Binder composition for lithium ion secondary battery, electrode comprising same, and lithium ion secondary battery comprising same

Abstract

The present invention relates to a binder composition for a lithium ion secondary battery, an electrode comprising same, and a lithium ion secondary battery comprising same. The binder composition for a lithium ion secondary battery comprises thermoplastic polyurethane having an ester-based structure and polyvinylidene fluoride, and can enhance the battery performance, including excellent adhesive strength between interfaces and low electrode resistance.

Description

Binder composition for lithium ion secondary battery, electrode comprising same, and lithium ion secondary battery comprising same

[0001] The present invention relates to a binder composition for a lithium ion secondary battery, an electrode comprising the same, and a lithium ion secondary battery comprising the same.

[0002]

[0003] Lithium-ion secondary batteries (LIBs) have been successfully commercialized since the 1990s in various applications across our daily lives, from portable home appliances to large-scale electric vehicles (EVs) and renewable energy storage systems (ESS). Lithium-ion secondary batteries use a cathode, which is made up of a mixture of a positive active material including lithium ions, a conductive agent, and an organic binder, fixed to the surface of a current collector made of metal foil, and a negative electrode, which is made up of a negative active material capable of inserting and removing lithium ions, a conductive agent, and an organic binder, fixed to the surface of a current collector made of metal foil.

[0004] Currently, lithium nickel cobalt manganese oxide (NCM) and lithium nickel cobalt aluminum oxide (NCA), which are high-nickel materials used as cathode active materials for LIBs, are receiving great attention as options to increase the energy density of LIBs due to their high-capacity and high-voltage characteristics.

[0005] However, the higher the nickel content of high-nickel materials, the higher the total alkaline content. Higher total alkaline content increases requirements for industrial production, storage, transportation, and battery preparation. Furthermore, these materials have low electronic conductivity, and their use alone cannot achieve sufficient battery performance. Therefore, attempts are being made to improve conductivity and reduce the internal resistance of the positive electrode by adding carbon materials such as carbon black (e.g., acetylene black) as a conductive additive. However, to effectively reduce internal resistance with less carbon black, both the positive electrode active material and the carbon conductive material must be uniformly mixed at a finer level within the electrode.

[0006] In addition, when forming a positive electrode mixture layer on a current collector such as a metal foil, there is a problem that the adhesion between the current collector and the positive electrode mixture layer, or between the positive electrode active material and the conductive material interface within the positive electrode mixture, deteriorates when charging and discharging are repeated multiple times, resulting in a decline in battery performance. This is because the positive electrode active material and the positive electrode mixture layer undergo repeated expansion and contraction due to the insertion and desorption of lithium ions during the charge and discharge process, which ultimately weakens the local adhesion between the positive electrode active material and the conductive material within the mixture. A binder is applied to strengthen the interfacial bonding between the positive electrode mixture layer, conductive material, and current collector. In particular, the mechanical stability of the electrode is determined by the characteristics of the electrode active material, the current collector, and the binder that provides adhesion between them. For example, the amount and type of active material used determines the amount of lithium ions that can bind to the active material. Therefore, a higher amount of active material and a material with a high specific capacity can yield a higher-capacity battery.

[0007] Furthermore, when the binder has good adhesion between the active materials and between the active materials and the current collector, the movement of electrons and lithium ions within the electrode is smooth, and the internal resistance of the electrode is reduced, thereby enabling high-efficiency charging and discharging. Furthermore, to maintain the battery's charge and discharge cycle for a long period of time, the binder must have low swelling properties in the electrolyte.

[0008] And recently, as the demand for high battery capacity has become stronger, there is a tendency to reduce the content of the binder component as a material constituting the electrode layer, and in addition, the electrode layer is being pressed during the electrode manufacturing process. However, in an electrode layer with a low content of the binder component, the electrode layer is easily peeled off from the current collector during the press processing. Therefore, not only does the electrode material cause contamination of the press processing machine, but the electrode is mounted on the battery with part of the electrode layer peeled off, which has been pointed out as a problem that the reliability of the battery performance is reduced. Since reducing the content of the binder component as in the above example is a factor that lowers the performance of the electrode or lowers the workability during processing, a binder that can suppress the internal resistance of the battery without reducing the content of the binder component is required.

[0009] Currently, polyvinylidene fluoride (PVDF) is primarily used as a binder in lithium secondary batteries, primarily due to its high adhesive strength, high thermal stability, and wide electrochemical window. However, polyvinylidene fluoride has weak adhesion to electrode current collectors. Therefore, increasing the polyvinylidene fluoride content to improve adhesion reduces the relative content of the active material, resulting in a decrease in battery capacity.

[0010] In the paper by Manthiram et al., polypyrrole and polyurethane were used to synthesize a polypyrrole-polyurethane (PPyPU) nanocomposite and used it as a binder to fabricate a flexible and independent self-supporting lithium-sulfur battery cathode material. However, in the binder, PU is used only as a substrate with good mechanical flexibility. In addition, the paper ACS Appl. Energy Mater. 2020, 3, 12494 discloses a comparison of the mechanical properties and battery performance of a PU-based binder compared to PVDF, and Korean Patent Publication No. 2022-0145002 discloses a technology to improve battery performance and adhesiveness by mixing polyurethane urea (PUU) and PVDF. However, they only observed changes in the physical properties of the manufactured mixture according to the binder composition, and there is a lack of clearly specifying the differences in battery performance according to the electrochemical analysis of the binder within the electrode. There is an urgent need for research on binder materials that can improve the performance of these lithium-ion secondary batteries and enhance the adhesive strength between interfaces.

