Patent Application
20230387382
The present disclosure relates to a negative electrode for a secondary battery, a manufacturing method thereof, and a secondary battery comprising the same.
Along with the technology development and increased demand for mobile devices, the demand for secondary batteries as energy sources is increasing rapidly. Among these secondary batteries, a lithium secondary battery having high energy density and a high operating voltage, a long cycle lifespan, and a low self-discharge rate is commercially available and widely used.
In recent years, as interest in environmental issues has increased, a considerable number of studies have been conducted on an electric vehicle (EV), a hybrid electric vehicle (HEV), and the like which can replace a vehicle using fossil fuels such as a gasoline vehicle and a diesel vehicle, which are one of the main causes of air pollution. As power sources for such electric vehicles (EV) and hybrid electric vehicles (HEV), lithium secondary batteries having high energy density, high discharge voltage, and output stability are mainly studied and used.
Generally, such a lithium secondary battery is manufactured by stacking or winding an electrode of a positive electrode and a negative electrode with a separator being interposed therebetween, and incorporating it into a secondary battery case together with an electrolyte.
At this time, the negative electrode is manufactured by applying an active material slurry containing a negative electrode active material onto a metal current collector, followed by drying, rolling, cutting with a unit electrode, and the like, but depending on the physical properties of the negative electrode, there is a problem that cracks and/or peeling off of the negative electrode active material layer occurs during the cutting.
However, such defects in the negative electrode may rapidly deteriorate the battery performance during subsequent manufacture of the secondary battery, and may also pose a problem in safety.
Therefore, there is a need to develop a technology that can solve these problems.
It is an object of the present disclosure to provide a negative electrode for a secondary battery that can prevent cracks and peeling off of a negative electrode active material layer when a negative electrode is cut during the manufacture of the negative electrode, a manufacturing method thereof, and a secondary battery comprising the same.
According to one embodiment of the present disclosure, there is provided a negative electrode for a secondary battery in which a negative electrode active material layer is formed on at least one surface of a current collector,
At this time, the continuous adhesive force of the Condition 1 is an adhesive force between active material layers per 10 μm in the thickness direction of the negative electrode.
Also, the negative electrode active material layer comprises a negative electrode active material, a conductive material, and a binder, wherein in order to allow the continuous adhesive strength of the Condition 1 to satisfy 30 gf/20 mm or more, the content of the binder contained in each active material layer per 10 μm in the thickness direction of the negative electrode includes 2% by weight or more based on the total weight of each active material layer having a thickness of 10 μm.
Moreover, when the negative electrode satisfies the Condition 1, the negative electrode may have a porosity of 28% or less, specifically 5% to 28%.
Most specifically, the negative electrode may satisfy both the Condition 1 and the Condition 2.
Meanwhile, according to another embodiment of the present disclosure, there is provided a method for manufacturing the negative electrode for a secondary battery as mentioned above, the method comprising the steps of:
The rolling stress σ may be a rolling load acting per unit area.
The negative electrode elastic modulus (Ee) may be obtained according to the following Equation 3:
According to yet another embodiment of the present disclosure, there is provided a secondary battery comprising the negative electrode for a secondary battery.
The terms and words used in the present specification and claims should not be interpreted as being limited to typical meanings or dictionary definitions, but should be interpreted as having meanings and concepts relevant to the technical scope of the present disclosure based on the rule according to which a person of ordinary skill in the art can appropriately define the terms and words as terms for describing most appropriately the best method he or she knows for carrying out the invention. Accordingly, the embodiments described herein and the configurations shown in the drawings are only most preferable embodiments of the present disclosure and do not represent the entire spirit of the present disclosure, so it should be appreciated that there may be various equivalents and modifications that can replace the embodiments and the configurations at the time at which the present application is filed, and the scope of the present invention is not limited to the embodiments described below.
According to one embodiment of the present disclosure, there is provided a negative electrode for a secondary battery in which a negative electrode active material layer is formed on at least one surface of a current collector,
Specifically, the continuous adhesive force of the Condition 1 means an adhesive force between active material layers per 10 μm in the thickness direction of the negative electrode. That is, when the adhesive force between the active material layers per 10 μm is measured, it is considered that the above Condition 1 is not satisfied if the adhesive force is less than 30 gf/20 mm in any section.
