Electrode for lithium secondary battery and lithium secondary battery comprising same

Abstract

The present disclosure relates to an electrode for a lithium secondary battery, which not only effectively suppresses heat generation or ignition, and thus has further improved stability, but also can provide a battery exhibiting excellent charge / discharge characteristics, and a lithium secondary battery comprising the same. An electrode for a lithium secondary battery includes a metal current collector; and an electrode active material and a conductive material, and further includes an active material layer formed on the metal current collector, in which the active material layer further includes a conductive polymer exhibiting a peak in a band of 1350 cm-1 to 1600 cm-1 during Raman spectroscopy, and exhibiting PTC (Positive Temperature Coefficient) characteristics, and wherein when a cross-section of the active material layer is subjected to Raman image analysis, the conductive polymer is distributed in an amount of 90 wt% or more within a region reaching 10% of a thickness of the active material layer from a surface of the metal current collector.

Description

Technical Field

[0001] Cross - reference to related applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2022-0151839, filed with the Korean Intellectual Property Office on November 14, 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, which not only effectively suppresses heat generation or ignition and thus has further improved stability, but also enables a battery to exhibit excellent charge / discharge characteristics, and a lithium secondary battery including the electrode. Background Art

[0004] As the demand for large and medium-sized devices such as electric vehicles and hybrid electric vehicles, and mobile devices such as smartphones and tablet computers has increased significantly, the demand for secondary batteries as an energy source required to drive such devices has also been rapidly increasing. In particular, as the data processing speed and usage time of mobile devices increase, lithium secondary batteries having higher energy density and working potential are being actively developed, which can maintain excellent characteristics for a long period of time and have a low self-discharge rate.

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

[0006] It is well known that the direct causes of fire, explosion, etc. in lithium secondary batteries are short circuits caused by direct contact between the positive and negative electrodes inside the secondary battery due to stimuli applied from the outside (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 increase, 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 and negative electrodes may come into contact, making a short circuit possible. When such a short circuit occurs, the movement of lithium ions and electrons may be concentrated through the contact portion between the positive and negative electrodes, enabling an overcurrent to occur, which may cause heat generation, gas generation, and volume expansion inside the battery, and thus may pose a risk of fire or explosion of the lithium secondary battery.

[0007] Therefore, in order to suppress ignition and explosion during a short circuit and improve the safety of a secondary battery, it is necessary to increase the resistance between electrodes and interrupt the current when a high temperature or an external shock 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 a high temperature is applied, thereby improving the safety of the secondary battery.

[0008] However, in the case of an electrode to which a previously known functional layer has been added, there are disadvantages in that it is difficult to sufficiently improve the safety of the lithium secondary battery, and thus there is still a possibility of ignition and explosion, or the functional layer partially suppresses the charge / discharge characteristics of the secondary battery. Summary of the Invention

[0009] Technical Problem

[0010] Therefore, an object of the present disclosure is to provide an electrode for a lithium secondary battery that not only effectively suppresses heat generation or ignition and thus has further improved stability, but also can provide a battery exhibiting excellent charge / discharge characteristics.

[0011] Another object of the present disclosure is to provide a lithium secondary battery including the electrode and thus exhibiting excellent stability and charge / discharge characteristics.

[0012] Technical Solution

[0013] Provided herein is an electrode for a lithium secondary battery, including a metal current collector; and an electrode active material and a conductive material, and including an active material layer formed on the metal current collector,

[0014] wherein the active material layer further includes a conductive polymer that exhibits a peak in a spectral band of 1350 cm -1 to 1600 cm -1 during Raman spectroscopic analysis and exhibits PTC (positive temperature coefficient) characteristics, and

[0015] wherein when Raman image analysis is performed on a cross-section of the active material layer, the conductive polymer is distributed in an amount of 90 wt% or more in a region reaching 10% of the thickness of the active material layer from the surface of the metal current collector.

[0016] Also provided herein is a lithium secondary battery including a positive electrode, a negative electrode, and a separator interposed therebetween, wherein the electrode is included as the positive electrode.

[0017] Now, an electrode for a lithium secondary battery and a lithium secondary battery including the same according to specific embodiments of the present disclosure will be described.

[0018] The terms or words used in this specification and claims should not be construed as limited to ordinary or dictionary terms, and this disclosure should be interpreted with meanings and concepts consistent with the technical idea of this disclosure based on the principle that the inventors can appropriately define terms so as to properly describe their own inventions in the best way.