[0011]

[0012] An object of the present invention is to provide a binder composition for a lithium ion secondary battery having excellent adhesive strength and capable of implementing low electrode resistance.

[0013] Another object of the present invention is to provide an electrode having a reduced thickness change rate during charge and discharge and a lithium ion secondary battery including the same.

[0014] The objectives of the present invention are not limited to those mentioned above. Other objectives and advantages of the present invention not mentioned above can be understood through the following description and will be more clearly understood through the embodiments of the present invention. Furthermore, it will be readily apparent that the objectives and advantages of the present invention can be realized by the means and combinations thereof set forth in the claims.

[0015]

[0016] In order to achieve the above-mentioned purpose, one embodiment of the present invention provides a binder composition for a lithium ion secondary battery comprising a thermoplastic polyurethane having an ester structure and polyvinylidene fluoride.

[0017] Another embodiment of the present invention provides an electrode comprising the above-described binder composition for a lithium ion secondary battery, a positive electrode active material, and a conductive material.

[0018] Another embodiment of the present invention provides a lithium ion secondary battery including the electrode described above.

[0019]

[0020] The binder according to the present invention possesses excellent adhesive properties, which can strengthen the bond between the electrode current collector and the electrode active material, and enhance adhesive strength through chemical and physical interactions with the electrode active material. Furthermore, the binder composition according to the present invention possesses flexibility and excellent bonding strength, which can mitigate thermal expansion and deformation of the electrode during its lifespan, thereby enhancing the stability of the electrode during the charge and discharge cycle of the battery.

[0021] In addition, the binder composition according to the present invention can provide effective electrical conductivity properties that can lower the electrical resistance of an electrode, and has high thermal stability, so that deformation and conductivity loss of the electrode due to heat generated inside the battery can be minimized, thereby improving the performance of the battery.

[0022]

[0023] FIG. 1 is a photograph showing the swelling characteristics of binder compositions manufactured according to Examples 1 to 2 and Comparative Example 3 according to the present invention.

[0024] FIG. 2 is a graph showing the electrochemical stability characteristics of binder compositions manufactured according to Examples 1 to 2 and Comparative Example 3 according to the present invention.

[0025] FIG. 3 and FIG. 4 are photographs showing the dispersion stability characteristics of positive electrode slurries manufactured according to Examples 3 to 6 and Comparative Example 3 according to the present invention and the results of the electrode dispersion evaluation.

[0026] FIG. 5 is a photograph showing the adhesive properties between the electrodes and the current collector manufactured according to Examples 3 to 6 and Comparative Example 5 according to the present invention.

[0027] Figure 6 is a graph showing the measured composite layer resistance and interface resistance of electrodes manufactured according to Examples 3 to 4 and Comparative Example 5 according to the present invention.

[0028] FIG. 7 is a graph showing the initial charge / discharge curves of batteries manufactured according to Examples 7 to 10 and Comparative Example 5 according to the present invention.

[0029]

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In general, the nomenclature used herein is well known and commonly used in the art.

[0031] Additionally, throughout this specification, whenever a part is said to "include" a component, this does not mean that it excludes other components, but rather that it may include other components, unless specifically stated otherwise.

[0032]

[0033] Binder composition for lithium-ion secondary batteries

[0034] The binder composition described herein is defined as having the same concept as a binder composed of two or more polymers, and the binder solution can be used as a binder for a lithium ion secondary battery electrode.

[0035]

[0036] Binders for lithium-ion secondary batteries exhibit excellent electrode adhesion (adhesion between the electrode current collector and the electrode active material, and adhesion between the electrode active materials), thereby preventing delamination of the electrode active material during charging and discharging. Consequently, electrode deterioration due to repeated charging and discharging can be prevented, resulting in secondary batteries with excellent cycle life characteristics. Furthermore, low electrode resistance can be ensured.

[0037] That is, the binder for lithium-ion secondary batteries is an important material that binds the active material and conductive material and fixes them to the current collector, and plays an important role in the production of thick-film electrodes and the performance of secondary batteries, so the role and interest in such binders are increasing.

[0038] Currently, poly(vinylidene fluoride; PVdF) is used as a cathode binder, but it has limitations such as weak bonding properties (Van der Waals force) with the active material and current collector and high price. Furthermore, existing cathode binder research has mainly focused on developing new binder functions and resolving issues with the cathode material itself, making practical electrode and battery applications difficult. Therefore, research is urgently needed to develop cathode binders that can meet the characteristics required for practical battery applications by controlling the structural and chemical properties of the polymers used as cathode binders to improve adhesive strength, swellability, and dispersibility.

[0039] The present invention has developed a cathode binder material that can be applied as a binder for lithium ion secondary batteries and simultaneously improves performance / economic efficiency / commercialization potential.

[0040] In the present invention, as a binder for a lithium-ion secondary battery that can overcome the limitations of the existing positive electrode binder described above, it is preferable to include a binder composition including a thermoplastic polyurethane having an ester structure and polyvinylidene fluoride in one embodiment of the present invention.

[0041]

[0042] The thermoplastic polyurethane having the above ester structure includes a hard segment containing aromatic and aliphatic urethane units and a soft segment containing polyester units.

