Power storage device and method for discharging power storage device

Abstract

Provided is a power storage device comprising: a positive electrode member having a manganese dioxide-containing positive electrode and a positive electrode electrolyte that is in contact with the positive electrode and contains a reduction mediator; a negative electrode member including a negative electrode and a negative electrode electrolyte that is in contact with the negative electrode; and a diaphragm disposed between the positive electrode member and the negative electrode member.

Description

Energy storage device, and method for discharging the energy storage device

[0001] The present invention relates to an energy storage device and a method for discharging an energy storage device.

[0002] In recent years, lithium-ion secondary batteries have been developed and widely used as energy storage devices. However, lithium, which is used in lithium-ion secondary batteries, is a rare metal, so zinc-manganese dioxide (Zn-MnO) is being considered as an alternative to lithium-ion secondary batteries. ２ The development of rechargeable batteries is desired.

[0003] Zinc-manganese dioxide (Zn-MnO) ２ ) For example, a secondary battery can be made of a Zn-based anode and MnO ２ A secondary alkaline battery using a cathode system has been proposed (see, for example, Patent Document 1).

[0004] However, while the nominal voltage of lithium-ion secondary batteries is 3.6V to 3.7V, zinc-manganese dioxide (Zn-MnO) ２ The nominal voltage of secondary batteries is 1.0V to 1.5V, therefore, zinc-manganese dioxide (Zn-MnO) has a high discharge capacity (mAh / g). ２ There is a problem in that it is difficult to replace lithium-ion rechargeable batteries unless they are rechargeable batteries.

[0005] Special table 2016-501425 publication

[0006] One aspect of the present invention aims to provide an energy storage device having a high discharge capacity (mAh / g).

[0007] A power storage device according to one aspect of the present invention as a means for solving the problem comprises: a positive electrode member having a positive electrode containing manganese dioxide and a positive electrode electrolyte containing a reducing mediator, in contact with the positive electrode; a negative electrode member having a negative electrode and a negative electrode electrolyte in contact with the negative electrode; and a diaphragm disposed between the positive electrode member and the negative electrode member.

[0008] A discharge method for an electricity storage device according to one aspect of the present invention as a means for solving the problem is a discharge method for an electricity storage device comprising: a positive electrode member including a positive electrode containing manganese dioxide, a positive electrode electrolyte in contact with the positive electrode and containing a reduction mediator; a negative electrode member including a negative electrode and a negative electrode electrolyte in contact with the negative electrode; and a separator disposed between the positive electrode member and the negative electrode member, the method comprising: an ionization step of ionizing manganese dioxide contained in the positive electrode into trivalent manganese ions; and a reduction step of reducing the trivalent manganese ions to divalent manganese ions by the reduction mediator.

[0009] According to one aspect of the present invention, an electricity storage device having a high discharge capacity (mAh / g) can be provided.

[0010] BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing an example of an electricity storage device according to an embodiment of the present disclosure. FIG. 2 is a schematic diagram showing the reaction mechanism of manganese in the discharge reaction of an electricity storage device according to an embodiment of the present disclosure. FIG. 3 is a schematic diagram showing the reaction mechanism of manganese in the discharge reaction of an electricity storage device when the positive electrode electrolyte does not contain a reduction mediator. FIG. 4 is a graph showing the discharge capacities of Example 1 and Comparative Example 1. FIG. 5 is a schematic diagram showing the discharge mechanism in a second embodiment. FIG. 6 is a schematic diagram showing the discharge mechanism when a positive electrode 11 does not contain bismuth (Bi). FIG. 7 is a schematic diagram showing the discharge capacity results in Examples 5 to 7.

[0011] Hereinafter, modes for carrying out the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments, and various modifications and substitutions can be made to the following embodiments without departing from the scope of the present invention.

