KR102496615B1 - Electrode for a secondary battery with improved rapid charging performance, a method of manufaturing the same and secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a secondary battery electrode, a manufacturing method thereof, and a secondary battery including the electrode, wherein the electrode includes a current collector; and an electrode active material layer positioned on at least one surface of the current collector, and satisfies relational expression 1.

Description

Electrode for secondary battery with improved rapid charging performance, method for manufacturing the same, and secondary battery including the same

The present invention relates to a secondary battery electrode with improved rapid charging performance, a manufacturing method thereof, and a secondary battery including the same.

Recently, in the case of large cells for high-energy EVs, the density of the electrode is high, so when the cell is charged with a high current density, there is a limit in diffusion of Li ions into the cathode, and in this case, Li ions are deposited on the surface of the anode. As a result, cell deterioration occurs.

In order to improve the above problem, it is important to keep the density of the negative electrode as low as possible or to lower the surface and internal resistance of the negative electrode so that Li ions can quickly diffuse into the negative electrode.

In order to improve cell resistance and rapid charging performance, a binder with high adhesive strength is developed, and a technology is being developed to lower the content of the binder using this, but there are limitations in reducing the type and content of binder with high adhesive strength, If it is lowered, a serious problem arises in that the electrode mixture layer is detached from the current collector during the notching process or the charging and discharging process of the cell.

Therefore, a technology for efficiently distributing the binder inside the anode is being developed. In this case, a high binder content is formed at the interface of the current collector, and it is possible to reduce the binder content on the negative electrode mixture layer and surface while suppressing desorption, thereby improving cell performance. can To this end, a technology has been developed to form an anode slurry with a high binder content in the lower layer with a dual layer and a cathode slurry with a low binder content in the upper layer, but in a general drying process, the binder particles move to the surface of the anode mixture layer. Due to the phenomenon, there are also limitations in implementing an ideal binder distribution.

In addition, in order to improve the adhesion between the current collector and the surface of the negative electrode and to maintain maximum electrical contact, a substrate coated with a conductive material such as carbon black or CNT is prepared on the current collector, and a negative electrode mixture layer is formed on the upper portion of the current collector. has been developed, but the effect of improving cell performance is not sufficient.

Accordingly, there is a need to develop a lithium secondary battery having a low interface resistivity between a current collector and an electrode mixture layer and improved rapid charging performance.

The purpose of the electrode for a lithium secondary battery of the present invention is to improve the interfacial adhesion between a current collector and an electrode active material layer, prevent defects in appearance in a process such as detachment of an electrode, and simultaneously improve rapid charging performance.

An object of the method for manufacturing an electrode for a lithium secondary battery of the present invention is to manufacture an electrode capable of rapid charging while improving interfacial adhesion and preventing process and appearance defects.

One embodiment of the present invention is a current collector; and an electrode active material layer disposed on at least one surface of the current collector, and satisfying the following relational expression 1, providing an electrode for a secondary battery.

[Relationship 1]

t ₂ ≤ t ₁ ≤ 8×t ₂

In the relational expression 1, t ₁ is the thickness of the electrode active material layer excluding the current collector on the side close to the current collector based on the position where the electrode is separated from the inside of the electrode active material layer when measuring the 90 ^° bending adhesive force of the electrode, and t ₂ is the electrode active material layer thickness. It is the particle size (D50) of the electrode active material included in the active material layer.

The electrode may further satisfy the following relational expression 2.

[Relationship 2]

1.5×t ₂ ≤ t ₁ ≤ 5×t ₂

In the relational expression 2, t ₁ is the thickness of the electrode active material layer excluding the current collector on the side close to the current collector based on the position where the electrode is separated from the inside of the electrode active material layer when the 90 ^° bending adhesive force of the electrode is measured, and t ₂ is the electrode active material layer thickness. It is the particle size (D50) of the electrode active material included in the active material layer.

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

The electrode active material layer may include 0.1 to 2% by weight of the binder based on the total weight.

The electrode may further satisfy the following relational expression 3.

[Relationship 3]

0.25 ≤ b ₂ /b ₁ < 0.7

In the relational expression 3, when the binder distribution is measured 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 binder in the 15% area of the total thickness of the electrode active material layer from the current collector is the weight of

The electrode may further satisfy the following relational expression 4.

[Relationship 4]

0.3 ≤ b ₂ /b ₁ < 0.5

In the relational expression 4, when the binder distribution is measured 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 binder in the 15% area of the total thickness of the electrode active material layer from the current collector is the weight of

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

The electrode may further satisfy the following relational expression 5.

[Relationship 5]

-30% ≤ (C-D)/D ≤ +30%

In Equation 5, C is the interfacial adhesive force between the current collector and the electrode active material layer measured at an arbitrary position selected in the width direction of the electrode active material layer, and D is the average value of the interfacial adhesive force between the current collector and the electrode active material layer.

The electrode may be a cathode.

Another embodiment of the present invention is a) applying a binder suspension to at least one surface of the current collector; b) applying an electrode slurry containing an electrode active material on top of the binder suspension; and c) drying the product of step b), wherein steps a) and b) are performed simultaneously or sequentially.

Step a) may be to uniformly apply the binder suspension to one surface of the current collector.

In step a), the coating thickness of the binder suspension may be 0.1 to 10 μm.

The binder suspension may include 30 wt% or more of the binder based on the total solid content, and the electrode slurry may include 2 wt% or less of the binder based on the total solid content.

Step c) may be performed at a temperature of 50 to 200° C. for 30 to 300 seconds.

Another embodiment includes 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 the product of step b), wherein steps a) and b) are performed simultaneously or sequentially.

Another embodiment is the electrode; separator; And it provides a lithium secondary battery containing an electrolyte solution.

The present invention can improve interfacial adhesion between a current collector and an electrode active material layer, improve process defects such as electrode separation, and improve rapid charging performance.

1 and 2 are cross-sectional SEM images in the thickness direction of cathodes prepared by Example 1 and Comparative Example 6;
3 and 4 are EDS mapping results in the thickness direction of the cathode prepared in Example 1,
5 is a schematic diagram showing a form in which the current collector and the negative electrode active material layer are separated by measuring the 90 ^° bending adhesive force of the negative electrodes prepared in Examples 1 to 4 and Comparative Examples 1 to 6;
6a and 6b are photos of an evaluation method for interfacial adhesion (90 ^° bending adhesion) between an electrode active material layer and a current collector;
Figure 6c is a photograph of the negative electrode separated after measuring the 90 ^° bending adhesive force for the negative electrode according to Example 1 and Comparative Example 1;
7 and 8 are graphs of interfacial adhesion between a current collector and an anode active material layer measured at positions in the width direction of the anode active material layer and a graph of the adhesion (%) distribution.

Advantages and features of the present invention, and methods of achieving them, will become clear with reference to the detailed description of the following embodiments taken in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in various different forms, only these embodiments make the disclosure of the present invention complete, and common knowledge in the art to which the present invention belongs. It is provided to fully inform the holder of the scope of the invention, and the present invention is only defined by the scope of the claims. With reference to the accompanying drawings below, specific details for the practice of the present invention will be described in detail. Like reference numbers refer to like elements, regardless of drawing, and "and/or" includes each and every combination of one or more of the recited items.

Unless otherwise defined, all terms (including technical and scientific terms) used in this specification may be used in a meaning commonly understood by those of ordinary skill in the art to which the present invention belongs. When it is said that a certain part "includes" a component throughout the specification, this means that it may further include other components, not excluding other components unless otherwise stated. In addition, singular forms also include plural forms unless specifically stated in the text.

