Electrochemical capacitor

Abstract

The present disclosure suppresses degradation in the float characteristics of an electrochemical capacitor, this electrochemical capacitor being provided with a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolytic solution. The electrolytic solution contains a lactone compound. The capacity of the positive electrode is larger than that of the negative electrode, and is 1.6 times or less the capacity of the negative electrode.

Description

[Technical Field]

[0001] This invention relates to an electrochemical capacitor. [Background technology]

[0002] An electrochemical capacitor comprises a pair of electrodes and an electrolyte, with at least one of the electrodes containing an active material capable of adsorbing and desorbing ions. An example of an electrochemical capacitor is an electric double-layer capacitor, which has a longer lifespan, enables rapid charging, and has superior output characteristics compared to secondary batteries, and is widely used as a backup power supply and the like.

[0003] Patent Document 1 describes a non-aqueous electrolyte for electric double-layer capacitors in which N-ethyl-N-methylpyrrolidinium tetrafluoroborate is dissolved as a quaternary ammonium salt, and K is used as an alkali metal cation. + 28.3 ppm and Na + An example containing 0.4 ppm is described (Example 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] International Publication No. 2016 / 092664 [Overview of the project]

[0005] Electrochemical capacitors tend to degrade under float charging conditions and require further improvement.

[0006] In view of the above, one aspect of the present invention relates to an electrochemical capacitor comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the electrolyte contains a lactone compound, and the capacity of the positive electrode is greater than the capacity of the negative electrode and 1.6 times or less the capacity of the negative electrode.

[0007] According to the present invention, the decrease in the float characteristics of an electrochemical capacitor can be suppressed. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is a cutaway perspective view of a portion of an electrochemical capacitor according to one embodiment of the present invention. [Figure 2] Figure 2 is a graph plotting the resistance increase rate after the float test of an electrochemical capacitor against the positive electrode potential (Ag / Ag+ reference) when charged at 3V. [Figure 3] Figure 3 is a graph plotting the capacitance degradation rate after a float test of an electrochemical capacitor against the positive electrode potential (Ag / Ag+ reference) when charged at 3V. [Modes for carrying out the invention]

[0009] [Electrochemical capacitor] An electrochemical capacitor according to one embodiment of the present invention comprises a positive electrode, a negative electrode, a separator interposed between the positive and negative electrodes, and an electrolyte. The electrolyte contains a lactone compound. The capacity of the positive electrode is greater than the capacity of the negative electrode, and is 1.6 times or less the capacity of the negative electrode. This configuration improves the float characteristics.

[0010] Float characteristics refer to the electrochemical processes that occur when float charging is performed using an external DC power supply to maintain a constant voltage. Capacitor This is an indicator of the degree of degradation. The smaller the capacity reduction during float charging and the smaller the increase in internal resistance, the better the float characteristics are considered to be.

[0011] Here, the positive electrode capacity is the maximum capacity that can be expressed at the positive electrode, and is a theoretical capacity determined by the amount of positive electrode active material, etc. Similarly, the negative electrode capacity is the maximum capacity that can be expressed at the negative electrode, and is a theoretical capacity determined by the amount of negative electrode active material, etc. The positive electrode capacity is roughly equal to the area (cm²) of the opposing surface between the positive and negative electrodes. 2 ) and the amount of positive electrode active material per unit area (g / cm³) 2The negative electrode capacity is obtained by multiplying the positive electrode capacity by the capacity per unit weight of the positive electrode active material (F / g). The negative electrode capacity is approximately the area (cm²) between the positive and negative electrodes. 2 ) and the amount of negative electrode active material per unit area (g / cm³) 2 This value is obtained by multiplying the positive electrode active material by the capacity per unit weight (F / g) of the negative electrode active material. The capacity (F) of the positive electrode active material and the negative electrode active material is determined from the amount of charge or stored charge when 3V is applied.

[0012] In an electrochemical capacitor, the larger the capacitance of the positive electrode is relative to the capacitance of the negative electrode, the lower the potentials of both the positive and negative electrodes become. Conversely, the larger the capacitance of the negative electrode is relative to the capacitance of the positive electrode, the higher the potentials of both the positive and negative electrodes become.

