JP2008243832A - Electrolyte for electric double-layer capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide electrolyte for reducing internal resistance of an electric double-layer capacitor. <P>SOLUTION: The electrolyte for electric double-layer capacitor is characterized in containing quarternary ammonium tetrafluoroborate, quarternary ammonium hexafluorophosphate, cyclic carbonate, and chain carbonate. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electrolytic solution for an electric double layer capacitor comprising quaternary ammonium tetrafluoroborate, quaternary ammonium hexafluorophosphate, cyclic carbonate, and chain carbonate.

  In recent years, there is an increasing demand for higher performance of electric double layer capacitors, and it is desired to reduce the internal resistance as much as possible, particularly when used in a low temperature environment such as an automobile.

  Conventionally, as an electrolytic solution for an electric double layer capacitor, an electrolytic solution in which a quaternary ammonium salt such as tetraethylammonium tetrafluoroborate or triethylmethylammonium tetrafluoroborate is dissolved in a solvent such as propylene carbonate or acetonitrile has been used. (Patent Document 1).

However, when propylene carbonate is used as a solvent, improvement has been demanded because of its relatively high internal resistance in a low temperature environment. On the other hand, when acetonitrile is used as a solvent, although performance under a low temperature environment is excellent, a toxic gas is generated in the event of a fire unexpectedly, and there is a problem in safety.
JP 2003-243260 A

  This invention is made | formed in view of the said subject, The objective is to provide the electrolyte solution for electric double layer capacitors excellent in low-temperature performance.

  The inventors of the present application have studied an electrolytic solution for an electric double layer capacitor in order to solve the conventional problems. As a result, the inventors have found that the object can be achieved by employing an electrolytic solution having the following specific composition, and have completed the present invention.

  That is, the electrolytic solution for an electric double layer capacitor of the present invention comprises quaternary ammonium tetrafluoroborate, quaternary ammonium hexafluorophosphate, cyclic carbonate, and chain carbonate in order to solve the above-mentioned problems. And

  In the above structure, it is preferable to use at least one of dimethyl carbonate and ethyl methyl carbonate as the chain carbonate.

    In the above structure, it is preferable to use at least one of ethylene carbonate and propylene carbonate as the cyclic carbonate.

    In the above configuration, the total content of the quaternary ammonium tetrafluoroborate and the quaternary ammonium hexafluorophosphate is preferably 15% by weight or more and 40% by weight or less.

    In the above configuration, the ratio of the content weight of the chain carbonate to the content weight of the cyclic carbonate is preferably 1 or more.

    In the above configuration, the ratio of the content weight of the quaternary ammonium tetrafluoroborate to the content weight of the quaternary ammonium hexafluorophosphate is preferably 1 or more.

  Moreover, it is preferable to use at least one of triethylmethylammonium and ethylmethylpyrrolidinium as the cation of the quaternary ammonium tetrafluoroborate and the quaternary ammonium hexafluorophosphate.

The present invention has the following effects by the means described above.
That is, the electrolytic solution for an electric double layer capacitor of the present invention, characterized by comprising quaternary ammonium tetrafluoroborate, quaternary ammonium hexafluorophosphate, cyclic carbonate, and chain carbonate, has been conventionally used. Compared with an electrolytic solution using only a quaternary ammonium salt as an electrolyte, the internal resistance of the electric double layer capacitor can be reduced. As for the internal resistance of the electric double layer capacitor, it is preferable that the electric conductivity of the electrolytic solution to be used is high. However, the magnitude of the electric conductivity does not necessarily coincide with the level of the internal resistance, and quaternary ammonium tetrafluoroborate. And quaternary ammonium hexafluorophosphate as an electrolyte, and using an electrolytic solution composed of a solvent having a specific composition, the internal resistance can be reduced.

Embodiments of the present invention will be described below.
The electrolytic solution for an electric double layer capacitor of the present invention comprises quaternary ammonium tetrafluoroborate, quaternary ammonium hexafluorophosphate, cyclic carbonate, and chain carbonate.

