JP2013225601A - Electrolyte and electric double-layer capacitor using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolyte whose reliability is enhanced by suppressing a pH change and creeping-up even when charging/discharging is performed in severe conditions such as high temperature, and to provide an electric double-layer capacitor using the same.SOLUTION: The electrolyte comprises an organic solvent, a solute contained in the organic solvent, and a compound represented by the following general formula. With this configuration, the compound preferentially acts on hydroxide ions generated from moisture in the electrolyte, thereby allowing a pH change in the electrolyte to be suppressed.

Description

  The present invention relates to an electrolytic solution used for various electronic devices and in-vehicle power storage devices, and an electric double layer capacitor using the electrolytic solution.

  FIG. 3 is a schematic view showing a current collector and a rubber seal used in a conventional electric double layer capacitor.

  Conventionally, an electric double layer capacitor has a positive electrode and a negative electrode in which an electrode layer containing activated carbon is formed on a metal foil, facing each other with a separator interposed therebetween, and the capacitor element is formed together with an electrolyte. It is accommodated in a metal case, and a rubber sealing body seals the opening of the metal case. And in order to pull out an electrode from a capacitor element, connecting members, such as a lead wire, are joined to each of a positive electrode and a negative electrode, and it is the structure pulled through the said rubber sealing body.

  A quaternary ammonium salt or a quaternary phosphonium salt is used as a solute in the electrolytic solution used for the electric double layer capacitor.

  However, these solutes have the property of being chemically active. Therefore, when electric double layer capacitors using these solutes are repeatedly charged and discharged under severe conditions such as high temperatures, the pH of the electrolyte in the vicinity of the negative electrode is biased to alkali, and the electrolyte is scuffing the surface of the negative electrode. There is a possibility that it will rise and leak outside from the rubber seal. Moreover, the lead wire which pulls out an electrode from a rubber sealing body or a negative electrode may be corroded by contacting with the electrolyte which goes up.

  In order to enhance the resistance to corrosion due to this leakage, in the electric double layer capacitor using lead wires as a method for drawing out the positive electrode and the negative electrode, the surfaces of the current collector 101 and the tab terminal 102 constituting the negative electrode have oxidation properties. A rubber sealing body 103 having a through hole 103a fitted into the tab terminal 102 and formed by vulcanization of butyl rubber with resin or peroxide is used to form a chemical film using a chemical agent. A configuration in which the hardness of the body 103 is in the range of 60 to 90 (according to a Wallace hardness meter) has been proposed.

  Thereby, even if a quaternary ammonium salt or a quaternary phosphonium salt is used as a solute of the electrolytic solution, the electrolytic solution can be prevented from leaking from the rubber sealing body 103 to the outside.

  As prior art document information relating to the invention of this application, for example, Patent Document 1 is known.

JP-A-7-122467

  Thus, when a quaternary ammonium salt or a quaternary phosphonium salt is used as the solute in the electrolytic solution of the electric double layer capacitor, measures such as pH fluctuations and liquid leakage in the electrolytic solution are taken into account for the case and the sealing rubber. It was necessary to take from the structural side. However, as an electric double layer capacitor, in order to overcome the above-mentioned problems and further improve reliability, an electrolytic solution in which fluctuations in pH and scooping that cause the liquid leakage are suppressed is required. ing.

  Therefore, in the present invention, there is provided an electrolytic solution and an electric double layer capacitor using the same, in which the fluctuation of pH and the rising of the electrolytic solution are suppressed even when charging / discharging under severe conditions such as high temperature is performed. The purpose is to provide.

  In order to solve the above-described problems, the electrolytic solution of the present invention and the electric double layer capacitor using the electrolytic solution include a quaternary phosphonium salt, and the cation of the quaternary phosphonium salt is It is characterized by being represented by 1).

(Where, P is represents phosphorus, R ^{1 to 3} represents a substituent group of the organic than 1 carbon atoms, these R ^{1 to 3} are different the same or at least one of its composition, R ^alpha is a number 1 or more carbon atoms There, an electron withdrawing group to the carbon atoms that constitute R ^alpha, at least one of the substituents is a hydrogen atom bonded to the nearest carbon atom to the phosphorus atom of the cation of the R ^{1 to 3} and R ^alpha doing.)

  With the above configuration, the electrolytic solution and the electric double layer capacitor in the present invention can suppress fluctuations in pH in the electrolytic solution. This is because the cation of the electrolytic solution of the present invention reacts with hydroxide ions that cause the pH fluctuation and is consumed in order to suppress the pH fluctuation. This is because, by providing an electron-withdrawing group on the cation during the reaction, it becomes easy to extract a hydrogen atom from the cation. The extracted hydrogen atoms neutralize hydroxide ions in the electrolytic solution, and the pH variation of the electrolytic solution is suppressed.

