Flow battery

A flow battery includes: a first liquid containing dissolved therein a condensed aromatic compound and lithium; a first electrode immersed in the first liquid; and a first circulator including a first container and a first passage prevention member. The first liquid containing the condensed aromatic compound dissolved therein has the property of causing the lithium to release solvated electrons and dissolve as cations. When the lithium dissolved in the first liquid precipitates on the first electrode, lithium precipitate particles are generated. The first circulator circulates the first liquid between the first electrode and the first container. The first circulator transfers the lithium precipitate particles generated on the first electrode to the first container. The first passage prevention member is disposed in a channel through which the first liquid flows from the first container to the first electrode. The first passage prevention member prevents passage of the lithium precipitate particles.

BACKGROUND

1. Technical Field

The present disclosure relates to a flow battery.

2. Description of the Related Art

Japanese Patent No. 5417441 discloses a redox flow battery that uses a negative electrode slurry solution containing a non-aqueous solvent and metal particles serving as solid negative electrode active material particles.

SUMMARY

There is a need in the related art to provide a high-energy density flow battery.

In one general aspect, the techniques disclosed here feature a flow battery including: a first liquid containing dissolved therein a condensed aromatic compound and lithium; a first electrode immersed in the first liquid; and a first circulator including a first container and a first passage prevention member. The first liquid containing the condensed aromatic compound dissolved therein has the property of causing the lithium to release solvated electrons and dissolve as cations. When the lithium dissolved in the first liquid precipitates on the first electrode, lithium precipitate particles are generated. The first circulator circulates the first liquid between the first electrode and the first container. The first circulator transfers the lithium precipitate particles generated on the first electrode to the first container, and the first passage prevention member is disposed in a channel through which the first liquid flows from the first container to the first electrode. The first passage prevention member prevents passage of the lithium precipitate particles.

The present disclosure can provide a high-energy density flow battery.

DETAILED DESCRIPTION

Embodiments of the present disclosure will next be described with reference to the drawings.

FIG. 1is a block diagram showing a general structure of a flow battery1000in embodiment 1.

The flow battery1000in embodiment 1 includes a first liquid110, a first electrode210, and a first circulator510.

The first liquid110contains dissolved therein a condensed aromatic compound and lithium.

The first electrode210is immersed in the first liquid110.

The first circulator510includes a first container511and a first passage prevention member512.

The first liquid110containing the condensed aromatic compound dissolved therein has the property of causing lithium to release solvated electrons and dissolve as cations.

When lithium dissolved in the first liquid110precipitates on the first electrode210, lithium precipitate particles700are generated.

The first circulator510circulates the first liquid110between the first electrode210and the first container511.

The first circulator510transfers the lithium precipitate particles700generated on the first electrode210to the first container511.

The first passage prevention member512is disposed in a channel through which the first liquid110flows from the first container511to the first electrode210(a pipe513in the example shown inFIG. 1).

The first passage prevention member512prevents passage of the lithium precipitate particles700.

The flow battery configured as above can have both a high energy density and a long cycle life.

Specifically, in the above structure, lithium can be present on the counter electrode side of the first electrode210in an amount exceeding the maximum amount dissolvable in the first liquid110containing the condensed aromatic compound dissolved therein. Since the lithium dissolved in the first liquid110and the lithium precipitate particles700are present, the amount of lithium is large, so that a high capacity density can be achieved. Therefore, a high energy density and a high capacity can be achieved.

In the above structure, the lithium precipitate particles700themselves are not circulated through the channel through which the first liquid110flows from the first container511to the first electrode210, but only the first liquid110containing lithium dissolved therein can be circulated. This can prevent the occurrence of, for example, clogging of pipes etc. with the lithium precipitate particles700. Therefore, the flow battery provided can have a long cycle life.

With the above structure, a material having a relatively low equilibrium potential (vs. Li/Li+) can be used as the condensed aromatic compound. This allows the flow battery negative electrode provided to have a lower potential. Therefore, the flow battery provided can have a high battery voltage (discharge voltage).

During charging of the flow battery1000in embodiment 1, (i.e., in a state in which electrons are supplied to the first electrode210from the outside of the flow battery1000), the condensed aromatic compound may be reduced on the first electrode210, and lithium dissolved in the first liquid110may precipitate on the first electrode210and form lithium precipitate particles700.

During discharging of the flow battery1000(i.e., in a state in which electrons are emitted from the first electrode210to the outside of the flow battery1000), the condensed aromatic compound may be oxidized on the first electrode210, and the lithium precipitate particles700may dissolve in the first liquid110as lithium.

With the above structure, a larger amount of lithium precipitate particles700can be generated during charging. Moreover, a large amount of lithium precipitate particles700can be used as a lithium source during discharging. This can increase the charge-discharge capacity.

For example, when the first liquid110comes into contact with the first electrode210, the condensed aromatic compound is oxidized or reduced on the first electrode210.

In the flow battery1000in embodiment 1, the condensed aromatic compound may be at least one selected from the group consisting of phenanthrene, biphenyl, o-terphenyl, trans-stilbene, triphenylene, and anthracene.

With the above structure, the condensed aromatic compound dissolved in the first liquid110provided can be electrochemically base. A solution containing the condensed aromatic compound (e.g., an ether solution) has the ability to dissolve lithium (e.g., lithium metal). Lithium easily releases electrons to form cations. Therefore, lithium donates electrons to the condensed aromatic compound in the solution and dissolves in the solution as cations. In this case, the condensed aromatic compound that has accepted the electrons solvates the electrons. The condensed aromatic compound solvating the electrons behaves as anions. Therefore, the condensed aromatic compound-containing solution itself has ion conductivity. In the condensed aromatic compound-containing solution, Li cations and electrons are present in equivalent amounts. Therefore, the condensed aromatic compound-containing solution itself can have strong reducing properties (in other words, can be electrochemically base).

For example, when an electrode that does not react with lithium is immersed in the first liquid110containing the condensed aromatic compound dissolved therein and then the potential with respect to lithium metal is measured, the potential measured is considerably low. The potential observed depends on the degree of solvation of electrons by the condensed aromatic compound (i.e., the type of condensed aromatic compound). Examples of the condensed aromatic compound that exhibits a low potential include phenanthrene, biphenyl, o-terphenyl, trans-stilbene, triphenylene, and anthracene.

FIG. 2is a table showing the results of measurement of the potentials of condensed aromatic compounds.

A 2×2 cm copper foil was wrapped with a polypropylene-made microporous separator, and the entire separator was wrapped with a large amount of lithium metal foil. A tab was attached to each of the copper foil and the lithium metal. Then a laminate exterior package was attached to the above product. 2MeTHF containing dissolved therein a condensed aromatic compound at a molar concentration (M) shown inFIG. 2was poured into the laminate exterior package, and the laminate exterior package was hermetically heat-sealed. A potential measurement cell was thereby prepared for each condensed aromatic compound.FIG. 2shows the potentials (V vs. Li/Li+) measured with respect to lithium metal using these potential measurement cells.

