JP2005191369A - Nonaqueous electrolyte for electric double layer capacitor and nonaqueous electrolyte electric double layer capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide nonaqueous electrolyte for electric double layer capacitor in which risk of ignition of aprotic organic solvent remaining in a capacitor and leaking to the outside through vaporization is reduced when temperature of the electric double layer capacitor rises abnormally. <P>SOLUTION: In the nonaqueous electrolyte for electric double layer capacitor comprising at least one kind of aprotic organic solvent and supporting electrolyte, compounds having a difference of boiling point of 25°C or below from that of the aprotic organic solvent and having phosphorus and/or nitrogen in a molecule are added to each aprotic organic solvent. <P>COPYRIGHT: (C)2005,JPO&NCIPI

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 more particularly to a non-aqueous electrolysis for an electric double layer capacitor with a greatly reduced risk of ignition in an emergency. It is about liquid.

  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. This is different from a 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 limelight as an environmentally friendly new energy storage product. Furthermore, since the electric double layer capacitor has the above-described characteristics, it has been spotlighted 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

  Although the non-aqueous electrolyte to which the phosphazene compound is added has greatly reduced the risk of ignition and ignition, the phosphazene compound is more than the aprotic organic solvent when the temperature of the electric double layer capacitor rises. If vaporized first, the remaining aprotic organic solvent is vaporized and decomposed alone to generate gas, or the generated gas and heat may cause the electric double layer capacitor to burst or ignite, or the aprotic organic solvent It is impossible to eliminate the danger of fire. Further, when the aprotic organic solvent is vaporized prior to the phosphazene compound, the vaporized aprotic organic solvent may leak out of the electric double layer capacitor and ignite.

  Therefore, the object of the present invention is to ignite the aprotic organic solvent remaining in the capacitor and the aprotic organic solvent leaking out of the capacitor due to vaporization when the temperature of the electric double layer capacitor is abnormally increased. The object is to provide a non-aqueous electrolyte for electric double layer capacitors with reduced risk of ignition. Another object of the present invention is to provide a non-aqueous electrolyte electric double layer capacitor comprising such a non-aqueous electrolyte and having reduced risk of ignition in the capacitor and outside the capacitor even if the temperature rises abnormally. Is to provide.

  As a result of diligent studies to achieve the above object, the inventors of the present invention, in a non-aqueous electrolyte solution for electric double layer capacitors containing at least one aprotic organic solvent, further correspond to each aprotic organic solvent. Then, by adding phosphorus and / or nitrogen-containing compounds having close boiling points, ignition and ignition of the aprotic organic solvent remaining in the capacitor and the aprotic organic solvent leaking out of the capacitor due to vaporization, etc. The present inventors have found that the danger can be significantly reduced and have completed the present invention.

  That is, the non-aqueous electrolyte for electric double layer capacitors of the present invention is a non-aqueous electrolyte for electric double layer capacitors containing at least one kind of aprotic organic solvent and a supporting salt. A difference in boiling point from the aprotic organic solvent with respect to the solvent is 25 ° C. or less, and each compound contains phosphorus and / or nitrogen in the molecule.

  In a preferred example of the non-aqueous electrolyte for an electric double layer capacitor of the present invention, the compound having phosphorus and / or nitrogen in the molecule has a phosphorus-nitrogen double bond. Here, as the compound having phosphorus and / or nitrogen in the molecule and having a phosphorus-nitrogen double bond, a phosphazene compound is particularly preferable.

  In another preferred embodiment of the non-aqueous electrolyte for electric double layer capacitor of the present invention, the aprotic organic solvent is at least one selected from the group consisting of propylene carbonate, γ-butyrolactone and acetonitrile.

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

  According to the present invention, phosphorous and / or nitrogen-containing compounds having close boiling points are added to the nonaqueous electrolytic solution containing at least one aprotic organic solvent, respectively, corresponding to each aprotic organic solvent. Thus, it is possible to provide a non-aqueous electrolyte for an electric double layer capacitor in which the risk of ignition and ignition of an aprotic organic solvent remaining in the capacitor or leaking outside the capacitor can be greatly reduced. In addition, it is possible to provide a non-aqueous electrolyte electric double layer capacitor that includes such a non-aqueous electrolyte and that greatly reduces the risk of ignition and the like inside and outside the capacitor even if the temperature rises abnormally. .

<Nonaqueous electrolyte for electric double layer capacitor>
Below, the non-aqueous electrolyte for electric double layer capacitors of this invention is demonstrated in detail. The non-aqueous electrolyte for an electric double layer capacitor of the present invention comprises at least one aprotic organic solvent and a supporting salt, and further has a difference in boiling point between each of the aprotic organic solvents and 25 ° C. or less in the molecule. Each containing a compound having phosphorus and / or nitrogen.

