Electrode for lithium secondary battery and lithium secondary battery comprising the same

Abstract

The present invention relates to an electrode for a lithium secondary battery and a lithium secondary battery including the same, which enables the provision of a battery that exhibits excellent charge / discharge characteristics while having excellent stability by including a uniform safety protection layer to suppress heat generation or ignition. The electrode for the lithium secondary battery comprises: a metal current collector; a safety functional layer formed to cover at least a portion of the metal current collector and comprising a polythiophene-based conductive polymer exhibiting PTC (positive temperature coefficient) characteristics and a thixotrophic agent; and an active material layer formed on the metal current collector and the safety functional layer, comprising an electrode active material and a conductive material.

Description

Technology Field

[0001] The present invention relates to an electrode for a lithium secondary battery and a lithium secondary battery including the same, which enables the provision of a battery having excellent stability and excellent charge / discharge characteristics by including a uniform safety protection layer to suppress heat generation or ignition. Background Technology

[0002] With the significant increase in demand for medium-to-large devices such as electric vehicles and hybrid electric vehicles, as well as mobile devices like smartphones and tablet PCs, the demand for secondary batteries as the energy source required to power these devices is rapidly rising. In particular, as the data processing speed and usage time of these mobile devices increase, there is active development of lithium secondary batteries that possess higher energy density and operating potential, maintain excellent performance over long periods, and have low self-discharge rates.

[0003] However, as the capacity and energy density of lithium secondary batteries have increased significantly, numerous ignition and explosion accidents have been reported in various mobile devices or electric vehicles containing them due to overcharging, exposure to high temperatures, or external impact. Accordingly, a major research task for lithium secondary batteries in recent years is to improve safety by suppressing such ignition and explosion.

[0004] It is known that the direct cause of ignition and explosion in the above-mentioned lithium secondary battery is a short circuit resulting from direct contact between the positive and negative electrodes inside the secondary battery due to external stimuli such as high temperature and external shock. For example, if a lithium secondary battery is overcharged or exposed to high temperature or external shock, the internal temperature of the secondary battery may rise rapidly, causing the separator to shrink or the internal structure of the secondary battery to be destroyed by external shock, which may result in the positive and negative electrodes coming into contact and causing a short circuit. When such a short circuit occurs, the movement of lithium ions and electrons may concentrate through the contact area between the positive and negative electrodes, causing an overcurrent. This can lead to heat generation, gas generation inside the battery, and volume expansion, thereby creating a risk of ignition and explosion of the lithium secondary battery.

[0005] Therefore, in order to improve the safety of the secondary battery by suppressing ignition and explosion during the above-mentioned short circuit, it is necessary to increase the resistance between the electrodes and cut off the current when the above-mentioned high temperature or external shock is applied. To this end, various attempts have been made to improve the safety of the secondary battery by adding various functional layers or functional materials to electrodes for lithium secondary batteries to increase the resistance when the above-mentioned high temperature is applied.

[0006] However, in the case of electrodes with previously known functional layers, there were disadvantages such as difficulty in sufficiently improving the safety of the lithium secondary battery or the inability to form the functional layer uniformly, which impairs basic battery performance, including charge and discharge characteristics of the secondary battery. The problem to be solved

[0007] Accordingly, the present invention provides an electrode for a lithium secondary battery that includes a uniform safety protection layer to suppress heat generation or ignition, thereby enabling the provision of a battery with excellent stability and excellent charge / discharge characteristics.

[0008] In addition, the present invention provides a lithium secondary battery including the electrode that exhibits excellent stability and charge / discharge characteristics. means of solving the problem

[0009] The present invention relates to a metal current collector; and

[0010] A safety functional layer formed to cover at least a portion of the metal current collector and comprising a polythiophene-based conductive polymer exhibiting PTC (positive temperature coefficient) characteristics and a thixotrophic agent; and

[0011] The present invention provides an electrode for a lithium secondary battery comprising an electrode active material and a conductive material, and an active material layer formed on the metal current collector and the safety protection layer.

[0012] The present invention also provides a lithium secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between them, wherein the electrode is included as the positive electrode.

[0014] Hereinafter, an electrode for a lithium secondary battery according to a specific embodiment of the invention and a lithium secondary battery including the same will be described.

[0015] Terms and words used in this specification and claims shall not be interpreted as being limited to their ordinary or dictionary meanings, but shall be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0016] The terms used in this specification are used merely to describe exemplary embodiments and are not intended to limit the invention. The singular expression includes the plural expression unless the context clearly indicates otherwise.

