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

Abstract

The present invention relates to a copolymer composition including a first copolymer containing vinyl alcohol monomer units and vinyl amine-based monomer units, a second copolymer containing vinyl alcohol monomer units and acrylic acid salt-based monomer units, and a crosslinker, as well as an anode slurry, an anode, and a secondary battery each containing the copolymer composition.

Description

[Technical Field]

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

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

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

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

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

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

[0007] Meanwhile, recent research has been conducted into suppressing the expansion of silicon-containing active materials by using partially cross-linked binders. However, the expansion suppression is still insufficient, leading to problems such as electrode detachment due to the continuous destruction and re-formation of the SEI layer and the depletion of lithium ions, which leads to reduced battery performance.

[0008] Therefore, there is a demand for a binder that can solve these problems and ensure the capacity retention rate of secondary batteries. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Republic of Korea Patent Publication No. 10-2016-0024921 Summary of the Invention [Problem to be solved by the invention]

[0010] Therefore, the present invention aims to provide a copolymer composition that can be used to prepare a slurry composition that is excellent in binding strength and ability to suppress electrode expansion.

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

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

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

[0014] The copolymer composition of the present invention can be used in a negative electrode slurry to increase the binding strength with the negative electrode current collector, suppress expansion of the negative electrode, and improve the capacity retention rate per cycle of a secondary battery. [Brief explanation of the drawings]

[0015] [Figure 1] 1 illustrates the crosslinking mechanism of the copolymer composition of the present application when glutaraldehyde is used as the crosslinking agent. DETAILED DESCRIPTION OF THE INVENTION

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

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

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

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

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

[0021] A copolymer composition according to one embodiment of the present application may include a first copolymer including a vinyl alcohol monomer unit and a vinyl amine-based monomer unit, a second copolymer including a vinyl alcohol monomer unit and an acrylic acid salt-based monomer unit, and a crosslinker.

[0022] The crosslinking agent crosslinks the first copolymer and the second copolymer, each having a polar functional group, thereby increasing the adhesive strength between the active material and the current collector and minimizing structural changes and damage to the electrode due to volumetric changes of the active material.

[0023] In one embodiment, the first copolymer may additionally include at least one selected from a vinyl acetate monomer unit and an N-vinylformamide-based monomer unit, and the second copolymer may additionally include at least one selected from an acrylate-based monomer unit and a vinyl acetate monomer unit.

[0024] The first copolymer contains a hydroxyl group and an amine group, and therefore, when used as a binder for a negative electrode slurry, it can form strong hydrogen bonds with silicon, which is a negative electrode active material, and can form coordinate bonds with the negative electrode current collector, thereby increasing the binding strength between the silicon and the current collector.

[0025] Meanwhile, the second copolymer, based on an ethylene skeleton, provides flexibility to the binder of the negative electrode slurry and suppresses volumetric changes in silicon, the negative electrode active material. The alkali metal ions substituted at the terminals of the acrylic acid salt-based monomer units contribute to improving ionic conductivity. Furthermore, the stretched chains interact with the negative electrode active material to form a dense, porous electrode, enabling the formation of a stable SEI layer.

[0026] The hydroxyl groups of the first copolymer and the carboxyl groups of the second copolymer can be chemically and / or physically crosslinked, thereby suppressing the volume change of silicon, which is the negative electrode active material.

[0027] In one embodiment, the vinylamine-based monomer unit of the first copolymer may be, but is not limited to, at least one selected from the group consisting of vinylamine and 1-methylvinylamine.

[0028] In addition, the acrylic acid salt-based monomer unit of the second copolymer may be, but is not limited to, at least one selected from the group consisting of acrylic acid and methacrylic acid.

[0029] In one embodiment, the N-vinylformamide-based monomer unit of the first copolymer may be, but is not limited to, one or more selected from the group consisting of N-vinylformamide and N-isopropenylformamide.

[0030] In addition, the acrylate-based monomer unit of the second copolymer may 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.

[0031] In one embodiment, the first copolymer may contain 50 mol% to 90 mol% of the vinyl alcohol monomer unit and 1 mol% to 50 mol% of the vinylformamide (N-vinylformamide)-based monomer unit, based on a total content of 100 mol%.

[0032] In one embodiment, the second copolymer may contain 1 mol% to 30 mol% of the vinyl alcohol monomer unit and 50 mol% to 90 mol% of the acrylic acid salt-based monomer unit, based on a total content of 100 mol%.

[0033] The content of the first copolymer and the second copolymer can be adjusted by changing the degree of hydrolysis during the manufacturing process of the first copolymer and the second copolymer.

[0034] In one embodiment, the first copolymer may include a monomer repeat unit represented by the following Formula 1, and the second copolymer may include a monomer repeat unit represented by the following Formula 2:

[0035] [ka] In the above Chemical Formula 1, 0≦x≦15 mol%, 50≦y≦90 mol%, 0≦m≦30 mol%, and 1≦n≦50 mol%. In the above formula 1, x, y, m, and n represent the mol% of each monomer unit.

