Binder for secondary battery negative electrode, secondary battery negative electrode comprising the binder, and lithium secondary battery comprising the negative electrode

Abstract

The present invention relates to a binder for a secondary battery negative electrode, a negative electrode comprising the same, and a secondary battery comprising the same. In more detail, it relates to a binder for a secondary battery negative electrode and a negative electrode for a secondary battery, which use a copolymer of the present invention as a binder, thereby improving adhesion while having excellent heat resistance and mechanical properties. Furthermore, according to the present invention, expansion and shrinkage of the negative electrode are effectively suppressed, thereby improving the charge / discharge cycle life characteristics and performance of the secondary battery.

Description

Technical Field

[0001] The present invention relates to a binder for a negative electrode of a secondary battery, a negative electrode containing the binder, and a secondary battery including the negative electrode.

[0002] More specifically, the present invention relates to a binder for a secondary battery negative electrode and a secondary battery negative electrode, which have excellent heat resistance and mechanical properties and improved adhesive strength by using the copolymer of the present invention as a binder.

[0003] Furthermore, according to the negative electrode of the present invention, expansion and contraction of the negative electrode are suppressed, thereby improving the charge and discharge life characteristics and performance of the secondary battery. Background Art

[0004] As the use of secondary batteries expands from small electronic devices to electric vehicles and power storage, the demand for electrode materials for secondary batteries with high safety, long life, high energy density and high power characteristics is increasing.

[0005] A lithium secondary battery refers to a battery that contains a non-aqueous electrolyte containing lithium ions in an electrode assembly comprising a positive electrode containing a positive electrode active material that can intercalate / deintercalate lithium ions, a negative electrode containing a negative electrode active material that can intercalate / deintercalate lithium ions, and a microporous separator interposed between the positive and negative electrodes.

[0006] The negative electrodes of lithium secondary batteries often have adverse effects on their performance after charge and discharge. In particular, silicon-based active materials, which expand to approximately 300% of their original size during charge and discharge due to the inherent properties of metals, face limitations in existing systems and significantly degrade charge and discharge performance when using existing binders.

[0007] In addition, as binders for negative electrode active materials, binders such as carboxymethylcellulose (CMC) and styrene butadiene rubber (SBR) can partially eliminate the volume expansion problem caused by the use of silicon-based active materials. However, due to their low binding strength, they become a major cause of deterioration in battery characteristics as charging and discharging proceed.

[0008] Therefore, there is a need for a new type of binder for secondary battery negative electrodes, which, through strong adhesion, can prevent deterioration caused by peeling and detachment of active materials even when the volume of the electrode changes with charge / discharge, and improve the structural stability of the electrode, inhibit the increase in resistance caused by volume expansion, thereby seeking to improve the life and performance of the battery.

[0009] [Prior art literature]

[0010] [Patent Literature]

[0011] (Patent Literature 0001) Korean Patent Laid-Open Publication No. 10-2012-0106041 (2012.09.26) SUMMARY

[0012] PROBLEMS TO BE SOLVED BY THE INVENTION

[0013] The object of the present invention for solving the problems as described above is to solve the above problems by providing a secondary battery negative electrode binder which improves mechanical properties and adhesive properties of a negative electrode active material binder containing a silicon-based negative electrode active material of a secondary battery and solves swelling problems, a negative electrode containing the binder, and a secondary battery including the negative electrode.

[0014] Accordingly, the object of the present invention is to provide a secondary battery negative electrode binder which suppresses swelling and shrinkage of a negative electrode, thereby improving charge-discharge cycle life characteristics and performance of a secondary battery, a negative electrode containing the binder, and a secondary battery including the negative electrode.

[0015] Further, the object of the present invention is to provide a secondary battery negative electrode binder which has excellent heat resistance and mechanical properties while adhesion is also improved, a negative electrode containing the binder, and a secondary battery including the negative electrode.

[0016] Further, the object of the present invention is to provide a secondary battery negative electrode composition in which peeling and detachment of a negative electrode active material are suppressed by improving coating properties and adhesion of a negative electrode binder, thereby improving battery performance.

[0017] TECHNICAL SOLUTION

[0018] One embodiment of the present invention for achieving the above technical problem provides a secondary battery negative electrode binder including a copolymer in which a repeating unit (A) of Chemical Formula 1, a repeating unit (B) of Chemical Formula 2, a repeating unit (C) of Chemical Formula 3, and a repeating unit (D) of Chemical Formula 4 form a main chain.

[0019] [Chemical Formula 1]

[0020]

[0021] [Chemical Formula 2]

[0022]

[0023] [Chemical Formula 3]

[0024]

[0025] [Chemical Formula 4]

[0026]

[0027] In the Chemical Formula 1 to Chemical Formula 4, R1 and R3 can each independently be a substituted or unsubstituted (C1-C10)hydrocarbyl group, R2 and R4 can each independently be hydrogen, a substituted or unsubstituted (C1-C10)hydrocarbyl group, M n+ may be a cation having an oxidation number of n other than a hydrogen ion, n can be an integer of 1 to 3.

[0028] In the binder for a negative electrode of a secondary battery according to one embodiment of the present application, the copolymer can be a random copolymer.

[0029] In the binder for a negative electrode of a secondary battery according to one embodiment of the present application, the (a+b):(c+d) of the copolymer contained therein can be 5:95 to 95:5. (At this time, the a represents the number of moles of the repeating unit (A), the b represents the number of moles of the repeating unit (B), the c represents the number of moles of the repeating unit (C), and the d represents the number of moles of the repeating unit (D).)