[0017] In this specification, terms such as “comprising,” “comprising,” or “having” are intended to specify the existence of the implemented features, numbers, steps, components, or combinations thereof, and should not be understood as precluding the existence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0018] According to one embodiment of the invention, a metal current collector; and

[0019] A safety functional layer formed to cover at least a portion of the metal current collector and comprising a polythiophene-based conductive polymer exhibiting PTC (positive temperature coefficient) characteristics and a thixotrophic agent; and

[0020] An electrode for a lithium secondary battery is provided, comprising an electrode active material and a conductive material, and an active material layer formed on the metal current collector and the safety protection layer.

[0021] The electrode of the above embodiment has a stability protection layer formed on the surface of the metal current collector, comprising a polythiophene-based conductive polymer exhibiting PTC characteristics and a thixotropic agent.

[0023] First, as the lithium secondary battery is activated, the above-mentioned polythiophene-based conductive polymer may exhibit conductivity as anions derived from the electrolyte of the secondary battery are doped onto the aromatic thiophene ring of the conductive polymer. Therefore, during the normal charging and discharging process of the secondary battery, the conductive polymer exhibits conductivity, thereby enabling the secondary battery to exhibit appropriate charge and discharge characteristics.

[0024] However, at temperatures above a certain level, the conductive polymer may be de-doped from the aromatic thiophene ring by anions derived from the electrolyte. As a result, the conductive polymer may act as an insulator, increasing resistance and exhibiting PTC characteristics that block the flow of current. Due to the action of such a polythiophene-based conductive polymer, an electrode of one embodiment having a safety protection layer formed therein can contribute to improving the stability of a lithium secondary battery as follows.

[0025] When an external stimulus, such as overcharging, high temperature, or external shock, is applied to a lithium secondary battery including the electrode above, causing the internal temperature of the battery to rise rapidly, such conductive polymers may be converted into insulators due to the de-doping of anions as described above. Accordingly, the resistance within the electrode is significantly increased, and the flow of current between the current collector and the active material layer is blocked, thereby preventing overcurrent caused by short circuits between electrodes, and suppressing heat generation, ignition, explosion, and gas generation in the secondary battery.

[0026] However, when the safety protection layer is coated and formed using only the polythiophene-based conductive polymer, it becomes difficult to form the safety protection layer uniformly due to the fluidity and / or viscosity of such polymer. In this case, the safety protection layer may be formed excessively thin or thick locally, and it may be difficult to form it well because the safety protection layer does not exhibit sufficient adhesion to metal current collectors, etc.

[0027] Due to the uneven formation of such safety protection layer, the safety protection layer may not be able to completely block local overcurrent even at temperatures above a certain level, making it difficult to guarantee the excellent safety of the lithium secondary battery, or the basic performance of the charging and discharging characteristics of the lithium secondary battery may be degraded as the safety protection layer has a large thickness in some local areas.

[0028] However, since the electrode of one embodiment includes a thixotropic agent together with the conductive polymer, the safety protection layer can be formed more uniformly. More specifically, the thixotropic agent can be added to the composition for forming the safety protection layer to improve its fluidity and enable uniform coating properties. Furthermore, after coating and drying the composition, the viscosity of the composition can be increased again, which can also contribute to the adhesion of the safety protection layer.

[0029] Due to the action of such a thixotropic agent, the electrode of one embodiment may include a uniform safety protection layer with a small overall thickness variation, and a uniform active material layer may be formed on the safety protection layer. Furthermore, in cases where electrode sheets are manufactured continuously and then punched to produce electrodes to be included in a plurality of lithium secondary batteries, the safety protection layer and the active material layer may have a uniform thickness in each electrode included in the plurality of lithium secondary batteries. For example, in each electrode included in one or more lithium secondary batteries manufactured from the same electrode sheet, the thickness of the safety protection layer may have a standard deviation of 60 nm or less, 50 nm or less, or 5 to 50 nm.

[0030] As a result, when a stimulus such as high temperature is applied to the lithium secondary battery, the safety protection layer of the electrode of one embodiment can effectively block overcurrent, thereby improving the safety of the lithium secondary battery. In addition, due to the formation of such a uniform safety protection layer, the degradation of basic performance, such as the charging and discharging characteristics of the lithium secondary battery, can be minimized.

[0031] Meanwhile, the polythiophene-based conductive polymer included in the electrode of the above embodiment exhibits the PTC characteristics described above, and the effective operating temperature at which this conductive polymer is converted into an insulator can be 70 to 130°C or 80 to 125°C. As the conductive polymer is converted into an insulator at this effective operating temperature, the conductive polymer can more effectively suppress ignition or explosion of the secondary battery when an external stimulus is applied, without hindering the normal charging and discharging process of the secondary battery.

