KR20150029090A - Binder of Improved Adhesive Force and Lithium Secondary Battery Comprising the Same - Google Patents

Abstract

The present invention relates to a binder having an improved adhesive force and to a lithium secondary battery comprising the same. The binder for a secondary battery according to the present invention comprises binder polymers composed of the copolymers of: (A) one or more kinds of monomers selected from the group which consists of (meth)acrylic acid ester-based monomers, acrylate-based monomers, vinyl-based monomers, and nitrile-based monomers; (b) unsaturated carboxylic acid-based monomers; and (C) isobornyl (meth)acrylate monomers.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a binder having improved adhesion and a lithium secondary battery including the binder,

The present invention relates to a binder having improved adhesion and a lithium secondary battery including the binder.

The increase in the price of energy sources due to the depletion of fossil fuels, the increase of interest in environmental pollution, and the demand for environmentally friendly alternative energy sources are becoming indispensable factors for future life. Various researches on power generation technologies such as nuclear power, solar power, wind power, and tidal power have been continuing, and electric power storage devices for more efficient use of such generated energy have also been attracting much attention.

Particularly, in the case of a lithium secondary battery, the demand for an energy source is rapidly increasing due to an increase in technology development and demand for a mobile device. Recently, the use of a lithium secondary battery as a power source for an electric vehicle (EV) and a hybrid electric vehicle , And the area of use is also expanding for applications such as power assisted power supply through gridization.

Generally, an electrode for a lithium secondary battery is manufactured by applying an electrode slurry containing an electrode active material, a binder, and a conductive material to an electrode current collector.

The binder is used for securing the adhesive force or binding force between the electrode active material and the electrode active material, between the electrode active material and the electrode current collector, but an excessive amount of binder is required to improve the adhesive force between the electrode current collector and the electrode active material.

However, an excessive amount of the binder has a problem that the capacity and conductivity of the electrode are lowered. In addition, there is a problem that the capacity is decreased due to a decrease in the amount of the active material.

On the other hand, insufficient adhesion causes electrode peeling phenomenon in drying process such as electrode drying and pressing process, thereby increasing the defect rate of the electrode. Further, the electrode having low adhesive force can be peeled off by an external impact, which adversely affects the performance of the battery. Specifically, such peeling of the electrode increases the contact resistance between the electrode material and the current collector, which may cause deterioration of the electrode output performance.

Therefore, it is necessary to design an electrode capable of achieving effective adhesion and performance between the electrode active material and the electrode active material, between the electrode active material and the electrode current collector.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described problems of the prior art and the technical problems required from the past.

The inventors of the present application have conducted intensive research and various experiments, and have found that when one or more monomers selected from the group consisting of a (meth) acrylic acid ester monomer, an acrylate monomer, a vinyl monomer and a nitrile monomer, When a binder containing a copolymer prepared by emulsion polymerization of an isobornyl (meth) acrylate monomer having a high glass transition temperature (Tg) is used, the amount of the electrode active material or the current collector The present invention has been accomplished on the basis of this finding.

Therefore, the binder used in the lithium secondary battery according to the present invention,

(A) at least one monomer selected from the group consisting of (meth) acrylic acid ester monomers, acrylate monomers, vinyl monomers and nitrile monomers; (B) an unsaturated carboxylic acid-based monomer; And (C) isobornyl (meth) acrylate monomers; And a binder polymer comprising a copolymer of ethylene and propylene.

More specifically, the binder used in the lithium secondary battery according to the present invention,

(A) a (meth) acrylic acid ester-based monomer; At least one monomer selected from the group consisting of acrylate monomers, vinyl monomers and nitrile monomers; (B) an unsaturated carboxylic acid-based monomer; And (C) isobornyl (meth) acrylate monomers; Of the binder polymer of the core-shell structure constituting the core portion and / or the shell portion.

That is, the binder according to the present invention may comprise a binder polymer of a single structure composed of the copolymer, and the binder may include a core-shell structure binder polymer in which the copolymer constitutes a core portion and / or a shell portion.

