Electrode assembly and method for manufacturing the same, and lithium secondary battery containing the same

Abstract

The present invention relates to a method for manufacturing an electrode assembly, the method including: (S1) applying a positive electrode slurry onto a positive electrode collector and drying the slurry to form a positive electrode active material layer; (S2) performing a primary rolling of the positive electrode 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) manufacturing an electrode assembly by stacking a negative electrode on the coating layer.

Description

Technical Field

[0001] Cross-reference to Related Applications (etc.) This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0067846 filed on June 2, 2022 and Korean Patent Application No. 10-2023-0069510 filed on May 30, 2023, and all the contents disclosed in the documents of the Korean patent applications are included as part of this specification.

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

Background Art

[0003] Recently, as the application fields of lithium secondary batteries have rapidly expanded not only to power supply for electronic devices such as electric, electronic, communication, and computers, but also to power storage supply for large-area devices such as automobiles and power storage devices, the demand for lithium secondary batteries with high capacity, high output, long life, and high stability has been increasing.

[0004] A lithium secondary battery generally includes a positive electrode, a negative electrode, a separator, and an electrolyte, and it is known in the art that it includes a positive electrode that can generate oxygen due to an unstable structure in a charged state. When oxygen is generated in this way, since the risk of ignition is high, research and development on methods to enhance the safety of lithium secondary batteries have been attempted.

[0005]

[0006] As an example, a general separator included in an existing lithium secondary battery, for example, a polyolefin-based porous separator, can shrink at a high temperature, and there is a possibility that a short circuit may occur between the positive electrode and the negative electrode due to the shrinkage of such a separator. When such a short circuit occurs, it may act with the oxygen generated by the unstable positive electrode, further increasing the risk of ignition.To reduce such fire risks and improve the safety of lithium secondary batteries, a lithium secondary battery can be considered that includes a porous coating layer interposed between the positive and negative electrodes, replacing or supplementing the separation membrane. Such a porous coating layer can electrically insulate the positive and negative electrodes from each other while allowing ion movement 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 become impregnated into the positive electrode, potentially causing the electrode active material layer to swell. 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 rupture during the formation of the coating layer.

[0008] Considering these problems, there is a continuing need for the development of technologies that can improve the safety of secondary batteries by forming a porous coating layer on the electrode that replaces or supplements the separation membrane, while also solving the problems in the coating layer formation process. [Overview of the Initiative] [Problems that the invention aims to solve]

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

[0010] The present invention also aims to provide a lithium secondary battery with improved safety, which includes the aforementioned electrode assembly. [Means for solving the problem]

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

[0012] Another embodiment of the invention provides an electrode assembly comprising 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 comprising a negative electrode current collector and a negative electrode active material layer, wherein the coating layer and the mixed layer comprise a polymer binder and polymer particles or ceramic particles dispersed on the polymer binder having an absolute value of 25 mV or more for their zeta potential, and the thickness of the mixed layer is less than 15 μm.

[0013] Another embodiment of the invention provides a lithium secondary battery comprising a battery case, a flame-retardant solvent having a flash point of 100°C or higher or having 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 the positive electrode active material layer, it is possible to successfully form a porous coating layer on the positive electrode active material layer that replaces or assists the existing separation membrane while suppressing damage to the positive electrode during the formation of the coating layer, and 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-ion secondary batteries that exhibit improved safety while also having high capacity characteristics and energy density. [Brief explanation of the drawing]

[0016] [Figure 1] Figure 1 shows a photograph of the cross-section of the positive electrode active material layer and coating layer of Example 1, observed through a scanning electron microscope (SEM). [Figure 2] Figure 2 shows photographs of the cross-sections of the positive electrode active material layer and coating layer of Comparative Example 4, observed through a scanning electron microscope (SEM). [Modes for carrying out the invention]

[0017] Hereafter, terms and words used in this specification and claims should not be interpreted in a manner limited to their ordinary or lexicographical meanings, but rather in a manner and concept consistent with the technical idea of ​​the present invention, based on the principle that inventors can appropriately define the concepts of terms in order to best describe their invention.

