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

Abstract

The present invention relates to an electrode for a lithium secondary battery that enables the provision of a battery having an excellent charge / discharge characteristic while including a uniform safety protection layer that suppresses heat generation or ignition and has excellent stability, and a lithium secondary battery including the same. The electrode for a lithium secondary battery includes a metal current collector; and a safety functional layer formed to cover at least a part of the metal current collector and including 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 and including an electrode active material and a conductive material.

Description

[Technical Field]

[0001] Cross-Citation of Related Applications This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0170925 dated December 8, 2022, and all contents disclosed in the documents of this Korean patent application are incorporated herein by reference.

[0002] The present invention relates to an electrode for a lithium secondary battery, which includes a uniform safety protective layer to suppress heat generation or ignition, thereby enabling the provision of a battery that has excellent stability and exhibits excellent charge / discharge characteristics, and a lithium secondary battery including the same. [Background technology]

[0003] As demand for medium- to large-sized devices such as electric vehicles and hybrid electric vehicles, and mobile devices such as smartphones and tablet PCs, has increased significantly, demand for secondary batteries as an energy source required to drive these devices has also increased sharply. In particular, as the data processing speed of these mobile devices has increased and their usage time has increased, active development has been underway for lithium secondary batteries that have higher energy density and working potential, can maintain excellent characteristics for a long period of time, and have a low self-discharge rate.

[0004] However, as the capacity and energy density of lithium secondary batteries have increased significantly, there have been numerous reports of fires and explosions caused by overcharging, high temperature exposure, external impact, etc., in various mobile devices or electric vehicles containing lithium secondary batteries. Therefore, the main research topic in recent years has been how to improve the safety of lithium secondary batteries by preventing such fires and explosions.

[0005] The direct cause of fires and explosions in lithium secondary batteries is known to be a short circuit caused by direct contact between the positive and negative electrodes inside the secondary battery due to external stimuli such as high temperature and external impact. For example, if a lithium secondary battery is overcharged or exposed to high temperature or external impact, the internal temperature of the secondary battery may rise rapidly, causing the separator to shrink, or the internal structure of the secondary battery may be destroyed by the external impact, resulting in contact between the positive and negative electrodes and a short circuit. When such a short circuit occurs, the movement of lithium ions and electrons may be concentrated at the contact points between the positive and negative electrodes, generating an overcurrent. This may result in heat generation, gas generation, and volume expansion inside the battery, which may increase the risk of fire or explosion in the lithium secondary battery.

[0006] Therefore, in order to improve the safety of secondary batteries by preventing fires and explosions due to short circuits, it is necessary to increase the resistance between electrodes to interrupt current when exposed to high temperatures or external impacts, etc. To this end, various attempts have been made to improve the safety of secondary batteries by adding various functional layers or functional materials to electrodes for lithium secondary batteries to increase resistance when exposed to high temperatures, etc.

[0007] However, in the case of electrodes to which previously known functional layers are added, it is difficult to sufficiently improve the safety of lithium secondary batteries, or the functional layers are not uniformly formed, which impairs basic battery performance such as the charge / discharge characteristics of the secondary battery. Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, the present invention provides an electrode for a lithium secondary battery that includes a uniform safety protective layer, thereby suppressing heat generation or ignition, and thus enabling the provision of a battery that has excellent stability and exhibits excellent charge / discharge characteristics.

[0009] The present invention also provides a lithium secondary battery that includes the electrode and exhibits excellent stability and charge / discharge characteristics. [Means for solving the problem]

[0010] The present invention relates to a metal current collector; a safety functional layer formed to cover at least a portion of the metal current collector and including a polythiophene-based conductive polymer exhibiting a positive temperature coefficient (PTC) characteristic and a thixotropic agent; and The present invention provides an electrode for a lithium secondary battery, which comprises an active material layer formed on the metal current collector and a safety protection layer, the active material layer including an electrode active material and a conductive material.

[0011] The present invention also provides a lithium secondary battery comprising a positive electrode, a negative electrode, and a separator interposed therebetween, the electrode being included as the positive electrode.

