Binder containing copolymer composition, negative electrode for secondary battery containing said binder, and secondary battery containing said negative electrode

Abstract

The present invention relates to a copolymer composition including a first copolymer containing acrylic acid-based monomer units and acrylamide-based monomer units, and a second copolymer containing vinylpyrrolidone-based units, hydroxyl-containing acrylate-based monomer units, and acrylamide-based monomer units, as well as a negative electrode slurry, a negative electrode, and a secondary battery each containing the copolymer composition.

Description

[Technical Field]

[0001] The present invention relates to a copolymer composition that can be used as a binder, and a slurry, an electrode, and a secondary battery that contain the same. [Background technology]

[0002] Lithium secondary batteries have a high energy density and are widely used in the electrical, electronic, communication, and computer industries. Following small lithium secondary batteries for portable electronic devices, their application fields are expanding to include high-capacity secondary batteries for hybrid vehicles, electric vehicles, etc.

[0003] As the range of applications expands, lithium secondary batteries are required to have not only higher capacity but also longer life. One method for increasing the capacity of lithium secondary batteries is to use an active material containing silicon atoms in the negative electrode.

[0004] The application of active materials containing silicon atoms, which have a higher lithium intercalation / deintercalation rate than conventional carbon-based active materials, is expected to improve battery capacity. However, silicon-containing active materials undergo large volume changes due to lithium intercalation / deintercalation, resulting in significant expansion and contraction of the negative electrode active material layer during charging and discharging.

[0005] As a result, the conductivity between negative electrode active materials decreases, or the conductive path between the negative electrode active material and the current collector is interrupted, resulting in problems such as a deterioration in the cycle characteristics of the secondary battery.

[0006] However, the various binders that have been developed so far (PAA, PAA / CMC, Na-PAA, crosslinked PAA, alginate, PVA, etc.) have insufficient adhesive strength or make the electrodes too brittle and lack durability, making it difficult to expect a solution to the volume expansion problem mentioned above.

[0007] Therefore, there is a demand for a binder that can solve these problems and ensure the capacity retention rate of secondary batteries. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Republic of Korea Patent Publication No. 10-2016-0024921 Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, an object of the present invention is to provide a copolymer composition having excellent flexibility.

[0010] The present invention also provides a slurry composition using the copolymer composition, which has excellent electrode expansion suppression ability, and improved coating properties and rheological properties.

[0011] In addition, the present invention provides an electrode (especially a negative electrode) having excellent performance to which the slurry composition is applied, and a secondary battery including the electrode and having excellent life characteristics (capacity retention rate per cycle).

[0012] However, the problems to be solved by the present application are not limited to those mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the following description. [Means for solving the problem]

[0013] One aspect of the present application is a copolymer comprising a first copolymer including an acrylic acid-based monomer unit and an acrylamide-based monomer unit; a second copolymer including a vinyl pyrrolidone-based unit, an acrylate-based monomer unit containing a hydroxyl group, and an acrylamide-based monomer unit; A copolymer composition is provided. Another aspect of the present application is a method for producing a polymerizable composition comprising the copolymer composition and a negative electrode active material, A negative electrode slurry is provided. Yet another aspect of the present application is a battery comprising: a current collector; a negative electrode active material layer formed on the current collector and including the copolymer composition, A negative electrode is provided. Yet another aspect of the present application is The negative electrode A secondary battery is provided. [Effects of the Invention]

[0014] The copolymer composition of the present invention has excellent flexibility, which improves the coating properties and rheological properties of the negative electrode slurry composition, and also suppresses the expansion of the negative electrode, thereby improving the life of the secondary battery. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a graph showing the change in viscosity with change in shear rate of negative electrode slurries of Examples 1 to 3 of the present invention and Comparative Examples 1 to 3. DETAILED DESCRIPTION OF THE INVENTION

[0016] The functions and effects of the present invention will be described in more detail below through specific examples of the present invention, but these examples are merely examples of the present invention and do not define the scope of the invention.

[0017] Prior to this, the terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, in accordance with the principle that an inventor can appropriately define the concept of a term in order to best describe his or her invention.

[0018] Therefore, it should be understood that the configuration of the embodiment described in this specification is merely one of the most preferred embodiments of the present invention and does not represent the entire technical idea of ​​the present invention, and that there may be various equivalents and modifications that can replace them at the time of this application.

[0019] In this specification, the singular includes the plural unless the context clearly indicates otherwise. It should be understood that in this specification, the terms "comprise," "comprise," or "have" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0020] In this specification, the terms "from" and "to" in "from a to b" and "a to b" that indicate a numerical range are defined as ≧a and ≦b.

[0021] A copolymer composition according to one embodiment of the present application may include a first copolymer including an acrylic acid-based monomer unit and an acrylamide-based monomer unit, and a second copolymer including a vinyl pyrrolidone-based unit, an acrylate-based monomer unit containing a hydroxyl group, and an acrylamide-based monomer unit.

