As described above, the positive electrode active material has a compound containing elemental sulfur, and may specifically be a sulfur-carbon composite. In addition, the sulfur-carbon composite may be formed by applying a compound containing elemental sulfur to the porous carbon, or may be formed by melting the compound and mixing with carbon. In this case, the content ratio of carbon and sulfur of the sulfur-carbon composite may be, for example, a ratio of 5:95 to 50:50 based on mass. The type of carbon may also be used without the above-mentioned graphite and the like without limitation.

The positive electrode active material may be included in the composition in a ratio of 30 to 95 parts by weight with respect 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 a desired battery.

The composition of the present application includes a conductive material. In the lithium-sulfur secondary battery, in order to overcome the problem caused by the low sulfur electrical conductivity, the active material should be included in the active layer, but if the amount of the conductive material is excessively large, aggregation of the conductive material may occur due to the deterioration of dispersion characteristics. Moreover, the energy density of the whole battery can also be reduced.

The composition of the present application includes an acrylic binder having an excellent dispersibility with respect to the conductive material, so that the conductive material may be contained in an amount within a range that does not lower the energy density of the entire battery without agglomeration of the conductive material. Can be.

In one example, the conductive material may be included in the composition in a ratio 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 with respect to 100 parts by weight of the composition solids.

In one example, the conductive material may be graphite such as natural graphite or artificial graphite; Carbon blacks 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 zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Alternatively, a conductive material such as polyaniline, polythiophene, polyacetylene, polypyrrole or polyphenylene derivative may be used, but is not limited thereto.

For example, the conductive material may have a particle diameter of 40 nm or less and use a surface area of 1,000 m 2 / g or more, 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 additionally providing solvent resistance to the electrolyte.

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

The non-acrylic binder may be included in the composition, for example, in a ratio of 0 to 20 parts by weight based on 100 parts by weight of the composition solids.

The composition may also further comprise a solvent.

The kind of solvent can be appropriately set in consideration of the desired performance and the like, and 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, dimethyl sulfoxide, formamide, dimethylformamide, acetonitrile, Nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxy methane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, propionic acid Organic solvents such as methyl or ethyl propionate and water can be used, but water is preferred in consideration of drying temperature and environmental influences.

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

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

In one example, the additive may be one or more 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 or Hg, and the like, the Group IIIA element may include, for example, Al, Ga, In or Ti, and the Group IVA element 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 above-described composition for forming an active layer, and includes an acrylic binder.

That is, the positive electrode for a lithium-sulfur secondary battery of the present application is a current collector; And an active layer formed on the current collector and including an acrylic binder. In addition, the acrylic binder includes a polymer 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. If necessary, the surface of the stainless steel may be treated with carbon, nickel, titanium, or silver. It may be performed.

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

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

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

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

In one example, the active layer is applied to the active layer forming composition comprising an acrylic binder, a positive electrode active material, a conductive material and other additives on the current collector using a known coating method, and then subjected to drying It can be formed on the current collector.

The coating step may be applied without any known coating method including, 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 an extrusion method.

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

In addition, a known process required for manufacturing the positive electrode before or after the active layer formation process, 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. The thickness range may be changed in consideration of desired performance. .

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

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

The negative electrode active material is a lithium alloy is an alloy of a metal 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 located 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. Such a separator may be an independent member such as a film, or may be a coating layer added to the anode and / or the cathode.

Materials of the separator include, but are not limited to, for example, polyolefins such as polyethylene and polypropylene, glass fiber filter paper, and ceramic materials, and the thickness thereof is about 5 to about 50 μm, specifically about 5 to about 25 μm. Can be.