[0013] Lactone compounds are used as solvents for electrolytes in electrochemical capacitors because they have low viscosity even at low temperatures. Examples of lactone compounds include β-propiolactone, γ-butyrolactone, γ-valerolactone, and δ-valerolactone. Among these, γ-butyrolactone (GBL) is the most preferred because it has low viscosity even at low temperatures, a high boiling point, and produces little gas due to side reactions.

[0014] However, lactone compounds can decompose when exposed to a highly oxidizing environment, especially when the positive electrode potential is high. In float charging, a high voltage is applied to the electrochemical capacitor for an extended period, resulting in a prolonged high positive electrode potential and making lactone compounds susceptible to oxidative decomposition. This is thought to lead to a decrease in float characteristics.

[0015] In contrast, according to one embodiment of the electrochemical capacitor, the positive electrode potential during charging can be lowered by making the positive electrode capacitance greater than the negative electrode capacitance. This suppresses the oxidative decomposition of lactone compounds and prevents a decrease in float characteristics.

[0016] The greater the capacity of the positive electrode relative to the capacity of the negative electrode, the greater the effect of suppressing oxidative decomposition of the lactone compound, and the greater the effect of suppressing the decrease in float characteristics. From the viewpoint of suppressing a decrease in float characteristics, the capacity of the positive electrode is preferably 1.1 times or more the capacity of the negative electrode.

[0017] On the other hand, as the capacity of the positive electrode is increased relative to the capacity of the negative electrode, the portion of the positive electrode capacity that does not contribute to the capacitance of the electric chemistry chemical capacitor increases, resulting in a decrease in the capacitance of the electrochemical capacitor. In addition, since the potential of the negative electrode decreases, the reducibility on the negative electrode side further increases. From the viewpoint of suppressing a decrease in the capacitance of the electrochemical capacitor and suppressing side reactions due to reductive decomposition at the negative electrode, the capacity of the positive electrode is set to 1.6 times or less the capacity of the negative electrode.

[0018] From the viewpoint of maintaining high capacitance of the electrochemical capacitor while suppressing a decrease in float characteristics, the capacity of the positive electrode is desirably 1.1 times or more and 1.6 times or less the capacity of the negative electrode.

[0019] The positive electrode of an electrochemical capacitor may be a polarizable electrode. The polarizable electrode may contain an active material capable of adsorbing and desorbing ions. In an electrochemical capacitor, capacitance is developed by adsorption of ions to the active material at least on the positive electrode side. Non-faradaic current flows when ions desorb from the active material. The negative electrode may be either a polarizable electrode or a non-polarizable electrode.

[0020] When both the positive electrode and the negative electrode are polarizable electrodes, the electrochemical capacitor may be an electric double-layer capacitor (EDLC) in which an electric double layer is formed by adsorption of ions to an active material. When the negative electrode is a non-polarizable electrode, the electrochemical capacitor may be a lithium ion capacitor (LIC) that develops capacitance through adsorption or desorption of lithium ions on the negative electrode side. In the case of an LIC, a negative electrode used in lithium ion secondary batteries may be used as the negative electrode. Note that in an electrochemical capacitor using a wound electrode body, the electrode body is usually configured such that the outermost periphery is the negative electrode.

[0021] A polarizing electrode comprises, for example, a current collector and a polarizing electrode layer supported on the current collector. When both the positive and negative electrodes are polarizing electrodes, the positive electrode comprises, for example, a positive electrode current collector and a polarizing electrode layer supported on the positive electrode current collector. The negative electrode comprises, for example, a negative electrode current collector and a polarizing electrode layer supported on the negative electrode current collector. The capacities of the positive and negative electrodes depend on the amount of active material contained in the polarizing electrode layer, and also on the specific surface area of ​​the active material if the capacity is generated by ion adsorption to the active material. However, it is easy to make the capacity of the positive electrode greater than that of the negative electrode by making the thickness of the polarizing electrode layer of the positive electrode greater than the thickness of the polarizing electrode layer of the negative electrode.

[0022] Alternatively, by compressing the polarization electrode layer of the positive electrode and increasing the density of the active material per unit area of ​​the polarization electrode layer supported on the positive electrode current collector, the capacity of the positive electrode may be made larger than that of the negative electrode while keeping the thicknesses of the positive and negative electrodes approximately the same.

