Electrode for secondary battery improving rapid charging performance, method for preparing the same, and secondary battery including the same

Abstract

The present application provides a secondary battery electrode, comprising: a current collector; and an electrode active material layer containing an electrode active material and a binder on at least one side of the current collector, the electrode active material layer, with the width direction as the reference, from one side of the electrode active material layer to the other side direction, includes a region a and a region b, the region a has a higher binder content than the region b, and the region a and the region b have a continuous binder content gradient at the boundary of each region. The present application can improve the interfacial adhesion between the current collector and the electrode active material layer, improve the process defects such as electrode peeling, and improve the rapid charging performance.

Description

TECHNICAL FIELD

[0001] The present application relates to an electrode for a secondary battery that improves rapid charging performance, a method for preparing the same, and a secondary battery including the same. BACKGROUND

[0002] Recently, in the case of large battery cells for high-energy electric vehicles (EVs), since the electrode density is high, when the battery cell is charged at a high current density, the diffusion of Li ions into the interior of the negative electrode is limited, and in this case, Li ions are deposited on the surface of the negative electrode, resulting in degradation of the battery cell.

[0003] In order to improve the above problem, it is important to maintain as low a density of the negative electrode as possible, or to reduce the resistance of the surface and interior of the negative electrode, so that Li ions can rapidly diffuse into the interior of the negative electrode.

[0004] In order to improve the resistance and rapid charging performance of the battery cell, an adhesive having high adhesion has been developed, and a technology for reducing the content of the adhesive using the same is being developed, but there are limitations in the type of the adhesive having high adhesion and in reducing the content of the adhesive, that is, if the content of the adhesive is excessively reduced, a serious problem of peeling of the electrode mixture layer from the current collector occurs in the notching process or the charging and discharging process of the battery cell.

[0005] Therefore, a technology for effectively distributing the adhesive within the negative electrode is being developed, and in this case, the adhesive content is high at the interface of the current collector, thereby suppressing peeling, and at the same time, the adhesive content of the negative electrode mixture layer and the surface can be reduced, and thus the performance of the battery cell can be improved. For this reason, a technology in which a dual layer, that is, a negative electrode slurry having a high binder content is formed in a lower layer, and a negative electrode slurry having a low binder content is prepared in an upper layer, is developed, but since the phenomenon in which the adhesive particles move to the surface of the negative electrode mixture layer occurs in a general drying process, there are limitations in achieving the ideal distribution of the adhesive.

[0006] Therefore, there is a need to develop a lithium secondary battery in which the interface resistance between the current collector and the electrode mixture layer is low and the rapid charging performance is improved. SUMMARY

[0007] TECHNICAL PROBLEM

[0008] The present application aims to improve the rapid charging performance by effectively distributing the adhesive within the electrode to improve the interface adhesion between the current collector and the electrode active material layer, and to prevent peeling of the electrode and other processes, and appearance defects.

[0009] TECHNICAL SOLUTION

[0010] One embodiment of the present application provides an electrode for a secondary battery, including: a current collector; and an electrode active material layer containing an electrode active material and a binder, provided on at least one side of the current collector, the electrode active material layer including, with respect to a width direction, an a region and a b region in a direction from one side to the other side of the electrode active material layer, the a region having a higher binder content than the b region, the a region and the b region having a continuous binder content gradient at a boundary of each region.

[0011] The electrode can satisfy the following relation 1.

[0012] [Relation 1]

[0013] 1.0 < A / B < 2.5

[0014] In the relation 1, A is the binder content of the a region, and B is the binder content of the b region.

[0015] The electrode active material layer can have a ratio (B L / B H ) of a binder content (B L ) at a 20% position in a thickness direction from a surface of the current collector to a surface direction of the electrode and a binder content (B H ) at an 80% position in the thickness direction of 1.5 to 3.5.

[0016] The a region and the b region of the electrode active material layer can each include a first layer (a1 and b1) and a second layer (a2 and b2) with respect to a thickness direction from a surface of the current collector to a surface direction of the electrode, the first layer having a higher binder content than the second layer, the first layer and the second layer having a continuous binder content gradient at a boundary of each layer.

[0017] The electrode can satisfy the following relation 2.

[0018] [Relation 2]

[0019] 2.0 < A1 / A2 < 3.0 and 1.0 < B1 / B2 < 2.5

[0020] In the relation 2, A1 and A2 are binder contents of the first layer and the second layer in the a region, respectively, and B1 and B2 are binder contents of the first layer and the second layer in the b region, respectively.

