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 - reference to related applications This application claims the benefit of priority based on Korean Patent Application No. 10 - 2022 - 0170925, filed on December 8, 2022, and all the contents disclosed in the literature of the Korean patent application are incorporated herein by reference in their entirety.

[0002] The present invention relates to an electrode for a lithium secondary battery and a lithium secondary battery including the same, which enable the provision of a battery having excellent charge / discharge characteristics while having excellent stability with a uniform safety protection layer that suppresses heat generation or ignition.

Background Art

[0003] As the 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, the demand for secondary batteries as an energy source required to drive these devices has been rapidly increasing. In particular, as the data processing speed and usage time of the mobile devices increase, the development of lithium secondary batteries having a higher energy density, operating potential, capable of maintaining excellent characteristics for a long time, and having a low self - discharge rate has been actively carried out.

[0004] However, as the capacity and energy density of lithium secondary batteries have increased significantly, many reports have been made on fire and explosion accidents due to overcharging, high - temperature exposure, or external impact of various mobile devices or electric vehicles including these batteries. Therefore, recently, the main research topic of lithium secondary batteries is to suppress such fires and explosions and improve safety.

[0005] In the lithium secondary battery, it is known that the direct causes of ignition, explosion, etc. are short circuits that occur when the positive and negative electrodes inside the secondary battery come into direct contact due to stimuli applied from the outside such as high temperature and external shock. For example, when the lithium secondary battery is overcharged or exposed to high temperature or external shock, the internal temperature of the secondary battery may rise rapidly, causing the separator to contract, or the internal structure of the secondary battery may be damaged by the external shock. As a result, the positive and negative electrodes may come into contact and a short circuit may occur. When such a short circuit occurs, the movement of lithium ions and electrons may be concentrated through the contact portion of the positive and negative electrodes, resulting in an overcurrent. This may cause heat generation, gas generation inside the battery, and volume expansion, etc., and there may be a risk of ignition and explosion of the lithium secondary battery.

[0006] Therefore, in order to suppress ignition, explosion, etc. during the short circuit and improve the safety of the secondary battery, it is necessary to increase the resistance between the electrodes and cut off the current when stimuli such as the high temperature or external shock are applied. For this purpose, various functional layers or functional materials have been added to electrodes for lithium secondary batteries, etc. to increase the resistance when stimuli such as high temperature are applied, and thus various attempts have been made to improve the safety of the secondary battery.

[0007] However, in the case of electrodes with previously known functional layers added, it is difficult to sufficiently improve the safety of the lithium secondary battery, or there are disadvantages that the functional layer is not formed uniformly and inhibits basic battery performance such as the charge / discharge characteristics of the secondary battery. Summary of the Invention Problems to be Solved by the Invention

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

[0009] In addition, the present invention provides a lithium secondary battery including the above electrode, which exhibits excellent stability and charge-discharge characteristics.

Means for Solving the Problems

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

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

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

[0013] The terms and words used in this specification and claims should not be construed as being limited to ordinary or preconceived meanings, and based on the principle that the inventor can appropriately define the concept of the terms in order to explain his own invention in the best way, they should be construed as meanings and concepts conforming to the technical idea of the present invention.

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

[0015] As used herein, terms such as "comprising", "including" or "having" are intended to specify the presence of implemented features, numbers, steps, components, or combinations thereof, and should not be construed as precluding the 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 part of the metal current collector, including a polythiophene-based conductive polymer exhibiting PTC (positive temperature coefficient) characteristics and a thixotrophic agent; and An electrode for a lithium secondary battery is provided, which includes an electrode active material and a conductive material, and an active material layer formed on the metal current collector and the safety functional layer.

[0017] The electrode of the above embodiment has a safety functional layer including a polythiophene-based conductive polymer and a thixotropic agent formed on the surface of the metal current collector.

[0018] First, as the lithium secondary battery is activated, anions derived from the electrolyte of the secondary battery can be doped onto the aromatic thiophene ring of the conductive polymer, enabling the polythiophene-based conductive polymer to exhibit conductivity. Therefore, during the normal charge and discharge process of the secondary battery, the conductive polymer can exhibit conductivity to enable the secondary battery to exhibit appropriate charge and discharge characteristics.

