KR102465822B1 - Electrode for secondary battery and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention is an electrode in which an electrode mixture layer containing a negative electrode active material, a conductive material, and a binder is formed on a current collector, and the electrode mixture layer is formed by using a Surface and Interfacial Cutting Analysis System (SAICAS). In the regression curve graph obtained by regression analysis of the shear strength data according to the cutting depth measured while obliquely cutting from the surface to the current collector until reaching the current collector, the maximum value (σ _max ) for the minimum value (σ _min ) of the shear strength It relates to an electrode for a secondary battery having a ratio of 1.7 or less.

Description

Electrode for secondary battery and lithium secondary battery including the same

The present invention relates to a secondary battery electrode and a lithium secondary battery including the same.

Recently, with the rapid spread of electronic devices using batteries such as mobile phones, notebook computers, and electric vehicles, demand for secondary batteries having a relatively high capacity and a small size is rapidly increasing. In particular, lithium secondary batteries are in the limelight as a driving power source for portable devices because they are lightweight and have high energy density. Accordingly, research and development efforts to improve the performance of lithium secondary batteries are being actively conducted.

A lithium secondary battery is an organic electrolyte solution or a polymer electrolyte solution charged between a positive electrode and a negative electrode containing an active material capable of intercalation and deintercalation of lithium ions, and when lithium ions are intercalated/deintercalated from the positive electrode and the negative electrode. Electrical energy is produced by oxidation and reduction reactions.

The electrode active material, the conductive material, and the current collector constituting the electrode of the lithium secondary battery are bound through a polymer binder, and the polymer binder can suppress the detachment phenomenon of the electrode during the coating, drying, and rolling processes of the electrode. In addition, electrical conductivity may be increased through the conductive material to improve output characteristics.

However, in order to increase the capacity of the electrode, it is desirable to minimize the content of the polymer binder having insulator properties within the limited space of the electrode. However, when the content of the polymer binder is reduced, cohesion failure may occur due to a decrease in binding force. Cohesive failure is caused by the loss of cohesive force between binder (adhesive) molecules due to particle breakage and tearing between particles and binders when stress is applied to the electrode due to expansion/contraction or external impact during charge/discharge cycles. This is a phenomenon in which cracks or separation occurs in the electrode mixture layer, and when cohesive failure occurs, battery performance deteriorates. In addition, the binder may be non-uniformly distributed depending on process conditions, and even in this case, cohesion failure may be caused.

In addition, in the case of a multi-layer electrode composed of multiple layers of electrode active materials that are advantageous to each of the battery life characteristics, capacity, and strength characteristics, cohesive failure between electrode layers may occur due to differences in strength and adhesive strength of different electrode active materials, and thus life characteristics are reduced. may be lowered

In particular, in the case of a multi-layered electrode formed of artificial graphite with excellent lifespan characteristics and capacity characteristics and natural graphite with excellent strength and adhesive strength, it is advantageous for cost reduction while improving battery characteristics, but the difference in strength and adhesive strength between artificial graphite and natural graphite Due to this, cohesive failure inside the electrode, in particular, cohesive failure between layers may occur, and thus, there is a problem in that life characteristics are deteriorated.

US Patent Publication No. 2014-0295248

The present invention is to solve the above problems, to suppress cohesive failure, especially interlayer cohesive failure, in the electrode during operation of a lithium secondary battery, to provide an electrode for a two-phase battery having excellent lifespan characteristics and capacity characteristics while having cost competitiveness. want to provide

The present invention is an electrode in which an electrode mixture layer containing a negative electrode active material, a conductive material, and a binder is formed on a current collector, and the electrode mixture layer is formed using a Surface and Interfacial Cutting Analysis System (SAICAS). The ratio of the maximum value (σ _max ) to the minimum value (σ _min ) of shear strength in the regression curve graph obtained by regression analysis of the shear strength data according to the cutting depth measured while obliquely cutting from the surface to the current collector until reaching the current collector. An electrode for a secondary battery having a ratio of 1.7 or less is provided.

In addition, the present invention provides a lithium secondary battery including the electrode for the secondary battery.