[0019] The terms used herein are provided to describe embodiments, but do not limit the inventive concept. Unless the context clearly indicates otherwise, the singular forms include the plural forms.

[0020] It should be understood that terms such as "comprising", "including", "having", etc. are used herein to indicate the presence of the stated features, integers, steps, components, or combinations thereof, but do not exclude the presence or addition of one or more other features, integers, steps, components, or combinations thereof.

[0021] According to one embodiment of the present disclosure, an electrode for a lithium secondary battery is provided, including: a metal current collector; and an electrode active material and a conductive material, and including an active material layer formed on the metal current collector.

[0022] wherein the active material layer further includes a conductive polymer, and the conductive polymer exhibits peaks in the spectral band of 1350 cm -1 to 1600 cm -1 during Raman spectroscopic analysis, and exhibits PTC (positive temperature coefficient) characteristics, and

[0023] wherein, when Raman image analysis is performed on a cross-section of the active material layer, the conductive polymer is distributed in an amount of 90 wt% or more in a region reaching 10% of the thickness of the active material layer from the surface of the metal current collector.

[0024] The electrode of one embodiment includes a conductive polymer mainly distributed in a region in contact with or adjacent to the surface of the metal current collector, wherein the conductive polymer can exhibit peaks in the spectral band of 1350 cm -1 to 1600 cm -1 or 1500 to 1600 cm -1 during Raman spectroscopic analysis, and can exhibit PTC (positive temperature coefficient) characteristics.

[0025] Characteristic peaks appearing in the bands of the Raman spectrum can define that the conductive polymer contains aromatic rings with a high content of a conjugated π - electron system in the repeating unit. And for example, it can indicate that the conductive polymer has a repeating unit containing an aromatic ring, which contains one or more or one or two hetero - elements such as nitrogen or sulfur, and the content is 50 mol% or more, or 70 mol% or higher, or 90 mol% to 100 mol% of the total repeating unit.

[0026] As the lithium secondary battery is activated, the conductive polymer containing an aromatic ring with a conjugated π - electron system can exhibit conductivity by doping anions derived from the secondary - battery electrolyte onto the aromatic ring. Thus, during the normal charge / discharge process of the secondary battery, the conductive polymer exhibits conductivity, which enables the secondary battery to exhibit appropriate charge / discharge characteristics.

[0027] However, at temperatures above a certain level, the conductive polymer can be de - doped by anions derived from the electrolyte from the aromatic ring. As a result, the conductive polymer can act as a non - conductor, thereby increasing the resistance and exhibiting PTC characteristics that interrupt the flow of current.

[0028] In particular, in one embodiment, the electrode for a lithium secondary battery adjusts properties such as the structure and solubility of the conductive polymer, coating thickness, and method, etc., which will be described later, so that such a conductive polymer can be mainly distributed in the region adjacent to the metal current collector. For example, in the region reaching 10% (or 7%) of the thickness of the active - material layer from the surface of the positive - electrode current collector. Specifically, the conductive polymer can be distributed in the region adjacent to the metal current collector in an amount of 90 wt% or more, or 90 wt% to 100 wt%, or 92 wt% to 98 wt% of the total conductive polymer.

[0029] Furthermore, in a specific embodiment, the minimum linear distance from the surface of the metal current collector to the region where the conductive polymer is distributed in an amount of 90 wt% or more can be 0 μm to 15 μm, or 0 μm to 10 μm.

[0030] Since the specific conductive polymer exhibiting PTC characteristics is concentrated and uniformly distributed in the region adjacent to the metal current collector in this way, the electrode of one embodiment can further improve the electrochemical characteristics such as safety and charge / discharge characteristics of the lithium secondary battery based on the following principle.

[0031] First, when an external stimulus such as overcharging, high temperature, or external shock is applied to a lithium secondary battery including an electrode and the temperature inside the battery rapidly increases, due to the de-doping of anions and the like, this conductive polymer can be transformed into a non-conductor. Therefore, the resistance within the electrode can be significantly increased and the flow of current can be interrupted to prevent overcurrent caused by a short circuit between the electrodes, and heat generation, ignition, explosion, gas generation, etc. in the secondary battery can be suppressed. In particular, in the electrode of one embodiment, since the conductive polymer is concentrated in the region adjacent to the metal current collector, the conductive polymer that is transformed into a non-conductor when an external stimulus is applied can very effectively interrupt the contact between the positive electrode active material and the electrode current collector, thereby further improving the safety of the lithium secondary battery.

