JP4511328B2 - Non-aqueous electrolyte for electric double layer capacitor and non-aqueous electrolyte electric double layer capacitor having the same - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte for an electric double layer capacitor and a non-aqueous electrolyte electric double layer capacitor including the same, and particularly to a non-aqueous electrolyte and a non-aqueous electrolyte for an electric double layer capacitor having excellent safety. The present invention relates to a multilayer capacitor.

  An electric double layer capacitor is a capacitor that uses an electric double layer formed between an electrode and an electrolyte, and the mass transfer is the cycle in which ions are electrically adsorbed from the electrolyte on the electrode surface. The battery is different from the battery in which the cycle of the oxidation-reduction reaction involving is a charge / discharge cycle. For this reason, the electric double layer capacitor has superior instantaneous charge / discharge characteristics as compared with the battery, and does not involve a chemical reaction. Therefore, even if charge / discharge is repeated, the instantaneous charge / discharge characteristics hardly deteriorate. Further, in the electric double layer capacitor, since there is no charge / discharge overvoltage at the time of charge / discharge, a simple and inexpensive electric circuit is sufficient. In addition, the remaining capacity is easy to understand, has durability temperature characteristics over a wide range of temperatures from -30 to 90 ° C, and is non-polluting. It is in the spotlight as an environmentally friendly new energy storage product. Furthermore, since the electric double layer capacitor has the above-described characteristics, it has been attracting attention as a power source for energy regeneration and engine start of electric vehicles, fuel cell vehicles and hybrid electric vehicles.

The electric double layer capacitor is an energy storage device having positive and negative electrodes and an electrolyte, and at the contact interface between the electrode and the electrolyte, positive and negative charges are arranged to face each other at a very short distance. Forming an electric double layer. Therefore, since the electrolyte plays a role as an ion source for forming the electric double layer, it is an important substance that influences the basic characteristics of the electric double layer capacitor, like the electrode. Conventionally known aqueous electrolytes include non-aqueous electrolytes, non-aqueous electrolytes, and solid electrolytes. From the viewpoint of improving the energy density of electric double layer capacitors, non-aqueous electrolytes that can set a high operating voltage. However, it is particularly in the spotlight and is being put to practical use. Examples of the non-aqueous electrolyte include high-dielectric constant aprotic organic solvents such as carbonate carbonate (ethylene carbonate, propylene carbonate, etc.), γ-butyrolactone, (C ₂ H ₅ ) ₄ P · BF ₄ , ( A mixed solution in which a solute (supporting salt) such as C ₂ H ₅ ) ₄ N · BF ₄ is dissolved has been put into practical use.

  However, since the aprotic organic solvent has a low flash point, for example, when an electric double layer capacitor ignites due to heat generation or the like, there is a high risk of ignition. In addition, the aprotic organic solvent has a high risk of vaporizing and decomposing to generate gas as the electric double layer capacitor generates heat, or causing explosion and ignition of the electric double layer capacitor by the generated gas and heat. .

  In contrast, a phosphazene compound is added to a non-aqueous electrolyte for an electric double layer capacitor to give the non-aqueous electrolyte non-flammability, flame retardancy, or self-extinguishing properties. Non-aqueous electrolyte electric double layer capacitors have been developed that greatly reduce the risk of ignition and ignition (see Patent Document 1).

JP 2001-217152 A

  When the non-aqueous electrolyte electric double layer capacitor becomes very hot due to a short circuit or the like, the phosphazene compound is thermally decomposed to release a substance that suppresses combustion of the aprotic organic solvent in the electrolyte, Although the safety of the double layer capacitor can be improved, there is a room for improvement because there is a part that does not contribute to combustion suppression depending on the molecular structure of the phosphazene compound used.

  Accordingly, an object of the present invention is to provide a non-aqueous electrolyte for an electric double layer capacitor having excellent safety, which is obtained by adding a compound having an excellent combustion suppression effect, and a highly safe non-aqueous solution comprising the non-aqueous electrolyte. The object is to provide an electrolytic electric double layer capacitor.

