Electrode for lithium secondary battery and lithium secondary battery including same

Abstract

The present disclosure relates to an electrode for a lithium secondary battery and a lithium secondary battery including the same, the electrode including a uniform safety functional layer to suppress heat generation or ignition, and capable of providing a battery exhibiting excellent charge / discharge characteristics while having excellent stability. The electrode for a lithium secondary battery includes: a metal current collector; the present invention relates to a lithium ion secondary battery comprising a metal current collector, a positive electrode layer formed on the metal current collector, a negative electrode layer formed on the metal current collector, a safety functional layer formed so as to cover at least a part of the metal current collector, and comprising a polythiophene-based conductive polymer exhibiting a positive temperature coefficient (PTC) characteristic and a thixotropic agent, and a positive electrode layer formed on the metal current collector, and a negative electrode layer formed on the metal current collector, and an active material layer including an electrode active material and a conductive material, and formed on the metal current collector and the safety function layer.

Description

Technical Field

[0001] Cross - reference to related applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2022-0170925, filed with the Korean Intellectual Property Office on Dec. 8, 2022, the disclosure of which is incorporated herein by reference in its entirety.

[0003] The present disclosure relates to an electrode for a lithium secondary battery and a lithium secondary battery including the electrode, the electrode including a uniform safety functional layer to suppress heat generation or ignition, and capable of providing a battery that exhibits excellent charge / discharge characteristics while having excellent stability. Background Art

[0004] With a significant increase in the demand for medium and large-sized devices such as electric vehicles and hybrid electric vehicles, and mobile devices such as smartphones and tablet computers, the demand for secondary batteries as an energy source required to drive such devices is rapidly increasing. In particular, with an increase in the data processing speed and usage time of mobile devices, lithium secondary batteries having a higher energy density and working potential, capable of maintaining excellent characteristics for a long time, and having a low self-discharge rate are being actively developed.

[0005] However, with a significant increase in the capacity and energy density of lithium secondary batteries, many ignition and explosion accidents caused by overcharging, exposure to high temperatures, external shocks, etc. of various mobile devices or electric vehicles including the devices have been reported. Therefore, recently, one of the main research tasks of lithium secondary batteries is to improve safety by suppressing ignition and explosion.

[0006] It is well known that the direct causes of ignition, explosion, etc. in lithium secondary batteries are short circuits caused by direct contact between the positive electrode and the negative electrode inside the secondary battery due to external stimuli such as high temperature and external shock. For example, when a lithium secondary battery is overcharged, or exposed to high temperature or external shock, the internal temperature of the secondary battery may rapidly rise, causing the separator to shrink, or the internal structure of the secondary battery may be damaged due to external shock. As a result, the positive electrode and the negative electrode may come into contact, and thus a short circuit may occur. When such a short circuit occurs, the movement of lithium ions and electrons may concentrate through the contact portion between the positive electrode and the negative electrode, and thus an overcurrent may occur, which may lead to heat generation, gas generation inside the battery, and volume expansion, and may therefore pose a risk of ignition or explosion of the lithium secondary battery.

[0007] Therefore, in order to suppress ignition and explosion during short circuit and improve the safety of secondary batteries, it is necessary to increase the resistance between electrodes and interrupt the current when high temperature or external impact is applied. For this purpose, various attempts have been made to add various functional layers or functional materials to the electrodes for lithium secondary batteries to increase the resistance when high temperature is applied, thereby improving the safety of secondary batteries.

[0008] However, in the case of electrodes to which previously known functional layers and the like are added, it is difficult to sufficiently improve the safety of lithium secondary batteries, or the functional layers are not formed uniformly, which provides the disadvantage of impairing the basic battery performance such as the charge / discharge characteristics of secondary batteries. Summary of the Invention

[0009] Technical Problem

[0010] An object of the present disclosure is to provide an electrode for a lithium secondary battery, which includes a uniform safety functional layer to suppress heat generation or ignition, and can provide a battery that exhibits excellent charge / discharge characteristics while having excellent stability.

[0011] Another object of the present disclosure is to provide a lithium secondary battery, which includes the electrode and thus exhibits excellent charge / discharge characteristics.

