JP2014075540A - Electrochemical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrochemical device arranged so that the expansion of an electrochemical device container, the drop in electrostatic capacitance, and the rise in internal resistance after continuous charging can be suppressed efficiently without worsening an initial property.SOLUTION: An electrochemical device is e.g. an electric double layer capacitor which comprises an electrolytic solution including a nonaqueous solvent and an electrolyte. The nonaqueous solvent preferably includes 20 wt% or more of a lactone compound expressed by the following formula (1) or (2). (In the formula, R-Rare as described in the specification hereof.)

Description

  The present invention relates to an electrochemical device such as an electric double layer capacitor or a secondary battery.

  Electrochemical devices such as electric double layer capacitors and secondary batteries using a non-aqueous electrolyte can increase the withstand voltage because of the high electrolysis voltage of the solvent, and can store a large amount of energy. The water content of the electrolytic solution is strictly controlled, and an electrolytic solution having a water content of several tens of ppm or less is usually used. However, it is difficult to completely remove the moisture mixed in the device because of moisture adsorbed on the pores of the active material of the electrode. The water present in the device reacts with a nonaqueous solvent such as carbonate ester or ester, and the nonaqueous solvent gradually decomposes. Decomposition of the non-aqueous solvent decreases the electrostatic capacity of the electric double layer capacitor and increases the internal resistance. In addition, decomposition of the non-aqueous solvent generates gas, and there is a concern that the pressure inside the container such as an electric double layer capacitor will increase, the container may expand, and may be damaged.

  In order to prevent such deterioration of the non-aqueous electrolyte due to water electrolysis, it has been proposed to use a chain sulfone solvent such as ethyl isopropyl sulfone as the non-aqueous solvent (Patent Document 1). Compared with ester solvents such as propylene carbonate, chain sulfone solvents are less prone to decomposition reactions due to water electrolysis. Therefore, the working voltage can be set high, and the energy density becomes high. However, chain sulfones have disadvantages such as higher viscosity and lower electrolyte solubility than ester solvents. Due to these drawbacks, when a chain sulfone solvent electrolyte is used, there is a problem that the internal resistance of the electric double layer capacitor is increased. Furthermore, when the internal resistance of the electric double layer capacitor is high, energy loss (IR drop) when discharging the electric double layer capacitor increases, and the energy that can be extracted decreases. Therefore, it is necessary to reduce the initial internal resistance of the electric double layer capacitor, and to reduce the capacity, increase the internal resistance, and expand the container of the electric double layer capacitor after continuous charging.

  In order to solve the above-mentioned problem, Patent Literature 2 is cited as a study on a lactone solvent that is a cyclic ester, and the literature is substituted with a hydrocarbon group in order to widen the potential window of the lactone solvent. The use of δ-lactones is disclosed.

JP 2011-44632 A Japanese Patent Laid-Open No. 9-50944

  In order to reduce the resistance of the electric double layer capacitor, it is one of effective means to use a carbonate solvent typified by propylene carbonate (PC) or a lactone solvent typified by γ-butyrolactone (GBL). . However, it has been clarified that when a high electrical load is applied to a cell using these solvents, the solvent is electrolyzed through hydrolysis near the electrodes. On the other hand, as described above, when a sulfone solvent is used, the decomposition of the solvent hardly occurs even when a high load is applied to the cell, but the internal resistance of the cell increases due to the high viscosity. The problem is to effectively suppress the expansion of the container of the electrochemical device after continuous charging, the decrease in capacitance, and the increase in internal resistance without deteriorating the initial characteristics of the electrochemical device.

As a result of intensive studies by the present inventors, the present invention having the following contents was completed.
According to the present invention, the electrolytic solution used in the electrochemical device has the following formula (1) or (2):

A non-aqueous solvent containing a lactone compound represented by the formula (1) and an electrolyte. R _{1 to} R ₃ will be described later. Preferably, the non-aqueous solvent contains 20 wt% or more of the lactone compound represented by the formula (1) or (2). In another preferred embodiment, the electrochemical device is an electric double layer capacitor, and the electric double layer capacitor includes a separator impregnated with the above-described electrolyte, and a positive electrode and a negative electrode positioned so as to sandwich the separator. .

  According to the present invention, γ-butyllactone having a predetermined chain alkyl group bonded to the α-position is contained as a solvent of the electrolytic solution, thereby effectively suppressing hydrolysis of the COO site of the lactone. The effect of effectively suppressing the expansion of the container of the electrochemical device after continuous charging, the decrease in capacitance, and the increase in internal resistance is exhibited without deteriorating the initial characteristics of the electrochemical device.

It is side surface sectional drawing of the electrochemical device of one Embodiment of this invention. It is a top view of the electrochemical device of one embodiment of the present invention.