[0012] Hereinafter, an electricity storage device according to an embodiment of the present disclosure (hereinafter referred to as "the present embodiment") will be described with reference to the drawings. All the drawings are schematic diagrams for explaining the configuration of the electricity storage device and the like according to the present embodiment, and do not accurately describe sizes and the like.

[0013] (Electricity Storage Device) - First Embodiment- As shown in FIG. 1, an electricity storage device 10 according to the first embodiment includes a positive electrode member 13 having a positive electrode 11 and a positive electrode electrolyte 12, a negative electrode member 16 having a negative electrode 14 and a negative electrode electrolyte 15, and a separator 17 disposed between the positive electrode member 13 and the negative electrode member 16.

[0014] Each member will be described below.

[0015] <Positive Electrode 11> The positive electrode contains manganese dioxide as an active material, and may optionally contain other components. It is sufficient that the active material contained in the positive electrode can oxidize and reduce manganese dioxide through an electrochemical reaction, and the content ratio of each element is not particularly limited.

[0016] There are no particular limitations on components other than manganese dioxide contained in the positive electrode, and they can be appropriately selected according to the purpose. For example, the positive electrode can further contain additives such as carbon, binders, current collectors, Bi, Sr, Ca, Ba, oxides thereof, hydroxides thereof, nitrates thereof, chlorides thereof, or combinations thereof.

[0017] There is no particular limitation on the thickness of the positive electrode, and it can be appropriately selected according to the purpose. For example, the thickness can be 0.1 mm or more and 1 mm or less.

[0018] <Positive Electrode Electrolyte 12> The positive electrode electrolyte contains an electrolyte and a reduction mediator. The positive electrode electrolyte may further contain an oxidation mediator.

[0019] The electrolyte promotes the movement of ions inside the electricity storage device and is responsible for electrical neutralization reactions during the discharge and charging processes. Examples of components of the electrolyte include alkaline aqueous solutions such as 6 M potassium hydroxide (KOH).

[0020] The reduction mediator is a compound that is stable in the positive electrode electrolyte and functions as a reducing agent. The reduction mediator reduces trivalent manganese ions desorbed from the positive electrode into the positive electrode electrolyte to divalent manganese ions during discharge of the electricity storage device.

[0021] Examples of trivalent manganese ions include trivalent ionized manganese hydroxide ([Mn(OH) ６ ３－ ), complex compounds with triethanolamine, and the like.

[0022] Examples of divalent manganese ions include divalent ionized manganese hydroxide ([Mn(OH) ４ ２－ ), complex compounds with triethanolamine, and the like.

[0023] The redox potential of the reducing mediator is lower than -0.25 V (based on a mercury-mercuric oxide electrode), which is the lower limit of the potential at which trimanganese tetroxide can be thermodynamically generated. This makes it possible to reduce trivalent manganese ions to divalent manganese ions while suppressing the generation of trimanganese tetroxide.

[0024] Examples of the reducing mediator include metal complex ions, oxygen-containing aromatic compounds, sulfur-containing compounds, nitrogen-containing aromatic compounds, and the like. These may be used alone singly, or two or more of these may be used in combination. The reducing mediator may be appropriately synthesized, or a commercially available product may be used.

[0025] Examples of the metal complex ions include nickel tetracyanide (redox potential E ｒｅｄ : -0.401 V), and the like.

[0026] The oxygen-containing aromatic compound includes, for example, organic molecules having a hydroquinone skeleton, a naphthoquinone skeleton, an anthraquinone skeleton, or the like. Examples thereof include 1,4-hydroquinone (redox potential E ｒｅｄ : -0.32 V (based on a mercury-mercuric oxide electrode)), 1,4-naphthoquinone (redox potential E ｒｅｄ : -0.33 V (based on a mercury-mercuric oxide electrode)), 1,8-dihydroxy-2,7-dicarboxymethyl-9,10-anthraquinone (redox potential E ｒｅｄ : -0.68 V (based on a mercury-mercuric oxide electrode)), and the like.