  In the non-aqueous electrolyte for electric double layer capacitor of the present invention, the compound having phosphorus and / or nitrogen in the molecule generates nitrogen gas and / or phosphate ester, etc., and makes the non-aqueous electrolyte non-flammable and flame retardant. Or self-extinguishing, has the effect of reducing the risk of ignition of the electric double layer capacitor. However, when the non-aqueous electrolyte containing the aprotic organic solvent does not contain phosphorus and / or nitrogen-containing compounds having a boiling point close to that of the aprotic organic solvent, the aprotic organic solvent is in either the gas phase or the liquid phase. Has a wide temperature range in which phosphorus and / or nitrogen-containing compounds do not coexist, so when the temperature of the electric double layer capacitor rises abnormally, the aprotic organic solvent vaporized or the aprotic organic solvent remaining in the capacitor The risk of ignition and ignition cannot be reduced. In contrast, the non-aqueous electrolyte for an electric double layer capacitor of the present invention includes an aprotic organic solvent and a phosphorus and / or nitrogen-containing compound having a boiling point close to that of the aprotic organic solvent, and the temperature of the capacitor is abnormal. When rising, the aprotic organic solvent and the phosphorus and / or nitrogen-containing compound are vaporized at a temperature close to the aprotic organic solvent, so that the aprotic organic solvent is present either as a liquid or as a gas. As a result, the risk of ignition and ignition of the non-aqueous electrolyte is greatly reduced.

  In addition, for example, when the non-aqueous electrolyte for an electric double layer capacitor of the present invention contains a low-boiling aprotic organic solvent and a high-boiling aprotic organic solvent, the low-boiling aprotic organic solvent is vaporized. Since the corresponding phosphorus and / or nitrogen-containing compound is vaporized in the vicinity of the temperature at which the vaporization occurs, the risk of ignition and ignition of the vaporized aprotic organic solvent can be reduced. In addition, after the low boiling point aprotic organic solvent and the phosphorus and / or nitrogen-containing compound having a boiling point close to that of the low boiling point aprotic organic solvent are vaporized, together with the high boiling point aprotic organic solvent, the high boiling point Since phosphorus and / or nitrogen-containing compounds having a boiling point close to that of the aprotic organic solvent are present in the electrolytic solution, the risk of ignition and ignition of the remaining nonaqueous electrolytic solution can also be reduced.

  The non-aqueous electrolyte for electric double layer capacitors of the present invention contains at least one aprotic organic solvent. The aprotic organic solvent 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; ether compounds such as 1,2-dimethoxyethane and tetrahydrofuran; dimethyl carbonate Preferred examples include ester compounds such as diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate (PC), diphenyl carbonate, γ-butyrolactone (GBL), and γ-valerolactone. 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.

  The nonaqueous electrolytic solution of the present invention contains a supporting salt. The supporting salt can be selected from conventionally known salts, but a quaternary ammonium salt is preferable from the viewpoint of good electrical conductivity in the electrolytic solution. 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 non-aqueous electrolyte for electric double layer capacitors of the present invention contains a compound having a difference in boiling point from an aprotic organic solvent contained in the electrolyte of 25 ° C. or less and having phosphorus and / or nitrogen in the molecule. When the difference in boiling point between the aprotic organic solvent and the phosphorus and / or nitrogen-containing compound contained in the non-aqueous electrolyte exceeds 25 ° C, the aprotic organic solvent is vaporized first, and the gaseous aprotic organic solvent Is ignited, and phosphorus and / or nitrogen-containing compounds are vaporized first, and the remaining liquid aprotic organic solvent is ignited. Here, from the viewpoint of further reducing the risk of ignition of the aprotic organic solvent, the difference in boiling point between the aprotic organic solvent and the compound having phosphorus and / or nitrogen in the molecule is 20 ° C. or less. preferable. The non-aqueous electrolyte for electric double layer capacitors of the present invention may contain at least phosphorus and / or nitrogen-containing compounds having a boiling point difference of 25 ° C. or less with respect to each of the aprotic organic solvents. Phosphorus and / or nitrogen-containing compounds with a difference exceeding 25 ° C. may be further included.

  Examples of the compound having phosphorus and / or nitrogen in the molecule include compounds having phosphorus in the molecule such as phosphate ester compounds, polyphosphate ester compounds, and condensed phosphate ester compounds; triazine compounds, guanidine compounds, pyrrolidine compounds, etc. Compound having nitrogen in molecule; and composite of phosphazene compound, isomer of phosphazene compound, phosphazane compound, compound exemplified as compound having phosphorus in molecule and compound exemplified as compound having nitrogen in molecule Examples thereof include compounds having phosphorus and nitrogen in the molecule such as compounds. Note that the compound having phosphorus and nitrogen in the molecule is also an example of a compound having phosphorus in the molecule and a compound having nitrogen in the molecule. The phosphorus and / or nitrogen-containing compound is appropriately selected according to the aprotic organic solvent used in the electrolytic solution.