[0032] In addition, since the conductive polymer does not diffuse into the active material layer but is concentrated in the region adjacent to the metal current collector, this conductive polymer does not interfere with the normal charge / discharge process of the secondary battery. In particular, during the normal charge / discharge process of the secondary battery, even if part of the conductive polymer is transformed into a non-conductor due to the application of local heat within the active material layer, this will be evenly distributed in the region adjacent to the metal current collector and thus will not interfere with the charge / discharge process and characteristics. As a result, a lithium secondary battery including the electrode of one embodiment can exhibit the same level or higher charge / discharge characteristics while exhibiting further improved safety.

[0033] At the same time, the region where the conductive polymer is distributed and the distribution content within the corresponding region can be confirmed and measured by performing Raman image analysis of the cross-section of the active material layer. In a specific embodiment, as Figure 1 shown, each component included in the active material layer, such as the electrode active material, the conductive material, and the above-mentioned conductive polymer, exhibits different Raman spectra from each other. Therefore, in the result of the Raman image analysis of the cross-section of the active material layer, each component distinguished by the Raman spectrum can be represented as different fluorescence. According to the result of this Raman image analysis, the region where the conductive polymer is distributed (for example, Figure 1 the region shown in red) can be confirmed, and the distribution content of the conductive polymer can be calculated by calculating the corresponding conductive polymer distribution region and its ratio of various cross-sections of the active material layer.

[0034] The conductive polymer included in the electrode of the above embodiment exhibits the above-mentioned PTC characteristics, and the effective operating temperature at which this conductive polymer is transformed into a non-conductor can be 70°C to 130°C, or 80°C to 125°C. Since the conductive polymer is transformed into a non-conductor at this effective operating temperature, the conductive polymer can more effectively suppress the ignition or explosion of the secondary battery when an external stimulus is applied without suppressing the normal charge / discharge process of the secondary battery.

[0035] The conductive polymers exhibiting the above-described effective working temperature and distribution characteristics will be described in more detail below.

[0036] Furthermore, the conductive polymer may be a polymer or copolymer containing repeating units having an aromatic ring, the aromatic ring containing one or more hetero elements, or one or two hetero elements, in an amount of 50 mol% or more, or 70 mol% or more, or 90 mol% to 100 mol% of the total repeating units, and more specifically, may be a polythienyl polymer or copolymer, the polythienyl polymer or copolymer mainly containing repeating units containing a substituted or unsubstituted thiophene ring as the aromatic ring.

[0037] In a more specific embodiment, the conductive polymer may be a polythienyl polymer or copolymer in which an oxyalkylene group is bonded to the thiophene ring in the repeating unit, for example, a homopolymer or copolymer containing the repeating unit of Chemical Formula 1 below:

[0038] [Chemical Formula 1]

[0039]

[0040] Wherein, in Chemical Formula 1, R 1 is a functional group of Chemical Formula 2 below,

[0041] [Chemical Formula 2]

[0042]

[0043] 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 in the range of 1 to 5000, or 10 to 2000, or 50 to 1000, wherein the alkylene group may be an alkylene group having 2 to 5 carbon atoms, and the alkyl group may be an alkyl group having 1 to 5 carbon atoms.

[0044] Furthermore, the weight average molecular weight of such a polythienyl polymer or copolymer may be, for example, 5000 g / mol to 100000 g / mol, or 10000 g / mol to 80000 g / mol.

[0045] In a more specific embodiment, the polythienyl polymer or copolymer may contain an amount of the repeating unit of Chemical Formula 1 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, or 30 mol% or less. At this time, the polythienyl polymer or copolymer may include the remaining content of alkylthienyl repeating units in addition thereto, for example, alkylthienyl repeating units in which the thiophene ring is substituted with an alkyl group having 1 to 20 carbon atoms or 3 to 15 carbon atoms.

[0046] Such polythiophene-based polymers or copolymers include substituted thiophene rings and can thus exhibit appropriate effective operating temperatures, etc. As a result, the polymer or copolymer not only does not impair the charge / discharge characteristics of the lithium secondary battery, but can be converted into a non-conductor when a high temperature above a certain level is applied, thereby improving the safety of the secondary battery.

