Electrode assembly including a positive electrode having an insulating coating layer formed thereon

Abstract

The present invention relates to an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode and the negative electrode each have a flat portion where an active material layer has a constant thickness and a slope portion where the thickness of the active material layer decreases from the flat portion, an insulating coating layer is formed on the slope portion of the positive electrode, the flat portion of the positive electrode has a porosity of 15 to 25%, and the slope portions of the positive electrode each have a porosity of 20 to 30%.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0172152 dated December 3, 2021, and all contents disclosed in the documents of that Korean patent application are incorporated herein by reference.

[0002] The present invention relates to an electrode assembly including a positive electrode having an insulating coating layer formed thereon. [Background technology]

[0003] As technological development and demand for mobile devices increases, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which exhibit high energy density and working potential, long cycle life, and low self-discharge rate, have been commercialized and are widely used.

[0004] In addition, recently, the design of electronic devices itself has become a very important factor in consumer product selection, and electronic devices are gradually becoming smaller and thinner according to consumer preferences. Therefore, in order to minimize unnecessary waste of internal space in electronic devices, lithium secondary batteries are also required to be smaller and thinner, and the demand for such batteries is increasing.

[0005] Such a lithium secondary battery is manufactured by manufacturing a positive electrode and a negative electrode, stacking them together with a separator to form an electrode assembly, and then incorporating the electrode assembly into a secondary battery case together with an electrolyte.

[0006] The secondary battery electrode is manufactured by applying an electrode active material slurry to a current collector. The electrode is divided into a coated portion where the slurry is applied and an uncoated portion where the slurry is not applied. The electrode active material slurry coating may cause a sliding phenomenon where the electrode is not coated perpendicularly at both ends of the boundary between the coated portion and the uncoated portion due to the concentration of the slurry, and the electrode is coated at an angle.

[0007] However, when this sliding phenomenon occurs, electrons move in the current collector due to the charging current in the inclined area, causing lithium ions to move from the positive electrode to the negative electrode. However, this causes a difference in the speed at which these electrons move, making it more likely that lithium deposition will occur. In addition, the distance between the positive electrode and negative electrode active material is greater than in other areas, increasing resistance and reducing cell performance.

[0008] Furthermore, the inclined portion is not well rolled and has a higher porosity than the other portions, which increases the resistance of the active material.

[0009] Therefore, there is a pressing need to develop a technology for secondary batteries that can solve these problems. Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention aims to provide a secondary battery that can eliminate resistance non-uniformity, improve overall performance of the secondary battery, and ensure safety by preventing problems such as lithium precipitation, by preventing the inclined portion of the active material layer from participating in the reaction and making the porosity of the inclined portion similar to the porosity of the flat portion. [Means for solving the problem]

[0011] An electrode assembly according to an embodiment of the present invention comprises: An electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, the positive electrode and the negative electrode each have a flat portion where the thickness of the active material layer is constant and an inclined portion where the thickness of the active material layer decreases from the flat portion, an insulating coating layer is formed on the inclined portion of the positive electrode; The flat portion of the positive electrode has a porosity of 15 to 25%, and each of the sloped portions of the positive electrode has a porosity of 20 to 30%.

[0012] Here, the inclined portion may include a first section in which the thickness of the active material layer gradually decreases from a flat section in a region close to the positive electrode tab in the extension direction of the positive electrode tab, or a second section in which the thickness of the active material layer gradually decreases from the flat section in the first section and a region far from the positive electrode tab.

[0013] The insulating coating layer may include inorganic particles and a binder polymer, and more specifically, may include one or more types of inorganic particles and two or more types of binder polymers.

[0014] Specifically, the inorganic particles may be a mixture of one or more selected from the group consisting of SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, Y2O3, Al2O3, TiO2, and SiC.

[0015] Specifically, the binder polymer may be a mixture of two or more selected from the group consisting of butyl acrylate, styrene, acrylic acid, hydroethyl acrylate, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR).

[0016] Most particularly, the insulating coating layer may be composed of Al2O3, carboxyl methyl cellulose (CMC) and styrene butadiene rubber (SBR).

[0017] The insulating coating layer may be formed to a thickness corresponding to a difference in thickness between the flat portion and the inclined portion so that the surface of the insulating coating layer is aligned with the surface of the flat portion.