[0022] The first copolymer and the second copolymer have higher flexibility than existing acrylic acid homopolymers, and can be added as a two-component type during preparation of electrode slurry to form a tertiary network within the electrode.

[0023] During the electrode manufacturing process, a covalent bond and / or a hydrogen bond can be formed between the first copolymer and the second copolymer in the electrode during the electrode drying step, and the expansion of silicon can be suppressed by interaction with silicon, which is an anode active material, thereby extending the life of the secondary battery.

[0024] That is, a tertiary network can be formed by cross-linking between the first copolymer and the second copolymer through covalent bonds and / or hydrogen bonds.

[0025] Specifically, the first copolymer contains a large amount of carboxyl groups via the acrylic acid-based monomer units, allowing it to react effectively with silicon and has rigidity, thereby effectively suppressing the expansion of silicon. Additionally, the first copolymer maintains the heat resistance of the electrode and interacts with the surface of silicon particles, which are the negative electrode active material, via the acrylamide-based monomer units.

[0026] Meanwhile, the second copolymer can reduce the rigidity and electrical resistance of the electrode through the vinylpyrrolidone-based unit, which exhibits a relatively high glass transition temperature.

[0027] In addition, the second copolymer can increase the flexibility of the copolymer through the hydroxyl-containing acrylate monomer unit, which has a relatively low glass transition temperature, and can form covalent bonds and / or hydrogen bonds within the electrode. The acrylamide monomer unit can maintain the heat resistance of the electrode and interact with the surface of silicon particles, which are the negative electrode active material.

[0028] In one embodiment, the acrylic acid-based monomer unit of the first copolymer may be formed by polymerizing an acrylic acid monomer, a methacrylic acid monomer, or a combination thereof, and the acrylamide-based monomer unit of the first copolymer may be formed by polymerizing an acrylamide monomer, a methacrylamide monomer, or a combination thereof.

[0029] Meanwhile, the hydroxyl group-containing acrylate monomer unit of the second copolymer may be formed by polymerizing a 2-hydroxyethyl acrylate monomer, a 2-hydroxyethyl methacrylate monomer, or a combination thereof, and the acrylamide monomer unit of the second copolymer may be formed by polymerizing an acrylamide monomer, a methacrylamide monomer, or a combination thereof.

[0030] That is, the hydroxyl group-containing acrylate-based monomer unit of the second copolymer may be formed by polymerizing a hydroxyl group-containing acrylate, a hydroxyl group-containing methacrylate, or a combination thereof.

[0031] Also, the vinylpyrrolidone-based unit of the second copolymer may be formed by polymerizing a vinylpyrrolidone monomer.

[0032] For example, the first copolymer may be formed by copolymerizing an acrylic acid monomer, a methacrylic acid monomer, and an acrylamide monomer.

[0033] The second copolymer may be formed by copolymerizing an acrylamide monomer, a hydroxyethyl acrylate monomer, and a vinylpyrrolidone monomer.

[0034] Meanwhile, the carboxylate group of the acrylic acid-based monomer unit of the first copolymer can bond with an alkali metal.

[0035] On the other hand, the weight ratio of the alkali metal to the copolymer (weight of one or more selected from the group consisting of the alkali metal and acetate salt compounds containing an alkali metal:weight of the copolymer) may be 0.1 to 15:100.

[0036] For example, the weight ratio of the alkali metal to the copolymer may be 1 to 10:100.

[0037] The alkali metal may be, for example, Li, Na, or K.

[0038] In one embodiment, the first copolymer may comprise 75 wt % or more and 95 wt % or less of the acrylic acid-based monomer units and 5 wt % or more and 25 wt % or less of the acrylamide-based monomer units, based on 100 wt % of the total weight of the first copolymer.

[0039] For example, the first copolymer may be prepared by copolymerizing 70 wt % of acrylic acid, 10 wt % of methacrylic acid, and 20 wt % of acrylamide, based on a total weight of the first copolymer (100 wt %).

[0040] If the content of the acrylic acid-based monomer unit exceeds the above range, flexibility may decrease, i.e., the copolymer may become brittle, and heat resistance and coating properties may decrease when applied to an electrode.

[0041] Furthermore, if the content of the acrylamide-based monomer unit is below the above range, flexibility may decrease and heat resistance may decrease when applied to an electrode.

[0042] In one embodiment, the second copolymer may comprise, based on a total weight of the second copolymer (100 wt%), 30 wt% to 50 wt% of the vinylpyrrolidone-based monomer units, 30 wt% to 50 wt% of the hydroxyl-containing acrylate-based monomer units, and 10 wt% to 30 wt% of the acrylamide-based monomer units.