  Among the compounds containing phosphorus and / or nitrogen in the molecule, compounds having phosphorus and nitrogen in the molecule are preferable from the viewpoint of cycle characteristics. Of the compounds having phosphorus and nitrogen in the molecule, compounds having a phosphorus-nitrogen double bond such as a phosphazene compound are particularly preferred from the viewpoint of improving thermal stability and improving high-temperature storage characteristics.

（式中、Ｒ¹、Ｒ²及びＲ³は、夫々独立して一価の置換基又はハロゲン元素を表し；Ｘ¹は、炭素、ケイ素、ゲルマニウム、スズ、窒素、リン、ヒ素、アンチモン、ビスマス、酸素、硫黄、セレン、テルル及びポロニウムからなる群から選ばれる元素の少なくとも１種を含む置換基を表し；Ｙ¹、Ｙ²及びＹ³は、夫々独立して２価の連結基、２価の元素又は単結合を表す。）

(ＮＰＲ⁴ ₂)_n ・・・ (II)
（式中、Ｒ⁴は夫々独立して一価の置換基又はハロゲン元素を表し；ｎは３〜１５を表す。） Specific examples of the phosphazene compound include a chain phosphazene compound represented by the following formula (I) and a cyclic phosphazene compound represented by the following formula (II).

(Wherein R ¹ , R ² and R ³ each independently represents a monovalent substituent or a halogen element; X ¹ represents carbon, silicon, germanium, tin, nitrogen, phosphorus, arsenic, antimony, bismuth) And represents a substituent containing at least one element selected from the group consisting of oxygen, sulfur, selenium, tellurium and polonium; Y ¹ , Y ² and Y ³ are each independently a divalent linking group, divalent Represents an element or a single bond.)

(NPR ⁴ ₂ ) _n ... (II)
(In the formula, each R ⁴ independently represents a monovalent substituent or a halogen element; n represents 3 to 15)

  Among the phosphazene compounds represented by formula (I) or formula (II), those which are liquid at 25 ° C. (room temperature) are preferable. The viscosity at 25 ° C. of the liquid phosphazene compound is preferably 300 mPa · s (300 cP) or less, more preferably 20 mPa · s (20 cP) or less, and particularly preferably 5 mPa · s (5 cP) or less. In the present invention, the viscosity is measured at 1 rpm, 2 rpm, 3 rpm, 5 rpm, 7 rpm, 10 rpm, 20 rpm and 50 rpm using a viscometer [R-type viscometer Model RE500-SL, manufactured by Toki Sangyo Co., Ltd.] The measurement was performed at a speed of 120 seconds, and the rotation speed when the indicated value reached 50 to 60% was set as an analysis condition, and the viscosity was measured at that time. When the viscosity of the phosphazene compound at 25 ° C. exceeds 300 mPa · s (300 cP), the supporting salt becomes difficult to dissolve, the wettability to electrodes, separators, etc. decreases, and the ionic conductivity increases due to the increase in the viscous resistance of the electrolyte. Remarkably lowered, and performance becomes insufficient particularly when used under low temperature conditions such as below freezing point. Further, since these phosphazene compounds are in a liquid state, they have the same conductivity as that of a normal liquid electrolyte, and exhibit excellent cycle characteristics when used in an electrolytic solution of a capacitor.

In the formula (I), R ¹ , R ² and R ³ are not particularly limited as long as they are monovalent substituents or halogen elements. Examples of the monovalent substituent include an alkoxy group, an alkyl group, a carboxyl group, an acyl group, and an aryl group. Among these, an alkoxy group is preferable because the viscosity of the electrolytic solution can be reduced. On the other hand, preferred examples of the halogen element include fluorine, chlorine, bromine and the like. R ^{1 to} R ³ may all be the same type of substituent, and some of them may be different types of substituents. Here, examples of the alkoxy group include a methoxy group, an ethoxy group, a propoxy group, a butoxy group, and an alkoxy-substituted alkoxy group such as a methoxyethoxy group and a methoxyethoxyethoxy group. Among these, a methoxy group, An ethoxy group, a methoxyethoxy group, and a methoxyethoxyethoxy group are preferable, and a methoxy group or an ethoxy group is particularly preferable from the viewpoint of low viscosity and high dielectric constant. 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. As the halogen element, fluorine, chlorine and bromine are preferred, fluorine is most preferred, and chlorine is then preferred. When the hydrogen element in the monovalent substituent is substituted with fluorine, the effect of improving the cycle characteristics of the capacitor tends to be greater than when the hydrogen element is substituted with chlorine.

