JP2023162465A - Electrolyte and redox flow battery - Google Patents

Abstract

To provide an electrolyte in which precipitated oxides can exist stably.SOLUTION: An electrolyte is used in a redox flow battery. The electrolyte includes a dispersoid and a dispersant in which the dispersoid is suspended. The dispersoid is a plurality of particles suspended in the dispersant under a specific condition. The specific condition is the moment immediately after the electrolyte has been left to stand for one hour. Each of the plurality of particles includes oxides, where the oxides include oxygen and elements to be active material ions.SELECTED DRAWING: Figure 2

Description

TECHNICAL FIELD This disclosure relates to electrolytes and redox flow batteries.

Patent Document 1 discloses a manganese-titanium-based redox flow battery using a positive electrode electrolyte containing manganese ions and a negative electrode electrolyte containing titanium ions. When the positive electrode electrolyte contains manganese ions, manganese oxide (MnO ₂ ) may be precipitated during charging and discharging. Patent Document 1 suppresses precipitation of manganese oxide by containing titanium ions in addition to manganese ions in the positive electrode electrolyte.

International Publication No. 2011/111254

Redox flow batteries with high energy density are desired.

In order to improve the energy density, for example, it is possible to increase the concentration of active material ions in the electrolytic solution. However, the ion concentration in the electrolyte is limited by solubility. Furthermore, when the ion concentration is increased, oxide precipitation tends to occur.

In redox flow batteries, from the viewpoint of increasing energy density, it is required to charge the electrolytic solution to a high state of charge (SOC) region. However, in a region where the SOC is high, the dissolved state of ions in the electrolytic solution is unstable, and oxides are likely to be generated. For example, manganese ions in the positive electrode electrolyte are oxidized from divalent ions (Mn ²⁺ ) to trivalent ions (Mn ³⁺ ) during charging. However, Mn ³⁺ is unstable and quickly reacts with water to turn into MnO ₂ . In particular, if charging and discharging are repeated over a long period of time or if the electrolytic solution is left on standby for a long time in a state where the SOC is high, the precipitated oxide may grow and become coarse.

When the oxides of the active material ions become coarse and precipitate in the tank storing the electrolyte, the amount of active material in the electrolyte decreases. The precipitated oxide is difficult to be redissolved in the electrolytic solution and used as an active material. If the amount of active material in the electrolyte decreases, the battery capacity will decrease, leading to a decrease in energy density. In addition, the coarse oxides adhere to electrodes, piping, etc., causing deterioration of the flow of the electrolytic solution. Therefore, it is desired that the above-mentioned oxide exists stably in the electrolyte.

One of the objects of the present disclosure is to provide an electrolytic solution in which precipitated oxides can exist stably. Moreover, one of the objects of the present disclosure is to provide a redox flow battery with high energy density.

The electrolyte solution of the present disclosure is
An electrolytic solution used in a redox flow battery,
a dispersoid,
a dispersion medium that suspends the dispersoid,
The dispersoid is a plurality of particles suspended in the dispersion medium under specific conditions,
The specific conditions are immediately after the electrolyte is left standing for 1 hour,
Each of the plurality of particles is made of an oxide containing oxygen and an element serving as an active material ion.

The redox flow battery of the present disclosure includes the electrolyte of the present disclosure.

In the electrolytic solution of the present disclosure, the precipitated oxide can exist stably.

The redox flow battery of the present disclosure has high energy density.

FIG. 1 is a schematic configuration diagram of a redox flow battery according to an embodiment. FIG. 2 is an enlarged view schematically showing the electrolytic solution of the embodiment. FIG. 3 is a flowchart showing the processing procedure for manufacturing the electrolytic solution of the embodiment. FIG. 4 is an explanatory diagram illustrating charging and discharging conditions for manufacturing the electrolytic solution of the embodiment. FIG. 5 is an explanatory diagram illustrating an example of a pulse waveform of pulse charging and discharging used in the process of manufacturing the electrolytic solution of the embodiment.

[Description of embodiments of the present disclosure]
First, embodiments of the present disclosure will be listed and described.

(1) The electrolytic solution according to one aspect of the present disclosure is
An electrolytic solution used in a redox flow battery,
a dispersoid,
a dispersion medium that suspends the dispersoid,
The dispersoid is a plurality of particles suspended in the dispersion medium under specific conditions,
The specific conditions are immediately after the electrolyte is left standing for 1 hour,
Each of the plurality of particles is made of an oxide containing oxygen and an element serving as an active material ion.

Each particle in the electrolyte solution of the present disclosure is generated by increasing the concentration of active material ions in the electrolyte solution. Therefore, the electrolytic solution containing the particles has a high concentration of active material ions. Due to the high concentration of active material ions in the electrolyte, the electrolyte of the present disclosure has high energy density.

Each particle in the electrolyte of the present disclosure is suspended in the dispersion medium even under certain conditions. "Floating" here means that even under specific conditions, almost no sedimentation due to gravity occurs. Each particle made of this buoyant oxide can stably exist in the electrolyte. Therefore, even if the electrolytic solution of the present disclosure contains a plurality of particles, it is possible to suppress deterioration of the flow of the electrolytic solution due to the oxide. By using the electrolytic solution of the present disclosure that has excellent flowability, a redox flow battery with high energy density can be constructed.

(2) As an example of the electrolyte of the present disclosure,
The plurality of particles may have a particle size of 1000 nm or more in a volume ratio of 20% or less.

In the above configuration, the proportion of coarse particles among the plurality of particles is small. Coarse particles tend to settle faster than fine particles due to gravity. Therefore, since the proportion of coarse particles is small, a plurality of particles are likely to float in the dispersion medium.

(3) As an example of the electrolyte of the present disclosure,
The plurality of particles may have an average particle size of 600 nm or less.