  Examples of the chain carbonate used in the present invention include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl butyl carbonate, and the like, and preferably dimethyl carbonate and ethyl methyl carbonate. These chain carbonates may be used alone or in admixture of two or more. Moreover, what substituted at least partially the hydrogen of the hydrocarbon group contained in the said chain carbonate molecule with a fluorine can also be used suitably.

  Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and preferably ethylene carbonate and propylene carbonate. These cyclic carbonates may be used alone or in admixture of two or more. Moreover, what substituted at least partially the hydrogen of the hydrocarbon group contained in the said cyclic carbonate molecule with fluorine can also be used suitably.

  In the chain carbonate and cyclic carbonate used in the electrolytic solution for electric double layer capacitor of the present invention, the ratio of the chain carbonate content to the cyclic carbonate content is preferably 1 or more, and 1.1 or more. Is more preferable, and it is especially preferable that it is 1.3 or more. If the ratio is less than 1, the viscosity of the electrolyte increases at low temperatures, which is not preferable. The upper limit of the ratio is not particularly limited as long as the electrolyte does not solidify or precipitate at a low temperature.

  The concentration of the quaternary ammonium tetrafluoroborate and quaternary ammonium hexafluorophosphate used as the electrolyte is preferably 15% by weight to 40% by weight of the total electrolyte weight, % To 35% by weight is more preferable. Particularly preferred is 25 to 30% by weight. If it is less than 15% by weight, the number of ions is insufficient, leading to a reduction in the capacity of the electric double layer capacitor. On the other hand, if it exceeds 40% by weight, the viscosity of the electrolytic solution increases, leading to an increase in the internal resistance of the electric double layer capacitor.

  Various cations can be used for the quaternary ammonium tetrafluoroborate and the quaternary ammonium hexafluorophosphate used in the present invention. Examples of quaternary ammonium cations used in the present invention include tetraalkylammonium cations, imidazolium cations, pyrazolium cations, pyridinium cations, triazolium cations, pyridazinium cations, thiazolium cations, and oxazolium cations. , Pyrimidinium cation, pyrazinium cation and the like, but not limited thereto.

  Tetraalkylammonium cations include tetraethylammonium, tetramethylammonium, tetrapropylammonium, tetrabutylammonium, triethylmethylammonium, trimethylethylammonium, dimethyldiethylammonium, trimethylpropylammonium, trimethylbutylammonium, dimethylethylpropylammonium, methylethylpropyl Butylammonium, N, N-dimethylpyrrolidinium, N-ethyl-N-methylpyrrolidinium, N-methyl-N-propylpyrrolidinium, N-ethyl-N-propylpyrrolidinium, N, N-dimethyl Piperidinium, N-methyl-N-ethylpiperidinium, N-methyl-N-propylpiperidinium, N-ethyl-N-propylpipe Dinium, N, N-dimethylmorpholinium, N-methyl-N-ethylmorpholinium, N-methyl-N-propylmorpholinium, N-ethyl-N-propylmorpholinium, trimethylmethoxymethylammonium, dimethyl Ethylmethoxymethylammonium, dimethylpropylmethoxymethylammonium, dimethylbutylmethoxymethylammonium, diethylmethylmethoxymethylammonium, methylethylpropylmethoxymethylammonium, triethylmethoxymethylammonium, diethylpropylmethoxymethylammonium, diethylbutylmethoxymethylammonium, dipropylmethyl Methoxymethylammonium, dipropylethylmethoxymethylammonium, tripropylmethoxymethylammonium , Tributylmethoxymethylammonium, trimethylethoxymethylammonium, dimethylethylethoxymethylammonium, dimethylpropylethoxymethylammonium, dimethylbutylethoxymethylammonium, diethylmethylethoxymethylammonium, triethylethoxymethylammonium, diethylpropylethoxymethylammonium, diethylbutylethoxymethyl Ammonium, dipropylmethylethoxymethylammonium, dipropylethylethoxymethylammonium, tripropylethoxymethylammonium, tributylethoxymethylammonium, N-methyl-N-methoxymethylpyrrolidinium, N-ethyl-N-methoxymethylpyrrolidinium N-propyl-N-methoxymethyl pyro Lidinium, N-butyl-N-methoxymethylpyrrolidinium, N-methyl-N-ethoxymethylpyrrolidinium, N-methyl-N-propoxymethylpyrrolidinium, N-methyl-N-butoxymethylpyrrolidinium, N-methyl-N-methoxymethylpiperidinium, N-ethyl-N-methoxymethylpyrrolidinium, N-methyl-N-ethoxymethylpyrrolidinium, N-propyl-N-methoxymethylpyrrolidinium, N- Examples thereof include, but are not limited to, methyl-N-propoxymethylpyrrolidinium.