[0023] When both the positive and negative electrodes are polarizable electrodes, the thickness of the polarizable electrode layer of the positive electrode is preferably greater than the thickness of the polarizable electrode layer of the negative electrode, and preferably 1.6 times or less the thickness of the polarizable electrode layer of the negative electrode. More preferably, the thickness of the polarizable electrode layer of the positive electrode is 1.1 times or more and 1.6 times or less the thickness of the polarizable electrode layer of the negative electrode.

[0024] By making the capacitance of the positive electrode greater than that of the negative electrode, the potential of the positive electrode decreases during float charging, while the potential of the negative electrode also decreases, creating a more reducing environment for the negative electrode. As a result, materials contained in the negative electrode (e.g., binders contained in the active material layer) may be reduced and decomposed, potentially degrading the characteristics of the electrochemical capacitor. Therefore, when binders are included in the positive electrode and / or negative electrode, it is preferable that the binder has high reduction resistance. Styrene-butadiene rubber (SBR) is an example of a binder with high reduction resistance. Styrene-butadiene rubber (SBR) includes styrene-butadiene copolymers and their modified forms.

[0025] When both the positive electrode and the negative electrode include polarizable electrode layers, styrene-butadiene rubber is preferably contained at least in the polarizable electrode layer of the negative electrode.

[0026] Quaternary ammonium ions are preferably used as ions (cations) contained in the electrolytic solution. The electrolytic solution may contain pyrrolidinium ions. Pyrrolidinium ions have high reduction resistance and are less likely to be decomposed at the negative electrode. Therefore, even when the capacity of the positive electrode is larger than the capacity of the negative electrode and the reducing property of the negative electrode is further enhanced, pyrrolidinium ions are stable, and generation of gas due to decomposition at the negative electrode is suppressed. Accordingly, a decrease in float characteristics can be further suppressed.

[0027] A pyrrolidinium ion is C₄H₈N + -R¹R² (where R¹ and R² are each a hydrocarbon group), which is a quaternary ammonium ion in which the nitrogen of the pyrrolidine ring is quaternized. R¹ and R² may each independently be a C1 to C4 alkyl group. Examples of the pyrrolidinium ion include N,N-dimethylpyrrolidinium (DMPy), N-methyl-N-ethylpyrrolidinium (MEPy), N,N-diethylpyrrolidinium (DEPy), and the like.

[0028] The quaternary ammonium ion is added to the electrolytic solution in the form of a salt with an anion. The anion is preferably a fluorine-containing anion. It is preferable to include an anion of a fluorine-containing acid. Examples of fluorine-containing anions include BF₄ - , PF₆ - and the like.

[0029] When an electrochemical capacitor is charged by applying a voltage of 3 V between the positive electrode and the negative electrode, the potential of the positive electrode is Ag / Ag + It is preferably +0.86 V or more and +0.96 V or less based on the potential (the potential of the negative electrode is -2.14 V or more and -2.04 V or less). In this case, an electrochemical capacitor in which a decrease in float characteristics is remarkably suppressed can be realized.

[0030] The potential of the positive (negative) electrode is determined by immersing the positive and negative electrodes, after charging at 3V, in a non-aqueous solution having the same composition as the electrolyte, with the active material layers (polarizable electrode layers) facing each other. The potential is then measured with the negative (positive) electrode as the counter electrode and the Ag electrode as the reference electrode. If the positive and negative electrodes have active material layers (polarizable electrode layers) on both sides, one side of the active material layer (polarizable electrode layer) is removed to avoid creating non-facing areas. For the Ag electrode, a solvent (GBL) is added to the electrolyte to achieve a salt concentration of 0.1 mol / L, and then Ag + A reference electrode can be used in which an internal solution for the reference electrode, obtained by adding AgBF4 to achieve an ion concentration of 0.1 mol / L, is filled into a glass tube, and a silver wire is immersed in the internal solution for the reference electrode.

[0031] The components of the electrochemical capacitor according to the embodiment of the present invention will be described in more detail below, using an electric double-layer capacitor as an example.

[0032] (Positive and negative electrodes) As the positive and / or negative electrodes of an electrochemical capacitor, for example, electrodes comprising an active layer (polarizable electrode layer) containing an active material and a current collector supporting the active layer are used as polarizable electrodes. The active material includes, for example, porous carbon particles. The active layer contains porous carbon particles, which are the active material, as an essential component, and may contain binders, conductive agents, etc., as optional components.