In this specification, when a part such as a layer, film, region, plate, etc. is said to be “on” or “on” another part, this includes not only the case where it is “directly on” the other part, but also the case where there is another part in the middle. do.

One embodiment of the present invention provides an electrode for a lithium secondary battery. The electrode for a lithium secondary battery may include a current collector; and an electrode active material layer positioned on at least one surface of the current collector, and satisfies the following relational expression 1.

[Relationship 1]

t ₂ ≤ t ₁ ≤ 8×t ₂

In the relational expression 1, t ₁ is the thickness of the electrode active material layer excluding the current collector on the side close to the current collector based on the position where the electrode is separated from the inside of the electrode active material layer when measuring the 90 ^° bending adhesive force of the electrode, and t ₂ is the electrode active material layer thickness. It is the particle size (D50) of the electrode active material included in the active material layer.

The electrode active material layer may be formed by applying a binder suspension on at least one surface of the current collector and coating the electrode slurry on the applied binder suspension, or by drying a result of simultaneously applying the binder suspension and the electrode slurry on at least one surface of the current collector. there is.

The binder suspension may be prepared by including a binder and a solvent. The suspension refers to a mixture in which binders are not dissolved and present in particulate form in a solvent, and a thickener, a conductive material, and the like may be additionally mixed and used as necessary.

The binder may include a styrene butadiene rubber (SBR)-based binder, and examples thereof include styrene-butyl rubber and styrene-butyl acrylate copolymer, but the present invention is not limited thereto. Accordingly, the electrode active material layer may include a styrene butadiene rubber (SBR)-based binder.

The electrode active material layer may 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, the total amount of binders included in the entire active material layer can be significantly reduced by distributing a plurality of binders at the interface between the current collector and the active material layer and reducing the binder content on the surface of the electrode. Accordingly, interface adhesion between the current collector and the active material layer may be improved, and at the same time, rapid charging performance may be improved.

When the SBR-based binder or the like is used, the viscosity of the binder suspension is very low because the binder is mixed in particulate form. In addition, since the size of the binder particles is composed of a small size of 200 nm or less, when the upper electrode slurry is applied and dried, the diffusion of the binder particles into the upper electrode active material layer is easy by osmotic pressure, and even after drying, the current collector and the active material layer A clear binder layer may not be formed therebetween. In addition, since the coating property with the current collector is good, it can be applied uniformly with a relatively thin thickness in the width direction of the current collector without forming a separate pattern, so that the adhesion between the current collector and the electrode active material layer can be uniformly improved. On the other hand, in the case of polyacrylic acid (PAA), polyvinylidene fluoride (PVdF), and carboxymethyl cellulose (CMC), which can be used as electrode binders in addition to SBR binders, they are applied in a solvent-dissolved state, unlike the SBR binders. When the electrode is dried, the solvent is sufficiently dried, and then the phase is separated to form a binder layer. Accordingly, the binder is not easily migrated into the upper electrode active material layer during the drying process, and furthermore, a distinct binder layer may be formed between the current collector and the active material layer. In addition, it is not uniformly distributed in the width direction of the current collector and forms a pattern (for example, an island type or a dot type), resulting in poor adhesion between the current collector and the active material layer and poor interface resistivity.

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.

The binder suspension may further include a thickening agent for creating a stable solution by imparting viscosity. For example, the thickener may be a mixture of one or more cellulose-based compounds, specifically, carboxymethyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose, or alkali metal salts thereof. As the alkali metal, Na, K or Li may be used.

The conductive material is used to impart conductivity to the electrode, and is not particularly limited as long as it is a conventional electronically conductive material that does not cause chemical change in the battery. For example, one selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanotube, and combinations thereof may be used, but is not limited thereto.

The current collector may be selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a conductive metal-coated polymer substrate, and combinations thereof. Not limited.

The viscosity of the binder suspension may be 1 to 10,000 cps, or 5 to 5,000 cps, or 10 to 2,000 cps. When using the binder suspension having the above viscosity, the binder suspension can be uniformly applied on the current collector, and the binder particles can be well migrated upward in the drying process.

The electrode active material layer is formed by applying a binder suspension and applying an electrode slurry on the coated binder suspension. Alternatively, it may be formed by simultaneously applying the binder suspension as a lower layer and the electrode slurry as an upper layer.

When the electrode is a positive electrode, the electrode active material may be used without limitation as long as it is a positive electrode active material commonly used in a secondary battery. For example, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiCoPO ₄ , LiFePO ₄ , LiNiMnCoO ₂ and LiNi _1-xyz Co _x M ¹ _y M ² _z O ₂ (M ¹ and M ² are each independently Al, Ni , Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg, and Mo, and x, y, and z are independently atomic fractions of oxide composition elements, 0 ≤ x < 0.5 , 0 ≤ y < 0.5, 0 ≤ z < 0.5, x + y + z ≤ 1).

When the electrode is a negative electrode, the electrode active material may be used without limitation as long as it is a negative electrode active material commonly used in a secondary battery. For example, it may be a carbon-based negative active material, a silicon-based negative active material, or a mixture thereof, but is not limited thereto. The carbon-based negative electrode active material may be one or more selected from artificial graphite, natural graphite, and hard carbon. Silicon-based negative electrode active materials include Si, SiO _x (0<x<2), Si-Q alloy (Q is 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, It is an element selected from the group consisting of rare earth elements and combinations thereof, but not Si), a Si-carbon composite, or a mixture of at least one of these and SiO ₂ .

The electrode slurry may further include a conductive material, a binder, a thickener, or a combination thereof as needed. The conductive material and the thickener may use materials used in the above-described binder suspension, and may be the same as or different from them, but the present invention is not limited thereto.

The electrode slurry may contain 90% by weight or more, preferably 90 to 99.5% by weight, 95 to 99.5% by weight, or 98 to 99.5% by weight of the electrode active material, based on the total solid content, and 2.0% by weight or less of the binder , or 1.5% by weight or less, 1.0% by weight or less, or may not be included, and may include the conductive material and the thickener as the balance. Even if the binder content of the electrode slurry is prepared low, the interface adhesion between the current collector and the electrode active material layer can be increased due to the migration of binder particles when the binder suspension is dried, and the resistance of the electrode surface can be lowered to improve rapid charging performance. .

An electrode for a lithium secondary battery according to an embodiment of the present invention is characterized in that it satisfies the following relational expression 1 when measuring the 90 ^o bending adhesive force of the electrode.

On the other hand, as a result of evaluating the 90 ^° bending adhesion of the electrode, at least 90 or more, or more than 95 electrodes out of 100 manufactured electrode samples may satisfy the following relational expression 1.

[Relationship 1]

t ₂ ≤ t ₁ ≤ 8×t ₂

In the relational expression 1, t ₁ is the thickness of the electrode active material layer excluding the current collector on the side close to the current collector based on the position where the electrode is separated from the inside of the electrode active material layer when measuring the 90 ^° bending adhesive force of the electrode, and t ₂ is the electrode active material layer thickness. It is the particle size (D50) of the electrode active material included in the active material layer.

In the relational expression 1, the electrode active material is one kind of electrode active material having the same particle diameter, and t ₂ may be the particle size (D50) of the electrode active material.

In the relational expression 1, the electrode active material is a mixture of two or more types of electrode active materials having different particle sizes, and t ₂ may be the particle size (D50) of one type of electrode active material included in the highest weight among the mixed electrode active materials. .