[0030] In the binder for a negative electrode of a secondary battery according to one embodiment of the present application, the a:b of the copolymer contained therein can be 5:95 to 60:40. (At this time, the a represents the number of moles of the repeating unit (A), and the b represents the number of moles of the repeating unit (B).)

[0031] In the binder for a negative electrode of a secondary battery according to one embodiment of the present application, the c:d of the copolymer contained therein can be 5:95 to 50:50. (At this time, the c represents the number of moles of the repeating unit (C), and the d represents the number of moles of the repeating unit (D).)

[0032] In the binder for a negative electrode of a secondary battery according to one embodiment of the present application, the saponification rate of the copolymer can satisfy Formula 1.

[0033] [Formula 1]

[0034] 0.45 < saponification rate = (b+d) / (a+b+c+d) < 1.00

[0035] (At this time, the a represents the number of moles of the repeating unit (A), the b represents the number of moles of the repeating unit (B), the c represents the number of moles of the repeating unit (C), and the d represents the number of moles of the repeating unit (D).)

[0036] In the binder for a negative electrode of a secondary battery according to one embodiment of the present application, the weight average molecular weight of the copolymer can be 100000-2000000 g / mole.

[0037] One embodiment of the present application can provide a secondary battery negative electrode composition comprising a negative electrode active material and the above-described secondary battery negative electrode binder.

[0038] In the secondary battery negative electrode composition of one embodiment of the present application, the negative electrode active material can include a silicon-based active material.

[0039] In the secondary battery negative electrode composition of one embodiment of the present application, the negative electrode active material can further include a graphite-based active material.

[0040] In the secondary battery negative electrode composition of one embodiment of the present application, the mass ratio of the silicon-based active material and the graphite-based active material in the negative electrode active material can be 97:3 to 3:97.

[0041] The solid content of the secondary battery negative electrode composition of one embodiment of the present application can be 45 wt% or more.

[0042] One embodiment of the present application can provide a secondary battery negative electrode including a current collector and a negative electrode active material layer provided over the current collector, and the negative electrode active material layer is manufactured using the above-described secondary battery negative electrode composition.

[0043] In the secondary battery negative electrode of one embodiment of the present application, the content of the secondary battery negative electrode binder in the negative electrode active material layer can be 0.5 to 30 wt%.

[0044] In the secondary battery negative electrode of one embodiment of the present application, the thickness of the negative electrode active material layer can be 10 to 120 μιη.

[0045] One embodiment of the present application can provide a secondary battery including the above-described negative electrode, a positive electrode, a separator provided between the positive electrode and the negative electrode, and an electrolyte.

[0046] In the secondary battery of one embodiment of the present application, the expansion rate of the secondary battery can be 65% or less.

[0047] In the secondary battery of one embodiment of the present application, the capacity retention rate of the secondary battery after charge and discharge for 50 cycles can be 80% or more.

[0048] Advantages

[0049] An object of the present application for solving the above-described problems is to provide a secondary battery negative electrode binder which improves mechanical properties and adhesive properties of a secondary battery negative electrode binder, a negative electrode including the binder, and a secondary battery including the negative electrode.

[0050] Therefore, the binder for a negative electrode according to one embodiment of the present invention, the negative electrode including the binder, and the secondary battery including the negative electrode prevent expansion and contraction of the negative electrode, thereby having the effect of improving the charge and discharge life characteristics and performance of the secondary battery.

[0051] A binder for a negative electrode according to an embodiment of the present invention, a negative electrode including the binder, and a secondary battery including the negative electrode have advantages of being excellent in heat resistance and mechanical properties while also having improved binding force.

[0052] The negative electrode binder according to one embodiment of the present invention has an effect of improving secondary battery performance by improving coating properties and adhesiveness to suppress peeling and separation of the negative electrode active material. DETAILED DESCRIPTION

[0053] The present invention will be described in more detail below. However, the following specific embodiments or implementations are merely a reference for describing the present invention in detail, and the present invention is not limited thereto and can be implemented through various implementations.

[0054] Furthermore, unless otherwise defined, all technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

[0055] The terms used in the description of the present invention are only used to effectively describe specific embodiments and are not intended to limit the present invention.

[0056] Furthermore, unless otherwise specifically stated, singular forms used in the specification and claims also include plural forms.

[0057] In addition, unless otherwise specifically stated, when a part is described as “including” or “comprising” a certain component, it means that other components may also be included, rather than excluding other components.

[0058] In order to solve the problems described above, the present invention relates to a novel secondary battery negative electrode binder, a secondary battery negative electrode, and a secondary battery including the negative electrode, which has excellent heat resistance and mechanical properties while improving adhesion and suppressing expansion even when a silicon-based negative electrode active material is used, by using a copolymer of one embodiment of the present invention as a binder.

[0059] The mechanical properties and adhesive properties of the binder for secondary battery negative electrodes are improved, thereby suppressing the peeling and separation of the negative electrode active material, thereby providing a secondary battery negative electrode composition with improved battery performance.

[0060] In addition, it was found that the expansion and contraction of the negative electrode containing the negative electrode binder of one embodiment of the present invention were suppressed, thereby providing a secondary battery negative electrode binder that improves the charge and discharge life characteristics and performance of the secondary battery, a negative electrode containing the binder, and a secondary battery including the negative electrode, thereby completing the present invention.

[0061] The binder for secondary battery negative electrodes according to one embodiment of the present invention includes a copolymer in which the repeating unit (A) of the following Chemical Formula 1, the repeating unit (B) of the following Chemical Formula 2, the repeating unit (C) of the following Chemical Formula 3, and the repeating unit (D) of the following Chemical Formula 4 are combined into a main chain.