According to the above embodiment, a plurality of particles are likely to float in the dispersion medium.

(4) As an example of the electrolyte of the present disclosure,
Each of the plurality of particles may have a size that changes depending on charging and discharging of the redox flow battery.

Each particle can function as an active material. Each particle changes in size as it functions as an active material. Each particle in the electrolyte of the present disclosure tends to float in the dispersion medium, even if the size varies.

(5) As an example of the electrolytic solution of the present disclosure described in (4) above,
The plurality of particles have an average particle size that changes due to charging and discharging of the redox flow battery,
An example is an embodiment in which the variation width of the average particle size is 1 nm or more and less than 600 nm.

According to the above embodiment, a plurality of particles are likely to float in the dispersion medium.

(6) As an example of the electrolyte of the present disclosure,
The plurality of particles may have an average particle diameter of 100 nm or less.

According to the above embodiment, the plurality of particles are more likely to float in the dispersion medium.

(7) As an example of the electrolyte of the present disclosure,
The electrolytic solution may include one or more ions selected from the group consisting of titanium ions, chromium ions, manganese ions, zinc ions, iron ions, cobalt ions, cerium ions, and vanadium ions.

The ions listed above serve as active material ions in the electrolyte. Each particle in the electrolytic solution of the present disclosure is made of an oxide containing an element that becomes an ion listed above. Each particle in the electrolytic solution of the present disclosure easily floats in the dispersion medium even if it is made of an oxide of the above element.

(8) As an example of the electrolyte of the present disclosure,
The electrolytic solution contains one or more ions selected from the group consisting of magnesium ions, cadmium ions, tin ions, indium ions, antimony ions, molybdenum ions, cerium ions, lead ions, bismuth ions, iron ions, and ammonium ions. Examples include forms that include.

When the electrolytic solution contains the ions listed above, it is easy to maintain the state in which each particle is suspended in the dispersion medium over time.

(9) The redox flow battery according to one aspect of the present disclosure includes:
The electrolytic solution according to any one of (1) to (8) above is provided.

The redox flow battery of the present disclosure has high energy density because it includes the electrolyte of the present disclosure. In particular, since the electrolytic solution of the present disclosure has excellent flowability, the redox flow battery of the present disclosure can easily maintain a high energy density state for a long period of time.

[Details of embodiments of the present disclosure]
Details of embodiments of the present disclosure will be described below with reference to the drawings. First, with reference to FIG. 1, a redox flow battery according to an embodiment will be described. Thereafter, with reference to FIG. 2, the electrolytic solution of the embodiment will be described. Further thereafter, a redox flow battery system capable of producing the electrolyte of the embodiment will be described with reference to FIGS. 1 to 5. Hereinafter, the redox flow battery will be referred to as an RF battery. The same reference numerals in the figures indicate the same names.

<RF battery>
The RF battery 1 is one of electrolyte circulation type storage batteries. As shown in FIG. 1, the RF battery 1 includes a battery cell 10 and a circulation mechanism that circulates an electrolyte through the battery cell 10. The RF battery 1 performs charging and discharging while supplying electrolyte to the battery cells 10.

The RF battery 1 is typically connected to a power generation unit 8 and a load 9 via a substation equipment 71 and an AC/DC converter 7 . The RF battery 1 is charged using the power generation unit 8 as a power supply source, and discharged using the load 9 as a power supply target. Examples of the power generation unit 8 include a solar power generator, a wind power generator, and other general power plants. Examples of the load 9 include a power system, a power consumer, and the like. The RF battery 1 is used for load leveling, voltage drop compensation, emergency power supply, and output smoothing of natural energy power generation such as solar power generation and wind power generation.

[Battery cell]
The battery cell 10 is separated into a positive electrode cell 12 and a negative electrode cell 13 by a diaphragm 11 . The positive electrode cell 12 has a built-in positive electrode 14 to which a positive electrolyte is supplied. The negative electrode cell 13 has a built-in negative electrode 15 to which a negative electrolyte is supplied. The battery cell 10 is sandwiched between a set of cell frames 5. The cell frame 5 includes a bipolar plate 51 and a frame 50. A positive electrode 14 is arranged on the front surface of the bipolar plate 51, and a negative electrode 15 is arranged on the back surface of the bipolar plate 51. The frame 50 is provided around the outer periphery of the bipolar plate 51. The frame 50 supports the bipolar plate 51. A sealing member 55 is arranged between the frames 50 in order to suppress leakage of electrolyte from the battery cells 10.

The RF battery 1 is typically used in a form called a cell stack 100 in which a plurality of battery cells 10 are stacked. The cell stack 100 includes a laminate in which a certain cell frame 5, a positive electrode 14, a diaphragm 11, a negative electrode 15, and another cell frame 5 are repeatedly stacked, a pair of end plates 101 that sandwich the laminate, and a fastening member 102. Equipped with Examples of the fastening member 102 include a connecting member such as a long bolt, a nut, and the like. The pair of end plates 101 are tightened by a fastening member 102. This tightening maintains the laminated state of the laminate. The cell stack 100 is typically used in a form in which a predetermined number of battery cells 10 are used as a substack (not shown), and a plurality of substackes are stacked. Supply/discharge plates (not shown) are arranged on the outside of the cell frame 5 located at both ends of the substack or cell stack 100 in the stacking direction of the battery cells 10 .