[0012] Technical Solution

[0013] According to one aspect of the present disclosure, there is provided an electrode for a lithium secondary battery, including:

[0014] A metal current collector;

[0015] A safety functional layer, the safety functional layer being 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 thixotropic agent; and

[0016] An active material layer, the active material layer including an electrode active material and a conductive material, and being formed on the metal current collector and the safety functional layer.

[0017] According to another aspect of the present disclosure, there is provided a lithium secondary battery, including a positive electrode, a negative electrode, and a separator interposed therebetween, wherein the electrode described above is included as the positive electrode.

[0018] Now, the electrode for a lithium secondary battery and the lithium secondary battery including the electrode according to the specific embodiments of the present disclosure will be described.

[0019] The terms or words used in this specification and claims should not be construed as being limited to ordinary terms or dictionary terms. The present disclosure should be interpreted based on the principle that the inventors can appropriately define the terms in order to best describe their own inventions in a proper manner, with meanings and concepts consistent with the technical idea of the present disclosure.

[0020] The terms used herein are provided to describe exemplary embodiments, but are not intended to limit the concepts of the present invention. Unless the context clearly indicates otherwise, the singular forms include the plural forms.

[0021] It should be understood that terms such as "comprising", "including", and "having" are used herein to specify the presence of the described features, integers, steps, components, or combinations thereof, but do not preclude the presence or addition of one or more other features, integers, steps, components, or combinations thereof.

[0022] According to one embodiment of the present disclosure, there is provided an electrode for a lithium secondary battery, comprising:

[0023] A metal current collector;

[0024] A safety functional layer, the safety functional layer being 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 thixotropic agent; and

[0025] An active material layer, the active material layer including an electrode active material and a conductive material and being formed on the metal current collector and the safety functional layer.

[0026] The electrode of this embodiment is configured such that a stability functional layer including a polythiophene-based conductive polymer exhibiting PTC characteristics and a thixotropic agent is formed on the surface of the metal current collector.

[0027] First, when the lithium secondary battery is activated, the polythiophene-based conductive polymer can exhibit conductivity by doping anions derived from the electrolyte of the secondary battery into the aromatic thiophene rings of the conductive polymer. Therefore, during normal charging and discharging of the secondary battery, the conductive polymer exhibits conductivity, allowing the secondary battery to exhibit appropriate charging / discharging characteristics.

[0028] However, the conductive polymer can be de-doped from the aromatic thiophene ring by anions derived from the electrolyte at a temperature above a certain level. As a result, the conductive polymer can act as a non-conductor, thereby increasing the resistance and exhibiting PTC characteristics that interrupt the current flow. Due to the action of this polythiophene-based conductive polymer, an electrode of one embodiment in which a safety protective layer containing the conductive polymer is formed can contribute to improving the stability of the lithium secondary battery, as described below.

[0029] When an external stimulus such as overcharging, high temperature, or external shock is applied to the lithium secondary battery including the electrode, the temperature inside the battery rises rapidly, and due to the de-doping of the above anions, etc., this conductive polymer can be converted into a non-conductor. Therefore, the resistance inside the electrode can be significantly increased and the current flow between the current collector and the active material layer can be interrupted, thereby preventing overcurrent caused by a short circuit between the electrodes and suppressing heat generation, ignition, explosion, gas generation, etc. of the secondary battery.

[0030] Incidentally, when only the polythiophene-based conductive polymer is used to coat and form the safety functional layer, it becomes difficult to uniformly form the safety functional layer due to the fluidity and / or viscosity of this polymer. In this case, the safety functional layer may be locally formed too thin or too thick, and the safety functional layer may not exhibit sufficient adhesion to the metal current collector, etc., which may make it difficult to form the layer well.

[0031] Due to the non-uniform formation of the safety functional layer, even at a temperature above a certain level, the safety functional layer cannot completely interrupt the local overcurrent. Therefore, it is difficult to ensure excellent safety of the lithium secondary battery, or the safety protective layer has a locally thick thickness, so that the basic performance of the charge / discharge characteristics of the lithium secondary battery may deteriorate.

[0032] However, since the electrode of one embodiment includes a thixotropic agent and a conductive polymer at the same time, the stability functional layer can be formed more uniformly. More specifically, a thixotropic agent can be added to the composition for forming the safety functional layer to improve its fluidity and exhibit uniform coating performance. In addition, after the composition is coated and dried, the viscosity of the composition can be increased again, thereby contributing to the adhesion of the safety functional layer.