  As a result of intensive studies to achieve the above object, the present inventors have found that a compound having a particularly excellent combustion inhibiting effect is present in the thermal decomposition product of the nonaqueous electrolytic solution to which the phosphazene compound is added. It has been found that the safety of the non-aqueous electrolyte and the non-aqueous electrolyte electric double layer capacitor can be significantly improved by adding to the non-aqueous electrolyte, and the present invention has been completed.

That is, the non-aqueous electrolyte for electric double layer capacitors of the present invention is characterized by containing a phosphine oxide compound having a PF bond and / or a P—NH ₂ bond in the molecule and a supporting salt.

  In a preferred example of the non-aqueous electrolyte for electric double layer capacitors of the present invention, the content of the phosphine oxide compound is 3% by volume or more, and more preferably 5% by volume or more.

In another preferred embodiment of the non-aqueous electrolyte for electric double layer capacitors of the present invention, the phosphine oxide compound is represented by the following formula (I):
O = PR ¹ ₃ ... (I)
(Wherein R ¹ is each independently a monovalent substituent or a halogen element, and at least one R ¹ is fluorine or an amino group). Wherein R ¹ in formula (I) is independently selected from the group consisting of fluorine, amino group, alkyl group and alkoxy group, and at least one R ¹ is fluorine or amino group. preferable. And a phosphine oxide compound in which at least one R ¹ in the formula (I) is fluorine and at least one R ^{1 in} the formula (I) is an amino group, and the total of R ¹ in the formula (I) In particular, phosphine oxide compounds which are fluorine or amino groups are particularly preferred.

  The non-aqueous electrolyte for electric double layer capacitors of the present invention preferably further contains an aprotic organic solvent. Here, as the aprotic organic solvent, nitrile compounds, cyclic and chain ester compounds, and chain ether compounds are preferable.

  Moreover, the non-aqueous electrolyte electric double layer capacitor of the present invention comprises the above non-aqueous electrolyte for electric double layer capacitor, a positive electrode, and a negative electrode.

According to the present invention, the non-aqueous electrolyte for an electric double layer capacitor includes a phosphine oxide compound having a PF bond and / or a P—NH ₂ bond in the molecule, and the risk of ignition and ignition is greatly suppressed. Can be provided. Moreover, the non-aqueous electrolyte electric double layer capacitor provided with the non-aqueous electrolyte and having significantly improved safety can be provided.

The present invention is described in detail below. The non-aqueous electrolyte for an electric double layer capacitor of the present invention contains a phosphine oxide compound having a PF bond and / or a P-NH ₂ bond in the molecule and a supporting salt, and if necessary, is aprotic. An organic solvent or the like may be included. When the present inventors investigated a thermal decomposition product at 800 ° C. of a nonaqueous electrolytic solution comprising a fluorine-containing phosphazene compound, a supporting salt and an aprotic organic solvent, a PF bond and / or It has been found that a phosphine oxide compound having a P—NH ₂ bond has an excellent combustion suppressing effect. Since the non-aqueous electrolyte of the non-aqueous electrolyte electric double layer capacitor of the present invention contains such a phosphine oxide compound, there is a risk of ignition and ignition of the non-aqueous electrolyte and the electric double-layer capacitor provided with the non-aqueous electrolyte. Sex is greatly suppressed.

The phosphine oxide compound used in the non-aqueous electrolyte for electric double layer capacitors of the present invention is not particularly limited as long as it has a PF bond and / or a P-NH ₂ bond in the molecule. Among such phosphine oxide compounds, phosphine oxide compounds represented by the above formula (I) are preferable. In the formula (I), each R ¹ is independently a monovalent substituent or a halogen element, and at least one R ¹ is fluorine or an amino group. Here, preferred examples of the halogen element include fluorine, chlorine, bromine, etc. Among these, fluorine is particularly preferred. On the other hand, examples of the monovalent substituent include an amino group, an alkoxy group, an alkyl group, a carboxyl group, an acyl group, and an aryl group. Among these, the effect of reducing the risk of ignition and ignition of the electrolyte From the viewpoint of superiority, an amino group and an alkoxy group are preferred. Examples of the alkoxy group include a methoxy group, an ethoxy group, a methoxyethoxy group, a propoxy group, and a phenoxy group. Examples of the alkyl group include a methyl group, an ethyl group, a propyl group, a butyl group, and a pentyl group. Examples of the acyl group include a formyl group, an acetyl group, a propionyl group, a butyryl group, an isobutyryl group, and a valeryl group. Examples of the aryl group include a phenyl group, a tolyl group, and a naphthyl group. The hydrogen element in these monovalent substituents is preferably substituted with a halogen element, and preferred examples of the halogen element include fluorine, chlorine, bromine, etc., with fluorine being most preferred, and then chlorine being preferred. .