In the formula (I), examples of the divalent linking group represented by Y ¹ , Y ² and Y ³ include CH ₂ group, oxygen, sulfur, selenium, nitrogen, boron, aluminum, scandium, gallium, Yttrium, indium, lanthanum, thallium, carbon, silicon, titanium, tin, germanium, zirconium, lead, phosphorus, vanadium, arsenic, niobium, antimony, tantalum, bismuth, chromium, molybdenum, tellurium, polonium, tungsten, iron, cobalt, And divalent linking groups containing at least one element selected from the group consisting of nickel. Among these, at least one element selected from the group consisting of CH ₂ groups and oxygen, sulfur, selenium, and nitrogen is included. A divalent linking group containing a seed is preferred, and a divalent linking group containing a sulfur and / or selenium element is particularly preferred. Y ¹ , Y ² and Y ³ may be a divalent element such as oxygen, sulfur or selenium, or a single bond. Y ^{1 to} Y ³ may all be the same type, or some of them may be different types.

［式(III)、式(IV)及び式(V)において、Ｒ⁵〜Ｒ⁹は、それぞれ独立に一価の置換基又はハロゲン元素を表し；Ｙ⁵〜Ｙ⁹は、それぞれ独立に２価の連結基、２価の元素又は単結合を表し；Ｚは２価の基又は２価の元素を表す。］ In the formula (I), X ¹ is a substituent containing at least one element selected from the group consisting of carbon, silicon, nitrogen, phosphorus, oxygen, and sulfur from the viewpoint of toxicity, environment, and the like. preferable. Of these substituents, substituents having a structure represented by the following formula (III), formula (IV) or formula (V) are more preferred.

[In Formula (III), Formula (IV) and Formula (V), R ^{5 to} R ⁹ each independently represents a monovalent substituent or a halogen element; Y ^{5 to} Y ⁹ each independently represents a divalent group. A linking group, a divalent element or a single bond; Z represents a divalent group or a divalent element. ]

In formula (III), formula (IV) and formula (V), R ^{5 to} R ⁹ are the same monovalent substituents or halogen elements as those described for R ^{1 to} R ³ in formula (I). Any of these is preferably mentioned. In addition, these may be the same type within the same substituent, or some of them may be different from each other. R ⁵ and R ^{6 in} formula (III) and R ⁸ and R ^{9 in} formula (V) may be bonded to each other to form a ring.

In the formula (III), formula (IV) and formula (V), the group represented by Y ^{5 to} Y ⁹ is a divalent linkage similar to that described for Y ^{1 to} Y ³ in the formula (I). In the same manner, a group containing a sulfur and / or selenium element is particularly preferable because the risk of ignition and ignition of the electrolyte is reduced. These may be the same type within the same substituent, or some of them may be different from each other.

In the formula (III), as Z, for example, CH ₂ group, CHR (R represents an alkyl group, an alkoxyl group, a phenyl group, etc .; the same shall apply hereinafter) group, NR group, oxygen, sulfur, selenium, Boron, aluminum, scandium, gallium, yttrium, indium, lanthanum, thallium, carbon, silicon, titanium, tin, germanium, zirconium, lead, phosphorus, vanadium, arsenic, niobium, antimony, tantalum, bismuth, chromium, molybdenum, tellurium, Examples include divalent groups containing at least one element selected from the group consisting of polonium, tungsten, iron, cobalt and nickel. Among these, in addition to CH ₂ groups, CHR groups, NR groups, oxygen, sulfur A divalent group containing at least one element selected from the group consisting of selenium is preferred. In particular, a divalent group containing an element of sulfur and / or selenium is preferable because the risk of ignition and ignition of the electrolyte is reduced. Z may be a divalent element such as oxygen, sulfur, or selenium.

  As these substituents, a substituent containing phosphorus as represented by the formula (III) is particularly preferable in that the risk of ignition / flammability can be particularly effectively reduced. Further, when the substituent is a substituent containing sulfur as represented by the formula (IV), it is particularly preferable in terms of reducing the interface resistance of the electrolytic solution.

In the formula (II), R ⁴ is not particularly limited as long as it is a monovalent substituent or a halogen element. Examples of the monovalent substituent include an alkoxy group, an alkyl group, a carboxyl group, an acyl group, and an aryl group. Among these, an alkoxy group is preferable because the viscosity of the electrolytic solution can be reduced. On the other hand, preferred examples of the halogen element include fluorine, chlorine, bromine and the like. Here, examples of the alkoxy group include a methoxy group, an ethoxy group, a methoxyethoxy group, a propoxy group, and a phenoxy group, and among these, a methoxy group, an ethoxy group, a methoxyethoxy group, and a phenoxy group are particularly preferable. The hydrogen element in these monovalent substituents is preferably substituted with a halogen element. Preferred examples of the halogen element include fluorine, chlorine, bromine and the like. Examples of the substituent substituted with a fluorine atom include Examples thereof include a trifluoroethoxy group.

