Aqueous electrolyte for energy storage devices, and energy storage devices containing the same

Abstract

The present invention provides an aqueous electrolyte for energy storage devices that has a wide potential window despite being an aqueous electrolyte, and also suppresses salt deposition, as well as an energy storage device containing the same. [Solution] The aqueous electrolyte for the energy storage device contains 0.1 to 40% by mass of smectite, such as hectorite or saponite. Furthermore, the potential window in the two-electrode system using glassy carbon electrodes is 5.7V or higher under 25°C conditions, and it contains sodium hydroxide and / or potassium hydroxide.

Description

Technical Field

[0001] The present invention relates to an aqueous electrolyte for a power storage device and a power storage device containing the same.

Background Art

[0002] With the spread of electric vehicles, digital devices, etc., the demand for power storage devices is increasing. Organic solvents are widely used as solvents for electrolytes for power storage devices. Since organic solvents generally have a wide difference (i.e., potential window) between the potential for oxidative decomposition and the potential for reductive decomposition, by using the organic solvent as the solvent for the electrolyte, it is possible to operate stably even at high voltages and provide a power storage device having a high energy density. On the other hand, organic solvents have low conductivity and pose problems in terms of safety and the environment due to their volatility and flammability.

[0003] Compared with organic electrolytes using organic solvents, aqueous electrolytes generally have high conductivity, excellent output characteristics, low cost, and the advantages of high safety and environmental friendliness. Therefore, it is expected to apply aqueous electrolytes to power storage devices. However, water (pure water) has a narrower potential window compared to organic solvents and theoretically decomposes at a voltage of 1.23 V. Therefore, conventional aqueous electrolytes have generally been applied to power storage devices assumed to be used at low voltages. On the other hand, in recent years, aqueous electrolytes with a wide potential window that enable the application of high voltages have been developed. In such aqueous electrolytes, the electrolyte concentration is often increased to near the saturation concentration, for example. For example, Patent Document 1 discloses an electrolyte for a power storage device containing water as a solvent, wherein the composition of the electrolyte is characterized in that the amount of the solvent is 4 mol or less per 1 mol of an alkali metal salt. The electrolyte described in Patent Document 1 is said to have a potential window of 2 V or more.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

[0005] When an aqueous solution containing a high concentration of alkali metal salt, such as the one disclosed in Patent Document 1, is used as the electrolyte for an energy storage device, the dissolved electrolyte tends to precipitate as salt during use, resulting in poor liquid stability. Furthermore, there is the problem of increased manufacturing costs due to the added electrolyte.

[0006] The present invention aims to provide an aqueous electrolyte for energy storage devices that has a wide potential window and can suppress salt deposition, and an energy storage device containing the same. [Means for solving the problem]

[0007] In view of the above problems, the inventors of this invention conducted extensive research and found that by adding smectite, a clay mineral, to water, the potential window of water can be sufficiently widened even while suppressing the content of other electrolyte components. By using this aqueous dispersion of smectite as the electrolyte for an energy storage device, an energy storage device that enables the application of higher voltages can be obtained. This invention was completed based on these findings.

[0008] In other words, the above-mentioned problems of the present invention were solved by the following means. [1] A water-based electrolyte for energy storage devices containing 0.1 to 40% by mass of smectite. [2] The aqueous electrolyte for an energy storage device according to [1] above, wherein the potential window in a two-electrode system using glassy carbon electrodes is 5.7V or higher under 25°C conditions. [3] The aqueous electrolyte for an energy storage device according to [1] or [2], wherein the smectite is one or more selected from hectorite, saponite, stivunsite, montmorillonite, bydelite, nontronite, soakonite, and fluorohectolite. [4] An aqueous electrolyte for an energy storage device according to any one of [1] to [3] above, comprising sodium hydroxide and / or potassium hydroxide. [5] A water-based electrolyte for an energy storage device according to any one of [1] to [4] above, wherein the energy storage device is a capacitor. [6] A water-based electrolyte for an energy storage device according to any one of [1] to [4] above, wherein the energy storage device is a secondary battery. [7] An energy storage device comprising an aqueous electrolyte for an energy storage device as described in any of [1] to [6] above. [Effects of the Invention]

[0009] The aqueous electrolyte for energy storage devices of the present invention has a wide potential window despite being aqueous, and can suppress salt precipitation. Furthermore, the energy storage device of the present invention can be subjected to higher voltages despite containing an aqueous electrolyte, and can achieve stable operation by suppressing salt precipitation in the electrolyte. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a graph showing the current-potential curves (cyclic voltammograms) for each electrolyte in Example 6 and Comparative Example 1. [Modes for carrying out the invention]

[0011] Preferred embodiments of the present invention will now be described, but the present invention is not limited to the embodiments described below, except as provided for in the present invention.

