Binder comprising copolymer composition, secondary battery negative electrode comprising same, and secondary battery comprising same

Abstract

The present invention relates to a copolymer composition comprising: a first copolymer comprising a vinyl alcohol (vinylcohol) monomer unit and a vinylamine (vinylamine)-based monomer unit, and a second copolymer comprising a vinyl alcohol (vinylcohol)-based monomer unit and a vinylamine (vinylamine)-based monomer unit; a second copolymer comprising a vinyl alcohol monomeric unit and an acrylic acid salt monomeric unit, and at least one of the vinyl alcohol monomeric unit and the acrylic acid salt monomeric unit; and a crosslinking agent. The invention also relates to a negative electrode paste, a negative electrode and a secondary battery containing the copolymer composition.

Description

Technical Field

[0001] The present invention relates to a copolymer composition useful as an adhesive, a slurry comprising the copolymer composition, an electrode, and a secondary battery. Background Art

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

[0003] As the application fields expand, lithium secondary batteries need to have higher capacity and longer life characteristics. One of the methods to increase the capacity of lithium secondary batteries is to use active materials containing silicon atoms as negative electrodes.

[0004] Compared with conventional carbon-based active materials, when using active materials containing silicon atoms, an increase in battery capacity can be expected because silicon materials can embed and extract more lithium. However, due to the large volume change of silicon-containing active materials during lithium embedding and extraction, the negative electrode active material layer will expand and shrink significantly during charging and discharging.

[0005] This results in a decrease in the conductivity between the negative electrode active materials, and the conductive path between the negative electrode active materials and the current collector is blocked, resulting in deterioration in the cycle characteristics of the secondary battery.

[0006] However, various adhesives developed in the past (such as PAA, PAA / CMC, Na-PAA, cross-linked PAA, alginate, PVA, etc.) are difficult to solve the volume expansion problem mentioned above due to insufficient adhesion or the electrodes are too fragile and lack durability.

[0007] In addition, some recent studies have used some cross-linking binders to inhibit the expansion of silicon-containing active materials, but the expansion inhibition effect is still insufficient, and the battery performance is reduced due to electrode detachment and consumption of lithium ions caused by the continuous destruction and reformation of the SEI layer.

[0008] Therefore, there is a need for an adhesive that can solve these problems and ensure the capacity retention rate of a secondary battery.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Korean Patent Publication No. 10-2016-0024921 Summary of the invention

[0012] Technical problem to be solved by the invention

[0013] The present invention aims to provide a copolymer composition capable of preparing a slurry composition having excellent adhesion and ability to inhibit electrode expansion.

[0014] In addition, the present invention also aims to provide an electrode (especially a negative electrode) with excellent performance using the slurry composition, and a secondary battery including the electrode with excellent cycle capacity retention rate.

[0015] However, the problems to be solved by the present invention are not limited to the problems, and other problems not mentioned will be clearly understood by those skilled in the art through the following description.

[0016] One aspect of the present invention provides a copolymer composition comprising:

[0017] A first copolymer, wherein the first copolymer comprises a vinyl alcohol monomer unit and a vinylamine monomer unit;

[0018] a second copolymer comprising vinyl alcohol monomer units and acrylic acid salt monomer units; and

[0019] Cross-linking agent.

[0020] Another aspect of the present invention provides a negative electrode slurry comprising:

[0021] The copolymer composition; and

[0022] Negative electrode active material.

[0023] Another aspect of the present invention provides a negative electrode, comprising:

[0024] a current collector; and

[0025] A negative electrode active material layer including the copolymer composition is formed on the current collector.

[0026] Another aspect of the present invention provides a secondary battery including the negative electrode.

[0027] The copolymer composition of the present invention can be used in negative electrode slurry to increase the adhesion to the negative electrode collector, inhibit negative electrode expansion, and improve the cycle capacity retention rate of the secondary battery.

[0028] Effects of the Invention

[0029] The copolymer composition of the present invention can be used in negative electrode slurry to increase the adhesion to the negative electrode collector, inhibit negative electrode expansion, and improve the cycle capacity retention rate of the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1The cross-linking mechanism of the copolymer composition of the present application using glutaraldehyde as a cross-linking agent is shown. DETAILED DESCRIPTION

[0031] The operation and effects of the present invention are described in detail below through specific embodiments of the present invention. However, these embodiments are only examples of the present invention, and the scope of the present invention is not limited thereto.

[0032] It should be noted that the terms and words used in this specification and claims should not be interpreted according to their ordinary or dictionary meanings, but should be interpreted according to the meanings and concepts that are consistent with the technical ideas of the present invention. The principle is that the inventor can define the concept of the term in the way he believes is most suitable for describing his invention.

[0033] Therefore, it should be understood that the embodiment described herein is only a preferred embodiment of the present invention, and is not the entirety of the technical concept of the present invention, and various equivalents and modifications may replace them when applying.

[0034] In this specification, unless the context clearly indicates otherwise, singular expressions include plural expressions. The terms "comprise", "include" or "have" are intended to indicate the presence of the described features, numbers, steps, components or combinations thereof, but do not exclude the possibility of the presence or addition of one or more other features, numbers, steps, components or combinations thereof.

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

[0036] According to one aspect of the present invention, the copolymer composition comprises:

[0037] A first copolymer, wherein the first copolymer comprises vinyl alcohol monomer units and vinylamine monomer units; a second copolymer, wherein the second copolymer comprises vinyl alcohol monomer units and acrylic acid salt monomer units; and a crosslinking agent.