In the flow battery1000in embodiment 1, the first liquid110may be an ether solution.

In the above structure, the first liquid110provided may be an electrolyte solution containing the condensed aromatic compound. Specifically, since the solvent for the condensed aromatic compound is an ether with no electron conductivity, the ether solution itself can have the properties of an electrolyte solution.

The ether used may be at least one of a commonly known cyclic ether and a commonly known chain ether. Tetrahydrofuran (THF), dioxane (DO), 2-methyltetrahydrofuran (2MeTHF), and 4-methyldioxane (4MeDO), etc. may be used as the cyclic ether. Glyme etc. may be used as the chain ether.

In the example inFIG. 1, the first electrode210is shown as a negative electrode, and a second electrode220is shown as a positive electrode.

When an electrode with a low relative potential is used as the second electrode220, the first electrode210can serve as a positive electrode.

Specifically, the first electrode210may be a positive electrode, and the second electrode220may be a negative electrode.

The first electrode210may have a surface serving as a reaction field for the condensed aromatic compound.

In this case, a material stable in the first liquid110may be used for the first electrode210. The material used for the first electrode210may be stable during an electrode reaction, which is an electrochemical reaction. For example, a metal (such as stainless steel, iron, copper, or nickel), carbon, etc. may be used for the first electrode210.

The first electrode210may have a structure with an increased surface area (e.g., a mesh, a nonwoven fabric, a surface roughened plate, or a sintered porous body). In this case, the first electrode210has a large specific surface area. This can facilitate the progress of the oxidation or reduction reaction of the condensed aromatic compound.

The second electrode220may have a structure including a current collector and an active material disposed on the current collector. In this case, for example, a high-capacity active material can be used. A compound having the property of reversibly occluding and releasing lithium ions may be used as the active material for the second electrode220.

Alternatively, the second electrode220may be lithium metal. When lithium metal is used for the second electrode220, dissolution and precipitation of the metal serving as the positive electrode can be easily controlled, and a high capacity can be achieved.

The flow battery1000in embodiment 1 may further include a separator400.

The separator400may be a microporous membrane (porous body) used for known secondary batteries.

Alternatively, the separator400may be a porous membrane such as glass paper, which is a nonwoven fabric including glass fibers woven thereinto.

Alternatively, the separator400may be a separation membrane having ion conductivity (lithium ion conductivity). For example, the separator400may be an ion exchange resin membrane (such as a cation exchange membrane or an anion exchange membrane), a solid electrolyte membrane, etc.

The first circulator510may be a mechanism including, for example, a pipe, a tank, a pump, a valve, etc.

In embodiment 1, the first container511may be, for example, a tank.

The first container511may contain the first liquid110containing the condensed aromatic compound dissolved therein.

As shown inFIG. 1, the flow battery1000in embodiment 1 may further include an electrochemical reaction section600, a positive electrode terminal221, and a negative electrode terminal211.

The electrochemical reaction section600is separated by the separator400into a negative electrode chamber610and a positive electrode chamber620.

An electrode serving as the negative electrode (the first electrode210in the example shown inFIG. 1) is disposed in the negative electrode chamber610.

The negative electrode terminal211is connected to the electrode serving as the negative electrode.

An electrode serving as the positive electrode (the second electrode220in the example shown inFIG. 1) is disposed in the positive electrode chamber620.

The positive electrode terminal221is connected to the electrode serving as the positive electrode.

The negative electrode terminal211and the positive electrode terminal221are connected to, for example, a charge-discharge device. The charge-discharge device applies a voltage between the negative electrode terminal211and the positive electrode terminal221or collects electric power through the negative electrode terminal211and the positive electrode terminal221.

As shown inFIG. 1, in the flow battery1000in embodiment 1, the first circulator510may include a pipe514, a pipe513, and a pump515.

One end of the pipe514is connected to the negative electrode chamber610or the positive electrode chamber620, whichever includes the first electrode210disposed therein (the negative electrode chamber610in the example shown inFIG. 1).

The other end of the pipe514is connected to an inlet of the first liquid110that is disposed in the first container511.

One end of the pipe513is connected to an outlet of the first liquid110that is disposed in the first container511.

The other end of the pipe513is connected to the negative electrode chamber610or the positive electrode chamber620, whichever includes the first electrode210disposed therein (the negative electrode chamber610in the example shown inFIG. 1).

The pump515is disposed, for example, in the pipe514. Alternatively, the pump515may be disposed in the pipe513.

The first passage prevention member512may be disposed, for example, at the joint between the first container511and the pipe513.

In the flow battery1000in embodiment 1, the first passage prevention member512may be a filter that can filter out the lithium precipitate particles700.

In this case, the filter may be formed of, for example, a glass fiber paper filter, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a metal mesh unreactive with lithium.

In the above structure, the lithium precipitate particles700are further prevented from flowing into the first electrode210side. This can further prevent clogging of a component (e.g., a pipe) of the first circulator510with the lithium precipitate particles700.

The filter may be a member having pores smaller than a prescribed diameter (e.g., a particle diameter causing clogging) of the lithium precipitate particles700. The material used for the filter may be a material unreactive with the lithium precipitate particles700, the first liquid110, etc.

With the above structure, even when the lithium precipitate particles700flow together with the first liquid110within the first container511, the lithium precipitate particles700are prevented from flowing out of the first container511.

In the example shown inFIG. 1, the first liquid110contained in the first container511passes through the first passage prevention member512and the pipe513and is then supplied to the negative electrode chamber610.

The condensed aromatic compound dissolved in the first liquid110is thereby oxidized or reduced on the first electrode210.

Then the first liquid110containing dissolved therein the condensed aromatic compound that has been oxidized or reduced passes through the pipe514and the pump515and is supplied to the first container511.

For example, the pump515may be used to control the circulation of the first liquid110. Specifically, the pump515may be used appropriately to start the supply of the first liquid110, stop the supply, or control the amount of the supply.

Alternatively, a mechanism (e.g., a valve) different from the pump515may be used to control the circulation of the first liquid110.

Electrolyte solutions (solvents) used in the negative electrode chamber610and the positive electrode chamber620separated by the separator400may have different compositions.

Alternatively, the electrolyte solutions (solvents) used in the positive electrode chamber620and the negative electrode chamber610may have the same composition

<Description of Charge and Discharge Processes>

Charge and discharge processes of the flow battery1000in embodiment 1 will next be described.

The charge and discharge processes will be specifically described while the following operation example is shown.

Specifically, in this operation example, the first electrode210is a negative electrode and is made of stainless steel.

In this operation example, the first liquid110is an ether solution containing dissolved therein the condensed aromatic compound.

In this operation example, the condensed aromatic compound is phenanthrene (hereinafter denoted by PNT).

