KR102779713B1 - Electrode for secondary battery and method for preparing the same - Google Patents

Abstract

The present invention relates to an electrode for a secondary battery, comprising: an electrode current collector; and an electrode composite layer formed on the electrode current collector and containing an electrode active material, wherein a rate of reduction in the residual stress of the electrode current collector or the electrode active material measured after immersing the electrode in an electrolyte is 15% or less, relative to the residual stress of the electrode current collector or the electrode active material measured after drying the impregnated electrode. According to the present invention, in particular, there is an effect of reducing the risk during mass production by confirming the limit density at which peeling of a high-density electrode does not occur at the electrode process stage prior to battery production.

Description

ELECTRODE FOR SECONDARY BATTERY AND METHOD FOR PREPARING THE SAME

The present invention relates to an electrode for a secondary battery and a method for manufacturing the same, and more specifically, to an electrode capable of significantly reducing the occurrence of electrode peeling and a method for manufacturing the same.

Secondary batteries, which have high applicability according to product group and electrical characteristics such as high energy density, are widely used not only in portable devices but also in electric vehicles (EV) or hybrid electric vehicles (HEV) that are driven by an electric drive source. These secondary batteries are receiving attention as a new energy source for improving environment friendliness and energy efficiency in that they can drastically reduce the use of fossil fuels and do not generate any by-products from energy use at all. The electrodes included in these secondary batteries are configured by adhesively coating electrode active materials on electrode current collectors. When coating the electrode active materials on the electrode current collectors, the adhesive strength of the electrode active materials is a major factor affecting the manufacturing process of the battery cell or the performance of the battery cell. Therefore, during the manufacturing process, a process of measuring the adhesive strength of the electrode active materials when adhesively coating the electrode active materials on the electrode current collectors is required.

Conventional electrode adhesion measuring devices measure the adhesive strength of such electrode active materials by peeling the electrode active material from the electrode current collector through a 90-degree peel test (JIS K6854-1:1999), thereby measuring the adhesive strength between the electrode current collector and the electrode active material and quantitatively comparing it with the peel strength. Such electrode adhesion measuring devices select the electrode adhesion standard based on the measured peel strength value.

However, electrode active materials are more frequently detached from the electrode collector by cutting or punching than by being peeled off from the electrode collector. In particular, in recent years when high-density electrodes are required, the difference in adhesive strength according to rolling density is very small, so it is difficult to determine whether or not there is peeling in high-density electrodes based on adhesive strength alone. Therefore, there is a need to find a method to more accurately evaluate the peeling phenomenon in order to increase the battery manufacturing yield.

The present invention has been made to solve the above problems, and aims to provide a method for manufacturing an electrode for a secondary battery, which can predict whether or not peeling will occur in advance at the electrode manufacturing stage prior to the production of a finished product, and apply the prediction to reduce the peeling phenomenon, and a secondary battery electrode according to the method.

According to one aspect of the present invention, in an electrode for a secondary battery, including an electrode current collector; and an electrode composite layer formed on the electrode current collector and containing an electrode active material, a reduction rate of a residual stress of the electrode current collector or electrode active material measured after impregnating the electrode in an electrolyte solution is 15% or less, compared to a residual stress of the electrode current collector or electrode active material measured after drying the impregnated electrode, is provided.

The above residual stress may be measured using X-ray diffraction analysis for either the (131) plane of the electrode collector or the (112) plane and (002) plane of the electrode active material.

The electrode density of the above electrode may be less than 1.9 g/cc.

According to another aspect of the present invention, a method for manufacturing an electrode for a secondary battery is provided, including the steps of: applying a slurry containing an electrode active material and a binder on an electrode collector and drying the slurry to manufacture an electrode; rolling the electrode; impregnating the rolled electrode with an electrolyte and measuring residual stress of the electrode collector or the electrode active material; and drying the electrode impregnated with the electrolyte and measuring residual stress of the electrode collector or the electrode active material.

With respect to the residual stress of the electrode collector or electrode active material of the electrode measured after impregnation in the above electrolyte, the reduction rate of the residual stress of the electrode collector or electrode active material measured after drying the impregnated electrode may be 15% or less.

The step of impregnating the above rolled electrode with an electrolyte can be performed at a temperature range of 25 to 100°C for 1 to 24 hours.

The above residual stress can be measured using X-ray diffraction analysis for either the (131) plane of the electrode collector or the (112) plane and (002) plane of the electrode active material.