[0033] Due to the action of this thixotropic agent, an electrode of one embodiment may include a uniform safety functional layer with a small overall thickness variation, and a uniform active material layer may be formed on the safety functional layer. In addition, when the electrodes included in multiple lithium secondary batteries are manufactured by continuously producing electrode sheets and then punching these electrode sheets, the safety functional layer and the active material may have a uniform thickness in each electrode included in the multiple lithium secondary batteries. For example, in each electrode included in one or more lithium secondary batteries manufactured from the same electrode sheet, the standard deviation of the thickness of the safety functional layer may be 60 nm or less, or 50 nm or less, or 5 nm to 50 nm.

[0034] As a result, in an electrode of one embodiment, when a stimulus such as high temperature is applied to the lithium secondary battery, the safety functional layer can effectively interrupt the overcurrent, and thus improve the safety of the lithium secondary battery. In addition, due to the formation of this uniform safety functional layer, the deterioration of the basic performance such as the charge / discharge characteristics of the lithium secondary battery can be minimized.

[0035] Meanwhile, the polythiophene-based conductive polymer included in the electrode of this embodiment exhibits the above-mentioned PTC characteristics, and the available operating temperature at which this conductive polymer is converted into a non-conductor can be 70 °C to 130 °C, or 80 °C to 125 °C. Since the conductive polymer is converted into a non-conductor at such an available operating temperature, the conductive polymer does not interfere with the normal charge and discharge processes of the secondary battery, and can more effectively suppress the ignition or explosion of the secondary battery when an external stimulus is applied.

[0036] In addition, the conductive polymer may be a polythiophene-based polymer, or a copolymer containing 50 mol% or more, 70 mol% or more, or 90 to 100 mol% of substituted or unsubstituted thiophene-based repeating units based on the total repeating units.

[0037] In a more specific example, the conductive polymer may be a polythiophene-based polymer, or a copolymer in which an alkylene oxide group is bonded to the thiophene ring of the repeating unit. For example, a homopolymer or copolymer containing the repeating unit of Chemical Formula 1 below:

[0038] [Chemical Formula 1]

[0039]

[0040] Among them, in Chemical Formula 1, R1 is a functional group of Chemical Formula 2 below,

[0041] [Chemical Formula 2]

[0042]

[0043] Among them, 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 from 1 to 5000, or from 10 to 2000, or from 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.

[0044] In addition, the weight-average molecular weight of such a polythienyl conductive polymer may be, for example, from 5000 g / mol to 100000 g / mol, or from 10000 g / mol to 80000 g / mol. Thus, the composition for forming the safety functional layer may exhibit excellent coating properties and may also exhibit appropriate adhesion to a metal current collector or the like.

[0045] In a more specific embodiment, the polythienyl conductive polymer may be included in an amount greater than 0 mol%, or 0.001 mol% or more, or 0.01 mol% or more, or 1 mol% or more, and 100 mol% or less, or 80 mol% or less, or 50 mol% or less of the repeating unit of Chemical Formula 1. Here, the polythienyl conductive polymer may include the remaining amount of alkylthienyl repeating units other than the repeating unit of Chemical Formula 1. For example, an alkylthienyl repeating unit in which an alkyl group having 1 to 20 carbon atoms, or 3 to 15 carbon atoms is substituted on the thiophene ring.

[0046] Such a polythienyl conductive polymer includes a substituted thiophene ring and can thus exhibit an appropriate usable working temperature, etc. As a result, when a high temperature higher than a certain level is applied, it can be converted into a non-conductor without impairing the charge / discharge characteristics of the lithium secondary battery, thereby improving the safety of the secondary battery.

[0047] In addition, because the polythienyl conductive polymer has the above-described predetermined structure, it can exhibit a relatively low affinity and solubility for organic solvents such as N-methylpyrrolidone mainly included in the slurry composition for forming the electrode active material layer, and can exhibit excellent adhesion to a metal current collector. Therefore, in the process of forming such a polythienyl conductive polymer on a metal current collector and then coating and drying the slurry composition to form an electrode active material layer, the phenomenon in which the conductive polymer dissociates and diffuses over a large area of the active material layer can be minimized. Therefore, by using such a polythienyl conductive polymer, a safety functional layer can be uniformly formed near the surface of the metal current collector. Thus, the conductive polymer can contribute to improving the safety of the secondary battery without impairing its basic performance.

