Binder comprising copolymer, secondary battery anode comprising binder, and secondary battery comprising anode

Abstract

The present invention relates to a copolymer, an anode slurry comprising same, an anode and a secondary battery, the copolymer comprising, on the basis of 100 mol% of the total weight of the copolymer, 65-99 mol% of an acrylic acid-based monomer unit or a combination of an acrylate-based monomer unit and an acrylic acid-based monomer unit and 1-35 mol% of a vinyl alcohol-based monomer unit or a combination of a vinyl acetate-based monomer unit and a vinyl alcohol-based monomer unit, and having a number average molecular weight of 150,000-230,000.

Description

A binder comprising a copolymer, a negative electrode for a secondary battery comprising the binder, and a secondary battery comprising the negative electrode

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

[0002] Lithium secondary batteries have high energy density and are widely used in the electrical, electronic, communications, and computer industries. Their application areas are expanding from small lithium secondary batteries for portable electronic devices to high-capacity secondary batteries for hybrid and electric vehicles.

[0003] As applications expand, demand for lithium secondary batteries is increasing, along with their higher capacity and longer lifespan. One method for increasing lithium secondary battery capacity is the use of active materials containing silicon atoms in the anode.

[0004] Compared to conventional carbon-based active materials, the application of active materials containing silicon atoms, which exhibit a higher lithium insertion / desorption capacity, can be expected to improve battery capacity. However, silicon-containing active materials exhibit significant volumetric changes associated with lithium insertion / desorption, resulting in significant expansion and contraction of the negative electrode active material layer during charge / discharge.

[0005] As a result, there was a problem in which the conductivity between the negative electrode active material and the negative electrode active material was reduced, the conductive path between the negative electrode active material and the current collector was blocked, and the cycle characteristics of the secondary battery deteriorated.

[0006] Additionally, cathode slurries containing silicon and carbon nanotubes may have high slurry viscosity, which may cause overloading of mixing equipment during slurry production, excessive time required for slurry filtering, or problems in transport and coating processes.

[0007] Therefore, there is a need for the development of a cathode binder that can suppress electrode expansion while having excellent binding and dispersibility while lowering the viscosity of a cathode slurry containing silicon and carbon nanotubes.

[0008] [Prior Art Literature]

[0009] [Patent Document]

[0010] (Patent Document 1) Republic of Korea Patent Publication No. 10-2013-0117901

[0011] Accordingly, the purpose of the present invention is to provide a copolymer for a negative electrode binder that can suppress electrode expansion while having excellent binding force and dispersibility while lowering the viscosity of a slurry (e.g., a negative electrode slurry including a silicon negative electrode active material, carbon nanotubes, or a combination thereof).

[0012] In addition, the present invention aims to provide a secondary battery having excellent performance, an electrode (particularly, a cathode) to which the slurry composition is applied, and excellent life characteristics (capacity retention rate) including the electrode.

[0013] However, the problems that the present invention seeks to solve are not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

[0014]

[0015] However, the problems that the present invention seeks to solve are not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

[0016] One aspect of the present invention is a copolymer comprising: an acrylic acid series monomer unit of 65 mol% or more and 99 mol% or less, based on 100 mol% of the total copolymer weight; and a combination of an acrylate series monomer unit and an acrylic acid series monomer unit; and

[0017] Containing 1 mol% or more and 35 mol% or less of a vinyl alcohol series monomer unit, or a combination of a vinyl acetate series monomer unit and a vinyl alcohol series monomer unit;

[0018] The number average molecular weight is 150,000 or more and 230,000 or less,

[0019] Provides a copolymer.

[0020] Another aspect of the present invention is the copolymer; and

[0021] Containing a negative active material;

[0022] Provides cathode slurry.

[0023] Another aspect of the main body is the whole house; and

[0024] A negative electrode active material layer including the copolymer formed on the above-mentioned collector;

[0025] Provides a cathode.

[0026] Another aspect of the present invention is that it comprises the cathode,

[0027] Provides secondary batteries.

[0028] The copolymer of the present invention can improve the life characteristics (capacity retention rate) of a lithium secondary battery by improving the binding force, dispersibility, and electrode expansion suppression ability while reducing the viscosity of a negative electrode slurry including a silicon negative electrode active material, carbon nanotubes, or a combination thereof.

[0029] Figure 1 shows the results of measuring the rheology of the negative electrode slurries prepared in Examples 1 and 2 and Comparative Examples 1 to 4, as measured by Evaluation Example 3 of the present invention.

[0030] Figure 2 is an SEM photograph taken to evaluate the dispersibility of the copolymer for negative electrode binder manufactured in Examples 1 and 2 and Comparative Examples 1 to 4, as measured by Evaluation Example 5 of the present invention.

[0031] Hereinafter, the functions and effects of the invention will be described in more detail through specific examples. However, these examples are provided merely as examples of the invention and do not define the scope of the invention.

