Electrode for secondary battery having improved rapid charging performance, method for preparing same, and secondary battery comprising same

Abstract

The present invention relates to an electrode for a secondary battery, a method of manufacturing the same, and a secondary battery including the electrode.SOLUTION: The electrode includes a current collector and an electrode active material layer disposed on at least one surface of the current collector, and satisfies Relational Expression 1.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrode for a secondary battery having improved rapid charging performance, a method for producing the same, and a secondary battery including the same. [Background technology]

[0002] Recently, in the case of large cells for high-energy EVs, when the cells are charged at high current density due to the high density of the electrodes, there is a limit to the diffusion of Li ions into the negative electrode, and in this case, Li ions are deposited on the surface of the negative electrode, causing cell deterioration.

[0003] To improve this problem, it is important to keep the density of the negative electrode as low as possible or to reduce the resistance on the surface and inside of the negative electrode so that Li ions can diffuse quickly into the inside of the negative electrode.

[0004] To improve cell resistance and fast charging performance, a technology is being developed to develop a binder with high adhesive strength and use it to reduce the binder content. However, there are limitations to the types of binders with high adhesive strength and the extent to which the binder content can be reduced. If the binder content is reduced too much, serious problems occur, such as detachment of the electrode mixture layer from the current collector during the notching process and the cell charge / discharge process.

[0005] Therefore, technologies for efficiently distributing the binder within the negative electrode are being developed, which would result in a high binder content at the interface of the current collector, suppressing detachment, and reducing the binder content in the negative electrode mixture layer and surface, thereby improving cell performance.To this end, technologies have been developed that form a dual layer, with a negative electrode slurry having a high binder content in the lower layer and a negative electrode slurry having a low binder content in the upper layer.However, there are still limitations to achieving ideal binder distribution due to the phenomenon of binder particles migrating to the surface of the negative electrode mixture layer during the typical drying process.

[0006] In addition, in order to improve adhesion between the negative electrode current collector and the surface and maximize electrical contact, a technology has been developed in which a substrate is prepared in which a conductive agent such as carbon black or CNT is coated on the current collector, and a negative electrode mixture layer is formed on top of this current collector, but this is not effective enough in improving cell performance.

[0007] Therefore, there is a demand for the development of a lithium secondary battery that has a low interfacial resistivity between the current collector and the electrode mixture layer and has improved rapid charging performance. Summary of the Invention [Problem to be solved by the invention]

[0008] The electrode for a lithium secondary battery of the present invention aims to improve the interfacial adhesion between the current collector and the electrode active material layer, prevent process defects and appearance defects such as electrode detachment, and improve rapid charging performance.

[0009] The method for producing an electrode for a lithium secondary battery of the present invention aims to produce an electrode that has improved interfacial adhesion, prevents defects in the process and appearance, and allows rapid charging. [Means for solving the problem]

[0010] An embodiment of the present invention provides an electrode for a secondary battery, which includes a current collector and an electrode active material layer located on at least one surface of the current collector, and satisfies the following Relational Formula 1: [Equation 1] t2≦t1≦8×t2 In the above Relational Formula 1, t1 is the thickness of the electrode active material layer other than the current collector closer to the current collector based on the position where the electrode active material layer is separated inside the electrode active material layer when measuring the 90° bending adhesive strength of the electrode, and t2 is the particle diameter (D50) of the electrode active material contained in the electrode active material layer.

[0011] The electrode may further satisfy the following relational expression 2: [Equation 2] 1.5×t1≦t1≦5×t2 In the above Relational Formula 2, t1 is the thickness of the electrode active material layer other than the current collector closer to the current collector based on the position where the electrode active material layer is separated inside the electrode active material layer when measuring the 90° bending adhesive strength of the electrode, and t2 is the particle diameter (D50) of the electrode active material contained in the electrode active material layer.

[0012] The electrode active material layer may include a styrene butadiene rubber (SBR)-based binder.

[0013] The electrode active material layer may contain the binder in an amount of 0.1 to 2% by weight based on the total weight.

[0014] The electrode may further satisfy the following relational expression 3: [Equation 3] 0.25≦b2 / b1<0.7 In the above Relational Formula 3, when measuring the binder distribution in the thickness direction of the electrode active material layer, b1 is the weight of the binder in the entire electrode active material layer, and b2 is the weight of the binder in a region from the current collector that is 15% of the total thickness of the electrode active material layer.

[0015] The electrode may further satisfy the following relational expression 4: [Equation 4] 0.3≦b2 / b1<0.5 In the above-mentioned Relational Formula 4, when measuring the binder distribution in the thickness direction of the electrode active material layer, b1 is the weight of the binder in the entire electrode active material layer, and b2 is the weight of the binder in a region from the current collector that is 15% of the total thickness of the electrode active material layer.

[0016] The electrode may have a continuous binder concentration in the thickness direction of the electrode.

[0017] The electrode may further satisfy the following relational expression 5: [Equation 5] -30%≦(CD) / D≦+30% In the above-mentioned Relational Formula 5, C is the interfacial adhesive strength between the current collector and the electrode active material layer measured at any position selected in the width direction of the electrode active material layer, and D is the average value of the interfacial adhesive strength between the current collector and the electrode active material layer.

[0018] The electrode may be a negative electrode.

[0019] Another embodiment of the present invention provides a method for manufacturing an electrode for a secondary battery, comprising: a) applying a binder suspension to at least one surface of a current collector; b) applying an electrode slurry containing an electrode active material on top of the binder suspension; and c) drying a resultant of step b), wherein steps a) and b) are performed simultaneously or sequentially.

[0020] The step a) may involve uniformly applying the binder suspension onto one surface of the current collector.

[0021] In the step a), the binder suspension may be applied to a thickness of 0.1 to 10 μm.

[0022] The binder suspension may contain 30% by weight or more of a binder based on the total weight of solids, and the electrode slurry may contain 2% by weight or less of a binder based on the total weight of solids.

[0023] The step c) may be carried out at a temperature of 50 to 200° C. for 30 to 300 seconds.

[0024] In yet another embodiment, there is provided an electrode for a secondary battery manufactured by a method including: a) applying a binder suspension to at least one surface of a current collector; b) applying an electrode slurry containing an electrode active material on top of the binder suspension; and c) drying a resultant of step b), wherein steps a) and b) are performed simultaneously or sequentially.

[0025] In yet another embodiment, a lithium secondary battery is provided, including the electrode, a separator, and an electrolyte. [Effects of the Invention]

[0026] According to the present invention, the interfacial adhesion between the current collector and the electrode active material layer is improved, which reduces process defects such as electrode detachment and improves fast charging performance. [Brief explanation of the drawings]

[0027] [Figure 1] 1 shows SEM images of cross sections in the thickness direction of negative electrodes prepared in Example 1 and Comparative Example 6. [Figure 2] 1 shows SEM images of cross sections in the thickness direction of negative electrodes prepared in Example 1 and Comparative Example 6. [Figure 3] FIG. 2 is a diagram showing the results of EDS mapping in the thickness direction of the negative electrode prepared in Example 1. [Figure 4] FIG. 2 is a diagram showing the results of EDS mapping in the thickness direction of the negative electrode produced in Example 1. [Figure 5] 1 is a schematic diagram showing the state in which the current collector and the negative electrode active material layer are separated when the 90° bending adhesive strength is measured for the negative electrodes produced in Examples 1 to 4 and Comparative Examples 1 to 6. FIG. [Figure 6a] 1 is a photograph showing a method for evaluating the interfacial adhesive strength (90° bending adhesive strength) between an electrode active material layer and a current collector. [Figure 6b] 1 is a photograph showing a method for evaluating the interfacial adhesive strength (90° bending adhesive strength) between an electrode active material layer and a current collector. [Figure 6c] 1 is a photograph of the negative electrodes according to Example 1 and Comparative Example 1, which were separated after measuring the 90° bending adhesive strength. [Figure 7] 1 is a graph showing the interfacial adhesive strength between a current collector and a negative electrode active material layer measured at different positions in the width direction of the negative electrode active material layer, and a graph showing the distribution (%) of the adhesive strength. [Figure 8] 1 is a graph showing the interfacial adhesive strength between a current collector and a negative electrode active material layer measured at different positions in the width direction of the negative electrode active material layer, and a graph showing the distribution (%) of the adhesive strength. DETAILED DESCRIPTION OF THE INVENTION