[0043] The above hard segment may include a urethane unit.

[0044] The above soft segment preferably includes a polyester unit. When the polyester is included, electrode adhesion can be improved, and changes in electrode thickness that occur during repeated charging and discharging can be alleviated.

[0045] The above urethane unit may be derived from the -OH group of a polyol (e.g., a diol) and a chain extender (e.g., an aliphatic diol) and the isocyanate group (-NCO) of a diisocyanate. In addition, the hard segment may include at least one of an aromatic and an aliphatic urethane unit. In this case, electrode adhesion may be further improved.

[0046] An aromatic urethane unit may refer to a urethane unit that includes an aromatic ring structure within its structure. An aliphatic urethane unit may refer to a urethane unit that includes an aliphatic (e.g., alkyl) structure within its structure.

[0047]

[0048] For example, the aromatic and aliphatic urethane units may be derived from the -OH group of a polyol and the isocyanate group (-NCO) of an aromatic diisocyanate (e.g., methylene diphenyl diisocyanate; MDI and hexamethylene diisocyanate; HDI). Furthermore, as described above, they may also be derived from aromatic and aliphatic diisocyanates and chain extenders.

[0049]

[0050] The polyester unit of the above soft segment may be derived from polyester polyol.

[0051] The polyester polyol may include at least one selected from the group consisting of random polyester polyols produced by the addition reaction of a polyfunctional carboxylic acid compound and a polyfunctional alcohol compound. As a specific example, the polyfunctional alcohol to the polyfunctional carboxylic acid are introduced into a batch reactor at a molar ratio of 1 to 1.05 to 1.50, mixed, and then heated from room temperature to 150°C, maintained at the first heating temperature of 150°C for about 90 minutes, then heated again from 150°C to 220°C, and maintained at the second heating temperature of 220°C for about 30 minutes. After applying a vacuum of 720 mmHg at the second heating temperature, the reaction is terminated when the acid value becomes 1 mgKOH / g or less, thereby producing a polyester polyol having a hydroxyl value of 40 to 230 mgKOH / g. The above hydroxyl value can be measured, for example, according to ASTM D 4274.

[0052]

[0053] The above multifunctional carboxylic acid compound may include at least one selected from the group consisting of sebacic acid, adipic acid, siberic acid, abelic acid, azelic acid, and dodecanedioic acid.

[0054]

[0055] The above polyfunctional alcohol compound may include at least one selected from the group consisting of ethylene glycol, diethylene glycol, butylene glycol, and hexylene glycol.

[0056]

[0057] The process for producing the above thermoplastic polyurethane may include both a pre-polymer process and a one-shot process, either a batch process or a continuous process. As a specific example, the polyurethane may be produced by reacting the polyurethane components together in a one-shot polymerization process, and the diisocyanate molar ratio for the polyester polyol or the mixture of the polyester polyol and the diol is 1.0 to 0.90 to 1.0, and the mixture is mixed in a pre-mixer and fed into a continuous reaction extruder heated to 180 to 240°C to react, and then the reactant in which the reaction is completed is extruded to produce a thermoplastic polyurethane in the form of pellets.

[0058]

[0059] The above diol compound may include at least one selected from the group consisting of ethylene glycol, diethylene glycol, butylene glycol and hexylene glycol.

[0060]

[0061] The above diisocyanate compound may include at least one selected from the group consisting of diphenylmethane diisocyanate (MDI), toluene diisocyanate (TDI), hexamethylene diisocyanate (HDI), dicyclohexylmethane diisocyanate (H12MDI), and isophorone diisocyanate (IPDI).

[0062]

[0063] The binder according to the present invention comprises a thermoplastic polyurethane having an ester structure including the above-described hard segment and polyester units.

[0064] Polyurethanes are classified as thermosetting polyurethanes and thermoplastic polyurethanes, depending on whether they can be melt-processed. While thermosetting polyurethanes can be dissolved in certain solvents to a limited extent, dissolution requires significant energy and time, making thermoplastic polyurethanes preferable.

[0065] Thermoplastic polyurethanes whose soft segment units are composed of ester polyols exhibit relatively superior mechanical properties and adhesive strength compared to those of ether polyols. Conversely, thermoplastic polyurethanes whose soft segment units are composed of ether polyols exhibit superior elasticity and hydrolysis resistance.

[0066]

[0067] The thermoplastic polyurethane according to the present invention may have a weight average molecular weight (Mw) of 50,000 to 300,000 g / mol, preferably 10,000 to 250,000 g / mol. If the weight average molecular weight is less than 50,000 g / mol, the mechanical properties and adhesive strength of the thermoplastic polyurethane may decrease. In addition, if it exceeds 300,000 g / mol, the economic feasibility of the polyurethane synthesis and manufacturing process may decrease, and the crystallinity may increase, reducing the solubility in a dispersing solvent.

[0068]

[0069] The thermoplastic polyurethane according to the present invention may have a glass transition temperature (Tg) of -35°C to 15°C and a melting point (Tm) of 80°C to 200°C. If the glass transition temperature and / or melting point are outside the range, swelling problems may occur in an electrolyte and solubility in polar solvents may be reduced.

[0070] The thermoplastic polyurethane according to the present invention may have a tensile strength of 150 to 450 kg / cm2, preferably 250 to 450 kg / cm2, and an elongation of 200 to 800%, preferably 300 to 800%, more preferably 450 to 800%.

