Electrochemical capacitor

Abstract

Disclosed is an electrochemical capacitor (10) comprising a first electrode (2), a second electrode (3), and an electrolytic solution. The electrolytic solution contains a lactone compound and a buffer that exhibits a buffering action in an acidic region of pH 3.0-5.0.

Description

electrochemical capacitor

[0001] The present disclosure relates to electrochemical capacitors.

[0002] Electrochemical capacitors have the advantages of long life and rapid charging, and various proposals have been made for electrochemical capacitors.

[0003] Claim 1 of Patent Document 1 (Japanese Patent No. 5162915) describes a capacitor comprising: "an element constructed by laminating or winding a pair of positive and negative electrodes, each of which has a polarizable electrode layer formed on a collector made of metal foil, with a separator interposed between them, with the electrode layers facing each other; a metal case containing the element together with a driving electrolyte; and a sealing member sealing the opening of the metal case; and ＋ ions and OH － The document discloses a "capacitor containing a weak acid and its salt, which has a buffering effect on the concentration of ions, and the weak acid and its salt are sodium dihydrogen phosphate and disodium hydrogen phosphate."

[0004] Patent No. 5162915

[0005] Currently, there is a demand for electrochemical capacitors that exhibit less degradation. One of the objects of the present disclosure is to provide an electrochemical capacitor that exhibits less degradation.

[0006] One aspect of the present disclosure relates to an electrochemical capacitor including a first electrode, a second electrode, and an electrolyte solution, wherein the electrolyte solution includes a lactone compound and a buffering agent that exhibits a buffering effect in an acidic region having a pH of 3.0 or more and 5.0 or less.

[0007] According to the present disclosure, an electrochemical capacitor with reduced degradation can be obtained. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0008] FIG. 1 is a perspective view schematically illustrating an example of an electrochemical capacitor according to a first embodiment.

[0009] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or greater and numerical value B or less." In the following description, when lower and upper limits for specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not greater than the upper limit. In the following description, when examples of components or methods are listed, only one of the listed examples may be used, or multiple of the listed examples may be used in combination, unless otherwise specified.

[0010] (Electrochemical Capacitor) An example of an electrochemical capacitor according to this embodiment will be described below. The electrochemical capacitor according to this embodiment may be referred to as an "electrochemical capacitor (C)" or a "capacitor (C)."

[0011] The capacitor (C) includes a first electrode, a second electrode, and an electrolyte solution containing a lactone compound and a buffering agent that exhibits a buffering effect in an acidic range having a pH of 3.0 or more and 5.0 or less.

[0012] Currently, there is a need to suppress the deterioration of capacitors (C). As a result of investigations, the present inventors have found that when an electrolyte solution containing a lactone compound is used, the lactone compound may decompose, causing the electrolyte solution to deteriorate. When the electrolyte solution deteriorates, the performance of the capacitor decreases. For example, γ-butyrolactone is hydrolyzed to hydroxybutyric acid in a solution with a pH of 6 or higher. As a result of further investigations, the present inventors have newly discovered that the use of a specific buffer agent can suppress the deterioration of capacitors. The present disclosure is based on this new finding.

[0013] (Buffer Agent) The buffer agent added to the electrolyte solution is a buffer agent that exhibits a buffering effect in the acidic region where the pH is 3.0 or more and 5.0 or less. Hereinafter, such a buffer agent may be referred to as "buffer agent (B)." Examples of acids that constitute buffer agent (B) include acetic acid, citric acid, succinic acid, barbituric acid, and citric acid. Buffer agent (B) is obtained by combining these acids with their salts in a predetermined ratio. From another perspective, buffer agent (B) is an additive that acts to maintain a predetermined pH of 3.0 or more and 5.0 or less. The acid that constitutes the buffer agent is preferably an acid that is easily soluble in a non-aqueous solvent. The solubility of phosphoric acid in a non-aqueous solvent is lower than the solubility of acetic acid in a non-aqueous solvent.

