Secondary battery material, active material, electrolyte and secondary battery

Abstract

The present invention relates to a material for a secondary battery containing at least one among an organic compound having a phenylenediamine structure in a molecule. The present invention further relates to an active material containing the material for a secondary battery, an electrolyte containing the active material, and a secondary battery containing the electrolyte.

Description

[Technical Field]

[0001] The present invention relates to a material for a secondary battery, an active material containing the material for a secondary battery, an electrolyte solution containing the active material, and a secondary battery containing the electrolyte solution. [Background technology]

[0002] As renewable energy sources increase their installed capacity, the introduction of large-scale secondary batteries (storage batteries) is being promoted to stabilize grid power. Redox flow batteries, which are expected to be large-scale secondary batteries, have both aqueous and non-aqueous electrolytes, with aqueous electrolytes being superior in terms of safety and cost. Therefore, the active material must have high solubility in water and an appropriate redox potential to achieve high energy density.

[0003] Currently, vanadium is used as the active material in redox flow batteries. However, the use of vanadium is limited due to resource constraints, and price fluctuations are an issue (Non-Patent Documents 1 and 2). Therefore, it is desirable to use a material that is abundant in resources as the active material. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Jan Winsberg al., Angew. Chem. Int. Ed. 2017, 56, 686-711 [Non-patent document 2] P. Leung et al., Journal of Power Sources 360 (2017) 243-283 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a secondary battery material that is less resource-constrained and enables improvements in energy density and cycle characteristics, as well as an active material, an electrolyte, and a secondary battery that contain the secondary battery material. [Means for solving the problem]

[0006] The secondary battery material according to this embodiment contains at least one organic compound having a phenylenediamine structure in the molecule. [Effects of the Invention]

[0007] According to the present invention, it is possible to realize a secondary battery material that is less restricted in terms of resources and that enables improvements in energy density and cycle characteristics, as well as an active material, an electrolyte, and a secondary battery that contain the secondary battery material. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 2 is a diagram showing charge and discharge curves of the redox flow battery 1 produced in Example 2. [Figure 2] FIG. 1 is a diagram showing charge and discharge curves of the redox flow battery 2 produced in Example 3. [Figure 3] FIG. 1 is a diagram showing charge and discharge curves of the redox flow battery 3 produced in Example 4. [Figure 4] FIG. 10 is a charge / discharge curve diagram of the redox flow battery 4 produced in Example 5. [Figure 5] FIG. 10 is a diagram showing charge and discharge curves of the redox flow battery 5 produced in Example 6. [Figure 6] FIG. 10 is a charge / discharge curve diagram of the redox flow battery 6 produced in Example 7. [Figure 7] FIG. 10 is a diagram showing charge and discharge curves of the redox flow battery 7 produced in Example 8. [Figure 8] FIG. 10 is a charge / discharge curve diagram of the redox flow battery 8 fabricated in Example 9. [Figure 9] FIG. 10 is a diagram showing charge and discharge curves of the redox flow battery 9 produced in Example 10. [Figure 10]FIG. 1 is a diagram showing charge and discharge curves of the redox flow battery 10 produced in Example 11. [Figure 11] FIG. 12 is a charge / discharge curve diagram of the redox flow battery 11 produced in Example 12. [Figure 12] FIG. 11 is a charge / discharge curve diagram showing the redox flow battery 12 produced in Example 13. [Figure 13] FIG. 11 is a charge / discharge curve diagram showing the redox flow battery 13 produced in Example 14. [Figure 14] FIG. 10 is a charge / discharge curve diagram showing the redox flow battery 14 produced in Example 15. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present invention will be described in detail below. In this specification, including the examples, all "parts" and "%" are by mass unless otherwise specified.

[0010] <Materials for secondary batteries> The secondary battery material according to this embodiment contains at least one organic compound having a phenylenediamine structure in the molecule. a R b N-Ph-NR c R d It is a diamine structure with a backbone and two amino groups (R a R b N- and R c R d N-) may be a primary amino group, a secondary amino group, or a tertiary amino group, and may be in any of the o-, m-, or p-positions. R a R b N-R a and R b may be bonded to each other to form a ring, and R c R d N-R c and R d may be bonded to each other to form a ring. a or R b But Rc or R d By using such an organic compound as a material for a secondary battery, preferably as an active material for an electrolyte solution for a redox flow battery, particularly as an active material for an aqueous electrolyte solution, it is possible to improve the energy density and cycle characteristics of the redox flow battery.

[0011] The organic compound having a phenylenediamine structure in the molecule is preferably selected from the group consisting of compounds represented by the following formula (1) and compounds represented by the following formula (2), and more preferably a compound represented by formula (1).

[0012] [ka]

[0013] Here, the term "secondary battery" in the context of secondary battery materials refers to a battery that can be used repeatedly by charging and discharging. Charging involves passing a current from an external power source into the battery, converting electrical energy into chemical energy, while discharging involves passing a current in the opposite direction to that of charging, allowing power to be supplied to the outside.

[0014] In the compound represented by the above formula (1), R 1 ~R 10 are each independently represented by a hydrogen atom, an alkyl group, a hydroxy group, an alkoxy group, a nitro group, an amino group, a mercapto group, a cyano group, a halogen group, an aryl group, a sulfone group, a carboxyl group, a carbonyl group, a sulfonyl group, or a heteroaryl group.

[0015] In addition, in the compound represented by the above formula (2), R 11 ~R 24 each independently represents a hydrogen atom, a sulfone group, an alkyl group, an aryl group, or a carbonyl group.

[0016] In the above formulas (1) and (2), examples of the alkyl group include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, and a t-butyl group.

[0017] In the above formula (1), examples of the alkoxy group include a methoxy group, an ethoxy group, an n-propoxy group, an isopropoxy group, an n-butoxy group, and a t-butoxy group.

[0018] In the above formula (1), examples of the amino group include an amino group (-NH2), a methylamino group, a dimethylamino group, an ethylamino group, and a diethylamino group.

[0019] In the above formula (1), examples of the halogen group include a fluorine group, a chlorine group, a bromine group, and an iodine group.

[0020] In the above formulas (1) and (2), examples of the aryl group include a phenyl group and a naphthyl group.

[0021] In the above formulas (1) and (2), examples of the carbonyl group include an acetyl group, a pivaloyl group, and a benzoyl group.

[0022] In the above formula (1), examples of the sulfonyl group include a methanesulfonyl group, a p-toluenesulfonyl group, an o-nitrobenzenesulfonyl group, and a trifluoromethanesulfonyl group.

[0023] In the above formula (1), examples of the heteroaryl group include a furyl group, a thienyl group, a pyrrolyl group, a pyridyl group, and other heterocycles containing a heteroatom as a ring component.

[0024] When the compound represented by the above formula (1) and the compound represented by the formula (2) each have an acidic group such as a hydroxy group, a sulfone group, or a carboxyl group, these acidic groups may be in the form of a free acid or a salt.

[0025] When the acidic group forms a salt, it may be, for example, an alkali metal salt such as lithium salt, sodium salt, or potassium salt, an alkaline earth metal salt such as calcium salt, or an ammonium salt such as ammonium salt or tetramethylammonium salt. When the compound represented by formula (1) and the compound represented by formula (2) each have a plurality of the acidic groups, they may all be free acids, all be salts, or some may be free acids and some may be salts. Furthermore, when a plurality of salts exist for the acidic group, the salts may be the same type of salt or different types of salts.

[0026] In the compound represented by the above formula (1), R 1 ~R 8 At least one of R is preferably a sulfone group or a salt thereof, 1 ~R 8 It is more preferable that at least two of R are sulfone groups or salts thereof. 1 ~R 8 Among these, at least one is preferably selected from the group consisting of an alkyl group, an alkoxy group, a halogen group, and a carboxyl group, and at least one is a sulfone group or a salt thereof, more preferably at least one is selected from the group consisting of an alkyl group, an alkoxy group, and a halogen group, and at least two are sulfone groups or a salt thereof, more preferably at least one is selected from the group consisting of a methyl group, a methoxy group, and a chlorine group, and at least two are sulfone groups or a salt thereof, even more preferably at least one is a methyl group and two are sulfone groups or a salt thereof, even more preferably at least two are alkyl groups and at least one is a sulfone group or a salt thereof, and particularly preferably at least two are methyl groups and at least two are sulfone groups or a salt thereof. 1 ~R 8 In the case where the sulfonic acid group is a salt, the sulfonic acid group is preferably an ammonium salt of the sulfonic acid group, and particularly preferably an ammonium sulfonate group (-SO3NH4). 1 ~R 8 Among them, R 2is preferably an alkyl group, an alkoxy group, a halogen group or a carboxyl group, more preferably an alkyl group, an alkoxy group or a halogen group, more preferably a methyl group, a methoxy group or a chlorine group, and particularly preferably a methyl group. 9 and R 10 At least one of R is preferably a hydrogen atom, 9 and R 10 It is more preferred that each of R is a hydrogen atom. 9 and R 10 When one of R is a hydrogen atom, the other is preferably an alkyl group, more preferably a methyl group. 9 ~R 10 At least one of the groups is an alkyl group, R 1 ~R 8 At least one of R is preferably a sulfone group or a salt thereof, 9 ~R 10 At least one of the groups is a methyl group, and R 1 ~R 8 It is more preferable that at least two of the groups be sulfone groups or salts thereof.