The electrode for a secondary battery according to the present invention can suppress cohesive failure inside the electrode mixture layer, in particular, cohesive failure between layers during operation of a lithium secondary battery, and exhibit excellent lifespan characteristics and capacity characteristics while having cost competitiveness.

In particular, in the case of a multilayer electrode formed of multiple layers of artificial graphite having excellent lifespan characteristics and capacity characteristics and natural graphite having excellent strength and adhesive strength, cohesive failure within the electrode caused by differences in strength and adhesive strength between artificial graphite and natural graphite, In particular, interlayer cohesive failure can be suppressed and life characteristics can be improved.

1 is a schematic diagram showing a process of cutting an electrode mixture layer using a surface-interface cutting analysis system.
Figure 2 is a graph showing the shear strength data and regression curve of the electrode according to Example 1.
3 is a graph showing shear strength data and regression curves of the electrode according to Example 2;
Figure 4 is a graph showing the shear strength data and regression curve of the electrode according to Example 3.
5 is a graph showing shear strength data and a regression curve of an electrode according to Comparative Example 1;
6 is a graph showing shear strength data and a regression curve of an electrode according to Comparative Example 2;
7 is a graph showing shear strength data and a regression curve of an electrode according to Comparative Example 3;

Hereinafter, the present invention will be described in more detail to aid understanding of the present invention. At this time, the terms or words used in this specification and claims should not be construed as being limited to ordinary or dictionary meanings, and the inventor appropriately defines the concept of terms in order to explain his/her invention in the best way. It should be interpreted as meaning and concept consistent with the technical idea of the present invention based on the principle that it can be done.

The electrode for a secondary battery of the present invention is an electrode in which an electrode mixture layer containing a negative electrode active material, a conductive material, and a binder is formed on a current collector, and the electrode mixture layer is formed on the current collector from the surface using a surface-interface cutting analysis system. In the regression curve graph obtained by regression analysis of the shear strength data according to the cutting depth measured while obliquely cutting until reaching the shear strength, the ratio of the maximum value (σ _max ) to the minimum value (σ _min ) of shear strength is 1.7 or less, preferably It is preferably 1 to 1.7, more preferably 1 to 1.6, and still more preferably 1.2 to 1.6.

The electrode for a secondary battery of the present invention may be a negative electrode, and specifically, may be an electrode having a structure in which an electrode mixture layer is formed on a current collector. The electrode mixture layer may include an anode active material, a conductive material, and a binder.

Meanwhile, the electrode mixture layer may be a single layer or may have a multilayer structure including a first electrode mixture layer and a second electrode mixture layer formed on the first electrode mixture layer. For example, the electrode mixture layer is formed on the current collector and includes a first electrode mixture layer including a first negative electrode active material, a first conductive material, and a first binder, and formed on the first electrode mixture layer. and a second electrode mixture layer including a second negative electrode active material, a second conductive material, and a second binder.

The first and second negative electrode active materials, the first and second conductive materials, and the first and second binders included in the first electrode mixture layer and the second electrode mixture layer may be the same or different from each other. may be

The current collector is not particularly limited as long as it does not cause chemical change in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, carbon on the surface of copper or stainless steel , those surface-treated with nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. may be used. In addition, the negative electrode current collector may have a thickness of typically 3 to 500 μm, and, like the positive electrode current collector, fine irregularities may be formed on the surface of the current collector to enhance bonding strength of the negative electrode active material. For example, it may be used in various forms such as films, sheets, foils, nets, porous materials, foams, and non-woven fabrics.

The anode active material may be a carbonaceous material such as natural graphite, artificial graphite, graphitized carbon fiber, or amorphous carbon; metallic compounds capable of being alloyed with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; metal oxides capable of doping and undoping lithium, such as SiO _β (0 < β < 2), SnO ₂ , vanadium oxide, and lithium vanadium oxide; Alternatively, it may be an anode active material commonly used in the art, such as a composite including the metallic compound and a carbonaceous material, such as a Si—C composite or a Sn—C composite.

