KR102197898B1 - Evaluation method for dispersibility of carbon nanotube in electrode - Google Patents

Abstract

The present invention provides a method for evaluating the dispersibility of CNTs in an electrode by analyzing the current distribution of the active material surrounded by the CNTs for an electrode sample including an active material, carbon nanotubes (CNT) and a binder.
According to the present invention, it is possible to efficiently evaluate the dispersibility of CNTs used in trace amounts in the electrode of a lithium ion battery by analyzing the current distribution in the electrode, in particular, by confirming the current value of the active material surrounded by CNTs.

Description

Method for evaluating the dispersibility of carbon nanotubes in the electrode {EVALUATION METHOD FOR DISPERSIBILITY OF CARBON NANOTUBE IN ELECTRODE}

The present invention relates to a method for evaluating the dispersibility of carbon nanotubes (CNTs) in an electrode, and provides a method for more precisely analyzing the dispersibility of CNTs present in a trace amount in an electrode.

The lithium ion battery has a structure in which an electrolyte including a lithium salt is impregnated with an electrode on which an active material is applied on a current collector, that is, an electrode assembly in which a porous separator is interposed between the positive electrode and the negative electrode, and the electrode is a foil ( It is generally manufactured by coating a slurry containing electrode materials such as an active material, a conductive material, and a binder on a current collector in the form of a foil), drying it, and forming an active material layer through a process of rolling. In the active material layer formed as described above, it is important that the active material through which lithium ions are intercalated/desorbed, the conductive material distributed around the active material, and the binder positioned between the active material and the conductive material to connect and fix them are uniformly dispersed. . In addition, pores are formed between the electrode materials distributed in the active material layer, and the porosity of the active material layer may be controlled according to a rolling rate during electrode manufacturing.

In general, a conductive material among the major electrode constituent materials is a component that imparts conductivity without causing chemical changes to the battery. Using a scanning spreading resistance microscopy (SSRM), a current or resistance After imaging the) value, the distribution can be checked through image processing.

However, in the case of carbon nanotubes (CNT) among the conductive materials used for the electrode, unlike point-type conductive materials such as carbon black, the amount used is less than 1% by volume, and CNTs uniformly surround the active material. Since CNT is thinner than one pixel size of the image when observed from a cross section, the probability of not being detected in the image of a scanning diffusion resistance microscope (SSRM) is high. There is a problem that it is difficult to grasp.

An object of the present invention is to provide a method capable of more efficiently and precisely evaluating the dispersibility of carbon nanotubes in an electrode.

According to an aspect of the present invention, as a method for evaluating the dispersibility of carbon nanotubes (CNT) in an electrode,

(S1) preparing an electrode sample including an active material, CNT and a binder;

(S2) obtaining an image of the electrode sample using a scanning spreading resistance microscope (SSRM), and extracting a current value therefrom;

(S3) creating a current distribution map from the current value and dividing the current distribution map into three current range sections; And

(S4) There is provided a method including the step of evaluating the dispersibility of CNTs in the active material by checking the current value of the active material corresponding to the intermediate range in the three sections.

The current distribution diagram can be expressed as a histogram using the measured current value and its counts, and can be created by graphing it on a log scale or a linear scale.

The SSRM image is obtained from 2 to 5 measurement positions in the electrode sample, and the current value can be measured by an average of them, and can represent an area of 35 μm×35 μm to 38 μm×38 μm.

The current value of the active material corresponding to the middle range in the current distribution diagram may be in the range of 1.E-4 (0.0001) to 4.E-1 (0.4) μA.

In addition, in the current distribution diagram, a current value in a low range may indicate pores in an electrode sample, and a current value in a high range may indicate CNT.

In the current distribution diagram, a current value of an active material that falls within an intermediate range may be proportional to the dispersibility of CNTs surrounding the active material.

Moreover, the average (A) and standard deviation (B) of the adjusted R-square obtained by Gauss fitting the current distribution of the active material for each measurement location of the electrode sample in the current distribution diagram are the following equations: When 1 is satisfied, the dispersibility of CNT in the electrode can be evaluated as excellent:

[Equation 1]

A ≥ 0.97

B ≤ 0.01

In the above equation, A and B are the average and standard deviation of the adjusted R-square obtained by Gauss fitting the current distribution of the active material for each measurement location in the current distribution diagram of the electrode sample, respectively.

The electrode may be a positive or negative electrode of a lithium ion battery.

