JP2010251442A - Method of manufacturing electrode for electric storage device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing an electrode for an electric storage device, the electrode being superior in shape preserving property of the electrode and facilitating collection/reuse of carbon nanotubes. <P>SOLUTION: The method of manufacturing the electrode for the electric storage device using carbon nanotubes includes a process of preparing single-layer carbon nanotubes of ≥10,000 in aspect ratio; a dispersion process (S10) of preparing a dispersion solution by dispersing the carbon nanotubes in solvent; a pressure-reduction filtering process (S20) of forming a carbon nanotube deposit body on a filter by filtering the dispersion solution through the filter under reduced pressure; and a heat treatment process (S40) of heat-treating the formed carbon nanotube deposit body. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for manufacturing an electrode for an electricity storage device. Specifically, the present invention relates to a method for manufacturing an electrode for an electricity storage device using carbon nanotubes as an electrode material.

  An electric double layer capacitor has little deterioration over time in characteristics and does not use an environmentally hazardous substance such as heavy metal as a material. Therefore, in recent years, an electric double layer capacitor has been gaining importance as a power storage device capable of obtaining clean energy.

  One typical configuration of the electric double layer capacitor includes a power generation element having a separator sandwiched between a pair of polarizable electrodes. In general, activated carbon having a large surface area is used as a material for the polarizable electrode constituting the power generation element. However, when activated carbon is used as an electrode material, the activated carbon has a low electrical conductivity, so that there is a problem that only the activated carbon increases the internal resistance of the polarizable electrode and cannot extract a large current. In order to eliminate such an increase in internal resistance of the polarizable electrode, it has been proposed to mix carbon nanotubes having higher electrical conductivity than activated carbon into the polarizable electrode. For example, in Patent Document 1, a porous conductive film is used as a current collector, and this is used as a filter. A porous conductive film is obtained by performing vacuum suction filtration of a polarizable electrode solution in which carbon nanotubes and activated carbon are uniformly dispersed. An electric double layer capacitor in which a substance containing carbon nanotubes is laminated on a film is disclosed. With this technique, a composite film of a current collector and a polarizable electrode can be produced by a simple method.

JP 2006-032371 A

  However, when an electric double layer capacitor is constructed using the polarizable electrode described in Patent Document 1, if the polarizable electrode is impregnated with an electrolyte having high dispersibility such as an ionic liquid, the current is collected on the current collector by suction filtration. In some cases, the carbon nanotubes laminated on the electrode were dispersed again by the electrolytic solution, and an appropriate shape as an electrode could not be maintained. Further, in the technique of Patent Document 1, an expensive porous conductive film is essential as a current collector, so that the component cost is increased and a composite film of the current collector and a polarizable electrode is formed. There has been a problem that it is difficult to recover and reuse carbon nanotubes from waste electrodes.

  The present invention has been made in view of such points, and its main object is to provide a method for producing an electrode for an electricity storage device that is excellent in electrode shape retention and that allows easy collection and reuse of carbon nanotubes. That is.

  The method provided by the present invention is a method for producing an electrode for an electricity storage device using carbon nanotubes (hereinafter also referred to as a carbon nanotube electrode). This method comprises a step of preparing single-walled carbon nanotubes having an aspect ratio of 10,000 or more, a dispersion step of preparing a dispersion solution by dispersing the carbon nanotubes in a solvent (typically in water), and the dispersion solution. It includes a vacuum filtration step of forming a carbon nanotube deposit on the filter by filtration under reduced pressure through a filter, and a heat treatment step of heat-treating the formed carbon nanotube deposit.

  According to the production method of the present invention, since single-walled carbon nanotubes having a large aspect ratio are intertwined closely in the process of vacuum filtration, strong mutual coupling occurs between the carbon nanotubes. The carbon nanotubes can be bonded to each other by this strong mutual bond. The carbon nanotube electrode thus obtained is firmly held in the electrode shape by the dense entangled structure between the carbon nanotubes. Therefore, even if immersed in the electrolytic solution, the shape of the carbon nanotube electrode does not collapse, and application to an electrolytic solution having high dispersibility, such as an ionic liquid, can be expected. Moreover, according to the production method of the present invention, carbon nanotubes can be bonded together without using a binder (typically a resin component). Therefore, it becomes easy to recover the carbon nanotubes from the waste electrode, and the carbon nanotubes can be regenerated (reused).