[0018] The insulating coating layer may be formed to cover the entire inclined portion, or may be formed to cover a portion of the positive electrode tab in the extension direction of the positive electrode tab, and may be formed to cover 5% to 80% of the entire length of the positive electrode tab.

[0019] Meanwhile, a method for manufacturing an electrode assembly according to another embodiment of the present invention includes: (a) applying a positive electrode slurry containing a positive electrode active material onto a positive electrode current collector and drying the slurry; (b) applying an insulating coating slurry to a sloping portion of the cathode slurry where the thickness of the cathode slurry gradually decreases, and drying the slurry to prepare a preliminary cathode; (c) rolling the preliminary positive electrode to produce a positive electrode having a positive electrode active material layer formed on the positive electrode current collector; and (d) manufacturing an electrode assembly by stacking the positive electrode, the negative electrode, and the separator.

[0020] Here, the insulating coating slurry may be a mixture of one or more inorganic materials selected from the group consisting of SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, Y2O3, Al2O3, TiO2, and SiC, and two or more binder materials selected from the group consisting of butyl acrylate, styrene, acrylic acid, hydroethyl acrylate, carboxymethyl cellulose (CMC), and styrene butadiene rubber (SBR) in a solvent.

[0021] Meanwhile, the rolling in step (b) may be performed so that the porosity of the flat portion of the active material layer containing the positive active material, where the thickness is uniform, is 15 to 25%.

[0022] The present invention also provides a lithium secondary battery in which the electrode assembly according to one embodiment and an electrolyte are contained in a battery case. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 is a cross-sectional schematic view of a positive electrode according to one embodiment of the present invention. [Figure 2] 1 shows a cross-sectional SEM photograph of a positive electrode according to Comparative Example 1 of the present invention, and an enlarged SEM photograph showing the porosity in each region. [Figure 3] 10 is a graph showing a charging profile according to Experimental Example 2 of the present invention. [Figure 4] 10 is a graph showing discharge profiles at different C rates according to Experimental Example 3 of the present invention. [Figure 5] 10 is a graph showing discharge profiles at different C rates according to Experimental Example 3 of the present invention. [Figure 6] 10 is a graph showing discharge profiles at different C rates according to Experimental Example 3 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] The terms and words used in this specification and claims should not be interpreted in their ordinary or dictionary sense, but should be interpreted in their meaning and concept consistent with the technical idea of ​​the present invention based on the principle that the inventor can appropriately define the concept of the term to best describe his / her invention. Therefore, the embodiments described in this specification and the configurations shown in the drawings are merely preferred embodiments of the present invention and do not represent the entire technical idea of ​​the present invention. Therefore, various equivalents and modifications that can replace them may exist at the time of filing this application, and the scope of the present invention is not limited to the embodiments described below.

[0025] The present invention will be described in detail below with reference to the drawings and embodiments. The terms and phrases used in the specification and claims are not to be construed as being limited to their ordinary or dictionary meanings, but are to be construed as meanings and concepts that correspond to the technical ideas of the present invention, based on the principle that the inventor can appropriately define the concepts of terms in order to best describe his / her invention.

[0026] Furthermore, it should be understood that the examples described in this specification and the configurations shown in the drawings are merely the most preferred embodiment of the present invention and do not represent the entire technical idea of ​​the present invention, and therefore that there may be various equivalents and modifications that can replace them at the time of this application.

[0027] According to one embodiment of the present invention, there is provided an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, the positive electrode and the negative electrode each have a flat portion where the thickness of the active material layer is constant and an inclined portion where the thickness of the active material layer decreases from the flat portion, an insulating coating layer is formed on the inclined portion of the positive electrode; The flat portion of the positive electrode has a porosity of 15 to 25%, and each of the sloped portions of the positive electrode has a porosity of 20 to 30%.

[0028] In this case, the inclined portion may include a first section in which the thickness of the active material layer gradually decreases from a flat section in a region close to the positive electrode tab in the extension direction of the positive electrode tab, or a second section in which the thickness of the active material layer gradually decreases from the flat section in the first section and a region far from the positive electrode tab.

[0029] That is, the inclined portion may be formed at one end where the tab is formed, or at both the one end and the other end.

[0030] Meanwhile, the flat portion and the sloped portion of the active material layer of the present invention may have similar porosities, and in this case, the flat portion of the positive electrode may have a porosity of 15 to 25%, and the sloped portion of the positive electrode may have a porosity of 20 to 30%.

