Electrode assembly, method of manufacturing the same, and lithium secondary battery including the same

Abstract

[Disclosure] [Technical Problem] An aspect of the present disclosure is to provide an electrode assembly and a method of manufacturing the same capable of improving safety of a secondary battery by forming a good porous coating layer on a positive electrode active material layer while preventing damage to the positive electrode active material layer during a process of forming the porous coating layer.SOLUTION: The method includes (S1) applying a positive electrode slurry on a positive electrode current collector and drying the positive electrode slurry to form a positive electrode active material layer, (S2) primarily rolling the positive electrode current collector and the positive electrode active material layer, (S3) applying a coating layer slurry on the positive electrode active material layer and drying the coating layer slurry to form a positive electrode laminate including a coating layer, (S4) secondarily rolling the positive electrode laminate, and (S5) stacking a negative electrode on the coating layer to manufacture an electrode assembly. To provide a method for manufacturing an electrode assembly.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Cross-citation of related applications (etc.) This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0067846, filed June 2, 2022, and Korean Patent Application No. 10-2023-0069510, filed May 30, 2023, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to an electrode assembly, a method for manufacturing the same, and a lithium secondary battery including the same. [Background technology]

[0003] Recently, the application areas of lithium secondary batteries have rapidly expanded from power supply for electronic devices such as electrical, electronic, communication, and computer equipment to power storage supply for large-area devices such as automobiles and power storage devices, and as a result, there has been an increasing demand for lithium secondary batteries with high capacity, high output, long life, and high stability.

[0004] Lithium secondary batteries generally include a positive electrode, a negative electrode, a separator, and an electrolyte, and are known in the art to include a positive electrode that generates oxygen due to an unstable structure in a charged state. Since the generation of oxygen poses a high risk of fire, research and development efforts are being conducted to improve the safety of lithium secondary batteries.

[0005] For example, a typical separator included in a conventional lithium secondary battery, such as a polyolefin-based porous separator, can shrink at high temperatures, which can cause a short circuit between the positive and negative electrodes. When such a short circuit occurs, the separator reacts with oxygen generated by the unstable positive electrode, further increasing the risk of fire.

[0006] In order to reduce the risk of fire and improve the safety of lithium secondary batteries, a lithium secondary battery including a porous coating layer interposed between the positive electrode and the negative electrode to replace or supplement the separator can be considered. Such a porous coating layer can electrically insulate the positive electrode and the negative electrode from each other while allowing ions to move between them.

[0007] However, during the process of forming a coating layer on the positive or negative electrode, the medium of the composition for forming the coating layer may penetrate into the positive electrode, causing problems such as swelling of the electrode active material layer. Furthermore, the additional formation of the coating layer may make it difficult to achieve an appropriate porosity in the positive electrode active material layer, or the active material layer may be broken during the process of forming the coating layer.

[0008] In consideration of these problems, there is a continuing need for the development of a technology that can effectively form a porous coating layer, which replaces or supplements a separator, on an electrode to improve the safety of secondary batteries while resolving the problems associated with the coating layer formation process. Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, the present invention provides an electrode assembly and a manufacturing method thereof that can form a good porous coating layer on a positive electrode active material layer to improve the safety of the secondary battery while suppressing problems such as damage to the positive electrode active material layer during the porous coating layer formation process.

[0010] Another object of the present invention is to provide a lithium secondary battery including the electrode assembly and having improved safety. [Means for solving the problem]

[0011] One embodiment of the present invention provides a method for manufacturing an electrode assembly, the method comprising: (S1) applying a positive electrode slurry onto a positive electrode current collector and drying the slurry to form a positive electrode active material layer; (S2) performing a primary rolling of the positive electrode current collector and the positive electrode active material layer; (S3) applying a coating layer slurry onto the positive electrode active material layer and drying the slurry to form a positive electrode laminate including a coating layer; (S4) performing a secondary rolling of the positive electrode laminate; and (S5) laminating a negative electrode on the coating layer to manufacture an electrode assembly, wherein the primary rolling is performed so that the positive electrode active material layer has a porosity of 35% to 45%, and the secondary rolling is performed so that the positive electrode active material layer has a porosity of 20% to 30%.

[0012] Another embodiment of the present invention provides an electrode assembly including a positive electrode current collector, a positive electrode active material layer disposed on the positive electrode current collector, a coating layer coated on the positive electrode active material layer, a mixed layer in which the positive electrode active material layer and the coating layer are mixed, and a negative electrode disposed on the coating layer and including a negative electrode current collector and a negative electrode active material layer, wherein the coating layer and the mixed layer include a polymer binder and polymer particles or ceramic particles dispersed in the polymer binder, the polymer particles having an absolute value of a zeta potential of 25 mV or more, and the mixed layer has a thickness of less than 15 μm.

[0013] Another embodiment of the present invention provides a lithium secondary battery including a battery case, a flame-retardant solvent having a flash point of 100°C or higher or no flash point, a flame-retardant electrolyte containing a lithium salt, and the electrode assembly. [Effects of the Invention]

[0014] According to the present invention, by optimizing the process of forming a porous coating layer on a positive electrode active material layer, it is possible to successfully form a porous coating layer on a positive electrode active material layer that replaces or supplements an existing separator while suppressing damage to the positive electrode during the formation of the coating layer, and it is possible to achieve appropriate porosity and mechanical properties of the positive electrode active material layer and the porous coating layer.

[0015] As a result, this can greatly contribute to the development of lithium secondary batteries that exhibit improved safety while also having high capacity characteristics and energy density. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a photograph of the cross section of the positive electrode active material layer and the coating layer of Example 1 observed through a scanning electron microscope (SEM). [Figure 2] FIG. 2 is a photograph of the cross section of the positive electrode active material layer and coating layer of Comparative Example 4 observed through a scanning electron microscope (SEM). DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, the terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of the term in order to best explain his or her invention.