  Here, as said single-walled carbon nanotube, it is good to use a thing with an aspect ratio (length / diameter ratio) of 10,000 or more. When carbon nanotubes having a small aspect ratio are used, the entangled carbon nanotubes are easily unraveled, and the electrode shape may not be appropriately maintained. Therefore, in the present invention, single-walled carbon nanotubes having an aspect ratio of 10,000 or more (more preferably 12000 or more) are used.

  In a preferred embodiment of the method disclosed herein, in the heat treatment step, the carbon nanotube deposit is heated at a temperature not lower than 400 ° C. By heating at a high temperature of 400 ° C. or higher, amorphous carbon contained as an impurity (typically, it can be mixed when synthesizing a single-walled carbon nanotube as a raw material) can be burned off. There can be obtained a good quality electrode with less.

  In a preferred embodiment of the method disclosed herein, before the filtration step, the dispersion solution is allowed to stand to aggregate at least a part of the carbon nanotubes dispersed in the solution. When the dispersion solution is allowed to stand, the aggregation of the single-walled carbon nanotubes starts, and the single-walled carbon nanotubes gather due to mutual intermolecular forces to form a carbon nanotube aggregate. At this time, the single-walled carbon nanotubes are not aggregated into one as a whole, but are aggregated into a plurality of carbon nanotube aggregates (for example, 3 to 5 carbon nanotube aggregates). When filtration under reduced pressure is performed in this state, a plurality of aggregates of carbon nanotubes enter each other's gaps and become entangled with each other, so that a more aggregated and densified carbon nanotube electrode can be formed. Since the carbon nanotube electrode obtained in this way increases the entanglement between the carbon nanotubes, the shape retention of the electrode can be further improved.

  In a preferred embodiment of the method disclosed herein, the obtained carbon nanotube deposit is subjected to acid cleaning (for example, hydrochloric acid treatment) after the filtration step. By including the acid washing step, it is possible to remove residual catalyst such as iron (typically, it can be mixed when synthesizing single-walled carbon nanotubes as a raw material) contained as impurities, resulting in less impurities. A high quality electrode can be obtained.

  Moreover, according to this invention, an electrical double layer capacitor provided with the electrode for electrical storage devices manufactured by one of the methods disclosed here is provided. Such an electric double layer capacitor is constructed using a carbon nanotube electrode with excellent shape retaining property that can retain its shape properly even when immersed in a highly dispersible electrolyte solution, so it has better performance (E.g., long life and little deterioration over time in characteristics). Such an electric double layer capacitor is suitable as an electric double layer capacitor mounted on a vehicle such as an automobile. Therefore, according to the present invention, there is provided a vehicle including the electric double layer capacitor disclosed herein.

The figure which shows the manufacturing flow of the carbon nanotube which concerns on one Embodiment of this invention. The figure which shows typically the suction filtration apparatus which concerns on one Embodiment of this invention. 1 is a cross-sectional view schematically showing an electric double layer capacitor according to an embodiment of the present invention. The observation figure by a scanning electron microscope (SEM). The observation figure by a transmission electron microscope (TEM).

  Embodiments according to the present invention will be described below with reference to the drawings. In the following drawings, members / parts having the same action are described with the same reference numerals. Note that the dimensional relationship (length, width, thickness, etc.) in each drawing does not reflect the actual dimensional relationship. In addition, matters other than the matters specifically mentioned in the present specification and necessary for the implementation of the present invention (for example, the configuration and manufacturing method of the current collector, the configuration and manufacturing method of the separator and the electrolyte, the electric double layer capacitor, etc. The general technology related to the construction of the electric double layer capacitor can be understood as a design matter of a person skilled in the art based on the prior art in the field.

  Although not intended to be particularly limited, the following mainly describes a method for manufacturing an electrode for an electricity storage device using carbon nanotubes according to the present embodiment, taking a polarizable electrode mounted on an electric double layer capacitor as an example. To do.