[0038] The cross-linking agent increases the adhesion between the active material and the current collector by cross-linking the first copolymer and the second copolymer having polar functional groups, and minimizes structural changes and damages of the electrode caused by volume changes of the active material.

[0039] In one embodiment, the first copolymer further comprises at least one selected from vinyl acetate monomer units and N-vinylformamide monomer units, and the second copolymer further comprises at least one selected from acrylate monomer units and vinyl acetate monomer units.

[0040] The first copolymer contains hydroxyl and amine groups, and when used as a binder for negative electrode slurry, can form strong hydrogen bonds with silicon (negative electrode active material) and coordinate bonds with a negative electrode collector, thereby increasing the cohesive force between silicon and the collector.

[0041] On the other hand, the second copolymer is based on an ethylene skeleton structure, which provides flexibility to the binder of the negative electrode slurry and can suppress the volume change of silicon (negative electrode active material). In addition, the alkali metal ions at the end of the acrylic acid salt monomer unit may help improve ionic conductivity. In addition, the stretched chain can interact with the negative electrode active material to form a porous electrode with a dense structure and can form a stable SEI layer.

[0042] The hydroxyl group of the first copolymer and the carboxyl group of the second copolymer may be chemically and / or physically cross-linked, thereby suppressing the volume change of silicon (negative electrode active material).

[0043] In one embodiment, the vinylamine-based monomer unit of the first copolymer may be at least one selected from the group consisting of vinylamine and 1-methylvinylamine, but is not limited thereto.

[0044] In addition, the acrylic acid salt monomer unit of the second copolymer may be at least one selected from the group consisting of acrylic acid and methacrylic acid, but is not limited thereto.

[0045] In one embodiment, the N-vinylformamide-based monomer unit of the first copolymer may be at least one selected from the group consisting of N-vinylformamide and N-isopropenylformamide, but is not limited thereto.

[0046] In addition, the acrylate monomer unit of the second copolymer can be at least one selected from the group consisting of 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, and ethylhexyl methacrylate, but is not limited thereto.

[0047] In one embodiment, based on the total content of the first copolymer being 100 mol %, the first copolymer comprises 50 mol % to 90 mol % of the vinyl alcohol monomer unit and 1 mol % to 50 mol % of the N-vinylformamide monomer unit.

[0048] In one embodiment, based on 100 mol % of the total content of the second copolymer, the vinyl alcohol monomer unit has a content of 1 mol % to 30 mol % and the acrylic acid salt monomer unit has a content of 50 mol % to 90 mol %.

[0049] The contents of the first copolymer and the second copolymer can be adjusted by changing the hydrolysis degree during the manufacturing process of the first copolymer and the second copolymer.

[0050] In one embodiment, the first copolymer may include a monomer repeating unit represented by the following Chemical Formula 1, and the second copolymer may include a monomer repeating unit represented by the following Chemical Formula 2.

[0051] [Chemical formula 1]

[0052]

[0053] In the chemical formula 1:

[0054] 0≤x≤15 mol%, 50≤y≤90 mol%, 0≤m≤30 mol%, 1≤n≤0 mol%.

[0055] In the chemical formula 1, x, y, m and n respectively represent the molar percentage of each monomer unit.

[0056] [Chemical formula 2]

[0057]

[0058] In the chemical formula 2:

[0059] R1 and R2 are the same or different and are each independently hydrogen or a straight or branched hydrocarbon having 1 to 5 carbon atoms;

[0060] R3 is a hydroxyl group (-OH);

[0061] M is an alkali metal;

[0062] 0≤a≤5mol%, 50≤b≤90mol%, 0≤c≤5mol%, 1≤d≤30mol%.

[0063] a, b, c and d in the chemical formula 2 represent the molar percentage of each monomer unit.

[0064] In addition, M in the chemical formula 2 may be any one selected from lithium (Li), potassium (K) and sodium (Na), but is not limited thereto.

[0065] On the other hand, R1 and R2 in the chemical formula 2 may each independently be any one of the group consisting of hydrogen, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl or n-pentyl, but are not limited thereto.

[0066] In one embodiment, based on 100 wt % of the total weight of the copolymer composition, the first copolymer may contain 10 wt % to 90 wt % and the second copolymer may contain 10 wt % to 90 wt %.

[0067] In the content range of the first copolymer and the second copolymer in the copolymer composition, the higher the content ratio of the first copolymer is, the more adhesion of the negative electrode can be improved when the first copolymer is used as a negative electrode binder.

[0068] In addition, in the copolymer composition, within the content range of the first copolymer and the second copolymer, the higher the content ratio of the second copolymer is, when used as a negative electrode binder, the dispersibility and stability of the negative electrode slurry can be further improved.

[0069] If the content of the first copolymer and the second copolymer in the copolymer composition exceeds the range of the present application, when used as a negative electrode binder, it may cause one or more of the dispersion stability of the negative electrode slurry composition, the adhesion of the negative electrode, and the characteristics of the secondary battery to be reduced.

[0070] In particular, if the adhesion between the electrode and the collector is low, the electrode may fall off during the drying process and rolling process, and when the rolling density of the electrode is increased, the coated slurry may be separated from the electrode. In addition, the low adhesion between the electrode and the plate during battery operation may cause the electrode wetted and swollen by the electrolyte to fall off, thereby reducing the operating stability of the battery.

[0071] In one embodiment, the first copolymer may be a random copolymer or a block copolymer, and the second copolymer may also be a random copolymer or a block copolymer.

