KR101141060B1 - Composite Binder having Conductivity and Secondary Battery Employing the Same - Google Patents

Abstract

The present invention provides a composite binder comprising a polyvinyl alcohol, polyvinylpyrrolidone, and conductive particles, wherein the conductive particles are uniformly distributed in the polymer resin, and a secondary battery including the same. By using the polymer and the conductive particles, which have very high adhesive strength and improved elongation, as the binder of the electrode mixture, they have excellent electrical conductivity and can maintain stable cycle characteristics between the active materials and the current collector. It is possible to reduce the volume change of the active materials. Based on this, in particular, a large capacity lithium secondary battery using a silicon or tin-based negative electrode active material and the like can be manufactured.

Description

Composite Binder with Conductivity and Secondary Battery Containing the Same {Composite Binder having Conductivity and Secondary Battery Employing the Same}

The present invention relates to a composite binder imparting conductivity and a secondary battery comprising the same, and more particularly, as a binder for electrode mixture of a secondary battery, including polyvinyl alcohol, polyvinylpyrrolidone, and conductive particles, The present invention relates to a composite binder and a secondary battery including the same, wherein the conductive particles are uniformly distributed in the polymer resin.

As technology development and demand for mobile devices increase, the demand for secondary batteries as energy sources is rapidly increasing, and lithium secondary batteries having high energy density and voltage are commercially used in such secondary batteries. Such lithium secondary batteries generally use lithium transition metal oxides as positive electrode active materials and graphite-based materials as negative electrode active materials.

The electrode of a lithium secondary battery is prepared by mixing such an active material and a binder resin component and dispersing it in a solvent to make a slurry, applying it to the surface of a current collector, and drying it to form a mixture layer. However, the charge and discharge cycle proceeds due to the volume change caused by the reaction with lithium during charge and discharge, the negative electrode active material is detached from the current collector during continuous charge and discharge, or the resistance increases due to the change of the contact interface between the negative electrode active materials. As a result, the capacity is drastically lowered, resulting in a shorter cycle life.

The binder gives a binding force between the active material as well as between the active material and the current collector, and has an important effect on battery characteristics by suppressing volume expansion caused by charging and discharging of the battery. However, when an excessive polymer is used as a binder to reduce the volume change during charging and discharging, it is possible to reduce the detachment of the active material from the current collector, but the electrical resistance of the negative electrode increases due to the electrical insulation of the binder, and relatively As the amount decreases, problems such as capacity reduction arise.

Polyvinylidene fluoride (PVdF) has been widely used as such a binder resin, but the adhesion is poor, there is a problem in safety, and by using an organic solvent during slurry production, problems such as environmental pollution are generated. In order to solve the problem of PVdF, a method of polymerizing styrene-butadiene rubber (SBR) in an aqueous phase and using a thickener and the like has been proposed. Such a binder has an advantage of being environmentally friendly and increasing battery capacity by reducing the amount of binder used. However, even in this case, the binding sustainability is improved by the elasticity of the rubber, but the binding force itself does not show a great effect, and the improvement of battery characteristics is also required. Many methods have been proposed to provide excellent binding power and to improve battery characteristics.

In this regard, some prior art has been proposed to solve the problem when using the SBR alone as a binder, which will be described as an example.

Korean Patent Application Publication No. 2003-0033595 discloses a method of mixing a mixture of PVdF and SBR at a predetermined ratio, and Korean Patent Registration No. 0582518 describes an ideal structure in which two or more polymers having different chemical structures form an ideal structure. The present invention relates to a battery binder containing composite polymer particles. Korean Patent Application Publication No. 2004-0078927 discloses a composite polymer particle having a two-phase or more structure containing a polymer which is respectively related to battery characteristics, adhesion and coating characteristics. US Patent No. 6,773,838 claims that high charge and discharge efficiency and stability can be achieved by using modified SBR binders and thickeners in a range of amounts. In addition, US Patent No. 6,881,517 discloses a method of using a polymer binder containing a butadiene structure in a certain ratio, US Patent No. 6,428,929 discloses a method of adding an acrylic acid derivative copolymer to a binder containing a butadiene structure. Is starting.

In addition, mixture binders of various compositions have been proposed in that the properties of the binder can be improved by properly combining the properties of the respective polymers used in the preparation of the binder. U. S. Patent No. 5,380, 606 discloses a composition comprising a polyamic acid, and U. S. Patent No. 6,399, 246 discloses a method of mixing styrene-butadiene copolymer or styrene-acrylate copolymer with polyacrylamide. Is disclosed.

However, in spite of these various technical proposals, binders which still exhibit desired levels of physical properties have not been developed.