[0047] Furthermore, due to the above-described predetermined structure, the polythiophene-based polymer or copolymer can exhibit relatively low affinity, solubility, etc. for the organic solvents mainly contained in the slurry composition for forming the electrode active material layer (e.g., solvents such as N-methylpyrrolidone), and can also exhibit excellent adhesion to the metal current collector. Therefore, in the process of forming such a polythiophene-based polymer or copolymer on the metal current collector and then coating and drying the slurry composition to form the electrode active material layer, the phenomenon of dissociation and diffusion of the conductive polymer over a large area of the active material layer can be minimized. Thus, by using such a polythiophene-based polymer or copolymer, this conductive polymer can be concentrated in the region adjacent to the metal current collector, and in the above-described Raman image analysis, 90 wt% or more can be distributed on the surface of the metal current collector up to a region 10% of the thickness of the active material layer. Ultimately, the polythiophene-based polymer or copolymer can contribute to improving the safety of the secondary battery without impairing its charge / discharge characteristics.

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

[0049] The conductive polymer having a repeating unit such as Chemical Formula 1 above 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, and then polymerizing this monomer alone or copolymerizing it with other monomers such as alkyl thiophene. The specific conditions for preparing such monomers and polymers are described in the preparation examples described later.

[0050] Furthermore, the conductive polymer can be formed on the metal current collector, for example, by coating a liquid composition dissolved or dispersed in an organic solvent such as chloroform, tetrahydrofuran (THF), toluene, or xylene at a concentration of about 0.1 wt% to 5 wt% on the metal current collector and drying it. Thereafter, the slurry composition described below is coated and dried to form the active material layer, and the active material layer can be rolled to produce an electrode for a lithium secondary battery of one embodiment.

[0051] At this time, in addition to the conductive polymer, the liquid composition for forming the conductive polymer may further include one or more additives selected from the group consisting of carbon-based conductive materials, conductive inorganic particles, binders, and esterified saccharides.

[0052] At this time, as the carbon-based conductive material and the binder, the same components as those contained in the active material layer can be used, and by adding these components, the conductivity of the electrode, the adhesion of the safety functional layer, or the mechanical properties of one embodiment can be further improved. Further, as the conductive inorganic particles, alumina or zirconia particles having a nano-sized particle diameter, for example, 5 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, or polysaccharides having acyl groups can be used as the esterified saccharide. This component generates gas when the secondary battery is overcharged, and can act to block 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.

[0053] On the other hand, the electrode of one embodiment further includes an active material layer formed on the metal current collector in the region where the conductive polymer is formed, and the active material layer may include an electrode active material, a conductive material, and optionally a binder. At this time, since it is preferable that the electrode on which the conductive polymer is formed is a positive electrode, such an example will be mainly described.

[0054] In the positive electrode for a lithium secondary battery, the metal current collector may generally have a thickness of 3 to 100 μm, and may be formed of any metal or alloy having excellent conductivity 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 surfaces are treated with carbon, nickel, titanium, or silver, etc. Further, the metal current collector may form fine protrusions and depressions on its surface to enhance the adhesion of the safety functional layer, etc., and may be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabric structures.

[0055] In addition, the positive electrode active material contained 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.

[0056] 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 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 (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-αZ2 (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).

[0057] In the above formulas, A is Ni, Co, Mn or a combination thereof; R is Al, Ni, Co, Mn, Cr, Fe, Mg, V, or a combination thereof; D is O, F, S, P or a combination thereof; E is Co, Mn or a combination thereof; Z is F, S, P or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, V or a combination thereof; Q is Ti, Mo, Mn or a combination thereof; T is Cr, V, Fe, Sc, Y or a combination thereof; and J is V, Cr, Mn, Co, Ni, Cu or a combination thereof.

[0058] Further, 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 coating elements, hydroxides of coating elements, hydroxyoxides of coating elements, carbonate oxides of coating elements, and basic carbonates of coating elements. The compounds constituting these coatings can be amorphous or crystalline compounds. As the coating elements contained 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.

[0059] Further, the conductive material used in the active layer is for imparting conductivity to the electrode, and a conductive material can be used without limitation as long as it has conductivity and does not cause chemical changes in the battery to be configured. Specific 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. Further, conductive materials such as polyphenylene derivatives can be used alone or in a mixture of one or more thereof.

[0060] Based on the total weight of the active material layer, the conductive material can be added in an amount of 1 wt% to 50 wt%, or 2 wt% to 20 wt%. Thereby, a preferred formation of the positive electrode can be ensured while imparting excellent electrical characteristics to the positive electrode.