[0019] However, at temperatures above a certain level, the anions derived from the electrolyte are de-doped from the aromatic thiophene ring in the conductive polymer. As a result, the conductive polymer can act as an insulator, increasing the resistance and exhibiting PTC characteristics that block the flow of current. Due to the action of such a polythiophene-based conductive polymer, an electrode of one embodiment having a safety protection layer formed thereon can contribute to improving the stability of a lithium secondary battery as follows.

[0020] When external stimuli such as overcharging, high temperature, or external shock are applied to the lithium secondary battery including the electrode and the temperature inside the battery rises rapidly, such a conductive polymer can be converted into an insulator by the above-described de-doping of anions. Therefore, the resistance inside the electrode can be greatly increased, the flow of current between the current collector and the active material layer can be blocked, overcurrent due to short circuit between electrodes can be prevented, and heat generation, ignition, explosion, and gas generation of the secondary battery can be suppressed.

[0021] However, when the safety protection layer is coated and formed only with the polythiophene-based conductive polymer, it becomes difficult to uniformly form the safety protection layer due to the fluidity and / or viscosity of such a polymer. In this case, the safety protection layer may be locally formed too thinly or thickly, and the safety protection layer may not exhibit sufficient adhesion to a metal current collector or the like, making it difficult to form it well.

[0022] Due to such non-uniform formation of the safety protection layer, it may be difficult to ensure excellent safety of the lithium secondary battery because the safety protection layer cannot completely block the overcurrent locally even at temperatures above a certain level, or the basic performance such as the charge and discharge characteristics of the lithium secondary battery may deteriorate because the safety protection layer has a locally large thickness.

[0023] However, the electrode of one embodiment includes a thixotropic agent together with the conductive polymer, so that the safety protection layer can be formed more uniformly. More specifically, the thixotropic agent can be added to the composition for forming the safety protection layer to improve its fluidity and exhibit uniform coating properties. Further, after the coating and drying of the composition, the viscosity of such a composition can be increased again to contribute to the adhesiveness of the safety protection layer and the like.

[0024] Due to the action of such a thixotropic agent, the electrode of one embodiment can include a uniform safety protection layer with a small thickness deviation overall, and a uniform active material layer can be formed on the safety protection layer. Further, after continuously manufacturing the electrode sheet, when punching out such an electrode sheet to manufacture the electrodes included in a plurality of lithium secondary batteries, the safety protection layer and the active material layer can have a uniform thickness in each electrode included in the plurality of lithium secondary batteries. For example, in each electrode included in one or more lithium secondary batteries manufactured from the same electrode sheet, the thickness of the safety protection layer can be 60 nm or less, or 50 nm or less, or have a standard deviation of 5 - 50 nm.

[0025] As a result, when a stimulus such as high temperature is applied to the lithium secondary battery for the electrode of one embodiment, the safety protection layer can effectively block the overcurrent and improve the safety of the lithium secondary battery. Further, due to the formation of such a uniform safety protection layer, the basic performance degradation such as the charge and discharge characteristics of the lithium secondary battery can be minimized.

[0026] On the other hand, the polythiophene-based conductive polymer included in the electrode of one embodiment exhibits the aforementioned PTC characteristics, and the effective operating temperature at which such a conductive polymer is converted into an insulator can be 70 - 130 °C, or 80 - 125 °C. By converting the conductive polymer into an insulator at such an effective operating temperature, while the conductive polymer does not inhibit the normal charge and discharge process of the secondary battery, it can more effectively suppress the ignition or explosion of the secondary battery when an external stimulus is applied.

[0027] Further, the conductive polymer may be a polythiophene-based polymer or copolymer containing a substituted or unsubstituted thiophene-based repeating unit in an amount of 50 mol% or more, or 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 containing the repeating unit of the following Chemical Formula 1: [Chemical Formula 1] [Chemical Structure] In Chemical Formula 1, R1 is a functional group of the following Chemical Formula 2, [Chemical Formula 2] [Chemical Structure] 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, and n is an integer in the range of 1 to 5000, or 10 to 2000, or 50 to 1000. 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] Also, such a polythiophene-based conductive polymer can have a weight average molecular weight of, for example, 5000 to 100000 g / mol, or 10000 to 80000 g / mol. Thereby, while the composition for forming the safety protection layer can exhibit excellent coating properties, it can also exhibit appropriate adhesiveness to a metal current collector or the like.