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

The concentration of the lithium salt is 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 charging and discharging conditions of the cell, the operating 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 be a single solvent or a mixture of two or more organic solvents. When using two or more mixed organic solvents, 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 those having a dielectric constant of less than 15 that can dissolve elemental sulfur among aryl compounds, bicyclic ethers, and acyclic carbonates; strong polar solvents include acyclic carbonates, sulfoxide compounds, lactone compounds, Among ketone compounds, ester compounds, sulfate compounds, and sulfite compounds, a dielectric constant capable of dissolving lithium poly sulfide is defined as greater than 15, and lithium metal protective solvents are saturated ether compounds, unsaturated ether compounds, N, O And a charge / discharge cycle efficiency (cycle efficiency) for forming a stable SEI (Solid Electrolyte Interface) on a lithium metal, such as a heterocyclic compound containing S or a combination thereof, is defined as a solvent of 50% or more.

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.

Specifically, the battery module may be used as a power source for an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or a power storage device.

The present application may provide an acrylic binder and a composition comprising the active layer of the lithium-sulfur secondary battery positive electrode that can effectively prevent the dissolution of the positive electrode active material, ultimately ensure excellent cycle characteristics.

In addition, the present application is excellent in dispersing characteristics, it can provide a positive electrode comprising 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 an active layer formed therefrom.

Hereinafter, although described with reference to the embodiments of the present application, the following examples are intended to illustrate the present application, and the scope of the present application is not limited by the following examples having ordinary skill in the art Self-explanatory

Physical properties shown in this Example and Comparative Example were evaluated in the following manner.

[One. Method of measuring conversion rate of binder]

Analytical Instruments

 Gas chromatography (PerkinElmer)

Analytical conditions

 Solvent: tetrahydrofuran

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

 Injection volume: 0.5 μl

Analytical Procedure

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

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

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

A _fin : Area relative ratio of toluene peak of monomer peak at the end of 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 result was fantastic using the standard polystyrene of Agilent system for the production of a calibration curve.

 <Measurement condition>

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

  Column: Connect 2 PL Mixed B

  Column temperature: 40 ℃

  Eluent: tetrahydrofuran or N, N-dimethylformaldehyde

  Flow rate: 1.0 mL / min

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

[3. Formation of Anode Active Layer]

Carbon-sulfur composite was obtained through a wet ball milling process in which the carbon powder: sulfur weight ratio was 10:90. 75.0 mass% of the carbon-sulfur composite: 20.0 mass% of super-P (conductive material): 5.0 mass% of a binder was added to the water composition to prepare a slurry, and then an aluminum current collector having a thickness of about 20 μm. on Coating to produce a positive electrode having a loading amount of 2.0mAh / cm ² .

[4. Lithium-sulfur secondary battery manufacturing]

A positive electrode prepared according to the above method of the present application was used, a lithium foil having a thickness of about 150 μm was used as a negative electrode, and a polyolefin membrane (Celgard® 2400) was used as a separator. Lithium-sulfur using an electrolyte solution containing 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) The manufacture of a secondary battery was completed.

[5. Evaluation of Cycle Characteristics]

Equipment: 100mA Class Charger

Charge: 0.1C, constant current / constant voltage mode

Discharge: 0.1C, Constant Current Mode (1.5V)

Cycle temperature: 25 ℃

[Resin Production Example 1]-Preparation of Acrylic Binder (A1)

7.5 g polyethylene oxide methyl ether methacrylate, 6.0 g N-vinyl-2-pyrrolidone, 1.5 g acrylonitrile and 60 g of water were added to a 250 mL round bottom flask, and the inlet was sealed. . After oxygen was removed through nitrogen bubbling for 30 minutes and the reaction flask was immersed in an oil bath heated to 60 ° C., 0.03 g of V-50 (Wako Chemical) was administered and the reaction was started. When the conversion 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 Production Examples 2 to 4]-Preparation of Acrylic Binders (A2, A3, A4)

An acrylic binder was prepared in the same manner as in Preparation Example 1, except that the type and content of the monomer used in the 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 (part 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 Fabrication of Lithium-Sulfur Secondary Battery

A lithium-sulfur secondary battery was manufactured by using a cathode having an active layer including an acrylic binder (A1) prepared according to Preparation Example 1. After evaluating charge / discharge at 100 cycles between 1.5V and 2.8V at 0.1C / 0.1C, the capacity retention rate was measured by calculating the remaining capacity at the second cycle and the remaining capacity at the 100th cycle compared to the initial capacity. The results are shown in Table 2.