[0062] [Chemical Formula 1]

[0063]

[0064] [Chemical Formula 2]

[0065]

[0066] [Chemical Formula 3]

[0067]

[0068] [Chemical Formula 4]

[0069]

[0070] In the chemical formulas 1 to 4, R1 and R3 may each independently be a substituted or unsubstituted (C1-C10) hydrocarbon group, R2 and R4 may each independently be hydrogen, a substituted or unsubstituted (C1-C10) hydrocarbon group, and M n+ It may be a cation having an oxidation number n other than a hydrogen ion, and n may be an integer from 1 to 3.

[0071] The hydrocarbon group may include (C1-C10)alkyl, (C6-C10)aryl, (C6)aryl(C1-C4)alkyl, (C2-C10)alkenyl, (C2-C10)alkynyl, (C3-C7)cycloalkyl, hetero(C5-C10)aryl, hetero(C3-C7)cycloalkyl, and the like.

[0072] More specifically, the R1 and R3 may each independently be a (C1-C5) alkyl group, more preferably selected from a methyl group, an ethyl group and a propyl group.

[0073] The R2 and R4 can each independently be hydrogen or (C1-C5) alkyl, more preferably selected from hydrogen, methyl, ethyl and propyl.

[0074] A specific embodiment of Chemical Formula 1 may be a unit derived from vinyl acetate, a specific embodiment of Chemical Formula 2 may be a unit derived from vinyl alcohol, a specific embodiment of Chemical Formula 3 may be a unit derived from (meth)acrylate, and a specific embodiment of Chemical Formula 4 may be a unit derived from (meth)acrylic acid salts.

[0075] As for the cation of the (meth)acrylate, when n is 1, it can be an alkali metal ion, preferably at least one selected from sodium ion, potassium ion and lithium ion, and can also be an ammonium ion (NH4 + ). In addition, when n is 2, it can be an alkaline earth metal ion, preferably Ca or Mg. When n is 3, it can be a metal such as Al or Ga, and is not limited as long as it is a metal ion known to those skilled in the art.

[0076] When a binder for a secondary battery negative electrode containing the units of Chemical Formulas 1 to 4 in the main chain is included, the binding force between the negative electrode current collector and the negative electrode active material layer or the binding force of the negative electrode active material layer can be improved, thereby suppressing the peeling and separation of the negative electrode active material while improving the strength and flexibility of the negative electrode active material layer.

[0077] Furthermore, in the case of a silicon-based negative electrode active material, it has the effect of suppressing expansion and stabilizing battery performance.

[0078] In the secondary battery negative electrode binder according to one embodiment of the present invention, the copolymer may be a random copolymer. Alternatively, it may be a block copolymer in which repeating unit blocks are linearly linked. As long as the four units form a backbone, either a random copolymer or a block copolymer may be used. However, random copolymers are more suitable for the purposes of the present invention and are therefore preferred.

[0079] Furthermore, the random copolymer containing units of Formulas 1 to 4 has high solubility in water, thereby improving processability when preparing a secondary battery negative electrode composition. Furthermore, the polymer backbone has sufficient tensile strength and flexibility, thereby improving the strength and flexibility of the negative electrode active material layer.

[0080] In the secondary battery negative electrode binder according to one embodiment of the present invention, the copolymer may have a ratio of (a+b):(c+d) of 5:95 to 95:5, specifically 10:90 to 90:10, and more specifically 20:80 to 80:20. Here, a represents the number of moles of the repeating unit (A), b represents the number of moles of the repeating unit (B), c represents the number of moles of the repeating unit (C), and d represents the number of moles of the repeating unit (D).

[0081] The ratio of a:b of the copolymer may be 5:95 to 60:40, specifically 10:90 to 50:50, and more specifically 15:85 to 45:55.

[0082] The c:d ratio of the copolymer may be 5:95 to 50:50, specifically 10:90 to 45:55, and more specifically 15:85 to 40:60.

[0083] Preferably, in the binder for secondary battery negative electrodes according to one embodiment of the present invention, the saponification rate of the copolymer satisfies Formula 1, which is more preferable in achieving the object of the present invention.

[0084] [Formula 1]

[0085] 0.45<Saponification rate=(b+d) / (a+b+c+d)<1.00

[0086] More specifically, the saponification rate may satisfy the range of 0.5 to 0.95, and more preferably may satisfy the range of 0.6 to 0.85.

[0087] A negative electrode binder having a saponification rate within the above range has the characteristics of being superior in tensile strength and adhesive force. When a secondary battery negative electrode composition containing the negative electrode binder is prepared, it has the effects of preventing agglomeration and having excellent coating properties, and is therefore more preferred.

[0088] Furthermore, the binding properties of the negative electrode binder can be improved, thereby achieving an effect of suppressing the negative electrode active material from being peeled off and separated from the current collector.

[0089] Furthermore, the negative electrode binder having the above-mentioned saponification ratio improves the binding force between the negative electrode active material and the current collector due to its excellent coating and adhesive properties, and prevents expansion and contraction of the negative electrode, thereby improving the charge and discharge life characteristics and performance of the secondary battery.

[0090] In the secondary battery negative electrode binder of one embodiment of the present invention, the weight average molecular weight of the copolymer may be 100,000-2,000,000 g / mole. For example, the weight average molecular weight of the copolymer may be 200,000-1,800,000 g / mole. Specifically, the weight average molecular weight of the copolymer may be 400,000-1,600,000 g / mole. More specifically, the weight average molecular weight of the copolymer may be 500,000-1,500,000 g / mole, but is not limited thereto. When the weight average molecular weight satisfies the above range, the adhesive force can be further improved.