Quantifying the electrode peeling phenomenon using only adhesive strength is limited to cases where the electrode density is below a certain level, so it is difficult to utilize it as an evaluation index. In terms of electrode design that requires higher energy density, the importance of an evaluation method that can prevent loss due to defective factors such as peeling in advance and an electrode manufactured using the same is expected to gradually increase. According to the present invention, in particular, there is an effect of reducing risk during mass production by confirming the limit density at which peeling does not occur in a high-density electrode at the electrode process stage prior to battery production.

Hereinafter, preferred embodiments of the present invention will be described with reference to various examples. However, the embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

The present invention relates to an electrode for a secondary battery and a method for manufacturing the same. In order to realize a high-capacity lithium secondary battery, it is essential to manufacture a high-density electrode. However, as the rolling density increases to manufacture a high-density electrode, a problem occurs in which the electrode active material layer is detached from the electrode current collector, which is a metal. Since this leads not only to a deterioration in battery performance but also to a decrease in mass productivity, accurately assessing whether or not there is detachment at the electrode manufacturing stage is very effective in terms of reducing mass production risk.

In the past, the adhesive strength of a semi-finished electrode was measured as a method of quantitatively evaluating peel strength, but since the deviation in adhesive strength according to the rolling density is very small, it is not suitable for determining whether or not a high-density electrode peels. In addition, it is difficult to correlate it with structural factors inside the battery resulting from swelling due to the electrolyte, relief of residual stress, and volume change during charge and discharge. Accordingly, the inventors of the present invention have perfected the idea that by measuring the adhesive strength of an electrode that is immersed in an electrolyte at a high temperature by simulating the aging process and then dried, the critical density at which peeling does not occur can be confirmed at the electrode process stage prior to battery production, thereby reducing mass production risk.

According to one aspect of the present invention, in an electrode for a secondary battery, including an electrode current collector; and an electrode composite layer formed on the electrode current collector and containing an electrode active material, the electrode current collector or electrode active material measured after impregnating the electrode in an electrolyte has a reduction rate of 15% or less in the residual stress of the electrode current collector or electrode active material measured after drying the impregnated electrode.

Residual stress is a stress that remains inside an object even though no external force is applied to the object. Residual stress is a stress generated inside a processed or heat-treated object, and depending on the processing history of the object, tensile stress or compressive stress may remain in the object. This residual stress may cause destruction or damage to the object. In the present invention, by simulating the aging process, a step of impregnating the rolled electrode with an electrolyte is included, and the residual stress of the electrode before and after impregnation is measured so that the limit density at which peeling does not occur can be confirmed at the electrode process stage prior to battery production. More specifically, after an electrode assembly such as a jelly roll (J/R) consisting of a cathode and anode and a separator is formed, the aging process is a process of injecting an electrolyte into the cell and sufficiently wetting it. In this process, in particular, in the case of the cathode, the swelling phenomenon due to solvent penetration into the pores increases the thickness by a certain amount, and in this process, the residual stress that the material had is relieved, but if the degree of relief is severe, it appears as a peeling phenomenon. According to the present invention, since the presence of peeling can be confirmed at a stage prior to cell assembly and electrode assembly manufacturing by simulating the aging process, the presence of peeling can be accurately evaluated at the electrode manufacturing stage, thereby significantly reducing mass production risk.

Accordingly, it is preferable that the reduction rate of the residual stress of the electrode collector or electrode active material measured after impregnating the electrode in the electrolyte is 15% or less, as compared to the residual stress of the electrode collector or electrode active material measured after drying the impregnated electrode. If the reduction rate of the residual stress exceeds 15%, the electrode may wrinkle or peel off due to the high energy state, and the adhesive strength between the electrode collector and the electrode composite layer may also be reduced.

The step of measuring the above residual stress can be performed using X-ray diffraction analysis. It can be confirmed through X-ray diffraction analysis that there is residual stress in the electrode due to the pressure generated during rolling during the electrode manufacturing process, and that the intensity of the residual stress increases as the density increases. That is, since such a high energy state appears as a peeling phenomenon in which the electrode wrinkles or lifts as the electrolyte impregnated inside the battery wets the binder particles, it is possible to predict whether the electrode peels by measuring the residual stress of the electrode.