[0021] The binder can include a styrene butadiene rubber (SBR) binder.

[0022] The electrode active material layer can include 0.1 to 2% by weight of the binder based on the total weight.

[0023] Another embodiment of the present application provides a method of manufacturing an electrode for a secondary battery, including: a) a step of coating a first electrode slurry and a second electrode slurry on at least one side of a current collector; and b) a step of drying the coated first electrode slurry and second electrode slurry, in the a) step, the first electrode slurry and the second electrode slurry are coated in a direction crossing from one side to the other side of the current collector based on a width direction of the current collector, while the first electrode slurry and the second electrode slurry are coated, the first electrode slurry has a higher binder content than the second electrode slurry.

[0024] In the a) step, the first electrode slurry can be coated from a position close to a surface of the current collector based on a thickness direction of the current collector, and the second electrode slurry can be coated from a position away from the surface of the current collector.

[0025] The first electrode slurry and the second electrode slurry respectively include an electrode active material and a binder, the first electrode slurry can include 1 to 5% by weight of the binder based on a solid content, and the second electrode slurry can include 0.1 to 1% by weight of the binder based on the solid content.

[0026] The b) step can be performed at a temperature of 50 to 200°C for 30 to 300 seconds.

[0027] Still another embodiment of the present application provides a secondary battery including the electrode.

[0028] Technical Effects

[0029] The present application can improve the interfacial adhesion between the current collector and the electrode active material layer, improve process defects such as electrode peeling, and improve rapid charging performance.

[0030] Also, the present application can optimize the binder composition within the electrode, thereby improving the electrode interfacial resistance without increasing the binder content and the conductive material. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1a and Figure 1b is a schematic diagram of a pattern coating core shape (1a) for coating the electrode slurry of the present application and a schematic diagram (1b) for providing a pattern coating core in the upper / lower portion;

[0032] Figure 2a and Figure 2b is a schematic diagram of a thickness direction cross-section (2a) of the electrode manufactured in Comparative Example 1 and Example 1 and a schematic diagram (2b) showing the distribution of SBR (binder). DETAILED DESCRIPTION

[0033] The advantages, features and implementation methods of the present application will become apparent from the following detailed description of the embodiments with reference to the drawings. However, the present application is not limited to the embodiments disclosed below, but can be implemented in various forms. The embodiments are provided to make the disclosure complete and to fully convey the scope of the present application to those skilled in the art to which the present application pertains. The present application is defined only by the scope of the claims. The specific contents for implementing the present application are described in detail below with reference to the drawings. The same reference numerals refer to the same elements regardless of the drawings, and "and / or" includes each of the mentioned items and all combinations of two or more of the mentioned items.

[0034] Unless otherwise defined, all terms (including technical and scientific terms) used in the present specification are used in the meaning commonly understood by those skilled in the art to which the present application pertains. In the present specification, when a certain part "includes" a certain component, unless otherwise specified, it means that other components can also be included, not excluding other components. Also, unless otherwise specified, the singular form includes the plural form.

[0035] In the present specification, when a certain part is described as being "on" or "above" another part, it includes not only the case where it is "directly on" or "directly above" the other part, but also the case where another part is interposed therebetween.

[0036] One embodiment of the present application provides an electrode for a secondary battery. The electrode is characterized by including: a current collector; and an electrode active material layer containing an electrode active material and a binder, which is provided on at least one side of the current collector, and which includes, in a width direction, an a region and a b region in a direction from one side to the other side of the electrode active material layer, the a region having a higher binder content than the b region, and the a region and the b region having a continuous binder content gradient at a boundary between the respective regions.

[0037] In the electrode active material layer, the b region having a relatively low binder content functions as a lithium ion channel, and thus the electrode content easily permeates Li-ions, diffuses to the side surface after permeation, and thus the intercalation speed of the ion conductivity can be increased. On the contrary, the a region having a relatively high binder content can improve the interface adhesion between the current collector and the electrode active material layer.

[0038] The electrode active material layer can be a structure formed by the intersection of a first a region, a first b region, a ka region, a kb region, and a na region, with the width direction as the reference, extending from one side of the electrode active material layer to the other. Here, k can be an integer from 2 to n-1, and n can be an integer from 3 to 10 or from 3 to 7. Furthermore, the width-direction length ratio of the a region to the b region can be 1:3 to 3:1, 1:2 to 2:1, or 1:1.2 to 1.2:1, and they can substantially have the same width-direction length ratio.

