Composition for forming electrode protection layer for lithium secondary battery, electrode and lithium secondary battery including the same

Abstract

The present invention relates to a composition for forming an electrode protective layer for a lithium secondary battery, which makes it possible to provide an electrode and a battery that have excellent stability by suppressing heat generation or ignition due to external impact, etc., while also having excellent conductivity and rate characteristics, and to an electrode for a lithium secondary battery and a lithium secondary battery including the same.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0169446 dated December 7, 2022, and all contents disclosed in the documents of that Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a composition for forming an electrode protective layer for a lithium secondary battery, which makes it possible to provide an electrode and a battery that have excellent stability by suppressing heat generation or ignition due to external impact, etc., while also having excellent conductivity and rate characteristics, and to an electrode for a lithium secondary battery 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 and usage time of these mobile devices have 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, exposure to high temperatures, external impacts, etc., in various mobile devices or electric vehicles containing these batteries. Therefore, the main research topic in recent years has been how to prevent such fires and explosions and improve safety.

[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 concentrate 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 lead to the risk of fire or explosion of the lithium secondary battery.

[0006] Therefore, in order to improve the safety of secondary batteries by preventing fires and explosions during short circuits, it is necessary to increase the resistance between electrodes and 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, conventional electrodes with functional layers have drawbacks, such as difficulty in sufficiently improving the safety of lithium secondary batteries, and the addition of such functional layers reduces the conductivity or rate characteristics of the lithium secondary battery itself. In particular, when the thickness of the functional layer is increased to improve the safety of the lithium secondary battery, the penetration of the electrolyte through such functional layer is difficult, resulting in a significant reduction in the conductivity or rate characteristics of the electrode itself.

[0008] Due to these problems, there is a continuing demand for the development of a technology that can further improve the safety of lithium secondary batteries while suppressing the deterioration of their conductivity and rate characteristics. Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, the present invention provides a composition for forming an electrode protective layer for a lithium secondary battery, which has excellent stability by suppressing heat generation or ignition due to external impact, etc., and makes it possible to provide an electrode and a battery that have excellent conductivity and rate characteristics.

[0010] The present invention provides an electrode for a lithium secondary battery and a lithium secondary battery that exhibit improved safety and have excellent conductivity and rate characteristics by including a safety protection layer formed from the composition. [Means for solving the problem]

[0011] The present invention provides a composition for forming an electrode protection layer for a lithium secondary battery, which comprises a polythiophene-based conductive polymer that exhibits PTC (positive temperature coefficient) characteristics and porous conductive carbon particles having a large number of pores with diameters of 10 to 300 nm.

[0012] The present invention also provides an electrode for a lithium secondary battery, comprising: a metal current collector; a safety functional layer formed so as to cover at least a portion of the metal current collector and made of the composition for forming an electrode protective layer; and an active material layer formed on the metal current collector and the safety functional layer, the active material layer including an electrode active material and a conductive material.

[0013] 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.

[0014] 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.

[0015] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, in accordance with the principle that the inventor can appropriately define the concept of the term in order to best describe his or her invention.

[0016] 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.

[0017] It should be understood that in this specification, the terms "comprises," "comprises," or "has" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0018] According to one embodiment of the present invention, there is provided a composition for forming an electrode protection layer for a lithium secondary battery, which comprises a polythiophene-based conductive polymer that exhibits PTC (positive temperature coefficient) characteristics and porous conductive carbon particles having a large number of pores with diameters of 10 to 300 nm.

[0019] The composition according to one embodiment is used to form, for example, an electrode protective layer (safety protective layer) on the surface of a metal current collector in an electrode for a lithium secondary battery, which is separate from an electrode active material layer, and includes a polythiophene-based conductive polymer exhibiting PTC characteristics and porous conductive carbon particles.

[0020] First, the polythiophene-based conductive polymer exhibits conductivity when anions derived from the electrolyte of the secondary battery are doped onto the aromatic thiophene rings of the conductive polymer upon activation of the lithium secondary battery. 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.