[0027] The sulfur-containing compound includes, for example, polysulfide (redox potential E ｒｅｄ : -0.51 V (based on a mercury-mercuric oxide electrode)), and the like.​​

[0028] Examples of nitrogen-containing aromatic compounds include organic molecules having a phenazine skeleton, such as benzo(a)hydroxyphenazine-7 / 8-carboxylic acid (redox potential E ｒｅｄ Examples include -0.87V (based on the mercury-mercury oxide electrode).

[0029] There are no particular restrictions on the concentration of the reducing mediator contained in the positive electrode electrolyte, and it can be appropriately selected depending on the purpose, but it is preferably 0.1 mM or more and 5 mM or less.

[0030] Oxide mediators are compounds that are stable in the positive electrode electrolyte and function as oxidizing agents. During the charging of an energy storage device, oxide mediators oxidize divalent manganese ions in the positive electrode electrolyte to trivalent manganese ions.

[0031] The oxidation-reduction potential of the oxide mediator is higher than -0.20 V (based on a mercury-mercury oxide electrode), which is the upper limit of the potential that trimanganese tetroxide can thermodynamically generate. This allows divalent manganese ions in the positive electrode electrolyte to be oxidized to trivalent manganese ions.

[0032] Examples of oxidative mediators include ferrocyanides, ferrocenes, bromides, methylene blue, and 2,2,6,6-tetramethylpiperidine-1-oxyl radicals. These may be used individually or in combination of two or more. Oxidative mediators may be synthesized as appropriate or commercially available products may be used.

[0033] <Negative electrode 14> The negative electrode contains zinc as an active material and may contain other components as needed. The active material contained in the negative electrode only needs to be able to oxidize and reduce zinc through an electrochemical reaction, and the proportions of each element are not particularly limited.

[0034] Other components besides zinc in the negative electrode are not particularly limited and can be appropriately selected depending on the purpose. For example, additives such as carbon, binders, current collectors, Bi, Sr, Ca, Ba, their oxides, their hydroxides, their nitrates, their chlorides, or combinations thereof may be included.

[0035] There are no particular restrictions on the thickness of the negative electrode, and it can be appropriately selected depending on the purpose, but for example, it can be 0.1 mm to approximately 1 mm thick.

[0036] <Negative electrode electrolyte 15> The negative electrode electrolyte contains an electrolyte and may optionally contain zinc oxide (ZnO). Examples of electrolyte components include alkaline aqueous solutions such as 6M potassium hydroxide (KOH).

[0037] <Diaphragm 17> The diaphragm is placed between the positive electrode member and the negative electrode member.

[0038] [Operating principle in the discharge reaction] The operating principle in the discharge reaction of the energy storage device according to this embodiment is as follows: Zinc (Zn) contained in the negative electrode is oxidized to zinc ions (Zn ２＋ ) is released into the negative electrode electrolyte (hereinafter sometimes referred to as the "anode reaction"). On the other hand, manganese dioxide (MnO) contained in the positive electrode is released into the negative electrode electrolyte. ２ ) is reduced, and electrons (e － ) receives (hereinafter sometimes referred to as the "cathode reaction"). Through these processes, current flows to the terminals of the energy storage device, and energy is supplied to the external circuit.

[0039] Anode and cathode reactions can be represented, for example, by the following equations: Anode reaction: Zn(s) → Zn ２＋ (aq) + 2e － Cathode reaction: Mno ２ (s) + 2e － +2H ２ O → MnOOH(s)+2OH － (ax)

[0040] Figure 2A is a schematic diagram showing the reaction mechanism of manganese in the discharge reaction of the energy storage device according to this embodiment. In the energy storage device according to this embodiment, the positive electrode electrolyte contains a reducing mediator, and as shown in Figure 2A, manganese dioxide (MnO) contained in the positive electrode ２ : tetravalent) manganese hydroxide (Mn(OH) ２ It is reduced to a divalent ion ([Mn(OH)]). The reduction mediator changes from the trivalent ion of manganese oxyhydroxide (α-MnOOH) to the divalent ion ([Mn(OH)]). ４ ] ２－ By promoting the reduction reaction to manganese hydroxide trivalent ion ([Mn(OH) ６ ] ３－ ) and divalent ionized manganese hydroxide ([Mn(OH) ４ ] ２－ This suppresses the mixing of (with) and inhibits the formation of trimanganese tetroxide.