[0036] [ka] In the above Chemical Formula 2, R1 and R2 are different from each other or the same, and each independently represents hydrogen or a linear or branched hydrocarbon having 1 to 5 carbon atoms; R3 is a hydroxyl (-OH) group; M is an alkali metal; 0≦a≦5 mol%, 50≦b≦90 mol%, 0≦c≦5 mol%, and 1≦d≦30 mol%. In the above Chemical Formula 2, a, b, c, and d represent the mol% of each monomer unit. Furthermore, M in the above Chemical Formula 2 may be any one selected from the group consisting of lithium (Li), potassium (K) and sodium (Na), but is not limited thereto. Meanwhile, R1 and R2 in Chemical Formula 2 may each independently be any one selected from the group consisting of hydrogen, methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl, tert-butyl, and n-pentyl, but are not limited thereto.

[0037] In one embodiment, the copolymer composition may contain 10 wt % or more and 90 wt % or less of the first copolymer and 10 wt % or more and 90 wt % or less of the second copolymer, based on a total weight of 100 wt % of the copolymer composition.

[0038] Within the range of the first copolymer and the second copolymer in the copolymer composition, the higher the content ratio of the first copolymer, the more improved the binding strength of the negative electrode when used as a negative electrode binder.

[0039] In addition, as the content ratio of the second copolymer increases within the content range of the first copolymer and the second copolymer in the copolymer composition, when the copolymer composition is used as a negative electrode binder, the dispersibility and stability of the negative electrode slurry can be further improved.

[0040] When the content of the first copolymer and the second copolymer in the copolymer composition is outside the range of the present application, when the copolymer composition is used as a negative electrode binder, one or more of the dispersion stability of the negative electrode slurry composition, the binding strength of the negative electrode, and the characteristics of the secondary battery may be reduced.

[0041] In particular, electrodes with low adhesion to the current collector may experience detachment during the drying and rolling processes, and when the rolling density of the electrode is increased, separation of the applied slurry from the electrode may occur. Furthermore, during battery operation, low adhesion to the electrode plate may cause the electrode to detach due to swelling caused by the electrolyte, resulting in reduced battery operation stability.

[0042] In one embodiment, the first copolymer may be a random or block copolymer, and the second copolymer may be a random or block copolymer.

[0043] 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.

[0044] Meanwhile, the first copolymer can be prepared by hydrolysis of a copolymer containing vinyl acetate monomer units and N-vinylformamide-based monomer units.

[0045] That is, the vinyl acetate monomer units and N-vinylformamide-based monomer units of the first copolymer can be hydrolyzed to vinyl alcohol monomer units and vinyl amine-based monomer units, respectively.

[0046] The second copolymer can be prepared by hydrolysis of a copolymer containing an acrylate-based monomer unit and a vinyl acetate monomer unit.

[0047] That is, the acrylate-based monomer unit and the vinyl acetate monomer unit of the second copolymer can be hydrolyzed into an acrylic acid salt-based monomer unit and a vinyl alcohol monomer unit, respectively.

[0048] The hydrolysis for producing the first copolymer and the second copolymer can be carried out using, but is not limited to, an alkali metal hydroxide.

[0049] In one embodiment, the cross-linking agent can contain two or more aldehyde groups.

[0050] For example, the cross-linking agent may be glutaraldehyde, succinaldehyde, glyoxal dialdehyde, adipic dialdehyde, or a combination thereof.

[0051] FIG. 1 shows the expected crosslinking mechanism of the copolymer composition of the present application when glutaraldehyde was used as the crosslinking agent.

[0052] In one embodiment, the crosslinking agent may be included in an amount of 0.7 wt % or more and 2.8 wt % or less, based on 100 wt % of the total weight of the copolymer composition.

[0053] For example, the copolymer may be contained in an amount of 1 wt %, 1.5 wt %, 2 wt %, or 2.5 wt %, based on the total weight of the copolymer composition (100 wt %).

[0054] The higher the content of the crosslinking agent, the higher the crosslinking rate may be when crosslinked under the same pH.

[0055] On the other hand, if the content of the crosslinking agent exceeds the content range of the present application, the binding strength of the copolymer composition may be significantly reduced when crosslinked under the same pH.

[0056] In addition, if the content of the crosslinking agent is below the content range of the present application, when crosslinked under the same pH, the electrode expansion rate may be significantly increased, which may shorten the life of a lithium secondary battery using the copolymer.

[0057] In one embodiment, the copolymer composition may have a pH of 6 or more and 12 or less.

[0058] As the pH of the copolymer composition increases, the crosslinking rate may decrease, and as the pH of the copolymer composition increases, the binding rate of a slurry composition using the copolymer composition may increase.