[0061] The binder plays a role in making the particles of the positive electrode active material adhere well to each other and further improving the binding property of the active material layer. As typical examples thereof, 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, etc. can be used.

[0062] Based on the total weight of the active material layer, the binder can be added in an amount of 1 wt% to 50 wt%, or 2 wt% to 30 wt%. Thereby, a positive electrode having excellent durability can be formed without impairing the electrical characteristics and / or capacity characteristics of the positive electrode.

[0063] The above active material layer can be formed by dissolving or dispersing each component such as a positive electrode active material, a conductive material, and a binder in a medium such as an organic solvent to form a slurry composition, and then coating, drying, and rolling the slurry composition on a metal current collector having a safety functional layer formed thereon.

[0064] Examples of the medium such as an organic solvent at this time include N-methyl-2-pyrrolidone (NMP), methoxypropyl acetate, butyl acetate, glycolic acid, butyl ester, butyl glycol, methylalkyl polysiloxane, alkylbenzene, propylene glycol, xylene, monophenyl glycol, etc. Among them, considering the dispersibility and processability of the above-mentioned positive electrode active material and conductive material, NMP, etc. can be appropriately used.

[0065] In this way, a region containing a conductive polymer and an active material layer are formed by separate compositions and processes, while using a polymer having a low solubility and affinity for NMP, for example, a polymer having a repeating unit of Chemical Formula 1, as the conductive polymer, so that the conductive polymer can be concentrated and uniformly distributed in the region adjacent to the surface of the metal current collector. As a result, a lithium secondary battery including an electrode of one embodiment can exhibit further improved safety and excellent charge / discharge characteristics.

[0066] At the same time, since the formation process and conditions of the active material layer except for the formation process of the conductive polymer can follow the general positive electrode formation process and situation, further description thereof will be omitted.

[0067] The active material layer formed by the above method can have a thickness of 5 μm to 200 μm, or 10 μm to 100 μm, and the region where the conductive polymer is distributed in an amount of 90% by weight or more can have a thickness of 0.01 μm to 20 μm, or 0.05 μm to 10 μm. This is because in the electrode of one embodiment, the conductive polymer is concentrated and uniformly distributed in the region adjacent to the surface of the metal current collector. Depending on the above thickness range, when an external stimulus such as an external impact is applied, the conductive polymer inhibits direct contact between the active material layer and the metal current collector, thereby ensuring improved safety of the secondary battery. Further, the inhibition of the charge / discharge characteristics of the secondary battery due to the formation of the conductive polymer distribution region being too thick can be minimized.

[0068] At the same time, 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 at the same time includes a positive electrode, a negative electrode, and a separator interposed therebetween.

[0069] In such a lithium secondary battery of another embodiment, the negative electrode is produced by coating, drying, and rolling a negative electrode active material on a negative electrode current collector, and may further include a conductive material and a binder as needed.

[0070] 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 where hexagonal honeycomb carbon planes are arranged in layers), hard carbon having a structure in which a low crystal structure is mixed with an amorphous part, carbon and graphite materials such as artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotubes, fullerenes, activated carbon; or metal composite oxides such as Li x Fe2O3 (0 ≤ x ≤ 1), Li x WO2 (0 ≤ x ≤ 1), Sn x Me 1-x Me' y O z (Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, elements in Groups 1, 2, 3 of the periodic table, halogens; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8); lithium metal; lithium alloy; silicon, silicon oxide or silicon-based alloy; tin-based alloy; conductive polymer such as polyacetylene; Li-Co-Ni-based material; titanium oxide; or lithium titanium oxide, etc.

[0071] In one embodiment, the negative electrode active material may include both 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, and the silicon (Si)-containing particles are particles containing silicon (Si) as a main component of the metal component, and may include silicon (Si) particles, silicon oxide particles, or a mixture of silicon (Si) particles and silicon oxide particles.

[0072] Furthermore, the conductive material and the binder that can be used together with the negative electrode active material may be the same components as the conductive material and the binder contained in the positive electrode active material layer.

[0073] Furthermore, the negative electrode active material layer containing the negative electrode active material may have a thickness of 100 μm to 200 μm, or 120 μm to 200 μm.

[0074] Moreover, there is no particular limitation on the negative electrode current collector 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. can be used, and in the case of copper or stainless steel, materials surface-treated with carbon, nickel, titanium, silver, etc. can be used.