[0021] However, the conductive polymer can de-dope anions derived from the electrolyte from the aromatic thiophene ring above a certain temperature level. 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 such polythiophene-based conductive polymers, lithium secondary battery electrodes having an electrode protection layer containing the same can contribute to improving the stability of lithium secondary batteries as follows:

[0022] 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 increase in the temperature inside the battery, the conductive polymer can be converted into a non-conductor by dedoping the anions, etc. As a result, the resistance inside the electrode is significantly increased, 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.

[0023] However, if the electrode protection layer is formed too thin, the generation of heat and other problems may not be sufficiently suppressed, resulting in insufficient safety for the lithium secondary battery. To address this issue, if the electrode protection layer is formed too thick, the electrolyte may not penetrate the electrode protection layer easily, reducing the doping effect of the conductive polymer. As a result, the conductivity of the electrode, including the electrode protection layer and the active material layer, may decrease, and the overall rate performance of the secondary battery may also decrease.

[0024] However, the porous conductive carbon particles contained in the composition of one embodiment, which have a large number of pores of 10 to 300 nm, facilitate electrolyte penetration due to their own porosity and can exhibit excellent electronic conductivity and thermal conductivity due to thermal diffusion. As a result, when an electrode protective layer is formed using the composition of one embodiment containing the porous conductive carbon particles, not only can the electrode protective layer, even if formed thick, exhibit excellent electrolyte permeability, but also can cause an electrode containing the electrode protective layer to exhibit improved electrical conductivity. Furthermore, due to the excellent thermal conductivity of the porous conductive carbon particles, an electrode including an electrode protective layer formed using the composition of one embodiment can effectively diffuse heat therein, thereby exhibiting further improved safety.

[0025] Therefore, the composition of one embodiment and the electrode protective layer formed therefrom can contribute to improving the safety of lithium secondary batteries, and even when formed thickly, the electrode and the lithium secondary battery including the electrode can exhibit excellent conductivity and rate characteristics.

[0026] Meanwhile, the polythiophene-based conductive polymer contained in the composition of one embodiment exhibits the above-mentioned PTC characteristics, and the effective operating temperature at which the conductive polymer is converted into a non-conductor may be 70 to 130° C., or 80 to 125° C. By converting the conductive polymer into a non-conductor 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 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, R 1 is a functional group of 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 5000, alternatively from 10 to 2000, or alternatively from 50 to 1000, 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] Furthermore, such a 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 electrode protection layer to exhibit excellent coating properties while also exhibiting appropriate adhesion to metal current collectors and the like.

[0030] In a more specific example, the polythiophene-based conductive polymer may contain the repeating unit of Formula 1 in a content of more than 0 mol%, or 0.001 mol% or more, or 0.01 mol% or more, or 1 mol% or more, or may contain the repeating unit of Formula 1 in a content of 100 mol% or less, or 80 mol% or less, or 50 mol% or less. In this case, the polythiophene-based conductive polymer may contain alkylthiophene-based repeating units, 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, excluding the repeating unit of Formula 1.

[0031] Such polythiophene-based conductive polymers contain the substituted thiophene ring and can exhibit an appropriate effective operating temperature. 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 the secondary batteries.

[0032] Furthermore, due to the specific structure described above, 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 is 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 electrode protection layer to be uniformly formed near the surface of the metal current collector. Therefore, the conductive polymer contributes to improving the safety of secondary batteries without impairing their basic performance.

[0033] The conductive polymer may be contained 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 electrode active material) contained in the active material layer, thereby providing the electrode of the embodiment with superior safety and charge / discharge characteristics.