On the other hand, as the cathode made of graphite-based material almost reaches its theoretical maximum capacity of 372 mAh / g (844 mAh / cc), it is limiting its ability to play a sufficient role as an energy source for the rapidly changing next generation mobile devices. have. Lithium metal, which has been examined as a negative electrode material, has a very high energy density to realize a high capacity, but has a problem of safety due to dendrite growth and short cycle life during repeated charging and discharging. In addition, attempts have been made to use carbon nanotubes as negative electrode active materials, but problems such as low productivity, high price, and low initial efficiency of 50% or less have been pointed out.

In this regard, many studies have recently been made, with silicon, tin, or alloys thereof known to be capable of reversibly occluding and releasing large amounts of lithium through compound formation reactions with lithium. It's going on. For example, silicon is promising as a high capacity cathode material because the theoretical maximum capacity is about 4020 mAh / g (9800 mAh / cc, specific gravity 2.23), which is much larger than graphite-based materials.

However, since silicon, tin, or alloys thereof have a large volume change of 200 to 300% due to reaction with lithium during charging and discharging, the negative electrode active material detaches from the current collector or contacts the negative electrode active materials during continuous charge and discharge. Due to the increase in resistance due to a large change in the interface, as the charge and discharge cycle proceeds, there is a problem that the capacity is drastically lowered and the cycle life is shortened. Due to these problems, when the conventional binder for graphite negative electrode active material, that is, PVdF, SBR, or the like is used as it is for a silicon-based or tin-based negative electrode active material, a desired effect cannot be obtained.

Therefore, in the conventional graphite-based lithium secondary battery, a silicon or tin-based negative electrode active material with a technology that can improve the capacity of the battery by securing the adhesion between the active material and the current collector and the active materials even when using a small amount of binder, In the lithium secondary battery using the need to develop an excellent binder that can maintain the adhesive force with the current collector while absorbing a large volume change of the negative electrode active material during charge and discharge.

On the other hand, in order to improve the electroconductivity of an electrode, conventionally, conductive materials, such as carbon black, are added to the electrode mixture of a lithium secondary battery. However, when the conductive material is included in the negative electrode mixture, such a conductive material cannot penetrate into the binder, which is an insulator, and thus has a limit in improving electrical conductivity.

In this regard, the present invention proposes a composite binder comprising polyvinyl alcohol, polyvinylpyrrolidone and conductive particles, as described below.

Some techniques for including such conductive particles in a binder are known. For example, Japanese Patent Application Laid-Open No. 2002-134115 discloses a technique for preparing a conductive binder liquid by dispersing conductive carbon fine powder in an organic binder. . However, the above technique does not effectively cope with the volume change of the negative electrode active material during charging and discharging by injecting conductive carbon fine powder into a binder such as polyvinylidene fluoride (PVdF) or styrene-butadiene rubber (SBR). There is a problem that the overall performance of the battery, such as to cause a decrease in. This fact can be more clearly confirmed through a comparative experiment with a binder including conductive carbon in the conventional PVdF binder composition described later.

In addition, Japanese Patent No. 3567618 discloses 30 to 98% by weight of butadiene bond and 20 to 95% by weight of carboxy-modified styrene butadiene copolymer latex and 50% by weight or more of primary particles to improve cycle characteristics of the battery. The conductive binder composition for secondary battery electrodes containing the conductive carbon of micrometer or less is disclosed. However, such a binder composition contains a relatively expensive conductive carbon in a large amount, leading to an increase in manufacturing cost, and the content of the binder polymer is relatively reduced, resulting in a decrease in overall battery performance such as deterioration of adhesion and cycle characteristics as mentioned above. Still have the problem.

SUMMARY OF THE INVENTION Accordingly, the present invention has been made to solve the above-mentioned problems of the prior art and the technical problems required from the past.

The inventors of the present application, after continuing in-depth research and various experiments, the composite binder containing the polyvinyl alcohol and polyvinylpyrrolidone physically mixed, and the conductive particles are uniformly distributed, the adhesion and elongation By the optimum combination of and excellent electrical conductivity, it was confirmed that the overall battery characteristics compared to the conventional binders, and came to complete the present invention.

Accordingly, the binder for electrode mixture of the secondary battery according to the present invention includes polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and conductive particles, and the conductive particles are uniformly distributed in the polymer resin. Consists of.

PVA, one component constituting the binder in the electrode mixture, provides an excellent binding force due to its high adhesiveness, especially for active materials having a large volume change during charging and discharging, and another component, PVP, is filled by its high elongation. By preventing the accumulation of stress generated during discharge, the cycle characteristics are greatly improved. On the other hand, as described above, when the conventional conductive material is added, it has considerable viscosity and intermolecular bonding force, and cannot be introduced into the electrically insulating binder, and thus electrical conductivity cannot be sufficiently improved. However, since the binder according to the present invention contains conductive particles, as defined above, the conductive particles are even distributed inside the binder, thereby improving the conductivity of the PVA and PVP mixture as the insulator, and consequently the conductivity of the electrode. It can increase greatly.