  Examples of the imidazolium cation include 1,3-dimethylimidazolium, 1-ethyl-3-methylimidazolium, 1,3-diethylimidazolium, 3-ethyl-1,2-dimethylimidazolium, 1,2-dimethyl- 3-propylimidazolium, 4-ethyl-1,2,3-trimethylimidazolium, 2,4-diethyl-1,3,4-trimethylimidazolium, 2-ethyl-1,3,4-trimethylimidazolium, 2,3-diethyl-1,4-dimethylimidazolium, 2,3,4-triethyl-1-methylimidazolium, 1,2-diethyl-3,4-dimethylimidazolium, 1,2,4-triethyl- Examples include, but are not limited to, 3-methylimidazolium.

  Examples of the pyrazolium cation include 1,2-dimethylpyrazolium, 1-methyl-2-ethylpyrazolium, 1-propyl-2-methylpyrazolium, 1-methyl-2-butylpyrazolium, and the like. This is not the case. Examples of the pyridinium cation include, but are not limited to, N-methylpyridinium, N-ethylpyridinium, N-propylpyridinium, N-butylpyridinium, and the like. Examples of the triazolium cation include, but are not limited to, 1-methyltriazolium, 1-ethyltriazolium, 1-propyltriazolium, 1-butyltriazolium, and the like. Examples of the pyridazinium cation include, but are not limited to, 1-methylpyridazinium, 1-ethylpyridazinium, 1-propylpyridazinium, 1-butylpyridazinium and the like. Examples of thiazolium cations include, but are not limited to, 1,2-dimethylthiazolium, 1,2-dimethyl-3-propylthiazolium, and the like. Examples of the oxazolium cation include, but are not limited to, 1-ethyl-2-methyloxazolium, 1,3-dimethyloxazolium, and the like. Examples of the pyrimidinium cation include 1,2-dimethylpyrimidinium and 1-methyl-3-propylpyrimidinium, but are not limited thereto. Examples of the pyrazinium cation include, but are not limited to, 1-ethyl-2-methylpyrazinium, 1-butylpyrazinium and the like. These may be used alone or in combination of two or more.

  In the quaternary ammonium tetrafluoroborate or quaternary phosphonium hexafluorophosphate, there is an ionic liquid that is liquid at room temperature. Since the ionic liquid is a liquid at room temperature, has high electrochemical stability, and exhibits good ionic conductivity, it can be suitably used as the electrolyte of the electrolytic solution for the electric double layer capacitor of the present invention.

  The ratio of the quaternary ammonium tetrafluoroborate content to the quaternary ammonium hexafluorophosphate content is preferably 1 or more, more preferably 1.5 or more, and more preferably 2 or more. Particularly preferred. When the ratio is smaller than 1, the effect of reducing the internal resistance of the electric double layer capacitor cannot be sufficiently obtained. On the other hand, the upper limit of the ratio is preferably 20 or less, more preferably 10 or less, and particularly preferably 5 or less. If it exceeds 20, the effect of reducing the internal resistance of the electric double layer capacitor cannot be sufficiently obtained.

  The electrolytic solution for electric double layer capacitors of the present invention can be used for various electric double layer capacitors. The shape is not particularly limited, and examples thereof include, for example, a cylindrical shape, a square shape, a laminate shape, and the like in addition to the coin-type cell described below. As shown in FIG. 1, a coin-type capacitor taken up as an example of an electric double layer capacitor has a positive electrode 1, a separator 3, and a negative electrode 2 in an internal space formed by a positive electrode can 4 and a negative electrode can 5 from the positive electrode can 4 side. , A structure in which a laminated body laminated in the order of the spacers 7 is accommodated. By interposing a spring 8 between the negative electrode can 5 and the spacer 7, the positive electrode 1 and the negative electrode 2 are appropriately pressed and fixed. The organic electrolyte is impregnated between the positive electrode 1, the separator 3 and the negative electrode 2. In a state where the gasket 6 is interposed between the positive electrode can 4 and the negative electrode can 5, the positive electrode can 4 and the negative electrode can 5 are sandwiched to bond them together, and the laminate is sealed.