  A carbon nanotube has a shape in which a single layer of graphene (graphene) with a continuous carbon 6-membered ring is wound in a cylindrical shape, and a single-walled carbon nanotube consisting of only one layer of graphene and two or more layers of graphene are concentric. There are multi-walled carbon nanotubes that overlap. The carbon nanotube used in the present embodiment is a single-walled carbon nanotube. Since single-walled carbon nanotubes are more likely to aggregate due to intermolecular forces than multi-walled carbon nanotubes, an electrode suitable for the purpose of the present invention can be produced.

  Here, the size of the single-walled carbon nanotube is not particularly limited, but the diameter (wire diameter) of the carbon nanotube may be, for example, in the range of 0.5 nm to 5 nm, and more preferably in the range of 1 nm to 3 nm. Further, the length of the carbon nanotube may be, for example, more than 5 μm, and more preferably more than 10 μm.

  In the present embodiment, single-walled carbon nanotubes having an aspect ratio (diameter / length ratio) of 10,000 or more (more preferably 12000 or more) are used. If the aspect ratio of the single-walled carbon nanotubes is too small, the entangled single-walled carbon nanotubes are likely to be unraveled, and thus the electrode shape may not be appropriately maintained. Accordingly, the aspect ratio of the single-walled carbon nanotube is desirably 10,000 or more, preferably 12,000 or more.

  The method for synthesizing single-walled carbon nanotubes is not particularly limited, and conventionally known methods such as an arc discharge method, a laser evaporation method, a vapor phase growth method, and a CVD method can be appropriately employed. In particular, when the arc discharge method is used, single-walled carbon nanotubes with few defects and good quality can be obtained. In the present embodiment, single-walled carbon nanotubes (diameter 1 nm to 3 nm, aspect ratio of about 12000) synthesized by an arc discharge method are prepared (synthesized, purchased, etc.).

  Next, an electrode manufacturing method using carbon nanotubes will be described with reference to FIG. As shown in the process flow of FIG. 1, this manufacturing method includes a dispersion step (S10), a vacuum filtration step (S20), an acid cleaning step (S30), and a heat treatment step (S40). Hereinafter, each process will be described.

  First, in the dispersion step of S10, carbon nanotubes are dispersed in a solvent (typically in water) to prepare a dispersion solution. As the solvent for dispersing the carbon nanotubes, water or a mixed solvent mainly containing water is preferably used. As a solvent component other than water constituting such a mixed solvent, one or more organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used. A particularly preferred example is an aqueous solvent substantially consisting of water.

  The method for dispersing the carbon nanotubes in the solvent is not particularly limited, and examples thereof include a method of stirring the dispersion solution at high speed with a mixer or the like. Single-walled carbon nanotubes have high agglomeration properties and usually exist as aggregates. By adopting the above method, aggregates can be appropriately loosened and dispersed. Note that the dispersion processing method is not limited to the high-speed stirring described above, and may be physical dispersion processing using ultrasonic waves or the like, for example. Moreover, you may add surfactant etc. which improve the hydrophilic property (in this example, hydrophilicity) of a carbon nanotube as needed. Thereby, the said dispersion | distribution process can be performed in a short time.

  Once the dispersion solution is prepared in this way, a vacuum filtration step (S20) is then performed. In the vacuum filtration step, the carbon nanotube dispersion is formed on the filter by filtering the dispersion solution of the carbon nanotube through a filter under reduced pressure. The vacuum filtration of the dispersion solution may be performed using, for example, a suction filtration device 50 as shown in FIG. In the suction filtration device 50 shown in FIG. 2, the carbon nanotube dispersion solution 10 is supplied to the surface of the filter (for example, filter paper) 20, and the suction bottle 30 is negatively pressured to remove the solvent (for example, water) from the back surface of the filter 20. By discharging, a carbon nanotube deposit 40 is formed on the filter 20. The suction force of the suction filtration may be performed, for example, by appropriately adjusting the negative pressure in the suction bottle 30.