[0048] Meanwhile, based on 100 parts by weight of the electrode active material (e.g., the positive electrode active material) included in the active material layer, the content of the conductive polymer can be 0.001 to 5 parts by weight, or 0.005 to 5 parts by weight. As a result, the electrode of one embodiment can have more excellent safety and charge / discharge characteristics.

[0049] The conductive polymer having the repeating unit of Chemical Formula 1 can be prepared, for example, by subjecting a halogenated thiophene compound and an alkylene diol compound to a substitution reaction to prepare a monomer bonded to the functional group of Chemical Formula 2, then polymerizing this monomer alone, or copolymerizing the monomer with other monomers such as alkyl thiophene. Specific conditions for preparing such monomers and polymers are described in the Preparation Examples below.

[0050] Meanwhile, in the electrode of one embodiment, the safety functional layer simultaneously includes a thixotropic agent and 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 functional layer and contribute to improving its coating performance and uniformly forming the safety functional layer. In addition, after the composition is coated and dried, the viscosity of the composition can be increased again so that the safety protection layer has excellent adhesion to the metal current collector.

[0051] As such a thixotropic agent, an organic substance or inorganic particles capable of causing a sol-gel state change or an increase or decrease in the fluidity of the polymer solution can be used. Specific examples of the thixotropic agent include hydrophilic fumed silica, aluminum salts, bentonite or its derivatives, cellulose-based compounds, polyvinyl compounds, polyacrylic acid-based compounds, modified urea, or maleic acid copolymers, etc. Considering the polythiophene-based conductive polymer, etc., a thixotropic agent in the form of modified urea or its organic solution can be appropriately used.

[0052] As the thixotropic agent, existing commercially available thixotropic agents can be used. Examples of the commercially available thixotropic agents include HL-200, HL-300, HL-380 (available from DKSH), GARAMITE-1958, RHEOBIK-410, 411, 7410, 605 (available from BYK), Alugel 28DG, Laevisil SP (available from Baerlocher), CAB-O-SIL H-300 (available from Cabot), Jaylink JL-106E (available from Dymax), Bentone 38, SD3, 34 (available from Elementis), and so on.

[0053] Based on 100 parts by weight of the polythiophene-based conductive polymer, such a thixotropic agent can 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, so as not to hinder the electrical properties of the electrode, etc., while appropriately controlling the fluidity and viscosity of the composition for forming the safety functional layer.

[0054] Meanwhile, the safety functional layer can be formed by dissolving or dispersing the above-mentioned conductive polymer and thixotropic agent in an organic solvent such as chloroform, tetrahydrofuran (THF), toluene, or xylene at a concentration of about 0.1 wt% to 5 wt% to form a liquid composition, and then coating and drying the composition on a metal current collector. After that, the following slurry composition is applied and dried to form an active material layer, and the active material layer can be roll-pressed to produce an electrode for a lithium secondary battery according to one embodiment.

[0055] At this time, in addition to the conductive polymer, the safety functional layer and the liquid composition for forming the safety functional layer may further include at least one additive selected from the group consisting of a carbon-based conductive material, conductive inorganic particles, a binder, and an esterified saccharide.

[0056] At this time, as the carbon-based conductive material and the binder, the same components as those included in the active material layer described below can be used, and by adding these components, the conductivity of an electrode according to one embodiment, the adhesion of the safety functional layer, or mechanical properties, etc. can be further improved. In addition, as the conductive inorganic particles, alumina or zirconia particles having a nano-scale particle size of, for example, 5 nm to 100 nm can be used, and by adding these particles, the conductivity of the electrode and the secondary battery can be further improved. In addition, monosaccharides, oligosaccharides, polysaccharides, etc. having an acyl group can be used as the esterified saccharide. This component can play a role in generating gas during overcharging of the secondary battery and interrupting the conduction path between the metal current collector and the electrode active material, and by adding this component, the safety of the secondary battery can be further improved.