[0041] Figure 2B is a schematic diagram showing the reaction mechanism of manganese in the discharge reaction of an energy storage device when the positive electrode electrolyte does not contain a reducing mediator. As shown in Figure 2B, manganese dioxide (MnO) contained in the positive electrode ２ The reaction mainly stops at the trivalent ion of manganese oxyhydroxide (α-MnOOH), with some of it becoming the trivalent ion of manganese hydroxide ([Mn(OH) ６ ] ３－ ) becomes. Here, since the reduction is not promoted by the reduction mediator, the trivalent ionized manganese hydroxide ([Mn(OH) ６ ] ３－ ) and divalent ionized manganese hydroxide ([Mn(OH) ４ ] ２－ ) and other substances are mixed together, and trimanganese tetroxide is formed. As a result, the amount of active material at the positive electrode decreases, and the discharge capacity decreases.

[0042] The discharge capacity of the energy storage device according to this embodiment is 300 mAh / g or more.

[0043] -Second Embodiment- In the second embodiment, an example of an energy storage device is shown in which the positive electrode 11 further contains bismuth (Bi) and carbon (C), unlike in the first embodiment. In the first embodiment, descriptions of components that are the same as those described in the previously described embodiments may be omitted.

[0044] Figure 4 is a schematic diagram showing the discharge mechanism in the second embodiment. By containing bismuth in the positive electrode 11, the discharge mechanism shown in Figure 4 (H ＋ ) δ-MnO ２ Suppresses the generation of (H ＋ , K ＋ ) δ-MnO ２ From [Mn(OH) ６ ] ３－ (Mn ３＋ It can improve solubility in [Mn(OH) ６ ] ３－ (Mn ３＋ ) from [Mn(OH) ４ ] ２－ (Mn ２＋ The rate of the reduction reaction to ) can be improved. These two effects result in a greater increase in discharge capacity. On the other hand, Figure 5 is a schematic diagram showing the discharge mechanism when the positive electrode 11 does not contain bismuth (Bi).

[0045] Bismuth is present in the positive electrode 11. ２ O ３ It is included as follows. There are no particular restrictions on the bismuth content, and it can be appropriately selected depending on the purpose, but it is preferably 10% by mass or more, more preferably 10% by mass or more and 30% by mass or less, and particularly preferably 10% by mass or more and 20% by mass or less, relative to the total amount of the positive electrode 11.

[0046] Carbon (C) can be included in the positive electrode 11 as KB, AB, CNT, AC, etc., with KB being preferred among these. Including KB in the positive electrode 11 improves the electron path formation ability and specific surface area within the positive electrode 11.

[0047] There are no particular restrictions on the carbon content, and it can be appropriately selected depending on the purpose, but it is preferably 10% by mass or more, and more preferably 10% by mass or more and 30% by mass or less, relative to the total amount of positive electrode 11.

[0048] (Discharge Method for Energy Storage Device) The discharge method for the energy storage device according to this embodiment is a discharge method for the energy storage device according to this embodiment, and includes an ionization step and a reduction step. In the discharge reaction of the energy storage device according to this embodiment, substantially no trimanganese tetroxide is produced. Substantially no trimanganese tetroxide is produced, which means that the amount of charge after 300 charge-discharge cycles of the energy storage device under the conditions of 0.2 ItA and 20 ± 5 degrees is so small that it can maintain at least 60% of the initial charge amount.

[0049] The following describes specific examples, but the embodiments are not limited to these examples.

