Redox flow battery

Abstract

This redox flow battery has a positive electrode electrolyte containing a positive electrode active material, and a negative electrode electrolyte containing a negative electrode active material, wherein: the amount of the positive electrode electrolyte and the amount of the negative electrode electrolyte are different from each other; among the positive electrode electrolyte and the negative electrode electrolyte, a liquid amount ratio of an electrolyte having a larger amount of liquid to an electrolyte having a smaller amount of liquid is 1.05 to 5.0; and a charged state of an electrolyte mixture, obtained by mixing the positive electrode electrolyte and the negative electrode electrolyte at the same ratio as the liquid amount ratio, is at least 2%.

Description

[Technical Field]

[0001] The present disclosure relates to redox flow batteries. This application claims priority based on Japanese Patent Application No. 2020-159077, filed on September 23, 2020, and incorporates by reference all of the contents of said Japanese application. [Background technology]

[0002] Patent Document 1 discloses a method for operating a redox flow battery in which, when the valence balance between the positive electrode electrolyte and the negative electrode electrolyte deviates from 3.5, the electrolyte in one tank is transferred to the other tank via a pipe.

[0003] Patent Document 2 discloses a method for determining electrolyte distribution in a flow battery. The flow battery is supplied with a positive electrode electrolyte and a negative electrode electrolyte having active materials. The method described in Patent Document 2 includes the following steps: determining the average oxidation states of the active materials in the negative electrode electrolyte and the positive electrode electrolyte; and adjusting the molar ratio of the active materials between the negative electrode electrolyte and the positive electrode electrolyte according to the determined average oxidation states. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-303611 [Patent Document 2] Special Publication No. 2017-505513 Summary of the Invention

[0005] The redox flow battery of the present disclosure comprises: a positive electrode electrolyte containing a positive electrode active material and a negative electrode electrolyte containing a negative electrode active material, the amount of the positive electrode electrolyte and the amount of the negative electrode electrolyte are different, a liquid volume ratio of the positive electrode electrolyte and the negative electrode electrolyte, which is a liquid volume ratio of the electrolyte having a larger liquid volume to the electrolyte having a smaller liquid volume, is 1.05 or more and 5.0 or less; The state of charge of a mixed electrolyte obtained by mixing the positive electrode electrolyte and the negative electrode electrolyte in the same liquid volume ratio as the above is 2% or more. [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of a redox flow battery according to an embodiment. [Figure 2] FIG. 2 is a schematic diagram showing an example of a cell stack.

[0007] [Problem to be solved by this disclosure] The inventions described in Patent Documents 1 and 2 both aim to minimize the decrease in battery capacity by transferring electrolyte to restore the original valence balance when the valence balance between the positive electrode electrolyte and the negative electrode electrolyte deviates from 3.5 during operation of a redox flow battery. Both inventions can be said to suppress the decrease in battery capacity caused by changes in the valence balance of the electrolyte due to electrolyte transfer or side reactions caused by repeated charging and discharging. In other words, the inventions described in Patent Documents 1 and 2 can be said to recover discharge capacity that has decreased from the initial state. Specifically, Patent Document 1 describes transferring the negative electrode electrolyte to a positive electrode tank when the valence balance exceeds 3.5, and transferring the positive electrode electrolyte to a negative electrode tank when the valence balance falls below 3.5.

[0008] From the viewpoint of improving the energy density of redox flow batteries, it is desirable to increase the discharge capacity in the initial state and maintain the discharge capacity at a high level over time.

[0009] An object of the present disclosure is to provide a redox flow battery with high energy density.

[0010] [Effects of this disclosure] The redox flow battery of the present disclosure has high energy density.

[0011] [Description of the embodiments of the present disclosure] The present inventors have conducted extensive research into improving the energy density of redox flow batteries and have come to the following conclusions.

[0012] In conventional redox flow batteries, the positive electrode tank for storing the positive electrode electrolyte and the negative electrode tank for storing the negative electrode electrolyte are identically designed to reduce costs and simplify the overall system. In this case, the volumes of the positive electrode tank and the negative electrode tank are substantially equal. Furthermore, to facilitate the adjustment of the liquid levels of the electrolytes in the positive electrode tank and the negative electrode tank, the liquid volumes of the positive electrode electrolyte and the negative electrode electrolyte are initially adjusted to be equal. Furthermore, the positive electrode electrolyte and the negative electrode electrolyte are initially adjusted in advance so that the state of charge (SOC) of the mixed electrolyte, which is mixed in the same ratio as the liquid volumes of the positive electrode electrolyte and the negative electrode electrolyte, is zero (0).

[0013] To increase the energy density, for example, it is possible to increase the concentration of the active materials contained in the positive and negative electrode electrolytes or to increase the utilization rate of the electrolytes, i.e., the utilization rate of the active materials. The utilization rate corresponds to the actual discharge capacity. The utilization rate corresponds to the difference between the amount of charge electricity at the upper SOC and the amount of charge electricity at the lower SOC. In other words, the utilization rate indicates the ratio of the active material used for charge and discharge relative to the active material concentration. The concentration of the active material contained in the electrolyte is limited by its solubility in the electrolyte solvent. Furthermore, increasing the active material concentration increases the viscosity of the electrolyte, resulting in a decrease in the fluidity of the electrolyte, a decrease in the operating efficiency of the battery, and an increase in costs. Therefore, there are limitations to increasing the active material concentration. On the other hand, expanding the SOC utilization range to increase the utilization rate raises the following problems.

[0014] Increasing the upper SOC limit increases internal resistance and reduces the efficiency of the redox reaction. Furthermore, a high SOC can easily cause side reactions, such as electrolysis of water in the electrolyte. For example, oxygen may be generated at the positive electrode and hydrogen may be generated at the negative electrode. Furthermore, increasing the upper SOC limit can result in precipitation of the active material, depending on the type and concentration of the active material. The active material precipitates may be unavailable for charge / discharge or may be difficult to use for charge / discharge due to its small surface area. For these reasons, if the active material precipitates in the electrolyte, it may no longer function as an active material. Therefore, precipitation of the active material can lead to a decrease in discharge capacity. On the other hand, decreasing the lower SOC limit increases internal resistance and reduces the efficiency of the redox reaction. Furthermore, depending on the type and concentration of the active material, precipitation of the active material may occur. Therefore, positive and negative electrode electrolytes are generally used within an SOC range that does not result in a decrease in the efficiency of the redox reaction, side reactions, or active material precipitation. In other words, the range of SOC usage is limited in order to prevent deterioration of battery performance and ensure reliability.

[0015] The optimal SOC range for each electrolyte may differ depending on the type of positive electrode active material contained in the positive electrode electrolyte and the type of negative electrode active material contained in the negative electrode electrolyte. In conventional redox flow batteries, as described above, the volume of the positive electrode electrolyte and the volume of the negative electrode electrolyte are equal, and the SOC of the mixed electrolyte is substantially zero. Therefore, the SOC ranges for the positive electrode electrolyte and the negative electrode electrolyte cannot be independently set. In other words, the positive electrode electrolyte and the negative electrode electrolyte are used within the same SOC range, and therefore, each electrolyte cannot be used within its optimal SOC range. Specifically, if the SOC range for one electrolyte is set to match the optimal SOC range for the other electrolyte, the actual SOC range for the other electrolyte may be lower or higher than the optimal SOC range. If the actual SOC range is lower than the optimal SOC range, the electrolyte utilization rate, i.e., the discharge capacity, decreases, and thus improvement in energy density cannot be achieved. On the other hand, if the actual SOC range is larger than the optimum SOC range, the efficiency of the oxidation-reduction reaction described above will decrease, and side reactions and precipitation of the active material will occur.

[0016] The present inventors propose that, in order to optimize the utilization range of the SOC of each of the positive electrode electrolyte and the negative electrode electrolyte, the liquid volume of the positive electrode electrolyte and the liquid volume of the negative electrode electrolyte are made different, and the SOC of the mixed electrolyte is adjusted to be equal to or greater than a predetermined value that is not zero.

[0017] First, embodiments of the present disclosure will be listed and described.

[0018] (1) A redox flow battery according to an embodiment of the present disclosure includes: a positive electrode electrolyte containing a positive electrode active material and a negative electrode electrolyte containing a negative electrode active material, the amount of the positive electrode electrolyte and the amount of the negative electrode electrolyte are different, a liquid volume ratio of the positive electrode electrolyte and the negative electrode electrolyte, which is a liquid volume ratio of the electrolyte having a larger liquid volume to the electrolyte having a smaller liquid volume, is 1.05 or more and 5.0 or less; The state of charge of a mixed electrolyte obtained by mixing the positive electrode electrolyte and the negative electrode electrolyte in the same liquid volume ratio as the above is 2% or more.

[0019] In the redox flow battery of the present disclosure, the liquid volumes of the positive electrode electrolyte and the negative electrode electrolyte are different, and the SOC of the mixed electrolyte is equal to or greater than a predetermined non-zero value, thereby optimizing the SOC utilization range of each electrolyte. Therefore, the redox flow battery of the present disclosure can improve energy density. Therefore, the discharge capacity in the initial state can be increased. In other words, the redox flow battery of the present disclosure actively differentiates the liquid volumes of the positive electrode electrolyte and the negative electrode electrolyte and sets the SOC of the mixed electrolyte to a predetermined non-zero value, thereby expanding the SOC utilization range of each electrolyte compared to conventional batteries. Furthermore, by controlling the liquid volume ratio between the positive electrode electrolyte and the negative electrode electrolyte and the SOC of the mixed electrolyte, the increase in discharge capacity in the initial state can be maintained over time. The definition of the SOC of the mixed electrolyte and the reason why the SOC utilization range can be optimized will be described later.