Such a continuous adhesive force is similar to the method of measuring the adhesive force of the manufactured negative electrode. Specifically, a double-sided tape is attached to a slide glass, a negative electrode punched to 30 mm×125 mm is placed thereon, and then a 20 mm wide scotch magic tape is attached to the upper surface of the negative electrode. And, in order to uniformly attach the negative electrode and the slide glass, and the negative electrode and the Scotch Magic tape, it is passed through a laminator at room temperature, and then the scotch magic tape is pulled at 100 mm/min using a UTM (from TA Instruments) device to measure the peeling force from the negative electrode. At this time, the measurement angle of the slide glass and the negative electrode is 90°. After that, the scotch magic tape is attached in a state in which the peeled upper active material layer was eliminated, and then pulled at 100 mm/min using a UTM (from TA Instruments) device to measure the peeling force from the negative electrode. The continuous adhesive force means a value measured by conducting an adhesion force test while continuously removing the peeled active material layer in this way.
If the adhesive force is less than 30 gf/20 mm even in any section of the values measured in this way, it is determined that the above Condition 1 is not satisfied.
That is, the present inventors confirmed that when the above Condition 1 is satisfied, cracks or peeling off of the negative electrode active material layer do not occur when cutting the negative electrode.
The methods for satisfying such Condition 1 may be diverse, and if the above Condition 1 is satisfied regardless of the method, cracks or peeling off of the negative electrode active material layer do not occur. However, as an example, in order to satisfy the above Condition 1, the negative electrode active material layer includes a negative electrode active material, a conductive material, and a binder, wherein the content of the binder contained in each active material layer per 10 μm in the thickness direction of the negative electrode may include 2% by weight or more based on the total weight of each active material layer having a thickness of 10 μm.
That is, the binder should be contained in an amount of 2% by weight or more in each active material layer having a thickness of 10 μm, which has a similar meaning that the binder should be uniformly distributed at 2% by weight or more anywhere. When the binder is contained in an amount of 2% by weight or more in any section in this way, the continuous adhesive force may satisfy the Condition 1.
Further, when the negative electrode satisfies the Condition 1, the porosity of the negative electrode may be 28% or less, specifically 5% to 28%, more specifically 10% to 25%.
The porosity may be determined by a process in which the surface of the negative electrode is magnified 2,500 times and measured using a scanning electron microscope (FE-SEM) (Hitachi S-4800 Scanning Electron Microscope), and then the area ratio of surface voids found in the randomly sampled range (10 um or more in width and 15 um or more in length) in the measured photograph is calculated relative to the total area and converted into a volume.
If the porosity is too large outside the above range, the adhesive force of the negative electrode may be lowered, which is not preferable.
Meanwhile, even if the Condition 1 is not satisfied, the present inventors confirmed that as described above, when the breaking stress is 3.6 N or more, which is the Condition 2, cracks or peeling off of the negative electrode active material layer do not occur.
Specifically, the breaking stress may be 3.6 N or more to 5 N or less.
The breaking stress may be a value measured by pressing the negative electrode active material layer with a linear tip having a width of 2.5 mm and a sharp end at a speed of 10 μm/s as described above, and more specifically, it is a value measured by vertically pressing the negative electrode in the 90 degree direction using DHR (from TA Instruments).
When the breaking stress satisfies the condition according to the present disclosure, cracks or peeling off of the negative electrode active material layer do not occur.
If the breaking stress is too small outside the scope of the present disclosure, cracks or peeling off of the negative electrode active material layer may occur, and when the breaking stress is too large, the flexibility is too low, which is not preferable.
Adjusting the breaking stress can be obtained by adjusting misfit stain during drying and rolling in the manufacturing step of the negative electrode, as will be described below.
Therefore, most specifically, the negative electrode may satisfy both the Condition 1 and the Condition 2.