[0033] Porous carbon particles can be produced, for example, by heat-treating a raw material to carbonize it, and then activating the resulting carbonized material to make it porous. The carbonized material may be crushed and sized before the activation treatment. The porous carbon particles obtained from the activation treatment may be pulverized. After pulverization, classification treatment may be performed. Examples of activation treatments include gas activation using gases such as water vapor, and chemical activation using alkalis such as potassium hydroxide.

[0034] Examples of raw materials include wood, coconut shells, pulp wastewater, coal or coal-based pitch obtained by its thermal decomposition, heavy oil or petroleum-based pitch obtained by its thermal decomposition, phenolic resin, petroleum-based coke, and coal-based coke. Among these, petroleum-based coke and coal-based coke are preferred as raw materials.

[0035] Petroleum-based coke or coal-based coke may be heat-treated, and the resulting carbides may be activated before the porous carbon particles are subjected to grinding. For example, a ball mill or jet mill can be used for the grinding process. The above grinding process yields fine porous carbon particles, the average particle size (D50) of which is, for example, 1 μm or more and 4 μm or less. In this specification, the average particle size (D50) refers to the particle size (median diameter) at which the integrated volume value in the volume-based particle size distribution measured by laser diffraction / scattering method becomes 50%.

[0036] The pore size distribution and particle size distribution of porous carbon particles can be adjusted by the raw material, heat treatment temperature, activation temperature during gas activation, degree of grinding, etc. Alternatively, the pore size distribution and particle size distribution of porous carbon particles may be adjusted by mixing two types of porous carbon particles made from different raw materials. The average particle size and particle size distribution of porous carbon particles are measured by laser diffraction / scattering. For example, the Microtrac MT3300EXII laser diffraction / scattering particle size distribution analyzer is used as the measuring device.

[0037] Examples of binders include resin materials such as polytetrafluoroethylene (PTFE), carboxymethylcellulose (CMC), and styrene-butadiene rubber (SBR). Examples of conductive agents include carbon black such as acetylene black.

[0038] The above electrode can be obtained, for example, by applying a slurry containing porous carbon particles, a binder and / or a conductive agent, and a dispersion medium to the surface of a current collector, drying the coating, and rolling it to form an active layer. For the current collector, a metal foil such as aluminum foil can be used.

[0039] When the electrochemical capacitor is an electric double-layer capacitor (EDLC), at least one of the positive and negative electrodes can be an electrode containing the above-mentioned porous carbon particles. When the electrochemical capacitor is a lithium-ion capacitor (LIC), an electrode containing the above-mentioned porous carbon particles can be used as the positive electrode, and a negative electrode used in lithium-ion secondary batteries can be used as the negative electrode. A negative electrode used in lithium-ion secondary batteries may, for example, include a negative electrode active material (e.g., graphite) capable of intercalating and releasing lithium ions.

[0040] (electrolyte) The electrolyte contains a solvent (non-aqueous solvent) and an ionic substance. The ionic substance is dissolved in the solvent and contains cations and anions. The ionic substance may include, for example, low-melting-point compounds (ionic liquids) that can exist as liquids at or near room temperature. The concentration of the ionic substance in the electrolyte is, for example, 0.5 mol / L or more, and 2.0 mol / L. below That is the case.

[0041] The solvent is preferably one with a high boiling point. The solvent contains a lactone compound and, if necessary, other solvents. Other solvents that can be used include, for example, cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate; linear carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; aliphatic carboxylic acid esters such as methyl formate, methyl acetate, methyl propionate, and ethyl propionate; 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.

[0042] Ionic substances include, for example, organic salts. An organic salt is a salt in which at least one of the anion and / or cation is an organic substance. Examples of organic salts in which the cation is an organic substance include quaternary ammonium salts. Examples of organic salts in which the anion (or both ions) is an organic substance include trimethylamine maleate, triethylamine borodisalicylate, ethyldimethylamine phthalate, mono-1,2,3,4-tetramethylimidazolinium phthalate, and mono-1,3-dimethyl-2-ethylimidazolinium phthalate.

[0043] From the viewpoint of improving dielectric strength, the anion preferably contains an anion of a fluorine-containing acid. Examples of fluorine-containing acid anions include BF4. - and / or PF6 - Examples include the following. The organic salt preferably contains, for example, a pyrrolidinium cation and an anion of a fluorine-containing acid. Specifically, examples include N,N-dimethylpyrrolidinium tetrafluoroborate (DMPyBF4), N-methyl-N-ethylpyrrolidinium tetrafluoroborate (MEPyBF4), and N,N-diethylpyrrolidinium tetrafluoroborate (DEPyBF4).