Therefore, the secondary battery including the binder according to the present invention is not only excellent in cycle characteristics, but also has excellent electrical conductivity, so that high capacity and high-speed charging are possible, and it is confirmed that a significant effect increase over the expected physical properties is obtained.

The conductive particles are a component for securing the conductivity of the insulating polymer binder, it is preferably contained in 0.01 to 20% by weight based on the total weight of the binder, more preferably contained in 5% by weight or less.

When the content of the conductive particles is too large, the content of the binder component is relatively reduced, and thus the adhesion between the electrode active materials or the current collector of the electrode mixture may be weakened. On the contrary, when the amount is too small, the desired electricity It is not preferable because the effect of improving conductivity is difficult to obtain.

The conductive particles are not particularly limited as long as they have conductivity without causing chemical changes in the battery or binder. Examples of the conductive particles include conductive whiskers such as graphite-based materials, conductive fibers, metal powders, zinc oxide, and potassium titanate, and conductivity. Metal oxides and other various conductive materials may be used, and preferably carbon-based particles.

The carbon-based particles are preferably carbon black such as carbon black and acetylene black; Carbon fibers, carbon nanotubes, and the like, but is not limited thereto. On the other hand, in the present invention, the conductive particles are a concept including all of the broad particulate form such as spherical particles, fibrous particles in terms of shape.

For example, the conductive particles may be uniformly distributed in the binder by being added to a mixture of PVA and PVP in the manufacturing process of the binder, and may be added to the binder composition as such, but are preferably UV ozone or acid. After the pretreatment process such as (acid) treatment may be dispersed in the binder composition.

Through such a pretreatment process, a functional group such as a carboxyl group is introduced at the terminal or surface of the conductive particles and mixed with the binder component in this state, thereby inducing physical or chemical bonding with the polymer binder component.

The method for producing the composite binder is not particularly limited. In a specific example, the conductive particles that have undergone the pretreatment process are dispersed in a solvent such as distilled water, dimethylformaldehyde, dimethylsulfoxide, methylethylketone, and the like. Prepared by forced reflux in an ultrasonic reactor for 2 minutes to 2 hours, preferably about 1 hour, so that the carbon-based particles are evenly dispersed in the solvent, and refluxed again for 1 hour by adding a mixture of PVA and PVP to the prepared solution. can do.

The PVA exhibits excellent adhesion to not only the electrode active material but also the surface of the current collector by the hydroxy group repeatedly formed in the polymer main chain. Therefore, it is possible to adhere the electrode active material to the surface of the current collector by adding a smaller amount as compared to the binder of the prior art, and by preventing the electrode active material from deviating from the surface of the current collector as the battery charge and discharge cycle proceeds. In addition, it can provide relatively high battery capacity and excellent cycle characteristics. In addition, since the polyvinyl alcohol has higher electrical conductivity than other polymers, the polyvinyl alcohol has a significantly lower electrical resistance compared to the same content in the electrode, thereby exhibiting excellent high rate charge / discharge characteristics.

Preferably, the PVA has a high degree of polymerization (DP) and a degree of saponification (DS). This is because when the degree of polymerization is too low, problems such as mechanical strength, adhesive strength, solvent resistance, and the like fall. On the other hand, polyvinyl alcohol having a high degree of saponification improves the adhesion between the current collector and the active material such as a metal foil by a hydroxy (OH) functional group, and the higher the degree of saponification is preferable as long as it can handle the solvent.

In view of this point, polyvinyl alcohol having a degree of polymerization of at least 3000 and a degree of saponification of at least 80% is more preferable. When the degree of polymerization of PVA is less than 3000, the resistance to the electrolyte is low (i.e., it is easy to be dissolved by the electrolyte), and partially dissolved in the electrolyte during the charge / discharge cycle of the battery, thereby increasing the resistance of the electrolyte and forming an electrode active material. Peeling from the current collector has a problem that the charge and discharge capacity is sharply lowered. This lowering of resistance to electrolytes is especially acute when the battery is operated at high temperatures above room temperature.

In addition, when the saponification degree of PVA is lower than 80%, the number of hydroxy groups repeatedly formed in the PVA polymer main chain is reduced, leading to a decrease in adhesive force, which is not preferable.

In general, PVA is prepared by hydrolyzing a PVA precursor (polyvinylacetate) obtained by polymerizing vinyl acetate (CH ₃ COOCHCH ₂ ) as in the following reaction scheme, and the degree of hydrolysis is referred to as 'blackness'.

