Binder for negative electrode of secondary battery, negative electrode for secondary battery comprising same, and lithium secondary battery comprising same

Abstract

The present invention relates to a binder for a secondary battery negative electrode, a negative electrode comprising the binder, and a secondary battery comprising the negative electrode. The present invention relates to a binder for a secondary battery negative electrode, and more particularly, to a binder for a secondary battery negative electrode and a secondary battery negative electrode, which use the copolymer of the present invention as a binder, thereby having excellent heat resistance and mechanical properties and improved adhesive force. Further, according to the present invention, expansion and contraction of the negative electrode are effectively suppressed, thereby improving charge / discharge life characteristics and performance of the secondary battery.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 202111429116.3, filed on November 29, 2021, entitled "Binder for Secondary Battery Anode, Anode for Secondary Battery Comprising the Binder, and Lithium Secondary Battery Comprising the Anode", and claims priority to Korean Application No. 10-2020-0164181. TECHNICAL FIELD

[0002] The present application relates to a binder for secondary battery anode, anode for secondary battery comprising the binder, and secondary battery comprising the anode.

[0003] More specifically, it relates to a binder for secondary battery anode and anode for secondary battery, which uses a copolymer of the present application as a binder, thereby improving adhesion while having excellent heat resistance and mechanical properties.

[0004] In addition, according to the anode of the present application, swelling and shrinking of the anode are inhibited, and thus the charge-discharge cycle life characteristics and performance of the secondary battery can be improved. BACKGROUND

[0005] As the use of secondary batteries is expanding from small electronic devices to electric vehicles and power storage, the demand for electrode materials for secondary batteries having high safety, long life, high energy density, and high power characteristics is increasing.

[0006] A lithium secondary battery refers to a battery in which a non-aqueous electrolyte containing lithium ions is contained in an electrode assembly including a cathode containing a cathode active material that can intercalate / deintercalate lithium ions, an anode containing an anode active material that can intercalate / deintercalate lithium ions, and a microporous separator interposed between the cathode and the anode.

[0007] In the anode of a lithium secondary battery, there are many cases where the characteristics shown after charge and discharge are adversely affected. In particular, in the case of an active material that is elongated to about 300% of its own size due to the characteristics of the metal when charge and discharge are performed, like a silicon-based active material, there are limitations in using existing systems, and when using existing binders, there is a problem that the charge and discharge characteristics are significantly deteriorated.

[0008] In addition, as a binder for an anode active material, a binder such as carboxymethylcellose (CMC) and styrene butadiene rubber (SBR) can partially eliminate the problem of volume expansion caused by the use of a silicon-based active material, but due to low adhesion, it becomes a major cause of deterioration of battery characteristics as charge and discharge are performed.

[0009] Therefore, there is a need for a new type of adhesive for the negative electrode of secondary batteries, which, through strong adhesive force, can prevent degradation caused by the peeling and detachment of active materials even when the volume of the electrode changes during charging / discharging, and improve the structural stability of the electrode, suppress the increase in resistance caused by volume expansion, thereby improving the battery's lifespan and performance.

[0010] [Existing technical documents]

[0011] [Patent Literature]

[0012] (Patent Document 0001) Korean Patent Publication No. 10-2012-0106041 (September 26, 2012) Summary of the Invention

[0013] Technical problems to be solved

[0014] The object of the present invention for solving the problems described above is to solve the above-mentioned problems by providing a secondary battery negative electrode adhesive that improves the mechanical and adhesive properties of the adhesive for the negative electrode active material containing the silicon-based negative electrode active material and solves the expansion problem, a negative electrode containing the adhesive, and a secondary battery including the negative electrode.

[0015] Therefore, the object of the present invention is to provide an adhesive for a secondary battery negative electrode that suppresses the expansion and contraction of the negative electrode, thereby improving the charge-discharge life characteristics and performance of the secondary battery, a negative electrode containing the adhesive, and a secondary battery including the negative electrode.

[0016] Furthermore, the present invention aims to provide an adhesive for a secondary battery negative electrode that has excellent heat resistance and mechanical properties while also improving adhesion, a negative electrode containing the adhesive, and a secondary battery including the negative electrode.

[0017] Furthermore, the object of the present invention is to provide a secondary battery negative electrode composition with improved battery performance by suppressing the peeling and detachment of the negative electrode active material by improving the coatability and adhesion of the adhesive for the negative electrode.

[0018] Technical solution

[0019] One embodiment of the present invention for addressing the above-mentioned technical problems provides an adhesive for the negative electrode of a secondary battery, the adhesive comprising a copolymer in which repeating units (A) of chemical formula 1, repeating units (B) of chemical formula 2, repeating units (C) of chemical formula 3, and repeating units (D) of chemical formula 4 form the main chain.

[0020] [Chemical Formula 1]

[0021]

[0022] [Chemical Formula 2]

[0023]

[0024] [Chemical Formula 3]

[0025]

[0026] [Chemical Formula 4]

[0027]

[0028] 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, and n can be an integer of 1 to 3.

[0029] 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.

[0030] 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).)

[0031] 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).)

[0032] 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).)

[0033] 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.