[0059] On the other hand, if the pH of the copolymer composition is below 6, the stability of the slurry containing the copolymer composition may be significantly reduced, making it unsuitable for production of products.

[0060] Furthermore, if the pH of the copolymer composition exceeds 12, the electrode expansion rate of the electrode using the copolymer composition may become significantly high, which may result in a decrease in the performance and life of the battery.

[0061] The pH of the copolymer composition can be adjusted by adding a pH adjuster to the copolymer composition.

[0062] 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 can be used.

[0063] For example, maleic acid, acrylic acid, or a combination thereof, which are monomers, can be used, and polyacrylic acid, which is a polymer, can be used.

[0064] The crosslinking rate of the first copolymer and the second copolymer in the copolymer composition may be 45% or more and 80% or less.

[0065] For example, it may be 50% or more and 80% or less.

[0066] According to another aspect of the present disclosure, a negative electrode slurry may include the copolymer composition and a negative electrode active material.

[0067] That is, the copolymer composition can be used as a binder for a negative electrode.

[0068] The peel strength between the negative electrode active material layer formed using the negative electrode slurry and the copper current collector was 10 dyne / cm 2 It may be 15 dyne / cm or more. 2 It may be the following:

[0069] The negative electrode active material may be a compound containing one or more 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 preferably, it may be silicon or a compound containing silicon.

[0070] Examples of the carbon-based material include, but are not limited to, artificial graphite, natural graphite, hard carbon, soft carbon, etc. The negative electrode active material containing silicon is not particularly limited as long as it is silicon or a compound containing silicon, and preferably, it is Si, SiO x (0 < x < 2), Si-Y alloy (where Y is an alkali metal, alkaline earth metal, group 13 element, group 14 element, transition metal, rare earth element, or a combination thereof, and is not Si), and one or more selected from the group consisting of Si-C composites may also be used.

[0071] When a negative electrode active material containing silicon and another negative electrode active material are mixed and used as the negative electrode active material, the negative electrode active material containing silicon may be contained at 8% by weight or more of the total weight of the negative electrode active material.

[0072] The negative electrode active material may be contained at 50 to 90% by weight, preferably 60 to 80% by weight, based on the total weight of the negative electrode active material layer.

[0073] When the negative electrode active material is contained at less than 50% by weight, the energy density decreases and a high-energy density battery cannot be manufactured. When it is contained at more than 90% by weight, the contents of the conductive material and the binder decrease, the electrical conductivity decreases, and the adhesion between the electrode active material layer and the current collector may decrease.

[0074] The copolymer composition binder of the present invention may be included in an amount of 1 to 35 wt % based on the total weight of the negative electrode slurry. If the copolymer is included in an amount less than 1 wt %, the physical properties of the negative electrode may be reduced, and the negative electrode active material and conductive material may fall off. If the copolymer is included in an amount more than 35 wt %, the ratio of the negative electrode active material to the conductive material may be relatively reduced, and the battery capacity may be reduced, and the electrical conductivity of the negative electrode may be reduced.

[0075] The negative electrode slurry may further include a polymer in addition to the copolymer composition of the present application, such as, but not limited to, polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyacrylic acid (PAA), metal polyacrylate (Metal-PAA), polymethacrylic acid (PMA), polymethyl methacrylate (PMMA), polyacrylamide (PAM), polymethacrylamide, polyacrylonitrile (PAN), polymethacrylonitrile, polyimide (PI), chitosan, starch, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, hydroxypropyl cellulose, regenerated cellulose, and various copolymers thereof.

[0076] A negative electrode according to yet another aspect of the present application may include a current collector and a negative electrode active material layer formed on the current collector, the negative electrode active material layer including the copolymer composition of the present application.

[0077] The negative electrode active material layer may further include a conductive material. The conductive material is used to further improve the conductivity of the negative electrode active material. The conductive material may be any material that is conductive without inducing chemical changes in the battery. Examples of the conductive material include graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives.

[0078] The conductive material may be contained in an amount of 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 contained in an amount less than 5 wt %, the electrical conductivity of the negative electrode will be low. If the conductive material is contained in an amount exceeding 30 wt %, the ratio of the silicon-based negative electrode active material to the binder will be relatively reduced, resulting in a decrease in battery capacity. Furthermore, since the binder content must be increased to maintain the negative electrode active material layer, the content of the negative electrode active material will be reduced, making it impossible to manufacture a high-energy density battery.

[0079] In the negative electrode of the present application, the negative electrode active material layer contains the copolymer composition of the present application, which makes it possible to suppress volume expansion of the negative electrode active material that occurs during charge and discharge of the secondary battery, and to improve the capacity retention rate per cycle.

[0080] The negative electrode can be manufactured by (a) preparing a composition for forming a negative electrode active material layer, the composition including a negative electrode active material and the copolymer composition of the present application, and (b) applying the composition for forming a negative electrode active material layer onto a negative electrode current collector and then drying the applied composition.