According to the method of the present invention, by analyzing the current distribution in the electrode, in particular, by checking the current value of the active material surrounded by CNTs, it is possible to efficiently evaluate the degree to which CNTs that are used in a relatively small amount and small in size are dispersed in the electrode compared to other components of the electrode. I can.

1 schematically shows a scanning spreading resistance microscopy (SSRM) system used for analysis of an electrode sample in the present invention.
2 is a picture of SSRM images obtained at three positions for three types of electrode samples in an embodiment of the present invention.
FIG. 3 illustrates a current distribution diagram of SSRM images acquired at three positions of each electrode sample, expressed as a histogram using the current values and their counts, and graphed them on a log scale.
Fig. 4 illustrates a current distribution diagram created from average current values at three positions of each electrode sample.
5 is a photograph of SEM images obtained at three locations for three types of samples in a comparative example.

Hereinafter, terms or words used in the present specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventor appropriately defines the concept of terms in order to describe his own invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of the present invention based on the principle that it can be done.

The present invention relates to a method for evaluating the dispersibility of carbon nanotubes (CNTs) used as a conductive component in an electrode included in a lithium ion battery.

In order to evaluate the dispersibility of the CNT, first, an electrode sample including an active material, CNT, and a binder is prepared (S1).

Specifically, the electrode sample is formed by coating a slurry containing an active material, CNT, and an electrode material of a binder on a current collector in the form of a foil, followed by drying and pressing to form an active material layer, It can be prepared by filling the pores formed between the electrode materials in the active material layer with resin and then performing ion milling.

The active material is not particularly limited as long as it is commonly used for a positive electrode or negative electrode included in a lithium ion battery.

For example, in the case of applying a positive electrode active material, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiCoPO ₄ , LiFePO ₄ , and LiNi _1-xyz Co _x M1 _y M2 _z O ₂ (M1 and M2 are independently of each other Al, Ni , Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg, and any one selected from the group consisting of Mo, and x, y and z are independently of each other as the atomic fraction of the oxide composition elements, 0=x<0.5 , 0≤=y<0.5, 0≤=z<0.5, 0<x+y+z=1). Any one selected from the group consisting of, or a mixture of two or more thereof may be used.

In addition, in the case of applying the negative active material, natural graphite, artificial graphite, carbonaceous material; Lithium-containing titanium composite oxide (LTO), Si, Sn, Li, Zn, Mg, Cd, Ce, Ni, or Fe metals (Me); Alloys composed of the metals (Me); Oxides of the metals (Me) (MeOx); And any one selected from the group consisting of a composite of the metals (Me) and carbon, or a mixture of two or more of them may be used.

The CNT is a material that can be used as a component that imparts conductivity distributed around an active material in an electrode layer, and is used in a relatively small amount compared to other electrode materials and has a small size. Therefore, it is difficult to accurately evaluate the dispersibility of CNTs.

Thereafter, an image is obtained for the electrode sample using a scanning spreading resistance microscope (SSRM), and a current value is extracted therefrom (S2).

1 schematically shows a scanning diffusion resistance microscope (SSRM) system used for analysis of an electrode sample in the present invention.

As can be seen in FIG. 1, a scanning diffusion resistance microscope (SSRM) is an analysis device based on an atomic force microscopy (AFM) device, and a conductive AFM probe provided with a pointed tip is used for the sample. A local area of the sample is scanned while moving the surface, and the current (resistance) flowing between the sample and the tip can be measured when a certain voltage is applied. The measurement is generally performed in a contact mode, and it is possible to measure a wide resistance range using a logarithmic amplifier, so that a topographic image of the sample surface can be obtained and electrical properties can also be obtained. It is possible to measure the current distribution for a sample in which several materials with different conductivity are mixed.

Using such an SSRM, a current image or a resistance image can be obtained as a surface image of an electrode sample, and in the present invention, a current image in which CNTs can clearly appear among the electrode components is utilized (see FIG. 2).

When referring to the SSRM image picture of FIG. 2, CNTs can be recognized only when they are clustered, and it is difficult to confirm the overall dispersion state of CNTs (e.g., Points 2 and 3 of Sample #1, Point 2 of Sample #2). ).

On the other hand, when CNT is hardly observed in FIG. 2 (e.g., sample #3), it can be seen that the CNT is rather well dispersed, and the color distribution of the active material occupying most of the pixels is relatively uniform. It is bright, and it can be considered that the current of the active material is relatively high. In other words, if CNTs are well dispersed around the active material to provide sufficient conductivity, the current value of the active material will increase.