[0072] In one embodiment, the number average molecular weight of the first copolymer may be 10,000 or more and 1,000,000 or less, and the number average molecular weight of the second copolymer may be 10,000 or more and 1,000,000 or less.

[0073] In one aspect, the first copolymer can be prepared by hydrolyzing a copolymer comprising vinyl acetate monomer units and N-vinylformamide-based monomer units.

[0074] That is, the vinyl acetate monomer units and N-vinylformamide monomer units of the first copolymer can be hydrolyzed into vinyl alcohol monomer units and vinylamine monomer units.

[0075] In addition, the second copolymer may be prepared by hydrolyzing a copolymer including an acrylate-based monomer unit and a vinyl acetate monomer unit.

[0076] That is, the acrylate monomer unit and the vinyl acetate monomer unit of the second copolymer can be hydrolyzed into an acrylic acid monomer unit and a vinyl alcohol monomer unit, respectively.

[0077] The first copolymer and the second copolymer may be prepared by hydrolysis using an alkali metal hydroxide, but are not limited thereto.

[0078] In one embodiment, the cross-linking agent may include two or more aldehyde groups.

[0079] For example, the cross-linking agent may be glutaraldehyde, succinaldehyde, glyoxaldialdehyde, adipic dialdehyde, or a combination thereof.

[0080] Figure 1 The expected crosslinking mechanism for the copolymer compositions of the present invention is shown when glutaraldehyde is used as the crosslinking agent.

[0081] In one embodiment, based on 100 wt % of the total weight of the copolymer composition, the content of the cross-linking agent may be greater than 0.7 wt % and less than 2.8 wt %.

[0082] For example, based on 100 wt % of the total weight of the copolymer composition, the content of the crosslinking agent may be 1 wt %, 1.5 wt %, 2 wt % or 2.5 wt %.

[0083] As the content of the cross-linking agent increases, the cross-linking rate may increase when cross-linking is performed under the same pH conditions.

[0084] On the other hand, if the content of the crosslinking agent is out of the range of the present application, the adhesion of the copolymer composition may be significantly reduced when crosslinking is performed under the same pH conditions.

[0085] In addition, if the content of the cross-linking agent is lower than the range of the present application, the electrode expansion rate may be significantly increased when cross-linking is performed under the same pH conditions, thereby reducing the life of the lithium secondary battery using the copolymer.

[0086] In one embodiment, the pH of the copolymer composition may be 6 or more and 12 or less.

[0087] As the pH of the copolymer composition increases, the crosslinking rate may decrease. In addition, as the pH of the copolymer composition increases, the adhesion rate of the slurry composition using the copolymer composition may increase.

[0088] On the other hand, if the pH of the copolymer composition is lower than 6, the stability of the slurry containing the copolymer composition may be significantly reduced, and may not be suitable for product production.

[0089] Furthermore, if the pH of the copolymer composition exceeds 12, the expansion rate of an electrode using the copolymer composition may be significantly increased, thereby reducing the performance and life of a battery.

[0090] The pH of the copolymer composition may be adjusted by adding a pH adjuster to the copolymer composition.

[0091] As the pH adjuster, any pH adjuster (particularly an acidic substance) that can adjust the pH of the copolymer composition to 6 or more and 12 or less may be used.

[0092] For example, maleic acid, acrylic acid, or a combination thereof, or polyacrylic acid may be used as the polymer.

[0093] In addition, the cross-linking rate of the first copolymer and the second copolymer of the copolymer composition may be 45% or more and 80% or less.

[0094] For example, the cross-linking rate may be 50% or more and 80% or less.

[0095] According to another aspect of the present application, a negative electrode slurry may include the copolymer composition and a negative electrode active material.

[0096] That is, the copolymer composition may be used as a binder for a negative electrode.

[0097] The peel strength between the negative electrode active material layer formed using the negative electrode slurry and the copper current collector can be 10 dyne / cm 215 dyne / cm or more 2 or less.

[0098] The negative electrode active material may be a compound containing at least one selected from the group consisting of carbon-based materials, silicon, alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, and rare earth elements, and is preferably silicon or a silicon-containing compound.

[0099] The carbon-based materials include artificial graphite, natural graphite, hard carbon, soft carbon, etc., but are not limited thereto. The type of the negative electrode active material containing silicon is not particularly limited as long as it is silicon or a silicon-containing compound, but is preferably selected from Si, SiO x (0 < x < 2), Si-Y alloy (Y is an alkali metal, alkaline earth metal, Group 13 element, Group 14 element, transition metal, rare earth element, or a combination thereof, but does not include Si), and at least one of Si-C composite materials.

[0100] In addition, when a mixture of a silicon-containing negative electrode active material and other negative electrode active materials is used as the negative electrode active material, the silicon-containing negative electrode active material may account for 8% by weight or more of the total weight of the negative electrode active material.

[0101] The content of the negative electrode active material may be 50 to 90% by weight, preferably 60 to 80% by weight, of the total weight of the negative electrode active material layer.

[0102] If the content of the negative electrode active material is less than 50% by weight, the energy density will decrease, and a battery with a high energy density cannot be manufactured; if it exceeds 90% by weight, the content of the conductive material and the binder will decrease, resulting in a decrease in conductivity, and the adhesion between the negative electrode active material layer and the current collector may decrease.