[0057] Meanwhile, an electrode according to one embodiment further includes an active material layer formed on the metal current collector and the safety functional layer, and this active material layer may include an electrode active material, a conductive material, and optionally a binder. At this time, since it is preferable to use an electrode formed of a conductive polymer as the positive electrode, the following will mainly explain these examples.

[0058] In a positive electrode for a lithium secondary battery, a metal current collector typically has a thickness of 3 μm to 100 μm and can be formed of any metal or alloy having excellent electrical conductivity and not causing 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 whose surface is treated with carbon, nickel, titanium, or silver, and the like. In addition, the metal current collector can form fine protrusions and depressions on its surface to enhance the adhesion of the safety functional layer, etc., and can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabric structures.

[0059] Furthermore, the positive electrode active material included in the active material layer is not particularly limited as long as it is a material capable of reversibly inserting and extracting lithium ions, and examples thereof include lithium metal composite oxides containing at least one metal element 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.

[0060] More specifically, a compound represented by any of the following formulas can be used as the positive electrode active material: 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, 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 Cob 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 (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 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 Mn 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 b E c G d O2 (where 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 (where 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 (where 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1); Li a CoG b O2 (where 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1); Li a MnG b O2 (where 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1); Li a Mn2G b O4 (where 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).

[0061] In the above 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.

[0062] In addition, those having a coating on the surface of the above compounds can be used, or a mixture of the above compounds and compounds having a coating can be used. The coating can include oxides of the coating element, hydroxides of the coating element, oxyhydroxides of the coating element, oxycarbonates of the coating element, and hydroxycarbonates of the coating element as compounds of the coating element. The compounds constituting these coatings can be amorphous or crystalline compounds. As the coating elements included in the coating, Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof can be used.

[0063] In addition, the conductive material used in the active layer is used to impart conductivity to the electrode, and the conductive material can be used without limitation as long as it has electronic conductivity without causing chemical changes in the battery to be configured. Examples thereof include natural graphite, artificial graphite, carbon black, carbon fiber, carbon nanotube, metal powders such as copper, nickel, aluminum, and silver, metal fibers, etc. In addition, conductive materials such as polystyrene derivatives can be used alone or as a mixture of one or more of them.

[0064] Based on the total weight of the active material layer, the addition amount of the conductive material can be 1 wt% to 50 wt%, or 2 wt% to 20 wt%. Thereby, while preventing the excellent electrical characteristics of the positive electrode from being impaired, the formation of a preferable positive electrode can be ensured.

[0065] The role of the binder is to make the particles of the positive electrode active material adhere well to each other and further improve the binding performance of the active material layer. As typical examples of the binder, the above-mentioned halogenated polyolefin-based 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, and the like can be used.

[0066] Based on the total weight of the active material layer, the addition amount of the binder can be 1 wt% to 50 wt%, or 2 wt% to 30 wt%. Thus, a positive electrode with excellent durability can be formed without hindering the electrical characteristics and / or capacity characteristics of the positive electrode.

[0067] The above-mentioned active material layer can be formed by dissolving or dispersing each component such as the above-mentioned 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 the slurry composition onto a metal current collector having a safety functional layer formed thereon.

[0068] At this time, examples of the medium such as an organic solvent include N-methyl-2-pyrrolidone (NMP), methoxypropyl acetate, butyl acetate, glycolic acid, butyl ester, butanediol, methylalkyl polysiloxane, alkylbenzene, propylene glycol, xylene, monophenol, or the like. Among these compounds, NMP or the like can be appropriately used in consideration of the dispersibility and processability of the above-mentioned positive electrode active material and conductive material.

[0069] Meanwhile, since the formation process and conditions of the active material layer can follow the general positive electrode formation process and conditions, further description thereof will be omitted.

[0070] The thickness of the active material layer formed by the above method can be 5 μm to 200 μm, or 10 μm to 100 μm, while the uniform thickness of the safety functional layer can be 0.01 μm to 20 μm, or 0.05 μm to 10 μm.

[0071] The uniform thickness can be defined as the standard deviation of the thickness of the above-mentioned safety functional layer. For example, in each electrode included in one or more lithium secondary batteries manufactured from the same electrode sheet, the standard deviation of the thickness of the safety functional layer can be 60 nm or less, or 50 nm or less, or 5 nm to 50 nm.