[0075] 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.

[0076] Furthermore, 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 this field. Specifically, chemically resistant and hydrophobic polypropylene; glass fiber; or a sheet or non-woven fabric made of polyethylene, etc. can be used. In some cases, a composite separator can be used, in which inorganic particles / organic particles are coated with an organic binder polymer on a porous polymer substrate such as a sheet or non-woven fabric. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte can also act as a separator. In addition, the separator may have an average pore size of 0.01 μm to 10 μm and an average thickness of 5 μm to 300 μm.

[0077] The above lithium secondary battery may further include an electrolyte, and such an electrolyte may be an electrolyte containing a non-aqueous organic solvent and a lithium salt, or an electrolyte film containing an organic or inorganic solid electrolyte, and these 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.

[0078] Beneficial Effects

[0079] 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, the overcurrent can be effectively interrupted by the conductive polymer concentrated near the metal current collector, thereby minimizing ignition and explosion.

[0080] Thereby, the lithium secondary battery can exhibit improved safety, and the deterioration of charge / discharge characteristics caused by the conductive polymer can be suppressed, thereby exhibiting excellent electrochemical characteristics. Brief Description of the Drawings

[0081] Figure 1 The Raman image analysis results of the active material layer of the electrode for a lithium secondary battery produced in Example 1 are shown, where Figure 1 The right figure shows the Raman spectrum analysis results of the conductive polymer prepared in the preparation example;

[0082] Figure 2is a diagram showing the process and results of calculating a probability density function based on the Raman image analysis results according to Figure 1 and deriving the content of the conductive polymer in the region where the conductive polymer is distributed;

[0083] Figure 3 is a diagram showing the process and results of calculating a probability density function based on the Raman image analysis results of Comparative Example 2 and deriving the content of the conductive polymer in the region where the conductive polymer is distributed; and

[0084] Figures 4a to 4c show the results of the nail penetration test of five lithium secondary batteries produced in Example 1 ( Figure 4a ), Comparative Example 1 (see Figure 4b ), and Comparative Example 2 (see Figure 4c ), respectively. DETAILED DESCRIPTION

[0085] 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 described herein.

[0086] Preparation example: Synthesis of monomers and conductive polymers

[0087] [Chemical Formula 3]

[0088]

[0089] After creating a nitrogen environment inside a three-way RBF (Round Bottom Flask) by flowing nitrogen, 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 a 60% Sodium Hydride in mineral oil solution 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-bromothophene was added, and the mixture was refluxed at about 100 °C for about 24 hours. The reaction solution was filtered through a decompression 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).

[0090] 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, and the mixture was subjected to a polymerization reaction with stirring at about 25 °C for 24 hours. The reaction solution was placed in a permeable membrane with an MWCO (molecular weight of cut-off) of 5000, and then immersed in 1500 ml of an 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. The weight-average molecular weight (Mw) of the conductive polymer was confirmed to be about 37,000 g / mol.

[0091] Example 1: Production of positive electrode and lithium secondary battery

[0092] (Production of the positive electrode)

[0093] Dissolve 20 g of the conductive polymer (Mw = 37,000 g / mol) obtained in the Preparation Example in 1,980 g of chloroform solvent to obtain a composition. Gravure coat and dry this composition on an aluminum (Al) thin film as the positive electrode current collector, thereby forming a conductive polymer-containing layer with a thickness of about 0.5 μm.

[0094] Add LiCoO2 as the positive electrode active material, a conductive material (carbon black), and a binder (polyvinylidene fluoride) in a weight ratio of 97.5:1:1.5 to N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode slurry (solid content: 60 wt%). Coat this slurry onto the conductive polymer-containing layer (however, based on a total of 100 weight parts of the positive electrode active material, conductive material, and binder, the weight of the conductive polymer is about 0.5 weight parts), dry, and then roll press to form an active material layer with a total thickness of 58 μm, thereby producing a positive electrode.

[0095] (Production of negative electrode)

[0096] Add a negative electrode active material (graphite), a binder (SBR-CMC), and a conductive material (carbon black) in a weight ratio of 95:3.5:1.5 to water as a solvent to prepare a negative electrode slurry (solid content: 60 wt%). Coat the negative electrode slurry on a copper (Cu) thin film with a thickness of 8 μm as the negative electrode current collector, dry, and then roll press to produce a negative electrode.