  By this effect, creeping up as an electric double layer capacitor is suppressed, and liquid leakage is suppressed.

The partial notch perspective view showing the electric double layer capacitor in the example of the present invention Schematic diagram of H-type cell used for performance evaluation test in the present invention Schematic showing a negative electrode with a tab terminal and a rubber seal used in a conventional electric double layer capacitor

  In the following, embodiments of the present invention and inventions according to all claims will be described with reference to the drawings.

  FIG. 1 is a cutaway perspective view of a capacitor according to an embodiment of the present invention.

  In FIG. 1, a capacitor element 1 is opposed to a positive electrode 2 and a negative electrode 3 each having a polarizable electrode layer 2b and 3b for adsorbing and desorbing ions on a surface of a current collector 2a and 3a made of metal. The separator 4 is wound or laminated with the separator 4 interposed between the positive electrode 2 and the negative electrode 3, and lead wires 5 a and 5 b are connected to the surfaces of the positive electrode 2 and the negative electrode 3 as lead members, respectively. In this state, the capacitor element 1 and an electrolytic solution (not shown) are accommodated in a bottomed outer case 6 that is an outer body, and lead wires 5a and 5b are exposed at the open ends of the outer case 6. Thus, the sealing member 7 is sealed.

  For the positive electrode 2 and the negative electrode 3, for example, a high-purity aluminum foil having a thickness of about 15 μm (containing 99% or more Al) is used as the current collectors 2a and 3a, and this aluminum foil is subjected to electrolytic etching in a chlorine-based etching solution. To roughen the surface.

  Then, the polarizable electrode layers 2b and 3b are respectively formed on the front and back surfaces of the roughened current collectors 2a and 3a. Examples of the material constituting the polarizable electrode layer 2b include activated carbon, a binder and a conductive additive.

  The activated carbon is, for example, a phenol resin activated carbon having an average particle diameter of 5 μm, the binder is, for example, an aqueous solution of carboxymethyl cellulose (CMC), and the conductive assistant is, for example, acetylene black, mixed in a weight ratio of 10: 2: 1. Use things. This mixture is kneaded with a kneader to adjust to a predetermined viscosity.

  This paste is applied to the front and back surfaces of the current collectors 2a and 3a, and dried in an air atmosphere at 100 ° C. to form a polarizable electrode layer 2b having a thickness of 40 μm. Thereafter, the collectors 2a and 3a provided with the polarizable electrode layers 2b and 3b are slit so as to have a predetermined width.

  Further, the polarizable electrode layers 2b and 3b formed on the front and back surfaces of the current collectors 2a and 3a are partially removed, and lead wires 5a and 5b are needle-clamped to the unformed portions of the polarizable electrode layers 2b and 3b. Connect with the method.

The positive electrode 2 and the negative electrode 3 are made to face each other, and a separator 4 is interposed between the positive electrode 2 and the negative electrode 3 and wound to complete the capacitor element 1. For the separator 4, for example, cellulosic paper or PTFE having a thickness of about 35 μm and a density of 0.45 g / cm ³ is used.

  As the electrolytic solution, one containing the quaternary phosphonium salt of the present invention having a cation represented by the above (Chemical Formula 1) is used.

As the solute anion, those containing a fluorine atom are preferable for increasing the withstand voltage, and BF ₄ ⁻ or PF ₆ ⁻ is particularly preferable. As the solvent, for example, γ-butyrolactone, which is an organic solvent, is used and mixed so that the concentration of the solute is 0.5 to 2.0 mol / l.

  For example, a metal such as aluminum, copper, or nickel is used for the outer case 6 from the viewpoint of heat dissipation.

  The sealing member 7 is not particularly limited as long as it is a rubber material having elasticity, for example, butyl rubber. In a state where the lead wires 5a and 5b protruding from the capacitor element 1 are passed through the through holes 7a provided in the sealing member 7, the sealing member 7 is disposed in the opening of the outer case 6 having a bottomed cylindrical shape, The sealing member 7 is crimped and gripped by drawing from the outer peripheral surface of the opening of the outer case 6 where the sealing member 7 is located toward the inside of the outer case 6 and by curling the opening end of the outer case 6. And fix. Thereby, sealing of the opening part of the exterior case 6 is completed.

  Thus, the electric double layer capacitor of the present invention is completed.