  In the phosphine oxide compound, 10% by mass or more in the molecule is preferably a halogen element, and more preferably 15% by mass or more in the molecule is a halogen element. Further, in the phosphine oxide compound, 7% by mass or more in the molecule is preferably fluorine, and more preferably 10% by mass or more in the molecule is fluorine. A phosphine oxide compound in which 10% by mass or more in the molecule is a halogen element is excellent in suppressing the combustion of the non-aqueous electrolyte, and a phosphine oxide compound in which 7% by mass or more in the molecule is fluorine is a non-aqueous electrolyte. It is particularly excellent in the effect of suppressing the combustion.

Examples of the phosphine oxide compound, at least one of fluorine and phosphine oxide compound is at least one amino group of R ¹ in formula (I) R ¹ in formula (I), as well as the formula (I) Particularly preferred are phosphine oxide compounds in which all of R ¹ are fluorine or amino groups. Since these phosphine oxide compounds have a high proportion of fluorine and amino groups in the molecule that contribute to the suppression of combustion of the electrolyte, they have an excellent combustion suppression effect.

Specific examples of the phosphine oxide compound of the above formula (I) include trifluorophosphine oxide [O = PF ₃ ], triaminophosphine oxide [O═P (NH ₂ ) ₃ ], aminodifluorophosphine oxide [O═PF. ₂ NH ₂ ], diaminofluorophosphine oxide [O═PF (NH ₂ ) ₂ ], dimethoxyfluorophosphine oxide [O═PF (OCH ₃ ) ₂ ], ethoxydifluorophosphine oxide [O═PF ₂ (OC ₂ H ₅ ) ], methoxy difluoro phosphine oxide _{[O = PF 2 (OCH 3} )], diethoxy-fluoro phosphine oxide _{[O = PF (OC 2 H} 5) 2] , and the like.

  The content of the phosphine oxide compound in the non-aqueous electrolyte is preferably 3% by volume or more, and more preferably 5% by volume or more. If the content of the phosphine oxide compound in the non-aqueous electrolyte is 3% by volume or more, the risk of ignition / flammability of the non-aqueous electrolyte can be sufficiently suppressed. In addition, the said phosphine oxide compound may be used individually by 1 type, and may mix and use 2 or more types.

  The supporting salt used in the non-aqueous electrolyte for electric double layer capacitors of the present invention can be selected from conventionally known salts, but quaternary ammonium salts are preferred from the viewpoint of good electrical conductivity in the electrolyte. The quaternary ammonium salt is a solute that plays a role as an ion source for forming an electric double layer in a nonaqueous electrolytic solution, and can effectively improve electrical characteristics such as electrical conductivity of the electrolytic solution. In view of the possibility, a quaternary ammonium salt capable of forming a multivalent ion is preferable.