When the copolymer constitutes a core part or a shell part, the remaining part (the core part in the case where the copolymer constitutes the core part and the core part in the case of forming the shell part) comprises (A) a (meth) acrylic acid ester monomer; At least one monomer selected from the group consisting of acrylate monomers, vinyl monomers and nitrile monomers; And (B) an unsaturated carboxylic acid-based monomer; ≪ / RTI >

In the present specification, the acrylate monomer means an acrylate monomer other than the isobonyl (meth) acrylate monomer.

In the binder polymer of the core-shell structure, the particle size of the core portion may be 50 to 150 nm, and the particle size of the binder polymer of the polymerized core-shell structure may be 200 nm to 300 nm.

When the particle diameter of the core portion is less than 50 nm, it can not serve as a seed, and when it is more than 150 nm, dispersion for the polymerization process of the shell portion becomes difficult.

On the other hand, the copolymer according to the present invention may specifically include the following combinations.

As non-limiting examples, (i) (meth) acrylic acid ester-based monomers and vinyl-based monomers; (B) an unsaturated carboxylic acid-based monomer; And (C) copolymers of isobonyl (meth) acrylate monomers,

(ii) (meth) acrylic acid ester monomers, vinyl monomers and acrylate monomers; (B) an unsaturated carboxylic acid-based monomer; And (C) a copolymer of isobonyl (meth) acrylate monomer, or

(iii) (meth) acrylic acid ester monomers, vinyl monomers, acrylate monomers and nitrile monomers; (B) an unsaturated carboxylic acid-based monomer; And (C) isobonyl (meth) acrylate monomers.

The inventors of the present application have found that the (meth) isobornyl acrylate and vinyl monomers contained in the copolymer have a low affinity with the electrolyte solution, thereby reducing the swelling phenomenon of the electrolyte solution and increasing the hardness of the binder to increase the modulus And that it is possible to improve the initial adhesive force when a binder containing the same is used.

In one specific example, the content of the isobonyl (meth) acrylate monomer is, based on the total weight of the above-mentioned binder polymer, in order to have improved adhesion between the electrode active material or between the electrode composition material and the electrode current collector, 1 to 10% by weight.

If the content of the isobornyl (meth) acrylate monomer is less than 1% by weight based on the total weight of the binder polymer described above, adhesion between the electrode active material or between the electrode forming material and the electrode current collector is deteriorated, , And when it is more than 10% by weight, the rate characteristic is deteriorated.

Wherein the copolymer comprises at least one monomer selected from the group consisting of a (meth) acrylic acid ester monomer, a vinyl monomer, an acrylate monomer and a nitrile monomer; Unsaturated carboxylic acid monomers; And an isobornyl (meth) acrylate monomer may be randomly distributed.

When the copolymer is produced, the vinyl monomer is 70 to 90 parts by weight based on 100 parts by weight of the (meth) acrylate monomer, and the isobonyl (meth) acrylate monomer is a 1 to 15 parts by weight based on 100 parts by weight of the unsaturated carboxylic acid monomer, and 10 to 20 parts by weight of the unsaturated carboxylic acid monomer based on 100 parts by weight of the (meth) acrylate monomer.

In addition, the copolymer may include a crosslinking agent to further improve the bond retention. In this case, the crosslinking agent is added in an amount of 1 to 5 parts by weight based on 100 parts by weight of the (meth) acrylic acid ester-based monomer.

The crosslinking agent may be at least one selected from the group consisting of ethylene glycol dimethacrylate, 1,3-butanediol dimethacrylate, 1,6-hexanediol dimethacrylate, aryl methacrylate (AMA), glycidyl methacrylate (GMA) (Meth) acrylate compounds such as aryl isocyanurate (TAIC), polyethylene glycol diacrylate, polypropylene glycol diacrylate, and polybutylene glycol diacrylate, or triallylamine (TAA) and diallylamine DAA) may be used, and they may be used alone or in combination of two or more.

The (meth) acrylic acid ester monomer may be at least one monomer selected from the group consisting of methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-amyl acrylate, isoamyl acrylate, n Ethylhexyl acrylate, 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, iso Butyl methacrylate, n-amyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate, n-ethylhexyl methacrylate, 2-ethylhexyl methacrylate, hydroxyethyl methacrylate, Hydroxypropyl methacrylate, and hydroxypropyl methacrylate.

