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

Abstract

The present disclosure relates to a composition for forming an electrode protective layer for a lithium secondary battery, and an electrode for a lithium secondary battery and a lithium secondary battery comprising the same, the composition not only suppressing heat generation or ignition caused by external impact, etc. Also, it is possible to provide an electrode and a battery having excellent conductivity and rate characteristics.

Description

Technical Field

[0001] Cross - reference to related applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2022-0169446, filed with the Korean Intellectual Property Office on December 7, 2022, the disclosure of which is incorporated herein by reference in its entirety.

[0003] The present disclosure relates to a composition for forming an electrode protective layer for a lithium secondary battery, an electrode for a lithium secondary battery, and a lithium secondary battery including the electrode. The composition not only inhibits heat generation or ignition caused by external shock or the like, and thus has excellent stability, but also enables an electrode and a battery having excellent conductivity and rate characteristics to be provided. 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 shock, 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 cause of ignition, explosion, etc. in lithium secondary batteries is a short circuit 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 cause heat generation, gas generation inside the battery, and volume expansion, and thus may pose a risk of ignition 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 electrodes to which previously known functional layers are added, there is a drawback that it is difficult to sufficiently improve the safety of lithium secondary batteries, or the conductivity or rate performance of the lithium secondary battery itself is reduced by adding such functional layers. In particular, when the thickness of the functional layer is increased to improve the safety of the lithium secondary battery, it is difficult for the electrolyte to penetrate the functional layer, resulting in a significant reduction in the conductivity and rate performance of the electrode itself.

[0009] Due to these problems, there is a continuous need to develop technologies that can further improve the safety of lithium secondary batteries while suppressing deterioration of their conductivity, rate performance, and the like. SUMMARY OF THE INVENTION

[0010] TECHNICAL PROBLEM

[0011] Accordingly, an object of the present disclosure is to provide a composition for forming an electrode protective layer for a lithium secondary battery, which not only suppresses heat generation or ignition caused by an external shock or the like and thus has excellent stability, but also enables the provision of an electrode and a battery having excellent conductivity and rate performance.

[0012] Another object of the present disclosure is to provide an electrode for a lithium secondary battery and a lithium secondary battery, which include a safety functional layer formed of the composition, thus exhibiting improved safety, and also having excellent conductivity and rate performance.

[0013] TECHNICAL SOLUTION

[0014] According to an aspect of the present disclosure, there is provided a composition for forming an electrode protective layer for a lithium secondary battery, including: a polythiophene-based conductive polymer exhibiting a PTC (positive temperature coefficient) characteristic; and porous conductive carbon particles having a plurality of pores with diameters ranging from 10 nm to 300 nm formed therein.

[0015] According to another aspect of the present disclosure, there is provided an electrode for a lithium secondary battery, including: a metal current collector; a safety functional layer formed to cover at least a part of the metal current collector and formed of the composition for forming an electrode protective layer; and an active material layer including an electrode active material and a conductive material and formed on the metal current collector and the safety functional layer.

[0016] According to still 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 is included as the positive electrode.

[0017] Now, a composition for forming an electrode protective layer for a lithium secondary battery according to a specific embodiment of the present disclosure, a lithium secondary battery including the composition, and the like will be described.

[0018] 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 terms in order to best describe their own inventions in an appropriate manner, with meanings and concepts consistent with the technical idea of the present disclosure.

[0019] The terms used herein are provided to describe 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.

[0020] 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.

[0021] According to an embodiment of the present disclosure, there is provided a composition for forming an electrode protective layer for a lithium secondary battery, including: a polythiophene-based conductive polymer exhibiting PTC (positive temperature coefficient) characteristics; and porous conductive carbon particles having a plurality of pores with diameters ranging from 10 nm to 300 nm formed therein.

[0022] The composition according to an embodiment is used to form, for example, an electrode protective layer (safety functional layer) different from the electrode active material layer on the surface of a metal current collector in an electrode for a lithium secondary battery, and the composition includes a polythiophene-based conductive polymer exhibiting PTC characteristics and porous conductive carbon particles.

[0023] First, when a lithium secondary battery is activated, the polythiophene-based conductive polymer can exhibit conductivity by doping anions from the electrolyte of the secondary battery into the aromatic thiophene rings of the conductive polymer. Thus, during the normal charge / discharge process of the secondary battery, the conductive polymer exhibits conductivity, which allows the secondary battery to exhibit appropriate charge / discharge characteristics.