[0028] The advantages and features of the present invention, as well as methods for achieving them, will become clearer with reference to the following detailed description of the embodiments in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various different forms. However, these embodiments are provided to fully disclose the present invention and fully convey the scope of the invention to those skilled in the art to which the present invention pertains. The present invention is defined only by the scope of the claims. Specific details for implementing the present invention will be described in detail below with reference to the accompanying drawings. The same element numbers refer to the same components regardless of the drawing, and "and / or" includes each and every combination of one or more of the referenced items.

[0029] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used in the same manner as commonly understood by a person of ordinary skill in the art to which this invention pertains. When a part of the entire specification "comprises" a certain element, this does not mean that it may further include other elements, but does not exclude other elements, unless otherwise specified. In addition, the singular form includes the plural form unless otherwise specified in the phrase.

[0030] In this specification, when a layer, film, region, plate, or other part is said to be "on" or "on top of" another part, this includes not only the case where it is "directly on top of" the other part, but also the case where there is another part in between.

[0031] An embodiment of the present invention provides an electrode for a lithium secondary battery, the electrode for a lithium secondary battery including a current collector and an electrode active material layer disposed on at least one surface of the current collector, and satisfying the following Relational Formula 1:

[0032] [Equation 1] t2≦t1≦8×t2

[0033] In the above Relational Formula 1, t1 is the thickness of the electrode active material layer other than the current collector closer to the current collector based on the position where the electrode active material layer is separated inside the electrode active material layer when measuring the 90° bending adhesive strength of the electrode, and t2 is the particle diameter (D50) of the electrode active material contained in the electrode active material layer.

[0034] The electrode active material layer may be formed by coating a binder suspension on at least one surface of the current collector, and then coating an electrode slurry on the coated binder suspension, or by simultaneously coating a binder suspension and an electrode slurry on at least one surface of the current collector and drying the resulting mixture.

[0035] The binder suspension may be prepared by including a binder and a solvent. The term "suspension" refers to a mixture in which the binder is not dissolved and exists in particulate form in the solvent, and may be further mixed with a thickener, a conductive material, etc., as needed.

[0036] The binder may include a styrene butadiene rubber (SBR)-based binder, such as, but not limited to, styrene-butadiene, styrene-butadiene acrylate copolymer, etc. Therefore, the electrode active material layer may include a styrene butadiene rubber (SBR)-based binder.

[0037] The electrode active material layer may contain the binder in an amount of 0.1 to 2 wt %, 0.1 to 1.8 wt %, 0.5 to 1.8 wt %, or 0.5 to 1.5 wt %, based on the total weight. In the present invention, by distributing a large amount of binder at the interface between the current collector and the active material layer and reducing the binder content on the surface side of the electrode, the total amount of binder contained in the entire active material layer can be significantly reduced. This can improve the interfacial adhesion between the current collector and the active material layer and also improve fast charging performance.

[0038] When using SBR-based binders, the binder is mixed in particulate form, resulting in a very low viscosity of the binder suspension. Furthermore, because the binder particles are small (200 nm or less), when the upper electrode slurry is applied and dried, the binder particles easily diffuse into the upper electrode active material layer due to osmotic pressure, eliminating the need for a solid binder layer between the current collector and the active material layer after drying. Furthermore, the binder spreads well on the current collector, allowing it to be applied uniformly at a relatively thin thickness without forming separate patterns across the width of the current collector, thereby improving the adhesion between the current collector and the electrode active material layer. Meanwhile, polyacrylic acid (PAA), polyvinylidene fluoride (PVdF), carboxymethyl cellulose (CMC), and other electrode binders can be used in addition to SBR-based binders. Unlike SBR-based binders, these binders are applied in a solvent and undergo phase separation during electrode drying after the solvent has fully dried, forming a binder layer. Therefore, the binder does not easily migrate into the upper electrode active material layer during drying, and a reliable binder layer can be formed between the current collector and the active material layer. Furthermore, the binder is not uniformly distributed across the width of the current collector, forming patterns (e.g., island type or dot type), resulting in poor adhesion and interfacial resistivity between the current collector and the active material layer.

[0039] The solvent may be at least one selected from the group consisting of water, pure water, deionized water, distilled water, ethanol, isopropanol, methanol, acetone, n-propanol, and t-butanol, but is not limited thereto.

[0040] The binder suspension may further include a thickener to provide viscosity and produce a stable solution. For example, the thickener may be a cellulose-based compound, specifically, carboxymethylcellulose, hydroxypropylmethylcellulose, methylcellulose, or an alkali metal salt thereof, and the like, or a mixture of two or more of these. The alkali metal may be sodium, potassium, or lithium.

[0041] The conductive material is used to impart conductivity to the electrode and is not particularly limited as long as it is a conventional electron-conductive material that does not cause a chemical change in the battery, and examples thereof include, but are not limited to, natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanotube, and combinations thereof.

[0042] The current collector may be, but is not limited to, 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.

[0043] The viscosity of the binder suspension may be 1 to 10,000 cps, 5 to 5,000 cps, or 10 to 2,000 cps. When a binder suspension with such a viscosity is used, the binder suspension can be uniformly applied onto the current collector, and the binder particles can migrate well to the top during the drying process.

[0044] The electrode active material layer may be formed by applying a binder suspension and then applying an electrode slurry on the applied binder suspension, or by simultaneously applying the binder suspension as a lower layer and the electrode slurry as an upper layer.

[0045] When the electrode is a positive electrode, the electrode active material can be any positive electrode active material generally used in secondary batteries, such as 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 2is any one selected independently from the group consisting of Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg, and Mo, and x, y, and z are independently, as atomic fractions of oxide composition elements, 0 ≦ x < 0.5, 0 ≦ y < 0.5, 0 ≦ z < 0.5, and x + y + z ≦ 1), and can include any one of the positive electrode active material particles or a mixture of two or more of these.

[0046] When the electrode is a negative electrode, the electrode active material can be used without limitation as long as it is a negative electrode active material usually used in 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 one or more selected from artificial graphite, natural graphite, and hard carbon. The silicon-based negative electrode active material is Si, SiO x (0 < x < 2), Si-Q alloy (where Q 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 Group 15 element, a Group 16 element, a transition metal, a rare earth element, and a combination thereof and is not Si), Si-carbon composite, or a mixture of at least one of these and SiO2.

[0047] The electrode slurry can further include a conductive material, a binder, a thickener, or a combination thereof, as needed. The conductive material and the thickener may use the substances used in the above binder suspension, and may be the same as or different from this, but the present invention is not limited thereto.