[0039] Regions a and b have a continuous binder content gradient at their boundaries. The first and second electrode slurries forming regions a and b are simultaneously applied at points where they intersect, based on the width direction of the current collector. This allows diffusion to the adjacent regions of the slurry and the binder contained within each slurry, resulting in a continuous binder content gradient at the interface between regions a and b. This invention can suppress cracking at the interface between regions a and b, thereby improving the width-direction durability of the electrode active material layer.

[0040] Regions a and b can satisfy the following relation 1.

[0041] [Relation 1]

[0042] 1.0

[0043] In Equation 1, A is the adhesive content in region a, and B is the adhesive content in region b.

[0044] Specifically, regions a and b can satisfy 1.5.

[0045] The electrode active material layer, with its thickness as a reference, extends from the surface of the current collector towards the surface of the electrode, and at a position where the binder content (B) is 20% of the thickness... L ) and the adhesive content at 80% of the thickness (B H The ratio of (B) L / B H The electrode active material layer can have a thickness of 1.5 to 3.5. Specifically, the electrode active material layer can satisfy 2. L / B H <3 or 2.2 L / B H ​​​​<2.8. In the present invention, even if the entire binder content of the electrode active material layer is the same as that of the prior art, the interface peeling and the like in the process / appearance can be improved and the rapid charging performance can be improved by adjusting the binder distribution in the thickness direction and the width direction of the electrode active material layer.

[0046] The electrode active material layer includes a first layer (a1 and b1) and a second layer (a2 and b2) in each of the a region and the b region with respect to the thickness direction from the surface of the current collector to the surface of the electrode. The first layer has a higher binder content than the second layer, and the first layer and the second layer can have a continuous binder content gradient at the boundary of each layer. In the present invention, even in the region (b region) in which the binder distribution is relatively small, the binder content is higher at the interface between the current collector and the electrode active material layer than at the electrode surface, and thus the electrode active material layer can improve the interface adhesion to the current collector as a whole. At the same time, the present invention employs a new process that has not been employed in the prior art, and thus the binder concentration gradient can be achieved in the width direction of the electrode as well.

[0047] The first layer and the second layer of the electrode active material layer can satisfy the following relation 2.

[0048] [Relation 2] 2.0 < A1 / A2 < 3.0 and 1.0 < B1 / B2 < 2.5

[0049] In the relation 2, A1 and A2 are the binder contents of the first layer and the second layer in the a region, and B1 and B2 are the binder contents of the first layer and the second layer in the b region.

[0050] Specifically, the electrode active material layer can satisfy 2 < A1 / A2 < 3 or 2.2 < A1 / A2 < 2.8, and can satisfy 1.5 < B1 / B2 < 2.5 or 1.8 < B1 / B2 < 2.3. Thereby, the above effects can be further improved.

[0051] The binder can include a styrene butadiene rubber (SBR) binder, for example, styrene-butyl rubber, styrene-butyl acrylate copolymer, or the like, but the present invention is not limited thereto. In this regard, the electrode active material layer can include a styrene butadiene rubber (SBR) binder.

[0052] The electrode active material layer can include 0.1 to 2% by weight, or 0.1 to 1.8% by weight, or 0.5 to 1.8% by weight, or 0.5 to 1.5% by weight of the binder based on the total weight. In the present invention, a large amount of the binder is distributed at the interface between the current collector and the active material layer, and the content of the binder is reduced at the electrode surface side, so that the total amount of the binder contained in the entire active material layer can be significantly reduced. Thereby, the interfacial adhesion between the current collector and the active material layer can be improved while the rapid charging performance is also improved.

[0053] In the case of using the SBR-based binder or the like, the binder is mixed in a particle form, and thus the viscosity of the electrode slurry is very low. Also, the size of the binder particles is configured to be a small size of 200 nm or less, and thus in the case of simultaneously coating the first and second electrode slurries and drying, the binder particles are easily diffused to the upper electrode active material layer by osmotic pressure, so that a clear upper / lower layer can not be formed at the interface between the upper and lower electrode active material layers.

[0054] In the case of the electrode being a positive electrode, the electrode active material can be used without limitation in the case of being a positive electrode active material generally used for a secondary battery. As an example, it can include any one positive electrode active material particle selected from the group consisting of LiCoO2, LiNiO2, LiMn2O4, LiCoPO4, LiFePO4, LiNiMnCoO2, and LiNi 1-x-y-z Co x M 1 y M 2 z O2(M 1 and M 2 are each independently any one selected from the group consisting of Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg, and Mo, x, y, and z are atomic fractions of oxide constituent elements, 0≤x<0.5, 0≤y<0.5, 0≤z<0.5, x+y+z≤1) or a mixture of two or more thereof.