[0043] For example, the second copolymer may contain 40 wt % of the vinylpyrrolidone-based monomer unit, 40 wt % of the hydroxyl group-containing acrylate-based monomer unit, and 20 wt % of the acrylamide-based monomer unit, based on 100 wt % of the total weight of the second copolymer.

[0044] If the content of the vinylpyrrolidone-based monomer unit is below the above range, the resistance characteristics of the battery may be reduced.

[0045] Furthermore, if the content of the hydroxyl group-containing acrylate monomer unit and / or the acrylamide monomer unit is below the above range, the adhesive strength may decrease.

[0046] In one embodiment, the acrylic acid-based monomer unit of the first copolymer may be formed by polymerizing an acrylic acid monomer and a methacrylic acid monomer in a weight ratio (weight of acrylic acid monomer:weight of methacrylic acid monomer) of 2 to 10:1.

[0047] When the weight ratio of the acrylic acid monomer to the methacrylic acid monomer in the first copolymer is above or below the above range, the copolymer may become brittle and have reduced rigidity.

[0048] In one embodiment, the first copolymer may include a monomer repeat unit represented by the following Formula 1, and the second copolymer may include a monomer repeat unit represented by the following Formula 2:

[0049] [ka]

[0050] In the above Chemical Formula 1, R and R′ are each independently hydrogen, an alkali metal, or a combination thereof; a+b+c=1.

[0051] [ka]

[0052] In the above formula 2, R″ is hydrogen, methyl, or a combination thereof; x+y+z=1.

[0053] In the above Chemical Formula 1, a, b, and c correspond to the weight fraction of each monomer unit, and the sum of the weight fractions of each monomer unit is 1.

[0054] In addition, x, y, and z in the above Chemical Formula 2 correspond to the weight fraction of each monomer unit, and the sum of the weight fractions of each monomer unit is 1.

[0055] In one embodiment, the copolymer composition may contain 10 wt % or more and 90 wt % or less of the first copolymer and 10 wt % or more and 90 wt % or less of the second copolymer, based on a total weight of 100 wt % of the copolymer composition.

[0056] For example, the copolymer composition may contain 20% by weight or more and 80% by weight or less of the first copolymer and 20% by weight or more and 80% by weight or less of the second copolymer, based on a total weight of 100% by weight of the copolymer composition.

[0057] If the content of the first copolymer and the second copolymer is above or below the above range, the viscosity may be low, resulting in reduced stability, or the electrode expansion rate may be high, resulting in reduced battery life characteristics.

[0058] Meanwhile, within the ranges of the first copolymer and the second copolymer in the copolymer composition, the higher the content ratio of the first copolymer, when used as a binder, the greater the effect of suppressing electrode expansion and the improved life characteristics of the secondary battery.

[0059] In one embodiment, the first copolymer may be a random or block copolymer, and the second copolymer may be a random or block copolymer.

[0060] In one embodiment, the weight average molecular weight of the first copolymer may be 10,000 or more and 1,000,000 or less, and the weight average molecular weight of the second copolymer may be 10,000 or more and 1,000,000 or less.

[0061] According to another aspect of the present disclosure, a negative electrode slurry may include the copolymer composition and a negative electrode active material.

[0062] That is, the copolymer composition can be used as a binder for a negative electrode, and in particular, may be an aqueous binder.

[0063] The negative electrode slurry uses distilled water as a solvent, contains 6 wt % of the copolymer, has a solid content of 48 to 58 wt %, and is measured at a shear rate of 1 s -1 The viscosity is 10,000 cp or more and 50,000 cp or less at a shear rate of 10 s -1 The viscosity at 2000 cp may be 4,000 cp or more and 10,000 cp or less.

[0064] The negative electrode active material may be a compound containing one or more elements selected from the group consisting of a carbon-based material, silicon, an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, and a rare earth element, and preferably may be silicon or a compound containing silicon.

[0065] The carbonaceous material includes, for example, artificial graphite, natural graphite, hard carbon, soft carbon, etc., but is not limited thereto. The negative electrode active material containing silicon is not particularly limited in its type as long as it is silicon or a compound containing silicon, but preferably Si, SiO x (0 < x < 2), Si-Y alloy (where Y is an alkali metal, alkaline earth metal, Group 13 element, Group 14 element, transition metal, rare earth element, or a combination thereof, and is not Si), and one or more selected from the group consisting of Si-C composites may also be used.

[0066] When the negative electrode active material containing silicon and another negative electrode active material are mixed and used as the negative electrode active material, the negative electrode active material containing silicon may be contained at 8% by weight or more of the total weight of the negative electrode active material.

[0067] The negative electrode active material may be contained at 50 to 98% by weight based on the total weight of the negative electrode active material layer.