[0032] In addition, the conductive polymer may be a polythiophene-based polymer or copolymer containing substituted or unsubstituted thiophene-based repeating units in an amount of 50 mol% or more, 70 mol% or more, or 90 to 100 mol% of the total repeating units.

[0033] In a more specific example, the conductive polymer may be a polythiophene-based polymer or copolymer in which an alkylene oxide group is bonded to a thiophene ring in the repeating unit, for example, a homopolymer or copolymer comprising a repeating unit of Formula 1 below:

[0034] [Chemical Formula 1]

[0035]

[0036] R in Chemical Formula 1 1 is a functional group of the following chemical formula 2, and

[0037] [Chemical Formula 2]

[0038]

[0039] In Chemical Formula 2, L1 is a single bond or an alkylene group

[0040] L2 is an alkylene group, R3 is hydrogen or an alkyl group, and n is an integer within the range of 1 to 5000, or 10 to 2000, or 50 to 1000, wherein the alkylene group may be an alkylene group having 2 to 5 carbon atoms, and the alkyl group may be an alkyl group having 1 to 5 carbon atoms.

[0041] In addition, such polythiophene-based conductive polymers may have a weight average molecular weight of, for example, 5,000 to 100,000 g / mol or 10,000 to 80,000 g / mol. As a result, the composition for forming the safety protective layer can exhibit excellent coating properties while also exhibiting appropriate adhesion to metal current collectors, etc.

[0042] In a more specific example, the polythiophene-based conductive polymer may contain the repeating unit of Formula 1 in an amount greater than 0 mol%, or 0.001 mol% or more, or 0.01 mol% or more, or 1 mol% or more, and may contain it in an amount of 100 mol% or less, or 80 mol% or less, or 50 mol% or less. At this time, the polythiophene-based conductive polymer may contain the remaining amount of alkyl thiophene repeating units excluding the repeating unit of Formula 1, for example, alkyl thiophene repeating units in which an alkyl group having 1 to 20 carbon atoms or 3 to 15 carbon atoms is substituted on a thiophene ring.

[0043] These polythiophene-based conductive polymers can exhibit appropriate effective operating temperatures by including the substituted thiophene ring, and as a result, can improve the safety of the secondary battery by converting to an insulator when a high temperature above a certain level is applied, without impairing the charge / discharge characteristics of the lithium secondary battery.

[0044] In addition, due to the predetermined structure described above, the polythiophene-based conductive polymer exhibits relatively low affinity and solubility for organic solvents, such as N-methylpyrrolidone, which are mainly included in the slurry composition for forming the electrode active material layer, and can exhibit excellent adhesion to the metal current collector. Therefore, in the process of forming the electrode active material layer by applying and drying the slurry composition after forming the polythiophene-based conductive polymer on the metal current collector, the phenomenon of the conductive polymer dissociating and spreading over a wide area of ​​the active material layer can be minimized. Accordingly, by using the polythiophene-based conductive polymer, the safety protection layer can be uniformly formed near the surface of the metal current collector. Thus, the conductive polymer can contribute to improving the safety of the secondary battery without impairing its basic performance.

[0045] Meanwhile, the conductive polymer may be included in an amount of 0.001 to 5 parts by weight, or 0.005 to 5 parts by weight, based on 100 parts by weight of the electrode active material (e.g., positive active material) included in the active material layer. As a result, the electrode of one embodiment may have superior safety and charge / discharge characteristics.

[0046] A conductive polymer having the repeating unit of Chemical Formula 1 described above can be prepared, for example, by substituting a thiophene halogenated compound with an alkylene glycol compound to produce a monomer to which the functional group of Chemical Formula 2 is bonded, and then polymerizing this monomer alone or copolymerizing it with other monomers such as alkyl thiophene. Specific conditions for preparing such monomers and polymers are described in the following manufacturing examples.

[0047] Meanwhile, in the electrode of the above-described embodiment, the safety protection layer comprises a thixotropic agent together with the polythiophene-based conductive polymer described above. The thixotropic agent is mixed with the conductive polymer to increase the fluidity of the composition for forming the safety protection layer, thereby contributing to improved coating properties and the uniform formation of the safety protection layer. Furthermore, after coating and drying the composition, the viscosity of the composition can be increased again to ensure that the safety protection layer has excellent adhesion to the metal current collector.

[0048] As such thixotropic agents, organic materials or inorganic particles capable of causing a change in the sol-gel state or an increase or decrease in fluidity of a polymer solution may be used. Specific examples of such thixotropic agents include hydrophilic fumed silica, aluminum salts, bentonite or its derivatives, cellulose-based compounds, polyvinyl-based compounds, polyacrylic acid-based compounds, modified urea or maleic acid copolymers, and, considering the above-mentioned polythiophene-based conductive polymer, the thixotropic agent in the form of the above-mentioned modified urea or an organic solution thereof may be appropriately used.