[0044] (Separator) A separator is typically interposed between the positive and negative electrodes. The separator is ion-permeable and physically separates the positive and negative electrodes to prevent short circuits. The separator can be made of cellulose fiber nonwoven fabric, glass fiber nonwoven fabric, polyolefin microporous membrane, woven fabric, or nonwoven fabric. The thickness of the separator is, for example, 8 to 300 μm, with 8 to 40 μm being preferred.

[0045] Hereinafter, an electrochemical capacitor according to an embodiment of the present invention will be described with reference to Figure 1. Figure 1 is a perspective view in which a part of the electrochemical capacitor according to an embodiment of the present invention is cut off. Note that the present invention is not limited to the electrochemical capacitor shown in Figure 1.

[0046] The electrochemical capacitor 10 in Figure 1 is an electric double-layer capacitor and comprises a wound-type capacitor element 1. The capacitor element 1 is constructed by winding a sheet-like first electrode (positive electrode) 2 and a second electrode (negative electrode) 3 with a separator 4 in between. The first electrode 2 and the second electrode 3 each have a metal first current collector and a second current collector, and a first active layer and a second active layer supported on their surfaces, and exhibit capacitance by adsorbing and desorbing ions. The first active layer and the second active layer include, for example, porous carbon particles.

[0047] For the current collector, for example, aluminum foil is used. The surface of the current collector may be roughened by a method such as etching. For the separator 4, for example, a nonwoven fabric mainly composed of cellulose is used. The first electrode 2 and the second electrode 3 are connected to a first lead wire 5a and a second lead wire 5b as lead members, respectively. The capacitor element 1 is housed in a cylindrical outer case 6 together with an electrolyte (not shown). The material of the outer case 6 may be a metal such as aluminum, stainless steel, copper, iron, or brass. The opening of the outer case 6 is sealed by a sealing member 7. The lead wires 5a and 5b are led out to the outside so as to pass through the sealing member 7. For the sealing member 7, a rubber material such as butyl rubber is used.

[0048] Although wound capacitors were described in the above embodiments, the scope of application of the present invention is not limited to those described above, and it can also be applied to capacitors of other structures, such as multilayer or coin-type capacitors.

[0049] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0050] Examples 1-9, Comparative Examples 1-9 In this embodiment, a wound-type electric double-layer capacitor was fabricated as the electrochemical capacitor. The specific manufacturing method of the electrochemical capacitor is described below.

[0051] (Electrode fabrication) A slurry was prepared by dispersing 88 parts by mass of porous carbon particles, which are the active material, 6 parts by mass of polytetrafluoroethylene (PTFE), which is the binder, and 6 parts by mass of acetylene black, which is the conductive agent, in water. The obtained slurry was applied to an aluminum foil (30 μm thick), the coating was dried with hot air at 110°C, and the mixture was rolled to form an active layer (polarizable electrode layer) of the thickness shown in Table 1, thereby obtaining a positive electrode and a negative electrode. The capacity of the positive electrode and the capacity of the negative electrode are proportional to the thickness of the active layer.

[0052] (Preparation of electrolyte solution) An electrolyte was prepared by dissolving pyrrolidinium salts in γ-butyrolactone (GBL), a lactone compound, as a non-aqueous solvent. The concentration of pyrrolidinium salt in the electrolyte was 1.0 mol / L. The pyrrolidinium salts consisted of pyrrolidinium cations and tetrafluoroborate anions, as shown in Table 1. BF 4 - ) and salt were used.

[0053] (Fabrication of electrochemical capacitors) A microporous film made of polypropylene (PP) was prepared as a separator. Lead wires were connected to the positive and negative electrodes, respectively, and cellulose Made A capacitor element was obtained by winding the material through a nonwoven fabric separator. The capacitor element was placed in a predetermined outer case along with the electrolyte, and the case was sealed with a sealing member to complete the electrochemical capacitor (electric double-layer capacitor). Subsequently, an aging treatment was performed at 60°C for 16 hours while applying the rated voltage.

