JP4957373B2 - Electric double layer capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric double layer capacitor which is superior in the charge/discharge cycle characteristics and the float test characteristics under a high voltage application condition and can operate at higher voltage as compared with conventional electric double layer capacitors. <P>SOLUTION: An electric double layer capacitor comprises: a positive electrode containing a carbon material; a negative electrode containing a carbon material and a titanium oxide; and an electrolytic solution containing an ammonium salt. The weight ratio of the titanium oxide to the carbon material contained in the negative electrode is between 2% by weight and 50% by weight. The combination of the electrode containing the titanium oxide and the electrolytic solution containing the ammonium salt uses no electrochemical reactions. Thus, the electric double layer capacitor is superior in the charge/discharge cycle characteristics and the float test characteristics under a high voltage application condition, and can operate at higher voltage as compared with conventional electric double layer capacitors. <P>COPYRIGHT: (C)2008,JPO&amp;INPIT

Description

  The present invention relates to an electric double layer capacitor.

  An electric double layer capacitor includes a positive electrode in which a polarizable electrode is in close contact with a positive current collector, a negative electrode in which a polarizable electrode is in close contact with a negative current collector, a separator, and an electrolyte. In the electric storage element, the positive electrode and the negative electrode are opposed to each other, and use the capacitance of the electric double layer formed on the surface of the polarizable electrode in the electrolytic solution. As a main component of the polarizable electrode of the electric double layer capacitor, a carbon-based material having a very large surface area per unit mass such as activated carbon is often used. The electric double layer capacitor configured as described above is characterized in that an extremely large capacitance can be obtained as compared with a capacitor such as an aluminum electrolytic capacitor. In addition, the electric double layer capacitor is charged and discharged by ions entering and exiting the pores of the activated carbon, unlike a storage element using an electrochemical reaction such as a secondary battery. Very fast charge / discharge is possible. Taking advantage of these features, electric double layer capacitors can be used as memory backup power sources for electronic devices, power storage for home appliances and copiers, power sources for start-up when automobiles are idle, power sources for hybrid vehicles, wind power generation, A wide range of uses such as peak shaving for solar power generation and power storage for leveling is considered, and it is expected to be a key device that helps to save energy and reduce carbon dioxide emissions.

  In electric double layer capacitors, improvement of stored energy, which is one of the main performances, is an important issue. Since the stored energy of the electric double layer capacitor is proportional to the square of the charging voltage, increasing the charging voltage leads to an improvement in the stored energy. In order to increase the charging voltage, it is necessary to increase the withstand voltage of the electric double layer capacitor. The withstand voltage of the electric double layer capacitor varies depending on the electrolyte used, but is about 2.3 to 2.7 V for non-aqueous systems. In order to increase the voltage, for example, a negative electrode having a polarizable electrode containing a carbon material or metal oxide capable of occluding and releasing lithium ions, a positive electrode having a polarizable electrode made of activated carbon, and an electrolyte containing lithium ions. Hybrid type electric double layer capacitors combined with an electrolytic solution have been proposed (see, for example, Patent Documents 1 to 4). In such a hybrid electric double layer capacitor, when lithium titanate is used for the polarizable electrode of the negative electrode, lithium titanate has a redox potential near 1.5 V with respect to lithium ions and occludes lithium ions. Since the volume expansion change is small with respect to the release, the cycle characteristics are improved. Also, as a method for improving the electrical resistance of the electrode, an electrode is proposed in which a trace amount of metal oxide of 1.25% or less by weight with respect to the activated carbon is added together with the binder to prevent the binder coating on the activated carbon surface. (For example, refer to Patent Document 5).

JP 2002-270175 (page 2) JP 2003-132945 A (2 pages) JP 2004-55541 A (page 3-4) Japanese Patent Laying-Open No. 2005-353651 (page 3) JP 2001-217162 A (page 2-3)

However, in an electric double layer capacitor that combines an electrolyte solution to which lithium ions are added and a negative electrode using a carbon material or metal oxide capable of occluding and releasing lithium ions, an electrochemical reaction is involved in charging and discharging, There was a problem of poor cycle characteristics and high temperature storage characteristics. For example, when lithium titanate is added to the electrode material for the negative electrode, irreversible deterioration of lithium titanate accompanying charging / discharging is avoided because the electrochemical reaction represented by the following formula (1) is used. In other words, the charge / discharge cycle stability is poor.
Li ₄ Ti ₅ O ₁₂ + XLi = Li _{4 + X} Ti ₅ O ₁₂ (1) Formula In the hybrid electric double layer capacitor in which lithium titanate is added to the negative electrode material, lithium titanate From the relationship of the oxidation-reduction potential, the operating voltage is about 2.5 V, and there is a problem that further increase in voltage cannot be expected.