In the relational expression 1, the electrode active material is a mixture of two or more types of electrode active materials having different particle sizes, and t ₂ may be a particle size (D50) of a large particle size electrode active material among the mixed electrode active materials. In this case, the large particle size electrode active material may mean an electrode active material having the largest particle size among two or more types of electrode active materials having different particle sizes.

More specifically, in the relational expression 1, t ₂ ≤ t ₁ ≤ 7×t ₂ , t ₂ ≤ t ₁ ≤ 6×t ₂ , t ₂ ≤ t ₁ ≤ 5×t ₂ , t ₂ ≤ t ₁ ≤ 4×t ₂ , t ₂ ≤ t ₁ ≤ 3×t ₂ or t ₂ ≤ t ₁ ≤ 2×t ₂ , 1.5×t ₂ ≤ t ₁ ≤ 7×t ₂ , 1.5×t ₂ ≤t ₁ ≤6× t ₂ or 1.5×t ₂ ≤ t ₁ ≤ 5.5×t ₂ .

Therefore, the electrode of the present invention that satisfies the relational expression 1 may be one in which separation occurs inside the electrode active material layer when the electrode's 90 ^° bending adhesive force is measured, and specifically, for example, when viewed in the electrode thickness direction, from the current collector It may be separated at a position corresponding to the thickness of the medium particle size (D50) of the electrode active material particle, or at a position corresponding to a thickness five times the medium particle size (D50). That is, when measuring the 90 ^° bending adhesive strength of the electrode, the separation position within the electrode active material layer may be a position where the thickness of the electrode active material layer closer to the current collector is t ₂ to 5×t ₂ .

Specifically, it can be understood that the electrode active material layer comprises (one) layer in which active material particles are uniformly arranged in the width direction (at this time, the layer thickness is D50 of the electrode active material), and such a plurality of layers are laminated in the thickness direction. That is, in the present invention, it is analyzed that the above result is derived from separation at the position of one thickness (D50) of one active material particle in the thickness direction when measuring the adhesive force at 90 ^° to five thicknesses (5 × D50) of the particle unit.

As a result, the electrode of the present invention can improve process/appearance defects such as interfacial separation of the negative electrode and improve rapid charging performance even if the binder content is significantly lowered than in the prior art in the negative electrode slurry forming the negative electrode active material layer.

The electrode may further satisfy the following relational expression 2, and thus, the above-described effect may be further improved.

[Relationship 2]

1.5×t ₂ ≤ t ₁ ≤ 5×t ₂

In the relational expression 2, t ₁ is the thickness of the electrode active material layer excluding the current collector on the side close to the current collector based on the position where the electrode is separated from the inside of the electrode active material layer when the 90 ^° bending adhesive force of the electrode is measured, and t ₂ is the electrode active material layer thickness. It is the particle size (D50) of the electrode active material included in the active material layer.

More specifically, in relational expression 2, 1.5×t ₂ ≤t ₁ ≤4×t ₂ , 1.5×t ₂ ≤t ₁ ≤3×t ₂ or 1.5×t ₂ ≤t ₁ ≤2×t ₂ may be satisfied.

On the other hand, the electrode active material layer is formed by applying a binder suspension on at least one surface of the current collector and applying the electrode slurry on the applied binder suspension, or applying the binder suspension on at least one surface of the current collector and applying the electrode slurry on the binder suspension It may be formed by simultaneously performing the steps of doing, and then drying the result.

Meanwhile, in the present invention, the electrode active material may have a particle size (D50) of 1 to 20 μm, 3 to 15 μm, 7 to 15 μm, or 9 to 15 μm, but is not limited thereto. The particle size (D50) may mean a particle diameter when the cumulative volume becomes 50% from a small particle diameter in the particle size distribution measurement by the laser scattering method. Here, D50 may take a sample according to KS A ISO 13320-1 standard and measure the particle size distribution using Malvern's Mastersizer3000. Specifically, after dispersing using ethanol as a solvent and using an ultrasonic disperser if necessary, the volume density can be measured.

The electrode may further satisfy the following relational expression 3.

[Relationship 3]

0.25 ≤ b ₂ /b ₁ < 0.7

In the relational expression 3, when the binder distribution is measured 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 binder in the 15% area of the total thickness of the electrode active material layer from the current collector is the weight of

In the relational expression 3, the b ₂ /b ₁ binder weight ratio is, when a styrene-butadiene rubber (SBR)-based binder is used as the binder, the content (at%) of the Os element can be applied by adsorbing Os gas to the binder. However, this is not limited to the Os element, and may be an element capable of representing the binder depending on the type of the binder.

Specifically, 0.25 ≤ b ₂ /b ₁ < 0.6 or 0.3 ≤ b ₂ /b ₁ < 0.6.

The electrode may further satisfy the following relational expression 4, and in this case, the above-described effect may be further improved.

[Relationship 4]

0.3 ≤ b ₂ /b ₁ < 0.5

In the relational expression 4, when measuring the distribution of the binder 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 binder in the 15% area of the total thickness of the electrode active material layer from the current collector is the weight of

An area of 15% of the total thickness of the electrode active material layer from the current collector may include 0.2 to 7 wt%, or 1 to 5 wt%, or 1 to 4.5 wt% of the binder based on the total solid weight of the area. there is.

In addition, the electrode active material layer is characterized in that the binder has a continuous concentration in the electrode thickness direction.

In the present specification, the distribution of the binder in a 'continuous' manner means that the binder suspension and the electrode slurry are not formed as separate and separated layers, but the binder is continuously continuously in the electrode active material layer, and accordingly, the concentration of the binder (% by weight) ) may mean that the value of the electrode active material layer is continuous in the thickness direction. On the other hand, the 'concentration of the binder' may mean the content (wt%) of the binder with respect to the total weight of the electrode active material layer of the unit volume (cross-sectional area in the width direction × unit thickness) of the electrode active material layer.

More specifically, the migration of the binder particles occurs from the interface with the current collector in the electrode active material layer to a specific region of the active material layer, and the binder may be densely distributed and continuously present in the corresponding region. However, since a small amount of binder included in the electrode slurry may exist in the region of the electrode active material layer where migration of binder particles does not occur from the interface, the binder is distributed at a relatively high concentration in the region where migration of the binder occurs possible. That is, it may exist in a continuous concentration in a specific region of the electrode active material layer due to migration of the binder particles in the binder suspension applied on the current collector.

For example, when the interface between the current collector and the electrode active material layer is viewed as 0% of the thickness, it is located on the current collector side, for example, in the area of 0 to 35% of the total thickness of the electrode active material layer, or in the area of 0 to 55%, or It may have a continuous binder concentration in the electrode thickness direction in the 0 to 75% or 0 to 95% region, or the 0 to 100% region (the entire electrode active material layer).

Accordingly, since adhesion (interfacial adhesion) at the interface between the current collector and electrode active material layer is stronger than cohesion (internal cohesion) inside the negative electrode active material layer, interfacial adhesion can be increased, and the insertion/desorption rate of lithium ions inside the electrode active material layer is improved. As is improved, the rapid charging performance of the battery can be improved.

The electrode may further satisfy the following relational expression 5.

[Relationship 5]

-30% ≤ (C-D)/D ≤ +30%

In Equation 5, C is the interfacial adhesive force between the current collector and the electrode active material layer measured at an arbitrary position selected in the width direction of the electrode active material layer, and D is the average value of the interfacial adhesive force between the current collector and the electrode active material layer.