[0030] In a more specific example, the polythiophene-based conductive polymer can contain the repeating unit of Chemical Formula 1 in an amount exceeding 0 mol%, or 0.001 mol% or more, or 0.01 mol% or more, or 1 mol% or more, and can contain it in an amount of 100 mol% or less, or 80 mol% or less, or 50 mol% or less. At this time, the polythiophene-based conductive polymer can contain an alkylthiophene-based repeating unit having a remaining content excluding the repeating unit of Chemical Formula 1, for example, an alkylthiophene-based repeating unit in which an alkyl group having 1 to 20 carbon atoms or 3 to 15 carbon atoms is substituted on a thiophene ring.

[0031] Such a polythiophene-based conductive polymer can exhibit an appropriate effective operating temperature and the like including the substituted thiophene ring. As a result, while not inhibiting the charge and discharge characteristics of the lithium secondary battery, it can be converted into an insulator when a high temperature above a certain level is applied, thereby improving the safety of the secondary battery.

[0032] In addition, the polythiophene-based conductive polymer exhibits relatively low affinity and solubility with respect to an organic solvent mainly contained in a slurry composition for forming an electrode active material layer, such as a solvent such as N-methylpyrrolidone, due to the predetermined structure as described above, and can exhibit excellent adhesion to the metal current collector. Therefore, after forming such a polythiophene-based conductive polymer on a metal current collector, in the process of applying and drying the slurry composition to form an electrode active material layer, the phenomenon in which the conductive polymer dissociates and diffuses into a wide area of the active material layer can be minimized. Therefore, by using such a polythiophene-based conductive polymer, the safety protection layer can be uniformly formed near the surface of the metal current collector. Therefore, the conductive polymer can contribute to improving the safety while not inhibiting the basic performance of the secondary battery.

[0033] On the other hand, 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 (for example, the positive electrode active material) contained in the active material layer. Thereby, the electrode of one embodiment can have more excellent safety and charge-discharge characteristics.

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

[0035] On the other hand, in the electrode of the above-described embodiment, the safety protection layer contains a thixotropic agent together with the above-mentioned polythiophene-based conductive polymer. The thixotropic agent can be mixed with the conductive polymer to increase the fluidity of the composition for forming the safety protection layer, contribute to the improvement of its coating property, and the uniform formation of the safety protection layer. Further, after the coating and drying of the composition, the viscosity of such a composition can be increased again so that the safety protection layer has excellent adhesion to the metal current collector.

[0036] As such a thixotropic agent, an organic substance or inorganic particles that can cause a sol-gel state change or an increase-decrease in fluidity of the polymer solution can be used. Specific examples of such a thixotropic agent include hydrophilic fumed silica, aluminum salts, bentonite or its derivatives, cellulose-based compounds, polyvinyl-based compounds, polyacrylic acid-based compounds, modified urea or maleic acid copolymers, etc. Considering the polythiophene-based conductive polymer, etc., the modified urea or its thixotropic agent in an organic solution state can be appropriately used.

[0037] As the thixotropic agent, existing commercialized thixotropic agents can be used. Examples of commercialized thixotropic agents include HL-200, HL-300, HL-380 (manufactured by DKSH), GARAMITE-1958, RHEOBYK-410, 411, 7410, 605 (manufactured by BYK), Alugel 28DG, Laevisil-SP (manufactured by Baerlocher), CAB-O-SIL H-300 (manufactured by Cabot), Jaylink JL-106E (manufactured by Dymax), Bentone38, SD3, 34 (manufactured by "Elements"), etc.

[0038] Such a thixotropic agent may be included in the safety protection layer in an amount of 0.1 to 5 parts by weight, or 0.2 to 3 parts by weight, or 0.5 to 2 parts by weight, based on 100 parts by weight of the polythiophene-based conductive polymer, while appropriately controlling the fluidity and viscosity of the composition for forming the safety protection layer and without inhibiting the electrical properties of the electrode, etc.

