JP2017103322A - Electrode sheet and electric double layer capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode sheet capable of effectively increasing electrostatic capacitance per unit volume of a capacitor.SOLUTION: The electrode sheet for use in a capacitor, includes a carbon material having a graphene layered structure and having an aspect ratio of 10 or more. When observing a cross section along a thickness direction of an electrode sheet, the plurality of carbon materials is laminated.SELECTED DRAWING: None

Description

  The present invention relates to an electrode sheet used for a capacitor and an electric double layer capacitor using the electrode sheet.

  Conventionally, carbon materials such as graphite, activated carbon, carbon nanofibers, and carbon nanotubes have been widely used as capacitor electrode materials from the environmental aspect.

For example, Patent Document 1 below discloses activated carbon for electrodes of electric double layer capacitors. In Patent Document 1, the activated carbon has uniform communication macropores, the center of the pore size distribution is in the range of 1.5 to 25 μm, the specific surface area is in the range of 1500 to 2300 m ² / g, and the micropores The volume is in the range of 0.4 to 1.0 ml / g, and the average micropore width is in the range of 0.7 to 1.2 nm.

International Publication No. 2013/140937

  In Patent Document 1, the capacitance of the capacitor is increased by using the activated carbon as described above for the electrode of the capacitor. However, when the activated carbon of Patent Document 1 is used as an electrode of a capacitor, the capacitance per unit volume of the capacitor cannot be sufficiently increased.

  An object of the present invention is to provide an electrode sheet and an electric double layer capacitor using the electrode sheet, which can effectively increase the capacitance per unit volume of the capacitor.

  An electrode sheet according to the present invention is an electrode sheet used for a capacitor, includes a carbon material having a graphene laminated structure and an aspect ratio of 10 or more, and observes a cross section along the thickness direction of the electrode sheet. Sometimes, a plurality of the carbon materials are laminated.

  On the specific situation with the electrode sheet which concerns on this invention, when the cross section of the said electrode sheet is observed, the average aspect-ratio of the carbon material contained in the said electrode sheet is 10 or more.

  In another specific aspect of the electrode sheet according to the present invention, the number of the carbon materials stacked per 1 μm in thickness of the electrode sheet is 10 or more.

  In another specific aspect of the electrode sheet according to the present invention, the carbon material is graphite or exfoliated graphite.

  In still another specific aspect of the electrode sheet according to the present invention, the carbon material is partially exfoliated exfoliated graphite having a structure in which graphite is partially exfoliated.

In still another specific aspect of the electrode sheet according to the present invention, a binder resin is further included.
Preferably, the binder resin is styrene butadiene rubber (SBR), polybutyral, polytetrafluoroethylene, or a fluorine-based polymer. Preferably, the fluorine-based polymer is polyvinylidene fluoride.

  In still another specific aspect of the electrode sheet according to the present invention, 0.3 to 40 parts by weight of the binder resin is included with respect to 100 parts by weight of the carbon material.

  The electric double layer capacitor according to the present invention includes an electrode sheet configured according to the present invention.

  The electrode sheet according to the present invention includes a carbon material having an aspect ratio of 10 or more, and a plurality of the carbon materials are stacked when a cross section along the thickness direction of the electrode sheet is observed. The capacitance per volume can be effectively increased.

It is a figure which shows the scanning electron microscope (SEM) photograph of 5000 times magnification in the cross section along the thickness direction of the electrode sheet obtained in Example 1. FIG. It is a figure which shows the scanning electron microscope (SEM) photograph of 5000 times magnification in the cross section along the thickness direction of the electrode sheet obtained by the comparative example 1. FIG.

[Electrode sheet]
The electrode sheet of the present invention is an electrode sheet used for a capacitor. The electrode sheet includes a carbon material having a graphene laminated structure and an aspect ratio of 10 or more. Moreover, when observing the cross section along the thickness direction of the said electrode sheet, multiple carbon materials are laminated | stacked.

  Since the electrode sheet of the present invention includes a carbon material having an aspect ratio of 10 or more, the orientation of the carbon material is excellent in the electrode sheet. Further, when a cross section along the thickness direction is observed, a plurality of carbon materials are stacked, so that the capacitance per unit volume of the capacitor can be effectively increased.

  In the present specification, the aspect ratio means the ratio of the maximum dimension in the direction of the laminated surface of the carbon material to the thickness of the carbon material.