Specifically -25% ≤ (C-D)/D ≤ +25%, or -20% ≤ (C-D)/D ≤ +20%, or -15% ≤ (C-D)/D ≤ +15%, or -10% ≤ (C-D)/D ≤ +10%. In the relational expression 4, C may be each measured at a position having a regular interval in the width direction of the electrode active material layer, for example, 0.1 to 0.5 mm, or 0.2 to 0.3 mm, for example, 0.25 mm interval, but the present invention is limited thereto. it is not going to be

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

Therefore, in the present invention, by using the binder suspension, specifically, a suspension containing a specific binder such as SBR, the high-content binder suspension is uniformly applied in the width direction of the current collector at a relatively thin thickness without forming a separate pattern, and at the same time As the binder particles migrate to a specific region of the electrode active material layer, adhesion between the current collector and the electrode active material layer may be further improved.

In addition, the electrode may have an interface resistivity value of 0.1 Ωcm ² or less, 0.05 Ωcm ² or less, or 0.03 Ωcm ² or less between the current collector and the electrode active material layer. Due to the effects of the present invention described above, the interface resistivity value can be remarkably reduced.

Another embodiment of the present invention provides a method for manufacturing the electrode for a lithium secondary battery. The electrode manufacturing method includes 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 the product of step b), wherein steps a) and b) are performed simultaneously or sequentially.

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

The types of the binder and the solvent and the current collector are as described above. A known method may be applied to the preparation method of the binder suspension. For example, it may be prepared by mixing a specific binder such as the SBR-based binder in a solvent and diluting it to have a suitable viscosity, but the present invention is limited thereto. It is not.

Step a) may be to uniformly apply the binder suspension to at least one surface of the current collector. Uniformly applying the binder suspension in the present specification means uniformly applying the binder suspension onto the current collector so that the binder does not form a specific pattern.

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. At this time, the coating thickness of the binder suspension represents a value obtained by measuring the thickness in a sufficiently dried state after coating only the binder suspension. If the thickness of the applied binder suspension is excessive, the binder suspension does not mix well with the electrode slurry, so that after drying, a clear distinction between the layers and the formation of an insulator binder layer may increase the interfacial resistance. On the other hand, when the coating thickness of the binder suspension is less than 0.1 μm, it may be difficult to achieve the intended purpose of the present invention. That is, it is possible to prevent an increase in interfacial resistance within the aforementioned thickness range, improve interfacial adhesion between the current collector and the electrode active material layer, and improve process defects such as electrode detachment.

According to one embodiment, the binder suspension may include 30 wt% or more of the binder based on the total solid content, and the electrode slurry may include 2 wt% or less of the binder based on 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 a binder based on the total solid content, and the electrode slurry contains 2 wt% or less, 1.5 wt% or less of the binder based on the total solid content. It may include less than 1% by weight or less by weight.

After step a), step b) applies an electrode slurry containing an electrode active material on top of the binder suspension.

The electrode active material is as described above, and any method known to be used for forming a known electrode slurry for a secondary battery may be used as a manufacturing method of the electrode slurry.

The coating of the binder suspension in step a) and the coating of the electrode slurry in step b) are non-limiting examples, and any coating method known to be generally used for forming a film by applying a liquid phase may be used. For example, spray coating, dip coating, spin coating, gravure coating, slot die coating, doctor blade coating, roll coating, inkjet printing, slot die coating, lexography printing, screen printing, electrostatic printing, micro contact printing, Imprinting, reverse offset printing, bar-coating, gravy offset printing, multi-layer simultaneous die coating, etc. may be used, but is not limited thereto.

Specifically, the binder suspension and the electrode slurry may be sequentially applied, or the binder suspension and the electrode slurry may be simultaneously applied by a multi-layer simultaneous die coating method. However, it may be preferable in terms of uniformity or quality of the surface of the electrode to apply the electrode slurry after applying the binder suspension.

In step c) after step b), the product of step b) is dried.

At this time, the drying is 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. . In addition, the drying is 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 the binder particles may be excessive and the interfacial adhesion may not be sufficiently implemented. In one embodiment, step c) may be performed at a temperature of 80 to 130 ° C for 30 to 300 seconds.

Subsequently, an electrode having an electrode active material layer formed on a current collector may be manufactured by rolling the dried electrode to an appropriate density. At this time, it is okay to apply the rolling conditions and rolling methods such as the rolling and known rolling density, and the present invention is not limited thereto.

In the method for manufacturing an electrode for a secondary battery 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 certain region of the electrode active material layer from the interface between the current collector and the electrode active material layer. As a result, it is possible to improve the problem of reducing the interface resistivity value and adhesive strength between the current collector and the active material layer, which is a problem that occurs when the conventional binder solution is applied.

Another embodiment of the present invention is a) applying a binder suspension to at least one surface of the current collector; b) applying an electrode slurry containing an electrode active material on top of the binder suspension; and c) drying the product of step b), wherein steps a) and b) are performed simultaneously or sequentially to provide an electrode for a lithium secondary battery manufactured by a method for manufacturing an electrode for a secondary battery.

At this time, the electrode is as described above.

In addition, the present invention the electrode; separator; And an electrolyte; it provides a secondary battery comprising a.

The electrode is as described above.

The separator is not particularly limited as long as it is a separator known in the art. For example, it may be selected from glass fibers, polyester, polyethylene, polypropylene, polytetrafluoroethylene, or combinations thereof, may be in the form of non-woven fabric or woven fabric, and may optionally be used in a single layer or multi-layer structure.

The electrolyte solution includes a non-aqueous organic solvent and an electrolyte salt. The non-aqueous organic solvent is ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), 1,2-dimethoxy Ten (DME), γ-butyrolactone (BL), tetrahydrofuran (THF), 1,3-dioxolane (DOL), diethylether (DEE), methyl formate (MF), methyl propio nate (MP), sulfolane (S), dimethyl sulfoxide (DMSO), acetonitrile (AN), or mixtures thereof, but is not limited thereto. The electrolytic salt is a material that dissolves in a non-aqueous organic solvent, acts as a source of electrolytic metal ions in a battery, enables basic operation of a secondary battery, and promotes the movement of electrolytic metal ions between an anode and a cathode. For a non-limiting example, when the electrolyte metal is lithium, the electrolyte salt is LiPF ₆ , LiBF ₄ , LiTFSI, LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li(CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , LiN(C _x F _2x+1 SO ₂ )(C _y F _2y+1 SO ₂ ) (x, y are natural numbers), LiCl, LiI Or it may be a mixture thereof, but is not limited thereto. In addition, the electrolyte salt may use a known material at a concentration suitable for the purpose, and may further include a known solvent or additive to improve charge/discharge characteristics, flame retardancy characteristics, etc., if necessary.

In the method for manufacturing a lithium secondary battery according to the present invention for achieving the above object, an electrode assembly is formed by sequentially stacking a negative electrode, a separator, and a positive electrode, and the manufactured electrode assembly is a cylindrical battery case or a prismatic battery case Then, a battery can be manufactured by injecting an electrolyte solution. Alternatively, it may be manufactured by laminating the electrode assembly, impregnating it with an electrolyte, and sealing the obtained result by putting it in a battery case.

The battery case used in the present invention may be one commonly used in the field, and there is no limitation on the external appearance according to the purpose of the battery, for example, a cylindrical shape using a can, a prismatic shape, a pouch shape, or a coin ( coin) type, etc.

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 can be preferably used as a unit cell in a medium-large battery module including a plurality of battery cells. Preferred examples of the medium or large-sized device include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, and the like.