In this operation example, the second electrode220is a positive electrode and includes a current collector (stainless steel) and lithium iron phosphate (LiFePO4) used as an active material disposed on the current collector.

In this operation example, lithium is present on the positive electrode side in an amount exceeding the maximum amount of lithium dissolvable in the first liquid110(the ether solution containing the condensed aromatic compound dissolved therein). The battery is designed such that this lithium amount controls the battery capacity.

[Description of Charge Process]

First, the charge reaction will be described.

A voltage is applied between the first electrode210and the second electrode220to perform charging.

(Reaction on Positive Electrode Side)

On the second electrode220serving as the positive electrode, the oxidation reaction of the positive electrode active material occurs when a voltage is applied. Specifically, lithium ions are released from the positive electrode active material. Electrons are thereby emitted from the second electrode220to the outside of the flow battery.

For example, in this operation example, the following reaction occurs.
LiFePO4→FePO4+Li++e−

Part of the lithium ions (Li+) generated can migrate to the first liquid110through the separator400.

When the battery assembled is in a discharged state, lithium is present on the positive electrode side. When the positive electrode is oxidized as described above, the lithium extracted migrates to the negative electrode side through the separator.

(Reaction on Negative Electrode Side)

When a voltage is applied, electrons are supplied from the outside of the flow battery to the first electrode210serving as the negative electrode. Lithium ions receive electrons on the first electrode210and become lithium atoms.

For example, in this operation example, the following reaction occurs.
Li++e−→Li

However, the lithium atoms dissolve in the first liquid110present therearound. Specifically, the reduction reaction of the condensed aromatic compound occurs on the first electrode210.

For example, in this operation example, the following reactions occur.
Li→Li++e−
PNT+Li++e−→PNT·Li

As the charging proceeds further, the concentration of lithium in the first liquid110reaches a saturated state. In this saturated state, the lithium formed cannot dissolve in the first liquid110. Therefore, the lithium formed in the saturated state precipitates on the first electrode210as lithium precipitate particles700.

For example, in this operation example, the following reaction occurs.
Li++e−→Li

The lithium precipitate particles700precipitated on the first electrode210are transferred (supplied) through the first circulator510to the first container511.

Specifically, a flow is always present around the first electrode210. Therefore, the lithium precipitate particles700formed migrate with the flow and are transferred to the first container511.

The lithium precipitate particles700transferred to the first container511are urged to flow from the first container511again to the first electrode210side with the flow. However, the lithium precipitate particles700are trapped by the first passage prevention member512disposed at the outlet of the first container511. Therefore, the lithium precipitate particles700are not transferred to the first electrode210side. The lithium precipitate particles700are stored in the first container511.

The above charge reaction may proceed until the positive electrode active material reaches its fully charged state.

Next, a discharge reaction from full charge will be described.

In the full charge, the positive electrode active material is in its charged state.

During the discharge reaction, electric power is outputted through the first electrode210and the second electrode220.

(Reaction on Positive Electrode Side)

During discharging of the battery, electrons are supplied from the outside of the flow battery to the second electrode220serving as the positive electrode. The reduction reaction of the active material thereby occurs on the second electrode220.

For example, in this operation example, the following reaction occurs.
FePO4+Li++e−→LiFePO4

Part of the lithium ions (Li+) may be supplied from the first liquid110through the separator400.

(Reaction on Negative Electrode Side)

During discharging of the battery, the oxidation reaction of the condensed aromatic compound occurs on the first electrode210serving as the negative electrode. Electrons are thereby emitted from the first electrode210to the outside of the flow battery.

For example, in this operation example, the following reaction occurs.
PNT·Li→PNT+Li++e−

Specifically, during discharging, lithium dissolved in the first liquid110releases electrons. The amount of lithium dissolved in the first liquid110thereby decreases.

The condensed aromatic compound oxidized on the first electrode210is transferred (supplied) through the first circulator510to the first container511.

The lithium precipitate particles700stored in the first container511dissolve in the first liquid110containing the condensed aromatic compound dissolved therein and oxidized on the first electrode210. Specifically, in the first container511, the reduction reaction of the condensed aromatic compound occurs.

For example, in this operation example, the following reactions occur.
Li→Li++e−
PNT+Li++e−→PNT·Li

As described above, the lithium precipitate particles700are excessively present in the first container511. Therefore, even when the amount of lithium in the first liquid110is decreased on the first electrode210, the decrease in the amount of lithium is immediately compensated by dissolution of the lithium precipitate particles700stored in the first container511in the first liquid110.

The condensed aromatic compound reduced in the first container511is transferred (supplied) through the first circulator510to a position in which the first electrode210is disposed. This causes the oxidation reaction of the condensed aromatic compound to occur again.

For example, in this operation example, the following reaction occurs.
PNT·Li→PNT+Li++e−

As described above, the amount of lithium in the first liquid110is always maintained in the saturated state until the late stage of discharging. This allows stable discharging.

The above discharge reaction may proceed until the positive electrode active material reaches its fully discharged state.

Embodiment 2 will be described. However, the description will be omitted as appropriate when it overlaps with that of embodiment 1 above.

In a structure shown in embodiment 2, electrolyte solutions circulate on both the first electrode side and the second electrode side.

FIG. 3is a block diagram showing a general structure of a flow battery2000in embodiment 2.

The flow battery2000in embodiment 2 includes, in addition to the structure of the flow battery1000in embodiment 1, the following structure.

Specifically, the flow battery2000in embodiment 2 further includes a second liquid120, the second electrode220, and a second active material320.

The second liquid120contains a second electrode-side mediator121dissolved therein.

The second electrode220is a counter electrode of the first electrode210. The second electrode220is immersed in the second liquid120.

The second active material320is immersed in the second liquid120.

The second electrode-side mediator121is oxidized and reduced on the second electrode220.

The second electrode-side mediator121is oxidized and reduced by the second active material320.

The flow battery configured as above can have both a higher energy density and a longer cycle life.

Specifically, in the flow battery configured as above, although the active materials are used, these active materials themselves are not circulated. Therefore, for example, a high-capacity powdery active material for charge and discharge reactions can be used as the second active material320. This allows a higher energy density and a higher capacity to be achieved. The battery capacity is determined by “positive electrode capacity density×negative electrode capacity density/(positive electrode capacity density+negative electrode capacity density).” Therefore, when a mediator-type flow battery structure is used for each of the first electrode210side and the second electrode220side, the capacity densities can be significantly improved.

In the above structure, only the second liquid120containing the second electrode-side mediator121dissolved therein can be circulated without circulation of the powdery active material itself. This can prevent the occurrence of, for example, clogging of pipes with the powdery active material. Therefore, the flow battery provided can have a longer cycle life.

In the above structure, when the second active material320used is an active material having a relatively high equilibrium potential (vs. Li/Li+) (e.g., lithium iron phosphate), a material having a relatively high equilibrium potential (vs. Li/Li+) (e.g., tetrathiafulvalene) can be used as the second electrode-side mediator121. In this case, the flow battery positive electrode provided can have a higher potential. Therefore, the flow battery provided can have a higher battery voltage (discharge voltage).