  Moreover, in this invention, when the cross section along the thickness direction of an electrode sheet is observed, it is preferable that the average aspect-ratio of the carbon material contained in the electrode sheet is 10 or more. By making the average aspect ratio of the carbon material more than the above lower limit, the orientation of the carbon material can be further enhanced in the electrode sheet, and the capacitance per unit volume of the capacitor can be further effectively increased. Can do.

  From the viewpoint of further increasing the capacitance per unit volume of the capacitor, the average aspect ratio of the carbon material contained in the electrode sheet is preferably 10 or more, and more preferably 30 or more. Moreover, from the viewpoint of obtaining the effect of orientation more reliably, the upper limit of the average aspect ratio of the carbon material contained in the electrode sheet is about 300.

  The average aspect ratio of the carbon material contained in the electrode sheet can be obtained as follows.

  First, the electrode sheet is cut in a direction along the thickness direction. Next, the carbon material enters a plurality of observation screens, and the obtained cross section is photographed with a scanning electron microscope (SEM) at a magnification of 5000 times. In the SEM image of the cross section taken in this way, an average value of the ratio of the length to the thickness of the carbon material existing in the field of view of the image is obtained. Thereby, the average aspect ratio of the carbon material can be obtained.

  Moreover, in the electrode sheet of this invention, it is preferable that the lamination | stacking number of the carbon material per 1 micrometer thickness of an electrode sheet is 5 layers or more. When the number of carbon material layers is equal to or greater than the above lower limit, the capacitance per unit volume of the capacitor can be further increased.

  From the viewpoint of further increasing the capacitance per unit volume of the capacitor, the number of carbon materials stacked per 1 μm thickness of the electrode sheet is more preferably 10 layers or more, and further preferably 20 layers or more. From the viewpoint of the thickness of the graphene single layer, the upper limit of the number of carbon materials stacked per 1 μm thickness of the electrode sheet is about 3000 layers.

  The number of carbon materials stacked per 1 μm thickness of the electrode sheet can be determined as follows.

  First, the electrode sheet is cut in a direction along the thickness direction. Next, the obtained cross section is photographed with a scanning electron microscope (SEM) at a magnification of 5000 times so that a plurality of carbon materials enter the observation screen. In the SEM image of the cross section photographed in this way, the number of carbon materials stacked per 1 μm thickness can be obtained.

  Hereinafter, the detail and manufacturing method of the electrode sheet of this invention are demonstrated.

(Carbon material)
The electrode sheet of the present invention includes a carbon material having a graphene laminated structure.

  Although it does not specifically limit as a carbon material which has a graphene laminated structure, Preferably, it is graphite or exfoliated graphite. More preferably, it is partially exfoliated graphite having a structure in which graphite is partially exfoliated.

  Graphite is a laminate of a plurality of graphene sheets. The number of graphite graphene sheets stacked is about 100,000 to 1,000,000. As graphite, natural graphite, artificial graphite, expanded graphite and the like can be used. Expanded graphite has a larger interlayer between graphene layers than normal graphite. Therefore, it is preferable to use expanded graphite as the graphite. Further, when expanded graphite is used, graphite can be more easily peeled off, so that exfoliated graphite described later can be obtained more easily.

  Exfoliated graphite is obtained by exfoliating the original graphite, and refers to a graphene sheet laminate that is thinner than the original graphite. The number of graphene sheets laminated in exfoliated graphite should be less than the original graphite.

  In the exfoliated graphite, the number of graphene sheets laminated is preferably 5 layers or more, more preferably 10 layers or more, preferably 1000 layers or less, and more preferably 500 layers or less.

  Partially exfoliated graphite includes graphite or primary exfoliated graphite and a resin, and a composition in which the resin is fixed to the graphite or primary exfoliated graphite by grafting or adsorption is prepared and included in the composition. The resin is thermally decomposed. Partially exfoliated graphite has a structure in which graphite is partially exfoliated. Such partially exfoliated graphite can be produced, for example, by the same method as the method for producing exfoliated graphite / resin composite material described in International Publication No. 2014/034156.

  However, in the present invention, the resin contained in the composition may be completely removed during the thermal decomposition, or may be thermally decomposed while a part of the resin remains. Therefore, in the partially exfoliated graphite, the resin may be completely removed or a part of the resin may remain. Moreover, in this invention, after obtaining partially exfoliated exfoliated graphite in which the resin remains by the above method, the resin may be removed by heating in another step.

  Such a resin is not particularly limited, but is preferably a polymer of a radical polymerizable monomer. The resin may be a copolymer of a plurality of types of radical polymerizable monomers or a homopolymer of one type of radical polymerizable monomer.