[0034] [Formula 1]

[0035] 0.45 < saponification rate = (b+d) / (a+b+c+d) < 1.00

[0036] (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)).

[0037] The secondary battery negative electrode binder of one embodiment of the present application can have a weight average molecular weight of 100,000 to 2,000,000 g / mol.

[0038] One embodiment of the present application can provide a secondary battery negative electrode composition including a negative electrode active material and the above-described secondary battery negative electrode binder.

[0039] The secondary battery negative electrode composition of one embodiment of the present application can include a silicon-based active material as the negative electrode active material.

[0040] The secondary battery negative electrode composition of one embodiment of the present application can include a graphite-based active material as the negative electrode active material.

[0041] The secondary battery negative electrode composition of one embodiment of the present application can have a mass ratio of the silicon-based active material to the graphite-based active material in the negative electrode active material of 97:3 to 3:97.

[0042] The secondary battery negative electrode composition of one embodiment of the present application can have a solid content of 45 wt% or more.

[0043] 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 formed using the above-described secondary battery negative electrode composition.

[0044] The secondary battery negative electrode of one embodiment of the present application can have a content of the secondary battery negative electrode binder in the negative electrode active material layer of 0.5 to 30 wt%.

[0045] The secondary battery negative electrode of one embodiment of the present application can have a thickness of the negative electrode active material layer of 10 to 120 μm.

[0046] One embodiment of the present application can provide a secondary battery including the above-described secondary battery negative electrode, a positive electrode, a separator provided between the positive electrode and the negative electrode, and an electrolyte.

[0047] The secondary battery of one embodiment of the present application can have an expansion rate of 65% or less.

[0048] The secondary battery of one embodiment of the present application can have a capacity retention rate of 80% or more after charge and discharge for 50 cycles.

[0049] Advantages

[0050] An object of the present application for solving the problems as described above is to provide a binder for a negative electrode of a secondary battery which improves mechanical properties and adhesive properties of a binder for a negative electrode of a secondary battery, a negative electrode comprising the binder, and a secondary battery including the negative electrode.

[0051] Therefore, the binder for a negative electrode, the negative electrode comprising the binder, and the secondary battery including the negative electrode of one embodiment of the present application prevent expansion and shrinkage of the negative electrode, and thus have an effect of improving charge-discharge cycle life characteristics and performance of the secondary battery.

[0052] The binder for a negative electrode, the negative electrode comprising the binder, and the secondary battery including the negative electrode of one embodiment of the present application have an advantage that adhesion is improved while heat resistance and mechanical properties are excellent.

[0053] The binder for a negative electrode of one embodiment of the present application suppresses peeling and separation of a negative electrode active material by improving coatability and adhesion, and thus has an effect of improving performance of a secondary battery. DETAILED DESCRIPTION

[0054] Hereinafter, the present application will be described in more detail. However, the following specific embodiments or examples are merely for detailed description of the present application and the present application is not limited thereto and can be implemented by various embodiments.

[0055] Further, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0056] The terms used in the specification of the present application are used only for effectively describing a specific embodiment and do not limit the present application.

[0057] Further, the singular forms used in the specification and claims include plural forms unless otherwise specified.

[0058] Further, unless otherwise specifically mentioned, the description and claims using singular forms also include plural forms.

[0059] In order to solve the problems as described above, the present application relates to a novel binder for a negative electrode of a secondary battery, a negative electrode for a secondary battery, and a secondary battery including the negative electrode, which have excellent heat resistance and mechanical properties while adhesion is improved and in which expansion is suppressed in the case of using a silicon-based negative electrode active material, by using the copolymer of one embodiment of the present application as a binder.

[0060] The mechanical and adhesive properties of the binder for a secondary battery negative electrode are improved, thereby suppressing peeling and separation of the negative electrode active material, and thus a composition for a secondary battery negative electrode with improved battery performance can be provided.

[0061] Further, it was found that the expansion and contraction of the negative electrode including the binder for a negative electrode according to one embodiment of the present application is suppressed, and thus a binder for a secondary battery negative electrode, a negative electrode including the binder, and a secondary battery including the negative electrode, which improve the charge / discharge cycle life characteristics and performance of the secondary battery, can be provided, and thus the present application was completed.

[0062] The binder for a secondary battery negative electrode according to one embodiment of the present application includes 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 are combined into a main chain.

[0063] [Chemical Formula 1]

[0064]

[0065] [Chemical Formula 2]

[0066]

[0067] [Chemical Formula 3]

[0068]

[0069] [Chemical Formula 4]

[0070]

[0071] In the Chemical Formula 1 to Chemical Formula 4, R1 and R3 can each independently be a substituted or unsubstituted (C1-C10)hydrocarbyl, R2 and R4 can each independently be hydrogen, a substituted or unsubstituted (C1-C10)hydrocarbyl, M n+ may be a cation having an oxidation number of n other than a hydrogen ion, and n can be an integer of 1 to 3.

[0072] The hydrocarbyl group can include a (C1-C10)alkyl group, a (C6-C10)aryl group, a (C6)aryl(C1-C4)alkyl group, a (C2-C10)alkenyl group, a (C2-C10)alkynyl group, a (C3-C7)cycloalkyl group, a hetero(C5-C10)aryl group, a hetero(C3-C7)cycloalkyl group, and the like.