[0048] The electrode slurry may contain the electrode active material in an amount of 90 wt % or more, preferably 90 to 99.5 wt %, 95 to 99.5 wt %, or 98 to 99.5 wt %, based on the total solid content, and may contain the binder in an amount of 2.0 wt % or less, 1.5 wt % or less, 1.0 wt % or less, or none, with the conductive material and thickener making up the remainder. Even when the binder content of the electrode slurry is low, the migration of binder particles during drying of the binder suspension can increase the interfacial adhesion between the current collector and the electrode active material layer, reducing the resistance of the electrode surface and improving fast charging performance.

[0049] The electrode for a lithium secondary battery according to an embodiment of the present invention is characterized in that it satisfies the following Relational Formula 1 when measuring the 90° bending adhesive strength of the electrode.

[0050] Meanwhile, the evaluation result of the 90° bending adhesive strength of the electrode may be such that at least 90 or 95 electrodes out of 100 manufactured electrode samples satisfy the following relational expression 1.

[0051] [Equation 1] t2≦t1≦8×t2

[0052] In the above Relational Formula 1, t1 is the thickness of the electrode active material layer other than the current collector closer to the current collector based on the position where the electrode active material layer is separated inside the electrode active material layer when measuring the 90° bending adhesive strength of the electrode, and t2 is the particle diameter (D50) of the electrode active material contained in the electrode active material layer.

[0053] In the relational expression 1, the electrode active material may be one type of electrode active material having the same particle size, and t2 may be the particle size (D50) of the electrode active material.

[0054] In the above Relational Formula 1, the electrode active material may be a mixture of two or more electrode active materials having different particle sizes, and t2 may be the particle size (D50) of one type of electrode active material contained by the highest weight among the mixed electrode active materials.

[0055] In the above Relational Formula 1, the electrode active material is a mixture of two or more electrode active materials having different particle sizes, and t2 may be the particle size (D50) of the large particle size electrode active material among the mixed electrode active materials. In this case, the large particle size electrode active material may refer to the electrode active material having the largest particle size among the two or more electrode active materials having different particle sizes.

[0056] More specifically, in the above relational expression 1, t2≦t1≦7×t2, t2≦t1≦6×t2, t2≦t1≦5×t2, t2≦t1≦4×t2, t2≦t1≦3×t2 or t2≦t1≦2×t2 may be satisfied, or 1.5×t2≦t1≦7×t2, 1.5×t2≦t1≦6×t2 or 1.5×t2≦t1≦5.5×t2 may be satisfied.

[0057] Therefore, in the electrode of the present invention that satisfies Relational Formula 1, separation may occur within the electrode active material layer when measuring the 90° bending adhesion strength of the electrode. Specifically, for example, separation may occur at a position corresponding to the thickness of the median particle size (D50) of the electrode active material particles from the current collector, or at a position corresponding to a thickness five times the median particle size (D50) when viewed in the thickness direction of the electrode. In other words, when measuring the 90° bending adhesion strength of the electrode, the position at which separation occurs within the electrode active material layer may be a position where the thickness of the electrode active material layer closest to the current collector is t2 to 5 × t2.

[0058] Specifically, the electrode active material layer can be understood as a single layer in which active material particles are uniformly arranged in the width direction (the thickness of the layer is D50 of the electrode active material), and multiple such layers are stacked in the thickness direction. That is, in the present invention, when measuring the 90° adhesive force, separation occurs at a position between the thickness of one active material particle (D50) and the thickness of five particle units (5 x D50), which is analyzed to be the reason for the above result.

[0059] As a result, the electrode of the present invention can improve process / appearance defects such as interface detachment of the negative electrode and improve fast charging performance even when the binder content in the negative electrode slurry forming the negative electrode active material layer is significantly reduced compared to conventional methods.

[0060] The electrode may further satisfy the following relational expression 2, which can further improve the above-mentioned effects.

[0061] [Equation 2] 1.5×t1≦t1≦5×t2

[0062] In the above Relational Formula 2, t1 is the thickness of the electrode active material layer other than the current collector closer to the current collector based on the position where the electrode active material layer is separated inside the electrode active material layer when measuring the 90° bending adhesive strength of the electrode, and t2 is the particle diameter (D50) of the electrode active material contained in the electrode active material layer.

[0063] More specifically, in the relational expression 2, 1.5×t2≦t1≦4×t2, 1.5×t2≦t1≦3×t2, or 1.5×t2≦t1≦2×t2 may be satisfied.

[0064] Meanwhile, the electrode active material layer may be formed by coating a binder suspension on at least one surface of the current collector and coating an electrode slurry on the coated binder suspension, or by simultaneously coating a binder suspension on at least one surface of the current collector and coating an electrode slurry on the binder suspension, and then drying the resultant.

[0065] Meanwhile, in the present invention, the particle diameter (D50) of the electrode active material may be, but is not limited to, 1 to 20 μm, 3 to 15 μm, 7 to 15 μm, or 9 to 15 μm. The particle diameter (D50) may refer to the particle diameter at which the cumulative volume becomes 50% from the smallest particle size when particle size distribution is measured using a laser scattering method. Here, D50 can be measured by collecting a sample in accordance with KS A ISO 13320-1 standard and measuring the particle size distribution using a Malvern Mastersizer 3000. Specifically, the volume density can be measured after dispersing the material using ethanol as a solvent and, if necessary, using an ultrasonic disperser.

[0066] The electrode may further satisfy the following relational expression 3:

[0067] [Equation 3] 0.25≦b2 / b1<0.7

[0068] In the above Relational Formula 3, when measuring the binder distribution in the thickness direction of the electrode active material layer, b1 is the weight of the binder in the entire electrode active material layer, and b2 is the weight of the binder in a region from the current collector that is 15% of the total thickness of the electrode active material layer.

[0069] In the above Relational Formula 3, when a styrene butadiene rubber (SBR)-based binder is used as the binder, the b2 / b1 binder weight ratio may be expressed as the content (at%) of Os element by adsorbing Os gas into the binder. However, this is not limited to Os element, and may be an element that can represent the binder depending on the type of binder.

[0070] Specifically, it may be 0.25≦b2 / b1<0.6, or 0.3≦b2 / b1<0.6.

[0071] The electrode may further satisfy the following relational expression 4, in which case the above-mentioned effects can be further improved.

[0072] [Equation 4] 0.3≦b2 / b1<0.5

[0073] In the above-mentioned Relational Formula 4, when measuring the binder distribution in the thickness direction of the electrode active material layer, b1 is the weight of the binder in the entire electrode active material layer, and b2 is the weight of the binder in a region from the current collector that is 15% of the total thickness of the electrode active material layer.

[0074] The region from the current collector to 15% of the total thickness of the electrode active material layer may contain the binder in an amount of 0.2 to 7 wt %, 1 to 5 wt %, or 1 to 4.5 wt %, relative to the total weight of the solid content of the region.

[0075] The electrode active material layer is characterized in that the binder has a continuous concentration in the thickness direction of the electrode.

[0076] In this specification, the term "continuously" distributing the binder may mean that the binder suspension and the electrode slurry are not formed as separate, discrete layers, but that the binder is continuous and uninterrupted in the electrode active material layer, and thus the binder concentration (wt%) is continuous in the thickness direction of the electrode active material layer. Meanwhile, the term "binder concentration" may mean the binder content (wt%) relative to the total weight of the electrode active material layer per unit volume (cross-sectional area in the width direction × unit thickness) of the electrode active material layer.

[0077] More specifically, binder particle migration occurs from the interface with the current collector in the electrode active material layer to specific regions of the active material layer, allowing the binder to exist in a densely distributed and continuous state in those regions. However, in regions of the electrode active material layer where binder particle migration does not occur from the interface, trace amounts of binder contained in the electrode slurry may be present, allowing the binder to be distributed at a relatively high concentration in regions where binder migration occurs. That is, the binder particles in the binder suspension applied to the top of the current collector can migrate to exist in a continuous concentration in specific regions of the electrode active material layer.