The acrylate monomer may be at least one selected from the group consisting of methacryloxyethyl ethylene urea,? -Carboxyethyl acrylate, aliphatic monoacrylate, dipropylene diacrylate, ditrimethylpropane tetraacrylate, hydroxyethyl acrylate, dipentaerythritol Triallyl acrylate, stearyl acrylate, lauryl methacrylate, cetyl methacrylate, stearyl methacrylate, stearyl methacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, lauryl acrylate, Acrylate, and the like.

The nitrile monomer may be at least one monomer selected from the group consisting of succinonitrile, sebaconitrile, fluoronitrile, nitrile chloride, acrylonitrile, and methacrylonitrile.

The vinyl-based monomer may be at least one monomer selected from the group consisting of styrene,? -Methylstyrene,? -Methylstyrene, p-t-butylstyrene, and divinylbenzene.

Wherein the unsaturated carboxylic acid monomer is at least one monomer selected from the group consisting of maleic acid, fumaric acid, methacrylic acid, acrylic acid, glutaconic acid, itaconic acid, tetrahydrophthalic acid, crotonic acid, isocrotonic acid, and nadic acid Lt; / RTI >

The copolymer can be produced by a polymerization method known in the art and is not limited thereto, and the gel content of the copolymer thus prepared after impregnation with a carbonate electrolyte is 98% or more.

Similarly, in the binder polymer of the core-shell structure, when the copolymer constitutes the core part or the shell part, the remaining part of the polymer may also be produced by polymerizing the materials described above according to a known polymerization method.

Meanwhile, the binder may be composed of only the binder polymer described above, and may further include binders other than the binder polymer. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluorine rubber, various copolymers and the like.

When the binder polymer is used in combination with another binder material, the adhesion may be improved. In Example 3, the adhesion of the binder blended with the styrene-butadiene rubber to 30 wt% of the total binder weight is increased by about 140% You can see it. In this case, the binder polymer is preferably 50% by weight or more based on the total weight of the binder.

The binder prepared by the above method is dissolved in a solvent in a range of 1% by weight to 30% by weight, preferably 2% by weight to 10% by weight based on the total weight of the electrode material, and the electrode active material and the electrode slurry May be coated on the electrode current collector and then dried to produce an electrode.

When the content of the binder is less than 1% by weight based on the total weight of the electrode material, it is not preferable because it can not exhibit desired effects such as high adhesive strength. If it exceeds 30% by weight, Which is undesirable.

The method of applying the paste of the electrode material to the metal material in a uniform manner can be selected from known methods in consideration of the characteristics of the material and the like or can be carried out by a new suitable method. For example, the paste can be uniformly dispersed by using a doctor blade or the like after being distributed on the current collector. In some cases, a method of performing the distribution and dispersion processes in a single process may be used. In addition, a die casting method, a comma coating method, a screen printing method, or the like may be used. Alternatively, the resin may be formed on a separate substrate, and then pressed or laminated by a pressing or laminating method. .

The paste applied on the metal plate is preferably dried in a vacuum oven at 50 to 200 DEG C, more specifically, at 50 to 150 DEG C within one day.

By providing the binder in a solvent as described above, a uniform composition for an electrode can be obtained. In addition, the viscosity of the electrode composition can be controlled so that the mixing process for the electrode composition and the coating process for the electrode collector can be easily performed .

Examples of the solvent used herein include organic solvents such as dimethylsulfoxide (hereinafter abbreviated as DMSO), N-methyl-2-pyrrolidinone (hereinafter abbreviated as NMP) and ethylene glycol, and distilled water. It is not.

The electrode may be a positive electrode or a negative electrode. The present invention provides a lithium secondary battery having a structure in which an electrode assembly having a separator interposed between the positive electrode and the negative electrode is impregnated with a non-aqueous electrolyte containing a lithium salt.

The lithium secondary battery according to the present invention can be used not only in a battery cell used as a power source of a small device but also as a unit cell in a middle- or large-sized battery module including a plurality of battery cells or a battery pack.