[0024] However, the conductive polymer can be de-doped of anions from the aromatic thiophene rings by the anions 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 for a lithium secondary battery in which an electrode protection layer containing the conductive polymer is formed can contribute to improving the stability of the lithium secondary battery, as described below.

[0025] 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 to interrupt the current flow between the current collector and the active material layer, to prevent overcurrent caused by a short circuit between the electrodes, and to suppress heat generation, ignition, explosion, gas generation, etc. in the secondary battery.

[0026] Incidentally, when the electrode protection layer is formed very thin, heat generation, etc. cannot be sufficiently suppressed, and the safety of the lithium secondary battery may be insufficient. When a thicker electrode protection layer is formed to solve this problem, the electrolyte becomes difficult to penetrate through the electrode protection layer, which reduces the doping effect of the conductive polymer. As a result, the conductivity of the electrode including the electrode protection layer and the active material layer deteriorates, and the overall rate performance of the secondary battery also deteriorates.

[0027] However, the porous conductive carbon particles having pores of 10 nm to 300 nm formed in a composition of one embodiment facilitate the penetration of the electrolyte solution due to their own porosity, and can exhibit excellent thermal conductivity and electronic conductivity through thermal diffusion. As a result, when the electrode protection layer is formed of a composition of one embodiment including porous conductive carbon particles, even if the electrode protection layer is formed thick, the electrode protection layer not only exhibits excellent electrolyte permeability, but also enables the electrode including the electrode protection layer to exhibit improved conductivity. In addition, due to the excellent thermal conductivity of the porous conductive carbon particles, the electrode including the electrode protection layer formed of a composition of one embodiment can effectively diffuse heat inside the electrode, thereby exhibiting further improved safety.

[0028] Therefore, the composition of one embodiment and the electrode protective layer formed therefrom can contribute to improving the safety of a lithium secondary battery. Even when formed thick, the electrode and the lithium secondary battery including the electrode can exhibit excellent conductivity and rate characteristics.

[0029] Meanwhile, the polythiophene-based conductive polymer included in the composition of one embodiment exhibits the above-described PTC characteristics, and the effective operating temperature at which such a 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 this effective operating temperature, when an external stimulus is applied, the conductive polymer can more effectively suppress the ignition or explosion of the secondary battery without inhibiting the normal charge / discharge process of the secondary battery.

[0030] In addition, the conductive polymer is a polythiophene-based polymer or a copolymer containing substituted or unsubstituted thiophene-based repeating units in an amount of 50 mol% or more, or 70 mol% or more, or 90 mol% to 100 mol% of the total repeating units.

[0031] In a more specific example, the conductive polymer can 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:

[0032] [Chemical Formula 1]

[0033]

[0034] In Chemical Formula 1, R1 is a functional group of Chemical Formula 2 below,

[0035] [Chemical Formula 2]

[0036]

[0037] In Chemical Formula 2, L1 is a single bond or an alkylene group, L2 is an alkylene group, R3 is hydrogen or an alkyl group, and n is an integer in the range of 1 to 5000, or 10 to 2000, or 50 to 1000. The alkylene group can be an alkylene group having 2 to 5 carbon atoms, and the alkyl group can be an alkyl group having 1 to 5 carbon atoms.

[0038] In addition, the weight average molecular weight of such a polythiophene-based conductive polymer can be, for example, 5000 g / mol to 100000 g / mol, or 10000 g / mol to 80000 g / mol. Thus, the composition for forming the electrode protective layer can exhibit excellent coating properties and can also exhibit appropriate adhesion to a metal current collector or the like.

[0039] In a more specific example, the polythiophene-based conductive polymer may contain the repeating unit of Chemical Formula 1 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 in an amount of 100 mol% or less, or 80 mol% or less, or 50 mol% or less. At this time, the polythiophene-based conductive polymer may include the remaining amount of alkylthiophene-based repeating units in addition to the repeating unit of Chemical Formula 1. For example, an alkylthiophene-based repeating unit in which a thiophene ring is substituted with an alkyl group having 1 to 20 carbon atoms, or 3 to 15 carbon atoms.

[0040] Such a polythiophene-based conductive polymer includes a substituted thiophene ring and can thus exhibit an appropriate effective working temperature, etc. As a result, when a high temperature higher than a certain level is applied, the conductive polymer 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.