[0027] As the compound represented by the above formula (1), R 1 ~R 10 A preferred combination of R 1 ~R 8 two of the groups are sulfone groups and the others are hydrogen atoms, and R 9 and R 10 are hydrogen atoms; R 2 is a methyl group, and R 1 and R 3 ~R 8 two of which are ammonium sulfonate groups and the others are hydrogen atoms, and R 9 and R 10 are hydrogen atoms; R 2 is a methoxy group, and R 1 and R 3 ~R 8 two of which are ammonium sulfonate groups and the others are hydrogen atoms, and R9 and R 10 are hydrogen atoms; R 1 ~R 8 two of them are methyl groups, two of them are ammonium sulfonate groups, and the others are hydrogen atoms; 9 and R 10 are hydrogen atoms; R 1 and R 2 are methyl groups, and R 3 ~R 8 two of which are ammonium sulfonate groups and the others are hydrogen atoms, and R 9 and R 10 are hydrogen atoms; R 2 is a chlorine atom, and R 1 and R 3 ~R 8 two of which are ammonium sulfonate groups and the others are hydrogen atoms, and R 9 and R 10 are hydrogen atoms; R 2 is a carboxyl group, and R 1 and R 3 ~R 8 one of which is a sulfo group and the other is a hydrogen atom, and R 9 and R 10 are each a hydrogen atom; and R 1 ~R 8 two of which are ammonium sulfonate groups and the others are hydrogen atoms, and R 9 and R 10 One of the groups is a methyl group and the other is a hydrogen atom. 1 ~R 8 two of the groups are sulfone groups and the others are hydrogen atoms, and R 9 and R 10 are hydrogen atoms; R 2 is a methyl group, and R 1 and R 3 ~R 8 two of which are ammonium sulfonate groups and the others are hydrogen atoms, and R 9 and R 10 are hydrogen atoms; R 2is a methoxy group, and R 1 and R 3 ~R 8 two of which are ammonium sulfonate groups and the others are hydrogen atoms, and R 9 and R 10 are hydrogen atoms; R 1 ~R 8 two of them are methyl groups, two of them are ammonium sulfonate groups, and the others are hydrogen atoms; 9 and R 10 are hydrogen atoms; R 1 and R 2 are methyl groups, and R 3 ~R 8 two of which are ammonium sulfonate groups and the others are hydrogen atoms, and R 9 and R 10 are hydrogen atoms; R 2 is a chlorine atom, and R 1 and R 3 ~R 8 two of which are ammonium sulfonate groups and the others are hydrogen atoms, and R 9 and R 10 are hydrogen atoms; R 2 is a carboxyl group, and R 1 and R 3 ~R 8 one of which is a sulfo group and the other is a hydrogen atom, and R 9 and R 10 are each a hydrogen atom, and the like.

[0028] The compound represented by the formula (1) may be used alone or in combination of two or more. When two or more types are used in combination, they can be used in any ratio.

[0029] In the compound represented by the above formula (2), R 17 ~R 24 It is preferable that at least one of R is a sulfone group or a salt thereof. 17 ~R 24In the case where the sulfonic acid group is a salt, the sulfonic acid group is preferably an ammonium salt of the sulfonic acid group, and particularly preferably an ammonium salt of sulfonic acid (—SO3NH4). 11 ~R 15 are preferably all hydrogen atoms.

[0030] The compound represented by the above formula (2) includes compounds represented by the formula (2) 17 ~R 24 At least one of the groups is a sulfone group or a salt thereof, and the others are all hydrogen atoms, and R 11 ~R 15 are preferably compounds in which each of R is a hydrogen atom, 20 is more preferably a sulfone group.

[0031] The compound represented by the formula (2) may be used alone or in combination of two or more. When two or more types are used in combination, they can be used in any ratio.

[0032] When the organic compound is selected from the group consisting of the compound represented by the above formula (1) and the compound represented by the following formula (2), any one of such organic compounds may be used alone or two or more may be used in combination. Furthermore, when two or more of the compound represented by the above formula (1) and the compound represented by the following formula (2) are used in combination, they may be used in any ratio.

[0033] The compound represented by the above formula (1) and the compound represented by the formula (2) may be isomers such as tautomers, stereoisomers, or optical isomers, and each may be any isomer or may be a mixture of different isomers.

[0034] The solubility of the compound represented by formula (1) and the compound represented by formula (2) in water is preferably in the range of 0.1 mol / L to 5 mol / L, with the lower limit being more preferably 0.2 mol / L and the upper limit being more preferably 3 mol / L. The solubility is expressed as the value (mol / L) when a sample aqueous solution is prepared by adding an acid or alkali to water containing the compound represented by formula (1) or the compound represented by formula (2), and the maximum amount of the compound represented by formula (1) or the compound represented by formula (2) is dissolved. Specifically, the absorbance of the prepared sample aqueous solution is measured, and the value calculated from a calibration curve prepared for the absorbance at the maximum absorption wavelength is expressed as the solubility.

[0035] <Active material> The active material according to this embodiment contains at least one of the organic compounds described above. By incorporating such an active material into an electrolyte, preferably an electrolyte for a redox flow battery, more preferably an aqueous electrolyte for a redox flow battery, the energy density and cycle characteristics of the redox flow battery can be improved. The active material may contain one or more of the materials for a secondary battery described above.

[0036] The active material is preferably a redox active material, and more preferably contained as a redox active material in the electrolyte for the redox flow battery. The active material may be contained in both the positive electrode electrolyte and the negative electrode electrolyte active material, or in either one of them.

[0037] <Electrolyte> The electrolyte solution according to this embodiment contains the active material described above. The active material according to this embodiment is preferably an electrolyte solution for a redox flow battery, since it enables the energy density and cycle characteristics of the redox flow battery to be improved. The electrolyte solution may contain one or more of the active materials described above. When two or more active materials are used in combination, they can be blended in any ratio. The concentration of the active material contained in the electrolyte solution is preferably in the range of 0.1 mol / L to 5 mol / L, with the lower limit being more preferably 0.2 mol / L and the upper limit being more preferably 3 mol / L.

[0038] The electrolytic solution may be an aqueous electrolytic solution containing water. When the electrolytic solution contains water, the water may be, for example, ion-exchanged water or Millipore water, and is preferably Millipore water.

[0039] The water content in the electrolytic solution can be set arbitrarily, but is preferably 1% by mass or more and 99% by mass or less, more preferably 5% by mass or more and 90% by mass or less, and particularly preferably 10% by mass or more and 80% by mass or less.

[0040] The electrolytic solution may further contain an antifoaming agent. Examples of antifoaming agents include alcohols such as methanol, ethanol, and propanol; ketones such as acetone and methyl ethyl ketone; polyhydric alcohols such as ethylene glycol, diethylene glycol, propylene glycol, and glycerin; and various commercially available antifoaming agents. Among these, the antifoaming agent is preferably an alcohol, and more preferably ethanol. The content of the antifoaming agent in the electrolytic solution is not particularly limited, but is preferably 0.1% by mass or more and 10% by mass or less, and more preferably 0.5% by mass or more and 8% by mass or less, relative to the amount of water contained in the electrolytic solution.

[0041] The electrolyte solution may further contain a pH buffer. The pH buffer is preferably an organic acid salt or an inorganic acid salt having a pKa of 2 or more and 13 or less. Examples of such pH buffers include lithium carbonate, sodium carbonate, potassium carbonate, ammonium carbonate, sodium bicarbonate, potassium bicarbonate, ammonium chloride, ammonium sulfate, lithium acetate, sodium acetate, potassium acetate, ammonium acetate, lithium formate, sodium formate, potassium formate, ammonium formate, trilithium phosphate, lithium dihydrogen phosphate, trisodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, tripotassium phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, triammonium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, acetic acid, phosphoric acid, formic acid, glycine, alanine, valine, leucine, isoleucine, serine, asparagine, glutamine, arginine, and lysine. Among these, the pH buffer is preferably ammonium chloride, ammonium dihydrogen phosphate, ammonium formate, ammonium acetate, or glycine, with ammonium acetate being particularly preferred. These pH buffers may be used alone or in combination of two or more. When two or more types are combined, they can be blended in any ratio.

[0042] The electrolytic solution may further contain any electrolyte, such as lithium hydroxide, sodium hydroxide, potassium hydroxide, lithium chloride, sodium chloride, potassium chloride, lithium sulfate, sodium sulfate, potassium sulfate, sulfuric acid, or hydrochloric acid, with sodium chloride or sodium hydroxide being preferred.