[0014] The pH value maintained by the buffer (B) can be changed by changing the mixing ratio of the acid and the acid salt. An example of preparing a buffer solution maintaining a predetermined pH using acetic acid and sodium acetate is described below. First, an aqueous solution of acetic acid with a concentration of 0.1 mol / L and an aqueous solution of sodium acetate with a concentration of 0.1 mol / L are prepared. Next, they are mixed in the volume ratio shown in Table 1 to obtain a buffer solution (200 mL) maintaining a predetermined pH.

[0015]

[0016] An example of preparing a buffer solution that maintains a predetermined pH using citric acid and sodium citrate is described below. First, a 0.1 mol / L aqueous citric acid solution and a 0.1 mol / L aqueous sodium citrate solution are prepared. Next, they are mixed in the volume ratio shown in Table 2 to obtain a buffer solution (100 mL) that maintains a predetermined pH.

[0017]

[0018] The pH at which the buffering agent (B) exhibits a buffering effect is 3.0 or higher, and may be 4.0 or higher. By maintaining the pH at 3.0 or higher, BF ４ － This can suppress the hydrolysis of BF and the dehydration and carbonization reaction of cellulose. ４ －When the pH of the buffer (B) is 3.0 or higher, the pH of the buffer (B) is preferably 3.0 or higher, so that the deterioration of the non-aqueous electrolyte and the separator can be suppressed.

[0019] The buffer (B) may contain at least one acid selected from the group consisting of acetic acid, citric acid, and succinic acid, and a salt of the at least one acid. The buffer (B) may contain at least one acid selected from the group consisting of acetic acid and citric acid, and a salt of the at least one acid. The buffer (B) may be composed of acetic acid and a salt of acetic acid, citric acid and a salt of citric acid, or succinic acid and a salt of succinic acid. The cation that constitutes the acid salt is not particularly limited. Examples of the cation include sodium ions, calcium ions, potassium ions, ammonium ions, magnesium ions, etc.

[0020] The buffer (B) may be a buffer that acts to maintain a predetermined pH in the range of 3.0 or more and 5.0 or less when an aqueous solution in the range of 100 to 200 mL is prepared.

[0021] The content Cb of the buffer (B) in the electrolyte may be 10 ppm or more (0.001% by mass or more), 20 ppm or more, 40 ppm or more, or 50 ppm or more, based on mass. The content Cb may be 200 ppm or less (0.02% by mass or less), 100 ppm or less, or 60 ppm or less, based on mass. High effects can be obtained by setting the content Cb to 20 ppm or more (e.g., 40 ppm or more, or 50 ppm or more). By setting the content Cb to 200 ppm or less (e.g., 100 ppm or less), adverse effects caused by an excessive amount of buffer (B) (e.g., the need for a long time to remove moisture) can be suppressed. The content Cb may be in the range of 20 ppm to 100 ppm, based on mass.

[0022] (Lactone Compound) Lactone compounds are preferred as solvents for the electrolyte solution because they have low viscosity even at low temperatures. Examples of lactone compounds include β-propiolactone, γ-butyrolactone, δ-valerolactone, ε-caprolactone, γ-valerolactone, and γ-caprolactone. The lactone compound may include γ-butyrolactone. γ-Butyrolactone (GBL) is particularly preferred because it has low viscosity even at low temperatures, a high boiling point, and emits little gas due to side reactions.

[0023] The proportion of the lactone compound (e.g., γ-butyrolactone) in the solvent of the electrolytic solution may be 50% by volume or more, 80% by volume or more, or 90% by volume or more. In order to enhance the effect of adding the buffer (B), it is preferable that the proportion of the lactone compound in the solvent of the electrolytic solution is high. The solvent of the electrolytic solution may be solely a lactone compound. For example, the solvent of the electrolytic solution may be γ-butyrolactone. Examples of solvents other than lactone compounds will be described later.

[0024] The components other than the essential components of the capacitor (C) are not particularly limited, and components used in known electrochemical capacitors may be applied. Examples of the components of the capacitor (C) are described below. However, the capacitor (C) is not limited to the examples described below.