[0012] Hereinafter, an electrode for a lithium secondary battery according to a specific embodiment of the present invention and a lithium secondary battery including the same will be described.

[0013] The terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or prior meanings, but should be interpreted as meanings and concepts that correspond to the technical idea of ​​the present invention, based on the principle that an inventor can appropriately define the concepts of terms in order to explain his or her invention in the best possible way.

[0014] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless the context clearly indicates otherwise.

[0015] In this specification, the terms "comprises," "comprises," or "has" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, and should be understood as not precluding the possible presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0016] According to one embodiment of the invention, a metal current collector; a safety functional layer formed to cover at least a portion of the metal current collector and including a polythiophene-based conductive polymer exhibiting a positive temperature coefficient (PTC) characteristic and a thixotrophic agent; and An electrode for a lithium secondary battery is provided, which comprises an active material layer including an electrode active material and a conductive material, and which is formed on the metal current collector and the safety protection layer.

[0017] In the electrode of the embodiment, a safety protection layer containing a polythiophene-based conductive polymer exhibiting PTC characteristics and a thixotropic agent is formed on the surface of the metal current collector.

[0018] First, the polythiophene-based conductive polymer exhibits conductivity as anions derived from the electrolyte of the secondary battery are doped onto the aromatic thiophene ring of the conductive polymer as the lithium secondary battery is activated. Therefore, during normal charging and discharging of the secondary battery, the conductive polymer exhibits conductivity, allowing the secondary battery to exhibit appropriate charging and discharging characteristics.

[0019] However, above a certain temperature, the conductive polymer de-dops anions derived from the electrolyte from the aromatic thiophene ring. As a result, the conductive polymer acts as a non-conductor, increasing resistance and blocking current flow, thereby exhibiting PTC characteristics. Due to the function of the polythiophene-based conductive polymer, an electrode according to one embodiment having a safety protection layer including the same can contribute to improving the stability of lithium secondary batteries as follows:

[0020] When an external stimulus such as overcharging, high temperature, or external impact is applied to a lithium secondary battery including the electrode, causing a sudden rise in the temperature inside the battery, the conductive polymer can be converted into a non-conductor by dedoping of the anions, etc. This significantly increases the resistance inside the electrode, blocking the flow of current between the current collector and the active material layer, preventing overcurrent due to short circuits between the electrodes, and suppressing heat generation, fire, explosion, and gas generation in the secondary battery.

[0021] However, when the safety protection layer is formed by coating only with the polythiophene-based conductive polymer, the fluidity and / or viscosity of the polymer make it difficult to form the safety protection layer uniformly. In this case, the safety protection layer may be formed to be excessively thin or thick in some areas, and the safety protection layer may not exhibit sufficient adhesion to the metal current collector, making it difficult to form the layer properly.

[0022] Due to the uneven formation of the safety protection layer, the safety protection layer may not be able to completely block overcurrent locally even at temperatures above a certain level, making it difficult to ensure the excellent safety of the lithium secondary battery. Alternatively, the safety protection layer may have a locally large thickness, which may degrade basic performance such as the charge / discharge characteristics of the lithium secondary battery.

[0023] However, in one embodiment, the electrode includes a thixotropic agent along with the conductive polymer, thereby enabling the safety protection layer to be formed more uniformly. More specifically, the thixotropic agent is added to the composition for forming the safety protection layer to improve its fluidity and provide a uniform coating. Furthermore, after the composition is coated and dried, the thixotropic agent can increase the viscosity of the composition, thereby contributing to the adhesion of the safety protection layer.

[0024] Due to the effect of the thixotropic agent, an electrode according to one embodiment can include a uniform safety protection layer with little overall thickness variation, and a uniform active material layer can be formed on the safety protection layer. Furthermore, when electrode sheets are continuously manufactured and then punched out to manufacture electrodes for use in multiple lithium secondary batteries, the safety protection layer and active material layer can have uniform thicknesses in each electrode for each of the multiple lithium secondary batteries. For example, the thickness of the safety protection layer in each electrode for one or more lithium secondary batteries manufactured from the same electrode sheet can have a standard deviation of 60 nm or less, or 50 nm or less, or 5 to 50 nm.