When the electrode mixture layer has a multilayer structure of two or more layers, the first negative electrode active material and the second negative electrode active material each include at least one selected from the group consisting of natural graphite and artificial graphite, and optionally graphitized carbon fiber and amorphous carbon. carbonaceous materials such as; metallic compounds capable of being alloyed with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; metal oxides capable of doping and undoping lithium, such as SiO _β (0 < β < 2), SnO ₂ , vanadium oxide, and lithium vanadium oxide; Alternatively, a composite including the metallic compound and a carbonaceous material, such as a Si—C composite or a Sn—C composite may be further included.

More preferably, the first negative electrode active material included in the first electrode mixture layer below the electrode includes natural graphite having excellent strength and adhesion, and the second negative electrode active material included in the second electrode mixture layer above the electrode has a lifespan It may include artificial graphite having excellent properties and capacity.

On the other hand, the conductive material, the first conductive material, and the second conductive material are used to impart conductivity to the electrode, and in the battery, any material having electronic conductivity without causing chemical change can be used without particular limitation. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like, and one of these may be used alone or in a mixture of two or more. The conductive material may be included in an amount of 1 to 30% by weight based on the total weight of the electrode mixture layer.

The binder, the first binder, and the second binder serve to improve adhesion between negative active material particles and adhesion between the negative active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC) ), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated-EPDM, and styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and the like may be used alone or in a mixture of two or more of them. The binder may be included in an amount of 1 to 30% by weight based on the total weight of the electrode mixture layer.

As an example, the electrode mixture layer is formed by applying a composition for forming an electrode including a negative electrode active material, a binder, and a conductive material on a current collector and drying it, or by casting the composition for forming an electrode on a separate support, and then removing the support from the support. It may also be manufactured by laminating the film obtained by peeling on a current collector.

Alternatively, the electrode may be manufactured by casting the composition for forming an electrode mixture layer on a separate support, and then laminating a film obtained by peeling from the support on a positive electrode current collector.

Meanwhile, the electrode for a secondary battery according to the present invention has a ratio of a maximum value (σ _max ) to a minimum value (σ _min ) of shear strength in a regression curve obtained through a shear strength analysis method to be described later, which is 1.7 or less.

According to the study of the present inventors, when the maximum value to the minimum value of the shear strength in the regression curve obtained through the shear strength analysis method satisfies the above ratio, the cohesive failure inside the electrode mixture layer is small, and thus when applied to a secondary battery , exhibiting excellent life characteristics.

In particular, in the case of a multi-layer electrode, cohesive failure within the electrode, in particular, interlayer cohesive failure, may be intensified due to differences in strength and adhesive strength of different electrode active materials, and thus lifespan characteristics may be deteriorated when a secondary battery is applied. However, in the regression curve obtained through the following shear strength analysis method, the multilayer electrode for a secondary battery of the present invention in which the ratio of the maximum value (σ _max ) to the minimum value (σ _min ) of shear strength satisfies 1.7 or less is the electrode during operation of the secondary battery. Cohesive failure within the mixture layer, particularly interlayer cohesive failure, is significantly reduced, and thus excellent life characteristics and capacity characteristics can be exhibited.

The shear strength analysis method of the present invention uses a Surface and Interfacial Cutting Analysis System (SAICAS) to cut the electrode mixture layer from the surface to the current collector while cutting the shear strength according to the cutting depth. It is performed by obtaining degree (σ) data, regression analysis of the shear strength (σ) data to obtain a regression curve, and then comparing the ratio of the maximum value to the minimum value of shear strength in the regression curve.

The surface-interface cutting analysis system (SAICAS) includes equipment for inclined cutting a coated thin film sample from the surface to the interface using a blade, and can measure the force applied to the blade during the cutting process. It is a system that can 1 is a view showing a process of cutting an electrode mixture layer using a surface-interface cutting analysis system (SAICAS). As shown in FIG. 1 , the blade 1 obliquely cuts the electrode mixture layer 20 formed on the current collector 10 at a specific cutting angle θ with respect to the surface. In this way, when cutting is performed with a cutting angle, cutting is performed while the blade descends obliquely in the depth direction of the electrode mixture layer. The cutting of the electrode mixture layer is performed until the blade reaches the current collector.

