KR102275858B1 - Method for estimating the dispersibility of conductive material dispersion - Google Patents

Abstract

The present invention comprises the steps of applying a conductive material dispersion by a spin coating method on a glass substrate; and drying the applied conductive material dispersion, wherein the conductive material dispersion relates to a dispersibility evaluation method of a conductive material dispersion including carbon nanotubes, a dispersant, and a dispersion medium.

Description

Dispersion evaluation method of conductive material dispersion {METHOD FOR ESTIMATING THE DISPERSIBILITY OF CONDUCTIVE MATERIAL DISPERSION}

The present invention relates to a dispersion evaluation method of a conductive material dispersion, specifically, the dispersion evaluation method comprising the steps of applying a conductive material dispersion by a spin coating method on a glass substrate; and drying the applied conductive material dispersion, wherein the conductive material dispersion may include carbon nanotubes, a dispersant, and a dispersion medium.

Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative energy or clean energy is increasing, and as a part of it, the fields most actively studied are power generation and electricity storage using electrochemical reactions.

Currently, a secondary battery is a representative example of an electrochemical device using such electrochemical energy, and its use area is gradually expanding. Recently, as technology development and demand for portable devices such as portable computers, portable phones, and cameras increase, the demand for secondary batteries as an energy source is rapidly increasing, and among such secondary batteries, high energy density, that is, high capacity lithium secondary batteries. A lot of research has been done on it, and it has been commercialized and widely used.

In general, a secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator, among which the positive electrode and the negative electrode may each include an active material layer. The active material layer may include active material particles and a conductive material. The conductive material is used to improve the conductivity of the active material layer, and a material having a conductive network is used, for example, carbon nanotubes may be used. The conductive material may be added to a solvent together with the active material particles, and a slurry may be formed through this, and then applied to a current collector to prepare a positive electrode or a negative electrode.

On the other hand, in order to improve the performance of the positive electrode and the negative electrode, techniques for increasing the dispersibility of the conductive material in the slurry are being tried. As one of them, a technique of preparing a slurry by preparing a conductive material dispersion including a conductive material, a dispersing agent, and a dispersion medium, and then adding active material particles thereto is used.

At this time, when the conductive material is well dispersed in the conductive material dispersion (high dispersibility), the conductive material may be well dispersed even in the slurry. Here, whether the conductive material dispersion has high dispersibility can be known through the particle size distribution of the conductive material in the conductive material dispersion. If the size distribution of the conductive material is too large, it means that the dispersion did not occur properly and some of the conductive materials aggregated with each other.

Conventionally, after diluting the conductive material dispersion in a solvent, particle size distribution was checked through a light-scattering method to evaluate the dispersibility of the conductive material dispersion. However, this method requires a separate solvent, and when it is diluted with the solvent, the size distribution of the conductive material cannot be accurately known due to a change in the force acting between the dispersed particles.

Therefore, there is a need for a method capable of evaluating the dispersibility of a conductive material dispersion by a simple method without adding a separate solvent.

One problem to be solved by the present invention is to provide a method for evaluating the dispersibility of a conductive material dispersion liquid in a simple manner without adding a separate solvent.

According to an embodiment of the present invention, the method comprising the steps of applying a conductive material dispersion on a glass substrate by a spin coating method; and drying the applied conductive material dispersion, wherein the conductive material dispersion is provided with a dispersibility evaluation method of the conductive material dispersion including carbon nanotubes, a dispersant, and a dispersion medium.

Since the dispersibility evaluation method of the conductive material dispersion according to an embodiment of the present invention can evaluate the dispersibility without adding a separate solvent, the evaluation process can be simplified. In addition, since dispersibility can be qualitatively evaluated with the naked eye from applying a conductive material dispersion to a glass substrate by spin coating, it is possible to first determine dispersibility before quantitative evaluation.

1 is a photograph of a conductive material dispersion applied on a glass substrate by a spin coating method.

Hereinafter, the present invention will be described in more detail to help the understanding of the present invention.

The terms or words used in the present specification and claims should not be construed as being limited to their ordinary or dictionary meanings, and the inventor may properly define the concept of the term in order to best describe his invention. It should be interpreted as meaning and concept consistent with the technical idea of the present invention based on the principle that there is.

The terminology used herein is used to describe exemplary embodiments only, and is not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly dictates otherwise.

In the present specification, terms such as "comprise", "comprising" or "have" are intended to designate the presence of an embodied feature, number, step, element, or a combination thereof, but one or more other features or It should be understood that it does not preclude the possibility of the presence or addition of numbers, steps, elements, or combinations thereof.