[0039] On the other hand, the safety protection layer can be formed by dissolving or dispersing the aforementioned conductive polymer and thixotropic agent in an organic solvent such as chloroform, tetrahydrofuran (THF), toluene or xylene at a concentration of about 0.1 to 5 wt% to form a liquid composition, and then coating and drying such a composition on the metal current collector. Thereafter, the slurry composition described below can be coated and dried to form an active material layer, and this can be rolled to manufacture an electrode for a lithium secondary battery in one embodiment.

[0040] At this time, the safety protection layer and the liquid composition for forming it may further contain one or more additives selected from the group consisting of carbon-based conductive materials, conductive inorganic particles, binders, and esterified saccharides in addition to the conductive polymer.

[0041] At this time, as the carbon-based conductive material and the binder, the same components as those contained in the active material layer described later can be used. By adding these components, the conductivity of the electrode, the adhesiveness of the safety protection layer, or the mechanical properties of the like in one embodiment can be further improved. Further, as the conductive inorganic particles, alumina or zirconia particles having a nano-scale particle size, for example, a particle size of 5 to 100 nm can be used, and the conductivity of the electrode and the secondary battery can be further improved by adding them. Further, as the esterified saccharides, monosaccharides, oligosaccharides or polysaccharides having an acyl group can be used. This component can play a role of generating gas during overcharging of the secondary battery to block the conduction path between the metal current collector and the electrode active material, and the safety of the secondary battery can be further improved by adding this component.

[0042] On the other hand, the electrode of one embodiment further includes an active material layer formed on the metal current collector and the safety protection layer. Such an active material layer may include an electrode active material, a conductive material, and optionally a binder and the like. At this time, since the electrode formed with the conductive polymer is preferably a positive electrode, the following description will be based on such an example.

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

[0044] In the case of the positive electrode active material contained in the active material layer, there is no particular limitation as long as it is a material capable of reversible insertion and extraction of lithium ions. For example, it can contain 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, as the positive electrode active material, a compound represented by any one of the following chemical formulas can be used. Li a A 1-b R b D2 (where 0.90 ≤ a ≤ 1.8 and 0 ≤ b ≤ 0.5); Li a E 1-b R b O 2-c D c (where 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, and 0 ≤ c ≤ 0.05); LiE 2-b R b O 4-c D c (where 0 ≤ b ≤ 0.5 and 0 ≤ c ≤ 0.05); Li a Ni 1-b-c Co b R c D α (where 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 (where 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 (where 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2); Li a Ni 1-b-c Mn b R c D α(In the above formula, 0.90 ≦ a ≦ 1.8, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05, and 0 < α ≦ 2); Li a Ni 1-b-c Mn b R c O 2-α Z α (In the above formula, 0.90 ≦ a ≦ 1.8, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05, and 0 < α < 2); Li a Ni 1-b-c Mn b R c O 2-α Z2 (In the above formula, 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 (In the above formula, 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 (In the above formula, 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 (In the above formula, 0.90 ≦ a ≦ 1.8 and 0.001 ≦ b ≦ 0.1); Li a CoG b O2 (In the above formula, 0.90 ≦ a ≦ 1.8 and 0.001 ≦ b ≦ 0.1); Li a MnG b O2 (In the above formula, 0.90 ≦ a ≦ 1.8 and 0.001 ≦ b ≦ 0.1); Li a Mn2G b O4 (In the above formula, 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; J is V, Cr, Mn, Co, Ni, Cu, or a combination thereof.

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

[0048] Also, the conductive material contained in the active material layer is used to impart conductivity to the electrode, and in the configured battery, any material can be used as long as it is an electron conductive material that does not cause a chemical change. Examples thereof include natural graphite, artificial graphite, carbon black, carbon fiber, carbon nanotube, copper, nickel, aluminum, metal powders such as silver, metal fibers, etc., and one or more conductive materials such as polyphenylene derivatives can be mixed and used.

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

[0050] The binder serves to well adhere the particles of the positive electrode active material to each other and enhance the adhesion of the active material layer. Typical examples thereof include the aforementioned halogenated polyolefin-based polymer binders, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon, etc., which can be used.

[0051] The binder can be added in an amount of 1 to 50% by weight, or 2 to 30% by weight, based on the total weight of the active material layer. This enables the formation of a positive electrode with excellent durability while not inhibiting the electrical properties and / or capacity properties, etc., of the positive electrode.