[0081] The composition for forming the negative electrode active material layer is prepared in the form of a negative electrode slurry. The solvent for preparing the slurry should be easy to dry, and most preferably, should be able to well dissolve the copolymer composition binder of the present invention and maintain the negative electrode active material in a dispersed state without dissolving it.

[0082] The solvent according to the present invention may be water or an organic solvent, and the organic solvent may be one or more selected from the group consisting of methylpyrrolidone, dimethylformamide, isopropyl alcohol, acetonitrile, methanol, ethanol, and tetrahydrofuran.

[0083] The composition for forming the negative electrode active material layer can be mixed by a conventional mixer, such as a latex mixer, a high-speed shear mixer, or a homomixer, using a conventional method.

[0084] The step (b) is a step of producing a negative electrode for a lithium secondary battery by applying the composition for forming a negative electrode active material layer produced in the step (a) onto a negative electrode current collector and then drying it.

[0085] Specifically, the negative electrode current collector may be made of a material selected from the group consisting of copper, stainless steel, titanium, silver, palladium, nickel, alloys thereof, and combinations thereof. The stainless steel may be surface-treated with carbon, nickel, titanium, or silver, and the alloy may be an aluminum-cadmium alloy. Other materials that may be used include calcined carbon, a non-conductive polymer surface-treated with a conductive material, and a conductive polymer.

[0086] The composition for forming a negative electrode active material layer prepared in step (a) is applied onto a negative electrode current collector, and can be coated onto the current collector to an appropriate thickness depending on the thickness to be formed, which can be suitably selected preferably within the range of 10 to 300 μm.

[0087] In this case, the method for applying the slurry-like composition for forming a negative electrode active material layer is not limited, and the negative electrode active material layer may be prepared by, for example, doctor blade coating, dip coating, gravure coating, slit die coating, spin coating, comma coating, bar coating, reverse roll coating, screen coating, cap coating, etc.

[0088] After coating, the coating is dried, and finally, a negative electrode for a secondary battery (particularly, a lithium secondary battery) having a negative electrode active material layer formed thereon can be produced.

[0089] A battery according to yet another aspect of the present application may include a current collector and a negative electrode having the negative electrode active material layer formed on the current collector.

[0090] The battery may be a secondary battery (particularly, a lithium secondary battery) including a positive electrode, the negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte solution.

[0091] The secondary battery may have a capacity retention rate of 80% or more after 500 charge / discharge cycles.

[0092] For example, it may be 83% or more, 85% or more, or 90% or more.

[0093] The secondary battery may have an electrode expansion rate of 60% or less after 500 charge / discharge cycles.

[0094] For example, it may be 55% or less, 50% or less, 45% or less, or 40% or less.

[0095] The positive electrode, separator, and electrolyte of the lithium secondary battery are not particularly limited in the present invention and may be any known material in the art.

[0096] The positive electrode includes a positive electrode active material formed on a positive electrode current collector.

[0097] The positive electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery, and examples thereof include stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. In this case, the positive electrode current collector may be in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric, having a finely textured surface to enhance adhesion to the positive electrode active material.

[0098] The positive electrode active material constituting the positive electrode active material layer can be any positive electrode active material used in the art. Specific examples of such positive electrode active materials include lithium metal; lithium cobalt oxides such as LiCoO; Li 1+x Mn 2-x O4 (where x is 0 to 0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxides such as Li2CuO2; vanadium oxides such as LiV3O8, LiFe3O4, V2O5, and Cu2V2O7; LiNi 1-x M x Lithium nickel oxide represented by the formula LiMnO2 (where M=Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x=0.01 to 0.3); 2-x M x Lithium manganese composite oxides represented by Li(Ni)O2 (where M = Co, Ni, Fe, Cr, Zn, or Ta, and x = 0.01 to 0.1) or Li2Mn3MO8 (where M = Fe, Co, Ni, Cu, or Zn); a Co b Mn c)O2 (where 0 < a < 1, 0 < b < 1, 0 < c < 1, and a + b + c = 1); sulfur or disulfide compounds; phosphates such as LiFePO4, LiMnPO4, LiCoPO4, LiNiPO4; Fe2(MoO4)3, etc. can be mentioned, but it is not limited to only these.

[0099] At this time, the positive electrode active material layer can additionally contain, in addition to the positive electrode active material, a binder, a conductive material, a filler, and other additives, etc. The conductive material is the same as the content described above in the negative electrode for the lithium secondary battery.

[0100] Also, the binder can include 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, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene - propylene - diene polymer (EPDM), sulfonated - EPDM, styrene - butadiene rubber (SBR), fluorine rubber, and various copolymers thereof, etc., but it is not limited to these.

[0101] The separator membrane may be composed of a porous substrate. The porous substrate can generally be any porous substrate used in electrochemical devices. For example, a polyolefin - based porous membrane or non - woven fabric can be used, but it is not particularly limited to this.