[0073] More specifically, R1 and R3 can each independently be a (C1-C5)alkyl group, and more preferably can be selected from a methyl group, an ethyl group, and a propyl group.

[0074] The R2 and R4 can each independently be hydrogen or (C1-C5)alkyl, more preferably can be selected from hydrogen, methyl, ethyl and propyl.

[0075] One specific embodiment of the Chemical Formula 1 can be a unit derived from vinyl acetate, one specific embodiment of the Chemical Formula 2 can be a unit derived from vinyl alcohol, one specific embodiment of the Chemical Formula 3 can be a unit derived from (meth)acrylate, and one specific embodiment of the Chemical Formula 4 can be a unit derived from (meth)acrylic acid salts.

[0076] As for the cation of the above (meth)acrylic acid salts, when n is 1, it can be an alkali metal ion, preferably can be at least one or more selected from the group consisting of 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 can be 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.

[0077] When the secondary battery negative electrode binder comprising a unit comprising the Chemical Formula 1 to the Chemical Formula 4 in the main chain is included, the adhesion between the negative electrode current collector and the negative electrode active material layer or the adhesion of the negative electrode active material layer can be improved, thereby suppressing the peeling and detachment of the negative electrode active material while improving the strength and flexibility of the negative electrode active material layer.

[0078] In addition, it also has the effect of suppressing expansion in the case of a silicon-based negative electrode active material, and can achieve stabilization of the battery performance.

[0079] The secondary battery negative electrode binder of one embodiment of the present application, the copolymer can be a random copolymer. And can also be a block copolymer in which the repeating unit blocks are linearly connected, but as long as the four units form the backbone, it can be a random copolymer or a block copolymer, but a random copolymer is more in line with the purpose of the present application, and is therefore more preferable.

[0080] And, the random copolymer satisfying the units of the Chemical Formula 1 to the Chemical Formula 4 has a high solubility in water, thereby improving the processability when preparing a secondary battery negative electrode composition. In addition, the polymer backbone has sufficient tensile strength and flexibility, and thus the strength and flexibility of the negative electrode active material layer can be improved.

[0081] The (a+b):(c+d) of the copolymer contained in the secondary battery negative electrode binder of one embodiment of the present application can be 5:95 to 95:5, specifically, 10:90 to 90:10, and more specifically, 20:80 to 80:20. 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).

[0082] The a:b of the copolymer contained can be 5:95 to 60:40, specifically, 10:90 to 50:50, and more specifically, 15:85 to 45:55.

[0083] The c:d of the copolymer contained can be 5:95 to 50:50, specifically, 10:90 to 45:55, and more specifically, 15:85 to 40:60.

[0084] Preferably, in the secondary battery negative electrode binder of one embodiment of the present application, the saponification rate of the copolymer satisfies Formula 1, which is more preferable in achieving the object of the present application.

[0085] [Formula 1]

[0086] 0.45 < saponification rate = (b+d) / (a+b+c+d) < 1.00

[0087] More specifically, the saponification rate can satisfy the range of 0.5 to 0.95, and more preferably, the range of 0.6 to 0.85.

[0088] The negative electrode binder satisfying the range of the saponification rate described above has more excellent characteristics of tensile strength and adhesion, and when a secondary battery negative electrode composition including the negative electrode binder is prepared, has the effect of not blocking and excellent coatability, and thus is more preferable.

[0089] Further, the adhesion of the negative electrode binder can be improved, and thus the effect of suppressing peeling and separation of the negative electrode active material from the current collector can be obtained.

[0090] Further, the negative electrode binder satisfying the saponification rate described above improves the adhesion between the negative electrode active material and the current collector by excellent coatability and adhesion, and prevents expansion and shrinkage of the negative electrode, and thus has the effect of improving the charge-discharge cycle life characteristics and performance of the secondary battery.

[0091] In the binder for a secondary battery negative electrode of one embodiment of the present application, the weight average molecular weight of the copolymer can be 100,000 to 2,000,000 g / mol. For example, the weight average molecular weight of the copolymer can be 200,000 to 1,800,000 g / mol. Specifically, the weight average molecular weight of the copolymer can be 400,000 to 1,600,000 g / mol. More specifically, the weight average molecular weight of the copolymer can be 500,000 to 1,500,000 g / mol, but there is no particular limitation thereto. The range of the weight average molecular weight satisfying the above range can further improve adhesion.

[0092] The binder can be prepared by various methods known as emulsion polymerization, suspension polymerization, bulk polymerization, or solution polymerization.

[0093] One embodiment of the present application provides a composition for a secondary battery negative electrode including the above binder for a secondary battery negative electrode and a negative electrode active material.

[0094] The negative electrode active material can be one or two or more selected from a graphite-based active material, platinum, palladium, a silicon-based active material, silver, aluminum, bismuth, tin, zinc, a silicon-carbon composite active material, or a combination thereof, and preferably, a silicon-based active material or a negative electrode active material including a silicon-based active material can show more excellent effects, and thus is preferred, but this is only in terms of suppression of expansion, and there is no limitation in terms of the excellence of binding force or electrical characteristics. As a preferred embodiment of the present application, 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 at a mass ratio of 97:3 to 3:97.