[0043] Examples of methods for preparing the electrolyte solution include a method in which an active material in an oxidized or reduced state is dissolved in a solution containing water or the like, and a method in which the valence of a solution in which the active material is dissolved is adjusted by electrolysis.

[0044] <Secondary battery> The secondary battery according to this embodiment contains the above-described electrolyte solution and is preferably a redox flow battery. The secondary battery preferably includes a positive electrode electrolyte solution, a negative electrode electrolyte solution, a positive electrode, a negative electrode, and a diaphragm. The positive electrode electrolyte solution and the negative electrode electrolyte solution may be the same electrolyte solution or different electrolyte solutions. When the positive electrode electrolyte solution and the negative electrode electrolyte solution are the same electrolyte solution, both electrolyte solutions are preferably the above-described electrolyte solution. When the positive electrode electrolyte solution and the negative electrode electrolyte solution are different electrolyte solutions, the positive electrode electrolyte solution and the negative electrode electrolyte solution may each contain the above-described electrolyte solution with different active materials, or one electrolyte solution may be the above-described electrolyte solution and the other electrolyte solution may be another electrolyte solution different from the above-described electrolyte solution. In the latter case, the electrolyte solution according to this embodiment is preferably the positive electrode electrolyte solution.

[0045] When the negative electrode electrolyte is an electrolyte other than the above-described electrolyte, i.e., an electrolyte containing the above-described organic compound as an active material, the negative electrode electrolyte is not particularly limited as long as it is an electrolyte containing an active material that functions as a negative electrode. Examples of the active material include compounds having an anthraquinone skeleton, compounds having a naphthoquinone skeleton, compounds having a benzoquinone skeleton, compounds having a phenazine skeleton, compounds having a viologen skeleton, vanadium, chromium, and zinc. Compounds having an anthraquinone skeleton, compounds having a phenazine skeleton, or zinc are preferred, with zinc being particularly preferred. Here, "zinc" refers to divalent zinc ions dissolved in the negative electrode electrolyte. The zinc ions, which are the active material of the negative electrode electrolyte, are reduced from divalent to zero during charging and oxidized from zero to divalent during discharging. Specific compounds present as "zinc ions" in the negative electrode electrolyte include zinc compounds such as zinc sulfate, zinc acetate, zinc formate, zinc chloride, zinc bromide, and zinc hydroxide, with zinc acetate, zinc hydroxide, and zinc chloride being preferred. The content of the zinc compound contained in the negative electrode electrolyte, calculated as the molar concentration of zinc ions, is preferably in the range of 0.1 M to 5.0 M, and more preferably in the range of 0.4 M to 3.0 M.

[0046] In addition to the above-described active materials, such a negative electrode electrolyte may optionally contain water, an antifoaming agent, a pH buffering agent, an electrolyte, etc. Such water, antifoaming agent, pH buffering agent, and electrolyte may contain the same substances as the water, antifoaming agent, pH buffering agent, and electrolyte described for the electrolyte solution according to this embodiment.

[0047] When the positive electrode electrolyte contains the organic compound described above as an active material and the negative electrode electrolyte contains the zinc ion described above as an active material, the difference between the oxidation-reduction potential of the organic compound and the oxidation-reduction potential of the zinc ion is not particularly limited, but is preferably 0.8 V or more and 2.1 V or less, and more preferably 1.0 V or more and 2.1 V or less.

[0048] The positive electrode and the negative electrode can be selected arbitrarily as long as they function as electrodes, but for example, carbon felt, carbon paper, and carbon cloth are preferable, and carbon felt is more preferable. Furthermore, when the negative electrode electrolyte contains zinc, a zinc plate may be further used for the negative electrode.

[0049] Any diaphragm can be used as long as it functions as a diaphragm between electrodes, but for example, an ion exchange membrane, a porous membrane, etc. is preferred, and an ion exchange membrane is more preferred. Examples of ion exchange membranes include Nafion (registered trademark).

[0050] The secondary battery may further include a current collector plate, a container, a sealant, screws, a bipolar plate, and the like, as needed.

[0051] The secondary battery material of the present invention can be synthesized as an organic compound, and therefore has few resource limitations. Furthermore, when used as an active material in an electrolyte for a secondary battery, particularly a redox flow battery, the secondary battery has a high energy density and good cycle characteristics. In particular, because it enables a redox flow battery using an aqueous electrolyte to achieve a high energy density, it is safer and easier to handle and maintain during the fabrication of the redox flow battery, electrolyte replacement, and the like, compared to electrolytes using organic solvents, etc. Furthermore, because the secondary battery material of the present invention has excellent solubility in water, it can be highly concentrated in an aqueous electrolyte, enabling a high energy density to be achieved.

[0052] Based on the above-described embodiments, the present invention relates to the following [1] to

[17] . [1] A secondary battery material comprising at least one organic compound having a phenylenediamine structure in the molecule. [2] The material for a secondary battery according to the above [1], wherein the organic compound is selected from the group consisting of compounds represented by the following formula (1) and compounds represented by the following formula (2): [ka] (In formula (1), R 1 ~R 10 are each independently a hydrogen atom, an alkyl group, a hydroxy group, an alkoxy group, a nitro group, an amino group, a mercapto group, a cyano group, a halogen group, an aryl group, a sulfone group, a carboxyl group, a carbonyl group, a sulfonyl group, or a heteroaryl group, and in formula (2), R 11 ~R 24 each independently represents a hydrogen atom, a sulfone group, an alkyl group, an aryl group, or a carbonyl group. [3] In the above formula (1), R 1 ~R 8 The material for a secondary battery according to the above [1] or [2], wherein at least one of the groups is a sulfonic group or a salt thereof. [4] In the above formula (1), R1 ~R 8 At least two of the above are sulfonic acid groups or salts thereof. [5] An active material comprising at least one secondary battery material according to any one of [1] to [4] above. [6] The active material according to [5] above, wherein the active material is a redox active material. [7] An electrolyte solution containing the active material according to [5] or [6] above. [8] The electrolyte solution according to [7] above, which is an electrolyte solution for a redox flow battery. [9] The electrolyte solution according to the above [7] or [8], further comprising a pH buffer.

[10] The electrolyte solution according to [9] above, wherein the pH buffer is an organic acid salt or an inorganic acid salt having a pKa of 2 or more and 13 or less.

[11] A secondary battery comprising the electrolyte solution according to any one of [7] to

[10] above.

[12] The secondary battery according to

[11] above, wherein the secondary battery is a redox flow battery.

[13] The secondary battery according to the above

[11] or

[12] , wherein the electrolytic solution is an electrolytic solution for a positive electrode.

[14] The secondary battery according to the above

[13] , further comprising a negative electrode electrolyte, a positive electrode, a negative electrode, and a diaphragm.

[15] The secondary battery according to

[14] above, wherein the negative electrode electrolyte contains zinc ions.

[16] The secondary battery according to

[15] above, wherein the difference between the oxidation-reduction potential of the organic compound and the oxidation-reduction potential of the zinc ion is 0.8 V or more and 2.1 V or less.

[17] The secondary battery according to any one of the above

[14] to

[16] , wherein the diaphragm is an ion exchange membrane. [Example]

[0053] The present invention will be described in further detail below, but the present invention is not limited thereto. Furthermore, unless otherwise specified, room temperature is within the range of 20°C ± 5°C.

[0054] [Synthesis Example 1] 25.2 parts of 2,3-dichloroquinoxaline and 13.7 parts of o-phenylenediamine were dissolved in 500 parts of N,N-dimethylformamide (DMF) and heated to 120°C. After stirring for 3 hours while maintaining 120°C, the mixture was allowed to cool to room temperature, and the precipitate was separated by filtration under reduced pressure to obtain 50 parts of a wet cake containing the compound represented by formula (I) below. 200 parts of acetone was added to this wet cake, and the mixture was stirred for 30 minutes. The precipitate was separated by filtration under reduced pressure to obtain a wet cake, which was further dried at 80°C for 1 day to obtain 23.4 parts of the compound represented by formula (I) below.

[0055] [ka]

[0056] [Synthesis Example 2] 2.5 parts of compound (I) obtained in Synthesis Example 1 above was added over 30 minutes to 30 parts of 15% fuming sulfuric acid cooled to below 10°C, and after the addition was completed, the mixture was stirred for 5 hours at below 15°C. After the reaction was completed, the reaction solution was added dropwise to 100 parts of ice water and stirred for 30 minutes at below 20°C. The precipitate was then filtered and separated under reduced pressure to obtain 10 parts of a wet cake containing a compound represented by formula (II) below. 100 parts of isopropanol was added to the wet cake, and the mixture was stirred for 30 minutes. The precipitate was then filtered and separated under reduced pressure to obtain a wet cake. The wet cake was further dried at 80°C for 1 day to obtain 3.8 parts of a sulfonated mixture containing a compound represented by formula (II) below.