By appropriately selecting R ^{1 to} R ⁹ , Y ^{1 to} Y ³ , Y ^{5 to} Y ⁹ , and Z in the formulas (I) to (V), it has more suitable viscosity, solubility suitable for addition / mixing, and the like. The electrolytic solution can be prepared. These phosphazene compounds may be used alone or in combination of two or more.

Among the phosphazene compounds of the above formula (II), from the viewpoint of lowering the viscosity of the electrolytic solution to improve the low temperature characteristics of the capacitor and further improving the deterioration resistance and safety of the electrolytic solution, it is represented by the following formula (VI). Preferred are phosphazene compounds.

(NPF ₂ ) _n ... (VI)
(In the formula, n represents 3 to 13.)

  The phosphazene compound represented by the formula (VI) is a low-viscosity liquid at room temperature (25 ° C.) and has a freezing point lowering action. For this reason, by adding the phosphazene compound of the formula (VI) to the electrolytic solution, it becomes possible to impart excellent low temperature characteristics to the electrolytic solution, and the viscosity of the electrolytic solution is reduced, and the low internal resistance and It becomes possible to provide a non-aqueous electrolyte electric double layer capacitor having high conductivity. For this reason, it is possible to provide a non-aqueous electrolyte electric double layer capacitor that exhibits excellent discharge characteristics over a long period of time even when used under low temperature conditions, particularly in regions and periods where the temperature is low.

  In the formula (VI), n is preferably 3 to 5, more preferably 3 to 4, and particularly preferably 3 in that it can impart excellent low-temperature characteristics to the electrolyte and can reduce the viscosity of the electrolyte. preferable. When the value of n is small, the boiling point is low, and the ignition prevention property at the time of flame contact can be improved. On the other hand, since the boiling point increases as the value of n increases, it can be used stably even at high temperatures. A plurality of phosphazenes can be selected and used in a timely manner in order to obtain the desired performance using the above properties.

  By appropriately selecting the n value in the formula (VI), it is possible to prepare an electrolytic solution having a more suitable viscosity, solubility suitable for mixing, low temperature characteristics, and the like. These phosphazene compounds may be used alone or in combination of two or more.

  The viscosity of the phosphazene compound represented by the formula (VI) is not particularly limited as long as it is 20 mPa · s or less, but is preferably 10 mPa · s or less from the viewpoint of improving conductivity and improving low-temperature characteristics, and 5 mPa · s. -S or less is more preferable.

Among the phosphazene compounds of the above formula (II), the phosphazene compounds represented by the following formula (VII) are preferable from the viewpoint of improving the deterioration resistance and safety of the electrolytic solution.

(NPR ¹⁰ ₂ ) _n ... (VII)
(In the formula, each R ¹⁰ independently represents a monovalent substituent or fluorine, at least one of all R ¹⁰ is a monovalent substituent or fluorine containing fluorine, and n represents 3 to 8) (However, not all R ¹⁰ are fluorine.)

If the phosphazene compound of the above formula (II) is contained, the electrolyte solution can be given excellent self-extinguishing properties or flame retardancy, and the safety of the electrolyte solution can be improved, but is represented by the formula (VII), If at least one of all R ¹⁰ contains a phosphazene compound which is a monovalent substituent containing fluorine, it is possible to impart superior safety to the electrolytic solution. Furthermore, if a phosphazene compound represented by the formula (VII) and at least one of all R ¹⁰ is fluorine is contained, further excellent safety can be imparted. That is, as compared with a phosphazene compound not containing fluorine, a phosphazene compound represented by the formula (VII), in which at least one of all R ¹⁰ is a monovalent substituent containing fluorine or fluorine, makes the electrolyte more difficult to burn. There is an effect, and it is possible to give further excellent safety to the electrolytic solution.

  Examples of the monovalent substituent in the formula (VII) include an alkoxy group, an alkyl group, an acyl group, an aryl group, and a carboxyl group, and an alkoxy group is preferable because it is particularly excellent in improving the safety of the electrolytic solution. is there. Here, examples of the alkoxy group include a methoxy group, an ethoxy group, an n-propoxy group, an i-propoxy group, a butoxy group, an alkoxy group-substituted alkoxy group such as a methoxyethoxy group, and the like. A methoxy group, an ethoxy group, and an n-propoxy group are particularly preferable in terms of excellent improvement in the above. Moreover, a methoxy group is preferable in terms of reducing the viscosity of the electrolytic solution.

  In the formula (VII), n is preferably 3 to 5 and more preferably 3 to 4 in that it can impart excellent safety to the electrolytic solution.