[0031] Here, the porosity can be determined as a percentage by dividing the packing density, which is the weight of the active material layer per unit area of ​​each corresponding portion of the positive electrode, by the thickness of the active material layer, by the density of the positive electrode slurry used to prepare the active material layer, and then subtracting the result from 1.

[0032] That is, it can be calculated using the following formula.

[0033] [Formula] (1 - positive electrode packing density / positive electrode slurry density) x 100

[0034] If the porosity is too low and deviates from the above range, the insertion and release of lithium ions is not smooth, and if the porosity is too high, the resistance of the active material increases, which is undesirable.

[0035] Furthermore, the difference in porosity between the flat and inclined portions may be within 10%, more specifically, within 5%. As will be explained below, in the present invention, the electrode is rolled after forming an insulating coating layer on the inclined portions, so the porosity at each of these positions can be made similar. On the other hand, the insulating tape or insulating layer formed on the conventional tab is formed after the positive electrode is manufactured, so the porosity of the inclined portion and the porosity of the flat portion are not formed to be similar. In this case, the intended effect of the present invention cannot be achieved.

[0036] Therefore, the insulating coating layer can be formed on the inclined portion at the position where the inclined portion is formed.

[0037] When an insulating coating layer is formed on the inclined portion, lithium ions cannot move, and therefore the inclined portion is not involved in the insertion and release of lithium ions during charging and discharging of the lithium secondary battery.

[0038] Therefore, it is possible to solve the problem of lithium deposition due to an increase in resistance caused by an increase in the distance of lithium ions moving in the inclined portion, and since this problem becomes more serious as fast charging progresses, it is possible to improve the fast charging characteristics of the lithium secondary battery.

[0039] In this case, the insulating coating layer may specifically include inorganic particles and a binder polymer, and more specifically, the insulating coating layer may include one or more types of inorganic particles and two or more types of binder polymers.

[0040] When inorganic particles are included as described above, heat resistance can be improved, and a binder polymer can be included to easily coat the sloped portion. All of these components do not affect the operation of the battery, and the insulating coating layer can be fully functioned.

[0041] Here, the types of the inorganic particles and the binder polymer are not limited, and any materials known in the art can be used as long as they are components that can be used in lithium secondary batteries. In particular, the inorganic particles can be a mixture of one or more materials selected from the group consisting of SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, YO3, Al2O3, TiO2, and SiC, and the binder polymer can be a mixture of two or more materials selected from the group consisting of butyl acrylate, styrene, acrylic acid, hydroethyl acrylate, carboxymethyl cellulose (CMC), and styrene butadiene rubber (SBR).

[0042] Although the inorganic particles and the binder polymer may contain only one type, it is more preferable to contain two or more types of binder polymers since this can improve adhesive strength and fulfill insufficient performance.

[0043] The inorganic particles and the binder polymer may be mixed in a ratio of 1:10 to 1:1 by weight, and more specifically, in a ratio of 1:5 to 1:3.

[0044] If the content of the inorganic particles is too low, the heat resistance may be reduced, and conversely, if the content is too high, the polymer content is too low, which weakens the adhesive strength between the inorganic particles and reduces the mechanical properties of the insulating coating layer.

[0045] Furthermore, although there is no limitation on the size of the inorganic particles, it is preferable that the size be in the range of 0.01 to 10 μm, more specifically, 0.1 to 10 μm, to form an insulating coating layer of uniform thickness and to have an appropriate void ratio between the inorganic particles. If the size is less than 0.01 μm, dispersibility decreases, making it difficult to control the physical properties of the insulating coating layer. If the size exceeds 10 μm, the thickness increases, mechanical properties decrease, and the excessively large pore size may prevent the insulating layer from functioning adequately, which is undesirable.

[0046] Considering various factors such as the above problems and the performance of the secondary battery, the insulating coating layer may be made of, in particular, Al2O3, carboxyl methyl cellulose (CMC), and styrene butadiene rubber (SBR).

[0047] The above combination is preferable for effectively achieving the intended effect of the present invention while also functioning as an insulating layer.

[0048] Meanwhile, the insulating coating layer may be formed to a thickness corresponding to a difference in thickness between the flat portion and the inclined portion so that the surface thereof is aligned with the surface of the flat portion.