[0025] (First and Second Electrodes) The first electrode and the second electrode are a pair of electrodes. The first electrode and the second electrode face each other with a separator interposed therebetween. The first electrode and the second electrode may be a positive electrode and a negative electrode, respectively. The configuration of the first electrode and the configuration of the second electrode may be the same or different.

[0026] The first and second electrodes may be polarizable electrodes. The polarizable electrodes may include an active material capable of adsorbing and desorbing ions. The adsorption of ions to the active material generates capacitance. The desorption of ions from the active material causes a non-Faradic current to flow. From one perspective, the electrochemical capacitor (C) is an electric double layer capacitor (EDLC) in which an electric double layer is formed by the adsorption of ions to the active material.

[0027] The polarizable electrode may include a current collector and a polarizable electrode layer disposed on the current collector. The positive electrode may include a positive electrode current collector and a polarizable electrode layer disposed on the positive electrode current collector. The negative electrode may include a negative electrode current collector and a polarizable electrode layer disposed on the negative electrode current collector.

[0028] The polarizable electrode layer contains an active material. The polarizable electrode layer may contain other components (such as a binder or a conductive material) as necessary. Examples of the active material include porous carbon particles (such as activated carbon).

[0029] Commercially available activated carbon may be used. Alternatively, activated carbon may be prepared by a predetermined method (e.g., a known method). Activated carbon may be prepared by heat treating a raw material to convert it into a carbonized product, and then activating the carbonized product to make it porous. Examples of raw materials include wood, coconut shells, pulp waste liquor, coal-based pitch, petroleum-based pitch, phenolic resin, petroleum coke, and coal coke.

[0030] The activation treatment may be gas activation using a gas (such as water vapor). Alternatively, the activation treatment may be zinc chloride (ZnCl ２ The activated carbon obtained by the activation treatment may be subjected to a pulverization treatment. After the pulverization treatment, classification may be performed. The pulverization treatment may be performed using a ball mill, a jet mill, or the like. After the pulverization treatment, the activated carbon may be heat-treated at a high temperature to remove functional groups on the surface of the activated carbon.

[0031] Examples of binders include polymers such as polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), etc. Examples of conductive materials include carbon black (acetylene black, ketjen black, etc.).

[0032] The current collector may be a conductive sheet or a metal foil (e.g., aluminum foil). For example, a metal foil such as aluminum foil is used. The surface of the current collector may be roughened by etching or other methods.

[0033] The method for forming the electrode is not particularly limited. The electrode may be formed by the following method. First, a slurry is prepared by mixing activated carbon, a binder and / or a conductive material, and a dispersion medium. Next, the slurry is applied to the surface of a current collector and dried to form a laminate of the current collector and the coating film (polarizable electrode layer). Next, the laminate is rolled as necessary. In this manner, an electrode including a current collector and a polarizable electrode layer disposed on the current collector is obtained.

[0034] (Electrolyte) The electrolyte contains a solvent (non-aqueous solvent), an ionic substance, and a buffer (B). The ionic substance contains a cation and an anion. The electrolyte can be prepared by dissolving the ionic substance in a solvent. The concentration of the ionic substance in the electrolyte may be 0.5 mol / L or more and may be 2.0 mol / L or more. The cation contained in the electrolyte may be one type or multiple types. The anion contained in the electrolyte may be one type or multiple types.

[0035] The solvent contains a lactone compound and may further contain other solvents as necessary. Examples of the solvent other than the lactone compound include cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate, polyhydric alcohols such as ethylene glycol and propylene glycol, cyclic sulfones such as sulfolane, amides such as N-methylacetamide, N,N-dimethylformamide, and N-methyl-2-pyrrolidone, ethers such as 1,4-dioxane, ketones such as methyl ethyl ketone, and formaldehyde.