[0068] When the negative electrode active material is contained less than 50% by weight, the energy density decreases and a high-energy density battery cannot be manufactured. When it is contained more than 98% by weight, the contents of the conductive material and the binder decrease, the electrical conductivity decreases, and the adhesive force between the electrode active material layer and the current collector may decrease.

[0069] On the other hand, the copolymer composition binder of the present application may be contained at 1 to 35% by weight based on the total weight of the negative electrode slurry. If the copolymer is less than 1% by weight, the physical properties of the negative electrode may deteriorate and the negative electrode active material and the conductive material may fall off. If it exceeds 35% by weight, the ratio of the negative electrode active material to the conductive material may relatively decrease, the battery capacity may decrease, and the electrical conductivity of the negative electrode may decrease.

[0070] The negative electrode slurry may further include a polymer in addition to the copolymer composition of the present application, such as, but not limited to, polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyacrylic acid (PAA), metal polyacrylate (Metal-PAA), polymethacrylic acid (PMA), polymethyl methacrylate (PMMA), polyacrylamide (PAM), polymethacrylamide, polyacrylonitrile (PAN), polymethacrylonitrile, polyimide (PI), chitosan, starch, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, hydroxypropyl cellulose, regenerated cellulose, and various copolymers thereof.

[0071] A negative electrode according to yet another aspect of the present application may include a current collector and a negative electrode active material layer formed on the current collector, the negative electrode active material layer including the copolymer composition of the present application.

[0072] The negative electrode active material layer may further include a conductive material. The conductive material is used to further improve the conductivity of the negative electrode active material. The conductive material may be any material that is conductive without inducing chemical changes in the battery. Examples of the conductive material include graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives.

[0073] The conductive material may be contained in an amount of 0.05 to 30 wt %, preferably 15 to 25 wt %, based on the total weight of the negative electrode active material layer. If the conductive material is contained in an amount less than 0.05 wt %, the electrical conductivity of the negative electrode will be reduced. If the conductive material is contained in an amount exceeding 30 wt %, the ratio of the silicon-based negative electrode active material to the binder will be relatively reduced, resulting in a reduced battery capacity. Furthermore, since the binder content must be increased to maintain the negative electrode active material layer, the content of the negative electrode active material will be reduced, making it impossible to manufacture a high-energy density battery.

[0074] In the negative electrode of the present application, the negative electrode active material layer contains the copolymer composition of the present application, which makes it possible to suppress volume expansion of the negative electrode active material that occurs during charge and discharge of the secondary battery, and to improve the capacity retention rate per cycle.

[0075] The negative electrode can be manufactured by (a) preparing a composition for forming a negative electrode active material layer, the composition including a negative electrode active material and the copolymer composition of the present application, and (b) applying the composition for forming a negative electrode active material layer onto a negative electrode current collector and then drying the applied composition.

[0076] The composition for forming the negative electrode active material layer is prepared in a negative electrode slurry state, and the solvent for preparing the slurry state must be easy to dry, and most preferably, be capable of dissolving the copolymer composition binder of the present invention well and maintaining the negative electrode active material in a dispersed state without dissolving it.

[0077] The solvent according to the present invention may be water or an organic solvent, and the organic solvent may be one or more selected from the group consisting of methylpyrrolidone, dimethylformamide, isopropyl alcohol, acetonitrile, methanol, ethanol, and tetrahydrofuran.

[0078] The composition for forming the negative electrode active material layer can be mixed by a conventional mixer, such as a latex mixer, a high-speed shear mixer, or a homomixer, using a conventional method.

[0079] The step (b) is a step of applying the composition for forming a negative electrode active material layer prepared in the step (a) onto a negative electrode current collector and then drying the applied composition to prepare a negative electrode for a lithium secondary battery.

[0080] Specifically, the negative electrode current collector may be made of a material selected from the group consisting of copper, stainless steel, titanium, silver, palladium, nickel, alloys thereof, and combinations thereof. The stainless steel may be surface-treated with carbon, nickel, titanium, or silver, and the alloy may be an aluminum-cadmium alloy. Other materials that may be used include calcined carbon, a non-conductive polymer surface-treated with a conductive material, and a conductive polymer.

[0081] The composition for forming a negative electrode active material layer prepared in step (a) is applied onto a negative electrode current collector, and can be coated onto the current collector to an appropriate thickness depending on the thickness to be formed, which can be suitably selected preferably within the range of 10 to 300 μm.

[0082] In this case, the method of applying the slurry-like composition for forming a negative electrode active material layer is not limited, and the negative electrode active material layer may be prepared by, for example, doctor blade coating, dip coating, gravure coating, slit die coating, spin coating, comma coating, bar coating, reverse roll coating, screen coating, cap coating, etc.

[0083] After coating, the coating is dried, and finally, a negative electrode for a secondary battery (particularly, a lithium secondary battery) having a negative electrode active material layer formed thereon can be produced.