[0054] The following evaluations were performed on each electrochemical capacitor obtained above. [evaluation] (1) Evaluation of float characteristics (Measurement of initial capacitance and initial internal resistance (DCR)) Under a -30°C environment, constant current charging was performed with a current of 2700mA until the voltage reached 3V, and then the 3V voltage was maintained for 7 minutes. Subsequently, under a -30°C environment, constant current discharge was performed with a current of 20mA until the voltage reached 0V.

[0055] During the discharge described above, the time t (sec) required for the voltage to drop from 2.16V to 1.08V was measured. Using the measured time t, the capacitance (initial capacitance) C1 (F) of the electrochemical capacitor before the float test was calculated using the following equation (1). Capacity C1=Id×t / V (1) In equation (1), Id is the discharge current (0.02A), and V is the value obtained by subtracting 1.08V from 2.16V (1.08V).

[0056] Using the discharge curve obtained from the above discharge (vertical axis: discharge voltage, horizontal axis: discharge time), a first-order approximation line was found in the range from 0.5 seconds to 2 seconds after the start of discharge, and the voltage VS at the intercept of this approximation line was determined. The value obtained by subtracting the voltage VS from the voltage V0 at the start of discharge (0 seconds after the start of discharge) (V0-VS) was calculated as ΔV. Using ΔV(V) and the discharge current value Id(0.02A), the internal resistance (DCR) R1(Ω) of the electrochemical capacitor before the float test was calculated from equation (2) below. Internal resistance R1=ΔV / Id (2)

[0057] (Measurement of capacitance and internal resistance (DCR) after float testing) Under a 65°C environment, the electrochemical capacitor was charged with a constant current of 1000mA until the voltage reached 3V, and then maintained at a voltage of 3.0V for 200 hours. In this way, the electrochemical capacitor was stored with a voltage of 3.0V applied. Subsequently, under a 65°C environment, a constant current discharge was performed with a current of 1000mA until the voltage reached 0V. Then, using the same method as for measuring the initial capacitance and initial internal resistance described above, the capacitor was charged and discharged under a -30°C environment, and the capacitance C2(F) and internal resistance R2(Ω) after the float test of the electrochemical capacitor were determined.

[0058] Using the capacitances C1 and C2 of the electrochemical capacitor obtained above, before and after the float test, the capacitance degradation rate was evaluated using the following formula. A smaller absolute value of the capacitance degradation rate indicates that the decrease in capacitance after the float test is suppressed. Capacity deterioration rate (%)=((C2 / C1)-1)×100

[0059] Using the internal resistances R1 and R2 of the electrochemical capacitor before and after the float test, obtained above, the rate of increase in resistance was calculated using the following formula. A smaller rate of increase indicates that the increase in internal resistance after the float test is suppressed. Resistance increase rate (%) = (R2 / R1) × 100

[0060] (2) Measurement of the potential of the positive and negative electrodes The manufactured electrochemical capacitor was disassembled, and the positive electrode, negative electrode, and separator were removed. One side of the polarizing electrode layer formed on both sides of the removed positive and negative electrodes was peeled off, and each was punched out to a diameter of 16 mm. The removed separator was punched out to a diameter of 24 mm. The punched-out positive electrode, negative electrode, and separator were stacked so that the polarizing electrode layers faced each other with the separator in between, and an evaluation cell was assembled. The evaluation cell was immersed in a non-aqueous solution having the same composition as the electrolyte of the electrochemical capacitor, and the Ag electrode prepared in the manner described above was placed as a reference electrode.

[0061] The evaluation cell was charged with a constant current of 1.8 mA at 25°C until the voltage reached 3 V. Then, a voltage of 3.0 V was applied and maintained for 10 minutes. The potentials of the positive and negative electrodes were measured after maintaining the 3.0 V voltage for 10 minutes.

[0062] Multiple electrochemical capacitors were fabricated and evaluated by varying the pyrrolidinium salt cations contained in the electrolyte, the thickness of the active layer at the positive electrode, and the thickness of the active layer at the negative electrode. The evaluation results are shown in Table 1. The electrochemical capacitors of Examples 1 to 9 are electrochemical capacitors A1 to A9 in Table 1. The electrochemical capacitors of Comparative Examples 1 to 9 are electrochemical capacitors B1 to B9 in Table 1. Table 1 also shows the binder used for each electrochemical capacitor, the thickness (μm) of the active layer at the positive and negative electrodes, and the thickness ratio Rd. In Table 1, DMPy, MEPy, and DEPy are N,N-dimethylpyrrolidinium cation, N-methyl-N-ethylpyrrolidinium cation, and N,N-diethylpyrrolidinium cation, respectively.