[0095] The silicon-based active material can include a silicon-based substance, for example, can include Si, SiO x (0 < x < 2), a Si-Q alloy (the Q is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof, and is not Si, C), a silicon-carbon composite. The silicon-carbon composite can include, for example, silicon carbide (SiC) or silicon-carbon particles having a core-shell structure. The silicon-carbon particles can be formed, for example, by depositing a silicon layer on the surface of a graphite core. In one embodiment, the silicon-carbon particles can be formed by coating a silicon layer on commercially available graphite particles by a chemical vapor deposition (CVD) process using a silicon precursor compound such as a Silane series compound. In some embodiments, the particles can further include, for example, an amorphous carbon coating layer.

[0096] The graphite-based active material can be artificial graphite or a mixture of artificial graphite and natural graphite. The graphite-based active material can have a particle size of 8 to 20 μm, and can be amorphous, plate-like, flake-like, spherical, or fibrous, but the present application 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 can be preferably the same as or more than that of the natural graphite. More preferably, artificial graphite and natural graphite can be contained in a weight ratio of 95:5 to 50:50, preferably in a weight ratio of 90:10 to 50:50, and more preferably in a weight ratio of 90:10 to 60:40. Thereby, 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 thereby improved, and thus are preferred.

[0097] The secondary battery negative electrode composition can further include a conductive agent and a solvent.

[0098] The conductive agent is used to impart conductivity to the electrode, and any conductive agent can be used as long as it is a conductive material that does not cause chemical changes in the battery constructed thereby, and at least one selected from a graphite-based conductive agent, a carbon black-based conductive agent, graphene, a carbon nanotube, a metal or metal compound-based conductive agent can be used. Examples of the graphite-based conductive agent include artificial graphite, natural graphite, etc., examples of the carbon black-based conductive agent include acetylene black, ketjen black, thermal black, channel black, etc., and examples of the metal or metal compound-based conductive agent include tin, tin oxide, tin phosphate (SnP04), titanium oxide, potassium titanate, perovskite materials such as LaSrCo03, LaSrMn03, etc. However, the conductive agent is not limited to those listed above.

[0099] The conductive agent is not particularly limited, and its content can be appropriately adjusted and used according to the purpose of use, but preferably, 1 to 30% by weight of the conductive agent can be used with respect to the negative electrode active material.

[0100] The solvent is a solvent 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 imparts an appropriate viscosity to the composition, in consideration of the coatability and the coating properties of the secondary battery negative electrode composition.

[0101] The solid content of the secondary battery negative electrode composition of one embodiment of the present application can be 45% by weight or more, and the upper limit is not limited, but can be 99.9% by weight, and is not limited thereto.

[0102] One embodiment of the present invention provides a negative electrode for a secondary battery, the negative electrode comprising a current collector and a negative electrode active material layer disposed on the current collector, the negative electrode active material layer being prepared from the above-described composition for a secondary battery negative electrode of the present invention.

[0103] The current collector may be selected from copper foil, nickel foil, stainless steel foil, titanium foil, foam nickel, foam copper, polymer substrate coated with conductive metal, and combinations thereof, but is not limited thereto, and may be any material known in the art.

[0104] 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 within a range that does not reduce the performance of the secondary battery. Within the above content range, the expansion of the negative electrode during charging and discharging can be suppressed, the detachment of the negative electrode active material can be suppressed, and the capacity and energy density of the secondary battery can be improved, thus making it a preferred option.

[0105] The thickness of the negative electrode active material layer can be 10-130 μm, specifically 10-120 μm, more preferably 10-90 μm, but is not limited thereto.

[0106] One embodiment of the present invention provides a secondary battery comprising: a negative electrode as described above; a positive electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte. The negative electrode is as described above.

[0107] The positive electrode includes a current collector and a positive electrode active material layer formed by coating the current collector with a positive electrode composition containing a positive electrode active material.

[0108] The current collector can be the aforementioned negative electrode current collector, or it can be a material known in the art, but the present invention is not limited thereto.

[0109] The positive electrode active material layer comprises a positive electrode active material, and may also optionally comprise a positive electrode binder and a conductive material. The positive electrode active material may be any positive electrode active material known in the art, for example, preferably a composite oxide of a metal selected from cobalt, manganese, nickel, and combinations thereof with lithium, but the invention is not limited thereto.

[0110] The positive electrode adhesive and conductive material can be the same as the negative electrode adhesive and conductive material described above, or can be any substances known in the art, but the present invention is not limited thereto.

[0111] For example, the separator can be selected from glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene, or combinations thereof, and can be in the form of nonwoven or 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 compositions containing ceramic components or polymeric substances can also be used to ensure heat resistance or mechanical strength. Single-layer or multi-layer structures can be selectively used, and separators known in the art can be used, but the present invention is not limited thereto.

[0112] The electrolyte is an electrolyte solution and contains an organic solvent and a lithium salt.