[0072] Since the safety functional layer is formed with the above-mentioned uniform thickness near the surface of the metal current collector, when an external stimulus such as an external impact is applied, the conductive polymer included in the safety functional layer prevents direct contact between the active material layer and the metal current collector, thereby ensuring improved safety of the secondary battery. In addition, the safety functional layer can be locally formed too thick, thereby minimizing the hindrance to the charge / discharge characteristics of the secondary battery.

[0073] Meanwhile, according to another embodiment of the present disclosure, a lithium secondary battery is provided, which includes an electrode of one embodiment as a positive electrode, and simultaneously includes the positive electrode, a negative electrode, and a separator interposed therebetween.

[0074] In such a lithium secondary battery according to another embodiment, the negative electrode is produced by coating, drying, and roll-pressing a negative electrode active material onto a negative electrode current collector, and may further include a conductive material and a binder as needed.

[0075] The negative electrode active material may include, for example, graphite having a complete layered crystal structure such as natural graphite; soft carbon having a low-crystalline layered crystal structure (graphene structure; in which the hexagonal honeycomb-shaped planes of carbon are arranged in layers); hard carbon having a structure in which a low-crystalline structure is mixed with an amorphous portion; carbon and graphite materials such as artificial graphite, expanded graphite, carbon fiber, graphitizable carbon, carbon black, carbon nanotubes, fullerenes, activated carbon; or 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, 2, 3 elements in the periodic table, halogen; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8); lithium metal; lithium alloy; silicon, silicon oxide, or silicon-based alloy; tin-based alloy; conductive polymers such as polyacetylene; Li-Co-Ni-based materials; titanium oxide; or lithium titanium oxide; and so on.

[0076] In one embodiment, the negative electrode active material may include graphite and silicon (Si)-containing particles. The graphite may include at least one of natural graphite having a layered crystal structure and artificial graphite having an isotropic structure. The silicon (Si)-containing particles are particles containing silicon (Si) as a main component, and may include silicon (Si) particles, silicon oxide particles, or a mixture of silicon (Si) particles and silicon oxide particles.

[0077] In addition, the conductive material and the binder that can be used together with the negative electrode active material may be the same components as those included in the positive electrode active material layer.

[0078] In addition, the thickness of the negative electrode active material layer containing the negative electrode active material may be 100 μm to 200 μm, or 120 μm to 200 μm.

[0079] In addition, the negative electrode current collector is not particularly limited as long as it has high conductivity and does not cause chemical changes in the battery. For example, copper, stainless steel, nickel, titanium, fired carbon, etc. may be used, and in the case of copper or stainless steel, materials surface-treated with carbon, nickel, titanium, silver, etc. may be used.

[0080] In addition, similar to the positive current collector, the negative current collector may have fine protrusions and depressions formed on its surface to enhance the adhesion of the negative active material layer, and may be formed in various forms such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabric structures. Further, considering the conductivity and total thickness of the negative electrode to be produced, the average thickness of the negative current collector can be appropriately applied in the range of 3 μm to 100 μm.

[0081] In addition, a separator is interposed between the positive electrode and the negative electrode, and a thin insulating 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 art. In particular, chemically resistant and hydrophobic polypropylene; glass fibers; or sheets or non-woven fabrics made of polyethylene; etc. can be used. In some cases, a composite separator can be used, in which inorganic particles / organic particles are coated onto a porous polymer substrate such as a sheet or non-woven fabric through an organic binder polymer. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte can also be used as the separator. Further, the average pore diameter of the separator can be 0.01 μm to 10 μm, and the average thickness can be 5 μm to 300 μm.

[0082] The above-described lithium secondary battery may further include an electrolyte, where such an electrolyte may be an electrolyte solution containing a non-aqueous organic solvent and a lithium salt, or an electrolyte film containing an organic or inorganic solid electrolyte, and these electrolytes can be mixed and used together. However, the types of electrolytes that can be used are well-known to those skilled in the art and are not particularly limited in the batteries of other embodiments, and thus, further description thereof will be omitted.