In the flow battery2000in embodiment 2, lithium may be dissolved in the second liquid120.

The second active material320may be a material having the property of occluding and releasing lithium.

During charging of the flow battery2000(i.e., in a state in which electrons are supplied to the first electrode210from the outside of the flow battery2000and electrons are emitted from the second electrode220to the outside of the flow battery2000), the second electrode-side mediator121may be oxidized on the second electrode220. Then the second electrode-side mediator121oxidized on the second electrode220may be reduced by the second active material320, and the second active material320may release lithium.

During discharging of the flow battery2000(i.e., in a state in which electrons are emitted from the first electrode210to the outside of the flow battery2000and electrons are supplied from the outside of the flow battery2000to the second electrode220), the second electrode-side mediator121may be reduced on the second electrode220. Then the second electrode-side mediator121reduced on the second electrode220may be oxidized by the second active material320, and the second active material320may occlude lithium.

In the above structure, the second active material320used may be, for example, an active material having the property of reversibly occluding and releasing lithium (e.g., lithium ions). In this case, the material design of the second active material320is facilitated. Moreover, a higher capacity can be achieved.

For example, when the second liquid120comes into contact with the second electrode220, the second electrode-side mediator121is oxidized or reduced on the second electrode220.

For example, when the second liquid120comes into contact with the second active material320, the second electrode-side mediator121is oxidized or reduced by the second active material320.

In the flow battery2000in embodiment 2, the redox potential range of the second electrode-side mediator121and the redox potential range of the second active material320may overlap each other.

In the above structure, the second active material320can oxidize and reduce the second electrode-side mediator121.

In the flow battery2000in embodiment 2, the upper limit of the redox potential range of the second electrode-side mediator121may be higher than the upper limit of the redox potential range of the second active material320.

In this case, the lower limit of the redox potential range of the second electrode-side mediator121may be lower than the lower limit of the redox potential range of the second active material320.

With the above structure, the capacity of the second active material320can be sufficiently utilized (e.g., up to nearly 100%). Therefore, the flow battery provided can have a higher capacity.

One redox species having a plurality of redox potentials may be used as the second electrode-side mediator121.

Alternatively, a mixture of a plurality of redox species each having a constant redox potential may be used as the second electrode-side mediator121.

In the flow battery2000in embodiment 2, the second electrode-side mediator121may be an organic compound having oxidizing and reducing properties.

With the above structure, the solubility of the second electrode-side mediator121in the second liquid120(e.g., a nonaqueous solvent) can be increased.

In the flow battery2000in embodiment 2, the second electrode-side mediator121may be an organic compound having multiple redox potentials (e.g., two or more redox potentials).

Examples of such an organic compound capable of multi-stage redox include organic compounds having π conjugated electron clouds such as tetrathiafulvalene derivatives, quinone derivatives, and TCNQ.

In the flow battery2000in embodiment 2, the second electrode-side mediator121may be tetrathiafulvalene.

With the above structure, the second electrode-side mediator121provided can have relatively high two redox potentials (a lower limit of about 3.4 V and an upper limit of about 3.7 V versus the lithium reference potential). The flow battery positive electrode provided can thereby have a higher potential. Therefore, the flow battery provided can have a high battery voltage (discharge voltage).

In the flow battery2000in embodiment 2, the second active material320may be a material having the property of reversibly occluding and releasing lithium ions. For example, the second active material320used may be a commonly known active material for secondary batteries (such as a transition metal oxide, a fluoride, polyanions, fluorinated polyanions, or a transition metal sulfide).

In the flow battery2000in embodiment 2, the second active material320may be lithium iron phosphate.

In the above structure, the second active material320can have a relatively high equilibrium potential (vs. Li/Li+). Therefore, a material having a relatively high equilibrium potential (vs. Li/Li+) (e.g., tetrathiafulvalene) can be used as the second electrode-side mediator121. The flow battery positive electrode provided can thereby have a higher potential. Therefore, the flow battery provided can have a high battery voltage (discharge voltage).

A compound containing iron, manganese, or lithium (such as LiFePO4or LiMnO2) and a vanadium-containing compound (such as V2O5) have a redox potential of 3.2 V to 3.7 V with respect to lithium. Therefore, when LiFePO4, for example, is used as the second active material320, tetrathiafulvalene may be used as the second electrode-side mediator121.

In the flow battery2000in embodiment 2, the second electrode-side mediator121may be a quinone derivative. The quinone derivative has multiple redox potentials of, for example, 1 V to 3 V with respect to lithium. In this case, a material having a redox potential of 1 V to 3 V with respect to lithium may be used as the second active material320. Examples of the material having a redox potential of 1 V to 3 V with respect to lithium include compounds containing titanium, niobium, or lithium (such as Li4Ti5O12and LiNbO3).

In the flow battery2000in embodiment 2, the second electrode-side mediator121may be metal-containing ions. Examples of the metal-containing ions include vanadium ions, manganese ions, and molybdenum ions that have multiple redox potentials. For example, vanadium ions have a variety of reaction stages (divalence and trivalence, trivalence and tetravalence, and tetravalence and pentavalence).

The second active material320used may be a powdery active material. By charging a tank with the second active material320in unprocessed powder form, production can be simplified, and the production cost can be reduced.

Alternatively, the second active material320used may be a pellet-like active material (prepared, for example, by forming a powder into pellets). By charging the tank with the second active material320in pellet form, the production can be simplified, and the production cost can be reduced.

Alternatively, the second active material320used may be an active material pelleted using a commonly known binder (such as polyvinylidene fluoride, polypropylene, polyethylene, or polyimide).

The second active material320used may be an active material in the form of a film fixed to a metal foil.

Alternatively, the second active material320used may be an active material mixed with a commonly known conductive assistant (such as carbon black or polyaniline) or an ion conductor (such as polymethyl methacrylate or polyethylene oxide).

The second active material320may be a material that does not dissolve (i.e., is insoluble) in the second liquid120. In the flow battery provided in this case, the second electrode-side mediator121is circulated together with the second liquid120, but the second active material320is not circulated.

The second liquid120may be, for example, a commonly known nonaqueous electrolyte solution for secondary batteries. In this case, the nonaqueous electrolyte solution is composed, for example, of a commonly known electrolyte salt (such as an electrolyte salt of a lithium ion and an anion) and a nonaqueous solvent containing the electrolyte salt dissolved therein.

The nonaqueous solvent used may be a commonly known nonaqueous solvent for secondary batteries. Specifically, the nonaqueous solvent used may be a cyclic or chain carbonate, a cyclic or chain ester, a cyclic or chain ether, a nitrile, a cyclic or chain sulfone, or a cyclic or chain sulfoxide.