[0071] Within the range of the above tensile strength and / or elongation, the bonding force between the materials within the slurry or the bonding force between the current collector and the slurry can be improved, thereby improving electrochemical stability and battery life, and the improved elasticity can prevent cracking or peeling in the slurry high-temperature lamination process.

[0072] According to another embodiment of the present invention, a binder composition may include the thermoplastic polyurethane having the ester structure described above and polyvinylidene fluoride in a weight ratio of thermoplastic polyurethane: polyvinylidene fluoride of 1 to 9:9 to 1, preferably 1 to 5:5 to 9, and more preferably 1 to 3:7 to 9. If the weight ratio is out of the above range, electrode adhesion and electrode resistance may be deteriorated. In addition, the electrode thickness change rate due to repeated charge / discharge of the secondary battery may increase, which may adversely affect the battery life. In particular, if the thermoplastic polyurethane having the ester structure described above and polyvinylidene fluoride are included within the above range, battery performance can be significantly improved due to a decrease in electrode resistance.

[0073] The binder composition for a lithium ion secondary battery according to the present invention is preferable in that it is a positive electrode binder applied to a positive electrode active material described below, and thus can realize high adhesive strength and high electrode electrical conductivity.

[0074] The positive electrode binder composition according to the present invention can improve the adhesion between the positive electrode active material layer and the current collector, which will be described later, because it has a low melting point and a functional group capable of inducing strong hydrogen bonding with the current collector. If the melting point (Tm) of the thermoplastic polyurethane of the binder composition is outside of 80°C to 200°C, not only will the solubility in N-methylpyrrolidone (NMP) decrease, but the adhesive strength may be insufficient or the binder composition may swell due to the electrolyte.

[0075] The electrolyte expansion of the above binder composition must be minimized because it weakens the adhesive strength during battery operation and dries out the electrolyte that should be present in the electrode, resulting in structural destruction of the electrode and loss of electrochemical performance. The positive electrode binder according to the present invention has the advantage of minimizing electrolyte expansion by controlling the melting point and improving the mechanical properties.

[0076]

[0077] electrode

[0078] According to another embodiment of the present invention, an electrode comprising the above-described binder composition for a lithium ion secondary battery, a positive electrode active material, and a conductive material is provided.

[0079] The above cathode active material is a layered compound such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium manganese oxide (LiMnO2) represented by the chemical formula Li1+xMn2-xO4 (wherein, x is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2, etc.; lithium copper oxide (Li2CuO2); vanadium oxide such as LiV3O8, LiFe3O4, V2O5, Cu2V2O7; lithium nickel oxide (lithiated nickel oxide) represented by the chemical formula LiNi1-xMxO2 (wherein, M is Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x is 0.01 to 0.3); A lithium manganese composite oxide represented by the chemical formula LiMn2-xMxO2 (wherein, M is Co, Ni, Fe, Cr, Zn or Ta, and x is 0.01 to 0.1) or Li2Mn3MO8 (wherein, M is Fe, Co, Ni, Cu or Zn); a lithium manganese composite oxide represented by the chemical formula LiMn2O4 in which a portion of the lithium in the chemical formula is replaced by an alkaline earth metal ion; a disulfide compound; a composite oxide formed by Fe2(MoO4)3 or a combination thereof.

[0080] The cathode active material according to the present invention is a high-nickel cathode active material system, for example, Li[Ni 1-x-y-z Co x Mn y Al z It may include at least one selected from O2(x=0.5~2, y=0.5~2, z=0~2)z. It is preferable that the positive electrode active material is an NCM-based metal oxide in terms of improving material capacity and battery energy density.

[0081] The electrode including the above positive electrode active material may contain 0.1 to 20 parts by weight, preferably 0.1 to 15 parts by weight, and more preferably 1 to 10 parts by weight of the binder for lithium ion secondary batteries, based on 100 parts by weight of the positive electrode active material. If the content of the binder is less than 0.1 parts by weight, there may be a problem of a detachment phenomenon occurring due to interfacial instability between the electrode layer and the current collector, and if it exceeds 20 parts by weight, there may be a problem of a loss of electrochemical performance due to the functioning of a resistance layer. The binder according to the present invention can implement excellent electrode characteristics even with a small amount of binder due to its strong adhesive strength and excellent slurry dispersing ability.

[0082]

[0083] The conductive material may include, but is not limited to, a carbon-based conductive material such as carbon black, carbon fiber, carbon nanotubes, or graphite. The carbon black may be selected from the group consisting of, for example, acetylene black, Ketjen black, Super P, channel black, furnace black, lamp black, and thermal black. The graphite may be natural graphite or artificial graphite.

[0084] The conductive material may be included in an amount of 0.1 to 20 parts by weight, 0.1 to 15 parts by weight, 0.1 to 10 parts by weight, or 1 to 10 parts by weight, based on 100 parts by weight of the positive electrode active material. If the content of the conductive material is less than 0.1 parts by weight, there is a risk of loss of electrochemical performance due to insufficient electronic conductivity, and if it exceeds 20 parts by weight, there is a risk of loss of energy of the secondary battery.

[0085] The above electrode may be a positive electrode including a positive electrode active material layer including the above-described binder, the above-described positive electrode active material, and the conductive material.