Hereinafter, the present invention will be described in detail through examples, but these are for explaining the present invention in more detail, and the scope of the present invention is not limited by the following examples.

Example

(Example 1)

<Manufacture of cathode>

A binder suspension was prepared by diluting a suspension of SBR (BM451B from Zeon) with pure water as a binder.

Artificial graphite (D50: 13㎛): Anode active material mixed with natural graphite (D50: 10㎛) in a weight ratio of 5:5, CMC thickener, and SBR binder are added to water in a weight ratio of 98.5:1:0.5 to obtain a viscosity of 5,000 cps. A negative electrode slurry was prepared.

The binder suspension and the negative electrode slurry prepared on one side of the copper current collector (8㎛ thick copper foil) are coated with a multi-layer simultaneous die coating method using a slot die, respectively, to a thickness of 1㎛ (based on the thickness after drying when the binder suspension is applied alone) and After applying to a thickness of 200㎛, it was dried, and after applying the same to another surface, it was dried. At this time, the drying conditions were those described in Table 5 below.

The dried negative electrode was rolled (rolling density: 1.68 g/cm ³ ) to prepare a negative electrode having a current collector negative active material layer formed thereon.

In the negative electrode prepared at this time, the solid composition of the negative electrode active material layer was 97.5% by weight of the negative electrode active material, 1.5% by weight of the SBR binder, and 1% by weight of the CMC thickener. In addition, the prepared negative electrode was formed with a copper foil thickness of 8 μm and a negative electrode active material layer thickness of 127 μm, and it was confirmed that the boundary between the binder layer and the negative active material layer was not clearly distinguished and formed as one negative active material layer on the SEM image. (see Figure 1).

<Manufacture of positive electrode>

A slurry was prepared by mixing Li[Ni _0.88 Co _0.1 Mn _0.02 ]O ₂ as a cathode 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 and vacuum dried to prepare a cathode for a secondary battery.

<Manufacture of secondary battery>

Positive and negative electrodes are laminated by notching to a predetermined size, and an electrode cell is formed with a separator (polyethylene, thickness 13㎛) interposed between the positive and negative electrodes, and then the tabs of the positive and negative electrodes are welded, respectively. did The welded anode/separator/cathode assembly was placed in a pouch and sealed on three sides except for the electrolyte injection side. At this time, the part with the electrode tab was included in the sealing part.

The electrolyte solution was injected through the remaining surface except for the sealing part, and after sealing the remaining surface, it was impregnated for 12 hours or more.

The electrolyte was prepared by dissolving 1M LiPF ₆ in a mixed solvent of EC/EMC/DEC (25/45/30; volume ratio), followed by 1 wt% of vinylene carbonate (VC) and 0.5 wt% of 1,3-propensultone (PRS). and 0.5wt% of lithium bis(oxalato)borate (LiBOB) was added.

Then, pre-charging was performed for 36 minutes with a current corresponding to 0.25C. After degasing after 1 hour and aging for more than 24 hours, chemical charging and discharging was performed (charging condition CC-CV 0.2C 4.2V 0.05C CUT-OFF, discharging condition CC 0.2C 2.5V CUTOFF).

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).

(Example 2)

In coating the binder suspension and the negative electrode slurry, the negative electrode, the positive electrode and the secondary battery were prepared in the same manner as in Example 1, except that the binder suspension was first applied through gravure coating and then the negative electrode slurry was applied using a slot die. did

(Example 3)

Negative, positive and secondary batteries were prepared in the same manner as in Example 1, except that a binder suspension was prepared by mixing and diluting SBR (Zeon's BM451B) and CMC (Daicel's D2200) at a solid content ratio of 97:3, respectively. was manufactured.

(Example 4)

The binder suspension and the negative electrode slurry prepared on one side of the copper current collector (8㎛ thick copper foil) are coated with a multi-layer simultaneous die coating method using a slot die to obtain a thickness of 7㎛ (based on the thickness after drying when the binder suspension is applied alone) and A negative electrode, a positive electrode, and a secondary battery were prepared in the same manner as in Example 1, except that the coating was applied to a thickness of 132 μm.

(Comparative Example 1)

Artificial graphite (D50: 13㎛): Anode active material mixed with natural graphite (D50: 10㎛) in a weight ratio of 5:5, SBR binder and CMC thickener are added to water in a weight ratio of 97.5: 1.5: 1 to obtain a viscosity of 5,000cps. A negative electrode slurry was prepared.

The negative electrode slurry prepared on one side of the copper current collector (copper foil having a thickness of 8 μm) was applied using a slot die in a die coating method, and then dried, and then applied to the other side in the same manner and then dried. The dried negative electrode was rolled (rolling density: 1.68 g/cm ³ ) to prepare a negative electrode having a negative electrode active material layer formed on a current collector.

At this time, the negative electrode was formed with a copper foil thickness of 8 μm and a negative electrode active material layer thickness of 126 μm.

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

(Comparative Example 2)

A first negative electrode slurry was prepared by adding a negative electrode active material in which artificial graphite:natural graphite was mixed in a weight ratio of 5:5, an SBR binder, and a CMC thickener to water in a weight ratio of 97:2:1.

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

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

(Comparative Example 3)

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

(Comparative Example 4)

A negative electrode, a positive electrode, and a secondary battery were prepared in the same manner as in Example 1, except that a binder suspension was prepared using PAA (Sumitomo Seika SW100) as a binder.

(Comparative Example 5)

CMC (Daicel D2200) and PAA (Sumitomo Seika SW100) were mixed and diluted at a solid content ratio of 1:3 to prepare a binder solution, but the negative electrode, positive electrode and secondary battery were proceeded in the same manner as in Example 1. was manufactured.

(Comparative Example 6)

The prepared binder suspension was applied to one surface of the copper current collector by a slot die coating method and then dried to form a binder layer. On the formed binder layer, a negative electrode slurry was applied by a slot die coating method and then dried to form a negative electrode active material layer. did Then, a negative electrode, a positive electrode, and a secondary battery were prepared in the same manner as in Example 1, except that a negative electrode having a binder layer and a negative active material layer formed on the current collector was prepared by rolling (rolling density: 1.68 g/cm ³ ).

[Evaluation Example 1] Evaluation of binder distribution characteristics in the cathode thickness direction and cathode surface direction by EDS (energy dispersive x-ray spectroscopy) mapping

The thickness direction cross-sectional SEM image results of the electrodes prepared in Example 1 and Comparative Example 6 are shown in FIGS. 1 and 2, respectively, and the EDS mapping results in the thickness direction of the electrodes prepared in Example 1 are shown in FIGS. shown in Figure 4. At this time, since the distribution of the SBR binder is difficult to distinguish by general EDS mapping, after sufficiently exposing Os gas to the electrode, the EDS mapping analysis of the Os element is shown in FIG. 3, and the SBR in the electrode thickness direction ( Os) content mass % profile graph is shown in FIG.

Referring to FIG. 1 , in the electrode prepared in Example 1, it can be seen that the distinction between the binder layer and the active material layer is not clear, which is analyzed because the SBR binder migrates into the electrode active material layer. By using the SBR binder suspension, the binder suspension can be applied with a sufficiently thin thickness, and as the SBR binder particles diffuse into the electrode slurry layer during the electrode drying process, the contact between the current collector and the active material layer is different from that of not additionally applying the binder layer. It can be confirmed by SEM image that they are formed very closely together. On the other hand, in FIG. 2, a binder layer was applied and dried to form a separate binder layer distinct from the electrode active material layer, and it can be seen from the SEM image that the binder layer and the active material layer were respectively formed on the current collector.