The middle- or large-sized devices include an electric vehicle and an electric power storage device including an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV) But the present invention is not limited thereto.

The structure of the battery module and the battery pack and the method of manufacturing the battery module and the battery pack are well known in the art, so a detailed description thereof will be omitted herein.

The negative electrode is prepared, for example, by applying a negative electrode active material, a conductive material and a mixture of the binder on the negative electrode current collector, followed by drying and rolling. If necessary, a filler may be further added to the mixture.

The negative electrode active material may be carbon such as non-graphitized carbon or graphite carbon; _{Li x Fe 2 O 3 (0≤x≤1} ), Li x WO 2 (0≤x≤1), Sn x Me 1-x Me 'y O z (Me: Mn, Fe, Pb, Ge; Me' : Metal complex oxides such as Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, Halogen, 0 < x < Lithium metal; Lithium alloy; Silicon-based alloys; Tin alloy; _{SnO, SnO 2, PbO, PbO} 2, Pb 2 O 3, Pb 3 O 4, Sb 2 O 3, Sb 2 O 4, Sb 2 O 5, GeO, GeO 2, Bi 2 O 3, Bi 2 O 4, Bi ₂ O ₅ and the like; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials; Titanium oxide; Lithium titanium oxide and the like can be used.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

The conductive material is usually added in an amount of 1 to 50 wt% based on the total weight of the mixture including the anode active material. Such a conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, for example, graphite such as natural graphite or artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The filler is not particularly limited as long as it is a fibrous material without causing a chemical change in the battery, and is used selectively as a component for suppressing the expansion of the negative electrode. Examples of the filler include an oligomer based polymer such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

Typical examples of the dispersion include isopropyl alcohol, N-methyl pyrrolidone (NMP), and acetone.

The positive electrode is prepared, for example, by coating a mixture of a positive electrode active material, a conductive material and a binder on a positive electrode current collector, and then drying the mixture. Optionally, a filler may be further added to the mixture.

The cathode active material may be a layered compound such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), or a compound substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + x} Mn _{2 -x} O ₄ (where x is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ and the like; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ and Cu ₂ V ₂ O ₇ ; A Ni-site type lithium nickel oxide expressed by the formula LiNi _1-x M _x O ₂ (where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga and x = 0.01 to 0.3); Formula _{LiMn 2-x M x O 2} ( where, M = Co, Ni, Fe , Cr, and Zn, or Ta, x = 0.01 ~ 0.1 Im) or Li ₂ Mn ₃ MO ₈ (where, M = Fe, Co, Ni, Cu, or Zn); A lithium manganese composite oxide having a spinel structure represented by LiNi _x Mn _2-x O ₄ ; LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with an alkaline earth metal ion; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ , and the like. However, the present invention is not limited to these.

The cathode current collector generally has a thickness of 3 to 500 mu m. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. Examples of the positive electrode current collector include stainless steel, aluminum, nickel, titanium, sintered carbon, aluminum or stainless steel A surface treated with carbon, nickel, titanium, silver or the like may be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the cathode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

The separation membrane is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally 0.01 to 10 mu m and the thickness is generally 5 to 300 mu m. Such separation membranes include, for example, olefinic polymers such as polypropylene, which are chemically resistant and hydrophobic; A sheet or nonwoven fabric made of glass fiber, polyethylene or the like; Kraft paper and the like are used. Representative examples currently on the market include the Celgard ^R 2400, 2300 (from Hoechest Celanese Corp.), polypropylene separator (from Ube Industries Ltd. or Pall RAI), and polyethylene series (from Tonen or Entek).

In some cases, a gel polymer electrolyte may be coated on the separation membrane to increase the stability of the cell. Representative examples of such a gel polymer include polyethylene oxide, polyvinylidene fluoride, and polyacrylonitrile.

When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

The lithium salt-containing non-aqueous electrolyte is composed of a non-aqueous electrolyte and a lithium salt. As the non-aqueous electrolyte, a non-aqueous electrolyte, an organic solid electrolyte, an inorganic solid electrolyte and the like are used.