[0032] Prior to this, the terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, and should be interpreted as meanings and concepts that conform to the technical idea of ​​the present invention based on the principle that the inventor can appropriately define the concept of the term to explain his or her own invention in the best way.

[0033] Accordingly, the configuration of the embodiments described in this specification is only one of the most preferred embodiments of the present invention and does not represent all of the technical ideas of the present invention, so it should be understood that various equivalents and modified examples that can replace them may exist at the time of filing this application.

[0034] In this specification, singular expressions include plural expressions unless the context clearly dictates otherwise. In this specification, terms such as "comprise," "include," or "have" should be understood to indicate the presence of a feature, number, step, component, or combination thereof, but not to preclude the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0035] In this specification, “a to b” and “a~b” indicating a numerical range are defined as “a to” and “~” as ≥ a and ≤ b.

[0036]

[0037] According to one aspect of the present invention, a copolymer may include, based on 100 mol% of the total copolymer weight, 65 mol% or more and 99 mol% or less of an acrylic acid series monomer unit, or a combination of an acrylate series monomer unit and an acrylic acid series monomer unit; and 1 mol% or more and 35 mol% or less of a vinyl alcohol series monomer unit, or a combination of a vinyl acetate series monomer unit and a vinyl alcohol series monomer unit.

[0038] Additionally, the number average molecular weight of the copolymer may be 150,000 or more and 230,000 or less.

[0039] For example, the copolymer may include 0 mol% or more and 5 mol% or less of an acrylate series monomer unit and / or 0 mol% or more and 5 mol% or less of a vinyl acetate series monomer unit, based on 100 mol% of the total copolymer weight.

[0040] If the above-mentioned number average molecular weight exceeds the range of the present invention, the viscosity of the binder copolymer and the slurry containing the binder copolymer may increase, which may significantly deteriorate electrode coating and processability, and if the above-mentioned number average molecular weight is lower than the range of the present invention, the binding force of the binder copolymer may significantly decrease, which may cause a delamination phenomenon of active materials, etc. from the electrode, making electrode production difficult, increasing the electrode expansion rate, and deteriorating the performance of the battery.

[0041] If the monomer unit of the above acrylate series exceeds or falls below the content range of the present invention, it may cause a decrease in bonding strength.

[0042] If the monomer unit of the acrylic acid series is above or below the content range of the present invention, it may cause polymer aggregation and precipitation or reduced adhesive strength.

[0043] If the above vinyl acetate monomer unit exceeds or falls below the content range of the present invention, stability problems such as storage stability may occur.

[0044] If the above vinyl alcohol series monomer unit exceeds or falls below the content range of the present invention, it may cause a decrease in adhesive strength, an increase in viscosity, and an increase in particle size.

[0045] In one embodiment, the copolymer can be prepared by copolymerizing and hydrolyzing an acrylate series monomer and a vinyl acetate series monomer.

[0046] The above hydrolysis may be alkaline hydrolysis.

[0047] In one embodiment, the copolymer can be produced by copolymerizing and hydrolyzing 65 mol% or more and 99 mol% or less of the acrylate series monomer and 1 mol% or more and 35 mol% or less of the vinyl acetate series monomer.

[0048] Meanwhile, the acrylate series monomer and the vinyl acetate series monomer can be copolymerized to form the acrylate series monomer unit and the vinyl acetate series monomer unit, respectively.

[0049] In one embodiment, the monomer unit of the acrylate series is methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, butyl acrylate, butyl methacrylate, sec-butyl acrylate, sec-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, ethyl hexyl acrylate, ethylhexyl It can be formed by polymerizing ethyl hexyl methacrylate, lauryl acrylate, lauryl methacrylate, stearyl acrylate, stearyl methacrylate, or a combination thereof.

[0050] Additionally, the above vinyl acetate series monomer unit can be formed by polymerization of vinyl acetate.

[0051] Meanwhile, some of the vinyl acetate series monomer units may be changed into vinyl alcohol series monomer units through the hydrolysis described above.

[0052] In addition, some of the monomer units of the acrylate series can be changed into monomer units of the acrylic acid series by the hydrolysis.

[0053] The degree of hydrolysis can be adjusted to, for example, 70% or more, 75% or more, 80% or more, 85% or more, 90% or more or 95% or more.

[0054] This means, for example, that 70 mol% or more, 75 mol% or more, 80 mol% or more, 85 mol% or more, 90 mol% or more, or 95 mol% or more of the total vinyl acetate series monomer units of the copolymer can be changed to vinyl alcohol series monomer units.

[0055] In addition, for example, it means that 70 mol% or more, 75 mol% or more, 80 mol% or more, 85 mol% or more, 90 mol% or more, or 95 mol% or more of the total acrylate series monomer units of the copolymer can be changed to the acrylic acid series monomer units.