[0102] The separation membrane may be a porous substrate made of any one selected from the group consisting of polyethylene, polypropylene, polybutylene, polypentene, polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, and polyethylene naphthalate, or a mixture of two or more thereof.

[0103] The electrolyte of the lithium secondary battery is a non-aqueous electrolyte containing a lithium salt, and is composed of a lithium salt and a solvent. The solvent may be a non-aqueous organic solvent, an organic solid electrolyte, or an inorganic solid electrolyte.

[0104] The lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte solution, and examples thereof include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB 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 carboxylates, and lithium 4-phenylborate imide can be used.

[0105] Examples of non-aqueous organic solvents that can be used include aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydroxyfuran (franc), 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, 4-methyl-1,3-dioxene, diethyl ether, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0106] Examples of the organic solid electrolyte that can be used include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyagitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and polymers containing secondary dissociating groups.

[0107] Examples of the inorganic solid electrolyte that can be used include nitrides, halides, and sulfates of Li, such as LiN, LiI, LiNI, LiN-LiI-LiOH, LiSiO, LiSiO-LiI-LiOH, LiSiS, LiSiO, LiSiO-LiI-LiOH, and LiPO-LiS-SiS.

[0108] The non-aqueous electrolyte solution may further contain other additives for the purpose of improving charge / discharge characteristics, flame retardancy, etc. Examples of the additives include pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, fluoroethylene carbonate (FEC), propene sultone (PRS), vinylene carbonate (VC), etc.

[0109] The lithium secondary battery according to the present invention can be manufactured by lamination stacking of separators and electrodes and folding processes in addition to the conventional winding process, and the battery case may be cylindrical, prismatic, pouch-shaped, or coin-shaped. [Example]

[0110] The present invention will be described in more detail below using examples, but the present invention is not limited to these examples.

[0111] [Production Example 1] Production of the first copolymer Vinyl acetate and N-vinyl formamide were continuously fed into a nitrogen-sparged reactor and reacted at 60°C to synthesize a copolymer of vinyl acetate and vinyl formamide (PVAc-co-PVNF).

[0112] The mixture containing the synthesized PVAc-co-PVNF was collected and poured into methanol containing dissolved KOH to hydrolyze the acetate functional groups of PVAc-co-PVNF, yielding a copolymer of vinyl alcohol and N-vinylformamide (PVOH-co-PVNF) in the form of a swollen gel.

[0113] The resulting gel was crushed into fine particles, washed with methanol, and then placed in methanol containing an alkaline catalyst for further hydrolysis. The soluble salts and by-products were removed by washing to obtain a first copolymer of vinyl alcohol and vinylamine (PVOH-co-PVAm).

[0114] [Production Example 2] Production of second copolymer 1,050 g of distilled water and 10 g of alkyldiphenyloxide disulfonate were placed in a reactor, and the mixture was stirred for 1 hour while blowing in nitrogen.

[0115] Then, 2.5g of potassium persulfate was added and the reactor was heated to 60°C. Then, 110g of vinyl acetate and 330g of ethyl acrylate were added dropwise over 3 hours, and the reaction was terminated by maintaining the temperature for 2 hours, resulting in a vinyl acetate-ethyl acrylate copolymer with a solid content of 30% by weight.

[0116] A reactor was charged with 100 g of vinyl acetate-ethyl acrylate copolymer with a solid content of 30%, 150 g of ethanol, hydroxide and organic salt, and hydrolysis was allowed to proceed with stirring at 60° C. for 4 hours.

[0117] After completion of the hydrolysis, the precipitated hydrolyzate was dissolved in distilled water, heated to 80° C., and stirred and stripped for 8 hours to produce a second copolymer.

[0118] [Production Example 3] Production of copolymer composition The first copolymer and the second copolymer were mixed in a weight ratio (weight of the first copolymer:weight of the second copolymer) of 70:30, and a pH adjuster (polyacrylic acid) was added to achieve the desired pH. Then, 0.5 to 3 wt % of glutaraldehyde, a crosslinking agent, was added based on 100 wt % of the total weight of the copolymer composition, and the mixture was stirred to produce a copolymer composition.

[0119] [Production Example 4] Production of lithium secondary battery A negative electrode slurry was prepared by mixing 80 g of artificial graphite as an electrode active material, 16 g of SiOx, 1 g of carbon nanotubes, 3 g of a binder containing the copolymer composition prepared in Preparation Example 3, and distilled water.

[0120] The prepared negative electrode slurry was uniformly coated on a copper current collector, dried at 110°C, and the resulting mixture was rolled and heated in a vacuum oven at 110°C for 4 hours or more to prepare a negative electrode.

[0121] Thereafter, a non-aqueous electrolyte solution containing a lithium salt was used as an electrolyte, and a polyolefin separator was interposed between the positive electrode and the negative electrode, and then a lithium secondary battery was manufactured without being classified into a pouch or coin cell type.