Therefore, it was confirmed that an electrode having no increase in interface resistivity can be manufactured even though a binder layer, which is an insulator, is uniformly applied in the width direction without applying a conventional pattern coating.

Referring to FIGS. 3 and 4 , it can be seen that the binder is continuously distributed at a relatively high concentration in some regions of the active material layer from the boundary between the current collector and the active material layer. From these results, not only can the adhesion of the boundary between the current collector and the electrode active material layer be greatly improved, but also the adhesion of a certain area under the electrode active material layer can be improved because the binder particles are diffused into the electrode active material layer during the drying process. ) can also be improved.

In addition, the results of analyzing the cross-sectional SEM image and EDS mapping of the electrode in the thickness direction for Examples 1 to 4 and Comparative Examples 1 to 6 are shown in Table 1 below. Specifically, the content of Os element a ₁ (at%) per unit thickness was measured for the entire area of the active material layer, and was converted to the content of Os element in the entire area (b ₁ ). The content a ₂ (at%) of the Os element per unit thickness was measured for the area corresponding to the 15% thickness from the current collector, and was converted to the total Os content of the 15% thickness (b ₂ ). Subsequently, b ₂ /b ₁ was calculated and shown in Table 1 below.

Entire area of the active material layer 15% thick area from the current collector b ₂ /b ₁ a ₁ (Os at% per unit thickness) _b1
(= a ₁ × 1.0) a ₂ (Os at% per unit thickness) _b2
(= a ₂ ×0.15) (wt ratio) Example 1 1.08 1.08 2.84 0.426 0.394 Example 2 1.04 1.04 3.02 0.453 0.436 Example 3 1.09 1.09 2.65 0.397 0.365 Comparative Example 1 1.00 1.00 0.92 0.138 0.138 Comparative Example 2 1.04 1.04 1.54 0.231 0.222 Comparative Example 3 not measurable Comparative Example 4 not measurable Comparative Example 5 not measurable Comparative Example 6 1.03 1.03 3.77 0.565 0.549 Example 4 2.86 2.86 12.38 1.857 0.649

In Examples 1 to 3, it was confirmed that a plurality of binders were distributed at the interface between the current collector and the active material layer, but in Comparative Example 1, due to the migration phenomenon of the binder, the average binder content of the entire active material layer was at the interface between the current collector and the active material layer. It can be seen that the distributed binder is insufficient. In Comparative Example 2, the binder content distributed at the interface increased compared to Comparative Example 1, but showed a lower value when compared to Examples. This means that the current collector and the active material layer are relatively more strongly bound in Examples 1 to 3, and also indicates that the binder content on the surface of the active material layer is low, which can be advantageous for fast charging performance.

[Evaluation Example 2] Evaluation of interlayer bonding strength and interface resistivity of electrodes

1) Measurement of 90° bending adhesion of the electrode

The 90 ° bending adhesive force was measured for the electrodes prepared in Examples 1 to 4 and Comparative Examples 1 to 6, and the measured adhesive force and the position separated from the electrode are shown in Table 2 below. In addition, the form in which the current collector and the electrode active material layer are separated is schematically shown in FIGS. 5A to 5D, respectively.

* Evaluation of interfacial adhesion between electrode active material layer and current collector

The cathodes prepared in Examples 1 to 4 and Comparative Examples 1 to 6 were cut into 18 mm in width / 150 mm in length, and a single-sided tape of 18 mm width was placed at 100 mm position except for about 50 mm in length as a joint part of a tensile tester on one side of the double-coated electrode ( manufactured by 3M), and then sufficiently bonded with a roller having a load of 2 kg. After attaching the double-sided tape to the bottom surface of the tensile tester, it was attached so that the double-sided tape surface and the single-sided tape surface of the prepared electrode faced each other. The part where the single-sided tape was not attached was fastened to the opposite side of the tensile tester, and the 90 ° bending adhesive force was measured (FIGS. 6a and 6b), and the results obtained by dividing the measured strength by the width of the tape are summarized in Table 2 below. In addition, after measuring the adhesion of the negative electrodes according to Example 1 and Comparative Example 1, a photograph of the separated negative electrodes is shown in FIG. 6C.

On the other hand, i) the average separation position (t ₁ ) measures the thickness of the part including the current collector among the two separated parts by measuring the 90 ^° bending adhesive force using a double-sided coated electrode, and then measuring the thickness of the current collector and the rear surface The thickness of the active material layer was subtracted from the value. The thickness of the electrode was measured using a 6.35pi tip micrometer (293 series, Mitsutoyo Co.) with a measuring force of 5N, and the measurement was performed 10 times, and the average value of 8 times excluding the upper and lower limit values was taken.

ii) The active material particle size (t ₂ ) is the graphite active material particle size (D50) used in Examples and Comparative Examples, and among them, the large particle size graphite active material particle size (D50) was used. At this time, the particle size of the large-diameter graphite active material was 13 μm.

Adhesion (N/cm) Average separation position (t ₁ )
(um) Active material particle size (t ₂ ) (D50) Whether relational expression 1 is satisfied
(t ₂ ≤ t ₁ ≤ 8×t ₂ ) Separation Position Classification Example 1 0.25 20.3 13 O Cohesion Example 2 0.27 21.5 13 O Cohesion Example 3 0.23 22.3 13 O Cohesion Comparative Example 1 0.10 4.2 13 X Adhesion (current collector - active material layer) Comparative Example 2 0.22 6.8 13 X Adhesion (current collector - active material layer) Comparative Example 3 0.03 9.1 13 X Adhesion (binder layer-active material layer) Comparative Example 4 0.02 9.2 13 X Adhesion (binder layer-active material layer) Comparative Example 5 0.03 9.6 13 X Adhesion (binder layer-active material layer) Comparative Example 6 0.14 11.8 13 X Adhesion (binder layer-active material layer) Example 4 0.29 33.7 13 O Cohesion

Figure 5a is a schematic diagram showing the results of measuring the 90 ^o bending adhesive strength of the electrodes prepared in Examples 1 to 3. Referring to Table 2 and FIG. 5A, it was found that since the cohesion inside the electrode active material layer is weaker than the adhesion at the interface between the current collector and the electrode active material layer, the separation occurs inside the negative electrode active material layer. That is, in order to be in the form of FIG. 5A, the content of the binder in the electrode active material layer is low, and this result can be seen that the content of the binder present in the electrode active material layer is low in relation to the binder distribution. A photograph of the separated cathode after measuring the adhesive force of Example 1 can be seen in FIG. 6C.

5B is an electrode manufactured by Comparative Examples 1 and 2, and the current collector and the electrode active material layer are separated when measuring the 90 ° bending adhesive force of a general electrode having an electrode active material layer formed by directly applying and drying the electrode slurry on top of the current collector The form is shown in a schematic diagram. Referring to Table 2 and FIG. 5B, in the above case, it was confirmed that separation occurred at the interface because the adhesion between the current collector and the electrode active material layer was lower than the cohesion inside the electrode active material layer.

Figure 5c is an electrode prepared by Comparative Examples 3 to 6, when an electrode active material layer is coated on a current collector dried after a binder layer is coated in advance (Comparative Example 6) or when mixed with a solvent such as SBR to implement a binder suspension It is a schematic diagram showing the form of measuring the 90 ° bending adhesive force of the electrode in the case of manufacturing using CMC and / or PAA, which is not a binder that can be used (Comparative Examples 3 to 5). In this case, separation occurs at the interface between the binder layer and the electrode active material layer. Referring to Table 2 and FIG. 5C, when CMC and PAA binders are used instead of SBR, and when the negative electrode active material layer is applied after drying the SBR binder suspension suggests that migration of the binder to the electrode active material layer is not sufficiently generated. Accordingly, it was confirmed that the binder layer and the electrode active material layer were clearly formed as individual layers, and since the adhesion between the formed binder layer and the negative electrode active material layer interface was the weakest, it was found that the corresponding position was separated.