  The carbon nanotubes deposited on the filter 20 are entangled with each other by the action of the suction force of the suction filtration. At this time, in the present embodiment, since single-walled carbon nanotubes that are likely to aggregate due to intermolecular force are used, single-walled carbon nanotubes are bundles in which a plurality of single-walled carbon nanotubes are gathered due to mutual intermolecular forces ( A bundle). In the present embodiment, since single-walled carbon nanotubes having an aspect ratio of 10,000 or more are used, the single-walled carbon nanotube bundles are thick and long fibers. Such a thick and long fibrous single-walled carbon nanotube bundle is closely intertwined in the suction filtration process, whereby strong mutual coupling occurs between the carbon nanotubes. The carbon nanotubes are bonded to each other by this strong mutual bond, and the carbon nanotube deposit 40 is held in a mat shape.

  After the carbon nanotube deposit 40 is formed in this way, the formed carbon nanotube deposit 40 is then removed from the filter 20, and the carbon nanotube deposit 40 is dried using an appropriate drying means. By this drying, the solvent is volatilized from the carbon nanotube deposit 40. Although the drying temperature varies depending on the type of solvent used, for example, when water is used as the solvent, it is preferable to heat at about 200 ° C.

  After the carbon nanotube deposit 40 is dried, an acid cleaning step (S30) is then performed. In the acid cleaning step, the carbon nanotube deposit is acid cleaned. By including an acid cleaning step, it is possible to remove residual catalyst (typically mixed when synthesizing raw single-walled carbon nanotubes) contained as impurities, and to obtain a high-quality electrode with few impurities. be able to. What is necessary is just to select the acid used for an acid washing suitably according to the kind of residual catalyst to remove. For example, when an iron component is included as a residual catalyst, it can be treated with hydrochloric acid.

  After the carbon nanotube deposit 40 is acid cleaned, a heat treatment step (S40) is performed next. In the heat treatment step, the carbon nanotube deposit is heat treated. In the heat treatment step, the carbon nanotube deposit is preferably heated at a temperature not lower than 400 ° C. By heating at a high temperature of 400 ° C. or higher, amorphous carbon contained as an impurity (typically, it can be mixed when synthesizing a single-walled carbon nanotube as a raw material) can be burned off. It is possible to obtain a carbon nanotube electrode with low quality and good quality. Further, in the heat treatment step, a compression (press) treatment may be performed in addition to the heat treatment. By compressing the carbon nanotube deposit in the thickness direction, the electrode can be formed to a desired thickness, and the entanglement between the carbon nanotubes can be further increased, and as a result, shape retention can be improved.

  Thus, the carbon nanotube electrode according to the present embodiment can be obtained through each process of the dispersion step (S10), the vacuum filtration step (S20), the acid washing step (S30), and the heat treatment step (S40). .

  According to the manufacturing method of the present embodiment, single-walled carbon nanotubes having a long fiber length and a large aspect ratio are intertwined in the process of vacuum filtration, so that strong mutual coupling occurs between the carbon nanotubes. The carbon nanotubes can be bonded to each other by this strong mutual bond. The carbon nanotube electrode thus obtained is firmly held in the electrode shape by the action of both the intermolecular force acting between the carbon nanotubes and the mechanical force due to the entanglement between the carbon nanotubes. For this reason, the shape of the carbon nanotube electrode does not collapse even when immersed in the electrolytic solution, and application to an electrolytic solution having high dispersibility, such as an ionic liquid, can be expected. Moreover, according to the manufacturing method of this embodiment, carbon nanotubes can be adhere | attached, without using a binder (typically resin component). Therefore, it becomes easy to collect the carbon nanotubes from the waste electrode, and the carbon nanotubes can be preferably regenerated (reused). Since carbon nanotubes are expensive materials, they are also highly technical in this respect.

  Further, according to the manufacturing method of the present embodiment, after the vacuum filtration step (S20), the formed carbon nanotube deposit is subjected to the acid cleaning (S30) and the heat treatment (S40). Impurities such as amorphous carbon and residual catalyst (for example, iron) contained in the body can be appropriately removed. Thereby, a high quality carbon nanotube electrode with few impurities can be manufactured.