[0056] In one embodiment, the acrylic acid series monomer unit can be combined with an alkali metal.

[0057] That is, the carboxylate group of the monomer unit of the acrylic acid series can be combined with an alkali metal.

[0058] Meanwhile, the weight ratio of the alkali metal and the copolymer (weight of the alkali metal: weight of the copolymer) may be 0.8 to 6.5:100.

[0059] When the weight ratio of the alkali metal and the copolymer exceeds or falls below the weight ratio of the present invention, the binding properties of the binder may deteriorate.

[0060] The bonding strength of the binder containing the above alkali metal can be determined by the strength of cohesion and repulsion between elements, and the adhesive strength can change due to changes in the order of cohesion, adhesion, and repulsion depending on changes in the content.

[0061] The binding force of the binder can improve the life characteristics of secondary batteries.

[0062] The copolymer of the present invention can improve the wettability of the copolymer by combining with an alkali metal and generating an alcohol functional group, and can maximize the adhesive strength by inducing an anchor effect.

[0063] In one embodiment, the copolymer may include a monomer repeating unit represented by the following chemical formula 1.

[0064]

[0065] [Chemical Formula 1]

[0066]

[0067]

[0068] In the above chemical formula 1,

[0069] R1 and R3 are each independently hydrogen, a linear or branched hydrocarbon having 1 to 4 carbon atoms, or a combination thereof, R2 and R4 are each independently a linear or branched hydrocarbon having 1 to 3 carbon atoms, R5 is -OH, M is an alkali metal, 0.65≤x+y≤0.99, 0.01≤m+n≤0.35, and x+y+m+n=1.

[0070] However, x can be 0 or greater than 0, and m can be 0 or greater than 0.

[0071] In the above chemical formula 1, x, y, m, and n correspond to the mole fraction of each monomer unit, and the sum of the mole fractions of each monomer unit is 1.

[0072] In one embodiment, R1 and R3 of the above chemical formula 1 may each independently be hydrogen, methyl, ethyl or a combination thereof, and R2 and R4 of the above chemical formula 1 may each independently be methyl, ethyl or a combination thereof.

[0073] Meanwhile, M in the above chemical formula 1 may be Li, Na or K, but is not limited thereto.

[0074] In one embodiment, the copolymer may be a random or block copolymer depending on the synthesis process.

[0075] In one embodiment, the pH of the copolymer may be 9 or more and 11 or less.

[0076] If the pH of the copolymer falls below the range of the present invention, the binding force of the copolymer for binder may decrease, the electrode expansion rate may increase, and the performance of the battery may deteriorate.

[0077] In addition, if the pH of the copolymer exceeds the range of the present invention, the dispersibility of the slurry may be reduced.

[0078] In one embodiment, the viscosity of the copolymer may be 1,100 cP or more and 2,000 cP or less.

[0079] For example, the viscosity of the copolymer may be 1,150 cP or more and 1,700 cP or less or 1,200 cP or more and 1,650 cP or less.

[0080] If the viscosity of the copolymer falls below the range of the present invention, the binding force may decrease, the electrode expansion rate may increase, and the performance of the battery may deteriorate.

[0081] In addition, if the viscosity of the copolymer exceeds the range of the present invention, the viscosity of the slurry containing the copolymer for binder increases, which may significantly deteriorate electrode coating and processability.

[0082] According to another aspect of the present invention, a negative electrode slurry may include the copolymer and a negative electrode active material.

[0083] That is, the copolymer can be used as a binder for a cathode, and in particular, can be an aqueous binder.

[0084] In one embodiment, the cathode slurry is heated at a shear rate of 10 s -1 The viscosity may be 2,000 cP or more and 6,000 cP or less.

[0085] If the viscosity of the cathode slurry falls below the range of the present invention, it may be difficult to control the loading amount of the composite layer when coating the cathode slurry.

[0086] In addition, if the viscosity of the cathode slurry exceeds the range of the present invention, it may be difficult to remove air bubbles that occur in the cathode slurry, the high viscosity may cause a strain on the mixing equipment during the production of the cathode slurry, and the transport of the cathode slurry may be difficult.

[0087] The above negative active material may be a compound including at least one selected from the group consisting of carbonaceous materials, silicon, alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, and rare earth elements, and preferably may be silicon or a compound including silicon.

[0088] The above carbon-based material may include, but is not limited to, artificial graphite, natural graphite, hard carbon, soft carbon, etc. The negative electrode active material including silicon is not particularly limited in type as long as it is silicon or a compound including silicon, but is preferably Si, SiO x (0 <x<2), Si-Y 합금(상기 Y는 알칼리 금속, 알칼리 토금속, 13족 원소, 14족 원소, 전이금속, 희토류 원소 또는 이들의 조합이며, Si은 아님.) 및 Si-C 복합체로 이루어진 군으로부터 선택되는 1종 이상일 수 있다.