[0078] As a specific example, when the interface between the current collector and the electrode active material layer is regarded as 0% of the thickness, the binder may be located on the current collector side, and may have a continuous binder concentration in the thickness direction of the electrode in a region of 0 to 35%, 0 to 55%, 0 to 75%, 0 to 95%, or 0 to 100% (entire electrode active material layer) of the total thickness of the electrode active material layer.

[0079] This makes it possible to increase the interfacial adhesion between the current collector and the electrode active material layer, as the adhesion (interface adhesion) is stronger than the cohesion (internal aggregation) inside the negative electrode active material layer. This improves the speed of lithium ion insertion / extraction inside the electrode active material layer, thereby improving the rapid charging performance of the battery.

[0080] The electrode may further satisfy the following relational expression 5:

[0081] [Equation 5] -30%≦(CD) / D≦+30%

[0082] In the above-mentioned Relational Formula 5, C is the interfacial adhesive strength between the current collector and the electrode active material layer measured at any position selected in the width direction of the electrode active material layer, and D is the average value of the interfacial adhesive strength between the current collector and the electrode active material layer.

[0083] Specifically, it may be -25%≦(CD) / D≦+25%, or -20%≦(CD) / D≦+20%, or -15%≦(CD) / D≦+15%, or -10%≦(CD) / D≦+10%. In Relational Formula 5, C may be measured at positions spaced apart at predetermined intervals in the width direction of the electrode active material layer, for example, at intervals of 0.1 to 0.5 mm, or 0.2 to 0.3 mm, for example, 0.25 mm, but the invention is not limited thereto.

[0084] According to one embodiment, the electrode may be a negative electrode.

[0085] Therefore, in the present invention, by using the binder suspension, specifically a suspension containing a specific binder such as SBR, a high-content binder suspension can be uniformly applied to a relatively thin thickness in the width direction of the current collector without forming a separate pattern, and the binder particles can migrate to specific regions of the electrode active material layer, thereby further improving the adhesive strength between the current collector and the electrode active material layer.

[0086] The electrode has an interface resistivity between the current collector and the electrode active material layer of 0.1 Ωcm 2 or less, or 0.05Ωcm 2 or less than 0.03Ωcm 2 The above-described effects of the present invention make it possible to significantly reduce the interface resistivity.

[0087] Another embodiment of the present invention provides a method for manufacturing an electrode for a lithium secondary battery, comprising: a) applying a binder suspension to at least one surface of a current collector; b) applying an electrode slurry containing an electrode active material on top of the binder suspension; and c) drying a resultant of step b), wherein steps a) and b) are performed simultaneously or sequentially.

[0088] In step a), a current collector is prepared, and a binder suspension is applied to at least one surface of the current collector.

[0089] The types of the binder and solvent and the current collector are as described above. The binder suspension may be prepared by a known method, for example, by mixing a specific binder, such as the SBR binder, with a solvent and diluting the mixture to a suitable viscosity, but the present invention is not limited thereto.

[0090] The step a) may involve uniformly applying the binder suspension to at least one surface of the current collector. In this specification, "uniformly applying the binder suspension" means applying the binder suspension uniformly on the current collector without forming a specific pattern.

[0091] According to one embodiment, in step a), the binder suspension may be applied to a thickness of 0.1 to 10 μm. More specifically, the coating thickness of the binder suspension may be 0.1 to 6 μm, 0.1 to 5 μm, 0.1 to 4 μm, 0.1 to 3 μm, 0.1 to 2 μm, or 0.1 to 1 μm. In this case, the coating thickness of the binder suspension refers to the thickness measured after the binder suspension is applied and thoroughly dried. If the binder suspension is applied too thick, it may not mix well with the electrode slurry. After drying, the separation between the two layers may be clear, forming an insulating binder layer and increasing interfacial resistance. On the other hand, if the binder suspension is applied to a thickness less than 0.1 μm, it may be difficult to achieve the intended objective of the present invention. That is, within the above thickness range, an increase in interfacial resistance is prevented, interfacial adhesion between the current collector and the electrode active material layer is improved, and process defects such as electrode detachment can be reduced.

[0092] According to one embodiment, the binder suspension may contain 30 wt % or more of the binder relative to the total solid content, and the electrode slurry may contain 2 wt % or less of the binder relative to the total solid content. More specifically, the binder suspension may contain 30 wt % or more, 50 wt % or more, or 70 wt % or more of the binder relative to the total solid content, and the electrode slurry may contain 2 wt % or less, 1.5 wt % or less, or 1 wt % or less of the binder relative to the total solid content.

[0093] After step a), in step b), an electrode slurry containing an electrode active material is applied on top of the binder suspension.

[0094] The electrode active material is as described above, and the electrode slurry may be produced by any known method used to produce an electrode slurry for a secondary battery.

[0095] The application of the binder suspension in step a) and the application of the electrode slurry in step b) may be performed using any coating method known to be commonly used for forming a film by applying a liquid, including, but not limited to, spray coating, dip coating, spin coating, gravure coating, slot die coating, doctor blade coating, roll coating, inkjet printing, slot die coating, flexography printing, screen printing, electrohydrodynamic printing, microcontact printing, imprinting, reverse offset printing, bar coating, gravure offset printing, and multi-layer simultaneous die coating.

[0096] Specifically, the binder suspension and the electrode slurry may be applied in this order, or the binder suspension and the electrode slurry may be applied simultaneously by a multi-layer simultaneous die coating method, although it is preferable to apply the electrode slurry after applying the binder suspension in terms of the uniformity and quality of the electrode surface.

[0097] After step b), step c) involves drying the resultant of step b).

[0098] In this case, the drying may be performed for 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. The drying may 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. If the drying temperature is too high or the drying time is too short, the migration of binder particles may be excessive, and the interfacial adhesion may not be sufficient. In one embodiment, step c) may be performed at a temperature of 80 to 130°C for 30 to 300 seconds.

[0099] The dried electrode may then be rolled at an appropriate density to produce an electrode having an electrode active material layer formed on a current collector. In this case, the above-described rolling conditions and rolling methods, such as known rolling densities, may be used, but the present invention is not limited thereto.

[0100] In the method for manufacturing a secondary battery electrode of the present invention, during drying in step c), the binder particles of the binder suspension migrate into the electrode active material layer and are densely present in a predetermined region of the electrode active material layer from the interface between the current collector and the electrode active material layer, thereby improving the problems of reduced interfacial resistivity and adhesive strength between the current collector and the active material layer that occur when applying a conventional binder solution.

[0101] Another embodiment of the present invention provides a method for manufacturing an electrode for a secondary battery, the method comprising: a) applying a binder suspension to at least one surface of a current collector; b) applying an electrode slurry containing an electrode active material on top of the binder suspension; and c) drying a resultant of step b), wherein steps a) and b) are performed simultaneously or sequentially.

[0102] In this case, the electrodes are as described above.

[0103] The present invention also provides a secondary battery comprising the electrode, a separator, and an electrolyte solution.

[0104] The electrodes are as described above.

[0105] The separator is not particularly limited as long as it is a separator known in the art, and may be made of, for example, glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene, or a combination thereof, and may be in the form of a nonwoven fabric or a woven fabric, and may be used in a single-layer or multi-layer structure.