[0103] Meanwhile, the copolymer composition binder of the present application may account for 1 to 35% by weight of the total weight of the negative electrode slurry. If the content of the copolymer is less than 1% by weight, the physical properties of the negative electrode may decrease, and the negative electrode active material and the conductive material may fall off; if the content of the copolymer exceeds 35% by weight, the ratio of the negative electrode active material and the conductive material will relatively decrease, resulting in a decrease in battery capacity, and the conductivity of the negative electrode may decrease.

[0104] In addition, the negative electrode slurry may include other polymers in addition to the copolymer composition of the present application. These polymers include 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, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, hydroxypropyl cellulose, regenerated cellulose and various copolymers thereof, but are not limited thereto.

[0105] According to another aspect of the present application, a negative electrode may include a current collector and a negative electrode active material layer including the copolymer composition of the present application formed on the current collector.

[0106] 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. These conductive materials are not particularly limited as long as they do not cause chemical changes in the battery and have conductivity. For example, the conductive material may 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 fibers and metal fibers; metal powders such as fluorocarbons, aluminum and nickel powders; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; polyphenylene derivatives, etc.

[0107] The content of the conductive material can be 5 to 30% by weight of the total weight of the negative electrode active material layer, preferably 15 to 25% by weight. If the content of the conductive material is less than 5% by weight, the conductivity of the negative electrode will be reduced; if it exceeds 30% by weight, the ratio of the silicon-based negative electrode active material and the binder will be relatively reduced, thereby reducing the battery capacity. In order to maintain the negative electrode active material layer, the content of the binder must be increased, which leads to a reduction in the content of the negative electrode active material, and a high energy density battery cannot be manufactured.

[0108] In the negative electrode of the present application, the negative electrode active material layer comprises the copolymer composition of the present application, so that the volume expansion of the negative electrode active material during the charge and discharge process of the secondary battery can be suppressed and the capacity retention rate per cycle can be improved.

[0109] The method for manufacturing the negative electrode comprises the following steps:

[0110] (a) preparing a negative electrode active material and a composition for forming a negative electrode active material layer comprising the copolymer composition of the present application; and

[0111] (b) coating the composition for forming the negative electrode active material layer on a negative electrode collector and drying it.

[0112] The composition for forming the negative electrode active material layer is prepared in the form of negative electrode slurry. The solvent used to prepare the slurry must be easy to dry and preferably a binder that can well dissolve the copolymer composition of the present application but will not dissolve the negative electrode active material and can maintain its dispersed state.

[0113] The solvent of the present application may be water or an organic solvent, and the organic solvent may be an organic solvent containing at least one selected from the group consisting of methylpyrrolidone, dimethylformamide, isopropanol, acetonitrile, methanol, ethanol and tetrahydrofuran.

[0114] The composition for forming the negative active material layer may be mixed in a conventional manner using a conventional mixer such as a speed mixer, a high-speed shear mixer, or a homomixer.

[0115] The step (b) is to manufacture a negative electrode of a lithium secondary battery by coating the composition for forming a negative electrode active material layer prepared in the step (a) on a negative electrode collector and drying the coating.

[0116] 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 the alloy may be an aluminum-cadmium alloy. In addition, calcined carbon, a non-conductive polymer surface treated with a conductive material, or a conductive polymer may be used.

[0117] The composition for forming the negative electrode active material layer prepared in step (a) is coated on the negative electrode collector, and an appropriate coating thickness can be selected according to the required thickness, preferably, it can be selected within the range of 10 to 300 μm.

[0118] At this time, the coating method of the above-mentioned slurry composition for forming a negative electrode active material layer is not particularly limited, 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 and the like can be used.

[0119] After coating, drying is performed, so that a negative electrode of a secondary battery (particularly a lithium secondary battery) having a negative electrode active material layer formed thereon can be finally manufactured.

[0120] According to another aspect of the present application, a battery may include a current collector and a negative electrode having a negative electrode active material layer formed on the current collector.

[0121] The battery may be a secondary battery (particularly a lithium secondary battery) including a positive electrode, the negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte.

[0122] After 500 charge and discharge cycles, the capacity retention rate of the secondary battery can be above 80%.

[0123] For example, the capacity retention rate may be 83% or more, 85% or more, or 90% or more.

[0124] In addition, after 500 charge and discharge cycles, the electrode expansion rate of the secondary battery can be less than 60%.

[0125] For example, the smaller electrode expansion ratio may be 55% or less, 50% or less, 45% or less, or 40% or less.

[0126] The compositions of the positive electrode, separator and electrolyte of the lithium secondary battery are not particularly limited in the present invention and follow the techniques known in the art.

[0127] The positive electrode includes a positive electrode active material formed on a positive electrode collector.

[0128] There is no particular limitation on the positive electrode collector, as long as it has high conductivity and does not cause chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or a material treated with carbon, nickel, titanium or silver on the surface of aluminum or stainless steel can be used. At this time, in order to increase the adhesion to the positive electrode active material, the positive electrode collector can be formed with a small uneven surface of the film, sheet, foil, net, porous material, foam and non-woven fabric in various forms.