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

[0053] At this time, 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, methyl alkyl polysiloxane, alkylbenzene, propylene glycol, xylene, or monophenyl glycol, etc. Among these, NMP, etc., can be appropriately used in consideration of the dispersibility and processability, etc., of the aforementioned positive electrode active material and conductive material.

[0054] On the one hand, since the formation process and conditions of the active material layer can follow general cathode formation processes and conditions, additional explanations regarding this are omitted.

[0055] The active material layer formed by the aforementioned method can have a thickness of 5 to 200 μm, or alternatively 10 to 100 μm, and the safety protection layer can have a uniform thickness of 0.01 to 20 μm, or alternatively 0.05 to 10 μm. Such a uniform thickness can be defined by the standard deviation of the thickness of the aforementioned safety protection layer. For example, for each electrode included in one or more lithium secondary batteries manufactured from the same electrode sheet, the thickness of the safety protection layer can have a standard deviation of 60 nm or less, or alternatively 50 nm or less, or 5 to 50 nm.

[0056] When the safety protection layer is formed with the aforementioned uniform thickness near the surface of the metal current collector, when an external stimulus such as an external impact is applied, the conductive polymer contained in the safety protection layer suppresses direct contact between the active material layer and the metal current collector, ensuring improved safety of the secondary battery. Also, it can be minimized that the safety protection layer is locally formed overly thick and inhibits charge and discharge characteristics of the secondary battery.

[0057] On the other hand, according to another embodiment of the invention, a lithium secondary battery is provided that includes the electrode of the aforementioned one embodiment as a positive electrode, together with a negative electrode and a separator interposed between the positive electrode and the negative electrode.

[0058] In such a lithium secondary battery of another embodiment, the negative electrode is manufactured by applying, drying, and rolling a negative electrode active material on a negative electrode current collector, and can further include a conductive material and a binder as 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, graphitization-resistant 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 polymers such as polyacetylene; Li-Co-Ni-based materials; 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 include 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 a mixture of the silicon (Si) particles and silicon oxide particles.

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

[0062] Further, 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] 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, fired carbon, etc. can be used. In the case of copper or stainless steel, those surface-treated with carbon, nickel, titanium, silver, etc. can also be used.

[0064] In addition, similar to the positive electrode current collector, the negative electrode current collector can also form fine irregularities on its surface to strengthen the bonding force with the negative electrode active material layer, and various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven bodies, etc. are possible. Also, the average thickness of the negative electrode current collector can be appropriately applied at 3 to 100 μm in consideration of the conductivity and total thickness of the manufactured negative electrode.

[0065] In addition, the separator is interposed between the positive electrode and the negative electrode, and an insulating thin film having high ion permeability and mechanical strength is used. The separator is not particularly limited as long as it is commonly used in the industry. Specifically, sheets or non-woven fabrics made of chemically resistant and hydrophobic polypropylene, glass fibers, or polyethylene, etc. can be used. In some cases, a composite separator in which inorganic particles / organic particles are coated on a porous polymer substrate such as the above-mentioned sheet or non-woven fabric with an organic binder polymer may also be used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte can also serve as the separator. In addition, the pore diameter of the separator may be 0.01 to 10 μm on average, and the thickness may be 5 to 300 μm on average.

[0066] The lithium secondary battery of the above-described other embodiments can further include an electrolyte. Such an electrolyte may be an electrolytic solution containing a non-aqueous organic solvent and a lithium salt, or an electrolyte membrane containing an organic or inorganic solid electrolyte, or these may be used in combination. However, the types of the usable electrolytes are well known to those skilled in the art and are not particularly limited in the batteries of other embodiments, so additional explanations regarding this are omitted.

Advantages of the Invention

[0067] As described above, even when external stimuli such as overcharging, high temperature, or external impact are applied to the lithium secondary battery including the electrode of the present invention and the temperature inside the battery rapidly rises, the overcurrent is effectively blocked by the safety protection layer uniformly formed near the metal current collector, and ignition and explosion can be minimized.

[0068] Therefore, the lithium secondary battery can exhibit excellent safety, and the decrease in charge-discharge characteristics due to the safety protection layer is suppressed, so that excellent electrochemical characteristics can be exhibited.