[0036] Cyclic carbonates (e.g., propylene carbonate) decompose in a solution with a pH of less than 5 or a solution with a pH of more than 6, generating carbon dioxide gas. Therefore, the buffer (B) used in the capacitor (C) cannot sufficiently suppress the decomposition of the cyclic carbonate. Therefore, it is preferable that the proportion of the cyclic carbonate in the electrolyte solution of the capacitor (C) is low. The proportion of the cyclic carbonate in the electrolyte solution of the capacitor (C) may be less than 50% by mass, 30% by mass or less, or 10% by mass or less. A preferred example of the electrolyte solution of the capacitor (C) does not contain a cyclic carbonate.

[0037] As a cation constituting an ionic substance, NR is preferred because it has high voltage resistance and high solubility in aprotic solvents. ４ ＋ Quaternary alkylammonium ions represented by (R is an alkyl group) are preferred. The four alkyl groups R bonded to N may be the same or different. Each of the four alkyl groups R may independently be an alkyl group having 1 to 4 carbon atoms. Each of the alkyl groups R may be a straight-chain alkyl group. Examples of the cation include tetramethylammonium ion, tetraethylammonium ion, diethyldimethylammonium ion, ethyltrimethylammonium ion, and triethylmethylammonium ion. Diethyldimethylammonium ion (N(C ２ H ５ ) ２ (CH ３ ) ２ ＋ ) is generated by decomposing a small amount of water. － This is preferable in that it easily reacts with the above and the pH of the electrolyte can be easily maintained constant.

[0038] Examples of anions that constitute the ionic substance include BF ４ － , P.F. ６ － , AsF ６ － , SbF ６ － , N(FSO ２ ) ２ － (FSI), N(F ３C Ｓ O ２ ) ２ － (TFSI) and others. ４ － and P.F. ６ － Fluorine-containing anions such as the above are preferred in that they improve the withstand voltage characteristics.

[0039] The organic salt may be composed of a quaternary alkyl ammonium cation and a fluorine-containing acid anion. Examples of such organic salts include diethyldimethylammonium tetrafluoroborate (DEDMABF ４ ), triethylmethylammonium tetrafluoroborate (TEMABF ４ ) etc.

[0040] The method for adding the buffer (B) to the electrolyte solution is not particularly limited. In one example of the manufacturing method, first, a buffer solution (aqueous solution) is prepared so as to maintain a predetermined pH. Next, the buffer solution is added to the electrolyte solution (or the non-aqueous solvent of the electrolyte solution), and then the water in the electrolyte solution (or the non-aqueous solvent) is removed. In this way, an electrolyte solution (or non-aqueous solvent) to which the buffer (B) has been added is obtained. When the buffer (B) has been added to the non-aqueous solvent, an ionic substance is further added to the non-aqueous solvent. The method for removing the water in the electrolyte solution (or the non-aqueous solvent) is not particularly limited, and a dehydrating agent may be used, or other methods (e.g., heating) may be used.

[0041] (Separator) A separator is usually disposed between the positive electrode and the negative electrode. The separator has ion permeability and insulating properties. The separator prevents short-circuiting between the first electrode and the second electrode. The separator may be a woven fabric, a nonwoven fabric, or a microporous membrane. Examples of separator materials include polymers and glass. Examples of polymers include polyolefins (such as polyethylene) and cellulose. Examples of separators include nonwoven fabrics made of cellulose fibers, nonwoven fabrics made of glass fibers, and microporous membranes made of polyolefins. The thickness of the separator may be in the range of 8 to 300 μm (e.g., in the range of 8 to 40 μm).

[0042] (Others) A capacitor element is formed by a positive electrode, a negative electrode, and a separator. For example, a wound capacitor element is formed by winding a positive electrode, a negative electrode, and a separator so that the separator is disposed between the positive electrode and the negative electrode. The electrochemical capacitor (C) includes an exterior body that houses a capacitor element and an electrolyte. The exterior body is not particularly limited, and a known exterior body may be used. The exterior body may include an exterior case and a sealing member that seals the opening of the exterior case. The exterior case may be formed of a metal such as aluminum, stainless steel, copper, iron, or brass. The sealing member may be formed of an elastic material such as rubber (butyl rubber). The capacitor (C) may include other members (such as lead wires) as necessary.