The monovalent substituent is preferably substituted with fluorine, and when at least one R ^{10 in} formula (VII) is not fluorine, at least one monovalent substituent contains fluorine.

  The fluorine content in the phosphazene compound of the formula (VII) is preferably 3 to 70% by mass, more preferably 7 to 45% by mass. If the fluorine content is 3 to 70% by mass, “excellent safety” can be particularly suitably imparted to the electrolytic solution.

  The phosphazene compound of the formula (VII) may contain a halogen element such as chlorine and bromine in addition to the above-mentioned fluorine. However, fluorine is most preferred, followed by chlorine. Those containing fluorine tend to have a greater effect of improving the cycle characteristics of the capacitor than those containing chlorine.

By appropriately selecting R ¹⁰ and n value in the formula (VII), it is possible to prepare an electrolytic solution having more suitable safety, viscosity, solubility suitable for mixing, and the like. These phosphazene compounds may be used alone or in combination of two or more.

  The viscosity of the phosphazene compound of the formula (VII) is not particularly limited as long as it is 20 mPa · s or less, but is preferably 10 mPa · s or less, and 5 mPa · s or less from the viewpoint of improving conductivity and improving low-temperature characteristics. Is more preferable.

Among the phosphazene compounds of the above formula (II), from the viewpoint of improving the deterioration resistance and safety of the electrolytic solution while suppressing the increase in the viscosity of the electrolytic solution, it is solid at 25 ° C. (room temperature) and has the following formula: A phosphazene compound represented by (VIII) is also preferred.

(NPR ¹¹ ₂ ) _n ... (VIII)
(In the formula, each R ¹¹ independently represents a monovalent substituent or a halogen element; n represents 3 to 6)

  Since the phosphazene compound represented by the formula (VIII) is solid at room temperature, when added to the electrolyte, it dissolves in the electrolyte and increases the viscosity of the electrolyte. However, if the addition amount is a predetermined amount, the rate of increase in the viscosity of the electrolyte is low, and a non-aqueous electrolyte electric double layer capacitor having low internal resistance and high conductivity is obtained. Further, since the phosphazene compound of the formula (VIII) is dissolved in the electrolytic solution, the long-term stability of the electrolytic solution is excellent.

In the formula (VIII), R ¹¹ is not particularly limited as long as it is a monovalent substituent or a halogen element. Here, preferred examples of the monovalent substituent include an alkoxy group, an alkyl group, a carboxyl group, an acyl group, and an aryl group, and preferred examples of the halogen element include fluorine, chlorine, bromine, and iodine. . Among these, an alkoxy group is preferable in that an increase in the viscosity of the electrolytic solution can be suppressed. The alkoxy group is preferably a methoxy group, an ethoxy group, a methoxyethoxy group, a propoxy group (i-propoxy group, n-propoxy group), a phenoxy group, a trifluoroethoxy group, or the like, and can suppress an increase in the viscosity of the electrolytic solution. In this respect, a methoxy group, an ethoxy group, a propoxy group (i-propoxy group, n-propoxy group), a phenoxy group, a trifluoroethoxy group, and the like are more preferable. The monovalent substituent preferably contains the aforementioned halogen element.

  In the formula (VIII), n is particularly preferably 3 or 4 from the viewpoint of suppressing an increase in the viscosity of the electrolytic solution.

The phosphazene compounds of the formula (VIII), the structure R ¹¹ in formula (VIII) is an n-a methoxy group 3, be at least one R ¹¹ is a methoxy group and phenoxy group in formula (VIII) a structure in which n is 4, a structure in which R ¹¹ is an ethoxy group and n is 4 in formula (VIII), and a structure in which R ¹¹ is an i-propoxy group and n is 3 or 4 in formula (VIII) A structure in which R ¹¹ is an n-propoxy group and n is 4 in formula (VIII), a structure in which R ¹¹ is a trifluoroethoxy group and n is 3 or 4 in formula (VIII), ), A structure in which R ¹¹ is a phenoxy group and n is 3 or 4 is particularly preferable in that an increase in the viscosity of the electrolytic solution can be suppressed.

  By appropriately selecting each substituent and n value in the formula (VIII), it is possible to prepare an electrolytic solution having a more suitable viscosity, solubility suitable for mixing, and the like. These phosphazene compounds may be used alone or in combination of two or more.

［式(IX)及び(X)において、Ｒ¹²、Ｒ¹³及びＲ¹⁴は、夫々独立して一価の置換基又はハロゲン元素を表し；Ｘ²は、炭素、ケイ素、ゲルマニウム、スズ、窒素、リン、ヒ素、アンチモン、ビスマス、酸素、硫黄、セレン、テルル及びポロニウムからなる群より選ばれる元素の少なくとも１種を含む置換基を表し；Ｙ¹²及びＹ¹³は、夫々独立して２価の連結基、２価の元素又は単結合を表す。］ Specific examples of the isomers of the phosphazene compound include compounds represented by the following formula (IX). The compound of formula (IX) is an isomer of a phosphazene compound represented by the following formula (X).