Examples of the nonaqueous electrolyte include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate , Gamma -butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, Diethyl ether, formamide, dimethyl formamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane A non-protonic organic solvent such as an ether, a methyl pyrophosphate, or an ethyl propionate is used as the solvent, a sulfone, a methyl sulfolane, a 1,3-dimethyl-2-imidazolidinone, a propylene carbonate derivative, a tetrahydrofuran derivative, .

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, Polymers containing ionic dissociation groups, and the like can be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides and sulfates of Li such as Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ can be used.

The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO 4, LiBF 4, LiB 10 Cl 10, LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, LiSCN, LiC (CF 3 SO 2) 3, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, 4-phenylborate, imide, and the like can be used.

For the purpose of improving the charge / discharge characteristics and the flame retardancy, the electrolytic solution is preferably mixed with an organic solvent such as pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, . In some cases, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further added to impart nonflammability. In order to improve the high-temperature storage characteristics, carbon dioxide gas may be further added. FEC (fluoro-ethylene carbonate, PRS (propene sultone), FPC (fluoro-propylene carbonate), and the like.

As described above, the binder for a lithium secondary battery according to the present invention includes a copolymer containing an isobonyl (meth) acrylate monomer. The adhesive force between the electrode active material and / or between the electrode forming material and the electrode current collector and the bonding holding force can be improved.

Therefore, the lithium secondary battery including the binder can exhibit the effect of increasing the capacity, improving the lifetime characteristics, and improving the rate characteristics.

Hereinafter, the present invention will be described in detail with reference to examples. The following examples are intended to assist the understanding of the present invention, and the scope of the present invention is not limited by the scope of the following examples.

≪ Example 1 >

Preparation of binder polymer seed (core part)

56 parts by weight of isobornyl acrylate, 37 parts by weight of styrene, 6 parts by weight of acrylic acid and itaconic acid, 1 part by weight of a crosslinking agent, 0.3 part by weight of an emulsifier and 0.4 part by weight of a buffer as a monomer of a binder polymer seed were mixed with potassium persulfate And the resulting mixture was mixed and polymerized at 60 DEG C for about 3 hours to prepare a seed. The average particle diameter of the obtained seed was 90 nm.

Preparation of binder polymer

6.5 parts by weight of the seed prepared above, 48 parts by weight of 2-ethylhexyl acrylate as a binder polymer shell part, 40 parts by weight of styrene, 2 parts by weight of hydroxyethyl acrylate, 8 parts by weight of acrylic acid and itaconic acid, 1 part by weight of a crosslinking agent, 0.4 part by weight of an emulsifier and 0.4 part by weight of NaHCO ₃ were added to water containing potassium persulfate as a polymerization initiator, and these were mixed and polymerized at 75 캜 for about 4 hours. Through the polymerization as described above, a binder polymer particle having a core-shell structure containing isobornyl acrylate in the core portion was prepared. The average particle diameter of the obtained polymer was 260 nm.

Preparation of electrode slurry and electrode

0.4 parts by weight of a conductive material, 1.5 parts by weight of a binder of the binder polymer, and 1.2 parts by weight of carboxymethyl cellulose as a thickening agent were mixed in 96.9 parts by weight of natural graphite with water as a dispersion medium so that the total solid content became 55% To prepare a negative electrode slurry. The negative electrode slurry was coated on a copper foil to a thickness of 100 탆, vacuum dried, and pressed to prepare a negative electrode.

A positive electrode slurry was prepared by mixing NMP (N-methyl-2-pyrrolidone) as a dispersion medium, 96 parts by weight of LiCoO _{2 as an} active material, ₂ parts by weight of a conductive material and 2 parts by weight of a PVDF binder, 100 탆 thick, dried and compressed to prepare a positive electrode.

Manufacture of lithium secondary battery

The prepared negative electrode plate was drilled at a surface area of 13.33 cm ² , and the positive electrode plate was drilled at a surface area of 12.60 cm ² to prepare a single cell (mono-cell). A tap was attached to the upper part of the positive electrode and the negative electrode, and the resultant was placed on the aluminum pouch with a separator made of a polyolefin microporous membrane between the negative electrode and the positive electrode. Then, 500 mg of the electrolytic solution was injected into the pouch. The electrolyte was prepared by dissolving the LiPF ₆ electrolyte at a concentration of 1M using a mixed solvent of ethyl carbonate (EC): diethyl carbonate (EMC) and ethyl methyl carbonate (EMC) in a volume ratio of 4: 3: 3.