[0106] The electrolyte solution includes a non-aqueous organic solvent and an electrolyte salt. The non-aqueous organic solvent may be, but is not limited to, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), 1,2-dimethoxyethene (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 a mixture thereof. The electrolyte salt is dissolved in the non-aqueous organic solvent and serves as a source of electrolyte metal ions within the battery, enabling basic secondary battery operation and promoting the movement of electrolyte metal ions between the positive and negative electrodes. By way of 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, LiN(C x F 2x+1 SO2)(C y F 2y+1 The electrolyte salt may be, but is not limited to, SO2 (where x and y are natural numbers), LiCl, LiI, or a mixture thereof. In addition, the electrolyte salt may be a known substance used at a concentration suited to the purpose, and may further contain a known solvent or additive to improve charge / discharge characteristics, flame retardancy, etc., as needed.

[0107] To achieve the above object, the method for manufacturing a lithium secondary battery according to the present invention may include stacking a manufactured anode, a separator, and a cathode in order to form an electrode assembly, placing the electrode assembly in a cylindrical or prismatic battery case, and injecting an electrolyte solution to manufacture a battery. Alternatively, the electrode assemblies may be stacked, and the resulting assembly may be impregnated with an electrolyte solution and placed in a battery case and sealed to manufacture a battery.

[0108] The battery case used in the present invention may be one commonly used in the art, and its shape is not limited depending on the intended use of the battery. For example, it may be a cylindrical can, a square can, a pouch can, or a coin can.

[0109] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for a small device, but also as a unit battery for a medium- to large-sized battery module including a plurality of battery cells. Preferred examples of the medium- to large-sized device include, but are not limited to, an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage system.

[0110] The present invention will be described in more detail below with reference to examples. However, these examples are provided for the purpose of explaining the present invention in more detail, and the scope of the present invention is not limited to the following examples.

[0111] Example Example 1 <Production of negative electrodes> A binder suspension was prepared by diluting a suspension of SBR (manufactured by Zeon, BM451B) as a binder with pure water.

[0112] A negative electrode active material, which was a mixture of artificial graphite (D50: 13 μm) and natural graphite (D50: 10 μm) in a weight ratio of 5:5, a CMC thickener, and an SBR binder were added to water in a weight ratio of 98.5:1:0.5 to prepare a negative electrode slurry with a viscosity of 5,000 cps.

[0113] The binder suspension and anode slurry were coated onto one side of a copper current collector (8 μm thick copper foil) using a slot die by multi-layer simultaneous die coating to thicknesses of 1 μm and 200 μm, respectively (based on the thickness after drying when the binder suspension was coated alone), and then dried. The other side was coated in the same manner and dried under the drying conditions listed in Table 5 below.

[0114] After drying, the negative electrode was rolled (rolling density: 1.68 g / cm 3 ) to produce a negative electrode having a current collector negative electrode active material layer formed thereon.

[0115] The solids composition of the negative electrode active material layer of the fabricated negative electrode was 97.5 wt% negative electrode active material, 1.5 wt% SBR binder, and 1 wt% CMC thickener. The fabricated negative electrode had a copper foil thickness of 8 μm and a negative electrode active material layer thickness of 127 μm. SEM images confirmed that the boundary between the binder layer and the negative electrode active material layer was not clearly distinguishable, and that they were formed into a single negative electrode active material layer (see Figure 1).

[0116] <Production of positive electrodes> As the positive electrode active material, Li[Ni 0.88 Co 0.1 Mn 0.02 A slurry was prepared by mixing 02, 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 coated on a 12 μm thick aluminum foil and dried under vacuum to prepare a positive electrode for a secondary battery.

[0117] <Secondary battery manufacturing> The positive and negative electrodes were stacked after being notched to a predetermined size. A separator (polyethylene, 13 μm thick) was placed between the positive and negative electrodes to form an electrode cell, and the tabs of the positive and negative electrodes were then welded together. The welded positive electrode / separator / negative electrode assembly was placed in a pouch and sealed on three sides except for the electrolyte injection area. At this time, the electrode tabs were included in the sealed area.

[0118] The electrolyte was poured through the remaining surface other than the sealed portion, and after sealing the remaining surface, the electrolyte was allowed to soak for 12 hours or more.

[0119] The electrolyte was prepared by dissolving 1M LiPF6 in a mixed solvent of EC / EMC / DEC (25 / 45 / 30 by volume), to which 1 wt% vinylene carbonate (VC), 0.5 wt% 1,3-propene sultone (PRS), and 0.5 wt% lithium bis(oxalato)borate (LiBOB) were added.

[0120] Next, pre-charging was performed for 36 minutes at a current corresponding to 0.25 C. After 1 hour, the cells were degassed and aged for more than 24 hours, after which chemical charge-discharge was performed (charge conditions: CC-CV 0.2 C, 4.2 V, 0.05 C, cut-off; discharge conditions: CC 0.2 C, 2.5 V, cut-off).

[0121] After that, standard charging and discharging was performed (charging condition CC-CV 0.33C 4.2V 0.05C CUT-OFF, discharging condition CC 0.33C 2.5V CUT-OFF).

[0122] Example 2 A negative electrode, a positive electrode, and a secondary battery were produced in the same manner as in Example 1, except that when coating the binder suspension and the negative electrode slurry, the binder suspension was first applied by gravure coating, and then the negative electrode slurry was applied using a slot die.

[0123] Example 3 A negative electrode, a positive electrode, and a secondary battery were manufactured in the same manner as in Example 1, except that a binder suspension was manufactured by mixing and diluting SBR (manufactured by Zeon Corporation, BM451B) and CMC (manufactured by Daicel Corporation, D2200) at a solid content ratio of 97:3.

[0124] Example 4 An anode, a cathode, and a secondary battery were manufactured in the same manner as in Example 1, except that the binder suspension and anode slurry were coated on one side of a copper current collector (copper foil with a thickness of 8 μm) using a slot die by a multi-layer simultaneous die coating method to thicknesses of 7 μm and 132 μm (based on the thickness after drying when the binder suspension is applied alone).

[0125] (Comparative Example 1) A negative electrode active material, which was a mixture of artificial graphite (D50: 13 μm) and natural graphite (D50: 10 μm) in a weight ratio of 5:5, SBR binder, and CMC thickener, was added to water in a weight ratio of 97.5:1.5:1 to prepare a negative electrode slurry with a viscosity of 5,000 cps.

[0126] The prepared negative electrode slurry was applied to one side of a copper current collector (8 μm thick copper foil) using a slot die by die coating and then dried. The other side was similarly coated and then dried. The dried negative electrode was rolled (rolling density: 1.68 g / cm 3 ) to produce a negative electrode in which a negative electrode active material layer was formed on a current collector.

[0127] In this case, the negative electrode was formed with a copper foil having a thickness of 8 μm and a negative electrode active material layer having a thickness of 126 μm.

[0128] A positive electrode and a secondary battery were manufactured in the same manner as in Example 1, except that the manufactured negative electrode was used.

[0129] (Comparative Example 2) A negative electrode active material, which was a mixture of artificial graphite and natural graphite in a weight ratio of 5:5, an SBR binder, and a CMC thickener, was added to water in a weight ratio of 97:2:1 to prepare a first negative electrode slurry.

[0130] A second negative electrode slurry was prepared by adding a negative electrode active material, which was a mixture of artificial graphite and natural graphite in a weight ratio of 5:5, an SBR binder, and a CMC thickener to water in a weight ratio of 98:1:1.

[0131] The first and second negative electrode slurries were applied to one side of a copper current collector (8 μm thick copper foil) using a slot die in a multi-layer simultaneous die coating method to form a 5:5 thickness, then dried and rolled (rolling density: 1.68 g / cm 3) to produce a negative electrode in which a first negative electrode active material layer and a second negative electrode active material layer were formed on a current collector. A positive electrode and a secondary battery were produced in the same manner as in Example 1, except that the produced negative electrode was used.