[0020] (2) As one form of the above redox flow battery, The state of charge of the mixed electrolyte solution is 20% or less.

[0021] The above configuration can sufficiently increase the energy density.

[0022] (3) As one form of the above redox flow battery, a positive electrode tank that stores the positive electrode electrolyte and a negative electrode tank that stores the negative electrode electrolyte, Of the positive electrode tank and the negative electrode tank, the ratio of the volume of the tank storing the electrolyte with a large amount to the volume of the tank storing the electrolyte with a small amount is 1.0 or more and 6.0 or less.

[0023] The volumes of the positive electrode tank and the negative electrode tank may be the same or different. When the volumes of both tanks are the same, i.e., when the volume ratio between the positive electrode tank and the negative electrode tank is 1.0, the cost of the tanks can be reduced and the design can be simplified. When the volumes of both tanks are different, it is easy to store different amounts of each electrolyte in each tank. Furthermore, when the volume ratio between the positive electrode tank and the negative electrode tank is different from the volume ratio between the positive electrode electrolyte and the negative electrode electrolyte, there is a high degree of freedom in the design, including the tanks and piping.

[0024] (4) As one form of the above redox flow battery, The positive electrode active material and the negative electrode active material may be metal ions of the same element.

[0025] In the above embodiment, even if metal ions move between the positive electrode electrolyte and the negative electrode electrolyte during operation of the redox flow battery, the effect on battery performance is small.

[0026] (5) As one embodiment of the redox flow battery described in (4), The metal ions include vanadium ions.

[0027] In the above embodiment, a high electromotive force can be obtained.

[0028] (6) As one embodiment of the redox flow battery according to any one of (1) to (3), The positive electrode active material and the negative electrode active material may be metal ions made of different elements.

[0029] In the above-mentioned embodiment, the effect of improving the energy density can be significantly achieved, and the above-mentioned embodiment also offers great advantages by optimizing the range of use of SOC for each electrolyte.

[0030] (7) As one embodiment of the redox flow battery described in (6) above, the positive electrode active material is at least one metal ion selected from the group consisting of iron ions, vanadium ions, and manganese ions, The negative electrode active material may be at least one metal ion selected from the group consisting of zinc ions, chromium ions, vanadium ions, and titanium ions.

[0031] The above configuration allows the construction of a redox flow battery with high energy density.

[0032] (8) As one embodiment of the redox flow battery described in (7), The positive electrode active material may contain manganese ions, and the negative electrode active material may contain titanium ions.

[0033] In the above embodiment, a high electromotive force can be obtained.

[0034] (9) As one embodiment of the redox flow battery described in (8), The positive electrode electrolyte and the negative electrode electrolyte contain both manganese ions and titanium ions.

[0035] In the above embodiment, a high electromotive force can be obtained. Also, in the above embodiment, the titanium ions can suppress the deposition of manganese ions, which are a positive electrode active material, in the positive electrode electrolyte.

[0036] (10) A redox flow battery according to an embodiment of the present disclosure includes: a positive electrode electrolyte containing a positive electrode active material and a negative electrode electrolyte containing a negative electrode active material, the amount of the positive electrode electrolyte and the amount of the negative electrode electrolyte are different, a liquid volume ratio of the positive electrode electrolyte and the negative electrode electrolyte, which is a liquid volume ratio of the electrolyte having a larger liquid volume to the electrolyte having a smaller liquid volume, is 1.05 or more and 5.0 or less; a state of charge of a mixed electrolyte obtained by mixing the positive electrode electrolyte and the negative electrode electrolyte in the same liquid volume ratio as the liquid volume ratio is 2% or more and 20% or less; The positive electrode active material is at least one metal ion selected from the group consisting of iron ions, vanadium ions, and manganese ions. The negative electrode active material is at least one metal ion selected from the group consisting of zinc ions, chromium ions, vanadium ions, and titanium ions.

[0037] Since the above redox flow battery includes the configuration of the redox flow battery described in (1), (2), and (7) above, the energy density can be increased.

[0038] [Details of Embodiments of the Present Disclosure] A specific example of the redox flow battery of the present disclosure will be described with reference to the drawings. Hereinafter, the redox flow battery may be referred to as an "RF battery" in some cases. The same reference numerals in the drawings indicate the same or corresponding parts. Note that the present invention is not limited to these examples, and is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0039] [Outline of RF Battery] Referring to FIG. 1, the RF battery 1 according to the embodiment will be described. The RF battery 1 performs charging and discharging using a positive electrode electrolyte 2 containing a positive electrode active material and a negative electrode electrolyte 3 containing a negative electrode active material. The positive electrode active material and the negative electrode active material are typically metal ions whose valence changes by oxidation-reduction.

[0040] The RF battery 1 is typically connected to the power grid 90 via an AC / DC converter 80 and power transformation equipment 81. The RF battery 1 can charge the power generated by the power generation unit 91 or discharge the charged power to the load 92. The power generation unit 91 is a power generation facility that utilizes natural energy such as solar power generation or wind power generation, or other general power plants. The RF battery 1 is used, for example, for load leveling, momentary voltage drop compensation, emergency power supply, and other applications, and for smoothing the output of natural energy power generation.

[0041] [Configuration of RF Battery] The RF battery 1 includes a battery cell 10 that performs charging and discharging, a positive electrode tank 12 that stores a positive electrode electrolyte 2, an negative electrode tank 13 that stores a negative electrode electrolyte 3, and circulation flow paths that circulate the positive electrode electrolyte 2 and the negative electrode electrolyte 3. The circulation flow paths include a positive electrode electrolyte circulation flow path that circulates the positive electrode electrolyte 2 between the positive electrode tank 12 and the battery cell 10, and an negative electrode electrolyte circulation flow path that circulates the negative electrode electrolyte 3 between the negative electrode tank 13 and the battery cell 10. The basic configuration of the RF battery 1 can be any known configuration as appropriate.

[0042] (battery cell) The battery cell 10 has a positive electrode 104, a negative electrode 105, and a diaphragm 101 interposed between the positive electrode 104 and the negative electrode 105. The battery cell 10 is separated into a positive electrode cell 102 and a negative electrode cell 103 by the diaphragm 101. The diaphragm 101 is, for example, an ion exchange membrane that allows hydrogen ions to pass through. The positive electrode 104 is built into the positive electrode cell 102. The negative electrode 105 is built into the negative electrode cell 103.

[0043] A positive electrode electrolyte 2 is supplied to the positive electrode cell 102. A negative electrode electrolyte 3 is supplied to the negative electrode cell 103. In this example, an outward pipe 108 and a return pipe 110 are provided to connect the battery cell 10 and the positive electrode tank 12, and an outward pipe 109 and a return pipe 111 are provided to connect the battery cell 10 and the negative electrode tank 13. Pumps 112 and 113 are provided on the outward pipes 108 and 109, respectively. The positive electrode electrolyte 2 is supplied from the positive electrode tank 12 to the positive electrode cell 102 through the outward pipe 108 by the pump 112. The positive electrode electrolyte 2, which has passed through the positive electrode cell 102 and been discharged from the positive electrode cell 102, is returned to the positive electrode tank 12 through the return pipe 110. The negative electrode electrolyte 3 is supplied from the negative electrode tank 13 to the negative electrode cell 103 through the outward pipe 109 by the pump 113. The negative electrode electrolyte 3 that has passed through the negative electrode cell 103 and is discharged from the negative electrode cell 103 is returned to the negative electrode tank 13 through the return pipe 111. In other words, the outward pipes 108, 109 and the return pipes 110, 111 form a circulation flow path.

[0044] An RF battery 1 typically uses a configuration called a cell stack 100, in which multiple battery cells 10 are stacked, as shown in FIG. 2. The cell stack 100 is configured by sandwiching a substack 20 between two end plates 22 on both sides, and fastening the end plates 22 on both sides with a fastening mechanism 23. FIG. 2 shows the cell stack 100 including multiple substacks 20. The substack 20 is configured by repeatedly stacking a cell frame 30, a positive electrode 104, a diaphragm 101, and a negative electrode 105 in this order, with supply and discharge plates 21 disposed on both ends of the stack. The supply and discharge plates 21 are connected to the outward pipes 108 and 109 and the return pipes 110 and 111 shown in FIG. 1, which constitute the circulation flow path described above. The number of stacked battery cells 10 in the cell stack 100 can be selected as appropriate.

[0045] As shown in FIG. 2 , the cell frame 30 has a bipolar plate 31 and a frame 32. The bipolar plate 31 is disposed between a positive electrode 104 and a negative electrode 105. The frame 32 is provided around the bipolar plate 31. The positive electrode 104 is disposed facing one side of the bipolar plate 31. The negative electrode 105 is disposed facing the other side of the bipolar plate 31. The positive electrode 104 and the negative electrode 105 are housed inside the frame 32, sandwiching the bipolar plate 31 between them. A single battery cell 10 is formed by disposing the positive electrode 104 and the negative electrode 105 between the bipolar plates 31 of adjacent cell frames 30, with a diaphragm 101 sandwiched between them.