[Reaction Scheme]

Techniques for producing PVA of high degree of polymerization and high degree of blackness are disclosed in Korean Patent Application No. 2005-0136273 of the applicant, the contents of which are incorporated by reference in the present invention. Referring to the method of manufacturing a high degree of polymerization and high degree of blackening PVA as follows.

After distilled water and a predetermined amount of PVA suspending agent (80% saponification) were added to the reactor, the mixture was stirred, and then purged with nitrogen from which oxygen and water were removed, followed by azobisbutyronitrile or azobisdimethylvaleronitrile. After dissolving and injecting the same radical initiator and vinyl acetate monomer, the temperature is raised to a predetermined temperature to carry out a polymerization reaction to prepare a polyvinylacetate having a high degree of polymerization. The polyvinylacetate is filtered, washed, and dried, dissolved in methanol, and subjected to saponification by adding a strong base of sodium hydroxide twice. The degree of polymerization of the prepared PVA can be adjusted to the desired level in the above range by the amount of the initiator and the reaction temperature, to obtain a PVA that is at least 99% saponified.

PVP, which is another component constituting the binder of the present invention, can be thoroughly mixed through physical mixing with PVA, thereby preventing the non-uniformity of the electrode due to the mixing of the two polymers, and increasing the elongation of the binder by the high elongation characteristic of PVP. Increase to buffer the volume change.

The molecular weight of the PVP is not particularly limited, but if the molecular weight is too small, it may be difficult to exhibit the excellent elongation of PVP in the binder, it is preferable to have a molecular weight of 1000 to 1,000,000.

The content of the PVP may preferably be included in an amount of 0.1 to 100 parts by weight, more preferably 1 to 50 parts by weight, based on 100 parts by weight of PVA. If the content of PVP is too small, the elongation of the binder may be insufficient to reduce the design capacity and the charge / discharge efficiency. On the contrary, if the content of PVP is too large, excess moisture due to high affinity with water is absorbed and swelled. This is because the adhesion and the performance of the battery can be reduced.

The present invention also provides an electrode mixture comprising an electrode active material, the composite binder, and optionally a conductive material.

The electrode active material may be both an active material for a positive electrode and a negative electrode mixture, and a negative electrode active material having a large change in volume during charging and discharging is particularly preferable.

Examples of the negative electrode active material include carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene, and activated carbon; Metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt and Ti which can be alloyed with lithium and compounds containing these elements; Complexes of metals and their compounds and carbon and graphite materials; Lithium-containing nitrides, and the like.

Among them, a carbon-based active material, a silicon-based active material, a tin-based active material, or a silicon-carbon based active material is more preferable, and these may be used singly or in combination of two or more.

In particular, a silicon-based active material, a tin-based active material, a silicon-carbon-based active material, or the like, which has a large volume change during charge and discharge despite its high theoretical capacity and is therefore limited in use as an active material, is more preferable.

The positive electrode 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; Li _{1 + x} Mn _{2 - x} O ₄ (Where x is 0 to 0.33), lithium manganese oxides such as LiMnO ₃ , LiMn ₂ O ₃ and LiMnO ₂ ; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ , Cu ₂ V ₂ O ₇ and the like; Ni-site type lithium nickel oxide represented by the formula LiNi _1-x M _x O ₂ , wherein 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 (wherein M = Co, Ni, Fe, Cr, Zn or Ta and x = 0.01 to 0.1) or Li ₂ Mn ₃ MO ₈ (wherein M = Fe, Co, Lithium manganese composite oxide represented by Ni, Cu or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ , and the like. However, the present invention is not limited to these.

The conductive material is added to further improve the conductivity of the electrode active material, and is typically added in an amount of 1 to 50 wt% based on the total weight of the mixture including the electrode active material. However, the electrode mixture according to the present invention may be selectively included since the binder itself contains conductive particles.

The conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery. Examples of the conductive material include graphite such as natural graphite and artificial graphite; Carbon blacks 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 powder, aluminum powder and nickel powder; Conductive whiskeys 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. Specific examples of commercially available conductive materials include Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, Ketjenblack and EC, which are acetylene black series. Family (Armak Company), Vulcan XC-72 (manufactured by Cabot Company) and Super P (manufactured by Timcal).

The binder included in the electrode mixture is preferably included in 1 to 50% by weight based on the total weight of the electrode mixture. If the content of the binder is too small, it may be difficult to withstand the volume change generated during charging and discharging, and on the contrary, the content of the binder is too high, which may cause a decrease in the capacity of the electrode and an increase in resistance.

In addition to the electrode active material and the composite binder, the electrode mixture may further include one or two or more materials selected from the group consisting of a crosslinking accelerator, a viscosity regulator, a filler, a coupling agent, and an adhesion promoter.