[0055] In the case of the electrode being a negative electrode, the electrode active material can be used without limitation in the case of being a negative electrode active material generally used for a secondary battery. As an example, it can be a carbon-based negative electrode active material, a silicon-based negative electrode active material, or a mixture thereof, but is not limited thereto. The carbon-based negative electrode active material can be selected from at least one of artificial graphite, natural graphite, and hard carbon. The silicon-based negative electrode active material can be Si, SiO x(0 < x < 2), Si-Q alloys (the Q is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof, other than Si), Si-carbon composites, and mixtures thereof with SiO2.

[0056] The electrode active material layer can further include, as necessary, a conductive material, a thickening agent, or a combination thereof. The conductive material is used to impart electrical conductivity to the electrode, and is not particularly limited as long as it is an existing electronic conductive material that does not cause chemical changes within the battery. As an example, one selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fibers, carbon nanotubes, and combinations thereof can be used, but is not limited thereto. The thickening agent is used to prepare a stable slurry by imparting viscosity, and as an example, a cellulose-based compound can be used, and in particular, at least one of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or alkali metal salts thereof, or the like can be used in combination. As the alkali metal, Na, K, or Li can be used.

[0057] The electrode active material layer can include 90% by weight or more of the electrode active material with respect to the total weight, and preferably, 90 to 99.9% by weight, 95 to 99.5% by weight, or 98 to 99.5% by weight can be included, 0.1 to 2% by weight, or 0.1 to 1.8% by weight, or 0.5 to 1.8% by weight, or 0.5 to 1.5% by weight of the binder can be included, and the balance of the conductive material and the thickening agent can be included. Even if an electrode slurry with a low binder content is prepared, the interfacial adhesion of the current collector and the electrode active material layer can be improved by the migration of the binder particles at the time of drying the binder suspension, and the resistance of the electrode surface can be reduced to improve the rapid charging performance.

[0058] The current collector can use a material selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof, but is not limited thereto.

[0059] Another embodiment of the present application provides a method of preparing an electrode for a secondary battery. The method includes: a) a step of coating a first and a second electrode slurry on at least one side of a current collector; and b) a step of drying the coated first and second electrode slurries.

[0060] The a) step is characterized in that the first and second electrode slurries are coated in a direction crossing from one side to the other side of the current collector with reference to the width direction of the current collector, wherein the first and second electrode slurries are simultaneously coated, and the first electrode slurry has a higher binder content than the second electrode slurry.

[0061] In the a) step, the first electrode slurry can be applied from a position close to the surface of the current collector, and the second electrode slurry can be applied from a position away from the surface of the current collector, with reference to the thickness direction of the current collector. Figure 1a and Figure 1b are a schematic diagram of a pattern coating core shape for applying the electrode slurry of the present application (1a) and a schematic diagram of a pattern coating core provided in the upper and lower portions (1b). See Figure 1a and Figure 1b The first electrode slurry is discharged from the lower portion of the pattern coating core, and the second electrode slurry is discharged from the upper portion of the pattern coating core. In this case, the slurry discharge portion and the slurry non-discharge portion of the upper and lower pattern coating cores are formed in a cross shape with reference to the width direction, so that the discharged first and second electrode slurries can be formed in a pattern with reference to the width direction of the electrode. In this case, the binder content is higher at the interface between the current collector and the electrode active material layer than at the electrode surface, and thus not only can the interface adhesion between the entire electrode active material layer and the current collector be improved, but also the b region, which has a relatively low binder content with reference to the width direction, can function as a lithium ion channel, Li-ion easily penetrates into the electrode and diffuses to the side surface after penetration, and the insertion speed of the ion conductivity can be improved, and, on the contrary, the a region, which has a relatively high binder content, can improve the interface adhesion between the current collector and the electrode active material layer. Figure 2a and Figure 2b are a schematic diagram of a thickness direction cross-sectional view of the electrode prepared in Comparative Example 1 and Example 1 (2a) and a schematic diagram showing the distribution of SBR (binder) (2b). See Figure 2a and Figure 2b The upper and lower pattern coating cores simultaneously discharge the first and second electrode slurries to perform coating (simultaneous double coating), and thus have a continuous content gradient of the binder at the interface between the a region and the b region of the current collector with reference to the width direction, and thus can inhibit the occurrence of cracks and improve the width direction durability of the electrode active material layer. Also, a continuous content gradient of the binder is present at the interface between the first layer (a1 and b1) and the second layer (a2 and b2) with reference to the thickness direction, and thus the occurrence of cracks can also be inhibited.