[0046] The frame body 32 of the cell frame 30 is formed with liquid supply manifolds 33 and 34, liquid discharge manifolds 35 and 36, liquid supply slits 33s and 34s, and liquid discharge slits 35s and 36s. In this example, the positive electrode electrolyte is supplied from the liquid supply manifold 33 to the positive electrode 104 via the liquid supply slit 33s. The positive electrode electrolyte supplied to the positive electrode 104 is discharged to the liquid discharge manifold 35 via the liquid discharge slit 35s. Similarly, the negative electrode electrolyte is supplied from the liquid supply manifold 34 to the negative electrode 105 via the liquid supply slit 34s. The negative electrode electrolyte supplied to the negative electrode 105 is discharged to the liquid discharge manifold 36 via the liquid discharge slit 36s. The liquid supply manifolds 33 and 34 and the liquid discharge manifolds 35 and 36 are provided to penetrate the frame body 32, and by stacking the cell frames 30, flow paths for the respective electrolytes are formed. These flow paths are connected to outward pipes 108, 109 and return pipes 110, 111 shown in Fig. 1, respectively, via the supply / discharge plate 21. The cell stack 100 is capable of circulating the positive electrode electrolyte and the negative electrode electrolyte to the battery cells 10 via the above-mentioned flow paths.

[0047] One of the features of the RF battery 1 of the embodiment is that the liquid volumes of the positive electrode electrolyte 2 and the negative electrode electrolyte 3 are different, and the SOC of the mixed electrolyte obtained by mixing the positive electrode electrolyte 2 and the negative electrode electrolyte 3 at the same ratio as the ratio of the liquid volumes of the positive electrode electrolyte 2 and the negative electrode electrolyte 3 is equal to or greater than a predetermined value other than zero. Specifically, the liquid volume ratio of the positive electrode electrolyte 2 to the negative electrode electrolyte 3 is 1.05 or more and 5.0 or less. The SOC of the mixed electrolyte is 2% or more.

[0048] (positive electrode electrolyte) The positive electrode electrolyte 2 contains a positive electrode active material. The positive electrode active material may be, for example, at least one metal ion selected from the group consisting of iron (Fe) ions, vanadium (V) ions, and manganese (Mn) ions. In the case of V ions, 4 Price or 5 The state of the valence ion ( V 4+ / V 5+ In the case of Fe ions, they exist mainly in the form of divalent or trivalent ions (Fe 2+ / Fe3+ ) In the case of Mn ions, they exist mainly in the form of divalent or trivalent ions (Mn 2+ / Mn 3+ ) exists.

[0049] (Negative electrode electrolyte) The negative electrode electrolyte 3 contains a negative electrode active material. The negative electrode active material may be, for example, at least one metal ion selected from the group consisting of zinc (Zn) ions, chromium (Cr) ions, V ions, and titanium (Ti) ions. In the case of V ions, 2 Price or 3 The state of the valence ion ( V 2+ / V 3+ ) In the case of Cr ions, they exist mainly in the state of divalent or trivalent ions (Cr 2+ / Cr 3+ In the case of Ti ions, they exist mainly in the form of trivalent or tetravalent ions (Ti 3+ / Ti 4+ ) exists as a tetravalent Ti ion (Ti 4+ ) is, for example, TiO 2+ In the case of Zn ions, it is mainly divalent ions (Zn 2+ ) Zn ions exist in the state of metallic zinc during charging.

[0050] In addition to metal ions, non-metallic organic active materials can also be used as the positive and negative electrode active materials. Examples of organic active materials include quinone compounds such as 2,6-dihydroxyanthraquinone and radicals such as 2,2,6,6-tetramethylpiperidine 1-oxyl.

[0051] The positive electrode active material and the negative electrode active material may be metal ions of the same element or different elements. When the positive electrode active material and the negative electrode active material are the same type of metal ion, a typical example is that both the positive electrode active material and the negative electrode active material contain V ions. When the positive electrode active material and the negative electrode active material are different types of metal ions, a typical example is that the positive electrode active material contains Mn ions and the negative electrode active material contains Ti ions.

[0052] The positive electrode active material and the negative electrode active material can be appropriately selected from the metal ions described above. Specific combinations of the positive electrode active material and the negative electrode active material are shown below. (1) Positive electrode active material: V ions ( V 4+ / V 5+ ), negative electrode active material: V ions ( V 2+ / V 3+ ) (2) Positive electrode active material: Fe ions (Fe 2+ / Fe 3+ ), negative electrode active material: Cr ions (Cr 2+ / Cr 3+ ) (3) Positive electrode active material: Mn ions (Mn 2+ / Mn 3+ ), negative electrode active material: Ti ions (Ti 3+ / Ti 4+ ) (4) Positive electrode active material: Fe ions (Fe 2+ / Fe 3+ ), negative electrode active material: Ti ions (Ti 3+ / Ti 4+ ) (5) Positive electrode active material: Mn ions (Mn 2+ / Mn 3+ ), negative electrode active material: Zn ions (Zn 2+ / Zn) (6) Positive electrode active material: V ions ( V 4+ / V 5+ ), negative electrode active material: Zn ions (Zn 2+ / Zn) (7) Positive electrode active material: V ions ( V 4+ / V 5+ ), negative electrode active material: Ti ions (Ti 3+ / Ti 4+ ) (8) Positive electrode active material: V ions ( V 4+ / V 5+ ), negative electrode active material: Cr ions (Cr 2+ / Cr 3+ ) (9) Positive electrode active material: Mn ions (Mn 2+ / Mn 3+ ), negative electrode active material: V ions ( V 2+ / V 3+ ) (10) Positive electrode active material: Mn ions (Mn 2+ / Mn 3+ ), negative electrode active material: Cr ions (Cr 2+ / Cr 3+ ) (11) Positive electrode active material: Fe ions (Fe 2+ / Fe 3+ ), negative electrode active material: V ions ( V 2+ / V 3+ ) (12) Positive electrode active material: Fe ions (Fe 2+ / Fe 3+ ), negative electrode active material: Zn ions (Zn 2+ / Zn)

[0053] The electromotive force is determined by the combination of the positive electrode active material and the negative electrode active material. Among the above combinations, for example, a combination in which both the positive electrode active material and the negative electrode active material are V ions, or a combination in which the positive electrode active material is Mn ions and the negative electrode active material is Ti ions, can obtain a high electromotive force.

[0054] The positive electrode electrolyte 2 and the negative electrode electrolyte 3 may contain the same type of metal ions. For example, the positive electrode active material and the negative electrode active material may contain metal ions of the same element, and at least one type of metal ion contained in the positive electrode electrolyte 2 and the negative electrode electrolyte 3 may be the same type. In particular, all of the metal ions contained in the positive electrode electrolyte 2 and the negative electrode electrolyte 3 may be the same type. In the RF battery 1, repeated charge and discharge over a long period of time may cause the metal ions contained in the positive electrode electrolyte 2 and the negative electrode electrolyte 3 to permeate the diaphragm 101 of the battery cell 10 and migrate between the positive electrode electrolyte 2 and the negative electrode electrolyte 3, or may cause liquid transfer, in which one electrolyte passes through the diaphragm 101 and moves to the other electrolyte. When the positive electrode electrolyte 2 and the negative electrode electrolyte 3 each contain the same type of metal ions, battery performance is easily maintained even if metal ions migrate or liquid transfer occurs between the positive electrode electrolyte 2 and the negative electrode electrolyte 3 due to repeated charge and discharge. If the positive electrode active material and the negative electrode active material are metal ions of the same element, the metal ions can function as active materials in both electrolytes. Furthermore, if the positive electrode electrolyte 2 and the negative electrode electrolyte 3 contain the same type of metal ions, the types of metal ions contained in the positive electrode electrolyte 2 and the negative electrode electrolyte 3 do not change even when the positive electrode electrolyte 2 and the negative electrode electrolyte 3 are mixed. To prevent liquid transfer, the positive electrode electrolyte 2 and the negative electrode electrolyte 3 can be easily mixed.

[0055] In a configuration in which the positive electrode active material is Mn ions and the negative electrode active material is Ti ions, for example, the positive electrode electrolyte 2 and the negative electrode electrolyte 3 may contain both Mn ions and Ti ions. In this case, Mn ions function as the positive electrode active material in the positive electrode electrolyte 2. Ti ions function as the negative electrode active material in the negative electrode electrolyte 3. When Mn ions are used as the positive electrode active material, the Mn ions may precipitate as Mn oxides in the positive electrode electrolyte 2. This is due to the trivalent Mn ions (Mn 3+ ) is unstable, so Mn 3+However, this is because manganese is likely to precipitate as manganese oxides such as MnO2. When positive electrode electrolyte 2 contains Ti ions, the Ti ions can suppress the precipitation of manganese ions. The Ti ions contained in positive electrode electrolyte 2 and the manganese ions contained in negative electrode electrolyte 3 do not function as active materials. The manganese ions contained in negative electrode electrolyte 3 are primarily used to equalize the metal ion species in both electrolytes.