  In addition, when such a hybrid type electric double layer capacitor is held at a high voltage state, decomposition products generated by the decomposition of the electrolytic solution and impurities cover the activated carbon surface, or gas generated by the decomposition is applied to the activated carbon surface. There is a problem that the electrostatic capacity of the electric double layer capacitor is lowered due to the adsorption and the charge / discharge cycle characteristics are lowered.

  Furthermore, an electrode in which a trace amount of metal oxide of 1.25% or less by weight with respect to the activated carbon is added together with a binder can be expected to increase the capacitance and reduce the electric resistance. There has been a problem that high temperature storage characteristics cannot be improved in a high voltage application state. Since the life of electric double layer capacitors is in units of several years, a float test in which the cell is stored at high temperature with voltage applied is performed as an accelerated test for life evaluation, and the life is diagnosed by accelerating deterioration. It is said that the lifetime will be halved when the temperature rises. Here, the high voltage indicates a potential of 3.0 V or higher, which is higher than 2.5 V to 2.7 V, which is a charging potential of a normal electric double layer capacitor.

  The present invention has been made to solve the above-described problems, and is excellent in charge / discharge cycle characteristics and float test characteristics in a high voltage application state, and can be increased in voltage compared to a conventional electric double layer capacitor. An object of the present invention is to provide an electric double layer capacitor.

The electric double layer capacitor according to the present invention is a titanium oxide selected from the group consisting of a positive electrode containing a carbon material, anatase type titanium dioxide, a mixture of anatase type titanium dioxide and rutile type titanium dioxide, or lithium titanate. The negative electrode formed by adding particles of the product to the carbon material and an electrolyte containing an ammonium salt, and the weight ratio of the titanium oxide to the carbon material contained in the negative electrode is 5 wt% or more and 35 wt% or less. This is the range.

In the electric double layer capacitor according to the present invention, since it is configured as described above, the electrochemical reaction is not used, so that the charge / discharge cycle characteristics and the float test characteristics in a high voltage application state are excellent, and the conventional electric capacitor is used. The voltage can be increased as compared with the double layer capacitor.

  In order to solve the problems of the conventional electric double layer capacitor, a polarizable electrode containing 2 to 50% by weight of a titanium oxide such as lithium titanate or titanium dioxide having a large specific surface area with respect to the carbon material is used as a negative electrode. When an electric double layer capacitor was prepared by adding an ammonium salt to the electrolyte and tested, it was found that it had better charge / discharge characteristics and high-temperature storage characteristics than a conventional electric double layer capacitor, and it was possible to increase the voltage. I found it. In addition, the electric double layer capacitor according to the present invention described above is a cycle even when compared with a conventional hybrid electric double layer capacitor in which a lithium salt is added to the electrolyte and lithium titanate is used as the polarizable electrode of the negative electrode. It was found that the characteristics are excellent. In the electric double layer capacitor, the titanium oxide such as lithium titanate contained in the negative electrode and the ammonium salt do not cause an electrochemical reaction and accumulate energy, so that the irreversible titanium oxide It is considered that deterioration does not occur, charge / discharge characteristics are excellent, and cycle stability is excellent. Furthermore, titanium oxides such as lithium titanate and titanium dioxide, which are added in a range of 2 to 50% by weight of the carbon material of the negative electrode, which is larger than the conventional amount, suppress the reaction between the carbon material and the electrolytic solution. It is considered that the effect was exhibited, the high temperature storage characteristics were improved, and a high voltage was made possible. In the present invention, even when the amount of titanium oxide added was 2% by weight or more based on the carbon material, the resistance of the electrode was only slightly increased.

  Hereinafter, embodiments of the electric double layer capacitor according to the present invention will be described.

Embodiment 1 FIG.
In Embodiment 1, a negative electrode polarizable electrode obtained by binding a negative electrode carbon material and a conductive additive with a binder is referred to as a negative electrode, and the positive electrode carbon material and a conductive auxiliary agent are combined. Is a positive electrode that is formed by integrating a positive polarizable electrode with a binder and a current collector. The negative electrode and the positive electrode are loaded in the exterior material, and an insulating separator is disposed between the negative electrode and the positive electrode to prevent a short circuit. The interior of the exterior material is sealed with an electrolyte solution. Leads are connected to the negative electrode and the positive electrode, respectively, and the leads are led out of the exterior material through a sealing body.

In the present embodiment, the carbon material contained in the positive electrode and the negative electrode can be activated carbon, nanogate carbon, nanostorage carbon, etc., and the specific surface area is preferably 500 to 3000 m ² / g, and the average particle diameter Is preferably 0.5 to 20 μm. The carbon material is preferably used by carbonizing and activating a carbon material raw material as described below. Examples of the carbon material raw material include phenol resin, petroleum coke, and palm. Examples of the activation method include a steam activation method and a molten alkali activation method.