Meanwhile, the residual stress can be measured on at least one of the crystal planes of the electrode current collector or the electrode active material. Although not particularly limited, the residual stress can be measured using X-ray diffraction analysis for either the (131) plane of the electrode current collector corresponding to the applied stress of the electrode current collector, or the (112) plane and (002) plane of the electrode active material corresponding to the applied stress and strain to the electrode active material. For example, in the negative electrode, although there is a difference in the cases where natural graphite and artificial graphite are used as the negative active material, it was confirmed that peeling occurred when the residual stress was 0.3 MPa or more and the strain was 0.18 or more based on the half width at half maximum (FWHM) value of the (002) plane of the graphite for the (112) plane of the electrode, which is the electrode active material of the electrode where peeling occurs, as a result of XRD measurement at 83.6 degrees.

Meanwhile, the rolling density of the electrode of the present invention is preferably less than 1.9.0 g/cc, and more preferably 1.6 g/cc to 1.8 g/cc. That is, it is possible to evaluate whether or not a high-density electrode is delaminated, and accordingly, provide an electrode capable of preventing the delamination phenomenon.

The electrode according to the present invention is not particularly limited, but is more preferably a cathode, and the composition and manufacturing method of the cathode are not particularly limited. For example, a thin plate made of copper, stainless steel, or nickel may be used as the cathode current collector, and a porous body such as a mesh or reticulated body may be used, and may be coated with an oxidation-resistant metal or alloy film to prevent oxidation.

In addition, the negative electrode active material included in the negative electrode composite layer may include a commonly used negative electrode active material, and as the negative electrode active material, a carbon-based material, silicon, silicon oxide, silicon-based alloy, silicon-carbon-based material composite, tin, tin-based alloy, tin-carbon composite, metal oxide or a combination thereof may be used, and may include lithium metal and/or lithium metal alloy.

The negative electrode composite layer may additionally contain a conductive material, if necessary. The conductive material is not particularly limited as long as it does not cause a chemical change in the secondary battery of the present invention and has conductivity. For example, graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, and lamp black; conductive fibers such as carbon fibers or metal fibers; fluorinated carbon; metal powders such as aluminum or nickel powder; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. can be used.

In addition, the negative electrode composite layer may include a binder to improve the bonding strength between the active material and the conductive material, and the binder may include, but is not limited to, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene butadiene rubber, fluororubber, and various copolymers.

The electrolyte is composed of an electrolyte and a lithium salt. A non-aqueous organic solvent can be used as the electrolyte, and any commonly used electrolyte can be used. For example, any one selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate, fluoroethylene carbonate (FEC), dimethyl sulfuroxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, and tetrahydrofuran, or a mixture of two or more thereof, can be used.

According to another aspect of the present invention, a method for manufacturing an electrode for a secondary battery is provided, including the steps of: applying a slurry containing an electrode active material and a binder on an electrode collector and drying the slurry to manufacture an electrode; rolling the electrode; impregnating the rolled electrode with an electrolyte and measuring residual stress of the electrode collector or the electrode active material; and drying the electrode impregnated with the electrolyte and measuring residual stress of the electrode collector or the electrode active material.

As described above, the present invention includes a step of immersing the rolled electrode in an electrolyte by simulating the aging process, and measures the residual stress of the electrode before and after the impregnation, so that the limit density at which peeling does not occur can be confirmed at the electrode process stage prior to battery production, and the step of measuring the residual stress can be performed using X-ray diffraction analysis, and it is preferable that the reduction rate of the residual stress of the electrode collector or electrode active material measured after drying the impregnated electrode is 15% or less with respect to the residual stress of the electrode collector or electrode active material measured after immersing the electrode in the electrolyte. When the reduction rate of the residual stress exceeds 15%, the electrode may wrinkle or peeling may occur due to a high energy state, and the adhesive strength between the electrode collector and the electrode composite layer may also be reduced.

Meanwhile, the step of impregnating the rolled electrode with the electrolyte can be performed at a temperature range of 25 to 100°C, and is more preferably performed at a temperature range of 60 to 100°C. If it is less than 25°C, the impregnation time must be increased until the stress is relieved, and if it is more than 100°C, the stress is completely relieved within a short period of time, which may lower the discrimination power according to the density.

The step of impregnating the electrode with the electrolyte can be performed for 1 to 24 hours, and is more preferably performed for 10 to 20 hours. If it is less than 1 hour, the electrolyte may not be sufficiently impregnated, and residual stress within the structure may not be relieved, and if it exceeds 24 hours, discrimination according to density may be reduced.

Hereinafter, the present invention will be described more specifically with reference to examples. The following examples are intended to more specifically explain the present invention, but the present invention is not limited thereto.