[0056] Aqueous solutions can be suitably used as the solvent for the positive electrode electrolyte 2 and the negative electrode electrolyte 3. Examples of the solvent include an aqueous solution of sulfuric acid (H2SO4), an aqueous solution of phosphoric acid (H3PO4), and an aqueous solution of nitric acid (HNO3). aqueous solution In particular, an aqueous solution of sulfuric acid is easy to use.

[0057] The concentration of the positive electrode active material contained in the positive electrode electrolyte 2 and the concentration of the negative electrode active material contained in the negative electrode electrolyte 3 can be, for example, 0.3 M or more and 5 M or less. The "M" above refers to molar concentration (mol / L). The "L" above refers to liters. 1 L is 10 -3 m 3 In the positive electrode electrolyte 2 and the negative electrode electrolyte 3, the active material concentration of 0.3 M or more facilitates increasing the energy density. Since the higher the active material concentration, the higher the energy density, the active material concentration is preferably 0.5 M or more, and more preferably 1.0 M or more. In consideration of the solubility in the solvent, the active material concentration may be 5 M or less, and even 2 M or less. When the positive electrode electrolyte 2 and the negative electrode electrolyte 3 contain both Mn ions and Ti ions, the concentration of Ti ions contained in the positive electrode electrolyte 2 and the concentration of Mn ions contained in the negative electrode electrolyte 3 may be, for example, 0.3 M or more and 5 M or less, and even 0.5 M or more and 2 M or less.

[0058] The concentration of the positive electrode active material contained in the positive electrode electrolyte 2 and the concentration of the negative electrode active material contained in the negative electrode electrolyte 3 may be the same or different. When the positive electrode active material and the negative electrode active material are the same type of metal ion, it is easy to adjust the concentrations of the respective active materials to be the same. When the positive electrode active material and the negative electrode active material are different types of metal ions, the solubilities differ depending on the type of metal ion, so it is possible to adjust the concentrations of the respective active materials to be different.

[0059] (Liquid volume ratio of positive electrode electrolyte to negative electrode electrolyte) The volume ratio of the positive electrode electrolyte 2 to the negative electrode electrolyte 3 is 1.05 or more and 5.0 or less. The volume ratio refers to the ratio of the volume of the electrolyte with a larger volume to the volume of the electrolyte with a smaller volume, among the positive electrode electrolyte 2 and the negative electrode electrolyte 3. The volume of the positive electrode electrolyte 2 refers to the volume of the positive electrode electrolyte 2. The volume of the negative electrode electrolyte 3 refers to the volume of the negative electrode electrolyte 3. The volume ratio can be set based on the concentrations of the positive electrode active material and the negative electrode active material, and the optimal SOC ranges of each electrolyte. When the volume ratio is 1.05 or more, the effect of improving energy density can be easily achieved. When the volume ratio is 5.0 or less, the volume of the electrolyte with a larger volume can be prevented from becoming too large. As a result, the tank for storing the electrolyte with a larger volume does not become too large. The volume ratio can be, for example, 1.1 or more and 4.0 or less, further 1.2 or more and 3.0 or less, or 1.3 or more and 2.5 or less.

[0060] (Volume ratio of positive electrode tank to negative electrode tank) The volume ratio between the positive electrode tank 12 and the negative electrode tank 13 is, for example, 1.0 or more and 6.0 or less. The volume ratio refers to the ratio of the volume of the tank storing the electrolyte with a larger volume to the volume of the tank storing the electrolyte with a smaller volume, among the positive electrode tank 12 and the negative electrode tank 13. The volumes of the positive electrode tank 12 and the negative electrode tank 13 may be set according to the volume of the positive electrode electrolyte 2 and the volume of the negative electrode electrolyte 3, respectively. The volumes of the positive electrode tank 12 and the negative electrode tank 13 may be the same or different. The volume ratio may be set based on the liquid volume ratio described above. When the volume ratio is 1.0 or more and 6.0 or less, it is easy to set the volume of each of the positive electrode tank 12 and the negative electrode tank 13 to a size corresponding to the volume of the positive electrode electrolyte 2 and the negative electrode electrolyte 3, respectively.

[0061] The volume ratio may be the same as or different from the liquid volume ratio. Even if the liquid volume ratio is 1.05 or greater, taking into consideration the volume of the gas phase above the liquid level of the positive electrode electrolyte 2 in the positive electrode tank 12 and the volume of the gas phase above the liquid level of the negative electrode electrolyte 3 in the negative electrode tank 13, the volume ratio does not necessarily need to be 1.05 or greater to match the liquid volume ratio. When the liquid volume ratio is sufficiently small, the volume ratio may be 1.0 to reduce the manufacturing cost of the tanks and simplify the design. In other words, it is acceptable for the volume of the positive electrode tank 12 and the volume of the negative electrode tank 13 to be the same. A sufficiently small liquid volume ratio refers to, for example, a liquid volume ratio less than 1.5. On the other hand, a certain amount of space may be provided in the gas phase portion of each tank for design reasons, such as the arrangement of the ends of the outbound pipes 108 and 109 and the return pipes 110 and 111. Therefore, even if the liquid volume ratio is 5.0, it may be desirable to design the volume ratio to be around 6.0.

[0062] The volume ratio may be, for example, 1.05 or more and 5.0 or less, further 1.1 or more and 4.0 or less, 1.2 or more and 3.0 or less, or 1.3 or more and 2.5 or less, depending on the liquid volume ratio. The volume ratio may be set depending on the volume of the gas phase portion in the positive electrode tank 12 and the volume of the gas phase portion in the negative electrode tank 13, and other design conditions.

[0063] In addition, not only the volume of the positive electrode tank 12 and the volume of the negative electrode tank 13, but also the volumes of the pipes connected to the positive electrode tank 12 and the negative electrode tank 13 may be different. Furthermore, to adjust the liquid level of the electrolyte in the positive electrode tank 12 and the liquid level of the electrolyte in the negative electrode tank 13, an object of an appropriate volume may be submerged inside at least one of the tanks. The positive electrode electrolyte 2 is stored not only in the positive electrode tank 12 but also in each of the pipes. The negative electrode electrolyte 3 is stored not only in the negative electrode tank 13 but also in each of the pipes. Therefore, the amount of positive electrode electrolyte 2 that can be stored is determined by the sum of the volume of the positive electrode tank 12 and the volume of the above-mentioned pipes. The amount of negative electrode electrolyte 3 that can be stored is determined by the sum of the volume of the negative electrode tank 13 and the volume of the above-mentioned pipes. Therefore, it is possible to store a predetermined liquid volume ratio of positive electrode electrolyte 2 and negative electrode electrolyte 3 not only by adjusting the volume ratio of positive electrode tank 12 and negative electrode tank 13 to the liquid volume ratio but also by adjusting the volume of each pipe. For example, by changing the length of each pipe or by submerging the object inside at least one of the tanks, the volume ratio can be selected in various ways regardless of the liquid volume ratio.

[0064] (SOC of mixed electrolyte) The SOC of the mixed electrolyte obtained by mixing the positive electrode electrolyte 2 and the negative electrode electrolyte 3 in the same ratio as the above liquid volume ratio is 2% or more. The SOC of the mixed electrolyte can be set based on the concentrations of the positive electrode active material and the negative electrode active material, the optimum SOC ranges of each electrolyte, and the above liquid volume ratio. When the SOC of the mixed electrolyte is 2% or more, the effect of improving energy density is enhanced. The SOC of the mixed electrolyte can be set appropriately, but a practical range is 20% or less. The SOC of the mixed electrolyte can be, for example, 2% or more and 20% or less, or further 3% or more and 10% or less.

[0065] <Definition of SOC for mixed electrolyte> Here, the SOC of the mixed electrolyte is defined as follows according to the state in which the positive electrode electrolyte 2 and the negative electrode electrolyte 3 are mixed in the same ratio as the liquid volume ratio described above.

[0066] First, consider the case where the redox reaction of the active material is a one-electron reaction. For the positive electrode active material, the concentration of the reductant is Cpr, the concentration of the oxidant is Cpo, and the total concentration is Cp. The total concentration Cp of the positive electrode active material is expressed as Cp = Cpr + Cpo. Similarly, for the negative electrode active material, the concentration Cnr is the reductant, the concentration Cno is the oxidant, and the total concentration Cn. The total concentration Cn of the negative electrode active material is expressed as Cn = Cnr + Cno. The ratio of the oxidant concentration to the reductant concentration is determined for each of the positive electrode active material and the negative electrode active material depending on the SOC of the RF battery. Here, it is assumed that the positive electrode electrolyte and the negative electrode electrolyte are mixed in an RF battery at a certain SOC. In an RF battery, when the volume vp(L) of the positive electrode electrolyte and the volume vn(L) of the negative electrode electrolyte are different, i.e., when vp≠vn, a mixed electrolyte is considered in which the positive electrode electrolyte and the negative electrode electrolyte are mixed in the same volume ratio as the positive electrode electrolyte and the negative electrode electrolyte. In other words, the mixed electrolyte is a mixture of the positive electrode electrolyte and the negative electrode electrolyte in a ratio of vp:vn.