[0122] The non-aqueous electrolyte used was a solution of LiPF6 electrolyte at a concentration of 1M in a solvent in which ethylene carbonate: ethyl methyl carbonate: diethyl carbonate were mixed in a volume ratio of 3:5:2.

[0123] [Example 1] When the copolymer composition was prepared according to Preparation Example 3, the pH was adjusted to 7, and 1.5 wt % of glutaraldehyde, a crosslinking agent, was added based on 100 wt % of the total weight of the copolymer composition. A lithium secondary battery was produced according to Production Example 4 using the produced copolymer composition.

[0124] [Example 2] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the pH of the copolymer composition prepared in Preparation Example 3 was adjusted to 9.

[0125] [Example 3] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the pH of the copolymer composition prepared in Preparation Example 3 was adjusted to 12.

[0126] [Comparative Example 1] A lithium secondary battery was manufactured in the same manner as in Example 1, except that when preparing the copolymer composition according to Preparation Example 3, 3 wt % of glutaraldehyde as a crosslinking agent was added, based on 100 wt % of the total weight of the copolymer composition.

[0127] Comparative Example 2 A lithium secondary battery was manufactured in the same manner as in Example 1, except that when preparing the copolymer composition according to Preparation Example 3, 0.5 wt % of glutaraldehyde as a crosslinking agent was added, based on 100 wt % of the total weight of the copolymer composition.

[0128] Comparative Example 3 A lithium secondary battery was manufactured in the same manner as in Example 1, except that the pH of the copolymer composition prepared in Preparation Example 3 was adjusted to 3.

[0129] Comparative Example 4 A lithium secondary battery was manufactured in the same manner as in Example 1, except that the pH of the copolymer composition prepared in Preparation Example 3 was adjusted to 5.

[0130] [Evaluation Example 1] Evaluation of crosslinking rate of copolymer composition The crosslinking rates of the first copolymer and the second copolymer of the copolymer compositions used in Examples 1 to 3 and Comparative Examples 1 to 4 were measured / calculated by the gel content measurement method.

[0131] First, the crosslinking rates of the first and second copolymers of the binder composition (copolymer composition) were measured by applying approximately 3 g of the copolymer compositions of Examples 1 to 3 and Comparative Examples 1 to 4 prepared in Preparation Example 3 onto a glass plate washed with MeOH, followed by coating with a glass rod.

[0132] Thereafter, the copolymer composition was crosslinked by vacuum heat treatment at 110° C. for 12 hours or more to prepare a film.

[0133] After drying, the film was peeled off with a razor, and 0.7 g of the film was placed in a 250 ml Erlenmeyer flask. The Erlenmeyer flask was placed in a hood, and 100 ml of distilled water was added, followed by leaving it in a thermostatic water bath at 70°C for 2 hours.

[0134] Thereafter, the Erlenmeyer flask was cooled in a low-temperature water bath for 5 minutes, and the Al dish was weighed and then placed on a hot plate.

[0135] The cooled solution in the Erlenmeyer flask was filtered into the prepared beaker using filter paper. 10 ml of the filtered solution was then pipetted onto an Al dish and dried at 165°C for 30 minutes, after which the mass was measured.

[0136] The crosslinking rate (gel content) was calculated using the following formula 1.

[0137] (Formula 1) Crosslinking rate (gel content) (%) = 100 - (mass of filtered solution after drying / mass of film obtained by crosslinking copolymer composition (0.7 g)) * 500

[0138] The mass of the filtered solution of Equation 1 after drying was calculated by subtracting the mass of the Al dish from the mass of the Al dish containing the filtered solution dried at 165° C. for 30 minutes.

[0139] [Evaluation Example 2] Evaluation of the stability of negative electrode slurry The negative electrode slurry containing the copolymer composition used in Examples 1 to 3 and Comparative Examples 1 to 4 prepared according to Preparation Example 4 was placed in a 30 ml vial and left at room temperature for 7 days, and then it was confirmed whether phase separation occurred, which was different from the initial state.

[0140] If phase separation occurred, the stability was calculated using the following equation 2.

[0141] (Formula 2) Stability of negative electrode slurry (%) = (height of phase-separated layer / height of initial slurry) * 100

[0142] [Evaluation Example 3] Evaluation of binder binding strength To measure the binding strength of the copolymer compositions (binders) used in Examples 1 to 3 and Comparative Examples 1 to 4, the copper current collector of the prepared negative electrode and the negative electrode slurry layer formed on the copper current collector were attached to an acrylic plate, and then peeled off at an angle of 180°, and the binding strength was measured using a UTM.

[0143] [Evaluation Example 4] Battery performance evaluation The lithium secondary batteries produced in Examples 1 to 3 and Comparative Examples 1 to 4 were charged and discharged three times at 25°C, a charge / discharge current density of 0.1C, a charge cut-off voltage of 4.8V, and a discharge cut-off voltage of 2.7V.