[0041] In addition, due to the above-described predetermined structure, the polythiophene-based conductive polymer can exhibit a relatively low affinity, solubility, etc. for organic solvents mainly contained in a slurry composition for forming an electrode active material layer, such as a solvent such as N-methylpyrrolidone, and can also exhibit excellent adhesion to a metal current collector. Therefore, in the process of forming such a polythiophene-based 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 polythiophene-based conductive polymer, an electrode protective layer can be uniformly formed near the surface of the metal current collector. Therefore, the conductive polymer can contribute to improving the safety of the secondary battery without impairing its basic performance.

[0042] Meanwhile, based on 100 parts by weight of the electrode active material (e.g., a positive electrode active material) included in the active material layer, the content of the conductive polymer may be 0.001 parts by weight to 5 parts by weight, or 0.005 parts by weight to 5 parts by weight. Therefore, an electrode according to one embodiment can have more excellent safety and charge / discharge characteristics.

[0043] A conductive polymer having the repeating unit of Chemical Formula 1, etc. can be prepared, for example, by subjecting a halogenated thiophene compound and an alkylene glycol 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 this monomer with other monomers such as alkylthiophene. Specific conditions for preparing such monomers and polymers are described in the preparation examples described later.

[0044] Meanwhile, a composition of an embodiment includes porous conductive carbon particles having a plurality of pores with diameters of 10 nm to 300 nm, or 30 nm to 250 nm, and the above-mentioned polythiophene-based conductive polymer. Such porous conductive carbon particles can be obtained, for example, from carbonaceous materials such as hard carbon and / or soft carbon, and are prepared by modifying or surface-treating the carbonaceous materials to have porosity, or can be commercially obtained and used as conductive carbon particles having the above porosity.

[0045] Such porous conductive carbon particles exhibit excellent dispersibility in an organic solvent together with the conductive polymer, so that a good electrode protective layer can be formed on a current collector with a composition of an embodiment containing the same. In addition, the porous conductive carbon particles contribute to the penetration of the electrolyte through the electrode protective layer, and can themselves exhibit excellent electrical conductivity and thermal conductivity.

[0046] Therefore, even if a thicker electrode protective layer is formed with a composition of an embodiment to further improve the safety of a lithium secondary battery, such an electrode protective layer exhibits excellent electrolyte permeability and excellent electrical conductivity. Therefore, the electrode protective layer not only contributes to improving the safety of the lithium secondary battery, but also contributes to improving its electrical conductivity and rate performance.

[0047] From the perspective of the excellent dispersibility and coatability in a solvent of a composition of an embodiment, the number average particle size (D50) of the porous conductive carbon particles is 0.5 μm to 20 μm, or 1 μm to 15 μm, or 1.5 μm to 10 μm.

[0048] In addition, from the perspective of the above electrolyte permeability and excellent electrical conductivity and thermal conductivity, such porous conductive carbon particles can have a porosity of 10% to 40%, or 15% to 35%, and can exhibit a porosity defined by a specific surface area of 20 m 2 / g to 600 m 2 / g, or 30 m 2 / g to 500 m 2 / g.

[0049] Based on 100 parts by weight of the polythiophene-based conductive polymer, the content of the above porous conductive carbon particles can be 0.1 part by weight to 80 parts by weight, or 0.5 part by weight to 60 parts by weight. Depending on the content of such porous conductive carbon particles, a good electrode protective layer can be formed with a composition of an embodiment, and the electrode protective layer can effectively contribute to improving the safety, rate performance, electrical conductivity, etc. of a lithium secondary battery.

[0050] Meanwhile, a composition of an embodiment can be prepared in the form of a liquid composition by dissolving or dispersing the above-mentioned conductive polymer and porous conductive carbon particles at a concentration of about 0.1 wt% to 5 wt% in an organic solvent such as chloroform, tetrahydrofuran (THF), toluene, or xylene. Such a composition can be coated on a metal current collector and dried to form an electrode protective layer. Subsequently, the slurry composition described later is coated and dried to form an active material layer, which can then be roll-pressed to prepare an electrode for a lithium secondary battery.

[0051] At this time, a composition of an embodiment may further include at least one additive selected from the group consisting of an additional carbon-based conductive material, a binder, and an esterified saccharide.

[0052] At this time, as the carbon-based conductive material and the binder, the same components as those included in the active material layer described later can be used. Adding these components can further improve the conductivity, adhesion, or mechanical properties of an electrode of an embodiment. For example, typical examples of the carbon-based conductive material include carbon black or carbon nanotubes.