  Examples of the resin used include polypropylene glycol, polyglycidyl methacrylate, polyvinyl acetate, polybutyral, polyacrylic acid, and polystyrene. Preferably, polypropylene glycol and polyvinyl acetate are used. When polypropylene glycol or polyvinyl acetate is used, the specific surface area of the partially exfoliated exfoliated graphite can be further increased.

  The amount of the resin remaining in the partially exfoliated graphite is preferably 5 parts by weight to 450 parts by weight, more preferably 15 parts by weight to 350 parts, with respect to 100 parts by weight of the partially exfoliated graphite. Part, more preferably 25 parts by weight to 300 parts by weight. By setting the amount of the residual resin within the above range, the specific surface area of the partially exfoliated exfoliated graphite can be further increased.

  In the partially exfoliated graphite, the graphene layer in the graphite or primary exfoliated graphite is expanded by thermal decomposition of the resin, whereby the graphite is partially exfoliated. In partially exfoliated graphite, the graphite is partially exfoliated from the edge to some extent inside.

  The partially exfoliated exfoliated graphite has many portions where graphite is exfoliated. The part where the graphite is exfoliated means a part of the graphite or primary exfoliated graphite where a part of the graphene laminate or graphene is partially peeled off.

  Further, the partially exfoliated exfoliated graphite has a structure in which graphene is laminated in the central portion, like the original graphite or primary exfoliated graphite. However, even in the central portion, there may be a portion where the graphene layer is expanded from the original graphite or primary exfoliated graphite due to the thermal decomposition of the resin.

  In the partially exfoliated exfoliated graphite, the abundance ratio between the edge part where the graphite is partially exfoliated and the unexfoliated central part is preferably 1: 4 to 1:60. In this case, the edge portion may be a left-right indefinite shape.

  In the edge portion, the number of graphene layers in the portion where the graphite is partially exfoliated and thinned is small. The number of graphene layers in the portion where the graphite is partially exfoliated is preferably 300 layers or less, more preferably 100 layers or less, and even more preferably 50 layers or less. When the number of laminated graphenes in the exfoliated portion is not more than the above upper limit, the compatibility with the binder resin described later can be further enhanced.

  In the partially exfoliated exfoliated graphite, the interlayer distance between the graphene layers is increased, and the number of graphene layers in the exfoliated portion of the edge portion is small, so that the specific surface area is large.

  Further, when using partially exfoliated exfoliated graphite, since the edge portion is open, different partially exfoliated exfoliated graphite can be stacked in the electrode sheet, and the number of laminated layers per 1 μm thickness can be further increased. Can be increased. Accordingly, when partially exfoliated exfoliated graphite is used, the orientation in the surface direction or thickness direction of the electrode sheet can be further increased, and the capacity per unit volume of the capacitor can be further increased.

  Furthermore, by removing the resin remaining in the partially exfoliated graphite, the conductivity can be further improved, and the output characteristics of the capacitor can be further improved. At this time, a removal method by heating, chemical treatment, or the like is possible, and a partial structure can be modified.

  In the present invention, the carbon material having such a graphene laminated structure has an aspect ratio of 10 or more. The aspect ratio of the carbon material refers to the ratio of the maximum dimension in the stacking surface direction of the carbon material to the thickness of the carbon material. If the aspect ratio of the carbon material is too low, the orientation of the carbon material may not be sufficiently increased in the electrode sheet. On the other hand, if the aspect ratio of the carbon material is too high, the effect may be saturated and a further reinforcing effect may not be expected. Therefore, the aspect ratio of the carbon material is preferably 50 or more, and preferably 200 or less.

(Binder resin)
The electrode sheet according to the present invention may further contain a binder resin.

  As the binder resin, polybutyral, polytetrafluoroethylene, styrene butadiene rubber, polyimide resin, fluorine-based polymer such as polyvinylidene fluoride (PVDF), water-soluble carboxymethyl cellulose, or the like can be used. Preferably, polytetrafluoroethylene can be used. When polytetrafluoroethylene is used, dispersibility and heat resistance can be further improved.

  About the compounding ratio of binder resin, it is preferable to set it as the range of 0.3-40 weight part with respect to 100 weight part of carbon materials which have a graphene laminated structure, and it is more preferable to set it as the range of 0.3-15 weight part. preferable. By setting the blending ratio of the binder resin within the above range, the capacitance of the capacitor can be further increased.

(Production method)
The electrode sheet which concerns on this invention can be manufactured by shape | molding the composition containing binder resin and a solvent as needed to the carbon material which has the said graphene laminated structure.