[0018] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used in a manner commonly understood by those skilled in the art to which the present invention pertains. Furthermore, terms defined in commonly used dictionaries should not be interpreted ideally or excessively unless explicitly defined otherwise.

[0019] The terms used in this specification are for the purpose of describing the embodiments and are not intended to limit the present invention. In this specification, the singular form includes the plural form unless otherwise specified in the context. As used in this specification, the words "comprises" and / or "comprising" do not exclude the presence or addition of one or more other elements other than the elements mentioned.

[0020] In this specification, when a part is said to include certain elements, this does not mean that it excludes other elements, unless otherwise specified, and it means that it may further include other elements.

[0021] In this specification, the phrase "A and / or B" means A or B or A and B.

[0022] Manufacturing method of electrode assembly A method for manufacturing an electrode assembly according to an embodiment of the present invention includes: (S1) applying a positive electrode slurry onto a positive electrode current collector and drying it to form a positive electrode active material layer; (S2) performing a primary rolling of the positive electrode current collector and the positive electrode active material layer; (S3) applying a coating layer slurry onto the positive electrode active material layer and drying it to form a positive electrode laminate including a coating layer; (S4) performing a secondary rolling of the positive electrode laminate; and (S5) laminating a negative electrode on the coating layer to manufacture an electrode assembly, wherein the primary rolling may be performed so that the positive electrode active material layer has a porosity of 35% to 45%, and the secondary rolling may be performed so that the positive electrode active material layer has a porosity of 20% to 30%.

[0023] In the method for manufacturing such an electrode assembly, the coating layer slurry may include, for example, a polymer binder and polymer particles or ceramic particles dispersed on the polymer binder, the polymer particles having an absolute value of a zeta potential of 25 mV or more, thereby electrically insulating the positive electrode and the negative electrode while allowing lithium ions to move during charge and discharge.

[0024] In one embodiment, when forming a porous coating layer that replaces or supplements an existing separator, the cathode active material layer and the porous coating layer are formed through multiple rolling steps. It has been confirmed that this process allows a very thin mixed layer, in which a portion of the porous coating layer penetrates and mixes with the cathode active material layer, to be formed.

[0025] The formation of such a thin porous coating layer can prevent short circuits or fires between electrodes due to thermal shrinkage of the separator, and the thin thickness of the porous coating layer can further increase the energy density of the unit secondary battery.

[0026] Furthermore, by performing the multiple rolling steps, a porosity suitable for the final positive electrode active material layer, for example, a porosity of 20% to 30%, can be achieved, while preventing damage to the positive electrode active material layer due to higher pressure in the rolling step for forming the coating layer. In addition, the multiple rolling steps can also reduce the problem of swelling of the positive electrode active material layer due to the liquid medium of the coating layer slurry.

[0027] Meanwhile, each component of the electrode assembly will be specifically described in the following description of the electrode assembly, and each step of the manufacturing method will be described below.

[0028] First, a positive electrode slurry is applied onto a positive electrode current collector and dried to form a positive electrode active material layer (S1).

[0029] The positive electrode slurry may be prepared by dissolving or dispersing a positive electrode active material, a binder, a conductive material, and an optional dispersant in a solvent.

[0030] The positive electrode slurry can be coated on a positive electrode current collector and dried to form a positive electrode active material layer. For example, the positive electrode slurry can be coated on a positive electrode current collector, and then the positive electrode current collector and the positive electrode slurry can be dried by passing the coated positive electrode current collector through a drying zone equipped with an exhaust fan at a constant speed in a continuous process. The drying zone can be divided into six zones, each about 2 m long, and the temperature in each drying zone can be about 50 to 95°C.

[0031] Thereafter, the positive electrode current collector and the positive electrode active material layer are subjected to a primary rolling (S2).

[0032] Such a first rolling step and a second rolling step described later can be performed using, for example, pressure rolls, and the thickness and / or porosity of the positive electrode current collector and the positive electrode active material layer can be controlled by appropriately setting the gap between the pressure rolls.

[0033] The thickness and / or porosity of the positive electrode current collector and the positive electrode active material layer can be changed within an appropriate range by the first rolling process.

[0034] After the primary rolling, the total thickness of the primarily rolled positive electrode current collector and the positive electrode active material layer may be 80% to 90%, or 83% to 87% of the total thickness of the non-rolled positive electrode current collector and the positive electrode active material layer before the primary rolling.

[0035] If the thickness ratio after the first rolling is less than 80% of the initial thickness, the positive electrode current collector and the positive electrode active material layer may be excessively rolled, which may cause damage such as wavy or other damage to the positive electrode active material layer on the positive electrode current collector. Conversely, if the thickness ratio after the first rolling exceeds 90% of the initial thickness, the positive electrode current collector and the positive electrode active material layer may not be substantially rolled, which may cause the liquid medium of the coating layer slurry to permeate the positive electrode active material layer in a subsequent step, resulting in swelling.

[0036] In a specific example, in an initial state before the primary rolling, the total thickness of the positive electrode current collector and the positive electrode active material layer may be 150 to 200 μm, or 160 to 180 μm, and the total thickness of the positive electrode current collector and the positive electrode active material layer after the primary rolling may be 120 to 180 μm, or 130 to 170 μm.

[0037] The primary rolling step may be performed by passing the positive electrode current collector and the positive electrode active material layer between the rolling rolls so that the porosity of the positive electrode active material layer is 35% to 45%, or 37% to 43%.

[0038] Here, the porosity (P, unit: %) of the positive electrode active material layer can be calculated by a method commonly used in the art, specifically, by the following Equation 1:

[0039] [Formula 1] P = (1-D) / T x 100 In Equation 1, D is the density of the positive electrode active material layer, and T is the true density of the positive electrode active material excluding the current collector. The true density refers to the inherent density of the positive electrode active material without voids.

[0040] The density D of the positive electrode active material layer can be calculated by the following formula 2.

[0041] [Formula 2] D=M / (S×H) In the above formula 2, M is the mass of the positive electrode active material layer, S is the area of ​​the positive electrode active material layer, and H is the thickness of the positive electrode active material layer.