[0053] In addition, a monosaccharide, oligosaccharide, or polysaccharide having an acyl group can be used as the esterified saccharide. This component generates gas when the secondary battery is overcharged and can play a role in blocking the conduction path between the metal current collector and the electrode active material. Adding this component can further improve the safety of the secondary battery.

[0054] Meanwhile, according to another embodiment of the present disclosure, there is provided an electrode for a lithium secondary battery, which includes an electrode protective layer (safety functional layer) formed of the composition of the above-mentioned embodiment. Such an electrode of another embodiment may include: a metal current collector; a safety functional layer formed to cover at least a part of the metal current collector and formed of the composition of the above-mentioned embodiment; and an active material layer including an electrode active material and a conductive material and formed on the metal current collector and the safety functional layer.

[0055] Such an electrode of another embodiment includes a safety functional layer formed of the composition of the above-mentioned embodiment and includes a conductive polymer and porous conductive carbon particles on the current collector, thereby being able to further improve safety, conductivity, and rate performance.

[0056] The electrodes of these other embodiments further include a metal current collector and an active material layer formed over a safety layer formed of a composition of one embodiment, where the active material layer may include an electrode active material, a conductive material, and optionally a binder. At this time, since it is preferred that the electrodes of another embodiment are positive electrodes, these examples will be mainly described.

[0057] In the positive electrode for a lithium secondary battery, the metal current collector typically may have a thickness of 3 μm to 100 μm and may 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 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.

[0058] In addition, 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 intercalating and deintercalating 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.

[0059] More specifically, a compound represented by any of the following formulas may 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 aNi 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-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); Lia 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).

[0060] In the above formulae, 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.

[0061] In addition, those having a coating on the surface of each compound 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 element 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.

[0062] In addition, the conductive material used in the active layer is for imparting 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. 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. In addition, conductive materials such as polyphenylene derivatives can be used alone or as a mixture of one or more of them.

[0063] 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 imparting excellent electrical characteristics to the positive electrode, the formation of a preferred positive electrode can be ensured.

[0064] The function 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, ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, and the like can be used.

[0065] 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 having excellent durability can be formed without imparting the electrical characteristics and / or capacity characteristics of the positive electrode.

[0066] The above-mentioned active material layer can be formed by dissolving or dispersing each component such as the positive electrode active material, the conductive material, and the binder in a medium such as an organic solvent to form a slurry composition, and then coating, drying, and roll-pressing the slurry composition onto a metal current collector having a safety functional layer formed thereon.

[0067] 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, monobenzene diol, or the like. Among these compounds, NMP and the like can be appropriately used in consideration of the dispersibility and processability of the above-mentioned positive electrode active material and conductive material.

[0068] Meanwhile, since the processes and conditions for forming the active material layer can follow the general positive electrode forming processes and conditions, further description thereof will be omitted.

[0069] 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, 0.05 μm to 10 μm, or 0.5 μm to 5 μm. Since the safety functional layer is formed to the above uniform thickness near the surface of the metal current collector, when an external stimulus such as an external shock is applied, the conductive polymer contained in the safety functional layer can inhibit the direct contact between the active material layer and the metal current collector, thereby ensuring an improvement in the safety of the secondary battery. In addition, since the safety functional layer includes porous conductive carbon particles, excellent conductivity of the electrode and excellent rate characteristics of the secondary battery can be ensured even if the safety functional layer is formed relatively thick.

[0070] Meanwhile, according to another embodiment of the present disclosure, a lithium secondary battery is provided, including: a positive electrode, a negative electrode, and a separator interposed therebetween, wherein the electrode described in other embodiments is included as the positive electrode.

[0071] In such a lithium secondary battery, the negative electrode is produced by coating, drying, and roll-pressing a negative electrode active material on a negative electrode current collector, and may further include a conductive material and a binder as needed.

[0072] 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; wherein 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 part; carbon and graphite materials such as artificial graphite, expanded graphite, carbon fiber, graphitization-resistant carbon, carbon black, carbon nanotube, fullerene, 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, 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 polymer such as polyacetylene; Li-Co-Ni-based material; titanium oxide; or lithium titanate oxide; and so on.

[0073] In one example, 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 of the metal component, and may include silicon (Si) particles, silicon oxide particles, or a mixture of silicon (Si) particles and silicon oxide particles.

[0074] 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 conductive material and the binder in the positive electrode active material layer.

[0075] 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.

[0076] In addition, the negative electrode current collector is not particularly limited as long as it has high electrical 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 that have been surface-treated with carbon, nickel, titanium, silver, etc. can be used.