  As the conductive aid, carbon black such as acetylene black, ketjen black, furnace black, carbon whisker, carbon fiber, natural graphite, artificial graphite and the like are preferable. The addition amount of the conductive assistant is preferably contained in the electrode in an amount of 1 to 20% by weight, and the average particle size is desirably 0.5 μm or less.

  As the binder, a water-based solvent, an organic solvent-based, or a powdery binder that does not use a solvent can be used. Polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), acrylic rubber, carboxymethyl cellulose (CMC) Hydroxypropyl cellulose, hydroxyethyl cellulose, polyvinyl alcohol, polyvinylidene fluoride (PVDF), polyamideimide, polyimide and the like are preferable. PVDF may be a homopolymer or a copolymer, and the cellulosic binder may be an ammonium salt or a Na salt. The binder may be used alone or in combination of several kinds. The amount of the binder is preferably 1 to 20% by weight in the polarizable electrode.

In the present embodiment, the negative electrode contains titanium oxide, and this titanium oxide is titanium such as lithium titanate (Li ₄ Ti ₅ O ₁₂ , Li ₂ TiO ₃ ), titanium dioxide (TiO ₂ ), or the like. Oxides are preferred. The specific surface area is ^{10m 2 ~400m} 2, it is preferred particle size of the primary particles is 10 to 1000 nm. This is because by using a titanium oxide having a small particle size and a large specific surface area, it can be uniformly mixed with the carbon material, and the effect of adding the titanium oxide is more exhibited. The amount of titanium oxide is preferably 2 to 50% by weight with respect to the carbon material of the polarizable electrode of the negative electrode. When titanium dioxide is used, the crystal structure of titanium dioxide is preferably an anatase type, and a mixture of anatase type and rutile type may be used.

  As a method for producing the positive electrode, for example, activated carbon powder and acetylene black as a conductive auxiliary agent are dispersed in an N-methylpyrrolidone (NMP) solution of PVDF, and this mixture is applied onto an aluminum current collector by a doctor blade method or the like. There is a method in which after drying, pressing is performed to form a positive electrode. In addition, polytetrafluoroethylene as a binder is mixed with a mixture of a carbon material and a conductive additive, kneaded and then formed into a sheet shape to form a polarizable electrode for a positive electrode, and this is used as a conductive adhesive on an aluminum current collector. There is also a method in which a positive electrode is manufactured by laminating together.

  The negative electrode can also be manufactured using a manufacturing method similar to that of the positive electrode. For example, after dispersing acetylene black as a conductive aid in an NMP solution of PVDF, an activated carbon powder and a titanium oxide powder are dispersed and mixed to prepare a slurry, and this mixture is made into an aluminum current collector by a doctor blade method or the like. There is a method in which the negative electrode is coated on the substrate, dried and then pressed to form a negative electrode. In addition, polytetrafluoroethylene as a binder is mixed with a mixture of a carbon material, titanium oxide, and a conductive additive, kneaded, and then formed into a sheet shape to form a polarizable electrode for a negative electrode. There is also a method in which a negative electrode is manufactured by bonding using a conductive adhesive.

An electrolytic solution in which a quaternary ammonium salt is dissolved in an organic solvent or an ionic liquid can be used, and a quaternary onium salt may be used in addition to the quaternary ammonium salt. Here, the quaternary ammonium salt means one containing a quaternary ammonium ion and a counter anion. Particularly preferred quaternary ammonium ions are tetraethylammonium ion, triethylmethylammonium ion, diethylmethyl-2-methoxyethylammonium ion, spirobipyrrolidinium ion and the like. As the quaternary onium salt, for example, a phosphonium salt, a sulfonium salt, a tetraethylphosphonium salt, or the like can be used. Moreover, as a counter anion, tetrafluoroborate ion (BF ₄ ⁻ ), hexafluorophosphate ion (PF ₆ ⁻ ), perchlorate ion (ClO ₄ ⁻ ), bistrifluoromethanesulfonimide ion (TFSI ⁻ ), One or more selected from bispentafluoroethanesulfonimide ions (BETI ⁻ ) and trifluoromethanesulfonic acid ions are preferred.

  Examples of the solvent of the electrolytic solution in the present embodiment include propylene carbonate (hereinafter abbreviated as PC), ethylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, sulfolane, dimethoxyethane, and the like. Or as a mixed solvent of two or more. Moreover, you may use an ionic liquid as a solvent.

  As separators, paper separators, polyolefin separators such as polyethylene and polypropylene, separators in which polyolefins are mixed with inorganic fillers such as silica, aluminum oxide or barium titanate, fluororesin separators, polyimide separators It is suitable that the thickness is 8 μm to 100 μm and the porosity is 30% to 95%.

  Next, examples for explaining the electric double layer capacitor according to the present invention in more detail will be described together with comparative examples.