[0049] The above thixotropic agent may be an existing commercially available thixotropic agent, and examples of commercially available thixotropic agents include HL-200, HL-300, HL-380 (all manufactured by DKSH), GARAMITE-1958, RHEOBYK-410, 411, 7410, 605 (all manufactured by BYK), Alugel 28DG, Laevisil-SP (all manufactured by Baerlocher), CAB-O-SIL H-300 (manufactured by Cabot), Jaylink JL-106E (manufactured by Dymax), Bentone 38, SD3, 34 (all manufactured by Element), etc.

[0050] Such a thixotropic agent may be included in the safety protective layer in an amount of 0.1 to 5 parts by weight, or 0.2 to 3 parts by weight, or 0.5 to 2 parts by weight, per 100 parts by weight of the polythiophene-based conductive polymer, so as to appropriately control the fluidity and viscosity of the composition for forming the safety protective layer while not impairing the electrical properties of the electrode.

[0051] Meanwhile, the safety protection layer described above may be formed by dissolving or dispersing the aforementioned conductive polymer and thixotropic agent in an organic solvent, such as chloroform, tetrahydrofuran (THF), toluene, or xylene, at a concentration of about 0.1 to 5 weight percent to form a liquid composition, and then coating and drying this composition on the metal current collector. Subsequently, an active material layer may be formed by applying and drying the slurry composition described below, and an electrode for a lithium secondary battery of one embodiment may be manufactured by rolling the same.

[0052] At this time, the safety protective layer and the liquid composition for forming the same may further include one or more additives selected from the group consisting of carbon-based conductive materials, conductive inorganic particles, binders, and esterified saccharides, in addition to the conductive polymer.

[0053] At this time, the carbon-based conductive material and binder may be the same as the components included in the active material layer described later, and by adding these components, the conductivity of the electrode of one embodiment, the adhesion of the safety protection layer, or mechanical properties may be further improved. In addition, the conductive inorganic particles may be nano-scale particles, such as alumina or zirconia particles having a particle size of 5 to 100 nm, and the conductivity of the electrode and secondary battery may be further improved by adding these. In addition, monosaccharides, oligosaccharides, or polysaccharides having acyl groups may be used as the esterified sugars. These components may generate gas when the secondary battery is overcharged to block the conductive path between the metal current collector and the electrode active material, and the safety of the secondary battery may be further improved by adding these components.

[0054] Meanwhile, the electrode of one embodiment further comprises an active material layer formed on the metal current collector and the safety protection layer, and this active material layer may include an electrode active material, a conductive material, and optionally a binder. In this case, since it is preferable that the electrode on which the conductive polymer is formed be an anode, the following description will be based on such an example.

[0055] In the positive electrode for the lithium secondary battery, the metal current collector may generally have a thickness of 3 to 100 μm and may be formed of any metal or alloy having excellent conductivity without causing chemical changes in the secondary battery. Examples of such metal current collectors include metal current collectors made of stainless steel, aluminum, copper, nickel, or titanium, or those with a surface treatment of aluminum or stainless steel with carbon, nickel, titanium, or silver. Additionally, the metal current collector may have fine irregularities formed on its surface to increase adhesion to a safety protection layer, and may have various forms such as a film, sheet, foil, net, porous body, foam, or nonwoven fabric.

[0056] In addition, the positive active material included in the above active material layer is not particularly limited as long as it is a material capable of reversible insertion and extraction of lithium ions, and may include, for example, a lithium metal composite oxide containing one or more metal elements selected from the group consisting of Co, Mn, Ni, W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg and Mo.

[0057] More specifically, as the positive electrode active material, a compound represented by any one of the following chemical formulas may be used. Li a A 1-b R b D2 (wherein 0.90 ≤ a ≤ 1.8 and 0 ≤ b ≤ 0.5); Li a E 1-b R b O 2-c D c (In the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, and 0 ≤ c ≤ 0.05); LiE 2-b R b O 4-c D c(In the above equation, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05); Li a Ni 1-b-c Co b R c D α (In the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05 and 0 < α ≤ 2); Li a Ni 1-b-c Co b R c O 2-α Z α (In the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2); Li a Ni 1-b-c Co b R c O 2-α Z2 (wherein 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05 and 0 < α < 2); Li a Ni 1-b-c Mn b R c D α (In the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05 and 0 < α ≤ 2); Li a Ni 1-b-c Mn b R c O 2-α Z α (In the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2); Li a Ni 1-b-c Mn b R c O 2-α Z2 (wherein 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05 and 0 < α < 2); Li a Ni b E c G dO2(wherein 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5 and 0.001 ≤ d ≤ 0.1); Li a Ni b Co c Mn d GeO2(wherein 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.5 and 0.001 ≤ e ≤ 0.1); Li a NiG b O2(wherein 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1); Li a CoG b O2(wherein 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1); Li a MnG b O2(wherein 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1); Li a Mn2G b O4 (wherein 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1); QO2; QS2; LiQS2; V2O5; LiV2O5; LiTO2; LiNiVO4; and Li (3-f) J2(PO4)3(0 ≤ f ≤ 2).