[0049] Therefore, since the insulating coating layer can fill the space in the inclined portion, the heights of the flat portion and the inclined portion can be made the same during the manufacturing of the positive electrode, so that the load due to rolling acts similarly. This prevents the active material layer from being properly rolled during the rolling process due to the inclination, and prevents active material resistance due to porosity imbalance caused by having a higher porosity than the flat portion, thereby eliminating performance degradation of the secondary battery and safety issues due to lithium precipitation caused by such imbalance.

[0050] To maximize this effect, the insulating coating layer may be formed to cover the entire slope portion.

[0051] Furthermore, the insulating coating layer may be formed to cover a portion of the positive electrode tab in the extension direction of the positive electrode tab.

[0052] Therefore, a separate insulating tape does not need to be attached to the tab of the positive electrode, and can be replaced with this insulating coating layer.

[0053] In this case, it is preferable that the insulating coating layer is formed sufficiently to replace the insulating tape typically formed on the tab. Specifically, the insulating coating layer may be formed to cover 5% to 80% of the entire length of the positive electrode tab.

[0054] Specifically, referring to FIG. 1, a cross-sectional view of a positive electrode according to one embodiment of the present invention is shown.

[0055] 1, a positive electrode 100 has a structure in which an active material layer 110 is formed on a current collector 101. The active material layer 110 has a flat portion 111 with a constant thickness and a sloped portion 112 where the thickness decreases from the flat portion 111.

[0056] An insulating coating layer 120 is formed on the inclined portion 112. At this time, the insulating coating layer 120 is formed to a thickness (t) corresponding to the thickness difference between the inclined portion 112 and the flat portion 111 so that the insulating coating layer 120 has the same height as the flat portion 111 of the active material layer 110, that is, so that the surface is on the same line as the surface of the flat portion 111.

[0057] The insulating coating layer 120 is formed so as to cover the entire inclined portion 112, and further so as to cover a portion of the positive electrode tab 102 in the extension direction of the positive electrode tab 102.

[0058] At this time, it may be formed to cover 5% to 80% of the entire length (l) of the positive electrode tab 102.

[0059] Meanwhile, according to another embodiment of the present invention, The method for manufacturing the electrode assembly includes: (a) applying a positive electrode slurry containing a positive electrode active material onto a positive electrode current collector and drying the slurry; (b) applying an insulating coating slurry to a sloping portion of the cathode slurry where the thickness of the cathode slurry gradually decreases, and drying the slurry to prepare a preliminary cathode; (c) rolling the preliminary positive electrode to produce a positive electrode having a positive electrode active material layer formed on the positive electrode current collector; and (d) A method for manufacturing an electrode assembly is provided, which includes the step of stacking the positive electrode, the negative electrode, and the separator to manufacture an electrode assembly.

[0060] In this case, the insulating coating slurry may be prepared by mixing inorganic particles and a binder polymer in a solvent, as described above, to contain the inorganic particles and the binder polymer. More specifically, the insulating coating slurry may be prepared by mixing, in a solvent, one or more inorganic mixtures selected from the group consisting of SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, YO3, Al2O3, TiO2, and SiC with two or more binder mixtures selected from the group consisting of butyl acrylate, styrene, acrylic acid, hydroethyl acrylate, carboxymethyl cellulose (CMC), and styrene butadiene rubber (SBR).

[0061] More specifically, Al2O3, carboxyl methyl cellulose (CMC) and styrene butadiene rubber (SBR) may be mixed in a solvent.

[0062] Other specific details are as explained above in connection with the insulating coating layer.

[0063] The insulating coating slurry and the positive electrode slurry may be applied simultaneously, or the insulating coating slurry may be applied after the positive electrode slurry is applied.

[0064] Thereafter, these are dried to volatilize the solvent of the positive electrode slurry and the solvent of the insulating coating slurry, so that the positive electrode slurry forms a positive electrode mixture, and the insulating coating slurry forms an insulating coating layer.

[0065] The rolling may be performed in step (b) so that the porosity of the flat portion of the active material layer containing the positive active material has a uniform thickness of 15 to 25%.

[0066] When done this way, the porosity of the slope can be 20-30%.

[0067] More specifically, the difference in porosity between the flat portion and the inclined portion may be within 10%, more specifically within 5%.