[0018] Unless otherwise defined, all terms used herein (including technical and scientific terms) should be used in a sense that is commonly understood by a person of ordinary skill 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 herein are for illustrative purposes only and are not intended to limit the invention. In this specification, the singular form includes the plural form unless otherwise specified. The terms “comprises” and / or “comprising” as used in this specification do not preclude the presence or addition of one or more other components beyond those mentioned.

[0020] In this specification, when a part is said to include a component, this does not mean that it excludes other components, unless otherwise stated, but rather that it may include other components.

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

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

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

[0024] In one embodiment of the manufacturing method, when forming a porous coating layer that replaces or supplements an existing separation membrane, the positive electrode active material layer and the porous coating layer are formed through multiple rolling steps. It has been confirmed that through this process, a mixed layer in which a portion of the porous coating layer has permeated and mixed with the positive electrode active material layer can be formed to a very thin thickness.

[0025] By forming such a thin porous coating layer, it is possible to suppress short circuits or ignition between electrodes due to thermal shrinkage of the separation membrane, and furthermore, the thin thickness of the porous coating layer can further increase the energy density of the unit secondary battery.

[0026] Furthermore, the progression of the multiple rolling steps makes it possible to ultimately achieve a porosity suitable for the positive electrode active material layer, for example, 20% to 30%, while suppressing damage to the positive electrode active material layer due to factors such as the higher pressure of the rolling step for forming the coating layer. In addition, the multiple rolling steps also reduce the problem of the liquid medium of the coating layer slurry swollen around the positive electrode active material layer.

[0027] On the other hand, each component of the electrode assembly will be described in detail in the following section describing the electrode assembly, and below, each step of the manufacturing method will be described.

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

[0029] The positive electrode slurry can be manufactured by dissolving or dispersing a positive electrode active material, a binder, a conductive material, and a selective dispersant in a solvent.

[0030] Such a positive electrode slurry can be applied to a positive electrode current collector and dried to form a positive electrode active material layer. For example, the positive electrode slurry can be applied to the positive electrode current collector, and the positive electrode current collector and the positive electrode slurry can be dried by passing them at a constant speed through a drying area equipped with an exhaust fan in a continuous process. The drying area may be divided into six sections with a length of about 2 m, and the temperature in each drying section may be about 50 to 95°C.

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

[0032] Such a primary rolling process and the secondary rolling process described later can be carried out, for example, by rolling rolls, and the thickness and / or porosity of the positive electrode current collector and positive electrode active material layer can be controlled by appropriately setting the gap between the rolling rolls.

[0033] The aforementioned primary rolling process can change the thickness and / or porosity of the positive electrode current collector and positive electrode active material layer within an appropriate range.

[0034] After such primary rolling, the sum of the thicknesses of the primary-rolled positive electrode current collector and the positive electrode active material layer may be 80% to 90%, or 83% to 87%, of the sum of the thicknesses of the unrolled positive electrode current collector and the positive electrode active material layer before primary rolling.

[0035] If the thickness ratio after the primary 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, potentially causing damage such as the positive electrode active material layer becoming wavy on the positive electrode current collector. Conversely, if the thickness ratio after the primary rolling exceeds 90% of the initial thickness, the positive electrode current collector and the positive electrode active material layer may not be substantially rolled, potentially allowing the liquid medium of the coating layer slurry to penetrate the positive electrode active material layer in subsequent steps, potentially causing swelling.

[0036] As a specific example, in the initial state before primary rolling, the combined 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, while the combined thickness of the primary rolled positive electrode current collector and the positive electrode active material layer may be 120 to 180 μm or 130 to 170 μm.

[0037] Furthermore, the primary rolling step can be carried out by passing the positive electrode current collector and the positive electrode active material layer between the rolling rolls such that the porosity of the positive electrode active material layer is 35% to 45%, or 37% to 43%.