[0144] Thereafter, the battery was charged and discharged 500 times with a charge / discharge current density of 1 C, a charge cut-off voltage of 4.8 V, and a discharge cut-off voltage of 2.7 V, and the capacity retention rate was measured.

[0145] All discharges were performed under constant current / constant voltage conditions, and the cut-off current for constant voltage discharge was 0.005C.

[0146] At this time, the capacity retention rate was calculated by the following formula 3.

[0147] (Formula 3) Capacity retention rate (%) = (discharge capacity after 500 cycles / discharge capacity after 3 cycles) * 100

[0148] After the charge / discharge evaluation, the cell was disassembled to check the change in thickness of the negative electrode, and the silicone expansion suppression effect of the copolymer composition binders used in Examples 1 to 3 and Comparative Examples 1 to 4 was compared.

[0149] At this time, the electrode expansion rate was calculated by the following equation 4.

[0150] (Formula 4) Electrode expansion rate (%) = (thickness of negative electrode after 200 cycles - thickness of negative electrode vacuum-dried before assembly) / thickness of negative electrode vacuum-dried before assembly * 100

[0151] [Evaluation Example 5] LiF content evaluation The lithium secondary batteries manufactured in Examples 1 to 3 and Comparative Examples 1 to 4 were disassembled after the initial three cycle discharge, and the LiF content on the negative electrode surface was measured by XPS.

[0152] Disassembly of the cell was performed in an Ar-filled glove box, and the negative electrode was rinsed with acetonitrile and transferred to a glove box connected to the XPS chamber by a vacuum tube, ensuring that the sample was not exposed to air.

[0153] XPS was performed on a Kratos Axis Supra XPS without charge neutralizers, with a 300 μm × 700 μm scan size and a 1.0 eV step size. High resolution scans were performed in the carbon 1s, sulfur 2p, and fluorine 1s regions with a 0.1 eV step size.

[0154] The crosslinking rate, stability of the negative electrode slurry, binding strength of the binder, capacity retention rate, electrode expansion rate, and LiF content measured in Evaluation Examples 1 to 5 are shown in Table 1 below.

[0155] [Table 1]

[0156] As shown in Table 1 above, it was confirmed that the crosslinking rate of the copolymer composition binders used in Examples 1 to 3 and Comparative Examples 1 to 4 decreased as the pH increased (from acidic to basic).

[0157] Furthermore, it was confirmed that at the same pH, the crosslinking rate increased as the content of the crosslinker increased.

[0158] On the other hand, it was confirmed that as the pH increased, the height of the phase-separated layer decreased, and therefore the stability of the slurry increased.

[0159] When the measured value of the slurry stability is 3% or more (i.e., the height of the phase-separated layer is high), as in the case of using the copolymer composition binders of Comparative Examples 3 and 4 with a pH of less than 6, the stability of the slurry is significantly reduced, which may result in a decrease in the processability of negative electrode production and make it difficult to apply to actual processes.

[0160] At the same pH, varying the content of cross-linker did not affect the stability of the slurries.

[0161] As the pH increased and the crosslinking rate decreased, the binding strength of the copolymer composition binder improved.

[0162] This is because as the crosslinking rate increases, the crosslinking rate of the functional group that improves the binding strength also increases, and the effect of the functional group that improves the binding strength in improving the binding strength decreases.

[0163] In particular, when the copolymer composition binders of Comparative Examples 3 and 4 with a pH of less than 6 were used, it was confirmed that the binding strength of the copolymer composition binder significantly decreased as the degree of crosslinking increased.

[0164] Furthermore, at the same pH, the binding strength of the copolymer composition binder decreased as the content of the crosslinking agent increased.

[0165] In particular, when the copolymer composition binder of Comparative Example 1, in which an excess crosslinker was used, was used, the binding strength was significantly reduced compared to when the copolymer composition binder of Example 1 was used under the same pH conditions.

[0166] It was measured that the capacity retention rate after 500 charge / discharge cycles of the batteries of Examples 1 to 3 and Comparative Examples 1 to 4 improved as the pH increased and the crosslinking rate decreased, and that the capacity retention rate decreased at pH 9 or higher.

[0167] In contrast, it was measured that the electrode expansion rate after 500 charge / discharge cycles of the batteries of Examples 1 to 3 and Comparative Examples 1 to 4 increased as the pH increased and the cross-linking rate decreased.

[0168] When the copolymer composition binders of Comparative Examples 3 and 4, which had a pH of less than 6, were used, the electrode expansion rate was lower and the electrode expansion suppression ability was excellent compared to when the copolymer composition binders of Examples 1 and 2, which had a pH of 6 or more, were used. However, as described above, the degree of crosslinking increased, and most of the functional groups that contribute to improving the binding strength were crosslinked, resulting in a decrease in the battery capacity retention rate.