[0077] In addition, similar to the positive electrode current collector, the negative electrode current collector can have fine protrusions and depressions formed on its surface to enhance the adhesion of the negative electrode active material layer, and can be formed in various forms such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabric structures. In addition, considering the electrical conductivity and total thickness of the negative electrode to be produced, the average thickness of the negative electrode current collector can be appropriately applied in the range of 3 μm to 100 μm.

[0078] In addition, the 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 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 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. In addition, 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.

[0079] The above lithium secondary battery can further include an electrolyte, and this electrolyte can be an electrolyte solution containing a non-aqueous organic solvent and a lithium salt, or an electrolyte membrane containing an organic or inorganic solid electrolyte, 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 therefore, further description thereof will be omitted.

[0080] Beneficial effects

[0081] As described above, even if the internal temperature of the battery rapidly increases due to external stimuli such as overcharging, high temperature, or external shock, the electrode including the electrode protection layer and the lithium secondary battery formed of the composition of the present disclosure can effectively interrupt the overcurrent and minimize ignition and explosion due to the relatively thick electrode protection layer (safety function layer) formed near the metal current collector. Therefore, the lithium secondary battery can exhibit excellent safety.

[0082] In addition, although the electrode protection layer is relatively thick, the lithium secondary battery can still exhibit excellent electrical conductivity and rate performance. Detailed Embodiments

[0083] 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.

[0084] Preparation examples: Synthesis of monomers and conductive polymers

[0085] [Chemical Formula 3]

[0086]

[0087] After forming the interior of a three-way RBF (Round Bottom Flask) into a nitrogen environment by a nitrogen stream, 2.34 g (0.01 mol) of copper(I) iodide and 50.36 g (0.31 mol) of triethyleneglycol were added. 3.68 g (0.096 mol) of sodium hydride, which is 60% in 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 decompression device, then washed with 100 mL of dichloromethane solution, and then washed successively with NH4Cl and brine. The solvent was removed by distillation under reduced pressure, and the crude product was purified by column chromatography (hexane:ethyl acetate = 60:40) to obtain about 9.0 g (yield: 60%) of the target compound (the monomeric compound of Chemical Formula 3).

[0088] 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 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. It has been confirmed that the weight-average molecular weight (Mw) of the conductive polymer is about 34,000 g / mol.

[0089] Example 1: Production of a positive electrode and a lithium secondary battery

[0090] (Production of the positive electrode)

[0091] Dissolve 20 g of the conductive polymer obtained in the Preparation Example (Mw = 34,000 g / mol) and 6 g (30 parts by weight based on 100 parts by weight of the conductive polymer) of porous conductive carbon particles (pore size: 10 nm to 300 nm, D50: 1 μm to 3 μm) in 1,980 g of chloroform solvent to obtain a composition. Gravure coat the composition on an aluminum (Al) thin film serving as a positive electrode current collector and dry it to form a safety functional layer with a thickness of about 5 μm.

[0092] Add LiCoO2 as a positive electrode active material, a conductive material (carbon black), and a binder (polyvinylidene fluoride) to N-methyl-2-pyrrolidone (NMP) in a weight ratio of 97.5:1:1.5 to prepare a positive electrode slurry (solid content: 60% by weight). Coat the solution on 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.

[0093] (Production of the negative electrode)

[0094] Add a negative electrode active material (graphite), a binder (SBR-CMC), and a conductive material (carbon black) to water as a solvent in a weight ratio of 95:3.5:1.5 to prepare a negative electrode slurry (solid content: 60% by weight). Coat the negative electrode slurry on a copper (Cu) thin film with a thickness of 8 μm serving as a negative electrode current collector, dry it, and then perform a roll press to produce a negative electrode.

[0095] (Production of the separator)

[0096] Add about 8.5% by weight of a polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP) binder to acetone and 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 (weight% ratio), and use a ball mill method to prepare the slurry for 12 hours or longer. Coat the slurry thus obtained onto 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.

[0097] (Production of the lithium secondary battery)

[0098] 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 (Press) 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) in a volume ratio of 30:70, and then inject an electrolyte in which 1.0 M LiPF6 is dissolved to produce a lithium secondary battery.

[0099] Example 2: Production of a positive electrode and a lithium secondary battery

[0100] Dissolve 20 g of the conductive polymer obtained in the Preparation Example (Mw = 34,000 g / mol) and 6 g (30 parts by weight based on 100 parts by weight of the conductive polymer) of porous conductive carbon particles (pore size: 10 nm to 300 nm, D50: 1 μm to 3 μm) in 1,980 g of chloroform solvent to obtain a composition. Gravure coat the composition on an aluminum (Al) thin film serving as a positive electrode current collector and dry it to form a safety functional layer with a thickness of about 10 μm.