Focusing on this point, the present invention evaluates the dispersibility of CNTs by checking the current value of the active material surrounded by CNTs.

In one embodiment of the present invention, the SSRM image may represent a large area of 35 μm×35 μm to 38 μm×38 μm.

For example, when the SSRM image has an area of 35 μm×35 μm, the size of the entire pixel is 1024×1024 and the size per pixel is 34.2 nm (=35,000/1024). Meanwhile, since the size of the carbon nanotubes (CNTs) is thinner than the size of one pixel of the SSRM image, there is a high probability that the CNTs are not detected within the measurement area of the SSRM image if they are uniformly dispersed.

Accordingly, in order to grasp the overall dispersion of CNTs present in trace amounts in the electrode cross-section having a thickness of several tens of µm, for example, 5 to 60 µm, it is recommended to acquire an SSRM image with as large an area as possible, but the pixel size increases accordingly. Measurement time may increase, making it difficult to measure. Therefore, the SSRM image can be adjusted to have an area of 35 μm×35 μm to 38 μm×38 μm.

In addition, the SSRM image may be measured from various positions of the electrode sample. For example, an image obtained by measuring SSRM at one or more, eg, 2 to 5 positions, from each sample may be obtained, and the average value thereof may be used.

By processing the SSRM image obtained as described above with software provided in the SSRM system, the current value for each pixel position can be extracted as text.

Subsequently, a current distribution map is created from the extracted current values, and the current distribution map is divided into three current range sections (S3).

The current distribution diagram is a Gaussian distribution diagram that is expressed as a histogram using the measured current value and its counts, and is graphed on a log scale or a linear scale. Specifically, the histogram may be expressed by setting a current value as a certain section and the number of pixels per section as a frequency (count).

In addition, the current distribution diagram can be prepared by using an average of current values measured from several positions of the electrode sample, for example, from 2 to 5 positions.

3 is a histogram using current values and their counts of SSRM images acquired at three positions of three electrode samples including CNT and a binder as an active material, a conductive material, and log It shows the current distribution chart created by graphing it with a scale.

The current distribution diagram represents the frequency of the corresponding current value in the current range of 1.E-5 (0.00001) to 1.E+1 (10) μA, and based on the current value, high, middle, It can be divided into a low section. At this time, a low current range indicates pores, a high current range indicates carbon nanotubes (CNTs), and an intermediate current range indicates an active material and a binder.

Specifically, a low current range may mean 8.E-5 (0.00008) μA or less, a high current range may mean 5.E-1 (0.5) μA or more, and a middle current range 1.E It may mean a range of -4 (0.0001) to 4.E-1 (0.4) μA. For example, a count measured in a current range of 5.E-4 (0.0005) to 3.E-1 (0.3) μA may indicate a distribution for the active material. In the current distribution diagram, the current value corresponding to the maximum frequency is indicated by the sum of the current value of the active material and the current value of the CNT surrounding the active material, and the greater the current value of this maximum frequency, the better the dispersion of CNTs. It can be determined, because the CNT imparting conductivity reflects the degree of surrounding the surface of the active material. That is, a current value corresponding to the maximum frequency in the current distribution diagram has a relationship proportional to the dispersibility of the carbon nanotubes surrounding the active material.

Therefore, by checking the current value corresponding to the maximum frequency of the active material in the middle range in the current distribution map, the dispersibility of CNTs in the active material can be evaluated (S4).

Meanwhile, FIG. 4 is an illustration of a current distribution diagram created from average current values at three positions of three electrode samples including CNT and a binder as an active material and a conductive material. Sample 3 with the largest current value corresponding to the maximum frequency It can be said that the CNT dispersibility of (#3) is the best.

In FIGS. 3 and 4, when the conductivity of the active material itself is changed according to the type of the active material, the current value distribution map moves left and right, and a current value corresponding to the maximum frequency may also change. Therefore, the current value of the maximum frequency can be used to evaluate the dispersibility of CNTs according to the electrode manufacturing method based on one type of electrode system.

In the current distribution diagram of FIG. 3, sample 3 (#3) has similar graphs for each measurement location and is overlapped, which can be understood that CNTs are uniformly distributed around the active material.