[0113] The organic solvent acts as a medium that allows ions participating in the electrochemical reaction of the battery to move. For example, carbonate-based, ester-based, ether-based, ketone-based, alcohol-based, or aprotic solvents can be used. The organic solvent can be used alone or in mixtures of two or more. When two or more are mixed, the mixing ratio can be appropriately adjusted according to the desired battery performance. In addition, organic solvents known in the art can be used, but the present invention is not limited thereto.

[0114] The lithium salt is dissolved in an organic solvent and acts as a lithium-ion supply source in the battery, enabling the lithium secondary battery to operate essentially. It also promotes the movement of lithium ions between the positive and negative electrodes. Examples of such lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiN(SO3C2F5)2, LiN(CF3SO2)2, LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, and LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2 (where x and y are natural numbers), LiCl, LiI, LiB(C2O4)2 or combinations thereof, but the present invention is not limited thereto.

[0115] The concentration of the lithium salt can be used in the range of 0.1-2.0 M. When the concentration of the lithium salt is within this range, the electrolyte has suitable conductivity and viscosity, thus exhibiting excellent electrolyte performance, and lithium ions can move effectively.

[0116] Furthermore, as needed, to improve charge / discharge characteristics, flame retardant properties, etc., the electrolyte may further contain pyridine, triethyl phosphate, triethanolamine, cyclic ethers, ethylenediamine, n-ethylene glycol dimethyl ether (glyme), triammonium hexaphosphate, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. Depending on the circumstances, to impart non-flammability, it may further contain halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride. To improve high-temperature storage characteristics, it may also contain fluoroethylene carbonate (FEC), propene sultone (PRS), fluoropropylene carbonate (FPC), etc.

[0117] The expansion rate of the secondary battery in one embodiment of the present invention can be 65% or less, more specifically 60% or less, and more preferably 58% or less. The expansion rate of the secondary battery in one embodiment of the present invention satisfies the above-mentioned value, thereby preventing deterioration caused by the separation of active materials even when the volume of the electrodes changes during charging / discharging due to strong adhesion, improving the structural stability of the electrodes, and suppressing the increase in resistance caused by volume expansion. Therefore, it has the effect of improving battery life and performance.

[0118] The secondary battery of one embodiment of the present invention can maintain a capacity retention rate of 80% or more after 50 charge-discharge cycles. Preferably, it can maintain a capacity retention rate of 82% or more, and more preferably, it can maintain a capacity retention rate of 85% or more. The secondary battery of one embodiment of the present invention can maintain a high capacity retention rate after charge-discharge and suppress the expansion and contraction of the negative electrode, thereby improving the charge-discharge life characteristics and performance of the secondary battery.

[0119] The present invention will now be described in more detail based on embodiments and comparative examples. However, the embodiments and comparative examples described below are merely examples for illustrating the present invention in more detail, and the present invention is not limited to the embodiments and comparative examples described below.

[0120] The physical properties of the following embodiments and comparative examples of the present invention were measured using the following methods.

[0121] [Weight-average molecular weight]

[0122] Measurements were performed using GPC (Agilent Technologies) with a Mixed C(x2ea) column from Agilent Technologies. Tetrahydrofuran was used as the solvent, and polystyrene was used as the standard. The analysis was conducted at a flow rate of 1 mL / min at room temperature.

[0123] [Adhesive strength]

[0124] For the negative electrode, the adhesive force was measured using a UTM device. Specifically, the pressed negative electrode surface was adhered to an adhesive tape, and the force required to tear it off at a 180° angle and a speed of 30 mm / min was measured to evaluate the adhesive force.

[0125] [Evaluation of tensile strength]

[0126] Tensile strength was measured according to the method described in ASTM D638. The prepared adhesive aqueous solution was applied to the substrate and dried to form a 50 μm thick film, which was then cut into type IV specimens. The tensile strength at deformation or fracture was measured using a universal testing machine (UTM) at a speed of 3 mm / min. The measured values ​​are shown in Table 1 below.

[0127] [Coating Properties of the Composition for the Negative Electrode of Secondary Batteries]

[0128] The composition for the negative electrode of a secondary battery was prepared and then coated onto copper foil. The coatability of the composition for the negative electrode of a secondary battery was evaluated according to the following criteria.

[0129] ◎: The coating is uniform and no aggregates were observed.

[0130] ○: The coating is uniform in shape (the number of aggregates observed per unit area (100×100mm) is less than 5).

[0131] △: A small number of agglomerates were observed (the number of agglomerates observed per unit area (100×100mm) was more than 5 but less than 20).

[0132] X: Multiple agglomerates were observed (more than 20 agglomerates were observed per unit area (100×100mm)).

[0133] [Battery lifespan characteristics]

[0134] Fabricate CR2016 coin-type half-cells and evaluate their electrochemical properties.

[0135] 1) Initial charging capacity, discharge capacity, and initial charge / discharge efficiency

[0136] The lithium secondary battery of the manufactured embodiment was subjected to a single charge-discharge cycle at 0.1C between 0.01 and 1.5V to measure the charge capacity, discharge capacity, and initial efficiency, respectively. The results are shown in Table 2 below.