[0062] The first and second electrode slurries can each independently include an electrode active material and a binder, and can also include a conductive material and a thickening agent, as necessary.

[0063] The first electrode slurry can include 1.0 to 3.0 wt% or 1.5 to 2.8 wt% of a binder, 88 to 98 wt% or 92 to 96 wt% of an electrode active material, with reference to the total amount of solid components, and can also include a conductive material and a thickening agent, as necessary.

[0064] The binder content of the first electrode slurry can be equal to or higher than that of the second electrode slurry, and thus can have a high viscosity.

[0065] The second electrode slurry can include 0 to 1.0 wt% or 0.4 to 0.6 wt% of a binder based on the total amount of solids, 88 to 98 wt% or 92 to 96 wt% of an electrode active material, and as necessary, the balance of a conductive material and a thickener.

[0066] The binder content of the second electrode slurry can be equal to or lower than that of the first electrode slurry, and thus can have a low viscosity.

[0067] The coating can be performed at room temperature, and when assembling the coating die, the patterned coating core can be assembled to be pattern-crossed. When the same amount of slurry is coated in the process of dual coating, the effect can be maximized when the lower layer having a relatively high binder content has a small load amount and the upper layer having a relatively low binder content has a high load.

[0068] The b) step is a step of drying the coated first and second electrode slurries, and can be performed at a temperature of 50 to 200°C for 30 to 300 seconds. It is preferable in the drying step that the coated first and second electrode slurries are dried at the same time.

[0069] Here, the drying can be performed for a period of 30 to 300 seconds, for example, 30 seconds or more, 40 seconds or more, 50 seconds or more, 60 seconds or more, 70 seconds or more, 80 seconds or more, or 90 seconds or more, and 300 seconds or less, 280 seconds or less, 260 seconds or less, 240 seconds or less, 220 seconds or less, 200 seconds or less, 180 seconds or less, 160 seconds or less, 150 seconds or less, 140 seconds or less, 130 seconds or less, 120 seconds or less, or 110 seconds or less. Also, the drying can be performed at a temperature of 50 to 200°C, for example, 50°C or more, 60°C or more, 70°C or more, 80°C or more, or 90°C or more, and 200°C or less, 190°C or less, 180°C or less, 170°C or less, 160°C or less, 150°C or less, 140°C or less, 130°C or less, 120°C or less, or 110°C or less. In the case of a too high drying temperature or a too short drying time, the migration of the binder particles is excessive, and thus the interface adhesion can be insufficiently achieved. As one embodiment, the b) step can be performed at a temperature of 80 to 130°C for 30 to 300 seconds.

[0070] During the drying process in step b), some of the binder particles in the first electrode slurry diffuse into the second electrode slurry, thereby improving the problem of decreased interfacial resistance and adhesion between the first and second layers of the electrode active material layer.

[0071] Next, the dried electrode is rolled to an appropriate density, thereby enabling the fabrication of an electrode with an electrode active material layer formed on the current collector. Here, the rolling process described herein, as well as known rolling densities and other rolling conditions and methods, are not required, and the present invention is not limited thereto.

[0072] Another embodiment of the present invention provides a secondary battery including electrodes.

[0073] The electrodes are as described above.

[0074] No particular limitation is made as long as the diaphragm is a diaphragm known in the art. For example, it may be selected from glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene or combinations thereof, and may be in the form of non-woven or woven fabric, and may be used selectively in a single-layer or multi-layer structure.

[0075] The electrolyte comprises a non-aqueous organic solvent and an electrolytic salt. The non-aqueous organic solvent may be ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), 1,2-dimethoxyethane (DME), γ-butyrolactone (BL), tetrahydrofuran (THF), 1,3-dioxolane (DOL), diethyl ether (DEE), methyl formate (MF), methyl propionate (MP), sulfolane (S), dimethyl sulfoxide (DMSO), acetonitrile (AN), or mixtures thereof, but is not limited thereto. The electrolytic salt is dissolved in the non-aqueous organic solvent and serves as a source of electrolytic metal ions in the battery, enabling the secondary battery to operate essentially and promoting the movement of electrolytic metal ions between the positive and negative electrodes. In a non-limiting example, when the electrolytic metal is lithium, the electrolytic salt may be LiPF6, LiBF4, LiTFSI, LiSbF6, LiAsF6, LiClO4, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, LiSbF6, LiAlO4, LiAlCl4, or LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2 (where x and y are natural numbers), LiCl, LiI, or mixtures thereof, but not limited thereto. Furthermore, the electrolytic salt may use known substances at concentrations appropriate for the purpose, and may also include, as needed, known solvents or additives for improving charge / discharge characteristics, flame retardant properties, etc.