Examples of the quaternary ammonium salt include (CH ₃ ) ₄ N · BF ₄ , (CH ₃ ) ₃ C ₂ H ₅ N · BF ₄ , (CH ₃ ) ₂ (C ₂ H ₅ ) ₂ N · BF _4. CH ₃ (C ₂ H ₅ ) ₃ N · BF ₄ , (C ₂ H ₅ ) ₄ N · BF ₄ , (C ₃ H ₇ ) ₄ N · BF ₄ , CH ₃ (C ₄ H ₉ ) ₃ N · BF ₄ , (C ₄ H ₉ ) ₄ N · BF ₄ , (C ₆ H ₁₃ ) ₄ N · BF ₄ , (C ₂ H ₅ ) ₄ N · ClO ₄ , (C ₂ H ₅ ) ₄ N · AsF ₆ , (C ₂ H ₅ ) ₄ N · SbF ₆ , (C ₂ H ₅ ) ₄ N · CF ₃ SO ₃ , (C ₂ H ₅ ) ₄ N · C ₄ F ₉ SO ₃ , (C ₂ H ₅ ) ₄ N · (CF ₃ SO ₂ ) ₂ N, (C ₂ H ₅ ) ₄ N · BCH ₃ (C ₂ H ₅ ) ₃ , (C ₂ H ₅ ) ₄ N · B (C ₂ H ₅ ) ₄ , (C ₂ H ₅ ) ₄ N · B (C ₄ H ₉ ) ₄ , (C ₂ H ₅ ) ₄ N · B (C ₆ H ₅ ) _{4 and the} like are preferable. Also preferred are hexafluorophosphates in which the anion portion (for example, • BF ₄ , • ClO ₄ , • AsF _6, etc.) of these quaternary ammonium salts is replaced with • PF ₆ . Among these, quaternary ammonium salts in which different alkyl groups are bonded to N atoms are preferable in that the solubility can be improved by increasing the polarizability. Furthermore, as the quaternary ammonium salt, for example, compounds represented by the following formulas (a) to (j) are also preferable. Here, in the formulas (a) to (j), Me represents a methyl group, and Et represents an ethyl group.

Among these quaternary ammonium salts, salts that can generate (CH ₃ ) ₄ N ⁺ , (C ₂ H ₅ ) ₄ N ^+, etc. as cations, in particular, from the viewpoint of ensuring high electrical conductivity. preferable. Further, a salt capable of generating an anion having a small formula weight is preferable. These quaternary ammonium salts may be used individually by 1 type, and may use 2 or more types together.

  The concentration of the supporting salt in the non-aqueous electrolyte for an electric double layer capacitor of the present invention is preferably 0.2 to 2.5 mol / L (M), more preferably 0.8 to 1.5 mol / L (M). If the concentration of the supporting salt is less than 0.2 mol / L (M), sufficient electrical properties such as the electrical conductivity of the electrolytic solution may not be ensured, and if it exceeds 2.5 mol / L (M), the viscosity of the electrolytic solution may be insufficient. The electrical characteristics such as electrical conductivity may be lowered.

  The aprotic organic solvent that can be used in the nonaqueous electrolytic solution of the present invention can reduce the viscosity of the electrolytic solution, and can easily achieve optimum ionic conductivity as an electric double layer capacitor. Specific examples of the aprotic organic solvent include nitrile compounds such as acetonitrile (AN), propiononitrile, butyronitrile, isobutyronitrile, and benzonitrile; 1,2-dimethoxyethane (DME), tetrahydrofuran (THF) and the like. Ether compounds: dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), diphenyl carbonate, γ-butyrolactone (GBL), γ-valerolactone, etc. The ester compound is preferably mentioned. Among these, propylene carbonate, γ-butyrolactone and acetonitrile are preferable. Note that the cyclic ester compound has a high relative dielectric constant and is excellent in the ability to dissolve the supporting salt, and the chain ester compound and the ether compound are suitable in terms of reducing the viscosity of the electrolyte because of low viscosity. It is. These may be used alone or in combination of two or more.

<Non-aqueous electrolyte electric double layer capacitor>
Next, the non-aqueous electrolyte electric double layer capacitor of the present invention will be described in detail. The non-aqueous electrolyte electric double layer capacitor of the present invention comprises the above-described non-aqueous electrolyte for electric double layer capacitor, a positive electrode, and a negative electrode, and in the technical field of an electric double layer capacitor such as a separator, if necessary. Other members that are normally used are provided.

  Although there is no restriction | limiting in particular as a positive electrode and a negative electrode of the nonaqueous electrolyte electric double layer capacitor of this invention, Usually, a porous carbon type polarizable electrode is preferable. The electrode is preferably one having characteristics such as a large specific surface area and bulk specific gravity, electrochemical inactivity, and low resistance. Here, activated carbon etc. are mentioned as said porous carbon.