[Circulation mechanism]
The circulation mechanism includes a cathode circulation mechanism that circulates the cathode electrolyte to the cathode cell 12 and a cathode circulation mechanism that circulates the anode electrolyte to the anode cell 13. The positive electrode circulation mechanism includes a positive electrode tank 22, an outgoing pipe 42, a return pipe 44, and a pump 46. The positive electrode tank 22 stores positive electrode electrolyte. The outbound pipe 42 and the return pipe 44 connect the positive electrode tank 22 and the positive electrode cell 12. The pump 46 is provided in the outgoing pipe 42 on the supply side. The negative electrode circulation mechanism includes a negative electrode tank 23 , an outgoing pipe 43 , a return pipe 45 , and a pump 47 . The negative electrode tank 23 stores negative electrode electrolyte. The outgoing piping 43 and the incoming piping 45 connect the negative electrode tank 23 and the negative electrode cell 13. The pump 47 is provided in the outgoing pipe 43 on the supply side.

The positive electrolyte is supplied from the positive electrode tank 22 to the positive electrode 14 via the outgoing piping 42, and is returned from the positive electrode 14 to the positive electrode tank 22 via the returning piping 44. The negative electrolyte is supplied from the negative electrode tank 23 to the negative electrode 15 via the outgoing pipe 43, and is returned from the negative electrode 15 to the negative electrode tank 23 via the return pipe 45. By circulating the positive electrolyte through the positive electrode 14 and the negative electrolyte through the negative electrode 15, charging and discharging are performed in accordance with the valence change reaction of active material ions in the electrolyte of each electrode.

As the basic configuration of the RF battery 1 described above, a known configuration can be used as appropriate.

<Electrolyte>
The electrolytic solution 3 of the embodiment is used in the RF battery 1 described above. As shown in FIG. 2, the electrolytic solution 3 of the embodiment includes a dispersoid and a dispersion medium 31 that suspends the dispersoid. The dispersion medium 31 is an aqueous solution containing active material ions. The active material ion is an ion of an element that becomes an active material. FIG. 2 shows an enlarged view of the electrolytic solution 3 surrounded by a broken line circle in the positive electrode tank 22 of FIG. One of the features of the electrolytic solution 3 of the embodiment is that a plurality of specific particles 30 are suspended in a dispersion medium 31 as a dispersoid.

Each particle 30 is made of an oxide containing an element that becomes an active material ion and oxygen. Each particle 30 is generated by increasing the concentration of active material ions contained in the dispersion medium 31. Each particle 30 is a precipitate generated from the active material ions. For example, when the electrolytic solution 3 contains manganese ions as active material ions, each particle 30 is manganese oxide (MnO ₂ ) derived from the manganese ions and precipitated.

Each particle 30 exists suspended in the dispersion medium 31 under specific conditions. In other words, each particle 30 hardly settles due to gravity under certain conditions. Each of these floating particles 30 can stably exist in the electrolytic solution 3. By floating each particle 30, even if a plurality of particles 30 are included, it is possible to suppress the flow of the electrolytic solution 3 from deteriorating. In other words, the plurality of particles 30 hardly obstruct the flow of the electrolytic solution 3. Therefore, the electrolytic solution 3 has excellent flowability. It can be visually confirmed that each particle 30 is floating.

The above specific conditions are immediately after the electrolytic solution 3 has been allowed to stand for one hour. Here, "immediately" refers to a short period of time within 3 minutes after the 1 hour has elapsed. The state in which each particle 30 is suspended in the dispersion medium 31 can be maintained even immediately after the electrolytic solution 3 is left to stand for 3 hours, further for 5 hours, for 8 hours, and especially for one day.

The sedimentation rate of each particle 30 in the dispersion medium 31 is preferably less than 1 mm/hour under the above specific conditions. Since the sedimentation rate is less than 1 mm/hour, each particle 30 tends to exist suspended in the dispersion medium 31 under specific conditions. Sedimentation velocity is determined by Stokes' equation. The Stokes equation is v={D _p ² (ρ _p −ρ _f )g}/18η. Each symbol in the formula is as follows. The symbol v is the sedimentation velocity. The symbol _Dp is the particle diameter of the particles 30. The symbol ρ _p is the density of the particles 30. The symbol ρ _f is the density of the dispersion medium 31. The symbol g is gravitational acceleration. The symbol η is the viscosity of the dispersion medium 31. The particle size of the particles 30 mainly depends on charging and discharging conditions. The density of particles 30 depends primarily on the composition of particles 30. The density of the dispersion medium 31 and the viscosity of the dispersion medium 31 mainly depend on the liquid composition of the electrolytic solution. The particle diameter of the particles 30, the density of the particles 30, the density of the dispersion medium 31, and the viscosity of the dispersion medium 31 can be measured using a known device.

For example, the volume percentage of the plurality of particles 30 having a particle size of 1000 nm or more is 20% or less. When the particle size becomes large, the particles 30 become difficult to float in the dispersion medium 31 and tend to settle due to gravity. Therefore, when the volume ratio of coarse particles having a particle size of 1000 nm or more is 20% or less, a plurality of particles 30 are likely to float in the dispersion medium 31. The volume ratio of the coarse particles is further preferably 15% or less, 10% or less, particularly 5% or less. The volume percentage of the coarse particles is determined from the scattering intensity obtained by particle size measurement using a dynamic light scattering method. An example of a particle size measuring device used in the dynamic light scattering method is Zeta Nanosizer manufactured by Malvern. When the coarse particles are contained in a volume ratio of 20% or less, the average sedimentation velocity of the entire particles 30 may be less than 1 mm/hour. When the average sedimentation speed is less than 1 mm/hour, the plurality of particles 30 are likely to float in the dispersion medium 31.

Further, the volume percentage of the plurality of particles 30 having a particle size of 800 nm or more is 30% or less. When the volume ratio of particles having a particle size of 800 nm or more is 30% or less, the plurality of particles 30 are more likely to float in the dispersion medium 31. The volume percentage of particles having a particle size of 800 nm or more is further 20% or less, 15% or less, particularly 10% or less.