[0012] [Water-based electrolyte for energy storage devices] The aqueous electrolyte for energy storage devices of the present invention (hereinafter also simply referred to as "the electrolyte of the present invention") is a smectite aqueous dispersion containing 0.1 to 40% by mass of smectite, and is suitably used as an electrolyte for energy storage devices that enable the application of high voltage. The smectite aqueous dispersion has a wider potential window than pure water, and the amount of other electrolyte components can be reduced even when they are included. Therefore, by using such a smectite aqueous dispersion as an electrolyte for energy storage devices, it is possible to set a high operating voltage, and an energy storage device can be obtained that achieves high energy density while suppressing salt precipitation and enabling stable operation.

[0013] The reason why smectite aqueous dispersions have a wider potential window compared to pure water is not clear, but it is thought that when a voltage is applied to a smectite aqueous dispersion, energy is preferentially consumed in the orientation polarization of smectite, a clay mineral in the aqueous dispersion; smectite forms a film on the electrode surface, making it difficult for water molecules to come into contact with the electrode; and water intercalates between the crystalline layers of swollen smectite, all of which suppress the decomposition of water. In particular, when the energy storage device is an electric double-layer capacitor (supercapacitor), it is possible not only to suppress the decomposition of water but also to improve the capacitance of the electric double-layer capacitor. More specifically, it is thought that polar smectite is oriented and polarized by the application of voltage and aligns near the electrodes, thereby increasing the capacitance of the electric double-layer capacitor. Figure 1 shows the current-potential curve (cyclic voltammogram) of the electrolyte of the present invention. As shown in Figure 1, in cyclic voltammetry using the electrolyte of the present invention, a peak is generated due to the orientation current associated with the orientation of the smectite, and it can be seen that the capacitance value of the cell has increased by that amount. Therefore, with the electrolyte of the present invention, stable energy storage and release derived from smectite becomes possible in an electric double-layer capacitor without relying on the oxidation-reduction Faraday reaction.

[0014] The conductivity of the electrolyte of the present invention is preferably 1 mS / cm or more. Since the electrolyte of the present invention contains 0.1 to 40% by mass of smectite, it has viscosity. Generally, conductivity is inversely proportional to viscosity. However, the electrolyte of the present invention exhibits high conductivity despite containing smectite and having viscosity as described above. From the viewpoint of imparting high output characteristics to the power storage device using the electrolyte of the present invention, the conductivity is more preferably 10 mS / cm or more, further preferably 50 mS / cm or more, still further preferably 100 mS / cm or more, and even more preferably 150 mS / cm or more. The conductivity of the electrolyte can also be adjusted by blending other electrolyte components shown below. The conductivity can be measured using an electric conductivity meter, for example, as described in the examples.

[0015] The component composition of the electrolyte of the present invention will be described below.

[0016] - Smectite - In the electrolyte of the present invention, smectite functions as an electrolyte component. Also, smectite functions as a thickener. Furthermore, since the smectite aqueous dispersion has thixotropy, using the smectite aqueous dispersion as the electrolyte can also reduce the risk of liquid leakage and the like. The type of the smectite is not particularly limited and can be appropriately set according to the purpose. Also, natural smectite or synthetic smectite can be used, and it is preferable to use synthetic smectite from the viewpoint of having a low impurity content. The synthetic smectite may be a smectite having a hydroxyl group at the crystal end, or may be a fluorinated smectite in which the hydroxyl group is substituted with a fluorine atom. The synthetic smectite can be produced by a conventional method. The smectite is preferably one or more selected from hectorite, saponite, stibnite, montmorillonite, beidellite, nontronite, sauconite, and fluorine hectorite, and from the viewpoint of further expanding the potential window, it is more preferably hectorite or saponite, and even more preferably hectorite.