[0068] Conventionally, due to the influence of factors such as the viscosity of the positive electrode slurry, the active material layer includes a flat portion and an inclined portion whose thickness is thinner than that of the flat portion. This means that the inclined portion cannot bear the same load as the flat portion during rolling, and the porosity of the inclined portion is always forced to be higher than the porosity of the flat portion.

[0069] However, when the porosity is increased in this way, the resistance of the active material increases in this area, causing problems such as lithium deposition due to the high resistance.

[0070] However, according to the present invention, an insulating coating layer is formed on the inclined portion of the active material layer, and the insulating coating layer is applied to a thickness similar to the height of the flat portion, and then rolling is performed at the same time. This results in a similar porosity between the inclined portion and the flat portion, and therefore, there is no large porosity deviation, and problems such as non-uniform resistance do not occur, which is preferable.

[0071] In the case of conventional insulating tapes or insulating layers formed on tabs, the active material layer is generally formed in its entirety and then rolled before the insulating tape or insulating layer is formed. As a result, there is still a large difference in porosity between the flat and inclined portions of the active material layer, and the effects of the present invention cannot be obtained.

[0072] On the other hand, in the present invention, after all of these coatings are applied, drying and rolling are performed, so the porosity can be adjusted to be similar.

[0073] Then, the cathode, anode, and separator are stacked to form an electrode assembly.

[0074] Here, the specific components and manufacturing methods of the positive electrode, negative electrode, and separator are well known in the art, and therefore, the description thereof will be omitted here.

[0075] Meanwhile, according to another embodiment of the present invention, there is provided a lithium secondary battery in which the electrode assembly and an electrolyte are housed in a battery case.

[0076] Other components of the lithium secondary battery are well known in the art, and therefore, a description thereof will be omitted here.

[0077] <Comparative Example 1> A slurry for the positive electrode active material layer was prepared by mixing LiCoO2 as a positive electrode active material, carbon black as a conductive material, and PVDF as a binder in a weight ratio of 97.6:1.1:1.3 in N-methylpyrrolidone solvent, and the slurry for the positive electrode active material layer was coated onto one side of a 10 μm-thick aluminum (Al) thin film positive electrode current collector using a slot die and dried at 130°C under vacuum for 2.5 hours to form an active material layer. The active material layer thus formed was rolled using a roll pressing method until the porosity of the flat portion of the active material layer was 17%, thereby preparing a positive electrode having an active material layer.

[0078] Here, the porosity can be calculated as a percentage by dividing the packing density, which is the weight of the active material layer per unit area of ​​the flat portion of the positive electrode divided by the thickness of the active material layer, by the density of the positive electrode slurry used to prepare the active material layer, and then subtracting the result from 1.

[0079] That is, it can be calculated using the following formula.

[0080] [Formula] (1 - positive electrode packing density / positive electrode slurry density) x 100

[0081] The cross section of the prepared positive electrode was photographed by SEM, and the results are shown in FIG.

[0082] Referring to FIG. 2, it can be seen that the flat areas corresponding to Area 2 and Area 3 were well-rolled and did not have many voids in the active material, whereas the sloped area corresponding to Area 1 was poorly rolled and had a large number of voids.

[0083] Therefore, it is expected that the positive electrode of Comparative Example 1 will have a high resistance at the inclined portion.

[0084] Example 1 LiCoO2 as a positive electrode active material, carbon black as a conductive material, and PVDF as a binder were mixed in an N-methylpyrrolidone solvent at a weight ratio of 97.6:1.1:1.3 to prepare a slurry for a positive electrode active material layer. The slurry for a positive electrode active material layer was then coated on both sides of a 10 μm-thick aluminum (Al) thin film serving as a positive electrode current collector using a slot die.

[0085] Next, a mixture of Al2O3 (Sumitomo Chemical's AES-11), carboxymethyl cellulose (CMC, LG Chem's BG-L01), and styrene butadiene rubber (SBR, Zeon's BM-L301) in a weight ratio of 20:0.2:79.8 was dispersed in a solvent (deionized water) to form an insulating coating slurry, which was then applied to the sloped portion of the positive electrode active material layer slurry and 30% of the total length of the tab, as shown in Figure 1.

[0086] The active material layer and insulating coating layer were then formed by drying at 130°C under vacuum for 2.5 hours. The active material layer and insulating coating layer were then rolled using a roll pressing method until the porosity of the flat portion of the active material layer was 17%, thereby producing a positive electrode with an active material layer. The porosity of the sloped portion was approximately 21%.

