KR20210015048A - Binder for improving a adhesion of positive electrode, positive electrode for lithium secondary battery including the same and lithium secondary battery including the positive electrode - Google Patents

Abstract

Disclosed are a binder, which can improve the performance of a battery by adsorbing lithium polysulfide while increasing adhesion of an electrode by adding a nonionic organic polymer interacting electrostatically with an acrylic acid group to an electrode binder, for improving adhesion of a positive electrode, a positive electrode for a lithium secondary battery including the same, and a lithium secondary battery including the positive electrode. The binder for improving adhesion of the positive electrode comprises: a nonionic organic polymer containing at least one nitrogen atom; a lithium-substituted polyacrylic acid-based polymer; and a dispersant.

Description

BACKGROUND OF THE INVENTION Binder for improving a adhesion of positive electrode, positive electrode for lithium secondary battery including the same and lithium secondary battery including the positive electrode}

The present invention relates to a binder for improving positive electrode adhesion, a positive electrode for a lithium secondary battery including the same, and a lithium secondary battery including the positive electrode, and more particularly, a nonionic organic polymer that interacts with an acrylic acid group and electrostatically is added to the electrode binder. The present invention relates to a binder for improving positive electrode adhesion, a positive electrode for a lithium secondary battery including the same, and a lithium secondary battery including the positive electrode, which can improve the performance of a battery by adsorbing lithium polysulfide while increasing the adhesion of the electrode.

As the interest in energy storage technology increases, the field of application of mobile phones, tablets, laptops, camcorders, and even electric vehicles (EVs) and hybrid electric vehicles (HEVs) is expanded. Research and development of chemical devices are gradually increasing. Electrochemical devices are the field that is receiving the most attention in this aspect, and among them, the development of lithium-based secondary batteries such as lithium-sulfur batteries capable of charging and discharging has become the focus of interest, and in recent years, capacity In order to improve the density and specific energy, research and development on the design of new electrodes and batteries are being conducted.

Lithium-sulfur (Li-S) batteries of such electrochemical devices, especially lithium secondary batteries, are batteries using sulfur as a positive electrode active material and lithium as a negative electrode active material, and have a theoretical high energy density, replacing lithium ion batteries. It is in the spotlight as a next-generation secondary battery that can be used. The lithium-the sulfur battery, the discharge when occurs the oxidation reaction of the reducing reaction with the lithium metal of sulfur, at this time, the sulfur lithium poly linear structure from the S ₈ of the ring structure sulfide _{(Li 2 S 2, Li 2} S 4, Li ₂ S ₆ , Li ₂ S ₈ ) is formed, and the lithium-sulfur battery is characterized by a stepwise discharge voltage until polysulfide (PS) is completely reduced to Li ₂ S.

The sulfur positive electrode of such a lithium-sulfur battery is non-conductive, and most of the positive electrode materials are mixed with sulfur that contributes to the electrochemical reaction and a conductive carbon-based material that is a support thereof (that is, In order to realize electrochemical activity, it is used as a sulfur-carbon composite composed of a porous carbon material with a large specific surface area), and the carbon-based material used at this time affects the reactivity, high rate characteristics, and life characteristics of sulfur. However, in the lithium-sulfur battery, Li ₂ S, the final reaction product of which sulfur is reduced, changes the electrode structure as the volume increases compared to S, and the intermediate product, polysulfide, is easily dissolved in the electrolyte, so it is continuously dissolved during the discharge reaction. The amount of the positive active material decreases. As a result, deterioration of the battery is accelerated, and long-term stability of the battery cannot be secured, such as deterioration of reactivity and life characteristics of the battery.

As one of the solutions to this problem, there is a method of using a large amount of binder to increase the binding force between porous carbon materials with a large specific surface area, but in this case, the resistance increases, which may rather act as a factor that impairs the performance of the battery. . As another method for improving the performance of the battery, a lithium ionic polymer binder may be used, or an adsorption functional element may be provided to suppress the elution of lithium polysulfide, which is a discharge intermediate product. To this end, a binder containing polyacrylic acid or lithium-substituted polyacrylic acid can be used to increase the adhesion of the electrode and also facilitate the movement of lithium ions. When a binder having a high viscosity such as acrylic acid is used, a slurry dispersion problem may occur, and when a dispersant is additionally applied, there is a concern that electrode adhesion may be deteriorated. Therefore, there is a need for an optimization method capable of improving the performance of a battery by combining a binder, a dispersant, and a functional element.

Accordingly, an object of the present invention is to increase the adhesion of the electrode by adding a nonionic organic polymer that interacts with an acrylic acid group electrostatically to an electrode binder, and at the same time, improve the performance of the battery by adsorbing lithium polysulfide. To provide a binder, a positive electrode for a lithium secondary battery including the same, and a lithium secondary battery including the positive electrode.

In order to achieve the above object, the present invention, a nonionic organic polymer containing at least one nitrogen atom; Lithium substituted polyacrylic acid polymer; It provides a binder for improving electrode adhesion comprising; and a dispersant.

In addition, the present invention provides a positive electrode for a lithium secondary battery comprising; a binder for improving the electrode adhesion.

In addition, the present invention, the positive electrode for the lithium secondary battery; Lithium-based negative electrode; An electrolyte interposed between the positive electrode and the negative electrode; It provides a lithium secondary battery comprising; and a separator.

According to the binder for improving positive electrode adhesion according to the present invention, a positive electrode for a lithium secondary battery including the same, and a lithium secondary battery including the positive electrode, a nonionic organic polymer that interacts with an acrylic acid group electrostatically is added to the electrode binder to improve the adhesion of the electrode. At the same time, there is an advantage of improving battery performance by adsorbing lithium polysulfide.