  As described above, the electrolytic solution in the present invention and the electric double layer capacitor using the electrolytic solution include a quaternary phosphonium salt having a cation represented by the above (Chemical Formula 1) in the electrolytic solution.

This is because the cation according to the present invention reacts with a hydroxide ion (OH ⁻ ) in the electrolyte solution that causes the fluctuation of pH, and from this reaction, the phosphorus represented by the following (Chemical Formula 2) is obtained. This is because phosphorus ylides having bonds of atoms and carbon atoms can be easily generated. By this reaction, when an electric double layer capacitor repeats charging / discharging, it suppresses that hydroxide ion increases in electrolyte solution, and suppresses that pH in electrolyte solution fluctuates to the alkali side.

(C represents carbon, P represents phosphorus, and Ra to ^e represent organic or inorganic substituents.)
In order for the present invention to exert its effect, it is necessary to react with hydroxide ions using the cation to produce the phosphorus ylide.

To generate this reaction, the cation of the present embodiment, at least one of the substituents of the substituents R ^{1 to 3} and R ^alpha existing around the phosphorus atom, the carbon atom directly bonded to the phosphorus atom Hydrogen atoms are provided and need to react with hydroxide ions in the electrolyte. That is, in order to facilitate the reaction between the hydrogen atom and the hydroxide ion, it is necessary to reduce the electron density of the hydrogen atom to create a state in which the hydrogen atom can be easily extracted in the cation.

As a method for reducing the electron density, in this embodiment, the number of carbon atoms represented by R ^α in the above (Chemical Formula 1) is 1 or more, and at least one electron withdrawing group is present on the carbon of R ^α The bonded substituent was provided on the cation.

By providing an electron withdrawing group on this R ^α , the electron density of hydrogen atoms involved in the formation of phosphorus ylide is lowered, and the hydrogen atoms are easily extracted. Thereby, it becomes possible to promote reaction with the hydroxide ion in the cation.

Note that the electron-withdrawing group, for example -F, -Cl, -Br, -OR, = O, -COR, -CO 2 R, -NO 2, -SO 2 R, -CN, -CR = CR ₂ , —C≡R, —CH _x F _y , —CH _v F _w —CH _x F _y (where —, =, ≡ represents the bonding state of atoms or molecules, and R represents an inorganic or organic substituent. And v and w are integers of 0 or more and 2 or less and satisfy v + w = 2, x and y are integers of 0 or more and 3 or less and x + y = 3 Is satisfied).

Among these electron withdrawing groups, —F, —COR, —NO ₂ , —CN and the like have high electron withdrawing properties and can promote the reaction with hydroxide ions.

Note that R ^{1 to 3} in (Chemical ^Formula 1) are organic substituents having 1 or more carbon atoms bonded to the phosphorus atom located at the center in the cation of the present invention. Further, these R ^{1 to 3} may have a structure having the electron withdrawing group. In addition, the effect of reducing the electron density may vary depending on the positional relationship between the electron-withdrawing group provided in the cation of this example and the hydrogen atom that is extracted during ylide formation. By providing, alkalization is suppressed compared with the conventional quaternary phosphonium salt without an electron withdrawing group.

  Therefore, when it is going to accelerate | stimulate iridation of the quaternary phosphonium salt of a present Example, the structure which combined the said electron withdrawing group to the carbon atom with the hydrogen atom pulled out is preferable.

  However, this phosphorus ylide is generally unstable and is difficult to produce stably. One means for solving this is to reduce the electron density of the carbon atom bonded to the phosphorus atom by the electron withdrawing group.

  This is because the electron density can be widely dispersed in the molecule by reducing the electron density of the carbon atom using the electron withdrawing group. Thereby, the decomposition of phosphorus ylide is suppressed, and the effect of stabilizing phosphorus ylide is achieved.

  By further stabilizing the phosphorus ylide in this way, the energy barrier existing in the process of reacting from the quaternary phosphonium salt to the phosphorus ylide becomes smaller, so that the reaction to the phosphorus ylide can be further promoted. Therefore, the hydroxide ion in the electrolytic solution and the quaternary phosphonium cation of this embodiment can react stably even under high temperature conditions, and the pH fluctuation can be further suppressed as a capacitor.

  Furthermore, this phosphorus ylide can react with a substance having an appropriate double bond. For example, it reacts with γ-butyrolactone having a carbonyl group, propylene carbonate, etc., and is converted into an electrically neutral substance that does not affect the electrical characteristics.