[0042] If the porosity of the positive electrode active material layer after the first rolling is less than 35%, the positive electrode current collector and the positive electrode active material layer may be excessively rolled, which may cause damage such as rippling of the positive electrode active material layer on the positive electrode current collector. Conversely, if the porosity of the positive electrode active material layer after the first rolling exceeds 45%, the positive electrode current collector and the positive electrode active material layer may not be substantially rolled, which may cause the liquid medium of the coating layer slurry to permeate into the positive electrode active material layer in subsequent steps, resulting in swelling. Furthermore, the permeation of the liquid medium contained in the coating layer slurry may increase the porosity of the positive electrode active material layer, which may result in the formation of non-uniform pores.

[0043] Thereafter, a coating layer slurry is applied onto the positive electrode active material layer and dried to form a positive electrode laminate including a coating layer (S3).

[0044] The coating layer slurry may be, for example, a slurry composition including a polymer binder, polymer particles or ceramic particles having an absolute value of a zeta potential of 25 mV or more, and a liquid medium, and may be used to form a porous coating layer (including a mixed layer) on the positive electrode active material layer.

[0045] The zeta potential of the polymer particles or ceramic particles reflects the surface polarity of the particles and defines the electrostatic repulsion or dispersibility between particles. Polymer particles or ceramic particles with a high absolute value of the zeta potential can be uniformly dispersed on the positive electrode active material layer, providing good and uniform coating properties, and can define a large number of fine and uniform pores between the particles that allow lithium ions to pass through. Furthermore, particles with such a high zeta potential can provide the porous coating layer with excellent impregnation properties for a flame-retardant electrolyte containing a flame-retardant solvent. The combination of the porous coating layer and the flame-retardant electrolyte can provide lithium secondary batteries with superior safety.

[0046] The zeta potential of the polymer particles or ceramic particles can be measured, for example, by electrophoretic light scattering using a dynamic light scattering device. The zeta potential can be measured by dispersing the polymer particles or ceramic particles in water or an alcohol-based solvent without the use of a separate dispersant. In one specific example, the zeta potential can be measured by dispersing the polymer particles or ceramic particles in water at a concentration of 0.1 wt % or less.

[0047] The absolute value of the zeta potential of the polymer particles or ceramic particles may be 25 mV or more, or 35 mV or more, or 45 mV or more, or 100 mV or less, or 90 mV or less, or 80 mV or less. Within this range, good coating properties of the coating layer slurry can be achieved, and high impregnation of the flame-retardant electrolyte can be ensured.

[0048] Specific examples of the polymer particles include one or more selected from the group consisting of polymethyl (meth)acrylate, polystyrene, polyvinyl chloride, polycarbonate, polysulfone, polyethersulfone, polyetherimide, polyphenylsulfone, polyamideimide, polyimide, polybenzimidazole, polyetherketone, polyphthalamide, polybutylene terephthalate, polyethylene terephthalate, and polyphenylene sulfide.

[0049] Specific examples of the ceramic particles include one or more selected from the group consisting of boehmite, aluminum oxide, titanium oxide, iron oxide, silicon oxide, zirconium oxide, cobalt oxide, tin oxide, nickel oxide, zinc oxide, vanadium oxide, and manganese oxide.

[0050] The zeta potential of the polymer particles or ceramic particles can be controlled not only by the type of particle but also by the particle size or surface properties of the particles. Thus, to achieve the zeta potential, dispersibility, or appropriate porosity of the porous coating layer of the polymer particles or ceramic particles, the polymer particles or ceramic particles may have a particle size of 50 nm to 3 μm, 50 nm to 1.5 μm, or 100 nm to 1 μm.

[0051] In addition, in order to control the surface properties of the polymer particles or ceramic particles and thereby adjust the zeta potential, the polymer particles or ceramic particles may be included in the coating layer slurry after being surface-treated with oxygen plasma or ion beam.

[0052] Meanwhile, in the coating layer slurry, the polymer binder may be the same polymer as the binder contained in the positive electrode active material layer, and specific examples thereof include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene (PE), polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, nitrile rubber, styrene-butadiene rubber, and fluororubber, and mixtures or copolymers of two or more selected from these may also be used. However, the specific composition of the polymer binder can be determined by those skilled in the art taking into account the type and characteristics of the polymer particles or ceramic particles, the method of forming the coating layer, and other factors.

[0053] In addition, in consideration of the good coating properties of the coating layer slurry and the good dispersibility of the particles, the coating layer slurry may contain the polymer binder:polymer particles or ceramic particles in a weight ratio of 5:95 to 40:60, or 10:90 to 35:65.

[0054] The coating layer slurry may further include a dispersant, and such a dispersant may include, for example, at least one of hydrogenated nitrile rubber (H-NBR) and tannic acid.

[0055] The coating layer slurry further includes a liquid medium for dispersing the aforementioned components, which may include an organic solvent such as N-methyl-2-pyrrolidone (NMP) or acetone, or water, and may be appropriately selected in consideration of the types of polymer particles, ceramic particles, or polymer binder.

[0056] The coating layer slurry may be applied to the first-rolled positive electrode active material layer and dried to form a porous coating layer on the positive electrode active material layer, and the resulting product may be defined as a positive electrode laminate.

[0057] After the porous coating layer is formed, the positive electrode laminate is subjected to a second rolling (S4). This second rolling process can be performed by appropriately setting the gap between the rolls, as in the first rolling process, and the thickness and / or porosity of the positive electrode active material layer and the porous coating layer can be appropriately achieved through the second rolling process.

[0058] After the second rolling, the total thickness of the second-rolled positive electrode current collector and the positive electrode active material layer may be 60% to 75%, or 63% to 72% of the total thickness of the unrolled positive electrode current collector and the positive electrode active material layer before the first rolling. In this case, the thickness of the second-rolled positive electrode active material layer may be calculated taking into account the thickness of the mixed layer containing the porous coating layer, and the thickness of each layer may be confirmed by, for example, electron microscope analysis.