[0129] The positive electrode active material constituting the positive electrode active material layer may be any positive electrode active material available in the art. Specific examples include lithium metal; lithium cobalt-based oxides such as LiCoO2; lithium manganese-based oxides such as Li 1+x Mn 2-x O4 (where x is 0 to 0.33), LiMnO3, LiMn2O3 and LiMnO2; lithium copper oxides such as Li2CuO2; vanadium oxides such as LiV3O8, LiFe3O4, V2O5, Cu2V2O7; lithium nickel-based oxides such as LiNi 1-x M xO2 (where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, x = 0.01 to 0.3); lithium manganese composite oxides such as LiMn 2-x M xO2 (where M = Co, Ni, Fe, Cr, Zn or Ta, x = 0.01 to 0.1) or Li2Mn3MO8 (where M = Fe, Co, Ni, Cu or Zn); lithium nickel manganese cobalt-based oxides such as Li(Ni a Co b Mn c )O2 (where 0 < a < 1, 0 < b < 1, 0 < c < 1, a + b + c = 1); sulfur or disulfide compounds; phosphates such as LiFePO4, LiMnPO4, LiCoPO4, LiNiPO4; Fe2(MoO4)3, etc., but are not limited to these.

[0130] At this time, in addition to the positive electrode active material, the positive electrode active material layer may further include a binder, a conductive material, a filler, and other additives, and the conductive material is the same as that described in the negative electrode of the lithium secondary battery.

[0131] In addition, the binder may be polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyacrylic acid (PAA), polymethacrylic acid (PMA), polymethyl methacrylate (PMMA), polyacrylamide (PAM), polymethacrylamide, polyacrylonitrile (PAN), polymethylacrylonitrile, 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, but are not limited thereto.

[0132] The separator may be made of a porous substrate. Any of the porous substrates commonly used in electrochemical devices can be used. For example, a polyolefin-based porous membrane or nonwoven fabric can be used, but is not particularly limited thereto.

[0133] The separator may be made of any one of polyethylene, polypropylene, polybutene, polyisopentene, polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene ether, polyphenylene sulfide, and polyethylene naphthalate or a mixture thereof.

[0134] 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 solvents used include non-aqueous organic solvents, organic solid electrolytes, and inorganic solid electrolytes.

[0135] The lithium salt is a material that is easily soluble in a non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 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 carboxylate, lithium tetraphenylborate imide, etc.

[0136] The non-aqueous organic solvent may be, for example, N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran (franc), 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, ethyl propionate and other aprotic organic solvents.

[0137] The organic solid electrolyte may include, for example, polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate polymers, polylysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, polymers containing secondary dissociative groups, and the like.

[0138] The inorganic solid electrolyte may be, for example, nitrides, halides, sulfates, and the like such as Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, LiSiO4, LiSiO4-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH, and Li3PO4-Li2S-SiS2.

[0139] In addition, the non-aqueous electrolyte may further contain other additives to improve charge and discharge characteristics, flame retardancy, etc. Examples of additives include pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glycol (glyme), hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidinones, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, fluoroethylene carbonate (FEC), propylene sultone (PRS), vinylene carbonate (VC), etc.

[0140] The lithium secondary battery according to the present invention can perform lamination stacking and folding processes of the separator and the electrode in addition to the conventional winding process. The battery case may be cylindrical, prismatic, pouch or coin-shaped.

[0141] The present application will be described in more detail below through examples, but the present application is not limited thereto.

[0142] [Preparation Example 1] Preparation of the first copolymer

[0143] Vinyl acetate and N-vinylformamide were continuously supplied to a nitrogen-purged reactor and reacted at 60° C. to prepare a copolymer of vinyl acetate and vinylformamide (PVAc-co-PVNF).

[0144] The mixture containing the synthesized PVAc-co-PVNF was recovered and added to methanol dissolved with KOH to hydrolyze the acetate functional groups of PVAc-co-PVNF to obtain a copolymer of vinyl alcohol and N-vinyl formamide (PVOH-co-PVNF), which was in a swollen gel state.

[0145] The obtained gel was crushed into fine particles, washed with methanol, added to methanol in which an alkaline catalyst was dissolved for further hydrolysis, and washed to remove soluble salts and by-products to obtain a first copolymer of vinyl alcohol and vinyl amine (PVOH-co-PVAm).

[0146] [Preparation Example 2] Preparation of the Second Copolymer

[0147] 1,050 g of distilled water and 10 g of alkyldiphenyloxidedisulfonate were added to the reactor and stirred for 1 hour under nitrogen purge.

[0148] After adding 2.5 g of potassium persulfate and raising the reactor temperature to 60° C., 110 g of vinyl acetate and 330 g of ethyl acrylate were added dropwise, reacted for 3 hours, and the temperature was maintained for another 2 hours to terminate the reaction, thereby obtaining a vinyl acetate-ethyl acrylate copolymer having a solid content of 30% by weight.

[0149] 100 g of ethylene acetate-ethyl acrylate copolymer with a solid content of 30%, 150 g of ethanol, hydroxide and organic salt were added into the reactor and stirred at 60° C. for 4 hours for hydrolysis.

[0150] After the hydrolysis was completed, the precipitated hydrolyzate was dissolved in distilled water, heated to 80° C., stirred and distilled for 8 hours to prepare a second copolymer.

[0151] [Preparation Example 3] Preparation of copolymer composition

[0152] The first copolymer and the second copolymer are mixed in a weight ratio of 70:30 (weight of the first copolymer: weight of the second copolymer), a pH adjuster (polyacrylic acid) is added to reach a desired pH, and then 0.5 to 3 weight % of a cross-linking agent glutaraldehyde is added based on 100 weight % of the total weight of the copolymer composition, and the copolymer composition is prepared by stirring.