[0076] The method for manufacturing the lithium secondary battery according to the present application for achieving the above object can be a method in which the prepared negative electrode, separator, and positive electrode are sequentially stacked to form an electrode assembly, and the prepared electrode assembly is put into a cylindrical battery case or an angular battery case, and then an electrolyte is injected to manufacture the battery. Alternatively, the electrode assembly can be stacked, and then the resultant is immersed in the electrolyte to obtain a final product, and then the final product is put into the battery case to be sealed to manufacture the battery.

[0077] The battery case used in the present application can be a case generally used in the art, and the shape thereof is not limited by the use of the battery, and for example, can be cylindrical, angular, pouch-shaped, coin-shaped, or the like using a can.

[0078] The lithium secondary battery according to the present application can be not only used as a battery cell used as a power source for a small device, but also preferably used as a unit battery of a medium-large battery module including a plurality of battery cells. Preferred examples of the medium-large device can include an electric vehicle, a hybrid vehicle, a plug-in hybrid vehicle, an electric power storage system, and the like, but are not limited thereto.

[0079] Hereinafter, the present application will be described in detail through examples, but these are for further detailed explanation of the present application, and the scope of the rights of the present application is not limited to the following examples.

[0080] Preparation Example

[0081] (Preparation Example 1) Preparation of 1st Negative Electrode Slurry

[0082] A 1st negative electrode slurry having a viscosity of 6,000 cps was prepared by adding a negative electrode active material using artificial graphite alone, a conductive material, a CMC thickener, and an SBR binder to water in a weight ratio of 95:1.0:1.2:2.8.

[0083] (Preparation Example 2) Preparation of 2nd Negative Electrode Slurry

[0084] A 2nd negative electrode slurry having a viscosity of 6,000 cps was prepared by adding a negative electrode active material using artificial graphite alone, a conductive material, a CMC thickener, and an SBR binder to water in a weight ratio of 97.6:1.0:1.2:0.2.

[0085] (Preparation Example 3) Preparation of 3rd Negative Electrode Slurry

[0086] A 1st negative electrode slurry having a viscosity of 6,000 cps was prepared by adding a negative electrode active material using artificial graphite alone, a conductive material, a CMC thickener, and an SBR binder to water in a weight ratio of 96.3:1.0:1.2:1.5.

[0087] Example

[0088] (Example 1)

[0089] The prepared first and second negative electrode slurries were applied to one face of a copper current collector (copper foil having a thickness of 6 μm) by a slot die coater at 10.6 mg / cm 2 The first and second slurries were applied to one face of a copper current collector (copper foil having a thickness of 6 μm) by a slot die coater at 10.6 mg / cm Figure 2b Figure 1a The first and second slurries were applied to one face of a copper current collector (copper foil having a thickness of 6 μm) by a slot die coater at 10.6 mg / cm

[0090] The dried negative electrode was calendered (calendering density: 1.68 g / cm 3 A negative electrode in which a negative electrode active material layer was formed on a current collector was produced.

[0091] In this case, the components of the solid portion of the negative electrode active material layer in the produced negative electrode were 96.3 wt% of the negative electrode active material, 1.0 wt% of the conductive material, 1.5 wt% of the SBR binder, and 1.2 wt% of the CMC thickener. Also, the produced negative electrode was formed to have a copper foil thickness of 6 μm and a negative electrode active material layer thickness of 132 μm, and it was confirmed from the SEM image that the boundary between the binder layer and the negative electrode active material layer was not clearly distinguished, and it was formed as one negative electrode active material layer.

[0092] A slurry was prepared by mixing Li[Ni 0.88 Co 0.1 Mn 0.02 ]O2 as a positive electrode active material, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 96.5:2:1.5. The slurry was uniformly applied to an aluminum foil having a thickness of 12 μm, vacuum-dried, and a positive electrode for a secondary battery was produced.