[0063] In electrochemical capacitors A1-A9 and B1-B9, the density of the active layer is the same at the positive and negative electrodes, and is also the same between electrochemical capacitors. Therefore, the ratio Rd of the thickness of the active layer at the positive electrode to the thickness of the active layer at the negative electrode is approximately equal to the ratio of the capacitance of the positive electrode to the capacitance of the negative electrode.

[0064] In electrochemical capacitors A1-A9 and B3, B6, and B9, the thickness of the active layer at the positive electrode is greater than the thickness of the active layer at the negative electrode, and Rd > 1. In this case, the capacitance of the electrochemical capacitor is limited by the capacitance of the negative electrode (thickness of the active layer at the negative electrode). In contrast, in electrochemical capacitors B1, B4, and B7, the thickness of the active layer at the positive electrode is less than the thickness of the active layer at the negative electrode, and Rd < 1. In this case, the capacitance of the electrochemical capacitor is limited by the capacitance of the positive electrode (thickness of the active layer at the positive electrode).

[0065] Examples 10-12, Comparative Examples 10-12 In preparing the electrolyte, diethyldimethylammonium tetrafluoroborate (DEDMABF4) was dissolved in γ-butyrolactone (GBL) instead of pyrrolidinium salt to prepare the electrolyte. The concentration of DEDMABF4 in the electrolyte was 1.0 mol / L.

[0066] The electrochemical capacitor was fabricated and evaluated in the same manner as in Example 1.

[0067] Multiple electrochemical capacitors were fabricated and evaluated while varying the thickness of the active layer at the positive electrode and the active layer at the negative electrode. The results are shown in Table 2. The electrochemical capacitors of Examples 10-12 are electrochemical capacitors A10-A12 in Table 2. The electrochemical capacitors of Comparative Examples 10-12 are electrochemical capacitors B10-B12 in Table 2. Table 2 also shows the binder used for each electrochemical capacitor, along with the thickness (μm) and thickness ratio Rd of the active layer at the positive and negative electrodes. In Table 2, DEDMA is diethyldimethylammonium cation.

[0068] In electrochemical capacitors A10-A12 and B10-B12, the density of the active layer is the same at the positive and negative electrodes, and is also the same between electrochemical capacitors. Therefore, the ratio Rd of the thickness of the active layer at the positive electrode to the thickness of the active layer at the negative electrode is approximately equal to the ratio of the capacitance of the positive electrode to the capacitance of the negative electrode.

[0069] In electrochemical capacitors A10-A12 and B12, the thickness of the active layer at the positive electrode is greater than the thickness of the active layer at the negative electrode, and Rd > 1. In this case, the capacitance of the electrochemical capacitor is limited by the capacitance of the negative electrode (thickness of the active layer at the negative electrode). In contrast, in electrochemical capacitor B10, the thickness of the active layer at the positive electrode is less than the thickness of the active layer at the negative electrode, and Rd < 1. In this case, the capacitance of the electrochemical capacitor is limited by the capacitance of the positive electrode (thickness of the active layer at the positive electrode).

[0070] [Table 1]

[0071] [Table 2]

[0072] As shown in Tables 1 and 2, in electrochemical capacitors A1 to A9, where the ratio of the thickness of the active layer at the positive electrode to the thickness of the active layer at the negative electrode (ratio of the capacitance of the positive electrode to the capacitance of the negative electrode), Rd, is greater than 1 and less than or equal to 1.6, the decrease in float characteristics was suppressed.

[0073] Tables 1 and 2 show that increasing the thickness ratio Rd lowers both the positive and negative electrode potentials. Consequently, in the range where Rd is greater than 1 and less than or equal to 1.6, oxidative decomposition of lactone compounds at the positive electrode is suppressed, resulting in smaller resistance increase and capacitance degradation rates, and maintaining high float characteristics. On the other hand, when Rd exceeds 1.6, the absolute values ​​of the resistance increase and capacitance degradation rates increase. This is thought to be because the decrease in negative electrode potential makes the constituent materials of the negative electrode or the solutes of the electrolyte more susceptible to reductive decomposition.