[0084] A battery according to yet another aspect of the present application may include a current collector and a negative electrode in which the negative electrode active material layer is formed on the current collector.

[0085] The battery may be a secondary battery (particularly, a lithium secondary battery) including a positive electrode, the negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte solution.

[0086] The secondary battery may have a capacity retention rate of 70% or more after 50 charge / discharge cycles.

[0087] The secondary battery may have an electrode expansion rate of 30% or less when subjected to 50 charge / discharge cycles.

[0088] The positive electrode, separator, and electrolyte of the lithium secondary battery are not particularly limited in the present invention and may be any known material in the art.

[0089] The positive electrode includes a positive electrode active material formed on a positive electrode current collector.

[0090] The positive electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery, and examples thereof include stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. In this case, the positive electrode current collector may be in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric, having a finely textured surface to enhance adhesion to the positive electrode active material.

[0091] The positive electrode active material constituting the positive electrode active material layer can be any positive electrode active material used in the art. Specific examples of such positive electrode active materials include lithium metal; lithium cobalt oxides such as LiCoO; Li 1+x Mn 2-xO4 (where x is from 0 to 0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, LiMnO2; lithium copper oxides such as Li2CuO2; vanadium oxides such as LiV3O8, LiFe3O4, V2O5, Cu2V2O7; LiNi 1-x M x O2 (where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga and x = 0.01 to 0.3), lithium nickel oxides represented by; LiMn 2-x M x O2 (where M = Co, Ni, Fe, Cr, Zn or Ta and x = 0.01 to 0.1) or lithium manganese composite oxides represented by Li2Mn3MO8 (where M = Fe, Co, Ni, Cu or Zn); Li(Ni a Co b Mn c )O2 (where 0 < a < 1, 0 < b < 1, 0 < c < 1, and a + b + c = 1), lithium-nickel-manganese-cobalt oxides; sulfur or disulfide compounds; phosphates such as LiFePO4, LiMnPO4, LiCoPO4, LiNiPO4; Fe2(MoO4)3, etc. are mentioned, but not limited to only these.

[0092] At this time, the positive electrode active material layer can additionally contain, in addition to the positive electrode active material, a binder, a conductive material, a filler, and other additives, etc. The conductive material is the same as the above-mentioned content in the negative electrode for the lithium secondary battery.

[0093] Examples of the binder include, but are not limited to, polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyacrylic acid (PAA), polymethacrylic acid (PMA), polymethyl methacrylate (PMMA), polyacrylamide (PAM), polymethacrylamide, polyacrylonitrile (PAN), polymethacrylonitrile, polyimide (PI), chitosan, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof.

[0094] The separator may be made of a porous substrate. The porous substrate may be any porous substrate commonly used in electrochemical devices, such as, but not limited to, a polyolefin-based porous membrane or nonwoven fabric.

[0095] The separation membrane may be a porous substrate made of any one selected from the group consisting of polyethylene, polypropylene, polybutylene, polypentene, polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, and polyethylene naphthalate, or a mixture of two or more thereof.

[0096] The electrolyte of the lithium secondary battery is a non-aqueous electrolyte containing a lithium salt, and is composed of a lithium salt and a solvent. The solvent may be a non-aqueous organic solvent, an organic solid electrolyte, or an inorganic solid electrolyte.

[0097] The lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte solution, and examples thereof include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB 10Cl 10 , LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiC4BO8, LiCF3CO2, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiN(SO2F)2, LiN(SO2C2F5)2, LiC4F9SO3, LiC(CF3SO2)3, (CF3SO2)·2NLi, lithium chloroborane, lower aliphatic lithium carboxylates, and lithium 4-phenylborate imide can be used.

[0098] Examples of non-aqueous organic solvents that can be used include aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydroxyfuran (franc), 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, 4-methyl-1,3-dioxene, diethyl ether, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0099] Examples of the organic solid electrolyte that can be used include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyagitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and polymers containing secondary dissociating groups.

[0100] Examples of the inorganic solid electrolyte that can be used include nitrides, halides, and sulfates of Li, such as LiN, LiI, LiNI, LiN-LiI-LiOH, LiSiO, LiSiO-LiI-LiOH, LiSiS, LiSiO, LiSiO-LiI-LiOH, and LiPO-LiS-SiS.

[0101] The non-aqueous electrolyte solution may further contain other additives for the purpose of improving charge / discharge characteristics, flame retardancy, etc. Examples of the additives include pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, fluoroethylene carbonate (FEC), propene sultone (PRS), vinylene carbonate (VC), etc.

[0102] The lithium secondary battery according to the present invention can be manufactured by lamination stacking of separators and electrodes and folding processes in addition to the conventional winding process, and the battery case may be cylindrical, prismatic, pouch-shaped, or coin-shaped. [Example]

[0103] The present invention will be described in more detail below using examples, but the present invention is not limited to these examples.