[0169] On the other hand, when the copolymer composition binder of Comparative Example 1, in which an excess crosslinker was used, was used, the electrode expansion rate was superior to that when the copolymer composition binder of Example 1 was used under the same pH conditions, but the capacity retention rate was reduced.

[0170] In addition, when the copolymer composition binder of Comparative Example 2, in which an amount of crosslinker was less than the appropriate amount, was used, the electrode expansion rate significantly increased and the capacity retention rate decreased, resulting in a decrease in the stability and lifespan characteristics of the battery, compared to when the copolymer composition binder of Example 1 was used under the same pH conditions.

[0171] After the formation, the LiF content on the surface of the negative electrode of each of the batteries of Examples 1 to 3 and Comparative Examples 1 to 4 was measured to be 80% or less.

[0172] Even after crosslinking, uncrosslinked carboxyl groups remain in the second copolymer, and these carboxyl groups can form stronger hydrogen bonds with fluorine than other functional groups. This promotes the decomposition of the electrolyte salt LiFSi to form the initial SEI layer, and then suppresses further decomposition of the SEI layer.

[0173] That is, the copolymer composition of the present invention allows the SEI layer to be formed stably at an early stage, thereby improving the performance of the secondary battery.

[0174] As a result, it was confirmed that the copolymer binder composition of the present application, which is a mixture of the first copolymer, the second copolymer, and a certain amount of crosslinker, can have a well-balanced dispersion stability of the negative electrode slurry composition, a negative electrode binding strength, and secondary battery properties (capacity retention rate and electrode expansion rate) within an appropriate range by appropriately adjusting the pH and crosslinking.

[0175] On the other hand, when a copolymer binder composition having a crosslinker content or a pH range outside the range of the present application is used, it has been found that one or more of the dispersion stability of the negative electrode slurry composition, the binding strength of the negative electrode, and the properties of the secondary battery are unsuitable for use in an actual secondary battery.

[0176] The scope of the present invention is indicated by the claims that follow rather than by the above detailed description, and all modifications and variations that fall within the meaning and scope of the claims and their equivalents should be interpreted as being included within the scope of the present invention. [Industrial Applicability]

[0177] The copolymer composition of the present invention can be used in a negative electrode slurry to increase the binding strength with the negative electrode current collector, suppress expansion of the negative electrode, and improve the capacity retention rate per cycle of a secondary battery.

Claims

1. a first copolymer including a vinyl alcohol monomer unit and a vinylamine-based monomer unit; a second copolymer containing vinyl alcohol monomer units and acrylic acid salt-based monomer units; a cross-linking agent; Copolymer composition.

2. The first copolymer additionally includes at least one selected from the group consisting of vinyl acetate monomer units and N-vinylformamide-based monomer units; The second copolymer additionally includes at least one selected from an acrylate-based monomer unit and a vinyl acetate monomer unit. The copolymer composition of claim 1 .

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

4. The N-vinylformamide-based monomer unit is at least one selected from the group consisting of N-vinylformamide and N-isopropenylformamide; The acrylate-based 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. The copolymer composition of claim 2.

5. The first copolymer contains 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-based monomer unit, based on a total content of 100 mol % of the first copolymer; The second copolymer contains 1 mol % to 30 mol % of the vinyl alcohol monomer unit and 50 mol % to 90 mol % of the acrylic acid salt-based monomer unit, based on a total content of 100 mol % of the second copolymer. The copolymer composition of claim 1 .

6. The first copolymer includes a monomer repeating unit represented by the following Chemical Formula 1: The second copolymer includes a monomer repeating unit represented by the following Chemical Formula 2: The copolymer composition of claim 1 . 【Chemical 1】 In the above Chemical Formula 1, 0≦x≦15 mol%, 50≦y≦90 mol%, 0≦m≦30 mol%, and 1≦n≦50 mol%. 【Chemistry 2】 In the above chemical formula 2, R 1 and R 2 are different from each other or the same, and each independently represents hydrogen or a linear or branched hydrocarbon having 1 to 5 carbon atoms; R 3 is a hydroxide (—OH) group, M is an alkali metal; 0≦a≦5 mol%, 50≦b≦90 mol%, 0≦c≦5 mol%, and 1≦d≦30 mol%.

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

8. The cross-linking agent contains two or more aldehyde groups. The copolymer composition of claim 1 .

9. The crosslinking agent is, based on 100% by weight of the total weight of the copolymer composition, Contains 0.7% by weight or more and 2.8% by weight or less, The copolymer composition of claim 1 .

10. The copolymer composition has a pH of 6 or more and 12 or less. The copolymer composition of claim 1 .

11. the cross-linking rate of the first copolymer and the second copolymer in the copolymer composition is 45% or more and 80% or less; The copolymer composition of claim 1 .

12. The copolymer composition according to any one of claims 1 to 11, a negative electrode active material, Anode slurry.

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

14. The negative electrode according to claim 13, Secondary battery.

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