  In addition, although the production | generation method of the quaternary phosphonium salt used for the electric double layer capacitor of a present Example is not specifically limited, The following method is mentioned as an example.

  Triphenylphosphine is dissolved in an appropriate solvent such as tetrahydrofuran in an inert atmosphere and reacted with an alkyl halide to obtain an alkyltriphenylphosphine halide. Next, alkyltriphenylphosphine tetrafluoroborate can be obtained by anion exchange reaction.

The solute cation according to the present invention is, in the above (Chemical Formula 1), among the four substituents including the R ^α that surrounds the phosphorus atom, two or three substituents having 1 carbon atom. Some configurations are preferred. This is because a quaternary phosphonium salt constructed in such a relationship with the number of carbons easily forms an ylide, and by providing an electron withdrawing group in such a quaternary phosphonium salt as in this embodiment. In addition to the ease of iridation, the stabilization of ylide can be improved, and as a result, pH fluctuation can be further suppressed as an electric double layer capacitor. Therefore, in consideration of the positional relationship between the effect of the electron withdrawing group and the hydrogen atom to be extracted, the R ^α preferably has a configuration with 1 carbon atom.

(Performance evaluation test)
The contents of the characteristic evaluation test for confirming the effect of the present invention using the electrolytic solution and the electric double layer capacitor described in the above examples are shown below.

The electrolyte solution 28 of Sample 1 is the electrolyte solution of this example, and a salt composed of a cation of (Chemical Formula 3) and tetrafluoroborate (BF ₄ ⁻ ) is used as a solute as a solute of the chemical formula (1) An electrolyte solution having a concentration of 1.0 mol / l using γ-butyrolactone is used.

  Sample 2 is an electrolytic solution 28 having the same configuration as Sample 1 except that a cation represented by (Chemical Formula 4) is used as an example of the cation of this example.

  Sample 3 is an electrolytic solution 28 having the same configuration as Sample 1 except that the cation represented by (Chemical Formula 5) is used as an example of the cation of this example.

  Sample 4 is an electrolytic solution having the same structure as Sample 1 except that tetraethylphosphonium tetrafluoroborate is used as the solute as the quaternary phosphonium salt.

Note the functional group of the above (Formula 3) to (Formula 5) in C ₆ H ₅ which is provided as substituents of the phosphorus atoms represented respectively is a phenyl group.

  Using these samples 1 to 4, tests were performed under the following conditions.

  FIG. 2 is a schematic diagram of an H-type cell used for measuring the pH variation of the electrolyte.

  The H-type cell used in this test is composed of an H-type glass case 26 including a pair of low and low-cylindrical cell portions 26a and a cylindrical relay portion 26b connecting the two cell portions 26a. And the positive electrode 22 and the negative electrode 23 made of Pt foil respectively accommodated in the cell part 26a, and the glass separator 24 arranged in the relay part 26b to isolate the positive electrode 22 and the negative electrode 23 from each other. Thus, the positive electrode 22 and the negative electrode 23 are electrically connected to the power source 29.

In this test, a current density of 1 mA / cm ² was continuously applied for 60 minutes to Samples 1 to 3 using this H-type cell in a dry room. Then, the pH of the negative electrode 23 in the electrolytic solution 28 was measured from the application start time to the 60 minute point, and the negative electrode pH at the stage where 20 minutes and 60 minutes had elapsed was compared in each sample.

  The results are shown as (Table 1) below.

  From (Table 1), the fluctuation | variation of pH is suppressed compared with the sample 4 which is the comparative example of the samples 1-3 using the cation of this invention.

  Next, a life test was conducted under the conditions of 60 ° C., 2.5 V, and 250 hours using the electric double layer capacitors using Samples 1 to 3, and the capacity of each electric double layer capacitor after 250 hours. Retention rates were compared. The results are shown as (Table 2) below.

  From Table 2, it can be seen that the electric double layer capacitors of Samples 1 to 3 using the electrolytic solution of this example have suppressed capacity deterioration compared to the electric double layer capacitor of Sample 4.

  In addition, the electrolyte solution in this invention is not limited to the structure of the said Example, If an electric double layer is formed in the positive electrode 2 and the negative electrode 3 in electrolyte solution, it will not be limited. Therefore, the quaternary phosphonium salt having the cations of (Chemical Formula 1) and (Chemical Formula 2) in the above examples is used as an additive for the purpose of suppressing alkalization, and the solute that is adsorbed and desorbed on the activated carbon surfaces of the positive electrode 2 and the negative electrode 3 separately. You may comprise the electrolyte solution containing.