[0017] The interlayer cation species of the smectite are not particularly limited, and are preferably sodium ions and / or lithium ions. By using sodium ions or lithium ions as the interlayer cations, excellent swelling properties and dispersion stability can be imparted to the smectite aqueous dispersion formed by dispersing the smectite in water. From the viewpoint of improving the dispersion stability during dispersion, the cation exchange capacity (CEC: Cation Exchange Capacity) of the smectite is preferably 15 meq (milliequivalent) / 100 g or more, more preferably 20 meq / 100 g or more, and even more preferably 25 meq / 100 g or more. Usually, the cation exchange capacity of the smectite that can be used in the present invention is 250 meq / 100 g or less.

[0018] The particle size of the smectite is not particularly limited and can be appropriately set according to the purpose. For example, the particle size (primary particle size) of the smectite can be 20 to 500 nm, may be 30 to 400 nm, may be 40 to 380 nm, or may be 50 to 370 nm. In the present invention, the "particle size" of the smectite in the dispersion liquid is the volume-based median diameter. This particle size can be determined, for example, by a laser diffraction / scattering type particle size distribution measuring device.

[0019] In the electrolyte of the present invention, the content of smectite is 0.1 to 40% by mass. From the viewpoint of further expanding the potential window of the aqueous electrolyte, the content of smectite contained in the electrolyte of the present invention is preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more. Also, from the viewpoint of improving the fluidity of the electrolyte, the content is preferably 37% by mass or less, more preferably 34% by mass or less, and even more preferably 30% by mass or less. When showing the content in a preferred range, the content of smectite contained in the electrolyte is preferably 0.5 to 37% by mass, more preferably 1 to 34% by mass, and even more preferably 2 to 30% by mass.

[0020] -Dispersion medium- The dispersion medium of the electrolyte of the present invention may be an aqueous solution containing water and, if necessary, a water-soluble solvent. There are no particular restrictions on the water used, but from the viewpoint of improving the dispersion stability of smectite in the electrolyte of the present invention, water in which ionic components in the water have been reduced or removed, such as distilled water, deionized water, or purified water, is preferred. In particular, water with an ionic conductivity of 10 μS / cm or less (preferably 5 μS / cm or less, more preferably 2 μS / cm or less) is more preferred. The dispersion medium of the electrolyte of the present invention is preferably water with an ionic conductivity of 10 μS / cm or less (preferably 5 μS / cm or less, more preferably 2 μS / cm or less).

[0021] -Other electrolyte components- From the viewpoint of improving the conductivity of the aqueous electrolyte, the electrolyte of the present invention may contain, in addition to the smectite, electrolyte components commonly used in aqueous electrolytes (hereinafter also referred to as "other electrolyte components"). Other electrolyte components used in the electrolyte of the present invention are not particularly limited as long as they are electrolyte components used in ordinary aqueous electrolytes, as described above. Since smectite, an electrolyte component in the electrolyte of the present invention, tends to exhibit weak alkalinity when dispersed in water, it is preferable that the other electrolyte components are materials that exhibit neutral to basic properties. Examples of other electrolyte components that exhibit neutral properties include perchlorates, nitrates, and sulfates. Examples of other electrolyte components that exhibit basic properties include metal hydroxides and metal carbonates. Examples of metal hydroxides include sodium hydroxide, potassium hydroxide, and lithium hydroxide, and as metal carbonates, sodium carbonate, potassium carbonate, and lithium carbonate. Among these, it is preferable to use sodium hydroxide and / or potassium hydroxide as the metal hydroxide.

[0022] If the electrolyte of the present invention contains the above-mentioned other electrolyte components, the content of the other electrolyte components in the electrolyte of the present invention is preferably 40% by mass or less, more preferably 35% by mass or less, even more preferably 30% by mass or less, even more preferably 25% by mass or less, and even more preferably 20% by mass or less. The content of the other electrolyte components may also be 0.1% by mass or more, 0.5% by mass or more, 1% by mass or more, 2% by mass or more, or 3% by mass or more. The preferred range for this content is 0.1 to 40% by mass, more preferably 0.5 to 35% by mass, even more preferably 1 to 30% by mass, even more preferably 2 to 25% by mass, and even more preferably 3 to 20% by mass.