[0089] In addition, when using a negative electrode active material including silicon as the negative electrode active material and another negative electrode active material as a mixture, the negative electrode active material including silicon may be included in an amount of 8 wt% or more of the total weight of the negative electrode active material.

[0090] The above negative electrode active material may be included in an amount of 50 to 99 wt%, preferably 60 to 80 wt%, based on the total weight of the negative electrode active material layer.

[0091] If the above-mentioned negative active material is included in an amount of less than 50 wt%, the energy density decreases, making it impossible to manufacture a high-energy-density battery. If it is included in an amount exceeding 99 wt%, the content of the conductive material and binder decreases, reducing electrical conductivity and possibly reducing the adhesive strength between the electrode active material layer and the current collector.

[0092] Meanwhile, the copolymer of the present invention may be included in an amount of 2 wt% or more and 5 wt% or less relative to the solid content of the negative electrode slurry. If the copolymer is included in an amount of less than 2 wt%, the physical properties of the negative electrode may deteriorate, causing the negative electrode active material and conductive material to fall off. If the copolymer is included in an amount of more than 5 wt%, the ratio of the negative electrode active material and conductive material may relatively decrease, resulting in a decrease in battery capacity and a decrease in electrical conductivity of the negative electrode.

[0093] In addition, the cathode slurry may additionally include a polymer in addition to the copolymer composition of the present disclosure. Specific examples of the polymer include, but are not limited to, polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyacrylic acid metal salt (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), fluoroelastomer, hydroxypropyl cellulose, regenerated cellulose, and various copolymers thereof.

[0094]

[0095] According to another aspect of the present invention, a negative electrode may include a current collector and a negative electrode active material layer including a copolymer of the present invention formed on the current collector.

[0096] The binding force of the cathode of this invention can be 7.0 to 8.0 gf / cm.

[0097] The above-described negative electrode active material layer may additionally include a conductive material. The conductive material is used to further improve the conductivity of the negative electrode active material. The conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery, and examples thereof include graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; conductive fibers such as carbon fiber or metal fiber; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives.

[0098] The conductive material may be included in an amount of 0.5 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 included in an amount of less than 0.5 wt%, the electrical conductivity of the negative electrode decreases. If the conductive material is included in an amount exceeding 30 wt%, the ratio of the silicon-based negative electrode active material and the binder relatively decreases, resulting in a decrease in battery capacity. In addition, since the binder content must be increased to maintain the negative electrode active material layer, the negative electrode active material content decreases, making it impossible to manufacture a high-energy-density battery.

[0099] The negative electrode of the present invention can suppress the volume expansion of the negative electrode active material that occurs during charging and discharging of a secondary battery by including the copolymer of the present invention in the negative electrode active material layer, improve the initial efficiency and capacity retention rate per cycle, and reduce the electrical resistance.

[0100] The above negative electrode can be manufactured through the steps of (a) preparing a composition for forming a negative electrode active material layer including a negative electrode active material and a copolymer composition of the present disclosure, and (b) applying the composition for forming a negative electrode active material layer on a negative electrode current collector and then drying it.

[0101] The composition for forming the above negative electrode active material layer is prepared in the form of a negative electrode slurry, and the solvent for preparing the slurry must be easy to dry, and it is most preferable that the solvent be able to dissolve the copolymer composition binder of the present disclosure well, but maintain the negative electrode active material in a dispersed state without dissolving it.

[0102] The solvent according to the present invention may be water or an organic solvent, and the organic solvent may be an organic solvent including at least one selected from the group consisting of methylpyrrolidone, dimethylformamide, isopropyl alcohol, acetonitrile, methanol, ethanol, and tetrahydrofuran.

[0103] The composition for forming the above negative electrode active material layer can be mixed by a conventional method using a conventional mixer, such as a rate mixer, a high-speed shear mixer, a homo mixer, etc.

[0104] The above step (b) is a step of manufacturing a negative electrode for a lithium secondary battery by applying a composition for forming a negative electrode active material layer manufactured in the above step (a) onto a negative electrode current collector and then drying it.

[0105] The negative electrode current collector may be specifically 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 an aluminum-cadmium alloy may be used as the alloy. In addition, calcined carbon, a non-conductive polymer surface-treated with a conductive material, or a conductive polymer may be used.

[0106] The composition for forming a negative electrode active material layer manufactured in the above step (a) is applied onto a negative electrode current collector, and can be coated on the current collector at an appropriate thickness depending on the thickness to be formed, and can preferably be appropriately selected within the range of 10 to 300 μm.

[0107] At this time, there is no limitation on the method of applying the composition for forming the negative electrode active material layer in the form of a slurry, and for example, it can be manufactured by performing a method such as doctor blade coating, dip coating, gravure coating, slit die coating, spin coating, comma coating, bar coating, reverse roll coating, screen coating, or cap coating.