1 is a real image showing the viscosity of a positive electrode binder for a lithium secondary battery (a) and a conventional positive electrode binder (b) for a lithium secondary battery according to an embodiment of the present invention.
2 is a graph showing a measurement of rheological properties of a positive electrode binder for a lithium secondary battery according to an embodiment of the present invention.
3 is a graph showing discharge capacity according to charge/discharge life of a lithium secondary battery and a typical lithium secondary battery according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail.

The binder for improving positive electrode adhesion according to the present invention includes a nonionic organic polymer including one or more nitrogen atoms, a lithium-substituted polyacrylic acid polymer, and a dispersant.

In order to improve the performance of the battery, conventionally, a large amount of binder is used to increase the binding force between porous carbon materials with a large specific surface area, a lithium ion polymer binder is used, or polyacrylic acid or lithium-substituted poly Although a binder containing acrylic acid (lithiated polyacrylic acid) was used, problems such as an increase in resistance, a slurry dispersion problem, and a decrease in electrode adhesion inevitably occur. Accordingly, the applicant of the present invention improves battery performance by adsorbing lithium polysulfide while increasing the adhesion of the electrode by adding a nonionic organic polymer that interacts with an acrylic acid group electrostatically to the electrode binder.

That is, the present invention is a polymer additive that induces electrostatic interaction in order to improve the adhesion of the electrode and the performance of the battery using a lithium-substituted polyacrylic acid type polymer. In order to alleviate the slurry dispersion issue due to the high viscosity of the polyacrylic acid-based polymer substituted with lithium, a dispersant such as PVA is used as a binder, and in this case, the adhesive strength of the electrode is rapidly deteriorated.

First, in the present invention, the nonionic organic polymer, which is a polymer additive that induces an electrostatic interaction, is applied as an additive to the binder for improving positive electrode adhesion of the present invention, and other components included in the binder, that is, lithium-substituted poly It is a component capable of improving the adhesion or binding power of an electrode by electrostatic interaction with an acrylic acid polymer (specifically, an acrylic acid group included in a lithium-substituted polyacrylic acid polymer, etc.). In addition, since the nonionic organic polymer is rich in nitrogen (N) atoms (particularly,'branched polyethyleneimine' to be described later), it has not only the adhesion of the electrode but also the function of adsorbing lithium polysulfide. .

Therefore, when the nonionic organic polymer additive of the present invention is mixed with a lithium-substituted polyacrylic acid polymer, the adhesion of the electrode is increased, the elution of lithium polysulfide is suppressed, and the performance of the battery such as capacity retention according to the charge/discharge life is improved. It can be improved or improved.

As described above, the nonionic organic polymer is one containing one or more nitrogen atoms, branched polyethyleneimine (Branched PEI), polyallylamine (poly allylamine, the number of nitrogen atoms per unit molecule is 1), derivatives thereof And mixtures thereof and the like can be exemplified (a non-polymer such as a monomolecular multiamine is not applicable to the present invention). However, as the number of nitrogen atoms per unit molecule increases, the capacity retention rate of the battery is relatively high, so it may be preferable to apply branched polyethyleneimine as a nonionic organic polymer rather than polyallylamine.

In addition, the weight average molecular weight (Mw) of the nonionic organic polymer is 10,000 to 800,000 (10k to 800k), and when the weight average molecular weight of the nonionic organic polymer is out of the above range, the adhesion improvement effect is insignificant or resistance Increasing problems may occur, etc. may deviate from the spirit of the present invention. In particular, when the weight average molecular weight of the nonionic organic polymer is excessively large, resistance may increase, thereby inhibiting the spirit of the present invention.

On the other hand, the content of the nonionic organic polymer is 0.1 to 2 parts by weight, preferably 0.2 to 1.5 parts by weight, more preferably 0.5 to 1.5 parts by weight based on 100 parts by weight of the lithium-substituted polyacrylic acid polymer to be described later. As an example, when the content of the nonionic organic polymer is out of the above range, there is a concern that the electrode adhesion is rather reduced.

Next, the lithium-substituted polyacrylic acid-based polymer includes two or more types of lithium-substituted polyacrylic acids (Li-PAA) having different molecular weights, and has a form in which hydrogen of a carboxyl group (COOH) in polyacrylic acid is substituted with lithium, and an electrode It is a component for increasing the adhesion of and smoothing the movement of lithium ions, and it is possible to prevent a slurry dispersion problem through electrostatic mutual bonding with the nonionic organic polymer. As such a lithium-substituted polyacrylic acid-based polymer (lithium substitution degree may be 70% or more, preferably 90% or more), lithium-substituted polyacrylic acid (Li-PAA), more specifically lithium-substituted acrylic acid as a monomer. A homopolymer, a block copolymer containing lithium-substituted acrylic acid as a monomer, and a mixture thereof can be exemplified. In the case of the block copolymer containing the lithium-substituted acrylic acid as a monomer, at least one hydrophobic monomer is included in addition to the lithium-substituted acrylic acid monomer.In this case, if it is included in an excessive amount, it is not dissolved in water and the value as a binder decreases. It may be desirable to ensure that the proportion of acrylic acid monomer is at least 60% or higher. On the other hand, the polyacrylic acid-based polymer, which is not lithium-substituted, causes agglomeration when it is mixed with the nonionic organic polymer additive, making it impossible to manufacture a slurry and an electrode.

When the molecular weight of the lithium-substituted polyacrylic acid-based polymer is described in more detail, the weight average molecular weight (Mw) of the lithium-substituted polyacrylic acid-based polymer may be 5,000 to 2,000,000. The lithium-substituted polyacrylic acid-based polymer may include two kinds of lithium-substituted polyacrylic acids having a difference in weight average molecular weight of 500,000 or more, preferably 500,000 to 800,000. If the lithium-substituted polyacrylic acid-based polymer does not contain two kinds of lithium-substituted polyacrylic acids having a difference in weight average molecular weight of 500,000 or more from each other, the effect of high molecular weight lithium-substituted polyacrylic acid and lithium substitution of low molecular weight It is difficult to simultaneously exhibit the effects of polyacrylic acid.