[0086] The above positive electrode can be manufactured by mixing and stirring the positive electrode binder, positive electrode active material, and conductive material with a dispersion solvent, etc. to manufacture a positive electrode slurry, and then applying, drying, and rolling the positive electrode slurry onto a positive electrode current collector.

[0087] The positive electrode current collector may include, for example, stainless steel, nickel, aluminum, titanium, copper or an alloy thereof, and preferably, aluminum or an aluminum alloy.

[0088] The above dispersion solvent is not particularly limited, and for example, N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), acetone, water, and mixtures thereof may be used. The content of the dispersion solvent may be 20 to 99 parts by weight based on 100 parts by weight of the positive electrode active material. The electrode slurry formation operation may be facilitated within the content range of the dispersion solvent.

[0089]

[0090] lithium-ion secondary battery

[0091] According to another embodiment of the present invention, a lithium ion secondary battery comprising the electrode described above is provided. The lithium ion secondary battery preferably comprises an electrolyte solution comprising an electrolyte salt and a non-aqueous electrolyte, in order to achieve the effects desired by the present invention.

[0092] The above lithium ion secondary battery may include the above-described positive electrode and the opposite negative electrode. In addition, it may include a separator interposed between the positive electrode and the negative electrode.

[0093]

[0094] The above separator can be wound or folded and accommodated in a battery case. An electrolyte can be injected into the battery case and sealed with a cap assembly to complete a lithium secondary battery. The battery case can be cylindrical, square, thin-film, or the like.

[0095]

[0096] The above separator may be a thin insulating film having high ion permeability and mechanical strength. The pore diameter of the separator is not limited thereto, but may be, for example, 0.01 to 10 μm, and the thickness may be 5 to 20 μm. The separator is not limited thereto, but may be, for example, a sheet or non-woven fabric made of an olefin-based polymer such as polypropylene; glass fiber or polyethylene; or the like. When a solid polymer electrolyte is used as the electrolyte, the solid polymer electrolyte may also function as the separator.

[0097]

[0098] The above electrolyte may include an electrolyte salt and a non-aqueous electrolyte.

[0099]

[0100] The electrolyte salt is not limited thereto, but may include, for example, an inorganic ion salt containing one kind of lithium (Li), sodium (Na), or potassium (K), such as LiClO4, LiBF4, LiAsF6, LiPF6, LiSCN, LiBr, LiI, Li2SO4, Li2B10Cl10, NaClO4, NaI, NaSCN, NaBr, KClO4, KSCN, etc. Also, as electrolyte salts, for example, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2)(C4F9SO2), LiC(CF3SO2)3, LiC(C2F5SO2)3, (CH3)4NBF4, (CH3)4NBr, (C2H5)4NClO4, (C2H5)4NI, (C3H7)4NBr, (n-C4H9)4NClO4, (n-C4H9)4NI, (C2H5)4N-maleate, (C2H5)4N-benzoate, (C2H5)4N-phthalate, lithium stearyl sulfate, lithium octyl sulfate, dodecylbenzenesulfonic acid Organic ion salts such as lithium dodecylbenzene sulfonate (Lithium Dodecylbenzene Sulphonate) can also be used. The electrolyte salts can be used alone or in combination of two or more. The concentration of the electrolyte salt is not particularly limited as long as it is a concentration applicable to a general lithium secondary battery. For example, the concentration of the electrolyte salt can be 0.5 mol / L to 2.0 mol / L.

[0101] The non-aqueous electrolyte may include, but is not limited to, cyclic carbonate esters such as propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate or vinylene carbonate, cyclic esters such as Y-butyrolactone or Y-valerolactone, chain carbonates such as dimethyl carbonate, diethylcarbonate or ethylmethyl carbonate, chain esters such as methyl formate, methyl acetate or methyl butyrate, tetrahydrofuran or its derivatives, Ethers such as 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane or methyldiglyme, nitriles such as acetonitrile or benzonitrile, dioxolane or its derivatives, ethylene sulfide, sulfolane, sultone or their derivatives, etc. can be used alone or in combination of two or more thereof.

[0102]

[0103] In the present invention, by applying the above-described binder composition, the adhesive strength of a lithium secondary battery electrode can be improved, thereby improving the life characteristics and stability of the lithium secondary battery.

[0104]

[0105] Hereinafter, the present invention will be described in detail through examples, but the following examples and experimental examples are merely illustrative of one form of the present invention, and the scope of the present invention is not limited by the following examples and experimental examples.

[0106]

[0107] Binder manufacturing

[0108] <Example 1>

[0109] A thermoplastic polyurethane having a hard segment content of about 32% was prepared by using a polyester polyol prepared by the method described above in the present invention using adipic acid and butane diol as the soft segment and MDI as the hard segment. The prepared TPU and polyvinylidene fluoride (PVDF) were added to an N-methyl-2-pyrrolidone (NMP) solvent at a weight ratio of 1:9, and mixed and stirred to prepare a binder solution. This was mixed with a positive electrode active material and a conductive material, applied to a current collector, dried, and used as a binder for secondary batteries.

[0110]

[0111] <Example 2>

[0112] The same method as Example 1 was performed except that the hard segment content was 26%.

[0113]

[0114] <Comparative Example 1>

[0115] A thermoplastic polyurethane having a hard segment content of 50% was manufactured using PTMEG (polytetramethylene ether glycol) polyether polyol as the soft segment and MDI and BDO as the hard segment.