[0058] In the above chemical formula, A is Ni, Co, Mn or a combination thereof; R is Al, Ni, Co, Mn, Cr, Fe, Mg, V or a combination thereof; D is O, F, S, P or a combination thereof; E is Co, Mn or a combination thereof; Z is F, S, P or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, V or a combination thereof; Q is Ti, Mo, Mn or a combination thereof; T is Cr, V, Fe, Sc, Y or a combination thereof; and J is V, Cr, Mn, Co, Ni, Cu or a combination thereof.

[0059] Additionally, the above compound may have a coating layer on its surface, or the above compound and the compound having the coating layer may be mixed and used. The coating layer may be a coating element compound and may include an oxide, hydroxide, oxyhydroxide, oxycarbonate, or hydroxycarbonate of a coating element. The compounds forming the coating layer may be amorphous or crystalline. The coating elements included in the coating layer may include Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof.

[0060] In addition, the conductive material included in the active material layer is used to impart conductivity to the electrode, and any electronically conductive material that does not cause chemical changes can be used in the battery being constructed. Examples include natural graphite, artificial graphite, carbon black, carbon fiber, carbon nanotube, metal powder such as copper, nickel, aluminum, and silver, metal fiber, etc., and one or more types of conductive materials such as polyphenylene derivatives can be used in combination.

[0061] The conductive material may be added in an amount of 1 to 50 weight% or 2 to 20 weight% based on the total weight of the active material layer. By doing so, excellent electrical properties can be imparted to the anode, while ensuring the desirable formation of the anode.

[0062] The above binder serves to adhere the particles of the positive electrode active material well to each other and also enhances the adhesion of the active material layer. Representative examples include the aforementioned halogenated polyolefin-based polymer binder, polyvinyl alcohol, carboxymethylcellulose, hydroxypropylcellulose, diacetylcellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0063] The binder may be added in an amount of 1 to 50 weight% or 2 to 30 weight% based on the total weight of the active material layer. This enables the formation of a positive electrode with excellent durability without impairing the electrical characteristics and / or capacitance characteristics of the positive electrode.

[0064] The above-described active material layer can be formed by dissolving or dispersing each component, such as the above-described positive active material, conductive material, and binder, in a medium such as an organic solvent to form a slurry composition, and then applying, drying, and rolling it onto a metal current collector on which the above-described safety protection layer is formed.

[0065] At this time, examples of the above-mentioned organic solvent or medium include N-methyl-2-pyrrolidone (NMP), methoxypropyl acetate, butyl acetate, glycol acid, butyl ester, butyl glycol, methylalkylpolysiloxane, alkylbenzene, propylene glycol, xylene, or monophenyl glycol, and among these, NMP, etc. can be appropriately used considering the dispersibility and processability of the above-mentioned cathode active material and conductive material.

[0066] Meanwhile, since the process and conditions for forming the above active material layer may follow general anode formation processes and conditions, further explanation regarding this will be omitted.

[0067] The active material layer formed by the method described above may have a thickness of 5 to 200 μm or 10 to 100 μm, and the safety protection layer may have a uniform thickness of 0.01 to 20 μm or 0.05 to 10 μm. This uniform thickness may be defined as the standard deviation of the thickness of the safety protection layer described above. For example, in each electrode included in one or more lithium secondary batteries manufactured from the same electrode sheet, the thickness of the safety protection layer may have a standard deviation of 60 nm or less, 50 nm or less, or 5 to 50 nm.

[0068] As the above safety protection layer is formed with the aforementioned uniform thickness near the surface of the metal current collector, when an external stimulus such as an external shock is applied, the conductive polymer included in the safety protection layer suppresses direct contact between the active material layer and the metal current collector, thereby ensuring improved safety of the secondary battery. In addition, the impairment of the charge / discharge characteristics of the secondary battery due to the safety protection layer being formed excessively thick locally can be minimized.

[0069] Meanwhile, according to another embodiment of the invention, a lithium secondary battery is provided that includes the electrode of the above-described embodiment as a positive electrode, together with a negative electrode and a separator interposed between the positive electrode and the negative electrode.