[0104] [Production Example 1] Production of the first copolymer 1.3 L of distilled water and ethylenediaminetetraacetic acid (EDTA) were added to a nitrogen-filled reactor and heated to 60°C. After the temperature reached 60°C, an initiator and a monomer mixture were added to the reactor and reacted to produce a first copolymer (PAA-PMA-PAM).

[0105] Meanwhile, the initiator was prepared by dissolving 0.1 g of Azo initiator in 60 g of distilled water.

[0106] In addition, the monomer mixture was prepared by mixing acrylic acid (AA), methacrylic acid (MA), and acrylamide (AM) in a weight ratio of 7:1:2, respectively, with a total solid content of 15 wt% or less.

[0107] The weight average molecular weight of the first copolymer produced was about 700,000 as measured by GPC.

[0108] [Production Example 2] Production of second copolymer A nitrogen-filled reactor was charged with 1 L of distilled water, EDTA, and 60 g of vinylpyrrolidine (VP), and the temperature was raised to 60°C. Once the temperature reached 60°C, the initiator and the remaining monomer mixture, excluding the vinylpyrrolidone already added, were added and reacted to produce a second copolymer (PVP-PHEA-PAM).

[0109] Meanwhile, the initiator was prepared by dissolving 0.075 g of Azo initiator in 45 g of distilled water.

[0110] A mixture of the remaining monomers, excluding PVP, was prepared by mixing 60 g of hydroxyethyl acrylate (HEA) and 30 g of acrylamide (AM), respectively.

[0111] That is, the weight ratio of vinylpyrrolidone, hydroxyethyl acrylate, and acrylamide in the second copolymer was 4:4:2.

[0112] The weight average molecular weight of the first copolymer produced was about 700,000 as measured by GPC.

[0113] [Production Example 3] Production of lithium secondary battery Based on 100 parts by weight of total slurry solids, 77.56 parts by weight of graphite as electrode active materials, 19.39 parts by weight of SiC, 0.05 parts by weight of single-walled carbon nanotubes (SWCNT) as a conductive material, and 3 parts by weight of a binder containing the first copolymer prepared in Preparation Example 1 and the second copolymer prepared in Preparation Example 2 were mixed, and distilled water was added to prepare a negative electrode slurry in which the slurry solids content was adjusted to 20 to 80% by weight.

[0114] The prepared negative electrode slurry was uniformly coated on a copper current collector and dried at 110°C. The resulting mixture was rolled using a roll press and heated in a vacuum oven at 110°C for 4 hours or more to prepare a negative electrode.

[0115] Thereafter, a non-aqueous electrolyte solution containing a lithium salt was used as an electrolyte, and a polyolefin separator was interposed between the positive electrode and the negative electrode, and then a lithium secondary battery was manufactured without being classified into a pouch or coin cell type.

[0116] The non-aqueous electrolyte used was a solvent in which ethylene carbonate: ethyl methyl carbonate: diethyl carbonate was mixed in a volume ratio of 2:1:7, to which 5 wt% FEC and 1 wt% LiP2FP were added, and LiPF6 electrolyte was dissolved at a concentration of 1.5M.

[0117] [Example 1] A lithium secondary battery was manufactured according to Manufacturing Example 3 using a binder prepared by mixing the first copolymer manufactured according to Manufacturing Example 1 and the second copolymer manufactured according to Manufacturing Example 2 in a weight ratio (weight % of the first copolymer:weight % of the second copolymer) of 40:60.

[0118] [Example 2] A lithium secondary battery was produced in the same manner as in Example 1, except that the weight ratio of the first copolymer to the second copolymer (weight % of the first copolymer:weight % of the second copolymer) was 50:50.

[0119] [Example 3] A lithium secondary battery was produced in the same manner as in Example 1, except that the weight ratio of the first copolymer to the second copolymer (weight % of the first copolymer:weight % of the second copolymer) was 60:40.

[0120] [Comparative Example 1] Instead of the composition of the first copolymer and the second copolymer, polyacrylic acid (PAA) alone was used to manufacture a lithium secondary battery in the same manner as in Example 1. The weight-average molecular weight of PAA measured by GPC was about 100,000.

[0121] The polyacrylic acid used in Comparative Example 1 was prepared by adding 1 L of distilled water and EDTA to a nitrogen-filled reactor, heating the reactor to 60°C, and then adding an initiator solution prepared by dissolving 0.0055 g of Azo initiator in 45 g of distilled water and acrylic acid, and reacting them.

[0122] Comparative Example 2 A lithium secondary battery was manufactured in the same manner as in Example 1, except that the binder was the first copolymer alone (a binder without the second copolymer).