[0057] [ka]

[0058] [Example 1] The solubility in water of the compound represented by formula (II) obtained in Synthesis Example 2 above was calculated from the absorbance. A UV-visible spectrophotometer (UV-1700, manufactured by Shimadzu Corporation) was used to measure the absorbance. A sample solution of known concentration was prepared using a standard buffer solution (Fujifilm Wako Pure Chemical Industries, neutral phosphate pH standard solution, pH 6.86 (25°C)), and the absorbance at the maximum absorption wavelength was measured using the UV-visible spectrophotometer in the wavelength range of 300 nm to 550 nm. A calibration curve was created from the obtained absorbance and concentration. Subsequently, a saturated solution of the sample was prepared using an aqueous solution containing an electrolyte (6N aqueous ammonia) and diluted with the standard buffer solution. The absorbance at the maximum absorption wavelength was measured, and the solubility (%) was calculated from the calibration curve. The density of the solution was 1.0 g / cm. 3 The solubility (mol / L) was calculated assuming the following. The results are shown in Table 1.

[0059] [Table 1]

[0060] [Synthesis Example 3] 17.6 parts of 1,2-phenylenediamine and 25 parts of 2,5-dihydroxy-1,4-benzoquinone were heated to reflux in 3,000 parts of water with stirring for 5 hours and 30 minutes, then cooled to room temperature and stirred overnight. A black wet cake was obtained from the resulting suspension by filtration and washed with water. This wet cake was dried under reduced pressure at 80°C to obtain 103.2 parts of a wet cake containing 0.163 mol of the compound represented by formula (III):

[0061] [ka]

[0062] [Synthesis Example 4] 51.2 parts of a wet cake containing 0.0808 moles of compound (III) obtained in Synthesis Example 3 above and 36.6 parts of 1,8-diazabicyclo[5,4,0]undec-7-ene were dissolved in 410 parts of dimethylformamide, and 30.2 parts of 1,3-propane sultone was added. The mixture was then heated to 120°C and stirred for 3 hours. The mixture was then cooled to room temperature, and an excess of 28% aqueous ammonia was added and stirred for 30 minutes. The resulting reaction solution was poured into 3.0 L of acetone, and the precipitated solid was separated by filtration to obtain a wet cake. This was dissolved in 70 parts of water, and 10 parts of 25% aqueous sodium hydroxide was added. The mixture was then poured into 1.5 L of ethanol, and the precipitated solid was separated by filtration to obtain a red wet cake. The wet cake was dried under reduced pressure at 80°C to obtain 34.8 parts of a compound represented by the following formula (IV):

[0063] [ka]

[0064] [Example 2] The compound represented by formula (II) above was dissolved in an aqueous solution (1.0 mol / L) of ammonium dihydrogen phosphate (special grade, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) to a concentration of 0.1 mol / L, and ammonia water was added to adjust the pH to 8.5, to prepare positive electrode electrolyte 1. Meanwhile, the compound represented by formula (IV) above was dissolved in an aqueous solution (1.0 mol / L) of ammonium dihydrogen phosphate (special grade, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) to a concentration of 0.1 (mol / L), and ammonia water was added to adjust the pH to 8.5, to prepare negative electrode electrolyte 1.

[0065] An ion-exchange membrane (Sigma-Aldrich Japan, LLC, "Nafion® NRE-212") was used as the diaphragm, and carbon felt (Toyobo Co., Ltd., "AAF304ZS", 10 mm × 50 mm × 4 mm) was used as the positive and negative electrodes. The carbon felt was placed in a 50 mm × 10 mm hole drilled in the center of a silicone gasket (80 mm × 30 mm × 3 mm), and the electrodes were assembled in the following order: current collector / positive electrode / diaphragm / negative electrode / current collector. The prepared cathode electrolyte 1 and anode electrolyte 1 were placed in vials, respectively, and the electrolytes were filled into the cell through the diaphragm using a peristaltic pump, thereby producing redox flow battery 1.

[0066] The redox flow battery 1 was placed in a glove box (manufactured by UNICO Corporation, "UL-1300A") equipped with a gas circulation purification device (manufactured by UNICO Corporation, "MF-100"), and electrochemical measurements were performed under conditions where the oxygen concentration was 1 ppm or less.

[0067] The positive electrode electrolyte 1 and the negative electrode electrolyte 1 of the redox flow battery 1 were circulated by a peristaltic pump connected to the outside of the battery, and testing was performed using a multi-electrochemical measurement system (HZ-Pro, manufactured by Hokuto Denko Corporation). The volumes of the positive electrode electrolyte 1 and the negative electrode electrolyte 1 were 6 mL and 7 mL, respectively, and a charge-discharge test was performed at a constant current of 105 mA with an upper voltage limit of 1.3 V and a lower voltage limit of 0.4 V. Figure 1 shows the charge-discharge curves for the first 10 cycles of the redox flow battery 1. As shown in Table 2, the coulombic efficiency at the 10th cycle was 100%, the voltage efficiency was 83%, and the energy density was 0.98 Wh / L, indicating high energy density and good cycle characteristics. Furthermore, as shown in Table 5, the average discharge voltage at the 5th cycle was 0.65 V and the energy density was 1.00 Wh / L.

[0068] [Table 2]

[0069] As shown in Table 2 and FIG. 1, it is clear that the redox flow battery 1 produced in Example 2 has a high energy density and good cycle characteristics.

[0070] [Example 3] The compound represented by formula (II) was dissolved in an aqueous solution (1.0 mol / L) of ammonium acetate (manufactured by Junsei Chemical Co., Ltd., special grade) to a concentration of 0.1 mol / L, and ammonia water was added to adjust the pH to 5.9 to prepare positive electrode electrolyte 2. Meanwhile, zinc chloride (manufactured by Tokyo Chemical Industry Co., Ltd., purity >98.0%) was dissolved in an aqueous solution (1.0 mol / L) of ammonium acetate (manufactured by Junsei Chemical Co., Ltd., special grade) to a concentration of 1.0 (mol / L), to prepare negative electrode electrolyte 2.

[0071] An ion-exchange membrane (Sigma-Aldrich Japan, LLC, "Nafion® NRE-212") was used as the diaphragm, and carbon felt (Toyobo Co., Ltd., "AAF304ZS", 10 mm × 50 mm × 4 mm) was used as the positive and negative electrodes. The carbon felt was placed in a hole drilled in the center of a silicone gasket (80 mm × 30 mm × 3 mm). For the negative electrode, a zinc plate (Sigma-Aldrich Japan, LLC, 10 mm × 50 mm × 0.25 mm, 99.9%) was sandwiched between the current collector and the electrode, and the cell was assembled in the following order: current collector / positive electrode / diaphragm / negative electrode / zinc plate / current collector. The prepared positive electrode electrolyte 2 and negative electrode electrolyte 2 were placed in respective vials as electrolytes, and the redox flow battery 2 was produced by filling each electrolyte inside the cell with a peristaltic pump across a diaphragm.

[0072] The positive electrode electrolyte 2 and the negative electrode electrolyte 2 of the redox flow battery 2 were circulated using a peristaltic pump connected to the outside of the battery, and testing was performed using a multi-electrochemical measurement system (HZ-Pro, manufactured by Hokuto Denko Corporation). The volume of the positive electrode electrolyte 2 and the negative electrode electrolyte 2 was 6 mL, and a charge-discharge test was performed at a constant current of 105 mA with an upper voltage limit of 1.7 V and a lower voltage limit of 0.5 V. Figure 2 shows the charge-discharge curves for redox flow battery 2 up to five cycles. As shown in Table 3, the average discharge voltage at the fifth cycle was 1.23 V, the coulombic efficiency was 100%, the voltage efficiency was 91%, and the energy density was 2.53 Wh / L, indicating high energy density and good cycle characteristics. Furthermore, as shown in Table 6, the average discharge voltages at the first and fifth cycles were 1.24 V and 1.23 V, respectively, with little change in the average discharge voltage from cycle to cycle.

[0073] [Table 3]

[0074] As shown in Table 3 and FIG. 2, it is clear that the redox flow battery 2 produced in Example 3 has a high energy density and good cycle characteristics.

[0075] [Synthesis Example 5] 13.0 parts of 2,3-dichloroquinoxaline and 8.00 parts of 3,4-diaminotoluene were dissolved in 130 parts of DMF and heated to 80°C. After stirring for 6 hours while maintaining the temperature at 80°C, the mixture was allowed to cool to room temperature, and the reaction solution was poured into 300 parts of water with stirring. The precipitate was separated by filtration under reduced pressure and washed with methanol and acetone to obtain a wet cake containing the compound represented by formula (V). This wet cake was dried under reduced pressure at 80°C for 2 hours to obtain 9.82 parts of the compound represented by formula (V) below.