For example, when the lithium-substituted polyacrylic acid-based polymer includes two types of lithium-substituted polyacrylic acid polymers having different molecular weights, the weight average molecular weight of one lithium-substituted polyacrylic acid may be 450,000, and the other lithium The weight average molecular weight of the substituted polyacrylic acid may be 1,250,000. In addition, when the lithium-substituted polyacrylic acid-based polymer includes two types of lithium-substituted polyacrylic acid polymers having different molecular weights, the weight ratio of one lithium-substituted polyacrylic acid and the other lithium-substituted polyacrylic acid is 1:1 to 5 : It may be 2, and if it is out of the weight ratio, there is a concern that the optimum combination may be deviated due to a trade-off between slurry dispersibility and electrode adhesion. In this case, the higher the fraction of the lithium-substituted polyacrylic acid polymer having a high molecular weight, the better the adhesion.

More specifically, the lithium-substituted polyacrylic acid-based polymer may include a low molecular weight lithium-substituted polyacrylic acid having a weight average molecular weight of 5,000 to 800,000, preferably 450,000 to 750,000. When the low molecular weight lithium-substituted polyacrylic acid has a weight average molecular weight of less than 5,000, it may negatively affect the adhesion properties of the electrode and the cycle performance of the battery. In contrast, when the low molecular weight lithium-substituted polyacrylic acid has a weight average molecular weight of more than 800,000, the effect of improving the processability of the binder and thus improving the performance of the battery according to the introduction of the low molecular weight lithium-substituted polyacrylic acid may be insignificant.

In addition, the lithium-substituted polyacrylic acid-based polymer may include a high molecular weight lithium-substituted polyacrylic acid having a weight average molecular weight of 1,000,000 to 2,000,000, and preferably 1,250,000 to 1,750,000. When the high molecular weight lithium-substituted polyacrylic acid has a weight average molecular weight of less than 1,000,000, the effect of improving the adhesion characteristics of the electrode and the cycle characteristics of the battery may be insignificant. On the other hand, when the high molecular weight lithium-substituted polyacrylic acid has a weight average molecular weight of more than 2,000,000, the processability of the binder is markedly deteriorated, and the processability may be difficult to recover even by mixing of lithium-substituted polyacrylic acid having a low molecular weight.

On the other hand, to explain in more detail about the electrostatic interaction between the nonionic organic polymer and the lithium-substituted polyacrylic acid polymer, the electrostatic interaction between the two polymers is the functional group of each polymer involved in the interaction. As a result, the number of functional groups capable of hydrogen bonding is reduced, which makes it difficult to penetrate moisture in the electrode, so that the moisture content in the electrode is relatively low, which means that gas generation inside the battery is reduced.

Finally, the dispersant is used to improve the dispersibility of the electrode slurry, and examples thereof include poly(vinyl alcohol), a compound having similar physical properties, and a mixture thereof. The weight average molecular weight (Mw) of the dispersant is preferably 10,000 to 100,000 (10k to 100k), and if the weight average molecular weight of the dispersant is out of the above range, problems of lowering adhesion and dispersibility may occur. .

Meanwhile, the content of the dispersant is 1 to 20 parts by weight, preferably 5 to 15 parts by weight based on 100 parts by weight of the lithium-substituted polyacrylic acid polymer, and the content of the dispersant is the lithium-substituted polyacrylic acid polymer If the content is less than 1 part by weight based on 100 parts by weight of, there may be a decrease in dispersibility, and if it exceeds 20 parts by weight, there may be a problem of increasing resistance along with a decrease in adhesion.

Next, a positive electrode for a lithium secondary battery according to the present invention will be described. The positive electrode for a lithium secondary battery includes the above-described binder for improving electrode adhesion, a positive electrode active material, and a conductive material, which may be formed on the positive electrode current collector.

The content of the binder in the positive electrode may be selected in consideration of the performance of a target battery, and in addition to the binder of the present invention, a binder generally used in the relevant technical field may be additionally used. As an exemplary additional binder, a fluororesin binder including polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE), styrene-butadiene rubber, acrylonitrile-butadiene rubber, styrene-isoprene One selected from the group consisting of rubber-based binders including rubber, polyalcohol binders, polyethylene, polyolefin-based binders including polypropylene, polyimide-based binders, polyester-based binders mussel adhesives, silane-based binders, and polyacrylate-based binders I can.

The content of the positive electrode active material may also be selected in consideration of the performance of a target battery. The positive electrode active material may be selected from elemental sulfur (S8), a sulfur-carbon composite, a sulfur-based compound, or a mixture thereof, but is not limited thereto. Specifically, the sulfur-based compound may be Li ₂ S _n (n≥1), an organosulfur compound or a carbon-sulfur polymer ((C ₂ S _x ) _n : x=2.5 to 50, n≥2). These are preferably applied in combination with a conductive material because the sulfur material alone has no electrical conductivity. In addition, the sulfur-carbon composite may be an aspect of a cathode active material in which carbon and sulfur are mixed to reduce the leakage of sulfur into the electrolyte and increase the electrical conductivity of the sulfur-containing electrode. The carbon material constituting the sulfur-carbon composite may be crystalline or amorphous carbon, or may be conductive carbon. Specifically, graphite, graphene, super p, carbon black, denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, carbon It may be selected from the group consisting of nanofibers, carbon nanotubes, carbon nanowires, carbon nanorings, carbon fabrics, and fullerenes (C60).