Meanwhile, according to another embodiment of the present disclosure, there is provided a method for manufacturing the negative electrode for secondary battery, the method comprising the steps of:
After repeated in-depth research by the present inventors, it has been found that as described above, in order for the breaking stress of the negative electrode for a secondary battery to have the above value, the misfit strain occurring during the manufacturing process satisfies at least one of the Conditions 3 and 4 above.
Therefore, when the negative electrode is manufactured so as to satisfy the above Conditions 3 and 4, the effect of the present disclosure can be achieved.
When the above Conditions are satisfied, cracks or peeling off of the negative electrode do not occur as an effect intended by the present disclosure.
Condition 1 is the same as described above.
The misfit strain of Conditions 3 and 4 is obtained as in Equation 1 above, wherein the drying or rolling stress and the method of obtaining the elastic modulus of the negative electrode are the same as described below.
Specifically, the drying stress (σ) can be a value obtained according to the following Equation 2:
Here, the thickness of the current collector and the negative electrode active material layer and the length of the current collector can be determined with the naked eye, and the elastic modulus and the Poisson's ratio of the current collector mean a value of the metal forming the current collector, which is a fixed value. For example, the elastic modulus of Al is 7.19×102, the Poisson's ratio may be 0.34, the elastic modulus of Cu is 1.25×103, and the Poisson's ratio may be 0.34. Further, the deflection of the current collector may measure the degree of bending of the negative electrode using a laser and a position sensor. The degree of bending is the value measured at the Z-axis height, in which the dried negative electrode is interposed in a flat holder and the negative electrode is bent downward or upward when viewed from the side.
Further, the rolling stress (σ) means a rolling load acting per unit area, unlike the drying stress.
The rolling load may be selected by the operator as the rolling applied to the negative electrode during rolling.
Further, the negative electrode elastic modulus (Ee) may be a value obtained according to the following Equation 3.
The current collector elastic modulus of the Ef, the thickness of the current collector, and the thickness of the negative active material layer are the same as described above, and the total elastic modulus of the Etot is a value measured using a DMA instrument (viscoelasticity measuring device).
When the drying or rolling stress and the negative electrode elastic modulus obtained in this way substitutes into Equation 1 above and any one of the misfit strains satisfies the condition of 0.1% or less, the breaking stress of the negative electrode becomes 3.6 N or more, and the effects intended by the present disclosure can be exhibited.
Therefore, it can be determined whether the conditions of the present technology are satisfied from the manufacturing stage, defects can be detected quickly, and cracks and/or peeling off of the negative electrode active material layer can be prevented, thereby preventing secondary battery performance deterioration and safety deterioration including the same.
Meanwhile, according to another embodiment of the present disclosure, there is provided a secondary battery including the negative electrode for a secondary battery.
Other manufacturing methods and components of the secondary battery are known in the art, and thus, a detailed description thereof is omitted here, which also falls under the scope of the present disclosure.
Hereinafter, preferred embodiments of the present disclosure will be mainly described with reference to Examples, Comparatives, and the accompanying drawings.
Artificial graphite as an active material, a binder (SBR and CMC were mixed in a weight ratio of 2:1), and carbon black as a conductive material was mixed in a weight ratio of 96:2.5:1.5. The mixture and the dispersion medium was mixed at a weight ratio of 1:2 using water as a dispersion medium to prepare a slurry for the active material layer.
Using a slot die, the slurry for the active material layer was coated onto one surface of a copper (Cu) thin film that is a negative electrode current collector having a thickness of 10 μm, and dried under vacuum at 130° C. for 1 hour to form an active material layer. The active material layer thus formed was rolled by a roll pressing method to manufacture a negative electrode having an active material layer having a thickness of 80 μm.
The porosity of the manufactured negative electrode was 30%.
The porosity is the value determined by a process in which the surface of the negative electrode was magnified 2,500 times and measured using a scanning electron microscope (FE-SEM) (Hitachi S-4800 Scanning Electron Microscope), and then the area ratio of surface voids found in the randomly sampled range (10 um or more in width and 15 um or more in length) in the measured photograph was calculated relative to the total area and converted into a volume.