[0097] (Production of separator)

[0098] 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 this binder solution such that Al2O3 / PVdF-HFP = 90 / 10 (wt% ratio), and use a ball mill method to prepare a slurry for 12 hours or longer. Coat the slurry prepared in this way on a polyolefin-based separator with a thickness of about 8 μm using a dip coating method. Adjust the coating thickness to about 4.5 μm to produce a porous separator.

[0099] (Production of lithium secondary battery)

[0100] The positive electrode, separator, and negative electrode are stacked in sequence, and then pressed using heat and pressure of 90 °C and 200 kPa to produce an electrode assembly composed of dual cells. The assembled electrode assembly is placed into a pouch-type battery case, and ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a volume ratio of 30:70, and then an electrolyte in which 1.0 M LiPF6 is dissolved is injected therein, thereby producing a lithium secondary battery.

[0101] Comparative Example 1

[0102] The positive electrode and lithium secondary battery of Comparative Example 1 were produced in the same manner as in Example 1, except that a conductive polymer was not used (a layer containing a conductive polymer was not formed).

[0103] Comparative Example 2

[0104] LiCoO2 as a positive electrode active material, a conductive material (carbon black), a binder (polyvinylidene fluoride), and the conductive polymer obtained in the Preparation Example were added to N-methyl-2-pyrrolidone (NMP) at a weight ratio of 97:1:1.5:0.5 to prepare a positive electrode slurry (solid content: 60 wt%).

[0105] This slurry was coated on an aluminum thin film (bare Al foil) and dried, and then roll-pressed to form an active material layer with a total thickness of 58 μm, and a positive electrode was produced.

[0106] Test example

[0107] Raman spectroscopy analysis and Raman image analysis

[0108] Regarding the positive electrodes produced in the Examples and Comparative Examples, cross-sections from the surface of the positive electrode to the current collector were ensured, and Raman images of the cross-sections were measured (equipment name: DXR3xi, Thermofisher Scientific, USA) to analyze the Raman spectra and distribution of the conductive polymer. As Figure 1 shown in the right figure of, the peaks on the Raman spectrum of the positive electrode produced in the Example are roughly divided into three parts. Among them, the Raman signal (peak center) of the conductive polymer was confirmed at 1350 cm -1 to 1600 cm -1 (specifically, 1350 cm -1 to 1500 cm -1 ), and the corresponding Raman signal appears due to the vibration mode of the aromatic ring derived from the conductive polymer. Based on this, the area of the region of 1350 cm -1 ~1500 cm -1 on the Raman image was calculated, andFigure 1 The area of the red region (conductive polymer) on the left figure of

[0109] *Analysis conditions : Excitation laser wavelength 532 nm, laser power 0.8 mW, detector exposure time (exposure time per unit analysis area) 0.15 sec, grating 1200 grooves / mm, pixel resolution 1 cm -1 , mapping size 30 μm x 65 μm, mapping pixel size 0.6 μm x 0.6 μm

[0110] In the measured Raman image, only the image of the red region (conductive polymer) is shown separately, and the content in the concentrated distribution region is calculated. The value represented by each Raman image pixel of the shown image is extracted using the Image J program, and then the probability density function is calculated using the extracted values as shown in Equation 1 below.

[0111] [Equation 1]

[0112]

[0113] In Equation 1, ρ(x) represents the probability density function, and I(x) represents the value of the Raman image pixel. The average pixel value in the thickness direction is obtained in the completely extracted Raman image region, and the values are normalized and standardized using the probability density function. Normalization is performed by calculating the integral of all pixel values in the denominator as shown in the above equation and dividing each pixel value by the corresponding integral. Finally, profile analysis is performed on the normalized values to confirm the proportion of the region occupied by the conductive polymer in the total thickness.

[0114] Figure 1 The Raman image analysis results of the active material layer of the electrode for a lithium secondary battery produced in Example 1 are shown, where Figure 1 The right figure of shows the Raman spectrum analysis results of the conductive polymer prepared in the preparation example. In addition, Figure 2 Schematically shows the process and results of calculating the probability density function based on the Raman image analysis results of Figure 1 and thereby deriving the content of the conductive polymer in the region where the conductive polymer is distributed. Further, Figure 3 Schematically shows the process and results of calculating the probability density function in the same manner as in Example 1 based on the Raman image analysis results of Comparative Example 2 and thereby deriving the content of the conductive polymer in the region where the conductive polymer is distributed.