[0038] In this case, the porosity (P, unit: %) of the positive electrode active material layer can be calculated by a method commonly used in the art, and specifically, it can be calculated by the following formula 1.

[0039] [Formula 1] P = (1-D) / T × 100 In Equation 1 above, 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 intrinsic density of the positive electrode active material without voids.

[0040] Furthermore, the density D of the positive electrode active material layer can be calculated by the following equation 2.

[0041] [Formula 2] D = M / (S × H) In equation 2 above, 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 primary rolling is less than 35%, the positive electrode current collector and the positive electrode active material layer may be excessively rolled, potentially causing damage such as the positive electrode active material layer becoming wavy on the positive electrode current collector. Conversely, if the porosity of the positive electrode active material layer after the primary rolling exceeds 45%, the positive electrode current collector and the positive electrode active material layer may not be substantially rolled, potentially allowing the liquid medium of the coating layer slurry to penetrate the positive electrode active material layer in subsequent steps, potentially causing swelling. Furthermore, the penetration of the liquid medium contained in the coating layer slurry may increase the porosity of the positive electrode active material layer, potentially forming non-uniform voids.

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

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

[0045] In this case, the zeta potential of the polymer particles or ceramic particles is a physical property that reflects the surface polarity of these particles and defines the electrostatic repulsion or dispersibility between particles. Polymer particles or ceramic particles with a large absolute value of the zeta potential can be uniformly dispersed on the positive electrode active material layer, exhibiting good and uniform coating properties, and can define a large number of fine and uniform pores between these particles that allow lithium ions to pass through. Furthermore, particles that satisfy such a zeta potential allow the porous coating layer to exhibit excellent impregnation properties with flame-retardant electrolytes containing flame-retardant solvents. With such a porous coating layer and flame-retardant electrolyte, lithium secondary batteries can exhibit superior safety.

[0046] The zeta potential of the polymer particles or ceramic particles can be measured, for example, by electrophoretic light scattering using dynamic light scattering equipment. In this case, the zeta potential of the polymer particles or ceramic particles can be measured while they are dispersed in water or an alcohol-based solvent without a separate dispersant. In a specific example, the zeta potential can be measured when the polymer particles or ceramic particles are dispersed in water at a concentration of 0.1% by weight or less.

[0047] The absolute value of the zeta potential of the polymer particles or ceramic particles may be 25mV or higher, 35mV or higher, or 45mV or higher, and may be 100mV or lower, 90mV or lower, or 80mV or lower. 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, polybendiimidazole, polyetherketone, polyphthalamide, polybutylene terephthalate, polyethylene terephthalate, and polyphenylene sulfide.

[0049] Furthermore, 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 adjusted not only by the type of particle but also by the particle size or surface characteristics of these particles. Therefore, in order to achieve the zeta potential of the polymer particles or ceramic particles, the dispersibility, or the appropriate porosity of the porous coating layer, the polymer particles or ceramic particles may have particle sizes of 50 nm to 3 μm, 50 nm to 1.5 μm, or 100 nm to 1 μm.

[0051] Furthermore, 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 in a state in which they have been surface-treated with oxygen plasma or an ion beam.

[0052] On the other hand, in the aforementioned coating layer slurry, the polymer binder may be the same type of polymer as the binder contained in the positive electrode active material layer. Specific examples include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene (PE), polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, nitrile rubber, styrene-butadiene rubber, or fluororubber. A mixture or copolymer of two or more selected from these may be used. However, the specific composition of the polymer binder can be determined as obvious to a person skilled in the art, taking into account the type and properties of the polymer particles or ceramic particles, and the method of forming the coating layer.

[0053] Furthermore, taking into consideration 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 and the polymer particles or ceramic particles in a weight ratio of 5:95 to 40:60, or 10:90 to 35:65.

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

[0055] Furthermore, the coating layer slurry further contains a liquid medium for dispersing each of the aforementioned components. This medium may include organic solvents such as N-methyl-2-pyrrolidone (NMP) or acetone, or water, and may be appropriately selected considering the types of polymer particles, ceramic particles, or polymer binders mentioned above.