[0091] The adhesive can be prepared by various known methods such as emulsion polymerization, suspension polymerization, bulk polymerization or solution polymerization.

[0092] One embodiment of the present invention provides a composition for a secondary battery negative electrode including the binder for a secondary battery negative electrode and a negative electrode active material.

[0093] The negative electrode active material can be one or more selected from graphite-based active materials, platinum, palladium, silicon-based active materials, silver, aluminum, bismuth, tin, zinc, silicon-carbon composite active materials, or combinations thereof. Preferably, it can show a more excellent effect on silicon-based active materials or negative electrode active materials containing silicon-based active materials, and is therefore preferred. However, this is only in terms of suppressing expansibility, and is not limited in terms of excellent bonding strength or electrical characteristics. As a preferred embodiment of the present invention, the negative electrode active material can include a silicon-based active material and a graphite-based active material, and the silicon-based active material and the graphite-based active material can be included in a mass ratio of 97:3 to 3:97.

[0094] The silicon-based active material may include silicon-based materials, for example, Si, SiO x (0 <x<2)、Si-Q合金(所述Q是选自碱金属、碱土金属、13族元素、14族元素、15族元素、16族元素、过渡金属、稀土类元素和它们的组合中的元素,并且不是Si、C)、硅-碳复合物。所述硅-碳复合物可以包含例如碳化硅(SiC)或具有核-壳(core-shell)结构的硅-碳颗粒。所述硅-碳颗粒例如可以通过在石墨核表面上沉积硅层而形成。在一个实施方案中,可以在商用化的石墨颗粒上通过使用诸如硅烷(Silane)系列化合物的硅前体化合物的化学气相沉积(CVD)工艺而涂覆硅层而形成硅-碳颗粒。在一些实施方案中,所述颗粒例如还可以包含非晶碳涂层。

[0095] The graphite-based active material can be a mixture of artificial graphite or artificial graphite and natural graphite. The graphite-based active material can have a particle size of 8-20 μm and can be amorphous, plate-like, flake-like, spherical or fibrous, but the present invention is not limited thereto. In addition, when the graphite-based active material is a mixture of artificial graphite and natural graphite, the content of the artificial graphite is preferably the same as or more than the content of natural graphite. More preferably, artificial graphite and natural graphite can be included in a weight ratio of 95:5 to 50:50, preferably in a weight ratio of 90:10 to 50:50, more preferably in a weight ratio of 90:10 to 60:40. Thus, the adhesion between the current collector and the active material layer is improved, and the high rate charge capacity retention rate and general life characteristics can be improved thereby, so it is preferred.

[0096] The secondary battery negative electrode composition may further include a conductive agent and a solvent.

[0097] The conductive agent is used to impart conductivity to the electrode. Any conductive material that does not cause chemical changes in the resulting battery can be used, and at least one selected from graphite-based conductive agents, carbon black-based conductive agents, graphene, carbon nanotubes, and metal or metal compound-based conductive agents can be used. Examples of graphite-based conductive agents include artificial graphite and natural graphite, examples of carbon black-based conductive agents include acetylene black, ketjen black, thermal black, and channel black, and examples of metal-based or metal compound-based conductive agents include tin, tin oxide, tin phosphate (SnPO4), titanium oxide, potassium titanate, and perovskite materials such as LaSrCoO3 and LaSrMnO3. However, the conductive agent is not limited to the conductive agents listed above.

[0098] The conductive agent is not particularly limited and may be used by appropriately adjusting its content according to the intended use. However, preferably, the conductive agent may be used in an amount of 1 to 30 wt % relative to the negative electrode active material.

[0099] The solvent is used to form the secondary battery negative electrode composition, and a water-soluble solvent such as water can be used. The solvent is preferably used in an amount that gives the composition an appropriate viscosity in consideration of the coating and application properties of the secondary battery negative electrode composition.

[0100] The solid content of the secondary battery negative electrode composition according to one embodiment of the present invention may be 45 wt % or more, with no upper limit, but may be 99.9 wt %, without being limited thereto.

[0101] One embodiment of the present invention may provide a negative electrode for a secondary battery, comprising a current collector and a negative electrode active material layer disposed on the current collector, wherein the negative electrode active material layer is prepared from the above-mentioned composition for a negative electrode for a secondary battery of the present invention.

[0102] The current collector may be made of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof, but is not limited thereto and can use materials known in the art.

[0103] The binder content in the negative electrode active material layer can be 0.5-30% by weight, specifically 1-20% by weight, and more specifically 1-10% by weight. However, the binder content is not particularly limited as long as it does not degrade the performance of the secondary battery. Within this range, the expansion of the negative electrode during charge and discharge can be suppressed, and the detachment of the negative electrode active material can be suppressed, thereby achieving an increased capacity and energy density of the secondary battery. Therefore, this is preferred.

[0104] The thickness of the negative electrode active material layer may be 10-130 μm, specifically 10-120 μm, more preferably 10-90 μm, but is not limited thereto.

[0105] One embodiment of the present invention provides a secondary battery, comprising: the negative electrode described above; a positive electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte.

[0106] The positive electrode includes a current collector and a positive electrode active material layer formed by coating a positive electrode composition including a positive electrode active material on the current collector.