On the other hand, Figure 5d is a negative electrode manufactured in Example 4, similar to Figure 5a, but shows a case where the boundary between the binder layer and the active material layer is clearly distinguished. Referring to Table 2 and FIG. 5D, when the SBR binder suspension is applied thicker than the standard value, the interfacial adhesion is improved due to the migration of the SBR particles, but the binder layer is formed thick and acts as an insulating layer.

On the other hand, referring to Table 2, it was confirmed that the electrode according to the present invention always has a thickness of 1.0 xt ₂ or more because the active material is separated in a state in which the active material remains in the form of a 'layer' on the current collector when the adhesive strength is measured. And when visually checking the photograph of Example 1 of FIG. 6C , it was found that the separated upper and lower layers were both active material layers. The average separation position satisfies relational expression 1 of the present invention.

Conversely, although the conventional electrode to which the technology of the present invention is not applied is said to be separated from the current collector and the interface, it was confirmed that the active material particles were not separated cleanly at the boundary and the active material particles were rugged and separated in particle units. Specifically, when visually checking the photograph of Comparative Example 1 of FIG. 6C, most of the Cu current collector is left in a small amount as if active material particles are scattered in some areas, which is different from remaining in the form of an 'active material layer' of Example 1. At this time, the average separation position thickness is 0 ≤ t ₁ < 1.0 x t ₂ , and does not satisfy relational expression 1 of the present invention. This result is analyzed to be because the tip size of the thickness meter is wide at 6.35 pi, so the average thickness is measured by including a large number of active material particles.

2) Measuring the interfacial resistivity of the electrode

For the electrodes prepared in Examples 1 to 4 and Comparative Examples 1 to 6, the interface resistance between the electrode and the current collector was measured using Hioki's interface resistivity value meter (Hioki's XF057), and the results are shown in Table 3 below. shown in

Interfacial resistivity value (ohm cm ² ) Example 1 0.007 Example 2 0.006 Example 3 0.008 Comparative Example 1 0.011 Comparative Example 2 0.012 Comparative Example 3 not measurable Comparative Example 4 not measurable Comparative Example 5 not measurable Comparative Example 6 0.083 Example 4 0.257

Referring to Table 3, the electrodes prepared in Examples 1 to 3 of the present invention had an interface resistivity value between the electrode and the current collector measured at a level similar to that of Comparative Examples 1 to 2 despite the application of the binder suspension. On the other hand, in Comparative Examples 3 to 5, reliable interfacial resistivity values could not be measured after rolling because sufficient adhesion was not secured. Comparative Example 6 suggests that the binder suspension is not mixed with the electrode active material, so that a binder insulator layer with clear division between these layers is formed after drying. Example 4 is analyzed to have the highest interfacial resistance as the insulator binder layer is formed thickly.

[Evaluation example 3] Quick charge performance evaluation

The secondary batteries prepared in Examples 1 to 4 and Comparative Examples 1 to 6 were charged at a C-rate of 2.5 C at a temperature of 25° C. and then discharged at a 1/3 C C-rate. After repeating 100 cycles and 200 cycles, the rapid charge capacity retention rate was measured, and the results are shown in Table 4 below.

Rapid charge capacity retention rate evaluation (%) 100 cycles 200 cycles Example 1 96 83 Example 2 97 84 Example 3 96 83 Comparative Example 1 79 58 Comparative Example 2 90 69 Comparative Example 3 Cell Manufacturing Fail Comparative Example 4 Cell Manufacturing Fail Comparative Example 5 Cell Manufacturing Fail Comparative Example 6 91 71 Example 4 82 53

Referring to Table 4, it was confirmed that the lithium secondary batteries prepared in Examples 1 to 3 had less decrease in cycle capacity retention (%) compared to Comparative Examples 1 to 3, and excellent fast charging performance was secured. In Comparative Examples 1 and 2, it was analyzed that the electrode resistance increased and the rapid charging characteristics deteriorated because the binder was uniformly dispersed on the surface of the electrode, and Comparative Examples 3 to 5 showed electrode active materials in the pressing and notching processes Cell manufacturing failed due to a defect in which the layer was partially detached from the current collector. In Comparative Example 6 and Example 4, as shown in Table 3, it was analyzed that the capacity characteristics deteriorated as the charge/discharge cycle progressed as the resistivity value of the current collector-active material layer interface increased.

[Evaluation Example 4] Evaluation of adhesion between the active material layer and current collector according to changes in electrode drying conditions

(Examples 5 to 9)

In step 2 of Example 1, an electrode was manufactured in the same manner as in Example 1, except that the drying process of the applied electrode slurry was described in Table 5 below.

(Comparative Example 7)

An electrode was prepared in the same manner as in Comparative Example 3, except that the drying process of Comparative Example 3 was described in Table 5 below.

(Assessment Methods)

Evaluation of interfacial adhesion between the active material layer and the current collector was performed in the same manner as in Evaluation Example 2, and the results are shown in Table 5 below.

division Binder type dry condition Adhesion (N/cm) Drying temperature (℃) Drying time (seconds) Example 1 SBR 120 60 0.25 Example 5 110 80 0.27 Example 6 100 100 0.28 Example 7 135 45 0.21 Example 8 150 30 0.17 Example 9 80 300 0.25 Comparative Example 7 CMC 100 100 0.05

Referring to Table 5, when the drying temperature is as low as 120 ° C or less (Examples 1, 5, 6 and 9), the drying time is increased to evaporate all the solvent, but the binder particles of the binder suspension migrate to the electrode active material layer. It can be seen that the adhesive force between the entire active material layer is increased. When the drying temperature is excessively high (Examples 7 and 8), the active material particles are easily exposed to the surface of the solvent during the drying process due to rapid drying, and it is analyzed that the adhesive force is slightly reduced due to excessive migration of the binder particles due to the capillary phenomenon. .

In the case of Comparative Example 7, despite performing the same drying process as in Example 6, which is the most excellent adhesive strength, it can be seen that it has the lowest adhesive strength.

[Evaluation Example 5] Binder Distribution Characteristics in the Width Direction of the Interface of the Electrode Active Material Layer

* Evaluation of interfacial adhesion between electrode active material layer and current collector

The negative electrode prepared in Example 1 was cut into 18 mm wide / 350 mm long with the width direction being the vertical direction, and a single-sided tape of 18 mm width at 300 mm position excluding about 50 mm vertical as a joint part of a tensile tester on one side of the double-coated electrode manufactured by 3M), and then sufficiently bonded with a roller having a load of 2 kg. After attaching the double-sided tape to the bottom surface of the tensile tester, it was attached so that the double-sided tape surface and the single-sided tape surface of the prepared electrode faced each other. On the opposite side of the tensile tester, the part where the single-sided tape was not attached was fastened, and the 90° bending adhesive force was measured for a total of 1201 areas with a gap of 0.25 mm in the width direction of the cathode, and the measured strength was divided by the width of the tape Results are shown in FIG. 7 . Subsequently, the distribution (%) characteristics of the adhesive strength measured in the 1201 areas are shown in FIG. 8. At this time, the distribution (%) was calculated as (individual value of adhesive force in each region - average value of adhesive force) / average value of adhesive force).