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

[0057] On the other hand, after the formation of the porous coating layer, the positive electrode laminate is subjected to secondary rolling (S4). This secondary rolling process can be carried out by appropriately setting the gap between the rolling rolls, similar to the primary rolling process, and the thickness and / or porosity of the positive electrode active material layer and the porous coating layer can be appropriately achieved by this secondary rolling process.

[0058] After the secondary rolling, the combined thickness of the secondary-rolled positive electrode current collector and the positive electrode active material layer may be 60% to 75%, or 63% to 72%, of the combined thickness of the unrolled positive electrode current collector and the positive electrode active material layer before the primary rolling. In this case, the thickness of the secondary-rolled positive electrode active material layer can be calculated considering the thickness of the mixed layer containing the porous coating layer, and the thickness of each such layer can be confirmed through electron microscopy analysis or the like.

[0059] If the thickness ratio after secondary rolling falls below 60% of the initial thickness, the positive electrode laminate is excessively rolled, and as the thickness of the porous coating layer becomes excessively thin, the positive and negative electrodes may easily short-circuit, or damage may occur to the positive electrode active material layer. Conversely, if the thickness ratio after secondary rolling exceeds 75% of the initial thickness, the positive electrode active material layer may not have an appropriate porosity, potentially degrading the battery performance.

[0060] Furthermore, the secondary rolling step can be carried out by passing the positive electrode laminate between the rolling rolls such 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 secondary rolling falls below 20%, the positive electrode laminate is excessively rolled, and as the thickness of the porous coating layer becomes excessively thin, the positive and negative electrodes may easily short-circuit, or damage may occur to the positive electrode active material layer. Conversely, if the porosity of the positive electrode active material layer after secondary rolling exceeds 30%, the positive electrode active material layer may not have an appropriate porosity, potentially degrading the battery's capacity characteristics.

[0062] Subsequently, an electrode assembly is manufactured by laminating a negative electrode onto the porous coating layer (S5).

[0063] The negative electrode may include a negative electrode current collector and a negative electrode active material layer, as described later. In the electrode assembly manufacturing step, the electrode assembly can be manufactured by laminating the negative electrode on the porous coating layer with the porous coating layer formed on the upper surface of the secondary-rolled positive electrode laminate.

[0064] Below, we will describe in detail an example of an electrode assembly that can be manufactured by the method described above.

[0065] electrode assembly An electrode assembly according to another embodiment of the invention 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 on the polymer binder having an absolute value of 25 mV or more for a zeta potential, and the thickness of the mixed layer may be less than 15 μm.

[0066] The coating layer may be, for example, a porous coating layer that includes a plurality of pores having a diameter of 10 nm or more, while exhibiting a porosity similar to that of the positive electrode active material layer. Such a porous coating layer can replace or supplement existing separation membranes by electrically insulating the positive and negative electrodes while enabling the movement of lithium ions during charging and discharging.

[0067] In the electrode assemblies of the other embodiments described above, the formation of such a porous coating layer can suppress short circuits or ignition between electrodes due to thermal shrinkage of the existing separation membrane, thereby enabling the provision of lithium secondary batteries 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 such other embodiment examples of electrode assemblies, 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 primary rolling described above.

[0069] Specifically, the minimum thickness of the mixed layer described above can be achieved by performing the primary rolling so that the sum of the thicknesses of the primary-rolled positive electrode current collector and the positive electrode active material layer is 80% to 90% of the sum of the thicknesses of the unrolled positive electrode current collector and the positive electrode active material layer. Furthermore, the minimum thickness of the mixed layer described above can be achieved during secondary rolling by performing the primary rolling process so that the porosity of the positive electrode active material layer is 35% to 45%.

[0070] By ensuring that the thickness of the mixed layer satisfies the numerical range, the thickness of the porous coating layer that replaces or assists the separation membrane can be reduced, thereby improving the coating properties on 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 a desired range.