At this time. The cutting angle θ may be greater than 0° and less than or equal to 10°, preferably 1° to 5.7°, and more preferably 3° to 5.7°. By setting the cutting angle (θ) within the above range, not only can the measurement time be shortened, but also the accuracy of the shear strength data can be improved.

In addition, when cutting the electrode mixture layer, the moving speed of the blade is preferably about 0.01 to 10 μm/s in the horizontal direction and about 0.001 to 1 μm/s in the vertical direction. More preferably, the blade advance speed is 0.1 to 1 μm/s in the horizontal direction and 0.01 to 0.1 μm/s in the vertical direction, more preferably 0.1 to 0.5 μm/s in the horizontal direction and 0.01 to 0.05 μm/s in the vertical direction. can be done with s. By setting the moving speed of the blade within the above range, the resolution of the shear strength data is improved and thus the accuracy of the data is improved.

On the other hand, the blade is not particularly limited as long as it has a higher hardness than the electrode mixture layer and is not deformed in shape during the cutting process. For example, diamond, cubic boron nitride (BN), steel, cermet , It may be made of a material such as ceramic, more preferably it may be made of cubic boron nitride (BN).

The width of the blade may be, for example, 0.1 to 4 mm, more preferably 0.3 to 1 mm.

On the other hand, using the surface-interface cutting analysis system (SAICAS), it is possible to measure the horizontal force (Fh) and the vertical force (Fv) applied to the blade during the cutting process. Therefore, the horizontal direction force (Fh) and the vertical direction force (Fv) applied to the blade according to the cutting depth (t ₀ ) are measured using the surface-interface cutting analysis system (SAICAS), and the measured values are used Thus, shear strength (σ) data according to the cutting depth can be obtained.

Specifically, the shear strength (σ) can be calculated through Equation 1 below.

[Equation 1]

In Equation 1, σ is the shear strength (N/mm ² ), F _h is the horizontal force applied to the blade (N) measured through the surface-interface cut analysis system, and Fv is the surface-interface cut analysis system is the vertical force (N) applied to the blade measured through, θ is the cutting angle (°), b is the blade width (mm), and t ₀ is the cutting depth (μm).

After obtaining shear strength data according to the cutting depth through the above process, a regression curve is obtained by performing regression analysis on the shear strength data. The shear strength (σ) data obtained through Equation 1 using the surface-interface ablation analysis system (SAICAS) previously has fluctuations due to deformation of electrode particles. By simplifying the shear strength data through regression analysis, the minimum and maximum values in the shear strength (σ) data can be clearly and simply selected.

In this case, the regression curve may be obtained through polynomial regression analysis, and more specifically, it may be obtained through a 4th order polynomial regression equation represented by Equation 2 below. At this time, the x-axis of the regression curve represents the cutting depth, and the y-axis represents the shear strength.

[Equation 2]

In Equation 2, σ is the shear strength (N/mm ² ), A ₀ is the shear strength when the cutting depth is 0 μm, and is a positive number, and A ₁ , A ₂ , A ₃ , and A ₄ are polynomial ) is a constant obtained by the quaternary equation, and d is the cutting depth (μm).

On the other hand, shear force is not applied to the blade at the interface between the electrode mixture layer and the current collector, and as a result, the shear strength is rapidly reduced. That is, the point where the shear strength value starts to decrease rapidly can be regarded as the interface between the electrode mixture layer and the current collector. If the shear strength value of the interface between the electrode mixture layer and the current collector is included, the maximum and minimum value data of the shear strength may be distorted, so it is preferable to obtain a regression curve excluding the shear strength data of the section where the shear strength value rapidly decreases.

Next, the minimum value (σ _min ) and maximum value (σ _max ) of shear strength are obtained from the regression curve obtained as described above, and the ratio of the maximum value to the minimum value (σ _max /σ _min ) is calculated.