[0025] As a result, in the electrode of one embodiment, when a stimulus such as high temperature is applied to the lithium secondary battery, the safety protection layer effectively blocks overcurrent, thereby improving the safety of the lithium secondary battery. Furthermore, the formation of such a uniform safety protection layer can minimize deterioration of basic performance such as the charge and discharge characteristics of the lithium secondary battery.

[0026] Meanwhile, the polythiophene-based conductive polymer included in the electrode of the embodiment exhibits the above-mentioned PTC characteristics, and the effective operating temperature at which the conductive polymer is converted into an insulator may be 70 to 130° C., or 80 to 125° C. Since the conductive polymer is converted into an insulator at such an effective operating temperature, the conductive polymer does not interfere with the normal charge / discharge process of the secondary battery, and can more effectively prevent the secondary battery from catching fire or exploding when an external stimulus is applied.

[0027] The conductive polymer may also be a polythiophene-based polymer or copolymer containing substituted or unsubstituted thiophene-based repeating units in a content of 50 mol % or more, 70 mol % or more, or 90 to 100 mol % of the total repeating units.

[0028] 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 including a repeating unit of the following Chemical Formula 1: [Chemical formula 1] [ka] In Chemical Formula 1, R1 is a functional group of Chemical Formula 2 below: [Chemical formula 2] [ka] In Chemical Formula 2, L1 is a single bond or an alkylene group, L2 is an alkylene group, R3 is hydrogen or an alkyl group, n is an integer ranging from 1 to 5,000, alternatively from 10 to 2,000, or alternatively from 50 to 1,000, and 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.

[0029] The polythiophene-based conductive polymer 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, which allows the composition for forming the safety protection layer to exhibit excellent coating properties and appropriate adhesion to metal current collectors, etc.

[0030] In a more specific example, the polythiophene-based conductive polymer may contain the repeating unit of Formula 1 in an amount of more than 0 mol%, or 0.001 mol% or more, or 0.01 mol% or more, or 1 mol% or more, or 100 mol% or less, or 80 mol% or less, or 50 mol% or less. In this case, the polythiophene-based conductive polymer may contain the remaining alkylthiophene-based repeating units excluding the repeating unit of Formula 1, for example, alkylthiophene-based repeating units in which a thiophene ring is substituted with an alkyl group having 1 to 20 carbon atoms or 3 to 15 carbon atoms.

[0031] Such polythiophene-based conductive polymers can exhibit an appropriate effective operating temperature by including the substituted thiophene ring. As a result, they do not impair the charge / discharge characteristics of lithium secondary batteries, and are converted into non-conductors when high temperatures above a certain level are applied, thereby improving the safety of secondary batteries.

[0032] Furthermore, due to its specific structure, the polythiophene-based conductive polymer exhibits relatively low affinity and solubility in organic solvents, such as N-methylpyrrolidone, that are typically used in slurry compositions for forming electrode active material layers, thereby providing excellent adhesion to the metal current collector. Therefore, when the polythiophene-based conductive polymer is formed on a metal current collector and then the slurry composition is applied and dried to form an electrode active material layer, the conductive polymer can be prevented from dissociating and diffusing over a wide area of ​​the active material layer. Therefore, the use of the polythiophene-based conductive polymer allows the safety protection layer to be uniformly formed near the surface of the metal current collector. Therefore, the conductive polymer can contribute to improving the safety of secondary batteries without impairing their basic performance.

[0033] 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., the positive electrode active material) included in the active material layer, thereby allowing the electrode of one embodiment to have better safety and charge / discharge characteristics.