[0108] After coating, drying is performed to finally form a negative electrode active material layer for a secondary battery (particularly, a lithium secondary battery), whereby a negative electrode for the secondary battery can be manufactured.

[0109] A battery according to another aspect of the present invention may include a current collector and a negative electrode having the negative electrode active material layer formed on the current collector.

[0110] The expansion rate of the above cathode may be 26.5% or less.

[0111] The above battery may be a secondary battery (particularly, a lithium secondary battery) including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.

[0112] Additionally, when the secondary battery is charged and discharged 200 times, the capacity retention rate may be 75% or higher.

[0113] The composition of the positive electrode, separator, and electrolyte of the above lithium secondary battery is not particularly limited in the present invention and follows what is known in the field.

[0114] The positive electrode includes a positive electrode active material formed on a positive electrode current collector.

[0115] The positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. At this time, the positive electrode current collector may be in various forms such as a film, sheet, foil, net, porous body, foam, non-woven fabric, etc. having fine irregularities formed on the surface so as to increase adhesion to the positive electrode active material.

[0116] The cathode active material constituting the cathode active material layer can be any cathode active material available in the relevant technical field. Specific examples of such cathode active materials include lithium metal; lithium cobalt oxides such as LiCoO2; Li 1+x Mn2-x Lithium manganese oxides such as O4 (where x is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxides such as Li2CuO2; vanadium oxides such as LiV3O8, LiFe3O4, V2O5, Cu2V2O7; LiNi 1-x M x Lithium nickel oxide represented by O2 (wherein, M=Co, Mn, Al, Cu, Fe, Mg, B or Ga and x=0.01 to 0.3); LiMn 2-x M x Lithium manganese composite oxide represented by O2 (wherein, M=Co, Ni, Fe, Cr, Zn or Ta and x=0.01 to 0.1) or Li2Mn3MO8 (wherein, M=Fe, Co, Ni, Cu or Zn); Li(Ni a Co b Mn c )O2 (wherein, 0<a<1, 0<b<1, 0<c<1, a+b+c=1); lithium-nickel-manganese-cobalt oxides; sulfur or disulfide compounds; phosphates such as LiFePO4, LiMnPO4, LiCoPO4, and LiNiPO4; Fe2(MoO4)3, but are not limited to these.

[0117] At this time, the positive electrode active material layer may additionally include a binder, a conductive material, a filler, and other additives in addition to the positive electrode active material, and the conductive material is the same as that described above for the negative electrode for the lithium secondary battery.

[0118] In addition, the binder may include, but is 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), fluoroelastomer, and various copolymers thereof.

[0119] The above separator may be formed of a porous substrate. Any porous substrate commonly used in electrochemical devices may be used as the porous substrate. For example, a polyolefin porous membrane or non-woven fabric may be used, but is not particularly limited thereto.

[0120] The above separation membrane may be a porous substrate made of one selected from the group consisting of polyethylene, polypropylene, polybutylene, polypentene, polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, and polyethylene naphthalate, or a mixture of two or more thereof.

[0121] The electrolyte of the above lithium secondary battery is a non-aqueous electrolyte containing a lithium salt, and is composed of a lithium salt and a solvent. Non-aqueous organic solvents, organic solid electrolytes, and inorganic solid electrolytes are used as the solvent.

[0122] The above lithium salt is a substance that is easy to dissolve in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl10 , 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, lithium lower aliphatic carboxylic acid, lithium 4-phenylborate imide, etc. can be used.

[0123] Non-aqueous organic solvents include, for example, N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, gamma-butyrolactone, 1,2-dimethoxy ethane, 1,2-diethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, 4-methyl-1,3-dioxene, diethyl ether, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxy methane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, Non-protic organic solvents such as tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate can be used.

[0124] As the organic solid electrolyte, for example, polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyagitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, polymers containing secondary dissociation groups, etc. can be used.

[0125] As the above inorganic solid electrolyte, for example, nitrides, halides, sulfates, etc. of Li, such as Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, LiSiO4, LiSiO4-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH, Li3PO4-Li2S-SiS2, etc. can be used.

[0126] In addition, the non-aqueous electrolyte may further include other additives for the purpose of improving charge / discharge characteristics, flame retardancy, etc. Examples of the additives include pyridine, triethylphosphite, triethanolamine, cyclic ethers, ethylene diamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine 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.

[0127] The lithium secondary battery according to the present invention can be formed by laminating and stacking a separator and electrodes and folding processes in addition to the conventional winding process. The battery case may be cylindrical, square, pouch-shaped, or coin-shaped.

[0128] Hereinafter, the present invention will be described in more detail using examples, but the present invention is not limited thereto.