[0123] Comparative Example 3 A lithium secondary battery was manufactured in the same manner as in Example 1, except that a binder containing only the second copolymer (a binder containing no first copolymer) was used.

[0124] The weight ratios of the first and second copolymers used in Examples 1 to 3 and Comparative Examples 1 to 3 are as shown in Table 1 below.

[0125] [Table 1]

[0126] [Evaluation Example 1] Evaluation of rheological properties of negative electrode slurry The rheological properties of the negative electrode slurries used to manufacture the lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 3 were measured at 25° C. using a rheometer.

[0127] During the measurement, distilled water was used as a solvent and the copolymer content was adjusted to 6 wt %.

[0128] As a result of the measurement, as shown in FIG. 1, the negative electrode slurries of Examples 1 to 3, in which the compositions of the first copolymer and the second copolymer were used, exhibited a shear thinning phenomenon in which the viscosity decreased with increasing shear rate.

[0129] In contrast, the negative electrode slurry containing only the PAA polymer in Comparative Example 1 showed a section in which the viscosity decreased and then increased again as the shear rate increased, which may lead to a decrease in coatability.

[0130] Meanwhile, it was confirmed that the negative electrode slurry in Comparative Example 2, in which the first copolymer was used alone, had a lower viscosity at low shear rates than the negative electrode slurries in Examples 1 to 3, in which the first copolymer and the second copolymer were used in combination. Such low viscosity at low shear rates may lead to a decrease in the stability of the binder.

[0131] In addition, it was confirmed that the negative electrode slurry in Comparative Example 3, in which the second copolymer was used alone, showed very little change in viscosity due to changes in shear rate.

[0132] [Evaluation Example 2] Battery performance evaluation The lithium secondary batteries produced in Examples 1 to 3 and Comparative Examples 1 to 3 were charged and discharged three times at 25°C, a charge / discharge current density of 0.1C, a charge cut-off voltage of 4.8V, and a discharge cut-off voltage of 2.7V.

[0133] Thereafter, the battery was charged and discharged 50 times with a charge / discharge current density of 1 C, a charge cut-off voltage of 4.8 V, and a discharge cut-off voltage of 2.7 V, and the capacity retention rate was measured.

[0134] All discharges were performed under constant current / constant voltage conditions, and the cut-off current for constant voltage discharge was 0.005C.

[0135] At this time, the capacity retention rate was calculated using the following formula 1. [Formula 1] Capacity retention rate (%) = (discharge capacity after 50 cycles / discharge capacity after 3 cycles) * 100

[0136] After the charge-discharge evaluation was completed, the cell was disassembled to check the change in thickness of the negative electrode, and the silicon expansion suppression effect of the binders used in Examples 1 to 3 and Comparative Examples 1 to 3 was compared.

[0137] At this time, the thickness change rate was calculated using the following equation 2. [Formula 2] Electrode expansion rate (%) = (thickness of negative electrode after 50 cycles - thickness of negative electrode vacuum-dried before assembly) / thickness of negative electrode vacuum-dried before assembly * 100

[0138] The capacity retention rates and electrode expansion rates of the batteries measured in Evaluation Examples 2 and 3 are shown in Table 2 below.

[0139] [Table 2]

[0140] On the other hand, in Examples 1 to 3 in which a binder in which the first copolymer and the second copolymer were mixed at a certain ratio was used, the capacity retention rate of the lithium secondary battery after 50 cycles was measured to be 70% or more.

[0141] Furthermore, it was measured that the higher the ratio of the first copolymer in the weight ratio of the first copolymer to the second copolymer, the higher the capacity retention rate of the lithium secondary battery.

[0142] In contrast, in Comparative Example 1 in which the PAA polymer alone was used as the binder, the capacity retention rate of the lithium secondary battery after 50 cycles was measured to be 45.7%.

[0143] In addition, in Comparative Examples 2 and 3 in which the first copolymer or the second copolymer alone was used as the binder, the capacity retention rates of the lithium secondary batteries after 50 cycles were measured to be 55.7% and 36.7%, respectively.

[0144] That is, when a binder in which the first copolymer and the second copolymer are mixed at a certain ratio is used, it was confirmed that the capacity retention rate of the lithium secondary battery is higher and the life characteristics of the lithium secondary battery are improved compared to when the binder is made of the PAA polymer, the first copolymer, or the second copolymer alone.

[0145] In terms of the electrode expansion rate after 50 cycles of the lithium secondary battery, it was confirmed that in Examples 1 to 3, in which a binder in which the first copolymer and the second copolymer were mixed at a certain ratio was used, the electrode expansion rate was suppressed to 30% or less.

[0146] In contrast, in Comparative Example 1, in which the PAA polymer alone was used as the binder, the electrode expansion rate after 50 cycles was as extremely high as 104%.