The crosslinking accelerator may be added in an amount of 0 to 50% by weight based on the weight of the binder as a material for promoting crosslinking of the binder. As such a crosslinking accelerator, amines such as diethylene triamine, triethylene tetramine, diethylamino propylamine, xylene diamine, and isophorone diamine , Acid anhydrides such as dodecyl succinic anhydride, phthalic anhydride and the like can be used. In addition, polyamide resin, polyselphite resin, phenol resin, or the like may be used.

The viscosity adjusting agent may be added up to 30% by weight based on the total weight of the electrode mixture, so as to control the viscosity of the electrode mixture so that the mixing process of the electrode mixture and the coating process on the collector may be easy. Examples of such viscosity modifiers include carboxymethylcellulose, polyvinylidene fluoride and the like, but are not limited thereto. In some cases, the above-described solvent may play a role as a viscosity adjusting agent.

The filler is an auxiliary component that suppresses the expansion of the electrode, and is not particularly limited as long as it is a fibrous material without causing chemical changes in the battery. Examples of the filler include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

The coupling agent is an auxiliary component for increasing the adhesion between the electrode active material and the binder, characterized in that it has two or more functional groups, it can be used up to 30% by weight based on the binder weight. Such coupling agents include, for example, one functional group reacting with a hydroxyl group or carboxyl group on the surface of a silicon, tin, or graphite-based active material to form a chemical bond, and the other functional group is chemically bonded through a reaction with a polymer binder. It may be a material forming a. Specific examples of the coupling agent include triethoxysilylpropyl tetrasulfide, mercaptopropyl triethoxysilane, aminopropyl triethoxysilane, and chloropropyl triethoxysilane ( chloropropyl triethoxysilane, vinyl triethoxysilane, methacryloxypropyl triethoxysilane, glycidoxypropyl triethoxysilane, isocyanatopropyl triethoxysilane, cyan Although silane coupling agents, such as atopropyl triethoxysilane, are mentioned, It is not limited only to these.

The adhesion promoter may be added in an amount of 10% by weight or less based on the binder, for example, oxalic acid, adipic acid, Formic acid, acrylic acid derivatives, itaconic acid derivatives, and the like.

The present invention also provides an electrode for secondary batteries in which the electrode mixture is coated on a current collector.

In the electrode, the current collector is a portion where electrons move in the electrochemical reaction of the active material, and a negative electrode current collector and a positive electrode current collector exist according to the type of electrode.

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.

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. For example, the surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with carbon, nickel, titanium, silver or the like can be used.

These current collectors may form fine concavities and convexities on the surface thereof to enhance the bonding strength of the electrode active material, and may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

The secondary battery electrode can be used for both the negative electrode and the positive electrode, of which the negative electrode is more preferred. In particular, a silicon-based active material, a tin-based active material, a silicon-carbon-based active material, or the like having a high capacity but having a large volume change during charge and discharge is more preferable.

The anode active materials are meant to include silicon (Si) particles, tin (Sn) particles, silicon-tin alloys, their respective alloy particles, composites, and the like. Representative examples of the alloy include a solid solution such as aluminum (Al), manganese (Mn), iron (Fe), titanium (Ti), intermetallic compounds, eutectic alloys, etc., but is not limited thereto. no. The composite can be used as a preferred example, a silicone / graphite composite according to the applicant's international patent application WO 2005/011030, the contents of which are incorporated by reference in the context of the present invention. Artificial graphite and natural graphite may be used as the graphite, and the shape of the graphite is not particularly limited, and may be amorphous, flat, flake, or granular.

The secondary battery electrode is manufactured by coating an electrode mixture, in which an electrode active material, a binder, and optionally a conductive material, a filler, and the like are mixed with a current collector. Specifically, the electrode mixture may be added to a predetermined solvent to prepare a slurry, and then coated on a current collector such as a metal foil, dried, and rolled to prepare a predetermined sheet-shaped electrode.

Preferred examples of the solvent used in the preparation of the electrode slurry include dimethyl sulfoxide (DMSO), N-methyl pyrrolidon (NMP), and the like, and the solvent is the total weight of the electrode mixture. Up to 400% by weight can be used and removed during drying.

The present invention also provides a lithium secondary battery comprising the electrode. The lithium secondary battery has a structure in which a lithium salt-containing non-aqueous electrolyte is impregnated into an electrode assembly having a separator interposed between a positive electrode and a negative electrode.

When the binder is used only in the positive electrode and the negative electrode, for example, the negative electrode, a general binder known in the art may be used for the positive electrode. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers, high polymer polyvinyl alcohol, and the like.