Best Mode for Carrying Out the Invention

[0069] Hereinafter, embodiments of the invention will be described so that those having ordinary knowledge in the technical field to which the invention pertains can easily implement it. However, the invention can be realized in various different forms and is not limited to the embodiments described herein.

Examples

[0070] Production Example: Synthesis of Monomer and Conductive Polymer [Chemical Formula 3]

Chem.

[0071] 3.0 g (12.2 mmol) of the monomer compound of Chemical Formula 3 and 47.8 g (243 mmol) of 3-octyl thiophene were added to a solution prepared by dissolving 124 g (767 mmol) of iron(III) chloride in 1000 ml of methylene chloride, and a polymerization reaction was carried out while stirring at about 25 °C for 24 hours. After putting the reaction solution into a permeation membrane with an MWCO (molecular weight of cut-off) of 5000, it was immersed in 1500 ml of acetonitrile solvent to remove unreacted iron(III) chloride and monomers, etc. The residue deposited inside the permeation membrane was washed with methanol and dried at about 25 °C to obtain the target conductive polymer. The weight average molecular weight (Mw) of the said conductive polymer was confirmed to be about 34,000 g / mol.

[0072] Example 1: Fabrication of a Cathode and a Lithium Secondary Battery (Fabrication of the Cathode) 20 g of the conductive polymer (Mw = 34,000 g / mol) obtained from the above manufacturing example and 0.2 g of a rheology modifier of RHEOBYK-411 (including a NMP solution of modified urea; manufactured by BYK) were dissolved in 1,980 g of a chloroform solvent to obtain a composition. This composition was gravure-coated and dried on an aluminum (Al) thin film, which is a positive electrode current collector, at about 0.5 μm to form a safety protection layer. The average thickness and its standard deviation of the finally formed safety protection layer are shown in Table 1 below.

[0073] LiCoO2 as a positive electrode active material, a conductive material (carbon black), and a binder (polyvinylidene fluoride) were added to N-methyl-2-pyrrolidone (NMP) at a weight ratio of 97.5:1:1.5 to produce a positive electrode slurry (solid content: 60% by weight). After this was applied and dried on the above conductive polymer-containing oil layer (however, the weight of the conductive polymer in the safety protection layer is about 0.5 parts by weight based on a total of 100 parts by weight of the positive electrode active material, conductive material, and binder), a roll press was performed to form an active material layer with a total thickness of 58 μm to produce a positive electrode.

[0074] (Manufacture of negative electrode) A negative electrode active material (graphite), a binder (SBR-CMC), and a conductive material (carbon black) were added to water, which is a solvent, at a weight ratio of 95:3.5:1.5 to produce a negative electrode slurry (solid content: 60% by weight). After the negative electrode slurry was applied and dried on a copper (Cu) thin film, which is a negative electrode current collector, with a thickness of 8 μm, a roll press was performed to produce a negative electrode.

[0075] (Manufacture of separator) After adding about 8.5 wt% of polyvinylidene fluoride - hexafluoropropylene copolymer (PVdF - HFP) binder to acetone, it was dissolved at a temperature of 50 °C for about 12 hours or more to produce a binder solution. Al2O3 powder was added to this binder solution so that Al2O3 / PVdF - HFP = 90 / 10 (wt% ratio), and a slurry was produced using the ball mill method for 12 hours or more. The slurry thus produced was coated on a polyolefin - based separator membrane with a thickness of about 8 μm using the dip coating method, and the coating thickness was adjusted to about 0.45 μm to produce a porous separator membrane.

[0076] (Manufacture of Lithium Secondary Battery) After sequentially laminating the positive electrode, separator membrane, and negative electrode, an electrode assembly composed of a bicell was manufactured by pressing using heat and pressure of 90 °C and 200 kPa. The assembled electrode assembly was housed in a pouch - type battery case, and after mixing ethylene carbonate (EC): ethyl methyl carbonate (EMC) at a volume ratio of 30:70, an electrolyte solution in which LiPF6 was dissolved to be 1.0 M was injected to manufacture a lithium secondary battery.