[0059] If the thickness ratio after the second rolling is less than 60% of the initial thickness, the positive electrode laminate is excessively rolled, and the thickness of the porous coating layer becomes too thin, which may easily cause a short circuit between the positive electrode and the negative electrode or damage to the positive electrode active material layer. Conversely, if the thickness ratio after the second rolling exceeds 75% of the initial thickness, the positive electrode active material layer may not have an appropriate porosity, which may result in a deterioration in battery performance.

[0060] The second rolling step may be performed by passing the positive electrode laminate between the rolling rolls so that the porosity of the positive electrode active material layer is 20% to 30%, or 22% to 28%.

[0061] If the porosity of the positive electrode active material layer after the second rolling is less than 20%, the positive electrode laminate may be rolled excessively, and the thickness of the porous coating layer may become too thin, which may easily cause a short circuit between the positive electrode and the negative electrode or damage to the positive electrode active material layer. Conversely, if the porosity of the positive electrode active material layer after the second rolling exceeds 30%, the positive electrode active material layer may not have an appropriate porosity, which may reduce the capacity characteristics of the battery.

[0062] Then, a negative electrode is stacked on the porous coating layer to manufacture an electrode assembly (S5).

[0063] The negative electrode may include a negative electrode current collector and a negative electrode active material layer, as described below. In addition, in the electrode assembly manufacturing step, the porous coating layer may be formed on the upper surface of the second rolled positive electrode laminate, and the negative electrode may be stacked on the porous coating layer to manufacture the electrode assembly.

[0064] Hereinafter, an example of an electrode assembly that can be manufactured by the method of the embodiment will be described in detail.

[0065] electrode assembly According to another embodiment of the present invention, an electrode assembly includes a positive electrode current collector, a positive electrode active material layer disposed on the positive electrode current collector, a coating layer coated on the positive electrode active material layer, a mixed layer in which the positive electrode active material layer and the coating layer are mixed, and a negative electrode disposed on the coating layer and including a negative electrode current collector and a negative electrode active material layer, wherein the coating layer and the mixed layer include a polymer binder and polymer particles or ceramic particles dispersed in the polymer binder, the polymer particles having an absolute value of a zeta potential of 25 mV or more, and the mixed layer may have a thickness of less than 15 μm.

[0066] The coating layer may be a porous coating layer having a plurality of pores with a diameter of, for example, 10 nm or more and a porosity comparable to that of the positive electrode active material layer. Such a porous coating layer electrically insulates the positive electrode and the negative electrode while allowing lithium ions to move during charge and discharge, thereby replacing or supplementing an existing separator.

[0067] In the electrode assembly of the other embodiment, the formation of such a porous coating layer can prevent short circuits or fires between electrodes due to thermal shrinkage of the existing separator, thereby providing a lithium secondary battery with improved safety. Furthermore, by minimizing the formation of the mixed layer, the porous coating layer can have a thinner thickness, thereby further increasing the energy density of the unit secondary battery.

[0068] In the electrode assembly of this other embodiment, the thickness of the mixed layer may be 0 μm or more and less than 15 μm, 0.1 μm or more and less than 10 μm, or 1 μm or more and less than 5 μm, specifically, 5 μm to 14 μm, or 7 μm to 12 μm. The thickness of the mixed layer can be adjusted by controlling the degree of the first rolling described above.

[0069] Specifically, the minimum thickness of the mixed layer can be achieved by performing the primary rolling so that the total thickness of the primary-rolled cathode current collector and the cathode active material layer is 80% to 90% of the total thickness of the unrolled cathode current collector and the cathode active material layer. Also, the minimum thickness of the mixed layer can be achieved by performing the primary rolling so that the porosity of the cathode active material layer is 35% to 45%.

[0070] When the thickness of the mixed layer satisfies the above range, the thickness of the porous coating layer that replaces or supports the separator can be reduced, improving the coating properties of the positive electrode active material layer. Furthermore, by controlling the thickness of the porous coating layer, the porosity and electrical properties of the positive electrode active material layer can be adjusted to desired ranges.

[0071] Meanwhile, the compositions of the mixed layer and the porous coating layer are the same as those described in the manufacturing method of the embodiment, so further description thereof will be omitted.

[0072] In addition, the positive electrode current collector in the electrode assembly is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, the current collector may be stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like. Such a positive electrode current collector may have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the positive electrode current collector to enhance adhesion to the positive electrode active material layer. For example, the positive electrode current collector may be in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0073] The positive electrode active material layer on the positive electrode current collector may contain a positive electrode active material, and may further contain a conductive material, a binder, and the like, as necessary.

[0074] The positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium, and specifically, may include a lithium metal oxide containing one or more metals such as cobalt, manganese, nickel, or aluminum and lithium. More specifically, the lithium metal oxide is a lithium-manganese-based oxide (e.g., LiMnO2, LiMn2O4, etc.), a lithium-cobalt-based oxide (e.g., LiCoO2, etc.), a lithium-nickel-based oxide (e.g., LiNiO2, etc.), a lithium-nickel-manganese-based oxide (e.g., LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-Z Ni Z O4 (where 0 < Z < 2), etc.), a lithium-nickel-cobalt-based oxide (e.g., LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), a lithium-manganese-cobalt-based oxide (e.g., LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-Z1 Co Z1 O4 (where 0 < Z1 < 2), etc.), a lithium-nickel-manganese-cobalt-based oxide (e.g., Li(Ni p Co q Mn r )O2 (where 0 < p < 1, 0 < q < 1, 0 < r < 1, p + q + r = 1) or Li(Ni p1 Co q1 Mn r1 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r1 < 2, p1 + q1 + r1 = 2), etc.) or a lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r2 M s2)O2 (where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r2, and s2 are, respectively, the atomic fractions of independent elements, 0 < p2 < 1, 0 < q2 < 1, 0 < r2 < 1, 0 < s2 < 1, and p2 + q2 + r2 + s2 = 1), etc.), and any one or two or more of these compounds may be included.