The separator is an insulating thin film interposed between the anode and the cathode and having high ion permeability and mechanical strength. The pore diameter of the separator is generally 0.01 to 10 mu m and the thickness is generally 5 to 300 mu m. As such a separation membrane, for example, a sheet or a nonwoven fabric made of an olefin-based polymer such as polypropylene which is chemically resistant and hydrophobic, glass fiber, polyethylene or the like is used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

The lithium salt-containing non-aqueous electrolyte consists of a non-aqueous electrolyte and a lithium salt. As the non-aqueous electrolyte, a liquid solvent, a solid electrolyte, an inorganic solid electrolyte, and the like are used.

As said liquid solvent, for example, N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, fluoro ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butylo lactone, 1, 2-dimethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxoron, acetonitrile, nitromethane, Methyl formate, methyl acetate, phosphate triester, trimethoxy methane, dioxoron derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether Aprotic organic solvents such as methyl pyroionate and ethyl propionate can be used.

Examples of the organic solid electrolyte include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyedgetion lysine, polyester sulfides, polyvinyl alcohols, polyvinylidene fluorides, Polymers containing ionic dissociating groups and the like can be used.

As the inorganic solid electrolyte, for example, 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, sulfates, and the like of Li, such as Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , and the like, may 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, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

In addition, for the purpose of improving charge / discharge characteristics, flame retardancy, etc., the non-aqueous electrolyte solution includes, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, and hexaphosphate triamide. Nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxy ethanol, aluminum trichloride, etc. It may be. In some cases, in order to impart nonflammability, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further included, or carbon dioxide gas may be further included to improve high temperature storage characteristics.

In the following Examples, Comparative Examples and Experimental Examples will be described in more detail, but the present invention is not limited thereto.

[Production Example 1]

In a reactor to which a baffle was attached, 1.149 g of PVA suspending agent (88% saponification) dissolved in 149 g of distilled water and 2% by weight was added and stirred at 200 rpm. Nitrogen from which oxygen and water were removed was purged in a reactor containing distilled water and suspending agent for 1 hour. Here, 0.05 g of azobisdimethylvaleronitrile is dissolved in 75 g of vinyl acetate monomer, added to the mixture, and the reaction is carried out by raising the polymerization temperature to 30 ° C. The final reaction time was 10 hours at which time the final conversion was 89%. The polymerized polymer was washed, filtered and dried to give polyvinylacetate. 80 g of a methanol solution of sodium hydroxide (concentration 3 wt%) was added to 500 g of a methanol solution of polyvinylacetate (concentration 10 wt%) and saponified at 40 ° C. for 1 hour. The solution was filtered and again mixed with 460 g of methanol, and 120 g of a methanol solution of sodium hydroxide (concentration 3 wt%) was added, followed by secondary saponification at 40 ° C for 1 hour. The saponified polyvinyl alcohol was measured by gel permeation chromatography (WATERS ultrahydrogel ^TM linear and 250 in series, pH 6.7 phosphate buffer, Polystyrenesulfonate standard) and the degree of saponification was confirmed by ¹ H-NMR. As a result of the measurement, the degree of polymerization was 4600 and the degree of saponification was 99%.

[Production Example 2]

A polyvinylpyrrolidone having a molecular weight of 300,000 was dissolved in dimethyl sulfoxide (DMSO) such that polyvinyl alcohol having a degree of polymerization of 4600 and saponification degree of 99% or more prepared by Preparation Example 1 was 5% by weight. The polyvinyl alcohol-polyvinylpyrrolidone polymer solution was prepared by adding 30% by weight of and physically mixing the mixture for a long time.

Example 1

As a carbon-based conductive material, multiwalled carbon nanotubes were pretreated using a UV ozone generator. In order to achieve a uniform UV contact effect during UV ozone treatment, after contacting for 20 minutes, the carbon nanotube sample was redispersed with a spatula, and the above procedure was repeated to UV ozone treatment for 180 minutes. It was dispersed in dimethyl sulfoxide.

The solution including the multi-walled carbon nanotubes was mixed with the solution prepared in Preparation Example 2 so that the carbon nanotubes were well dispersed in the polyvinyl alcohol and polyvinylpyrrolidone mixture. At this time, the multi-walled carbon nanotubes were 1% to the weight of the polyvinyl alcohol and polyvinylpyrrolidone mixture. The solution was coated on a copper foil with a doctor blade to a thickness of 500 μm, dried at 130 ° C. for 2 hours, and then the copper foil was removed to prepare a polymer composite film.

[Example 2]

A polymer composite film was prepared in the same manner as in Example 1, except that the multi-walled carbon nanotubes were added to 5% by weight of the polyvinyl alcohol and polyvinylpyrrolidone mixture.