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

[0035] The composition of the present embodiment includes porous conductive carbon particles having a large number of pores with diameters of 10 to 300 nm or 30 to 250 nm, together with the polythiophene-based conductive polymer. Such porous conductive carbon particles can be obtained from, for example, a carbonaceous material such as hard carbon and / or soft carbon, and can be produced by modifying or surface-treating the carbonaceous material to have porosity, or the porous conductive carbon particles can be commercially obtained and used.

[0036] The porous conductive carbon particles, together with the conductive polymer, exhibit excellent dispersibility in organic solvents, allowing a composition containing the porous conductive carbon particles to form an excellent electrode protective layer on a current collector. Furthermore, the porous conductive carbon particles facilitate the permeation of an electrolyte through the electrode protective layer, and can exhibit excellent electrical and thermal conductivity.

[0037] Therefore, even if a thicker electrode protective layer is formed using the composition of one embodiment to further improve the safety of the lithium secondary battery, such an electrode protective layer exhibits excellent electrolyte permeability and excellent conductivity, which can contribute to improving not only the safety but also the conductivity and rate characteristics of the lithium secondary battery.

[0038] The porous conductive carbon particles may have a number average particle size (D50) of 0.5 to 20 μm, or 1 to 15 μm, or 1.5 to 10 μm, in order to provide excellent dispersibility in the solvent and coatability of the composition of one embodiment.

[0039] In addition, such porous conductive carbon particles have a porosity of 10 to 40%, or 15 to 35%, and a thickness of 20 to 600 mm, in view of the above-mentioned electrolyte permeability, excellent electrical conductivity, and thermal conductivity. 2 / g, or 30-500m 2 The porosity defined as the specific surface area in g / g can be shown.

[0040] The porous conductive carbon particles may be contained in an amount of 0.1 to 80 parts by weight, or 0.5 to 60 parts by weight, per 100 parts by weight of the polythiophene-based conductive polymer. This amount of porous conductive carbon particles allows the composition of one embodiment to form a good electrode protective layer, which can effectively contribute to improving the safety, rate characteristics, and conductivity of the lithium secondary battery.

[0041] The composition of the present embodiment can be prepared in the form of a liquid composition by dissolving or dispersing the conductive polymer and porous conductive carbon particles in an organic solvent such as chloroform, tetrahydrofuran (THF), toluene, or xylene at a concentration of about 0.1 to 5 wt %. This composition can be coated on a metal current collector and dried to form an electrode protection layer. Thereafter, a slurry composition described below can be coated and dried to form an active material layer, which can then be rolled to produce an electrode for a lithium secondary battery.

[0042] In this case, the composition of the embodiment may further include one or more additives selected from the group consisting of an additional carbon-based conductive material, a binder, and an esterified saccharide.

[0043] In this case, the carbon-based conductive material and binder may be the same components as those contained in the active material layer described below, and the addition of these components may further improve the conductivity of the electrode, the adhesiveness or mechanical properties of the safety protection layer, etc. Representative examples of carbon-based conductive materials include carbon black and carbon nanotubes.

[0044] The esterified sugar may be a monosaccharide, oligosaccharide, or polysaccharide having an acyl group, which generates gas during overcharging of the secondary battery, thereby blocking the conductive path between the metal current collector and the electrode active material. The addition of this component can further improve the safety of the secondary battery.

[0045] Meanwhile, according to another embodiment of the present invention, there is provided an electrode for a lithium secondary battery, including an electrode protective layer (safety functional layer) formed from the composition of the above-described embodiment. The electrode of this other embodiment includes a metal current collector, a safety functional layer formed from the composition of the above-described embodiment so as to cover at least a portion of the metal current collector, and an active material layer formed on the metal current collector and the safety functional layer, the active material layer including an electrode active material and a conductive material.

[0046] The electrode of this other embodiment is formed from the composition of the above-described embodiment, and includes a safety protection layer on a current collector, the safety protection layer including the conductive polymer and porous conductive carbon particles, thereby exhibiting improved safety, conductivity, rate characteristics, and the like.