[0083] Advantageous Effects

[0084] As described above, in the lithium secondary battery including the electrode of the present disclosure, even when external stimuli such as overcharging, high temperature, or external shock are applied and the temperature inside the battery rapidly rises, an overcurrent can be effectively interrupted by the safety functional layer uniformly formed near the metal current collector, thereby minimizing ignition, explosion, and similar events.

[0085] Accordingly, the lithium secondary battery can exhibit excellent safety, and can suppress the deterioration of charge / discharge characteristics caused by the safety functional layer, thereby exhibiting excellent electrochemical characteristics. Detailed Embodiments

[0086] Hereinafter, various embodiments of the present disclosure will be described in detail so that those skilled in the art can easily implement them. However, the present disclosure can be modified in various different ways and is not limited to the embodiments set forth herein.

[0087] Preparation Examples: Synthesis of Monomers and Conductive Polymers

[0088] [Chemical Formula 3]

[0089]

[0090] After forming a nitrogen environment inside a three-way RBF (Round Bottom Flask) through a nitrogen stream, 2.34 g (0.01 mol) of copper(I) iodide and 50.36 g (0.31 mol) of triethylene glycol were added. 3.68 g (0.096 mol) of sodium hydride in 60% mineral oil was slowly added to the RBF, and the mixture was stirred while maintaining the nitrogen environment. After stirring for about 1 hour, 10.0 g (0.06 mol) of 3-bromothiophene was added, and the mixture was refluxed at about 100 °C for about 24 hours. The reaction solution was filtered through a pressure-reducing device, then washed with 100 mL of dichloromethane solution, and then washed successively with NH4Cl and brine. The solvent was removed by vacuum distillation, and the crude product was purified by column chromatography (hexane∶ethyl acetate = 60∶40) to obtain about 9.0 g (yield: 60%) of the target compound (the monomeric compound of Chemical Formula 3).

[0091] 124 g (767 mmol) of iron(III) chloride was dissolved in 1000 ml of dichloromethane, and 3.0 g (12.2 mmol) of the monomeric compound of Chemical Formula 3 and 47.8 g (243 mmol) of 3-octyl thiophene were added to the solution. The mixture was stirred at about 25 °C for 24 hours to carry out a polymerization reaction. The reaction solution was placed in a permeable membrane with a MWCO (molecular weight of cut-off) of 5000, and then immersed in 1500 ml of acetonitrile solvent to remove unreacted iron(III) chloride, monomers, etc. The residue precipitated inside the permeable membrane was washed with methanol and dried at about 25 °C to obtain the desired conductive polymer. It has been confirmed that the weight-average molecular weight (Mw) of the conductive polymer is about 34,000 g / mol.

[0092] Example 1: Production of Cathodes and Lithium Secondary Batteries

[0093] (Production of the positive electrode)

[0094] Dissolve 20 g of the conductive polymer obtained in Preparation Example (Mw = 34,000 g / mol) and 0.2 g of the thixotropic agent RHEOBYK-411 (an NMP solution containing modified urea; available from BYK) in 1,980 g of chloroform solvent to obtain a composition. Gravure coat the composition onto an aluminum (Al) thin film serving as a positive current collector and dry it to about 0.5 μm to form a safety functional layer. The average thickness and standard deviation of the finally formed safety functional layer after drying are shown in Table 1 below.

[0095] Add LiCoO2 as a positive electrode active material, a conductive material (carbon black), and a binder (polyvinylidene fluoride) to N-methyl-2-pyrrolidone (NMP) at a weight ratio of 97.5:1:1.5 to prepare a positive electrode slurry (solid content: 60 wt%). Coat the slurry onto the layer containing the conductive polymer and dry it (however, based on a total of 100 parts by weight of the positive electrode active material, conductive material, and binder, the weight of the conductive polymer in the safety functional layer is about 0.5 parts by weight), and then perform a roll press to form an active material layer with a total thickness of 58 μm, thereby producing a positive electrode.

[0096] (Production of negative electrode)

[0097] Add a negative electrode active material (graphite), a binder (SBR-CMC), and a conductive material (carbon black) to water as a solvent at a weight ratio of 95:3.5:1.5 to prepare a negative electrode slurry (solid content: 60 wt%). Coat the negative electrode slurry onto a copper (Cu) thin film with a thickness of 8 μm serving as a negative current collector, dry it, and then perform a roll press to produce a negative electrode.