[0093] The positive electrode and the negative electrode were notched to predetermined sizes and stacked with a separator (polyethylene, thickness of 13 μm) interposed therebetween to form an electrode unit, and the tab portions of the positive electrode and the negative electrode were welded, respectively. The assembly of the welded positive electrode / separator / negative electrode was put in a pouch, and three faces other than the face for injecting electrolyte were sealed. In this case, the portion having the electrode tab was included in the sealed portion. After injecting electrolyte into the remaining face other than the sealed portion and sealing the remaining face, the assembly was immersed for 12 hours or more. An electrolyte in which 1M LiPF6 was dissolved in a mixed solvent of EC / EMC / DEC (25 / 45 / 30; volume ratio) and 1 wt% of vinylene carbonate (VC), 0.5 wt% of 1,3-propane sultone (PRS), and 0.5 wt% of lithium bis(oxalato)borate (LiBOB) were further added was used.

[0094] ​After that, pre-charging was performed for 36 minutes at a current equivalent to 0.25C. After degasing for 1 hour, aging treatment was performed for 24 hours or more, and then formation charge-discharge (charge condition CC-CV 0.2C 4.2V 0.05C CUT-OFF, discharge condition CC 0.2C 2.5V CUT-OFF) was performed. After that, standard charge-discharge (charge condition CC-CV 0.33C 4.2V 0.05C CUT-OFF, discharge condition CC 0.33C 2.5V CUT-OFF) was performed to prepare a secondary battery.

[0095] (Comparative Example 1)

[0096] The prepared 3rd negative electrode slurry was coated on one side of a copper current collector (copper foil having a thickness of 6 μm) by a slot die coater. The coating core of the slot die coater used a general coating core in which the discharge portion was open (the non-discharge portion of the upper and lower layers of the slurry was changed to the discharge portion). Other than this, the negative electrode, the positive electrode, and the secondary battery were prepared in the same manner as in Example 1. Figure 1b

[0097] (Comparative Example 2)

[0098] The prepared 1st and 2nd negative electrode slurries were coated on one side of a copper current collector (copper foil having a thickness of 6 μm) by a slot die coater using a multilayer simultaneous mold coating method. The coating core of the slot die coater used a general coating core in which the discharge portion was open (the non-discharge portion of the upper and lower layers of the slurry was changed to the discharge portion). Other than this, the negative electrode, the positive electrode, and the secondary battery were prepared in the same manner as in Example 1. Figure 1b

[0099] Evaluation Example

[0100] [Example 1]: SBR distribution analysis by SEM-EDS analysis

[0101] In order to evaluate the binder content at each negative electrode position of the examples and the comparative examples, the binder content of the 1st layer (current collector surface, al, bl) and the 2nd layer (electrode surface, a2, b2) was analyzed by SEM-EDS analysis, and the binder content in the width direction (a, b) was analyzed by TGA analysis.

[0102] [Table 1]

[0103]

[0104] (The respective values of the binder distribution in the width direction and the thickness direction in the table 1 are weight ratios)

[0105] ​​In Example 1, it was confirmed that the adhesive concentration gradient occurred in both the width direction and the thickness direction when the coating method of the present application was applied. On the contrary, it was analyzed that in Comparative Example 1, during the process of drying the coated slurry when preparing the negative electrode, as the adhesive particles moved to the surface of the negative electrode active material layer, the thickness direction adhesive gradient was generated, and the adhesive concentration in the width direction was constant, without the concentration gradient. In addition, in the case of Comparative Example 2, the simultaneous die coating (double coating) method was used, but the adhesive concentration gradient was not achieved in the width direction.

[0106] [Example 2]: Performance verification comparison after preparation of electrode unit

[0107] The adhesion of the negative electrode prepared in Example 1 and Comparative Examples 1 to 2 and the resistance of the secondary battery were analyzed, and the results are shown in Table 2 below.

[0108] (Evaluation of electrode adhesion)

[0109] The electrode adhesion was measured using an adhesion measuring device (IMADA Z Link 3.1) according to the measurement standard (after attaching the electrode to a tape, the force when peeled off at an angle of 90° was measured).

[0110] (Evaluation of interface resistance)

[0111] The interface resistance between the negative electrode and the current collector was measured using an interface specific resistance value measuring device (XF057 of Hioki) for the prepared negative electrode.