  The electrode generally contains porous carbon such as activated carbon, and contains other components such as a conductive agent and a binder as necessary. There is no restriction | limiting in particular as a raw material of the activated carbon which can be used suitably for the said electrode, For example, various heat resistant resins, pitch, etc. other than a phenol resin are mentioned suitably. Preferable examples of the heat resistant resin include polyimide, polyamide, polyamideimide, polyetherimide, polyethersulfone, polyetherketone, bismaleimide triazine, aramid, fluororesin, polyphenylene, polyphenylene sulfide and the like. These may be used alone or in combination of two or more. The activated carbon is preferably in the form of powder, fiber cloth or the like from the viewpoint of increasing the specific surface area and increasing the charge capacity of the non-aqueous electrolyte electric double layer capacitor. Further, these activated carbons may be subjected to treatment such as heat treatment, stretch molding, vacuum high temperature treatment, and rolling for the purpose of increasing the charge capacity of the electric double layer capacitor.

  The conductive agent used for the electrode is not particularly limited, and examples thereof include graphite and acetylene black. The binder used for the electrode is not particularly limited, and examples thereof include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), and the like. . These additives can be used at a blending ratio similar to the conventional one.

  The non-aqueous electrolyte electric double layer capacitor of the present invention preferably includes a separator, a current collector, a container and the like in addition to the above-described electrodes (positive electrode and negative electrode) and non-aqueous electrolyte, and moreover, a normal electric double layer capacitor. Each known member used can be provided. Here, the separator is interposed between the positive and negative electrodes for the purpose of preventing a short circuit of the non-aqueous electrolyte electric double layer capacitor. There is no restriction | limiting in particular as this separator, Usually, the well-known separator used as a separator of a nonaqueous electrolyte electric double layer capacitor is used suitably. As a material for the separator, for example, a microporous film, a nonwoven fabric, paper, and the like are preferably exemplified. Specifically, a nonwoven fabric made of a synthetic resin such as polytetrafluoroethylene, polypropylene, and polyethylene, a thin layer film, and the like are preferable. Among these, a microporous film made of polypropylene or polyethylene having a thickness of about 20 to 50 μm is particularly suitable.

  There is no restriction | limiting in particular as said collector, The well-known thing normally used as a collector of a nonaqueous electrolyte electric double layer capacitor is used suitably. The current collector is preferably one having excellent electrochemical corrosion resistance, chemical corrosion resistance, workability, mechanical strength, and low cost, such as a current collector layer of aluminum, stainless steel, conductive resin, etc. Is preferred. Moreover, there is no restriction | limiting in particular as said container, The well-known thing normally used as a container of a nonaqueous electrolyte electric double layer capacitor is mentioned suitably. As the material of the container, for example, aluminum, stainless steel, conductive resin and the like are suitable.

  There is no restriction | limiting in particular as a form of the non-aqueous-electrolyte electric double layer capacitor of this invention, Well-known forms, such as a cylinder type (cylindrical type and a square type), a flat type (coin type), are mentioned suitably. These non-aqueous electrolyte electric double layer capacitors are, for example, main power supplies or auxiliary power supplies for electric vehicles and fuel cell vehicles, memory backup devices for various electronic devices, industrial devices, aircraft devices, toys, cordless devices, etc. It is suitably used as a power source for electromagnetic holding such as industrial equipment, gas equipment and instantaneous water heater equipment, and for watches such as watches, wall clocks, solar watches, AGS watches and the like.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples.

(Relation between pyrolysis products of non-aqueous electrolyte and critical oxygen index)
90% by volume of propylene carbonate (PC) and the following formula (II):
(NPR ² ₂ ) ₃ ... (II)
In 6 R ² , 1 is an ethoxy group and 5 is a fluorine cyclic phosphazene compound A [viscosity at 25 ° C .: 1.2 mPa · s] 10% by volume of tetraethylammonium tetrafluoroborate [ Et ₄ N · BF ₄ ] (supporting salt) was dissolved to 1 mol / L (M) to prepare a nonaqueous electrolytic solution A.