[0132] (Comparative Example 3) A negative electrode, a positive electrode, and a secondary battery were produced in the same manner as in Example 1, except that the binder suspension was produced using CMC (Daicel, D2200) as the binder.

[0133] Comparative Example 4 A negative electrode, a positive electrode, and a secondary battery were produced in the same manner as in Example 1, except that the binder suspension was produced using PAA (SW100, manufactured by Sumitomo Seika Chemicals Co., Ltd.) as the binder.

[0134] (Comparative Example 5) A negative electrode, a positive electrode, and a secondary battery were manufactured in the same manner as in Example 1, except that a binder solution was prepared by mixing and diluting CMC (Daicel, D2200) and PAA (Sumitomo Seika Chemicals, SW100) in a solid content ratio of 1:3.

[0135] (Comparative Example 6) The binder suspension was applied to one side of a copper current collector by a slot die coating method and then dried to form a binder layer. The negative electrode slurry was then applied to the formed binder layer by a slot die coating method and then dried to form a negative electrode active material layer. Next, the negative electrode active material layer was rolled (rolling density: 1.68 g / cm). 3 ) to produce a negative electrode in which a binder layer and a negative electrode active material layer were formed on a current collector, and the same procedures as in Example 1 were carried out to produce a negative electrode, a positive electrode, and a secondary battery.

[0136] [Evaluation Example 1] Evaluation of binder distribution characteristics in the thickness direction and surface direction of the negative electrode using EDS (energy dispersive x-ray spectroscopy) mapping Cross-sectional SEM images of the electrodes prepared in Example 1 and Comparative Example 6 in the thickness direction are shown in Figures 1 and 2, respectively. EDS mapping results of the electrode prepared in Example 1 in the thickness direction are shown in Figures 3 and 4. Since it is difficult to distinguish the distribution of the SBR binder using conventional EDS mapping, the electrode was sufficiently exposed to Os gas, and then EDS mapping analysis of the Os element was performed. This image is shown in Figure 3, and a graph of the SBR (Os) content mass % profile in the electrode thickness direction is shown in Figure 4.

[0137] Referring to Figure 1, the electrode prepared in Example 1 does not clearly separate the binder layer and active material layer. This is believed to be due to migration of the SBR binder into the electrode active material layer. The SBR binder suspension allows the binder suspension to be applied to a sufficiently thin layer. During the electrode drying process, the SBR binder particles diffuse into the electrode slurry layer, resulting in a very close contact between the current collector and the active material layer, as if no additional binder layer had been applied, as can be seen in the SEM image. In contrast, Figure 2 shows the electrode prepared in Example 1, where a binder layer was applied and dried to form a separate binder layer distinct from the electrode active material layer. The SEM image also confirms the formation of a binder layer and an active material layer on the current collector.

[0138] Therefore, it was confirmed that an electrode with no increase in interfacial resistivity can be manufactured even though the insulating binder layer is uniformly applied in the width direction without applying conventional pattern coating.

[0139] 3 and 4, it can be seen that the binder is continuously distributed at a relatively high concentration from the boundary between the electrode current collector and the active material layer to a portion of the active material layer. These results suggest that not only is the adhesion at the boundary between the current collector and the electrode active material layer significantly improved, but the diffusion of binder particles into the electrode active material layer during the drying process also improves the cohesion in a specific region below the electrode active material layer.

[0140] Furthermore, for Examples 1 to 4 and Comparative Examples 1 to 6, cross-sectional SEM images and EDS mapping analysis results in the electrode thickness direction are shown in Table 1 below. Specifically, the Os element content a1 (at%) per unit thickness was measured for the entire region of the active material layer, and this was used to convert to the Os element content for the entire region (b1). The Os element content a2 (at%) per unit thickness was measured for the region corresponding to 15% of the thickness from the current collector, and this was used to convert to the Os content for the entire 15% thickness (b2). Next, b2 / b1 was calculated, and the results are shown in Table 1 below.

[0141] [Table 1]

[0142] In Examples 1 to 3, a large amount of binder was found to be distributed at the interface between the current collector and the active material layer, while in Comparative Example 1, the binder distributed at the interface between the current collector and the active material layer was insufficient relative to the average binder content of the entire active material layer due to binder migration. In Comparative Example 2, the binder content distributed at the interface was higher than in Comparative Example 1, but was still lower than in the Examples. This indicates that in Examples 1 to 3, the current collector and the active material layer were bonded more strongly, and that the low binder content on the surface of the active material layer resulted in advantageous fast charging performance.

[0143] [Evaluation Example 2] Evaluation of interlayer bonding strength and interface resistivity of electrodes 1) Measurement of electrode 90° bending adhesive strength The 90° bending adhesive strength was measured for the electrodes produced in Examples 1 to 4 and Comparative Examples 1 to 6, and the measured adhesive strength and the position at which the electrode was separated are shown in Table 2. The manner in which the current collector and the electrode active material layer were separated is also shown in schematic diagrams in Figures 5a to 5d, respectively.

[0144] *Evaluation of the interfacial adhesion between the electrode active material layer and the current collector The negative electrodes prepared in Examples 1 to 4 and Comparative Examples 1 to 6 were cut into 18 mm wide x 150 mm long pieces. An 18 mm wide single-sided tape (manufactured by 3M) was attached to one side of the double-coated electrode at a location 100 mm from the vertical line, approximately 50 mm apart, at the clamping portion of the tensile tester, and then thoroughly bonded using a roller with a 2 kg load. The double-sided tape was attached to the bottom of the tensile tester, and the double-sided tape was attached so that the single-sided tape side of the prepared electrode faced the double-sided tape side. The portion without the single-sided tape was clamped to the opposite side of the tensile tester, and the 90° bending adhesive strength was measured (Figures 6a and 6b). The measured strength was divided by the tape width, and the results are summarized in Table 2 below. Figure 6c shows a photograph of the negative electrodes separated after adhesive strength measurements for Example 1 and Comparative Example 1.

[0145] Meanwhile, i) the average separation position (t1) was determined by measuring the 90° bending adhesive strength using a double-sided coated electrode, measuring the thickness of the part including the current collector between the two separated parts, and then subtracting the thickness of the current collector and the active material layer on the rear surface. The electrode thickness was measured using a 6.35 pi tip micrometer (293 series, Mitutoyo Corporation) with a measuring pressure of 5 N, and the average of the 8 measurements excluding the upper and lower limits was calculated as the thickness.

[0146] ii) The active material particle diameter (t2) was the particle diameter (D50) of the graphite active material used in the examples and comparative examples, and the particle diameter (D50) of the large-grain graphite active material, which had a particle diameter of 13 μm.

[0147] [Table 2]

[0148] Figure 5a is a schematic diagram showing the results of measuring the 90° bending adhesive strength of the electrodes prepared in Examples 1 to 3. Referring to Table 2 and Figure 5a, it was found that separation occurred within the negative electrode active material layer because the cohesion within the electrode active material layer was weaker than the adhesion at the interface between the current collector and the electrode active material layer. In other words, the structure shown in Figure 5a requires a low binder content within the electrode active material layer, and this result is related to the binder distribution and indicates that the binder content in the electrode active material layer is low. Figure 6c shows a photograph of the negative electrode separated after measuring the adhesive strength in Example 1.