[Example 1]
Planetary mixer with 100% by weight of activated carbon with an average particle size of 15 μm and specific surface area of 2000 m ² / g, 8% by weight of acetylene black as a conductive additive and 12% by weight of PVDF as a binder in NMP A well-mixed and paste-like material is applied to a 50 μm aluminum current collector with a Peker applicator, dried at 120 ° C. to evaporate the solvent, and then pressed to separate a positive electrode having a thickness of about 100 μm. A polar electrode was prepared. A current collecting tab portion was formed on the positive electrode, and the positive electrode was cut so that the area of the polarizable electrode was 30 mm square.

100% by weight of activated carbon having an average particle size of 15 μm and a specific surface area of 2000 m ² / g, 25% by weight of lithium titanate having an average particle size of 0.1 μm and a specific surface area of 10 m ² / g, and 8 acetylene blacks as a conductive assistant Add 12 wt% of PVDF as binder and 12 wt% of binder to NMP, mix well with a planetary mixer, apply the paste to a 50 μm aluminum current collector with a Peker applicator, and 120 ° C After drying and evaporating the solvent, a negative polarizable electrode having a thickness of about 110 μm was produced by pressing. A current collecting tab portion was formed on the negative electrode, and the negative electrode was cut so that the area of the polarizable electrode was 30 mm square.

  These negative electrode and positive electrode were vacuum-dried at 150 ° C. for 24 hours and then used as electrodes. The tab portions of the positive electrode current collector and the negative electrode current collector were led out of the exterior material as leads. At this time, in order to prevent the electrolyte from leaking to the outside and to prevent water and air from entering from the outside, a modified polypropylene film is previously used as a sealing body in the sealing portion of the positive electrode tab portion and the negative electrode tab portion. Heat-welded. A 35 μm paper separator was used as the separator. The positive electrode, the negative electrode, and the separator were packaged with an aluminum laminate film exterior material, and the remaining sides except for one side were heat-welded to produce a capacitor element.

Next, an electrolytic solution was injected. As the electrolytic solution, a solution in which tetraethylammonium tetrafluoroborate (TEABF ₄ ) as an electrolyte was dissolved in PC so as to have a concentration of 1 mol / l was used. About 2 ml of electrolytic solution is put through the opening of the bag-shaped outer film. Thereafter, the capacitor element filled with the electrolytic solution was put in a vacuum vessel and evacuated to about 4 × 10 ³ Pa or less, whereby the positive electrode, the negative electrode, and the separator were impregnated with the electrolytic solution.

  Next, in order to remove impurities such as moisture present in a minute amount in the electrolytic solution or in the electrode, charging was performed by applying a voltage of 3.2 V for 30 minutes between the positive electrode and the negative electrode. After completion of charging, a load was connected between the positive electrode and the negative electrode, and the battery was completely discharged until the voltage was 1.5 V or less.

Next, in order to remove the gas generated by charging / discharging and further impregnate the electrolyte into the electrode, vacuuming is again performed to about 4 × 10 ³ Pa or less, and one side of the opened exterior material is removed. An electric double layer capacitor was obtained by welding.

[Example 2]
The composition of the polarizable electrode of the negative electrode is 100% by weight of activated carbon having an average particle diameter of 15 μm and a specific surface area of 2000 m ² / g, and 50% by weight of lithium titanate having an average particle diameter of 0.8 μm and a specific surface area of 10 m ² / g. An electric double layer capacitor was produced in the same manner as in Example 1 except that 8% by weight of acetylene black as a conductive additive and 12% by weight of PVDF as a binder were prepared. It was.

[Example 3]
The composition of the polarizable electrode of the negative electrode is 100% by weight of activated carbon having an average particle diameter of 15 μm and a specific surface area of 2000 m ² / g, and 10% by weight of lithium titanate having an average particle diameter of 0.8 μm and a specific surface area of 10 m ² / g. An electric double layer capacitor was prepared in the same manner as in Example 1 except that 8% by weight of acetylene black as a conductive additive and 12% by weight of PVDF as a binder were prepared. It was.

[Example 4]
The composition of the negative electrode of the polarizable electrode, the average particle diameter of 15 [mu] m, and 100% by weight of activated carbon having a specific surface area of 2000 m ² / g, an average particle diameter of 0.02 [mu] m, titanium dioxide having a specific surface area of ^{50 m} 2 / g (anatase, rutile An electric double layer capacitor was prepared and carried out in the same manner as in Example 1 except that 50% by weight of the mixture), 8% by weight of acetylene black as the conductive auxiliary agent, and 8% by weight of PVDF as the binder. The electric double layer capacitor of Example 4 was obtained.