[0075] Among these, in terms of being able to enhance the capacity characteristics and safety of the battery, the lithium metal oxide is LiCoO2, LiMnO2, LiNiO2, lithium nickel manganese cobalt oxide (for example, Li(Ni 1 / 3 Mn 1 / 3 Co 1 / 3 )O2, Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2 and Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc.), lithium nickel cobalt aluminum oxide (for example, Li(Ni 0.8 Co 0.15 Al 0.05 )O2, etc.) or lithium nickel manganese cobalt aluminum oxide (for example, Li(Ni 0.86 Co 0.05 Mn 0.07 Al 0.02 )O2), etc., and any one or two or more of these mixtures may be used.

[0076] The positive electrode active material may be contained at 60 to 99% by weight, or 70 to 99% by weight, or 80 to 98% by weight based on the total weight of the positive electrode active material layer.

[0077] Furthermore, the conductive material is a component for further improving the conductivity of the positive electrode active material, and such a conductive material is not particularly limited as long as it does not induce chemical changes in the battery and has conductivity. For example, carbon powders such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powders such as natural graphite, artificial graphite, or graphite with a highly developed crystal structure; conductive fibers such as carbon fibers and metal fibers; carbon fluoride powder; conductive powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives may be used.

[0078] Typically, the conductive material may be included in an amount of 1 to 20 wt %, or 1 to 15 wt %, or 1 to 10 wt %, based on the total weight of the positive electrode active material layer.

[0079] On the other hand, the binder is a component that assists in binding the positive electrode active material to the conductive material and the like and in binding the positive electrode current collector.

[0080] Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene (PE), polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers.

[0081] Typically, the binder may be included in an amount of 1 to 20 wt %, 1 to 15 wt %, or 1 to 10 wt %, based on the total weight of the positive electrode active material layer.

[0082] Meanwhile, the negative electrode may include a negative electrode current collector and a negative electrode active material layer.

[0083] The negative electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, baked carbon, copper or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like, an aluminum-cadmium alloy, or the like may be used.

[0084] The negative electrode current collector may typically have a thickness of 3 μm to 500 μm, and like the positive electrode current collector, the negative electrode current collector may have a surface with fine irregularities to strengthen the binding force of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0085] The negative electrode active material layer may contain a negative electrode active material, and may further contain a conductive material, a binder, and the like, as necessary.

[0086] The negative electrode active material may include at least one selected from the group consisting of lithium metal, a carbon material capable of reversibly intercalating / deintercalating lithium ions, a metal or an alloy of such a metal and lithium, a metal composite oxide, a material capable of doping and dedoping lithium, and a transition metal oxide.

[0087] The carbonaceous material capable of reversibly intercalating / deintercalating lithium ions may be any carbonaceous negative electrode active material commonly used in lithium ion secondary batteries, and representative examples thereof include crystalline carbon, amorphous carbon, or a combination of these. Examples of crystalline carbon include graphite, such as amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite. Examples of amorphous carbon include soft carbon (low-temperature calcined carbon), hard carbon, mesophase peach carbide, and calcined coke.

[0088] As the metal or an alloy of these metals and lithium, a metal selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn or an alloy of these metals and lithium may be used.

[0089] Examples of the metal composite oxide include PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, Bi2O5, Li x Fe2O3(0 < x ≦ 1), Li x WO2(0 < x ≦ 1) and Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen; 0 < x < 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8) may be selected from the group consisting of and used.

[0090] Examples of the substance capable of doping and undoping lithium include Si, SiO x (0 < x ≦ 2), Si - Y alloy (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof and is not Si), Sn, SnO2, Sn - Y (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof and is not Sn), etc. Also, at least one of these and SiO2 may be mixed and used. Examples of the element Y may be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

[0091] Examples of the transition metal oxide include lithium-containing titanium composite oxide (LTO), vanadium oxide, and lithium vanadium oxide.

[0092] The negative electrode active material may be included in an amount of 60 to 99 wt %, 70 to 99 wt %, or 80 to 98 wt % based on the total weight of the negative electrode active material layer.

[0093] Meanwhile, the types and contents of the binder and conductive material that may be included in the negative electrode active material layer are substantially the same as those described above for the positive electrode active material layer, and therefore, further description thereof will be omitted.

[0094] Lithium secondary battery According to a further embodiment of the present invention, there is provided a lithium secondary battery including a battery case, a flame-retardant solvent having a flash point of 100°C or higher or no flash point, a flame-retardant electrolyte containing a lithium salt, and the electrode assembly described above.

[0095] The lithium secondary battery includes an electrode assembly including the porous coating layer and a flame-retardant electrolyte containing a flame-retardant solvent. As described above, the porous coating layer includes polymer particles or ceramic particles having specific properties and exhibits excellent impregnation with the flame-retardant solvent and electrolyte. Therefore, the combination of the porous coating layer and the flame-retardant electrolyte more effectively prevents fires due to short circuits between electrodes, further improving the safety of the lithium secondary battery and achieving excellent electrical characteristics.

[0096] Such a flame-retardant solvent is a solvent with low volatility and flammability and can be defined by a predetermined flash point. For example, the flame-retardant solvent can include organic solvents that are substantially non-flammable and have no flash point, as well as organic solvents with a high flash point of 100°C or higher, or 100 to 250°C, or 110 to 200°C, and low volatility. The lithium secondary battery containing such a flame-retardant solvent and a flame-retardant electrolyte containing a lithium salt can exhibit excellent safety and stability. Furthermore, since the flame-retardant electrolyte can be uniformly impregnated into the porous coating layer, the lithium secondary battery can exhibit excellent electrochemical properties. The flash point defining the flame-retardant solvent can be measured using a closed or open-type method according to the standard methods of ASTM D93 or ASTM D1310.