[0077] Comparative Example 1 The positive electrode and lithium secondary battery of Comparative Example 1 were manufactured in the same manner as in Example 1, except that the rheology modifier RHEOBYK - 411 was not used.

[0078] Test Example Measurement of Thickness Profile (Standard Deviation) Using a Confocal laser scanning microscope, the thickness of the finally formed safety protection layer was measured point - by - point in the TD (transverse direction) of FIG. 1 below, and the average thickness and standard deviation with respect to the measured values were confirmed. [Figure 1] JPEG2025521013000004.jpg1927

[0079] High - rate Discharge Characteristic Evaluation The lithium secondary batteries manufactured in the examples and comparative examples were charged under constant current (0.7C) and constant voltage (4.47V, 0.025C cut-off) conditions, then rested for 10 minutes, and discharged until reaching 3V under constant current (0.1C, 0.2C, 0.5C, 1.0C, 1.5C) conditions. That is, when the charge-discharge cycle recovery increased, the discharge rates were periodically changed to 0.1C, 0.2C, 0.5C, 1.0C, and 1.5C respectively to evaluate the high-rate discharge characteristics (rate capability) of each battery. At this time, the high-rate discharge characteristics at 1.5C are shown in Table 1 below.

[0080] Nail Penetration Test Five lithium secondary batteries manufactured in the examples and comparative examples were each prepared, fully charged to SOC100% at 4.47V (0.05C cut off) under CC / CV, 0.5C conditions at 25°C, and then stored at room temperature for 24 hours. Each lithium secondary battery was placed on a flat plate, and a nail made of stainless steel with a diameter of 3 ± 0.2 mm and a length of 100 mm was penetrated through the center of the cell at a vertical angle and a penetration speed of 100 mm / sec with a penetration distance of 30 mm to measure the presence or absence of ignition. The number of non-ignited batteries among the five batteries is described in Table 1 below. [Table 1]

Table 1

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

Claims

1. A metal current collector; A safety functional layer formed to cover at least a part of the metal current collector, including a polythiophene-based conductive polymer exhibiting PTC (positive temperature coefficient) characteristics and a thixotropic agent; and An electrode for a lithium secondary battery, including an electrode active material and a conductive material, and including an active material layer formed on the metal current collector and the safety functional layer.

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

3. The electrode for a lithium secondary battery according to Claim 1, wherein the polythiophene-based conductive polymer includes a homopolymer or copolymer including a repeating unit of the following Chemical Formula 1: [Chemical Formula 1] [Chemical Formula 4] In Chemical Formula 1, R １ is a functional group represented by the following Chemical Formula 2, [Chemical Formula 2] [Chemical Formula 5] In Chemical Formula 2, L １ is a single bond or an alkylene group, and L ２ is an alkylene group, R ３ is hydrogen or an alkyl group, and n is an integer within the range of 1 to 5000.

4. 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. The electrode for a lithium secondary battery according to Claim 1, wherein the thixotropic agent includes one or more selected from the group consisting of hydrophilic fumed silica, aluminum salts, bentonite or its derivatives, cellulose-based compounds, polyvinyl-based compounds, polyacrylic acid-based compounds, modified urea, and maleic acid copolymers.

6. The electrode for a lithium secondary battery according to Claim 1, wherein the thixotropic agent is included in a content of 0.1 to 5 parts by weight with respect to 100 parts by weight of the polythiophene-based conductive polymer.

7. The electrode for a lithium secondary battery according to Claim 1, wherein the safety functional layer further includes one or more additives selected from the group consisting of carbon-based conductive materials, conductive inorganic particles, binders, and esterified saccharides.

8. The active material layer has a thickness of 5 to 200 μm, The electrode for a lithium secondary battery according to Claim 1, wherein the safety functional layer has a thickness of 0.01 to 20 μm.

9. The electrode for a lithium secondary battery according to Claim 8, wherein a standard deviation of the thickness of the safety functional layer is 60 nm or less.

10. The electrode for a lithium secondary battery according to Claim 1, wherein the electrode for a lithium secondary battery is a positive electrode.

11. A lithium secondary battery including a positive electrode, a negative electrode, and a separator interposed therebetween, wherein the electrode for a lithium secondary battery according to any one of claims 1 to 10 is included as the positive electrode.

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