  The positive electrode 1 and the negative electrode 2 are composed of a current collector and an active material layer.

As the activated carbon, known activated carbon that is usually used can be used and is not particularly limited. Normally, activated carbon is obtained by carbonizing a carbonaceous raw material at a temperature of 900 ° C. or less and then activating treatment. Carbonaceous raw materials include wood, coconut shell, sawdust, coal, pitch, coke, phenol resin, furan resin, polyvinyl chloride resin, polyvinylidene chloride resin, polyimide resin, polyamide resin, polycarbodiimide resin, waste plastic Examples include, but are not limited to, waste tires. The activation method includes a gas activation method in which a carbonized raw material is reacted at a high temperature at 600 ° C. to 1000 ° C. in a gas atmosphere such as water vapor, carbon dioxide gas, oxygen, and the carbonized raw material with zinc chloride, potassium hydroxide, There is a chemical activation method in which chemicals such as sodium hydroxide are mixed and heat-treated in an inert atmosphere, but any method may be used. By the activation treatment, a large number of pores are formed in the carbon material, and the specific surface area is increased. The specific surface area of the activated carbon obtained in this manner is 1000 to 2000 ² / g, is greater by 3000 m ² / g.

  The current collector may be a conductive material that is electrochemically and chemically corrosion resistant. More specifically, a plate or foil of stainless steel, aluminum, titanium, tantalum or the like can be used. Of these, stainless steel or aluminum plates and foils are preferred current collectors in terms of both performance and cost.

  The positive electrode 1 and the negative electrode 2 may be formed by press-molding the above-described activated carbon together with a known conductive aid or binder if necessary, or pyrrolidone together with a known conductive aid or binder if necessary. It can be obtained by mixing a paste in a solvent and applying the paste to a current collector such as an aluminum foil, followed by drying.

  Examples of the conductive assistant include, but are not limited to, graphite, carbon black, needle coke and the like. These conductive assistants may be used alone or in combination of two or more.

  The amount of the conductive auxiliary agent contained in the active material layer is usually preferably 0.01% by weight to 20% by weight, more preferably 0.1% by weight to 15% by weight, and particularly preferably 1% by weight to 10% by weight. preferable. If the amount of the conductive additive is less than 0.01% by weight, the conductivity may be insufficient. On the other hand, if it exceeds 20% by weight, the battery capacity may be reduced.

  The binder is not particularly limited as long as it is a material that is stable with respect to the nonaqueous solvent used in the electrolyte and the solvent used during electrode production. Specific examples include polyethylene, polypropylene, polyethylene terephthalate, and cellulose. , Styrene-butadiene rubber, isoprene rubber, butadiene rubber, fluorine rubber, acrylonitrile-butadiene rubber, vinyl acetate copolymer, polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, polytetrafluoroethylene, alkali metal ion conductivity Include a functional polymer. These solvents can be used alone or in combination of two or more.

  The amount of the binder added in the active material layer is usually preferably 0.1% by weight to 30% by weight, more preferably 1% by weight to 20% by weight, and particularly preferably 5% by weight to 10% by weight. When the added amount of the binder is less than 0.1% by weight, the mechanical strength of the active material layer is insufficient, and the performance as a device may be deteriorated. On the other hand, if it exceeds 30% by weight, the capacity may be insufficient or the electrical resistance may be increased.

  The solvent for forming the paste is not particularly limited as long as it can dissolve or disperse the active material, the binder and the conductive additive and can be easily removed by subsequent drying. Examples include water, alcohol, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, tetrahydrofuran, toluene, acetone, dimethyl ether, dimethyl sulfoxide, benzene, xylene, hexane, and the like. In addition, these solvents are used individually by 1 type or in mixture of 2 or more types.