  In addition, you may perform the process which aggregates at least one part of the carbon nanotube currently disperse | distributed in a dispersion solution before a vacuum filtration process (S20). In that case, after the carbon nanotubes are dispersed in a solvent in the dispersion step (S10) to prepare a dispersion solution, stirring of the dispersion solution may be stopped and left as it is. When the dispersion solution is allowed to stand for several minutes, the aggregation of the single-walled carbon nanotubes starts, and the single-walled carbon nanotubes gather due to mutual intermolecular forces to form a carbon nanotube aggregate. At this time, the single-walled carbon nanotubes are not aggregated into one as a whole, but are aggregated into a plurality of carbon nanotube aggregates (for example, 3 to 5 carbon nanotube aggregates). When filtration under reduced pressure is performed in this state, a plurality of aggregates of carbon nanotubes enter and intertwine with each other, and can be formed into a more aggregated and densified mass (mat shape). Since the carbon nanotube electrode obtained in this way increases the bonding (entanglement) between the carbon nanotubes, the shape retention of the electrode can be further improved.

  As described above, the carbon nanotube electrode obtained by the manufacturing method of the present embodiment is excellent in electrode shape retention as described above, and therefore can be preferably used as a constituent electrode of various types of power storage devices. For example, it can be preferably used as a component (typically a polarizable electrode) of an electric double layer capacitor including a carbon nanotube electrode manufactured by applying the present invention.

  FIG. 3 shows a structure of the electric double layer capacitor 100 including the carbon nanotube electrode 110 manufactured by using the method of the present embodiment. In the electric double layer capacitor 100 shown in FIG. 3, a power generating element is constructed by sandwiching a separator 130 between a pair of carbon nanotube electrodes 110a and 110b. This power generation element is fixed to the outside of the Teflon (registered trademark) case 140 provided with current collectors 120a and 120b for taking out current to the outside (wiring connected to the outside) by the elastic force of the spring mechanisms 150 and 160, The electrolytic double layer capacitor 100 according to the present embodiment is constructed by injecting an electrolyte solution (not shown) into the case 140 and impregnating the power generation element, and then sealing the case with a resin gasket 170. Is done.

  The separator 130 sandwiched between the carbon nanotube electrodes 110a and 110b is not particularly limited as long as it is the same as that used in a typical electric double layer capacitor. For example, a porous separator made of synthetic resin (for example, made of polyolefin such as polyethylene) or paper is preferably used. For the current collectors 120a and 120b, a metal suitable for the current collector of the electric double layer capacitor, such as aluminum or copper, is preferably used. The electrolyte solution is not particularly limited as long as it is a conventionally known one, but an electrolyte solution with high dispersibility such as an ionic liquid (for example, imidazolium series) is particularly preferably used. Since the carbon nanotube electrodes 110a and 110b of this embodiment are excellent in shape retention, the electrode shape does not collapse even when such an ionic liquid having high dispersibility is used.

  The electric double layer capacitor constructed in this way is constructed using carbon nanotube electrodes with excellent shape retentivity that can hold the shape appropriately even when immersed in a highly dispersible electrolyte as described above. Therefore, it may show better performance (for example, longer life and less deterioration of characteristics over time).

  In order to confirm that a carbon nanotube electrode excellent in shape retention can be obtained by using the production method of the present invention, the following experiment was conducted as an example.

  First, single-walled carbon nanotubes (diameter 1 nm to 3 nm, aspect ratio 12000) were synthesized by an arc discharge method, and the obtained carbon nanotubes were dispersed in an appropriate amount of water to prepare a dispersion solution. Next, the dispersion solution is allowed to stand for several minutes to start agglomeration of the carbon nanotubes dispersed in the solution, and is then agglomerated on the filter paper by performing vacuum suction filtration using a general suction filtration device. A carbon nanotube deposit with deposits was obtained. Next, the carbon nanotube deposit was heated to 200 ° C. on a hot plate to volatilize water, and then washed with hydrochloric acid to remove the iron component of the residual catalyst in the carbon nanotube deposit. Thereafter, the carbon nanotube deposit was fired in the atmosphere at 400 ° C. for several hours to remove amorphous carbon (impurities) in the carbon nanotube deposit, thereby producing a carbon nanotube electrode. The firing process was performed while press-molding to a desired thickness using a metal plate. The carbon nanotube electrode thus obtained was used as an example.