[0149] Figure 5b is a schematic diagram illustrating the separation of the current collector and electrode active material layer during 90° bending adhesion strength measurement for a typical electrode fabricated according to Comparative Examples 1 and 2, in which an electrode active material layer is formed by immediately applying and drying electrode slurry on top of the current collector. Referring to Table 2 and Figure 5b, it was confirmed that in these cases, separation occurred at the interface because the adhesion between the current collector and the electrode active material layer was lower than the cohesion within the electrode active material layer.

[0150] Figure 5c is a schematic diagram showing the 90° bending adhesion strength of electrodes fabricated according to Comparative Examples 3 to 6. The electrodes were fabricated using a binder layer pre-coated and then dried on a current collector, and the electrodes were fabricated using CMC and / or PAA instead of a binder that can be mixed with a solvent such as SBR to form a binder suspension (Comparative Example 6) and Comparative Examples 3 to 5. In these cases, separation occurs at the interface between the binder layer and the electrode active material layer. However, Table 2 and Figure 5c suggest that when using CMC and PAA binders instead of SBR, or when the SBR binder suspension is dried and then coated with the negative electrode active material layer, binder migration into the electrode active material layer is insufficient. This confirms that the binder layer and electrode active material layer are clearly formed as separate layers, and that separation occurs at this location because adhesion is weakest at the interface between the formed binder layer and the negative electrode active material layer.

[0151] Meanwhile, Figure 5d shows a negative electrode prepared according to Example 4, which is similar to Figure 5a, but shows a case in which the boundary between the binder layer and the active material layer is clearly defined. Referring to Table 2 and Figure 5d, when the SBR binder suspension is applied thicker than the standard value, the interfacial adhesion improves due to migration of SBR particles, but the binder layer is formed too thick and can act as an insulating layer.

[0152] Meanwhile, referring to Table 2, it was confirmed that the electrode according to the present invention always had a thickness of 1.0 x t2 or more because the active material remained in the form of a "layer" on the current collector during the adhesive strength measurement. Furthermore, when the photograph of Example 1 in Figure 6c was visually inspected, it was confirmed that both the upper and lower separated layers were active material layers. The average separation position satisfied Relational Equation 1 of the present invention.

[0153] Conversely, while conventional electrodes not incorporating the technology of the present invention are expected to separate at the interface with the current collector, we observed that the separation was not clean and clean, with the active material particles separating unevenly on a particle-by-particle basis. Specifically, when visually inspecting the photograph of Comparative Example 1 in Figure 6c, it is apparent that the Cu current collector is the majority, with a small amount of scattered active material particles remaining in some areas, which differs from the "active material layer" form observed in Example 1. In this case, the average thickness at the separation location is 0 ≦ t1 < 1.0 × t2, which does not satisfy Relational Equation 1 of the present invention. This result is analyzed to be due to the fact that the tip size of the thickness gauge is 6.35 pi, which is rather wide, and therefore contains a large number of active material particles, resulting in an average thickness being measured.

[0154] 2) Measurement of electrode interface resistivity The interfacial resistance between the electrode and the current collector was measured for the electrodes manufactured in Examples 1 to 4 and Comparative Examples 1 to 6 using an interfacial resistivity measuring instrument manufactured by Hioki Corporation (Hioki Corporation, XF057), and the results are shown in Table 3 below.

[0155] [Table 3]

[0156] Referring to Table 3, the electrodes manufactured according to Examples 1 to 3 of the present invention had similar electrode-current collector interfacial resistivity values ​​to those of Comparative Examples 1 and 2, despite the application of a binder suspension. On the other hand, Comparative Examples 3 to 5 did not have sufficient adhesive strength, making it impossible to measure reliable interfacial resistivity values ​​after rolling. Comparative Example 6 suggests that the binder suspension was not mixed with the electrode active material, resulting in the formation of a distinct binder-insulator layer between these layers after drying. Example 4 was analyzed to have the highest interfacial resistance due to the formation of a thick insulator binder layer.

[0157] [Evaluation Example 3] Evaluation of rapid charging performance The secondary batteries manufactured in Examples 1 to 4 and Comparative Examples 1 to 6 were subjected to rapid charging evaluation by charging at a C-rate of 2.5 C at a temperature of 25° C. and discharging at a C-rate of 1 / 3 C. After 100 and 200 cycles, the rapid charging capacity retention rate was measured, and the results are shown in Table 4 below.

[0158] [Table 4]

[0159] Referring to Table 4, it was confirmed that the lithium secondary batteries manufactured according to Examples 1 to 3 had less decrease in cycle capacity retention rate (%) compared to Comparative Examples 1 to 3, and thus ensured excellent fast charge performance. It was analyzed that Comparative Examples 1 and 2 had a binder uniformly dispersed on the electrode surface, which increased electrode resistance and deteriorated fast charge characteristics. In Comparative Examples 3 to 5, cell manufacturing failed due to a defect in which the electrode active material layer partially detached from the current collector during the pressing and notching processes. It was analyzed that Comparative Example 6 and Example 4 had a deterioration in capacity characteristics as the interfacial resistivity between the current collector and active material layer increased as shown in Table 3, with the increase in interfacial resistivity.

[0160] [Evaluation Example 4] Evaluation of adhesive strength between active material layer and current collector due to changes in electrode drying conditions Examples 5 to 9 An electrode was produced in the same manner as in Example 1, except that in step 2 of Example 1, the drying step of the applied electrode slurry was carried out as shown in Table 5 below.

[0161] (Comparative Example 7) An electrode was manufactured in the same manner as in Comparative Example 3, except that the drying step in Comparative Example 3 was performed as shown in Table 5 below.

[0162] (Evaluation method) The interfacial adhesive strength between the active material layer and the current collector was evaluated in the same manner as in Evaluation Example 2, and the results are shown in Table 5 below.

[0163] [Table 5]

[0164] Referring to Table 5, when the drying temperature is low (120°C or less) (Examples 1, 5, 6, and 9), the drying time increases to evaporate all of the solvent, but the binder particles in the binder suspension migrate into the electrode active material layer, increasing the adhesive strength between the current collector and the active material layer. When the drying temperature is excessively high (Examples 7 and 8), rapid drying causes the active material particles to be easily exposed to the surface of the solvent during the drying process, which is analyzed to be due to excessive binder particle migration due to capillary action, resulting in a slight decrease in adhesive strength.

[0165] In the case of Comparative Example 7, it can be seen that the adhesive strength is the lowest even though the same drying process as in Example 6, which is the condition for the best adhesive strength, is carried out.

[0166] [Evaluation Example 5] Binder distribution characteristics in the width direction of the electrode active material layer interface *Evaluation of the interfacial adhesion between the electrode active material layer and the current collector The negative electrode prepared in Example 1 was cut into a 18 mm wide x 350 mm long strip, with the width aligned vertically. An 18 mm wide single-sided tape (manufactured by 3M) was attached to one side of the double-coated electrode at a location 300 mm from the vertical line, approximately 50 mm apart, using the clamping mechanism of a tensile tester. The tape was then firmly attached using a roller with a 2 kg load. The double-sided tape was attached to the bottom of the tensile tester, and the double-sided tape was then attached so that the single-sided tape side faced the single-sided tape side of the prepared electrode. The portion of the negative electrode without the single-sided tape was clamped to the opposite side of the tensile tester. 90° bending adhesive strength measurements were performed on a total of 1,201 regions spaced 0.25 mm apart across the width of the negative electrode. The measured strength was divided by the tape width, and the results are shown in Figure 7. The dispersion (%) of the adhesive strength measured in the 1,201 regions is shown in Figure 8. The dispersion (%) was calculated by dividing the average adhesive strength by the individual adhesive strength value in each region.

[0167] 7 and 8, it was confirmed that the active material layer of the electrode prepared in Example 1 was formed with SBR binder particles uniformly distributed in the width direction.