[0097] In a specific example, the flame-retardant solvent may be an organic solvent having low volatility and a functional group that can contribute to flame retardancy or non-flammability, for example, a functional group selected from the group consisting of a sulfone-based functional group, a fluorine-containing functional group such as a fluorine-substituted hydrocarbon group, a phosphorus-containing functional group such as a phosphate group or a phosphonate group, and a nitrile-based functional group, or one or more of these organic solvents may be mixed and used. More specifically, the flame-retardant solvent may include one or more organic solvents selected from the group consisting of sulfone-based compounds, nitrile-based compounds, phosphoric acid-based compounds, and fluorine-substituted carbonate-based compounds.

[0098] Among these, the sulfone-based compound may be a cyclic sulfone-based compound or a linear sulfone-based compound, and specifically may include at least one selected from the group consisting of sulfolane, ethyl methyl sulfone, dibutyl sulfone, ethyl vinyl sulfone, methyl propyl sulfone, ethyl-i-propyl sulfone, ethyl-i-butyl sulfone, i-propyl-i-butyl sulfone, i-propyl-s-butyl sulfone, and butyl-i-butyl sulfone.

[0099] The nitrile compound may include at least one selected from the group consisting of malononitrile, succinonitrile, glutaronitrile, adiponitrile, suberonitrile, and sebaconitrile.

[0100] The phosphoric acid compound may include at least one selected from the group consisting of dimethyl methyl phosphate, trimethyl phosphate, triethyl phosphate, tributyl phosphate, diethyl ethyl phosphate, dimethyl methyl phosphate, dimethyl (2-methoxyethoxy) methyl phosphonate, diethyl (2-methoxyethoxy) methyl phosphonate, and triphenyl phosphate.

[0101] In addition, the fluorine-substituted carbonate compounds include bis(2,2,3,3-tetrafluoropropyl)carbonate, methyl-2,2,2-trifluoroethyl carbonate, ethyl-2,2,2-trifluoroethyl carbonate, propyl-2,2,2-trifluoroethyl carbonate, methyl-2,2,2,2',2',2'-hexafluoro-i-propyl carbonate, ethyl-2,2,2,2',2',2'-hexafluoro-i-propyl carbonate, The fluorocarbon or fluoroisopropyl carbonate may include one or more selected from the group consisting of di-2,2,2-trifluoroethyl carbonate, 2,2,2-trifluoroethyl-N,N-dimethyl carbonate, hexafluoro-i-propyl-N,N-dimethyl carbonate, 4-(2,2,3,3-tetrafluoropropoxymethyl)-[1,3]-dioxolan-2-one, and bis(2,2,3,3-pentafluoropropyl)carbonate.

[0102] On the other hand, the lithium salt contained in the flame-retardant electrolyte is used as a medium for transferring ions within the lithium secondary battery.

[0103] Lithium salts, for example, contain Li as the cation. + Includes F - , Cl - , Br - , I - , NO3- , N(CN)2 - , BF4 - , ClO4 - , B 10 Cl 10 - , AlCl4 - , AlO2 - , PF6 - , CF3SO3 - , CH3CO2 - , CF3CO2 - , AsF6 - , SbF6 - , CH3SO3 - , (CF3CF2SO2)2N - , (CF3SO2)2N - , (FSO2)2N - , BF2C2O4 - , BC4O8 - , PF4C2O4 - , PF2C4O8 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , C4F9SO3 - , CF3CF2SO3 - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , CF3(CF2)7SO3 - and SCN - The anion may also contain an anion selected from the group consisting of:

[0104] Specifically, the lithium salts include LiCl, LiBr, LiI, LiBF4, LiClO4, LiB 10 Cl 10, LiAlCl4, LiAlO2, LiPF6, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6, LiCH3SO3, LiFSI (lithium bis(fluorosulfonyl)imide, LiN(SO2F)2), LiBETI (lithium bis(perfluoroethanesulfonyl)imide, LiN(SO2CF2CF3)2), and LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, LiN(SO2CF3)2), but those containing LiN(SO2CF3)2 are preferred in view of their excellent stability. In addition to these, lithium salts commonly used in electrolytes for lithium secondary batteries can be used without any particular restriction.

[0105] The concentration of the lithium salt can be appropriately varied within a range that is normally usable, but to obtain an optimal effect of forming a corrosion-preventing coating on the electrode surface, the lithium salt may be contained in the flame-retardant electrolyte at a concentration of 0.5 M to 6 M, or 1 M to 3 M, or 1 M to 2.5 M. When the concentration of the lithium salt satisfies the above range, the effect of improving the cycle characteristics during high-temperature storage of the lithium secondary battery is sufficient, and the viscosity of the flame-retardant electrolyte is appropriate, thereby improving the impregnation of the flame-retardant electrolyte.

[0106] In addition, the flame-retardant electrolyte may further contain an electrolyte additive as needed to prevent the electrolyte from being decomposed in a high-power environment, thereby causing anode collapse, or to improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, or suppression of battery expansion at high temperatures.

[0107] Representative examples of such electrolyte additives include one or more compounds selected from the group consisting of cyclic carbonate compounds, halogen-substituted carbonate compounds, sultone compounds, sulfate compounds, phosphate compounds, borate compounds, nitrile compounds, benzene compounds, amine compounds, silane compounds, and lithium salt compounds.

[0108] Among these, the cyclic carbonate compounds include vinylene carbonate (VC) and vinylethylene carbonate. The halogen-substituted carbonate compounds include fluoroethylene carbonate (FEC). The sultone compounds include compounds selected from the group consisting of 1,3-propane sultone (PS), 1,4-butane sultone, ethene sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene sultone. Examples of the sulfate-based compound include ethylene sulfate (Esa), trimethylene sulfate (TMS), and methyltrimethylene sulfate (MTMS). Examples of the phosphate-based compound include one or more selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tetramethyltrimethylsilylphosphate, and tris(2,2,2-trifluoroethyl)phosphate.