[0023] <Potential window> The electrolyte of the present invention exhibits a wide potential window despite being an aqueous electrolyte. Therefore, it is possible to apply a high voltage (a voltage of 1.23V or higher, which is the theoretical decomposition voltage of water) to an energy storage device to which the electrolyte of the present invention is applied, resulting in an energy storage device with high energy density. The potential window of an electrolyte is usually measured using a three-electrode system with a reference electrode such as a standard hydrogen electrode (SHE). However, in this invention and specification, a voltage may be applied using a two-electrode system without a reference electrode, and the starting voltage for electrolysis of water contained in the electrolyte (or the difference between the oxygen generation cell voltage and the hydrogen generation cell voltage) may be used as the potential window. Even in such a two-electrode system, the measured potential window will roughly correlate with the potential window measured using a reference electrode. That is, the magnitude of the potential window obtained by the two-electrode system will roughly correlate with the magnitude of the potential window obtained by the three-electrode system. As a measurement method using the two-electrode system, for example, a measurement method using cyclic voltammetry can be mentioned. Specifically, using cyclic voltammetry, with measurement conditions at 25°C, the difference between the oxygen generation cell voltage and the hydrogen generation cell voltage in a two-electrode cell using glassy carbon electrodes can be used as the potential window. Therefore, the value measured and calculated by the method described in the examples can be used as the potential window.

[0024] [Method for producing the electrolyte solution of the present invention] The method for producing the electrolyte of the present invention (preparation method) is not particularly limited. For example, one method involves adding a predetermined amount of other electrolyte components and smectite to a water-based solvent (dispersion medium) and performing a dispersion treatment by stirring, or a method in which a mixture (aqueous solution) of water and other electrolyte components is used as the dispersion medium, and smectite is added to the dispersion medium and dispersed. In the method for producing the electrolyte of the present invention, the smectite may be smectite powder, or it may be a clay gel that has been swollen and gelled in water beforehand. In particular, from the viewpoint of further improving the dispersibility of smectite in the electrolyte of the present invention, it is preferable that the smectite used in preparing the electrolyte of the present invention is a clay gel that has been swollen and gelled in water beforehand.

[0025] [Energy storage devices] The energy storage device of the present invention can have the same configuration as a conventional energy storage device, except that it contains the electrolyte of the present invention as the electrolyte. The energy storage device of the present invention may be a capacitor or a secondary battery. Examples of the capacitor include electric double-layer capacitors, such as electrochemical capacitors, hybrid capacitors, and electrolytic capacitors. Examples of the secondary battery include lead-acid batteries, nickel-metal hydride batteries, lithium-ion batteries, and next-generation secondary batteries such as metal-air batteries and metal anode batteries. An example of the configuration of the energy storage device of the present invention, other than the electrolyte, is described below.

[0026] (positive electrode) As the positive electrode of the energy storage device of the present invention, a conventional electrode configuration used in general energy storage devices can be employed. For example, if the energy storage device of the present invention is an electric double-layer capacitor, a polarizable electrode material can be used as the positive electrode. Typical polarizable electrode materials include gold, platinum, nickel, mercury, and carbon materials, and a polarizable electrode can be made containing at least one of these. Furthermore, the electrode may be made in a porous form to provide an electrode with a higher charge storage capacity. For example, at least one material such as activated carbon, porous carbon nanofiber, or porous carbon can be used as the electrode. In particular, a high specific surface area (e.g., 2000 m²) is used for both electrodes. 2 It is also preferable to use activated carbon with a content of approximately / g.

[0027] In the case of a metal secondary battery, nickel oxide or manganese oxide can be used as the positive electrode active material, and in the case of a metal-air battery, oxygen can be used.

[0028] (Negative electrode) As the negative electrode of the energy storage device of the present invention, a conventional electrode configuration used in general energy storage devices can be employed. For example, if the energy storage device of the present invention is an electric double-layer capacitor, a polarizable electrode material can be used as the negative electrode. Typical polarizable electrode materials include gold, platinum, nickel, mercury, and carbon materials, and a polarizable electrode can be made containing at least one of these. Furthermore, the electrode may be made in a porous form to provide an electrode with a higher charge storage capacity. For example, at least one material such as activated carbon, porous carbon nanofiber, or porous carbon can be used as the electrode. In particular, a high specific surface area (e.g., 2000 m²) is used for both electrodes. 2 It is also preferable to use activated carbon with a content of approximately / g.