[0067] When the concentration of the oxidized form of the positive electrode active material, Cpo (mol / L), and the concentration of the reduced form of the negative electrode active material, Cnr (mol / L), are zero, i.e., when Cpo = Cnr = 0, the amount of electricity oxidized or reduced in the mixed electrolyte is equivalent to (Cpo·vp-Cnr·vn) / (vp+vn)×F [C], where F is Faraday's constant. F = 96485 [C / mol]. Here, if we set m = (Cpo·vp-Cnr·vn) / (vp+vn) [mol / L], the symbol m in this formula corresponds to the concentration of the active material that carries the oxidized or reduced electrical charge. When m>0, the SOC of the mixed electrolyte is defined as (m / Cp)×100[%]. When m<0, the SOC of the mixed electrolyte is defined as (m / Cn)×100[%].

[0068] The same definition can be used when an active material undergoes a two-electron or more reaction and both one-electron and two-electron or more reactions coexist. Assume that the positive electrode active material undergoes an Np-electron reaction, and the negative electrode active material undergoes an Nn-electron reaction. The symbols Np and Nn are both natural numbers greater than or equal to 2. In this case, the total concentration Cp of the positive electrode active material and the total concentration Cn of the negative electrode active material are expressed as follows: Cp=Cpr+ΣCpo i (i=1,2,...,Np) Cn=ΣCnr i +Cno (i=1,2,...,Nn) Therefore, m={vpΣ(Cpo i ×i)-vnΣ(Cnr i ×i)} / (vp+vn).

[0069] [Example 1] In Example 1, an explanation will be given of a case where an electrolyte containing 1 mol / L of Ti ions and 1 mol / L of Mn ions is used as the positive electrode electrolyte and the negative electrode electrolyte. In an RF battery using this electrolyte, Mn, which is a reduced form, is used as the positive electrode active material. 2+ and the oxidized form Mn 3+ As the negative electrode active material, the oxidized form of Ti 4+ and the reduced form Ti 3+ Assume that the RF battery is operating under the following conditions: Cpo = 0.42 (mol / L), Cpr = 0.58 (mol / L), Cnr = 0.40 (mol / L), Cno = 0.60 (mol / L), vp = 1.05 (L), vn = 1.00 (L). In this case, m = (Cpo·vp-Cnr·vn) / (vp+vn) = (0.42×1.05-0.40×1.00) / (1.05+1.00) = 0.02. That is, when the positive electrode electrolyte and the negative electrode electrolyte are mixed according to the ratio of the liquid volume of the positive electrode electrolyte to the liquid volume of the negative electrode electrolyte, the mixed electrolyte contains Mn 3+ The charge equivalent to 0.02 mol / L is charged. The SOC of the mixed electrolyte is calculated as 0.02 / Cp = 0.02 / (0.42 + 0.58) = 2%.

[0070] [Example 2] In Example 2, an RF battery using the same electrolyte as in Example 1 also contains MnO2, an oxidant that undergoes a two-electron reaction, as a positive electrode active material. This RF battery is operated under the following conditions: Cpo1 = 0.22 (mol / L), Cpo2 = 0.10 (mol / L), Cpr = 0.68 (mol / L), Cnr = 0.40 (mol / L), Cno = 0.60 (mol / L), vp = 1.05 (L), and vn = 1.00 (L). Cpo1 is Mn 3+ Cpo2 corresponds to the concentration of MnO2. In this case, m={vpΣ(Cpo i ×i)-(Cnr·vn)} / (vp+vn)={1.05×(0.22+(0.10×2))-0.40×1.00} / (1.05+1.00)=0.02 。 That is, the mixed electrolyte contains Mn 3+ The charge equivalent to 0.02 mol / L is charged. The SOC of the mixed electrolyte is calculated as 0.02 / Cp = 0.02 / (0.22 + 0.10 + 0.68) = 2%.

[0071] [Example 3] In Example 3, an electrolyte solution containing only 1 mol / L of V ions as the active material is used as the positive electrode electrolyte and the negative electrode electrolyte. In an RF battery using this electrolyte, V, which is a reduced form, is used as the positive electrode active material. 3+ and the oxidized form V 4+ As the negative electrode active material, the oxidized form V 4+ and the reduced form V 3+ In RF batteries, V is commonly used as the positive electrode active material. 4+ is the reduced form, V 5+ is treated as an oxidant, and V is used as the negative electrode active material. 3+ is the oxidized form, V 2+ However, since we are focusing on the state of the mixed electrolyte, we should assume the SOC in the initial state of the RF battery. Therefore, as in the former case, the reduced V is used as the positive electrode active material. 3+ and the oxidized V 4+ and V as the negative electrode active material. 4+and the reduced form V 3+ It is reasonable to consider that the following exist: Assume that this RF battery is operated under the following conditions: Cpo = 0.51 (mol / L), Cpr = 0.49 (mol / L), Cnr = 0.49 (mol / L), Cno = 0.51 (mol / L), vp = 1.05 (L), vn = 1.00 (L). In this case, m = (Cpo vp - Cnr vn) / (vp + vn) = (0.51 × 1.05 - 0.49 × 1.00) / (1.05 + 1.00) = 0.022 。 The SOC of the mixed electrolyte is calculated as 0.022 / Cp=0.022 / (0.51+0.49)=2.2%. When vp=vn=1, the SOC of the mixed electrolyte is 1%. In this case, the mixed electrolyte is V 3+ 0.49M, V 4+ It contains 0.51M of . In this case, it is sometimes written as "average valence of 3.51."

[0072] The SOC of the mixed electrolyte corresponds to the imbalance between the valence of the positive electrode electrolyte 2 and the valence of the negative electrode electrolyte 3. When the SOC of the mixed electrolyte is equal to or higher than a certain level, it means that in the state in which the positive electrode electrolyte 2 and the negative electrode electrolyte 3 are mixed, a positive electrode active material in an oxidized state or a negative electrode active material in a reduced state is present at a certain level or higher.

[0073] The SOC of the mixed electrolyte is calculated as the SOC of the entire electrolyte when the positive electrode electrolyte 2 and the negative electrode electrolyte 3 are mixed. The SOC here is expressed assuming that the positive electrode active material or the negative electrode active material undergoes a one-electron reaction. For example, if the positive electrode active material is contained in the positive electrode electrolyte 2 at a concentration of 1 mol / L, and the positive electrode active material is present in an oxidized state when the positive electrode electrolyte 2 and the negative electrode electrolyte 3 are mixed, the SOC of the mixed electrolyte is defined as 2% when the amount of electricity obtained after the positive electrode electrolyte 2 and the negative electrode electrolyte 3 are mixed is equivalent to 0.02 mol / L. In this case, the SOC of the mixed electrolyte is 2% when the positive electrode active material is oxidized to an amount equivalent to 96485 (C / mol) × 0.02 (mol / L) = 1929.7 (C / L), where Faraday's constant is 96485 (C / mol). Furthermore, when the quantity of electricity is equivalent to 0.2 mol / L, the "SOC of the mixed electrolyte is 20%." In this case, "the SOC of the mixed electrolyte is 20%" means that the positive electrode active material is oxidized by an amount equivalent to 96,485 (C / mol) × 0.2 (mol / L) = 19,297 (C / L). When the negative electrode active material is contained in the negative electrode electrolyte 3 at a concentration of 1 mol / L, the negative electrode active material is in a reduced state when the positive electrode electrolyte 2 and the negative electrode electrolyte 3 are mixed. Similarly, when the quantity of electricity is equivalent to 0.02 mol / L when the positive electrode electrolyte 2 and the negative electrode electrolyte 3 are mixed, the "SOC of the mixed electrolyte is 2%." In this case, "the SOC of the mixed electrolyte is 2%" means that the negative electrode active material has been reduced by an amount equivalent to 96485 (C / mol) × 0.02 (mol / L) = 1929.7 (C / L). Furthermore, when the amount of electricity is equivalent to 0.2 mol / L, "the SOC of the mixed electrolyte is 20%" is similarly defined. In this case, "the SOC of the mixed electrolyte is 20%" means that the negative electrode active material has been reduced by an amount equivalent to 96485 (C / mol) × 0.2 (mol / L) = 19297 (C / L).

[0074] The SOC of the mixed electrolyte obtained by mixing the positive electrode electrolyte 2 and the negative electrode electrolyte 3 can be adjusted by oxidizing or reducing the active material based on the concentration of the active material in either the positive electrode electrolyte 2 or the negative electrode electrolyte 3. The SOC of the mixed electrolyte can be adjusted by adjusting the SOC of each electrolyte, i.e., the valence of the active material in each electrolyte, when preparing the positive electrode electrolyte 2 and the negative electrode electrolyte 3. Alternatively, the positive electrode active material in the positive electrode electrolyte 2 can be oxidized or the negative electrode active material in the negative electrode electrolyte 3 can be reduced after the electrolytes are prepared. Examples of methods for oxidizing the positive electrode active material include natural oxidation using air and chemical oxidation using an oxidizing agent. Examples of oxidizing agents that can be used include hydrogen peroxide solution. Examples of methods for reducing the negative electrode active material include chemical reduction using a reducing agent. Examples of reducing agents that can be used include hydrogen, sulfurous acid, and oxalic acid. Alternatively, the valence of the positive electrode electrolyte 2 and the negative electrode electrolyte 3 can be adjusted to be different from each other by charging the positive electrode electrolyte 2 and the negative electrode electrolyte 3 to a predetermined SOC, oxidizing the positive electrode active material and reducing the negative electrode active material, and then leaving one of the electrolytes with an adjusted SOC and replacing the other electrolyte with an electrolyte having a different SOC. In other words, the SOC of the mixed electrolyte can be adjusted.