[0070] In a lithium secondary battery of this other embodiment, the negative electrode is manufactured by applying, drying, and rolling a negative electrode active material on a negative electrode current collector, and may further include a conductive material and a binder as needed.

[0071] The above-mentioned cathode active material may include, for example, carbon and graphite materials such as graphite having a completely formed layered crystal structure like natural graphite, soft carbon having a low-crystallinity layered crystal structure (graphene structure; a structure in which hexagonal honeycomb-shaped planes of carbon are arranged in layers), hard carbon in which such structures are mixed with amorphous parts, artificial graphite, expanded graphite, carbon fiber, non-graphitized carbon, carbon black, carbon nanotubes, fullerene, and activated carbon; or LixFe2O3(0≤x≤1), LixWO2(0≤x≤1), SnxMe1-xMe'yOz (Me: Mn, Fe, Pb, Ge; Me', Al, B, P, Si, elements of Group 1, Group 2, and Group 3 of the periodic table, halogens; 0 <x≤1; 1≤y≤3; 1≤z≤8) 등의 금속 복합 산화물; 리튬 금속; 리튬 합금; 규소, 규소 산화물 또는 규소계 합금; 주석계 합금; 폴리아세틸렌 등의 도전성 고분자; Li-Co-Ni계 재료; 티타늄 산화물; 또는 리튬 티타늄 산화물 등을 사용할 수 있다.

[0072] In one example, the cathode active material may include graphite and silicon (Si)-containing particles together, and the graphite may include one or more of natural graphite having a layered crystal structure and artificial graphite having an isotropic structure, and the silicon (Si)-containing particles may include silicon (Si) particles, silicon oxide particles, or a mixture of silicon (Si) particles and silicon oxide particles as particles containing silicon (Si) as a main component as a metal component.

[0073] In addition, the conductive material and binder that can be used together with the above-mentioned cathode active material may have the same components as the conductive material and binder included in the anode active material layer.

[0074] In addition, the cathode active material layer including the above cathode active material may have a thickness of 100㎛ to 200㎛, or 120㎛ to 200㎛.

[0075] In addition, the above-mentioned negative current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, nickel, titanium, calcined carbon, etc. may be used, and in the case of copper or stainless steel, surface-treated carbon, nickel, titanium, silver, etc. may be used.

[0076] In addition, the above-mentioned negative current collector, like the positive current collector, may form fine irregularities on its surface to strengthen the bonding force with the negative active material layer, and can take various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics. Furthermore, the average thickness of the above-mentioned negative current collector can be appropriately applied in the range of 3 to 100 μm, taking into consideration the conductivity and total thickness of the manufactured negative electrode.

[0077] In addition, the separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The separator is not particularly limited as long as it is commonly used in the industry, but specifically, a sheet or nonwoven fabric made of chemically resistant and hydrophobic polypropylene; glass fiber; or polyethylene may be used, and in some cases, a composite separator in which inorganic particles / organic particles are coated by an organic binder polymer on a porous polymer substrate such as the sheet or nonwoven fabric may be used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as the separator. Furthermore, the pore diameter of the separator may be an average of 0.01 to 10 μm, and the thickness may be an average of 5 to 300 μm.

[0078] The lithium secondary battery of the other embodiment described above may further include an electrolyte, and such electrolyte may be an electrolyte solution containing a non-aqueous organic solvent and a lithium salt, or an electrolyte membrane containing an organic or inorganic solid electrolyte, and these may be mixed together. However, the types of usable electrolytes are well known to those skilled in the art and are not particularly limited in the battery of the other embodiment, so further explanation regarding this is omitted. Effects of the invention

[0079] As described above, in a lithium secondary battery including the electrode of the present invention, even if the internal temperature of the battery rises rapidly due to the application of external stimuli such as overcharging, high temperature, or external shock, the overcurrent is effectively blocked by a safety protection layer uniformly formed near the metal current collector, thereby minimizing ignition and explosion.