[In the formulas (IX) and (X), R ¹² , R ¹³ and R ¹⁴ each independently represents a monovalent substituent or a halogen element; X ² represents carbon, silicon, germanium, tin, nitrogen, Represents a substituent containing at least one element selected from the group consisting of phosphorus, arsenic, antimony, bismuth, oxygen, sulfur, selenium, tellurium and polonium; Y ¹² and Y ¹³ each independently represent a divalent linkage Represents a divalent element or a single bond. ]

R ¹² , R ¹³ and R ¹⁴ in formula (IX) are not particularly limited as long as they are monovalent substituents or halogen elements, and are the same as those described for R ^{1 to} R ³ in formula (I) above. Suitable examples include monovalent substituents and halogen elements. In the formula (IX), the divalent linking group or divalent element represented by Y ¹² and Y ¹³ may be the same divalent as described for Y ^{1 to} Y ³ in the formula (I). Suitable examples include a linking group or a divalent element. Further, in the formula (IX), as the substituent represented by X ² , any of the same substituents as those described for X ¹ in the formula (I) can be preferably exemplified.

  The isomer of the phosphazene compound represented by the formula (IX) and represented by the formula (X) can exhibit extremely low temperature characteristics in the electrolyte when added to the electrolyte. Deterioration resistance and safety can be improved.

  The isomer represented by the formula (IX) is an isomer of the phosphazene compound represented by the formula (X), for example, the degree of vacuum when producing the phosphazene compound represented by the formula (X) and / or It can be manufactured by adjusting the temperature. The content (volume%) of the isomer of the phosphazene compound can be measured by gel permeation chromatography (GPC) or high performance liquid chromatography (HPLC).

  Specific examples of the phosphate ester include alkyl phosphates such as triphenyl phosphate, tricresyl phosphate, tris (fluoroethyl) phosphate, tris (trifluoroneopentyl) phosphate, alkoxy phosphate, and derivatives thereof.

  In the non-aqueous electrolyte for an electric double layer capacitor of the present invention, the content of the compound containing phosphorus and / or nitrogen in the molecule is preferably 3% by volume or more from the viewpoint of improving the safety of the electrolyte, A volume% or more is more preferable.

<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. .

  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.

(Example 1)
90% by volume of propylene carbonate (PC, boiling point 242 ° C.) and additive A [in the formula (II), n is 3, 3 out of 6 R ⁴ are methoxy group (CH ₃ O—), 3 are fluorine A mixed solution consisting of 10% by volume of a cyclic phosphazene compound having a viscosity at 25 ° C. of 3.9 mPa · s and a boiling point of 230 ° C. is prepared, and tetraethylammonium tetrafluoroborate [TEATFB, (C ₂ H ₅ ) _{4 is} added to the mixed solution. N · BF ₄ ] (supporting salt) was dissolved at a concentration of 1 mol / L (M) to prepare a non-aqueous electrolyte. Moreover, the safety | security of the obtained non-aqueous electrolyte was evaluated by the following method. The results are shown in Table 1.

(1) Safety of electrolyte solution The safety of the non-aqueous electrolyte solution was evaluated from the combustion behavior of flames ignited in an atmospheric environment by the method of arranging the UL94HB method of UL (Underwriting Laboratory) standard. At that time, ignitability, combustibility, formation of carbides, and secondary ignition phenomena were also observed. Specifically, based on the UL test standard, a non-combustible quartz fiber was impregnated with 1.0 mL of the electrolytic solution, and a test piece of 127 mm × 12.7 mm was produced. Here, when the test flame does not ignite the test piece (combustion length: 0 mm), it is “non-flammable”, and when the ignited flame does not reach the 25 mm line and the fallen object is not ignited, “flame retardant” The case where the ignited flame was extinguished on the 25 to 100 mm line and the fallen object was not ignited was evaluated as “self-extinguishing”, and the case where the ignited flame exceeded the 100 mm line was evaluated as “combustible”.

Next, 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 is assembled using the above 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). Dried. The cell was impregnated with the above non-aqueous electrolyte to produce a non-aqueous electrolyte electric double layer capacitor. A heating test was performed on the obtained electric double layer capacitor by the following method. The results are shown in Table 1.

(2) Heating test Place a fully charged electric double layer capacitor in the oven, heat to 160 ° C at a rate of 5 ± 2 ° C / min, hold at 160 ° C for 60 minutes, and check if the capacitor ignites. Observed.