[0043] The shape of the capacitor (C) is not particularly limited. The capacitor (C) may be a cylindrical capacitor including a wound capacitor element. The capacitor (C) may be a prismatic capacitor including a stacked capacitor element. Alternatively, the capacitor (C) may be a coin-type capacitor.

[0044] (Method for producing electrochemical capacitor (C)) Except for using an electrolytic solution containing a buffer (B), the method for producing the capacitor (C) is not particularly limited. Except for using an electrolytic solution containing a buffer (B), the capacitor (C) may be produced by a known method.

[0045] An example of a capacitor (C) according to the present disclosure will be described below with reference to the drawings. The components described above can be applied to the components of the example described below. The components of the example described below can be modified based on the above description. The matters described below may also be applied to the above embodiment. In the example described below, components that are not essential for the capacitor (C) according to the present disclosure may be omitted.

[0046] Embodiment 1 An electrochemical capacitor 10 according to embodiment 1 is shown schematically in Fig. 1. Fig. 1 is a perspective view of the electrochemical capacitor 10 with a portion cut away.

[0047] Electrochemical capacitor 10 is an electric double layer capacitor. Electrochemical capacitor 10 includes a wound capacitor element 1. Capacitor element 1 is formed by winding a first electrode (positive electrode) 2, a second electrode (negative electrode) 3, and a separator 4 therebetween. First electrode 2, second electrode 3, and separator 4 are each strip-shaped.

[0048] A lead wire 5a is connected to the first electrode 2. A lead wire 5b is connected to the second electrode 3. The capacitor element 1 is housed in a cylindrical outer case 6 with a bottom together with an electrolyte (not shown). The opening of the outer case 6 is sealed with a sealing member 7. The lead wires 5a and 5b pass through the sealing member 7.

[0049] (Additional Notes) The above description discloses the following technologies. (Technology 1) An electrochemical capacitor including a first electrode, a second electrode, and an electrolyte solution, wherein the electrolyte solution contains a lactone compound and a buffering agent that exhibits a buffering effect in an acidic region having a pH of 3.0 or more and 5.0 or less. (Technology 2) The electrochemical capacitor according to Technology 1, wherein the buffering agent contains at least one acid selected from the group consisting of acetic acid and citric acid, and a salt of the at least one acid. (Technology 3) The electrochemical capacitor according to Technology 1 or 2, wherein the content of the buffering agent in the electrolyte solution is in the range of 20 ppm to 100 ppm by mass. (Technology 4) The electrochemical capacitor according to any one of Technology 1 to 3, wherein the lactone compound contains γ-butyrolactone.

[0050] The electrochemical capacitor according to the present disclosure will be described in more detail below with reference to examples. The present disclosure is not limited to the following examples. In these examples, a plurality of electrochemical capacitors were fabricated and evaluated.

[0051] (Capacitor A1) Capacitor A1 (electric double layer capacitor) was fabricated using the following procedure. (1) Electrode Fabrication Activated carbon (active material), conductive material (acetylene black: AB), dispersant (carboxymethyl cellulose: CMC), and binder (styrene butadiene rubber: SBR) were dispersed in water in a mass ratio of activated carbon: AB: CMC: SBR = 100: 6.6: 5.6: 1.0 to prepare a slurry. The slurry was then applied to a current collector and dried to form a laminate consisting of the current collector and a coating film (polarizable electrode layer). Aluminum foil (thickness: 20 μm) whose surface had been roughened by etching was used as the current collector. The resulting laminate was rolled to obtain an electrode sheet including a current collector and polarizable electrode layers (thickness: 70 μm) formed on both sides of the current collector. The electrode sheet was cut to a predetermined size to obtain positive and negative electrodes. Aluminum leads were connected to each electrode.

[0052] (2) Preparation of Capacitor Element The positive electrode and negative electrode were wound with a separator interposed therebetween to form a capacitor element. The negative electrode was wound around the outermost periphery of the capacitor element. A cellulose nonwoven fabric (thickness: 35 μm) was used as the separator.