[0080] Thus, the above lithium secondary battery can exhibit excellent safety, and the degradation of charge and discharge characteristics by the safety protection layer is suppressed, thereby exhibiting excellent electrochemical characteristics. Specific details for implementing the invention

[0081] Hereinafter, embodiments of the invention are described so that those skilled in the art can easily practice the invention. However, the invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0083] Preparation Example: Synthesis of Monomer and Conductive Polymer

[0084] [Chemical Formula 3]

[0085]

[0086] After creating a nitrogen environment inside a 3-way Round Bottom Flask (RBF) through nitrogen flowing, 2.34 g (0.01 mol) of Copper(I) Iodide and 50.36 g (0.31 mol) of Triethylene Glycol were added. Additionally, 3.68 g (0.096 mol) of Sodium Hydride 60% in mineral oil was slowly added to the RBF, and the mixture was stirred while maintaining a nitrogen environment. After stirring for about 1 hour, 10.0 g (0.06 mol) of 3-Bromothophene was added, and the mixture was refluxed at approximately 100°C for about 24 hours. The reaction solution was filtered through a vacuum device, washed with 100 mL of dichloromethane solution, and then washed in the order of NH4Cl and brine. The solvent was removed by vacuum distillation, and the crude product was purified by column chromatography (Hexane:ethyl acetate=60:40) to obtain approximately 9.0 g (yield: 60%) of the target compound (monomer compound of Chemical Formula 3).

[0087] 124 g (767 mmol) of iron(III) chloride was dissolved in 1000 ml of methylene chloride, to which 3.0 g (12.2 mmol) of the monomer compound of Formula 3 and 47.8 g (243 mmol) of 3-octylthiophene were added, and the polymerization reaction was carried out at approximately 25°C for 24 hours while stirring. The reaction solution was placed in an osmotic membrane with a molecular weight of cut-off (MWCO) of 5000, and then immersed in 1500 ml of acetonitrile solvent to remove unreacted iron(III) chloride and monomers. The residue precipitated inside the osmotic membrane was washed with methanol and dried at approximately 25°C to obtain the desired conductive polymer. The weight-average molecular weight (Mw) of the conductive polymer was confirmed to be approximately 34,000 g / mol.

[0089] Example 1: Preparation of a positive electrode and a lithium secondary battery

[0090] (Manufacturing of the anode)

[0091] A composition was obtained by dissolving 20 g of the conductive polymer (Mw=34,000 g / mol) obtained in the above preparation example and 0.2 g of the thixotropic agent RHEOBYK-411 (containing an NMP solution of modified urea; manufactured by BYK) in 1,980 g of chloroform solvent. This composition was gravure-coated to a thickness of approximately 0.5 μm onto an aluminum (Al) thin film serving as an anode current collector and dried to form a safety protective layer. The average thickness of the safety protective layer finally formed after drying and its standard deviation are shown in Table 1 below.

[0092] An anode slurry (solid content: 60 wt%) was prepared by adding LiCoO2 as an anode active material, a conductive material (carbon black), and a binder (polyvinylidene fluoride) to N-methyl-2-pyrrolidone (NMP) in a weight ratio of 97.5:1:1.5, and then applied and dried on the conductive polymer-containing layer (provided that the weight of the conductive polymer in the safety protection layer is about 0.5 wt parts based on 100 wt parts of the total anode active material, conductive material, and binder), and then a roll press was performed to form an active material layer with a total thickness of 58 μm and an anode was manufactured.

[0093] (Manufacturing of the cathode)

[0094] A cathode slurry (solid content: 60 wt%) was prepared by adding a cathode active material (graphite), a binder (SBR-CMC), and a conductive material (carbon black) to water, which is a solvent, in a weight ratio of 95:3.5:1.5. The cathode slurry was coated onto a copper (Cu) thin film, which is a cathode current collector with a thickness of 8 μm, dried, and then a roll press was performed to manufacture a cathode.

[0095] (Manufacturing of separator membranes)

[0096] A binder solution was prepared by adding approximately 8.5 wt% of a polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP) binder to acetone and dissolving it at a temperature of 50°C for at least 12 hours. Al2O3 powder was added to this binder solution in a ratio of Al2O3 / PVdF-HFP = 90 / 10 (wt% ratio), and a slurry was prepared using a ball mill method for at least 12 hours. The slurry prepared in this manner was coated onto a polyolefin-based separator with a thickness of approximately 8 μm using a dip coating method, and a porous separator was prepared by adjusting the coating thickness to approximately 0.45 μm.

[0097] (Manufacturing of lithium secondary batteries)

[0098] An electrode assembly composed of bicells was manufactured by sequentially stacking the above positive electrode, separator, and negative electrode, and then pressing them using heat and pressure of 90°C and 200 kPa. The assembled electrode assembly was placed in a pouch-type battery case, and a lithium secondary battery was manufactured by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70 and then injecting an electrolyte solution in which LiPF6 was dissolved to a concentration of 1.0 M.

[0100] Comparative Example 1

[0101] The positive electrode and lithium secondary battery of Comparative Example 1 were prepared in the same manner as in Example 1, except that the thixotropic agent of the above RHEOBYK-411 was not used.