Example

Manufacturing of cathode

A negative electrode slurry was prepared by using a mixture of artificial graphite and natural graphite as a negative electrode active material and an aqueous binder containing styrene-butadiene rubber (SBR) and 0.6 wt% to 1.5 wt% of carboxymethyl cellulose (CMC) dispersed in water as a binder, which was then applied to a copper foil and dried.

The above copper foil was rolled by controlling the rolling density as shown in Table 1, and impregnated in a PC electrolyte at a temperature of 40°C for 20 hours. The residual stress and the residual stress reduction rate before and after impregnation were measured, and are shown in Table 1.

Meanwhile, the measurement of residual stress was performed by the following method. XRD analysis was performed on the electrodes of Examples 1 to 3 and Comparative Examples 1 and 2. More specifically, the degree of crystal diffraction for the sample plane at various angles was measured while tilting the sample (φ-scan), and the interplanar distance of planes with different crystal directions was measured through XRD measurement in a specific 2theta range, The residual stress value was calculated by plotting the 131-surface, 89.9-degree copper foil as a reference.

Evaluation items Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Rolled density (g/cc) 1.6 1.7 1.8 1.85 1.9 residual stress
(MPa) Before impregnation 19.3 27.5 32.2 35.6 39.4 After impregnation 17.6 25.0 27.4 29.8 32.1 Decrease rate (%) 9 9 15 16 19 Whether peeling occurs X X X O O

As shown in Table 1, in Examples 1 to 3, the residual stress reduction rate was 15% or less before and after electrolyte impregnation, so that no peeling occurred between the copper foil and the cathode composite layer. On the other hand, in Comparative Examples 1 and 2, when the residual stress reduction rate exceeded 15%, peeling was confirmed to occur.

That is, when a heterogeneous material is deposited by rolling, stress occurs and some of the stress remains internally depending on the properties of the material, but if it exceeds it appears as strain. In the case of an electrode, there are residual stresses of the active material and residual stresses of the substrate (foil), and the residual stress can be relieved by external conditions such as solvents or heat. This experiment was conducted based on the correlation that as the rolling density increases, the difference in adhesive strength is similar but the size of the residual stress increases, and it was found that when the residual stress inside the electrode was relieved by impregnation in an electrolyte, peeling occurred at the rolling density at which the standard deviation in the adhesive strength measurement began to significantly increase. This can be more accurate than the method of predicting peeling using adhesive strength alone, and can reduce the loss that may occur in the post-process after electrode manufacturing.

Although the embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and it will be apparent to those skilled in the art that various modifications and variations are possible within a scope that does not depart from the technical spirit of the present invention described in the claims.

Claims (8)

An electrode for a secondary battery comprising an electrode current collector; and an electrode composite layer formed on the electrode current collector and containing an electrode active material,
Regarding the residual stress of the electrode current collector or electrode active material measured after immersing the above electrode in the electrolyte,
An electrode for a secondary battery, wherein the reduction rate of the residual stress of the electrode current collector or electrode active material measured after drying the impregnated electrode is 15% or less.
In the first paragraph,
An electrode for a secondary battery, wherein the above residual stress is measured using X-ray diffraction analysis for either the (131) plane of the electrode current collector or the (112) plane and (002) plane of the electrode active material.
In the first paragraph,
An electrode for a secondary battery, characterized in that the electrode density of the above electrode is less than 1.9 g/cc.
In any one of claims 1 to 3,
The above electrode is a negative electrode for a secondary battery.
A step of manufacturing an electrode by applying a slurry containing an electrode active material and a binder on an electrode collector and drying it;
A step of rolling the above electrode;
A step of immersing the above rolled electrode in an electrolyte and measuring the residual stress of the electrode collector or electrode active material; and
A method for manufacturing an electrode for a secondary battery, comprising the steps of drying an electrode impregnated in the above electrolyte and measuring the residual stress of an electrode collector or an electrode active material.
In paragraph 5,
Regarding the residual stress of the electrode collector or electrode active material of the electrode measured after immersion in the above electrolyte,
A method for manufacturing an electrode for a secondary battery, wherein the reduction rate of the residual stress of an electrode current collector or electrode active material measured after drying an electrode impregnated in the above electrolyte is 15% or less.
In paragraph 5,
A method for manufacturing an electrode for a secondary battery, wherein the step of impregnating the rolled electrode with an electrolyte is performed at a temperature range of 25 to 100°C for 1 to 24 hours.
In paragraph 5,
A method for manufacturing an electrode for a secondary battery, wherein the above residual stress is measured using X-ray diffraction analysis for either the (131) plane of the electrode current collector or the (112) plane and (002) plane of the electrode active material.