[0029] When the energy storage device of the present invention is a metal secondary battery, metals such as zinc, aluminum, iron, magnesium, lithium, sodium, and calcium can be used as the negative electrode active material. Furthermore, these metals may be used as elemental metals or in the form of alloys or oxides containing these metals. Furthermore, by using hydrogen storage alloys, hydrogen can also be used as the negative electrode active material.

[0030] (Current collector) The energy storage device of the present invention may also be provided with a pair of current collectors. However, if the polarizing electrode also functions as a current collector, a configuration without a current collector is also possible. The material of the current collector is not particularly limited as long as it is a conductor. For example, metals such as aluminum, copper, titanium, silver, lead, iron, zinc, and nickel, or alloys such as stainless steel, brass, cast iron, steel, and aluminum alloys, or graphite can be used. The current collector can be formed into a sheet, for example.

[0031] (Separator) By interposing a separator between the positive and negative electrodes, the electrolyte can be supported on the separator while preventing contact between the positive and negative electrodes. There are no particular restrictions on the type of separator; for example, nonwoven fabrics mainly composed of synthetic fibers, glass fibers, olefin resins, PVDF (polyvinylidene fluoride) resins, PAN (polyacrylonitrile copolymer) resins, PMMA (polymethyl methacrylate) resins, etc., can be used. Specifically, the separator may be composed of a porous membrane made of synthetic resin such as polyester fibers, nylon fibers, rayon fibers, polyacrylonitrile resin, vinylidene fluoride resin, acrylic resin and urethane resin, polyethylene or polypropylene, or a porous membrane made of natural fibers or ceramics, or a configuration in which two or more of these porous membranes are laminated.

[0032] [Method for manufacturing the energy storage device of the present invention] The energy storage device of the present invention can be manufactured in the same manner as conventional energy storage devices that use ordinary aqueous electrolytes, except that the electrolyte of the present invention is used as the electrolyte. [Examples]

[0033] The present invention will be described in more detail below based on examples, but the present invention is not limited thereto.

[0034] <Example of electrolyte preparation> The materials used to prepare each electrolyte are as follows: (Clay minerals) • Smecton-SWN (product name): Synthetic clay, hectorite, interlayer cation sodium ion, cation exchange capacity 43 meq / 100g, primary particle size 75 nm, manufactured by Kunimine Industries Co., Ltd. • Smecton-SA (product name): Synthetic clay, saponite, interlayer cation sodium ion, cation exchange capacity 66 meq / 100g, primary particle size 100 nm, manufactured by Kunimine Industries Co., Ltd. (electrolyte) • Sodium hydroxide: Wako Special Grade, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. • Potassium hydroxide: Wako Special Grade, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. (Gelling agent) • Sodium carboxymethylcellulose: Practical Grade, manufactured by Fujifilm & Wako Pure Chemical Industries, Ltd. • Carboxyvinyl polymer: Manufactured by Lubrizol. (solvent) • Distilled water: Conductivity 0.2 μS / cm or less

[0035] The smecton-SWN and smecton-SA were thoroughly washed with a diluted solution of 2-propanol (first grade, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) (2-propanol:water = 7:3, volume ratio) to remove impurities before use.

[0036] (Examples 1-7, Comparative Examples 1-6) Using the materials listed above, the electrolytes for Examples 1-7 and Comparative Examples 1-6 were prepared by mixing and stirring them in the proportions shown in Table 1 below.

[0037] <Performance Test> (Measurement of potential window) Using the electrolytes obtained in Examples 1-7 and Comparative Examples 1-6, a two-electrode cell using a glassy carbon electrode was constructed, and the potential window was measured. Inner volume 16cm 3 Each of the electrolytes from Examples 1-7 and Comparative Examples 1-6 was filled to 50% of its volume in a polypropylene cup. Two glassy carbon disk electrodes (GC electrodes) (product name: glassy carbon electrodes, OD: 6.0 mm, ID: 3.0 mm, manufactured by BAS) were inserted and fixed in place as polarizing electrodes with current collectors, with a distance of 1 cm between electrodes, up to halfway from the liquid surface, to form a two-electrode cell.