[0076] [ka]

[0077] [Synthesis Example 6] 10.0 parts of the compound represented by formula (V) obtained in Synthesis Example 5 above was added over 30 minutes to 120 parts of 15% fuming sulfuric acid cooled to 0°C, and the mixture was stirred at 0°C for 30 minutes. The temperature was gradually raised to room temperature over 30 minutes, and the mixture was stirred at room temperature for 18 hours. After the reaction was completed, the reaction solution was added dropwise to 500 parts of ice with stirring, and the mixture was stirred at room temperature for 30 minutes. The precipitate was separated by filtration under reduced pressure and washed with cold water and acetic acid to obtain a wet cake containing a compound represented by formula (VI) below. This wet cake was added to 30 parts of 15% aqueous ammonia, and the resulting aqueous solution was poured into 500 parts of acetone. The precipitate was separated by filtration under reduced pressure and washed with acetone and ethanol. The obtained wet cake was dried under reduced pressure at 80°C for 6 hours to obtain 3.40 parts of a sulfonated mixture containing a compound represented by formula (VI) below.

[0078] [ka]

[0079] [Example 4] The compound represented by the formula (VI) above was dissolved in water to a concentration of 0.1 mol / L, ammonium acetate (manufactured by Junsei Chemical Co., Ltd., special grade) to a concentration of 0.5 mol / L, and acetic acid (manufactured by Junsei Chemical Co., Ltd., special grade) to a concentration of 0.5 mol / L, respectively, to prepare a positive electrode electrolyte 3. Meanwhile, zinc acetate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity 99.9%) was dissolved in water to a concentration of 0.2 mol / L, ammonium acetate (manufactured by Junsei Chemical Co., Ltd., special grade) to a concentration of 0.5 mol / L, and acetic acid (manufactured by Junsei Chemical Co., Ltd., special grade) to a concentration of 0.5 mol / L, respectively, to prepare a negative electrode electrolyte 3.

[0080] An ion-exchange membrane (Sigma-Aldrich Japan, LLC, "Nafion® NRE-212") was used as the diaphragm, and carbon felt (Toyobo Co., Ltd., "AAF304ZS", 10 mm × 50 mm × 4 mm) was used as the positive and negative electrodes. The carbon felt was placed in a hole drilled in the center of a silicone gasket (80 mm × 30 mm × 3 mm). For the negative electrode, a zinc plate (Sigma-Aldrich Japan, LLC, 10 mm × 50 mm × 0.25 mm, 99.9%) was sandwiched between the current collector and the electrode, and the cell was assembled in the following order: current collector / positive electrode / diaphragm / negative electrode / zinc plate / current collector. The prepared positive electrode electrolyte 3 and negative electrode electrolyte 3 were placed in respective vials as electrolytes, and the redox flow battery 3 was produced by filling each electrolyte inside the cell with a peristaltic pump across a diaphragm.

[0081] The positive electrode electrolyte 3 and the negative electrode electrolyte 3 of the redox flow battery 3 were circulated using a peristaltic pump connected to the outside of the battery, and testing was performed using a multi-electrochemical measurement system (HZ-Pro, manufactured by Hokuto Denko Corporation). The volume of the positive electrode electrolyte 3 and the negative electrode electrolyte 3 was 6 mL, and a charge-discharge test was performed at a constant current of 105 mA with an upper voltage limit of 2.1 V and a lower voltage limit of 0.5 V. Figure 3 shows the charge-discharge curves for the first five cycles of the redox flow battery 3. As shown in Table 4, the average discharge voltage at the fifth cycle was 1.22 V, the coulombic efficiency was 100%, the voltage efficiency was 84%, and the energy density was 1.68 Wh / L, indicating high energy density and favorable cycle characteristics. Furthermore, as shown in Table 6, the average discharge voltages at the first and fifth cycles were 1.24 V and 1.22 V, respectively, with little change in the average discharge voltage from cycle to cycle.

[0082] [Table 4]

[0083] As shown in Table 4 and FIG. 3, it is clear that the redox flow battery 3 produced in Example 4 has a high energy density and good cycle characteristics.

[0084] [Example 5] The compound represented by formula (II) was dissolved in an aqueous solution (1.0 mol / L) of ammonium formate (manufactured by Junsei Chemical Co., Ltd., special grade) so that the concentration was 0.1 mol / L, and ammonia water was added to adjust the pH to 5, to prepare positive electrode electrolyte 4. Meanwhile, zinc chloride (manufactured by Tokyo Chemical Industry Co., Ltd., purity >98.0%) was dissolved in an aqueous solution (1.0 mol / L) of ammonium formate (manufactured by Junsei Chemical Co., Ltd., special grade) so that the concentration was 1.0 (mol / L), to prepare negative electrode electrolyte 4.

[0085] An ion-exchange membrane (Sigma-Aldrich Japan, LLC, "Nafion® NRE-212") was used as the diaphragm, and carbon felt (Toyobo Co., Ltd., "AAF304ZS", 10 mm × 50 mm × 4 mm) was used as the positive and negative electrodes. The carbon felt was placed in a hole drilled in the center of a silicone gasket (80 mm × 30 mm × 3 mm). For the negative electrode, a zinc plate (Sigma-Aldrich Japan, LLC, 10 mm × 50 mm × 0.25 mm, 99.9%) was sandwiched between the current collector and the electrode, and the cell was assembled in the following order: current collector / positive electrode / diaphragm / negative electrode / zinc plate / current collector. The prepared positive electrode electrolyte 4 and negative electrode electrolyte 4 were placed in respective vials as electrolytes, and the redox flow battery 4 was produced by filling each electrolyte into the cell with a peristaltic pump across a diaphragm.

[0086] The positive electrode electrolyte 4 and the negative electrode electrolyte 4 of the redox flow battery 4 were circulated using a peristaltic pump connected to the outside of the battery, and testing was performed using a multi-electrochemical measurement system (HZ-Pro, manufactured by Hokuto Denko Corporation). The volume of the positive electrode electrolyte 4 and the negative electrode electrolyte 4 was 6 mL, and a charge-discharge test was performed at a constant current of 105 mA with an upper voltage limit of 1.7 V and a lower voltage limit of 0.5 V. Figure 4 shows the charge-discharge curves for the first five cycles of the redox flow battery 4. As shown in Table 5, the average discharge voltage at the fifth cycle was 1.28 V, and the energy density was 2.64 Wh / L, indicating high energy density and good cycle characteristics. Furthermore, as shown in Table 6, the average discharge voltages at the first and fifth cycles were both 1.28 V, with no change in average discharge voltage from cycle to cycle. Furthermore, the coulombic efficiency was 100% and the voltage efficiency was 91%.

[0087] [Example 6] A compound represented by the following formula (VII) (5-amino-2-[(p-aminophenyl)amino]benzenesulfonic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was dissolved in an aqueous solution (3.0 mol / L) of sodium hydroxide (manufactured by Junsei Chemical Co., Ltd., special grade) to a concentration of 0.1 mol / L, to prepare electrolyte solution 5 for the positive electrode. Meanwhile, zinc acetate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 99.9%) was dissolved in an aqueous solution (3.0 mol / L) of sodium hydroxide (manufactured by Junsei Chemical Co., Ltd., special grade) to a concentration of 0.1 (mol / L), to prepare electrolyte solution 5 for the negative electrode.

[0088] [ka]

[0089] An ion-exchange membrane (Sigma-Aldrich Japan, LLC, "Nafion® NRE-212") was used as the diaphragm, and carbon felt (Toyobo Co., Ltd., "AAF304ZS", 10 mm × 50 mm × 4 mm) was used as the positive and negative electrodes. The carbon felt was placed in a hole drilled in the center of a silicone gasket (80 mm × 30 mm × 3 mm). For the negative electrode, a zinc plate (Sigma-Aldrich Japan, LLC, 10 mm × 50 mm × 0.25 mm, 99.9%) was sandwiched between the current collector and the electrode, and the cell was assembled in the following order: current collector / positive electrode / diaphragm / negative electrode / zinc plate / current collector. The prepared positive electrode electrolyte 5 and negative electrode electrolyte 5 were placed in respective vials as electrolytes, and the redox flow battery 5 was produced by filling each electrolyte with a peristaltic pump inside the cell across a diaphragm.

[0090] The positive electrode electrolyte 5 and the negative electrode electrolyte 5 of the redox flow battery 5 were circulated using a peristaltic pump connected to the outside of the battery, and testing was performed using a multi-electrochemical measurement system (HZ-Pro, manufactured by Hokuto Denko Corporation). The volume of the positive electrode electrolyte 5 and the negative electrode electrolyte 5 was 6 mL, and a charge-discharge test was performed at a constant current of 105 mA with an upper voltage limit of 1.5 V and a lower voltage limit of 0.5 V. Figure 5 shows the charge-discharge curve for the first five cycles of the redox flow battery 5. As shown in Table 5, the average discharge voltage at the fifth cycle was 1.04 V and the energy density was 1.23 Wh / L, demonstrating high energy density and favorable cycle characteristics. The coulombic efficiency was 90%, and the voltage efficiency was 83%.