[Example 5]
The composition of the negative electrode of the polarizable electrode, the average particle diameter of 15 [mu] m, and 100% by weight of activated carbon having a specific surface area of 2000 m ² / g, an average particle diameter of 0.02 [mu] m, and 35% by weight of titanium dioxide having a specific surface area of ^{50 m} 2 / g, An electric double layer capacitor was produced in the same manner as in Example 4 except that 8% by weight of acetylene black as a conductive additive and 12% by weight of PVDF as a binder were produced. did.

[Example 6]
The composition of the negative electrode of the polarizable electrode, the average particle diameter of 15 [mu] m, and 100% by weight of activated carbon having a specific surface area of 2000 m ² / g, an average particle diameter of 0.02 [mu] m, and the titanium dioxide having a specific surface area of ^{50 m} 2 / g 25 wt%, An electric double layer capacitor was prepared in the same manner as in Example 4 except that 8% by weight of acetylene black as a conductive additive and 12% by weight of PVDF as a binder were prepared. did.

[Example 7]
The composition of the negative electrode of the polarizable electrode, the average particle diameter of 15 [mu] m, and 100% by weight of activated carbon having a specific surface area of 2000 m ² / g, an average particle diameter of 0.02 [mu] m, and the titanium dioxide having a specific surface area of ^{50 m} 2 / g 15 wt%, An electric double layer capacitor was produced in the same manner as in Example 4 except that 8% by weight of acetylene black as a conductive additive and 12% by weight of PVDF as a binder were prepared. did.

[Example 8]
The composition of the negative electrode of the polarizable electrode, the average particle diameter of 15 [mu] m, and 100% by weight of activated carbon having a specific surface area of 2000 m ² / g, an average particle diameter of 0.02 [mu] m, titanium dioxide having a specific surface area of ^{50 m} 2 / g and 10 wt%, An electric double layer capacitor was prepared in the same manner as in Example 4 except that 8% by weight of acetylene black as a conductive additive and 12% by weight of PVDF as a binder were produced. did.

[Example 9]
The composition of the negative electrode of the polarizable electrode, the average particle diameter of 15 [mu] m, and 100% by weight of activated carbon having a specific surface area of 2000 m ² / g, an average particle diameter of 0.02 [mu] m, and the titanium dioxide having a specific surface area of ^{50 m} 2 / g 5 wt%, An electric double layer capacitor was prepared in the same manner as in Example 4 except that 8% by weight of acetylene black as a conductive additive and 12% by weight of PVDF as a binder were produced. did.

[Example 10]
The composition of the negative electrode of the polarizable electrode, the average particle diameter of 15 [mu] m, and 100% by weight of activated carbon having a specific surface area of 2000 m ² / g, an average particle diameter of 0.02 [mu] m, and the titanium dioxide having a specific surface area of ^{50 m} 2 / g 2 wt%, An electric double layer capacitor was prepared in the same manner as in Example 4 except that 8% by weight of acetylene black as a conductive additive and 12% by weight of PVDF as a binder were produced. did.

[Example 11]
The composition of the positive electrode and the negative electrode of the polarizable electrodes, an average particle diameter of 15 [mu] m, and 100% by weight of activated carbon having a specific surface area of 2000 m ² / g, an average particle diameter of 0.02 [mu] m, titanium dioxide having a specific surface area of ^{50 m} 2 / g 25 wt% An electric double layer capacitor was prepared in the same manner as in Example 4 except that 8% by weight of acetylene black as a conductive additive and 8% by weight of PVDF as a binder were prepared. A capacitor was used.

[Example 12]
The composition of the polarizable electrode of the negative electrode is 100% by weight of activated carbon having an average particle diameter of 15 μm and a specific surface area of 2000 m ² / g, and 25% by weight of lithium titanate having an average particle diameter of 1 μm and a specific surface area of 2 m ² / g. An electric double layer capacitor was produced in the same manner as in Example 1 except that 8% by weight of acetylene black as an auxiliary agent and 12% by weight of PVDF as a binder were prepared. .

[Comparative Example 1]
The negative polarizable electrode is made of the same polarizable electrode as the positive electrode, that is, a material not containing lithium titanate, and an electric double layer capacitor is manufactured in the same manner as in Example 1 except that the electric double layer capacitor of Comparative Example 1 is used. A multilayer capacitor was obtained.

[Comparative Example 2]
The composition of the polarizable electrode of the negative electrode is 100% by weight of lithium titanate having an average particle size of 0.8 μm and a specific surface area of 10 m ² / g, 8% by weight of acetylene black as a conductive additive, and PVDF as a binder. 12% by weight. An electric double layer capacitor was prepared in the same manner as in Example 1 except that a solution of LiBF ₄ as an electrolyte dissolved in PC was used as the electrolytic solution, and a concentration of 1 mol / l was used. It was.

[Comparative Example 3]
An electric double layer capacitor was produced in the same manner as in Example 1 except that LiBF ₄ as an electrolyte was dissolved in PC as an electrolytic solution so as to have a concentration of 1 mol / l. A capacitor was used.