[0087] <Example 2> The same procedure as in Example 1 was used to prepare a positive electrode having an active material layer, except that the active material layer was rolled so that the porosity of the flat portion was 24%. In this case, the porosity of the sloped portion was about 28%.

[0088] <Comparative Example 2> The same procedure as in Example 1 was used to prepare a positive electrode having an active material layer, except that the active material layer was rolled so that the porosity of the flat portion was 30%. In this case, the porosity of the sloped portion was about 35%.

[0089] <Experimental Example 1> The active material resistance and interface resistance of the positive electrodes prepared in Examples 1 and 2 and Comparative Example 2 were measured, and the results are shown in Table 1 below.

[0090] At this time, the active material resistance and the interface resistance were measured at room temperature using a HIOKI RM2610 by positioning a probe inside and at the interface of each active material layer.

[0091] The active material resistance and interface resistance can be calculated by inputting a) the thickness of the electrode active material, b) the thickness of the current collector (Al), and c) the specific volume resistance of the current collector.

[0092] [Table 1]

[0093] <Experimental Example 2> The positive electrodes prepared in Example 1 and Comparative Example 2 were used.

[0094] A first negative electrode was produced as follows.

[0095] A slurry for the active material layer was prepared by mixing a mixture of artificial graphite as the negative electrode active material, a binder (a mixture of SBR and CMC in a weight ratio of 2:1), and carbon black as the conductive material in a weight ratio of 96.4:0.5:3.1, and water as the dispersant in a weight ratio of 1:2.

[0096] The active material layer slurry was coated on both sides of a copper (Cu) thin film (8 μm thick) serving as a negative electrode current collector using a slot die, and then dried for 12 hours under vacuum at 130° C. The active material layer thus formed was rolled using a roll pressing method until the porosity of the flat portion of the active material layer became 28%, thereby producing a first negative electrode having an active material layer.

[0097] The second negative electrode was fabricated in the same manner as the first negative electrode, except that the porosity of the flat portion of the active material layer of the first negative electrode was set to 36%.

[0098] An electrode assembly was prepared by interposing a porous polyethylene separator between the positive electrode and the first negative electrode prepared as described above, and the electrode assembly was placed inside a case. An electrolyte solution was then injected into the case to prepare a first lithium secondary battery (positive electrode-first negative electrode in Example 1: A-1, positive electrode-first negative electrode in Comparative Example 2: B-1).

[0099] An electrode assembly was prepared by interposing a porous polyethylene separator between the positive electrode and the second negative electrode prepared as described above, and the electrode assembly was placed inside a case. An electrolyte solution was then injected into the case to prepare a second lithium secondary battery (positive electrode-second negative electrode in Example 1: A-3, positive electrode-first negative electrode in Comparative Example 2: B-3).

[0100] The electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1.15M in an organic solvent consisting of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (EC / DMC / EMC mixed volume ratio = 3 / 4 / 3).

[0101] The four lithium secondary batteries were charged at 25° C. under constant current / constant voltage (CC / CV) conditions up to 4.35 V / 38 mA at 1.5 C. The charging graph is shown in FIG.

[0102] 3, it can be seen that even when the present invention is applied, when the porosity of the positive electrode is 30% or more, the CC charging time is shortened and the total charging time is increased, whereas when the porosity is not 30%, it can be seen that the charging time can be shortened.

[0103] <Experimental Example 3> The four lithium secondary batteries (A-1, A-3, B-1, and B-3) prepared in Experimental Example 2 were charged at 1.5 C to 4.35 V / 38 mA under constant current / constant voltage (CC / CV) conditions at 25° C., and then discharged at different C rates (0.5 C, 1.0 C, and 1.5 C). The results are shown in FIGS. 4 to 6.

[0104] 4 to 6, when examining the discharge profile, it can be seen that when the porosity of the positive electrode is 30% or more, the discharge capacity is significantly reduced during rapid discharge.

[0105] Those skilled in the art will appreciate that various modifications and variations within the scope of the present invention can be made based on the above description. [Industrial Applicability]

[0106] The electrode assembly according to the present invention has an insulating coating layer formed on the sloped portion where the thickness of the positive electrode active material layer decreases, thereby preventing the sloped portion from participating in the reaction. This prevents the problem of increased resistance due to the difference in distance across the sloped portion during fast charging, and prevents lithium deposition in the portion, thereby improving the safety of a secondary battery including the same.