Referring to FIGS. 7 and 8 , it was confirmed that the active material layer of the electrode prepared in Example 1 was formed by uniformly distributing SBR binder particles in the width direction.

[Evaluation Example 6] Evaluation of bonding strength between electrodes by active material particle size

(Example 10)

A binder suspension was prepared by diluting a suspension of SBR (BM451B from Zeon) with pure water as a binder.

An anode slurry having a viscosity of 5,000 cps was prepared by adding a SiOx-based anode active material (D50: 6um), a CMC thickener, and an SBR binder to water in a weight ratio of 98:1:1.0.

After the prepared binder suspension was first applied on one side of the copper current collector (8 μm-thick copper foil) through gravure coating, the negative electrode slurry was applied using a slot die, and proceeded in the same manner as in Example 1, A negative electrode, a positive electrode and a secondary battery were prepared. At this time, the thickness of the binder suspension applied through the gravure coating was 2um (based on the thickness after drying when the binder suspension was applied alone), and the thickness of the final coated electrode was 204um.

The dried negative electrode was rolled (rolling density: 1.68 g/cm ³ ) to prepare a negative electrode having a current collector negative active material layer formed thereon.

(Example 11)

A binder suspension was prepared by diluting a suspension of SBR (BM451B from Zeon) with pure water as a binder.

An anode slurry having a viscosity of 5,000 cps was prepared by adding a SiC-based anode active material (D50: 2um), a CMC thickener, and an SBR binder to water in a weight ratio of 98:1:1.0.

After the prepared binder suspension was first applied on one side of the copper current collector (8 μm-thick copper foil) through gravure coating, the negative electrode slurry was applied using a slot die, and proceeded in the same manner as in Example 1, A negative electrode, a positive electrode and a secondary battery were prepared. At this time, the thickness of the binder suspension applied through the gravure coating was 2um (based on the thickness after drying when the binder suspension was applied alone), and the thickness of the final coated electrode was 200um.

The dried negative electrode was rolled (rolling density: 1.68 g/cm ³ ) to prepare a negative electrode having a current collector negative active material layer formed thereon.

* Evaluation of interfacial adhesion between electrode active material layer and current collector

Evaluation of interfacial adhesion between the electrode active material layer and the current collector was performed in the same manner as in Evaluation Example 2. The measured 90 ° bending adhesive force and the position of separation from the electrode are shown in Table 6 below.

adhesion
(N/cm) Average separation position (t ₁ )
(um) active material particle size
(t ₂ , D50) (um) ₅ ×t2
(um) ₈ ×t2
(um) Example 1 0.25 20.3 13 65 104 Example 10 0.22 38.3 6 30 48 Example 11 0.15 13.6 2 10 16

(In Table 6, t ₂ is the particle size (D50) of the negative electrode active material with a large particle size among the mixed negative electrode active materials.)

Referring to Table 6, it was confirmed that the electrode according to the present invention always has a thickness of 1.0 xt ₂ or more because the active material is separated in a state in which the active material remains in the form of a 'layer' on the current collector when the adhesive strength is measured. In addition, it was confirmed that Example 1, in which the average separation position was within the preferred range, exhibited better adhesive strength than Examples 10 to 11.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, but can be manufactured in various different forms, and those skilled in the art to which the present invention belongs can understand the technical spirit of the present invention. However, it will be understood that it may be embodied in other specific forms without changing its essential features. Therefore, the embodiments described above should be understood as illustrative in all respects and not limiting.

1: current collector
3: binder layer
5: electrode active material layer
11: binder

Claims (17)

current collector; and
Including an electrode active material layer located on at least one surface of the current collector,
Satisfies the following relational expression 1,
Having a continuous binder concentration in the thickness direction of the electrode,
The electrode active material layer contains 0.1 to 2% by weight of the binder based on the total weight, a secondary battery electrode:
[Relationship 1]
t ₂ ≤ t ₁ ≤ 8×t ₂
In the relational expression 1, t ₁ is the thickness of the electrode active material layer excluding the current collector on the side close to the current collector based on the position where the electrode is separated from the inside of the electrode active material layer when measuring the 90 ^° bending adhesive force of the electrode, and t ₂ is the electrode active material layer thickness. It is the particle size (D50) of the electrode active material included in the active material layer. According to claim 1,
The electrode further satisfies the following relational expression 2, a secondary battery electrode:
[Relationship 2]
1.5×t ₂ ≤ t ₁ ≤ 5×t ₂
In the relational expression 2, t ₁ is the thickness of the electrode active material layer excluding the current collector on the side close to the current collector based on the position where the electrode is separated from the inside of the electrode active material layer when the 90 ^° bending adhesive force of the electrode is measured, and t ₂ is the electrode active material layer thickness. It is the particle size (D50) of the electrode active material included in the active material layer. According to claim 1,
The electrode active material layer includes a styrene butadiene rubber (SBR)-based binder, a secondary battery electrode. delete According to claim 1,
The electrode further satisfies the following relational expression 3, a secondary battery electrode:
[Relationship 3]
0.25 ≤ b ₂ /b ₁ < 0.7
In the relational expression 3, when the binder distribution is measured 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 binder in the 15% area of the total thickness of the electrode active material layer from the current collector is the weight of According to claim 5,
The electrode further satisfies the following relational expression 4, a secondary battery electrode:
[Relationship 4]
0.3 ≤ b ₂ /b ₁ < 0.5
In the relational expression 4, when measuring the distribution of the binder 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 binder in the 15% area of the total thickness of the electrode active material layer from the current collector is the weight of delete According to claim 1,
The electrode further satisfies the following relational expression 5, a secondary battery electrode:
[Relationship 5]
-30% ≤ (CD)/D ≤ +30%
In Equation 5, C is the interfacial adhesive force between the current collector and the electrode active material layer measured at an arbitrary position selected in the width direction of the electrode active material layer, and D is the average value of the interfacial adhesive force between the current collector and the electrode active material layer. According to claim 1,
The electrode is a negative electrode, an electrode for a secondary battery. a) applying a binder suspension to at least one surface of the current collector;
b) applying an electrode slurry containing an electrode active material on top of the binder suspension; and
c) drying the product of step b);
Steps a) and b) are performed simultaneously or sequentially,
The binder suspension does not contain an active material and a conductive material,
The binder suspension contains 30% by weight or more of a binder based on the total solid content, and the electrode slurry contains 2% by weight or less of a binder based on the total solid content,
Having a continuous binder concentration in the thickness direction of the electrode,
A method of manufacturing a secondary battery electrode comprising 0.1 to 2% by weight of the binder based on the total weight of the electrode active material layer dried in step c). According to claim 10,
Wherein step a) is to uniformly apply the binder suspension on one surface of the current collector, a method for manufacturing a secondary battery electrode. According to claim 10,
In step a), the coating thickness of the binder suspension is 0.1 to 10 μm, a method for producing a secondary battery electrode. delete According to claim 10,
Step c) is performed at a temperature of 50 to 200 ° C. for 30 to 300 seconds, a method for manufacturing a secondary battery electrode. According to claim 10,
Step c) is performed at a temperature of 80 to 130 ° C. for 30 to 300 seconds, a method for manufacturing a secondary battery electrode. An electrode for a secondary battery manufactured by the manufacturing method according to claim 10. The electrode of any one of claims 1 to 3, 5, 6, 8 and 9; separator; and a lithium secondary battery comprising an electrolyte solution.

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