[0050] [Example 1] A positive electrode component consisting of a positive electrode (manganese dioxide: AB: PTFE = 80% by mass: 14% by mass: 6% by mass), a positive electrode electrolyte (a mixture of 6M potassium hydroxide and 1 mM 1,4-naphthoquinone), and a reference electrode (Hg / HgO), a negative electrode component consisting of a negative electrode (zinc) and a negative electrode electrolyte (a mixture of 6M potassium hydroxide and zinc oxide), and a diaphragm (FAAM-PK-75, manufactured by Fumap) were placed in a reaction vessel to fabricate an energy storage device. The fabricated energy storage device was discharged and its discharge capacity was measured. The results are shown in Figure 3 and Table 1.

[0051] [Example 2] Discharge of the energy storage device was performed in the same manner as in Example 1, except that the reducing mediator was changed from 1,4-naphthoquinone to 1,4-hydroquinone. The measured values ​​of the discharge capacity are shown in Table 1.

[0052] [Example 3] The energy storage device was discharged in the same manner as in Example 1, except that the reducing mediator was changed from 1,4-naphthoquinone to 1,8-dihydroxy-2,7-dicarboxymethyl-9,10-anthraquinone. The measured discharge capacity is shown in Table 1.

[0053] [Example 4] The energy storage device was discharged in the same manner as in Example 1, except that the reducing mediator was changed from 1,4-naphthoquinone to benzo(a)hydroxyphenazine-7 / 8-carboxylic acid. The measured discharge capacity is shown in Table 1.

[0054] [Comparative Example 1] Discharge of the energy storage device was performed in the same manner as in Example 1, except that a reduction mediator was not used. The measured values ​​of the discharge capacity are shown in Table 1.

[0055]

[0056] As shown in Figure 3 and Table 1, the energy storage device of Example 1 was found to have improved discharge capacity compared to the energy storage device of Comparative Example 1, which did not contain a reduction mediator. This is because, by using 1,4-naphthoquinone as a reduction mediator, the reduction from trivalent manganese ions to divalent manganese ions was promoted compared to Comparative Example 1, suppressing the formation of trimanganese tetroxide and manganese hydroxide (Mn(OH)) ２ This is thought to be because the ions were reduced to a divalent state, which improved the discharge capacity.

[0057] Furthermore, as shown in Table 1, it was confirmed that the energy storage devices of Examples 2 to 4 also showed improved discharge capacity compared to Comparative Example 1. From this, it became clear that 1,4-hydroquinone, 1,8-dihydroxy-2,7-dicarboxymethyl-9,10-anthraquinone, and benzo(a)hydroxyphenazine-7 / 8-carboxylic acid can similarly improve the discharge capacity of energy storage devices as reduction mediators.

[0058] [Example 5] In Example 1, the mass fraction of manganese dioxide in the positive electrode was set to 70% by mass, and Bi ２ O ３ The mass fraction of Bi is such that it is 10% by mass. ２ O ３ Except for adding and further adding KB, the energy storage device was discharged in the same manner as in Example 1. The measured discharge capacity is shown in Figure 6. Note that the amount of KB in the positive electrode did not change, and by adjusting the mass fraction of manganese dioxide, the amount of Bi in the positive electrode２ O ３ The mass fraction of was set to 10% by mass.

[0059] [Example 6] In Example 5, the mass fraction of manganese dioxide in the positive electrode was set to 60% by mass, and Bi ２ O ３ Discharge of the energy storage device was performed in the same manner as in Example 1, except that the mass fraction of was changed to 20% by mass. The measured discharge capacity is shown in Figure 6. Note that the amount of KB in the positive electrode did not change, and by adjusting the mass fraction of manganese dioxide, the amount of Bi in the positive electrode was changed. ２ O ３ The mass fraction of was set to 20% by mass.