  Hereinafter, the present invention will be described in detail with appropriate reference to the drawings. However, the present invention is not limited to the illustrated embodiment, and in the drawings, the characteristic portions of the invention may be emphasized and expressed, so that the accuracy of the scale is not necessarily guaranteed in each part of the drawings. Not.

  The electrochemical device of the present invention contains a specific lactone compound as a non-aqueous solvent. The specific lactone compound is a compound in which one or two alkyl groups are bonded to the α-position of γ-butyrolactone (GBL), and specifically, a compound represented by the above formula (1) or (2) It is.

Here, R ₁ and R ₂ in the compound of the formula (1) (hereinafter also referred to as the compound (1)) may be the same or different from each other, each having 1 to 6 carbon atoms, Preferably a C1-C3 linear alkyl group is shown. A part of carbon atoms in the linear alkyl group may be replaced with a hetero atom. The hetero atom is preferably an oxygen atom. More specifically, a methylene group that forms part of a linear alkyl structure may be replaced by -O-. Specific examples of the compound (1) include, but are not limited to, compounds of the following formulas (3) to (7) (hereinafter also referred to as compounds (3) to (7)).

The compound of the above formula (2) (hereinafter also referred to as compound (2)) is a compound in which only one alkyl group R ₃ is bonded to the α-position of GBL, where R ₃ has 3 to 3 carbon atoms. 7, preferably a branched alkyl group having 3 to 5 carbon atoms. Specific examples of the compound (2) include, but are not limited to, compounds of the following formulas (8) to (11) (hereinafter also referred to as compounds (8) to (11)).

  These compounds (1) and (2) are generally available as industrial chemical materials from, for example, Tokyo Chemical Industry Co., Ltd., as well as generally available γ-butyrolactone and α-methyl-γ-. After the butyrolactone is treated with a base, it can be synthesized by a method such as reacting a corresponding alkyl halide.

  According to this invention, the above-mentioned compound (1) or compound (2) is contained in the nonaqueous solvent in the electrolytic solution. One type of compound (1) or compound (2) may be contained in the nonaqueous solvent, or two or more types of compound (1) or compound (2) may be contained. The non-aqueous solvent may contain a solvent other than the compound (1) or the compound (2). The total amount of compound (1) or compound (2) in the non-aqueous solvent is preferably 15 wt% or more, more preferably 20 wt% or more, still more preferably 50 wt% or more, and particularly preferably 80 wt% or more. The non-aqueous solvent may be composed entirely of the compound (1) or the compound (2) (that is, 100 wt%).

  Examples of the solvent that may be contained in the nonaqueous solvent together with the compound (1) or the compound (2) include, but are not limited to, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate, dimethyl Chain carbonates such as carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, γ-butyrolactone (GBL), γ-valerolactone (GVL), 3-methyl-γ-butyrolactone, 2-methyl- Cyclic esters such as γ-butyrolactone, chain esters such as methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, methyl butyrate, methyl valerate, 1,4-dioxane, 1,3-dioxolane Tetrahydr Furan, 2-methyltetrahydrofuran, 3-methyl-1,3-dioxolane, cyclic ethers such as 2-methyl-1,3-dioxolane, 1,2-dimethoxyethane, 1,2-diethoxyethane, dimethyl-2, 5-dioxahexanedioate, chain ethers such as dipropyl ether, nitrile compounds such as acetonitrile, propanenitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, 3-methoxypropionitrile, and sulfolane, dimethylsulfone, diethylsulfone And sulfur-containing compounds such as ethyl methyl sulfone, ethyl propyl sulfone, ethyl isopropyl sulfone, and dimethyl sulfoxide. These may be used in combination.

According to the present invention, the electrolyte contains an electrolyte in addition to the non-aqueous solvent described above. The concentration of the electrolyte in the electrolytic solution is not particularly limited, and is preferably 1.0 to 2.5 mol / L. The type of the electrolyte is not particularly limited, and a compound or the like usually used in an electrochemical device can be used as appropriate. Examples of the electrolyte include quaternary ammonium salts, quaternary phosphonium salts, and lithium compounds. These electrolytes can be used alone or in combination of two or more. Examples of quaternary ammonium salts and quaternary phosphonium salts include tetraethylammonium tetrafluoroborate, tetrabutylammonium tetrafluoroborate, triethylmethylammonium tetrafluoroborate, 1,1′-spirobipyrrolidinium tetrafluoroborate, tetraethylammonium hexafluoro. Examples include phosphate, tetrabutylammonium hexafluorophosphate, triethylmethylammonium hexafluorophosphate, tetraethylphosphonium tetrafluoroborate, tetrabutylphosphonium tetrafluoroborate, tetraethylphosphonium hexafluorophosphate, tetrabutylphosphonium hexafluorophosphate, and the like. Lithium compounds include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiB (C ₂ O ₄ ) _{2 and the} like. Among the above electrolytes, tetraethylammonium tetrafluoroborate, triethylmethylammonium tetrafluoroborate, and 1,1′-spirobipyrrolidinium tetrafluoroborate are preferable.