[0034] A conductive polymer having the repeating unit of Formula 1 can be prepared by, for example, subjecting a halogenated thiophene compound to a substitution reaction with an alkylene glycol compound to prepare a monomer having a functional group of Formula 2 attached thereto, and then polymerizing the monomer alone or copolymerizing it with another monomer such as an alkylthiophene. Specific conditions for preparing the monomer and the polymer are described in the Preparation Examples below.

[0035] Meanwhile, in the electrode according to the embodiment, the safety protection layer includes a thixotropic agent in addition to the polythiophene-based conductive polymer. The thixotropic agent, when mixed with the conductive polymer, increases the fluidity of the composition for forming the safety protection layer, thereby contributing to improved coating properties and uniform formation of the safety protection layer. Furthermore, after coating and drying the composition, the thixotropic agent can increase the viscosity of the composition, thereby ensuring excellent adhesion of the safety protection layer to the metal current collector.

[0036] The thixotropic agent may be an organic or inorganic particle capable of causing a sol-gel state change or increasing or decreasing the fluidity of the polymer solution. Specific examples of the thixotropic agent include hydrophilic fumed silica, aluminum salt, bentonite or its derivatives, cellulose-based compounds, polyvinyl-based compounds, polyacrylic acid-based compounds, modified urea or maleic acid copolymers. In consideration of the polythiophene-based conductive polymer, the modified urea or its organic solution state thixotropic agent may be suitably used.

[0037] The thixotropic agent may be any commercially available thixotropic agent. Examples of commercially available thixotropic agents include HL-200, HL-300, and HL-380 (manufactured by DKSH), GARAMITE-1958, RHEOBYK-410, 411, 7410, and 605 (manufactured by BYK), Alugel 28DG and Laevisil-SP (manufactured by Baerlocher), CAB-O-SIL H-300 (manufactured by Cabot), Jaylink JL-106E (manufactured by Dymax), and Bentone 38, SD3, and 34 (manufactured by Elements).

[0038] The thixotropic agent may be contained in the safety protection layer in an amount of 0.1 to 5 parts by weight, 0.2 to 3 parts by weight, or 0.5 to 2 parts by weight, relative to 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 protection layer while not impairing the electrical properties of the electrode.

[0039] The safety protection layer can be formed by dissolving or dispersing the 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 wt % to form a liquid composition, and then coating and drying the composition on the metal current collector. A slurry composition, which will be described later, is then applied and dried to form an active material layer, which can then be rolled to produce an electrode for a lithium secondary battery according to one embodiment.

[0040] In this case, the safety protection layer and the liquid composition for forming the safety protection layer may further include, in addition to the conductive polymer, one or more additives selected from the group consisting of a carbon-based conductive material, conductive inorganic particles, a binder, and an esterified saccharide.

[0041] The carbon-based conductive material and binder may be the same as those contained in the active material layer described below. Addition of these components can further improve the conductivity of the electrode, the adhesiveness of the safety protection layer, or the mechanical properties of the electrode. The conductive inorganic particles may be alumina or zirconia particles having a nanoscale particle size, for example, 5 to 100 nm, and addition of these particles can further improve the conductivity of the electrode and secondary battery. The esterified sugars may be monosaccharides, oligosaccharides, or polysaccharides having an acyl group. These components can generate gas during overcharge of the secondary battery, thereby blocking the conductive path between the metal current collector and the electrode active material. Addition of these components can further improve the safety of the secondary battery.

[0042] Meanwhile, the electrode of one embodiment further includes an active material layer formed on the metal current collector and the safety protection layer, and the active material layer may include an electrode active material, a conductive material, and optionally a binder, etc. In this case, the electrode on which the conductive polymer is formed is preferably a positive electrode, and therefore, the following description will be based on this example.

[0043] In the positive electrode for the lithium secondary battery, the metal current collector generally has a thickness of 3 to 100 μm and can be formed from any metal or alloy that has excellent conductivity while not inducing chemical changes in the secondary battery. Examples of such metal current collectors include metal current collectors such as stainless steel, aluminum, copper, nickel, or titanium, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver. In addition, the metal current collector can have fine irregularities on its surface to enhance the adhesive strength of a safety protection layer, and can have various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0044] Furthermore, the positive electrode active material contained in the active material layer is not particularly limited as long as it is a material that allows reversible insertion and desorption of lithium ions, and can 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.