[0071] On the other hand, the composition of the mixed layer and the porous coating layer is the same as that described in relation to the manufacturing method of one embodiment, so no further explanation is provided therein.

[0072] Furthermore, the positive electrode current collector in the electrode assembly is not particularly limited, as long as it does not induce a chemical change in the battery and is conductive. For example, the current collector may be made of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. Such a positive electrode current collector can have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the positive electrode current collector to increase adhesion to the positive electrode active material layer. For example, it can be used in a variety of forms such as film, sheet, foil, net, porous material, foam, nonwoven fabric, etc.

[0073] Furthermore, the positive electrode active material layer on the positive electrode current collector may contain positive electrode active material, and may further contain conductive material, binder, etc., as needed.

[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 s2 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 the atomic fractions of independent elements, respectively, 0 < p2 < 1, 0 < q2 < 1, 0 < r2 < 1, 0 < s2 < 1, and p2 + q2 + r2 + s2 = 1), etc.), and one 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 (e.g., 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 (e.g., Li(Ni 0.8 Co 0.15 Al 0.05 )O2, etc.) or lithium nickel manganese cobalt aluminum oxide (e.g., Li(Ni 0.86 Co 0.05 Mn 0.07 Al 0.02 )O2), etc., and a mixture of one or more of these 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 a chemical change in the battery and is conductive. For example, carbon powder such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powder such as natural graphite, artificial graphite, or graphite with a highly developed crystalline structure; conductive fibers such as carbon fibers or metal fibers; fluorinated carbon 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% by weight, 1 to 15% by weight, or 1 to 10% by weight, 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 the bonding of the positive electrode active material to the conductive material and to the positive electrode current collector.

[0080] Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, 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 present in an amount of 1 to 20% by weight, 1 to 15% by weight, or 1 to 10% by weight, based on the total weight of the positive electrode active material layer.

[0082] On the other hand, 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, calcined carbon, copper or stainless steel surface treatment with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy may be used.

[0084] The negative electrode current collector can typically have a thickness of 3 μm to 500 μm, and, similar to the positive electrode current collector, fine irregularities can be formed on the surface of the negative electrode current collector to strengthen the bonding force of the negative electrode active material. For example, it can be used in a variety of forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0085] Furthermore, the negative electrode active material layer may contain a negative electrode active material, and may further contain conductive materials, binders, etc., as needed.

[0086] The negative electrode active material may include at least one selected from the group consisting of lithium metal, carbon material capable of reversibly intercalating / deintercalating lithium ions, metal or alloys of these metals with lithium, metal composite oxides, lithium-doped and dedoped materials, and transition metal oxides.

[0087] The carbon material that can reversibly intercalate / deintercalate lithium ions can be any carbon-based negative electrode active material commonly used in lithium-ion secondary batteries, and typical examples include crystalline carbon, amorphous carbon, or a combination thereof. Examples of crystalline carbon include graphite such as amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, while 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] As the metal composite oxide, 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 and used.

[0090] As the substance capable of doping and undoping lithium, Si, SiO x (0 < x ≦ 2), Si - Y alloy (where Y is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, and a combination thereof and is not Si), Sn, SnO2, Sn - Y (where Y is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, and a combination thereof and is not Sn), etc. may be mentioned, and at least one of these and SiO2 may be mixed and used. As the element Y, it 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 a combination thereof.

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

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

[0093] On the other hand, the types of binders and conductive materials that may be included in the negative electrode active material layer, and their content, are substantially the same as those described above for the positive electrode active material layer, so no further explanation is provided.

[0094] Lithium-ion battery An additional embodiment of the invention provides a lithium secondary battery comprising a battery case, a flame-retardant solvent having a flash point of 100°C or higher or having no flash point, a flame-retardant electrolyte containing a lithium salt, and the aforementioned electrode assembly.