[0074] Table 1 shows that in electrochemical capacitors A1-A9 and B1-B9, which use pyrrolidinium ions as cations in the electrolyte, the positive and negative electrode potentials decrease when the thickness ratio Rd is increased, compared to electrochemical capacitors A10-A12 and B10-B12, which use DEDMA as shown in Table 2. As a result, oxidative decomposition of lactone compounds at the positive electrode is further suppressed in electrochemical capacitors A1-A9. Furthermore, because pyrrolidinium ions have high reduction resistance, reductive decomposition is suppressed even when the negative electrode potential decreases. Consequently, when the thickness ratio Rd is the same, electrochemical capacitors A1-A9 have smaller absolute values ​​for resistance increase and capacitance degradation than electrochemical capacitors A10-A12, and maintain higher float characteristics.

[0075] Figure 2 shows a graph plotting the resistance increase rate of an electrochemical capacitor against the positive electrode potential. Figure 3 shows a graph plotting the capacitance degradation rate of an electrochemical capacitor against the positive electrode potential. From Figures 2 and 3, it can be seen that when the positive electrode potential is Ag / Ag + It can be seen that when the potential is within the range of +0.86V to +0.96V relative to the reference potential, the absolute values ​​of the resistance rise rate and capacitance degradation rate can be made smaller, and the float characteristics can be maintained at a higher level. [Industrial applicability]

[0076] The electrochemical capacitor according to the present invention is suitably used in applications requiring high capacity and excellent float characteristics. [Explanation of Symbols]

[0077] 1: Capacitor element, 2: First electrode, 3: Second electrode, 4: Separator, 5a: First lead wire, 5b: Second lead wire, 6: Outer casing, 7: Sealing member, 10: Electrochemical capacitor

Claims

1. A positive electrode having a polarizable electrode layer containing porous carbon particles, A negative electrode having a polarizable electrode layer containing porous carbon particles, A separator interposed between the positive electrode and the negative electrode, Equipped with an electrolyte, The electrolyte comprises a lactone compound and pyrrolidinium ions. The lactone compound comprises γ-butyrolactone, The capacity of the positive electrode, obtained by multiplying the opposing area of ​​the positive electrode and the negative electrode by the amount of positive electrode active material per unit area in the polarizing electrode layer of the positive electrode and the capacity per unit weight of the positive electrode active material, is 1.1 times or more and 1.6 times or less of the capacity of the negative electrode, obtained by multiplying the opposing area by the amount of negative electrode active material per unit area in the polarizing electrode layer of the negative electrode and the capacity per unit weight of the negative electrode active material. An electrochemical capacitor in which the potential of the positive electrode when charged at 3V is between +0.86V and +0.96V, with respect to the Ag / Ag+ potential.

2. The electrochemical capacitor according to claim 1, wherein the thickness of the polarizing electrode layer of the positive electrode is greater than the thickness of the polarizing electrode layer of the negative electrode.

3. The electrochemical capacitor according to claim 2, wherein the thickness of the polarizing electrode layer of the positive electrode is 1.1 times or more and 1.6 times or less the thickness of the polarizing electrode layer of the negative electrode.

4. The electrochemical capacitor according to claim 2 or 3, wherein the polarizing electrode layer of the negative electrode comprises styrene-butadiene rubber.

5. The positive electrode contains porous carbon particles, The electrochemical capacitor according to any one of claims 1 to 4, wherein the average particle size (D50) of the porous carbon particles is 1 μm or more and 4 μm or less.

6. The electrochemical capacitor according to any one of claims 1 to 5, wherein the separator is a nonwoven fabric made of cellulose fibers, and its thickness is 8 μm or more and 40 μm or less.

7. The electrolyte comprises a solvent and an ionic substance dissolved in the solvent. The electrochemical capacitor according to any one of claims 1 to 6, wherein the concentration of the ionic substance in the electrolyte is 0.5 mol / L or more and 2.0 mol / L or less.

8. A capacitor element in which the positive electrode and the negative electrode are wound around the separator, A first lead connected to the positive electrode, A second lead connected to the aforementioned negative electrode, An outer case for housing the capacitor element together with the electrolyte, The outer casing comprises a sealing member that seals the opening, The electrochemical capacitor according to any one of claims 1 to 7, wherein the first lead and the second lead are led out to the outside so as to penetrate the sealing member.

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