Comparative Example 1

The solution prepared in Preparation Example 2 was coated on a copper foil with a doctor blade to a thickness of 500 μm, and then dried at 130 ° C. for 2 hours, after which the copper foil was removed to prepare a polymer film.

Comparative Example 2

After mixing the multi-walled carbon nanotubes to 1% by weight to the polyvinyl alcohol prepared in Preparation Example 1 to prepare a polymer composite film in the same manner as in Example 1.

Comparative Example 3

A polymer composite film was prepared by dissolving polyvinylidene fluoride (polyvinylidene fluoride), which is used as a binder of a conventional lithium secondary battery in NMP (N-methyl-2-pyrrolidone) as a dispersion medium, in 10% by weight. Prepared.

Example 3

88 g of the silicon-graphite composite active material, 13 g of the composite binder of Example 1, and 2 g of carbon black as the conductive material were mixed with dimethyl sulfoxide (DMSO) as a solvent, and the total solid content was 30% by weight so that the slurry was mixed. Prepared. The slurry was coated on a copper foil to a thickness of 100 μm using a doctor blade, then placed in a dry oven at 130 ° C., dried for 30 minutes, and then rolled to a suitable thickness to prepare a negative electrode.

Example 4

A negative electrode was prepared in the same manner as in Example 3, except that the composite binder of Example 2 was used instead of the composite binder of Example 1.

[Comparative Example 4]

A negative electrode was prepared in the same manner as in Example 3, except that 13 wt% of the binder of Comparative Example 1 was used instead of the composite binder of Example 1.

[Comparative Example 5]

A negative electrode was prepared in the same manner as in Example 3, except that 13 wt% of the binder of Comparative Example 2 was used instead of the composite binder of Example 1.

[Comparative Example 6]

A negative electrode was prepared in the same manner as in Example 3, except that NMP was used instead of DMSO as a solvent, and 13 wt% of the binder of Comparative Example 3 was used instead of the composite binder of Example 1.

[Example 5]

A negative electrode plate prepared in Example 3 was drilled in a circular shape with a surface area of 1.49 cm ² to form a working electrode, and a coin-type half cell was produced using a metallic lithium foil drilled in a surface area of 1.77 cm ² as a jackpot. It was. A separator made of a polyolefin microporous membrane was interposed between the working electrode and the counter electrode, and then EC (ethyl carbonate): DEC (diethyl carbonate): EMC (ethyl-metyl carbonate) = 4: 3: 3 (volume ratio) mixed solvent was used. An electrolyte solution in which a LiPF ₆ electrolyte was dissolved at a concentration of 1 M was added thereto to complete a lithium secondary battery.

[Example 6]

A lithium secondary battery was completed in the same manner as in Example 5, except that the negative electrode completed in Example 4 was used.

[Comparative Example 7]

A lithium secondary battery was completed in the same manner as in Example 5, except that the negative electrode prepared in Comparative Example 4 was used.

Comparative Example 8

A lithium secondary battery was completed in the same manner as in Example 5, except that the negative electrode prepared in Comparative Example 5 was used.

[Comparative Example 9]

A lithium secondary battery was completed in the same manner as in Example 5, except that the negative electrode prepared in Comparative Example 6 was used.

The following experiments were performed to analyze the properties of the polymer film and the electrode prepared according to the present invention.

Experimental Example 1

In order to measure the elongation of the polymer films according to Examples 1 and 2 and Comparative Examples 1 and 2, an experiment was performed based on the ASTM D638 method, and the results are shown in Table 1 below. Here, the evaluation took the average value after measuring the elastic force with respect to five or more samples.

[Experimental Example 2]

In order to compare the electrical conductivity of the polymer films according to Examples 1 and 2 and Comparative Examples 1 and 2, the surface resistance of the prepared films was measured, and the results are shown in Table 1 below.

TABLE 1

As shown in Table 1, since the polymer films according to Examples 1 and 2 include conductive particles, the polymer films of Comparative Examples 1 and 3 have extremely low internal resistance, and thus exhibit very good electrical conductivity. Able to know. In addition, it can be seen that the polymer film according to Examples 1 and 2 exhibits excellent elasticity since it has a significantly improved elongation compared to the polymer films of Comparative Examples 2 and 3 that do not contain polyvinylpyrrolidone.

[Experimental Example 3]

In order to measure the adhesive force between the electrode active material and the current collector when the polymer film according to the present invention was used as a negative electrode binder of a lithium secondary battery, the prepared electrode surface was cut to a fixed size and fixed to a slide glass, and then the current collector. Stripping strength was measured by peeling off 180 and the results are shown in Table 2 below. Evaluation was made as an average value by measuring 5 or more peeling strengths.