[0116]

[0117] Comparative Example 2

[0118] A polyurethane urea (PUU) having a urethane group by polyether polyol and isocyanate and a urea group by diamine and isocyanate, a structure represented by the following chemical formula, and a hard segment content of 24% was manufactured.

[0119]

[0120] The above-mentioned PUU and polyvinylidene fluoride (PVDF) were added in a weight ratio of 5:5 to an N-methyl-2-pyrrolidone (NMP) solvent, mixed, and stirred to prepare a binder solution. This was mixed with a positive electrode active material and a conductive material, applied to a current collector, dried, and used as a binder for secondary batteries.

[0121]

[0122] <Comparative Example 3>

[0123] The same procedure as Example 1 was followed, except that only PVDF was used.

[0124]

[0125] Comparative Example 4

[0126] The same procedure as Example 1 was followed, except that a polyol having a molecular weight twice that of adipic acid and butane diol was used.

[0127]

[0128] Evaluation of physical properties of binder compositions

[0129] The physical properties of the binder materials manufactured according to Examples 1 to 2 and Comparative Examples 1 to 4 were measured, and the results are shown in Table 1.

[0130]

[0131] Item Example 1 (TPU) Example 2 (TPU) Comparative Example 1 (TPU) Comparative Example 2 (PUU) Comparative Example 3 (PVDF) Comparative Example 4 (TPU) Mw (g / mol) 130,000 110,000 130,000 500,000 280,000 70,000 Tg (℃) -5-22-30 N / A-355 Tm (℃) 110 133 180 N / A 173 203

[0132] Evaluation of electrical properties of binder compositions

[0133] The electrical properties of the binders manufactured according to Examples 1 and 2 and the binders manufactured according to Comparative Examples 1 to 4 were measured and evaluated by the following method.

[0134]

[0135] 1. Swelling Evaluation: After manufacturing a free-standing polymer film without a substrate, cut it into 1 cm X 1 cm pieces. Then, it is impregnated with electrolyte for a certain period of time. The weight change before and after electrolyte impregnation (weight after impregnation / weight before impregnation X 100, weight changes) and the film size and area changes (dimension changes) are measured.

[0136] 2. Electrochemical Stability Evaluation: A polymer binder solution is coated and dried onto an aluminum current collector to produce a film of a certain thickness. The polymer binder film on the aluminum current collector is vacuum-dried, and a battery containing the electrolyte is manufactured in an argon-atmosphere glove box. After this, an electrochemical stability evaluation is conducted.

[0137]

[0138] Table 2 below shows the swelling evaluation measurement results, and Figure 1 is a photograph showing the swelling characteristics of binder compositions manufactured according to Examples 1 and 2 and Comparative Examples 3 and 4.

[0139]

[0140] Item Example 1 (TPU) Example 2 (TPU) Comparative Example 1 (TPU) Comparative Example 2 (PUU) Comparative Example 3 (PVDF) Comparative Example 4 (TPU) Weight changes 131% 137% Dissolved (not measurable) Dissolved (not measurable) 113% Dissolved (not measurable) Dimension changes 107% 105% Dissolved (not measurable) Dissolved (not measurable) 111% Dissolved (not measurable)

[0141] As shown in Table 2 and Fig. 1 above, the binders according to Examples 1 and 2 showed weight changes before and after impregnation in the electrolyte at a level comparable to that of the conventional binder PVdF. From this, it was confirmed that the binder satisfies the swelling conditions for lithium-ion secondary batteries.

[0142] Figure 2 is a graph showing the electrochemical stability characteristics of binders manufactured according to Examples 1 and 2 and Comparative Example 3. As shown in Figure 3, Examples 1 and 2, like Comparative Example 3, were confirmed to secure excellent electrochemical stability up to around 4.5 V. This can be interpreted as the basis for Examples 1 and 2 being able to be stably used in active materials that are charged and discharged up to 4.3 V.

[0143]

[0144] Electrode manufacturing

[0145] <Example 3>

[0146] The positive electrode used NCM (NCM811 positive active material), the negative electrode used 200 μm lithium foil, and both electrodes used carbon black as a conductive material. The positive electrode slurry and negative electrode slurry were prepared at a ratio of 96 (active material): 2 (conductive material): 2 (binder).

[0147] Thereafter, the positive electrode slurry was uniformly applied onto an aluminum current collector (20 μm thick), dried, and then rolled to manufacture a positive electrode. At this time, the binder used was a binder composition manufactured according to Example 1.

[0148]

[0149] <Example 4>

[0150] It was manufactured in the same manner as Example 3, except that the binder composition manufactured according to Example 2 was used.

[0151]

[0152] <Example 5>

[0153] The same method as Example 3 was used, except that a binder solution was prepared by adding the manufactured TPU and polyvinylidene fluoride (PVDF) in a weight ratio of 2:8 to an N-methyl-2-pyrrolidone (NMP) solvent and mixing and stirring.

[0154]

[0155] <Example 6>

[0156] The same method as Example 3 was used, except that a binder solution was prepared by adding the manufactured TPU and polyvinylidene fluoride (PVDF) in a weight ratio of 3:7 to an N-methyl-2-pyrrolidone (NMP) solvent and mixing and stirring.

[0157]

[0158] Comparative Example 5

[0159] It was manufactured in the same manner as Example 3, except that the binder composition manufactured according to Comparative Example 3 was used.