[0129]

[0130] [Manufacturing Example 1] Manufacturing of copolymer

[0131] After adding 1,051 g of distilled water and 10 g of alkyldiphenyloxide disulfonate to the reactor, nitrogen was added and stirring was performed for 1 hour.

[0132] Afterwards, the temperature was raised to the first reaction temperature, 3.01 g of potassium persulfate was added to the reactor, 130 g of vinyl acetate and 350 g of ethylacrylate were added in portions over 2 hours, and the temperature was maintained for 2 hours to obtain a vinyl acetate-ethylacrylate copolymer having a solid content of 29 wt%.

[0133] 100 g of a prepared vinyl acetate-ethyl acrylate copolymer with a solid content of 29 wt%, 150 g of ethanol, a hydroxide, and an organic salt were placed in a reactor and hydrolysis was performed while stirring at 60°C for 4 hours.

[0134] After hydrolysis was completed, the precipitated hydrolyzate was dissolved in distilled water, heated to 80°C, stirred, and stripped for 8 hours to prepare a copolymer for a negative electrode binder.

[0135]

[0136] [Manufacturing Example 2] Manufacturing of a lithium secondary battery

[0137] An anode slurry was prepared by mixing 23.7 g of artificial graphite as an electrode active material, 17.7 g of Si active material, 0.03 g (based on solid content) of single-walled carbon nanotubes (SWCNTs), and 0.9 g (based on solid content) of the anode binder prepared according to Manufacturing Example 1 with distilled water.

[0138] The manufactured negative electrode slurry was uniformly applied onto a copper current collector, dried at 120°C for 10 minutes, and the resulting electrode was rolled and vacuum-dried in a 110°C vacuum oven for more than 4 hours to manufacture a negative electrode.

[0139] Afterwards, a non-aqueous electrolyte 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 distinguishing the form as a pouch or coin cell type.

[0140] As the above non-aqueous electrolyte, 5 wt% FEC and 1 wt% LiPO2F2 were added to a solvent in which ethylene carbonate: ethyl methyl carbonate: diethyl carbonate were mixed in a volume ratio of 2:1:7, and LiPF6 electrolyte was dissolved at a concentration of 1.5 M.

[0141]

[0142] [Example 1]

[0143] A copolymer for a negative electrode binder having a pH of 10 and a number average molecular weight of 180,000 was manufactured according to Manufacturing Example 1 at a first reaction temperature of 68°C, and then a negative electrode slurry, a negative electrode, and a lithium secondary battery containing 3.5 g of the copolymer for a negative electrode binder based on solid content were manufactured according to Manufacturing Example 2.

[0144]

[0145] [Example 2]

[0146] A copolymer for a negative electrode binder having a number average molecular weight of 180,000 was prepared in the same manner as in Example 1, except that an organic acid was added to make the pH 8, and then a negative electrode slurry, a negative electrode, and a lithium secondary battery including the copolymer for a negative electrode binder were prepared according to Manufacturing Example 2.

[0147] As the organic acid, lactic acid, citric acid, acetic acid, malic acid, or a combination thereof can be used.

[0148]

[0149] [Comparative Example 1]

[0150] A copolymer for a negative electrode binder having a pH of 10 and a number average molecular weight of 130,000 was prepared in the same manner as in Example 1, except that the primary reaction temperature was set to 75°C. Then, a negative electrode slurry, a negative electrode, and a lithium secondary battery including the copolymer for a negative electrode binder were prepared according to Manufacturing Example 2.

[0151]

[0152] [Comparative Example 2]

[0153] Except that the first reaction temperature was 60°C, a copolymer for a negative electrode binder having a pH of 10 and a number average molecular weight of 300,000 was manufactured in the same manner as in Example 1. Then, a negative electrode slurry, a negative electrode, and a lithium secondary battery including the copolymer for a negative electrode binder were manufactured according to Manufacturing Example 2.

[0154]

[0155] [Comparative Example 3]

[0156] A copolymer for a negative electrode binder having a pH of 10 and a number average molecular weight of 400,000 was prepared in the same manner as in Example 1, except that the primary reaction temperature was set to 55°C. Then, a negative electrode slurry, a negative electrode, and a lithium secondary battery including the copolymer for a negative electrode binder were prepared according to Manufacturing Example 2.

[0157]

[0158] [Comparative Example 4]

[0159] A copolymer for a negative electrode binder having a number average molecular weight of 180,000 was prepared in the same manner as in Example 1, except that an organic acid was added to make the pH 7, and then a negative electrode slurry, a negative electrode, and a lithium secondary battery including the copolymer for a negative electrode binder were prepared according to Manufacturing Example 2.

[0160]

[0161] The number average molecular weight of the copolymers for negative electrode binders of Examples 1 and 2 and Comparative Examples 1 to 4 was measured by gel filtration chromatography (GFC) using water as a solvent.