[0101] After forming the safety functional layer, perform the same procedures as in Example 1 to produce the positive electrode and lithium secondary battery of Example 2.

[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 porous conductive carbon particles are not used.

[0104] Comparative Example 2

[0105] Produce the positive electrode and lithium secondary battery of Comparative Example 2 in the same manner as in Example 1, except that 6 g (30 parts by weight based on 100 parts by weight of the conductive polymer) of carbon black conductive material is used instead of porous conductive carbon particles.

[0106] Comparative Example 3

[0107] Produce the positive electrode and lithium secondary battery of Comparative Example 3 in the same manner as in Example 1, except that a safety functional layer is not formed on the aluminum (Al) thin film serving as a positive electrode current collector.

[0108] Test example

[0109] 1. Evaluation of high - rate discharge characteristics

[0110] The lithium secondary batteries produced in the examples and comparative examples were charged under constant current (0.7C) and constant voltage (4.47V, cutoff at 0.025C), then rested for 10 minutes, and discharged under constant current conditions (0.1C, 0.2C, 0.5C, 1.0C, 1.5C) until the voltage reached 3V. That is, as the number of charge / discharge cycles increased, the discharge rate was periodically changed to 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. These high-rate discharge characteristics were evaluated at a temperature of 45°C.

[0111] At this time, the high-rate discharge characteristics at 1.5C are shown in Table 1 below.

[0112] 2. Impact test

[0113] For each test, 10 lithium secondary batteries produced in the examples and comparative examples were prepared and placed on a flat plate. After placing an iron rod with a diameter of 15.8 mm, a 9.1 kg weight was freely dropped from a height of 61 cm above the iron rod onto the lithium secondary battery.

[0114] After the free fall, it was measured whether any of the 10 batteries would catch fire, and the number of non-ignited batteries was shown as the test result in Table 1 below.

[0115] [Table 1]

[0116]

[0117] Referring to Table 1, it has been confirmed that due to the formation of the safety functional layer, Examples 1 and 2 are superior to Comparative Example 3 and exhibit a safety level equivalent to or higher than that of Comparative Examples 1 and 2. In particular, it has been confirmed that Example 2 shows higher safety compared to Comparative Examples 1 and 2.

[0118] In addition, it has been confirmed that Examples 1 and 2 include porous conductive carbon particles in the safety functional layer, and thus are equivalent to Comparative Example 3 in which no safety functional layer is formed, and exhibit more excellent rate characteristics (high-rate discharge characteristics) than Comparative Examples 1 and 2.

Claims

1. A composition for forming an electrode protective layer for a lithium secondary battery, comprising: A polythiophene-based conductive polymer exhibiting PTC (positive temperature coefficient) characteristics; and Porous conductive carbon particles having a plurality of pores with diameters ranging from 10 nm to 300 nm formed therein.

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

3. The composition for forming an electrode protective layer for a lithium secondary battery according to claim 1, wherein the polythiophene-based conductive polymer comprises a homopolymer or copolymer containing repeating units represented by Chemical Formula 1: [Chemical Formula 1] Among them, In Chemical Formula 1, R1 is a functional group represented by 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.

4. The composition for forming an electrode protective layer for a lithium secondary battery according to claim 1, wherein the polythiophene-based conductive polymer has a weight-average molecular weight of 5000 g / mol to 100000 g / mol.

5. The composition for forming an electrode protective layer for a lithium secondary battery according to claim 1, wherein the number-average particle diameter (D50) of the porous conductive carbon particles is 0.5 μm to 20 μm.

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

7. The composition for forming an electrode protective layer 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 porous conductive carbon particles is 0.1 part by weight to 80 parts by weight.

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

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

10. An electrode for a lithium secondary battery, comprising: A metal current collector; A safety functional layer formed to cover at least a part of the metal current collector and formed of the composition according to any one of claims 1 to 9; and An active material layer comprising an electrode active material and a conductive material and formed on the metal current collector and the safety functional layer.

11. The electrode for a lithium secondary battery according to claim 10, 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.

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

13. A lithium secondary battery, comprising: A positive electrode, a negative electrode, and a separator interposed therebetween, wherein the electrode according to claim 10 is included as the positive electrode.

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