[0137] 2) Lifetime characteristics

[0138] The lithium secondary batteries of the manufactured examples were subjected to 0.5C (one charge-discharge cycle) conditions between 0.01 and 1.5V to evaluate lifetime characteristics. Lifetime characteristics were evaluated based on discharge capacity retention, which is expressed as a percentage (%) of the capacity after 50 repeated charge-discharge cycles relative to the initial capacity. The results are shown in Table 1 below.

[0139] [Evaluation of battery expansion rate]

[0140] The thickness (t1) of the negative electrode of the manufactured secondary battery is measured. After charging the coin-shaped half-cell to 0.01V at a rate of 0.1C, the half-cell is disassembled, and the thickness (t2) of the negative electrode after charging is measured, thereby measuring the expansion rate of the negative electrode. The expansion rate can be expressed by the following formula 2.

[0141] [Equation 2]

[0142] Expansion rate: (t2-t1) / t1×100

[0143] t1 is the thickness of each negative electrode before the experiment, and t2 is the thickness of the negative electrode after charging (measured by disassembling the half-cell after charging the coin-shaped half-cell to 0.01V at a rate of 0.1C). The expansion rate is calculated according to Equation 2 and is shown in Table 2.

[0144] [Example 1]

[0145] [Synthesis of copolymers]

[0146] 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 the reaction was carried out under a nitrogen atmosphere. Next, the temperature was raised to 65 °C, and an aqueous solution of 0.05 g of potassium persulfate dissolved in 2 g of water was added. Immediately afterwards, 30.0 g of vinyl acetate and 13.6 g of methyl acrylate monomers were added dropwise over 3 hours, and the mixture was stirred further at 65 °C for 2 hours to complete the reaction. Then, the polymerization solution was added to 450 g of saturated sodium chloride aqueous solution to cause the copolymer to coagulate. The solid was then filtered and dried to obtain 39 g of vinyl acetate / methyl acrylate copolymer. The obtained polymer was dissolved in THF and filtered. The weight-average molecular weight was determined to be 770 kDa using a molecular weight measurement device (GPC, RI detector).

[0147] [Saponification reaction of copolymers]

[0148] Add 150 ml of water, 150 ml of methanol, 12.5 g of sodium hydroxide, and 30 g of the copolymer prepared in the reaction into a round-bottom flask. Then, heat up to 60 °C and stir overnight to complete the saponification reaction. After that, add the copolymer saponification solution into 1 L of ethanol to precipitate the copolymer saponification product, and then filter and dry the solid to obtain 18 g of vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer. The composition ratio of the obtained copolymer is confirmed by 13 C-NMR, and the molar ratio of vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer is 4 / 64 / 2 / 30.

[0149] [Preparation of Aqueous Solution of Copolymer Adhesive]

[0150] Add 10 g of the copolymer undergoing saponification reaction and 90 g of water into a round-bottom flask. Then, heat up to 60 °C and stir for 6 hours to prepare an aqueous solution of copolymer adhesive. The physical properties such as tensile strength and adhesion measured using the prepared aqueous solution of adhesive are listed in Table 1 below.

[0151] [Manufacture of Negative Electrode of Secondary Battery]

[0152] Mix the prepared aqueous solution of adhesive for 180 minutes at 70 °C and 1500 rpm using a high-speed mixer to prepare a dispersion liquid with 10.0 wt% of adhesive dispersed. Add 25 g of CNT series conductive material solution (1 wt%) and 30 g of water into 30 g of the adhesive dispersion liquid, and disperse using a high-speed mixer (Homomixer). Add 95.75 g of a mixed active material (negative electrode active material) composed of 22% of 6 μm SiO x (0 < x < 2) + 78% of graphite into the dispersed solution, and mix using a planetary mixer at 45 rpm for 40 minutes to prepare a composition for negative electrode of secondary battery. Add 10 g of the remaining adhesive solution and 10 g of water into the composition for negative electrode of secondary battery, and mix again using a planetary mixer at 45 rpm for 40 minutes. The composition for negative electrode of secondary battery prepared as above is a mixed solution (solid content is 49.8 wt%) in which the negative electrode active material, conductive material, and adhesive are mixed at a weight ratio of 95.75:0.25:4.0. Coat the manufactured composition for negative electrode of secondary battery on a negative electrode current collector with a thickness of 10 μm so that the electrode loading per unit area (mg / cm 2 ) is 5.6 mg, and dry in a vacuum oven at 70 °C for 10 hours, and then press between rollers heated to 50 °C at a pressure of 15 MPa to manufacture a negative electrode with a final thickness (current collector + active material layer) of 50 μm. The physical properties are listed in Table below.

[0153] [Manufacturing of Lithium-ion Secondary Batteries]

[0154] A positive electrode composition was prepared by mixing NMC (nickel-based composite oxide, nickel-manganese-cobalt, NMC(LiNiMnCoO2)), a carbon black-based conductive material, and PVDF powder as a binder in a weight ratio of 92:2:6 in N-methyl-2-pyrrolidone as a solvent. The prepared positive electrode composition was then coated onto a 15 μm thick positive electrode current collector to achieve an electrode loading per unit area (mg / cm²). 2 The concentration of the anode material was 23.4 mg, and it was dried in a vacuum oven at 120°C for 10 hours. Then, it was 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 fabricated negative electrode, positive electrode, and porous polyethylene separator were assembled using a stacking method. 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 manufacture a lithium secondary battery. The physical properties are listed in Table 2 below.