  The carbon nanotube electrode of the example produced as described above was observed with an electron microscope (SEM and TEM). The results are shown in FIG. 4 (SEM) and FIG. 5 (TEM), respectively. As shown in FIG. 4, the obtained carbon nanotube electrode has a mat shape in which long-fiber single-walled carbon nanotubes having a large aspect ratio are intertwined closely. Further, as shown in FIG. 5, the single-walled carbon nanotube constituting the carbon nanotube electrode has a bundle structure in which a plurality of single-walled carbon nanotubes are gathered by mutual intermolecular forces. Ions can be stored in the gap (mesospace).

  In Comparative Examples 1 and 2, carbon nanotube electrodes were prepared using single-walled carbon nanotubes having different aspect ratios. Specifically, the carbon nanotube electrode was produced by reducing the aspect ratio to 4000 and 8000 in the order of Comparative Examples 1 and 2. It was produced under the same conditions as in the examples except that the aspect ratio of the carbon nanotube used was changed.

  The carbon nanotube electrodes of Examples and Comparative Examples 1 and 2 prepared as described above were impregnated with imidazolium-based ionic liquid (EMIBF4), and the electrode states after being left for 1 hour were observed.

  As a result, in the electrodes using the carbon nanotubes having the aspect ratios of 4000 and 8000 (Comparative Examples 1 and 2), the carbon nanotubes are dispersed in the ionic liquid when left standing after being impregnated with the ionic liquid, and the electrode shapes are separated. It was. On the other hand, the electrode using the carbon nanotubes having an aspect ratio of 12000 (Example) did not collapse even when left standing after impregnation with the ionic liquid, and was able to maintain the electrode shape appropriately. From this, it was confirmed that by using single-walled carbon nanotubes having an aspect ratio of 12000 or more, it is possible to produce a carbon nanotube electrode excellent in electrode shape retention.

  In addition, since the electric double layer capacitor using the carbon nanotube electrode obtained by the method of the present invention has excellent characteristics as described above, it is suitable as an electric double layer capacitor mounted on a vehicle such as an automobile. . Therefore, according to the present invention, there is provided a vehicle including the electric double layer capacitor disclosed herein.

  As mentioned above, although this invention was demonstrated by suitable embodiment, such description is not a limitation matter and of course various modifications are possible.

  For example, the carbon nanotube electrode obtained by the method of the present invention is not limited to the electric double layer capacitor described above, and can be used as a constituent electrode of various power storage devices. For example, you may use as a constituent electrode of the lithium secondary battery (typically lithium ion battery) which uses a carbon nanotube electrode as at least one of a positive electrode and a negative electrode.

DESCRIPTION OF SYMBOLS 10 Dispersion solution 20 Filter 30 Suction bottle 40 Carbon nanotube deposit body 50 Suction filtration apparatus 100 Electric double layer capacitor 110a, 110b Carbon nanotube electrode 120a, 120b Current collector 130 Separator 140 Teflon (registered trademark) case 150, 160 Spring mechanism 170 gasket

Claims (6)

A method for producing an electrode for an electricity storage device using carbon nanotubes,
Preparing a single-walled carbon nanotube having an aspect ratio of 10,000 or more;
A dispersion step of dispersing the carbon nanotubes in a solvent to prepare a dispersion solution;
A vacuum filtration step of forming a carbon nanotube deposit on the filter by filtering the dispersion solution through a filter under reduced pressure;
And a heat treatment step of heat-treating the carbon nanotube deposit formed as described above.   The manufacturing method according to claim 1, wherein, in the heat treatment step, the carbon nanotube deposit is heated at a temperature not lower than 400 ° C.   The production method according to claim 1 or 2, wherein, before the vacuum filtration step, the dispersion solution is allowed to stand to aggregate at least a part of the carbon nanotubes dispersed in the solution.   The manufacturing method according to any one of claims 1 to 3, further comprising a step of acid cleaning the carbon nanotube deposit after the vacuum filtration step.   An electric double layer capacitor provided with the electrode for electrical storage devices obtained by the method as described in any one of Claim 1 to 4. A vehicle comprising the electric double layer capacitor according to claim 5.