A method for evaluating dispersibility of a conductive material dispersion according to an embodiment of the present invention includes: applying a conductive material dispersion on a glass substrate by a spin coating method; and drying the applied conductive material dispersion, wherein the conductive material dispersion may include carbon nanotubes, a dispersant, and a dispersion medium.

The carbon nanotube corresponds to a conductive material.

The carbon nanotube may have a diameter of 9 nm to 12 nm. The carbon nanotubes may have a length of 10 μm to 70 μm. The BET specific surface area of the carbon nanotubes may be 180 m ² /g to 260 m ² /g. However, the dispersibility evaluation method of the present invention is not applied only when the above ranges are satisfied.

The dispersant is at least one selected from the group consisting of hydrogenated nitrile butadiene rubber (H-NBR), polyvinylpyrrolidone (PVP), and polyvinyl butyral (PVB). may include. Specifically, the dispersant may be a hydrogenated nitrile butadiene-based rubber.

The dispersion medium is N-methyl-2-pyrrolidone (N-Methyl-2-Pyrrolidone, NMP), N,N-dimethylacetamide (N,N-Dimethyl Acetamide, DMAc), N-dimethylformamide (N- Dimethyl Formamide, DMF), and may be at least one selected from the group consisting of an alcohol dispersion medium, specifically N-methyl-2-pyrrolidone.

The viscosity of the conductive material dispersion may be 7,000 cps to 25,000 cps at 20° C. to 35° C., specifically 12,000 cps to 18,000 cps, and more specifically 14,000 cps to 16,000 cps. When the above range is satisfied, spin coating for checking dispersibility may be smoothly performed.

The conductive material dispersion may be finally formed by adding carbon nanotubes and a dispersing agent to the dispersion medium, performing pre-mixing, and performing a milling process to further increase dispersibility.

The step of applying the conductive material dispersion on the glass substrate by a spin coating method includes rotating the glass substrate and spraying the conductive material dispersion on the center of the glass substrate to apply the conductive material dispersion on the glass substrate can do.

In the drying of the applied conductive material dispersion, the drying may include heat-treating the glass substrate to which the conductive material dispersion is applied. The heat treatment may be performed at a temperature of 110° C. to 150° C., specifically, at a temperature of 125° C. to 135° C. When the above range is satisfied, the solvent in the dispersion can be effectively removed, and an appropriate coating for confirming dispersibility can be made. The heat treatment may be performed for 0.5 hour to 1 hour.

After the drying of the applied conductive material dispersion, the dispersibility of the conductive material dispersion may be first qualitatively determined. When the carbon nanotubes are not evenly dispersed in the conductive material dispersion, a large number of particles formed by agglomeration of the carbon nanotubes exist, and the carbon nanotube size distribution is wide. When the dispersion of a conductive material with low dispersibility is applied on the glass substrate by spin coating, the dispersion of the conductive material is thinly applied by centrifugal force, and at the same time, large-sized particles formed by agglomeration of carbon nanotubes are freed by centrifugal force. It leaves a linear trace as it is pushed outward from the center of the substrate. On the other hand, when the conductive material is evenly dispersed on the conductive material dispersion liquid, aggregation between the carbon nanotubes is minimized, and thus the linear trace can be minimized even during spin coating. That is, dispersibility can be qualitatively evaluated according to the presence and amount of the trace.

The dispersibility evaluation method of the conductive material dispersion may further include measuring an optical density of the applied conductive material dispersion.

The optical density may be measured using a film densitometer (X-rite densitometer). Specifically, after irradiating visible light to the conductive material dispersion applied on the glass substrate through the film densitometer, the optical density is measured through the degree of light transmission. At this time, after measuring the optical density of several portions of the applied and dried conductive material dispersion, the average value is calculated by averaging the measured optical densities. When the conductive material, for example, carbon nanotubes, is not smoothly dispersed in the conductive material dispersion, the glass substrate may be partially exposed to the outside between the applied and dried conductive material dispersion, or a part of the conductive material dispersion is compared to other parts. It may be applied to a relatively thin thickness and in a dried state. Therefore, when the conductive material is not smoothly dispersed, the light incident on the conductive material dispersion liquid through the film densitometer can easily be transmitted, so that the measured and calculated optical density is low. On the other hand, when the conductive material is smoothly dispersed, the applied and dried conductive material dispersion may cover the glass substrate with a relatively constant thickness. Accordingly, in this case, since light incident on the conductive material dispersion is relatively difficult to be transmitted through, the measured and calculated optical density is high. Therefore, dispersibility can be evaluated through the relative comparison of the derived optical densities. Alternatively, in a situation in which the optical density of the conductive material dispersion in which dispersion is smoothly made is known, dispersibility may be evaluated by comparing the optical density with the optical density of the conductive material dispersion prepared using the optical density as a reference value. For example, if the reference value optical density is 3, it is possible to determine that the prepared conductive material dispersion has high dispersibility with respect to an optical density of 3 or more.