In the electrode for a secondary battery of the present invention, the ratio of the maximum value (σ _max ) to the minimum value (σ _min ) of the shear strength (σ) obtained through the above process satisfies 1.7 or less. More preferably, the ratio of the maximum value (σ _max ) to the minimum value (σ _min ) of the shear strength (σ) may be 1 to 1.7, more preferably 1 to 1.6, and even more preferably 1.2 to 1.6. can be When the ratio of the maximum value (σ _max ) to the minimum shear strength value (σ _min ) exceeds 1.7, the shear strength is non-uniform in the thickness direction of the electrode mixture layer, so when cycle repetition and external impact occur, the electrode mixture Cohesive failure easily occurs in the layer, and as a result, life characteristics are rapidly deteriorated. In particular, in the case of a multilayer electrode including two or more electrode mixture layers having different compositions (components and/or contents), cohesive failure may be intensified at the interface between layers due to differences in strength of different electrode active materials and adhesion between layers. However, when an electrode satisfying the shear strength ratio of the present invention is used, the shear strength variation in the thickness direction of the electrode mixture layer is small, and cohesive failure within the electrode mixture layer is minimized, thereby obtaining excellent lifespan characteristics.

On the other hand, the ratio of the maximum value (σ _max ) to the minimum value (σ _min ) of the shear strength is the type or content of the components (ie, active material, conductive material, binder) constituting the electrode mixture layer, the strength of the particles, the electrode Factors such as the thickness of the mixture layer, porosity, electrode manufacturing process conditions such as rolling and drying temperatures, etc. are comprehensively determined. Therefore, an electrode having a desired shear strength ratio can be manufactured by appropriately adjusting the composition of the electrode mixture layer and/or electrode process conditions.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention may be embodied in many different forms and is not limited to the embodiments described herein.

Example 1

Natural graphite as the first anode active material, carbon black as the first conductive material, carboxylmethyl cellulose (CMC) and styrene butadiene rubber (SBR) as the first binder in a weight ratio of 96.85:0.5:1.15 in distilled water as a solvent : 1.5 to prepare a composition for forming a first negative electrode (solid content: 40% by weight).

In addition, artificial graphite as a second negative electrode active material, carbon black as a second conductive material, carboxylmethyl cellulose (CMC) and styrene butadiene rubber (SBR) as a second binder were mixed in distilled water as a solvent at a weight ratio of 95.95:0.5. : 1.15: added to distilled water at a ratio of 2.4 and then mixed to prepare a composition for forming a second negative electrode (solid content: 40% by weight).

After coating the composition for forming the first negative electrode and the composition for forming the second negative electrode on one surface of the copper current collector, drying at 110 ° C. for 2 minutes and rolling, the current collector, the first negative electrode mixture layer, and the second negative electrode mixture layer (mixture A negative electrode consisting of a total layer thickness of 150 μm) was prepared.

Example 2

A composition for forming a first negative electrode was prepared by using artificial graphite as the first negative electrode active material, and using the first negative electrode active material, the first conductive material, and the first binder in a weight ratio of 95.35:0.5:1.15:3.0, A composition for forming a second negative electrode was prepared by using artificial graphite as the negative electrode active material, and using the second negative electrode active material, the second conductive material, and the second binder in a weight ratio of 97.45: 0.5: 1.15: 0.9, After coating the composition for forming the first negative electrode and the composition for forming the second negative electrode on one surface, drying at 120° C. for 1 minute and rolling, the current collector, the first negative electrode mixture layer, and the second negative electrode mixture layer (the total thickness of the mixture layer is 150 ㎛) was prepared.

Example 3

A negative electrode was prepared in the same manner as in Example 1, except that the composition for forming the first negative electrode and the composition for forming the second negative electrode were coated on one surface of the copper current collector, and then dried at 110° C. for 1 minute and 30 seconds to prepare the negative electrode. manufactured.