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

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

[Comparative Examples 1 and 2]-Fabrication of lithium-sulfur secondary battery

Polyvinylidene fluoride (PVDF) binder or 1: 1 mixture of styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) instead of acrylic binder (A1, A2, A3, A4) Then, a battery was manufactured in the same manner as in Example 1, the capacity retention rate was evaluated, and is 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 including a polymer unit of a polymerizable monomer having a polar functional group interacting with sulfur as a positive electrode active material in the active layer. The elution phenomenon could be suppressed, so the capacity retention rate was high as the cycle progressed.

Claims (22)

It is an acrylic binder for lithium-sulfur secondary battery positive electrode containing the polymer unit of the polymerizable monomer which has the alkylene oxide group which is a polar functional group which interacts with a positive electrode active material,
Acrylic binder for a lithium-sulfur secondary battery positive electrode further comprising at least one polar functional group selected from nitrogen-containing functional group, hydroxy group and alkoxy silyl group in addition to the alkylene oxide group. It is an acrylic binder for lithium-sulfur secondary battery positive electrode containing the polymerization unit of the polymerizable monomer which has a polar functional group which interacts with a positive electrode active material,
The acrylic binder is a polymer unit of a polymerizable monomer having an alkylene oxide group as a polar functional group; And a polymerized unit of a polymerizable monomer having a nitrogen-containing functional group which is a polar functional group. The method according to claim 1 or 2,
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 . The method according to claim 1 or 2,
The polymerizable monomer having an alkylene oxide group is poly (ethylene oxide) methyl ether methacrylate acrylic binder for a lithium-sulfur secondary battery positive electrode. The method according to claim 1 or 2,
The polymerizable monomer having a polar functional group is an acrylic binder for a lithium-sulfur secondary battery positive electrode contained in a polymerization unit ratio of 30 to 100 parts by weight. The method according to claim 1 or 2,
The positive electrode active material is an acrylic binder for lithium-sulfur secondary battery positive electrode having a compound containing elemental sulfur The method of claim 6,
The positive electrode active material is an acrylic binder for a lithium-sulfur secondary battery positive electrode which is a sulfur-carbon composite. The method according to claim 1 or 2,
The interaction is an acrylic binder which is the interaction between the polar functional group and the elemental sulfur. The method according to claim 1 or 2,
Acrylic binder for lithium-sulfur secondary battery positive electrode further comprising a polymerization unit of alkyl (meth) acrylate The method of claim 9,
Alkyl (meth) acrylate is an acrylic binder for lithium-sulfur secondary battery positive electrode contained in a polymerization unit ratio of 5 to 30 parts by weight. The method according to claim 1 or 2,
An acrylic binder for lithium-sulfur secondary battery positive electrode having a particle diameter of 10 nm or less. The method according to claim 1 or 2,
Acrylic binder for lithium-sulfur secondary battery positive electrode whose glass transition temperature is in the range of -80 degreeC-50 degreeC. The method according to claim 1 or 2,
Acrylic binder for lithium-sulfur secondary battery positive electrode whose weight average molecular weight is in the range of 5,000-3,000,000. Composition for forming an active layer of a lithium-sulfur secondary battery positive electrode comprising the acrylic binder, the positive electrode active material and the conductive material according to claim 1 The method of claim 14,
Acrylic binder is a composition for forming an active layer of a lithium-sulfur secondary battery positive electrode is contained in a ratio 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, which is included in a ratio 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,
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, which is contained in a ratio of 2 to 70 parts by weight based on 100 parts by weight of the solid content of the composition. Current collector;
A cathode 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 lithium-sulfur secondary battery positive electrode in the range of 1 to 200㎛. A lithium-sulfur secondary battery comprising the positive electrode of claim 20.