[0168] [Evaluation Example 6] Evaluation of interlayer bonding strength of electrodes according to active material particle size Example 10 A binder suspension was prepared by diluting a suspension of SBR (manufactured by Zeon, BM451B) as a binder with pure water.

[0169] A SiOx-based negative electrode active material (D50: 6 μm), a CMC thickener, and an SBR binder were added to water in a weight ratio of 98:1:1.0 to prepare a negative electrode slurry with a viscosity of 5,000 cps.

[0170] An anode, a cathode, and a secondary battery were fabricated in the same manner as in Example 1, except that the binder suspension was first coated on one side of a copper current collector (8 μm thick copper foil) by gravure coating, and then the anode slurry was coated using a slot die. In this case, the thickness of the binder suspension coated by gravure coating was 2 μm (based on the thickness after drying when the binder suspension is coated alone), and the thickness of the final coated electrode was 204 μm.

[0171] After drying, the negative electrode was rolled (rolling density: 1.68 g / cm 3 ) to produce a negative electrode having a current collector and a negative electrode active material layer formed thereon.

[0172] Example 11 A binder suspension was prepared by diluting a suspension of SBR (manufactured by Zeon, BM451B) as a binder with pure water.

[0173] A negative electrode slurry with a viscosity of 5,000 cps was prepared by adding SiC-based negative electrode active material (D50: 2 μm), CMC thickener, and SBR binder to water in a weight ratio of 98:1:1.0.

[0174] An anode, a cathode, and a secondary battery were fabricated in the same manner as in Example 1, except that the binder suspension was first coated on one side of a copper current collector (8 μm thick copper foil) by gravure coating, and then the anode slurry was coated using a slot die. In this case, the thickness of the binder suspension coated by gravure coating was 2 μm (based on the thickness after drying when the binder suspension is coated alone), and the thickness of the final coated electrode was 200 μm.

[0175] After drying, the negative electrode was rolled (rolling density: 1.68 g / cm 3 ) to produce a negative electrode having a current collector and a negative electrode active material layer formed thereon.

[0176] *Evaluation of the interfacial adhesion between the electrode active material layer and the current collector The interfacial adhesive strength between the electrode active material layer and the current collector was evaluated in the same manner as in Evaluation Example 2. The measured 90° bending adhesive strength and the position at which the electrode was separated are shown in Table 6 below.

[0177] [Table 6] (In Table 6, t2 is the particle diameter (D50) of the large particle diameter negative electrode active material among the mixed negative electrode active materials.)

[0178] Referring to Table 6, it was confirmed that the electrodes according to the present invention always had a thickness of 1.0 x t2 or more when measuring the adhesive strength because the active material remained on the current collector in the form of a "layer." In addition, it was confirmed that Example 1, in which the average separation position fell within the preferred range, exhibited superior adhesive strength compared to Examples 10 and 11.

[0179] Although the present invention has been described above with reference to the preferred embodiments, the present invention is not limited to the preferred embodiments and can be implemented in various different forms. Those skilled in the art will appreciate that the present invention can be embodied in other specific forms without changing the technical concept or essential features of the present invention. Therefore, the above-described preferred embodiments should be understood to be illustrative in all respects and not limiting. [Explanation of symbols]

[0180] 1 Current collector 3. Binder layer 5 Electrode active material layer 11. Binder

Claims

1. A current collector; an electrode active material layer located on at least one surface of the current collector, An electrode for a secondary battery, which satisfies the following relational expression 1: [Relationship 1] t ２ ≦t １ ≦8×t ２ In the above-mentioned relational formula 1, t １ is the thickness of the electrode active material layer other than the current collector closer to the current collector based on the position where the electrode active material layer is separated when measuring the 90° bending adhesive strength of the electrode, and t ２ is the particle diameter (D50) of the electrode active material contained in the electrode active material layer.

2. The electrode for a secondary battery according to claim 1 , wherein the electrode further satisfies the following relational expression 2: [Relationship 2] 1.5×t １ ≦t １ ≦5×t ２ In the above-mentioned relational formula 2, t １ is the thickness of the electrode active material layer other than the current collector closer to the current collector based on the position where the electrode active material layer is separated when measuring the 90° bending adhesive strength of the electrode, and t ２ is the particle diameter (D50) of the electrode active material contained in the electrode active material layer.

3. The electrode for a secondary battery according to claim 1 , wherein the electrode active material layer comprises a styrene butadiene rubber (SBR)-based binder.

4. 4. The electrode for a secondary battery according to claim 3, wherein the electrode active material layer contains the binder in an amount of 0.1 to 2% by weight based on the total weight.

5. The electrode for a secondary battery according to claim 1 , wherein the electrode further satisfies the following relational expression 3: [Relationship 3] 0.25≦b ２ / b １ <0.7 In the above-mentioned Relational Formula 3, when measuring the binder distribution in the thickness direction of the electrode active material layer, b １ is the weight of the binder in the entire electrode active material layer, and b ２ is the weight of the binder in an area from the current collector to 15% of the total thickness of the electrode active material layer.

6. The electrode for a secondary battery according to claim 5 , wherein the electrode further satisfies the following relational expression 4: [Relationship 4] 0.3≦b ２ / b １ <0.5 In the relational expression 4, when measuring the binder distribution in the thickness direction of the electrode active material layer, b １ is the weight of the binder in the entire electrode active material layer, and b ２ is the weight of the binder in an area from the current collector to 15% of the total thickness of the electrode active material layer.

7. The electrode for a secondary battery according to claim 1 , wherein the electrode has a continuous binder concentration in the thickness direction of the electrode.

8. The electrode for a secondary battery according to claim 1 , wherein the electrode further satisfies the following relational expression 5: [Relationship 5] -30%≦(CD) / D≦+30% In the above-mentioned Relational Formula 5, C is the interfacial adhesive strength between the current collector and the electrode active material layer measured at any position selected in the width direction of the electrode active material layer, and D is the average value of the interfacial adhesive strength between the current collector and the electrode active material layer.

9. The electrode for a secondary battery according to claim 1 , wherein the electrode is a negative electrode.

10. a) applying a binder suspension to at least one surface of a current collector; b) applying an electrode slurry containing an electrode active material on top of the binder suspension; c) drying the result of step b), The method for manufacturing an electrode for a secondary battery, wherein the steps a) and b) are carried out simultaneously or sequentially.

11. The method for manufacturing an electrode for a secondary battery according to claim 10, wherein the step a) comprises uniformly coating the binder suspension on one surface of the current collector.

12. 11. The method for manufacturing an electrode for a secondary battery according to claim 10, wherein in step a), the coating thickness of the binder suspension is 0.1 to 10 μm.

13. The binder suspension contains 30% by weight or more of a binder based on the total amount of solids, The method for producing an electrode for a secondary battery according to claim 10 , wherein the electrode slurry contains 2% by weight or less of a binder based on the total weight of solids.

14. The method for manufacturing an electrode for a secondary battery according to claim 10, wherein the step c) is performed at a temperature of 50 to 200° C. for 30 to 300 seconds.

15. The method for manufacturing an electrode for a secondary battery according to claim 10, wherein the step c) is performed at a temperature of 80 to 130° C. for 30 to 300 seconds.

16. a) applying a binder suspension to at least one surface of a current collector; b) applying an electrode slurry containing an electrode active material on top of the binder suspension; c) drying the result of step b), An electrode for a secondary battery, produced by a method in which the steps a) and b) are carried out simultaneously or sequentially.

17. A secondary battery comprising the electrode according to any one of claims 1 to 9, a separator, and an electrolyte solution.

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