(Examples 2-6 and Comparative Examples 1-6)
A mixed solution having the composition shown in Table 1 was prepared, and tetraethylammonium tetrafluoroborate [TEATFB, (C ₂ H ₅ ) ₄ N · BF ₄ ] (supporting salt) was added to the mixed solution at a concentration of 1 mol / L (M). A non-aqueous electrolyte was prepared by dissolution. The safety of the obtained nonaqueous electrolytic solution was evaluated in the same manner as in Example 1. A non-aqueous electrolyte electric double layer capacitor was produced using the non-aqueous electrolyte in the same manner as in Example 1, and a heating test was performed on the capacitor. The results are shown in Table 1.

In Table 1, GBL represents γ-butyrolactone (boiling point 204 ° C.), and AN represents acetonitrile (boiling point 82 ° C.). Additive B is a cyclic phosphazene compound (25 ° C.) in which n is 4 in formula (II), 2 out of 8 R ⁴ are ethoxy groups (CH ₃ CH ₂ O—) and 6 are fluorine. In the formula (II), n is 3, and two of the six R ⁴ are ethoxy groups (CH ₃ CH ₂ O—). ) A cyclic phosphazene compound in which four are fluorine (viscosity at 25 ° C .: 1.2 mPa · s, boiling point 195 ° C.); additive D is a compound of formula (II) where n is 3 and 6 R ⁴ A cyclic phosphazene compound (viscosity at 25 ° C .: 1.7 mPa · s, boiling point 195 ° C.), one of which is a phenoxy group (PhO—) and five of which is fluorine; a 3, contact one of chlorine of the six R ^4, 5 one is cyclic phosphazene compound which is fluorine (in 25 ° C. That viscosity: 0.8 mPa · s, there at the boiling point 82 ° C.); additives F, in formula (II), A n is 4, cyclic phosphazene compounds all eight R ⁴ is fluorine (25 ° C. In the formula (II), n is 3 and 3 out of 6 R ⁴ are ethoxy groups (CH ₃ CH ₂ O—). ) Three cyclic phosphazene compounds (viscosity at 25 ° C .: 4.0 mPa · s, boiling point over 300 ° C.), which is fluorine.

  The non-aqueous electrolyte of the example added with the phosphazene compound having a boiling point close to that of the aprotic organic solvent is high in safety, and the non-aqueous electrolyte electric double layer capacitor of the example using the non-aqueous electrolyte is heated. The test did not ignite, and it was confirmed that the safety was high even in an emergency.

  On the other hand, the electric double layer capacitors of Comparative Examples 2, 4 and 6 that did not contain phosphorus and / or nitrogen-containing compounds and used a combustible non-aqueous electrolyte ignited in the heating test. Further, the capacitor of Comparative Example 1 using a non-aqueous electrolyte containing no phosphazene compound having a boiling point close to PC and that does not contain phosphorus and / or nitrogen having a boiling point close to PC, phosphorus having a boiling point close to GBL, and / or Capacitor of Comparative Example 3 using a non-aqueous electrolyte containing a phosphazene compound that does not contain a nitrogen-containing compound and does not have a boiling point close to GBL, does not contain phosphorus and / or a nitrogen-containing compound that has a boiling point close to AN, and AN and boiling point The capacitor of Comparative Example 5 using a non-aqueous electrolyte containing a phosphazene compound that was not close ignited in a heating test although the electrolyte was highly safe.

  From the above results, the safety of the non-aqueous electrolyte can be improved by adding a compound having a boiling point close to that of the aprotic organic solvent constituting the non-aqueous electrolyte and having phosphorus and / or nitrogen in the molecule. It can also be seen that the safety of the capacitor in an emergency can be remarkably improved by using the non-aqueous electrolyte in the electric double layer capacitor.

Claims (5)

In a non-aqueous electrolyte for an electric double layer capacitor comprising at least one aprotic organic solvent and a supporting salt,
Furthermore, each of the aprotic organic solvents contains a compound having a difference in boiling point from the aprotic organic solvent of 25 ° C. or less and having phosphorus and / or nitrogen in the molecule. Non-aqueous electrolyte for electric double layer capacitors.   The non-aqueous electrolyte for an electric double layer capacitor according to claim 1, wherein the compound having phosphorus and / or nitrogen in the molecule has a phosphorus-nitrogen double bond.   The non-aqueous electrolyte for an electric double layer capacitor according to claim 2, wherein the compound having phosphorus and / or nitrogen in the molecule is a phosphazene compound.   2. The non-aqueous electrolyte for an electric double layer capacitor according to claim 1, wherein the aprotic organic solvent is at least one selected from the group consisting of propylene carbonate, γ-butyrolactone, and acetonitrile.   A non-aqueous electrolyte electric double layer capacitor comprising the non-aqueous electrolyte according to claim 1, a positive electrode, and a negative electrode.