[0045] More specifically, the positive electrode active material may be a compound represented by any one of the following chemical formulas: 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 (wherein 0.90≦a≦1.8, 0≦b≦0.5, and 0≦c≦0.05); LiE 2-b R b O 4-c D c (wherein 0≦b≦0.5 and 0≦c≦0.05); Li a Ni 1-b-c Co b R c D α (wherein 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 (wherein 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(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 O 2-α Z α (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 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 d O2 (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).

[0046] 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.

[0047] The compound may also have a coating layer on its surface, or the compound may be mixed with a compound having a coating layer. The coating layer is a coating element compound and may include an oxide, hydroxide, oxyhydroxide, oxycarbonate, or hydroxycarbonate of the coating element. The compound forming the coating layer may be amorphous or crystalline. The coating element contained in the coating layer may be Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof.

[0048] The conductive material contained in the active material layer is used to impart conductivity to the electrode, and any material that does not cause a chemical change in the battery that is constructed and is electronically conductive can be used. Examples of such materials that can be used include natural graphite, artificial graphite, carbon black, carbon fiber, carbon nanotubes, metal powders and metal fibers such as copper, nickel, aluminum, and silver, and also conductive materials such as polyphenylene derivatives can be used alone or in combination of two or more.

[0049] The conductive material may be added in an amount of 1 to 50 wt %, or 2 to 20 wt %, based on the total weight of the active material layer, thereby ensuring favorable formation of the positive electrode while imparting excellent electrical properties to the positive electrode.

[0050] The binder serves to effectively adhere particles of the positive electrode active material to each other and to enhance the adhesion of the active material layer. Representative examples of the binder include the above-mentioned halogenated polyolefin polymer binder, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, etc.

[0051] The binder may be added in an amount of 1 to 50 wt %, or 2 to 30 wt %, based on the total weight of the active material layer, thereby enabling the formation of a positive electrode having excellent durability without impairing the electrical and / or capacity characteristics of the positive electrode.

[0052] The active material layer can be formed by dissolving or dispersing each component, such as the positive electrode active material, conductive material, and binder, in a medium such as an organic solvent to form a slurry composition, and then applying the slurry onto the metal current collector on which the safety protection layer is formed, drying, and rolling the slurry.

[0053] In this case, examples of the medium such as the organic solvent include N-methyl-2-pyrrolidone (NMP), methoxypropyl acetate, butyl acetate, glycol acid, butyl ester, butyl glycol, methylalkylpolysiloxane, alkylbenzene, propylene glycol, xylene, and monophenyl glycol. Among these, NMP may be appropriately used in consideration of the dispersibility and processability of the positive electrode active material and the conductive material.

[0054] Meanwhile, the process and conditions for forming the active material layer may follow the process and conditions for forming a general positive electrode, and therefore, further explanation regarding this will be omitted.

[0055] The active material layer formed by the above method 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 can be defined as the standard deviation of the thickness of the safety protection layer. For example, the thickness of the safety protection layer in each electrode included in one or more lithium secondary batteries manufactured from the same electrode sheet may have a standard deviation of 60 nm or less, or 50 nm or less, or 5 to 50 nm.

[0056] The safety protection layer is formed with a uniform thickness near the surface of the metal current collector, and when an external stimulus such as an external impact is applied, the conductive polymer contained in the safety protection layer prevents direct contact between the active material layer and the metal current collector, thereby ensuring improved safety of the secondary battery. In addition, the safety protection layer can be prevented from being locally excessively thick, which could impair the charge / discharge characteristics of the secondary battery.

[0057] Meanwhile, according to another embodiment of the present invention, there is provided a lithium secondary battery including the electrode of the above-described embodiment as a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

[0058] In this other embodiment of the lithium secondary battery, the negative electrode is prepared by coating a negative electrode active material on a negative electrode current collector, drying, and rolling, and may further include a conductive material and a binder, if necessary.