[0107] The current collector may be the above-mentioned negative electrode current collector, or any material known in the technical field, but the present invention is not limited thereto.

[0108] The positive electrode active material layer includes a positive electrode active material and may optionally include a positive electrode binder and a conductive material. The positive electrode active material may be any known positive electrode active material in the art, for example, preferably a composite oxide of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof, but the present invention is not limited thereto.

[0109] The positive electrode binder and the conductive material may be the above-mentioned negative electrode binder and negative electrode conductive material, and may be materials well known in the technical field, but the present invention is not limited thereto.

[0110] For example, the separator can be selected from glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene or a combination thereof, and can be in the form of a non-woven fabric or a woven fabric. For example, polyolefin-based polymer separators such as polyethylene and polypropylene can be mainly used in lithium secondary batteries, and separators coated with a composition containing a ceramic component or a polymer substance can also be used to ensure heat resistance or mechanical strength. A single layer or multilayer structure can be selectively used, and separators known in the art can be used, but the present invention is not limited thereto.

[0111] The electrolyte is an electrolytic solution and contains an organic solvent and a lithium salt.

[0112] The organic solvent functions as a medium in which ions participating in electrochemical reactions of the battery can move, and for example, a carbonate-based, ester-based, ether-based, ketone-based, alcohol-based, or aprotic solvent can be used, alone or in a mixture of two or more, and the mixing ratio when used in a mixture of two or more can be appropriately adjusted depending on the desired battery performance. In addition, an organic solvent known in the art can be used, but the present application is not limited thereto.

[0113] The lithium salt is dissolved in the organic solvent and functions as a lithium ion supply source in the battery to enable the lithium secondary battery to substantially operate, and is a substance that facilitates movement of lithium ions between the positive electrode and the negative electrode. As examples of the lithium salt, LiPF6, LiBF4, LiSbF6, LiAsF6, LiN(SO3C2F5)2, LiN(CF3SO2)2, LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2)(x and y are natural numbers), LiCl, LiI, LiB(C2O4)2, or a combination thereof can be cited, but the present application is not limited thereto.

[0114] The concentration of the lithium salt can be used in the range of 0.1 to 2.0 M. When the concentration of the lithium salt is in the range, the electrolyte has appropriate electrical conductivity and viscosity, and thus can exhibit excellent electrolyte performance, and lithium ions can move effectively.

[0115] Further, as needed, in order to improve charge-discharge characteristics, flame retardant characteristics, and the like, the electrolyte can further include pyridine, triethyl phosphate, triethanolamine, a cyclic ether, ethylenediamine, n- glyme, triamide hexaphosphate, a nitrobenzene derivative, sulfur, a quinonimine dye, an N-substituted oxazolidinone, an N,N-substituted imidazolidine, a glycol dialkyl ether, an ammonium salt, a pyrrole, 2-methoxyethanol, aluminum trichloride, and the like. Depending on the case, in order to impart non-flammability, a halogen-containing solvent such as carbon tetrachloride, ethylene trifluoride, and the like can be further included, and in order to improve high-temperature storage characteristics, fluoro-ethylene carbonate (FEC), propene sultone (PRS), fluoro-propylene carbonate (FPC), and the like can be included.

[0116] The expansion ratio of a secondary battery according to one embodiment of the present invention can be 65% or less, more specifically 60% or less, and more preferably 58% or less. Satisfying the aforementioned expansion ratio allows the secondary battery according to one embodiment of the present invention to prevent degradation caused by separation of active materials even when the volume of the electrode changes with charge and discharge due to strong bonding force, thereby improving the structural stability of the electrode and suppressing the increase in resistance caused by volume expansion. This has the effect of improving the battery's lifespan and performance.

[0117] The capacity retention rate of a secondary battery according to one embodiment of the present invention after 50 cycles of charge and discharge can also be 80% or higher. Preferably, it can be 82% or higher, and more preferably, it can be 85% or higher. The secondary battery according to one embodiment of the present invention can maintain a high capacity retention rate after charge and discharge, and suppress expansion and contraction of the negative electrode, thereby improving the charge and discharge life characteristics and performance of the secondary battery.

[0118] Hereinafter, the present invention will be described in more detail based on Examples and Comparative Examples. However, the following Examples and Comparative Examples are merely examples for describing the present invention in more detail, and the present invention is not limited to the following Examples and Comparative Examples.

[0119] The physical properties of the following Examples of the present invention and Comparative Examples were measured by the following methods.

[0120] [Weight average molecular weight]

[0121] The measurement was performed by GPC (Agilent) using Agilent Mixed C (x2ea) as the GPC column, tetrahydrofuran as the solvent, and polystyrene as the standard substance. The analysis was performed at room temperature at a flow rate of 1 mL / min.

[0122] [Adhesion]

[0123] The adhesive force of the negative electrode was measured using a UTM device. Specifically, the pressed negative electrode surface was adhered to a tape, and then the force when the tape was torn off at an angle of 180° and a speed of 30 mm / min was measured to evaluate the adhesive force.

[0124] [Evaluation of tensile strength]

[0125] Tensile strength was measured according to the method described in ASTM D638. After the prepared adhesive aqueous solution was coated on a substrate and dried, a film having a thickness of 50 μm was formed, cut into a type IV, and thus a test piece was prepared. A universal testing machine (UTM) was used to stretch the sample at a speed of 3 mm / minute, and thus the tensile strength at which the sample was deformed or broken was measured. The measured values are shown in Table 1 below.

[0126] [Coatability of composition for secondary battery anode]

[0127] After the composition for secondary battery anode was prepared, it was coated on a copper foil, and the coatability of the composition for secondary battery anode was evaluated according to the following criteria.