Thereafter, the pouch was sealed using a vacuum packing machine, kept at room temperature for 12 hours, filled with a constant current at a rate of about 0.05 C, and subjected to a tangential pressure charging process in which the voltage was maintained until it was about 1/6 of the current . At this time, since the gas is generated inside the cell, degassing and resealing are performed to complete the lithium secondary battery.

≪ Example 2 >

A lithium secondary battery was produced in the same manner as in Example 1, except that isobornyl methacrylate was used instead of isobornyl acrylate as a monomer of the seed.

≪ Example 3 >

A lithium secondary battery was produced in the same manner as in Example 1, except that a binder obtained by mixing 70 wt% of the binder polymer and 30 wt% of the styrene-butadiene rubber was used as the binder of Example 1.

<Example 4>

48 parts by weight of 2-ethylhexyl acrylate, 42 parts by weight of styrene, 3.6 parts by weight of isobornyl acrylate, 2 parts by weight of hydroxyethyl acrylate, 8 parts by weight of acrylic acid and itaconic acid, 0.4 part by weight of an emulsifier and 0.4 part by weight of NaHCO ₃ were added to water containing potassium persulfate as a polymerization initiator, and these were mixed and polymerized at 75 캜 for about 4 hours. A lithium secondary battery was produced in the same manner as in Example 1, except that binder polymer particles having a single structure including isobornyl acrylate were prepared through the polymerization as described above. The average particle size of the obtained polymer was 260 nm.

&Lt; Comparative Example 1 &

A lithium secondary battery was produced in the same manner as in Example 1, except that styrene-butadiene rubber was used as the binder of Example 1.

&Lt; Comparative Example 2 &

A lithium secondary battery was produced in the same manner as in Example 1, except that styrene was used instead of isobornyl acrylate in the monomer of the seed of Example 1.

<Experimental Example 1>

<Tensile test>

Tensile tests of the binders specified in Examples 1 to 4 and Comparative Examples 1 and 2 were carried out. Specifically, a binder dispersed in a solvent was applied and dried on a PET film to a predetermined thickness, and a binder film was cut to 1 cm * 4 cm to prepare a test piece. Stress-strain curve shows the modulus, tensile strain, and tensile strength (tensile strength) of a binder through a stress applied to the binder, ), And the results are shown in Table 1 below.

Elongation at break (%) Breaking strength (N / mm ² ) Modulus (N / mm ² ) Example 1 230 14 38 Example 2 334 14 49 Example 3 432 9 31 Example 4 341 15 45 Comparative Example 1 766 6 3 Comparative Example 2 264 14 26

As shown in Table 1, the binders of Examples 1 to 4 according to the present invention have improved modulus as compared with the binders of Comparative Example 1 and Comparative Example 2. This is because the binder according to the present invention contains a (meth) isobornyl acrylate monomer to increase the hardness. The modulus is an important factor for the initial adhesion properties between the electrode active material and the current collector during the electrode drying process.

<Experimental Example 2>

<Adhesion Test>

Experiments were conducted to measure the adhesive force between the negative electrode slurry and the current collector when the binder specified in Examples 1 to 4 and Comparative Examples 1 and 2 was used.

The negative electrode plate prepared according to Examples 1 to 4 and Comparative Example 1 and 2 was cut into a predetermined size and fixed on a slide glass. The current collector was peeled off and the 180 ° peel strength was measured. The results are shown in Table 2 below . The evaluation was made by measuring the peel strengths of 5 or more and calculating the average value.

Adhesion (gf / cm) Example 1 17 Example 2 18 Example 3 19 Example 4 18 Comparative Example 1 14 Comparative Example 2 6

As shown in Table 2, it was confirmed that the adhesive strength of the cathodes of Examples 1 to 4 according to the present invention exhibited higher adhesive force than the cathodes according to Comparative Examples 1 and 2.