[0059] Examples of the negative electrode active material include graphite with a completely formed layered crystal structure such as natural graphite, soft carbon having a low crystalline layered crystal structure (graphene structure; a structure in which hexagonal honeycomb planes of carbon are arranged in layers), and hard carbon in which such a structure is mixed with an amorphous portion, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene, activated carbon, and other carbon and graphite materials; metal composite oxides such as LixFe2O3 (0 ≦ x ≦ 1), LixWO2 (0 ≦ x ≦ 1), SnxMe1-xMe'yOz (Me: Mn, Fe, Pb, Ge; Me', Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen; 0 < x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8); lithium metal; lithium alloy; silicon, silicon oxide or silicon-based alloy; conductive polymer such as polyacetylene; Li-Co-Ni-based material; titanium oxide; or lithium titanate, etc. can be used.

[0060] In one example, the negative electrode active material can contain both graphite and silicon (Si)-containing particles. The graphite can contain any one or more of natural graphite having a layered crystal structure and artificial graphite having an equilateral structure. The silicon (Si)-containing particles are particles containing silicon (Si) as a main component as a metal component, and can include silicon (Si) particles, silicon oxide particles, or a mixture of the silicon (Si) particles and silicon oxide particles.

[0061] And, as the conductive material and binder that can be used together with the negative electrode active material, the same components as those of the conductive material and binder contained in the positive electrode active material layer can be used.

[0062] Also, the negative electrode active material layer containing the negative electrode active material can have a thickness of 100 μm to 200 μm, or 120 μm to 200 μm.

[0063] The negative electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity. For example, copper, stainless steel, nickel, titanium, calcined carbon, etc. may be used. In the case of copper or stainless steel, those surface-treated with carbon, nickel, titanium, silver, etc. may also be used.

[0064] In addition, like the positive electrode current collector, the negative electrode current collector may have fine irregularities on its surface to strengthen the bonding force with the negative electrode active material layer, and may be in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc. In addition, the average thickness of the negative electrode current collector may be appropriately set to 3 to 100 μm, taking into consideration the conductivity and total thickness of the negative electrode to be manufactured.

[0065] The separator is a thin insulating membrane interposed between the positive and negative electrodes, exhibiting high ion permeability and mechanical strength. The separator can be any material commonly used in the art, including sheets or nonwoven fabrics made from chemically resistant and hydrophobic materials such as polypropylene, glass fiber, or polyethylene. In some cases, a composite separator may be used, in which inorganic or organic particles are coated on a porous polymer substrate such as a sheet or nonwoven fabric with an organic binder polymer. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte can also function as the separator. The separator may have an average pore diameter of 0.01 to 10 μm and an average thickness of 5 to 300 μm.

[0066] The lithium secondary battery of the above-described other embodiment may further include an electrolyte, which 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, or a mixture of these. However, the types of electrolytes that can be used are well known to those skilled in the art and are not particularly limited in the battery of the other embodiment, so further description thereof will be omitted. [Effects of the Invention]

[0067] As described above, even if the internal temperature of a lithium secondary battery including the electrode of the present invention is rapidly increased due to the application of an external stimulus such as overcharging, high temperature, or external impact, the safety protection layer uniformly formed near the metal current collector effectively blocks overcurrent, thereby minimizing fire and explosion.

[0068] Therefore, the lithium secondary battery can exhibit excellent safety, and the deterioration of charge / discharge characteristics due to the safety protection layer is suppressed, thereby exhibiting excellent electrochemical characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0069] The following description of the preferred embodiments of the present invention will be given in order to enable those skilled in the art to easily implement the present invention, but the present invention may be embodied in many different forms and is not limited to the preferred embodiments set forth herein. [Example]