[0095] Such a lithium secondary battery includes an electrode assembly containing the aforementioned porous coating layer, along with a flame-retardant electrolyte containing a flame-retardant solvent. As already mentioned, the porous coating layer contains polymer particles or ceramic particles having predetermined properties, and can exhibit excellent impregnation properties with the flame-retardant solvent and electrolyte. Therefore, with such a combination of porous coating layer and flame-retardant electrolyte, ignition due to short circuits between electrodes can be more effectively suppressed, the safety of the lithium secondary battery can be further improved, and excellent electrical properties can be achieved.

[0096] Such flame-retardant solvents are solvents with low volatility and flammability, and can be defined through a predetermined flash point. For example, the flame-retardant solvent can include organic solvents that are substantially non-flammable and have no flash point, and organic solvents with high flash points of 100°C or higher, or 100 to 250°C, or 110 to 200°C, and low volatility. Including such a flame-retardant solvent and a flame-retardant electrolyte containing a lithium salt, the lithium secondary battery can exhibit excellent safety and stability. Furthermore, since the flame-retardant electrolyte can be uniformly impregnated into the porous coating layer described above, excellent electrochemical properties of the lithium secondary battery can be achieved. The flash point defining the flame-retardant solvent can be measured by a closed or open 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, such as a sulfone 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 phosphonate group, and a nitrile functional group, and one or more such organic solvents may be used in mixture form. More specifically, the flame-retardant solvent may contain one or more organic solvents selected from the group consisting of sulfone compounds, nitrile compounds, phosphoric acid compounds, and fluorine-substituted carbonate compounds.

[0098] Among these, the sulfone compound may be a cyclic sulfone compound or a linear sulfone compound, and specifically, it may contain one or more selected from the group consisting of sulfolane, ethylmethylsulfone, dibutylsulfone, ethylvinylsulfone, methylpropylsulfone, ethyl-i-propylsulfone, ethyl-i-butylsulfone, i-propyl-i-butylsulfone, i-propyl-s-butylsulfone, and butyl-i-butylsulfone.

[0099] Furthermore, the nitrile compound may include one or more selected from the group consisting of malononitrile, succinonitrile, glutalonitrile, adiponitrile, suberonitrile, and sebaconitrile.

[0100] Furthermore, the phosphate compound may include one or more selected from the group consisting of dimethylmethyl phosphate, trimethyl phosphate, triethyl phosphate, tributyl phosphate, diethylethyl phosphate, dimethylmethyl 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, and ethyl-2,2,2,2',2',2'-hexafluoropropyl It may contain one or more selected from the group consisting of oro-i-propyl carbonate, 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 a lithium secondary battery.

[0103] Lithium salts, for example, have Li as a 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 - may together contain an anion selected from the group consisting of.

[0104] Specifically, the lithium salt is LiCl, LiBr, LiI, LiBF4, LiClO4, LiB 10 Cl 10It may also contain one or more selected from the group consisting of 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 for superior stability. In addition to these, lithium salts commonly used as electrolytes in lithium secondary batteries can be used without any special restrictions.

[0105] The concentration of the lithium salt can be appropriately changed within a normally usable range, but in order to obtain the optimal effect of forming a protective film to prevent corrosion on the electrode surface, it may be included in the flame retardant electrolyte at a concentration of 0.5 M to 6 M, 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 cycle characteristics is sufficient when the lithium secondary battery is stored at high temperatures, the viscosity of the flame retardant electrolyte is appropriate, and the impregnation of the flame retardant electrolyte can be improved.

[0106] Furthermore, the aforementioned flame-retardant electrolyte may also contain additional electrolyte additives as needed, taking into consideration low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, or battery swelling suppression effects at high temperatures.

[0107] Typical 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, vinylene carbonate (VC) or vinylethylene carbonate can be cited as the cyclic carbonate compound. Fluoroethylene carbonate (FEC) can be cited as the halogen-substituted carbonate compound. Furthermore, compounds selected from the group consisting of 1,3-propanesultone (PS), 1,4-butanesultone, ethensortone, 1,3-propensultone (PRS), 1,4-butensultone, and 1-methyl-1,3-propensultone can be cited as the sultone compound. Examples of the sulfate compound include ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyltrimethylene sulfate (MTMS), and examples of the phosphate compound include one or more selected from the group consisting of lithium difluoro(bisoxalato) phosphate, lithium difluorophosphate, tetramethyltrimethylsilyl phosphate, and tris(2,2,2-trifluoroethyl) phosphate.