The plurality of particles 30 may have an average particle size of 600 nm or less. Since the average particle diameter of the plurality of particles 30 is 600 nm or less, the plurality of particles 30 can easily float in the dispersion medium 31. Moreover, since the average particle diameter of the plurality of particles 30 is 600 nm or less, it is easy to maintain the state in which the plurality of particles 30 are suspended in the dispersion medium 31. The finer each particle 30 is, the easier it is to float in the dispersion medium 31, and the easier it is to maintain a suspended state in the dispersion medium 31. The average particle diameter of the plurality of particles 30 may further be 300 nm or less, 200 nm or less, 100 nm or less, or 80 nm or less. The average particle size of the plurality of particles 30 is determined from the average particle size of the population using a dynamic light scattering method. The population refers to an aggregate of all particles 30.

It is preferable that the plurality of particles 30 have an average particle size of more than 1 nm. If the particles 30 are too fine, the particles 30 that are too fine will tend to aggregate with each other. When particles 30 aggregate, they grow into coarse particles and tend to settle. Since the average particle diameter of the plurality of particles 30 is more than 1 nm, aggregation of the particles 30 can be suppressed. The average particle diameter of the plurality of particles 30 is further preferably 10 nm or more, particularly 20 nm or more.

The plurality of particles 30 may have an average particle diameter of more than 1 nm and less than 600 nm, more preferably more than 10 nm and less than 300 nm, and especially more than 20 nm and less than 100 nm.

The plurality of particles 30 may be dispersed in a dispersion medium 31. The plurality of particles 30 being dispersed in the dispersion medium 31 refers to a state in which the plurality of particles 30 are uniformly present in the dispersion medium 31 without agglomerating. For example, adjacent particles 30 are floating with a certain distance between them. In other words, adjacent particles 30 are floating apart from each other. The state in which the plurality of particles 30 are dispersed in the dispersion medium 31 can be maintained over time. For example, the plurality of particles 30 may be dispersed in the dispersion medium 31 under the above specific conditions. The state in which the plurality of particles 30 are dispersed in the dispersion medium 31 can be maintained even immediately after the electrolytic solution 3 is left to stand for 3 hours, further for 5 hours, for 8 hours, and especially for one day. The dispersion state of the plurality of particles 30 in the dispersion medium 31 can be determined by a dynamic light scattering method.

The number of particles 30 contained per unit volume of the dispersion medium 31, that is, the number density of particles 30, is preferably 10 ¹⁰ particles/cm ³ or more and 10 ²² particles/cm ³ or less. It can be said that the larger the number of particles 30, the higher the concentration of active material ions in the dispersion medium 31. The fact that the number density of particles 30 is 10 ¹⁰ particles/cm ³ or more means that many particles 30 are present in the dispersion medium 31 . Therefore, when the number density of particles 30 is 10 ¹⁰ particles/cm ³ or more, an improvement in energy density can be expected. On the other hand, when the number density of the particles 30 is 10 ²² particles/cm ³ or less, the average particle diameter of the plurality of particles 30 is likely to exceed 1 nm. When the average particle size of the plurality of particles 30 is more than 1 nm, it is easy to prevent too fine particles 30 from agglomerating and settling together. Therefore, when the number density of the particles 30 is 10 ²² pieces/cm ³ or less, each particle 30 can stably exist in the electrolytic solution 3, so that an improvement in energy density can be expected. The number density of the particles 30 is further preferably 10 ¹³ particles/cm ³ or more and 10 ²⁰ particles/cm ³ or less, particularly 10 ¹⁵ particles/cm ³ or more and 10 ¹⁷ particles/cm ³ or less. The number density of particles 30 is determined by a calculation formula expressed as A×B/(C×D). Each symbol in the calculation formula is as follows. Symbol A is the concentration of active material ions. Symbol B is the molecular weight of each particle 30. Symbol C is the density of each particle 30. The symbol D is 4/3×π×r ³ . The symbol r is 1/2 of the average particle diameter of the plurality of particles 30. The concentration of active material ions is determined, for example, from the amount of charged electricity and the measured potential using the Nernst equation. The concentration of active material ions can also be determined by measuring the concentration using centrifugation or the like.

The size of each particle 30 can change depending on charging and discharging of the RF battery 1. That is, each particle 30 can function as an active material. Specifically, each particle 30 becomes larger when the RF battery 1 is charged, and becomes smaller when the RF battery 1 is discharged. The change in size of each particle 30 is based on the change in the valence of active material ions in the dispersion medium 31. For example, the average particle size of each particle 30 changes as the RF battery 1 is charged and discharged. The range of change in average particle size is preferably 1 nm or more and less than 600 nm. The range of change in average particle size is the difference between the average particle size of the plurality of particles 30 at the time of full charge and the average particle size of the plurality of particles 30 at the time of discharge. Here, full charge is when the state of charge is 100%, and discharge is when the state of charge is 30%. When the variation width of the average particle size is 1 nm or more, it is possible to suppress agglomeration and sedimentation of oxides that may be newly generated during charging. Moreover, since the variation width of the average particle diameter is 1 nm or more, each particle 30 can be actively used as an active material. On the other hand, since the variation width of the average particle diameter is less than 600 nm, coarsening of each particle 30 can be easily suppressed. The range of change in the average particle size may be 2 nm or more and 300 nm or less, 2 nm or more and 200 nm or less, particularly 5 nm or more and 100 nm or less.

The electrolyte 3 contains titanium (Ti) ions, chromium (Cr) ions, manganese (Mn) ions, zinc (Zn) ions, iron (Fe) ions, cobalt (Co) ions, cerium (Ce) ions, and vanadium ( V) Containing one or more ions selected from the α group consisting of ions. The ions listed in the α group serve as active materials in the electrolytic solution 3. The ions listed in the α group react with water contained in an aqueous solution to generate oxides during charging. For example, trivalent Mn ions can react with water during charging to generate MnO ₂ which is an oxide. Each particle 30 may be made of an oxide of an element constituting an ion listed in the α group.