[0162]

[0163] [Evaluation Example 1] Evaluation of Copolymer Bonding Strength for Negative Electrode Binder

[0164] In order to measure the bonding strength of the negative electrodes manufactured in Examples 1 and 2 and Comparative Examples 1 to 4, the negative electrode composite layers manufactured in Examples 1 and 2 and Comparative Examples 1 to 4 were attached to an acrylic plate, and then a 180° peeling test was performed using a UTM to measure the bonding strength.

[0165]

[0166] [Evaluation Example 2] Evaluation of viscosity of copolymer for cathode binder

[0167] The viscosity of the copolymers for negative electrode binders manufactured in Examples 1 and 2 and Comparative Examples 1 to 4 was measured at 25°C using a Brookfield viscometer (63 spindle, 15 rpm).

[0168] After pouring the copolymer for negative binder into a 50 ml transparent vial at a distance of 5 cm from the bottom, measurement was performed, and the value measured after 1 minute was taken as the viscosity.

[0169]

[0170] [Evaluation Example 3] Evaluation of cathode slurry rheology

[0171] The rheological properties of the cathode slurries manufactured in Examples 1 and 2 and Comparative Examples 1 to 4 were measured using a rheometer to measure the change in viscosity according to the change in shear rate using 3 g of the cathode slurry.

[0172] The shear rate range is 0.01 to 1000 s -1 and was measured at 25℃.

[0173]

[0174] [Evaluation Example 4] Evaluation of capacity retention rate and cathode expansion rate of secondary batteries

[0175] The lithium secondary batteries manufactured in Examples 1 and 2 and Comparative Examples 1 to 4 were charged and discharged three times at 25°C with the end-of-charge voltage set to 4.8 V and the end-of-discharge voltage set to 2.7 V, and the charge and discharge current densities of each charge and discharge cycle were adjusted to -0.1 C, 0.2, and 0.5 C, respectively.

[0176] Afterwards, the capacity retention rate was measured by performing 200 charge / discharge cycles with a charge / discharge current density of 1C, a charge end voltage of 4.8 V, and a discharge end voltage of 2.7 V.

[0177] All charges were performed under constant current / constant voltage conditions, and the terminal current of the constant voltage discharge was 0.005 C.

[0178] The capacity retention rate and electrode expansion rate were calculated according to the following mathematical equations 1 and 2, respectively.

[0179]

[0180] [Mathematical Formula 1]

[0181] Capacity retention (%) = (discharge capacity after 200 cycles / discharge capacity after initial 3 cycles) Х 100

[0182]

[0183] [Equation 2]

[0184] Electrode expansion ratio (%) = [(Cathode thickness measured after one charge at 100% SOC - Initial cathode thickness after vacuum drying) / Initial cathode thickness after vacuum drying] Х 100

[0185]

[0186] [Evaluation Example 5] Evaluation of copolymer dispersibility for cathode binder

[0187] The cathodes manufactured in Examples 1 and 2 and Comparative Examples 1 to 4 were analyzed by SEM to measure the dispersibility of the copolymer for cathode binder in the cathode.

[0188]

[0189] The measured values ​​of the bonding force, viscosity, capacity retention rate, and electrode expansion rate evaluated by Evaluation Examples 1, 2, and 4 are shown in Table 1 below.

[0190]

[0191] Adhesion (gf / cm) Viscosity (cP) 200 cycle capacity retention rate (%) Electrode expansion rate (%) Example 17.21, 5507826.2 Example 27.61, 2607825.7 Comparative example 14.61, 0407228.3 Comparative example 28.42, 2507927.0 Comparative example 39.03, 5307625.9 Comparative example 46, 39807427.4

[0192]

[0193] As shown in Table 1 above, the binding strength of the copolymer for binder manufactured by Examples 1 and 2 was measured to be 7.0 to 8.0 gf / cm, and the viscosity was measured to be 1,100 to 2,000 cP.

[0194] In addition, the capacity retention rate of the lithium secondary batteries manufactured by Examples 1 and 2 was 75% or more, and the electrode expansion rate was measured to be 26.5% or less.

[0195] Compared to Examples 1 and 2, Comparative Example 1, which had a low number average molecular weight, had a low viscosity of the copolymer for the binder, but the binding force was reduced, resulting in a detachment phenomenon, making it difficult to manufacture the electrode, and the capacity retention characteristics of the battery were reduced.

[0196] In addition, compared to Examples 1 and 2, Comparative Examples 2 and 3, which had a high number average molecular weight, had high binding power of the copolymer for binder, but the viscosity of the copolymer for binder was too high, making electrode coating difficult and significantly reducing the processability.

[0197] Meanwhile, in Comparative Example 4, which had a low pH compared to Examples 1 and 2, the viscosity of the copolymer for the binder was low, but the binding force and capacity retention rate were reduced, and the electrode expansion rate was increased, so the capacity retention rate characteristics of the battery were also reduced.