Further, instead of cyclic phosphazene compound A, cyclic phosphazene compound B [in the formula (II), a cyclic phosphazene compound in which two of six R ² are ethoxy groups and four are fluorines at 25 ° C .: 1.2 mPa · s ] To prepare a non-aqueous electrolyte B, and instead of cyclic phosphazene compound A, cyclic phosphazene compound C [in formula (II), one of six R ² is a phenoxy group (PhO—) and five are Nonaqueous electrolytic solution C was prepared using a cyclic phosphazene compound which is fluorine, and a viscosity at 25 ° C .: 1.7 mPa · s (1.7 cP)].

Furthermore, tetraethylammonium tetrafluoroborate [Et ₄ N · BF ₄ ] (supporting salt) was dissolved in γ-butyrolactone (GBL) so as to be 1 mol / L (M) to prepare a nonaqueous electrolytic solution D.

With respect to the non-aqueous electrolytes A to D, the critical oxygen index was measured by the following method, further pyrolyzed at 800 ° C., and each pyrolysis product was analyzed by GC-MS. In addition, the generation amount of each thermal decomposition product was indexed with the peak height of GC of O = PF ₃ generated when the nonaqueous electrolytic solution A was pyrolyzed as 100. The results are shown in Table 1.

(1) Limiting oxygen index of electrolyte solution The limiting oxygen index of the electrolyte solution was measured according to JIS K7201. The larger the limiting oxygen index, the more difficult the electrolyte is to burn. Specifically, a SiO ₂ sheet (quartz filter paper, non-combustible) 127 mm × 12.7 mm can be reinforced with a U-shaped aluminum foil so that it can be self-supporting, and the SiO ₂ sheet is impregnated with 1.0 mL of the electrolyte solution, and a test piece Was made. The test piece was perpendicular to the test piece support, and a combustion cylinder (with an inner diameter of 75 mm, a height of 450 mm, and a diameter of 4 mm was uniformly filled with a thickness of 100 ± 5 mm from the bottom, and a metal net was placed thereon. It is attached so that it is located at a distance of 100 mm or more from the upper end of the object), and then oxygen (JIS K 1101 or equivalent) or nitrogen (JIS K 1107 grade 2 or equivalent or more) is attached to the combustion cylinder. The test piece was ignited under predetermined conditions (the heat source was JIS K 2240 Type 1 No. 1), and the combustion state was examined. However, the total flow rate in the combustion cylinder is 11.4 L / min. This test was performed three times, and the average value is shown in Table 2. The oxygen index refers to the value of the minimum oxygen concentration expressed by the volume percent necessary for the material to continue burning. In this application, the test piece burns continuously for 3 minutes or longer, From the minimum oxygen flow rate required for burning 50 mm or more and the nitrogen flow rate at that time, the following formula:
Critical oxygen index = (oxygen flow rate) / [(oxygen flow rate) + (nitrogen flow rate)] × 100 (volume%)
The limiting oxygen index was calculated according to

  Next, the correlation coefficient between the generation amount (index) of each thermal decomposition product and the critical oxygen index (volume%) of each non-aqueous electrolyte was calculated, and the results shown in Table 1 were obtained. In addition, the larger the correlation coefficient value, the stronger the positive correlation between the generation amount of the thermal decomposition product and the critical oxygen index of the non-aqueous electrolyte solution, that is, the harder it is to burn.

From Table 1, it can be seen that the generation amount of O = PF ₂ (OCH ₃ ), O = PF (OCH ₃ ) ₂ , and O = PF ₂ NH _{2 in} the pyrolysis product and the critical oxygen index of the non-aqueous electrolyte are positive. The correlation is strong and it can be seen that these phosphine oxide compounds contribute to the suppression of combustion of the electrolyte.

(Comparative Example 1 and Examples 1-3)
Next, 10% by volume of the thermal decomposition product or cyclic phosphazene compound A and 90% by volume of propylene carbonate (PC) are mixed, and tetraethylammonium tetrafluoroborate [Et ₄ N · BF ₄ ] (supporting salt). Was dissolved to 1 mol / L to prepare a non-aqueous electrolyte. The critical oxygen index of the obtained non-aqueous electrolyte was measured by the above method, and the results shown in Table 2 were obtained.