The electrolytic solution 3 may be an Mn--Ti based electrolytic solution in which the positive electrode active material is Mn ions and the negative electrode active material is Ti ions. Another example is an all-V electrolyte in which both the positive electrode active material and the negative electrode active material are V ions. Another example is a Fe--Cr electrolytic solution in which the positive electrode active material is Fe ions and the negative electrode active material is Cr ions.

The electrolyte 3 contains magnesium (Mg) ions, cadmium (Cd) ions, tin (Sn) ions, indium (In) ions, antimony (Sb) ions, molybdenum (Mo) ions, cerium (Ce) ions, and lead (Pb). ) ion, bismuth (Bi) ion, iron (Fe) ion, and ammonium (NH ₄ ) ion. The ions listed in the β group are different from active material ions. It is thought that by including ions listed in the β group in the electrolytic solution 3, it is possible to suppress the aggregation of the oxides. By including ions listed in the β group in the electrolytic solution 3, it is easy to maintain a state in which each particle 30 is suspended in the dispersion medium 31 over time.

The total concentration of the ions listed in the β group may be 0.001 mol/L or more and 1 mol/L or less. When the total concentration of the ions listed in the β group is 0.001 mol/L or more, it is easy to maintain the state in which each particle 30 is suspended in the dispersion medium 31 over time. On the other hand, when the concentration of ions listed in the β group is 1 mol/L or less, it is possible to suppress a relative decrease in the proportion of active material ions. The total concentration of the ions listed in the β group may be 0.01 mol/L or more and 0.5 mol/L or less.

When multiple types of ions listed in the β group are included, the concentration ratio of each ion may be the same or different.

As the aqueous solution constituting the dispersion medium 31, for example, a sulfuric acid (H ₂ SO ₄ ) aqueous solution, a phosphoric acid (H ₃ PO ₄ ) aqueous solution, a nitric acid (HNO ₃ ) aqueous solution, etc. can be used. The concentration of the aqueous solution may be 1 mol/L or more and 5 mol/L or less, further 1 mol/L or more and 4.5 mol/L or less, particularly 1 mol/L or more and 4 mol/L or less.

A plurality of particles 30 are likely to be generated in the positive electrode electrolyte. Therefore, the electrolytic solution 3 containing the plurality of particles 30 described above is preferably used as a positive electrode electrolyte.

<RF battery system>
The RF battery system 1α includes the above-described RF battery 1 and the control unit 6, as shown in FIG. The control unit 6 controls charging and discharging using the electrolyte. The control unit 6 controls at least part of the charging to be performed at a current density of 250 mA/cm ² or more. The control unit 6 of this example controls charging to start at a current density of 250 mA/cm ² or more using an electrolytic solution whose state of charge is 60% or more and 100% or less. Specifically, the control unit 6 of this example performs first charging to bring the electrolyte to a charged state of 60% or more and 100% or less, and then performs second charging at a current density of 250 mA/cm ² or more. The operation of the RF battery 1 is controlled so as to perform the following steps. Moreover, the control unit 6 of this example controls the battery cell 10 to perform the first charging and the second charging.

For example, a computer can be used as the control unit 6. A computer typically includes a processor 60 and memory (not shown). Processor 60 is, for example, a CPU. The control unit 6 is operated by the processor 60 executing a control program stored in the memory.

Known techniques can be used to determine the state of charge of the electrolyte. For example, the state of charge of the electrolyte can be grasped using a monitor cell to which the same electrolyte as the battery cell 10 is supplied.

The electrolytic solution 3 described above can be produced, for example, by sequentially performing first charging and second charging on the battery cell 10 using the RF battery system 1α. As shown in FIG. 3, the method for operating an RF battery using the RF battery system 1α described above includes sequentially performing a first step S1 and a second step S2. FIG. 4 is an explanatory diagram illustrating the first step and the second step. In the graph shown in FIG. 4, the vertical axis represents the state of charge, and the horizontal axis represents time. In FIG. 4, "I" represents the first step, and "II" represents the second step.

[First step]
In the first step, the first charging is performed until the state of charge of the electrolyte becomes 60% or more and 100% or less, although it depends on the type of ions contained in the electrolyte. For example, when the electrolytic solution contains Mn ions as active material ions, in the first charging, charging is performed until the state of charge of the electrolytic solution becomes 60% or more and 70% or less. When the electrolytic solution contains Ti ions in addition to Mn ions, in the first charging, charging is performed until the state of charge of the electrolytic solution becomes 70% or more and 100% or less. In the first charging, as shown in FIG. 4, the state of charge uniformly increases over time.

For the first charging, known charging conditions can be applied. For example, in the first charging, the current density may be set to 40 mA/cm ² or more and 200 mA/cm ² or less, further 70 mA/cm ² or more and 200 mA/cm ² or less, particularly 100 mA/cm ² or more and 200 mA/cm ^{2 or} less. It will be done. The state of charge can be determined from the amount of electricity charged.