[0091] [Example 7] Positive electrode electrolyte 1, negative electrode electrolyte 1, and redox flow battery 1 were prepared using the same procedures as in Example 2. These were designated positive electrode electrolyte 6, negative electrode electrolyte 6, and redox flow battery 6, respectively. The positive electrode electrolyte 6 and negative electrode electrolyte 6 of redox flow battery 6 were circulated using a peristaltic pump connected to the outside of the battery via piping, and testing was performed using a multi-electrochemical measurement system (HZ-Pro, manufactured by Hokuto Denko Corporation). The volumes of positive electrode electrolyte 6 and negative electrode electrolyte 6 were 6 mL and 7 mL, respectively, and a charge-discharge test was performed at a constant current of 105 mA with an upper limit voltage of 1.3 V and a lower limit voltage of 0.4 V. Figure 6 shows the charge-discharge curves for redox flow battery 6 up to five cycles. As shown in Table 5, the average discharge voltage at the fifth cycle was 0.65 V, and the energy density was 1.00 Wh / L.

[0092] [Table 5]

[0093] As shown in Table 5 and FIGS. 4 to 6, redox flow batteries 4 and 5, which were fabricated using a negative electrode electrolyte containing zinc ions as the active material, exhibited higher energy density and higher cycle characteristics.

[0094] [Example 8] Redox flow battery 7 was fabricated in the same manner as in Example 3, except that the ammonium acetate used as the pH buffer in Example 3 was replaced with ammonium chloride and glycine was further added. A charge-discharge test was also performed using redox flow battery 7 in the same manner as in Example 3. Figure 7 shows the charge-discharge curves for redox flow battery 7 up to the fifth cycle. As shown in Table 6, the average discharge voltages for the first and fifth cycles were 1.35 V, respectively, demonstrating good cycle characteristics and no change in the average discharge voltage between cycles.

[0095] [Example 9] Redox flow battery 8 was fabricated in the same manner as in Example 3, except that ammonium acetate used as the pH buffer in Example 3 was replaced with ammonium sulfate. A charge-discharge test was also performed using redox flow battery 8 in the same manner as in Example 3. Figure 8 shows the charge-discharge curves for redox flow battery 8 up to the fifth cycle. As shown in Table 6, the average discharge voltages for the first and fifth cycles were 1.41 V and 1.35 V, respectively, and a slight decrease in the average discharge voltage was observed with each cycle.

[0096] [Table 6]

[0097] 2 to 4, 7, and 8, it was observed that the average discharge voltage for each cycle changed depending on the type of pH buffer used. In particular, the redox flow batteries 2 to 4 and 7 fabricated in Examples 3 to 5 and 8 showed little or no difference in the average discharge voltage for each cycle, and exhibited superior cycle characteristics.

[0098] [Synthesis Example 7] 14.6 parts of 2,3-dichloroquinoxaline and 9.67 parts of 3,4-diaminoanisole were dissolved in 140 parts of 1-butanol and refluxed. After stirring for 6 hours while maintaining reflux, the mixture was allowed to cool to room temperature, 80 parts of ethanol was added, and the precipitate was separated by filtration under reduced pressure to obtain a wet cake containing a compound represented by formula (VIII) below. This wet cake was washed with methanol, ethanol, and acetone, and the precipitate was separated by filtration under reduced pressure. The wet cake obtained was heated and dried under reduced pressure at 70°C for 1.5 hours, and then dried under reduced pressure for 15.5 hours to obtain 16.5 parts of a compound represented by formula (VIII) below.

[0099] [ka]

[0100] [Synthesis Example 8] 10.0 parts of compound (VIII) obtained in Synthesis Example 7 above was added over 30 minutes to 125 parts of 15% fuming sulfuric acid cooled to below 10°C. After the addition, the mixture was stirred at below 15°C for 25 hours. After the reaction was completed, the reaction solution was added dropwise to 250 parts of ice water and stirred at below 20°C for 30 minutes. The precipitate was then filtered and separated under reduced pressure to obtain a wet cake containing a compound represented by formula (IX): This wet cake was washed with acetic acid, and the precipitate was filtered and separated under reduced pressure. The resulting wet cake was neutralized with 100 parts of 15% aqueous ammonia to obtain a basic aqueous solution. This aqueous solution was added dropwise to 550 parts of acetone, and the precipitate was filtered and separated under reduced pressure to obtain a wet cake. 160 parts of ethanol was added to the wet cake, and the mixture was stirred for 30 minutes. The precipitate was then filtered and separated under reduced pressure to obtain a wet cake. This procedure was repeated twice, and the resulting wet cake was washed with ethanol and acetone. This wet cake was dried under reduced pressure at 80°C for 5 hours to obtain 14.0 parts of a sulfonated mixture containing a compound represented by formula (IX):

[0101] [ka]

[0102] [Synthesis Example 9] 12.1 parts of 3,4-diaminotoluene was dissolved in 80 parts of 4 M hydrochloric acid and refluxed. After stirring for 3 hours while maintaining reflux, the mixture was allowed to cool to room temperature, and the precipitate was separated by filtration under reduced pressure to obtain a wet cake containing a compound represented by the following formula (X). This wet cake was washed with water, and the precipitate was separated by filtration under reduced pressure. The wet cake obtained was heated and dried under reduced pressure at 80°C for 8 hours to obtain 17.3 parts of a quinoxaline compound represented by the following formula (X).

[0103] [ka]

[0104] [Synthesis Example 10] 10.6 parts of the quinoxaline compound obtained in Synthesis Example 9 above and 0.4 parts of DMF were dissolved in 63 parts of 1,2-dichloroethane and refluxed. 17.1 parts of thionyl chloride were added, and the mixture was stirred for 4 hours while maintaining reflux. After that, the mixture was ice-cooled to 10°C or below, and water was added to stop the reaction. The reaction product was extracted with chloroform, and the solvent was distilled off under reduced pressure. The resulting solid was dried under reduced pressure for 1 day to obtain 9.40 parts of a quinoxaline compound represented by the following formula (XI):

[0105] [ka]

[0106] [Synthesis Example 11] 9.40 parts of the quinoxaline compound obtained in Synthesis Example 10 above and 5.13 parts of 3,4-diaminotoluene were dissolved in 70 parts of 1-butanol and refluxed. After stirring for 6 hours while maintaining reflux, the mixture was allowed to cool to room temperature, and the precipitate was separated by filtration under reduced pressure to obtain a wet cake containing a compound represented by formula (XI) below. This wet cake was washed with ethanol, and the precipitate was separated by filtration under reduced pressure to obtain a wet cake that was heated and dried under reduced pressure at 80°C for 1 hour, and then further dried under reduced pressure for 15.5 hours to obtain 7.40 parts of a compound represented by formula (XII) below.

[0107] [ka]

[0108] [Synthesis Example 12] 7.00 parts of compound (XII) obtained in Synthesis Example 11 above was added over 30 minutes to 90 parts of 15% fuming sulfuric acid cooled to below 10°C. After the addition, the mixture was stirred at below 15°C for 2 hours. After the reaction was completed, the reaction solution was added dropwise to 200 parts of ice water and stirred at below 20°C for 30 minutes. The precipitate was then filtered and separated under reduced pressure to obtain a wet cake containing a compound represented by formula (XIII). The wet cake was washed with acetic acid, and the precipitate was filtered and separated under reduced pressure. The resulting wet cake was neutralized with 15% aqueous ammonia and filtered and separated under reduced pressure to obtain a basic aqueous solution. The solvent from this aqueous solution was evaporated under reduced pressure, and the resulting solid was separated by reverse-phase column chromatography. The solvent from the resulting solution was evaporated under reduced pressure to obtain 2.40 parts of a sulfonated mixture containing a compound represented by formula (XIII).

[0109] [ka]

[0110] [Synthesis Example 13] 5.99 parts of 2,3-dichloroquinoxaline and 4.10 parts of 3,4-dimethyl-o-phenylenediamine were dissolved in 12 parts of 1-butanol and refluxed. After stirring for 3 hours while maintaining reflux, the mixture was allowed to cool to room temperature, 5 parts of methanol was added, and the precipitate was separated by filtration under reduced pressure to obtain a wet cake containing a compound represented by formula (XIV) below. This wet cake was washed with methanol and water, and the precipitate was separated by filtration under reduced pressure to obtain a wet cake. The wet cake was dried by heating at 80°C for 4 days to obtain 4.00 parts of a compound represented by formula (XIV) below.

[0111] [ka]

[0112] [Synthesis Example 14] 3.80 parts of compound (XIV) obtained in Synthesis Example 13 above was added over 10 minutes to 40 parts of 15% fuming sulfuric acid cooled to below 10°C. After the addition, the mixture was stirred at below 25°C for 2.5 hours. After the reaction was completed, the reaction solution was added dropwise to 200 parts of ice water and stirred at below 20°C for 3 days. The precipitate was then filtered under reduced pressure to obtain a wet cake containing a compound represented by formula (XV). The wet cake was washed twice with 40 parts of isopropanol (IPA), and the precipitate was filtered under reduced pressure to obtain a wet cake. The wet cake was neutralized with 14 parts of 15% aqueous ammonia to obtain a basic aqueous solution. This aqueous solution was added dropwise to 160 parts of acetone, and the precipitate was filtered under reduced pressure to obtain a wet cake, which was then washed with acetone. The wet cake was dried by heating at 80°C for 18 hours to obtain 1.92 parts of a sulfonated mixture containing a compound represented by formula (XV).