[0038] The electrolysis voltage of each two-electrode cell constructed as described above was measured by cyclic voltammetry using an electrochemical analyzer (BAS Model 600E). The measurement conditions were an ambient temperature of 25°C, a sweep rate of 10 mV / s, and a sweep range of +4 to -4 V. From the cyclic voltammogram obtained by cyclic voltammetry, the voltage at which the current value sharply increased on the anode side was defined as the oxygen generation cell voltage, and the voltage at which the current value sharply decreased on the cathode side was defined as the hydrogen generation cell voltage. The respective voltage values ​​are shown in Table 1. Next, the potential window in the two-electrode cell was calculated by determining the difference between the measured cell voltages (Equation 1). The calculated values ​​are shown in Table 1. In this measurement, a GC electrode was used as the electrode, so the potential window of water itself is higher than the theoretical value of 1.23V. However, in this invention, this measurement system was adopted as a model test system to demonstrate the effect of various additives on widening the potential window of water. Potential window (V) in a two-electrode cell = [Oxygen generation cell voltage (V)] - [Hydrogen generation cell voltage (V)] ... (Equation 1)

[0039] (Measurement of electrical conductivity) The conductivity of each electrolyte in Examples 1-7 and Comparative Examples 1-6 was measured using a compact electrical conductivity meter, LAQUAtwin (manufactured by HORIBA, EC-33B). The measured values ​​are shown in Table 1 below.

[0040] [Table 1]

[0041] In Comparative Examples 1-6, the electrolytes, which did not contain smectite and instead contained other electrolyte components such as sodium hydroxide or potassium hydroxide, all had potential windows of 5.5V or less in a two-electrode cell. The electrolyte of Comparative Example 3 contained carboxymethylcellulose and had high viscosity. Compared to the electrolyte of Comparative Example 1, which did not contain carboxymethylcellulose, the conductivity decreased, but no change was observed in the potential window. Similarly, the electrolyte of Comparative Example 4, which contained carboxyvinyl polymer, did not show a significant change in the potential window compared to the electrolyte of Comparative Example 1, and the conductivity decreased even further.

[0042] In contrast, in the electrolytes of Examples 1 to 7, which contained smectite as an electrolyte component, the potential window in the two-electrode cell was 5.8V or higher in all cases, regardless of the presence or absence of other electrolyte components. Furthermore, in the electrolytes of Examples 4 to 7, which contained other electrolyte components, it was shown that the conductivity could be increased to 150 mS / cm or higher. In addition, a comparison between Example 4-Comparative Example 1, Example 5-Comparative Example 2, Example 6-Comparative Example 1, and Example 7-Comparative Example 5 showed that although the conductivity remained almost unchanged in the corresponding examples and comparative examples, the potential window was widened in the examples containing smectite. This indicates that smectite, as an electrolyte component, expands the potential window without substantially changing the resistance in the electrolyte. Furthermore, the electrolytes in Examples 1 to 7 did not show salt precipitation even after being left standing at 25°C for 72 hours.

[0043] Figure 1 shows the cyclic voltammograms obtained above for each electrolyte in Example 6 and Comparative Example 1. As is clear from Figure 1, in cyclic voltammetry using the electrolyte of Example 6, which is the electrolyte of the present invention, a current peak originating from smectite was observed at a potential of approximately ±1.0. In addition, current peaks originating from smectite were observed in the other examples (Examples 1 to 5 and 7) in the same manner as in Example 6. From this, it is shown that the capacitance of an electric double-layer capacitor can be increased by using the electrolyte of the present invention.

Claims

1. A water-based electrolyte for energy storage devices containing 0.1 to 40% by mass of smectite.

2. The aqueous electrolyte for an energy storage device according to claim 1, wherein the potential window in a two-electrode system using glassy carbon electrodes is 5.7 V or higher under 25°C conditions.

3. The aqueous electrolyte for an energy storage device according to claim 2, wherein the smectite is one or more selected from hectorite, saponite, stivunsite, montmorillonite, bydelite, nontronite, soakonite, and fluorohectolite.

4. The aqueous electrolyte for an energy storage device according to claim 3, comprising sodium hydroxide and / or potassium hydroxide.

5. The aqueous electrolyte for an energy storage device according to claim 4, wherein the energy storage device is a capacitor.

6. The aqueous electrolyte for an energy storage device according to claim 4, wherein the energy storage device is a secondary battery.

7. A power storage device comprising an aqueous electrolyte for a power storage device according to any one of claims 1 to 6.

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