In addition, the degree to which CNTs are distributed around the active material in the electrode can be evaluated by comparing the value of the adjusted R-square obtained by Gauss fitting the current distribution of the active material at each measurement location. have. The term'adjusted R-square', which is a term used in the present invention, refers to a statistical value representing the degree of approximation between a fitting model and an actual data point in nonlinear curve fitting including Gaussian fitting. .

More specifically, the average (A) and standard deviation (B) of the adjusted R-square obtained by Gauss fitting the current distribution of the active material at each measurement location in the current distribution diagram of each electrode sample When the following Equation 1 is satisfied, it can be evaluated as having excellent dispersibility of CNTs in the electrode.

[Equation 1]

A ≥ 0.97

B ≤ 0.01

In the above equation, A and B are the average and standard deviation of the adjusted R-square obtained by Gauss fitting the current distribution of the active material for each measurement location in the current distribution diagram of the electrode sample, respectively.

At this time, the average (A) and standard deviation (B) of the modified coefficient of determination may be the average and standard deviation of the values measured at various locations. The closer the value of the corrected coefficient of determination is to 1, the better the approximation accuracy. Therefore, it can be evaluated that CNTs are uniformly distributed around the active material.

In this way, by checking the current value of the maximum frequency that appears for each measurement location in the current distribution diagram of the electrode sample, in particular, by using the value of the modified coefficient of determination obtained by Gauss fitting the current distribution of the active material, the dispersibility of the CNT included in the electrode sample. Can be evaluated efficiently.

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 implemented in various different forms and is not limited to the embodiments described herein.

Example 1: Evaluation of dispersibility of CNTs in electrodes using a scanning diffusion resistance microscope (SSRM)

In order to compare the dispersibility of carbon nanotubes (CNT), the mixing method is different as shown in Table 1 below, but the composition of the active material (LiCoO ₂ , Nichia), the conductive material (bundled CNT) and the binder (PVdF) is After coating the same three electrode slurries on an Al foil, drying and rolling to form an active material layer, the pores formed between the electrode materials in the active material layer are formed by resin, that is, epoxy resin. The electrode filled with was ion milled (Hitachi, IM4000) to prepare a cross-sectional sample.

division Mixing method Sample 1 A Use of a regular mixer Sample 2 B bead mill used Sample 3 A+B

In Table 1, A and B refer to a mixing method, specifically A is a general mixer (mixer) and B is a bead mill (bead mill). In addition, A+B means using both a general mixer and a bead mill mixing method.

Then, after fixing each electrode sample to a sample holder of an SSRM (Park Systems, NX10) as shown in FIG. 1, the AFM probe was operated and at three positions (#1, #2, and #3). The measurement was carried out. When measuring the SSRM, a diamond tip having a large k value was used, the applied bias voltage was 2V, the area of the measurement area was 35 µm × 35 µm, the pixel size was 1024 × 1024, and the scan rate was 0.5 Hz or less.

For each sample, the SSRM image (Fig. 2) acquired at three positions (#1, #2, and #3) is processed with the software (XEI program) provided in the SSRM system, and the current value for each pixel position is text ) Was extracted. Then, the extracted current value was set as a certain section, and the histogram was expressed by using the number of pixels for each section as the frequency (count), and the current distribution diagram (FIGS. 3 and 4) was created by graphing this on a log scale. Subsequently, the current distribution diagram was divided into high, middle, and low current range sections, and current values in each range were checked.

2 shows SSRM images acquired at three positions (Points 1, 2, 3) for Sample 1 (#1), Sample 2 (#2), and Sample 3 (#3), respectively, and Sample 1 (# In the images of 1) and sample 2 (#2), it was possible to recognize only when the CNTs were clustered, and it was difficult to confirm the overall dispersion state of CNTs, and almost no CNTs were observed in the image of sample 3 (#3). . In addition, since the color distribution of the active material is relatively uniform and bright, the dispersibility of CNTs around the active material is highest, so that the current of the active material is relatively high.

On the other hand, as can be seen in the current distribution diagram of FIG. 3, when the current range of 1.E-5 (0.00001) to 1.E+1 (10) μA is divided into upper, middle, and lower, the low current range is pore ( pore), the high current range indicates carbon nanotubes (CNT), and the middle current range indicates the active material and the binder. Sample 3(#) has a maximum current value in the middle range. You can see the biggest one. The part that occupies the largest area in the cross section of the electrode sample corresponds to the active material, and accordingly, the part that occupies most of the frequency in the current histogram corresponds to the active material. Even though the active material of the same type, the measured mid-range current value is The reason for the difference in the three samples can be interpreted as a difference in the degree to which CNTs that impart conductivity surround the surface of the active material.