  The shaping of the composition can be performed, for example, by forming into a sheet with a rolling roller and then drying. It can also be carried out by applying a coating liquid comprising the carbon material, binder resin and solvent onto a current collector and then drying it. As the solvent, ethanol, N-methylpyrrolidone (NMP), water, or the like can be used.

  Moreover, you may obtain the electrode sheet of this invention by pressing with a roll press machine after shaping | molding of the said composition. In that case, the orientation of the carbon material can be further increased, and the capacitance per unit volume of the capacitor can be further increased.

  In addition, as said press temperature, it is preferable that it is 10 to 100 degreeC. Moreover, a preferable clearance width is 20 μm to 250 μm, and a preferable press time is 5 seconds to 60 seconds.

[Electric double layer capacitor]
The electric double layer capacitor of the present invention includes an electrode sheet configured according to the present invention. Therefore, the electric double layer capacitor of the present invention has an increased capacitance per unit volume. The capacitor electrode material of the present invention can be shaped into the capacitor electrode and used for an electric double layer capacitor.

  As an electrolytic solution of the electric double layer capacitor, an aqueous system or a non-aqueous system (organic system) may be used.

  Examples of the aqueous electrolyte include an electrolyte using water as a solvent and sulfuric acid or potassium hydroxide as an electrolyte.

On the other hand, as the nonaqueous electrolytic solution, for example, an electrolytic solution using the following solvent or electrolyte can be used. Specifically, examples of the solvent include propylene carbonate (PC), acetonitrile (AN), ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). As electrolytes, lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), tetraethylammonium tetrafluoroborate (TEABF ₄ ), or triethylmethylammonium tetrafluoroborate (TEMAF) ₄ ).

  Next, the present invention will be clarified by giving specific examples and comparative examples of the present invention. In addition, this invention is not limited to a following example.

Example 1
Preparation of partially exfoliated exfoliated graphite;
20 g of expanded graphite (trade name “PF powder 8”, manufactured by Toyo Tanso Co., Ltd., BET specific surface area = 22 m ² / g) and azodicarbonamide (ADCA, manufactured by Eiwa Chemical Co., Ltd., product name “AC” as a thermally decomposable foaming agent 40 g of # R-K3 ”, thermal decomposition temperature 210 ° C., 400 g of polypropylene glycol (manufactured by Sanyo Kasei Co., Ltd., product number“ Sanix GP-3000 ”, number average molecular weight = 3000), and 400 g of tetrahydrafuran as a solvent. The raw material composition was prepared by mixing. The raw material composition was irradiated with ultrasonic waves for 5 hours at 100 W and an oscillation frequency of 28 kHz using an ultrasonic treatment apparatus (manufactured by Honda Electronics Co., Ltd.). Polypropylene glycol (PPG) was adsorbed on expanded graphite by ultrasonic treatment. In this way, a composition in which polypropylene glycol was adsorbed on expanded graphite was prepared.

  After the ultrasonic irradiation, the composition was molded by a solution casting method, maintained at a drying temperature of 80 ° C. for 2 hours, and then maintained at a temperature of 110 ° C. for 1 hour. Thereafter, the temperature was maintained at 150 ° C. for 1 hour, and further maintained at 230 ° C. for 2 hours. Thereby, the ADCA was thermally decomposed and foamed in the composition.

  Next, the heating process which maintains at the temperature of 430 degreeC for 1.5 hours was implemented. Thereby, the polypropylene glycol was thermally decomposed to obtain partially exfoliated exfoliated graphite (EEXG). The aspect ratio of partially exfoliated exfoliated graphite, which is a carbon material, was 50.

Production of electrode sheets;
The partially exfoliated exfoliated graphite obtained as described above is dispersed in N-methylpyrrolidone, and 1 part by weight of PVDF as a binder resin is added to 9 parts by weight of the partially exfoliated exfoliated graphite and mixed. It was. The obtained coating liquid was applied onto an aluminum foil and dried. Next, an electrode sheet was obtained by pressing for 30 seconds with a roll press machine (model number “BLUD90A” manufactured by Toho Co., Ltd.) under the conditions of a temperature of 25 ° C. and a clearance width of 80 μm.

Measurement of average aspect ratio;
First, a part of the obtained electrode sheet was cut in a direction along the thickness direction with a cross section polisher (manufactured by JEOL Ltd., model number “IB-19500CP”). Next, a plurality of partially exfoliated graphite, that is, a plurality of carbon materials, entered the observation screen, and the obtained cross section was scanned with an electron microscope (SEM, manufactured by Hitachi High-Technologies Corporation, model number “S-3400”. ) At a magnification of 5000 times. In the SEM image of the cross section taken as described above, the average value of the ratio of the length to the thickness of the carbon material existing in the field of view of the image was determined, and the average aspect ratio of the carbon material was 50.