[0155] [Example 2]

[0156] The copolymer was synthesized using the same method as in Example 1, and the binder aqueous solution was prepared using the same method, except that 11.2 g of sodium hydroxide was used in the subsequent saponification reaction. At this point, the composition ratio of the synthesized vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer was 9 / 59 / 2 / 30 in molar ratio. Subsequently, the secondary battery negative electrode and the secondary battery were manufactured using the same method, and their physical properties are listed in Tables 1 and 2 below.

[0157] [Example 3]

[0158] The copolymer was synthesized using the same method as in Example 1, and the binder aqueous solution was prepared using the same method, except that 8.4 g of sodium hydroxide was used in the subsequent saponification reaction. At this point, the composition ratio of the synthesized vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer was 19 / 49 / 3 / 29 in molar ratio. Subsequently, the secondary battery negative electrode and the secondary battery were manufactured using the same method, and their physical properties are listed in Tables 1 and 2 below.

[0159] [Example 4]

[0160] The copolymer was synthesized using the same method as in Example 1, and the binder aqueous solution was prepared using the same method, except that 3.7 g of sodium hydroxide was used in the subsequent saponification reaction. At this point, the composition ratio of the synthesized vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer was 35 / 33 / 10 / 22 in molar ratio. Subsequently, the secondary battery negative electrode and the secondary battery were manufactured using the same method, and their physical properties are listed in Tables 1 and 2 below.

[0161] [Example 5]

[0162] Except for changing the vinyl acetate content to 30.0 g and the methyl acrylate content to 7.3 g in Example 1, the copolymer was synthesized using the same method. This time, the synthesized vinyl acetate / methyl acrylate copolymer had a weight-average molecular weight of 1000 kDa. The binder aqueous solution was prepared using the same method, except that 8.4 g of sodium hydroxide was used in the subsequent saponification reaction. The composition ratio of the synthesized vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer was 21 / 60 / 2 / 17 in molar ratio. Subsequently, the secondary battery negative electrode and the secondary battery were manufactured using the same method, and their physical properties are listed in Tables 1 and 2 below.

[0163] [Example 6]

[0164] Except that the content of vinyl acetate was changed to 20g and the content of methyl acrylate was changed to 26g in Example 1, the copolymer was synthesized using the same method. This time, the weight-average molecular weight of the synthesized vinyl acetate / methyl acrylate was 760kDa. And except that 8.4g of sodium hydroxide was used in the subsequent saponification reaction, the binder aqueous solution was prepared using the same method. At this time, the composition ratio of the synthesized vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer, in molar ratio, was 13 / 31 / 13 / 43. Subsequently, the secondary battery negative electrode and the secondary battery were manufactured using the same method, and the physical properties are listed in Tables 1 and 2 below.

[0165] [Example 7]

[0166] Except for changing the vinyl acetate content to 15.0 g and the methyl acrylate content to 33.0 g in Example 1, the copolymer was synthesized using the same method. This time, the synthesized vinyl acetate / methyl acrylate copolymer had a weight-average molecular weight of 820 kDa. The binder aqueous solution was prepared using the same method, except that 8.4 g of sodium hydroxide was used in the subsequent saponification reaction. The composition ratio of the synthesized vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer was 13 / 20 / 22 / 45 in molar ratio. Subsequently, the secondary battery negative electrode and the secondary battery were manufactured using the same method, and their physical properties are listed in Tables 1 and 2 below.

[0167] [Comparative Example 1]

[0168] The copolymer was synthesized using the same method as in Example 1, and the binder aqueous solution was prepared using the same method, except that 13.9 g of sodium hydroxide was used in the subsequent saponification reaction. At this point, the composition ratio of the synthesized vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer was 0 / 67 / 0 / 33 in molar ratio. Subsequently, the secondary battery negative electrode and the secondary battery were manufactured using the same method, and their physical properties are listed in Tables 1 and 2 below.

[0169] [Comparative Example 2]

[0170] The copolymer was synthesized using the same method as in Example 1, and the binder aqueous solution was prepared using the same method, except that 2.1 g of sodium hydroxide was used in the subsequent saponification reaction. At this point, the composition ratio of the synthesized vinyl acetate / vinyl alcohol / methyl acrylate / sodium acrylate copolymer was 43 / 25 / 17 / 15 in molar ratio. Subsequently, the secondary battery negative electrode and the secondary battery were manufactured using the same method, and their physical properties are listed in Tables 1 and 2 below.

[0171] [Comparative Example 3]

[0172] Styrene-butadiene rubber and sodium CMC (carboxymethyl cellulose sodium salt) were purchased from Sigma-Aldrich and mixed in a 1:1 ratio to prepare an aqueous adhesive solution using the same method as in Example 1. Subsequently, a secondary battery negative electrode and a secondary battery were manufactured using the same method, and their physical properties are listed in Tables 1 and 2 below.