<Experimental Example 3>

&Lt; Electrolyte Swelling Test >

The electrolytic solution swelling test of the binders specified in Examples 1 to 4 and Comparative Examples 1 and 2 was carried out to measure the volume change, and the results are shown in Table 3 below. The electrolytic solution swelling test was performed by applying a binder dispersed in a solvent on a PET film to a predetermined thickness and drying it. The binder film was cut into 1.5 cm * 8 cm to prepare a specimen. The prepared specimen was impregnated with an electrolytic solution, , And the stretched length of the film was checked and converted into the volume ratio. At this time, a carbonate-based mixed solvent (EC: DEC: EMC = 4: 3: 3) except for the lithium salt (LiPF6) was used as the electrolytic solution.

Electrolyte swelling (vol%) Example 1 49 Example 2 53 Example 3 57 Example 4 47 Comparative Example 1 67 Comparative Example 2 71

As shown in Table 3, it was confirmed that the binder of Examples 1 to 4 according to the present invention had a reduced electrolyte swelling phenomenon as compared with the binders of Comparative Examples 1 and 2. This is because of the characteristics of the polymer, and the (meth) isobornyl acrylate and the styrene monomer contained in the binder polymer have a low affinity with the carbonate electrolyte.

<Experimental Example 4>

<Battery Test>

Charging and discharging tests of the batteries manufactured in Examples 1 to 4 and Comparative Examples 1 and 2 were carried out. Charging and discharging tests were carried out twice at a charging / discharging current density of 0.2 C, a charging end voltage of 4.2 V (Li / Li +) and a discharge end voltage of 2.5 V (Li / Li +). Charging and discharging tests were carried out 48 times at a charging / discharging current density of 1 C and a charging end voltage of 4.2 V (Li / Li +) and a discharge end voltage of 2.5 V (Li / Li +). All charging was performed at constant current / constant voltage, and the end current of constant voltage charging was 0.05C. After completing a total of 50 cycles, the charge / discharge efficiency (initial efficiency and 50 cycle capacity retention rate) of the first cycle was obtained. Then, the capacity ratio (50th / 1st) of dividing the charge capacity of 50 cycles by the charge capacity of the first cycle was calculated and regarded as the capacity retention rate. The results are shown in Table 4 below.

Initial efficiency (%) 50 cycle capacity retention rate (%) Example 1 93.1 84 Example 2 93.3 85 Example 3 93.0 85 Example 4 93.2 84 Comparative Example 1 90.4 80 Comparative Example 2 91.0 78

As shown in Table 4, the batteries using the binder polymer containing isobornyl (meth) acrylate as the binder (Examples 1 to 4) were superior in terms of initial efficiency and cycle capacity retention (See Table 3 and Experiment 3 above).

This is because the adhesive strength of the binder polymer containing isobornyl (meth) acrylate is superior to that of the polymer binder used in the conventional carbon-based active material as described in Experimental Example 2. In other words, since the binder according to the present invention includes the (meth) isobornyl acrylate monomer and the modulus (hardness) is increased, the initial adhesive strength is increased and the acrylic copolymer containing the functional group is introduced, Results.

While the present invention has been described with respect to certain specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications may be made therein without departing from the scope of the present invention

Claims (17)