  In order to prevent a short circuit between the positive electrode and the negative electrode, a separator 3 is usually interposed between the positive electrode and the negative electrode. The material and shape of the separator 3 are not particularly limited, but it is preferable that the above-described organic electrolytic solution easily pass therethrough and is an insulator and a chemically stable material. Examples thereof include microporous films, sheets and nonwoven fabrics made of various polymer materials. Specific examples of the polymer material include polyolefin polymers such as nylon (registered trademark), nitrocellulose, polyacrylonitrile, polyvinylidene fluoride, polyethylene, and polypropylene. From the viewpoints of electrochemical stability and chemical stability, polyolefin polymers are preferred.

  Hereinafter, preferred embodiments of the present invention will be described in detail by way of example. However, the materials, blending amounts, and the like described in the examples are not intended to limit the scope of the present invention only to them, but are merely illustrative examples, unless otherwise specified.

Example 1
Triethylmethylammonium tetrafluoroborate (TEMA.BF4), triethylmethylammonium hexafluorophosphate (TEMA.PF6), ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) (EC, EMC, DMC: Kishida Chemical Co., Ltd., lithium battery grade) was mixed with the composition shown in Table 1 in an argon atmosphere dry box with a dew point of −60 ° C. or lower. The water content of the solution was measured with a Karl Fischer moisture meter (Hiranuma Sangyo Co., Ltd., Hiranuma trace moisture measuring device AQ-7) and confirmed to be 30 ppm or less.

(Example 2)
Ethylmethylpyrrolidinium tetrafluoroborate (EMP · BF4), ethylmethylpyrrolidinium hexafluorophosphate (EMP · PF6), ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) (EC, EMC DMC: manufactured by Kishida Chemical Co., Ltd., lithium battery grade) was mixed in an argon atmosphere dry box having a dew point of −60 ° C. or less with the composition shown in Table 1. The water content of the solution was measured with a Karl Fischer moisture meter (Hiranuma Sangyo Co., Ltd., Hiranuma trace moisture measuring device AQ-7) and confirmed to be 30 ppm or less.

(Comparative Example 1)
Triethylmethylammonium tetrafluoroborate, propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC) (PC, EC, DMC: manufactured by Kishida Chemical Co., Ltd., lithium battery grade) with a dew point of −60 ° C. or less The compositions shown in Table 1 were mixed in an argon atmosphere dry box. The water content of the solution was measured with a Karl Fischer moisture meter (Hiranuma Sangyo Co., Ltd., Hiranuma trace moisture measuring device AQ-7) and confirmed to be 30 ppm or less.

<Evaluation of electrolyte>
An electric double layer capacitor was prepared as follows, and the electrolytic solution was evaluated.
First, with respect to 100 parts by weight of activated carbon whose specific surface area was confirmed to be 1700 m ² / g by the BET method, polyvinylidene fluoride powder as a binder (manufactured by Kureha Chemical Industry Co., Ltd., KF polymer # 1100) After 2.5 parts by weight and Denka black (manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive aid were mixed at a ratio of 5.5 parts by weight, N-methylpyrrolidone was added and kneaded to obtain an electrode paste. This paste was applied to one side of an aluminum foil having a thickness of 38 μm, which is an electrode current collector, with a uniform thickness using an applicator for electrode coating (manufactured by Tester Sangyo Co., Ltd.). Subsequently, after vacuum drying for 2 hours under heating at 130 ° C., the total thickness of the active material layer was adjusted to 82 μm by a roll press to produce a sheet-like electrode. The obtained sheet-like electrode was cut into a circular shape to obtain a positive electrode and a negative electrode.