[0128] ◎: Coated in a uniform form as a whole and no agglomerates were observed

[0129] O: Coated in a uniform form as a whole (the number of agglomerates observed per unit area (100 x 100 mm) was less than 5)

[0130] Δ: A small amount of agglomerates were observed (the number of agglomerates observed per unit area (100 x 100 mm) was 5 or more and less than 20)

[0131] X: A large number of agglomerates were observed (the number of agglomerates observed per unit area (100 x 100 mm) was 20 or more)

[0132] [Lifetime characteristics of battery]

[0133] CR2016 coin-type half-batteries were manufactured and electrochemical characteristics were evaluated.

[0134] 1) Initial charge capacity, discharge capacity, and initial charge-discharge efficiency

[0135] The lithium secondary batteries of the manufactured examples were subjected to one charge-discharge between 0.01 and 1.5 V at 0.1 C, and thus the charge capacity, discharge capacity, and initial efficiency were measured, respectively. The results thereof are shown in Table 2 below.

[0136] 2) Lifetime characteristics

[0137] The lithium secondary batteries of the manufactured examples were subjected to one charge-discharge between 0.01 and 1.5 V at 0.5 C, and thus the lifetime characteristics were evaluated. The lifetime characteristics were evaluated according to the discharge capacity retention, which is shown as a percentage (%) with respect to the initial capacity after the charge-discharge was repeated for 50 cycles. The results thereof are shown in Table 1 below.

[0138] [Evaluation of expansion rate of battery]

[0139] The thickness (t1) of the manufactured secondary battery negative electrode was measured, the coin-type half-cell was disassembled after charging to 0.01 V at 0.1 C-rate (C-rate), the thickness (t2) of the charged negative electrode was measured, and thus the expansion rate of the negative electrode was measured. The expansion rate can be represented by Equation 2 below.

[0140] [Equation 2]

[0141] Expansion rate: (t2-t1) / t1 x 100

[0142] The t1 is the thickness of each negative electrode before the experiment, and the t2 is the thickness of the charged negative electrode measured after disassembling the half-cell after charging to 0.01 V at 0.1 C-rate. The expansion rate was calculated according to Equation 2 and is shown in Table 2.

[0143] [Example 1]

[0144] [Synthesis of Copolymer]

[0145] In a round bottom flask, 90 g of water, 0.91 g of sodium dodecylbenzenesulfonate (SDBS), and 0.13 g of sodium bicarbonate were added and replaced with a nitrogen atmosphere. Next, after warming to 65°C, a monomer solution of 30.0 g of vinyl acetate and 13.6 g of methyl acrylate dissolved in 2 g of water was immediately added dropwise over 3 hours, and further stirring was performed at 65°C for 2 hours, thereby completing the reaction. Thereafter, the polymerization solution was added to 450 g of a saturated sodium chloride aqueous solution, thereby causing the copolymer to coagulate, and then the solid was filtered and dried, thereby obtaining 39 g of a vinyl acetate / methyl acrylate copolymer. The obtained polymer was dissolved in THF, and then filtered with a filter, and the weight average molecular weight was 770 kDa as measured by a molecular weight measuring device (GPC, RI detector).

[0146] [Saponification reaction of copolymer]

[0147] In a round bottom flask, 150 ml of water, 150 ml of methanol, 12.5 g of sodium hydroxide, and 30 g of the copolymer prepared in the reaction were added. Next, after warming to 60°C, stirring was performed overnight, thereby completing the saponification reaction. Thereafter, the copolymer saponate solution was added to 1 L of ethanol, the copolymer saponate was precipitated, and then the solid was filtered and dried, thereby obtaining 18 g of a vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer. The composition ratio of the obtained copolymer was confirmed by C-NMR, and the molar ratio of the vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer was 4 / 64 / 2 / 30. 13 C-NMR confirmation, and the molar ratio of the vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer was 4 / 64 / 2 / 30.

[0148] [Preparation of an aqueous solution of a copolymer adhesive]

[0149] A round-bottom flask was charged with 10 g of the saponified copolymer and 90 g of water. The mixture was then heated to 60°C and stirred for 6 hours to prepare an aqueous copolymer adhesive solution. Physical properties of the prepared aqueous adhesive solution, such as tensile strength and adhesive strength, were measured and are listed in Table 1 below.

[0150] [Manufacturing of Secondary Battery Negative Electrode]

[0151] The prepared binder aqueous solution was mixed for 180 minutes using a high-speed mixer at 70°C and 1500 rpm to prepare a 10.0 wt% binder dispersion. 25 g of a CNT series conductive material solution (1 wt%) and 30 g of water were added to 30 g of the binder dispersion and dispersed using a high-speed mixer (Homomixer). 95.75 g of 22% 6 μm SiO x (0 <x<2)+78%的石墨组成的混合活性物质(负极活性物质),并利用行星式(Planetary)搅拌机在45rpm下混合40分钟,从而制备二次电池负极用组合物。二次电池负极用组合物中加入10g的剩余的粘合剂溶液和10g的水,并再次利用行星式搅拌机在45rpm下混合40分钟。如上所述制备的二次电池负极用组合物是负极活性物质、导电材料和粘合剂以95.75:0.25:4.0的重量比混合的混合溶液(固形物含量为49.8重量%)。将制造的二次电池负极用组合物涂布在厚度为10μm的负极集流体上,以使每单位面积的电极负载(mg / cm 2 ) was 5.6 mg and dried in a vacuum oven at 70°C for 10 hours, and then pressed between rollers heated to 50°C at a pressure of 15 MPa to produce a negative electrode with a final thickness (current collector + active material layer) of 50 μm. The physical properties are listed in Table 1 below.