Different solvents may be used for the first liquid110and the second liquid120, or the same solvent may be used.

In the example inFIG. 3, the first electrode210is shown as a negative electrode, and the second electrode220is shown as a positive electrode.

When an electrode with a low-relative potential is used as the second electrode220, the first electrode210can server as a positive electrode.

Specifically, the first electrode210may be a positive electrode, and the second electrode220may be a negative electrode.

The second electrode220may have a surface serving as a reaction field for the second electrode-side mediator121.

In this case, a material stable in the solvent for the second liquid120and the supporting electrolyte therefor may be used for the second electrode220. The material used for the second electrode220may be stable during an electrode reaction, which is an electrochemical reaction. For example, a metal (such as stainless steel, iron, copper, or nickel), carbon, etc. may be used for the second electrode220.

The second electrode220may have a structure with an increased surface area (e.g., a mesh, a nonwoven fabric, a surface roughened plate, or a sintered porous body). In this case, the second electrode220has a large specific surface area. This can facilitate the progress of the oxidation or reduction reaction of the second electrode-side mediator121.

Different electrode materials may be used for the first electrode210and the second electrode220, or the same electrode material may be used.

The flow battery2000in embodiment 2 may further include the separator400.

The separator400separates the first electrode210and the first liquid110from the second electrode220and the second liquid120.

The separator400used may have the structure shown in embodiment 1 above.

The flow battery2000in embodiment 2 may further include a second circulator520.

The second circulator520is a mechanism that circulates the second liquid120between the second electrode220and the second active material320.

With the above structure, the second electrode-side mediator121can be circulated together with the second liquid120between the second electrode220and the second active material320. This allows the oxidation and reduction reactions of materials to proceed more efficiently.

The second circulator520may be a mechanism including, for example, a pipe, a tank, a pump, a valve, etc.

In the flow battery2000in embodiment 2, the second circulator520may include a second container521.

The second active material320and the second liquid120may be contained in the second container521.

The second circulator520may circulate the second liquid120between the second electrode220and the second container521.

In the second container521, the second active material320may come into contact with the second liquid120. This allows the second electrode-side mediator121to be oxidized and reduced by the second active material320.

In the above structure, the second liquid120and the second active material320can come into contact with each other in the second container521. In this case, for example, the area of contact between the second liquid120and the second active material320can be increased. Moreover, the time of contact between the second liquid120and the second active material320can be increased. This allows the oxidation and reduction reactions of the second electrode-side mediator121by the second active material320to proceed more efficiently.

In embodiment 2, the second container521may be, for example, a tank.

In the second container521, the second liquid120containing the second electrode-side mediator121dissolved therein may be accommodated, for example, in spaces in the second active material320placed in the second container521.

As shown inFIG. 3, in the flow battery2000in embodiment 2, the second circulator520may include a pipe523, a pipe524, and a pump525.

One end of the pipe524is connected to the positive electrode chamber620or the negative electrode chamber610, whichever includes the second electrode220disposed therein (the positive electrode chamber620in the example shown inFIG. 3).

The other end of the pipe524is connected to an inlet of the second liquid120that is disposed in the second container521.

One end of the pipe523is connected to an outlet of the second liquid120that is disposed in the second container521.

The other end of the pipe523is connected to the positive electrode chamber620or the negative electrode chamber610, whichever includes the second electrode220disposed therein (the positive electrode chamber620in the example shown inFIG. 3).

The pump525is disposed, for example, in the pipe524. Alternatively, the pump525may be disposed in the pipe523.

In the flow battery2000in embodiment 2, the second circulator520may include a second passage prevention member522.

The second passage prevention member522prevents passage of the second active material320.

The second passage prevention member522is disposed in a channel through which the second liquid120flows from the second container521to the second electrode220(the pipe523in the example shown inFIG. 3).

The above structure can prevent the second active material320from flowing out of the second container521(e.g., flowing into the second electrode220side). Specifically, the second active material320stays in the second container521. Therefore, the flow battery provided can have a structure in which the second active material320itself is not circulated. This can prevent clogging of a component (e.g., a pipe) of the second circulator520with the second active material320. Moreover, the occurrence of resistance loss caused by the second active material320flowing into the second electrode220side can be prevented.

The second passage prevention member522may be disposed, for example, at the joint between the second container521and the pipe523.

The second passage prevention member522may be, for example, a filter that can filter out the second active material320. In this case, the filter may be a member having pores smaller than the minimum diameter of the particles of the second active material320. A material unreactive with the second active material320, the second liquid120, etc. can be used as the material of the filter. The filter may be, for example, a glass fiber paper filter, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a metal mesh unreactive with lithium metal.

With the above structure, even when the second active material320flows together with the second liquid120within the second container521, the second active material320is prevented from flowing out of the second container521.

In the example shown inFIG. 3, the second liquid120contained in the second container521passes through the second passage prevention member522and the pipe523and is then supplied to the positive electrode chamber620.

The second electrode-side mediator121dissolved in the second liquid120is thereby oxidized or reduced on the second electrode220.

Then the second liquid120containing the oxidized or reduced second electrode-side mediator121dissolved therein passes through the pipe524and the pump525and is supplied to the second container521.

Then the second electrode-side mediator121dissolved in the second liquid120is oxidized or reduced by the second active material320.

For example, the pump525may be used to control the circulation of the second liquid120. Specifically, the pump525may be used appropriately to start the supply of the second liquid120, stop the supply, or control the amount of the supply.

Alternatively, a mechanism (e.g., a valve) different from the pump525may be used to control the circulation of the second liquid120.

<Description of Charge and Discharge Processes>

Charge and discharge processes of the flow battery2000in embodiment 2 will next be described.

The charge and discharge processes will be specifically described while the following operation example is shown.

Specifically, in this operation example, the first electrode210is a negative electrode and is made of stainless steel.

In this operation example, the first liquid110is an ether solution containing dissolved therein a condensed aromatic compound.

In this operation example, the condensed aromatic compound is phenanthrene (hereinafter denoted by PNT).

In this operation example, the second electrode220is a positive electrode and is made of stainless steel.

In this operation example, the second liquid120is an ether solution containing the second electrode-side mediator121dissolved therein.

In this operation example, the second electrode-side mediator121is tetrathiafulvalene (hereinafter denoted by TTF).

In this operation example, the second active material320is lithium iron phosphate (LiFePO4).

In this operation example, the separator400is a lithium ion conductive solid electrolyte membrane.

In this operation example, lithium is present on the positive electrode side in an amount exceeding the maximum amount of lithium dissolvable in the first liquid110(the ether solution containing the condensed aromatic compound dissolved therein). The battery is designed such that this lithium amount controls the battery capacity.

[Description of Charge Process]

First, the charge reaction will be described.

A voltage is applied between the first electrode210and the second electrode220to perform charging.