[Comparative Example 4]
An electric double layer capacitor was produced in the same manner as in Example 4 except that LiBF ₄ as an electrolyte was dissolved in PC as an electrolytic solution so as to have a concentration of 1 mol / l. A capacitor was used.

[Comparative Example 5]
An electric double layer capacitor was produced in the same manner as in Example 1 except that the positive electrode and the negative electrode in Example 1 were replaced, and the electric double layer capacitor in Comparative Example 5 was obtained.

[Comparative Example 6]
An electric double layer capacitor was produced in the same manner as in Example 4 except that the positive electrode and the negative electrode in Example 4 were replaced, and the electric double layer capacitor of Comparative Example 6 was obtained.

[Comparative Example 7]
The composition of the negative electrode of the polarizable electrode, the average particle diameter of 15 [mu] m, and 100% by weight of activated carbon having a specific surface area of 2000 m ² / g, an average particle diameter of 0.02 [mu] m, titanium dioxide having a specific surface area of ^{50 m} 2 / g 1.0 wt% An electric double layer capacitor was prepared in the same manner as in Example 4 except that 8% by weight of acetylene black as a conductive additive and 12% by weight of PVDF as a binder were prepared. A capacitor was used.

[Comparative Example 8]
The composition of the negative electrode of the polarizable electrode, the average particle diameter of 15 [mu] m, and 100% by weight of activated carbon having a specific surface area of 2000 m ² / g, an average particle diameter of 0.02 [mu] m, titanium dioxide having a specific surface area of ^{50 m} 2 / g 0.5 wt% An electric double layer capacitor was prepared in the same manner as in Example 4 except that 8% by weight of acetylene black as a conductive additive and 12% by weight of PVDF as a binder were prepared. A capacitor was used.

[Comparative Example 9]
The composition of the negative electrode of the polarizable electrode, the average particle diameter of 15 [mu] m, and 100% by weight of activated carbon having a specific surface area of 2000 m ² / g, an average particle diameter of 0.02 [mu] m, and the titanium dioxide having a specific surface area of ^{50 m} 2 / g 60 wt%, An electric double layer capacitor was produced in the same manner as in Example 4 except that 8% by weight of acetylene black as a conductive additive and 12% by weight of PVDF as a binder were produced. did.

[Comparative Example 10]
The composition of the polarizable electrode of the negative electrode is 100% by weight of activated carbon having an average particle size of 15 μm and a specific surface area of 2000 m ² / g, and 100% by weight of titanium dioxide having an average particle size of 0.02 μm and a specific surface area of 50 m ² / g, An electric double layer capacitor was produced in the same manner as in Example 4 except that 8% by weight of acetylene black as a conductive additive and 12% by weight of PVDF as a binder were produced. did.

  Using the electric double layer capacitors of Examples 1 to 12 and Comparative Examples 1 to 10, initial capacitance and internal resistance were measured.

Here, assuming that the capacitance is C [F], the accumulated discharge power is P [W · sec], the discharge start voltage is V ₁ [V], and the discharge end voltage is V ₂ [V], the accumulated discharge power P is ,
P = 1 / 2C · V ₁ ² −1 / 2C · V ₂ ² ... (2) Since this is expressed by the equation (2), the capacitance C at this time is
C = 2P / (V ₁ ² −V ₂ ² ) (3) The capacitance C can be calculated. Using a charging / discharging device (ACD-10APS-05N manufactured by Asuka Electronics Co., Ltd.), the integrated discharge power P, the discharge start voltage V ₁ , and the discharge end voltage V ₂ were measured, and the capacitance C was calculated. Further, this capacitance was converted to a capacitance [F / g] per unit weight normalized by dividing the total weight of the positive electrode active material weight and the negative electrode active material weight. As charging / discharging conditions at the time of capacitance measurement, charging is performed at a current value of 3.3 mA / cm ² up to 2.7 V, and after reaching 2.7 V, constant current-constant voltage charging and discharging are performed at a constant voltage. Was performed by constant current discharge discharging to 0 V at a current value of 3.3 mA / cm ² .

The internal resistance R [Ω] is defined as a voltage drop δE [V] at the start of discharge and a discharge current I [A].
R = δE / I · · · · · · · · · ················· (4) Here, the voltage drop δE is a difference between the pre-discharge start voltage V ₃ [V] and the discharge start voltage V ₁ [V].
δE = (V ₃ −V ₁ ) (5) Equation (4) and (5)
R = (V ₃ −V ₁ ) / I (6) The internal resistance R was calculated by measuring the pre-discharge start voltage V ₃ , the discharge start voltage V ₁ and the discharge current I with a charge / discharge device.