[0115] In addition, through the above process, based on the results of Raman image analysis, for each example and comparative example, the content (wt%) of the conductive polymer present in the region reaching 10% of the thickness of the active material layer from the surface of the positive current collector was calculated and shown together in Table 1 below.

[0116] Evaluation of high - rate discharge characteristics

[0117] The lithium secondary batteries produced in the examples and comparative examples were charged under the conditions of constant current (0.7C) and constant voltage (4.47V, cut-off at 0.025C), then left standing for 10 minutes, and discharged under the conditions of constant current (0.1C, 0.2C, 0.5C, 1.0C, 1.5C) until the voltage reached 3V. That is, when the number of charge / discharge cycles increased, the discharge rates periodically became 0.1C, 0.2C, 0.5C, 1.0C, and 1.5C, respectively. Thus, the high-rate discharge characteristics (rate capability) of each battery were evaluated. At this time, the high-rate discharge characteristics at 1.5C are shown in Table 1 below.

[0118] Nail penetration test

[0119] Five lithium secondary batteries produced in the examples and comparative examples were respectively prepared, fully charged to 100% SOC at 4.47V (cut-off at 0.05C) under the conditions of CC / CV and 0.5C 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 30 mm was penetrated into the center of the battery at a vertical angle and a penetration speed of 100 mm / sec to measure the presence of ignition. Table 1 below lists the number of non-ignited batteries among the five batteries. Figures 4a to 4c Photographs of each battery pack after the above-mentioned nail penetration test are shown.

[0120] [Table 1]

[0121]

[0122]

[0123] 1). The results of Raman image analysis represent the content (wt%) of the conductive polymer present in the region reaching 10% of the thickness of the active material layer from the surface of the positive current collector.

[0124] Referring to Table 1, it can be confirmed that even though the lithium secondary battery of Example 1 contains a conductive polymer for improving safety, it is similar to the lithium secondary battery of Comparative Example 1 and shows significantly improved rate discharge characteristics compared to Comparative Example 2.

[0125] In addition, it was confirmed that the lithium secondary battery of Example 1 did not catch fire even when subjected to a large external impact, thus showing excellent safety compared to Comparative Examples 1 and 2.

Claims

1. An electrode for a lithium secondary battery, comprising: Metal current collector; and an electrode active material and a conductive material, and including an active material layer formed on the metal current collector, wherein the active material layer further comprises a conductive polymer, the conductive polymer exhibiting a peak in a band of 1350 cm -1 to 1600 cm -1 and exhibiting a PTC (positive temperature coefficient) characteristic, and wherein when Raman image analysis is performed on a cross-section of the active material layer, the conductive polymer is distributed in an amount of 90% by weight or more in a region reaching 10% of the thickness of the active material layer from the surface of the metal current collector.

2. The electrode for a lithium secondary battery according to claim 1, wherein an effective operating temperature of the conductive polymer exhibiting the PTC characteristics is 70°C to 130°C.

3. The electrode for a lithium secondary battery according to claim 1, wherein the conductive polymer includes a repeating unit containing an aromatic ring having a conjugated π electron system.

4. The electrode for a lithium secondary battery according to claim 3, wherein the conductive polymer includes a polythiophene-based polymer or copolymer.

5. The electrode for a lithium secondary battery according to claim 3, wherein the conductive polymer includes a homopolymer or copolymer containing a repeating unit represented by the following Chemical Formula 1: [Chemical Formula 1] Wherein, In Chemical Formula 1, R 1 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 in the range of 1 to 5000.

6. The electrode for a lithium secondary battery according to claim 4, wherein the conductive polymer has a weight-average molecular weight of 5000 g / mol to 100000 g / mol.

7. The electrode for a lithium secondary battery according to claim 1, wherein the active material layer has a thickness of 5 μm to 200 μm, and a region where the conductive polymer is distributed in an amount of 90% by weight or more has a thickness of 0.01 μm to 20 μm.

8. The electrode for a lithium secondary battery according to claim 1, wherein a minimum linear distance from the surface of the metal current collector to a region where the conductive polymer is distributed in an amount of 90% by weight or more is 0 μm to 15 μm.

9. The electrode for a lithium secondary battery according to claim 1, further comprising one or more additives selected from the group consisting of a carbon-based conductive material, conductive inorganic particles, a binder, and an esterified saccharide in a region where the conductive polymer is distributed in an amount of 90% by weight or more.

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

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

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