The concentration of active material ions contained in the electrolytic solution is 0.3 mol/L or more and 5 mol/L or less, although it depends on the type of active material ions. When a plurality of active material ions are included, the total concentration may satisfy the above concentration range. When the concentration of active material ions is 0.3 mol/L or more, the energy density of the electrolytic solution can be increased. Since the higher the concentration of active material ions, the higher the energy density, the concentration of active material ions is 0.5 mol/L or more, further 0.7 mol/L or more, and 0.9 mol/L or more. . From the viewpoint of solubility in an aqueous solution, etc., the concentration of active material ions is 5 mol/L or less, and further 2 mol/L or less. In other words, the concentration of the active material ions may be 0.5 mol/L or more and 5 mol/L or less, further 0.7 mol/L or more and 5 mol/L or less, and 0.9 mol/L or more and 2 mol/L. When using Mn ions as active material ions, the concentration of Mn ions is preferably 0.8 mol/L or more and 2.0 mol/L or less, and further 1.0 mol/L or more and 1.5 mol/L or less. The concentration of active material ions in the electrolytic solution is approximately maintained even after both the first step and the second step are performed. That is, the concentration of active material ions is approximately the same before performing the first step and after performing the second step.

When using Mn ions as active material ions, the electrolytic solution may include Ti ions in addition to Mn ions. Ti ions have the effect of making the MnO ₂ oxide that precipitates during charging into fine particles. In a positive electrode electrolyte containing Mn ions as active material ions, Ti ions in the positive electrode electrolyte do not function as a positive electrode active material. When Ti ions are included in addition to Mn ions, the concentration of Ti ions is preferably 0.8 mol/L or more and 2.0 mol/L or less, and further 1.0 mol/L or more and 1.8 mol/L or less. The concentration of Mn ions and the concentration of Ti ions may be the same or different.

[Second process]
The second step includes a charging step of performing second charging at a current density of 250 mA/cm ² or more. The charging process is started using an electrolytic solution whose state of charge is 60% or more and 100% or less. That is, the state of charge of the electrolytic solution at the start of the second step is 60% or more and 100% or less. The charging process includes performing pulse charging alternately with pulse discharge. That is, the second charging may be pulse charging. In this case, in the second step, a charging step of performing pulse charging and a discharging step of performing pulse discharge are alternately repeated. Overall, the second step does not significantly change the state of charge P (FIG. 4) obtained in the first charging. In the second step, as shown in FIG. 4, the charging state has a small zigzag waveform due to pulse charging and discharging, but overall there is no change in the charging state.

Pulse charging and discharging can be performed such that the current pulse waveforms during charging and discharging are symmetrical, as shown in the current pulse waveform shown in the upper diagram of FIG. The lower diagram in FIG. 5 shows a voltage pulse waveform corresponding to the current pulse waveform in the upper diagram in FIG. The charging current density per cycle in pulse charging and discharging is the above-mentioned current density of 250 mA/cm ² or more. One cycle in pulse charging and discharging is a unit of a current pulse waveform represented by one pulse of charging and one pulse of discharging. In other words, symmetrical current pulse waveforms during charging and discharging mean that the waveform of one pulse of current during charging and the waveform of one pulse of current during discharging are symmetrical. Charging in pulse charging and discharging has a higher output than general constant current charging. When the current density is 250 mA/cm ² or more, the electrolytic solution 3 (FIG. 2) containing the plurality of particles 30 described above can be manufactured. Specifically, it is possible to produce an electrolytic solution 3 that includes a dispersoid and a dispersion medium 31 that suspends the dispersoid, and in which a plurality of specific particles 30 are suspended in the dispersion medium 31 as the dispersoid. Each particle 30 is a precipitate made of an oxide generated from active material ions.

The current density mainly affects the amount of precipitation of each particle 30. The higher the current density, the more likely the amount of particles 30 to precipitate increases. On the other hand, the lower the current density, the easier it is to reduce the amount of particles 30 precipitated. Therefore, in order to increase the amount of precipitation of each particle 30, it is preferable that the current density is high. Therefore, the current density may further be set to 300 mA/cm ² or more, 400 mA/cm ² or more, or 500 mA/cm ² or more. However, if the current density is too high, the deposited particles 30 tend to become coarse. The coarse particles 30 are less likely to float in the dispersion medium 31 and more likely to settle due to gravity. Therefore, it is preferable that the current density is low to some extent. Therefore, the current density may be 1000 mA/cm ² or less, further 800 mA/cm ² or less, or 600 mA/cm ² or less. The current density shall be 250 mA/ ^cm2 or more and 1000 mA/ ^cm2 or less, 300 mA/ ^cm2 or more and 800 mA/ ^cm2 or less, 400 mA/ ^cm2 or more and 700 mA/ ^cm2 or less, and 500 mA/ ^cm2 or more and 600 mA/cm2 ^or less. can be mentioned.

The charging time t1 (FIG. 5) per cycle in pulse charging and discharging may be 1 second or more and 10 minutes or less. The charging time t1 per cycle in pulse charging and discharging is the charging time of one pulse of the unit of the current pulse waveform represented by one pulse of charging and one pulse of discharging. When the charging time t1 per cycle is 1 second or more, it is easy to manufacture the electrolytic solution 3 (FIG. 2) described above. The charging time t1 per cycle affects the size of each particle 30 (FIG. 2) produced. The longer the charging time t1 per cycle is, the larger each particle 30 that is generated tends to be. The shorter the charging time t1 per cycle, the more likely each particle 30 to be generated becomes fine. By setting the charging time t1 per cycle to 10 minutes or less, coarsening of each particle 30 can be suppressed. The charging time t1 per cycle may further be 1 second or more and 300 seconds or less, particularly 5 seconds or more and 100 seconds or less.

Similarly to the charging time t1, the discharge time t2 (FIG. 5) per cycle in pulse charging and discharging can be set to 1 second or more and 10 minutes or less, further 1 second or more and 300 seconds or less, particularly 5 seconds or more and 100 seconds or less. Can be mentioned.

Pulse charging and discharging may provide a standby time between charging and discharging per cycle. The waiting time may be set to 0 seconds or more and 60 seconds or less.