Comparative Example 1

A composition for forming a first negative electrode was prepared by using natural graphite as the first negative electrode active material, and using the first negative electrode active material, the first conductive material, and the first binder in a weight ratio of 95.35:0.5:1.15:3.0. A composition for forming a second negative electrode was prepared by using artificial graphite as the negative electrode active material, and using the second negative electrode active material, the second conductive material, and the second binder in a weight ratio of 97.45: 0.5: 1.15: 0.9, After coating the composition for forming the first negative electrode and the composition for forming the second negative electrode on one surface, drying at 120° C. for 1 minute and rolling, the current collector, the first negative electrode mixture layer, and the second negative electrode mixture layer (the total thickness of the mixture layer is 150 ㎛) was prepared.

Comparative Example 2

A negative electrode was carried out in the same manner as in Example 1, except that the weight ratio of the composition for forming the first negative electrode was 96.5:0.5:1:2 and the weight ratio of the composition for forming the second negative electrode was 97.5:0.5:1.0:1.0. was manufactured.

Comparative Example 3

A negative electrode was prepared in the same manner as in Example 1, except that the composition for forming the first negative electrode and the composition for forming the second negative electrode were coated on one surface of the copper current collector, and then dried at 120 ° C. for 1 minute to prepare the negative electrode. .

[Experimental Example 1: Shear Strength (σ) Analysis]

(1) Measurement of shear strength (σ) data

For each of the cathodes prepared according to Examples 1 to 3 and Comparative Examples 1 to 3, a surface-interface cutting analysis system (SAICAS, SAICAS-DN, Dipla Wintes, Japan) was used to measure the force applied to the blade according to the cutting depth while performing oblique cutting of the negative electrode mixture layer, and the shear strength (σ) according to the cutting depth was obtained through Equation 1 below.

At this time, the inclined cutting is performed by placing the blade parallel to the surface of the electrode mixture layer and positioning it until a vertical force of 0.03 N is applied, and then in a constant velocity mode, a horizontal speed of 0.5 μm/s and a vertical speed of 0.05 μm/s s, the blade was advanced at a cutting angle of 5.7°.

Then, the shear strength (σ) data obtained above was analyzed through a 4th order polynomial regression equation to obtain a regression curve.

Shear strength data and regression curves of each electrode obtained through the above method are shown in the graphs of FIGS. 2 to 5.

2 (Example 1), 3 (Example 2), 4 (Example 3), 5 (Comparative Example 1), 6 (Comparative Example 2), and 7 (Comparative Example 3). From the curve, the minimum value (σ _min ), maximum value (σ _max ) and the ratio of the maximum value (σ _max ) to the minimum value (σ _min ) of shear strength were obtained, which are shown in Table 1 below.

Maximum value (σ _max ) (N/mm ² ) minimum value (σ _min ) (N/mm ² ) σ _max/ σ _min Example 1 1.79 1.20 1.49 Example 2 1.43 1.17 1.22 Example 3 1.45 0.93 1.56 Comparative Example 1 1.77 0.38 4.66 Comparative Example 2 1.55 0.33 4.70 Comparative Example 3 2.36 1.31 1.80

Referring to Table 1, the negative electrodes of Examples 1 to 3 satisfy the ratio of the maximum value (σ _max ) to the minimum value (σ _min ) of 1.7 or less, whereas the negative electrodes of Comparative Examples 1 to 3 exceed 1.7. can confirm that

[Experimental Example 2: Evaluation of life characteristics]

LiCoO ₂ cathode active material, carbon black conductive material, and PVdF binder were mixed in a weight ratio of 96:2:2 in an N-methylpyrrolidone solvent to prepare a cathode composite material (viscosity: 5000 mPa s), which was then used as an aluminum current collector. After coating on one side of the, dried at 130 ℃, and then rolled to prepare a positive electrode.

As the negative electrode, the negative electrodes prepared in Examples 1 to 3 and Comparative Examples 1 to 3 were used, respectively.