In addition, in the current distribution diagram of FIG. 3, after Gauss fitting the current distribution of the active material for each measurement location of the electrode sample, the value of the adjusted R-square obtained therefrom, and the average (A) and standard thereof The deviation (B) is shown in Table 2 below.

Modified coefficient of determination
(Adjusted R-square) Sample 1 Sample 2 Sample 2 Point1 0.9873 0.9492 0.9839 Point2 0.9713 0.9640 0.9858 Point3 0.9010 0.9327 0.9781 Average (A) 0.9532 0.9486 0.9826 Standard deviation (B) 0.0459 0.0157 0.0040

In Table 2, the average (A) and standard deviation (B) of the modified coefficient of determination for Sample 3 satisfy the following Equation 1, so that the CNTs are uniformly distributed around the active material in Sample 3 It can be evaluated as the best in the dispersibility of CNT.

[Equation 1]

A ≥ 0.97

B ≤ 0.01

In the above formula, A and B are each

In the above equation, A and B are the average and standard deviation of the adjusted R-square obtained by Gauss fitting the current distribution of the active material for each measurement location in the current distribution diagram of the electrode sample, respectively.

Comparative Example 1: Cross-section observation using SEM

SEM images of the same sample used in Example 1 were taken, and are shown in FIG. 5. In FIG. 5, #1, #2, and #3 denote sample 1, sample 2, and sample 3, respectively.

As shown in Fig. 5, it is difficult to directly determine the dispersibility from the region corresponding to the CNT+binder (highlighted in blue).

In other words, CNT dispersibility evaluation using SEM cross-sectional images is the same problem as the pixel limit in SSRM measurement mentioned above, so when measuring a large area at low magnification, only the area where CNTs are clustered cannot be observed. It is possible. In addition, since both CNTs and binders are observed in similar black color, there is a limitation in that they cannot be distinguished.

As described above, it was confirmed that the analysis of the current distribution in the electrode according to the present invention, in particular, the method of evaluating the dispersibility of CNTs through the current value of the active material surrounded by the CNTs can obtain a result with high accuracy.

Claims (10)

As a method for evaluating the dispersibility of carbon nanotubes (CNT) in an electrode,
(S1) preparing an electrode sample including an active material, CNT and a binder;
(S2) obtaining an image of the electrode sample using a scanning spreading resistance microscope (SSRM), and extracting a current value for the entire SSRM image;
(S3) creating a current distribution map from the current value and dividing the current distribution map into three current range sections; And
(S4) including the step of evaluating the dispersibility of CNT to the active material by checking the current value of the active material corresponding to the middle range in the three sections,
In the current distribution diagram, as the current value of the active material corresponding to the middle range is in the range of 5.E-4 (0.0005) to 3.E-1 (0.3) μA, the larger the current value corresponding to the maximum frequency A method of evaluating as having excellent dispersibility. The method of claim 1,
The current distribution diagram is expressed as a histogram using the measured current value and its frequency (counts), and is created by graphing it on a log scale or a linear scale. The method of claim 1,
The SSRM image is obtained from 2 to 5 measurement positions in the electrode sample, and a method of measuring a current value by an average of these. The method of claim 1,
The SSRM image represents an area of 35 μm×35 μm to 38 μm×38 μm. delete The method of claim 1,
In the current distribution diagram, the current value in the low range indicates pores in the electrode sample. The method of claim 1,
The method in which the current value in the high range in the current distribution diagram represents CNT. The method of claim 1,
In the current distribution diagram, the current value of the active material corresponding to the middle range is proportional to the dispersibility of CNTs surrounding the active material. The method of claim 1,
In the current distribution diagram, the average (A) and standard deviation (B) of the adjusted R-square obtained by Gauss fitting the current distribution of the active material for each measurement location of the electrode sample are expressed in Equation 1 below. When satisfied, the method of evaluating that the dispersibility of CNTs in the electrode is excellent:
[Equation 1]
A ≥ 0.97
B ≤ 0.01
In the above equation, A and B are the average and standard deviation of the adjusted R-square obtained by Gauss fitting the current distribution of the active material for each measurement location in the current distribution diagram of the electrode sample, respectively. The method of claim 1,
The electrode is a method of a positive or negative electrode of a lithium ion battery.

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