(Reaction of Positive Electrode Side)

On the second electrode220serving as the positive electrode, the oxidation reaction of the second electrode-side mediator121occurs when a voltage is applied. Specifically, the second electrode-side mediator121is oxidized on the surface of the second electrode220. Electrons are thereby emitted from the second electrode220to the outside of the flow battery.

For example, in this operation example, the following reactions occur.
TTF→TTF++e−
TTF+→TTF2++e−

The second electrode-side mediator121oxidized on the second electrode220is transferred (supplied) through the second circulator520to a position in which the second active material320is disposed.

In this case, the second electrode-side mediator121oxidized on the second electrode220is reduced by the second active material320. In other words, the second active material320is oxidized by the second electrode-side mediator121. The second active material320thereby releases lithium.

For example, in this operation example, the following reaction occurs.
LiFePO4+TTF2+→FePO4+Li++TTF+

The second electrode-side mediator121reduced by the second active material320is transferred (supplied) through the second circulator520to a position in which the second electrode220is disposed.

In this case, the second electrode-side mediator121is oxidized on the surface of the second electrode220.

For example, in this operation example, the following reaction occurs.
TTF+→TTF2++e−

Part of the lithium ions (Li+) generated may be transferred to the first liquid110through the separator400.

When the battery assembled is in a discharged state, lithium is present on the positive electrode side. When the positive electrode is oxidized as described above, the lithium extracted migrates to the negative electrode side through the separator.

As described above, the second electrode-side mediator121is unchanged in the overall reaction including circulation.

However, the second active material320located in the position spaced apart from the second electrode220is brought to a charged state.

In a fully charged state, TTF2+is present in the second liquid120, and the second active material320is in the form of FePO4. In this case, the charge potential is determined by the potential for oxidation to TTF2+.

(Reaction of Negative Electrode Side)

When a voltage is applied, electrons are supplied from the outside of the flow battery to the first electrode210serving as the negative electrode. Lithium ions receive electrons on the first electrode210and become lithium atoms.

For example, in this operation example, the following reaction occurs.
Li++e−→Li

However, the lithium atoms dissolve in the first liquid110present therearound. Specifically, the reduction reaction of the condensed aromatic compound occurs on the first electrode210.

For example, in this operation example, the following reactions occur.
Li→Li++e−
PNT+Li++e−→PNT·Li

As the charging proceeds further, the concentration of lithium in the first liquid110reaches a saturated state. In this saturated state, the lithium formed cannot dissolve in the first liquid110. Therefore, the lithium formed in the saturated state precipitates on the first electrode210as lithium precipitate particles700.

For example, in this operation example, the following reaction occurs.
Li++e−→Li

The lithium precipitate particles700precipitated on the first electrode210are transferred (supplied) through the first circulator510to the first container511.

Specifically, a flow is always present around the first electrode210. Therefore, the lithium precipitate particles700migrate with the flow and are transferred to the first container511.

The lithium precipitate particles700transferred to the first container511are urged to flow from the first container511again to the first electrode210with the flow. However, the lithium precipitate particles700are trapped by the first passage prevention member512disposed at the outlet of the first container511. Therefore, the lithium precipitate particles700are not transferred to the first electrode210side. The lithium precipitate particles700are stored in the first container511.

The above charge reaction may proceed until the second active material320reaches its fully charged state.

Next, a discharge reaction from full charge will be described.

In the full charge, the second active material320is in its charged state.

During the discharge reaction, electric power is outputted through the first electrode210and the second electrode220.

(Reaction of Positive Electrode Side)

During discharging of the battery, electrons are supplied from the outside of the flow battery to the second electrode220serving as the positive electrode. The reduction reaction of the second electrode-side mediator121thereby occurs on the second electrode220. Specifically, the second electrode-side mediator121is reduced on the second electrode220.

For example, in this operation example, the following reactions occur.
TTF2++e−→TTF+
TTF++e−→TTF

The second electrode-side mediator121reduced on the second electrode220is transferred (supplied) through the second circulator520to the position in which the second active material320is disposed.

In this case, the second electrode-side mediator121reduced on the second electrode220is oxidized by the second active material320. In other words, the second active material320is reduced by the second electrode-side mediator121. The second active material320thereby occludes lithium.

For example, in this operation example, the following reaction occurs.
FePO4+Li++TTF→LiFePO4+TTF+

The second electrode-side mediator121oxidized by the second active material320is transferred (supplied) through the second circulator520to the position in which the second electrode220is disposed.

In this case, the second electrode-side mediator121is reduced on the surface of the second electrode220.

For example, in this operation example, the following reaction occurs.
TTF++e−→TTF

Part of the lithium ions (Li+) may be supplied from the first liquid110through the separator400.

As described above, the second electrode-side mediator121is unchanged in the overall reaction including circulation.

However, the second active material320located in the position spaced apart from the second electrode220is brought to a discharged state.

In a fully discharged state, TTF is present in the second liquid120, and the second active material320is in the form of LiFePO4. In this case, the discharge potential is determined by the potential for reduction to TTF.

(Reaction of Negative Electrode Side)

During discharging of the battery, the oxidation reaction of the condensed aromatic compound occurs on the first electrode210serving as the negative electrode. Electrons are thereby emitted from the first electrode210to the outside of the flow battery.

For example, in this operation example, the following reaction occurs.
PNT·Li→PNT+Li++e−

Specifically, during discharging, lithium dissolved in the first liquid110releases electrons. The amount of lithium dissolved in the first liquid110thereby decreases.

The condensed aromatic compound oxidized on the first electrode210is transferred (supplied) through the first circulator510to the first container511.

The lithium precipitate particles700stored in the first container511dissolve in the first liquid110containing the condensed aromatic compound dissolved therein and oxidized on the first electrode210. Specifically, in the first container511, the reduction reaction of the condensed aromatic compound occurs.

For example, in this operation example, the following reactions occur.
Li→Li++e−
PNT+Li++e−→PNT·Li

As described above, the lithium precipitate particles700are excessively present in the first container511. Therefore, even when the amount of lithium in the first liquid110is decreased on the first electrode210, the decrease in the amount of lithium is immediately compensated by dissolution of the lithium precipitate particles700stored in the first container511in the first liquid110.

The condensed aromatic compound reduced in the first container511is transferred (supplied) through the first circulator510to a position in which the first electrode210is disposed. This causes the oxidation reaction of the condensed aromatic compound to occur again.

For example, in this operation example, the following reaction occurs.
PNT·Li→PNT+Li++e−

As described above, the amount of lithium in the first liquid110is always maintained in the saturated state until the late stage of discharging. This allows stable discharging.

The above discharge reaction may proceed until the second active material320reaches its fully discharged state.

Embodiment 3 will be described. However, the description will be omitted as appropriate when it overlaps with that of embodiment 1 or 2 above.

FIG. 4is a table showing the results of measurement of the potentials of condensed aromatic compounds.