[0070] Manufacturing example: Synthesis of monomers and conductive polymers [Chemical formula 3] [ka] After creating a nitrogen environment through nitrogen flow inside a 3-way round bottom flask (RBF), 2.34 g (0.01 mol) of copper(I) iodide and 50.36 g (0.31 mol) of triethylene glycol were added. An additional 3.68 g (0.096 mol) of sodium hydride 60% in mineral oil was slowly added to the RBF and stirred while maintaining a nitrogen atmosphere. After stirring for approximately 1 hour, 10.0 g (0.06 mol) of 3-bromothiophene was added and refluxed at approximately 100°C for approximately 24 hours. The reaction solution was filtered through a vacuum device and washed with 100 mL of dichloromethane solution, followed by NH4Cl and brine. The solvent was removed by distillation under reduced pressure, and the crude product was purified by column chromatography (hexane:ethyl acetate=60:40) to obtain about 9.0 g (yield: 60%) of the target compound (monomer compound of Formula 3).

[0071] To a solution of 124 g (767 mmol) of iron(III) chloride in 1000 ml of methylene chloride, 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 polymerized at approximately 25°C for 24 hours with stirring. The reaction solution was placed in a membrane with a molecular weight cut-off (MWCO) of 5000 and immersed in 1500 ml of acetonitrile to remove unreacted iron(III) chloride and monomer. The residue precipitated inside the 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.

[0072] Example 1: Fabrication of positive electrode and lithium secondary battery (Cathode manufacturing) 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 RHEOBYK-411 (a modified urea solution in NMP; manufactured by BYK) thixotropic agent in 1,980 g of chloroform solvent. This composition was gravure coated to a thickness of approximately 0.5 μm on an aluminum (Al) thin film, which served as a positive electrode current collector, and then dried to form a safety protection layer. The average thickness and standard deviation of the safety protection layer finally formed after drying are shown in Table 1 below.

[0073] A cathode slurry (solid content: 60 wt%) was prepared by adding LiCoO2 as a cathode 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. This was then applied to the conductive polymer oil-impregnated layer and dried (the weight of the conductive polymer in the safety protection layer was approximately 0.5 parts by weight based on 100 parts by weight of the total of the cathode active material, conductive material, and binder). A roll press was then performed to form an active material layer with a total thickness of 58 μm, thereby preparing a cathode.

[0074] (Manufacturing of negative electrodes) Anode active material (graphite), binder (SBR-CMC), and conductive material (carbon black) were mixed in a weight ratio of 95:3.5:1.5 with water as a solvent to prepare anode slurry (solid content: 60 wt%). The anode slurry was applied to an 8 μm-thick copper (Cu) thin film as anode current collector, dried, and then roll-pressed to prepare anode.

[0075] (Separation membrane manufacturing) A binder solution was prepared by adding approximately 8.5 wt% polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP) binder to acetone and dissolving it at 50°C for approximately 12 hours. Al2O3 powder was added to the binder solution at a weight ratio of Al2O3 / PVdF-HFP = 90 / 10 and ball milled for 12 hours to prepare a slurry. The resulting slurry was then coated onto an approximately 8μm thick polyolefin separator using a dip coating method, resulting in a porous separator with a thickness of approximately 0.45μm.

[0076] (Lithium secondary battery manufacturing) The positive electrode, separator, and negative electrode were stacked in order and then pressed under heat and pressure at 90°C and 200 kPa to produce an electrode assembly consisting of a bicell. The assembled electrode assembly was placed in a pouch-type battery case, and an electrolyte solution prepared by mixing ethylene carbonate (EC):ethyl methyl carbonate (EMC) in a volume ratio of 30:70 and dissolving LiPF6 to a concentration of 1.0 M was injected to produce a lithium secondary battery.

[0077] Comparative Example 1 A positive electrode and a lithium secondary battery of Comparative Example 1 were prepared in the same manner as in Example 1, except that the RHEOBYK-411 thixotropic agent was not used.