Examples of such a sulfur-carbon composite include a sulfur-carbon nanotube composite. Specifically, the sulfur-carbon nanotube composite includes a carbon nanotube aggregate having a three-dimensional structure, and a sulfur or sulfur compound provided on at least a portion of an inner surface and an outer surface of the carbon nanotube aggregate. Since the sulfur-carbon nanotube composite contains sulfur in the three-dimensional structure of the carbon nanotube, even if soluble polysulfide is produced by an electrochemical reaction, if it can be located inside the carbon nanotube, when polysulfide is eluted Also, the structure that is entangled in three dimensions is maintained, so that the collapse of the anode structure can be suppressed. As a result, the lithium-sulfur battery including the sulfur-carbon nanotube composite has an advantage that a high capacity can be realized even in high loading. In addition, the sulfur or sulfur-based compound may be provided in the inner pores of the carbon nanotube aggregate. The carbon nanotube means linear conductive carbon, and specifically, carbon nanotube (CNT), graphitic nanofiber (GNF), carbon nanofiber (CNF), or activated carbon fiber (ACF) Can be used, single-walled carbon nanotubes (SWCNT) or multi-walled carbon nanotubes (MWCNT) can be used.

According to an example of the present invention, the sulfur-carbon composite is prepared by impregnating sulfur or a sulfur-based compound into the outer surface and the inner surface of carbon, and optionally, the diameter of carbon before, after, or before and after the impregnation step. You can go through the steps of adjusting. The impregnating step may be carried out by mixing carbon and sulfur or sulfur-based compound powder, followed by heating, and impregnating the molten sulfur or sulfur-based compound into carbon, and during such mixing, a dry ball mill method, a dry jet mill method, or a dry type The dyno mill method can be used.

The content of the conductive material may also be selected in consideration of the performance of a target battery. The conductive material is graphite such as natural graphite or artificial graphite; Carbon black, such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; Conductive fibers such as carbon fibers or metal fibers; Metal powders such as carbon fluoride, aluminum or nickel powder; Conductive whiskey such as zinc oxide or potassium titanate; Conductive metal oxides such as titanium oxide; Alternatively, it may be selected from polyphenylene derivatives, but is not limited thereto.

The positive electrode may further include other components in addition to the above-described binder, positive electrode active material, and conductive material. Components that can be additionally applied to the positive electrode include a crosslinking agent or a conductive material dispersant. The crosslinking agent may be one having two or more functional groups capable of reacting with the crosslinkable functional group of the polymer in order to allow the polymer of the binder to form a crosslinking network. The crosslinking agent is not particularly limited, but may be selected from an isocyanate crosslinking agent, an epoxy crosslinking agent, an aziridine crosslinking agent, or a metal chelate crosslinking agent. The crosslinking agent may be an isocyanate crosslinking agent. In addition, the conductive material dispersant helps to disperse the non-polar carbon-based conductive material to form a paste. The conductive material dispersant is not particularly limited, but carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, a cellulose compound including regenerated cellulose, polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP ) Can be selected from. The conductive material dispersant may be preferably polyvinyl alcohol (PVA).

Meanwhile, a solvent may be used in the manufacture of the positive electrode. The type of solvent may be appropriately selected in consideration of the performance of a target battery. The solvent is N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyllolactone, 1,2- Dimethoxy ethane, tetrahydrofuran, 2-methyl tetrahydrofuran, dimethyl sulfoxide, formamide, dimethylformamide, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate tryester, trimethoxy methane, sulfolane, Methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, organic solvents such as methyl propionate or ethyl propionate, and water may be selected from. Among these, when water is used as a solvent, it may be advantageous in terms of drying temperature or environmental aspects.

The positive electrode current collector is not particularly limited as long as it is generally used in the manufacture of a positive electrode. The positive electrode current collector may be one or more materials selected from stainless steel, aluminum, nickel, titanium, calcined carbon and aluminum, and if necessary, the surface of the material may be treated with carbon, nickel, titanium, or silver. . The shape of the positive electrode current collector may be selected from a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric. Further, the thickness of the positive electrode current collector is not particularly limited, and may be set in an appropriate range in consideration of the mechanical strength of the positive electrode, productivity, or capacity of a battery.

Finally, a lithium secondary battery according to the present invention will be described. The lithium secondary battery includes the above-described positive electrode for a lithium secondary battery, a lithium-based negative electrode, an electrolyte and a separator interposed between the positive electrode and the negative electrode, and the lithium secondary battery is a lithium-based secondary battery known in the art such as a lithium-sulfur battery. It can be a battery.

The negative electrode may be manufactured according to a conventional method known in the art. For example, a negative electrode active material, a conductive material, a binder, and if necessary, a filler, etc., are dispersed and mixed in a dispersion medium (solvent) to make a slurry, which is coated on the negative electrode current collector, and then dried and rolled to prepare a negative electrode. . As the negative active material, lithium metal or a lithium alloy (eg, an alloy of lithium and a metal such as aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, or indium) may be used. The anode current collector includes platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), ruthenium (Ru), nickel (Ni), stainless steel (STS), copper (Cu). ), molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W), ITO (In doped SnO ₂ ), FTO (F doped SnO ₂ ), and alloys thereof , Copper (Cu) or stainless steel may be surface-treated with carbon (C), nickel (Ni), titanium (Ti), or silver (Ag), but is not limited thereto. The negative electrode current collector may be in the form of a foil, a film, a sheet, a punched one, a porous body, or a foam.

As the electrolyte or electrolyte, carbonate, ester, ether, or ketone may be used alone or in combination of two or more as a non-aqueous electrolyte (non-aqueous organic solvent), but is not limited thereto. For example, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methylethyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, n-methyl acetate, n- Such as ethyl acetate, n-propyl acetate, phosphoric acid tryster, dibutyl ether, N-methyl-2-pyrrolidinone, 1,2-dimethoxy ethane, tetrahydroxy franc (Franc), 2-methyl tetrahydrofuran Tetrahydrofuran derivatives, dimethylsulfoxide, formamide, dimethylformamide, dioxolone and derivatives thereof, acetonitrile, nitromethane, methyl formate, methyl acetate, trimethoxy methane, sulfolane, methyl sulfolane, 1,3 -An aprotic organic solvent such as dimethyl-2-imidazolidinone, methyl propionate, and ethyl propionate may be used, but is not limited thereto.