[0103] Test example

[0104] Thickness profile measurement (standard deviation)

[0105] Using a confocal laser scanning microscope, the thickness of the final formed safety protective layer was measured point by point in the transverse direction (TD) of Figure 1 below, and the average thickness and standard deviation of the measured values ​​were confirmed.

[0106] [Figure 1]

[0107]

[0109] High-rate discharge characteristic evaluation

[0110] The lithium secondary batteries prepared in the examples and comparative examples were charged under constant current (0.7C) and constant voltage (4.47V, 0.025C cut-off) conditions, rested for 10 minutes, and then discharged until they reached 3V under constant current (0.1C, 0.2C, 0.5C, 1.0C, 1.5C) conditions. That is, the high-rate discharge characteristics (rate capability) of each battery were evaluated by periodically changing the discharge rate to 0.1C, 0.2C, 0.5C, 1.0C, and 1.5C, respectively, as the number of charge-discharge cycles increased. At this time, the high-rate discharge characteristics at 1.5C are shown in Table 1 below.

[0112] Nail penetration test

[0113] Five lithium secondary batteries prepared in the examples and comparative examples were each fully charged to 100% SOC at 4.47V (0.05C cut off) under CC / CV and 0.5C conditions at 25℃, and then stored at room temperature for 24 hours. Each lithium secondary battery was placed on a flat plate, and a stainless steel nail with a diameter of 3±0.2mm and a length of 100mm was inserted through the center of the cell with a penetration distance of 30mm under conditions of a vertical angle and a penetration speed of 100mm / sec to measure whether ignition occurred. The number of batteries that did not ignite among the five batteries is listed in Table 1 below.

[0115] Average thickness (standard deviation) High-rate discharge characteristics (standard deviation) Nail penetration test (number of unignited / total number) Example 1 457.3 nm (16.8 nm) 91.3% (0.45%) 5 / 5 Comparative Example 1 451.2 nm (87.7 nm) 89.0 % (4.5%) 3 / 5

[0116] Referring to Table 1 above, it was confirmed that the safety protection layer in the lithium secondary battery of Example 1 was formed with a more uniform thickness, and as a result, it was confirmed that it exhibited excellent safety and more uniform and superior high-rate discharge characteristics compared to Comparative Example 1.

Claims

Claim 1 An electrode for a lithium secondary battery comprising: a metal current collector; and a safety functional layer formed to cover at least a portion of the metal current collector and comprising a polythiophene-based conductive polymer exhibiting PTC (positive temperature coefficient) characteristics and a thixotrophic agent; and an electrode active material and a conductive material, and comprising an active material layer formed on the metal current collector and the safety functional layer, wherein the polythiophene-based conductive polymer comprises a homopolymer or copolymer containing repeating units of the following chemical formula 1, and has a weight average molecular weight of 5,000 to 100,000 g / mol: [Chemical Formula 1] R in Chemical Formula 1 1 is a functional group of the following chemical formula 2, and [Chemical Formula 2] In Chemical Formula 2, L1 is a single bond or an alkylene group having 2 to 5 carbon atoms, L2 is an alkylene group having 2 to 5 carbon atoms, R3 is hydrogen or an alkyl group having 1 to 5 carbon atoms, and n is an integer in the range of 1 to 5000. Claim 2 An electrode for a lithium secondary battery according to claim 1, wherein the effective operating temperature of the polythiophene-based conductive polymer is 70 to 130°C. Claim 3 delete Claim 4 delete Claim 5 An electrode for a lithium secondary battery according to claim 1, wherein the thixotropic agent comprises one or more selected from the group consisting of hydrophilic fumed silica, aluminum salt, bentonite or its derivative, cellulose-based compound, polyvinyl-based compound, polyacrylic acid-based compound, modified urea, and maleic acid copolymer. Claim 6 An electrode for a lithium secondary battery according to claim 1, wherein the thixotropic agent is included in an amount of 0.1 to 5 parts by weight per 100 parts by weight of the polythiophene-based conductive polymer. Claim 7 An electrode for a lithium secondary battery according to claim 1, wherein the safety protection layer further comprises one or more additives selected from the group consisting of carbon-based conductive materials, conductive inorganic particles, binders, and esterified saccharides. Claim 8 An electrode for a lithium secondary battery according to claim 1, wherein the active material layer has a thickness of 5 to 200 μm and the safety protection layer has a thickness of 0.01 to 20 μm. Claim 9 In claim 8, an electrode for a lithium secondary battery in which the standard deviation of the thickness of the safety protection layer is 60 nm or less. Claim 10 In claim 1, the electrode is an electrode for a lithium secondary battery that is a positive electrode. Claim 11 A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between them, wherein the electrode of claim 1 is included as the positive electrode.

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