[0075] Optimization of SOC utilization range, relationship between liquid volume ratio and SOC of mixed electrolyte As described above, in the RF battery 1 of the embodiment, the liquid amounts of the positive electrode electrolyte 2 and the negative electrode electrolyte 3 are different and the valences are different, so that the SOC utilization range of both the positive electrode electrolyte 2 and the negative electrode electrolyte 3 can be optimized. The SOC utilization range refers to the range of SOC actually used when charging and discharging. The reason why the SOC utilization range can be optimized will be explained below.

[0076] The optimal SOC ranges for the positive and negative electrode electrolytes are primarily determined by the type of metal ions used in the positive and negative electrode active materials. The optimal SOC ranges for each electrolyte may also vary depending on the active material concentration and solvent concentration. Assuming no influence from metal ion migration or liquid transfer between the two electrolytes, the optimal SOC ranges for each electrolyte do not change depending on the combination of positive and negative electrode active materials. The upper limit of the optimal SOC range for each electrolyte is set to an SOC value that does not cause problems such as increased internal resistance, side reactions, or active material precipitation at the end of charging. The lower limit of the optimal SOC range for each electrolyte is set to an SOC value that does not cause problems such as increased internal resistance or active material precipitation at the end of discharging. First, consider the case where the volume of the positive electrode electrolyte and the volume of the negative electrode electrolyte are equal and the SOC when the two electrolytes are mixed is 0%. A 0% SOC when the two electrolytes are mixed means that the valence of the positive and negative electrolytes is consistent, i.e., the SOC of the mixed electrolyte is 0%. For example, suppose the optimal SOC range of the positive electrolyte is half that of the negative electrolyte. In this case, if charging and discharging are performed to match the optimal SOC range of the positive electrolyte, the actual SOC range of the negative electrolyte is limited by the SOC range of the positive electrolyte. In this example, the actual SOC range of the negative electrolyte is limited to at most half the optimal SOC range. Therefore, the SOC range of the negative electrolyte is reduced. Because the SOC range of the negative electrolyte is limited by the positive electrolyte, an improvement in energy density cannot be achieved. Conversely, if charging and discharging are performed to match the actual SOC range of the negative electrolyte to the optimal SOC range, the actual SOC range of the positive electrolyte is twice as large as the optimal SOC range. In other words, at least one of the SOCs at the end of charge and the end of discharge of the positive electrode electrolyte falls outside the optimal SOC range. If the SOC of the positive electrode electrolyte at the end of charge is higher than the optimal SOC range, side reactions may occur or the positive electrode active material may precipitate. The precipitated positive electrode active material may be unable to be used for charge / discharge or may be difficult to use for charge / discharge.If the SOC of the positive electrode electrolyte at the end of discharge is lower than the optimum SOC range, the internal resistance increases and the reaction efficiency decreases.

[0077] In the above example, consider a situation in which the SOC of the mixed electrolyte remains 0% when the positive and negative electrode electrolytes are mixed, but only the volume of the positive electrode electrolyte is increased. For example, assume that the volume ratio of the positive electrode electrolyte to the negative electrode electrolyte is 2:1, resulting in a volume ratio of 2. If the optimal SOC range for the positive electrode electrolyte is half that of the negative electrode electrolyte, doubling the volume of the positive electrode electrolyte would enable the positive and negative electrode electrolytes to charge and discharge amounts of electricity corresponding to their optimal SOC ranges. This would increase the energy density. However, simply adjusting the volume ratio does not guarantee that the actual SOC ranges for both the positive and negative electrode electrolytes will be within the optimal SOC ranges. The SOC ranges for the positive and negative electrode electrolytes may deviate from the optimal SOC ranges. Therefore, in one or both of the positive electrode electrolyte and the negative electrode electrolyte, when the SOC utilization range is higher than the optimal SOC range, side reactions and active material precipitation may occur, while when the SOC utilization range is lower than the optimal SOC range, internal resistance increases and reaction efficiency decreases.

[0078] In the above example, by increasing the amount of the positive electrode electrolyte and adjusting the valence of the positive electrode electrolyte and the negative electrode electrolyte, it is possible to adjust the SOC utilization range of each electrolyte to an optimal SOC range. For example, if the SOC of the positive electrode electrolyte at the end of discharge is lower than the optimal SOC range, a portion of the positive electrode active material in the positive electrode electrolyte is oxidized in advance, and the valence is adjusted so that an oxidized positive electrode active material exists when the positive electrode electrolyte and the negative electrode electrolyte are mixed. By adjusting the valence in this way so that an oxidized positive electrode active material exists, the SOC of the positive electrode electrolyte at the end of discharge can be brought closer to the optimal SOC range. This eliminates the problem of reduced reaction efficiency. Furthermore, if the SOC of the positive electrode electrolyte at the end of charge is higher than the optimal SOC range, a portion of the negative electrode active material in the negative electrode electrolyte is reduced in advance, and the valence is adjusted so that a reduced negative electrode active material exists when the positive electrode electrolyte and the negative electrode electrolyte are mixed. By adjusting the valence so that the negative electrode active material is in a reduced state, the SOC of the positive electrode electrolyte at the end of charge can be brought closer to the optimal SOC range. Therefore, by adjusting the liquid volume ratio between the positive electrode electrolyte and the negative electrode electrolyte and the SOC of the mixed electrolyte, the effect of improving energy density can be further enhanced.

[0079] In the above explanation, a case where the optimal SOC range of the positive electrode electrolyte is smaller than the optimal SOC range of the negative electrode electrolyte has been described as an example. The same can be considered when the optimal SOC range of the negative electrode electrolyte is smaller than the optimal SOC range of the positive electrode electrolyte. Specifically, the energy density can be improved by increasing the amount of the negative electrode electrolyte and adjusting the valence of the positive electrode electrolyte and the negative electrode electrolyte.

[0080] <Action and effect> In the RF battery 1 of the above-described embodiment, for example, in an initial state, the liquid volumes of the positive electrode electrolyte 2 and the negative electrode electrolyte 3 are different and the valences are different, so that the utilization range of the SOC of both the positive electrode electrolyte 2 and the negative electrode electrolyte 3 can be optimized. This makes it possible to improve the energy density, and an RF battery with a high energy density can be obtained. In particular, when the liquid volume ratio of the positive electrode electrolyte 2 to the negative electrode electrolyte 3 is 1.05 or more and 5.0 or less, and further, the SOC of the mixed electrolyte is 2% or more, the effect of improving the energy density is further enhanced. The initial state includes, for example, a state before the RF battery 1 starts operating.

[0081] As described above, the optimal SOC range for the positive electrode electrolyte 2 and the optimal SOC range for the negative electrode electrolyte 3 may differ. In conventional RF batteries, in the initial state, the liquid volumes of the positive electrode electrolyte 2 and the negative electrode electrolyte 3 are equal, and the valences of the positive electrode electrolyte 2 and the negative electrode electrolyte 3 do not differ. Therefore, the SOC usage ranges of the positive electrode electrolyte 2 and the negative electrode electrolyte 3 are both limited to only overlapping ranges within their respective optimal SOC ranges. In other words, in conventional RF batteries, the positive electrode electrolyte 2 and the negative electrode electrolyte 3 cannot be used within their respective optimal SOC ranges. For example, if the SOC usage range is set to match the optimal SOC range of one electrolyte, the SOC usage range of the other electrolyte may be lower or higher than the optimal SOC range. In contrast, in the RF battery 1 of the embodiment, by controlling the liquid volume ratio and the SOC of the mixed electrolyte, the utilization range of the SOC of the positive electrode electrolyte 2 and the utilization range of the SOC of the negative electrode electrolyte 3 can be made to approach or coincide with the respective optimal SOC ranges.

[0082] When the positive electrode active material and the negative electrode active material are metal ions composed of different elements, it is highly likely that the optimal SOC range for use of the positive electrode electrolyte 2 and the optimal SOC range for use of the negative electrode electrolyte 3 will differ. Furthermore, the solubility of metal ions in the positive electrode electrolyte 2 and the negative electrode electrolyte 3 will differ depending on the type of metal ion. Due to the difference in solubility, the concentrations of the active materials contained in the positive electrode electrolyte 2 and the negative electrode electrolyte 3 are adjusted to be different. Therefore, in an embodiment in which the positive electrode active material and the negative electrode active material are composed of different types of metal ions, it is believed that there will be significant benefits to be obtained by optimizing both the SOC range for use of the positive electrode electrolyte 2 and the SOC range for use of the negative electrode electrolyte 3.

[0083] (Calculation example 1) The concentration of the positive electrode active material is set to 1.0 mol / L, and the concentration of the negative electrode active material is set to 1.0 mol / L. The optimum SOC range for the positive electrode electrolyte is set to 20% at the lower limit and 80% at the upper limit. The optimum SOC range for the negative electrode electrolyte is set to 17% at the lower limit and 80% at the upper limit. In other words, the SOC usable range for the positive electrode electrolyte is limited to 20% or more and 80% or less, and the SOC usable range for the negative electrode electrolyte is limited to 17% or more and 80% or less. The SOC is calculated assuming that each active material undergoes a one-electron reaction. The discharge capacity under these conditions was calculated. The discharge capacity was determined for each of the following cases. The discharge capacity for each case is shown in Table 1.