[0098] (Production of separator)

[0099] Add about 8.5 wt% of a polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP) binder to acetone, and then dissolve it at a temperature of 50 °C for about 12 hours or longer to prepare a binder solution. Add Al2O3 powder to the binder solution such that Al2O3 / PVdF-HFP = 90 / 10 (wt% ratio), and use a ball mill method to prepare the slurry for 12 hours or longer. Coat the slurry thus prepared onto a polyolefin-based separator with a thickness of about 8 μm using a dip coating method. Adjust the coating thickness to about 0.45 μm to produce a porous separator.

[0100] (Production of lithium secondary battery)

[0101] Stack the positive electrode, separator, and negative electrode in sequence, and then press them using heat at 90 °C and a pressure of 200 kPa to produce an electrode assembly composed of dual cells. Accommodate the assembled electrode assembly in a pouch-type battery case, mix ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 30:70, and then inject an electrolyte solution in which 1.0 M LiPF6 is dissolved, thereby producing a lithium secondary battery.

[0102] Comparative Example 1

[0103] Produce the positive electrode and lithium secondary battery of Comparative Example 1 in the same manner as in Example 1, except that the thixotropic agent of RHEOBYK-411 is not used.

[0104] Test Examples

[0105] Measurement of Thickness Profile (Standard Deviation)

[0106] Using a confocal laser scanning microscope, measure the thickness of the finally formed safety functional layer point by point in the TD (transverse direction) shown in Figure 1 below, and confirm the average thickness and standard deviation of the measured values.

[0107] [Figure 1]

[0108]

[0109] Evaluation of High - speed Discharge Characteristics

[0110] Charge the lithium secondary batteries produced in the examples and comparative examples under conditions of constant current (0.7 C) and constant voltage (4.47 V, cut-off at 0.025 C), then rest for 10 minutes, and discharge under conditions of constant current (0.1 C, 0.2 C, 0.5 C, 1.0 C, 1.5 C) until the voltage reaches 3 V. That is, when the number of charge / discharge cycles increases, the discharge rate is periodically changed to 0.1 C, 0.2 C, 0.5 C, 1.0 C, and 1.5 C respectively. Thus, the high-rate discharge capability of each battery was evaluated. At this time, the high-rate discharge capability at 1.5 C is shown in Table 1 below.

[0111] Nail - penetration Test

[0112] Five lithium secondary batteries produced in the examples and comparative examples were respectively prepared, fully charged to 100% SOC at 4.47 V (cut-off at 0.05C) under CC / CV and 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 stainless steel nail with a diameter of 3 ± 0.2 mm and a length of 100 mm was penetrated through the center of the battery at a vertical angle and a penetration speed of 100 mm / s to measure whether ignition occurred. Table 1 below lists the number of non-ignited batteries among the five batteries.

[0113] [Table 1]

[0114]

[0115] Referring to Table 1, it has been confirmed that in the lithium secondary battery of Example 1, the safety functional layer is formed to have a more uniform thickness. As a result, it has been confirmed that compared with Comparative Example 1, it not only exhibits excellent safety but also exhibits uniform and excellent high-rate discharge characteristics.

Claims

1. An electrode for a lithium secondary battery, comprising: A metal current collector; A safety functional layer, the safety functional layer being 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 thixotropic agent; and An active material layer, the active material layer including an electrode active material and a conductive material and being formed on the metal current collector and the safety functional layer.

2. The electrode for a lithium secondary battery according to claim 1, wherein the available operating temperature of the polythiophene-based conductive polymer is 70°C 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 containing repeating units of the following Chemical Formula 1: [Chemical Formula 1] Among them, In Chemical Formula 1, R1 is a functional group of the following Chemical Formula 2, [Chemical Formula 2] wherein, 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 from 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 5000 to 100000 g / mol.

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

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

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

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

9. The electrode for a lithium secondary battery according to claim 8, wherein the 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 is used as a positive electrode.

11. A lithium secondary battery, comprising a positive electrode, a negative electrode, and a separator interposed therebetween, wherein the electrode according to claim 1 is included as the positive electrode.

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