[0153] [Manufacturing Example 4] Manufacture of lithium secondary battery

[0154] As an electrode active material, 80 g of artificial graphite, 16 g of SiOx, 1 g of carbon nanotubes, 3 g of a binder including the copolymer composition prepared in Preparation Example 3, and distilled water were mixed to prepare a negative electrode slurry.

[0155] The prepared negative electrode slurry was uniformly coated on a copper current collector, dried at 110° C., and the obtained mixture was roll-pressed, and then heated in a vacuum oven at 110° C. for more than 4 hours to prepare a negative electrode.

[0156] Subsequently, a non-aqueous electrolyte containing a lithium salt is used as an electrolyte, a polyolefin separator is placed between the positive electrode and the negative electrode, and a lithium secondary battery is manufactured, regardless of its form as a pouch cell or a button cell.

[0157] As a non-aqueous electrolyte, LiPF6 electrolyte was dissolved at a concentration of 1 M in a mixed solvent of ethylene carbonate:ethylmethyl carbonate:diethyl carbonate in a volume ratio of 3:5:2.

[0158] [Example 1]

[0159] When preparing the copolymer composition in Preparation Example 3, the pH was adjusted to 7, and 1.5 wt % of a cross-linking agent, glutaraldehyde, was added based on 100 wt % of the total weight of the copolymer composition.

[0160] A lithium secondary battery was manufactured according to Preparation Example 4 using the prepared copolymer composition.

[0161] [Example 2]

[0162] When preparing the copolymer composition in Preparation Example 3, the pH was adjusted to 9, and the rest was the same as in Example 1 to manufacture a lithium secondary battery.

[0163] [Example 3]

[0164] When preparing the copolymer composition in Preparation Example 3, the pH was adjusted to 12, and the rest was the same as in Example 1 to manufacture a lithium secondary battery.

[0165] [Comparative Example 1]

[0166] When preparing the copolymer composition in Preparation Example 3, 3 wt % of crosslinking agent glutaraldehyde was added based on 100 wt % of the total weight of the copolymer composition, and the rest was the same as in Example 1, thereby manufacturing a lithium secondary battery.

[0167] [Comparative Example 2]

[0168] When preparing the copolymer composition in Preparation Example 3, 0.5 wt % of crosslinking agent glutaraldehyde was added based on 100 wt % of the total weight of the copolymer composition, and the rest was the same as in Example 1, thereby manufacturing a lithium secondary battery.

[0169] [Comparative Example 3]

[0170] In Preparation Example 3, the copolymer composition was prepared in the same manner as in Example 1 except that the pH was adjusted to 3, thereby manufacturing a lithium secondary battery.

[0171] [Comparative Example 4]

[0172] In Preparation Example 3, the copolymer composition was prepared in the same manner as in Example 1 except that the pH was adjusted to 5, thereby manufacturing a lithium secondary battery.

[0173] [Evaluation Example 1] Evaluation of Crosslinking Rate of Copolymer Composition

[0174] The crosslinking rates of the first and second copolymers of the copolymer compositions used in Examples 1 to 3 and Comparative Examples 1 to 4 were measured and calculated by a gel content measurement method.

[0175] First, in order to determine the crosslinking rate of the first and second copolymers of the adhesive composition (copolymer composition), about 3 g of the copolymer compositions of Examples 1 to 3 and Comparative Examples 1 to 4 prepared according to Preparation Example 3 were taken and coated on a glass plate washed with MeOH, and then coated with a glass rod.

[0176] Subsequently, the copolymer composition was cross-linked by vacuum heat treatment at 110° C. for 12 hours or more, thereby preparing a film.

[0177] After drying, tear off the film with a razor, take 0.7 g of the film and put it into a 250 ml conical flask. Put the conical flask into a fume hood, add 100 ml of distilled water, and place it in a 70°C constant temperature water bath for 2 hours.

[0178] Subsequently, the conical flask was cooled in a low-temperature water bath for 5 min, and the aluminum pan was weighed and placed on a hot plate.

[0179] The solution in the cooled conical flask was filtered into the prepared beaker with filter paper. Subsequently, 10 ml of the filtrate was taken with a pipette, placed in an aluminum pan, dried at 165°C for 30 minutes, and the mass was measured.

[0180] The cross-linking rate (gel content) was calculated according to the following Mathematical Formula 1.

[0181] [Mathematical formula 1]

[0182] Cross-linking rate (gel content) (%) = 100-(mass of filtrate after drying / mass of film of cross-linked copolymer composition (0.7 g))*500

[0183] The mass of the filtrate after drying in the above Mathematical Formula 1 is obtained by subtracting the mass of the aluminum pan from the mass of the aluminum pan containing the filtrate dried at 165° C. for 30 minutes.

[0184] [Evaluation Example 2] Evaluation of the stability of negative electrode slurry

[0185] The negative electrode slurry including the copolymer composition used in Examples 1 to 3 and Comparative Examples 1 to 4 prepared according to Preparation Example 4 was put into a 30 ml vial and left at room temperature for 7 days to check whether phase separation occurred.

[0186] If phase separation occurs, the stability is calculated according to the following mathematical formula 2.

[0187] [Mathematical formula 2]

[0188] Negative electrode slurry stability (%) = (phase separation layer height / initial slurry height) * 100

[0189] [Evaluation Example 3] Evaluation of Adhesive Adhesion

[0190] In order to measure the adhesion of the copolymer composition (binder) used in Examples 1 to 3 and Comparative Examples 1 to 4, the copper collector of the manufactured negative electrode and the negative electrode slurry layer formed on the copper collector were attached to an acrylic plate and then peeled off at 180°, and the adhesion was measured using UTM.