Next, the activated carbon [AC, trade name: Kuractive-1500, manufactured by Kuraray Chemical Co., Ltd.], acetylene black (conductive agent), and polyvinylidene fluoride (PVDF) (binder) are each in a mass ratio (activated carbon: acetylene black: PVDF) was mixed to 8: 1: 1 to obtain a mixture. 100 mg of the obtained mixture was sampled, put into a 20 mmφ pressure-resistant carbon container, and compacted under conditions of a pressure of 150 kgf / cm ² and a normal temperature to prepare a positive electrode and a negative electrode (electrode). A cell was assembled using the obtained electrodes (positive electrode and negative electrode), an aluminum metal plate (current collector) (thickness: 0.5 mm), and a polypropylene / polyethylene plate (separator) (thickness: 25 μm), and vacuum dried. Dry thoroughly. The cell was impregnated with the above non-aqueous electrolyte to produce a non-aqueous electrolyte electric double layer capacitor. The cycle characteristics and low temperature characteristics of the obtained electric double layer capacitor were tested by the following methods. The results are shown in Table 2.

(2) Cycle characteristics of the electric double layer capacitor For the obtained non-aqueous electrolyte electric double layer capacitor, the initial discharge capacity and the discharge capacity after 1000 cycles were measured at 20 ° C. From the discharge capacity of the following formula:
Capacity retention ratio S = discharge capacity after 1000 cycles / initial discharge capacity × 100 (%)
The capacity retention rate S was calculated according to the above and used as an index of the cycle characteristics of the capacitor.

(3) Low temperature characteristics of electric double layer capacitor The obtained non-aqueous electrolyte electric double layer capacitor was measured for discharge capacity after 1000 cycles of charge / discharge in each environment of 20 ° C and -20 ° C. From the discharge capacity after 1000 cycles at 20 ° C and the discharge capacity after 1000 cycles at -20 ° C, the following formula:
Capacity maintenance ratio L = discharge capacity (-20 ° C.) / Discharge capacity (20 ° C.) × 100 (%)
The capacity retention ratio L was calculated according to the above and used as an index of the low temperature characteristics of the capacitor.

As shown in Table 2, the critical oxygen index of the non-aqueous electrolyte can be improved by adding a phosphine oxide compound having a PF bond and / or a P—NH ₂ bond in the molecule to the electrolyte. It was confirmed that the safety of the non-aqueous electrolyte can be improved.

Claims (10)

A non-aqueous electrolyte for an electric double layer capacitor, comprising a phosphine oxide compound having a PF bond and / or a P-NH ₂ bond in a molecule, and a supporting salt.   The non-aqueous electrolyte for an electric double layer capacitor according to claim 1, wherein the content of the phosphine oxide compound is 3% by volume or more.   The non-aqueous electrolyte for an electric double layer capacitor according to claim 2, wherein the content of the phosphine oxide compound is 5% by volume or more. The phosphine oxide compound is represented by the following formula (I):
O = PR ¹ ₃ ... (I)
2. wherein R ¹ is independently a monovalent substituent or a halogen element, and at least one R ¹ is fluorine or an amino group. Non-aqueous electrolyte for electric double layer capacitors. R ¹ in the formula (I) is independently selected from the group consisting of fluorine, amino group, alkyl group and alkoxy group, and at least one R ¹ is fluorine or amino group. The non-aqueous electrolyte for electric double layer capacitors according to claim 4. The non-electric double layer capacitor capacitor according to claim 5, wherein at least one R ¹ in the formula (I) is fluorine, and at least one R ^{1 in} the formula (I) is an amino group. Water electrolyte. 6. The non-aqueous electrolyte for an electric double layer capacitor according to claim 5, wherein R ¹ in the formula (I) is each independently fluorine or an amino group.   The non-aqueous electrolyte for electric double layer capacitors according to claim 1, further comprising an aprotic organic solvent.   The non-aqueous electrolyte for an electric double layer capacitor according to claim 8, wherein the aprotic organic solvent contains a nitrile compound, a cyclic or chain ester compound, or a chain ether compound.   A non-aqueous electrolyte electric double layer capacitor comprising the non-aqueous electrolyte for an electric double layer capacitor according to claim 1, a positive electrode, and a negative electrode.