A negative electrode was manufactured in the same manner as in Comparative Example 1, except that in Comparative Example 1, active material:binder:conductive material were mixed in a weight ratio of 96:2.8:1.2 and rolled so that the active material layer had a thickness of 75 a during rolling. The porosity of the manufactured negative electrode was 25%.
A negative electrode was manufactured in the same manner as in Comparative Example 1, except that in Comparative Example 1, the slurry for active material layer was dried under vacuum at 100° C. for 30 minutes, then dried under vacuum at 110° C. for 30 minutes, and again dried under vacuum at 130° C. for 30 minutes to form an active material layer. The porosity of the manufactured negative electrode was 30%.
A negative electrode was manufactured in the same manner as in Comparative Example 1, except that in Comparative Example 1, during rolling, the active material layer was primarily rolled to a thickness of 90 μm, and then secondarily rolled again to a thickness of 80 μm. The porosity of the manufactured negative electrode was 30%.
The continuous adhesive force of the negative electrodes manufactured in Comparative Example 1 and Examples 1 to 3 was measured, and shown in
A double-sided tape was attached to a slide glass, a negative electrode punched to 30 mm×125 mm was placed thereon, and then a 20 mm wide scotch magic tape was attached to the upper surface of the negative electrode. And, in order to uniformly attach the negative electrode and the slide glass, and the negative electrode and the Scotch Magic tape, it was passed through a laminator at room temperature, and then the scotch magic tape was pulled at 100 mm/min using a UTM (from TA Instruments) device to measure the peeling force from the negative electrode. At this time, the measurement angle of the slide glass and the negative electrode was 90°. After that, the scotch magic tape was attached in a state in which the peeled upper active material layer was eliminated, and then pulled at 100 mm/min using a UTM (from TA Instruments) device to measure the peeling force from the negative electrode. The continuous adhesive force of the negative electrode means a value measured by conducting an adhesion force test while continuously removing the peeled active material layer in this way.
Reviewing
Misfit strains in the drying and rolling processes of the negative electrodes manufactured in Comparative Example 1 and Examples 1 to 3 were calculated, and shown in Table 1 below.
Referring to Table 1, it can be confirmed that in Comparative Example 1 and Example 1, both drying and rolling misfit strains are 0.1% or more, while in Examples 2 and 3, at least any one is 0.1% or less.
The stress and elastic modulus are obtained from Equation 2, the rolling load and Equation 3, and the misfit strain is obtained from Equation 1 based on the above values.
The breaking stresses of the negative electrodes manufactured in Comparative Example 1 and Examples 1 to 3 were measured, and are shown in Table 2 below.
The breaking stress is a value measured by vertically pressing the negative electrode active material layer with a linear tip having a width of 2.5 mm and a sharp end at a speed of 10 μm/s in the 90 degree direction using DHR (from TA Instruments).
Reviewing Table 2, it can be confirmed that when at least one of the misfit strains of the drying process or the rolling process is 0.1% or less in comparison with Experimental Example 2, the breaking stress is 3.6 N or more.
The negative electrodes manufactured in Comparative Example 1 and Examples 1 to 3 were notched in half to examine the presence or absence of cracks.
The presence or absence of cracks was confirmed with the naked eye by taking a photomicrograph of the upper part of the negative electrode in a vertical direction, and the results are shown in
Reviewing
Therefore, it can be seen that when any one of the above Conditions 1 to 4 of the present disclosure is satisfied, the effects intended by the present disclosure can be achieved.
A person having ordinary knowledge in the field to which the present disclosure belongs can make various applications and modifications departing the sprit and scope of the present disclosure based on the above contents.
Since the negative electrode for a secondary battery according to the present disclosure satisfies a specific condition, no cracks or peeling off of the negative electrode active material layer occurs when the negative electrode is cut, whereby it is possible to solve the problem of performance deterioration or safety deterioration of the secondary battery including such a negative electrode.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2021-0112407 | Aug 2021 | KR | national |
This application is a national phase entry under 35 U.S.C. § 371 of International Application No. PCT/KR2022/012632, filed Aug. 24, 2022, which claims priority from Korean Patent Application No. 10-2021-0112407, filed on Aug. 25, 2021, the entire contents of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2022/012632 | 8/24/2022 | WO |