An electrode assembly was prepared by interposing a porous polyethylene separator between the positive electrode and the negative electrode prepared as described above, the electrode assembly was placed inside a case, and an electrolyte solution was injected into the case to prepare a lithium secondary battery. At this time, the electrolyte is ethylene carbonate / ethyl methyl carbonate / diethyl carbonate / (mixed volume ratio of EC / EMC / DEC = 3/4/3) in an organic solvent consisting of 1.0M concentration of lithium hexafluorophosphate (LiPF ₆ ) is dissolved It was prepared by

Each of the prepared lithium secondary batteries having a capacity of 50 mAh was charged at 2.5 V with a constant current of 0.33 C until the voltage reached 4.25 V, and then charged at a constant voltage of 4.25 V, and charging was terminated when the charging current reached 2.5 mA. Thereafter, after leaving it for 30 minutes, the initial capacity was measured by discharging until it became 2.5V with a 0.33C constant current. Thereafter, the secondary battery was charged at 25° C. at 2.5V to 4.25V at a constant current of 1C, and then charged at a constant voltage of 4.25V to terminate charging when the charging current reached 2.5mA. Thereafter, the battery was allowed to stand for 30 minutes and then discharged until it became 2.5V with a 1C constant current, and the capacity retention rate was measured when the charge/discharge experiment was conducted 200 times. The results are shown in Table 2.

Capacity retention rate (%) (@200 cycles) Example 1 96.4 Example 2 95.0 Example 3 92.5 Comparative Example 1 82.3 Comparative Example 2 84.9 in comparison 3 89.2

Referring to Table 2, the secondary batteries using the negative electrodes of Examples 1 to 3 having a ratio of the maximum value (σ _max ) to the minimum shear strength value (σ _min ) of 1.7 or less have the maximum value of the minimum shear strength value (σ _min ). It can be seen that the lifespan characteristics are remarkably superior to the secondary batteries using the negative electrodes of Comparative Examples 1 to 3 in which the ratio of (σ _max ) exceeds 1.7.

1: blade
10: entire collector
20: electrode mixture layer
100: electrode

Claims (11)

An electrode in which an electrode mixture layer containing a negative electrode active material, a conductive material, and a binder is formed on a current collector,
Using the Surface and Interfacial Cutting Analysis System (SAICAS), regression analysis of the shear strength data according to the cutting depth measured while obliquely cutting the electrode mixture layer from the surface to the current collector until it reaches the current collector In the regression curve graph obtained by doing, the ratio of the maximum value (σ _max ) to the minimum value (σ _min ) of shear strength is 1.7 or less,
Shear strength (σ) data according to the cutting depth is a secondary battery electrode that is calculated through Equation 1 below.
[Equation 1]

In Equation 1, σ is the shear strength (N/mm2), Fh is the horizontal force (N) applied to the blade measured through the surface-interface cut analysis system, and Fv is the surface-interface cut analysis system Measured through is the vertical force (N) applied to the blade, θ is the cutting angle (°), b is the blade width (mm), and t ₀ is the cutting depth (μm).
According to claim 1,
A secondary battery electrode wherein the ratio of the maximum value (σ _max ) to the minimum value (σ _min ) of shear strength in the regression curve graph is 1 to 1.6.
According to claim 1,
When using the surface-interface cutting analysis system (SAICAS), the electrode mixture layer is obliquely cut at a cutting angle of more than 0 ° and less than 10 °.
According to claim 1,
When using the surface-interface cutting analysis system (SAICAS), the blade travel speed is 0.01 to 10 μm / s in the horizontal direction and 0.001 to 1 μm / s in the vertical direction.
delete According to claim 1,
The regression curve is an electrode for a secondary battery that is obtained by performing a 4th order polynomial regression analysis on shear strength data.
According to claim 1,
The electrode for a secondary battery is a negative electrode for a secondary battery.
According to claim 1,
The electrode mixture layer,
a first electrode mixture layer formed on the current collector and including a first negative electrode active material, a first conductive material, and a first binder; and
An electrode for a secondary battery comprising a second electrode mixture layer formed on the first electrode mixture layer and including a second negative electrode active material, a second conductive material, and a second binder.
According to claim 8,
The electrode for a secondary battery, wherein the first negative electrode active material and the second negative electrode active material are different from each other.
According to claim 8,
Wherein the first negative active material includes natural graphite, and the second negative active material includes artificial graphite.
A lithium secondary battery comprising the secondary battery electrode according to claim 1 .

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