Experimental example

(1) Preparation of sample (conductive material dispersion)

1) Preliminary sample preparation

70 g of carbon nanotubes having an average diameter of 12 nm and 14 g of H-NBR were added to 3,416 g of NMP to form a mixture. Thereafter, 3,500 g of the mixture was pre-stirred for 4 hours through a preliminary stirrer (BTM-50, Inoue Co., Ltd.) to prepare a preliminary sample.

2) Preparation of Sample 1

Sample 1 was prepared by milling 1,000 g of the preliminary sample through a bead mill (machine name, company name) for 0.5 hour.

3) Preparation of Sample 2

Sample 2 was prepared by milling 1,000 g of the preliminary sample through a bead mill (machine name, company name) for 1 hour.

(2) Qualitative evaluation of spin coating progress, drying and dispersibility

In a state in which the glass substrate was rotated at a speed of 2,500 rpm, 2 g of the samples 1 and 2 were dropped to the center of the glass substrate for 1 minute, respectively. Thereafter, the sample-coated glass substrate was dried in an oven at 130° C. for 1 hour. The results are shown in FIG. 1 .

(3) Optical density evaluation

The optical density was evaluated for Samples 1 and 2 applied on each glass substrate. Specifically, each of the coated samples 1 and 2 is cut to 2 mm × 2 mm to make a specimen, and the specimen is divided into 9 divisions having a certain area (3 × 3), and a film densitometer (331C, 331C, X-rite) to measure optical density (OD, Optical Density). Then, the average of the nine measured values was calculated to derive the final optical density.

(4) resistance evaluation

The resistance was evaluated by a 4-point probe method for Samples 1 and 2 applied on each glass substrate. The results are shown in Table 1. When the carbon nanotubes are evenly dispersed in the conductive material dispersion, aggregation between the carbon nanotubes is minimized, and the carbon nanotubes are cut to a relatively uniform size so that the contact point between the carbon nanotubes increases, so that the conductivity is reduced. This will increase the resistance.

Optical density (D) Resistance (Ω/sq) sample 1 1.99 50 sample 2 3.04 100

Considering the manufacturing process of Sample 1 and Sample 2, Sample 2, which has a relatively long milling time, has greater dispersibility. In order to confirm this, as a result of deriving the optical density by the above method, the optical density of Sample 2 was high, which seems to be because the coated and dried conductive material dispersion liquid relatively uniformly covers the glass substrate. Furthermore, in the case of sample 2 with high dispersibility, since the carbon nanotubes were cut to a uniform size, it was confirmed that the resistance was measured to be high due to an increase in contact points between the carbon nanotubes.

Therefore, it can be seen that the dispersibility of the conductive material dispersion can be confirmed through the above method.

Claims (7)

applying a conductive material dispersion on a glass substrate by a spin coating method; and
Drying the applied conductive material dispersion,
The conductive material dispersion is a dispersibility evaluation method of a conductive material dispersion including carbon nanotubes, a dispersing agent, and a dispersion medium.
The method according to claim 1,
In the step of drying the applied conductive material dispersion,
The drying is a method for evaluating dispersibility of a conductive material dispersion comprising heat-treating the glass substrate coated with the conductive material dispersion at a temperature of 110° C. to 150° C. for 0.5 hour to 1 hour.
The method according to claim 1,
Dispersibility evaluation method of the conductive material dispersion further comprising the step of measuring the optical density of the applied conductive material dispersion.
4. The method according to claim 3,
The optical density is a dispersibility evaluation method of a conductive material dispersion liquid measured using a film densitometer.
The method according to claim 1,
The dispersing agent is a method for evaluating dispersibility of a conductive material dispersion comprising at least one selected from the group consisting of hydrogenated nitrile butadiene rubber, polyvinylpyrrolidone, and polyvinylbutyral.
The method according to claim 1,
The dispersion medium is at least one selected from the group consisting of N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N-dimethylformamide, and alcohol dispersion medium. Dispersibility evaluation method of a conductive material dispersion.
The method according to claim 1,
The viscosity of the conductive material dispersion is 7,000 cps to 25,000 cps at 20° C. to 35° C. Dispersibility evaluation method of the conductive material dispersion.

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