As a binder used in a lithium secondary battery,
(A) a (meth) acrylic acid ester-based monomer; At least one monomer selected from the group consisting of acrylate monomers, vinyl monomers and nitrile monomers;
(B) an unsaturated carboxylic acid-based monomer; And
(C) isobornyl (meth) acrylate monomers; Wherein the binder polymer comprises a binder polymer comprising a copolymer of ethylene and propylene. As a binder used in a lithium secondary battery,
(A) a (meth) acrylic acid ester-based monomer; At least one monomer selected from the group consisting of acrylate monomers, vinyl monomers and nitrile monomers;
(B) an unsaturated carboxylic acid-based monomer; And
(C) isobornyl (meth) acrylate monomers; Wherein the binder comprises a core-shell structure binder polymer in which the copolymer constitutes the core portion and / or the shell portion. 3. The method of claim 2, wherein the copolymer comprises a core portion,
(A) a (meth) acrylic acid ester-based monomer; At least one monomer selected from the group consisting of acrylate monomers, vinyl monomers and nitrile monomers; And
(B) an unsaturated carboxylic acid-based monomer; Wherein the binder is selected from the group consisting of polyvinyl alcohol, polyvinyl alcohol, and polyvinyl alcohol. 3. The method of claim 2, wherein the copolymer comprises a shell part,
(A) a (meth) acrylic acid ester-based monomer; At least one monomer selected from the group consisting of acrylate monomers, vinyl monomers and nitrile monomers; And
(B) an unsaturated carboxylic acid-based monomer; Wherein the binder is selected from the group consisting of polyvinyl alcohol, polyvinyl alcohol, and polyvinyl alcohol. The binder for a lithium rechargeable battery according to claim 2, wherein the core part has a particle diameter of 50 to 150 nm. The binder for a lithium secondary battery according to claim 2, wherein the binder polymer particle diameter of the core-shell structure is 200 to 300 nm. The binder for a lithium secondary battery according to claim 1 or 2, wherein the content of the isobonyl (meth) acrylate monomer is 1 to 10% by weight based on the total weight of the binder polymer. The acrylic pressure-sensitive adhesive composition according to claim 1 or 2, wherein the (meth) acrylic acid ester monomer is at least one selected from the group consisting of methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, Acrylate, isoamyl acrylate, n-ethylhexyl acrylate, 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate , n-butyl methacrylate, isobutyl methacrylate, n-amyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate, n-ethylhexyl methacrylate, Hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxypropyl methacrylate, and hydroxypropyl methacrylate. Binder for lithium secondary battery. The acrylic resin composition according to claim 1 or 2, wherein the acrylate monomer is at least one monomer selected from the group consisting of methacryloxyethyl ethylene urea,? -Carboxyethyl acrylate, aliphatic monoacrylate, dipropylene diacrylate, ditrimethylpropane tetraacrylate, But are not limited to, hydroxyethyl acrylate, dipentaerythritol hexaacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, lauryl acrylate, seryl acrylate, stearyl acrylate, lauryl methacrylate , Cetyl methacrylate, and stearyl methacrylate. The binder for a lithium secondary battery according to claim 1, wherein the binder is at least one selected from the group consisting of cetyl methacrylate, stearyl methacrylate and stearyl methacrylate. The method of claim 1 or 2, wherein the nitrile monomer is at least one monomer selected from the group consisting of succinonitrile, cevanonitrile, fluorinated nitrile, nitrile chloride, acrylonitrile, and methacrylonitrile Wherein the binder is a binder for lithium secondary batteries. The method according to claim 1 or 2, wherein the vinyl monomer is at least one monomer selected from the group consisting of styrene,? -Methylstyrene,? -Methylstyrene, pt-butylstyrene, and divinylbenzene A binder for a lithium secondary battery. 3. The composition of claim 1 or 2, wherein the unsaturated carboxylic acid monomer is selected from the group consisting of maleic acid, fumaric acid, methacrylic acid, acrylic acid, glutaconic acid, itaconic acid, tetrahydrophthalic acid, crotonic acid, isocrotonic acid, and nadic acid Wherein the binder is at least one monomer selected from the group consisting of: An electrode slurry obtained by mixing an electrode material including a binder for a lithium secondary battery, an electrode active material, and a conductive material according to claim 1 on an electrode current collector. An electrode according to claim 2, wherein an electrode slurry obtained by mixing an electrode material including a binder for a lithium secondary battery, an electrode active material, and a conductive material is applied onto an electrode current collector. 15. The electrode according to claim 13 or 14, wherein the content of the binder for the lithium secondary battery is 1 wt% to 30 wt% based on the total weight of the electrode material. An electrode assembly comprising an electrode and a separator according to claim 13, wherein the electrode assembly is impregnated with an electrolyte solution and sealed. The lithium secondary battery according to claim 14, wherein the electrode assembly including the electrode and the separator is impregnated with the electrolyte solution.