[0109] Examples of the borate-based compound include tetraphenylborate, lithium oxalyl difluoroborate (LiODFB), and lithium bis(oxalato)borate (LiB(C2O4)2, LiBOB). Examples of the nitrile-based compound include succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile. Examples of the benzene-based compound include fluorobenzene. Examples of the amine-based compound include triethanolamine and ethylenediamine. Examples of the silane-based compound include tetravinylsilane. The lithium salt-based compound is a compound different from the lithium salt contained in the flame-retardant electrolyte, and examples thereof include lithium nitrate, lithium difluorophosphate (LiDFP), LiPO2F2, and LiBF4.

[0110] The electrolyte additive may be included in an amount of 0.1 to 10 wt %, 0.2 to 8 wt %, or 0.5 to 8 wt % based on the total weight of the flame-retardant electrolyte, and may contribute to improving ionic conductivity or cycle characteristics.

[0111] The lithium secondary battery may have a form in which the electrode assembly is housed in a case and the flame-retardant electrolyte is injected and impregnated therein, and may be a cylindrical, prismatic, pouch-type, or coin-type battery depending on the type of the case, etc. In this case, the battery case may be one commonly used in the art, and there is no limitation on the outer shape depending on the use of the battery, and may be, for example, but not limited to, a cylindrical, prismatic, pouch-type, or coin-type.

[0112] The lithium secondary battery can be fabricated by placing the electrode assembly in a suitable battery case and then injecting a flame-retardant electrolyte, or by stacking the electrode assemblies, impregnating them with a flame-retardant electrolyte, and then placing the resulting assembly in a battery case and sealing it.

[0113] The lithium secondary battery described above can be used not only as a battery cell used as a power source for a small device, but also as a unit battery for a medium- to large-sized battery module containing a number of battery cells. Preferred examples of the medium- to large-sized device include an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and an energy storage system (ESS).

[0114] The present invention will be described in more detail below through specific examples, but the following examples are merely illustrative for understanding the invention and are not intended to limit the scope of the invention.

[0115] <Examples and Comparative Examples> First, in the following examples, the zeta potential of polymer particles or ceramic particles was measured by electrophoretic light scattering using a dynamic light scattering device (product name: ELS-Z) at a temperature of 25°C, with the polymer particles or ceramic particles dispersed in an aqueous solvent at a concentration of 0.1 wt% or less.

[0116] Example 1 S1: Li(Ni) was deposited on an aluminum current collector as a positive electrode active material. 0.8 Mn 0.1 Co 0.1 A cathode slurry containing the cathode active material was applied and dried to form a cathode active material layer. The total thickness of the current collector and the cathode active material layer was 176 μm.

[0117] S2: Using a pilot rolling machine, primary rolling was performed so that the total thickness of the current collector and the positive electrode active material layer was 150 μm, and the porosity of the positive electrode active material layer was 40%. The porosity of the positive electrode active material layer was calculated from the true density of the positive electrode active material and the mass, area, and thickness of the positive electrode active material layer using Equations 1 and 2.

[0118] S3: A coating layer slurry was applied onto the primarily rolled positive electrode active material layer.

[0119] The coating layer slurry was prepared by dispersing polymethyl methacrylate (PMMA) polymer particles with a particle size of 1 μm and a zeta potential of -50 mV in an N-methyl-2-pyrrolidone (NMP) solvent so that the mass ratio of PMMA: hydrogenated nitrile rubber (H-NBR): polyvinylidene fluoride (PVDF) was 7:1:2 (solid content 21 wt%).

[0120] The coating layer slurry was dried to form a positive electrode stack including a coating layer.

[0121] S4: The positive electrode laminate was subjected to secondary rolling using a pilot rolling machine. The secondary rolling was performed so that the porosity of the positive electrode active material layer was 26%, and the total thickness of the positive electrode current collector and the positive electrode active material layer after the secondary rolling was 125 μm.

[0122] S5: An electrode assembly was prepared by stacking a negative electrode on the coating layer.

[0123] The negative electrode used had a copper current collector and contained graphite as the negative electrode active material.

[0124] Comparative Example 1 S1: Performed in the same manner as in Example 1 above.

[0125] S2: The same procedure as in Example 1 was carried out, except that the primary rolling was performed so that the porosity of the positive electrode active material layer was 26%, and the total thickness of the current collector and the positive electrode active material layer after the primary rolling was 125 μm.

[0126] S3: Carried out in the same manner as in Example 1.

[0127] S4: No secondary rolling is performed.

[0128] S5: The same procedure as in Example 1 was carried out.

[0129] Comparative Example 2 S1: Performed in the same manner as in Example 1 above.

[0130] S2: The same procedure as in Example 1 was carried out, except that the primary rolling was performed so that the porosity of the positive electrode active material layer was 30%, and the total thickness of the current collector and the positive electrode active material layer after the primary rolling was 132 μm.

[0131] S3: Carried out in the same manner as in Example 1.

[0132] S4: The same procedure as in Example 1 was carried out, except that secondary rolling was performed so that the porosity of the positive electrode active material layer was 26%, and the total thickness of the positive electrode current collector and the positive electrode active material layer after the secondary rolling was 125 μm.

[0133] S5: The same procedure as in Example 1 was carried out.

[0134] Comparative Example 3 S1: Performed in the same manner as in Example 1 above.

[0135] S2: The same procedure as in Example 1 was carried out, except that the primary rolling was performed so that the porosity of the positive electrode active material layer was 48%, and the total thickness of the current collector and the positive electrode active material layer after the primary rolling was 173 μm.

[0136] S3: Carried out in the same manner as in Example 1.

[0137] S4: The same procedure as in Example 1 was carried out, except that secondary rolling was performed so that the porosity of the positive electrode active material layer was 26%, and the total thickness of the positive electrode current collector and the positive electrode active material layer after the secondary rolling was 125 μm.

[0138] S5: The same procedure as in Example 1 was carried out.