A lithium salt may be further added to the electrolyte solution (so-called lithium salt-containing non-aqueous electrolyte solution), and the lithium salt is a well-known one that is soluble in a non-aqueous electrolyte solution, such as 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 , LiPF ₃ (CF ₂ CF ₃ ) ₃ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, lithium chloroborane, lithium lower aliphatic carboxylic acid, lithium 4 phenyl borate, and imide, but are not limited thereto. In the (non-aqueous) electrolyte solution, for the purpose of improving charge/discharge properties and flame retardancy, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide , Nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol, aluminum trichloride, etc. May be. If necessary, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further included in order to impart non-flammability, and carbon dioxide gas may be further included in order to improve high-temperature storage characteristics.

The separator is interposed between the positive electrode and the negative electrode to prevent a short circuit therebetween and serves to provide a passage for lithium ions to move. As the separator, olefin-based polymers such as polyethylene and polypropylene, glass fibers, etc. may be used in the form of sheets, multi-membrane, microporous films, woven fabrics and non-woven fabrics, but are not limited thereto. On the other hand, when a solid electrolyte such as a polymer (eg, organic solid electrolyte, inorganic solid electrolyte, etc.) is used as the electrolyte, the solid electrolyte may also serve as a separator. Specifically, an insulating thin film having high ion permeability and mechanical strength is used. The separator may have a pore diameter of generally 0.01 to 10 μm, and a thickness of 5 to 300 μm.

On the other hand, the lithium secondary battery of the present invention can be manufactured according to a conventional method in the art. For example, it can be prepared by putting a porous separator between the positive electrode and the negative electrode, and adding a non-aqueous electrolyte. The lithium secondary battery according to the present invention can be particularly suitably used as a unit cell of a battery module, which is a power source for medium and large devices, as well as applied to a battery cell used as a power source for a small device. In this respect, the present invention also provides a battery module including two or more lithium secondary batteries electrically connected (series or parallel). It goes without saying that the number of lithium secondary batteries included in the battery module may be variously adjusted in consideration of the use and capacity of the battery module.

Furthermore, the present invention provides a battery pack in which the battery modules are electrically connected according to conventional techniques in the art. The battery module and the battery pack may include a power tool; Electric vehicles including electric vehicles (EV), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); Electric truck; Electric commercial vehicles; Alternatively, it can be used as a power supply for any one or more medium and large devices among the power storage systems, but is not limited thereto.

Hereinafter, preferred embodiments are presented to aid in the understanding of the present invention, but this is only illustrative of the present invention, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and spirit of the present invention. It is natural that the modifications belong to the appended claims. On the other hand, it should be noted that in the following Examples and Comparative Examples, the'molecular weight' mark is for convenience only, and actually means'weight average molecular weight'.

[Example 1] Preparation of positive electrode for lithium secondary battery

First, a lithium-substituted polyacrylic acid-based polymer by mixing a first lithium-substituted polyacrylic acid (Sigma-Aldrich, molecular weight: 450,000) and a second lithium-substituted polyacrylic acid (Sigma-Aldrich, molecular weight: 1,250,000) in a weight ratio of 5:2. After (Li-PAA) was prepared, 0.5 parts by weight of a nonionic organic polymer polyallylamine (PallAM, Sigma-Aldrich, molecular weight: 15,000) was added to 100 parts by weight of a lithium-substituted polyacrylic acid polymer, Subsequently, 7 parts by weight of polyvinyl alcohol (PVA, Sigma-Aldrich, molecular weight: 15,000) as a dispersant was added to 100 parts by weight of a lithium-substituted polyacrylic acid polymer to prepare a binder for improving positive electrode adhesion.

Separately, sulfur (Sigma-Aldrich) was mixed with carbon nanotubes using a ball mill and then heat-treated at 155°C to prepare a sulfur-carbon composite positive electrode active material, and as a conductive material, VGCF (Vapor-grown Carbon Fiber ) Was prepared.

After adding the above positive electrode active material, conductive material, and binder to water as a solvent and mixing with a mixer (the mixing ratio is a weight ratio, positive electrode active material: conductive material: binder = 88:5:7), then apply it to an aluminum foil current collector and apply at 50℃ Drying at for 2 hours to prepare a positive electrode.

[Example 2] Preparation of positive electrode for lithium secondary battery

The mixing ratio (weight ratio) of the first lithium-substituted polyacrylic acid and the second lithium-substituted polyacrylic acid is changed from 5:2 to 1:1, and as a nonionic organic polymer, a polyallylamine having a molecular weight of 65,000 instead of a polyallylamine having a molecular weight of 15,000. Except for using, a positive electrode was manufactured in the same manner as in Example 1.

[Example 3] Preparation of positive electrode for lithium secondary battery

The mixing ratio (weight ratio) of the first lithium-substituted polyacrylic acid and the second lithium-substituted polyacrylic acid is changed from 5:2 to 1:1, and as a nonionic organic polymer, a polyallylamine having a molecular weight of 65,000 instead of a polyallylamine having a molecular weight of 15,000. Was used, and a positive electrode was manufactured in the same manner as in Example 1, except that the content of the polyallylamine was changed from 0.5 parts by weight to 1 part by weight based on 100 parts by weight of the lithium-substituted polyacrylic acid polymer.

[Example 4] Preparation of positive electrode for lithium secondary battery

The mixing ratio (weight ratio) of the first lithium-substituted polyacrylic acid and the second lithium-substituted polyacrylic acid is changed from 5:2 to 1:1, and as a nonionic organic polymer, a polyallylamine having a molecular weight of 65,000 instead of a polyallylamine having a molecular weight of 15,000. Was used, and a positive electrode was manufactured in the same manner as in Example 1, except that the content of the polyallylamine was changed from 0.5 parts by weight to 1.5 parts by weight based on 100 parts by weight of the lithium-substituted polyacrylic acid polymer.