[0152] [Manufacturing of lithium secondary batteries]

[0153] The positive electrode active material NMC (nickel-based composite oxide, nickel-manganese-cobalt, NMC (LiNiMnCoO2)), a carbon black series conductive material, and PVDF powder as a positive electrode binder were mixed in a weight ratio of 92:2:6 in N-methyl-2-pyrrolidone as a solvent to prepare a positive electrode composition. The prepared positive electrode composition was coated on a positive electrode current collector with a thickness of 15 μm to make the electrode load per unit area (mg / cm2 ) was 23.4 mg and dried in a vacuum oven at 120°C for 10 hours, and then pressed between rollers heated to 80°C at a pressure of 15 MPa to produce a positive electrode with a final thickness (current collector + active material layer) of 74.0 μm. The negative electrode, the positive electrode, and the porous polyethylene separator were assembled using a stacking method, and an electrolyte (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 1 / 2 (volume ratio)) and lithium hexafluorophosphate (LiPF6, 1 mol) were injected into the assembled battery to produce a lithium secondary battery. The physical properties are listed in Table 2 below.

[0154] [Example 2]

[0155] A copolymer was synthesized using the same method as in Example 1, and an aqueous binder solution was prepared using the same method, except that 11.2 g of sodium hydroxide was used in the subsequent saponification reaction. The resulting vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer had a molar ratio of 9 / 59 / 2 / 30. Secondary battery negative electrodes and secondary batteries were then manufactured using the same method, and their physical properties are listed in Tables 1 and 2 below.

[0156] [Example 3]

[0157] A copolymer was synthesized using the same method as in Example 1, and an aqueous binder solution was prepared using the same method, except that 8.4 g of sodium hydroxide was used in the subsequent saponification reaction. The resulting vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer had a molar ratio of 19 / 49 / 3 / 29. Secondary battery negative electrodes and secondary batteries were then manufactured using the same method, and their physical properties are listed in Tables 1 and 2 below.

[0158] [Example 4]

[0159] A copolymer was synthesized using the same method as in Example 1, and an aqueous binder solution was prepared using the same method, except that 3.7 g of sodium hydroxide was used in the subsequent saponification reaction. The resulting vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer had a molar ratio of 35 / 33 / 10 / 22. A secondary battery negative electrode and secondary battery were then manufactured using the same method, and their physical properties are listed in Tables 1 and 2 below.

[0160] [Example 5]

[0161] A copolymer was synthesized using the same method as in Example 1, except that the vinyl acetate content was changed to 30.0 g and the methyl acrylate content was changed to 7.3 g. The weight-average molecular weight of the synthesized vinyl acetate / methyl acrylate was now 1000 kDa. The binder aqueous solution was also prepared using the same method, except that 8.4 g of sodium hydroxide was used in the subsequent saponification reaction. The resulting vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer had a molar ratio of 21 / 60 / 2 / 17. Secondary battery negative electrodes and secondary batteries were then manufactured using the same method, and their physical properties are listed in Tables 1 and 2 below.

[0162] [Example 6]

[0163] A copolymer was synthesized using the same method as in Example 1, except that the vinyl acetate content was changed to 20 g and the methyl acrylate content was changed to 26 g. The resulting vinyl acetate / methyl acrylate solution had a weight-average molecular weight of 760 kDa. The binder aqueous solution was also prepared using the same method, except that 8.4 g of sodium hydroxide was used in the subsequent saponification reaction. The resulting vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer had a molar ratio of 13 / 31 / 13 / 43. Secondary battery negative electrodes and secondary batteries were then manufactured using the same method, and their physical properties are listed in Tables 1 and 2 below.

[0164] [Example 7]

[0165] A copolymer was synthesized using the same method as in Example 1, except that the vinyl acetate content was changed to 15.0 g and the methyl acrylate content was changed to 33.0 g. The resulting vinyl acetate / methyl acrylate solution had a weight-average molecular weight of 820 kDa. The binder aqueous solution was also prepared using the same method, except that 8.4 g of sodium hydroxide was used in the subsequent saponification reaction. The resulting vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer had a molar ratio of 13 / 20 / 22 / 45. Secondary battery negative electrodes and secondary batteries were then manufactured using the same method, and their physical properties are listed in Tables 1 and 2 below.

[0166] [Comparative Example 1]

[0167] A copolymer was synthesized using the same method as in Example 1, and an aqueous binder solution was prepared using the same method, except that 13.9 g of sodium hydroxide was used in the subsequent saponification reaction. The resulting vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer had a molar ratio of 0 / 67 / 0 / 33. Secondary battery negative electrodes and secondary batteries were then manufactured using the same method, and their physical properties are listed in Tables 1 and 2 below.

[0168] [Comparative Example 2]

[0169] A copolymer was synthesized by the same method as in Example 1, and an adhesive aqueous solution was prepared by the same method except that 2.1 g of sodium hydroxide was used in the subsequent saponification reaction. At this time, the composition ratio of the synthesized vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer was 43 / 25 / 17 / 15 in terms of molar ratio. Thereafter, a secondary battery negative electrode and a secondary battery were manufactured by the same method, and the physical properties were listed in Tables 1 to 2 below.