[0078] Test Example Thickness profile measurement (standard deviation) The thickness of the finally formed safety protection layer was measured at each point in the transverse direction (TD) of FIG. 1 using a confocal laser scanning microscope, and the average thickness and standard deviation of the measured values ​​were determined. [Figure 1] JPEG0007786028000004.jpg1927

[0079] High-rate discharge characteristic evaluation The lithium secondary batteries prepared in the examples and comparative examples were charged at a constant current (0.7 C) and constant voltage (4.47 V, 0.025 C cutoff), rested for 10 minutes, and then discharged to 3 V at constant currents (0.1 C, 0.2 C, 0.5 C, 1.0 C, 1.5 C). As the charge-discharge cycle recovery increased, the discharge rate was periodically changed to 0.1 C, 0.2 C, 0.5 C, 1.0 C, and 1.5 C, respectively, to evaluate the high-rate discharge capability of each battery. The high-rate discharge capability at 1.5 C is shown in Table 1 below.

[0080] Nail penetration test Five lithium secondary batteries were prepared for each of the examples and comparative examples. They were fully charged to 4.47 V (0.05 C cut-off) at 25°C under CC / CV and 0.5 C conditions to 100% SOC and then stored at room temperature for 24 hours. Each lithium secondary battery was placed on a flat plate and a stainless steel nail, 3 ± 0.2 mm in diameter and 100 mm in length, was penetrated 30 mm into the center of the cell at a vertical angle at a penetration speed of 100 mm / sec. The number of batteries that did not ignite is listed in Table 1 below. [Table 1] [Table 1]

[0081] Referring to Table 1 above, it was confirmed that the safety protection layer was formed with a more uniform thickness in the lithium secondary battery of Example 1, and as a result, it was confirmed that the lithium secondary battery of Example 1 exhibited excellent safety and more uniform and superior high-rate discharge characteristics compared to Comparative Example 1.

Claims

1. Metal current collector; a safety functional layer formed to cover at least a portion of the metal current collector and including a polythiophene-based conductive polymer exhibiting positive temperature coefficient (PTC) properties and a thixotropic agent; and an active material layer including an electrode active material and a conductive material, the active material layer being formed on the metal current collector and the safety protection layer; the active material layer has a thickness of 5 to 200 μm, the safety protection layer has a thickness of 0.01 to 20 μm; The standard deviation of the thickness of the safety protection layer is 60 nm or less; Electrodes for lithium secondary batteries.

2. 2. The electrode for a lithium secondary battery according to claim 1, wherein the polythiophene-based conductive polymer has an effective operating temperature of 70 to 130°C.

3. 2. The electrode for a lithium secondary battery according to claim 1, wherein the polythiophene-based conductive polymer comprises a homopolymer or copolymer having a repeating unit of the following Formula 1: [Chemical formula 1] 【Chemistry 4】 In chemical formula 1, R １ is a functional group of the following chemical formula 2, [Chemical formula 2] 【Transformation 5】 In chemical formula 2, L １ is a single bond or an alkylene group, and L ２ is an alkylene group, and R ３ is hydrogen or an alkyl group, and n is an integer ranging from 1 to 5,000.

4. 2. The electrode for a lithium secondary battery according to claim 1, wherein the polythiophene-based conductive polymer has a weight average molecular weight of 5,000 to 100,000 g / mol.

5. 2. The electrode for a lithium secondary battery according to claim 1, wherein the thixotropic agent comprises at least one selected from the group consisting of hydrophilic fumed silica, aluminum salt, bentonite or a derivative thereof, a cellulose-based compound, a polyvinyl-based compound, a polyacrylic acid-based compound, modified urea, and a maleic acid copolymer.

6. 2. The electrode for a lithium secondary battery according to claim 1, wherein the thixotropic agent is contained in an amount of 0.1 to 5 parts by weight based on 100 parts by weight of the polythiophene-based conductive polymer.

7. 2. The 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 a carbon-based conductive material, a conductive inorganic particle, a binder, and an esterified saccharide.

8. 2. The electrode for a lithium secondary battery according to claim 1, wherein the electrode for a lithium secondary battery is a positive electrode.

9. A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator interposed therebetween, the positive electrode comprising the electrode for a lithium secondary battery according to claim 1 .

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