[0112] (Evaluation of cell resistance)

[0113] The direct current resistance of the prepared secondary battery was calculated from the current difference and voltage difference when different currents were applied, and the voltage measurement result after resting for 30 minutes in the charged state SOC 50 was calculated as V0, and the value V1 after discharging at a constant current for 10 seconds, and the direct current resistance was calculated using Ohm's law AR=AV(V1-V0) / AI.

[0114] [Table 2]

[0115]

[0116] For the adhesive content of the negative electrode of Example 1, the adhesive content at the interface between the current collector and the electrode active material layer was higher than that on the electrode surface, so the interface adhesion between the electrode active material layer as a whole and the current collector was improved compared to Comparative Example 1. Also, the adhesive content in the width direction of the negative electrode was not uniform, so Li-ion easily penetrated into the electrode content, and after penetration, diffused to the side surface, and the insertion speed of the ion conductivity could be increased, so it could be confirmed that the cell resistance of the secondary battery was significantly improved compared to Comparative Example 2.

[0117] The above described embodiments of the present application are merely illustrative of the many possible specific arrangements that would not depart from the scope of the present application. It should be understood that the present application is not limited to the described embodiments, and that many modifications and variations are possible without departing from the spirit or scope of the present application. Accordingly, it is therefore intended that the present application embrace all such arrangements as come within the scope of the appended claims, and that the present application be constructed as broadly as legally possible.

Claims

1. An electrode for a secondary battery, comprising: Current collector; as well as An electrode active material layer containing electrode active material and binder is located on at least one side of the current collector. The electrode active material layer, with its width as a reference, extends from one side to the other, intersecting with regions a and b. Region a has a higher adhesive content than region b. The boundaries of regions a and b have a continuous gradient of adhesive content. Regions a and b of the electrode active material layer, with the thickness direction as a reference, extend from the surface of the current collector to the surface of the electrode, and include a first layer and a second layer. The first layer is composed of regions a1 and b1 that are staggered along the width direction, and the second layer is composed of regions a2 and b2 that are staggered along the width direction. The first layer has a higher binder content than the second layer, and the first and second layers have a continuous binder content gradient at the boundaries of each layer. Furthermore, the electrodes for the secondary battery satisfy the following relationship 2: [Relation 2] 2.0 < A1 / A2 < 3.0 and 1.0 < B1 / B2 < 2.5 In Equation 2, A1 and A2 are the adhesive contents of the first and second layers in region a, respectively, and B1 and B2 are the adhesive contents of the first and second layers in region b, respectively.

2. The electrode for a secondary battery according to claim 1, wherein: The electrodes for the secondary battery satisfy the following relationship 1: [Relation 1] 1.0 < A / B < 2.5 In Equation 1, A is the adhesive content in region a, and B is the adhesive content in region b.

3. The electrode for a secondary battery according to claim 1, wherein: The electrode active material layer extends from the surface of the current collector to the surface of the electrode, with the thickness direction as a reference. Adhesive content B at 20% thickness L Adhesive content B at 80% thickness H Ratio B L / B H It has a value of 1.5 to 3.

5.

4. The electrode for a secondary battery according to claim 1, wherein: The adhesive includes styrene-butadiene rubber (SBR) based adhesives.

5. The electrode for a secondary battery according to claim 1, wherein: The electrode active material layer comprises 0.1 to 2% by weight of the adhesive.

6. A method for preparing an electrode for a secondary battery according to claim 1, comprising: a) The step of coating at least one side of the current collector with a first electrode paste and a second electrode paste; as well as b) The step of drying the coated first electrode paste and the second electrode paste. In step a), taking the width direction of the current collector as a reference, the first electrode paste and the second electrode paste are applied crosswise from one side of the current collector to the other side, wherein the first electrode paste and the second electrode paste are applied simultaneously. The first electrode paste has a higher binder content than the second electrode paste; Furthermore, in step a), the first electrode paste is applied from a position close to the surface of the current collector, and the second electrode paste is applied from a position away from the surface of the current collector, with the thickness direction of the current collector as the reference.

7. The method for preparing an electrode for a secondary battery according to claim 6, wherein: The first electrode slurry and the second electrode slurry each separately comprise an electrode active material and a binder. The first electrode paste includes 1 to 5% by weight of binder based on the solid portion, and the second electrode paste includes 0.1 to 1% by weight of binder based on the solid portion.

8. The method for preparing an electrode for a secondary battery according to claim 6, wherein: Step b) is performed at a temperature of 50 to 200°C for 30 to 300 seconds.

9. A secondary battery comprising the electrode according to any one of claims 1-5.

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