[Example 5] Preparation of positive electrode for lithium secondary battery

Branched polyethylene having a molecular weight of 25,000 instead of polyallylamine having a molecular weight of 15,000 as a nonionic organic polymer by changing the mixing ratio (weight ratio) of the first lithium-substituted polyacrylic acid and the second lithium-substituted polyacrylic acid from 5:2 to 1:1 Except for using imine (PEI), a positive electrode was manufactured in the same manner as in Example 1 above.

[Example 6] Preparation of positive electrode for lithium secondary battery

Branched polyethylene having a molecular weight of 25,000 instead of polyallylamine having a molecular weight of 15,000 as a nonionic organic polymer by changing the mixing ratio (weight ratio) of the first lithium-substituted polyacrylic acid and the second lithium-substituted polyacrylic acid from 5:2 to 1:1 A positive electrode was prepared in the same manner as in Example 1, except that imine was used and the amount of the nonionic organic polymer was changed from 0.5 parts by weight to 1 part by weight based on 100 parts by weight of the lithium-substituted polyacrylic acid polymer.

[Example 7] Preparation of positive electrode for lithium secondary battery

Branched polyethylene having a molecular weight of 25,000 instead of polyallylamine having a molecular weight of 15,000 as a nonionic organic polymer by changing the mixing ratio (weight ratio) of the first lithium-substituted polyacrylic acid and the second lithium-substituted polyacrylic acid from 5:2 to 1:1 A positive electrode was manufactured in the same manner as in Example 1, except that imine was used and the amount of the nonionic organic polymer was changed from 0.5 parts by weight to 1.5 parts by weight based on 100 parts by weight of the lithium-substituted polyacrylic acid polymer.

[Example 8] Preparation of positive electrode for lithium secondary battery

Branched polyethylene having a molecular weight of 750,000 instead of polyallylamine having a molecular weight of 15,000 as a nonionic organic polymer by changing the mixing ratio (weight ratio) of the first lithium-substituted polyacrylic acid and the second lithium-substituted polyacrylic acid from 5:2 to 1:1 A positive electrode was prepared in the same manner as in Example 1, except that imine was used and the amount of the nonionic organic polymer was changed from 0.5 parts by weight to 0.2 parts by weight based on 100 parts by weight of the lithium-substituted polyacrylic acid polymer.

[Example 9] Preparation of positive electrode for lithium secondary battery

Branched polyethylene having a molecular weight of 750,000 instead of polyallylamine having a molecular weight of 15,000 as a nonionic organic polymer by changing the mixing ratio (weight ratio) of the first lithium-substituted polyacrylic acid and the second lithium-substituted polyacrylic acid from 5:2 to 1:1 A positive electrode was manufactured in the same manner as in Example 1, except that imine was used.

[Example 10] Preparation of positive electrode for lithium secondary battery

Branched polyethylene having a molecular weight of 750,000 instead of polyallylamine having a molecular weight of 15,000 as a nonionic organic polymer by changing the mixing ratio (weight ratio) of the first lithium-substituted polyacrylic acid and the second lithium-substituted polyacrylic acid from 5:2 to 1:1 A positive electrode was prepared in the same manner as in Example 1, except that imine was used and the amount of the nonionic organic polymer was changed from 0.5 parts by weight to 1 part by weight based on 100 parts by weight of the lithium-substituted polyacrylic acid polymer.

[Example 11] Preparation of positive electrode for lithium secondary battery

Branched polyethylene having a molecular weight of 750,000 instead of polyallylamine having a molecular weight of 15,000 as a nonionic organic polymer by changing the mixing ratio (weight ratio) of the first lithium-substituted polyacrylic acid and the second lithium-substituted polyacrylic acid from 5:2 to 1:1 A positive electrode was prepared in the same manner as in Example 1, except that imine was used and the amount of the nonionic organic polymer was changed from 0.5 parts by weight to 2 parts by weight based on 100 parts by weight of the lithium-substituted polyacrylic acid polymer.

[Comparative Example 1] Preparation of positive electrode for lithium secondary battery

The same as in Example 1, except that the mixing ratio (weight ratio) of the first lithium-substituted polyacrylic acid and the second lithium-substituted polyacrylic acid was changed from 5:2 to 1:1, and a nonionic organic polymer was not used. To prepare a positive electrode.

[Comparative Example 2] Preparation of positive electrode for lithium secondary battery

A positive electrode was manufactured in the same manner as in Example 1, except that a nonionic organic polymer was not used.

[Comparative Example 3] Preparation of positive electrode for lithium secondary battery

A positive electrode was manufactured in the same manner as in Example 1, except that tetraethylene tetraamine (TETAm) was used instead of polyallylamine having a molecular weight of 15,000 as a nonionic organic polymer.

[Comparative Example 4] Preparation of positive electrode for lithium secondary battery

A lithium-substituted polyacrylic acid polymer (Li-PAA) is used instead of a lithium-unsubstituted polyacrylic acid (PAA), and as a nonionic organic polymer, a branched polyethyleneimine having a molecular weight of 25,000 is used instead of a polyallylamine having a molecular weight of 15,000. Except, a positive electrode was manufactured in the same manner as in Example 1.

The configurations of the binders used in Examples 1 to 11 and Comparative Examples 1 to 4 are shown in Table 1 below.