[0053] (3) Preparation of Electrolyte Solution An electrolyte solution was prepared by the following procedure. First, an organic salt (diethyldimethylammonium tetrafluoroborate: DEDMA) was added to γ-butyrolactone (GBL). ＋ BF ４ － ) was dissolved to prepare an electrolyte solution. The concentration of the organic salt in the electrolyte solution was 1 mol / L. A buffer solution (aqueous solution of a buffering agent) was added to the obtained electrolyte solution. The buffer solution was prepared by mixing an aqueous solution of acetic acid and an aqueous solution of sodium acetate in a predetermined ratio (the ratio shown in Table 1) so that the buffering agent would maintain the pH shown in Table 3.

[0054] The mass of the buffer solution added to the electrolyte (mass including water) was 0.5% of the mass of the electrolyte solution before the buffer solution was added. Next, the water content in the electrolyte solution to which the buffer solution had been added was removed using a dehydrating agent (molecular sieve 3A) to adjust the water content in the electrolyte solution to 100 ppm or less (by mass). The content of the buffer agent in the electrolyte solution was determined using the mass of the buffer agent in the buffer solution (mass excluding water).

[0055] (4) Assembly of Capacitor The capacitor element was housed inside the outer case. A cylindrical case with a bottom (made of aluminum) was used as the outer case. Next, the above-mentioned electrolyte solution was poured into the case, thereby impregnating the capacitor element with the electrolyte. Next, a sealing material was placed at the opening of the outer case so that the lead wires passed through the insertion holes provided in the sealing material. Next, the opening edge of the outer case was drawn to seal the opening of the outer case with the sealing material. The capacitor thus formed was subjected to an aging treatment at 60°C for 5.5 hours while applying the rated voltage. In this manner, capacitor A1 (electrochemical capacitor) was obtained.

[0056] (Evaluation of Resistance Change Rate) The change rate of the internal resistance of capacitor A1 was evaluated by the following method. (1) Initial Internal Resistance (Initial DC Resistance) Capacitor A1 was charged at a constant current of 100 mA in an environment of −30° C. until the voltage reached 2.7 V. Next, the state in which a voltage of 2.7 V was applied to capacitor A1 was maintained for 7 minutes. Thereafter, capacitor A1 was discharged at a constant current of 20 mA in an environment of −30° C. until the voltage reached 0 V. The discharge voltage during discharge was measured to obtain a discharge curve (vertical axis: discharge voltage, horizontal axis: discharge time).

[0057] From the linear approximation line of the discharge curve in the range of 0.5 seconds to 2 seconds after the start of discharge, the voltage V at the intercept of the approximation line Ｓ The voltage V at the start of discharge (0 seconds after the start of discharge) was calculated. ０ minus the voltage Vs (V ０ -V Ｓ) was calculated as ΔV. Using ΔV (V) and the current value Id (A) during discharge, the initial internal resistance R1 (Ω) of the capacitor A1 was calculated by the following formula: Initial internal resistance R1 = ΔV / Id

[0058] (2) Internal Resistance Retention Rate Capacitor A1 was charged at a constant current of 100 mA in a 50°C environment until the voltage reached 2.7 V. Next, capacitor A1 was held at 50°C for 750 hours with a voltage of 2.7 V applied (float test). In this manner, the electrochemical capacitor was stored with a voltage of 2.7 V applied. Thereafter, capacitor A1 was discharged at a constant current of 20 mA in a 25°C environment until the voltage reached 0 V. Next, the internal resistance R2 (Ω) of capacitor A1 after the float test was determined using the same method as for measuring the initial internal resistance. Next, the resistance retention rate was calculated using the initial internal resistance R1 and the internal resistance R2 according to the following formula. The closer the resistance retention rate is to 100, the smaller the change in internal resistance and the more excellent the float characteristics. Resistance retention rate (%) = (R2 / R1) x 100

[0059] Capacitor A2 was fabricated in the same manner and under the same conditions as Capacitor A1, except that the buffering agent was changed to a combination of acetic acid and calcium acetate. The buffer solution was prepared by mixing an aqueous solution of acetic acid and an aqueous solution of calcium acetate in a predetermined ratio so that the buffering agent would maintain the pH shown in Table 3.