[0047] The electrode of this other embodiment further includes an active material layer formed on the metal current collector and the safety protection layer formed from the composition of the embodiment, and the active material layer may include an electrode active material, a conductive material, and optionally a binder, etc. In this case, the electrode of this other embodiment is preferably a positive electrode, and therefore, the following description will be based on this example.

[0048] 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 of any metal or alloy that has excellent conductivity without 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. Furthermore, 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.

[0049] 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.

[0050] 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 CoGb 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).

[0051] 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.

[0052] 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 may contain a coating element compound such as an oxide, hydroxide, oxyhydroxide, oxycarbonate, or hydroxycarbonate of the coating element. These coating layer compounds 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.

[0053] The conductive material contained in the active material layer is used to impart conductivity to the electrode, and any material that is electronically conductive without causing a chemical change in the battery that is constructed can be used. Examples of such a material include natural graphite, artificial graphite, carbon black, carbon fiber, carbon nanotubes, metal powders and metal fibers such as copper, nickel, aluminum and silver, and conductive materials such as polyphenylene derivatives can be used alone or in combination of two or more.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] The active material layer can be formed by dissolving or dispersing the components, 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, followed by drying and rolling.

[0058] 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 above-mentioned positive electrode active material and conductive material.

[0059] Meanwhile, the process and conditions for forming the active material layer may be the same as those for forming a general positive electrode, and therefore, further explanation regarding this will be omitted.

[0060] The active material layer formed by the above method can have a thickness of 5 to 200 μm, or 10 to 100 μm, and the safety protection layer can have a uniform thickness of 0.01 to 20 μm, or 0.05 to 10 μm, or 0.5 to 5 μm. By forming the safety protection layer near the surface of the metal current collector to the above uniform thickness, the conductive polymer contained in the safety protection layer can prevent direct contact between the active material layer and the metal current collector when an external stimulus such as an external impact is applied, thereby ensuring improved safety of the secondary battery. Furthermore, by including porous conductive carbon particles in the safety protection layer, excellent electrode conductivity and excellent rate characteristics of the secondary battery can be ensured even when the safety protection layer is formed relatively thick.

[0061] Meanwhile, according to yet another embodiment of the present invention, there is provided a lithium secondary battery including the electrode of the other embodiment described above as a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

[0062] In such a 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.

[0063] As the negative electrode active material, for example, graphite having a complete layered crystal structure like natural graphite, soft carbon having a low-crystalline layered crystal structure (graphene structure; a structure in which hexagonal honeycomb-shaped planes of carbon are arranged in layers), and hard carbon in which such a structure is mixed with an amorphous part, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene, activated carbon and other carbon and graphite materials; 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) and other metal composite oxides; 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 and the like can be used.

[0064] 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 isometric 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 those in which the silicon (Si) particles and silicon oxide particles are mixed.

[0065] 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.

[0066] 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.

[0067] In addition, the negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, nickel, titanium, calcined carbon, etc. may be used. In the case of copper or stainless steel, it may be surface-treated with carbon, nickel, titanium, silver, etc.

[0068] In addition, the negative electrode current collector, like the positive 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.

[0069] The separator is a thin insulating membrane interposed between the positive and negative electrodes and exhibits high ion permeability and mechanical strength. The separator can be any commonly used material in the industry, but examples include sheets or nonwoven fabrics made of chemically resistant and hydrophobic materials such as polypropylene, glass fiber, or polyethylene. In some cases, a composite separator may be used in which inorganic particles 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 may also function as the separator. The separator may have an average pore size of 0.01 to 10 μm and an average thickness of 5 to 300 μm.