  The electrochemical device of the present invention is not particularly limited as long as it is a device that utilizes the above-described electrochemical reaction of the electrolytic solution, and includes an electrochemical capacitor, a secondary battery, and the like. Examples include an electric double layer capacitor, a lithium ion capacitor, a redox capacitor, and a hybrid capacitor.

  FIG. 1 is a side sectional view of an electric double layer capacitor as an example of an electrochemical device. FIG. 2 is a plan view thereof. The electric double layer capacitor includes, for example, a positive electrode 10, a negative electrode 20, a storage element B having a separator 30 between the positive electrode 10 and the negative electrode 20, a nonaqueous electrolytic solution in which an electrolyte is dissolved in a nonaqueous solvent, and a laminate film. It is formed. The electric double layer capacitor has a terminal 50 whose one end is connected to the storage element B and the other end is led out from the film package 40. The film package 40 encloses the storage element B and a non-aqueous electrolyte. Polarizable electrode layers 12 and 22 are formed on the surfaces of current collectors 11 and 21 made of metal foil via a conductive adhesive (not shown), respectively. For example, the positive electrode 10 and the negative electrode 20 are disposed so that the polarizable electrode layer 12 of the positive electrode 10 and the polarizable electrode layer 22 of the negative electrode 20 face each other.

  The separator is formed from a material that can be impregnated with a non-aqueous electrolyte, such as cellulose, polypropylene, polyethylene, and fluorine resin. The separator 30 is disposed between the polarizable electrode layers 12 and 22 of the positive electrode 10 and the negative electrode 20 facing each other. A known structure used in a film package type electric double layer capacitor can be applied to the storage element B and the film package 40.

  As the polarizable electrode layers 12 and 22, those having a known material and structure used in the polarizable electrode layer of the electric double layer capacitor can be used. The polarizable electrode layers 12 and 22 include active materials such as polyacene (PAS), polyaniline (PAN), activated carbon, carbon black, graphite, and carbon nanotube, and are used for the polarizable electrode layer of the electric double layer capacitor. Other components such as a conductive agent and a binder may be included as necessary. Examples of the activated carbon material include sawdust, coconut shell, phenol resin, various heat resistant resins, and pitch. Examples of the heat resistant resin include polyimide, polyamide, polyamideimide, polyetherimide, polyethersulfone, polyetherketone, bismaleimide triazine, aramid, fluororesin, polyphenylene, polyphenylene sulfide and the like. These can be used alone or in combination of two or more.

  A lithium ion capacitor as another example of an electrochemical device includes, for example, a positive electrode, a negative electrode, a power storage element having a separator between the positive electrode and the negative electrode, a non-aqueous electrolyte solution in which an electrolyte is dissolved in a non-aqueous solvent, and a laminate. Formed from film. One end of the lithium ion capacitor is connected to the power storage element, and the other end has a terminal that is led out from the film package (the power storage element and the non-aqueous electrolyte are enclosed). For example, a polarizable electrode layer is formed on the surface of a positive electrode current collector made of an aluminum metal foil via a conductive adhesive. As the polarizable electrode layer, one having the same material and structure as those used in the electric double layer capacitor can be used. Moreover, what has the well-known material and structure used with the polarizable electrode layer of the positive electrode of a lithium ion capacitor can be used. An active material layer is formed on the surface of a negative electrode current collector made of, for example, copper metal foil. The positive electrode and the negative electrode are arranged, for example, so that the polarizable electrode layer of the positive electrode and the active material layer of the negative electrode face each other. A lithium metal sheet is disposed in the vicinity of the negative electrode. As a result, lithium in the lithium metal sheet is dissolved in the non-aqueous electrolyte, and the lithium ions are pre-doped in the active material layer of the negative electrode, so that the potential of the negative electrode is 3 V, for example, compared with the potential of the positive electrode before charging. The level is lowered. The separator is formed from a material that can be impregnated with a nonaqueous electrolytic solution, such as cellulose, polypropylene, polyethylene, and fluorine resin. The separator is disposed between the positive polarizable electrode layer and the negative active material layer facing each other. A known structure used in a film package type lithium capacitor can be applied to the power storage element and the film package.

  As the active material layer of the negative electrode, a material having a known material and structure used in the active material layer of a lithium ion capacitor can be used. For example, active materials such as non-graphitizable carbon, graphite, tin oxide, silicon oxide, etc. And a conductive auxiliary such as carbon black and metal powder, and a binder such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and styrene butadiene rubber (SBR) as necessary. .