[0198]

[0199] The results of the evaluation of the rheology of the cathode slurry measured by Evaluation Example 3 are shown in Fig. 1.

[0200] Compared to the viscosity of the cathode slurry of Examples 1 and 2, the viscosity of the cathode slurry of Comparative Examples 2 to 4 was measured to be high, and the viscosity of the cathode slurry of Comparative Example 1 was measured to be low.

[0201] The results of the dispersibility evaluation of the copolymer for negative electrode binder measured by Evaluation Example 5 are shown in Fig. 2.

[0202] The dispersion of the copolymer for the negative electrode binder in Examples 1 and 2 was good.

[0203] In comparison, Comparative Examples 1 and 4 showed good dispersion of the copolymer for the negative electrode binder within the negative electrode, but the negative electrode active material was observed to not be sufficiently wrapped due to the low molecular weight of the copolymer for the negative electrode binder, and the connection between the negative electrode active materials was also observed to be insufficient.

[0204] Meanwhile, Comparative Examples 2 and 3 showed insufficient dispersion of the copolymer for the cathode binder within the cathode, and sporadically agglomerated copolymers for the cathode binder were observed.

[0205]

[0206] That is, it was confirmed that changes in the number average molecular weight and pH of the copolymer for the negative electrode binder affect the characteristics and performance of the negative electrode slurry, negative electrode, and secondary battery.

[0207]

[0208] The scope of the present invention is indicated by the claims described below rather than the detailed description above, and all changes or modifications derived from the meaning and scope of the claims and their equivalent concepts should be interpreted as being included in the scope of the present invention.

[0209] The copolymer of the present invention can improve the life characteristics (capacity retention rate) of a lithium secondary battery by improving the binding force, dispersibility, and electrode expansion suppression ability while reducing the viscosity of a negative electrode slurry including a silicon negative electrode active material, carbon nanotubes, or a combination thereof.

Claims

1. Based on 100 mol% of the total copolymer weight, 65 mol% or more and 99 mol% or less of acrylic acid series monomer units, or a combination of acrylate series monomer units and acrylic acid series monomer units; and Containing 1 mol% or more and 35 mol% or less of a vinyl alcohol series monomer unit, or a combination of a vinyl acetate series monomer unit and a vinyl alcohol series monomer unit; The average molecular weight is 150,000 or more and 230,000 or less, Copolymer.

2. In paragraph 1, Manufactured by copolymerizing and hydrolyzing acrylate series monomers and vinyl acetate series monomers. Copolymer.

3. In paragraph 2, A copolymer manufactured by copolymerizing and hydrolyzing 65 mol% or more and 99 mol% or less of the above acrylate series monomer and 1 mol% or more and 35 mol% or less of the above vinyl acetate series monomer. Copolymer.

4. In paragraph 1, The above acrylic acid series monomer unit is combined with an alkali metal, Copolymer.

5. In paragraph 4, The weight ratio of the above alkali metal and the above copolymer (weight of the above alkali metal: weight of the above copolymer) is 0.8 to 6.5:

100. Copolymer.

6. In paragraph 1, Comprising a monomer repeating unit represented by the following chemical formula 1, Copolymer. [Chemical Formula 1] In the above chemical formula 1, R1 and R3 are each independently hydrogen, a linear or branched hydrocarbon having 1 to 4 carbon atoms, or a combination thereof, R2 and R4 are each independently a linear or branched hydrocarbon having 1 to 3 carbon atoms, R5 is -OH, M is an alkali metal, 0.65≤x+y≤0.99, 0.01≤m+n≤0.35, x+y+m+n=1. (but x can be 0 or greater than 0, and m can be 0 or greater than 0) 7. In paragraph 6, R1 and R3 in the above chemical formula 1 are each independently hydrogen, methyl, ethyl or a combination thereof, In the above chemical formula 1, R2 and R4 are each independently methyl, ethyl or a combination thereof. Copolymer.

8. In paragraph 1, The above copolymer is a random or block copolymer, Copolymer.

9. In paragraph 1, The pH of the above copolymer is 9 or more and 11 or less. Copolymer.

10. In paragraph 1, The viscosity of the above copolymer is 1,100 cP or more and 2,000 cP or less. Copolymer.

11. A copolymer according to any one of claims 1 to 10; and Containing a negative active material; Cathode slurry.

12. In paragraph 11, Shear speed 10 s -1 In which the viscosity is 2,000 cP or more and 6,000 cP or less, Cathode slurry.

13. The entire house; and A negative electrode active material layer comprising a copolymer of any one of claims 1 to 10 formed on the entire body of the above-mentioned electrode; cathode.

14. Containing the negative electrode of Article 13, Secondary battery.

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