[0191] [Evaluation Example 4] Battery Performance Evaluation

[0192] The lithium secondary batteries prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were charged and discharged three times at 25° C. with a charge and discharge current density of 0.1 C, a charge termination voltage of 4.8 V, and a discharge termination voltage of 2.7 V.

[0193] Subsequently, the charge and discharge current density was set to 1C, the charge end voltage was 4.8V, and the discharge end voltage was 2.7V. 500 charge and discharge cycles were performed to measure the capacity retention rate.

[0194] All discharges were carried out under constant current / constant voltage conditions, and the termination current of the constant voltage discharge was 0.005C.

[0195] At this time, the capacity retention rate is calculated according to the following mathematical formula 3.

[0196] [Mathematical formula 3]

[0197] Capacity retention rate (%) = (discharge capacity after 500 cycles / discharge capacity after 3 cycles) * 100

[0198] Furthermore, after the charge and discharge evaluation was completed, the battery was disassembled to confirm the change in the thickness of the negative electrode, and to compare the inhibitory effect of the copolymer composition binder used in Examples 1 to 3 and Comparative Examples 1 to 4 on silicon expansion.

[0199] At this time, the electrode expansion rate is calculated according to the following mathematical formula 4.

[0200] [Formula 4]

[0201] Electrode expansion rate (%) = (negative electrode thickness after 200 cycles - negative electrode thickness after vacuum drying before assembly) / negative electrode thickness after vacuum drying before assembly * 100

[0202] [Evaluation Example 5] LiF content evaluation

[0203] After the initial 3-cycle discharge of the lithium secondary batteries manufactured in Examples 1 to 3 and Comparative Examples 1 to 4, the batteries were disassembled and the LiF content on the surface of the negative electrode was measured using XPS.

[0204] The battery disassembly was carried out in a glove box filled with Ar. The anode was rinsed with acetonitrile and transferred to the glove box connected to the XPS chamber through a vacuum tube. The sample was not exposed to air.

[0205] A Kratos Axis Supra XPS was used for XPS, with measurements of 300 μm x 700 μm without charge neutralizer and scanned with a 1.0 eV step size. High-resolution scans of the carbon 1s, sulfur 2p, and fluorine 1s regions were performed with a step size of 0.1 eV.

[0206] Table 1 shows the crosslinking rate, negative electrode slurry stability, binder adhesion, capacity retention rate, electrode expansion rate, and LiF content measured in Evaluation Examples 1 to 5.

[0207] [Table 1]

[0208]

[0209]

[0210] As shown in Table 1, as the pH increases (from acidic to alkaline), the crosslinking rates of the copolymer composition adhesives used in Examples 1 to 3 and Comparative Examples 1 to 4 decrease.

[0211] In addition, at the same pH, the cross-linking rate increased with the increase of cross-linker content.

[0212] On the other hand, with the increase of pH, the height of the phase separation layer decreased, thus improving the stability of the slurry.

[0213] If the measured value of the slurry stability exceeds 3% (i.e., the height of the phase separation layer increases), for example, when the copolymer composition binder of Comparative Examples 3 and 4 with a pH less than 6 is used, the stability of the slurry is significantly reduced, the uniformity of negative electrode manufacturing may deteriorate, and it is difficult to apply to actual processes.

[0214] At the same pH, the change of crosslinker content had no effect on the slurry stability.

[0215] As the pH increases and the crosslinking rate decreases, the adhesion of the copolymer composition adhesive improves.

[0216] This is because as the cross-linking rate increases, the cross-linking rate of the adhesion-improving functional group also increases, thereby reducing the adhesion-improving effect of the adhesion-improving functional group.

[0217] In particular, when the copolymer composition adhesives of Comparative Examples 3 and 4 having a pH of less than 6 were used, the adhesion of the copolymer composition adhesive was significantly reduced as the degree of crosslinking increased.

[0218] In addition, at the same pH, as the crosslinker content increases, the adhesion of the copolymer composition adhesive decreases.

[0219] In particular, when the copolymer composition adhesive of Comparative Example 1 having an excessively high crosslinking agent content was used, the adhesive force was significantly reduced compared to the case where the copolymer composition adhesive of Example 1 was used under the same pH conditions.

[0220] The capacity retention rates of the batteries of Examples 1 to 3 and Comparative Examples 1 to 4 after 500 charge and discharge cycles increased with increasing pH and decreasing crosslinking rate, but decreased at pH 9 or above.

[0221] In contrast, the electrode expansion ratios of the batteries of Examples 1 to 3 and Comparative Examples 1 to 4 after 500 charge and discharge cycles increased with increasing pH and decreasing crosslinking rate.

[0222] When the copolymer composition binder of Comparative Examples 3 and 4 having a pH of less than 6 is used, the electrode expansion rate is lower than the case of using the copolymer composition binder of Examples 1 and 2 having a pH of more than 6, and thus the ability to suppress electrode expansion is excellent. However, as described above, as the degree of crosslinking increases, most of the functional groups that contribute to improving adhesion are crosslinked, resulting in a decrease in the capacity retention rate of the battery.

[0223] On the other hand, when the copolymer composition binder of Comparative Example 1 having an excessively high crosslinking agent content was used, the electrode expansion ratio was better than that of the copolymer composition binder of Example 1 under the same pH conditions, but the capacity retention ratio was reduced.