  FIG. 1 is a scanning electron micrograph of a magnification of 5000 times in a cross section along the thickness direction of the electrode sheet obtained in Example 1. FIG. 1 shows that in the electrode sheet obtained in Example 1, the carbon material is oriented in the direction along the surface direction of the electrode sheet. Further, FIG. 1 shows that in Example 1, a plurality of carbon materials are laminated.

  In Example 1, the number of carbon materials stacked per 1 μm thickness was determined from a scanning electron micrograph at a magnification of 5000 times in the cross section of the electrode sheet, and was 10 layers.

(Comparative Example 1)
Activated carbon (manufactured by Wako Pure Chemical Industries, Ltd.) was dispersed in N-methylpyrrolidone, and 1 part by weight of PVDF as a binder resin was added to 9 parts by weight of activated carbon and mixed. The obtained coating liquid was applied onto an aluminum foil and dried. Next, an electrode sheet was obtained by pressing with a roll press machine under conditions of a temperature of 25 ° C. and a clearance width of 80 μm. In addition, when the average aspect-ratio of the activated carbon which is a carbon material was measured by the method similar to Example 1, the average aspect-ratio of the carbon material of the comparative example 1 was 3.

  FIG. 2 is a scanning electron micrograph of a magnification of 5000 times in a cross section along the thickness direction of the electrode sheet obtained in Comparative Example 1. FIG. 2 shows that the activated carbon is not oriented in the direction along the surface direction of the electrode sheet in the electrode sheet obtained in Comparative Example 1.

(Comparative Example 2)
By changing the carbon material to a different activated carbon (manufactured by Osaka Gas Chemical Company), an electrode sheet was obtained in the same manner as in Comparative Example 1 except that the average aspect ratio of the carbon material was set as shown in Table 1 below. It was.

(Capacitor evaluation using 1M TEABF4 propylene carbonate electrolyte)
The electrode sheet obtained in Example 1 and Comparative Examples 1 and 2 is punched out with φ1 mm, and the separator is sandwiched by using it as a working electrode and a symmetric electrode, and impregnated with 1M TEABF4 propylene carbonate electrolytic solution to produce an electric double layer capacitor. did.

  The capacitance of the electric double layer capacitor is calculated from the following formula based on the measurement result of the repeated charge / discharge characteristics between 0V and 2V (calculation range: 1.6-0.8V, current value 10 mA / g or current value 40 mA / g). It calculated using.

Capacitance Fm per weight;
Fm = I / (ΔV / Δt) / 2m
m: Weight per electrode (g)
Capacitance Fv per volume;
Fv = I / (ΔV / Δt) / (S × h)
S: electrode area (cm ² ), h: electrode thickness (cm)

  The capacitance was determined according to the following criteria.

[Capacitance criteria]
○: 0.7 ≦ Fv / Fm
×: 0.5 ≦ Fv / Fm <0.7
XX: 0 <Fv / Fm <0.5

  The results are shown in Table 1 below.

Claims (10)

An electrode sheet used for a capacitor,
A carbon material having a graphene laminated structure and an aspect ratio of 10 or more,
An electrode sheet in which a plurality of the carbon materials are laminated when a cross section along the thickness direction of the electrode sheet is observed.   The electrode sheet according to claim 1, wherein an average aspect ratio of a carbon material contained in the electrode sheet is 10 or more when a cross section of the electrode sheet is observed.   The electrode sheet according to claim 1 or 2, wherein the number of the carbon materials laminated per 1 µm in thickness of the electrode sheet is 10 or more.   The electrode sheet according to claim 1, wherein the carbon material is graphite or exfoliated graphite.   The electrode sheet according to any one of claims 1 to 4, wherein the carbon material is partially exfoliated exfoliated graphite having a structure in which graphite is partially exfoliated.   The electrode sheet according to claim 1, further comprising a binder resin.   The electrode sheet according to claim 6, wherein the binder resin is styrene-butadiene rubber (SBR), polybutyral, polytetrafluoroethylene, or a fluorine-based polymer.   The electrode sheet according to claim 7, wherein the fluoropolymer is polyvinylidene fluoride.   The electrode sheet according to any one of claims 6 to 8, comprising 0.3 to 40 parts by weight of the binder resin with respect to 100 parts by weight of the carbon material.   An electric double layer capacitor comprising the electrode sheet according to any one of claims 1 to 9.