[0060] [Example 7] In Example 5, the mass fraction of manganese dioxide in the positive electrode was set to 50% by mass, and Bi ２ O ３ Discharge of the energy storage device was performed in the same manner as in Example 1, except that the mass fraction of was changed to 30% by mass. The measured discharge capacity is shown in Figure 6. Note that the amount of KB in the positive electrode did not change, and by adjusting the mass fraction of manganese dioxide, the amount of Bi in the positive electrode was changed. ２ O ３ The mass fraction of was set to 30% by mass.

[0061] As shown in Figure 5, it was confirmed that the discharge capacity was improved in Examples 5 to 7, which contained bismuth and carbon. Furthermore, from the results of Example 6, it was confirmed that the discharge capacity was further improved when the bismuth content was 20% by mass. For reference, Figure 6 also shows a reference example without Bi (biosilicon).

[0062] The embodiments of the present invention have been described in detail above. The embodiments described above are merely examples of how to implement the present invention. The content of the embodiments does not limit the technical scope of the present invention, and many design changes, such as changes, additions, and deletions of components, are possible as long as they do not depart from the spirit of the invention as defined in the claims. The new embodiments to which design changes have been made combine the effects of the respective embodiments and modifications. In the embodiments described above, the content in which such design changes are possible is emphasized with notations such as "in this embodiment" or "in this embodiment," but design changes are also permitted even if there are no such notations. Any combination of the above components is also valid as an embodiment of the present invention.

[0063] This embodiment may be specified by the following items: [Item 1] A positive electrode member having a positive electrode containing manganese dioxide and a positive electrode electrolyte containing a reducing mediator, in contact with the positive electrode; a negative electrode member having a negative electrode and a negative electrode electrolyte in contact with the negative electrode; and a diaphragm disposed between the positive electrode member and the negative electrode member. [Item 2] The energy storage device according to Item 1, wherein the oxidation-reduction potential of the reducing mediator is lower than the oxidation-reduction potential of trimanganese tetroxide. [Item 3] The energy storage device according to Item 2, wherein the oxidation-reduction potential of the reducing mediator is -0.25 V (based on a mercury-mercury oxide electrode) or less. [Item 4] The energy storage device according to Item 1 or 2, wherein the reducing mediator is at least one compound selected from the group including metal complex ions, oxygen-containing aromatic compounds, sulfur-containing compounds, and nitrogen-containing aromatic compounds. [Item 5] The energy storage device according to Item 4, wherein the reducing mediator is at least one compound selected from the group comprising 1,4-hydroquinone, 1,4-naphthoquinone, 1,8-dihydroxy-2,7-dicarboxymethyl-9,10-anthraquinone, and benzo(a)hydroxyphenazine-7 / 8-carboxylic acid. [Item 6] The energy storage device according to Item 1 or 2, wherein the reducing mediator reduces trivalent manganese ions to divalent manganese ions when the energy storage device is discharged. [Item 7] The energy storage device according to Item 1 or 2, having a discharge capacity of 300 mAh / g or more. [Item 8] The energy storage device according to Item 1 or 2, wherein the positive electrode electrolyte contains an oxidizing mediator that oxidizes divalent manganese ions to trivalent manganese ions when the energy storage device is charged. [Item 9] The energy storage device according to Item 8, wherein the oxidizing mediator oxidizes divalent manganese ions to trivalent manganese ions when the energy storage device is charged. [Item 10] The energy storage device according to item 1 or 2, wherein the diaphragm is an anion exchange membrane. [Item 11] The energy storage device according to item 1 or 2, wherein the negative electrode contains zinc. [Item 12] The energy storage device according to item 1 or 2, wherein the positive electrode contains bismuth and carbon.[Item 13] A method for discharging an energy storage device having a positive electrode member having a positive electrode containing manganese dioxide and a positive electrode electrolyte containing a reducing mediator in contact with the positive electrode; a negative electrode member having a negative electrode and a negative electrode electrolyte in contact with the negative electrode; and a diaphragm disposed between the positive electrode member and the negative electrode member, the method comprising: an ionization step of ionizing the manganese dioxide contained in the positive electrode to trivalent manganese ions; and a reduction step of reducing the trivalent manganese ions to divalent manganese ions using the reducing mediator. [Item 14] The method for discharging an energy storage device according to Item 13, wherein substantially no trimanganese tetroxide is produced. [Item 15] The method for discharging an energy storage device according to Item 14, wherein the ionization step and the reduction step are performed at a rate that does not produce trimanganese tetroxide.