A 2×2 cm copper foil was wrapped with a polypropylene-made microporous separator, and the entire separator was wrapped with a large amount of lithium metal foil. A tab was attached to each of the copper foil and the lithium metal. Then a laminate exterior package was attached to the above product. 2MeTHF containing dissolved therein a condensed aromatic compound at a molar concentration (M) shown inFIG. 4was poured into the laminate exterior package, and the laminate exterior package was hermetically heat-sealed. A potential measurement cell was thereby prepared for each condensed aromatic compound.FIG. 4shows the potentials (V vs. Li/Li+) measured with respect to lithium metal using these potential measurement cells. In this measurement, the ether used was 2MeTHF, but other ethers can also be used.

A flow battery in embodiment 3 includes, in addition to the structure of the flow battery in embodiment 1 or 2, the following structure.

Specifically, in the flow battery in embodiment 3, the condensed aromatic compound is at least one selected from the group consisting of phenanthrene, biphenyl, o-terphenyl, trans-stilbene, triphenylene, anthracene, butyrophenone, valerophenone, acenaphthene, acenaphthylene, fluoranthene, and benzil.

In the above structure, the condensed aromatic compound dissolved in the first liquid110can be electrochemically base. A solution containing the condensed aromatic compound (e.g., an ether solution) has the ability to dissolve lithium (e.g., lithium metal). Lithium easily releases electrons to form cations. Therefore, lithium donates electrons to the condensed aromatic compound in the solution and dissolves in the solution as cations. In this case, the condensed aromatic compound that has accepted the electrons solvates the electrons. The condensed aromatic compound solvating the electrons behaves as anions. Therefore, the condensed aromatic compound-containing solution itself has ion conductivity. In the condensed aromatic compound-containing solution, Li cations and electrons are present in equivalent amounts. Therefore, the condensed aromatic compound-containing solution itself can have strong reducing properties (in other words, can be electrochemically base).

Embodiment 4 will be described. However, the description will be omitted as appropriate when it overlaps with that of any of embodiments 1 to 3 above.

A flow battery in embodiment 4 includes, in addition to the structure of the flow battery in any of embodiments 1 to 3, the following structure.

Specifically, the flow battery in embodiment 4 includes the first liquid110, the first electrode210, the second liquid120, the second electrode220, and an electrolyte salt.

The first liquid110contains a condensed aromatic compound dissolved therein.

The first electrode210is immersed in the first liquid110.

The second electrode220is a counter electrode of the first electrode210. The second electrode220is immersed in the second liquid120.

The electrolyte salt is dissolved in at least one of the first liquid110and the second liquid120.

The concentration of the electrolyte salt in the first liquid110is equal to or lower than the concentration of the condensed aromatic compound in the first liquid110.

With the above structure, the flow battery provided can have a high energy density. Specifically, when the concentration of the electrolyte salt in the first liquid110is set to be equal to or lower than the concentration of the condensed aromatic compound, the equilibrium potential of the condensed aromatic compound can be easily maintained. In other words, a significant increase in the equilibrium potential of the condensed aromatic compound (an increase caused by the electrolyte salt) can be prevented. This can prevent the equilibrium potential of the condensed aromatic compound from exceeding the equilibrium potential on the second electrode220side.

In the above structure, the electrolyte salt is dissolved in at least one liquid of the first liquid110and the second liquid120. This can increase the ion conductivity of the at least one liquid.

In embodiment 4, the concentration of the electrolyte salt in the first liquid110may be lower than the concentration of the condensed aromatic compound in the first liquid110.

With the above structure, the equilibrium potential of the condensed aromatic compound can be maintained easily. In other words, a significant increase in the equilibrium potential of the condensed aromatic compound (an increase caused by the electrolyte salt) can be further prevented. Therefore, the flow battery provided can have a higher energy density.

The flow battery in embodiment 4 may further include the separator400.

The separator400separates the first electrode210and the first liquid110from the second electrode220and the second liquid120.

In this case, the concentration of the electrolyte salt in the first liquid110may be lower than the concentration of the electrolyte salt in the second liquid120.

In the above structure, the equilibrium potential of the condensed aromatic compound can be easily maintained (a change in the equilibrium potential can be further reduced). Therefore, the flow battery provided can have a high battery voltage (discharge voltage). Moreover, the flow battery provided can have a high energy density.

In embodiment 4, the electrolyte salt may be dissolved in the second liquid120. In this case, the electrolyte salt may not be dissolved in the first liquid110. Specifically, the concentration of the electrolyte salt in the first liquid110may be 0M.

In the above structure, the equilibrium potential of the condensed aromatic compound can be easily maintained (a change in the equilibrium potential can be further reduced). Therefore, the flow battery provided can have a higher battery voltage (discharge voltage). Moreover, the flow battery provided can have a higher energy density.

In embodiment 4, the first electrode210may be a negative electrode, and the second electrode220may be a positive electrode.

In embodiment 4, the electrolyte salt may be a lithium salt. The lithium salt used may be LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, or LiN(SO2F)2. The lithium salt used may be one selected from these lithium salts. Alternatively, a mixture of two or more selected from these lithium salts may be used.

In embodiment 4, the electrolyte salt may be at least one selected from the group consisting of LiBF4, LiN(SO2CF3)2, LiN(SO2F)2, and LiCF3SO3.

FIGS. 5, 6, 7, and 8are tables showing the results of measurement of the potentials of biphenyl solutions.

Each sample was prepared by dissolving biphenyl, i.e., a condensed aromatic compound, and an electrolyte salt in a 2-methyltetrahydrofuran (2MeTHF) solution at molar concentrations (M) shown in one of the tables. In samples shown inFIG. 5, LiBF4was used as the electrolyte salt. In samples shown inFIG. 6, LiN(SO2CF3)2was uses as the electrolyte salt. In samples shown inFIG. 7, LiN(SO2F)2was used as the electrolyte salt. In samples shown inFIG. 8, LiCF3SO3was used as the electrolyte salt. Potential measurement cells containing respective samples poured therein were produced, and the potential of each cell was measured. The potentials (V vs. Li/Li+) measured with respect to lithium metal are shown in the tables.

As can be seen fromFIGS. 5 to 8, when the concentration of the electrolyte salt is higher than the concentration of biphenyl, the equilibrium potential of biphenyl with respect to the potential of lithium metal increases as the concentration of the electrolyte salt increases.

However, when the concentration of the electrolyte salt is equal to or lower than the concentration of biphenyl, a significant increase in the equilibrium potential of biphenyl is prevented. For example, when the concentration of the electrolyte salt is equal to or lower than the concentration of biphenyl, the equilibrium potential of biphenyl can be maintained at 0.2 V vs. Li/Li+or lower.

FIG. 9is a table showing the results of measurement of the potentials of trans-stilbene solutions.