[0160]

[0161] <Comparative Example 6>

[0162] It was manufactured in the same manner as Example 3, except that the binder composition manufactured according to Comparative Example 1 was used.

[0163]

[0164] Comparative Example 7

[0165] It was manufactured in the same manner as Example 3, except that the binder composition manufactured according to Comparative Example 2 was used.

[0166]

[0167] Comparative Example 8

[0168] It was manufactured in the same manner as Example 3, except that the binder composition manufactured according to Comparative Example 4 was used.

[0169]

[0170] Electrode characteristic evaluation

[0171] The electrical properties of the electrodes manufactured according to Examples 3 to 6 and the electrodes manufactured according to Comparative Example 5 were measured and evaluated using the following method.

[0172]

[0173] 1-1. Dispersion safety evaluation: After manufacturing the electrode slurry, the same weight of slurry was placed in a vial or container, and the phase separation or particle settling phenomenon was confirmed over time.

[0174] 1-2. Electrode dispersion evaluation: After slurry coating and drying in a hot air oven, the presence of particle agglomeration on the electrode surface or abnormalities in the electrode appearance was checked.

[0175]

[0176] 2. Electrode adhesion evaluation: Cut a piece with an equal area of ​​10π or more and measure the weight using a scale. Then, immerse it in a container containing the same amount of 1M LiPF6 EC / EMC (3 / 7 v / v) electrolyte. After exposing it to ultrasonic waves for a certain period of time using ultrasonic cleaning equipment, measure the weight using a scale and compare the rate of change (electrode weight after ultrasonic exposure / electrode weight before ultrasonic exposure X 100).

[0177]

[0178] 3. Electrode resistance measurement: Measure the weight using a scale and a cutting board with an equal area of ​​10π or more. Measure the electrode resistance of five different locations within each electrode and calculate the average and standard deviation. The measuring equipment used was a Hioki XF057 probe unit, and the measurement conditions were a current range of 10μA-50mA and a voltage range of 0.1-10V.

[0179] 4. Measurement of electrode slurry solids and viscosity: The solids and viscosity of the positive electrode slurry were measured. The solids are the weight ratio of the solid components in the slurry to the total weight of the slurry, and are calculated as (weight of solid components) / (weight of solid components + weight of liquid components) according to the actual mixing amount used. The viscosity was measured and compared at 25℃ at the same shear rate or according to shear rate using a viscometer or rheometer.

[0180]

[0181] Figure 3 is a photograph showing the dispersion stability characteristics of the positive electrode slurries manufactured according to Examples 3 to 6 and Comparative Example 5. As can be seen in Figure 3, the positive electrode slurries of Examples 3 to 6, which applied the binders manufactured according to Examples 1 to 2, all showed no phase separation over time, confirming that the slurry viscosity and dispersion stability were excellent.

[0182] Figure 4 is a photograph showing the evaluation of dispersion and appearance abnormalities of the positive electrodes manufactured according to Examples 3 to 6 and Comparative Example 5. As seen in Figure 4, all of the positive electrodes of Examples 3 to 6, which applied the binder manufactured according to Examples 1 to 2, did not exhibit particle agglomeration or detachment after high-temperature drying, confirming excellent electrode dispersion.

[0183] FIG. 5 is a photograph showing the adhesive properties and adhesive strength test process between the electrodes and current collectors manufactured according to Examples 3 to 6 and Comparative Example 5. As seen in FIG. 5, the positive electrodes of Examples 3 to 4, which applied the binders manufactured according to Examples 1 to 2, showed excellent adhesive strength compared to Comparative Example 5, but Examples 5 to 6 showed relatively weak adhesive strength compared to Comparative Example 5. However, it was confirmed that Examples 5 to 6 satisfied the adhesive strength required in the process of manufacturing the electrode and designing the battery.

[0184] In addition, the electrodes manufactured according to Comparative Examples 6 to 8 had a problem of peeling due to a significantly reduced adhesive strength between the current collector and the composite layer, making battery evaluation impossible.

[0185] Fig. 6 is a graph showing the resistance characteristics of electrodes manufactured according to Examples 3 to 4 and Comparative Example 5. As can be seen in Fig. 6, the electrodes of Examples 3 to 4, which applied the binder manufactured according to Examples 1 to 2, showed low electrode resistance in the electrode layer excluding the current collector and low resistance at the current collector-electrode interface.

[0186]

[0187] Lithium secondary battery manufacturing

[0188] <Example 7>

[0189] A 2032 coin cell was manufactured using a polyethylene separator (20 μm thick) between the positive electrode and the Li metal negative electrode (200 μm) manufactured according to Example 3.

[0190] As the electrolyte of the above coin cell, a 1 M LiPF6 solution (EC / EMC mixed solvent with a volume ratio of 30:70) was prepared, and then 10 wt% of FEC (Fluoroethylene carbonate) and 2 wt% of VC (Vinylethylene carbonate) were added and mixed based on the total weight of the electrolyte.

[0191]

[0192] <Example 8>

[0193] It was manufactured in the same manner as Example 7, except that the positive electrode manufactured according to Example 4 was used.

[0194]

[0195] <Example 9>

[0196] It was manufactured in the same manner as Example 7, except that the positive electrode manufactured according to Example 5 was used.