Next, a charge / discharge cycle test was performed in a range of 2.7 V to 1.5 V at a charge / discharge current of 10 mA / cm ² , the capacitance after 50,000 cycles was measured, and the initial capacitance was set to 100 The retention rate when calculated as% (cycle test characteristics) was calculated. Further, using the electric double layer capacitors of other Examples 1 to 12 and Comparative Examples 1 to 10, the battery was stored in a charged state of 3.0 V in an environment of 70 ° C., returned to room temperature after 500 hours, and electrostatic capacity And the internal resistance were measured. (Float test characteristics). The results of these measured values are shown in FIG.

  In the electric double layer capacitor, the higher the capacitance maintenance ratio in the cycle test characteristics, the better the charge / discharge cycle characteristics and the higher the voltage, and the higher the capacitance after 500 hours in the float test characteristics, the higher the temperature retention. Excellent characteristics and long life expectancy. If the capacitance maintenance ratio in the cycle test characteristics is 85% or more and the capacitance maintenance ratio after 500 hours in the float test characteristics is 15 F / g or more, it is determined that there is no practical problem. If the electrostatic capacity maintenance ratio is 90% or more and the electrostatic capacity after 500 hours in the float test characteristics is 18 F / g or more, the voltage characteristics and the life characteristics will be satisfactory for all uses of the electric double layer capacitor. .

  As shown in FIG. 1, Examples 1-11 using a negative electrode in which titanium oxide such as lithium titanate and titanium dioxide was added to activated carbon and an electrolyte solution in which an ammonium salt was dissolved were added with titanium oxide. Both the charge / discharge cycle test characteristics and the float test characteristics are superior to those of Comparative Example 1 that is not.

  Moreover, Examples 1-12 are the comparative example 2 using the electrolyte solution which melt | dissolved lithium salt, and the negative electrode comprised only with lithium titanate, the electrolyte solution which melt | dissolved lithium salt, and the negative electrode which added the titanium oxide. Both the charge / discharge cycle test characteristics and the float test characteristics are superior to the comparative examples 3 and 4 used.

  In addition, Examples 1 to 12 are superior in both charge / discharge cycle test characteristics and float test characteristics as compared with Comparative Examples 5 and 6 in which an electrolytic solution in which an ammonium salt is dissolved is used and titanium oxide is added to the positive electrode. . Example 11 uses an electrolytic solution in which an ammonium salt is dissolved, and titanium oxide is added to the positive electrode. However, since the effect of adding titanium oxide to the negative electrode is high, titanium oxide is added only to the positive electrode. Both the charge / discharge cycle test characteristics and the float test characteristics are improved as compared with the added Comparative Example 6.

  Moreover, Examples 1-12 which added the titanium oxide to the negative electrode 2 to 50 weight% with respect to 100 weight% of activated carbon were the comparative examples 7 which added the titanium oxide to the negative electrode 1.0 weight% with respect to 100 weight% of activated carbon. Both charge / discharge cycle test characteristics and float test characteristics are superior, and both charge / discharge cycle test characteristics and float test characteristics are superior to Comparative Example 9 in which 60% by weight of titanium oxide is added to 100% by weight of activated carbon. Yes. Therefore, the amount of titanium oxide added to the activated carbon is preferably in the range of 2 wt% to 50 wt%.

Further, Examples 1 to 11 in which a titanium oxide having a specific surface area of 10 m ² or more was added to the negative electrode were more charge / discharge cycle test characteristics and float tests than Example 12 in which a titanium oxide having a specific surface area of 2 m ² was added to the negative electrode. Both properties are excellent. Therefore, when the specific surface area of titanium oxide is 10 m ² / g or more, the effect of suppressing deterioration during storage at high voltage and high temperature is high.

  When Example 1 and Comparative Example 3 are compared, the electrolyte salt is different between an ammonium salt and a lithium salt, but the lithium salt is inferior in cycle test characteristics to the ammonium salt and has a high deterioration rate during high-voltage storage.

FIG. 2 uses Examples 4 to 10 and Comparative Examples 7 to 10 and titanium oxide having a specific surface area of 50 m ² / g was used as the titanium oxide, and the weight ratio of the titanium oxide to the negative electrode activated carbon was changed. It is the characteristic view which compared the value of the initial stage of a case, and the value after 500 hours.

  As shown in FIG. 2, in the capacitance per unit weight of the polarizable electrode, since titanium oxide does not have capacitance, the weight ratio of titanium oxide increases and capacitance (F / g). Will decline. However, the electrostatic capacity after 500 hours clearly exceeds the electrostatic capacity when titanium oxide is not added (0% by weight) at 2% by weight or more and 50% by weight or less. That is, the life characteristics of the electric double layer capacitor are improved at 2 wt% or more and 50 wt% or less. That is, if the weight ratio of the titanium oxide to the activated carbon is in the range of 2 to 50% by weight, the electrostatic capacity after 500 hours in the float test characteristics is 15 F / g or more, and it is in the range of 5 to 35% by weight. For example, the electrostatic capacity after 500 hours in the float test characteristics is 18 F / g or more.