[0224] In addition, when the copolymer composition binder of Comparative Example 2 having a cross-linking agent content lower than an appropriate amount is used, the electrode expansion rate is significantly increased and the capacity retention rate is reduced compared to the case where the copolymer composition binder of Example 1 is used under the same pH conditions, thereby resulting in reduced stability and life characteristics of the battery.

[0225] After Formation, the LiF content on the surface of the negative electrode of the batteries of Examples 1 to 3 and Comparative Examples 1 to 4 was measured to be less than 80%.

[0226] Even after cross-linking, the uncross-linked carboxyl groups of the second copolymer still exist, and the carboxyl groups can form stronger hydrogen bonds with fluorine than other functional groups. As a result, the decomposition of the electrolyte salt LiFSi is promoted to generate an initial SEI layer, which can subsequently inhibit the decomposition of additional SEI layers.

[0227] That is to say, the copolymer composition of the present application can stably form a SEI layer at an early stage, thereby improving the performance of the secondary battery.

[0228] In summary, the copolymer binder composition of the present application, which is a mixture of a first copolymer, a second copolymer and a certain amount of a cross-linking agent, is cross-linked by appropriately adjusting the pH, thereby having the dispersion stability of a negative electrode slurry composition, the adhesion of the negative electrode, and the characteristics of a secondary battery (capacity retention rate and electrode expansion rate) that form a balance within an appropriate range.

[0229] On the other hand, when a copolymer binder composition having a crosslinking agent content or a pH range outside the scope of the present application is used, one or more of dispersion stability of the negative electrode slurry composition, adhesion of the negative electrode, and characteristics of the secondary battery may be insufficient for actual secondary batteries.

[0230] The scope of the present invention is specified by the following claims rather than the above detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalent concepts should be construed as being included in the scope of the present invention.

[0231] Industrial Applicability

[0232] The copolymer composition of the present invention can be used in negative electrode slurry to increase the adhesion to the negative electrode collector, inhibit negative electrode expansion, and improve the capacity retention rate per cycle of the secondary battery.

Claims

1. A copolymer composition comprising: A first copolymer, wherein the first copolymer comprises a vinyl alcohol monomer unit and a vinylamine monomer unit; a second copolymer comprising vinyl alcohol monomer units and acrylic acid salt monomer units; and Cross-linking agent.

2. The copolymer composition according to claim 1, wherein: The first copolymer further comprises at least one selected from ethylene acetate monomer units and N-vinylformamide monomer units; The second copolymer further comprises at least one selected from acrylate monomer units and vinyl acetate monomer units.

3. The copolymer composition according to claim 1, wherein: The vinylamine monomer unit is at least one selected from the group consisting of vinylamine and 1-methylvinylamine; The acrylic acid salt monomer unit is at least one selected from the group consisting of acrylic acid and methacrylic acid.

4. The copolymer composition according to claim 2, wherein: The N-vinylformamide monomer unit is at least one selected from the group consisting of N-vinylformamide and N-isopropenylformamide; The acrylate monomer unit is at least one selected from the group consisting of 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, and ethylhexyl methacrylate.

5. The copolymer composition according to claim 1, wherein: Based on 100 mol % of the total content of the first copolymer, the first copolymer comprises 50 mol % or more and 90 mol % or less of the vinyl alcohol monomer unit and 1 mol % or more and 50 mol % or less of the vinylamine monomer unit; The second copolymer comprises 1 mol % or more and 30 mol % or less of the vinyl alcohol monomer unit and 50 mol % or more and 90 mol % or less of the acrylic acid salt monomer unit based on 100 mol % of the total content of the second copolymer.

6. The copolymer composition according to claim 1, wherein: The first copolymer comprises a monomer repeating unit represented by the following chemical formula (1); The second copolymer comprises a monomer repeating unit represented by the following chemical formula (2); [Chemical formula 1] In the chemical formula 1: 0≤x≤15mol%, 50≤y≤90mol%, 0≤m≤30mol%, 1≤n≤50mol%; [Chemical formula 2] In the chemical formula 2: R1 and R2 are the same or different and are each independently hydrogen or a straight or branched hydrocarbon having 1 to 5 carbon atoms; R3 is a hydroxyl group (-OH); M is an alkali metal; 0≤a≤5mol%, 50≤b≤90mol%, 0≤c≤5mol%, 1≤d≤30mol%.

7. The copolymer composition according to claim 1, wherein: Based on 100 wt % of the total weight of the first copolymer and the second copolymer, the first copolymer is contained in an amount of 10 wt % to 90 wt % and the second copolymer is contained in an amount of 10 wt % to 90 wt %.

8. The copolymer composition according to claim 1, wherein: The cross-linking agent contains two or more aldehyde groups.

9. The copolymer composition according to claim 1, wherein: Based on 100 wt % of the total weight of the copolymer composition, the content of the crosslinking agent is greater than or equal to 0.7 wt % and less than or equal to 2.8 wt %.

10. The copolymer composition according to claim 1, wherein: The pH of the copolymer composition is 6 or more and 12 or less.

11. The copolymer composition according to claim 1, wherein: The cross-linking rate of the first copolymer and the second copolymer is 45% or more and 80% or less.

12. A negative electrode slurry comprising: The copolymer composition according to any one of claims 1 to 11; and Negative electrode active material.

13. A negative electrode comprising: a current collector; and A negative electrode active material layer including the copolymer composition according to any one of claims 1 to 11 is formed on the current collector. 14 . A secondary battery comprising the negative electrode according to claim 13 .

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