[0113] [ka]

[0114] [Synthesis Example 15] 8.72 parts of 2,3-dichloroquinoxaline and 6.25 parts of 4-chloro-o-phenylenediamine were dissolved in 28 parts of 1-butanol and refluxed. After stirring for 2 hours while maintaining reflux, the mixture was allowed to cool to room temperature, and the precipitate was separated by filtration under reduced pressure to obtain a wet cake containing a compound represented by formula (XVI): The wet cake was washed with methanol and water, and the precipitate was separated by filtration under reduced pressure. The wet cake obtained was dried by heating at 80°C for 1 day to obtain 9.35 parts of a compound represented by formula (XVI):

[0115] [ka]

[0116] [Synthesis Example 16] 2.00 parts of compound (XVI) obtained in Synthesis Example 15 above was added over 10 minutes to 50 parts of 15% fuming sulfuric acid cooled to below 10°C. After the addition, the mixture was stirred at below 50°C for 7 hours. After allowing the reaction mixture to cool, it was added dropwise to 300 parts of ice water and stirred at below 20°C for 4 hours. The precipitate was then filtered and separated under reduced pressure to obtain a wet cake containing a compound represented by formula (XVII). The wet cake was washed twice with 80 parts of IPA, and the precipitate was filtered and separated under reduced pressure. The resulting wet cake was neutralized with 20 parts of 15% aqueous ammonia to obtain a basic aqueous solution. This aqueous solution was added dropwise to 200 parts of acetone, and the precipitate was filtered and separated under reduced pressure to obtain a wet cake. The wet cake was washed twice with 40 parts of acetone. The wet cake was dried by heating at 80°C for 1 day to obtain 1.97 parts of a sulfonated mixture containing a compound represented by formula (XVII).

[0117] [ka]

[0118] [Synthesis Example 17] 25.0 parts of 2,3-dichloroquinoxaline and 19.1 parts of 3,4-diaminobenzoic acid were dissolved in 260 parts of DMF and stirred at 140°C. After stirring for 2 hours, the mixture was allowed to cool to room temperature, and the precipitate was separated by filtration under reduced pressure to obtain a wet cake containing a compound represented by formula (XVIII): The wet cake was washed with DMF and acetone, and the precipitate was separated by filtration under reduced pressure to obtain a wet cake. The wet cake was dried by heating at 80°C for 1 day to obtain 14.5 parts of a compound represented by formula (XVIII):

[0119] [ka]

[0120] [Synthesis Example 18] 4.00 parts of compound (XVIII) obtained in Synthesis Example 17 above was added over 10 minutes to 120 parts of 15% fuming sulfuric acid cooled to below 10°C, and after addition, the mixture was stirred at below 45°C for 30 hours. After allowing the reaction mixture to cool, it was added dropwise to 300 parts of ice water and stirred at below 20°C for 6 hours. The precipitate was then filtered under reduced pressure to obtain a wet cake containing the compound represented by formula (XIX): The wet cake was washed with IPA and water, and the precipitate was dried by heating at 80°C for 1 day to obtain 3.53 parts of a sulfonated mixture containing the compound represented by formula (XIX):

[0121] [ka]

[0122] [Synthesis Example 19] 19.9 parts of 2,3-dichloroquinoxaline and 19.1 parts of N-methyl-1,2-phenylenediamine dihydrochloride were dissolved in 100 parts of 1-pentanol and refluxed. After stirring for 10 hours while maintaining reflux, the mixture was allowed to cool to room temperature. The precipitate was filtered under reduced pressure and washed with ethanol to obtain a wet cake containing a compound represented by the following formula (XX). This wet cake was suspended in a 9:1 water:ethanol mixed solvent, ultrasonicated for 10 minutes, and then filtered under reduced pressure to obtain a wet cake. This procedure was repeated three times, and the resulting wet cake was washed with ethanol and dried under reduced pressure for 18 hours. The resulting solid was suspended in 2 M hydrochloric acid, ultrasonicated for 20 minutes, and then filtered under reduced pressure to obtain a wet cake. This procedure was repeated twice, and the resulting wet cake was heated at 80°C for 4 hours and dried under reduced pressure to obtain 4.47 parts of a compound represented by the following formula (XX).

[0123] [ka]

[0124] [Synthesis Example 20] 3.00 parts of compound (XX) obtained in Synthesis Example 19 above was added over 10 minutes to 60 parts of 15% fuming sulfuric acid cooled to below 10°C, and after the addition was completed, the mixture was stirred at below 45°C for 1 day. After allowing the reaction solution to cool, it was added dropwise to 145 parts of ice water and stirred at below 20°C for 2 hours. The precipitate was then filtered and separated under reduced pressure to obtain a wet cake containing the compound represented by formula (XXI): This wet cake was neutralized with 25% aqueous ammonia to obtain a basic aqueous solution, which was then separated by reverse-phase column chromatography. The solvent from the resulting solution was distilled off under reduced pressure to obtain 0.72 parts of a sulfonated mixture containing the compound represented by formula (XXI):

[0125] [ka]

[0126] [Example 10] The compound represented by formula (IX) above was dissolved in an aqueous solution (1.0 mol / L) of ammonium formate (manufactured by Junsei Chemical Co., Ltd., special grade) to a concentration of 0.1 mol / L, to prepare positive electrode electrolyte 9. Meanwhile, zinc acetate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 99.9%) was dissolved in an aqueous solution (1.0 mol / L) of ammonium formate (manufactured by Junsei Chemical Co., Ltd., special grade) to a concentration of 1.0 (mol / L), to prepare negative electrode electrolyte 9.

[0127] An ion-exchange membrane (Sigma-Aldrich Japan, LLC, "Nafion® NRE-212") was used as the diaphragm, and carbon felt (Toyobo Co., Ltd., "AAF304ZS", 10 mm × 50 mm × 4 mm) was used as the positive and negative electrodes. The carbon felt was placed in a hole drilled in the center of a silicone gasket (80 mm × 30 mm × 3 mm). For the negative electrode, a zinc plate (Sigma-Aldrich Japan, LLC, 10 mm × 50 mm × 0.25 mm, 99.9%) was sandwiched between the current collector and the electrode, and the cell was assembled in the following order: current collector / positive electrode / diaphragm / negative electrode / zinc plate / current collector. The prepared positive electrode electrolyte 9 and negative electrode electrolyte 9 were placed in respective vials as electrolytes, and the redox flow battery 9 was produced by filling each electrolyte inside the cell with a peristaltic pump across a diaphragm.

[0128] The positive electrode electrolyte 9 and the negative electrode electrolyte 9 of the redox flow battery 9 were circulated by a peristaltic pump connected to the outside of the battery via piping, and testing was performed using a multi-electrochemical measurement system (HZ-Pro, manufactured by Hokuto Denko Corporation). The volume of the positive electrode electrolyte 9 and the negative electrode electrolyte 9 were both 6 mL, and a charge / discharge test was performed at a constant current of 105 mA with an upper voltage limit of 1.7 V and a lower voltage limit of 0.5 V. Figure 9 shows the charge / discharge curve for the first five cycles of the redox flow battery 9. As shown in Table 7, the average discharge voltage at the fifth cycle was 1.16 V, the coulombic efficiency was 100%, the voltage efficiency was 85%, and the energy density was 1.76 Wh / L, indicating high energy density and good cycle characteristics.

[0129] [Example 11] A redox flow battery 10 was fabricated in the same manner as in Example 10, except that the compound represented by formula (IX) used as the positive electrode active material in Example 10 was replaced with the compound represented by formula (XIII). A charge-discharge test was also performed using the redox flow battery 10 in the same manner as in Example 10. Figure 10 shows the charge-discharge curves for the first five cycles of the redox flow battery 10. As shown in Table 7, the average discharge voltage at the fifth cycle was 1.22 V, the coulombic efficiency was 100%, the voltage efficiency was 89%, and the energy density was 2.53 Wh / L, indicating that the energy density was high and favorable cycle characteristics were obtained.

[0130] [Example 12] A redox flow battery 11 was fabricated in the same manner as in Example 10, except that the compound represented by formula (IX) used as the positive electrode active material in Example 10 was replaced with the compound represented by formula (XV) and the positive electrode electrolyte solution was prepared to have a concentration of 0.05 mol / L. A charge / discharge test was also performed using the redox flow battery 11 in the same manner as in Example 10. FIG. 11 shows the charge / discharge curves for the first five cycles of the redox flow battery 11. As shown in Table 7, the average discharge voltage at the fifth cycle was 1.13 V, the coulombic efficiency was 100%, the voltage efficiency was 84%, and the energy density was 0.85 Wh / L, indicating high energy density and favorable cycle characteristics.