Further, pulse charging and discharging may be performed so that the pulse waveforms of the current during charging and discharging are asymmetrical. For example, pulse charging and discharging includes alternating high-output charging and low-output discharging. When the state of charge P (FIG. 4) is low, for example, when the state of charge P is less than 60%, when trying to generate a plurality of particles 30, the charging current density per cycle is increased and charging is performed at a high current value. It turns out. When discharging according to this high current value, there is a possibility that the plurality of particles 30 generated during charging may melt in a low charging state. Therefore, when the state of charge P is low, charging may be performed at a high current value and discharging may be performed at a low current value. At this time, in the pulse waveform, the area represented by the product of the current value during charging in one cycle and the charging time, and the area represented by the product of the current value during discharging in one cycle and the discharging time. are the same. Therefore, charging and discharging in one cycle includes charging at high output for a short time and discharging at low output and for a long time.

Another example of pulse charging and discharging is to alternately repeat low-output charging and high-output discharging. When the state of charge P (Fig. 4) is high, for example, when the state of charge P is 60% or more, increasing the charging current density per cycle and charging at a high current value may damage the battery cell 10 (Fig. 1). There is a risk of Therefore, when the state of charge P is high, the charging current density per cycle may be lowered to perform charging at a lower current value. On the other hand, in a highly charged state, when discharging at a high current density, if a plurality of particles 30 generated during charging adhere to the electrode, it can be expected that the plurality of particles 30 will be peeled off from the electrode. At this time, in the pulse waveform, the area represented by the product of the current value during charging in one cycle and the charging time, and the area represented by the product of the current value during discharging in one cycle and the discharging time. are the same. Therefore, charging and discharging in one cycle include charging at low output and for a long time, and discharging at high output and for a short time.

Pulse charging and discharging can also be performed by chronoamperometry or chronopotentiometry, which controls by voltage and time or quantity of electricity, instead of controlling by current value and time as described above.

Pulse charging and discharging may be performed for 0.5 hours or more and 24 hours or less. The pulse charge/discharge time is the total time from the start to the end of pulse charge/discharge. For example, when repeating multiple cycles of pulse charging and discharging, the pulse charging and discharging time is the total time of all cycles. By performing pulse charging and discharging for 0.5 hours or more, it is easy to manufacture the electrolytic solution 3 (FIG. 2) containing the plurality of particles 30 described above. The time of pulse charging and discharging affects the size of particles 30 produced. The longer the pulse charging/discharging time is, the more each particle 30 generated tends to become coarse. On the other hand, the shorter the pulse charging/discharging time, the more likely each particle 30 to be generated becomes fine. By setting the pulse charging/discharging time to 24 hours or less, coarsening of each particle 30 can be suppressed. The pulse charging/discharging time may further be 1 hour or more and 12 hours or less, particularly 2 hours or more and 6 hours or less. In the second step, when charging is performed once without pulse charging and discharging, the charging time for this single charge is 0.5 hours or more and 24 hours or less, further 1 hour or more and 12 hours or less, particularly 2 hours or more and 6 hours. time or less.

The above-described RF battery operating method may be performed, for example, at an initial stage before operating the RF battery 1 (FIG. 1). By performing this at an early stage before operating the RF battery 1, the RF battery 1 can be operated using the electrolytic solution 3 (FIG. 2) containing the plurality of particles 30 described above. In the electrolytic solution 3 containing the plurality of particles 30 described above, each particle 30 exists stably. Moreover, the electrolytic solution 3 containing the plurality of particles 30 described above has a high energy density. Therefore, by operating the RF battery 1 using the electrolytic solution 3 containing the plurality of particles 30 described above, the RF battery 1 with high energy density can be constructed.

As described above, the electrolytic solution 3 containing the plurality of particles 30 can be obtained by charging at a current density of 250 mA/cm ² or more using an electrolytic solution whose state of charge is 60% or more and 100% or less. The plurality of particles 30 are likely to be generated when the state of charge of the electrolyte is relatively high, such as 60% or more. Each particle 30 generated in a relatively high state of charge easily passes through the electrodes during operation of the RF battery 1. Therefore, the plurality of particles 30 generated during operation of the RF battery 1 is difficult to reduce.

The above-described RF battery operating method can also be performed, for example, during maintenance of the RF battery 1. Maintenance is performed, for example, when a considerable period of time has passed since the RF battery 1 started operating. The corresponding period is a period of three days or more after the start of operation of the RF battery 1. When the RF battery 1 is operated using the electrolytic solution 3 containing the plurality of particles 30 described above, the plurality of particles 30 may adhere to the electrodes. Therefore, by performing the above-described second step during maintenance of the RF battery 1, it can be expected that the plurality of particles 30 attached to the electrodes will be peeled off. It is considered that the plurality of particles 30 attached to the electrode are mainly peeled off by the discharge of pulse charging and discharging in the second step. The plurality of particles 30 attached to the electrodes are difficult to peel off during normal discharge during operation of the RF battery 1. When the second step is performed during maintenance, a plurality of particles 30 are newly generated. Each particle 30 generated during maintenance can also float in the electrolytic solution 3.

[Test example]
In the test example, the particle size and floating state of particles generated in the positive electrode electrolyte were investigated assuming an RF battery that would actually be used. In this example, a battery cell having a single cell structure including one positive electrode cell and one negative electrode cell was used.