[0070] The lithium secondary battery described above may further include an electrolyte, which may be an electrolyte solution containing a non-aqueous organic solvent and a lithium salt, an electrolyte membrane containing an organic or inorganic solid electrolyte, or a mixture of these. However, types of usable electrolytes are well known to those skilled in the art and are not particularly limited in the batteries of other embodiments, so further description thereof will be omitted. [Effects of the Invention]

[0071] As described above, in an electrode and a lithium secondary battery including an electrode protective layer formed from the composition of the present invention, even if the temperature inside the battery rises rapidly due to the application of an external stimulus such as overcharging, high temperature, or external impact, the electrode protective layer (safety protective layer) formed relatively thickly near the metal current collector effectively blocks overcurrent, minimizing fire and explosion, etc. As a result, the lithium secondary battery exhibits excellent safety.

[0072] Furthermore, the lithium secondary battery can exhibit excellent conductivity and rate characteristics despite the electrode protection layer being formed relatively thick. DETAILED DESCRIPTION OF THE INVENTION

[0073] The following description of the preferred embodiments of the present invention will be made in a manner that will 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.

[0074] Manufacturing example: Synthesis of monomers and conductive polymers [Chemical formula 3] [ka]

[0075] After creating a nitrogen environment inside a 3-way round bottom flask (RBF) with a nitrogen flow, 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 60% sodium hydroxide in mineral oil was slowly added to the RBF and stirred while maintaining the nitrogen environment. After stirring for approximately 1 hour, 10.0 g (0.06 mol) of 3-bromothiophene was added and the mixture was refluxed at approximately 100°C for approximately 24 hours. The reaction solution was filtered using a vacuum device and then 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).

[0076] 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 poured into a 5000 MWCO (molecular weight of cut-off) permeation membrane and immersed in 1500 ml of acetonitrile to remove unreacted iron(III) chloride and monomer. The residue deposited inside the permeation 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.

[0077] 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 6 g (30 parts by weight based on 100 parts by weight of the conductive polymer) of porous conductive carbon particles (pore diameter: 10-300 nm, D50: 1-3 μm) in 1,980 g of chloroform solvent. This composition was gravure coated onto a thin aluminum (Al) film of a positive electrode current collector and dried to form a safety protection layer with a thickness of approximately 5 μm.

[0078] 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. The cathode slurry was then coated on the conductive polymer-containing 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 cathode active material, conductive material, and binder). The resulting mixture was then roll pressed to form an active material layer with a total thickness of 58 μm, thereby preparing a cathode.

[0079] (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 anode current collector, dried, and then roll-pressed to prepare anode.

[0080] (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 an Al2O3 / PVdF-HFP weight ratio of 90 / 10, and the mixture was ball milled for 12 hours to prepare a slurry. The resulting slurry was then applied to an approximately 8 μm thick polyolefin separator using a dip coating method, resulting in a porous separator with a coating thickness of approximately 4.5 μm.

[0081] (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 comprising a bi-cell. 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 poured into the case to produce a lithium secondary battery.

[0082] Example 2: Fabrication of positive electrode and lithium secondary battery A composition was obtained by dissolving 20 g of the conductive polymer (Mw = 34,000 g / mol) obtained in the above Preparation Example and 6 g (30 parts by weight based on 100 parts by weight of the conductive polymer) of porous conductive carbon particles (pore diameter: 10-300 nm, D50: 1-3 μm) in 1,980 g of chloroform solvent. This composition was gravure coated onto a thin aluminum (Al) film of a positive electrode current collector and dried to form a safety protection layer with a thickness of approximately 10 μm.

[0083] After forming the safety protection layer, the same process as in Example 1 was carried out to manufacture a positive electrode and a lithium secondary battery of Example 2.

[0084] Comparative Example 1 A positive electrode and a lithium secondary battery of Comparative Example 1 were produced in the same manner as in Example 1, except that the porous conductive carbon particles were not used.

[0085] Comparative Example 2 A positive electrode and a lithium secondary battery of Comparative Example 2 were manufactured in the same manner as in Example 1, except that 6 g of a carbon black conductive material (30 parts by weight based on 100 parts by weight of the conductive polymer) was used instead of the porous conductive carbon particles.