Lithium-substituted polyacrylic acid polymer Nonionic organic polymer (Mw)
/Addition amount Dispersant Example 1 Li-PAA (5: 2) PallAm (15K) / 0.5 parts by weight PVA Example 2 Li-PAA (1: 1) PallAm (65K) / 0.5 parts by weight PVA Example 3 Li-PAA (1: 1) PallAm (65K) / 1 part by weight PVA Example 4 Li-PAA (1: 1) PallAm (65K) / 1.5 parts by weight PVA Example 5 Li-PAA (1: 1) PEI (25K) / 0.5 parts by weight PVA Example 6 Li-PAA (1: 1) PEI (25K) / 1 part by weight PVA Example 7 Li-PAA (1: 1) PEI (25K) / 1.5 parts by weight PVA Example 8 Li-PAA (1: 1) PEI (750K) / 0.2 parts by weight PVA Example 9 Li-PAA (1: 1) PEI (750K) / 0.5 parts by weight PVA Example 10 Li-PAA (1: 1) PEI (750K) / 1 part by weight PVA Example 11 Li-PAA (1: 1) PEI (750K) / 2 parts by weight PVA Comparative Example 1 Li-PAA (1: 1) - PVA Comparative Example 2 Li-PAA (5: 2) - PVA Comparative Example 3 Li-PAA (5: 2) TETAm / 0.5 parts by weight PVA Comparative Example 4 PAA (5:2) PEI (25K) / 0.5 parts by weight PVA

[Experimental Example 1] Evaluation of Adhesion of Anode

The anodes prepared in Examples 1 to 11 and Comparative Examples 1 to 4 were dried at 50° C. for 2 hours, and then cut into a size of 15 cm×2 cm, and then on a slide glass with double-sided tape. A sample for a peel test was prepared through adhesion to the anode side and lamination. Subsequently, the peeling test sample was loaded onto a UTM capable of measuring adhesion, and a 90° peel test was performed to measure the applied peel resistance (gf/cm), and the adhesion of each positive electrode was calculated, and the results are as follows. It is shown in Table 2.

<Analysis conditions>

-Sample width: 20mm

-Propagation speed: 300 mm/min

-Data valid calculation section: 10 ~ 40 mm

Anode adhesion (gf/cm) Example 1 6 Example 2 14 Example 3 17 Example 4 16 Example 5 12 Example 6 13 Example 7 6 Example 8 13 Example 9 17 Example 10 16 Example 11 7 Comparative Example 1 4 Comparative Example 2 3 Comparative Example 3 One Comparative Example 4 Cannot manufacture anode

Through the measurement and calculation method as described above, as a result of evaluating the adhesion of the positive electrodes prepared in Examples 1 to 11 and Comparative Examples 1 to 4, a lithium-substituted polyacrylic acid-based polymer and polyallylamine or branched polyethyleneimine It was confirmed that the positive electrode of Examples 1 to 11, in which a nonionic organic polymer (ie, a polymer of multi-amine having a branched form) was mixed, had excellent positive electrode adhesion compared to the positive electrode of Comparative Examples 1 to 4, which was not.

More specifically, through Example 1 and Comparative Example 2, it can be seen that adhesion is increased when a lithium-substituted polyacrylic acid-based polymer and a nonionic organic polymer are mixed, and Examples 7 and 11 are compared to Comparative Examples. It has excellent adhesion, but it shows that if nonionic organic polymer is added in an excessive amount, the adhesion is rather low.

In addition, through Comparative Example 1 and Comparative Example 2, it can be seen that the higher the fraction of Li-PAA having a high molecular weight, the higher the basic adhesion, and through Comparative Example 3, the single molecule multiamine (TETAm) improves the adhesion. In the case of using polyacrylic acid in which lithium is not substituted as in Comparative Example 4, aggregation occurs when mixing with a nonionic organic polymer, so that it is not possible to manufacture a slurry and an electrode. there was. That is, FIG. 1 shows the viscosity of a positive electrode binder for a lithium secondary battery (FIG. 1 a) and a typical positive electrode binder for a lithium secondary battery (FIG. 1 b) according to an embodiment (specifically, Example 9) of the present invention. 2 is a graph showing the rheological properties of the positive electrode binder for a lithium secondary battery according to an exemplary embodiment of the present invention. The binder included in the positive electrode of Comparative Example 4 is agglomeration phenomenon as shown in b of FIG. On the other hand, it was confirmed that the binder included in the positive electrode of Example 9 was gelled as shown in FIG. 1A. When a (positive binder of Example 9) of FIG. 1 is visually observed, the binder is gelled and it seems that there is difficulty in coating.However, in the rheological property measurement graph of FIG. 2, the viscosity decreases when shear is applied. It can be seen that uniform coating is possible, and through this, it can be seen that the binder included in the positive electrode of the present invention has an appropriate thixo property.

In addition, through the examples, it was confirmed that when the molecular weight of the nonionic organic polymer increases, the positive electrode adhesion generally increases under the same addition conditions. However, when the molecular weight of the nonionic organic polymer is excessively large, the resistance is rather It was found that the positive electrode adhesion was decreased by increasing (confirmed through Examples 2 to 4 or 5 to 7 or Examples 8 to 11).

[Experimental Example 2] Evaluation of moisture content in positive electrode

The positive electrode prepared in Examples 1, 5, 9 and Comparative Example 1 was used with a KF 831 Coulometer (Metrohm) to establish an appropriate curve through moisture measurement in a completely dried vial, and then in the vial containing the sample. The moisture content was confirmed through moisture measurement, and the results are shown in Table 3 below.

Anode moisture content (ppm) Example 1 2300±450 Example 5 2060±500 Example 9 1645±150 Comparative Example 1 3000±500

As a result of evaluating the moisture content of the positive electrode prepared in Examples 1, 5, 9 and Comparative Example 1 through the above measurement, Examples 1 and 5 in which a lithium-substituted polyacrylic acid polymer and a nonionic organic polymer were mixed. It was confirmed that the positive electrode of and 9 had less moisture content than the positive electrode of Comparative Example 1, which was not. This is because the functional groups of the lithium-substituted polyacrylic acid-based polymer interact with the functional groups of the nonionic organic polymer, and the portions capable of hydrogen bonding decrease. Therefore, it is difficult to penetrate moisture in the positive electrodes of Examples 1, 5 and 9, Compared to Example 1, the moisture content is relatively low.