<Test specimen>
Each test specimen was tested in an RF battery in which a positive electrolyte was circulated between a positive tank and a positive cell, and a negative electrolyte was circulated between a negative tank and a negative cell. and the second step.
In each test specimen, the composition of the positive electrode electrolyte and the composition of the negative electrode electrolyte were made the same.
Each of the positive electrode electrolyte and the negative electrode electrolyte contains Mn ions with a concentration of 1 mol/L, Ti ions with a concentration of 1 mol/L, sulfuric acid with a concentration of 5 mol/L, and additive ions with a concentration of 0.2 mol/L. including. When multiple types of added ions are included, the total concentration of added ions is 0.2 mol/L. Note that the test specimen 1 does not contain added ions.
The types of added ions in each test specimen are as follows.
・Test specimen 1: Does not contain added ions ・Test specimen 2: Bi ions ・Test specimen 3: Sn ions ・Test specimen 4: Fe ions ・Test specimen 5: Al ions ・Test specimen 6: Bi ions and Sn ions ・Test Body 7: Bi ions and Fe ions ・Test body 8: Sn ions and Fe ions ・Test body 9: Fe ions and Al ions In each test body, the composition of the electrolyte was different, and the first step and second step The conditions were the same.
In the first step, first charging was performed at a current density of 70 mA/cm ² until the state of charge of the positive electrode electrolyte reached 70%.
In the second step, pulse charging and discharging was performed after the first step. As shown in FIG. 5, pulse charging and discharging were performed by repeating charging and discharging alternately and symmetrically. In pulse charging and discharging, the current density per cycle was 250 mA/cm ² . The charging time per cycle in pulse charging and discharging was 8 seconds. Pulse charging and discharging was performed for a total of 4 hours.
After completing the second step, the positive electrode electrolyte was charged to a state of charge of 100%.

The average particle diameter of a plurality of particles contained in the electrolyte solution of each test specimen obtained was measured by a dynamic light scattering method. The measurement conditions were: refractive index of dispersoid: 1.59, absorption rate of dispersoid: 0.01, refractive index of dispersion medium: 1.33, attenuator: 10, measurement position: automatic, temperature: 25°C, viscosity of dispersion medium. : The viscosity of the electrolyte. The average particle size of the plurality of particles was measured immediately after the electrolytic solution was allowed to stand for a predetermined period of time after stopping pulse charging and discharging. The predetermined times were 1 hour, 3 hours, and 24 hours. The results are shown in Table 1. The unit of average particle size is nm. "*" in Table 1 indicates that sedimentation was confirmed in a plurality of particles. The "-" in Table 1 indicates that the average particle size was not measured because sedimentation was confirmed in a plurality of particles.

As shown in Table 1, immediately after the electrolytic solution was allowed to stand for 1 hour, test specimens 2 to 4 and test specimens 6 to 9, in which the average particle diameter of the plurality of particles was 1000 nm or less, Almost no sedimentation was observed. In particular, test specimens 2, 3, 6, 7, and 8, which had an average particle size of 50 nm or less immediately after the electrolyte was left to stand for 1 hour, had multiple particles even after 24 hours had passed. Almost no sedimentation was observed in the particles. In each sample in which sedimentation of multiple particles was not observed, the sedimentation rate of each particle immediately after 1 hour and further 24 hours was less than 1 mm/hour.

For test specimen 2, test specimen 3, specimen 6, specimen 7, and specimen 8, the volume percentage of particles having a particle size of 1000 nm or more was measured by a dynamic light scattering method. As a result, all test specimens had a content of 20% or less.

The present invention is not limited to these examples, but is indicated by the scope of the claims, and is intended to include all changes within the meaning and scope equivalent to the scope of the claims. For example, in the test example, the types of active material ions can be changed, and the types and combinations of added ions can be changed. Further, in the test example, the charging state in the first charging can be changed, and the conditions of pulse charging and discharging in the second step can be changed.

1 RF battery, 1α RF battery system 10 battery cell 11 diaphragm 12 positive electrode cell, 13 negative electrode cell 14 positive electrode, 15 negative electrode 22 positive electrode tank, 23 negative electrode tank 3 electrolyte, 30 particles, 31 dispersion medium 42, 43 outward piping, 44, 45 return piping 46, 47 pump 5 cell frame, 50 frame body, 51 bipolar plate, 55 seal member 100 cell stack, 101 end plate, 102 fastening member 6 control unit, 60 processor 7 AC/DC converter, 71 substation equipment, 8 power generation section, 9 load t1 charging time, t2 discharging time

Claims (9)

An electrolytic solution used in a redox flow battery,
a dispersoid,
a dispersion medium that suspends the dispersoid,
The dispersoid is a plurality of particles suspended in the dispersion medium under specific conditions,
The specific conditions are immediately after the electrolyte is left standing for 1 hour,
Each of the plurality of particles is made of an oxide containing an element serving as an active material ion and oxygen,
Electrolyte. The electrolytic solution according to claim 1, wherein the volume proportion of the plurality of particles having a particle size of 1000 nm or more is 20% or less. The electrolytic solution according to claim 1 or 2, wherein the plurality of particles have an average particle size of 600 nm or less. The electrolyte solution according to any one of claims 1 to 3, wherein each of the plurality of particles changes in size depending on charging and discharging of the redox flow battery. The plurality of particles have an average particle size that changes due to charging and discharging of the redox flow battery,
The electrolytic solution according to claim 4, wherein the variation width of the average particle size is 1 nm or more and less than 600 nm. The electrolytic solution according to any one of claims 1 to 5, wherein the plurality of particles have an average particle size of 100 nm or less. 7. The electrolytic solution contains one or more ions selected from the group consisting of titanium ions, chromium ions, manganese ions, zinc ions, iron ions, cobalt ions, cerium ions, and vanadium ions. The electrolytic solution according to any one of the above. The electrolytic solution contains one or more ions selected from the group consisting of magnesium ions, cadmium ions, tin ions, indium ions, antimony ions, molybdenum ions, cerium ions, lead ions, bismuth ions, iron ions, and ammonium ions. The electrolytic solution according to any one of claims 1 to 7, comprising: comprising the electrolytic solution according to any one of claims 1 to 8;
redox flow battery.

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