[0086] Comparative Example 3 A positive electrode and a lithium secondary battery of Comparative Example 3 were manufactured in the same manner as in Example 1, except that a safety protection layer was not formed on the aluminum (Al) thin film of the positive electrode current collector.

[0087] Test Example 1. 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 number of charge-discharge cycles 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. These high-rate discharge capabilities were evaluated at a temperature of 45°C. The high-rate discharge characteristics at 1.5 C are shown in Table 1 below.

[0088] 2. Impact Test Ten lithium secondary batteries each manufactured in the examples and comparative examples were prepared and placed on a flat plate. An iron rod having a diameter of 15.8 mm was placed on top of the iron rod, and then a 9.1 kg weight was dropped from a height of 61 cm onto the lithium secondary batteries. After the free fall, the 10 batteries were checked for ignition, and the number of batteries that did not ignite is shown in Table 1 below as the test results.

[0089] [Table 1]

[0090] Referring to Table 1, it was confirmed that Examples 1 and 2 were superior to Comparative Example 3 due to the formation of the safety protection layer, and exhibited safety at a level equal to or higher than Comparative Examples 1 and 2. In particular, it was confirmed that Example 2 exhibited superior safety compared to Comparative Examples 1 and 2.

[0091] Furthermore, it was confirmed that Examples 1 and 2, due to the inclusion of porous conductive carbon particles in the safety protection layer, are equivalent to Comparative Example 3, in which no safety protection layer is formed, and exhibit better rate characteristics (high-rate discharge characteristics) than Comparative Examples 1 and 2.

Claims

1. a polythiophene-based conductive polymer exhibiting PTC (positive temperature coefficient) characteristics; and porous conductive carbon particles having a large number of pores with diameters of 10 to 300 nm formed therein.

2. 2. The composition for forming an electrode protection layer 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 composition for forming an electrode protection layer for a lithium secondary battery according to claim 1, wherein the polythiophene-based conductive polymer comprises a homopolymer or copolymer having a repeating unit represented by the following Chemical Formula 1: [Chemical formula 1] 【Chemistry 4】 In chemical formula 1, R １ is a functional group of formula 2 below: [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 in the range of 1 to 5000.

4. 2. The composition for forming an electrode protection layer 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 composition for forming an electrode protection layer for a lithium secondary battery according to claim 1, wherein the porous conductive carbon particles have a number average particle size (D50) of 0.5 to 20 μm.

6. The porous conductive carbon particles have a porosity of 10 to 40% and a surface roughness of 20 to 600 m ２ The composition for forming an electrode protection layer for a lithium secondary battery according to claim 1 , having a specific surface area of ​​1 / g.

7. 2. The composition for forming an electrode protection layer for a lithium secondary battery according to claim 1, wherein the porous conductive carbon particles are contained in an amount of 0.1 to 80 parts by weight per 100 parts by weight of the polythiophene-based conductive polymer.

8. The composition for forming an electrode protection layer for a lithium secondary battery according to claim 1 , further comprising a carbon-based conductive material such as carbon black or carbon nanotubes.

9. The composition for forming an electrode protection layer for a lithium secondary battery according to claim 1 , further comprising at least one of a binder or an esterified saccharide.

10. a metal current collector; a safety functional layer formed so as to cover at least a portion of the metal current collector and made of the composition for forming an electrode protective layer for a lithium secondary battery according to any one of claims 1 to 9; An electrode for a lithium secondary battery, comprising 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.

11. the active material layer has a thickness of 5 to 200 μm, 11. The electrode for a lithium secondary battery according to claim 10, wherein the safety protection layer has a thickness of 0.01 to 20 μm.

12. The electrode for a lithium secondary battery according to claim 10, which serves as a positive electrode.

13. A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator interposed therebetween, the lithium secondary battery comprising the electrode for lithium secondary batteries according to claim 10 as the positive electrode.

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