[0109] Examples of the borate compound include tetraphenyl borate, lithium oxalyl difluoroborate (LiODFB), or lithium bisoxalate borate (LiB(C2O4)2, LiBOB). Examples of the nitrile compound include one or more selected from the group consisting of succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentane cabonitrile, cyclohexane cabonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile. In addition, examples of the benzene compound include fluorobenzene, examples of the amine compound include triethanolamine or ethylenediamine, and examples of the silane compound include tetravinylsilane. Furthermore, the lithium salt compound is a compound different from the lithium salt contained in the flame-retardant electrolyte, and examples include lithium nitrate, lithium difluorophosphate (LiDFP), LiPO2F2, or LiBF4.

[0110] The aforementioned electrolyte additives are included in amounts of 0.1 to 10% by weight, 0.2 to 8% by weight, or 0.5 to 8% by weight, based on the total weight of the flame-retardant electrolyte, and can contribute to improving ionic conductivity or cycle characteristics.

[0111] The lithium secondary battery described above may be in a form in which the electrode assembly is housed in a case and the flame-retardant electrolyte is injected and impregnated, and depending on the form of the case, it may be cylindrical, rectangular, pouch type, or coin type battery. In this case, the battery case may be one that is commonly used in this field, and there are no restrictions on the external shape according to the application of the battery, and it may be cylindrical, rectangular, pouch type, or coin type, but is not limited thereto.

[0112] The aforementioned lithium secondary battery can be manufactured by placing the electrode assembly into a suitable battery case and then injecting a flame-retardant electrolyte. Alternatively, the electrode assembly can be stacked, impregnated with a flame-retardant electrolyte, and the resulting product can be placed in a battery case and sealed.

[0113] The aforementioned lithium secondary batteries can be used not only as battery cells for powering small devices, but also preferably as unit batteries in medium- and large-sized battery modules containing a large number of battery cells. Preferred examples of such medium- and large-sized devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems (ESS).

[0114] The invention will be described in more detail below through specific examples. However, the following examples are merely illustrative for understanding the invention and do not 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 dynamic light scattering equipment (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% by weight or less.

[0116] Example 1 S1: Li(Ni) is used as the positive electrode active material on an aluminum current collector. 0.8 Mn 0.1 Co 0.1 O2 was used, and a positive electrode slurry containing such positive electrode active material was applied and dried to form a positive electrode active material layer. The sum of the thickness of the current collector and the thickness of the positive electrode active material layer was 176 μm.

[0117] S2: Primary rolling was performed using pilot rolling equipment so that the sum of the thickness of the current collector and the thickness of the positive electrode active material layer was 150 μm, and the porosity of the positive electrode active material layer was 40%. At this time, the porosity of the positive electrode active material layer was calculated from the true density of the positive electrode active material, 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 primary rolled positive electrode active material layer.

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

[0120] The coating layer slurry was dried to form a positive electrode laminate containing the coating layer.

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

[0122] S5: An electrode assembly was manufactured by laminating a negative electrode onto the coating layer.

[0123] The negative electrode used a copper current collector and a negative electrode active material containing graphite.

[0124] Comparative Example 1 S1: The procedure was carried out in the same manner as in Example 1 above.

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

[0126] S3: The procedure was carried out in the same manner as in Example 1 above.

[0127] S4: No secondary rolling was performed.

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

[0129] Comparative Example 2 S1: The procedure was carried out in the same manner as in Example 1 above.

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

[0131] S3: The procedure was carried out in the same manner as in Example 1 above.

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

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

[0134] Comparative Example 3 S1: The procedure was carried out in the same manner as in Example 1 above.