TABLE 2

As shown in Table 2, when measuring the adhesive force of the fresh electrode, the composite binder (Examples 3 and 4) according to the present invention exhibits almost the same adhesive strength as the case containing only PVA (Comparative Example 5) Bar, the physical mixing with polyvinylpyrrolidone showed little effect on the adhesion of the electrode. In addition, the adhesive force of the binder of Example 4, which increased the content of the conductive particles compared to the binder of Example 3, appeared to be further increased, which was induced by physical bonding with the binder component by the conductive particles that had been pretreated. It is assumed that this is because the bonding force between materials is further improved. On the other hand, it can be seen that the adhesive strength of the composite binder according to the present invention is improved by 2.5 times or more compared to the electrode (Comparative Example 6) using the PVdF binder that is used in the past.

[Experimental Example 4]

In order to evaluate the performance of the coin-type battery, the battery was repeatedly charged and discharged two times with 0.1 C constant current / constant voltage method and 50 cycles with 0.5 C constant current / constant voltage method. The expansions were compared respectively. Evaluation was made into the average value after evaluating five or more coin type batteries about the same binder composition. The results are shown in Table 3 below.

TABLE 3

As shown in Table 3, when the composite binder according to the present invention is used (Examples 5 and 6), the volume expansion is greatly reduced by including PVP, and the battery using the binder containing conductive particles in PVA (Comparative Example 8) Alternatively, the design capacity of the battery can be significantly increased as compared with the battery (Comparative Example 9) using a binder containing conductive particles in the conventional PVdF, and the cycle characteristics of the battery were also significantly improved. As described above, it is possible to physically mix polyvinylpyrrolidone having excellent elongation (see Table 1) with high polymerization polyvinyl alcohol having good adhesion (see Table 2) to maintain adhesion and cracking of electrodes during charge and discharge cycles. It can be seen from the volume expansion rate that this is because it minimized.

In addition, it was confirmed that the battery of the present invention significantly improved the electrical conductivity by including the conductive particles (see Table 1), thereby significantly reducing the charge and discharge time of the battery up to 0.5 hours or more (see Table 3). Fast charging is possible.

Therefore, the secondary battery including the binder according to the present invention has excellent battery characteristics by improving cycle characteristics and electrical conductivity.

Those skilled in the art to which the present invention pertains will be able to perform various applications and modifications within the scope of the present invention based on the above contents.

As described above, the composite binder of polyvinyl alcohol and polyvinylpyrrolidone including the conductive particles according to the present invention, and a lithium secondary battery including the same, add the conductive particles to a polymer binder having a very high adhesive strength and an improved elongation. By improving the electrical conductivity, it is possible to maintain a stable cycle characteristics due to the bonding strength between the active material and the current collector, it is possible to improve the charge and discharge rate can have excellent overall battery characteristics.

Claims (15)

A binder for mixing electrodes of a secondary battery, the binder comprising polyvinyl alcohol, polyvinylpyrrolidone, and conductive particles pretreated with UV ozone or acid, wherein the conductive particles are uniformly distributed in the polymer resin. Composed composite binder. The composite binder according to claim 1, wherein the conductive particles are contained in an amount of 0.01 to 20% by weight based on the total weight of the binder. The composite binder according to claim 1, wherein the conductive particles are carbon-based particles. The composite binder of claim 3, wherein the carbon-based particles are carbon black, carbon fiber, or carbon nanotubes. delete The composite binder of claim 1, wherein the polyvinyl alcohol has a degree of polymerization of at least 3000 and a degree of saponification of at least 80%. The composite binder of claim 1, wherein the polyvinylpyrrolidone has a molecular weight of 1000 to 1,000,000. The composite binder of claim 1, wherein the polyvinylpyrrolidone is included in an amount of 0.1 to 100 parts by weight based on 100 parts by weight of polyvinyl alcohol. An electrode mixture comprising an electrode active material, the composite binder according to any one of claims 1 to 4 and 6 to 8, and optionally a conductive material. 10. The electrode mixture of claim 9, wherein the binder is included in an amount of 1 to 50 wt% based on the total weight of the electrode mixture. 10. The electrode mixture according to claim 9, wherein the mixture further comprises one or more substances selected from the group consisting of a crosslinking accelerator, a viscosity modifier, a filler, a coupling agent, and an adhesion promoter. The electrode for secondary batteries in which the electrode mixture of Claim 9 is apply | coated to the electrical power collector. The electrode of claim 12, wherein the electrode is a negative electrode. The secondary battery electrode according to claim 13, wherein the negative electrode comprises a carbon-based active material, a silicon-based active material, a tin-based active material, or a silicon-carbon-based active material. A lithium secondary battery comprising the electrode according to claim 12.