[0084] (a) When the volume of the positive and negative electrolytes is equal and the SOC of the mixed electrolyte is zero (b) When the volume ratio of the positive electrode electrolyte to the negative electrode electrolyte is 1.05 and the SOC of the mixed electrolyte is zero (c) When the volume of the positive and negative electrolytes is equal and the SOC of the mixed electrolyte is 2% (d) When the volume ratio of the positive electrode electrolyte to the negative electrode electrolyte is 1.05 and the SOC of the mixed electrolyte is 2% The liquid volume ratio is defined as [volume of positive electrode electrolyte / volume of negative electrode electrolyte]. In other words, when the liquid volume ratio is 1.05, the volume of the positive electrode electrolyte is 1.05 times the volume of the negative electrode electrolyte. The SOC of the mixed electrolyte is expressed as the SOC of the positive electrode electrolyte when the positive electrode electrolyte and the negative electrode electrolyte are mixed in the same volume ratio. In the above cases (a) and (c), the positive electrode electrolyte and the negative electrode electrolyte are mixed in a 1:1 ratio. In the above cases (b) and (d), the positive electrode electrolyte and the negative electrode electrolyte are mixed in a 1.05:1 ratio.

[0085] The discharge capacity is determined as follows. If the ratio of the volume of the positive electrode electrolyte to the volume of the negative electrode electrolyte is r, then the ratio of the volume of the positive electrode electrolyte to the total volume of the positive electrode electrolyte and the negative electrode electrolyte is {r / (r+1)}. The discharge capacity is expressed as follows using the volume ratio r: Discharge capacity (Ah / L) = Faraday constant (C / mol) × positive electrode active material concentration (mol / L) × positive electrode SOC range (%) × {r / (r+1)} / 3600 The Faraday constant is 96485 (C / mol). The positive electrode SOC range is the range in which the SOC of the positive electrode electrolyte can be utilized during charging and discharging.

[0086] The positive electrode SOC range is determined as follows. Here, we consider the valence shift toward the positive electrode electrolyte. The concentration of the positive electrode active material contained in the positive electrode electrolyte is A (mol / L), and the SOC of the mixed electrolyte is x (%). When the positive electrode electrolyte and the negative electrode electrolyte are mixed, the positive electrode active material is assumed to be present in an oxidized state in an amount corresponding to X (mol / L), which is calculated by [X (mol / L) = A (mol / L) × x (%)]. Let B (mol / L) be the concentration of the negative electrode active material contained in the negative electrode electrolyte, and let b1 (%) to b2 (%) be the optimal SOC range for the negative electrode electrolyte. If we temporarily ignore the optimal SOC range for the positive electrode electrolyte, the SOC at the end of discharge and the SOC at the end of charge for the positive electrode electrolyte can be expressed as follows: SOC at the end of discharge (%) = {X + (B × b1 + X) / r} / A (%) SOC at the end of charging (%) = {X + (B × b2 + X) / r} / A (%) If the SOC of the positive electrode electrolyte falls outside the optimum range, problems such as side reactions and precipitation of the active material will occur. Therefore, if the optimum SOC range for the positive electrode electrolyte is from a1 (%) to a2 (%), the SOC at the end of discharge and the SOC at the end of charge of the positive electrode electrolyte will each satisfy the following: The SOC (%) at the end of discharge is the larger of [{X + (B × b1 + X) / r} / A] and [a1]. The SOC (%) at the end of charging is the smaller of [{X + (B × b2 + X) / r} / A] and [a2]. The positive electrode SOC range is a value calculated by [SOC at the end of charging - SOC at the end of discharging].

[0087] [Table 1]

[0088] As shown in Table 1, in case (d) where the liquid volume ratio of the positive electrode electrolyte to the negative electrode electrolyte is 1.05 and the SOC of the mixed electrolyte is 2%, the discharge capacity can be increased, i.e., the energy density can be improved.

[0089] (Calculation example 2) The concentration of the positive electrode active material is 1.0 mol / L, and the concentration of the negative electrode active material is 1.0 mol / L. The optimum SOC range for the positive electrode electrolyte is set to a lower limit of 10% and an upper limit of 20%. The optimum SOC range for the negative electrode electrolyte is set to a lower limit of 30% and an upper limit of 88%. The discharge capacity under these conditions was calculated in the same manner as in Calculation Example 1. The discharge capacity was determined for each of the following cases. Table 2 shows the discharge capacity for each case.

[0090] (e) When the volume of the positive electrode electrolyte and the negative electrode electrolyte are equal and the SOC of the mixed electrolyte is zero (f) When the volume ratio of the positive electrode electrolyte to the negative electrode electrolyte is 5.0 and the SOC of the mixed electrolyte is zero (g) When the volume of the positive and negative electrolytes is equal and the SOC of the mixed electrolyte is 2% (h) When the volume ratio of the positive electrode electrolyte to the negative electrode electrolyte is 5.0 and the SOC of the mixed electrolyte is 2%

[0091] [Table 2]

[0092] In this example, cases (e) and (g) were not allowed to be charged or discharged because the SOC range of the positive or negative electrode electrolyte was outside the optimal SOC range. As shown in Table 2, case (h), in which the volume ratio of the positive electrode electrolyte to the negative electrode electrolyte was set to 5.0 and the SOC of the mixed electrolyte was set to 2%, was able to increase the discharge capacity.

[0093] (Calculation example 3) The concentration of the positive electrode active material is 1.0 mol / L, and the concentration of the negative electrode active material is 1.0 mol / L. The optimum SOC range for the positive electrode electrolyte is set to a lower limit of 60% and an upper limit of 80%. The optimum SOC range for the negative electrode electrolyte is set to a lower limit of 22% and an upper limit of 43%. The discharge capacity under these conditions was calculated in the same manner as in Calculation Example 1. The discharge capacity was determined for each of the following cases. The discharge capacity for each case is shown in Table 3.

[0094] (i) When the volume of the positive electrode electrolyte and the negative electrode electrolyte are equal and the SOC of the mixed electrolyte is zero (j) When the volume ratio of the positive electrode electrolyte to the negative electrode electrolyte is 1.05 and the SOC of the mixed electrolyte is zero (k) When the volume of the positive electrode electrolyte and the negative electrode electrolyte are equal and the SOC of the mixed electrolyte is 20% (l) When the volume ratio of the positive electrode electrolyte to the negative electrode electrolyte is 1.05 and the SOC of the mixed electrolyte is 20%

[0095] [Table 3]

[0096] In this example, cases (i) and (j) were not allowed to be charged or discharged because the SOC range of the positive or negative electrode electrolyte was outside the optimal SOC range. As shown in Table 3, case (l), in which the volume ratio of the positive electrode electrolyte to the negative electrode electrolyte was set to 1.05 and the SOC of the mixed electrolyte was set to 20%, was able to increase the discharge capacity.

[0097] (Calculation example 4) The concentration of the positive electrode active material is 1.0 mol / L, and the concentration of the negative electrode active material is 1.0 mol / L. The optimum SOC range for the positive electrode electrolyte is set to a lower limit of 30% and an upper limit of 40%. The optimum SOC range for the negative electrode electrolyte is set to a lower limit of 30% and an upper limit of 80%. The discharge capacity under these conditions was calculated in the same manner as in Calculation Example 1. The discharge capacity was determined for each of the following cases. Table 4 shows the discharge capacity for each case.

[0098] (m) When the volume of the positive electrode electrolyte and the negative electrode electrolyte are equal and the SOC of the mixed electrolyte is zero (n) When the volume ratio of the positive electrode electrolyte to the negative electrode electrolyte is 5.0 and the SOC of the mixed electrolyte is zero (o) When the volume of the positive and negative electrolytes is equal and the SOC of the mixed electrolyte is 20% (p) When the volume ratio of the positive electrode electrolyte to the negative electrode electrolyte is 5.0 and the SOC of the mixed electrolyte is 20%

[0099] [Table 4]

[0100] In this example, cases (n) and (o) were not allowed to be charged or discharged because the SOC range of the positive or negative electrode electrolyte was outside the optimal SOC range. As shown in Table 4, case (p), in which the volume ratio of the positive electrode electrolyte to the negative electrode electrolyte was set to 5.0 and the SOC of the mixed electrolyte was set to 20%, was able to increase the discharge capacity.

[0101] (Calculation example 11) The concentration of the positive electrode active material was set to 1.0 mol / L, and the concentration of the negative electrode active material was set to 1.05 mol / L. The optimum SOC range for the positive electrode electrolyte was set to a lower limit of 20% and an upper limit of 80%. The optimum SOC range for the negative electrode electrolyte was set to a lower limit of 16.1% and an upper limit of 76.1%. The discharge capacity under these conditions was calculated in the same manner as in Calculation Example 1. The discharge capacity was determined for each of the above cases (a) to (d). Table 5 shows the discharge capacity for each case.

[0102] [Table 5]

[0103] As shown in Table 5, in case (d) where the liquid volume ratio of the positive electrode electrolyte to the negative electrode electrolyte is 1.05 and the SOC of the mixed electrolyte is 2%, the discharge capacity can be increased.