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the embodiments. Nonaqueous solvent compounds used in each example and comparative example were those obtained from Tokyo Chemical Industry Co., Ltd. or the like, or those appropriately synthesized. The electrolyte triethylmethylammonium tetrafluoroborate was obtained from Tokyo Chemical Industry Co., Ltd.

[Example 1]
An electrolytic solution having a concentration of 1.5 mol / L was prepared using triethylmethylammonium tetrafluoroborate as an electrolyte and the above-mentioned compound (3) as a nonaqueous solvent. A slurry containing activated carbon as an active material and carboxymethyl cellulose and styrene butadiene rubber as a binder was prepared, and this slurry was applied on an aluminum foil to prepare a sheet-like polarizable electrode. A separator made of cellulose was sandwiched between polarizable electrodes, a lead terminal was attached by ultrasonic welding, and vacuum drying was performed at about 180 ° C. to obtain a device. An element was put into a sealing material cut into an electrode size, and an electrolytic solution was further injected. Then, the sealing material was heat-sealed using the sealing material, and a cell having a size of about 20 mm × 26 mm was produced. As the sealing material, a laminate film of nylon / aluminum / CPP (unstretched polypropylene) was used.

[Examples 2 to 9]
An electrochemical capacitor was produced in the same manner as in Example 1 except that the compounds (4) to (11) were used as shown in Table 1 instead of the compound (3) in the production of the electrolytic solution.

[Examples 10 to 15]
An electrochemical capacitor was produced in the same manner as in Example 1 except that the mixed solvent as shown in Table 1 was used instead of the compound (3) in the production of the electrolytic solution. Here, the mixing ratio of Table 1 regarding the mixed solvent is a weight ratio.

[Comparative Examples 1-5]
An electrochemical capacitor was produced in the same manner as in Example 1 except that the compound or mixed solvent shown in Table 1 was used instead of compound (3) in the production of the electrolytic solution. Here, the mixing ratio of Table 1 regarding the mixed solvent is a weight ratio.

[Evaluation method]
For the electrochemical capacitors produced in each Example and Comparative Example, the capacitance, DC resistance, and capacitor container thickness (T1) were measured as initial characteristics. Capacitance was discharged when a cell discharged at room temperature for 30 minutes was charged at 100 mA to 2.5 V for 10 minutes and then discharged at 10 mA to 0 V using a charge / discharge tester (Toyo System Co., Ltd. TOSCAT-3200). Calculated from the slope of the curve. DC resistance is a voltage drop when a charge / discharge tester (TOSCAT-3200 manufactured by Toyo System Co., Ltd.) is used and a cell discharged at room temperature for 30 minutes is charged at 100 mA to 2.5 V for 10 minutes and then discharged at 2 A to 0 V Calculated from The thickness of the capacitor container was measured using a micrometer and the thickness of the central portion of the cell.

Next, the battery was continuously charged at a voltage of 2.5 V for 1000 hours in a constant temperature bath at 60 ° C. Then, after cooling to room temperature and discharging to 0 V, the capacitance, DC resistance, and capacitor container thickness (T2) were measured. For each measured value, the percentage of the measured value after continuous charge / discharge with respect to the measured value (100%) as the initial characteristic was calculated.
The production conditions and results of each example and comparative example are summarized in Table 1.

[Evaluation results]
Compared to Comparative Example 1, in Examples 1 to 9, characteristic deterioration after continuous charging was suppressed. Specifically, it was confirmed that the capacitance was larger and the resistance value was lower than that of Comparative Example 1. It was also confirmed that the thickness of the capacitor cell was small. This effect was confirmed even when a mixed solvent was used, and in Examples 10 to 15 compared to Comparative Examples 2 to 5, the characteristic deterioration after continuous charging was suppressed. The above effects are considered to be based on the action of the substituent introduced into the compounds (3) to (11) described in the present invention effectively suppressing hydrolysis of the COO site of the lactone.

DESCRIPTION OF SYMBOLS 10 Positive electrode, 20 Negative electrode, 11.21 Current collector, 12.22 Polarization electrode layer, 30 Separator, 40 Film package, 50 terminal, B electrical storage element.

Claims (3)

Following formula (1) or (2):

(In the formula, R ₁ and R ₂ are each independently a straight-chain alkyl group having 1 to 6 carbon atoms, in which some of the carbon atoms may be replaced by hetero atoms, and R ₃ has ₃ carbon atoms. An electrochemical device using an electrolytic solution containing a non-aqueous solvent containing a lactone compound represented by (7) and an electrolyte.   The electrochemical device according to claim 1, wherein the non-aqueous solvent contains 20 wt% or more of the lactone compound represented by the formula (1) or (2).   The electrochemical device according to claim 1 or 2, which is an electric double layer capacitor comprising a separator impregnated with the electrolytic solution, and a positive electrode and a negative electrode positioned so as to sandwich the separator.

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