FIG. 3 shows that, using Examples 4 to 10 and Comparative Examples 7 to 10, titanium oxide having a specific surface area of 50 m ² / g was used as the titanium oxide, and the weight ratio of the titanium oxide to the negative electrode activated carbon was changed. It is the characteristic view which compared the value of the internal resistance in the case with the value of 500 hours after.

  As shown in FIG. 3, the internal resistance decreased as the weight ratio of titanium oxide increased despite the fact that titanium oxide was a semiconductor. However, in the case of 60% by weight, the internal resistance turned to a clear increase. Therefore, in the range of 2% by weight to 50% by weight, the initial internal resistance is clearly lower when titanium oxide is not added (0% by weight). Although the internal resistance also increases due to deterioration, as shown in FIG. 2, the addition of titanium oxide improves the life performance, so the internal resistance after 500 hours is 2 wt% or more and 50 wt% or less. , The internal resistance when no titanium oxide is added (0% by weight) is clearly below.

  The cause of the decrease in internal resistance when the amount of titanium oxide added is in the range of 2 wt% to 50 wt% will be described with reference to FIGS. 4 and 5. FIG. 4 is an enlarged schematic view of the inside of a polarizable electrode assuming that the addition amount of titanium oxide is 2 wt%, and FIG. 5 is an assumption that the addition amount of titanium oxide is 50 wt%.

  When the addition amount of titanium oxide is 2% by weight, as shown in FIG. 4, semiconducting titanium oxide particles 2 exist around the portion where activated carbon particles 1 are in direct contact with each other, and activated carbon 1 is in direct contact with each other. Even in a portion that is not, it is expected that electrons are transferred through the titanium oxide 2 and the electrical conductivity is supplementarily improved. On the other hand, when the addition amount of titanium oxide reaches 50% by weight, the titanium oxide particles 2 enter between the activated carbon particles 1 as shown in FIG. This is thought to increase.

As described above, a negative electrode obtained by adding a titanium oxide such as lithium titanate or titanium dioxide to a carbon material such as activated carbon in the range of 2% by weight to 50% by weight, and an electrolytic solution in which an ammonium salt is dissolved are used. As a result, since the electrostatic capacity after 500 hours in the float test characteristics is 15 F / g or more, it is possible to produce an electric double layer capacitor that is excellent in charge and discharge cycles and has a low electrostatic capacity deterioration rate during high voltage storage. did it. In particular, when the specific surface area of the titanium oxide to be added is 10 m ² / g or more, the effect of suppressing deterioration during high voltage and high temperature storage is enhanced.

  Moreover, if the weight ratio of the titanium oxide to the carbon material such as activated carbon is in the range of 5 to 35% by weight, the electrostatic capacity after 500 hours in the float test characteristics becomes 18 F / g or more, so that a higher voltage In addition, the effect of suppressing deterioration during high-temperature storage becomes significant.

  In this example, PVDF is used as the binder. However, when a mixture of SBR and CMC or a mixture of acrylic rubber and CMC is used as the binder, similar results can be obtained even when PTFE is used as the binder. It was.

  In this embodiment, titanium dioxide is used as the titanium oxide, but titanium oxide reduced by hydrogen reduction or the like may be used. In this case, the electron conduction resistance of titanium oxide becomes small, and the effect of reducing the internal resistance becomes even more remarkable. Further, an effect of trapping oxygen is also produced, and an effect of mitigating deterioration by taking in oxygen that adversely affects the deterioration can be obtained.

It is a characteristic comparison figure of the Example for implementing this invention, and a comparative example. It is a characteristic view of the Example for implementing this invention, and a comparative example. It is a characteristic comparison figure of the Example for implementing this invention, and a comparative example. It is an internal expansion schematic diagram of the polarizable electrode for demonstrating the change of internal resistance. It is an internal expansion schematic diagram of the polarizable electrode for demonstrating the change of internal resistance.

Explanation of symbols

1 Activated carbon particles 2 Titanium oxide particles

Claims (2)

A positive electrode containing a carbon material;
An anatase-type titanium dioxide, a mixture of anatase-type titanium dioxide and rutile-type titanium dioxide, or a negative electrode formed by adding particles of titanium oxide selected from the group consisting of lithium titanate to a carbon material ;
An electrolyte containing an ammonium salt;
Have
The weight ratio of the titanium oxide particles to the carbon material contained in the negative electrode is
An electric double layer capacitor characterized by being in the range of 5 wt% to 35 wt% . The specific surface area of anatase-type titanium dioxide, a mixture of anatase-type titanium dioxide and rutile-type titanium dioxide, or titanium oxide particles selected from the group consisting of lithium titanate is 10 m ² / g or more. The electric double layer capacitor according to claim 1.

Images