[0147] In addition, in Comparative Examples 2 and 3, in which the first copolymer or the second copolymer alone was used as the binder, the electrode expansion rate of the lithium secondary battery after 50 cycles was extremely high at 84% and 128%, respectively.

[0148] A high electrode expansion rate shortens the lifespan of lithium secondary batteries and can lead to serious malfunctions such as fires.

[0149] That is, when a binder consisting solely of the first copolymer or the second copolymer is used, the capacity retention rate of the lithium secondary battery may decrease and the electrode expansion rate may increase, resulting in a deterioration in battery characteristics, compared to a binder in which the first copolymer and the second copolymer are mixed at a certain ratio.

[0150] As a result, it was confirmed that the copolymer binder composition of the present invention, in which the first copolymer and the second copolymer are mixed at a certain ratio, has appropriate ranges of rheological properties of a negative electrode slurry composition and secondary battery properties.

[0151] On the other hand, when the first copolymer or the second copolymer is used alone or mixed with an existing polymer to be used as a binder, it was found that one or more of the rheological properties of the negative electrode slurry composition, the expansion coefficient of the negative electrode, and the properties of the secondary battery are unsuitable for use in actual secondary batteries.

[0152] The scope of the present invention is indicated by the claims that follow rather than by the above detailed description, and all modifications and variations that fall within the meaning and scope of the claims and their equivalents should be interpreted as being included within the scope of the present invention. [Industrial Applicability]

[0153] The copolymer composition of the present invention has excellent flexibility, which improves the coating properties and rheological properties of the negative electrode slurry composition, and also suppresses the expansion of the negative electrode, thereby improving the life of the secondary battery.

Claims

1. a first copolymer containing acrylic acid-based monomer units and acrylamide-based monomer units; a second copolymer containing vinylpyrrolidone-based units, acrylate-based monomer units containing a hydroxyl group, and acrylamide-based monomer units; Copolymer composition.

2. The acrylic acid-based monomer unit of the first copolymer is formed by polymerizing an acrylic acid monomer, a methacrylic acid monomer, or a combination thereof; the acrylamide-based monomer unit of the first copolymer is formed by polymerizing an acrylamide monomer, a methacrylamide monomer, or a combination thereof; the hydroxyl group-containing acrylate monomer unit of the second copolymer is formed by polymerizing a 2-hydroxyethyl acrylate monomer, a 2-hydroxyethyl methacrylate monomer, or a combination thereof; The acrylamide-based monomer unit of the second copolymer is formed by polymerizing an acrylamide monomer, a methacrylamide monomer, or a combination thereof. The copolymer composition of claim 1 .

3. The first copolymer contains 75 wt % or more and 95 wt % or less of the acrylic acid-based monomer unit and 5 wt % or more and 25 wt % or less of the acrylamide-based monomer unit, based on 100 wt % of the total weight of the first copolymer. The copolymer composition of claim 1 .

4. The second copolymer contains, based on a total weight of the second copolymer (100 wt%), 30 wt% to 50 wt% of the vinylpyrrolidone-based monomer unit, 30 wt% to 50 wt% of the hydroxyl-containing acrylate-based monomer unit, and 10 wt% to 30 wt% of the acrylamide-based monomer unit. The copolymer composition of claim 1 .

5. The acrylic acid-based monomer unit of the first copolymer is formed by polymerizing an acrylic acid monomer and a methacrylic acid monomer in a weight ratio (weight of the acrylic acid monomer:weight of the methacrylic acid monomer) of 2 to 10:

1. The copolymer composition of claim 1 .

6. The first copolymer includes a monomer repeating unit represented by the following Chemical Formula 1: The second copolymer includes a monomer repeating unit represented by the following Chemical Formula 2: The copolymer composition of claim 1 . 【Chemical 1】 In the formula 1, R and R′ are each independently hydrogen, an alkali metal, or a combination thereof; a+b+c=1. 【Chemistry 2】 In the formula 2, R″ is hydrogen, methyl, or a combination thereof; x+y+z=1.

7. The copolymer composition contains 10% by weight or more and 90% by weight or less of the first copolymer and 10% by weight or more and 90% by weight or less of the second copolymer, based on a total weight of 100% by weight of the copolymer composition. The copolymer composition of claim 1 .

8. the first copolymer is a random or block copolymer; The second copolymer is a random or block copolymer. The copolymer composition of claim 1 .

9. the weight average molecular weight of the first copolymer is 10,000 or more and 1,000,000 or less; The weight average molecular weight of the second copolymer is 10,000 or more and 1,000,000 or less. The copolymer composition of claim 1 .

10. The copolymer composition according to any one of claims 1 to 9, a negative electrode active material, Anode slurry.

11. A current collector; a negative electrode active material layer formed on the current collector and comprising the copolymer composition according to any one of claims 1 to 9; Negative electrode.

12. The negative electrode according to claim 11, Secondary battery.

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