[0139] Comparative Example 4 S1: Performed in the same manner as in Example 1 above.

[0140] S2: No primary rolling was performed.

[0141] S3: Carried out in the same manner as in Example 1.

[0142] S4: The same procedure as in Example 1 was carried out, except that the positive electrode active material layer was rolled to have a porosity of 26%, and the total thickness of the positive electrode current collector and the positive electrode active material layer after the rolling was 125 μm.

[0143] S5: The same procedure as in Example 1 was carried out.

[0144] <Experimental Example 1> Check for damage to the electrode assembly The electrode assemblies manufactured in Example 1 and Comparative Examples 1 to 4 were visually inspected for damage and judged as follows: The measurement results are shown in Table 1 below.

[0145] The presence or absence of damage to the electrode assembly was determined by visually determining whether waviness occurred at the boundary between the region of the electrode assembly where the positive electrode active material layer was formed after the first rolling in step (S2) and the current collector region where the positive electrode active material layer was not formed. If waviness occurred, it was determined that the electrode assembly was damaged.

[0146] In addition, it was visually determined whether or not cracks occurred on the surface of the coating layer, and if cracks occurred, it was determined that the electrode assembly was damaged.

[0147] O: Waviness is observed on the current collector, or cracks are observed on the surface of the coating layer.

[0148] X: No waviness is observed on the current collector, and no cracks are observed on the surface of the coating layer.

[0149] [Table 1]

[0150] In the case of Example 1, no waviness occurred at the boundary between the region where the positive electrode active material layer was formed after the first rolling in step (S2) and the current collector region where the positive electrode active material layer was not formed, and no cracks were found in the coating layer.

[0151] In the case of Comparative Examples 1 and 2, waviness occurred at the boundary between the region where the positive electrode active material layer was formed after the primary rolling in step (S2) and the current collector region where the positive electrode active material layer was not formed.

[0152] In Comparative Example 3, no waviness was observed in step (S2), but the organic solvent in the coating layer slurry penetrated into the positive electrode active material layer when the coating layer slurry was applied to the first-rolled positive electrode active material layer in step (S3). As a result, the positive electrode active material layer was easily detached from the electrode assembly, and cracks were observed on the surface of the dried coating layer.

[0153] In Comparative Example 4, no waviness was observed in step (S2), but the organic solvent in the coating layer slurry penetrated into the positive electrode active material layer when the coating layer slurry was applied to the positive electrode active material layer in step (S3). As a result, the positive electrode active material layer was easily detached from the electrode assembly, and cracks were observed on the surface of the dried coating layer.

[0154] <Experimental Example 2> Check the thickness of the mixed layer of the electrode assembly The thickness of the mixed layer was confirmed from photographs of the cross sections of the positive electrode and coating layer of Example 1 and Comparative Example 4 taken using a scanning electron microscope (SEM).

[0155] The thickness of the mixed layer was defined as the maximum depth of the coating layer penetrating into the positive electrode active material layer from the boundary between the positive electrode active material layer and the coating layer in a photograph of the cross section of the positive electrode and the coating layer.

[0156] SEM photographs of Example 1 and Comparative Example 4 are shown in FIG. 1 and FIG. 2, respectively, and the measured thicknesses of the mixed layers of Example 1 and Comparative Example 4 are shown in Table 2 below.

[0157] [Table 2]

[0158] Unlike Comparative Example 4, Example 1 was subjected to primary rolling so that the positive electrode active material layer had an appropriate porosity. It was confirmed that Example 1 had a very thin mixed layer of the positive electrode active material layer and the coating layer.

Claims

1. a positive electrode current collector; a positive electrode active material layer disposed on the positive electrode current collector; a coating layer coated on the positive electrode active material layer; a mixed layer in which the positive electrode active material layer and the coating layer are mixed; a negative electrode disposed on the coating layer, the negative electrode including a negative electrode current collector and a negative electrode active material layer, The coating layer and the mixed layer each include a polymer binder and polymer particles or ceramic particles dispersed on the polymer binder, the polymer particles having an absolute value of a zeta potential of 25 mV or more, and the mixed layer has a thickness of less than 15 μm.

2. 2. The electrode assembly of claim 1, wherein the thickness of the mixed layer is 5 to 14 μm, and the total thickness of the mixed layer and the coating layer is 5 to 50 μm.

3. The electrode assembly of claim 1 , wherein the polymer particles or ceramic particles have a particle size of 50 nm to 3 μm.

4. 2. The electrode assembly of claim 1, wherein the coating layer and the mixed layer contain the polymer binder and the polymer particles or ceramic particles in a weight ratio of 5:95 to 40:

60.

5. 2. The electrode assembly of claim 1, wherein the polymer particles comprise at least one selected from the group consisting of polymethyl (meth)acrylate, polystyrene, polyvinyl chloride, polycarbonate, polysulfone, polyethersulfone, polyetherimide, polyphenylsulfone, polyamideimide, polyimide, polybenzimidazole, polyetherketone, polyphthalamide, polybutylene terephthalate, polyethylene terephthalate, and polyphenylene sulfide.

6. 2. The electrode assembly of claim 1, wherein the ceramic particles comprise at least one selected from the group consisting of boehmite, aluminum oxide, titanium oxide, iron oxide, silicon oxide, zirconium oxide, cobalt oxide, tin oxide, nickel oxide, zinc oxide, vanadium oxide, and manganese oxide.

7. A lithium secondary battery comprising: a flame-retardant solvent having a flash point of 100° C. or higher or no flash point; a flame-retardant electrolyte containing a lithium salt; and the electrode assembly according to claim 1 .

8. 8. The lithium secondary battery according to claim 7, wherein the flame-retardant solvent includes one or more organic solvents selected from the group consisting of sulfone-based compounds, nitrile-based compounds, phosphoric acid-based compounds, and fluorine-substituted carbonate-based compounds.

9. The lithium secondary battery according to claim 7 , further comprising a battery case that houses the electrode assembly and the flame-retardant electrolyte.

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