[0060] (Capacitors A3 to A6) Capacitors A3 to A6 were fabricated using the same method and conditions as those for fabricating Capacitor A1, except that the type and amount of buffering agent were changed as shown in Table 3. A combination of citric acid and sodium citrate was used as the buffering agent. The buffer solution was prepared by mixing an aqueous citric acid solution and an aqueous sodium citrate solution in a predetermined ratio so that the buffering agent would maintain the pH shown in Table 3.

[0061] (Capacitors C1 to C4) Capacitors C1 to C4 were fabricated using the same method and conditions as those for Capacitor A1, except that the solvent of the electrolyte, the type of buffer, and the amount of buffer added were changed as shown in Table 3. In the fabrication of Capacitor C1, no buffer was added to the electrolyte. In the fabrication of Capacitor C2, a combination of sodium dihydrogen phosphate and disodium hydrogen phosphate was used as the buffer. The buffer solution for Capacitor C2 was prepared by mixing a 0.2 mol / L sodium dihydrogen phosphate solution and a 0.2 mol / L disodium hydrogen phosphate solution in a predetermined ratio so that the buffer would maintain the pH shown in Table 3.

[0062] Capacitor C3 was fabricated using the same method and conditions as capacitor A1, except that the solvent for the electrolyte was changed. Capacitor C4 was fabricated using the same method and conditions as capacitor A4, except that the solvent for the electrolyte was changed. The solvent for the electrolyte of capacitors C3 and C4 was a mixture of propylene carbonate (PC) and dimethyl carbonate (DMC) in a volume ratio of PC:DMC = 60:40.

[0063] The internal resistance retention rate of each of the produced capacitors was determined using the method described above. Table 3 shows some of the production conditions and evaluation results for each capacitor. The "solvent" in Table 3 refers to the solvent of the electrolyte solution. In Table 3, the pH value of the buffering agent refers to the pH value that the buffering agent aims to maintain (at least the pH value that the buffering agent aims to maintain in an aqueous solution). In Table 3, the "amount of buffer solution" is the value when the mass of the electrolyte solution before the buffering agent is added is taken as 100% by mass. As described above, the closer the resistance retention rate is to 100, the smaller the change in internal resistance and the more excellent the float characteristics.

[0064]

[0065] Capacitors A1 to A6 are examples of electrochemical capacitors (C) according to the present disclosure. Capacitors C1 to C4 are comparative examples. As shown in Table 3, the resistance maintenance ratios of capacitors A1 to A6 were close to 100 and were good compared to the resistance maintenance ratios of capacitors C1 to C4.

[0066] The present disclosure can be used for electrochemical capacitors. Although the present invention has been described with reference to presently preferred embodiments, such disclosure should not be construed as limiting. Various modifications and alterations will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Therefore, the appended claims should be construed to cover all modifications and alterations without departing from the true spirit and scope of the present invention.

[0067] 1: Capacitor element 2: First electrode 3: Second electrode 4: Separator 6: Outer case 7: Sealing member 10: Electrochemical capacitor

Claims

1. An electrochemical capacitor comprising a first electrode, a second electrode, and an electrolyte solution, wherein the electrolyte solution comprises a lactone compound and a buffering agent that exhibits a buffering effect in an acidic region having a pH of 3.0 or more and 5.0 or less.

2. The electrochemical capacitor of claim 1, wherein the buffering agent comprises at least one acid selected from the group consisting of acetic acid and citric acid, and a salt of the at least one acid.

3. The electrochemical capacitor according to claim 1, wherein the content of the buffer in the electrolyte is in the range of 20 ppm to 100 ppm by mass.

4. The electrochemical capacitor according to any one of claims 1 to 3, wherein the lactone compound includes γ-butyrolactone.

Images