  A power storage element was created using a test cell having a structure shown in FIG. 2 (Tomcell TJ-AC, manufactured by Nippon Tomcell Co., Ltd.). In FIG. 2, the bolt 9, the caulking washer 10, the container 11, the lid 14, the nut 16, the round plate 20, and the spring 21 are all made of stainless steel, and the separator 18 is made of polypropylene and cut into a circular shape. used. The assembly of the electricity storage element was performed in an argon dry box having a dew point of −60 ° C. or less. First, the bolt 9, the caulking washer 10, the container 11, the spacer 12, the O ring 13, the lid 14, the bush 15, the nut 16, the electrode 17, the round plate 20, and the spring 21 are vacuum-dried for 24 hours under heating at 120 ° C. After that, it was brought into the dry box. The separator 18 was vacuum dried under heating at 60 ° C. for 24 hours, and the circularly cut electrode sheet was vacuum dried at 130 ° C. for 12 hours and then brought into a dry box.

  Next, the electrode and separator 18 were impregnated under reduced pressure with the electrolyte solutions prepared in the examples and comparative examples, and then the positive electrode 17, the separator 18 and the spacer 12 were mounted on the container body 11 in the order shown in FIG. Then, an appropriate amount of the electrolyte was poured. Subsequently, after the negative electrode 19, the round plate 20 and the spring 21 were installed in the order shown in FIG. 1, the lid 14 was covered from above, and the container body was sealed with the bolt 9 and the nut 16 to produce an electric double layer capacitor. .

<Evaluation of characteristics of storage element>
With respect to the electric double layer capacitors using the electrolytic solutions prepared in the above Examples and Comparative Examples, charge / discharge tests were performed, and the internal resistance was measured. The charge / discharge test was carried out in a thermostatic chamber (Espec Corp., TEMPERATURE CARBINET LU-112) maintained in an atmosphere of −20 ° C. After holding the test cell in a thermostat for 2 hours, constant current charging with a current density of 1.0 mAcm ⁻² was performed, and switching to constant voltage charging was performed when the voltage reached 2.5V. After holding at 2.5 V for 10 minutes, a constant current discharge of 1.0 mAcm ⁻² was performed. When the voltage reached 0 V, switching to constant voltage discharge was performed, and the voltage was held at 0 V for 10 minutes. This cycle was repeated 10 times, and the iR drop during the discharge at the 10th cycle was measured. Table 1 shows the values of each example based on the iR drop when the electrolytic solution of Comparative Example 1 was used.

  As is clear from Table 1, in the example, compared with the case of Comparative Example 1, iR drop was reduced, and the internal resistance of the electric double layer capacitor was reduced.

It is a cross-sectional schematic diagram which shows the outline of the electric double layer capacitor which concerns on one Embodiment of this invention. It is a cross-sectional schematic diagram which shows the outline of the electric double layer capacitor used in the Example of this invention.

Explanation of symbols

9 Bolt 10 Caulking Washer 11 Container 12 Spacer 13 O-ring 14 Lid 15 Bush 16 Nut 17 Positive 18 Separator 19 Negative 20 Round Plate 21 Spring

Claims (7)

  An electrolytic solution for an electric double layer capacitor comprising quaternary ammonium tetrafluoroborate, quaternary ammonium hexafluorophosphate, cyclic carbonate, and chain carbonate.   The electrolytic solution for an electric double layer capacitor according to claim 1, wherein the chain carbonate is at least one of dimethyl carbonate and ethyl methyl carbonate.   The electrolytic solution for an electric double layer capacitor according to claim 1 or 2, wherein at least one of ethylene carbonate and propylene carbonate is used as the cyclic carbonate.   The total content of the quaternary ammonium tetrafluoroborate and the quaternary ammonium hexafluorophosphate is 15 wt% or more and 40 wt% or less, according to any one of claims 1 to 3. Electrolyte for electric double layer capacitor of description.   5. The electrolytic solution for an electric double layer capacitor according to claim 1, wherein the ratio of the content of the chain carbonate to the content of the cyclic carbonate is 1 or more.   6. The electricity according to claim 1, wherein the ratio of the content of the quaternary ammonium tetrafluoroborate to the content of the quaternary ammonium hexafluorophosphate is 1 or more. Electrolyte for double layer capacitors.   The cation of the quaternary ammonium tetrafluoroborate or the quaternary ammonium hexafluorophosphate uses at least one of triethylmethylammonium and ethylmethylpyrrolidinium, according to any one of claims 1 to 6. Electrolyte for electric double layer capacitors as described in 2.

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