[0170] [Comparative Example 3]

[0171] Styrene butadiene rubber and CMC sodium salt (carboxymethyl cellulose sodium salt) were purchased from Sigma Aldrich Co., and an adhesive aqueous solution was prepared by the same method as in Example 1 at a mixing ratio of 1:1. Thereafter, a secondary battery negative electrode and a secondary battery were manufactured by the same method, and the physical properties were listed in Tables 1 to 2 below.

[0172] [Table 1]

[0173]

[0174]

[0175] In the Table 1, it can be seen that the saponification ratio of the examples of the present application satisfies a value of 0.45 to less than 1, and the negative electrode adhesive having the ratio in the range has excellent tensile strength and adhesion, and when prepared as a negative electrode composition, has no caking, excellent coatability, and improved adhesion of the negative electrode adhesive, so that an effect of suppressing peeling and separation of the negative electrode active material can be obtained.

[0176] [Table 2]

[0177]

[0178] In the Table 2, compared to the comparative examples, the negative electrode adhesive of the present application improves the binding force between the negative electrode current collector and the negative electrode active material by excellent coatability and adhesion, thereby showing a significantly low expansion rate, and a capacity retention rate after 50 cycles of charge and discharge is high. In contrast, the expansion rates of Comparative Example 1 having a saponification ratio of 1, Comparative Example 2 having a saponification ratio of 0.4, and Comparative Example 3 using a mixed adhesive of the existing SBR / CMC significantly increase, so that the safety of the battery can be greatly reduced, and also shows a low charge / discharge cycle life characteristic and a performance retention rate.

[0179] Thus, the negative electrode binder of the present application can improve the binding force between the negative electrode current collector and the negative electrode active material, thereby achieving the effect of suppressing the peeling and separation of the negative electrode active material, and effectively suppressing the expansion and shrinkage of the negative electrode, thereby having the effect of improving the charge / discharge cycle life characteristics and performance of the secondary battery.

[0180] As described above, the present application is described through specific contents and limited embodiments, but this is only provided to help more fully understand the present application, and the present application is not limited to the above-described embodiments, and a person having ordinary skill in the art to which the present application pertains can make various modifications and changes through such description.

[0181] Therefore, the idea of the present application should not be limited to the described embodiments, and all the claims of the present application and the entire contents equivalent or having equivalent modifications to the claims are within the scope of the idea of the present application.

Claims

1. A binder for a secondary battery negative electrode, comprising a copolymer whose main chain is formed by repeating units A of the following chemical formula 1, repeating units B of the following chemical formula 2, repeating units C of the following chemical formula 3, and repeating units D of the following chemical formula 4, [Chemical Formula 1] [Chemical Formula 2] [Chemical Formula 3] [Chemical Formula 4] In the chemical formulas 1 to 4, R1 and R3 are each independently a substituted or unsubstituted (C1-C10) hydrocarbon group, R2 and R4 are each independently hydrogen, a substituted or unsubstituted (C1-C10) hydrocarbon group, and M n+ is a cation other than hydrogen ion having an oxidation number n, where n is an integer from 1 to 3, and The saponification rate of the copolymer satisfies Formula 1, [Formula 1] 0.55≤Saponification rate=(b+d) / (a+b+c+d)≤0.94 wherein a represents the molar number of the repeating unit A, b represents the molar number of the repeating unit B, c represents the molar number of the repeating unit C, and d represents the molar number of the repeating unit D. The ratio of (a+b):(c+d) of the copolymer contained therein is 5:95 to 95:5, wherein, a represents the molar number of the repeating unit A, b represents the molar number of the repeating unit B, c represents the molar number of the repeating unit C, and d represents the molar number of the repeating unit D. The a:b ratio of the copolymer is 5:95 to 60:40, wherein a represents the molar number of the repeating unit A, and b represents the molar number of the repeating unit B. The c:d ratio of the copolymer is 5:95 to 50:50, wherein c represents the molar number of the repeating unit C, and d represents the molar number of the repeating unit D. The weight average molecular weight of the copolymer is 100,000-2,000,000 g / mole.

2. The binder for secondary battery negative electrodes according to claim 1, wherein The copolymer is a random copolymer. 3 . A composition for a secondary battery negative electrode, comprising a negative electrode active material and the binder for a secondary battery negative electrode according to claim 1 .

4. The secondary battery negative electrode composition according to claim 3, wherein The negative electrode active material includes a silicon-based active material.

5. The secondary battery negative electrode composition according to claim 4, wherein The negative electrode active material further includes a graphite-based active material.

6. The secondary battery negative electrode composition according to claim 5, wherein The mass ratio of the silicon-based active material to the graphite-based active material is 97:3 to 3:

97.

7. The secondary battery negative electrode composition according to claim 3, wherein The solid content of the secondary battery negative electrode composition is 45% by weight or more. 8 . A negative electrode for a secondary battery, comprising a current collector and a negative electrode active material layer provided on the current collector, wherein the negative electrode active material layer is formed from the composition for a negative electrode for a secondary battery according to claim 3 .

9. The negative electrode for a secondary battery according to claim 8, wherein The content of the binder for secondary battery negative electrodes in the negative electrode active material layer is 0.5-30 wt %.

10. The negative electrode for a secondary battery according to claim 8, wherein The thickness of the negative electrode active material layer is 10-120 μm.

11. A secondary battery comprising: The negative electrode for a secondary battery according to claim 8; the positive electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte.

12. The secondary battery according to claim 11, wherein The expansion rate of the secondary battery is less than 65%.

13. The secondary battery according to claim 11, wherein The capacity retention rate of the secondary battery after 50 cycles of charge and discharge is 80% or more.

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