[0064] This application claims priority based on Japanese Patent Application No. 2025-030537, filed with the Japan Patent Office on 27 February 2025, and includes the entire contents of that Japanese Patent Application.

[0065] 10 Energy storage device 11 Positive electrode 12 Positive electrode electrolyte 13 Positive electrode component 14 Negative electrode 15 Negative electrode electrolyte 16 Negative electrode component 17 Diaphragm

Claims

1. An energy storage device comprising: a positive electrode member having a positive electrode containing manganese dioxide and a positive electrode electrolyte containing a reducing mediator, in contact with the positive electrode; a negative electrode member having a negative electrode and a negative electrode electrolyte in contact with the negative electrode; and a diaphragm disposed between the positive electrode member and the negative electrode member.

2. The energy storage device according to claim 1, wherein the oxidation-reduction potential of the reduction mediator is lower than the oxidation-reduction potential of trimanganese tetroxide.

3. The energy storage device according to claim 2, wherein the oxidation-reduction potential of the reduction mediator is -0.25 V (based on the mercury-mercury oxide electrode) or less.

4. The energy storage device according to claim 1 or 2, wherein the reducing mediator is at least one compound selected from the group consisting of metal complex ions, oxygen-containing aromatic compounds, sulfur-containing compounds, and nitrogen-containing aromatic compounds.

5. The energy storage device according to claim 4, wherein the reducing mediator is at least one compound selected from the group comprising 1,4-hydroquinone, 1,4-naphthoquinone, 1,8-dihydroxy-2,7-dicarboxymethyl-9,10-anthraquinone, and benzo(a)hydroxyphenazine-7 / 8-carboxylic acid.

6. The energy storage device according to claim 1 or 2, wherein the reduction mediator reduces trivalent manganese ions to divalent manganese ions when the energy storage device is discharged.

7. The energy storage device according to claim 1 or 2, having a discharge capacity of 300 mAh / g or more.

8. The energy storage device according to claim 1 or 2, wherein the positive electrode electrolyte includes an oxide mediator that oxidizes divalent manganese ions to trivalent manganese ions when the energy storage device is charged.

9. The energy storage device according to claim 8, wherein the oxide mediator oxidizes divalent manganese ions to trivalent manganese ions when the energy storage device is charged.

10. The energy storage device according to claim 1 or 2, wherein the diaphragm is an anion exchange membrane.

11. The energy storage device according to claim 1 or 2, wherein the negative electrode contains zinc.

12. The energy storage device according to claim 1 or 2, wherein the positive electrode comprises bismuth and carbon.

13. A method for discharging an energy storage device having a positive electrode member having a positive electrode containing manganese dioxide and a positive electrode electrolyte containing a reducing mediator in contact with the positive electrode; a negative electrode member having a negative electrode and a negative electrode electrolyte in contact with the negative electrode; and a diaphragm disposed between the positive electrode member and the negative electrode member, the method comprising: an ionization step of ionizing the manganese dioxide contained in the positive electrode into trivalent manganese ions; and a reduction step of reducing the trivalent manganese ions into divalent manganese ions using the reducing mediator.

14. A method for discharging an energy storage device according to claim 13, wherein virtually no trimanganese tetroxide is generated.

15. The method for discharging an energy storage device according to claim 14, wherein the ionization step and the reduction step are performed at a rate such that trimanganese tetroxide is not produced.

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