Each sample was prepared by dissolving trans-stilbene, i.e., a condensed aromatic compound, and LiBF4, i.e., an electrolyte salt, in a 2-methyltetrahydrofuran (2MeTHF) solution at molar concentrations shown inFIG. 9. Potential measurement cells containing respective samples poured therein were produced, and the potential of each cell was measured.FIG. 9shows the potentials (V vs. Li/Li+) measured with respect to lithium metal.

As can be seen fromFIG. 9, when the concentration of the electrolyte salt is higher than the concentration of trans-stilbene, the equilibrium potential of trans-stilbene with respect to the potential of lithium metal increases as the concentration of the electrolyte salt increases.

However, when the concentration of the electrolyte salt is equal to or lower than the concentration of trans-stilbene, a significant increase in the equilibrium potential of trans-stilbene is prevented. For example, when the concentration of electrolyte salt is equal to or lower than the concentration of trans-stilbene, the equilibrium potential of trans-stilbene can be maintained within the range of 0.2 to 0.6 V vs. Li/Li+.

Embodiment 5 will be described. However, the description will be omitted as appropriate when it overlaps with that of any of embodiments 1 to 4 above.

A flow battery in embodiment 5 includes, in addition to the structure of the flow battery in any of embodiments 1 to 3 above, the following structure.

In a flow battery in embodiment 5, the first liquid110contains dissolved therein a condensed aromatic compound and an electrolyte salt.

The electrolyte salt dissolved in the first liquid110is LiPF6.

With the above structure, the flow battery provided can have a high energy density. Specifically, when the electrolyte salt in the first liquid110is LiPF6, the equilibrium potential of the condensed aromatic compound can be maintained even when the concentration of the electrolyte salt is set arbitrarily relative to the concentration of the condensed aromatic compound. In other words, the ion conductivity can be improved by the addition of a sufficient amount of LiPF6, while a significant increase in the equilibrium potential of the condensed aromatic compound (an increase caused by the electrolyte salt) is prevented. Specifically, the ion conductivity can be increased while the equilibrium potential of the condensed aromatic compound is prevented from exceeding the equilibrium potential on the second electrode220side.

The flow battery in embodiment 5 may further include the second liquid120and the second electrode220.

The second electrode220is a counter electrode of the first electrode210. The second electrode220is immersed in the second liquid120.

In this case, LiPF6, i.e., the electrolyte salt, may be dissolved in the second liquid120.

In the above structure, LiPF6, i.e., the electrolyte salt, is dissolved in at least one liquid of the first liquid110and the second liquid120, and the ion conductivity of the at least one liquid can thereby be increased.

The flow battery in embodiment 5 may further include the separator400. The separator400separates the first electrode210and the first liquid110from the second electrode220and the second liquid120.

In the above structure, the equilibrium potentials of the condensed aromatic compound can be easily maintained (a change in the equilibrium potential can be further reduced). Therefore, the flow battery provided can have a higher battery voltage (discharge voltage). Moreover, the flow battery provided can have a higher energy density.

In embodiment 5, the first electrode210may be a negative electrode, and the second electrode220may be a positive electrode.

FIG. 10is a table showing the results of measurement of the potentials of biphenyl solutions.

Each sample was prepared by dissolving biphenyl, i.e., a condensed aromatic compound, and LiPF6, i.e., an electrolyte salt, in a 2-methyltetrahydrofuran (2MeTHF) solution at molar concentrations (M) shown inFIG. 10. Potential measurement cells containing respective samples poured therein were produced, and the potential of each cell was measured.FIG. 10shows the potentials (V vs. Li/Li+) measured with respect to lithium metal. As shown inFIG. 10, even when the concentration of the electrolyte salt LiPF6is higher (and also lower) than the concentration of biphenyl, the equilibrium potential of biphenyl with respect to the potential of lithium metal can be maintained at 0.2 V vs. Li/Li+or lower.

FIG. 11is a table showing the results of measurement of the potentials of trans-stilbene solutions.

Each sample was prepared by dissolving trans-stilbene, i.e., a condensed aromatic compound, and LiPF6, i.e., an electrolyte salt, in a 2-methyltetrahydrofuran (2MeTHF) solution at molar concentrations (M) shown inFIG. 11. Potential measurement cells containing respective samples poured therein were produced, and the potential of each cell was measured.FIG. 11shows the potentials (V vs. Li/Li+) measured with respect to lithium metal. As shown inFIG. 11, even when the concentration of the electrolyte salt LiPF6is higher (and also lower) than the concentration of trans-stilbene, the equilibrium potential of trans-stilbene with respect to the potential of lithium metal can be maintained at around 0.3 V vs. Li/Li+.

Embodiment 6 will be described. However, the description will be omitted as appropriate when it overlaps with that of any of embodiments 1 to 5 above.

A flow battery in embodiment 6 includes, in addition to the structure of the flow battery in any of embodiments 1 to 5 above, the following structure.

Specifically, in the flow battery in embodiment 6, the first liquid110is prepared by dissolving a condensed aromatic compound in at least one selected from the group consisting of tetrahydrofuran, 2-methyltetrahydrofuran, 1,2-dimethoxyethane, 2,5-dimethyltetrahydrofuran, diethoxyethane, dibutoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 3-methylsulfolane, and tetrahydrofurfurylamine.

In the above structure, the first liquid110provided can have the function of stabilizing solvated electrons released from lithium and paired with the condensed aromatic compound and can dissolve lithium (e.g., lithium metal).

FIG. 12is a table showing samples of the first liquid.

Biphenyl used as the condensed aromatic compound was dissolved in the samples (solvents) shown inFIG. 12at a concentration of 0.1M to prepare solutions, and lithium metal pieces were added to the prepared solutions. Each solution was left to stand, and dissolution of lithium metal was visually checked.

When lithium metal gives solvated electrons to a colorless solution and dissolves as lithium ions, the solution is colored. The occurrence of dissolution of lithium metal can be determined by disappearance of the lithium metal.

InFIG. 12, “◯ (yes)” denotes a sample with dissolution of lithium metal. InFIG. 12, “× (no)” denotes a sample with no dissolution of lithium metal.

FIG. 13is a table showing other samples of the first liquid.

Each sample shown inFIG. 13was prepared by mixing a solvent X and a solvent Y shown inFIG. 13at a volume mixing ratio shown inFIG. 13.

For each of the samples shown inFIG. 13, the same experiment as that for the samples shown inFIG. 12was performed to check dissolution of lithium metal.

As shown inFIG. 12, when dibutoxymethane, anisole, or phenetole was used alone, the solvent did not exhibit the ability to dissolve lithium metal. However, as shown inFIG. 13, mixtures of dibutoxyethane with dibutoxymethane, anisole, and phenetole exhibited the ability to dissolve lithium metal.

It was found that when a solvent having the ability to dissolve lithium metal coexists, the ability to dissolve lithium metal can be imparted to a solvent having no ability to dissolve lithium metal, as described above.

The structures described in embodiments 1 to 6 above may be appropriately combined.

The flow battery of the present disclosure can be preferably used as an electricity storage device.