[0131] [Example 13] Redox flow battery 12 was fabricated in the same manner as in Example 10, except that the compound represented by formula (IX) used as the positive electrode active material in Example 10 was replaced with the compound represented by formula (XVII). Furthermore, a charge / discharge test was performed using redox flow battery 12 in the same manner as in Example 10, except that the upper limit voltage of the charge / discharge test was changed to 1.8 V. FIG. 12 shows the charge / discharge curves for redox flow battery 12 up to the fifth cycle. As shown in Table 7, the average discharge voltage at the fifth cycle was 1.28 V, the coulombic efficiency was 100%, the voltage efficiency was 87%, and the energy density was 3.12 Wh / L, indicating high energy density and favorable cycle characteristics.

[0132] [Example 14] The compound represented by the formula (XIX) above was dissolved in an aqueous solution (0.5 mol / L) of ammonium dihydrogen phosphate (special grade, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) to a concentration of 0.1 mol / L, and ammonia water was added to adjust the pH to 9, to prepare a positive electrode electrolyte 13. On the other hand, the compound represented by the formula (IV) above was dissolved in an aqueous solution (0.5 mol / L) of ammonium dihydrogen phosphate (special grade, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) to a concentration of 0.1 (mol / L), and ammonia water was added to adjust the pH to 9, to prepare a negative electrode electrolyte 13.

[0133] An ion-exchange membrane (Sigma-Aldrich Japan, LLC, "Nafion® NRE-212") was used as the diaphragm, and carbon felt (Toyobo Co., Ltd., "AAF304ZS", 10 mm × 50 mm × 4 mm) was used as the positive and negative electrodes. The carbon felt was inserted into a 50 mm × 10 mm hole drilled in the center of a silicone gasket (80 mm × 30 mm × 3 mm), and the electrodes were assembled in the following order: current collector / positive electrode / diaphragm / negative electrode / current collector. The prepared positive electrode electrolyte 13 and negative electrode electrolyte 13 were placed in vials, respectively, and the electrolytes were filled into the cell through the diaphragm using a peristaltic pump, thereby producing a redox flow battery 13.

[0134] The redox flow battery 13 was placed in a glove box (manufactured by UNICO, "UL-1300A") equipped with a gas circulation purification device (manufactured by UNICO, "MF-100"), and electrochemical measurements were carried out under conditions where the oxygen concentration was 1 ppm or less.

[0135] The positive electrode electrolyte 13 and the negative electrode electrolyte 13 of the redox flow battery 13 were circulated by a peristaltic pump connected to the outside of the battery through piping, and testing was performed using a multi-electrochemical measurement system (HZ-Pro, manufactured by Hokuto Denko Corporation). The volumes of the positive electrode electrolyte 13 and the negative electrode electrolyte 13 were 6 mL and 7 mL, respectively, and a charge / discharge test was performed at a constant current of 105 mA with an upper voltage limit of 1.2 V and a lower voltage limit of 0.3 V. Figure 13 shows the charge / discharge curves for the first five cycles of the redox flow battery 13. As shown in Table 7, the average discharge voltage at the fifth cycle was 0.63 V, the coulombic efficiency was 100%, the voltage efficiency was 82%, and the energy density was 0.99 Wh / L, indicating high energy density and favorable cycle characteristics.

[0136] [Example 15] The fluoroflavin compound represented by the above formula (XXI) was dissolved in water to a concentration of 0.1 mol / L, ammonium acetate (manufactured by Junsei Chemical Co., Ltd., special grade) to a concentration of 0.4 mol / L, and acetic acid (manufactured by Junsei Chemical Co., Ltd., special grade) to a concentration of 0.4 mol / L, respectively, to prepare a positive electrode electrolyte 14. Meanwhile, zinc acetate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity 99.9%) was dissolved in water to a concentration of 0.2 mol / L, and ammonium acetate (manufactured by Junsei Chemical Co., Ltd., special grade) to a concentration of 1.0 mol / L, respectively, to prepare a negative electrode electrolyte 14.

[0137] An ion-exchange membrane (Sigma-Aldrich Japan, LLC, "Nafion® NRE-212") was used as the diaphragm, and carbon felt (Toyobo Co., Ltd., "AAF304ZS", 10 mm × 50 mm × 4 mm) was used as the positive and negative electrodes. The carbon felt was placed in a hole drilled in the center of a silicone gasket (80 mm × 30 mm × 3 mm). For the negative electrode, a zinc plate (Sigma-Aldrich Japan, LLC, 10 mm × 50 mm × 0.25 mm, 99.9%) was sandwiched between the current collector and the electrode, and the cell was assembled in the following order: current collector / positive electrode / diaphragm / negative electrode / zinc plate / current collector. The prepared positive electrode electrolyte 14 and negative electrode electrolyte 14 were placed in respective vials as electrolytes, and the redox flow battery 14 was produced by filling each electrolyte inside the cell with a peristaltic pump across a diaphragm.

[0138] The redox flow battery 14 was placed in a glove box (manufactured by UNICO, "UL-1300A") equipped with a gas circulation purification device (manufactured by UNICO, "MF-100"), and electrochemical measurements were carried out under conditions where the oxygen concentration was 1 ppm or less.

[0139] The positive electrode electrolyte 14 and the negative electrode electrolyte 14 of the redox flow battery 14 were circulated by a peristaltic pump connected to the outside of the battery via piping, and testing was performed using a multi-electrochemical measurement system (HZ-Pro, manufactured by Hokuto Denko Corporation). The volume of the positive electrode electrolyte 14 and the negative electrode electrolyte 14 were both 6 mL, and a charge / discharge test was performed at a constant current of 105 mA with an upper voltage limit of 1.65 V and a lower voltage limit of 1.0 V. Figure 14 shows the charge / discharge curves for the first five cycles of the redox flow battery 14. As shown in Table 7, the average discharge voltage at the fifth cycle was 1.39 V, the coulombic efficiency was 81%, the voltage efficiency was 86%, and the energy density was 0.22 Wh / L, indicating high energy density and favorable cycle characteristics.

[0140] [Table 7]

[0141] As shown in Table 7 and FIGS. 9 to 14, it is clear that the redox flow batteries 9 to 14 fabricated in Examples 10 to 15 have high energy density and good cycle characteristics. [Industrial Applicability]

[0142] The electrolyte solution containing the secondary battery material of the present invention and the redox flow battery using the same can achieve high energy density and provide good cycle characteristics. Furthermore, since the electrolyte solution is an aqueous electrolyte solution, it is safer and easier to handle than organic solvent-based electrolyte solutions, and can be used in a wide range of applications.

Claims

1. A material for a redox flow secondary battery, comprising at least one organic compound selected from the group consisting of a compound represented by the following formula (1) and a compound represented by the following formula (2): 【Chemistry 1】 (In formula (1), R １ ~R １０ are each independently a hydrogen atom, an alkyl group, a hydroxy group, an alkoxy group, a nitro group, an amino group, a mercapto group, a cyano group, a halogen group, an aryl group, a sulfone group, a carboxyl group, a carbonyl group, a sulfonyl group, or a heteroaryl group, and in formula (2), R １１ ~R ２４ each independently represents a hydrogen atom, a sulfone group, an alkyl group, an aryl group, or a carbonyl group.

2. In the above formula (1), R １ ~R ８ 2. The material for a redox flow secondary battery according to claim 1, wherein at least one of the groups is a sulfonic acid group or a salt thereof.

3. In the above formula (1), R １ ~R ８ 2. The material for a redox flow secondary battery according to claim 1, wherein at least two of the groups are sulfonic acid groups or salts thereof.

4. An active material comprising at least one material for a redox flow secondary battery according to claim 1 .

5. The active material of claim 4 , wherein the active material is a redox active material.

6. An electrolyte solution containing the active material according to claim 4 or 5.

7. The electrolyte according to claim 6, which is an electrolyte for a redox flow battery.

8. The electrolyte solution according to claim 6 , further comprising a pH buffer.

9. 9. The electrolyte solution according to claim 8, wherein the pH buffer is an organic acid salt or an inorganic acid salt having a pKa of 2 or more and 13 or less.

10. A redox flow secondary battery comprising the electrolyte solution according to claim 6.

11. The redox flow secondary battery according to claim 10 , wherein the electrolyte is a positive electrode electrolyte.

12. The redox flow secondary battery according to claim 11, further comprising a negative electrode electrolyte, a positive electrode, a negative electrode, and a diaphragm.

13. The redox flow secondary battery according to claim 12 , wherein the negative electrode electrolyte contains zinc ions.

14. 14. The redox flow secondary battery according to claim 13, wherein a difference between an oxidation-reduction potential of the organic compound and an oxidation-reduction potential of the zinc ion is 0.8 V or more and 2.1 V or less.

15. The redox flow secondary battery according to claim 12 , wherein the diaphragm is an ion exchange membrane.

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