[Examples 1, 5, 9 / Comparative Example 1] Preparation of lithium secondary battery

After placing the positive electrode prepared in Examples 1, 5, 9 and Comparative Example 1 to face the negative electrode (Li metal foil, thickness: about 40 μm), a polyethylene separator was interposed therebetween, and then, ethylene glycol ethyl A lithium secondary battery was prepared by injecting an electrolyte in which 0.75 M concentration of LiFSI and 3% by weight of LiNO ₃ were added to a mixed solvent (2:1, v/v) of methyl ether and 2-methyltetrahydrofuran.

[Experimental Example 3] Evaluation of capacity retention rate of lithium secondary battery

Charging and discharging were performed by repeating the lithium secondary batteries prepared according to the above Examples and Comparative Examples under the following analysis conditions, respectively. During charging and discharging, the discharge capacity in the first cycle and the discharge capacity in the fourth cycle were measured, and the results are shown in Table 4 below.

<Analysis conditions>

-Device: 100 mA class charger and discharger

-Charging: 0.3C, constant current/constant voltage mode

-Discharge: 0.5C, constant current/constant voltage mode, 1.8 V

-Cycle temperature: 25℃

1 ^st discharge capacity@0.1
(mAh/g) 4 ^th discharge capacity @0.2
(mAh/g) Capacity retention rate
(%@4 ^th /1 ^st ) Example 1 1065 850 82.0 Example 5 1060 901 85.0 Example 9 1052 909 86.4 Comparative Example 1 1070 842 78.7

3 is a graph showing discharge capacity according to charge/discharge life of a lithium secondary battery and a typical lithium secondary battery according to an embodiment of the present invention. Through the measurement as described above, as a result of evaluating the capacity retention rate of the lithium secondary batteries prepared in the Examples and Comparative Examples, a battery including a positive electrode having excellent adhesion and low moisture content (i.e., a lithium-substituted polyacrylic acid polymer and A battery including a positive electrode mixed with an onogenic organic polymer), it was confirmed through Table 4 and FIG. 3 that the capacity retention rate was superior to that of a battery that did not.

On the other hand, in the case of Example 1 in which polyallylamine containing nitrogen atoms (ie, having only one nitrogen atom per unit molecule) was applied to the polymer side branch, the initial capacity retention rate was compared to Comparative Example 1 as shown in FIG. Although it was slightly higher, it was closer to Comparative Example 1 as the cycle progressed, whereas in Examples 5 and 9, in which polyethyleneimine containing abundant nitrogen atoms was applied, relatively high capacity retention even when the cycle proceeded. Through this, it was found that strong networking by the interaction between the lithium-substituted polyacrylic acid-based polymer and the nonionic organic polymer stabilized the electrode structure, and that the elution of lithium polysulfide was suppressed by the nitrogen atom of the nonionic organic polymer.

Claims (15)

Nonionic organic polymers containing one or more nitrogen atoms;
Lithium substituted polyacrylic acid polymer; And
A binder for improving electrode adhesion comprising a dispersant. The method according to claim 1, wherein the binder for improving electrode adhesion is characterized in that the moisture content is small by reducing the functional group capable of hydrogen bonding through electrostatic interaction between the nonionic organic polymer and the lithium-substituted polyacrylic acid polymer. Binder for improving electrode adhesion. The binder for improving electrode adhesion according to claim 1, wherein the nonionic organic polymer is selected from the group consisting of branched polyethyleneimine, polyallylamine, derivatives thereof, and mixtures thereof. The binder according to claim 3, wherein the nonionic organic polymer is a branched polyethyleneimine containing a large amount of nitrogen atoms. The binder according to claim 1, wherein the nonionic organic polymer has a weight average molecular weight of 10,000 to 800,000. The binder for improving electrode adhesion according to claim 1, wherein the content of the nonionic organic polymer is 0.1 to 2 parts by weight based on 100 parts by weight of the lithium-substituted polyacrylic acid-based polymer. The method according to claim 1, wherein the lithium-substituted polyacrylic acid-based polymer comprises two or more lithium-substituted polyacrylic acids having different molecular weights, the binder for improving electrode adhesion. The method according to claim 7, wherein the lithium-substituted polyacrylic acid-based polymer is characterized in that it comprises two kinds of lithium-substituted polyacrylic acid having a difference in weight average molecular weight of 500,000 or more from each other, electrode adhesion improvement binder. The method according to claim 8, wherein the lithium-substituted polyacrylic acid-based polymer comprises a low molecular weight lithium-substituted polyacrylic acid having a weight average molecular weight of 5,000 to 800,000, and a high molecular weight lithium-substituted polyacrylic acid having a weight average molecular weight of 1,000,000 to 2,000,000. Characterized in that, a binder for improving electrode adhesion. The binder according to claim 8, wherein the two kinds of lithium-substituted polyacrylic acid are included in a weight ratio of 1:1 to 5:2. The method according to claim 1, wherein the lithium-substituted polyacrylic acid-based polymer is a binder for improving electrode adhesion, characterized in that selected from the group consisting of a homopolymer, a block copolymer and a mixture thereof containing lithium-substituted acrylic acid as a monomer. The method according to claim 11, wherein the block copolymer containing the lithium-substituted acrylic acid as a monomer, characterized in that the lithium-substituted acrylic acid monomer in a ratio of 60% or more, the binder for improving electrode adhesion. The binder according to claim 1, wherein the dispersant is polyvinyl alcohol. A positive electrode for a lithium secondary battery comprising a; binder for improving electrode adhesion of claim 1. The positive electrode for a lithium secondary battery of claim 14; Lithium-based negative electrode; An electrolyte interposed between the positive electrode and the negative electrode; And a separator; lithium secondary battery comprising a.

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