[0104] (Calculation example 12) The concentration of the positive electrode active material was set to 1.0 mol / L, and the concentration of the negative electrode active material was set to 5.0 mol / L. The optimum SOC range for the positive electrode electrolyte was set to a lower limit of 20% and an upper limit of 80%. The optimum SOC range for the negative electrode electrolyte was set to a lower limit of 17.6% and an upper limit of 77.6%. The discharge capacity under these conditions was calculated in the same manner as in Calculation Example 1. The discharge capacity was determined for each of the above cases (e) to (h). Table 6 shows the discharge capacity for each case.

[0105] [Table 6]

[0106] In this example, cases (e) and (g) were not allowed to be charged or discharged because the SOC range of the positive or negative electrode electrolyte was outside the optimal SOC range. As shown in Table 6, case (h), in which the volume ratio of the positive electrode electrolyte to the negative electrode electrolyte was set to 5.0 and the SOC of the mixed electrolyte was set to 2%, was able to increase the discharge capacity.

[0107] (Calculation Example 13) The concentration of the positive electrode active material was set to 1.0 mol / L, and the concentration of the negative electrode active material was set to 1.05 mol / L. The optimum SOC range for the positive electrode electrolyte was set to a lower limit of 50% and an upper limit of 80%. The optimum SOC range for the negative electrode electrolyte was set to a lower limit of 11% and an upper limit of 41%. The discharge capacity under these conditions was calculated in the same manner as in Calculation Example 1. The discharge capacity was determined for each of the above cases (i) to (l). Table 7 shows the discharge capacity for each case.

[0108] [Table 7]

[0109] In this example, cases (i) and (j) were not allowed to be charged or discharged because the SOC range of the positive or negative electrode electrolyte was outside the optimal SOC range. As shown in Table 7, case (l), in which the volume ratio of the positive electrode electrolyte to the negative electrode electrolyte was set to 1.05 and the SOC of the mixed electrolyte was set to 20%, was able to increase the discharge capacity.

[0110] (Calculation Example 14) The concentration of the positive electrode active material was set to 1.0 mol / L, and the concentration of the negative electrode active material was set to 5.0 mol / L. The optimum SOC range for the positive electrode electrolyte was set to a lower limit of 50% and an upper limit of 80%. The optimum SOC range for the negative electrode electrolyte was set to a lower limit of 26% and an upper limit of 56%. The discharge capacity under these conditions was calculated in the same manner as in Calculation Example 1. The discharge capacity was determined for each of the above cases (m) to (p). Table 8 shows the discharge capacity for each case.

[0111] [Table 8]

[0112] In this example, cases (m) and (o) were not allowed to be charged or discharged because the SOC range of the positive or negative electrode electrolyte was outside the optimal SOC range. As shown in Table 8, case (p), in which the volume ratio of the positive electrode electrolyte to the negative electrode electrolyte was set to 5.0 and the SOC of the mixed electrolyte was set to 20%, was able to increase the discharge capacity.

[0113] From the above-mentioned Trial Calculation Examples 1 to 4 and Trial Calculation Examples 11 to 14, it can be seen that the discharge capacity of an RF battery can be improved by setting the liquid volume ratio of the positive electrode electrolyte to the negative electrode electrolyte to be 1.05 or more and 5.0 or less, and the SOC of the mixed electrolyte to be 2% or more and 20% or less. The above-mentioned trial calculation examples are merely examples. In an RF battery, by setting the liquid volume ratio and the SOC of the mixed electrolyte within the above-mentioned specific ranges according to the concentration of the active material of each of the positive electrode electrolyte and the negative electrode electrolyte and the range of use of SOC, it is possible to increase the discharge capacity in the initial state. In other words, the effect of improving energy density can be obtained.

[0114] In the above-mentioned example calculation, it is possible to replace the positive electrode active material with the negative electrode active material, and the positive electrode electrolyte with the negative electrode electrolyte. In addition, in the above-mentioned example calculation, it was assumed that each active material undergoes a one-electron reaction, but if the active material undergoes a two-electron or more reaction, the maximum SOC value can be calculated as [100% × number of reaction electrons]. [Explanation of symbols]

[0115] 1 Redox flow battery (RF battery) 2 Positive electrode electrolyte, 3 Negative electrode electrolyte 10 battery cells 101 diaphragm, 102 positive electrode cell, 103 negative electrode cell 104 positive electrode, 105 negative electrode 12 positive electrode tank, 13 negative electrode tank 108,109 Outward piping, 110,111 Return piping 112,113 Pump 100 cell stacks 20. Substack 21 Supply / discharge plate 22 end plate, 23 tightening mechanism 30 Cell Frame 31 bipolar plate, 32 frame 33,34 Fluid supply manifold, 35,36 Fluid drain manifold 33s, 34s ​​Liquid supply slit, 35s, 36s Liquid drain slit 80 AC / DC converter, 81 substation equipment 90 Power system, 91 Power generation section, 92 Load

Claims

1. a positive electrode electrolyte containing a positive electrode active material and a negative electrode electrolyte containing a negative electrode active material, the amount of the positive electrode electrolyte and the amount of the negative electrode electrolyte are different, a liquid volume ratio of the positive electrode electrolyte and the negative electrode electrolyte, which is a liquid volume ratio of the electrolyte having a larger liquid volume to the electrolyte having a smaller liquid volume, is 1.05 or more and 5.0 or less; a state of charge of a mixed electrolyte obtained by mixing the positive electrode electrolyte and the negative electrode electrolyte in the same liquid volume ratio as the liquid volume ratio is 2% or more and 20% or less; The state of charge of the mixed electrolyte is When m>0, it is expressed as (m / Cp)×100[%], When m<0, it is expressed as (m / Cn)×100[%], The m is a value given by m = (Cpo vp - Cnr vn) / (vp + vn) [mol / L], Cp is the total concentration of the positive electrode active material and is expressed as Cp = Cpr + Cpo, Cn is the total concentration of the negative electrode active material and is expressed as Cn = Cnr + Cno, Cpr is the concentration (mol / L) of the reduced form of the positive electrode active material, Cpo is the concentration (mol / L) of the oxidant of the positive electrode active material, Cnr is the concentration (mol / L) of the reduced form of the negative electrode active material, Cno is the concentration (mol / L) of the oxidant of the negative electrode active material, vp is the liquid volume (L) of the positive electrode electrolyte, The vn is the liquid volume (L) of the negative electrode electrolyte. Redox flow battery.

2. a positive electrode tank that stores the positive electrode electrolyte and a negative electrode tank that stores the negative electrode electrolyte, 2. The redox flow battery according to claim 1, wherein a ratio of a volume of the tank storing the electrolyte with a larger amount of liquid to a volume of the tank storing the electrolyte with a smaller amount of liquid, of the positive electrode tank and the negative electrode tank, is 1.0 or more and 6.0 or less.

3. 3. The redox flow battery according to claim 1, wherein the positive electrode active material and the negative electrode active material are metal ions of the same element.

4. 4. The redox flow battery according to claim 3, wherein the metal ions include vanadium ions.

5. 3. The redox flow battery according to claim 1, wherein the positive electrode active material and the negative electrode active material are metal ions of different elements.

6. the positive electrode active material is at least one metal ion selected from the group consisting of iron ions, vanadium ions, and manganese ions, 6. The redox flow battery according to claim 5, wherein the negative electrode active material is at least one metal ion selected from the group consisting of zinc ions, chromium ions, vanadium ions, and titanium ions.

7. a positive electrode electrolyte containing a positive electrode active material and a negative electrode electrolyte containing a negative electrode active material, the amount of the positive electrode electrolyte and the amount of the negative electrode electrolyte are different, a liquid volume ratio of the positive electrode electrolyte and the negative electrode electrolyte, which is a liquid volume ratio of the electrolyte having a larger liquid volume to the electrolyte having a smaller liquid volume, is 1.05 or more and 5.0 or less; a state of charge of a mixed electrolyte obtained by mixing the positive electrode electrolyte and the negative electrode electrolyte in the same liquid volume ratio as the liquid volume ratio is 2% or more and 20% or less; the positive electrode active material is at least one metal ion selected from the group consisting of iron ions, vanadium ions, and manganese ions, the negative electrode active material is at least one metal ion selected from the group consisting of zinc ions, chromium ions, vanadium ions, and titanium ions, The state of charge of the mixed electrolyte is When m>0, it is expressed as (m / Cp)×100[%], When m<0, it is expressed as (m / Cn)×100[%], The m is a value given by m = (Cpo vp - Cnr vn) / (vp + vn) [mol / L], Cp is the total concentration of the positive electrode active material and is expressed as Cp = Cpr + Cpo, Cn is the total concentration of the negative electrode active material and is expressed as Cn = Cnr + Cno, Cpr is the concentration (mol / L) of the reduced form of the positive electrode active material, Cpo is the concentration (mol / L) of the oxidant of the positive electrode active material, Cnr is the concentration (mol / L) of the reduced form of the negative electrode active material, Cno is the concentration (mol / L) of the oxidant of the negative electrode active material, vp is the liquid volume (L) of the positive electrode electrolyte, The vn is the liquid volume (L) of the negative electrode electrolyte. Redox flow battery.

Images