[0135] S2: The procedure was carried out in the same manner as in Example 1, except that the positive electrode active material layer was primary rolled to have a void ratio of 48%, and the sum of the thickness of the current collector and the thickness of the positive electrode active material layer after the primary rolling was 173 μm.

[0136] S3: The procedure was carried out in the same manner as in Example 1 above.

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

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

[0139] Comparative Example 4 S1: The procedure was carried out in the same manner as in Example 1 above.

[0140] S2: Primary rolling was not performed.

[0141] S3: The procedure was carried out in the same manner as in Example 1 above.

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

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

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

[0145] To determine whether the electrode assembly was damaged, in step (S2), a visual inspection was performed to see if undulation occurred at the boundary between the region of the electrode assembly where the positive electrode active material layer was formed after the primary rolling and the current collector region where the positive electrode active material layer was not formed. If undulation occurred, it was determined that the electrode assembly was damaged.

[0146] Furthermore, a visual inspection was conducted to determine whether or not cracks were occurring on the surface of the coating layer. If cracks were found, it was determined that the electrode assembly was damaged.

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

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

[0149] [Table 1]

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

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

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

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

[0154] <Experimental Example 2> Confirmation of the thickness of the mixed layer in the electrode assembly. For Example 1 and Comparative Example 4, the thickness of the mixed layer was confirmed by taking photographs of the cross-sections of the cathode and coating layers using a scanning electron microscope (SEM).

[0155] The thickness of the mixed layer was defined as the maximum depth of the coating layer that penetrated into the interior of the positive electrode active material layer, starting from the boundary point between the positive electrode active material layer and the coating layer, as seen in photographs of the cross-sections of the positive electrode and the coating layer.

[0156] SEM images of Example 1 and Comparative Example 4 are shown in Figures 1 and 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, in Example 1, the primary rolling was performed so that the positive electrode active material layer had an appropriate porosity, and it was confirmed that in Example 1, a mixed layer containing the positive electrode active material layer and the coating layer existed only at a very thin thickness.

Claims

1. (S1) A step of applying a positive electrode slurry onto a positive electrode current collector and drying it to form a positive electrode active material layer, (S2) A step of primary rolling the positive electrode current collector and the positive electrode active material layer, (S3) A step of applying a coating layer slurry onto the positive electrode active material layer and drying it to form a positive electrode laminate including the coating layer, (S4) The step of secondary rolling the positive electrode laminate, (S5) The step of manufacturing an electrode assembly by laminating a negative electrode on the coating layer, A method for manufacturing an electrode assembly, wherein the primary rolling step is performed so that the porosity of the positive electrode active material layer is 35% to 45%, and the secondary rolling step is performed so that the porosity of the positive electrode active material layer is 20% to 30%.

2. The method for manufacturing an electrode assembly according to claim 1, wherein the coating layer slurry comprises a polymer binder, polymer particles or ceramic particles having an absolute value of 25 mV or more for zeta potential, and a liquid medium.

3. The method for manufacturing an electrode assembly according to claim 2, wherein the coating layer slurry contains the polymer binder and the polymer particles or ceramic particles in a weight ratio of 5:95 to 40:

60.

4. The method for manufacturing an electrode assembly according to claim 2, wherein the polymer particles or ceramic particles have a particle size of 50 nm to 3 μm.

5. The method for manufacturing an electrode assembly according to claim 1, wherein the sum of the thicknesses of the primary-rolled positive electrode current collector and the positive electrode active material layer is 80% to 90% of the sum of the thicknesses of the positive electrode current collector and the positive electrode active material layer before primary rolling.

6. The method for manufacturing an electrode assembly according to any one of claims 1 to 5, wherein the sum of the thicknesses of the secondary-rolled positive electrode current collector and the positive electrode active material layer is 60% to 75% of the sum of the thicknesses of the positive electrode current collector and the positive electrode active material layer before primary rolling.

7. The method for manufacturing an electrode assembly according to claim 2, further comprising the step of plasma treatment of polymer particles or ceramic particles to adjust the absolute value of the zeta potential to 25 mV or more.

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