[0197]

[0198] <Example 10>

[0199] It was manufactured in the same manner as Example 7, except that the positive electrode manufactured according to Example 6 was used.

[0200]

[0201] <Comparative Example 6>

[0202] It was manufactured in the same manner as Example 7, except that the positive electrode manufactured according to Comparative Example 5 was used.

[0203]

[0204] Lithium secondary battery characteristic evaluation

[0205] The electrical characteristics of the lithium ion secondary batteries manufactured according to Examples 7 to 10 and the lithium ion secondary batteries manufactured according to Comparative Example 6 were measured and evaluated using the following method.

[0206] 1. Initial capacity: The initial discharge capacity and initial coulombic efficiency (initial discharge capacity / initial charge capacity X 100) were calculated by performing CC / CV charging (0.1C, 4.3V 0.05C cut-off) and CC discharging (0.1C, 3.0V cut-off) at room temperature (25℃).

[0207] 2. Initial capacity and life characteristics evaluation: At room temperature (25℃), CC / CV charging (0.5C, 4.3V 0.05C cut-off) and CC discharge (0.5C, 3.0V cut-off) were repeated 50 times to calculate the capacity retention rate (100th discharge capacity / 1st discharge capacity X 100).

[0208]

[0209] Figure 7 is a graph showing the initial capacity of lithium secondary batteries manufactured according to Examples 7 to 10 and Comparative Example 6.

[0210] Table 3 below shows the results comparing the characteristics of batteries tested according to Examples 7 to 10 and Comparative Example 6.

[0211]

[0212] Item Example 7 Example 8 Example 9 Example 10 Comparative Example 6 Initial discharge capacity (mAh / g) 174.23 1173.37 171.34 163.42 171.596 Initial charge capacity (mAh / g) 201.22 2197.95 197.89 187.75 197.633 Initial coulombic efficiency (%) 86.59 87.58 86.58 87.04 86.82

[0213] It was found that Examples 7 and 9 manufactured according to the present invention had initial coulombic efficiency values ​​that were equivalent to those of Comparative Example 6, and that Examples 8 and 10 had improved initial coulombic efficiency values ​​compared to Comparative Example 6.

[0214]

[0215] The binder according to the present invention possesses excellent adhesive properties, which can strengthen the bond between the electrode current collector and the electrode active material, and enhance adhesive strength through chemical and physical interactions with the electrode active material. Furthermore, the binder composition according to the present invention possesses flexibility and excellent bonding strength, which can mitigate thermal expansion and deformation of the electrode during its lifespan, thereby enhancing the stability of the electrode during the charge and discharge cycle of the battery.

[0216] In addition, the binder composition according to the present invention can provide effective electrical conductivity properties that can lower the electrical resistance of an electrode, and has high thermal stability, so that deformation and conductivity loss of the electrode due to heat generated inside the battery can be minimized, thereby improving the performance of the battery.

Claims

1. A binder composition for a lithium ion secondary battery comprising a binder composition comprising thermoplastic polyurethane having an ester structure and polyvinylidene fluoride.

2. A binder composition for a lithium ion secondary battery, characterized in that in claim 1, the thermoplastic polyurethane comprises a hard segment comprising aromatic and aliphatic urethane units and a soft segment comprising polyester units.

3. A binder composition for a lithium ion secondary battery in the first paragraph, wherein the thermoplastic polyurethane has a weight average molecular weight of 50,000 to 300,000 g / mol, a glass transition temperature (Tg) of -35°C to 15°C, and a melting point (Tm) of 80°C to 200°C.

4. A binder composition for a lithium ion secondary battery, characterized in that in the first paragraph, the thermoplastic polyurethane has a tensile strength of 150 to 450 kg / cm2 and an elongation of 200 to 800%.

5. An electrode comprising a binder composition for a lithium ion secondary battery according to any one of claims 1 to 4, a positive electrode active material, and a conductive material.

6. In the fifth paragraph, the positive electrode active material is a layered compound such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium manganese oxide (LiMnO2) represented by the chemical formula Li1+xMn2-xO4 (wherein, x is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2, etc.; lithium copper oxide (Li2CuO2); vanadium oxide such as LiV3O8, LiFe3O4, V2O5, Cu2V2O7; lithium nickel oxide (lithiated nickel oxide) represented by the chemical formula LiNi1-xMxO2 (wherein, M is Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x is 0.01 to 0.3); An electrode characterized by including a lithium manganese composite oxide represented by the chemical formula LiMn2-xMxO2 (wherein, M is Co, Ni, Fe, Cr, Zn or Ta, and x is 0.01 to 0.1) or Li2Mn3MO8 (wherein, M is Fe, Co, Ni, Cu or Zn); LiMn2O4 in which a part of the lithium in the chemical formula is substituted with an alkaline earth metal ion; a disulfide compound; Fe2(MoO4)3 or a composite oxide formed by a combination thereof.

7. An electrode according to claim 6, characterized in that the positive electrode active material is an NCM-based metal oxide.

8. An electrode characterized in that in paragraph 5, the binder composition for secondary batteries is included in an amount of 0.1 to 20 parts by weight based on 100 parts by weight of the positive electrode active material.

9. A lithium ion secondary battery comprising an electrode according to Article 5.

10. A lithium ion secondary battery according to claim 9, characterized in that the lithium secondary battery includes an electrolyte solution including an electrolyte salt and a non-aqueous electrolyte.

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