[0173] [Table 1]

[0174]

[0175] As shown in Table 1, the saponification rate of the embodiments of the present invention satisfies a value of 0.45 to less than 1. The adhesive for the negative electrode within this range has excellent tensile strength and adhesion. When prepared as a composition for the negative electrode, it does not clump, has excellent coatability, and improves the adhesion of the adhesive for the negative electrode, thereby achieving the effect of inhibiting the peeling and detachment of the negative electrode active material.

[0176] [Table 2]

[0177]

[0178] In Table 2, compared to the comparative examples, the adhesive for the negative electrode of the present invention improves the bonding force between the negative electrode current collector and the negative electrode active material through excellent coatability and adhesion, thereby exhibiting a significantly low expansion rate and high capacity retention after 50 charge-discharge cycles. In contrast, the expansion rates of Comparative Example 1 (with a saponification rate of 1), Comparative Example 2 (with a saponification rate of 0.4), and Comparative Example 3 (using a conventional SBR / CMC hybrid adhesive) are significantly increased, which may substantially reduce battery safety, and also exhibit low charge-discharge life characteristics and performance retention.

[0179] Therefore, the adhesive for the negative electrode of the present invention can improve the bonding force between the negative electrode current collector and the negative electrode active material, thereby achieving the effect of inhibiting the peeling and detachment of the negative electrode active material, and effectively inhibiting the expansion and contraction of the negative electrode, thus improving the charge and discharge life characteristics and performance of the secondary battery.

[0180] As described above, the present invention has been described with specific content and limited embodiments, but this is only provided to help to understand the present invention more fully. The present invention is not limited to the above embodiments, and those skilled in the art can make various modifications and variations based on such description.

[0181] Therefore, the concept of this invention should not be limited to the described embodiments, and all contents of the claims of this invention and those equivalent to or having equivalent variations thereof fall within the scope of the concept of this invention.

Claims

1. An adhesive for the negative electrode of a secondary battery, comprising a copolymer comprising repeating unit A of chemical formula 1, repeating unit B of chemical formula 2, repeating unit C of chemical formula 3, and repeating unit D of chemical formula 4 forming the main chain. [Chemical Formula 1] [Chemical Formula 2] [Chemical Formula 3] [Chemical Formula 4] In 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 a hydrogen, substituted or unsubstituted (C1-C10) hydrocarbon group, and M n+ It is a cation with an oxidation number of n other than hydrogen ions, where n is an integer from 1 to 3, and The saponification rate of the copolymer satisfies Equation 1. [Formula 1] 0.45 < Saponification rate = (b+d) / (a+b+c+d) < 1.00 Where a represents the number of moles of repeating unit A, b represents the number of moles of repeating unit B, c represents the number of moles of repeating unit C, and d represents the number of moles of repeating unit D.

2. The adhesive for the negative electrode of a secondary battery according to claim 1, wherein, The copolymer is a random copolymer.

3. The adhesive for the negative electrode of a secondary battery according to claim 1, wherein, The copolymer contained therein has a (a+b):(c+d) ratio of 5:95 to 95:

5. Wherein, a represents the number of moles of repeating unit A, b represents the number of moles of repeating unit B, c represents the number of moles of repeating unit C, and d represents the number of moles of repeating unit D.

4. The adhesive for the negative electrode of a secondary battery according to claim 1, wherein, The copolymer contained therein has an a:b ratio of 5:95 to 60:

40. Where a represents the number of moles of repeating unit A, and b represents the number of moles of repeating unit B.

5. The adhesive for the negative electrode of a secondary battery according to claim 1, wherein, The c:d ratio of the copolymer contained therein is from 5:95 to 50:

50. Where c represents the number of moles of the repeating unit C, and d represents the number of moles of the repeating unit D.

6. The adhesive for the negative electrode of a secondary battery according to claim 1, wherein, The copolymer has a weight-average molecular weight of 100,000-2,000,000 g / mol.

7. A composition for a secondary battery negative electrode, comprising a negative electrode active material and a binder for a secondary battery negative electrode according to any one of claims 1 to 6.

8. The composition for a secondary battery negative electrode according to claim 7, wherein, The negative electrode active material contains silicon-based active material.

9. The composition for a secondary battery negative electrode according to claim 8, wherein, The negative electrode active material also includes graphite-based active material.

10. The composition for a secondary battery negative electrode according to claim 9, wherein, The mass ratio of the silicon-based active material to the graphite-based active material is 97:3 to 3:

97.

11. The composition for a secondary battery negative electrode according to claim 7, wherein, The solid content of the composition for the negative electrode of the secondary battery is 45% by weight or more.

12. 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 formed by the composition for a secondary battery negative electrode according to claim 7.

13. The negative electrode for a secondary battery according to claim 12, wherein, The content of the adhesive for the secondary battery negative electrode in the negative electrode active material layer is 0.5-30% by weight.

14. The negative electrode for a secondary battery according to claim 12, wherein, The thickness of the negative electrode active material layer is 10-120 μm.

15. A secondary battery, comprising: The negative electrode and positive electrode as described in claim 12; A membrane, which is located between the positive electrode and the negative electrode; and an electrolyte.

16. The secondary battery according to claim 15, wherein, The expansion rate of the secondary battery is less than 65%.

17. The secondary battery according to claim 15, wherein, The capacity retention rate of the secondary battery after 50 charge-discharge cycles is more than 80%.

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