  In that case, the solute is preferably an onium salt, in which a quaternary ammonium salt is preferred, in which at least one of the four substituents surrounding the nitrogen atom, such as ethyltrimethylammonium, is substituted. A quaternary ammonium salt having a different structure from other substituents is preferred.

  This is because the use of the quaternary ammonium salt as described above is superior to other quaternary ammonium salts in that it suppresses the electrolyte from creeping up in the electric double layer capacitor. .

The organic solvent used in the electrolytic solution is not limited to the above-mentioned γ-butyrolactone, and other lactones may be γ-caprolactone, γ-valerolactone, etc. Other than the lactones, propylene carbonate, ethylene carbonate, etc. Similar effects can be obtained with carbonates and sulfolanes. The anion is not limited to the above BF ₄ ⁻ , and PF ₆ ⁻ , ClO ₄ ⁻ , trifluoromethyl sulfonate, bis (trifluoromethylsulfonyl) imide, tris (trifluoromethylsulfonyl) methide and the like may be used. Good.

  Similarly, the activated carbon used for the positive electrode and the negative electrode has also been described using a phenol resin-based activated carbon in this example, but is not limited thereto, and is not limited to this, coconut shell, wood powder, paper, petroleum coke, petroleum pitch, The material is not particularly limited as long as it is a porous material that adsorbs and desorbs ions on the surface thereof, such as a carbon material made from a raw material.

  Furthermore, the amount of water contained in the electrolytic solution is preferably small so that unstable phosphorus ylide is difficult to decompose. The amount of water is at least 1 wt% or less, preferably 500 ppm or less.

  As described above, the electrolytic solution of the present invention and the electric double layer capacitor using the electrolytic solution may occur while repeating charge / discharge by forming phosphorus ylide by acting on hydroxide ions as a compound in the electrolytic solution. Variations in pH in the electrolytic solution can be suppressed.

  In the electrolytic solution and the electric double layer capacitor in the present invention, fluctuations in pH are suppressed, and leakage of the electrolytic solution to the outside is suppressed. Therefore, reliability as a power storage device can be improved. As a result, it is expected to be used in in-vehicle and electronic devices where high reliability is required.

DESCRIPTION OF SYMBOLS 1 Capacitor element 2 Positive electrode 2a, 3a Current collector 2b, 3b Polarization electrode layer 3 Negative electrode 4 Separator 5a, 5b Lead wire 6 Exterior case 7 Sealing member 7a Through-hole 22 Positive electrode 23 Negative electrode 24 Glass separator 26 Glass case 26a Cell part 26b Relay unit 28 Electrolyte 29 Power supply

Claims (5)

An electrolytic solution containing a quaternary phosphonium salt, wherein the quaternary phosphonium salt is composed of a cation represented by (Chemical Formula 1).

(Where, P is represents a phosphorus atom, R ^{1 to 3} represents a substituent group of the organic than 1 carbon atoms, these R ^{1 to 3} are different the same or at least one of its composition, the R ^alpha 1 or more carbon atoms , and the has an electron-withdrawing group on the carbon atoms that constitute R ^alpha, at least one of the substituents among R ^{1 to 3} and R ^alpha is a hydrogen atom closest carbon atom to the phosphorus atom of the cationic Combined.) It said electron withdrawing group is, -F, -Cl, -Br, -OR , = O, -COR, -CO 2 R, -NO 2, -SO 2 R, -CN, -CR = CR 2, - C≡R, —CH _x F _y , —CH _v F _w —CH _x F _y (where —, =, ≡ represents the bonding state of atoms or molecules, R represents an inorganic or organic substituent, Except for represents an atom based on the periodic table, and v and w are integers of 0 or more and 2 or less and satisfy v + w = 2, and x and y are integers of 0 or more and 3 or less and satisfy x + y = 3. The electrolytic solution according to claim 1, wherein The electrolytic solution according to claim 1, wherein R ^α has 1 carbon atom, and a hydrogen atom is bonded to a carbon atom constituting R ^α . 2. The electrolytic solution according to claim 1, wherein R ^{1 to 3} and R ^α are substituents having 2 or less carbon atoms, and 3 or 2 of these 4 substituents have 1 carbon atom. Capacitor element in which a positive electrode capable of adsorbing / desorbing anions and a negative electrode capable of adsorbing / desorbing cations are opposed to each other with a separator interposed therebetween, an exterior body containing the capacitor element together with an electrolyte, and an opening of the exterior body is sealed A sealing member at least,
The said electrolyte solution is an electrical double layer capacitor comprised from the electrolyte solution of Claim 1.

Images