[0107] In addition, since the rolling is performed with the insulating coating layer formed on the inclined portion, there is no difference in porosity between the inclined portion and the flat portion of the positive electrode, and the porosity of the active material layer in the inclined portion can also be sufficiently reduced. This prevents the occurrence of porosity imbalance, sufficiently reduces the porosity, and reduces the active material resistance, thereby shortening the rapid charging time of the secondary battery including the same, as well as achieving uniform other performance characteristics.

Claims

1. An electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, the positive electrode and the negative electrode each have a flat portion where the thickness of the active material layer is constant and an inclined portion where the thickness of the active material layer decreases from the flat portion, an insulating coating layer is formed on the inclined portion of the positive electrode; the flat portion of the positive electrode has a porosity of 15 to 25% and the sloped portion of the positive electrode has a porosity of 20 to 30%; The electrode assembly, wherein the porosity of the inclined portion of the positive electrode is greater than the porosity of the flat portion of the positive electrode, the difference being within 10%.

2. 2. The electrode assembly of claim 1, wherein the inclined portion includes a first section in which the thickness of the active material layer gradually decreases from a flat section in a region close to the positive electrode tab in an extension direction of the positive electrode tab, or a second section in which the thickness of the active material layer gradually decreases from a flat section in a region far from the positive electrode tab.

3. The electrode assembly of claim 1 or 2, wherein the insulating coating layer comprises inorganic particles and a binder.

4. The electrode assembly of claim 3 , wherein the inorganic particles include one or more types, and the binder includes two or more types.

5. The inorganic particles are SrTiO ３ , SnO ２ , CeO ２ , MgO, NiO, CaO, ZnO, ZrO ２ , Y ２ O ３ , Al ２ O ３ , TiO ２ 4. The electrode assembly according to claim 3, wherein the silicon dioxide is one or a mixture of two or more selected from the group consisting of silicon dioxide and silicon carbide.

6. 4. The electrode assembly according to claim 3, wherein the binder is a mixture of two or more selected from the group consisting of carboxyl methyl cellulose (CMC) and styrene butadiene rubber (SBR).

7. The insulating coating layer is made of Al ２ O ３ 3. The electrode assembly according to claim 1, wherein the electrode assembly is made of carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR).

8. 3. The electrode assembly of claim 1, wherein the insulating coating layer is formed to a thickness corresponding to a thickness difference between the flat portion and the inclined portion so as to have a surface on the same line as a surface of the flat portion.

9. The electrode assembly according to claim 1 or 2, wherein the insulating coating layer is formed to cover the entire inclined portion.

10. The electrode assembly according to claim 1 or 2, wherein the insulating coating layer is formed to cover a portion of the positive electrode tab in an extension direction of the positive electrode tab.

11. The electrode assembly of claim 10 , wherein the insulating coating layer is formed to cover 5% to 80% of the entire length of the positive electrode tab.

12. 2. The method for manufacturing an electrode assembly according to claim 1, (a) applying a positive electrode slurry containing a positive electrode active material onto a positive electrode current collector and drying the slurry; (b) applying an insulating coating slurry to a sloping portion of the cathode slurry where the thickness of the cathode slurry gradually decreases, and drying the slurry to prepare a preliminary cathode; (c) rolling the preliminary positive electrode to produce a positive electrode having a positive electrode active material layer formed on the positive electrode current collector; (d) A method for manufacturing an electrode assembly, comprising: stacking the positive electrode, the negative electrode, and a separator to manufacture an electrode assembly.

13. The insulating coating slurry is SrTiO ３ , SnO ２ , CeO ２ , MgO, NiO, CaO, ZnO, ZrO ２ , Y ２ O ３ , Al ２ O ３ , TiO ２ 13. The method for manufacturing an electrode assembly according to claim 12, wherein the inorganic mixture is one or more inorganic substances selected from the group consisting of carboxymethyl cellulose (CMC) and SiC, and a binder mixture of two or more binders selected from the group consisting of carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR).

14. The method of claim 12, wherein the rolling in step (c) is performed so that the porosity of the flat portion of the positive active material layer having a uniform thickness is 15 to 25%.

15. A lithium secondary battery comprising the electrode assembly according to claim 1 and an electrolyte solution housed in a battery case.

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